Platinum(IV) DMSO Complexes: Synthesis, Isomerization, and Agostic

Mar 24, 2010 - Oxidation of Pt(II) complexes with S-bound DMSO ligands initially results in Pt(IV) complexes with ligands in the same relative positio...
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Organometallics 2010, 29, 1966–1976 DOI: 10.1021/om901087m

Platinum(IV) DMSO Complexes: Synthesis, Isomerization, and Agostic Intermediates Sarah H. Crosby, Guy J. Clarkson, Robert J. Deeth, and Jonathan P. Rourke* Department of Chemistry, Warwick University, Coventry, U.K. CV4 7AL Received December 18, 2009

Oxidation of cyclometalated Pt(II) complexes with S-bound DMSO ligands initially results in Pt(IV) complexes that retain the S-bound DMSO ligands in the same relative position. Isomerization reactions result in a rearrangement of the ligands to give O-bound DMSO complexes, with the DMSO trans to a cyclometalated carbon. X-ray structures representing the only two known examples of Pt(IV) complexes with O-bound DMSO ligands have been solved. The rate of isomerization of complexes without a pendant alkyl chain is strongly solvent dependent, consistent with the need to stabilize a coordinatively unsaturated intermediate. Pt(IV) complexes with a pendant alkyl chain show little dependence on isomerization rate with solvent, with solution NMR data strongly suggesting the presence of agostic complexes. DFT calculations provide support for the presence of agostic complexes, with the same interactions being used to account for the loss of DMSO from the O-bound DMSO complexes.

We have recently been studying the synthesis and reactivity of high oxidation state organometallic platinum complexes,1,2 with a view to elucidating some further details of the cyclometalation reaction.3 Our interest in the cyclometalation reaction comes about because of its role in the functionalization of C-H bonds,4-8 for which there is, as yet, no general method. The selective and general transformation of unreactive C-H bonds to other functional groups is of tremendous worldwide interest and significance,9-11 and cyclometalation reactions,3 such as those of phenylpyridines,1,2,12 are seen as intramolecular analogues of intermolecular C-H activations. Cyclometalated complexes

have uses in their own right,13-16 for instance as sensors17 or as models for the Fischer-Tropsch synthesis.18 The oxidation of platinum centers is also of fundamental importance to Shilov-type functionalizations of alkanes,19,20 and the Pt(II)/Pt(IV) redox cycle has been investigated as a model for the activation of C-H bonds.21,22 Agostic complexes are widely invoked as intermediates in the activation of C-H bonds in organometallic chemistry23,24 and have considerable precedent in the stabilization of coordinatively unsaturated metal centers. We have recently reported a platinum(II) agostic interaction within a complex with ligands very similar to those we discuss in this paper.25 This complex, and other formally 14e T-shaped Pt(II) complexes,26-28 contain electrophilic platinum centers that ought

*Corresponding author. E-mail: [email protected]. (1) Newman, C. P.; Casey-Green, K.; Clarkson, G. J.; Cave, G. W. V.; Errington, W.; Rourke, J. P. Dalton Trans. 2007, 3170. (2) Mamtora, J.; Crosby, S. H.; Newman, C. P.; Clarkson, G. J.; Rourke, J. P. Organometallics 2008, 29, 5559. (3) Ryabov, A. D. Chem. Rev. 1990, 90, 403. (4) Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H. Acc. Chem. Res. 1995, 28, 154. (5) Dyker, G. Angew. Chem., Int. Ed. 1999, 38, 1699. (6) Guari, Y.; Sabo-Etienne, S.; Chaudret, B. Eur. J. Inorg. Chem. 1999, 1047. (7) Ritleng, V.; Sirlin, C.; Pfeffer, M. Chem. Rev. 2002, 102, 1731. (8) Ryabov, A. D. Synthesis 1985, 233. (9) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507. (10) Bergman, R. G. Nature 2007, 446, 391. (11) Goldman, A. S.; Goldberg, K. I. In Activation and Functionalization of C-H Bonds; Goldberg, K. I., Goldman, A. S., Eds.; ACS Symposium Series 885; ACS: Washington, DC, 2004. (12) Newman, C. P.; Cave, G. W. V.; Wong, M.; Errington, W.; Alcock, N. W.; Rourke, J. P. J. Chem. Soc., Dalton Trans. 2001, 2678. (13) Develay, S.; Blackburn, O.; Thompson, A. L.; Williams, J. A. G. Inorg. Chem. 2008, 47, 11129. (14) Dupont, J.; Consorti, C. S.; Spencer, J. Chem. Rev. 2005, 105, 2527. (15) van der Boom, M. E.; Milstein, D. Chem. Rev. 2003, 103, 1759. (16) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed. 2001, 40, 3750.

(17) Thomas, S. W.; Venkatesan, K.; Muller, P.; Swager, T. M. J. Am. Chem. Soc. 2006, 128, 16641. (18) Reinartz, S.; Brookhart, M.; Templeton, J. L. Organometallics 2002, 21, 247. (19) Shilov, A. E.; Shul’pin, G. B. Activation and Catalytic Reactions of Hydrocarbons in the Presence of Metal Complexes; Kluwer: Dordrecht, 2000; Vol. 21. (20) Weinberg, D. R.; Labinger, J. A.; Bercaw, J. E. Organometallics 2007, 26, 167. (21) Dick, A. R.; Kampf, J. W.; Sanford, M. S. Organometallics 2005, 24, 482. (22) Hull, K. L.; Lanni, E. L.; Sanford, M. S. J. Am. Chem. Soc. 2006, 128, 14047. (23) Brookhart, M.; Green, M. L. H.; Parkin, G. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 6908. (24) Clot, E.; Eisenstein, O. Agostic interactions from a computational perspective: One name, many interpretations. In Principles and Applications of Density in Inorganic Chemistry II; Springer-Verlag: Berlin, 2004; Vol. 113, p 1. (25) Crosby, S. H.; Clarkson, G. J.; Rourke, J. P. J. Am. Chem. Soc. 2009, 131, 14142. (26) Carr, N.; Dunne, B. J.; Orpen, A. G.; Spencer, J. L. J. Chem. Soc., Chem. Commun. 1988, 926. (27) Carr, N.; Mole, L.; Orpen, A. G.; Spencer, J. L. J. Chem. Soc., Dalton Trans. 1992, 2653. (28) Ingleson, M. J.; Mahon, M. F.; Weller, A. S. Chem. Commun. 2004, 2398.

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Scheme 1

Scheme 2

to be ideally set up for subsequent C-H activation. The work we report here builds on this previous work with agostic complexes and that on C-H activation with platinum(II) species, work that lead to the only high-yielding route to CNC triply coordinating complexes29,30 and to the unusual C-H activation that gave a carbene species, rather than a cyclometalated species.31,32 This paper reports and discusses the synthesis of new platinum(IV) species, assesses the intermediacy of agostic complexes in their subsequent isomerization reactivity, and suggests implications for future work on C-H activation.

Results and Discussion Synthesis and Isomerization of the Pt(IV) DMSO Complexes. We have previously described the oxidation and subsequent reactivity of two related classes of cyclometalated platinum(II) complexes with a pyridine or phenyl pyridine co-ligand.1,2 In this paper we look at the oxidation and isomerization of a new class of cyclometalated platinum(II) complex: those with a dimethylsulfoxide ligand, 1, Scheme 1. Three variants of these complexes of 2-(4-fluorophenyl)pyridine are discussed: one with a 6-(n-propyl) substituent, complex 1c, one with a 6-(ethyl) substituent, complex 1b, and one without any substituent, complex 1a. The synthesis of complexes 1 proceeded cleanly, and in high yield, using routes we1,2 and others33 have described before. We have previously reported the use of hydrogen peroxide as an oxidant for platinum complexes; however complications arose from the large number of products that were formed (some of which could be identified as hydroxide complexes),1 and we have subsequently reported the use of the chlorine-based oxidant iodobenzene dichloride, whereupon reactions are much cleaner and such complications are avoided.2 Although others34 have recently reported the use (29) Cave, G. W. V.; Alcock, N. W.; Rourke, J. P. Organometallics 1999, 18, 1801. (30) Cave, G. W. V.; Fanizzi, F. P.; Deeth, R. J.; Errington, W.; Rourke, J. P. Organometallics 2000, 19, 1355. (31) Cave, G. W. V.; Hallett, A. J.; Errington, W.; Rourke, J. P. Angew. Chem., Int. Ed. 1998, 37, 3270. (32) Newman, C. P.; Clarkson, G. J.; Alcock, N. W.; Rourke, J. P. Dalton Trans. 2006, 3321. (33) Mdleleni, M. M.; Bridgewater, J. S.; Watts, R. J.; Ford, P. C. Inorg. Chem. 1995, 34, 2334. (34) Zucca, A.; Petretto, G. L.; Stoccoro, S.; Cinellu, M. A.; Manassero, M.; Manassero, C.; Minghetti, G. Organometallics 2009, 28, 2150.

