Reversible Inter- and Intramolecular Carbon–Hydrogen Activation

Communication pubs.acs.org/IC

Reversible Inter- and Intramolecular Carbon−Hydrogen Activation, Hydrogen Addition, and Catalysis by the Unsaturated Complex Pt(IPr)(SnBut3)(H) Anjaneyulu Koppaka and Burjor Captain* Department of Chemistry, University of Miami, Coral Gables, Florida 33124, United States S Supporting Information *

transition-metal complexes, where the pendant groups on tin control both the geometry and reactivity.4 Recent studies of the hydrogen activation of platinum(II) complexes showed that the stability of the platinum(IV) dihydride complex formed depended critically on the nature of the R group of the stannane ligand.4 This work was recently extended to investigate smallmolecule activation by [Pt(IBut)(SnBut3)(μ-H)]2 in which each platinum center was bound to only one bulky tin ligand but also to the N-heterocyclic carbene (NHC) ligand IBut [IBut = N,N′di-tert-butylimidazol-2-ylidene].5 The crystal structure of the bridging hydride complex in eq 1 was determined, and it served as

ABSTRACT: The complex Pt(IPr)(SnBut3)(H) (1) was obtained from the reaction of Pt(COD)2 with But3SnH and IPr [IPr = N,N′-bis(2,6-diisopropylphenyl)imidazol-2ylidene]. Complex 1 undergoes exchange reactions with deuterated solvents (C6D6, toluene-d8, and CD2Cl2), where the hydride ligand and the methyl hydrogen atoms on the isopropyl group of the IPr ligand have been replaced by deuterium atoms. Complex 1 reacts with H2 gas reversibly at room temperature to yield the complex Pt(IPr)(SnBut3)(H)3 (2). Complex 2 also undergoes exchange reactions with deuterated solvents as in 1 to deuterate the hydride ligands and the methyl hydrogen atoms on the isopropyl group of the IPr ligand. Complex 1 catalyzes the hydrogenation of styrene to ethylbenzene at room temperature. The reaction of 1 with 1 equiv of styrene at −20 °C yields the η2-coordinated product Pt(IPr)(SnBut3)(η2-CH2CHPh)(H) (3), and with 2 equiv of styrene, it forms Pt(IPr)(η2-CH2CHPh)2 (4).

a potential source of the more reactive monomeric fragment Pt(IBut)(SnBut3)(H), which was not detected; however, the observed reactivity pattern was in keeping with reactions such as the oxidative addition of dihydrogen, shown in eq 1, proceeding via this intermediate species. The trihydride complex shown in eq 1 could not be isolated but was characterized spectroscopically. This Communication extends those studies and reports that replacement of the NHC bound to platinum from IBut to IPr serves to destabilize the dimeric structure to the point that in this case the equilibrium corresponding to eq 1 but with IBut replaced by IPr is shifted to favor the quantitative formation of the formally 14-electron divalent monomer Pt(IPr)(SnBut3)(H) (1), which can be isolated and structurally characterized and whose reactions are probed directly. The room temperature, and as well as low-temperature, reaction of Pt(COD)2 with IPr and But3SnH furnished the 14electron mononuclear complex 1. Its structure in the solid state is shown in Figure 1 (left). The compound contains one hydride ligand (δ = −12.83, 1JPt−H = 547 Hz, 2JSn−H = 144 Hz), and as seen in the structure, the Sn1−Pt−C1 bond angle is not 180° but considerably less by about 30°. (There are two molecules present in the asymmetric crystal unit: molecule 1, 149°; molecule 2, 152°). The hydride ligand was not located crystallographically, and the line structure in Figure 1 is shown to depict a hydride ligand as well as to show the shortest methyl hydrogen to

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lectronic unsaturation in metal complexes plays a crucial role in their reactivity toward other molecules and in catalytic homogeneous reactions.1 One of the best ways to induce electronic unsaturation in metal complexes is by using sterically bulky ligands that occupy a large spatial area around the metal center. The most frequently used bulky ligands have historically been tritertiary phosphine ligands. The presence of at least one vacant site at a metal center is essential to small-molecule activation and homogeneous catalysis. One of the best examples of that is the development of the DuPont process for hydrocyanation of butadiene to adiponitrile.2 At the conclusion of his seminal paper defining the “cone angle” of phosphine ligands, Tolman concluded that “electronic effects play only a secondary role compared to steric effects in determining the stability of the Ni(0) complexes studied”.3 This statement referred to an understanding of how variation in the phosphine substituent R influenced the equilibrium between catalytically inert Ni(PR3)4 versus catalytically active Ni(PR3)3 complexes. This understanding was key to the work at DuPont in achieving the fine balance between the catalyst stability and catalyst reactivity, both of which are essential in any practical catalyst. While a large number of new ligand systems have been developed since then, the goal of achieving a stable complex (or precursor to it) that contains a vacant site for small-molecule activation remains a central goal in catalyst design and development. We have been actively pursuing the chemistry of bulky tin ligands in © XXXX American Chemical Society

