The synthesis of several binuclear, oxobis(carboxylato)-bridged dimanganese(III) complexes, [Mn2O(O2CR)2(HB(pz)3)2] where R = CH3(1), C2H5(2), or H (3), and HB(pz)3“ is the hydrotris(l-pyrazolyl)borate ligand, is described. X-ray structural studies of 1•CH3CN and 14CH3CN reveal two six-coordinate manganese atoms bridged by a M-oxo [av Mn-0,1.78 υ; Mn-O-Mn, 125.1°] and two μ-acetato (av Mn-O, 2.07 Å) groups and capped by two tridentate HB(pz)3“ ligands. Each high spin, d4Mn(III) center has its empty d-orbital directed toward the short Mn-Ooxobond axis, the consequences of which are a shortening of Mn-N bonds trans to the p-oxo group and markedly reduced antiferromagnetic coupling of the two Mn(III) centers compared to the two high spin, d5Fe(III) centers in the [Fe2O(O2CCH3)2(HB(pz)3)2] analogue. In the latter, the spin exchange coupling constant J = -121 cm-1, whereas XM vs- T measurements over the range 4.2 < T < 300 K for 1 reveal a J value of ~ -0.5 cm-1. The greater paramagnetism and rapid spin relaxation of the manganese complexes leads to large isotropic shifts and narrow lines in their proton NMR spectra. All protons were observed in the 67 to -56 ppm region, and most could be assigned on the basis of deuterium substitution. The methyl resonance of the bridging acetate ligands in 1 occurs at +65.6 ppm, which should be useful for identifying the (Mn2O(O2CCH2R)2)2+core in biology. These results suggest that substitution of Mn(III) for Fe(III) in metalloproteins such as hemerythrin or ribonucleotide reductase, that are known or believed to contain such cores, would provide a powerful NMR structural probe. The results of UV-vis, Raman, and infrared spectral studies are reported, including work on isotopically substituted 1, from which the symmetric and asymmetric Mn-O-Mn bridge bond stretching frequencies are assigned at 558 and 717 cm-1, respectively. Electrochemical studies of 1 reveal a quasi-reversible one-electron oxidation at 0.51 V vs. the Fc+/Fc couple to form the mixed valence Mn2(III,IV) complex. The ESR spectrum of a species, which was chemically generated from 1, exhibits a 16-line55Mn hyperfine pattern that is typical of the Mn2(III,IV) trapped valence state. This spectrum closely matches that observed for the 300 K form of the S2state of the manganese complex involved in photosynthetic water oxidation in green plants. Further oxidation of the mixed valence species reveals a second, quasi-reversible wave at 1.22 V vs. Fc+/Fc, tentatively assigned as the Mn2(IV,IV) complex.
ASJC Scopus subject areas
- Colloid and Surface Chemistry