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Abstract: Magnetic interactions in a new copper(II) carboxylate complex {Cu2(O2CH)4}{Cu(O2CH)2(2-methylpyridine)2} were studied with the aim of better understanding magneto-structural correlation in a quasi 1-D spin arrangement. The only magnetic ions in the structure (Cu2+, S=1/2) form chains of dimer–monomer units. Copper ions in dimers are bridged by four carboxylate groups with an antiferromagnetic exchange coupling J/k B=−320K. Susceptibility of the monomer copper ions follows Curie 1/T law down to 20K. Below 20K the susceptibility abruptly decreases due to the antiferromagnetic coupling between monomers. [Copyright &y& Elsevier]

A new ionic compound (C5H6NO)2 [Cu2(μ-O2CH)4(O2CH)2], 1 formed of 4-hydroxypyridinium cations and a complex anion was synthesized. The anion is a paddle-wheel dicopper carboxylate complex with four syn,syn-bridging and two axial anionic methanoato ligands. The XRD structure determination of 1 reveals that the molecular structure is stabilized by two H-bonds between the cations and the axial paddle-wheel anions (N-H…O 2.755(3), O-H…O 2.489(2) Å). The compound exhibits a very strong (2J = 500 cm-1) intra-binuclear antiferromagnetic interaction noticed already at room temperature attributed to the methanoato intra-binuclear bridges. The typical EPR S = 1 spin system signals of the dicopper paddle-wheel complexes at 90 and 450-700 mT are found in the room temperature spectrum, but they are poorly seen in the 110 K spectrum. These signals are of very low intensity and are accompanied by a dominant signal at 320 mT, all closely related to a very strong antiferromagnetic interaction present in 1. [ABSTRACT FROM AUTHOR]

Abstract: Several copper(II) methanoato complexes, namely mononuclear [Cu(O2CH)2(2-mpy)2] (1) (2-mpy=2-methylpyridine), binuclear [Cu2(μ-O2CH)4(2-mpy)2] (2), and the polynuclear {[Cu(μ-O2CH)2(2-mpy)2][Cu2(μ-O2CH)4]} n (3) and {Na2[Cu(μ-O2CH)2(O2CH)2][Cu2(μ-O2CH)4]} n (4), have been synthesized. The mononuclear complex 1 is formed by two asymmetric chelate methanoate anions and two 2-methylpyridine molecules, giving a highly distorted ‘elongated octahedral’ coordination sphere. Complex 1 decomposes outside the mother-liquid, transforming into a regular isolated binuclear paddle-wheel complex 2 with four intra-binuclear bridging methanoates and two axial 2-mpy ligands. The polynuclear complex 3 is formed of alternate mononuclear and binuclear building blocks resembling the central cores of 1 and 2, but with significant differences, especially for the methanoates of the mononuclear units. The oxygen atom of the mononuclear unit in the octahedral axial position in 3 is simultaneously coordinated to the axial position of the binuclear paddle-wheel central core, thus enabling a chain type of structure. A chain of alternate mononuclear and binuclear building blocks, as in the neutral compound 3, are found as well in the ionic polymeric compound 4, though two types of bridges are found in 4, while there is only one type in 3. Namely, the axial position of the octahedral mononuclear unit in 4 is occupied by the methanoate oxygen atom that is already a part of the binuclear paddle-wheel unit, while one equatorial methanoate from the mononuclear unit serves as a triatomic bridge to the axial position of the binuclear building block. A very strong antiferromagnetic interaction is found for all the complexes with the paddle-wheel building blocks [Cu2(μ-O2CH)4] 2–4 (−2J =444–482cm−1), attributed to the methanoate intra-binuclear bridges. On the other hand, this strong antiferromagnetism, found already at room temperature, reduces the intensity of the EPR S =1 spin signals reported for the isolated paddle-wheel complex 2. For the polymeric 3, only the spin S =½ signals are found in the EPR spectra, and they are assigned to the mononuclear building blocks. No signals with a clear origin are however seen in the room temperature EPR spectrum of the polymeric analogue 4, only the S =½ signals in the low temperature spectra. This feature is suggested to be due to a specific influence between the adjacent S =1 (binuclear) and S =½ (mononuclear) species via their bridges. [Copyright &y& Elsevier]

Abstract: A novel dinuclear complex [Cu2(μ-L)4(HL)2] (1) was isolated from starting 2-pyridone (HL) via a resonance and a tautomeric transformation. Each copper centre is in a square-pyramidal coordination sphere, defined by two oxygen atoms (Cu–O4 1.978(5), Cu–O11 1.964(4)Å) and two nitrogen atoms (Cu–N2 2.003(5), Cu–N3 2.007(5)Å) of four bridging deprotonated pyridin-2-olates and an oxygen atom on the top from a neutral 2-pyridone (Cu–O2 2.227(5)Å), analogous to tetracarboxylate paddle-wheel complexes. Compound 1 was compared with mixed pyridin-2-olato/methanoato analogues [Cu2(μ-HCO2)2(μ-L)2(HL)2]·2CH3CN (2) and [Cu2(μ-HCO2)2(μ-L)2(HL)2] (2a) (2a is an air stable form obtained from 2 outside mother-liquid). The EPR spectra of air stable 1 and 2a show three signals H z1, H ⊥2 and H z2, typical for the binuclear systems with spin S =1, both revealing strong antiferromagnetism 2J =−334 (1) and −324cm−1 (2a). Interestingly, only for 1 additional H ⊥1 signal at 100mT is noticed (D(1)=0.293cm−1 < hν =0.320cm−1 < D(2a)=0.347cm−1). On the other hand, several broad signals in the 100–450mT region, only in the high temperature spectrum for 2a are observed. These results are in agreement with the magnetic susceptibility analysis. [Copyright &y& Elsevier]