Unexpectedly High Barriers to M–P Rotation in Tertiary Phobane Complexes: PhobPR Behavior That Is Commensurate with ^tBu₂PR

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The four isomers of 9-butylphosphabicyclo[3.3.1]nonane, s-PhobPBu, where Bu = n-butyl, sec-butyl, isobutyl, tert-butyl, have been prepared. Seven isomers of 9-butylphosphabicyclo[4.2.1]nonane (a₅-PhobPBu, where Bu = n-butyl, sec-butyl, isobutyl, tert-butyl; a₇-PhobPBu, where Bu = n-butyl, isobutyl, tert-butyl) have been identified in solution; isomerically pure a₅-PhobPBu and a₇-PhobPBu, where Bu = n-butyl, isobutyl, have been isolated. The σ-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 = ⁿBu (1a), ⁱBu (1b), ^sBu (1c), ^tBu (1d)); trans-[PtCl₂(a₅-PhobPR)₂] (R = ⁿBu (2a), ⁱBu (2b)); trans-[PtCl₂(a₇-PhobPR)₂] (R = ⁿBu (3a), ⁱBu (3b)); trans-[PdCl₂(s-PhobPR)₂] (R = ⁿBu (4a), ⁱBu (4b)); trans-[PdCl₂(a₅-PhobPR)₂] (R = ⁿBu (5a), ⁱBu (5b)); trans-[PdCl₂(a₇-PhobPR)₂] (R = ⁿBu (6a), ⁱBu (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 and two show a syn conformation. Solution variable-temperature ³¹P NMR studies reveal that all of the Pt and Pd complexes are fluxional on the 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 ^tBu₂PR 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 [^tBu₂PR-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···Cl clashes involving the Pd–Cl groups with H atoms on the α- or β-carbon in the R substituent and H atoms in 1,3-axial sites within the phosphabicycle.