Mesityl Groups Research Articles

Revisiting phosphorus analogues of phthalimides and naphthalimides: syntheses and comparative studies.

A series of phosphorus analogues of aromatic-fused monoimides (phthalimides and naphthalimides) bearing a mesityl group on the P center have been synthesized. In a comparison of their photophysical, electrochemical, and thermal properties with those of the corresponding imides, the impact of P incorporation was revealed. Furthermore, theoretical studies using DFT methods were conducted to understand their properties.

Selective Sensitization of Eu(III) and Tb(III) Emission with Triarylboron-Functionalized Dipicolinic Acids

Three triarylboron- (Mes2BAr-) functionalized dipicolinic acids, namely, 4-(4-(dimesitylboranyl)-2,3,5,6-tetramethylphenyl)pyridine-2,6-dicarboxylic acid (H2L1), 4-(4-(4-dimesitylboranyl)-2,3,5,6-tetramethylphenyl)-1H-1,2,3-triazol-1-yl)pyridine-2,6-dicarboxylic acid (H2L2), and 4-(4-(4-dimesitylboranyl) phenyl)-1H-1,2,3-triazol-1-yl)pyridine-2,6-dicarboxylic acid (H2L3), have been designed and synthesized. Lanthanide(III) complexes with the general formula of [NBu4]3[LnL3] (Ln = Eu or Tb; L = L1, L2, or L3) were obtained. The new triarylboron-functionalized ligands were found to be effective in the selective sensitization of the emissions of Eu(III) and Tb(III) ions with a high quantum efficiency (e.g., 0.54 for [NBu4]3[TbL13] in the solid state) upon excitation at ∼330 nm. An intraligand charge-transfer (ILCT) transition from the mesityl or aryl group to the boron or boron-aryl unit was found to play a key role in the activation of the Eu(III) and Tb(III) emissions, based on TD-DFT computational data and luminescence titration experiments performed using fluoride and cyanide ions.

An Al/P-based frustrated Lewis pair as an efficient ambiphilic ligand: coordination of boron trihalides, rearrangement, and formation of HBX₂ complexes (X = Br, I).

The Al/P-based frustrated Lewis pair (FLP) Mes2P-C(═CH-Ph)-Al(CMe3)2 (1) reacted with boron halides BX3 (X = F, Cl, Br, I) as an ambiphilic ligand to form complexes (2-5) in which the boron atoms were coordinated to phosphorus and one of the halogen atoms to aluminum. Nonplanar five-membered heterocycles resulted that had five different ring atoms (AlCPBX). The distance of the bridging halogen atoms to the AlCPB plane increased steadily with the radius of the halogen atoms. Only the BF3 adduct showed a dynamic behavior in solution at room temperature with equivalent tert-butyl or mesityl groups in the NMR spectra, while in other cases, the rigid conformation led to the magnetic inequivalence of the substituents at Al and P with well-resolved signals for each group. The BBr3 and BI3 complexes underwent in solution at room temperature a spontaneous stereoselective rearrangement with the concomitant release of isobutene. The obtained products, Mes2P-(μ-C═CH-Ph)(μ-HBX2)-AlX(CMe3) (6 and 7) may be viewed as unique adducts of a modified new Al/P-based FLP, Mes2P-C(═CH-Ph)-AlX(CMe3) (X = Br, I), with dihalogenboranes, HBX2. The trapped boranes are either completely unknown (X = I) or unstable in the free form. Quantum-chemical calculations suggest an ionic rearrangement mechanism via the formation of a borenium cation, β-hydride elimination, and hydride transfer. The bromine migration from boron to aluminum corresponds to a formal suprafacial 1,3-sigmatropic rearrangement.

Efficient copper mediators based on bulky asymmetric phenanthrolines for DSSCs.

