Syntheses and chemistry of tris(2-pyridyl)phosphine complexes of Group VI transition metals. X-ray structural studies of the molybdenum complexes

Treatment of P(2-py)(3) with Mo(CO)(6) or M(CO)(3)(CH3CN)(3) (M=Cr, W) in C H3CN affords compounds of the type, P(2-py)(3)M(CO)(3) (1a, M = Cr; 1b, M = Mo; 1c, M = W). Complex P(2-py)(3)Mo(CO)(3). CH2Cl2 was structurally chara cterized, and its ORTEP drawing shows an almost perfect octahedral arrangem ent around the Mo center, and the P(2-py)(3) ligand occupying the facial po sition of the octahedron through the coordination of three pyridyl nitrogen atoms. Substitution of CO ligands can take place when treating compounds I b and Ic with one or two equiv, of NOBF4 in CH3NO2 solvent to afford [P(2-p y)(3)M(CO)(3-) n(NO)(n)](BF4)(n) (2a, M = Mo, n = 1; 2b, M = W, n = 1; 3a, M = Mo, n = 2; 3b, M = W, n = 2). These singly and doubly charged cationic species have been characterized by H-1-, C-13-, P-31-, F-19-NMR and IR spec troscopy, as well as elemental analysis. The CO lability of 3a and 3b gener ated the 16-electron species, [P(2-py)(3)M(NO)(2)](2+), which binds to most Lewis basic donor ligands to give complexes of the type [P(2-py)(3)M(L)(NO )(2)](BF4)(2) (L = nitriles, aldehydes). A single crystal of [P(2-py)(3)M(C H3CH2CN)(NO)(2)](BF4)(2). CH3NO2 was also isolated and structurally charact erized. The crystallographic analysis indicated an octahedral arrangement a bout the Mo metal center. The H-1-NMR downfield shift of the metal-coordina ted crotonaldehyde in compounds 4c and 5c is used as a qualitative measurem ent of relative Lewis acidity, and the acid strength of [P(2-py)(3)M(NO)(2) ](2+) is comparable to that of TiCl4 when M=Mo and to BF3 and AlCl3 when M= W. The Diels-Alder reactions between cyclohexadienes and methyl vinyl keton e were catalyzed by 0.3 mol% of [P(2-py)(3)W(CO)(NO)(2)](BF4)(2) (3b) to af ford an average 94% conversion and 83% isolated yield. (C) 1999 Elsevier Sc ience S.A. All rights reserved.