Activation of Dioxygen by Dimethylplatinum(II) Complexes

Nov 3, 2017 - The ligands react with [Pt2Me4(SMe2)2] to give an equilibrium mixture, with the major constituent being [PtMe2(κ2-N,N′-L)] (1a, L = L...
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Activation of Dioxygen by Dimethylplatinum(II) Complexes Mahmood Azizpoor Fard, Ava Behnia, and Richard J. Puddephatt* Department of Chemistry, University of Western Ontario, London, Ontario, Canada N6A 5B7 S Supporting Information *

ABSTRACT: The ligands RN(CH2-2-C5H4N)2 (L1, R = CH2CH2OH; L2, R = CH2CH2CH2OH; L3, R = 2-C6H4OH) have been designed to give dimethylplatinum(II) complexes that can activate dioxygen in the absence of a protic solvent. The ligands react with [Pt2Me4(SMe2)2] to give an equilibrium mixture, with the major constituent being [PtMe2(κ2-N,N′-L)] (1a, L = L1; 1b, L = L2; 1c, L = L3). In the absence of air, 1a reacts with solvent CH2Cl2 to give [PtMe2(CH2Cl)(κ3-N,N′,N″L1]Cl, while 1c decomposes with loss of methane to give [PtMe(κ3-N,N′,N″-L3-H)] and then, by reaction with solvent, the binuclear complex [{PtMe(κ3-N,N′,N″-L3-H)}2(μ-H)]Cl. In the presence of oxygen the complexes 1 in CH2Cl2 solution react to give [Pt(OH)Me2(κ3-N,N′,N″-L)]Cl, when L = L1 or L2, or [Pt(OH)Me2(κ3-N,N′,OL-H)], when L = L3. The complex [Pt(OH)Me2(κ3-N,N′,N″-L2)]Cl decomposed in the presence of air to give the binuclear complex [{PtMe2(2-C5H4NCO2)(μ-OH)}2]. The factors influencing reactivity and mechanism in these reactions are elucidated, and the presence of both a free pyridyl donor (push group) and a free hydroxyl (pull group) is suggested to give a synergy for dioxygen activation by dimethylplatinum(II) complexes.

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INTRODUCTION Enormous efforts have been directed toward the use of dioxygen as a selective oxidant in the catalysis of hydrocarbon oxidation, but major challenges remain in controlling the selectivity and reactivity in the oxidation of alkanes to alcohols.1 In alkane oxidation using either metallic platinum or platinum complexes, a key step is the initial activation of dioxygen, and this has already stimulated much research into the reactivity of organoplatinum complexes with dioxygen.2 Several groups have shown that dialkylplatinum(II) complexes, which are stable to air in the solid state or in nonprotic organic solvents, can be oxidized in protic solvents.2 The reaction of dimethylplatinum(II) complexes (A, Scheme 1, NN = bidentate nitrogen donor ligand such as 2,2′-bipyridine) with dioxygen in methanol occurs in two steps, each of which can be considered as a proton-coupled two-electron transfer, first to give a hydroperoxide and then a hydroxide complex (Scheme 1, B, C).2−4 It has also been shown that ligands with an additional Lewis base group can enhance the reactivity in these oxidation reactions (Scheme 1, D−F).2,5 In addition, if the ligand contains an appended phenol or alcohol substituent, this can enhance reactivity in nonprotic solvents (Scheme 1, G−J).6 These effects can be understood in terms of stabilizing an intermediate platinum(IV) peroxide complex containing the Pt(IV)+−O− O− unit. The Lewis base can provide stabilization by donation to the incipient five-coordinate platinum(IV) center, while the phenol or alcohol substituent can provide stabilization by acting as a hydrogen bond donor to the peroxide group as it forms. In continuing this search to achieve facile oxidation of platinum(II) complexes by dioxygen, we have studied dimethylplatinum(II) complexes with the tridentate nitrogen donor ligands L1−L3, which contain an appended alcohol (L1 and L2, differing in the length of the alcohol spacer group) or phenol (L3) group (Scheme 2). It was anticipated that these © XXXX American Chemical Society

Scheme 1. Reactions of Dimethylplatinum(II) Complexes with O2a

a

NN = diimine or diamine ligand; X = N or O.

would form square-planar complexes [PtMe2(κ2-L)], using the amine and one pyridine group as donors. This would leave the remaining pyridine donor to act as a Lewis base and the alcohol or phenol to act as a hydrogen bond donor (Lewis acid) in stabilizing potential platinum(IV) peroxide intermediates such Received: August 10, 2017

A

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below). They were therefore characterized in solution by their H NMR spectra and used in situ for further reactions. The peaks assigned to the complexes 1 in the 1H NMR spectra were broad at room temperature, while peaks for other compounds in the equilibrium mixture remained sharp. This suggested that the complexes were fluxional, and indeed, the spectra were sharper at −20 °C. As an example, the 1H NMR spectra for complex 1b in CD3CN solution are shown in Figure 1. At low temperature, two methylplatinum resonances were

Scheme 2. Ligands Employed in This Study and a Proposed Reaction with Dioxygen

1

as K, and so give dual stabilization effects.5,6 The ligands have been reported previously and used in coordination complexes, mostly with first-row transition metals, as catalysts or as mimics for enzymic catalysts.7,8 The results are reported below.

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RESULTS AND DISCUSSION Platinum(II) Precursors. The ligands L1−L3 (Scheme 2) were not good ligands for the dimethylplatinum(II) unit and reacted with [Pt2Me4(μ-SMe2)2], using 2:1 stoichiometry, to give an equilibrium mixture of the product 1 with [PtMe2(SMe2)2] and free L and SMe2, as shown in Scheme 3. The reactions in CD2Cl2 or CD3CN solution were

Figure 1. 1H NMR spectra (400 MHz, CD3CN solution, peaks labeled according to Scheme 4) of complex 1b in the regions for aromatic protons (left) and methylplatinum protons (right) at (bottom) 25 °C and (top) −20 °C. Peaks marked with * are due to free ligand L2, and the peak marked with # is due to cis-[PtMe2(SMe2)2].

