Influence of counterions on the structure of bis(oxazoline)copper(ii) complexes; an EPR and ENDOR investigation

Abstract

X- and Q-band EPR and ENDOR spectroscopy was used to study the structure of a series of heteroleptic and homoleptic copper bis(oxazoline) complexes, based on the (-)-2,2'-isopropylidenebis[(4S)-4-phenyl-2-oxazoline] ligand and bearing different counterions (chloride versus triflate); labelled [Cu(II)(1a-c)]. The geometry of the two heteroleptic complexes, [Cu(II)(1a)] and [Cu(II)(1c)], depended on the choice of counterion. Formation of the homoleptic complex was only evident when the Cu(II)(OTf)(2) salt was used (Cu(II)(Cl)(2) inhibited the transformation from heteroleptic to homoleptic complexes). The hyperfine and quadrupole parameters for the surrounding ligand nuclei were determined by ENDOR. Well resolved (19)F and (1)H couplings confirmed the presence of both coordinated water and TfO(-) counterions in [Cu(1a)].

Chiral Copper(II) Bis(oxazoline) Complexes Directly Coordinated to Amine‐Functionalized Phenylene/Biphenylene Periodic Mesoporous Organosilicas as Heterogeneous ­Catalysts

“…Nevertheless, the value is higher than that for CuCl 2 , which indicates the presence of nitrogen in the . This may be due to a change in the geometry of the coordination sphere from distorted octahedral to square planar [1] and/or coordination to the nitrogen atom of the amine-modified mesoporous NH 2 PhPMO. The g z values of the copper(II) complexes immobilized on the amine-modified mesoporous organosilicas are similar and lower than the value for the corresponding copper(II) salts, which is consistent with the incorporation of nitrogen into their coordination spheres.…”

“…Nevertheless, the results of the simulation of the spectra by using the WinEPR software are compiled in Table 2, together with parameters from the free copper(II) salts and complexes with related bis(oxazoline) ligands found in the literature. [1] The spectra are axial or almost axial with A z higher than the corresponding free copper(II) salts, which is indicative of the presence of nitrogen in the coordination sphere. Furthermore, the A z values are high in the case of the CuMe 2 PhBox-(OTf ) 2 @NH 2 PhPMO and CuPhBox(OTf ) 2 @NH 2 BphPMO materials and are typical of square-planar structures.…”

“…Copper(II) complexes with bis(oxazoline) ligands are usually generated in situ by combining suitable amounts of the ligand and the copper(II) salt. [1,7,8] Herein, the complexes were allowed to form in a ligand/copper ratio of 1:1 until the solution stabilized, as determined by UV/Vis spectroscopy. The amine-modified mesoporous organosilicas were then added to the complexes.…”

“…There is a wide range of synthetic catalysts based on a diverse class of chiral organic ligands that can achieve excellent levels of enantioselectivity for many different types of reactions. [1] Bis(oxazoline)s are among this class of privileged ligands. [2,3] They are bidentate organic ligands with a N 2 coordination sphere and are able to coordinate several types of transition metals ( Figure 1).…”

The Role of Low Valent Transition Metal Complexes in Homogeneous Catalysis: An EPR Investigation

“…For a number of years we have exploited a number of advanced EPR and hyperfine based techniques to explore the structure-function relationships in transition metal based homogeneous catalysis, including Fe(I) for catalytic C-C bond formation [8,9], Cr(I) for ethylene oligomerisation [10], Co(II) for hydrolytic kinetic resolution [11], Ni(I) for cross coupling reactions [12] and Cu(II) for asymmetric aziridination [13]. We have demonstrated how the EPR approach can be used not only to identify the key oxidation states involved in these reactions, but also how the technique can help to unravel the complementary role of the ligands in controlling the selectivity and stability of the intermediates formed.…”