J(Si,H) Coupling Constants of Activated Si–H Bonds

Abstract

We outline in this combined experimental and theoretical NMR study that sign and magnitude of J(Si,H) coupling constants provide reliable indicators to evaluate the extent of the oxidative addition of Si-H bonds in hydrosilane complexes. In combination with experimental electron density studies and MO analyses a simple structure-property relationship emerges: positive J(Si,H) coupling constants are observed in cases where M → L π-back-donation (M = transition metal; L = hydrosilane ligand) dominates. The corresponding complexes are located close to the terminus of the respective oxidative addition trajectory. In contrast negative J(Si,H) values signal the predominance of significant covalent Si-H interactions and the according complexes reside at an earlier stage of the oxidative addition reaction pathway. Hence, in nonclassical hydrosilane complexes such as CpTi(PMe)(HSiMeCl) (with n = 1-3) the sign of J(Si,H) changes from minus to plus with increasing number of chloro substituents n and maps the rising degree of oxidative addition. Accordingly, the sign and magnitude of J(Si,H) coupling constants can be employed to identify and characterize nonclassical hydrosilane species also in solution. These NMR studies might therefore help to reveal the salient control parameters of the Si-H bond activation process in transition-metal hydrosilane complexes which represent key intermediates for numerous metal-catalyzed Si-H bond activation processes. Furthermore, experimental high-resolution and high-pressure X-ray diffraction studies were undertaken to explore the close relationship between the topology of the electron density displayed by the η(Si-H)M units and their respective J(Si,H) couplings.

Pressure‐Enhanced C–H Bond Activation in Chloromethane Platinum(II) Complexes

“…This methodology has been introduced recently by one of us to study the related bonding scenario of σ agostic interactions and σ silane complexes. [28a], We will show in the following that application of hydrostatic pressure can also be used to compress the η 1 (C–H)Pt moieties in single crystals of 2 to push the C–H br bond of the chloroform ligand closer to the metal. As a consequence we might systematically enhance the Pt(d z ² )→σ*(C–H) back donation and Pt(RY)←σ(C–H) σ donation to trigger the C–H bond activation upon increasing pressure similar to the electronic situation in σ‐agostic d 8 Ni complexes .…”

Manganese Silyl Dihydride Complexes: A Spectroscopic, Crystallographic, and Computational Study of Nonclassical Silicate and Hydrosilane Hydride Isomers

“…These nonclassical hydrosilane complexes have been the subject of a number of reviews, − and from a molecular orbital perspective, they are typically described using a modification of the Dewar–Chatt–Duncanson bonding model, with concomitant σ donation and π backdonation to/from the metal (Figure ). A variety of terms have been employed to describe these intermediate complexes, , and comprehensive studies by a number of groups, including those of Nikonov, − Sabo-Etienne, ,− and Scherer, − have shown that a continuum exists between σ-hydrosilane complexes involving minimal π-backdonation, and classical silyl hydride complexes. This continuum follows the oxidative addition/reductive elimination reaction coordinate, with progressively weakening Si–H interactions being indicative of a greater degree of oxidative addition.…”

“…Analyses of nonclassical hydrosilane complexes have naturally focused on the strength of the Si–H interaction, from which the degree of oxidative addition can be inferred. These strengths have often been determined computationally, ,,,,,− while experimental studies have primarily involved solid-state structural determination of the Si–H distance. However, care must be taken when discussing Si–H distances determined by X-ray crystallography, due to difficulties associated with accurately locating hydrogen atoms from the difference map.…”

“…However, it is important to recognize that 1-bond Si–H coupling has a negative sign, whereas 2-bond Si–H coupling has a positive sign. Thus, J Si,H passes through zero on the continuum from a σ-hydrosilane to a classical silyl hydride complex, and proper placement of a complex along this continuum requires knowledge both of the magnitude and sign of J Si,H . ,,, …”

“…Recently, Scherer reported an alternative spectroscopic method for determining the sign (and magnitude) of J Si,H in silyl hydride/hydrosilane complexes where the silicon center also features a terminal Si–H bond. This method relies upon 2D 1 H– 1 H COSY data, using the negative coupling constants of the terminal Si–H bond as an internal reference to determine definitely the sign of J Si,H from the orientation of 29 Si satellites associated with a cross-peak of interest ( vide infra ). , While there is still some debate, J Si,H values ranging from 0 to −70 Hz are commonly considered to indicate activated Si−H bonds in nonclassical hydrosilane complexes, where 1 J Si,H coupling dominates over 2 J Si,H coupling (i.e., σ-donation dominates over π-backdonation). Values more negative than −70 Hz are indicative of σ-hydrosilane complexes with limited π-backdonation, and positive values are indicative of complexes with very weak or non-existent Si−H interactions). ,, …”