Ultrafast Photophysics of a Dinitrogen-Bridged Molybdenum Complex

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Ultrafast Photophysics of a Dinitrogen Bridged Molybdenum Complex Shahnawaz Rafiq, Máté J. Bezdek, Marius Koch, Paul J. Chirik, and Gregory D. Scholes J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b00890 • Publication Date (Web): 02 May 2018 Downloaded from http://pubs.acs.org on May 2, 2018

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Shahnawaz Rafiq, Máté J. Bezdek, Marius Koch, Paul J. Chirik*, Gregory D. Scholes* Frick Chemistry Laboratory, Princeton University, NJ 08544 Molybdenum dinitrogen complex, MLCT, excited state dynamics, electronic localization, two-dimensional electronic spectroscopy, internal conversion, intersystem crossing, transition metal photophysics ABSTRACT: Among the many metal-dinitrogen complexes synthesized, the end-on bridging (M-NN-M) coordination mode is notoriously unreactive for nitrogen fixation. This is principally due to the large activation energy for ground-state nitrogenelement 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-toligand 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 430–1600 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 *-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 * orbitals to the Mo-N=N-Mo core. Implication of the observed dynamics to the potential N-H bond forming reactivity are discussed.

INTRODUCTION The synthesis of ammonia from its elements, N2 and H2 has been a long-standing challenge in chemical synthesis.1 Bacterial nitrogen fixation and the industrial hydrogenation of N2 using the Haber-Bosch process, both catalyzed by transition metals, are responsible for the vast majority of fixed nitrogen on Earth.1-4 The latter is one of the most important technological innovations of the 20th century having transformed global food production and altered the natural nitrogen cycle.5 Since the discovery of the first transition metal dinitrogen complex by Allen and Senoff,6 there has been continued interest in developing well-defined molecular catalysts for ammonia synthesis.7-14 Motivations for these studies include gaining an understanding of reaction intermediates and pathways in both the biological and heterogeneously catalyzed processes and ultimately, developing new catalysts for batch ammonia synthesis from renewable rather than fossil-derived H2.15 When acting as a ligand for transition metals, dinitrogen adopts a range of coordination modes with various degrees of activation (Fig. 1).16 Among these, side-on coordination (2, 2, 2) with early transition metals offers some of the richest stoichiometric functionalization chemistry including hydrogenation,17-18 silylation,17, 19 various other modes of hydrofunctionalization and N-C bond formation20-23 including CO-induced N2 cleavage.24 By contrast, end-on bridging N2 (2, 1, 1) is historically far more common, yet typically involves weak activation, inert character with respect to functionalization, and tendency to dissociate.16 Dinitrogen complexes of molybdenum are of particular

Figure 1. The coordination modes of N2 in transition metal complexes, highlighting the strategy reported in this work.

interest given the observation of N2 coordination over multiple oxidation states, presence in the most ubiquitous class of nitrogenase enzymes,25 demonstrated ability to bind and cleave N2,26-27 and recent examples of pre-catalysts that promote NH3 synthesis by addition of protons and electrons,7-8 albeit with high chemical overpotential (Fig. 1).15, 28 A bimetallic molybdenum complex bridged by an activated dinitrogen (N2) ligand and supported by a mixed terpyridine and phosphine ligand environment,

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[{(PhTpy)(PPh2Me)2Mo}2(2-N2)][BArF24]2 ([1-N2]2+; PhTpy = 4-Ph-2,2,6,2-terpyridine) was synthesized by the Chirik laboratory (Fig. 2a).29 The design motivation was to explore the cooperative metal ligand redox events enabled by the redox-active tridentate pincer, with the goal of lowering the activation barrier associated with hydrogenolysis of important, potentially catalytically relevant intermediates (e.g. M-NH2).30-31 Each molybdenum atom in the dimer is best described as Mo(II) bridged by a modestly activated [N2]2- ligand. A rich redox chemistry was observed and dinitrogen complexes across five oxidation states were identified. Despite the presence of a modestly activated N2 ligand, the exposure of the [1-N2]2+ to strong acid and reducing agents produced no detectable quantity of ammonia consistent with the resistance of 2, 1, 1-N2 ligands to functionalization.16 The lack of ground state reactivity inspired the study of alternative modes for promoting N2 functionalization chemistry in [1-N2]2+. Because terpyridine ligands are wellknown chromophores,32 enabling new reactivity from visible-light accessed photoexcited states is attractive. Herein, ultrafast electronic spectroscopies and theoretical calculations were implemented to understand the photoexcited behavior of the dinitrogen bridged [1–N2]2+ complex. The character of the electronic transitions was established, and a mechanism for photophysical relaxation of the excited state is proposed. Notably, in the excited state, the electron density is transferred from the terpyridine ligands to the dimolybdenum dinitrogen bridge within 2-3 ps and remains localized in the Mo-NN-Mo core until the ground state fully recovers. The localization of electron density on

