Stereoelectronic effects in phosphorus dichloride cation/pyridine complexes

Abstract

The kinetic method is applied to order the relative affinities of a group of substituted pyridines towards PCl + 2 and to seek relationships with the affinities towards other cations. The absolute affinities are estimated with the aid of AMI molecular orbital calculations while the PCl + 2 affinity of pyridine itself is also estimated by ab initio calculations at MP2/6-31G(d,p)//631G(d,p) level to be 76.0 kcal mol −1. The experiments employ the PCl + 2-bound dimer of two pyridines generated via ion/molecule reactions between the mass-selected PCl + 2 ion and a mixture of pyridines. The dimers, examined using MS 3 experiments, fragment exclusively to yield the pyridine/PCl + 2 monomers and this is consistent with ab initio RHF/6-31G(d,p) and AMI molecular orbital calculations which show a tetrahedral complex with a N-P-N angle of 129°. For meta- and parasubstituted pyridines, there is an excellent linear correlation (slope 0.69) between the logarithm of the ratio of the two fragment ion abundances and the proton affinity of the corresponding substituted pyridine. Similar correlations are observed for other cations (SiC + 3, Cl +, SF + 3 and SiCl +) and it is shown that both the number of degrees of freedom in the dimer and the cation affinity control this correlation. Dimers comprising ortho-substituted pyridines show decreased affinities due to stereoelectronic interactions between the ortho-substituted alkyl group and the central PCl + 2 cation. A set of gas phase stereoelectronic parameters ( S k ) is determined and ordered as 2-MePy (−0.38) < 2,4-diMePy (−0.84) < 2,6-diMePy (−0.86) < 2,5-diMePy (−1.08) < 2,3-diMePy (−1.26). AMI calculations show that the eclipsed conformation of 2-methylpyridine/PCl + 2 adduct is more stable than the staggered conformation by approx. 3 kcal mol −1 and this is suggested to be due to a favorable agostic interaction between the hydrogen of the ortho methyl group and the central phosphorus atom. The most stable conformation is found when the two chlorines face the two hydrogens of the ortho methyl substituent in a “face-to-face” interaction. This novel type of interaction is also the reason for the relatively small magnitude of S k , the stereoelectronic parameter, in 2,6-dimethylpyridine. The overall stereoelectronic effects of the ortho-substituent(s) on PCl + 2 affinities indicate that steric effects dominate electronic effects in this system. The PCl + 2 ion behaves similarly in its steric and agostic effects to SF + 3 and very differently to SiCl + which displays uniquely strong agostic effects.