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

Abstract

Chiral cis -MoO 2 (acac) 2 racemizes via four pathways that agree with and extend upon Muetterties’ topological analysis for dynamic MX 2 (chel) 2 complexes. Textbook Ray–Dutt and Bailar twists are the least favored with barriers of 27.5 and 28.7 kcal/mol, respectively. Rotating both acac ligands of the Bailar structure by 90° gives the lower Conte–Hippler twist (20.0 kcal/mol), which represents a valley–ridge inflection that invokes the trans isomer. The most favorable is a new twist that was found by 90° rotation of only one acac ligand of the Bailar structure. The gas-phase barrier of 17.4 kcal/mol for this Dhimba–Muller–Lammertsma twist further decreases upon inclusion of the effects of solvents to 16.3 kcal/mol (benzene), 16.2 kcal/mol (toluene), and 15.4 kcal/mol (chloroform), which are in excellent agreement with the reported experimental values.

Chiral‐at‐Molybdenum – A Computational Study on N‐Substituted cis‐MoO2(nacnac)2

“…All four racemization pathways resemble those of the Mo‐complex with two acac ligands, [25] but the magnitudes of the barriers differ markedly for two of them. This is the case for the Bailar twist (14.9 kcal/mol) which is much lower than that for MoO 2 (acac) 2 (24.0 kcal/mol).…”

“…This is indeed what is found, but unexpectedly only to a limited degree. Racemization can occur by four distinct pathways, i. e., the Bailar, [31] Rây-Dutt, [32] Conte-Hippler (CH), [26] and Dhimba-Muller-Lammertsma (DML) [25] twists, all of which were fully confirmed by intrinsic reaction coordinate (IRC) calculations (see Supporting Information). The relative ωB97XÀ D energies of the minima (Figure 1) and the transition structures (Figure 2) are given in Table 1 together with those for MoO 2 (acac) 2 using a smaller (S) 6-31G(d) and a larger (L) 6-311 + G(2d,p) basis set (LANL2DZ for Mo).…”

“…[24] Non-dissociative racemization occurs readily for the established bis-chelated olefin epoxidation catalyst cis-MoO 2 (acac) 2 (acac = acetylacetonate) because the barrier for this process is only 16 kcal/mol. [25,26] The challenge is to design a chiral-atmolybdenum catalyst with ligands that resists racemization.…”

Chiral‐at‐Metal Racemization Unraveled for MX2(a‐chel)2 by means of a Computational Analysis of MoO2(acnac)2

“…The trans(OO) isomer is 24.1 kcal/ mol less stable and for trans(NO) the energy difference is even 54.5 kcal/mol, which compares to the cis-trans energy differences of both MoO 2 (acac) 2 and MoO 2 (nacnac) 2 . [8,9] Chiral trans(OO) has its ligand NÀ H atoms on the same side of the MoO 2 unit but tilted by 15°from ligand planarity in opposite directions to minimize H … H repulsion. Trans(NO) has C s symmetry at 6-311 + + G(2d,p) with both acnac ligands oriented in the same plane with the dioxo ligands of the orthogonal MoO 2 plane bend by 140 °towards one of the acnac ligands.…”

“…Recently, we reported on the racemization of MoO 2 (acac) 2 (acac = acetylacetonate) [8] and MoO 2 (nacnac) 2 (nacnac = βdiketiminate), [9] which are 6-coordinate with two symmetrical bidentate ligands. Most surprisingly, we found two pathways that are more facile than the well-established Bailar (B) [10] and Ray-Dutt (RD) [11] twists and named them the Conte-Hippler (CH) [12] and Dhimba-Muller-Lammertsma (DML) [8,9] twists (Figure 1). This theoretical finding is in harmony with the topological analysis of Muetterties for MX 2 (chel) 2 with symmetrical chelates.…”