Copper(II) Complexes of 2,2′:6′,2″-Terpyridine Derivatives for Catalytic Aerobic Alcohol Oxidations - Observation of Mixed-Valence Cu I Cu II Assemblesby Guoqi Zhang, E Liu, Chengxiong Yang, Li Li, James A. Golen, Arnold L. Rheingold

Eur. J. Inorg. Chem.

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Year
2015
DOI
10.1002/ejic.201403140
Subject
Inorganic Chemistry

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Job/Unit: I43140 /KAP1 Date: 28-01-15 13:03:04 Pages: 10

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DOI:10.1002/ejic.201403140

Copper(II) Complexes of 2,2:6,2-Terpyridine

Derivatives for Catalytic Aerobic Alcohol Oxidations –

Observation of Mixed-Valence CuICuII Assembles

Guoqi Zhang,*[a] E Liu,*[b] Chengxiong Yang,[b] Li Li,[b]

James A. Golen,[c] and Arnold L. Rheingold[c]

Keywords: Nitrogen heterocycles / Copper / Mixed-valent compounds / Oxidation / Homogeneous catalysis

The reactions of copper(II) salts with simple terpyridine (tpy) ligands gave mononuclear monoligand complexes 1–3, of which two new structures have been characterized by X-ray crystallography. These complexes were applied as catalysts for the oxidation of benzylic alcohol to benzaldehyde in air in the presence of the radical 2,2,6,6-tetramethylpiperidinyl1-oxyl (TEMPO). Copper complexes 1 and 2 are efficient catalysts for the reactions in water at 70 °C with 4-dimethylaminopyridine (DMAP) as an extra base. The influence of

TEMPO on the formation of new reactive intermediates during the catalytic reactions was tentatively investigated by introducing TEMPO into the reaction solutions of tpy derivaIntroduction

The oxidation of alcohols to aldehydes or ketones is an important organic transformation in synthetic chemistry.[1]

The development of new methodologies for atomeconomic, environmentally friendly, catalytic aerobic alcohol oxidations has been a key research topics in recent decades,[2,3] and transition metal catalyst systems are particularly effective for such transformations. Amongst the active metal catalysts reported for aerobic oxidations, those based on nonprecious, earth-abundant copper have received great attention from chemists owing to their low cost and environmental sustainability.[4–7] In 1984, Semmelhack and co-workers reported the first example of a copper-catalyzed aerobic oxidation of an alcohol mediated by 2,2,6,6tetramethylpiperidinyl-1-oxyl (TEMPO), that is, the effective aerobic oxidation of benzylic and allylic alcohols catalyzed by a CuCl/TEMPO system in N,N-dimethylformamide (DMF).[8] Recent progress has also indicated that the [a] Department of Sciences, John Jay College and

The Graduate Center, City University of New York,

New York, NY 10019, USA

E-mail: guzhang@jjay.cuny.edu [b] College of Chemical Engineering and Pharmacy,

Jingchu University of Technology,

Jingmen 448000, China

E-mail: liue2011@126.com [c] Department of Chemistry, University of California San Diego,

La Jolla, CA 92093, USA

Eur. J. Inorg. Chem. 0000, 0–0 © 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim1 tives with copper dichloride. Two new complexes with interesting solid-state structures resulting from these reactions have been isolated and characterized. The coordination reactions in the presence of TEMPO led to new mixed-valence

CuICuII supramolecular assembles (4 and 5), although they adopt either discrete complex or 1D polymeric structures.

Further catalytic studies indicated that the mixed-valence assembles 4 and 5 displayed higher catalytic activity than those of the mononuclear complexes under milder conditions. The relationship between the molecular structures of diverse copper complexes and their reactivity is discussed on the basis of the results obtained. combination of a variety of CuI or CuII salts with different organic ligands, typically involving bidentate N,N or N,O ligands, represented a class of best-performing catalysts for the oxidations of a range of alcohol substrates under mild conditions. Systems with various nitrogen-containing ligands, including 2,2-bipyridine (Bpy), 1,4-diazabicyclo[2.2.2]octane (DABCO), and 4,4-trimethylenedipyridine (TMDP), were recently developed by the Koskinen,

Sheldon, and Stahl groups[9–11] and effectively promote copper-catalyzed aerobic oxidation; of particular note are highly efficient copper/2,2-bipyridine/TEMPO systems, which have mainly been explored by the Stahl group.[12–15]

However, the known copper-containing catalyst systems are still limited, and structural studies relevant to any catalyst precursors and reactive intermediates have rarely been disclosed.[11–18] 2,2:6,2-Terpyridine (tpy) is an archetypical tridentate nitrogen ligand, and the use of tpy and its derivatives in metal coordination chemistry has been extensively studied since its initial observation.[19] Although the great potential of tpy and its derivatives in a variety of catalytic processes has been implicated,[20–26] they have scarcely been exploited as ancillary ligands for the aerobic oxidations of alcohols.

To the best of our knowledge, only one example concerning the copper(II) complexes of 4-phenyl-2,2:6,2-terpyridine (4-Phtpy) for the catalytic oxidation of benzylic alcohols was reported by the Pombeiro group very recently.[16]

Job/Unit: I43140 /KAP1 Date: 28-01-15 13:03:04 Pages: 10 www.eurjic.org FULL PAPER

In continuation of our recent studies of copper-catalyzed aerobic alcohol oxidation reactions,[27] we were interested in extending the known nitrogen ligands to tpy and its derivatives for the development of effective catalyst systems.

In this work, we wish to report the structures of several new copper(II) complexes with tpy, 4-chloro-2,2:6,2-terpyridine (4-Cltpy), and 4-cyano-2,2:6,2-terpyridine (4CNtpy) as well as their applications as catalyst precursors for catalytic alcohol oxidations (Scheme 1). Moreover, the structures of mixed-valence CuICuII/tpy supramolecular assembles resulting from TEMPO-mediated reactions involving tpy-type ligands are reported for the first time. Our catalytic studies suggest that the CuICuII/tpy catalyst precursors performed better for the oxidation of benzylic alcohol than their mononuclear CuII/tpy analogues.

Scheme 1. The molecular structures of 2,2:6,2-terpyridine (tpy), 4-chloro-2,2:6,2-terpyridine (4-Cltpy), and 4-cyano2,2:6,2-terpyridine (4-CNtpy).

Results and Discussion

Structures of the CuII Complexes

The reaction of tpy with copper(II) chloride in CH2Cl2/