The validity of approximate equations for k c in dynamic nuclear magnetic resonance

Reaction of a stoichiometric amount of Hzedt/NaHedt (edt = ethane-l,2-dithiolate) with the appropriate metal amide, followed by cation exchange, gave the complexes [NEt412 [M(edt),] (M = Ti, 1; Hf, 2) and [AsPh,] [Nb(edt),].CH,CN, 3. The structure of 3 has been determined crystallographically (space group; cell dimensions; Z; unique data (I 2 3u (I); R/R,): p i ; a = 9.095(1), b = 14.609(3), c = 15.639(3) A and a = 60.09(2), P = 80.07(3), y = 89.89(2)"; z = 2; 1488; 4.3/4.5. The coordination geometry of 3 is severely distorted from the trigonal prismatic (TP) structure found in the analogous Nb(1,2-S2C6H4)3-r which contains an unsaturated 1,2-dithiolate ligand system. The average twist angle between triangular faces is 33.3". However, on the basis of VT 'H NMR spectroscopy it is argued that the TP form is at most 9-10 kcal mol-' less stable than the observed geometry. A comparison between the structures of tris-chelate complexes of the early transition metals containing saturated and unsaturated 1,2-dithiolates and related ligands is made, and the factors thought to be responsible for TP vs. octahedral geometry are reviewed and their relative importance assessed.Key words: structure, trigonal prismatic, 1,2-dithiolate, early transition metals. . La gComCtrie de coordination du composC 3 est fortement deformCe par rapport a la structure prismatique trigonale (PT) qui a kt6 observCe antkrieurement avec I'analogue Nb(l,2-SZC6H4)3-qui contient un ligand 1,2-dithiolate insaturk. L'angle moyen de dCviation entre les faces triangulaires est de 33,3". Toutefois, sur la base de la RMN du 'H a tempkrature variable, on suggkre que la difference de stabilitk entre la forme PT et la gComCtrie observke ne favorise cette dernikre que par 9-10 kcal mol-I. On a comparC les structures des complexes tris chClates des mCtaux de transition du debut contenant des ligands 1,2-dithiolates saturts et insaturks ainsi que d'autres ligands apparentis; on passe en revue les facteurs que I'on croit responsable des structures PT vs. les gComCtries octaCdriques et on Cvalue leurs importances relatives. Mots clis : structure, prismatique trigonal, 1,2-dithiolate, mCtaux de transition du dCbut.[Traduit par la revue] Introduction The possibility of trigonal prismatic (TP) geometry in highly covalent do transition metal complexes was predicted as early as 1932 by Hultgren (I), based on the concept of metal orbital hybridization presenting a more favorable bonding picture for TP than for octahedral geometry. This result has been corroborated by Hoffman et al. (2) on the basis of extended Hiickel MO calculations.In 1965 the first observation of a discrete TP complex, Re(l ,2,-S2C2Ph2)3 (3), rekindled interest in TP geometry and since then a host of factors, such as (a) ligand constraint (4, 5), (b) ligand field stabilization energy (2,4,6), (c) interdonor atom bonding (3, 7-9), (d) proper matching of ligand and metal orbital energies (1, 2, 10-12), (e) the symmetry of ligand n orbitals (2, 8, 13), and (f) the overall charge of the complex, have bee...

Section: Bond Distancesmentioning

confidence: 77%

Trigonal prismatic versus octahedral coordination geometry of transition metal dithiolates: Comparison between saturated and unsaturated early transition metal tris(1,2-dithiolate) complexes

Martin¹,

Takats²

1989

“…In addition, the sinall impurity peak, identified with a star in Fig. 1 adds to T, is estimated at -140" and a AG' value of 6.2 kcal/mol is calculated at this temperature (8).…”

Section: Resultsmentioning

confidence: 99%

On the Mechanism of Ring Inversion in Cycloheptene and Its Derivatives

St‐Jacques¹,

Vaziri²

1973

Free energy barriers to ring inversion of 3,3-dimethyl-, 4,4-dimethyl-, and 5,s-dimethylcycloheptene were found to be much smaller than those of corresponding benzocycloheptene derivatives but show the same trend. This observation strongly suggests that the chair conformationof both types of molecules interconverts through the same mechanism. On the other hand, the AGf value for 4,4,6,6-tetramethylcycloheptene is about 3 kcal/mol out of line from that expected from a consideration of the behavior of the corresponding benzocycloheptene derivative. An explanation is offered.Les barrikres d'tnergie libre pour l'inversion des dimethyl-3,3, dimethyl-4,4 et dimethyl-5,5 cycloheptimes, tout en conservant la m&me tendance, sont plus faibles que celles qui caractkrisent les derives correspondants du benzocycloheptkne. Cette observation indique que la conformation chaise des deux classes de molecules s'inverse par le m&me mecanisme. Par contre, la valeur de AG+ pour le tetramethyl-4,4,6,6 cycloheptene est environ 3 kcal/mol plus elevke que celle attendue aprks examen des benzocycloheptknes correspondants. Une explication est presentee.

“…We therefore report the results of our study of the conformation of 1,5-benzodioxepin (2) and its 3,3-dimethyl derivative (7). Our nmr observations provide more complete insight into the conformational and dynamic features characterizing this class of heterocyclic molecules.…”

mentioning

confidence: 99%

Conformational dynamic investigation of 1,5-benzodioxepin and its dimethyl derivative by ¹H and ¹³C nmr

Menard¹,

St‐Jacques²

1981

The conformational and dynamic properties of 1,5-dioxa-6,7-benzocycloheptene (2; 1,5-benzodioxepin) and its dimethyl derivative (7) have been investigated by dynamic 1H and 13C nmr methods. Analysis of the spectra at low temperatures (below coalescence) indicates that the most stable seven-membered ring conformations detected for solutions in CHF2Cl are a mixture of the chair (C, 80%) and the twist-boat (TB, 20%) for 2 and a mixture of TB (64%) and C (36%) for 7. Free energy barriers were determined for the chair inversion of 2 (6.5 kcal/mol) and 7 (7.5 kcal/mol) as well as for the twist-boat pseudorotation of 7 (6.8 kcal/mol). The conformational properties of these seven-membered heterocycles are discussed and compared to those known for 2,4-benzodioxepins.