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Organometallics 2007, 26, 2402-2413
Facile Aerobic Oxidation of dpms-Platinum(II) Ethylene Complexes (dpms ) di(2-pyridyl)methanesulfonate) Julia R. Khusnutdinova, Peter Y. Zavalij, and Andrei N. Vedernikov* Department of Chemistry and Biochemistry, UniVersity of Maryland at College Park, College Park, Maryland 20742 ReceiVed January 12, 2007
Di(2-pyridyl)methanesulfonato hydroxo platinum(II) ethylene complex 4b, LPtII(C2H4)OH, reacts readily with oxygen in aqueous solution to cleanly produce unsymmetrical 2-hydroxyethyl platinum(IV) complex unsym-6b, LPtIV(C2H4OH)(OH)2. The latter eliminates ethylene oxide and ethylene glycol in virtually quantitative yield in neutral aqueous solution at 80 °C and produces the dinuclear µ-hydroxo platinum(II) complex 7 as another reaction product. The oxidation reaction was shown to proceed via an anionic 2-hydroxyethyl hydroxo platinum(II) intermediate, 5b. The chloro analogues 4a and 5a are inert toward dioxygen but can be converted to 6b under air in the presence of 1 equiv of NaOH. As established by DFT calculations, both the formal charge on the platinum(II) center and the nature of ligands coordinated to it have a crucial effect on the energy of the HOMO of the complexes, which may be related to their ability to undergo an aerobic oxidation. Introduction The use of atmospheric oxygen for selective oxidation of organic compounds is a challenging but practical goal.1 In particular, oxygen is an attractive oxidant for the platinummediated alkane functionalization involving organoplatinum intermediates.2-5 Recently we have reported facile oxidation of aqua methylplatinum(II) complex 1 by atmospheric oxygen (Scheme 1-a) and clean elimination of methanol from the resulting dihydroxo methylplatinum(IV) unsym-2 in acidic (Scheme 1-b) and basic aqueous solutions,6 all enabled by di(2-pyridyl)methanesulfonate ligand (dpms).7 Importantly, platinum(II) complex 3 resulting from the elimination reaction is inert toward dioxygen. Hence, the presence of a hydrocarbyl attached to the PtII center is essential for facile oxidation reactions.6 To the best of our knowledge, such selective direct aerobic functionalization of monomethyl platinum(II) complexes involving methyl platinum(IV) intermediates is currently the unique feature of the dpms-based system. The scope of reactions presented in Scheme 1, allowing for the use of dioxygen for functionalization of hydrocarbyl platinum(II) complexes, is currently a focus of our research. In this work we report facile aerobic functionalization of ethylene dpms (L) complexes 4, which leads cleanly to a mixture of ethylene oxide and ethylene glycol (Scheme 2). Our data * Corresponding author. E-mail:
[email protected]. (1) Stahl, S. S. Angew. Chem., Int. Ed. 2004, 43, 3400. (2) (a) Gol’dshleger, N. F.; Es’kova, V. V.; Shilov, A. E.; Shteinman, A. A. Zh. Fiz. Khim. 1972, 46, 1353. (b) Shilov, A. E.; Shulpin, G. B. Chem. ReV. 1997, 97, 2879. (c) Shilov, A. E.; Shulpin, G. B. ActiVation and Catalytic Reactions of Saturated Hydrocarbons in the Presence of Metal Complexes; Kluwer: Boston, 2000. (3) Stahl, S.; Labinger, J. A.; Bercaw, J. E. Angew. Chem., Int. Ed. 1998, 37, 2181. (4) Rostovtsev, V. V.; Henling, L. M.; Labinger, J. A.; Bercaw, J. E. Inorg. Chem. 2002, 41, 3608. (5) Weinberg, D. R.; Labinger, J. A.; Bercaw, J. E. Organometallics 2007, 26, 167. (6) Vedernikov, A. N.; Binfield, S. A.; Zavalij, P. Y.; Khusnutdinova, J. R. J. Am. Chem. Soc. 2006, 128, 82. (7) Vedernikov, A. N.; Fettinger, J. C.; Mohr, F. J. Am. Chem. Soc. 2004, 126, 11160.
