SYNTHESIS, STRUCTURE AND REACTIVITY OF ETA(4)(5E)-BUTADIENYL SUBSTITUTED MOLYBDENUM COMPLEXES

Reaction of lithium halides with the cationic complexes [Mo(NCMe)(eta( 2)-alkyne)(2)L] (L = eta-C5H5 or eta(5)-(CH7)-H-9) afforded the haloge no-bis(alkyne) substituted molybdenum complexes [MoX(eta(2)-alkyne)(2) L] (X = Cl, Br or I). A single-crystal X-ray diffraction study of the complex [MoI(eta(2)-MeC(2)Me)(2)(eta-C5H5)] showed that the two alkyne ligands lie approximately parallel to the Mo-I vector and the plane o f the eta-C5H5 ligand. Reaction of [MoX-(eta(2)-RC(2)R)(2)(eta-C5H5)] with HBF4 . Et(2)O afforded excellent yields of the aqua complexes {=C (R)-eta(3)-[C(R)C(R)CHR]}X(OH2)(eta-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-crys tal X-ray diffraction study of the cation 11 confirmed the presence of co-ordinated H2O and of a eta(4)(5e)-butadienyl fragment in an anti-s upine conformation, the water occupying a co-ordination position trans to the Mo=C bond. The H2O ligand in these cations can be displaced by acetonitrile allowing the synthesis of the complexes (R)-eta(3)-[C(R) C(R)CHR]}X(NCMe)(eta-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-o rdinated MeCN also occupies a position trans to the Mo=C bond. Treatme nt of the aqua complexes with LiX resulted in the formation of the neu tral dihalogeno complexes [Mo{=C(R)-eta(3)-[C(R)C(R)CHR]}X(2)(eta-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)-eta(3)-[C(Et)C(Et)CHEt]}CII(eta-C5H5)] 21, is forme d in high yield on reaction of the acetonitrile-substituted complex 15 with LiCl. Reaction of trimethyl phosphite with the aqua- or acetonit rile-substituted cations resulted in the stereoselective formation of the complexes -eta(3)-[C(R)C(R)CHR]}X(P(OMe)(3)}(eta-C5H5)][BF4] (X = Br, R = Me 23; X = Cl, R = Me 24 and X = Br, R = Et 25). A single-crys tal X-ray study of 23 confirmed the presence of a cisoid anti-supine e ta(4)(5e)-butadienyl ligand and also showed that the P(OMe)(3) ligand occupies a position cis to the Mo=C bond. In contrast, treatment of th e aqua complexes with the poorer pi-acceptor PMe(3) afforded isomeric mixtures of substitution products. However, reaction of complex 14 wit h PMe(3) afforded a complex which was structurally identified by X-ray crystallography as eta(3)-[C(Et)C(Et)CHEt]}Br(PMe(3))(eta-C5H5)][BF4] 26a where the phosphine ligand is cis to the Mo=C bond. The base, Li[ N(SiMe(3))(2)], reacted with 24 to give the X-ray crystallographically identified, air-sensitive, eta(4)-vinylallene complex (4)-CH(Me)=C(Me )C(Me)=C=CH2}{P(OMe)(3)}(eta-C5H5)] 28, which upon treatment with HBF4 . Et(2)O reformed the eta(4)(5e)-butadienyl complex 24.When 23 was re acted with AlHBu(2)(1), the 1,3-diene complex [MoBr{eta(4)-CH(Me)=C(Me )C(Me)=CH(Me)} {P(OMe)(3)}(eta-C5H5)] 29 was formed. Reaction of this air-sensitive molecule with [Ph(3)C][BF4] regenerated 23. The structur es and mechanisms of formation of these various new types of complexes are discussed.