A Modular Access to Divinyldiphosphenes with a Strikingly Small HOMO-LUMO Energy Gap

The olefinic C-H bond functionalization of (NHC)CHPh (NHC=IPr=C{(NAr)CH}(2) 1; SIPr=C{(NAr)CH2}(2) 2; Ar=2,6-iPr(2)C(6)H(3)), derived from classical N-heterocyclic carbenes (NHCs), with PCl3 affords the dichlorovinylphosphanes {(NHC)C(Ph)}PCl2 (NHC=IPr 3, SIPr 4). Two-electron reduction of 3 and 4 with magnesium leads to the formation of the divinyldiphosphenes [{(NHC)C(Ph)}P](2) (NHC=IPr 5, SIPr 6) as crystalline solids. Unlike literature-known diphosphenes, which are mostly yellow or orange, 5 is a green whereas 6 is a purple solid. Although the P=P bond lengths of 5 (2.062(1)) and 6 (2.055(1) angstrom) are comparable to those of the known diphosphenes (2.02-2.08 angstrom), the C-P bond lengths of 5 (1.785(1)) and 6 (1.797(1) angstrom) are, however, considerably shorter than a Csp2-P single bond length (1.85 angstrom), indicating a considerable pi-conjugation between C=C and P=P moieties. The HOMO-LUMO energy gap for 5 (4.15) and 6 (4.52 eV) is strikingly small and thus the narrowest among the diphosphenes (>4.93 eV) reported as yet. Consequently, 5 readily undergoes P=P bond cleavage at room temperature on treatment with sulfur to form the unique dithiophosphorane {(IPr)C(Ph)}P(S)(2) 7. Interestingly, reaction of 5 with selenium gives the selenadiphosphirane [{(IPr)C(Ph)}P](2)Se 8 with an intact P-P bond.