A Ruthenium cis-Dihydride with 2-Phosphinophosphinine Ligands Catalyses the Acceptorless Dehydrogenation of Benzyl Alcohol

The first ruthenium dihydride complex featuring a phosphinine ligand cis-[Ru(H)2(2-PPh2-3-Me-6-SiMe3-PC5H2)2] was synthesised exclusively as the cis-isomer. When formed in situ from the reaction of cis-[Ru(Cl)2(2-PPh2-3-Me-6-SiMe3-PC5H2)2] with two equivalents of Na[BHEt3], as demonstrated by 31P and 1H NMR spectroscopy, the catalysed acceptorless dehydrogenation of benzyl alcohol was observed leading to benzyl benzoate in up to 70% yield.

Homogeneous ruthenium catalysts with phosphine ligands have proven extremely useful over many decades including those with the small bite-angle ligand bis(diphenylphosphinomethane) (dppm). 39 For example, the pre-catalyst cis-[Ru(Cl)2(dppm)2] achieved an initial TOF of 180,000 h -1 for the reaction of CO2 with hydrogen in an amine solution leading to the production of an amine-formate adduct, thus demonstrating facile CO2 conversion. 40 The development of phosphinines, the phosphorus analogue of pyridine, as effective ligands for homogeneous catalysis has grown over the last 25 years, [41][42][43] and now includes applications such as Rh-catalysed hydroformylation 44 and hydrogenation, [45][46] Au-catalysed cycloisomerisations, 47 Cr-catalysed ethylene oligomerisation 48 and the hydroboration of carbonyls. 49 The application of homogeneous metal-phosphinine catalysts to confront energy challenges has also been explored, including in water oxidation reactions 50 and the catalytic upgrading of alcohols to advanced biofuels. 46 Previous work in our group has demonstrated that Ru catalysts based on the bidentate chelating phosphinophosphinine ligand 2-PPh2-3-Me-6-SiMe3-PC5H2 (PPʹ) catalysed the room temperature transfer hydrogenation of acetophenone and the H-borrowing upgrading of alcohol fuels. 46,51 Ruthenium hydrides are implicated in many catalytic reactions, yet the first Ru complex comprising of both a hydride and phosphinine ligand has only been described recently by us. 52 Herein, we describe the first ruthenium phosphinine dihydride complex and compare its catalytic activity to that of classical diphosphine-supported complexes for AD reactions.
Ruthenium dihydride fragments supported by phosphine ligands are well known, and have proven to be successful catalysts for numerous reactions, as well as suitable precursors for photochemical C-H activation. 53

The formation of cis-[Ru(H)2(PPʹ)2]
(2) was evident by an immediate colour change of orange to bright red upon the addition of Na[HBEt3] to 1. 31 P{ 1 H} NMR spectroscopy confirmed the presence of only four resonances, two at high chemical shift indicative of phosphinine groups, and two at lower chemical shift from the diphenylphosphino groups. Two doublet-ofdoublet-of-doublets resonances were observed with a large trans-coupling (261 Hz), which has decreased compared to the trans 2 JPP coupling in 1 (425 Hz). The 1 H NMR spectrum displayed resonances for two inequivalent PPʹ ligands as well as two multiplet resonances at -6.55 and -7.04 for the two Ru-H. ‡ Slow evaporation of a petroleum ether solution of 2 in a glovebox yielded crystals suitable for structural characterisation using X-ray diffraction. § The crystal structure of 2, Figure 1, shows Ru(1) in a slightly distorted octahedral geometry with two phosphinophosphinine and cis-dihydride ligands, which were located in the Fourier difference map and refined ( Figure  1). The phosphinine donors are cis-disposed, due to their strong π-accepting character, and this explains why only one isomer of 2 was formed (as a racemic mixture). The bond lengths of the phosphinine phosphorus atoms P(1) and P(3) to Ru(1) (2.2795(5) and 2.2634(5) Å respectively) are shorter than the distances from the diphenylphosphino phosphorus atoms P(2) and P(4) to Ru(1) (2.3071(5) and 2.3889(5) Å respectively). The angles between phosphorus atoms on the same ligand, e.g. P(1)-Ru (1)  Despite the clean synthetic route, attempts to isolate 2 were problematic whenever 2 was dried under vacuum. Upon exposure to vacuum, the resulting 31 P{ 1 H} NMR spectra displayed many resonances of weak intensity instead of the anticipated product. Analysis by mass spectrometry (APCI) corroborated these results with only very low intensity signals for masses above 300 Da. This was indicative that drying under vacuum led to the decomposition of 2. Isolation of 2 by evaporation of the solvent under a stream of nitrogen led to the successful isolation of 2 in modest yields ( Four benzyl alcohols with different electron donating and withdrawing substituents were screened to test the reactivity of 2. These reactions were compared to the catalytic activity of [Ru(H)2(dppm)2] (3). Base line reactions with no Ru catalyst gave no formation of benzyl benzoate, and when only Na[HBEt3] was added, there was only a small amount of benzaldehyde produced (0.5 %). Using 1 mol% of 2, a 28% yield of ester was evident after 45 hours at 140 o C (run 1), which is higher than for 3 (19%, run 9). Doubling the catalyst concentration and increasing the reaction time led to better conversions, thereby increasing yields (run 3: 61% for 2, run 11: 38% for 3 after 94 hours) up to 70% benzyl benzoate for 2. The addition of base (NaO t Bu) did not increase yields. Substitution of the benzyl alcohol with a para-methoxy group led to significant production of the substituted benzaldehyde as the favoured product (30%, run 6). Electron withdrawing substituents inhibit the acceptorless dehydrogenation reaction (para-NO2: 12%, run 8; para-Br: 11%, run 7), although the Br-substituted aldehyde byproduct is also observed to a considerable extent (11%). In comparison [RuH2(dppm)2] produced similar yields and outcomes for substituted benzaldehydes.
Comparisons between different catalysts can be made for the coupling of benzyl alcohol. The mixture of [RuCl2(pcymene)(IiPr)] (2.5 mol%), KOH (10 mol%) and PCy3 (4.5 mol%) at 163°C for 18hrs gave a yield of 31% benzyl benzoate; benzyl alcohols with para-OMe and Me groups led to lower yields. 58 A RuCl2 complex with an aryl-tethered phosphine ligand in combination with 2 equiv. NaO t Bu gave benzaldehyde as the product at 110°C (2 mol%, 36 h, 80% yield). 59 [RuH2(PPh3)4] (2 mol%) at 180 o C in refluxing mesitylene gave 60% benzyl benzoate. 15 We observed that use of [RuCl2(PPh3)3] did not lead to substantial amounts of ester formation (2.4%) even with the addition of KO t Bu. Yields were found to be similar to 2 after 120 h using an imidazolyl phosphine ruthenium dichloride complex with the addition of base (KO t Bu). 60 Morton and Cole-Hamilton achieved high rates of hydrogen production from aliphatic alcohols using [RuH2(N2)(PPh3)3], albeit at 150°C. 61 Aldehydes were synthesised from primary alcohols in moderately high yields with ruthenium triazolylidene dichloride complexes. 62 In our work, an electron donating OMe substituent produced mixtures of ester:aldehyde products (run 6, 1:6; run 13, 1.6:2) in a higher ratio of aldehyde compared to an Ir-phosphine pincer catalyst (3:1). 24 In

Conflicts of interest
There are no conflicts to declare.