Isomeric Complexes Research Articles

Theoretical study of the structure and stability of oxovanadate complexes with MO4 n− tetraoxo anions in the inner and outer spheres of the V20O50 cluster

The energies and structural and spectroscopic characteristics of endohedral and exohedral isomers of oxovanadate complexes with MO4 n− tetraoxo anions located in the inner and outer spheres of the V20O50 cluster have been calculated by the density functional theory method (B3LYP). It has been demonstrated that, among the endohedral MO4@V20O50 n− clusters with strong multiply charged anions (VO4 3−, CrO4 2−, PO4 3−, SO4 2−, BO4 5−, etc.), the isomer in which a distorted “guest” tetrahedron MO4 is displaced from the cluster center and bonded to the V20O50 cage through two internal oxygen bridges M−O*−V is the most favorable one. Among the exohedral analogues MO4 · V20O50 n−, the most favorable isomer contains the guest in the capping position bonded to the cage from the outside through three external M−O−V bridges. The latter isomer is more favorable than the former; however, the “exo–endo” transition between them for the clusters with multiply charged anions is accompanied by small changes in energy of ∼2−10 kcal/mol. Singly charged perchlorate and perbromate anions ClO4 - and BrO4 − in the inner sphere of V20O50 are unstable: during optimization, they are “reduced” with decomposition into atoms and concurrent “inner-sphere oxidation” of V20O50 to V20O54 with four peroxide cage bridges V−O−O−V. Charge transfer and specific features of the host–guest interaction in oxovanadate complexes and trends in the properties of endo- and exohedral isomers as a function of the charge and bond character of guest anions are discussed.

(π-Allyl)palladium coupling of 2-(tributylstannyl)cyclopent-2-enone for the synthesis of jasmonoid analogs

The coupling of 2-(tributylstannyl)cyclopent-2-enone with several (π-allyl)palladium complexes derived from allylic electrophiles was investigated as the key step in the synthesis of jasmonoids. These compounds have an important role in plant development, triggering direct and indirect responses when harmed to induce pest resistance. Palladium-catalyzed coupling conditions to obtain a jasmonoid library are described. The retention of geometry of the olefin in the allyl group is not always observed due to syn-anti isomerization of the (π-allyl)palladium complex. The methodology was employed for the synthesis of a simplified jasmonic acid analog.

Ligand Design for Site-Selective Metal Coordination: Synthesis of Transition-Metal Complexes with η6 -Coordination of the Central Ring of Anthracene.

A polycyclic aromatic ligand for site-selective metal coordination was designed by using DFT calculations. The computational prediction was confirmed by experiments: 2,3,6,7-tetramethoxy-9,10-dimethylanthracene initially reacts with [(C5 H5 )Ru(MeCN)3 ]BF4 to give the kinetic product with a [(C5 H5 )Ru]+ fragment coordinated at the terminal ring, which is then transformed into the thermodynamic product with coordination through the central ring. These isomeric complexes have markedly different UV/Vis spectra, which was explained by analysis of the frontier orbitals. At the same time, the calculations suggest that electrostatic interactions are mainly responsible for the site selectivity of the coordination.

Ligand Design for Site‐Selective Metal Coordination: Synthesis of Transition‐Metal Complexes with η6‐Coordination of the Central Ring of Anthracene

AbstractA polycyclic aromatic ligand for site‐selective metal coordination was designed by using DFT calculations. The computational prediction was confirmed by experiments: 2,3,6,7‐tetramethoxy‐9,10‐dimethylanthracene initially reacts with [(C5H5)Ru(MeCN)3]BF4 to give the kinetic product with a [(C5H5)Ru]+ fragment coordinated at the terminal ring, which is then transformed into the thermodynamic product with coordination through the central ring. These isomeric complexes have markedly different UV/Vis spectra, which was explained by analysis of the frontier orbitals. At the same time, the calculations suggest that electrostatic interactions are mainly responsible for the site selectivity of the coordination.

