Dimeric Rare‐Earth BINOLate Complexes: Activation of 1,4‐Benzoquinone through Lewis Acid Promoted Potential Shifts

Dimeric Rare Earth BINOLate Complexes: Activation of 1,4–Benzoquinone through Lewis-Acid Promoted Potential Shifts Jerome R. Robinson, [a] Corwin H. Booth, [b] Patrick J. Carroll, [a] Patrick J. Walsh, [a] * and Eric J. Schelter [a] * Abstract: Reaction of p–benzoquinone (BQ) with a series of rare earth/alkali metal/1,1′–BINOLate (REMB) complexes (RE: La, Ce, Pr, Nd; M: Li) results in the largest recorded shift in reduction potential observed for BQ upon complexation. In the case of cerium, the formation of a 2:1 [Ce]:[BQ] complex shifts the two electron reduction of BQ by ≥ 1.6 V to a more favourable potential. Reactivity investigations were extended to other RE III (RE = La, Pr, Nd) where the resulting highly electron deficient quinone ligands afforded isolation of the first lanthanide quinhydrone-type charge transfer complexes. The large reduction potential shift associated with the formation of 2:1 [Ce]:[BQ] complexes illustrate the potential of Ce Introduction Electron transfer (ET) reactions play an important role in chemical and biological systems. [1] In many cases, ET is enabled in Nature or in synthetic systems through the action of Lewis acidic metal cations. [2] Coordination of a Lewis basic electron acceptor to Lewis acidic metal cations can impact both the thermodynamic potential of the electron acceptor and the reorganization energy associated with the ET process. [2e-g] For the purposes of this work, we refer to the positive shift in the thermodynamic reduction potential of the electron acceptor upon coordination to a Lewis acid as Lewis Acid Promoted Potential Shift (LAPPS). Trivalent rare earth (RE III ) cations display the largest recorded LAPPS due the high Lewis [a] Department of Chemistry, University of Pennsylvania 231 S. 34 th St. Philadelphia, PA 19104 (USA) Fax: (+) (215) 573-2112 Tel: (+) (215) 898-8633, (215) 573-2875 E-mail: pwalsh@sas.upenn.edu, schelter@sas.upenn.edu [b] 1 Cyclotron Rd. MS70A-1150 Chemical Sciences Division, Lawrence Berkeley National Laboratory Berkeley, CA 94720, United States Fax: + (510) 486-5596 Tel (510) 486-6079 E-mail:chbooth@lbl.gov Supporting information for this article is available on the WWW under http://www.chemeurj.org/ or from the author. complexes to function both as a Lewis acids and electron sources in redox chemistry and organic substrate activation. Keywords: lanthanides •oxidation • electrochemistry • magnetism • spectroscopy acidity associated with these ions. [3] In the most dramatic examples, an ET process that is unfavorable (endergonic) in the absence of a Lewis acid occurs at an appreciable rate in the presence of a Lewis acid. [2f, g] For example, the oxidation of cobalt tetraphenylphorphyrin, E° ox = 0.35 V vs SCE in MeCN, with p– benzoquinone (BQ), E° red = –0.51 V vs SCE in MeCN, is endergonic; however, addition of strong Lewis acids such as Sc(OTf) 3 results in a ~1 V shift in the oxidation potential of BQ and a rapid ET event. [3a] While there are several examples of RE III inducing LAPPS, [2f, g, 3] there has been no investigation of RE III cations acting as both a Lewis acid in the activation of electron acceptors and as the electron donor. Cerium is the only rare earth ion with appreciable chemistry in the +4 oxidation state, which is necessary for the RE III to act as an electron donor. [4] To investigate the impact of LAPPS with Ce III complexes, we were interested in systems where (1) the cerium(III) ion is accessible to coordination by the Lewis basic oxidant, and (2) the formal potentials of the donor and acceptor alone are endergonic. With this in mind, we chose to investigate the reactivity of the electron-rich heterobimetallic framework [Li 3 (THF) 4 ][(BINOLate) 3 Ce–(THF)], 1–Ce, E pa = –0.45 V vs Fc in THF, [5] with the formally weak oxidant, p–benzoquinone, BQ, E BQ/BQ - = –1.03 V and E BQ - / BQ2 - = –2.05 vs Fc in THF. We hypothesized that the sterically accessible cerium(III) center of 1– Ce could induce activation of BQ, especially through dual coordination of BQ to form a 2:1 Ce:BQ complex. Herein we report the successful isolation of a rare, stable dimeric Ce IV complex, which displays the largest LAPPS observed for complexation by lanthanide cations (≥ 1.6 V). We also extended our reactivity investigations of BQ to Pr III and Nd III ions in the same