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H/D Exchange Under Mild Conditions in Arenes and Unactivated Alkanes with C6D6 and D2O Using Rigid, Electron-Rich Iridium PCP Pincer Complexes

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H/D Exchange Under Mild Conditions in Arenes and Unactivated Alkanes with C6D6 and D2O Using Rigid, Electron-Rich Iridium PCP Pincer Complexes doi.org/10.26434/chemrxiv.12276632.v1 H/D Exchange Under Mild Conditions in Arenes and Unactivated Alkanes with C6D6 and D2O Using Rigid, Electron-rich Iridium PCP Pincer Complexes Joel D. Smith, George Durrant, Daniel Ess, Warren Piers Submitted date: 09/05/2020 • Posted date: 11/05/2020 Licence: CC BY-NC-ND 4.0 Citation information: Smith, Joel D.; Durrant, George; Ess, Daniel; Piers, Warren (2020): H/D Exchange Under Mild Conditions in Arenes and Unactivated Alkanes with C6D6 and D2O Using Rigid, Electron-rich Iridium PCP Pincer Complexes. ChemRxiv. Preprint. https://doi.org/10.26434/chemrxiv.12276632.v1 The synthesis and characterization of an iridium polyhydride complex (Ir-H4)supported by an electron-rich PCP framework is described. This complex readily loses molecularhydrogen allowing for rapid room temperature hydrogen isotope exchange (HIE) at the hydridicpositions and the α-C-H site of the ligand with deuterated solvents such as benzene-d6, toluene-d8and THF-d8. The removal of 1-2 equivalents of molecular H2 forms unsaturated iridium carbenetrihydride (Ir-H3) or monohydride (Ir-H) compounds that are able to create further unsaturationby reversibly transferring a hydride to the ligand carbene carbon. These species are highly activehydrogen isotope exchange (HIE) catalysts using C6D6 or D2O as deuterium sources for thedeuteration of a variety of substrates. By modifying conditions to influence the Ir-Hn speciation,deuteration levels can range from near exhaustive to selective only for sterically accessible sites.Preparative level deuterations of select substrates were performed allowing for procurement of>95% deuterated compounds in excellent isolated yields; the catalyst can be regenerated bytreatment of residues with H2 and is still active for further reactions. File list (1) Text221_ChemRxiv.pdf (5.79 MiB) view on ChemRxiv download file H/D Exchange Under Mild Conditions in Arenes and Unactivated Alkanes with C6D6 and D2O Using Rigid, Electron-rich Iridium PCP Pincer Complexes Joel D. Smith,a George Durrant,b Daniel H. Essb and Warren E. Piersa* aDepartment of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada bDepartment of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602, USA KEYWORDS: PCP Pincer, Polyhydride, C-H Activation, H/D exchange, Iridium. ABSTRACT: The synthesis and characterization of an iridium polyhydride complex (Ir-H4) supported by an electron-rich PCP framework is described. This complex readily loses molecular hydrogen allowing for rapid room temperature hydrogen isotope exchange (HIE) at the hydridic positions and the α-C-H site of the ligand with deuterated solvents such as benzene-d6, toluene-d8 and THF-d8. The removal of 1-2 equivalents of molecular H2 forms unsaturated iridium carbene trihydride (Ir-H3) or monohydride (Ir-H) compounds that are able to create further unsaturation by reversibly transferring a hydride to the ligand carbene carbon. These species are highly active hydrogen isotope exchange (HIE) catalysts using C6D6 or D2O as deuterium sources for the deuteration of a variety of substrates. By modifying conditions to influence the Ir-Hn speciation, deuteration levels can range from near exhaustive to selective only for sterically accessible sites. Preparative level deuterations of select substrates were performed allowing for procurement of >95% deuterated compounds in excellent isolated yields; the catalyst can be regenerated by treatment of residues with H2 and is still active for further reactions. 1 Introduction Metal-catalyzed hydrogen isotope exchange (HIE) is a key fundamental process for preparing isotopically labeled organic compounds necessary for studying reaction mechanisms and biological metabolic processes.1 Since the seminal work on homogeneous HIE by Garnett and Hodges2, 3 as well as Shilov and co-workers,4, 5 advances in analytical biology and chemistry have created an ever growing demand for deuterium6 and tritium7 labeled molecules.8 Consequently, an important body of research devoted to studying the homogeneously catalyzed exchange of hydrogen and deuterium atoms (H/D exchange) has developed over the last 25 years.9-12 Pioneering work by the Bergman group on pentamethylcyclopentadienyl supported iridium hydrides demonstrated low temperature C-H activation13-15 and it was subsequently shown that mild H/D exchange by cationic iridium complexes16-18 occurred under relatively mild conditions using C6D6 as a deuterium source. Most of these transformations involved selective deuterium incorporation into arene substrates10, 11 with only a handful of catalysts capable of H/D exchange on unactivated alkane substrates under more forcing conditions.16, 17, 19-24 