Pyridyl azine Schiff-base ligands exhibiting unexpected bonding modes towards ruthenium, rhodium and iridium half-sandwich complexes: Synthesis and structural studies

The reaction of multidentate azine Schiff-base ligands was investigated towards p-cymene ruthenium, Cp*Rh and Cp*Ir complexes. The reaction of [(arene)MCl2]2 (arene = p-cymene, Cp*; M = Ru, Rh and Ir) with azine Schiff-base ligands (L1-L3) in 1:2 M ratio led to the formation of mononuclear complexes of the type [(arene)M(L)Cl]+ whereas the reaction of [Cp*MCl2]2 (M = Rh/Ir) with azine Schiff-base ligands (L1-L3) in 1:1 M ratio afforded dinuclear rhodium and iridium complexes bearing formula [{Cp*MCl}2(μ-L)]+ (M = Rh/Ir). The ligands in dinuclear complexes exhibited interesting coordination modes towards the metal atom. The reaction of ligand L3 with [Cp*IrCl2]2 dimer yielded two coordination isomers whereas with [Cp*RhCl2]2 yielded exclusively one isomer. The molecular structure of both the isomers of the iridium complex has been established. In rhodium and iridium complexes the ligand L3 acted as uninegative pentadentate bridging ligand coordinating one metal center in a tridentate fashion and the other metal in a bidentate fashion. In the other isomer the ligand L3 behaved as uninegative tetradentate bridging ligand coordinating both iridium centers in a bidentate fashion. In the mononuclear complexes, the ligands are coordinated to the metal atom in a bidentate N∩N fashion through pyridine nitrogen and azine nitrogen. The ligand (L1 and L2) acted as tetradentate bridging ligand in dinuclear complexes. All these complexes were isolated and characterized by spectroscopic and analytical techniques.

Multiscale Modelling of Lytic Polysaccharide Monooxygenases.

Lytic polysaccharide monooxygenase (LPMO) enzymes have attracted considerable attention owing to their ability to enhance polysaccharide depolymerization, making them interesting with respect to production of biofuel from cellulose. LPMOs are metalloenzymes that contain a mononuclear copper active site, capable of activating dioxygen. However, many details of this activation are unclear. Some aspects of the mechanism have previously been investigated from a computational angle. Yet, either these studies have employed only molecular mechanics (MM), which are inaccurate for metal active sites, or they have described only the active site with quantum mechanics (QM) and neglected the effect of the protein. Here, we employ hybrid QM and MM (QM/MM) methods to investigate the first steps of the LPMO mechanism, which is reduction of CuII to CuI and the formation of a CuII–superoxide complex. In the latter complex, the superoxide can bind either in an equatorial or an axial position. For both steps, we obtain structures that are markedly different from previous suggestions, based on small QM-cluster calculations. Our calculations show that the equatorial isomer of the superoxide complex is over 60 kJ/mol more stable than the axial isomer because it is stabilized by interactions with a second-coordination-sphere glutamine residue, suggesting a possible role for this residue. The coordination of superoxide in this manner agrees with recent experimental suggestions.

Silybum marianum cell cultures stably transformed with Vitis vinifera stilbene synthase accumulate t-resveratrol in the extracellular medium after elicitation with methyl jasmonate or methylated β-cyclodextrins.

The growing demand for t-resveratrol for industrial uses has generated considerable interest in its production. Heterologous resveratrol production in plant cell suspensions, apart from requiring the introduction of only one or two genes, has the advantage of high biomass yield and a short cultivation time, and thus could be an option for large-scale production. Silybum marianum is the source of the flavonolignan silymarin. Phenylpropanoid synthesis in cultures of this species can be activated by elicitation with methyl jasmonate and methylated β-cyclodextrins, with products of the pathway (coniferyl alcohol and some isomers of the silymarin complex) being released into the medium. Given that stilbene synthase shares the same key precursors involved in flavonoid and /or monolignol biosynthesis, we explored the potential of metabolically engineered S. marianum cultures for t-resveratrol production. Cell suspensions were stably transformed with Vitis vinifera stilbene synthase 3 and the expression of the transgene led to extracellular t-resveratrol accumulation at the level of milligrams per litre under elicitation. Resveratrol synthesis occurred at the expense of coniferyl alcohol. Production of silymarin was less affected in the transgenic cultures, since the flavonoid pathway is limiting for its synthesis, due to the preferred supply of precursors for the monolignol branch. The fact that the expressed STS gene took excessively produced precursors of non-bioactive compounds (coniferyl alcohol), while keeping the metabolic flow for target secondary compounds (i.e. silymarin) unaltered, opens a way to extend the applications of plant cell cultures for the simultaneous production of both constitutive and foreign valuable metabolites.

