Infrared Spectra of Platinum Insertion and Methylidene Complexes

1358

Organometallics 2009, 28, 1358–1368

Infrared Spectra of Platinum Insertion and Methylidene Complexes Prepared in Oxidative C-H(X) Reactions of Laser-Ablated Pt Atoms with Methane, Ethane, and Halomethanes Han-Gook Cho and Lester Andrews* Department of Chemistry, UniVersity of Incheon, 177 Dohwa-dong, Nam-ku, Incheon, 402-749, South Korea, and Department of Chemistry, UniVersity of Virginia, P.O. Box 400319, CharlottesVille, Virginia 22904-4319 ReceiVed NoVember 11, 2008

Reactions of laser-ablated Pt atoms with CH4 and C2H6 produce CH3-PtH and CH3CH2-PtH via oxidative C-H insertion and show that the Pt atom is an effective C-H insertion agent. The methylidene CH2dPtH2 is also detected in methane experiments. Methyl fluoride, CH3F, on the other hand, yields C-H as well as C-F insertion products (CH2F-PtH and CH3-PtF). The CH3-PtF molecule exists in photochemical equilibrium with an agostic CH2(H)-PtF isomer in the matrix, which is unique for an insertion product, and the CH2dPtHF methylidene appears to participate in the rearrangement process. Moreover, CHFdPtHCl, CHFdPtHF, and CF2dPtHF are primary products from the reactions of CH2FCl, CH2F2, and CHF3, while CHCldPtCl2 is generated from CHCl3. The surprising preference of the Pt-H over the Pt-F bond in reactions of fluorine containing precursors is traced to the similar Pt-H and Pt-F bond energies. The formation of small Pt carbenes also reveals that high oxidation-state complexes are favored in reactions with di- and trihalomethanes as well as in previously studied reactions of Pt and tetrahalomethanes. Calculations show that endothermic HF elimination from CHFdPtHF can produce the HCtPtF molecule, which contains a platinum-carbon triple bond. Introduction The C-H bond activation and following rearrangements are key steps of numerous synthetic reactions and catalytic processes, providing the possibility for transformation of alkanes to more valuable products.1-3 Previous investigations have also shown that not only metal cations but also neutral metal atoms are facile as well in the reactions with small alkanes and methyl halides.4-7 Many transition metals, lanthanides, and actinides form C-H(X) insertion products and also high oxidation-state complexes with a carbon-metal multiple bond following subsequent H(X) migration.7 These simple metal complexes, small cousins of metal complexes with large ligands, have * Author to whom correspondence should be addressed. E-mail: [email protected]. (1) (a) Campos, K. R. Chem. Soc. ReV. 2007, 36, 1069. (b) Dı´az-Requejo, M. M.; Pe´rez, P. Chem. ReV. 2008, 108, 3379. (2) (a) Siegbahn, P. E. M.; Blomberg, M. R. A.; Svensson, M. J. Am. Chem. Soc. 1993, 115, 4191. (b) Porembski, M.; Weisshaar, J. C. J. Phys. Chem. A 2000, 104, 1524. (c) Proctor, D. L.; Davis, H. F. Proc. Nat. Acad. Sci. U.S.A. 2008, 105, 12673. (3) (a) Crabtree, R. H. Chem. ReV. 1995, 95, 987. (b) Wada, K.; Pamplin, C. B.; Legzdins, P.; Patrick, B. O.; Tsyba, I.; Bau, R. J. Am. Chem. Soc. 2003, 125, 7035. (c) Olson, D. E.; Du Bois, J. J. Am. Chem. Soc. 2008, 130, 11248. (d) Davies, H. M.; Beckwith, R. E. J. Chem. ReV. 2003, 103, 2861. (4) (a) Davis, S. C.; Klabunde, K. J. J. Am. Chem. Soc. 1978, 100, 5973. (b) Billups, W. E.; Konarski, M. M.; Hauge, R. H.; Margrave, J. L. J. Am. Chem. Soc. 1980, 102, 7393. (5) (a) Cho, H.-G.; Andrews, L. Organometallics 2007, 26, 4089. (b) Cho, H.-G.; Andrews, L. Inorg. Chem. 2008, 47, 1653. (c) Cho, H.-G.; Andrews, L. Organometallics 2008, 27, 1786. (d) Cho, H.-G.; Andrews, L. Dalton Trans. 2008,in press ( DOI: 10.1039/b811805a). (6) (a) Zhang, X.-G.; Liyanage, R.; Armentrout, P. B. J. Am. Chem. Soc. 2001, 123, 5563. (b) Heinemann, C.; Wesendrup, R.; Schwarz, H. Chem. Phys. Lett. 1995, 239, 75. (7) Andrews, L.; Cho, H.-G. Organometallics 2006, 25, 4040, and references cited therein (Review article). .

unique structures owing to the agostic interaction and often show intersting photochemical properties.7,8 Recently, Wang et al. reported formation of CH3-RhH in the reaction of Rh atoms with CH4 during annealing in a cold Ar matrix.9 These workers also produced a Pd complex with methane but found, on the other hand, no evidence for Pt reaction with the smallest alkane in excess argon on sample deposition, on annealing, and on broadband irradiation. In contrast, the reaction of ground-state Pt atoms with CH4 at 300 K has been monitored through laser-induced fluoresence.10 The C-H bond insertion process by Pt has been investigated theoretically and its activation barrier predicted to be relatively low (2 kcal/mol).11 Previous studies have also shown that high oxidation-state complexes (particularly carbynes) are most favored in reactions of group 6-8 metals and on going down a family column.7 In this regard, laser-ablated Re and Os form methylidyne complexes exclusively,5 while W yields the methylidyne complex along with the insertion and methylidene products.7 However, the metal complexes with M-H(X) and C-M bonds are expected to be less favored for later transition metals as the d-orbitals become filled more. Recent studies have shown that only Pt among group 10 metals forms C-H insertion products in reactions with C2H2 and C2H4.12 More recently, small Pt (8) (a) von Frantzius, G.; Streubel, R.; Brandhorst, K.; Grunenberg, J. Organometallics 2006, 25, 118. (b) Alikhani, M. E.; Reinhardt, P.; Berkaine, N. Chem. Phys. 2008, 343, 241. (9) Wang, G.; Chen, M.; Zhou, M. Chem. Phys. Lett. 2005, 412, 46. (10) Carroll, J. J.; Weisshaar, J. C.; Siegbahn, P. E. M.; Wittborn, C. A. M.; Blomberg, M. R. A. J. Phys. Chem. 1995, 99, 14388. (11) Wittborn, A.; M, C.; Costas, M.; Blomberg, M. R. A.; Siegbahn, P. E. M. J. Chem. Phys. 1997, 107, 4318, and references therein. . (12) (a) Wang, X.; Andrews, L. J. Phys. Chem. A 2004, 108, 4838. (b) Cho, H.-G.; Andrews, L. J. Phys. Chem. A 2004, 108, 6272.

10.1021/om801077x CCC: $40.75  2009 American Chemical Society Publication on Web 02/11/2009

Infrared Spectra of Platinum Insertion

methylidene complexes with PtdC bonds computed in the 1.810-1.816 Å range have been produced from reactions of Pt with tetrahalomethanes.13 It is, therefore, intriguing to determine whether or not C-H bond insertion and the following rearrangement to higher oxidation-state complexes occur in reactions of Pt with saturated small hydrocarbons and mono-, di-, and trihalomethanes. Here, we report the IR spectra of isotopic products from reactions of laser-ablated Pt atoms with methane, ethane, and halomethanes. The products are identified by isotopic substitution and electronic state calculations. The structures of the Pt insertion and methylidene complexes are also reported, and the interesting preference of the Pt-H bond over the Pt-F bond in the insertion reaction is also investigated. A report has appeared on the Pt activation of methane.14

