Syn Structure Research Articles

Probing bilin-protein interaction in the protochromic photocycle of cyanobacteriochrome RcaE by site-directed mutagenesis.

Cyanobacteriochromes (CBCRs) are members of the phytochrome superfamily of photosensor proteins that bind a bilin chromophore. CBCRs exhibit substantial diversity in their absorption wavelengths through a variety of bilin-protein interactions. RcaE is the first discovered cyanobacteriochrome as a regulator of chromatic acclimation, where cyanobacteria optimize the absorption wavelength of their photosynthetic antenna. RcaE undergoes a reversible photoconversion between a green-absorbing (Pg) and a red-absorbing (Pr) states, where the bilin chromophore adopts a deprotonated C15-Z,anti and a protonated C15-E,syn structures, respectively. This photocycle is designated as "protochromic photocycle" as the change of the bilin protonation state is responsible for the large absorption shift. With the guidance of recently determined Pg and Pr structures of RcaE, in this study, we investigated bilin-chromophore interaction by site-directed mutagenesis on three key residues referred to as a protochromic triad and also other conserved residues interacting with the bilin. Among the protochromic triad residues, Glu217 and Lys261 are critical for the formation of the Pr state, while Leu249 is critical for the formation of both Pg and Pr states. Substitution in other conserved residues, including Val218, Phe219, and Pro220 in the wind-up helix and Phe252, Phe214, and Leu209 in a part of the bilin-binding pocket, had less substantial effects on the spectral sensitivity in RcaE. These data provide insights into our understanding of the bilin-chromophore interaction in the protochromic photocycle and also its evolution in the CBCRs.

Green/red light-sensing mechanism in the chromatic acclimation photosensor.

Certain cyanobacteria alter their photosynthetic light absorption between green and red, a phenomenon called complementary chromatic acclimation. The acclimation is regulated by a cyanobacteriochrome-class photosensor that reversibly photoconverts between green-absorbing (Pg) and red-absorbing (Pr) states. Here, we elucidated the structural basis of the green/red photocycle. In the Pg state, the bilin chromophore adopted the extended C15-Z,anti structure within a hydrophobic pocket. Upon photoconversion to the Pr state, the bilin is isomerized to the cyclic C15-E,syn structure, forming a water channel in the pocket. The solvation/desolvation of the bilin causes changes in the protonation state and the stability of π-conjugation at the B ring, leading to a large absorption shift. These results advance our understanding of the enormous spectral diversity of the phytochrome superfamily.

NMR evidence for hydrogen bonding stabilized anti conformation of 1-methoxy-1-methyl-3-phenylurea and the concentration detection by SERS

Phenylurea-based herbicides have been widely used to control the plant growth. Due to their toxicity, strict regulations to limit their usage have been applied. Developing trace detection protocols of substituted phenylureas has attracted more attention, especially with the availability of a wide range of spectroscopic and analytical techniques. In this study, the conformational and structural properties of 1-methoxy-1-methyl-3-phenylurea (MMPU) which belongs to the family of phenylurea-based herbicides have been characterized. The anti-configuration, facilitated by the intramolecular hydrogen bonding between the oxygen atom of the methoxy moiety and the acidic amide proton, was observed using the nuclear magnetic resonance (NMR) spectroscopy through the Selective Nuclear Overhauser Effect (SelNOE) approach. The density functional theory (DFT) calculations suggested that the anti form is at least 9 kcal/mol more stable than the corresponding syn structure. Possible degradation pathways of such a class of herbicides can be explored when a firm understanding of their electronic and conformational behaviors is established. The detection of trace MMPU has been successfully developed using the surface-enhanced Raman scattering (SERS) spectroscopic tool. The most enhanced peak associated with the carbonyl group stretching vibration which appears nearly at 1600 cm−1 has presented a Raman enhancement factor of 3.24 × 105 and a micrommolar limit of detection.

