Deuterated N2Py2 Ligands : Building More Robust Non-Heme Iron Oxidation Catalysts

Fe(N2Py2)/H 2 O 2 /AcOH catalytic systems provide powerful tools for efficient C-H and C=C bond oxidations (N2Py2 = bis-alkylamine-bis-pyridine ligand). Yet, the stability of these catalysts under the oxidizing conditions still remains a problem. The generally accepted catalyst decomposition pathway of Fe(N2Py2) complexes is through oxidative dimerization to form inactive oxo-bridged Fe 2 (μ-O)(N2Py2) 2 dimers. Detailed ESI-MS analysis has now shown a catalyst decomposition pathway of ligand oxidation via C-H oxidation on the 2-pyridinylmethylene sites, followed by dissociation of the oxidized ligand from the iron center. By deuterating the 2-pyridinylmethylene sites of a series of N2Py2 ligands with variations on both alkylamine and pyridine fragments, providing access to the corresponding Fe(N2Py2-D 4 ) complexes, longer catalysts lifetimes are achieved in catalytic oxidation reactions with all complexes. As a consequence, improved substrate conversions and product yields were consistently observed in both aliphatic C-H oxidations and alkene epoxidations. Kinetic and catalytic studies revealed that deuteration does not change the intrinsic reactivity and product selectivity of Fe(N2Py2) complexes. In addition, different Fe(N2Py2-D 4 ) complexes provide different improvements in catalytic performances and lifetimes, responding to the differences in ligand rigidity and robustness of the corresponding nondeuterated N2Py2 ligands. Accordingly, these improvements are more pronounced for ligands with a more flexible bis-alkylamine backbone. These observations provide insights into the development of more robust ligands for homogeneous oxidation catalysis

Tracking On-Surface Chemistry with Atomic Precision

The field of on-surface synthesis has seen a tremendous development in the past decade as an exciting new methodology towards atomically well-defined nanostructures. A strong driving force in this respect is its inherent compatibility with scanning probe techniques, which allows one to ‘view’ the reactants and products at the single-molecule level. In this article, we review the ability of noncontact atomic force microscopy to study on-surface chemical reactions with atomic precision. We highlight recent advances in using noncontact atomic force microscopy to obtain mechanistic insight into reactions and focus on the recently elaborated mechanisms in the formation of different types of graphene nanoribbons

P'CP'-Pincer palladium complex-catalyzed allylation of N,N-dimethylsulfamoyl-protected aldimines

The P'CP'-pincer palladium complex-catalyzed allylation of N,N-dimethylsulfamoyl-protected aldimines with allyl(tributyl)stannane is investigated for the preparation of N-homoallylic sulfamides. The desired N,N-dimethylsulfamoyl-protected products are obtained in moderate to high yields in DMF under very mild conditions and a high yielding and convenient deprotection of the N,N-dimethylsulfamoyl group is also demonstrated.

Tracking On-Surface Chemistry with Atomic Precision

The field of on-surface synthesis has seen a tremendous development in the past decade as an exciting new methodology towards atomically well-defined nanostructures. A strong driving force in this respect is its inherent compatibility with scanning probe techniques, which allows one to ‘view’ the reactants and products at the single-molecule level. In this article, we review the ability of noncontact atomic force microscopy to study on-surface chemical reactions with atomic precision. We highlight recent advances in using noncontact atomic force microscopy to obtain mechanistic insight into reactions and focus on the recently elaborated mechanisms in the formation of different types of graphene nanoribbons

