1,3-Diphospholene-4-ylidene chromium (tungsten) pentacarbonyl complexes formed by CO insertion into the ring of a 1,3-diphosphacyclobutane-2,4-diyl-2-ide-complexes of a phosphanyl carbene or a phosphonium ylide?

摘要

Reaction of the 1,3-diphosphacyclobutane-2,4-diyl-2-ide 1 with chromium or tungsten hexacarbonyl afforded the anionic complexes [cyclo-{P(Mes)-C(SiMe_3)-P(Mes)-C(O)-C[M(CO)_5]}]~- (3a,b: M = Cr, W) by the formal insertion of CO into the four membered ring. Computational analysis suggests that this reaction proceeds via two intermediates that can be formulated as a cyclic metal acyl and an acyclic ketenyl complex.The anionic complexes 3a, b further reacted with electrophiles to afford the neutral complexes [cyclo-{P-(Mes)-C(SiMe_3)-P(Mes)-C(OR)-C[M(CO)_5]}] (4a,b: M = Cr, W,R, = Me; 5,6: M = Cr, R = SiMe_3, H). All products were characterized by standard spectroscopic (NMR and MS) techniques, and 4a,b further by extensive one-and two-dimensional multinuclear (~1H, ~(13)C, ~(31)P, ~(183)W) NMR studies. From these investigations, an unequivocal assignment of chemical shifts and coupling constants was derived, confirming unusually large shielding for the formal carbenic carbon atoms which exceed even those in complexes of imidazoyl carbenes. Single-crystal X-ray diffraction analyses of 3a, 4a, b and 5 revealed that all of these compounds contain planaer P_2C_3 rings. The phosphorus atoms are slightly pyramidal, and the carbonmetal distances (C-Cr 218 pm, C-W 230 pm) suggest low bond orders. Comparison of the structural parameters of 3a with those of the O-substitution products 4a, 5 revealed substantial changes inendocyclic P-C bond lengths and the degree of pyramidal character of bonding at the phosphorus atoms. In line with the spectrooscopic and computational results, these effects were interpreted in terms of a considerable reorganization of #pi# electrons in the ring, which induces a substantial degree of aromatic character in the neutral complexes 4-6.