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the dinitrogen in the triplet state may be a key for N2-based photo-reactivity. A short lifetime of the photoexcited complex was identified and methods for increasing it are discussed. RESULTS AND DISCUSSION Localized or Delocalized Ground State. The synthesis of [1–N2]2+ by halide abstraction from the corresponding molybdenum chloride, 1-Cl in the presence of N2 gas was previously reported (Fig. 2a).29 The absorption spectrum of [1–N2]2+ reveals that this metal complex uniquely absorbs a broad spectral range of light from UV to the near IR region. In Et2O, [1–N2]2+ exhibits three prominent electronic transitions with maxima at 517, 733 and 1094 nm (Fig. 2b) with extinction coefficients of 31508, 11392, 18373 M-1cm-1, respectively. The metal chloride precursor, 1-Cl has three absorption bands peaking at 475, 612, and 882 nm with extinction coefficients of 13780, 10915, 1978 M-1cm-1 respectively. 1-Cl exhibits a low-spin Mo(I) configuration and the singly occupied molecular orbital (SOMO) is molybdenumrather than ligand-based.29 In [1-N2]2+, each molybdenum atom is best described as low spin closed-shell octahedral Mo(II) and the DFT computed highest occupied molecular orbitals are principally molybdenum-based.29 To determine the character of the observed absorption bands of [1-N2]2+, natural transition orbital (NTO) analysis was performed on the three most prominent allowed singlet transitions.33 It establishes ground states with electron density predominantly localized on the four atom Mo-N-NMo core. However, the excited states have predominantly

Figure 2. (a) Synthesis of the dinitrogen bridged bimetallic Molybdenum complex [1–N2]2+ from 1-Cl. (b) Experimental absorption spectra of the 1-Cl (solid red) and [1–N2]2+ complex (violet shaded curve). (c) Theoretically computed energies of frontier molecular orbitals as a function of separation between the two monomeric units presented as logarithmic x-axis. The encircled point corresponds to optimum N-N bond length. Inset shows the theoretically computed absorption spectra of [1–N2]2+ (3Å N-N bond length) and 1-Cl.


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terpyridine * character (Fig. S1). These findings support that the electronic transitions have significant metal-to-ligand charge transfer (MLCT) character. Similarly, NTO analysis of 1-Cl also showed MLCT character of the absorption bands. The structural difference between dimeric [1–N2]2+ and monomeric 1-Cl raises the question of the role of bridging dinitrogen ligand in the electronic structure and the extent of delocalization of electron density in the ground- and excited state. As per a strictly qualitative approach,29 the central core Mo-N-N-Mo molecular orbitals of  symmetry can be constructed by the linear combination of Mo dxz, Mo dyz, Mo dxy, N px, N py orbitals and energetically ordered based on the number of nodes.29, 34-36 DFT computed molecular orbitals of the [1–N2]2+ validate this classical view. They demonstrate that the highest occupied orbitals consist of molybdenum centers electronically coupled through dinitrogen orbitals (Fig. S1). This interaction, facilitated by the N2 ligand, was further demonstrated by predicting the effect of increasing N=N bond length on the energetics of the HOMO and LUMO (Fig. 2c). The energies of these frontier orbitals vary significantly as a function of increasing N=N bond length. In the absence of electronic coupling at long bond lengths, the computed absorption spectrum of [1– N2]2+ resembles that of 1-Cl (Fig. 2c inset). This resemblance is further established by the monomer-like MLCT transition for elongated [1–N2]2+, in comparison to the delocalized transition in the equilibrium-structure [1– N2]2+ (Fig. S2). Localized/delocalized transition have specifically been elaborated in the context of intervalence type charge transfer metal complexes.37-39 Furthermore, from the experimental absorption spectra, the lowest electronic transition at 1100 nm is significantly lower in energy in the [1–N2]2+ complex than in 1-Cl where it appears at 882 nm (Fig. 1b). This observation suggests that dinitrogen promotes strong coupling, which results in delocalized electronic states of primarily metal-to-ligand charge transfer character and, which lowers the energy of the electronically excited states. In other words, the electronic excited states of the [1–N2]2+ have excitonic character with the electronically excited singlet state wavefunction delocalized over the two terpyridine ligands. ULTRAFAST TRANSIENT ABSORPTION SPECTROSCOPY

