Thorium Metallocene Cation Chemistry: Synthesis and

Article Cite This: Organometallics 2018, 37, 454−458

Thorium Metallocene Cation Chemistry: Synthesis and Characterization of the Bent [(C5Me5)2Th(C6H5)(THF)][BPh4] and the Parallel Ring [(C5Me5)2Th(NCR)5][BPh4]2 (R = Me, Ph) Complexes Ryan R. Langeslay, Cory J. Windorff, Megan T. Dumas, Joseph W. Ziller, and William J. Evans* Department of Chemistry, University of California, Irvine, California 92697-2025, United States S Supporting Information *

ABSTRACT: Attempts to synthesize the base-free dication [(C 5 Me 5 ) 2 Th] 2+ by reaction of the bis(allyl) complex (C5Me5)2Th(C3H5)2 with 2 equiv of [Et3NH][BPh4] in benzene yielded a cationic phenyl complex that, in the presence of THF, crystallized from toluene as [(C5Me5)2Th(C6H5)(THF)][BPh4]. The reaction of the dimethyl complex (C5Me5)2ThMe2 with [Et3NH][BPh4] in toluene in the presence of nitriles RCN generates cations of the formula [(C5Me5)2Th(NCR)5][BPh4]2 (R = Me, Ph) in 40−55% crystalline yield. The molecular structures reveal the first examples of thorium cyclopentadienyl metallocene complexes with parallel rings.

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INTRODUCTION Recent studies of the pentamethylcyclopentadienyl chemistry of thorium have yielded the cationic complex [(C5Me5)3Th][BPh4], which is unusual in many respects. It is the first (C5Me5)3M complex (M = rare-earth metal or actinide) that does not ring-open THF, a universal reaction for these sterically crowded complexes.1−9 Instead, this complex forms a base adduct with THF, [(C5Me5)3Th(THF)][BPh4], which is the first (C5Me5)3M complex that does not have the three (C5Me5)− ring centroids in a trigonal-planar geometry. [(C5Me5)3Th][BPh4] also forms the first molecular thorium carbonyl complex, [(C5Me5)3Th(CO)][BPh4].10 Given the unusual nature of this pentamethylcyclopentadienyl thorium cation, exploration of other pentamethylcyclopentadienyl thorium cations was pursued. The methyl metallocene cation [(C5Me5)2ThMe][BPh4] had been known since 1987.11 However, to our knowledge no [(C5Me5)2Th]2+ cations have been reported in the literature. We previously had experience with the lanthanide metallocene cations [(C5Me5)2Ln][(μ-Ph)2BPh2],12−15 which are excellent precursors to many classes of organolanthanide complexes due to the weakly coordinating [(μ-Ph)2BPh2)]− counteranion. Accordingly, we sought to synthesize a Th4+ analogue. A decade ago Ephritikhine and co-workers made uranium metallocene dications in acetonitrile that were isolated with five coordinated nitriles, [(C5Me5)2U(NCMe)5][X]2 X = I−, OTf−, (BPh4)− (OTf = OSO2CF3)16−18 (Scheme 1). These were remarkable complexes in that the rings were parallel: previously, all f-element cyclopentadienyl metallocenes had a bent (ring centroid)−M−(ring centroid) geometry. Only the bis(cyclooctatetraenyl) complexes of the actinides19 and [K(18crown-6)][U(η7-C7H7)2]20 had parallel rings. These parallel ring actinide complexes provided an interesting contrast with © 2018 American Chemical Society

transition-metal metallocenes. With transition metals, all of the initially isolated examples were the parallel ring analogues of ferrocene, (C5H5)2Fe, and bis(benzene)chromium, (C6H6)2Cr, and the bent transition-metal metallocenes were discovered later.21 Interestingly, parallel plane uranium metallocenes were obtained with cationic complexes of U4+ (vide supra), but not for cationic complexes of U3+ such as [(C5Me5)2U(NCMe)3][BPh4], which has a bent metallocene structure.17,18 Attempts to synthesize parallel ring metallocenes with +3 lanthanide ions suggested the importance of oxidation state, since these reactions gave only bent metallocene cations, [(C5Me5)2Ln(NCR)3][BPh4] (Ln = La, Gd, Y; R = Me, tBu, Ph).22 To our knowledge, no thorium analogues of the [(C5Me5)2U(NCMe)5][X]2 complexes have been reported. Our efforts in this area have led to the synthesis of the unusual cationic phenyl complex [(C5Me5)2Th(C6H5)(THF)][BPh4] and the isolation of the nitrile-solvated cations [(C5Me5)2Th(NCR)5][BPh4]2 (R = Me, Ph), which have parallel cyclopentadienyl rings.

