Synthesis,structure and fluxional behaviour of isomeric bis(trimethylstannyl)dihydropentalenes

Interaction of dihydropentalene (IV) with trimethylstannyldiethylamide in molar ratios of 1:1 or 1:2 leads to mono- or bis-organotin derivatives of IV, respectively. X-ray analysis of trans(E)-bis(trimethylstannyl)dihydropentalene (Va) has been carried out, R = 3.4%. Molecules of Va are centrosyommetric. The parmeters of monoclinic cell: a 8.680(1), b 7.322(1), c 13.073(2) Å, β 97.74(1)°, space group P2₁/c, Z = 2. Geometrical parameters of Va have been determined and their values are discussed in comparison with the same parameters for η¹-cyclopentadienyl compounds of elements. Chemical shifts ¹³C, ¹H and ¹¹⁹Sn and coupling constants ¹³C-¹¹⁹Sn and ¹¹⁷Sn-¹¹⁹Sn of bis- and tris-organotin derivatives of IV have been determined. Rapid intramolecular suprafacial metallotropic rearrangement, proceeding as a [1,5]-sigmatropic shift of SnMe₃ group, has been found in Va and cis-(Z)-bis(trimethylstannyl)dihydropentalene (Vb). Activation parameters have been evaluated by the analysis of temperature dependence of ¹³C NMR spectra within the framework of degenerate two-site exchange in the isomers Va and Vb, E_A: 42.2±0.9 and 31.2±0.6 kJ · mole⁻¹; ΔH^≠₂₉₈: 39.8±0.9 and 27.7±0.6 kJ · mole⁻¹; ΔS^≠₂₉₈: −6.2±4.0 and −87.0±3.1 J · mole⁻¹ · K⁻¹ and ΔG^≠₂₉₈: 41.6±1.5 and 54.6±1.1. kJ δ mole⁻¹, respectively.     

On the fluxional behaviour of a polycyclic (4.4.2) propella-2,4-diene

Existence of bicyclo (4.2.0) octa-2,4-diene ? 1,3,5-cyclooctatriene type of valence isomerisation in a polycyclic propellane framework has been demonstrated through cycloaddition and thermolysis reactions.     

Structure and fluxional behavior of pentamethoxycarbonylcyclopentadiene

Quantum-chemical calculations by the density functional theory method at the B3LYP/6- 311++G** level have shown that the fulvene form of pentamethoxycarbonylcyclopentadiene 1 in the gas phase is more favorable in energy than cyclopentadiene form 2 by ΔE_ZPE = 7.8 kcal/mol. The fluxional behavior of fulvene 1, detected by dynamic NMR can be explained by the mechanism of circular low-barrier 1,9-O,O'-H shifts accompanied by rotations of the hydroxymethoxymethylene substituent about the С=С bond with the activation barrier ΔE_ZPE= 23.5 (gas) and 20.9 (CH₂Cl₂) kcal/mol. This reaction path is 18.6 kcal/mol more favorable in energy than transition of fulvene 1 to cyclopentadiene 2 with subsequent 1,5- sigmatropic hydrogen shifts in the five-membered ring.     

The synthesis, X-ray structure and fluxional behaviour of an ytterbium(II) phenalenide complex

The first lanthanoid complexes containing delocalized phenalenide anions are reported and the rapid migration of the eta(3)-bonded metal centre between the three rings of the pi-system is the first report of a degenerate haptotropic rearrangement in organolanthanoid chemistry.     

Structures and fluxional behaviour of transition metal cluster carbonyls

Conclusion We have examined the structures of transition metal cluster carbonyls M_m(CO)n from the point of view of the geometrical arrangement of the carbonyl ligands, and, in particular, the polyhedra defined by their oxygen atoms. Provided that the metal atom cluster is reasonably spherical (which is presumed to be predetermined by electronic factors), these polyhedra correspond well to the ideal forms calculated by optimisation of mutual repulsions between points on a sphere. Thus the concepts of coordination polyhedra about single central atoms may be successfully applied to the coordination of carbonyl ligands about entire clusters of metal atoms. This is in contrast to the more usual emphasis on the environment of individual metal atoms in the cluster. In the fine balance of factors governing the structures of these molecules and anions, we believe that steric interaction between carbonyl groups is of greater importance than previously thought. The steric crowding of ligands around the central metal atom cluster has important implications for the reactivity and catalytic activity of cluster carbonyls. Additionally, a new approach to the fluxionality of these species, based on the idea that carbonyl mobility may occur by rearrangements of the entire ligand polyhedron, has led to new insights into the behaviour of a number of systems.     

