Can a Formally Zwitterionic Rhodium(I) Complex Emulate the Charge Density of a Cationic Rhodium(I) Complex? A Combined Synchrotron X-ray and Theoretical Charge-Density Study

Abstract

The molecular electron densities of structurally related cationic ([(κ(2)-3-P(i)Pr(2)-2-NMe(2)-indene)Rh(COD)](CF(3)SO(3)), [1c](CF(3)SO(3))) and formally zwitterionic ([(κ(2)-3-P(i)Pr(2)-2-NMe(2)-indenide)Rh(COD)], 1z) complexes were accurately determined using synchrotron bright-source X-ray radiation at 30 K followed by multipolar refinement (COD = η(4)-1,5-cyclooctadiene). The densities were also obtained from density functional theory calculations with a large, locally dense basis set. A 28-electron ([Ar]3d(10)) core of the Rh atom was modeled by an effective core potential to obtain a density that was then augmented with relativistic cores according to the Keith-Frisch approximation. Calculations were performed at the experimental geometry and after vacuum-phase geometry optimization starting from the experimental geometry. Experimental and calculated geometries and electron-density distributions show that the electron density and electronic structure in the region of the Rh center are not significantly altered by protonation of the aromatic ring and that formal removal of CF(3)SO(3)H from [1c](CF(3)SO(3)) affords a complex 1z possessing substantial zwitterionic character (with a charge separation of ca. 0.9 electronic charge) featuring a negatively charged aromatic indenide framework. Further, the molecular electrostatic potentials of 1c and 1z exhibit similar topography around the metal, despite being drastically different in the vicinity of the indene or indenide portion of the cation (1c) and zwitterion (1z), respectively. Collectively, these observations obtained from high-level experimental and theoretical electron-density analysis confirm, for the first time, that appropriately designed zwitterionic complexes can effectively emulate the charge distribution found within ubiquitous cationic platinum-group metal catalyst complexes, in keeping with recent catalytic investigations.

Iridium(I) Complexes with Anionic N-Heterocyclic Carbene Ligands as Catalysts for the Hydrogenation of Alkenes in Nonpolar Media

“…2028 cm –1 ) also match those measured for the neutral analog with average values of 2032 and 2037 cm –1 , indicating at best a marginal increase of the donor ability by borate substitution. Accordingly, one may conclude that, in zwitterions such as 4 and 8 , the negative charge at the borate ion is well separated from the cationic metal complex fragment and that these complexes emulate the charge distribution and reactivity of their cationic analogs, notwithstanding their potentially quite different solubility ( vide infra ).…”

Intermolecular Metal-Involving Pnictogen Bonding: The Case of σ-(SbIII)-Hole···dz2[PtII] Interaction

“…Analysis of relatively large electron densities (0.021–0.028 e/bohr 3 ), positive Laplacian values, small but negative energy density (from −0.004 to −0.001 hartree/bohr 3 ), as well as 1 < | V |/ G < 2 indicated that the Sb···Pt interactions are closed-shell but still exhibit some degree of the covalency . Notably, experimentally obtained ρ values in BCPs of conventional Rh–P, − Pd–N, , and Zn–N coordination pnictogen-metal bonds are in the 0.050–0.111 e/bohr 3 range according to the high-resolution XRD data.…”

Contemporary X-ray electron-density studies using synchrotron radiation

“…A challenging 30 K ED study using SR on two related Rhcontaining complexes was recently carried out by Bendeif et al (2012). The challenge comes from the fact that rhodium is a second-row transition metal, with a large core scattering contribution.…”

“…Experimentally determined molecular EP at an isosurface with |V(r)| = 0.05 a.u., showing the similarity of the form of the EP around the Rh atom (red denotes negative EP and blue positive EP). Reprinted with permission fromBendeif et al (2012). Copyright (2012) American Chemical Society.…”