Synthesis and reactivity studies of cyclopentadienyl derivatives of ruthenium iridium and osmium iridium mixed metal clusters 1

Chapter 1 Heteronuclear carbonyl clusters containing groups 8 and 9 metals 1.1 Transition metal carbonyl clusters A transition metal cluster has been defined as a molecular compound wi

Chapter 1 Heteronuclear carbonyl clusters containing

groups 8 and 9 metals

1.1 Transition metal carbonyl clusters

A transition metal cluster has been defined as a molecular compound with at least

three transition metal atoms held together by some metal-metal bonding interactions

[1] Cluster compounds of early transition metal elements are generally associated

with π-donor ligands, which donate additional electrons to the metal bonding orbitals,

and these clusters are termed π-donor clusters or high oxidation state clusters as the

metals are in a high formal oxidation state In contrast, transition metals in groups

7-10 form clusters with π-acceptor ligands which are able to withdraw electron density

from the cluster and thereby depopulate skeletal molecular orbitals The metals are in

low oxidation states and these clusters are termed π-acceptor clusters

π-acceptor clusters can be neutral, cationic or anionic and the most common structural

entity is the metal triangle Some of the π-acceptor ligands which are capable of

stabilizing cluster compounds are carbonyls, isonitriles, nitric oxide, phosphines and

polyenes (in particular cyclopentadienyl and benzene) These ligands help to produce

the most favourable condition by inducing the greatest overlap between the atomic

orbitals of the metals In these clusters, the metal-ligand and the metal-metal

combinations must be kinetically inert towards bond dissociation otherwise

fragmentation or colloid formation can take preference over cluster formation

1.2 Electron counting rules in clusters

The structures adopted by clusters could be rationalized in terms of the Effective Atomic Number Rule (EAN) which is an extension of the 18-electron rule [2] The EAN count for a cluster of nuclearity x and having y metal-metal bonds is defined by [3]

EAN count = 18x – 2y

Trinuclear ruthenium and osmium clusters have electron counts of 48 whereas tetranuclear clusters have electron counts of 60 to 64 depending on the shapes adopted These values are arrived at by adding the metal valence electrons and the electrons donated by the ligands giving allowance for the charge if the cluster is ionic Each metal-metal bond is regarded as a two-centre two-electron bond and the cluster

is said to be “electron precise” and the skeletal geometries of these can be interconverted by appropriate addition or removal of electron pairs This interconversion and the relation between the resulting clusters are illustrated in Scheme 1.1

+CO -CO

Wing-tip

Hinge

+CO -CO

Wade’s rule, is employed This rule is based on the concept of isolobal fragments and the general formula used is given by [1, 4]

S = (Total number of valence electrons -12n) / 2

where

S - Number of skeletal electron pairs

n - Number of skeletal metal atoms

The value 12 is arrived at based on the assumption that each skeletal metal atom uses six of its available atomic orbitals for metal-ligand bonding

1.3 Homonuclear carbonyl clusters of osmium, ruthenium, iridium and rhodium

Some knowledge of the well-documented chemistries of the trinuclear and tetranuclear clusters of the heavier groups 8 and 9 metals will be useful to gain a better understanding of their heteronuclear clusters

1.3.1 Trinuclear clusters of osmium and ruthenium

A trinuclear cluster may be considered as a complex of three metals If the metal atoms forming the cluster are the same then they are called homonuclear clusters Osmium and ruthenium are two of the most abundant cluster-forming elements It is customary to consider the chemistries of triosmium and triruthenium together due to their similarity The interest in the clusters of these metals are attributed to the following properties [5]

1 These clusters are robust (especially osmium), thermally stable at room temperature and inert to water and oxygen thereby facilitating chemical handling considering the cost of these metals

2 It is rather easy to introduce organic and inorganic ligands into these clusters

3 Since chromatographic methods can be utilized for separation of the compounds and crystals suitable for X-ray can be grown without much difficulty, the analysis can be quite straightforward

Despite their similarities there are, however, some differences in their properties

