Chemistry

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

Peter J. Heard et al. — Mon, 20 Jun 2011 06:12:26 PDT

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.

Main Group Dithiocarbamate Complexes

Peter J. Heard — Mon, 13 Jun 2011 06:49:51 PDT

Dithiocarbamates, R₂NC(S)S⁻, are highly versatile ligands towards main group metals; they can stabilise a wide variety of oxidation states and co-ordination geometries, and seemingly small modifications to the ligand can lead to significant changes in the structure/behaviour of the complexes formed. The literature on main group dithiocarbamate complexes has not been reviewed as a whole since the 1970s, despite the large number of publications that have appeared subsequently. This review focuses primarily on structural aspects of all main group dithiocarbamate complexes, covering the essential literature from 1978 to 2003.

NMR of Organomagnesium Compounds

Peter J. Heard — Mon, 13 Jun 2011 06:49:49 PDT

Since their discovery, the exact solution composition of Grignard compounds has been

the subject of considerable debate, and given the power of NMR in elucidating chemical

structure it is unsurprising that it was applied to the study of Grignard and other organomagnesium compounds from its earliest days. However, the complex nature of the solution behaviour of such compounds and the low magnetic field strengths then available often frustrated proper analysis of the data, and the first reported NMR studies were generally inconclusive. Worse, the interpretation of early NMR spectra was often based on preconceived (and as it is now realized incorrect) notions as to the nature of the compounds in solution, so caution must be exercised when considering much of the pre-1970’s data.

Although the advent of higher field NMR instruments and our increasing understanding

of the solution behaviour of organomagnesium reagents have greatly improved the veracity

of NMR studies, detailed NMR reports on such compounds remain relatively sparse. The bulk of the literature that has been published was done so prior to the 1980’s. The reasons

for the paucity of reported NMR studies are probably three-fold: (i) organomagnesium

reagents are generally highly sensitive, making the isolation of sufficiently pure samples

problematic; (ii) different preparations can apparently give quite different NMR spectra;

(iii) the exact solution behaviour depends on a number of factors, making it difficult to

draw any general conclusions.

This paper reviews the current state of the literature on the NMR of Grignard and other organomagnesium compounds.

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

Peter J. Heard et al. — Thu, 02 Dec 2010 07:40:03 PST

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.

A detailed NMR study of the solution stereodynamics in tricarbonylrhenium(I) halide complexes of the non-racemic chiral ligand 2,6-bis[(4R,5R)-4,5-dimethyl-1,3-dioxolan-2-yl]pyridine (L¹) and the molecular structure of fac-[ReBr(CO)₃(L¹)]

Peter J. Heard et al. — Thu, 02 Dec 2010 07:21:37 PST

1 Tricarbonylrhenium(I) halide complexes of the non-racemic chiral ligand 2,6-bis[(4R, 5R)-dimethyl-1,3-dioxan-2-yl]pyridine (L¹), namely fac-[ReX(CO)₃(L¹)] (X = Cl, Br or I), have been prepared. In these complexes the ligand is bound in a bidentate fashion, with the N atom of the pyridine ring and an O atom of one of the acetal rings co-ordinated to the octahedral metal centre. The bidentate mode is confirmed by the X-ray structure of fac-[ReBr(CO)₃(L¹)]. There are four possible diastereoisomers, depending on the configuration at the metal centre and at the acetal-carbon atom of the co-ordinated ring; the X-ray structure of fac-[ReBr(CO)₃(L¹)] shows that the SR diastereoisomer is present in the solid state. In solution, three of the four possible diastereoisomers are observed, namely SR, RR and RS; their relative populations are in the order SR > RR > SS. Above ambient temperature the complexes are stereochemically non-rigid. The fluxional kinetics have been measured by a combination of standard band shape analysis and selective inversion experiments. Two distinct processes are present: an acetal ring flip and exchange of the pendant and co-ordinated acetal rings. The latter process occurs via two independent mechanisms, namely tick-tock and rotation pathways. The activation energies for the stereodynamics are in the ranges 72 – 73 kJ mol⁻¹ (tick-tock), 77 – 78 kJ mol⁻¹ (acetal ring flip) and 83 – 90 kJ mol⁻¹ (rotation) at 298 K.

Dynamic Stereochemical Rearrangements in Chira Organometallic

Peter J. Heard — Wed, 17 Nov 2010 07:51:37 PST

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 ).

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

Peter J. Heard et al. — Tue, 16 Nov 2010 08:50:40 PST

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.

