A strong organic electron donor incorporating highly π-donating triphenylphosphonium ylidyl substituents

The π-electron donor strength of a triphenylphosphonium ylidyl group (Ph 3 P&z.dbd;CH-) was explored through its substitution onto a bispyridinylidene (BPY) scaffold. Electrochemical studies revealed that the new triphenylphosphonium ylidyl-substituted BPY is the most reducing di-substituted derivative reported to date ( E 1/2 = −1.55 V vs. SCE). By using a previously established correlation between the redox potential of the substituted BPYs and the corresponding substituent, a Hammett constant for the Ph 3 P&z.dbd;CH- group was determined ( σ p + = −2.33), establishing it as the most donating neutral substituent currently quantified. The BPY is readily oxidized by hexachloroethane to produce the corresponding dicationic bipyridinium salt as a mixture of isomers owing to hindered C ylidyl -C pyridyl bond rotation. In preliminary tests of the BPY as a reductant, dichlorotricyclohexylphosphorane and chlorodiphenylphosphine were reduced to the corresponding phosphine and diphosphine, respectively. The use of Ph 3 P&z.dbd;CH-, the most donating substituent yet quantified, makes B more reducing than D or E and causes hindered bond rotation in A and C .