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EXPERIMENTAL DETAILS

All manipulations and syntheses described below were conducted with the rigorous exclusion of air and water using standard Schlenk, highvacuum-line, and glovebox techniques under an argon atmosphere. Solvents were sparged with UHP argon and dried by passage through columns containing Q-5 and molecular sieves prior to use. C6D6 and THF-d8 were dried over sodium benzophenone ketyl, degassed by three freeze−pump−thaw cycles, and vacuum-transferred before use. MeCN-d3 and pyridine-d5 were dried over molecular sieves and Received: November 28, 2017 Published: January 31, 2018 454

DOI: 10.1021/acs.organomet.7b00855 Organometallics 2018, 37, 454−458

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Organometallics Scheme 1. Formation of [(C5Me5)2U(NCMe)5][X]2 (X = I−, OTf−, (BPh4)−)18

degassed by three freeze−pump−thaw cycles before use. 1H and 13 C{1H} NMR spectra were recorded on Bruker GN500 or CRYO500 spectrometers operating at 500 and 125 MHz, respectively, at 298 K unless otherwise noted, and the spectra were referenced internally to residual protio-solvent resonances. FT-IR samples were prepared as KBr pellets and analyzed using a Jasco 4700 FT-IR spectrometer. Elemental analyses were conducted on a PerkinElmer 2400 Series II CHNS elemental analyzer, and yields reported are on isolated crystalline samples. PhCN (Aldrich) was dried over molecular sieves and degassed by three freeze−pump−thaw cycles before use. (C5Me5)2ThMe2,23 (C5Me5)2Th(C3H5)2,1 and [Et3NH][BPh4]24 were prepared according to the literature. [(C5Me5)2Th(C6H5)(THF)][BPh4] (1). In a glovebox containing noncoordinating solvents, [Et3NH][BPh4] (28 mg, 0.066 mmol) was added to a stirred pale yellow solution of (C5Me5)2Th(C3H5)2 (19 mg, 0.032 mmol) in benzene (1.5 mL). The resulting pale yellow slurry was stirred overnight before being filtered. Toluene (0.2 mL) was added to aid in solvent removal, and the mixture was dried under reduced pressure. This reaction proved to give variable results, and on the basis of the solubility of the products in arenes, it did not appear to form the desired metallocene dication. However, in one case, the 1H NMR spectrum of the crude product in C6D6 contained a single main C5Me5 resonance at δ 1.46 ppm (see the Supporting Information for a crude reaction spectrum) and was examined further. The resulting material was washed with hexane, dissolved in THF, and dried under reduced pressure. A small amount of X-ray-quality crystals of 1 (approximately 5 mg, 18%) were grown from toluene at −30 °C. [(C5Me5)2Th(NCMe)5][BPh4]2 (2). In a glovebox, solid [Et3NH][BPh4] (975 mg, 2.29 mmol) was added to a concentrated mixture of (C5Me5)2ThMe2 (607 mg, 1.24 mmol) in toluene (8 mL) and acetonitrile (3 mL). The solution was stirred for 45 min before being dried under reduced pressure to yield pale yellow solids that were washed with THF (4 × 10 mL) and dried to give 2 as a white solid (711 mg, 46%). X-ray-quality crystals were grown from vapor diffusion of an Et2O solution into a concentrated MeCN solution of 2 at ambient temperature (2-RT) and at −15 °C (2-cold). 1H NMR (MeCN-d3): δ 7.30 (m, 16H, o-BPh4), 7.02 (t, 3JHH = 7 Hz, 16H, mBPh4), 6.87 (t, 3JHH = 7 Hz, 8H, p-BPh4), 2.00 (s, 30H, C5Me5), 1.97 (s, 15H, MeCN). 13C{1H} NMR (MeCN-d3): δ 136.7 (o-BPh4), 127.5 (C5Me5), 126.6 (m-BPh4), 122.8 (p-BPh4), 13.3 (C5Me5). No 13C signals were observed for the bound acetonitrile moieties due to exchange with the solvent. FT-IR: 3345w, 3219w, 3191w, 3159w, 3125w, 3060s, 2989s, 2920s, 2728w, 2306m, 2275s [νCN], 2184w, 1945w, 1882w, 1822w, 1763, 1639w, 1588m, 1487s, 1435s, 1370m, 1274w, 1190w, 1152w, 1073m, 1040m, 939w, 915w, 852m, 741s, 712s, 616m cm−1. Anal. Calcd for C78H85B2N5Th: C, 69.59; H, 6.36; N, 5.20. Found: C, 69.64; H, 6.56; N, 5.04. [(C5Me5)2Th(NCPh)5][BPh4]2 (3). In a glovebox, (C5Me5)2ThMe2 (120 mg, 0.225 mmol) was dissolved in toluene (5 mL) to form a pale yellow solution to which benzonitrile (3 mL) was added, causing the solution to turn orange. Solid [Et3NH][BPh4] (192 mg, 0.451 mmol) was added to the reaction mixture, causing the solution to become pale yellow again. The solution was stirred for 5 h before removing the toluene under reduced pressure followed by addition of hexane (100 mL), causing the product to separate from the excess benzonitrile as a yellow oil. The yellow oil was dissolved in THF (18 mL) and dried