1 Ruthenium compounds in general tend to be more reactive than osmium compounds Thus synthetic methods should be tailored accordingly for preparing the derivatives of each metal For example, the stable as well as highly reactive osmium compound Os3(μ-H)2(CO)10 could be synthesized easily while the ruthenium analogue, Ru3(μ-H)2(CO)10, has not been isolated although it could be generated in situ by hydrogenation of Ru3(CO)12 [6]; it is highly unstable and is readily converted to Ru4(μ-H)4(CO)12

2 Ruthenium compounds generally prefer to adopt structures with bridging CO

3 Generally, fluxional intramolecular processes are faster for ruthenium than for osmium compounds and it is common not to be able to obtain the limiting NMR spectra for ruthenium compounds at low temperatures A notable exception is CO fluxionality involving movement of a bridging CO to a terminal site, which can be slower for ruthenium

1.3.2 Tetranuclear clusters of osmium and ruthenium

Tetranuclear clusters of osmium and ruthenium are interesting because they act as a bridge between the trinuclear clusters and higher nuclearity clusters They are generally synthesized from lightly stabilized derivatives of the parent carbonyls

Ru3(CO)12 and Os3(CO)12 There are no reports of neutral binary carbonyls containing four ruthenium atoms For osmium, the binary carbonyls [Os4(CO)n] [n = 14-16] are

known The tetranuclear hydrido clusters of both osmium and ruthenium have been synthesized in high yields from the hydrogenation of Ru3(CO)12 and Os3(CO)12 [7, 8]

4[M 3 (CO) 12 ] + 6H 2 3[M 4 ( μ-H) 4 (CO) 12 + 12CO

(1)

The addition of a mononuclear compound to an activated trinuclear complex also provides a high yield route for the synthesis of tetranuclar clusters of osmium [9-11] Treatment of Os3(CO)10(COE)2 in hexane at 0 ºC with Os(CO)5 yields Os4(CO)15 in good yield Refluxing a hexane solution of Os4(CO)15 with a nitrogen purge readily affords Os4(CO)14 The puckered square cluster Os4(CO)16 is isolated from the reaction of Os4(CO)15 in CH2Cl2 with CO (1 atm) at 0 ºC

Os 3 (CO) 10 (NCMe)+ Os(CO) 4 (PMe 3 )

Os 3 ( μ-H) 2 (CO) 10 + Os(CO) 4 (PMe 3 )

(2)

(3) COE = cyclooctene

1.3.3 Tetranuclear clusters of iridium and rhodium

The homoleptic tetranuclear clusters M4(CO)12 (M = Rh, Ir) are all based on a tetrahedral metal core In Ir4(CO)12, all the carbonyls are terminal whereas in

Rh4(CO)12, nine carbonyls are terminal and three carbonyls are found bridging one face of the cluster (Figure 1.1)

Figure 1.1 Comparison of tetranuclear clusters of Ir4(CO)12 and Rh4(CO)12

The low solubility of Ir4(CO)12 and Rh4(CO)12 in organic solvents below 100 ºC is a severe constraint for an extensive investigation of its substitution reactions The drastic conditions required for the substitution reactions in Ir4(CO)12 mainly promote the activation of C-H bonds in the unsaturated organic substrates However, the use of TMNO as an oxidative decarbonylating agent has allowed the formation of derivatives of Ir4(CO)12 under mild conditions Alkene derivatives of Ir4(CO)12 are useful synthetic intermediates The reaction of [Ir4(CO)11Br]⎯ with an alkene and AgBF4 affords Ir4(CO)11(η2-alkene) (alkene = C2H4, C3H6, cyclooctadiene or norbornadiene) (Scheme 1.2) These derivatives are quite useful in cluster build-up reactions [12, 13]

Br

C C

H H

H H

Scheme 1.2

1.4 Heteronuclear transition metal carbonyl clusters

Clusters containing two or more different types of metals are termed heteronuclear or mixed metal clusters (Figure 1.2) [3]

