Alkoxo and Aryloxo Derivatives of Metals Elsevier ppt

It is only since the early 1980s thatdefinitive X-ray structural elucidation has become feasible and increasingly revealing.The rapidly advancing applications11 – 16of metal alkoxides for

Alkoxo and Aryloxo Derivatives of Metals

2 - Homometallic Alkoxides, Pages 3-181

3 - Heterometallic Alkoxides, Pages 183-228

4 - X-Ray Crystal Structures of Alkoxo Metal Compounds, Pages 229-382

5 - Metal Oxo-alkoxides, Pages 383-443

6 - Metal Aryloxides, Pages 445-669

7 - Industrial Applications, Pages 671-686

Index, Pages 687-704

The value of a book may well be judged by the number of times a person has tobuy it, for, while many books once read gather dust upon a shelf, those more oftensought can sometimes be seldom found Over 20 years ago, I was fortunate to receive

a complimentary copy of “Metal Alkoxides” by Bradley, Mehrotra and Gaur As one

interested in alkoxide metal chemistry, this proved a valuable reference for me and

my research group In fact, I had to purchase two subsequent copies and probablywould have purchased more were it not for the fact that the book became out of printand unavailable except through the library Now I have received the galley proofs of

the second edition entitled “Alkoxo and Aryloxo Derivatives of Metals” by Bradley,

Mehrotra, Rothwell and Singh After 20 years, virtually every field of chemistry musthave changed to the extent that a new edition would be appropriate However, it isunlikely that any field of chemistry, save computational chemistry, will have changed

as much as that of the chemistry of metal alkoxides and aryloxides during the period1978–2000 The explosion of interest in metal alkoxides has arisen primarily for tworeasons First and foremost, we have witnessed the tremendous growth of materialschemistry spurred on by the discovery of high temperature superconducting oxidesand by the increasingly important role of other metal oxides to technology Metalalkoxides, mixed metal alkoxides and their related complexes have played an essentialrole in the development of new routes to these materials either by sol-gel or chemicalvapor deposition techniques In a second area of almost equal magnitude, we haveseen the growth of a new area of organometallic chemistry and catalysis supported

by alkoxide or aryloxide ancillary ligands As a consequence of these major changes

in chemistry, virtually any issue of a current chemistry journal will feature articlesdealing with metal alkoxides and aryloxides Thus, although the present book owes itsorigins, and to some extent its format, to the first edition, its content is largely new Forexample, while the first edition reported on but a handful of structurally characterizedmetal alkoxides, this second edition carries a whole chapter dealing with this topic, achapter with over 500 references to publications The second edition is therefore mosttimely, if not somewhat overdue, and will be a most valuable reference work for thisrapidly expanding field of chemistry I only hope that I can hold on to my copy moresuccessfully than I did in the first instance

Malcolm H Chisholm FRS

Distinguished Professor of Mathematical and Physical Sciences

The Ohio State University

Department of Chemistry

Columbus, OH 43210-1185 USA

January 2001

1 Introduction

In 1978 the book entitled “Metal Alkoxides” was published.1 It contained over onethousand references and attempted to summarize most of what was known about metalalkoxides up to that time A striking feature was the dearth of X-ray crystal struc-tures and so structural aspects necessarily involved speculation based on the results ofmolecular weight determinations, combined where possible with spectroscopic data.The intervening years have witnessed a spectacular advance in our knowledge ofthe chemistry of the metal alkoxides, a development which has been driven primarily

by research activity resulting from the realization that these compounds have greatpotential as precursors for the deposition of metal oxide films for microelectronicdevice applications and in bulk for producing new ceramic materials Simultaneously

a tremendous advance occurred in X-ray crystallography with the advent of controlled automated diffractometers and with improvements in the techniques forgrowing and mounting single crystals of the air sensitive metal alkoxides Consequentlythe number of structures solved has become so large that in this book a separate chapterwith over 500 references has been devoted to crystal structures with much of the datasummarized in tabular form In addition, considerable advances have been made in thesynthesis and characterization of a range of new alkoxides of the alkali metals, alkalineearths, yttrium and the lanthanides which together with other new developments hasled to a chapter on Homometallic Alkoxides containing well over 1000 references.Similarly the chapter on Heterometallic Alkoxides (previously described as DoubleMetal Alkoxides) has been expanded to include many novel compounds, with particularemphasis on the recently authenticated species containing two, three and even fourdifferent metals in one molecule

computer-Another area that has expanded in recent years concerns the Industrial Applications

of metal alkoxides Besides the previously mentioned deposition of metal oxides in themicroelectronic and ceramics industries there have also been major developments inthe catalytic activity of early transition metal alkoxo compounds in several importanthomogeneous reactions This has stimulated a growing interest in the mechanisms ofreactions catalysed by metal alkoxides

Metal Oxo Alkoxides are implicated as intermediates in the hydrolysis of metalalkoxides to metal oxides and their importance in the sol–gel process has led to muchresearch activity in this area Accordingly we have allocated a whole chapter to theMetal Oxo Alkoxides

In the 1978 book very little space was devoted to metal aryloxides because thisarea had received scant attention, but the intervening years have seen a resurgence ofactivity involving the synthesis and characterization of many novel compounds and

studies on their catalytic activity Therefore we have added a separate chapter dealingwith this important topic.

In this book we are giving the relevant references at the end of the seven ters rather than placing them all at the end of the text in the hope that this will bemore convenient for the reader Finally, the authors acknowledge their indebtedness toall of their former research students, postdoctoral assistants, and colleagues for theirinvaluable contributions to the research which has provided much of the informationcollected in this publication

chap-REFERENCE

1. D.C Bradley, R.C Mehrotra, and D.P Gaur, Metal Alkoxides, Academic Press, London

(1978).

2 Homometallic Alkoxides

on molecular association, volatility, chemical reactivity, and spectroscopic (IR, NMRand electronic) as well as magnetic properties It is only since the early 1980s thatdefinitive X-ray structural elucidation has become feasible and increasingly revealing.The rapidly advancing applications11 – 16of metal alkoxides for synthesis of ceramicmaterials by sol–gel/MOCVD (metallo-organic chemical vapour deposition) processes(Chapter 7) have more recently given a new impetus to intensive investigations onsynthetic, reactivity (including hydrolytic), structural, and mass-spectroscopic aspects

of oxo-alkoxide species.17 – 21

Some of the exciting developments since 1990 in metal alkoxide chemistry have beenfocussing on the synthesis and structural characterization of novel derivatives involvingspecial types of alkoxo groups such as (i) sterically demanding monodentate (OBut,OCHPri2, OCHBut2, OCMeEtPri, OCBut3) as well as multidentate (OCR0CH2OPri2)(R0DBut or CF3), OCR00

2CH2X (R00DMe or Et, X D OMe, OEt, NMe2) ligands,21 – 24

(ii) fluorinated tertiary alkoxo (OCMeCF32, OCMe2CF3, OCCF33, etc.) ties,21 – 23 and (iii) ligands containing intramolecularly coordinating substituents(OCBut2CH2PMe2, OCH2CH2X (X D OMe, OEt, OBun, NR2, PR2)).21,22 Compared

moie-to simple alkoxo groups, most of these chelating/sterically demanding ligands possessthe inherent advantages of enhancing the solubility and volatility of the products bylowering their nuclearities owing to steric factors and intramolecular coordination.Solubility and volatility are the two key properties of metal alkoxides which provideconvenient methods for their purification as well as making them suitable precursorsfor high-purity metal oxide-based ceramic materials

It is noteworthy that the homoleptic platinum group metal (Ru, Rh, Pd, Os, Ir, Pt)alkoxides are kinetically more labile possibly owing to ˇ-hydrogen elimination9,10,21

type reaction(s) (Eq 2.1):

exclude organometallic alkoxides and a considerable range of metal-organic compoundscontaining alkoxo groups, as in these systems the alkoxo groups play only a subsidiaryrole in determining the nature of the molecule

Metal alkoxides in general are highly moisture-sensitive Stringent precautions are,therefore, essential during their synthesis and handling; these involve drying of allreagents, solvents, apparatus, and the environment above the reactants and products.Provided that these precautions are taken, the preparation of metal alkoxides, althoughsometimes tedious and time consuming, is relatively straightforward

The method employed for the synthesis3,4,8,17,21 of any metal/metalloid alkoxidedepends generally on the electronegativity of the element concerned Highly elec-tropositive metals with valencies up to three (alkali metals, alkaline earth metals, andlanthanides) react directly with alcohols liberating hydrogen and forming the corre-sponding metal alkoxides The reactions of alcohols with less electropositive metalssuch as magnesium and aluminium, require a catalyst (I2 or HgCl2) for successfulsynthesis of their alkoxides The electrochemical synthesis of metal alkoxides by anodicdissolution of metals (Sc, Y, Ti, Zr, Nb, Ta, Fe, Co, Ni, Cu, Pb) and even metalloids(Si, Ge) in dry alcohols in the presence of a conducting electrolyte (e.g tetrabutylam-monium bromide) appears to offer a promising procedure (Section 2.2) of considerableutility It may be worthwhile to mention at this stage that the metal atom vapour tech-nique, which has shown exciting results in organometallics, may emerge as one of thepotential synthetic routes for metal alkoxides also in future

For the synthesis of metalloid (B, Si) alkoxides, the method generally employedconsists of the reaction of their covalent halides (usually chlorides) with an appropriatealcohol However, the replacement of chloride by the alkoxo group(s) does not appear toproceed to completion, when the central element is comparatively more electropositive

In such cases (e.g titanium, niobium, iron, lanthanides, thorium) excluding the stronglyelectropositive s-block metals, the replacement of halide could in general be pushed

to completion by the presence of bases such as ammonia, pyridine, or alkali metalalkoxides.

