Design of dinuclear copper species with carboranylcarboxylate ligands: study of their steric and electronic effects.

A series of new mononuclear and carboranylcarboxylate-bridged dinuclear copper(II) compounds containing the 1-CH(3)-2-CO(2)H-1,2-closo-C(2)B(10)H(10) carborane ligand (LH) has been synthesized. Reaction of different copper salts with LH at room temperature leads to dinuclear compounds of the general formula [Cu(2)(μ-L)(4)(L(t))(2)] (L(t) = thf (1), L(t) = H(2)O (1')). The reaction of 1 and 1' with different terminal pyridyl (py) ligands leads to the formation of a series of structurally analogous complexes by substitution of the terminal ligand thf or H(2)O (L(t) = py (2), p-CF(3)-py (3), p-CH(3)-py (4), pz (6), and 4,4'-bpy (7)), which maintain the structural Cu(2)(μ-O(2)CR)(4) core in the majority of the cases except for o-(CH(3))(2)-py, where a mononuclear compound (5) is exclusively obtained. These compounds have been characterized through analytical, spectroscopic (NMR, IR, UV-visible, ESI-MS) and magnetic techniques. X-ray structural analysis revealed a paddle-wheel structure for the dinuclear compounds, with a square-pyramidal geometry around each copper ion and the carboranylcarboxylate ions bridging two copper atoms in syn-syn mode. The mononuclear complex obtained with the o-(CH(3))(2)-py ligand presents a square-planar structure, in which the carboranylcarboxylate ligand adopts a monodentate coordination mode. The magnetic properties of the dinuclear compounds 1, 3, 4, and 6 show a strong antiferromagnetic coupling in all cases (J = -261 (1), -255 (3), -241 (4), -249 cm(-1) (6)). Computational studies based on hybrid density functional methods have been used to study the magnetic properties of the complexes and also to evaluate their relative stability on the basis of the strength of the bond between each Cu(II) and the terminal ligand.