Organometallic clusters as precursors of metallic nanoparticles 3

Chapter 2 Deposition of osmium clusters onto inorganic oxide surfaces Over the past two decades or so there has been sustained interest in the synthesis, characterization and chemistry

Chapter 2 Deposition of osmium clusters onto inorganic oxide surfaces

Over the past two decades or so there has been sustained interest in the synthesis, characterization and chemistry of transition metal clusters, which provide an opportunity to examine the evolution of properties associated with the bulk state.1Metal clusters have found many important applications in homo and heterogeneous catalysis.2 One example is their use as precursors for the deposition of particularly small and well-defined homonuclear or heteronuclear metal particles on oxide supports.3 In addition, clusters can act as valuable models for the structure and reactivity of reagents and fragments that are bound to metal surfaces during reactions Direct interaction of carbonyl clusters with high-surface-area oxides can give rise to chemisorbed derivatives, and in most cases anchoring of osmium clusters to inorganic oxides can been studied by comparison of their IR spectra with known osmium clusters Most often, cluster attachment has been effected by simple ligand association and ligand exchange

Attachment of osmium clusters with inorganic oxide surfaces proceeds through reactions with the groups on the surfaces The surface species Os3(CO)10(μ-H)(μ-OSi≡) (where Si≡ represents surface) from the reaction between Os3(CO)124 Os3(CO) (NCCH ) ,10 3 2 5 or Os3(CO) (μ-H)10 26 with silica support has been confirmed by many characterization methods, such as IR, Raman, XPS, EXAFS, UV-visible spectroscopies and TEM Oxidative fragmentation of the clusters Os3(CO) adsorbed on MgO powder was investigated by X-ray absorption 12spectroscopy and scanning transmission electron microscopy (STEM) Exposure of the clusters to air leads to their fragmentation, oxidation of the osmium, and formation

of ensembles consisting of three Os atoms.7 [Os5 14

supported species were characterized by IR, C NMR, and EXAFS spectroscopies.13 8The attachment of osmium clusters on functionalized silica support (ligand used =

PCH CH Si(OEt) (OSi≡), HS(CH ) Si(OMe) (OSi≡))or the interaction between Os (CO) (μ-H)[μ-S(CH ) Si(OMe) ] with silica gel have been well studied using IR spectroscopy.9 Bimetallic catalysts of gold-osmium supported on silica have also been studied.10 Some of these supported osmium clusters with well-defined structures are listed in Table 2.1

Table 2.1 Supported osmium clusters with well-defined structures on functionalized silica surface

Metal framework

composition

Support Surface structures Characterization method

Os3 Ph2P-Si≡ H2Os3(CO)9(Ph2P-Si≡) IR

Os3 Ph2P(CH2)2-Si≡ Os3(CO)9(μ-Cl)PPh2(CH2)-Si≡ IR

AuOs3 Ph2P(CH2)2-Si≡

Ph 2 P(CH 2 ) 2 -Si≡

HAuOs3(CO)10Ph2P(CH2)-Si≡

ClAuOs 3 (CO) 10 Ph 2 P(CH 2 )-Si≡

IR

Os3 HS-Si≡ HOs3(CO)10S(CH2)3-Si≡ IR

Os3 SiO2 HOs3(CO)10-O-Si≡ IR, Raman, TEM, XPS,

Os3(CO)10(μ-OR)2, respectively;11 and with thiols and carboxylic acids to form hydridothiolato or hydridocarboxylato derivatives, respectively.12 The reaction between Os3(CO)10(NCCH3)2 and dicarboxylic acids afforded linked clusters with the general formula [{Os3(CO)10(μ-H)}2(μ-L’)].13 With HSCH2CH2SH or diols, only clusters with one triosmium moiety, viz., Os3(CO)10(μ-H)(μ-SCH2CH2SH) or Os3(CO)10(μ-H)[μ-O(CH2)nOH], were obtained respectively However, the reaction of Ru3(CO)12 with HSCH2COOH gave only Ru3(CO)10(μ-H)(μ-SCH2COOH).14

