Manganese−Bipyridine Complex Incorporated into Mesoporous MCM-41 Molecular Sieves

Siliceous (SiMCM-41) and aluminosilicate mesoporous molecular sieves (AlMCM-41) with variable framework Si/Al ratios have been used as supports to immobilize manganese 2,2‘-bipyridine (L) complex cations, [MnL 2 ] 2+, via incipient-wetness impregnation or ion exchange with MnL 2 (NO 3 ) 2 in acetonitrile. Electron spin resonance (ESR), diffuse reflectance ultraviolet−visible (UV−vis), and Fourier transform infrared (FTIR) spectroscopies, thermal gravimetric (TG) analysis, and electron probe microanalysis have been employed to investigate the chemical environment of manganese in these materials and its transformation with oxidation−reduction treatments. [MnL 2 ] 2+ cation loading by impregnation is quantitatively controllable. At low loadings corresponding to Mn/Si ratios less than 2.4 × 10 - 3 for AlMCM-41 with Si/Al = 30.4, the Mn 2+ complex cations are monatomically dispersed with a sextet ESR signal at g = 2.00 and 95 G hyperfine coupling. They have a low thermal decomposition temperature of about 160 °C. With increasing loading, the Mn 2+ complexes interact as indicated by loss of resolution of the ESR sextet of Mn 2+ . The thermal decomposition temperature increases to 215−265 °C which is still lower than 307 °C for MnL 2 (NO 3 ) 2 crystals. For impregnation with washing by acetonitrile, the Mn 2+ complex cation loading approaches a maximum of Mn/Si = 5.7 × 10 - 3 for AlMCM-41 with Si/Al = 30.4. By ion exchange, a comparable maximum manganese complex loading of Mn/Si = 6.6 × 10 - 3 is reached for the same sample. For this ion-exchanged AlMCM-41 the ESR shows a strong singlet at g = 2.00 with 132 G line width which overlaps a poorly resolved Mn 2+ sextet at g = 2.00. This 132 G wide line is assigned to Mn 4+ complex cations. TG analysis shows that this Mn 4+ complex cation is stable to 225 °C. The ion exchange capacity of AlMCM-41 is enhanced by increasing the framework aluminum content. SiMCM-41 shows some ion exchange capacity due to silanol groups. FTIR suggests that the immobilized Mn 2+ complex cations maintain a cis-bipyridyl molecular configuration as in solution, and UV−vis shows a lower charge-transfer energy at 510 nm in comparison with 380 nm for MnL 2 (NO 3 ) 2 microcrystals. This is consistent with the absence of nitrate ligands in the coordination sphere of the Mn 2+ on the MCM-41 wall surfaces. Upon oxidation with hydrogen peroxide in acetonitrile, the ESR signal of the immobilized Mn 2+ complex cations disappears and the ESR siginal of Mn 4+ complex cations increases while the total spin concentration remains constant. This change is reversed by reduction by oxalic acid in acetonitrile. This suggests that immobilized manganese complex cations may serve as effective active sites for oxidation−reduction reactions at low temperature.