Another generally applicable method, particularly in the case of electronegativeelements, is the esterification of their oxyacids or oxides (acid anhydrides) with alcohols(Section 2.6), and removing the water produced in the reaction continuously

In addition to the above, alcoholysis or transesterification reactions of metal ides themselves have been widely used for obtaining the targeted homo- and heterolepticalkoxide derivatives of the same metal Since the 1960s, the replacement reactions

alkox-of metal dialkylamides with alcohols has provided a highly convenient and versatileroute (Section 2.9) for the synthesis of homoleptic alkoxides of a number of metals,particularly in their lower valency states

The metal–hydrogen and metal–carbon bond cleavage reactions have also beenexploited in some instances (Section 2.10.2)

The following pages present a brief summary of the general methods used for thesynthesis of metal and metalloid alkoxides applicable to specific systems Tables 2.1and 2.2 in Section 2.1 (pp 6–14) list some illustrative compounds along with theirpreparative routes and characterization techniques

2.1 Reactions of Metals with Alcohols (Method A)

The facility of the direct reaction of a metal with an alcohol depends on both theelectropositive nature of the metal and the ramification of the alcohol concerned

In view of the very feeble acidic character of nonfluorinated alcohols [even

are CH3OH(15.8), CH3CH2OH(15.9), CH32CHOH(17.1), CH33COH(19.2),

CF3CH2OH(12.8), CH3CF32COH(9.6), CF32CHOH(9.3), CF33COH(5.4)], thisroute is more facile with lower aliphatic and fluorinated alcohols

2.1.1 s-Block Metals

2.1.1.1 Group 1 metals (Li, Na, K, Rb, Cs)

The more electropositive alkali metals react vigorously with alcohols by replacement

of the hydroxylic hydrogen (Eq 2.2):

Table 2.1 Examples of some homoleptic alkoxides

NMR; X-ray

422 [LiOCBu t

2 CH 2 PPh 2 ] 2 Bu t

NMR; X-ray

422 [MOMe] 1

[BefOCCF 3 g2] 3 OEt 2 E-2 1 H, 19 F NMR; MW 396

[Ca-ORORthf] 2 toluene 2 E-2 IR; 1 H, 13 C NMR;

48 Ba[OCH 2 CH 2 xCH 3 ] 2

NMR; MS

345 [Y3OAm t 9HOAm t 2] I IR; 1 H, 13 C, 89 Y

NMR; MS

345

(continued overleaf )

345 [LaOR 3 ] 2

[PrfOCMeCF32g3NH32]2 I IR; MS; X-ray 349 [PrfOCMeCF 3 2g 3 NH 3 4] I IR; 1 H NMR; X-ray 349b

R D CMe3, CMe2Et, CMeEt2,

CMe2Pr n , CMe2Pr i , CEt3,

Table 2.1 (Continued )

Method of Characterization

Th 2 OBu t 8HOBu t  J-3 IR; 1 H, 13 C NMR;

MeCH2CH2CH2(and its isomers),

MeCH2CH2CH2CH2(and its

isomers)

(continued overleaf )

Table 2.1 (Continued )

Method of Characterization

CrOCHBu t

Table 2.1 (Continued )

Method of Characterization

W 4 OPr i 12/W 2 OPr i 6 Crystallization of

W 2 OPr i 6from dimethoxyethane

solvent)

IR; 1 H NMR;

UV-Vis; X-ray

351a

MS; X-ray

178 PtOCMe2CH2PPh22 E-2 IR; 1 H, 31 P NMR;

X-ray

178a PtOCMe 2 CH 2 PPh 2 .3.5H 2 O I 1 H, 31 P NMR; X-ray 178b

Group 11

[CuOCHBu t

E-3 IR; 1 H NMR; ESR n, o

CufOCHCF 3 2g PPh 3 3 J-2 IR; 1 H NMR; X-ray p

(continued overleaf )

L D tmeda, teed, bipy, (py)2

E-2, G UV-Vis; ESR; X-ray

(L D tmeda)

175 Cu[OCMeCF32]2L

L D tmeda, bipy, (py)2

E-2, G UV-Vis; ESR; X-ray

R D Pr i , Bu t , CMe 2 Et, CEt 3

I IR; 1 H, 13 C NMR;

MW; X-ray (R D Pr i ,

[BiOC 2 H 4 OMe 3 ] E-2; I IR; 1 H, 13 C NMR;

MS; X-ray, MW

205, 339 [BiOCHCF 3 2] 3 thf] 2 E-2 IR; 1 H, 19 F NMR;

Ł Lv D Latent heat of vaporization.

1 bpy D 2,2 0 -bipyridine; diglyme D bis (2-methoxyethyl) ether (ligand); dppe D 1,2-bis (diphenylphosphino)ethane, Py D pyridine (ligand); teed D N,N,N 0 ,N 0 -tetraethylethylenediamine (ligand); tmeda D N,N,N 0 ,N 0 -tetramethylethylenediamine (ligand); 2 Methods A – J (J-1 – J-7) as described in text; 3 ESR D electron spin resonance;  eff D magnetic moment; MS D mass spectrum; MW D molecular weight; UV-Vis D ultraviolet and visible.

a E.Weiss, Helv Chim Acta, 46, 2051 (1963); b E Weiss and W Bi¨ucher, Angew Chem., 75,

1116 (1963);c E Weiss, Z Anorg Allg Chem., 332, 197 (1964); d E Weiss and H Alsdorf, Z.

Anorg Allg Chem., 372, 2061 (1970); e J.E Davies, J Kopf, and E Weiss, Acta Crystallogr., 38,

2251 (1982);f E.Weiss, Angew Chem., Int Ed Engl., 32, 1501 (1993); gE Weiss, H Alsdorf,

and H K¨uhr, Angew Chem Int Ed Engl., 6, 801 (1967); h R.A Andersen, Inorg Nucl Chem.,

Lett., 15, 57 (1979); i B.D Murray, H Hope, and P.P Power, J Am Chem Soc., 107, 169

(1985); j M.H Chisholm, D.L Clark, J.C Huffman, and M Hampden-Smith, J Am Chem.

Soc., 109, 7750 (1987); kM.H Chisholm, K.Folting, C.E Hammond, M.J Hampden-Smith, and

K.G Moodley, J Am Chem Soc., 111, 5300 (1989); lB Horvath, R Moseler, and E.G Horvath,

Z Anorg Allg Chem., 449, 41 (1979); m T Greiser and E Weiss, Chem Ber., 104, 3142 (1976);

n T Tsuda, T Hashimoto, and T Saegusa, J Am Chem Soc., 94, 658 (1972); oT.H Lemmen,

G.V Goeden, J.C Huffman, R.L Geerts, and K.G Caulton, Inorg Chem., 29, 3680 (1990); pK.

Osakada, T Takizawa, M Tanaka, and T Yamamoto, J Organomet Chem., 473, 359 (1994);

q J.M Canich, G.L Gard, and J.M Shreeve, Inorg Chem., 23, 441 (1984); rN.Temple and W.

Schwarz, Z Anorg Allg Chem., 474, 157 (1981); sD.B Denny, D.Z Denny, P.T Hammond,

and Y.F Hsu, J Am Chem Soc., 103, 2340 (1981).

Table 2.2 Examples of a few selected heteroleptic alkoxides

[CeOCBu t

C Bu t OOBu t [NdOCBu t

M D Ti, Zr

R D CH2CCl3, CH2CF3, CH2CH2Cl

[WOCH2CF3 2 Cl2PMe2Ph2] WCl4PMe2Ph2 IR; 1 H, 13 C, 31 P NMR; X-ray 171

C TlOCH2CF3

elimination from Re3OPr i 9

C BunSnOMe2

1 acac D acetylacetonate; py D pyridine; thf D tetrahydrofuranligand; 2 For methods see text (Section 2);

3 For abbreviations see footnote of Table 2.1.

a C.J Burns, D.C Smith, A.P Sattelberger, and H.B Gray, Inorg Chem., 31, 3724 (1992); bM.H Chisholm,

C.E Hammond, M Hampden-Smith, J.C Huffman, and W.G Van der Sluys, Angew Chem., Int Ed Engl.,

26, 904 (1987).

reactivity sequence of alcohols, MeOH > EtOH > PriOH > ButOH, towards an alkalimetal This order of reactivity is understandable from an electronic viewpoint whichpredicts a decrease in the acidity of the hydroxyl hydrogen in the same order.

