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

Abstract

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.

Racemization Pathway for MoO2(acac)2 Favored over Ray–Dutt, Bailar, and Conte–Hippler Twists

“…22b Conte and Hippler determined by variable 1 H NMR spectroscopy a modest activation energy E a of 16.9 kcal/mol for the racemization of cis -MoO 2 (acac) 2 in benzene, 13.7 kcal/mol in chloroform, and 15.1 kcal/mol in toluene, indicating a small solvation effect. 24 These barriers are similar to those reported by the group of Wise in 1971. 25 SOGGA11-X/LANL2DZ+G** calculations by Conte and Hippler gave E 0 barriers of 26.7 and 27.2 kcal/mol for the Ray–Dutt and Bailar twists and a lower, but still sizable, barrier of 19.4 kcal/mol for a different pathway; the heights of the barriers were not affected by inclusion of the effect of solvents.…”

“…The calculated barriers for these different solvent systems compare exceptionally well with the experimental E a values of 16.9 kcal/mol (benzene), 15.1 kcal/mol (toluene), and 13.7 kcal/mol (chloroform), which were determined by Conte and Hippler. 24 Evidently, this new twist represents the most favorable pathway by which the enantiomers of cis -MoO 2 (acac) 2 racemize.…”

Synthesis, Characterization, and Catalytic Exploration of Mononuclear Mo(VI) Dioxido Complexes of (Z)-1-R-2-(4′,4′-Dimethyl-2′-oxazolin-2′-yl)-eth-1-en-1-ates

“…The treatment of alcoholic solutions (L n H; n = 1, 2 and 4: MeOH; L 3 H: 95% aq. EtOH) of LH (∼2 equivalents) with cis-MoO2(acac)2 (acac = κ 2 -O,O'-acetylacetonato) [17][18][19] leads to the facile formation of complexes 1-4, presumably involving the loss of acacH (Scheme 2), in good to moderate yields (see Section 4). Complexes 1-4 are air-and moisture-stable solids and range in color from yelloworange to orange to red.…”

“…This observation is in agreement with the spectroscopic characterization of the isolated complexes 1-4. Hence, it is likely that 1-4 retain the cis-orientation of the two Mo=O units known to be present in the starting complex MoO 2 (acac) 2 [17][18][19]. Of the cis-dioxido isomers, form III (trans-N ox : ox = oxazoline) was found to be the most stable (relative values of ∆H f = 0.00; +56.8 and +44.5 kJ mol -1 respectively, for III, IV and V).…”

The Problem of Dynamic Process Manifestation in Identification of Organic Compounds by NMR Spectroscopy

“…Есть два пути решения проблемы отрицательных динамических эффектов: увеличение скорости динамических процессов или изменение заселенности положений путем смещения динамического 1 равновесия. Основными факторами, влияющими на скорость динамического процесса и динамическое равновесие, являются температура и растворитель 2 [5][6][7][8][9]. Принято считать, что, варьируя эти параметры ЯМР-эксперимента, можно достичь соответствия между числом сигналов в спектре и числом структурных фрагментов, продуцирующих сигнал, и существенно улучшить качество спектра за счет увеличения разрешенности сигналов.…”