Bulky Carboxylate Research Articles

Colloidal ruthenium catalysts for selective quinaldine hydrogenation: Ligand and solvent effects.

Colloidal Ru nanoparticles (NP) display interesting catalytic properties for the hydrogenation of (hetero)arenes as they proceed efficiently in mild reaction conditions. In this work, a series of Ru based materials was used in order to selectively hydrogenate quinaldine and assess the impact of the stabilizing agent on their catalytic performances. Ru nanoparticles stabilized with polyvinylpyrrolidone (PVP) and 1-adamantanecarboxylic acid (AdCOOH) allowed to obtain 5,6,7,8-tetrahydroquinaldine with a remarkable selectivity in mild reaction conditions by choosing the suitable solvent. The presence of a carboxylate ligand on the surface of the Ru NP led to an increase in the activity when compared to Ru/PVP catalyst. The stabilizing agent had also an impact on the selectivity, as carboxylate ligand modified catalysts promoted the selectivity towards 1,2,3,4-tetrahydroquinaldine, with bulky carboxylate displaying the highest ones.

Front Cover: Paddle‐Wheel Dirhodium Complexes Bearing Bulky Carboxylate Ligands: Synthesis and Catalysis in Carbene Insertion Reactions (Chem. Asian J. 11/2023)

Paddle-Wheel dirhodium complexes bearing bulky carboxylate ligands were synthesized and characterized. The steric bulk of carboxylate ligands could affect the reaction selectivity in Rh-catalyzed intramolecular competitive carbene insertions: Rh catalysts with bulky carboxylates could provide five-membered ring products preferentially via the insertion into a carbon–hydrogen bond while conventional Rh catalysts gave six-membered ring products via the insertion of a carbon-carbon double bond. More information can be found in the Research Article by Tetsuaki Fujihara et al.

Paddle-Wheel Dirhodium Complexes Bearing Bulky Carboxylate Ligands: Synthesis and Catalysis in Carbene Insertion Reactions.

Dirhodium complexes bearing bulky carboxylate ligands are synthesized and characterized. The steric bulk of carboxylate ligands could affect the reaction selectivity in Rh-catalyzed intramolecular reactions: Rh catalysts with bulky carboxylate ligands provided five-membered ring products preferentially via the insertion into a carbon-hydrogen bond. Meanwhile, six-membered ring products were obtained using conventional Rh catalysts via the insertion of a carbon-carbon double bond.

Biomimetic Cubane‐Type Manganese Complex: Structurally Inspired by Photosystem II

A biomimetic system comprising a cubane‐type tetranuclear complex with a [Mn4O4] center similar to the core of the oxygen‐evolving complex (OEC) of photosystem II (PSII), which is responsible for producing molecular oxygen, was synthesized from bulky carboxylate ligands and manganese by an expedient process. The synergistic effect of the exterior ligands attached to the core and the interatomic interactions led to a cubane‐type morphology. This study lays the foundation for further study of metallic complexes analogous to biotic matter and contributes to structural knowledge of the second coordination sphere. In addition, the findings may shed light on the mechanism of oxygen evolution during photosynthesis.

Quasi-Solid Electrolytes Comprising Sulfide Electrolyte and Carboxylate Esters: Investigation of the Influence of the Carboxylate Ester Structure

Composite quasi-solid electrolytes composed of Li3PS4 (LPS) glass and a carboxylate ester were prepared via planetary ball milling, and the influence of the molecular structure of the carboxylate esters on their ion-conduction was investigated. The results revealed that filling the voids in the solid electrolyte with a bulky carboxylate ester, which hardly reacted with the LPS glass, effectively maintains the high ionic conductivity of the LPS glass even when the relative density of LPS decreases. The use of this composite quasi-solid electrolyte is an effective approach to maintain the ion-conduction in not only an electrolyte separator layer but also a composite electrode layer in an all-solid-state battery.