Scheme 3. Formation of the Dimethylplatinum(II) Complexes 1a−c

observed at δ 0.32 and 0.40, each with broad satellites with 2 J(PtH) = 84 Hz, in the range expected for methylplatinum(II) complexes. Separate resonances were observed for the free and coordinated 2-pyridylmethyl groups (Figure 1), supporting the proposed structure. The complex 1b has no symmetry; therefore, each of the five CH2 groups (two for the pyridylmethyl groups and three for the 3-hydroxypropyl group) has nonequivalent CHaHb protons and most of these were clearly resolved. The nonequivalent pyridyl and methylplatinum protons exchange at about the same rate, and a likely mechanism of fluxionality is shown in Scheme 4. The free and coordinated pyridyl arms exchange by an associative mechanism, likely to involve square-pyramidal and trigonal-

monitored by 1H NMR spectroscopy. It is well-known that there is a facile equilibrium between the complex cis[PtMe2(SMe2)2] and the binuclear complex [Pt2Me4(μSMe2)2] and free SMe2.9 In the present case, it is convenient to consider the reaction to take place in two stages. The first stage gives 1 equiv of the product 1 and cis-[PtMe2(SMe2)2] and, under normal NMR concentrations, proceeds essentially to completion. The second stage, which involves the further reaction of cis-[PtMe2(SMe2)2] with L to give another 1 equiv of 1 and 2 equiv of free SMe2, is an equilibrium, and so the final reaction mixture contains the product 1, the mononuclear complex cis-[PtMe2(SMe2)2], free SMe2, and unreacted ligand L. The equilibrium constants for this second step at 25 °C were approximately equal for the formation of 1a,b, with Keq = [9(1)] × 10−3 M, and were considerably higher at low temperature (K = 0.2 M for 1a at −20 °C). Attempts to isolate the complexes 1 (Scheme 3) in pure form from the reaction mixtures have been unsuccessful, since they decomposed or reacted with solvent during attempted recrystallization (see

Scheme 4. A Likely Mechanism of Fluxionality and NMR Labels

B

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coupling constants 2J(PtH) = 67 and 68 Hz, in the range expected for platinum(IV). The PtCH2Cl group also gives two resonances, at δ 3.93 (2J(PtH) = 79 Hz) and 3.78 (2J(PtH) = 33 Hz) and each of the three CH2N groups also gives two resonances for the nonequivalent CHaHb protons. Several other dimethylplatinum(II) complexes are known to oxidatively add dichloromethane, and this can limit the usefulness of this and other chlorinated solvents for these compounds.11 The phenol group in the ligand L3 is acidic, and complex 1c decomposed slowly in dichloromethane solution by protonolysis of one of the methylplatinum groups. The initial zwitterionic product 3 then adds HCl, derived from the solvent during recrystallization, to give crystals of the binuclear complex [{PtMe(κ3-N,N′,N″-L3-H)}2H]+Cl− (4) (Scheme 6). Complex 4 in CD2Cl2 solution gave a single methylplatinum

bipyramidal intermediates in which both pyridyl groups are coordinated in an 18-electron intermediate.10 The more symmetrical trigonal-bipyramidal intermediate 1* can collapse by dissociation of either pyridyl group, and this naturally leads to exchange of both the pyridyl and the methylplatinum group environments (Scheme 4). Reactions under Nitrogen Atmosphere. The dimethylplatinum(II) complexes 1a−c could be generated in solution under an inert atmosphere, but they were reactive to air, as described below, and they also decomposed slowly under an inert atmosphere. Two of these reactions have been studied in detail. Complex 1a reacted slowly with dichloromethane solvent to give a mixture of products, one of which was shown to be formed by oxidative addition (Scheme 5). The ionic complex [PtMe2(CH2Cl)(κ3-N,N′,N″-L1)]Cl (2) was isolated by recrystallization and characterized by structure determination (Figure 2).

Scheme 6. Formation of Complex 4

Scheme 5. Oxidative Addition of Dichloromethane

resonance in the 1H NMR spectrum at δ 1.13, with 2J(PtH) = 81 Hz (Figures S1 and S2 in the Supporting Information). Complexes 3 and 4 have effective Cs symmetry, with the mirror plane bisecting the two pyridylmethyl groups; therefore, in contrast to 1 and 2, only a single set of pyridyl resonances was observed. However, the methylene protons are diastereotopic and, in complex 4, gave two doublet resonances at δ 5.40 and 5.28. When the reaction in CD2Cl2 solution was monitored by 1 H NMR spectroscopy, the characteristic resonance for methane grew slowly over a period of about 1 day as 4 was formed and if the ligand L3-d, prepared by exchange of the phenol proton with excess D2O, was used, then CH3D was formed, but no CH2D2 or CHD3 was detected. Protonolysis of methylplatinum groups often occurs by an oxidative addition/ reductive elimination sequence involving a hydridoplatinum(IV) intermediate.12 In this case, no hydride intermediate could be detected; thus, the mechanism of protonolysis is not directly determined. However, DFT calculations indicate that the intramolecular SE2 mechanism is disfavored by geometrical constraints, whereas a hydride intermediate is viable. Complex 4 could be prepared in higher yield by direct reaction of trans[PtClMe(SMe2)2] with L3 (Scheme 6).

Figure 2. Structure of complex 2, showing 30% probability ellipsoids. Selected bond parameters: Pt(1)C(2) 2.078(6), Pt(1)C(1) 2.083(5), Pt(1)C(3) 2.045(7), Pt(1)N(2) 2.168(5), Pt(1)N(1) 2.155(4), Pt(1)N(3) 2.168(5), C(3)Cl(1) 1.742(7) Å; Pt(1)C(3)Cl(1) 118.0(4), C(1)Pt(1)C(2) 86.0(2), C(2)Pt(1)N(1) 91.1(2), C(2)Pt(1)C(3) 91.4(3), N(1)Pt(1)N(3) 84.2(2), N(2)Pt(1)C(1) 176.8(2)°.

