Oxidative Addition of MeI to a Rollover Complex of ... - ACS Publications

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Oxidative Addition of MeI to a Rollover Complex of Platinum(II): Isolation of the Kinetic Product Luca Maidich,*,†,‡ Antonio Zucca,† Guy J. Clarkson,‡ and Jonathan P. Rourke*,‡ †

Dipartimento di Chimica e Farmacia, Università di Sassari, Via Vienna, 2, I-07100 Sassari, Italy Department of Chemistry, University of Warwick, CV4 7AL Coventry, United Kingdom

‡

S Supporting Information *

ABSTRACT: A pair of new Pt(II)/Pt(IV) 2,2′-bipyridine cyclometalated rollover complexes have been synthesized and characterized. [Pt(bpy-H)(CH3)(PMe3)] (1), where bpy-H = κ2N,C-2,2′-bipyridine, was obtained from the electron-rich precursor cis-[Pt(CH3)2(DMSO)2] with a one-pot, two step synthesis; its reactivity has been tested with CH3I, giving the corresponding Pt(IV) complex cis-[Pt(bpy-H)(CH3)2(I)(PMe3)] (2), which was fully characterized. Crystals suitable for X-ray analysis were obtained and allowed the determination of the structure of isomer 2A which is the product of the trans addition of CH3I, usually thought of as the kinetic product.

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INTRODUCTION 2,2-Bipyridine is one of the most used ligands in organometallic chemistry,1 and some years ago a different coordination motif, i.e. κ2N,C rather than the ubiquitous κ2N,N, was exploited to synthesize a new class of platinum(II) complexes.2−4 The growing interest toward these so-called rollover species is highlighted in a recent review.5

Figure 1. Synthesis of complexes 1 and 2A.

H)(CH3)(L)] in an efficient manner. The success of the reaction was determined by the analysis of the 1H and 31P{1H} NMR spectra: in the aliphatic region of the 1H spectrum the coordination of the methyl appears as a doublet with satellites due to the coupling with phosphorus and 195Pt (3H, 0.85 ppm, 3 JP−H = 8 Hz, 2JPt−H = 84 Hz); another doublet with satellites is ascribable to the coordinated PMe3 (9H, 1.57 ppm, 2JP−H = 8 Hz, 3JPt−H = 21 Hz). The aromatic region is diagnostic because there are seven different signals, each integrating to one proton, for the cyclometalated ligand rather than the starting four, with each one integrating two protons. Characteristic protons are those which couple with the metal center, i.e. the broad doublet with satellites that corresponds to H6 (8.86 ppm, 3JPt−H = 22 Hz), a ddd with satellites that corresponds to H4′ (coupling with H5′, H6′, and the phosphorus atom trans to the metalated carbon), and H5′, which is also a ddd with satellites (4JPt−H = 15 Hz). A 195Pt−1H HMQC experiment confirms that all these signals are due to protons bonded to the same platinum, i.e. they belong to the same species, that which gives a doublet at

Cyclometalated complexes have been studied in depth, and almost every d-block transition metal has its own representative;6 partially this is because cyclometalation reactions are envisaged as intramolecular analogues of intermolecular C−H activation/functionalization in solution7−10 and in the gas phase.11−13 In the case of rollover cyclometalation the process is slightly different and is possible only with ligands that are at least bidentate; the reaction must involve partial decomplexation of the ligand, rotation and the activation of the C−H bond, and formation of the metallacycle.5 Oxidation of mononuclear platinum(II) rollover complexes has yet to be reported, and as our interests lie in the reactivity (including oxidation) of cyclometalated complexes,14−23 we sought to establish the results of oxidation of rollover complexes with the use of CH3I.24−26

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RESULTS The new complex 1 was obtained as a yellow solid in high yield via the improvement of a synthetic method previously used to isolate analogous compounds;27 the improved route avoids the troublesome workup of the intermediate [Pt(bpy-H)(CH3)(DMSO)] (Figure 1) and thus allows us to obtain [Pt(bpy© 2013 American Chemical Society

