Dinitrogen as Double Lewis Acid: Structure and Bonding of Triphenylphosphinazine N2(PPh3)2

摘要

Much progress has been made in recent years in the synthesis and isolation of small low-valent, low oxidation state main-group molecules M which are stabilized by N-heterocyclic carbene ligands (NHC) in donor-acceptor complexes (NHC→)_nM. In particular, diatomic species E2 of Group 13-15 elements E which cannot be isolated as free molecules have been synthesized as NHC→E2←NHC species. Robinson et al. reported in 2008 the isolation of the silicon complex Si2(NHC)2 which has a trans-planar arrangement of the CSiSiC moiety featuring a Si=Si double bond. A similar structure was found by us for the heavier homoloeues Ge2(NHC)2 and Sn2(NHC)2. Theoretical studies suggest that the NHC→E2←NHC donation in both these compexes is into the vacant in-plane 1π_u and 1π_g valence orbitals of E2 in the excited (1)1A? state which yields two lone-pairs at the | E = E | moiety (Figure 1 a). An even more dramatic stabilization is found in the recently synthesized boron complex NHC→B2←NHC by Braunschweig et al.which has a boron-boron triple bond and a linear CBBC arrange- ment. The charge donation of the NHC ligands occurs into the vacant 1σ_u and 2σ_g valence orbitals of the highly (double) excited (3)~1Σ_g~+ state of B2 which leads to an electron configuration that is similar to the ground-state configuration of N2 (Figure 1b). Theoretical studies predict that the heavier Group 13 homologues NHC→E2←NHC where E = Al-In have an anti-periplanar structure. The Al-In complexes could not become synthesized to date.