Structural, Spectroscopic, and Electrochemical Studies of Binuclear Manganese(II) Complexes of Bis(pentadentate) Ligands Derived from Bis(1,4,7-triazacyclononane) Macrocycles

Abstract

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, [Mn2(tmpdtne)Cl2](ClO4)2·2DMF, 2), propyl (tmpdtnp, [Mn2(tmpdtnp)Cl2](ClO4)2· 3H2O, 3), butyl (tmpdtnb, [Mn2(tmpdtnb)Cl2](ClO4)2·DMF·2H2O, 4), m-xylyl (tmpdtn-m-X, [Mn2(tmpdtn-m-X)-Cl2](ClO4)2, 5) and 2-propanol (tmpdtnp-OH, [Mn2(tmpdtnp-OH)Cl2](ClO4)2, 6) groups. 1 crystallizes in the orthorhombic space group P212121 (No. 19) with a = 7.959(7) Å, b = 12.30(1) Å, and c = 21.72(2) Å; 2, in the monoclinic space group P2(1/c) (No. 14) with a = 11.455(4) Å, b = 15.037(6) Å, c = 15.887(4) Å, and β = 96.48(2)°; 3, in the monoclinic space group P2(1/c) (No. 14) with a = 13.334(2) Å, b = 19.926(2) Å, c = 18.799-(1) Å, and β = 104.328(8)°; and [Mn2(tmpdtnb)Cl2](ClO4)2·4DMF·3H2O (4'), in the monoclinic space group P2(1/n) (No. 14) with a = 13.361(3) Å, b = 16.807(5) Å, c = 14.339(4) Å, and β = 111.14(2)°. Significant distortion of the Mn(II) geometry is evident from the angle subtended by the five-membered chelate (ca. 75°) and the angles spanned by trans donor atoms (<160°). 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(II) 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 → Mn(II)Mn(III) → Mn(III)2} and then to the Mn(IV) state {Mn(III)2 → Mn(III)Mn(IV) → Mn(IV)2}. 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(III)2 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 Mn(II)Mn(IV) state at +0.89 V and the Mn(IV)2 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 oxo-bridged mixed-valent Mn(III)Mn(IV) intermediate with a characteristic 16-line ESR signal.