DOI: 10.1002/chem.201405184

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& Iron Carbene Photochemistry | Very Important Paper |

A Heteroleptic Ferrous Complex with Mesoionic Bis(1,2,3-triazol-5ylidene) Ligands: Taming the MLCT Excited State of Iron(II) Yizhu Liu,[a, b] Kasper S. Kjær,[a, c] Lisa A. Fredin,[d] Pavel Chbera,[a] Tobias Harlang,[a] Sophie E. Canton,[e] Sven Lidin,[b] Jianxin Zhang,[b] Reiner Lomoth,[f] Karl-Erik Bergquist,[b] Petter Persson,[d] Kenneth Wrnmark,*[b] and Villy Sundstrçm*[a]

Abstract: Strongly s-donating N-heterocyclic carbenes (NHCs) have revived research interest in the catalytic chemistry of iron, and are now also starting to bring the photochemistry and photophysics of this abundant element into a new era. In this work, a heteroleptic FeII complex (1) was synthesized based on sequentially furnishing the FeII center with the benchmark 2,2’-bipyridine (bpy) ligand and the more strongly s-donating mesoionic ligand, 4,4’-bis(1,2,3-triazol-5-ylidene) (btz). Complex 1 was comprehensively characterized by electrochemistry, static and ultrafast spectroscopy, and quantum chemical calculations and compared to

Introduction Manipulation of the metal–ligand interaction is central to transition-metal (TM) chemistry. It directly influences the electronic density on the metal center and the energy level ordering of different states of the complex, thus playing a critical role in [a] Dr. Y. Liu, Dr. K. S. Kjær, Dr. P. Chbera, T. Harlang, Prof. V. Sundstrçm Department of Chemical Physics, Lund University P.O. Box 124, 22100 Lund (Sweden) E-mail: [email protected] [b] Dr. Y. Liu, Prof. S. Lidin, Dr. J. Zhang,+ Dr. K.-E. Bergquist, Prof. K. Wrnmark Centre for Analysis and Synthesis Department of Chemistry, Lund University P.O. Box 124, 22100 Lund (Sweden) E-mail: [email protected] [c] Dr. K. S. Kjær Department of Physics, Technical University of Denmark 2800 Kongens Lyngby (Denmark) [d] Dr. L. A. Fredin, Prof. P. Persson Theoretical Chemistry Division, Lund University P.O. Box 124, 22100 Lund (Sweden) [e] Dr. S. E. Canton Department of Synchrotron Instrumentation, Lund University P.O. Box 124, 22100 Lund (Sweden) [f] Prof. R. Lomoth Department of Chemistry—ngstrçm Laboratory, Uppsala University P.O. Box 523, 75120 Uppsala (Sweden) [+] Current address: School of Environment and Chemical Engineering Tianjin Polytechnic University Tianjin, 300087 (China) Supporting information for this article (including experimental details) is available on the WWW under http://dx.doi.org/10.1002/chem.201405184. Chem. Eur. J. 2015, 21, 3628 – 3639

[Fe(bpy)3](PF6)2 and (TBA)2[Fe(bpy)(CN)4]. Heteroleptic complex 1 extends the absorption spectrum towards longer wavelengths compared to a previously synthesized homoleptic FeII NHC complex. The combination of the mesoionic nature of btz and the heteroleptic structure effectively destabilizes the metal-centered (MC) states relative to the triplet metal-to-ligand charge transfer (3MLCT) state in 1, rendering it a lifetime of 13 ps, the longest to date of a photochemically stable FeII complex. Deactivation of the 3MLCT state is proposed to proceed via the 3MC state that strongly couples with the singlet ground state.

determining a wide spectrum of fundamental properties.[1] A prominent example is the photophysics and photochemistry of TM complexes, in which the metal-to-ligand charge transfer (MLCT), metal-centered (MC), and ligand-centered (LC) states can be substantially manipulated relative to each other depending on the geometric configuration of the surrounding ligands and their s-donor/p-acceptor strengths.[2] The first-row TM elements are abundant and environmentally benign, but intrinsically possess much less ligand-field splitting compared to their second- and third-row congeners. For the former, the coordination environment is of special importance, as the above-mentioned states can be very sensitively tuned depending on the variation of the ligand-field strength imposed by a specific type of ligation. This opens up possibilities to manipulate the way the molecules absorb and dissipate the photon energy. The octahedral polyimine complexes of d6 FeII, for example, have been most commonly characterized by the low-lying quintet MC state (5MC),[3] which is a direct result of the smaller ligand-field splitting of Fe[4] compared to its second- and thirdrow congeners.[1] Under these circumstances, the d electrons overcome the energy gap between the t2g and eg* orbitals to avoid the repulsion of pairing, yielding the high-spin electronic configuration that may be either the ground state (GS) or a state accessible from the singlet GS through thermal fluctuation or photophysical cascades following photoexcitation, the latter being termed as light-induced excited-state spin trapping (LIESST).[3a] While such properties have potential applications in displays or memory storage devices,[5] they generate a series of unfavorable features in the context of

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Full Paper solar energy harvesting, such as ligand lability and ultra-shortlived photoactive MLCT manifolds.[6] Recently, however, we have demonstrated that by incorporating strongly s-donating N-heterocyclic carbene (NHC) ligands, the lifetime of the 3 MLCT excited state of FeII complexes can be extended by two orders of magnitude from about 100 fs to 9 ps in photochemically stable FeII complexes.[7] This was rationalized by the intact bonding between the FeII center and the NHC carbon that effectively destabilizes the 3MC and 5MC states.[8] This result was rewarding as a long-lived 3MLCT state is the foundation of many photochemical applications, and a FeII complex with an extended lifetime would allow this earth-abundant, inexpensive, and environmentally benign metal to be utilized for this purpose. Therefore, we are encouraged to further exploit the limits of such interaction using strong-field ligands to achieve even longer-lived 3MLCT states. Iron NHC chemistry has been extensively developed in the field of catalysis[9] since its first report in 2000.[10] Therein the superiority of NHC ligands has been manifested by increasing the electron density on Fe and thus its reactivity towards organic substrates.[9a] Interestingly, the electron density also dictates the excited-state properties of FeII complexes, making the little explored photophysics of Fe NHC complexes promising.[7] While in our previous report, bis(tridentate) FeII NHC complexes were investigated in comparison to [Fe(tpy)2](PF6)2 (tpy = 2,2’:6’:2“-terpyridine),[7] herein we explore the other standard octahedral ligand approach, namely the tris(bidentate) configuration. In general, bidentate ligands allow for more freedom towards ideal octahedral geometry, and therefore a stronger ligand field could be expected than with tridentate ligands, as demonstrated by [RuII(bpy)3](PF6)2 (bpy = 2,2’-bipyridine) versus [RuII(tpy)2](PF6)2.[11] Furthermore, by developing heteroleptic tris(bidentate) structures, one can selectively choose the number of s-donating or p-accepting ligation sites, thus allowing more fine-tuning of the electronic states and promoting versatility and functionality. To realize such a strategy, bis(NHC) ligands that can be regarded as analogues of the benchmark bpy[12] and 1,10-phenanthroline (phen)[13] ligands are considered for the constructing of heteroleptic FeII complexes. In this study, we focus on the 4,4’-bis(1,2,3-triazol-5-ylidenes) (btz) ligand. The carbene moiety herein is mesoionic in nature[14] and is believed to be even more strongly s-donating compared to normal NHC ligands owing to the formal negative charge on the carbene carbon in the classical drawings of the resonance structure.[12b, 15] The 1,2,3-triazol-5-ylidene moiety has been previously coupled with heterocycles to construct (C^N)[16] and (C^C)[17] heterobidentate complexes. However, there are few reports on homobidentate complexes incorporating two triazolylidene moieties in one ligand (btz).[12b, d] Therefore, we aimed for a heteroleptic complex 1 (Scheme 1) containing one bpy ligand and two btz ligands. The bpy ligand ensures the presence of MLCT transitions in this complex while the btz ligands are intended to tune the electronic properties compared to traditional FeII polyimine complexes. To the best of our knowledge, this is the first six-coordinate octahedral TM complex based on the btz-type ligands. As a NHC analogue of the benchmark complex [Fe(bpy)3](PF6)2, it is the Chem. Eur. J. 2015, 21, 3628 – 3639

