Five polymorphous frameworks of cobalt(II) imidazolates (1-5) have been prepared by solvatothermal syntheses. Of these, compound 3 has already been synthesized in a gas-phase reaction by Seel et al. in 1969 and structurally characterized by Sturm et al. in 1975. The new synthetic strategy affords four polymorphous frameworks of cobalt(II) imidazolates (1, 2, 4, 5) of crystalline substances, of which the compound 4 (a = b = 23.450(3), c = 12.460(3) A, tetragonal, I4(1)cd, Z = 16) is an isomorphous compound of [Zn(im)(2)]( infinity ), which was also synthesized in a gas-phase reaction in 1980. The frameworks of compounds 1 and 2 are porous and isostructural; they have the same framework topology that represents a novel uninodal (6,4)-net: 1: a = 18.513(4), b = 24.368(5), c = 9.2940(19) A, orthorhombic, Fdd2, Z = 16; 2: a = 17.635(4), b = 27.706(6), c = 9.0810(18) A, orthorhombic, Fdd2, Z = 16. The framework of compound 5 exhibits a topology of zeolitic structure with the unit-cell parameters: a = 24.3406(8), b = 9.4526(3), c = 24.8470(8) A, beta = 91.977(1) degrees, monoclinic, P2(1)/n, Z = 4. All polymorphous frameworks of cobalt(II) imidazolates reflect the structural features of silica (SiO(2)) and also exhibit different magnetic behaviors, although the imidazolates transmit the antiferromagnetic coupling between the cobalt(II) ions in all cases. However, the uncompensated antiferromagnetic couplings arise from spin-canting are sensitive to the structures: compound 1 is an antiferromagnet with T(N) = 13.11 K; compounds 2-4 are weak ferromagnets (canted antiferromagnets): 2 shows a very weak ferromagnetism below 15 K, 3 exhibits a relatively strong ferromagnetism below 11.5 K and a coercive field (H(C)) of 1800 Oe at 1.8 K, and 4 displays the strongest ferromagnetism of the three cobalt imidazolates and demonstrates a T(C) of 15.5 K with a coercive field, H(C), of 7300 Oe at 1.8 K. However, compound 5 seems to be a hidden canted antiferromagnet with a magnetic ordering temperature of 10.6 K.
Despite wide potential applications of Gd-clusters in magnetocaloric effect (MCE) owing to f electron configuration of Gd(III), the structural improvement in order to enhance MCE remains difficult. A new approach of the situ hydrolysis of acetonitrile is reported, and the slow release of small ligand CHCOO is realized in the design and synthesis of high-nuclearity lanthanide clusters. A large lanthanide-exclusive cluster complex, [Gd(CO)(CHCOO)(μ-OH)(μ-OH)(HO)](NO3)Br(dmp)·30CHOH·20Hdmp (1-Gd), was isolated under solvothermal conditions. To the best our knowledge, cluster 1 possesses the high metal/ligand ratio (magnetic density) and the largest magnetic entropy change (- Δ S = 48.0 J kg K at 2 K for Δ H = 7 T) among previously reported high-nuclearity lanthanide clusters.
Two novel complexes trans-[Fe(MBPT) 2 (NCS) 2 ] (1) and cis-[Fe(mMBPT) 2 (NCS) 2 ] (2) have been synthesized and their structure characterized at 293 K by single-crystal X-ray crystallography [MBPT (or mMBPT) ) 4-p(or m)-methylphenyl-3,5-bis(pyridin-2-yl)-1,2,4triazole]. Complexes 1 and 2 are isomers, and 1 crystallizes in the triclinic space group P1 h with Z ) 1, a ) 9.120(3) Å, b ) 9.2563(18) Å, c ) 11.576(2) Å, R ) 76.256(15)°, β ) 76.405-(18)°, γ ) 84.983(19)°, and V ) 922.1(4) Å 3 in contrast to 2 in the monoclinic space group C2/c with Z ) 4, a ) 21.851(3) Å, b ) 13.4113(15) Å, c ) 16.859(3) Å, β ) 128.153( 9)°, and V ) 3885.0(9) Å 3 . Both complexes have a similar pseudo-octahedral [FeN 6 ] core with the NCSgroups in the trans arrangement in 1 but cis in 2. Variable-temperature magnetic susceptibility and IR spectral measurements reveal that 1 shows an abrupt high-spin T low-spin (HS T LS) transition centered at T 1/2 around 231 K, in contrast to 2 staying in a high-spin state in the observed temperature range of 75-300 K. The extended X-ray absorption fine structures (EXAFS) spectra confirm that the average Fe-N distance for the HS state in 1 is ≈0.2 Å longer than that of the LS state. Complex 1 represents the first spin-crossover iron(II) complex with triaryltriazole and trans-thiocyanate ligands.
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