Molecular cyclo-P3 complexes of the rare-earth elements via a one-pot reaction and selective reduction

Synthesis of new organo-lanthanide polyphosphides with an aromatic cyclo-[P4]2− moiety and a cyclo-[P3]3− moiety is presented. For this purpose, the divalent LnII-complexes [(NON)LnII(thf)2] (Ln = Sm, Yb) ((NON)2− = 4,5-bis(2,6-diisopropylphenyl-amino)-2,7-di-tert-butyl-9,9-dimethylxanthene) and trivalent LnIII-complexes [(NON)LnIIIBH4(thf)2] (Ln = Y, Sm, Dy) were used as precursors in the reduction process of white phosphorus. While using [(NON)LnII(thf)2] as a one-electron reducing agent the formation of organo-lanthanide polyphosphides with a cyclo-[P4]2− Zintl anion was observed. For comparison, we investigated a multi-electron reduction of P4 by a one-pot reaction of [(NON)LnIIIBH4(thf)2] with elemental potassium. As products molecular polyphosphides with a cyclo-[P3]3− moiety were isolated. The same compound could also be obtained by reducing the cyclo-[P4]2− Zintl anion within the coordination sphere of SmIII in [{(NON)SmIII(thf)2}2(μ-η4:η4-P4)]. Reduction of a polyphosphide within the coordination sphere of a lanthanide complex is unprecedented. Additionally, the magnetic properties of the dinuclear DyIII-compound bearing a bridging cyclo-[P3]3− moiety were investigated.


General Methods:
All manipulations of air-sensitive materials were performed under rigorous exclusion of oxygen and moisture in flame-dried Schlenk-type glassware either on a dual manifold Schlenk line, interfaced to a high vacuum pump (10 -3 mbar), or in an argon-filled MBraun glove box. Hydrocarbon solvents were predried using an MBraun solvent purification system (SPS-800), degassed and stored in vacuo over LiAlH4. Tetrahydrofuran was additionally distilled under nitrogen over potassium before storage in vacuo over LiAlH4. THF was dried over K and degassed by freeze-pump-thaw cycles. Elemental analyses were carried out with an Elementar Vario MICRO Cube. NMR spectra were recorded on Bruker spectrometers (Avance III 300 MHz, Avance Neo 400 MHz, or Avance III 400 MHz). Chemical shifts are referenced internally using signals of the residual protio solvent ( 1 H, 1 H{ 11 B}) or the solvent ( 13 C{ 1 H}) and are reported relative to tetramethylsilane ( 1 H, 13 C{ 1 H}), phosphoric acid (85%) ( 31 P{ 1 H}) or BF3-Et2O (15%) in CDCl3 ( 11 B). All NMR spectra were measured at 298 K. The multiplicity of the signals is indicated as s = singlet, d = doublet, t = triplet, sept. = septet, m = multiplet, and br = broad. IR spectra were obtained on a Bruker Tensor 37 spectrometer equipped with a room temperature DLaTGS detector and a diamond ATR (attenuated total reflection) unit. In terms of their intensity, the signals were classified into different categories (vs = very strong, s = strong, m = medium, w = weak).
Note: To ensure the best possible purity and reliability of all compounds, only crystalline material was isolated. Hence all yields and analytics refer to isolated crystalline samples, whereas yields are generally lower compared to bulk samples.
Potential Hazards: Potassium may violently react with moisture, water and air, particular attention need to be paid. P4 is pyrophoric.

Synthesis of [(NON)Ln II (thf)2] (1-Ln)
In a general procedure 20 mL of THF was added to a mixture of 500 mg [K2(NON)] (0.667 mmol, 1.00 eq.) and the corresponding [Ln II I2(thf)2] (0.667 mmol, 1.00 mmol, Ln = Sm: 366 mg, Yb: 381 mg). After stirring the reaction mixture at room temperature for 16 h, all volatiles were removed in vacuo. Afterwards, the remaining solid was extracted with 30 mL of hot n-heptane and filtered while still hot. The residue was washed with another 20 mL of hot n-heptane to increase the yields.
Upon cooling the filtrate to room temperature, single crystals suitable for X-ray diffraction could be obtained. The crystalline yield could be increased by reducing the amount of solvent in vacuo and storing the filtrate at -20°C. The products were isolated by separating the crystals from the mother liquor as highly crystalline black (Ln = Sm) and orange (Ln = Yb) solids. Elemental analysis indicates removal of one molecule of THF from 1-Sm upon drying in vacuo. Thus, above depiction of compound 1-Sm reflects their molecular structure in the solid state as determined by single crystal X-ray diffraction, since the exact structure of the isolated solvatefree compounds was not investigated. For 1-Yb the 1 H NMR spectrum shows no THF resonance after drying the substance in vacuuo.

Magnetism
Ab initio [9,7]-CASSCF/RASSI-SO/SINGLE_ANISO type calculations were performed using the OpenMolcas Package. 10 The input structure was modified as described in the main article but otherwise used as obtained from crystal structure refinement. 21 sextets, 128 quartets and 130 doublets were taken into account for the RASSI routine of the calculation. The applied basis sets were the relativistic ANO-RCC sets taken from the Molcas library. For Dy and P valence triple zeta with polarisation, for Y, O, N, K and the C's coordinating towards K valence double zeta with polarisation and for the remaining C's and H's valence double zeta was chosen. The simulation of the molar susceptibility and magnetisation data was performed using the PHI program package. 11 Magnetic measurements were carried out on a polycrystalline sample of 4-Dy mixed with eicosane in a flame sealed NMR tube. All data were corrected for the diamagnetic contributions of the sample holder, the eico and the underlying diamagnetism of the compound. T-dependent DC susceptibility and AC data were collected on a QuantumDesign MPMS-XL SQUID magnetometer. Field-dependent magnetisation data up to 7 T and hysteresis loops were collected on a QuantumDesign MPMS3-VSM SQUID magnetometer.

Quantum Chemical Calculations
The quantum chemical RI-DFT calculations were performed by means of the program system TURBOMOLE 12 using the RI-BP86 functional. [13][14][15] The basis sets for each atom were of def-SV(P) quality. 16,17 For Sm 18 effective core potentials (ecp) containing 51 electrons including the f-electrons were taken. Population analyses based on occupation numbers were performed to calculate shared electron numbers (SEN) as reliable measures for covalent bonding. 19,20 Cartesian Coordinates of the molecules under discussion (given in a.u.) are summarized in a separated file.