Conformational studies on chiral rhodium complexes by ECD and VCD spectroscopy

Abstract

This article reports vibrational circular dichroism (VCD) and electronic circular dichroism (ECD) spectroscopic studies in acetonitrile on the chiral Rh(2)(O-Phe-Cbz)(1)(OAc)(3) and Rh(2)(O-Phe-Ac)(1)(OAc)(3) complexes (abbreviated Rh(2)Z(1) and Rh(2)Ac(1) , respectively; Phe, L-phenylalanine; Cbz, benzyloxycarbonyl; Ac, acetyl) supported by theoretical calculations. The ECD spectra of the complexes depend on temperature that indicates the conformational mobility of the chiral ligands. Calculations of the VCD spectra were performed at ab initio (DFT) level of theory using Gaussian 03 [B3LYP functional combined with the LANL2DZ basis set for the dirhodium core and the 6-31G(d) basis set for other atoms]. The population-weighted sums of the computed VCD spectra of the conformers are in excellent agreement with the experimental VCD spectra. The combination of the VCD and ECD spectroscopic methods led us to the structural characterization of the complexes.

Chelate Structure of a Dirhodium–Amino Acid Complex Identified by Chiroptical and NMR Spectroscopy

“…This is followed by a weaker negative couplet at around 1554 cm –1 , assigned to ν as (COO) of the acetate, in which the positive component band (observed at 1568 cm –1 ) contains some coupling contribution from the aromatic ring vibration. The wavenumber of the ν as (COO) band of the amino acid ligand is unusually high in comparison with the similar band at 1599 cm –1 of [Rh 2 (O‐Phe‐Cbz)(OAc) 3 ],21b which can be explained by a different binding mode of the chiral ligand. The positive band at 1444 cm –1 and the very weak negative band at 1424 cm –1 are assigned to ν s (COO) of the acetate, the latter being coupled with δ s (CH 3 ).…”

Chiral Dirhodium Catalysts: A New Era for Asymmetric Catalysis

“…Recently, Szilvagyi et al [106] reported the vibrational circular dichroism (VCD) and electronic circular dichroism (ECD) spectroscopic studies for the chiral Rh 2 (O-Phe-Cbz). (OAc) 3 and Rh 2 (OPhe-Ac).…”

Structure analysis of proteins, peptides and metal complexes by vibrational circular dichroism

“…20 The ECD spectra of the complexes depend on temperature that indicates the conformational mobility of the chiral ligands. The agreement between the measured and calculated VCD spectra gave unique information on the steric structure of complexes Rh 2 (OPhe-Cbz) 1 (OAc) 3 (5) or Rh 2 (O-Phe-Ac) 1 (OAc) 3 (6).…”