Carbyne-carbyne coupling at a single metal center. Synthesis structure of bis(diethylamino)acetylene complexes of molybdenum(II) and tungsten(II)

A high-yield synthesis of Mo^II and W^II bis(diethylamino)acetylene complexes starting from M(CO)₆ (1, 2) is reported. This begins with the stepwise transformation of M(CO)₆ to the M^II isocyanide complexes M(CO)₂(CNR)₃(Br)₂ (7a-8b) (7: M Mo; 8: M W; a: R Et; b: R ^tBu). Complexes 7a-8b react with Et₂NCCNEt₂ to give the cationic alkyne complexes mer-[Br(CO)(RNC)₃M(η²-Et₂NCCNEt₂)]Br (9a-10b) (9: M Mo; 10: M W). Thermal decarbonylation of 9a-10b with RNC affords the carbonyl-free derivatives trans-[Br(RNC)₄M(η²-Et₂NCCNEt₂)]Br (11a-12b) (11: M Mo; 12: M W). Complexes 11a and 12a can be also obtained from the homoleptic isocyanide complexes M(CNEt)₆ (13: M Mo; 14: M W) by a sequence of reactions resulting in coupling of two ethyl isocyanide ligands to give a bis(diethylamino)acetylene ligand. This sequence is initiated by the regioselective ethylation of M(CNEt)₆ with one equivalent of [Et₃O]BF₄ to give the diethylaminocarbyne complexes [(EtNC)₅MCNEt₂]BF₄ (15: M Mo; 16: M W). Addition of another equivalent [Et₃O]BF₄ to 15 and 16 results in the formation of the bis-diethylaminocarbyne complexes [(EtNC)₄M(CNEt₂)₂(BF₄)₂ (17: M Mo; 18: M W). On treatment with [NEt₄]Br these undergo a carbyne-carbyne coupling reaction to give the alkyne complexes 11a and 12a, respectively. Spectroscopic data for the novel transition-metal complexes of bis(diethylamino)acetylene 9a-12b indicate substantial bond delocalization in the metal-alkyne fragment and restricted rotation of the diethylamino groups around the C_alkyne-N bonds. The alkyne ligand acts as a four-electron donor and allows the metal center to achieve a closed-shell configuration. These observations are supported by the results of a single-crystal X-ray diffraction study of the complex trans-[Br(^tBuNC)₄W(η²-Et₂NCCNEt₂)]PF₆ (12b′), obtained from 12b and TIPF₆.