Complexes with substituted 2,5-dihydroxy- p-benzoquinones. Hydrogen bonding of Ca[C 6(C 2H 5) 2O 4] · 3H 2O: A neutron diffraction study

Abstract

Orthorhombic crystals of Ca[C 6(C 2H 5) 2O 4] · 3H 2O were grown in an aqueous silica gel. Lattice constants were a = 503.1(1) pm, b = 1561.9(4) pm, c = 1613.6(4) pm, Z = 4, space group P2 12 12 1. Anisotropic refinement on single-crystal neutron diffraction data led to R w = 0.038. Infinite, corrugated chains formed by Ca 2+ and the bis-chelating [C 6(C 2H 5) 2O 4] 2− ions are the main feature of the crystal structure. These chains extend along [010] and are linked to each other by asymmetric hydrogen bonds. Ca 2+ is coordinated by three water molecules and four oxygen atoms of the [C 6(C 2H 5) 2O 4] 2− ions. Six CaO distances are within the range from 237.7 to 243.0 pm. The seventh oxygen atom of the coordination sphere is more remote from Ca 2+ (CaO(1) 254.9(5) pm). Although the oxygen atoms of the dianion are not equally bound to Ca 2+ CO and CC bond lengths do not indicate a severe perturbation of the π electron delocalization. In one-half of the dianion the CO bond lengths are nearly identical (C(2)O(2) 126.1(4) and C(4)O(4) 125.5(4) pm) and the CC bonds differ by only 1.6 pm (C(2)C(3) 141.2(4) and C(3)C(4) 139.6(4) pm). In the other half containing O(1) which is more weakly bound to Ca 2+ than the other oxygen atoms of the dianion there is a very slight tendency toward a system of alternating double bonds (C(1)O(1) 128.4(4), C(1)C(6) 138.2(4), C(6)C(5) 142.3(4), and C(5)O(5) 124.7(4) pm). Obviously hydrogen bonding prevents a greater perturbation of resonance stabilization of the dianion by compensating for deficient coordination by Ca 2+ ions of the partially negatively charged oxygen atoms, since O(1) is involved in three hydrogen bonds, whereas each of the three other oxygen atoms of the dianion which are more tightly bound to Ca 2+ occurs as proton acceptor only in one hydrogen bond.