The Metaphosphite (PO₂⁻) Anion as a Ligand

Abstract

The utilization of monomeric, lower phosphorous oxides and oxoanions, such as metaphosphite (PO2−), which is the heavier homologue of the common nitrite anion but previously only observed in the gas phase and by matrix isolation, requires new synthetic strategies. Herein, a series of rhenium(I–III) complexes with PO2− as ligand is reported. Synthetic access was enabled by selective oxygenation of a terminal phosphide complex. Spectroscopic and computational examination revealed slightly stronger σ‐donor and comparable π‐acceptor properties of PO2− compared to homologous NO2−, which is one of the archetypal ligands in coordination chemistry.

The Metaphosphite (PO2−) Anion as a Ligand

“…Interestingly, computational evaluation by density functional theory (DFT) indicated that the equilibrium between monomeric 2 and dimeric 2 2 (Scheme 1) is close to thermoneutral. [15,18] 31 P NMR chemical shifts were computed to clarify solution speciation. The calculated PO 2 chemical shift for monomeric 2 (d P (PO 2 ) = 260 ppm, d P (PNP) = 46 ppm) distinctly differs from dimeric 2 2 (d P (P 2 O 4 ) = 122 ppm, d P (PNP) = 43 ppm) and closely resembles the experimental data (d P = 246 and 13 ppm).…”

“…Electrochemical characterization of 2 by cyclic voltammetry (CV) features a reversible one-electron reduction at E 1 0 = À1.81 V (potentials are reported vs. FeCp 2 +/0 ). [15] Another reduction at very low potential (E 2,pc = À2.9 V), which is irreversible at all scan rates (v = 0.05-3 V s À1 ) and associated with an irreversible feature in the reverse scan at (Figure 3) exhibits a 6-line signal, which could be satisfactorily simulated with g iso = 2.046 and large hyperfine interaction (HFI) with a single low-spin rhenium(II) ion (A iso ( 185/187 Re) = 770 MHz). [20] The absence of resolved 31 P HFI suggests little spin delocalization onto the metaphosphite ligand.…”

“…However, the lower formal metal oxidation state coincides with the replacement of a p-donor (12). [15] Angewandte Chemie Zuschriften 23782 www.angewandte.de (amide) by a competing p-acceptor (imine) ligand, which precludes direct comparison.…”

“…Bonding of the metaphosphite ligand to the metal was further examined by natural bond orbital (NBO) analysis. [15,24] The Re-PO 2 interaction is dominated by the s-bonding natural localized molecular orbital (NLMO), which is slightly polarized towards phosphorus (2: 36% Re, 61% P; 2 À : 35 % Re, 62 % P), yet more covalent with respect to the computed nitro analogue [Re(k-N-NO 2 )(k-N 2 -PyrPz)(PNP)] (2 NO2 : 21 % Re, 78 % N). Re!…”

“…Heating the terminal phosphide complex 1 in benzene at 70 8C in the presence of pyridine-N-oxide results in the formation of a new diamagnetic rhenium species, which could be isolated in 77 % yield (Scheme 1). [15] Full conversion of 1 requires two equivalents of the OAT reagent. Intermediates were not observed.…”