Di-<i>tert</i>-butyl Phosphate Complexes of Cobalt(II) and Zinc(II) as Precursors for Ceramic M(PO₃)₂ and M₂P₂O₇ Materials: Synthesis, Spectral Characterization, Structural Studies, and Role of Auxiliary Ligands

Abstract

Reaction of the metal acetates M(OAc)2xH2O with di-tert-butyl phosphate (dtbp-H) (3) in a 4:6 molar ratio in methanol or tetrahydrofuran followed by slow evaporation of the solvent results in the formation of metal phosphate clusters [M4(mu 4-O)(dtbp)6] (M = Co (4, blue); Zn (5, colorless)) in nearly quantitative yields. The same reaction, when carried out in the presence of a donor auxiliary ligand such as imidazole (imz) and ethylenediamine (en), results in the formation of octahedral complexes [M(dtbp)2(imz)4] (M = Co (6); Ni (7); Zn (8)) and [Co(dtbp)2-(en)2] (9). The tetrameric clusters 4 and 5 could also be converted into mononuclear 6 and 8; respectively, by treating them with a large excess of imidazole. The use of slightly bulkier auxiliary ligand 3,5-dimethylpyrazole (3,5-dmp) in the reaction between cobalt acetate and 3 results in the isolation of mononuclear tetrahedral complex [Co(dtbp)2(3,5-dmp)2] (10) in nearly quantitative yields. Perfectly air- and moisture-stable samples of 4-10 were characterized with the aid of analytical, thermoanalytical, and spectroscopic techniques. The molecular structures of the monomeric pale-pink compound 6, colorless 8, and deep-blue 10 were further established by single-crystal X-ray diffraction studies. Crystal data for 6: C28H52CoN8O8P2, a = 8.525(1) A, b = 9.331(3) A, c = 12.697(2) A, alpha = 86.40(2) degrees, beta = 88.12(3) degrees, gamma = 67.12(2) degrees, triclinic, P1, Z = 1. Crystal data for 8: C28H52N8O8P2Zn, a = 8.488(1) A, b = 9.333(1) A, c = 12.723(2) A, alpha = 86.55(1) degrees, beta = 88.04(1) degrees, gamma = 67.42(1) degrees, triclinic, P1, Z = 1. Crystal data for 10: C26H52CoN4O8P2, a = b = 18.114(1) A, c = 10.862(1) A, tetragonal, P4(1), Z = 4. The Co2+ ion in 6 is octahedrally coordinated by four imidazole nitrogens which occupy the equatorial positions and oxygens of two phosphate anions on the axial coordination sites. The zinc derivative 8 is isostructural to the cobalt derivative 6. The crystal structure of 10 reveals that the central cobalt atom is tetrahedrally coordinated by two phosphate and two 3,5-dmp ligands. In all structurally characterized monomeric compounds (6, 8, and 10), the dtbp ligand acts as a monodentate, terminal ligand with free P=O phosphoryl groups. Thermal studies indicate that heating the samples at 171 (for 4) or 93 degrees C (for 5) leads to the loss of twelve equivalents of isobutene gas yielding carbon-free [M4(mu 4-O)(O2P(OH)2)6], which undergoes further condensation by water elimination to yield a material of the composition Co4O19P6. This sample of 4 when heated above 500 degrees C contains the crystalline metaphosphate Co(PO3)2 along with amorphous pyrophosphate M2P2O7 in a 2:1 ratio. Similar heat treatment on samples 6-8 results in the exclusive formation of the respective metaphosphates Co(PO3)2, Ni(PO3)2, and Zn(PO3)2; the tetrahedral derivative 10 also cleanly converts into Co(PO3)2 on heating above 600 degrees C.