Narrowband transient absorption spectroscopy,40 a technique applied to probe excited state processes, was used to selectively promote population into different MLCT states in [1–N2]2+. For this purpose, five different wavelengths of 440, 520, 610, 730 and 1150 nm were used; the observed transient dynamics following excitation by 520 and 1150 nm pump pulses is described in detail here. Population Dynamics of the Highest-lying 1MLCT State. Following excitation at 520 nm, excited state absorption (ESA) bands appear instantly at 430, 660, 800, and 1600 nm (Fig. 3a–b). The ESA bands at 430 and 660 nm decay within ca. 100 fs, being replaced by strong ground state bleach (GSB) signals peaking at 517, 610 and 730 nm. The ESA band spanning from ca. 750–1000 nm continues to grow until ca. 8 ps and then decays until the GSB is fully recovered. This ESA signal overlaps strongly with the higher

wavelength GSB peaking at 1100 nm. The additional overlap between bleach and ESA is observed towards the 1600 nm region. At later delay times, in the 610 nm region, the GSB signal is overwhelmed by a new ESA band appearing after 12 ps, which continues to remain until the GSB fully recovers within 150 ps. Global analysis of transient absorption spectra, recorded in both the visible and the near-IR region, was performed in GLOTARAN based on the unbranched and unidirectional kinetic model.41 In this model, back-reactions are ignored on the assumption that energy losses are large enough that the back-reactions are negligible, which seems highly applicable to the complex under study.42 The data required a minimum of four time-constants for adequate reproduction of the experimental decays. Evolution associated spectra of the four time-constants are shown in figure 3c–d and the corresponding decay associated spectra are shown in figure S3. These four time-constants are 7510 fs, 2.60.22 ps, 110.7 ps, 231.6 ps for the visible region and 15515 fs, 2.2 0.23 ps, 110.6 ps, 231.8 ps for the near-IR region. To authenticate the shortest time-constant, transient absorption dynamics were also measured with a 12-fs pump laser pulse in the visible region. Global analysis of the short-pulse transient absorption data reproduced all the 4 time-constants that were obtained with the narrowband transient absorption measurements. Population Dynamics of the Lowest-lying 1MLCT State. After excitation at 1150 nm (populating the lowestlying 1MLCT state), the same transient spectral features were observed as those that were obtained at 520 nm excitation (Fig. 4a-b). The instant appearance of the ESA signal in the 430 and 660 nm region and its decay within 100 fs are similar for both excitation conditions. Likewise, all other transient signals behave identically as can be seen by comparing figure 4a-b with 3a-b. Global analysis (Fig. 4c–d) required again a minimum of four time-components to adequately fit the experimental data. The four time-components are 7510 fs, 3.80.31 ps, 16.00.8 ps, 261.5 ps in the visible probe region and 15020 fs, 3.10.28 ps, 14.80.7 ps, 261.4 ps in the near-IR probe region. Representative kinetic traces obtained at 520 and 1150 nm excitation wavelengths are shown in figure 5 and the fitted time-constants are summarized in table S1. The average lifetime of the fitted kinetic traces is found to be slightly larger for 1150 nm excitation relative to excitation at 520 nm. However, this small difference does not seem to originate from an intermediate state as it is reflected in all time constants for 520 and 1150 nm excitations. We note that two-photon absorption effects may arise due to the relative disposition of energy states at 1100 and 520 nm transition energies. In order to exclude this possibility, we performed transient absorption measurements with 10 nano-Joules (nJ), 50 nJ and 300 nJ pulse energies of 1150 nm pump wavelength. The dynamics were found to be independent of the pulse energies, thus ruling out two-photo absorption effects (Figure S4). The transient absorption measurements obtained with other selective excitation wavelengths: 440, 610, and 630 nm did not show any noticeable differences and the dynamics was similar to the above-discussed cases. This

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Figure 3. Transient absorption maps of [1–N2]2+ obtained upon excitation at 520 nm and probing in the (a) visible and (b) near-infrared spectral region. The data is shown as filled contours with log scale of the time axis. Negative and positive signals represent ground state bleach (GSB) and excited state absorption (ESA) resp. (c) and (d) are the evolution associated difference spectra (EADS) in the visible and nearinfrared regions obtained by global analysis of the TA data. DAS are shown in figure S3 of SI.

Figure 4. Transient absorption maps in the (a) visible and (b) near-infrared probing region obtained upon excitation at 1150 nm. (c) and (d) are the evolution associated difference spectra (EADS) obtained from global analysis of the respective spectral regions. For DAS, see fig. S3 of SI.

excitation-wavelength independence is distinctly notable, especially when the energy separation between the lowest and the highest MLCT state is of the order of ca. 10,000 cm 1 (ca. 1.2 eV).

