VT 1H NMR Research Articles

Towards robust alkane oxidation catalysts: electronic variations in non-heme iron(ii) complexes and their effect in catalytic alkane oxidation

A series of non-heme iron(ii) bis(triflate) complexes containing linear and tripodal tetradentate ligands has been prepared. Electron withdrawing and electron donating substituents in the para position of the pyridine ligands as well as the effect of pyrazine versus pyridine and sulfur or oxygen donors instead of nitrogen donors have been investigated. The electronic effects induced by these substituents influence the strength of the ligand field. UV-vis spectroscopy and magnetic susceptibility studies have been used to quantify these effects and VT (1)H and (19)F NMR spectroscopy as well as X-ray diffraction have been used to elucidate structural and geometrical aspects of these complexes. The catalytic properties of the iron(ii) complexes as catalysts for the oxidation of cyclohexane with hydrogen peroxide have been evaluated. In the strongly oxidising environment required to oxidise alkanes, catalyst stability determines the overall catalytic efficiency of a given catalyst, which can be related to the ligand field strength and the basicity of the ligand and its propensity to undergo oxidation.

Solid and solution state flexibility of sterically congested bis(imino)bipyridine complexes of zinc(ii) and nickel(ii)

Two new sterically demanding bis(imino)bipyridine ligands, 6,6'-{(2,6-i-Pr(2)C(6)H(3))N=R}(2)C(10)H(6)N(2) (R = H (L1), Me (L2)), have been prepared in high yield by the condensation reaction of 2,6-diisopropylaniline with 6,6'-(O=R)(2)C(10)H(6)N(2) (R = H, Me). Palladium(0)-mediated cross coupling of 2-(Bu(3)Sn)-6-{C(Me)OCH(2)CH(2)O}C(5)H(3)N with 2-Br-6-{C(Me)OCH(2)CH(2)O}C(5)H(3)N, followed by an acid-mediated deprotection, has been employed as an efficient route to the precursor 6,6'-bis(acetyl)-2,2'-bipyridine. Reaction of aldimino L1 with two equivalents of MX(2) [MX(2) = ZnCl(2), NiCl(2) or (DME)NiBr(2)] in n-BuOH at elevated temperature gives the five-coordinate mononuclear complexes [(L1)MX(2)] (M = Zn, X = Cl 1; M = Ni, X = Cl 2a; M = Ni, X = Br 2b) as the sole products, in which one imine group is bound and the other uncoordinated (endo-exo). In the case of diamagnetic 1, VT (1)H NMR spectroscopy reveals a fast exchange process operating between the two possible forms of the endo-exo isomer which is likely to proceed via an exo-exo intermediate (DeltaH(double dagger)(interconversion) = 35.6 +/- 1.5 kJ mol(-1), DeltaG(double dagger)(298) = 47.4 +/- 3.3 kJ mol(-1)). In contrast, treatment of ketimino L2 with MCl(2) (MCl(2) = ZnCl(2) or NiCl(2)) affords bimetallic [(L2)Zn(2)Cl(4)] (4) and the six-coordinate monometallic species [(L2)NiCl(2)] (5), respectively; in 4, L2 adopts a bis(bidentate) bonding mode (endo-endo) while in 5 it acts as tetradentate ligand (endo-endo). Prolonged standing of 1 in chlorinated solvents results in partial hydrolysis and the formation of the 6-imino-6'-formyl-2,2'-bipyridine zinc complex, [(6-{(2,6-i-Pr(2)C(6)H(3))N=Me)-6'-(CH=)C(10)H(6)N(2))ZnCl(2)] (3) (endo-imine, exo-formyl). Single crystal X-ray structures are reported for L1,1 , 2a, 3, 4 and 5.

Synthesis, characterization and role of a novel dimeric α-diimine iron(II) complex in ATRP of styrene

A novel dimeric α-diimine iron(II) complex, [(dab)FeCl-(μ-Cl)]2 (dab=1,4-bis-cyclohexyl-1,4-diazabutadiene) was synthesized and structurally characterized by single crystal X-ray diffraction and VT 1H NMR spectroscopy. The performances of this complex in atom transfer radical polymerization (ATRP) of styrene have been studied and related to the molecular structure.