In complex 2, the platinum(IV) center has octahedral stereochemistry with L1 acting as a fac-tridentate ligand. The complex has no symmetry, since one of the methyl groups is trans to the amine and the other trans to one of the pyridine donors. The chloromethyl group is trans to the other pyridyl group. This is the stereochemistry expected if the chloromethyl group adds to one side of the square-planar platinum(II) center in 1a while the free pyridine coordinates at the opposite face. The chloride ion in 2 is hydrogen-bonded to the hydroxyethyl group (Figure 2), and this interaction might accelerate the oxidative addition reaction by stabilizing the chloride leaving group. Because of the absence of symmetry, the 1H NMR spectrum of 2 contains two methylplatinum resonances, with C

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derived from solvent is necessary to give 5a. Complex 1b displayed similar reactivity in forming the product 5b (Scheme 7). The stereochemistry of 5a,b was deduced from the 1H and 13 C NMR spectra (Figures S3−S6 in the Supporting Information), which contain a single methylplatinum resonance and a single set of pyridylmethyl resonances, as expected for a complex with effective Cs symmetry. For example, complex 5a in CD3OD solution gave a single methylplatinum resonance in the 1H NMR spectrum at δ 1.32, with 2J(PtH) = 70 Hz, in the range expected for platinum(IV) complexes. The pyCH2 methylene protons are diastereotopic and gave well-separated resonances in the 1H NMR spectrum at δ 5.06 and 4.87, with coupling 2J(HaHb) = 17 Hz. Note that the initial reaction is expected to give the unsymmetrical complex with the hydroxo ligand trans to pyridine, and isomerization is likely to occur after dissociation of one pyridyl arm to give a five-coordinate intermediate.13 The structure of complex 5a was determined crystallographically and is shown in Figures 4 and 5. L1 binds as a fac-

The structure of complex 4 is shown in Figure 3. The ligand coordinates in the mer-κ3-N,N′,N″ bonding mode and so acts as

Figure 3. Structure of the cation in complex 4, showing 30% probability ellipsoids. Selected bond parameters: Pt(1)C(1) 2.035(8), Pt(1)N(1) 2.003(6), Pt(1)N(3) 2.015(6), Pt(1)N(2) 2.124(7) Å; C(1)Pt(1)N(3) 95.8(3), N(1)Pt(1)C(1) 97.4(3), N(2)Pt(1)N(3) 82.8(2)°. Symmetry equivalent atoms: x, y, z; 1 − x, 1 − y, 1 − z.

a pincer ligand to the square-planar platinum(II) center. There is one chloride ion for each dimer, and so one of the ligands is present in deprotonated form. There is a short contact O(1)··· O(1A) = 2.40 Å, indicating a strong hydrogen bond O(1)···H··· (O1A). The two symmetry-equivalent units in Figure 3 are related by an inversion center, and one proton is shared between them. This proton was not directly located but must be either at the inversion center or disordered on either side of it. Reactions under Air or Oxygen Atmosphere. The reaction of complex 1a in dichloromethane solution with oxygen occurred easily at room temperature to give complex 5a (Scheme 7). The reaction in CD2Cl2 solution was monitored

Figure 4. Structure of complex 5a, showing 30% probability ellipsoids. Selected bond parameters: Pt(1)C(1) 2.050(3), Pt(1)C(2) 2.040(3), Pt(1)O(1) 1.989(2), Pt(1)N(2) 2.153(3), Pt(1)N(1) 2.098(3), Pt(1)N(3) 2.131(3) Å; C(2)Pt(1)C(1) 91.1(1), C(1)Pt(1)N(2) 94.3(1), C(2)Pt(1)N(3) 92.0(1), N(3)Pt(1)N(2) 82.6(1)°.

tridentate ligand with the methylplatinum groups trans to the pyridyl groups and the hydroxide trans to the amine. The chloride anion is hydrogen-bonded to the PtOH group (O(1)··· Cl(1) 3.08 Å). In this geometry, the PtOH and CH2CH2OH groups are remote from one another and cannot interact directly, as had been expected. The formation of 5a requires one of the methylplatinum groups to switch from its position trans to the amine donor in 1a to a site trans to pyridine in 5a. Complex 5a forms a supramolecular polymer in the solid state through intermolecular hydrogen bonding between the alcohol group (as an H-bond donor) and the Pt−OH group (as an Hbond acceptor), with O(1)···O(2A) = O(2)···O(1B) = 2.60 Å (Figure 5). The reaction of complex 1c with oxygen occurred to give the complex [Pt(OH)Me2{κ3-N,N′,O-(L3-H)}] (6) (Scheme 8). When the reaction in CD2Cl2 solution was monitored by 1H NMR spectroscopy, several intermediate complexes were formed but the reaction ultimately gave a mixture of 6, formed by reaction with oxygen, and complex 4, formed by protonolysis of a methylplatinum group (Scheme 6). Complex 6 has no symmetry, and in the 1H NMR spectrum (Figures S7 and S8 in the Supporting Information) it gave two

Scheme 7. Formation of Complexes 5a,b

by 1H NMR spectroscopy. Several intermediate compounds were formed, but the spectra were too complex to allow them to be structurally characterized. The formation of 5a requires the abstraction of HCl from the solvent. Complex 5a was first obtained by the reaction of 1a with oxygen in acetone solution, followed by recrystallization of the initial product from dichloromethane. Complex 5a was also prepared by reaction of complex 1a with hydrogen peroxide, and again a chloride ion D

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Figure 6. Structure of complex 6 in the hydrogen-bonded dimer, showing 30% probability ellipsoids. Selected bond parameters: Pt(1)C(1) 2.034(2), Pt(1)C(2) 2.038(2), Pt(1)O(2) 2.001(1), Pt(1)O(1) 2.0105(9), Pt(1)N(2) 2.218(1), Pt(1)N(1) 2.178(1) Å; C(2)Pt(1)C(1) 86.23(7), N(1)Pt(1)N(2) 77.26(5), O(2)Pt(1)O(1) 176.11(5)°. Figure 5. Polymeric structure of complex 5a formed by intermolecular hydrogen bonding.

