Electronic and Steric Manipulation of the Preagostic Interaction in Isoquinoline Complexes of Rh^I

Abstract

Structures and properties were obtained by density functional calculations for the complexes [RhCl(CO)2(L)] (L = isoquinoline ligands) containing close Rh····H separations. Electrostatic repulsion keeps the atoms apart and Rh····C separations are prevented from developing much further by ligand inflexibility. Natural bond orbital analysis shows small components of C–Hσ to Rh (agostic) donation and Rh to C–Hσ* backdonation. Both σ‐ and π‐electron‐withdrawing substituents produce an increase in the Rh····H separation, while ligand inflexibility prevents σ‐ and π‐electron‐donating substituents from decreasing the Rh····C separation. Substitution at C2 by Me closes up the Rh····H separation and CHMe2 and CMe3 groups cause a structural change which minimises the interaction. Substitution at C2 by a Ph group gives the shortest Rh····H separation (2.187 Å) and largest Rh to C–Hσ* backdonation. Electron donation at C7 or at C7 and C9 increases the Rh····H separation, as the ligand flexes substantially to accommodate a decrease in the Rh····C separation. On the basis of the minimal C–Hσ to Rh (agostic) donation involved, the complexes are best described as preagostic.

Weak M···H–C Interactions in Neutral Complexes, Anions and Cations of Palladium(II) and Rhodium(I) Containing the iso‐Quinoline Ligand – Anagostic or Preagostic?

“…Comparing this structure with the relaxed C–H bond structure (Figure b), it is seen that the iso ‐quinoline ligand is rotated about the Pd–N bond such that the aromatic ring systems sit much less upright over the coordination plane (C 14 –Pd–N–C 13 torsion angles 131.3 and 81° respectively). This type of rotation was already found in the complex [RhCl(CO) 2 ( iso ‐quinoline)] reported earlier by us . A further distortion that occurs in energy minimised structure 1 but not seen in the X‐ray result is that the nitrogen atom is not face‐on to the metal centre but is bent away, which is indicated by the O–Pd–N–C 3 torsion angle changing from 92.0° in a to 60.8° in 1 (see Table ).…”

“…For the above‐plane interactions, these orbital overlaps simply do not develop to an appreciable extent. However, as seen in the present work and our other studies in this field,, small overlaps do develop and it is possible that all above‐plane interactions will have a component of covalency, in which case there would be no anagostic complexes or at least very few if the electrostatic description is rigorously applied.…”

“…A further question to be answered is whether the C–H bond/antibonding and metal molecular orbitals can become resonant to an extent whereby sufficient covalency can be introduced so that the complexes can be termed preagostic . As part of an effort to understand the effect of forcing very close above‐plane M ··· H–C separations,, we show in the present work how changing the overall charge of a complex containing the relatively inflexible iso ‐quinoline ligand influences the separation distance and effects covalency.…”

“…Bearing in mind that anagostic interactions are regarded as being repulsive, the charges present on the C 11 , H 11 and Pd atoms in 1 would then appear to be a reasonable assessment criterion for the local repulsion involved in the interaction and we have used this approach in understanding anagostic/preagostic separations in our previous studies , . Thus for 1 , QTAIM charges show that there are partial positive charges on the H atom and the metal [ q (H) value, 0.045 e; q (Pd) value, 0.508 e ], which represents a repulsive electrostatic interaction that could keep the two entities apart.…”

“…A key finding of this study was that flexibility in the ligand system was a major factor in allowing the hydrogen to avoid very close contact with the metal, so that the anagostic separation did not become smaller than 2.548 Å. Using a much less‐flexible ligand system in the same type of complex, we turned to the iso ‐quinoline ligand (see Figure ), which gave complexes of the short variety but even in this case, sufficient distortion of the ligand occurred to influence the separation distance …”