Electronic and Molecular Structure of the Transient Radical

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Electronic and Molecular Structure of the Transient Radical Photocatalyst Mn(CO)5 and Its Parent Compound Mn2(CO)10 Hana Cho,†,‡,∥ Kiryong Hong,†,‡,# Matthew L. Strader,‡,⊥ Jae Hyuk Lee,‡ Robert W. Schoenlein,*,‡,∇ Nils Huse,*,§ and Tae Kyu Kim*,† †

Department of Chemistry and Chemistry Institute of Functional Materials, Pusan National University, Busan 46241, Republic of Korea ‡ Ultrafast X-ray Science Laboratory, Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States § Department of Physics, University of Hamburg, Max Planck Institute for the Structure and Dynamics of Matter, and Center for Free-Electron Laser Science, 22761 Hamburg, Germany S Supporting Information *

ABSTRACT: We present a time-resolved X-ray spectroscopic study of the structural and electronic rearrangements of the photocatalyst Mn2(CO)10 upon photocleavage of the metal−metal bond. Our study of the manganese K-edge fine structure reveals details of both the molecular structure and valence charge distribution of the photodissociated radical product. Transient X-ray absorption spectra of the formation of the Mn(CO)5 radical demonstrate surprisingly small structural modifications between the parent molecule and the resulting two identical manganese monomers. Small modifications of the local valence charge distribution are decisive for the catalytic activity of the radical product. The spectral changes reflect altered hybridization of metal-3d, metal-4p, and ligand-2p orbitals, particularly loss of interligand interaction, accompanied by the necessary spin transition due to radical formation. The spectral changes in the manganese pre- and main-edge region are well-reproduced by time-dependent density functional theory and ab initio multiple scattering calculations.

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INTRODUCTION The binuclear metal complex Mn2(CO)10 is one of the simplest room-temperature stable metal carbonyl compounds and has long served as an example for the photochemistry of transition metal carbonyls with metal−metal bonds. This complex is frequently used in controlled photoactivated organic synthesis where specific bonds in the complex are broken depending on the excitation wavelength. Especially, photogenerated Mn(CO)5 species are employed in various reactions under mild conditions as a photocatalyst for initiating alkene dimerization and polymerization as well as photoinduced enzyme reactions.1−5 As the reaction mechanism leading to the cleavage of specific bonds in the binuclear metal complex is of experimental and practical interest, the photolysis of Mn2(CO)10 has been extensively studied, both experimentally6−19 and theoretically20−29 in solid matrices, in the gas phase, and in solution. Results on excited-state kinetics and the subsequent relaxation processes have been reported on femtoto picosecond time scales. However, it is still challenging to experimentally probe molecular and valence electronic structures of intermediates due to the lack of atomically resolved experimental methods; optical probes ranging from the ultraviolet (UV) to the infrared (IR) are neither element © 2016 American Chemical Society

specific nor as informative for electronic structure as core-level transitions. The detailed characterization of atomic and electronic rearrangements in photoinduced species is crucial information for understanding of metal−metal bonds and more general optimization of the functionality of photocatalysts. The UV−visible absorption spectrum of Mn2(CO)10 in 2propanol has one prominent absorption band centered at 336 nm with additional absorption shoulders at 266 and 374 nm as shown in Figure 1. Mn2(CO)10 undergoes two distinct wavelength-dependent dissociation pathways in solution.8,10,13,15,18 The short wavelength absorption band in the UV region has been assigned to a metal-to-ligand chargetransfer (MLCT) state that ultimately induces single carbonyl group loss, producing Mn2(CO)9 in solution. The lower energy band at 374 nm is attributed to the dπ → σ* transition between metal d-orbitals, resulting in enhanced homolytic cleavage of Mn−Mn bonds. Excitation at 400 nm yields predominantly Mn(CO)5 species with little to no carbonyl loss (depending on excitation fluence and duration). The formation dynamics of square pyramidal Mn(CO)5 radicals following Mn−Mn bond Received: January 28, 2016 Published: June 1, 2016 5895