Ligand Oxidation and a Binuclear Hydroxoplatinum(IV) Complex. A solution of complex 5b in methanol in air decomposed slowly at room temperature, and crystals of the binuclear complex [{PtMe2(2-C5H4NCO2)(μ-OH)}2] (7) were deposited in two different crystalline forms, identified as 7·4MeOH (7a) and 7·2MeOH (7b). The reaction clearly involves oxidation of one pyridylmethyl arm of the ligand L2 to picolinate (Scheme 9). We have not investigated the reaction in

Scheme 8. Formation of Complex 6

Scheme 9. Unanticipated Oxidation Reaction

methylplatinum resonances at δ 1.85 (2J(PtH) = 71 Hz) and 1.56 (2J(PtH) = 72 Hz), as expected for methylplatinum(IV) groups trans to nitrogen. Two sets of pyridylmethyl group resonances were observed. When this reaction was carried out in acetone solution, a mixture of 3 and 6 was formed and crystallization gave the mixed complex 3·6·3H2O (Figures S9 and S10 in the Supporting Information). The structure of complex 6 is shown in Figure 6. Each chiral, octahedral platinum(IV) center is coordinated by the factridentate deprotonated ligand L3-H, two methyl groups, and one hydroxide ligand. As in the precursor complex 1c, one methyl group is trans to pyridine and the other trans to the amine group, while the hydroxide and phenoxide groups are mutually trans. The crystals contain equal amounts of each enantiomer, and pairs of like enantiomers are connected into hydrogen-bonded dimers by a bridging water molecule. Figure 6 shows a pair of AA enantiomers. The major difference from 5a,b (Figure 5) is that the phenol group of ligand L3 is deprotonated and coordinated to platinum while one pyridyl group is not coordinated in 6, whereas both pyridyl groups are coordinated and the alcohol group is not coordinated in 5a,b. The difference can be attributed to the greater acidity of the phenol in L3 in comparison to the alcohol group in L1 and L2.

detail, but we note that the products have interesting structures and that the unanticipated oxidation illustrates a limitation of the potential use of these pyridylmethylamine ligands in oxidation catalysis. The 1H NMR spectrum of 7 in methanol showed two methylplatinum resonances, as expected (Figure S11 in the Supporting Information). There are few examples of binuclear organoplatinum(IV) hydroxide complexes known; therefore, the characterization of both 7a and 7b represents a considerable advance.14 The molecular structures of 7a,b are shown in Figure 7. In each case there is a crystallographic inversion center relating the halves of the molecule, so that the PtO2Pt core is strictly planar. Each bridging hydroxide group is trans to a methyl group (high trans influence) at one platinum center and a pyridine group (low trans influence) at the other; thus, there is a significant difference in the associated bond distances (Figure 7). In both 7a and 7b, two methanol molecules act as hydrogen bond E

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Figure 7. Structures of complexes 7a (top) and 7b (bottom), showing 30% probability ellipsoids. Selected bond parameters for 7a: Pt(1)C(2) 2.041(1), Pt(1)C(1) 2.029(2), Pt(1)O(2) 2.162(1), Pt(1)N(1) 2.012(1), Pt(1)O(1) 2.020(1), Pt(1)O(1) 2.206(1) Å; C(1)Pt(1)C(2) 90.14(7), O(1)Pt(1)O(1A) 79.64(4), C(1)Pt(1)O(1) 93.40(6)°.

Figure 8. Supramolecular structures of complexes 7a (top, polymer, pinacolate-bound methanol molecules are not shown) and 7b (bottom, sheet structure, the methanol molecules do not participate in intermolecular bonding between dimers in 7b and are not shown, for clarity).

donors to the two bridging hydroxide groups (O(1)···O(2S) 2.70 Å in 7a, O(1)···O(1S) 2.80 Å in 7b) and, in 7a, the extra two methanol molecules act as hydrogen bond donors to the pinacolate groups (O(3)···O(1S) 2.70 Å). Complexes 7a,b have different supramolecular structures, formed through intermolecular hydrogen bonding (Figure 8). Complex 7a forms a one-dimensional polymer in which each dimer is further connected to two neighbors through bridging methanol molecules, in which each methanol molecule and PtOH unit acts as both a hydrogen bond donor and acceptor. In complex 7b, the intermolecular hydrogen bonds occur between PtOH groups as donors and picolinate groups as acceptors, with O(1)···O(3) 2.76 Å (Figure 8). Each dimer forms four intermolecular hydrogen bonds to four different neighbors, with two H-bond donors and two H-bond acceptors. Propagation of this motif gives a two-dimensional sheet structure (Figure 8). Computational Studies. Some calculations by DFT (see the Experimental Section for details) were carried out to gain further insight into the reaction mechanisms. A likely mechanism for the reaction of 1c with dioxygen to form 6 is shown in Figure 9. The calculated structure of 1c suggests the presence of a hydrogen bond with the phenol group as donor and the electron-rich platinum center as acceptor, with Pt···H = 2.03 Å, but with no significant interaction between the free pyridyl group and platinum. The initial activation of dioxygen gives the proposed intermediate M and is aided by the push− pull effect, with the “electron push” by coordination of the pyridyl group to the site trans to the forming peroxide group and the “pull” by hydrogen bonding of the peroxide group to the phenol proton. The calculated structure has Pt−O 2.02 Å,

