Di-tert-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^†

Reaction of the metal acetates M(OAc)₂·xH₂O 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(μ₄-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) Å, b = 9.331(3) Å, c = 12.697(2) Å, α = 86.40(2)°, β = 88.12(3)°, γ = 67.12(2)°, triclinic, P1̄, Z = 1. Crystal data for 8:  C₂₈H₅₂N₈O₈P₂Zn, a = 8.488(1) Å, b = 9.333(1) Å, c = 12.723(2) Å, α = 86.55(1)°, β = 88.04(1)°, γ = 67.42(1)°, triclinic, P1̄, Z = 1. Crystal data for 10:  C₂₆H₅₂CoN₄O₈P₂, a = b = 18.114(1) Å, c = 10.862(1) Å, 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 [M₄(μ₄−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 °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.