Structural, spectroscopic, and electrochemical studies of binuclear manganese(II) complexes of bis(pentadentate) ligands derived from bis(1,4,7-triazacyclononane) macrocycles

Structural, electrochemical, ESR, and H2O2 reactivity studies are reported for [Mn(dmptacn)Cl]ClO4 (1, dmptacn = 1,4-bis(2-pyridylmethyl)-1,4,7-triazacyclononane) and binuclear complexes of bis(pentadentate) ligands, generated by attaching 2-pyridylmethyl arms to each secondary nitrogen in bis(1,4,7-triazacyclononane) macrocycles and linked by ethyl (tmpdtne, [Mn-2(tmpdtne)Cl-2](ClO4)(2). 2DMF, 2), propyl (tmpdtnp,[Mn-2(tmpdtnp)Cl-2](ClO4)(2). 3H(2)O, 3), butyl (tmpdtnb, [Mn-2(tmpdtnb)Cl-2](ClO4)(2). DMF . 2H(2)O, 4), m-xylyl (tmpdtn-m-X,[Mn-2(tmpdtn-m-X)Cl-2](ClO4)(2), 5) and 2-propanol (tmpdtnp-OH, [Mn-2(tmpdtnp-OH)Cl-2](ClO4)(2), 6) groups. 1 crystallizes in the orthorhombic space group P2(1)2(1)2(1) (No. 19) with a = 7.959(7) Angstrom, b = 12.30(1) Angstrom, and c = 21.72(2) Angstrom; 2, in the monoclinic space group P2(1)/c (No. 14) with a = 11.455(4) Angstrom, b = 15.037(6) Angstrom, c = 15.887(4) Angstrom, and beta = 96.48(2)degrees; 3, in the monoclinic space group P2(1)/c (No. 14) with a = 13.334(2) Angstrom, b = 19.926(2) Angstrom, c = 18.799-(1) Angstrom, and beta = 104.328(8)degrees; and [Mn-2(tmpdtnb)Cl-2](ClO4)(2). 4DMF . 3H(2)O (4'), in the monoclinic space group P2(1)/n (No. 14) with a = 13.361(3) Angstrom, b = 16.807(5) Angstrom, c = 14.339(4) Angstrom, and beta = 111.14(2)degrees. Significant distortion of the Mn(II) geometry is evident from the angle subtended by the five-membered chelate (ca. 75 degrees) and the angles spanned by trans donor atoms (<160 degrees). The Mn geometry is intermediate between octahedral and trigonal prismatic, and for complexes 2-4, there is a systematic increase in M...M distance with the length of the alkyl chain. Cyclic and square-wave voltammetric studies indicate that 1 undergoes a 1e(-) oxidation from Mn(TI) to Mn(III) followed by a further oxidation to Mn-IV at a significantly more positive potential. The binuclear Mn(II) complexes 2-5 are oxidized to the Mn(III) state in two unresolved 1e(-) processes {Mn-II(2)-->(MnMnIII)-Mn-II-->Mn-2(III)} and then to the Mn-IV state {Mn-2(III)-->(MnMnIV)-Mn-III-->Mn-2(IV)}. For 2, the second oxidation process was partially resolved into two 1e(-) oxidation processes under the conditions of square-wave voltammetry. In the case of 6, initial oxidation to the Mn-2(III) state occurs in two overlapping 1e(-) processes as was found for 2-5, but this complex then undergoes two further clearly separated 1e(-) oxidation processes to the (MnMnIV)-Mn-III state at +0.89 V and the Mn-2(IV) state at +1.33 V (vs Fc/Fc(+)). This behavior is attributed to formation of an alkoxo-bridged complex. Complexes 1-6 were found to catalyze the disproportionation of H2O2. Addition of H2O2 to 2 generated an ore-bridged mixed-valent (MnMnIV)-Mn-III intermediate with a characteristic 16-line ESR signal.