Tris Complexes Research Articles

Highly covalent metal-ligand π bonding in chelated bis- and tris(iminoxolene) complexes of osmium and ruthenium.

The bis(aminophenol) 2,2'-biphenylbis(3,5-di-tert-butyl-2-hydroxyphenylamine) (ClipH4) forms trans-(Clip)Os(py)2 upon aerobic reaction of the ligand with {(p-cymene)OsCl2}2 in the presence of pyridine and triethylamine. A more oxidized species, cis-β-(Clip)Os(OCH2CH2O), is formed from reaction of the ligand with the osmium(vi) complex OsO(OCH2CH2O)2, and reacts with Me3SiCl to give the chloro complex cis-β-(Clip)OsCl2. Octahedral osmium and ruthenium tris-iminoxolene complexes are formed from the chelating ligand tris(2-(3',5'-di-tert-butyl-2'-hydroxyphenyl)amino-4-methylphenyl)amine (MeClampH6) on aerobic reaction with divalent metal precursors. The complexes' structural and electronic features are well described using a simple bonding model that emphasizes the covalency of the π bonding between the metal and iminoxolene ligands rather than attempting to dissect the parts into discrete oxidation states. Emphasizing the continuity of bonding between disparate complexes, the structural data from a variety of Os and Ru complexes show good correlations to π bond order, and the response of the intraligand bond distances to the bond order can be analyzed to illuminate the polarity of the bonding between metal and the redox-active orbital on the iminoxolenes. The osmium compounds'π bonding orbitals are about 40% metal-centered and 60% ligand-centered, with the ruthenium compounds' orbitals about 65% metal-centered and 35% ligand-centered.

Characterization of Terminal Iron(III)-Oxo and Iron(III)-Hydroxo Complexes Derived from O2 Activation.

O2 activation at nonheme iron centers is a common motif in biological systems. While synthetic models have provided numerous insights into the reactivity of high-valent iron-oxo complexes related to biological processes, the majority of these complexes are synthesized using alternative oxidants. This report describes O2 activation by an iron(II)-triflate complex of the imino-functionalized tris(pyrrol-2-ylmethyl)amine ligand framework, H3[N(piCy)3]. Initial reaction conditions result in the formation of a mixture of oxidation products including terminal iron(III)-oxo and iron(III)-hydroxo complexes. The relevance of these species to the O2 activation process is demonstrated through reactivity studies and electrochemical analysis of the iron(III)-oxo complex.

Employing Aryl‐Linked Bis‐mesoionic Carbenes as a Pincer‐Type Platform to Access Ambient‐Stable Palladium(IV) Complexes

AbstractThe study of palladium(IV) species has great implications for PdII/PdIV‐mediated catalysis. However, most of the PdIV complexes rapidly decompose under ambient conditions, which makes the isolation, characterization and further reactivity study very challenging. The reported ancillary ligand platforms to stabilize PdIV species are dominated by chelating N‐donors such as bipyridines. In this work, we present two PdIV complexes with scarcely used C‐donors as the supporting platform. The anionic aryl donor and MIC (MIC=mesoionic carbene) are combined in a [CC′C]‐type pincer framework to access a series of ambient‐stable PdIV tris(halido) complexes. Their synthesis, solid‐state structures, stability, and reactivity are presented. To the best of our knowledge, the work presented herein reports the first isolated PdIV–MIC as well as the first PdIV carbene‐based aryl pincer.

Employing Aryl-Linked Bis-mesoionic Carbenes as a Pincer-Type Platform to Access Ambient-Stable Palladium(IV) Complexes.

The study of palladium(IV) species has great implications for PdII /PdIV -mediated catalysis. However, most of the PdIV complexes rapidly decompose under ambient conditions, which makes the isolation, characterization and further reactivity study very challenging. The reported ancillary ligand platforms to stabilize PdIV species are dominated by chelating N-donors such as bipyridines. In this work, we present two PdIV complexes with scarcely used C-donors as the supporting platform. The anionic aryl donor and MIC (MIC=mesoionic carbene) are combined in a [CC'C]-type pincer framework to access a series of ambient-stable PdIV tris(halido) complexes. Their synthesis, solid-state structures, stability, and reactivity are presented. To the best of our knowledge, the work presented herein reports the first isolated PdIV -MIC as well as the first PdIV carbene-based aryl pincer.

