Ligand Steric Effects Research Articles

Ligand Steric and Electronic Effects on β-Ketiminato Neutral Nickel(II) Olefin Polymerization Catalysts

A series of novel neutral nickel complexes 3a-g and 4a-d bearing the beta-ketiminato ligands [(2,6-(Pr2C6H3)-Pr-i)N=CHCHC(R)O]Ni(R')(L) (for 3a-g, R' = Me, L = Py, and R = Bu-t (3a), Ph (3b), 1-naphthyl (3c), 9-anthryl (3d), PhNMe2(p) (3e), PhOMe(p) (3f), PhCF3(p) (3g); for 4a-d, R' = Ph, L = PPh3, and Bu-t (4a), Ph (4b), 1-naphthyl (4c), 9-anthryl (4d)) have been synthesized and characterized. The molecular structures of 3b-d,f,g and 4a,c were further confirmed by X-ray crystallographic analysis. These complexes were employed in ethylene polymerization to systematically investigate ligand steric and electronic effects on the catalytic properties, including activity, molecular weight (MW), and branching number of the polyethylene obtained. The complexes bearing more bulky ligands showed higher activities and produced more branched polyethylene. Electron-deficient ligands were found to increase the catalytic activity, decrease the MW, and enhance the branching content of the polyethylene. In addition, phosphine Ni-II-Ph complexes 4a-d proved to be more active than the corresponding pyridine Ni-II-Me complexes 3a-d, probably due to the easier dissociation of PPh3 relative to a pyridine from a nickel center.

Complexation studies of Cm(III), Am(III), and Eu(III) with linear and cyclic carboxylates and polyaminocarboxylates

Solvent extraction and potentiometric titration methods have been used to measure the stability constants of Cm(III), Am(III), and Eu(III) with both linear and cyclic carboxylates and polyaminocarboxylates in an ionic strength of 0.1 mol L−1 (NaClO4). Luminescence lifetime measurements of Cm(III) and Eu(III) were used to study the change in hydration upon complexation over a range of concentrations and pH values. Aromatic carboxylates, phthalate (1,2 benzene dicarboxylates, PHA), trimesate (1,3,5 benzene tricarboxylates, TSA), pyromellitate (1,2,4,5 tetracarboxylates, PMA), hemimellitate (1,2,3 benzene tricarboxylates, HMA), and trimellitate (1,2,4 benzene tricarboxylates, TMA) form only 1 : 1 complexes, while both 1 : 1 and 1 : 2 complexes were observed with PHA. Their complexation strength follows the order: PHA∼TSA>TMA>PMA>HMA. Carboxylate ligands with adjacent carboxylate groups are bidentate and replace two water molecules upon complexation, while TSA displaces 1.5 water molecules of hydration upon complexation. Only 1 : 1 complexes were observed with the macrocyclic dicarboxylates 1,7-diaza-4,10,13-trioxacyclopentadecane-N,N′-diacetate (K21DA) and 1,10-diaza-4,7,13,16-tetraoxacyclooctadecane-N,N′-diacetate (K22DA); both 1 : 1 and 1 : 2 complexes were observed with methyleneiminodiacetate (MIDA), hydroxyethyleneiminodiacetate (HIDA), benzene-1,2-bis oxyacetate (BDODA), and ethylenediaminediacetate (EDDA), while three complexes (1 : 1, 1 : 2, and 1 : 3) were observed with pyridine 2,6 dicarboxylates (DPA) and chelidamate (CA). The complexes of M-MIDA are tridentate, while that of M-HIDA is tetradentate in both 1 : 1 and 1 : 2 complexes. The M-BDODA and M-EDDA complexes are tetradentate in the 1 : 1 and bidentate in the 1 : 2 complexes. The complexes of M-K22DA are octadentate with one water molecule of hydration, while that of K21DA is heptadentate with two water molecules of hydration. Simple polyaminocarboxylate 1,2 diaminopropanetetraacetate (PDTA) and ethylenediamine N,N′-diacetic-N,N′-dipropionate (ENDADP) like ethylenediaminetetraacetate (EDTA) form only 1 : 1 complexes and their complexes are hexadentate. Polyaminocarboxylates with additional functional groups in the ligand backbone, e.g., ethylenebis(oxyethylenenitrilo) tetraacetate (EGTA), and 1,6 diaminohexanetetraacetate (HDTA) or with additional number of groups in the carboxylate arms diethylenetriamine pentaacetato-monoamide (DTPA-MA), diethylenetriamine pentaacetato-bis-methoxyethylamide (DTPA-BMEA), and diethylenetriamine pentaacetato-bis glucosaamide (DTPA-BGAM) are octadentate with one water molecule of hydration, except N-methyl MS-325 which is heptadentate with two water molecules of hydration and HDTA which is probably dimeric with three water molecules of hydration. Macrocyclic tetraaminocarboxylate, 1,4,7,10-tetraazacyclododecanetetraacetate (DOTA) forms only 1 : 1 complex which is octadentate with one water molecule of hydration. The functionalization of these carboxylates and polycarboxylates affect the complexation ability toward metal cations. The results, in conjunction with previous results on the Eu(III) complexes, provide insight into the relation between ligand steric requirement and the hydration state of the Cm(III) and Eu(III) complexes in solution. The data are discussed in terms of ionic radii of the metal cations, cavity size, basicity, and ligand steric effects upon complexation.