Energetics of the S2 State Spin Isomers of the Oxygen-Evolving Complex of Photosystem II.

The S2 redox intermediate of the oxygen-evolving complex in photosystem II is present as two spin isomers. The S = 1/2 isomer gives rise to a multiline electron paramagnetic resonance (EPR) signal at g = 2.0, whereas the S = 5/2 isomer exhibits a broad EPR signal at g = 4.1. The electronic structures of these isomers are known, but their role in the catalytic cycle of water oxidation remains unclear. We show that formation of the S = 1/2 state from the S = 5/2 state is exergonic at temperatures above 160 K. However, the S = 1/2 isomer decays to S1 more slowly than the S = 5/2 isomer. These differences support the hypotheses that the S3 state is formed via the S2 state S = 5/2 isomer and that the stabilized S2 state S = 1/2 isomer plays a role in minimizing S2QA- decay under light-limiting conditions.

The Cobalt cyclo-P4 Sandwich Complex and Its Role in the Formation of Polyphosphorus Compounds.

A synthetic approach to the sandwich complex [Cp′′′Co(η4‐P4)] (2) containing a cyclo‐P4 ligand as an end‐deck was developed. Complex 2 is the missing homologue in the series of first‐row cyclo‐Pn sandwich complexes, and shows a unique tendency to dimerize in solution to form two isomeric P8 complexes [(Cp′′′Co)2(μ,η4:η2:η1‐P8)] (3 and 4). Reactivity studies indicate that 2 and 3 react with further [Cp′′′Co] fragments to give [(Cp′′′Co)2(μ,η2:η2‐P2)2] (5) and [(Cp′′′Co)3P8] (6), respectively. Furthermore, complexes 2, 3, and 4 thermally decompose forming 5, 6, and the P12 complex [(Cp′′′Co)3P12] (7). DFT calculations on the P4 activation process suggest a η3‐P4 Co complex as the key intermediate in the synthesis of 2 as well as in the formation of larger polyphosphorus complexes via a unique oligomerization pathway.

A theoretical study on the mechanistic highlights behind the Brønsted-acid dependent mer-fac isomerization of homoleptic carbenic iridium complexes for PhOLEDs.

Recently, a successful Brønsted-acid mediated geometric isomerization of the meridional homoleptic carbenic iridium(iii) complexes tris-(N-phenyl,N-methyl-benzimidazol-2-yl)iridium(iii) (1) and tris-(N-phenyl,N-benzyl-benzimidazol-2-yl)iridium(iii) (2) into their facial form has been reported. In the present work the pronounced acid-dependency of this particular isomerization procedure is revisited and additional mechanistic pathways are taken into account. Moreover, the acid-induced material decomposition is addressed. All calculations are carried out using density functional theory (DFT) while the environmental effects in solution are accounted for by the COSMO-RS model. The simulated results clearly reveal the outstanding importance of the complex interplay between acid strength, coordinating power of the corresponding base and the steric influence of the ligand system in contrast to the plain calculation of minimum energy pathways for selected complexes. Eventually, general rules to enhance the material-specific reaction yields are provided.

Detection of a higher energy isomer of the CO-N2O van der Waals complex and determination of two of its intermolecular frequencies.

