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

The Ph 2 P-functionalized phosphoniobenzo[c]phospholide 5 reacts with [CrCO) 5 (cyclooctene)] and [Cr(CO) 3 (naphthalene)] under site-selective n(P)-complexation at the Ph 2 P moiety and π-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, BH 3 ) on the π-complex 7 occurs at the pendant Ph 2 P 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)-coordinated benzophospholide moiety and an adjacent sulfur atom (for E = S) or a BH-σ-bond (for E = BH 3 ). 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 π-donor power of the ligand with increasing phosphonium character of the substituents at the benzophospholide moiety. The observed spontaneous π(η 5 )/κ 2 -P,σ-BH-coordination isomerization of a benzophospholide implies that the unusual coordination mode through a phosphorus lone pair and a BH-σ-bond is thermodynamically preferable to π-coordination through the five-membered ring.