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Title: Ultrafast Photophysics of a Dinitrogen-Bridged Molybdenum Complex

Abstract

Among the many metal–dinitrogen complexes synthesized, the end-on bridging (μ2, η1, η1—N2) coordination mode is notoriously unreactive for nitrogen fixation. This is principally due to the large activation energy for ground-state nitrogen–element bond formation and motivates exploration of the photoexcited reactivity of this coordination mode. To provide the foundation for this concept, the photophysics of a dinitrogen-bridged molybdenum complex was explored by ultrafast electronic spectroscopies. The complex absorbs light from the UV to near-IR, and the transitions are predominantly of metal-to-ligand charge transfer (MLCT) character. Five excitation wavelengths (440, 520, 610, 730, and 1150 nm) were employed to access MLCT bands, and the dynamics were probed between 430 and 1600 nm. Despite the large energy space occupied by electronic states (ca. 1.2 eV), the dynamics were independent of the excitation wavelength. In the proposed kinetic model, photoexcitation from a Mo–N=N–Mo centered ground state populates the π*-state delocalized over two terpyridine ligands. Due to a large terpyridine–terpyridine spatial separation, electronic localization occurs within 100 fs, augmented by symmetry breaking. The subsequent interplay of internal conversion and intersystem crossing (ISC) populates the lowest 3MLCT state in 2–3 ps. Decay to the ground state occurs either directly or via a thermally activated metal-centeredmore » (3MC) trap state having two time constants (10–15 ps, 23–26 ps [298 K]; 103 ps, 612 ps [77 K]). ISC between 1MLCT and 3MLCT involves migration of energized electron density from the terpyridine π* orbitals to the Mo–N=N–Mo core. As a result, implication of the observed dynamics for the potential N–H bond forming reactivity are discussed.« less

Authors:
ORCiD logo [1];  [1]; ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [1]
  1. Princeton Univ., Princeton, NJ (United States)
Publication Date:
Research Org.:
Princeton Univ., NJ (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1485273
Grant/Contract Number:  
SC0006498
Resource Type:
Accepted Manuscript
Journal Name:
Journal of the American Chemical Society
Additional Journal Information:
Journal Volume: 140; Journal Issue: 20; Journal ID: ISSN 0002-7863
Publisher:
American Chemical Society (ACS)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY

Citation Formats

Rafiq, Shahnawaz, Bezdek, Máté J., Koch, Marius, Chirik, Paul J., and Scholes, Gregory D. Ultrafast Photophysics of a Dinitrogen-Bridged Molybdenum Complex. United States: N. p., 2018. Web. doi:10.1021/jacs.8b00890.
Rafiq, Shahnawaz, Bezdek, Máté J., Koch, Marius, Chirik, Paul J., & Scholes, Gregory D. Ultrafast Photophysics of a Dinitrogen-Bridged Molybdenum Complex. United States. doi:10.1021/jacs.8b00890.
Rafiq, Shahnawaz, Bezdek, Máté J., Koch, Marius, Chirik, Paul J., and Scholes, Gregory D. Wed . "Ultrafast Photophysics of a Dinitrogen-Bridged Molybdenum Complex". United States. doi:10.1021/jacs.8b00890. https://www.osti.gov/servlets/purl/1485273.
@article{osti_1485273,
title = {Ultrafast Photophysics of a Dinitrogen-Bridged Molybdenum Complex},
author = {Rafiq, Shahnawaz and Bezdek, Máté J. and Koch, Marius and Chirik, Paul J. and Scholes, Gregory D.},
abstractNote = {Among the many metal–dinitrogen complexes synthesized, the end-on bridging (μ2, η1, η1—N2) coordination mode is notoriously unreactive for nitrogen fixation. This is principally due to the large activation energy for ground-state nitrogen–element bond formation and motivates exploration of the photoexcited reactivity of this coordination mode. To provide the foundation for this concept, the photophysics of a dinitrogen-bridged molybdenum complex was explored by ultrafast electronic spectroscopies. The complex absorbs light from the UV to near-IR, and the transitions are predominantly of metal-to-ligand charge transfer (MLCT) character. Five excitation wavelengths (440, 520, 610, 730, and 1150 nm) were employed to access MLCT bands, and the dynamics were probed between 430 and 1600 nm. Despite the large energy space occupied by electronic states (ca. 1.2 eV), the dynamics were independent of the excitation wavelength. In the proposed kinetic model, photoexcitation from a Mo–N=N–Mo centered ground state populates the π*-state delocalized over two terpyridine ligands. Due to a large terpyridine–terpyridine spatial separation, electronic localization occurs within 100 fs, augmented by symmetry breaking. The subsequent interplay of internal conversion and intersystem crossing (ISC) populates the lowest 3MLCT state in 2–3 ps. Decay to the ground state occurs either directly or via a thermally activated metal-centered (3MC) trap state having two time constants (10–15 ps, 23–26 ps [298 K]; 103 ps, 612 ps [77 K]). ISC between 1MLCT and 3MLCT involves migration of energized electron density from the terpyridine π* orbitals to the Mo–N=N–Mo core. As a result, implication of the observed dynamics for the potential N–H bond forming reactivity are discussed.},
doi = {10.1021/jacs.8b00890},
journal = {Journal of the American Chemical Society},
number = 20,
volume = 140,
place = {United States},
year = {2018},
month = {5}
}

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