Article pubs.acs.org/Organometallics
Oxidation of Dimethylplatinum(II) Complexes with a Dioxirane: The Viability of Oxoplatinum(IV) Intermediates Kyle R. Pellarin, Matthew S. McCready, and Richard J. Puddephatt* Department of Chemistry, University of Western Ontario, London, Canada N6A 5B7 S Supporting Information *
ABSTRACT: The complexes [PtMe2(NN)] (NN = 2,2′-bipyridine = bipy, 1a; NN = di-2-pyridylamine = dpa, 1b; NN = di-2-pyridyl ketone = dpk, 1c) react with dimethyldioxirane in moist acetone to give the hydroxoplatinum(IV) complexes [Pt(OH)2Me2(NN)] (NN = bipy, 2a; NN = dpa, 2b, or [Pt(OH)Me2(dpkOH)], 3). Complex 2a crystallizes as the hydrate 2a·7H2O, which has a complex supramolecular network structure formed through hydrogen bonding between PtOH groups and water molecules. Attempts to trap a potential oxoplatinum(IV) intermediate in these reactions were unsuccessful, and computational studies suggest that oxoplatinum(IV) intermediates are improbable. It is suggested that oxygen atom transfer from the dioxirane to platinum is coupled to proton addition to give the hydroxoplatinum group directly.
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INTRODUCTION The oxidation of dimethylplatinum(II) complexes with oxygen, peroxides, or other oxygen atom donors is of much current interest, mostly because it could constitute a key step in potential catalytic cycles for the oxidation of methane to methanol.1−3 Recently, a reactive oxoplatinum(IV) complex was reported,4 and this has stimulated a discussion on whether oxoplatinum(IV) complexes might be intermediates in the above chemistry and in other platinum-catalyzed oxidation reactions (A, B, Scheme 1).4,5 The proposal is attractive because oxo complexes are already known to be involved in important biological C−H bond activation reactions and in model systems.6 However, theory predicts that MO double bonds cannot exist in octahedral transition metal complexes if the metal ion has the d6 or higher electron configuration, and platinum(IV) has the 5d6 electron configuration.4−8 Experimental evidence for an oxoplatinum(IV) complex of the type [Pt(O)(OH2)(LL)2]16−, C, where LL is a chelating polyoxometalate ligand [PW9O34]9−, which violates the theoretical prediction, was reported, but the claim has recently been withdrawn.8,9 Complexes A and B (Scheme 1) are not forbidden by theory, because they are formulated as four- or five-coordinate complexes with an available 6pπ orbital to take part in the proposed PtO π-bonding.4,5 Platinum oxides, hydroxides, and peroxides are important intermediates in other catalytic reactions, and so there is general interest in the synthesis, structure, and reactivity of complexes of this type.1−10 The reagent dimethyldioxirane, dmdo, was attractive for further studies of oxidation of dimethylplatinum(II) complexes because it is reactive in oxygen atom transfer and has been used in forming new EO double bonds (Scheme 2)11 as well as in forming the oxoplatinum(IV) complex A (Scheme 1).4 By analogy, it seemed possible that an oxoplatinum(IV) complex D (Scheme 2) might be formed as a reactive intermediate. © 2012 American Chemical Society
Scheme 1. Synthesis of the Oxoplatinum(IV) Complex A (S = solvent), a Possible Oxoplatinum(IV) Intermediate B, and the Withdrawn Polyoxotungstate Complex C
Received: July 3, 2012 Published: August 6, 2012 6388