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Scheme 1. Chemical structures of complexes 1 and 2.

first heteroleptic FeII NHC complex bearing spatially separated four NHC sites and a bpy ligand. It is also the first heteroleptic FeII NHC complex of which detailed excited-state photophysics is comprehensively investigated and reported. Interestingly, Gros et al.[18] also very recently reported heteroleptic FeII NHC complexes comprised of a tpy derivative and a NHC ligand that is very close to our previous report.[7] However, the work of Gros et al. was motivated by extending the absorption spectrum of the series of complexes and there was no investigation of the excited-state dynamics. The difficulty of studying the excited-state photophysics of heteroleptic complexes lies in the possible multiple isoenergetic MLCT states.[19] In relation to complex 1, while the mesoionic nature should enhance the s-donor strength of the btz ligand, the increased number of N atoms may lower its p* energy compared to normal NHC ligands, so that it could lie at a similar level as polypyridines.[16e] However, owing to the lack of related complexes as stated above, there is no reference to whether the btz-type ligands are electrochemically active or photoredox-active. To address this issue, comparison was made to (nBu4N)2[Fe(bpy)(CN)4], complex 2, which is also shown in Scheme 1. First of all, the ancillary cyanide ligands are also strong-field ligands in traditional transition-metal chemistry.[1] More importantly, the 3 MLCT state can only be localized on bpy as it is the only site within the molecule to accommodate the excited electron in a MLCT state.

Results and Discussion Synthesis of complex 1 The synthesis of the heteroleptic complex 1 is shown in Scheme 2 and described in detail in the Supporting Information. Generally speaking, the synthesis of heteroleptic FeII sixcoordinate complexes is not as straightforward as for their RuII congeners, especially when the ligand bears considerable donor strength that may eventually induce the disproportionation of the coordinatively unsaturated intermediates.[22] This tendency can be remedied by kinetic barriers for the formation of homoleptic complexes,[23] or if the partially coordinated synthetic intermediates can be readily isolated due to solubility issues.[24] In the recent work of Gros et al. on a heteroleptic FeII NHC complex, Fe(tpy)Cl3 was employed as the intermediate.[18] However, the employment of the NHC ligand generated in situ as a reductant to bring FeIII back to FeII significantly limited the yield. In our approach, the heteroleptic FeII structure could be

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Scheme 2. Synthetic pathways for complex 1. a) Ethanol, 10 equiv FeCl2, 60 8C,[20] 82 % based on bpy; b) K2CO3, CuSO4, Na ascorbate, pyridine, tBuOH/H2O, RT,[21] 70 %; c) 1) MeOTf, ClCH2CH2Cl, 78 8C to RT to 100 8C,[12d] 2) NH4PF6(aq.), 85 %; d) tBuOK, THF, 78 8C to RT, 45 %.

Figure 1. X-ray crystal structure of complex 1 Br.[48] Ellipsoids are set at 30 % probability; solvent molecules, counterions, and hydrogen atoms are omitted for clarity.

failed by all means for complex 1. However, after changing the counter ion from PF6 to Br , high-quality crystals could be readily obtained by slow diffusion of diethyl ether into the corresponding methanolic solution of the bromide complex (1 Br). Figure 1 shows the resulting X-ray structure of the coordination cation (see the Supporting Information for details). The complex adopts a distorted octahedral configuration as expected. The p-tolyl N-substituents of the two btz ligands stay in close to perpendicular conformation relative to the btz planes, so that they behave like intertwining clamps, holding the whole coordination sphere together. For each btz ligand, one p-tolyl group is almost parallel to and vertically overlapping the cis-pyridine rings of the bpy ligand. The interplanar distances fall in the range of 3.3–3.6 , indeed suggesting p–p interactions between the aromatic planes. The other p-tolyl group is on top of the cis-triazolylidene ring of the other btz ligand at a slightly tilted angle to avoid the steric hindrance coming from the other p-tolyl group on the latter. This demonstrates that the p-tolyl group is of modest size so as not to congest the coordination sphere. The key structural parameters of [Fe(bpy)3](PF6)2[28] and complexes 1 and 2[29] are listed in Table 1. The bonding between the Fe center and bpy ligand is almost identical for 1 and 2 in terms of the FeN bond distances and the N-Fe-N bite angle. Owing to the stronger trans influence of btz as a stronger-field ligand compared to bpy, the bpy ligand in 1 is slightly pushed away from the Fe center compared to in [Fe(bpy)3](PF6)2, so that longer FeN bond distances and smaller N-Fe-N bite angles are seen. In 1, the FeC bonds that are trans to the pyridine rings are slightly shorter by 0.03–0.05  than those trans to the triazolylidene rings owing to the larger trans influence of the later, giving rise to nonequivalent halves within each btz ligand. A similar situation is seen in 2 as CN is also a stronger-

achieved by directly ligating the FeII center with first bpy and then the btz ligand generated in situ, by the isolation of Fe(bpy)Cl2.[20, 25] This is also different from the not-executed opposite approach, namely assembling the NHC first and the other ancillary ligand later, which would either have involved the air-sensitive intermediate FeCl2(PPh3)2[26] or be limited by the electron stoichiometry (two electrons per Fe for two NHC sites) based on an electrochemical method.[27] Before successfully obtaining complex 1 bearing the p-tolyl-pendants, various attempts were performed to utilize 1-alkyl-substituted btz-type ligands with both primary (n-octyl) and secondary (3-pentyl) alkyl substituents, which are sufficiently large to stabilize the azide intermediate in the click chemistry (Scheme 2) but sterically allow the realization of the six-coordinate structure. Unfortunately, the resulting complexes were not stable enough. They existed transiently after isolation of the crude product and decomposed gradually during the workup and could only be characterized by mass spectrometry. In contrast, complex 1 is robust. It precipitated from the reaction mixture as a hexafluorophosphate (PF6) salt and just filtration and washing were sufficient to obtain the analytically pure compound. Such a discrepancy could be due to the additional stabilization of the whole coordination sphere introduced by the rigid p-tolyl group, which makes the two btz ligands like clamps, with the possibility of holding each other as well as the third bpy ligand through p–p interactions. It should be noted that during the characterization stage we observed temperatureand concentration-dependent peak broadening in the 1H NMR spectra of complex 1, which is not completely understood yet. However, the 1H NMR spectrum at a concentration below 1 mm at room temperature is well-defined. Therefore, NMR Table 1. Selected structural parameters of [Fe(bpy)3](PF6)2, 1, and 2. spectra (see the Supporting Information) were taken at 1 mm rFe–Ccis rFe–Ctrans N-Fe-N Ccis-Fe-Ctrans N-Fe-Ctrans Ccis-Fe-Ccis rFe–N [] [] [] [8] [8] [8] [8] and the concentration of the solution samples used in all of the [Fe(bpy)3](PF6)2 [a] 1.97 81.8 1 1.99, 2.0 1.99, 2.02 1,96, 1.97 80.5 79.3, 80.0 172.7, 178.1 172.6 measurements described below [b] 1.99, 2.0 1.93, 1.94 1.89, 1.91 80.6 88.4–91.1 174.2, 175.6 178.3 2 were also restricted to be below [a] The structural data of [Fe(bpy)3](PF6)2 are taken from reference [28]. [b] The structural data of 2 are taken this threshold. from reference [29]. Potassium (K + ) instead of tetra-n-butylammonium (TBA + ) was used as the counter-ion Efforts to grow crystals suitatherein. ble for X-ray diffraction analysis Chem. Eur. J. 2015, 21, 3628 – 3639

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Full Paper field ligand than bpy. However, without the structural constrain imposed by the conjugated system as in the btz ligands in 1, the cyanide ligands in 2 can be freely arranged around the Fe center. Therefore, the FeC bonds in the latter are generally shorter by 0.06–0.08  compared to those in 1, and the C-Fe-C “bite angles”, if termed likewise, are very close to 908 in comparison with those of about 808 in 1.