Experimental and Computational Methods Laser-ablated Pt atoms were reacted with CH4 (Matheson, UHP grade), 13CH4, CD4, CH2D2 (Cambridge Isotopic Laboratories), C2H6 (Matheson), C2D6 (MSD Isotopes), CH3F (Matheson), CD3F (synthesized from CD3Br and HgF2)), 13CH3F (Cambridge Isotopic Laboratories, 99%), CH2F2 and CH2FCl (Du Pont), CHF3 (PCR Research), CHCl3, and CDCl3 (Aldrich, reagent) in excess argon during condensation at 8-10 K using a closed-cycle refrigerator (Air Products Displex). These methods have been described in detail elsewhere.15 Reagent gas mixtures ranged from 0.5 to 2.0% in argon. After reaction, infrared spectra were recorded at a resolution of 0.5 cm-1 using a Nicolet 550 spectrometer with an MCT-B detector. Samples were later irradiated for 20 min periods by a mercury arc street lamp (175 W) with the globe removed using a combination of optical filters and subsequently annealed to allow further reagent diffusion. Following earlier work,7,13 complementary density functional theory (DFT) calculations were carried out using the Gaussian 03 package,16 the B3LYP density functional,17 6-311++G(3df,3pd) basis sets for C, H, F, and Cl,18 and SDD pseudopotential and basis set19 for Pt to provide a consistent set of vibrational frequencies for the reaction products. Geometries were fully relaxed during optimization, and the optimized geometry was confirmed by (13) Cho, H.-G.; Andrews, L. J. Am. Chem. Soc. 2008, 130, 1583615841 (Pt + CX4). (14) Cho, H.-G.; Andrews, L. J. Phys. Chem. A 2008, 112, 12293 (Pt + CH4 letter). (15) (a) Andrews, L.; Citra, A. Chem. ReV. 2002, 102, 885, and references cited therein. (b) Andrews, L. Chem. Soc. ReV. 2004, 33, 123, and references cited therein. (16) Frisch, M. J. ; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A. Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M. ; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, ReVision C.02; Gaussian, Inc.: Pittsburgh, PA, 2003. (17) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, Y.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (18) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80, 3265. (19) Andrae, D.; Haeussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1990, 77, 123.

Organometallics, Vol. 28, No. 5, 2009 1359 vibrational analysis. Additional BPW91,20 MP2,21 and more rigorous CCSD22 calculations were done to support the B3LYP results. We use the B3LYP frequencies as the basis of most of our discussion because we have more experience with this method and more confidence in its comparison with experimental results for the identification of new molecules. The vibrational frequencies were calculated analytically except for the CCSD calculation, and zero-point energies are included in the determination of binding energies.

Results and Discussion Infrared spectra and density functional calculations of the products of laser-ablated Pt atom reactions with methane, ethane, methyl fluoride, chlorofluoromethane, methylene fluoride, fluoroform, and chloroform will be presented in turn. Pt + CH4. Infrared spectra from reactions of Pt with CH4, CD4, 13CH4, and CH2D2 (Figure S1, Supporting Information) have been described in a recent report.14 The major product absorptions (marked i for insertion) increase 10% on UV (240 < λ < 380 nm) photolysis and later decrease in the process of annealing. Radiation from the laser ablation plume on the metal surface also produces methyl radical and the trapped proton ArnH+, and the intrinsically strong absorptions of PtNN are detected from the reaction with nitrogen impurity.23 The signature absorption at 2315.9 cm-1 shows no 13C shift and a D shift to 1661.7 cm-1 (H/D ratio of 1.394), and these diagnostic bands are assigned to Pt-H and Pt-D stretching modes. These frequencies are just below PtH2 stretching frequencies at 2365.7 and 2348.9 cm-1 and PtD2 at 1697.7 and 1683.3 cm-1,24 although binary Pt dihydride absorptions are not observed in this study. The single new Pt-H stretching absorption provides strong evidence for the Pt into C-H insertion complex, CH3-PtH. A strong characteristic CH3 deformation absorption for the insertion complex is observed at 1225.9 cm-1, with weaker CH3 rocking bands at 828.4 and 804.1 cm-1 (Table S1, Supporting Information).14 In the high frequency region, two C-H stretching frequencies are also observed at 2947.6 and 2874.6 cm-1, with D shifts to 2205.0 and 2099.0 cm-1 (H/D frequency ratios of 1.337 and 1.370), and 13C counterparts at 2939.8 and 2869.6 cm-1. The six observed absorptions are the strongest bands predicted by DFT calculation for CH3-PtH in its ground 1A′ state (Table S1, Supporting Information). The observed frequencies are in very good agreement with the computed frequencies: the C-H and Pt-H stretching frequencies are calculated 4.6, 3.8, and 4.2% higher and the methyl modes 2.5, 2.1, and 1.0% higher using the B3LYP functional, which is the range of agreement expected for harmonic calculated and anharmonic observed frequencies.25 As expected, the BPW91 density functional predicts slightly lower frequencies, which are 2.5, 1.6, and 4.4% (20) (a) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (b) Perdew, J. P.; Burke, K.; Wang, Y. Phys. ReV. B 1996, 54, 16533, and references cited therein. See also: (c) Becke, A. D. J. Chem. Phys. 1997, 107, 8554. (d) Schmider, H. L.; Becke, A. D. J. Chem. Phys. 1998, 108, 9624. (21) Frisch, M. J.; Head-Gordon, M.; Pople, J. A. Chem. Phys. Lett. 1990, 166, 281. (22) Purvis, G. D.; Bartlett, R. J. J. Chem. Phys. 1982, 76, 1910. (23) (a) Jacox, M. E. J. Mol. Spectrosc. 1977, 66, 272. (b) Wight, C. A.; Ault, B. S.; Andrews, L. J. Chem. Phys. 1976, 65, 1244, and references cited therein. (c) Citra, A.; Wang, X.; Bare, W. D.; Andrews, L. J. Phys. Chem. A 2001, 105, 7799(Pt + N2). (24) Andrews, L.; Wang, X.; Manceron, L J. Chem. Phys. 2001, 114, 1559 (Pt + H2). (25) (a) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502. (b) Andersson, M. P.; Uvdal, P. L. J. Phys. Chem. A 2005, 109, 3937.

1360 Organometallics, Vol. 28, No. 5, 2009

Cho and Andrews

high for stretching and 0.9, 0.2, and 1.5% low for bending modes, and the CCSD wave function based method computes slightly higher frequencies, but the relationships among the frequencies are the same. In contrast, the CH2dPtH2 product is 31 kcal/mol higher in energy, and it has two strong calculated Pt-H stretching modes much lower at 2130 and 1918 cm-1. These favorable comparisons between calculated and argon matrix observed frequencies and isotopic shifts substantiate the formation and identification of the insertion product, CH3-PtH. Further support comes from the Pt reaction with the mixed isotopic precursor CH2D2 (Figure S1, Supporting Information). The diagnostic 2316.1 and 1661.6 cm-1 product absorptions are essentially the same as observed with CH4 and CD4, and these bands are 2/1 in relative intensity. Our calculations (Table S1, Supporting Information) find the Pt-H infrared absorption to be double the intensity of the Pt-D absorption, which shows that equal yields of CH2D-PtD and CHD2-PtH are produced.14 Two structures, Cs and C1, are possible for the mixed isotopic products, and our calculations find the two isomer Pt-H modes within 0.24 cm-1 and the two isomer Pt-D modes within 0.05 cm-1, which cannot be resolved. Two CH2D, CHD2 deformation modes are observed at 1151.2 and 1004.6 cm-1, which are within the pure isotopic values. The observed bands are listed with the Cs structures in Table S2, Supporting Information. Additional weak product absorptions are observed at 1901.6 cm-1 in the CH4 and 13CH4 spectra (labeled m for methylidene, Figure S1, Supporting Information) and at 1365.8 cm-1 in the CD4 spectra (not shown) (H/D frequency ratio 1.392). They are compared with the B3LYP calculated frequencies of 1917.8 and 1364.5 cm-1 for the very strong, PtH2 antisymmetric stretching mode of CH2dPtH2 and the deuterated isotopomer (Table S3, Supporting Information). These bands increase about 40% on vis (λ > 420 nm), decrease 30% on UV (240 < λ < 380 nm) irradiation, restore on annealing to 20 K, and they are assigned to the strongest infrared absorption of the higher oxidation-state CH2dPtH2 complex. The Pt-H stretching band absorbance ratio for the i and m products, 8/1, normalized by the computed infrared intensities, suggests that about 1% of the CH3-PtH undergoes R-H transfer to CH2dPtH2 during relaxation in the matrix. The analogous mechanism has been proposed for the formation of CCl2dPtCl2 from the reaction with CCl4.13 It appears that uv irradiation excites CH2dPtH2 to an excited state which connects to the transition state described by Carroll et al.10 for H2 elimination, which then falls back to the more stable CH2dPtH2 molecule on annealing the argon matrix.

