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Carbon−Hydrogen versus Nitrogen−Oxygen Bond Activation in Reactions of N‑Oxide Derivatives of 2,2′-Bipyridine and 1,10Phenanthroline with a Dimethylplatinum(II) Complex Mohamed E. Moustafa,†,‡ Paul D. Boyle,† and Richard J. Puddephatt*,† †

Department of Chemistry, University of Western Ontario, London, Ontario, Canada N6A 5B7 Department of Chemistry, Faculty of Education, Suez Canal University, Arish, Egypt

‡

S Supporting Information *

ABSTRACT: The reactions of the potential oxygen atom donor ligands 1,10-phenanthroline N-oxide (phenO) and 2,2′-bipyridine N-oxide (bipyO) with the dimethylplatinum(II) complex [Pt2Me4(μ-SMe2)2] are reported. The reaction with the more rigid ligand phenO gave [PtMe2(κ2N,O-phenO)], which underwent oxidative addition with 4-tBu-C6H4CH2Br to give the platinum(IV) complex [PtBrMe2(CH2C6H4-4t-Bu)(phenO)]. The complex [PtMe2(phenO)] reacted with methanol in air to give [Pt(OH)(OMe)Me2(phenO)], but under an inert atmosphere it gave [Pt(OH)(OMe)Me2(phen)], in a reaction involving N−O bond activation. In contrast, the reaction of [Pt2Me4(μ-SMe2)2] with bipyO occurred by C−H bond activation to give methane and [PtMe(κ2N,CC5H4N-C5H3NO)(SMe2)], which underwent ligand substitution with pyridine, triphenylphosphine, or bis(diphenylphosphino)methane (dppm) to give [PtMe(κ2N,C-C5H4N-C5H3NO)(NC5H5)], [PtMe(κ2N,C-C5H4N-C5H3NO)(PPh3)], or the binuclear [{PtMe(κ2N,CC5H4N-C5H3NO)}2(μ-dppm)], respectively. With bis(diphenylphosphino)ethane (dppe), ligand substitution gave [PtMe(κ1CC5H4N-C5H3NO)(dppe)], which contains a monodentate metalated bipyO ligand. The mechanisms of the key reactions are discussed.

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INTRODUCTION The selective oxidation of organic compounds, especially by molecular oxygen, is a continuing challenge for catalysis. For example, the oxidative upgrading of methane to methanol under mild conditions would have enormous economic and environmental impact.1 In this context, the combination of C− H bond activation of methane by platinum(II) complexes followed by oxidation of the product methylplatinum(II) complexes by dioxygen or hydrogen peroxide can give rise to methyl(hydroxo)platinum(IV) complexes, which have the potential to complete a catalytic cycle by reductive elimination of methanol.1,2 In the oxidation of platinum complexes by dioxygen or hydrogen peroxide, several functional groups with Pt−O bonds are invoked as intermediates or products, including Pt−OH, Pt−O−OH, Pt−OH2, Pt-OR, Pt−O−OR, Pt−O2, or PtO groups, and so there is current interest in the synthesis, structure, and properties of platinum complexes containing these functional groups.1−7 Terminal oxo complexes of late transition metals are rare,2,4,5,7,8 and only one oxoplatinum(IV) complex has been characterized (A, eq 1).7 One approach to the formation of terminal oxoplatinum(IV) complexes, as stable compounds or transient intermediates, is to react organoplatinum(II) complexes with potential oxygen atom donors, as exemplified by the use of dimethyldioxirane in the formation of A.7 © 2014 American Chemical Society

However, reactions of electron-rich dimethylplatinum(II) complexes with potential oxygen atom donors only occurred in the presence of a source of protons (Scheme 1), and it was suggested that, in these reactions, there was coupling of the oxygen atom transfer with proton transfer so as to avoid formation of a high-energy terminal oxoplatinum(IV) complex.5 Pyridine or tertiary amine N-oxide derivatives can act as oxygen atom donors, though they are less reactive than the reagents shown in Scheme 1. Thus, pyridine can be oxidized to pyridine N-oxide by dimethyldioxirane, oxaziridines, or mchloroperbenzoic acid, while pyridine N-oxide can act as an oxygen atom donor to tertiary phosphines or PCl3.9 The pyridine N-oxides have an extensive coordination chemistry as oxygen donors with hard metal ions,10 but there are few examples of oxygen atom transfer to metal ions or of Received: July 10, 2014 Published: September 18, 2014 5402

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Scheme 1. Reactions of Dimethylplatinum(II) Complexes with Oxygen Atom Donors (R = H, t-Bu, Ar = 3-ClC6H4)

Scheme 3. Potential Reactions of Complexes 3a,b

Scheme 4. Complex 3a and Its Oxidative Addition Reaction with 4-t-BuC6H4CH2Br coordination complexes with soft metal ions.4,11 While this work was in progress, a report of complexes formed by reaction of an electrophilic phenylplatinum(II) complex with pyridine N-oxide was published (Scheme 2).4 The aim was to study if Scheme 2. Pyridine N-Oxide Complex of Platinum(II) (R = t-Bu)

isolated as a red solid, the color being due to a low-energy MLCT transition which is typical of dimethylplatinum(II) complexes with diimine and related ligands with low-energy π* molecular orbitals.16 Complex 3a was characterized by its 1H NMR spectrum, which contained two equal-intensity methylplatinum resonances at δ 0.76 (2JPtH = 90 Hz) and 1.02 (2JPtH = 85 Hz) assigned to the methylplatinum groups trans to oxygen and nitrogen, respectively. These high coupling constants are characteristic for platinum(II) rather than platinum(IV) complexes. The expected resonances for the ligand 2a were also observed. However, complex 3a was reactive and difficult to crystallize; therefore, it was further characterized by its reaction chemistry. Complex 3a reacted with 4-tert-butylbenzyl bromide to give complex 4, as a mixture of isomers in an approximately 1:10 ratio. The product of selective cis oxidative addition 4b, with the 4-tert-butylbenzylplatinum group trans to nitrogen, is dominant in this reaction, whereas trans oxidative addition of benzylic halides to platinum(II) complexes is more common.17 The dominant isomer 4b gave two methylplatinum resonances at δ 1.27 (2JPtH = 80 Hz) and 1.49 (2JPtH = 72 Hz), assigned to the methylplatinum groups trans to oxygen and bromide, respectively, and two resonances for the diastereotopic PtCHAHB protons of the 4-tert-butylbenzylplatinum group at δ 3.10 (d, 3JHH = 9 Hz, 2JPtH = 54 Hz) and 4.19 (d, 3 JHH = 9 Hz, 2JPtH = 104 Hz), respectively. The second isomer might have the structure 4a or 4c, and since both have only C1 symmetry, they cannot easily be distinguished. Two methylplatinum resonances were resolved at δ 1.36 (2JPtH = 72 Hz) and 1.60 (2JPtH = 70 Hz), neither of which has the high coupling constant expected for a methyl group trans to oxygen

