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Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Unusual Rearrangement of an N‑Donor-Functionalized N‑Heterocyclic Carbene Ligand on Group 8 Metals Qiuming Liang, Andrew Salmon, Patrick Jinhyung Kim, Linfan Yan, and Datong Song* Davenport Chemical Research Laboratories, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada S Supporting Information *
Scheme 1. Compound Conversions
ABSTRACT: We report an unexpected rearrangement of a deprotonated picolyl-functionalized N-heterocyclic carbene (NHC) ligand from N,C-chelate to N,N-chelate in three-legged piano-stool Fe(II) and Ru(II) complexes. The reaction mechanism has been explored for one of the Fe(II) complexes. Experimental and computational studies suggest an unusual intermediate featuring a fourmembered chelate ring, where the NHC and the α-carbon of one of the N-substituents coordinate to the Fe(II) center. A possible Fe−alkylidene intermediate has also been predicted by computations.
isolation of 2[Fe] and 2[Ru] as reddish brown and red crystals in 76% and 72% yields, respectively. When the bulky tBuNC ligand is used instead of CO, the rearrangement reactions speed up drastically, e.g., the complete conversions can be achieved at 90 °C in 5 and 15 h for 3[Fe] and 3[Ru], respectively (Scheme 1). The molecular structures of the rearrangement products 2[Fe]/ [Ru] and 4[Fe]/[Ru] were established with X-ray crystallography. As shown in Figure 1, 2[Fe]/[Ru] and 4[Fe]/[Ru] still retain the piano-stool coordination geometry, but the N,Cchelate ligand in 1[Fe]/[Ru] and 3[Fe]/[Ru] has isomerized into a N,N-chelate ligand in all cases. The C(5)−C(6)−C(7) angles in 2[Fe]/[Ru] and 4[Fe]/[Ru] are in the range of 123− 125°, consistent with the sp2 hybridization of C(6). The C(5)− C(6) and C(6)−C(7) bond lengths in 2[Fe]/[Ru] and 4[Fe]/ [Ru] are all ∼1.40 Å, which suggest a more complete delocalization of π-electrons in the N(1)−C(5)−C(6)−C(7)− N(2) moiety compared to the N,C-chelate in 1[Fe]/[Ru] and 3[Fe]/[Ru]. Within each rearrangement product, the M−N(2) bond is slightly shorter than the M−N(1) bond (e.g., 1.971(2) vs 2.100(5) Å in 2[Fe]), suggesting a stronger M−N bond to the imidazole ring. In C6D6, the pyridylic methine of 2[Fe] resonates at 4.22 and 74.1 ppm in the 1H and 13C NMR spectra, respectively, upfield shifted compared to those of 1[Fe] (at 5.46 and 94.4 ppm). The imidazole-C2 of 2[Fe] resonates at 149.5 ppm, upfield shifted with respect to the carbene carbon signal in 1[Fe] at 168.0 ppm. The CO stretch signals in the IR spectra of 1[Fe] and 2[Fe] are 1893 and 1912 cm−1, respectively, suggesting that the N,Nchelate is less electron donating than the N,C-chelate. The diagnostic spectroscopic data for 3[Fe]/[Ru] and 4[Fe]/[Ru] show a similar trend (see Table S2). The rearrangement reaction
T
he N-heterocyclic carbenes (NHCs) are widely used ancillary ligands in coordination chemistry.1 Despite the generally assumed role as spectator ligands, NHCs have shown noninnocent behaviors, including the ring opening and expansion of the N-heterocycle, C−H activation at the backbone, C−H and C−C activation at N-substituents, loss of an Nsubstituent followed by the coordination of imidazole N-donor, ring expansion of an N-bound aryl group, reductive elimination, and insertion of an unsaturated group into M−C(NHC) bonds.2−5 Although some of the noninnocent behaviors of NHCs result in undesired ligand decompositions, the reversible transformations (such as C−H cleavage) of the N-substituents can be utilized in a productive manner toward small molecule activation and catalysis through metal−ligand cooperation.4b,c,6 In particular, pincer complexes featuring picolyl−NHC moieties can break and form C−H or C−C bonds reversibly at the pyridylic positions accompanied by the dearomatization-rearomatization of the pyridine ring.6 These picolyl−NHCcontaining pincer complexes arguably stemmed from Milstein’s versatile pincer systems containing picolyl−phosphine moieties; NHCs have been used in lieu of phosphines for more electronrich and robust late metal complexes because of their superb σdonating ability and the soft nature of the carbon donor. The success of picolyl-NHC-containing pincer ligands prompted us to explore the reactivity of group 8 complexes of bidentate picolyl−NHC ligands,4d,e,7 where similar metal−ligand cooperation was observed. We herein report an unexpected rearrangement of group 8 piano-stool complexes of a deprotonated picolyl−NHC ligand with a dearomatized pyridine ring. Heating 1[Fe]4e in C6D6 at 90 °C for 24 h resulted the complete conversion into a new compound 2[Fe] (Scheme 1), whereas the complete conversion of 1[Ru]4e converts to 2[Ru] at 90 °C took 7 days. Larger scale reactions in toluene led to the © XXXX American Chemical Society
Received: December 11, 2017 Published: January 10, 2018 A
DOI: 10.1021/jacs.7b13097 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society
Figure 1. Molecular structures of 2[Fe] (top left), 2[Ru] (top right), 4[Fe] (bottom left), and 4[Ru] (bottom right). Ellipsoids are shown at 50% probability.
