Intermolecular Oxidative Addition of Aryl Halides to Platinum(II) Alkyl Complexes
We report the first well-defined example of intermolecular aryl halide oxidative addition (OA) to Pt(II). Complexes of the type (IMes)PtMe2(L) and (IMes')PtMe(L) (L = SMe2, pyridine; IMes = N, N-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene; IMes' = cyclometalated IMes) undergo intermolecular OA of phenyl iodide (PhI) at 60 °C, producing toluene via reductive elimination from a proposed Pt(IV)-phenyl species. Isolation of a model Pt(IV) OA product provides evidence for a Pt(II)/(IV) pathway. The OA of PhI is not limited to Pt(II)-IMes complexes;analogous reactions also proceed with phosphine-ligated Pt(II) dialkyl complexes, demonstrating that this reaction is feasible for a variety of electron-rich Pt(II) complexes bearing labile ligands.
<!>Scheme 1. Proposed catalytic cycle for methane functionalization.<!>Scheme 3. Reactivity of cyclometalated 1a',b' with PhI.
<p>Oxidative addition (OA) is a fundamental reaction that is a critical step in catalytic processes such as Suzuki-Miyaura coupling, 1 Buchwald-Hartwig amination, 2 olefin hydrogenation, 3 and the Monsanto process. 4 The OA of a variety of oxidants to low valent metals is well established; the aforementioned processes often rely on OA of carbon-(pseudo)halide (C-X) bonds to M(0) or M(I) species. 5 The OA of alkyl halides to group 10 M(II) species has been well studied stoichiometrically and catalytically. 4,[6][7][8][9][10][11][12][13][14][15][16] Despite the increasing prevalence of M(II)/M(IV) (M = Ni, Pd) catalytic processes invoking Csp 2 -X OA in the literature, 17 few well-defined examples of such reactivity have been reported. [18][19][20][21][22][23] While many Pt(II) complexes undergo facile SN2-type Csp 3 -X OA, these complexes are inactive toward intermolecular Csp 2 -X OA which typically occurs in a concerted fashion. 24 With the exception of a poorly defined example from 1967, 25 efforts to achieve intermolecular C(aryl)-X oxidative addition have necessitated incorporation of the oxidant into the ligand scaffold (intramolecular OA). 16,22,[26][27][28][29][30][31][32] As part of our ongoing efforts in Pt(II)/(IV) catalysis, [33][34][35][36] we were interested in exploring the potential of aryl halides as oxidants in a proposed process for methane functionalization (Scheme 1). Our initial efforts in this area were reliant on a strategy developed by Puddephatt and coworkers, wherein Csp 2 -X activation is facilitated by tethering an aryl halide to the ancillary ligand scaffold. 22,[26][27][28][29][30][31]37 This approach provides well-defined Pt(IV) intermediates from which C-C reductive elimination (RE) can be induced, but tethering the aryl group prevents product dissociation and facilitates decomposition via ligand C-H activation. 38 As intermolecular Csp 2 -X activation would obviate this problem, it became crucial to develop a Pt(II) system capable of intermolecular oxidative addition of aryl halides. In this work we describe a N-heterocyclic carbene (NHC) ligated dimethyl Pt(II) complex capable of intermolecular oxidative addition of PhX (X= I, Br). Through a combined experimental and computational study, we demonstrate the feasibility of concerted Csp 2 -X OA to electron-rich Pt(II) complexes and extend this reactivity to several other L2PtMe2 complexes featuring common ligands.</p><!><p>We initially hypothesised that intermolecular Csp 2 -X OA would be possible at sufficiently electron rich Pt(II) centres. As such, we aimed to prepare a series of Pt(II) dialkyl complexes using N-heterocyclic carbene (NHC) ligands. Complexes bearing an NHC in addition to SMe2 (1a) or pyridine (1b) donors were readily prepared from Pt2Me4(µ-SMe2)2 and PtMe2COD, respectively. Heating 1a,b at 60 °C in the presence of one equivalent of PhI generated toluene and new Pt(II) complexes 2a,b and 3a,b (Scheme 2). Deuterium labelling experiments with PhI-d5 revealed exclusive formation of toluene-d5, indicating that the phenyl group in the observed toluene was derived from PhI.</p><p>In-situ 1 H NMR spectroscopic analysis of the reaction mixtures of 1a,b with PhI resulted in the detection of toluene and a single new Pt-CH3 signal in each case. The Pt-CH3 coupling constants (2a; 2 JPt-H = 80 Hz, 2b; 2 JPt-H = 84 Hz) are consistent for methyl and NHC ligands being in a cis configuration. Additionally, the detection of methane in the 1 H NMR suggested that a C-H activation event occurred, which was confirmed by the presence of diastereotopic methylene Scheme 2. Reaction of complexes 1a,b with PhI at 60 °C.