Pt(excited by laser ablation or UV) + CH4 f [CH3 - PtH*] f CH3 - PtH (1) fCH2dPtH2

(2)

CH2dPtH2 + UV f CH(H)PtH2

(3)

CH(H)PtH2 + annealing f CH2dPtH2

(4)

Pt + C2H6. Figure 1 shows IR spectra from reactions of Pt with C2H6 and C2D6. The product absorptions are marked with “i”, and their intensities increase slightly on UV photolysis and slightly further in the early stage of annealing. The observed frequencies are listed and compared with calculated frequencies in Table 1. The strongest product CH2 wagging absorption at 1195.1 cm-1 and CD2 absorption at 978.7 cm-1 (H/D ratio 1.221) are the most distinctive features. Computed intensities in Table 1 also show that the CH2 wagging band of CH3CH2-PtH is predicted to be remarkably strong due to combination with stretching of the polarized C-Pt bond, while

Figure 1. IR spectra in the product absorption regions from the laser-ablated Pt atom reactions with C2H6 isotopomers in excess argon at 8 K: (a) Pt + 1.0% C2H6 in Ar codeposited for 1 h; (b) as in (a) after photolysis (λ > 420 nm); (c) as in (b) after photolysis (240 < λ < 380 nm); (d) as in (c) after photolysis (λ > 220 nm); (e) as in (d) after annealing to 28 K; (f) Pt + 1.0% C2D6 in Ar; (g) As in (f) after photolysis (240 < λ < 380 nm); (h) As in (g) after photolysis (λ > 220 nm); (i) As in (h) after annealing to 28 K. The label i denotes the new insertion product absorptions, c identifies product absorptions common to this precursor in other metal experiments, and P indicates unreacted ethane precursor absorptions. PtNN, CO, and C2H5 absorptions are also designated.

the other bands are all much weaker, including the Pt-H stretching mode. Additional bands are detected for PtNN and ethyl radical.23,26 Photolysis product absorptions that are common to this precursor and other laser-ablated metals are labeled “c”, and unreacted precursor bands are labeled “P” in the figures. The i Pt-H stretching absorption is observed at 2243.5 cm-1, which is 72.4 cm-1 lower than that of the CH4 insertion complex, and its D counterpart is at 1612.5 cm-1 (H/D ratio 1.391) (not shown). Our calculations predict a 14 cm-1 lowering of the metal-hydrogen stretching frequency by substitution of a CH3 group for H. We anticipate that the large difference between the experiment and theory is due to a stronger matrix interaction for the larger molecule. The i absorption at 1449.3 cm-1 has its D counterpart at 1125.5 cm-1 (H/D ratio of 1.288) and is designated to the CH3 deformation mode. Another i absorption split at 578.9 and 574.8 cm-1 has D counterparts at 531.2 and 527.5 cm-1 (H/D ratios of both 1.090) are assigned to the mostly C-Pt stretching mode on the basis of the frequency and moderate D shifts. A CH2 stretching absorption is observed at 2860.7 cm-1 along with its D counterpart at 2063.2 cm-1 (H/D ratio of 1.387). The i absorption at 710.9 cm-1 is assigned to (26) Cho, H.-G.; Andrews, L. J. Phys. Chem. A 2008, 112, 1519 (group 4 + ethane).


Organometallics, Vol. 28, No. 5, 2009 1361

Table 1. Observed and Calculated Fundamental Frequencies of CH3CH2-PtH Isotopomers in the 1A ground statea CH3CH2-PtH approx description CH3 str. CH3 str. CH3 str. CH2 str. Pt-H str. CH3 deform CH2 wag CH2 rock C-Pt str. HCPt bend

obs

2860.7 2243.5 1448.3 1195.1 710.9 578.9

CD3CD2-PtD

B3LYP

int

BPW91

CCSD

3115.5 3080.7 3003.3 2944.2 2398.2 1483.3 1232.4 735.5 581.8 447.1

17 14 26 22 19 11 108 10 13 25

3068.4 3030.2 2947.9 2877.9 2400.7 1434.9 1188.1 717.7 577.9 441.9

3139.9 3110.8 3037.3 2978.9 2580.8 1515.1 1287.8 749.7 625.3 494.1

obs

B3LYP

int

BPW91

CCSD

2063.2 1612.5 1125.5 978.7

2308.8 2270.8 2166.0 2143.9 1700.8 1150.8 1005.2 690.3 549.2 333.8

8 7 9 12 10 7 79 2 5 17

2273.5 2233.5 2125.3 2095.4 1702.6 1117.7 971.0 668.9 535.1 329.1

2326.2 2299.5 2188.0 2167.4 1830.4 1187.4 1054.0 720.7 560.4 370.5

531.2

a Frequencies and intensities are in cm-1 and km/mol. Observed in an argon matrix. Frequencies and intensities computed with 6-311++G(3df, 3pd) are for harmonic calculations and the SDD core potential and basis set are used for Pt. Calculation results shown only for observable bands (frequency >400 cm-1 and intensity >10 km/mol). CH3CH2-PtH has a C1 structure.

the CH2 rocking mode on the basis of the frequency without observation of the D counterpart. As shown in Table 1, the i absorptions are consistent with the bands predicted to be strong for CH3CH2-PtH in its ground 1A state. We also considered the C-C bond insertion product, H3C-Pt-CH3. Although this possible product is about 2 kcal/ mol lower in energy, we believe that C-C insertion has a smaller reaction probability than C-H insertion owing to the number of bonds, and such C-C insertion products have not been observed in our previous work.26 Furthermore, the strongest infrared absorption for H3C-Pt-CH3, the antisymmetric CH3 deformation mode, is computed at 1245 cm-1 (intensity 55 km/ mol), just 13 cm-1 above the insertion product value, and the former band is not observed. The symmetric deformation (computed 1263 cm-1 (16 km/mol) and the methyl rocking modes (calculated 745 cm-1 (20 km/mol) and 809 cm-1 (29 km/mol) are not detected either. We therefore conclude that the C-C insertion product is not formed in observable quantity in these experiments. The observation of CH3-PtH and C2H5-PtH shows that the neutral Pt atom is also an efficient C-H insertion agent, and because CH4 and C2H6 are in general the least reactive for C-H insertion, similar reactions are expected to occur for other hydrocarbons. Pt + CH3F. Figure 2 shows the IR spectra in the product absorption regions from reactions of Pt with CH3F isotopomers. Unlike the cases of CH4 and C2H6, three groups of product absorptions marked i, i′, and m are observed, and the observed frequencies are listed in Table 2 and compared with calculated product frequencies in Tables 3, 4 and S4 (Supporting Information). The strong i absorption at 580.3 cm-1 has D and 13C counterparts at 579.8, and 580.2 cm-1, and a weak i absorption on the blue side at 600.2 cm-1 has D and 13C counterparts at 598.4, and 600.1 cm-1. The frequencies are compared with the previously observed Pt-F stretching frequencies of 595 and 565 cm-1 for (PNP)2[PtF4] and 695 cm-1 for PtF6.27 The observed frequencies and small isotopic shifts, therefore, strongly suggest that they originate from the Pt-F stretching mode of a major product. The i absorption at 580.3 cm-1 decreases slightly (10%) on visible irradiation while the weak i absorption at 600.2 cm-1 increases 3-fold, and their intensities return to original values on UV irradiation. The sum of the absorption intensities, (27) (a) Preetz, W.; Uttecht, J.-G.; Peters, G. Z. Anorg. Allg. Chem. 2003, 629, 410. (b) Claassen, H. H.; Selig, H. Israel J. Chem. 1969, 7, 499. (c) Holloway, J. H.; Stanger, G.; Hope, E. G.; Levason, W.; Ogden, J. S. J. Chem. Soc., Dalton Trans. 1988, 1341. (d) Richardson, A. D.; Hedberg, K. H.; Lucier, G. M Inorg. Chem. 2000, 39, 2787 (Pt-F frequencies).