Received: January 7, 2016

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DOI: 10.1021/acs.inorgchem.6b00048 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

cycling process can be repeated several times, with minor decomposition attributed to the highly air-sensitive nature of 1 and 2. Pure crystals of 2 were obtained by crystallizing solutions of 1 under an H2 or D2 atmosphere. Single-crystal X-ray diffraction analysis revealed the structure of 2, as shown in Figure 2. Complex 2 is a 16-electron complex with platinum formally in

Figure 1. Left: ORTEP6 of 1 showing 40% probability thermal ellipsoids. The hydride ligand is not shown. Right: Line structure showing the hydride ligand along with the closest Pt···H distance to the methyl groups on the IPr ligand.

platinum distances. The geometry of this complex may be best described as a distorted “T-shaped” structure, with the hydride ligand occupying one of three equatorial sites of a distorted trigonal bipyramid, or as expanded Y-shapes (half way between T-shape and trigonal-planar).7 Complex 1 is coordinatively unsaturated and is stabilized by the bulky ligand groups. Additionally, electronic unsaturation may also be relieved by interactions of the methyl hydrogen atoms on the isopropyl group with platinum, which are close at 2.87(1) Å (molecule 1) and 2.76(1) Å (molecule 2).8 This coordination presumably also plays a role in isotopic substitution reactions observed in the NMR spectra of 1. Dissolution of 1 in deuterated solvents like C6D6, toluene-d8, or CD2Cl2 results in a decrease in the NMR signal assigned to Pt−H, the hydrido ligand, but also at a significantly slower rate than the C−H signal of the pendant methyl hydrogen atoms of the IPr ligand. Examination by 2 H{1H} NMR shows that isotopic exchange has occurred at both places and that an increase in the proton signal of the deuterated solvent has also occurred. Several possible mechanisms for this hydrogen/deuterium exchange, which occurs in the absence of added H2 or D2 gas, exist, with the simplest being the reversible C−D oxidative addition/reductive elimination process shown in Scheme 1.9 Reactions, as shown in Scheme 1, or possibly a closely

Figure 2. Left: ORTEP6 of 2 showing 30% probability thermal ellipsoids. Right: Hydride ligands located using XHYDEX in the WINGX suite of programs.10 Crystal data for 2 obtained from 1 + D2 in C6D6. Single crystals were also obtained from 1 + H2 in C6H6, as well as from 1 + H2 in C6H14. See the Supporting Information.

the 4+ oxidation state. While the structures of both 1 and 2 are expected to be similar, the Sn1−Pt−C1 bond angle is slightly less at 144°, and the Pt1−Sn1 distance is also longer at 2.6587(3) Å compared to 2.5955(3) Å [2.6124(3) Å molecule 2] in 1. The 1H NMR spectrum shows a single resonance at −3.73 ppm (1JPt−H = 687 Hz and 2JSn−H = 119 Hz), indicating that the hydrogen ligands are equivalent on the NMR time scale. The rapid equilibrium in eq 2 provides a simple and rapid pathway for the uptake of deuterium upon exposure of 2 to D2 gas, which generated resonances fully consistent with the H2D, HD2, and D3 isotopologues of 2. These proton resonances did not exhibit any observable coupling to deuterium, suggesting that at room temperature the compound is indeed a trihydrido complex. Variable-temperature 1H NMR spectroscopy (see Figure S1) for 2 at low temperatures was performed; however, no new peaks were observed, and only some broadening of the hydride resonance started to occur at −73 °C, rather than a limiting spectrum displaying a large H−D coupling. Furthermore, both proton-coupled 195Pt and 119Sn{1H} NMR spectroscopies indicate unequivocally the presence of three hydrogen ligands directly bonded to platinum, which are all equivalent at room temperature; see Figure S3. Thus, the formulation of three hydrides, as shown in Figure 2, is supported by the NMR data. We can also rule out that complex 2 could conceivably be a complex containing one hydrido ligand in rapid exchange with a coordinated molecular dihydrogen ligand as in the complex Pt(IPr)(SnBut3)(H)(H2), based on NMR. In addition, for the reaction of 2 and D2, HD gas is detected in the NMR, indicating that 2 could serve as a catalyst for HD formation from H2/D2 mixtures.4 In the absence of added D2 gas but in the presence of deuterated solvents, complex 2, like complex 1, was found to slowly incorporate deuterium atoms into the isopropylmethyl groups of the coordinated IPr ligand. The detailed mechanisms of these exchanges are not known at this time and will be the subject

