Photochemical CâC Bond Formation in Luminescent Zirconium

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

pubs.acs.org/Organometallics

Photochemical C−C Bond Formation in Luminescent Zirconium Complexes with CNN Pincer Ligands Yu Zhang, Jeffrey L. Petersen, and Carsten Milsmann* C. Eugene Bennett Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506, United States

Downloaded via KAOHSIUNG MEDICAL UNIV on August 16, 2018 at 14:07:04 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: The complexes Zr(HCNN)2 and Zr(MeCNN)2 were prepared via reaction of the ligand precursors 2-phenyl-6-(5methyl-3-phenyl-1H-pyrrol-2-yl)pyridine, H2HCNN, and 2-(3,5-dimethyl-phenyl)-6-(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine, H2MeCNN, with tetrabenzyl zirconium. Both complexes are photoluminescent upon excitation with visible light and exhibit remarkably long emission lifetimes of tens to hundreds of microseconds in solution at room temperature. The nature of the emissive state was investigated using density functional theory, which allowed the assignment as a predominantly intraligand triplet state with significant ligand-to-metal charge transfer contributions (3IL/3LMCT). Electrochemical studies revealed two fully reversible one-electron reductions for each complex and an additional reversible oxidative EC process for the more sterically protected complex Zr(MeCNN)2. On the basis of the optical and electrochemical properties, the utility of the two zirconium complexes as photosensitizers for photoredox catalytic transformations was investigated. While Zr(MeCNN)2 readily promotes the photochemical homocoupling of benzyl bromide in the presence of a sacrificial benzimidazolium hydride reductant, Zr(HCNN)2 undergoes an intramolecular photochemical reaction with formation of a new carbon−carbon bond between the phenyl units of the two HCNN ligands.

■

INTRODUCTION

While the well-established MLCT pathway for photoinduced charge separation has been successfully adapted for low-valent isocyanide complexes of group 6 and 7 elements,14−19 the generally electron-deficient nature of early transition metals requires the study of alternative excitation mechanism. Recent reports of long-lived d−d excited states in luminescent CrIII (d3) complexes20−22 and the successful use of emissive f−d states in lanthanide complexes for photocatalysis23−27 highlight the potential of unconventional approaches. Inspired by reports of ligand-to-metal charge transfer (LMCT) as a design principle for the generation of luminescent metallocene complexes of group 3 and 4 elements,28−35 we recently reported the synthesis of a luminescent zirconium complex with strongly π-donating pyridine dipyrrolate (PDP) pincer ligands.36 The complex Zr(MePDP)2 exhibits a remarkably long excited state lifetime of 325 μs and is a competent photosensitizer for reductive photoredox catalytic transformations. Detailed studies estab-

Transition metal photosensitizers play an important role in the development of new technologies for solar energy conversion1−3 and in the discovery of novel photochemical reactions.4−9 For several decades, the field of photoactive metal complexes has primarily been the domain of electronrich late transition metals and particularly precious metals. In combination with strong π acceptor ligands, often derived from polypyridine or phenylpyridine architectures, the central d6, d8, or d10 metal ions exhibit strong metal-to-ligand charge transfer (MLCT) transitions that feature high extinction coefficients and readily undergo intersystem crossing resulting in long-lived triplet excited states.10−12 Motivated by a long-standing interest to develop photoactive metal complexes based on earth-abundant elements,13 early transition metals have become the focus of more recent research efforts. In addition to their higher abundance in the Earth’s crust and the associated lower costs, early transition metals often have the added benefit of decreased toxicity compared to their late transition metal counterparts, which could be particularly attractive for photochemical applications in the production of pharmaceuticals. © XXXX American Chemical Society

Special Issue: Organometallic Complexes of Electropositive Elements for Selective Synthesis Received: June 6, 2018

A

DOI: 10.1021/acs.organomet.8b00388 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

°C resulted in clean formation of Zr(HCNN)2, which was isolated in excellent yield. The synthesis of Zr(MeCNN)2 employing H2MeCNN required more forcing conditions of 130 °C in a pressure tube and provided only a modest yield of the desired homoleptic zirconium complex after workup. The more challenging C−H activation step for H2MeCNN is consistent with the increased steric profile imposed by the additional methyl groups in the ortho-position of the newly formed Zr−C bond. The 1H and 13C NMR spectra of Zr( H CNN) 2 and Zr( Me CNN) 2 are consistent with C 2 symmetric molecules with equivalent pincer ligands in solution. Most prominently, both complexes exhibit a significantly downfield shifted resonance for the ipso-carbon of the Zr-Ph moiety, which can be found at 191.5 and 191.9 ppm for Zr(HCNN)2 and Zr(MeCNN)2, respectively. Both compounds are moisture-sensitive in the solid state and in solution and react with water to yield the ligand precursors and ZrO2. The complexes are stable under an atmosphere of dry oxygen in the absence of light but react with O2 upon irradiation with visible light in solution furnishing dark red solids that have so far eluded characterization due to their insolubility in organic solvents. Single crystals of Zr(HCNN)2·Et2O suitable for crystallographic analysis by X-ray diffraction were obtained by cooling a saturated solution of the complex in diethyl ether to −35 °C. The compound crystallizes in the monoclinic space group C2/c and a representation of the molecular structure of Zr(HCNN)2 is shown in Figure 1. Important bond lengths and angles are

lished photoinduced single electron transfer (SET) as the mechanism for photocatalysis.37 Here we describe the synthesis and characterization of two photoluminescent zirconium complexes with 2-phenyl-6(pyrrol-2-yl)pyridine (CNN) ligands. These ligands can be viewed as a hybrid between the strongly π-accepting 2phenylpyridine (ppy) ligands most prominently employed in the design of luminescent iridium complexes11 and the πdonating PDP ligands featured in our previous studies (Scheme 1). The physical properties of the new complexes Scheme 1. Relationship between Previously Studied Pyridine Dipyrrolate and 2-Phenylpyridine Ligands and the CNN Ligand Framework Used in this Study

were established via a combined experimental and computational approach and the photochemical reactivity was investigated. Minor changes to the substitution pattern of the ligand backbone resulted in a dramatic shift in the primary reactivity of the complexes from photoinduced outersphere electron transfer to photochemical intramolecular C−C bond formation.

■

RESULTS AND DISCUSSION Synthesis and Characterization of the Complexes Zr(HCNN)2 and Zr(MeCNN)2. The two ligand precursors 2phenyl-6-(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine, H2HCNN, and 2-(3,5-dimethyl-phenyl)-6-(5-methyl-3-phenyl1H-pyrrol-2-yl)pyridine, H2MeCNN, were prepared from commercially available starting materials via a straightforward three-step protocol outlined in Scheme 2. The direct reaction of H2HCNN with 0.5 equiv of ZrBn4 in benzene solution at 80 Scheme 2. Synthesis of Zr(HCNN)2 and Zr(MeCNN)2a

Figure 1. Solid-state molecular structure of Zr(HCNN)2 at 30% probability ellipsoids. Hydrogen atoms were omitted for clarity.

summarized in Table 1. The coordination environment around the central zirconium ion is best described as a distorted octahedron. The most significant deviation from idealized octahedral geometry is imposed by the small bite angles of the pincer ligand with Npy−Zr−CPh and Npy−Zr−Npyrrole angles being slightly below 70°. A second, smaller distortion is apparent from the Npy−Zr−Npy angle of 170.50(6)° for the trans-coordinating pyridine units. The two planes defined by the donor atoms of the pincers are nearly perpendicular with an angle of 85.66°, and both ligand backbones exhibit only minor deviations from planarity. While the molecule does not lie on a crystallographic C2 axis, the differences between the two ligands are generally within experimental error and the solid-state structure is consistent with the C2 symmetry in solution observed by NMR spectroscopy.

a Reagents and conditions: (a) Pd(PPh3)4 (3 mol %), Na2CO3 (3 equiv), toluene/methanol/water (2:1:1), reflux; (b) (1) 3-benzyl-5(2-hydroxyethyl)-4-methyl-thiazolium chloride (50 mol %), NaOtBu (50 mol %), EtOH, reflux; (2) NH4OAc, EtOH, reflux; (c) C6H6, 80 °C (R = H) or 130 °C (R = Me).

