Crystal structures of the carborane dianions [1,4-(PhCB₁₀H₁₀C)₂C₆H₄]²⁻ and [1,4-(PhCB₁₀H₁₀C)₂C₆F₄]²⁻ and the stabilizing role of the para-phenylene unit on 2n+3 skeletal electron clusters

Abstract

While carboranes with 2 n+2 and 2 n+4 (n=number of skeletal atoms) skeletal electrons (SE) are widely known, little has been reported on carboranes with odd SE numbers. Electrochemical measurements on two-cage assemblies, where two C-phenyl-ortho-carboranyl groups are linked by a para-phenylene or a para-tetrafluorophenylene bridge, revealed two well separated and reversible two-electron reduction waves indicating formation of stable dianions and tetraanions. The salts of the dianions were isolated by reduction with sodium metal and their unusual structures were determined by X-ray crystallography. The diamagnetic dianions contain two 2 n+3 SE clusters where each cluster has a notably long carborane C-carborane C distance of ca 2.4 Å. The π conjugation within the phenylene bridge plays an important role in the stabilization of these carboranes with odd SE counts.

Keywords: carborane; conjugation; electrochemistry; phenylene; quinoidal structures.

Scheme 1

Diphenyl-ortho-carborane 1 and the two-cluster assemblies 2, 3 and 4.

Figure 1

Molecular structures of the dianions [3]²⁻ and [4]²⁻ in the crystals. The tris(dimethoxyethane)sodium cations and hydrogen atoms are omitted for clarity.

Scheme 2

Generic structures for [3]²⁻ and [4]²⁻ (left) and 5 (right) with atom numbering. The Me₃Si groups attached to the carbon atoms in 5 are omitted for clarity.

Scheme 3

Structures of disilaquinodimethane 6 and diboraquinodimethane 7.