is surprising in that the strong bonds between NHC and late metals are broken in low-spin d6 complexes with electronically saturated metal centers and the N-substituent of the NHC and the metal center swap their binding sites on the five-membered N-heterocycle. To the best of our knowledge, this type of reactivity is only known for group 1 complexes of O-donor functionalized NHCs, where the coordination bonds are weak, and the metals prefer the harder N-donor.8 To shed light on the mechanism of the unexpected rearrangement reaction, the conversion of 1[Fe] to 2[Fe] in toluene-d8 at 90 °C was monitored with 1H NMR spectroscopy (Figure 2).9 The consumption of 1[Fe] follows a first-order decay curve (see Supporting Information), suggesting that the rearrangement is unimolecular. During the course of reaction, the pyridylic methine signal (at 4.65 ppm) of a new species (5[Fe]) grew in fast initially, maximized to ∼20 mol %, and slowly decayed away.10 For the formation of 2[Fe], there is an induction period initially and the timeline of the inflection point coincides with that of the peak concentration of 5[Fe], suggesting that 5[Fe] is an intermediate of the rearrangement reaction. The kinetics were modeled using COPASI software11 as a nonlimiting case of 1[Fe] ⇌ 5[Fe] → 2[Fe] where k1, k−1, and k2 are all similar (see Supporting Information). The rate constants at different temperatures were obtained, and the activation parameters were determined through an Eyring analysis (Table 1). The conversion of 1[Fe] to 5[Fe] is slightly exergonic, with a free energy change of −0.5 kcal·mol−1. Gratifyingly, by stopping the reaction when 5[Fe] reached its peak concentration, we were able to isolate a small amount of 5[Fe] from the reaction mixture through repeated recrystallization.12 The 1H NMR spectrum of 5[Fe] in C6D6 displays a pyridylic methine proton signal at 4.74 ppm (or 4.65 ppm in toluene-d8). The downfield chemical shifts of the proton signals on the C5N ring compared to those of 1[Fe] and 2[Fe] suggest the C5N ring has been rearomatized. The imidazole-C2 at 172.1 ppm in the 13C NMR spectrum is consistent with a coordinating carbene carbon. The pyridylic methine carbon signal at 46.0 ppm suggests sp3 hybridization. The 1H−13C HMBC experiment showed a cross-peak between the Cp* proton and the pyridylic
Figure 2. Top: Partial 1H NMR (600 MHz, toluene-d8, 20 mM, 90 °C) demonstrating the conversion of 1[Fe] to 2[Fe] through intermediate 5[Fe] at t = 0.13, 0.87, 1.61, 2.35, 3.09, 3.83, 4.57, 5.31, 6.05, 6.80, 7.54, 8.28, 9.02, 9.76, 12.23, 14.70, 17.17, 19.64, 22.35, 23.59 h starting from bottom; Bottom: Experimentally measured concentrations (circles) and simulated data (lines) based on 1[Fe] ⇌ 5[Fe] → 2[Fe] kinetic model.
