Département de chimie moléculaire

An unprecedented atom connectivity, MnIV($μ$-O)MnIV($μ$-O)2MnIV($μ$-O)MnIV, is found in [MnIV4O4(EtO-terpy)4(OH)2(OH2)2](ClO4)6·8H2O (EtO-terpy = 4'-ethoxy-2,2':6',2''-terpyridine), which was characterized by x-ray crystallog., x-ray powder diffraction, EPR spectroscopy, and magnetic studies. This complex is the 1st example of a compd. where a MnIV ion is coordinated by all three types of water-derived ligands: oxo, hydroxo, and aqua. Bond distances and angles for this complex are consistent with a MnIV4 oxidn. state assignment. The di-$μ$-oxo- and mono-$μ$-oxo-bridged Mn-Mn distances are 2.80 and 3.51 {\AA}, resp. The variable-temp. magnetic susceptibility data for this complex, at 10-300 K, are consistent with an S = 0 ground state and were fit using the spin Hamiltonian HHDvV = -J1S2S1-J2S1S1A-J1S1AS2A (S1 = S1A = S2 = S2A = 3/2) with J1 = -432 cm-1 and J2 = -164 cm-1 (J1 and J2 are exchange consts. through the mono-$μ$-oxo and the di-$μ$-oxo bridges, resp.). The 1st excited spin state of this tetramer is a spin triplet state at 279 cm-1 above the diamagnetic ground state. The next spin states are the S = 1 and S = 2 levels at ∼700 and 820 cm-1 above the S = 0 ground state, resp. These large energy gaps are consistent with the absence of an EPR signal for this complex, even at high temp. [on SciFinder(R)]