DOI: 10.1021/acs.inorgchem.6b00208 Inorg. Chem. 2016, 55, 5895−5903

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can be directly addressed with atomic resolution and elemental specificity by static and time-resolved X-ray absorption spectroscopy (TRXAS).33,34 The extended X-ray absorption fine structure (EXAFS) at the Mn K-edge can be used to accurately determine metal−ligand and metal−metal bond distances and coordination numbers.35−37 The pre-edge and Xray absorption near-edge structure (XANES), at the Mn Kedge, contain information about the electronic structure, spinstate, and symmetry of metal-sites. Changes of electron density in molecular orbitals (MOs) can result in shifts of the continuum edge onset and intensity modulations in X-ray absorption. Thus, TRXAS offers a unique opportunity to investigate molecular structure and valence electron distributions of the photocatalyst Mn2(CO)10 in solution following dπ → σ* excitation. Herein, we present the corresponding preedge, XANES, and EXAFS spectra and interpret the observed spectral features and their changes upon excitation using (multi)scattering theory and time-dependent density functional theory (TD-DFT) calculations. Our comprehensive characterization of geometrical and electronic information provides insight into the electronic and molecular structure and the photocatalytic activity of Mn2(CO)10 via the Mn(CO)5 radical.

Figure 1. Optical absorption spectrum of Mn2(CO)10 in 2-propanol solution at room temperature and related electronic transition assignments: dπ → σ* (374 nm), σ → σ* (336 nm), σ → π* (303 nm), and dπ → π* (266 nm) in cyclohexane solution.14,17,19 The green arrow indicates the 400 nm excitation wavelength in the present study. All experimental spectra acquired in 2-propanol only, but related electronic transition assignments are reported in cyclohexane solution.14,17,19

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cleavage has been studied by various time-resolved methods. In particular, ultrafast IR spectroscopy, which monitors the course of specific intermediates via the absorption bands of the carbonyl groups, has provided details of electronically excitedstate dynamics such as cooling rates and orientational relaxation of reactive intermediates on multiple time scales (intramolecular vibrational relaxation, ∼1 ps; orientational diffusion, ∼5−10 ps; vibrational energy relaxation to the solvent, ∼100 ps).30,31 However, the change in molecular structure (bond length and angle changes) and the distributions of valence electrons (in terms of valence charge density and orbital interactions) of photogenerated Mn(CO)5 radicals have never been directly observed. Upon cleavage of the Mn−Mn bond, significant changes in charge and spin density at the metal center of the catalytic Mn(CO)5 radical are anticipated. Such information needs to be provided for a comprehensive understanding of the chemistry of this class of molecules, allowing for optimal design of multimetal center photocatalytic materials such as hydrogenase cofactors.32 The valence electron distributions and geometries of the parent compound and reactive intermediate species Mn(CO)5

EXPERIMENTAL AND COMPUTATIONAL METHODS

Experimental Details. The time-resolved laser-pump/X-ray probe experiments have been described in detail elsewhere.38 We have conducted steady-state XAS measurements at the Mn K-edge by total fluorescence yield and TRXAS measurements in transmission mode at the hard X-ray beamline (BL) 6.0.1 of the Advanced Light Source (ALS). A 4-kHz-repetition rate amplified femtosecond Ti:sapphire laser system is coupled with this undulator-based beamline for timeresolved X-ray spectroscopy. The experimental layout is shown in Figure 2. Ultrashort pulses are generated in a laser oscillator (Coherent Vitara) and stretched before being amplified in a home-built regenerative preamplifier and subsequent double-pass power amplifier. The 400 nm excitation pulses (160 μJ) are obtained by frequencydoubling 800 nm pulses in a 0.5 mm thick BBO crystal. Pulses are subsequently stretched in a fused silica prism pair to 0.5 ps (mitigating multiphoton absorption and liquid jet ablation) and focused to 250 × 250 μm2 at the sample. During the study the ALS fill pattern consisted of two so-called camshaft electron bunches (with constant current of 5 mA each) placed in 50 ns gaps within a continuous train of microbunches, allowing for individual detection of the X-ray pulses produced by the camshaft bunches (cf., Figure 2). The femtosecond laser system is

