Unsupported Ti−Co and Zr−Co Bonds in Heterobimetallic Complexes:  A Theoretical Description of Metal−Metal Bond Polarity

Abstract

The synthesis, structural, and theoretical characterization of heterobimetallic complexes [CH3Si{SiMe2N(4-CH3C6H4)}3M−Co(CO)3(L)] (M = Ti, Zr; L = CO, PPh3, PTol3) with unsupported metal−metal bonds between cobalt atoms and titanium or zirconium atoms is being reported. The synthesis of the dinuclear compounds was achieved by salt metathesis of the chlorotitanium and zirconium complexes and the alkalimetal carbonylates. X-ray crystal structure analyses of four of these heterobimetallic complexes established the unsupported metal−metal bonds [M = Ti, L = CO (3): 2.554(1) Å; M = Ti, L = PTol3 (4b): 2.473(4) Å; M = Zr, L = CO (5): 2.705(1) Å; M = Zr, L = PPh3 (6a): 2.617(1) Å] as well as the 3-fold molecular symmetries. Upon axial phosphine substitution, a metal−metal bond contraction of ca. 0.08 Å is observed, which also results in the quantum chemical structure optimizations performed on the model compounds [(H2N)3Ti−Co(CO)4] (3x) and [(H2N)3Ti−Co(CO)3(PH3)] (4x) using gradient-corrected and hybrid density functionals. A theoretical study of the homolytic dissociation of the metal−metal bonds focuses on the relaxation energies of the complex fragments and indicates that the geometrical constraints imposed by the tripod ligand lead to a major thermodynamic contribution to the stability of the experimentally investigated complexes. The central question of the polarity of the metal−metal bond is addressed by detailed analysis of the calculated electron charge distribution using natural population analysis (NPA), charge decomposition analysis (CDA), Bader's atoms in molecules (AIM) theory, and the electron localization function (ELF). Both the orbital-based NPA and CDA schemes and the essentially orbital-independent AIM and ELF analysis suggest a description of the Ti−Co bond as being a highly polar covalent single bond. The combination of AIM and ELF is employed for the first time to analyze metal−metal bond polarity and appears to be a powerful theoretical tool for the description of bond polarity in potentially ambiguous situations.

Photochemical Route to Actinide-Transition Metal Bonds: Synthesis, Characterization and Reactivity of a Series of Thorium and Uranium Heterobimetallic Complexes

“…33.44°, presumably to help minimize steric repulsions upon formation of the heterobimetallics. 36 The bond lengths and angles observed for 4 4 and 5 5 (Table S1) compare well with related uranium isocarbonyls, as well as a Yb compound having an analogous isocarbonyl bridged cobalt unit.…”

“…The CO stretch of the carbonyl trans to the metal-metal bond provides a probe of the interaction strength and is indeed the only stretch with a significant disparity between the two complexes. 36 The stretch is weaker in the thorium example, which could be attributed to a stronger metal-metal interaction in the uranium case, causing in turn a weaker backbonding from the cobalt to the carbonyl. Additionally, the stretches for 6 6 and 7 7 are weaker than for those of their isocarbonyl counterparts and consistent with a polarization of charge from the cobalt to the actinide center and a corresponding decrease in Co-CO backbonding.…”

Highlights in the Renaissance of Amidometal Chemistry

“…[125] Rotation barriers, however, can also be dictated by the steric interactions of the ligand peripheries. A good qualitative description of the bond polarity in unbridged Ti ± Co or Zr ± Co bimetal complexes [129] is also possible by Bader analysis of the electron density (Baders AIM) [130] and electron localization functions (ELF). [131] The high partial charges of the complex fragments emphasize the strong polar character of the Ti ± Co bond.…”

Cooperative Reactivity of Early-Late Heterodinuclear Transition Metal Complexes with Polar Organic Substrates

“…This situation is responsible for an additional thermodynamic stabilization of the dinuclear compounds. [10] Some preliminary studies into their reactive behaviour have been carried out indicating the general pattern described above. [11,12] In this paper we give a comprehensive account of the reactive behaviour of such heterodinuclear complexes towards a wide variety of organic substrates most of which contain unsaturated functional groups.…”