The possible reason for such similar excited state relaxation kinetics, even when different electronic states are accessed, is credited to the MLCT character of the electronically excited states. Their direct excitation results in the instantaneous transfer of electron density from the metal


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Figure 5. Comparison of the kinetic traces at three probe wavelengths; 520 (a), 820 (b), and 1100 (c) nm obtained for 520 and 1150 nm excitation. The kinetic traces are plotted on a logarithmic scale. Table S1 in the supporting information summarizes all fitting parameters of traces at these and other additional wavelengths.

towards the terpyridine ligand centers.43 Consequently, the dynamics are defined predominantly by electronic relaxation at the ligand center. The transient absorption data are hence a signature of the dynamics of the transient electron density on the ligand centers and hence shows insensitivity towards different excitation wavelengths. The second aspect is about the photophysical mechanisms controlling the relaxation of [1–N2]2+ from excited to the ground state and will be dealt with in the discussion to follow. Monomer Dynamics. A control transient absorption experiment was performed on the 1-Cl metal complex by exciting at 400 nm and probing across the 340-760 nm region (Fig. S5). The transient signals decay within ca. 50 ps and are described by two global time-constants, i.e. 2.3 ps and 13 ps. There are two points worth noting here. First, only two time-constants are required for the global fitting of the 1-Cl data instead of four in the [1–N2]2+ complex, and second, the average excited state lifetime of the monomer and the bimetallic complex are similar. The latter observation suggests that complexation of the two mononuclear metal complexes by a dinitrogen bridge does not affect the relaxation dynamics significantly. TWO-DIMENSIONAL ELECTRONIC SPECTROSCOPY Two-dimensional electronic spectroscopy (2DES) provides additional insights by spreading the transient absorption data along the excitation (1) and detection (3) axes, wherein each excitation wavelength is correlated to the detection wavelength.44-47 These 2D absorptive population maps are presented here on a frequency scale (THz) rather than in the wavelength scale. In addition to population dynamic, 2DES contains an abundance of information related to the oscillatory coherent dynamics, which will not be discussed here. Pulse spectrum 1 shown in figure 6a was used to cover simultaneously the electronic transitions at 580 THz (517 nm) and 490 THz (610 nm), while pulse spectrum 2 covers 490 THz (610 nm) and 410 THz (730 nm) electronic transitions (Fig. 6a). The dynamics was measured up to delay times, 𝑡2𝑚𝑎𝑥 of 440 fs. Maps obtained with the two different pulse spectra focusing on different sets of transitions contain two peaks along the diagonal corresponding to the

ground state bleach (GSB) and stimulated emission (SE) signals of the respective transitions. 2D Populations Maps. With pulse spectrum 1, the diagonal peaks appear at 1 = 3 = 570 THz and at 1 = 3 = 490 THz (Fig. 6b–d). In simpler terms, the diagonal peak positions correspond to the peaks in the linear absorption spectrum. The diagonal peak at 490 THz is weak due to its smaller oscillator strength, as seen in the absorption spectrum. In addition to the two diagonal peaks, 2D maps also show a pair of cross peaks on the antidiagonal axis for correlated signals, similar to 2D-NMR. Here, the two electronic states share a common ground state and consequently, the upper cross peak is a manifestation of GSB and vanishes upon completion of the photo-cycle. This is illustrated by the double-sided Feynman diagrams in figure S4.48-50 The lower cross peak is generated when the laser pulses photoexcite higher energy electronic state and probe at the energy of the lower electronic state, as shown in the doublesided Feynman representation (Fig. S6).48-51 The amplitude of the lower cross peak grows in with time owing to population relaxation from a higher state to the lower, as shown in figure 6e. The oscillatory nature of the trace results from coherent wavepacket modulations (not discussed here). Exponential fitting of the amplitude of lower cross peak as a function of waiting time up to 440 fs gives a time-constant of 140±10 fs. From the amplitude of the lower cross peak, it appears that the relaxation is not completed within the 440 fs t2 window, so the actual time constant can be longer than this. For the set of 490 THz (610 nm) and 410 THz (730 nm) electronic transitions with pulse 2, a similar pattern of two diagonal and two antidiagonal cross peaks is observed (Fig. S7a–c). At short delay times, the overlap of ESA and GSB signals in this spectral region results in approximately zero net signal, making it difficult to phase the 2D data up to ca. 150 fs. Nevertheless, the overall examination of the amplitude of the lower cross peak clearly suggests a transfer of population from the high energy state (490 THz) to the lower state (410 THz). The fit from the cross-peak results in a 150±10 fs rise time (Fig. S5d). This time-constant is similar to the population relaxation time between the 580 THz and 490 THz states.



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Figure 6. 2DES of the [1–N2]2+ complex measured using two different pulse spectra as shown in (a). 2DES are reported in THz energy units which makes it easy to follow the correlation of the electronic states. (b-d) represent the 2D absorptive maps at 100, 220, and 440 fs waiting times obtained with pulse 1. (e) Lower crosspeak extracted from the 2D maps (red square) and plotted as a function of delay time, t2.

