Trans-dicyanobis(acetylacetonato)ruthenate(III) as a precursor to build novel cyanide-bridged RuIII–MII bimetallic compounds [M = Co and Ni]

The use of the mononuclear complex trans-[Ru(acac)2(CN)2]− as a ligand towards the preformed species [Ni2L(H2O)2Cl2], [Co(dmphen)](NO3)2, [Ni(dmphen)](NO3)2 and [Co(H2O)6](NO3)2 afforded the novel cyanide-bridged bimetallic compounds of formula [{Ru(acac)2(CN)2}{Ni2(L)(H2O)2}]{Ru(acac)2(CN)2}·2H2O (1), [{Ru(acac)2(CN)2}{Co(dmphen)(NO3)}]·H2O (2) and [{Ru(acac)2(CN)2}{Ni(dmphen)(NO3)}]·H2O (3) and [{Ru(acac)2(CN)2}2Co] (4) [Hacac = acetylacetone, dmphen = 2,9-dimethylphenanthroline and H2L = 11,23-dimethyl-3,7,15,19-tetrazatricyclo[19.3.1.19,13]hexacosa-2,7,9,11,13(26),14,19,21(25),22,24-decaene-25,26-diol]. Their syntheses, X-ray crystal structures and magnetic properties are reported here. The structure of 1 consists of cationic cyanide-bridged chains of formula [{Ru(acac)2(CN)2}{Ni2(L)(H2O)2}]+ and mononuclear anions [Ru(acac)2(CN)2]−. Compounds 2 and 3, which are isostructural, are made up of cyanide-bridged neutral chains [{Ru(acac)2(CN)2}{M(dmphen)(NO3)}] with regular alternating ruthenium(III) and cobalt(II) (2)/nickel(II) (3) ions. Each cobalt atom in 4 is tetrahedrally coordinated by four [RuIII(acac)2(CN)2]− ions through the cyano nitrogen atoms to afford a two-fold interpenetrated (6,4) three-dimensional network which is formed by 12 gon cycles having six ruthenium and six cobalt atoms. The metal–metal separations through the cyanide bridge are 5.2491(1) Å for 1, 5.2038(12) and 5.2426(12) Å for 2, 5.2130(12) and 5.2253(12) Å for 3 and 5.1155(15) and 5.1100(15) Å for 4. The magnetic properties of 1–4 together with those of the mononuclear precursor trans-PPh4[Ru(acac)2(CN)2] (PPh4+ = tetraphenylphosphonium cation and Hacac = acetylacetone) were investigated in the temperature range 1.9–295 K. The cyano-bearing ruthenium(III) precursor exhibits the magnetic behavior expected for a low-spin distorted octahedral ruthenium(III) system with spin-orbit coupling of the 2T2g ground term. Compound 1 exhibits an overall antiferromagnetic behavior, the magnetic coupling between the nickel(II) ions through the double phenoxo group and that between adjacent ruthenium(III) and nickel(II) centers through the single cyano bridge being −50.0 and +6.6 cm−1, respectively. Compounds 2 and 3 behave as ferromagnetic chains. The intrachain magnetic coupling in 3 is +3.2 cm−1 whereas that in 2 could not be evaluated because of the lack of a suitable model. No magnetic ordering is observed in 2 and 3 down to 1.9 K. Compound 4 shows an overall ferromagnetic behavior and it exhibits ferromagnetic ordering at Tc = 5.4 K. Theoretical calculations based on density functional theory (DFT) have been employed on the trans-[Ru(acac)2(CN)2]− mononuclear complex and on dinuclear fragments of the compounds 1–3 in order to analyze the efficiency of the exchange pathways through the double phenoxo (1) and single cyano bridge (1–3) and to substantiate the exchange coupling parameters involved.