Octanuclearity in Copper(II) Chemistry: Preparation, Characterization, and Magnetochemistry of [Cu8(dpk·OH)8(O2CCH3)4](ClO4)4·9H2O (dpk·H2O = the Hydrated, gem-Diol Form of Di-2-pyridyl Ketone)

Abstract

The complexes [Cu8(dpk·OH)8(O2CMe)4](ClO4)4·9H2O (1) and [Cu(dpk·H2O)2](O2CMe)(ClO4)·2H2O (2), where dpk·H2O is the hydrated, gem-diol form of di-2-pyridyl ketone, have been prepared. Complex 1 crystallizes in triclinic space group P1̄ with the following unit cell dimensions at 25 °C: a = 18.396(1) Å, b = 16.720(1) Å, c = 19.171(1) Å, α = 96.10(1)°, β = 87.68(1)°, γ = 99.14(1)°, Z = 2. Crystal structure data for 2 at room temperature are as follows: monoclinic, P21/c, a = 13.000(2) Å, b = 8.008(1) Å, c = 27.095(3) Å, β = 93.19(1)°, Z = 4. The two centrosymmetrically related cubanes in the tetracation of 1 are doubly-bridged with two syn, anti acetate groups bridging two CuII atoms. The monoanion dpk·OH- functions as a η1:η3:η1:μ3 ligand. Three CuII atoms have distorted octahedral coordination geometries with CuO3N3 and CuNO5 chromophores, while the fourth CuII center displays a distorted square pyramidal geometry; a terminal monodentate acetate is ligated to this latter CuII atom. In the mononuclear [Cu(dpk·H2O)2]2+ cation of 2, the four pyridyl nitrogens can be viewed as strongly coordinating to the metal (Cu−N = 2.013(4)−2.022(4) Å), while one of the hydroxyl oxygens on each ligand forms a weak bond to CuII (Cu−O = 2.417(4), 2.352(3) Å). Variable-temperature magnetic susceptibility studies on 1 are in line with both an overall antiferromagnetic interaction between CuII atoms and the magnetic behavior of a simple cubane. Exchange parameters, J, derived by using a four-J magnetic model, are found to be J1 = 6 cm-1, J2 = −144 cm-1, J3 = −14 cm-1, J4 = 3 cm-1 and g = 2.29 (adjustable parameter) by least-squares fitting to the spin Hamiltonian H = −2∑i<jJijSi·Sj. The thus derived energy level spectrum shows a S = 1 ground state, further supported by the solid-state and solution EPR spectra of 1. Insight concerning the effect of structural parameters on the magnitude of the magnetic exchange interactions was gained through EHMO calculations performed on a model Cu(OR)2Cu moiety. Accordingly, estimates of the J parameters, experimentally derived, were in close agreement both with known magneto−structural correlations established for planar Cu(OR)2Cu moieties and a criterion established by us, holding for the magneto−structural correlations in symmetrical roof-shaped, alkoxo-bridged Cu(OR)2Cu moieties.