oxygen transfer with insertion into the phenyl−platinum bond might occur (E to F, Scheme 2), by analogy to organometallic Baeyer−Villiger reactions, 12 but no such reaction was observed.4 We adopted a complementary approach by studying the reaction of the mono-N-oxide of 2,2′-bipyridine13 and 1,10phenanthroline13,14 with more electron-rich dimethylplatinum(II) complexes. It was anticipated that initial coordination would occur and that further reaction might occur according to Scheme 3, with oxidation to the platinum(IV) complex G (compare Scheme 1) or by oxygen atom insertion into a methyl−platinum bond to give the methoxoplatinum(II) complex H (compare Scheme 2), since the reactions might be further promoted by formation of the stable Pt(bipy) or Pt(phen) chelate groups. However, the chemistry proved to be more complex, as described below, with the reactions with 2b dominated by C−H bond activation and not N−O bond activation.

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RESULTS AND DISCUSSION 1,10-Phenanthroline N-Oxide System. The reaction of [Pt2Me4(μ-SMe2)2] (1)15 with 1,10-phenanthroline N-oxide (2a) in acetone solution occurred easily at room temperature to give complex 3a, according to Scheme 4. Complex 3a was 5403

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(80 Hz for 4b). This suggests that the structure is 4c rather than 4a, but the assignment is tentative. The isomers equilibrate only slowly in solution. The structure of complex 4b is shown in Figure 1. It confirms the presence of an octahedral platinum(IV) center with an

At room temperature, in acetone solution, complex 3a shows no tendency to rearrange to complex H (Scheme 3) by transfer of the oxygen atom into the Pt−Me bond, but it does react easily with methanol. In the presence of air, the reaction occurs to give the complex [Pt(OH)(OMe)Me2(phenO)] (5) (Scheme 5), in which the phenO ligand remains intact. Scheme 5. Oxidation of Complex 3a and [PtMe2(phen)] (7) in Methanol

Figure 1. Structure of complex 4b. Selected bond parameters (distances in Å and angles in deg): Pt(1)C(1), 2.051(4); Pt(1)C(2), 2.051(4); Pt(1)C(3), 2.049(4); Pt(1)N(2), 2.195(3); Pt(1)O(1), 2.151(3); Pt(1)Br(1), 2.6180(9); O(1)N(1), 1.338(4); O(1)Pt(1)C(3), 97.25(14); O(1)Pt(1)N(2), 77.08(11); C(1)Pt(1)N(2), 100.95(13); Pt(1)O(1)N(1), 114.2(2).

Complex 5 was formed along with several minor impurities, and it has not been possible to grow single crystals suitable for structure determination; therefore, it was characterized spectroscopically by comparison with several known related compounds.2h,3b,6a,20 The 1H NMR spectrum (Figure 2) contained two equal-intensity methylplatinum resonances at δ(1H) 1.41 (2JPtH = 70 Hz) and 1.74 (2JPtH = 71 Hz), with

intact 1,10-phenanthroline N-oxide ligand (2a). The benzyl group is trans to the nitrogen donor and is oriented toward the oxygen donor of the ligand 2a (the torsion angle O(1)Pt(1)C(3)C(4) is 16°). The complex has only C1 symmetry, and so it is chiral at platinum(IV), but the lattice contains equal amounts of the C and A enantiomers. There is a significant twist of the phenanthroline skeleton, with the donor atoms O(1) and N(2) displaced farthest from the best plane of the phenanthroline N-oxide group by +0.40 and −0.29 Å, respectively. A similar conformation, but with a smaller distortion, has been observed in the chelate complex of phenanthroline N-oxide trans-[CuCl2(2b)2], in which the corresponding displacements from the best plane are 0.23 and −0.25 Å.18 The ligand bite angle O(1)Pt(1)N(2) is 77.08(11)° in 4b, but the corresponding angle OCuN is 86.7(2)° in trans-[CuCl2(2b)2].18 The angle between the plane defined by Pt(1)C(1)C(3)O(1)N(2) and the atoms of the ligand 2a is 42°, with the ligand displaced toward the bromide ligand (Figure 1). All of these data indicate strain in the sixmembered chelate ring of 4b, whose distorted structure can be compared to the near-planarity of the five-membered Pt(phen) chelate ring, found in many platinum(IV) complexes.19

Figure 2. Partial 1H NMR spectrum (400 MHz) of complex 5 in CD3OD solution. 195Pt satellite spectra are indicated by *. 5404

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Hz) in a 1:2 ratio. There were only seven aromatic resonances for 8, compared to eight such resonances in the ligand 2b. There was resolved 195Pt−1H coupling to the H6 proton of the pyridine ring and to both the H4 and H5 protons of the pyridine N-oxide ring of 8, indicating binding of platinum to both rings. Another useful diagnostic feature is the unusual chemical shift of the H3 proton of the pyridyl ring, which is oriented toward the NO group of the rigid bidentate ligand and which occurred at δ 9.95 in 8. The structure of complex 8 was determined and is shown in Figure 4. There are two independent, but very similar, molecules in the lattice, and each forms a loose dimer with an equivalent molecule (Figure 4). Each platinum center has square-planar stereochemistry with coordination to a methyl group, a dimethyl sulfide ligand, and a nitrogen and carbon atom of the metalated ligand 2b. There is a relatively short intermolecular Pt···N distance (Pt(1)N(2A), 3.69 Å; Pt(2)N-

