Direct Observation of Metal Ketenes Formed by Photoexcitation of a

Article pubs.acs.org/Organometallics

Direct Observation of Metal Ketenes Formed by Photoexcitation of a Fischer Carbene using Ultrafast Infrared Spectroscopy Son C. Nguyen,†,‡,∥ Justin P. Lomont,†,‡,∥ Matthew C. Zoerb,†,‡,⊥ Phong V. Pham,†,‡,# James F. Cahoon,§ and Charles B. Harris*,†,‡ †

Department of Chemistry, University of California, Berkeley, California 94720, United States Chemical Science Division, Lawrence Berkeley National Laboratory, Berkeley California 94720, United States § Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States ‡

S Supporting Information *

ABSTRACT: Fischer carbenes are commonly used as reagents in the synthesis of new carbon−carbon bonds, a reaction made possible by the unique chemistry of the formal metal−carbon double bond. Nevertheless, the photoinduced reactions of these complexes are relatively poorly understood. For instance, it has been postulated but not confirmed that visible irradiation leads to photocarbonylation, in which a CO ligand inserts into the metal−carbon bond to form a metal ketene intermediate. Here, we report the first direct observation of this intermediate following 400 nm photoexcitation of the model group 6 Fischer carbene Cr(CO)5[CCH3(OCH3)]. Using ultrafast time-resolved infrared spectroscopy (TRIR), we observe the formation of three distinct metal ketene structures, which we assign as a singlet and two isoenergetic triplet excited-state structures. The singlet relaxes to the ground state on a time scale of ∼35 ps, whereas the two triplets are longlived (>2 ns). TRIR of the tungsten analogue yields no evidence for a metal ketene structure, consistent with the limited reactivity of this complex. The results directly elucidate the fundamental role of triplet metal ketenes in the photoreactivity of Fischer carbene complexes.

1. INTRODUCTION Fischer carbene transition-metal complexes have been developed as reagents for a variety of reactions that form new carbon−carbon bonds and that exploit the chemistry of the formal double bond between the metal and the carbene carbon.1,2 This carbon atom is electrophilic and readily undergoes nucleophilic addition reactions, resulting, for example, in benzannulation between unsaturated alkoxycarbenes and alkynes.3 These types of thermal reactions are a longstanding area of investigation; however, Fischer carbenes are also useful for an entirely different set of unique but poorly understood photoinduced reactions. Hegedus and co-workers discovered that photoexcitation of group 6 Fischer carbenes leads to products consistent with the reactivity expected from ketenes.4−7 For example, carbenes readily react with imines after visible photoexcitation to form βlactams, the four-membered ring common to all penicillin antibiotics. This reactivity led Hegedus to propose an intermediate metal ketene structure in which a carbonyl has inserted into the metal−carbene bond to form the ketene moiety (a process termed photocarbonylation; see Scheme 1). A band in the UV/vis spectrum at ca. 400 nm has been assigned to a metal-to-ligand charge-transfer (MLCT) transition.8 Nevertheless, there exists no direct evidence for this metal ketene structure, despite previous attempts to observe it in © 2014 American Chemical Society

Scheme 1. Photocarbonylation Proposed for Cr and Mo Fischer Carbene Complexes

cryogenic matrices and with microsecond time-resolved spectroscopy.8−10 Interestingly, this reactivity was observed for complexes of Cr and Mo but not W.4 For many years after the seminal report by Hegedus in 1982, the mechanistic details surrounding the reactivity of the group 6 Fischer carbenes remained elusive. Recent work by Fernández, ́ Sierra, and co-workers led to the proposal of a Cossio, mechanism for the photocarbonylation reaction involving initial excitation into a triplet state which, in the presence of coordinating solvents, undergoes a solvent-induced spin crossover to yield a metal ketene in which the solvent coordinates to the metal center to stabilize the vacant site.11−13 A new photochemical reaction was also experimentally uncovered through which tungsten-based complexes, in Received: August 2, 2014 Published: October 2, 2014 6149