Rh-Catalyzed Intermolecular Reactions of α-Alkyl-α-Diazo Carbonyl Compounds with Selectivity over β-Hydride Migration.

Rh-carbenes derived from α-diazocarbonyl compounds have found broad utility across a remarkable range of reactivity, including cyclopropanation, cyclopropenation, C-H insertions, heteroatom-hydrogen insertions, and ylide forming reactions. However, in contrast to α-aryl or α-vinyl-α-diazocarbonyl compounds, the utility of α-alkyl-α-diazocarbonyl compounds had been moderated by the propensity of such compounds to undergo intramolecular β-hydride migration to give alkene products. Especially challenging had been intermolecular reactions involving α-alkyl-α-diazocarbonyl compounds. This Account discusses the historical context and prior limitations of Rh-catalyzed reactions involving α-alkyl-α-diazocarbonyl compounds. Early studies demonstrated that ligand and temperature effects could influence chemoselectivity over β-hydride migration. However, effects were modest and conflicting conclusions had been drawn about the influence of sterically demanding ligands on β-hydride migration. More recent advances have led to a more detailed understanding of the reaction conditions that can promote intermolecular reactivity in preference to β-hydride migration. In particular, the use of bulky carboxylate ligands and low reaction temperatures have been key to enabling intermolecular cyclopropenation, cyclopropanation, carbonyl ylide formation/dipolar cycloaddition, indole C-H functionalization, and intramolecular bicyclobutanation with high chemoselectivity over β-hydride migration. Cyclic α-diazocarbonyl compounds have been shown to be particularly resilient toward β-hydride migration and are the first class of compounds that can engage in intermolecular reactivity in the presence of tertiary β-hydrogens. DFT calculations were used to propose that for cyclic α-diazocarbonyl compounds, ring constraints relieve steric interaction for intermolecular reactions and thereby accelerate the rate of intermolecular reactivity relative to intramolecular β-hydride migration. Enantioselective reactions of α-alkyl-α-diazocarbonyl compounds have been developed using bimetallic N-imido-tert-leucinate-derived complexes. The most effective complexes were found by computation and X-ray crystallography to adopt a "chiral crown" conformation in which all of the imido groups are presented on one face of the paddlewheel complex in a chiral arrangement. Insight from computational studies guided the design and synthesis of a mixed ligand paddlewheel complex, Rh2(S-PTTL)3TPA, the structure of which bears similarity to the chiral crown complex Rh2(S-PTTL)4. Rh2(S-PTTL)3TPA engages substrate classes (aliphatic alkynes, silylacetylenes, α-olefins) that are especially challenging in intermolecular reactions of α-alkyl-α-diazoesters and catalyzes enantioselective cyclopropanation, cyclopropenation, and indole C-H functionalization with yields and enantioselectivities that are comparable or superior to Rh2(S-PTTL)4. The work detailed in this Account describes progress toward enabling a more general utility for α-alkyl-α-diazo compounds in Rh-catalyzed carbene reactions. Further studies on ligand design and synthesis will continue to broaden the scope of their selective reactions.

Unexpected Role of Anionic Ligands in the Ruthenium‐Catalyzed Base‐Free Selective Hydrogenation of Aldehydes

Bigger and better: The replacement of anionic chloride ligands in Noyori-type [(diamine)(diphosphine)RuCl2 ] catalysts with bulky carboxylate ligands enabled the efficient selective hydrogenation of a variety of aldehydes under base-free conditions. Turnover numbers of up to 100 000 were reached in the presence of a bulky carboxylic acid co-catalyst. This type of catalytic system probably operates through an inner-sphere mechanism.

