Transfer Hydrogenation of Carbonyl Groups, Imines and N‐Heterocycles Catalyzed by Simple, Bipyridine‐Based MnI Complexes

Utilization of hydroxy‐substituted bipyridine ligands in transition metal catalysis mimicking [Fe]‐hydrogenase has been shown to be a promising approach in developing new catalysts for hydrogenation. For example, MnI complexes with 6,6′‐dihydroxy‐2,2′‐bipyridine ligand have been previously shown to be active catalysts for CO2 hydrogenation. In this work, simple bipyridine‐based Mn catalysts were developed that act as active catalysts for transfer hydrogenation of ketones, aldehydes and imines. For the first time, Mn‐catalyzed transfer hydrogenation of N ‐heterocycles was reported. The highest catalytic activity among complexes with variously substituted ligands was observed for the complex bearing two OH groups in bipyridine. Deuterium labeling experiments suggest a monohydride pathway

Proton-responsive naphthyridinone-based RuII complexes and their reactivity with water and alcohols

We report the synthesis and reactivity of RuII complexes with a new naphthyridinone-substituted phosphine ligand, 7-(diisopropylphosphinomethyl)-1,8-naphthyridin-2(1H)-one (L-H), which contains two reactive sites that can potentially be deprotonated by a strong base: an NH proton of naphthyridinone and a methylene arm attached to the phosphine. In the absence of a base, the stable bis-ligated complex Ru(L-H)2Cl2 (1) containing two NH groups in the secondary coordination sphere is formed. Upon further reaction with a base, a doubly deprotonated, dimeric complex is obtained, [Ru2(L*-H)2(L)2] (2), in which two of the four ligands undergo deprotonation at the NH (L), while the other two ligands are deprotonated at the methylene groups (L*-H) as confirmed by an X-ray diffraction study; intramolecular hydrogen bonding is present between the NH group of one ligand and an O-atom of another ligand in the dimeric structure, which stabilizes the observed geometry of the complex. Complex 2 reacts with protic solvents such as water or methanol generating aqua Ru(L)2(OH2)2 (3) or methanol complexes Ru(L)2(MeOH)2 (4), respectively, both exhibiting intramolecular H-bonded patterns with surrounding ligands at least in the solid state. These complexes react with benzyl alcohols to give aldehydes via base-free acceptorless dehydrogenation

Aryl–X Bond-Forming Reductive Elimination from High-Valent Mn–Aryl Complexes

C–X bond reductive elimination and oxidative addition are key steps in many catalytic cycles for C–H functionalization catalyzed by precious metals; however, engaging first row transition metals in these overall 2e– processes remains a challenge. Although high-valent Mn aryl species have been implicated in Mn-catalyzed C–H functionalization, the nature and reactivity of such species remain unelucidated. In this work, we report rare examples of stable, cyclometalated monoaryl MnIII complexes obtained through clean oxidative addition of Ar–Br to MnI(CO)5Br. These isolated MnIII–Ar complexes undergo unprecedented 2e– reductive elimination of the Ar–X (X = Br, I, and CN) bond and MnII induced by 1e– oxidation, presumably via transient reactive MnIV species. Mechanistic studies suggest a nonradical pathway

Cobalt(III) and copper(II) hydrides at the crossroad of catalysed chain transfer and catalysed radical termination: a DFT study

International audienceMetal complexes that mediate radical polymn. may also lead to catalyzed chain transfer (CCT) or to catalyzed radical termination (CRT), both processes occurring via the same type of hydride intermediate. What leads these intermediates to prefer reacting with the monomer, leading to CCT, or with radicals, leading to CRT, was unclear. We report here a DFT investigation of the comparative reactivity of two different hydride complexes, [(TMP)CoIII(H)] (TMP = tetramesitylporphyrin) and [(TPMA)CuII(H)]+ (TPMA = tris(2-pyridylmethyl)amine), generated from [CoII(TMP)] and [CuI(TPMA)]+, vs. the monomer and radical, using thė CH(CH3)(COOCH3) anḋ C(CH3)2(COOCH3) radicals as models for the growing PMA and PMMA radical chains. The unsubstituted porphyrin was used as a model for full quantum mech. (QM) calcns., but selected calcns. on the full TMP system were also carried out by the hybrid QM/MM approach, treating the mesityl substituents at the mol. mechanics (MM) level. The calcns. provide a basis for rationalizing the exptl. obsd. strong activity of the cobalt system in catalyzed chain transfer (CCT) polymn. without a reported activity so far for catalyzed radical termination (CRT), whereas the copper system leads to CRT but does not promote CCT. In essence, the key factors in favor of CCT for the cobalt system are a very low barrier for H transfer to the monomer and the much greater concn. of the monomer relative to the radical, yielding vCCT > vCRT. For the copper system, on the other hand, the greater barrier for H transfer to the monomer makes the CCT rate much slower, while the CRT quenching pathway favorably takes place through an electronically barrierless pathway with incipient stabilization at long C···H distances. The different spin states of the two systems (spin quenching along the CCT pathway for the Co system and along the CRT pathway for the Cu system) rationalize the obsd. behavior. The new acquired understanding should help design more efficient systems