Received: April 9, 2013 Published: May 28, 2013 3371

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−4106 ppm. 31P{1H} NMR spectroscopy confirms the presence of only one phosphorus bound to Pt, showing a singlet with satellites (JPt−P = 2112 Hz) at −18.6 ppm; the value of the coupling constant is typical of a phosphorus coordinated trans to a group having strong trans influence that in this case turns out to be the aromatic carbon.28 A solution of complex 1 in acetone-d6 was treated with an excess of CH3I at room temperature, and the reaction was monitored by 1H and 31P NMR techniques. As soon as the reagent was added, the solution became paler and almost colorless; correspondingly, the spectra changed. Major changes are visible in the aliphatic region of the 1H spectrum, where the signals of the PMe3 protons are deshielded, and there are now two doublets with satellites both integrating for three protons at 1.40 (3JP−H = 7 Hz, 2JPt−H = 69 Hz) and at 0.82 (3JP−H = 7 Hz, 2 JPt−H = 69 Hz). A single resonance in the 31P spectrum at −44.6 ppm with Pt coupling was seen; a 195Pt−1H correlation spectrum shows the presence of only one platinum resonance at −3429 ppm. We can thus formulate the product as [Pt(bpyH)(CH3)2(I)(PMe3)] (2; see the Supporting Information for more details. There are, in principle, seven possible isomers of the single product that forms (Figure 2). We can quickly discount two of

(H6). The fact that the phosphine has influence only on a specific proton of the cyclometalated ring is a probe that the two must be close in space: comparing this observation with the possible structures left (Figure 2), we realize that the only one that fits these data is 2A. We should note that we might not expect isomers 2B−D on the basis of trans-phobia,29,30 leading to 2E being the only other realistic isomeric form. However, we can easily distinguish our product as 2A, as the NOE from the PMe3 group would affect only one Me (i.e., that trans to the nitrogen) and nonspecific aromatic protons in 2E (see the Supporting Information for more details). Thus, the product obtained derives from the trans addition of CH3I to complex 1, the PMe3 is in the plane of the cyclometalated ligand (“equatorial”), and the iodide is perpendicular to the aforementioned plane (“axial” position) trans to a methyl group. The 31P resonance shows a coupling to Pt of 1400 Hz, diagnostic of a carbon group trans to it, as in the precursor 1. The decrease in JPt−P coupling constant is attributable mainly to the increase in the oxidation number of the metal center; this lowers the s character in the Pt−P bond from 0.25 to 0.16, considering “pure” dsp2 and d2sp3 hybrid orbitals, respectively. It is also worth noting that the ratio between JPt−P in 2A and 1 is 0.66, in line with the expected value of 0.67, which represents the ratio of s character present in the PtIV−P and PtII−P bonds.31 In order to get some clues of the mechanism of the reaction, we treated 1 with CD3I under the same conditions (acetone-d6, room temperature). As before, the solution became paler and eventually almost colorless, and all the signals were the same, apart from the fact that one methyl resonance, that at 0.82 ppm, was not present. The absence of this signal is due to the stereoselective incorporation in the axial position of the CD3, which is “mute” in the 1H NMR. This observation confirms that in this case the well-established SN2 mechanism for the reaction of Pt(II) complexes is operating.26 The alkyl halide undergoes nucleophilic attack by the electron pair formally occupying the dz2 orbital on platinum; iodide is the leaving group that can trap one of the two possible square-planar intermediates, i.e. PMe3 in an equatorial or axial position, and lead to the final six-coordinate saturated Pt(IV) complex. Additional confirmation of the geometry of 2 comes from the X-ray structure: the solved structure is illustrated in Figure 4 and clearly shows the six-coordination of the platinum(IV)

Figure 2. Possible isomers of complex 2 and their relative stabilities (ΔH ZPE corrected in kJ mol−1, 298 K, in vacuo).

