From end-on coordination of acetonitrile molecules to crosswise bridging ; formation of iminophosphino and acetamidate ligands in a dimolybdenum complex by further reactions with nucleophiles

Three complexes from the reaction of bis(diphenylphosphino)ethane (dppe), water and acetonitrile have been structurally characterized. Reaction of the quadruply M - M bonded complex [Mo₂(CH₃CN)₈ (ax-CH₃CN)] [BF₄J₄ (1a), with dppe produces [Mo₂(μ,η²-CH₃CN)(μη ¹-NC(CH₃)P(Ph₂)CH₂CH ₂PPh₂) (CH₃CN)₇][BF₄]₄ (2), in which an acetonitrile molecule is bound to both Mo atoms in a perpendicular mode. A new chelating ligand, namely NC(CH₃)P(Ph₂)CH₂CH₂PPh₂, is formed by coupling of a dppe to an acetonitrile molecule. It bridges both Mo atoms with its N atom and chelates to one of the Mo atoms with both N and P atoms. When a trace amount of water is introduced into 2 at low temperature, the compound [Mo₂(μ,η²-CH₃CNBF ₃)(μ,η¹-N(H)COCH₃)(CH ₃CN)₈][BF₄]₃ (3), is obtained. It has a crosswise bridging acetonitrile moiety ; the nitrogen atom of this bridging acetonitrile is also coordinated to a BF₃ group in the solid state. An acetamidate ligand, instead of the coupled dppe group, bonds to both Mo atoms symmetrically using only the N atom. Hydrogen bonding is observed between the amide NH group and one F atom of one of the BF^-₄ counteranions. Both compounds 2 and 3 have edge-sharing bioctahedra and elongated Mo - Mo distances. By adding an excess of water to the solution of 2, complex [Mo₂(μ-N(H)COCH₃)₂(CH₃CN) ₄] [BF₄]₂ (4) is produced. The crystal structure of 4 shows two cis bridging acetamidate ligands. The N and O atoms of each acetamido group coordinate to the Mo atoms and they are evenly distributed on two positions. The results are rationalized as nucleophilic attacks of dppe or water molecules on coordinated acetonitrile ligands on the dimolybdenum complexes.