In the context of the development of new electron mediators for dye-sensitized solar cells that could efficiently substitute the more common I3(-)/I(-) redox couple, we have prepared a series of Cu(I)- phenanthroline complexes through an easy synthetic route. The novel [Cu(2-mesityl-4,7-dimethyl-1,10-phenanthroline)2]PF6 (3) is of particular interest because the presence of the bulky mesityl groups protects as a kiss lock the Cu center, leading to superior dye regeneration kinetics with respect to simpler phenanthroline-based Cu(I) complexes and to a lower dark current with respect the I3(-)/I(-). The exploitation of a dual-component electrolyte constituted by (3) and a Fe(II) comediator, in combination with the [Ru(4,4'-dicarboxy-2,2'-bipyridine)2(4,4'-dinonyl-2,2'-bipyridine)](PF6)2 sensitizer, allowed us to increase the performance of the cell, reaching Jsc values of 4.0 mA cm(-2), comparable to that recorded with I3(-)/I(-) (3.8 mA cm(-2)).

Reactions of silver thiolates with bidentate phosphanes

A series of reactions of different silver thiolates, AgSR (R=Mes (mesityl), Nph (naphthyl), Cy (cyclohexyl)), with bidentate phosphanes was performed yielding a bunch of new compounds, which were characterized by single crystal X-ray diffraction.Depending on the nature of the thiolate and the phosphane the solid state structure of the products varies from discrete complexes or clusters to a 3-dimensional coordination network. The mesityl group favors the formation of trinuclear units which are then linked in one, e.g. [Ag3(SMes)3(dpph)2] (5), two, e. g. [Ag3(SMes)3 (dpppt)2] (4), or three dimensions [Ag3(SMes)3(dppb)2] (3), via the phosphanes. With naphthyl and cyclohexyl groups mostly clusters are formed which are either isolated with some phosphanes in their ligand sphere, e.g. [Ag12(SCy)12(dpppt)2] (13), or additionally linked by the phosphanes to form 1-dimensional chains, e.g. [Ag12(SCy)12(dppo)] (17).

Preparation via Sterically Promoted SN2′ Substitution and Structural Characteristics of a Mes*‐Substituted Tertiary Ethynylphosphine

ABSTRACTThe reaction of a sterically encumbered 2‐bromo‐3‐silyloxyphosphapropene [(Z)‐Mes*‐P = C(Br)CH(OSiMe3)Mes; Mes* = 2,4,6‐tBu3C6H2, Mes = 2,4,6‐Me3C6H2] and butyllithium afforded a bulky ethynylphosphine [Mes*P(nBu)C≡CMes] via SN2′ reaction of the alkyllithium and subsequent beta‐elimination of HBr. The orbital‐controlled reaction and prohibition of the halogen–metal exchange would be induced by the steric encumbrance of the mesityl group. Structural characteristics of the novel ethynylphosphine were also discussed in views of steric encumbrance as well as stereochemistry. The remarkable distortion of Mes* aromatic ring was further discussed according to DFT calculations.

A two-coordinate boron cation featuring C-B+-C bonding.

Two-coordinate boron cations (R2B(+)), referred to as borinium ions, are chemical species in which the boron bears only four valence electrons, and that are isoelectronic with hypothetical carbon dications (R2C(2+)). Although lone-pair-donating substituents such as amino groups have enabled the isolation of several borinium ions, diarylated and dialkylated borinium derivatives remain entirely unexplored. Here, we present the synthesis, structure and reactivity of the dimesitylborinium ion, which displays unexpectedly high thermal stability. X-ray crystallography and (11)B NMR spectroscopy, supported by density functional theory calculations, reveal that the borinium ion adopts a linear two-coordinate structure in both the solid state and in solution. The boron centre is stabilized by pπ bonding from the mesityl groups and is free from coordination by the counterion or solvent molecules. This diarylborinium ion possesses exceptional Lewis acidity, accepting a pair of electrons from CO2 to cause an unusual deoxygenation reaction.