Electrochemistry and steady-state absorption spectroscopy Cyclic voltammetry (CV; Figure 2) was performed in acetonitrile (MeCN) to evaluate the energy levels of the three compounds. The key parameters are listed in Table 2 for comparison. In the cathodic region, [Fe(bpy)3](PF6)2 shows a one-electron reversible redox process at + 0.68 V vs. the ferrocenium/ferrocene (Fc + /Fc) redox couple and is assigned to the oxidation of the FeII center.[30] The potential of this process is drastically shifted down to 0.35 V and 0.63 V vs. Fc + /Fc in 1 and 2, respectively, the latter in excellent agreement with a previous report.[31] Considering the structural analogy among these complexes, it is evident that substitution of two bpy ligands for C-based ligands significantly altered the electronic properties of the FeII center. A similar cathodic shift was observed for RuIII/II with both triazolylidene-containing[16c] and CN[32] ligands. A direct comparison between the two is, however, not possible owing to the more intimate interaction between the Fe center and the donating carbon atoms in 2 in terms of the shorter FeC bond distances and the closer-to-perfect octahedral geometry. Although it is tempting to correlate the more energetic FeII center with the superior donor strength of the C-based ligands, the oxidation of the FeII center is nonetheless formally removing an electron from the dp (t2g) orbital, which in prin-

Figure 2. Cyclic voltammograms of [Fe(bpy)3](PF6)2 (L.), 1 (c), and 2 (d) in deaerated MeCN.

ciple do not interact with the ligand s-orbitals[33] and thus should not necessarily be destabilized by the latter. In this regard, the indeed widely observed cathodic shift of the metal oxidation potential upon coordination with NHC ligands,[16c, e, 34] including our previous[7] and present studies, may point to a synergistic effect of the poor p-accepting ability of NHC and a non-perfect octahedricity that attenuates the dpM–p*L overlap.[4] In concert with the negatively shifted oxidation potential of the Fe center, the energy levels of the bpy ligands are also destabilized in the sequence [Fe(bpy)3](PF6)2 to 1 to 2. In the anodic region, [Fe(bpy)3](PF6)2 shows two closely separated one-electron reversible redox processes at 1.75 and 1.94 V vs. Fc + /Fc, which can be ascribed to the sequential reduction of two bpy ligands in the tris(bpy) complexes of d6 transition metals.[30, 32] In complex 1, ligand-based reduction happens at a much more negative potential. It is essentially irreversible, which has been similarly observed for other related NHC-containing complexes.[7, 34b] Differential pulse voltammetry (DPV) was carried out to help to resolve the potentials of the ligandbased reduction processes, and the result is shown in the Supporting Information, Figure S12. Three closely spaced reduction processes are found at 2.28, 2.42, and 2.64 V vs. Fc + /Fc, respectively. Although an anodic shift of more than 500 mV is seen for the lowest-energy ligand-based reduction, this is to a lesser degree than observed for its FeIII/II oxidation. The overall result is therefore a decreased MLCT transition energy, consistent with the red-shift of the lowest-energy MLCT band from 520 nm of [Fe(bpy)3](PF6)2 to 609 nm of 1. This improves the overlap with the solar spectrum compared to our first-generation FeII NHC complexes (Figure 3 and Table 2).[7] In com-

Figure 3. a) The steady-state UV/Vis absorption spectra of [Fe(bpy)3](PF6)2 (L.), 1 (c), and 2 (d) in MeCN. The inset shows the normalized MLCT bands. b) The solvatochromism of complex 1.

Table 2. Redox potentials and spectroscopic parameters of [Fe(bpy)3](PF6)2, 1, and 2 in MeCN.

[Fe(bpy)3](PF6)2 1 2

Eox[a] [V]

Ered1[a] [V]

Ered2[a] [V]

lmax [nm] (e [103 L mol1 cm1])

+ 0.68 0.35 0.63

1.75 2.28[b] 2.47[c]

1.94 2.42[b] –

248 (27.5), 298 (62.2), 349 (6.03), 486 (6.88, sh), 520 (7.98) 243 (23.1, sh), 300 (29.5), 432 (5.08), 609 (3.26) 244 (15.5), 298 (14.9), 438 (5.51), 678 (3.32[d))

[a] Potential values are reported vs. the Fc + /Fc redox couple. [b] Irreversible peaks. [c] Quasi-reversible peak. [d] From reference [35].

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Full Paper plex 2, reduction did not happen until the potential was as negative as 2.47 V vs. Fc + /Fc. As bpy is the only redox-active ligand in this complex, such a reduction is ascribed to being centered on bpy. Although 2 has been studied intermittently for several decades, this bpy-based reduction has never been reported. A somewhat astonishing point here is that such an energetic reduction is quasi-reversible, whereas reductions of the ligands in NHC-containing complexes, not only of the complex 1 reported here, but also of our first-generation Fe NHC complexes,[7] are essentially irreversible. This in turn indicates the instability of the reduced NHC radical anion, and that the ligand-based reduction processes in 1 are an intermingling of the two different types (btz as NHC and bpy, respectively) of ligands. Another interesting point to note is that, in accordance with the higher electron density on the bpy ligand in complexes 1 and 2 compared to in [Fe(bpy)3](PF6)2, the chemical shifts of the protons on the bpy ligands are also positioned at a higher field (Supporting Information, Figure S10). However, with complex 2 holding a more energetic bpy ligand, the protons are less-shielded compared to those of bpy in complex 1. This can be explained based on the crystal structure as a result of the bpy ligand being sandwiched between two p-tolyl groups so that extra shielding is imposed. Figure 3 a illustrates the steady-state UV/Vis absorption of the three complexes in MeCN. In the UV region, characteristic ligand-centered bands have more or less identical peak positions (Table 2). In the visible region, 1 and 2 show similar MLCT bands in terms of shape and extinction coefficient,[35] both redshifted compared to [Fe(bpy)3](PF6)2 with less than half of the intensity. A further red-shift is observed from 1 to 2 which can be understood by a roughly 300 mV anodic shift of the Febased oxidation potential but only 200 mV anodic shift of the ligand-based reduction. Complex 2 is extremely solvatochromic (Supporting Information, Figure S13),[35] but different solvents show very little effect on the absorption maxima of the MLCT bands of complex 1 (Figure 3 b, except for a significantly distinct spectrum in H2O, which is probably related to solubility issues). Although solvatochromism could be expected for com-

plex 1, owing to the dipolar nature of the MLCT state,[36] the absence of a significant effect may lie in the planar btz fragments in 1 acting as an insulator between the FeII center and the solvent molecules compared to the rather linear and mediating CN in 2. As will be discussed in the next section, different solvents also exert drastically less influence on the excited-state dynamics of 1 compared to that reported for 2.[37] As a consequence, complex 1 is advantageous in terms of the versatility of applications. Femtosecond transient absorption spectroscopy To investigate the excited-state dynamics of 1, femtosecond (fs) transient absorption (TA) spectroscopic studies were carried out. Considering the above-mentioned possibility of multiple MLCT states localized on different types of ligands in 1 as mentioned above, 2 was studied for comparison as a complex where the MLCT states could only be localized on the bpy ligand. In 1987, Winkler et al. reported that in acetone, complex 2 displayed an excited state that was MLCT in nature with a lifetime within the instrumental response (of 25 ps).[37] Strangely, there was no follow-up since that study until recently when Gaffney et al. unambiguously identified the excited state in DMSO as a 3MLCT state with a lifetime of 18 ps using fs X-ray emission spectroscopy (XES).[38] With this result as a basis, it is much easier to understand the behavior of complex 1 by comparing with 2. Figure 4 shows the time-resolved differential absorption spectra of complexes 1 and 2 excited at 615 and 705 nm, respectively, in MeCN. Both wavelengths were chosen as slightly lower in energy than their lowest-energy MLCT absorption maxima. The gaps seen in the spectra are due to the strong scattering at the pump wavelengths (615 nm for complex 1 and 705 nm for complex 2) and the primary laser source (around 800 nm). Despite the speculation about multiple MLCT states of complex 1, a striking similarity is observed at first sight when comparing the two. Upon excitation, both complexes instantaneously show two well-defined ground-state

