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

Chapter Heteronuclear carbonyl clusters containing groups and 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 710 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. Wing-tip +CO +CO -CO -CO Hinge Tetrahedral 60-e Butterfly 62-e Rhombus 64-e Scheme 1.1. As the nuclearity of the cluster increases, EAN rule alone is not sufficient to explain the electron count and Polyhedral Skeletal Electron Pair Theory (PSEPT), also called 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) / 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 and 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 without changing the nuclearity of the cluster. 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[M3(CO)12] + 6H2 3[M4(μ-H)4(CO)12 + 12CO M=Ru, Os 80-90% (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 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 ºC. Os3(CO)10(NCMe)+ Os(CO)4(PMe3) 40% Os4(CO)15(PMe3) 57% Os3(μ-H)2(CO)10+ Os(CO)4(PMe3) Os3(CO)10(COE)2+ Os(CO)5 0C hexane COE = cyclooctene Os4(CO)15 Os4(CO)15 (2) reflux hexane CO 0C hexane Os4(CO)15 73% (3) Os4(CO)14 95% (4) Os4(CO)16 86% (5) 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). Ir Ir Rh Ir Ir Rh Rh Rh 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]. H Br H C C H + C2H4 H AgBF4 [Ir4(CO)11(η2-C2H4)] [Ir4(CO)11Br] 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]. Ru Co Ru Ru C Os Os Os H H CoOs3(μ-H)2(CO)10(η5-C5H5) Ru Ru Pd Ru5Pd(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). Ru Ru Ru Ru Pt Ru Pt Ru Ru Pt Ru Ru Pt Ru + Ru Ru Ru Ru Ru Ru 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]. Os [Pt2Os3(CO)10(cod)2] -cod +2CO cod Pt Os Pt Pt Os Os Os Pt Os 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]. Os [Pt(cod)2] + [Os3(CO)10(NCMe)2] Os Pt Pt 298 K cod [Pt(cod)2] + [Os6(CO)16(NCMe)2] Os 298 K cod Os Os Os Os Pt Pt cod Os Os cod Scheme 1.5. 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(μH)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). δH = -20.17 ppm Ru Ru Os H Os Os Os H H δH = -21.17 ppm H Os H Os (I) (IIa) Ru Os Os Os δH = -20.95 ppm δH = -20.04 ppm (III) H Os Os H Ru H Os (IIb) Scheme 1.6. Another interesting example is the addition of Rh(CO)2(η5-C9H7) to W(CO)2(ηC5H5)(=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]. 10 C W OC CO Co2(CO)8 C Co W OC OC CO CO OC Co W(CO)2(η5-C5H5) ( CTol) CO C C Rh OC CO W(CO)2(η5-C5H5) (CO)8 (μ3CTol) Rh(CO)2(η5-C9H7) W CO Ir(CO)2(η5−C9H7) OC CO C Ir W OC C Rh O O WRhIr(CO)3(η5-C5H5)(-μ 3-CTol)(η -C9H7)2 WRh(CO)3(η5-C5H5)(μ-CTol) ( η5- C9H7) 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]. 11 Rh Os H H H Os Os 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][Os4(μH)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]. 12 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 13 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. o c o c o c o c c c c o cc c o o o o o c o Terminal Edge-bridging (μ-CO) Face-capping (μ3-CO) Figure 1.4. Common CO Bonding modes. O C μ3-η2 [2] O O C μ4 [29] C μ4-η2 [30] Figure 1.5. Various other bonding modes of CO. 14 1.6.2 Hydrides Hydride ligands play a very important role in cluster chemistry since they are capable of displaying a variety of bonding modes like terminal (H), bridging (μ-H) and face capping (μ3-H). They can sometimes occupy interstitial sites to exhibit tetrahedral (μ4H) and octahedral (μ6-H) bonding modes (Figure 1.6). H H H µ-H µ3-H µ4-H H µ6-H Figure 1.6. Bonding modes of hydrides. The 1H NMR technique is most useful for detecting metal hydrides because they resonate to high field of TMS in a region (δ to 60 ppm) normally free of other ligand resonances. Hydrides couple with phosphines which serve as a valuable tool in elucidating the structure of a cluster. Inequivalent hydrides can also couple with each other (J = 1-10 Hz). Hydrides are usually observed in the 1500-2200 cm-1 region in the infrared spectrum [2]. The intensities of these hydride signals are generally very weak and therefore IR studies are not conclusive evidence for their presence. X-ray crystallography has been successful to some extent in determining the position of these ligands. Neutron diffraction is a more effective technique for locating hydrides [31]. 