Naphthalene and anthracene cobaltates(1-): Useful storable sources of an atomic cobalt anion

Reductions of CoBr₂ or cobaltocene by 3 equiv of potassium anthracene radical anion in tetrahydrofuran (THF) afford 60-80% yields of bis(anthracene)cobaltate(1-) (1), of interest as a readily accessible and quite labile source of spin-paired atomic Co^-. Although the unsolvated potassium salt of 1 is thermally unstable at 20 °C, the [K(18-crown-6)(THF) ₂]⁺ salt of 1 functions as a useful storable crystalline reagent for Co^- in several reactions. Previously known classic cobaltates, [CoL₄]^-, for L = 1/2 (1,3-butadiene) (2), PF₃ (3), and P(OiPr)₃ (5), were obtained directly from 1 and structurally characterized for the first time. Anion 3 is noteworthy because it appears to possess the shortest known Co-P distance, av = 2.012(4) Å. Although the naphthalene analogue of 1 is not yet available as a pure substance, low-temperature reductions of CoBr₂ or cobaltocene by naphthalene radical anion in the presence of 1,5-cyclooctadiene (COD) afford variable yields (80-90% from CoCp₂) of (naphthalene)(COD)cobaltate(1-) (10). Ready displacement of naphthalene in 10 by L = 1,3-butadiene, 2,2'-bipyridine, and COD occurs to give good yields of the respective [Co(L)(COD)]^-, all of which have been structurally characterized. Both ligands in 10 are displaced by tert-butylisocyanide to afford [Co(CNtBu)₄]^- (16), the first isolable and structurally characterized homoleptic alkylisocyanometalate. The molecular structure of 16 shows unprecedented bending of the isocyanides, av C-N-C = 137(2)°, for homoleptic isocyanide complexes.