Figure 4. The transient absorption (TA) spectra of a) complex 1 excited at 615 nm and b) complex 2 excited at 705 nm in MeCN. Chem. Eur. J. 2015, 21, 3628 – 3639

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Full Paper bleach (GSB) bands which reflect their respective steady-state absorption spectra. Also present are three excited-state absorption (ESA) features, one with a well-defined peak at around 370 nm, one between the two GSB bands at around 500 nm, and one with a featureless broad band in the near-infrared (NIR) region. For complex 1, the circa 500 nm ESA feature is less pronounced owing to its more negatively biasing GSB in that region, which can be deduced from the inset of Figure 3 a. The peak at 370 nm is widely regarded as characteristic for the reduced bpy anion, which is a signature of the MLCT excited state.[6a, c, 30a, 39] The transient spectra of 2 resemble those reported previously for this complex[37] and decay monotonically at first sight without significant evolution into other features. All this together with the established 3MLCT identity of 2 according to the fs XES studies as mentioned above suggests that complex 1 also ends up with a 3MLCT state before decaying back to the ground state. Singular value decomposition (SVD) analysis of the data was carried out as an unbiased analysis of the temporal evolution of the spectral features.[6b, 40] The six most significant singular vectors, each containing a spectral and temporal trace, are plotted in Figure 5. From this it is evident that, for both complexes, the TA spectra after the very initial few hundred femtoseconds can be well described with only two major singular vectors (vector 1, shown in black; vector 2, shown in gray). The spectral traces of vector 1 describe the main spectral features of the recorded TA spectra for the two complexes. The corre-

Figure 5. The singular value decomposition (SVD) of the transient spectra of complexes a) 1 and b) 2 measured in Figure 4. Chem. Eur. J. 2015, 21, 3628 – 3639

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sponding temporal traces reach maximum intensity almost instantaneously, followed by a multi-exponential decay. Vector 2 describes the blue-shift of the TA spectra. A larger amplitude of the spectral trace of this vector for complex 1 is consistent with the clearly visible blue-shift of the circa 375 and 720 nm ESA and circa 440 nm GSB bands in Figure 4 a. For both complexes, vector 2 exhibits a rise-time, followed by a decay with a similar rate as that of vector 1. Quantitative analysis was carried out by fitting a bi-exponential decay function to the temporal traces of the two vectors. For complex 2, both vectors deliver a short component of 3.4–3.7 ps and a longer component of 18.1 ps. The former is ascribed to solvation and vibrational cooling, which usually happen in similar time ranges,[6d] of the complex while the latter is in very good agreement with the 3MLCT lifetime measured by femtosecond XES,[38] and is thus ascribed to the decay of the 3MLCT excited state. For complex 1 the situation is slightly more complicated. Although both vectors share a common long component of 14.4 ps, the shorter component is different for the two vectors. Vector 1 has a fast component of 4.1 ps while that of vector 2 is 0.8 ps. Considering the possibility of multiple MLCT states, the 0.8 ps component is tentatively assigned to the localization of the initially populated Franck–Condon (FC) MLCT excited state into a bpy-dominated 3MLCT excited state.[11, 19d–f, 36, 41] The 4.1 ps component of vector 1 is ascribed to solvation and vibrational cooling of the 3MLCT state, similar to complex 2. The 14.4 ps common component for both vectors is ascribed to the lifetime of the thermally equilibrated 3MLCT state, and a more detailed assignment will be discussed below. This value is slightly shorter than that of complex 2. However, in our experiments, significant photobleaching (indicative of decomposition) was observed for 2 after only 15 min in the laser beam, whereas complex 1 was robust even after overnight measurements (Supporting Information, Figures S14 and S15). The results from the above-mentioned analysis are employed as a guidance for the global analysis (GA), which is a simultaneous fitting of the spectral and temporal information in the TA spectra.[6b] The results are shown in Figure 6. A multiexponential model is fitted to the data for both complexes. For complex 2, a decent fitting can be achieved with only two exponents of 3.4 and 18.2 ps, which matches with the SVD analysis very well according to Figure 5. The corresponding decay-associated spectra (DAS) and species-associated spectra (SAS) are plotted in Figure 6 c, d, respectively. The GSB bands of DAS1 are considerably distorted compared to the ground-state absorption spectrum, which implies involvement of strong ESA features of this component. On the other hand, DAS2 shows well-defined GSB bands that reflect the ground-state absorption spectrum. Furthermore, the three ESA features discussed in SVD analysis can be distinguished. The whole spectrum is blue-shifted relative to DAS1, which is attributed to the blueshift of ESA. The two SASs reconstructed from the DASs are very similar. Therefore, the 3.4 ps lifetime is assigned to the decay of the vibrationally hot 3MLCT state to the relaxed 3 MLCT state, which then decays with an 18 ps time constant. For complex 1, three components are needed to obtain decent GA (Figure 6 a, b). This is in accordance with the three

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Figure 6. The decay-associated spectra (DAS; (a), (c)) and species-associated spectra (SAS; (b), (d)) of complexes 1 and 2, respectively, from the global analysis.

time constants extracted from the SVD analysis. DAS1 with a lifetime of 0.9 ps, like that for complex 2, is also relatively featureless, especially in the GSB region. Following DAS1 are DAS2 and DAS3 with lifetimes of 4.2 and 12.7 ps, respectively. All these lifetimes agree very well with those obtained from the SVD analysis. Compared to DAS1, for DAS2 and DAS3 there is significant attenuation of the amplitude in the blue region and a systematic blue-shift of the whole spectrum. The blueshift is so significant in the NIR region that it looks like the spectrum is drastically regaining intensity there. This is in agreement with the fast grow-in (0.8 ps) of vector 2, which has a substantial negative amplitude in the blue region and positive amplitude between 600 and 700 nm according to the SVD analysis. This results in the same features in the SASs (Figure 6 b). It should be noted that in the third component, ESA is still present at many wavelengths where DAS3 and SAS3 have apparently zero amplitude. For example, at around 350 and 500 nm positive ESA has to contribute to the spectrum to balance the negative GSB that must be there, owing to the substantial ground-state absorption at these wavelengths. Apart from that, the results of the TA measurements indicate overall Chem. Eur. J. 2015, 21, 3628 – 3639

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similarity of the photophysical behaviors between complexes 1 and 2 following energetically comparable photoexcitation in MeCN. The validity of the above-mentioned SVD and GA is corroborated by the kinetic fitting at single wavelengths for both complexes (Supporting Information, Figure S17), which delivered three time constants of 0.9, 3.2–4.2, and 12.5–15.4 ps for 1 and 2.9–4.2 and 16.1–18.4 ps for 2, respectively, depending on the specific wavelengths. It should also be noted that the excitedstate properties of 1 is little affected by the nature of the solvent as demonstrated by the similar TA dynamics acquired in methanol (Supporting Information, Figures S18–S20). This is in sharp contrast to 2 which lost the long-lived 3MLCT state in protic solvents.[37] Spectroelectrochemistry To further assist in the assignment of the excited species of complex 1 resulting from GA, spectroelectrochemistry was performed in MeCN to gain insights into the expected appearance of the MLCT states. Complex 1 was thus constantly electrolyzed