coupling constants typical of platinum(IV) complexes, and a methoxyplatinum resonance at δ(1H) 2.81 (3JPtH = 32 Hz). There were eight equal-intensity resonances for the unsymmetrical phenO ligand. A similar reaction in methanol-d4 gave complex 5* (Scheme 5), whose 1H NMR spectrum was essentially the same as that for complex 5 except for the absence of the methoxyplatinum resonance, thus showing that the methoxy group in 5 is derived from the solvent and not from a methylplatinum group. When complex 3a was dissolved in methanol or methanol-d4 in the absence of air, the major product was complex 6 or 6* (Scheme 5), respectively, and this reaction clearly does involve oxygen atom transfer from the phenO ligand. Complex 6 could not be isolated in pure form from this reaction, but it was positively identified by comparison with the known complex 6, prepared by reaction of [PtMe2(phen)] (7) with methanol in the presence of air (Scheme 5).2h,3b,20 Complex 6 has Cs symmetry; therefore, its 1 H NMR spectrum contains only one methylplatinum and four aromatic CH resonances and it is easily distinguished from the less symmetrical complex 5. 2,2′-Bipyridine N-Oxide System. The reaction of complex 1 with 2,2′-bipyridine N-oxide (2b) in acetone solution occurred rapidly at room temperature, with loss of methane, to give a stable yellow product, identified as the cyclometalated platinum(II) complex [PtMe(κ2C,N-2b-H)(SMe2)], 8 (Scheme 6). The structure of 8 was determined Scheme 6. Metalation of the 2,2′-Bipyridine N-Oxide Ligands

initially by the 1H NMR spectra (Figure 3). For example, complex 8 gave singlet resonances for the methylplatinum (δ 1.01, 2JPtH = 81 Hz) and methylsulfur protons(δ 2.51, 3JPtH = 28

Figure 4. Structure of complex 8, showing the stacked dimers formed by each of the independent molecules. Selected bond parameters (distances in Å and angles in deg): Pt(1)C(1), 2.054(3); Pt(1)C(8), 2.003(2); Pt(1)N(1), 2.105(2); Pt(1)S(1), 2.3379(9); N(2)O(1), 1.310(3); Pt(2)C(14), 2.043(3); Pt(2)C(21), 1.999(2); Pt(2)N(3), 2.106(2); Pt(2)S(2), 2.3434(9); N(4)O(2), 1.310(3); C(8)Pt(1)N(1), 80.36(8); C(21)Pt(2)N(3), 80.15(9).

Figure 3. 1H NMR spectrum (400 MHz) of complex 8: (top) methyl region; (bottom) aromatic region. The 195Pt satellite spectra are indicated by *, and the peak marked # is due to residual chloroform in the CDCl3 solvent. 5405

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4) and 2b (Scheme 6), is whether or not the 2,2′-bipyridine Noxide complex 3b (Scheme 3) might be formed as an intermediate in the reaction of 1 with 2a, followed by rollover cyclometalation to give 8.23 To check for this, the reaction was monitored by low-temperature 1H NMR spectroscopy in acetone-d6 solution. The reagents were mixed at −78 °C, and spectra were obtained as the sample temperature was raised slowly to room temperature. Most of the reaction occurred in the region from 0 to 20 °C, and the final products, 8 and methane, were formed in essentially quantitative yield. At intermediate stages, free dimethyl sulfide (δ(MeS) 2.10), cis[PtMe2(SMe2)2]15 (δ(MePt) 0.45, 2J(PtH) = 84 Hz; δ(MeS) 2.33, 3J(PtH) = 24 Hz) and methane (δ(CH4) 0.16) were formed and two new complexes, 9 and 10, were detected, both of which gave a single methylplatinum resonance, with a coupling constant in the range expected for a methylplatinum(II) complex. Complex 3b, as well as I (Scheme 7), should give two equal-intensity methylplatinum resonances, and neither was formed in a detectable amount. Complex 9, which was the major intermediate, gave a methylplatinum resonance (δ(MePt) 0.69, 2J(PtH) = 83 Hz) and 15 aromatic resonances. Complex 10 gave a methylplatinum resonance (δ(MePt) 0.87, 2 J(PtH) = 82 Hz) and a bridging dimethyl sulfide resonance (δ(MeS) = 2.74, 3J(PtH) = 21 Hz) (characterized by a 1:8:18:8:1 intensity ratio, arising from coupling to 195Pt) in a 1:1 ratio. The observations are interpreted in terms of Scheme 7. The initial reaction is likely to occur by ligand addition and splitting of the dimethyl sulfide bridge to give I, though this complex was not detected. Dissociation of dimethyl sulfide from I could occur to give 3b (Scheme 3), but instead it leads to C−H bond activation, probably through intermediates J− L.23,24 Displacement of methane from L appears to occur nonselectively to give mostly a mixture of complex 8, which arises by attack by Me2S released in an earlier step, complex 9, which arises from attack by free ligand 2b, and, to a minor extent, complex 10, which is formed from attack by the PtSMe2 group of complex 8. cis-[PtMe2(SMe2)2], which is observed at

(4A), 3.55 Å) in each dimer, which could be interpreted as a weak attraction between the electron-rich platinum(II) center and the electron-poor nitrogen atom. The point of metalation at C3 of the pyridine N-oxide ring is as expected for directed metalation, after initial coordination of the pyridyl group,21 but we note that it is an unusual point of attack by either electrophiles or nucleophiles on pyridine Noxide derivatives.9,22 Chart 1 illustrates important resonance Chart 1. Some Resonance Forms of Pyridine N-Oxide

forms for the parent pyridine N-oxide, which are useful in accounting for the activating effect of the NO group for the attack of either electrophiles or nucleophiles at the ortho or para positions.9 However, the directed metalation of the 2,2′bipyridine N-oxide ligands (Scheme 6) occurs selectively at the position meta to the NO group. Given the growing importance of pyridine N-oxide derivatives in organic synthesis, especially in carbon−carbon bond coupling steps, this could be a significant finding.9,22 From the dimer structure of Figure 4, it can be envisaged that either intermolecular oxidative addition of the N−O bond to platinum(II) or intermolecular metalation at C6 of the pyridine N-oxide ring might occur, but neither of these reactions is observed and complex 5 is stable at room temperature. An interesting question, arising from the different products of reaction of complex 1 with the N-oxide derivatives 2a (Scheme Scheme 7. Proposed Mechanism of Formation of Complex 8

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intermediate stages, is formed by the reaction of complex 1 with dimethyl sulfide (Scheme 7).15 At room temperature, and as the concentration of free ligand 2b decreases through the cyclometalation, an equilibrium is established which strongly favors complex 8. There is a resemblance of this chemistry to that shown in Scheme 8, in which there can be a competition between C−H Scheme 8. Competition between C−H and C−X Bond Activation

Figure 5. Structure of complex 11. Selected bond parameters (distances in Å and angles in deg): Pt(1)C(1), 2.0493(14); Pt(1)C(2), 1.9804(12); Pt(1)N(2), 2.0976(12); Pt(1)N(3), 2.0936(11); N(1)O(1), 1.3087(13); C(2)Pt(1)N(2), 80.55(4).

bond activation (observed when X = H, F) and C−X bond activation (observed when X = Cl, Br, SMe).23,24 In the present case, C−H bond activation takes precedence over chelate formation to give 3b or N−O bond activation to give an oxoplatinum(IV) complex. Ligand Substitution in Complex 8. The dimethyl sulfide ligand in complex 8 is easily displaced, and derivatives 11 and 12, with the monodentate ligands pyridine and triphenylphosphine, respectively, are shown in Scheme 9, with the structures

Figure 6. Structure of complex 12. Selected bond parameters (distances in Å and angles in deg): Pt(1)C(1), 2.049(3); Pt(1)C(2), 2.035(3); Pt(1)N(2), 2.118(2); Pt(1)P(1), 2.2981(9); N(1)O(1), 1.302(3); C(2)Pt(1)N(2) 79.59(9).

Scheme 9. Synthesis of Complexes 11 and 12 weak O···H−C hydrogen bond formed by the nucleophilic Noxide group. The bidentate ligands bis(diphenylphosphino)methane (dppm) and bis(diphenylphosphino)ethane (dppe) reacted with complex 8 according to Scheme 10 to give complexes 13 and 14, respectively. The structures of the complexes are shown in Figures 7 and 8. Complex 13 contains a bridging dppm ligand, and there is strong π stacking of the N-oxide ligands and a particularly close contact between the two platinum atoms, with Pt(1)··Pt(1A) 3.1501(13) Å, indicating the presence of a metallophilic bond.25 A water molecule is hydrogen-bonded to the oxygen atom of each N-oxide group with a O(1)O(1W) hydrogen bond distance of 2.761(5) Å. The molecule contains a 2-fold rotation axis which passes through the bridging carbon atom C(12) and the oxygen atom of the water molecule O(1W). In complex 14 (Figure 8) the pyridine donor in 8 has been displaced, forming the monodentate metalated ligand, and the dppe ligand acts as a chelate. The N-oxide group is

shown in Figures 5 and 6. Complex 11 (which can be considered as a model for complex 9; Scheme 7) forms loosely associated dimers in the solid state, with relatively short intermolecular Pt···N contacts of 3.60 Å, very similar to those for the dimers observed for complex 8 (Figure 4). Complex 12 also forms dimers in the solid state, but the π stacking is offset in comparison with 8 or 11, apparently as a result of steric effects of the bulky triphenylphosphine group. There is a short contact O(1)···C(25A) of 3.21 Å, which probably represents a 5407

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Scheme 10. Synthesis of Complexes 13 and 14

Figure 8. Structure of complex 14. Selected bond parameters (distances in Å and angles in deg): Pt(1)C(1), 2.100(2); Pt(1)C(2), 2.0537(19); Pt(1)P(1), 2.2745(8); Pt(1)P(2), 2.2897(6); N(1)O(1), 1.318(2); P(1)Pt(1)P(2), 84.82(2); H-bond distance O(1)O(1W), 2.834(3).

Figure 7. Structure of complex 13. Selected bond parameters (distances in Å and angles in deg): Pt(1)C(1), 2.063(4); Pt(1)C(2), 2.049(4); Pt(1)N(2), 2.125(4); Pt(1)P(1), 2.2971(13); Pt(1)Pt(1A), 3.1501(13); N(1)O(1), 1.313(5); C(2)Pt(1)N(2), 79.53(17); Hbond distance O(1)O(1W), 2.761(5).

hydrogen-bonded to a water molecule, with a O(1)O(1W) distance of 2.834(3) Å. Computational Studies. DFT calculations were carried out (BLYP functional, double-ζ basis set, first-order scalar relativistic corrections)26 on some of the compounds and reaction steps proposed in order to gain further insights. First, the relative energies of the three isomers of [PtBrMe2(CH2Ph)(phenO)] (4a′−c′; Figure 9) were calculated as models for 4a−c of Scheme 4. The isomer 4b′ was calculated to be more stable by only 2 kJ mol−1 than 4a′,c′, which were equal in energy. Thus, the theory correctly predicts the most stable isomer but it does not distinguish between the isomers 4a′ and 4c′. In the complex [PtBrMe2(CH2Ph)(bipy)], the trans adduct, analogous to 4a′, is most stable.17 In this case, π stacking of the benzyl group above the bipy ligand favors this isomer, while Figure 9 shows that this effect is absent in 4a′ because of the orientation of the phenO ligand. The possible isomerization and hydration reactions of complex 3a are shown in Scheme 11, while the calculated structures are shown in Figure 10. The relative stabilities of the isomers are calculated to be M > N > 3a > O (Scheme 11), and only the axial oxo complex, O, is higher in energy than 3a (the equatorial isomer N is expected to be stabilized by p bonding between the oxygen and platinum pz orbitals).5,7 The oxo

Figure 9. Calculated structures of isomers of [PtBrMe2(CH2Ph)(phenO)] as models for 4a−c.

complexes N and O, if they could be formed, are expected to be very easily hydrolyzed to the corresponding dihydroxo complexes P and Q. None of the isomerization products are easily formed, for reasons discussed in the context of pyridine N-oxide complexes of platinum(II), as illustrated in Scheme 2.4 The calculated relative energies of the complexes shown in Scheme 5 are shown in Figure 11. Complex 3a reacts rapidly with methanol in air to give complex, 5 but in the absence of dioxygen, it gives 6 as the major product. Complex 6 is also formed by reaction of [PtMe2(phen)] (7) with methanol in air.2h,3b,20 The reactions in air are very likely to occur by the known mechanism, involving a hydroperoxide intermediate.3b,6f 5408

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Scheme 11. Potential Isomerization and Hydration Reactions of Complex 3aa