B


Article

Organometallics

is related to these irreversible oxidations and is absent in CVs with turnaround potentials at 0 V. The peak separation between the two oxidation events is solvent-dependent with slightly larger peak separation and potentials at 0.46 and 0.68 V in THF solution. In both solvents, the reversibility of the reduction events is negatively affected if the CV scans are conducted with an initial sweep direction toward positive potential which highlights the irreversibility of the oxidation processes. On first glance, the redox properties of Zr(MeCNN)2 seem nearly identical to those of Zr(HCNN)2. Two reversible reductions can be detected at −2.26 and −2.63 V indicating that the introduction of electron-donating methyl substituents has only a limited effect on the reduction potentials of the complex compared to the unsubstituted system. In contrast, a significant change is observed upon oxidation of the complex. A single, irreversible oxidation with an anodic peak maximum at 0.51 V is observed for Zr(MeCNN)2. While the oxidation potential is comparable to those for the two events observed for Zr(HCNN)2, this redox process has no negative influence on the two reduction events when scans are run toward positive potentials first. This behavior is consistent with a reversible electrochemical/chemical (EC) process and is reflected in the well-behaved reductive feature with a cathodic peak potential of −1.06 V. While the nature of the reversible chemical process following oxidation of Zr(MeCNN)2 is unknown at this point, it is likely that the steric protection introduced by the methyl substituents confers increased stability to the product compared to Zr(HCNN)2 and enables reversible (electro)chemistry. Optical Properties. The optical properties of Zr(HCNN)2, Zr(MeCNN)2, and their corresponding ligand precursors H2RCNN (R = H, Me) were studied in benzene solution at room temperature. The electronic absorption spectra of all four compounds are shown in Figure 3. Both ligand precursors exhibit a single absorption band in the UV region of the spectrum with a peak maximum at 341 nm (ε = 19 300 M−1 cm−1) and 340 nm (ε = 20 200 M−1 cm−1) for H2HCNN and H2MeCNN, respectively. Deprotonation of the pyrrole ring by addition of 1 equiv of n-BuLi resulted only in a small red-shift of the absorption maximum for both ligand precursors (Figure S15; LiHHCNN: 360 nm; LiHMeCNN: 361 nm). In contrast, both Zr(HCNN)2 and Zr(MeCNN)2 possess a single strong absorption band in the visible region of the spectrum with maxima at 470 nm (ε = 14 300 M−1 cm−1) and 471 nm (ε = 12 400 M−1 cm−1), respectively. An additional absorption band is observed at 325 nm (ε = 25 600 M−1 cm−1) for Zr(HCNN)2. A similar feature for Zr(MeCNN)2 is split into two overlapping signals with maxima at 340 nm (ε = 22 800 M−1 cm−1) and 316 nm (ε = 22,400 M−1 cm−1). Analogous to our previous studies for the closely related complex Zr(MePDP)2,36,37 the occurrence of a new absorption band with high extinction coefficient in the visible region illustrates significant impact of the central zirconium ion on the optical properties of the compounds likely due to ligand-to-metal charge transfer contributions. The steady-state emission spectra obtained upon excitation at the lowest energy absorption maximum for each compound mirror the trends observed for the absorption spectra (Figure 3). The emission profiles for the free ligand precursors are almost indistinguishable with emission maxima at 412 and 410 nm for H2HCNN and H2MeCNN, respectively. Similar to the absorption maxima, deprotonation of the pyrrole rings resulted

Table 1. Selected Bond Lengths (Å) and Angles (deg) for Zr(HCNN)2 Zr(HCNN)2 Zr(1)−N(1) Zr(1)−N(2) Zr(1)−C(15) Zr(1)−N(3) Zr(1)−N(4) Zr(1)−C(37)

2.179(2) 2.330(2) 2.256(2) 2.169(2) 2.345(2) 2.268(2)

N(1)−Zr(1)−N(2) N(1)−Zr(1)−C(15) N(2)−Zr(1)−C(15)

69.70(6) 138.02(7) 69.50(7)

N(3)−Zr(1)−N(4) N(3)−Zr(1)−C(37) N(4)−Zr(1)−C(37)

68.60(6) 137.56(7) 69.85(7)

N(2)−Zr(1)−N(4)

170.50(6)

Electrochemistry. The electrochemical properties of Zr(HCNN)2 and Zr(MeCNN)2 were studied via cyclic voltammetry (CV). All CV experiments were conducted with [N(n-Bu)4]PF6 as the supporting electrolyte (0.1 M) and all potentials were referenced against the Fc+/Fc couple using ferrocene (Fc) as an internal standard. The CVs for the two zirconium complexes measured in 1,2-difluorobenzene solution are shown in Figure 2. Zr(HCNN)2 undergoes two fully reversible redox events at −2.27 and −2.64 V, which were assigned to one-electron reductions of the complex. Two irreversible redox processes are observed with anodic peak potentials of 0.41 and 0.53 V and are likely due to ligandcentered oxidation events. An additional feature around −1 V

Figure 2. Cyclic voltammograms of Zr(HCNN)2 (black, top) and Zr(MeCNN)2 (red, bottom) in 1,2-difluorobenzene (glassy carbon working electrode, 0.1 M [n-Bu4N][PF6], scan rate 200 mV/s, 295 K). C


Article

Organometallics

(MeCNN)2 can be readily fit by a single exponential decay with a lifetime of τ = 412 μs, the data for Zr(HCNN)2 is more complicated and can only be fit satisfactorily using a biexponential approach. This behavior is consistent with two separate decay pathways, in which the majority of the molecules in the excited state decay to the ground state via a faster process with a lifetime of τ1 = 77 μs, while a smaller portion can access a slower process with τ2 = 296 μs. This significant change in excited state lifetime is also reflected in the emission quantum yields of Φ = 0.03 and 0.18 obtained via comparative method for Zr(HCNN)2 and Zr(MeCNN)2, respectively. With experimental values of Φ and τ in hand, the rate constants for radiative decay were determined via kr = Φ/τ as 390 s−1 and 437 s−1 for Zr(HCNN)2 and its methylsubstituted analog, respectively. The similarity between the obtained kr values suggests that the methyl substitutents in Zr(MeCNN)2 exert little influence on the rate of phosphorescent emission but are essential for minimizing nonradiative decay pathways by lowering knr (τ = 1/(kr + knr)). The limited influence of these substituents on the electronic properties established via steady-state absorption and emission spectroscopy implies that this outcome is primarily due to steric effect. To further clarify the role of the central zirconium ion in Zr(RCNN)2, time-resolved emission data for both ligand precursors and their corresponding monolithium salts were collected (Figure S15). All four species exhibit relatively short luminescence lifetimes in the low nanosecond range (H2HCNN: τ = 9 ns; H2MeCNN: τ = 7 ns; LiHHCNN: τ = 4 ns; LiHMeCNN: τ = 5 ns). We note that these values are close to the detection limit of the available instrumentation (∼2 ns). Nevertheless, the recorded values clearly establish that the zirconium ion has a pronounced effect on the nature and properties of the emissive state. On the basis of the dramatic increase in excited state lifetime by several orders of magnitude, it is likely that the zirconium center plays a key-role in intersystem crossing to the triplet manifold. Density Functional Theory. To gain further insight into the frontier molecular orbitals and the resulting optical transitions, ground state and time-dependent density functional theory, (TD-)DFT, calculations were performed for Zr(HCNN)2. All computations were conducted at the B3LYP level using the crystallographically determined geometry of the untruncated complex as the starting point for geometry optimization. The geometric parameters after optimization shown in Table 2 are in excellent agreement with the experimental data and reproduce the approximately C2 symmetric structure observed in the solid state. The largest deviation between corresponding bond distances between the two ligands is observed for the Zr−Npyrrole bonds (2.2 pm). For all remaining bonds, the deviation is less than 1 pm. As is typically observed for DFT calculations,38 the computed zirconium-ligand bond lengths are slightly overestimated by 4−5 pm, while all intraligand bond distances are within 1−2 pm of the experimental values. The optimized structure reproduces the slight deviation from 180° for the Npy−Zr−Npy angle of the trans-coordinating pyridine rings providing a value of 168.4° (exp. 170.5°). The angle between the two planes defined by the three donor atoms of each pincer ligand is also well-reproduced at 87.1° (exp. 85.7°). The electronic absorption spectrum obtained via TD-DFT calculations based on the optimized geometry is shown in Figure 4. The conductor-like screening model (COSMO) was used to

Figure 3. Optical properties of Zr(HCNN)2 (top, black), Zr(MeCNN)2 (bottom, red), and the corresponding ligand precursors H2RCNN (blue) recorded in benzene solution at room temperature. Solid lines represent the UV−vis absorption profiles and dotted lines show the emission spectra obtained upon excitation at the lowest absorption maximum (λex = 470 nm for Zr(HCNN)2 and Zr(MeCNN)2; λex = 341 nm for H2RCNN). The insets show timeresolved emission data detected at 565 nm after excitation at 456 nm.