Figure 2. Schematic layout of beamline 6.0.1 at the ALS used for performing laser-pump/X-ray-probe experiments. X-ray pulses from a storage ring are overlapped with laser pulses at the sample position both in time and space. While the ALS ring is filled with electron microbunches spaced by 2 ns, the so-called camshaft bunch is placed in a 50 ns gap. A time-gated detection scheme in transmission mode records the X-ray pulse intensities produced by the camshaft bunch (see text for more details). 5896

DOI: 10.1021/acs.inorgchem.6b00208 Inorg. Chem. 2016, 55, 5895−5903

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2.0 eV. The ORCA program package49 was used for all DFT and TDDFT calculations.

synchronized to the X-ray pulses from the synchrotron by locking the laser oscillator to the 500 MHz radio frequency at which the electron bunches are driven through the synchrotron’s storage ring. A 4 kHz Xray chopper reduces heat and radiation load on beamline optics and the sample and serves as a trigger for the laser amplifiers while a pulse picker transmits every second laser pulse for laser-on−laser-off/X-ray probe measurements. To achieve spatial overlap, 400 nm pump and Xray probe beams pass through a 100 μm diameter pinhole at the sample plane. The angle between both is about 3°. The temporal overlap of Ceq−Oeq). This behavior indicates that the axial metal−ligand bond is strengthened by increased electronic population of the COax π* antibonding orbital through Mn− COax π-back-donation and a decreasing trans-effect of the Mn− Mn bond. Conversely, the axial metal−ligand bond lengthens because of Mn−Mn bond cleavage. Since the second pre-edge transition has a much larger COax contribution compared with the first one (cf., Figures S5 and S6), the pre-edge changes (B → B′) are further amplified. In summary, the differential pump−probe spectrum is dominated in the XANES region by altered 4p-MOs but a more complete description of the absolute spectra in the preedge region shows that additional contributions from Mn 1s → 3d transitions are vital. The interpretation of the second preedge feature on the basis of MO-based pictures leads to the conclusion that the interaction between Mn-3d and CO-2p(π*) orbitals bonded to the other Mn atom has to be considered. We found that the σ and π contributions can be quantified

Figure 7. Experimental (open black and red circles for Mn2(CO)10 and the Mn(CO)5 radical, respectively) and TD-DFT calculated (green solid lines) of both Mn2(CO)10 and the Mn(CO)5 radical in the pre-edge region of the Mn K-edge. The individual transitions are plotted as stick spectra with green circles. Details of the computed transitions are found in Tables S1 and S2.

region of the Mn K-edge (open black and red circles) along with ORCA simulation results (green circles). The computed transitions were convolved with Gaussian of 2.0 eV fwhm (green lines) for both Mn2(CO)10 and the Mn(CO)5 radical. The experimental spectrum of Mn2(CO)10 and the Mn(CO)5 radical exhibits two distinguishable pre-edge features, a lower energy peak (A and A′) at ∼6541 eV and a higher energy peak (B and B′) at ∼6548 eV. The overall absorption structures and energy positions of pre-edge peaks are quite similar for Mn2(CO)10 and the Mn(CO)5 radical. This spectral similarity is a manifestation of similar electronic structures. The ORCA simulations can generate MO pictures for each transition contributing to the pre-edge features to help visualize the orbital interactions as depicted in Figure 8. In both Mn2(CO)10 and the Mn(CO)5 radical, strong metal-to-ligand back-bonding interactions lead to mixing of metal d- and p-orbitals with empty ligand orbitals.21,24 Hence, the observed spectral features in the pre-edge region represent transitions from metal 1s-core electrons into delocalized MOs that are shared between the metal center and the ligands. The intensity of the first pre-edge transition (A and A′) at ∼6541 eV results from transitions into Mn d-orbitals mixed with equatorial (COeq) ligand orbitals in both Mn2(CO)10 and the Mn(CO)5 radical. Axial (COax) ligand contributions are negligible in both cases, i.e., the first pre-edge represents transitions into Mn d-orbitals with COeq 2p-orbital character. Bond cleavage has little to no effect on these transitions. As is 5901