Scheme 1
Scheme 2
suggest that similar to the case of monomethyl platinum(II) complex 1 (Scheme 1), aerobic oxidation of 4 occurs via an alkyl platinum(II) intermediate, 5b (Schemes 2-a, 2-b; X ) OH), where the alkyl is 2-hydroxyethyl. Similarly to the chemistry presented in Scheme 1, the oxidation of 5b leads to an isolable platinum(IV) compound, unsym-6b, unsym-LPtIV(C2H4OH)(OH)2 (Scheme 2-b). In neutral aqueous solution unsym-6b produces ethylene oxide, ethylene glycol, and a dinuclear hydroxo-bridged complex L2PtII2(µ-OH)2,6 7 (Scheme 2-c). Neither acid nor base additives are required for the C-O reductive elimination from 6b. No less remarkably, the aerobic oxidation of hydroxo ethylene complex 4b also proceeds readily in pure water. In contrast, an aerobic oxidation of 4a is
10.1021/om070033o CCC: $37.00 © 2007 American Chemical Society Publication on Web 03/30/2007
Facile Aerobic Oxidation of dpms-Pt(II) Ethylene Complexes
impossible in neutral solution but is facile in the presence of 1 equiv of a strong base (Scheme 2-b). The ability of platinum(II) ethylene complexes 4 to add hydroxide anion and form 5 reversibly (Scheme 2-a) can be readily demonstrated in alkaline solutions and is similar to the reversible nucleophilic addition of alkoxide anions to the ethylene ligand of Zeise’s anion, according to a recent report.8 The question of whether or not the direct aerobic oxidation involves 4b or 5b is more complex and will be discussed here. Altogether, the reactions presented in Scheme 2 lead to noncarbonyl/ non-vinylic type products that are typical for the wellestablished Wacker chemistry.9-11 Another type of aerobic olefin oxidation includes amidate-bridged polynuclear platinum(III) complexes as catalysts. It leads to ketones and epoxides and includes PtIII alkyl intermediates.10,11 It may be important to emphasize that, in contrast to those systems, the reactions reported in this work involve the PtIV/PtII couple. Chemical transformations presented by Scheme 2, the effect of the ligand X on nucleophilic addition (2-a), aerobic oxidation (2-b), and subsequent C-O elimination (2-c) chemistry, will be discussed below.
Results and Discussion Synthesis of LPtII(C2H4)X Complexes (X ) Cl, OH). LPtII(C2H4)Cl, 4a. Preparation of chloro ethylene LPtII complex 4a was achieved by reacting equivalent amounts of Zeise’s salt and K(L) in aqueous solution at room temperature:
K[PtIICl3(η2-C2H4)] + K(L) f LPtII(η2-C2H4)Cl + 2KCl (1) 4a A white precipitate of the poorly water-soluble target compound formed after several minutes with concomitant disappearance of the yellow color of Zeise’s salt. The product was isolated in analytically pure form in 80% yield. Complex 4a was sparingly soluble in water and methanol, stable in aqueous and methanolic solutions at room temperature under an argon and O2 atmosphere, but decomposed slowly upon heating in water, releasing ethylene. Electrospray ionization mass spectra (ESI-MS) of a dilute aqueous solution of 4a showed the presence of 4a‚H+ ion as the only Pt-containing species. Low molecular symmetry of complex 4a containing a dpms ligand with two nonequivalent pyridine rings was evident from the observation of a pattern of eight multiplets of equal intensity in the aromatic region of the 1H NMR spectra of 4a. Platinum-coordinated ethylene exhibited two multiplets with the platinum-195 satellites at 4.9 and 5.1 ppm (2JPtH ) 55 Hz) integrating as 2H each per one dpms ligand. The ethylene resonances were downfield shifted as compared to the 4.6 ppm signal observed for Zeise’s salt.8,12 Single crystals of LPtII(CH2CH2)Cl suitable for X-ray diffraction analysis were prepared by slow crystallization from a reaction mixture. The platinum atom in 4a is four-coordinate (8) Benedetti, M.; Fanizzi, F. P.; Maresca, L.; Natile, G. Chem. Commun. 2006, 1118. (9) (a) Homogeneous Catalysis, Understanding the Art; Leeuwen, P. W. N. M., Ed.; Kluwer Academic Publishers: Boston, 2004. (b) Helfer, D. S.; Atwood, J. D. Organometallics 2004, 23, 2412. (10) Matsumoto, K.; Nagai, Y.; Matsunami, J.; Mizuno, K.; Abe, T.; Somazawa, R.; Kinoshita, J.; Shimura, H. J. Am. Chem. Soc. 1998, 120, 2900. (11) Matsumoto, K.; Mizuno, K.; Abe, T.; Kinoshita, J.; Shimura, H. Chem. Lett. 1994, 1325. (12) Hahn, C. Chem.-Eur. J. 2004, 10, 5888.
