Stabilization of Hypophosphite in the Binding Pocket of a Dinuclear Macrocyclic Complex: Synthesis, Structure, and Properties of [Ni2L(μ-O2PH2)]BPh4(L = N6S2Donor Ligand)

Abstract

The dinickel(II) complex [Ni2L(ClO4)]ClO4 (1), where L(2-) represents a 24-membered macrocyclic hexaamine-dithiophenolate ligand, reacts with [nBu4N]H2PO2 to form the hypophosphito-bridged complex [Ni2L(μ-O2PH2)](+), which can be isolated as an air-stable perchlorate [Ni2L(μ-O2PH2)]ClO4 (2) or tetraphenylborate [Ni2L(μ-O2PH2)]BPh4 (3) salt. 3·MeCN crystallizes in the triclinic space group P1̅. The bisoctahedral [Ni2L(μ-O2PH2)](+) cation has a N3Ni(μ1,3-O2PH2)(μ-S)2NiN3 core structure with the hypophosphito ligand attached to the two Ni(II) ions in a μ1,3-bridging mode. The hypophosphito ligand is readily replaced by carboxylates, in agreement with a higher affinity of the [Ni2L](2+) dication for more basic oxoanions. Treatment of [Ni2L(μ-O2PH2)]ClO4 with H2O2 or MCPBA results in the oxidation of the bridging thiolato to sulfonato groups. The hypophosphito group is not oxidized under these conditions due to the steric protection offered by the supporting ligand. An analysis of the temperature-dependent magnetic susceptibility data for 3 reveals the presence of ferromagnetic exchange interactions between the Ni(ii) (S = 1) ions with a value for the magnetic exchange coupling constant J of +22 cm(-1) (H = -2JS1S2). These results are additionally supported by DFT (density functional theory) calculations.