Deuterated N2Py2 Ligands: Building More Robust Non-Heme Iron Oxidation Catalysts

Fe(N2Py2)/H 2 O 2 /AcOH catalytic systems provide powerful tools for efficient C-H and C=C bond oxidations (N2Py2 = bis-alkylamine-bis-pyridine ligand). Yet, the stability of these catalysts under the oxidizing conditions still remains a problem. The generally accepted catalyst decomposition pathway of Fe(N2Py2) complexes is through oxidative dimerization to form inactive oxo-bridged Fe 2 (μ-O)(N2Py2) 2 dimers. Detailed ESI-MS analysis has now shown a catalyst decomposition pathway of ligand oxidation via C-H oxidation on the 2-pyridinylmethylene sites, followed by dissociation of the oxidized ligand from the iron center. By deuterating the 2-pyridinylmethylene sites of a series of N2Py2 ligands with variations on both alkylamine and pyridine fragments, providing access to the corresponding Fe(N2Py2-D 4 ) complexes, longer catalysts lifetimes are achieved in catalytic oxidation reactions with all complexes. As a consequence, improved substrate conversions and product yields were consistently observed in both aliphatic C-H oxidations and alkene epoxidations. Kinetic and catalytic studies revealed that deuteration does not change the intrinsic reactivity and product selectivity of Fe(N2Py2) complexes. In addition, different Fe(N2Py2-D 4 ) complexes provide different improvements in catalytic performances and lifetimes, responding to the differences in ligand rigidity and robustness of the corresponding nondeuterated N2Py2 ligands. Accordingly, these improvements are more pronounced for ligands with a more flexible bis-alkylamine backbone. These observations provide insights into the development of more robust ligands for homogeneous oxidation catalysis

A multi-O2 complex derived from a copper(I) dendrimer

The high-pressure reaction of 2-vinylpyridine with the primary amines of four consecutive generations of poly(propylene imine) dendrimers (DAB-dendr- (NH2)(n)) (n = 4, 8, 16, 32) yielded dendrimers with bis[2-(2- pyridyl)ethyl]-amine (PY2) ligands. The complexation of the new dendrimers with metal ions was investigated by a variety of techniques. The reaction of the first- and fourth-generation dendrimers (n=4 and 32, respectively) with Zn(II)(ClO4)2 was studied by 1H NMR titration, and the complexation of all new dendrimers with Cu(II)(ClO4)2 was investigated by UV/Vis and EPR spectroscopy. Quantitative coordination of one metal ion per PY2 group was demonstrated in all cases. A UV/Vis titration of the fourth-generation dendrimer DAB-dendr-(PY2)32 in dichloromethane with [Cu(I)(CH3CN)4](ClO4) in acetonitrile revealed that approximately 30 Cu(I) ions were bound. Low-temperature UV/Vis spectroscopy of this complex in dichloromethane at -85°C in the presence of dioxygen showed that approximately 60-70% of the copper centers can bind dioxygen, corresponding to 10-11 of these molecules per dendrimer molecule. This complex can be considered a synthetic analogue of hemocyanin, the copper-containing oxygen transport protein from the hemolymph of molluscs and arthropods

Hydrogen Evolution Electrocatalysis with a Molecular Cobalt Bis(alkylimidazole)methane Complex in DMF: a critical activity analysis

Abstract: [Co(HBMIMPh2)2](BF4)2 (1), (HBMIMPh2 = bis(1-methyl-4,5-diphenyl-1H-imidazol-2-yl)methane), was investigated for its electrocatalytic hydrogen evolution performance in DMF using voltammetry and during controlled potential/current electrolysis (CPE/CCE) in a novel in-line product detection setup. Performances were benchmarked against three reported molecular cobalt HER electrocatalysts: [Co(dmgBF2)2(solv)2] (2), (dmgBF2 = difluoroboryldimethylglyoximato), [Co(TPP)] (3), (TPP = 5,10,15,20-tetraphenylporphyrinato) and [Co(bapbpy)Cl](Cl) (4), (bapbpy = 6,6’-bis-(2-aminopyridyl)-2,2’-bipyridine) showing distinct performances differences with 1 being the runner up in H2 evolution during CPE and the best catalyst in terms of overpotential and FE during CCE. After bulk electrolysis with all of the complexes a deposit on the glassy carbon electrode was observed and post electrolysis XPS analysis of the deposit formed from 1 demonstrated only a minor cobalt contribution (0.23%), mainly consisting of Co2+. Rinse tests on the deposits derived from 1 and 2 showed that the initially observed distinct activity is (partly) preserved for the deposits. These observations indicate that the molecular design of the complexes dictates the features of the formed deposit and therewith the observed activity

A [4Fe-4S] Cluster Dimer Bridged by Bis(2,2':6',2"-terpyridine-4'-thiolato)iron(II)

