Mesityl Groups Research Articles

Catalytic Activity of Biscobalt Porphyrin-Corrole Dyads Toward the Reduction of Dioxygen

A series of biscobalt cofacial porphyrin-corrole dyads bearing mesityl substituents at the meso positions of the corrole ring were investigated as to their electrochemistry, spectroelectrochemistry, and CO binding properties in nonaqueous media and then applied to the surface of a graphite electrode and tested as electrocatalysts for the reduction of dioxygen to water or hydrogen peroxide in air-saturated aqueous solutions containing 1 M HClO(4). The catalytic reduction of O(2) with the same dyads was also investigated in the homogeneous phase using 1,1'-dimethylferrocene as a reductant in PhCN containing HClO(4). The examined compounds are represented as (PMes(2)CY)Co(2), where P = a porphyrin dianion, Mes(2)C = a corrole trianion with two mesityl groups in trans meso-positions of the macrocycle, and Y is one of three bridging groups separating the two metallomacrocycles in a face-to-face arrangement, either with 9,9-dimethylxanthene, dibenzofuran, or diphenylether as linkers. Cyclic voltammetry and rotating disk electrode voltammetry revealed that the examined compounds are all catalytically active toward the electroreduction of dioxygen in acid media giving H(2)O(2) or H(2)O depending upon the type of linkage (Y) and the initial site of electron transfer which, in nonaqueous media, could be switched between the corrole and the porphyrin metal center by variations of substituents on the corrole macrocycle or the gas above the solution. The homogeneous reduction of dioxygen via a two- or four-electron transfer process was also investigated using 1,1'-dimethylferrocene as reductant in PhCN containing HClO(4).

Oxybis(dimesitylborane) dichloromethane hemisolvate

The title compound, C36H44B2O·0.5CH2Cl2, contains an almost linear O—B—O linkage [177.23 (15)°] and approximately orthogonal [interplanar angles 89.49 (5) and 80.77 (4)°] trigonal planar B centers, consistent with the previously reported non-solvated structure [Cardinet al. (1983). J. Chem. Res. (S), p. 93]. Inter­molecular C—H⋯π inter­actions exist between mesityl groups, with a C—H⋯centroid separation of 3.6535 (18) Å. The dichloromethane mol­ecules lie on twofold rotation axes.

Synthesis and Structural Characterization of Tin Analogues of N-Heterocyclic Carbenes

The synthesis and X-ray crystal structures of five N-heterocyclic stannylenes are reported. These compounds, containing a variety of backbones, were prepared by the salt metathesis of the appropriate dilithiated diamide with SnCl(2) and show a high degree of thermal stability compared to the corresponding species with unsaturated backbones. If bulky diisopropylphenyl groups are attached to the nitrogen centers then the structures are monomeric, but when the less bulky mesityl groups are employed the solid-state structure was shown to be dimeric.

Attainment of new Te II and Te IV iodides by unexpected removal of the mesityl group of [mesTeI 2] −: Synthesis and structural features of [TeI 4][Co( en) 3] · I, [TeI 6][Co( en) 3] · I, [{TeI 6}{Te 2I 10}][Co(NH 3) 6] 2 · 2I · 6H 2O and [(mesTeI 2) 2(μ-I 2)][Co(bpy) 3] · I 3 · 2CHCl

(MesTe) 2 reacts with I 2 and [CoL n ]I 3 {L = (NH 3) 6, ( en) 3} in a biphasic system (chloroform and water) to give, in the interface of the two phases, crystals of [TeI 4][Co( en) 3] · I − ( 1), [TeI 6][Co( en) 3] · I − ( 2) and [{TeI 6}{Te 2I 10}] [Co(NH 3) 6] 2 · 2I − · 6H 2O ( 3), in a high yield. If the crystallization is carried out with [Co(bpy) 3]I 3 under addition of THF, the mesityl groups are retained and the final product is [(mesTeI 2) 2(μ-I 2)][Co(bpy) 3] · I 3 - · 2CHCl 3 ( 4), which crystallizes in the organic phase. The already known species {mesTeI⋯I(mes)Te−Te(mes) 2} appears as subproduct in the syntheses of 1, 2 and 3, in the organic phase. The X-ray structural characterization of the new complexes is discussed, together with some synthetic features related with the abstraction of the mesityl groups in the three first syntheses.