Figure 2. IR spectra in the product absorption regions from the laser-ablated Pt atom reactions with CH3F isotopomers in excess argon at 8 K: (a) Pt + 0.5% CH3F in Ar codeposited for 1 h; (b) As in (a) after photolysis (λ > 420 nm); (c) As in (b) after photolysis (240 < λ < 380 nm); (d) As in (c) after annealing to 28 K; (e) As in (d) after annealing to 34 K; (f) Pt + 0.5% CD3F in Ar; (g) As in (e) after photolysis (240 < λ < 380 nm); (h) As in (f) after photolysis (240 < λ < 380 nm); (i) As in (g) after annealing to 28 K; (j) As in (i) after annealing to 38 K; (k) Pt + 0.5% 13CH3F in Ar; (l) As in (k) after annealing to 28 K; (m) As in (l) after annealing to 36 K. Again, i denotes the insertion product absorption, m identifies methylidene product, and c and P denote product absorptions common to all metals with this precursor and unreacted methyl fluoride precursor absorptions, respectively. H2O and CO2 impurity absorptions are also designated.

however, apparently remains almost the same during the photolysis sequence, suggesting that the two absorptions show-


Cho and Andrews

Table 2. Frequencies of Product Absorptions Observed from Reactions of Pt with Fluoromethane Isotopomers in Excess Argona CH3F i i′

m

1223.5, 1203.5 600.2, 580.3 2301.5 1250.2 956.9 553.9

CD3F b

598.6, 579.8 1651.4 970.3 932.7 557.1

13

CH3F

1216.4, 1197.5 600.1, 580.2 2301.5 1243.8 935.8 553.9

description CH3 deform Pt-F str Pt-H str CH2 wag C-F str Pt-F str

a All frequencies are in cm-1. Description gives major vibrational coordinate. b Region covered by precursor band.

ing reversible intensity changes on the irradiation cycles arise from two interconvertible species containing a Pt-F moiety. It is notable that calculations also show that there are two almost equally stable configurations for the Pt-F insertion complex (CH3-PtF and CH2(H)-PtF illustrated in Figure 3 on an energy profile diagram). The frequencies for the normal open methyl and H-bridged Pt-F insertion complexes (30.9 and 31.4 kcal/ mol more stable than the reactants) are only slightly different from one another (Table 3). In fact, the bridged structure is suggestive of agostic distortion, which is unique for an insertion complex.7 The frequency of the strong i absorption is consistent with the predicted value of 601.8 cm-1 for the Pt-F stretching mode of CH3-PtF (obs/cal ratios of 0.964), and the observed D and 13 C isotopic shifts of -0.5 and -0.1 cm-1 are also reproduced exactly. The frequencies of the weaker i absorptions are, on the other hand, more consistent with the predicted Pt-F stretching frequency (612.6 cm-1) for CH2(H)-PtF. Other i absorptions are observed at 1223.5 and 1203.6 cm-1. Unfortunately, the D counterparts are covered by the strong precursor bands at 1020-940 cm-1, while the weak 13C counterparts are observed at 1216.4 and 1197.5 cm-1. They are assigned to the CH3 deformation modes of the two conformers (CH3-PtF and CH2(H)-PtF). Other i absorptions are too weak to be observed in these experiments. Another group of product absorptions marked i′ increase about 15 and 45% on visible and UV photolysis (60% increase in total) and later slightly increase on annealing to 28 K. The i and i′ absorptions are clearly distinguishable due to the different intensity variations during photolysis. The most distinct strong i′ absorption is at 956.9 cm-1 with its D and 13C counterparts at 932.7 and 935.8 cm-1 (H/D and 12/13 ratios of 1.026 and 1.023). The frequency and relatively small isotopic shifts suggest that it arises from a mostly C-F stretching mode of another major product. The i′ absorption in the Pt-H stretching region at 2301.5 cm-1 shows essentially no 13C shift and a large D shift to 1651.4 cm-1 (H/D ratio of 1.394), providing evidence that the product responsible for the i′ absorptions contains a Pt-H moiety. The i′ absorption at 1250.2 cm-1 has its D and 13C counterparts at 970.3 and 1243.8 cm-1 (H/D and 12/13 ratios of 1.288 and 1.005), and it is designated to the CH2 bending mode on the basis of the frequency and considerable D shift. The product responsible for the observed i′ absorptions, therefore, contains C-F, Pt-H, and CH2 moieties, but not a Pt-F bond. Interestingly enough, the most stable product among the plausible ones from the Pt + CH3F reaction is CH2F-PtH, the C-H insertion product (40.9 kcal/mol lower than reagents). The observed i′ frequencies are compared with the predicted values for CH2F-PtH isotopomers, in Table 5, and the good agreement between the observed and predicted values (e.g., obs/calc ratios of 0.966, 0.987, and 0.976 for CH2F-PtH) substantiates formation of the Pt into C-H insertion complex.

Still another weak absorption is observed at 553.9 cm-1 on UV irradiation (labeled m), and this band increases slightly on annealing to 28 and 36 K at the expense of the 580.3 cm-1 band. Notice the uncommon blue shift to 557.1 cm-1 in the CD3F experiment and no shift with the carbon-13 sample. These observations suggest another Pt-F stretching mode, and the remaining species with comparable energy is the CH2dPtHF methylidene (34.2 kcal/mol lower than reagents). Our B3LYP calculation reveals one very strong band, at 572.8 cm-1, which mixes slightly with a C-Pt-H bending mode at 639.3 cm-1. When this coupling is diminished on deuterium substitution, the strong band is calculated at 575.7 cm-1 and the C-Pt-D bending mode at 453.1 cm-1 (Table S4, Supporting Information). The observed blue shift is 3.2 cm-1 for the 553.9 cm-1 band, which identifies the PtHF functional group from our calculation, and substantiates the identification of CH2dPtHF as a minor reaction product. Formation of the C-H insertion product (CH2F-PtH) from the reaction of Pt with CH3F is counterintuitive because normally the metal-hydrogen bond is expected to be weaker than the metal-halogen bond. It is also noteworthy that no C-H insertion complexes have been identified from reactions of other transition metals with fluoromethanes.7 The lone electron pairs on the halogen atom attract the electron-deficient transitionmetal atom to form the C-X insertion product first, which in most cases is isolated in the matrix.7 Subsequent molecular rearrangements via H(X) migration lead to higher oxidationstate complexes if energetically allowable.13,14 In the reaction of Pt with CH3F, it is thus believed that C-F insertion occurs first, producing CH3-PtF, but it is possible that direct C-H insertion also occurs to give some CH2F-PtH. Relative intensities of the Pt-F mode of CH3-PtF and the C-F and Pt-H stretching modes of CH2F-PtH, normalized by the calculated infrared intensities, suggests that four times as much CH3-PtF is formed on sample deposition as CH2F-PtH. The following rearrangement to CH2F-PtH, however, requires migration of H to Pt as well as F to C. The presence of CH2(H)-PtF (the H bridged C-F insertion complex) suggests that H migration from C to Pt occurs first and F migration follows later in reverse probably going through the CH2dPtHF methylidene. Simple annealing decreases the strong CH3-PtF band and increases the CH2dPtHF absorption such that there is three times as much of the former as the latter after annealing to 36 K. The R-H transfer process has been observed for a large number of insertion products, in particular CH3-TiF, and this process is photochemically reversible.7,28 Similarly, R-F transfer has been observed more recently, but not reversibly, as the first examples attained substantially lower energies with F bonded to the metal centers.29,30 Calculations also show the relative stability of the Pt into C-H insertion product (CH2F-PtH), CH2dPtHF, CH3-PtF, and CH2(H)-PtF, which are 40.9, 34.2 30.9, and 31.4 kcal/ mol more stable than the reactants at the B3LYP level of theory, and the new absorptions in Figure 2 indicate a reasonable production yield. The B3LYP reaction energies of Pt(3D) + H2 f PtH2(S) and Pt(3D) + F2 f PtF2(S) are -48.5 and -115.0 kcal/mol, and the bond energies of H-H and F-F are 104 and 38 kcal/mol31 (B3LYP values are 103.8 and 34.8 kcal/mol). (28) Cho, H.-G.; Andrews, L. J. Phys. Chem. A 2004, 108, 6294 (Ti + CH3F). (29) Lyon, J. T.; Andrews, L Inorg. Chem. 2006, 112, 45 (Ti + CF4). (30) Lyon, J. T.; Cho, H.-G.; Andrews, L Organometallics 2007, 26, 6373 (group 6 + CX4). (31) Wall, F. T. Chemical Thermodynamics, 3rd ed.; W. H. Freeman: San Francisco, 1974; p 63.



