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, [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 P21/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 P21/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
P21/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 MnIV 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 {MnII2 → MnIIMnIII →
MnIII2} and then to the MnIV state {MnIII2 → MnIIIMnIV → MnIV2}. 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 MnIII2 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 MnIIIMnIV state at +0.89 V
and the MnIV2 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 MnIIIMnIV intermediate with a characteristic 16-line ESR signal.