Unexpected structural and electronic effects of internal rotation in diruthenium paddlewheel complexes containing bulky carboxylate ligands

A series of Ru 2 II,III complexes containing the bulky carboxylate ligand 2,4,6-triisopropylbenzoate (TiPB) of type trans-[Ru 2(TiPB) 2(O 2CCH 3) 2X] [X = Cl ( 1), PF 6 ( 2)] and [Ru 2(TiPB) 4X] [X = Cl ( 3), PF 6 ( 4)] have been synthesised. The corresponding Ru 2 II,II complexes trans-[Ru 2(TiPB) 2(O 2CCH 3) 2] ( 5) and [Ru 2(TiPB) 4] ( 6) were also isolated. Magnetic susceptibility measurements indicate that the diruthenium cores have the expected three ( 1– 4) or two ( 5 and 6) unpaired electrons consistent with σ 2π 4δ 2(δ ∗π ∗) 3 and σ 2π 4δ 2δ ∗2π ∗2 electronic configurations. Compounds 1– 4 and 6 were structurally characterised by X-ray crystallography, and show the expected paddlewheel arrangement of carboxylate ligands around the diruthenium core. The diruthenium cores of complexes 3, 4 and 6 are all distorted to minimise steric interactions between the bulky carboxylate ligands. The Ru–Ru bond length in the Ru 2 II,II complex 6 [2.2425(6) Å] is the shortest observed for a diruthenium tetracarboxylate and, surprisingly, is 0.014 Å shorter than in the analogous Ru 2 II,III complex 4, despite an increase in the formal Ru–Ru bond order from 2.0 ( 6) to 2.5 ( 4). This is rationalised in terms of the extent of internal rotation, or distortion, about the diruthenium core. This was supported by density functional theory calculations on the model complexes [Ru 2(O 2CH) 4] and [Ru 2(O 2CH) 4] +, that demonstrate the relationship between Ru–Ru bond length and internal rotation. Electrochemical and electronic absorption data were recorded for all complexes in solution. Comparison of the data for the ‘ bis– bis’ ( 1, 2 and 5) and tetra-substituted ( 3, 4 and 6) complexes indicates that the shortening of the Ru–Ru bond length results in a small increase in energy of the near-degenerate δ ∗ and π ∗ orbitals.

Recognition of Carboxylate Anions and Carboxylic Acids by Selenium-Based New Chromogenic Fluorescent Sensor: A Remarkable Fluorescence Enhancement of Hindered Carboxylates

A selenium metal-based new fluorescence sensor 5-pivaloylamino-1,2,5-selenodiazolo[3,4-d]pyrimidin-7-(6H)-one (receptor 1) has been reported for the recognition of monocarboxylic acids and carboxylate anions both by UV-vis and fluorescence methods. Receptor 1 recognizes carboxylate anions more than monocarboxylic acids and it is a selective sensor for carboxylates with specially hindered carboxylate anions. The changes of fluorescence intensity are remarkably enhanced with red shift in presence of bulky carboxylate anions. The X-ray crystal structure of receptor 1 with pivalic acid has been reported.

Polymeric Frameworks Constructed from Bulky Carboxylates and 4,4′-Bipyridine Linkages: Synthesis, Crystal Structures, and Properties

Six novel coordination polymers, {[M2(L1)2(μ2-L1)2(4,4′-bipy)2]·CH3OH}n (HL1 = 3-(4-carboxylphenylamino)-1-ferrocenyl-2-butylen-1-one, 4,4′-bipy = 4,4′-bipyridine) (M = Zn (1) or Cd (2)), {[Cd2(η2-L2)2(μ2-η2-L2)2(4,4′-bipy)2]·CH3OH}n (HL2 = N-phenylanthranilic acid) (3), {[Cd2(μ-L3)(μ2-L3)(η2-L3)(4,4′-bipy)2(H2O)2]·NO3}n (HL3 = cinnamic acid) (4), {[Cd2(μ2-L4)4(4,4′-bipy)2]·2H2O}n (HL4 = hippuric acid) (5), and {[Cd2(μ2-η2-L5-η2)2(SO4)(4,4′-bipy)2(H2O)2]·5H2O}n (HL5 = 1-(carboxymethyl)-1,3-benzimidazol-3-ium-3-acetate) (6), have been synthesized and structurally characterized by single-crystal X-ray diffraction method. Polymers 1−4 exhibit one-dimensional (1-D) ladder-like structures, in which carboxylate ligands bridge two metal ions forming dinuclear cores, and 4,4′-bipy ligands link these cores. In polymers 5 and 6, the carboxylate-linked chains [Cd2(μ2-L4)4]n or mixed carboxylate and SO42− linked [Cd2(μ2-η2-L5-η2)2(SO4)]n chains are bridged by 4,4′-bipy ligands to form two-dimensional (2-D) sheet stru...