CoOs 3 ( μ-H) 2 (CO) 10 (η 5 -C 5 H 5 ) Ru 5 Pd(μ6 -C)(CO) 16

Figure 1.2 Examples of mixed-metal clusters

Research into heteronuclear clusters is stimulated by the belief that disparate metals within one molecule may lead to new and interesting reactivity It is believed that the combination of different metals in the same complex can give rise to enhanced catalytic activity The importance of heterometallic clusters towards catalysis has been attributed to the following [5] :

1 Adjacent metal centers offer the possibility for co-operative reactivity and the intrinsic polarity of the heterometallic bonds can provide bi- or multifunctional activation and direct the selectivity of substrate-cluster interactions

2 The metal core of these clusters resembles a molecular micro alloy and can therefore be used as a precursor of novel heterogeneous catalysts

Metal cluster compounds can be regarded as intermediate between coordination complexes and bulk metal surfaces or particles It has been argued that study of their chemistries will provide an insight into reactions occurring on metal surfaces Johnson

and coworkers, and others, have reported that high nuclearity mixed-metal clusters can act as precursors for bimetallic nanoparticle catalysts that can be anchored inside mesoporous silica [14-17]

1.5 Synthetic strategies to heteronuclear carbonyl clusters

Systematic and well designed syntheses have been developed for mixed-metal clusters A brief survey of the methods employed for the synthesis of heteronuclear clusters will be discussed in this section

1.5.1 Condensation reactions

Thermal condensation between small clusters to form large clusters (pyrolysis or thermolysis) was one of the first methods used to synthesize high nuclearity clusters Metal-metal bond formation occurs due to the thermal elimination of CO or other two electron ligand This ligand loss serves as the driving force for the reaction [4] For example, Ru4(μ-H)4(CO)12 reacts with Pt2Ru4(CO)18 to form the decanuclear cluster

Pt2Ru8(μ-H)2(CO)23 The structure can be viewed as two condensed octahedra sharing

a common Pt-Pt edge (Scheme 1.3)

The pentanuclear complex [Pt2Os3(CO)10(cod)2] (cod = 1,5-cyclooctadiene) condenses in the presence of CO to give the decanuclear complex [Pt4Os6(CO)22(cod)] at 401 K (Scheme1.4) [5, 18]

Pt

Pt cod

-cod +2CO

Scheme 1.4

Heteronuclear clusters can also be synthesized by condensing two compounds that have weakly co-ordinated or labile ligands These reactions can be carried out under relatively mild conditions The compounds Pt2Os3(CO)10(cod)2 and [Pt2Os6(CO)16(cod)2] were synthesized by the reactions of [Pt(cod)2] with activated osmium complexes (Scheme 1.5) [3, 19-21]

[Pt(cod) 2 ] + [Os 3 (CO) 10 (NCMe) 2 ] 298 K

1.5.2 Addition reactions

Clusters containing metal-metal multiple bonds undergo addition reactions These compounds can add small metal fragments to the cluster framework without elimination of a ligand However, the metal fragment added must be unsaturated or possess labile ligands Recently, a high yield synthesis of the cluster, RuOs3(μ-H)2(CO)13 has been reported [22] It has been obtained by reacting the unsaturated triosmium hydrido cluster, Os3(μ-H)2(CO)10, with Ru(CO)4(C2H4) The weakly coordinated ethylene ligand is eliminated during the reaction, along with CO, to yield the RuOs3 mixed metal cluster It has also been reported that the cluster exists as at least three isomers which rapidly interconvert in solution (Scheme 1.6)

H

Ru

Os H H

Ru

Os H

Another interesting example is the addition of Rh(CO)2(η5-C9H7) to W(CO)2(η-

C5H5)(=CTol) to produce WRh(CO)3(η5-C5H5)(μ-CTol) which was subsequently reacted with Ir(CO)2(η5-C9H7) to give WRhIr(CO)3(η5-C5H5)(μ3-CTol)(η5-C9H7)2 This cluster has a chiral tetrahedral WRhIrC core and the optical isomers were resolved by chromatography using a chiral support (Scheme 1.7) [3]