in a red-shift of the emission maxima (LiHHCNN: 523 nm; LiHMeCNN: 521 nm). The emission profiles for the zirconium species are significantly red-shifted compared to the ligand precursors but are nearly identical between the two complexes with maxima at 562 and 565 nm for Zr(HCNN)2 and Zr(MeCNN)2, respectively. It is again notable that the introduction of electron-donating substituents to the phenyl moieties has a negligible effect on the absorption and emission profiles. Combined with the observations from the electrochemical data this suggests that the phenyl units should have limited contributions to the frontier molecular orbitals involved in the optical transitions as well as outersphere electron transfer. While the absorption and emission profiles are mostly unperturbed by the addition of methyl substituents, the excited state lifetimes reveal dramatic differences between Zr(HCNN)2 and Zr(MeCNN)2. As shown in the insets in Figure 3, both complexes exhibit long-lived emissive states consistent with phosphorescence. The absence of fast decay pathways upon termination of the excitation pulse is consistent with efficient intersystem crossing to a long-lived triplet state in both complexes. While the time-resolved emission data for ZrD


Article

Organometallics Table 2. Selected Bond Distances for the S0 and T1 Geometries of Zr(HCNN)2 Obtained via DFT Calculations

singlet (RKS)a

triplet (UKS3)a

ligand 1

ligand 2

ligand 1

ligand 2

Zr(1)−N(1) Zr(1)−N(2) Zr(1)−C(15)

2.208 2.376 2.287

2.230 2.366 2.290

2.229 2.380 2.299

2.266 2.290 2.286

N(1)−C(1) C(1)−C(2) C(2)−C(3) C(3)−C(4) N(1)−C(4) C(4)−C(5)

1.369 1.384 1.416 1.400 1.390 1.439

1.369 1.387 1.411 1.397 1.392 1.434

1.367 1.384 1.416 1.401 1.390 1.440

1.386 1.403 1.387 1.479 1.371 1.416

N(2)−C(5) C(5)−C(6) C(6)−C(7) C(7)−C(8) C(8)−C(9) N(2)−C(9)

1.359 1.404 1.380 1.393 1.390 1.354

1.358 1.405 1.379 1.395 1.388 1.354

1.360 1.404 1.380 1.392 1.391 1.355

1.397 1.401 1.383 1.405 1.389 1.365

C(9)−C(10) C(10)−C(11) C(11)−C(12) C(12)−C(13) C(13)−C(14) C(14)−C(15) C(10)−C(15)

1.475 1.398 1.389 1.391 1.393 1.399 1.410

1.474 1.397 1.389 1.391 1.394 1.399 1.410

1.476 1.399 1.388 1.391 1.393 1.401 1.411

1.469 1.399 1.388 1.392 1.393 1.400 1.412

Figure 4. Electronic absorption spectrum of Zr(CNN)2 in benzene obtained via TD-DFT calculations (red line, fwhm of 2500 cm−1). The stick plot indicates the positions and relative intensities of individual transitions. The major contributions to each numbered state are listed in Table 3. The experimental spectrum is shown as a dotted black line for comparison.

Table 3. Vertical Electronic Excitation Energies and Main Excitations Contributing to the Absorption Bands of Zr(HCNN)2 Obtained via TD-DFT Calculations TDDFT state

energy/cm−1 (λ/nm)

fosc

excitations (weight)a,b

1

18820.3 (531.3)

0.031

181 → 182 (0.98)

2

20089.5 (497.8)

0.004

3

20268.4 (493.4)

0.062

4

21286.9 (469.8)

0.050

180 181 181 180 180

5

22811.1 (438.4)

0.058

181 → 184 (0.86)

6

23829.7 (419.6)

0.054

180 → 184 (0.86)

→ → → → →

182 183 183 182 183

(0.80) (0.19) (0.77) (0.18) (0.94)

character (% LMCT) 1

IL/1LMCT (26%) 1 IL/1LMCT (26%) 1

IL/1LMCT (20%)

1

IL/1LMCT (20%) 1 IL/1LMCT (30%) 1 IL/1LMCT (30%)

a

Only excitations with a weight larger than 0.1 are shown. bHOMO 181, LUMO 182.

a

No symmetry constraints were applied during geometry optimization.

20% for the LUMO+1 to 30% for the LUMO+2. On the basis of this analysis, the transitions giving rise to the absorption band in the visible region are best described as an intraligand (IL) transition with significant ligand-to-metal charge transfer (LMCT) transitions character of 20−30%. A closer inspection of the frontier MOs also provides an explanation for the limited influence of the substituents on the phenyl rings on the visible absorption profile of Zr(HCNN)2 compared to Zr(MeCNN)2: Neither the donor nor the acceptor orbitals show significant contributions from the π-system of the phenyl rings. The maximum contribution can be found for the LUMO and amounts to less than 10% phenyl character for this orbital. The UV region exhibits a more complex pattern of transitions with a multitude of states giving rise to a single broad absorption feature consistent with the experimental data.

simulate a dielectric medium corresponding to the solvent benzene used during experimental data acquisition.39 The calculated spectrum is in good agreement with the experimental data. Six TD-DFT states contribute to the absorption band observed in the visible region of the spectrum. The main single-electron excitations contributing to each state are listed in Table 3. A molecular orbital diagram depicting the frontier molecular orbitals (HOMO−1 to LUMO+2) involved in these excitations is provided in Figure 5. Consistent with a central zirconium ion in the +IV oxidation state (d0) the doubly occupied donor orbitals (HOMO and HOMO−1) exhibit no contributions from the metal d orbitals and are exclusively ligand centered. The three main acceptor orbitals contain varying amounts of zirconium character ranging from E


Article

Organometallics

Figure 6. Spin density distribution for the lowest energy triplet state of Zr(HCNN)2 obtained via Mulliken population analysis. Figure 5. Frontier molecular orbital diagram showing the donor and acceptor orbitals contributing to the TD-DFT excitations computed in the visible region of the electronic absorption spectrum. The gray numbers to the right correspond to the MO number with 181 being the HOMO and 182 being the LUMO.

Zr(MePDP)2.37 On the basis of these results, we propose that emission from the triplet to the singlet state occurs from a symmetric structure closer to the equilibrium geometry of the singlet ground state in which spin−orbit coupling is maximized due to contributions from a degenerate set of metal d orbitals. According to the computed asymmetric structure of the triplet state, such a transition would likely have to occur from a vibrationally excited state of the triplet state, which could provide an explanation for the long phosphorescence lifetime observed experimentally. More detailed spectroscopic studies supporting this hypothesis are currently underway in our laboratory. In an attempt to gain more insight into the ground state redox properties of Zr(RCNN)2 DFT calculations were also performed for the one-electron reduced and oxidized species [Zr(HCNN)2]1− and [Zr(HCNN)2]1+, respectively. The spin density distributions obtained via Mulliken population analysis for both complexes assuming doublet ground states (S = 1/2) are shown in Figure 7. On the basis of these data, the additional electron in the one-electron reduced complex occupies an orbital that is highly delocalized over both ligand and metal. Hence, the electronic structure of the anion is best described by the two resonance structures [ZrIV(HCNN2−)(HCNN1−•)]1− and [ZrIII(HCNN2−)2]1− with a majority contribution of 65% from the first resonance form. The high degree of delocalization is consistent with the nature of the LUMO in the neutral complex Zr(HCNN)2. Consequently, a clear assignment as ligand- versus metal-centered reduction is not possible. In contrast, the SOMO of the one-electron oxidized species is completely localized on the pincer ligands with major contributions from the pyrrole rings analogous to the HOMO of the neutral compound. The pure organic radical character of the SOMO in [Zr(HCNN)2]1+ explains the increased reactivity of this species reflected in the irreversibility of the one-electron oxidation of Zr(HCNN)2 Photoredox Properties of Zr(HCNN)2 and Zr(MeCNN)2. By combining the information obtained from the electrochemical and steady-state emission measurements, the excited state redox potentials of the two zirconium complexes were estimated. The emission maxima were used as first-order approximations for the energy difference, E0, between the triplet excited state and the ground state. With E0 values of 2.21 and 2.19 eV and ground state redox potentials, E1/2([Zr]/ [Zr]1−), of −2.27 and −2.26 V, the excited state potentials, E1/2([Zr]*/[Zr]1−), were calculated as −0.06 and −0.07 V for Zr(HCNN)2 and Zr(MeCNN)2, respectively, via E1/2([Zr]*/ [Zr]1−) = E1/2([Zr]/[Zr]1−) + E0.40 These potentials are on