Infrared spectra in the carbon monoxide CO stretch region (∼2150 cm-1) and in the ν3 asymmetric stretch region of N2O (∼2223 cm-1) are assigned to the previously unobserved O-bonded form of the CO-N2O dimer ("isomer 2"). This van der Waals complex has a planar skewed T-shaped structure like that of the previously observed C-bonded form ("isomer 1"), but with the CO rotated by 180°. The effective intermolecular distance between the centers of mass is 3.51 Å for isomer 2 as compared to 3.88 Å for isomer 1. In addition to the fundamental band, two combination bands are observed for isomer 2, yielding values for two intermolecular vibrational modes: 14.502(5) cm-1 for the coupled disrotatory motion or the uncoupled CO rock and 21.219(5) cm-1 for the out-of-plane rock. We show that the published ab initio study on this system is inadequate in predicting the intermolecular frequencies for isomer 2 of CO-N2O.

Capillary electrophoretic separation of cis/trans isomers of bis(o-diiminobenzoquinonato)platinum(ii) complexes using β-cyclodextrins as the selector

Capillary electrophoresis was employed to successfully resolve the cis/trans isomers of PtII-diradical complexes using β-cyclodextrins as the selector, allowing for the estimation of the inclusion constant and the representation of the inclusion complex through mobility.

Alkyne activation and polyhedral reorganization in benzothiazolate-capped osmium clusters on reaction with diethyl acetylenedicarboxylate (DEAD) and ethyl propiolate

The reactivity of the face-capped benzothiazolate clusters HOs3(CO)9[μ3-C7H3(R)NS] (1a, R = H; 1b, R = 2-CH3) with alkynes has been investigated. 1a reacts with DEAD at 67 °C to furnish the isomeric alkenyl clusters Os3(CO)9(μ-C7H4NS)(μ3-EtO2CCCHCO2Et) (2a and 3a). X-ray crystallographic analyses of 2a and 3a have confirmed the stereoisomeric relationship of these products and the regiospecific polyhedral expansion that follows the formal transfer of the hydride to the coordinated alkyne ligand in HOs3(CO)9(μ-C7H4NS)(η2-DEAD). The significant structural differences between the two isomers, as revealed by the solid-state structures, derives from the regiospecific cleavage of one of the three Os-Os bonds in the intermediate alkenyl cluster Os3(CO)9(μ-C7H4NS)(η1-EtO2CCCHCO2Et), which follows hydride transfer to the coordinated alkyne ligand in the pi compound HOs3(CO)9(μ-C7H4NS)(η2-DEAD). Control experiments confirm the reversibility of the reaction leading to the formation of 2a and 3a. Whereas heating either isomer in refluxing THF or benzene affords a binary mixture containing 2a and 3a, thermolysis in refluxing toluene leads to the activation of the alkenyl ligand and formation of the new cluster Os3(CO)9(μ-C7H4NS)(μ3-EtO2CCCH2) (4). 4 was independently synthesized from 1a and ethyl propiolate at room temperature. The computed mechanisms that account for the formation of 2a and 3a are presented, along with the mechanism for the reaction of 1a with ethyl propiolate to give 4.

Cisplatin Primary Complex with l-Histidine Target Revealed by IR Multiple Photon Dissociation (IRMPD) Spectroscopy.

The primary complex obtained from cisplatin and l-histidine in water has been detected and isolated by electrospray ionization. The so-obtained cis-[PtCl(NH3 )2 (histidine)]+ complex has been characterized in detail by high-resolution mass spectrometry (MS), tandem MS, IR multiple photon dissociation (IRMPD) spectroscopy, and by quantum chemical calculations. The structural features revealed by IRMPD spectroscopy indicate that platinum binds to the imidazole group, which presents tautomeric forms. Thus, depending on the position of the amino acid pendant on the imidazole ring, isomeric complexes are formed that are remarkably different with respect to the ease with which they undergo fragmentation when activated either by energetic collisions or by multiple IR photon absorption. It is shown here how IRMPD kinetics can allow their relative proportions to be estimated.

