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

The molecular electron densities of structurally related
cationic
([(κ2-3-PiPr2-2-NMe2-indene)­Rh­(COD)]­(CF3SO3), [1c]­(CF3SO3)) and formally zwitterionic ([(κ2-3-PiPr2-2-NMe2-indenide)­Rh­(COD)], 1z) complexes were accurately
determined using synchrotron bright-source X-ray radiation at 30 K
followed by multipolar refinement (COD = η4-1,5-cyclooctadiene).
The densities were also obtained from density functional theory calculations
with a large, locally dense basis set. A 28-electron ([Ar]­3d10) 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 CF3SO3H from [1c]­(CF3SO3) 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.