Cycloaddition Reactions of Terminal Alkynes and Phosphaalkynes

These 5-coordinate complexes are inert toward many ligands, but they do reluctantly .... Terminal Alkyne Addition to the 5-Coordinate Platinum(IV) Com...
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Organometallics 2009, 28, 1425–1434

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Cycloaddition Reactions of Terminal Alkynes and Phosphaalkynes with an Isolable 5-Coordinate β-Diiminate Platinum(IV) Dihydrosilyl Complex Nathan M. West,† Peter S. White,† Joseph L. Templeton,*,† and John F. Nixon*,‡ Department of Chemistry, UniVersity of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, and Chemistry Department, School of Life Sciences, UniVersity of Sussex, Falmer, Brighton, U.K. BN19QJ ReceiVed October 8, 2008

Combining (Cl-nacnac)Pt(H)(1-pentene) (Cl-nacnac ) N-p-Cl-phenyl-β-diiminate) and HSiR3 (SiR3 ) SiPh3, SiEt3, SiHPh2) leads to rapid displacement of 1-pentene; oxidative addition of the Si-H bond produces a stable 5-coordinate Pt(IV) silyl dihydride complex which has been isolated and characterized crystallographically. The 5-coordinate silyl dihydride platinum(IV) complex undergoes a cycloaddition reaction with either acetylene or phosphaalkyne to form a vinyl or phosphaalkenyl Pt(IV) product, respectively. One acetylenic carbon adds to the central methine carbon of the Cl-nacnac ligand and the other alkyne carbon or phosphorus coordinates to the vacant site of the d6 metal. Phosphaalkynes are polarized such that the carbon is at the negative end of the dipole; however we observe a final product that reflects a reversal of this normal polarity. The phosphorus is nucleophilic even after the cycloaddition reaction is complete, and its lone pair can be methylated by MeOTf (OTf ) trifluoromethanesulfonate) or protonated by addition of HBF4 · Et2O. Introduction Sixteen-electron 5-coordinate d6 complexes play an important role as intermediates in both bond activation and reductive elimination reactions involving group 10 transition metals. Formation of a 5-coordinate intermediate has been shown to be the first step along the reaction pathway leading to reductive elimination from a Pt(IV) center. Although 5-coordinate d6 species have long been postulated, only a few examples have been isolated and structurally characterized. Several 5-coordinate derivatives of the PtMe3 moiety with chelating nitrogen donors, typified by (nacnac)PtMe3, (anilide-imine)PtMe3, and (pyridylpyrrolide)PtMe3, have been reported by Goldberg since 2000.1-4 Wang recently described a 5-coordinate (1,2-bis(N-7-azaindolyl)benzene)PtMe3 complex.5 Cationic dihydrosilyl Pt complexes containing the protonated Tp′ (Tp′ ) hydrotris(3,5dimethylpyrazolyl)borate) ligand ([κ2-((Hpz*)BHpz*2)Pt(H)2(SiR3)][BAr′4]; Ar′ ) 3,5-trifluoromethylphenyl) have been characterized.6 The (pyridine-indolyl)Pt(SiMe3)(Me)2 complex recently reported by Tilley is proposed to be 5-coordinate, although no X-ray structure could be obtained.7 A 5-coordinate * To whom correspondence should be addressed. E-mail: [email protected] (J.L.T.); [email protected] (J.F.N.). † University of North Carolina at Chapel Hill. ‡ University of Sussex. (1) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem. Soc. 2001, 123, 6423. (2) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem. Soc. 2003, 125, 15286. (3) Kloek, S. M.; Goldberg, K. I. J. Am. Chem. Soc. 2007, 129, 3460. (4) Luedtke, A. T.; Goldberg, K. I. Inorg. Chem. 2007, 46, 8496. (5) Zhao, S.-B.; Wu, G.; Wang, S. Organometallics 2008, 27, 1030. (6) Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J. L. J. Am. Chem. Soc. 2001, 123, 6425. (7) Karshtedt, D.; McBee, J. L.; Bell, A. T.; Tilley, T. D. Organometallics 2006, 25, 1801.

dimethylsilyl Pt(IV) complex supported by the bis(8-quinolyl)methylsilyl ligand was also reported by Tilley.8 A convenient synthesis of β-diiminate Pt olefin hydride complexes, in particular (nacnac)Pt(H)(1-pentene), is available by dehydrogenation of alkanes or ethers with a (nacnac)PtMe fragment formed in situ.9,10 Unlike related β-diiminate Pt complexes that contain bulky substituents in the ortho positions of the phenyl rings on the nacnac ligand, these Pt(II) complexes can undergo rapid associative exchange of the olefin for other ligands.2 The lability of the olefin has previously allowed us to explore the reactivity of the (nacnac)Pt(H) fragment toward alkynes.11 Having observed C-H activation with the (nacnac)Pt framework, we now report efforts to probe reactivity patterns of Si-H bonds. We report here that the olefin hydride Pt(II) reagent reacts with HSiR3, and olefin displacement is followed by cleavage of the Si-H bond. The resulting compound is a stable 5-coordinate 16-electron d6 Pt(IV) silyl dihydride complex, (Clnacnac)Pt(SiR3)(H)2, that has been characterized by NMR, IR, and X-ray crystallography. These 5-coordinate complexes are inert toward many ligands, but they do reluctantly bind trialkylphosphines trans to the silyl ligand. Addition of acetylene or phosphaalkynes results in cycloaddition across the Cl-nacnac central CH and Pt, yielding an anionic tridentate N,N,C or N,N,P ligand. It is interesting to note that in the cycloaddition products the geometry has changed from having both hydride ligands trans to N to having one trans to N and one trans to the newly formed anionic ligand. The phosphaalkenyl complex represents the first report of such a ligand bound to Pt(IV). While free (8) Sangtrirutnugul, P.; Tilley, T. D. Organometallics 2008, 27, 2223. (9) West, N. M.; White, P. S.; Templeton, J. L. J. Am. Chem. Soc. 2007, 129, 12372. (10) West, N. M.; Templeton, J. L. Can. J. Chem. 2009, 87, 288. (11) West, N. M.; White, P. S.; Templeton, J. L. Organometallics 2008, 27, 5252.

10.1021/om800968v CCC: $40.75  2009 American Chemical Society Publication on Web 02/11/2009

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West et al.

Figure 1. X-ray structure of (Cl-nacnac)Pt(H)2(SiPh3) (2a). Thermal ellipsoids are given at the 50% probability level. The platinum hydrides were not located in the difference Fourier map. Selected bond distances in Å: Pt1-Si1 ) 2.286, Pt1-N1 ) 2.099, Pt1-N5 ) 2.076. Selected angles in deg: N1-Pt1-Si1 ) 108.98, N5-Pt1-Si1 ) 114.36, C3-Pt1-Si1 ) 116.26, N1-Pt1-N5 ) 90.41.

phosphaalkynes are polarized in the sense Pδ+-Cδ ,12-14 the insertion reaction of the phosphaalkyne with the platinum(IV) Cl-nacnac silyl dihydride reagent binds the phosphorus to the electrophilic platinum.

Table 1. Platinum Hydride 1H NMR Data for (Cl-nacnac)Pt(H)2(SiR3) Complexes

_

Results and Discussion Synthesis of (Cl-nacnac)Pt(H)2(SiR3) (2). Addition of triphenylsilane to (Cl-nacnac)Pt(H)(1-pentene) (1; Cl-nacnac ) N-pCl-phenyl-β-diiminate) resulted in quantitative formation of a new species in less than 1 min. The product was shown to be (Cl-nacnac)Pt(H)2(SiPh3) (2a). It has Cs symmetry, as evidenced by its 1H NMR, which exhibits a single resonance for both Clnacnac CH3 groups and one resonance at -15.62 ppm (1JPt-H ) 959 Hz) for the two equivalent Pt hydrides (eq 1). The hydride 1 H NMR resonance is similar to that observed for the cationic silyl dihydride [κ2-((Hpz*BHpz*)2Pt(H)2(SiPh3)][BAr′4] (-15.05 ppm (1JPt-H ) 986 Hz)).6 A single infrared absorption for both Pt hydrides appears at 2190 cm-1 in the solution IR spectrum, consistent with Pt-H stretching and incompatible with an η2silane structure. The symmetric nature of the complex indicates that the hydrides are probably located roughly trans to the Clnacnac nitrogens and the open site of the octahedron is trans to the silyl ligand. Presumably this is the thermodynamic isomer, due to the high ligand field strength of the silyl ligand.