Synthesis and Characterization of Bis(dithiolene) Tungsten(VI), ‐(V), and ‐(IV) Complexes and Their Reactivities in Coupled Electron–Proton Transfer: A New Series of Active Site Models of Tungstoenzymes

AbstractNew tungsten(VI), ‐(V), and ‐(IV) complexes containing 3,6‐dichloro‐1,2‐benzenedithiolate (bdtCl2), (Et4N)2[WVIO2(bdtCl2)2] {(Et4N)2[1]}, (Et4N)[WVO(bdtCl2)2] {(Et4N)[2]}, and (Et4N)2[WIVO(bdtCl2)2] {(Et4N)2[3]} were synthesized and characterized as active site models of tungstoenzymes.(Et4N)2[1] was prepared in high yield by the reaction of WO2Cl2 with 2bdtCl2 at low temperature, and its crystal structure was determined. Isomerization between octahedral structures (Δ and Λ forms) of the tungsten center of(Bu4N)2[1] was characterized by variable‐temperature 1H NMR (VT 1H NMR) spectral analysis, which offers the first example of six‐coordinate bis(dithiolene) tungsten(VI) complexes. The isomerization of the molybdenum center of (Bu4N)2[MoVIO2(bdtCl2)2] {(Bu4N)2[4]} was similarly characterized. A comparison of their activation energies indicated that the tungsten center underwent slower isomerization than that of the molybdenum one. (Et4N)2[1] underwent an irreversible reduction at –2.1 V vs. SCE in CH3CN by a coupled electron–proton transfer (CEPT) process to yield(Et4N)2[3], while (Et4N)2[3] underwent an irreversible oxidation by a CEPT process at –0.27 V in the presence of 2 equiv. of Et4NOH in CH3CN to yield (Et4N)2[1]. (Et4N)[2] disproportionated into 0.5 equiv. of (Et4N)2[1] and 0.5 equiv. of (Et4N)2[3] when treated with 1 equiv. of Et4NOH in CH3CN.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Conformational Interconversions in [2]Catenanes Containing a Wide Rigid Bis(p-benzyl)methyl Spacer

The conformational interconversions of four [2]catenanes (1-4) containing a dibenzo-34-crown-10 ether (BPP34C10) interlocked with rings containing two 4,4'-dipyridiniums tethered by 1,3-bis(ethyloxy)phenyl and bis(p-benzyl)methyl spacers have been studied by VT 1H NMR spectroscopy. Symmetrically placed blocking groups on thickened tethers enabled either pathway for circumrotation of the BPP34C10 between isoenergetic sites to be blocked. On the basis of chemical shifts of the BPP34C10, its internal p-hydroquinone forms pi-pi-stacking interactions with only one 4,4'-dipyridinium ring at a time. The activation barrier for migration along either open tether was approximately 11.5 kcal/mol. This study demonstrates an ability to select the pathway for conformational interconversions in these [2]catenanes containing the rigid bis(p-benzyl)methyl tether and the lowering the barrier for interconversion through destabilization of the ground state structures.

Synthesis of aluminum alkoxide and bis-alkoxide compounds containing bidentate pyrrolyl ligands

A series of aluminum alkoxide and bis-alkoxides compounds were synthesized and characterized. Reacting 1 with 1 and 2 equiv. of t-butanol in methylene chloride generates [C 4H 3N(CH 2NMe 2)-2] 2Al(O- t-Bu) ( 2) and [C 4H 3N(CH 2NMe 2)-2-H-C 4H 3N(CH 2NMe 2)-2]Al(O- t-Bu) 2 ( 3) in 47% and 54% yield, respectively. The 1H NMR spectrum of 2 exhibits two singlets for NMe 2 and CH 2N at δ 2.52 and 3.84, respectively, representing the symmetrical manner of molecular structure 2 in a solution. Compound 3 is not thermal stable in solution which decompose into substituted pyrrole ligand C 4H 4N(CH 2NMe 2)-2 and unknown aluminum alkoxides. Reacting 1 with 2 equiv. of triphenylsilanol in methylene chloride generates a tetra-coordinated aluminum “ate” compound [C 4H 3N(CH 2NMe 2)-2-H- C 4H 3N(CH 2NMe 2)-2]Al(OSiPh 3) 2 ( 4) in 49% yield. The 1H NMR spectra of 4 at room temperature show a broad signal at δ 1.57 for NMe 2 fragments and the signals for CH 2N were not observed. VT 1H NMR spectra of 4 show the NMe 2 fragments became two singlets ( δ 1.27 and 2.12) and the CH 2N exhibited two doublets ( δ 2.44 and 3.56) at 240 K. The fluxional energy barrier (Δ G ‡) is estimated at ca. 50 kJ/mol. The molecular structures of compounds 3 and 4 are determined by single-crystal X-ray diffractometer.