dx.doi.org/10.1021/om500795b | Organometallics 2014, 33, 6149−6153

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along a parallel path through a computer-controlled spectrograph with entrance slits set at 70 μm (Acton Research Corp., SpectraPro-150) and detected by a 2 × 32 element MCT-array IR detector (InfraRed Associates, Inc.) and a high-speed signal acquisition system and data acquisition software (Infrared Systems Development Corp.) with a resolution of ca. 2.5 cm−1. Collected signals are averaged over 2 × 104 laser shots to correct for shot-to-shot fluctuations. Differences in optical density as small as 5 × 10−5 are observable after 1 s of data collection. 2.3. Data Analysis. Kinetic data in this work result from spectra measured at delay times between 0 and 1700 ps between the UV/ visible-pump and IR-probe pulses. The kinetic data were then fit to exponential curves using Origin Pro 8 software.22 Errors on experimental time constants are reported as 95% confidence intervals. 2.4. DFT Modeling. Density functional theory (DFT) calculations have been carried out to facilitate assignment of the absorptions observed in the TRIR spectra. Calculations were carried out using the BP86 functional23,24 in either the Gaussian03 or Gaussian09 package25 using the 6-31+g(d,p) basis set26 for C, O, and H atoms and the lanl2dz basis set27,28 for Cr. Geometry optimizations were followed by a frequency analysis for use in interpreting the TRIR results and to ensure that all geometries were genuine local minima.

addition to their chromium counterparts, were able to undergo a 1,2-dyotropic rearrangement to yield imine products, thus demonstrating that tungsten-based Fischer carbenes can indeed be photoreactive.14 Photochemical investigations into both chromium and tungsten complexes demonstrated that UV excitation also leads to anti → syn isomerization of the methoxy substituent as one of the primary photochemical processes, with CO loss also occurring to a lesser extent.15,16 Despite these new insights into the photochemistry of group 6 Fischer carbenes, the key photocarbonylation mechanism has, until now, not been directly characterized by experiments. Here, we resolve several of the longstanding questions on the photochemistry of these complexes using picosecond TRIR measurements on the complexes Cr(CO)5[CCH3(OCH3)] (A) and W(CO)5[CCH3(OCH3)] (B). For the Cr complex, we report the first direct evidence of metal ketene intermediates, and we show that the same intermediates do not form for the W complex.

2. METHODS 2.1. Sample Preparation. Cr(CO)5[CCH3(OCH3)] and W(CO)5[CCH3(OCH3)] were synthesized according to literature precedent,17,18 and cyclohexane (Sigma-Aldrich Co.) was used without further purification. Solutions of the Fischer carbene complexes prepared for the TRIR experiments were dilute (ca. 3 mM) and contained ca. 3% relative concentration of the precursor metal hexacarbonyl (Cr(CO)6 or W(CO)6), which could not be easily removed during synthesis (see Figure S4 in the Supporting Information). These metal hexacarbonyls do not influence the TRIR spectra collected following 400 nm excitation, as they do not absorb significantly at this wavelength.19,20 Although the sample cell used for the TRIR experiments was not rigorously air free, samples were purged with argon and sealed after dissolution of the carbene complex to minimize any potential decomposition due to air exposure. 2.2. Ultrafast UV Pump−IR Probe Spectroscopy. The experimental setup consists of a Ti:sapphire regenerative amplifier (SpectraPhysics, Spitfire) seeded by a Ti:sapphire oscillator (SpectraPhysics, Tsunami) to produce a 1 kHz train of 100 fs pulses centered at 800 nm with an average pulse power of 1.1 mJ. The output of this commercial system is split, and 30% of the output is used to generate 400 and 267 nm pump pulses (80 and 6 μJ per pulse at sample, respectively) via second- and third-harmonic generation. The other 70% is used to pump a home-built two-pass BBO-based optical parametric amplifier (OPA),21 the output of which is mixed in a AgGaS2 crystal to produce mid-IR probe pulses tunable from 3.0 to 6.0 μm with a 200 cm−1 spectral width and a ca. 100 fs pulse duration. The 400 and 267 nm pulses pass through a 25 cm silica rod, which stretches the pulses in time to 1 ps and gives a cross correlation of the mid-IR and 400 or 267 nm pulses of 1.1 ps at the sample. The stretched 400 and 267 nm pulses are necessary to achieve high pump fluence without generating products resulting from multiphoton excitation. The stretched pulses also reduce artifacts resulting from nonlinear optical effects in the sample cell windows. The polarization of the pump beam is held at the magic angle (54.7°) with respect to the mid-IR probe beam to eliminate effects from rotational diffusion. A computer-controlled translation stage (Newport) allows for variable time delays up to ca. 1.7 ns between pump and probe pulses. The sample is flowed using a mechanical pump through a stainless steel cell (Harrick Scientific) fitted with 2 mm thick CaF2 windows separated by 100−930 μm spacers. Thicker spacers were used to obtain better signals at longer delay times. The pump and probe beams are spatially overlapped at the sample and focused so that the beam diameters are ca. 200 and 100 μm, respectively. The sample cell is moved by computer-controlled translational stages (Standa) during the course of data collection so that absorptions are not altered by any accumulation of photoproduct on the sample windows. Reference and signal mid-IR beams are sent