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is naturally present in the dmdo solution in acetone, which cannot be rigorously dried.11 Complex 2a has been reported previously, but it is difficult to crystallize and its precise structure has been debated. In the first report, the complex was tentatively formulated as a hydrate, [Pt(OH)(OH2)Me2(bipy)]OH, with one hydroxo and one aqua ligand, primarily because it acts as close to a 1:1 conductor in methanol solution.12 However, in later reports the complex is formulated as the dihydroxo complex 2a, Scheme 3.1,2 The dihydroxo complexes are easily protonated, and the complex [Pt(OH)(OH2)Me2(4,4′-dimethyl-2,2′-bipyridine)]+ is about as acidic as CHCl2CO2H (pKa = 6.38).2a In the known structures of the dihydroxo complexes, the hydroxo groups are strongly involved in hydrogen bonding. Thus, [Pt(OH)2Me2(4,4′-dimethyl-2,2′-bipyridine)] forms a solvate with methanol containing Me-O-H···O(H)Pt groups, with the PtOH groups acting as hydrogen bond acceptors.2a Complex 2b forms a zeolite-like network structure, formed through intermolecular NH···O(H)Pt and PtOH···O(H)Pt hydrogen bonds, with the PtOH groups acting as both hydrogen bond donor and acceptor.13 In the complex [Pt(OH)(OMe)Me2(tmeda)], there is a PtOH···O(Me)Pt hydrogen bond, with the PtOH group acting only as a hydrogen bond donor.1b The complex [Pt(OH)Me 2 (tacn)][Pt(OH 2 )Me 2 (tacn)][BF4]3·MeOH contains both an aqua and a hydroxo complex, and the PtOH2 group acts as a hydrogen bond donor to the PtOH group.1e Complex 2a finally crystallized as the hydrate [Pt(OH)2Me2(bipy)]·7H2O, one of the most hydrated organometallic compounds known,14 and it has a correspondingly unusual structure, which is illustrated in Figures 1−3. There are
Scheme 2. Typical Oxidation with dmdo and Potential for Formation of an Oxoplatinum(IV) Complexa
a
NN = chelating nitrogen-donor ligand. The oxo group is shown as Pt+-O− rather than PtO to be consistent with theory.
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RESULTS AND DISCUSSION Synthesis and Characterization. The new reactions occurred according to Scheme 3. In a typical reaction, Scheme 3. Oxidation Reactions with dmdo
dimethyldioxirane11 was prepared as an approximately 0.1 M solution in acetone and added slowly to a solution of [PtMe2(2,2′-bipyridine)], 1a,12 in acetone. The reaction occurred rapidly at room temperature, and the color changed from the characteristic red color of 1a to colorless as the dmdo was added. The product 2a precipitated as a white solid. Complex 2a has been prepared previously by oxidation of complex 1a with dioxygen in moist solvent or with hydrogen peroxide, so it could be characterized by its 1H NMR spectrum.1b,12 The structural assignment was confirmed by an X-ray structure determination in this case. Complex 2b was prepared in a similar way and characterized by comparison of its 1H NMR spectrum and mass spectrum with that of a sample prepared by reaction of 1b with hydrogen peroxide.13 Finally, complex 3 was prepared by reaction of dmdo with the complex [PtMe2(dpk)], 1c. Complex 3 is also formed by reaction of 1c with hydrogen peroxide and is considered as a derivative of the ketal.2e In each of these reactions, there was no direct evidence for formation of an oxoplatinum(IV) complex (Scheme 2), which is expected to be intensely colored,4 even as a short-lived reaction intermediate. Experiments were carried out with 1a and dmdo in the presence of alkenes such as acrylonitrile or butadiene in order to trap a possible oxoplatinum(IV) intermediate,10 but only 2a was isolated. The formation of 2a, 2b, or 3 requires the presence of adventitious water, which
Figure 1. Structure of the Pt(1) molecule and its interactions with neighboring Pt(1) molecules. Selected bond distances (Å): Pt(1)− O(1) 2.019(5); Pt(1)−O(2) 2.031(5); Pt(1)−C(12) 2.036(7); Pt(1)−C(11) 2.045(7); Pt(1)−N(1) 2.170(5); Pt(1)−N(2) 2.178(6); O(1)···O(16A) 2.784(7); O(2)···O(10) 2.733(8).