the possible isomers, i.e. those having two methyls in reciprocal trans positions (2F and 2G), because we would have seen only one resonance for the two Pt-bound methyls integrating for six protons in the 1H NMR, whereas we see two signals, each integrating for three protons. In order to identify which isomer is present in solution, we carried out extensive NMR characterization. From the proton NMR spectrum it is evident that the reaction did not affect the cyclometalated ligand (see the Supporting Information). A series of NOE-1d experiments identified clearly which isomer is present in solution (Figure 3). Irradiation of the PMe3 protons at 1.84 ppm leads to enhancements detected at 0.82 and 1.40 ppm (the two methyl groups attached to Pt) and 8.92 ppm

Figure 4. ORTEP view of the crystal structure of complex 2A. Thermal ellipsoids are drawn at 50% probability, and hydrogens are removed for clarity.

Figure 3. NOE contacts for complex 2A. 3372

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trap the intermediate before the addition is complete: i.e., before the coordination of the iodide.42 If the intermediate lives long enough before being trapped by the iodide, it can possibly rearrange to a more stable isomer, according to the characteristics, both steric and electronic, of the ligands. Once the octahedral complex is formed, isomerization can happen according to two main mechanisms: (a) on the sixcoordinate species via a Bailar or Rây-Dutt twist or (b) on a five-coordinate species arising from dissociation of the most labile ligand through a Berry or turnstile twist. All these passages are shown in Figure 5.

metal center and PMe3 in the equatorial position. The heteroleptic nature of the complex leads to a slightly distorted octahedral geometry with the plane of the cyclometalated 2,2′bipyridine not very far from the ideal, as highlighted by the angles between P−Pt−C(sp2) and N−Pt−CH3(eq) that are both close to 180°. To the best of our knowledge, this is the first crystal structure of a Pt(IV) rollover complex with bpy. Crystal structures of a dimethylplatinum(IV) cyclometalated complex are rather rare in the literature, and all of the examples that we were able to find have the neutral ligand in an axial position. The only cases having an arrangement similar to that presented in this work, i.e. methyl and halide in mutually trans positions, are those arising from C−H activation of ((dimethylamino)ethyl)benzy lim in es o f t he ty pe Me2NCH2CH2NCHAr with Ar = 3,5-C6H3Cl2, 4-(C6H5)C6H4 synthesized by Crespo and co-workers,32,33 where the C,N,N binding mode of the ligand is assumed to force a planar conformation. All the other structures found have halogenated benzylimine derivatives as cyclometalating units and PPh334−36 or a sulfide37,38 as the neutral ligand. As we show below, complex 2A is actually the kinetic product of the reaction, and the thermodynamically favored isomer is that with the PMe3 in the axial position. A series of DFT calculations (see the Supporting Information) was performed on all seven possible isomers of complex 2, depicted in Figure 2 with their respective relative ΔH values (in kJ mol−1, ZPE corrected, in vacuo). A validation of the theoretical results can be obtained by comparing bond distances and angles in 2A, which show a very good agreement. As expected, all of the calculated distances are longer than the real ones, except for Pt−CH3(eq), due to the approximations used in DFT calculations: only one molecule in vacuo at 0 K. Comparing the experimental data and the different values obtained for ΔH, we can rule out the isomers that have both methyls in a trans arrangement because (a) they would have displayed the same chemical shift in the 1H NMR and (b) they lie at very high energy, 70 and 111 kJ mol−1 for 2G and 2F, respectively. It is worth noting that even in the case of these high-energy isomers the one more in agreement with the rules of trans effects in the plane of the cyclometalated ligand (i.e., 2G) has the lowest energy. The enthalpy values obtained from the calculations respect the trans-phobia concept, with the lowest lying isomers being those with the highest trans-influence groups not in reciprocal trans position: i.e., 2A,E. Though the trend is not exact, it would appear that those isomers with an C(sp3) to C(sp3) trans arrangement are higher in energy in comparison with those having a C(sp2) to C(sp3) trans arrangement, suggesting that the methyl group has a stronger trans influence than the metalated sp2 aryl ring. More interesting is the fact that complex 2A is found to be ca. 26 kJ mol−1 less stable than 2E, which is in this case the thermodynamic isomer, in agreement with other calculations on similar systems performed by Rashidi et al.26 It thus appears that the crystal structure obtained is the kinetic isomer, which is, to the best of our knowledge, the first and only case present in the literature regarding this kind of reaction. Oxidative addition of CH 3I to Pt(II) square-planar complexes is a well-studied reaction, and almost everything has been elucidated.39,40 The reaction starts with the nucleophilic attack of the Pt(II) complex at the alkyl halide in an SN2 fashion, leading to inversion of the configuration if optically active halides are used.41 In some cases it is possible to

Figure 5. Possible isomerization pathways following the nucleophilic attack of complex 1 at CH3I.