Organometallics, Vol. 26, No. 9, 2007 2403
Figure 1. ORTEP drawings of complex 4a, 50% probability ellipsoids. Selected bond distances, Å: Pt1-O1, 2.890; Pt1-C12, 2.180; Pt1-C11, 2.155; Pt1-N11, 2.062; Pt1-N12, 2.013; Pt1Cl1, 2.295; C11-C12, 1.378.
and has a square planar environment, with an angle between the platinum coordination plane and the mean plane of the ethylene ligand of 89.9° (Figure 1). The length of the C-C bond in the coordinated olefin, 1.378 Å, is greater than in free ethylene (1.337 Å) and is almost the same as in Zeise’s salt, 1.375 Å.12 The platinum-sulfonate oxygen distance, Pt1-O11, is slightly shorter than the sum of van der Waals radii of these atoms. LPtII(C2H4)OH, 4b. Preparation of hydroxo ethylene LPtII complex LPtII(CH2CH2)OH, 4b, was performed by reacting the chloro analogue 4a with 1 equiv of silver(I) oxide under an argon atmosphere in carefully deaerated water at room temperature:
2LPtII(η2-C2H4)Cl + Ag2O + H2O f 4a 2LPtII(η2-C2H4)OH + 2AgCl (2) 4b According to 1H NMR spectroscopy, the reaction was complete in 24 h and produced the target hydroxo complex 4b in a virtually quantitative yield. Pure 4b was isolated from the pale yellow solution, obtained upon centrifugation of silver compounds, in 76% yield. Aqueous solutions of analytically pure complex 4b were slightly alkaline (pH ≈ 8) and contained 3-4% of 2-hydroxoethyl LPtII complex 5b, according to 1H NMR spectroscopy. The elevated pH value was due to the weakly basic nature of the PtII(OH) fragment. A potentiometric titration of a pure sample of 4b in a standardized 0.0970 M HNO3 with a standardized 0.1000 M NaOH gave the Kb value of 1.0 × 10-11 (see Supporting Information for more details). On the basis of this result, the pH of a 50 mM solution of 4b in water should be equal to 7.8, matching our observations. Solutions of 4b in pure water decomposed very slowly under an argon atmosphere at room temperature. This process could be slowed down significantly at 5-6 °C. When exposed to air, 4b produced unsym-6b in virtually quantitative yield, according to 1H NMR spectroscopy. The low symmetry of complex 4b, featuring two nonequivalent pyridine rings, was confirmed by 1H and 13C NMR. Platinum-coordinated ethylene exhibited two multiplets at 4.7 and 5.0 ppm in 1H NMR spectrum of aqueous 4b, slightly more upfield than for the chloro analogue 4a, each multiplet integrating as 2H per one dpms ligand. Electrospray ionization mass spectra of a dilute aqueous solution of 4b showed the presence of the 4b‚H+ ion. Nucleophilic Addition of OH- to LPtII(C2H4)X Complexes to Produce Na[LPtII(C2H4OH)X] (X ) Cl, OH). Na[LPtII(CH2CH2OH)Cl], 5a. Nucleophilic addition of hydroxide to
2404 Organometallics, Vol. 26, No. 9, 2007
KhusnutdinoVa et al.
