Selective coordination of cyanate and thiocyanate in the end-on mode: synthesis, structures and properties of [Ni II2L(μ1,1-NCO)]+ and [Ni II2L(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 [Ni^II₂L(μ_1,1-NCO)]BPh₄ (3) and [Ni^II₂L(μ_1,1-NCS)]BPh₄ (4) have been synthesized by addition of KOCN or KSCN to [Ni^II₂L](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)(μ_1,1-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.