Vicinal diphosphoniums: electrostatic repulsion under covalent constraint.

A series of five vicinal bis(alkyl-triarylphosphoniums) derived from o-bis(diphenyl-phosphino)benzene (o-dppb) is described. Each of them have been prepared by specific methods, and their formal electrostatic and possible van der Waals strain is compared through the P+...P+ distances in the crystal state. According to X-ray diffraction analyses, while the conformations of the dimethyl dication is C2 symmetric, the conformation of alka-1,n-diyl-diphosphoniums (n = 1, 2, 3) is pseudo-Cs symmetric. The solution structure of the dimethyldiphosphonium 2 was studied by NMR techniques after metathesis of the triflate counterions with enantiomerically pure X-PHAT chiral anions (X = TRIS, BIN). [(Delta)-BINPHAT] was indirectly shown to discriminate between the enantiomers of the diphosphonium, but no effective enantio-differentiation could be measured down to -70 degrees C in CD2Cl2. When associated to [(Delta)-TRISPHAT] counterions, progressive cleavage of the diphosphonium afforded the corresponding methyl-triphenyl-monophosphonium salt. In the bridged series, the diphosphoniacyclopentene dication was found to be stable in spite of the very short transannular P+...P+ distance (2.83 A). The naturally more relaxed homologous diphosphoniacyclohexene dication (P+...P+ = 3.28 A) was found to be flexible in solution, as indicated by the equivalence of the four protons of the +P-CH2CH2-P+ bridge from 20 degrees C to -80 degrees C in acetone-d6. The two diphosphoniacycloheptene dications exhibit no P+...P+ steric compression (ca 3.64 A), just as the dimethyl dication (3.70 A). In solution, the seven-membered ring of the +P-CH2CR2CH2-P+ dications (R = H, Me) remains rigidly Cs symmetric: no interconversion of the pseudo-axial and pseudo-equatorial R groups is evidenced at the NMR time scale at -95 degrees C (coalescence at -20 degrees C for R = H, at -65 degrees C for R = Me). According to DFT calculations at the B3PW91/6-31G** level in 1,2-dichloroethane (DCE: PCM, epsilon = 10.36), the lowest energy conformation is indeed Cs-symmetric. However a quasi-isoenergetic C2-symmetric conformation which is the most stable in the gas phase is also found, and is thus a likely intermediate in the observed Cs <==> Cs interconversion. This +P-CH2CMe2CH2-P+ diphosphonium was obtained by dialkylation of o-dppb with 2,2-dimethylpropan-1,3-diyl bistriflate in DCE at 80 degrees C. The use of this solvent was the key for the synthesis of this dication, which was however competitively produced along with some +P-CH2CH2-P+ dication. The latter was also obtained by a totally different route, by heating a sulfinylethyl monophosphonium salt of o-dppb in the presence of the cationic complex [Rh(cod)2][PF6]. A mechanism for this peculiar process is proposed.