Synthesis and preliminary DNA-binding studies of diimineplatinum(ii) complexes containing 3- or 4-pyridineboronic acid

A series of platinum(II) complexes of the type [Pt(NN)(pyB)2](NO3)2 (NN = bipy, phen; pyB = 3- or 4-pyridineboronic acid) were successfully prepared and fully characterised by 1D- and 2D-multinuclear NMR spectroscopy and ESI-MS. Using VT 1H NMR spectroscopy, rotational isomers for [Pt(NN)(3-pyB)2](NO3)2 were identified and the free energies of activation for rotation of 3-pyB about the Pt-N bond were determined to be DeltaG++310) = 69.2 +/- 0.1 kJ mol(-1) and DeltaG++(305) = 66.0 +/- 0.1 kJ mol(-1) for [Pt(bipy)(3-pyB)2](NO3)2 and [Pt(phen)(3-pyB)2](NO3)2, respectively. The 3- and 4-pyB ligands readily deboronate in boiling H2O to afford [Pt(NN)(py)2](NO3)2; the structure of [Pt(phen)(py)2](2+) (as its PF6- salt) was confirmed by X-ray crystallography. Preliminary thermal denaturation studies revealed only minimal interactions between [Pt(NN)(pyB)2](NO3)2 and calf-thymus DNA and is attributed to hydroxylation of the boronic acid groups at pH 7.4 to afford the corresponding zwitterionic boronate species. This was confirmed by means of variable pH 1H and 11B{1H} NMR spectroscopy.

Synthesis, Structure, and Hydroamination Kinetics of (2,2‘-Diaryldipyrrolylmethane)- and Bis(2-arylpyrrolyl)titanium Complexes

The trifluoroacetic acid catalyzed reaction of acetone and 2-arylpyrroles results in the formation of 2,9-diaryl-5,5-dimethyldipyrrolylmethanes. Ligands bearing 3,5-(CF3)2C6H3 (H2dmpm3,5-CF3) and mesityl (H2dmpmmes) arenes were prepared in 72% and 68% yields using this procedure. The new dipyrrolylmethanes react with Ti(NMe2)4 to form Ti(NMe2)2(dmpm3,5-CF3) and Ti(NMe2)2(dmpmmes) in 92% and 30% yields, respectively. The solid-state structures of these complexes are quite similar to that of the sterically smaller 5,5-dimethyldipyrrolylmethane complex Ti(NMe2)2(dmpm) with one η5- and one η1-pyrrolyl; however, the substituted derivatives display much lower barriers to pyrrolyl conformational exchange, as judged by VT 1H NMR spectroscopy. Also synthesized were bis(pyrrolyl) complexes without the methylene connector. Reactions of 2-arylpyrroles where the aryl group was 3,5-(CF3)2C6H3, mesityl, 4-(CF3)C6H4, and p-tolyl generated Ti(NMe2)2(pyrr3,5-CF3)2, Ti(NMe2)2(pyrrmes)2, Ti(NMe2)2(pyrr4-CF3)2, and Ti(NMe2)2(...