A more detailed analysis of the most intense transitions in this area is provided in the Supporting Information. The electronic structure of the emissive triplet state was also explored by DFT calculations. The optimized geometry obtained for the lowest energy triplet state shows a substantial distortion from the close to C2 symmetric structure computed for the singlet ground state (Table 2). The most significant changes are a shortening of the Zr−Npy bond length by almost 10 pm and distortions within the pyridine pyrrole moiety for one of the two pincer ligands. The structural parameters for the second pincer are very similar to those found in the singlet ground state. Among the intraligand distances, the most characteristic difference between the two ligands is found for the C(3)−C(4) bond length (Δ(C(3)−C(4)) = 7.8 pm) in the pyrrole ring and the N(2)−C(5) bond length of the pyridine ring (Δ(N(2)−C(5)) = 3.7 pm). Both bonds are elongated compared to the second pincer ligand of the triplet state and the corresponding bonds in the singlet ground state. Altogether, these structural changes are consistent with a triplet excited state that contains a majority contribution from an intraligand charge transfer (3ILCT) process, in which an electron is transferred from the predominantly pyrrolecentered HOMO of the singlet ground state (C(3)−C(4) bonding) to the mixed pyridine- and zirconium-based LUMO (N(2)−C(5) antibonding, Zr(1)−N(2) bonding). The partial metal character of one of the two resulting SOMOs in the triplet excited state is reflected in the spin density distribution (20% Zr) shown in Figure 6. To computationally approximate the emissive triplet-tosinglet transition responsible for the observed phosphorescence of Zr(HCNN)2 TD-DFT calculations including spinforbidden transitions were performed. Initial calculations were conducted based on the structure obtained for the triplet excited state assuming emission from the triplet following the Franck−Condon principle. To our surprise, the lowest tripletto-singlet transition was predicted to occur at 723 nm, significantly red-shifted with respect to the experimental emission maximum at 562 nm. However, calculations using the same approach based on the more symmetric singlet geometry provided a more accurate value of 568 nm for the lowest triplet-to-singlet transition. A similar situation was observed in our previous study for the closely related complex F


Article

Organometallics

Figure 7. Spin density distributions for [Zr(HCNN)2]1+ (left) and [Zr(HCNN)2]1− (right) obtained via Mulliken population analysis.

par with the excited state redox potential of Zr(MePDP)2, which can act as a photosensitizer for reductive photoredox reactions.36 Notably, the more negative ground state redox potentials of Zr(RCNN)2 compared to Zr(MePDP)2 are offset by the blue-shifted emission profiles, which indicate increased energy storage in the excited state. With the excited state potentials in hand, Zr(HCNN)2 and Zr(MeCNN)2 were tested as photosensitizers in the reductive homocoupling of benzyl bromide to bibenzyl. This wellstudied reaction has been employed by several research groups as a benchmark system for reductive photoredox transformations using various precious metal and earth-abundant photosensitizers.14,23,41 Using our previously reported conditions 37 with 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-7methoxybenzo[d]imidazole, MeOBIH, as the sacrificial reductant and triethylamine as the base the desired coupling product was obtained in 40% yield after irradiation with blue light (LED, λmax = 462 nm) for 20 h in the presence of 5 mol % Zr(MeCNN)2 (Scheme 3). The outcome of the reaction

conversion was detected for the methyl derivative in the same time. Furthermore, the methyl substituted complex Zr(MeCNN)2 reacts only sluggishly in the presence of 10 equiv of benzyl bromide even upon prolonged irradiation with blue light. Analysis of the reaction mixture by 1H NMR spectroscopy after 4 days of irradiation revealed a complex mixture of products in addition to unreacted starting material (Figure S28). In contrast, clean conversion to a new diamagnetic species was observed under similar conditions for Zr(HCNN)2 within 4 h of irradiation on a small scale in an NMR tube (Figure S27). An equimolar amount of bibenzyl was detected as the only byproduct of the reaction. Preparative scale reactions required longer irradiation times of up to 24 h but allowed isolation of the new zirconium species in 64% yield. The most remarkable feature of this complex immediately detectable by 13C NMR spectroscopy is the absence of a downfield shifted signal indicative of a Zr−Ph bond, which suggests photochemical Zr−C bond cleavage. The identity of the zirconium complex Zr(DPDP)Br2 (H4DPDP = 2,2′-bis(6-(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridin-2-yl)biphenyl) was established by X-ray diffraction and a representation of the molecular structure is shown in Figure 8. The structure confirms the absence of a Zr−C bond and reveals C−C bond formation between the two ipso-C atoms of the Zr(HCNN)2 starting material. It is noteworthy that the solid-state molecular structure of Zr(DPDP)Br2 exhibits C1 symmetry, which is inconsistent with the data obtained via NMR spectroscopy in solution that suggests

Scheme 3. Photoredox Catalytic Homocoupling of Benzyl Bromide with Zr(MeCNN)2 as Photosensitizer

changed dramatically when Zr(HCNN)2 was used as the photosensitizer under otherwise identical conditions. While only a small amount of bibenzyl (10% yield) was obtained, 1H NMR spectroscopy indicated complete decomposition of Zr(HCNN)2. Further experiments showed that catalyst decomposition occurred rapidly within only 2 h of irradiation and without consumption of sacrificial reductant but providing a consistent amount of bibenzyl (10%). Inspired by these observations, direct photoreactions between benzyl bromide and Zr(RCNN)2 were investigated. Initial control experiments conducted in C6D6 solution established that both complexes are photostable upon irradiation with blue light (72 h) and that neither zirconium complex reacts directly with benzyl bromide at room temperature in the absence of light. Some reactivity between benzyl bromide and either complex was observed upon prolonged heating to 120 °C, but proceeded with low selectivity yielding several unidentified products. Complete decomposition of the metal complex upon heating in the dark was observed after 2 days for Zr(HCNN)2, while only 20%

Figure 8. Solid-state molecular structure of Zr(DPDP)Br2 at 30% probability ellipsoids. Hydrogen atoms were omitted for clarity. G


Article

Organometallics

process upon visible light excitation that is not available to the more sterically protected complex. However, the long-time photostability of Zr(HCNN)2 established by NMR spectroscopy after prolonged irradiation in the absence of oxidants indicates that this process must be fully reversible. More detailed studies supporting this hypothesis are currently underway in our laboratory.

higher symmetry with equivalent pyrrole and pyridine heterocycles. The appearance of higher symmetry in solution is likely due to a fast dynamic process that renders the heterocycles equivalent on the NMR time scale. While the mechanism of the overall transformation (Scheme 4) is not Scheme 4. Photochemical Reaction of Zr(HCNN)2 with Benzyl Bromide

■

CONCLUDING REMARKS The luminescent early transition metal complexes Zr(HCNN)2 and Zr(MeCNN)2 were successfully synthesized and characterized and their electrochemical and optical properties were established. The complexes exhibit rich electron-transfer chemistry with two reversible single-electron reductions at chemically accessible potentials. The introduction of methyl groups adjacent to the zirconium−carbon bond of the phenyl ring enables an additional reversible EC process in Zr(MeCNN)2. Both compounds can access long-lived excited states upon excitation with visible light resulting in phosphorescent emission with lifetimes in the microsecond range and luminescence quantum yields of 3% for Zr(HCNN)2 and 18% for Zr(MeCNN)2 in benzene solution at room temperature. Density functional theoretical calculations allowed the assignment of the absorption bands in the visible region as predominantly IL transitions with significant LMCT character of 20−30%. Computational studies of the lowest energy triplet state are consistent with a mixed 3IL/3LMCT state and revealed a significant distortion from the C2 symmetric structure of the singlet ground state that likely contributes to the remarkably long emission lifetimes. The absence of methyl substituents adjacent to the Zr−C bond in Zr(HCNN)2 compared to Zr(MeCNN)2 has a pronounced effect on the photochemical reactivity of the complexes. While the predominant mode of reactivity for Zr(MeCNN)2 is photoinduced outersphere electron transfer that enables photoredox catalysis, Zr(HCNN)2 undergoes C− C reductive elimination upon photoexcitation in the presence of mild oxidants resulting in the formation of Zr(DPDP)X2 (X = Br or SBn) complexes. A clear difference in the photophysics between the two complexes is also apparent from time-resolved emission studies that show a simple single-exponential decay for Zr(MeCNN)2 and a more complex biexponential behavior for Zr(HCNN)2.

clear at this point and likely proceeds via single-electron steps, Zr(DPDP)Br2 can be viewed as the product of a formal C−C reductive elimination with concomitant oxidation by two equivalents of benzyl bromide. Formal aryl−aryl reductive eliminations in zirconium complexes have been reported previously for diaryl zirconocene derivatives42−45 and a diphenyl zirconium complex with redox-active ligands.46 However, these examples require either activation by UV light or occur under thermal conditions upon treatment with strong single electron oxidants. To the best of our knowledge, the system presented herein represents the first example for photochemical aryl−aryl bond formation at zirconium using visible light. On the basis of the observed photochemical reactivity of Zr(HCNN)2, dibenzyl disulfide was employed as an alternative mild oxidant to promote oxidative C−C bond formation between the two HCNN ligands. Upon irradiation with blue light, clean conversion to Zr(DPDP)(SBn)2 was observed and the complex was isolated in 75% yield (Scheme 5). Control experiments in the absence of light confirmed the photochemical nature of this transformation as no reaction was observed under dark conditions. Scheme 5. Photochemical Reaction of Zr(HCNN)2 with Dibenzyl Disulfide

