Catalytic properties of nickel bis(phosphinite) pincer complexes in the reduction of CO 2 to methanol derivatives Sumit Chakraborty, Yogi J. Patel, Jeanette A. Krause, Hairong Guan ⇑ Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, OH 45221, USA article info Article history: Available online 4 May 2011 Keywords: Nickel Pincer complex Carbon dioxide Reduction Methanol Borane abstract A new nickel bis(phosphinite) pincer complex [2,6-(R 2 PO) 2 C 6 H 3 ]NiCl (L R NiCl, R = cyclopentyl) has been prepared in one pot from resorcinol, ClP(C 5 H 9 ) 2 , NiCl 2 , and 4-dimethylaminopyridine. The reaction of this pincer compound with LiAlH 4 produces a nickel hydride complex, which is capable of reducing CO 2 rap- idly at room temperature to give a nickel formate complex. X-ray structures of two related nickel formate complexes L R NiOCHO (R = cyclopentyl and isopropyl) have shown an ‘‘in plane’’ conformation of the for- mato group with respect to the coordination plane. The stoichiometric reaction of nickel formate com- plexes L R NiOCHO (R = cyclopentyl, isopropyl, and tert-butyl) with catecholborane has suggested that the reaction is favored by a bulky R group. L R NiOCHO (R = tert-butyl) does not react with PhSiH 3 at room temperature; however, it reacts with 9-borabicyclo[3.3.1]nonane and pinacolborane to generate a meth- anol derivative and a boryl formate species, respectively. The catalytic reduction of CO 2 with catecholbo- rane is more effectively catalyzed by a more sterically hindered nickel pincer hydride complex with bulky R groups on the phosphorus donor atoms. The nickel pincer hydride complexes are inactive catalysts for the hydrosilylation of CO 2 with PhSiH 3 . Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Catalytic reduction of CO 2 to liquid fuels such as methanol and hydrocarbons is one of the key strategies to mastering carbon man- agement [1]. Homogeneous systems of such a process are exceed- ingly rare in the literature [2–4], despite the widespread use of homogeneous catalysts in many other reactions. To make metha- nol or its derivatives in particular, from CO 2 , one has to use an appropriate reagent to abstract one of the two oxygen atoms in CO 2 . Successful examples have involved the use of boron- [5] or aluminum-based [6] frustrated Lewis acid–base pairs, largely due to the oxophilic nature of these reagents. Simple silanes [7,8] and boranes [9] may also be used, although a catalyst must be em- ployed to facilitate the reduction. We have recently reported that in the presence of a catalytic amount of L tBu NiH (Fig. 1), the reac- tion of CO 2 with HBcat affords CH 3 OBcat along with catBOBcat as the oxygen-containing byproduct [9]. DFT calculations of our cata- lytic system have implied that the steric environment around the nickel center is critical to the efficiency of the catalysis [10]. The choice of reductant is also crucial; the same computational study has predicted that, if PhSiH 3 is used instead, the reduction of CO 2 will not occur [10]. To validate these computational results, we re- port herein the synthesis of related nickel bis(phosphinite) pincer complexes with different sizes of alkyl groups on the phosphorus donor atoms, and the catalytic properties of these complexes in the reduction of CO 2 with various boranes and PhSiH 3 . 2. Results and discussion 2.1. Synthesis of L cPe NiH We have previously prepared L R NiH (R = tBu and iPr) through the reaction of L R NiCl with LiAlH 4 in toluene [11]. Unfortunately, using the same approach for the synthesis of L Ph NiH and L Me NiH produced intractable products. It is likely that the smaller R groups do not provide enough steric protection to stabilize the hydride, or perhaps in these complexes the more exposed pincer P–O bonds are susceptible to cleavage by an adventitious base [12]. We were, however, able to isolate L cPe NiH in 55% yield from the reaction between LiAlH 4 and L cPe NiCl, which was obtained following a similar procedure described in the literature [13–17], or alterna- tively from a one-pot synthesis as shown in Scheme 1. The 1 H NMR spectrum of L cPe NiH in C 6 D 6 displays a characteristic hydride resonance at À7.88 ppm as a triplet ( 2 J P–H = 54.4 Hz), and the 31 P{ 1 H} NMR spectrum exhibits a singlet at 199.66 ppm. 0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2011.04.030 Abbreviations: HBcat, catecholborane; DFT, density functional theory; tBu, tert-butyl; iPr, isopropyl; cPe, cyclopentyl; DMAP, 4-dimethylaminopyridine; 9-BBN, 9-borabicyclo[3.3.1]nonane; HBpin, pinacolborane; TON, turnover number. ⇑ Corresponding author. Tel.: +1 513 556 6377; fax: +1 513 556 9239. E-mail address: hairong.guan@uc.edu (H. Guan). Polyhedron 32 (2012) 30–34 Contents lists available at ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly