Di-tert-butyl Phosphate Complexes of Cobalt(II) and Zinc(II) as Precursors for Ceramic M(PO_3)_2 and M_2P_2O_7 Materials: Synthesis, Spectral Characterization, Structural Studies, and Role of Auxiliary Ligands

摘要

Reaction of themetal acetates M(OAc)_2.xH_2O with di-tert-butyl phosphate (dtbp-H) (3) in a 4:6 molar ratio iii methanol or tetrahydrofuran followed by slow evaporation of the solvent results in the formation of metal phosphate' clusters (~(u4-0)(dtbp)6J (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); resul~s in the formation of octahedral complexes [M(dtbp)~(imz)4J (M = Co (6); Ni (7); Zn (8?) and [Co(dtbph.: (en)2J (9). The tetrameric clusters 4 and 5 could also be converted into mononuclear 6 and 8, respectively, by treating them with a large exce~s 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(dtbph(3,5-dmphJ (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: C28Hs2CoN808P~, a = 8.525( I) A. b = 9.331(3) A. c = 12.697(2) A, a= 86.40(2)0. /3 = 88.12(3)0, y =67.12(2)0, triclinic, pI. Z = 1. Crystal data for 8: C28Hs~N808P2Z!.l, a = 8.488(1) A, b = 9.333(1) A. c= 12.723(2) ;.., a= 86.55(1)0, p = 88.04(1)0. y ~ 67.42(1)0, triclinic. Pl. Z = I. Crystal data for 10: C~~s2CoN4O8P2, a = b = 18.114(1) A, c = 10.862(1) A. tetragonal. P41, 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 °C (for 5) leads to the loss of twelve equivalents of isobutene gas yielding carbon-free [M-I(U-l-O)(O2P(OH)2)6]. which undergoes further condensation by water elimination to yie.ld a material of the composition CO40!9P6. This sample of 4 when heated above 500 °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 °C.