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)CH 2 } 2 2 ; Ar=2,6‐ i Pr 2 C 6 H 3 ), derived from classical N‐heterocyclic carbenes (NHCs), with PCl 3 affords the dichlorovinylphosphanes {(NHC)C(Ph)}PCl 2 (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) Å) are comparable to those of the known diphosphenes (2.02–2.08 Å), the C−P bond lengths of 5 (1.785(1)) and 6 (1.797(1) Å) are, however, considerably shorter than a Csp2−P single bond length (1.85 Å), indicating a considerable π‐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.