Crystal Structure of the 2:1 Charge-Transfer Salt Derived from Hexamethylenetetraselenafulvalene (HMTSF) and 7,7,8,8-Tetracyano-p-Quinodimethane (TCNQ), (HMTSF)₂(TCNQ)

Abstract

The 2:1 charge-transfer salt (HMTSF)₂(TCNQ) crystallizes (from mesitylene) in the monoclinic system, space group P2₁/c, with the following cell constants: a = 13.993(4) Å, b = 10.723(3) Å, c = 13.032(3) Å, β = 112.71(2)°, V = 1803.8 ų, D _meand = 2.10(1) g cm⁻³, D _caled = 2.114 g cm⁻³ (for two formula units per cell). The crystal structure of (HMTSF)₂(TCNQ) consists of linear chains of alternating donor diads and acceptor monads propagating along the crystallographic b axis. Within these mixed stacks, the donor/donor overlap is of the ring-over-double bond type (mean separation 3.72 Å), while the donor/acceptor overlap (mean separation 3.46 Å) is such that the HMTSF molecule is very nearly parallel to and directly centered above the adjacent TCNQ molecule. Strong interstack interactions of the types Se(HMTSF)…Se(HMTSF), 3.64 Å, and Se(HMTSF)…N(TCNQ), 3.29 Å, serve to couple adjacent mixed stacks. The crystal structure of this 2:1 salt is compared to that of its 1:1 stoichiometric congener, HMTSF-TCNQ, whose crystal structure exhibits uniform segregated stacks of donor and acceptor molecules.