Übergangsmetall-methylen-komplexe IL. Alkyliden-cobalt-komplexe:synthese, konstitution, struktur und folgereaktionen

The dinuclear cobalt complex [(η⁵-C₅Me₅)Co(η-(CO)]₂ (Me CH₃; 1) adds, when treated with the diazoalkanes 2a-2n, under mild reaction conditions (-80... + 25°C), (CRR′) of different structures to the metal-metal double bond. The novel η-alkylidene complexes 3c-3l, 4a,b,e,n and 5m are formed in mostly quantitative yields following elimination of nitrogen. The triply bridged derivatives 3 are generally more stable than in the series of the analogous rhodium compounds described earlier; usually they form at room temperature the isolated final products. Consecutive structural isomerization and/or decarbonylation yield the secondary products of type 4 and 5. The η-alkylidene complexes 5 open the hitherto best and most straightforward method for the stepwise construction of multiply alkylidene-bridged organometallics: for example, this bis(μ-alkylidene) derivatives 6 and 7 are quantitatively formed when the precursors 5a and 5c, respectively, are treated with diazomethane or diazoethane. The first phosphorylmethylene complex 4n has been obtained by reaction of the cobalt complex 1 with the argentio diazohydrocarbon 2n via elimination of dinitrogen and hydrolysis. The μ-methylene complex (μ-CH₂)[(η⁵-C₅Me₅)Co(CO)]₂ occurs as a mixture of the singly- and the triply-bridged isomers 4a and 3a, both in the solid state and in solution (IR evidence). According to a single-crystal X-ray analysis, trans-oriented sets of equivalent ligands (CO/CO) and C₅Me₅/C₅Me₅) are present in the molecule. The dicobaltacyclopropane geometry is characterized by a nearly equilateral Co(1),C,Co(2) triangle having the following bond parameters: d(COC) 190.7(10) and 194.3(8) pm, d(CoCo) 250.2(2) pm (crystal data for 4a: monoclinic, C⁵_2h-P2₁/c; a 966.9(4), b 1688.2(7), c 1419.9(6) pm, β 109.86(3)°; Z 4).