Highly fluxional tetrahapto-cyclooctatetraene complexes of ruthenium(0)

In complexes of the type Ru(arene)(cot) (cot = cyclooctatetraene) the cyclooctatetraene ring is 1—4-η-bonded as shown by X-ray diffraction study of the hexamethylbenzene derivative, and the barrier to intramolecular exchange of the bound and unbound halves of the eight-membered ring is unusually low (< 6 kcal/mol).     

Theoretical analysis of the fluxional behaviour of cyclooctatetraene Ru and Ni complexes

Cyclooctatetraene's (COT) behaviour in some of its Ni and Ru complexes is studied by means of DFT methods (B3LYP and BLYP). The experimentally observed COT fragment conformational changes (tub-shaped or planar) are analysed, together with the different locations of metal–carbon bonds in the complexes. These phenomena are studied using both static and molecular dynamics calculations. The results allow us to predict changes in the COT fragment, which goes from aromatic to totally antiaromatic, depending on its environment. This situation favours the electronic flow through the metal atoms into the organometallic molecules, giving place to different geometries that generate the dynamic fluxional behaviour.     

Synthesis and fluxional behaviour of new “heavy fluorous” cyclopentadienes

The new “heavy fluorous” cyclopentadienes C₅H_6−n[M(C₂H₄C₆F₁₃)₃]_n (M = Si, n = 1 (3); n = 2 (4) and M = Sn, n = 1 (10)) were synthesized by reaction of cyclopentadienyl lithium with BrSi(C₂H₄C₆F₁₃)₃ (2) or commercial BrSn(C₂H₄C₆F₁₃)₃. Fluorous cyclopentadienes prepared in this manner contain three or six C₆F₁₃ groups, which significantly increase their solubility in perfluorinated solvents. They also provide intermediates for titanium complexes suitable for fluorous biphase catalysis. All three isomers of silylcyclopentadienes 3 and 4 were identified and fully characterized by two dimensional NMR spectroscopy, which was performed at low temperature. The allylic isomers 3a and 4a undergo degenerate metallotropic rearrangement. This fluxional behaviour was compared with the behaviour of previously prepared cyclopentadienes 6 and 7 (C₅H_6−n[SiMe₂(C₂H₄C₈F₁₇)]_n where n = 1, 2, respectively). The presence of allylic isomers 6a and 7a was further confirmed by Diels-Alder cycloaddition of the strong dienophile tetracyanoethylene (TCNE), providing compounds 8 and 9.     

A theoretical and experimental study of the fluxional behaviour of molybdenum dihydrobis- and hydrotris-pyrazolylborates

The fluxional barrier of (dicarbonyl)[dihydrobis(3,5-dimethylpyrazol-1-yl)borato][eta-(1,2,3)-2-methylpropen-1-yl]-molybdenum (1) has been measured and a complete assignment of its (1)H, (13)C and (15)N NMR signals has been carried out. Theoretical calculations at the B3LYP/LANL2DZ level including GIAO absolute shieldings (sigma) have allowed to analyze the molecular contributions to the barrier as well as to assign some signals involved in the fluxional process.     

Synchronized fluxional motion and cyclometalation of ligands in platinum(II) complexes

Oscillation of the 2,9-dimethyl-1,10-phenanthroline (dmphen) ligand between nonequivalent exchanging sites in [Pt(Me)(dmphen)(P(o-tolyl)3)]+ and phosphane rotation around the Pt-P bond take place at the same rate. Thus, this cationic complex behaves as a molecular gear, exhibiting a fascinating synchronism between two otherwise independent fluxional motions. The process (DeltaG(3330)(#) = 68.5 +/- 0.2 kJ mol(-1)) was found to be unaffected by (i) the nature of various counteranions (X = PF6- 1, SbF6- 2, CF3SO3- 3, BF4- 4, BArf- 5), (ii) the polarity or the electron-donor properties of the solvent and, (iii) the addition of weak nucleophiles. Restricted phosphane rotation around the Pt-P bond impedes free dmphen oscillation in a 14-electron three-coordinate T-shaped intermediate, containing eta1-coordinated dmphen, generated by easy Pt-N bond dissociation from [Pt(Me)(dmphen)(P(o-tolyl)3)]+. 1-5 undergo easy orthoplatination, leading to new [Pt(dmphen){CH2C6H4P(o-tolyl)2-kappaC,P}]X cyclometalated Pt(II) compounds (X = PF6- 1, SbF6- 2, CF3SO3- 3, BF4- 4, BArf- 5). The kinetics of the cyclometalation of 3 and 4 were followed in tetrachloroethane by both 1H NMR and spectrophotometric techniques (kobs = 1.7 x 10-4 s(-1) at 333 K, DeltaH = 59.3 +/- 3 kJ mol(-1), and DeltaS = -141 +/- 8 J K(-1) mol(-1)). Ring opening of dmphen is again a prerequisite for C-H bond activation, which takes place through a multistep oxidative-addition reductive-elimination pathway. The molecular structure of cyclometalated 10 shows a butterfly shape with two o-tolyl rings projected above and below the coordination plane. Variable-temperature 1H NMR spectra revealed hindered rotation around the P-Cipso(o-tolyl) bonds at rather mild temperatures (DeltaG(3330)(#) = 55.2 +/- 0.4 kJ mol(-1)). Dmphen oscillation results very slowly and is dependent on the nature of the counteranions, of the solvents, and is strongly accelerated by the presence of weak nucleophiles that act as catalysts, according to an associative mode of activation.     