We have found that the reaction of 1 in refluxing toluene, or 2 at ambient

temperature in dichloromethane, with bifunctional ligands afforded two types of clusters (Scheme 2.1 and 2.2, respectively) The major product is of the general formula Os3(CO)10(μ-H)(μ-L ͡ L’H)(type ‘a’) In some cases, a minor product of the general formula Os3(CO)10(μ-H)(μ-L ͡ L’)Os3(CO)10(μ-H) (type ‘b’) was also

obtained The reaction of 1 with 2,5-dimercapto-1,3,4-thiadiazol afforded a complex

with a more complex bonding mode, viz Os3(CO)10(μ-H)(μ-SC=NNCSS-μ3, η2) Os3(CO)9(μ-H), 8

HL L'H

HS(CH2)3SH SH HS

CH2SH HS(CH2)8SH HSCH2CH2OH HSCH2CH2CH2OH

L' Os

L

type 'b'

or +

L' Os

Os Os H

L

type 'b' or

Scheme 2.2

With the exception of 8, 15b, 17b and 18b, the products show similar patterns in

the carbonyl region of their IR spectra, which are comparable to those of other triosmium clusters of the type Os3(CO)10(μ-H)(μ-SR), indicating that the metal core

of these products are structurally similar; that for 17a is shown in Figure 2.1 The

similar patterns of the IR spectra suggest that the anchoring of the organic fragments has little structural effect on the triosmium framework In the case of products containing two triosmium moieties bridging by thiolato-carboxylato ligands, more complicated patterns are observed Their IR spectra can be regarded as the overlap of that due to a thiolato-bridged Os3(CO)10(μ-H) moiety, which has the highest energy band at 2108-2110 cm-1, and a carboxylato-bridged Os3(CO)10(μ-H) moiety, which has the highest energy band at 2113-2114 cm-1

In the 1H NMR spectra, the thiolato bridged osmium clusters exhibit a hydride resonance at about -17 ppm For carboxylato bridged clusters, this appears at about -10 ppm, and for alkoxy bridged clusters at about -12 ppm There is also an upfield shift in the hydride resonances of osmium clusters containing an aromatic ring compared to the alkyl analogues about 0.3-0.7 ppm for thiolato bridged clusters, and about 0.3 ppm for carboxylato bridged clusters The resonances for organic fragments nearer to the clusters are also shifted upfield compared to the free ligands This suggests that the clusters exert some electronic effect on the organic substrates

1960 1970 1980 1990 2000 2020 2040 2060 2080 2100 2120 2140 2160

1/cm 20

40 60 80

Whether products containing one or two triosmium moieties (type ‘a’) are formed depend on the nature of the ligand The methylene chain length of the ligands has a strong influence A longer methylene chain favors a product containing two triosmium moieties while a shorter chain favors the formation of a product containing one triosmium moiety An aromatic ring spacer also favors the formation of a product containing two triosmium moieties Furthermore, propensity of the functionalities to bond to osmium follows the order SH > COOH > OH

+ HS COOH Ru

14a(Ru) (31 %)

15a(Ru) (61 %)

16a(Ru) (27 %) 17a(Ru) (43 %) 18a(Ru) (80 %)

Scheme 2.3

In the reactions with Ru3(CO)12, 3, only products of type ‘a’ moiety were formed (Scheme 2.3) This showed that the metal has an important influence on the product formed The IR spectra of the products also showed patterns in the metal carbonyl region that are similar to the type ‘a’ osmium clusters, and the metal hydride resonances are at ~ -15 ppm, which indicated that it is the SH which has reacted with

the triruthenium core.14

Os

Os

(2)(3)(1)

Table 2.2 Selected bond lengths (Å) and bond angles (o) for 5a, 5b, 9a, 13a, 14a and

Bond angles (o)

Os(1)-S(1)-Os(2) 72.63(3) 72.47(5) 73.46(4) 72.77(5) 72.60(8) 73.00(5)

