NMR chemical shift analysis decodes olefin oligo- and polymerization activity of d ⁰ group 4 metal complexes

Abstract

d metal-alkyl complexes (M = Ti, Zr, and Hf) show specific activity and selectivity in olefin polymerization and oligomerization depending on their ligand set and charge. Here, we show by a combined experimental and computational study that the C NMR chemical shift tensors of the α-carbon of metal alkyls that undergo olefin insertion signal the presence of partial alkylidene character in the metal-carbon bond, which facilitates this reaction. The alkylidene character is traced back to the π-donating interaction of a filled orbital on the alkyl group with an empty low-lying metal d-orbital of appropriate symmetry. This molecular orbital picture establishes a connection between olefin insertion into a metal-alkyl bond and olefin metathesis and a close link between the Cossee-Arlmann and Green-Rooney polymerization mechanisms. TheC NMR chemical shifts, the α-H agostic interaction, and the low activation barrier of ethylene insertion are, therefore, the results of the same orbital interactions, thus establishing chemical shift tensors as a descriptor for olefin insertion.

C−H Activation and Olefin Insertion in d8 and d0 Complexes: Same Elementary Steps, Different Electronics

“…In the ethylene insertion transition state Sc‐INS , δ 11 is still perpendicular to the plane and becomes much more deshielded (200 ppm) than in 1‐Sc or Sc‐SBM . As previously discussed, this strong deshielding arises from the magnetic coupling of a high‐lying σ (M−C) orbital with a low‐lying empty orbital of π *(M−C) symmetry in the approximate horizontal plane . This empty π *(M−C) orbital indicates the presence of a filled π (M−C) orbital, thus implying partial alkylidene character of the M−C bond, as further supported by the acute M−C−H a angle of the α ‐agostic C−H a bond.…”

“…Figure ,a shows a significant difference between the respective M−C and C−H bond contributions to σ 11 ( para ). In 1‐Sc , the σ (M−C) orbital presents the single dominant contribution to σ 11 , resulting from the coupling of a high‐lying filled σ (M−C) orbital with a low‐lying vacant π *(M−C) orbital . The contribution of this orbital is much less pronounced in systems with d 8 electron count: 1‐Pd shows a dominant deshielding contribution from σ (C−H a ) of the coplanar C−H a bond.…”

“…In previous studies, olefin insertion with d 0 complexes has been described as isolobal to metallacyclobutane formation in olefin metathesis . Both processes can be considered as [2+2] cycloadditions, that mainly differ in the degree of M−C alkylidene character, as shown in Figure ,a .…”

“…Recently, a detailed investigation of the electronic structures of d 0 ‐metallocene complexes based on NMR chemical shift analysis has shown that olefin insertion into the metal−carbon bond and C−H bond activation through σ ‐bond metathesis are isolobal reactions for these compounds ( Figure ,a ). Both elementary steps are particularly favored when the M−C bond features a partial π ‐character, which is evidenced by a deshielded α ‐carbon chemical shift and in some cases by an acute M−C−H angle of the α ‐agostic C−H bond ( Figure ,b ) . The deshielding of the α ‐carbon is mainly due to a highly deshielded δ 11 component of the chemical shift tensor that is perpendicular to the M−C axis and the pseudo horizontal plane of the Cp 2 M fragment.…”

“…For d 0 systems, the two reactions are isolobal. b) Previous work on d 0 metallocene systems showed that olefin insertion and C−H activation ( σ ‐bond metathesis) are enabled by a π ‐character of the M−C bond, evidenced by analysis of the α ‐carbon NMR chemical shift tensors . c) Selected experimental NMR data of Cp* 2 ScMe ( 1‐Sc ) and ether adducts of α ‐diimine Pd(II) and Pt(II) methyl complexes ( 1‐Pd‐OEt , 1‐Pt‐OEt , Ar=2,6‐(C 6 H 3 ( i Pr) 2 )) that are active in C−H activation and olefin insertion upon ether decoordination.…”