Controlled redox conversion of new X-ray-characterized mono- and dinuclear heptacoordinated Mn(ll) complexes into Di-μ-oxo-dimanganese core complexes

Two heptacoordinated Mn(II) complexes are isolated and X-ray characterized using the well-known tpen ligand (tpen = N,N,N',N'-tetrakis(2-pyridylmethyl)-1,2-ethanediamine): [(tpen)Mn(OH2)](ClO4)(2) (1(ClO4)(2)) and [(tpen)Mn-(mu-OAc)Mn(tpen)](ClO4)(3).2H(2)O (2(ClO4)(3).2H(2)O). Crystallographic data for 1(ClO4)(2) at 110(2) K (respectively at 293-(2) K): monoclinic, space group C2/c, a = 15.049(3) Angstrom (15.096(3) Angstrom), b = 9.932(2) Angstrom (10.105(2) Angstrom), c = 19.246(4) Angstrom (19.443(4) Angstrom), beta = 94.21(3)degrees (94.50(3)degrees), Z = 4. Crystallographic data for 2(ClO4)(3).0.5(C2H5)(2)O at 123(2) K: triclinic, space group P (1) over bar, a = 12.707(3) Angstrom, b = 12.824(3) Angstrom, c = 19.052(4) Angstrom, alpha = 102.71(3)degrees, beta = 97.83(3)degrees, gamma = 98.15(3)degrees, Z = 2. Investigation of the variation upon temperature of the molar magnetic susceptibility of compound 2(ClO4)(3).2H(2)O reveals a weak antiferromagnetic exchange interaction between the two high-spin Mn(II) ions (J = -0.65 +/- 0.05 cm(-1), H = -J (S) over cap (1). (S) over cap (2)). EPR spectra are recorded on powder samples and on frozen acetonitrile solutions, demonstrating the maintenance upon dissolution of the heptacoordination of Mn in complex 1 while complex 2 partially dissociates. Electrochemical responses of complexes 1 and 2 are investigated in acetonitrile, and bulk electrolyses are performed at oxidative potential in the presence of various amounts of 2,6-lutidine (0-2.65 equiv per Mn ion). The formation from either 1 or 2 of the mixed-valent complex [(tpen)Mn-III(mu-O)(2)Mn-IV(tpen)](3+) (3) is established from mass spectrometry and EPR and IR spectroscopy measurements. When reaction is started from 2, formation of [(tpen)Mn-IV(mu-O)(2)(mu-OAc)Mn-IV](3+) (4) is evidenced from cyclic voltammetry, EPR, and UV-vis data. The Mn vs tpen ratio in the electrogenerated complexes is accurately controlled by the quantity of additional 2,6-lutidine. The role of tpen as a base is discussed.