2.1.1.2 Group 2 metals (Be, Mg, Ca, Sr, Ba)

Group 2 metals, being less electropositive than group 1 metals, react sluggishly evenwith sterically compact alcohols and require a catalyst (iodine or mercury(II) chloride)particularly in cases of lighter group 2 metals (Be and Mg)30 – 34 to yield insoluble,polymeric, and nonvolatile metal dialkoxides

The reaction of magnesium with methanol had been reported26to form solvates of ferent compositions: MgOCH32.3CH3OH and MgOCH32.4CH3OH,35,36which havebeen shown by X-ray diffraction studies to have the compositions MgOCH32.2CH3OH37

dif-and MgOCH32.3.5CH3OH,38respectively

With sterically less demanding alcohols, alkoxides of the heavier alkaline earthmetals (Ca, Sr, Ba) [M(OR)2]n R D Me, Et, Pri had been prepared by a number

of workers39 – 44 by reactions of metals with alcohols These are also oligomeric orpolymeric, and nonvolatile

Interest in the synthesis and chemistry of soluble and volatile alkaline earth metalalkoxides experienced a sudden upsurge in the 1990s,21 – 23owing to the discovery ofsuperconducting ceramics45,46 containing Ba and Ca

Reactions of sterically demanding monodentate alcohols47with heavier alkaline earthmetals (M0) have been reported to yield soluble derivatives:

M0DBa; R0DCMe3,CEt3,CHMe2,CHCF32.M0DCa, Sr; R0DCCF33

By contrast, reaction of barium granules with Ph3COH does not appear to take place,even in the presence of I2 or HgCl2 as a catalyst, in refluxing tetrahydrofuran (THF)over three days However, the same reaction in the presence of ammonia as a catalystyields X-ray crystallographically characterized dimeric derivative [H3Ba6OOBut11

OCEt2CH2O(thf)3].48 It may be inferred that ammonia reacts initially with barium

to form BaNH22, which undergoes proton transfer and anion metathesis to yield thedesired alkoxide derivative

Although the reactions of heavier alkaline earth metals with alcohols are generallystraightforward, yielding the expected homoleptic derivatives, in some instances ithas been reported that the reaction follows a different course to yield an intriguingproduct as in the case of the formation of X-ray crystallographically characterized49

oxo-alkoxide cluster of the composition H3Ba6OOBut11OCEt2CH2O(thf), in thereaction of Ba with ButOH in THF The reasons for the formation of such an unusualproduct in a simple reaction of the above type (Eq 2.3) are not yet well understood,but it tends to indicate that either adventitious hydrolysis or alkene/ether eliminationmay be the main factor Furthermore, the formation of OCEt2CH2O ligated product

in this reaction indicates that the diolate ligand is probably formed in a side-reactioninvolving the solvent tetrahydrofuran molecules

2-Methoxyethanol (a chelating alcohol) has been shown50 to react with calcium

filings in refluxing n-hexane to yield an X-ray crystallographically authenticated product

according to the following reaction:

Ca C 4HOC2H4OMe!n-hexane 19[Ca9OC2H4OMe18].2HOC2H4OMe C H2 " 2.4

By contrast, a similar reaction with barium granules followed a different course51

to yield [H4Ba6OOCH2CH2OMe14] which has been characterized by single-crystalX-ray diffraction studies

Recently, it has been reported that monomeric Ba[OCH2CH2OnCH3]2(n D 2 or 3)

products are obtained in the reactions of barium granules with an oligoether alcohol52

2.1.2 Group 3 and the f-block Metals

The method involving direct reaction of a metal with alcohol was extended by

Mazdi-yasni et al 54 for the formation of scandium, yttrium, and lanthanide alkoxides usingmercuric chloride (103–104 mol per mol of metal) as a catalyst:

Ln C 3PriOH(excess)

Ln D Sc, Y, Dy, and Yb

Mercuric chloride appears to form an amalgam with the metal which reacts with

isopropyl alcohol to yield the triisopropoxide Mazdiyasni et al 54also noticed that theuse of HgCl2 in stoichiometric ratio resulted in the formation of alkenoxide contami-nated with chloride For example, the reaction of yttrium metal, isopropyl alcohol, andmercuric chloride in 1:3:4 molar ratio yielded yttrium isopropeneoxide55and hydrogenchloride:

Y C 3HOCHCH32C4HgCl2!Y[OCCH3DCH2]3C4Hg C 8HCl C12H2

2.8The above route has also been utilized for the synthesis of neodymium56 andyttrium57 alkoxides as shown by Eqs (2.9) and (2.10):

4Nd C 16PriOH ! [NdOPri3.PriOH]4C6H2" 2.9

By contrast, interesting oxo-isopropoxides of the type Ln5OOPri13, where Ln D

Sc,58 Y,58,59 Nd,60 and Yb58 have been isolated from the reaction mixtures resultingfrom the interaction of metal chips and isopropyl alcohol, out of which, the last threehave been characterized by X-ray crystallography

In restrospect, the isolation of oxo-alkoxide products, in the straightforward reactions

of metals with alcohols has generated a new interest in metal oxo-alkoxide products(Chapter 5), a large number of which have been reported in the extensive investiga-

tions of Turova et al 61 – 63 employing the solubility and vapour pressure studies of

Bradley et al 58 have suggested that the formation of Ln5OOPri13 occurs bythe mechanism of metal alkoxide decomposition involving elimination of an ether,64according to Eq (2.11):

5LnOPri3 !Ln5OOPri13CPri2O 2.11Obviously, more quantitative work is essential to explore the extent and course

of side-reactions in the interactions of metals with different alcohols under varyingexperimental conditions

2.1.3 p-Block Elements

Aluminium alkoxides may be prepared65 – 74 by reaction of an alcohol with aluminiumactivated by I2, HgCl2, or SnCl4 under refluxing conditions, for example:

2Al C 6ROH(excess)

1%HgCl2

!

where R D primary, secondary, or tertiary alkyl groups

Aluminium triethoxide was first prepared in 1881 by Gladstone and Tribe65 bythe reaction of aluminium metal with ethanol in the presence of iodine as a catalyst.Wislicenus and Kaufman66in 1893 reported an alternative method of preparing normal

as well as isomeric higher alkoxides of aluminium by reacting amalgamated aluminiumwith excess of refluxing alcohol Hillyer67 prepared aluminium trialkoxides by thereaction of metal with alcohols in the presence of SnCl4as a catalyst Tischtschenko68

in 1899, however, pointed out that the above reactions involving catalysts were usefulfor the preparation of primary and secondary alkoxides of aluminium, but the reaction

of metal with tert-butyl alcohol was very slow even in the presence of catalysts A successful synthesis of aluminium tri-tert-butoxide described by Adkins and Cox71

in 1938, involved the reaction of amalgamated aluminium with refluxing tert-butyl

Tl2O C EtOH ! TlOEt C TlOH 2.14

2.2 Electrochemical Technique (Method B)

The possibility of synthesizing metal alkoxides by the anodic dissolution of metalsinto alcohols containing conducting electrolytes was demonstrated for the first time

by Szilard76in 1906 for the methoxides of copper and lead Since then this techniquehas proved to be most promising For example, the electrochemical method for thepreparation of ethoxides of Ti, Zr, Ta, Si, and Ge77 was patented by the Monsanto

Corporation in 1972, and was later applied by Lehmkuhl et al 78 for the synthesis ofFe(II), Co, and Ni alkoxides M(OR)2 (R D Me, Et, Bun, and But)

Turova et al 79have substantially widened the scope of this technique by the synthesis

of a wide variety of homoleptic metal alkoxides and oxo-metal alkoxides: (i) soluble

M(OR)n, M D Sc, Y, La, lanthanide,80 Ti, Zr, Hf, Nb,79 Ta81 when R D Me, Et, Pri,

Bun; MO(OR)4, M D Mo, W when R D Me, Et, Pri;82 – 86 2-methoxyethoxides of Y,lanthanide, Zr, Hf, Nb, Ta, Fe(III), Co, Ni, Sn(II),87 and (ii) insoluble metal alkoxides

such as Bi(OMe)3;88Cr(OR)3, R D Me, Et, MeOC2H4;89V(OR)3;86Ni(OR)2, R D Me,

Prn, Pri;90Cu(OR)2, R D Bun, C2H4OMe;91Re4O2(OMe)16.92

Besides the above, Banait et al have also employed the electrochemical reactions

of some (including polyhydroxy) alcohols for the synthesis of alkoxides of copper93and mercury.94