Table 3. Calculated Fundamental Frequencies of CH3-PtF and CH2(H)-PtF Isotopomers in the 1A′ Ground Statesa CH3-PtF approx description A′′CH3 str A′ CH3 str A′ CH3 str A′ CH3 scis A′ CH3 deform A′ CH3 deform A′′ CH3 rock A′ CH3 rock A′ Pt-F str A′ C-Pt str A′′ CH3 distort A′ HPtC bend

obs

1203.5

580.3

CD3-PtF

CH2(H)-PtF

B3LYP int 3150.9 3089.2 2860.4 1461.8 1363.0 1260.6 807.2 768.9 601.8 548.7 192.4 84.8

obs

0 13 18 2 4 37 1223.5 13 8 73 600.2 4 0 10

B3LYP int 3162.2 3085.6 2378.9 1466.2 1211.2 1292.2 733.3 830.1 612.6 567.6 354.1 80.0

obs

0 9 21 8 3 b 79 8 15 82 579.8 21 2 13

CH3-PtF

13

CD2(D)-PtF

B3LYP int 2339.4 2259.6 2066.3 1064.7 994.5 982.0 595.6 620.1 601.3 476.1 140.1 81.8

obs

0 11 7 1 3 b 20 6 0 75 598.4 4 0 10

B3LYP int 2351.1 2239.7 1729.1 1075.5 891.2 1006.8 534.5 680.8 612.1 487.7 263.6 66.9

obs

0 8 11 0 1 39 1197.5 3 1 83 580.2 18 1 12

13

CH2(H)-PtF

B3LYP int 3138.8 3082.2 2854.8 1458.2 1359.4 1251.8 803.9 759.8 601.7 534.5 192.4 84.7

obs

0 12 18 2 4 36 1216.4 13 9 74 600.1 4 0 10

B3LYP int 3149.7 3080.0 2373.8 1462.7 1207.0 1280.4 731.1 817.6 612.5 554.5 353.7 78.5

0 9 21 8 3 78 8 16 83 18 2 13

a Frequencies and intensities are in cm-1 and km/mol. Observed in an argon matrix. Frequencies and intensities are computed with 6-311++G(3df, 3pd), and the SDD core potential and basis set are used for Pt. CH3-PtF and CH2(H)-PtF (bridged) have Cs structures. The symmetry notations are based on the Cs structure. b Region covered by precursor band.

Table 4. Calculated Fundamental Frequencies of CH2F-PtH Isotopomers in the 1A′ Ground Statea CH2F-PtH approx description A′′ CH2 as. str A′ CH3 a. str A′ Pt-H str A′ CH2 scis A′ CH2 wag A′′ CH2 twist A′ C-F str. A′′ CH3 rock A′ C-Pt str. A′ HPtC bend A′′ Pt-H tort A′ FCPt bend

obs

2301.5 1250.2 956.9

13

CD2F-PtD

B3LYP

int

BPW91

CCSD

3127.0 3045.2 2382.3 1467.7 1266.1 1204.4 980.9 803.9 692.8 594.9 237.3 202.8

3 15 22 1 68 4 203 9 8 28 18 12

3052.3 2968.6 2388.0 1414.4 1219.3 1160.4 944.1 783.4 684.6 589.0 220.2 199.9

3163.7 3084.9 2572.8 1498.3 1314.9 1229.4 1025.9 828.2 751.3 640.2 243.9 222.7

obs

1651.4 b

932.7

CH2F-PtH

B3LYP

int

BPW91

CCSD

2329.0 2202.2 1689.6 1092.0 988.9 901.4 949.4 608.8 595.8 460.0 172.3 195.9

2 11 11 14 92 2 156 5 7 8 10 12

2273.7 2145.8 1693.6 1051.3 952.6 869.7 915.0 592.8 591.0 454.3 160.4 192.8

2354.0 2233.5 1824.7 1118.7 1042.0 916.9 980.1 630.7 635.0 499.9 215.6 177.7

obs

2301.8 1243.8 935.8

B3LYP

int

BPW91

CCSD

3114.2 3040.3 2382.3 1463.9 1258.5 1200.7 959.0 798.2 683.0 585.3 237.1 201.8

3 14 22 2 63 4 196 9 6 29 18 12

3039.7 2963.9 2388.0 1410.8 1212.1 1156.8 923.0 777.8 674.2 580.1 220.0 198.9

3151.0 3079.7 2572.8 1494.3 1306.3 1226.1 1003.2 821.9 742.3 629.0 243.5 221.6

a Frequencies and intensities are in cm-1 and km/mol. Observed in an argon matrix. Frequencies and intensities are computed with 6-311++G(3df, 3pd), and the SDD core potential and basis set are used for Pt. Intensities are calculated with B3LYP. CH2F-PtH has a Cs structure with two equal C-H bonds at levels of theory used in this study. The symmetry notations are based on the Cs structure. b Region covered by precursor band.

Therefore, the Pt-H and Pt-F bond energies are both estimated to be about 77 kcal/mol. Due to similar product Pt-H and Pt-F bond energies, the initial C-H and C-F bond energies of 99 and 117 kcal/mol31 become the determining factor instead, if other thermochemical differences for the complexes are negligible. Thus, CH2F-PtH is then predicted to be 18 kcal/mol more stable from the bond energies than CH3-PtF, which is compared with the 10 kcal/mol difference in B3LYP energies for the two complexes. Formation of the Pt-H over the Pt-F bond is chemically profound in reactions of Pt with fluoromethane. Pt + CH2F2. Two weak (A ) absorbance ) 0.002), new sharp bands are observed from the reaction of Pt and methylene fluoride, at 1192.1 and 552.7 cm-1 (labeled ‘m’ for methylidene for the upper band in Figure 4), which decrease slightly on UV and restore on full arc irradiations. Other transient species such as CF2, CH2F, and CHF2 are also produced by laser plume photolysis of the precursor.32 Following the methyl fluoride reactions, our calculations find similar products for the CH2F2 reaction, namely CHF2-PtH, CHFdPtHF, and CH2F-PtF, which are 40.3, 40.0, and 26.6 kcal/mol lower energy than reactants. Again, we expect CH2F-PtF to be formed first, but the strongest calculated band at 904 cm-1 is not observed. However, R-H-transfer to form the lower energy CHFdPtHF methylidene is a straightforward next step, and the 1192.1 and 552.7 cm-1 bands are in good agreement with strongly absorbing frequencies calculated at 1214.2 and 579.3 cm-1 (Table 6). We have no evidence for the CHF2-PtH species in this experiment. (32) Jacox, M. E. J. Phys. Chem. Ref. Data 1994, Monograph 3; 1998, 27, 115 and references cited therein.

Figure 3. Energy profile of products from the Pt and CH3F reaction relative to the reagent energy computed at the B3LYP level of theory.


Cho and Andrews

Table 5. Frequencies of Product Absorptions Observed from Reactions of Pt with Di- and Trihalomethanes in Excess Argona CH2F2

CH2FCl

CHF3

1371.1

CHCl3 1190.7, 1184.1

CDCl3

Description

1033.7, 1031.5

C-H IP bend CF2 sym str CF2 as str C-F str C-Cl str C-H OOP bend CdPt bend Pt-F str

1293.3 1198.8,1195.7

1192.1 1201.1, 1193.6

947.2, 945.1, 942.6 708.4 688.1 552.7 a

798.2, 795.9 604.4

571.5

All frequencies are in cm-1. Stronger absorptions in a set are bold. Description gives major coordinate.

Figure 4. IR spectra in the product absorption regions from the laser-ablated Pt atom reactions with CH2FCl, CH2F2, and CHF3 in excess argon at 8 K: (a) Pt + 0.5% CH2FCl in Ar codeposited for 1 h; (b) as in (a) after photolysis (λ > 420 nm); (c) As in (b) after photolysis (240 < λ < 380 nm); (d) As in (c) after photolysis (λ > 220 nm); (e) Pt + 0.5% CH2F2 in Ar codeposited for 1 h; (f) As in (e) after photolysis (λ > 420 nm); (g) As in (f) after photolysis (240 < λ < 380 nm); (h) As in (g) after photolysis (λ > 220 nm); (i) Pt + 0.5% CHF3 in Ar codeposited for 1 h; (j) As in (i) after photolysis (λ > 420 nm); (k) As in (j) after photolysis (240 < λ < 380 nm); (l) As in (k) after photolysis (λ > 220 nm). Again, m denotes the methylidene product absorption and c and P denote product absorptions that are common to these precursors in other experiments including unreacted precursor absorptions. CH4 impurities from cracking diffusion pump oil, CF2, CF3, and CHF2 absorptions from precursor photolysis are also designated.

