A motif for heteronuclear CE (E = Si, Ge, Sn, Pb) bonding: Lewis acid–base pair strategy

Abstract

The design and characterization of the heteronuclear group 14 CE (E = Si, Ge, Sn, Pb) triple bonds have attracted intensive interest in the past few decades. In the current work, utilizing the advantages of N-heterocyclic carbenes (NHCs) and Lewis acid–base pair strategy, we theoretically designed a new class of compounds III-1, i.e., (NHC^AR)CE(Al(C₆F₅)₃). Quantum chemical calculations showed that these singlet compounds possess very favourable isomerization, fragmentation and dimerization stabilities at the B3LYP/def2-TZVPP//B3LYP/def2-SVP level. The calculated bond lengths of CE in III-1 are 1.63 Å for Si, 1.70 Å for Ge, 1.91 Å for Sn and 2.01 Å for Pb, respectively, which are close to or even shorter than the known CE bond lengths. In addition, the significant Mayer bond order values, two orthogonal π orbitals and one σ orbital between the C and E atoms also indicate the characteristics of triple bonds. Based on several bonding analyses, strong delocalization is found to exist between the CE core and NHC^AR forming a weak CC double bond. Hence, such obtained CE species also can be described by their resonace structures as cunmulene analogs. In all, III-1 proposed here not only presents a universal CE motif for all the heavier group 14 elements, but also provides a new strategy for the design and synthesis of heteronuclear group 14 triple bonds in the future.