Effect of α- and β-H/F substitution on the homolytic bond strength in dormant species of controlled radical polymerization: OMRP vs. ITP and RAFT

International audienceThe X-C bond dissociation energies (BDEs) for five series of X-CH2-nFnCH3-mFm molecules (n = 0, 1, 2; m = 0, 1, 2, 3) with X = H, I, SC(S)OEt, Co(acac)2 or Mn(CO)5 were calculated using a DFT approach, yielding results in good agreement with the few experimentally determined values. Calculations were also carried out on the simpler (CO)5Mn-CFnH3-n molecules (n = 0, 1, 2, 3), for which experimental data are available. The BDE trends as n and m vary are different for different X groups: BDE increases as n increases (particularly from 0 to 1) for X = H, I and SC(S)OEt, but decreases (particularly from 1 to 2) for X = Co(acac)2 and Mn(CO)5. The effective charge analysis suggests that the effect of the bond polarity on the ionic component of the bond energy is a major contribution to these trends. These results rationalize the limited control, for the polymerization of vinylidene fluoride (VDF), by the iodine transfer polymerization (ITP) and reversible addition-fragmentation chain-transfer (RAFT) polymerization approaches. They also predict a better controlled process for this monomer by organometallic mediated radical polymerization (OMRP), mediated by Co(acac)2. They also allow predictions for the performance of the same processes for other fluorinated monomers. The results for X = Mn(CO)5 suggest that the (CO)5Mn-CH2-nFnCH3-mFm molecules cannot be thermally activated at significant rates. Therefore, these molecules either do not form or are photochemically reactivated during the Mn2(CO)10-assisted ITP polymerization of VDF

Catalyzed radical termination in the presence of tellanyl radicals

International audienceThe decomposition of the diazo initiator dimethyl 2,2′‐azobis(isobutyrate) (V‐601), generating the Me2C.(CO2Me) radical, affords essentially the same fraction of disproportionation and combination in media with a large range of viscosity (C6D6, [D6]DMSO, and PEG 200) in the 25–100 °C range. This is in stark contrast to recent results by Yamago et al. on the same radical generated from Me2C(TeMe)(CO2Me) and on other X‐TeR systems (X=polymer chain or unimer model; R=Me, Ph). The discrepancy is rationalized on the basis of an unprecedented RTe.‐catalyzed radical disproportionation, with support from DFT calculations and photochemicaL V‐601 decomposition in the presence of Te2Ph2

The cyclooctadiene ligand in [IrCl(COD)]2 is hydrogenated under transfer hydrogenation conditions: a study in the presence of PPh3 and a strong base in isopropanol

International audienceThe interaction of [IrCl(COD)]2 with PPh3 in isopropanol has been investigated for various P/Ir ratios, in the absence or presence of a strong base (KOtBu), at room temperature and at reflux. At room temperature, PPh3 adds to the metal center to yield [IrCl(COD)(PPh3)] and additional PPh3 only undergoes rapid degenerative ligand exchange. Subsequent addition of KOtBu affords [IrH(COD)(PPh3)2] as the main compound, even for high P/Ir ratios, although very minor amounts of products having a “HIr(PPh3)3” core are also generated. Warming to the solvent reflux temperature results in a rapid (< 1 h) and quantitative COD removal from the system as hydrogenated products (54.4% of cyclooctene plus 32.2% of cyclooctane according to a quantitative GC analysis) and in the eventual generation of [IrH3(PPh3)3]. The latter is observed as a mixture of the fac and mer isomers in solvent-dependent proportions. Other minor products, one of which is suggested to be mer-cis-[IrH2(OiPr)(PPh3)3] by the NMR characterization, are also generated. These results show that, contrary to certain previously published assumptions, systems of this kind are unlikely to function via a COD-containing active species in transfer hydrogenation catalyses conducted in hot isopropanol in the presence of a strong base

A noninnocent cyclooctadiene (COD) in the reaction of an “Ir(COD)(OAc)” precursor with imidazolium salts