Pt + CH2FCl. Figure 4 also illustrates product absorptions from the Pt reaction with CH2FCl. These absorptions marked “m” increase slightly on visible photolysis, but halve and almost disappear on UV and full arc (λ > 220 nm) photolysis. No other product absorptions of interest are observed. The relatively weak absorption at 1371.1 cm-1 most likely originates from a C-H bending mode of the product on the basis of the frequency, whereas the two stronger absorptions, matrix split at 1201.1 and 1193.6 cm-1, arise from the C-F stretching mode. Calculations were carried out for the plausible products such asCH2F-PtCl,CH2Cl-PtF,CHFCl-PtH,CH2dPtFCl,CHFdPtHCl, and CHCldPtHF. Among them, only CHFdPtHCl shows consistent vibrational characteristics with the observed values as shown in Table 6. The CH2F-PtCl molecule would show

only one strong band at around 900 cm-1, and CHFCl-PtH would give strong absorptions at around 1070 and 1030 cm-1, which are not observed. Moreover, CHFdPtHCl and CH2F-PtCl are the most stable and second most stable among the plausible products (57.5 and 53.1 kcal/more more stable than the reactants), but CH2Cl-PtF and CH2dPtFCl are, on the other hand, only 44.5 and 44.0 kcal/ mol more stable than the reactants. After C-Cl bond insertion by the Pt atom, the excess reaction energy activates H migration from C to Pt instead of F migration. This result reconfirms the preference of the Pt-H over the Pt-F bond. It has recently been shown that Pt readily forms simple methylidenes in reactions with tetrahalomethanes, which exhibit strong dπ-pπ bonding between the carbon and platinum atoms.13 The present results reveal that dihalomethanes also produce high oxidation-state complexes33 in the reaction with Pt while small alkanes and CH3F form primarily insertion complexes. Pt + CHX3. Figure 4 also shows the product IR absorptions from reaction of Pt with CHF3. The product absorptions are relatively weak, and their vibrational characteristics are somewhat similar to those in the Pt + CH2FCl spectra. The product absorptions marked “m” slightly increase on visible photolysis but decrease on UV and full arc photolysis. No other absorptions of interest are observed, though as in other experiments with fluoroform, CF3 and CHF2 radical absorptions are produced.32 The two product absorptions at 1293.3 and 1198.8 cm-1 in the C-F stretching region are consistent with the predicted symmetric and antisymmetric stretching frequencies for CF2dPtHF shown in Table 7. A weaker 572.8 cm-1 band (not shown) is also appropriate for the expected Pt-F stretching mode. Other plausible Pt products including CHF2-PtF, CF3-PtH, and CHFdPtF2 do not reproduce the observed vibrational characteristics; for example, CHF2-PtF would show absorptions at around 1180 and 860 cm-1 and CHFdPtF2 would have a strong absorptions at around 1180 cm-1. In addition, CF2dPtHF is the most stable among the plausible products; CF2dPtHF and CF3-PtH (45 and 41 kcal/mol more stable than the reactants) are considerably more stable than the other products (e.g., CHF2-PtF and CHFdPtF2 are 18 and 19 kcal/mol more stable than the reactants). The Pt high oxidation-state complex with a Pt-H bond, namely CF2dPtHF, is again favored over CHFdPtF2 in reaction with CHF3. Shown in Figure 5 are the IR spectra from reactions of Pt with chloroform isotopomers. The product absorptions marked “m” slightly increase on visible photolysis and decrease on UV and full arc photolysis. Calculations find that the most stable among the plausible products is CHCldPtCl2, which is 76 kcal/ (33) (a) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds; John Wiley and Sons: New York, 1988. (b) Herndon, J. W. Coord. Chem. ReV. 2008, 252, in press. (c) Herndon, J. W. Coord. Chem. ReV. 2007, 251, 1158. (d) Herndon, J. W. Coord. Chem. ReV. 2006, 250, 1889. (e) Herndon, J. W. Coord. Chem. ReV. 2005, 249, 999. (f) Herndon, J. W Coord. Chem. ReV. 2004, 248, 3. and earlier review articles in this series.



Table 6. Observed and Calculated Fundamental Frequencies of CHFdPtHF and CHFdPtHCl in the 1A′ Ground Statesa CHFdPtHF approx sescription

obs

A′ CH str A′ Pt-H str A′ C-H IP bend A′ C-F str A′′ C-H OOP bend A′ C-Pt str A′ Pt-H IP bend A′ Pt-X str A′ PtCF IP bend A′′ Pt-H OOP bend A′ CPtX IP bend A′′ CPtX OOP bend

1192.1

552.7

CHFdPtHCl

B3LYP

int

BPW91

MP2

3024.3 2457.4 1401.6 1214.0 931.3 805.5 649.9 580.7 365.1 350.4 113.0 83.4

12 2 65 358 3 14 10 130 2 1 5 1

2953.4 2438.6 1343.6 1164.6 890.4 798.1 628.1 567.9 367.6 358.4 99.5 76.9

3090.8 2708.0 1414.6 1236.9 989.8 931.4 735.4 603.9 416.8 496.3 111.3 42.3

obs

1371.1 1201.1, 1193.6

B3LYP

int

BPW91

MP2

3021.4 2439.8 1396.1 1218.3 898.2 770.1 635.3 359.7 341.0 299.1 77.3 51.1

11 3 86 440 2 15 1 52 7 3 1 0

2948.4 2418.6 1340.0 1170.5 858.8 765.9 620.3 361.4 336.9 323.9 77.1 57.6

3082.9 2665.3 1409.2 1238.2 967.1 893.9 710.4 450.2 401.4 365.6 73.6 40.9

a Frequencies and intensities are in cm-1 and km/mol. Observed in an argon matrix. Frequencies and intensities are computed with 6-311++G(3df, 3pd), and the SDD core potential and basis set are used for Pt. Intensities are calculated with B3LYP. Both CHFdPtHF and CHFdPtHCl have planar (Cs) structures at all levels of theory used in this study. The symmetry notations are based on the Cs structure.

Table 7. Observed and Calculated Fundamental Frequencies of CF2dPtHF in the 1A′ Ground Statea CF2dPtHF approx description A′ C-H str A′ CF2 s str A′ CF2 as str A′ Pt-H IP bend A′ CF2 bend A′′ CF2 wag A′ Pt-F str A′ CdPt str A′ CF2 rock A′′ Pt-H OOP bend A′ CPtF IP bend A′′ CPtF OOP bend

obs 1293.3 1198.8

572.8

B3LYP

int

BPW91

MP2

2459.8 1300.2 1206.6 724.3 700.2 671.6 596.4 370.3 340.0 243.5 127.5 91.8

3 582 292 5 2 7 129 1 0 2 6 3

2442.2 1254.9 1141.2 690.3 671.2 652.0 587.5 366.6 331.4 252.5 122.3 85.1

2746.3 1353.2 1206.4 789.4 728.4 721.5 616.1 419.4 376.4 373.8 103.0 79.8

a Frequencies and intensities are in cm-1 and km/mol. Observed in an argon matrix. Frequencies and intensities are computed with 6-311++G(3df, 3pd), and the SDD core potential and basis set are used for Pt. Intensities are calculated with B3LYP. CF2dPtHF has a planar (Cs) structure at all levels of theory used in this study. The symmetry notations are based on the planar structure.

mol lower in energy than the reactants. The CCl2dPtHCl methylidene isomer and CHCl2-PtCl and CCl3-PtH insertion products are 68, 61, and 44 kcal/mol lower energy than the reactants. The product absorption at 1190.7 cm-1 has its D counterpart at 798.2 cm-1, and the stronger product absorption at 945.1 cm-1 has its D counterpart at 1033.7 cm-1. They are assigned to the interacting C-H in-plane bending and C-Cl stretching mode pair, which reverse order on deuterium substitution and mix in different proportions as shown by the intensity changes. This is reminiscent of the CHCl2 and CDCl2 radical infrared spectra in this region.34 The 945.1 cm-1 band is due to mixed C-Cl stretching and CdPt stretching modes with less contribution from the Cl-C-H bending coordinate. However, deuteration shifts the resulting Cl-C-D bend below the C-Cl stretching mode position, resulting in an increase in the C-Cl stretching frequency and more contribution from Cl-C-D bending and less contribution from CdPt stretching in the normal mode. In this regard, our calculations find the CdPt stretching mode for CH2dPtH2 at 868 cm-1 (Table S3, Supporting Information), but this mode is also mixed slightly with CH2 bending coordinates. Only CHCldPtCl2 among the plausible products reproduces the observed vibrational characteristics and their variations with deuteration as shown in Table 8. The higher energy methylidene isomer CCl2dPtHCl would have two strong absorptions in the 1000-800 cm-1 region, based on our calculations and the observed spectrum of CCl2dPtCl2.13 The less stable CHCl2PtCl insertion intermediate would have even lower frequency (34) Carver, T. G.; Andrews, L. J. Chem. Phys. 1969, 50, 4235.

Figure 5. IR spectra in the product absorption regions from the laser-ablated Pt atom reactions with CHCl3 isotopomers in excess argon at 8 K: (a) Pt + 0.5% CHCl3 in Ar codeposited for 1 h; (b) As in (a) after vis (λ > 420 nm) photolysis; (c) As in (b) after uv (240 < λ < 380 nm) photolysis; (d) As in (c) after annealing to 28 K; (e) Pt + 0.5% CDCl3 in Ar codeposited for 1 h; (f) As in (e) after vis (λ > 420 nm) photolysis; (g) As in (f) after uv (240 < λ < 380 nm) photolysis; (h) As in (g) after annealing to 28 K. m denotes the methylidene product absorption. c and P denote product absorptions common to this precursor in other experiments and unreacted precursor absorptions.


