High-spin molecules:: Hexanuclear MnIII clusters with [Mn6O4X4]6+ (X = Cl-, Br-) face-capped octahedral cores and S=12 ground states

Reaction of R(2)dbmH (R = H, Me, Et; dbmH is dibenzoylmethane) with MnCl3 (generated in situ from MnCl2 and MnO4-) in MeCN yields the five-coordinate, square-pyramidal complexes [MnCl(R(2)dbm)(2)] (R = H, 1; Me, 4; Et, 7). The same reaction with MnBr2 yields [MnBr(R(2)dbm)(2)] (R = H, 2; Me, 5; Et, 8). Slow hydrolysis of 4 or 7 in CH2Cl2/MeCN over two weeks gives [Mn6O4Cl4(R(2)dbm)(6)] (R = Me, 9; Et, 11); the crystal structure of 9 reveals a novel [Mn-6(mu(3)-O)(4)(mu(3)-Cl)(4)](6+) core comprising an Mn-6(III) octahedron whose faces are capped by O2- or Cl- ions. Similar slow hydrolysis of 5 or 8 gives [Mn6O4Br4(R(2)dbm)(6)] (R = Me, 10; Et, 12); the crystal structure of 10 is identical with that of 9 except for the Br--for-Cl- substitution. The H-1 NMR spectra of 1-12 in CDCl3 show paramagnetically shifted and broadened R(2)dbm resonances. The spectra are as expected for retention of the solid-state structures, the Mns complexes exhibiting effective T-d solution symmetry with no evidence of fragmentation to [MnX(R(2)dbm)], [Mn(R(2)dbm)(3)], or any other species. The effective magnetic moment (mu(eff)) per Mn-6 for 9/12 slowly increases from 16.01/16.73 mu(B) at 320/300 K to a maximum of 24.37/24.60 mu(B) at 30.1/40.0 K, and then decreases to 13.69/13.86 mu(B) at 2.00 K. Fitting of the data to the theoretical equation derived for a Mn-6(III) octahedron gives (for 9/12) J(cis) = 8.6/8.5 cm(-1) and g = 1.97/1.98 with J(trans) and TIP held fixed at 0 cm(-1) and 1200 x 10(-6) cm(3) mol(-1), respectively. These values indicate a S = 12 ground state. Fitting of magnetization vs field data collected in the 2.00-15.0 K and 0.500-50.0 kG ranges confirmed S = 12 ground states with D approximate to 0 cm(-1).