Distinguishing Isomeric Peptides: The Unimolecular Reactivity and Structures of (LeuPro)M+ and (ProLeu)M+ (M = Alkali Metal).

The unimolecular chemistries and structures of gas-phase (ProLeu)M+ and (LeuPro)M+ complexes when M = Li, Na, Rb, and Cs have been explored using a combination of SORI-CID, IRMPD spectroscopy, and computational methods. CID of both (LeuPro)M+ and (ProLeu)M+ showed identical fragmentation pathways and could not be differentiated. Two of the fragmentation routes of both peptides produced ions at the same nominal mass as (Pro)M+ and (Leu)M+, respectively. For the litiated peptides, experiments revealed identical IRMPD spectra for each of the m/z 122 and 138 ions coming from both peptides. Comparison with computed IR spectra identified them as the (Pro)Li+ and (Leu)Li+, and it is concluded that both zwitterionic and canonical forms of (Pro)Li+ exist in the ion population from CID of both (ProLeu)Li+ and (LeuPro)Li+. The two isomeric peptide complexes could be distinguished using IRMPD spectroscopy in both the fingerprint and the CH/NH/OH regions. The computed IR spectra for the lowest energy structures of each charge solvated complexes are consistent with the IRMPD spectra in both regions for all metal cation complexes. Through comparison between the experimental spectra, it was determined that in lithiated and sodiated ProLeu, metal cation is bound to both carbonyl oxygens and the amine nitrogen. In contrast, the larger metal cations are bound to the two carbonyls, while the amine nitrogen is hydrogen bonded to the amide hydrogen. In the lithiated and sodiated LeuPro complexes, the metal cation is bound to the amide carbonyl and the amine nitrogen while the amine nitrogen is hydrogen bonded to the carboxylic acid carbonyl. However, there is no hydrogen bond in the rubidiated and cesiated complexes; the metal cation is bound to both carbonyl oxygens and the amine nitrogen. Details of the position of the carboxylic acid C═O stretch were especially informative in the spectroscopic confirmation of the lowest energy computed structures.

Differentiating Two Nitrosylruthenium Isomeric Complexes by Steady-State and Ultrafast Infrared Spectroscopies.

The [Ru(II)-NO+] group affects the structure and chemical reactivity of nitrosylruthenium(II) complexes. A characteristic infrared absorption band due to the nitrosyl (NO) stretching motion is shown in the frequency region 1800-1900 cm-1. In this work, linear infrared (IR) and nonlinear IR methods, including pump-probe and two-dimensional (2D) IR, were utilized to study the structures and dynamics of two isomeric nitrosylruthenium complexes [Ru(OAc)(2mqn)2NO] (H2mqn = 2-methyl-8-quinolinol) in cis and trans isomeric configurations in a weak polar solvent (CDCl3). Using the NO stretching mode as a vibrational probe, information about local structural dynamics of the Ru complex as well as solvent fluctuation dynamics was obtained. In particular, a "structured" solvent environment is believed to form in the vicinity of the NO group in the case of the cis isomer with the aid of a neighboring OAc ligand, which is the reason for more efficient vibrational relaxation but more inhomogeneously distributed solvent and thus associated slower spectral diffusion. Our results also suggest a more anharmonic potential surface for the NO stretching mode in the less stable trans isomer.

Structure and properties of the volatile isomers of bis(1,1,1-trifluoro-5,5-dimethylhexane-2,4-dionato) of platinum(II)