two crystallographically nonequivalent platinum complexes in the structure and 14 independent water molecules. Most of the hydrogen atoms could not be located directly, so the nature of the hydrogen bonding must be deduced primarily from the positions of the oxygen atoms. The coordination of the Pt(1) molecules is shown in Figure 1. It confirms the trans-dihydroxo stereochemistry proposed for 2a and shows the likely hydrogen bonding to neighboring water molecules. Single hydrogen atoms on O(1) and O(2) are thought to act as H-bond donors to O(16A) and O(10), respectively, with O(1)···O(16A) = 6389
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of the motif gives a supramolecular polymer of Pt(2) molecules linked by bridging water molecules. The chains of Pt(1) and Pt(2) molecules are connected by a further array of hydrogen-bonded water molecules to give a very complex network structure, a small section of which is illustrated in Figure 3. Because the hydrogen-bonding network is so extensive, and because few of the H atoms were directly located in the X-ray structure determination, it is still not possible to determine with certainty whether the hydrated 2a should be considered as [Pt(OH)2Me2(bipy)]·7H2O or as [Pt(OH)Me2(OH2)(bipy)][OH]·6H2O. Acid−base considerations and the similarity of the Pt−O distances suggest that the former formulation is preferred.2a Theoretical Considerations. It has not been possible to avoid the incorporation of water in the reactions of Scheme 3, but it is possible to discuss potential anhydrous reactions of 1a with dmdo on the basis of DFT calculations, with the aim of predicting the stability of the axial oxo complex D or its equatorial isomer E (Scheme 4). Some potential reactions are
Figure 2. Structure of the Pt(2) molecule and its interactions with a neighboring Pt(2) molecule. Selected bond distances (Å): Pt(2)− O(4) 2.022(5); Pt(2)−O(3) 2.029(5); Pt(2)−C(24) 2.038(7); Pt(2)−C(23) 2.058(7); Pt(2)−N(4) 2.169(6); Pt(2)−N(3) 2.185(6); O(3)···O(8) 2.760(8); O(4)···O(17) 2.673(7).
Scheme 4. Potential Routes to and Reactions of Oxo Complexes D and E
Figure 3. Small section of the network structure of hydrated complex 2a, illustrating the connection of Pt(1) and Pt(2) chains. Pt(1) molecules and directly H-bonded water molecules are shown in red, while Pt(2) molecules and directly H-bonded water molecules are shown in blue (compare Figures 1 and 2). The O atoms of other water molecules are shown in black. Only the NCCN atoms of the bipy groups are shown, for clarity.
shown in Scheme 4, and the corresponding calculated energies are in Figure 4. The initial interaction is expected to occur by nucleophilic attack from the 5dz2 orbital of 1a on the σ*(OO) orbital of dmdo to give F (Scheme 2, Figure 5).11 Loss of acetone could then give the oxo complex D. However, the
2.784(7) Å and O(2)···O(10) = 2.733(8) Å; so the O(1) and O(2) atoms are presumed to act as hydrogen bond acceptors in the other interactions with water molecules, with O(1)···O(18) = 2.79 Å, O(1)···O(11) = 2.92 Å, O(1)···O(11B) = 2.95 Å, O(2A)···O(16A) = 2.72 Å, and O(2A)···O(18) = 2.89 Å. Some of these distances represent weak hydrogen-bonding interactions15 but, if all are counted, then propagation of this motif leads to formation of a supramolecular double-stranded polymer of Pt(1) molecules linked by bridging water molecules. The structure of the Pt(2) molecule is similar, as shown in Figure 2. Again, one hydrogen atom for each PtOH group is thought to act as a H-bond donor, with O(3)···O(8) = 2.760(8) Å and O(4)···O(17) = 2.673(7) Å. The other interactions involve O(3) and O(4) as H-bond acceptors, with O(3)···O(14A) = 2.87 Å, O(3)···O(17A) = 2.75 Å, O(4)···O(13) = 3.09 Å, and O(4)···O(14) 2.78 Å. Propagation
Figure 4. Calculated energies of complexes shown in Scheme 4. 6390
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Figure 5. Proposed initial interaction of 1a with dmdo.