methine carbon, suggesting the coordination of the pyridylic methine carbon to the Fe center, i.e., four bonds away from the Cp* proton. The IR spectrum of 5[Fe] has a CO stretch at 1888 cm−1, lower than those in 1[Fe] and 2[Fe], comparable to those of [FeCp*(L′IMes-κC,κC′)(CO)]4b (where L′IMes-κC,κC′ = (2(3-mesityl-imidazol-3-ium-2-id-1-yl)-3,5-dimethylphenyl)methanyl) and [FeCp*(L-κC,κC′)(CO)]4e (where L-κC,κC′ = (2-(3-picoline-imidazol-3-ium-2-id-1-yl)-3,5-dimethylphenyl)methanyl) where the first coordination sphere consists of carbon B
DOI: 10.1021/jacs.7b13097 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society Table 1. Activation Parameters for 1[Fe] ⇌ 5[Fe] → 2[Fe] (toluene-d8, 20 mM) Steps
ΔH‡ (kcal·mol−1)
ΔS‡ (cal·mol−1·K−1)
ΔG‡ (298 K) (kcal·mol−1)
1[Fe] → 5[Fe] 5[Fe] → 1[Fe] 5[Fe] → 2[Fe]
31.3 33.6 24.2
9.3 15.2 −9.8
28.6 29.1 27.2
donors only. Based on the spectroscopic evidence, we propose the structure of 5[Fe] with a dangling rearomatized pyridine ring and a four-membered chelate ring via the NHC and the pyridylic methine carbon donors. The four-membered Fe−C−N−C chelate ring is a known structural motif in a few Fe complexes where the C,C-chelate ligands are derived from isocyanides.13 Attempts to obtain X-ray-quality single crystals of 5[Fe] were unsuccessful. By reacting 5[Fe] with BH3·THF, we were able to isolate X-ray-quality single crystals of 6[Fe], the BH3 adduct of 5[Fe]. Compared to that of 5[Fe], the pyridylic methine 1H NMR signal of 6[Fe] in C6D6 significantly downfield shifted to 5.59 ppm. In the 13C NMR spectrum of 6[Fe], the pyridylic methine carbon and imidazole-C2 signals are at 40.3 and 172.7 ppm, respectively. The CO stretch of 6[Fe] (1887 cm−1) is similar to that of 5[Fe]. The X-ray structure of 6[Fe] is shown in Figure 3. The Fe(1)−C(7) bond length is 1.949(9) Å. The
Figure 4. Proposed mechanism (top) and computed energetics (bottom).
ring. DFT shows that the first step 1[Fe] ⇌ 5[Fe] is the ratedetermining step with the ΔG‡ for the forward and backward reactions of 26.7 and 27.2 kcal·mol−1, respectively. The ratedetermining step involves the dissociation of the N-donor, which is consistent with the fact that complexes with bulky tBuNC ancillary ligand rearrange faster than those with CO ligand. The second step 5[Fe] → A has a slightly lower barrier of 25.3 kcal· mol−1. The high-energy intermediate A (17.1 kcal·mol−1 with respect to 1[Fe]) sits in a shallow energy well and converts to B (−15 kcal·mol−1 with respect to 1[Fe]) quickly with a barrier of 2.9 kcal·mol−1, followed by another fast transformation to 2[Fe], which is 30.1 kcal·mol−1 more stable than 1[Fe]. Intermediates A and B are both unobserved experimentally and compounded in the irreversible 5[Fe] → 2[Fe] step in our kinetic model. The computed energetics match with those deduced from the kinetic experiments within 2 kcal·mol−1. The transformation of an NHC ligand into an N-bound imidazole ligand in the coordination sphere of a transition metal is scarcely known.15 The known transformations were either triggered by the loss of an N-substituent,15a−f or initiated by the insertion of an alkyne into metal−carbon (NHC) bond.15g The reactivity reported herein is distinct from all the abovementioned in that it is an isomerization reaction. Such reactivity might be relevant to the pincer complexes with picolyl−NHC structure motif, where the pyridylic CH2 group is deprotonated for metal−ligand cooperation in small molecule activation and
Figure 3. Synthesis (top) and molecular structure (bottom) of 6[Fe]. Ellipsoids are shown at 50% probability.
