Self-assembly, metal binding ability, and magnetic properties of dinickel(ii) and dicobalt(ii) triple mesocates

Abstract

Two metallacyclic complexes of general formula Na₈[M^II₂L₃]·xH₂O [M = Ni (4) and Co (5) with x = 15 (4) and 17 (5)] have been self-assembled in aqueous solution from N,N′-1,3-phenylenebis(oxamic acid) (H₄L) and M²⁺ ions in a ligand/metal molar ratio of 3 : 2 in the presence of NaOH acting as base. X-Ray structural analyses of 4 and 5 show triple-stranded, dinuclear anions of the meso-helicate-type (so-called mesocates) with C_3h molecular symmetry. The two octahedral metal–tris(oxamate) moieties of opposite chiralities (Δ,Λ form) are connected by three m-phenylene spacers at intermetallic distances of 6.822(2) (4) and 6.868(2) Å (5) to give a metallacryptand core. In the crystal lattice, the binding of these heterochiral dinickel(II) and dicobalt(II) triple mesocates to sodium(I) ions leads to oxamato-bridged heterobimetallic three-dimensional open-frameworks with a hexagonal diamond architecture having small pores of 17.566(4) (4) and 17.640(2) Å (5) in diameter where the crystallization water molecules and the sodium(I) countercations are hosted. Variable temperature (2.0–300 K) magnetic susceptibility measurements reveal relatively anisotropic S = 2 Ni^II₂ (4) and S = 3 Co^II₂ (5) ground states resulting from the moderate to weak intramolecular ferromagnetic coupling between the two high-spin Ni^II (S_Ni = 1) or Co^II (S_Co = 3/2) ions across the m-phenylenediamidate bridges [J = +3.6 (4) and +1.1 cm⁻¹ (5); H = −JS₁·S₂]. A simple molecular orbital analysis of the electron exchange interaction identifies the π-type pathways of the meta-substituted phenylene spacers involving the d_z² and d_x²−y² pairs of magnetic orbitals of the two trigonally distorted octahedral high-spin M^II ions (M = Ni and Co) as responsible for the overall ferromagnetic coupling observed in 4 and 5 in agreement with a spin polarization mechanism. The decrease of the overall ferromagnetic coupling from 4 to 5 is in turn explained by the additional antiferromagnetic exchange contribution involving the d_xy pair of magnetic orbitals of the two trigonally distorted octahedral high-spin Co^II ions across the σ-type pathway of the meta-substituted phenylene spacers.