VALENCE TAUTOMERISM FOR QUINONE COMPLEXES OF MANGANESE - MEMBERS OF THE MN-IV(N-N)(CAT)(2)-MN-III(N-N)(SQ)(CAT)-MN-II(N-N)(SQ)(2) SERIES

Intramolecular electron transfer has been investigated for members of the Mn(N-N)(3,6-DBQ)(2) tautomeric series for complex molecules with both cis and trans structures prepared with N-donor coligands of varying basicity. Characteristic structural features of d(3) Mn(IV), d(4) Mn(III), and d(5) Mn(II) have been used to assign charge distribution for complexes characterized crystallographically. Mn-IV(2,2'-bpy)(3,6-DBCat)(2) [monoclinic, C2/c, a 10.713(2) Angstrom, b = 29.880(4) Angstrom, c 12.208(2) Angstrom, beta = 113.51(1)degrees, V = 3583(1) Angstrom(3), Z = 4, R = 0.043], trans-Mn-III(4,4'-bpy)(2)(3 ,6-DBSQ)(3,6-DBCat) [monoclinic, P2(1)/n, a 14.428(2) Angstrom, b = 10.070(2) Angstrom, c 19.976(3) Angstrom, beta = 109.16(1)degrees, V = 2741.5(8) Angstrom(3), Z = 2, R = 0.064], and Mn-II(NO2-phen)(3,6-DBSQ)(2) [monoclinic, Pc, a = 12.753(3) Angstrom, b = 11.288(3) Angstrom, c = 17.950(4) Angstrom, beta = 105.21(2)degrees, V = 2493(1) Angstrom(3), Z = 2, R = 0.049] are examples of the three tautomers that result from differences in intramolecular Mn-quinone charge distribution. Mn(phen)(3,6-DBQ)(2) [monoclinic, C2/c, a = 10.502(2) Angstrom, b = 31.151(5) Angstrom, c = 12.324(2) Angstrom, beta = 114.02(1)degrees, V = 3683(1) Angstrom(3), Z = 4, R = 0.052] exists as an equilibrium mixture of Mn(IV) and Mn(III) tautomers in the solid state at room temperature. An intense transition appears characteristically for the Mn(III) tautomers in the 2100 nm region of the infrared. The appearance of a band in this region for Mn(2,2'-bpy)(3,6-DBCat)(2) is indicative of a shift to the Mn(III) charge distribution at 350 K in the solid state, and a decrease in the intensity at 2100 nm for Mn(4,4'-bpy)(2)(3,6-DBSQ)(3,6-DBCat) at 350 K indicates a shift in the Mn(III)/Mn(II) equilibrium toward the Mn(II) tautomer at increased temperature. The Mn(IV) tautomer is favored at low temperature and for hard donor coligands. Shifts in equilibrium to the high-spin Mn-II(SQ)(2) tautomer at increased temperature are thought to be driven entropically, primarily by the increase in low-frequency vibrational activity that results from the addition of charge to the octahedral e(g) sigma-antibonding orbital.