Air-stable yellow crystals of 2a (98% isolated yield) were grown by dissolving 1 and an excess of HSiPh3 in a minimum volume of pentane and cooling the solution to 0 °C for 2 h. (12) Weber, L. Coord. Chem. ReV. 2005, 249, 741. (13) Pombeiro, A. J. L. J. Organomet. Chem. 2001, 632, 215. (14) Dillon, K. B.; Mathey, F.; Nixon, J. F. Phosphorus: The Carbon Copy; Wiley: Chichester, U.K., 1998.

SiR3

δ(Pt-H) (ppm)

SiPh3 (2a) SiEt3 (2b) SiHPh2 (2c)

-15.62 -16.67 -15.65

a

JPt-H (Hz)

ν(Pt-H) (cm-1)

959 1006 949

2190a 2205a 2194b

1

CH2Cl2 solution IR. b 9:1 hexanes-CH2Cl2 solution IR.

A single-crystal X-ray analysis of 2a (Figure 1) confirmed the proposed structure. Although the hydrides were not located in the difference Fourier map, the silyl ligand is roughly orthogonal to the plane of the Cl-nacnac ligand, as expected for a square-based-pyramidal or a distorted-square-basedpyramidal structure. The C3-Pt1-Si1 angle of 116.26° indicates that the silyl ligand is tilted away from the Cl-nacnac ligand, probably because of steric interactions between the phenyl groups. The hydrides could be directly trans to the nitrogens, resulting in a square-based-pyramidal structure, or could be symmetrically distorted out of the square plane, resulting in a distorted-square-based-pyramidal structure; both possibilities would yield symmetrical hydrides in the 1H NMR. While it is conceivable that one hydride could be trans to the silyl and the other hydride in the plane of the Cl-nacnac ligand in a trigonal-bipyramidal structure, we believe this is unlikely. The two distinct hydride sites would produce dramatically different chemical shifts, and although there is no evidence for a fluxional process that would equilibrate the two hydrides even upon low-temperature NMR at 193 K, we cannot rule out a fluxional process occurring with a barrier too low to freeze out at this temperature. Reaction of 1 with HSiEt3 or H2SiPh2 results in rapid conversion to the corresponding (Cl-nacnac)Pt(H)2(SiR3) (SiR3 ) SiEt3 (2b), SiHPh2 (2c)). The NMR data for these complexes are similar to the data for complex 2a; the Pt-H data for all of these complexes are tabulated in Table 1. While 2a,b form stable solids from pentane, all three decompose over time in CH2Cl2; the diphenyl species 2c decomposes the fastest. No exchange between the silyl proton and platinum hydrides is observed in 2c, and in addition no exchange between bound and free silane occurs for 2a-c, suggesting that these complexes are static as Pt(IV) dihydrides in solution rather than η2-silanes. Addition of D2SiPh2 to 1 resulted in formation of (Cl-nacnac)Pt(H)-

Cycloaddition to a 5-Coordinate Pt(IV) Complex

(D)(DSiPh2) (2c-d2). Complex 2c-d2 exhibits a singlet for the hydride which is shifted about 0.1 ppm downfield from the corresponding dihydride chemical shift for 2c. No H-D coupling is observed in the proton NMR, thus reinforcing the identity of the complex as a classical dihydride rather than an η2-H2 species. In this reaction the 1-pentene ligand is rapidly displaced by the silane en route to the oxidative addition reaction that yields the silyl dihydride complex. It is worth noting that this synthetic method is in contrast with other 5-coordinate Pt(IV) complex preparations. The PtMe3 derivatives are formed from an intact [PtMe3]+ fragment by reaction with the respective ligand.1,3-5 The Pt(SiMe3)Me2 complex is generated by an SN2 type reaction between an anionic Pt center and TMSOTf rather than via an Si-H bond activation process.7 The (bis(8-quinolyl)methylsilyl)PtMe2 complex becomes 5-coordinate upon abstraction of a halide or triflate ion trans to the silyl ligand.8 The cationic Tp′-based silyl dihydrides are generated by oxidative addition of a Si-H bond, but they are formed from a [(N-N)Pt(solvent)(H)]+ precursor complex, and thus the silane has only to displace a weakly coordinated solvent molecule.6 In the present Cl-nacnac case there is a strong π-acid ligand, 1-pentene, displaced by the Si-H bond. One might expect the displaced pentene to bind to the open coordination site on the d6 metal in the 5-coordinate product; we see no evidence for interaction between the olefin and the metal. This is true even if excess ethylene, which is a more strongly binding olefin than pentene, is added to a solution of 2. The strong trans influence of the silyl ligand and high oxidation state of the metal evidently disfavor the binding of olefins. Trimethylphosphine Addition to the 5-Coordinate Platinum(IV) Complex 2. Given that the 5-coordinate Pt(IV) complex does not bind the 1-pentene released in forming 2 from 1, we explored interactions with other potential ligands. One would anticipate that this electron-deficient coordinatively unsaturated Pt(IV) species would be electrophilic and bind η1 σ-donor ligands such as nitriles. However, we have observed no indication that nitriles bind to this Pt(IV) center even upon dissolving 2a in neat acetonitrile. We also see no evidence for binding of carbon monoxide or ethylene to 2a. Only with strongly σ-donating phosphines were we able to observe ligand binding. Compound 2a reacts with PMe3 to form a 6-coordinate compound 3 (eq 2). Trimethylphosphine is a strong ligand, but here it appears to be bound only weakly due to the trans influence of the silyl ligand. When excess PMe3 is present, we observe rapid exchange on the NMR time scale between the bound and the free phosphine. Also, the Pt-P coupling constant of 745 Hz is unusually low even for a Pt(IV)-P bond, which will have a smaller coupling constant than a Pt(0) or Pt(II) complex due in part to less Pt s orbital contribution to the bond in the Pt(IV) case; a typical 1JPt-P coupling constant for a Pt(IV)-PMe3 adduct would be at least 1200 Hz.15-18

Terminal Alkyne Addition to the 5-Coordinate Platinum(IV) Complex 2. A surprising reaction was observed when the triphenylsilyl complex 2a was exposed to acetylene in CD2Cl2 at low temperature. The 1H NMR of the resulting product, 4a, showed a C1-symmetric species with two inequiva-

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lent hydrides that exhibited two-bond coupling of about 3 Hz to each other. This was unexpected, because in the PMe3 addition reaction the phosphine simply fills the vacant octahedral site and does not perturb the fundamental framework of the reagent, thus maintaining Cs symmetry and leaving the two hydrides equivalent. With the acetylene product the two hydrides have widely disparate chemical shifts and significantly different 1 JPt-H coupling constants, clearly indicating that they are trans to different ligands. One hydride resonates at -5.69 ppm with a coupling to Pt of 773 Hz, and the other resonates at -18.72 ppm and has a 1JPt-H coupling constant of 1242 Hz (Figure 2). The hydride at -5.7 is trans to a strong ligand as indicated by both its low coupling constant and its low field chemical shift. Platinum hydrides trans to alkyl or aryl ligands typically resonate downfield of -10 ppm and have coupling constants with magnitudes of 600-800 Hz,19-21 while Pt hydrides trans to L type nitrogen ligands typically resonate upfield of -15 ppm and have coupling constants greater than 1000 Hz.22-25 The hydride at -18.7 has a chemical shift and one-bond coupling constant typical for a hydride trans to nitrogen. These data suggest that the silyl ligand has moved into the plane of the Cl-nacnac along with one hydride and that the other hydride is now located cis to the Cl-nacnac ligand and is trans to the new ligand. Importantly, the 1H NMR also contains an informative doublet at 8.14 ppm which has an H-H coupling of 8.8 Hz and Pt coupling of 74 Hz; this doublet is surely due to one of the original acetylene protons. The mated proton is difficult to observe due to overlap with aromatic signals, but it can be identified in the 2D H-H COSY NMR. The second proton resonates at 6.52 ppm, but it appears as a multiplet rather than as a simple doublet that would be expected for a static η2-acetylene ligand. In addition to being coupled to the proton that resonates at 8.14 ppm, the resonance at 6.52 ppm is coupled to a doublet at 5.36 ppm with 16 Hz coupling. The other proton signals for this complex are assigned to aromatic protons and to two inequivalent Cl-nacnac methyl groups. Note that we no longer observe a simple singlet for the central Cl-nacnac methine proton; it has presumably become the doublet at 5.36 ppm. We postulate that cycloaddition of acetylene across platinum and the methine carbon has occurred to form a tridentate N,N,C bis-imine vinyl ligand (eq 3). This type of reaction has been observed recently with acetylene and a ruthenium β-diiminate complex.26 There have also been examples of cycloaddition of alkenes and alkynes across the CH and Al atoms in a [(nacnac)AlR]+ complex reported by Jordan.27 In another recent example involving a main-group-element cycloaddition of (15) Williams, B. S.; Goldberg, K. I. J. Am. Chem. Soc. 2001, 123, 2576. (16) Hitchcock, P. B.; Nixon, J. F.; Sakarya, N. Chem. Commun. 2000, 1745. (17) Jones, R.; Kelly, P. F.; Williams, D. J.; Woollins, J. D. J. Chem. Soc., Dalton Trans. 1988, 1569. (18) Goggin, P. L.; Goodfellow, R. J.; Haddock, S. R.; Knight, J. R.; Reed, F. J. S.; Taylor, B. F. J. Chem. Soc., Dalton Trans. 1974, 523. (19) Arnold, D. P.; Bennett, M. A. Inorg. Chem. 1984, 23, 2110. (20) Arnold, D. P.; Bennett, M. A. Inorg. Chem. 1984, 23, 2117. (21) Ros, R.; Michelin, R. A.; Bataillard, R.; Roulet, R. J. Organomet. Chem. 1977, 139, 355. (22) Church, M. J.; Mays, M. J. J. Chem. Soc. A 1968, 3074. (23) Holtcamp, M. W.; Henling, L. M.; Day, M. W.; Labinger, J. A.; Bercaw, J. E. Inorg. Chim. Acta 1998, 270, 467. (24) Reinartz, S.; Baik, M. H.; White, P. S.; Brookhart, M.; Templeton, J. L. Inorg. Chem. 2001, 40, 4726. (25) Chen, G. S.; Labinger, J. A.; Bercaw, J. E. Proc. Natl. Acad. Sci. USA 2007, 104, 6915. (26) Phillips, A. D.; Laurenczy, G.; Scopelliti, R.; Dyson, P. J. Organometallics 2007, 26, 1120. (27) Radzewich, C. E.; Coles, M. P.; Jordan, R. F. J. Am. Chem. Soc. 1998, 120, 9384.