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Figure 7. The spectral changes of complex 1 electrolyzed at a) 0.1 V and b) 2.28 V vs. Fc + /Fc in de-aerated MeCN for 10 min. c) Linear combination of the differential spectra coming from these two conditions generates the simulated differential spectrum of the 3MLCT state. d) Comparison of the ESA spectrum of the third long-time GA component (a, recovered by subtracting the scaled ground-state absorption spectrum (d) from SAS3 (black c)) and the differential spectrum for reduction (gray c).

at 0.1 V and 2.28 V vs. Fc + /Fc, corresponding to the FeII to FeIII oxidation and ligand-based reduction, respectively. The spectral changes are plotted in Figure 7 a,b. The potential chosen for oxidative electrolysis can be thermodynamically sufficient considering the well-defined behavior of the FeIII/II redox processes. The potential chosen for reductive electrolysis, on the other hand, was the peak position of the first reduction wave owing to the small separation of the series of reduction processes. Even so, it is not possible to guarantee complete exclusion of involvement of the second process, and therefore, one has to be careful to judge the identity of the obtained species owing to the likely mingling of the two different types of ligand states as mentioned above. The spectral change observed for the FeII to FeIII oxidation is relatively simple. It is mainly comprised of the loss of intensity in the MLCT and LC bands as expected, with a slight increase in the UV region. The reduction electrolysis, which was carried out for an irreversible event, is more complex and needs more caution. However, considering that little driving force was employed in this case, and that only one set of isosbestic points (289, 318, 562, 654 nm) with a single trend of nicely resolved spectral evolution were observed, it is indicated that only one species was developed in such a condition. With constant reduction, the complex shows significant increases of absorbance in three wavelength regions in the window covered by Chem. Eur. J. 2015, 21, 3628 – 3639

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TA measurements, namely, between 320 and 420 nm (between the LC and second MLCT bands), in the visible region centered at 500 nm (between the two MLCT bands), and from 650 nm to 900 nm. Coupled with these is the slight attenuation in the lowest-energy (first) MLCT band. By a linear combination of the differential spectra from oxidation and reduction, it is possible to generate the simulated differential spectrum of the 3 MLCT state[6a, 42] for 1 (Figure 7 c). By comparing Figure 6 b and Figure 7 c, it is evident that the first and second components from the GA are essentially MLCT in nature owing to their clear resemblance with the simulated differential spectrum. Although the third long-time component does not completely match the simulated differential spectrum in Figure 7 c, the three spectral fingerprints observed for reduction are still present, namely, the embedded ESA in the blue (350 nm) and visible (500 nm) regions as discussed above, and the considerable positive ESA in the red region. This is illustrated more quantitatively by comparing the ESA contribution of this component (obtained by subtracting a scaled ground state absorption spectrum from SAS3 (Figure 6 b) with the differential absorption spectrum of reduction, as shown in Figure 7 d. Therefore, it is strongly suggested that the third SAS3 component also possesses considerable MLCT identity, and the assignment of the time constants obtained from the GA is consistent with that from SVD.

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Full Paper DFT and TD-DFT calculations Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were carried out to assist in the understanding of the properties of complex 1. The ground-state structure was optimized using Gaussian 09[43] with PBE0 j 6-311G(d,p) and with a complete acetonitrile polarizable continuum model (PCM). The structure is parameterized by the average distances between Fe and either bpy (qbpy) or btz (qbtz), and the octahedricity (O-value), which is a measure of the mean absolute deviation of the set of all metal–ligand bond angles from the values in an “ideal” octahedral structure, as listed in Table 3.

Table 3. Calculated structural properties of complex 1 for the GS, 3MLCT, 3 MC, and 5MC states using PBE0 j 6 311G(d,p) j PCM(MeCN).[a]

E [eV] Mulliken Spin on Fe qbpy[c] qbtz[c] R[d] O-value[e]

X-ray[b]

GS

3

3

– –

0.00 –

1.00 1.14

0.67 2.09

0.46 3.79

2.00 1.99 1.99  0.02 6.30

1.99 2.00 2.00  0.01 7.94

1.98 2.01 2.00  0.02 8.41

2.18 2.09 2.12  0.09 9.36

2.23 2.25 2.24  0.02 9.89

MLCT

MC

5

MC

[a] Distances in  and angles in degrees. Deviations are calculated as sn values. [b] Calculated from crystallographic data. [c] qbpy and qbtz are the composite reaction coordinates for ligands the bpy and btz ligands, respectively. They are calculated as the average of the FeC/N distances. qbpy = [D(FeN1) + D(FeN2)]/2 and qbtz = [D(FeC1) + D(FeC2) + D(Fe C3) + D(FeC4)]/4. [d] R is the average of all metal coordinating atom bond distances, where the error is the standard deviation. [e] Average deviation = (S j ideal angle-measured angle j)/n.

The calculated structure matches very well with the experimentally measured X-ray crystal structure. The molecular-orbital structure of the optimized ground state geometry was examined in detail and the first five highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) are plotted in Figure 8 a. Detailed orbital distributions for more levels are listed in the Supporting Information, Table S1. The three highest HOMO levels are mainly distributed over the Fe center, with slight occupation on the btz ligands. The LUMO level is almost exclusively localized on the bpy ligand, while the LUMO + 1 and LUMO + 2 levels are localized on the five-membered rings of the btz ligands. These three LUMO levels are very close in energy; the LUMO + 1 level is calculated to lie about 0.15 eV above the LUMO level, and the LUMO + 2 level is another 0.1 eV above LUMO + 1, in excellent agreement with the consecutive reduction processes observed in Figure 2 and the Supporting Information, Figure S12. Also illustrated in Figure 8 is the ground-state absorption spectrum calculated by TD-DFT. The nature of the corresponding transitions was assigned based on the dominant contributions, and the details are listed in the Supporting Information, Table S2. The generated spectrum clearly shows the MLCT bands and a p–p* LC band in the expected region. Notably, at the maximum of the lowest-energy MLCT band, two dominating transitions with comparable oscillator strengths are found Chem. Eur. J. 2015, 21, 3628 – 3639

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Figure 8. The structural and electronic properties of complex 1 at the ground state geometry: a) the first five HOMO and LUMO levels and b) the computed spectra for PBE0 j 6-311G(d,p) j PCM(MeCN) TD-DFT in the 200– 600 nm range.

(450.6 and 449.6 nm; Figure 8 b; Supporting Information, Table S2). These energetically intimate transitions show dramatically different origin. The most contributing transition at 450.6 nm is HOMO!LUMO+1 (72 % weight) while that at 449.6 nm is HOMO1!LUMO (57 % weight). As shown in the Supporting Information, Table S2, the LUMO and LUMO+1 levels are localized on bpy and btz, respectively. Therefore, HOMO!LUMO+1 can be regarded as an MLCT transition from the FeII center to btz, while HOMO1!LUMO as the transition to bpy. The optical transition at the MLCT band maximum is thus expected to be substantially mixed. In the TA measurements, complex 1 was excited at a slightly longer wavelength than the maxima (615 nm relative to the 609 nm maximum). Therefore, it is highly probable that the FC state is not exclusively bpy-localized but rather shares a substantial distribution over the btz ligands. This could explain the much more significant blue-shift of the TA spectra of complex 1 as well as the extra 0.8 ps component which is assigned as the localization process. To better understand the photophysical cascade, the relevant excited states were optimized using spin-unrestricted DFT

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Full Paper (uDFT) calculations. The lowest triplet (3MC, 3MLCT) and quintet (5MC, 5MLCT) states can be assigned as MLCT or MC based on their Fe spin density, which is shown in the Supporting Information, Table S3. The key structural and energetic parameters are listed in Table 3. Specifically, qbpy and qbtz of these states are plotted as the gray dots in the inset of Figure 9. From this, the