Table 1. DFT-Calculated Energy of the HOMO of Cationic, Neutral, and Anionic Platinum(II) dpms Complexes, eV LPtII(C2H4)OH2+
LPtII(C2H4)Cl
LPtII(C2H4)OH
[LPtII(C2H4OH)Cl]
[LPtII(C2H4OH)OH]
-9.08
-5.51
-5.26
-1.95
-1.51
LPtII(CH2CH2)Cl was accomplished by reacting solid 4a with 4 equiv of 0.2 M sodium hydroxide in deaerated D2O under an argon atmosphere at room temperature. The solid dissolved quickly upon stirring to produce a clear, colorless solution (eq 3):
LPt (η -C2H4)Cl + NaOH h 4a II
2
Na[LPtII(CH2CH2OH)Cl] (3) 5a
At 1 h after preparation the ratio of LPtII(CH2CH2)Cl, LPtII(CH2CH2OH)Cl-, and LPtII(CH2CH2)OH was 6:84:10. Upon mixing of the solution with 1 equiv of aqueous HBF4, 4a was produced (eq 6):
Na[LPtII(CH2CH2OH)Cl] + HBF4 f 5a LPtII(CH2CH2)Cl + NaBF4 + H2O (6) 4a
According to 1H NMR spectroscopy, the conversion of the starting material 4a was complete in 10 min. Two dpmscontaining species detected by the NMR were 2-hydroxyethyl chloro platinum(II) complex 5a (96 mol %), which resulted from a nucleophilic attack of OH- at the ethylene ligand in 4a, and 2-hydroxyethyl hydroxo complex 5b (4 mol %), which resulted from the Cl/OH ligand exchange in 5a (eq 4):
According to 1H NMR spectroscopy, the mixture contained complexes 4a and 4b in an 85:15 ratio, the resulting solution being slightly acidic (pH ) 3). The ratio did not change after 1 day. Hence, we could conclude that the nucleophilic addition of hydroxide to 4a (eq 3) is fully reversible. Na[LPtII(CH2CH2OH)OH], 5b. A chloride-free solution of the hydroxo analogue 5b could be prepared by reacting ethylene hydroxo complex 4b with a 1.4-fold excess of an aqueous sodium hydroxide solution under an argon atmosphere (eq 7):
Na[LPtII(C2H4OH)Cl] + NaOH f 5a Na[LPtII(C2H4OH)OH] + NaCl (4) 5b
LPtII(η2-C2H4)OH + NaOH h 4b Na[LPtII(CH2CH2OH)OH] (7) 5b
A 1H NMR spectrum of the major product 5a showed the presence of a dpms ligand with two nonequivalent pyridine rings and two sets of signals of diastereotopic hydrogen atoms of the 2-hydroxyethyl ligand. The signals of hydrogens attached to the R-carbon were well resolved, centered at 1.7 and 1.9 ppm, and had broad platinum-195 satellites with 2JPtH ) 41 and 38 Hz, respectively. In the 13C NMR spectrum of 5a the R-carbon atom exhibited a singlet with two platinum-195 satellites and 1JPtC ) 728 Hz. The presence of 5a in freshly prepared alkaline aqueous solutions of 4a (pH ) 10) was also confirmed by ESI mass spectrometry. A slow Cl/OH ligand exchange (eq 4) was further seen in the reaction mixture, causing the 5b/5a ratio to increase over time with a half-life of 16 h under the above conditions. The ligand exchange was complete after 8 days to produce 5b as the only water-soluble product. Formation of a small amount of dark precipitate was also evident after that time. Reversibility of the Nucleophilic Attack of Hydroxide Anion at the Ethylene Ligand in LPtII(CH2CH2)Cl. Reversibility of the nucleophilic addition of hydroxide to 4a leading to 5a (eq 3) was confirmed in a separate experiment. A 4.0 mM solution of Na[LPtII(CH2CH2OH)Cl] was prepared from 4a and approximately 1 equiv of NaOH. The solution contained 4a, 5a, and 4b in a 6:87:7 molar ratio as established by 1H NMR integration. The presence of complex 4a and some sodium hydroxide in the mixture could result from the reverse reaction given by eq 3. A partial Cl for OH ligand substitution in 4a then led to 4b (eq 5):
According to 1H NMR spectroscopy, the resulting clear yellowish solution contained the target complex as the only dpms-derived species. Thus, the conversion of 4b to 5b was virtually quantitative. 