Steric and Electronic Effects of Tetradentate Nickel(II) and Palladium(II) Complexes toward the Vinyl Polymerization of Norbornene

A series of Ni(II) and Pd(II) complexes bearing N4-type tetradentate ligands, [Ni(-6-) 1] and [Pd(-6-) 2]; 6- = N,N'-(o-phenylene)bis(6-methylpyridine-2-carboxamidate), = Cl, H, or , = , Cl, F, H, , or ) were designed, synthesized, and characterized to investigate electronic and steric effects of ligand on the norbornene polymerization catalysts. Using modified methylaluminoxanes as an activator, the complexes exhibited high catalytic activities for the polymerization of norbornene and the nickel complexes exhibited better catalytic activity the palladium complexes. Ni complex 1a with group on the benzene ring showed the highest catalytic activity of g of PNBEs/ and molecular weight of g/mol with PDI 100 ), and tend to afford higher polymerization productivities than the ones having electron-donating groups. Amorphous polynorbornenes were obtained with good solubility in halogenated aromatic solvents. A vinyl addition mechanism has been proposed for the catalytic polymerization.

Investigation of ligand steric effects on a highly cis-selective Rh(i) cyclopropanation catalyst

Four new Rh(I) complexes bearing chelating imine-functionalized N-heterocyclic carbene ligands have been synthesized and characterized. The catalytic activity of these new Rh(I) complexes has been tested in the cyclopropanation reaction between ethyl diazoacetate and styrene. One of the new complexes, having ethyl groups on the ligand N-aryl ring, exhibited a reactivity and a cis-diastereoselectivity that were comparable to our previously reported Rh(I) cyclopropanation catalyst of this type, and a higher yield and cis-diastereoselectivity were obtained at lower catalyst loadings and higher temperatures. The other new Rh(I) complexes were found to be inferior to the previously reported Rh(I) cyclopropanation catalyst. The catalytic study gave important information about the effect that changing the steric requirements of the substituents at the ligand system has on the efficiency and cis-diastereoselectivity of the complexes as cyclopropanation catalysts.

Recent Research Progress of Branched and Functional Branched Polyethylene Prepared Using Ni- and Pd–Based Catalysts

In this paper, the recent development of branched and functional branched polyethylene prepared using Ni- and Pd-based catalysts are reviewed. The influence of ligand species, steric effect, electronic effect and polymerization condition on branching density of resultant polymer prepared using Ni- and Pd-based catalysts is discussed. In addition, the review is also focused on functional branched polyethylene produced by copolymerization of ethylene and polar monomer and copolymerization mechanism.

