VALENCE TAUTOMERISM FOR QUINONE COMPLEXES OF MANGANESE - MEMBERS OF THE -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 b oth cis and trans structures prepared with N-donor coligands of varyin g 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 fo r complexes characterized crystallographically. Mn-IV(2,2'-bpy)(3,6-DB Cat)(2) [monoclinic, C2/c, a 10.713(2) Angstrom, b = 29.880(4) Angstro m, 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, P c, a = 12.753(3) Angstrom, b = 11.288(3) Angstrom, c = 17.950(4) Angst rom, beta = 105.21(2)degrees, V = 2493(1) Angstrom(3), Z = 2, R = 0.04 9] 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(II I) tautomers in the solid state at room temperature. An intense transi tion 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) ch arge distribution at 350 K in the solid state, and a decrease in the i ntensity 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 l ow temperature and for hard donor coligands. Shifts in equilibrium to the high-spin Mn-II(SQ)(2) tautomer at increased temperature are thoug ht to be driven entropically, primarily by the increase in low-frequen cy vibrational activity that results from the addition of charge to th e octahedral e(g) sigma-antibonding orbital.