Theoretical Description of Triplet Silylenes Evolved from H2Si═Si

Abstract

Computations describe the dependence of the H2M═Si triplet electronic structure on the α-substituent. Whereas silylidenes H2C═Si and H2Si═Si benefit from a π1p1 triplet state, the electronegative nitrogen of HN═Si prefers an n1p1 triplet. CCSD(T) and B3LYP calculations predict R2Si═Si triplet silylenes are stabilized by π-donor/σ-acceptor R substituents which compensate for electron deficiency in the singly occupied π orbital of the π(1)p(1) triplet state. (NH2)2Si═Si, (OH)2Si═Si, F2Si═Si, (NH2)HSi═Si, and (OH)HSi═Si all are triplet ground states. In particular, (NH2)2Si═Si and (OH)2Si═Si have singlet–triplet energy gaps (ΔES-T = ET – ES) of −10.2 and −10.3 kcal/mol, respectively. More practical results are achieved via cyclization of (NH2)2Si═Si, which eliminates the probability of rearrangement. Unsaturation of the resulting cyclic structure to give (NHCHCHNH)Si═Si leads to a more favorable triplet silylene with a ΔES-T value of −19.6 kcal/mol. Similar to the common approach of bulky substitution in the synthesis of singlet Arduengo-type N-heterocyclic silylenes, triplet (NRCH2CH2NR)Si═Si and (NRCHCHNR)Si═Si could be experimentally achievable.