Five clusters of type ‘a’ (5a, 5b, 9a, 13a, 14a, and 15a) and two of type ‘b’ (5b and

15b), together with 8, were characterized crystallographically The molecular

structures of 5a and 5b are shown in Figures 2.2 and 2.3 respectively The overall structure of 5b is very similar to that of the methanedithiolate osmium analog of

[Os3(CO)10(μ-H)]2(μ-η2-SCH2S) and propanedithiolato osmium/ruthenium analogs of [M3(CO)10(μ-H)]2(μ-η2-SCH2CH2CH2S) (M = Os, Ru).13b, 13e Structurally, 5b is equivalent to two moieties of 5a and hence it is discussed together with the other type

‘a’ clusters Selected bond parameters and a common atomic number scheme for 5a,

5b, 9a, 13a, 14a, and 15a are shown in Table 2.2 In all these structures, the Os-S

distances [2.399(3)-2.4185 Å] is similar to reported values The Os-Os distances

[2.8347(4)-2.8694(4) Å] are also similar to reported values for μ-S bridging clusters However, it is interesting to note that the latter does not show any trend with respect

to the other Os-Os bond lengths

Figure 2.2 ORTEP diagram for 5a; thermal ellipsoids were drawn at the 50%

probability level and aromatic hydrogens have been omitted

igure 2.3 ORTEP diagram for 5b; thermal ellipsoids were drawn at the 50%

robability level

F

p

Compound 15b is a type ‘b’ cluster in which L ≠ L’ The molecular structure of ompound 15b and selected bond parameters are given in Figure 2.4 The Os-Os bond

he same unit [2.9244(10), 2.8659(7), 2.8758(8) Å] This effect is ell-known and is sensitive to the other donor atoms coordinated to the osmium

% : Os(1)-Os(2)

Figure 2.4 ORTEP diagram for 15b, with thermal ellipsoids drawn at the 50

probability level Selected bond lengths (Å) and bond angles (o) for 15

2.8708(7), Os(2)-Os(3) 2.8746(8), Os(3)-Os(1) 2.8450(7), Os(4)-Os(5) 2.9244(10)Os(5)-Os(6) 2.8659(7), Os(6)-Os(4) 2.8758(8), Os(1)-S(1) 2.418(3), Os(2)-S(1) 2.426(3)

Os(1)-S(1)-Os(2) 72.69(8)

The structure of 8 is shown below (Figure 2.5) It is structurally similar to 5b,

except that an additional nitrogen atom has also coordinated to one of the triosmium

moiety in place of a carbonyl The Os(1)-Os(2) distance [2.8067 Å] in this particular unit is shorter compared to the others in the same cluster [2.8413(9)-2.8503(10) Å], and the thiolato-bridged Os-Os bond length is similar to those of structurally similar clusters such as Os3(CO)9(μ-H)(μ-pyS) [2.874(1) Å] and [{Os3(CO)9(μ-H)}(μ-SC5H3NCO2){Os3(CO)10(μ-H)}] [2.853(1) Å].16, b15

Figure 2.5 ORTEP diagram and selected bond lengths (Å) and angles (o) for 8

(thermal ellipsoids were drawn at the 50% probability level): Os(1)-Os(2) 2.8067(10), Os(2)-Os(3) 2.8318(10), Os(3)-Os(1) 2.8083(10), Os(4)-Os(5) 2.8464(9), Os(5)-Os(6) 2.8413(9), Os(6)-Os(4) 2.8503(10), Os(1)-S(1) 2.426(5), Os(2)-S(1) 2.453(4), Os(4)-S(2) 2.413(4), Os(5)-S(2) 2.417(2), S(1)-C(1) 1.733(17), S(2)-C(2) 1.791(17), Os(1)-S(1)-Os(2) 70.23(12), Os(4)-S(2)-Os(5) 72.11(11)