■

EXPERIMENTAL SECTION

Materials. All air- and moisture-sensitive manipulations were carried out using standard Schlenk line and cannula techniques or in an MBraun inert atmosphere drybox containing an atmosphere of purified nitrogen. Solvents for air- and moisture-sensitive manipulations were dried and deoxygenated using a Glass Contour Solvent Purification System and stored over 4 Å molecular sieves. Tetrabenzyl zirconium,47 2-phenyl-6-pyridincarboxaldehyde,48−50 and 6-(3,5dimethylphenyl)-2-pyridinecarboxyaldehyde51 were prepared following literature procedures. All solids were dried under high vacuum; all liquids were dried over CaH2 and vacuum transferred into oven-dried glassware in order to bring into the glovebox. Deuterated benzene (C6D6) for NMR spectroscopy was distilled from sodium metal. Physical Measurements. Electronic absorption spectra were recorded using a Shimadzu UV-1800 spectrophotometer in gas-tight quartz cuvettes with a 10 mm path length fitted with J. Young valves. Extinction coefficients are reported with three significant figures. Emission spectra were obtained in 10 mm path length gas-tight quartz cuvettes with J. Young valves using a Shimadzu RF-5301 PC spectrofluorophotometer. Time-resolved emission data were collected

As a final control experiment the reaction between both Zr(RCNN)2 complexes and iodine was explored to address whether the C−C bond formation could be induced in the absence of light by sufficiently strong oxidants. Both compounds react readily but unselectively with I2 providing a complex mixture of products and highlighting the remarkable selectivity of the photoreactivity of Zr(HCNN)2 with mild oxidants. The combination of the photochemical experiments with the data from time-resolved emission spectroscopy (vide supra) provides an interesting starting point for mechanistic considerations. The biexponential decay profile and reduced lifetime for Zr(HCNN)2 compared to Zr(MeCNN)2 is consistent with an additional intramolecular photochemical H


Article

Organometallics using a Horiba Jobin Yvon Fluorolog-3 Spectrofluorometer equipped with a single-photon-counting module in multichannel scaler mode and a 456 nm spectraLED pulsed excitation light source. Emission lifetimes were determined using the provided decay analysis software package, DAS v6.1. 1H and 13C {1H} NMR spectra were recorded on an Agilent 400 MHz spectrometer, JEOL 400 MHz YH spectrometer or a Varian INOVA 600 MHz spectrometer. All chemical shifts are reported relative to SiMe4 using 1H (residual) chemical shifts of the solvent as a secondary standard. To obtain NMR spectra free of solvent molecules present in the crystal lattice, which could overlap with signals from the complex in question, the compounds were first dissolved in a small amount of deuterated solvent, which was then removed under vacuum to coevaporate the cocrystallized solvent molecules. Fresh deuterated solvent was then added to the residue to obtain a (close to) solvent-free NMR spectrum. High-resolution mass spectra were obtained on a Thermo Finnigan Linear Trapping Quadrupole mass spectrometer. Elemental analyses were performed at Robertson Microlit Laboratories, Inc., in Ledgewood, NJ. Cyclic voltammetry measurements were conducted under nitrogen atmosphere inside an MBraun drybox using a Gamry Interface 1000 electrochemical workstation in a single compartment cell using 1 mM sample solutions in THF or 1,2-difluorobenzene with 0.1 M tetrabutylammonium hexafluorophosphate as supporting electrolyte. A three electrode setup was employed with a glassy carbon electrode as working electrode, a platinum sheet as the counter electrode and a silver wire as a quasi-reference electrode. Ferrocene was added as an internal standard after completion of the measurements and all potentials are referenced versus the Fc+/Fc couple. Preparation of 2-Phenyl-6-(5-methyl-3-phenyl-1H-pyrrol-2yl)pyridine, H2HCNN. 2-Phenyl-6-pyridincarboxaldehyde (2.00 g, 10.92 mmol, 1.00 equiv), 4-phenyl-3-buten-2-one (1.76 g, 12.01 mmol, 1.10 equiv), and 3-benzyl-5-(2-hydroxyethyl)-4-methyl-thiazolium chloride (1.47 g, 5.46 mmol, 0.5 equiv) were mixed in a 250 mL Schlenk flask under an argon atmosphere. Then, 20 mL of absolute ethanol were added. A solution of sodium tert-butoxide (525 mg, 5.46 mmol, 0.5 equiv) in ethanol (20 mL) was added via syringe and the mixture was heated to 80 °C for 1 h during which the 1,4-diketone intermediate precipitated. The reaction was cooled to room temperature. The solid was collected via filtration and washed three times with cold ethanol. Then, the 1,4-diketone intermediate was mixed with ammonium acetate (8.41 g, 109.17 mmol, 10.00 equiv) in 10 mL of absolute ethanol and the reaction mixture heated to reflux. After 12 h, the solvent was removed. The resulting residue was diluted with H2O, and the product was extracted with ethyl acetate three times. The combined organic layers were washed with saturated NaHCO3 solution, brine, and dried over Na2SO4. Removal of ethyl acetate and drying under high vacuum yielded the desired product as a brown solid (Yield: 2.456 g, 72%). Mp 112−113 °C. 1H NMR (400 MHz, C6D6; δ, ppm): 9.30 (br, 1H), 8.06 (d, J = 7.2 Hz, 2H), 7.61 (d, J = 7.2 Hz, 2H), 7.40−7.10 (m, 7H), 7.04 (d, J = 7.6 Hz, 1H), 6.89 (t, J = 7.6 Hz, 1H), 6.08 (s, 1H), 1.82 (s, 3H). 13C NMR (101 MHz, C6D6; δ, ppm): 156.89, 151.39, 140.41, 138.59, 136.93, 129.90, 129.34, 129.05, 128.89, 128.89, 127.41, 126.81, 126.31, 126.06, 118.09, 117.10, 111.54, 12.60. HRMS (ESI) calcd for C22H19N2+ [M + H]+ m/z 311.15428. Found 311.15409. Preparation of 2-(3,5-Dimethyl-phenyl)-6-(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine, H2MeCNN. 6-(3,5-Dimethylphenyl)-2pyridinecarboxyaldehyde (2.60 g, 12.31 mmol, 1.00 equiv), 4-phenyl3-buten-2-one (1.89 g, 12.92 mmol, 1.05 equiv), and 3-benzyl-5-(2hydroxyethyl)-4-methyl-thiazolium chloride (1.68 g, 6.15 mmol, 0.5 equiv) were mixed in a 250 mL Schlenk flask under an argon atmosphere. Then, 10 mL of absolute ethanol were added. A solution of sodium tert-butoxide (591 mg, 6.15 mmol, 0.5 equiv) in ethanol was added via syringe, and the mixture was heated to 70 °C for 1 h during which the 1,4-diketone intermediate precipitated. The reaction was cooled to room temperature. The solid was collected via filtration and washed three times with cold ethanol. Then, the 1,4-diketone intermediate was mixed with ammonium acetate (9.49 g, 123.07 mmol, 10.00 equiv) in 10 mL of absolute ethanol, and the reaction mixture was heated to reflux. After 12 h, the solvent was removed, and

