Synthesis, crystal structures and magnetic properties of cyanide- and phenolate-bridged [MIIINiII]2 tetranuclear complexes (M = Fe and Cr)

Abstract

The binuclear complex Ni^II₂L(H₂O)₂(ClO₄)₂ (1) and the neutral tetranuclear bimetallic compounds [{M^III(phen)(CN)₄}₂{Ni^II₂L(H₂O)₂}]·2CH₃CN with M = Fe (2) and Cr (3) [H₂L = 11,23-dimethyl-3,7,15,19-tetraazatricyclo[19.3.1.1^9,13]hexacosa-2,7,9,11,13(26),14,19,21(25),22,24-decaene-25,26-diol] have been synthesized and the structures of 2 and 3 determined by single crystal X-ray diffraction. 2 and 3 are isostructural compounds whose structure is made up of centrosymmetric binuclear cations [Ni₂(L)(H₂O)₂]²⁺ and two peripheral [M(phen)(CN)₄]⁻ anions [M = Fe (2) and Cr (3)] acting as monodentate ligands towards the nickel atoms through one of their four cyanide nitrogen atoms. The environment of the metal atoms in 2 and 3 is six-coordinated: two phen-nitrogen and four cyanide-carbon atoms at the iron and chromium atoms and a water molecule, one cyanide-nitrogen and two phenolate-oxygens and two imine-nitrogens from the binucleating ligand L²⁻ at the nickel atom build distorted octahedral surroundings. The values of the Fe⋯Ni and Cr⋯Ni separations through the single cyanide bridge are 5.058(1) and 5.174(2) Å respectively, whereas the Ni–Ni distances across the double phenolate bridge are 3.098(2) (2) and 3.101(1) Å (3). The magnetic properties of 1–3 have been investigated in the temperature range 1.9–290 K. The magnetic behaviour of 1 corresponds to that of an antiferromagnetically coupled nickel(II) dimer with J = −61.0(1) cm⁻¹, the Hamiltonian being defined as Ĥ = −J Ŝ_A·Ŝ_B. An overall antiferromagnetic behaviour is observed for 2 and 3 with a low-lying singlet spin state. The values of the intramolecular magnetic couplings are J_Fe–Ni = +17.4(1) cm⁻¹ and J_Ni–Ni(a) = −44.4(1) cm⁻¹ for 2 and J_Cr–Ni = +11.8(1) cm⁻¹ and J_Ni–Ni(a) = −44.6(1) cm⁻¹ for 3 [Ĥ = −J_M–Ni (Ŝ_M·Ŝ_Ni + Ŝ_Ma·Ŝ_Nia) −J_Ni–NiaS_NiS_Nia]. Theoretical calculations using methods based on density functional theory (DFT) have been employed on 2 in order to analyze the efficiency of the exchange pathways involved and also to substantiate the exchange coupling parameters.