under reduced pressure to yield 3 as a yellow solid (205 mg, 55%). Yellow needles suitable for X-ray diffraction were obtained via vapor diffusion of pentane into a concentrated THF solution of 3 at room temperature. 1H NMR (pyridine-d5): δ 8.13 (br s, 5H, p-NCPh), 7.95 (d, 3JHH = 5 Hz, 10H, o-NCPh), 7.34 (t, 3JHH = 5 Hz, 10H, m-NCPh), 7.29 (t, 3JHH = 7 Hz, 16H, o-BPh4), 7.19 (t, 3JHH = 5 Hz, 16H, mBPh4), 7.16 (t, 3JHH = 10 Hz, 8H, p-BPh4), 2.15 (s, C5Me5). 1H NMR (THF-d8): δ 7.32 (d, 3JHH = 5 Hz, 5H, p-NCPh), 7.16 (m, 3JHH = 7 Hz, 10 H, o-NCPh), 6.98 (br s, 16H, o-BPh4), 6.54 (t, 3JHH = 7 Hz, 16H, m-BPh4), 6.40, (t, 3JHH = 7 Hz, 8H, p-BPh4), 6.32 (br s, 10 H, mNCPh), 1.39 (s, 30 H, C5Me5). FT-IR: 3052m, 3034m, 2996m, 2980m, 2964m, 2922m, 2862m, 2237s [νCN], 1591m, 1579m, 1480m, 1446m, 1426m, 1262w, 1175w, 1062w, 1030w, 1022w, 998w, 839w, 755m, 731m, 704s, 680m, 611m, 556m cm−1. Anal. Calcd for C103H95B2N5Th·5THF: C, 74.19; H, 6.11; N, 3.87. Found: C, 73.85; H, 5.66; N, 4.13. X-ray Data Collection, Structure Determination, and Refinement. Crystallographic details for compounds [(C5Me5)2Th(C6H5)(THF)][BPh4] (1), [(C5Me5)2Th(NCMe)5][BPh4]2 (2-RT and 2cold), and [(C5Me5)2Th(NCPh)5][BPh4]2 (3) are summarized in the Supporting Information.

