Dynamic Stereochemical Rearrangements in Chira Organometallic

Organometallic complexes have long been known to display a wide variety of dynamic stereochemical processes. Classic examples of such processes include the exchange of axial and equatorial environments in trigonal bipyramidal complexes, such as Fe(CO)₅, and the migration of the metal moiety round the periphery of the cyclopentadiene ring in η¹-bound Cp complexes. The systematic study of fluxional processes is of interest because it can not only help provide a detailed, quantitative ‘picture’ of the bonding between the metals and ligands involved, but it can also help to rationalise chemical reactivity patterns. The introduction of chirality into organometallic complexes, usually in the form of a non-racemic chiral ligand, has led to an explosion in the importance such species, particularly with regard to their applications in organic functional group transformations. The presence of a chiral centre can also provide an excellent spectroscopic handle on the complex in question, enabling both novel fluxional processes to be observed and new light to be shed on old (unresolved) problems. In this critical review (101 references) the literature on metal-centred fluxional rearrangements in chiral transition and main group organometallic complexes is reviewed, complementing the recent review by Faller (see reference )

Synthesis And NMR Investigation of 1-Phenyl-1,2-propanedione-2-oxime Complexes of Trimethylplatinum(IV).

The ionised monoxime, 1-phenyl-1,2-propanedione-2-oxime (ppdm) reacts smoothly with trimethylplatinum(IV) sulfate in aqueous acetone, to form the complex fac-[PtMe₃ (ppdm)(H₂O)] 1 in high yield. 1 Reacts with 3,5-lutidine and 2,2'-bipyridine to form stable 1:1 adducts, viz. fac-[PtMe₃(ppdm)(3,5-lut)] 2 and fac-[PtMe₃(ppdm)(bipy)] 3. In complexes 1 and 2, the ionised monoxime behaves in a N/O bidentate chelate fashion, whereas in 3, ppdm is co-ordinated to the metal moiety in a monodentate fashion, via the oximate N donor atom. The parent complex, 1, dissolves in polar solvents to form species of general formulae fac-[PtMe₃(ppdm)(solv)] (solv = DMSO, methanol or acetone), which undergo an intramolecular "windscreen-wiper" fluxional rearrangement. The stereodynamics of the fluxional process have been measured in CD₃OD and [D₆]DMSO solution by two-dimensional exchange spectroscopy; ΔG‡ (298 K) is 73.6 kJ mol⁻¹ and 88.5 kJ mol⁻¹, respectively. The effects of the solvent on the energetics and a possible mechanism for the fluxional process are discussed

2,6-Bis(diphenylphosphinosulfide)pyridine (L) as a facial terdentate ligand: synthesis and characterisation of the tricarbonylrhenium(I) complexes fac-[Re(CO)₃L]⁺ [Re₂(CO)₆(µ-X)₃]⁻ (X = Cl, Br or I) and fac-[Re(CO)₃L]⁺[SbF₆]⁻.

The halogenopentacarbonylrhenium(I) compounds react with 2,6-bis(diphenylphosphinosulfide) pyridine (L) under mild conditions to yield ionic complexes of general formulae, fac-[Re₂(CO)₃L]⁺ [Re2(CO) ₆(μ-X) )₃]⁻ (X=Cl, Br or I), in which the ligand adopts a facial terdentate bonding mode. A synthesis of [Re(CO)₃L]⁺ [SbF₆]⁻ was carried out to establish the presence of the cation, fac-[Re(CO)₃L]⁺, in the complexes. The character of the anions was confirmed by negative ion MALDI-TOF mass spectrometry. The cation is fluxional; the P–phenyl rings oriented towards the metal moiety exhibit restricted rotation at low temperature. The free energy of activation, ΔG †, for hindered rotation is ca. 47 kJ mol⁻¹ for all complexes. Solid-state₃ ¹P NMR data are reported for the free ligand and for the complexes, [Re(CO)₃L][SbF₆] and [Re(CO)₃L][Re₂(CO)₆(μ-X)₃] (X=Cl, Br or I)

Trimethylplatinum(IV) complexes of anionic N/O and O/O donor ligands: synthesis, NMR and fluxional behaviour

