Understanding Copper(I)–Ethylene Bonding Using Cu K-Edge X-ray Absorption Spectroscopy

Abstract

Copper plays many important roles in ethylene chemistry, thus generating significant interest in understanding the structures, bonding, and properties of copper­(I)-ethylene complexes. In this work, the ethylene binding characteristics of a series of isolable Cu­(I)-ethylene compounds supported by a systematic set of fluorinated and nonfluorinated bis- and tris­(pyrazolyl)­borate and the related bis­(pyrazolyl)­methane ligands have been investigated. Through a combination of X-ray absorption spectroscopy and quantum chemical calculations, we characterize their geometric and electronic structures and the role that fluorinated ligands play in lowering the electron density at Cu sites. Such ligands increase the ethylene-to-Cu σ-donor interaction and, correspondingly, decrease the Cu-to-ethylene π back-bonding. This latter interaction leads to a partial vacancy in the Cu 3d level, which manifests experimentally as a low-energy feature in the Cu K pre-edge, allowing for its direct observation and comparison within a series of Cu­(I) compounds. The pre-edge feature is reproduced by TD-DFT calculations, and its energy position and total intensity are used to quantitatively probe Cu–ethylene bonding. The variations in the Cu electronic structure influence the stability and overall ethylene bonding strength of these compounds, ultimately showing how substituents on the supporting ligands have a notable effect on their physical and chemical properties.

Coinage metal-ethylene complexes of sterically demanding 1,10-phenanthroline ligands

“…We have been utilizing nitrogen-based chelators such as bis(pyrazolyl)borates, 26–36 bis(pyrazolyl)methanes, 30,34,37 and bis(pyridyl)borates 38,39 in coinage metal (Cu, Ag, Au) chemistry, in particular, to develop homogeneous catalysts, 27,29,40 achieve olefin-paraffin separations, 35,36 and to stabilize molecules with CO, ethylene and acetylene, and larger alkenes and alkynes on these metal ions 26,28–30,36,37,40–42 for bonding investigations and to serve as models for reaction intermediates in catalytic processes involving these metal-gas combinations. In this work, we report the use of two sterically demanding phen ligands, 2,9-bis(2,4,6-triisopropylphenyl)-1,10-phenanthroline ( L1 ) and 2,9-bis(2,4,6-trifluoromethylphenyl)-1,10-phenanthroline ( L2 ) in a systematic study of coinage metal ethylene chemistry (Fig.…”