Figure 9. Projected potential energy surfaces (PPESs) versus q. Gray points are optimized minima, and black points are single-point energies calculated at the minimum geometries (· S0, singlet state;  T1, triplet states; + Q1, quintet states). The gray lines schematically show the PPESs. Inset: the structural differences between calculated minima geometries in terms of qbpy and qbtz. The dashed diagonal is used as the horizontal axis (average iron–ligand distance q) of the main figure, to which the gray points in the inset are projected.

structural distortion of the excited states relative to the ground state can be easily ascertained. With the projection of these gray dots along the diagonal direction of the inset (average Fe–ligand distance q) as the horizontal coordinates and their corresponding energy as the vertical coordinates, a projected potential energy surface (PPES) diagram is plotted in Figure 9. The 3MLCT state is structurally akin to the ground state, lying only 1 eV above the latter, which is much smaller than calculated for [Fe(bpy)3]2 + in previously reported studies,[44] but consistent with the previous conclusion of reduced excitation energy for complex 1. The 3MLCT state is largely localized on bpy (Supporting Information, Table S3), which is consistently predicted to be lower in energy than btz according to the ordering of the LUMO levels. This supports the assignment of the longest component from both SVD and GA as a bpy-localized 3MLCT state. The lowest-energy triplet MC state (3MC) shows a typical structural distortion owing to the population of the eg* antibonding orbitals. Compared to the ground and 3MLCT states the average Fe–bpy distance is elongated by 0.2 , which is only seen in the structural distortion of the quintet MC state (5T2) of [Fe(bpy)3]2 + , and is much larger than that of the triplet MC states (3T1, 3T2, and so on) therein.[44, 45] This can be understood as being due to the much larger electron density on the Fe center. On the other hand, the Fe–btz bonds also stretch by around 0.1 , and the bite angles are smaller, as manifested by the increased deviation from ideal octahedricity (higher OChem. Eur. J. 2015, 21, 3628 – 3639

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value). The 3MC minimum lies 0.67 eV above the ground state, which is less than the corresponding [Fe(bpy)3]2 + 3MC (1.24 eV).[44] However, the difference between the energy minima of the 3MLCT and 3MC is reduced to only 0.33 eV compared to 1.26 eV for [Fe(bpy)3]2 + .[44] In other words, the 3MC state is significantly destabilized relative to the 3MLCT state in complex 1, and its minimum is moved even further away compared to [Fe(bpy)3]2 + . A similar situation is seen for the lowestenergy quintet excited state (5MC). An increase of 0.25  in the average Fe–ligand distances is observed, accompanied with an even larger distortion from ideal octahedricity. The energy difference between the 3MLCT and 5MC minimum is lowered to 0.54 eV compared to 1.81 eV in [Fe(bpy)3]2 + .[44] For common FeII polyimine complexes, the 5MC state is usually regarded as the main state for deactivating the photochemically interesting 3MLCT state,[6d] while for most RuII analogues, the thermally activated access to the triplet 3MC state is the main deactivation mechanism.[11, 46] Such a discrepancy clearly arises from the weaker ligand interaction of FeII compared to RuII. For this reason, in FeII complexes such as [Fe(bpy)3]2 + , the triplet 3T1 state is so short-lived that only an ultrafast (femtosecond) X-ray emission technique[6f] and theoretical calculations[44] can shed light on its dynamics. With increased ligand field strength, however, the 3T1 state may begin to dictate the deactivation cascade.[4] Recently, the role of the triplet MC states in the mechanism of the 3MLCT deactivation cascade has received increased attention.[4, 6f, 44, 47] Indeed, in our first-generation FeII NHC complex, the 3MC state is shown to be critical to the major 3MLCT deactivation pathway. In this work,[8] the strong s-donation imposed by the NHC ligands was shown to effectively destabilize the 5MC state and push it far away along the molecular coordinate compared to common FeII polypyridine complexes. The consequence of this is that the 3MLCT state is first intercepted by the 3MC surface before the 5MC surface crosses the 3MLCT surface. In the present study, an even stronger s-donation than our first generation FeII NHC complex is achieved in complex 1 thanks to the combination of the mesoionic nature of btz and its heteroleptic structure. Therefore, complex 1 features an even smaller 3 MLCT–3MC energy gap (0.33 eV) and an even larger structural distortion in its 3MC state, which contribute to its longer 3 MLCT lifetime measured by TA spectroscopy. Notably, according to Figure 9, the energy of the singlet ground state at the optimized 3MC structure is very close to that of the 3MC state. Therefore, it is expected that once complex 1 is in the 3MC state, it will quickly decay to the ground state.

Conclusions We have successfully synthesized the first heteroleptic tetrakis(NHC) FeII bpy complex (1) using the bis(1,2,3-triazol-5-ylidene) ligand and the uncommon Fe(bpy)Cl2 intermediate. This is also the first octahedral TM complex based on this type of btz ligand. A strong ligand field is demonstrated to be imposed in complex 1 without sacrificing stability and solvent compatibility, yielding a remarkably electron-rich Fe center. Electrochemistry, ultrafast spectroscopy, and computational

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Full Paper studies revealed that in such a heteroleptic structure, the btz ligand is also redox- and photoredox-active, like the benchmark bpy ligand. Understanding of the photophysical behavior of complex 1 is aided by the comparison with complex 2 and [Fe(bpy)3]2 + , as well as theoretical calculations. The employment of the strongly s-donating btz ligand effectively destabilizes both the triplet and quintet metal-centered states and moves their minima far away from that of the photochemically valuable 3MLCT state along the molecular coordinate (first coordination sphere bond distances). This results in a long-lived 3 MLCT state of 13 ps compared to the 130 fs of that for [Fe(bpy)3]2 + , and such a success achieved from a completely different approach, namely the tris(bidentate) instead of the bis(tridentate) configuration, which was adopted in our previous report,[7] demonstrates that Fe NHC chemistry is indeed a viable strategy for improving the photophysical properties of FeII complexes. The MLCT lifetime is the longest for a photochemically stable FeII complex to date, raising hopes that replacing expensive and rare RuII for inexpensive and earth-abundant FeII, for the purpose of solar energy harvesting, will be possible. In this respect, it is important that the heteroleptic complex 1 also increases the overlap with the solar spectrum to above 750 nm, a bathochromic shift of 200 nm compared to our previously synthesized homoleptic FeII NHC complex. The quantum chemical calculations also suggest a 3MLCT deactivation pathway that proceeds through the short-lived 3MC state. Thus, the photophysical mechanism of complex 1 resembles that of most RuII complexes, although the MLCT excited state lifetime is still significantly shorter. Therefore, the knowledge and principles developed for RuII complexes may now be applied to FeII complexes, turning research on the latter in a new direction that is amenable to further improvement with the ever enlarged library of photochemically interesting compounds.

