Article pubs.acs.org/Organometallics
Ligand Attachment Chemistry in the Preparation of PCsp3P and PCsp2P Complexes of Rhodium Jessamyn R. Logan, Warren E. Piers,* Javier Borau-Garcia, and Denis M. Spasyuk Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4 S Supporting Information *
ABSTRACT: The attachment of electron-rich PCP pincer ligands bis(2-(dialkylphosphino)phenyl)methane (alkyl = isopropyl, tert-butyl) to rhodium via reactions with [(COE)2Rh(μ-X)]2 (X = Cl, OSO2CF3) through C−H bond activations is reported. The first C−H activation to produce PCsp3P derivatives is facile and favors products wherein the remaining benzylic C−H and the Rh−H hydrogens are transdisposed across the new Rh−C bond. For the less bulky isopropyl-substituted ligand, chlorido- or triflato-bridged dinuclear products are favored, while for the tert-butyl-adorned ligand, monomers are formed. The favoring of both trans C− H/Rh−H and dinuclear systems hampers the second C−H activation, necessary to form the (more desirable) PCcarbeneP derivatives. Through spectroscopic and structural investigations, the factors that influence the ligand attachment chemistry through successive C−H activations in these ligands are discussed.
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INTRODUCTION Pincer ligands are tridentate chelating ligands that provide a tunable donor environment for elements from across the transition-metal and main-group series in the periodic table.1,2 Variations in donor atom, linker, and formal charge provide a vast pallet of possibilities, and this basic design has been exploited in the development of catalysts for many important reactions.3−6 Within the large number of ligands reported, those characterized by two flanking phosphines and anchored by a strongly σ donating carbon atom occupy a privileged position, in terms of both history and effective performance. From the early designs of Shaw7,8 to more recent examples,9−12 these “PCP” ligand frameworks are electron-rich donor arrays that facilitate a variety of bond activations through the stabilization of low-coordinate reactive intermediates and higher oxidation state derivatives. One of the methods used to prepare metal complexes of these ligands is via protocols that utilize C−H activation processes, and so later transition metals that commonly engage in C−H activation have well-developed chemistry with these ligands.13,14 A subset of this PCP pincer ligand family include those in which the anchoring carbon donor has carbene character.8,15−19 These moieties not only are strong donors but also introduce the possibility for ligand cooperativity20−24 in which the MC unit participates in small-molecule activation processes. A case in point are the bis(2-(dialkylphosphino)phenyl)methane ligands I (Scheme 1), recently introduced by our group,25 wherein complexes of iridium25−28 are prepared via successive C−H bond activations.29 In this chemistry, the first benzylic C−H bond addition produces five-coordinate Ir(III) hydrido chloride complexes of the PCsp3P donors (II), which are © 2016 American Chemical Society
Scheme 1. C−H Activation for PCP Ligand Attachment
somewhat ill-defined due to dynamic processes in solution and the propensity to undergo a further C−H activation/H2 elimination sequence that yields the square-planar Ir(I) carbene chlorides III (Scheme 1).30,31 Related PCcarbeneP complexes of nickel32−34 and palladium35−38 can be prepared using C−H activation followed by a dehydrohalogenation; the MC moiety in these compounds also engages in the cooperative activation of wide variety of small molecules. Here, we report an extension of the coordination chemistry of these ligands to rhodium, which provides insight into the details concerning the successive C−H activations necessary to generate PCsp3P and PCsp2P complexes of this ligand with group 9 metals and the dynamic behavior of the former complexes through the isolation and characterization of intermediates.
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RESULTS AND DISCUSSION Chelating phosphines can be smoothly coordinated to Rh(I) using the readily available dimer [(COE)2Rh(μ-Cl)]239 (COE = cyclooctene) as the starting material. For the ligand where R = iPr, IiPr, the reaction with [(COE)2Rh(μ-Cl)]2 proceeds Received: February 24, 2016 Published: April 21, 2016 1279
DOI: 10.1021/acs.organomet.6b00155 Organometallics 2016, 35, 1279−1286
Article
Organometallics
Figure 1. Low-temperature 243 MHz 31P{1H} NMR spectrum of the mixture of 1trans and 1cis isomers taken in toluene-d8 at 218 K. Since we are unable to assign which isomer is the major species, only the trans isomer is depicted in the inset; see Scheme 2 for a depiction of the cis isomer.