Early Metal Di(pyridyl) Pyrrolide Complexes with Second Coordination Sphere Arene-π Interactions: Ligand Binding and Ethylene Polymerization.

Early metal complexes supported by hemilabile, monoanionic di(pyridyl) pyrrolide ligands substituted with mesityl and anthracenyl groups were synthesized to probe the possibility of second coordination sphere arene−π interactions with ligands with potential for allosteric control in coordination chemistry, substrate activation, and olefin polymerization. Yttrium alkyl, indolide, and amide complexes were prepared and structurally characterized; close contacts between the anthracenyl substituents and Y-bound ligands are observed in the solid state. Titanium, zirconium, and hafnium tris(dimethylamido) complexes were synthesized, and their ethylene polymerization activity was tested. In the solid state structure of one of the Ti tris(dimethylamido) complexes, coordination of Ti to only one of the pyridine donors is observed pointing to the hemilabile character of the di(pyridyl) pyrrolide ligands.

X-ray crystal structure, spectroscopic and DFT computational studies of H-bonded charge transfer complexes of tris (hydroxymethyl)aminomethane (THAM) with chloranilic acid (CLA)

The crystal and molecular structures of two new synthesized charge transfer complexes produced from the reaction of tris(hydroxymethyl)aminomethane, THAM and 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (chloranilic acid, CLA) in the mole ratios (2:1) [I] and (1:1) [II], have been studied. The; FT-IR and 1H NMR spectral data of the complexes are confirmed by single-crystal X-ray crystallography. The single-crystal X-ray crystallography, indicated the formation of both inter- and intra-molecular hydrogen bonding as well as the proton transfer from CLA to THAM. The electronic transition spectra of these two complexes as acetonitrile (AN), methanol (MeOH), ethanol (EtOH) solutions are recorded. The molecular compositions of the formed complexes in the three solvents determined by Job's method, support 1:1 mol ratio complex, whatever the polarity of the solvent. The high values of charge transfer complex formation constants, KCT determined by applying the minimum–maximum absorbance method, indicated the formation of stable CT-complexes, with the order Ethanol > Methanol ≫ Acetonitrile. Computational study incorporated the use of TD-DFT with PCM as solvation model are applied for energy minimization and estimation of the stabilization energy for the produced complexes. The geometrical parameters (bond lengths and angles) of the formed CT complexes are computed and analyzed. Furthermore, Mulliken atomic charges, molecular potential energy surface, HOMO and LUMO molecular orbitals of the formed complexes are presented. A good agreement between experimental and DFT computational analysis has been found.

Luminescence studies of uranyl-aliphatic dicarboxylic acid complexes in acetonitrile medium

Abstract The uranyl (UO2 2+)-aliphatic dicarboxylic acid complexes are studied by luminescence and UV-Vis spectroscopy in acetonitrile (MeCN) medium. The ligands used are malonic acid (MA), succinic acid (SA), glutaric acid (GA), adipic acid (AA) and pimelic acid (PA). The complexes of UO2 2+ with the above ligands showed well resolved luminescence spectra at pH 4.0 with M/L = 5. Both luminescence and UV-Vis spectra indicated the formation of 1:2 and 1:3 complexes of UO2 2+ with MA and GA, AA, PA, respectively. DFT computations indicated the formation of 1:2 chelate complex of UO2 2+ with MA and two types of 1:3 complexes of UO2 2+ with SA, GA, AA and PA. Furthermore, the effect of solvent (water and acetonitrile) on the UO2 2+-ligand complexes has been performed using COSMO model. The present study demonstrates, for the first time, the formation of tris complexes of uranyl with these ligands in acetonitrile medium.

Solute-solvent electronic interaction is responsible for initial charge separation in ruthenium complexes [Ru(bpy)3]2+ and [Ru(phen)3]2+

Origin of the initial charge separation in optically-excited Ruthenium(II) tris(bidentate) complexes of intrinsic D3 symmetry has remained a disputed issue for decades. Here we measure the femtosecond two-photon absorption (2PA) cross section spectra of [Ru(2,2′-bipyridine)3]2 and [Ru(1,10-phenanthroline)3]2 in a series of solvents with varying polarity and show that for vertical transitions to the lower-energy 1MLCT excited state, the permanent electric dipole moment change is nearly solvent-independent, Δμ = 5.1–6.3 D and 5.3–5.9 D, respectively. Comparison of experimental results with quantum-chemical calculations of complexes in the gas phase, in a polarizable dielectric continuum and in solute-solvent clusters containing up to 18 explicit solvent molecules indicate that the non-vanishing permanent dipole moment change in the nominally double-degenerate E-symmetry state is caused by the solute-solvent interaction twisting the two constituent dipoles out of their original opposite orientation, with average angles matching the experimental two-photon polarization ratio.

