Reactions with electrophiles control isomerization from η5(π)- to η1(P)-coordination of phosphinyl-substituted benzophospholide ligands

The Ph2P-functionalized phosphoniobenzo[c]phospholide 5 reacts with [Cr(CO)(5)(cyclooctene)] and [Cr(CO)(3)(naphthalone)] under site-selective n(P)-complexation at the Ph2P moiety and pi-complexation at the five-membered heterocycle, respectively, to give complexes 6 and 7, which were characterized by spectroscopic data and an X-ray diffraction study of 6. The attack of electrophiles E (E = alkyl(+), Ag+, S, BH3) on the pi-complex 7 occurs at the pendant Ph2P substituent to give unstable quaternization products that were in some cases detectable by NMR. The initial products decay either via decomplexation to metal-free quaternization products (for E = alkyl(+)) or via a combination of decomplexation/isomerization reactions to give mixtures of free quaternization products and complexes 10a and 13a containing a Cr(CO)(4) unit that is chelated by a n(P)-coo'rdinated benzophospholide moiety and an adjacent sulfur atom (for E = S) or a BH-sigma-bond (for E = BH3). These products as well as their tungsten analogues 10b and 13b were likewise accessible from the appropriate ligands,and [M(CO)(4)(norbornadiene)], and 10a,b and 13b were isolated from these reactions and characterized by spectroscopic and X-ray diffraction studies. The course of the studied reactions of 7 with electrophiles suggests that the, destabilization of the primary quaternization' products is controlled by increased weakening of the pi-donor power of the ligand with increasing phosphonium character of the substAuents at the benzophospholide moiety. The observed spontaneous pi(eta(5))/kappa(2)-P,sigma-BH-coordination isomerization of a benzophospholide implies that the, unusual coordination mode through a phosphorus lone pair and a BH-sigma-bond, is thermodynamically preferable to pi-coordination through the five-membered ring.