Figure 10. Calculated structures, with selected distances and Hirshfeld atomic charges, of potential isomers and hydration products of 3a illustrated in Scheme 11. Hydrogen atoms are not shown. Calculated relative energies for M−Q with respect to 3a are given in kJ mol−1. Energies for hydrates P and Q are corrected appropriately.

a

As proposed for reactions with other oxygen atom transfer reactions, it is unlikely that the reaction of 3a with methanol in the absence of air occurs via the oxo complex N or O but that the oxygen transfer is coupled with protonation.5 A simplified mechanism is shown in Scheme 12. It is difficult to model the proposed intermediates R and S because the H and OMe groups of methanol will be components of methanol molecules that will certainly be involved in strong hydrogen bonding to other methanol solvent molecules.5 A gas-phase calculation on the proposed intermediate R led to collapse to the product 6, suggesting that the reaction has a low activation energy once significant hydrogen bonding of methanol to the NO oxygen atom is initiated. Calculated structures and relative energies in comparison to 3b for some proposed intermediates and products of Schemes 7 and 9 are shown in Figure 12. The possible intermediate 3b is calculated to be 26 kJ mol−1 more stable than the σ-CH complex J and 62 kJ mol−1 more stable than the dimethyl(hydrido)platinum(IV) complex K. Reductive elimination from K to give the methane complex L is favorable, and L is calculated to be 25 kJ mol−1 more stable than 3b. When the reaction of 1 with 2b was carried out in CD3OD solution, no deuterium incorporation into either the methane or methylplatinum group of 8 was detected, indicating that the reductive elimination from K is not reversible under the experimental conditions.1,6 The displacement of methane from L by the ligands dimethyl sulfide and pyridine is calculated to be favorable, as expected (Figure 12).

Figure 11. Calculated relative energies (kJ mol−1) of complexes of Schemes 5 and 12

Scheme 12. Possible Mechanism for the Reaction of 3a with Methanol To Give Complex 6a

The H and OMe groups in R and S are components of methanol molecules in this simplified illustration.

a

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CONCLUSIONS Webb and co-workers recently showed that oxygen atom transfer did not occur in electrophilic pyridine N-oxide 5409

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While 1,10-phenanthroline N-oxide has broad potential as an oxygen transfer reagent to metals in low oxidation states, aided by the formation of stable 1,10-phenanthroline chelate complexes, it is unlikely that 2,2′-bipyridine N-oxide will find similar applications because of the preference for C−H over N−O bond activation.

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EXPERIMENTAL SECTION

All reactions were carried out under nitrogen using standard Schlenk techniques, unless otherwise specified. 1H NMR spectra were recorded by using a Varian Mercury 400 spectrometer or Varian INOVA 600 or 400 MHz spectrometers. The NMR labeling system is defined in Chart 2. Assignments were confirmed by recording COSY spectra. The

Chart 2. NMR Labeling Scheme

complexes [Pt2Me4(μ-SMe2)2] and [PtMe2(phen)] were prepared according to the literature.15,20b The phenO complexes had limited thermal stability, and analytical data could not be obtained; they are characterized by accurate mass determination and by the NMR spectra, which are given in the Supporting Information. DFT Calculations. DFT calculations (gas phase, without zero point energy corrections) were carried out by using the Amsterdam Density Functional (ADF) program based on the GGA:BLYP functional, with double-ζ basis set and first-order scalar relativistic corrections.26 Energy minima were confirmed by vibrational analysis, and atomic coordinates for the calculated structures are included in the Supporting Information. Transition states were not determined. X-ray Structure Determinations: 27 In a typical experiment, a sample was mounted on a Mitegen polyimide micromount with a small amount of Paratone N oil. All X-ray measurements were made using a Bruker Kappa Axis Apex2 diffractometer at a temperature of 110 K. The data collection strategy was a number of ω and φ scans. The frame integration was performed using SAINT. The resulting raw data were scaled and absorption-corrected using a multiscan averaging of symmetry equivalent data with SADABS. The structure was solved by direct methods. The hydrogen atoms were introduced at idealized positions and were allowed to ride on the parent atom. The structural model was fit to the data using full-matrix least squares based on F2. The calculated structure factors included corrections for anomalous dispersion. The structure was refined using the SHELXL-2013 program from the SHELXTL program package. Details are given in the CIF files (Supporting Information). 1,10-Phenanthroline N-Oxide and 2,2′-Bipyridine N-Oxide (2a,b). These were prepared by the literature methods.13,14 NMR in CDCl3: 2a, δ(1H) 7.43 (t, 1H, 3JHH = 7 Hz, H8), 7.66 (m, 1H, H3), 7.72 (d, 1H, 3JHH = 8 Hz, H6), 7.74 (d, 3JHH = 8 Hz, H5), 7.78 (d, 1H, 3 JHH = 7 Hz, H7), 8.23 (d, 1H, 3JHH = 9 Hz, H4), 8.73 (d, 1H, 3JHH = 7 Hz, H9), 9.30 (d, 1H, 3JHH = 6 Hz, H2); 2b, δ(1H) 7.27 (t, 1H, 3JHH = 6 Hz, H5), 7.38 (m, 2H, H4,H5′), 7.84 (t, 1H, 3JHH = 7 Hz, H4′), 8.19 (d, 3JHH = 7 Hz, H3), 8.33 (d, 1H, 3JHH = 7 Hz, H3′), 8.74 (d, 1H, 3 JHH = 8 Hz, H6), 8.91 (d, 1H, 3JHH = 6 Hz, H6′). [PtMe2(phenO)] (3a). To a solution of 1,10-phenanthroline Noxide (PhenO) (0.050 g, 0.255 mmol) in acetone (10 mL), was added [Pt2Me4(μ-SMe2)2] (1; 0.074 g, 0.128 mmol). The color changed

Figure 12. Calculated structures of intermediates and products of C− H activation of 2b. Calculated energies relative to 3b are given in kJ mol−1. Energies of 8 and 11 are corrected for addition of the ligand (SMe2 or C5H5N) and loss of methane. H atoms are shown only when necessary for clarity.