the resulting residue was diluted with H2O. The product was extracted with ethyl acetate three times, and the combined organic layers were washed with saturated NaHCO3 solution, brine, and dried over Na2SO4. Removal of the ethyl acetate and drying under high vacuum yielded the desired product as a light brown solid (Yield: 3.02 g, 73%). Mp 148−150 °C. 1H NMR (400 MHz, C6D6; δ, ppm): 9.58 (br, 1H), 7.79 (s, 2H), 7.63 (dm, J = 6.8 Hz, 2H), 7.39 (d, J = 8.0 Hz, 1H), 7.26 (tm, J = 7.6 Hz, 1H), 7.16 (tt, J = 8.0, 1.2 Hz, 1H), 7.07 (d, J = 7.6 Hz, 1H), 6.92 (t, J = 8.0 Hz, 1H), 6.88 (s, 1H), 6.06 (d, J = 3.2 Hz, 1H), 2.22 (s, 6H), 1.77 (s, 3H). 13C NMR (101 MHz, C6D6; δ, ppm): 157.60, 151.52, 140.50, 138.70, 138.16, 136.84, 130.84, 129.91, 129.44, 128.89, 126.76, 126.23, 126.11, 125.54, 118.23, 117.48, 111.48, 21.49, 12.48. HRMS (ESI) calcd for C24H23N2+ [M + H]+ m/ z 339.18558. Found 339.18514. Preparation of Zr(HCNN)2. In the glovebox, a solution of tetrabenzyl zirconium (587 mg, 1.29 mmol, 0.50 equiv) in 3 mL of benzene was added to a 50 mL thick-walled glass vessel containing a solution of H2CNN (800 mg, 2.58 mmol, 1.00 equiv) in 5 mL of benzene. The thick-walled vessel was sealed with a PTFE screw cap and heated to 80 °C overnight, resulting in a brown solution. After cooling to room temperature, the reaction vessel was brought back into the glovebox and the reaction mixture was concentrated in vacuum until a large amount of orange solid precipitated. The solid was collected by filtration, washed three times with Et2O, and dried in vacuum. The product was collected as a light orange solid (Yield: 856 mg, 94%). 1H NMR (400 MHz, C6D6; δ, ppm): 7.73 (d, J = 6.4 Hz, 2H), 7.64 (d, J = 6.8 Hz, 4H), 7.43 (d, J = 8.4 Hz, 2H), 7.34−7.22 (m, 6H), 7.19 (d, J = 7.2 Hz, 2H), 6.91 (t, J = 7.6 Hz, 2H), 6.86−6.77 (m, 4H), 6.73 (d, J = 7.6 Hz, 2H), 5.93 (s, 2H), 2.00 (s, 6H). 13C NMR (101 MHz, C6D6; δ, ppm): 191.45, 164.37, 157.09, 145.99, 141.94, 141.50, 138.09, 135.31, 134.13, 131.39, 129.97, 129.81, 129.09, 128.81, 127.13, 122.65, 116.31, 114.04, 112.10, 14.33. Anal. Calcd for C44H32N4Zr·Et2O: C, 73.71; H, 5.41; N, 7.16. Found: C, 73.70; H, 5.54; N, 7.13. Single crystals suitable for X-ray crystallographic analysis were grown from a saturated solution of Zr(HCNN)2 in Et2O at −35 °C. Preparation of Zr(MeCNN)2. In the glovebox, a solution of tetrabenzyl zirconium (202 mg, 443 mmol, 0.50 equiv) in 3 mL of benzene was added to a 50 mL thick-walled glass vessel containing a solution of H2(MeCNN) (300 mg, 886 mmol, 1.00 equiv) in 5 mL of benzene. The thick-walled vessel was sealed with a PTFE screw cap and heated to 130 °C for 36 h, resulting in a brown solution. After cooling to room temperature, the reaction vessel was brought back into the glovebox and the solvent was removed in vacuum. The crude product was redissolved in 3 mL of Et2O. Then, pentane was added to precipitate the solid product, which was collected by filtration. The resulting solid was washed 10 times with Et2O and dried in vacuum. The product was collected as an orange solid (Yield: 145 mg, 43%). 1 H NMR (400 MHz, C6D6; δ, ppm): 7.62 (d, J = 7.2 Hz, 4H), 7.38 (dd, J = 7.2, 1.6 Hz, 2H), 7.28 (tm, J = 7.2 Hz, 4H), 7.20−7.16 (m, 4H), 6.85−6.77 (m, 4H), 6.45 (s, 2H), 5.81 (s, 2H), 2.42 (s, 6H), 2.08 (s, 6H), 2.00 (s, 6H). 13C NMR (101 MHz, C6D6; δ, ppm): 191.87, 164.62, 156.84, 146.33, 143.57, 141.66, 141.43, 138.35, 138.28, 136.03, 132.55, 130.96, 129.96, 128.78, 127.05, 120.75, 116.03, 114.50, 112.31, 23.02, 21.56, 14.69. Anal. Calcd for C48H40N4Zr·Et2O: C, 74.51; H, 6.01; N, 6.68. Found: C, 74.37; H, 5.59; N, 6.97. Preparation of Zr(DPDP)Br2. Benzyl bromide (169 mg, 989 mmol, 7.00 equiv) was added to a 50 mL thick-walled glass vessel containing a solution of Zr(CNN)2 (100 mg, 141 mmol, 1.00 equiv) in 10 mL of benzene. The thick-walled vessel was sealed with a PTFE screw cap and irradiated with blue LED light (λmax = 462 nm) for 24 h, resulting in a dark red solution. The reaction vessel was brought back into the glovebox, and the solvent was removed in vacuo. After evaporation of benzene, the crude product was recrystallized from THF and Et2O cooled to −35 °C. The analytically pure product was collected as red crystals (Yield: 79 mg, 64%). 1H NMR (400 MHz, C6D6; δ, ppm): 7.48 (d, J = 7.2 Hz, 4H), 7.22−7.10 (m, 10H), 6.90 (t, J = 7.2 Hz, 2H), 6.85 (t, J = 7.2 Hz, 2H), 6.71(d, J = 7.2 Hz, 2H), 6.59 (t, J = 8.4 Hz, 2H), 6.36 (d, J = 6.8 Hz, 2H), 6.02 (s, 2H), 2.34 I


Organometallics

■

(s, 6H). 13C NMR (101 MHz, C6D6; δ, ppm): 158.21, 156.22, 143.47, 140.21, 140.12, 139.43, 137.89, 133.33, 130.82, 130.41, 129.91, 129.86, 128.96, 128.89, 127.47, 127.32, 120.62, 117.42, 115.40, 17.70. Anal. Calcd for C44H32Br2N4Zr: C, 60.90; H, 3.72; N, 6.46. Found: C, 61.10; H, 4.04; N, 6.16. X-ray quality crystals of Zr(DPDP)2Br2 were grown from THF and Et2O solution cooled to −35 °C. Preparation of Zr(DPDP)(SBn)2. Dibenzyl disulfide (88 mg, 356 mmol, 1.20 equiv) was added to a 50 mL thick-walled glass vessel containing a solution of Zr(CNN)2 (210 mg, 297 mmol, 1.00 equiv) in 10 mL of benzene. The thick-walled vessel was sealed with a PTFE screw cap and irradiated with blue LED light (λmax = 462 nm) for 38 h, resulting in a bright orange solution. The reaction vessel was brought back into the glovebox and the solvent was removed in vacuum. After evaporation of benzene, the resulting solid was washed multiple times with Et2O and gave analytically pure product as a light orange solid (Yield: 211 mg, 75%). 1H NMR (400 MHz, C6D6; δ, ppm): 7.61 (d, J = 7.2 Hz, 4H), 7.32 (d, J = 7.2 Hz, 2H), 7.28 (d, J = 7.2 Hz, 4H), 7.23 (t, J = 7.6 Hz, 4H), 7.18−7.11 (m, 4H), 7.02 (t, J = 7.6 Hz, 4H), 6.92 (t, J = 7.6 Hz, 4H), 6.69 (td, J = 7.6, 1.2 Hz, 2H), 6.64 (dd, J = 8.4, 7.6 Hz, 2H), 6.45 (d, J = 7.6 Hz, 2H), 6.30 (d, J = 7.2 Hz, 2H), 6.21 (s, 2H), 4.34 (d, J = 14.0 Hz, 2H), 4.24 (d, J = 14.0 Hz, 2H), 2.40 (s, 6H). 13C NMR (101 MHz, C6D6; δ, ppm): 158.03, 154.99, 144.87, 143.72, 141.45, 139.41, 138.43, 138.32, 133.35, 129.82, 129.73, 129.61, 129.33, 129.17, 129.07, 128.82, 128.59, 127.49, 127.03, 126.22, 119.21, 115.97, 115.86, 40.45, 17.24. Anal. Calcd for C58H46N4S2Zr: C, 72.99; H, 4.86; N, 5.87. Found: C, 72.32; H, 4.98; N, 5.83. Elemental analysis results for this complex showed consistently low values for carbon, which is likely due to the formation of zirconium carbide species during combustion analysis even in the presence of combustion aid. Hydrogen and nitrogen values were reproducible within ±0.2 of the calculated value. Density Functional Theory Calculations. All DFT calculations were performed with the ORCA program package (version 3.0.2).52 Geometry optimizations of the complexes and single-point calculations on the optimized geometries were carried out at the B3LYP level of DFT.53−55 The all-electron Gaussian basis sets were those developed by the Ahlrichs group.56−58 Triple-ζ quality basis sets def2TZVP with one set of polarization functions on the metal and on the atoms directly coordinated to the metal center were used. For the carbon and hydrogen atoms, slightly smaller polarized split-valence def2-SVP basis sets were used that were of double-ζ quality in the valence region and contained a polarizing set of d functions on the non-hydrogen atoms. Auxiliary basis sets to expand the electron density in the resolution-of-the-identity (RIJCOSX) approach59−61 were chosen to match the orbital basis.62−64 The conductor-like screening model (COSMO) was applied to model solvent effects.39 All molecular orbital and spin density plots were generated using the program Gabedit.65

■

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yu Zhang: 0000-0001-8948-1434 Carsten Milsmann: 0000-0002-9249-5199 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS West Virginia University, the Don and Linda Brodie Resource Fund for Innovation, and the National Science Foundation (CHE-1752738) are acknowledged for financial support. This work used X-ray crystallography (CHE-1336071) and NMR (CHE-1228336) equipment funded by the National Science Foundation. The WVU High Performance Computing facilities are funded by the National Science Foundation EPSCoR Research Infrastructure Improvement Cooperative Agreement #1003907, the state of West Virginia (WVEPSCoR via the Higher Education Policy Commission), the WVU Research Corporation, and faculty investments.