15 1.6.3 Cyclopentadienyl ligands The Cp ligand is regarded as the most important member of the polyene family. It has been described as a mostly inert, firmly bound, spectator ligand. It acts as a 5electron donor exhibiting η5-coordination mode in which all carbon atoms are bonded to a single metal center. However, η5 – η3 – η1 slippage has been proposed to be an important step in substitution reactions (Figure 1.7) [2]. H M M η η M η1 Figure 1.7. Ring slippage from η5- η3- η1. Typical η5- Cp 1H resonance occurs between δ 3.5 – 5.5 ppm. For η1-Cp, the α hydrogen resonance occurs at about δ 3.5 ppm, and the β and γ resonances occur between δ 5-7 ppm. The most exciting characteristic of Cp ligands is their ability to sterically impose a different geometry to that predicted electronically. The pentamethylcyclopentadienyl ligand (Cp*) is a significant modification of Cp. It is more bulky and more electron releasing than Cp. The Cp* ligand can stabilize a wide range of complexes and it has been employed in general to produce stable versions of interesting Cp compounds. Its derivatives are also more soluble than the Cp analogues. 1.6.4 Alkynes Alkynes exhibit a wide variety of bonding modes as both their π - bonds can simultaneously react with different metal centers. Their geometry is drastically affected upon coordination to a metal cluster due to the back donation of the electron 16 density from the metals into the alkyne π* orbitals. It is usually difficult to control the reaction with simple substitution and oligomerization is often observed. There are four basic types of alkynes found in cluster chemistry, namely, symmetrical alkynes with aromatic substituents (eg., C2Ph2), alkyl substituted internal alkynes (eg., C2Et2) terminal alkynes (HC2R), and phosphinoalkynes (eg., Ph2PC2R) [32]. The coordination of alkynes to metal clusters has been shown to depend on both the metal and the substituents on the alkyne. The interactions of alkynes with trinuclear clusters of the iron triad can be divided into three different types: 1. Symmetrical alkynes, i.e., alkynes having symmetrical groups on both sides of the C-C triple bond (such as C2Ph2) are coordinated without rearrangement. 2. Alkyl - substituted internal alkynes, such as EtCCEt or MeCCEt, are coordinated without drastic change in some instances, but usually isomerise to allenyl and allylic ligands with transfer of one hydrogen onto the cluster. For example the reaction of Ru3(CO)12 with 3-hexyne afforded a triruthenium cluster containing a pseudo π-allyl moiety, Ru3(CO)9(μ-H)(C6H9) (Scheme 1.9) [33]. Ru3(CO)12 + CH3CH2C CCH2CH3 reflux C6H6 H H3C C C Ru C CH2CH3 Ru Ru Scheme 1.9. 3. Terminal alkynes, HCCR, give cluster derivatives containing a bridging hydride and multibound acetylides. For example, the reaction of Os3(CO)12 with HCCPh yields the products Os3(CO)9[HCC(Ph)COC(Ph)CH], and 17 Os3(CO)10(HCCPh), both of which are converted to the acetylide complex Os3(μ-H)(CO)9(CCPh), upon heating [34]. The variety of bonding interaction of a single alkyne molecule (HC2R, RC2R or RC2R’) with two to four metal centers is summarized in Figure 1.8. R R C C R' R R' C M M R C C M M C R' C C M M M M R' M M M M M A B C D Figure 1.8. Modes of alkyne interaction with two to four metal centers. 1.6.5 Phosphines Phosphines are good σ-donors and poor π-acceptors and generally occupy terminal coordination sites. In the case of CO it is the π* orbital which accepts the electron density from metals whereas in phosphines, it has been argued that the P-C σ* orbitals accept the back donation. The π-acid character of phosphines follows the order PMe3 ~ P(NR2)3 < PAr3 < P(OMe)3 < PCl3 < CO ~ PF3 For electron counting purposes they are regarded as two-electron donors. The electronic effect of PR3 ligands as a function of the R group has been reported by Tolman. He has measured the νCO frequencies of a series of Ni(CO)3(PR3) complexes and defned an electronic parameter (χ) in terms of them. [35]. Another important aspect of the PR3 series of ligands is that electronic effects may be changed without changing the steric effects, for example, by moving from PBu3 to P(OiPr)3; steric effects can also be changed without affecting the electronic effects. This proves to be extremely useful, especially in catalysis. 