Relationship between metal–metal bond length and internal rotation in diruthenium tetracarboxylate paddlewheel complexes

The Ru-Ru bond length for Ru2II,III and Ru2II,II paddlewheel complexes containing the bulky carboxylate ligand 2,4,6-triisopropylbenzoate was found to decrease despite a reduction in Ru-Ru bond order, due to increased internal rotation.

Spin Switching via Targeted Structural Distortion

The deliberate "stepwise" structural distortion of the [MnIII6O2(sao)6(O2CR)2L4] (S = 4, Ueff = 28 K) family of SMMs (where sao2- is the dianion of salicylaldoxime or 2-hydroxybenzaldeyhyde oxime and L = MeOH, EtOH) via the use of derivatized oxime ligands and bulky carboxylates leads to a family of single-molecule magnets with larger spin ground states and enhanced blocking temperatures. Replacing sao2- and HCO2- in the molecule [MnIII6O2(sao)6(O2CH)2(MeOH)4] (1), with Et-sao2- (Et-saoH2 = 2-hydroxypropiophenone oxime) and Me3CCO2- (pivalate), produces the complex [MnIII6O2(Et-sao)6(O2CCMe3)2(EtOH)5] (2) that displays an S = 7 ground state with Ueff = 30 K. Replacing Me3CCO2- with PhCO2- produces the complex [MnIII6O2(Et-sao)6(O2CPh)2(EtOH)4(H2O)2] (3) that displays an S = 12 ground state with Ueff = 53 K. The ligand substitution invokes a subtle structural distortion to the core of the molecule evidenced by an increased "twisting" of the oxime moiety (Mn-N-O-Mn) and a change in carboxylate ligation, which, in turn, invokes a dramatic change in the observed magnetic properties by switching weak antiferromagnetic exchange to weak ferromagnetic exchange.

A new class of oxo-bridged high-valent hexamanganese clusters supported by sterically hindered carboxylate ligands.

A new class of oxo-bridged high-valent hexamanganese (Mn6) clusters containing a novel (Mn6O8)6+ core, [MnIV(4)MnIII2(mu-O)4(mu3-O)4(dmb)6(O2CR)2]4+ (where dmb=4,4'-dimethyl-2,2'-bipyridine, and RCO2=2,6-di(p-tolyl)benzoate (Ar(Tol)CO2-) (3) or 2,6-di(4-tert-butylphenyl)benzoate (Ar(4-tBuPh)CO2-) (4)), was synthesized using sterically hindered m-terphenyl-derived carboxylate ligands. These complexes can be synthesized by oxidizing the MnII mononuclear complexes, [Mn(dmb)2(OH2)(O2CR)]+ (where RCO2=Ar(Tol)CO2- (1) or Ar(4-tBuPh)CO2- (2)) with (n-Bu4N)MnO4, by direct Mn(II) + Mn(VII) in situ comproportionation reactions, or by ligand substitution on the dinuclear manganese (III,IV) or (IV,IV) complexes, [(Mn2(mu-O)2(dmb)4)](3+/4+). The compound [MnIV4MnIII2(mu-O)4(mu3-O)4(dmb)6(Ar(Tol)CO2)2](OTf)4 [3(OTf)4] crystallizes in the monoclinic space group P2(1)/n, with the cell parameters a=15.447(1) A, b=15.077(2) A, c=27.703(2) A, beta=91.68(2) degrees, V=6449.3(6) A3, and Z=2. The X-ray structure reveals that there are three different bridging modes for the oxo groups: mu, "pyramidal" mu3, and "T-shaped" mu3. Solid-state variable temperature magnetic susceptibility studies suggest that the Mn centers are net antiferromagnetically coupled to yield a diamagnetic ST=0 ground spin state with a large number of low-lying, thermally accessible states with ST>0. 1H NMR spectra were recorded for both Mn6 clusters and selected resonances assigned. The electronic and redox properties of these complexes along with the effect of the presence of the bulky carboxylate ligands are also described here.

