Selective coordination of cyanate and thiocyanate in the end-on mode: Synthesis, structures and properties of [Ni II 2 L(μ1,1-NCO)]+ and [Ni II 2 L(μ1,1-NCS)]+ (L2- = macrocyclic N6S2 ligand)

Abstract

The ability of the dinickel complex [Ni2L]2+ supported by the binucleating hexaaza-dithiophenolate macrocycle (L)2- to bind linear (OCN-, SCN-) and bent triatomic anions (NO2-) has been examined. The complexes [NiII2L(μ1,1-NCO)]BPh4 (3) and [NiII2L(μ1,1-NCS)]BPh4 (4) have been synthesized by addition of KOCN or KSCN to [NiII2L](ClO4)2 (1) and characterized by IR, ESI-MS, UV-vis, SQUID magnetometry, X-ray crystallography (3) and computational studies. [Ni2L][ClO4]2 (1) binds cyanate and thiocyanate ions specifically end-on via their N atoms yielding face-sharing bis(octahedral) N3Ni(μ-S2)(μ1,1-X)NiN3 core structures (X = NCO or NCS), while no reactions occur with NO2-. The X-ray structure of 3∗ and accompanying DFT calculations imply that the selective binding of the pseudohalide ions is governed by repulsive host-guest CH⋯(anion)π interactions. 3 and 4 exhibit an S = 2 ground state that is attained by competing antiferromagnetic and ferromagnetic exchange interactions via the thiolato and cyanato (or thiocyanato) bridges, respectively, with a value for the magnetic exchange coupling constant J of 12.6 and 9.8 cm-1 (H = -2JS1S2). The affinity of 1 for cyanate determined by absorption spectrometry in MeCN/MeOH (1/1 v/v) at I = 0.01 M is ca. two orders of magnitude higher than for thiocyanate (log K11 = 3.92(1) (3), 2.16(1) (4)). The value for 3 was found to compare favorably well with the equilibrium constant derived by isothermal titration calorimetry.