2.2 Deposition of osmium clusters on silica

Silica gel is the support which is most commonly used in catalysis because it is cheap, ery stable and available with high surface areas A number of triosmium clusters were acted with silica gel or a silicon wafer at room temperature for 24 h and analyzed by spectroscopy and ToF-SIMS ToF-SIMS is potentially very useful for probing the

ause of its low detection limit down to

)10(μ-H)]+ (cluster

v

re

IR

composition and structure of surface species bec

1 ppb However, only one report describing the analysis of an organometallic compound, Ru5C(CO)14Pt(C8H12), with ToF-SIMS has appeared.17

Figure 2.6(a) shows the ToF-SIMS spectrum of a clean silicon wafer that has been reacted with Os3(CO)12, 1 in refluxing octane Because osmium has a unique isotopic pattern, it is easy to distinguish a triosmium moiety from other molecular fragments Each assignment was confirmed by a comparison of the simulated isotope pattern with that recorded at high resolution The cluster fragment [Os3(CO

of peaks around m/z 853) can be discerned, as well as [Os3(CO)8(μ-H)(μ-O)]+corresponding to the loss of two CO ligands This result suggests that a surface-anchored cluster species Os3(CO)10(μ-H)(μ-OSi≡) (where Si≡ represents the silica surface) was formed on the silicon wafer A fragment corresponding to [Os3CO)10(μ-H)(μ-OSi≡)]+ was not observed, which may be attributed to the fact that

the cleavage of the Os-O bond is more facile than that of the Si-O bond, and is consistent with the suggested mechanism for the formation of Os3(CO)10(μ-H)(μ-OH) from the reaction of Os3(μ-H)(CO)10(μ-OSi≡) with water.18 A similar spectrum was

obtained from a silicon wafer soaked in a dichloromethane solution of Os3 (CO)10(μ-H)2, 22 (Figure 2.6b) suggesting that the species Os3(CO)10(μ-H)(μ-OSi≡)

was also formed with 22.

824 813

669

796 740

684 712 656

Mass (m/z)

a

-CO -CO -CO -CO -CO

[Os3(CO)8] + 768

[Os3(CO)10(μ-H)] + [Os3(CO)8(μ-O)] +

712 726

796 784

[Os3(CO)8] +

Figure 2.6 ToF-SIMS spectra (positive ion mode) of (a) Os3(CO)10(μ-H)(μ-OSi≡); (b)

silicon wafer modified by 22

700 800 0

-CO -CO -CO -CO -CO -CO

684

656

Figure 2.7 ToF-SIMS spectra of a silicon wafer soaked in a dichloromethane solution

of 20a in the mass range of (a) 600-900 (negative ion mode), (b) 40-80 (positive ion

Figure 2.7 shows the ToF-SIMS spectra of Os3(CO)10(μ-H)[μ-O(CH2)4OH], 20a mode)

supported on a silicon wafer Two series of cluster fragments attributed to [Os3(CO)x(μ-H)(μ-OH)]- (x ≦ 6)(m/z = 757, 729, 701, 673, 655) and [Os3(CO)x]- (x

≦ 9) (m/z = 824, 795, 768, 740, 712, 684, 656, 628) can be observed in the negative

ion mode The positive ion mode shows the organic fragment [HO(CH2)4]+

Similarly, for the clusters Os3(CO)10(μ-H)(μ-SRLH) (R = alkyl chain or an aromatic ring , L = COO, S and O), the ToF-SIMS spectra show fragments assignable

to the series [Os3(CO)x(μ-H)(μ-S)]- (x ≦ 9) (Figure 2.9) These results are similar to

those for the ZnO and In2O3 surfaces (see later)

Table 2.3 shows the IR spectra of the various osmium clusters grafted onto silica

gel The pale yellow solids all exhibit a vCO pattern which is similar to that of the

species Os3(CO)10(μ-H)(μ-OSi≡) The spectrum for grafted 12a shown in Figure 2.8 is typical For ligands containing the same functional group bonded to the cluster, a similar pattern and vibrational frequencies were obtained with different chain lengths

or an aromatic ring Thus the triosmium clusters anchored onto silica gel via the free functional group

