Ring Slippage and Dissociation of Pentamethylcyclopentadienyl

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Ring Slippage and Dissociation of Pentamethylcyclopentadienyl Ligand in an (η5‑Cp*)Ir Complex with a κ3‑O,C,O Tridentate Calix[4]arene Ligand under Mild Conditions Takuya Kuwabara, Ryogen Tezuka, Mikiya Ishikawa, Takuya Yamazaki, Shintaro Kodama, and Youichi Ishii* Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551 Japan S Supporting Information *

ABSTRACT: The first examples of κ3-O,C,O tridentate calix[4]arene transition-metal complexes [Cp*M(p-tBucalix[4]arene(2−)-κ3-O,C,O)] (M = Ir, Rh) that are stable both in the solid state and in solution were synthesized. The reaction of the Ir complex with 2 equiv of xylyl isocyanide afforded the (η1-Cp*)Ir(CNXy)2 complex in which the Cp* ligand is slipped from an η5 to an η1 fashion, although ring slippage of an η5-Cp* ligand is very uncommon in comparison with that of Cp. The reaction of the (η1-Cp*)Ir complex with additional 1 equiv of isocyanide resulted in the liberation of the Cp* ligand as Cp*H, which was confirmed by 1H NMR. This study provides the first observation of stepwise dissociation of a Cp* ligand from group 4−10 transition-metals via η1 coordination. This unique phenomenon seems to be facilitated by flexible coordination of an OH group in the C-metalated calix[4]arene ligand.

C

Cp* ligand from an (η5-Cp*)Ir hydride complex upon addition of bis(dimethylphosphino)methane (dmpm) ligand to form catalytically active iridium species, in which the dissociation mechanism is proposed to be ring slippage of the Cp* from an η5 to an η1 fashion, followed by β-H elimination.13 However, no direct observation of this stepwise dissociation process of Cp* ligand has yet been achieved. We report herein the clean observation of stepwise dissociation of a Cp* ligand in welldefined group 9 metal calix[4]arene complexes that exhibit a rare type of coordination mode of calix[4]arene. Since calix[n]arenes have phenolic oxygens and arene units as coordination sites, transition-metal complexes of calixarene, so-called metallocalixarenes, have been extensively investigated to reveal their structural diversity (Chart 1b).14 In contrast to many reports on their phenoxo and π-arene complexes, utilization of the methylene bridge as a coordination site is rather limited. Although phenolic OH-directed activation of a benzylic CH bond is expected to be a promising route to realize the κ2-O,C or κ3-O,C,O coordination of a calixarene molecule where the methylene bridge serves as the donor atom,15 very few examples of CH bond activation of calix[4]arene have been reported in spite of the recent growing interest in CH bond activation chemistry.16 Parkin and co-workers successfully

yclopentadienyl anion (C5H5; Cp) has played a central role as a ligand for transition-metal complexes since the discovery of the first sandwich complex, ferrocene (Cp2Fe).1 Cp complexes have been widely investigated in various fields including catalytic chemistry,2 materials science,3 and maingroup chemistry.4 Although Cp and related ligands including indenyl usually coordinate to a transition-metal in an η5 fashion, they sometimes undergo ring slippage to change their coordination mode into η3 and η1 fashions (Chart 1a).5 This haptotropic shift of Cp and indenyl ligands generates a new coordination site at the transition-metal center, which enables two characteristic reactions; one is ligand exchange reactions widely known as the indenyl effect,6 and the other is various catalytic reactions.7 In contrast, ring slippage of a pentamethylcyclopentadienyl ligand (C5Me5; Cp*) has been much less common due to its stronger electron-donating character in comparison to Cp and indenyl ligands.8 Hence, there have been limited examples of a haptotropic shift9,10 or dissociation of Cp* under mild conditions (especially for late-transition-metal complexes),11 although degradation of Cp* under harsh oxidation conditions has been becoming common recently.12 Boag and co-workers reported the formation of an (η1-Cp*)Pt complex that is proposed as an intermediate for nucleophilic substitution at an (η5-Cp*)Pt complex, although no solid-state structure of the (η1-Cp*)Pt complex was documented.9b Recently, Hintermair and Crabtree observed dissociation of a © XXXX American Chemical Society

Received: April 24, 2018

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DOI: 10.1021/acs.organomet.8b00257 Organometallics XXXX, XXX, XXX−XXX

