Some problems in the stereochemistry of coordination compounds

Bailar, John C.

doi:10.1016/0022-1902(58)80179-7

Cited by 223 publications

(71 citation statements)

References 24 publications

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“…Since the two oxygen ligands stay cis, as if they would be held in place by being part of a bidentate, the situation resembles chiral tris-chelate, octahedral ligands ML3, which are known to racemize via the Ray-Dutt or the Bailar twist mechanisms. [58][59][60] Similar twist mechanisms may thus be expected to apply to MoO2(acac)2 as well. Looking at the planes of each bidentate ligand, in the Bailar mechanism of racemization, each plane turns synchronously in one direction (say clockwise) through an achiral, trigonal prismatic transition state of D3 symmetry.…”

Section: Racemization Mechanisms In Moo2(acac)2mentioning

confidence: 88%

Dynamic NMR and Quantum-Chemical Study of the Stereochemistry and Stability of the Chiral MoO₂(acac)₂ Complex in Solution

Conte

Hippler

2016

J. Phys. Chem. A

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The stereochemistry and dynamics of MoO2(acac)2 in benzene, chloroform and toluene was investigated by variable temperature 1 H NMR, density functional theory (SOGGA11-X, B3LYP) and ab initio (MP2) methods. In solution, an equilibrium between two chiral enantiomers with C2 symmetry was identified, Λ-cis-MoO2(acac)2 and ∆-cis-MoO2(acac)2. The two enantiomers are connected via achiral cis transition states that switch the enantiomeric conformations via a RayDutt, Bailar and a newly described racemization twisting mechanism. All three mechanisms have similar calculated activation energies. Activation parameters Ea, ∆H ‡ , and ∆S ‡ were experimentally determined for the exchange process, with a small, negative ∆S ‡ , and a positive ∆H ‡ of 68.1 kJ mol -1 in benzene, 54.9 kJ mol -1 in chloroform, and 60.6 kJ mol -1 in toluene, in reasonable general agreement with the calculations. trans configurations of MoO2(acac)2 are very much higher in energy than cis and are not relevant in the temperature range experimentally studied, 243-340 K. The enantiomers interconvert within seconds near room temperature, and much faster at elevated temperatures. Racemization will thus prevent the use of enantiomerically pure MoO2(acac)2 for chiral catalysis under practical conditions.

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Section: Racemization Mechanisms In Moo2(acac)2mentioning

confidence: 88%

Dynamic NMR and Quantum-Chemical Study of the Stereochemistry and Stability of the Chiral MoO₂(acac)₂ Complex in Solution

Conte

Hippler

2016

J. Phys. Chem. A

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“…2+ give close-lying features on the Oh-TP shape map (red in Figure 7b), a much larger area of the map is covered by the ls and the hs state structures of [FeL] 2+ (blue in Figure 7b). Furthermore, both the ls and the hs state structures of [FeL] 2+ match the implications of Bailar's trigonal twist pathway (line in Figure 7b; [28] 2+ . In agreement with the structural argumentation Figure 7b; [28] 2+ .…”

Section: Discussionmentioning

confidence: 64%

“…Furthermore, both the ls and the hs state structures of [FeL] 2+ match the implications of Bailar's trigonal twist pathway (line in Figure 7b; [28] 2+ . In agreement with the structural argumentation Figure 7b; [28] 2+ . In agreement with the structural argumentation outlined above, we associate the qualitatively different thermal barriers with qualitatively modulated reaction coordinates.…”

Section: Discussionmentioning

confidence: 64%

“…Projection of the coordination sphere along the (pseudo) C 3 axis reveals significant distortion from a regular octahedron (right in Figure 1). The trigonal twist angle θ was introduced by Hendrickson et al in order to quantify such Bailar-type [28,36] distortions of L 6 coordination spheres. The trigonal twist angle θ amounts to 60 • for a regular octahedron and gives 0 • for a trigonal prism.…”

