Unexpectedly High Barriers to M-P Rotation in Tertiary Phobane Complexes: PhobPR Behavior That Is Commensurate with t-Bu2PR

The four isomers of 9-butylphosphabicyclo[3.3.1]nonane, 5-PhobPBu, where Bu = n-butyl, sec-butyl, isobutyl, tert-butyl, have been prepared. Seven isomers of 9-butylphosphabicyclo[4.2.1]nonane (a(5)-PhobPBu, where Bu = n-butyl, sec-butyl, isobutyl, tert-butyl; a(7)-PhobPBu, where Bu = n-butyl, isobutyl, tert-butyl) have been identified in solution; isomerically pure a(5)-PhobPBu and a(7)-PhobPBu, where Bu = n-butyl, isobutyl, have been isolated. The sigma-donor properties of the PhobPBu ligands have been compared using the J(PSe) values for the PhobP(=Se)Bu derivatives. The following complexes have been prepared: trans-[PtCl2(s-PhobPR)(2)] (R = Bu-n (1a), Bu-i (1b), Bu-s (1c), Bu-t (1d)); trans-[PtCl2(a(5)-PhobPR)(2)] (R = Bu-n (2a), Bu-t (2b));.trans-[PtCl2(a(7)-PhobPR)(2)] (R = Bu-n (3a), Bu-i (3b)); trans-[PdCl2(5-PhobPR)(2)] (R = Bu-n (4a), Bu-t (i)(4b)); trans-[PdCl2(a(5)-PhobPR)(2)] (R = Bu-n (5a), Bu-i (5b)); trans-[PdCl2(a(7)-PhobPR)(2)] (R = Bu-n (6a), Bu-i (6b)). The crystal structures of 1a-4a and 1b-6b have been determined, and of the ten structures, eight show an anti conformation with respect to the position of the ligand R groups conformation. Solution variable-temperature P-31 NMR studies reveal that all of the Pt and Pd complexes NMR time scale. In each case, two species are present (assigned to be the syn and anti conformers) which interconvert with kinetic barriers in the range 9 to >19 kcal mol(-1). The observed trend is that, the greater the bulk, the higher the barrier. The magnitudes of the barriers to M-P bond rotation for the PhobPR complexes are of the same order as those previously reported for (Bu2PR)-Bu-t complexes. Rotational profiles have been calculated for the model anionic complexes [PhobPR-PdCl3](-) using DFT and these faithfully reproduce the trends seen in the NMR studies of trans-[MCl2(PhobPR)(2)]. Rotational profiles have also been calculated for [(Bu2PR)-Bu-t-PdCl3](-), and these show that the greater the bulk of the R group, the lower the rotational barrier: i.e., the opposite of the trend for [PhobPR-PdCl3](-). Calculated structures for the species at the maxima and minima in the M-P rotation energy curves indicate the origin of the restricted rotation. In the case of the PhobPR complexes, it is the rigidity of the bicycle that enforces unfavorable H center dot center dot center dot Cl clashes involving the Pd-Cl groups with H atoms on the alpha- or beta-carbon in the R substituent and H atoms in 1,3-axial sites within the phosphabicycle.