NMR Study of Hyperbranched Polyphenylenes from the AB2, (AB2 + AB) and (A2 + B3) Methods

AbstractSummary: The 1H NMR and 13C NMR spectra of hyperbranched polyphenylenes synthesised from AB2, (AB2 + AB) and (A2 + B3) monomers (A: ethynyl group; B: cyclopentadienonyl group) were analysed with respect to the characteristic substructures of these polymers. The broad and overlapping NMR spectra were studied by a combination of 1D and 2D NMR techniques. Furthermore, appropriate model compounds were synthesised, and their 1H and 13C NMR spectra were fully assigned. The signal assignments achieved allow to substantiate the different hyperbranched polyphenylene structures. Steric hindrance in densely packed di‐ and trihexaarylphenyl substituted units of the (A2 + B3) polyphenylenes results in the decrease of the rotation frequency of phenyl rings in these structures to such an extent that the motion is slow on the 1H NMR time scale. This can be proved both by EXSY and variable‐temperature experiments. Steric constraints were also deduced for the AB2 polyphenylenes from signal line shape.Selected spectra from a VT 1H NMR experiment on polymer 7b (solvent: C2D2Cl4; *–13C satellites of the solvent; signal at 7.3 ppm due to traces of CHCl3).magnified imageSelected spectra from a VT 1H NMR experiment on polymer 7b (solvent: C2D2Cl4; *–13C satellites of the solvent; signal at 7.3 ppm due to traces of CHCl3).

Dynamic behaviour of 1, n′-disubstituted ferrocenes

1, n′-Disubstituted ferrocenes with ketone/phthalimido ( 2) and ketone/amine substituents ( 3) were synthesised and characterised by IR, 1H NMR, 13C NMR, VT 1H NMR, UV/Vis spectroscopy, mass spectrometry and cyclic voltammetry. The molecular structure of 2 was confirmed by X-ray crystal structure determination. The dynamic behaviour was experimentally studied in solution and theoretically by DFT calculations. The thermal stability of the ketone/amine derivative 3 was investigated using thermal analyses.

Neutral and Cationic Heteroscorpionate Aluminum Complexes: Synthesis, Structure, and Ring-Opening Polymerization of ε-Caprolactone

Treatment of 2,4-di-tert-butyl-6-(bis(3,5-dimethylpyrazol-1-yl)methyl)phenol (bpzmp-H) as tridentate ligand with aluminum trialkyls affords the corresponding heteroscorpionate aluminum complexes (bpzmp)AlR2 (R = Me (1); R = Et (2); R = iBu (3)) in high yield. In the solid state, complexes 1−3 were isolated as racemic mixtures in which each stereoisomer adopts a tetrahedral structure with the bpzmp ligand κ2-coordinated to the metal via the phenoxy group and the imino nitrogen of one of pyrazolyl rings. The investigation of the solution structure of 1−3 by means of VT 1H NMR spectroscopy revealed fluxional exchange between coordinated and noncoordinated pyrazolyl rings, producing interconversion between the two enantiomers. The activation enthalpy (ΔH#) for racemization processes (10.1 ± 0.2 kcal·mol-1 (1); 11.6 ±0.3 kcal·mol-1 (2); 14.1 ± 0.2 kcal·mol-1 (3)) was dependent on steric hindrance at the aluminum center. Reaction of 1 or 2 with B(C6F5)3 proceeds through net alkyl abstraction, forming the expected cationic aluminum complexes [(bpzmp)AlR]+[RB(C6F5)3]- (R = Me (4); R = Et (5)), in which the bpzmp fragment acts as a tridentate ligand. The ionization reaction of 3 with Lewis acidic compounds (B(C6F5)3 or [Ph3C][B(C6F5)4]) proceeds by net β-H abstraction from the isobutyl group, forming [(bpzmp)AliBu]+[HB(C6F5)3]- (6) along with isobutene; in contrast, reaction with the Brönsted acid [HNMe2Ph][B(C6F5)4] proceeds by protonolysis of the isobutyl group. Complex 4 is active in ring-opening polymerization of ε-caprolactone (ε-CL), producing high-molecular-weight polymers. The 1H NMR monitoring of the reaction between 4 and ε-CL in 1:1 molar ratio showed that the initiation involves monomer insertion into the Al−O bond of the bpzmp fragment, forming the intermediate [(bpzphe)AlMe]+[MeB(C6F5)4]- (bpzphe = 2,4-di-tert-butyl-6-(bis(3,5-dimethylpyrazol-1-yl)methyl)phenyl 6-hydroxyhexanoate (8). The growth of the polymer chain occurs through continuous insertions of the monomer in the Al−alkoxide bond.

