Nickel coordination compounds of the cyanamido squarate ligand. The crystal structures of the 2,4-bis(cyanamido)cyclobutane-1,3-dione dianion (2,4-NCNsq 2−) and of three polymers with general formula [Ni(Him) x(H 2O) 4 − x(2,4-NCNsq) · yH 2O] n ( x = 2–4; y = 1–3; Him = imidazole)

Abstract

Disodium and ditetraphenylphosphonium salts of 2,4-bis(cyanamido)cyclobutane-1,3-dione, Na 2(2,4-NCNsq) · 2H 2O ( 1) and (Ph 4P) 2(2,4-NCNsq) · 4H 2O ( 2) have been synthesized and used to prepare three nickel(II) complexes: [Ni(Him) 4(2,4-NCNsq) · 2H 2O] n ( 3), [Ni(Him) 3(H 2O)(2,4-NCNsq) · H 2O] n ( 4) and [Ni(Him) 2(H 2O) 2(2,4-NCNsq) · 3H 2O] n ( 5). Compounds 1– 5 have been synthesized and characterized by electronic absorption and IR spectroscopies and by electrochemical studies. The structures of the salt 2 and of the complexes 3– 5 have been determined by X-ray crystallography. The dianion of 2 presents a centro-symmetric square planar structure with two NCN functions deviated from the ring plane by an angle of 20.6°. Complexes 3– 5 are polynuclear nickel(II) compounds containing 2,4-NCNsq 2− bridging ligands. These ligands are coordinated by the nitrile nitrogen atoms of the cyanamido functions and lie in cis (complex 4) or trans (complexes 3 and 5) positions in the coordination sphere of the nickel(II). The magnetic properties of the three complexes have been investigated in the 2–300 K temperature range (μ eff = 2.79–3.34 μ B for 3, 2.76–3.25 μ B for 4 and 3.14–3.46 μ B for 5). Their redox properties are discussed and compared to those of the free 2,4-NCNsq 2− dianion.