Reinvestigation of arylmanganese chemistry – Synthesis and molecular structures of [(thf) 4Mg(μ-Cl) 2Mn(Br)Mes], [Mes(thf)Mn(μ-Mes)] 2, and (MnPh 2) ∞ (Ph = C 6H 5; Mes = mesityl, 2,4,6-Me 3C 6H 2)

The reaction of 2,4,6-trimethylphenylmagnesium bromide (MesMgBr) with manganese(II) chloride in an equimolar ratio yields [(thf) 4Mg(μ-Cl) 2Mn(Br)Mes] ( 1) with two bridging chloro ligands and a Mn–C bond length of 2.136(5) Ǻ. A molar ratio of 2:1 leads to a metathesis reaction and the formation of [Mes(thf)Mn(μ-Mes)] 2 ( 2) with two bridging mesityl groups. Whereas THF-free [MnMes 2] 3 crystallizes as a trimer from toluene solution, the reduced bulkiness of the phenyl groups in (MnPh 2) ∞ ( 3) leads to the formation of a chain-like structure with Mn–C bond lengths of 2.245(3) Ǻ.

Chemistry of Boryllithium: Synthesis, Structure, and Reactivity

A series of lithium salts of boryl anion, boryllithiums, were synthesized and characterized by NMR spectroscopy and crystallographic analysis. In addition to the parent boryllithium compound 35a, structural modification of boryllithium, using saturated C-C and benzannulated C=C backbones in the five-membered ring and mesityl groups on the nitrogen atoms, also allowed generation of the corresponding boryllithium. The solid state structures of boryllithium showed that the boron-lithium bond is polarized where the boron atom is anionic in all (35a x DME)(2), 35a x (THF)(2), 35b x (THF)(2), and 35c x (THF)(2) when compared to the structures of hydroborane 38a-c and optimized free boryl anion opt-46a-c. Dissolution of the isolated single crystals of (35a x DME)(2) and 35a x (THF)(2) in THF-d(8) showed that the boron-lithium bond remained in solution and free DME or THF molecules were observed. Temperature-dependent (11)B NMR chemical shift changes of 35a were observed in THF-d(8) or methylcyclohexane-d(14), suggesting a change of chemical shift anisotropy around the boron center. The HOMO of opt-35a x (THF)(2) had a lone pair character on the boron atom, as observed for phenyllithium, whereas the HOMO of hydroborane 38a corresponds to the pi-orbital of the boron-containing five-membered heterocycle. The polarity of the B-Li bond, estimated by AIM analysis, was similar to that of alkyllithium. Boryllithiums 35a and 35b behave as a base or a boron nucleophile in reaction with organic electrophiles via deprotonation, S(N)2-type substitution, halogen-metal exchange or electron-transfer, 1,2-addition to a carbonyl group, and S(N)Ar reaction. In the case of the reaction with CO(2), intramolecular cyclization followed by CO elimination from borylcarboxylate anion and subsequent protonation gave hydroxyboranes 64a and 64b. The characters of the carbonyl groups in the borylcarbonyl compounds 60a, 60b, 61, 62, and 63a, which were obtained from the reaction of boryllithiums 35a and 35b, were investigated by X-ray crystallography, IR, and (13)C NMR spectroscopy to show that the boryl substituent weakened the C=O bond when compared to carbon substituted analogues.

Sterically Crowded Terphenylselenium Compounds

AbstractExtremely bulky terphenyltriselanes RSeSeSeR with R = 2,6‐(2,4,6‐Me3C6H2)2C6H3 and 2,6‐(2,4,6‐iPr3C6H2)2C6H3 were prepared. The chlorination with sulfuryl chloride resulted in the formation of the corresponding selenenyl chlorides RSeCl. In the case of the methyl substituted derivative, also a chlorination of the mesityl groups was observed and the derivative could be isolated. The molecular structure of the isopropyl substituted triselane [2,6‐(2,4,6‐iPr3C6H2)2C6H3Se]2Se and that of 2,6‐(2,4,6‐Me3C6Cl2)2C6H3SeCl, have been determined by X‐Ray diffraction.