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[4]arene and [Cp*Rh(OAc)2] in mesitylene at 165 °C for 13 h. When the latter reaction was carried out at room temperature, monoacetate complex 2, which is an intermediate for the formation of 1b, was obtained. Indeed, refluxing a mesitylene solution of 2 generated 1b in 77% yield, presumably via a carboxylate-assisted CMD/AMLA process.17 Notably, each 1H NMR spectrum of 1a,b displays a singlet signal assignable to the proton on the metalated carbon atom at 4.79 and 5.12 ppm, respectively. Moreover, a characteristic doublet signal (62.9 ppm, 1JRh−C = 25 Hz) was found in 13C NMR of 1b. These NMR data demonstrate that complexes 1a,b maintain the κ3O,C,O tridentate coordination mode in solution. It should also be noted that the 1H NMR spectra of 1a,b exhibited only four doublet signals (1a, δ 4.30 (2H), 4.27 (1H), 3.42 (2H), 3.37 (1H); 1b, δ 4.32, (2H), 4.29 (1H), 3.41 (1H), 3.35 (2H)) for the methylene protons and two singlets (1a, δ 1.21 (18H), 1.19 (18H); 1b, δ 1.22 (18H), 1.20 (18H)) for the protons of the t Bu groups, suggesting an apparent Cs symmetry over the temperature range of −60 °C to room temperature. This observation indicates fast proton shift within the OH···OH··· OH···O hydrogen bond network in 1a,b. Figure 1 illustrates the solid-state structures of 1b and 2, while that of 1a is shown in Figure S1. The unit cell of 1b

Chart 1. (a) Ring Slippage of η5-Cp and Related Ligands in Transition-Metal Complexes and (b) Three Different Coordination Sites in Calix[4]arene

synthesized the first calixarene complex in which the calix[4]arene ligand coordinates to the transition-metal as a κ3-O,C,O tridentate ligand by utilizing CH bond activation.14f However, the C-metalated calix[4]arene was in equilibrium with its agostic isomer in solution, suggesting that utilization of the benzylic methylene as a stable coordination site is difficult. In this study, we have synthesized the first example of C-metalated calix[4]arenes that are stable even in solution. Moreover, sequential addition of xylyl isocyanide to the complex resulted in stepwise dissociation of Cp* ligand. The C-metalated p-tBucalix[4]arenes 1a,b were synthesized in moderate to good yields, as shown in Scheme 1. Refluxing a THF solution of p-tBucalix[4]arene and [Cp*Ir(OAc)2(dmso)] for 22 h resulted in the formation of 1a, while 1b was synthesized by the reaction of a potassium salt of p-tBucalix-

Figure 1. ORTEP drawings of 1b (left) and 2 (right) with 50% probability ellipsoids. The tBu groups, the solvent molecules (dichloromethane and diethyl ether for 1b and 2, respectively), and all of the hydrogen atoms except for those of the OH groups were omitted for clarity. Selected bond lengths of 1b (Å): Rh−C(1) 2.086(3), Rh−O(1) 2.096(3), Rh−O(2) 2.158(3).

Scheme 1. Synthesis of Cp*M Complexes Having Calix[4]arene as a κ3-O,C,O Tridentate Ligand (M = Ir, Rh)

contains two independent molecules, which have similar structural characteristics. Each of the complexes has a typical piano-stool structure with the Cp* ligand, two oxygen atoms of the calix[4]arene, and the benzylic carbon atom (1b) or with the Cp*, an oxygen atom of the calix[4]arene, and the two oxygen atoms of the acetate group (2). The Rh−C(1) bond length of 1b is 2.086(3) Å, which is within the normal range of Rh(III)−C(sp3) bonds18 and much shorter than the distance between these two atoms in 2 (ca. 4.0 Å). The reactivities of 1a toward isocyanide, CO, PMe3, pyridine, and 2,2′-bipyridine were next investigated (Scheme 2 and Scheme S1). Treatment of 1a with 1 equiv of xylyl isocyanide at −20 °C provided the isocyanide adduct 3, in which the calix[4]arene ligand coordinates to the iridium center as a κ2O,C bidentate ligand (Figure S2a). Thus, substitution of the phenolic OH group in 1a is facile. Interestingly, however, complex 3 undergoes disproportionation to the starting material 1a and a new compound 4, and the 1H NMR spectrum of the latter suggested the existence of two different xylyl isocyanides.19 In accordance with this observation, reaction of 1a and 2 equiv of xylyl isocyanide afforded complex B