Cho and Andrews

Table 8. Calculated Fundamental Frequencies of CHCldPtCl2 Isotopomers in the 1A′ Ground Statea CHCldPtCl2 approx description A′ C-H str A′ C-H IP bend A′ C-Cl str A′′ C-H OOP bend A′ CdPt A′′ PtCl2 as bend A′ PtCl2 s bend A′ ClCPt bend A′′ PtCCl2 deform A′ PtCl2 wag A′ PtCl2 scis A′′ CHCl tort

obs 1190.7 945.1 708.4 688.1

CDCldPtCl2

B3LYP

int

BPW91

MP2

3129.8 1223.4 950.5 750.9 689.0 368.8 350.7 234.2 182.5 106.6 98.8 45.7

12 86 214 18 20 68 8 0 4 1 0 0

3062.8 1166.9 931.3 700.3 675.3 371.6 352.7 228.3 194.0 107.4 98.8 51.2

3154.2 1256.9 1073.8 749.8 727.3 405.9 387.0 248.8 229.4 110.0 105.0 54.4

obs 798.2 1033.7 604.4

B3LYP

int

BPW91

MP2

2303.8 811.3 1044.5 615.3 653.6 368.5 350.7 233.0 160.1 106.6 98.7 45.3

6 52 253 7 11 68 8 0 3 1 0 0

2253.9 786.7 1007.4 572.5 640.3 371.4 352.7 227.1 170.3 107.3 98.8 50.8

2323.2 804.3 1204.7 616.7 700.2 387.0 387.0 247.3 200.1 109.9 105.0 54.0

a Frequencies and intensities are in cm-1 and km/mol. Observed in an argon matrix. Frequencies and intensities are computed with 6-311++G(3df, 3pd), and the SDD core potential and basis set are used for Pt. Intensities are calculated with B3LYP. CHCldPtCl2 has a Cs structure at all levels of theory. The symmetry notations are based on the Cs structure.

absorptions. Another product absorption at 708.4 cm-1 is designated to the C-H out-of plane bending mode without observation of the D counterpart. The product absorption at 688.1 cm-1 along with the D counterpart at 604.4 cm-1 is assigned to the predominantly CdPt stretching mode of the methylidene complex, whose frequency is relatively low due to mixing with the C-Cl stretching mode. The good agreement with the predicted frequencies as shown in Table 8 supports formation of CHCldPtCl2, which is compared with CF2dPtHF produced from the reaction of CHF3. The bond energies of Cl-Cl and C-Cl are 58 and 79 kcal/ mol,31 and the B3LYP reaction energy of Pt + Cl2 f PtCl2 is -76.0 kcal/mol; therefore, the Pt-Cl bond energy is estimated to be about 67 kcal/mol, some 10 kcal/mol lower than the Pt-H and Pt-F bond energies. However, in this case, the C-Cl bond energy is 20 kcal/mol lower than the C-H bond energy,31 and as a result, the product with a C-H bond is more stable. If we neglect other thermochemical factors, CHCldPtCl2 is expected to be 10 kcal/mol more stable using bond energies than CCl2dPtHCl, which is compared with the B3LYP energy difference of 8 kcal/mol. Structures. The computed structures of the identified products from reactions of Pt with small alkanes and halomethanes are illustrated in Figure 6. CH3-PtH has a Cs structure at all levels of theory used in this study. The C-Pt bond length of 1.948 Å is slightly shorter than those of typical Pt complexes, such as 1.994 Å for cis-dichloro(1-methyl-3-(2-bromobenzyl)imidazol-2-ylidine)(DMSO)platinum(II).35 The CH3CH2PtH complex, on the other hand, has a C1 structure, and the C-Pt bond length (1.953 Å) is slightly longer than that of CH3-PtH. Substitution of a methyl group for H evidently leads to decreases in the effective bond order and polarity of the C-Pt bond (Table 9), both of which slightly weaken the bond. The methylidene CH2dPtH2 has the C2v structure with perpendicular CH2 and PtH2 planes and 1.752 Å (CCSD) or 1.790 Å (B3LYP) CdPt bond length similar to that of CCl2dPtCl2.13 The products from Pt + CH3F reaction, CH2F-PtH, CH2dPtHF, CH3-PtF, and CH2(H)-PtF, all have Cs symmetry. The C-Pt bond lengths (1.927 and 1.934 Å) are slightly shorter than those for CH3-PtH and CH3CH2-PtH; substitution of F for H obviously strengthens the carbon-metal bond, consistent with the previous results for Pt tetrahalocarbenes.13 In the (35) Newman, C. P.; Deeth, R. J.; Clarkson, G. J.; Rourke, J. P. Organometallics 2007, 26, 6225. (a) Fang, M.; Jones, N. D.; Ferguson, M. J.; McDonald, R.; Cavell, R. G. Angew Chem., Int. Ed. 2005, 44, 2005. (b) Hanks, T. W.; Ekeland, R. A.; Emerson, K.; Larsen, R. D.; Jennings, P. W. Organometallics 1987, 6, 28.

bridged structure of CH2(H)-PtF, the relatively short C-Pt bond and elongated C-H bond indicate that there are considerable interactions between the C-H bond and metal atom, leading to the substantial distortion of the methyl group, an evidence of strong agostic interaction. While considerable agostic distortions are reported from many transition-metal methylidenes,7 this is the first example of agostic distortion in a small transition-metal insertion complex. Figure 6 also shows that CHFdPtHCl and CF2dPtHF have planar structures, whereas CHCldPtCl2 has an allene-type structure with the CHCl group perpendicular to the PtCl2 group, similar to those for previously studied CX2dPtX2 molecules.13 Notice the short CdPt bonds of CHFdPtHCl, CF2dPtHF, and CHCldPtCl2 (1.857, 1.857, and 1.805 Å) revealing that they are indeed carbon-metal double bonds. They are much shorter than those of Pt(II) carbene complexes (1.94-1.99 Å)36 and comparable with those for the recently discovered CX2dPtX2 methylidenes (1.810-1.816 Å).13 The computed Pt-Cl bond lengths (2.289 and 2.275 Å for CHFdPtHCl and CHCldPtCl2) are also compared with those measured for previous Pt(II) carbene complexes in the 2.367-2.377 Å range. The shorter CdPt and Pt-Cl bond lengths suggest that the Pt atom in these methylidene complexes are in an effectively higher oxidation state, in line with the CX2dPtX2 cases. Finally, note that the CdPt-H bond angle is near 84° for the Pt methylidene complexes, which contrasts 105-111° CdM-H angles for the group 4 CH2dMHX complexes7 and reflects different d orbital participations for the early and late transition metals. The HCtPtF molecule is also illustrated in Figure 6 for its carbon-platinum triple bond and much shorter bond length of 1.696 Å. Although this molecule was not observed here owing to its higher energy, it is calculated to be the more stable HF elimination product of the methylidene CHFdPtHF, where the HF elimination process is 19 kcal/mol endothermic. The alternative FCdPtH molecule is cis-planar with a 1.761 Å double bond and 9 kcal/mol higher energy. For comparison, the FCtPtF molecule also has a carbon-platinum triple bond and short bond length of 1.695 Å. The natural bond orders, bonding minus antibonding occupancies divided by 2, for CH3-PtH, CH3CH2-PtH, CH2(H)-PtF, CH2F-PtH, CHFdPtHCl, CF2dPtHF, and (36) (a) Gosavi, T.; Wagner, C.; Merzweiler, K.; Schmidt, H.; Steinborn, D. Organometallics 2005, 24, 533. (b) Vuzman, D.; Poverenov, E.; DiskinPosner, Y.; Leitus, G.; Shimon, L. J. W.; Milstein, D. Dalton Trans. 2007, 5692. (c) Harrak, Y.; Blaszykowski, C.; Bernard, M.; Cariou, M.; Mainetti, E.; Mouries, V.; Dhimane, A.-L.; Fensterbank, L.; Malacria, M. J. Am. Chem. Soc. 2004, 126, 8656.



