Hexa-μ-chloro-μ₄-oxo-tetrakis{[5-(2,4,6-trimethylphenyl)pyrazole-κN²]copper(II)}

The [Cu₄(µ₄-O)(µ-X)₆L₄]^n- structural type is well known, where X⁻ can be Cl⁻, Br⁻ or I⁻ and L is a mondentate ligand. The terminal ligand L can be a halide (giving n = 4), or a variety of neutral monodentate N or O donors (giving n = 0), such as imidazoles (Atria et al., 1999; Clegg et al., 1988), thiazolines (Bolos & Christidis, 2002), pyridines (Gill & Sterns, 1970; Händler, 1990; Kilbourn & Dunitz, 1967), pyrazines (Näther & Jeß, 2002), amines (Pavlenko et al., 1993), sulfimides (Kelly et al., 1999), phosphine oxides (Jorík et al., 1996) or sulfoxides (Guy et al., 1988) (not a complete list). Only one previous example containing a pyrazole terminal ligand has been structurally characterized, namely [Cu₄(µ₄-O)(µ-Cl)₆(L¹)₄] (L¹ is 3,4-dimethyl-5-phenylpyrazole; Keij et al., 1991). As part of our studies of the reactions of substituted pyrazoles with Cu^II salts (Liu et al., 2001; Liu, McAllister et al., 2002; Liu, McLaughlin et al., 2002), we have now isolated the title compound, (I), as a second example with a similar formulation, namely [Cu₄(µ₄-O)(µ-Cl)₆(L²)₄] [L² is 5-(2,4,6-trimethylphenyl)pyrazole]. \sch

The approximately tetrahedral cluster core in (I) has crystallographic C₂ symmetry, with the C₂ axis at (1/2, y, 1/4) passing through atoms Cl34, O3 and Cl35. The two unique Cu centres have very similar distorted trigonal-bipyramidal stereochemistries, with τ values of 0.7198 (14) (Cu1) and 0.7560 (12) (Cu2), which are closer to the ideal value of 1 for a trigonal bipyramid than the value of 0 expected for an ideal square pyramid (Addison et al., 1984). The Cu—N, Cu—O and Cu—Cl distances in the molecule lie within the usual ranges, but show small differences between the two Cu centres.

Interestingly, the Cu—Cl bonds to the two ligands that accept N—H···Cl hydrogen bonds (see below) are not notably longer than the other Cu—Cl bonds in the molecule, showing that the small Cu—Cl bond-length variations cannot be attributed to this feature of the structure. The Cu—O—Cu angles in the molecule also show small distortions from the tetrahedral angle. These small deviations from ideal tetrahedral molecular symmetry are common in [Cu₄(µ₄-O)(µ-X)₆L₄] complexes, and have a significant effect on their magnetic properties (Blake et al., 1997, and references therein). The four unique Cu···Cu distances in the structure range from 3.0570 (4)–3.1689 (5) Å.

The four L² ligands in the molecule form two pairs of intramolecular hydrogen bonds, with N19—H19 and N19ⁱ—H19ⁱ [symmetry code: (i) 1 − x, y, 1/2 − z] interacting with atom Cl34, and N5—H5 and N5ⁱ—H5ⁱ with atom Cl35. This is the same pattern of intramolecular hydrogen bonding shown by [Cu₄(µ₄-O)(µ-Cl)₆(L¹)₄] (Keij et al., 1991).

The molecules in the crystal associate through two unique π–π interactions. One is between the aryl groups C23—C28 and C9ⁱⁱ—C14ⁱⁱ [symmetry code: (ii) 1 − x, y + 1, 1/2 − z]. The least-squares planes of these two groups have a dihedral angle between them of 5.89 (11)°, and are separated by an average of 3.72 Å. The centroids of the two interacting aryl rings are offset by 1.69 Å. The second π–π interaction is between the opposite face of C23—C28 and C23ⁱⁱⁱ—C28ⁱⁱⁱ [symmetry code: (iii) 1 − x, 2 − y, 1 − z]. These two rings are coplanar by symmetry and separated by 3.70 Å, with centroids offset by 2.59 Å. The effect of these interactions is to associate the molecules into discrete π–π-stacked tetrads, which zigzag along the crystallographic [110] direction. There are no other significant intermolecular interactions in the lattice. Request from coeditor - please insert references to Figs. 1 and 2 at appropriate places.