Molecular Conformation, Organizational Chirality, and Iron Metalation of meso-Tetramesitylporphyrins on Copper(100)

We report on the conformation and self-assembly properties of meso-tetramesitylporphyrin on Cu(100). The results show that the presence of the mesityl groups limits the interaction between the porphyrin ring and the copper surface, contributing to the high porphyrin mobility at room temperature. At low temperatures it is the substrate which determines the molecule orientation. The intermolecular interaction is also very weak, and only for high coverages do the porphyrins self-assemble to form large islands with two different mirror symmetric unit cells. The porphyrins can be Fe metalated by sublimation of Fe at room temperature on a porphyrin overlayer deposited on the copper surface.

Phosphine Selenide Arenetellenyliodide Complexes and Telluroselenophosphonium Salts

Trialkylphosphine selenides tBu n iPr3− n PSe (n = 3, 2, 1, 0) and bidentate phosphine selenides (dppmSe2, dppeSe2) react with arenetellurenyl iodides (RTeI) furnishing complexes tBu n iPr3− n PSeTe(R)I (R = p-tolyl, p-anisyl, mesityl [Mes]), dppm[SeTe(Mes)I]2 and dppe[SeTe(Mes)I]2 that contain Se-Te(R)-I moieties (10-Te-3). Dppe[SeTe(Mes)I]2 exhibits supramolecular soft-soft interactions by “T-stacking” of the Se-Te(R)-I functions in the solid. When trialkylphosphine selenides tBu n iPr3− n PSe react with two equivalents of RTeI, novel trialkyl(aryltelluroseleno)phosphonium diiodotellurate(II) salts are formed. In solid [(tBu2iPrPSeTe-p-Tol)+(p-TolTeI2)−] the Te atom from the (p-TolTeI2)−anion behaves as soft donor towards the formally uncharged Te atom in the cation (tBu2iPrPSeTe-p-Tol)+exhibiting a novel type of Te…Te anion → cation donor-acceptor interaction (Te…Te 318 pm). Loss of mesityl groups in course of reactions with MesTeI led to a unique iodotelluroselenophosphonium salt {[(iPr3PSeTeI)+]2(TeI6)2−} with chelating cation-anion Te II …I-Te IV contacts, to the six-membered chelate complex dppmSe2TeI2 with a square planar cis-Se2TeI2 group, and to dppeSe2TeI2 with trans-Se2TeI2 groups in a helical chain structure.

Crystals of an Iron Nitride

CHEMISTRY Both industrial and enzymatic nitrogen reduction catalysts rely on iron centers. However, high valent molecular iron nitride complexes (FeN) have stubbornly eluded crystallographic characterization, in contrast to analogous terminal oxo structures (Fe=O). Vogel et al. now find that a tridentate ligand comprising three coordinating N-heterocyclic carbene moieties offers the solution. Reaction of the ligand (bearing either xylyl or mesityl groups for steric protection) with ferrous chloride followed by reduction with sodium amalgam and treatment with trimethylsilylazide yields a cationic Fe-N3 complex that loses dinitrogen under xenon lamp photolysis to afford the terminal iron nitride. Air-stable purple crystals of the compound were characterized by x-ray diffraction, revealing a short FeN bond length of 1.53 . Mssbauer spectroscopy further suggested an iron center more electron-rich than a previously prepared phosphorus-coordinated iron nitride characterized in solution; the authors attribute the difference to -donation from the carbene ligands. JSY Angew. Chem. Int. Ed. 47 , 10.1002/anie.200800600 (2008).

A new mechanism for the addition of alcohols to disilenes revealed by DFT and ONIOM calculations

Density functional and ONIOM calculations of alcohol and phenol additions to two (tetramethyl and tetramesityl) disilenes were carried out. The dimer of MeOH adds to Me 2Si SiMe 2 more readily than the monomer. The trimer does not afford the adduct, but a zwitter-ionic intermediate. In the (CF 3OH) 2 addition to Me 2Si SiMe 2, H⋯Si bond formation is more advanced than O⋯Si bond formation in the transition state (TS). Addition of seven phenol derivatives to Me 2Si SiMe 2 was examined, and the dimer reactions were found to be superior to the monomer reaction regardless of the substituents on the benzene rings. (MeOH) 2 reacts also with Mes 2Si SiMes 2 favorably, and an isomer of the reactant-like complex (precursor) may afford an adduct of different stereochemistry via internal rotations. Generally, the dimer of the alcohol or phenol is the reactant toward the disilenes. Exceptionally, a monomer of p-(dimethylamino)phenol reacts with Mes 2Si SiMes 2 owing to steric congestion by the four mesityl groups.