In 1998, the anodic oxidation of molybdenum and tungsten95in alcohols in the ence of LiCl (as electroconductive additive) was found to yield a variety of interestingoxo-metal alkoxide complexes, some of which have been authenticated by single-crystal X-ray crystallograpy

pres-The electrode ionization reactions of alcohols and anode polarized metals in thepresence of an electroconductive additive, followed by the interaction of the generatedintermediate species and the formation of the final products can by illustrated96by thefollowing reactions (Eqs 2.16 and 2.17):

where M D anode metal and ROH D an appropriate alcohol

This process has great promise for the direct conversion of the less electropositivemetals to their alkoxides owing to its simplicity and high productivity as well as itscontinuous and non-polluting character (with hydrogen as the major by-product).The electrochemical technique appears to have been successfully employed in Russiafor the commercial production96 of alkoxides of Y, Ti, Zr, Nb, Ta, Mo, W, Cu, Ge,

Sn, and other metals

2.3 Reactions of Metal Atom Vapours with Alcohols (Method C)

Although the development of metal atom vapour technology over the past three decadeshas shown tremendous utility for the synthesis of a wide range of organometalliccompounds (many of which were inaccessible by conventional techniques),97 the use

of this technique for the synthesis of metal alkoxides and related derivatives does notappear to have been fully exploited.98In 1990, Lappert et al 99demonstrated the utility

of this technique for the synthesis of M—O—C bonded compounds by the isolation

of alkaline earth metal aryloxides

2.4 Direct Reactions of Metal Halides with Alcohols (Method D)

By far the most common synthetic technique for metal alkoxides (Eq 2.18) is thereplacement of halides from an appropriate metal halide by alkoxo groups

MClnCx C y ROH   MClnx(OR)x(ROH)yCx HCl " 2.18Halides of alkaline earth, lanthanide, actinide, and later 3d (Mn, Fe, Co, Ni) metals oninteractions with alcohols form crystalline molecular adducts like MgBr2.6MeOH,100

CaBr2.6MeOH,100 LnCl3.3PriOH101 – 103 where Ln is a lanthanide metal,ThCl4.4EtOH,104,105MCl2.2ROH (M D Mn, Fe, Co, Ni; R D Me, Et, Prn, Pri).106Apartfrom the alkaline earth metal (Ca, Sr, Ba) halides, all of these undergo alcoholysis inthe presence of a suitable base to yield the corresponding homoleptic alkoxide orchloride-alkoxide derivatives (Sections 2.5.1, 2.5.2, and 2.5.3)

Interesting variations in the extent of alcoholysis reactions of tetravalent metal (Ti,

Zr, Th, Si) chlorides may be represented107 by Eqs (2.20–2.23), to which CCl4 hasbeen added for comparison

2ZrCl4C6ROH (excess) !ZrCl2(OR)2.ROH

CZrCl3(OR).2ROH C 3HCl " 2.22

Depending on the nature of the metal (M), the initial metal chloride (MCln) or aproduct MClx y(OR)y forms an addition complex with alcohol molecules (ROH)without enough perturbation of electronic charges for the reaction to proceed further.The reactions of metal tetrachlorides MCl4 (M D Ti, Zr, Th) towards ethyl alcoholshow a gradation TiCl4>ZrCl4 >ThCl4.107

Although no clear explanation is available for the varying reactivity of different metalchlorides with alcohols, it is interesting to note that final products of similar compo-sitions have been isolated in the reactions of tetraalkoxides of these metals with HCl.For example, the reaction of TiOPri4 with HCl leads finally to TiOPri2Cl2.PriOH(Section 4.11.2)

Specific intermediate products according to Eq (2.18) may be isolated by controllingthe conditions (solvent, stoichiometry, or temperature) For example, the equimolarreaction of TiCl4 with PriOH in dichloromethane at room temperature has beenshown108 to yield the dimeric complex [TiCl3HOPri-Cl]2 The above reaction

in 2–3 molar ratios gives the dimeric complexes [TiCl2OPriHOPri-Cl]2 and[TiCl2OPriHOPri-OPri]2 as outlined in Scheme 2.1 on the basis of X-raystructures of the products

− 2HCl

Cl Ti Cl Ti O H

O H Cl Cl

Cl Cl

Cl H

O H Cl O

U C 2I2C4PriOH Pr

i OH

!UI2OPri2PriOH2C2HI 2.24

This type of reaction appears to have considerable promise for the preparation of otherpolyvalent metal–iodide–isopropoxide complexes

Out of the p-block elements, anhydrous chlorides of electronegative elementsboron,110 – 112 silicon,2,113 – 117 and phosphorus118,119 react vigorously with alcohols toyield homoleptic alkoxo derivatives [M(OR)x] (Eq 2.25) Although no detailed studieshave been made, AlCl3120,121and NbCl5122,123 undergo only partial substitution, whileGeCl4124,125 does not appear to react at all with alcohols

MClx Cx ROH ! M(OR) xCnHCl " 2.25

M D B, x D 3; Si, x D 4; P, As x D 3

The reactions indicated above occur with primary and secondary alcohols only, andhave been studied mainly with ethyl and isopropyl alcohols With a tertiary alcohol(ButOH), silicon tetrachloride yields almost quantitatively Si(OH) and ButCl.126 This

has been shown by Ridge and Todd127 to be due to facile reactivity of HCl initiallyevolved to yield ButCl and H2O, which hydrolyses SiCl4.

The reaction of AsCl3with an excess of CF3CH2OH128gives AsOCH2CF33, whichcould be oxidized with chlorine in the presence of CF3CH2OH to AsOCH2CF35 asshown by the following reaction (Eq 2.27):

AsCl3C3CF3CH2OH !

3HCl AsOCH2CF33 CCl 2 , C2HOCH 2 CF 3

!AsOCH2CF35C2HCl " 2.27

Following the earlier observations of Fischer,129Klejnot130observed that the reaction

of WCl6 with ethyl alcohol can be represented by Eqs (2.28) and (2.29):

WCl6C2C2H5OH ! WCl3(OEt)2C12Cl2C2HCl " 2.28

Chloro-alkoxo derivatives of W(V) can be prepared by the direct reactions of WCl5

with alcohols at 70ŽC.131,132 The reaction between WCl4 and the alcohols ROH(R D Me, Et) leads to the (WDW8C-containing derivatives W2Cl4(OR)4(HOR)2,133which have been characterized by X-ray crystallography

2.5 Reactions of Simple and Complex Metal Chlorides or Double Nitrates with Alcohols in the Presence of a Base (Method E)

On the basis of the earlier observations,3,4,26it appears that except for a few metal(loid)halides, most of these undergo only partial solvolysis or no solvolysis even underrefluxing conditions Thus in order to achieve the successful preparation of purehomoleptic metal alkoxides, the use of a base such as ammonia, pyridine, trialkyl-amines, and alkali metal alkoxides appears to be essential While alkali alkoxidesprovide anions by direct ionization, the role of other bases (ž

B) could be to increasethe concentration of alkoxide anions according to Eqs (2.30)–(2.32):

reac-2.5.1 The Ammonia Method (E-1)

The addition of a base, typically ammonia, to mixtures of metal(loid) halides andalcohols allows the synthesis of homoleptic alkoxides for a wide range of metals andmetalloids Anhydrous ammonia appears to have been employed for the first time byNelles134 in 1939 for the preparation of titanium tetra-alkoxides (Eq 2.33):

The ammonia method has, therefore, been successfully employed3,4,21,26 for thesynthesis of a large number of alkoxides of main-group and transition metals according

to the following general reaction (Eq 2.34):

strin-Benzene has been reported to be a good solvent for the preparation of metal alkoxides

by the ammonia method, as its presence tends to reduce the solubility of ammoniumchloride, which has a fair solubility in ammoniacal alcohols In addition, the ammo-nium chloride precipitated tends to be more crystalline under these conditions, makingfiltration easier and quicker Although most of the earlier laboratory preparations havebeen carried out in benzene, the recently emphasized carcinogenic properties of thissolvent suggests that the use of an alternative solvent should be explored

2.5.1.1 Group 3 and f-block metals

To date no unfluorinated alkoxides of scandium, yttrium, and lanthanides18,21 in thecommon C3 oxidation state appear to have been prepared by the ammonia method

By contrast, yttrium and lanthanides (Ln) fluoroalkoxide derivatives of the typesLnfOCHCF32g3138 and LnfOCHCF32g3.2NH3139 have been isolated by this route