Figure 1. X-ray crystal structure of 2a showing hydrogen bonding to a chloroform solvent molecule; thermal ellipsoids at 50% probability. Selected bond lengths (A˚) and angles (deg): Pt(1)C(108) 2.045(4); Pt(1)-N(101) 2.062(3); Pt(1)-Cl(1) 2.3215(8); Pt(1)-S(202) 2.3235(9); Pt(1)-Cl(2) 2.3253(8); Pt(1)-Cl(3) 2.4508(9); S(202)-O(202) 1.469(3); C(108)-Pt(1)-N(101) 80.81(13); C(108)-Pt(1)-Cl(1) 86.81(10); N(101)-Pt(1)-Cl(1) 88.44(8); C(108)-Pt(1)-S(202) 100.01(11); N(101)-Pt(1)-S(202) 178.98(8); Cl(1)-Pt(1)-S(202) 92.21(3); C(108)-Pt(1)-Cl(2) 89.91(10); N(101)-Pt(1)-Cl(2) 88.42(8); Cl(1)-Pt(1)-Cl(2) 175.78(3); S(202)-Pt(1)-Cl(2) 90.97(3); C(108)-Pt(1)-Cl(3) 174.18(10); N(101)-Pt(1)-Cl(3) 93.83(8); Cl(1)-Pt(1)-Cl(3) 90.79(3); S(202)-Pt(1)-Cl(3) 85.37(3); Cl(2)-Pt(1)-Cl(3) 92.23(3); O(202)-S(202)-Pt(1) 118.35(11).

of Au(III) to oxidize Pt(II), in this work we continue with the use of iodobenzene dichloride to oxidize the DMSO complexes 1, where we see that reaction in chloroform gives a clean and rapid conversion to the platinum(IV) species 2, Scheme 1. The oxidations of complexes 1 proceed to completion within five minutes at room temperature and initially lead to a single product. In the 1H NMR a single new DMSO peak is seen (with a reduced 3JH-Pt compared with complexes 1). Together with a new set of peaks for the cyclometalated phenylpyridine, a new peak is seen in the 19F NMR (again with reduced 4JF-Pt, compared with complexes 1). 195Pt chemical shifts of -1459, -1236, and -1237 ppm are observed for the products of oxidation of 1a, 1b, and 1c, respectively, confirming the oxidation to Pt(IV). The geometry of this initial product was identified as still having the DMSO co-ligand trans to the nitrogen donor of the pyridine by a sequence of NOE experiments: irradiation of the proton ortho to the cyclometalated carbon led to an enhancement of the methyl protons of the DMSO and vice versa, whereas irradiation of the ortho proton on the pyridine ring of 2a had no effect on the DMSO protons. Sulfur coordination of the DMSO ligand is indicated in the 1H NMR spectrum by the observation of platinum satellites on the DMSO methyl proton signals. The proposed geometry depicted in Scheme 2 was confirmed by solving the crystal structure of 2a, Figure 1. Crystals of 2a formed directly from the reaction of 1a with PhICl2, with redissolution of these crystals showing that they did indeed represent the bulk sample. Spectroscopic data for 2a, 2b, and 2c were very similar; therefore an equivalent geometry can be assigned to 2b and 2c.

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Figure 2. X-ray crystal structure of 3b; thermal ellipsoids at 50% probability. Selected bond lengths (A˚) and angles (deg): Pt(1)-C(12) 2.009(4); Pt(1)-N(1) 2.098(3); Pt(1)-O(15) 2.205(3); Pt(1)-Cl(2) 2.3030(10); Pt(1)-Cl(3) 2.3143(10); Pt(1)-Cl(1) 2.3201(10); N(1)-C(2) 1.363(6); N(1)-C(6) 1.371(6); O(15)-S(16) 1.550(3); C(12)-Pt(1)-N(1) 81.14(16); C(12)Pt(1)-O(15) 175.94(14); N(1)-Pt(1)-O(15) 102.24(13); C(12)Pt(1)-Cl(2) 88.50(12); N(1)-Pt(1)-Cl(2) 88.29(10); O(15)Pt(1)-Cl(2) 89.35(9); C(12)-Pt(1)-Cl(3) 93.45(13); N(1)Pt(1)-Cl(3) 174.59(10); O(15)-Pt(1)-Cl(3) 83.17(9); Cl(2)-Pt(1)-Cl(3) 91.63(4); C(12)-Pt(1)-Cl(1) 93.00(12); N(1)-Pt(1)-Cl(1) 89.32(10); O(15)-Pt(1)-Cl(1) 89.31(9); Cl(2)-Pt(1)-Cl(1) 176.96(4); Cl(3)-Pt(1)-Cl(1) 90.93(4); S(16)-O(15)-Pt(1) 116.92(17); O(15)-S(16)-C(17) 103.9(2); O(15)-S(16)-C(18) 103.8(2); C(17)-S(16)-C(18) 99.0(3).

While solid samples of complexes 2 do not appear to change with time, some solutions do. Both acetone and chloroform solutions of 2b and 2c begin to change with time, with a new compound appearing in the NMR spectra of solutions of each of 2b and 2c. Both the new complexes have a single DMSO resonance in the 1H NMR spectrum, indicating that the methyl groups of the DMSOs are equivalent. However, no platinum satellites were seen on these peaks, implying the DMSOs are no longer coordinated via the sulfur atom. The 4JF-Pt values and the platinum chemical shifts of the new complexes are typical of Pt(IV) species. We were able to grow crystals of the new materials that formed from both 2b and 2c and to establish unambiguously that the new compounds were isomers of 2, with an oxygen-bound DMSO ligand trans to the cyclometalated carbon, 3b and 3c, Scheme 2 and Figures 2 and 3. It is interesting to note that 3b and 3c are the first crystallographically characterized Pt(IV) complexes to contain an O-bound DMSO, with only two other crystallographically characterized platinum complexes containing an O-bound DMSO.35,36 DFT calculations (see later) confirm that the O-bound DMSO isomer is indeed the lowest energy form. Additionally we note that the rate of formation of the new complexes does not vary significantly between acetone and chloroform solutions and is slightly retarded on addition of extra DMSO. (35) Cotton, F. A.; Falvello, L. R.; Han, S. Inorg. Chem. 1982, 21, 2889.

Crosby et al.

The isomerization of 2a is rather more complicated and strongly solvent dependent: chloroform solutions remain essentially unchanged with time (no change after one month), whereas acetone solutions begin to show changes within five minutes. Within a few minutes of the oxidation of an acetone solution of 1a to give 2a, two new DMSO signals of equal intensity begin to appear in the 1H NMR spectrum, together with a new set of peaks representing the cyclometalated phenylpyridine and one new peak in the 19F NMR spectrum. The growth of only one peak in the 19F NMR spectrum suggests only a single new compound, and a 1H-195Pt correlation experiment was used to established that both the two new DMSO signals correlate to the same Pt(IV) nucleus (at -1609 ppm). This suggests that the two methyl groups of the DMSO have been rendered inequivalent due to a loss of symmetry within the new complex. This reaction of 2a occurs with analytically pure samples, suggesting the new product is simply an isomer of 2a. It is apparent to us that the DMSO remains coordinated via the sulfur donor since platinum satellites are still observed on the DMSO resonances in the 1H NMR spectrum. We therefore propose a structure of the new complex in which the DMSO ligand is out of the plane of the phenylpyridine ligand, with the chlorines in a fac configuration, 4a, Scheme 3. Scheme 3

The formation of 4a from 2a in acetone was never complete before another new complex began to be seen in solution; consequently we were never able to isolate pure samples of 4a. This new species became obvious in acetone solutions of 2a after about 24 h. Eventually all the material ended up as this third species, with the reaction taking about 7 days at room temperature; during this time the relative proportions of 2a and 4a appeared to stabilize at approximately one-third 2a and two-thirds 4a. We believe these data indicate that, compared with the reaction through to what is presumably the thermodynamic product of this reaction, complexes 2a and 4a are effectively in rapid equilibrium, with the relative proportions indicating only a small difference in energy between them. The rates of both reactions are retarded by addition of DMSO to the reaction solution. The final product of the reaction has very similar spectroscopic properties to the previously characterized 3b and 3c: in particular no platinum satellites were seen on the DMSO resonance in the 1H NMR spectrum and the 4JF-Pt value (28 Hz for 3a, 3b, and 3c) and the platinum chemical shift (-774 ppm for 3a, -613 ppm for 3b, and -654 ppm for 3c) were similar. We are thus happy to assign an equivalent structure to this new complex and characterize this material as another O-bound complex, 3a, Scheme 4. DFT calculations (see later) confirm that the O-bound DMSO isomer is indeed the lowest energy form and also indicate a close balance in energy between isomers 2a and 4a. (36) Elding, L. I.; Oskarsson, A. Inorg. Chim. Acta 1987, 130, 209.