Synthesis and Structure of Boron Compounds Bearing Tridentate Ligands with 1,3-Bicarbonylbenzene Skeleton

Abstract In order to examine differences in structure due to the electronic nature of the oxygen ligand, several organoboron compounds bearing ester 8a–8c and amide 13a and 13b ligands were prepared and crystallographically analyzed. In the ester ligand systems, the pinacolato derivative 8a and catecholato derivative 8b took a pentacoordinate structure, while fluorenyl derivative 8c took a tetracoordinate structure instead of pentacoordinate due to the strong electrophilic nature of the boron atom. In contrast to the ester ligand systems, no pentacoordinate species were found in the amide ligand systems. The pinacolato derivative 13a is tricoordinated, and catecholato derivative 13b is tetracoordinated. These results could be due to the steric effect of the diisopropylamide ligand together with the electronic effects. The VT NMR study and the DFT calculations reveal that the energy difference between the tetracoordinate and pentacoordinate structure in 8c is very small, and indicated that the electronic nature and steric effects of the ligand greatly affect the coordination state of the boron atom.

To Bend or Not to Bend: Electronic Structural Analysis of Linear versus Bent M−H−M Interactions in Dinickel Bis(dialkylphosphino)methane Complexes

The M-H-M bonding in the dinuclear complexes Ni(2)(mu-H)(mu-P(2))(2)X(2) (P(2) = R(2)PCH(2)PR(2), R = iPr, Cy; X = Cl, Br) has been investigated. These dinickel A-frames were studied via density functional theory (DFT) calculations to analyze the factors that influence linear and bent M-H-M bonding. The DFT calculations indicate that the bent geometry is favored electronically, with ligand steric effects driving the formation of the linear M-H-M structures.

Percent buried volume for phosphine and N-heterocyclic carbene ligands: steric properties in organometallic chemistry

Electronic and steric ligand effects both play major roles in organometallic chemistry and consequently in metal-mediated catalysis. Quantifying such parameters is of interest to better understand not only the parameters governing catalyst performance but also reaction mechanisms. Nowadays, ligand molecular architectures are becoming significantly more elaborate and existing models describing ligand sterics prove lacking. This review presents the development of a more general method to determine the steric parameter of organometallic ligands. Two case studies are presented: the tertiary phosphines and the N-heterocyclic carbenes.

Cyclopentadienyl Chromium β-Diketiminate Complexes: Initiators, Ligand Steric Effects, and Deactivation Processes in the Controlled Radical Polymerization of Vinyl Acetate

The new compounds CpCr(nacnacAr,Ar′) with nacnacAr,Ar′ = Ar−N---C(Me)---CHC(Me)---N−Ar′ (Ar = Ar′ = C6H2Me3-2,4,6 or mes, 2; C6H3Et2-2,6 or dep, 3; Ar = C6H3Me2-2,6 or xyl and Ar′ = C6H3iPr2-2,6 or dipp, 4) have been synthesized and used in polymerization experiments in addition to the previously known analogues with Ar = Ar′ = xyl, 1, or dipp, 5. The compounds were used as moderators for the polymerization of vinyl acetate (VAc) initiated by V-70, according to an OMRP mechanism. The alkylchromium(III) thermal initiator CpCr(nacnacxyl,xyl)(CH2CMe3) (8) was synthesized from CpCr(nacnacxyl,xyl)(OTs) (7) and Mg(CH2CMe3)2(dioxane), while 7 was obtained from CpCr(nacnacxyl,xyl)Cl (6) and AgOTs. The polymerizations carried out with (1−5)/V-70/VAc at elevated temperatures yielded rapid deactivation, suggestive of irreversible radical trapping. On the other hand, room-temperature polymerizations carried out with 6/VAc proceeded, albeit slowly, to greater conversions. A labilizing effect of the Ar/Ar′ steric bulk is suggested by QM/MM calculations of CrIII−C BDE for models of the OMRP dormant species with 1, 5, and the parent system where Ar = Ar′ = Ph. Thermal deactivation of the nacnacxyl,xyl system has been evidenced, with formation of the acetate complex CpCr(nacnacxyl,xyl)(OAc), 9, as confirmed by an UV−vis study and by independent synthesis. This product is proposed to form by β-acetate transfer from the growing radical chain, triggered by a head−head monomer insertion. Compounds 2, 3, 6, 7, 8, and 9 have been structurally characterized by X-ray diffraction methods.

