Laura Turculet

Rhodium and Iridium Amido Complexes Supported by Silyl Pincer Ligation: Ammonia N−H Bond Activation by a [PSiP]Ir Complex

Ir silyl pincer complexes insert into the N-H bonds of anilines and ammonia under mild conditions to form isolable [Cy-PSiP]Ir(H)(NHR) complexes that are resistant to N-H bond reductive elimination at room temperature, even in the presence of arenes, alkenes, and phosphines.

Mild reduction of carbon dioxide to methane with tertiary silanes catalyzed by platinum and palladium silyl pincer complexes.

The large-scale combustion of fossil fuels (coal, petroleum, natural gas) has led to a significant and alarming rise in levels of atmospheric CO2 over the past several decades, and this trend is anticipated to continue. As a greenhouse gas, CO2 is a significant contributor to global warming, and thus the current high level of anthropogenic CO2 in the atmosphere is of great concern. Although efforts to capture and sequester CO2 are being pursued, such efforts have so far proven relatively costly and remain in their infancy. A complementary strategy for addressing both the high levels of atmospheric CO2 and the dwindling supply of fossil fuels available to meet our energy needs is the recycling of CO2 by conversion into a hydrocarbon fuel that is suitable for use in our current energy infrastructure. As such, there is significant interest in the development of efficient methodologies for the reduction of CO2 to methanol and/or methane, with the ultimate goal of achieving a carbon-neutral catalytic process. 4] In this context, the pursuit of homogeneous catalysts for the conversion of CO2 to methanol and/or methane is an area of growing interest, and to date, only a handful of such catalyst systems have been developed. With respect to methanol formation, Milstein and co-workers recently demonstrated the facile hydrogenation of CO2-derived organic carbonates, carbamates, and formates to methanol utilizing PNN pincer Ru complexes as catalysts. Building on this pioneering work, Sanford and co-workers reported a three component cascade catalysis route for the hydrogenation of CO2 to form methanol, [9d] and Leitner and co-workers demonstrated that a single Ru phosphine catalyst is also effective in this transformation. There has also been significant interest in the development of efficient reduction pathways that do not rely on hydrogen for the reduction of CO2 to the methoxide level. Utilizing a POCOP pincer Ni II hydride catalyst, Guan and co-workers demonstrated the hydroboration of CO2 with catecholborane to give methoxyboryl species with a relatively high turnover frequency. Furthermore, although the Ir-catalyzed hydrosilylation of CO2 to form methoxysilane species was initially reported over twenty years ago, albeit with low efficiency, more recently Ying and co-workers reported a metal-free N-hetACHTUNGTRENNUNGero ACHTUNGTRENNUNGcyclic carbene catalyzed process for the reduction of CO2 with diphenylsilane to form the corresponding methoxysilane with significantly improved turnover numbers and turnover frequencies. c] Focusing on the complete reduction of CO2 to methane, prior to our commencing work in this area, only three examples of such homogeneous catalyst systems had been reported. 15] Matsuo and Kawaguchi disclosed the use of bisACHTUNGTRENNUNG(phenoxo) Zr alkyl complexes, which in conjunction with the highly Lewis acidic borane B ACHTUNGTRENNUNG(C6F5)3, catalyzed the reaction of CO2 with hydrosilanes to form methane. [14a] Piers and co-workers subsequently disclosed a catalyst system based on the frustrated Lewis pair (FLP) 2,2,6,6-tetraACHTUNGTRENNUNGmethylpiperidine (TMP)/BACHTUNGTRENNUNG(C6F5)3, which reacts with CO2 and Et3SiH to form a formatoborate species that is subsequently hydrosilylated in the presence of excess B ACHTUNGTRENNUNG(C6F5)3 to form methane. Lastly, Wehmschulte and co-workers recently reported that the highly reactive Lewis acid Et2Al + catalyzes the conversion of CO2 to methane as well as other hydrocarbons upon reaction with hydrosilanes, albeit at elevated temperatures and long reaction times. In an effort to diversify the catalyst portfolio available for the reduction of CO2 to methane, we became interested in identifying a complementary late metal catalyst system for this transformation. We envisioned that a platinum group metal catalyst might offer increased stability and decreased sensitivity to protic impurities relative to a d metal alkyl or a cationic Group 13 alkyl species, while still achieving suitACHTUNGTRENNUNGable activity in CO2 reduction chemistry. We were particularly encouraged by the utility of such late metal species in the reduction of CO2 to methanol, and considered that the identification of an appropriately configured late metal catalyst could provide an entry point towards complementary proACHTUNGTRENNUNGcesses leading to formation of methane. Herein we report our results detailing the utility of soluble, well-defined Group 10 (Pd, Pt) silyl pincer complexes, in combination with B ACHTUNGTRENNUNG(C6F5)3, for the efficient catalytic conversion of CO2 to methane using tertiary silanes as the reductant. While this manuscript was in preparation, Brookhart and co-workers reported a related example of a cationic [(POCOP)Ir] species that functions as a catalyst for the conversion of CO2 to methane using hydrosilanes. [15] [a] S. J. Mitton, Prof. Dr. L. Turculet Department of Chemistry, Dalhousie University 6274 Coburg Road, P.O. Box 15000 Halifax, NS, B3H 4R2 (Canada) Fax: (+1) 902-494-1310 E-mail : laura.turculet@dal.ca Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201203226.