Insight into the molecular mechanism underlying the inhibition of α-synuclein aggregation by hydroxytyrosol

Parkinson’s disease (PD) is the second most prevalent neurodegenerative disease in the elderly people. To date, drugs able to reverse the disease are not available; the gold standard is levodopa that only relieves clinical symptoms, yet with severe side effects after prolonged administration. Many efforts are underway to find alternative targets for PD prevention or treatment, the most promising being α-synuclein (Syn). Recently, we reported that oleuropein aglycone (OleA) interferes with amyloid aggregation of Syn both stabilizing its monomeric state and inducing the formation of harmless, off-pathway oligomers.This study is focused at describing the interaction between Syn and hydroxytyrosol (HT), the phenolic moiety and main metabolite of OleA, and the interferences with Syn aggregation by using biophysical and biological techniques. Our results show that HT dose-dependently inhibits Syn aggregation and that covalent and non-covalent binding mediate HT-Syn interaction. HT does not modify the natively unfolded structure of Syn, rather, it stabilizes specific regions of the molecule leading to inhibition of protein fibrillation. Cellular assays showed that HT reduces the toxicity of Syn aggregates. Moreover, Syn aggregates interaction with the cell membrane, an important factor for prion-like properties of Syn on-pathway oligomers, was reduced in cells exposed to Syn aggregates grown in the presence of HT.

Bicyclo[3.2.0]heptane as a Core Structure for Conformational Locking of 1,3-Bis-Pharmacophores, Exemplified by GABA.

The synthesis and X-ray crystal structures of syn and anti 4-N-Boc-aminobicyclo[3.2.0]heptane-1-carboxylic acids are described. The placement of the N-Boc-amino groups in the two stereoisomers in either pseudo-equatorial or pseudo-axial positions renders the molecules conformationally locked, with N-Boc-protected γ-aminobutyric acid (GABA) embedded within the bicyclic core. Despite the different conformations of the urethane and distinct crystal packing, the bicyclic core units of the two stereoisomers adopt virtually identical structures. They correspond to in silico models of the parent bicyclic core and a systematic array of disubstituted derivatives. The study documents an intrinsic property of the bicyclo[3.2.0]heptane core to favor adoption of a boat-like conformation, which is largely unaffected by various substitution patterns. The structural concepts are useful in the design of molecules with spatial and directional fixation of pharmacophoric groups.

Conformational and Molecular Structures of α,β-Unsaturated Acrylonitrile Derivatives: Photophysical Properties and Their Frontier Orbitals.

We report single crystal X-ray diffraction (hereafter, SCXRD) analyses of derivatives featuring the electron-donor N-ethylcarbazole or the (4-diphenylamino)phenyl moieties associated with a -CN group attached to a double bond. The compounds are (2Z)-3-(4-(diphenylamino)-phenyl)-2-(pyridin-3-yl)prop-2-enenitrile (I), (2Z)-3-(4-(diphenylamino)phenyl)-2-(pyridin-4-yl)-prop-2-enenitrile (II) and (2Z)-3-(9-ethyl-9H-carbazol-3-yl)-2-(pyridin-2-yl)enenitrile (III). SCXRD analyses reveal that I and III crystallize in the monoclinic space groups P2/c with Z’ = 2 and C2/c with Z’ = 1, respectively. Compound II crystallized in the orthorhombic space group Pbcn with Z’ = 1. The molecular packing analysis was conducted to examine the pyridine core effect, depending on the ortho, meta- and para-positions of the nitrogen atom, with respect to the optical properties and number of independent molecules (Z’). It is found that the double bond bearing a diphenylamino moiety introduced properties to exhibit a strong π-π-interaction in the solid state. The compounds were examined to evaluate the effects of solvent polarity, the role of the molecular structure, and the molecular interactions on their self-assembly behaviors. Compound I crystallized with a cell with two conformers, anti and syn, due to interaction with solvent. DFT calculations indicated the anti and syn structures of I are energetically stable (less than 1 eV). Also electrochemical and photophysical properties of the compounds were investigated, as well as the determination of optimization calculations in gas and different solvent (chloroform, cyclohexane, methanol, ethanol, tetrahydrofuran, dichloromethane and dimethyl sulfoxide) in the Gaussian09 program. The effect of solvent by PCM method was also investigated. The frontier HOMO and LUMO energies and gap energies are reported.

Spectroscopic and Thermodynamic Properties of Some Transition Metal Complexes Derived from 2-(Hydroxyimino)-1-(2-hydroxyphenyl)butane-1,3-dione

