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-C(7)H(11))[(i-Pr)NCHPhCH(2)CMe=CHCMe=CH(2)] (C(7)H(11) = dimethylpentadienyl), have been derived from accurate synchrotron X-ray diffraction measurements. The molecular structure reveals asymmetric coordination of Zr by the pentadienyl (2,4-C(7)H(11)) ligand ( = 2.56(6) A), the butadiene fragment ( = 2.43(5) A), and the amide nitrogen atom (Zr-N = 2.0312(5) A) 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 pi-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 sigma-donation from the conjugated pi 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 C(17)NH(24) 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) degrees. 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 d(0) complexes.