Citation: Jia, X.; Tian, S.; Shivokevich, P.J.; Harman, W.D.; Dickie, D.A.; Gunnoe, T.B. Electron-Deficient Ru(II) Complexes as Catalyst Precursors for Ethylene Hydrophenylation. Inorganics 2022, 10, 76. https:// doi.org/10.3390/inorganics10060076 Academic Editors: Wolfgang Linert, Duncan H. Gregory, Richard Dronskowski, Vladimir Arion, Claudio Pettinari and Torben R. Jensen Received: 10 May 2022 Accepted: 27 May 2022 Published: 31 May 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). inorganics Article Electron-Deficient Ru(II) Complexes as Catalyst Precursors for Ethylene Hydrophenylation Xiaofan Jia , Songyuan Tian, Philip J. Shivokevich, W. Dean Harman, Diane A. Dickie and T. Brent Gunnoe * Department of Chemistry, University of Virginia, Charlottesville, VA 22904, USA; xiaofan.jia@yale.edu (X.J.); sst2nh@virginia.edu (S.T.); pjs6kb@virginia.edu (P.J.S.); wdh5z@virginia.edu (W.D.H.); dad8v@virginia.edu (D.A.D.) * Correspondence: tbg7h@virginia.edu Abstract: Ruthenium(II) complexes with the general formula TpRu(L)(NCMe)Ph (Tp = hydrido(trisp yrazolyl)borate, L = CO, PMe 3 , P(OCH 2 ) 3 CEt, P(pyr) 3 , P(OCH 2 ) 2 (O)CCH 3 ) have previously been shown to catalyze arene alkylation via Ru-mediated arene C–H activation including the conversion of benzene and ethylene to ethylbenzene. Previous studies have suggested that the catalytic performance of these TpRu(II) catalysts increases with reduced electron-density at the Ru center. Herein, three new structurally related Ru(II) complexes are synthesized, characterized, and studied for possible catalytic benzene ethylation. TpRu(NO)Ph 2 exhibited low stability due to the facile elimination of biphenyl. The Ru(II) complex (Tp Br3 )Ru(NCMe)(P(OCH 2 ) 3 CEt)Ph (Tp Br3 = hydridotris(3,4,5-tribromopyrazol- 1-yl)borate) showed no catalytic activity for the conversion of benzene and ethylene to ethylbenzene, likely due to the steric bulk introduced by the bromine substituents. (Ttz)Ru(NCMe)(P(OCH 2 ) 3 CEt)Ph (Ttz = hydridotris(1,2,4-triazol-1-yl)borate) catalyzed approximately 150 turnover numbers (TONs) of ethylbenzene at 120 ◦ C in the presence of Lewis acid additives. Here, we compare the activity and features of catalysis using (Ttz)Ru(NCMe)(P(OCH 2 ) 3 CEt)Ph to previously reported catalysis based on TpRu(L)(NCMe)Ph catalyst precursors. Keywords: olefin hydroarylation; ruthenium; catalysis; ethylbenzene; C–H activation 1. Introduction Alkyl arenes serve as precursors for a wide range of products, including polymers, pharmaceuticals, and surfactants. For example, ethylbenzene is produced on a scale of approximately 40 million tons each year [1]. The current industrial synthesis of alkyl arenes is accomplished either by a traditional Friedel–Crafts alkylation, which is catalyzed by a Lewis acid (e.g., AlCl 3 ) in the presence of a Brønsted acid (e.g., HF), or by using an acidic zeolite catalyst [2–6]. Due to the nature of the acid-mediated electrophilic arene substitution reaction, important reaction outcomes are dictated by the mechanism of the reaction. For example, the alkyl arene product is generally more reactive than the arene substrate, which can result in polyalkylation at even low or modest conversions [3]. Moreover, reactions using α-olefins exclusively give products with x-aryl alkanes where x ≥ 2. The selective synthesis of 1-aryl alkanes is not viable with current commercial catalytic processes for arene alkylation [7–10]. Catalytic arene alkylation has been reported using molecular complexes and homo- geneous catalysts based on iridium [11,12], platinum [13–19], and ruthenium [20–28], and direct arene alkenylation has been reported using rhodium [29–39], ruthenium [40,41], and palladium [42,43] catalysts. These catalytic processes provide routes for the synthesis of alkyl and alkenyl arenes that provide complementary selectivity to acid-catalyzed arene alkylation reactions [3]. Recent success for Rh-catalyzed arene alkenylation to give anti- Markovnikov products and the newly reported Ni catalysis by the Hartwig and Eisenstein groups for the formation of 1-aryl alkanes or alkenes, which we have termed super linear Inorganics 2022, 10, 76. https://doi.org/10.3390/inorganics10060076 https://www.mdpi.com/journal/inorganics