tetrahedral geometry at the pyridylic methine carbon C(6) is consistent with sp3 hybridization. The Fe(1)−C(6) bond length of 2.184(9) Å is slightly longer than a typical Fe−alkyl distance due to the ring strain. The bite angle C(6)−Fe(1)−C(7) is 66.8(3)°, similar to those of the four-membered Fe−C−N−C chelate rings.13 To our knowledge, compound 6[Fe] is the first NHC iron complex that cyclometalates through the α-carbon atom of one of the N-substituents. Combining the experimental data summarized above with DFT studies,14 we propose the following mechanism (Figure 4): (1) N-bound to C-bound tautomerization by twisting the picolyl arm to form 5[Fe] accompanied by the rearomatization of the C5N ring; (2) C−N bond cleavage to form an iron alkylidene complex A; (3) insertion of alkylidene into Fe−C bond and coordination of imidazole nitrogen to form complex B; (4) Cbound to N-bound tautomerization by twisting the picolyl arm to form 2[Fe] accompanied by the dearomatization of the C5N C
DOI: 10.1021/jacs.7b13097 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society
Trans. 2017, 46, 7791. (d) Schneider, H.; Krahfuß, M. J.; Radius, U. Z. Anorg. Allg. Chem. 2016, 642, 1282. (4) For recent examples on C−H activation on the N-substituents, see: (a) Danopoulos, A. A.; Pugh, D.; Wright, J. A. Angew. Chem., Int. Ed. 2008, 47, 9765. (b) Ohki, Y.; Hatanaka, T.; Tatsumi, K. J. Am. Chem. Soc. 2008, 130, 17174. (c) Liu, H.-J.; Raynaud, C.; Eisenstein, O.; Tilley, T. D. J. Am. Chem. Soc. 2014, 136, 11473. (d) Liang, Q.; Osten, K. M.; Song, D. Angew. Chem., Int. Ed. 2017, 56, 6317. (e) Liang, Q.; Song, D. Inorg. Chem. 2017, 56, 11956. (5) For recent examples on insertion into M−C(NHC) bond, see: (a) Brown, C. C.; Plessow, P. N.; Rominger, F.; Limbach, M.; Hofmann, P. Organometallics 2014, 33, 6754. (b) Hu, X.; Meyer, K. J. Am. Chem. Soc. 2004, 126, 16322. (6) For review articles, see: (a) Khusnutdinova, J. R.; Milstein, D. Angew. Chem., Int. Ed. 2015, 54, 12236. (b) Peris, E. Chem. Rev. 2017, DOI: 10.1021/acs.chemrev.6b00695. (c) Pugh, D.; Danopoulos, A. A. Coord. Chem. Rev. 2007, 251, 610. (7) Liang, Q.; Janes, T.; Gjergji, X.; Song, D. Dalton Trans. 2016, 45, 13872. (8) (a) Steiner, G.; Krajete, A.; Kopacka, H.; Ongania, K.-H.; Wurst, K.; Preishuber-Pflügl, P.; Bildstein, B. Eur. J. Inorg. Chem. 2004, 2004, 2827. (b) Wang, Z.-G.; Sun, H.-M.; Yao, H.-S.; Yao, Y.-M.; Shen, Q.; Zhang, Y. J. Organomet. Chem. 2006, 691, 3383. (c) Zhang, D.; Kawaguchi, H. Organometallics 2006, 25, 5506. (d) Zhang, D.; Aihara, H.; Watanabe, T.; Matsuo, T.; Kawaguchi, H. J. Organomet. Chem. 2007, 692, 234. (9) The concentrations of 1[Fe], 5[Fe], and 2[Fe] were determined based on the integrations of pyridylic methine 1H signals (5.54, 4.65 and 4.08 ppm, respectively in toluene-d 8 ) with respect to 1,3,5trimethoxybenzene internal standard (3.42 ppm). (10) An intermediate was also observed for the conversion of 3[Fe] to 4[Fe]. In contrast, no intermediate was observed for the conversion of 1[Ru] to 2[Ru] or 3[Ru] to 4[Ru] under the same conditions (i.e., 20 mM in toluene-d8, 90 °C), suggesting different relative rates in the kinetic model or a distinct mechanism. (11) Hoops, S.; Sahle, S.; Gauges, R.; Lee, C.; Pahle, J.; Simus, N.; Singhal, M.; Xu, L.; Mendes, P.; Kummer, U. Bioinformatics 2006, 22, 3067 ; COPASI 4.22 (Build 170) was downloaded from http://copasi. org/. (12) During this process, we identified a trace amount of [FeCp*(CO)2]2 a decomposition product. (13) (a) Miller, J.; Balch, A. L.; Enemark, J. H. J. Am. Chem. Soc. 1971, 93, 4613. (b) Riera, V.; Ruiz, J.; Tiripicchio, A.; Camellini, M. T. J. Organomet. Chem. 1987, 327, C5. (c) Cardaci, G.; Bellachioma, G.; Zanazzi, P. J. Chem. Soc., Chem. Commun. 1984, 650. (d) Fehlhammer, W. P.; Hirschmann, P.; Stolzenberg, H. J. Organomet. Chem. 1982, 224, 165. (e) Bellachioma, G.; Cardaci, G.; Zanazzi, P. Inorg. Chem. 1987, 26, 84. (14) All computations were performed using Gaussian 16, Revision A.03 with PBEPBE exchange-correlation functional and TZVP basis set. All structures were optimized with PCM solvent correction (solvent = toluene) and the D3 version of Grimme’s dispersion correction with the original D3 damping function. For more details, see Supporting Information. (15) (a) Häller, L. J. L.; Page, M. J.; Erhardt, S.; Macgregor, S. A.; Mahon, M. F.; Naser, M. A.; Vélez, A.; Whittlesey, M. K. J. Am. Chem. Soc. 2010, 132, 18408. (b) Burling, S.; Mahon, M. F.; Powell, R. E.; Whittlesey, M. K.; Williams, J. M. J. J. Am. Chem. Soc. 2006, 128, 13702. (c) Day, B. M.; Pugh, T.; Hendriks, D.; Guerra, C. F.; Evans, D. J.; Bickelhaupt, F. M.; Layfield, R. A. J. Am. Chem. Soc. 2013, 135, 13338. (d) Day, B. M.; Pal, K.; Pugh, T.; Tuck, J.; Layfield, R. A. Inorg. Chem. 2014, 53, 10578. (e) Wang, X.; Chen, H.; Li, X. Organometallics 2007, 26, 4684. (f) Cabeza, J. A.; del Río, I.; Miguel, D.; Sánchez-Vega, M. G. Angew. Chem., Int. Ed. 2008, 47, 1920. (g) Fernández, F. E.; Puerta, M. C.; Valerga, P. Inorg. Chem. 2013, 52, 6502.
