Noncovalent Interactions in Organometallic Chemistry: From Cohesion to Reactivity, a New Chapter

ConspectusNoncovalent
interactions (NCIs) have long interested a vast community
of chemists who investigated their “canonical categories”
derived from descriptive crystallography, e.g., H-bonds, π–π
interactions, halogen/chalcogen/tetrel bonds, cation−π
and C–H−π interactions, metallophilic interactions
in the broad sense, etc. Recent developments in theoretical chemistry
have enabled the treatment of noncovalent interactions under new auspices:
dispersion-force-inclusive density functionals have emerged, which
are reliable for modeling small to large molecular systems. It is
possible to perform the full analysis of the contributions of London,
Debye, and Keesom forces, i.e., the main components of van der Waals
forces, by the DFT-D and ab initio methods at a reasonable computational
cost. Our research has been focusing for now 15 years on the role
of NCIs in the cohesion of organometallic complexes. NCIs are not
only effective in Werner’s secondary coordination sphere but
also in the metal’s primary one. The stabilization of electron-unsaturated
transition metal complexes by hemichelation, metal–metal donor–acceptor
complexes, and self-aggregation of cationic Rh­(I) chromophores have
indeed outlined the significance of the London dispersion force as
an attractive force operating throughout the whole molecule or molecular
assembly. The recent outburst of interest in C–H bond functionalization
led us to address the broader question of reaction and catalyst engineering:
although one can now satisfactorily analyze bonding and molecular
cohesion in transition-metal-based organometallic systems, can modern
theoretical methods guide reactivity exploration and the engineering
of novel catalytic systems? We addressed this question by investigating
the ambiphilic metal–ligand activation/concerted metalation–deprotonation
mechanism involved in transition-metal-catalyzed directed C–H
bond functionalization. This endeavor was initiated having in scope
the construction of a rationale for the transposition of 4–5d
metal chemistry to earth-abundant 3d metals. In this base-assisted
mechanism of C–H bond metalation, agostic interactions are
necessary but not sufficient because C–H bond breaking actually
relies on the attractive NCI coding of a proton-transfer step and
the minimization of metal–H repulsion. This Account introduces
the recent shift of our research toward the construction of an NCI-inclusive
paradigm of chemical reactivity engineering based on experimental
efforts propped up by state-of-the-art theoretical tools.