S. M. Wahidur Rahaman

Bimetallic Catalysis Involving Dipalladium(I) and Diruthenium(I) Complexes

Dipalladium(I) and diruthenium(I) compounds bridged by two [{(5,7-dimethyl-1,8-naphthyridin-2-yl)amino}carbonyl]ferrocene (L) ligands have been synthesized. The X-ray structures of [Pd(2)L(2)][BF(4)](2) (1) and [Ru(2)L(2)(CO)(4)][BF(4)](2) (2) reveal dinuclear structures with short metal-metal distances. In both of these structures, naphthyridine bridges the dimetal unit, and the site trans to the metal-metal bond is occupied by weakly coordinating oxygen from the amido fragment. The catalytic utilities of these bimetallic compounds are evaluated. Compound 1 is an excellent catalyst for phosphine-free, Suzuki cross-coupling reactions of aryl bromides with arylboronic acids and provides high yields in short reaction times. Compound 1 is also found to be catalytically active for aryl chlorides, although the corresponding yields are lower. A bimetallic mechanism is proposed, which involves the oxidative addition of aryl bromide across the Pd-Pd bond and the bimetallic reductive elimination of the product. Compound 1 is also an efficient catalyst for the Heck cross-coupling of aryl bromides with styrenes. The mechanism for aldehyde olefination with ethyl diazoacetate (EDA) and PPh(3), catalyzed by 2, has been fully elucidated. It is demonstrated that 2 catalyzes the formation of phosphorane utilizing EDA and PPh(3), which subsequently reacts with aldehyde to produce a new olefin and phosphine oxide. The efficacy of bimetallic complexes in catalytic organic transformations is illustrated in this work.

Metal–Ligand Cooperation on a Diruthenium Platform: Selective Imine Formation through Acceptorless Dehydrogenative Coupling of Alcohols with Amines

Metal-metal singly-bonded diruthenium complexes, bridged by naphthyridine-functionalized N-heterocyclic carbene (NHC) ligands featuring a hydroxy appendage on the naphthyridine unit, are obtained in a single-pot reaction of [Ru2(CH3COO)2(CO)4] with 1-benzyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)imidazolium bromide (BIN⋅HBr) or 1-isopropyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)imidazolium bromide (PIN⋅HBr), TlBF4, and substituted benzaldehyde containing an electron-withdrawing group. The modified NHC-naphthyridine-hydroxy ligand spans the diruthenium unit in which the NHC carbon and hydroxy oxygen occupy the axial sites. All the synthesized compounds catalyze acceptorless dehydrogenation of alcohols to the corresponding aldehydes in the presence of a catalytic amount of weak base 1,4-diazabicyclo[2.2.2]octane (DABCO). Further, acceptorless dehydrogenative coupling (ADHC) of the alcohol with amines affords the corresponding imine as the sole product. The substrate scope is examined with 1 (BIN, p-nitrobenzaldehyde). A similar complex [Ru2(CO)4(CH3COO)(3-PhBIN)][Br], that is devoid of a hydroxy arm, is significantly less effective for the same reaction. Neutral complex 1 a, obtained by deprotonation of the hydroxy arm in 1, is found to be active for the ADHC of alcohols and amines under base-free conditions. A combination of control experiments, deuterium labeling, kinetic Hammett studies, and DFT calculations support metal-hydroxyl/hydroxide and metal-metal cooperation for alcohol activation and dehydrogenation. The bridging acetate plays a crucial role in allowing β-hydride elimination to occur. The ligand architecture on the diruthenium core causes rapid aldehyde extrusion from the metal coordination sphere, which is responsible for exclusive imine formation.

RAFT synthesis of well-defined PVDF-b-PVAc block copolymers

RAFT polymerization of vinylidene fluoride (VDF), leading to relatively well defined poly(vinylidene fluoride) (PVDF), is negatively affected by chain inversion resulting in less easily reactivatable PVDFT-XA dormant chains (terminated with the tail end of an inversely added VDF unit; XA = xanthate moiety). Although slow reactivation of these chains by PVDF˙ radicals (in contrast to general belief) was recently demonstrated, slow radical exchange leads to progressive loss of chain growth control. This article deals with the possibility of synthesizing block copolymers from PVDF-XA macroCTAs by sequential addition. The investigations show that only PVDFH-XA (chains terminated with the head end of regularly added VDF) can be reactivated by PNVP˙ (poly(N-vinylpyrrolidone)) radicals and that PVDFT-XA chains are completely unreactive in the presence of PNVP˙, PB˙ (poly(butylacrylate)) or PDM˙ (poly(N,N′-dimethylacrylamide)). However, both PVDFH-XA and PVDFT-XA can be reactivated by PVAc˙ (poly(vinyl acetate)) radicals. The reactivation of the PVDFT-XA, albeit slower than that of the PVDFH-XA, is sufficiently fast to allow the synthesis of unprecedented well-defined PVDF-b-PVAc block copolymers with relatively high end-group fidelity. DFT calculations rationalize this behavior on the basis of faster radical exchange in the order PVDFH-XA/VAc > PVDFH-XA/NVP > PVDFT-XA/VAc ≫ PVDFT-XA/NVP. The success of the chain extension also relies on faster activation relative to homopropagation of the chain extending monomer, as well as fast addition of the released and to the monomer.