DOI: 10.1021/acs.organomet.8b00257 Organometallics XXXX, XXX, XXX−XXX

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internal angles of 539.71° and exhibits clear C−C bond alternation (1.342(8)−1.516(7) Å). The 1H NMR of 4 showed five methyl singlets which are attributable to the η1-Cp* ligand, indicating that the chiral structure of the Ir center is maintained in solution.21 It is worth mentioning that 3 readily undergoes disproportionation to give the ring-slipped compound 4 and 1a as the main products even at room temperature. Obviously, ring slippage of the Cp* ligand from five-electron-donating η5 to one-electron-donating η1 generates two vacant coordination sites at the iridium center, but they are spontaneously occupied by a phenolic OH group nearby in the calix[4]arene ligand and a second XyNC ligand. Thus, we consider that the flexible coordination structure change of calix[4]arene between κ2-O,C bidentate and κ3-O,C,O tridentate facilitates the unusual ring slippage of the Cp* ligand. On the other hand, reaction of 1a with CO and PMe3 resulted in the formation of mono-CO adduct 6 and monoPMe 3 adduct 7, respectively, but no further product accompanied by ring slippage was observed (Scheme S1). Furthermore, 1a was found to be inert toward tBuNC, pyridine, and 2,2′-bipyridine under mild conditions (Scheme S1). In stark contrast, the rhodium analogue 1b failed to react with XyNC and CO. It is interesting to point out that the stability of the κ3-O,C,O coordination to Cp*RhIII is much higher than that of Cp*IrIII, although we must await further investigation to clarify whether the lower reactivity of the rhodium complex is a thermodynamic or kinetic phenomenon. Finally, addition of 5 equiv of xylyl isocyanide to 1a afforded tris(isocyanide) adduct 5 in 51% yield. The Cp* group was dissociated as Cp*H, as evidenced by the 1H NMR spectrum of the crude reaction product obtained from 4 and 1 equiv of XyNC (Figure S3). The proton of the Cp*H is derived from the calix[4]arene OH groups, which is confirmed by 1H NMR of 5; unlike the case for complexes 1, 3, and 4, only two protons assignable to the OH groups were detected. The structure of complex 5 was determined by preliminary X-ray diffraction analysis to confirm that the η1-Cp* ligand in 4 was replaced by a xylyl isocyanide (Figure S2b). The iridium atom of 5 has an octahedral geometry with three isocyanide ligands and the tridentate calix[4]arene(3−). In summary, we have successfully synthesized (η5-Cp*)Ir and Rh complexes incorporating a κ3-O,C,O tridentate calix[4]arene ligand, which features stable coordination with the benzylic carbon both in the solid state and in solution. The sequential addition of xylyl isocyanide to 1a resulted in stepwise dissociation of the Cp* ligand. The mono-, bis-, and tris(isocyanide) adducts 3−5 were satisfactorily characterized by spectroscopic and X-ray diffraction analysis, which allowed us to observe directly the dissociation process of Cp* ligand from η5 to η1 and Cp*H. This unusual haptotropic shift followed by dissociation of the Cp* ligand under mild conditions is likely facilitated by the flexible coordination behavior of the calix[4]arene ligand between κ2-O,C bidentate and κ3-O,C,O tridentate coordination modes.

Scheme 2. Reactions of 1a with Xylyl Isocyanide in Various Ratiosa

a Legend: (a) THF, −20 °C, 15 h, up to 93% yield but could not be isolated in pure form; (b) THF, room temperature., 20 h, 74% yield; (c) p-xylene reflux, 15 h, 51% yield; (d) THF, room temperature, 20 h, 20% yield; (e) toluene, 100 °C, 20 h, 19% yield.

4 as the major product, which was isolated in 74% yield. Unexpectedly, its single-crystal X-ray diffraction analysis unambiguously revealed that the product 4 is the bis(isocyanide) complex [Cp*Ir(p-tBucalix[4]arene(2−)-κ2-O,C)(CNXy)2], in which the Cp* ligand coordinates to the iridium center in an η1 fashion (Figure 2). To the best of our knowledge, complex 4 is the first example of a structurally characterized η1-Cp* complex of group 4−10 transitionmetals.20 The η1-Cp* ligand is located trans to the ArOH ligand, while the two XyNC ligands reside in the trans positions of ArO and CH. The Cp* ring is planar with a sum of the

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ASSOCIATED CONTENT

S Supporting Information *

Figure 2. ORTEP drawing of 4 with 50% probability ellipsoids. The t Bu groups of the calix[4]arene, the THF molecules, and all of the hydrogen atoms except for those of the OH groups are omitted for clarity. Selected bond lengths (Å): Ir−C(1) 2.119(4), Ir−C(2) 2.177(5), Ir−C(3) 1.879(6), Ir−C(4) 2.011(6), Ir−O(1) 2.083(3), Ir−O(2) 2.228(4).