Synthesis, Crystal Structures, and Skeletal Inversion of Dithienodiphosphorines

ABSTRACTtrans‐ and cis_endo‐Diphenyl[c]dithienodiphosphorines 1a and 1b were successfully synthesized as phosphorus atoms are trivalent. Two stereoisomers, trans form 1a and cis_endo form 1b, were isolated by preparative gel permeation chromatography (GPC) and their butterfly structures were shown by X‐ray crystallographic analyses of the single crystals from hexane/CHCl3 and CH3CN/CHCl3, respectively. The VT‐31P{1H} NMR spectra in CD2Cl2 showed that the signal due to cis_endo form 1b appeared at δP −45.0 with 9.5 Hz of half‐width at −90°C, which is almost same as that at room temperature, while broadening of the signal due to trans form 1a was observed and finally the signal appeared at δP −33.7 with 55.5 Hz of half‐width at −90°C. The results indicated that the skeletal inversion of a tricyclic system in solution is a fast process on the NMR timescale. On the other hand, X‐ray crystallographic analysis showed that trans‐dimesityl[c]dithienodiphosphorine 2a has a planar structure, which is considered as the corresponding transition state of the skeletal inversion of trans form 1a. The 31P signal due to 2a in CD2Cl2 did not change at all even at −90°C, which was in a different manner from 1a at the same temperature, showing that its planar structure is also kept in solution because of the release of the steric repulsion between bulky mesityl groups and fused thiophene rings in the butterfly structure. The density functional theory calculations suggested the low‐energy barriers of the skeletal inversion of trans–trans and cis_endo–cis_exo forms, which were estimated to be 3.79 and 3.31 kcal/mol by searching for transition state using the synchronous transit‐guided quasi‐Newton (STQN) methods.

Porphyrin dyes on TiO2 surfaces with different orientations: a photophysical, photovoltaic, and theoretical investigation.

Porphyrin dyes with a triphenylamino group as an electron donor, para- or meta-benzoic acids as electron acceptors, and hydrogen (H) or mesityl (M) substituents on the meso position as auxiliary groups were synthesized. Their photophysical properties and photovoltaic performance in dye-sensitized solar cells were investigated. All four porphyrins exhibited similar photophysical properties in the solution and dye-loading densities on the surface of TiO2 nanoparticles; however, the p-benzoic acid functionalized porphyrins, p-H(M)PZn, gave better photovoltaic performance than m-benzoic acid functionalized porphyrins, m-H(M)PZn. Theoretical calculations indicated that the electron density on the frontier molecular orbital was more delocalized to p-benzoic acid than to m-benzoic acid. Absorption spectra indicated the stronger H-aggregation in m-H(M)PZn than that in p-H(M)PZn on the surface of TiO2 nanoparticles. The mesityl groups in the meso positions reduced the dye-loading density due to steric hindrance between dyes. As a result, the p-MPZn exhibited the best energy conversion efficiency among the four porphyrins studied. This efficiency was further enhanced when a complementary dye BET was used.

Photophysical and Photochemical Trends in Tricarbonyl Rhenium(I) N-Heterocyclic Carbene Complexes

A family of tricarbonyl Re(I) complexes of the formulation fac-[Re(CO)3(NHC)L] has been synthesized and characterized, both spectroscopically and structurally. The NHC ligand represents a bidentate N-heterocyclic carbene species where the central imidazole ring is substituted at the N3 atom by a butyl, a phenyl, or a mesityl group and substituted at the N1 atom by a pyridyl, a pyrimidyl, or a quinoxyl group. On the other hand, the ancillary L ligand alternates between chloro and bromo. For the majority of the complexes, the photophysical properties suggest emission from the lowest triplet metal-to-ligand charge transfer states, which are found partially mixed with triplet ligand-to-ligand charge transfer character. The nature and relative energy of the emitting states appear to be mainly influenced by the identity of the substituent on the N3 atom of the imidazole ring; thus, the pyridyl complexes have blue-shifted emission in comparison to the more electron deficient pyrimidyl complexes. The quinoxyl complexes show an unexpected blue-shifted emission, possibly occurring from ligand-centered excited states. No significant variations were found upon changing the substituent on the imidazole N3 atom and/or the ancillary ligand. The photochemical properties of the complexes have also been investigated, with only the complexes bound to the pyridyl-substituted NHC ligands showing photoinduced CO dissociation upon excitation at 370 nm, as demonstrated by the change in the IR and NMR spectra as well as a red shift in the emission profile after photolysis. Temperature-dependent photochemical experiments show that CO dissociation occurs at temperatures as low as 233 K, suggesting that the Re-C bond cleaves from excited states of metal-to-ligand charge transfer nature rather than thermally activated ligand field excited states. A photochemical mechanism that takes into account the reactivity of the complexes bound to the pyridyl-NHC ligand as well as the stability of those bound to the pyrimidyl- and quinoxyl-NHC ligands is proposed.