The protonation constants of the syn ketoxime, 2-(hydroxyimino)-1-(2-hydroxyphenyl)butane-1,3-dione (HPDO) were determined by a pH-metric method in solutions of dioxane, methanol and dimethylformamide (75 % v/v) at 20 ± 0.1 °C and ionic strength of 0.1 mol·L−1 supported by KNO3. The complexation reaction of the ligand HPDO with the transition metal ions Cu(II), Ni(II), Co(II), Cr(III), Fe(III), and Mn(II) in solution has been also studied potentiometrically. The stability constants of the respective species were determined and show that their values decrease in the order: 1,4-dioxane < DMF < methanol. The stability of the chelates is highly affected by the relative permittivity (er) and the donor number of the solvents. In addition, the complexes of [Cu(PDO)(Cl)]2·EtOH 1, [Ni(PDO)(H2O)(Cl)]2·2H2O 2, [Co(PDO)(Cl)]2·EtOH 3, [Fe(PDO)(Cl)2(EtOH)3]·EtOH 4, and [VO(PDO)H2O]2SO4·4H2O 5 were prepared and characterized by elemental analysis, IR, UV/Vis spectra, magnetic measurements and thermal analysis. Magnetic and spectral data suggest the formation of (i) neutral polymerized molecules accompanied by inclusion of lattice or coordinated water molecules, and (ii) a paramagnetic octahedral structure around metal ions in all cases. The geometry optimization of the syn structure of the ligand HPDO has been also calculated by HyperChem 8.0 software.

Stereolability of chiral ruthenium catalysts with frozen NHC ligand conformations investigated by dynamic-HPLC.

The stereolability of chiral Hoveyda-Grubbs II type ruthenium complexes bearing N-heterocyclic carbene (NHC) ligands with Syn-phenyl groups on the backbone and Syn- or Anti-oriented o-tolyl N-substituents was studied by resorting to dynamic high-performance liquid chromatography (D-HPLC). A complete chromatographic picture of the involved stereoisomers (four for Anti- and two for Syn-complexes) was achieved at very low temperatures (-53°C and -40°C respectively), at which the NHC-Ru bond rotations were frozen out. Inspection of the chromatographic profiles recorded at higher temperatures revealed the presence of plateau zones between the couples of either Syn or Anti stereoisomers, attesting to the active interconversion between the eluted species. Such dynamic chromatograms were successfully simulated through procedures based on both theoretical plate and classical stochastic models. The good superimposition achieved between experimental and simulated chromatographic profiles allowed determination of the related isomerization energy barriers (ΔGisom (#) ), all derived by rotation around the NHC-Ru bond. The obtained diastereomerization barriers between the Anti isomers were found in very good agreement with those previously measured by experimental nuclear magnetic resonance (NMR) and assessed through Density Functional Theory (DFT) calculations. With the same approach, for the first time we also determined the enantiomerization barrier of the Syn isomer. Focused changes to the structure of complex Syn, studied by a molecular modeling approach, were found suitable to strongly reduce the stereolability arising from rotation around the NHC-Ru bond.

HAl(OH)2 molecular structures and reaction paths. Post-Hartree–Fock, DFT calculations and infrared spectroscopic

Quantum chemical calculations were performed to determine the structures and formation path for the HAl(OH)2 molecule. The analyses of the anharmonic frequencies as well the potential energy surface (PES) using coupled cluster, Møller–Plesset and density functional calculations, support the idea that the Syn structure proposed in the literature for HAl(OH)2 is not right. Unlike the Syn conformer, the Endo–Exo structure is consistent with the PES, the statistical thermodynamic results and infrared spectra. Different reaction channels for the reaction of laser-ablated Al atoms with a H2+O2 mixture in gas phase were analyzed. The results show that there are three possible reaction channels that involve two possible intermediates for the reaction of Al with H2+O2.

Electron diffraction analysis for the molecules with multiple large-amplitude motions. 3-nitrostyrene--a molecule with two internal rotors.

Dynamic structural analysis of the molecules possessing large-amplitude degrees of freedom has been attempted by many researchers; however, so far, electron diffraction investigations involved only one large-amplitude coordinate (internal rotation or bending). The current state of computational facilities allows extending of the general dynamic approach to the systems possessing two or more large-amplitude motions. This paper presents the first practical implementation of the theoretical method developed previously by the authors for solving the dynamic-structural problem with two or more large-amplitude coordinates; the procedure is applied to a molecule of 3-nitrostyrene. The molecule is represented as a set of pseudoconformers built on a two-dimensional grid corresponding to both internal rotation coordinates present in the molecule (with 10-30° steps by each angle); altogether, up to 342 pseudoconformers were used. Structural analysis was based on the experimental electron diffraction data supported by quantum chemical calculations (at the MP2 and B3LYP levels of theory) and molecular spectroscopy data. Quantum chemistry predicts the planar structure of both syn- and anti- stable conformations with close energies and weak interaction between internal rotations of nitro and vinyl groups. The gas-phase electron diffraction experimental data are compatible with the quantum chemical predictions. The principal equilibrium geometry parameters of the molecule (syn- conformation) have been determined as follows: r(e)(C-C)(ring, avg.) = 1.391(1) Å, r(e)(C-C) = 1.477(5) Å, r(e)(C═C) = 1.333(7) Å, r(e)(C-N) = 1.463(5) Å, r(e)(N═O) = 1.227(3) Å, ∠e(O═N═O) = 124.3 (4)°. Experimental data for this molecule are insufficient to make estimates of the barrier heights of internal rotation; the population ratio of syn- and anti- conformations is evaluated as 50 ± 20%. Results of our investigation confirm the presence of significant internal rotations in the 3-nitrostyrene molecule.

