Steric Influence Research Articles

Electronic and Steric Effects in Rollover C–H Bond Activation

Steric and electronic factors in rollover C–H bond activation of substituted 2,2′-bipyridines, mediated by platinum(II), have been investigated by comparing the influence of two substituents, CH3 and CF3, on the progress of the reaction. The substituents were chosen to have similar steric hindrance but different electronic effects and were placed in position 6 (i.e., near one of the nitrogen atoms) or in position 5, which allows, in part, electronic and steric influence to be distinguished. The ligands studied, 6-methyl-2,2′-bipyridine, 5-methyl-2,2′-bipyridine, 6-trifluoromethyl-2,2′-bipyridine, and 5-trifluoromethyl-2,2′-bipyridine, were compared to unsubstituted 2,2′-bipyridine in the reaction with the electron-rich complex [Pt(Me)2(DMSO)2]. The electron-withdrawing CF3 group was found to have a significant effect in accelerating the cyclometalation reaction. The substituent in position 6 influences the stability of the intermediate adduct [Pt(N,N)(Me)2] (N,N = chelated bipyridine), as indicated by density functional theory calculations. The steric hindrance of substituted bipyridines was also evaluated by defining and measuring the angle ζ in [Pt(N,N)(Me)2] adducts. The presence of a substituent in position 6 causes destabilization of the adduct, acceleration of the cyclometalation reaction, and regioselectivity of C–H bond activation

Asymmetric Base‐Free Michael Addition at Room Temperature with Nickel‐Based Bifunctional Amido‐Functionalized N‐Heterocyclic Carbene Catalysts

AbstractA series of nickel‐based chiral bifunctional catalysts (1d–3d) with N‐heterocyclic carbene (NHC) ligands derived from (1R)‐(–)‐menthol, (1S)‐(–)pinene, and (1R)‐(+)‐camphor have been successfully designed for asymmetric Michael addition reactions under base‐free conditions. The NHC complexes, namely, [1‐R‐3‐{N‐(phenylacetamido)}imidazol‐2‐ylidene]2Ni [R = (1S)‐menthyl (1d), (1S)‐pinane (2d), and (1R)‐isobornyl (3d)], bearing chiral ancillaries on the amido‐functionalized side arms of the NHC ligands, performed the bifunctional catalysis of the asymmetric base‐free Michael addition reaction of the α‐methyl cyano ester substrates ethyl 2‐cyanopropanoate (4), isopropyl 2‐cyanopropanoate (5), and tert‐butyl 2‐cyanopropanoate (6) with the activated olefinic substrates methyl vinyl ketone (7) and acrylonitrile (8) in 63–98 % yields with enantiomeric excess (ee) values of 2–75 % at room temperature in 8 h. More interestingly, only the longest of the three catalysts, the menthol derivative 1d, showed significant chiral induction of up to 75 % ee; this has been attributed to the reduction of the steric influence owing to the relatively distant dispositions of the chiral ancillaries from the catalytically active metal center that arise as a consequence of the cis geometries of 1d–3d.

Tribromobenzene on Cu(111): Temperature-dependent formation of halogen-bonded, organometallic, and covalent nanostructures.

The temperature-controlled surface-assisted synthesis of halogen bonded, organometallic, and covalent nanostructures based on 1,3,5-tribromo-benzene (TriBB) was studied with scanning tunneling microscopy and X-ray photoemission spectroscopy in ultrahigh vacuum. Vapor deposition of TriBB onto a Cu(111) surface held at 90 K leads to the formation of large domains of a honeycomb-like organic monolayer structure stabilized by triangular nodes with Br⋯Br intermolecular bonds. Upon annealing the organic monolayer to ∼140 K, a new hexagonal close-packed structure with intact TriBB molecules connected by Cu adatoms is formed. Further warming up the sample to 300 K gives rise to the scission of C-Br bonds and formation of C-Cu-C bonds between phenyl fragments such that stable dendritic organometallic networks are formed. Larger islands of organometallic networks are obtained by maintaining the temperature of Cu(111) at 420 K during deposition of TriBB. Simultaneously, large islands of Br atoms are formed around the organometallic networks. Annealing the more extended organometallic network (prepared at 420 K) to 520 K leads to the formation of a branched covalent organic framework (COF) which comprises structural elements of porous graphene and is surrounded by Br islands. These organometallic networks and COFs appear as small dendritic and branched domains, most likely due to the steric influence exerted by the Br islands.