Reaction of 1 with CH3I in acetone-d6 at room temperature leads only to 2A with no other detectable species in solution, and with CD3I we ascertained the selectivity of the process (i.e. CD3 is incorporated in an “axial” position) and thus confirmed that the reaction mechanism proposed by Rashidi et al. for phenylpyridine cyclometalated complexes is followed even in our case.26 During the synthesis, we observe only the kinetic isomer deriving from the trans addition of CH3I to 1 with no further evolution to the thermodynamic isomer with the bulkier PMe3 perpendicular and the iodide planar with the cyclometalated ligand. Extended heating of a sample of 2A (5 days at 60 °C) does not result in any significant change. We believe that the rationale for this observation lies in the following explanations. First of all, isomerization in octahedral complexes (for example 2A → 2E) is predicted to be highly energy demanding, due to the conversion through a trigonal-prismatic transition state.40 Second, on the basis of preliminary DFT calculations,43 dissociation of one of the ligands from 2A requires a not negligible amount of energy (around 90 kJ mol−1 in the gas phase). Third, the Tolman electronic parameter and steric hindrance of PMe3 are not extreme (ν = 2064.1 cm−1 and θ = 118°44) so that there is not a strong preference for the axial site that is wider but also trans to a methyl. These two factors possibly result in a slower equilibrium between the two isomers of the intermediate (Iax → Ieq in Figure 5); thus the iodide likely attacks the P-trans-C(sp2) isomer. 3373

