Reversible Spin-State Switching and Tuning of Nuclearity and Dimensionality via Nonlinear Pseudohalides in Cobalt(II) Complexes

The self-assembly of a macrocyclic tetradentate ligand, cobalt­(II) tetrafluoroborate, and nonlinear pseudohalides (dicyanamide and tricyanomethanide) has led to two cobalt­(II) complexes, {[Co­(L)­(μ1,5-dca)]­(BF4)·MeOH} n (1) and [Co2(L)2(μ1,5-tcm)2]­(BF4)2 (2) (L = N,N′-di-tert-butyl-2,11-diaza­[3,3]­(2,6)­pyridinophane; dca– = dicyanamido; tcm– = tricyanomethanido). Both complexes were characterized by single-crystal X-ray diffraction, spectroscopic, magnetic, and electrochemical studies. Structural analyses revealed that 1 displays a one-dimensional (1D) coordination polymer containing [Co­(L)]2+ repeating units bridged by μ1,5-dicyanamido groups in cis positions, while 2 represents a discreate dinuclear cobalt­(II) molecule bridged by two μ1,5-tricyanomethanido groups in a cis conformation. Both complexes have a CoN6 coordination environment around each cobalt center offered by the tetradentate ligand and cis coordinating bridging ligands. Complex 1 exhibits a high-spin (S = 3/2) state of cobalt­(II) in the temperature range of 2–300 K with a weak ferromagnetic coupling between two dicyanamido-bridged cobalt­(II) centers. Interestingly, complex 2 exhibits reversible spin-state switching associated with spin–spin coupling. Complexes 1 and 2 also exhibit interesting redox-stimuli-based reversible paramagnetic high-spin cobalt­(II) to diamagnetic low-spin cobalt­(III) conversion, offering an additional way to switch magnetic properties. A detailed theoretical calculation was consistent with the stated results.