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Organometallics, Vol. 29, No. 8, 2010 Scheme 4

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redissolution/acetone removal eventually results in the near total loss of any DMSO resonances. No change in the pattern of the other 1H signals is seen, and the 19F signal broadens substantially. Addition of DMSO to such solutions regenerates 3b and 3c instantly. By comparison, complex 3a showed little tendency to lose DMSO. These results suggested to us that we were making a five-coordinate complex, possibly stabilized via an agostic interaction from the alkyl groups. We therefore attempted an independent synthesis of these new materials via the oxidation of the chloride-bridged dimers 5, Scheme 5. Scheme 5

At no point did we observe any evidence for the existence of equivalent fac complexes, 4b and 4c: complexes 2b and 2c simply did not isomerize in this fashion. Recall that these isomerization reactions of 2a were easily observed in acetone solutions of 2a (the rates of both isomerizations are retarded by addition of extra DMSO) but were not seen in chloroform solution. This suggests to us a reaction whereby the coordinated DMSO dissociates before recoordinating in a different site at the metal, with the coordinatively unsaturated intermediate being stabilized by interaction with an acetone molecule. A dissociative isomerization reaction is exactly the sort expected for 18e Pt(IV),37 and a number of five-coordinate Pt(IV) complexes have previously been isolated and characterized.38-48 The relatively rapid isomerization of the ethyl and propyl derivatives, even in noncoordinating chloroform, suggests that an additional stabilization of the unsaturated intermediate is present in these derivatives. We would suggest that an agostic interaction from the alkyl group could be the factor that stabilizes the coordinately unsaturated intermediate and favors such reactions. We present further evidence for the existence of agostic interactions later in this paper, while noting that although unambiguous examples of Pt(IV) agostic complexes appear to be absent, others have invoked them as potential intermediates.49 Five-Coordinate Intermediates. The removal of solvent from acetone solutions of 3b and 3c under vacuum results in a broadening of the 1H NMR spectra, together with a slight reduction in the integral of the DMSO peak. Repeated (37) Bernhardt, P. V.; Gallego, C.; Martinez, M. Organometallics 2000, 19, 4862. (38) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem. Soc. 2001, 123, 6423. (39) Fekl, U.; Goldberg, K. I. J. Am. Chem. Soc. 2002, 124, 6804. (40) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem. Soc. 2003, 125, 15286. (41) Karshtedt, D.; McBee, J. L.; Bell, A. T.; Tilley, T. D. Organometallics 2006, 25, 1801. (42) Kloek, S. M.; Goldberg, K. I. J. Am. Chem. Soc. 2007, 129, 3460. (43) Luedtke, A. T.; Goldberg, K. I. Inorg. Chem. 2007, 46, 8496. (44) Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J. L. J. Am. Chem. Soc. 2001, 123, 6425. (45) Sangtrirutnugul, P.; Tilley, T. D. Organometallics 2008, 27, 2223. (46) West, N. M.; White, P. S.; Templeton, J. L.; Nixon, J. F. Organometallics 2009, 28, 1425. (47) Zhao, S. B.; Wu, G.; Wang, S. Organometallics 2008, 27, 1030. (48) McBee, J. L.; Tilley, T. D. Organometallics 2009, 28, 3947. (49) Baber, A.; Fan, C.; Norman, D. W.; Orpen, A. G.; Pringle, P. G.; Wingad, R. L. Organometallics 2008, 27, 5906.

Dimers 5b and 5c are the direct product of reaction of 2-(4fluorophenyl)-6-ethylpyridine or 2-(4-fluorophenyl)-6-propylpyridine with potassium tetrachloroplatinate, and the X-ray structure of 5c is reported here for the first time, Figure 4. Oxidation of the chloride-bridged dimers 5b and 5c in chloroform gave precipitates that analyzed as the formally five-coordinate complexes 6b and 6c. These new complexes might simply be platinum(IV) dimers, but they were soluble in acetone and gave identical NMR spectra to those obtained via the removal of solvent from 3b and 3c. DFT calculations (see later) indicate that a dimer is higher in energy than other monomeric possibilites. In addition, more careful analysis of the NMR spectra revealed more features. While the 19F and 195Pt NMR spectra of complex 6c remain essentially unchanged with temperature, this is not the case with the 1H NMR spectrum. At about -60 °C a broad new peak (relative integral two) appears about 6.5 ppm; further cooling to -80 °C results in this peak sharpening considerably, with it moving to about 7.5 ppm. Addition of D2O to the sample results in the loss of this new peak, suggesting strongly that it is in fact a coordinated water. This new peak does not show obvious Pt satellites, but confirmation of the coordination of this water to the platinum center comes from a 1H-195Pt correlation experiment, which shows a distinct correlation of this new peak in the 1H to the same platinum that is giving rise to satellites on the cyclometalated phenyl ring. An NOE experiment at -80 °C also shows distinct enhancements of the propyl chain when the coordinated water peak is irradiated (and vice versa), suggesting the water is coordinated trans to the carbon of the cyclometalated ring. In order to make sense of the NMR data, we need to assume the bound water ligand is rapidly exchanging with water in solution. However attempts to dry the acetone sufficiently to prevent the formation of this water complex were not successful. The NMR spectra of the ethyl derivative 6b were more complex still. At room temperature, the 1H and 19F NMR spectra (on a 400 MHz spectrometer) were significantly broader than one might expect (we did not run a 195Pt NMR spectrum at this temperature), but sharpened to give a single set of peaks consistent with a cyclometalated system such as 6b

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Figure 3. X-ray crystal structure and atom numbering of the two crystallographic independent molecules in the asymmetric unit of 3c; thermal ellipsoids at 50% probability. Selected bond lengths (A˚) and angles (deg) for one of the independent molecules: Pt(1)-C(108) 1.997(3); Pt(1)-N(101) 2.069(3); Pt(1)-O(116) 2.197(2); Pt(1)-Cl(1) 2.3091(9); Pt(1)-Cl(2) 2.3096(9); Pt(1)-Cl(3) 2.3170(9); O(116)-S(117) 1.547(3); C(108)-Pt(1)-N(101) 81.30(13); C(108)-Pt(1)-O(116) 175.13(12); N(101)-Pt(1)-O(116) 99.16(10); C(108)-Pt(1)-Cl(1) 92.29(11); N(101)-Pt(1)-Cl(1) 89.67(9); O(116)-Pt(1)-Cl(1) 92.56(7); C(108)-Pt(1)-Cl(2) 94.61(11); N(101)-Pt(1)-Cl(2) 175.76(8); O(116)-Pt(1)-Cl(2) 85.01(7); Cl(1)-Pt(1)-Cl(2) 89.37(3); C(108)-Pt(1)-Cl(3) 88.30(11); N(101)-Pt(1)-Cl(3) 89.29(9); O(116)-Pt(1)-Cl(3) 86.86(7); Cl(1)-Pt(1)-Cl(3) 178.71(3); Cl(2)-Pt(1)-Cl(3) 91.73(3); S(117)O(116)-Pt(1) 116.74(13); O(116)-S(117)-C(118) 102.98(19); O(116)-S(117)-C(119) 103.85(19).