reaction is calculated to be thermodynamically unfavorable and to have a high activation energy (Figure 4). An alternative reaction of F would be to form the platinum(IV) metallacycle G, analogous to known platina(IV)cyclobutanes or platina(II)oxetanes.10c,d,16 Loss of acetone from G could lead directly to the equatorial oxo complex E, and formation of E and acetone from 1a and dmdo is calculated to be thermodynamically favorable (Figure 4). It is also conceivable that G could rearrange to give an intermediate octahedral oxo-acetone complex, [Pt(O)Me2(OCMe2)(bipy)], J, but this is less favored than direct loss of acetone. Assuming that D or E could be formed, would they be stable in the absence of water? Two potential reactions were considered, and both were found to be thermodynamically favored. Rearrangement of either D or E could occur by reductive coupling to give the methoxoplatinum(II) complex, I, but a high barrier is expected for such reactions.17 A much easier reaction would be dimerization of the equatorial isomer E to give the μ-oxo complex H, and since there is no significant steric barrier,4 this would be the most likely product of the reaction of 1a and dmdo. Many μ-oxo complexes of platinum are known,10,18 and a diprotonated analogue of H, namely, the dihydroxo complex [Pt2(μ-OH)2Me4(dpa)2]2+, has been structurally characterized.13 The calculated structures and some frontier orbitals of the oxo complexes D and E are shown in Figure 6, and a partial energy correlation diagram is shown in Figure 7. The calculated PtO distances are 2.04 Å for D (Figure 6a) and 1.92 Å for E (Figure 6e), suggesting no PtO double bonding in D but some double-bond character in E. For comparison, the calculated PtO distance for A (Scheme 1) is 1.81 Å.4 For complex D, both the HOMO and HOMO−1 (Figure 6b, c) are antibonding combinations of oxygen 2pπ and platinum 5dπ orbitals, and they are both at high energy and close to the lowest pπ* MO of the bipy ligand (Figures 6d and 7). For complex E, the HOMO is still at high energy (Figure 6f), but the HOMO−1 (Figure 6g) is at lower energy (Figure 7) and has some Pt−O π-bonding character, using the available 6pπ orbital on platinum. These findings are in accord with established theory and give insight into the calculated preference for E over D.4,7 Nevertheless, even in the more stable isomer D, the oxo group has much Pt+−O− character, and so the oxygen atom is expected to be strongly nucleophilic. The calculated structures of some of the other potential intermediates discussed above are shown in Figure 8. Note in particular the very long Pt−O bond to acetone in J, such that
Figure 6. Calculated structure of (a) D and (e) E and the corresponding HOMO (b and f), HOMO−1 (c and g), and LUMO (d and h).
Figure 7. Energy correlation diagram for formation of D or E from 1a and O. Left, frontier orbitals for 1a and O; center, for D; right, for E.
the coordination of acetone to E is unfavorable. Normally, fivecoordinate, 16-electron, platinum(IV) complexes are not favored, but the PtO bonding in E can lead to it being considered as an 18-electron complex.19 One route to the dihydroxo complex 2a would be through hydration of an intermediate oxo complex, as shown in Scheme 5, which shows only the sequence in which water first coordinates to platinum, followed by proton transfer to the oxo group to give the dihydroxo complex. The opposite sequence is also possible. As expected, the reactions are strongly favored overall, with the greater energy gain in the proton transfer step (Figure 9). The trans and cis isomers 2a and M are calculated to have very similar energies, and since E is expected to be easily formed from the less stable D, the reaction would be expected to give M in preference to the observed product 2a as the kinetic product. The structures and relative energies of the oxo(aqua) and dihydroxo complexes are shown in Figure 10, illustrating the strong preference for the dihydroxo over the oxo(aqua) isomers. There is an analogy between K and the controversial 6391
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donor, π-acceptor, neutral, and anionic ligands including OH2, NH3, PH3, CNMe, OH−, F−, Cl−, and CN−. In some cases, minima could not be found for the oxo(aqua) isomer, but in all cases where a direct comparison was possible, the dihydroxo isomer was significantly more stable. Given the ease of proton transfer reactions, it therefore seems unlikely that compounds of general type C could be isolated. The theory indicates that the reaction of 1a with dioxirane is unlikely to lead to formation of an oxoplatinum(IV) complex. The high energy of the potential intermediate F and the corresponding absence of a low-energy route from 1a to D, by elimination of acetone from dmdo, can both be traced to the inability of platinum(IV) to form an axial PtO double bond.7 The question still remains as to how complex 2a is formed so easily by reaction of 1a, dmdo, and water. Some insight can be gained by considering the addition of water to the complex F (Figure 11). Bringing up a water molecule to the oxygen atom
Figure 8. Calculated structures of possible intermediates and products of Scheme 4. Selected distances: F, Pt−O 2.21 Å, O−O 2.02 Å (O−O in free dmdo is calculated to be 1.60 Å); G, Pt−O(trans to N) 2.07 Å, Pt−O(trans to C) 2.17 Å; H, Pt−O(trans to N) 2.05 Å, Pt−O(trans to C) 2.18 Å; J, Pt−O(oxo) 1.94 Å, Pt···O(acetone) 2.84 Å.