The solution structure and fluxional behaviour of (η4-enone)Fe(CO)2L complexes (L = phosphine,phosphite)

(η⁴-enone)Fe(CO)₂L complexes (enone = benzylideneacetone, cinnamaldehyde; L = PPh_3-xMe_x (x = 0–2), P(OPh)₃) exist as interconverting isomeric mixtures in solution in which L occupies either the axial or basal position of a square pyramidal structure. The ratio of isomers is dependent on the steric properties of both L and the enone.     

Structure and IR spectroscopic behaviour of 1,8-bis-(dimethylamino)naphthalene 2,4-dinitroimidazolate

The crystal structure of 1,8-bis(dimethylamino)naphthalene (DMAN) 2,4-dinitroimidazolate, C₁₇H₂₀N₆O₄, has been determined by X-ray diffraction. The crystals are monoclinic, P2 ₁/c, a = 13.426(4), b = 10.465(3), c = 15.915(4) », β = 126.12(4)°, Z = 4. The structure was solved by direct methods, and refined to an R value of 0.033 for 2291 non-zero independent amplitudes. The [NN⋯N]⁺ bridges of 2.606(3) » with ∠NHN = 160(3)° are characterized by an asymmetric proton density distribution. The IR protonic absorption is located in two regions at about 650 and 1950 cm⁻¹ showing relatively small intensity. The isotopic ratio ν(NHN/ν(NDN) for the low frequency region is almost unity. It seems that hydrogen bonds in protonated DMAN are characterized by a flat asymmetric single minimum potential for the proton motion.     

The molecular dynamics of fluxional organometallic molecules in the solid state: tetracarbonylbis(cyclooctatetraene)triruthenium(0)

The “wide line” NMR spectrum of tetracarbonylbis(cyclooctatetraene)triruthenium(0) is markedly temperature dependent, indicating that the cyclooctatetraene rings are reorienting in the solide state. An energy of activation of 5.15 kcal/mol for ring rearrangement is found from T₁ measurements. A comparison of the experimental second moment, 0.72 G² at 304 K, with theoretical second moments calculated for several different modes of reorientation supports a rearrangement process through a series of 1,2 shifts. The results are discussed in relation to the known crystal structure data for the complex.     

Structural properties and dissociative fluxional motion of 2,9-dimethyl-1,10-phenanthroline in platinum(II) complexes

A dynamic 1H NMR study has been carried out on the fluxional motion of the symmetric chelating ligand 2,9-dimethyl-1,10-phenanthroline (Me2-phen) between nonequivalent exchanging sites in a variety of square-planar complexes of the type [Pt(Me)(Me2-phen)(PR3)]BArf, 1-14, (BArf = B[3,5-(CF3)2C6H3]4). In these compounds, the P-donor ligands PR3 encompass a wide range of steric and electronic characteristics [PR3 = P(4-XC6H4)3, X = H 1, F, 2, Cl 3, CF3 4, MeO 5, Me 6; PR3 = PMe(C6H5)2 7, PMe2(C6H5) 8, PMe3 9, PEt3 10, P(i-Pr)3 11, PCy(C6H5)2 12, PCy2(C6H5) 13, PCy3 14]. All complexes have been synthesized and fully characterized through elemental analysis, 1H and 31P{1H} NMR. X-ray crystal structures are reported for the compounds 8, 11, 14, and for [Pt(Me)(phen)(P(C6H5)3)]PF6 (15), all but the last showing loss of planarity and a significant rotation of the Me2-phen moiety around the N1-N2 vector. Steric congestion brought about by the P-donor ligands is responsible for tetrahedral distortion of the coordination plane and significant lengthening of the Pt-N2 (cis to phosphane) bond distances. Application of standard quantitative analysis of ligand effects (QALE) methodology enabled a quantitative separation of steric and electronic contributions of P-donor ligands to the values of the platinum-phosphorus 1J(PtP) coupling constants and of the free activation energies DeltaG++ of the fluxional motion of Me2-phen in 1-14. The steric profiles for both 1J(PtP) and DeltaG++ show the onset of steric thresholds (at cone angle values of 150 degrees and 148 degrees , respectively), that are associated with an overload of steric congestion already evidenced by the crystal structures of 11 and 14. The sharp increase of the fluxional rate of Me2-phen can be assumed as a perceptive kinetic tool for revealing ground-state destabilization produced by the P-donor ligands. The mechanism involves initial breaking of a metal-nitrogen bond, fast interconversion between two 14-electron three-coordinate T-shaped intermediates containing eta1-coordinated Me2-phen, and final ring closure. By use of the results from QALE regression analysis, a free-energy surface has been constructed that represents the way in which any single P-donor ligand can affect the energy of the transition state in the absence of aryl or pi-acidity effects.     

