Metal Complexes of a Structurally Embellished Phosphinane Ligand: An Assessment of Stereoelectronic Effects



The stereoelectronic features of the pentacyclic phosphane (1S,4R,4aS,5aR,6R,9S,9aS,10aR)-4,6,11,11,12,12-hexamethyl-10-phenyldodecahydro-1,4:6,9-dimethanophenoxaphosphinine (phenop) have been explored through a range of empirical methods including single-crystal X-ray structure determinations of the sulfide derivative phenopS (1), the selenide phenopSe (2), [Fe(CO)41-phenop)] (3), [W(CO)51-phenop)] (4) and trans-[Rh(κ1-phenop)(CO)Cl] (5). Cone angles derived from the structural data range from 164–203° with the smaller values being observed for the compounds possessing a phenyl group that is orthogonal to the P–Z bond and the larger values for the compounds expressing a parallel phenyl ring orientation. The cone angle data suggest a moderately bulky phosphane comparable, in steric terms, to PCy3. This is further borne out on inspection of the M–P bond lengths which tend towards the longer end of the known scale. Some flexibility is observed in the central ring which approximates to a boat conformation at one extreme and an envelope at the other depending on the nature of the P-substituent. The electronic properties of κ1-phenop have been assessed using a combination of infrared and NMR spectroscopy. The absolute value of the one-bond coupling constants 1JP-Se and 1JP-Rh are very close to those reported for PPh3, suggesting a close analogy between κ1-phenop and the well known triphenylphosphane. In addition, relevant υ(CO) stretches in the IR spectra of the metal carbonyl complexes also closely mimic those for the analogous complexes containing PPh3. These conclusions are supported by molecular electrostatic potential calculations at the DFT level which place phenop close to PPh3 in terms of lone pair availability.