Isomeric Complexes Research Articles

Selective Synthesis of Secondary and Tertiary Amines by Reductive N‐Alkylation of Nitriles and N‐Alkylation of Amines and Ammonium Formate Catalyzed by Ruthenium Complex

AbstractA new ruthenium catalytic system for the syntheses of secondary and tertiary amines via reductive N‐alkylation of nitriles and N‐alkylation of primary amines is proposed. Isomeric complexes 8 catalyze transfer hydrogenation and N‐alkylation of nitriles in ethanol to give secondary amines. Unsymmetrical secondary amines can be produced by N‐alkylation of primary amines with alcohols via the borrowing hydrogen methodology. Aliphatic amines were obtained with excellent yields, while only moderate conversions were observed for anilines. Based on kinetic and mechanistic studies, it is suggested that the rate determining step is the hydrogenation of intermediate imine to amine. Finally, ammonium formate was applied as the amination reagent for alcohols in the presence of ruthenium catalyst 8. Secondary amines were obtained from primary alcohols within 24 hours at 100 °C, and tertiary amines can be produced after prolonged heating. Secondary alcohols can only be converted to secondary amines with moderate yield. Based on mechanistic studies, the process is suggested to proceed through an ammonium alkoxy carbonate intermediate, where carbonate acts as an efficient leaving group.

Isomers of Coumarin-Based Cyclometalated Ir(III) Complexes with Easily Tuned Phosphorescent Color and Features for Highly Efficient Organic Light-Emitting Diodes.

Three Ir(C∧N)2(acac)-type and one Ir(C1∧N)(C2∧N)(acac)-type coumarin-based cyclometalated Ir(III) complex isomers (IrC5, IrC7, IrC7-A, and IrC8) have been obtained using three coumarin-based isomers of 2-phenylpyridine (ppy)-type cyclometalating ligands (L-C5, L-C7, and L-C8). Two coordination isomers emerging as principal products (IrC7 and IrC7-A) are obtained in the synthesis of corresponding coumarin-based cyclometalated Ir(III) complexes because of two different coordination sites in ligand L-C7 to form a C-Ir bond. To the best of our knowledge, there are no such isomers reported to date. Interestingly, a broad range of phosphorescent color tuning from green (IrC8, λ = 516 nm) to red (IrC5, λ = 608 nm) has been realized through variation of the pyridyl substitution positions on the fused phenyl ring of the coumarin skeleton. In addition, based on natural transition orbital (NTO) analyses, features of the lowest triplet excited states (T1) from these coumarin-based cyclometalated Ir(III) complex isomers can be tuned easily by these ligand isomers as well. IrC5, IrC7, and IrC7-A show prevailing 3MLCT character associated with their T1 states which emit the phosphorescent signals, while the T1 state of IrC8 exhibits the dominant ligand-centered π-π* transition feature. Importantly, owing to the strong rigidity of the coumarin skeleton, all the coumarin-based cyclometalated Ir(III) complex isomers can show high phosphorescent quantum yields Φp (ca. 0.4-1). Together with the improved electron-injection/electron-transport (EI/ET) ability, all the phosphorescent emitters display impressive electroluminescence (EL) performance. The device based on IrC8 gives the highest EL efficiencies of external quantum efficiency (ηext) 22.7%, current efficiency (ηL) 79.7 cd A-1, and power efficiency (ηP) 58.2 lm W-1, representing the most state-of-the-art EL ability ever achieved by coumarin-based phosphorescent emitters. All these encouraging data definitely suggest the great potential of the coumarin skeleton in both easy tuning of the photophysical properties of ppy-type Ir(III) phosphorescent complexes and developing high-performance phosphorescent emitters.

