The Nature of β‐Agostic Bonding in Late‐Transition‐Metal Alkyl Complexes

Abstract

In general, C À H bonds can be considered chemically inert as a result of their strength, nonpolar nature, and low polarizability. Since the pioneering work of La Placa and Ibers in 1965, who reported the close approach of a CÀH bond to a transition-metal center, there have been many attempts to trace the microscopic control parameters of such C À H activation processes by metal atoms in general.[1] In particular, complexes containing side-on-coordinated (h 2 -CH) moieties next to a transition metal are the focus of intensive research as they allow the systematic study of the CÀH activation phenomenon in molecules and solids in their electronic ground states. Furthermore, M···H À C interactions (M = transition metal) play a key role in the performance of several industrially relevant catalytic processes, such as olefin polymerization.[2]In the course of a systematic analysis of such M···HÀC interactions, Brookhart and Green coined the expression agostic interactions to "discuss the various manifestations of covalent interactions between C À H groups and transitionmetal centers in organometallic compounds". [3a,b] In case of d 0 early-transition-metal alkyl or amido complexes, the strength of agostic interactions is mainly controlled by 1) the local Lewis acidity of the metal center, 2) the extent of negative hyperconjugative delocalization of the M À C/M À N bonding electrons, and 3) to a smaller degree by s(M ! H À C) donation.[3c, 4] For agostic late-transition-metal complexes, however, the control parameters are less clear. We therefore synthesized a variety of new Spencer-type [5] nickel alkyl cations 2 b-d by protonation of the corresponding olefin complexes 1 b-d to study the nature of their pronounced agostic interactions by combined experimental and theoretical charge density studies (Scheme 1).Re-examination of the classic Spencer-type complex [EtNi(dtbpe)]showed a fast rotation of the b-agostic methyl moiety in solution[6] and a systematic crystallographic disorder in the solid state, thus preventing a detailed investigation of the bonding properties of this agostic textbook example by experimental charge-density studies. We therefore replaced the ethylene moiety in 1 a by the sterically more demanding norbornyl (nbe) and dicyclopentadienyl (DCp) ligands. Protonation of 1 b-d yielded the agostic complexes 2 b-d, which all have a significantly reduced fluxional behavior in solution. Furthermore, single crystals of excellent quality could be obtained for 2 b-d, which even allowed an experimental charge-density analysis of 2 c.

Physical Nature of Interactions in ZnIIComplexes with 2,2′-Bipyridyl: Quantum Theory of Atoms in Molecules (QTAIM), Interacting Quantum Atoms (IQA), Noncovalent Interactions (NCI), and Extended Transition State Coupled with Natural Orbitals for Chemical Valence (ETS-NOCV) Comparative Studies

“…. [84][85][86][87] In these contacts an existence of the interaction is manifested by decrease in 1 J(C-H) coupling constants due to the interaction of C-H bond with adjacent moiety (metal, lone pair, or, like in our case, another C-H bond). Such interactions result in weakening of C-H bonds (either due to charge depletion from occupied (C-H) orbital or due to population of the empty *(C-H)).…”

“…Such interactions result in weakening of C-H bonds (either due to charge depletion from occupied (C-H) orbital or due to population of the empty *(C-H)). [84][85][86][87] It can clearly be seen from Table S14 Stabilizing contribution of these diatomic intramolecular interactions toward the overall electronic energy of complexes increases from ZnL to ZnL 3 , but an opposite trend is observed for both coordination bonds (as found from QTAIM, IQA, NCI, and ETS-NOCV).…”

Anagostic Interactions under Pressure: Attractive or Repulsive?

“…Accordingly, the electron density accumulation at the M···H' ago bond critical point (BCP) in 2 is rather small (1(r) exp = 0.082(4); 1(r) calc = [0.10] e À3 ) and a BCP is even lacking in case of 4. [18] The weakness of the agostic interaction in 2 also becomes obvious by direct comparison with the d 8 -Ni complex [(DCpH)Ni(d t bpe)] + [BF 4 ] À (6, DCpH = dicyclopentenyl), [19] which is our benchmark system for compounds displaying strong agostic interactions (Figure 4 b): in that case the Ni···H BCP is well pronounced (1(r) = 0.553(4) e À3 ), the NiÀH' ago bond distance (1.671 (9) ) is significantly shorter than in 2 (2.376 ), and the C À H activation is large (r(C À H) = 1.20(1) ). [19] Apparently, complexes 2, 4, and 6 represent benchmark systems for three different scenarios of agostic interactions (Figure 4).…”

“…[19] Apparently, complexes 2, 4, and 6 represent benchmark systems for three different scenarios of agostic interations (Figure 4). In the final step of our analysis we will focus on the NMR properties of 1 and 2 which display in 1 H NMR experiments rather different chemical shifts of the agostic protons (Scheme 2).The agostic protons in 1 are clearly shifted upfield ( ( 1 H) = -5.88 ppm) but downfield in 2 ( ( 1 H) = +6.1 ppm).…”

Agostic Interactions in Early Transition‐Metal Complexes: Roles of Hyperconjugation, Dispersion, and Steric Effect

“…For late transition‐metal complexes, Scherer et al. showed that β‐agostic interactions can be described in terms of the Dewar–Chatt–Duncanson model, which involves π donation and backdonation between the metal atom and the alkyl backbone …”