STEREOELECTRONIC EFFECTS IN PHOSPHORUS DICHLORIDE CATION PYRIDINE COMPLEXES

The kinetic method is applied to order the relative affinities of a gr oup of substituted pyridines towards PC2+ relationships with the affin ities towards other cations. The absolute affinities are estimated wit h the aid of AM1 molecular orbital calculations while the PCl2+ affini ty of pyridine itself is also estimated by ab initio calculations at M P2/6-31G(d,p)//6-31G(d,p) level to be 76.0 kcal mol(-1). The experimen ts employ the PCl2+-bound dimer of two pyridines generated via ion/ mo lecule reactions between the mass-selected PCl2+ ion and a mixture of pyridines. The dimers, examined using MS3 experiments, fragment exclus ively to yield the pyridine/PCl2+ monomers and this is consistent with ab initio RHF/6-31G(d,p) and AM1 molecular orbital calculations which show a tetrahedral complex with a N-P-N angle of 129 degrees. For met a- and parasubstituted pyridines, there is an excellent linear correla tion (slope 0.69) between the logarithm of the ratio of the two fragme nt ion abundances and the proton affinity of the corresponding substit uted pyridine. Similar correlations are observed for other cations (Si Cl3+, Cl+, SF3+ and SiCl+) and it is shown that both the number of deg rees of freedom in the dimer and the cation affinity control this corr elation. Dimers comprising ortho-substituted pyridines show decreased affinities due to stereoelectronic interactions between the ortho-subs tituted alkyl group and the central PCl2+ cation. A set of gas phase s tereoelectronic 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). AM1 calculations show that the eclipsed conforma tion of 2-methylpyridine/PCl2+ adduct is more stable than the staggere d conformation by approx. 3 kcal mol(-1) and this is suggested to be d ue to a favorable agostic interaction between the hydrogen of the orth o methyl group and the central phosphorus atom. The most stable confor mation is found when the two chlorines face the two hydrogens of the o rtho methyl substituent in a ''face-to-face'' interaction. This novel type of interaction is also the reason for the relatively small magnit ude of S-k, the stereoelectronic parameter, in 2,6-dimethylpyridine. T he overall stereoelectronic effects of the ortho-substituent(s) on PCl 2+ affinities indicate that steric effects dominate electronic effects in this system. The PCl2+ ion behaves similarly in its steric and ago stic effects to SF3+ and very differently to SiCl+ which displays uniq uely strong agostic effects. (C) 1997 Elsevier Science B.V.