Coordinative control of photoinduced electron transfer: bulky carboxylates as molecular curtains

An intramolecular photoinduced electron transfer which takes place in a ZnII polyamine complex can be interrupted through coordination of a bulky carboxylate anion, acting as a curtain.

Induction of smectic layering in nematic liquid crystals using immiscible components IV. The effect of bulky lateral carboxyl substituents on the thermotropic behaviour of 2,5-bis[4-(n-perfluoroheptyloctyloxy)benzoyloxy]toluene

Bulky lateral carboxylate substituents were introduced at the benzylic position of 2,5-bis[4-(n-perfluoroheptyloctyloxy)benzoyloxy]toluene by esterification of the corresponding benzyl bromide with potassium carboxylates. In spite of the bulky lateral substituents, none of the 2,5-bis[4-(n-perfluoroheptyloctyloxy)benzoyloxy]benzyl carboxylates exhibit a nematic mesophase in addition to, or instead of, the smectic mesophases. All of the crystalline and SmC-SmA transition temperatures and, with the exception of the 9-anthracene carboxylate derivative, all of the isotropization temperatures of the resulting 2,5-bis[4-(n-perfluoroheptyloctyloxy)benzoyloxy]benzyl carboxylates are lower than those of the parent toluene compound. The SmC-SmA transition decreases the most, thereby stabilizing the SmA mesophase. In most cases, the SmC mesophase is destabilized from an enantiotropic to a monotropic mesophase. There is no correlation between any of the transition temperatures and the size of the lateral substituent.

Covalent Conversion of Cyclic Onium Salt End Groups of Poly(tetrahydrofuran) by Bulky Counteranions in the Absence and Presence of Macrocyclic Compounds

A series of bulky carboxylates was introduced as a counteranion for N-methylpyrrolidinum salt end groups of poly(tetrahydrofuran), poly(THF), having molecular weights of ca. 5000. 4-(7,7,7-Triphenylheptoxy)benzoate was found to remain intact as a counteranion at ambient temperature but to cause the ring-opening reaction of pyrrolidinium groups at 90 °C to produce poly(THF) with bulky stopper groups in a pure form in 71% yield. The relevant ring-opening reaction also took place in the presence of such macrocyclics as 30-crown-10 and cyclodextrins, but an effective entrapping of cyclic components in polyrotaxanes was not achieved presumably because of the entropic repulsion between the two components.

Preparation of Biscarboxylato Imido Alkylidene Complexes of Molybdenum and Cyclopolymerization of Diethyldipropargylmalonate To Give a Polyene Containing only Six-Membered Rings

We wanted to prepare a catalyst that would yield polymers having exclusively one ring size and therefore began a search for new initiators containing `ancillary` ligands other than alkoxides. We report here that a new class of catalysts containing bulky carboxylate ligands can be prepared which encourage regioselective addition of the first triple bond to an alkylidene to give only a {beta}-subsituted metallacyclobutene and therefore polyenes that contain only six-membered rings in the polymer backbone as a consequence of specific head-to-tail cyclopolymerization. 14 refs., 1 fig., 2 tabs.