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Directed Synthesis of the Triangular Mixed-Metal Cluster H2RhRe2Cp*(CO)9: Ligand Fluxionality and Facile Cluster Fragmentation in the Presence of CO, Halogenated Solvents, and Thiols

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posted on 11.01.2010, 00:00 by Shih-Huang Huang, William H. Watson, Carl J. Carrano, Xiaoping Wang, Michael G. Richmond
The reaction of H2Re2(CO)8 (1) with Cp*Rh(CO)2 (2) in refluxing hexane affords the mixed-metal clusters H2RhRe2Cp*(CO)9 (4, major product), HRh2ReCp*2(CO)6 (5), and HRhRe3Cp*(CO)14 (6). 4 and 5 are electron-precise 48e clusters and display triangular metallic cores, while 6 contains 64 valence electrons and exhibits a spiked-triangular core having a pendant Re(CO)5 moiety. Heating 1 with Cp*2Rh2(CO)2 (3) gives 4 and 5 as the principal products, in addition to H2Rh2Re2Cp*2(CO)8 (7) in low yield. Cluster 7 possesses 60e and contains a tetrametallic core with two face-capping CO and hydride groups. Heating 4 under CO leads to cluster fragmentation and formation of Re2(CO)10 and 2 in essentially quantitative yield, as assessed by IR spectroscopy. The kinetics for the fragmentation of 4 in toluene under CO have been investigated over the temperature range 325−349 K by UV−vis spectroscopy. On the basis of the first-order rate constants and the Eyring activation parameters (ΔH = 25.0(8) kcal/mol; ΔS = −2.6(3) eu), a rate-limiting step involving a polyhedral opening of 4 is supported. 4 is thermally and photochemically sensitive, and reactions conducted in the presence of chlorinated solvents furnish the face-shared bioctahedral compound Cp*Rh(μ-Cl)3Re(CO)3 (8). Heating 4 and H2S in benzene at ca. 60 °C furnishes the 48e triangular cluster S2Rh3Cp*(CO)4 (9), which contains two Rh(CO)2 moieties and two face-capping sulfide groups. The reaction of 4 with p-methylbenzenethiol gives the sulfido-bridged dimer Cp*Rh(μ-SC6H4Me-p)3Re(CO)3 (11). The dinuclear compounds Cp*Rh(μ-Cl)(μ-SC6H4Me-p)2Re(CO)3 (10) and 11 are formed when 8 is allowed to react with p-methylbenzenethiol. Treatment of 8 and 10 with excess p-methylbenzenethiol yields 11 at elevated temperature in toluene. Compounds 411 have been isolated and fully characterized by IR and NMR spectroscopy and by X-ray crystallography. The reactivity displayed by 4 is contrasted with that of the known indenyl-substituted cluster H2Re2Ir(η5-ind)(CO)9 prepared earlier by Shapley and co-workers.