catalysis. Such a rearrangement may contribute to catalyst deactivation. On the other hand, our DFT calculation suggests the possible involvement of an interesting iron alkylidene intermediate. Possibilities of redirecting the rearrangement reaction into a synthetic method for iron−alkylidene species for reactivity chemistry are being explored in our laboratory.
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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/jacs.7b13097. Experimental procedures, NMR and IR spectra, along with computational details and computed atomic coordinates and energies of species involved in the proposed reaction mechanism (PDF) Crystallographic data (CIF)
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AUTHOR INFORMATION
Corresponding Author
*
[email protected] ORCID
Datong Song: 0000-0001-6622-5980 Notes
The authors declare no competing financial interest. CCDC 1590127−1590135 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.
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ACKNOWLEDGMENTS We thank Natural Science and Engineering Research Council (NSERC) of Canada for funding. Q.L. thanks the Ontario government for an Ontario Graduate Scholarship. We thank Jack Sheng and Darcy Burns for invaluable help with NMR spectroscopic experiments. This research was enabled in part by support provided by Sharcnet (www.sharcnet.ca) and Compute Canada (www.computecanada.ca).
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REFERENCES
(1) (a) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122. (b) Poyatos, M.; Mata, J. A.; Peris, E. Chem. Rev. 2009, 109, 3677. (c) Schuster, O.; Yang, L.; Raubenheimer, H. G.; Albrecht, M. Chem. Rev. 2009, 109, 3445. (d) Díez-González, S.; Marion, N.; Nolan, S. P. Chem. Rev. 2009, 109, 3612. (e) Arnold, P. L.; Casely, I. J. Chem. Rev. 2009, 109, 3599. (f) Lin, J. C. Y.; Huang, R. T. W.; Lee, C. S.; Bhattacharyya, A.; Hwang, W. S.; Lin, I. J. B. Chem. Rev. 2009, 109, 3561. (g) Nelson, D. J.; Nolan, S. P. Chem. Soc. Rev. 2013, 42, 6723. (h) Hopkinson, M. N.; Richter, C.; Schedler, M.; Glorius, F. Nature 2014, 510, 485. (i) Johnson, C.; Albrecht, M. Coord. Chem. Rev. 2017, 352, 1. (2) For reviews on noninnocent behaviors of NHCs, see: (a) Würtemberger-Pietsch, S.; Radius, U.; Marder, T. B. Dalton Trans. 2016, 45, 5880. (b) Lake, B. R. M.; Chapman, M. R.; Willans, C. E. Organomet. Chem. 2016, 40, 107. (3) For recent examples on C−H activation at the NHC backbone, see: (a) Schnee, G.; Nieto Faza, O.; Specklin, D.; Jacques, B.; Karmazin, L.; Welter, R.; Silva Lopez, C.; Dagorne, S. Chem. - Eur. J. 2015, 21, 17959. (b) Ellul, C. E.; Mahon, M. F.; Saker, O.; Whittlesey, M. K. Angew. Chem., Int. Ed. 2007, 46, 6343. (c) Ghadwal, R. S.; Rottschafer, D.; Andrada, D. M.; Frenking, G.; Schurmann, C. J.; Stammler, H. G. Dalton
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NOTE ADDED AFTER ASAP PUBLICATION Figure 4 was corrected on January 18, 2018.
D
DOI: 10.1021/jacs.7b13097 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
