Investigation of Zr−C, Zr−N, and Potential Agostic Interactions in an Organozirconium Complex by Experimental Electron Density Analysis

The crystal structure and electron density (ED) distribution of an imine coupling product with an
open zirconocene, Zr(2,4-C7H11)[(i-Pr)NCHPhCH2CMeCHCMeCH2] (C7H11 = dimethylpentadienyl), have
been derived from accurate synchrotron X-ray diffraction measurements. The molecular structure reveals
asymmetric coordination of Zr by the pentadienyl (2,4-C7H11) ligand (〈Zr−C〉 = 2.56(6) Å), the butadiene
fragment (〈Zr−C〉 = 2.43(5) Å), and the amide nitrogen atom (Zr−N = 2.0312(5) Å) of the second ligand.
The study of the ED and its topological analysis affords new insight into the bonding and electronic structure
of the title zirconium complex. The interactions between the metal center and the ligands are represented
by a Zr−N bond path and one Zr−C bond path with each of the pentadienyl and butadiene moieties, contrary
to the usually depicted global metal−ligand bonding. The butadiene and pentadienyl groups exhibit a
polarization of the corresponding π-like ED in the C−Zr directions, indicating that the whole conjugated
systems are nonetheless involved. The 4d atomic orbitals of Zr exhibit unusual populations according to
ligand field considerations, which reveal a high degree of σ-donation from the conjugated π systems of
both ligands. As deduced from numerical integration over the topologically defined atomic basins, the Zr
to ligand charge transfer is 1.48 e to the C17NH24 ligand and 0.68 e to dimethylpentadienyl. Topological
analysis of a short intramolecular Zr···(C,H) contact provides no indication of the presence of agostic
interactions, despite a small Zr−N−C angle of 102.87(4)°. Thus, no bond path and BCP (bond critical
point) of the ED are found in the Zr···(C,H) region, which would have provided evidence for such direct
interactions, nor is there any evidence for charge accumulation between the Zr and H atoms, or for
lengthening of the C−H bond involved in the putative interaction. These characteristics, similar to those in
other distorted situations, may be common for other electron-deficient d0 complexes.