Figure 4. Solid-state structure of the chloride-bridged dimer 5c with only the key atoms of the asymmetric unit labeled; thermal ellipsoids at 50% probability. The dimer lies on an inversion center. Selected bond lengths (A˚) and angles (deg): Pt(1)-C(8) 1.970(2); Pt(1)-N(1) 2.0737(19); Pt(1)-Cl(1)#1 2.3100(6); Pt(1)-Cl(1) 2.4620(6); C(8)-Pt(1)-N(1) 81.45(9); C(8)-Pt(1)Cl(1)#1 92.55(7); N(1)-Pt(1)-Cl(1)#1 173.99(6); C(8)-Pt(1)Cl(1) 170.84(7); N(1)-Pt(1)-Cl(1) 107.40(6); Cl(1)#1-Pt(1)Cl(1) 78.61(2); Pt(1)#1-Cl(1)-Pt(1) 101.39(2).

at 40 °C. Cooling the sample to -20 °C resulted in the separation of peaks into two sets, each consistent with a cyclometalated structure, in the ratio approximately 1:5. Two distinct Pt correlations are now seen in the 1H-195Pt correlation experiment, and all evidence points to two separate complexes. The spectra of the minor component remained essentially un-

changed upon further cooling to -90 °C, whereas the spectra of the major component did show further change. This change was similar to that seen for complex 6c, whereby a new peak (relative integral two) appeared at about -60 °C; again D2O exchange suggests strongly a coordinated water, and again it was shown to correlate to the same platinum as the rest of the molecule. Once again NOE experiments proved informative: the major component (the water complex) showed enhancements between the water and the ethyl group (in particular the CH2 of the ethyl group). Interestingly, irradiation of the CH2 of the ethyl group did not give rise to any enhancement of the adjacent H on the pyridine ring, suggesting a conformation whereby the CH3 group of the ethyl chain is positioned away from the coordinated water. Irradiation of the CH3 group of the ethyl chain does result in enhancement of the nearest hydrogen on the pyridine ring, providing further evidence for this conformation. The minor component also shows interesting NOE effects: irradiation of both the CH2 and the CH3 groups of the ethyl chain results in enhancement of the adjacent H on the pyridine ring, suggesting that (at least part of the time) the hydrogens of the CH2 group are pointing up toward the pyridine. We suggest that an agostic interaction between either the CH2 or the CH3 group of the ethyl group and the Pt center would account for this observation. We will return to a discussion of this potential agostic interaction in the next section of this paper. It should be noted that we looked for evidence of acetone coordination: we ran NMR spectra in a mixture of H6 and D6 acetone. Even at -100 °C we saw no indication of peaks representing coordinated acetone. Our experimental evidence, therefore, is consistent with formally five-coordinate oxidation products dissolving in acetone and forming complexes that contain either a weakly coordinated water ligand or an agostic interaction in the sixth coordination site. DFT Calculations. DFT calculations using the Amsterdam Density Functional program version 200850 were carried out

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Organometallics, Vol. 29, No. 8, 2010 Scheme 6

for all six possible isomers of 2, for both the R = H series, a, and the R = Et series, b. The energies of each member of each series, relative to the lowest energy member of that series, are shown below the structures (in kJ mol-1), Scheme 6. The calculations correspond nicely with our experimental results on a number of points. First, the O-bound isomers 3 are predicted to be the lowest energy isomers by a substantial margin and thus should be the thermodynamic products of our oxidation reactions, as we indeed see. Second, for the R = H derivatives (the a series), the isomerization of 2a to 4a is predicted to be favored by only 1.2 kJmol-1, implying an equilibrium constant of 1.6 in favor of 4a, close to what we observe; all other isomers are predicted to be higher in energy than the initial product 2a, so we would not expect to see any of them. Third, for the R = Et derivatives (the b series), the isomerization of 2b to 4b is shown to be disfavored, with only 3b having a lower energy than 2b, thus predicting the isomerization of 2b should go straight to 3b, as seen. We can, therefore, look at the energies of the isomers we did not experimentally observe with some confidence. Thus we can see that the unobserved S-bound 4b is a little bit higher in energy than 2b; presumably the steric interaction of the ethyl group with an adjacent DMSO ligand destabilizes it somewhat. Likewise the S-bound complex 9a with the DMSO trans to the carbon is somewhat higher in energy than the starting 2a complex. However, the equivalent 9b complex is predicted to minimize to a form in which the DMSO is essentially dissociated (although residual interactions keep it weakly bound to the periphery of the molecule), presumably because the steric interactions with the adjacent ethyl group are now significant. More pertinently, the energies of the three O-bound isomers are very significantly different: the two O-bound DMSO complexes that were not observed experimentally (i.e., 7 and 8) are predicted to be higher in energy than any of the S-bound complexes. Since we would expect the S-bound (50) Baerends, E. J. B., A.; Bo, C.; Boerrigter, P. M.; Cavallo, L.; Deng, L.; Dickson, R. M.; Ellis, D. E.; Fan, L.; Fischer, T. H.; Fonseca Guerra, C.; van Gisbergen, S. J. A.; Groeneveld, J. A.; Gritsenko, O. V.; Harris, F. E.; van den Hoek, P.; Jacobsen, H.; van Kessel, G.; Kootstra, F.; van Lenthe, E.; Osinga, V. P.; Philipsen, P. H. T.; Post, D.; Pye, C. C.; Ravenek, W.; Ros, P.; Schipper, P. R. T.; Schreckenbach, G.; Snijders, J. G.; Sola, M.; Swerhone, D.; te Velde, G.; Vernooijs, P.; Versluis, L.; Visser, O.; van Wezenbeek, E.; Wiesenekker, G.; Wolff, S. K.; Woo, T. K.; Ziegler, T. ADF 2008.01; Scientific Computing and Modelling NV: Free University, Amsterdam, 2008.

1971

Scheme 7

isomers to be most affected by steric interactions, the high energy of these O-bound isomers must be largely due to the electronic effects of the bonding and, in particular, the group trans to the ligand. If we think of the O atom of the DMSO as both a σ and a π donor, we can immediately see why putting such a group trans to chloride (also both a σ and a π donor) would not be energetically favorable; putting the O trans to the cyclometalated carbon (a σ donor and a good π acceptor) becomes the favored option. Intermediate in energy between these two extremes would be putting the O trans to the pyridine N (a σ donor and a weak π acceptor), as predicted by the calculations. This, then, provides the reason for the lack of previously characterized O-bound DMSO ligands in Pt(IV) complexes: they need to be opposite a good π acceptor, something that was not true for the five previously characterized sulfoxide complexes of Pt(IV).51-54 If we return to look at the notionally five-coordinate complexes that result from dissociation of the DMSO in the ethyl derivatives, we can further calculate relative energies of a variety of forms, including agostically stabilized ones. Scheme 7 shows the relative energies of the complexes (in kJ mol-1) relative to the isolated 3b (values are corrected for the presence or absence of DMSO, water, and acetone, as appropriate). We can thus see that the lowest energy form is indeed predicted to be an agostic complex, though it is not greatly preferred over the water complex. We can also note that an acetone complex is predicted to be disfavored, as is a chloride-bridged dimer (not shown), which is calculated to be 7.3 kJ mol-1 higher in energy than 3b. By contrast, the calculated energy of the water complex 10a and the five-coordinate complex 6a for the non-ethyl pyridine are 4.1 and 23.1 kJ mol-1, respectively, higher in energy than the O-bound DMSO complex 3a. This would suggest that, while invoking a water complex as a reactive intermediate might be sensible, invoking a fully dissociated five-coordinate complex for the unsubstituted pyridines is not. Thus these calculations suggest we should not expect dissociation and coordination isomerism of the DMSO from (51) Kaplan, S. F.; Kukushkin, V. Y.; Pombeiro, A. J. L. J. Chem. Soc., Dalton Trans. 2001, 3279. (52) Kukushkin, V. Y.; Belskii, V. K.; Aleksandrova, E. A.; Pankova, E. Y.; Konovalov, V. E.; Yakovlev, V. N.; Moiseev, A. I. Zh. Obshch. Khim. 1991, 61, 318. (53) Kukushkin, Y. N.; Krylov, V. K.; Kaplan, S. F.; Calligaris, M.; Zangrando, E.; Pombeiro, A. J. L.; Kukushkin, V. Y. Inorg. Chim. Acta 1999, 285, 116. (54) Vicente, J.; Chicote, M. T.; Guerrero, R.; Vicente-Hernandez, I.; Jones, P. G.; Bautista, D. Inorg. Chem. 2006, 45, 5201.

