A sixteen-valence-electron carbon-group 13 family with global penta-atomic planar tetracoordinate carbon: an ionic strategy

Abstract

The design of planar tetracoordinate carbon (ptC) has always been a challenge due to its unique bonding mode that necessitates the perfect balance between the carbon center and surrounding ligands both electronically and mechanically. A unique type of 18-valence-electron (18ve) template, i.e., CAl₄²⁻, has been found to be very effective in designing various novel 18ve-species upon skeletal substitution. In this work, we showed that though ptC is not the global structure for the parent 16ve-CAl₄, suitable skeletal substitution can allow for a series of global minimum ptC species. Theoretical calculations at the level of CCSD(T)/def2-QZVP//B3LYP/def2-QZVP for 35 carbon-group 13 systems with 16-ve, i.e., CX_aY_bZ_cK_d (X, Y, Z, K = Al/Ga/In/Tl; 0 ≤ a, b, c, d ≤ 4, a + b + c + d = 4), showed that 9 systems (CAl₃Tl, CGa₃Tl, CGa₂Tl₂, CAl₂GaTl, CAl₂InTl, CGa₂InTl, CAlGa₂Tl, CGa₂InTl and CAlGaInTl) possess global minimum ptC and 2 systems (CAl₃In and CAl₂Tl₂) have quasi-GM ptC. Except for CAl₃Tl and CAl₃In, all the ptCs were predicted for the first time. All these stable ptC structures have the same skeleton and can be described as the same ionic sub-structure, i.e., [A⁻]B⁺. This study not only enriches 16ve-ptC, but also directly demonstrates that utilizing an ionic strategy, non-ptC CAl₄ also can be used as a template to extend the ptC family.