Table 2.3 vCO data for supported osmium clusters

1960 1980

2000 2040

2080 2120

2160 2200

1/cm 30

800 1000 12000

[Os3(CO)10(μ-H)(μ-S)]

-745

829 801 773

-CO -CO -CO -CO -CO -CO

857 885 829

801

773 7

689

745 17

-857 829 801

773 745 717 689

-CO -CO -CO -CO -CO -CO -CO

Figure 2.9 ToF-SIMS spectra (negative ion mode) of silicon wafer soaked in a

dichloromethane solution of (a) 12a, (b) 18a and (c) 5a

2.3 Deposition of clusters on ZnO and In2O3 surfaces

The nature of the support can have a dramatic influence on the activity and selectivity of some catalytic reactions.20 ZnO and In2O3 are used as supports in some catalytic reactions.21 An example is the epoxidation of styrene over gold supported on In2O3.22

2.3.1 Deposition of Os3(CO)10(μ-H)(μ-OH) or Os3(CO)10(μ-H)2 on ZnO or In2O3

The reaction of a dichloromethane solution of Os3(CO)10(μ-H)(μ-OH), 1 or Os3(CO)10(μ-H)2, 22 with a ZnO or In2O3 substrate for 24 h at room temperature leads

to the attachment of triosmium moieties; the proposed structure of the cluster species

on these supports is illustrated in Scheme 2.4

supports were the same; Figure 2.10 shows the positive ion mass spectrum of 1 on the

ZnO The molecular fragment [Os3(CO)8(μ-H)]+ (m/z = 797) together with the expected fragments corresponding to successive loss of CO ligands can be observed clearly Although the characteristic feature of the spectrum is a broad distribution of peaks rather than isolated clusters of peaks, we can still observe some peaks with relatively higher intensities than the background, such as those clustered around m/z =

813, 825, 841 and 853, which corresponded to [Os3(CO)8(μ-H)(μ-O)]+,

lear that the surface species are triosmium clusters rather than mononuclear

or dinuclear species

850 400

[Os3(CO)9(μ-H)]+, [Os3(CO)9(μ-H)(μ-O)]+ and [Os3(CO)10(μ-H)]+, respectively Very intense signals at m/z = 65 and 81 assigned to Zn and ZnO respectively can also be observed However, no peaks attributable to [Os3(CO)9(μ-H)(μ-Zn)]+ was observable

-CO

-CO -CO

Mass (m/z)

797 743

[Os3(CO)9(μ-H)(μ-O)] +

785 769

813 825

853

-CO

841 [Os3(CO)9(μ-H)] +

2029 and 1942 cm-1 respectively These C-O vibration frequencies are all within the

Figure 2.10 ToF-SIMS spectrum (positive ion mode) of 1 anchored onto ZnO

Further information on the structure of the grafted osmium cluster species was provided by FTIR spectroscopy Figure 2.11 shows the IR spectra in the carbonyl

region for all the triosmium functionalized supports, together with 1 and 22 The

functionalized surfaces clearly showed IR spectra with the same pattern and similar vibrational bands, that is, one weak and two moderately intense νCO bands at 2121,

terminal carbonyl range (1800-2125 cm-1) This pattern matches with that of 1, which

has a set of vibrations with similar relative intensities at 2109, 2063, 2008 and 1970

cm-1 Hence, the grafted osmium cluster species should have a similar structure The surface-anchored cluster species Os3(CO)10(μ-H)(μ-OSi≡), which has been shown to

be bonded to silica gel via an oxygen atom, has vibrational frequencies at 2115m, 2080(s), 2067(s), 2031(s), 2013(sh) and 1995(sh) cm-1 Hence, it is likely that the triosmium species is anchored onto the metal oxide surfaces via a Zn or an In atom rather than an oxygen atom On the basis of IR and ToF-SIMS analyses, it was thus concluded that the surface species was Os3(CO)10(μ-H)(μ-Zn) or Os3(CO)10(μ-H)(μ-In) This result is different from that reported by Gates, et al,23who reported that triosmium clusters anchor to the ZnO surface via the oxygen atom which was proven via IR spectroscopy In their work, the surface-anchored clusters obtained by refluxing an n-octane solution of Os3(CO)12 with ZnO showed vibrational bands at 2107(m), 2067(s), 2050(s), 2030(vs), 2006(m) and 2000(sh) cm-1 They interpreted these patterns and values as that the surface osmium species were formed

