An insight into transfer hydrogenation reactions catalysed by iridium(III) bis-N-heterocyclic carbenes

A variety of [M(L)2(L′)2{κC,C′-bis(NHC)}]BF4 complexes (M = Rh or Ir; L = CH3CN or wingtip group; L′ = I– or CF3COO–; NHC=N-heterocyclic carbene) have been tested as pre-catalysts for the transfer hydrogenation of ketones and imines. The conversions and TOF's obtained are closely related to the nature of the ligand system and metal centre, more strongly coordinating wingtip groups yielding more active and recyclable catalysts. Theoretical calculations at the DFT level support a classic stepwise metal-hydride pathway against the concerted Meerwein–Ponndorf–Verley (MPV) mechanism. The calculated catalytic cycle involves a series of ligand rearrangements due to the high trans effect of the carbene and hydrido ligands, which are more stable when situated in mutual cis positions. The reaction profiles obtained for the complexes featuring an iodide or a trifluoroacetate in one of the apical positions agree well with the relative activity observed for both catalysts.The authors would like to acknowledge the support by the Ministry of Higher Education, Saudi Arabia, in establishment of the Centre of Research Excellence in Petroleum Refining & Petrochemicals at KFUPM (KACST-funded project ART-32-68). The support under the KFUPM–University of Zaragoza research agreement is also highly appreciated. This work was further supported by the Spanish Ministry of Economy and Competitiveness (MINECO/FEDER) (CONSOLIDER INGENIO CSD2009-0050, CTQ2011-27593 and CTQ2012-35665 projects) and the Diputación General de Aragón (DGA/FSE-E07).Peer Reviewe

Early Metal Di(pyridyl) Pyrrolide Complexes with Second Coordination Sphere Arene−π Interactions: Ligand Binding and Ethylene Polymerization

Early metal complexes supported by hemilabile, monoanionic di(pyridyl) pyrrolide ligands substituted with mesityl and anthracenyl groups were synthesized to probe the possibility of second coordination sphere arene−π interactions with ligands with potential for allosteric control in coordination chemistry, substrate activation, and olefin polymerization. Yttrium alkyl, indolide, and amide complexes were prepared and structurally characterized; close contacts between the anthracenyl substituents and Y-bound ligands are observed in the solid state. Titanium, zirconium, and hafnium tris(dimethylamido) complexes were synthesized, and their ethylene polymerization activity was tested. In the solid state structure of one of the Ti tris(dimethylamido) complexes, coordination of Ti to only one of the pyridine donors is observed pointing to the hemilabile character of the di(pyridyl) pyrrolide ligands

Olefin Polymerization by Dinuclear Zirconium Catalysts Based on Rigid Teraryl Frameworks: Effects on Tacticity and Copolymerization Behavior

Toward gaining insight into the behavior of bimetallic catalysts for olefin polymerization, a series of structurally related binuclear zirconium catalysts with bisamine bisphenolate and pyridine bisphenolate ligands connected by rigid teraryl units were synthesized. Anthracene-9,10-diyl and 2,3,5,6-tetramethylbenzene-1,4-diyl were employed as linkers. Bulky Si^iPr_3 and SiPh_3 substituents were used in the position ortho to the phenolate oxygen. Pseudo-C_s and C_2 symmetric isomers are observed for the binuclear complexes of bisamine bisphenolate ligands. In general, binuclear catalysts show higher isotacticity compared to the monozirconium analogues, with some differences between isomers. Amine bisphenolate-supported dizirconium complexes were found to be moderately active (up to 1.5 kg mmol_(Zr)^(–1) h^(–1)) for the polymerization of 1-hexene to isotactically enriched poly-1-hexene (up to 45% mmmm) in the presence of stoichiometric trityl or anilinium borate activators. Moderate activity was observed for the production of isotactically enriched polypropylene (up to 2.8 kg mmol_(Zr)^(–1) h^(–1) and up to 25.4% mmmm). The previously proposed model for tacticity control based on distal steric effects from the second metal site is consistent with the observed behavior. Both bisamine bisphenolate and pyridine bisphenolate supported complexes are active for the production of polyethylene in the presence of MAO with activities in the range of 1.1–1.6 kg mmol_(Zr)^(–1) h^(–1) and copolymerize ethylene with α-olefins. Little difference in the level of α-olefin incorporation is observed between mono- and dinuclear catalysts supported with the pyridine bisphenolate catalysts. In contrast, the size of the olefin affects the level of incorporation differently between monometallic and bimetallic catalysts for the bisamine bisphenolate system. The ratio of the incorporation levels with dinuclear vs mononuclear catalysts decreases with increasing comonomer size. This effect is attributed to steric pressure provided by the distal metal center on the larger olefin in dinuclear catalysts