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Organometallics, Vol. 29, No. 8, 2010

Figure 5. Calculated structure of the γ-agostic form of 6b. The distance between the platinum center and the agostic hydrogen is 2.54 A˚.

complex 2a to occur in noncoordinating solvents such as chloroform. A close look at the calculated geometry of the predicted lowest energy form of 6b (the “γ-agostic” complex in Scheme 7) is shown in Figure 5. This figure shows that the interaction of one of the hydrogens of the CH2 group with the Pt center is such that it brings this hydrogen as close as possible to the Pt center and causes the other hydrogen to rotate up toward the pyridine ring (and causes the CH3 group to rotate somewhat away from the pyridine ring). This is consistent with our NMR data: in this conformation we would expect an NOE interaction between the nonagostic CH2 and the CH3 hydrogens with the pyridine ring. The energy barrier to the exchange of the CH2 hydrogens between agostic coordination and pointing toward the pyridine ring would be far too low to freeze out on the NMR time scale, even at -90 °C. Thus it seems entirely reasonable to us to invoke agostic intermediates as providing stabilization for dissociative coordination isomerization reaction for the ethyl derivatives. Although we have not carried out calculations on the propyl derivatives, c, we can easily see that similar agostic interactions should not be dramatically disfavored and that, once again, we can invoke agostic intermediates in coordination isomerization reactions. Solid-State Structures. In the course of the work reported in this paper, we solved the X-ray structure of one S-bound sulfoxide complex of Pt(IV) and two O-bound sulfoxide complexes of Pt(IV). In general it would be expected that sulfur would be the favored donor atom of sulfoxides in the low oxidation state metal complexes, whereas oxygen-bound species would be favored with higher oxidation states.55 Such a conclusion is based on the relative donor/acceptor abilities of the S and O atoms: the S atom has suitable orbitals to accept π electron density from an electron-rich (low oxidation state) metal center, whereas the O atom does not: in sulfoxides it is a σ donor with little π-accepting ability. The low oxidation state metal with an S-bound sulfoxide, together with an O-bound form with the high oxidation state, is seen for a number of metals,55 but has not previously been seen for platinum. A search of the Cambridge Crystallographic Database56 reveals 296 structures containing a sulfoxide bound via the S atom to a four-coordinate Pt(II) center and five to a Pt(IV) center.51-54 Oxygen-bound sulfoxide (55) Calligaris, M.; Carugo, O. Coord. Chem. Rev. 1996, 153, 83. (56) Fletcher, D. A.; McMeeking, R. F.; Parkin, D. J. Chem. Inf. Comput. Sci. 1996, 36, 746.

Crosby et al.

ligands are very rare for Pt complexes: there are only two reported crystal structures; one is a Pt(II) complex36 and the other is a Pt(III) dimer.35 Thus, the examples in this paper represent the only crystallographically characterized examples of an oxygen-bound DMSO bonded to a Pt(IV) center. Sterically, an O-bound DMSO ligand requires less space than an S-bound one, and this presumably is part of the reason for the O-bound isomers 3 to be favored over the S-bound isomers 2. Further light is shed upon the issue by the DFT calculations we performed (see previous section). If we look in a bit more detail at the X-ray structure of the S-bound complex 2a, we see that the asymmetric unit contains the complex composed of the cycloplatinated fluorophenylpyridine ligand, three chlorines, and a DMSO bound to platinum in an octahedral arrangement; all bond lengths and angles are typical for such complexes. It is pertinent to note that though the Pt-S distance of 2.3235(9) A˚ is significantly longer than that in the Pt(II) precursor 1a (2.2161(16) A˚),1 this is in line with those reported in the Cambridge Crystallographic Database, where the average Pt(IV)-S distance in sulfoxides is 2.31 A˚ and the average Pt(II)-S distance in sulfoxides is 2.22 A˚. There is also a molecule of chloroform hydrogen bonded to one of the chlorido ligands (C301-Cl1 3.610(4) A˚), and the bound DMSO molecule has reciprocal short contacts with a symmetry-related bound DMSO molecule (C-O distances of 3.354(4) and 3.531(5) A˚), Figure 6; there is a short contact of 3.2103(25) A˚ between F110 and Cl33 of the chloroform. The X-ray structures of 3b and 3c show the same gross features as each other (O trans to C, methyl groups of the sulfoxide rotated away from the ethyl or propyl chain) together with very similar bond lengths and angles (Pt-C distances of 2.009(4) and 1.997(3) A˚; Pt-O distances of 2.205(3) and 2.197(2) A˚; S-O distances of 1.550(3) and 1.547(3) A˚; S-O-Pt angles of 116.92(17)° and 116.74(13)° in 3b and 3c, respectively). It is instructive to compare the S-O and O-Pt distances and Pt-O-S angles with the other two examples published. In the Pt(III) dimer,35 the DMSO is trans to the Pt-Pt bond, with the S-O distance at 1.554(6) A˚, the O-Pt distance at 2.126(6) A˚, and the Pt-O-S angle at 118.5°. In the Pt(II) complex,36 two different O-bound DMSO ligands are present, each trans to S-bound DMSO ligands; the S-O distances are 1.532(11) and 1.561(9) A˚, the Pt-O distances are 2.051(9) and 2.040(10) A˚, and the Pt-O-S angles are 123.5(6)° and 121.5(5)°. For comparison, the S-O distance in DMSO itself is reported as 1.513(5) A˚.57 Thus, while we see a progressive lengthening of the Pt-O bond as we go from Pt(II) to Pt(III) to Pt(IV), we see relatively little difference in the S-O distances between the complexes. An additional hydrogen bonding (Cl(2)-H(17F) 3.646(5) A˚) and an S-S (3.5376(20) A˚) interaction are also present in the structure of 3b, resulting in the formation of a network traveling along the a axis of the crystal, Figure 7. The X-ray structure of 5c shows the core of the molecule (the cyclometalated phenylpyridines and the Pt2Cl2 core) to be flat with no significant nonbonding interactions.

Conclusions Oxidation of cyclometalated Pt(II) complexes with sulfurbound DMSO ligands initially results in Pt(IV) complexes (57) Thomas, R.; Shoemaker, C. B.; Eriks, K. Acta Crystallogr. 1966, 21, 12.

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Organometallics, Vol. 29, No. 8, 2010

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Figure 6. X-ray crystal structure of 2a showing hydrogen bonding between two DMSO ligands and between a chloroform solvent molecule with a chloride ligand; thermal ellipsoids at 50% probability.

Figure 7. X-ray crystal structure of 3b showing hydrogen bonding between DMSO ligands and chlorides, together with an S-S interaction of 3.5376(20) A˚; thermal ellipsoids at 50% probability.

that retain the S-bound DMSO ligands in the same relative position. With time, isomerization reactions result in a rearrangement of the ligands to give oxygen-bound DMSO complexes, where the O-bound DMSO has moved to a position trans to a cyclometalated carbon. X-ray structures of two analogues of these O-bound ligands have been solved: these represent the only known examples bound to Pt(IV). Isomerization reactions of Pt(IV) complexes would be expected to proceed via a dissociative pathway, and all our experimental results back this up. We see a dramatic difference in reaction rate with solvent for some complexes, consistent with the need to stabilize a coordinatively unsaturated intermediate. Where the Pt(IV) complexes have a pendant alkyl chain potentially capable of forming an agostic interaction with the coordinatively unsaturated center, we see little dependence on isomerization rate with solvent. Solution NMR data also strongly suggest the presence of agostic complexes for one of our derivatives, with calculations providing support for this model. The same agostic interactions can be used to account for our experimental observation of the loss of DMSO from the O-bound DMSO complexes with alkyl chains on the pyridine.

Experimental Section General Procedures. All chemicals were used as supplied, unless noted otherwise. All NMR spectra were obtained on a Bruker Avance 300, 400, 500, or 600 MHz spectrometer and are referenced to external TMS, assignments being made with the use of decoupling, NOE, and DEPT and COSY pulse sequences. 1H-19F and 1H-195Pt correlation spectra were recorded using a variant of the HMBC pulse sequence. 19F chemical shifts are quoted from the directly observed signals (referenced to external CFCl3), whereas the 195Pt chemical shifts quoted are taken from the 2D HETCOR spectra (referenced to external Na2PtCl6). Mass spectra were run on a Bruker MaXis using electrospray ionization. All elemental analyses were performed by Warwick Analytical Service. Because of the propensity of many of the complexes to lose DMSO, we were unable to obtain satisfactory elemental analyses for many complexes. Starting platinum complex 1a was prepared as previously reported.1 Synthesis of DMSO Complex 1b. Complex 5b (0.011 g, 1.28  10-5 mol) was dissolved in DMSO (10 mL, excess) and the solution stirred at 40 °C for 6-8 h. Distilled water (50 mL) was added and the product extracted with DCM. The solution was

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Organometallics, Vol. 29, No. 8, 2010