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RESULTS AND DISCUSSION Isolation of a Cationic Phenyl Complex. Isolation of a dicationic compound of thorium with the general formula [(C5Me5)2Th][BPh4]2 was attempted by reaction of 2 equiv of [ Et 3 NH][BPh 4 ] w it h (C 5 Me 5 ) 2 Th(C 3 H 5 ) 2 , 2 5 and (C5Me5)2ThMe2.23 The reactions proved to be complicated with variable results, and no evidence for the formation of a dication was observed, since the products were arene soluble. However, in one reaction of [Et3NH][BPh4] with (C5Me5)2Th(C3H5)2, 1H NMR evidence for a phenyl complex was observed. After dissolution of the reaction mixture in THF and crystallization from toluene, the phenyl complex [(C5Me5)2Th(C6H5)(THF)][BPh4], (1) could be isolated and crystallographically characterized (Figure 1). This reaction does not reproducibly form 1 as the major product and several other unidentified products of similar solubility are also produced. We include the structure here to demonstrate that this type of cation can form in [Et3NH][BPh4] reactions. It is likely that the phenyl ligand in 1 arises from the (BPh4)− counteranion, as has been observed in other cases.26 Although numerous thorium phenyl complexes have been reported in the literature,23,27,28 complex 1 is only the third example of a crystallographically characterizable thorium−phenyl compound: previously, only the structures of the perphenyl complexes [Li(Lx)]2[Th(C6H5)6] (Lx = (DME)3, (12-crown4)(THF))29 have been reported. The 2.408(5) and 2.416(5) Å Th−Cipso bond lengths from the two crystallographically independent units are shorter than the 2.553(3)−2.648(3) Å lengths for the analogous perphenyl compounds.29 455

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[BPh4]2, solutions of 2 and 3 are stable to air for at least 24 h. The 1H NMR spectra of 2 and 3 were unexceptional, with (C5Me5)− resonances at δ 2.00 and 1.99 ppm, respectively, in the range found for bent metallocenes in C6D6: e.g., [(C5Me5)2ThPh2] (1.77 ppm),25 (C5Me5)2Th(C3H5)2 (1.85 ppm),25 [(C5Me5)2Th(CH2Ph)2] (1.87 ppm),23 [(C5Me5)2ThMe2] (1.92 ppm),23 [(C5Me5)2ThEt2] (1.93 ppm), 2 3 [(C 5 Me 5 ) 2 Th(CH 2 SiMe 3 ) 2 ] (1.98 ppm), 2 3 [(C5Me5)2Th(neopentyl)2] (2.05 ppm),23 and [(C5Me5)2ThH2]2 (2.20 ppm).23 Hence, the NMR spectra do not distinguish the linear metallocenes from bent complexes. Two CN stretching frequencies, νCN 2306 and 2275 cm−1, are observed in 2 and are similar to the 2269 and 2262 cm−1 values for [(C5Me5)2U(NCMe)5][BPh4]2.17,22 These absorptions are higher in energy than the νCN 2254 cm−1 value of free MeCN, which is consistent with the ligand acting as a σ donor to the metal center.17,22 The IR spectrum of 3 contains one feature corresponding to νCN 2237 cm−1, which is also higher in energy than the 2231 cm−1 absorption of free PhCN.7 Structure. Table 1 compares the metrical data of 2-RT and 3 with those of the uranium acetonitrile analogues [(C5Me5)2U(NCMe)5][X]2 (3-X; X = (BPh4)−,16,17 OTf−,17 I−,16,17). 2-RT and 3 contain formally 11-coordinate thorium centers with two cyclopentadienyl ligands opposite and parallel to each other with 179.1 and 178.4° (centroid)−Th−(centroid) angles, respectively. The dihedral angles between the two ring planes are 0.39 and 0.66°, respectively. These angles are similar to the 178.0−179.9 and 0.5−2.8° ranges respectively observed for uranium complexes containing the [(C5Me5)2U(NCMe)5]2+ unit.16,17 The rings are nearly eclipsed with the average dihedral angle between the planes containing the methyl, the ring carbon it is attached to, and the thorium center being 2.68° for 3 and a surprisingly larger value of 9.31° for 2-RT. In both 2RT and 3, the nitriles are staggered with respect to the methyl groups of the (C5Me5)− ligands, minimizing steric interactions. The Th−(ring centroid) and Th−N(RCN) distances are similar in 2-RT and 3. They are slightly larger than those in the uranium complexes, which is consistent with the difference in ionic radii: the 12-coordinate Shannon ionic radius of Th is 1.21 Å in comparison to 1.17 Å for U. The 2.594−2.603 Å range of Th−(C5Me5 ring centroid) distances for 2-RT and 3 are longer than those for typical bis(pentamethylcyclopentadienyl) thorium metallocenes and start to approach the 2.62 Å values of the sterically crowded tris(pentamethylcyclopentadienyl) complexes (C5Me5)3ThMe and [(C5Me5)3Th(THF)][BPh4].10 A characteristic of the sterically crowded tris(pentamethylcyclopentadienyl) complexes is at least one large displacement of the methyl substituents from each C5Me5 plane by 0.48 Å or more.30 However, in 2-RT and 3, these methyl displacements are not unusual and range from 0.187 to 0.247 Å and from 0.200 to 0.239 Å, respectively. The 2.555(2)−2.610(3) Å range of Th− N distances for 2-RT and 3 spans the 2.58(3) Å Th−N distance in [(C5H5)3U(NCMe)2][(C5H5)ThCl4(NCMe)],31 but the distances are much shorter than the 2.802(3) Å d i s t a n c e i n ( M e s D A B M e ) ( M e s D A B M e 2 ) T h I (N C Me ) , ((MesDABMe) = [MesNC(Me)C(Me)NMes], (MesDABMe2) = [MesNC(Me)C(Me)2NMes], Mes = 2,4,6-trimethylphenyl).32 There are no crystallographically characterized examples of thorium benzonitrile complexes in the literature.

