Dinitrogen, butadiene and related complexes of molybdenum. Crystal structures of [Mo(N2)(PMe3)5] and [Mo(η3-CH3CHCHCH2)(η4-C4H6)(PEt3)2][BF4]

Abstract

The sodium amalgam reduction of [MoCl₃(PEt₃)₃] prepared in situ, under 2–3 atm of N₂, yielded the dimeric complex [{Mo(N₂)₂(PEt₃)₃}₂(µ-N₂)]1, which reacted with 3 and with 4 equivalents of PMe₃ to produce trans-[Mo(N₂)₂(PMe₃)₃(PEt₃)]2 and trans-[Mo(N₂)₂(PMe₃)₄]3, respectively. In the presence of an excess of PMe₃(>5 equivalents, under Ar) the known [Mo(N₂)(PMe₃)₅]4a was the final product. This synthetic methodology allowed the preparation, for the first time, of the pure trans-bis(dinitrogen) compound 3. Complexes of the type [Mo(N₂)(PMe₃)₃(L–L)](L–L = Me₂PCH₂PMe₂4b, Me₂PCH₂CH₂PMe₂4c or Et₂PCH₂CH₂PEt₂4d) and [Mo(N₂)(PMe₃){N(CH₂CH₂PMe₂)₃}]4e were also synthesized and structurally characterized by spectroscopic methods. Upon reaction with C₂H₄, under mild conditions, 1 gave the bis(butadiene) derivative [Mo(η⁴C₄H₆)₂(PEt₃)₂]5a. Protonation of 5a, and of its PMe₂Ph analogue 5b, produced the cationic but-2-enyls [[graphic omitted]CHCHCH₂)(η⁴-C₄H₆)L₂][BF₄](L = PEt₃6a or PMe₂Ph 6b) in which the butenyl ligand exhibits the classical η³-allylic co-ordination in addition to an agostic Me–Mo interaction. Compounds 4a and 6a were structurally characterized by X-ray crystallography. The Mo atom of 4a resides on a crystallographic mirror plane and is bound to the N₂ moiety with a Mo–N separation of 2.02(3)Å. The Mo–C and Mo–H separations within the agostic unit of 6a are 2.48(2) and 1.9(2)Å, respectively.