Synthesis, spectroscopic, magnetic properties and X-ray crystal structure of di-mu-hydroxo-bis(mu-perchlorato-O,O ')bis-[bis(2-amino-4-methylpyrimidine)copper(II)]: bridging perchlorate results in significant deviation from the predicted magnetic exchange

A new copper(II) complex, [Cu2L4(OH)(2)(ClO4)(2)], was prepared in a one-st ep synthesis, where L = 2-amino-4-methylpyrimidine, and characterized by ch emical analyses, infrared spectra, ligand-field spectra, magnetic susceptib ility, electron paramagnetic resonance measurements and X-ray crystal struc ture determination The compound crystallizes in the orthorhombic space grou p Pbca. Z = 4, a = 13.6282(2), b = 12.7193(3), c = 18.1459(4) Angstrom, V = 3145.43(11) Angstrom(3). The least-squares refinement of the structure led to R-1 = 0.036 and wR(2) = 0.0848. Each copper(II) ion is in a distorted t etragonal environment consisting of two nitrogen atoms from the ligands. tw o oxygen atoms from the hydroxo bridges and two trans oxygens from bridging perchlorates in semi-coordination. The equatorial Cu-O and Cu-N distances vary from 1.924(2) to 1.936(2) and from 1.991(2) to 2.004(2) Angstrom, resp ectively. The weak axial Cu-O coordination distances to the bridging perchl orates are 2.602(2) and 2.673(2) Angstrom. The Cu-Cu distance is 2.9414(5) Angstrom with a Cu-O-Cu angle of 99.30(9)degrees. A sharp peak at 3640 cm(- 1) in the IR spectrum is assigned to the bridging OH group. Cryomagnetic in vestigations (5-266 K) revealed the operation of antiferromagnetic spin-spi n interactions through the hydroxo bridges for the complex. PL singlet-trip let energy gap (2J) of - 68.1 cm(-1) was determined for the compound, a val ue significantly higher than the value of -131 cm(-1) predicted from the Ha tfield-Hodgson formula. The X-band electron paramagnetic resonance studies in the solid state showed only a weak signal of a monomeric impurity. No tr iplet EPR signal was found between 0 and 0.8 T, suggesting a large zero-hel d splitting. Frozen-solution EPR spectra indicate that the complex dissocia tes into mononuclear species when dissolved in dmf or dmso. (C) 1999 Elsevi er Science S.A. All rights reserved.