Transient Intermediates from Mn(salen) with Sterically Hindered Mesityl Groups: Interconversion between MnIV-Phenolate and MnIII-Phenoxyl Radicals as an Origin for Unique Reactivity

In order to reveal structure-reactivity relationships for the high catalytic activity of the epoxidation catalyst Mn(salen), transient intermediates are investigated. Steric hindrance incorporated to the salen ligand enables highly selective generation of three related intermediates, OMnIV(salen), HO-Mn IV(salen), and H2O-MnIII(salen (+*)), each of which is thoroughly characterized using various spectroscopic techniques including UV-vis, electron paramagnetic resonance, resonance Raman, electrospray ionization mass spectrometry, 2H NMR, and X-ray absorption spectroscopy. These intermediates are all one-electron oxidized from the starting MnIII(salen) precursor but differ only in the degree of protonation. However, structural and electronic features are strikingly different: The Mn-O bond length of HO-MnIV(salen) (1.83 A) is considerably longer than that of OMnIV(salen) (1.58 A); the electronic configuration of H2O-MnIII(salen (+*)) is MnIII-phenoxyl radical, while those of OMnIV(salen) and HO-MnIV(salen) are MnIV-phenolate. Among OMnIV(salen), HO-MnIV(salen), and H2O-MnIII(salen (+*)), only the OMnIV(salen) can transfer oxygen to phosphine and sulfide substrates, as well as abstract hydrogen from weak C-H bonds, although the oxidizing power is not enough to epoxidize olefins. The high activity of Mn(salen) is a direct consequence of the favored formation of the reactive OMnIV(salen) state.

Synthesis, structure and isomerization of arylphosphoranes with anti-apicophilic bonding modes using a novel bidentate ligand with two C2F5 groups

A series of anti-apicophilic pentacoordinate phosphoranes (with one chelating substituent in an O-equatorial, C-apical bonding mode at pentacoordinated phosphorus atom) bearing a para-substituted aryl group (-C(6)H(4)(p-X); X = H, CF(3), F, OMe) or a mesityl (2,4,6-trimethylphenyl) group were isolated using a novel bulky bidentate ligand with two C(2)F(5) groups. These phosphoranes were stable to isomerization at room temperature, and quantitatively converted into the corresponding more stable isomers (O-apical) at elevated temperatures in solution. On the basis of a kinetic study, the free energy of activation (DeltaG(double dagger)) of the stereomutation of the O-equatorial mesitylphosphorane to its O-apical isomer was higher than that of the CF(3) derivative by 2.6 kcal mol(-1), giving rise to a further example of the steric effect of the C(2)F(5) group to freeze the isomerization of the pentacoordinate phosphorus compounds. Kinetic measurements of the isomerization of the O-equatorial ortho-unsubstituted derivatives (-C(6)H(4)(p-X)) to the corresponding O-apical isomers suggested that the O-equatorial isomers were stabilized by the pi --> sigma*(P-O) interaction in the ground state.

Acylmethyl(aryl)tellurium(iv,ii) derivatives: intramolecular secondary bonding and steric rigidity

