SYNTHESIS, STRUCTURE, AND SPECTRAL AND MAGNETIC-PROPERTIES OF (MU-CARBONATO) (MU-HYDROXO)-BIS[N,N'-(BIS(2-AMINOETHYL)ETHANE-1,2-DIAMINE)CHROMIUM(III)] PERCHLORATE

The mu-carbonato mu-hydroxo complex [(tren)Cr(mu-CO3)(mu-OH)Cr(tren)](ClO4)3.2H2O has been prepared from both Cr(II) and Cr(III) precursors and characterized by structural analysis, magnetic susceptibility measurements, and luminescence spectroscopy. The complex crystallizes in the monoclinic space group P2(1/c) (No. 14) with a = 7.022 (1) angstrom, b = 25.679 (8) angstrom, c = 8.646 (2) angstrom, beta = 102.27 (2)-degrees, V = 1523.4 (7) angstrom 3, and Z = 2. Least-squares refinement led to final R and R(w) values of 0.054 and 0.043, respectively, for 1857 observed reflections with F greater-than-or-equal-to 6-sigma(F). In the complex cation, the two Cr(III) centers are in a distorted octahedral environment and are linked by a carbonate and a hydroxide bridge. Within the bridging moiety, Cr-O(CO3) = 1.954 (4) angstrom, Cr-O(OH) = 1.944 (2) angstrom, Cr-OH-Cr = 131.5 (3)-degrees, and O-Cr-O = 94.5 (2)-degrees, while the Cr-N distances are 2.081 (4) angstrom. Magnetic susceptibility measurements in the temperature range 4.2-300 K indicate that the Cr centers are weakly antiferromagnetically coupled (J = -16.8 cm-1). The luminescence spectrum of the complex at 1.5 K showed four transitions from the luminescent lowest excited state (S* = 2) to the ground state (S = 0, 1, 2, 3), and from the splitting pattern, a J value of -18.5 (+/- 0.1) cm-1 was determined, consistent with the susceptibility data. The degree of coupling between the metal centers is similar to that observed in related complexes containing hydroxo bridges. A correltion between coupling constant and the average of the shortest Cr-O distance in binuclear Cr(III) complexes similar to that reported for Fe(III) complexes cannot be developed at this stage because of an insufficient number of oxo-bridged complexes with short Cr-O distances.