Binuclear Manganese Compounds of Potential Biological Significance. 1. Syntheses and Structural, Magnetic, and Electrochemical Properties of Dimanganese(II) and -(II,III) Complexes of a Bridging Unsymmetrical Phenolate Ligand

Abstract

Reactions of the unsymmetrical phenol ligand 2-(bis(2-pyridylmethyl)aminomethyl)-6-((2-pyridylmethyl)(benzyl)aminomethyl)-4-methylphenol with Mn(OAc)(2).4H(2)O or Mn(H(2)O)(6)(ClO(4))(2) in the presence of NaOBz affords the dimanganese(II) complexes 1(CH(3)OH), [Mn(2)(L)(OAc)(2)(CH(3)OH)](ClO(4)), and 2(H(2)O), [Mn(2)(L)(OBz)(2)(H(2)O)](ClO(4)), respectively. On the other hand, reaction of the ligand with hydrated manganese(III) acetate furnishes the mixed-valent derivative 3(H(2)O), [Mn(2)(L)(OAc)(2)(H(2)O)](ClO(4))( 2). The three complexes have been characterized by X-ray crystallography. 1(CH(3)OH) crystallizes in the monoclinic system, space group P2(1)/c, with a = 10.9215(6) A, b = 20.2318(12) A, c = 19.1354(12) A, alpha = 90 degrees, beta = 97.5310(10) degrees, gamma = 90 degrees, V = 4191.7 A(3), and Z = 4. 2(H(2)O) crystallizes in the monoclinic system, space group P2(1)/n, with a = 10.9215(6) A, b = 20.2318(12) A, c = 19.1354(12) A, alpha = 90 degrees, beta = 97.5310(10) degrees, gamma = 90 degrees, V = 4191.7 A(3), and Z = 4. 3(H(2)O) crystallizes in the monoclinic system, space group P2(1)/c, with a = 11.144(6) A, b = 18.737(10) A, c = 23.949(13) A, alpha = 90 degrees, beta = 95.910(10) degrees, gamma = 90 degrees, V = 4974(5) A(3), and Z = 4. Magnetic measurements revealed that the three compounds exhibit very similar magnetic exchange interactions -J = 4.3(3) cm(-)(1). They were used to establish tentative magneto-structural correlations which show that for the dimanganese(II) complexes -J decreases when the Mn-O(phenoxo) distance increases as expected from orbital overlap considerations. For the dimanganese(II,III) complexes, crystallographic results show that the Mn(II)-O(phenoxo) and Mn(III)-O(phenoxo) bond lengths are inversely correlated. An interesting magneto-structural correlation is found between -J and the difference between these bond lengths, delta(Mn)(-)(O) = d(Mn)()II(-)(O) - d(Mn)()III(-)(O): the smaller this difference, the larger -J. Electrochemical studies show that the mixed-valence state is favored in 1-3 by ca. 100 mV with respect to analogous complexes of symmetrical ligands, owing to the asymmetry of the electron density as found in the analogous diiron complexes.

Spectroscopic and mechanistic studies of dinuclear metallohydrolases and their biomimetic complexes

“…24,96 In some cases the ligands have one structural variation in one arm, 23,24,96 in others one donor arm has been omitted. 19,[104][105][106] Often the vacant coordination site is found to be occupied by water or solvent molecules in the complex. 105,106 …”

“…19,[104][105][106] Often the vacant coordination site is found to be occupied by water or solvent molecules in the complex. 105,106 …”

Asymmetric zinc(ii) complexes as functional and structural models for phosphoesterases

“…It has been proposed that these asymmetric complexes are not only more appropriate functional models for the active site of phosphoesterase enzymes, but also that they exhibit enhanced catalytic rates compared with their symmetric counterparts. [1][2][3] Ligands used to generate purple acid phosphatase, 1,4-10 phosphoesterase, 11 urease, 12,13 catechol oxidase 14 and manganese catalase biomimetics 15,16 have been reported. Some ligands engender both a hard and a soft coordination site resulting in heterodinuclear complexes as, for example, models for purple acid phosphatase metalloenzymes.…”

“…Some ligands engender both a hard and a soft coordination site resulting in heterodinuclear complexes as, for example, models for purple acid phosphatase metalloenzymes. 4,7 In other cases the ligands have a structural variation in one arm 4,6,7 whilst in others one donor arm has been omitted, 1,[14][15][16] with the vacant coordination site often found to be occupied by water or solvent molecules in the complex. 15,16 By careful design the asymmetric ligand can offer both differentiated metal binding sites, and/or sites for further synthetic elaboration.…”

Crystal structure of the mononuclear complex [Mn(mesalim)2Cl] and electrochemical studies of manganese complexes of the ligand Hmesalim

“…The principal mechanism of defence of living cells makes use of superoxide dismutase or catalase enzymes to protect the cell structure against harmful and reactive oxygen species, such as superoxide radicals or dihydrogen peroxide. There is an increasing interest in understanding the chemistry of dinuclear manganese complexes, as much structural and mechanistic information regarding the activity of the enzyme could be obtained through studies on model complexes [4,5]. Recently, we have reported good catalase activity of two manganese complexes of the ligands Hmesalim and Hetsalim ( Fig.…”