Electrophilic substitution of acylmethanes (methyl ketones), RCOCH3 (R = i-Pr, 1; Et, 2; Me, 3) with aryltellurium trichlorides, ArTeCl3 (Ar = 1-C10H7, Np, A; 2,4,6-Me3C6H2, Mes, B; 4-MeOC6H4, Anisyl, C) under mild conditions affords the corresponding acylmethyl(aryl)tellurium dichlorides (RCOCH2)ArTeCl2. Reduction of the dichlorides, gives tellurides, (i-PrCOCH2)ArTe, 1A-1C, which give the corresponding dihalides, (i-PrCOCH2)ArTeX2 (X = Cl, 1Aa-1Ca; Br, 1Ab-1Cb; I, 1Ac-1Cc) when reacted in situ with SO2Cl2, Br2 or I2. The unsymmetric tellurides are labile towards disproportionation and attempts to obtain them lead to the isolation of Ar2Te2 except in the case of (i-PrCOCH2)MesTe (1B), which represents an interesting example of a kinetically stable aryl(alkyl)telluride. All the dihalomesityltellurium(IV) derivatives show separate 1H and 13C NMR signals for the ortho methyls irrespective of the sizes of R and X ligands. The telluride, 1B with free rotation about Te-C(mesityl) bond shows, like the unsymmetric diorganotellurium(IV) dihalides, only one 125Te NMR signal. The 1,4-chelating behavior of the acyl ligand among diorganotellurium(IV) compounds is inferred from the X-ray diffraction data for 1Aa, 1Ac, 1Ba, 1Bb, 1cA and 1Cc which are indicative of the presence of intramolecular Te...O secondary bonding interactions (SBIs) at least in the solid state. As a consequence, steric repulsion in case of the mesityltellurium(IV) derivatives, 1Ba and 1Bb, reaches the threshold so as to cause loss of two-fold rotational symmetry of the mesityl group about the Te-C(mesityl) bond axis. Intermolecular C-HO...O H-bonding interactions appears to stabilize such an orientation of the aryl ligand at least in the solid state.

Oxidation of adamantane catalysed by imidazolylporphyrinatoiron(III) complexes and structural studies of 5-coordinating iron(III) porphyrin

Oxidation of adamantane with phenylperacetic acid was carried out in the presence of three imidazolyltriarylporphyrinatoiron(III) complexes having pentafluorophenyl, phenyl, and mesityl (2,4,6-trimethylphenyl) groups as meso-substituents and three corresponding tetraarylporphyrinatoiron(III) complexes. The yield of 1- and 2-adamantanols was 76% in the case of chloro-5-(1-methyl-2-imidazolyl)-10,15,20-tri(pentafluorophenyl)porphyrinatoiron(III) (ImTPFPP–Fe(III)Cl), whereas the yield was only 26% in the case of chloro-5,10,15,20-tetra(pentafluorophenyl)porphyrinatoiron(III) in the presence of 100 eq. N-methylimidazole. The apparent effect of the appended imidazolyl group is discussed in terms of a 5-coordinated dimer of ImTPFPP–Fe(III)Cl, which was observed in the 1H and 19F NMR, and UV–vis spectra.

Light-Induced Geometrical Changes in Acyclic Ruthenium(II) Complexes and Their Ruthena−Macrocyclic Analogues

The two ligands 1 (4'-(3-anisylphenyl)-2,2';6',2' '-terpyridine) and 2 (2-mesityl-8-anisyl-1,10-phenanthroline) (Scheme 2) were synthesized and coordinated to ruthenium. The corresponding complexes Ru(1)(2)(L)n+, where L = Cl-, CH3CN, or C5H5N, have been fully characterized. Notably, the hindering mesityl group of the phenanthroline ligand was shown to lie opposite to the monodentate ligand L both in solution and in the solid state. Upon irradiation in acetonitrile or pyridine, quantitative isomerization of the complex occurred, which consisted of a 90 degrees rotation of the bidentate chelate. In the new isomers the mesityl group was shown to pi stack to the coordinated monodentate ligand with the anisyl group of the phen (1,10-phenanthroline) lying on the other side of the ruthenium atom. The back reaction was performed by heating the photochemical isomers of the complexes in DMSO and exchanging the DMSO with chloride anion, acetonitrile, or pyridine. The stability of the ruthenium(II)-pyridine bond was used in order to inscribe the Ru(terpy)(phen) motif in a molecular ring. Functionalization of the ligands and subsequent cyclization reaction on the complex were performed on the two isomers of Ru(1)(2)(C5H5N)2+. Four macrocyclic complexes including the Ru(terpy)(phen)(py)n+ moiety were obtained and characterized. A (CH2)18 alkane chain or polyethylene glycol chain formed the flexible part of the ruthena-macrocycles. Upon visible light irradiation a dramatic geometrical changeover of the cyclic complex took place, which could be reversed thermally.