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West et al.

Figure 2. Hydride region of the 1H NMR (400 MHz, 243 K) of acetylene product 4a.

acetylene and phenylacetylene to a dianionic diiminate based germylene complex was reported by Driess and co-workers.28 Iron, titanium, and copper nacnac complexes have been reported to add oxygen- and nitrogen-containing unsaturated molecules across the metal and ligand CH.29-31 Acetylacetonate complexes of Pd(II), Rh(I), and Ir(I) added the activated alkyne hexafluoro2-butyne across the M and central CH as well, but unactivated alkynes did not react, and unlike our system, these are all d8 metal reagents.32,33 A number of macrocyclic compounds of Co, Rh, Fe, and Ru with ligands based on multiple β-diiminate units have been reported to undergo cycloadditions.34-39

Most relevant to this work is the report by Dyson and coworkers that acetylene reacts with a cationic β-diiminate Ru(II) (28) Yao, S.; van Wullen, C.; Driess, M. Chem. Commun. 2008, 5393. (29) Gregory, E. A.; Lachicotte, R. J.; Holland, P. L. Organometallics 2005, 24, 1803. (30) Basuli, F.; Huffman, J. C.; Mindiola, D. J. Inorg. Chem. 2003, 42, 8003. (31) Yokota, S.; Tachi, Y.; Itoh, S. Inorg. Chem. 2002, 41, 1342. (32) Russell, D. R.; Tucker, P. A. J. Chem. Soc., Dalton Trans. 1975, 1743. (33) Russell, D. R.; Tucker, P. A. J. Chem. Soc., Dalton Trans. 1975, 1749. (34) Weiss, M. C.; Goedken, V. L. J. Am. Chem. Soc. 1976, 98, 3389. (35) Weiss, M. C.; Gordon, G. C.; Goedken, V. L. J. Am. Chem. Soc. 1979, 101, 857. (36) Kofod, P.; Moore, P.; Alcock, N. W.; Clase, H. J. J. Chem. Soc., Chem. Commun. 1992, 1261. (37) Setsune, J.; Ikeda, M.; Kitao, T. J. Am. Chem. Soc. 1987, 109, 6515. (38) Riley, D. P.; Stone, J. A.; Busch, D. H. J. Am. Chem. Soc. 1977, 99, 767. (39) Cotton, F. A.; Czuchajowska, J. J. Am. Chem. Soc. 1991, 113, 3427.

d6 16-electron arene complex to form a cycloaddition product.26 Comparison of the 13C NMR data for our Pt product and the Ru acetylene insertion product shows a consistency that suggests that the complexes are analogous. Complex 4a shows 13C signals at 167.2, 115.7, and 71.5 ppm for the Pt-CR, CRdCβ, and Cβ-Cnacnac carbons, respectively (eq 3). The Ru acetylene complex displays signals at 169.4, 116.7, and 63.1 ppm for the congruent carbons. In addition, the chemical shift for the Clnacnac imine carbon shifts about 30 ppm downfield because the electrons are now localized in the C)N unit of the ligand. The Pt couplings for CR, Cβ, and Cnacnac are 478, 44, and 64 Hz, respectively, reflecting the one-, two-, and three-bond couplings. It is also interesting to note that we observe coupling between the hydrides and the vinyl protons. The hydride at -5.7 couples to the proton on Cβ with a four-bond coupling constant of 3.6 Hz, but no coupling is observed to the proton on CR. This strong four-bond coupling is probably observed because there is strong orbital coupling between the trans hydride across the Pt atom into the Pt-CR bond which is aligned in a zigzag fashion with the proton on Cβ, facilitating coupling through the σ-bonding framework analogous to trans alkene coupling. The sp2 hybridization of CR causes poor coupling to the proton on CR analogous to geminal alkene coupling. The hydride at -18.7 ppm exhibits a small three-bond coupling to the proton on CR, on the order of 1 Hz, just enough to broaden the peaks and to show up in the H-H COSY NMR. The coupling between the hydride at -18.7 ppm and the proton on CR may be a throughspace coupling, because we know that this cis hydride is orthogonal to the Pt-CR bond and should not couple to the vinyl group as strongly as the trans hydride. All of these couplings constants support the proposed geometry of the complex. The arrangement of the vinyl ligand trans to the silyl ligand is unfavorable because of the strong trans influence of each ligand. However, we believe that the rearrangement of the silyl group into the nacnac plane is primarily driven by the distortion of

Cycloaddition to a 5-Coordinate Pt(IV) Complex

the nacnac ligand which forces the N-aryl groups towards the SiPh3 group. These unfavorable steric interactions are relieved by moving the SiPh3 group trans to N. The phosphine complex 3 does not distort the nacnac ligand and therefore no rearrangement occurs. The reaction with acetylene is fast and results in complete conversion of 2a to 4a. The compound is stable in solution below -30 °C, but it decomposes in 1-2 h at room temperature. This is not surprising, as it is a cis alkenyl hydride complex and probably undergoes facile reductive elimination. Addition of phenylacetylene also results in cycloaddition, but the reaction is considerably slower and requires a large excess of alkyne to drive the reaction toward products. The phenylacetylene product 4b is the isomer in which the terminal carbon binds to the Pt and the phenyl-bound carbon binds to the Cl-nacnac CH. This product is sterically favored over the other possibility, in which the phenyl group would be forced into the SiPh3 group. No insertion products are observed when internal alkynes are used. Also, no reaction is observed with terminal alkyl alkynes, including propyne. This may be because the electron-donating nature of the alkyl substituent deactivates the alkyne toward nucleophilic attack by the Cl-nacnac CH. Less likely is the possibility is that the planar phenyl group allows easier access to the alkyne carbon than an alkyl group. Calculations on the cycloaddition of alkynes and alkenes to [(nacnac)AlMe]+ reflected the same pattern, as electron-donating methyl groups on the olefins inhibit cycloaddition.40 The diphenylsilyl platinum species 2c reacts with phenylacetylene to form the analogous cycloaddition product, 4c. However, if the triethylsilyl derivative 2b is used, the reaction is significantly inhibited. No reaction with phenylacetylene is observed with 2b, and the reaction with acetylene is slow and results in only a trace amount of the cycloaddition product, even in the presence of excess acetylene. It is probable that the more electron-rich triethylsilyl ligand stabilizes the electron-deficient metal, thus decreasing its Lewis acidity and thereby inhibiting the reaction. This insertion reaction is extremely sensitive to the nature of the alkyne and the metal complex. Phosphaalkyne Addition to the 5-Coordinate Platinum(IV) Complex 2. After observing the propensity for the 5-coordinate Pt(IV) complex to undergo cycloaddition reactions, we turned to a more exotic substrate that has been shown to be well suited for insertion reactions: phosphaalkynes. Phosphaalkynes readily participate in cyclization reactions with both organic substrates and transition-metal complexes. Reactions of Pt complexes with phosphaalkynes have been limited to platinum in the oxidation states of 0, I, or II, and in all but one case free phosphaalkynes simply coordinate η2 to the Pt; there is one example of a phosphaalkyne bridging across two platinum atoms.41 Phosphorus is a soft donor atom that stabilizes lowoxidation-state transition metals, and π-acid ligands such as alkynes and phosphaalkynes bind more strongly to lower oxidation state metals. To date there have been no literature reports of Pt(IV) phosphaalkyne reactions. Addition of tert-butyl phosphaalkyne to 2a at -78 °C resulted in a rapid color change from light yellow to bright red. As the solution was warmed to room temperature, the color faded back to light yellow, before turning black as the compound decomposed over 1 h at room temperature. Proton NMR suggested that the product, 5a, was a cycloaddition product analogous to that observed from the insertion of acetylene. The signature of an insertion between the Cl-nacnac methine and the platinum (40) Ariafard, A.; Lin, Z.; Jordan, R. F. Organometallics 2005, 24, 5140. (41) Al-Resayes, S. I.; Nixon, J. F. Inorg. Chim. Acta 1993, 212, 265.

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center is two drastically different hydride signals for the complex: a doublet at -4.64 ppm with two-bond phosphorus coupling of 62 Hz and one-bond Pt coupling of 929 Hz, and a singlet at -17.8 ppm with one-bond Pt coupling of 1179 Hz and no visible phosphorus coupling (Figure 3). There were also signals in the 1H NMR for two inequivalent Cl-nacnac methyl groups, one phosphaalkyne t-Bu group, and the Cl-nacnac CH as a doublet at 5.24 ppm with 8 Hz three-bond coupling to phosphorus. The hydride at -4.6 is trans to the strong phosphaalkenyl ligand, resulting in the unusual chemical shift and the low Pt-H coupling constant; the strong coupling to P is surely due to the trans orientation of the ligands, resulting in heavy orbital overlap. The hydride at -17.8 ppm is cis to the new P donor atom and has a chemical shift and Pt coupling constant typical for a hydride trans to a nitrogen; note also that there is no coupling to P due to the orthogonal orientation of the H and P atoms. The phosphaalkenyl phosphorus appears as a broad singlet at 371 ppm in the 31P NMR spectrum and displays 95 Hz coupling to platinum. The low-field chemical shift is in the range of an η2-phosphaalkyne or an unstabilized η1-phosphaalkene, and the small coupling to platinum is typical of π-bound phosphaalkynes, as the bonding orbital on phosphorus is almost entirely p in character. In general, the more s character in the molecular orbital the larger the magnitude of the nuclear spin-spin coupling.42,43 This means that the remaining lone pair on the P atom has mostly s character and is not strongly nucleophilic. In sum the 1H and 31P NMR data indicate that the phosphorus has coordinated to the Pt while the carbon has coupled to the Cl-nacnac methine carbon (eq 4).

Free phosphaalkynes are typically polarized such that the phosphorus is electrophilic and the carbon is nucleophilic, RCδ-tPδ+, due to the greater electronegativity of carbon.12-14 This polarization is manifested in the addition of hydrohalogens across the triple bond such that the carbon is protonated and the halide attacks at phosphorus, forming products of the type (R)(H)CdPX.13,14,44,45 Thus, one might expect the nucleophilic CH to bind to the phosphorus and the electrophilic metal to bind to the carbon, but this is not observed. The 62 Hz coupling to the trans hydride at -4.6 ppm is too large to be three-bond coupling and is large even for two-bond coupling, probably due to the strong orbital overlap because of the trans nature of the ligands. The platinum coupling constant to the hydride at -4.6 ppm is about 140 Hz larger than for the analogous hydride in the alkyne addition product 4. The larger coupling indicates that the hydride is trans to a weaker ligand, which again suggests that the P atom is bound to Pt as it is typically a weaker ligand than a C atom. If the C of the phosphaalkyne were bound to Pt, the hydride should have a similar one-bond Pt coupling (42) Muller, N.; Pritchard, D. E. J. Chem. Phys. 1959, 31, 768. (43) Maciel, G. E., Jr.; Ostlund, N. S.; Pople, J. A. J. Am. Chem. Soc. 1970, 92, 1. (44) Hitchcock, P. B.; Lemos, M. A. N. D. A.; Meidine, M. F.; Nixon, J. F.; Pombeiro, A. J. L. J. Organomet. Chem. 1991, 402, C23. (45) Hitchcock, P. B.; Johnson, J. A.; Lemos, M. A. N. D. A.; Meidine, M. F.; Nixon, J. F.; Pombeiro, A. J. L. J. Chem. Soc., Chem. Commun. 1992, 645.

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Figure 3. Hydride resonances of the t-BuCtP addition product 5a (1H NMR, 263 K).

to the alkyne addition product. While one would expect the harder carbon atom to better stabilize the hard Pt(IV) atom, we believe that this binding motif is not observed because of the unfavorable steric interaction between the t-Bu group and the SiPh3 group that it would cause. If the cycloaddition were to occur in a way that maintained the geometry at platinum, that is, with the Pt-SiPh3 bond axis still orthogonal to the Clnacnac ligand, the opposite insertion product would be expected. There would be fewer unfavorable steric interactions if the alkyne substituent were located over the platinum hydrides rather than over the Cl-nacnac ligand. The outcome suggests that isomerization at platinum occurs either before or concomitant with the insertion reaction such that the product geometry determines the direction of insertion. Reaction of the 5-coordinate Pt(IV) reagent 2a with the bulky adamantyl phosphaalkyne (AdCtP) resulted in cycloaddition to form the six-coordinate phosphaalkenyl species (5b) in analogy to the reaction with t-BuCtP (eq 4). The NMR data for this species are similar to those of the t-Bu analogue. The rate of reaction is qualitatively the same as for the t-Bu analogue; therefore, the effect of the adamantyl group relative to tertbutyl is minimal. If 2a is reacted with phosphaalkyne in an NMR tube at -78 °C and the reaction is monitored by variable-temperature NMR from 193 to 273 K, several intermediates can be observed before cycloaddition forms complex 5. For the adamantyl derivative three distinct hydride resonances can be observed in a ratio of ca. 1:1.5:2 at -17.3, -18.1, and -18.2 ppm in the 1H NMR at 210 K. At the same time there are two signals at 393.6 and 358.1 ppm in the bound phosphaalkyne region of the 31P NMR as well as free phosphaalkyne at -70 ppm. The hydride at -18.2 ppm and the phosphorus at 393.6 ppm correspond to the major species, while the hydride at -17.3 ppm and the phosphorus at

358.1 ppm correspond to the minor species. The hydrides at -18.2 and -17.3 ppm (8′ and 8′′) have platinum coupling constants of 1154 and 1172 Hz, respectively; phosphorus coupling to platinum could not be determined due to the broadness of the peaks. The coupling constant should be small, as the Pt(0) phosphaalkyne complex (PPh3)2Pt0(t-BuCtP) has a Pt-P coupling constant of only 62 Hz to the phosphaalkyne and the coupling to a Pt(IV) center is expected to be smaller because there is less contribution of the Pt s orbital to the Pt-P bond.42,43,46,47 The peak at -18.1 ppm has a platinum coupling constant of 997 Hz; this species is not bound to a phosphaalkyne, as there is no corresponding phosphorus signal. Furthermore, free phosphaalkyne is present although no excess was added, and ultimately it is all consumed in the reaction. This signal is attributed to complex 2a, which may be in equilibrium with a datively bound phosphaalkyne complex. We have attributed species 8′ and 8′′ to the two possible rotamers of a complex in which the phosphaalkyne is bound in an η2 fashion to the vacant site of 2a with the reagent Cs symmetry maintained such that the hydrides are equivalent. The major species, 8′, is most likely the rotamer in which the adamantyl group is oriented over the platinum hydrides and the phosphorus atom is toward the Clnacnac ligand. The minor species, 8′′, is the opposite rotamer, in which the adamantyl group is oriented over the Cl-nacnac ligand and the phosphorus is toward the platinum hydrides. Correlation between the adamantyl group and the Cl-nacnac phenyl rings of 8′ in the 2D NOESY NMR support these assignments. Species 8′′ is less favored because of steric interactions between the adamantyl group and the Cl-nacnac ligand. Even though 8′′ is the less favored rotamer, it is the one (46) Burckett-St. Laurent, J. C. T. R.; Hitchcock, P. B.; Kroto, H. W.; Nixon, J. F. J. Chem. Soc., Chem. Commun. 1981, 1141. (47) Clark, H. C.; Manzer, L. E. Inorg. Chem. 1974, 13, 1291.