RELAXATION MECHANISMS Internal Conversion or Intersystem Crossing. The population relaxation from the higher to the lower electronic states can either occur in the singlet manifold via rapid internal conversions (IC) or in the triplet manifold accessed via intersystem crossing (ISC).52-56 If the dynamics were to follow the singlet manifold route, then rapid IC from higher 1MLCT states will bring the population down to the lowest electronic state. Thereby the transient ESA signal observed in the TA measurements must originate from the lowest 1MLCT state. In such a hypothetical scenario, excitation into higher-lying electronic states by 440/520/610/730 nm laser pulses should result in a growth of the ESA signal, whose timescale must correspond to the decay rate of the higher-lying states. Once the excitation wavelength is switched to 1150 nm, which populates the lowest electronic state, this growth of the ESA signal should be absent, because 1150 nm pump directly accesses the lower 1MLCT state. However, the TA experiments show a consistent growth of the ESA signal around 800 and 1600 nm region irrespective of the lower or higher excitation wavelength (Fig. 5b). Also, the number of time-components required to globally fit the data obtained with the higher excitation wavelength should be fewer than what is needed for fitting the data obtained with lower excitation wavelengths, which is not observed. These observations indicate

that the transient spectroscopic signals do not originate from the 1MLCT state. However, that does not mean internal conversion between 1MLCT states is not at all operational during the relaxation. Lower cross-peak growth in the 2DES provides evidence that a small percentage of population relaxes via IC between the 1MLCT states. However, it only brings that small population down to the lowest 1MLCT state. The predominant relaxation mechanism is ISC which channels the population to the lowest 3MLCT electronic state. The spectroscopic signature is the persistent57 growth of ESA signal around 800 and 1600 nm (irrespective of the pump wavelength) in the TA data. This ESA signal is hence proposed to originate from the lowest 3MLCT state. In general, ISC is, in fact, a predominant relaxation mechanism in most of the transition metal complexes used for processes like photosensitization, spin crossover, photocatalysis, etc.58-61 The second time-component of 2-3 ps obtained in the global analysis of the [1–N2]2+ complex, matching with the first global time-constant of the monomer (2.3 ps), is assigned to this interplay of IC and ISC processes, which concentrates the population in the lowest triplet 3MLCT state (Scheme 1). The independence of this time-constant on the excitation wavelength predicates the occurrence of multiple ISC events between the accessed 1MLCT state and the near-degenerate 3MLCT states (Scheme 1). Otherwise, the


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presence of a single triplet state should manifest in the form of different timescales of ISC, depending on which electronic state is photoexcited. However, this is not observed. Additionally, any significant contribution of vibrational relaxation or cooling can be ruled out based on the independence of global time-constants on the excitation wavelength as well as the lack of time-dependent peak shift and spectral narrowing. Natural Transition Orbital Analysis of [1–N2]2+. As mentioned earlier, the electronic transitions in [1–N2]2+ show a predominant MLCT character. The electron density in the singlet excited states is distributed toward the * terpyridine ligands and along the two Mo–N bonds (Fig. S1). These singlet excited states are anti-bonding with respect to dinitrogen and the ground state has a bonding character along the dinitrogen bond. While the singlet states have a strong electron density towards the terpyridine ligands, the NTOs of the triplet states calculated at the TDDFT level does not show any electron density over the terpyridine ligands (Fig. S8). The electron density in the lowest triplet states is predominantly delocalized over the two metal centers and the dinitrogen bridge (Fig. S8). This suggests that the ISC, besides changing the spin, also shifts electron density from the terpyridine ligands toward the center of the bimetallic complex. Hence ligand character of the 1MLCT states comes from terpyridine, whereas it originates from dinitrogen for 3MLCT. Electronic localization. Due to the strong electronic coupling in the bimetallic complex established above, electronic excitation likely occurs from a delocalized ground state into an excitonic state, which is distributed over the terpyridine ligands. However, the terpyridine moieties are spatially well separated from each other (9.3 Å, DFT computed value), minimizing the extent of coupling in the electronic excited state. By contrast, strong excitonic interactions are observed at much shorter distance (< 5 Å) representative of strong coupling in well-known light harvesting systems.62 Therefore, electronic excitation is expected to immediately induce less coupling in the excited state and hence the delocalized excitonic states tend to localize the electron density on either of the terpyridine moiety (Fig. S9). This electronic localization in the excited state occurs in the 1MLCT state. We propose that the first global time-constant of ca. 100 fs corresponds to this process of electronic localization in the excited state. This localization is likely to occur in all 1MLCT states and hence the 100 fs time-constant is observed for all excitation wavelengths. Logically, this component is missing in the monomer. Due to this electronic localization towards a terpyridine moiety in the bimetallic complex, the dynamics will be determined by the transient electron density on terpyridine moiety in both cases. This reasoning explains why the IC/ISC time-constant (2-3 ps) is the same for the monomer and the bimetallic complex. Furthermore, the charge localization on one of two identical ligands is a manifestation of symmetry breaking. This is usually accompanied by minor structural changes in the molecular structure, introduced either by bath or intramolecular fluctuations.63-64 Using TDDFT, two electronically excited states of [1–N2]2+ were optimized. The calculations