In-Reactor Polypropylene Functionalization-The Influence of Catalyst Structures and Reaction Conditions on the Catalytic Performance

To unravel the relationship between silylene-bridged metallocene catalyst structures and polymerization conditions and their effect on the performance in in-reactor functionalization of polypropylene, the behaviors of rac-Me2Si(2-Me-4-Ph-Ind)2ZrCl2/MMAO, rac-Me2Si(Ind)2ZrCl2, rac-Me2Si(2-Me-4-Ph-Ind)2HfCl2, and rac-Me2Si(Ind)2HfCl2 in propylene/aluminum alkyl-passivated 10-undecen-1-ol copolymerization were compared. Kinetic analysis revealed higher catalytic activities for zirconocenes compared to analogous hafnocenes. Both the zirconocene and hafnocene with substituted indenyl ligands afforded a higher molecular weight capability, improved stereo-selectivity, and enhanced ability to incorporate functionalized comonomers compared to their non-substituted congeners. An in-depth study of polypropylene functionalization using the best performing catalyst system, rac-Me2Si(2-Me-4-Ph-Ind)2ZrCl2/MMAO, at temperatures ranging from 40 to 100 °C, revealed a linear inversely proportional correlation of polymerization temperature with functionalized comonomer reactivity (↑Tp → ↓ r1), copolymer molecular weight (↑Tp → ↓Mn), and melting temperature (↑Tp → ↓Tm). While performing well under standard laboratory polymerization conditions, rac-Me2Si(2-Me-4-Ph-Ind)2ZrCl2/MMAO showed limited molecular weight and stereo-selectivity capabilities under high-temperature (130-150 °C) solution process conditions. Although immobilization of rac-Me2Si(2-Me-4-Ph-Ind)2ZrCl2 onto silica, allowing it to be used under industrially relevant slurry and gas-phase conditions, led to an active catalyst, it failed to incorporate any functionalized comonomer

Early Metal Di(pyridyl) Pyrrolide Complexes with Second Coordination Sphere Arene−π Interactions: Ligand Binding and Ethylene Polymerization

Early metal complexes supported by hemilabile, monoanionic di(pyridyl) pyrrolide ligands substituted with mesityl and anthracenyl groups were synthesized to probe the possibility of second coordination sphere arene−π interactions with ligands with potential for allosteric control in coordination chemistry, substrate activation, and olefin polymerization. Yttrium alkyl, indolide, and amide complexes were prepared and structurally characterized; close contacts between the anthracenyl substituents and Y-bound ligands are observed in the solid state. Titanium, zirconium, and hafnium tris(dimethylamido) complexes were synthesized, and their ethylene polymerization activity was tested. In the solid state structure of one of the Ti tris(dimethylamido) complexes, coordination of Ti to only one of the pyridine donors is observed pointing to the hemilabile character of the di(pyridyl) pyrrolide ligands