Cycloaddition to a 5-Coordinate Pt(IV) Complex Scheme 1. Observation of Pt(IV) η2-Phosphaalkyne Intermediates by Low-Temperature NMR

Scheme 2. Addition of Electrophiles to the Phosphorus Lone Pair of 5

which will ultimately lead to products, as it is the one that will give the observed product geometry. When the sample is warmed, 8′′ broadens and disappears first. The species without phosphaalkyne and 8′ separate slightly upon warming, appearing at -17.8 and -18.0 ppm at 260 K. These species disappear as the product 5b grows in, and eventually only 5b remains (Scheme 1). The reaction can be monitored by a color change as well; at low temperature the solution is bright red, but as 5b forms, the solution turns back to yellow. Addition of Electrophiles to the Phosphaalkenyl Species 5. Addition of MeOTf (OTf ) trifluoromethanesulfonate) to 5 at 250 K results in rapid methylation of the phosphorus atom, forming the cationic η1-phosphaalkene complex (κ3(R)CdP(Me)(CdNAr)2(CH))}Pt(SiPh3)(H)2][OTf] (6) (R ) t-Bu (6a), R ) Ad (6b)) (Scheme 2). Rapid alkylation of the phosphorus atom at low temperature indicates that the lone pair is nucleophilic. Phosphaalkenes, like phosphaalkynes, typically react with electrophiles at the RR′CdP carbon, except in cases where R or R′ is a good π-donor group such as NMe2, which reverses the polarization ((NMe2)(R)Cδ+dPδ-(R′)).48 Phosphaalkenes bound to transition metals in a bent fashion, as in complex 5, have been shown to react with electrophiles at (48) Weber, L.; Scheffer, M. H.; Stammler, H.-G.; Neumann, B.; Schoeller, W. W.; Sundermann, A.; Laali, K. K. Organometallics 1999, 18, 4216.

Organometallics, Vol. 28, No. 5, 2009 1431

phosphorus.12,14,49,50 In the present case electrophilic attack on the phosphorus is observed; this could be due to either a change in the polarization of the phosphaalkene because of its coordination to Pt or a kinetic preference for the phosphorus lone pair because of the steric bulk surrounding the CdP carbon. The 1H NMR of 6a displayed a doublet at 1.78 ppm with 16.4 Hz phosphorus coupling due to the CdP-CH3 methyl group. The hydride resonance trans to P shifted upfield to -7.78 ppm and displayed a two-bond phosphorus coupling of 266 Hz, more than 4 times the coupling constant of the neutral species 5a, and the platinum coupling constant increased to 1143 Hz. The cis hydride resonance shifted slightly downfield to -17.33 ppm and had 8.8 Hz phosphorus coupling as well as 1154 Hz coupling to platinum. The phosphorus resonance shifted upfield from 371.5 to 216.9 ppm upon methylation; the platinum coupling constant also changed dramatically from 95 to 1359 Hz. A large increase in coupling constant was expected for the methylation reaction, because after methylation the Pt-P bond is essentially donation by the phosphorus lone pair, which is predominately s orbital based, whereas before this bond was predominately phosphorus p orbital based. The phosphorus atom of 5b can be protonated by reaction with HBF4 · Et2O at -78 °C (Scheme 2). The resulting phosphaalkene complex 7 is stable at temperatures below 0 °C. As in the methyl phosphaalkene species 6b, the hydride trans to the phosphorus in the protonated species shifted upfield to -7.46 ppm and the phosphorus coupling constant increased from 60 Hz in the neutral species to 282 Hz in the protonated species. Large three-bond coupling of 23 Hz between the trans hydride and the proton on phosphorus was evident as well. The cis hydride resonates at -17.09 ppm and has only 7 Hz coupling to phosphorus; no coupling to the phosphorus proton is observed. The one-bond platinum coupling constants are 1271 and 1161 Hz for the trans and cis hydrides, respectively; unlike the the case for the methylated species the phosphorus is now a weaker field ligand than the Cl-nacnac nitrogens, as evidenced by the 110 Hz larger platinum coupling constant for the trans hydride than for the cis hydride. This difference is reflected in the 31P NMR spectrum of the species as well; the protonated phosphaalkene P resonates at 166.7 ppm with 1231 Hz coupling to Pt, whereas the methylated phosphaalkene P resonates at 218.7 ppm with 1362 Hz coupling to Pt. The protonated species has less electron density than the methylated analogue and therefore binds more weakly to the metal, which results in the smaller coupling to platinum. The only other example of a Pt hydride trans to a phosphaalkene ligand is the platinum(II) compound (DPCB)Pt(H)(SiMe2Ph) (DPCB ) diphosphinidenecyclobutene), and it displayed a 2JP-H value of 230 Hz, similar to the value observed for our cationic Pt(IV) complexes 6 and 7.51 The platinum coupling constants for the hydrides are similar, reflecting the fact that they are all trans to neutral two-electron donor ligands. The platinum coupling constants to the hydrides provide a quantitative comparison of the trans influence of the η1-phosphaalkenyl and η1-phosphaalkene ligands compared to that of the more heavily studied η1-vinyl and η1-imine ligands. The ordering from highest to lowest field strength is η1-vinyl > η1-phosphaalkenyl . η1-phosphaalkene ≈ η1-imine. Anec(49) Bedford, R. B.; Hill, A. F.; Jones, C. Angew. Chem., Int. Ed. 1996, 35, 547. (50) Bedford, R. B.; Hill, A. F.; Jones, C.; White, A. J. P.; Wilton-Ely, J. D. E. T. J. Chem. Soc., Dalton Trans. 1997, 139. (51) Ozawa, F.; Ishiyama, T.; Yamamoto, S.; Kawagishi, S.; Murakami, H. Organometallics 2004, 23, 1698.

1432 Organometallics, Vol. 28, No. 5, 2009

dotal evidence for a strong trans effect of η1-phosphaalkenyl ligands has been reported, but quantitative comparisons are needed.49 The large phosphorus coupling to the trans hydride and the observable phosphorus coupling to the cis hydride indicate more s character for the Pt-P bond after electrophilic addition to phosphorus. The P-Pt bond can be considered a dative bond made using the lone pair on the phosphorus, which is where most of the s orbital character resides. The phosphorus signal in the 31P NMR shifted from 371 ppm in neutral 5a to 217 ppm in methylated 6a, which is compatible with a report of the methylation of the Ru η1-phosphaalkenyl complex (CO)(Cl)(PPh3)2Ru(PdC(H)(tBu)) with MeI to yield (CO)(Cl)(PPh3)2Ru((Me)PdC(H)(tBu))(I),in which the phosphorus resonance shifts from 450 ppm to 225 ppm.50 Hill and Jones also reported protonation of the Ru η1-phosphaalkenyl complex (CO)(Cl)(RNC)(PPh3)2Ru(PdC(H)(tBu)) to yield [(CO)(Cl)(RNC)(PPh3)2Ru((H)PdC(H)(tBu))][BF4], in which the phosphaalkene P resonates at 164.3 ppm.49 This is only a difference of 2.4 ppm from the 31P resonance of complex 7, and of course both Ru(II) and Pt(IV) are d6 metals. To the best of our knowledge these Pt(IV) phosphaalkenyl complexes are only the second examples of Pt phosphaalkenyl compounds and the first example in the Pt(IV) oxidation state. The previous example was formed by oxidative addition of the P-I bond in (Me3Si)2CdP-I to Pt(0) in (Ph3P)2Pt(C2H4).52 There are many examples of neutral η1-phosphaalkene complexes of Pt(0) or Pt(II), though virtually all of them are kinetically stabilized by bulky groups, most commonly the super mesityl group, bound to phosphorus.51,53-63 We are not aware of any other Pt(IV) complexes with neutral phosphaalkenes. This is consistent with the fact that soft phosphine ligands typically stabilize softer low-valent metals, whereas harder nitrogen ligands typically stabilize high-valent metal atoms.