Scheme 1. Schematic Illustration of the excited state relaxation dynamics of the dinuclear complex at room temperature interpreted as Jablonski diagram.

predict distortion of the two monomeric planes along the four atom Mo–N=N–Mo core (Fig. S10). In one state, the two planes are distorted by additional 15 torsional angle with respect to the ground state geometry, while in the other, the distortion is ca. 8 (Fig. S10). This structural relaxation could enhance the localization of the electron density on the ligand centers.54 One must bear in mind that this distortion around the dinitrogen bridge predicted by adiabatic calculations is structural relaxation, however, the symmetry lowering may be indicative of Jahn-Teller distortion often found in copper(I) MLCT complexes.43, 58, 65-68 Electron-Hole Recombination. In [1–N2]2+, so far, we have hypothesized that sequential events of electronic localization in the 1MLCT take place in ca. 100 fs and ISC populates the lowest triplet 3MLCT state in 2-3 ps. In certain metal complexes, the electron in the 3MLCT state recombines with the hole on the ground state radiatively, leading to photoluminescence.69-71 In such cases, the 3MLCT state has a long lifetime. In cases where no photoluminescence is observed, a rapid non-radiative decay of the 3MLCT population is operational. This has previously been ascribed to the presence of two parallel non-radiative pathways in related bimetallic ruthenium terpyridine complexes and others.7276 Pathway (i) corresponds to the direct ISC between the 3MLCT and the ground state, wherein the smaller energy gap “energy gap law” and a large density of states accessible to the 3MLCT state (Fermi-Golden rule) causes rapid decay. Energy gap law usually dominates in cases where energy gap between electronic states is small and the density of vibronic levels corresponding to the ground state is sufficiently large to act as a dissipative quasi-continuum.77-79 Pathway (ii) involves decay of the 3MLCT state into the ground state via a high-energy empty metal-centered (3MC) state, also termed as ligand-field excited states. These metal



centered states significantly control the nonadiabatic dynamics of the charge transfer states and are strongly coupled to the ground state, hence act as electron traps.80 Short excited state lifetime and availability of empty d-orbitals due to strong field terpyridine ligands suggest the presence of metal-centered states. NTO analysis, discussed above, also shows that the lowest 3MLCT states (Fig. S8) are primarily localized on the Mo-N=N-Mo moiety. Thereby the transition from the 3MLCT state into the metal-centered (3MC) state, should not involve any significant spatial redistribution of electron density, which eases the recombination via 3MC states. Therefore, we propose that electronhole recombination in the mono- and bimetallic complexes occurs via both pathways. In the bimetallic complex, however, due to the delocalization of the 3MLCT state, its energy will be pushed below the 3MC state as shown in scheme 1.72 This will cause heterogeneity in the recombination rate. The direct decay of the 3MLCT state into the ground state occurs in ca. 10-15 ps and the thermally activated decay via the intermediate 3MC state occurs in ca. 23-26 ps. The slower rate of recombination via the 3MC state is due to thermal activation, needed to populate the 3MC state from the lower-energy 3MLCT state.72-74 In the mononuclear complex, the possibility of the near-degenerate energy of 3MLCT and 3MC states may result in a faster decay rate, which appears as a single time-constant of ca. 13 ps. Because the nonradiative decay via MC trap states is proposed to have an energy barrier. The decrease in temperature should thus result in increase in the lifetime of the excited state through lesser nonradiative contributions. TA dynamics was measured by freezing a 2-methyl-THF solution of the metal complex to 77 K. The dynamics of the complex at 77 K slowed drastically as compared to the room temperature (Figure S11). The four global time-constants increase from 100 fs, 2 ps, 10 ps, 25 ps at room temperature to 15010 fs, 6.20.2 ps, 1035 ps and 61210 ps at 77 K, respectively. This increase in lifetime is an indication of less energy available to the electron to be trapped in the MC states. Thereby confirming the presence of activated MC states as intermediate states in the excited state relaxation. The differential increase in the lifetime (10 times for 3 rd time-constant and ca. 25 times for 4th time-constant) with decrease in temperature from 298 K to 77 K is a further indication of the involvement of two non-radiative pathways operational in the relaxation of lowest 3MLCT state. Photo-induced Chemical Reactivity Prospects. The above discussion reveals that photo-excitation of [1–N2]2+ into 1MLCT state predominantly populates the * orbitals of terpyridine ligands. Following electronic localization in 100 fs, 1MLCT state undergoes ISC to 3MLCT state in 2–3 ps. This state-crossing results in the transfer of electron density from the terpyridine * orbitals to four atom Mo-N=N-Mo core (Fig. S12). The transient electron density on the N2 bridge makes dinitrogen potentially more basic and nucleophilic, a favorable feature for N-H bond formation by protonation.14 In one view, the terpyridine ligands act as strong light harvesters and the crossing between 1MLCT and 3MLCT states funnels the energized electron density towards the four-atom core, which is the primary site for