Synthesis of 3-(1H-1,2,3-Triazol-1-yl)-2-(arylselanyl)pyridines by Copper-Catalyzed 1,3-Dipolar Cycloaddition of 2-(Arylselanyl)-3-azido-pyridines with Terminal Alkynes

We present here our results in the synthesis of eleven new 3-(1H-1,2,3-triazol-1-yl)-2-(arylselanyl)pyridines by copper-catalyzed azide-alkyne cycloaddition reactions. The reactions were performed in the presence of catalytic amount of copper(II) acetate and sodium ascorbate using a mixture of tetrahydrofuran/water (1:1) as solvent at room temperature in air. The reaction is tolerant to different functional groups such as substituted-benzene rings, alcohol and ester and none electronic or steric influence was observed. All the products were obtained in good to excellent yields. Alternatively to the conventional oil bath heating, the use of microwave irradiation or ultrasound methods is also presented as alternative energy sources.

α-Alkylation of a norbornene-derived tricyclic ketone: are steric factors really in control?

Density functional theory (DFT) has been used to investigate the α-alkylation of a chiral tricyclic ketone. These calculations reveal that torsional strain and a strong conformational preference, rather than steric influences, are responsible for the high levels of observed stereoselectivity and suggest a way by which alternative stereoisomers could be accessed.

ChemInform Abstract: Steric and Electronic Factors Influence Regio‐Isomeric Thiazole Formations Between Substituted Benzothioamides and Ethyl Bromopyruvate.

ChemInform Abstract: Steric and Electronic Factors Influence Regio‐Isomeric Thiazole Formations Between Substituted Benzothioamides and Ethyl Bromopyruvate.

Steric and Electronic Effects in the Synthesis and Regioselective Hydrolysis of Unsymmetrical Imides

The AgI-promoted coupling reaction of thioamides and carboxylic acids is shown to be a useful method for the generation of unsymmetrical imides. The reaction proceeds efficiently with unhindered and electron-rich or neutral coupling partners, but not with hindered thioamides (such as thiopivalamides) or electron deficient thioamides (such as trifluorothioacetamides). Intriguingly, thioformamides are also ineffective coupling partners, despite having minimal steric or electronic influence. Hindered carboxylic acid coupling partners (such as pivalic acid) are tolerated, but electron deficient acids, such as trifluoroacetic acid, are ineffective coupling partners. Furthermore, an interplay of both steric and electronic effects is observed in the subsequent hydrolysis of unsymmetrical imides. Imides with a dimethoxybenzoyl group give high regioselectivity upon hydrolysis, favouring cleavage of the distal acyl group. Imides with a p-nitrobenzoyl or pivaloyl group give reversed selectivity, favouring cleavage of the proximal acyl group.

Tuning steric and electronic effects in transition-metal β-diketiminate complexes.

β-Diketiminates are widely used supporting ligands for building a range of metal complexes with different oxidation states, structures, and reactivities. This Perspective summarizes the steric and electronic influences of ligand substituents on these complexes, with an eye toward informing the design of new complexes with optimized properties. The backbone and N-aryl substituents can give significant steric effects on structure, reactivity and selectivity of reactions. The electron density on the metal can be tuned by installation of electron withdrawing or donating groups on the β-diketiminate ligand as well. Examples are shown from throughout the transition metal series to demonstrate different types of effects attributable to systematic variation of β-diketiminate ligands.

The influence of organic structure and rare earth metal cation on the corrosion efficiency observed on AS1020 steel compared with La(4OHCin)3

Whilst the corrosion protection of steel in aqueous chloride environments by the rare earth inhibitor lanthanum 4-hydroxycinnamate is well known, the influence of the structural variation of the organic component as well as the nature of the metal centre has not previously been addressed. Herein we show that praseodymium 4-hydroxy cinnamate is comparable to its lanthanum counterpart in aqueous solution. On the other hand, cerium 4-hydroxycinnamate and lanthanum 2-hydroxycinnamate show poor corrosion protection performance while lanthanum 3-hydroxycinnamate provides a level of inhibition between these. These differences are shown to be related to the speciation in solution and are postulated to be linked to steric influences which are likely to affect the bonding environment within the rare earth compound itself, as well as its bonding with the steel substrate.