rapidly as indicated by 31P NMR spectroscopy, which shows that the ligand resonance at −5 ppm disappears within 30 min. The spectrum of the resulting product solution, however, is featureless and uninformative at room temperature, exhibiting a broad resonance centered around 50 ppm (ν1/2 ≈ 1250 Hz). Warming the sample to 343 K resulted in the emergence of two broad signals at ∼48 and 53 ppm, but decomposition and/or conversion to other species ensued with more heating. Lowering the temperature of the sample, however, resulted in the emergence of a spectrum at 218 K of more well-resolved signals (Figure 1). Evidenced in this spectrum are two sets of four resonances, present in an approximately 4:3 ratio. The pattern of 1JRh−P coupling constants indicates that, within each set of four resonances, there are two phosphorus nuclei coupled to Rh(I) centers (1JRh−P ≈ 195 Hz) and two phosphorus nuclei coupled to formally Rh(III) centers (1JRh−P ≈ 116−119 Hz).40 Furthermore, the large 2JP−P coupling constants for the phosphorus atoms on the Rh(III) centers (∼670 Hz) point to a trans arrangement, while the much smaller couplings of ∼40 Hz for the 2JP−P values on the Rh(I) centers are consisted with a cis orientation of the inequivalent phosphorus atoms.41 The 31P NMR data are broadly consistent with a situation where the COE ligands in [(COE)2Rh(μ-Cl)]2 have been substituted with the bis-phosphine ligand, and one of them engages in an initial C−H activation to form the chloridobridged dinuclear Rh(III)−Rh(I) species 1 (Scheme 2). The temperature dependence of the 31P NMR spectrum indicates that this C−H activation is rapidly reversible on the NMR time scale. We propose that the two isomers of 1 are a result of two possible trajectories of approach for the C−H bond that is activated on the central carbon of the ligand. If the approach is on the inner side of the geminal C−H bonds (depicted as an endo transition state on the left side of Scheme 2), the resulting hydrido ligand on the rhodium necessarily ends up in a trans relationship to the benzylic C−H group that remains on the ligand carbon atom, forming 1trans. This isomer of 1 has been crystallographically characterized (see below). If the C−H bond approaches from the outside of the methylene group (the exo
Scheme 2. IiPr Ligand Attachment to Rhodium using [(COE)2Rh(μ-Cl)]2
transition state depicted on the right side of Scheme 2), then the two hydrogen atoms assume a cis disposition in the Rh(III)/Rh(I) dinuclear product, resulting in the isomer 1cis. This hypothesis is supported by the 1H NMR data obtained for the isomers of compound 1 at various temperatures (Figure S1 in the Supporting Information). As might be expected, the spectra are rather featureless in the aromatic and aliphatic regions, but the resonances for the hydrogens associated with 1280
DOI: 10.1021/acs.organomet.6b00155 Organometallics 2016, 35, 1279−1286
Article
Organometallics
bite-angle diphosphines of this type.42,43 Here, the ligand assumes a “boat-like” conformation that places the dibenzylic CH2 group (C26) into close proximity of the apical site on Rh2, the nonbonding distance being 3.31 Å. The C−H bond that points toward the metal center is not close enough to be considered an agostic44 interaction; rather, these kinds of interactions have been referred to as being “preagostic” or “anagostic”45,46 and typically result in a downfield shift of the resonance for this proton in the 1H NMR spectrum. In 1, this is the resonance at 9.93 ppm; as mentioned above, the other resonates upfield at 3.80 ppm. This disparity in chemical shift for these diastereotopic protons has also been observed in a palladium complex of a related ligand I, where R = C6H5.47 Such interactions are typically observed in square-planar d8 complexes wherein the C−H bond is coerced into proximity with the metal centers primarily by steric factors.45 This appears to be the case for both isomers of 1 and accounts for the diastereotopic nature of the two dibenzylic C−H bonds in the unactivated diphosphine ligand. In an effort to encourage full pincer ligand coordination via C−H activation, the ligand IiPr was reacted with a triflatebridged COE dimer of rhodium, [(COE)2Rh(μ-OTf)]2,48 the assumption being that the more weakly coordinating triflate anion would encourage C−H activation at the more electrophilic rhodium center. This reaction produced the symmetric pincer dimer 2trans as a pale yellow solid in 67% isolated yield (Scheme 3). Dropwise addition of a solution of the ligand to
the methylene protons of the ligand and the Rh−H moiety are informative. At room temperature, four broad and featureless resonances integrating in a 1:1:1:1 ratio at −20.25 (Rh−H), 3.80, 5.32, and 9.93 ppm are observed. As the sample is cooled, each of these resonances coalesce and re-emerge as two signals present in a 4:3 ratio, indicative of the two isomers of 1. 1H−1H COSY and 1H−13C HMQC experiments indicate that the resonances at 3.80 and 9.93 ppm can be assigned to the two protons on the unactivated methylene bridge; it follows that the signal at 5.32 ppm is due to the dibenzylic C−H atom of the PCsp3P ligand on the Rh(III) center. A 1H−1H EXSY experiment indicates that the rhodium hydride proton is in exchange with this dibenzylic proton on this time scale, suggesting that reductive elimination of C−H from the Rh(III) center is rapid. In theory, all four of these protons should be in exchange, but the rates of these processes are outside of the experiment’s time scale. Nonetheless, all of these protons (and all of the inequivalent phosphorus atoms) presumably undergo exchange, the nexus being the symmetrical diphosphine ligated chlorido bridged dimer labeled IV in Scheme 2. Attempts to observe these exchanges by heating the samples resulted in spectral changes suggestive of more rapid exchange, but at high temperatures, competitive elimination of H2 (see below) complicated matters and so a complete study was precluded. Crystals suitable for X-ray analysis were grown from pentane solutions of 1 via slow evaporation at −35 °C. The structure determination showed it to be the 1trans isomer (Figure 2). In
Scheme 3. IiPr Ligand Attachment to Rhodium using [Rh(COE)2(μ-OTf)]2
the starting material gave an immediate color change to a purple species that rapidly faded to yellow, followed by precipitation of the product, which was sparingly soluble in toluene. We surmise that the purple color is due to intermediates akin to the isomers of compound 1, or perhaps monomeric (P2)RhOTf compounds48 formed prior to ratelimiting C−H activation, but we did not pursue a detailed characterization of this material. The yellow product could be dissolved in C6D5Br, and it was apparent already from spectroscopy that the compound was more symmetrical than the product mixture seen in the reaction with [(COE)2Rh(μCl)]2. A doublet (1JRhP = 120.5 Hz) at 55.7 ppm was consistent with equivalent Rh(III) centers; this resonance drifted slightly upfield (
