Catalysis Science & Technology Accepted Manuscript This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. www.rsc.org/catalysis Page 1 of 9 Catalysis Science & Technology Catalysis Science and Technology RSC Publishing ARTICLE Oxidation Catalysis in Air with Cp*Ir: Influence of Added Ligands and Reaction Conditions on Catalytic Cite this: DOI: 10.1039/x0xx00000x Activity and Stability Ahmet Gunay, Mark A. Mantell, Kathleen D. Field, Wenbo Wu, Michael Chin, Manuscript Received 00th January 2012, and Marion H. Emmert a*, Accepted 00th January 2012 DOI: 10.1039/x0xx00000x We describe the systematic evaluation of Cp*Ir catalysts for the aerobic oxidation of alcohols. Our results demonstrate turnover numbers up to 270 per [Cp*IrCl ] which have not been www.rsc.org/ 2 2 previously achieved for this reaction. Using air as the sole oxidant under base-free conditions, the effects of solvent systems and additives on the catalytic activity are documented systematically. We further elucidate the role of additives in catalyst decomposition processes and establish a novel buffer system which results in significant catalyst stabilization upon prolonged reaction times. Accepted Introduction reactions with O 2 are rather rare. Few reports of O 2 coupled catalysis using Cp*Ir complexes have been published13,14 and Cp*Ir complexes have received considerable attention lately as all of them exhibit fundamental drawbacks: high catalyst catalyst precursors for reactions relevant to sustainable energy loadings (5-10 mol % [Ir]) indicating significant catalyst 1 2 and green chemistry: water oxidation, O2 reduction, H2 decomposition or low efficiencies of the used catalysts, 3 4 5 5 t activation, CO 2 hydrogenation, , formic acid decomposition, stoichiometric amounts of strong bases (KO Bu, NEt 3, 6 acceptorless dehydrogenation, ammonia alkylation with Na 2CO 3), or a large excess of O 2 as oxidant (open-flask, air 7 6a,8 9 alcohols, transfer (de)hydrogenation, and C-H activation. balloon, pure O 2 gas). The latter issue is particularly important The majority of these reactions are oxidation/reduction when considering large scale aerobic oxidation reactions, as an Technology reactions, and as such the use of sustainable reductants and excess of O 2 in the presence of large amounts of organic 15 oxidants (i.e., H 2 and O 2) is an important goal for the solvent constitutes a considerable explosion hazard. The role development of green catalytic methodologies. 2 of ancillary ligands in these Cp*Ir catalyst systems is not well & Studying Cp*Ir complexes as oxidation catalysts has so far understood, as they exhibit a wide variety of structural 10 mainly focused on reactions in the presence of strong oxidants differences (no ligands vs. bidentate N,N/C,N ligands; or on alcohol dehydrogenation reactions which produce H 2 as secondary amines vs. pyridines) and the used conditions are not side product. 6,11 Catalysis with strong oxidants is often not comparable from one protocol to another. well-defined, as ligand oxidation and subsequent ligand loss One recent report elucidates the mechanism of aerobic alcohol can occur, leading to nanoparticles which can also be oxidations with simple [Cp*IrCl 2]2 in the presence of NEt 3 as 10b 14 catalytically active. Dehydrogenative catalysis, on the other additive. In these studies, employing 1 atm of O 2 and 10 mol hand, suffers from the drawback that it needs to be performed % [Ir] are crucial for high yields (39-86%, depending on the substrate). The use of NEt (20-100 mol %) has been under conditions that successfully drive off H 2; as such, high 3 Science temperatures are inherently necessary in these reactions to rationalized with the need to deprotonate the coordinated provide a thermodynamic driving force. In contrast to alcohol substrate in order to achieve β-H elimination (Scheme dehydrogenative Cp*Ir catalysis, aerobic oxidations have in 1). The proposed mechanism also speculates that the by- principle the potential to enable mild catalytic conditions due to product H 2O2 decomposes to 0.5 O2 and H 2O under the reaction the higher thermodynamic driving force of oxidative reactions. conditions (toluene, 80 ºC) based on the O 2 uptake Additionally, excellent atom economies can be realized with stoichiometry and fast H 2O2 disproportionation