3. RESULTS AND DISCUSSION 3.1. Picosecond TRIR Spectroscopy of Cr(CO)5[CCH3(OCH3)] in Cyclohexane Solution. Figure 1A shows TRIR difference spectra in the region of the CO stretching vibrations for dilute A in cyclohexane following 400 nm excitation. Negative signals, referred to as bleaches, result

Figure 1. TRIR spectra and kinetics of Cr(CO)5[CCH3(OCH3)] following 400 nm excitation in cyclohexane solution. (A) Spectra at time delays of (top) 1 ps (red), 5 ps (green), and 50 ps (blue) and (bottom) 1700 ps. Note that spectra from 1800 to 1880 cm−1 are omitted for clarity, as denoted by the dashed vertical line. (B) Normalized kinetic traces for the parent complex at 1947 cm−1 (black circles) and for new absorptions at 1990 cm−1 (blue squares) and integrated absorptions across the 1771−1791 cm−1 range (red triangles). Dashed lines represent fits to exponential functions (see text). 6150


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that study, and the complex could warrant additional investigation. 3.2. Density Functional Theory Investigation of the Metal Ketene Intermediates. DFT calculations were performed to identify the structures of the metal ketene intermediates observed experimentally. A recent DFT study of Cr carbene complexes by Fernández et al. showed that the lowest-lying triplet state possesses a metal ketene structure, suggesting that this state is responsible for the unique photochemistry of the compounds.12 Our calculations on the lowest triplet (T1) state of A found two nearly isoenergetic triplet geometries with metal ketene structures (see Figure 2;