Titanocene(III) complexes with 2-phosphinoaryloxide ligands for the catalytic dehydrogenation of dimethylamine borane.

A study of the dehydrogenation of dimethylamine borane using different titanocene(III) complexes with 2-phosphinoaryloxide ligands is presented. Complexes Cp2Ti(κ(2)-O, P-O-C6H4-PR2) (3a: R = i-Pr, 3b: R = Ph) (Cp = η(5)-cyclopentadienyl) and Cp*2Ti(κ(1)-O-O-C6H4-PR2) (5a: R = i-Pr, 5b: R = Ph) (Cp* = η(5)-pentamethylcyclopentadienyl) were prepared by reactions of the 2-phosphinophenol ligand with different titanocene sources and fully characterised. Their catalytic activity depends on the steric influence of the cyclopentadienyl ligand, the coordination mode of the 2-phosphinoaryloxide ligand and on the used solvent. Complex 3a showed a turnover number of 43.2 in the neat substrate after 24 hours. EPR investigations were used to elucidate the fate of the Ti(III) catalyst.

Structure determination of bound nitrogen-based adducts with copper(II) acetylacetonato; an EPR, ENDOR and DFT study.

The adducts of bis(acetylacetonato)–copper(II), [Cu(acac)2], formed with a range of nitrogen heterocycles including pyridine (2), methylpyridines (3,4,5), amino-methylpyridines (6,7) and diazines (8,9,10) were investigated in frozen solution using X-band EPR and 1H ENDOR spectroscopy. The small perturbations to the EPR spin Hamiltonian parameters (g and CuA) were consistent with the axial coordination of the nitrogen bases to Cu(II), and found to be dependent on both the basicity and steric influence of the coordinating substrate. The detailed structure of two adducts was then investigated by angular selective (1)H ENDOR and DFT. For the [Cu(acac)2](pyridine) adduct, axial coordination of the substrate was found to occur via the pyridine nitrogen as expected, producing a characteristic (1)H hyperfine coupling ((H)Ai = −2.6, −2.04, 4.7 MHz; β = 36°; aiso = 0.2 MHz) arising from the ortho-(1)H in the ring 2 or 6 position. These results were confirmed by DFT. However, in the [Cu(acac)2](2-amino-6-methyl-pyridine) adduct, the ENDOR data revealed a substantially different (1)H hyperfine coupling ((H)Ai = −4.52, −3.35, 6.47 MHz; β = 14°; aiso = −0.47 MHz) arising from the –NH2 amino protons. Analysis of this experimentally derived tensor in conjunction with the calculated DFT tensors, revealed that the 2-amino-6-methyl-pyridine substrate binds to Cu(II) via the exocyclic amino pyridine nitrogen, but with a tilt angle of 20° of the pyridine ring away from the geometry optimised structure. These results reveal how important structural information on the coordination geometry of Cu(II) adducts can be obtained by (1)H ENDOR, but only when the complete angular dependency profile of the ENDOR data is thoroughly considered.

Investigation of Steric Factors Involved in the Formation of Terminal Cationic Platinum Arylborylene Complexes

The abstraction of halido ligands from PtII diphosphine boryl complexes has previously been shown to yield one of two isomeric products: either T-shaped cationic boryl complexes or square-planar cationic borylene complexes. However, the latter product has only been observed in one case, that of a mesitylboryl ligand, which converts to a mesitylborylene ligand upon halido abstraction. In an effort to test the efficacy of this reaction in the presence of different steric and electronic influences, PtII diphosphine boryl complexes were prepared with both 4-tert-butylphenyl and duryl (2,3,5,6-tetramethylphenyl) groups. Halide abstraction from the 4-tert-butylphenyl complex resulted in a T-shaped cationic boryl complex. However, subjecting the duryl-substituted complexes to the same conditions exclusively results in terminal cationic borylene complexes, a difference we attribute to the greater steric hindrance between the boron-bound bromide and the methyl groups at the 2- and 6-positions of the duryl group. T...