C OC

OC

Co 2 (CO) 8

C

OC OC

Co

CO CO

CO CO CO

C O O

WRh(CO) 3 (η5-C 5 H 5 )( μ-CTol)( η5- C 9 H 7 ) WRhIr(CO) 3 (η5-C 5 H 5 )(-μ 3-CTol)(η5-C 9 H 7 ) 2

W(CO) 2 (η5-C 5 H 5 ) (CO) 8 (μ3CTol) ( CTol)

W

W

W

Rh Ir

CO

Rh

Scheme 1.7

1.5.3 Salt elimination or ionic coupling reactions

The synthesis of a wide variety of clusters has been accomplished by this method These reactions are believed to proceed via an associative nucleophilic substitution mechanism (SN2) Wong and coworkers have reported the coupling reaction of [(Ph3P)2N][Os3(μ-H)(CO)11] with [Rh(COD)2]+ to yield the mixed metal cluster,

Os3Rh(μ-H)3(CO)10(COD), in 25% yield (Figure 1.3) [23]

Os

H H

H

Figure 1.3 Molecular structure of Os3Rh(μ-H)3(CO)10(COD)

Os3Ir(μ-H)3(CO)12 has been synthesized by Johnson and coworkers in 40% yield from the reaction between the anionic cluster [(Ph3P)2N][Os3(μ-H)(CO)11] and [(Ir(COD)Cl)2] in the presence of Tl[PF]6 [24] The structure has, however, not been reported

A recent development has been the systematic cluster build-up via ionic coupling between pre-formed cluster anions and mono- or di-nuclear metal cations Clegg and coworkers have reported the synthesis of penta and hexanuclear mixed-metal clusters

Os4Rh(μ-H2)(CO)13 and Os4Rh2(μ-H)2(CO)11(η5-Cp*)2 by reacting [K][Os4H)4(CO)11] with [BF4]2[Rh(η5-Cp*)(MeCN)3] [25] Of late, the cation [Ru(C5H5)(MeCN)3]+ has been widely used to introduce the cyclopentadienyl ligand into mixed-metal clusters, as in the synthesis of [Os3RuH(CO)11(η5-C5H5)] and [Os3Ru2(CO)9(μ3-CO)2(η5-C5H5)2] (Scheme 1.8) [26] This monocationic ruthenium species has two advantages over the dicationic species: Electron transfer to a cluster dianion must occur in two steps, which limits redox activity, and the reaction of a dianion with such a monocationic species provides the opportunity to increase the nuclearity of a neutral product by two metal units [16, 27, 28]

(μ-Scheme 1.8

1.6 Short introduction to ligands and substrates

As the focus of the research is on the synthesis and reactivity of mixed metal carbonyl clusters, a short review of some of the ligands like CO, hydrides, and cyclopentadienyls (Cp, Cp*), which generally form an integral part of the ligand sphere of the clusters, will be made The bonding modes of substrates like phosphines, isonitriles, alkynes and chalcogens will also be discussed in this section

1.6.1 Carbon monoxide

The most widely found ligand in π-acceptor clusters is carbon monoxide, which can adopt terminal, edge-bridging, or face-capping bonding modes in a cluster For electron counting purposes all the three bonding types are considered as two-electron

donors (Figure 1.4) [1] CO is regarded as the most important ligand in transition

metal carbonyl cluster chemistry owing to its versatility, range of bonding modes

exhibited, property of stabilizing metals in low oxidation states and also its small size

which allows a large number of it to surround the metal In addition to these, CO has

been found to bond in a variety of other ways as illustrated in Figure 1.5

Bridging carbonyls are generally better π- acceptors than terminal carbonyls because

of the effective overlap between the d orbitals of two metals with the π* orbitals of the

carbonyls and the increased π* back-bonding is reflected in the νCO values which are

lower than those of the terminal ligands Bridging tendency decreases on descending a

transition metal sub-group

c

c c c c

c o

Terminal Edge-bridging ( μ-CO) Face-capping (μ 3 -CO)

μ4

C O

μ4-η 2

[2] [29] [30] Figure 1.5 Various other bonding modes of CO