Synthesis of a rhodium(iii) dinitrogen complex using a calix[4]arene-based diphosphine ligand

The synthesis and characterisation of the rhodium(iii) dinitrogen complex [Rh(2,2′-biphenyl)(CxP2)(N2)]+ are described, where CxP2 is a trans-spanning calix[4]arene-based diphosphine and the dinitrogen ligand is projected into the cavity of the macrocycle.

All manipulations were performed under argon using standard Schlenk line and glove box techniques unless otherwise stated. Glassware was oven dried at 150 °C overnight and flamed under vacuum prior to use. 3 Å molecular sieves were activated by heating at 300 °C in vacuo overnight prior to use. Anhydrous solvents were obtained from commercial sources. Hexane was further dried over Na/K2 alloy, vacuumdistilled, and freeze-pump-thaw degassed before being placed under argon over a potassium mirror.
CH2Cl2 was further dried over CaH2 overnight, vacuum-distilled, and freeze-pump-thaw degassed three times before being placed under argon over activated 3 Å molecular sieves. THF was further dried over Na/benzophenone, vacuum-distilled, and freeze-pump-thaw degassed three times before being placed under argon over activated 3 Å molecular sieves. Fluorobenzene was stirred over neutral alumina, filtered, stirred over CaH2 overnight, vacuum-distilled, and freeze-pump-thaw degassed three times before being placed under argon over activated 3 Å molecular sieves. 1 CD2Cl2 was placed over activated 3 Å molecular sieves and freeze-pump-thaw degassed three times before being placed under argon. trans-[Rh(biph)(PPh3)2Cl] 2,3 and CxP2 4 were prepared using literature procedures, or minor variations thereof.
For convenience, the full multi-step procedure for the latter is reported below. All other reagents and solvents are commercial products and were used as received. NMR spectra were recorded on Bruker spectrometers at 298 K unless otherwise stated. Chemical shifts are quoted in ppm and coupling constants in Hz. Virtual coupling constants are reported as the separation between the first and third lines. 5 1 H and 31 P{ 1 H} NMR spectra recorded in fluorobenzene were referenced using an internal sealed capillary of a 25 mM solution of trimethylphosphate in C6D6 (δ1H 3.38, 3 JPH = 11 Hz; δ31P = 3.7). 6 The C(CF3)3 carbon resonance of the [Al(OR F )4]anion was not observed in any 13 C{ 1 H} NMR spectra described herein. Highresolution ESI mass spectra (HR ESI-MS) were recorded on a Bruker Maxis Plus instrument. Lowresolution ESI mass spectra (LR ESI-MS) were recorded on an Agilent 6130B single Quad instrument.
Solution-phase IR spectroscopy was performed on a PerkinElmer Spectrum 100 spectrometer. ATR-IR spectroscopy was performed on a Bruker ALPHA FT-IR spectrometer. Microanalyses were performed by Stephen Boyer at London Metropolitan University.

Preparation of CxP2
Method A: To a solution of C7 (1.0 g, 1.5 mmol) in THF (25 mL) cooled to -78 °C was added a solution of KPPh2 (0.5 M in THF,8.7 mL,4.4 mmol) dropwise with stirring. The reaction was warmed to ambient temperate overnight and then refluxed at 70 °C for 2 h. The resulting suspension was washed with NH4Cl(aq) (satd., 3×30 mL, argon-sparged), H2O (3×30 mL, argon-sparged), then dried over MgSO4 and filtered. The solution was to ca. 5 mL in vacuo and the product precipitated by addition of excess hexane (ca. 100 mL). Additional material was obtained by concentrating the supernatant to ca. 50 mL in vacuo.
The product was isolated by filtration and the combined batches dried in vacuo. Yield: 1.00 g (1.01 mmol, 67%; white crystalline solid).

Method B:
To solution of C8 (0.45 g, 0.58 mmol) in THF (15 mL) cooled to -78 °C was added a solution of KPPh2 (0.5 M in THF, 3.5 mL, 1.75 mmol) dropwise with stirring. The reaction was warmed to ambient temperate overnight, and the resulting suspension washed with NH4Cl(aq) (satd., 3×20 mL, argon-sparged), H2O (3×20 mL, argon-sparged), then dried over MgSO4 and filtered. The solution was reduced to ca. 5 mL in vacuo and the product precipitated by addition of excess hexane (ca. 100 mL). Additional material was obtained by concentrating the supernatant to ca. 50 mL in vacuo. The product was isolated by filtration and the combined batches dried under high vacuum. Yield: 463 mg (46.8 mmol, 81%; white microcrystalline solid).
Spectroscopic data are consistent with previous reports.

Stability of 2-DCM under dinitrogen.
A solution of 2-DCM (8.7 mg, 5.     Whilst not systematically prepared, the crystal structure of the [BAr F 4] salt has previously been reported.

