Versatile Reactivity of a Four-Coordinate Scandium Phosphinidene Complex: Reduction, Addition, and CO Activation Reactions

The
four-coordinate scandium phosphinidene complex, [LSc­(μ-PAr)]2 (L = (MeC­(NDIPP)­CHC­(Me)­(NCH2CH2N­(iPr)2)), DIPP = 2,6-(iPr)2C6H3; Ar = 2,6-Me2C6H3) (1), has been synthesized
in good yield, and its reactivity has been investigated. Although 1 has a bis­(μ-phosphinidene)­discandium structural unit,
this coordinatively unsaturated complex shows high and versatile reactivity
toward a variety of substrates. First, two-electron reduction occurs
when substrates as 2,2′-bipyridine, elemental selenium, elemental
tellurium, Me3PS, or Ph3PE (E
= S, Se) is used, resulting in the oxidative coupling of two phosphinidene
ligands 2­[PAr]2– into a diphosphene ligand [ArP-PAr]2–. Complex 1 easily undergoes nucleophilic
addition reactions with unsaturated substrates, such as benzylallene,
benzonitrile, tert-butyl isocyanide, and CS2. This complex also shows a peculiar reactivity to CO and Mo­(CO)6, that includes C–P bond formation, C–C coupling
and C–O bond cleavage of CO, to afford novel phosphorus-containing
products. In the last two types of reactivity, reaction profiles have
been computed (for the insertion of tBuNC
and the CO activation by 1) at the DFT level. The unexpected/surprising
sequence of steps in the latter case is also revealed.