It might appear that the ammonia method is applicable to the synthesis of a largenumber of metal alkoxides, but there are certain limitations For example, metal chlo-rides (such as LaCl3)140 tend to form a stable and insoluble ammoniate MNH3yClninstead of the corresponding homoleptic alkoxide derivative Difficulties may also arise

if the metal forms an alkoxide which has a base strength comparable with or greater thanthat of ammonia Thorium provides a good example of this type where the ammoniamethod has not been found to be entirely satisfactory.141 For example, during thepreparation of thorium tetra-alkoxides from ThCl and alcohols, Bradley et al 142could

obtain only thorium trialkoxide monochlorides owing to the partial replacement of rides These workers observed that the alcoholic solutions of Th(OEt)4 or ThOPri4

chlo-were alkaline to thymolphthalein On the other hand anhydrous ammoniacal alcoholswere acidic to this indicator Thus thorium tetra-alkoxides tend to be more basic thanammonia and the following feasible equilibria (Eqs 2.35 and 2.36) may be responsiblefor the formation of Th(OR)3Cl instead of the expected tetra-alkoxides

Bradley et al 145 had earlier reported that dipyridinium hexachlorozirconate

C5H6N2ZrCl6, which can be prepared from the commonly available ZrOCl2.8H2O,also reacted smoothly with alcohol in the presence of ammonia to form the tetra-alkoxides Zr(OR)4

During an attempt to prepare tetra-tert-alkoxides of zirconium and cerium by the

reactions of C5H6N2MCl6 (M D Zr, Ce) with tert-butyl alcohol, Bradley and

co-workers143,144 had noticed the formation of MClOBut3.2C5H5N as represented by

Eq (2.38):

C5H6N2MCl6C3ButOH C 5NH3 !MClOBut3.2C5H5N C 5NH4Cl # 2.38

As the product reacts with primary alcohols (Eq 2.39) in the presence of ammonia togive heteroleptic alkoxides, M(OR)OBut3, steric reasons have been suggested as a

possible explanation for the partial replacement reactions with tert-butyl alcohol:

MClOBut3.2C5H5N C EtOH C NH3 !M(OEt)OBut3C2C5H5N C NH4Cl #

2.39

It is, however, somewhat intriguing that dipyridinium hexachloro derivatives ofzirconium and cerium146 undergo complete replacement with Cl3C.CMe2OH, whichshould apparently be an even more sterically hindered alcohol than ButOH:

C5H6N2MCl6C4Cl3C.CMe2OH C 6NH3

Reactions of MCl4(M D Se, Te) with a variety of alcohols (MeOH, EtOH, CF3CH2OH,

ButCH2OH, Me2CHOH) in 1:4 molar ratio in THF using Et3N as a proton acceptor affordcorresponding tetra-alkoxides.146a

2.5.2 The Sodium (or Potassium) Alkoxide Method (E-2)

This procedure, sometimes referred to as transmetallation or (metathesis or elimination) reaction, is by far the most versatile synthetic method for a wide range of d-and p-block metal alkoxide complexes The alkali metal (usually sodium or potassium)alkoxide is treated in the presence of excess alcohol with the corresponding metal(loid)halide either in a hydrocarbon (generally benzene) or an ether solvent (Eq 2.41):

where M D a metal or metalloid and M0DNa or K

Although this procedure normally results in complete substitution, except for thesterically more demanding alkoxo groups, 100% synthetic predictability is not likely

to be achieved The generality and limitations of Eq 2.41 for a wide variety of elementsmay be reflected by the group-wise discussion that follows

BaI2C2KOCButCH2OPri2

2.5.2.2 Group 3 and f-block metals

The addition of a sodium (or potassium) isopropoxide to an appropriate LnCl3.3PriOH

in a medium of isopropyl alcohol and benzene results in the precipitation of NaCl(or KCl), which is removed by filtration From the filtrate, quantitative yields of[LnOPri3]x can be isolated (Eq 2.44):103,148 – 158

the products, an impressive series of structurally novel species have been obtained via

reactions of alkali metal alkoxides with the appropriate lanthanide chloride or cericammonium nitrate, as illustrated below

Interesting partial substitution reactions between yttrium trichloride and sodium

tert-butoxide in different (1:2 and 1:3) molar ratios have been reported (Eqs 2.45 and 2.46)

by Evans and co-workers:160,161

3YCl3C7NaOButTHF!Y3OBut7Cl2thf2C7NaCl #

X-ray structural determinations have shown complexes A and B to have structures

which can be represented as [Y33-OBut3-Cl2-OBut3OBut3Cl(thf)2] and[Y33-OBut3-Cl2-OBut3OBut4(thf)2], respectively

These workers further showed161 that the complex A could be converted into B by

treating with the requisite amount of NaOBut, but further reaction led to insolubleproducts:

The above findings are rather unusual and intriguing in view of the general trends ofmetal alkoxide chemistry and are somewhat at variance with the earlier findings (mainly

on isopropoxide derivatives) from the research groups of Mehrotra103,148 – 150,156 – 158

and Mazdiyasni.54,55 Also, although most of the 1:3 reactions of lanthanide ides and alkali alkoxides (mainly methoxides, ethoxides, isopropoxides and even 2-methoxyethoxides57 have been reported to be quantitative, yet a product (with incom-plete chloride substitution) had been reported159 as Nd6OPri17Cl with an interestingstructure, in the reaction of NdCl3 with three equivalents of NaOPri

trichlor-By contrast, the reaction of LaCl3 with three equivalents of NaOBut in THFwas reported160 to be straightforward, yielding the homoleptic alkoxide complex,[LaOBut3]3.2thf, as represented by Eq (2.49):

3LaCl3C9NaOButTHF!La33-OBut22-OBut3OBut4thf2C9NaCl 2.49

Starting with the readily available (NH4)2Ce(NO3)6 (CAN), synthesis of Ce(IV)alkoxides, Ce(OR)4, has been reported162,163by the reaction represented by Eq (2.50):

NH42CeNO36C4ROH C 6NaOMe ! CeOR4C6NaNO3C2NH3C6MeOH

2.50where R D Me, Et, Pri, or n-octyl.

The above convenient method was extended by Evans and co-workers164 in 1989

to the synthesis of a series of ceric tert-butoxide complexes with the general formula,

Ce(OBut)n(NO3)4n(solvent) by the reactions of CAN with NaOButin the appropriatesolvent (S D THF or ButOH):

NH42CeNO36C2 C nNaOBut

THF or Bu t OH

The reactions in THF have been reported to be cleaner with higher yields in all the

cases when n D 1–4 The formation of heterobimetallic alkoxides, Na2Ce(OR)6 andNaCe2(OR)9 was also reported, when n D 6 or 4.5, respectively.

The above novel synthesis of cerium(IV) tert-butoxide nitrate complexes appears

to be the only report using a nitrate salt as the starting material for the preparation

of tertiary butoxo derivatives It may, therefore, be worthwhile to investigate similar

routes for alkoxo (particularly t-butoxo) derivatives of other metals (especially in their

higher oxidation states) starting with their nitrate salts The chances of success in theseefforts may be higher in cases where the nitrate groups are bonded predominantly in

a monodentate manner

The X-ray crystallographically characterized165 tetrameric thorium(IV) isopropoxidecomplex, Th4(OPri)16(py)2, has been prepared in 80% yield by the (1:4) reaction ofThBr4(thf)4 with KOPri in THF followed by addition of excess pyridine (py):

By contrast, the alkali alkoxide route appears to be inapplicable for the synthesis ofzirconium tetra-alkoxides or niobium (tantalum) penta-alkoxides, as these tend to formheterobimetallic alkoxides with alkali metal alkoxides (Chapter 3, Section 3.2.1.1),which volatilize out during final purification, whereas alkali titanium alkoxides, even

if formed, dissociate readily to give volatile titanium alkoxides

The alkali alkoxide method has been extended167 to the preparation of alkoxides

of the hexanuclear niobium and tantalum cluster units, e.g., [M6X12](OMe)2.4MeOH(where M D Nb or Ta and X D Cl or Br), and M2[Ta2Cl12](OMe)6.6MeOH

The chromium(IV) alkoxides, Cr(OR)4 (where R D OBut or 1-adamantoxide), can

be prepared by the reaction of CrCl3(thf)3 with 4 equivalents of the corresponding K(or Na) alkoxide in the presence of cuprous chloride in THF:

CrCl3thf3C4MOR C CuClTHF!CrOR4C4MCl # C Cu # 2.54

The heteroleptic chloride tert-butoxide of rhenium Re3(OBut)6Cl3 was prepared by

Wilkinson et al 172by the reaction in THF of Re3(-Cl)3Cl6(thf)3 with NaOButin 1:6molar ratio:

Re3-Cl3Cl6thf3C6NaOButTHF!Re3OBut6Cl3C6NaCl # 2.56

In an attempt to prepare Re3(OPri)6Cl3, Hoffman et al 173,174 reacted Re3Cl)3Cl6(thf)3 with NaOPri in 1:6 molar ratio in THF and isolated the X-raycrystallographically characterized174 green complex Re3(-OPri)3(OPri)6.13PriOH in avery low (18%) yield Naturally the unreacted Re3(-Cl)3Cl6(thf)3 had to be removedfrom the reaction medium

(-By contrast, an isopropyl alcohol-free product Re3(-OPri)3(OPri)6has been prepared,but again in a low (31%) yield according to Eq (2.57):

Re3-Cl3Cl6thf3C9NaOPriTHF!Re3-OPri3OPri6C9NaCl # 2.57

The yield of Re3(-OPri)3(OPri)6 (Eq 2.57) could be improved (i.e from 31%

to 53%) considerably by the addition of a few drops of acetone to the solvent ofcrystallization.174

The synthesis of simple generally insoluble alkoxides, M(OR)2(M D Mn, Fe, Co, Ni,

Cu, Zn; R D Me, Et, Pri), was found not to be feasible owing to the difficulty of

sepa-rating them from the insoluble alkali metal (Na, K) chlorides (cf preparation through

LiOR in Section 2.5.3) However, a soluble copper(II) fluoroalkoxide, Cu(ORf)2(py)2

where RfDCHCF32 or C(CF3)3,175 has been synthesized as shown in Eq (2.58):

where RfDCHCF32 or C(CF3)3

In the absence of ancillary ligands such as C5H5, CO, PR3, and R2PCH2CH2PR2

(R D alkyl or aryl) there are relatively few stable platinum group metal (Ru, Rh, Pd;

Os, Ir, Pt) alkoxides9,10,21 because these metals prefer softer donor ligands (relative

to the hard (oxygen) donor alkoxo groups) and the alkoxide ligands when bonded toplatinum group metals are labile to thermal decomposition (Section 2.1), typically by

a ˇ-hydrogen elimination pathway However, with the use of some special (fluorinatedand/or donor-functionalized) type of alkoxo ligands,21 the synthesis of hydrocarbon-soluble and monomeric alkoxides of later transition metals including palladium(II) andplatinum(II) can be achieved (Eqs 2.59–2.62):

cis-R3P2MCl2C2NaOCHCF32!cis-R3P2MfOCHCH32g2C2NaCl #

2.59

M D Ni, R D Et;176 M D Pt, R D Ph176

(dppe)PtCl2C2NaOCH3 !C6H6/MeOH

slight excess (dppe)PtOCH32C2NaCl #177 (2.60)2KOH

M{OC(CF3)2CH2PPh2}2

where M D Co, Ni, Pd, Pt.10,24

PtCl2(NCBut)2+ 2 R′′2PCH2CRR′OH (i) NaOH in MeOH Pt(OCRR′CH2PR′′2)2.(H2O)x

However, NaSn2(OEt)9 on further treatment with hydrogen chloride or the alcoholate

of tin trichloride monoethoxide afforded tin tetraethoxide:

3NaSn2OEt9CSnCl3OEt.EtOH ! 7SnOEt4C3NaCl # C EtOH 2.64Interestingly, homometallic alkoxides of gallium,181 – 185 indium,186,187 german-ium,124,188tin,189 – 191lead,192 – 194arsenic,195 – 198antimony,195 – 197,199 – 201bismuth,202 – 206and tellurium207,208can easily be prepared according to the general reaction (Eq 2.65):

where M D metal(loid) of groups 13, 14, 15, and 16; X D halide (usually chloride);

M0DNa or K; R D a simple or substituted alkyl group; x D valency of the metal(loid).

The tendency for the formation of heterobimetallic alkoxides with alkalimetals,179,180,195 has generally restricted the applicability of this procedure for thesynthesis of homometallic alkoxides of many of these metals, except in cases where theheterobimetallic alkoxides are thermally labile and dissociate to yield the correspondingvolatile homometallic alkoxides of p-block metals However, under suitable conditions,when addition of an excess of alkali metal alkoxide is avoided the chances of theformation of binary alkoxides in an excellent yield is high

In spite of the above difficulties, this route has been found convenient even forthe synthesis of heteroleptic alkoxides such as M(OBut)Cl (M D Ge, Sn)209 andAsCl(OEt)2.195

The stoichiometric reactions of alkyltin chlorides with sodium alkoxides generallyyield the alkyltin alkoxide derivatives The method was finally extended to the prepa-ration of BunSn(OR)3,210,211 in spite of earlier unsuccessful attempts:212

BunSnCl3C3NaOR!benzene

isopropyl alcohol

2.5.3 The Lithium Alkoxide Method (E-3)

2.5.3.1 Synthesis of insoluble metal alkoxides (usually methoxides)

Although ammonia and sodium (potassium) alkoxides appear to be convenientlyemployed as proton acceptors for the preparation of metal alkoxides, the method is

not conveniently applicable for the synthesis of insoluble metal methoxides, owing todifficulties in their separation from insoluble ammonium or/and alkali halides A new

method was, therefore, developed by Gilman et al 213 for the preparation of insolubleuranium tetramethoxide by the reaction of uranium tetrachloride with lithium methoxide

in methanol:

The advantage of this method over the others described in the preceding sections

is that the lithium chloride is soluble in methanol, making the separation of insolublemethoxides easily possible The method was later extended by other workers for thepreparation of methoxides of beryllium,214 lanthanum,215 uranium,216 neptunium,217

vanadium,218 iron,219,220 and copper.221

By contrast, pure binary methoxide of zinc222 and ethoxides223 of zinc, cadium,and mercury could not be isolated by the lithium alkoxide method possibly becauseheterobimetallic alkoxide complexes were formed

Interestingly, Talalaeva et al 224 have prepared Zn(OBut)2 by the reaction of zinc

chloride with lithium tert-butoxide in ether solution.

The lithium alkoxide method has been extensively exploited,7,225 by Mehrotra andco-workers for the synthesis of insoluble alkoxides of later transition metals (Eqs 2.68and 2.69)

CrCl3thf3C3LiORROH/C6H!6 CrOR3 # C3LiCl 2.68

where R D Me, Et, Bun; when R D But, pure product is obtained only when free

tert-butyl alcohol is carefully excluded.

MCl2C2LiORROH/C6H!6 MOR2 # C2LiCl 2.69

(M D Co, R D Me, Et, or Pri; M D Ni, R D Me, Et, Prn, Pri, Bus, But, t-C5H11, or

t-C6H13; M D Cu, R D Me, Et, Pri, or But)

2.5.3.2 Synthesis of soluble metal alkoxides

The sterically compact metal alkoxides prepared by the lithium alkoxide methodare generally insoluble and nonvolatile, owing to the formation of oligomers orpolymers involving alkoxo bridging Minimizing molecular oligomerization, and hencelattice cohesive energies, by saturating the metal coordination sphere with stericallyencumbered (mono- and/or multi-dentate) or halogenated (preferably fluorinated)and/or donor-functionalized alkoxo ligands is an attractive strategy for the design andsynthesis of hydrocarbon-soluble and volatile derivatives (Table 2.1) In this contextthe use of lithium derivatives of such ligands,21 which are conveniently prepared bythe interaction of an alcohol with butyllithium, has played an important role Forexample, the reactions of LiOR (R D a sterically demanding alkyl, halogenated alkyl,

or alkyl with donor functionalities) with metal halides yield derivatives generallywith unprecedented structural and reactivity features The synthesis of such solublederivatives is generally carried out in Et2O or THF solvents, in which LiCl tends to

be precipitated (Eqs 2.70–2.74):

x D 3, L D THF, n D 0, y D 0, m D 1; M D U,228 R D CHBut2, x D 4, L D THF,

n D 0, y D 0, m D 1).

The above reactions when carried out in 1:4 molar ratio with CrCl3(thf)3and FeCl3226

result in heterobimetallic alkoxide complexes, respectively of the types OCHBut2)2Cr(OCHBut2)2] and [(HOCBut2)Li(-OCHBut2)2Fe(OCHBut2)2]

Mo2Cl6dme C 6LiORDME!Mo2OR6C6LiCl # C DME 2.74

where R D But, CMe2Et.170

2.6 Preparation of Alkoxide Derivatives from Metal(loid) Hydroxides and Oxides (Method F)

Hydroxides and oxides of non-metals behave as oxyacids or acid anhydrides and fore react with alcohols to form esters (alkoxides of these non-metallic elements) andwater:

In view of the reversible nature of the reactions, a continuous removal of waterduring these reactions is necessary to yield the final alkoxide products This has beenaccomplished by fractionating out water with organic solvents (benzene, toluene, andxylene) using a Dean–Stark assembly The preparation of ethoxides by this methodhas an additional advantage that ethanol forms a minimum boiling ternary azeotrope(water–ethanol–benzene) which helps in the fractionation of water.

