Synthesis, structure and reactivity of η4(5e)-butadienyl substituted molybdenum complexes

Abstract

Reaction of lithium halides with the cationic complexes [Mo(NCMe)(η²-alkyne)₂L](L =η-C₅H₅ or η⁵-C₉H₇) afforded the halogeno-bis(alkyne) substituted molybdenum complexes [MoX(η²-alkyne)₂L](X = Cl, Br or I). A single-crystal X-ray diffraction study of the complex [MoI(η²-MeC₂Me)₂(η-C₅H₅)] showed that the two alkyne ligands lie approximately parallel to the Mo–I vector and the plane of the η-C₅H₅ ligand. Reaction of [MoX(η²-RC₂R)₂(η-C₅H₅)] with HBF₄·Et₂O afforded excellent yields of the aqua complexes [Mo{C(R)-η³[C(R)C(R)CHR]}X(OH₂)(η-C₅H₅)][BF₄](X = Cl, R = Me 9; X = Cl, R = Et 10; X = Br, R = Et 11 and X = I, R = Et 12); a single-crystal X-ray diffraction study of the cation 11 confirmed the presence of co-ordinated H₂O and of a η⁴(5e)-butadienyl fragment in an anti-supine conformation, the water occupying a co-ordination position trans to the MoC bond. The H₂O ligand in these cations can be displaced by acetonitrile allowing the synthesis of the complexes [Mo{C(R)-η³-[C(R)C(R)CHR]}X(NCMe)(η-C₅H₅)][BF₄](X = Br, R = Me 13; X = Br, R = Et 14 and X = I, R = Et 15). A single-crystal structure determination of 14 confirmed the overall geometry of the complex and showed that the co-ordinated MeCN also occupies a position trans to the MoC bond. Treatment of the aqua complexes with LiX resulted in the formation of the neutral dihalogeno complexes [Mo{C(R)-η³-[C(R)C(R)CHR]}X₂(η-C₅H₅)](X = Cl, R = Me 16; X = Cl, R = Et 17; X = Br, R = Et 18; X = I, R = Et 19 and X = Br, R = Me 20). The structure of 18 was confirmed by X-ray crystallography, and it was also found that the mixed dihalogeno complex [Mo{C(Et)-η³-[C(Et)C(Et)CHEt]}ClI(η-C₅H₅)]21, is formed in high yield on reaction of the acetonitrile-substituted complex 15 with LiCl. Reaction of trimethyl phosphite with the aqua- or acetonitrile-substituted cations resulted in the stereoselective formation of the complexes [Mo{C(R)-η³-[C(R)C(R)CHR]}X{P(OMe)₃}(η-C₅H₅)][BF₄](X = Br, R = Me 23; X = Cl, R = Me 24 and X = Br, R = Et 25). A single-crystal X-ray study of 23 confirmed the presence of a cisoid anti-supine η⁴(5e)-butadienyl ligand and also showed that the P(OMe)₃ ligand occupies a position cis to the MoC bond. In contrast, treatment of the aqua complexes with the poorer π-acceptor PMe₃ afforded isomeric mixtures of substitution products. However, reaction of complex 14 with PMe₃ afforded a complex which was structurally identified by X-ray crystallography as [Mo{C(Et)-η³-[C(Et)C(Et)CHEt]}Br(PMe₃)(η-C₅H₅)][BF₄]26a where the phosphine ligand is cis to the MoC bond. The base, Li[N(SiMe₃)₂], reacted with 24 to give the X-ray crystallographically identified, air-sensitive, η⁴-vinylallene complex [MoCl{η⁴-CH(Me)C(Me)C(Me)CCH₂}{P(OMe)₃}(η-C₅H₅)]28, which upon treatment with HBF₄·Et₂O reformed the η⁴(5e)-butadienyl complex 24. When 23 was reacted with AlHBuⁱ₂ the 1,3-diene complex [MoBr{η⁴-CH(Me)C(Me)C(Me)CH(Me)}{P(OMe)₃}(η-C₅H₅)]29 was formed. Reaction of this air-sensitive molecule with [Ph₃C][BF₄] regenerated 23. The structures and mechanisms of formation of these various new types of complexes are discussed.