Room temperature benzene C-H activation by a new [PSiP]Ir pincer complex

The synthesis and reactivity of coordinatively unsaturated Rh and Ir complexes supported by the new bis(phosphino)silyl pincer ligand [kappa(3)-(2-Cy(2)PC(6)H(4))(2)SiMe](-) ([Cy-PSiP](-)) are reported, including the first example of facile, room temperature intermolecular arene C-H bond activation mediated by a silyl pincer complex.

(N‐Phosphinoamidinate)cobalt‐Catalyzed Hydroboration: Alkene Isomerization Affords Terminal Selectivity

Herein we establish the utility of a three-coordinate (N-phosphinoamidinate)cobalt(amido) pre-catalyst that is capable of effecting challenging alkene isomerization/hydroboration processes at room temperature, leading to the selective terminal addition of the boron group.

A Manganese Pre-Catalyst: Mild Reduction of Amides, Ketones, Aldehydes, and Esters

A new (N-phosphinoamidinate)manganese complex is shown to be a useful pre-catalyst for the hydrosilative reduction of carbonyl compounds, and in most cases at room temperature. The Mn-catalyzed reduction of tertiary amides to tertiary amines, with a useful scope, is demonstrated for the first time by use of this catalyst, and is competitive with the most effective transition-metal catalysts known for such transformations. Ketones, aldehydes, and esters were also successfully reduced under mild conditions by using this new Mn catalyst.

Synthesis and Reactivity of a Neutral, Three‐Coordinate Platinum(II) Complex Featuring Terminal Amido Ligation

A crystallographically characterized three-coordinate, formally 14 electron Pt(II) complex 1 featuring terminal amido ligation is reported. Computational analysis revealed relatively weak π donation from the amide lone pair to platinum and supports a 14-electron assignment for 1. Stoichiometric reactivity studies confirmed the viability of net O-H and C-H addition across, as well as isonitrile insertion into, the terminal platinum-amido linkage of 1.

Synthesis and characterization of five-coordinate, 16-electron RuII complexes supported by tridentate bis(phosphino)silyl ligation

The synthesis of 16-electron complexes of the type (Cy-PSiP)RuX(L) (Cy-PSiP = κ3-(2-Cy2PC6H4)2SiMe) is reported. Treatment of (Cy-PSiP)H with one equiv. RuCl2(PPh3)3 in the presence of Et3N afforded a mixture of (Cy-PSiP)RuCl(PPh3) (1) and [(Cy-PSiP)RuCl]2 (2), which exists in a temperature-dependent equilibrium process involving reversible dissociation of PPh3 from 1 to form 2. While treatment of this equilibrium mixture with PMe3 afforded (Cy-PSiP)RuCl(PMe3) (3) cleanly, this product was more straightforwardly prepared by the addition of PMe3 to 2 (97% yield). Attempts to form an alkyl complex of the type (Cy-PSiP)RuMe(PMe3) via treatment of 3 with MeMgBr led to the dehydrogenation and cyclometalation of a cyclohexyl phosphino group to give the isolable 18-electron complex [MeSi(C6H4PCy2)(C6H4PCy(η3-C6H8))]RuPMe3 (4, 78% yield). The hydrido complex (Cy-PSiP)RuH(PMe3) (5) was identified as a minor by-product in this dehydrogenative process. Whereas simple 16-electron alkyl complexes of the type (Cy-PSiP)RuR(PMe3) remained elusive, the isolable allyl complex (Cy-PSiP)Ru(η3-C3H5) (9) was successfully prepared from 2 and (C3H5)MgCl (79% yield). Treatment of 3 with Me3SiN3 or NaN3 did not generate a five-coordinate azido species. However, the putative dinuclear complex [(Cy-PSiP)RuN3]2 (10) was observed to react with one equiv. of PMe3 to afford the isolable five-coordinate azido complex (Cy-PSiP)Ru(N3)(PMe3) (11; 94% yield). Crystallographic data for 3, 4, 5, 9 and 11 are presented.

Dehydrogenative B−H/C(sp3)−H Benzylic Borylation within the Coordination Sphere of Platinum(II)

The first examples of stoichiometric dehydrogenative B-H/C(sp3 )-H benzylic borylation reactions, which are of relevance to catalytic methylarene (di)borylation, are reported. These unusual transformations involving a (κ2 -P,N)Pt(η3 -benzyl) complex, and either pinacolborane or catecholborane, proceed cleanly at room temperature. Density functional calculations suggest that borylation occurs via successive σ-bond metathesis steps, whereby a PtII -H intermediate engages in C(sp3 )-H bond activation-induced dehydrogenation.