Nitrogenase-Relevant Reactivity of a Synthetic Iron-Sulfur-Carbon Site.

Simple synthetic compounds with only S and C donors offer a ligation environment similar to the active site of nitrogenase (FeMoco) and thus demonstrate reasonable mechanisms and geometries for N2 binding and reduction in nature. We recently reported the first example of N2 binding at a mononuclear iron site supported by only S and C donors. In this work, we report experiments that examine the mechanism of N2 binding in this system. The reduction of an iron(II) tris(thiolate) complex with 1 equiv of KC8 leads to a thermally unstable intermediate, and a combination of Mössbauer, EPR, and X-ray absorption spectroscopies identifies it as a high-spin (S = 3/2) iron(I) species that maintains coordination of all three sulfur atoms. DFT calculations suggest that this iron(I) intermediate has a pseudotetrahedral geometry that resembles the S3C iron coordination environment of the belt iron sites in the resting state of the FeMoco. Further reduction to the iron(0) oxidation level under argon causes the dissociation of one of the thiolate donors and gives an η6-arene species which reacts with N2. Thus, in this system the loss of thiolate and binding of N2 require reduction beyond the iron(I) level to the iron(0) level. Further reduction of the iron(0)-N2 complex gives a reactive, formally iron(-I) species. Treatment of the putative iron(-I) complex with weak acids gives low yields of ammonia and hydrazine, demonstrating that these nitrogenase products can be generated from N2 at a synthetic Fe-S-C site. Catalytic N2 reduction is not observed, which is attributed to protonation of the supporting ligand and degradation of the complex via ligand dissociation. Identification of the challenges in this system gives insight into the design features needed for functional biomimetic complexes.

Eu3+ -tetrakis β-diketonate complexes for solid-state lighting application.

Eu3+ -β-diketonate complexes are used, for example, in solid-state lighting (SSL) or light-converting molecular devices. However, their low emission quantum efficiency due to water molecules coordinated to Eu3+ and low photostability are still problems to be addressed. To overcome such challenges, we synthesized Eu3+ tetrakis complexes based on [Q][Eu(tfaa)4 ] and [Q][Eu(dbm)4 ] (Q1=C26 H56 N+ , Q2=C19 H42 N+ , and Q3=C17 H38 N+ ), replacing the water molecules in the tris stoichiometry. The tetrakis β-diketonates showed desirable thermal stability for SSL and, under excitation at 390nm, they displayed the characteristic Eu3+ emission in the red spectral region. The quantum efficiencies of the dbm complexes achieved values as high as 51%, while the tfaa complexes exhibited lower quantum efficiencies (28-33%), but which were superior to those reported for the tris complexes. The structures were evaluated using the Sparkle/PM7 model and comparing the theoretical and the experimental Judd-Ofelt parameters. [Q1][Eu(dbm)4 ] was used to coat a near-UV light-emitting diode (LED), producing a red-emitting LED prototype that featured the characteristic emission spectrum of [Q1][Eu(dbm)4 ]. The emission intensity of this prototype decreased only 7% after 30h, confirming its high photostability, which is a notable result considering Eu3+ complexes, making it a potential candidate for SSL.

Synthesis, Structure, and Haptotropic Interconversions of Tungsten Cycloheptatrienyl–Acetonitrile–Carbonyl Complexes

Tungsten cycloheptatrienyl complexes (η7-C7H7)W(CO)2I (I), [(η3-C7H7)W(CO)2(CH3CN)3]PF6 (II), and [(η7-C7H7)W(CO)2(CH3CN)]PF6 (III) (CIF files CCDC nos. 1875096 (I), 1875097 (II), 1875098 (III)), characterized earlier only by spectroscopic methods, were prepared and studied by X-ray diffraction. The tris(acetonitrile) complex II can lose two acetonitrile ligands in both coordinating and non-coordinating solvents with the η3 → η7 change in the hapticity of the cycloheptatrienyl ring to be converted to III. The reverse ligand attachment process with the η7 → η3 change in the hapticity of the cycloheptatrienyl ring takes place upon the dissolution of monoacetonitrile complex III in acetonitrile. The electron-compensating η3 ↔ η7 change in the hapticity of the tropylium ligand is reversible.

