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.