Reactivity and Electronic Properties of a Ferrocene Molecule Bearing an N,C-Chelated BMes2 Unit

A dimesitylboron-functionalized benzimidazolylferrocene, B(2-ferrocenyl-N-Me-benzimidazolyl)Mes2 (1), has been synthesized and fully characterized. The B–N bond in 1 was found to undergo a dynamic dissociation/association process in solution, leading to a dynamic exchange of the two mesityls bound to the boron atom and slow hydrolysis of 1 under ambient conditions. The hydrolyzed product 2-BMes(OH)-1-(N-methylbenzimidazol-2-yl)ferrocene (2) was isolated and characterized, in which the boron center has a trigonal-planar geometry with one of the mesityls being replaced by an OH– group. The photoisomerization process of the boron unit in 1 was fully inhibited by the low lying d–d/MLCT states of the ferrocene unit. Compound 1 can be oxidized readily by I2, forming the ferrocenium species [1+]I3- (3). NMR and EPR data for 3 indicated a notable spin delocalization through space from the Fe(III) center to a flanking mesityl group.

Hydrosilylation of Aldehydes and Ketones Catalyzed by Half‐Sandwich Manganese(I) N‐Heterocyclic Carbene Complexes

AbstractEasily available manganese(I) N‐heterocyclic carbene (NHC) complexes, Cp(CO)2Mn(NHC), obtained in one step from industrially produced cymantrene, were evaluated as pre‐catalysts in the hydrosilylation of carbonyl compounds under UV irradiation. Complexes with NHC ligands incorporating at least one mesityl group led to the most active and selective catalytic systems. A variety of aldehydes (13 examples) and ketones (11 examples) were efficiently reduced under mild conditions [Cp(CO)2Mn(IMes) (1 mol%), Ph2SiH2 (1.5 equiv.), hν (350 nm), toluene, 25 °C, 1–24 h] with good functional group tolerance.magnified image

Remote Impacts of Methyl Substituents on the Guest‐Binding Ability of Self‐Assembled Cages

We synthesized self-assembled coordination cages in which 1,10-phenanthroline derivatives serve as capping ligands. Substituents at the 2,9-positions of the phenanthroline ligand covered the outside of the cage but had an impact on the guest binding inside the cage. Introduction of methyl groups at the 2,9-positions allowed the cage to accommodate tetraphenylsilane. Bulky mesityl groups overhanging the cage framework significantly shrunk the cage cavity through π-π interactions with the aromatic panels of the cage. The p-methyl group of the mesityl substituent was a determinant of the restricted motion of 4,4'-dimethoxybenzil inside the cage at high temperature. Thus, the presence or absence of one methyl group, which is far from the guest-binding site, makes a significant difference in the guest species and motions inside the cage.

Mechanistic Studies of Alkene Isomerization Catalyzed by CCC-Pincer Complexes of Iridium

Iridium complexes containing CCC-pincer m-phenylene-bridged N-heterocyclic carbene ligands were examined as catalysts for alkene isomerization. Complexes containing either mesityl or adamantyl side groups were found to catalyze the isomerization of a number of alkenes to the internal isomers, including 1-octene, vinylcyclohexane, and allylbenzene. Mechanistic studies indicate a surprising dichotomy, apparently caused by ligand steric effects. For the mesityl-substituted catalyst, several lines of evidence provide strong support for isomerization via an iridium allyl hydride intermediate: (1) H–D crossover experiments indicate that 1,3-hydrogen migration is exclusively intramolecular, (2) the catalyst resting state, a π-allyl hydride species, was isolated and serves as a kinetically competent catalyst, (3) NMR experiments indicate that the π-allyl hydride resting state undergoes reversible C–H reductive elimination that is rapid relative to catalytic turnover, and (4) kinetic studies indicate that the isomerization reaction is first order in substrate and catalyst, consistent with turnover-limiting ligand substitution. H–D crossover experiments for alkene isomerization catalyzed by the adamantyl-substituted complex show selectivity for a 1,3-deuterium shift, as well as the intermolecular transfer of hydrogen. These results are consistent with an insertion/elimination mechanism proceeding selectively through a secondary metal–alkyl or with a π-allyl-type mechanism with an unknown pathway for intermolecular hydrogen crossover.