Supramolecular structures in oxovanadium(IV) compounds with pyrid-2-one and pyrid-4-one ligands

The aim of this study is to perform an analysis of the spectroscopic and electronic properties of three vanadium(IV) complexes formed from bis(acetylacetonato)oxovanadium(IV) [VO(acac)2] and pyrid-2-one (2pyon) or pyrid-4-one (4pyon) ligands: [VO(acac)2(2pyon)] (1), [VO(acac)2(4pyon)] (2) and [VO(acac)2(H2O)]·4pyon (3). In 1 and 2, the pyridone ligand coordinates neutral and monodentate to the vanadium center, with the oxygen atom placed in the axial trans position to the oxo ligand. In 3, the 4pyon ligand remains outside the metal first coordination sphere but bound to the aqua ligand through hydrogen bonds that led to the formation of a supramolecular structure. Moreover, the Density Functional Theory (DFT) were applied to answer two overarching aspects. First, in complexes 1 and 2, the DFT analysis was used to establish the most stable configuration among the vanadium pyridone complex isomers, comparing the ligand via O-bound pyridone and N-bound hydroxypyridine chelating modes. Second, DFT was also performed to enable understanding of the intermolecular interactions between the 4pyon and the [VO(acac)2(H2O)] moiety to elucidate the intermolecular interactions in 3. Further insights were also obtained by applying the Natural Bond Orbital analysis (NBO) along with a topological perspective via the use of the Quantum Theory of Atoms in Molecules (QTAIM).

Effects of intramolecular and intermolecular interactions on excited state properties of two isomeric Cu complexes with AIE and TADF mechanisms in solid phase: A QM/MM study

Abstract Molecular photophysical properties are not only related to the basic structures but also affected by their packing modes. In this work, excited state properties of two isomeric Cu complexes (Complex 1 and Complex 2) with aggregation induced emission (AIE) and thermally activated delayed fluorescence (TADF) features are theoretically studied. The surrounding environments for molecule in solution and solid phase are considered by polarizable continuum model (PCM) and the combined quantum mechanics and molecular mechanics (QM/MM) method respectively. In addition, the intermolecular interaction and intramolecular interaction are visualized by the reduced density gradient (RDG) function. Our investigations show that the geometrical changes between ground state (S0) and lowest singlet excited state (S1) are hindered in solid phase by the restricted intramolecular rotation (RIR) effect, which brings smaller Huang-Rhys factors. Moreover, the decreased reorganization energy from solution to solid phase, is mainly contributed by the reduction of dihedral angle in low frequency (

Optical Stereoisomers of Geometric Isomers of the Octahedral Complex TiF4[Ph2P(O)CH2CH(OH)Me)]2 in CH2Cl2

Some peculiarities of the effect of enantiomers of the monodentate ligand on the stereoisomerism of the mixed octahedral tetrafluoride complexes of d0-transition metals are established by the 19F{1Н} and 31Р{1Н} NMR methods for the complex formation of TiF4 with Ph2P(O)CH2CH(OH)Me (L) having the asymmetric carbon atom in the aliphatic hydrocarbon radical and being a racemic mixture of enantiomers in СН2Сl2. The compositions of the complexes formed in solutions are determined. The conclusion about the relative stereochemical configuration of the chiral and meso-stereoisomers of the geometric isomers of the TiF4L2 octahedral complex is based on an analysis of the 19F NMR spectra using the heterotropic concept.

Quantum Chemical Predictions of Substituent Effect on the Stability, Electronic Structure, and 14N NQR Parameters of a Ruthenium Azirinyl Complex

Using quantum chemical calculations, we report structures, energetic, natural bond analysis, and 14N NQR parameters in ruthenium azirinyl complex isomers (I, II, III). The MPW1PW91 method indicates the most stability for I-isomer. The effects of various donor and acceptor substituents (H, OH, OMe, Me, CCl3, CN, NO2, NO) on the stability, frontier orbitals energies, HOMO–LUMO gap, thermochemical parameters, structure and 14N NQR parameters of these isomers was investigated. NBO analysis was used for exploring bonding properties in these isomers.

Heterodinuclear Diphosphane-Bridged Iron–Platinum Diyne Complexes as Metalloligands for the Assembly of Polymetallic Systems (Fe, Pt, Co)

Treatment of [(OC)3Fe(μ-C=O)(µ-dppm)Pt(PPh3)] (1a) (dppm=Ph2PCH2PPh2) with 1,7-octadiyne affords the isomeric dimetallacyclopentenones [(OC)2Fe(µ-dppm)(µ-C(=O)C{(CH2)4C≡CH}=CH)Pt(PPh3)] (2a) and [(OC)2Fe(µ-dppm)(µ-C(=O)C(H)=C{(CH2)4C≡CH})Pt(PPh3)] (2a′). In a similar manner, the reaction with 1,3-diethynylbenzene and 1,4-diethynylbenzene yields [(OC)2Fe(μ-C(=O)C{C6H4–C≡CH}=CH)(μ-dppm)Pt(PPh3)] (2b,c); using [(OC)3Fe(μ-C=O)(µ-dppa)Pt(PPh3)] (1b) (dppa=Ph2PNHPPh2) as starting material provides [(OC)2Fe(μ-C(=O)C{p-C6H4–C≡CH}=CH)(μ-dppa)Pt(PPh3)] (2d). The isomeric μ-vinylidene complexes [(OC)3Fe(µ-dppm){(µ-C=CH(C6H4C≡CH)})Pt(PPh3)] (3b,c) result from treatment of [(OC)3Fe{Si(OMe)3}(μ-dppm)Pt(H)(PPh3)] with the two aromatic diynes. Reaction of 2a with a further equivalent of 1a yields the structurally characterized tetranuclear species [(OC)2Fe(µ-dppm)(µ-C(=O)C{(CH2)2}=CH)Pt(PPh3)]2 (4a). A dissymmetric heterotetranuclear dimetallacyclopentenone 4b is produced by reacting 2b with [(OC)3Fe(μ-C=O)(µ-dppa)Pt(PPh3)] (1b). An unusual heterotetranuclear dimetallacyclopentenone-μ-vinylidene compound 4c results from reaction of 3c with 1 equivalent of 1a. The dimetallacyclopentenone-dicobaltatetrahedrane cluster [(OC)2Fe(µ-dppm)(µ-C(=O)C{(CH2)4(η2-HC2Co2(CO)6}=CH)Pt(PPh3)] (5a) is formed by addition of [Co2(CO)8] to 2a. Likewise, [(OC)2Fe(µ-dppm)(µ-C(=O)C{C6H4(η2-HC2Co2(CO)4(µ-dppm)}=CH)Pt(PPh3)] (5b) is produced by mixing [Co2(CO)6(µ2-dppm)] with 2b. When attempting to prepare a heterotetranuclear complex by reacting 2a with [Co2(CO)6{P(OPh)3}2], the targeted compound was not formed. Instead, the substitution product [(OC)2Fe(µ-dppm)(µ-C(=O)C{(CH2)4C≡CH}=CH)Pt{P(OPh)3}] (6a) was isolated. 2b reacts in an analogous manner yielding [(OC)2Fe(μ-C(=O)C{C6H4–C≡CH}=CH)(μ-dppm)Pt{P(OPh)3}] (6b).

Homo- and Heterodinuclear Head-to-Head or Head-to-Tail Complexes of Rhodium(I) and Iridium(I) with C2,N3 or C8,N9 Bridging Azolato Ligands

Reaction of in situ lithiated N-methylimidazole or 1,5,6-trimethylbenzimidazole with [M(cod)(μ-Cl)]2 (M = Rh, Ir) led to the exclusive formation of the homodinuclear head-to-head complexes [2]–[5], where the metal centers are bridged by two C,N-coordinated azolato ligands. The two metal centers are each coordinated by one cod ligand and either two carbon or two nitrogen donors of the azolato ligands. Access to a heterodinuclear Ir/Rh complex [6] was achieved via the lithiated iridium complex [7] and addition of an equimolar amount of [Rh(cod)(μ-Cl)]2. Deprotonation of caffeine with lithium diisopropylamide and addition of [M(cod)(μ-Cl)]2 (M = Rh, Ir) led to mixtures of the head-to-head (M = Ir: [8H–H]; M = Rh: [9H–H]) and head-to-tail isomers (M = Ir: [8H–T]; M = Rh: [9H–T]) of the homodinuclear complexes. The molecular structures of the homo- and heterodinuclear complexes [3], [5]·2C6H5F, [6]·2C6H5F, and [9H–T]·C4H8O have been determined by X-ray diffraction studies.

Spectroscopic and 1O2 Sensitization Characteristics of a Series of Isomeric Re(bpy)(CO)3Cl Complexes Bearing Pendant BODIPY Chromophores.

Two new Re(I)bipyridyltricarbonyl chloride complexes, Re(BB3)(CO)3Cl and Re(BB4)(CO)3Cl, featuring BODIPY groups appended to the 5,5'- or 6,6'-positions of the bipyridine ligand, respectively, were synthesized as structurally isomeric compliments to a previously reported 4,4'-substituted homologue, Re(BB2)(CO)3Cl. X-ray crystal structures of the compounds show that the 4,4'-, 5,5'-, and 6,6'-substitution patterns place the BODIPY groups at progressively shorter distances of 9.43, 8.39, and 5.56 Å, respectively, from the complexes' Re centers. The photophysical properties of the isomeric complexes were investigated to ascertain the manner in which the heavy rhenium atom might induce intersystem crossing of the pendant BODIPY moieties positioned at progressively shorter through-space distances. Electronic absorption spectroscopy revealed that the three metal complexes retain the strong visible absorption features characteristic of the bpyBODIPY (BB2-BB4) ligands; however, the fluorescence of the parent borondipyrromethane appended ligands is attenuated by more than an order of magnitude in Re(BB2)(CO)3Cl and Re(BB3)(CO)3Cl and by more than two orders of magnitude in Re(BB4)(CO)3Cl. Furthermore, phosphorescence from Re(BB4)(CO)3Cl is observed under a nitrogen atmosphere, consistent with highly efficient ISC to the triplet-excited state. Singlet oxygen sensitization studies confirm that all three complexes produce singlet oxygen with quantum yields that increase as the distance of the BODIPY groups to the heavy rhenium center is decreased. The trends observed across the series of rhenium complexes with respect to emission and 1O2 sensitization properties can be rationalized in terms of the varied distal separation between the metal center and BODIPY groups in each system.

Photocrystallographic Studies on Transition Metal Nitrito Metastable Linkage Isomers: Manipulating the Metastable State.

ConspectusThe design of solid-state materials whose properties and functions can be manipulated in a controlled manner by the application of light is an important objective in modern materials chemistry. When the material changes property or function, it is helpful if a simple measurable response, such as a change in color, can be detected. Potential applications for such materials are wide ranging, from data storage to smart windows. With the growing emphasis on solid-state materials that have two or more accessible energy states and which exhibit bistability, attention has turned to transition metal complexes that contain ambidentate ligands that can switch between linkage isomeric forms when activated by light. Suitable ligands that show promise in this area include nitrosyls, nitro groups, and coordinated sulfur dioxide molecules, each of which can coordinate to a metal center in more than one bonding mode. A nitrosyl normally coordinates through its N atom (η1-NO) but when photoactivated can undergo isomerism and coordinate through its O atom (η1-ON). At a molecular level, converting between these two configurations can act as an “on/off” switch. The analysis of such materials has been aided by the development of photocrystallographic techniques, which allow the full three-dimensional structure of a single crystal of a complex, under photoactivation, to be determined, when it is in either a metastable or short-lived excited state. The technique effectively brings the dimension of “time” to the crystallographic experiment and brings us closer to being able to watch solid-state processes occur in real time.In this Account, we highlight the advances made in photocrystallography for studying solid-state, photoactivated linkage isomerism and describe the factors that favor the switching process and which allow complete switching between isomers. We demonstrate that control of temperature is key to achieving either a metastable state or an excited state with a specific lifetime. We draw our conclusions from published work on the formation of photoactivated metastable states for nitrosyl and sulfur dioxide complexes and from our own work on photoactivated switching between nitro and nitrito groups. We show that efficient switching between isomers is dependent on the wavelength of light used, on the temperature at which the experiment is carried out, on the flexibility of the crystal lattice, and on both the electronic and steric environment of the ambidentate ligand undergoing isomerism. We have designed and prepared a number of nitro/nitrito isomeric metal complexes that undergo reversible 100% conversion between the two forms at temperatures close to room temperature. Through our fine control over the generation of the metastable states, it should be possible to effectively “dial up” a suitable temperature to give a metastable or an excited state with a desired lifetime.

Fingerprints of microscopic superfluidity in HHen+ clusters

ABSTRACTThe structures and the vibrational dynamics of the complexes HHe are investigated experimentally (via mass spectrometry (MS)) and at high levels of electronic-structure theory. The MS measurements reveal interesting trends about the stability of the starting members of the HHe family. The computations establish that the basically linear, strongly bound, symmetric triatomic molecular ion He(H)He, with an equilibrium H–He distance of 0.925 Å and about 2/3 but at least 1/2 of the positive charge on H, is the molecular core of all of the complexes. Definitive quantum-chemical results are obtained for HHe and HHe, including the proton affinity of He (computed to be cm−1 via the focal-point analysis (FPA) scheme), the FPA isomerisation energy between the two linear isomers of HHe ( cm−1), and the dissociation energy of the HHe HHe + He reaction, with an FPA estimate of cm−1. The structural isomers of the He-solvated complexes are discussed up to n=18. A useful notation, [k−l−m]-HHe, is introduced to characterise qualitatively the three possible belts around the He–H–He core in HHe (), where l denotes the number of He atoms in the central belt and denote the number of He atoms in the top and bottom belts. Capping He atoms attached to the belts can be indicated by sub- and superscripts. Several possible indicators of microscopic superfluidity are investigated: He evaporation energies, rotational constants, and vibrational fundamentals.

Engineered amine dehydrogenase exhibits altered kinetic mechanism compared to parent with implications for industrial application

Leucine amine dehydrogenase (L-AmDH) was engineered from the leucine dehydrogenase (LeuDH) from Bacillus stearothermophilus through the introduction of two key mutations to the substrate binding pocket, thus removing affinity for keto-acids in favor of methyl ketones and enabling the production of chiral amines, important intermediates for pharmaceuticals. The present work examines the effects of the mutations on the kinetic properties of these two enzymes. We conclude that L-AmDH exhibits a random order of association of ketone and cofactor in the reductive amination direction, compared to the ordered sequential mechanism of LeuDH. In the oxidative deamination direction, an ordered sequential mechanism was confirmed for LeuDH, while a more complicated situation was found for L-AmDH. Kinetic solvent viscosity effects revealed an isomerization of enzyme-substrate complexes for both LeuDH reactions, but not for L-AmDH. Inhibition experiments showed stronger product inhibition for L-AmDH than for LeuDH, an important consideration for large-scale synthesis.

Computer Design of Fe-M-Fe (M = Co, Ni, Cu, Zn) Complexes with Bis-Salicylaldiminate Linker Functionalized with 1,10-Phenanthroline

Density functional theory method UB3LYP*/6-311++G(d,p) has been applied to the study of geometry, energy, and magnetic characteristics of the isomers of trinuclear complexes FeII-MII-FeII (M = Co, Ni, Cu, Zn) designed from two terminal iron-containing fragments connected via a linker based on bis-salicylaldiminate of transition metal functionalized with 1,10-phenanthroline. It has been shown that the investigated complexes are capable of two-step spin crossover on iron ions. In the systems with cobalt and nickel bis-chelates, the possibility of intramolecular rearrangements via the mechanism of configurational isomerism has been predicted.