from species depleted by the 400 nm excitation pulse and correspond to the absorptions of A, while positive signals result from new species present after photoexcitation. At early delay times, strong positive features are observed in the 1985−1995 and 1765−1795 cm−1 spectral ranges. The latter absorptions are weak at the earliest time delays and grow in within the first few picoseconds. The kinetics in this spectral region (see Figure 1B) are well fit to the sum of an exponential rise and decay with time constants of 5 ± 1 and 33 ± 5 ps, respectively. The peaks at 1985−1995 cm−1 as well as the bleaches from A exhibit similar decay time constants of 36 ± 1 and 33 ± 2 ps, respectively (see Figure 1B), and instrument-limited rise times (not shown). The similarity of the decay time constants suggests that a majoritybut not allof the transient species relax back to the ground-state structure A on a time scale of ∼35 ps. The low frequency of the 1765−1795 cm−1 bands suggests that these absorptions are the result of CO ligands in a metal ketene coordination geometry (confirmed by density functional theory (DFT) calculations; see below and Table S1 (Supporting Information)). In this spectral range, one highamplitude (1777 cm−1) and two low-amplitude peaks (1771 and 1791 cm−1) are apparent at early time delays; however, only the two low-amplitude peaks remain at long time delays. This observation suggests that three metal ketene structures are formed upon photoexcitation, and the structure in highest yield (at 1777 cm−1) relaxes to the ground state, A, in ∼35 ps. The other two peaks persist beyond the ∼2 ns time scale for the TRIR experiment and are likely to be responsible for the observed photoreactivity of the complex. On the basis of the decay time constants noted above as well as DFT calculations (see Table S1), we can also assign the absorption bands at 1990 and 1988 cm−1 to terminal CO vibrations of metal ketene structures. Additional bands are apparent in Figure 1A that can be assigned to previously reported side reactions. Peaks at 1930, 1953, and 2070 cm−1 are a result of anti → syn isomerization of the methoxy substituent in A.8,15,16 Similarly, bands at 1899 and 2044 cm−1 are assigned to a CO-loss product. These side reactions have been reported previously.15,16 Changes in the intensities of these bands on the picosecond time scale are attributed to spectral overlap with the absorptions of the metal ketenes and bleaches of the parent complex, all of which exhibit dynamics on the picosecond time scale. Note that we report the spectral position that corresponds to the maximum observed intensity of each feature in the TRIR spectra; however, these positions may be slightly shifted from the true center frequency of each band due to complex spectral overlap with other species. For CO-loss products, the band assignments were verified by comparison to analogous experiments using 267 nm excitation, in which CO loss is expected to occur in significantly higher yield. Additional bands of the CO-loss product were also visible at 1951 and 1965 cm−1 following 267 nm excitation; however, these peaks were obscured by other features in the spectra collected with 400 nm excitation. The metal ketene peaks observed following 400 nm excitation were not observed following 267 nm excitation. Note that a recent study using picosecond TRIR to examine the CO-release characteristics of an amino carbene complex, Cr(CO)5[CCH3(NC4H8)], under 400 nm excitation did not report evidence for metal ketene intermediates.29 However, the characteristic low-frequency peaks from a metal ketene may have been outside the spectral window (>1850 cm−1) used in

Figure 2. DFT energetics as a function of the carbene−chromium− carbonyl C−Cr−C bond angle for the S0 and T1 electronic levels involved in photoexcitation of Cr(CO)5[CCH3(OCH3)]. Insets show the calculated molecular structures at various points, and the associated bond angles are given in the lower right corners of the insets. Data points for T1 correspond to the structure shown in inset 2. Structures 1 and 2 were obtained by geometry optimization in the lowest triplet state. Structure 3 corresponds to an optimized local minimum located for the alkane-solvated singlet structure, and structure 4 represents an optimized, unsolvated geometry on the S0 surface with a constrained C−Cr−C bond angle. Also shown are the relative energies of singlet (S1, S2) and triplet (T1, T2, T3) excited states (obtained from TD-DFT calculations) accessible through photoexcitation with a 400 nm photon (depicted by the blue arrow).

insets 1 and 2). The calculated metal ketene stretching frequencies for these two triplet structures are 1749 and 1772 cm−1, which are in good agreement with the low-amplitude and long-lived absorptions at 1771 and 1791 cm−1. The DFTcalculated shift in vibrational frequency between the complexes, 23 cm−1, is in excellent agreement with the experimental shift of 20 cm−1. Furthermore, the lifetime (>2 ns) of these two bands is consistent with assignment as triplet spin states, which are well-known to have long lifetimes, because relaxation to the singlet ground state is spin forbidden. The calculated terminal CO stretching frequencies for both structures (see Table S1 (Supporting Information)) also agree reasonably well with the experimentally observed band at 1988 cm−1. Note that an alternative assignment of these bands could be an oxygencoordinated metal ketene, as previously proposed by Fernández 6151