by the reaction of Os3(CO)12 with surface OH groups, i.e., Os3(CO)10(μ-H)(μ-OZn) They also reported that the grafting of Os3(CO)10(μ-H)2 onto TiO2 at 125 oC led to a monoosmium species which has a structure analogous to the polymeric complex [Os(CO)2I2]n They reported the vibrational frequencies for the new surface species to

be 2118, 2040 and 1955 cm-1 The IR spectra for the different surface species in our study here are also similar to [Os(CO)2I2] (2119, 2045 and 1988 cm ) However, the ToF-SIMS analysis shows that these species are trinuclear clusters This is also consistent with the fact that no heat was involved in our case, and the stability of triosmium core is well established

-1

2100 2000 1900 18002200

≦ 9) An example is that for 7a on ZnO (Figure 2.12a) This result is consistent with

observations that the C-S bond in clusters can be fairly easily cleaved.24 Thus the

b c d

f

Figure 2.11 Solid state FTIR spectra (vCO region) for (a) 22, (b) 22 on In2O3, (c) 22

2 3

2.3.2 Deposition of osmium clusters containing a free functional group

For comparison, triosm clusters containing a linker and a functional group capable of interacting with the surfaces were used as precursors to deposit onto the

Os3(CO)10 μ-H)[μ-S(CH2)8SH], 7a, and Os3(CO)10(μ-H)[μ-S(CH2)2SH], 23a, which

all have a free –SH group; Os3(CO)10(μ-H)[μ-S(CH2)3OH], , which has a free –OH group; and Os3(CO)10(μ-H)(μ-SCH2COOH), 13a, which has a free –COOH

group

mode showed that the main fragments can be attributed to [Os

most stable species, [Os3(CO)x(μ-H)(μ-S) (x ≦ 9), were observed These results

suggest that the triosmium core was retained, which is further supported by their IR

spectra; the samples show similar patterns and vibrational frequencies in the vCO

region (2116w, 2066s, 2020s for 23a on ZnO)

The linker group is important in determining the fragmentation under ToF-SIMS A species with a longer alkyl chain tends to show more fragments due to break-up of the alkyl chain while for a species with a shorter alkyl chain, the fragment corresponding

to cleavage of the S-support bond shows up more clearly For example, the signals due to the molecular ions [Os3(CO)10(μ-H)(μ-S(1,3-C6H4)S)-In]- and [Os3(CO)10(μ-H)(μ-S(1,3-C6H4)S)]- respectively, for 5a on In2O3 are clearly visible (Figure 2.12c), while that for 10a on ZnO shows a complicated envelope in the mass

range greater than about 875 (Figure 2.13) In general, the ToF-SIMS spectra show that osmium clusters containing a free thiol functionality are bound to the surfaces via sulfur atoms

In some cases, rearrangement of the molecular fragments may have occurred For

example, for 23a on ZnO, the negative mode mass spectrum showed fragments at m/z

= 879, 907, 935, 963 and 991 which may be attributed to the fragments [Os (CO)x(μ-SCH CH S)(μ-SCH CH OH)]- (5≦x≦8) These could, however, also

be ascribed to the presence of a surface species like that depicted in Figure 2.12b

the IR spectrum as such a species is expected to show very similar IR pattern with

700 8000

-CO -CO

906 879

829 716

857

801

773 745

801 745

[Os (CO) {S(CH ) SH){S(CH ) O]

-Os Os

Os

O

C2H4S

-CO

Figure 2.12 ToF-SIMS spectra (negative ion mode) of (a) 7a anchored to ZnO, (b) 5a

anchored on In2O3 and (c) 23a anchored onto ZnO