Biomedical subjects
Publications and source records attributed to Xavier Solans.
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The first example of spin crossover iron(II) complexes based on dihydrobis(1-pyrazolyl)borate are presented here. The complexes {Fe[H(2)B(Pz)(2)](2)(phen)} (phen = 1,10-phenanthroline), 1, and {Fe[H(2)B(Pz)(2)](2)(bipy)} (bipy = 2,2'-bipyridine), 2, have been synthesized and their structures determined by X-ray diffraction methods. Crystals 1 and 2 are monoclinic, space group C2/c, Z = 4 with a = 17.448(4) Å, b = 16.101(4) Å, c = 10.611(2) Å, and beta = 112.47(2) degrees for 1 and a = 16.307(2) Å, b = 15.075(4) Å, c = 11.024(4) Å, and beta = 114.95(5) degrees for 2 at 293 K. The crystal structure of 2 was also determined at 139 K in order to detect the structural changes associated with the S = 0 <--> S = 2 spin conversion. 2 retains the same space group upon spin conversion with a = 16.086(6) Å, b = 14.855(6) Å, c = 10.812(2) Å, and beta = 114.18(3) degrees. The structures of 1 and 2 are made up of mononuclear neutral species where the positive charge of iron(II) is neutralized through the coordination of two chelate bidentate dihydrobis(pyrazolyl)borate anions, and phen or bipy neutral ligands are used to fill the iron(II) coordination sphere. The molecular structures for both compounds are very similar, with Fe-N bond lengths in the 2.212-2.158 Å range for the high-spin phase. The structural modifications associated with the spin change in 2 mainly consist of a large reorganization of the metal environment: the Fe-N decreases by 0.15 Å (mean value) when the temperature is lowered from 290 to 139 K and a more regular shape of the [FeN(6)] octahedron is achieved through a slight modification of the trigonal deformation angle from 5.3 degrees to 3.2 degrees along with remarkable variations of the N-Fe-N angles. The thermodynamic model of Slichter and Drickamer was applied to account for the magnetic data. The intermolecular interaction parameter and the enthapy and entropy changes associated with the spin transition were estimated as Gamma = 3.3 kJ mol(-)(1), DeltaH = 13.4 kJ mol(-)(1), and DeltaS = 81.9 J mol(-)(1) K(-)(1) for 1 and Gamma = 1.7 kJ mol(-)(1), DeltaH = 13.4 kJ mol(-)(1), and DeltaS = 83.9 J mol(-)(1) K(-)(1) for 2, respectively.
The trinuclear complex (&mgr;(3)-CO(3))[Ni(3)(Medpt)(3)(NCS)(4)] was obtained by reaction of basic solutions of nickel(II), Medpt (bis(3aminopropyl)methylamine) and thiocyanate ligand with atmospheric CO(2) or by simple reaction with carbonate anion. (&mgr;(3)-CO(3))[Ni(3)(Medpt)(3)(NCS)(4)] crystallizes in the triclinic system, space group P&onemacr;, with a = 12.107(5) Å, b = 12.535(7) Å, c = 16.169(9) Å, alpha = 102.69(5) degrees, beta = 92.91(5) degrees, gamma = 118.01(4) degrees, Z = 2, and R = 0.043. The three nickel atoms are asymmetrically bridged by one pentadentate carbonato ligand, which shows a novel coordination mode. The (&mgr;(3)-CO(3))[Ni(3)(Medpt)(3)(NCS)(4)] compound shows a very strong antiferromagnetic coupling. Fit as irregular triangular arrangement gave J(1) = -88.4, J(2) = -57.7, and J(3) = -9.6 cm(-)(1), which is the strongest AF coupling observed to date for Ni(3) compounds. The magnetic behavior of the carbonato bridge is discussed.
Two new nickel(II) end-to-end azido-bridged compounds, cis-catena-[NiL(2)(&mgr;-N(3))](n)()(ClO(4))(n)().nH(2)O (1) and [Ni(2)L(4)(&mgr;-N(3))(2)](PF(6))(2) (2), were synthesized and characterized; L is 2-(aminoethyl)pyridine. The crystal structures of 1 and 2 were solved. Complex 1: monoclinic system, space group P2(1)/a, a = 8.637(2) Å, b = 18.9995(7) Å, c = 12.3093(7) Å, beta = 105.92(2) degrees, Z = 4. Complex 2: triclinic system, space group P&onemacr;, a = 9.139(7) Å, b = 10.124(3) Å, c = 12.024(2) Å, alpha = 70.407(14) degrees, beta = 84.19(2) degrees, gamma = 67.67(4) degrees, Z = 1. In the two complexes the nickel atom is situated in a similarly distorted octahedral environment. The two complexes are different; 1 is a one-dimensional helicoidal complex with the two L ligands and the two end-to-end azido bridges in a cis arrangement while complex 2 is a dinuclear system with two end-to-end azido bridges, indicating the extreme importance of the counteranion present (ClO(4)(-) for 1 and PF(6)(-) for 2). The magnetic properties of the two compounds were studied by susceptibility measurements vs temperature. The chi(M) vs T plot for 1 shows the shape for a weakly antiferromagnetically coupled nickel(II) one-dimensional complex without a maximum until 4 K. In contrast, for complex 2 the shape of the chi(M) vs T curve shows a maximum near 40 K, indicating medium antiferromagnetic coupling. From the spin Hamiltonian -J(ij)()S(i)()S(j)(), J values for 1 and 2 were less than -1 and -29.1 cm(-)(1), respectively. The magnetic behavior for 1 and 2 may be explained in terms of the overlap between magnetic orbitals, taking into account the torsion of the Ni(II) atoms and azido-bridging ligands in the two structures.
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