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and co-workers.30 However, DFT calculations (see Table S2 (Supporting Information)) indicate that this structure exclusively possesses CO vibrational modes above 1900 cm−1, which is inconsistent with assignment to the peaks observed experimentally below 1800 cm−1. With the long-lived peaks assigned to triplets, we assign the high-amplitude but short-lived peaks at 1777 and 1990 cm−1 to a singlet metal ketene structure. DFT calculations indicate that association of a solvent molecule can stabilize a metal ketene structure in the S0 state (see Figure 2, inset 3) with predicted vibrational frequencies at 1754 and 1932 cm−1. This solvent interaction is expected to be similar to other reported cases in which an alkane coordinates to a coordinatively unsaturated metal center as a token ligand.31 However, the barrier to decoordination of the solvent is low (ca. 3 kcal/mol; see Figure S3 (Supporting Information)). Potential energy surface (PES) scans along the carbene−chromium−carbonyl C−Cr−C bond angle (Figure 2) show that the S0 PES surface is steeply downhill to A for the unsolvated metal ketene structure. These results explain the relatively fast ∼35 ps time scale for relaxation of the singlet metal ketene. Fernández et al. suggested that a solvated singlet metal ketene is responsible for the photoreactivity of Fischer carbenes and postulated that the singlet forms by a solvent-induced spin crossover of a triplet.11−13 To investigate this possibility, we performed TRIR experiments analogous to those shown in Figure 1 in methanol solution (see the Supporting Information for spectra). A single broad peak was observed at 1760 cm−1, which decayed on a time scale of 31 ± 4 ps. No long-lived absorptions were observed to indicate a spin-crossover reaction, and the dynamics otherwise appeared nearly identical with those in cyclohexane solution. However, peak broadening in the polar methanol solution may have resulted in peak amplitudes below our detection threshold; therefore, the reactivity in coordinating solvents warrants future investigation. To understand the appearance of both singlet and triplet metal ketenes, we performed excited-state DFT calculations, as shown in Figure 2. The calculations show five electronic levels accessible with the energy of a 400 nm photon (71.5 kcal/mol): three triplet states (T1, T2, T3) and two singlet states (S1, S2) obtained from time-dependent density functional theory (TDDFT) calculations. Because spin-allowed absorption cross sections are typically larger than those for spin-forbidden excitations, photoexcitation should promote A to a singlet electronic state (S1 or S2), and the molecule can subsequently relax, branching between T1 (or T2) and S0 states. PES scans on T1 (see Figure 2) reveal a downhill pathway for formation of the two nearly isoenergetic triplet metal ketene structures. The C−Cr−C bond angle changes by ∼43° between structure A (inset 5) and triplet ketene structures, and the distance between the carbene carbon and carbonyl carbon shortens from 2.75 to 1.63 Å. Thus, the ∼5 ps rise time of the low-frequency metal ketene bands most likely corresponds to the time scale for insertion of a CO into the metal−carbene bond. Considering that the peak at 1990 cm−1 shows no rise but is broad at early time delays, this band likely results from a distribution of photoexcited A molecules which may or may not have yet formed the metal ketene structure but already possess terminal CO absorptions in this spectral region. 3.3. Picosecond TRIR Spectroscopy of W(CO)5[CCH3(OCH3)] in Cyclohexane Solution. We also investigated the analogous W Fischer carbene complex W(CO)5[CCH3(OCH3)] using picosecond TRIR spectrosco-

Figure 3. TRIR spectra of W(CO)5[CCH3(OCH3)] following 400 nm excitation in cyclohexane solution: (top) spectra at time delays of 5 ps (green) and 50 ps (blue); (bottom) spectrum at a time delay of 1700 ps.

py. TRIR difference spectra following 400 nm excitation in cyclohexane solution are shown in Figure 3. All spectral features can be assigned to either anti → syn isomerization of the methoxy substituent or to excited-state absorptions that decay in ∼23 ± 1 ps. Similar to the case for the Cr analogue, the anti → syn isomerization of the methoxy substituent has been observed previously; bands observed in Figure 3 at 1938, 1948, 1982 (weak), and 2069 (weak) cm−1 are assigned to the product of the isomerization.8,15,16 Most importantly, we observe no transient absorption signals from 1750 to 1900 cm−1, indicating the absence of any metal ketene structures formed upon photoexcitation of this metal complex. The absence is consistent with the lack of photochemical reactivity typically observed for W Fischer carbenes in synthetic applications.13 It has been suggested that high occupation of the pz atomic orbital of the carbene carbon may inhibit photoinsertion of the CO ligand, either making photocarbonylation unfavorable or presenting a substantial kinetic barrier.13