Figure 6. Structures of the complexes identified from reactions of Pt with small alkanes and halomethanes in this study. The CH3-PtH, C2H5-PtH, and CH2F-PtH structures are calculated with CCSD/6-311++G(3df,3pd)/SDD, while the other structures are calculated with B3LYP/6-311++G(3df,3pd)/SDD. C2H5-PtH has a C1 structure, but the other insertion products have Cs structures. Notice the bridged structure of CH2(H)-PtF, the planar structures of CHFdPtHCl and CF2dPtHF, and nonplanar structure of CHCldPtCl2 containing mutually perpendicular CHCl and PtCl2 planes. The bond lengths and angles are in angstroms and degrees. Table 9. Parameters from Natural Bond Orbital Analysis of Pt Insertion, Methylidene, and Methylidyne Productsa parameters CH3-PtH CH3CH2-PtH CH3-PtF CH2(H)-PtF CH2F-PtH CH2dPtHF CHFdPtHF CHFdPtHCl CF2dPtHF CHCl)PtCl2 CHtPtF σ(C-Pt)b σ*(C-Pt)b π(C-Pt)b π*(C-Pt)b EBOc q(C)d q(Pt)d r(C-Pt)e

1.959 0.059

1.948 0.082

1.943 0.059

1.939 0.073

1.953 0.100

0.95 -0.66 0.05 1.948

0.93 -0.40 0.03 1.953

0.94 -0.65 0.63 1.965

0.93 -0.55 0.56 1.897

0.93 0.03 0.05 1.927

1.950 0.293 1.997 0.079 1.79 -0.32 0.59 1.848

1.948 0.301 1.995 0.209 1.72 0.25 0.53 1.843

1.997 0.233 1.945 0.394 1.66 0.25 0.35 1.857

1.997 0.313 1.954 0.310 1.66 0.71 0.49 1.857

1.920 0.194 1.565 0.332 1.48 -0.14 0.66 1.805

1.909 0.194 1.996× 2 0.084× 2 2.81 0.00 0.49 1.696

a Molecular parameters regarding the effective bond order of the identified complexes from Pt reactions with small alkanes and halomethanes. Natural bond orbital analysis in Gaussian 03, ref 16. b Occupation number. c Effective bond order calculated from occupation numbers. d Natural charge. e C-Pt bond length.

CHCldPtCl2 are 0.95, 0.93, 0.94, 0.93, 1.65, 1.66, and 1.48. They are compared with those for Pt tetrahalomethylidenes ranging from 1.41-1.71. Evidently, there is substantial π-bonding, and introduction of F to these Pt methylidene increases the effective bond order, similar to the cases of F substitution for Cl in CX2dPtX2. The more electronegative F contracts the platinum d orbitals and makes them more compatible for bonding (particularly π-bonding) to carbon valence orbitals. The higher effective bond orders of the F containing Pt carbenes are, however, counteracted by the lower ionic-bonding characters due to the less polarized C-Pt bond (Table 9), and it is considered at least part of the reason for the longer CdPt bond. Finally, the HCtPtF molecule contains a platinum-carbon

triple bond of natural bond order 2.81. For this linear molecule, the two π molecular orbitals are degenerate. Mechanism. As described above, CH4 and C2H6 form predominantly the C-H bond insertion complex in reactions with Pt. Although the corresponding carbene complexes are higher in energy than the insertion complex, CH2dPtH2 is detected with a population of about 1% of the insertion product.14 More evidence for H(X) migration following C-X insertion, however, is observed from reactions of halomethanes with Pt, as found for the earlier transition metals.7 The halogen lone electron pairs are believed to attract the electron deficient Pt atom and to form an insertion complex, and the excess reaction energy then activates H(X) migration.


Cho and Andrews

The ultimate formation of a C-H insertion complex (CH2F-PtH), in addition to the C-F insertion complex in metal reactions with CH3F is unique. The stability of the platinum C-H insertion complex relative to the C-F insertion complex is certainly the driving force for generating this rare species. After formation of the initial energized insertion complex, [CH3-PtF]*, rearrangement is expected to occur in two steps, first H to Pt through the bridged intermediate [CH2(H)-PtF]* and then F back to C migration. The methylidene, CH2dPtHF is probably involved in this rearrangement as well. After final annealing to 36 K, our sample still contains approximately 1.5 times as much CH3-PtF as CH2F-PtH.

Pt * + CH3F f [CH3 - PtF] * f [CH2 - Pt(H)F] * f [CH2dHF] * f CH2F-PtH (5a) If we assume that intersystem crossing occurs during the initial reagent encounter and reaction (5a) proceeds on the singlet potential energy surface, small transition state barriers are computed between [CH3-PtF] and [CH2-Pt(H)F], 0.03 kcal/ mol, between [CH2-Pt(H)F] and [CH2dPtHF], 2 kcal/mol, and a larger barrier between [CH2dPtHF] and [CH2F-PtH], 26 kcal/ mol (see Figure S2, Supporting Information). Recall that these rearrangements are driven by visible and uv irradiation so the formation of CH2dPtHF and its rearrangement to CH2F-PtH involve higher electronic states in a photochemical process. Finally, in view of the fact that CH2F-PtH is 10 kcal/mol more stable than CH3-PtF, some of this more stable CH2F-PtH product may be formed directly in the initial reaction as suggested in reaction (5b).

Pt * + CH3F f [(Pt*)(CH3F)] f CH2F-PtH

(5b)

The reaction with methylidene fluoride is also believed to start with C-F insertion, followed by H migration to the lower energy methylidene complex.

Pt * + CH2F2 f [(CH2F-PtF)] * f CHF d PtHF (6) Likewise, the formation of CHFdPtHF, CHFdPtHCl, and CF2dPtHF involves H (instead of F) migration from C to Pt after initial C-X insertion (X ) H, Cl, or F). This contrasts previous results showing that X migration is favored over H migration, due to the stronger M-X bond.30

Pt * + CHFX2 f [(CHFX-PtF)] * f CFX d PtHX (7) On the other hand, CHCldPtCl2 is formed in the reaction of CHCl3 via C-Cl insertion and following Cl migration. In addition, the product yield is higher with chlorocarbon precursors, which suggests that the initial insertion of Pt into C-Cl is more favorable and probably faster than into C-F bonds, based on comparable effective reaction periods for energized Pt during the condensation processes.

Pt * + CHCl3 f [(CHCl2-PtCl)] * f CHCl-PtCl2 (8) The identification of Pt complexes in singlet ground states also indicates that intersystem crossings occur readily from the

starting triplet state (Pt(3D)) to the more stable singlet states formed in these Pt systems.

Conclusions Reactions of laser-ablated Pt atoms and small alkanes and halomethanes have been investigated and the matrix IR spectra examined. The C-H insertion complexes (CH3-PtH and CH3CH2-PtH) are produced from methane and ethane, showing that the late-transition-metal atom is an effective C-H bond insertion agent. Based on infrared intensities, about 1% of the CH3-PtH undergoes R-H transfer to CH2dPtH2 during relaxation in the matrix. Methyl fluoride, on the other hand, forms the C-H as well as C-F insertion products (CH2F-PtH and CH3-PtF). The computation results also show the stability of the C-H insertion complex over the C-F insertion complex. Likewise, CH2FCl, CH2F2, and CHF3 generate primary products containing a Pt-H bond (CHFdPtHCl, CHFdPtHF, and CF2dPtHF), reconfirming the preference of the Pt-H bond in reaction of Pt with fluoroalkanes. The CHCl3 precursor, on the other hand, forms only CHCldPtCl2. Finally, Pt is more reactive with C-Cl than with C-F bonds. The interesting competition for Pt-H over Pt-F bond formation is believed to originate from the similar Pt-H and Pt-F bond energies, and therefore, the difference between the C-H and C-F bond energies plays the primary role in determining the major Pt and CH3F reaction product. Partition between these two reaction channels is a subject for future theoretical analysis. The identified Pt carbenes also reveal that the high oxidationstate products are favored in reactions of Pt with di- and trihalomethanes as well as tetrahalomethanes.13 The short (1.80-1.86 Å) C-Pt bond lengths of the carbene complexes along with the high effective bond orders are signs of strong π-bonding between the C and Pt atoms. The short C-Pt bond lengths of these small carbene complexes relative to those of previously studied Pt(II) complexes (1.94-1.99 Å) also suggests that the metal atom is in fact in a higher oxidation state. Finally, we calculate a much shorter 1.696 Å bond length in the HCtPtF molecule, which contains a platinum-carbon triple bond.

Acknowledgment. We gratefully acknowledge financial support from NSF Grant No. CHE 03-52487 to L.A., and the use of the computing system of the KISTI Supercomputing Center (KSC-2008-S02-0001) is also greatly appreciated. Supporting Information Available: Tables S1-S4 comparing observed and calculated frequencies, Figure S1 containing Pt and methane reaction product spectra, and Figure S2 containing Pt and CH3F reaction product energies and barriers. This material is available free of charge via the Internet at http://pubs.acs.org. OM801077X

Infrared Spectra of Platinum Insertion and Methylidene Complexes

Recommend Documents