complexes of platinum(II), as illustrated in Scheme 2.4 This work has shown that the nucleophilic 1,10-phenanthroline Noxide complex [PtMe2(phenO)] is also unreactive to Baeyer− Villiger type oxygen transfer and to formation of an oxoplatinum(IV) complex. However, oxygen atom transfer coupled to protonation has been shown to occur in the reaction of [PtMe2(phenO)] with methanol to give [Pt(OH)(OMe)Me2(phen)], which had previously been prepared by reaction of [PtMe2(phen)] with dioxygen and methanol.2h,3b,20 The oxygen atom transfer from [PtMe2(phenO)] complements earlier reactions with oxygen atom transfer reagents such as dioxiranes and oxaziridines, in which oxygen atom transfer is also coupled with protonation.5 An extensive but completely different chemistry, giving selective C−H bond activation, is observed in analogous reactions with 2,2′-bipyridine N-oxide. 5410

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was added [Pt2Me4(μ-SMe2)2] (0.084 g, 0.146 mmol). The color changed rapidly to yellow, followed by precipitation of the product as a yellow solid. The reaction mixture was stirred at room temperature for 1 h. The yellow solid was filtered, washed with pentane (3 × 3 mL), and dried under vacuum. Yield: 75%. NMR in CDCl3: δ(1H) 1.01 (s, 3H, 2JPtH = 81 Hz, MePt), 2.51 (s, 6H, 3JPtH = 28 Hz, SMe), 7.12 (t, 1H, 3JHH = 7 Hz, 4JPtH = 20 Hz, H5), 7.40 (dd, 1H, 3JHH = 8 Hz, 6 Hz, H5′), 7.76 (d, 1H, 3JHH = 7 Hz, 3JPtH = 66 Hz, H4), 8.01 (t, 1H, 3JHH = 8 Hz, H4′), 8.08 (d, 1H, 3JHH = 6 Hz, H6), 9.06 (d, 1H, 3JHH = 6 Hz, 3 JPtH = 22 Hz, H6′), 9.95 (d, 1H, 3JHH = 8 Hz, H3′). HRMS (ESI): calcd for C13H16N2OPtS [M + Na]+ 465.05077, found 465.0488. Anal. Calcd for C13H16NOPtS: C, 35.21; H, 3.64; N, 6.32; S, 7.23. Found: C, 35.45; H, 3.59; N, 6.20; S, 7.12. Single crystals were grown by slow diffusion of n-pentane into a solution of the ligand dissolved in dichloromethane. The formation of complex 8 was monitored by 1H NMR spectroscopy in acetone-d6 solution at both 400 and 600 MHz. The reagents were mixed at −78 °C, and spectra were obtained at intervals as the sample was warmed to room temperature. COSY spectra were recorded at key points to assist the assignments. NMR in acetone-d6: 9, δ(1H) 0.69 (s, 2JPtH 83 Hz, MePt), 6.95 (t, 1H, 3JHH = 7 Hz, 4JPtH = 20 Hz, H5), 7.15 (t, 1H, 3JHH = 7 Hz, H5A), 7.27 (t, 1H, 3JHH = 7 Hz, H5′), 7.29 (t, 1H, 3JHH = 7 Hz, H4A), 7.42 (d, 1H, 3JHH = 7 Hz, 3JPtH = 60 Hz, H4), 7.75 (t, 1H, 3JHH = 7 Hz, H5A′), 7.88 (d, 1H, 3JHH = 7 Hz, H6), 7.92 (d, 1H, 3JHH = 7 Hz, H6A), 7.97 (t, 1H, 3JHH = 7 Hz, H4′), 8.01 (d, 1H, 3JHH = 7 Hz, H3A), 8.08 (d, 1H, 3JHH = 7 Hz, H3A′), 8.18 (d, 1H, 3JHH = 7 Hz, 3JPtH = 21 Hz, H6′), 8.26 (t, 1H, 3JHH = 7 Hz, H4A′), 8.98 (d, 1H, 3JHH = 7 Hz, 3JPtH = 21 Hz, H6A′), 9.72 (d, 1H, 3 JHH = 7 Hz, H3′); 10, δ(1H) 0.87 (s, 6H, 2JPtH 82 Hz, MePt), 2.74 (s, 6H, 3J(PtH) = 21 Hz, MeS), 7.67 (t, 2H, 3JHH = 7 Hz, H5′), 8.28 (t, 2H, 3JHH = 7 Hz, H4′), 8.45 (d, 2H, 3JHH = 7 Hz, H3′), 9.20 (d, 2H, 3 JHH = 7 Hz, 3JPtH = 21 Hz, H6′). Resonances for the C5H3NO ring were not resolved. [PtMe(C5H4N-C5H3NO)(C5H5N)] (11). To a stirred suspension of complex 8 (0.050 g, 0.113 mmol) in acetone (5 mL) was added excess pyridine (0.1 mL). The reaction mixture was stirred for 1 h at room temperature to give a clear yellow solution. The solvent was concentrated to 1 mL, and pentane (5 mL) was added to precipitate the product as a yellow solid, which was collected, washed with pentane (3 × 2 mL) and ether (3 × 2 mL), and dried under vacuum. Yield: 79%. NMR in CDCl3: δ(1H) 0.92 (s, 3H,, 2JPtH = 82 Hz, MePt), 7.07 (t, 1H, 3JHH = 7 Hz, H5), 7.19 (t, 1H, 3JHH = 7 Hz, H5′), 7.54 (t, 2H, 3JHH = 8 Hz, Hm), 7.79 (d, 1H, 3JPtH = 64 Hz, 3JHH = 7 Hz, H4), 7.92 (d, 1H, 3JPtH = 22 Hz,3JHH = 6 Hz, H6′), 7.97 (m, H, Hp, H4′), 8.07 (d, 1H, 3JHH = 7 Hz, H6), 8.83 (d, 2H, 3JPtH = 28 Hz, 3JHH = 8 Hz, Ho), 9.92 (d, 1H, 3JHH = 8 Hz, H3′). HRMS (ESI): calcd for C16H16N3OPt [M + H]+ 460.0906, found 460.0920. Anal. Calcd for C16H15N3OPt: C, 41.74; H, 3.28; N, 9.13. Found: C, 41.55; H, 3.17; N, 9.02. Single crystals were grown by slow diffusion of n-pentane into a solution of the complex dissolved in dichloromethane. [PtMe(C5H4N-C5H3NO)PPh3] (12). To a stirred solution of complex 8 (0.050 g, 0.113 mmol) in acetone (5 mL) was added PPh3 (0.031 g, 0.113 mmol). The reaction mixture was stirred for 3 h at room temperature, and then the prcipitated solid was collected, washed with pentane (3 × 2 mL) and ether (3 × 2 mL), and dried under vacuum. Yield: 88% (0.064 g). NMR in CDCl3: δ(1H) 0.72 (s, 3H, 3JPH = 8 Hz, 2JPtH = 88 Hz, MePt), 6.69 (t, 1H, 3JHH = 6 Hz, H5′), 7.22 (t, 1H, 3JHH = 7 Hz, H5), 7.42 (m, 9H, Ph), 7.74 (m, 6H, Ph), 7.82 (t, 1H, 3JHH = 9 Hz, 3JPtH = 46 H4′), 7.89 (t, 1H, 3JHH = 6 Hz, H4), 7.96 (d 1H, 3JHH = 6 Hz, 3JPtH = 22 Hz, H6′), 8.14 (d, 1H, 3JHH = 7 Hz, H6), 9.98 (d, 1H, 3JHH = 9 Hz, H3′); δ(31P) 32.06 (s,1JPtP = 2304 Hz, PPh3). HRMS (ESI): calcd for C29H25N2OPPt [M]+ 643.1409, found 643.1396. Anal. Calcd for C29H25N2OPPt: C, 54.12; H, 3.92; N, 4.35. Found: C, 54.14; H, 3.80; N, 4.29. Single crystals were grown by slow diffusion of n-pentane into a solution of the complex dissolved in dichloromethane. [{PtMe(C5H4N-C5H3NO)}2(μ-dppm)] (13). To a stirred solution of complex 8 (0.050 g, 0.113 mmol) in acetone (5 mL) was added dppm (0.022 g, 0.056 mmol). The solution color changed rapidly from yellow to orange with precipitation of an orange solid. The reaction

