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

The binuclear complex (Ni2L)-L-II(H2O)(2)(ClO4)(2) (1) and the neutral tetranuclear bimetallic compounds [{M-III(phen)(CN)(4)}(2){(Ni2L)-L-II(H2O)(2)}]center dot 2CH(3)CN 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 [Ni-2(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 center dot center dot center dot Ni and Cr center dot center dot center dot Ni separations through the single cyanide bridge are 5.058(1) and 5.174(2) angstrom respectively, whereas the Ni-Ni distances across the double phenolate bridge are 3.098(2) (2) and 3.101(1) angstrom (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 (H) over cap = -J (S) over cap (A).(S) over cap (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 [(H) over cap = -J(M-Ni) ((S) over cap (M).(S) over cap (Ni) + (S) over cap (Ma).(S) over cap (Nia)) -J(Ni-Nia) SNiSNia]. 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.