in independent H O as the only by-product of catalysis. Furthermore, ligand experiments. However, no information is provided regarding 2 16 decomposition can be expected to be less problematic than with the speciation of the Ir catalyst after its reaction with O 2, the other oxidants, since O 2 is a kinetically hindered oxidant. effects of other bases on catalysis, or regarding catalytic Despite these interesting features of aerobic oxidations and activity in the absence of bases. Due to these limitations, recent advances that elucidate stoichiometric reactions of O 2 general predictions for the design of efficient and stable Cp*Ir Catalysis with organometallic Ir complexes, 2,12 Cp*Ir catalysts in catalysts in aerobic transformations cannot be made. This journal is © The Royal Society of Chemistry 2013 J. Name ., 2013, 00 , 1-3 | 1 Catalysis Science & Technology Page 2 of 9 ARTICLE Journal Name In order to address these issues (requirement for 1 atm of O 2, ligands bound to Ir, we reacted 1 mol % [Cp*IrCl 2]2 with 4 mol high catalyst loadings, and base; lack of information on additive % of several Ag salts in situ (Scheme 3). This approach has effects), we have performed a comparative study of additives been used widely in the literature to synthesize Ir compounds that can potentially serve as ligands to Ir and reaction conditions for the air oxidation of alcohols at low catalyst with different X-type ligands from chloride precursor 23 loadings. The herein described investigations reveal that water complexes and to evaluate the activity of Cp*Ir complexes content, solvents, and additives all play a crucial role for with different X-type ligands for C-H activation under catalyst stability and need to be taken into account when catalytically relevant conditions. 19,24 establishing efficient aerobic oxidation protocols. In contrast to 12,13 previous reports the presence of a strong base is not OH O required for high activity in our systems. 1 mol % [Cp*IrCl2]2 RCH OH 2 mol % L 2 1 2 [Cp*IrCl ] [(Cp*IrCl 2)2(RCH 2OH)] 2 2 100 ºC, air, toluene H O + 2 H O 2 2 NEt 3 0.5 O2 + NEt 3 Manuscript HNEt 3Cl HNEt 3Cl + RCHO + O2 [(Cp*IrCl) 2(µ-H)( µ-Cl)] Scheme 1. Previously Proposed Mechanism of [Cp*IrCl 2]2 Catalyzed Aerobic Alcohol Oxidation. Results and discussion 1.1 L-type Additive Effects on Catalytic Activity. Initial studies focused on evaluating the effect of various pyridine and amine additives on the activity of [Cp*IrCl ] ,17 as these bases 2 2 Accepted i t represent the most common catalyst modifications used in the L none NEt3 HNEt2 HN Pr2 H2N Bu literature for Cp*Ir catalyzed aerobic oxidations (Scheme N N OH 13a,b,18 N 2). Previous detailed studies of [Cp*IrCl 2]2 and related complexes ,19 suggest that addition of pyridines in non- Scheme 2. L-Type Additive Effects on Catalytic Activity of coordinating solvents result in formation of complexes of the [Cp*IrCl 2]2. formula [Cp*IrCl 2(pyridine)], which might be the active catalysts in the reactions in Scheme 2. 1-Phenyl-1-propanol ( 1) was chosen as a substrate for this study due to its low volatility OH O and electron-neutral character, which is expected to help in the 1 mol % [Cp*IrCl 2]2 development of a widely applicable catalyst system.14 An AgX additional advantage of using 1 as substrate is a simplified GC 1 4 mol % 2 100 ºC, air, toluene analysis of the reaction mixture, as ketone 2 is the only Technology obtained oxidation product. 20 In order to benefit catalyst development, all subsequent reactions involving Ir catalysts & were performed at a low catalyst loading of 1 mol % [Cp*IrCl 2]2 (2 mol % [Ir]) using sealed vials in toluene as solvent. 21 Reaction yields were measured after 2 and 24 h in order to account for initial reactivity and long-term stability. Employing simple [Cp*IrCl 2]2 as the catalyst under these conditions resulted in formation of 53% of 2 after 24 h with the reaction yielding only 10% after 2 h (Scheme 2). 22 Addition of 2 mol % of L-type additives to form 2 mol % of Cp*IrCl 2(L) Science (3) in situ did not result in higher yields after 24 h. However, none AgBF4 AgO2CCF3 AgNO3 i AgX several primary and secondary amine bases (HNEt 2, HN Pr 2, AgOTf AgPF6 AgOAc t H2N Bu) increased the short-term reactivity of the catalyst, as shown by higher yields of 2 after 2 h.