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H4′), 7.44 (ddd, 1H, JH−H = 7.4, 5.5, 1.5 Hz, H5), 7.16 (ddd sat, 1H, JH−H = 7.4, 4.5, 1.9 Hz, 4JPt−H = 15.2 Hz, H5′), 1.57 (d sat, 9H, 2JP−H = 8.1 Hz, 3JPt−H = 21.0 Hz, P(CH3)3), 0.85 (d sat, 3H, 3JP−H = 8.0 Hz, 2 JPt−H = 84.0 Hz, Pt-CH3). 13C{1H} NMR (ppm, acetone-d6, 100.6 MHz, 298 K): 151.8 (s sat, 3JPt−C = 4 Hz, C6), 145.0 (s sat, 4JPt−C = 15 Hz, C6′), 141.2 (s, C4), 139.9 (s sat, 2JPt−C = 78 Hz, C4′), 138.7 (br), 125.7 (s sat, 3JPt−C = 13 Hz, C5), 124.7 (d sat, 4JP−C = 6 Hz, 3JPt−C = 49 Hz, C5′), 122.4 (s sat, 3JPt−C = 20 Hz, C3), 15.0 (d sat, JP−C = 29 Hz, 2 JPt−C = nr, P(CH3)3), −17.0 (d sat, 2JP−C = 7 Hz, JPt−C = nr, Pt-CH3). 31 1 P{ H} NMR (ppm, acetone-d6, 202.4 MHz, 298 K): −18.6 (s sat, 1P, JPt−P = 2112 Hz). 195Pt−1H HMQC (ppm, acetone-d6, 298 K): −4107 (d, JPt−P = 2112 Hz). ESI-MS (m/z): found 442.1008, calcd for C14H20N2P196Pt 442.0935. cis-[Pt(bpy-H)(CH3)2(I)(PMe3)] (2A). At room temperature, in an NMR tube, 10 mg of 1 was dissolved in acetone-d6 and 3 drops (excess) of CH3I were added to the solution. The reaction was followed by NMR and was complete in ca. 10 min; during this time the solution became paler and almost colorless. The reaction is completely clean by NMR criteria. 1H NMR (ppm, acetone-d6, 500 MHz, 298 K): 8.91 (d sat, 1H, JH−H = 5.5 Hz, 3JPt−H = 10.0 Hz, H6), 8.51 (d br, 1H, JH−H = 8.1 Hz, H3), 8.32 (d br, 1H, JH−H = 4.4 Hz, H6′), 8.11 (t br, 1H, JH−H = 7.8 Hz, H4), 7.99 (t br sat, 1H, JH−H = 7.9 Hz, 3JPt−H = 30.3 Hz, H4′), 7.52 (t br, 1H, JH−H = 6.7 Hz, H5), 7.24 (ddd sat, 1H, JH−H = 7.7,4.7,2.4 Hz, 4JPt−H = 10 Hz, H5′), 1.84 (d sat, 9H, 2JP−H = 9.6 Hz, 3JPt−H = 12.2 Hz, P(CH3)3), 1.40 (d sat, 3H, 3JP−H = 7.4 Hz, 2JPt−H = 69.2 Hz, Pt-CH3(eq)), 0.82 (d sat, 3H, 3JP−H = 7.4 Hz, 2JPt−H = 69.4 Hz). 13C{1H} NMR (ppm, acetone-d6, 125.7 MHz, 298 K): 150.4, 145.0, 138.7, 137.4 (JPt−C = 38 Hz), 124.8 (JPt−C = 12 Hz), 122.9 (JPt−C = 15 Hz), 11.8 (JP−C = 32 Hz, JPt−C = 13), 2.88 (JP−C = 3, JPt−C = 593 Hz), −6.45 (JP−C = 4 Hz). 31P{1H} NMR (ppm, acetone-d6, 202.4 MHz, 298 K): −46.4 (s sat, 1P, JPt−P = 1467 Hz). 195 Pt−1H HMQC (ppm, acetone-d6, 298 K): −3429 (d, JPt−P = 1467 Hz). ESI-MS (m/z): found 455.1146, calcd for C15H22N2P194Pt 455.1142. Crystals of 2A suitable for X-ray analysis were grown from the slow evaporation of solvent from an acetone solution. X-ray diffraction data were obtained on an Oxford Diffraction Gemini fourcircle system with a Ruby CCD area detector controlled by the CrysAlisPro software60 using Mo Kα radiation. The crystals were mounted in oil and held at 150(2) K with the Oxford Cryosystem Cryostream Cobra. Absorption corrections were applied using ABSPACK.60 The structures were solved by direct methods using SHELXS (TREF)61 with additional light atoms found by Fourier methods. Refinement used SHELXL 97.62 H atoms were placed at geometrically calculated positions and refined riding on their parent atoms. X-ray crystallographic data in CIF format have been deposited at the Cambridge Crystallographic Data Center as supplementary publication no. CCDC 933129 (2A). Full details are given in the Supporting Information.

CONCLUSIONS We have reported here the synthesis and characterization of the new complex [Pt(bpy-H)(CH3)(PMe3)] (1) and its reaction with CH3I, leading to the isolation of the kinetic isomer [Pt(bpy-H)(CH3)2(I)(PMe3)] (2A) resulting from the trans addition of the alkyl halide. The structure proposed on the basis of NMR investigations was clearly confirmed by single-crystal X-ray diffraction. Reaction with CD3I allowed us to distinguish clearly that the methyl is selectively incorporated perpendicularly to the cyclometalated ligand, suggesting that a SN2 mechanism is operative. Theoretical calculations confirmed that 2A is not the thermodynamically favored isomer, being 26.4 kJ mol−1 less stable than the expected isomer with the PMe3 in an axial position.

