Rhenium Complexes Bearing Tridentate and Bidentate Phosphinoamine Ligands in the Production of Biofuel Alcohols via the Guerbet Reaction

We report a variety of rhenium complexes supported by bidentate and tridentate phosphinoamine ligands and their use in the formation of the advanced biofuel isobutanol from methanol and ethanol. Rhenium pincer complexes 1–3 are effective catalysts for this process, with 2 giving isobutanol in 35% yields, with 97% selectivity in the liquid fraction, over 16 h with catalyst loadings as low as 0.07 mol %. However, these catalysts show poorer overall selectivity, with the formation of a significant amount of carboxylate salt solid byproduct also being observed. Production of the active catalyst 1d has been followed by 31P NMR spectroscopy, and the importance of the presence of base and elevated temperatures to catalyst activation has been established. Complexes supported by diphosphine ligands are inactive for Guerbet chemistry; however, complexes supported by bidentate phosphinoamine ligands show greater selectivity for isobutanol formation over carboxylate salts. The novel complex 7 was able to produce isobutanol in 28% yield over 17 h. The importance of the N–H moiety to the catalytic performance has also been established, giving further weight to the hypothesis that these catalysts operate via a cooperative mechanism.

Infrared spectra were recorded on a Perkin Elmer Spectrum Two FT-IR spectrometer as solid samples in air. Mass spectra (Nanospray) were recorded on a Thermo Scientific Orbitrap Elite.

Synthesis of 2-[(Re(CO) 3 (PNP Ph )]Br
Bis[(2-diphenylphosphino)ethyl]ammonium chloride (0.1 g, 0.209 mmol) was suspended in toluene (4 mL) in a round bottom flask. NaOH (2 mL, 10 wt% in water) was added sequentially while stirring until all starting material has dissolved. The aqueous layer was then removed, and the organic layer was washed with distilled water (3 x 2 mL) and dried over sodium sulphate. Toluene was removed in vacuo giving bis[(2-diphenylphosphino)ethyl]amine as a clear oil.
[Re(CO) 3 (H 2 O) 3 ]Br (0.081 g, 0.200 mmol) was measured out in a Schlenk flask and suspended in dichloromethane (2 mL). The ligand was dissolved in DCM (4 mL) and added dropwise to the complex while stirring, the Schlenk was then washed through with DCM (2 mL). The suspension was heated to 40 o C for 18 hours, giving a colourless liquid and a fine grey S3 suspension. The solution was filtered, reduced in volume by approx. 50%, and triturated with pentane giving a fine white powder. This was isolated by filtration and dried in vacuo (0.072 g, 45.5%). The NMR data was consistent with the literature. 3 31 P{ 1 H} NMR (122 MHz, CDCl 3 ): (δ, ppm) 23.09
Upon heating the suspension dissolved to give a clear, colourless solution. After reflux, a paleyellow solution and an off-white solid were observed, the solution was cooled in an ice-water bath causing further precipitation. The solid was isolated by filtration, washed with hexane (3 x 8 mL), and dried in vacuo giving an off-white powder (0.305 g, 62%). Single crystals for use in X-ray diffraction were grown from a layered solution of methanol with Et 2 O.
The suspension was heated to reflux for 18 hours. Upon heating the suspension dissolved to give a clear, colourless solution. The solution was allowed to cool and toluene was removed in vacuo. The crude solid was dissolved in THF (12 mL) and filtered, giving a golden solution.
The solution was reduced to 1.5 mL and triturated with hexane (20 mL). The off-white solid produced was isolated by filtration and dried in vacuo (0.147 g, 56.7%). Single crystals for use in X-ray diffraction were grown from layering a DCM solution with hexane.

Solid analysis
The post reaction mixture (
Samples were heated at reflux for 3 days. Analysis by 31 P NMR spectroscopy.              after being heated to reflux in methanol with 100 eqv. NaOMe.

S19
Crystallographic data X-ray diffraction experiments on 7, 10 and 11 were carried out at 100(2) K on a Bruker APEX II diffractometer using Mo-Kα radiation (λ = 0.71073 Å). Data collections were performed using a CCD area detector. Intensities were integrated in SAINT 9 and absorption corrections based on equivalent reflections were applied using SADABS. 10 Structures 7, 10 and 11 were solved using ShelXT 11 and refined by full matrix least squares against F2 in ShelXL 12,13 using Olex2. 14 All of the non-hydrogen atoms were refined anisotropically. While all of the hydrogen atoms were located geometrically and refined using a riding model, apart from the none-disordered N-H protons in 7, 10 and 11 which were located in the difference map and refined with isotropic displacement parameters Uiso(H) = 1.2Ueq(N). In the case of 11 there are two molecules present in the asymmetric unit and one displayed disorder, disorder was also present in one of the bromide counter ions and one of the water solvent molecules. The occupancies of the fragments were determined by refining them against a free variable with the sum of the two sites set to equal 1. SADI and SIMU were used to maintain sensible geometries and thermal parameters. Crystal structure and refinement data are given in Table S3. Crystallographic data for compounds 7, 10