Electronic and Steric Ligand Effects in the Radical Polymerization of Vinyl Acetate Mediated by β‐Ketoiminate Complexes of Cobalt(II)

Complexes Co[OC(Ph)CHC(Me)NAr](2) [Ar=Ph, 1; o,o'-C(6)H(3)Me(2) (Xyl), 2; p-C(6)H(4)CF(3), 3] are tested in the polymerization of vinyl acetate (VAc) initiated by V-70 (0.8 equiv) at 30 degrees C. Polymerization occurs without any notable induction time yielding PVAc with relatively low polydispersity, but with higher than expected M(n) values, which indicates inefficient trapping processes. The apparent polymerization rate constant varies in the order 2>1>3. Controlled polymer growth is also observed when the polymerization is conducted in the presence of a much higher V-70/1 ratio, demonstrating that this system can also function as a transfer agent in a degenerative transfer process. Competition between chain growth and catalyzed chain transfer (CCT) is also observed, the latter prevailing at higher temperatures. Comparison of these results with previous reports on bis(beta-diketonato) complexes allows a separate assessment of ligand electronic and steric effects in the ability to control polymerization.

The AZARYPHOS Family of Ligands for Ambifunctional Catalysis: Syntheses and Use in Ruthenium‐Catalyzed anti‐Markovnikov Hydration of Terminal Alkynes

The family of AZARYPHOS (aza-aryl-phosphane) phosphane ligands, containing a phosphine unit and sterically shielded nitrogen lone pairs in the ligand periphery, is introduced as a tool for developing ambifunctional catalysis by the metal center and nitrogen lone pairs in the ligand sphere. General synthetic strategies have been developed to synthesize over 25 examples of structurally diverse (6-aryl-2-pyridyl)phosphanes (ARPYPHOS), (6-alkyl-2-pyridyl)phosphanes (ALPYPHOS), 4,6-disubsituted 1,3-diazin-2-ylphosphanes or 1,3,5-triazin-2-ylphosphanes, quinazolinylphosphanes, quinolinylphosphanes, and others. The scalable syntheses proceed in a few steps. The incorporation of AZARYPHOS ligands (L) into complexes [RuCp(L)(2)(MeCN)][PF(6)] (Cp = cyclopentadienyl) gives catalysts for the anti-Markovnikov hydration of terminal alkynes of the highest known activities. Electronic and steric ligand effects modulate the reaction kinetics over a range of two orders of magnitude. These results highlight the importance of using structurally diverse ligand families in the process of developing cooperative ambifunctional catalysis by a metal and its ligand.

Studies on the Mechanism of Allylic Coupling Reactions: A Hammett Analysis of the Coupling of Aryl Silicate Derivatives

Hammett analysis of the palladium-catalyzed allyl-aryl coupling reaction has demonstrated that the rate of the coupling reaction is enhanced by electron-withdrawing groups on the aryl siloxane. The positive slope of the Hammett plot indicated involvement of a charged transition state in which negative charge on the aryl ring is stabilized inductively. This result is consistent with either transmetalation or reductive elimination being the rate-determining step in the coupling process. Furthermore, the influence of ligand on the metal site has been assessed from competition studies as a function of ligand type, cone angle, and electronic effects. From the relative ratios of coupling products produced in the Hammett study, it is possible to gather insight into the role of the electronic as well as the steric effects of ligands on the mechanism of the coupling reaction.

A Solution Study of Complex Formation of Some Diamines with Lanthanones

To study the metal ligand equilibrium in aqueous solution, the well known Irving‐Rossotti titration method was used. The temperature selected is 30±0.10C at ionic strength 0.2 M (NaClO4) which was maintained constant through out the work. The binary metal complex (ML2) formation was studied. The metals selected are Sm3+, Gd3+, Dy3+ and Yb3+. The diamine ligands taken are ethylenediamine, 1,2 diamino propane, 1,3 diamino propane, N-N diethyl ethylenediamine and N-N -dimethyl ethylenediamine. Factors that affected the stability of the complexes are size and ionic potential of lanthanone ions, basicity of ligands, ring size and steric effect of ligands.