Scheme 1

related σ-bond scission can account for the incorporation of deuterium at the Pt−H bond. There must also be a corresponding process, albeit at a slower rate, in which the C−H bond of one of the coordinated methyl groups of the IPr ligand undergoes reversible oxidative addition/reductive elimination, resulting in ultimately full deuterium incorporation into the methyl group of the coordinated IPr ligand. Exposure of 1 to H2 gas at 1 atm and room temperature yields the adduct Pt(IPr)(SnBut3)(H)3 (2). This process is readily reversible, and evacuation of 2 regenerates 1, while reexposure to H2 produces 2, as depicted in eq 2. Equation 2 is rapid, and the B

DOI: 10.1021/acs.inorgchem.6b00048 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

complex Pt(IBut)(SnBut3)(H)3 can be prepared by hydrogenation of [Pt(IBut)(SnBut3)(μ-H)]2, it loses H2 much more readily than does the corresponding IPr derivative 2. Further, the reaction landscape observed with respect to both inter- and intramolecular C−H activation appears to be restricted to the IPr series rather than the IBut series of compounds. It seems likely that the bulky tin ligand SnBut3 serves to synergistically amplify the subtle differences between the IPr and IBut ligands. Additional synthetic and mechanistic as well as physical studies aimed at mapping out the factors controlling the stability and reactivity in complexes in which two different sterically demanding ligands interact are planned.

of future studies. What is known is that deuterium is rapidly incorporated at the Pt−H bond and also at the methyl C−H bonds of this complex and that the source of deuterium can be either D2 or C6D6 or other deuterated solvents, indicating that 1 is capable of both intra- and intermolecular C−H activation. In order to investigate the potential catalytic activity of 1, hydrogenation of styrene was attempted using 150 equiv of styrene in a toluene solution at room temperature and 1 atm of H2. Under these conditions, a 33% conversion of styrene to ethylbenzene was achieved in 24 h. The reaction was also conducted in the presence of mercury, and no detectable change in the catalytic activity of 1 was observed to rule out the possibility of heterogeneous processes in the catalytic cycle.11 Further catalytic studies utilizing 1 are in progress. Stoichiometric reactions of 1 and styrene provide insight into the catalytic mechanism as well as a potential deactivating step. The reaction of 1 equiv of styrene with 1 yields the η2-alkenecoordinated compound Pt(IPr)(SnBut3)(η2-CH2CHPh)(H) (3), as shown in Figure 3 (left). Complex 3 most likely represents

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00048. Experimental details and characterization data (PDF) CIF files for each of the structural analyses (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The National Science Foundation (Grant CHE-1300206) is gratefully acknowledged for support of this work. We also thank Manuel Temprado and Carl D. Hoff for valuable discussions.

Figure 3. Left: ORTEP6 of 3 at 30% probability thermal ellipsoids. Right: ORTEP6 of 4 at 40% probability thermal ellipsoids. There are two molecules present in the asymmetric crystal unit.

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REFERENCES

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the first step in the catalytic hydrogenation of styrene. While this complex was able to be isolated and characterized crystallographically, it proved to be unstable in solution. Dissolution of pure crystalline 3, which exists as a single isomer in the solid-state structure, yields a complex NMR spectrum suggestive of the formation of isomeric structures as well as decomposition to multiple products, as discussed further in the Supporting Information. The reaction of 1 with 2 equiv or more of styrene yields the complex Pt(IPr)(η2-CH2CHPh)2 (4), in which But3SnH has been reductively eliminated. An ORTEP diagram of 4 is given in Figure 3 (right). Exposure of complex 4 to H2 does not lead to the hydrogenation of coordinated styrene, indicating that it is a “dead end” complex with respect to the catalytic activity in this system. The major conclusions from this work are that ligand modification of the NHC from IBut to IPr in the previously reported dimeric complex [Pt(IBut)(SnBut3)(μ-H)]2 increases the steric strain to the point that only the monomeric complex exists in both solution and the solid state, allowing isolation and structural characterization of both the novel 14-electron complex 1 and also its H2 addition product 2. The chemistry of 1 indicates that it is effective at both intra- and intermolecular C−H activation, as demonstrated by hydrogen/deuterium labeling experiments. In addition, 1 has proven to catalytically hydrogenate styrene, and new coordinated complexes of styrene were isolated and structurally characterized, giving insight into a potential reactive intermediate and also a deactivation product for this process. It is worth noting that, whereas the trihydride C

DOI: 10.1021/acs.inorgchem.6b00048 Inorg. Chem. XXXX, XXX, XXX−XXX