Acknowledgements This work was supported by the Crafoord Foundation, the Swedish Research Council (VR), the Knut and Alice Wallenberg (KAW) Foundation, the European Research Council (ERC, 226136-VISCHEM) and the Swedish Energy Agency. We thank Dr. Erling Thyrhaug for the assistance in measuring highconcentration UV/Vis absorption spectroscopy and Sofia Essn for measuring HR-MS. P.P. acknowledges support from the NSC and LUNARC supercomputing facilities. Keywords: iron · femtosecond spectroscopy · N ligands · metal-to-ligand charge transfer · N-heterocyclic carbenes [1] B. N. Figgis, M. A. Hitchman, Ligand Field Theory and Its Applications, Wiley-VCH, New York, 2000. [2] V. Balzani, G. Bergamini, S. Campagna, F. Puntoriero, Top. Curr. Chem. 2007, 280, 1 – 36. [3] a) A. Hauser, Top. Curr. Chem. 2004, 234, 155 – 198; b) M. A. Halcrow, Polyhedron 2007, 26, 3523 – 3576. [4] L. L. Jamula, A. M. Brown, D. Guo, J. K. McCusker, Inorg. Chem. 2014, 53, 15 – 17. [5] O. Kahn, Science 1998, 279, 44 – 48. Chem. Eur. J. 2015, 21, 3628 – 3639

www.chemeurj.org

[6] a) J. E. Monat, J. K. McCusker, J. Am. Chem. Soc. 2000, 122, 4092 – 4097; b) W. Gawelda, A. Cannizzo, V. T. Pham, F. van Mourik, C. Bressler, M. Chergui, J. Am. Chem. Soc. 2007, 129, 8199 – 8206; c) C. Consani, M. Premont-Schwarz, A. Elnahhas, C. Bressler, F. van Mourik, A. Cannizzo, M. Chergui, Angew. Chem. 2009, 121, 7320 – 7323; Angew. Chem. Int. Ed. 2009, 48, 7184 – 7187; d) A. Cannizzo, C. J. Milne, C. Consani, W. Gawelda, C. Bressler, F. van Mourik, M. Chergui, Coord. Chem. Rev. 2010, 254, 2677 – 2686; e) K. Haldrup, G. Vanko, W. Gawelda, A. Galler, G. Doumy, A. M. March, E. P. Kanter, A. Bordage, A. Dohn, T. B. van Driel, K. S. Kjaer, H. T. Lemke, S. E. Canton, J. Uhlig, V. Sundstrom, L. Young, S. H. Southworth, M. M. Nielsen, C. Bressler, J. Phys. Chem. A 2012, 116, 9878 – 9887; f) W. Zhang, R. Alonso-Mori, U. Bergmann, C. Bressler, M. Chollet, A. Galler, W. Gawelda, R. G. Hadt, R. W. Hartsock, T. Kroll, K. S. Kjaer, K. Kubicek, H. T. Lemke, H. W. Liang, D. A. Meyer, M. M. Nielsen, C. Purser, J. S. Robinson, E. I. Solomon, Z. Sun, D. Sokaras, T. B. van Driel, G. Vanko, T. C. Weng, D. Zhu, K. J. Gaffney, Nature 2014, 509, 345 – 348. [7] Y. Liu, T. Harlang, S. E. Canton, P. Chbera, K. Surez-Alcntara, A. Fleckhaus, D. A. Vithanage, E. Gçransson, A. Corani, R. Lomoth, V. Sundstrçm, K. Wrnmark, Chem. Commun. 2013, 49, 6412 – 6414. [8] L. A. Fredin, M. Ppai, E. Rozslyi, G. Vank, K. Wrnmark, V. Sundstrçm, P. Persson, J. Phys. Chem. Lett. 2014, 5, 2066 – 2071. [9] a) K. Riener, S. Haslinger, A. Raba, M. P. Hogerl, M. Cokoja, W. A. Herrmann, F. E. Kuhn, Chem. Rev. 2014, 114, 5215 – 5272; b) M. J. Ingleson, R. A. Layfield, Chem. Commun. 2012, 48, 3579 – 3589. [10] J. Louie, R. H. Grubbs, Chem. Commun. 2000, 1479 – 1480. [11] S. Campagna, F. Puntoriero, F. Nastasi, G. Bergamini, V. Balzani, Top. Curr. Chem. 2007, 280, 117 – 214. [12] a) S. Schick, T. Pape, F. E. Hahn, Organometallics 2014, 33, 4035 – 4041; b) S. Hohloch, L. Suntrup, B. Sarkar, Organometallics 2013, 32, 7376 – 7385; c) J. M. Aizpurua, M. Sagartzazu-Aizpurua, Z. Monasterio, I. Azcune, C. Mendicute, J. I. Miranda, E. Garcia-Lecina, A. Altube, R. M. Fratila, Org. Lett. 2012, 14, 1866 – 1868; d) G. Guisado-Barrios, J. Bouffard, B. Donnadieu, G. Bertrand, Organometallics 2011, 30, 6017 – 6021; e) S. Sanz, A. Azua, E. Peris, Dalton Trans. 2010, 39, 6339 – 6343; f) M. Poyatos, W. McNamara, C. Incarvito, E. Clot, E. Peris, R. H. Crabtree, Organometallics 2008, 27, 2128 – 2136; g) D. H. Jeong, W. J. Park, J. H. Jeong, D. G. Churchill, H. Lee, Inorg. Chem. Commun. 2008, 11, 1170 – 1173. [13] a) V. Gierz, A. Seyboldt, C. Maichle-Mçssmer, K. W. Tçrnroos, M. T. Speidel, B. Speiser, K. Eichele, D. Kunz, Organometallics 2012, 31, 7893 – 7901; b) V. Gierz, C. Maichle-Mçssmer, D. Kunz, Organometallics 2012, 31, 739 – 747. [14] a) B. Schulze, U. S. Schubert, Chem. Soc. Rev. 2014, 43, 2522 – 2571; b) K. F. Donnelly, A. Petronilho, M. Albrecht, Chem. Commun. 2013, 49, 1145 – 1159; c) R. H. Crabtree, Coord. Chem. Rev. 2013, 257, 755 – 766; d) A. Kr ger, M. Albrecht, Aust. J. Chem. 2011, 64, 1113; e) J. D. Crowley, A.-L. Lee, K. J. Kilpin, Aust. J. Chem. 2011, 64, 1118 – 1132; f) G. GuisadoBarrios, J. Bouffard, B. Donnadieu, G. Bertrand, Angew. Chem. 2010, 122, 4869 – 4872; Angew. Chem. Int. Ed. 2010, 49, 4759 – 4762; g) M. Heckenroth, E. Kluser, A. Neels, M. Albrecht, Dalton Trans. 2008, 6242 – 6249. [15] a) H. V. Huynh, G. Frison, J. Org. Chem. 2013, 78, 328 – 338; b) J. C. Bernhammer, G. Frison, H. V. Huynh, Chem. Eur. J. 2013, 19, 12892 – 12905; c) D. Yuan, H. V. Huynh, Organometallics 2012, 31, 405 – 412; d) H. V. Huynh, Y. Han, R. Jothibasu, J. A. Yang, Organometallics 2009, 28, 5395 – 5404; e) D. G. Gusev, Organometallics 2009, 28, 6458 – 6461; f) A. K. Phukan, A. K. Guha, S. Sarmah, R. D. Dewhurst, J. Org. Chem. 2013, 78, 11032 – 11039. [16] a) S. Sinn, B. Schulze, C. Friebe, D. G. Brown, M. Jager, E. Altuntas, J. Kubel, O. Guntner, C. P. Berlinguette, B. Dietzek, U. S. Schubert, Inorg. Chem. 2014, 53, 2083 – 2095; b) A. Bolje, S. Hohloch, D. Urankar, A. Pevec, M. Gazvoda, B. Sarkar, J. Kosˇmrlj, Organometallics 2014, 33, 2588 – 2598; c) V. Leigh, W. Ghattas, R. Lalrempuia, H. Muller-Bunz, M. T. Pryce, M. Albrecht, Inorg. Chem. 2013, 52, 5395 – 5402; d) D. G. Brown, P. A. Schauer, J. Borau-Garcia, B. R. Fancy, C. P. Berlinguette, J. Am. Chem. Soc. 2013, 135, 1692 – 1695; e) D. G. Brown, N. Sanguantrakun, B. Schulze, U. S. Schubert, C. P. Berlinguette, J. Am. Chem. Soc. 2012, 134, 12354 – 12357; f) B. Schulze, D. Escudero, C. Friebe, R. Siebert, H. Gorls, U. Kohn, E. Altuntas, A. Baumgaertel, M. D. Hager, A. Winter, B. Dietzek, J. Popp, L. Gonzalez, U. S. Schubert, Chem. Eur. J. 2011, 17, 5494 – 5498. [17] a) A. Petronilho, J. A. Woods, S. Bernhard, M. Albrecht, Eur. J. Inorg. Chem. 2014, 708 – 714; b) R. Lalrempuia, H. Muller-Bunz, M. Albrecht,