Peculiar Extraction Behavior of Nickel(II) Originated in Steric Effect of Schiff Base Ligands in Ion-pair Extraction Systems

The Steric effect of two di-Schiff base ligands having phenylene skeleton as extractant on ion-pair extraction of divalent metal cations into nitorbenzene with picrate anion was investigated. In these extraction systems, all of the extraction except for nickel was equilibrated within 1 h, and ratio of metal: ligand: picrate in the extracted species was 1: 2: 2 or 1: 3: 2. In the extraction of nickel, 2 or 10 hours needed to be equilibrated and the Steric effect of ligands resulted in the change of the extracted species. Namely, a difference between the favorable extracted species for kinetics and equilibrium appeared by the steric effect. Consequently, nickel was extracted quantitatively and the highly selectivity for nickel was obtained.

Ligand steric effect on the coordination mode of nitrite ion: Syntheses and X-ray single crystal structures of trans-[NiL 2(ONO) 2] and cis-[NiL′ 2(O 2N)](NO 2)·H 2O [L = 2-(aminomethyl)pyridine and L′ = (2-aminoethyl)pyridine

The steric effects of the co-ligands originated from the pi-cloud of the pyridine moiety and from increased aliphatic chain length have been utilized to synthesize two different binding modes of nitrite ion. A red-violet nitrito isomer trans-[NiL 2(ONO) 2] ( 1) and a blue-violet nitrito (O,O) isomer cis-[NiL′ 2(O 2N)](NO 2)·H 2O ( 2) are obtained by adding L and L′ to the methanolic suspension of K 4[Ni(NO 2) 6]·H 2O [L = 2-(aminomethyl)pyridine and L′ = 2-(aminoethyl)pyridine]. No other isomer of complex 1 or 2 is yielded in solution irrespective of the nature of the solvent used or in the solid state. They have been characterized by routine physicochemical studies as well as by X-ray single crystal structure analysis.

Steric Effects in the Aerobic Oxidation of π-Allylnickel(II) Complexes with N-Heterocyclic Carbenes

Pi-allylchloro(NHC)nickel(II) complexes were synthesized and their reactions with O2 were studied. Ligand steric effects were found to determine the difference between rapid oxidation of the allyl group to produce bis-mu-hydroxonickel complexes and no observable reaction. The ability of the metal-NHC bond to rotate correlates with the ability of the complex to react with O2. In the limiting cases, conformationally restricted complexes are stable to O2 and complexes with rapid Ni-NHC bond rotation react rapidly with O2. Complexes with intermediate conformational flexibility were found to exhibit lesser reactivity with O2. On the basis of the observed inertness of complexes with saddle-shaped ligands to O2, we propose the adoption of a nonplanar geometry upon reaction with O2 to be required. The issue of conformational flexibility versus rigidity is expected to directly impact the catalytic behavior of metal-NHC complexes.

An improved method for the computation of ligand steric effects based on solid angles

An improved algorithm has been designed to characterize ligand interactions in organometallic and coordination complexes in terms of the percentage of the metal coordination sphere shielded by a given ligand. The computations for ligand solid angles are performed numerically and employ introduced atomic radii that are larger than covalent but smaller than van der Waals radii. This approach enables facile evaluation of steric congestion in the metal coordination sphere, quantification of unfavorable interligand contacts, and in some cases prediction of the complex composition or ligand coordination on purely geometrical grounds.

The Importance of Ligand Steric Effects on Transmetalation

A very striking phosphine steric effect on the rate of transmetalation of phenyl zinc bromide to platinum complexes has been observed: most Pt(II) complexes of type [Pt(diphosphine)(Ph)Br] react w...