Titanium Complexes Bearing a Hemilabile Heteroscorpionate Ligand: Synthesis, Reactivity, and Olefin Polymerization Activity

A series of titanium complexes (bpzmp)TiR1R2R3 (bpzmp = (3,5-tBu2-2-phenoxo)bis(3,5-Me2-pyrazol-1-yl)methane; R1 = R2 = R3 = Cl (2); R1 = R2 = R3 = NMe2 (3); R1 = Cl; R2 = R3 = NMe2 (4); R1 = R2 = Cl; R3= NMe2 (5); R1 = R2 = R3 = Me (6)) has been synthesized and characterized by VT NMR spectroscopy and X-ray diffraction analysis (2, 3, 5). The complexes 2−6 adopt in the solid state an octahedral structure in which the bpzmp ligand is κ3-coordinated to the metal via the phenoxy group and the imino nitrogens of the two pyrazolyl rings. The investigation of the solution structure of 3 by means of VT 1H NMR spectroscopy revealed a fluxional behavior of the bpzmp ligand that produces an equilibrium between the octahedral and tetrahedral form of the titanium complex, the latter resulting from the η1-coordination of the ligand through exclusively the phenolate group. The thermodynamic and kinetic parameters of this process were evaluated by VT NMR spectroscopy. The selective replacement of chloride for dimethylamide in 4 and 5 shifts the equilibrium toward the octahedral complex, which is the favorite configuration at room temperature. Site exchange of the nonequivalent methyl groups in 6 was observed at room temperature in the slow-regime 1H NMR time scale: ΔH⧧ and ΔS⧧ values of 22.3 ± 1.1 kcal·mol-1 and −19.6 ± 3.7 cal mol-1 K-1 were respectively determined for the isomerization process occurring with a rate constant of 3 s-1 and ΔG⧧ of 28.1 ± 0.1 kcal·mol-1 at 293 K. Complexes 2 and 6 are active olefin polymerization catalysts after activation with MAO or [Ph3C][B(C6F5)4]. Linear polyethylene and atactic polypropylene were obtained in the polymerization experiments catalyzed by 6-[Ph3C][B(C6F5)4]. Highest polymerization activities were found with the 2-MAO catalyst, where leaching of the ligand due to MAO excess was suspected. Reaction of 6 with [Ph3C][B(C6F5)4] in the presence of THF readily produces the ionic complex [(bpzmp)TiMe2(THF)][B(C6F5)4], proposed as a model of the active species in this class of olefin polymerization catalysts.

Mechanistic investigation of the thermal decomposition of Biphen(OP i-Pr)PtEt 2: An entrance into C–C single bond activation?