In the work, isomeric complexes of platinum(II) with the (ptac)–1 pivaloyltrifluoroacetonate ion (Pt((CH3)3–CO–CH–CO–CF3)2) are studied. The synthesis and chromatographic separation of Pt(ptac)2 isomers are described, TGA data for the separated isomers are given, and the crystal structures of the solid phases are studied. The cis-Pt(ptac)2 complex crystallizes in the space group P-1, a = 10.7091(4) A, b = 12.7787(6) A, c = 16.0154(8) A, α = 92.389(2)°, β = 90.868(2)°, γ = 112.1260(10)°, V = 2027.39(16) A3, Z = 4, dcalc = 1.918 g/cm3. The trans-Pt(ptac)2 complex crystallizes in the space group C2/m, a = 13.3235(5) A, b = 8.5515(3) A, c = 9.6694(3) A, β = 118.5880(10)°, V = 967.38(6) A3, Z = 2, dcalc = 2.010 g/cm3. The structures of the complexes are molecular, the Pt atom has a square coordination of four oxygen atoms of two ligands; for cis-Pt(ptac)2, the Pt–Oav distance is 1.968 A, for trans-Pt(ptac)2 it is 1.980 A.

Long-Lived Five-Coordinate Platinum(IV) Intermediates: Regiospecific C–C Coupling

Three different phosphine derivatives of doubly cyclometalated diphenypyridine complexes of Pt(II), 1, were reacted with methyl iodide to give octahedral Pt(IV) complexes, 2, as two isomers. Treatment of either isomer of complexes 2 with AgBF4, to abstract iodide, gave long-lived five-coordinate complexes 3, which could be trapped as pyridine adducts. Complexes 3 underwent a C–C coupling reaction, at a rate that depended on phosphine size, to give a methyl group attached to the original diphenylpyridine. Recylometalation was then performed, and the cycle of reactions was repeated to give a diphenylpyridine doubly methylated on only one phenyl, with complete regiospecificity. NMR was used to demonstrate the geometry of all complexes in solution, with multiple X-ray crystal structures confirming these assignments.

Light-triggered Supramolecular Isomerism in a Self-catenated Zn(II)-organic Framework: Dynamic Photo-switching CO2 Uptake and Detection of Nitroaromatics

A self-catenated Zn(II)-organic framework formulated as [Zn2(3,3′-bpeab)(oba)2]·DMF (1) exhibiting a six-connected 44·610·8 topology has been successfully synthesized through the mixed-ligand of kinked 3,3′-bis[2-(4-pyridyl)ethenyl]azobenzene (3,3′-bpeab) and 4,4′-oxybis-benzoic acid (H2oba) under solvothermal condition. UV light triggers isomerization of complex 1 in a single-crystal-to-single-crystal (SCSC) manner, giving rise to a conformational supramolecular isomer 1_UV through the pedal motion of photoresponsive double bonds. Dynamic photo-switching in the obtained light-responsive supramolecular isomers leads to instantly reversible CO2 uptake. Furthermore, the ligand originated fluorescence emission of water-resistant complex 1 is selectively sensitive to 4-nitrotoluene (4-NT) owing to a higher quenching efficiency of the perilous explosive over other structurally similar nitroaromatics, prefiguring the potentials of 1 as a fluorescence sensor towards 4-NT in aquatic media.

Mechanism of Alkyl Migration in Diorganomagnesium 2,6-Bis(imino)pyridine Complexes: Formation of Grignard-Type Complexes with Square-Planar Mg(II) Centers

Dialkylmagnesium compounds [MgR2L2] (R = n-Bu, L = none or R = Bn, L = THF) react with 2,6-bis(imino)pyridines (BIP) to afford different types of Mg(II) alkyl complexes, depending on the nature of R. For R = n-Bu, thermally stable products resulting from selective alkyl transfer to the pyridine nitrogen (N1) atom are obtained. However, NMR studies showed that the reaction of [Mg(Bn)2THF2] with iPrBIP at −65 °C leads to a thermally unstable product arising from benzyl migration to position C2 in the pyridine ring. Above +5 °C, this compound rearranges, cleanly yielding a mixture of two isomeric complexes, in which the benzyl group has migrated to positions C3 or C4 of the central ring, respectively. Similar isomeric mixtures were obtained when [Mg(Bn)2THF2] was reacted with iPrBIP or MesBIP at room temperature. Such mixtures are thermally stable below 80 °C, but at this temperature, the 3-benzyl isomer converts into the thermodynamically favored 4-benzyl product, albeit not quantitatively. An alternate rou...