4.
Synthesis and reactivity of rhodium CxP2 complexes

Analysis of 1-N2 by IR spectroscopy
A solution of 1-OH2 (11.1 mg,5.0 µmol) and [ZrCp2Me2] (3.1 mg, 12.5 µmol) in C6H5F (0.5 mL) was prepared within a J. Young valve NMR tube under dinitrogen (1 atm; passed over activated 3 Å molecular sieves) as described above, monitoring by NMR spectroscopy until full conversion of 1-OH2 into 1-N2 was observed. In a dinitrogen glovebox (<0.1 ppm H2O and O2), an aliquot of the sample was transferred into a pre-dried KBr transmission cell, sealed with two Teflon stopcocks, and then transported to an external IR spectrometer within a sealed ziplock plastic bag under dinitrogen. The cell was removed from the bag and the sample immediately analysed by IR spectroscopy, background correcting for C6H5F solvent. A very low intensity ν(N2) band was observed at 2290 cm -1 and this assignment was supported by (a) exposure of the sample to air, which resulted in complete conversion of 1-N2 into 1-OH2 within 5 seconds, (b) analysis by ATR-IR spectroscopy, where the sample was added directly onto the reflectance crystal of a spectrometer housed within the dinitrogen filled glovebox using rigorously dried glassware and allowed to evaporate; and (c) computational analysis.

Crystallography
Data were collected on a Rigaku Oxford Diffraction SuperNova AtlasS2 CCD diffractometer using graphite monochromated Mo Kα (λ = 0.71073 Å) or CuKα (λ = 1.54184 Å) radiation and an Oxford Cryosystems N-HeliX low temperature device [150(2) K]. Data were collected and reduced using CrysAlisPro and refined using SHELXT, 13 through the Olex2 interface. 14 Both complexes are isomorphous and crystallise with 1.5 equivalents of hexane in the low symmetry P-1 space group. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed in calculated positions and refined using the riding model. Disorder of two of the cation OCH2CH2CH3 substituents was treated by modelling the propyl chains over two sites and restraining their geometry.
Disorder of three of the anion OC(CF3)3 substituents was treated by modelling the CF3 groups over two sites and restraining their geometry. Disorder of both hexane solvent molecules was treated by modelling them over two sites and restraining their geometry. Restraints to thermal parameters were applied where necessary in order to maintain sensible values.
Full details about the collection, solution, and refinement are documented in CIF format, which have been deposited with the Cambridge Crystallographic Data Centre under CCDC 2225292 (1-OH2) and 2225293 (1-N2).

Computational details
The geometry optimizations followed by the harmonic frequency calculations were carried out at the B3LYP-D3(BJ)/def2-SVP or PBE-D3(BJ)/def2-SVP level of theory using the Gaussian 16 suit of programs. 15,16 Effective core potentials were used for the 28 core electrons of Rh. Superfine integration grid is considered for all cases. Cartesian coordinates at the latter level of theory are provided in XYZ format.
The energy decomposition analysis (EDA) 17 in combination with the natural orbital for chemical valence (NOCV) 18 method was performed at the PBE-D3(BJ)-ZORA/TZ2P//PBE-D3(BJ)/def2-SVP level using the ADF (2018.105) program package. 19,20 The zeroth-order regular approximation (ZORA) 21 was used to include scalar relativistic effects. All electrons were considered in the computations. In the EDA method, the interaction energy (ΔΕint) between two prepared fragments is divided into three energy terms, viz., the electrostatic interaction energy (ΔEelstat), which represents the quasiclassical electrostatic interaction between the unperturbed charge distributions of the prepared fragments, the Pauli repulsion (ΔEPauli), which is the energy change associated with the transformation from the superposition of the unperturbed electron densities of the isolated fragments to the wavefunction that properly obeys the Pauli principle through explicit antisymmetrization and renormalization of the product wavefunction, and the orbital interaction energy (ΔEorb), which is originated from the mixing of orbitals, charge transfer and polarization between the isolated fragments. Use of DFT-D3 with the Becke-Johnson type damping function (BJ) gives S25 additional dispersion interaction energy (ΔΕint(disp)) between two interacting fragments. Therefore, the total interaction energy (ΔΕint) between two fragments can be defined as: ΔΕint(elec) = ΔEelstat + ΔEPauli + ΔEorb (2) The EDA-NOCV combination allows the partition of ΔEorb into pairwise contributions of the orbital interactions, which gives important information about bonding. The deformation density Δρk(r) which originates from the mixing of the NOCVs k(r) and -k(r), resulting from diagonalizing the deformation density matrix, gives the charge flow due to orbital interactions (equation 3), and the corresponding ΔEk orb reflects the amount of orbital interaction energy coming from such interaction which sums to the total orbital energy (equation 4).