Novel homogeneous Ir-catalysts: Ligand design, applications and mechanisms

Resumen del trabajo presentado a la 2nd World Chemistry Conference and Exhibition (WCCE), celebrada en Valencia (España) del 9 al 11 de julio de 2018.This presentation will deal with two main subjects: (i) the preparation ofthe frrst PCP-type ligand based on an N-heterocyclic olefm (NHO) scaffold, accompanied by an evaluation of the impact of this type of ligand in the activity of iridium complexes in several relevant catalytic processes; and (ii) the development of well- defined Ir-NHC complexes as catalysts for the dehydrogenative silylation of aromatic C-H bonds. (i) A great variety of pincer complexes has been reported in the literature. In particular, transition metal complexes containing PCP pincer ligands have shown remarkable activities in relevant catalytic processes. Recent work by us on this subject has resulted in the preparation of an ewPCP-type ligand based on an N- heterocyclic olefin (NHO) scaffold. The flexibille coordination of this NHO-based PCP-ligand can be attributed to the dual nature (ylide-olefm) oftbe NHO. Iridium(I) complexes featuring this ligand show excellent activities in transfer hydrogenation reactions. The active species ([Ir(KP,C,P'-NHO-PPh2)(iPrO)]), formed via COD dissociation and subsequent isopropoxide coordination, features an NHO moiety that behaves as a hemilabile ligand, which allows the catalyst to adopt stabJe square planar geometries in the transition states, thus reducing the energetic barrier of the process. More recently, we have tested the activity ofthese complexes in the dehydrogenation of formic acid, showing outstanding activities in water and in neat formicacid. (ii) The preparation of fine chernicals by the catalytic functionalization of C-H bonds has seen an outstanding development in recent years, with borylation and silylation reactions being prominent examples of this chemistry. In this regard, the versatility of oganosilicon compounds can be attributed to the low cost and non-toxic nature of silicon reagents, together with their straight forward functionalization by various reactions. Moreover, conjugated organosilicon materials are attractive targets per se owing to their unique properties, which permit a widespread applicability in the field of organic electronics and photonics. Most of the catalysts employed so far for this reaction are generated >in situ> from commercial metal precursors and ligands. Hence, we have focused on the development of well-defined organometallic catalysts bearing appropriate ligands in order to improve the efficiency of current silylation catalysts. In particular, the use of NHC-Ir (III) complex [Ir(H)2(IPr)(py)3][BF4] (IPr = 1 ,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene) as a catalyst has allowed for the preparation of a wide range of aryl- and heteroarylsilanes. The directed and non-directed functionalization of C-H bonds has been accomplisbed successfully using the areneas tbe limiting reagent and a variety of hydrosilanes, including Et3SiH, Ph2MeSiH, PhMe2SiH, Ph3SiH and(Et0)3SiH.Peer Reviewe

Intermolecular hydroamination versus stereoregular polymerization of phenylacetylene by rhodium catalysts based on N-O bidentate ligands

N-O bidentate ligands, such as 8-quinolinol and aminoacids, in combination with the dinuclear precursor [{Rh(μ-OMe)(COD)}2] are versatile catalytic systems. Thus, stereoregular polymerization of phenylacetylene (PA) is observed in the presence of secondary amines. Interestingly, the outcome of the catalysis changes drastically on addition of strong coordinating phosphines, giving the product of the intermolecular anti-Markovnikov hydroamination of phenylacetylene. © 2013 Elsevier B.V. All rights reserved.The authors would like to acknowledge the support from the Ministry of Higher Education, Saudi Arabia, in establishment of the Center of Research Excellence in Petroleum Refining & Petrochemicals at King Fahd University of Petroleum and Minerals (KFUPM). The support of the KFUPM under the KACST funded project (T-K-11-630) and the KFUPM-University of Zaragoza research agreement are also highly appreciated. The author (MAC) thankfully acknowledges the support from the project CTQ2012-35665.Peer Reviewe

Solvent-free iridium-catalyzed CO2 hydrosilylation: experiments and kinetic modeling

This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.The iridium(iii) complex [Ir(H)(CF3SO3)(NSiN)(coe)] (NSiN = bis(pyridine-2-yloxy)methylsilyl, coe = cyclooctene) has been demonstrated to be an active catalyst for the solvent-free hydrosilylation of CO2 with 1,1,1,3,5,5,5-heptamethyltrisiloxane (HMTS) under mild reaction conditions (3 bar). The activity of this catalytic system depends on the reaction temperature. The best catalytic performance has been achieved at 75 °C. A kinetic study at variable temperature (from 25 °C to 75 °C) and constant pressure (3 bar) together with kinetic modeling has been carried out. The results from such a study show an activation energy of 73.8 kJ mol-1 for the process.The authors express their appreciation to the Ministry of Higher Education, Saudi Arabia, for the establishment of the Center of Research Excellence in Petroleum Refining & Petrochemicals at King Fahd University of Petroleum & Minerals (KFUPM) and the KFUPM-University of Zaragoza research agreement for their support. Financial support from MINECO/FEDER (projects CONSOLIDER INGENIO-2010 MULTICAT CSD2009-00050 and CTQ2011-27593) and DGA/FSE (group E07) is also acknowledged.Peer Reviewe