O−O 1.53 Å, OO−H 1.03 Å, and CO−H 1.60 Å. The distance O−O = 1.53 Å is similar to the distance in H2O2 of 1.56 Å, calculated using the same DFT parameters, and is much longer than the calculated distance for triplet dioxygen of 1.29 Å. The observation that the OO−H bond is significantly shorter than the CO−H bond suggests that M is best considered as a platinum(IV) hydroperoxide complex that forms a hydrogen bond to the phenoxide group.2,5,6 The next proposed step is the comproportionation reaction of M with 1c to give the intermediate complex N.2 The calculated distances in N are Pt−O 2.01 Å, PtO−H 1.02 Å, and CO−H 1.80 Å, indicating that it best considered as a hydroxoplatinum(IV) complex hydrogen-bonded to a phenoxide group. The final step involves substitution of the phenoxide group for one of the pyridyl donors in N to give the product 6. Substitution reactions at platinum(IV) occur by a dissociative mechanism, and this is perhaps accompanied by a turnstile motion in forming complex 6 from N. All of these steps are calculated to be favorable (Figure 9). The complex M, proposed to be formed by the initial activation of dioxygen (Figure 9), is not directly observed. It is interesting to estimate how much of the energy of formation is contributed by the “push” and “pull” effects of the free pyridine and the phenol group, respectively, and this is addressed computationally in the reactions shown in Scheme 10, with results summarized in Table 1. Along the sequence M, R, S, T, U, the calculated energy of reaction becomes less favorable (ΔE = −110 kJ mol−1 for M and +55 kJ mol−1 for U), the Pt−O F

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Table 1. Calculated Distances (Å) and Energies (ΔE, kJ mol−1) of Reactions of Scheme 10 reagent/product ΔE Pt−O O−O OO−H CO−H

1c/M

1a/R

O/S

P/T

Q/U

−110 2.02 1.53 1.03 1.60

−61 2.03 1.52 1.08 1.45

−9 2.07 1.47

+31 2.09 1.44 1.48 1.06

+55 2.19 1.40

distance increases (Pt−O = 2.02 Å for M and 2.19 Å for U), and the O−O bond distance decreases (O−O = 1.53 Å for M and 1.40 Å for U). For comparison, the calculated distance for the superoxide anion, O2•−, is 1.46 Å. Complexes M and R have both push and pull substituents, but M has the stronger phenol pull group. Complex S has the pyridyl push substituent but no pull group, while T has the phenol pull group but no push group. The calculated stronger interaction for S over T (Table 1) indicates that the push effect should be greater than the pull effect in these examples. Complex U has neither push nor pull substituent and gives the weakest Pt−O2 interaction. There is clearly a synergy between the push and pull effects which favors formation of M and R. It is instructive to compare the calculated structures of M and T, which both contain the phenol pull substituent while only M has the extra pyridyl push group. The calculated hydrogen bond distances are very different with OO−H = 1.03 and 1.48 Å and CO−H = 1.60 and 1.06 Å in M and T, respectively. The absence of a push substituent in T leads to less negative charge on the PtOO group and so a much less marked transfer of the phenol proton to the PtOO group. M approximates to a PtOOH group with a weak hydrogen bond to phenoxide, PtOO−H···OC6H4, while in T the corresponding group is more accurately written as PtOO···H−OC6H4, with the stronger, shorter O−H bond to the phenol. Figure 10 shows some frontier orbitals for M and U which have the strongest and weakest interactions with dioxygen, respectively. The orbitals for U give insight into the reaction mechanism. Recall that free triplet O2 has single-electron occupation of each of the orthogonal 2pπ* orbitals. The HOMO for U has largely 2pπ* character and is doubly occupied, consistent with a single-electron transfer from the HOMO of precursor Q, which is the filled 5dz2 orbital of platinum(II), to dioxygen, analogous to the proposed first step in dioxygen coordination to palladium(0).4 The Pt−O σ bond is then formed by overlap of the other singly occupied 2pπ* orbital of dioxygen with the now singly occupied 5dz2 orbital of platinum. In U, this interaction is weak and the HOMO-1 and LUMO represent the Pt−O σ orbital and σ* orbital, respectively (Figure 10). In M, the interaction is much stronger and the Pt−O σ-orbital and σ*-orbital are at lower and higher energy, respectively. The HOMO in M is mostly associated with the phenoxide group, and the LUMO is a π* orbital associated with the push pyridyl group. The acidity of the phenol group makes it a good “pull” group in the activation of dioxygen by complex 1c, but it also leads to competitive protonolysis of a methylplatinum group to give 3 and 4 (Scheme 6). Protonolysis of electron-rich methylplatinum(II) complexes usually occurs by oxidative addition to give a platinum(IV) hydride followed by C−H reductive elimination.12 A likely sequence of reactions involves reaction of 1c to give V, followed by C−H reductive

Figure 9. Calculated structures and relative energies for the reaction of 1c with dioxygen to give complex 6.

Scheme 10. Model Compounds for Estimating Substituent Effects on Dioxygen Activation

G

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the reactivity to oxygen activation and also to the undesired protonolysis.

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CONCLUSIONS The aim of this research was to determine if square-planar dimethylplatinum(II) complexes with ligands designed to have a free Lewis base on one side and a free Lewis acid on the other could activate dioxygen. This was indeed shown to be feasible by the synthesis of complexes 5 and 6 (Schemes 7 and 8) by reaction of the appropriate dimethylplatinum(II) precursor complex 1a−c (Scheme 3) with dioxygen. The initial reaction with dioxygen is proposed to give a hydroperoxide intermediate, and the more acidic phenol substituent in 1c leads to higher reactivity. A synergic effect between the Lewis base and Lewis acid groups in forming the hydroperoxide intermediate is supported by DFT calculations. Some limitations to this approach have been identified. In particular, the complexes 1a−c are reactive in other ways, and they may be oxidized by reaction with dichloromethane and 1c can undergo intramolecular protonolysis of a methylplatinum group. In addition, unexpected oxidation of the ligand L3 occurred from complex 5b. Further optimization of ligand design is likely to lead to more reactive and selective organoplatinum compounds to model a key step in many catalytic oxidation reactions.