18 1.6.6 Isonitriles (RNC) Isonitriles are two-electron donors and are isoelectronic with CO. They are better σdonors but poorer π-acceptors than CO. In contrast to CO their bridging tendency is low. The CN stretching vibration is often lowered on complexation and this effect is more pronounced than in CO [2]. Isonitriles are usually capable of replacing one or two carbonyls in cluster compounds. 1.6.7 Chalcogens The group 16 elements, oxygen, sulfur, selenium, tellurium and polonium are called chalcogens. Their compounds, especially sulfides, selenides and tellurides, are collectively known as chalcogenides (ER). They can act as bridging or capping ligands and have been utilized in cluster chemistry to facilitate the stabilization of some unusual cluster geometries [36, 37]. Clusters with unique structural features and unusual reactivities have been observed by using chalcogen atoms (S, Se, Te) as bridging ligands [38]. They are found to exhibit various bonding modes in which they contribute two to six electrons to the cluster as shown in Table 1.1. They usually bridge a metal-metal bond thereby acting as three-electron donors (μ-ER). The elements themselves act as 4-electron donors by either capping triangular metal faces (μ3-E) or square metal faces (μ4-E). Quadruply bridging chalcogens can serve either as or 6-electron donors. The chalcogen ligands have been employed as bridging ligands in cluster growth reactions. The new geometries and coordination modes exhibited by the clusters containing bonds between transition metals and chalcogen ligands have been argued to serve as useful models and precursors for the synthesis of new materials [39, 40]. Figure 1.9 shows the cluster core structure of the mixed metal cluster, 19 Fe2Mo2(CO)6(C5H5)2(μ3-Se)(μ4-Te) which has three different chalcogens in two common bonding modes [41]. Table 1.1. Some examples of different chalcogen bonding modes. Bonding Mode Example R E R HRu3(CO)10(μ-SEt2) [42] R E Os3(CO)11(SEt2) [43] E Os3(CO)9(μ3-Se)PPh3 [44] E Co4(CO)10(μ4-S)2 E [45] [HPtOs3(CO)8(μ4-S)(μ3-S)(PPh2C6H4)]2 [46] Te Fe Mo Fe Mo Se S Figure 1.9. Cluster core of Fe2Mo2(CO)6(C5H5)2(μ3-Se)(μ4-Te). 20 1.7 Motivation and aim of the project Heteronuclear clusters are getting overwhelming attention due to their synergistic catalytic potential. The disproportionate increase in reaction rate observed upon mixing multiple catalytic systems has created a resurgence of interest in the study of mixed metal clusters, in particular those of groups and 9. For example, it has been shown that the carbonylation of methanol to give acetic acid is effectively catalysed by various iridium complexes, especially when promoted by ruthenium or osmium compounds. Notable results have been reported for a ruthenium:iridium and osmium:iridium ratio of 3:1. The tremendous potential for ruthenium-iridium and osmium-iridium clusters in catalytic chemistry can be envisaged from the above results. The Cp ligands, especially the pentamethylcyclopentadienyl ligand, are known to act as spectator ligands. However, C-H activation of the ring methyl groups has been reported under thermolytic conditions. Since Cp*Ir(CO)2 is well-known to react with hydrocarbons, it is of interest to see whether C-H activation can be brought about by Cp*IrRu3 and Cp*IrOs3 clusters. Another interesting feature of the Cp and Cp* ligands is that they can be derivatised easily. Taking advantage of this property, several alkyl and silyl substituted Cp derivatives have been reported in literature. The coordinative potential of amino-functionalized cyclopentadienyl ligands has gained considerable attention in catalytic reactions. It is hoped that Cp* ligand can not only stabilize the cluster core, but also lead to some novel rearrangements during reactivity studies with various organic and inorganic substrates due to its strong donor properties. To date the clusters Cp*IrRu3(μ-H)4(CO)9 and CpIrOs3(μ-H)2(CO)10 are the only known examples of Ru3Ir and Os3Ir clusters with a Cp ligand. There has also been no 21 report on the reactivity of these clusters with organic and inorganic substrates. This may be partly due to the low yields obtained for these clusters. The objective of this research is to therefore synthesize Cp, Cp*IrRu3 and Cp*IrOs3 clusters in high yields, and to investigate their reactivity with various classes of organic and inorganic substrates. 