Figure 1. Thermal ellipsoid plot of the cationic component of [(C5Me5)2Th(C6H5)(THF)][BPh4] (1) drawn at the 50% probability level. Only one cation of the two crystallographically independent units is shown. Hydrogen atoms, except those on the phenyl ligand, and the (BPh4)− anion are omitted for clarity. Selected bond lengths (Å) and angles (deg) in 1: Th−Cipso avg = 2.412, Th−(centroid) = 2.509, Th−O = 2.476(3), (centroid)−Th−(centroid) = 136.3, O− Th−Cipso avg = 108.5, Th−C21−C22 = 98.6(3), Th−C21−C26 = 146.2(4), Th(1)···C(22) = 2.960(5); Th···H−C22 = 2.70; Th···H− C26 = 3.89.

Pentanitrile Thorium Metallocenes. Synthesis. Due to the difficulty in isolating a complex containing the [(C5Me5)2Th]2+ moiety in the absence of coordinating solvents, the reaction of (C5Me5)2ThMe2 with 2 equivalents of [Et3NH][BPh4] in the presence of nitriles was examined. With MeCN and PhCN, this cleanly produced the pentanitrile adducts of the metallocene dications [(C5Me5)2Th(NCR)5][BPh4]2 (R = Me (2), Ph (3)) in 40−55% yield (eq 1). X-ray

crystallography revealed that both 2 (Figure 2) and 3 (Figure 3) have parallel cyclopentadienyl rings analogous to the uranium complexes of Ephritikhine et al. (Scheme 1).16−18 Samples of 2 grown at different temperatures crystallized with different unit cell parameters and different spatial arrangements of the (BPh4)− counteranions (see the Supporting Information), but the crystals grown at ambient glovebox temperature (2-RT) or at −15 °C (2-cold) were not isomorphous with the uranium analogue [(C5Me5)2U(NCMe)5][BPh4]2.16,18 Since the crystal data on 2-cold were not of sufficient quality to discuss metrical parameters due to disorder, only 2-RT will be analyzed below along with 3. Complex 2 is soluble in MeCN and pyridine but not in arenes or ethers, while 3 dissolves in THF without loss of the coordinated nitrile. As is the case for [(C5Me5)2U(NCMe)5]456

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Figure 2. Thermal ellipsoid plots of the cationic portion of [(C5Me5)2Th(NCMe)5][BPh4]2 (2-RT) grown at ambient glovebox temperature, viewed side-on (left) and down the ring centroid−Th−ring centroid axis (right). Thermal ellipsoids are drawn at the 50% probability level, and hydrogen atoms, cocrystallized solvent, and (BPh4)− anions are omitted for clarity.

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CONCLUSION Attempts to isolate the base free dication [(C5Me5)2Th][BPh4]2 yielded the cationic phenyl complex [(C5Me5)2Th(C6H5)(THF)][BPh4] (1), which is a rare example of a crystallographically characterizable thorium phenyl complex. In the presence of nitriles, synthesis of a thorium parallel plane metallocene dication by reaction of (C5Me5)2ThMe2 with 2 equiv of [Et3NH][BPh4] led to the isolation of [(C5Me5)2Th(NCR)5][BPh4] (R = Me (2), Ph (3)) complexes, which are rare examples of thorium nitriles. These complexes are the first thorium metallocenes with parallel cyclopentadienyl rings and show that the linear metallocene structural motif identified for uranium is also possible with the first member of the actinide series, thorium.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00855. NMR spectrum of crude [(C5Me5)2Th(C6H5)(THF)][BPh4], crystal structures and crystallographic discussions, and crystallographic details (PDF) Accession Codes

CCDC 1588074−1588077 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.