Section: Structural Characteristics Ofmentioning

confidence: 99%

“…In this picture, slow exchange prevails, when the reaction coordinate consists of two components; (i) trivial isotropic "breathing" of the coordination sphere due to (de)population of antibonding eg-type orbitals in the (ls) hs state; (ii) anisotropic torsion of the coordination sphere along a (pseudo) C3 axis, akin to Bailar's trigonal twist [28]. Actually, L and derived ligands are known to facilitate the synthesis of trigonal prismatic complexes [29][30][31].…”

Section: Introductionmentioning

confidence: 99%

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Structural Dynamics of Spin Crossover in Iron(II) Complexes with Extended-Tripod Ligands

et al. 2017

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Selective manipulation of spin states in iron(II) complexes by thermal or photonic energy is a desirable goal in the context of developing molecular functional materials. As dynamic spin-state equilibration in isolated iron(II) complexes typically limits the lifetime of a given spin state to nanoseconds, synthetic strategies need to be developed that aim at inhibited relaxation. Herein we show that modulation of the reaction coordinate through careful selection of the ligand can indeed massively slow down dynamic exchange. Detailed structural analysis of [FeL] 2+ and [ZnL] 2+ (L: tris(1-methyl-2-{[pyridin-2-yl]-methylene}hydrazinyl)phosphane sulfide) with crystallographic and computational methods clearly reveals a unique trigonal-directing effect of the extended-tripod ligand L during spin crossover, which superimposes the ubiquitous [FeN 6 ] breathing with trigonal torsion, akin to the archetypal Bailar twist. As a consequence of the diverging reaction coordinates in [FeL] 2+ and in the tren-derived complex [Fe(tren)py 3 ] 2+ , their thermal barriers differ massively, although the spin crossover energies are close to identical. As is shown by time-resolved transient spectroscopy and dynamic 1 H-NMR line broadening, reference systems deriving from tren (tris-(2-aminoethyl)amine), which greatly lack such trigonal torsion, harbor very rapid spin-state exchange.

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Quantitative 2D EXSY and Dynamic19F-NMR Studies of the Dynamic Behaviour of the Bidentate Chelate Complex Ga(fox)3 (fox = 5-fluoro-8-hydroxyquinoline)

Gromova¹,

Jarjayes²,

Hamman³

et al. 2000

Eur. J. Inorg. Chem.

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Some problems in the stereochemistry of coordination compounds

Cited by 223 publications

References 24 publications

Dynamic NMR and Quantum-Chemical Study of the Stereochemistry and Stability of the Chiral MoO₂(acac)₂ Complex in Solution

Dynamic NMR and Quantum-Chemical Study of the Stereochemistry and Stability of the Chiral MoO₂(acac)₂ Complex in Solution

Structural Dynamics of Spin Crossover in Iron(II) Complexes with Extended-Tripod Ligands

Quantitative 2D EXSY and Dynamic19F-NMR Studies of the Dynamic Behaviour of the Bidentate Chelate Complex Ga(fox)3 (fox = 5-fluoro-8-hydroxyquinoline)

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Some problems in the stereochemistry of coordination compounds

Cited by 223 publications

References 24 publications

Dynamic NMR and Quantum-Chemical Study of the Stereochemistry and Stability of the Chiral MoO2(acac)2 Complex in Solution

Dynamic NMR and Quantum-Chemical Study of the Stereochemistry and Stability of the Chiral MoO2(acac)2 Complex in Solution

Structural Dynamics of Spin Crossover in Iron(II) Complexes with Extended-Tripod Ligands

Quantitative 2D EXSY and Dynamic19F-NMR Studies of the Dynamic Behaviour of the Bidentate Chelate Complex Ga(fox)3 (fox = 5-fluoro-8-hydroxyquinoline)

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Dynamic NMR and Quantum-Chemical Study of the Stereochemistry and Stability of the Chiral MoO₂(acac)₂ Complex in Solution

Dynamic NMR and Quantum-Chemical Study of the Stereochemistry and Stability of the Chiral MoO₂(acac)₂ Complex in Solution