Tridentate “pincer” ligands have played a prominent role in homogeneous catalysis.25, 26 For example, polyhydridic pincer complexes of the late transition metals (particularly Ru and Ir) 27 28, 29 are known to be exemplary alkane dehydrogenation and CO2 reduction catalysts, but recent work has demonstrated their promise as competent catalysts for promoting homogenous H/D exchange. Leitner, Milstein and co-workers were the first to show that a Ru based PNP 30, 31 polyhydride, I Chart 1, could use C6D6 or D2O for arene deuteration at 50°C. Shortly after, Zhou and Hartwig revealed that the aliphatic iridium PCP polyhydride II could be used as a pre- catalyst to efficiently perform H/D exchange between olefins and C6D6 at room temperature 2 without isomerization.32 Following these reports, PNP (III) and PCP (IV) complexes synthesized by Grubbs33 and Wendt34, respectively, were also tested as HIE mediators. The iridium dihydride Chart 1. Contextualization of this study. Leitner & Milstein Hartwig 2007 • Selective H/D Exchange • Selective H/D Exchange 2008 t t P( Bu)2 of Arenes of Olefins and N-Heterocycles P( Bu)2 0 - 97% D H2 • Mild Temperatures • Mild Temperatures N < 5% > 90% 92% N Ru H • Deuteration with C6D6 & D2O • Deuteration with C6D6 H Ir (H)4 D < 5% R H 95% t > 84% t D P Bu2 > 84% P Bu2 95% 28% 0 - 97% I 50 - 75 °C Hydrogen Isotope Exchange II RT - 50 °C 72 hrs with Pincer Complexes 1 min to 60 hrs 1 1 R [Cat] R R2 C H R2 C D R3 Source: R3 Grubbs D2O and/or C6D6 Wendt 2011 2016 t 15% P(iPr) • H/D Exchange at P( Bu)2 2 1 R unactivated Csp3-H sites H O H • Selective H/D Exchange H 9% R2 Si D Selective H/D Exchange 25% N Ir of Arenes & Silanes • Ir 3 • Mild Temperatures of Arenes H H R 9% > 95% • High Temperature t i > 95% • Deuteration with C D & D O P( Bu)2 P( Pr)2 6 6 2 > 95% • Deuteration with C6D6 80 °C 150 °C III 72 hrs IV 24 hrs Piers 2012 Piers 2016 This Work <_ N 27% _ _ i (iPr) i < 94% N < 94% ( Pr)2 2 ( Pr)2 P P P S <_ 94% <_ 94% H H H Ir (H)4 Ir (H)4 Ir (H)4 <_ 95% <_ 95% S P P P i i <_ ( Pr)2 (iPr) ( Pr)2 37% 2 N O Averaged <_ 95% V VI Ir-H4 <_ 95% <_ 63% • No observed Deuteration • Observed Deuteration of • H/D Exchange of unactivated Csp3-H sites and Alkanes of Hydrides & Ligand C-Hs Hydrides & Ligand C-Hs • Selective and Exhaustive H/D Exchange of Arenes with toluene-d8 • Mild Temperatures • Deuteration with C6D6 and D2O species III was able to selectively deuterate aromatic compounds and silanes, while the cyclohexyl-based PCP complex IV was found to successfully deuterate arenes and to a limited extent Csp3-H sites, albeit at elevated temperatures of 150 °C. Design elements of the ligands II/IV and III have been incorporated into a family of PCP ligands V introduced by our group35 and also explored by others.36-39 An attractive feature of these ligands is their ability to assume PCsp3P or PCcarbeneP forms by the addition or removal of 3 a hydrogen atom from the anchoring carbon of the pincer framework. This has precedence in the ligands II and IV but in the aryl-linked PCP systems V (and the related benzothiophene-based system VI40) this is potentially more controllable because of the lack of β-hydrogens in the PCP linker framework, and through manipulation of the electronic properties of the ligand via substitution on the aryl linkers and/or rigidification of the ligand via additional linking of the backbone aryl groups.41, 42 With these two structural levers, a library of PCP ligands has been + 43 prepared and a substantial range of νCO values has been found for [(PCcarbeneP)IrCO] species. 40 We have shown that the rate of addition of N2O to the C=Ir bond can therefore be tuned 44 substantially. Furthermore, the rigidification of the ligand tends to favor the PCcarbeneP form of the ligand vs. the PCsp3P form, while also providing greater protection against C-C bond cleavage reactions that destroy the integrity of the ligand in unlinked systems akin to V.45 The ability of the carbon atom in these PCP pincers to engage in ligand cooperativity has been exploited in hydrogen,35, 46 silane47-49 and other small molecule40, 50-53 activations in various systems. Furthermore, in polyhydridic systems, the dibenzylic hydrogen is actively involved in exchange with the metal hydrides,34, 35 increasing the “hydrogen capacity” of these compounds relative to other (for example PNP) polyhydridic pincer complexes and potentially providing new pathways for catalysis. In both the parent, unsubstituted, unlinked system V,35 and that incorporating the benzothiophene-based ligand VI,46 the Ir polyhydride is only stable under an atmosphere of dihydrogen but it was observed that slow H/D exchange in the sterically exposed C-H bonds of the ligand aryl backbone occurred in C6D6. We hypothesized that a more electron rich system would accelerate this process and indeed, the linked dihydroanthracene PCP ligand framework fitted with electron donating dimethylamino groups para to the PCP carbon (bottom of Chart 1) 4 supports a stable, isolable polyhydride species Ir-H4 (Chart 1).
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