Reaction of pentane-2,4-dione, pyridine-2-carboxylic acid or pyridine-2,6-dicarboxylic acid with trimethylplatinum(IV) gives dimeric complexes of general formulae fac-[PtMe₃L]₂, in which the ionised ligand acts in a chelating and a bridging fashion. High-resolution solid-state ¹⁹ ;Pt NMR data shows that the two platinum atoms are equivalent ; the chemical shielding anisotropy and the principal components of the shielding tensor are reported. The complexes are soluble in co-ordinating solvents, yielding monomeric species of general formulae fac-[PtMe₃L(solvent)], which are fluxional. The pyridine adducts, fac-[PtMe₃L(py)] (L = pentane-2,4-dionato or pyridine-2-carboxylato), are also stereochemically non-rigid. The energetics of the dynamic processes have been studied by standard ¹H NMR band shape analysis techniques ; ΔG ‡ (298 K) is in the range 69–86 kJ mol⁻¹. Solid-state ¹³C, and solution-state ¹³C and ¹⁹;Pt NMR data are also reported

Stereochemical rearrangements in tricarbonylrhenium(I) halide complexes of the non-racemic chiral ligand 2-[(4R),(5R)-dimethyl-1,3-dioxan-2-yl]pyridine (L): a dynamic NMR study

Tricarbonylrhenium(I) halide complexes of the non-racemic chiral ligand 2-[(4R),(5R)-dimethyl-1,3-dioxan-2-yl]pyridine (L), namely fac-[ReX(CO)3L] (X = Cl, Br or I), have been prepared and their latent fluxionality studied by dynamic NMR techniques in the slow and intermediate exchange regimes. In solution, these complexes give rise to four diastereoisomers, depending on the configuration at the metal and at the acetal-carbon atom, respectively; the relative populations are in the order SR > RR >> RS > SS. At moderate temperatures, a reversible ‘acetal ring flip’ leads to formal inversion of configuration at the acetal-carbon atom; the free energies of activation are in the range 84 - 88 kJ mol-1 at 298 K. Above ca. 370 K, reversible ligand dissociation also occurs, leading to an exchange of all four diastereoisomers on the NMR chemical shift time-scale

Synthesis and Reactivity of N,N,N’,N’-Tetramethyldiaminomethane (TMDM) Complexes of Tricarbonylrhenium(I). X-Ray Molecular Structures of [ReBr(CO)₃(TMDM)] and [{Re(bipy)(CO)₃}₂(μ-OH)][SbF₆].

N,N,N′,N′-Tetramethyldiaminomethane (TMDM) is known to act as a source of Me₂NCH in carbonyl substitution reactions, but the reaction of TMDM with the neutral halogenopentacarbonylrhenium(I) compounds gave unexpectedly fac-[ReX(CO)₃(TMDM)] (X=Cl, Br or I), in which the intact TMDM ligand acts in a chelating fashion. The complexes are stable both in the solid-state and in solution, but rapidly decompose on dehalogenation, yielding Re metal. Under anaerobic conditions, the reaction of TMDM with [Re(CO)₃(bipy)]⁺also leads to decomposition. In the presence of oxygen the system is stable. Three Re(bipy) containing species were identified in the reaction mixture: [{Re(CO)₃(bipy)}₂(μ-OH)][SbF₆] (1), which was characterised by X-ray crystallography, [{Re(CO)₃(bipy)}₂(μOH)₂][SbF₆]₂(2) and [Re(OH)(CO)₃(bipy)] (3). Graphical Abstract Reaction of neutral [ReX(CO)₅] (X=Cl, Br or I) with N,N,N′,N′-tetramethyldiaminomethane ((TMDM)) gave unexpectedly fac-[ReX(CO)₃(TMDM)] (X=Cl, Br or I), in which the intact TMDM ligand acts in a chelating fashion. Reaction of TMDM with [Re(CO)₃(bipy)]⁺ under aerobic conditions yields three complexes: [{Re(CO)₃(bipy)}₂(μ-OH)][SbF₆] (1), which was characterised by X-ray crystallography, [{Re(CO)₃(bipy)}₂(μOH₂)][SbF₆]₂(2) and [Re(OH)(CO)₃(bipy)] (3)

Tricarbonylrhenium(I) halide complexes of chiral non-racemic 2-(dioxolanyl)-(dioxanyl)pyridine ligands: synthesis, NMR and DFT calculations.

The chiral non-racemic O/N/O donor ligands 2-[(4R,5R)-4,5-dimethyl-1,3-dioxolan-2-yl]-6-[(4R,6R)-4,6-dimethyl-1,3-dioxan-2-yl]pyridine and 2-[(4R,5R)-4,5-dimethyl-1,3-dioxolan-2-deuteryl]-6-[(4R,6R)-4,6-dimethyl-1,3-dioxan-2-yl]pyridine were prepared in a stepwise fashion form 2,6-dibromopyridine. Reaction with the pentacarbonylhalogenorhenium(I) compounds yields the complexes [ReX(CO)3L], in which the ligands act in a N/O bidentate chelate fashion. There are eight possible diastereoisomers, three of which are observable in solution. DFT calculations indicate that the relative stability of the diastereoisomers is SR5>RR5>SS5≈RS5>RS6>SS6>RR6>SR6. Above ambient temperature, a dynamic process leads to the exchange of 2 of the 3 diastereoisomers: the free energy of activation is ca. 79 kJ mol−1. The results of the DFT calculations and the magnitude of ΔG‡ suggest the dynamic process to be the flip of the co-ordinated acetal ring. DFT calculations on the [ReX(CO)3] complexes of chiral non-racemic 2-(dioxolanyl)-6-(dioxanyl)pyridines, in which the ligands coordinate in a bidentate N/O fashion, indicate that binding of the five-membered dioxolanyl ring is strongly favoured over that of the six-membered dioxanyl ring. In solution 3 of the 8 possible diastereoisomers are observed, two of which undergo exchange above ambient temperature

Successes and Challenges Associated with Solution Processing of Kesterite Cu2ZnSnS4 Solar Cells on Titanium Substrates

Roll-to-roll (R2R) processing of solution-based Cu2ZnSn(S,Se)4 (CZT(S,Se)) solar cells on flexible metal foil is an attractive way to achieve cost-effective manufacturing of photovoltaics. In this work we report the first successful fabrication of solution-processed CZTS devices on a variety of titanium substrates with up to 2.88% power conversion efficiency (PCE) collected on flexible 75 μm Ti foil. A comparative study of device performance and properties is presented aiming to address key processing challenges. First, we show that a rapid transfer of heat through the titanium substrates is responsible for the accelerated crystallisation of kesterite films characterised with small grain size, a high density of grain boundaries and numerous pore sites near the Mo/CZTS interface which affect charge transport and enhance recombination in devices. Following this, we demonstrate the occurrence of metal ion diffusion induced by the high temperature treatment required for the sulfurization of the CZTS stack: Ti4+ ions are observed to migrate upwards to the Mo/CZTS interface whilst Cu1+ and Zn2+ ions diffuse through the Mo layer into the Ti substrate. Finally, residual stress data confirm the good adhesion of stacked materials throughout the sequential solution process. These findings are evidenced by combining electron imaging observations, elemental depth profiles generated by secondary ion mass spectrometry, and x-ray residual stress analysis of the Ti substrate

Trimethylplatinum(IV) complexes of dithiocarbamato ligands: an experimental NMR study on the barrier to C-N bond rotation.

The trimethylplatinum(IV) complexes of a number of dithiocarbamato ligands have been prepared. The complexes are dimeric in the solid-state and in solution, with the ligands acting in both a bridging and chelating fashion. Restricted rotation about the ligand C-N bonds in solution leads to the formation of four rotomers. The kinetics of the restricted rotation were measured by a variety of dynamic NMR techniques in the slow and intermediate exchange regimes. Two distinct processes are observed, namely the independent rotation about each C-N bond and correlated rotation about both C-N bonds. The free energies of activation, which are strongly dependent on the nature of the ligand substituents, are in the range 65 – 88 kJ mol-1 at 298 K. The origins of the barrier to rotation and the effects of the N substituents are discussed. The X-ray structures of fac-[PtMe3(Me2NCS2)]2 (2) and fac-[PtMe3(Ph(H)NCS2)]2 , (6) are reported