Conclusions Silanes react with the (Cl-nacnac)Pt(H)(1-pentene) complex 1 to displace pentene, and oxidative addition of the Si-H bond forms stable 5-coordinate Pt(IV) silyl dihydride complexes. The 5-coordinate d6 Pt(IV) complex only weakly binds the strong σ-donor ligand PMe3. On the other hand, acetylene and phospaalkynes react rapidly with the unsaturated 5-coordinate complex 2a by adding across the Cl-nacnac CH and Pt to form (52) Gudat, D.; Meidine, M. F.; Nixon, J. F.; Niecke, E. J. Chem. Soc., Chem. Commun. 1989, 1206. (53) Eshtiagh-Hosseini, H.; Kroto, H. W.; Nixon, J. F.; Maah, M. J.; Taylor, M. J. J. Chem. Soc., Chem. Commun. 1981, 199. (54) Al-Resayes, S. I.; Klein, S. I.; Kroto, H. W.; Meidine, M. F.; Nixon, J. F. J. Chem. Soc., Chem. Commun. 1983, 930. (55) Van der Knaap, T. A.; Bickelhaupt, F.; Kraaykamp, J. G.; Van Koten, G.; Bernards, J. P. C.; Edzes, H. T.; Veeman, W. S.; De Boer, E.; Baerends, E. J. Organometallics 1984, 3, 1804. (56) Kroto, H. W.; Klein, S. I.; Meidine, M. F.; Nixon, J. F.; Harris, R. K.; Packer, K. J.; Reams, P. J. Organomet. Chem. 1985, 280, 281. (57) Kraaijkamp, J. G.; Van Koten, G.; Van der Knaap, T. A.; Bickelhaupt, F.; Stam, C. H. Organometallics 1986, 5, 2014. (58) Brauer, D. J.; Liek, C.; Stelzer, O. J. Organomet. Chem. 2001, 626, 106. (59) Boubekeur, L.; Ricard, L.; Le Floch, P.; Mezailles, N. Organometallics 2005, 24, 3856. (60) Ozawa, F.; Okamoto, H.; Kawagishi, S.; Yamamoto, S.; Minami, T.; Yoshifuji, M. J. Am. Chem. Soc. 2002, 124, 10968. (61) Ozawa, F.; Kawagishi, S.; Ishiyama, T.; Yoshifuji, M. Organometallics 2004, 23, 1325. (62) Benvenutti, M. H. A.; Cenac, N.; Nixon, J. F. Chem. Commun. 1997, 1327. (63) Dugal-Tessier, J.; Dake, G. R.; Gates, D. P. Organometallics 2007, 26, 6481.

West et al.

new monoanionic N,N,C or N,N,P ligands containing either a vinyl or a phosphaalkenyl group. We believe that the alkyne addition reaction occurs because of the Lewis acidic nature of the Pt and Lewis basic nature of the Cl-nacnac CH. The phosphaalkyne addition product is counterintuitive, on the basis of the polarization of free phosphaalkynes in which the carbon is nucleophilic. We believe the isomer with phosphorus bound to platinum is the thermodynamically preferred one. This is probably due to the severe steric interactions that would occur in the other isomer. The phosphaalkenyl lone pair showed significant nucleophilicity and reacted cleanly with MeOTf to form a cationic methyl phosphaalkene complex. The phosphaalkenyl complex 5 can also be protonated cleanly by reaction with HBF4 · Et2O. The trans influence of the phosphaalkene ligand in the cationic complexes was significantly lower than that of the phosphaalkenyl ligand in the neutral complex, as assessed by the platinum hydride coupling constant values for the corresponding trans hydrides.

Experimental Section General Procedures. Unless otherwise stated, all reactions were performed under an atmosphere of dry nitrogen or argon using standard Schlenk and drybox techniques. Argon and nitrogen were purified by passage through columns of BASF R3-11 catalyst and 4 Å molecular sieves. All glassware was flame-dried under vacuum and cooled under N2 before use. Diethyl ether, methylene chloride, toluene, and pentane were purified under an argon atmosphere and passed through a column packed with activated alumina.64 Tetrahydrofuran was distilled from sodium/benzophenone ketyl. Methylene chloride-d2 was vacuum-transferred from calcium hydride and degassed by several freeze-pump-thaw cycles. Complex 1, (Cl-nacnac)Pt(H)(1-pentene), was synthesized via published procedures.11 The t-Bu phosphaalkyne was provided by John Nixon. All other reagents, including adamantyl phosphaalkyne, were used as received from Sigma Aldrich. 1 H NMR and 13C NMR spectra were recorded on Bruker AMX 300 MHz, Bruker Avance 400 MHz, and Bruker DRX 500 MHz spectrometers. 1H NMR and 13C NMR chemical shifts were referenced to residual 1H and 13C signals of the deuterated solvents; 31 P NMR spectra were referenced to external 85% H3PO4. Infrared spectra were recorded on an ASI ReactIR 1000. (Cl-nacnac)Pt(SiPh3)(H)2 (2a). In a vial in open air 30 mg (0.0514 mmol) of 1 and 16 mg (0.0617 mmol, 1.2 equiv) of HSiPh3 were dissolved in 3 mL of pentane. The vial was capped and placed in the refrigerator for 2 h, yielding 39 mg (0.0503 mmol, 98% yield) of yellow block crystals suitable for X-ray diffraction. IR (CH2Cl2): νPt-H 2190 cm-1 (br). 1H NMR (δ, CD2Cl2, 300 MHz): 7.31 (t, 3H, 3JH-H ) 7.4 Hz, p-SiPh3); 7.19 (t, 6H, 3JH-H ) 7.5 Hz, m-SiPh3); 7.05 (d, 6H, 3JH-H ) 6.9 Hz, o-SiPh3); 6.95 (d, 4H, 3JH-H ) 8.4 Hz, m-nacnacPh); 6.23 (d, 4H, 3JH-H ) 8.4 Hz, o-nacnacPh); 5.25 (s, 1H, nacnac-CH); 1.75 (s, 6H, nacnac-CH3); -15.62 (s, 2H, 1JPt-H ) 959 Hz, Pt-(H)2). Anal. Calcd for C35H32N2Cl2PtSi: C, 54.26; H, 4.16; N, 3.62. Found: C, 54.07; H, 3.97; N, 3.40. (Cl-nacnac)Pt(SiEt3)(H)2 (2b). The same procedure was used as for 2a, but HSiEt3 was used instead of HSiPh3. A yellow solid was obtained from a pentane solution of 2b at -30 °C. IR (CH2Cl2): νPt-H 2205 cm-1 (br). 1H NMR (δ, CD2Cl2, 300 MHz): 7.25 (d, 4H, 3JH-H ) 8.4 Hz, m-nacnacPh); 6.96 (d, 4H, 3JH-H ) 8.4 Hz, o-nacnacPh); 5.14 (s, 1H, nacnac-CH); 1.84 (s, 6H, nacnac-CH3); 0.56 (s, 15H, Si(CH2CH3)3); -16.73 (s, 2H, 1JPt-H ) 1006 Hz, Pt-(H)2). Anal. Calcd for C23H32N2Cl2PtSi: C, 43.81; H, 5.11; N, 4.44. Found: C, 43.85; H, 4.84; N, 4.35. (Cl-nacnac)Pt(SiHPh2)(H)2 (2c). The same procedure was used as for 2a, but H2SiPh2 was used instead of HSiPh3. A yellow solid (64) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 1996, 15, 1518.