Biphen(OP i-Pr) and (COD)PtCl 2 give Biphen(OP i-Pr)PtCl 2 which upon treating with ethyl Grignard forms Biphen(OP i-Pr)PtEt 2. The thermal decomposition of Biphen(OP i-Pr)PtEt 2 was investigated in the temperature range of 353–383 K. The clean and quantitative formation of the Pt(Ethene) adduct was observed. X-ray structures of a molecule in the solid state of all three reaction products and two further related complexes with phenyl fingers instead of i-Pr have been determined. For the complexes with i-Pr fingers a decisive deviation from a square plane is observed in contrast to the complexes with phenyl fingers. The P–Pt–P angle increases from about 95° in Biphen(OP i-Pr)PtCl 2 to about 120° in Biphen(OP i-Pr)Pt(Ethene), forcing the bridging C–C single bond of the biphenyl fragment as near as 4.17 Å to the Pt center. No through-space coupling between the bridging C atoms and the Pt center could be observed in 13C NMR spectroscopy. No bond lengthening of the bridging C–C single bond in the biphenyl fragment was observed in Biphen(OP i-Pr)Pt(Ethene) in comparison to the precursor complexes. The thermal decomposition of Biphen(OP i-Pr)PtEt 2 can be described by a first-order kinetic and the activation parameters were determined (temperature range: 353–383 K; Δ H ‡ = 173.8 ± 16.2 kJ/mol and Δ S ‡ = 104.7 ± 44.1 J/(mol K)). The reaction kinetics were also measured for perdeuterated ethyl groups yielding in a kinetic isotopic effect of 1.56 ± 0.14 which was almost temperature-independent. Selective deuteration at α and β position of the ethyl group, respectively, showed that β-H elimination takes place fast in comparison to the complete thermolysis. In the temperature range of 333–353 K only a scrambling of the deuterium atoms was found without further decomposition (temperature range: 333–353 K; Δ scram H ‡ = 76.1 ± 15.2 kJ/mol, Δ scram S ‡ = −80.7 ± 45.5 J/(mol K) for Biphen(OP i-Pr)PtEt 2-d 6). The ethene is not lost during the scrambling process. The scrambling process is connected with a primary KIE decisively larger than 1.56. Biphen(OP i-Pr)Pt(Ethene) exchanges the coordinated ethene with ethene in solution as proven by labeling experiments. Both a dissociative and an associative mechanism could be shown to take place as ethene exchange reaction by means of VT 1H NMR spectroscopy via line shape analysis (temperature range: 333–373 K; Δ ass H ‡ = 26.9 ± 29.6 kJ/mol, Δ ass S ‡ = −148.0 ± 87.5 J/(mol K), Δ diss H ‡ = 86.0 ± 6.5 kJ/mol, Δ diss S ‡ = 5.4 ± 17.8 J/(mol K)). The Pt(0) complex formed during the dissociative loss of ethene activates several substrates among them: O 2, H 2, H 2SiPh 2 via Si–H activation, MeI presumably via forming a cationic methyl adduct and ethane via C–H activation but it was proven that the bridging C–C single bond of the biphenyl fragment is not even temporarily broken. The materials were characterized by means of 1H NMR, 13C NMR, 31P NMR, 195Pt NMR, EA, MS, IR, X-ray analysis and polarimetric measurement where necessary.

Simultaneous freezing of chirality and in-out conformation of a macropentacyclic cryptand by protonation.

Compound 1, a cryptand-derived macropentacycle, is a flexible molecule that encompasses many conformations (symmetrical, unsymmetrical, and chiral ones) depending on the observation temperature (VT 1H NMR). Selective monoprotonation of this molecule leads to a totally unsymmetrical, rigidly chiral species in solution (1H NMR). Helical chirality and in-out conformation of monoprotonated 1 are observed in the solid state by X-ray diffraction analysis, as well as the proton location. The latter is bound to the endo bridgehead nitrogen atom and involved in hydrogen-bonding interactions with the three closest sulfurs. Significant induction of chirality is triggered by reaction of 1 with the optically active (R)-(-)-1,1'-binaphthyl-2,2'-diylphosphoric acid. It proceeds with a maximum 24% diastereomeric excess, as shown by the splitting, in the 62:38 intensity ratio, of several 1H NMR signals. These correspond to the two indistinguishable diastereomeric ion pairs: (Lambda-[1-H])((R)-(-)-BNP) and (Delta-[1-H])((R)-(-)-BNP).

Main Chain Ferrocenyl Amides from 1‐Aminoferrocene‐1′‐carboxylic Acid

AbstractThe non‐natural amino acid 1‐aminoferrocene‐1′‐carboxylic acid is synthesised from ferrocene in eight steps. The folding and association phenomena of amido‐substituted ferrocenes in the crystal as well as in solution are studied by X‐ray crystallography, IR and NMR spectroscopy and DFT calculations. The amino acid is selectively protected at the amino group with the use of the fluorenyl‐9‐methoxycarbonyl (Fmoc) group. An amide‐linked ferrocene dimer is prepared using the HOBt/DCC protocol for amide formation. In the crystal the dimer forms a hydrogen‐bonded sheet structure, while in solution dynamic intramolecular hydrogen bonds are observed by VT 1H NMR and IR spectroscopy. The dynamic flipping process has been rationalised by DFT calculations. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Chemistry of zwitterionic penta-coordinate silicon compounds of the (ammoniomethyl)bis[glycolato(2–)-O1,O2]silicate type in aqueous and nonaqueous solution

