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.

Synthesis of 3 K . Complex 2 (7.0 mg, 9.3 µmol, 1.0 e) and 18-crown-6 (2.5 mg, 9.3 µmol, 1.0 eq) are mixed in benzene (1 mL) and stirred for 5 min. Then KO t Bu (1.0 mg, 9.3 µmol, 1.0 eq) is added and the reaction is stirred for 20 min at RT. The deep red solution is evaporated in vacuo to dryness and the crude product is washed with pentane (3 x 1 mL) and extracted with Et2O (4 x 0.5 mL). Removal of the solvent and lyophilization from benzene yields 3 K as deep red powder (6.8  Re N P t Bu 2 P t Bu 2 P N C 4 C 5 C 6 C 7 C 8

Figure S15
Comparison of experimental (top) and DFT calculated (RI-M06L/def2-SVP, bottom) IR spectra of complexes 2 (left), 2 K (middle) and 3 K (right) and assignments of the asymmetric (magenta) and symmetric (blue) stretching and symmetric bending (green) modes of the PO2 ligands.

SUPPORTING INFORMATION
The number of transferred electrons per Re was calculated by: = * Q: electric charge (C), F: Faraday constant (C mol -1 ) , n: amount of substance (mol).

Crystallographic Details
Suitable single crystals for X-ray structure determination were selected from the mother liquor under an inert gas atmosphere and transferred in protective perfluoro polyether oil on a microscope slide. The selected and mounted crystals were transferred to the cold gas stream on the diffractometer. The diffraction data were obtained at 100 K on a Bruker D8 three-circle diffractometer, equipped with a PHOTON 100 CMOS detector and an INCOATEC microfocus source with Quazar mirror optics (Mo-Kα radiation, l = 0.71073 Å). The data obtained were integrated with SAINT and a semi-empirical absorption correction from equivalents with SADABS was applied. The structure was solved and refined using the Bruker SHELX 2014 software package. [5] All non-hydrogen atoms were refined with anisotropic displacement parameters. All C-H hydrogen atoms were refined isotropically on calculated positions by using a riding model with their Uiso values constrained to 1.5 Ueq of their pivot atoms for terminal sp 3 carbon atoms and 1.2 times for all other atoms.

Computational Details
Structure optimizations and single point calculations including electronic structure analyses were performed within the ORCA 4.2.1 program suite. [7] The thermodynamics of the dimerization equilibrium (2 2 ⇌ 22) were evaluated by three methods employing different DFT functionals: The molecular structure was either optimized applying the PBE functional [8] with the RIJ approximation [9] to minimize computational costs and Ahlrichs' revised def2-SVP basis sets in combination with the corresponding auxiliary basis sets, which include an all electron basis for all elements but Re, for which a Stuttgart-Dresden 60 electron core potential replaces the inner shell 1s-4f orbitals.
[10] To increase numerical accuracy and facilitate optimization of the molecular structure of 22, tight convergence criteria in the SCF procedure and a fine integration grid (Grid6 and GridX6) were applied. Additionally, single point calculations were conducted with the same method but without RIJ or RIJCOSX approximation, using Ahlrichs' def2-TZVP basis for all atoms, again replacing the 60 core electrons of Re with the SD(60,MWB) effective core potential. The influence of the solvent (THF) was accounted for by Truhlar's SMD solvation model. [11] Thermodynamic corrections were evaluated by means of analytical vibrational analyses at the same level of theory as the geometry optimization applying Grimme's quasi-RRHO approach which treats low energy frequencies below 35 cm −1 as free rotors instead of harmonic vibrations for the vibrational partition function. [12] In a second approach, the PBE0 [13] functional was employed in both the optimizations and single point calculations. As PBE0 includes 25 % Hartree-Fock exchange, the RIJCOSX [14] integral evaluation routine was used instead of RIJ. Otherwise the procedure was identical with method 1.
In a third method, Truhlar's strongly parametrized M06L density functional [15] was used for the structure optimizations to evaluate the influence of Grimme's dispersion correction, applying the RIJ approximation and Ahlrich's def2-SVP basis. The thermodynamics are slightly biased by two low negative vibrational modes of 22 which could not be eliminated, even in repeated optimizations. Finally, single point energies were calculated with the M06 functional, [16] Ahlrichs' def2-TZVP basis set and the SMD solvation model. !"# = $%! + 1.89 kcal/mol NBO analyses were conducted using the D3BJ-RIJCOSX-PBE0/def2-TZVP single point calculations with the NBO 6.0 software of Landis and Weinhold. [17] In order to ensure comparability between 2, 2and 2 NO2 , all analyzed structures exhibit the same backbone conformation. In case of 2, the lowest energy conformation deviates but NBO analyses of both conformers are very similar. The NBO Lewis structures of the PO2/NO2 groups were enforced with the CHOOSE command (see figure). The strength of π-backdonation was examined by investigating the interaction of the filled Re lone pairs and the P=O or N=O π * -orbital by second order perturbation theory.
The structures obtained from method 2 were used to calculate the 31 P NMR shifts of 2 and 22, using the PBE0 functional, the RIJCOSX approximation and the GIAOs (Gauge Including Atomic Orbitals) approach. [18] As in the single point calculations, the def2-TZVP basis sets in combination with the def/2 auxiliary basis were employed except for the phosphorous and oxygen atoms, for which Jensen's special pcSseg-4 basis sets were utilized which were specifically developed for nuclear magnetic shielding calculations. [19] Very tight convergence criteria and a fine Grid (Grid6 and GridX6) were applied. The solvent (benzene) was included by means of the conductorlike polarizable continuum model (C-PCM) [20] as implemented in ORCA. As reference, the 31 P NMR shift of H3PO4 (0 ppm) calculated in water was used. The relative chemical shifts of the two 31 P signals computed for 2 (DdP) are in excellent agreement with experiment but differ from the absolute values by about 70 ppm, which partially arises from the missing inclusion of relativistic effects. Therefore, 31 P NMR chemical shift calculations were also performed with the NMR module [21] of the ADF program, [22] which accounts for relativistic spin-orbit contributions to nuclear magnetic shielding constants by means of the two-component ZORA formalism implemented in ADF. [23] The PBE0 hybrid functional was employed in combination with the TZ2P Slater type orbital basis set. [24] Solvent effects were taken into account with the COSMO model implemented in ADF. [25] The absolute chemical shifts obtained from this method are significantly closer (about 20 ppm) to the experimental results (see below).

Thermodynamics of the equilibrium of 2 and 22
The free energy of the equilibrium between 2 and 22 is either slightly exergonic, thermoneutral or endergonic depending on the applied density functional. The transition state of the reaction could not be located. Taking into account, that in solution the 31 P signal of 22 cannot be detected, a ratio of at least 1:40 appears reasonable. This corresponds to a free energy of ∆G = +2.2 kcal·mol -1 , which is in good agreement with the computed value of method 3.