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EXPERIMENTAL SECTION

NMR spectra were recorded using Varian Mercury 400 NMR, Inova 400, and Inova 600 spectrometers. MALDI-TOF mass spectra were recorded using an AB Sciex 5800 TOF/TOF mass spectrometer using pyrene as the matrix in a 20/1 matrix/substrate molar ratio. 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. Single-crystal X-ray diffraction measurements were made using a Bruker APEX-II CCD diffractometer with graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation. Single crystals of the complexes were immersed in paraffin oil and mounted on MiteGen micromounts. The structures were solved using direct methods and refined by the full-matrix least-squares procedure of SHELXTL. Crystallographic data are given in Table S1 in the Supporting Information, and more complete X-ray data are given in the CIF files (CCDC 1570529−1570534, 1577968). The complex [Pt2Me4(μ-SMe2)2] and the ligands 2-hydroxyethylbis(2-pyridylmethyl)amine (L1), 3-hydroxypropyl-bis(2pyridylmethyl)amine (L2), and 2-hydroxyphenyl-bis(2pyridylmethyl)amine (L3) were synthesized according to literature procedures.7−9 Formation of [PtMe2(κ2-N,N′-L1)] (1a). To a solution of [Pt2Me4(μ-SMe2)2] (0.036 g, 0.062 mmol) in dry CD3CN (0.5 mL) under a nitrogen atmosphere in an NMR tube was added a solution of L1 (0.030 g, 0.124 mmol) in dry CD3CN (0.5 mL). The 1H NMR spectrum after 1 h. indicated an equilibrium was present with 1a as major component. NMR in CD3CN (600 MHz, −20 °C): δ(1H) 8.66 (d, 1H, 3J(HH) = 5 Hz, 3J(PtH) = ca. 15 Hz, H6a), 8.52 (d, 1H, 3 J(HH) = 5 Hz, H6b), 7.92 (dd, 1H, 3J(HH) = 7 Hz, 8 Hz, H4a), 7.78 (dd, 1H, 3J(HH) = 7 Hz, 8 Hz, H4b), 7.74 (d, 1H, 3J(HH) = 8 Hz, H3b), 7.40 (d, 1H, 3J(HH) = 8 Hz, H3a), 7.33 (dd, 1H, 3J(HH) = 5 Hz, 7 Hz, H5b), 7.29 (dd, 1H, 3J(HH) = 5 Hz, 7 Hz, H5a), 4.09 (m, 2H, H7a, H7b), 3.85 (m, 2H, H7a′, H7b′), 3.06 (m, 2H, H2), 2.50 (m, 2H, H1), 0.59 (s, 3H, 2J(PtH) = ca. 80 Hz, MePt), 0.48 (s, 3H, 2J(PtH) = ca. 80 Hz, MePt). The solution was then exposed to air and allowed to evaporate slowly. The 1H and 13C NMR spectra of the residue in CD3OD proved the formation of the oxidized complex [Pt(OH)Me2(L1)]+, 5a. Complex 1b was prepared in solution similarly. NMR in CD3CN (400 MHz, −20 °C): δ(1H) 8.58 (d, 1H, 3J(HH) = 6 Hz, 3J(PtH) = ca. 20 Hz, H6a), 8.42 (d, 1H, 3J(HH) = 5 Hz, H6b), 7.82 (t, 1H, 3 J(HH) = 8 Hz, H4a), 7.62 (t, 1H, 3J(HH) = 8 Hz, H4b), 7.37 (d, 1H,

Figure 10. Frontier orbitals for complexes M and U.

elimination (probably after dissociation of one arm of the ligand to give a five-coordinate intermediate)13 to give the methane complex W, followed by rapid displacement of methane by the free pyridyl arm of W to give 3 (Figure 11). No hydride intermediate was detected, but no viable intermediate for direct intramolecular SE2 cleavage could be found by DFT, and so this alternative mechanism is considered unlikely. The reaction of 1c to give 3 does indicate the limitation on the use of more acidic pull substituents in oxygen activation. There is evidently a fine balance between increasing

Figure 11. Potential hydridoplatinum(IV) intermediates in the protonolysis reaction. H

