1. Monoclinic hydroxyapatite
J C Elliott, P E Mackie, R A Young Science. 1973 Jun 8;180(4090):1055-7. doi: 10.1126/science.180.4090.1055.
The existence of a monoclinic phase of hydroxyapatite, Ca(2)(PO(4))(4)OH, has been confirmed, by single-crystal structure analysis (weighted "reliability" factor = 3.9 percent on |F|(2)). The structure has space group P21/b, a = 9.4214(8) angstroms, b = 2a, c = 6.8814(7) angstroms, and gamma = 120 degrees , and is analogous to that of chlorapatite. The distortions from the hexagonal structure with which the monoclinic structure is pseudosymmetric are similar to those in chlorapatite, including enlargement of that triangular array of oxygen atoms in which the chlorine ion or, in hydroxyapatite, the hydroxyl hydrogen ion is approximately centered. The hydroxyapatite specimen was prepared by the conversion of a single crystal of chlorapatite in steam at 1200 degrees C, was mimetically twinned, and was approximately 37 percent monoclinic.
2. Poly[tetra-kis(μ-cyclo-hexa-ne-1,4-di-carboxyl-ato)di-μ-hydroxido-penta-zinc(II)]
Jin-Xi Chen, Wei-Wei Meng Acta Crystallogr Sect E Struct Rep Online. 2009 Dec 4;66(Pt 1):m16-7. doi: 10.1107/S1600536809051666.
In the title coordination polymer, [Zn(5)(μ(3)-OH)(2)(1,4-CDC)(4)](n) (1,4-CDCH(2) = 1,4-cyclo-hexa-nedicarboxylic acid) or [Zn(5)(C(8)H(10)O(4))(4)(OH)(2)](n), the asymmetric unit comprises one half of an octa-hedrally coordinated ZnO(6) complex unit (site symmetry ) and two five-coordinate ZnO(5) complex units, together with two μ(3)-bridging hydroxido ligands and four 1,4-CDC ligands (comprising two whole mol-ecules and four inversion-related half-molecules). The ZnO(6) unit consists of four carboxyl-ate O donors (two bridging) and two hydroxido O donors (both bridging three Zn centres) [Zn-O range 2.065 (3)-2.125 (3) Å]. Each of the ZnO(5) units [one capped tetra-hedral, the other square-pyrimidal; Zn-O range 1.928 (3)-2.338 (3) Å] has one hydroxido O donor and four carboxyl O donors from three different 1,4-CDC carboxyl-ate O donors (one bridging). Infinite (ZnO)(n) inorganic chains run parallel to the a axis and are interconnected by the organic ligands into a three-dimensional structure.
3. Poly[(μ(2)-4,4'-bipyridine)bis-(μ(4)-5-tert-butyl-isophthalato)bis-(μ(3)-5-tert-butyl-isophthalato)di-μ(3)-hydroxido-penta-zinc(II)]
Dong-Sheng Zhou, Di Sun, Shi-Yao Yang, Rong-Bin Huang Acta Crystallogr Sect E Struct Rep Online. 2009 Aug 15;65(Pt 9):m1078-9. doi: 10.1107/S1600536809031365.
The asymmetric unit of the title compound, [Zn(5)(C(12)H(12)O(4))(4)(OH)(2)(C(10)H(8)N(2))](n), consists of three Zn(II) ions (one of which is located on a twofold rotation axis), two 5-tert-butyl-isophthalate ligands, one 4,4'-bipyridine ligand and one hydroxide group. The five Zn(II) ions form a penta-nuclear zinc cluster, which is further bridged by ten organic ligands, forming two-dimensional sheets. The central zinc ion of the cluster has site symmetry 2 and is octahedrally coordinated in a N(2)O(4) donor set, whereas the other four zinc atoms are tetrahedrally coordinated by four O atoms. The coordination modes for the 5-tert-butyl-isophthalates are bis-(bidentate) or bidentate-monodentate. Hydrogen bonds are formed between adjacent sheets through the hydroxide groups and the O atoms of the monodentate carboxyl-ate groups. The two tert-butyl groups are disordered over two positions with ratios of 0.64 (2):0.36 (2) and 0.85 (3):0.15 (3).
