Zinc Di(<i>tert</i>-butyl)phosphate Complexes as Precursors to Zinc Phosphates. Manipulation of Zincophosphate Structures

The reaction of ZnEt2 with HO(O)P(OtBu)2 gives the insoluble polymer {Zn[O2P(OtBu)2]2}n (1). In the presence of slight amounts of water, this reaction produces good yields of the oxo-centered tetranuclear cluster Zn4(μ4-O)[O2P(OtBu)2]6 (2), which has been characterized by X-ray crystallography. Compound 2 is thermally labile and eliminates isobutene and water over the temperature range 130−220 °C. The ceramic yield at 900 °C corresponds to the theoretical yield for a Zn4P6O19 material, and the observed products at this temperature are α-Zn2P2O7 and β-Zn(PO3)2 (by XRD). When heated in ethanol at 85 °C for 30 h, 2 converts to polymer 1 and ZnO. This transformation is facilitated by acids, which allow the conversion to occur at room temperature. Polymer 1, characterized by X-ray crystallography, adopts a zigzag structure with zinc atoms linked alternately by one and then three bridging phosphate groups. This structure is therefore different from that adopted by the other two organozincophosphate {Zn[O2P(OR)2]2}n polymers that are known, which exist as linear chains with the zinc atoms bridged by two phosphate groups. Polymer 1 undergoes a quantitative pyrolytic conversion to β-Zn(PO3)2. Diffusion of a toluene solution of 2 into a dichloromethane solution of 1,6-hexanediamine produces a coordination network with the formula {Zn[O2P(OtBu)2]2[H2N(CH2)6NH2]}n (3), with elimination of ZnO. The network structure of 3 consists of {Zn[O2P(OtBu)2]2[H2N(CH2)6NH2]}n polymer strands interconnected via hydrogen bonds between the N−H and PO groups to form layers stacked along the crystallographic b axis. Each polymer chain contains four-coordinate zinc atoms bonded to two monodentate di-tert-butylphosphate ligands and linked by 1,6-hexanediamine groups. Slabs of the layered structure are held together by a dense array of hydrogen bonds involving the N−H and PO functionalities. These layers possess zinc phosphate/1,6-hexanediamine cores and are coated with tert-butyl groups such that there are only van der Waals interactions between layers. Thermogravimetric analysis and XRD studies show that 3 undergoes thermolysis to a mixture of crystalline α-Zn2P2O7 and β-Zn(PO3)2.