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00257. Details of experiments and X-ray diffraction analysis and NMR spectra of new compounds (PDF) C

DOI: 10.1021/acs.organomet.8b00257 Organometallics XXXX, XXX, XXX−XXX

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(11) (a) Pedersen, A.; Tilset, M. Organometallics 1993, 12, 3064− 3068. (b) Hughes, R. P.; Lindner, D. C.; Rheingold, A. L.; Yap, G. P. A. Organometallics 1996, 15, 5678−5686. (12) (a) Zuccaccia, C.; Bellachioma, G.; Bolaño, S.; Rocchigiani, L.; Savini, A.; Macchioni, A. Eur. J. Inorg. Chem. 2012, 2012, 1462−1468. (b) Hintermair, U.; Sheehan, S. W.; Parent, A. R.; Ess, D. H.; Richens, D. T.; Vaccaro, P. H.; Brudvig, G. W.; Crabtree, R. H. J. Am. Chem. Soc. 2013, 135, 10837−10851. (13) Campos, J.; Hintermair, U.; Brewster, T. P.; Takase, M. K.; Crabtree, R. H. ACS Catal. 2014, 4, 973−985. (14) For reviews, see: (a) Redshaw, C. Coord. Chem. Rev. 2003, 244, 45−70. (b) Homden, D. M.; Redshaw, C. Chem. Rev. 2008, 108, 5086−5130. (c) Chinta, J. P.; Ramanujam, B.; Rao, C. P. Coord. Chem. Rev. 2012, 256, 2762−2794. (d) Li, Y.; Zhao, K.-Q.; Redshaw, C.; Ortega, B. A. M.; Nuñez, A. Y.; Hanna, T. A., Coordination Chemistry and Applications of Phenolic Calixarene−metal Complexes. In PATAI’S Chemistry of Functional Groups, Zabicky, J., Ed.; Wiley, 2014. For selected examples of calix[4]arene transition-metal complexes, see: (e) Ishii, Y.; Onaka, K.-i.; Hirakawa, H.; Shiramizu, K. Chem. Commun. 2002, 1150−1151. (f) Buccella, D.; Parkin, G. J. Am. Chem. Soc. 2006, 128, 16358−16364. (g) Kotzen, N.; Goldberg, I.; Lipstman, S.; Vigalok, A. Inorg. Chem. 2006, 45, 5266−5268. (h) Aronica, C.; Chastanet, G.; Zueva, E.; Borshch, S. A.; ClementeJuan, J. M.; Luneau, D. J. Am. Chem. Soc. 2008, 130, 2365−2371. (i) Redshaw, C.; Rowan, M.; Homden, D. M.; Elsegood, M. R. J.; Yamato, T.; Pérez-Casas, C. Chem. - Eur. J. 2007, 13, 10129−10139. (j) Arbaoui, A.; Redshaw, C.; Elsegood, M. R. J.; Wright, V. E.; Yoshizawa, A.; Yamato, T. Chem. - Asian J. 2010, 5, 621−633. (k) Espinas, J.; Pelletier, J.; Jeanneau, E.; Darbost, U.; Szeto, K. C.; Lucas, C.; Thivolle-Cazat, J.; Duchamp, C.; Henriques, N.; Bouchu, D.; Basset, J.-M.; Chermette, H.; Bonnamour, I.; Taoufik, M. Organometallics 2011, 30, 3512−3521. (l) Czakler, M.; Artner, C.; Maurer, C.; Schubert, U. Z. Naturforsch., B: J. Chem. Sci. 2014, 69, 1253. (15) Rhodium and platinum calix[4]arene complexes possessing an anagostic interaction with the bridging methylene group have been reported; see refs 14e and g. (16) Gibson, V. C.; Redshaw, C.; Clegg, W.; Elsegood, M. R. J. Chem. Commun. 1997, 1605−1606. (17) (a) Davies, D. L.; Macgregor, S. A.; McMullin, C. L. Chem. Rev. 2017, 117, 8649−8709. (b) Musaev, D. G.; Figg, T. M.; Kaledin, A. L. Chem. Soc. Rev. 2014, 43, 5009−5031. (18) (a) Sánchez-Sordo, I.; Salinas, S. M. n. d.; Díez, J.; Lastra, E.; Gamasa, M. P. Organometallics 2015, 34, 4581−4590. (b) Campos, J.; Carmona, E. Organometallics 2015, 34, 2212−2221. (19) Complex 3 could not be obtained as a pure product due to its thermal instability, although tentative 1H NMR and X-ray diffraction data for 3 were in full agreement with the formulation shown in Scheme 2. For a preliminary solid-state structure of 3, see Figure S2a. (20) Only two isolable η1-Cp* complexes of group 4−10 transitionmetals ([(η1-Cp*)PtX(CO)(PR3)] (X = Cl, Br) and [(η1-Cp*)(η5Cp*)WO2]) have been reported, although no crystallographic data have been described for them. See refs 9a and b. (21) This indicates that the structure of 1 is more stable than that of the corresponding bis(phenoxo)−C−H agostic isomer.

CCDC 1824327−1824330 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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AUTHOR INFORMATION

Corresponding Author

*E-mail for Y.I.: [email protected]. ORCID

Takuya Kuwabara: 0000-0002-5259-0124 Shintaro Kodama: 0000-0003-4190-9539 Youichi Ishii: 0000-0002-1914-7147 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI (15K05460). REFERENCES

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DOI: 10.1021/acs.organomet.8b00257 Organometallics XXXX, XXX, XXX−XXX