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EXPERIMENTAL SECTION

All chemicals were used as supplied, unless noted otherwise. All NMR spectra were obtained on Bruker spectrometers (500 or 400 MHz) and are referenced to the residual peak of the solvent, i.e. acetone-d6, assignments being made with the use of NOE, DEPT, and COSY pulse sequences. 13C−1H and 195Pt−1H correlation spectra were recorded using a variant of the HMBC pulse sequence and 195Pt chemical shifts quoted are taken from here (referenced to external Na2PtCl6. The starting material cis-[Pt(CH3)2(DMSO)2] was prepared according to known procedures.45,46 DFT calculations were carried out with the Firefly QC package,47 which is partially based on the GAMESS (US)48 source code using the hybrid PBE0 functional developed by Perdew, Burke, and Ernzerhof49,50 and modified in its hybrid version by Adamo and Barone.51 Ahlrichs and co-workers52 def2-SVP basis sets, used as found in the EMSL basis set library,53,54 were used for all lighter atoms (H, C, N, and P) while for iodine and platinum the same basis set was integrated with an effective core potential (ECP) removing 28 and 60 core electrons, respectively. Convergence criteria used were the default ones: i.e., 10−5 for the absolute energy change in density between two subsequent iterations in the SCF cycle and 10−5 hartree bohr−1 for the largest component of the gradient. Harmonic analysis at the same level of theory (PBE0/def2-SVP) was carried out on all the equilibrium geometries to confirm their nature of minima, i.e. absence of imaginary frequencies, on the potential energy surface (PES). Before this final step a survey of the PES was carried out with a lower level of theory, i.e. using the Vosko−Wilk−Nusair functional V (VWN5)55 and the SBKJC VDZ ECP basis set,56−59 in order to find the phosphine’s rotational angle affording the minimum energy; the geometries thus obtained were then used as a starting guess for the following calculations. Assignment of the cyclometalated 2,2′-bipyridine protons follows the scheme depicted below:

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Text, tables, figures and a CIF file giving details for DFT calculations, full details of the X-ray analysis and crystallographic data for 2A, and DFT-optimized coordinates for all isomers of complex 2 along with energies. This material is available free of charge via the Internet at http://pubs.acs.org.

[Pt(bpy-H)(CH3)(PMe3)] (1). Under an N2 atmosphere cis[Pt(CH3)2(DMSO)2] (47.6 mg, 0.1248 mmol, 1 equiv) was dissolved in 2 mL of distilled toluene and then, to the colorless solution, an excess of 2,2′-bipyridine was added (50.5 mg, 0.3233 mmol, 2.6 equiv), yielding a red solution that was degassed twice and then heated to reflux for 3 h. A few drops of PMe3 were added (excess) under N2 flow, giving a yellow solution that was left to react for 30 min. The reaction mixture was then evaporated to dryness and purified by chromatography on silica gel, with Et2O as eluent, and evaporating to dryness the colored fraction (32.9 mg, 0.0745 mmol). Yield: 60%. 1H NMR (ppm, acetone-d6, 500 MHz, 298 K): 8.86 (d sat, 1H, JH−H = 5.6 Hz, 3JPt−H = 22.0 Hz, H6), 8.33 (ddd, JH−H = 7.9, 1.5, 0.7 Hz, H3), 8.28 (ddd, 1H, JH−H = 4.6, 1.8, 0.9 Hz, H6′), 8.10 (td, 1H, JH−H = 7.9, 1.5 Hz, H4), 8.09 (ddd sat, 1H, JH−H = 5.6, 3.6, 1.7 Hz, 3JPt−H = 44.4 Hz,

Corresponding Author

*E-mail: [email protected] (L.M.); j.rourke@warwick. ac.uk (J.P.R.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS L.M. gratefully acknowledges a Ph.D. fund, financed by POR/ FSE 2007-2013, from the Regione Autonoma della Sardegna 3374

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and the resources given by the Cybersar Project managed by the “Consorzio COSMOLAB”. We acknowledge support from Advantage West Midlands (AWM) (partially funded by the European Regional Development Fund) for the purchase of a high-resolution mass spectrometer and the XRD system that was used to solve the crystal structure of 2A.

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REFERENCES

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dx.doi.org/10.1021/om400300n | Organometallics 2013, 32, 3371−3375