Predicted organic noble-gas hydrides derived from acrylic acid.

The Ar-, Kr-, and Xe-insertion compounds into acrylic acid, i.e., C2H3COONgH (Ng = Ar, Kr, and Xe), have been studied by ab initio calculations. The geometry optimization, frequency calculation, and stability were investigated at the MP2/aug-cc-pVTZ (aug-cc-pVTZ-PP) level of theory. Two configuration isomers, i.e., s-cis syn (isomer A) and s-cis anti (isomer B), were optimized for each molecule. Using the s-cis syn structure (isomer A) as an example, we performed natural bond orbital (NBO) analysis, natural energy decomposition analysis (NEDA), and atom-in-molecules (AIM) analysis to investigate the bonding nature of these noble-gas compounds. Our study predicts the existence of Kr- and Xe-derivatives of acrylic acid and the instability of Ar-related compounds.

Strongly Luminous Tetranuclear Gold(I) Complexes Supported by Tetraphosphine Ligands, meso‐ or rac‐Bis[(diphenylphosphinomethyl)phenylphosphino]methane

A series of tetragold(I) complexes supported by tetraphosphine ligands, meso- and rac-bis[(diphenylphosphinomethyl)phenylphosphino]methane (meso- and rac-dpmppm) were synthesized and characterized to show that the tetranuclear Au(I) alignment varies depending on syn- and anti-arrangements of the two dpmppm ligands with respect to the metal chain. The structures of syn-[Au4 (meso-dpmppm)2X]X'3 (X = Cl; X' = Cl (4 a), PF6 (4 b), BF4 (4 c)) and syn-[Au4 (meso-dpmppm)2]X4 (X = PF6 (4 d), BF4 (4 e), TfO (4 f); TfO = triflate) involved a bent tetragold(I) core with a counter anion X incorporated into the bent pocket. Complexes anti-[Au4 (meso-dpmppm)2]X4 (X = PF6 (5 d), BF4 (5 e), TfO (5 f)) contain a linearly ordered Au4 string and complexes syn-[Au4 (rac-dpmppm)2X2]X'2 (X = Cl, X' = Cl (6 a), PF6 (6 b), BF4 (6 c)) and syn-[Au4 (rac-dpmppm)2]X4 (X = PF6 (6 d), BF4 (6 e), TfO (6 f)) consist of a zigzag tetragold(I) chain supported by the two syn-arranged rac-dpmppm ligands. Complexes 4 d-f, 5 d-f, and 6 d-f with non-coordinative large anions are strongly luminescent in the solid state (λmax = 475-515 nm, Φ = 0.67-0.85) and in acetonitrile (λmax = 491-520 nm, Φ = 0.33-0.97); the emission was assigned to phosphorescence from (3) [dσ*σ*σ* pσσσ] excited state of the Au4 centers on the basis of DFT calculations as well as the long lifetime (a few μs). The emission energy is predominantly determined by the HOMO and LUMO characters of the Au4 centers, which depend on the bent (4), linear (5), and zigzag (6) alignments. The strong emissions in acetonitrile were quenched by chloride anions through simultaneous dynamic and static quenching processes, in which static binding of chloride ions to the Au4 excited species should be the most effective. The present study demonstrates that the structures of linear tetranuclear gold(I) chains can be modified by utilizing the stereoisomeric tetraphosphines, meso- and rac-dpmppm, which may lead to fine tuning of the strongly luminescent properties intrinsic to the Au(I) 4 cluster centers.