immediately from yellow-green to red, with precipitation of a redbrown solid. The reaction mixture was stirred at room temperature for 1 h. The solid was collected, washed with pentane (3 × 3 mL), and dried under vacuum. NMR in CDCl3: δ(1H) 0.76 (s, 3H, 2JPtH = 90 Hz, PtMe), 1.02 (s, 3H, 2JPtH = 85 Hz, Pt-Me), 7.68 (m, 1H, H3), 7.75 (t, 1H, 3JHH = 7 Hz, H8), 7.85 (d, 1H, 3JHH = 8 Hz, H6), 7.90 (d, 1H, 3 JHH = 8 Hz, H5), 8.13 (d, 1H, 3JHH = 8 Hz, H7), 8.55 (d, 1H, 3JHH = 8 Hz, H9), 8.91 (d, 1H, 3JHH = 7 Hz, H4) 9.48 (d, 1H, 3JPtH = 28 Hz, 3 JHH = 5 Hz, H2). HRMS (ESI): calcd for C14H16N2OPtNa 446.0808, found 446.0810. [PtBrMe2(CH2C6H4-4-t-Bu)(phenO)] (4). To a solution of [PtMe2(PhenO)] (0.050 g, 0.118 mmol) in acetone (15 mL) was added 4-tert-butylbenzyl bromide (0.027 g, 0.118 mmol). Within 5 min, the color changed to yellow with precipitation of the product as a yellow solid. The solid was separated by filtration, washed with ether (3 × 2 mL) and pentane (3 × 2 mL), and dried under vacuum. Yield: 78% (0.060 g). NMR in CDCl3: 4b, δ(1H) 1.27 (s, 3H, 2JPtH = 80 Hz, PtMe),1.43 (s, 9H, t-Bu), 1.49 (s, 3H, 2JPtH = 72 Hz, PtMe), 3.10 (d, 1H, 2JHH = 9 Hz, 2JPtH = 54 Hz, PtCH2), 4.19 (d, 1H, 2JHH = 9 Hz, 2 JPtH = 104 Hz, PtCH2), 6.97 (d, 1H, 3JHH = 7 Hz, H9), 7.39 (d, 2H, 3 JHH = 8 Hz, Hm), 7.47 (dd, 1H, 3JHH = 6 Hz, 8 Hz, H8), 7.67 (d, 2H, 3 JHH = 8 Hz, 4JPtH = 4 Hz, Ar−H), 7.85 (m, 2H, H6,H3), 7.91 (d, 1H, 3 JHH = 8 Hz, H5), 8.09 (d, 1H, 3JHH = 8 Hz, H7), 8.48 (d, 1H, 3JHH = 7 Hz, H4) 9.10 (d, 1H, 3JPtH = 22 Hz, 3JHH= 6 Hz, H2); minor isomer 4a, δ(1H) 1.20 (s, 9H, t-Bu), 1.36 (s, 3H, 2JPtH = 72 Hz, PtMe), 1.60 (s, 3H, 2JPtH = 70 Hz, PtMe), 3.10 (d, 1H, 2JHH = 9 Hz, PtCH2), 4.19 (d, 1H, 2JHH = 9 Hz, PtCH2), aromatic peaks not assigned. HRMS (ESI): calcd for C25H29N2OPt [M − Br]+ 567.19065, found 567.19039. Single crystals were grown from dichloromethane/pentane solution. [Pt(OH)(OMe)Me2(phenO)] (5). Method A. [PtMe2(phenO)], (0.030 g, 0.071 mmol) was dissolved in methanol (5 mL), and the solution was stirred at room temperature for 3 h. The solvent was evaporated to yield the product as a brown solid, which was washed with pentane and dried under vacuum. Yield: 77%. Method B. To a solution of 1,10-phenanthroline N-oxide (phenO; 0.030 g, 0.153 mmol) in methanol (10 mL) was added [Pt2Me4(μSMe2)2] (1; 0.044 g, 0.076 mmol). The reaction mixture was stirred at room temperature for 3 h. The solvent was evaporated under vacuum to afford the product as a brown solid, which was washed with pentane (3 × 3 mL) and dried under vacuum. Yield: 69%. NMR in CD3OD: δ(1H) 1.41 (s, 3H, 2JPtH = 70 Hz, PtMe), 1.74 (s, 3H, 2JPtH = 71 Hz, Pt-Me), 2.81 (s, 3H, 3JPtH = 32 Hz, PtOMe), 7.73 (m, 1H, 3JHH = 9 Hz, 7 Hz, H8), 7.88 (d, 1H, 3JHH = 9 Hz, H6), 7.91 (d, 1H, 3JHH = 9 Hz, H5), 7.99 (m, 1H, 3JHH = 7 Hz, 9 Hz, H3), 8.49 (d, 1H, 3JHH = 9 Hz, H7), 8.72 (d, 1H, 3JHH = 9 Hz, H4), 8.84 (d, 1H, 3JHH = 6 Hz, H9), 9.14 (d, 1H, 3JPtH = 22 Hz, 3JHH = 7 Hz, H2). [Pt(OD)(OCD3)Me2(phenO)] (5*). [PtMe2(phenO)] (0.020 g, 0.047 mmol) was dissolved in CD3OD (0.75 mL), and the solution was stirred at room temperature for 3 h and then transferred to an NMR tube. NMR in CD3OD: δ(1H) 1.41 (s, 3H, 2JPtH = 70 Hz, PtMe), 1.73 (s, 3H, 2JPtH = 71 Hz, Pt-Me), 7.71 (m, 1H, 3JHH = 7 Hz, 9 Hz, H8), 7.80 (m, 1H, 3JHH = 7 Hz, 9 Hz, H3), 7.91 (d, 1H, 3JHH = 9 Hz, H6), 7.95 (d, 1H, 3JHH = 9 Hz, H5), 8.13 (d, 1H, 3JHH = 9 Hz, H7), 8.44 (d, 1H, 3JHH = 9 Hz, H4), 8.79 (d, 1H, 3JHH = 6 Hz, H9), 9.12 (d, 1H, 3JPtH = 22 Hz, 3JHH = 7 Hz, H2). [Pt(OD)(OCD3)Me2(phen)] (6). [PtMe2(phenO)] (0.020 g, 0.047 mmol) was dissolved in degassed CD3OD (0.75 mL), and the solution was stirred under a nitrogen atmosphere at room temperature for 3 h and then transferred to an NMR tube. NMR in CD3OD: 6, δ(1H) 1.88 (s, 6H, 2JPtH = 72 Hz, PtMe), 8.18 (m, 2H, 3JHH = 5 Hz, 8 Hz, H3, H8), 8.25 (s, 2H, H5, H6), 8.86 (d, 2H, 3JHH = 8 Hz, H4, H7), 9.38 (d, 2H, 3JHH = 5 Hz, 3JPtH = 12 Hz, H2, H9). A similar reaction in methanol, followed by isolation and redissolution in CD3OD gave an additional PtOMe resonance at δ(1H) 2.61 (s, 3J(PtH) = 40 Hz, OMe). The same spectra were obtained by reaction of complex 7 in CD3OD or MeOH in air.3b [PtMe(C5H4N-C5H3NO)(SMe2)] (8). To a solution of 2,2′bipyridine N-oxide (2b; 0.050 g, 0.29 mmol) in acetone (10 mL) 5411

