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.
A new heterotrinuclear complex of formula [Cu(2)Zn(Hdmg)(2)(dmg)(2)(H(2)O)].0.5H(2)dmg.H(2)O (C(18)H(34)Cu(2)N(9)O(11)Zn, 1) (H(2)dmg = dimethylglyoxime) has been synthesized and its crystal structure determined by single-crystal X-ray diffraction. It crystallizes in the triclinic system, space group P&onemacr;, with a = 11.414(1) Å, b = 11.992(3) Å, c =12.567(10) Å, alpha = 91.27(6) degrees, beta = 111.46(2) degrees, gamma = 112.24(2) degrees, and Z = 2. The structure consists of a chain of neutral [Cu(2)Zn(Hdmg)(2)(dmg)(2)(H(2)O)] trinuclear units and noncoordinated H(2)dmg and water molecules. The configuration around the zinc atom is distorted trigonal bipyramidal with four oximate oxygens from two [Cu(Hdmg)(dmg)](-) fragments (each one acting as a bidentate ligand through its deprotonated oximate oxygens in cis positions) occupying one axial and the three equatorial positions and an additional oximate oxygen from a symmetry-related [Cu(Hdmg)(dmg)](-) fragment filling the remaining axial position. The environment around Cu(1) and Cu(2) is distorted square pyramidal with four oximate nitrogen atoms building the equatorial plane. An oxygen atom [O(9)] from a water molecule and an oximate oxygen from a symmetry-related [Cu(Hdmg)(dmg)](-) fragment occupy the apical position of the square pyramids around Cu(1) and Cu(2), respectively. The trinuclear units are repeated through inversion centers standing at the middle of the Zn(1).Zn(1)(i) and Cu(2).Cu(2)(ii) vectors leading to a chain which runs parallel to the diagonal of the ac-plane. The Cu(1).Cu(2), Zn(1).Zn(1)(i), and Cu(2).Cu(2)(ii) separations are 5.506(2), 3.390(2), and 3.930(2) Å, respectively. 1 exhibits a characteristic ferromagnetic behavior with a continuous increase of the chi(M)T product as the temperature is lowered from 300 to 2.0 K. The field dependence of the magnetization at 2.0 K is consistent with a low-lying quintet state. The only efficient exchange pathways responsible for the overall ferromagnetic coupling in 1 are the diamagnetic zinc(II) ion and the out-of-plane double-oximato bridge, the magnitude of the magnetic coupling between the copper(II) ions through these bridging units being 3.9 and 5.1 cm(-)(1), respectively.
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