The reactions between [Ir(COD)(μ-OAc)]2 and the functionalized imidazolium salt 1-mesityl-3-(pyrid-2-yl)imidazolium bromide (MesIPy·HBr) or 1-benzyl-3-(5,7-dimethylnaphthyrid-2-yl)imidazolium bromide (BnIN·HBr) at room temperature afford the COD-activated IrIII–N-heterocyclic carbene (NHC) complexes [Ir(1-κ-4,5,6-η-C8H12)(κ2C,N-MesIPy)Br] (1) and [Ir(1-κ-4,5,6-η-C8H12)(κ2C,N-BnIN)Br] (2), respectively. In contrast, the methoxy analogue [Ir(COD)(μ-OMe)]2 on reaction with MesIPy·HBr under identical conditions affords the IrI–NHC complex [Ir(COD)(κ2C,N-MesIPy)Br]. Treatment of [Ir(COD)(κ2C,N-MesIPy)Br] with sodium acetate does not lead to COD activation. Further, use of 2,2′-bipyridine (bpy) with [Ir(COD)(μ-X)]2 (X = MeO or AcO) in the presence of [nBu4N][BF4] affords exclusively [Ir(bpy)(COD)][BF4] (3). Metal-bound acetate is shown to be an essential promoter for activation of the COD allylic C–H bond. An examination of products reveals the following transformations of the precursor components: cleavage of the imidazolium C2–H and subsequent NHC metalation, metal oxidation from IrI to IrIII, and 2e reduction of COD, effectively resulting in 1-κ-4,5,6-η-C8H12 coordination to the metal. Mechanistic investigation at the DFT/B3LYP level of theory strongly suggests that NHC metalation precedes COD allylic C–H activation. Two distinct pathways have been examined which involve initial C2–H oxidative addition to the metal followed by acetate-assisted allylic C–H activation (path A) and the reverse sequence, i.e., deprotonation of C2–H by the acetate and subsequent allylic C–H oxidative addition to the metal (path B). The result is an IrIII–NHC–hydride−κ1, η2-C8H11 complex. Subsequent intramolecular insertion of the COD double bond into the metal–hydride bond followed by isomerization gives the final product. An acetate-assisted facile COD allylic C–H bond activation, in comparison to oxidative addition of the same to Ir, makes path A the favored pathway. This work thus raises questions about the innocence of COD, especially when metal acetates are used for the synthesis of NHC complexes from the corresponding imidazolium salts

Carbon monoxide induced double cyclometalation at the iridium center

Bubbling of CO into a dichloromethane solution of [Ir(COD)(CH<SUB>3</SUB>CN)<SUB>2</SUB>][BF<SUB>4</SUB>] followed by the addition of 2-phenyl-1,8-naphthyridine (LH) at room temperature results in the bis-cyclometalated IrIII complex [Ir(C<SUP>∧</SUP>N)<SUB>2</SUB>(CO)(LH)][BF<SUB>4</SUB>] (C<SUP>∧</SUP>N = L). The observed cyclometalation contradicts the classical role of CO, which is to hinder oxidative addition by lowering electron density on the metal. DFT calculations reveal that the first cyclometalation involves oxidative addition of the ligand. Subsequently, preferential electrophilic activation of the second ligand followed by elimination of dihydrogen affords the bis-cyclometalated Ir<SUP>III</SUP> complex

Limits of Vinylidene Fluoride RAFT Polymerization

International audienceThe investigations reported in this article probe the behavior of the RAFT polymerization of vinylidene fluoride (VDF) when degrees of polymerization higher than 50 are targeted: they demonstrate that higher molar mass PVDF (11 000 g mol–1) can indeed be prepared by RAFT polymerization, but only at rather low monomer conversions (<33%). This study more carefully examines the behavior of the reputedly nonreactive −CF2CH−XA chain ends (where XA designates the xanthate group) formed by inverse VDF addition and known to accumulate in the reaction medium during the polymerization. A combination of 1H and 19F NMR spectroscopic monitoring and comprehensive DFT calculations of the various exchange and propagation reactions at work explains the unexpected behavior of this polymerization. The present study disproves entirely the generally accepted belief that −CF2CH2–XA-terminated PVDF chains are “dead” and shows how these chains are reactivated, albeit slowly, throughout the polymerization. This activation occurs prevalently and counterintuitively through degenerative exchange by the minority PVDF–CF2CH2• radicals. The resulting kinetic scheme rationalizes the experimentally observed absence, after conversion of all the dormant chains into the less reactive −CF2CH2–XA end-group, of the longer polymer chains expected from a free radical polymerization mechanism