Structure and swelling-release behaviour of poly(vinyl pyrrolidone) (PVP) and acrylic acid (AAc) copolymer hydrogels prepared by gamma irradiation

Copolymer network hydrogels were prepared by gamma irradiation of aqueous solutions of poly(vinyl pyrrolidone) (PVP) and acrylic acid monomer (AAc). The composition of the final hydrogels compared to the composition of the initial preparation solutions of hydrogels was determined. The chemical structure and nature of bonding was characterized by IR spectroscopy analysis, while the thermal durability of the prepared hydrogels was assessed by thermogravimetric analysis (TGA). The kinetic swelling in water and the pH-sensitivity of PVP/AAc copolymer hydrogels was studied. The drug release properties of PVP/AAc hydrogels taking methyl orange indicator as a drug model was investigated. The IR spectra indicate the formation of copolymer networks, whereas the TGA study showed that the PVP/AAc hydrogels possess higher thermal stability than pure PAAc and lower than PVP hydrogels. The kinetic swelling in water showed that all the hydrogels reached equilibrium after 24 h and that the degree of swelling increases with increasing the ratio of AAc in the initial feeding solutions. It was found that the degree of swelling of PVP/AAc hydrogels increases greatly within the pH range 4-7 depending on composition.     

Combining NMR of dynamic and paramagnetic molecules: fluxional high-spin nickel(II) complexes bearing bisguanidine ligands

A detailed nuclear magnetic resonance (NMR) study was carried out on a series of paramagnetic, tetrahedrally coordinated nickel(II) dihalide complexes featuring chelating guanidine ligands. A complete assignment of the NMR signals for all complexes was achieved by sophisticated NMR experiments, including correlation spectra. The effects of halide exchange, as well as the variation in the guanidine-metal bite angles on the paramagnetic shifts, were assessed. The paramagnetic shift was derived with the aid of the diamagnetic NMR spectra of the analogous Zn complexes, which were synthesized for this purpose. The experimentally derived paramagnetic shift was then compared with the values obtained from quantum chemical (DFT) calculations. Furthermore, variable-temperature NMR studies were recorded for all complexes. It is demonstrated that NMR spectroscopy can be applied to evaluate the rate constants of fast fluxional processes within paramagnetic and catalytically active metal complexes.     

Disk aromaticity of the planar and fluxional anionic boron clusters B20(-/2-)

The presence of excess electrons modifies the structural landscape and tends to extend the planarity of boron clusters. While the neutral B(20) is tubular, both the anion and dianion B(20)(-/2-) become planar. Geometrical features of the stable anions suggest the existence of a new type of cluster that is planar and doubly cyclic with one atom located at the center (see figure), as well as being fluxional.     

Carbon-fluorine bond cleavage in the preparation of Osmium(III) and Osmium(IV) fluorothiolate complexes. Fluorine by fluorine NMR-assignment and fluxional processes

Reactions of OsO4 with HSR (R=C6F5, C6F4H-4,) in refluxing ethanol afford [Os(SC6F5)3(SC6F4(SC6F5)-2)] (1) and [Os(SC6F4H-4)3(SC6F3H-4-(SC6F4H-4)-2)] (2), which involve the rupture of C-F bonds. At room temperature, the compound [Os(SC6F5)3(PMe2Ph)2] or [Os(SC6F5)4(PMe2Ph)] reacts with KOH(aq) in acetone, giving rise to [ Os(SC6F5)(SC6F4(SC6F4O-2)-2)(PMe2Ph)2] (3), through a process involving the rupture of two C-F bonds, while the compound [Os(SC6F4H)4(PPh3)] reacts with KOH(aq) in acetone to afford [Os(SC6F4H-4)2(SC6F3H-4-O-2)(PPh3)] (4), which also implies a C-F bond cleavage. Single-crystal X-ray diffraction studies of 1, 2, and 4 indicate that these compounds include five-coordinated metal ions in essentially trigonal-bipyramidal geometries, whereas these studies on the paramagnetic compound 3 show a six-coordinated osmium center in a distorted octahedral geometry. 19F, 1H, 31P{1H}, and COSY 19F-19F NMR studies for the diamagnetic 1, 2, and 4 compounds, including variable-temperature 19F NMR experiments, showed that these molecules are fluxional. Some of the activation parameters for these dynamic processes have been determined.