Benz[c]indeno[2,1-a]fluorene: a 2,3-naphthoquinodimethane incorporated into an indenofluorene frame

Synthesis of 13,14-dimesitylbenz[c]indeno[2,1-a]fluorene (9b), a 2,3-naphthoquinodimethane derivative isolated for the first time, was achieved. The X-ray crystallographic structure and spectroscopic and electrochemical properties of 9b indicate substantial singlet biradical character which is higher than that of 11,12-dimesitylindeno[2,1-a]fluorene (8b), in accord with the quantum chemical study. Cycloadditions with oxygen and dichlorodicyano-p-benzoquinone took place at the inner naphthalene carbons rather than the most reactive exocyclic carbons because of the effective steric protection by the mesityl groups at the latter positions. [4 + 2] Cyclodimerization also took place at the inner naphthalene position as a 4π component and at the outer naphthalene ring as a 2π component.

1,2,3-Triazolylidene ruthenium(II)(η⁶-arene) complexes: synthesis, metallation and reactivity.

Three bis(1,2,3-triazolylidene) silver(I) complexes were synthesized, and the ruthenium complexes ([RCH2N2(NMe)C2Ph)]RuCl2(p-cymene) (R = C6H2Me3 4a₁, C6H2iPr3 4b₁) were isolated as major products with the minor C(sp(2))-H activated products ([RCH2N2(NMe)C2C6H4)]RuCl(p-cymene) (R = C6H2Me3 4a₂, C6H2iPr3 4b₂). In the related case where R = Ph, the species ([PhCH2N2(NMe)C2Ph)]RuCl2(p-cymene) 4c₁ was obtained with two C(sp(2))-H activated products [PhCH2N2(NMe)C2C6H4)]RuCl(p-cymene) 4c₂ and [(C6H4)CH2N2(NMe)C2Ph)]RuCl(p-cymene) 4c₃ derived from metallation of the N and C-bound arene rings. Heating a solution of 4a₁ at 45 °C over three weeks resulted in a ruthenium(II)(1,2,3-triazolylidene) complex [(C6H2Me3)CH2N2(NMe)C2Ph)]RuCl2 5a, where the pendant mesityl group on the triazolylidene moiety displaced the p-cymene ligand. The complexes 4a₁, 4b₁, 4c₁ and 5a displayed moderate catalytic activities in base-free oxidation of benzyl alcohols to benzaldehydes and oxidative homocoupling of benzyl amines to imines using oxygen as oxidant.

Blue phosphorescent nitrile containing C^C* cyclometalated NHC platinum(ii) complexes

Since C^C* cyclometalated Pt(II) complexes with N-heterocyclic carbene (NHC) ligands have been identified as potential emitter materials in organic light-emitting devices (OLEDs), very promising results regarding quantum yields, colour and stability have been presented. Herein, we report on four nitrile substituted complexes with a chelating NHC ligand (1-(4-cyanophenyl)-3-isopropyl-1H-benzo[d]imidazole or 4-(tert-butyl)-1-(4-cyanophenyl)-3-methyl-1H-imidazole) and a bidentate monoanionic auxiliary ligand (acetylacetone or dimesitoylmethane). The complexes have been fully characterized including extensive 2D NMR studies (COSY, HSQC, HMBC, NOESY, (195)Pt NMR), three of them also by solid-state structures. Photophysical measurements in amorphous PMMA films and pure emitter films at room temperature reveal the impact of the mesityl groups in the auxiliary ligand, which led to a significant increase of the quantum yield, while the decay lifetimes decreased. The electron withdrawing nitrile groups shift the emission towards blue colour coordinates.