Reactivity of [PtCl(η2-C2H4)(N-N)]+, N-N = diimine ligand, with phenol derivatives and first comparison between single crystal X-ray structures of syn- and anti-[Pt(N-N)(phenolate)2] rotamers in the solid state

In this work it is described a new synthetic pathway for the synthesis of bis-cresolate Pt(II) complexes with (N-N) dinitrogen ligands. In particular, we were able, for the first time, to isolate, in the solid state, both anti-[Pt(N-N)(OC6H4-4-Me)2], N-N=1,10-phenanthroline, (head to tail) and syn-[Pt(N-N)(OC6H4-4-Me)2], N-N=2,2′-bipyridyl, (head to head) rotamers, which were characterized as different rotamers in the solid state by single crystal X-ray diffraction.

Stereoisomerization of α-hydroxy-β-sulfenyl-α,β-dimethyl naphthoquinones controlled by nonbonded sulfur–oxygen interactions

The anti α-hydroxy-β-sulfenyl-α,β-dimethyl naphthoquinones isomerize in basic media into syn/anti mixtures of isomers, giving the syn isomer as the major product. Conversely, anti α-hydroxy-β-alkoxy-α,β-dimethyl naphthoquinones isomerize to furnish the anti isomer as the major product. The crystal structure of syn α-hydroxy-β-phenylsulfenyl-α,β-dimethyl naphthoquinone has been determined. The X-ray and experimental work demonstrated that an attractive 1,4 intramolecular interaction of divalent sulfur with hydroxyl oxygen is the driving force for the aforementioned stereochemical preference.

Position matters: High resolution spectroscopy of 6-methoxyindole

The structures of syn and anti 6-methoxyindole have been determined in the electronic ground and excited states using rotationally resolved electronic spectroscopy and high level ab initio calculations. Second order coupled cluster theory predicts the lowest excited singlet states to be heavily mixed and the transition dipole moments to depend strongly on the geometries. From the analysis of the rovibronic spectra of seven isotopomers, the absolute orientation of the transition dipole moment within the principle axis frame was determined to be L(b)-like for both conformers.

Influence of pH on the structural optical and magnetic properties of Zn1−xMnxO thin films grown by sol–gel method

In this research, Zn1−xMnxO (0⩽x⩽0.2) thin films were synthesized by sol–gel technique for ferromagnetic and paramagnetic responses at high and low temperatures. In this respect, the produced films were characterized through X-ray diffraction (XRD), scanning electron microscope (SEM), UV–vis spectrometer and superconducting quantum interference device (SQUID) magnetometer. Based on the characterizations and measurements, the influence of pH on the structural, optical and magnetic properties of a Zn1−xMnxO thin films was scrutinized in detail. The analyzed results indicate that the films possess a good crystal quality and homogeneous, smooth surfaces with a pure hexagonal zincsite syn structure of ZnO without any Mn based phases. By increasing the Mn content, the films exhibited increase or decrease in both the c-axis lattice constant and fundamental band gap energy depending on their respective pH. It is clear that the Zn1−xMnxO (x=0.02 and 0.05) thin films prepared at pH of 4.6 exhibited both a ferromagnetic at high temperatures (100, 200, and 300K) and a paramagnetic response at low temperature (5K), whilst Zn1−xMnxO (0.01⩽x⩽0.2) films prepared at pH value of 7.56 have paramagnetic behavior and antiferromagnetic contributions. The observed room temperature ferromagnetic behavior of the Zn1−xMnxO (x=0.02 and 0.05) thin films at pH of 4.6 may be related to the interaction between Mn doping and the oxygen vacancies, an oxygen-vacancy-stabilized metastable phase Mn2−xZnxO3−d or clustering of Mn ions.

Z-Selective Metathesis Homocoupling of 1,3-Dienes by Molybdenum and Tungsten Monoaryloxide Pyrrolide (MAP) Complexes

Molybdenum or tungsten monoaryloxide pyrrolide (MAP) complexes that contain OHIPT as the aryloxide (hexaisopropylterphenoxide) are effective catalysts for homocoupling of simple (E)-1,3-dienes to give (E,Z,E)-trienes in high yield and with high Z selectivities. A vinylalkylidene MAP species was shown to have the expected syn structure in an X-ray study. MAP catalysts that contain OHMT (hexamethylterphenoxide) are relatively inefficient.