4. CONCLUSIONS In conclusion, we have monitored the formation of the metal ketene intermediates formed upon visible excitation of a prototypical chromium Fischer carbene complex. Photoexcitation leads to rapid formation of both singlet and triplet metal ketene structures. The singlet metal ketene relaxes to reform the parent complex in tens of picoseconds, whereas the triplet structures are long-lived and are most likely responsible for the photoreactivity of these complexes. Analogous experiments carried out on a prototypical tungsten Fischer carbene do not show evidence for metal ketene intermediates, an observation that is consistent with the lack of photochemical reactivity observed for these species in synthetic applications. These results clarify the fundamental role of triplet metal ketene intermediates in the photoreactivity of Fischer carbene complexes and provide the first direct evidence for the metal ketene intermediate proposed by Hegedus long ago.

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ASSOCIATED CONTENT

S Supporting Information *

Figures, tables, and xyz files giving DFT z matrix coordinates for structures depicted in Figure 2, TRIR spectra of 6152


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(17) Fischer, E. O.; Schubert, U.; Kleine, W.; Fischer, H.; Darst, K. P.; Lukehart, C. M.; Warfielo, L. T.; Darensbourg, D. J.; Burch, R. R.; Froelich, J. A. In Inorganic Syntheses; Shriver, D. F., Ed.; Wiley: New York, 1979; pp 164−172. (18) Aumann, R.; Fischer, E. O. Angew. Chem., Int. Ed. Engl. 1967, 6, 879−880. (19) Lundquist, R. T.; Cais, M. J. Org. Chem. 1962, 27, 1167−1172. (20) Graham, M. A.; Rest, A. J.; Turner, J. J. J. Organomet. Chem. 1970, 24, C54−C56. (21) Hamm, P.; Kaindel, R.; Stenger, J. Opt. Lett. 2000, 25, 1798− 1800. (22) OriginPro, version 8.5; OriginLab, Northampton, MA, 2007. (23) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (24) Perdew, J. P. Phys. Rev. B 1986, 33, 8822−8824. (25) (a) Frisch, M. J. et al. Gaussian03, Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (b) Frisch, M. J. et al. Gaussian09, Revision B.01; Gaussian, Inc.: Wallingford, CT, 2009. . (26) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257−2261. (27) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299−310. (28) Dunning, T. H.; Hay, P. J. Methods of Electronic Structure Theory; Plenum: New York, 1977; Vol. 2. (29) McMahon, S.; Rochford, J.; Halpin, Y.; Manton, J. C.; Harvey, E. C.; Greetham, G. M.; Clark, I. P.; Rooney, A. D.; Long, C.; Pryce, M. T. Phys. Chem. Chem. Phys. 2014, 16, 21230−21333. (30) Fernández, I.; Sierra, M. A.; Mancheño, M. J.; Gómez-Gallego, M.; Cossío, F. P. J. Am. Chem. Soc. 2008, 130, 13892−13899. (31) Dobson, G. R.; Hodges, P. M.; Healy, M. A.; Poliakoff, M.; Turner, J. J.; Firth, S.; Asali, K. J. J. Am. Chem. Soc. 1987, 109, 4218− 4224.

Cr(CO)5[CCH3(OCH3)] in ethanol solution in the CO stretching region, kinetic traces for infrared absorptions, FTIR spectra of the samples measuring the TRIR spectra, and the full version of ref 25. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for C.B.H.: [email protected]. Present Addresses ⊥

Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093, United States. # Ngo Thi Thuan Laboratory of Catalysis, Division of Organic Chemistry, VNU-University of Science, 19 Le Thanh Tong, Hanoi, Vietnam. Author Contributions ∥

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSF grant CHE-1213135. The authors acknowledge use of the Molecular Graphics and Computation Facility at UC-Berkeley (grants CHE-0840505 and CHE-0233882) and the National Energy Research Scientific Computing Center, supported by the Office of Science of the U.S. DOE under Contract No. DE-AC0205CH11231. S.C.N. acknowledges support through a VIED fellowship. J.P.L. acknowledges support through an NSF graduate research fellowship. P.V.P. acknowledges support through a VNU Dept. of Chem. research fellowship.

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REFERENCES

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Direct Observation of Metal Ketenes Formed by Photoexcitation of a

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