Electrocatalytic Reduction of CO2 to Methanol by Iron Tetradentate Phosphine Complex Through Amidation Strategy.

The iron complex of tetradentate tris[2-(diphenylphosphino) ethyl]phosphine (PP3 ), [Fe(PP3 )(MeCN)2 ](BF4 )2 , was able to electrocatalytically reduce CO2 to formate with a Faradaic efficiency (FE) of approximately 97.3 % in acetonitrile. Upon addition of diethylamine as a cocatalyst, electrocatalytic reduction to methanol was achieved with an FE of 68.5 %, and other products were formamide and formate. A mechanistic study suggested that the [FeH(PP3 )](BF4 ) hydride complex was the active species in the electrocatalysis. Added amine as cocatalyst could react with CO2 to form carbamate, which could then be reduced to formamide and further to methanol. By contrast, free CO2 could only be reduced to formate as the end-product.

Inhibition of acid phospholipase A1 activity by the complex of tris(2-hydroxyethyl)amine with zinc bis(2-methylphenoxyacetate)

Studies in the atherosclerosis model in rabbits showed that the complex of tris(2-hydroxyethyl)amine with zinc bis(2-methylphenoxyacetate) (zincatrane) modulates lipid metabolism in the liver and reduces activity of a lysosomal lipolytic enzyme of the hydrolase class. Zincatrane was proposed for use as an acid phospholipase A1 inhibitor to increase the resistance of the vascular system to cholesterol and prevent atherosclerosis.

A triangular palladium(II) supramolecular coordination complex based on 1,4-bis(1H-imidazol-1-yl)benzene and (2,2'-bipyridyl)palladium(II) nitrate: synthesis and crystal structure.

The self-assembly of ditopic bis(1H-imidazol-1-yl)benzene ligands (LH) and the complex (2,2'-bipyridyl-κ2N,N')bis(nitrato-κO)palladium(II) affords the supramolecular coordination complex tris[μ-bis(1H-imidazol-1-yl)benzene-κ2N3:N3']-triangulo-tris[(2,2'-bipyridyl-κ2N,N')palladium(II)] hexakis(hexafluoridophosphate) acetonitrile heptasolvate, [Pd3(C10H8N2)3(C12H10N4)3](PF6)6·7CH3CN, 2. The structure of 2 was characterized in acetonitrile-d3 by 1H/13C NMR spectroscopy and a DOSY experiment. The trimeric nature of supramolecular coordination complex 2 in solution was ascertained by cold spray ionization mass spectrometry (CSI-MS) and confirmed in the solid state by X-ray structure analysis. The asymmetric unit of 2 comprises the trimetallic Pd complex, six PF6- counter-ions and seven acetonitrile solvent molecules. Moreover, there is one cavity within the unit cell which could contain diethyl ether solvent molecules, as suggested by the crystallization process. The packing is stabilized by weak inter- and intramolecular C-H...N and C-H...F interactions. Interestingly, the crystal structure displays two distinct conformations for the LH ligand (i.e. syn and anti), with an all-syn-[Pd] coordination mode. This result is in contrast to the solution behaviour, where multiple structures with syn/anti-LH and syn/anti-[Pd] are a priori possible and expected to be in rapid equilibrium.

Iridium-Promoted B–B Bond Activation: Preparation and X-ray Diffraction Analysis of a mer-Tris(boryl) Complex

The tris(boryl) complex Ir(Bcat)3{κ3-P,O,P-[xant(PiPr2)2]} [Bcat = catecholboryl; xant(PiPr2)2 = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene] has been prepared and characterized by X-ray diffraction analysis. The boryl ligands are disposed in a mer arrangement. The Ir-B bonds situated mutually trans are ∼0.1 Å longer than that disposed cis to the other two. An energy decomposition analysis method coupled to natural orbitals for chemical valence has revealed that the level of π-back-donation from the metal to the p z atomic orbital of the boron atom decreases ∼43% in the longer bonds with respect to the shorter one, while the level of σ-bonding interaction diminishes by only ∼8%.