Vibrational properties of some elusive members of the R[sbnd]N[dbnd]S[dbnd]O family: CF3[sbnd]N[dbnd]S[dbnd]O and SF5[sbnd]N[dbnd]S[dbnd

The vibrational spectra of N-(trifluoromethyl)sulfinylamine, CF3NSO and N-(pentafluorsulfanyl)sulfinylamine, SF5NSO have been recorded and assigned. According to quantum chemical calculations (B3LYP and MP2 with 6-31G(d) and 6-311+G(df) basis sets) the sterically unfavorable syn (Z-) conformers are 2.6–4.1kcal/mol (CF3NSO) and 3.1–4.6kcal/mol (SF5NSO) lower in free energy than the anti conformers. Such ΔG° values determine an equilibrium of 98.8% (syn) and 1.2% (anti) for CF3NSO, and ≈99% (syn) and<1% (anti) for SF5NSO (B3LYP and MP2 with 6-311+G(df) basis set). The presence of a single conformer is confirmed by the vibrational spectra. The syn structure of RNSO compounds is stabilized by strong anomeric effects between the N and S electron lone pairs and the opposite NR and SO σ* bonds. The bonding properties of some RNSO and RNS(O)F2 compounds are compared.

Complete Family of Mono-, Bi-, and Trinuclear ReI(CO)3Cl Complexes of the Bridging Polypyridyl Ligand 2,3,8,9,14,15-Hexamethyl-5,6,11,12,17,18-hexaazatrinapthalene: Syn/Anti Isomer Separation, Characterization, and Photophysics

The syn and anti isomers of the bi- and trinuclear Re(CO)(3)Cl complexes of 2,3,8,9,14,15-hexamethyl-5,6,11,12,17,18-hexaazatrinapthalene (HATN-Me(6)) are reported. The isomers are characterized by (1)H NMR spectroscopy and X-ray crystallography. The formation of the binuclear complex from the reaction of HATN-Me(6) with 2 equiv of Re(CO)(5)Cl in chloroform results in a 1:1 ratio of the syn and anti isomers. However, synthesis of the trinuclear complex from the reaction of HATN-Me(6) with 3 equiv of Re(CO)(5)Cl in chloroform produces only the anti isomer. syn-{(Re(CO)(3)Cl)(3)(μ-HATN-Me(6))} can be synthesized by reacting 1 equiv of Re(CO)(5)Cl with syn-{(Re(CO)(3)Cl)(2)(μ-HATN-Me(6))} in refluxing toluene. The product is isolated by subsequent chromatography. The X-ray crystal structures of syn-{(Re(CO)(3)Cl)(2)(μ-HATN-Me(6))} and anti-{(Re(CO)(3)Cl)(3)(μ-HATN-Me(6))} are presented both showing severe distortions of the HATN ligand unit and intermolecular π stacking. The complexes show intense absorptions in the visible region, comprising strong π → π* and metal-to-ligand charge-transfer (MLCT) transitions, which are modeled using time-dependent density functional theory (TD-DFT). The energy of the MLCT absorption decreases from mono- to bi- to trinuclear complexes. The first reduction potentials of the complexes become more positive upon binding of subsequent Re(CO)(3)Cl fragments, consistent with changes in the energy of the MLCT bands and lowering of the energy of relevant lowest unoccupied molecular orbitals, and this is supported by TD-DFT. The nature of the excited states of all of the complexes is also studied using both resonance Raman and picosecond time-resolved IR spectroscopy, where it is shown that MLCT excitation results in the oxidation of one rhenium center. The patterns of the shifts in the carbonyl bands upon excitation reveal that the MLCT state is localized on one rhenium center on the IR time scale.

Influence of arene dissociation and phosphine coordination on the catalytic activity of [RuCl( κ2-triphos)( p-cymene)]PF 6

The catalytic activity of a ruthenium(II)- p-cymene complex containing a partially coordinated triphosphine ligand, [RuCl( κ 2-triphos)( p-cymene)]PF 6 1, has been investigated in the hydrogenation of styrene to ethylbenzene. The influence of arene dissociation and coordination of the free phosphine donor group on the catalytic activity have been probed directly and indirectly by comparison to structural analogues. Analogues of 1 containing in a diphosphine ligand, [RuCl( κ 2-dppp)( p-cymene)]PF 6 2, or a labile arene ligand, [RuCl( κ 2-triphos)( η 6-PhCO 2Et)]PF 6 3, show significantly enhanced catalytic activity – demonstrating the importance of ligand coordination/dissociation dynamics in ruthenium(II)-arene compounds during catalysis. These observations are supported by thermolysis reactions of 1 in DMSO. In addition, improved syntheses of 1 and 2 are reported together with the solid-state structures of syn- 1, syn- 3 and [Ru( η 3-C 8H 13)( κ 3-triphos)]PF 6.