■

REFERENCES

(1) Grätzel, M. Dye-Sensitized Solar Cells. J. Photochem. Photobiol., C 2003, 4, 145−153. (2) Nazeeruddin, M. K.; Baranoff, E.; Grätzel, M. Dye-Sensitized Solar Cells: A Brief Overview. Sol. Energy 2011, 85, 1172−1178. (3) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. DyeSensitized Solar Cells. Chem. Rev. 2010, 110, 6595−6663. (4) Tucker, J. W.; Stephenson, C. R. J. Shining Light on Photoredox Catalysis: Theory and Synthetic Applications. J. Org. Chem. 2012, 77, 1617−1622. (5) Xuan, J.; Xiao, W.-J. Visible-Light Photoredox Catalysis. Angew. Chem., Int. Ed. 2012, 51, 6828−6838. (6) Zeitler, K. Photoredox Catalysis with Visible Light. Angew. Chem., Int. Ed. 2009, 48, 9785−9789. (7) Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis. Chem. Rev. 2013, 113, 5322−5363. (8) Lang, X.; Zhao, J.; Chen, X. Cooperative Photoredox Catalysis. Chem. Soc. Rev. 2016, 45, 3026−3038. (9) Yoon, T. P.; Ischay, M. A.; Du, J. Visible Light Photocatalysis as a Greener Approach to Photochemical Synthesis. Nat. Chem. 2010, 2, 527−532. (10) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; von Zelewsky, A. Ru(II) Polypyridine Complexes: Photophysics, Photochemistry, Electrochemistry, and Chemiluminescence. Coord. Chem. Rev. 1988, 84, 85−277. (11) Dixon, I. M.; Collin, J.-P.; Sauvage, J.-P.; Flamigni, L.; Encinas, S.; Barigelletti, F. A Family of Luminescent Coordination Compounds: Iridium(III) Polyimine Complexes. Chem. Soc. Rev. 2000, 29, 385−391. (12) Mara, M. W.; Fransted, K. A.; Chen, L. X. Interplays of Excited State Structures and Dynamics in Copper(I) Diimine Complexes: Implications and Perspectives. Coord. Chem. Rev. 2015, 282−283, 2− 18. (13) Larsen, C. B.; Wenger, O. S. Photoredox Catalysis with Metal Complexes Made from Earth-Abundant Elements. Chem. - Eur. J. 2018, 24, 2039−2058. (14) Büldt, L. A.; Guo, X.; Vogel, R.; Prescimone, A.; Wenger, O. S. A Tris(Diisocyanide)Chromium(0) Complex Is a Luminescent Analog of Fe(2,2’-Bipyridine)32+. J. Am. Chem. Soc. 2017, 139, 985− 992. (15) Büldt, L. A.; Guo, X.; Prescimone, A.; Wenger, O. S. A Molybdenum(0) Isocyanide Analogue of Ru(2,2′-Bipyridine)32+: A

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00388. Additional experimental procedures, spectroscopic and electrochemical data, , and computational details (DOCX) Crystallographic data for Zr(HCNN)2·Et2O and Zr(DPDP)Br2·2Et2O (XYZ) Accession Codes

CCDC 1847795−1847796 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. J


Article

Organometallics Strong Reductant for Photoredox Catalysis. Angew. Chem., Int. Ed. 2016, 55, 11247−11250. (16) Büldt, L. A.; Wenger, O. S. Chromium(0), Molybdenum(0), and Tungsten(0) Isocyanide Complexes as Luminophores and Photosensitizers with Long-Lived Excited States. Angew. Chem., Int. Ed. 2017, 56, 5676−5682. (17) Larsen, C. B.; Wenger, O. S. Photophysics and Photoredox Catalysis of a Homoleptic Rhenium(I) Tris(Diisocyanide) Complex. Inorg. Chem. 2018, 57, 2965−2968. (18) Sattler, W.; Henling, L. M.; Winkler, J. R.; Gray, H. B. Bespoke Photoreductants: Tungsten Arylisocyanides. J. Am. Chem. Soc. 2015, 137, 1198−1205. (19) Sattler, W.; Ener, M. E.; Blakemore, J. D.; Rachford, A. A.; Labeaume, P. J.; Thackeray, J. W.; Cameron, J. F.; Winkler, J. R.; Gray, H. B. Generation of Powerful Tungsten Reductants by Visible Light Excitation. J. Am. Chem. Soc. 2013, 135, 10614−10617. (20) Otto, S.; Grabolle, M.; Förster, C.; Kreitner, C.; Resch-Genger, U.; Heinze, K. [Cr(Ddpd)2]3+: A Molecular, Water-Soluble, Highly NIR-Emissive Ruby Analogue. Angew. Chem., Int. Ed. 2015, 54, 11572−11576. (21) Stevenson, S. M.; Shores, M. P.; Ferreira, E. M. Photooxidizing Chromium Catalysts for Promoting Radical Cation Cycloadditions. Angew. Chem., Int. Ed. 2015, 54, 6506−6510. (22) Barbour, J. C.; Kim, A. J. I.; DeVries, E.; Shaner, S. E.; Lovaasen, B. M. Chromium(III) Bis-Arylterpyridyl Complexes with Enhanced Visible Absorption via Incorporation of Intraligand ChargeTransfer Transitions. Inorg. Chem. 2017, 56, 8212−8222. (23) Yin, H.; Carroll, P. J.; Anna, J. M.; Schelter, E. J. Luminescent Ce(III) Complexes as Stoichiometric and Catalytic Photoreductants for Halogen Atom Abstraction Reactions. J. Am. Chem. Soc. 2015, 137, 9234−9237. (24) Yin, H.; Carroll, P. J.; Manor, B. C.; Anna, J. M.; Schelter, E. J. Cerium Photosensitizers: Structure−Function Relationships and Applications in Photocatalytic Aryl Coupling Reactions. J. Am. Chem. Soc. 2016, 138, 5984−5993. (25) Yin, H.; Jin, Y.; Hertzog, J. E.; Mullane, K. C.; Carroll, P. J.; Manor, B. C.; Anna, J. M.; Schelter, E. J. The Hexachlorocerate(III) Anion: A Potent, Benchtop Stable, and Readily Available Ultraviolet A Photosensitizer for Aryl Chlorides. J. Am. Chem. Soc. 2016, 138, 16266−16273. (26) Qiao, Y.; Sergentu, D.-C.; Yin, H.; Zabula, A. V.; Cheisson, T.; McSkimming, A.; Manor, B. C.; Carroll, P. J.; Anna, J. M.; Autschbach, J.; Schelter, E. J. Understanding and Controlling the Emission Brightness and Color of Molecular Cerium Luminophores. J. Am. Chem. Soc. 2018, 140, 4588−4595. (27) Guo, J. J.; Hu, A.; Chen, Y.; Sun, J.; Tang, H.; Zuo, Z. Photocatalytic C−C Bond Cleavage and Amination of Cycloalkanols by Cerium(III) Chloride Complex. Angew. Chem., Int. Ed. 2016, 55, 15319−15322. (28) Pfennig, B. W.; Thompson, M. E.; Bocarsly, A. B. A New Class of Room Temperature Luminescent Organometallic Complexes: Luminescence and Photophysical Properties of Permethylscandocene Chloride in Fluid Solution. J. Am. Chem. Soc. 1989, 111, 8947−8948. (29) Pfennig, B. W.; Thompson, M. E.; Bocarsly, A. B. Luminescent d0 Scandocene Complexes: Photophysical Studies and Electronic Structure Calculations on Cp*2ScX (X = Cl, I, Me). Organometallics 1993, 12, 649−655. (30) Loukova, G. V.; Vasiliev, V. P.; Milov, A. A.; Smirnov, V. A.; Minkin, V. I. Unraveling Electronic Properties of an Organometallic Solute: Lippert-Mataga and Quantum-Chemical Extensive Study. J. Photochem. Photobiol., A 2016, 327, 6−14. (31) Loukova, G. V.; Smirnov, V. A. Phosphorescent Ligand-toMetal Charge-Transfer Excited States in the Group IVB Metallocene Triad. Chem. Phys. Lett. 2000, 329, 437−442. (32) Loukova, G. V.; Starodubova, S. E.; Smirnov, V. A. Triplet Energy Transfer Insight in Coordination of Unsaturated Hydrocarbons by d0 Bent Metallocenes (Zr, Hf). J. Phys. Chem. A 2007, 111, 10928−10937.