The above technique has been quite successful for the synthesis of alkoxides of a

number of s- and p-block metals and metalloids such as sodium,235,236 boron,237,238

thallium,239,240 silicon,241 – 243and arsenic.244 – 246

Although synthesis of VO(OR)3by the reaction of V2O5with alcohols was described

as early as 1913,247 only a few more vanadium oxo-alkoxides248,249 appear to havebeen synthesized as shown by Eqs (2.77) and (2.78):

In view of the importance of alkoxysilanes and alkoxysiloxanes as precursors forglasses and ceramic materials, a process of obtaining these from portland cementand silicate minerals was described in 1990.251 Under the mild reaction conditionsemployed, the silicon–oxygen framework in the original mineral tends to be retained

in the final alkoxysilane or alkoxysiloxane obtained, e.g.:

Ca3SiO4O C EtOH !

tricalcium silicate(based on SiO4 framework)

HO4Si.x EtOHHC!SiOEt4 (2.79)

Preparation of organometal alkoxides of germanium,253 – 255tin,256 – 266and lead267,268

have also been described by the reactions of an appropriate organometal oxide orhydroxide with alcohols The reaction between an organometal oxide and a dialkyl

carbonate has also been found to yield the corresponding organometal alkoxides oftin261 and mercury.269

2.7 Alcohol Interchange (or Alcoholysis) Reactions of Metal(loid)

Alkoxides (Method G)

One of the characteristic features of metal(loid) alkoxides is their ability to exchangealkoxo groups with alcohols, and this has been widely exploited for the synthesis of new

homo- and heteroleptic alkoxide derivatives of various s- , d- , f- , and p-block elements

such as beryllium,219 yttrium,103,148 titanium,270 – 272 zirconium,135,273 – 276 hafnium,277vanadium,249,278 niobium,279 – 281 tantalum,280 iron,282,283 copper,7,284,285 zinc,222cerium,286,287 praseodymium,151 – 153 neodymium,151 – 153 samarium,155 gadolin-ium,103,156,157 erbium,103,156,157 ytterbium,103,148 thorium,141,287,288 uranium,289 – 291

boron,111,237,292 aluminium,293,294 gallium,184,185 indium,186 germanium,113,125,295

tin,296 – 300antimony,301and tellurium208,302according to the general reaction (Eq 2.82):

MORx CnR0OH   MORx nOR0n CnROH " 2.82

If the alcohol R0OH has a higher boiling point than ROH, then the desired productcan be easily obtained by shifting the equilibrium of Eq (2.82) by removing ROH(preferably as an azeotrope with benzene) by fractional distillation

Many of the final products (particularly those containing sterically congested andchelating alkoxo ligands) prepared by this route assume special significance because

of their reduced molecularity, enhanced solubility (in organic solvents), and volatility

as well as novelty in structural features (Chapter 4)

There are three important factors that influence the extent of substitution303,304

in alcoholysis reactions: (i) the steric demands of the alkoxo groups (OR and OR0)involved, (ii) the relative O—H bond energies of the reactant and product alcohols,and (iii) the relative bond strengths of the metal–alkoxo bonds of the reactant andproduct alkoxides, and, in view of their wide applicability, it may be appropriate todiscuss briefly the general conditions which govern such equilibria (Eq 2.82) employedfor synthetic purposes

2.7.1 Interchangeability of Different Alkoxy Groups

In general, the facility for interchange of alkoxy groups increases from tertiary tosecondary to primary groups Verma and Mehrotra270 tried to determine the extent ofsuch equilibria in the case of titanium alkoxides, Ti(OR)4and found the following order

in the interchangeability of alkoxo groups in alcoholysis reactions: MeO >EtO>

PriO >ButO

Such an alcoholysis reaction from a more branched alkoxide to a less branchedalkoxide is sometimes facilitated even further, if the product is significantly moreassociated than the reactant alkoxide For example, Mehrotra305 has shown that the

reactions of monomeric zirconium tetra-tert -butoxide with methanol and ethanol in

lower stoichiometric ratios are highly exothermic, resulting in almost instantaneouscrystallization of the dimeric mixed alkoxide products:

2.7.2 Steric Factors

The high rate of alcohol interchange has some interesting mechanistic implicationsespecially in view of the strong (100–110 kcal mol1) metal–oxygen bonds306,307 intitanium alkoxides The presence of vacant d-orbitals in most of the metals offers afacile initial step in a nucleophilic attack (SN2) of an alcohol molecule on the metalalkoxide (Eq 2.84), and thus a low activation energy for alcohol interchange involving

a four-membered cyclic transition state seems reasonable:

R′OM

ORRO

an attempt to measure the kinetics of alcohol interchange in titanium and zirconiumalkoxides, Bradley3observed that an equilibrium was established between the reactantswhen they were mixed at room temperature This was later confirmed by 1H NMRspectra which indicated that a mixture of titanium tetraethoxide and ethanol gave onlyone type of ethoxy signal, thus indicating the rapidity of the exchange of ethoxy groups

A similar observation has been made by Mehrotra and Gaur308who found only one type

of isopropyl protons in the 1H NMR spectrum of titanium dibromide diisopropoxide

in isopropyl alcohol solution indicating a dynamic equilibrium (Section 3.4) On the

other hand, a mixture of titanium tert -butoxide and tert -butyl alcohol showed different

types of methyl signals indicating that the rate of exchange is slow, which may beascribed to steric factors.26

2.7.3 Fractionation of More Volatile Product

Even in cases where reactions are rather slow, the equilibrium can be pushed tocompletion if the alcohol produced in the reaction is continuously fractionated out.For example, alcoholysis of aluminium isopropoxide with primary as well as secondarybutyl alcohols can be completed by fractionating out the isopropyl alcohol produced:293

AlOPri3C3BunOH ! AlOBun3C3PriOH " 2.85

However, in the case of tert -butyl alcohol even after careful fractionation, a maximum

of only two isopropoxy groups per aluminium atom appears to be replaced:293

AlOPri3C2ButOH ! 12[AlOBut2OPri]2C2PriOH " 2.86

As the product Al(OPri)(OBut)2 is found to be dimeric it was suggested that

alumi-nium atoms on being surrounded by bulky isopropoxy and tert -butoxy groups in a

structure of the type (2-I), are shielded so effectively that the lone pair orbital of

the oxygen atom of another tert -butyl alcohol molecule cannot approach sufficiently

close to the ‘d’ orbitals of aluminium for the interaction to be initiated A finer ence in susceptibilities to steric factors was further demonstrated by the fact that withaluminium ethoxide, some further (albeit extremely slow) replacement was possible,

resulting in a final product of the composition Al2(OEt)(OBut)5 It appears, fore, that the corresponding [Al(OEt)(OBut)2]2 may also have (2-II) as one possiblestructure,293 in which further replacement at the less shielded of the two aluminiumatoms may lead to further reaction leading to a structure of the type (2-III)

there-AlO

ButO

ButO

EtAl

ButO

ButO

EtAl

OBut

But

(2-III)

2.7.4 Fractionation in the Presence of an Inert Solvent

In order to complete an alcoholysis reaction, an excess of the higher boiling reactantalcohol would be required, so that the lower boiling alcohol could be fractionated out;this can sometimes be achieved by use of a solvent which is higher boiling than eventhe reactant as well as the product alcohols When the original alkoxide is ethoxide orisopropoxide, the use of an inert solvent such as benzene offers an added advantage

by virtue of the formation of a lower boiling azeotrope with ethyl alcohol or isopropylalcohol, which facilitates the removal of the liberated alcohol by fractional distillation.For example, zirconium isopropoxide on treatment with alcohols was converted to purealkoxides by the removal of the isopropyl alcohol–benzene azeotrope continuously on

a fractionating column:135

Zr(OPri4.PriOH C x ROH ! Zr(OPri4xORx Cx C1PriOH " 2.87

Mehrotra4,308 – 310 observed that benzene is a good solvent for the alcoholysis tions for the following reasons:

reac-(a) The reactant alcohol can be used economically since only the stoichiometricamount is required for complete replacement

(b) When the reaction is carried out with a higher boiling alcohol, the refluxingtemperature can be lowered in the presence of benzene and the side reactions arethus minimized.

(c) The technique has the added advantage that it can be used with different metric ratios of the reactants Thus the mixed alkoxides of many elements havebeen prepared

stoichio-(d) A simple oxidimetric method developed for estimation of ethyl alcohol orisopropyl alcohol present in the azeotrope makes it possible to follow the progress

of the reaction quantitatively

2.7.5 Solubility Factor

The solubility factor has been found to be of some use for the synthesis ofinsoluble alkoxides For example, when titanium ethoxide or isopropoxide is treatedwith methanol, an instantaneous reaction occurs with the separation of insolublemethoxide.270The fractionation process is not necessary in cases where such insolublederivatives are obtained

The insoluble methoxides are generally not preferred as starting materials for holysis reactions, because they requires a longer refluxing period for completion of the

alco-reaction Bradley et al 274 and Mehrotra,305 however, made the interesting observation

that zirconium methoxide, after prolonged refluxing with excess tert-butyl alcohol274

or tert-amyl alcohol305 in the presence of benzene, finally yielded monomethoxide

tri-tert-alkoxide only:

Zr(OMe)4C3RtOH

(excess)

!Zr(OMe)(ORt3C3MeOH " 2.88

where RtDtert -butyl or tert-amyl.

