| Abstract
| - The reaction of ethanolic/water solutions ofNiX2·6H2O (X =NO3-, ClO4-),bidentate amine N,N,N‘,N‘-tetramethylethylenediamine (tmen) andN,N,N‘-trimethylethylenediamine (trimen) with[Cu(oxpn)] (oxpn = N,N‘-bis(3-aminopropyloxamide) and ammonium thiocyanate, in the presenceof ammonium hexafluorophosphate whenX is NO3-, leads to the high-yield formationof new heterotetranuclear (Cu−Ni)2 complexes of generalformulation[Cu(oxpn)Ni(μ-NCS)(H2O)(aa)]2(X)2(1, X = PF6-, aa = tmen;2, X = ClO4-, aa = tmen;3, X = ClO4-; aa=trimen; 4 , X = PF6-, aa =trimen). The crystal structures of 1, 2, and3 were determined. Complex 1crystallizesin the triclinic system, space group P1̄ witha = 9.059(8) Å, b = 12.604(2) Å,c = 12.743(4) Å, α =95.27(6)°,β = 99.22(2)°, γ = 108.86(7)°, and Z= 2. Complex 2 crystallizes in the triclinic system,space group P1̄ witha = 8.7586(7) Å, b = 12.580(4) Å,c = 12.6318(12) Å, α = 94.630(14)°, β= 98.486(8)°, γ = 107.963(13)°, andZ = 2. Complex 3 crystallizes in thetriclinic system, space group P1̄ with a =8.407(7) Å, b = 12.779(4) Å,c = 14.081(3) Å, α = 63.25(2)°, β =88.62(3)°, γ = 79.06(4)°, and Z = 2.The cationic parts of the threecomplexes are very similar, consisting of two [Cu(oxpn)Ni]moieties linked by two SCN- bridging ligands.TheCuII ions have square-pyramidal geometry with the S atom ofthe thiocyanato ligand in the apical position atca.3 Å. The NiII ions are distorted octahedrallycoordinated. Variable-temperature magnetic susceptibilitystudieswere performed on 1−4 in the temperature range2−300 K. Satisfactory fits to the observed susceptibilitydatawere obtained by assuming isotropic magnetic exchange interactions andusing the appropriate spin Hamiltoniansconsidering the complexes first as two dinuclear entities and second asa single tetranuclear unit. In the formerthe susceptibility equation is easy to derive, but for the latterfull-matrix diagonalization was needed. TheJvalue for the Cu(oxpn)Ni entity is close to −110cm-1 in all four cases. By using thefull-matrix diagonalizationmethod it was possible to derive both J1,Cu(oxpn)Ni and J2, Cu−SCN−Nicoupling constants. J1 is verysimilarto that derived assuming a dinuclear entity (ca. −115cm-1) and J2 is verysmall but positive, ∼0.03 cm-1(ferromagnetic coupling). Theoretical calculations were performedfor different J2 values (from 10 to −10cm-1)assuming a high J1 coupling constant(J1 = −115 cm-1).Such calculations revealed the influence of thisJ2value on the susceptibility curves at low temperatures. The EPRspectra measured at different temperatures(from 4 to 300 K) on polycrystalline samples of1−4 show the typical band centered atg ≈ 2.2 due to the spintransition |1/2,−1/2> → |1/2,1/2> of the spin ground-stateST = 1/2.
- The crystal structureof the first three heterotetranuclear complexes[Cu(oxpn)Ni(μ-NCS)(H2O)(tmen)]2(PF6)2(1),[Cu(oxpn)Ni(μ-NCS)(H2O)(tmen)]2(ClO4)2(2), and[Cu(oxpn)Ni(μ-NCS)(H2O)(trimen)]2(ClO4)2(3) is reported. They have analogous structures: two[Cu(oxpn)Ni] dinuclear entities linked by twoSCN- bridging ligands. The S atom is linked to theCuII ion, and the N atom, to NiII ion. Themagnetic susceptibility measurements were fitted firstly by assumingisolated [CuNi] entities and, secondly, as a whole tetranuclearentity by full-diagonalization matrixmethod.
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