(33) Loukova, G. V. The First Experimental Approach to Probing Frontier Orbitals and HOMO-LUMO Gap in Bent Metallocenes. Chem. Phys. Lett. 2002, 353, 244−252. (34) Loukova, G. V.; Huhn, W.; Vasiliev, V. P.; Smirnov, V. A. Ligand-to-Metal Charge Transfer Excited States with Unprecedented Luminescence Yield in Fluid Solution. J. Phys. Chem. A 2007, 111, 4117−4121. (35) Pritchard, V. E.; Thorp-Greenwood, F. L.; Balasingham, R. G.; Williams, C. F.; Kariuki, B. M.; Platts, J. A.; Hallett, A. J.; Coogan, M. P. Simple Polyphenyl Zirconium and Hafnium Metallocene RoomTemperature Lumophores for Cell Imaging. Organometallics 2013, 32, 3566−3569. (36) Zhang, Y.; Petersen, J. L.; Milsmann, C. A Luminescent Zirconium(IV) Complex as a Molecular Photosensitizer for Visible Light Photoredox Catalysis. J. Am. Chem. Soc. 2016, 138, 13115− 13118. (37) Zhang, Y.; Lee, T. S.; Petersen, J. L.; Milsmann, C. A Zirconium Photosensitizer with a Long-Lived Excited State: Mechanistic Insight into Photo-Induced Single Electron Transfer. J. Am. Chem. Soc. 2018, 140, 5934−5947. (38) Neese, F. A Critical Evaluation of DFT, Including TimeDependent DFT, Applied to Bioinorganic Chemistry. J. Biol. Inorg. Chem. 2006, 11, 702−711. (39) Klamt, A.; Schüürmann, G. COSMO: A New Approach to Dielectric Screening in Solvents with Explicit Expressions for the Screening Energy and Its Gradient. J. Chem. Soc., Perkin Trans. 2 1993, 799−805. (40) Arias-Rotondo, D. M.; McCusker, J. K. The Photophysics of Photoredox Catalysis: A Roadmap for Catalyst Design. Chem. Soc. Rev. 2016, 45, 5803−5820. (41) Park, G.; Yi, S. Y.; Jung, J.; Cho, E. J.; You, Y. Mechanism and Applications of the Photoredox Catalytic Coupling of Benzyl Bromides. Chem. - Eur. J. 2016, 22, 17790−17799. (42) Erker, G. The Reaction of Intermediate Zirconocene-Aryne Complexes with C-H Bonds in the Thermolysis of Diarylzirconocenes. J. Organomet. Chem. 1977, 134, 189−202. (43) Margulieux, G. W.; Semproni, S. P.; Chirik, P. J. Photochemically Induced Reductive Elimination as a Route to a Zirconocene Complex with a Strongly Activated N2 Ligand. Angew. Chem., Int. Ed. 2014, 53, 9189−9192. (44) Erker, G.; Kropp, K.; Atwook, J. L.; Hunter, W. E. Reactions of Vinylzirconocene Complexes with a Zirconium Hydride. The Unexpected Formation of [μ-(β-η1:α,β-η2-Styryl)](μ-Chloro)Bis[Bis(η5-Cyclopentadienyl)Zirconium], a Binuclear (Olefin)Zirconocene Complex. Organometallics 1983, 2, 1555−1561. (45) Tung, H.-S.; Brubaker, C. H., Jr. Photochemical Decomposition of (Diphenyl)Bis(η5-Cyclopentadienyl)Titanium, (Diphenyl)Bis(η5Pentamethylcyclopentadienyl)Titanium and the Zirconium Analogs. Inorg. Chim. Acta 1981, 52, 197−204. (46) Haneline, M. R.; Heyduk, A. F. C-C Bond-Forming Reductive Elimination from a Zirconium(IV) Redox-Active Ligand Complex. J. Am. Chem. Soc. 2006, 128, 8410−8411. (47) Radlauer, M. R.; Agapie, T. Bimetallic Zirconium Amine Bis(Phenolate) Polymerization Catalysts: Enhanced Activity and Tacticity Control for Polyolefin Synthesis. Organometallics 2014, 33, 3247−3250. (48) Holub, J.; Vantomme, G.; Lehn, J. M. Training a Constitutional Dynamic Network for Effector Recognition: Storage, Recall, and Erasing of Information. J. Am. Chem. Soc. 2016, 138, 11783−11791. (49) Morales-Cerón, J. P.; Lara, P.; López-Serrano, J.; Santos, L. L.; Salazar, V.; Á lvarez, E.; Suárez, A. Rhodium(I) Complexes with Ligands Based on N-Heterocyclic Carbene and Hemilabile Pyridine Donors as Highly E Stereoselective Alkyne Hydrosilylation Catalysts. Organometallics 2017, 36, 2460−2469. (50) Chuang, C. L.; Lim, K.; Chen, Q.; Zubieta, J.; Canary, J. W. Synthesis, Cyclic Voltammetry, and X-Ray Crystal Structures of Copper(I) and Copper(II) Complexes of Tris((6-Phenyl-2-Pyridyl)Methyl)Amine (TPPA). Inorg. Chem. 1995, 34, 2562−2568. K


Article

Organometallics (51) Makowska-Grzyska, M. M.; Szajna, E.; Shipley, C.; Arif, A. M.; Mitchell, M. H.; Halfen, J. A.; Berreau, L. M. First Row Divalent Transition Metal Complexes of Aryl-Appended Tris((Pyridyl)Methyl)Amine Ligands: Syntheses, Structures, Electrochemistry, and Hydroxamate Binding Properties. Inorg. Chem. 2003, 42, 7472−7488. (52) Neese, F. The ORCA Program System. Wiley Interdisciplinary Reviews: Computational Molecular Science 2012, 2, 73−78. (53) Becke, A. D. Density Functional Calculations of Molecular Bond Energies. J. Chem. Phys. 1986, 84, 4524−4529. (54) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (55) Lee, C.; Yang, W.; Parr, R. G. Development of the ColleSalvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785− 789. (56) Schäfer, A.; Horn, H.; Ahlrichs, R. Fully Optimized Contracted Gaussian Basis Sets for Atoms Li to Kr. J. Chem. Phys. 1992, 97, 2571−2577. (57) Schäfer, A.; Huber, C.; Ahlrichs, R. Fully Optimized Contracted Gaussian Basis Sets of Triple Zeta Valence Quality for Atoms Li to Kr. J. Chem. Phys. 1994, 100, 5829−5835. (58) Weigend, F.; Ahlrichs, R. Balanced Basis Sets of Split Valence, Triple Zeta Valence and Quadruple Zeta Valence Quality for H to Rn: Design and Assessment of Accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297−3305. (59) Neese, F.; Wennmohs, F.; Hansen, A.; Becker, U. Efficient, Approximate and Parallel Hartree-Fock and Hybrid DFT Calculations. A ‘chain-of-Spheres’ Algorithm for the Hartree-Fock Exchange. Chem. Phys. 2009, 356, 98−109. (60) Kossmann, S.; Neese, F. Comparison of Two Efficient Approximate Hartee-Fock Approaches. Chem. Phys. Lett. 2009, 481, 240−243. (61) Neese, F. An Improvement of the Resolution of the Identity Approximation for the Formation of the Coulomb Matrix. J. Comput. Chem. 2003, 24, 1740−1747. (62) Eichkorn, K.; Treutler, O.; Ö hm, H.; Häser, M.; Ahlrichs, R. Auxiliary Basis Sets to Approximate Coulomb Potentials. Chem. Phys. Lett. 1995, 240, 283−289. (63) Eichkorn, K.; Treutler, O.; Ö hm, H.; Häser, M.; Ahlrichs, R. Erratum: Auxiliary Basis Sets to Approximate Coulomb Potentials (Chem. Phys. Letters 240 (1995) 283). Chem. Phys. Lett. 1995, 242, 652−660. (64) Eichkorn, K.; Weigend, F.; Treutler, O.; Ahlrichs, R. Auxiliary Basis Sets for Main Row Atoms and Transition Metals and Their Use to Approximate Coulomb Potentials. Theor. Chem. Acc. 1997, 97, 119−124. (65) Allouche, A.-R. Software News and Updates GabeditA Graphical User Interface for Computational Chemistry Softwares. J. Comput. Chem. 2011, 32, 174−182.

L


Photochemical CâC Bond Formation in Luminescent Zirconium

Recommend Documents

Photochemical CâC Bond Formation in Luminescent Zirconium