Cycloaddition to a 5-Coordinate Pt(IV) Complex Table 2. Crystal Data and Structure Refinement Details for 2a empirical formula mol wt temp, K wavelength, Å cryst syst space group a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 Z calcd density, Mg/m3 µ, mm-1 F(000) cryst size, mm3 2θ range, deg index ranges no. of rflns collected no. of indep rflns no. of data/restraints/params goodness of fit on F2 final R indices (I > 2σ(I)) R indices (all data) largest diff peak and hole, e Å-3

C35H32Cl2N2PtSi 774.71 100(2) 1.541 78 triclinic P1j 9.841(2) 11.172(2) 14.862(3) 88.797(9) 82.253(7) 73.627(6) 1553.2(6) 2 1.657 10.606 764 0.20 × 0.10 × 0.10 3.00-70.06 -11 e h e 11 -13 e k e 13 -15 e l e 17 33 592 5693 (R(int) ) 0.0389) 5693/2/380 1.126 R1 ) 0.0236 wR2 ) 0.0756 R1 ) 0.0250 wR2 ) 0.0875 1.127 and -0.628

was obtained from a pentane solution of 2c at -30 °C. 1H NMR (δ, CD2Cl2, 300 MHz): 7.17 (d, 4H, 3JH-H ) 8.4 Hz, m-nacnacPh); 6.61 (d, 4H, 3JH-H ) 8.4 Hz, o-nacnacPh); 5.23 (s, 1H, nacnacCH); 1.79 (s, 6H, nacnac-CH3); -15.66 (s, 2H, 1JPt-H ) 948 Hz, Pt-(H)2). IR (9:1 hexanes-CH2Cl2): νPt-H 2194; νSi-H 2121 cm-1. This compound proved too unstable to obtain satisfactory elemental analysis. (Cl-nacnac)Pt(PMe3)(SiPh3)(H)2 (3). Crystals of 2a (30 mg, 0.039 mmol) were dissolved in 2 mL of CH2Cl2 under nitrogen. Trimethylphosphine (4.4 µL, 1.1 equiv) was added, and the solution was shaken for 30 s, after which the solvent was evacuated. The resultant yellow oil was taken up in 2 mL of pentane, and the solvent was evacuated to yield a yellow solid (32.9 mg 100% yield). 1 H NMR (δ, CD2Cl2, 400 MHz): 7.59 (d, 2H, 3JH-H ) 8.0 Hz, nacnacPh); 7.49 (d, 1H, 3JH-H ) 8.0 Hz, nacnacPh); 7.40 (m, 4H, nacnacPh); 7.28 (t, 1H, 3JH-H ) 8.0 Hz, nacnacPh); 7.18 (t, 3H, 3 JH-H ) 7.2 Hz, p-SiPh3); 7.07 (t, 6H, 3JH-H ) 7.2 Hz, m-SiPh3); 6.96 (d, 6H, 3JH-H ) 7.2 Hz, o-SiPh); 4.63 (s, 1H, nacnac-CH); 1.61 (s, 6H, nacnac-CH3); 1.56 (d, 9H, 2JP-H ) 7.2 Hz, P(CH3)3); -19.26 (s, 2H, 1JPt-H ) 1038 Hz, Pt-(H)2). 31P NMR (δ, CD2Cl2, 162 MHz): -27.73 (s, JPt-P ) 745 Hz, PMe3). Anal. Calcd for C38H41N2Cl2PPtSi: C, 53.65; H, 4.86; N, 3.29. Found: C, 53.93; H, 5.06; N, 3.11. (K3-CHdCH-(Cl-nacnac))Pt(SiPh3)(H)2 (4a). In a J-Young NMR tube was dissolved 10 mg (0.013 mmol) of 2a in 0.6 mL of CD2Cl2. The solvent was freeze-pump-thawed, and 0.13 mmol (10 equiv) of acetylene was condensed into the tube at 77 K. The tube was thawed out and shaken at room temperature before being placed in the NMR probe cooled to 243 K. 1H NMR (δ, CD2Cl2, 400 MHz, 243 K): 8.14 (d, 1H, 3JH-H ) 8.8 Hz, 2JPt-H ) 74 Hz, CHCdCHPt); 7.54 (d, 1H, 3JH-H ) 6.4 Hz, nacnacPh); 7.5-7.3 (m, 5H, nacnacPh); 7.23 (m, 3H, SiPh3); 7.16 (m, 12H, SiPh3); 6.94 (d, 2H, 3JH-H ) 8.0 Hz, nacnacPh); 6.52 (m, 1H, CHCdCHPt); 5.36 (d, 1H, 3JH-H(acetylene) ) 16 Hz, nacnac-CH); 2.24, 2.11 (s, 3H, nacnac-CH3); -5.69 (dd, 1H, 1JPt-H ) 773 Hz, Pt-Htrans-C); -18.72 (d, 1H, 2JH-H ) 2.8 Hz, 1JPt-H ) 1242 Hz, Pt-Htrans-N). 13C NMR (δ, CD2Cl2, 100 MHz, 243 K): 172.4 (2JPt-C ) 16 Hz, (Me)(CH)CdNAr); 172.4 (2JPt-C ) 22 Hz, (Me)(CH)CdNAr); 167.2 (1JPt-C ) 478 Hz, CdC(H)(Pt)); 150.1 (N-Ph ipso C); 148.4

Organometallics, Vol. 28, No. 5, 2009 1433 (2JPt-C ) 11 Hz, N-Ph ipso C); 141.1 (2JPt-C ) 48 Hz, Si-Ph ipso C); 136.1 (3JPt-C ) 17 Hz, Si-Ph ipso C); 135.9, 130.9, 130.5, 129.7, 129.0, 128.7, 128.7, 128.0 (N--Ph C); 127.6 (Si-Ph p-C); 126.8 (Si-Ph m-C); 122.9, 122.9 (N-Ph C-Cl); 115.7 (2JPt-C ) 44 Hz, (H)(C)CdC(H)(Pt)); 71.5 (3JPt-C ) 64 Hz, (C)(H)CdC); 24.9, 24.0 ((H3C)CdN). (K3-(Ph)CdCH-(Cl-nacnac))Pt(SiPh3)(H)2 (4b) in Situ. To a sample of 10 mg (0.013 mmol) of 2a in 0.6 mL of CD2Cl2 in an NMR tube was added several equivalents of phenylacetylene. The tube was shaken and the NMR recorded. 1H NMR (δ, CD2Cl2, 400 MHz, room temp): 8.41 (s, 1H, 2JPt-H ) 71 Hz, (HC)(Ph)CdCHPt); 7.6-6.9 (m, 28H, Ph); 5.83 (s, 1H, nacnac-CH); 2.36, 2.17 (s, 3H, nacnac-CH3); -5.74 (d, 1H, 1JPt-H ) 762 Hz, Pt-Htrans-C); -18.57 (s, 1H, 1JPt-H ) 1252 Hz, Pt-Htrans-N). (K3-(Ph)CdCH-(Cl-nacnac))Pt(SiHPh2)(H)2 (4c) in Situ. To a sample of 10 mg (0.013 mmol) of 2c in 0.6 mL of CD2Cl2 in an NMR tube was added several equivalents of phenylacetylene. The tube was shaken and the NMR recorded. 1H NMR (δ, CD2Cl2, 500 MHz, 260 K): 8.25 (s, 1H, 2JPt-H ) 70 Hz, (HC)(Ph)CdCHPt); 7.6-6.9 (m, 23H, Ph); 5.80 (s, 1H, nacnac-CH); 2.30, 2.27 (s, 3H, nacnac-CH3); -6.01 (s, 1H, 1JPt-H ) 760 Hz, Pt-Htrans-C); -18.52 (s, 1H, 1JPt-H ) 1260 Hz, Pt-Htrans-N). (K3-(t-Bu)CdP-(Cl-nacnac))Pt(SiPh3)(H)2 (5a). In the drybox 10 mg (0.013 mmol) of 2a was placed in an NMR tube and 0.6 mL of CD2Cl2 was added. The tube was capped with a septum and removed from the box. The tube was cooled to -78 °C, and 2.1 µL of tBuCP was syringed in. The tube was shaken, at which time it rapidly turned from a light yellow to bright red solution. The sample was placed in an NMR probe precooled to 213 K. NMR spectra were taken as the sample was warmed to 263 K. Upon removal of the sample after complete conversion to 5a, the solution had become light yellow. 1H NMR (δ, CD2Cl2, 400 MHz, 263 K): 7.27 (br m, 3H, Ph); 7.18 (br d, 4H, Ph) 7.11 (m, 12H, Ph); 6.75 (br, 2H, Ph); 6.42 (d, 1H, Ph); 6.38 (s, 1H, Ph); 5.24 (d, 1H, 3JP-H ) 8 Hz, nacnac-CH); 2.20 (s, 3H, nacnac-CH3); 2.13 (s, 3H, nacnacCH3); 1.42 (s, 9H, (CH3)3CP); -4.64 (d, 1H, 2JP-H ) 62.4 Hz, 1 JPt-H ) 929 Hz, Pt-Htrans-P); -17.85 (s, 1H, 1JPt-H ) 1179 Hz, Pt-Htrans-N). 31P{1H} NMR (δ, CD2Cl2, 162 MHz, 263 K): 371.5 (s, 1P, 1JPt-P ) 95 Hz, t-BuCP). (K3-(adamantyl)CdP-(Cl-nacnac))Pt(SiPh3)(H)2 (5b). In the drybox 10 mg (0.013 mmol) of 2a and 2.5 mg (0.014 mmol, 1.1 equiv) of adamantyl phosphaalkyne were placed in an NMR tube. The tube was capped with a septum and removed from the box. The tube was cooled to -78 °C, and 0.6 mL of CD2Cl2 was syringed in. The tube was shaken, at which time it rapidly turned from a light yellow to a bright red solution. The sample was placed in an NMR probe precooled to 213 K. NMR spectra were taken as the sample was warmed to 263 K. Upon removal of the sample after complete conversion to 5a the solution became light yellow. 1 H NMR (δ, CD2Cl2, 500 MHz, 250 K): 7.30 (m, 3H, Ph); 7.19 (m, 5H, Ph) 7.12 (t, 7H, Ph); 7.07 (d, 6H, Ph); 6.39 (m, 2H, Ph); 5.22 (dd, 1H, 3JP-H ) 8.3 Hz, 5JH-H ) 2.3 Hz, nacnac-CH); 2.18 (s, 3H, nacnac-CH3); 2.12 (s, 3H, nacnac-CH3); 2.12 (s, 3H, (CH)3AdCP); 1.97 (s, 6H, (CH2)3AdCP); 1.77 (d, 3H, 2JH-H ) 12 Hz, (CH2)3AdCP); 1.73 (d, 3H, 2JH-H ) 12 Hz, (CH2)3AdCP); -4.58 (d, 1H, 2JP-H ) 60 Hz, 1JPt-H ) 927 Hz, Pt-Htrans-P); -17.82 (s, 1H, 1JPt-H ) 1176 Hz, Pt-Htrans-N). 31P{1H} NMR (δ, CD2Cl2, 202 MHz, 250 K): 380.8 (s, 1P, 1JPt-P ) 96 Hz, AdCP). [(K3-(t-Bu)CdP(Me)-(Cl-nacnac))Pt(SiPh3)(H)2][OTf] (6a). To an NMR solution of 5a (0.013 mmol) in CD2Cl2 was added 2.5 uL of MeOTf at 195 K. The tube was shaken and placed in an NMR probe precooled to 250 K. 1H NMR (δ, CD2Cl2, 400 MHz, 250 K): 7.41 (d, 2H, Ph); 7.34 (t, 3H, Ph) 7.21 (t, 7H, Ph); 7.05 (d, 6H, Ph); 7.01 (m, 2H, Ph); 6.92 (s, 1H, Ph); 6.63 (s, 1H, Ph); 6.05 (s, 1H, Ph); 5.99 (d, 1H, 3JP-H ) 18.4 Hz, nacnac-CH); 2.41, 2.38 (s, 3H, nacnac-CH3); 1.78 (d, 3H, 2JPt-H ) 16.4 Hz, C)P(CH3)); 1.45 (s, 9H, (CH3)3CdP); -7.78 (d, 1H, 2JP-H ) 266 Hz, 1JPt-H ) 1143