CO2 Activation with Bulky Neutral and Cationic Phenoxyalanes

The sterically crowded aluminum diphenolate (2,6-Mes2C6H3O)2AlEt (2; Mes =2,4,6-Me3C6H2−) was converted into the ionic species [(2,6-Mes2C6H3O)2Al]+[CHB11Cl11]− (6) by ethide abstraction with the silylium salt [Et3Si][CHB11Cl11] or by a combination of β-hydride abstraction and concomitant ethylene elimination with the trityl salt [Ph3C][CHB11Cl11]. Compound 6 consists of solvent-separated ions, and the cation features very short Al···C contacts involving the flanking mesityl groups, leading to an overall distorted-tetrahedral coordination geometry around the aluminum center. Analogous reactions with the diphenolate (2,6-tBu2-4-MeC6H2O)2AlEt (1) led to tert-butyl transfer to the benzene solvent to afford tert-butylbenzene as the main product. A 1:1 mixture of the triphenolate (2,6-Ph2C6H3O)3Al and tri-tert-butylphosphine forms a frustrated Lewis pair, which reacts with CO2 at room temperature to give the compound (2,6-Ph2C6H3O)3AlOC(O)Pt-Bu3 (8). The high Lewis acidity of 6 catalyzes the scrambling of the ...

(Cyclopentadienyl)iron(II) Complexes of N-Heterocyclic Carbenes Bearing a Malonate or Imidate Backbone: Synthesis, Structure, and Catalytic Potential in Hydrosilylation

The backbone-functionalized anionic carbenes maloNHC (1R; malonate backbone) and imidNHC (2; imidate backbone) were generated in situ from their respective zwitterionic precursors and treated with FeCp(CO)2I to afford the zwitterionic complexes {FeCp(CO)2(1R)} (3R; 59–84% yield), and {FeCp(CO)2(2)} (4; 77% yield), respectively. Methylation of the malonate complex 3Me takes place at one of the backbone oxygen atoms to give the cationic adduct [FeCp(CO)2(1MeMe)](OTf) ([5Me](OTf); 96% yield), whereas methylation of 4 takes place at the imidate nitrogen atom to produce the cationic adduct [FeCp(CO)2(2Me)](OTf) ([6Me](OTf); 84% yield). All of the complexes were characterized by NMR and IR in solution, while X-ray structure analyses were carried out for 3Me, 4, and [6Me](OTf). In addition, a detailed experimental and theoretical investigation of the electron density within the archetypal zwitterionic complex 3Me was carried out. The observation of short intramolecular contacts between Cipso or Cortho of the mesityl groups of the carbene and the proximal carbonyl groups is rationalized in terms of a noncovalent “through space” π–π* interaction involving a two-electron delocalization of the occupied π(Cipso═Cortho) molecular orbital (MO) of the aryl ring into one vacant π*(C≡O) MO of the carbonyl ligand. A theoretical analysis carried out on dissymmetrical model complexes reveals that the magnitude of such an interaction is correlated with the donor properties of aryl group substituents. A catalyst screening of the above complexes in the hydrosilylation of benzaldehyde under visible light irradiation revealed a dramatic effect of the electronic donor properties of these carbenes on the performances of their complexes, with the more nucleophilic carbene 1tBu– in the zwitterionic species 3tBu appearing as the most efficient. This complex shows good efficiency and excellent chemoselectivity in the hydrosilylation of various aldehydes bearing reactive functional groups. It is also moderately active in the hydrosilylation of a few ketone substrates and exhibits very good performance in the hydrosilylation of representative aldimines and ketimines.

Experimental and theoretical study of Hoveyda–Grubbs catalysts modified by perfluorohexyl ponytail in the alkoxybenzylidene ligand

The alkoxybenzylidene ligand of Hoveyda–Grubbs 1st and 2nd generation catalysts was modified with one or two perfluorohexyl groups by Ullmann reaction with the aim to improve both the fluorophilicity and activity of the catalyst. While bis(perfluorohexylation) resulted in inability of the ligand to exchange tricyclohexylphosphine ligand of parent Grubbs catalysts, mono(perfluorohexylation) and subsequent ligand exchange resulted in the formation of complexes of light fluorous properties and substantially higher activity in model RCM reactions. Modification of the mesityl group of the unsaturated NHC ligand by polyfluoroalkyl ponytails resulted in the formation of ruthenium precatalyst furnishing active catalytic intermediate with light fluorous properties. DFT computations of the model initiation reaction of ethene with Hoveyda–Grubbs 2nd generation catalyst or its pentafluoroethylated counterpart revealed that in the latter, the intermediate ruthenacyclobutane can form and decompose with significantly lower energies, thus explaining its higher activity.