</p><p>(*) Indicates the presence of an additional isomer (see experimental section of SI for details). Percentages correspond to sum of yields of both isomers. Major isomers shown except for 3b. 50 resonances characteristic of IMes cyclometallation at the ortho-methyl position (3a,b). 39,40 The independent synthesis and full characterization ( 1 H, 13 C{ 1 H} NMR spectroscopy, ESI-MS) of all Pt(II) species presumed to have formed in the reaction of 1a,b with PhI corroborated our assignments. The conversion of PhI to toluene is consistent with a mechanism in which PhI OA is followed by Csp 2 -Csp 3 RE to provide 2a,b, but the mechanism by which 3a,b are formed is less clear. Cyclometalation of organoplatinum(II) NHC complexes via Csp 3 -H activation and subsequent reductive elimination of methane is well established. [39][40][41][42][43] Thus we reasoned that 3a,b was formed directly from 1a,b or from the less electron-rich products, 2a,b. To probe these possibilities, product mixtures containing 2a,b and 3a,b were heated for two hours at 100 °C. No changes in product ratios were observed. Heating an independently prepared sample of 2b at 100 °C for one hour did not afford any conversion to 3b, nor did heating in the presence of PhI. These results indicate that cyclometalated complexes are formed by an initial C-H activation process at 1a,b, followed by PhI OA and reductive elimination of toluene. Gratifyingly, heating 1a,1b at 60 °C for two hours in C6H6 was found to provide cyclometalated complexes 1a' and 1b' in 71% and 78% yield, respectively (see SI for solid state structures). Comparison of τ4 values for 1a (0.057) and 1a' (0.070) shows that both complexes adopt square planar geometries 44 as expected for Pt(II) complexes. Cyclometallation results in minor distortions of the square plane for 1b (0.034) and 1b' (0.061).</p><p>The isolated complexes 1a',b' were heated at 60 °C for 1 hour in the presence of PhI, resulting in the formation of toluene in excellent yields (80, 99% by 1 H NMR spectroscopy, respectively). Phenyl bromide also reacts with 1a' to form toluene in ~37% yield, but this reaction is slower (16 h) and requires an excess of PhBr to achieve appreciable conversion (further details in Figure S57). The reaction of PhI with SMe2 complex 1a' provides toluene quantitatively if longer reaction times are employed (2 h). The higher reactivity of pyridine derivative 1b' compared to SMe2 complex 1a' may be attributed to the ability of nitrogen to stabilize the Pt(IV) oxidation state, or to a higher barrier for SMe2 dissociation due to the soft nature of sulfur. To explore this, the reaction of 1a' with PhI was performed in the presence of excess SMe2 at 60 °C. Added SMe2 was found to inhibit the reaction, affording toluene and 3a in only 4% yield over a one-hour period (c.f. 80% yield in the absence of added SMe2). Similar inhibition was observed when 1b and PhI were heated in the presence of excess pyridine (See SI for details). These observations are consistent with a mechanism in which dissociation of L creates a coordinative vacancy for interaction with PhI. The steric bulk of NHC ligands is known to stabilize low-coordinate, T-shaped complexes. 39,41,45 Furthermore, Crespo and Martinez have shown with several studies that dissociation of ligand, to create a vacant site, must occur prior to OA of the Csp 2 -X bond. 24,28,37 On this basis, we tentatively propose a mechanism in which ligand L dissociates to facilitate the concerted OA of PhI (Scheme 4a). DFT calculations support this proposal, establishing that a concerted OA process is energetically feasible for both complexes 1a and 1a'. Starting from 1a', the dissociation of SMe2 results in the formation of a weak PhI adduct 4', 4.9 kcal•mol -1 uphill in energy due to the relatively weak donor ability of PhI. These interactions have previously been demonstrated through computational calculations. [46][47][48] The oxidative addition barrier (G ‡ ) was calculated to be 14.9 kcal•mol -1 relative to 1a' in benzene, which is in general agreement with our experimental observations. The OA transition state geometry 4'/5' TS is characterized by a 'face-on' approach of the aryl halide to Pt and a single imaginary frequency corresponding to cleavage of the Ph-I bond. The Csp 2 -I addition occurs in a manner that positions the Ph group trans to a vacant coordination site in the resultant Pt(IV) complex 5', which exhibits a square pyramidal geometry (5 = 0.04). Toluene reductive elimination from Pt(IV) intermediate 5' proceeds through transition state 5' TS with a barrier of 14.6 kcal•mol -1 , accounting for the lack of Pt(IV) intermediates detected experimentally.