Crosby et al. 8.2 Hz, 4J = 2.4 Hz), 3.62 (6H, s, 3JH-Pt = 23 Hz, DMSO), 3.58 (2H, t, 3J = 7.3 Hz, CH2), 1.89 (2H, sextet, 3J = 7.3 Hz, CH2), 1.00 (3H, t, 3J = 7.3 Hz, CH3) ppm. 13C NMR (298 K, CDCl3): 167.7, 164.8 (C), 161.9 (d, 1JC-F = 253 Hz), 142.8, 142.0, 139.8, 125.5 (d, 3JC-F = 9.2 Hz), 122.6 (3JC-Pt = 22 Hz), 120.2 (d, 2JC-F = 20.7 Hz), 115.5 (3JC-Pt = 34 Hz), 112.2 (d, 2JC-F = 23.8 Hz), 45.9, 41.0, 24.5, 13.8. 19F NMR (298 K, CDCl3): -108.42 (4JF-Pt = 64 Hz). 195Pt NMR (298 K, CDCl3): -3579. HR-MS (ESI): m/z 486.0795 calcd for C16H19FNO194PtS = (M - Cl)þ 486.0793. Oxidation of DMSO Complex 1a to Give 2a, 3a, and 4a. Complex 1a (0.006 g, 1.179  10-5 mol, 1 equiv) was dissolved in acetone, and (dichloroiodo)benzene (0.0032 g, 1.179  10-5 mol, 1 equiv) was added at room temperature. After 10 min the solvent was removed, and the product washed with pet ether (40-60) to give pure compound 2a. Yield: 0.0058 g (1.063  10-5 mol, 90%). Crystals of 2a suitable for single-crystal analysis precipitated directly from the chloroform reaction solution used to synthesize 2a (data given in Table 2). Acetone solutions of 2a left at room temperature slowly transformed to a mixture of 2a, 3a, and 4a, before eventually isomerizing completely to 3a (seven days); pure solutions of 4a could not be obtained, and therefore only some characteristic spectroscopic data can be given. 2a. 1H NMR (298 K, acetone-d6): 9.75 (1H, d, 3J = 6 Hz, 3 JH-Pt = 25 Hz, H ortho to N), 8.31-8.36 (2H, m, pyridine), 8.20 (1H, dd, 3JH-F = 9.8 Hz, 4J = 2.4 Hz, 3JH-Pt = 34 Hz, H ortho to Pt and F), 8.11 (1H, dd, 3J = 8.8 Hz, 4JH-F = 5.5 Hz), 7.73 (1H, td, 3J = 6.7 Hz, 4J = 2 Hz, pyridine), 7.19 (1H, td, 3 JH-H, H-F = 8.5 Hz, 4J = 2.4 Hz), 4.04 (6H, s, 3JH-Pt = 14.3 Hz, DMSO). 13C NMR (298 K, acetone-d6): 163.6 (d, 1JC-F = 254 Hz), 162.1, 149.6, 143.4, 128.4 (d, 3JC-F = 9 Hz, 3JC-Pt = 38 Hz), 125.3 (3JC-Pt = 24 Hz), 122.5 (JC-Pt = 25 Hz), 117.2 (d, 2JC-F = 25 Hz), 115.0 (d, 2JC-F = 22 Hz), 44.2 (2JC-Pt = 28 Hz). 19F NMR (298 K, acetone-d6): -105.7 (4JF-Pt = 30 Hz). 195 Pt NMR (298 K, acetone-d6): -1459. HR-MS (ESI): m/z 571.9285 calcd for C13H1335Cl3FNNaO194PtS = (M þ Na)þ 571.9286. 3a. 1H NMR (298 K, acetone-d6): 8.97 (1H, d, 3J = 8 Hz, 3 JH-Pt = 33 Hz, H ortho to N), 8.20-8.30 (2H, m, pyridine), 7.95 (1H, dd, 3J = 8.5 Hz, 4JH-F = 5.6 Hz), 7.64 (1H, td, 3J = 8.7 Hz, 4J = 2.5 Hz, pyridine), 7.59 (1H, dd, 3JH-F = 9.5 Hz, 4 J = 2.5 Hz, H ortho to Pt and F), 7.10 (1H, td, 3JH-H, H-F = 8.6 Hz, 4J = 2.6 Hz), 3.09 (6H, s, DMSO). 19F NMR (298 K,

dried over MgSO4 and the solvent removed to give 1b. Yield: 0.011 g (2.15  10-5 mol, 84%). 1 H NMR (298 K, CDCl3): 7.93 (1H, dd, 3JH-F = 11 Hz, 4J = 2.6 Hz, 3JH-Pt = 52 Hz, H ortho to F and Pt), 7.69 (1H, t, 3J = 7.8 Hz, pyridine), 7.44 (1H, br d, 3J = 8 Hz, pyridine), 7.38 (1H, dd, 3J = 8.5 Hz, 4JH-F = 5.5), 7.07 (1H, br d, 3J = 7.6 Hz, pyridine), 6.80 (1H, td, 3JH-H, H-F = 8.5 Hz, 4JH-H = 2.6 Hz), 3.51 (6H, s, 3JH-Pt = 12 Hz, DMSO), 3.49 (2H, q, 3J = 7.5 Hz, CH2), 1.30 (3H, t, 3J = 7.5 Hz, CH3). 13C NMR (298 K, CDCl3): 168.0, 163.5, 161.3 (d, 1JC-F = 252 Hz), 141.7 (d, 4JC-F = 7 Hz), 140.0 (d, 3JC-F = 3 Hz), 139.0, 124.5 (d, 3JC-F = 9 Hz, 3 JC-Pt = 51 Hz), 120.9 (3JC-Pt = 26 Hz, 119.0 (d, 2JC-F = 22 Hz, 2JC-Pt = 77 Hz), 114.5 (3JC-Pt = 33 Hz), 111.2 (d, 2JC-F = 24 Hz), 44.8 (2JC-Pt = 63 Hz), 31.5, 14.1. 19F NMR (298 K, CDCl3): -108.6 (4JF-Pt = 65 Hz). 195Pt NMR (298 K, CDCl3): -3582. HR-MS (ESI): m/z 472.0639 calcd for C15H17FNO194PtS = (M - Cl)þ 472.0642. Synthesis of DMSO Complex 1c. Complex 5c (0.0542 g, 6.09  10-5 mol) was dissolved in DMSO (10 mL, excess) and the solution stirred at 40 °C for 20 h. The yellow solution was cooled to room temperature and water (25 mL) added dropwise to cause precipitation. The resulting solid was filtered and washed with 40-60 pet ether. Yield: 0.0183 g, (3.50  10-5 mol, 57%). 1 H NMR (298 K, CDCl3): 8.06 (1H, dd, 3JH-F = 11 Hz, 4J = 2.4 Hz, 3JH-Pt = 52 Hz, H ortho to Pt and F), 7.76 (1H, t, 3J = 7.9 Hz, pyridine), 7.52 (1H, dd, 3J = 8.2 Hz, 4J = 1.2 Hz, pyridine), 7.47 (1H, dd, 3J = 8.8 Hz, 4JH-F = 5.8 Hz), 7.13 (1H, dd, 3J = 7.9 Hz, 3J = 1.2 Hz, pyridine), 6.91 (1H, td, 3JH-H, H-F = Table 1. Computed Energies for the Complexes complex

E/kcal

E/kJ

complex

E/kcal

E/kJ

2a 3a 4a 6a 7a 8a 9a 10a 11a DMSO H2O(aq) acetone

-4613.82 -4616.42 -4614.17 -3450.63 -4611.07 -4607.54 -4613.41 -3789.43 -4744.30 -1160.28 -334.27 -1293.38

-19378.04 -19388.96 -19379.51 -14492.65 -19366.49 -19351.67 -19376.32 -15915.61 -19926.06 -4873.18 -1403.93 -5432.20

2b 3b 4b 6b þ DMSO 6b_delta_agostic 6b_gamma_agostic 7b 8b 10b 11b Et_dimer

-5360.39 -5365.51 -5362.43 -5367.51 -4206.03 -4210.12 -5360.96 -5355.62 -4541.92 -5496.02 -8408.73

-22513.64 -22535.14 -22522.21 -22543.54 -17665.33 -17682.50 -22516.03 -22493.60 -19076.06 -23083.28 -35316.67

Table 2. X-ray Data for Complexes 2a, 3b, 3c, and 5c

cryst form dimens/mm emp. formula Mw cryst syst space group a/A˚ b/A˚ c/A˚ R/deg β/deg γ/deg U/A˚3 T/K Z Dcalc/Mg m-3 F(000) μ(Mo KR)/mm-1 θmax/deg reflns measd unique data R1 [I > 2σ(I)] wR2 data/restr/params

2a

3b

3c

5c

yellow plate 0.20  0.10  0.08 C14H14Cl6FNOPtS 671.11 triclinic P1 8.4226(2) 10.1642(2) 13.2591(3) 72.2980(10) 78.8930(10) 67.7890(10) 997.22(4) 120(2) 2 2.235 636 7.957 27.52 20 425 4554 0.0256 0.0551 4554/0/228

colorless block 0.52  0.36  0.32 C16H18Cl6FNOPtS 699.16 monoclinic P2(1)/c 8.18830(10) 13.9875(3) 19.7052(4) 90 101.8530(10) 90 2208.79(7) 120(2) 4 2.102 1336 7.189 27.50 5051 5051 0.0314 0.0787 5051/0/247

yellow block 0.20  0.12  0.10 C16H19Cl3FNOPtS 593.82 triclinic P1 8.6007(3) 14.1997(5) 16.4235(6) 105.974(3) 96.165(3) 95.714(3) 1899.59(12) 100(2) 4 2.076 1136 7.931 29.10 14 886 8682 0.0244 0.0527 8682/358/439

yellow block 0.30  0.10  0.04 C14H13ClFNPt 444.79 orthorhombic Pbcn 15.3935(3) 7.33660(10) 22.4051(4) 90 90 90 2530.34(8) 100(2) 8 2.335 1664 11.293 29.38 15 969 3203 0.0176 0.0317 3203/0/164