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mixture was stirred for 1 h at room temperature, and then the solid was collected, washed with pentane (3 × 2 mL) and ether (3 × 2 mL), and dried under vacuum. Yield: 86% (0.056 g). NMR in CD2Cl2: δ(1H) 0.53 (m, 6H, 2JPtH = 81 Hz, 3JPH = 6 Hz, MePt), 4.07 (t, 2H, 3 JPH = 6 Hz, CH2), 6.42 (t, 2H, 3JHH = 6 Hz, H5′), 6.90 (t, 2H, 3JHH = 7 Hz, H5), 7.18 (t, 2H, 3JHH = 7 Hz, 3JPtH = 55 Hz, H4), 7.29 (m, 8H, Ph), 7.35 (m, 4H, Ph), 7.61 (t, 2H, 3JHH = 8 Hz, H4′), 7.79 (m, 8H, Ph), 7.84 (m, 4H, 3JHH = 7 Hz, H6,H6′), 9.62 (d, 2H, 3JHH = 8 Hz, H3′). δ(1H) 21.60 (s, 1JPtP = 2296 Hz, 3JPtP = 41 Hz, 2JPP = 40 Hz, dppm). HRMS (ESI): calcd for C47H42N4O2P2Pt2 [M]+ 1145.2114, found 1145.2104. Anal. Calcd for C47H42N4O2P2Pt2: C, 49.22; H, 3.69; N, 4.88. Found: C, 48.50; H, 3.37; N, 4.67. Single crystals were grown by slow diffusion of n-pentane into a solution of the complex in dichloromethane. [PtMe(C5H4N-C5H3NO)(dppe)] (14). To a stirred solution of complex 8 (0.050 g, 0.113 mmol) in acetone (5 mL) was added dppe (0.045 g, 0.113 mmol). The solution color changed from yellow to colorless. The reaction mixture was stirred for 1 h at room temperature. The volume of solvent was reduced to 1 mL, and pentane was added to precipitate the product as a white solid, which was collected, washed with pentane (3 × 2 mL) and ether (3 × 2 mL), and dried under vacuum. Yield: 72% (0.063 g). NMR in CDCl3: δ(1H) 0.34 (t, 3H, 2JPtH = 70 Hz, 3JPH = 8 Hz, MePt), 2.02−2.30 (m, 4H, CH2), 6.80 (t, 1H, 3JHH = 7 Hz, H5′), 6.88 (d, 1H, 3JHH = 7 Hz, H4′), 7.13 (t, 1H, 3JHH = 6 Hz, H5), 7.25 (m, 6H, Ph and H4), 7.39 (m, 10H, Ph), 7.49 (m, 3H, Ph), 7.30 (m, 2H, Ph), 7.98 (d, 1H, 3JHH = 6 Hz, H6), 8.49 (d, 1H, 3JHH = 7 Hz, H3′). δ(31P) = 35.60 (s, 1JPtP = 723 Hz, Pa), 35.98 (s, 1JPtP = 829 Hz, Pb). HRMS (ESI): calcd for C37H34N2OP2Pt [M]+ 779.1851, found 779.1859. Anal. Calcd for C37H34N2OP2Pt·CH2Cl2: C, 52.79; H, 4.20; N, 3.24. Found: C, 52.70; H, 3.86; N, 3.78.

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AUTHOR INFORMATION

S Supporting Information *

Figures and CIF and xyz files giving X-ray data for 4, 8, and 11−14, computed Cartesian coordinates of all of the molecules reported in this study, and NMR spectra of new compounds. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail for R.J.P.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the NSERC (Canada) for financial support. REFERENCES

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