Tricarbonylrhenium(I) halide complexes of 2-[(4R,6R)-4,6-dimethyl-1,3-dioxan-2-yl]pyridine (L¹) and 2,6-bis[(4R,6R)-4,6-dimethyl-1,3-dioxan-2-yl]pyridine (L²): structure and solution stereodynamics

Peter J. Heard et al. — Mon, 15 Nov 2010 08:58:37 PST

The non-racemic chiral ligands 2-[(4R,6R)-4,6-dimethyl-1,3-dioxan-2-yl]pyridine (L¹) and 2,6-bis[(4R,6R)-4,6-dimethyl-1,3-dioxan-2-yl]pyridine (L²) reacted with the [ReX(CO)₅] compounds (X = Cl, Br, or I) to form fac-[ReX(CO)3(L¹)] and fac-[ReX(CO)₃(L²)], respectively. The ligands co-ordinate to the metal moiety in a bidentate fashion via the N donor of the pyridine ring and one of the acetal O atoms. The bonding modes are confirmed by the molecular structures of [ReI(CO)₃(L¹)] and [ReBr(CO)₃(L²)]. There are four possible diastereoisomers, depending on the configuration at the metal and the acetal carbon atom. In the solid state [ReI(CO)₃(L¹)] and [ReBr(CO)₃(L²)] exist exclusively as the RR and SR diastereoisomers, respectively. In solutions of the complexes of L¹ all four diastereoisomers are observed in varying concentrations, with the SR species being dominant. Only two of the four possible species are observed in solutions of the complexes of L², with the SR diastereoisomer being present in ca. 96% diastereomeric excess over the RR diastereoisomer. Above ambient temperatures both series of complexes are fluxional. A reversible flip of the acetal ring in the complexes [ReX(CO)₃(L¹)] leads to the pair-wise exchange of diastereoisomers as a result of the formal inversion of configuration at the acetal carbon atom; the free energies of activation are in the range 78–82 kJ mol⁻¹. The ring flip process appears to occur via a pseudo seven-co-ordinate transition state, in which both O atoms of the acetal ring are loosely bound to the metal. The pendant and co-ordinated acetal rings in the complexes of L² are exchanged via the tick-tock mechanism; the free energies of activation are in the range 75–79 kJ mol⁻¹.

Isolation and characterisation of the pyridazine (pydz) bridged halogenotrimethylplatinum(IV) complexes [(PtXMe₃)₂(μ-pydz)] (X = Cl, Br or I). X-Ray structure of [(PtIMe₃)₂(μ-pydz)]

Edward W. Abel et al. — Mon, 15 Nov 2010 08:18:26 PST

The bridged dinuclear platinum(IV) complexes [(PtXMe₃)₂( μ-pydz)] (X = Cl, Br, I) were isolated as minor products from the reaction between pyridazine (pydz) and the halogenotrimethylplatinum(IV) tetramers. The X-ray crystal structure of the iodo complex reveals both a double halide bridge and a pyridazine bridged structure with a Pt Pt non-bonded distance of 3.5937(5) Å. The platinum atoms show only a slight deviation from an idealized octahedral geometry, but the structure is folded in the equatorial plane by 51.5° to accommodate the iodines between the two platinum moieties.

A dynamic NMR study of 1,2-metallotropic shifts in trimethylplatinum(IV) halide complexes of 3-methylpyridazine.

Edward W. Abel et al. — Mon, 15 Nov 2010 07:52:49 PST

3-Methylpyridazine (3-Mepydz) reacts with halogenotrimethylplatinum(IV), [PtXMe₃)₄], to afford complexes of type fac-[PtXMe₃(3-Mepydz)₂] (X = Cl, Br or I). In solution these complexes undergo 1,2-metallotropic shifts between the contiguous nitrogen donors producing different linkage isomers. Solution abundances of these ismers are strongly influenced by the steric effects of the 3-methyl group on the pyridazine ring, with the (1,1)-isomer always being predominant (≈ 95%). Activation energies for the fluxion were determined by variable temperature ¹H NMR spectroscopy, values of the (1,1) → (1,2) isomer shift being in the range 74–78 kJ mol⁻¹.

Formation of dimethylamine complexes of trimethylplatinum(IV) from reactions with N,N,N’,N’-tetramethyldiaminomethane

Edward W. Abel et al. — Fri, 15 Oct 2010 06:37:30 PDT

Attempts to prepare complexes of trimethylplatinum(IV) with N,N,N′N′-tetramethyldiaminomethane (TMDM) as either a chelating or bridging ligand gave only the complexes of dimethylamine [PtMe₃X(HNMe₂))]₂(X = Cl, Br and I ; 1–3 and [PtMe₃((2,2′-bipyridyil) (HNMe₂))]⁺[[BF₄]]⁺ (4)). ¹HH NMR evidence showed that only one of the two possible isomers was formed for 1–3.

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].

Peter J. Heard et al. — Wed, 13 Oct 2010 01:48:14 PDT

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).

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]⁻.

Peter J. Heard et al. — Mon, 11 Oct 2010 07:25:54 PDT

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 dithiocarbamato ligands: an experimental NMR study on the barrier to C-N bond rotation.

Peter J. Heard et al. — Mon, 11 Oct 2010 02:42:50 PDT

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.

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

Peter J. Heard et al. — Mon, 11 Oct 2010 02:25:41 PDT

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.