The final product was found to be a stable dimeric species [RtO3OMe2ZrORt3], which did not undergo further alcoholysis for steric reasons

Zr-In the light of the above factors, the alcoholysis reactions of a few metal ides may be briefly summarized In the alcoholysis reactions of boron alkoxideswith primary alcohols, Mehrotra and Srivastava292 observed that reactions proceed to

alkox-completion conveniently, but with tertiary alcohols, (tert-butyl alcohol) mono-alkoxy di-tert-butoxide was the final product The non-replaceability of the last alkoxy group with tert-butyl alcohol was ascribed to steric factors It was observed that the reaction

was fast in the beginning but slowed down at the later stages after the formation ofthe mixed alkoxide Presumably steric hindrance of the mixed alkoxide B(OR)(OBut)2

prevented the close approach of another molecule of tert-butyl alcohol.

The alcoholysis reactions of tin(IV) alkoxides are comparatively faster than those ofthe silicon and germanium analogues and proceed to completion without any catalyst.Bradley26thus prepared a number of primary, secondary and tertiary alkoxides by thealcoholysis reactions of tin tetraisopropoxide isopropanolate with various alcohols inthe presence of benzene (Eq 2.89):

Sn(OPri4.PriOH C 4ROH ! Sn(OR)4C5PriOH " 2.89However, in the alcoholysis reaction of tin tetraisopropoxide isopropanolate

with tert-heptyl alcohol in refluxing toluene or benzene, Gupta311 could isolate

only monoisopropoxide tri-tert-heptyloxide (Eq 2.90) which on distillation in vacuo disproportionated into tetraisopropoxide and tetra-tert-heptyloxide.

Sn(OPri4.PriOH C 3Me2BuCOH ! Sn(OPriOCBuMe23C4PriOH " 2.90

In attempts to prepare tantalum pentaisopropoxide and penta-tert-butoxide by alcoholysis of tantalum pentamethoxide with isopropyl alcohol or tert-butyl alcohol, respectively, Bradley et al 312 could isolate only monomethoxide tetra-isopropoxide or

tetra-tert-butoxide:

Ta(OMe)5C4PriOH(ButOH ! Ta(OMe)(OPri/OBut4C4MeOH " 2.91Similarly, the reaction of niobium pentaethoxide with isopropyl alcohol yieldedmonoethoxide tetra-isopropoxide:312

Nb(OEt)5C4PriOH ! Nb(OEt)(OPri4C4EtOH " 2.92

In contrast to earlier transition metals, some of the later 3d metal (Ni and Co)alkoxides exhibit an interesting variation in their alcoholysis reactions; for example,secondary and tertiary alkoxides of these metals undergo facile alcoholysis with primaryalcohols, whereas their primary alkoxides do not appear to undergo alcoholysis withtertiary, secondary, or even other primary alcohols.7,225

The alcoholysis reactions of tetra-alkoxysilanes, Si(OR)4, are generally very slowand the reactions of Si(OEt)4 with tertiary alcohols were unsuccessful even in thepresence of a variety of catalysts,313,314 whereas similar reactions with germaniumanalogues125,293,315 are quite facile It may be mentioned that compared to the tetra-alkoxysilanes, the alkylalkoxysilanes appear to offer less steric hindrance to alcoholysisreactions, which have been carried out successfully in a number of cases with the

help of catalysts such as sodium, p-toluene sulphonic acid, hydrogen chloride, and

sulphuric acid.316 In the alcoholysis reactions of dialkylgermanium dialkoxides with

primary, secondary, and tertiary alcohols, Mathur et al 317,318 observed that reactionswith primary alcohols proceed easily but those with secondary and tertiary alcohols

are completed only in the presence of p-toluene sulphonic acid as a catalyst.

The alcoholysis reactions of tellurium isopropoxide with primary, secondary andtertiary alcohols readily yielded the corresponding tetra-alkoxides:302

On the other hand when diethyl selenite reacted with primary alcohols it gave dialkylselenite by alcohol interchange but with secondary and tertiary alcohols only partialreplacement was observed.319

The differences in solubility of metal isopropoxides and their alcoholates haveled to the crystallization of pure isopropoxide isopropanolates of some tetravalentelements For example, zirconium tetraisopropoxide is a viscous supercooled liquidwhich dissolves in excess of isopropyl alcohol and crystallizes out in the form of purewhite crystals which were characterized as Zr(OPri)4.PriOH.135 Similarly tin tetraiso-propoxide and cerium tetraisopropoxide have been obtained as crystalline alcoholates,Sn(OPri4.PriOH320 and Ce(OPri4.PriOH287 which were used as the starting materialsfor the synthesis of a number of new alkoxides

2.8 Transesterification Reactions of Metal Alkoxides (Method H)

As early as 1938, Baker showed that aluminium alkoxides undergo facile esterification reactions, which can be represented by Eq (2.94):

trans-Al(OR)3C3CH3COOR0  Al(OR 0

where R0Da primary or secondary alkyl group

He also observed that the reactions of Al(OEt)3 with CH3COOBut yielded only theheteroleptic alkoxide, Al(OEt)(OBut)2

Mehrotra293 in 1954 confirmed Baker’s observation and extended the above dure for the preparation321 of higher alkoxides of titanium, zirconium, and hafnium;the alcoholysis of the convenient starting alkoxide, Zr(OPri)4.PriOH, with ButOH wasextremely slow owing to the small difference in the boiling points (in parentheses) of

proce-PriOH (82ŽC) and ButOH (83ŽC), whereas the much larger difference in the boilingpoints of CH3COOPri(89ŽC) and CH3COOBut(98ŽC) facilitated the reaction consid-erably, providing the first convenient method for the preparation of Zr(OBut4:

vanadium,322 niobium,323 tantalum,324 iron,283 and gallium.184,185

Following the transesterification technique, Bradley and Thomas325,326extended thetechnique for a more convenient preparation of trialkylsiloxides of titanium, zirconium,and other metals:

M(OPri4C4R3SiOCOCH3!M(OSiR34C4CH3COOPri" 2.97

Transesterification reactions have the following advantages over alcoholysis tions:

reac-(a) The tert-butoxide derivatives of elements can be easily prepared from the

corre-sponding isopropoxides as there is a significant difference in the boiling points

of their organic esters (¾9ŽC) compared with the corresponding small difference

in the boiling points of the two alcohols (¾0.2ŽC); this makes the fractionation

of the more volatile ester much easier

(b) In some cases the esters (e.g silyl acetate) are much more stable than the

corresponding alcohols (silanol), which sometimes undergo self-condensation

(siloxane) or which are oxidized more readily (e.g in the preparation of higher

vanadyl alkoxides from the ethoxide)

(c) The method appears to be less prone to steric factors compared with the ysis technique, and hence tertiary alkoxides may easily be obtained

alcohol-The utility of transesterification reactions has been extended considerably by the use

of an inert solvent like cyclohexane (b.p 80.8ŽC) which forms a convenient azeotropewith ethyl acetate (b.p 72.8ŽC) and isopropyl acetate (b.p 78.9ŽC) Using this modifiedtechnique, it is now possible to prepare mixed alkoxides also by taking the reactants

in the desired stoichiometric ratios:

As stated earlier, the method has been extended for the preparation of alkoxides andmixed alkoxides of a large number of metals However, Mehrotra and Srivastava327made the interesting observation that boron alkoxides do not appear to undergo trans-esterification reactions at all, although they do undergo alcoholysis reactions

There has been some conjecture about the mechanism of these transesterificationreactions The most obvious mode of reaction could be represented by Eq (2.99) inwhich the alkoxy oxygen atom of the ester molecule coordinates to the metal atom:

OROR

ROMRO

OR

ROMRO

OR

OR′

R′ OC

H3CO

Alternatively, the coordination may occur through the carbonyl oxygen atom, givingthe metal atom a negative and the carbonyl carbon atom a positive charge, and thistype of transition state would probably take the route represented by Eq (2.100):

ROM

OR

CH3COOR +

COMO

R′O

H3C

OROROROR

ROM

ORC

OR

OR′

H3C

ROM

OR

COMORO

H3C

OROR

R′OR

In view of the importance of alkoxysilanes and alkoxysiloxanes as precursors forglasses and ceramic materials, a process of obtaining these from portland cementand silicate