Binuclear nickel complexes with an edge sharing bis(square-pyramidal) N3Ni(μ-S2)NiN3 core: synthesis, characterization, crystal structure and magnetic properties

Abstract

The synthesis of the novel macrocyclic octadentate amino-thiophenolate ligand H₂L2 (3,7,11,19,23,27-hexaaza-33,34-dithiol-15,31-di(tert-butyl)-tricyclo[27,3,1^13.17]-tetratriaconta-1(32),13,15,17(34),29,30-hexane) and its ability to support binuclear nickel(II) complexes with dithiolato-bridged square-pyramidal Ni^II ions are reported. H₂L2 is obtained as the hexahydrobromide salt from a Schiff-base condensation reaction between 1,2-bis(4-tert-butyl-2,6-diformylphenylthio)ethane and bis(3-aminopropyl)amine followed by two successive reductions with NaBH₄ and Na/NH₃. The ligand forms a green, paramagnetic, binuclear nickel(II) complex dication [Ni^II₂L2]²⁺, which can be isolated as a ClO₄⁻ (4) or BPh₄⁻ salt (5). The binuclear nickel(II) complex contains a central N₃Ni(μ-S)₂NiN₃ core with two square-pyramidal coordinated Ni^II ions. The [Ni₂L2]²⁺ dication does not bind further coligands, in striking contrast to the behaviour of the parent [Ni₂L1]²⁺ dication supported by the smaller (L1)²⁻ macrocycle (containing diethylenetriamine in place of the dipropylenetriamine units) which readily binds a variety of other coligands (L′) to form bisoctahedral [Ni₂L1(L′)]⁺ structures. The unusual behaviour of 4 relates to two different N configurations which leads to a steric shielding of the third bridging position by the CH₂-groups of the dipropylenetriamine chains. An analysis of the temperature-dependent magnetic susceptibility data of 5 reveals the presence of a weak antiferromagnetic exchange interaction between the spins of the nickel(II) ions with a value for the magnetic exchange coupling constant J of −23.5 cm⁻¹ (H = −2JS₁S₂). These results are further substantiated by DFT calculations.