Article acetone-d6): -106.9 (4JF-Pt = 28 Hz). 195Pt NMR (298 K, acetone-d6): -774. HR-MS (ESI): m/z 571.9307, calcd for C13H1335Cl3FNNaO194PtS = (M þ Na)þ 571.9286, 493.9158 calcd for C11H735Cl3FNNa194Pt = (M - DMSO þ Na)þ 493.9152, 457.9391 calcd for C11H635Cl2FNNa194Pt = (M DMSO - Cl þ Na)þ 457.9380. Characteristic Spectroscopic Data of Complex 4a. 1H NMR (298 K, acetone-d6): 9.77 (1H, d, 3J = 6 Hz, 3JH-Pt = 30 Hz, H ortho to N), 3.79 (3H, s, 3JH-Pt = 15 Hz, DMSO), 3.24 (3H, 3 JH-Pt = 20 Hz, DMSO). 19F NMR (298 K, acetone-d6): -105.2 (4JF-Pt = 25 Hz). 195Pt NMR (298 K, acetone-d6): -1609. Oxidation of DMSO Complex 1b to Give Complexes 2b and 3b. Complex 1b (0.013 g, 2.555  10-5 mol, 1 equiv) was dissolved in chloroform, and (dichloroiodo)benzene (0.0070 g, 2.555  10-5 mol, 1 equiv) was added at room temperature. After 10 min the solvent was removed, and the product washed with pet ether (40-60) to give pure compound 2b. Yield: 0.0127 g (2.197  10-5 mol, 86%). 2b isomerized in chloroform solution to give pure 3b over a period of 10 days at room temperature. Crystals of 3b suitable for single-crystal analysis were formed by slow evaporation of solvent from a chloroform solution (data available in Table 2). 2b. 1H NMR (298 K, CDCl3): 7.99 (1H, dd, 3JH-F = 10.4 Hz, 4 J = 2.4 Hz, 3JH-Pt = 31.4 Hz, H ortho to Pt and F), 7.79 (1H, t, 3 J = 7.7 Hz, pyridine), 7.59 (1H, d, 3J = 8.2 Hz, pyridine), 7.49 (1H, dd, 3J = 8.2 Hz, 4JH-F = 5.6 Hz), 7.16 (1H, d, 3J = 7.9 Hz, pyridine), 6.94 (1H, td, 3JH-H, H-F = 8.3 Hz, 4J = 2.4 Hz), 3.87 (6H, s, 3JH-Pt = 15.5 Hz, DMSO), 3.84 (2H, q, 3J = 7.4 Hz, CH2), 1.32 (3H, t, 3J = 7.4 Hz, CH3). 19F NMR (298 K, CDCl3): -103.7 (4JF-Pt = 31 Hz). 195Pt NMR (298 K, CDCl3): -1236. 3b. 1H NMR (298 K, CDCl3): 7.76 (1H, t, 3J = 8 Hz, pyridine), 7.66 (1H, dd, 3JH-F = 9.4 Hz, 4J = 2.4 Hz), 7.60 (1H, d, 3J = 8 Hz, pyridine), 7.42 (1H, dd, 3J = 8.5 Hz, 4JH-F = 5.5 Hz), 7.15 (1H, d, 3 J = 8 Hz, pyridine), 6.89 (1H, td, 3JH-H, H-F = 8.5 Hz, 4J = 2.4 Hz), 3.49 (2H, q, 3J = 7.6 Hz, CH2), 2.80 (6H, br s, DMSO), 1.35 (3H, t, 3J = 7.6 Hz, CH3) ppm. 13C NMR (298 K, CDCl3): 139.4, 125.0 (d, 3JC-F = 10 Hz), 122.9, 119.4 (d, 2JC-F = 25 Hz), 117.1, 113.4 (d, 2JC-F = 24 Hz), 38.6 (DMSO), 28.3, 13.6. 19F NMR (298 K, CDCl3): -103.3 (4JF-Pt = 28 Hz). 195Pt NMR (298 K, CDCl3): -613. HR-MS (ESI): m/z 521.9462 calcd for C13H1135Cl3FNNa194Pt = (M - DMSOþNa)þ 521.9460, 463.9864 calcd for C13H1135Cl2FN194Pt = (M - DMSO - Cl)þ 463.9874. Oxidation of DMSO Complex 1c to Give Complexes 2c and 3c. Complex 1c (0.016 g, 3.060  10-5 mol, 1 equiv) was dissolved in acetone, and (dichloroiodo)benzene (0.0084 g, 3.060  10-5 mol, 1 equiv) was added at room temperature. After 10 min the solvent was removed, and the product washed with pet ether (40-60) to give pure compound 2c. Yield: 0.0162 g (2.723  10-5 mol, 89%). 2c isomerized in chloroform solution to give pure 3c over a period of three days at room temperature. Crystals of 3c suitable for single-crystal analysis were formed by slow evaporation of solvent from a chloroform solution (data available in Table 2). 2c. 1H NMR (298 K, CDCl3): 7.99 (1H, dd, 3JH-F = 10. Hz, 4 J = 2.3 Hz, 3JH-Pt = 31 Hz, H ortho to Pt and F), 7.77 (1H, t, 3 J = 7.8 Hz, pyridine), 7.58 (1H, dd, 3J = 7.9 Hz, 4J = 1.3 Hz, pyridine), 7.49 (1H, dd, 3J = 8.5 Hz, 4JH-F = 5.5 Hz), 7.16 (1H, dd, 3J = 7.7 Hz, 4J = 1.5 Hz), 6.94 (1H, td, 3JH-H, H-F = 8.3 Hz, 4J = 2.5 Hz), 3.88 (6H, s, 3JH-Pt = 15.2 Hz, DMSO), 3.74-3.78 (2H, m, CH2), 1.75 (2H, sextet, 3J = 7.5 Hz, CH2), 0.97 (3H, t, 3J = 7.3 Hz, CH3). 19F NMR (298 K, CDCl3): -103.9 (4JF-Pt = 29 Hz). 195Pt NMR (298 K, CDCl3): -1237. 3c. 1H NMR (298 K, CDCl3): 7.79 (1H, t, 3J = 7.8 Hz, pyridine), 7.71 (1H, dd, 3J(H-F) = 9.7 Hz, 4J = 2.4 Hz), 7.65 (1H, br d, 3J = 8 Hz, pyridine), 7.47 (1H, dd, 3J = 8.6 Hz, 4 J(H-F) = 5.4 Hz), 7.18 (1H, br d, 3J = 7.8 Hz, pyridine), 6.93 (1H, td, 3J(H-H), (H-F) = 8.4 Hz, 4J = 1.9 Hz), 3.50 (2H, m, CH2), 2.87 (6H, s, DMSO), 1.85 (2H, m, CH2), 1.10 (3H, t, 3J =