1H and 13C NMR spectra of 5b exhibited a set of signals characteristic of an unsymmetrical dpms complex. Two sets of diastereotopic hydrogen atoms of the 2-hydroxyethyl ligand with platinum-195 satellites (2JPtH ) 37 Hz for both R-CH protons) were seen in the 1H NMR spectrum of 5b. The complex decomposed slowly in strongly alkaline solutions (pH > 10) with a half-life of ca. 28 h at room temperature, leading to formation of an unidentified dark precipitate. Complex 5b was air-sensitive and produced unsym6b when oxygen was admitted to its solution. The identity of 5b prepared in deaerated H2O was confirmed by ESI-MS, which showed the presence of the target anionic complex. Reversibility of the Nucleophilic Attack of Hydroxide Anion at the Ethylene Ligand of LPtII(CH2CH2)OH. Nucleophilic addition of hydroxide to the ethylene ligand of hydroxo ethylene complex 4b was demonstrated in the following experiment. A dilute solution of LPtII(CH2CH2)OH in deaerated D2O was combined with approximately 1 equiv of sodium hydroxide under an argon atmosphere. According to 1H NMR spectroscopy, 10 min after preparation the reaction mixture contained [LPtII(CH2CH2OH)OH]- and LPtII(CH2CH2)OH in a 77:23 ratio. This ratio did not change in the course of 10 h, thereby suggesting that a dynamic equilibrium had been reached (eq 7). When this mixture was combined with 1 equiv of HBF4 in D2O, LPtII(CH2CH2)OH was found to be the only dpmsderived species present in solution (eq 8):
LPtII(η2-C2H4)Cl + NaOH f 4a LPtII(η2-CH2CH2)OH + NaCl (5) 4b
Na[LPtII(CH2CH2OH)OH] + HBF4 f 5b LPtII(CH2CH2)OH + NaBF4 + H2O (8) 4b
Facile Aerobic Oxidation of dpms-Pt(II) Ethylene Complexes
Figure 2. Transformations of a mixture of LPtII(CH2CH2OH)Cl-, 5a (empty triangles), LPtII(CH2CH2)Cl, 4a (filled diamonds), and LPtII(CH2CH2)OH, 4b (filled circles), prepared from equimolar amounts of 4a and NaOH in D2O under an argon atmosphere; [Pt]0 ) 50 mM, [NaOH]0 ) 50 mM.
On the basis of the established fact of reversibility of addition of OH- to 4b, we were able to estimate the equilibrium constant for reaction 7. A potentiometric determination of [D+]13 combined with 1H NMR measurements of the [5b]/[4b] ratio was performed in alkaline D2O solutions of 4b at several pD values to give the equilibrium constant K of (3 ( 1) × 106 at 23 °C (see Supporting Information for details). The 2-hydroxyethyl:ethylene complex ratios observed in the mixtures containing 4 and approximately 1 equiv of sodium hydroxide were slightly different for complexes 4b and 4a. More precisely, the 77:23 5b/4b ratio for the hydroxo ethylene complex 4b was lower than the 87:6 5a/4a ratio for the chloro analogue 4a, suggesting that the chloro ethylene complex 4a was more electrophilic than the hydroxo complex 4b. Relative Electrophilicity of LPtII(CH2CH2)Cl and LPtII(CH2CH2)OH Complexes. The greater electrophilicity of the chloro complex 4a as compared to its hydroxo analogue 4b could be estimated more reliably in an experiment when LPtII(CH2CH2)Cl was allowed to react with 1 equiv of aqueous NaOH. Two fast reversible OH- addition reactions (eqs 3 and 7) and two slow irreversible Cl for OH ligand exchange reactions (eqs 4 and 5) determined the composition of the reaction mixture. Therefore, the 5a/5b ratio measured at any moment of reaction should reflect the relative electrophilicity of the chloro and hydroxo ethylene complexes 4a and 4b. The composition of a fresh clear solution prepared from solid LPtII(CH2CH2)Cl and NaOH in D2O was monitored by 1H NMR spectroscopy. Concentrations of complexes LPtII(CH2CH2)OH, LPtII(CH2CH2)Cl, and LPtII(CH2CH2OH)Cl- were calculated from the integral intensity of the coordinated ethylene signals at 4.7 and 5.1 ppm for the first two compounds and from the integral intensity of the signals of the 2-hydroxyethyl ligand at 1.4-2.3 ppm for 5a, respectively. Complex 5b was not detected in the course of the whole experiment. The plot of concentrations of complexes 4a, 4b, and 5a versus time is given in Figure 2. The 2-hydroxyethyl chloro complex 5a was the major component in the beginning of this experiment along with less than 10% of complexes 4a and 4b combined. The concentration of 5a decreased over time due to reversible elimination of OH(13) (a) Glasoe, P. K.; Long, F. A. J. Phys. Chem. 1960, 64, 188. (b) Fu, X.; Wayland, B. B. J. Am. Chem. Soc. 2006, 128, 8947.
Organometallics, Vol. 26, No. 9, 2007 2405
with concomitant formation of 4a (eq 3). The reaction was driven by an irreversible Cl/OH ligand exchange in complex 4a (eq 5), which consumed NaOH released by 5a. Since no 2-hydroxyethyl platinum(II) complex 5b was observed by 1H NMR spectroscopy, the electrophilicity of 4b was not high enough to compete with 4a for the nucleophilic hydroxide anion. Trace amounts (