DOI: 10.1021/acs.organomet.7b00614 Organometallics XXXX, XXX, XXX−XXX

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Organometallics 3

J(HH) = 8 Hz, H3b), 7.27 (d, 1H, 3J(HH) = 8 Hz, H3a), 7.18 (m, 1H, H5b), 7.16 (m, 1H, H5a), 4.16 (d, 1H, 2J(HH) = 13 Hz, H7b), 4.10 (d, 1H, 2J(HH) = 15 Hz, H7a), 4.08 (d, 1H, 2J(HH) = 13 Hz, H7b′), 3.75 (d, 1H, 2J(HH) = 15 Hz, H7a′), 3.2−3.3 (m, 2H, H3,H3′) 2.75 (m, 1H, H1), 2.41 (m, 1H, H1′), 1.95−2.05 (m, 2H, H2,H2′), 0.40 (s, 3H, 2 J(PtH) = 84 Hz, PtMe), 0.32 (s, 3H, 2J(PtH) = 84 Hz, PtMe). The solution was then exposed to air and allowed to evaporate slowly. The 1 H and 13C NMR spectra of the residue in CD3OD proved the formation of the complex [Pt(OH)Me2(L2)]+ (5b). [PtMe2(CH2Cl)(κ3-N,N′,N″-L1)]Cl (2). To a stirred solution of [Pt2Me4(μ-SMe2)2] (0.098 g, 0.170 mmol) in dry CH2Cl2 (5 mL) was added a solution of ligand L1 (0.083 g, 0.341 mmol) in dry CH2Cl2 (5 mL). The solution was stirred for 36 h under a nitrogen atmosphere. The solution was filtered, and the product was crystallized from the filtrate by slow diffusion of pentane (50 mL). The single crystals were collected, washed with pentane, and dried in vacuo. Yield: 0.046 g, 24%. Anal. Calcd for C17H25Cl2N3OPt: C, 36.90; H, 4.55; N, 7.59. Found: C, 36.81; H, 4.57; N, 7.65. MALDI MS: calcd for [2]+ m/z 517.1, obsd m/z 517.9 (Figure S12 in the Supporting Information). NMR in CD3OD (400 MHz, 25 °C): δ(1H) 8.59 (m, 2H, H6a, H6b), 8.00 (m, 2H, H4a, H4b), 7.63 (m, 2H, H3a, H3b), 7.51 (m, 2H, H5a, H5b), 5.04 (d, 1H, 2J(HH) = 16 Hz, H7a), 4.96 (d, 1H, 2J(HH) = 17 Hz, H7b), 4.90 (d, 1H, 2J(HH) = 16 Hz, H7a′), 4.73 (d, 1H, 2J(HH) = 17 Hz, H7b′), 4.16 (t, 2H, 3J(HH) = 5 Hz, H2), 3.93 (d, 1H, 2J(HH) = 9 Hz, 2J(PtH) = 79 Hz, PtCHaHbCl), 3.78 (d, 1H, 2J(HH) = 9 Hz, 2 J(PtH) = 33 Hz, PtCHaHbCl), 3.66 (dt, 1H, 2J(HH) = 14 Hz, 3J(HH) = 5 Hz, H1), 3.55 (dt, 1H, 2J(HH) = 14 Hz, 3J(HH) = 5 Hz, H1′), 1.52 (s, 3H, 2J(PtH) = 67 Hz, MePt), 0.71 (s, 3H, 2J(PtH) = 68 Hz, MePt). [{PtMe(L3-H)}2(μ-H)]Cl (4). To a solution of [Pt2Me4(μ-SMe2)2] (0.084 g, 0.145 mmol) in CH2Cl2 (5 mL) was added a solution of ligand L3 (0.085 g, 0.291 mmol) in CH2Cl2 (5 mL). The solution was allowed to react for 72 h, and then the product was crystallized as red block crystals by slow diffusion of pentane. Yield: 0.030 g, 20%. MALDI MS: calcd for [PtMe(L3)]+ m/z 501.1, obsd m/z 501.4. NMR in CD3OD (400 MHz, 25 °C): δ(1H) 8.68 (d, 2H, 3J(HH) = 5 Hz, H6a), 8.68 (d, 1H, 3J(HH) = 8 Hz, H3), 8.00 (dd, 2H, 3J(HH) = 7 Hz, 8 Hz, H4a), 7.47 (dd, 2H, 3J(HH) = 5 Hz, 7 Hz, H5a), 7.45 (d, 2H, 3 J(HH) = 8 Hz, H3a), 7.12 (dd, 1H, 3J(HH) = 8 Hz, 9 Hz, H5), 6.96 (d, 1H, 3J(HH) = 9 Hz, H6), 6.60 (t, 1H, 3J(HH) = 8 Hz, H4), 5.40 (d, 2H, 2J(HH) = 16 Hz, H7a), 5.28 (d, 2J(HH) = 16 Hz, H7a′), 1.13 (s, 3H, 2J(PtH) = 81 Hz, MePt); δ(13C) 167.4, 153.9, 150.0, 140.7, 134.6, 131.4, 130.6, 126.5, 125.3, 120.0, 118.9, 67.0 (CH2), −11.7 (PtMe). [Pt(OH)Me2(L1)]Cl (5a). To a stirred solution of [Pt2Me4(μSMe2)2] (0.056 g, 0.098 mmol) in acetone (5 mL) was added a solution of ligand L1 (0.048 g, 0.196 mmol) in acetone (5 mL). The solution was stirred for 12 h under an oxygen atmosphere to give the product as an off-white precipitate. Pentane (40 mL) was added, and the solid product was separated, washed with pentane, dried in vacuo, and then recrystallized from CH2Cl2/pentane to give colorless plate crystals. Yield: 0.046 g, 44%. Mp: 243−245 °C. Anal. Calcd for C16H24ClN3O2Pt: C, 36.89; H, 4.64; N, 8.07. Found: C, 36.83; H, 4.57; N, 7.86. MALDI MS: calcd for 5a+ m/z 485.1, obsd m/z 485.4. NMR in CD3OD (400 MHz, 25 °C): δ(1H) 8.72 (d, 2H, 3J(HH) = 5 Hz, H6a), 8.02 (t, 2H, J(HH) = 8 Hz, H4a), 7.63 (d, 2H, 3J(HH) = 8 Hz, H3a), 7.57 (dd, 2H, 3J(HH) = 5 Hz, 8 Hz, H5a), 5.06 (d, 2H, 2 J(HH) = 17 Hz, H7a), 4.87 (d, 2H, 2J(HH) = 17 Hz, H7a′), 4.09 (t, 2H, 3J(HH) = 5 Hz, H2), 3.57 (t, 2H, 3J(HH) = 5 Hz, H1), 1.32 (s, 6H, 2J(PtH) = 70 Hz, MePt); δ(13C) 159.1, 146.3, 141.7, 126.7, 124.6, 70.7 (ArCH2), 66.8 (CH2N), 58.6 (CH2OH), −2.6 (PtMe). The OH resonance was not resolved. [Pt(OH)Me2(L2)]Cl (5b). This was prepared as for 5a from [Pt2Me4(μ-SMe2)2] (0.050 g, 0.087 mmol) and ligand L2 (0.045 g, 0.174 mmol). Yield: 0.036 g, 45%. MALDI MS: calcd for 5b+ m/z 499.1, obsd m/z 499.5. NMR in CD3OD (400 MHz, 25 °C): δ(1H) 8.72 (d, 2H, 3J(HH) = 6 Hz, H6a), 8.02 (dd, 2H, 3J(HH) = 7 Hz, 8 Hz, H4a), 7.63 (d, 2H, 3J(HH) = 8 Hz, H3a), 7.57 (dd, 2H, 3J(HH) = 6 Hz, 7 Hz, H5a), 4.89 (d, 2H, 2J(HH) = 16 Hz, H7a), 4.77 (d, 2H, 2J(HH) = 16 Hz, H7a′), 3.72 (t, 2H, 3J(HH) = 6 Hz, H3), 3.56 (m, 2H, H1), 2.15 (m, 2H, H2) 1.35 (s, 6H, 2J(PtH) = 70 Hz, MePt); δ(13C) 157.2,

