A (31)P-NMR study of the H(+)-MoO(4)(2-)-(HP)O(3)(2-)-HPO(4)(2-)-(C(6)H(5)P)O(3)(2-)-(CH(3)P)O(3)(2-) system at low Mo(tot)/P(tot) ratio - Formation of mixed-hetero X2M5-type polyanions.
Formation of X2M5-type heteropolyanions in the aqueous mixed-hetero H(+)-MoO(4)(2-)-(HP)O(3)(2-)-HPO(4)(2-)-(C(6)H(5)P)O(3)(2-)-(CH(3)P)O(3)(2-) system has been studied by (31)P-NMR at [Mo](tot)/[P(all)](tot) = 1.5 at 298(1) K in 0.600 M Na(Cl) ionic medium. The -log[H(+)] range 1.4-6.2 has been covered. The pK(a) values of methylphosphonate were determined from (31)P-NMR chemical shift data in the range of 0 < -log[H(+)] < 13.2 as H(CH(3)P)O(3)(-) 7.30 and H(2)(CH(3)P)O(3) 2.05. All four-component H(+)-MoO(4)(2-)-P1-P2 subsystems (P1 and P2 are two of the four phosphorus-containing components), except H(+)-MoO(4)(2-)-(HP)O(3)(2-)-HPO(4)(2-), which has been studied previously, were examined instead of the full six-component system in order to simplify the evaluation of the NMR spectra. The concentrations were adjusted to [Mo](tot) = 60 mM and [P1](tot) = [P2](tot) = 20 mM in the measurements. All conceivable P2M5 and (P1)(P2)M5 heteropolyanions were identified. The following novel X2M5-type polyanions were found and their formation constants are given in logarithmic values with 3sigma in parentheses: (CH(3)P)(2)Mo(5)O(21)(4-) 69.68(7), (HP)(C(6)H(5)P)Mo(5)O(21)(4-) 67.09(4), (C(6)H(5)P)PMo(5)O(22)(5-) 63.23(5), H(C(6)H(5)P)PMo(5)O(22)(4-) 68.11(4), (HP)(CH(3)P)Mo(5)O(21)(4-) 67.85(4), (CH(3)P)PMo(5)O(22)(5-) 64.14(7), H(CH(3)P)PMo(5)O(22)(4-) 68.89(4) and (C(6)H(5)P)(CH(3)P)Mo(5)O(21)(4-) 69.18(4). The equilibrium conditions are illustrated in distribution diagrams. Phosphite proved to bind more weakly in X2M5-type polyoxometallates than phosphate or phosphonates, which are almost equally favoured in complexation.
Department of Material Science and Chemistry, Faculty of Systems Engineering, Wakayama University, 930 Sakaedani, Wakayama, 640-8510, Japan. mh1043@sys.wakayama-u.ac.jp
Masato
M
Hashimoto
Ingegärd
I
Andersson
Lage
L
Pettersson