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7.2 Hz, CH3). 13C NMR (298 K, CDCl3): 139.2, 124.9 (d, 3JC-F = 8.5 Hz), 123.4, 119.3 (d, 2JC-F = 19 Hz), 117.2, 113.3 (d, 2JC-F = 24 Hz), 38.7, 38.4, 22.2, 12.9. 19F NMR (298 K, CDCl3): -103.9 (4JF-Pt = 28 Hz). 195Pt NMR (298 K, CDCl3): -654. HR-MS (ESI): m/z 535.9616 calcd for C14H1335Cl3FNNa194Pt = (M DMSO þ Na)þ 535.9616. Synthesis of Dimer 5b. 2-Ethyl-6-(4-fluorophenyl)pyridine (0.0648 g, 3.17  10-4 mol, 2.8 equiv), K2PtCl4 (0.0470 g, 1.13  10-4 mol, 1 equiv), and tetrabutylammonium bromide (0.001 g) were mixed in glacial ethanoic acid (20 mL). The solution was stirred at 90 °C until all the red K2PtCl4 had disappeared (ca. 48 h). The yellow mixture was allowed to cool to room temperature and filtered, yielding 5b as a yellow solid. Yield: 0.0171 g, (1.98  10-5 mol, 35%). 1 H NMR (298 K, CDCl3): 7.67 (1H, t, 3J = 7.7 Hz, pyridine), 7.34 (1H, br d, 3J = 7.9 Hz, pyridine), 7.24 (1H, dd, 3J = 8.9 Hz, 4 JH-F = 5.5 Hz), 7.05 (1H, dd, 3JH-F = 10.3 Hz, 4J = 2.4 Hz), 6.98 (1H, br dd, 3J = 7.6 Hz, 4J = 1.4 Hz), 6.74 (1H, td, 3JH-H, H-F = 8.4 Hz, 4J = 2.5 Hz), 3.29 (2H, q, 3J = 7.6 Hz, CH2), 1.38 (3H, t, 3 J = 7.6 Hz, CH3). 19F NMR (298 K, CDCl3): -109.2 (4JF-Pt = 54 Hz). HR-MS (ESI): m/z 823.0705 calcd for C26H2235ClF2N2194 Pt2 = (M - Cl)þ 823.0687. Anal. Found (expected for C26H22Cl2F2N2Pt2): C 36.00 (36.25); H 2.36 (2.57); N 3.04 (3.25). Synthesis of Dimer 5c. This complex was synthesized in a similar fashion to 5b; yield 42%. A crystal suitable for X-ray diffraction was grown by slow evaporation of solvent from a chloroform solution. Details are included in Table 2. 1 H NMR (298 K, CDCl3): 7.64 (1H, t, 3J = 7.8 Hz, pyridine), 7.33 (1H, br d, 3J = 8 Hz, pyridine), 7.23 (1H, dd, 3J = 8.7 Hz, 4 JH-F = 5.6 Hz), 7.05 (1H, dd, 3JH-F = 10 Hz, 4J = 2.6 Hz), 6.96 (1H, dd, 3J = 7.5 Hz, 4J = 1.3 Hz, pyridine), 6.74 (1H, td, 3 JH-H, H-F = 8.5 Hz, 4J = 2.6 Hz), 3.23 (2H, m, CH2), 1.89 (2H, quin, 3J = 7.7 Hz, CH2), 1.01 (3H, t, 3J = 7.2 Hz, CH3). 19F NMR (375 MHz, 298 K, CDCl3): -109.2 ppm (br satellites). HR-MS (ESI): m/z 851.1002 calcd for C28H2635ClF2N2194Pt2 = (M - Cl)þ 851.1000. Anal. Found (expected for C28H26Cl2F2N2Pt2): C 37.36 (37.80); H 3.25 (2.95); N 2.85 (3.15). Oxidation of Platinum Chloride Bridged Dimer 5b. To a chloroform solution of 5b was added (dichloroiodo)benzene (1 equiv) and the resulting precipitate filtered and washed with chloroform. HR-MS (ESI): m/z 521.9459 calcd for C13H1135Cl3FN194 PtNa = (M þ Na)þ 521.9460. Anal. Found (expected for C13H11Cl3FNPt): C 30.44 (31.12); H 2.32 (2.21); N 2.50 (2.79). NMR data were then recorded in acetone-d6. 1H NMR (298 K, acetone-d6): 8.00 (2H, br m, pyridine), 7.75 (1H, br dd, 3J = 8.3 Hz, 4JH-F = 5.9 Hz), 7.51 (1H, br d, 3JH-F = 10 Hz, H ortho to Pt and F), 7.41 (1H, br d, pyridine), 6.98 (1H, br t, 3J = 8.3 Hz), 3.38 (2H, br q, CH2), 1.29 (3H, t, 3J = 7.4 Hz, CH3) ppm. 19 F NMR (298 K, acetone-d6): -107.3 ppm (broad satellites). Major Component. 1H NMR (193 K, acetone-d6): 8.29 (1H, br d, 3J = 7.6 Hz, pyridine), 8.22 (1H, t, 3J = 7.8 Hz, pyridine), 8.06 (1H, dd, 3J = 8.7 Hz, 4JH-F = 5.8 Hz), 7.97 (2H, s, H2O), 7.65 (1H, dd, 3JH-F = 9.7 Hz, 4J = 2.5 Hz, H ortho to Pt and F), 7.62 (1H, br d, 3J = 7.5 Hz, pyridine), 7.25 (1H, td, 3JH-H, H-F = 8.5 Hz, 4J = 2.3 Hz), 3.61 (2H, q, 3J = 7.4 Hz, CH2), 1.43 (3H, t, 3 J = 7.5 Hz, CH3). 19F NMR (203 K, acetone-d6): -107.2 (4JF-Pt = 27 Hz). 195Pt NMR (193 K, acetone-d6): -647. Minor Component. 1H NMR (223 K, acetone-d6): 7.96 (1H, br d, 3J = 7.8 Hz, pyridine), 7.91 (1H, t, 3J = 7.8 Hz, pyridine), 7.79 (1H, dd, 3J = 8.5 Hz, 4JH-F = 5.6 Hz), 7.74 (1H, dd, 3JH-F = 10 Hz, 4J = 2.6 Hz, H ortho to Pt and F), 7.30 (1H, br d, 3J = 7.6 Hz, pyridine), 6.92 (1H, td, 3JH-H, H-F = 8.6 Hz, 4J = 2.6 Hz), 4.03 (2H, q, 3J = 7.4 Hz, CH2), 1.22 (3H, t, 3J = 7.5 Hz, CH3). 19F NMR (203 K, acetone-d6): -109.5 (4JF-Pt = 24 Hz). 195Pt NMR (193 K, acetone-d6): -578. Oxidation of Platinum Chloride Bridged Dimer 5c. To a chloroform solution of 5c was added (dichloroiodo)benzene (1 equiv) and the resulting precipitate filtered and washed with chloroform.

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Organometallics, Vol. 29, No. 8, 2010

HR-MS (ESI): m/z 535.9621 calcd for C14H1335Cl3FNPtNa = (M þ Na)þ 535.9616. Anal. Found (expected for C14H13Cl3FNPt): C 31.20 (32.61); H 2.30 (2.54); N 2.41 (2.72). NMR data were then recorded in acetone-d6. 1H NMR (298 K, acetone-d6): 8.13-8.15 (2H, m, pyridine), 7.90 (1H, dd, 3J = 8.5 Hz, 4JH-F = 5.5 Hz), 7.65 (1H, br dd, 3JH-F = 10 Hz, 4J = 2 Hz, H ortho to Pt and F), 7.56 (1H, br m, pyridine), 7.14 (1H, td, 3JH-F = 3J = 8.5 Hz, 4J = 2.4 Hz), 3.44 (2H, br m, CH2), 1.93 (2H, sextet, 3J = 7.5 Hz, CH2), 1.09 (3H, t, 3J = 7.5 Hz, CH3). 19F NMR (298 K, acetone-d6): -107.2 (4JF-Pt = 32 Hz). 1 H NMR (213 K, acetone-d6): 8.12 (1H, br d, 3J = 7.8 Hz, pyridine), 8.07 (1H, t, 3J = 7.8 Hz, pyridine), 7.90 (1H, dd, 3J = 8.7 Hz, 4JH-F = 5.7 Hz), 7.66 (2H, s, H2O), 7.55 (1H, dd, 3JH-F = 9.8 Hz, 4J = 2.4 Hz, H ortho to Pt and F), 7.51 (1H, dd, 3J = 7.6 Hz, 4J = 1.3 Hz, pyridine), 7.10 (1H, m), 3.41 (2H, m, CH2), 1.80 (2H, m, CH2), 1.00 (3H, t, 3J = 7.2 Hz, CH3). 195Pt NMR (213 K, acetone-d6): -641. DFT Calculations. All DFT calculations used the Amsterdam density functional (ADF) code version 2008.01.50 The general features available in ADF have been described.58 Here, we have used scalar zero-order regular approximation (ZORA) relativi194

(58) Velde, G. t.; Baerends, E. J. J. Comput. Phys. 1992, 99, 84.

Crosby et al. sitic corrections with the OPBE functional59,60 and the supplied frozen-core, triple-ζ plus polarization ZORA basis sets. Solvation effects were included via the conductor-like screening model (COSMO) as implemented in ADF. Default SCF and geometry optimization convergence criteria were used.

Acknowledgment. We thank Warwick University for a WPRS award to S.H.C., the EPSRC UK National Crystallographic Service for collecting the diffraction data for the crystals of 2a and 3b, and support from Advantage West Midlands (partly funded by the European Regional Development Fund) for the purchase of an XRD system that was used to solve the crystal structures of 3c and 5c. Supporting Information Available: CIFs of all X-ray structures together with calculated coordinates and relative energies of all our DFT calculations are available free of charge via the Internet at http://pubs.acs.org. (59) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. (60) Handy, N. C.; Cohen, A. J. Mol. Phys. 2001, 99, 403.