The zwitterionic spirocyclic λ5Si-silicates bis[glycolato(2–)-O1,O2][(trimethylammonio)methyl]silicate ([OCH2C(O)O]2SiCH2NMe3, 3) and bis[2-methyllactato(2–)-O1,O2][(trimethylammonio)methyl]silicate ([OCMe2C(O)O]2SiCH2NMe3, 4) were synthesized by treatment of the zwitterionic λ5Si-silicate F4SiCH2NMe3 with 2 mol. equiv. of Me3SiOCR2C(O)OSiMe3 (R = H, Me) (formation of 4 mol. equiv. of Me3SiF). Compounds 3 (isolated as 3•2/3MeCN after drying in vacuo) and 4 were studied by solution 1H, 13C, and 29Si NMR spectroscopy and by solid-state VACP/MAS 13C, 15N, and 29Si NMR experiments, and the solvates 3•MeCN and 4•2MeCN were structurally characterized by single-crystal X-ray diffraction. These experimental investigations were complemented by computational studies of the zwitterions 3 and 4 and the anionic model species bis[2-methyllactato(2–)-O1,O2]hydridosilicate(1–) ([OCMe2C(O)O]2SiH–, 8). Compounds 3•2/3MeCN and 4 and their already known derivatives [OCH2C(O)O]2SiCH2NMe2H (1) and [OCMe2C(O)O]2SiCH2NMe2H (2) were studied for their dynamic behavior (Λ/Δ-enantiomerization) in solution by VT 1H NMR spectroscopy. In addition, compounds 1, 2, 3•2/3MeCN, and 4 were studied in CD3OD and D2O with 1H, 13C, and 29Si NMR experiments.Key words: penta-coordinate silicon, zwitterionic λ5Si-silicates, Λ/Δ-enantiomerism, NMR spectroscopy, computational chemistry.

Synthesis, characterization and structures of zirconocene complexes of sterically demanding pentaphenylcyclopentadienyl and tetraphenyl- m-tolyl cyclopentadienyl ligands

The synthesis, characterization and catalytic activity of zirconium complexes containing bulky pentaarylcyclopentadienyl ligands are reported. The monocyclopentadienyl 12-electron complex, trichloropentaphenylcyclopentadienylzirconium(IV), C 5Ph 5ZrCl 3 ( I), has an unusual monomeric structure in the solid state and has short Zr–ligand distances suggesting that the complex has significant Lewis acidity. The phenyl rings adopt a propeller arrangement in the solid state; VT 1H-NMR measurements of the analogous trichloro(tetraphenyl- m-tolylcyclopentadienyl)zirconium(IV) ( II) indicate that there is rapid rotational motion of the phenyl rings. The Lewis acidity of this complex is manifested in its ability to catalyze the [4+2] cycloadditions of acrolein and methyl acrylate to cyclopentadiene. The sandwich complex, C 5Ph 5CpZrCl 2 ( III), has more typical Zr–ligand distances, but has a more parallel arrangement of the two cyclopentadienyl rings and does not display the reactivity shown by I and II.

Synthesis and structures of monocyclopentadienylniobium complexes with bi- and tri-dentate dithiolate ligandsElectronic supplementary information (ESI) available: NMR data for compounds 1 and 2 in d8-toluene; cyclic voltammogram of compound 1; ESR spectrum of reduced compound 1. See http://www.rsc.org/suppdata/dt/b2/b209823g/