</p><!><p>Consistent with observed differences in reactivity between 1a and 1a', the Csp 2 -I OA pathway starting from non-cyclometallated 1a proceeds with a significantly higher OA barrier (G ‡ = 23.1 kcal•mol -1 relative to 1a) than 1a'. As the free energy (G) for formation of Pt(IV) intermediates 5/5' is similar in both systems (G = -10.3 kcal•mol -1 , 5; -9.0 kcal•mol -1 , 5'), we attribute the difference in transition state energies to greater steric repulsion between the NHC and PhI in the non-cyclometalated complex (Scheme 4b). We hypothesized that a chelating aryl iodide could be employed to trap the putative Pt(IV) OA product by disfavouring the formation of 5-coordinate Pt(IV) species that undergo C-C RE. To this end, we treated 1a' with 2-(2-iodophenyl)pyridine in a 1:1 ratio in C6H6. After 36 h at room temperature, octahedral Pt(IV) complex 6 was isolated from the reaction mixture. An X-ray crystal structure of 6 was obtained, unambiguously confirming OA of the Csp 2 -X bond (Figure 1). We are aware of only one report providing evidence for intermolecular OA of aryl halides to Pt(II), reported by Kistner and coworkers in 1967. 25 In this paper, a solution of Pt(o-tol)2(pyridine)2 (8; o-tol = ortho-tolyl) in neat PhI was reported to produce PtI(o-tol)(pyridine)2 over a period of 6 d at 25 °C. Only limited characterization data were provided for the Pt product, and the formation of the organic product was inferred from that data. We reasoned that the formation of the observed PtI(o-tol)(pyridine)2 complex could be attributed to a Csp 2 -I OA/Csp 2 -Csp 2 RE process, which would also result in formation of the presumed organic product. As this would suggest that the intermolecular Csp 2 -I reaction is generalizable to other electron-rich Pt(II) complexes with labile ligands, we sought to reproduce the reported reactivity. Indeed, we found that the treatment of 8 with one equivalent of PhI afforded 2methylbiphenyl in 42% yield (Table 1), confirming the hypothesis of the original paper.</p><p>To further establish the generality of this reaction, we prepared a series of mono-and diphosphine complexes (9 -11, Table 1). Treatment of monophosphine complexes 9 and 10 with one equivalent of PhI at 60 and 100 °C, respectively, resulted in the formation of toluene in ~50% yield ( 1 H NMR spectroscopy, 1,3,5-trimethoxybenzene internal standard) along with methane, ethane, and ethylene. Pt, as well as methane and ethylene; no ethane or toluene were observed. d Gaseous products identified by comparison with literature data. 49 Unexpectedly, we found that changing the electron density at Pt(II) did not inhibit toluene formation completely (complexes 9 and 10, Table 1). Notably, however, higher temperatures were required to see any toluene formation with complex 10, indicating a higher activation barrier for less electron rich metal centres. According to our proposed mechanism, replacement of the labile SMe2 ligand with a second strongly donating phosphine was expected to inhibit toluene formation. This hypothesis was supported by experiments with bisphosphine complex 11 (Table 1), which does not react with PhI at temperatures up to 100 °C.</p><p>In summary, we have demonstrated that simple Pt(II) complexes featuring NHC, pyridine and phosphine ligands indeed undergo intermolecular aryl halide oxidative addition and that this process is more general than previously thought. DFT calculations were performed to demonstrate the feasibility of a concerted OA pathway. Trapping of a Pt(IV) complex was achieved using a chelating aryl iodide, providing experimental evidence for a Pt(II)/(IV) pathway. The results described herein pave the way for the development of new reactions utilizing simple aryl halides as oxidants in Pt(II)/Pt(IV) catalytic cycles.</p>