144.7, 140.2, 125.2, 122.8, 68.6 (ArCH2), 61.7 (CH2N), 58.4 (CH2OH), 25.6 (CH2), −4.3 (PtMe). [Pt(OH)Me2(L3-H)] (6). This was prepared as for 5a from [Pt2Me4(μ-SMe2)2] (0.088 g, 0.154 mmol) and ligand L3 (0.090 g, 0.308 mmol). Block colorless crystals were obtained after 5 days. Yield: 0.101 g, 62%. Anal. Calcd for C20H23N3O2Pt·0.5H2O·CH2Cl2: C, 40.26; H, 4.18; N, 6.71. Found: C, 40.48; H, 4.21; N, 6.71. NMR in CDCl3 (400 MHz, 25 °C): δ(1H) 8.80 (d, 1H, 3J(HH) = 5 Hz, H6b), 8.70 (d, 1H, 3J(HH) = 5 Hz, 3J(PtH) ca. 15 Hz, H6a), 7.73 (dd, 1H, 3 J(HH) = 8 Hz, 7 Hz, H4b), 7.70 (dd, 1H, 3J(HH) = 8 Hz, 7 Hz, H4a), 7.39 (dd, 1H, 3J(HH) = 5 Hz, 8 Hz, H5b), 7.35 (dd, 1H, 3J(HH) = 5 Hz, 8 Hz, H5a), 7.30 (d, 1H, 3J(HH) = 7 Hz, H3a), 7.25 (d, 1H, 3 J(HH) = 7 Hz, H3b), 6.76 (dd, 1H, 3J(HH) = 7 Hz, 8 Hz, H5), 6.68 (d, 1H, 3J(HH) = 8 Hz, H3), 6.31 (d, 1H, 3J(HH) = 8 Hz, H6), 6.15 (dd, 1H, 3J(HH) = 8 Hz, 7 Hz, H4), 5.53 (d, 1H, 2J(HH) = 14 Hz, H7a), 5.02 (d, 1H, 2J(HH) = 14 Hz, H7a′), 4.92 (d, 1H, 2J(HH) = 14 Hz, H7b), 5.02 (d, 1H, 2J(HH) = 14 Hz, H7b′), 1.85 (s, 3H, 2J(PtH) = 71 Hz, MePt), 1.56 (s, 3H, 2J(PtH) = 72 Hz, MePt); δ(13C) 168.3, 158.9, 155.3, 149.5, 146.1, 138.9, 136.6, 135.5, 129.0, 127.1, 125.3, 124.5, 123.4, 122.9, 119.9, 114.7, 65.8 (CH2), 64.9 (CH2), −3.3 (PtMe), −5.3 (PtMe). The OH resonance was not resolved. [Pt2(μ-OH)2Me4(C5H4N-2-CO2)2] (7). A sample of complex 5b was prepared from [Pt2Me4(μ-SMe2)2] (0.050 g, 0.087 mmol) and ligand L2 (0.045 g, 0.174 mmol), as above, and then dissolved in MeOH (6 mL). Colorless needle and plate crystals of 7a,b were obtained by slow evaporation of the methanol solution. The crystals were separated, washed with pentane, and dried in vacuo. Yield: 0.024 g, 38%. Anal. Calcd for C16H20N2O6Pt2·CH3OH: C, 26.92; H, 3.19; N, 3.69. Found: C, 26.53; H, 3.51; N, 3.78. NMR in CD3OD (400 MHz, 25 °C): δ(1H) 8.86 (d, 2H, 3J(HH) = 5 Hz, H6), 8.32 (dd, 2H, 3J(HH) = 8 Hz, 7 Hz, H4), 8.23 (d, 2H, 3J(HH) = 7 Hz, H3), 8.00 (dd, 2H, 3J(HH) = 5 Hz, 8 Hz, H5), 1.67 (s, 6H, 2J(PtH) = 70 Hz, MePt), 1.50 (s, 6H, 2 J(PtH) = 83 Hz, MePt); δ(13C) 149.6, 146.2, 142.2, 130.7, 129.1, −3.7 (PtMe), −5.6 (PtMe).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00614. epresentative 1H NMR and MALDI-MS spectra and Xray data (PDF) Cartesian coordinates of calculated structures (XYZ) Accession Codes

CCDC 1570529−1570534 and 1577968 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

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

Richard J. Puddephatt: 0000-0002-9846-3075 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the NSERC (Canada) for financial support. The authors declare no competing financial interests. I

DOI: 10.1021/acs.organomet.7b00614 Organometallics XXXX, XXX, XXX−XXX

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(13) (a) Grice, K. A.; Scheuermann, M. L.; Goldberg, K. I. Top. Organomet. Chem. 2011, 35, 1. (b) Puddephatt, R. J. Angew. Chem., Int. Ed. 2002, 41, 261. (14) (a) Lippert, B.; Miguel, P. J. S. Coord. Chem. Rev. 2016, 327− 328, 333.

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