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 MnCl 3 (generated in situ from MnCl 2 and MnO 4 - ) 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 MnBr 2 yields [MnBr(R 2 dbm) 2 ] (R = H, 2; Me, 5; Et, 8 ). Slow hydrolysis of 4 or 7 in CH 2 Cl 2 /MeCN over two weeks gives [Mn 6 O 4 Cl 4 (R 2 dbm) 6 ] (R = Me, 9; Et, 11 ); the crystal structure of 9 reveals a novel [Mn 6 (μ 3 -O) 4 (μ 3 -Cl) 4 ] 6+ core comprising an octahedron whose faces are capped by O 2 - or Cl - ions. Similar slow hydrolysis of 5 or 8 gives [Mn 6 O 4 Br 4 (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 1 H NMR spectra of 1 − 12 in CDCl 3 show paramagnetically shifted and broadened R 2 dbm resonances. The spectra are as expected for retention of the solid-state structures, the Mn 6 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 (μ eff ) per Mn 6 for 9 / 12 slowly increases from 16.01/16.73 μ B at 320/300 K to a maximum of 24.37/24.60 μ B at 30.1/40.0 K, and then decreases to 13.69/13.86 μ B at 2.00 K. Fitting of the data to the theoretical equation derived for a 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 × 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 ≈ 0 cm - 1 .