CORRELATION OF ELECTRON-DENSITY AND SPIN-EXCHANGE INTERACTION IN DIMERIC COPPER(II) FORMATES, ACETATES AND SILANECARBOXYLATES

The structures of six dimeric copper(II) formates and acetates with picoline as an axial ligand, (I)-(VI), have been determined at room temperature and electron density distributions in lithium acetate dihydrate, (VII), and copper(II) formate diurea dihydrate, (VIII), have been studied at 120 K. Mo K-alpha radiation was used throughout (lambda = 0.71073 angstrom): tetrakis(mu-formato-O,O')-bis(alpha-picoline)discopper(II), [Cu(HCOO)2-alpha-pic]2, (I), M(r) = 493.42, triclinic, P1BAR, a = 7.310 (1), b = 10.493 (2), c = 7.291 (1) angstrom, a = 91.35 (2), beta = 113.93 (1), gamma = 109.24 (2)-degrees, V = 474.5 (1) angstrom 3, Z = 1, D(x) = 1.73 Mg m-3, mu = 2.29 mm-1, F(000) = 250, R = 0.029 for 1675 reflections; [Cu(HCOO)2-beta-pic]2, (II), triclinic, P1BAR, a = 10.922 (3), b = 13.123 (2), c = 7.208 (2) angstrom, alpha = 98.64 (2), beta = 109.11 (2), gamma = 83.39 (2)-degrees, V = 962.6 (3) angstrom 3, Z = 2, D(x) = 1.70 Mg m-3, mu = 2.26 mm-1, F(000) = 500, R = 0.032 for 2629 reflections; [Cu(HCOO)2-gamma-pic]2, (III), monoclinic, P2(1)/c, a = 10.695 (2), b = 11.373 (2), c = 7.755 (1) angstrom, beta = 90.13 (2)-degrees, V = 942.7 (3) angstrom 3, Z = 2, D(x) = 1.74 Mg m-3, mu = 2.31 mm-1, F(000) = 500, R = 0.027 for 1474 reflections; tetrakis(mu-acetato-O,O')-bis(alpha-picoline)dicopper(II), [Cu(CH3COO)2-alpha-pic]2, (IV), M(r) = 549.53, monoclinic, P2(1)/c, a = 7.697 (1), b = 20.021 (3), c = 8.226 (1) angstrom, beta = 116.0 (1)-degrees, V = 1139.4 (3) angstrom 3, Z = 2, D(x) = 1.60 Mg m-3, mu = 1.92 mm-1, F(000) = 564, R = 0.051 for 1691 reflections; [Cu(CH3COO)2-beta-pic]2, (V), triclinic, P1BAR, a = 8.315 (1), b = 20.242 (2), c = 7.789 (1) angstrom, alpha = 93.21 (1), beta = 117.36 (1), gamma = 92.74 (1)-degrees, V = 1158.5 (2) angstrom 3, Z = 2, D(x) = 1.58 Mg m-3, mu = 1.89 mm-1, F(000) = 564, R = 0.063 for 3041 reflections; [Cu(CH3COO)2-gamma-pic]2, (VI), monoclinic, P2(1)/c, a = 10.499 (1), b = 13.031 (2), c = 8.880 (1) angstrom, beta = 102.16 (1)-degrees, V = 1187.6 (2) angstrom 3, Z = 2, D(x) = 1.55 Mg m-3, mu = 1.84 mm-1, F(000) = 564, R = 0.034 for 1832 reflections; lithium acetate dihydrate, LiCH3COO.2H2O, (VII), M(r) = 102.01, orthorhombic, Cmmm, a = 6.7291 (7), b = 10.8764 (7), c = 6.5734 (5) angstrom, V = 481.09 (7) angstrom 3, Z = 4, D(x) = 1.41 Mg m-3, mu = 0.125 mm-1, F(000) = 216, R = 0.035 for 1357 reflections; copper(II) formate diurea dihydrate, Cu(HCOO)2.2CO(NH2)2.2H2O, (VIII), M(r) = 309.71, monoclinic, P2(1)/c, a = 8.1584 (9), b = 8.3683 (4), c = 8.0088 (5) angstrom, beta = 95.813 (7)-degrees, V = 543.96 (7) angstrom 3, Z = 2, D(x) = 1.89 Mg m-3, mu = 2.05 mm-1, F(000) = 318, R = 0.024 for 5427 reflections. The structures of the picoline adducts (I)-(VI) show no important geometrical differences in the Cu2(COO)4 cage between the copper acetates and formates. The observed deformation-density distributions in the CH3COO- (VII) and HCOO- (VIII) ions are in accord with theoretical expectations. Mulliken population analysis of eight carboxylate ions using STO-6G basis sets indicates that the electron population of the carboxylate group of the bridging ligand correlates with the -2J values of the binuclear copper(II) carboxylates. The key point that determines the strength of the spin-exchange interaction is the 2p(x) orbital population of the carboxylate C atom (x is parallel to the C-R bond axis in the RCOO- ion). The electron population on a carboxylate O atom is shown to exhibit a roughly linear correlation with the pK(a) value of the parent carboxylic acid. The conundrum of why formate and fluoroacetate are exceptions to the correlation of -2J with pK(a) value has been solved in this paper.