Reactions of Electron-Deficient Triosmium Clusters with Diazomethane:  Electrochemical Properties and Computational Studies of Charge Distribution

The 46-electron quinoline triosmium clusters (μ-H)Os3(CO)9(μ3-η2-C9H5(R)N) (6, R = 4-CH3; 7, R = H) react with excess CH2N2 at 0 to 25 °C to give (μ-H)2Os3(CO)9(μ3-η2-CHC9H5(R)N) (10, R = 4-CH3; 11, R = H), formed by insertion and subsequent C−H oxidative addition of a CH2 moiety into the ring C(8)−Os bond. In contrast, the related 46-electron quinoxaline compound (μ-H)Os3(CO)9(μ3-η2-C8H5N2) (8) reacts with excess CH2N2 at 0 to 25 °C to give (μ-H)2Os3(CO)9(μ3-η2-CHC8H4(5-CH3)N2) (12), where two CH2 groups have inserted into the C(5)−H bond as well as the C(8)−Os bond, and (μ-H)2Os3(CO)9(μ3-η2-CHC8H5N2) (13), the analogue of 11. The 5-methylquinoxaline compound (μ-H)Os3(CO)9(μ3-η2-C8H4(5-CH3)N2) (9) reacts with diazomethane at 0 to 25 °C to give 12 and Os3(CO)9(μ3-η2-C8H4(5-CH3)N2)(μ-CH2)(CH3) (14), containing an edge-bridging methylene group and a σ-bound methyl group. Thermolysis of 10 and 11 at 98 °C yields (μ-H)3Os3(CO)8(μ3-η2-C(C9H5)(R)N) (15, R = 4-CH3; 16, R = H), (μ-H)Os3(CO)9(μ3-η2-C(C9H5)(R)N) (17, R = 4-CH3; 18, R = H), and (μ-H)Os4(CO)11(μ3-η2-C(C9H5)(R)N) (19, R = 4-CH3; 20, R = H). The trihydrides 15 and 16 are formed by a further C−H activation of the coordinated methylidene group of 10 and 11, respectively. Thermolysis of 10 and 11 in the presence of H2 at 1 atm at 80 °C gives 15 and 16 in high yields. The solid-state structure of 17 reveals that the methylidyne carbon atom has a significant bonding interaction with the osmium atom coordinated to nitrogen. Thermolysis of 12 and 13 at 98 °C gives (μ-H)Os3(CO)9(μ3-η2-CC8H4(R)N2) (21, R = 5-CH3; 22, R = H), whereas a similar thermolysis in the presence of H2 (1 atm) at 80 °C gives (μ-H)3Os3(CO)8(μ3-η2-CC8H4(5-CH3)N2) (23, R = 5-CH3; 24, R = H). The compounds 12, 13, 23, and 24 all show one-electron reversible reductions, and the reduction potentials are similar to previously studied quinoxaline clusters. DFT calculations show that most of the spin density is confined to the heterocyclic ligand, and this explains the similarity between the observed reduction potentials for the products with different bonding modes to cluster but containing the same heterocycle. The qualitative correlation between the calculated natural charges and the distribution of unpaired spin densities in 12 and 13 corroborate this conclusion. The molecular structures of 10, 12, 13, 17, and 20 have been determined by single-crystal X-ray diffraction studies.