Spectroscopic and magnetic properties of cobalt(II) and nickel(II) clusters obtained from 1-(hydroxymethyl)-3,5-dimethylpyrazole. X-Ray structure of tetrakis[chloro(µ3-3,5-dimethyl-N-oxymethylpyrazolato-N2,µ3-O)(ethanol)nickel(II)]

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Abstract
The synthesis, and spectroscopic and magnetic characterisation of four isomorphous cluster compounds of general formula [{MX(L)(EtOH)}4][L = 3,5-dimethyl-1-oxymethylpyrazolate(1–); X = Cl or Br; M = CoII or NiII] is described. The compounds are formed upon reaction of 1-(hydroxymethyl)-3,5-dimethylpyrazole (HL) with the metal(II) halides in ethanol in the presence of one equivalent of KOH. The deprotonated ligand co-ordinates as a bidentate ligand through its pyrazole N2 nitrogen and the oxy-part. The oxygen atom bridges between three metal ions, thereby forming a M4O4 cubane-type cluster. In the case of M = Ni the metal ions are ferromagnetically coupled, whereas in the case of M = Co antiferromagnetic coupling occurs. The presence of the cubane-type structure was confirmed by the crystal structure determination of [{NiCl(L)(EtOH)}4]. The complex crystallizes in space group P21/n with a= 12.936(6), b= 18.152(5), c= 20.79(1)Å, β= 106.23°, and Z= 4. Standard least-squares refinement gave R= 0.039 (R′= 0.047). The Ni atom has a NiO4NCl chromophore in a distorted octahedral geometry. Important averaged distances are: Ni–N = 2.06, Ni–Cl = 2.35, Ni–O(EtOH)= 2.08, Ni–O = 2.08 and, Ni ⋯ Ni = 3.18 Å. Intramolecular hydrogen bonds between Cl (bound to one Ni) and EtOH (bound to another Ni) with average O ⋯ Cl contacts of 3.01 Å stabilize the cluster.

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