Influence of Ligand Substitution Pattern on Structure in Cobalt(II) Complexes of Bulky N , N′ ‐Diarylformamidinate N ‐Oxides

Cobalt(II) complexes of bulky N,N′-diarylformamidinate N-oxide ligands were synthesized and structurally characterized. The cobalt(II) bis(chelates) are square-planar (low spin) in the solid state, according to XRD and magnetic measurements (μeff = 1.8 to 2.1 μB), but show square-planar (low spin) to tetrahedral (high spin: μeff = 3.5 to 4.7 μB) isomerization in solutions of noncoordinating solvents, as demonstrated by different spectroscopic techniques. The isomerization equilibrium is highly sensitive to the substitution pattern on the ligand due to a combination of steric and electronic influences.

Uranyl–Organic Hybrids Designed from Hydroxyphosphonate

AbstractA series of phosphonate‐based uranyl–organic hybrids has been assembled based on the reaction of uranyl cations with 2,5‐dihydroxy‐1,4‐benzenediphosphonic acid in the presence of a variety of alkali metal and organoammonium cations: K[(UO2)2F2{H0.5O3PC6H2(OH)2PO3H0.5}](H2O)2 (1), Cs[(UO2)2{H0.75O3PC6H2(OH)2PO3H0.75}](H2O)4·2H2O (2), [(CH3)4N]2[UO2{HO3PC6H2(OH)2PO3H}2] (3), [(CH3CH2)4N][UO2F{HO3PC6H2(OH)2PO3H}] (4), and [(CH3CH2)2N(CH3)2][UO2F{HO3PC6H2(OH)2PO3H}] (5). In compounds 1, 4, and 5, chains of UO5F2 pentagonal bipyramids are linked by the hydroxyphosphonate moiety into three‐dimensional frameworks, and compound 4 is isostructural with 5. However, the structures of 2 and 3 are composed of monomeric UO7 pentagonal bipyramids assembled to form layered structural units. Additional steric influences from the –OH groups appended on the diphosphonate species play a vital role in directing the structure topologies.

Aqua substitution by biologically relevant nucleophiles in dinuclear platinum(II) complexes linked by diamino linkers with cyclohexyl groups

Substitution of the aqua ligand from three platinum(II) complexes, namely [Pt(H2O)(N,N-bis(2-pyridylmethyl)cyclohexylamine](ClO4)2 (Pt1); [{Pt(H2O)}2(N,N,N′,N′-tetrakis(2-pyridylmethyl)-trans-1,4-cyclohexyldiamine)](ClO4)4 (Pt2); and [{Pt(H2O)}2(N,N,N′,N′-tetrakis(2-pyridylmethyl)-4,4′-ethylenedicyclohexyldiamine)](ClO4)4 (Pt3), by three biologically relevant nucleophiles [viz., glutathione (Glu); DL-penicillamine (Pen); and l-histidine (His)] was studied in aqueous 0.1 M perchloric acid medium. The substitutions were investigated under pseudo-first-order conditions as a function of nucleophile concentration and temperature using UV–visible spectrophotometry. The reactions of these complexes with the nucleophiles proceeded via a single step whose reactivity decreased in the order: Pt1 > Pt3 > Pt2 and was controlled by steric and electronic influences of the complexes. Sulfur donor nucleophiles (Glu and Pen) reacted much faster than the nitrogen bearing His, with Glu appearing to be more nucleophilic than Pen. The large and negative activation entropies and low but positive enthalpies of activation affirm an associative mode of activation.

Molybdenum(0)‐N2 Complexes Supported by the Tripod Ligand 1,1,1‐Tris(Diphenyl‐Phosphinomethyl)Ethane: Steric Influence on the Coordination of Mono‐ and Diphosphine Coligands

AbstractThe syntheses and physicochemical properties of the three dinitrogen complexes [Mo(N2)2(tdppme)(PMe3)], [Mo(N2)(tdppme)(PMe3)2] and [Mo(N2)2(tdppme)(PMe2Ph)] supported by the tripodal ligand 1,1,1‐tris(diphenylphosphinomethyl)ethane (tdppme) are reported. The complexes are characterized by 31P NMR‐, IR‐ and Raman spectroscopy. The influence of the ligand tdppme on the coordination of PMe3, PMe2Ph, PMePh2, PPh3 and dcpm (bis(dicyclohexylphosphine) is investigated and compared to earlier results involving dppm and dmpm as co‐ligands.