22 1.8 References 1. B.F.G. Johnson, Transition Metal Clusters, J. Wiley: New York, 1980. 2. R.H. Crabtree, The Organometallic Chemistry of the Transition Metals, 3rd Edition John Wiley & Sons Inc.: New York ; Singapore, 2001. 3. P.J. Dyson, and J.S. 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Andy Hor, Organometallics, 1984, 3, 1915. 26 [...]... and R.K Pomeroy, Journal of the American Chemical Society, 19 86, 10 8, 338 11 L.R Martin, F.W.B Einstein, and R.K Pomeroy, Organometallics, 19 88, 7, 294 12 R Ros, F Canziani, and R Roulet, Journal of Organometallic Chemistry, 19 84, 267, C9 23 13 R Ros, A Scrivanti, and R Roulet, Journal of Organometallic Chemistry, 19 86, 303, 273 14 B.F.G Johnson, Journal of Organometallic Chemistry, 19 94, 475, 31 15... metal units [16 , 27, 28] 12 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,... 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][Os4(μH)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. .. Tolman, Chemical Reviews, 19 77, 77, 313 36 H Vahrenkamp, Angewandte Chemie International Edition, 19 75, 87, 363 37 L.C Roof, and J.W Kolis, Chemical Reviews, 19 93, 93, 10 37 38 K.H Whitmire, Journal of Coordination Chemistry, 19 88, 17 , 95 39 M.L Steigerwald, Polyhedron, 19 94, 13 , 12 45 40 P Mathur, M.M Hossain, P.B Hitchcock, and J.F Nixon, Organometallics, 19 95, 14 , 310 1 25 41 P Mathur, M.M Hossain,... Liable-Sands, and G.P.A Yap, Organometallics, 19 96, 15 , 18 98 42 M.R Churchill, J.W Ziller, and J.B Keister, Journal of Organometallic Chemistry, 19 85, 297, 93 43 N.K Kiriakidou-Kazemifar, E Kretzschmar, H Carlsson, M Monari, S Selva, and E Nordlander, Journal of Organometallic Chemistry, 20 01, 623, 19 1 44 W.K Leong, W.L.J Leong, and J Zhang, Journal of the Chemical Society, Dalton Transactions, 20 01, 10 87... Cotton, Catalysis by Di- and Polynuclear Metal Cluster Complexes, Wiley: New York, 19 98 20 R.D Adams, G Chen, J.C Lii, and W Wu, Inorganic Chemistry, 19 91, 30, 10 07 21 C Couture, and D.H Farrar, Journal of the Chemical Society, Dalton Transactions, 19 86, 13 95 22 L Pereira, W.K Leong, and S.Y Wong, Journal of Organometallic Chemistry, 2000, 609, 10 4 23 W.-T Wong, Organometallics, 19 99, 18 , 3474 24 P Sundberg,... Shields, and S.J Teat, New Journal of Chemistry, 19 98, 22, 11 11 31 A.G Orpen, Journal of the Chemical Society Dalton Transactions, 19 80, 2509 32 E Sappa, A Tiripicchio, and P Braunstein, Chemical Reviews, 19 83, 83, 203 33 O Gambino, M Valle, S Aime, and G.A Vaglio, Inorganica Chimica Acta, 19 74, 8, 71 34 O Gambino, R.P Ferrari, M Chinone, and G.A Vaglio, Inorganica Chimica Acta, 19 75, 12 , 15 5 35 C.A... a resurgence of interest in the study of mixed metal clusters, in particular those of groups 8 and 9 For example, it has been shown that the carbonylation of methanol to give acetic acid is effectively catalysed by various iridium complexes, especially when promoted by ruthenium or osmium compounds Notable results have been reported for a ruthenium: iridium and osmium :iridium ratio of 3 :1 The tremendous... F Grepioni, and B.F.G Johnson, Chemical Reviews, 19 94, 94, 15 85 16 J Lewis, C.A Morewood, P.R Raithby, and M.C Ramirez de Arellano, Journal of the Chemical Society, Dalton Transactions, 19 96, 4509 17 J Lewis, C.K Li, P.R Raithby, and W.T Wong, Journal of the Chemical Society, Dalton Transactions, 19 93, 999 18 R.D Adams, J.C Lii, and W Wu, Inorganic Chemistry, 19 92, 31, 2556 19 R.D Adams, and F.A Cotton,... yields obtained for these clusters The objective of this research is to therefore synthesize Cp, Cp*IrRu3 and Cp*IrOs3 clusters in high yields, and to investigate their reactivity with various classes of organic and inorganic substrates 22 1. 8 References 1 B.F.G Johnson, Transition Metal Clusters, J Wiley: New York, 19 80 2 R.H Crabtree, The Organometallic Chemistry of the Transition Metals, 3rd Edition John . 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. 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. Rh Rh Figure 1. 1. Comparison of tetranuclear clusters of Ir 4 (CO) 12 and Rh 4 (CO) 12 . The low solubility of Ir 4 (CO) 12 and Rh 4 (CO) 12 in organic solvents below 10 0 ºC is a severe