Isomeric Palladium Complexes Bearing Imidazopyridine-Based Abnormal Carbene Ligands: Synthesis, Characterization, and Catalytic Activity in Direct C–H Arylation Reaction

A series of “abnormal” NHC ligand precursors based on imidazopyridines and their corresponding NHC-palladium diiodide complexes were prepared. Our study included the first report on a palladium complex with an abnormal NHC ligand derived from imidazo[1,5-a]pyridine. DFT calculation on the free ligands confirmed the higher electron-donating properties of the abnormal NHC ligands than their normal NHC isomers. The structural properties of all the complexes were studied in detail by single-crystal X-ray diffraction studies, revealing that, in general, Pd–carbene bond distances in palladium complexes with abnormal NHCs were longer than those in their isomers with normal NHCs. A significant trans influence is observed in the palladium complexes with abnormal NHC ligands. But in the complexes with normal NHC ligands, both Pd–carbene and Pd–N bond distances are short. Catalytic studies revealed that the palladium complexes were very efficient in catalyzing direct C–H arylation between 1,2-dimethylimidazole and a...

Cu(C6Cl2F3)(tht)]4: An Extremely Efficient Catalyst for the Aryl Scrambling between Palladium Complexes

The copper compound [Cu(C6Cl2F3)(tht)]4 (1) (tht: tetrahydrothiophene) is an excellent catalyst for the trans-to-cis isomerization of complexes [PdAr2(tht)2](Ar: fluoroaryl) and also for the exchange of aryls between those palladium complexes. Herein, we also communicate the synthesis and characterization of 1 and some of its derivatives. The X-ray structure of 1 shows a linear chain of copper atoms, supported by fluoroaryl rings. The nuclear magnetic resonance 19F diffusion-ordered spectroscopy study indicates that this structure is preserved in solution, although it is in equilibrium with other species.

Mechanism of Coupling of Alcohols and Amines To Generate Aldimines and H2 by a Pincer Manganese Catalyst

Acceptorless dehydrogenative coupling (ADC) of alcohols and amines using a manganese-based catalyst is able to produce aldimines, which have a wide reactivity, without underestimating the generation of a clean fuel, since as a result of this coupling, molecular hydrogen is also obtained. Therefore, the aldimine synthesis represents an interesting reaction from chemical and clean energy points of view. In this work, the computational study, via density functional theory (DFT) calculations, of a manganese-based catalyst for the acceptorless dehydrogenative coupling of alcohols and amines, together with the elucidation of its catalytic cycle using benzene as a solvent, is carried out. Calculations provided insight not only into the catalytic pathway but also into the noncatalyzed organic transformation to reach the desired aldimine, on the basis of the aldehyde generated in the catalytic cycle. The importance of the alternative isomers of the Mn-pincer complex, previously characterized experimentally, and th...

Study of Intermolecular Interactions between 2-Chloroaniline Isomeric Butanol Complexes in Gas Phase by Using DFT, NBO, QTAIM and RDG Analysis

Density functional theoretical (DFT) studies on intermolecular hydrogen bond interactions between self and cross-associated molecular complexes of 2-chloroaniline and isomeric butanols (e.g., 2-methyl-2-propanol, 2-methyl-1-propanol, 2-butanol and1-butanol) have been analyzed in gas phase. Thirteen 2-chloroaniline isomeric butanol complexes are analyzed at B3LYP/6-311++G(d,p) level regarding their geometries, bond characteristics and interaction energies. The second-order perturbation stabilization energy has been calculated by natural bond orbitals analysis. Barder's quantum theory of atoms in molecules are employed to elucidate electron density (ρ) as well as its Laplacian (∇2ρ) of the complexes. Further to evaluate the strong and weak interactions between the selected molecular complexes non-covalent interactions plots we used the reduced gradient method.

Steric and electronic control of an ultrafast isomerization.