Scheme 5. Potential Route to Dihydroxo Complex 2a or M
Figure 11. Calculated structures of compounds formed by interaction of F with water. Selected distances (Å): N, Pt−O; PtO−H; HO−H; O, Pt−O.
bound to platinum leads to spontaneous loss of acetone in N, while bringing up a proton to the Pt−O oxygen atom and a hydroxide in the trans position leads to loss of acetone and formation of the stable dihydroxo complex in O. The precise sequence of events leading to formation of 2a is not determined, but we suggest that a key step is the coupling of proton addition with the acetone elimination step, which leads directly to a hydroxoplatinum(IV) complex rather than the unfavorable oxoplatinum(IV) complex. If a nucleophile, such as acetone, water, or hydroxide can be added trans to the incipient hydroxo group, then a very low barrier to the reaction is expected. There is an analogy to the easy reaction of 1a with dioxygen and water to give 2a, though 1a is stable to oxygen in dry aprotic solvents.1b,g The concerted proton−electron transfer may therefore be a general phenomenon.20 Dioxygen can insert into the methylplatinum bond in some complexes, but this reaction does not occur easily with complex 1a.21
Figure 9. Calculated energies of the intermediates and products of Scheme 5.
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CONCLUSIONS Both the experimental and computational work reported here suggest that the reaction of complex 1a with dmdo does not proceed by the simple oxygen atom transfer mechanism observed in reactions of dmdo with organic sulfides, amines, or phosphines,11 because the axial oxoplatinum(IV) intermediate that would be formed is too high in energy (Scheme 2).7 Instead, it is suggested that the oxygen atom transfer is coupled with protonation to give a hydroxoplatinum group directly, as shown in Scheme 6. The reaction might be assisted further by coordination of a second water molecule or a solvent molecule as the positive charge on platinum develops. There is at least one related case in which a protonation should accompany oxidation. Thus, the reaction of dimethylplatinum(II) complexes with dioxygen in methanol gives an intermediate hydroperoxo complex, and the simplest
Figure 10. Calculated structures of complexes 2a, K, L, and M. Selected distances (Å): 2a, Pt−O 2.10; K, Pt−O(oxo) 2.00, Pt− O(aqua) 2.04; L, Pt−O(oxo) 2.00, Pt−O(aqua) 2.39; M, Pt−O(trans Me) 2.15, Pt−O (trans N) 2.06.
trans-oxo(aqua) complex trans-[Pt(O)(OH2)(LL)2]16−, C, LL = [PW9O34]9−.8,9 In this context, the following question can be asked: Is it possible to find a ligand L for which the complex trans-[Pt(O)(OH2)L4]2+ will be more stable than trans[Pt(OH)2L4]2+? Calculations were carried out with several π6392
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washed with pentane, and dried in vacuo. Yield: 84%. NMR in CD3OD: δ(1H) 1.74 (s, 6H, 2J(PtH) = 70 Hz, MePt), 7.83 (ddd, 3 J(H5H6) = 6 Hz, 3J(H5H4) = 8 Hz, 4J(H5H3) = 1 Hz, H5), 8.27 (ddd, 2H, 3J(H4H5) = 8 Hz, 3J(H3H4) = 8 Hz, 4J(H4H6 = 1 Hz, H4), 8.86 (d, 2H, 3J(H3H4) = 8 Hz, H3), 9.02 (dd, 2H, 3J(H6H5) = 6 Hz, 4J(H6H4 = 1 Hz, H6). The spectrum was identical to that of an authentic sample.1b,12 [Pt(OH)2Me2(dpa)], 2b. This was prepared in a similar way from [PtMe2(dpa)] (9.8 mg, 0.0247 mmol) and dmdo in acetone. Yield: 60%. NMR in methanol-d4: δ(1H) 1.54 (s, 6H, 2J(PtH) = 64 Hz, MePt), 7.19 (dd, 2H, 3J(H5H6) = 6 Hz, 3J(H5H4) = 7 Hz, H5), 7.31 (d, 2H, 3J(H3H4) = 9 Hz, H3), 7.90 (ddd, 2H, 3J(H4H5) = 7 Hz, 3J(H4H3) = 9 Hz, 4J(H4H6) = 1 Hz, H4), 8.45 (d, 2H, 3J(H6H5) = 6 Hz, H6). ESI-MS: m/z = 429.1; calcd for [2b-H]+ 429.1. The spectrum was identical to that of an authentic sample.13 [Pt(OH)Me2(dpkOH)], 3. This was prepared in a similar way from [PtMe2(dpk)] (40 mg, 0.076 mmol) and dmdo in acetone. Yield: 80%. NMR in methanol-d4: δ(1H) 1.51 (s, 6H, 2J(Pt−H) = 73 Hz, MePt), 7.53 (ddd, 