Structurally Homologous β- andmeso-Alkynyl Amidinium Porphyrins

Alkynylamidinium groups have been introduced at the beta and meso positions of a nickel(II) porphyrin (PNi(II)) framework. The modification permits the distance between the amidinium-amidine acid-base group and porphyrin to be increased while effectively maintaining pi conjugation between the porphyrin macrocycle and the acid-base functionality. Use of an ethynyl spacer as a linker (i) extends the amidinium functionality away from the sterically bulky mesityl groups of the porphyrin, allowing it to be nearly planar with respect to the porphyrin ring, and (ii) draws the pi-orbital character of the porphyrin out toward the amidinium functionality, thereby engendering sensitivity of the electronic properties of the porphyrin macrocycle to the protonation state of the amidinium. The barrier for rotation of the amidinium group, as calculated by time-dependent density functional theory (TDDFT), is approximately 8.5 kT (5 kcal/mol) for both porphyrins. Analysis of UV-visible absorption profiles for the beta- and meso-alkynylamidinium PNi(II) upon deprotonation enables accurate determination of the amidinium acidity constants for the ground state (pK(a)(beta) = 7.03 +/- 0.1, pK(a)(meso) = 7.74 +/- 0.1 in CH(3)CN) and excited state (pK(a)*(beta) = 6.89 +/- 0.1, pK(a)*(meso) = 8.37 +/- 0.1 in CH(3)CN) porphyrins. Whereas pK(a)* < pK(a) for the beta-alkynylamidinium porphyrin, pK(a)* > pK(a) for the meso-alkynylamidinium porphyrin, indicating that beta-alkynylamidinium PNi(II) is a photoacid and meso-alkynylamidinium PNi(II) is a photobase. These divergent behaviors are supported by analysis of the frontier molecular orbitals of the homologous pair with TDDFT.

Quantum chemical investigation on one- and two-photon absorption properties for a series of donor-π-acceptor-type compounds with trivalent boron as an acceptor

In this paper, one- and two-photon absorption properties as well as the transition nature of a series of donor-π-acceptor-type compounds with trivalent boron as an acceptor have been theoretically studied by using INDO/SDCI method. Our calculations indicate that the four o-methyl moieties on the two mesityl groups play an important part in protecting the trivalent boron from being attacked by oxygen in the air. The trivalent boron containing group can be an all-right electron-acceptor with some bulky groups attached to it. On the basis of geometry optimization and UV–vis spectra, the positions and strengths of two-photon absorption for these molecules were reported.

Meso-13C-Labeled Porphyrins for Studies of Ground-State Hole Transfer in Multiporphyrin Arrays

Understanding electronic communication among interacting chromophores provides the foundation for a variety of applications. The ground-state electronic communication in diphenylethyne-linked zinc-porphyrin dyads has been investigated by a novel molecular design strategy that entails introduction of a 13C-atom (*) at specific sites of the porphyrins where there is substantial electron density in the relevant frontier (highest occupied) molecular orbital. The site of 13C substitution is at a meso-position, either the site of attachment of the linker (proximal, "P") or the site trans to the linker (distal, "D"). The substituents (R) at the non-linking meso-positions are mesityl, tridec-7-yl ("swallowtail"), or p-tolyl groups. Altogether five isotopically labeled porphyrin dyads have been prepared. The hole/electron-transfer properties of one-electron oxidized dyads have been examined by electron paramagnetic resonance (EPR) spectroscopy. The introduction of the meso-13C label provides a "clock" (via the hyperfine interactions) that allows investigation of a time scale for hole transfer that is 3-4 times shorter than that provided by the natural abundance 14N nuclei of the pyrrole nitrogen atoms. The EPR studies indicate that the hole transfer, which has been previously shown to be fast on the time scale of the 14N hyperfine clock ( approximately 220 ns), remains fast on the time scale of the 13C hyperfine clock ( approximately 50 ns).

Revisiting the Stability of Hexacenes

The Strating-Zwanenberg photodecarbonylation was used to prepare hexacene (1). Compound 1 was found to be extremely unstable in solution, undergoing dimerization and oxidation. However, when generated in a polymer matrix, 1 survived for more than 12 h under ambient conditions. Hexacenes substituted at the 6 and 15 positions with the phenyl, p-tert-butylphenyl, and mesityl groups were synthesized using the quinone reduction method, but these compounds were also shown to be unstable in solution.