Copper(I)−Olefin Complexes: The Effect of the Trispyrazolylborate Ancillary Ligand in Structure and Reactivity

Structural diversity of copper(I)-ethylene complexes with 2,4-bis(2-pyridyl)pyrimidine directed by anions

The 3 : 1 reaction of [Cu(C2H4)n]ClO4 with 2,4-bis(2-pyridyl)pyrimidine (bpprd) in Me2CO under C2H4 afforded yellow prism crystals of the dinuclear Cu(I)-C2H4 complex [Cu2(bpprd)(η2-C2H4)2(ClO4)2] (1). The 3 : 1 reaction of [Cu(C2H4)n]NO3 with bpprd in Me2CO under C2H4 afforded yellow plate crystals of the tetranuclear Cu(I)-C2H4 complex [Cu4(bpprd)2(η2-C2H4)4(μ-NO3)2](NO3)2 (2). The 10 : 1 reaction of [Cu(C2H4)n]BF4 with bpprd in Me2CO under C2H4 afforded yellow plate crystals of the dinuclear Cu(I)-C2H4 complex [Cu2(bpprd)(η2-C2H4)2(BF4)]BF4 (3). The 3 : 1 reaction of [Cu(C2H4)n]BF4 with bpprd in Me2CO under C2H4 afforded red prism crystals of the polymeric Cu(I)-C2H4 complex {[Cu6(bpprd)4(η2-C2H4)2(μ-η2:η2-C2H4)(μ-BF4)2](BF4)4}n (4). The X-ray crystal structures of complexes 1-4 have been determined. The structural diversity of Cu(I)-C2H4 complexes bridged by bpprd with different anions was demonstrated. The 1D Cu(I)-bpprd/C2H4 coordination polymer 4 bridged by unusual μ-η2:η2-C2H4 and the μ-BF4- anion is of particular significance. Complex 1 exhibited relatively well-resolved 1H NMR signals of bpprd and C2H4 (δ = 4.97 ppm) in (CD3)2CO at 23 °C.

Spectroscopic and QM/MM studies of the Cu(I) binding site of the plant ethylene receptor ETR1

Herein, we present, to our knowledge, the first spectroscopic characterization of the Cu(I) active site of the plant ethylene receptor ETR1. The x-ray absorption (XAS) and extended x-ray absorption fine structure (EXAFS) spectroscopies presented here establish that ETR1 has a low-coordinate Cu(I) site. The EXAFS resolves a mixed first coordination sphere of N/O and S scatterers at distances consistent with potential histidine and cysteine residues. This finding agrees with the coordination of residues C65 and H69 to the Cu(I) site, which are critical for ethylene activity and well conserved. Furthermore, the Cu K-edge XAS and EXAFS of ETR1 exhibit spectroscopic changes upon addition of ethylene that are attributed to modifications in the Cu(I) coordination environment, suggestive of ethylene binding. Results from umbrella sampling simulations of the proposed ethylene binding helix of ETR1 at a mixed quantum mechanics/molecular mechanics level agree with the EXAFS fit distance changes upon ethylene binding, particularly in the increase of the distance between H69 and Cu(I), and yield binding energetics comparable with experimental dissociation constants. The observed changes in the copper coordination environment might be the triggering signal for the transmission of the ethylene response.

Binding Interactions in Copper, Silver and Gold π-Complexes

The copper(I), silver(I), and gold(I) metals bind π‐ligands by σ‐bonding and π‐back bonding interactions. These interactions were investigated using bidentate ancillary ligands with electron donating and withdrawing substituents. The π‐ligands span from ethylene to larger terminal and internal alkenes and alkynes. Results of X‐ray crystallography, NMR, and IR spectroscopy and gas phase experiments show that the binding energies increase in the order Ag<Cu<Au and the binding energies are slightly higher for alkynes than for alkenes. Modulation of the electron density at the metal using substituents on the ancillary ligands shows that the π‐back bonding interaction plays a dominant role for the binding in the copper and gold complexes.

