om050942i_si_007.cif (19.51 kB)

A Less Carbocentric View of Agostic Interactions:  The Complexes [Rh(η4-cod){H2A(mt)2}] (A = B, C+; mt = Methimazolyl)

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posted on 24.04.2006, 00:00 by Ian R. Crossley, Anthony F. Hill, Elizabeth R. Humphrey, Matthew K. Smith
The reaction of [Rh2(μ-Cl)2(cod)2] (cod = 1,5-cyclooctadiene) with Na[H2B(mt)2] (mt = methimazolyl) provides the complex [Rh(cod){H2B(mt)2}] (1), which in turn reacts with CO or CNC6H3Me2-2,6 to provide the derivatives [RhL2{H2B(mt)2}] (L = CO (2), CNC6H3Me2-2,6 (3)). In a similar manner [Rh2(μ-Cl)2(cod)2] reacts with H2C(mt)2 to provide the cationic complex [Rh(cod){H2C(mt)2}]+ (4+), which was isolated after counteranion metathesis as either 4·BF4 or 4·PF6. Carbonylation of 4·PF6 provides [Rh(CO)2{H2C(mt)2}]PF6 (5·PF6); however, the reaction is reversible in the presence of cod, regenerating 4·PF6 under reduced pressure. The reactions of 4·PF6 with CNC6H3Me2-2,6 or CNCMe3 provide respectively the salts [Rh(CNC6H3Me2-2,6)2{H2C(mt)2}]PF6 (6·PF6) and [Rh(CNCMe3)4]PF6. The crystal structures of 1, 4·BF4, 4·PF6, and 6·BF4 reveal long B−H···Rh or C−H···Rh interactions; however, such an interaction is effectively absent for 5·PF6·CHCl3, while solution spectroscopy (1H NMR and IR) indicates that none of these interactions persist in solution. The complexes 1 and 4+ represent the first structurally characterized isoelectronic pair involving, albeit weak, three-center−two-electron B−H−metal or C−H−metal interactions.