Systematic Investigation of Zinc Aminoalkylphosphonates: Influence of the Alkyl Chain Lengths on the Structure Formation

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Abstract
With the high-throughput (HT) methodology, the bifunctional aminoalkylphosphonic acids (AAPA) linker molecules 2-aminoethyl- (AEPA), 3-aminopropyl- (APPA), and 4-aminobutylphosphonic acid (ABPA) [HO3P–CnH2n–NH2 (n = 2–4)] and zinc nitrate were used to synthesize new metal phosphonates in order to investigate the influence of the alkyl chain length on the structure formation. The systematic investigations led to one known (ZnO3PC2H4NH2) and six new compounds: one using AEPA, three using APPA, and two using ABPA. The crystal structures of five compounds were determined by single crystal X-ray diffraction, using X-ray powder diffraction (XRPD) data as well as structure modeling employing force field methods. For compound 1, Zn(O3P–C2H4–NH3)(NO3)(H2O) (monoclinic, Cc, a = 4.799(1) Å, b = 29.342(6) Å, c = 5.631(1) Å, β = 91.59(3)°, V = 792.7(3) Å3, Z = 4), and compound 2, Zn2(OH)(O3P–C3H6–NH3)(NO3) (monoclinic, P2/c, a = 12.158(2) Å, b = 5.0315(10) Å, c = 13.952(3) Å, β = 113.23(3)°, V = 784.3(3) Å3, Z = 2), the structures were determined using single crystal X-ray diffraction data. The crystal structures of [Zn(O3P–C3H6–NH2)]·H2O (3) (monoclinic, P21/c, a = 9.094(2) Å, b = 5.0118(7) Å, c = 16.067(4) Å, β = 90.38(2)°, V = 732.3(2) Å3, Z = 4) and Zn(O3P–C4H8–NH2) (5) (monoclinic, P21/c, a = 8.570(7) Å, b = 8.378(4) Å, c = 9.902(6) Å, β = 90.94(5)°, V = 710.9(8) Å3, Z = 4) were determined using XRPD data. The structural model for compound 6, Zn(O3P–C4H8–NH3)(NO3)(H2O), was established using lattice parameters from XRPD data and following crystal structure modeling employing force field methods. The structures depend strongly on the alkyl chain length n. For n = 2 and 4 isoreticular compounds are observed, while n = 3 leads to new structures. Larger amounts of all compounds were obtained employing scale-up syntheses in a conventional oven as well as in a microwave reactor system. In addition, in situ energy dispersive X-ray diffraction (EDXRD) experiments at 130 °C were performed at beamline F3 at HASYLAB, DESY, Hamburg, to investigate the formation of compounds 2 and 3 as well as the phase transformation of 2 into 3 upon addition of NaOH. All compounds were characterized in detail using X-ray powder diffraction, IR/Raman spectroscopy, and thermogravimetric and elemental analysis.

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