Extremely bulky copper(i) complexes of [HB(3,5-{1-naphthyl}2pz)3]- and [HB(3,5-{2-naphthyl}2pz)3]- and their self-assembly on graphene

The synthesis and characterization, using NMR (¹H and ¹³C), infrared spectroscopy, and X-ray crystallography, of the ethene and carbon monoxide copper(i) complexes of hydridotris(3,5-diphenylpyrazol-1-yl)borate ([Tp^Ph₂]^-) and the two new ligands hydridotris(3,5-bis(1-naphthyl)pyrazol-1-yl)borate ([Tp^(1Nt)₂]^-) and hydridotris(3,5-bis-(2-naphthyl)pyrazol-1-yl)borate ([Tp^(2Nt)₂]^-) are described. X-ray crystal structures are presented of [Cu(Tp^Ph₂)(C₂H₄)] and [Cu(Tp^(2Nt)₂)(C₂H₄)]. The compound [Cu(Tp^Ph₂)(C₂H₄)] features interactions between the protons of the ethene ligand and the π-electron clouds of the phenyl substituents that make up the binding pocket surrounding the copper(i) center. These dipolar interactions result in strongly upfield shifted signals of the ethene protons in ¹H-NMR. [Cu(Tp^(1Nt)₂)(CO)] and [Cu(Tp^(2Nt)₂)(CO)] were examined using infrared spectroscopy and were found to have CO stretching vibrations at 2076 and 2080 cm^-1 respectively. The copper(i) carbonyl complexes form self-assembled monolayers when drop cast onto HOPG and thin multilayers of a few nanometers thickness when dip coated onto graphene. General macroscopic trends such as the different tendencies to crystallize observed in the complexes of the two naphthyl-substituted ligands appear to extend well to the nanoscale where a well-organized monolayer could be observed of [Cu(Tp^(2Nt)₂)(CO)].

Synthesis and Properties of "Sandwich" Diimine-Coinage Metal Ethylene Complexes

Synthesis and full characterization of cationic isostructural "sandwich" diimine-coinage metal ethylene complexes are reported. Ethylene self-exchange kinetics proceeds by an associative exchange mechanism for Cu and Au complexes. The fastest ligand exchange was observed for Ag complex 8a. The upper limit of ΔG‡, assuming associative ligand exchange, was found to be ca. 5.0 kcal/mol. Ethylene self-exchange in Cu complex 7b proceeds with ΔG298‡ = 12.9 ± 0.1 kcal/mol, while the exchange is the slowest in Au complex 9, with ΔG298‡ = 16.7 ± 0.1 kcal/mol. Copper complex 7b is unusually stable and can survive in air for years.

Copper-induced ammonia N–H functionalization

The Tp^MsCu core has been found to bind ammonia as well as to promote stoichiometric or catalytic modification of the ammonia N–H bond.

A computational study of the effects of ancillary ligands on copper(i)–ethylene interaction

Structural and electronic calculations on a ligand database are used to quantify the influence of the ancillary ligands and coordination mode on the electronic structure of Cu(i) ethylene complexes.

Highly tunable fluorinated trispyrazolylborates [HB(3-CF3-5-{4-RPh}pz)3]− (R = NO2, CF3, Cl, F, H, OMe and NMe2) and their copper(i) complexes

A spectroscopic series of fluorinated trispyrazolylborate ligands was used to make copper(i) complexes with carbon monoxide and ethene. The tunable ligands induce clear trends in infrared and NMR spectra of the copper compounds.

Structural diversity of copper(I)-N-heterocyclic carbene complexes; ligand tuning facilitates isolation of the first structurally characterised copper(I)-NHC containing a copper(I)-alkene interaction

The preparation of a series of imidazolium salts bearing N-allyl substituents, and a range of substituents on the second nitrogen atom that have varying electronic and steric properties, is reported. The ligands have been coordinated to a copper(I) centre and the resulting copper(I)-NHC (NHC=N-heterocyclic carbene) complexes have been thoroughly examined, both in solution and in the solid-state. The solid-state structures are highly diverse and exhibit a range of unusual geometries and cuprophilic interactions. The first structurally characterised copper(I)-NHC complex containing a copper(I)-alkene interaction is reported. An N-pyridyl substituent, which forms a dative bond with the copper(I) centre, stabilises an interaction between the metal centre and the allyl substituent of a neighbouring ligand, to form a 1D coordination polymer. The stabilisation is attributed to the pyridyl substituent increasing the electron density at the copper(I) centre, and thus enhancing the metal(d)-to-alkene(π*) back-bonding. In addition, components other than charge transfer appear to have a role in copper(I)-alkene stabilisation because further increases in the Lewis basicity of the ligand disfavours copper(I)-alkene binding.