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productive N2 fixation. These cascadic events of light harvesting and subsequent migration of electron density in this bimetallic complex are analogous to natural photosynthetic systems, where antenna chlorophylls absorb sun’s energy that is funneled to the reaction center for charge separation.62 These features of this bridged bimetallic complex suggest an attractive design for exploring photoinduced states towards chemical reactivity. Usually, the triplet states in transition metal complexes decay on a much slower timescale with lifetimes ranging from nanoseconds to microseconds.69-70 The longer lifetime is key to the use of transition metal-based complexes as photosensitizers and spin-crossover precursors.69-70, 80-82 The molybdenum dinitrogen complex studied here, contrarily decays at a much more rapid rate, with an average excited state lifetime of ca. 40 ps. It presents a limitation for harnessing bimolecular reactivity in the photoinduced states. Design modification in the ligand architecture could prove pivotal in increasing the lifetime of the 3MLCT state for practical applications. The modifications must alter the relative energetics of the 3MLCT and 3MC states in a way that the metal-centered states are not accessible. For example, a gain of two orders of magnitude in lifetime was obtained using [Ru(tpy)(CN)3]- instead of the corresponding [Ru(tpy)2]2+ complex, wherein one terpyridine ligand was replaced by cyano ligands.73 An alternative route of increasing the lifetime of the 3MLCT state is by attaching electron accepting groups on the 4’ position of the terpyridine, which have previously been suggested to significantly increase the excited state lifetime. For example, the photoluminescence lifetime of [Ru(tpy-MeSO2)(tpy)]2+ at room temperature is 36 ns compared to the 250 ps relaxation of [Ru(tpy)2]2+.83-84 The second route is more noninvasive for increasing the lifetime of the bimetallic complex in the electronically excited state. While making modification at the terpyridine ligand centers, an additional challenge will be to achieve this without perturbing the localization of electron density on the four-atom Mo-N=N-Mo core, likely key for N2 functionalization. CONCLUSION Excited-state dynamics of a bimetallic molybdenum complex bridged by an activated N2 ligand and supported by mixed terpyridine and phosphine ligands, [1–N2]2+, was explored by ultrafast transient absorption and two-dimensional electronic spectroscopy. The current work lays the foundation for exploring the possibilities for functionalization of dinitrogen in the electronically excited state to facilitate the hydrogenation of the dinitrogen bridge, which is the first step in the catalytic synthesis of ammonia. Excited electronic states are found to have dominantly MLCT character, as indicated by natural transition orbital analysis of the electronic transitions. Transient dynamics of these MLCT states are elucidated on ultrafast timescales using five different pump wavelengths (440, 520, 610, 730, and 1150 nm) and probed from the visible to the near-IR regime (430–1600 nm). The excited state dynamics are found to be independent of excitation wavelength even


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though the four prominent electronic states are spread over an energy range of ca. 10,000 cm-1 (1.2 eV). Global analysis uses four time-constants to describe the dynamics in the bimetallic complex (75–160 fs, 2–3 ps, 10–15 ps, 23–26 ps), while two time-constants (2.3 ps and 13 ps) are adequate for fitting the monomer data. The dynamics are characterized by immediate electronic localization towards the terpyridine moieties possibly enhanced by symmetry breaking due to structural distortion along Mo–N=N–Mo axis in 100 fs. Simultaneous interplay of IC and ISC processes is found to occur with a collective timeconstant of ca. 2-3 ps. Singlet to triplet ISC results in migration of electron density from the terpyridine ligands towards the center dinitrogen bridge, which could make dinitrogen a site for facile electrophilic attack. The final triplet 3MLCT state relaxes back to the ground state which is equivalent to electron-hole recombination. This recombination process from the 3MLCT state is governed by two rapid nonradiative decay pathways. Direct decay of the 3MLCT state into the ground state controlled by the energy gap law and Fermi-Golden rule (10-15 ps) and indirect decay via an intermediate high-energy metal-centered state (23-26 ps). Short excited state lifetime presents a limitation for harnessing bimolecular reactivity in the photoinduced states. Design modifications for the ligand architecture were proposed which could increase the lifetime of the 3MLCT state for practical applications. The modifications must aim at altering the relative energetics of the 3MLCT and 3MC states in a way that the metal-centered trap states are not accessible.