Vibrational Spectroscopy and Structures of Ni+(C2H2)n (n =1−4) Complexes

Nickel cation-acetylene complexes of the form Ni(+)(C(2)H(2))(n), Ni(+)(C(2)H(2))Ne, and Ni(+)(C(2)H(2))(n)Ar(m) (n = 1-4) are produced in a molecular beam by pulsed laser vaporization. These ions are size-selected and studied in a time-of-flight mass spectrometer by infrared laser photodissociation spectroscopy in the C-H stretch region. The fragmentation patterns indicate that the coordination number is 4 for this system. The n = 1-4 complexes with and without rare gas atoms are also investigated with density functional theory. The combined IR spectra and theory show that pi-complexes are formed for the n = 1-4 species, causing the C-H stretches in the acetylene ligands to shift to lower frequencies. Theory reveals that there are low-lying excited states nearly degenerate with the ground state for all the Ni(+)(C(2)H(2))(n) complexes. Although isomeric structures are identified for rare gas atom binding at different sites, the attachment of rare gas atoms results in only minor perturbations on the structures and spectra for all complexes. Experiment and theory agree that multiple acetylene binding takes place to form low-symmetry structures, presumably due to Jahn-Teller distortion and/or ligand steric effects. The fully coordinated Ni(+)(C(2)H(2))(4) complex has a near-tetrahedral structure.

Modulation of the Ring Size and Nuclearity of Metallamacrocycles via the Steric Effect of Ligands: Preparation and Characterization of 18-Membered Hexanuclear, 24-Membered Octanuclear, and 30-Membered Decanuclear Manganese Metalladiazamacrocycles with α- and β-Branched N-Acylsalicylhydrazides

Hexanuclear, octanuclear, and decanuclear manganese metalladiazamacrocycles have been prepared by reacting a series of pentadentate ligands, N-acylsalicylhydrazides (N-(3-methylbutanoyl)salicylhydrazide (H(3)3-mbshz), N-(phenylacetyl)salicylhydrazide (H(3)pashz), N-(3,3-dimethylbutanoyl)salicylhydrazide (H(3)3-dmbshz), N-(2-methylpropanoyl)salicylhydrazide (H(3)2-mpshz), N-((R,S)-2-methylbutanoyl)salicylhydrazide (H(3)RS-2-mbshz), N-((S)-2-methylbutanoyl)salicylhydrazide (H(3)S-2-mbshz), and N-(2,2-dimethylpropanoyl)salicylhydrazide (H(3)2-dmpshz)), with manganese(II) acetate tetrahydrate. The self-assembled, supramolecular complexes assume a nearly planar cyclic structure with an -(Mn-N-N)(n) backbone and measure approximately 2.1, approximately 2.3, and approximately 2.6 nm in outer diameters for n = 6, 8, and 10, respectively. The chiralities of the manganese centers on the metalladiazamacrocycle occur in alternating ...LambdaDeltaLambdaDelta... configurations. While beta-branched N-acylsalicylhydrazides (H(3)3-mbshz, H(3)pashz, H(3)3-dmbshz) with a sterically flexible Calpha methylene group yield 18-membered hexanuclear manganese metalladiazamacrocycles of S(6) point group symmetry, alpha-branched N-acylsalicylhydrazides lead to 24-membered octanuclear manganese metalladiazamacrocycles or 30-membered decanuclear manganese metalladiazamacrocycles depending on the size of the N-acyl substituents. The alpha-branched H(3)2-mpshz ligand with the sterically least demanding isopropyl tail at the N-acyl position yields a 24-membered octanuclear manganese metalladiazamacrocycle of S(8) point group symmetry, but other alpha-branched N-acylsalicylhydrazides such as H(3)RS-2-mbshz, H(3)S-2-mbshz, and H(3)2-dmpshz lead to 30-membered decanuclear manganese metalladiazamacrocycles of S(10) point group symmetry. The magnetic properties of the metalladiazamacrocycles are characterized by a weak antiferromagnetic exchange interaction, with J(eff) = -8.5 to -3.8 K between the Mn(3+) ion spins with S = 2 in the cyclic system.