DOI: 10.1021/acs.inorgchem.6b00208 Inorg. Chem. 2016, 55, 5895−5903

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experimentally from the spectral shapes using ORCA predictions (details can be found in the Supporting Information). We further note that our results agree with previous theoretical studies21,24 represented by the MO diagram of Figure 4. Overall, the valence charge distribution remains similar in the parent and product compounds which is also reflected in the small structural changes stabilized by πback-bonding through overlap of Mn-4pz and COeq-2π orbitals. Interestingly, there is no structural similarity between the Mn(CO)5 radical and Fe(CO)5. The latter is a d6-system with trigonal bipyramidal symmetry and rich photochemistry.73,74 We verified that upon hypothetical addition of an electron DFT also predicts trigonal bipyramidal symmetry for the a d6-system [Mn(CO)5]−, while electron abstraction to [Mn(CO)5]+ leaves the symmetry unchanged but reduces the d-electrons to that of Cr(CO)6 which is octahedral in symmetry.

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Center for Inorganic Analysis, Division of Metrology for Quality of Life, Korea Research Institute of Standards and Science, Daejeon 34113, Republic of Korea. ⊥ Stanford Synchrotron Radiation Light Source, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States. # PULSE Institute, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States. ∇ Linac Coherent Light Source and PULSE Institute, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States. Author Contributions

H.C. and K.H. contributed equally.

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Notes

The authors declare no competing financial interest.

CONCLUSIONS Our study is the first experimental work to provide detailed structural and electronic information at the atomic level for Mn2(CO)10 and the photocatalyst Mn(CO)5 in solution. We present XANES and EXAFS spectra in the Mn K-edge region for both Mn2(CO)10 and photogenerated Mn(CO)5 radicals upon 400 nm excitation. We find good agreement between the computational predictions and the experimental XAS spectra, suggesting that combining DFT electronic structure calculations as implemented in the program package ORCA and ab initio multiple scattering calculations using the FEFF program package provides a useful route for distilling a unique combination of information from X-ray absorption spectra. Changes in the XANES and EXAFS spectral regions of the manganese centers are associated with changes of valence charge distribution and slight structural modifications during the photodissociation reaction. In particular, we find an interligand bridging interaction, the loss of which upon photodissociation manifests most clearly in pre-edge changes. This approach can be applied to many types of photoinitiated reactions for which only indirect structural and electronic information can be gained from transient optical spectroscopy. Future work will concentrate on improving this method as well as using complementary metal-2p and ligands-1s core-level transitions as time-resolved probes75−79 to understand chemical reaction pathways, magnetic interactions, and electron delocalization on ultrafast time scales.

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ACKNOWLEDGMENTS

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REFERENCES

This research was supported by the Director, Office of Science, Office of Basic Energy Sciences, the Chemical Sciences, Geosciences, and Biosciences Division under the Department of Energy, Contract No. DE-AC02-05CH11231 (H.C., M.L.S, J.H.L, and R.W.S.) Experiments were conducted at the Advanced Light Source (LBNL), a DOE Office of Science User Facility. This work was supported by National Research Foundation of Korea (NRF) grants, funded by the Korean government (MEST and MSIP) (2013S1A2A2035406, 2013R1A1A2009575, and 2014R1A4A1001690). This work was also supported by Max Planck POSTECH/Korea Research Initiative Program [Grant 2011-0031558] through the National Research Foundation of Korea (NRF) funded by Ministry of Science, ICT & Future Planning. N.H. acknowledges funding from the Max Planck Society and the City of Hamburg.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00208. FEFF-simulated XANES and EXAFS spectra of Mn2(CO)10 and Mn(CO)5 radical, TD-DFT X-ray transition energies, oscillator strengths, molecular orbitals, and pre-edge absorption (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. 5902

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DOI: 10.1021/acs.inorgchem.6b00208 Inorg. Chem. 2016, 55, 5895−5903