Tert-Butyl(cyclopentadienyl) Ligands Will Stabilize Nontraditional +2 Rare-Earth Metal Ions

Reduction of the tris(tert-butylcyclopentadienyl) complexes, Cpt3Ln (Cpt = C5H4CMe3), with potassium in the presence of 18-crown-6 forms the crystallographically characterizable Ln(II) complexes, {...

Synthesis and Characterization of a Tripodal Tris(nitroxide) Aluminum Complex and Its Catalytic Activity toward Carbonyl Hydroboration

An aluminum complex of a tripodal tris(nitroxide)-based ligand has been prepared and characterized. The complex has the ability to participate in metal–ligand cooperative catalysis, which has been exploited for the hydroboration of both aldehydes and ketones. This represents new, transition-metal-like reactivity for an earth-abundant metal.

DFT and TD-DFT Study of [Tris(dithiolato)M]3- Complexes[M= Cr, Mn and Fe]: Electronic Structures, Properties and Analyses

Using Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) methods, transition metal complexes of benzene-1, 2-dithiolate (L2-) ligand from Cr to Fe have been studied theoretically. The ground state geometries, binding energies, UV-Visible spectra (UV-Vis), frontier molecular orbitals (FMOs) analysis, charge analysis and natural bond orbital (NBO) have been calculated. The structural parameters are in good accord with the experimental data. The metal-ligand binding energies are one (1) order of magnitude higher than the physisorption energy of a benzene-1, 2-dthiolate molecule on a metallic surface. In accordance with experiment the calculated electronic spectra of these tris complexes show bands at 565, 559 and 546 nm for Cr3+, Mn3+, and Fe3+ respectively which are mainly qualified to ligand-to metal charge transfer (LMCT) transitions. The electronic properties analysis demonstrate that the highest occupied molecular orbital (HOMO) is mostly centered on metal coordinated sulfur atoms whereas the lowest unoccupied molecular orbital (LUMO) is mainly located on the metal surface. By calculating natural bond orbital (NBO), the intramolecular interactions and electron delocalization was obtained. The results of NBO analysis illustrated the significant charge transfer from sulfur to central metal ions, as well as to the benzene of the complex. The calculated charges on metal ions are also reported at various charge schemes. The calculations show encouraging agreement with the available experimental data.
 Dhaka Univ. J. Sci. 67(1): 63-68, 2019 (January)

Synthesis, characterization, photoluminescence and intensity parameters of high quantum efficiency pure-red emitting Eu(iii) fluorinated β-diketone complexes

The high quantum efficiency mononuclear 7-, 8- and 9-coordinate complexes of tris(heptafluoro-dimethyl-octanedionato)Eu(iii) and 1H-indazole, 2-(1H-imidazol-2-yl)pyridine and 2,4,6-tris(2-pyridyl)-s-triazine are synthesized and thoroughly characterized.

Synthesis and structural, redox and photophysical properties of tris-(2,5-di(2-pyridyl)pyrrolide) lanthanide complexes.

A first series of lanthanide complexes of tris(dipyridyl)pyrrolide ligands has been prepared. The [Ln(dppR1,R2)3] complexes (Ln = La(iii), Sm(iii), Eu(iii), Gd(iii) and Yb(iii); and dppR1,R2 = 2,5-di(2-pyridyl-3-(R1)-4-(R2)) pyrrolide) have been isolated and their structures and photophysical and redox properties characterised, both in the solid-state and in solution. In the complexes, the three dpp- ligands form a distorted tricapped trigonal prismatic coordination geometry about the lanthanide ions, with the antiparallel isomer observed in the solid state for non-symmetric (dppCO2Me,Me)-. However, 1H NMR spectroscopy of the diamagnetic and paramagnetic [Ln(dppR1,R2)3] complexes in d6-benzene solution reveal evidence for a statistical distribution of all possible isomers. Time-resolved luminescence studies suggest that the dpp- ligand (with triplet excited state T1 energy at 18 622 cm-1) sensitises red emission from [Eu(dppCO2Me,Me)3] and near-infrared emission from [Yb(dppCO2Me,Me)3] through the antenna effect. Cyclic voltammetry reveals three consecutive, reversible, one-electron oxidation processes for each [Ln(dppR1,R2)3] complex, corresponding to oxidations of each dpp- ligand between 0.3-0.8 V vs. E1/2 (Fc+/0), and for [Eu(dppCO2Me,Me)3] the EuIII/II couple was -2.099 V vs. E1/2 (Fc+/0).