Synthetic control of the influence of steric and electronic factors on the ultrafast (picosecond) isomerization of penta-coordinate ruthenium dithietene complexes (Ru((CF3)2C2S2)(CO)(L)2, where L = a monodentate phosphine ligand) is reported. Seven new ruthenium dithietene complexes were prepared and characterized by single crystal X-ray diffraction. The complexes are all square pyramidal and differ only in the axial vs. equatorial coordination of the carbonyl ligand. Fourier Transform Infrared (FTIR) spectroscopy was used to study the ν(CO) bandshapes of the complexes in solution, and these reveal rapid exchange between two or three isomers of each complex. Isomerization is proposed to follow a Berry psuedorotation-like mechanism where a metastable, trigonal bipyramidal (TBP) intermediate is observed spectroscopically. Electronic tuning of the phosphine ligands L = PPh3, P((p-Me)Ph)3, ((p-Cl)Ph)3, at constant cone angle is found to have little effect on the kinetics or thermodynamic stabilities of the axial, equatorial and TBP isomers of the differently substituted complexes. Steric tuning of the phosphine ligands over a range of phosphine cone angles (135 < θ < 165°) has a profound impact on the isomerization process, and in the limit of greatest steric bulk, the axial isomer is not observable. Temperature dependence of the FTIR spectra was used to obtain the relative thermodynamic stabilities of the different isomers of each of the seven ruthenium dithietene complexes. This study details how ligand steric effects can be used to direct the solution state dynamics on the picosecond time scale of discrete isomers energetically separated by <2.2 kcal mol-1. This work provides the most detailed description to date of ultrafast isomerization in the ground states of transition metal complexes.

Photoisomerization and thermal isomerization of ruthenium aqua complexes with chloro-substituted asymmetric bidentate ligands.

A series of ruthenium complexes with chloro-substituted bidentate ligands, proximal-[Ru(tpy)(Cl-pyqu)L]n+ [n = 1 for L = Cl, and n = 2 for L = OH2, tpy = 2,2′;6′,2′′-terpyridine, pyqu = 2-(2′-pyridyl)quinoline] were synthesized and their reversible photoisomerizations and thermal isomerizations were investigated experimentally. The crystal structures of the complexes indicated that introduction of a chloro substituent at the 4- or 4′-position of the pyqu ligand did not change the structure around the metal center from that of the non-substituted complex, proximal-[Ru(tpy)(pyqu)L]n+. In contrast, the 6′-substituted complexes had sterically hindered environments around the metal center. The ruthenium aqua complexes showed reversible photoisomerization between the proximal and distal isomers. The quantum yield for photoisomerization of the 6′-substituted ruthenium aqua complex was almost twice as large as those of the other derivatives. This is explained by weakening of the ligand field on the ruthenium center by introduction of a chloro substituent at the 6′-position. Thermal back isomerization from the distal isomer to the proximal one was observed for the 6′-substituted complex, but such reactions were not observed for the other derivatives. The steric hindrance in the 6′-substituted aqua complex enhanced both thermal isomerization and photoisomerization.

Direct observation of the intermediate in an ultrafast isomerization.

Using a combination of two-dimensional infrared (2D IR) and variable temperature Fourier transform infrared (FTIR) spectroscopies the rapid structural isomerization of a five-coordinate ruthenium complex is investigated. In methylene chloride, three exchanging isomers were observed: (1) square pyramidal equatorial, (1); (2) trigonal bipyramidal, (0); and (3) square pyramidal apical, (2). Exchange between 1 and 0 was found to be an endergonic process (ΔH = 0.84 (0.08) kcal mol-1, ΔS = 0.6 (0.4) eu) with an isomerization time constant of 4.3 (1.5) picoseconds (ps, 10-12 s). Exchange between 0 and 2 however was found to be exergonic (ΔH = -2.18 (0.06) kcal mol-1, ΔS = -5.3 (0.3) eu) and rate limiting with an isomerization time constant of 6.3 (1.6) ps. The trigonal bipyramidal complex was found to be an intermediate, with an activation barrier of 2.2 (0.2) kcal mol-1 and 2.4 (0.2) kcal mol-1 relative to the equatorial and apical square pyramidal isomers respectively. This study provides direct validation of the mechanism of Berry pseudorotation - the pairwise exchange of ligands in a five-coordinate complex - a process that was first described over fifty years ago. This study also clearly demonstrates that the rate of pseudorotation approaches the frequency of molecular vibrations.