2H, 2J(H5H4) = 8 Hz, 2J(H5H6) = 5 Hz, 2J(H5H3) = 2 Hz, H5), 7.89 (dd, 2H, 2J(H3H4) = 8 Hz, 3J(H3H5) = 2 Hz, 8.04 (ddd, 2H, 2 J(H4H3) = 8 Hz, 2J(H4H5) = 8 Hz, 3J(H4H6) = 2 Hz, H4), 8.82 (dd, 2H, 2J(H6H5) = 5 Hz, 3J(H6H4) = 2 Hz, H6). The spectrum was identical to that of an authentic sample.2e Xray Structure Determination. The crystal was mounted on a glass fiber, and data were collected at 150(2) K by using a Bruker Smart Apex II CCD diffractometer. The unit cell parameters were calculated and refined from the full data set, and the structure was solved and refined by using the SHELX software.24 Crystal data and refinement parameters: [Pt(OH) 2 Me 2 (bipy)]·7H 2 O, formula C12H18N2O9Pt, fw 529.37, λ = 0.71073 Å, monoclinic, P21/n, a = 7.271(3) Å, b = 17.379(8) Å, c = 30.265(13) Å, β = 95.533(8)°, V = 3807(3) Å3, Z = 8, μ = 7.415 mm−1, R1 [I > 2σ(I)] = 0.0429, wR2 (all data) = 0.0896. A search for hydrogen atoms was made using highangle reflection data. Few H-atoms were located, and these could not be refined so were placed in fixed positions. Water molecules for which the hydrogen atoms were not located were treated as oxygen atoms only, because it is not possible to predict the positions and so to use the “riding” model.
Scheme 6. Proposed Mechanism of Formation of Complex 2a
way that this could react with a second dimethylplatinum(II) complex would give an axial oxoplatinum(IV) complex, as shown in Scheme 7. Again, it is more likely that the “oxo wall”7 would be avoided by coupling the second step with protonation of the incipient oxo group by solvent methanol.1 Scheme 7. Potential Oxoplatinum(IV) Complex from Dioxygen
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ASSOCIATED CONTENT
S Supporting Information *
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Crystallographic data in electronic CIF form only is available free of charge via the Internet at http://pubs.acs.org.
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EXPERIMENTAL SECTION
Reactions were carried out using standard Schlenk techniques, unless otherwise stated. NMR spectra were recorded using a Varian Mercury 400 or Varian INOVA 400 or 600 MHz spectrometer. 1H NMR chemical shifts are reported relative to TMS. Mass spectrometric analysis was carried out using an electrospray PE-Sciex mass spectrometer (ESI-MS) coupled with a TOF detector. The complexes [Pt2Me4(μ-SMe2)2] and 1a−c were prepared according to the literature.2e,12,22 Dimethyldioxirane was prepared as a solution in acetone by using the known method.11e DFT calculations were carried out by using the Amsterdam Density Functional program based on the BLYP functional, with double-ζ basis set and first-order scalar relativistic corrections.23 The reported results are from gas phase calculations, but selected calculations were also carried out for acetone solution, using the conductor-like screening model (COSMO) to treat solvation effects,23 and gave qualitatively similar results. The energy minima were confirmed by vibrational frequency analysis in each case, with no imaginary frequency for high-energy intermediates F and J. [PtMe2(OH)2(bpy)], 2a. A solution of dimethyldioxirane (∼0.1 M) was added dropwise to a stirring solution of [PtMe2(bipy)] (20 mg, 0.052 mmol) in acetone (5 mL), until the initial red color dissipated and a clear, colorless solution was obtained. Almost immediately, a white precipitate began to form in the reaction flask. The reaction was allowed to stir for 10 min, and pentane was added to complete the precipitation of the product as a white solid, which was separated,
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS We thank the NSERC (Canada) for financial support. REFERENCES
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Organometallics
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dx.doi.org/10.1021/om300616x | Organometallics 2012, 31, 6388−6394