Two organophosphorus pesticides: methyl parathion and dicapthon.

Structural studies performed in this laboratory of organophosphorus pesticides continue with these related compounds. The -NO2 groups of methyl parathion (systematic name: dimethyl 4-nitrophenyl phosphorothioate, C8H10NO5PS) and dicapthon (systematic name: 2-chloro-4-nitrophenyl dimethyl phosphorothioate, C8H9ClNO5PS) make dihedral angles of 10.67 (8) and 5.8 (1)°, respectively, with the planes of their attached rings, which accompanies angular distortion at the ring C atoms to which the -NO2 groups are attached. Similar distortions are observed at the C atom to which the thiophosphate groups are attached. Significant differences in distances and angles around the phenolic O, versus the -OMe groups, explain why it is the site of hydrolysis for these compounds. A comparison of a torsion angle involving the thiophosphate group and phenolic O atom with similar pesticide structures is given and indicates steric influences on that angle.

Spectral and structural studies on Ni(II) dithiocarbamates: Nickel sulfide nanoparticles from a dithiocarbamate precursor

Three new planar Ni(II) dithiocarbamate complexes; [Ni(4-dpmpzdtc)2] (1), [Ni(4-dpmpzdtc)(PPh3)(NCS)] (2) and [Ni(bupcbzdtc)(PPh3)(NCS)] (3) (where, 4-dpmpzdtc=4-(diphenylmethyl)piperazinecarbodithioato anion, bupcbzdtc=N-butyl-N-(4-chlorobenzyl))dithiocarbamato anion and PPh3=triphenylphosphine) with “NiS4” and “NiS2PN” chromophore units were synthesized and characterized by single crystal X-ray structural analysis as well as UV–Vis, IR and NMR (1H, 13C and 31P) spectroscopy. For 2, 1H–1H COSY spectrum was also recorded. Single crystal X-ray structural analysis of 1–3, reveals a slightly distorted square planar geometry in all three complexes wherein the steric and electronic influence of auxiliary ligands PPh3 and NCS− induces change in the bonding parameters of the latter two complexes (especially in 3). Nickel sulfide nanoparticles were produced from complex 1 by solvothermal synthesis. The morphology and a qualitative phase analysis of the nanoparticles are also viewed in the study.

Correlating net charges and the activity of bis(imino)pyridylcobalt complexes in ethylene polymerization

A series of unsymmetrical bis(imino)pyridylcobalt complexes was investigated by DFT–QEq method to correlate the net charges of cobalt center and their catalytic activities. The effective net charge values on cobalt atom, calculated by QEq method at doublet spin state, reflected the influences of substituents within their ligands. The catalytic activities of these cobalt complexes in ethylene polymerization continuously increase along with higher values of the effective net charges on cobalt atom. The energy differences at doublet and quartet states kept the constant tendency for the cobalt complexes without the steric influences of substituents within ligands.

Understanding the role of flexible 4′-functionalized polyethylene glycoxy chains on the behavior of platinum(II) (4′-(ethylene glycoxy)-2,2′:6′,2′′-terpyridine: a kinetic and a mechanistic study

The ligand substitution kinetics of 4′-functionalized mononuclear Pt(II) (4′-(ethylene glycoxy)-2,2′:6′,2′′-terpyridine complexes, [Pt(nY-tpy)Cl)Cl] (where Y = ethylene glycoxy, n = number of ethylene, glycoxy units = 1, 2, 3, and 4, and tpy = 2,2′:6′,2′′-terpyridine), with thiourea, 1,3-dimethyl-2-thiourea, 1,1,3,3-tetramethyl-2-thiourea, and iodide were investigated under pseudo-first-order conditions as a function of concentration and temperature by conventional stopped-flow technique. The observed first-order rate constants followed the simple rate law kobs = k2[Nu]. The data obtained show that the ethylene glycoxy pendant, trans to the leaving group, acts as a σ-donor into the terpyridine ligand and is effective only up to n = 1, beyond which the substitution reactivity of the complexes are controlled by the steric influence of the appended ethylene glycoxy pendant units, which decreases with increase in the number of ethylene glycoxy units. The activation parameters obtained support an associative mechanism, where bond formation in the transition state is favored. The observed reactivity trends were supported by density functional theory calculations.