Supporting Information. The supporting information is available free of charge on the ACS Publications website at DOI: Sample preparation, Experimental Methods, NTO analysis of the singlet transitions. Molecular orbitals calculated at elongated bond length in [1–N2]2+. Time-resolved and decay associated spectra. Table containing a list of fitting parameters for the kinetic traces. Pulse-energy dependence. Transient absorption map of the monomer, 1-Cl. Double-sided Feynman diagrams. 2DES of [1–N2]2+ obtained with pulse 2. NTO analysis of the triplet states. A schematic illustration of electronic localization. Optimized geometries of [1–N2]2+ in the two electronically excited states showing structural relaxation. 77 K measurements. Molecular orbital depiction of electron migration from 1MLCT to 3MLCT state.

*[email protected]; *[email protected];

Shahnawaz Rafiq: 0000-0002-9660-4495 Mate J. Bezdek: 0000-0001-7860-2894 Marius Koch: 0000-0001-8721-7613 Paul J. Chirik: 0000-0001-8473-2898 Gregory D. Scholes: 0000-0003-3336-7960

The authors declare no competing financial interests

S. R, M. K. and G. D. S. acknowledge support from Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U. S. Department of Energy through Grant No. DE-SC0015429. M.K. further acknowledges support from a Swiss National Science Foundation postdoctoral fellowship. M. J. B. and P. J. C. acknowledge support from the U. S. Department of Energy, Office of Science, Basic Energy Science (DE-SC0006498). M. J. B. thanks the Natural Sciences and Engineering Research Council of Canada for a predoctoral fellowship (PGS-D) as well as Princeton University for an Edward C. Taylor Fellowship. S. R thanks Daniel Oblinsky for assistance with low-temperature pump-probe measurements.

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TOC Graphic


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Figure 1. The coordination modes of N2 in transition metal complexes, highlighting the strategy reported in this work. 186x134mm (150 x 150 DPI)



Figure 2. (a) Synthesis of the dinitrogen bridged bimetallic Molybdenum complex [1–N2]2+ from 1-Cl. (b) Experimental absorption spectra of the 1-Cl (solid red) and [1–N2]2+ complex (violet shaded curve). (c) Theoretically computed energies of frontier molecular orbitals as a function of separation between the two monomeric units presented as logarithmic x-axis. The encircled point corresponds to optimum N-N bond length. Inset shows the theoretically computed absorption spectra of [1–N2]2+ (3Å N-N bond length) and 1Cl. 182x150mm (300 x 300 DPI)


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Figure 3. Transient absorption maps of [1–N2]2+ obtained upon excitation at 520 nm and probing in the (a) visible and (b) near-infrared spectral region. The data is shown as filled contours with log scale of the time axis. Negative and positive signals represent ground state bleach (GSB) and excited state absorption (ESA) resp. (c) and (d) are the evolution associated difference spectra (EADS) in the visible and nearinfrared regions obtained by global analysis of the TA data. DAS are shown in figure S3 of SI. 223x148mm (300 x 300 DPI)



Figure 4. Transient absorption maps in the (a) visible and (b) near-infrared probing region obtained upon excitation at 1150 nm. (c) and (d) are the evolution associated difference spectra (EADS) obtained from global analysis of the respective spectral regions. For DAS, see fig. S3 of SI. 219x153mm (300 x 300 DPI)


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Figure 5. Comparison of the kinetic traces at three probe wavelengths; 520 (a), 820 (b), and 1100 (c) nm obtained for 520 and 1150 nm excitation. The kinetic traces are plotted on a logarithmic scale. Table S1 in the supporting information summarizes all fitting parameters of traces at these and other additional wavelengths. 250x79mm (300 x 300 DPI)



Figure 6. 2DES of the [1–N2]2+ complex measured using two different pulse spectra as shown in (a). 2DES are reported in THz energy units which makes it easy to follow the correlation of the electronic states. (b-d) represent the 2D absorptive maps at 100, 220, and 440 fs waiting times obtained with pulse 1. (e) Lower cross-peak extracted from the 2D maps (red square) and plotted as a function of delay time, t2. 220x166mm (300 x 300 DPI)


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Scheme 1. Schematic Illustration of the excited state relaxation dynamics of the dinuclear complex at room temperature interpreted as Jablonski diagram. 99x133mm (300 x 300 DPI)



85x39mm (300 x 300 DPI)


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Ultrafast Photophysics of a Dinitrogen-Bridged Molybdenum Complex

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