1434 Organometallics, Vol. 28, No. 5, 2009 Hz, Pt-Htrans-P); -17.33 (s, 1H, 2JP-H ) 8.8 Hz, 1JPt-H ) 1154 Hz, Pt-Htrans-N). 31P{1H} NMR (δ, CD2Cl2, 162 MHz, 250 K): 216.9 (s, 1P,1JPt-P ) 1359 Hz, t-BuCdP(Me)). [(K3-(adamantyl)CdP(Me)-(Cl-nacnac))Pt(SiPh3)(H)2][OTf] (6b). To an NMR solution of 5d (0.013 mmol) in CD2Cl2 was added 2.5 uL of MeOTf at 195 K. The tube was shaken and placed in an NMR probe precooled to 250 K. 1H NMR (δ, CD2Cl2, 500 MHz, 250 K): 7.45 (d, 2H, 3JH-H ) 9 Hz, nacnac Ph); 7.36 (t, 3H, 3JH-H ) 7.5 Hz, p-SiPh3) 7.24 (t, 7H, Ph); 7.07 (d, 8H, Ph); 6.95 (d, 1H, 3 JH-H ) 9 Hz, nacnac Ph); 6.65 (d, 1H, 3JH-H ) 8.5 Hz, nacnac Ph); 6.06 (d, 1H, 3JP-H ) 31 Hz, nacnac-CH); 6.05 (d, 1H, 3JH-H ) 9 Hz, nacnac Ph); 2.44 (s, 3H, nacnac-CH3); 2.42 (s, 3H, nacnacCH3); 2.20 (s, 3H, (CH)3AdCP); 2.05 (d, 3H, 2JH-H ) 11.5 Hz, (CH2)3AdCP); 1.99 (d, 3H, 2JH-H ) 11.5 Hz, (CH2)3AdCP); 1.86 (d, 3H, 2JP-H ) 16 Hz, CdP(CH3)); 1.81 (s, 6H, (CH2)3AdCP); -7.74 (d, 1H, 2JP-H ) 266 Hz, 1JPt-H ) 1134 Hz, Pt-Htrans-P); -17.27 (s, 1H, 2JP-H ) 8.5 Hz, 1JPt-H ) 1150 Hz, Pt-Htrans-N). 31 P{1H} NMR (δ, CD2Cl2, 202 MHz, 250 K): 218.7 (s, 1P, 1JPt-P ) 1362 Hz, AdCdP(Me)). [(K3-(adamantyl)CdP(H)-(Cl-nacnac))Pt(SiPh3)(H)2][BF4] (7). To an NMR solution of 5d (0.013 mmol) in CD2Cl2 was added 2.5 uL of HBF4 · Et2O at 195 K. The tube was shaken and placed in an NMR probe precooled to 250 K. 1H NMR (δ, CD2Cl2, 500 MHz, 220 K): 7.45 (m, 3H, nacnac Ph); 7.35 (t, 3H, 3JH-H ) 7.5 Hz, p-SiPh3); 7.26 (m, 9H, Ph); 7.11 (d, 6H, Ph); 6.51 (d, 1H, 3JH-H ) 8 Hz, nacnac Ph); 6.39 (dd, 1, 1JP-H ) 390 Hz, 3JH-H ) 23 Hz, CdPH); 6.13 (d, 1H, 3JP-H ) 29 Hz, nacnac-CH); 5.65 (d, 1H, 3 JH-H ) 8 Hz, nacnac Ph); 2.41 (s, 3H, nacnac-CH3); 2.32 (s, 3H,

West et al. nacnac-CH3); 2.19 (s, 3H, (CH)3AdCP); 1.86-1.6 (m, 12H, AdCP); -7.46 (dd, 1H, 2JP-H ) 282 Hz, 3JH-H ) 23 Hz, 1JPt-H ) 1271 Hz, Pt-Htrans-P); -17.09 (s, 1H, 2JP-H ) 7 Hz, 1JPt-H ) 1161 Hz, Pt-Htrans-N). 31P{1H} NMR (δ, CD2Cl2, 202 MHz, 220 K): 166.7 (s, 1P, 1JPt-P ) 1239 Hz, AdCdP(H)). (Cl-nacnac)Pt(η2-adamantylCtP)(SiPh3)(H)2 (8) in Situ. The reaction of 1 and phosphaalkyne in an NMR tube as described above forms these intermediates at 210 K. Upon warming to 270 K they funnel to the cycloaddition product 5b. Spectral data for 8′: 1H NMR (δ, CD2Cl2, 500 MHz, 210 K) -18.17 (s, 1H, 1JPt-H ) 1154 Hz, Pt-H2); 31P{1H} NMR (δ, CD2Cl2, 202 MHz, 210 K) 393.6 (s, 1P, AdCtP). Spectral data for 8′′: 1H NMR (δ, CD2Cl2, 500 MHz, 210 K) -17.28 (s, 1H, 1JPt-H ) 1172 Hz, Pt-H2); 31P{1H} NMR (δ, CD2Cl2, 202 MHz, 210 K) 358.1 (s, 1P, AdCtP). Crystallographic Data for 2a. Crystal data and structure refinement details for 2a are given in Table 2.

Acknowledgment. We thank Elise Traversa for assistance in acquiring analytical data for complex 3. This research was supported by funding from the National Science Foundation (Grant No. CHE-0717086). J.F.N. thanks the EPSRC for continuing financial support. Supporting Information Available: A CIF file giving structural data for complex 2a This material is available free of charge via the Internet at http://pubs.acs.org. OM800968V