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Cp* Non-Innocence Leads to a Remarkably Weak C−H Bond Via View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Caltech Authors Subscriber access provided by Caltech Library Article Cp* Non-innocence Leads to a Remarkably Weak C-H Bond via Metallocene Protonation Matthew J. Chalkley, Paul H. Oyala, and Jonas C. Peters J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b00193 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 21, 2019 Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts. is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties. Page 1 of 9 Journal of the American Chemical Society 1 2 3 4 5 6 7 Cp* Non-innocence Leads to a Remarkably Weak C–H Bond via Me- 8 tallocene Protonation 9 10 Matthew J. Chalkley, Paul H. Oyala,* and Jonas C. Peters* 11 12 Division of Chemistry and Chemical Engineering, California Institute of Technology (Caltech), Pasadena, California 91125, 13 United States 14 ABSTRACT Metallocenes, including their permethylated variants, are extremely important in organometallic chemistry. In partic- 15 + ular, many are synthetically useful either as oxidants (e.g., Cp2Fe ) or as reductants (e.g., Cp2Co, Cp*2Co and Cp*2Cr). The latter 16 have proven to be useful reagents in the reductive protonation of small molecule substrates, including N2. As such, understanding 17 the behavior of these metallocenes in the presence of acids is paramount. In the present study, we undertake the rigorous characteri- 18 zation of the protonation products of Cp*2Co using pulse EPR techniques at low temperature. We provide unequivocal evidence for 4 + 19 the formation of the ring protonated isomers Cp*(exo/endo-η -C5Me5H)Co . Variable temperature Q-Band (34 GHz) pulse EPR 4 + 20 spectroscopy, in conjunction with DFT predictions, are key to reliably assigning the Cp*(exo/endo-η -C5Me5H)Co species. We 21 also demonstrate that exo-protonation selectivity can be favored by using a bulkier acid and suggest this species is thus likely a 22 relevant intermediate during catalytic nitrogen fixation given the bulky anilinium acids employed. Of further interest, we provide physical data to experimentally assess the C−H bond dissociation free energy (BDFE ) for Cp*(exo-η4-C Me H)Co+. These ex- 23 C−H 5 5 perimental data support our prior DFT predictions of an exceptionally weak C–H bond (<29 kcal mol−1), making this system among 24 the most reactive (with respect to C–H bond strength) to be thoroughly characterized. These data also point to the propensity of 25 4 − Cp*(exo-η -C5Me5H)Co to mediate hydride (H ) transfer. Our findings are not limited to the present protonated metallocene sys- 26 tem. Accordingly, we outline an approach to rationalizing the reactivity of arene-protonated metal species, using decamethylnickel- 27 ocene as an additional example. 28 29 30 31 1. INTRODUCTION ates have not been reliably identified and characterized. It has been presumed that the direct reactions of acids with reducing 32 Metallocenes such as ferrocene, chromocene, and cobaltocene have enjoyed a privileged role in the development of organo- metallocenes are deleterious to selectivity for N2RR versus H2 33 generation.4,15 34 metallic chemistry and serve as useful reagents owing to their high compositional stabilities and accessible redox couples.1,2 Our lab became interested in metallocenes following the ob- 35 Indeed, many chemists first encounter metallocenes in the servation that Cp* Co could serve as the electron source for 36 2 context of their one-electron redox chemistry, with the Fe-mediated N2RR in the presence of anilinium acids and an 37 +/0 +/0 +/0 B B Cp2Fe , Cp2Cr , and Cp2Co couples, and those of their iron catalyst, P3 Fe (P3 = tris(o-diisopropylphosphinophenyl)- 8,9 38 related permethylated variants, being some of the most com- borane). Indeed, the selectivity for N2RR under these condi- 3 39 monly exploited in all of synthetic chemistry. tions proved far more efficient for NH3 formation (up to 78%) 40 than our originally published conditions using KC and An area where divalent metallocene reductants (e.g., Cp2*Cr, 8 [H(OEt ) ][BArF ] (HBArF , BArF = tetrakis(3,5- 41 Cp2*Co) have been proven particularly effective is catalytic 2 2 4 4 4 4,5,6 bis(trifluoromethyl)phenyl)borate)).16,17 However, contrary to 42 N2-to-NH3 conversion (N2RR). Schrock first identified their F 43 utility in this context via the discovery of a molybdenum our mechanistic experiments with HBAr 4, reaction of P BFeN − with anilinium acids led neither to the observation of 44 tris(amido)amine ([HIPTN3N]Mo, HIPTN3N = [(3,5-(2,4,6- 3 2 i 3− relevant intermediates (e.g., P BFe(NNH )+) in freeze-quench 45 Pr3C6H2)2C6H3NCH2CH2)3N] ) N2RR catalyst system em- 3 2 4 spectroscopic methods, nor to the observation of fixed-N ploying lutidinium as the acid and Cp2*Cr as the reductant. 46 products upon annealing.9,18,19 47 Since that discovery, other labs have exploited related cock- tails that pair a metallocene reductant with an acid to drive The apparent need for both acid and reductant to be present to 48 N2RR using a range of metal catalysts, with selectivities and achieve productive N–H bond formation is reminiscent of 49 5,6,7,8,9 turnover numbers that continue to improve. Schrock and coworkers’ observations when attempting to 50 The protonation chemistry of metallocenes is well studied, functionalize [HIPTN3N]Mo–N2 with catalytically relevant 51 20 especially among Group 810,11 and 1012,13 metallocenes. Relat- reagents. In both cases, we have hypothesized that metallo- 52 ed studies on more reducing Group 6 and 9 metallocenes (e.g., cene-mediated proton-coupled electron transfer reactions may 53 7,9,21 Cp*2Cr, Cp2Co, Cp*2Co), which are relevant to the aforemen- play a key role in N–H bond-forming steps. Furthermore, 54 tioned proton-coupled reduction of N2, have been much more given the ubiquity of these metallocene reagents in N2RR, we 55 limited. While studies have shown that the release of H2 is wondered whether metallocene-mediated N–H bond forming 56 highly favorable on both thermodynamic and kinetic steps might provide a contributing, or even dominant, mecha- 57 grounds,14 protonated Group 6 and 9 metallocene intermedi- nistic pathway. 58 59 60 ACS Paragon Plus Environment Journal of the American Chemical Society Page 2 of 9 Density functional theory (DFT) studies by our group support- 2.1 Pulse Electron Paramagnetic Resonance Spectros- 1 ed the notion that protonation of metallocenes such as Cp2*Co copy on Protonated Cp*2Co. To interrogate the reaction of 2 or Cp2*Cr by catalytically relevant acids is thermodynamically Cp*2Co with HOTf, we employed Q-band pulse EPR experi- 3 favorable. To our surprise, these DFT studies also predicted ments at very low temperatures. Electron spin-echo (ESE) 4 that ring protonation is thermodyamically favored versus pro- detected, field-swept spectra at Q-band, performed at 6 K and 8 5 tonation at the metal (to form a hydride). Such selectivity 10 K, clearly identify the presence of two different species 6 would contrast with the classic case of ferrocene, where proto- with dramatically different g-anisotropy in the precipitated nation at iron has been firmly established.10,22 The Cp *Co and solid (Figure 2). Fortuitously, measurement of the approxi- 7 2 Cp* Cr ring-protonated species are predicted to have remarka- mate spin-lattice relaxation rates via inversion recovery (see 8 2 bly weak C–H bond dissociation enthalpies (BDE <37 kcal SI) reveals that the two species have significantly different T1’ 9 mol−1), which should in turn make them excellent PCET do- times. The species with higher g-anisotropy (g = [2.625, 2.349, 10 nors.7,9 These observations intimate that protonated metallo- 1.984]) exhibits a much shorter T1’ than the species exhibiting 11 cene intermediates might thereby offer viable pathways for N– a narrower spectrum (g = [2.170, 2.085, 2.005]), even at 6 K. 12 H bond formation via PCET (or even hydride transfer; vide This difference in relaxation rates becomes more dramatic 13 infra), in addition to the more commonly presumed pathway upon warming the sample to 10 K; at this temperature, T1’ for 14 for deleterious H2 evolution (Figure 1).
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