Figure 3. Thermal ellipsoid plots of the cation in [(C5Me5)2Th(NCPh)5][BPh4]2 (3), viewed side-on (top) and down the ring centroid−Th−ring centroid axis (bottom). Thermal ellipsoids are drawn at the 50% probability level, and hydrogen atoms, cocrystallized solvent, and (BPh4)− anions are omitted for clarity.

Table 1. Bond Distance (Å) and Angle (deg) Data on [(C5Me5)2Th(NCR)5][BPh4]2 (R = Me (2-RT), Ph (3)) and [(C5Me5)2U(NCMe)5][X]2 (4-X, X = (BPh4)−,16,17OTf−,17 I−,16,17)a 2-RT space group An−centroid distance (Å) An−N range (Å) An−N avg (Å) Cent−An−Cent (deg) a

3

4-BPh416,17

4-OTf17

4-I16

4-I16,17

P1̅ 2.594

P1̅ 2.601

P21/c 2.528

Pnma 2.532

C2/c 2.539

P21/c 2.531

2.601 2.555(2)−2.610(3) 2.59(2) 179.1

2.603 2.567(2)−2.599(2) 2.58(1) 178.4

2.540 2.521(4)−2.576(3) 2.55(2) 178.6

2.541 2.539(7)−2.563(8) 2.55(1) 179.5

2.541 2.535(4)−2.556(4) 2.547(7) 179.9

2.549 2.518(2)−2.577(3) 2.55(2) 178.0

Abbreviations: Cent = (C5Me5)− centroid, An = actinide. 457

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(24) Berthet, J.-C.; Villiers, C.; Le Maréchal, J.-F.; Delavaux-Nicot, B.; Lance, M.; Nierlich, M.; Vigner, J.; Ephritikhine, M. J. Organomet. Chem. 1992, 440 (1−2), 53−65. (25) Langeslay, R. R.; Walensky, J. R.; Ziller, J. W.; Evans, W. J. Inorg. Chem. 2014, 53 (16), 8455−8463. (26) Evans, W. J.; Miller, K. A.; Hillman, W. R.; Ziller, J. W. J. Organomet. Chem. 2007, 692 (17), 3649−3654. (27) Lin, Z.; Marks, T. J. J. Am. Chem. Soc. 1987, 109 (26), 7979− 7985. (28) England, A. F.; Burns, C. J.; Buchwald, S. L. Organometallics 1994, 13 (9), 3491−3495. (29) Pedrick, E. A.; Hrobarik, P.; Seaman, L. A.; Wu, G.; Hayton, T. W. Chem. Commun. 2016, 52 (4), 689−692. (30) Evans, W. J.; Kozimor, S. A.; Ziller, J. W. Inorg. Chem. 2005, 44, 7960−7969. (31) Rebizant, J.; Apostolidis, C.; Spirlet, M. R.; Kanellakopulos, B. Inorg. Chim. Acta 1987, 139 (1−2), 209−210. (32) Mrutu, A.; Barnes, C. L.; Bart, S. C.; Walensky, J. R. Eur. J. Inorg. Chem. 2013, 2013 (22−23), 4050−4055.

AUTHOR INFORMATION

Corresponding Author

*E-mail for W.J.E.: [email protected]. ORCID

Ryan R. Langeslay: 0000-0003-2915-9309 Megan T. Dumas: 0000-0001-7000-2130 William J. Evans: 0000-0002-0651-418X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Chemical Sciences, Geosciences, and Biosciences Division of the Office of Basic Energy Sciences of the Department of Energy (DE-SC0004739) for support and Dr. Jason R. Jones for assistance with X-ray crystallography.

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REFERENCES

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DOI: 10.1021/acs.organomet.7b00855 Organometallics 2018, 37, 454−458