Organometallic Chemistry of the Sassari University Unit

The research activity of the unit of Sassari is focalized on the synthesis, characterization and applications of cyclometalated complexes of transition metals, specifically noble metals. The applications concern especially the use of these complexes as catalysts, as potential biological agents and, finally, as models for the activation and functionalization of C-H bonds. In particular, our attention has been focused on the so-called “rollover” cyclometalation by means of platinum(II) electron-rich derivatives. We have investigated substituted 2,2’-bipyridines and related ligands, such as 2-(2’-pyridyl)quinoline, evaluating the effect of electronic and steric factors on the behaviour of these ligands. Our attention has been recently dedicated to the effect of a series of substituents (e.g. CF3, CH3, OCH3, Ph) on the reactivity and the reversibility of the rollover process, which offers the possibility to design catalytic cycles based on “rollover” and “retrorollover” reactions, where the bidentate donor may serve as a hydrogen atom reservoir. Rollover cyclometalation constitute an excellent route for the activation of and functionalization of C-H bonds usually not easily accessible, such as the C3 position in 2,2’-bipyridines. 1-7 Part of study has been focused to potentially terdentate cyclometalated ligands. Cyclometalation of 6-(1-phenylbenzyl)-2,2′-bipyridine directly generates a stereogenic carbon atom to give Pt(II) and Pd(II) terdentate N,N,C systems containing a six-membered ring with a boat-like conformation. Under different experimental conditions rollover cyclometalation occurs. Furthermore a series of new, potentialy terdentate pincer ligands, such as 1-(pyridin-2-yl)-3arylimidazo[1,5-a]pyridine and 1-(pyridin-2-yl)-3-benzylimidazo[1,5-a]pyridine, has been studied. Despite several attempts only chelated N,N Pt(II) and Pd(II) complexs have been isolated and characterized. There is also a great interest in multitasking catalysts that are able to efficiently promote different organic transformations by careful switching of the reaction parameters. In this context, osmium complexes have shown the ability to catalyze efficiently different types of reactions involving hydrogen, proving high thermal stability and simple synthesis. In the catalytic reduction of C═X (X = O, N) bonds by both hydrogenation (HY) and transfer hydrogenation (TH) reactions, the interest has been focused on homogeneous systems based on rhodium, iridium, and in particular ruthenium catalysts. By contrast, osmium catalysts have received much less attention being considered less active on account of their slower ligand exchange kinetics. Thus, this area remained almost neglected until recent studies refuted these prejudices. Some of us have demonstrate that osmium complexes are emerging as powerful catalysts for asymmetric and non-asymmetric syntheses, showing a remarkably high catalytic activity in HY and TH reactions of ketones, aldehydes, imines, and esters as well in DHY reactions of alcohols. Thus, for instance, the introduction of ligands with an NH function, possibly in combination with a pyridine ring, led to a new family of [OsCl2(PP)(NN)] (NN = diamine, 2-aminomethylpyridine; PP = diphosphine) and pincer [OsCl(CNN)(PP)] (HCNN = 6-aryl-2-aminomethylpyridine, 2-aminomethylbenzo[h]quinoline) complexes, which are outstanding catalysts for (asymmetric) HY and TH of carbonyl compounds and DHY of alcohols with turnover numbers and turnover frequencies up to 105 and 106 h –1 , respectively. In addition, PNN osmium complexes containing the 2-aminomethylpyridine motif have been found to be among the most active catalysts for HY of esters. These complexes have shown catalytic activities that are comparable and in some cases superior to those reported for analogous ruthenium systems. These results give an idea of the potential of Os complexes for the 2015 Activity. University of Sassari Bologna, March 11, 2016 *Delegate: Sergio Stoccoro Tel: +39 079229545; fax: +39 079229559; e-mail address: stoccoro@uniss.it; design of new highly productive and robust catalysts for the synthesis of chiral and nonchiral alcohols and amines as well as ketones from alcohols. Thus, we hope that this report will promote increased interest in the chemistry of these metal complexes, opening novel opportunities for new catalytic processes as well as the improvement of existing ones. 8 References [1] Rollover-assisted C(sp2)-C(sp3) bond formation. A. Zucca, L. Maidich, L. Canu, G. L. Petretto, S. Stoccoro, M. A. Cinellu, G. J. Clarkson, J. P. Rourke; Chem. – Eur. J., 2014, 20, 5501-5510. [2] Cyclometalated complexes of platinum(II) with 2-vinylpyridine. A. Zucca, L. Maidich, V. Carta, G. L. Petretto, S. Stoccoro, M. A. Cinellu, M. I. Pilo, G. J. Clarkson, Eur. J. Inorg. Chem., 2014, 2278-2287. [3] Synthesis and Characterization of Palladium(II) and Platinum(II) Adducts and Cyclometalated Complexes of 6,6’-Dimethoxy-2,2’-bipyridine – C(sp3)-H and C(sp2)-H Bond Activations. F. Cocco, A. Zucca, S. Stoccoro, M. Serratrice, A. Guerri, Maria A. Cinellu, Organometallics 2014, 33, 3414-3424. [4] Assembly of symmetrical and unsymmetrical platinum(II) rollover complexes with bidentate phosphine ligands. L. Maidich, G. Zuri, S. Stoccoro, M. A. Cinellu and A. Zucca, Dalton Trans. 2014, 43, 14806-14815. [5] Electronic and steric effects in rollover C-H bond activation. L. Maidich, G. Dettori, S. Stoccoro, M. A. Cinellu, J. P. Rourke, A. Zucca, Organometallics 2015, 34, 817-828. [6] Chiral cyclometalation of 6-(1-phenylbenzyl)-2,2′-bipyridine. S. Stoccoro, L. Maidich, T. Ruiu, M.A. Cinellu, G.J. Clarkson and A. Zucca, Dalton Trans., 2015, 44, 18001-18011. [7] Rollover cyclometalation of 6-methoxy-2,2′-bipyridine. L. Maidich, M.A. Cinellu, F. Cocco, S. Stoccoro, M. Sedda, S. Galli and A. Zucca. Submitted to J. of Organometallic Chemistry (2.02.2016). [8] Recent advances in osmium-catalyzed hydrogenation and dehydrogenation reactions. G. Chelucci, S. Baldino, W. Baratta, Account of Chemical Research, 2015, 48, 363-379.