The use of 2,2':6',2"-terpyridine-4'-thiol (tpySH) was explored as a bridging ligand for the formation of stable assemblies containing both [4Fe-4S] clusters and single metal ions. Reaction of tpySH (2 equiv) with (NH4)2Fe(SO4)2 · 6H2O generated the homoleptic complex [Fe(tpySH)2]2+, which was isolated as its PF6- salt. The compound could be fully deprotonated to yield neutral [Fe(tpyS)2], and the absorption spectrum is highly dependent on the protonation state. Reaction of [Fe(tpySH)2](PF6)2 with the new 3:1 site-differentiated cluster (n-Bu4N)2[Fe4S4(TriS)(SEt)] yielded the first metal-bridged [4Fe-4S] cluster dimer, (n-Bu4N)2[{Fe4S4(TriS)(µ-Stpy)}2Fe]. Electrochemical studies indicate that the [4Fe-4S] clusters in the dimer act as independent redox units, while UV–vis spectroscopy provides strong evidence for a thioquinonoid electron distribution in the bridging tpyS- ligand. TpySH thus acts as a directional bridging ligand between [4Fe-4S] clusters and single metal ions, thereby opening the way to the synthesis of larger, more complex assemblies.

Highly efficient epoxidation of vegetable oils catalyzed by a manganese complex with hydrogen peroxide and acetic acid

Epoxidized vegetable oils (EVOs) are versatile building blocks for lubricants, plasticizers, polyvinyl chloride (PVC) stabilizers, and surface coating formulations. In this paper, a catalytic protocol for the efficient epoxidation of vegetable oils is presented that is based on a combination of a manganese catalyst, H 2 O 2 as an oxidant, and acetic acid as an additive. This protocol relies on the use of a homogeneous catalyst based on the non-noble metal manganese in combination with a racemic mixture of the N,N′-bis(2-picolyl)-2,2′-bispyrrolidine ligand (rac-BPBP). The optimized reaction conditions entail only 0.03 mol% of the manganese catalyst with respect to the number of double bonds (ca. 0.1 wt% with respect to the oil) and ambient temperature. This epoxidation protocol is highly efficient, since it allows most of the investigated vegetable oils, including cheap waste cooking oil, to be fully epoxidized to EVOs in more than 90% yield with excellent epoxide selectivities (>90%) within 2 h of reaction time. In addition, the protocol takes place in a biphasic reaction medium constituted by the vegetable oil itself and an aqueous acetic acid phase, from which the epoxidized product can be easily separated via simple extraction. In terms of efficiency and reaction conditions, the current epoxidation protocol outperforms previously reported catalytic protocols for plant oil epoxidation, representing a promising alternative method for EVO production

Enantioselective C-H Lactonization of Unactivated Methylenes Directed by Carboxylic Acids

The formidable challenges of controlling site-selectivity, enantioselectivity, and product chemoselectivity make asymmetric C-H oxidation a generally unsolved problem for nonenzymatic systems. Discrimination between the two enantiotopic C-H bonds of an unactivated methylenic group is particularly demanding and so far unprecedented, given the similarity between their environments and the facile overoxidation of the initially formed hydroxylation product. Here we show that a Mn-catalyzed C-H oxidation directed by carboxylic acids can overcome these challenges to yield γ-lactones in high enantiomeric excess (up to 99%) using hydrogen peroxide as oxidant and a Brønsted acid additive under mild conditions and short reaction times. Coordination of the carboxylic acid group to the bulky Mn complex ensures the rigidity needed for high enantioselectivity and dictates the outstanding γsite-selectivity. When the substrate contains nonequivalent γ-methylenes, the site-selectivity for lactonization can be rationally predicted on the basis of simple C-H activation/deactivation effects exerted by proximal substituents. In addition, discrimination of diastereotopic C-H bonds can be modulated by catalyst design, with no erosion of enantiomeric excess. The potential of this reaction is illustrated in the concise synthesis of a tetrahydroxylated bicyclo[3.3.1]nonane enabled by two key, sequential γ-C-H lactonizations, with the latter that fixes the chirality of five stereogenic centers in one step with 96% ee