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[18] [19]

[20] [21] [22] [23] [24]

[25] [26] [27] [28]

[29] [30]

[31] [32] [33]

Angew. Chem. 2011, 123, 10144 – 10148; Angew. Chem. Int. Ed. 2011, 50, 9969 – 9972; c) D. B. Grotjahn, D. B. Brown, J. K. Martin, D. C. Marelius, M. C. Abadjian, H. N. Tran, G. Kalyuzhny, K. S. Vecchio, Z. G. Specht, S. A. Cortes-Llamas, V. Miranda-Soto, C. van Niekerk, C. E. Moore, A. L. Rheingold, J. Am. Chem. Soc. 2011, 133, 19024 – 19027; d) Y. Wei, A. Petronilho, H. M ller-Bunz, M. Albrecht, Organometallics 2014, 33, 5834 – 5844. T. Duchanois, T. Etienne, M. Beley, X. Assfeld, E. A. Perp te, A. Monari, P. C. Gros, Eur. J. Inorg. Chem. 2014, 3747 – 3753. a) R. Ghosh, D. K. Palit, Phys. Chem. Chem. Phys. 2014, 16, 219 – 226; b) J. T. Hewitt, P. J. Vallett, N. H. Damrauer, J. Phys. Chem. A 2012, 116, 11536 – 11547; c) W. Henry, C. G. Coates, C. Brady, K. L. Ronayne, P. Matousek, M. Towrie, S. W. Botchway, A. W. Parker, J. G. Vos, W. R. Browne, J. J. McGarvey, J. Phys. Chem. A 2008, 112, 4537 – 4544; d) S. Wallin, J. Davidsson, J. Modin, L. Hammarstrçm, J. Phys. Chem. A 2005, 109, 4697 – 4704; e) G. B. Shaw, D. J. Styers-Barnett, E. Z. Gannon, J. C. Granger, J. M. Papanikolas, J. Phys. Chem. A 2004, 108, 4998 – 5006; f) G. B. Shaw, C. L. Brown, J. M. Papanikolas, J. Phys. Chem. A 2002, 106, 1483 – 1495. F. F. Charron, W. M. Reiff, Inorg. Chem. 1986, 25, 2786 – 2790. J. T. Fletcher, B. J. Bumgarner, N. D. Engels, D. A. Skoglund, Organometallics 2008, 27, 5430 – 5433. W. M. Reiff, B. Dockum, M. A. Weber, R. B. Frankel, Inorg. Chem. 1975, 14, 800 – 806. R.-A. Fallahpour, M. Neuburger, M. Zehnder, New J. Chem. 1999, 23, 53 – 61. a) H. V. Phan, P. Chakraborty, M. Chen, Y. M. Calm, K. Kovnir, L. K. Keniley, Jr., J. M. Hoyt, E. S. Knowles, C. Besnard, M. W. Meisel, A. Hauser, C. Achim, M. Shatruk, Chem. Eur. J. 2012, 18, 15805 – 15815; b) Z. Ni, A. M. McDaniel, M. P. Shores, Chem. Sci. 2010, 1, 615 – 621; c) D. L. Reger, C. A. Little, A. L. Rheingold, R. Sommer, G. J. Long, Inorg. Chim. Acta 2001, 316, 65 – 70. G. Albertin, S. Antoniutti, M. Bortoluzzi, Inorg. Chem. 2004, 43, 1328 – 1335. O. Kaufhold, F. E. Hahn, T. Pape, A. Hepp, J. Organomet. Chem. 2008, 693, 3435 – 3440. B. Liu, Y. Zhang, D. Xu, W. Chen, Chem. Commun. 2011, 47, 2883 – 2885. a) A. Hauser, C. Enachescu, M. L. Daku, A. Vargas, N. Amstutz, Coord. Chem. Rev. 2006, 250, 1642 – 1652; b) S. Dick, Z. Kristallogr. New Cryst. Struct. 1998, 213, 356. M. Nieuwenhuyzen, B. Bertram, J. F. Gallagher, J. G. Vos, Acta Crystallogr. Sect. C 1998, 54, 603 – 606. a) P. S. Braterman, J.-I. Song, R. D. Peacock, Inorg. Chem. 1992, 31, 555 – 559; b) C.-T. Lin, W. Bçttcher, M. Chou, C. Creutz, N. Sutin, J. Am. Chem. Soc. 1976, 98, 6536. M. Yang, D. W. Thompson, G. J. Meyer, Inorg. Chem. 2002, 41, 1254 – 1262. A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser, A. von Zelewsky, Coord. Chem. Rev. 1988, 84, 85 – 277. H. Jacobsen, A. Correa, A. Poater, C. Costabile, L. Cavallo, Coord. Chem. Rev. 2009, 253, 687 – 703.

Chem. Eur. J. 2015, 21, 3628 – 3639

www.chemeurj.org

[34] a) S. U. Son, K. H. Park, Y. S. Lee, B. Y. Kim, C. H. Choi, M. S. Lah, Y. H. Jang, D. J. Jang, Y. K. Chung, Inorg. Chem. 2004, 43, 6896 – 6898; b) L. H. Chung, K. S. Cho, J. England, S. C. Chan, K. Wieghardt, C. Y. Wong, Inorg. Chem. 2013, 52, 9885 – 9896. [35] H. E. Toma, M. S. Takasugi, J. Solution Chem. 1983, 12, 547 – 561. [36] E. M. Kober, B. P. Sullivan, T. J. Meyer, Inorg. Chem. 1984, 23, 2098 – 2104. [37] J. R. Winkler, C. Creutz, N. Sutin, J. Am. Chem. Soc. 1987, 109, 3470 – 3471. [38] K. Gaffney, private communication. [39] C. Creutz, M. Chou, T. L. Netzel, M. Okumura, N. Sutin, J. Am. Chem. Soc. 1980, 102, 1309 – 1319. [40] I. H. van Stokkum, D. S. Larsen, R. van Grondelle, Biochim. Biophys. Acta 2004, 1657, 82 – 104. [41] a) A. T. Yeh, C. V. Shank, J. K. McCusker, Science 2000, 289, 935 – 938; b) R. A. Malone, D. F. Kelley, The Journal of Chemical Physics 1991, 95, 8970 – 8976; c) L. F. Cooley, P. Bergquist, D. F. Kelley, J. Am. Chem. Soc. 1990, 112, 2612 – 2617. [42] A. M. Brown, C. E. McCusker, J. K. McCusker, Dalton Trans. 2014, 43, 17635 – 17646. [43] Gaussian 09, Revision D.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, . Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, and D. J. Fox, Gaussian, Inc., Wallingford CT, 2009. [44] C. Sousa, C. de Graaf, A. Rudavskyi, R. Broer, J. Tatchen, M. Etinski, C. M. Marian, Chem. Eur. J. 2013, 19, 17541 – 17551. [45] M. Ppai, G. Vank, C. de Graaf, T. Rozgonyi, J. Chem. Theory Comput. 2013, 9, 509 – 519. [46] Q. Sun, S. Mosquera-Vazquez, L. M. Daku, L. Guenee, H. A. Goodwin, E. Vauthey, A. Hauser, J. Am. Chem. Soc. 2013, 135, 13660 – 13663. [47] a) I. M. Dixon, F. Alary, M. Boggio-Pasqua, J. L. Heully, Inorg. Chem. 2013, 52, 13369 – 13374; b) A. Marino, P. Chakraborty, M. Servol, M. Lorenc, E. Collet, A. Hauser, Angew. Chem. Int. Ed. 2014, 53, 3863 – 3867; Angew. Chem. 2014, 126, 3944 – 3948. [48] CCDC 1018263 (1) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Received: September 8, 2014 Published online on December 11, 2014

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