In this study, carried out in collaboration with the research groups of C. F. Matta (Department of Chemistry and Physics, Mount Saint Vincent University, Halifax), M. Stradiotto (Department of Chemistry, Dalhousie University, Halifax, Canada) and C. Lecomte (CRM2, Institut Jean Barriol, Nancy), the detailed information obtained from high resolution X-ray diffraction experiments is used as an alternative way of determining the physico-chemical properties of rhodium Rh (I) complexes and, in particular, their reaction mechanism. Establishing their physico-chemical properties is a key stage in analyzing structure-function relationships in these complexes and provides information that will lead to greater understanding of their catalytic properties. Rhodium complexes are involved notably during organic synthesis used in some processes in the chemical industry.
The first high resolution diffraction experiments at 30 K performed on the CRISTAL beamline allowed the crystal structure determination, as well as the electron density distribution of the rhodium complex C25H37NPRh. Diffraction intensities from a single crystal were measured on CRISTAL’s four-circle diffractometer, using monochromatic radiation with energy E = 22.651 keV (λ = 0.54737 Å), just below the K edge of Rh, in order to minimise absorption effects. Over 9,700 diffraction frames (812429 Bragg reflections) were collected up to a resolution of sinθ/λ = 1.37 Å-1 (d= 0.37 Å) in 20 hours of measurements.
The structural parameters (atomic positions and displacements) obtained from the spherical model* were used as input for a multipolar refinement** of the electron density, carried out using the MoPro1 software. Detailed analysis, based on the multipolar model2, allows the accurate modelling of valence electron density, the parameters of which were used for topological analysis*** and the calculation of electrostatic properties. Figure 1 shows the distribution of static deformation** electron density of this rhodium complex. This illustrates electron density accumulation centred on the atomic bonds of the organic ligands. The displacement of the maximum density reveals the polarity of the atomic bonds. It can also be observed that the deformation densities of the C—H bonds are polarized towards the hydrogen atoms. In agreement with chemical properties, the nitrogen atom N1 coordinating the rhodium (Rh1) presents an electron lone pairs directed towards it (Figure 1-b).
Figure 1. Static deformation electron density map:
(a) in the (C1-C7A-C3A) plane,
(b) in the (N1-Rh1-P) plane.
The electron density around the rhodium atom, the determination of which was the most difficult part of the experiment, is clearly defined. Figure 2 shows the overlaps in electron densities that appear between rhodium and its ligands. It can be observed that the rhodium-phosphorus interaction (Rh1—P1) is stronger than that involving the nitrogen atom (N1). In addition, topological analysis around the rhodium atom has revealed the nature of the interactions between this atom and the six atoms of its coordination sphere. This approach has also revealed two types of Rh—C=C interaction, contrary to the results expected from a simple geometric analysis.
Figure 2. Three-dimensional representation of the total (static) electron density of the (C25H37NPRh) complex.
* Atomic electron clouds represented as neutral and independent spheres.
** Deformation of spherical electron clouds for modelling interatomic electron density.
*** Method of characterizing the nature of the chemical bond.
Référence :
[1] Guillot, B., Viry L., Guillot, R., Lecomte, C. & Jelsch, C. (2001). J. Appl. Cryst. 34, 214-223.
[2] Hansen, N. K. & Coppens, P. (1978). Acta Cryst. A34, 909-921.