Reactions of cyclopentadienyl and methylcyclopentadienyl-tetrachloroniobium with thallium(I) 1,3-propanedithiolate yield compounds [NbCpx(1,3-S2C3H6)2] [1, Cpx = η-C5H5 (Cp); 2, Cpx = η-C5H4Me (Cp′)]; similar reactions of the thallium(I) derivative of 2,2′-thiodiethanethiol afford [NbCpxCl2{(SC2H4)2S}] (3, Cpx = Cp; 4, Cpx = Cp′). Reaction of [NbCp′Cl4] with thallium(I) 2,2′-oxydiethanethiolate produces both [NbCp′Cl2{(SC2H4)2O}] 5 and [NbCp′O{(SC2H4)2O}] 6. Products have been characterised analytically and by NMR spectroscopy; the solid-state structures of 1, 3, 5 and 6 have been established by XRD. Compound 1 is five-coordinate, with a distorted four-legged piano-stool geometry, compound 3 is six-coordinate with tridentate thiolate and cis-equatorial Cl ligands, compound 5 is also six-coordinate but with trans-equatorial Cl ligands, and compound 6 is five-coordinate, approximating to a trigonal bipyramidal geometry with an equatorial oxo ligand. Compounds 1 and 2 are fluxional in solution due to non-rigidity of the twist conformations of the six-membered chelate rings, as shown by VT 1H NMR spectroscopic studies with ΔG‡ = 42.7 ± 1.4 kJ mol−1 (at 231 K) and ΔG‡ = 43.1 ± 1.3 kJ mol−1 (at 233 K), respectively, for ring inversion in toluene solutions. Cyclic voltammetric studies on 1 and 2 showed two consecutive, quasi-reversible, reduction processes, attributable to the sequential formation of mono- and di-anionic derivatives; reductions of the other compounds showed less evidence for reversibility. ESR spectra of the initial products of chemical reduction of 1 and 2 in thf, using either a sodium film or cobaltocene as reductant, are typical of those from Nb(IV) species and assigned to the respective mono-anions.

Dianionic complexes with hexacoordinate silicon(IV) or germanium(IV) and three bidentate ligands of the salicylato(2–) type: Syntheses and structural characterization in the solid state and in solution

Reaction of tetramethoxysilane or tetramethoxygermane with salicylic acid and morpholine (molar ratio 1:3:2) in tetrahydrofuran yielded morpholiniummer-tris[salicylato(2–)-O1,O3]silicate(mer-5) and morpholiniummer-tris[salicylato(2–)-O1,O3]germanate (mer-8), respectively. Treatment of tetramethoxysilane with 5-chlorosalicylic acid and piperidine (molar ratio 1:3:2) in tetrahydrofuran afforded piperidinium mer-tris[5-chlorosalicylato(2–)-O1,O3]silicate–ditetrahydrofuran (mer-6·2THF). Triethylammonium mer-tris[3-methylsalicylato(2–)-O1,O3]silicate (mer-7) was obtained analogously by reaction of tetramethoxysilane with three molar equivalents of 3-methylsalicylic acid and two molar equivalents of triethylamine in dichloromethane/diethyl ether. The racemic compounds mer-5, mer-6· 2THF,mer-7, and mer-8 were characterized by elemental analyses (C, H, N), single-crystal X-ray diffraction, as well as solid-state (29Si) and solution(1H, 13C, 29Si) NMR studies. The structural characterizationwas complemented by computational studies (HF studies, TZVP level) of thefac- and mer-tris[salicylato(2–)-O1,O3]silicatedianion. In addition, the behavior of mer-7 in solution was studied by VT 1HNMR experiments.

Zwitterionic spirocyclic 5Si-silicates with two cis-1,2-diphenylethene-1,2-diolato(2–) ligands: Synthesis and structural characterization

The zwitterionic λ 5Si-silicates bis[cis-1,2-diphenylethene-1,2-diolato(2–)](morpholiniomethyl)silicate (2) and bis[cis-1,2-diphenylethene-1,2-diolato(2–)][(2,2,6,6-tetramethylpiperidinio)methyl]silicate (3) were synthesized by various methods, including remarkableSi–C cleavage reactions with benzoin. Treatment of trimethoxy(morpholinomethyl)silane (4), dimethoxy(morpholinomethyl)phenylsilane (5), or dimethoxy(methyl)(morpholinomethyl)silane (6) with two molar equivalents of benzoin in acetonitrile yielded 2. Compound 3 was synthesized by treatment of trimethoxy[(2,2,6,6-tetramethylpiperidino)methyl]silane (7) with two molar equivalents of benzoin in 1,4-dioxane/n-pentane and was isolated as the 1,4-dioxane solvate 3 ·3/2C4H8O2. Compounds 2 and 3 ·3/2C4H8O2 were structurally characterized by solution and solid-state NMR spectroscopy and by single-crystal X-ray diffraction. In addition, the dynamic behavior of 3 in solution was studied by VT 1H NMR experiments.