Organometallic‐Mediated Alternating Radical Copolymerization of tert‐Butyl‐2‐Trifluoromethacrylate with Vinyl Acetate and Synthesis of Block Copolymers Thereof

Organometallic-mediated radical polymerization (OMRP) has given access to well-defined poly(vinyl acetate-alt-tert-butyl-2-trifluoromethacrylate)-b-poly(vinyl acetate) and poly(VAc-alt-MAF-TBE) copolymers composed of two electronically distinct monomers: vinyl acetate (VAc, donor, D) and tert-butyl-2-trifluoromethacrylate (MAF-TBE, acceptor, A), with low dispersity (≤1.24) and molar masses up to 57 000 g mol-1 . These copolymers have a precise 1:1 alternating structure over a wide range of comonomer feed compositions. The reactivity ratios are determined as rVAc = 0.01 ± 0.01 and rMAF-TBE = 0 at 40 °C. Remarkably, from a feed containing >50% molar VAc content, poly(VAc-alt-MAF-TBE)-b-PVAc block copolymers are produced via a one-pot synthesis. Such diblock copolymers exhibit two glass transition temperatures attributed to the alternating and homopolymer sequences. The OMRP of this fluorine-containing alternating monomer system may provide access to a wide range of new polymer materials.

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

Metal complexes that mediate radical polymerisation may also lead to catalysed chain transfer (CCT) or to catalysed 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)]+, versus the monomer and radical, using the ˙CH(CH3)(COOCH3) and ˙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 mechanical (QM) calculations, but selected calculations on the full TMP system were also carried out by the hybrid QM/MM approach, treating the mesityl substituents at the molecular mechanics (MM) level. The calculations provide a basis for rationalizing the experimentally observed strong activity of the cobalt system in catalysed chain transfer (CCT) polymerization without a reported activity so far for catalysed radical termination (CRT), whereas the copper system leads to CRT but does not promote CCT. In essence, the key factors in favour of CCT for the cobalt system are a very low barrier for H transfer to the monomer and the much greater concentration 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 favourably 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 observed behavior. The new acquired understanding should help design more efficient systems.

Olefin Oxygenation by Water on an Iridium Center

Oxygenation of 1,5-cyclooctadiene (COD) is achieved on an iridium center using water as a reagent. A hydrogen-bonding interaction with an unbound nitrogen atom of the naphthyridine-based ligand architecture promotes nucleophilic attack of water to the metal-bound COD. Irida-oxetane and oxo-irida-allyl compounds are isolated, products which are normally accessed from reactions with H2O2 or O2. DFT studies support a ligand-assisted water activation mechanism.

Contrasting reactivity of 2-mesityl-1,8-naphthyridine (Mes-Np) with singly-bonded [Rh II Rh II ] and [Ru I –Ru I ] compounds

Reaction of cis-[Rh2(CH3COO)2(CH3CN)6](BF4)2 with two equivalents of 2-mesityl-1,8-naphthyridine (Mes-NP) affords trans-[Rh2(CH3COO)2(Mes-NP)2](BF4)2 (1). X-ray structure reveals weak Rh–C(ipso) interaction, and a short Rh–Rh distance. The same ligand, in contrast, oxidatively cleaves the Ru–Ru bond in cis-[Ru2(CO)4(CH3CN)6](BF4)2 and results in trans-[Ru(Mes-NP)2(CH3CN)2](BF4)2 (2). Both compounds adopt trans geometry to relieve the steric strain. Compound 2 exhibits moderate activity for the alcohol oxidation and aldehyde olefination reactions.

A Triflamide-tethered NHC‒Rh(I) Catalyst for Hydroalkoxylation Reactions: Ligand Promoted Nucleophilic Activation of Alcohols

A triflamide‐tethered N‐heterocyclic carbene (NHC)‐bound RhI dicarbonyl catalyst was highly effective for both intermolecular hydroalkoxylation and intramolecular heteroannulation reactions. The involvement of both the amido nitrogen and triflato oxygen atoms of the triflamide functionality for alcohol activation and 1,2‐hydrogen shift, respectively, was proposed.