Solvent-free iridium-catalyzed reactivity of CO2 with secondary amines and hydrosilanes

The complex [Ir(H)(CFSO)(NSiN)(coe)] (NSiN=bis(pyridine-2-yloxy)methylsilyl fac-coordinated) (1) is an effective catalyst precursor for the solvent-free synthesis of silyl carbamates from reaction of aliphatic secondary amines with CO and HSiMe(OSiMe). The preferential formation of the silyl carbamate instead of the expected formamide or methylamine has proven to be consequence of an iridium-catalyzed dehydrogenative Si-N coupling between the silane and the amine to afford the corresponding silyl amine, which under the reaction conditions reacts with CO to give the corresponding silyl carbamate.The authors express their appreciation to the support from the MINECO/FEDER project CTQ2012–35665, Gobierno de Aragón (Spain) DGA/FSE-E07 and the Ministry of Higher Education, Saudi Arabia, in establishment of the Center of Research Excellence in Petroleum Refining & Petrochemicals at King Fahd University of Petroleum & Minerals (KFUPM) and the support from KFUPM-University of Zaragoza research agreement.Peer Reviewe

Experimental and computational studies on the reactivity and binding mode of thiophene with N-heterocyclic carbene iridium complexes

The reactivity of thiophene (T), 2-methylthiophene (2-MeT), and benzothiophene (BT) with [Ir(cod)(IPr)(L)]BF complexes (L = acetone (1), pyridine (2) or dimethylphenylphosphine (3); IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) in the presence of molecular hydrogen has been investigated. Under these conditions the 1,5-cyclooctadiene ligand is hydrogenated to cyclooctane, which renders an unsaturated species (Ir-IPr-L) able to coordinate the thiophene moiety. The coordination mode of T, 2-MeT, and BT depends on the nature of the substrate and the ligand trans to the IPr (L). The reaction of 1 with T and 2-MeT leads to dissociation of the acetone ligand to afford the complexes [Ir(H)(IPr)(η--T)]BF (4) and [Ir(H)(IPr)(η--2-MeT)(η-S-2-MeT)]BF (5), respectively, but no stable complex is observed on reaction with BT. Analogously to 1, complex 2 does not give a stable complex on reaction with BT, while reaction with 2-MeT yields complex 5 again. Conversely, reaction with T affords a mixture of complexes, [Ir(H)(IPr)(η--T)(Py)]BF (6) and [Ir(H)(IPr)(η-S-T)(Py)]BF (6′), both featuring a coordinated pyridine ligand. The reaction of 3 with T, 2-MeT, and BT yields in all cases η-S complexes, namely [Ir(H)(IPr)(η-S-T)(PPhMe)]BF (7), [Ir(H)(IPr)(η-S-2-MeT)(PPhMe)]BF (8) in equilibrium with [Ir(H)(IPr)(η-S-2-MeT)(PPhMe)]BF (8′), and [Ir(H)(IPr)(η-S-BT)(PPhMe)]BF (9). Finally, DFT calculations were employed to rationalize the coordination modes of T, 2-MeT, and BT, as well as the tendency of these complexes to undergo hydrogenation instead of hydrogenolysis of the thiophene moiety under catalytic conditions.The authors acknowledge the support by the Ministry of Higher Education, Saudi Arabia, in establishment of the Centre of Research Excellence in Petroleum Refining & Petrochemicals at KFUPM (KACST-funded project ART-32-68). The support of the KFUPM under the KACST funded project (ART-32-68) and the KFUPM−University of Zaragoza research agreement are also highly appreciated. This work was further supported by the Spanish Ministry of Economy and Competitiveness (MINECO/FEDER) (CONSOLIDER INGENIO CSD2009-0050, CTQ2011-27593, and CTQ2012-35665 projects) and the Diputación General de Aragón (DGA/FSE-E07).Peer Reviewe