First Stabilization of 14‐Electron Rhodium(I) Complexes by Hemichelation

Abstract

Hemichelation is emerging as a new mode of coordination where non‐covalent interactions crucially contribute to the cohesion of electron‐unsaturated organometallic complexes. This study discloses an unprecedented demonstration of this concept to a Group 9 metal, that is, RhI. The syntheses of new 14‐electron RhI complexes were achieved by choosing the anti‐[(η6:η6‐fluorenyl){Cr(CO)3}2] anion as the ambiphilic hemichelating ligand, which was treated with [{Rh(nbd)Cl}2] (nbd=norbornadiene) and [{Rh(CO)2Cl}2]. The new T‐shaped RhI hemichelates were characterized by analytical and structural methods. Investigations using the methods of the DFT and electron‐density topology analysis (NCI region analysis, QTAIM theory) confirmed the closed‐shell, non‐covalent and attractive characters of the interaction between the RhI center and the proximal Cr(CO)3 moiety. This study shows that, by appropriate tuning of the electronic properties of the ambiphilic ligand, truly coordination‐unsaturated RhI complexes can be synthesized in a manageable form.

Effect of Enhanced Electron Withdrawal on the Cohesion of Cr‐Pd Hemichelates

“…Figure displays the NCI plots of models of 4a and 4b characterized by the red‐colored isosurfaces that materialize some attractive interaction between the Cr(CO) 3 moiety and the arene ligands, and the attractive noncovalent interaction existing between the Pd centre and the proximal Cr(CO) 3 moiety. The rather spread out intermetallic attractive NCI isosurface is similar to all hemichelates synthesized to date, where the attractive NCI component of the intermetallic interaction is only weakly covalent and largely dominated by electrostatics and electron correlation , , . The blue isosurfaces materialize either repulsive (steric) interactions or non‐bonded so‐called van der Waals contacts.…”

“…In the present study, the compounds of interest were derived from mono and bistricarbonyl(η 6 :fluorene)chromium complexes 1a and 1b (Scheme ) . The associated benzylic anion used in the synthesis of hemichelates showed in several instances that the formation of Pd and Rh hemichelates with tricarbonylchromium‐bound complexes was possible without any risk of developing alternative coordination scenarios such as the η 3 bonding of the metal center reported by Ceccon et al and by us[3c] with indene derivatives. In previous reports fluorene complex 1a was shown to readily provide hemichelate 4a by the low temperature deprotonation‐lithiation of the former and the reaction of the resulting lithio intermediate with the palladacyclic dimer of N , N ‐dimethylbenzylamine.…”

“…The best ways of considering such a benzylic anion such as deprotio‐ 1a is like an ambiphilic Lewis base possessing not less than five sites of high charge density potentially capable of interacting with a Lewis acid. [3c] Of course, if one considers benzylic anion deprotio‐ 1b , the negative charge is expected to delocalize over two Cr(CO) 3 moieties. Therefore, the question is to what extent the formation and the cohesion of a hemichelate will be affected by the expected enhanced “stabilization” of the benzylic anion by delocalization.…”

“…Non‐covalent interactions play a central role in Chemistry and particularly in the cohesion of small to large molecular coordination and organometallic complexes Their role in organometallic chemistry is growingly being investigated to rationalize the interplay between the ligand retinue and its metal centre in important processes of homogeneous catalysis such as C‐H bond activation. [1c], In 2013, we defined the concept of “hemichelation”; as a genuine mode of coordination of polytopic ligands to metal centres allying covalent and non‐covalent binding interaction. In this situation, at least one site of coordination is formally left covalently‐unbound and favoured over other coordination scenarios .…”

“…[1b] Thus it opens new perspectives in the design of new architectures of interest in catalysis and allows the study of otherwise difficult processes such as the rarely observed trans‐cis isomerization of T‐shaped complexes . A number of subsequent reports have shown that although the dominating non‐covalent character of the interaction is directly responsible for the dynamic behavior of complexes in solution, hemichelates of Mn(I), Re(I), Pd(II[3a], [3b] and I), Pt(II)[3a] and Rh(I)[3c] could be generally isolated as moderately air sensitive compounds and characterized in the solid state. A recent independent investigation of the molecular dynamics of a Pd(II) hemichelate further confirms the evanescent and mostly non‐covalent nature of the interaction of the Pd(II) centre with the Cr(CO) 3 fragment, which exhibits fluxionality, the Pd centre binding alternatively the two proximal Cr‐bound CO ligands and the rotation of the Cr(CO) 3 moiety being largely slower than this rapid coordinative exchange equilibrium.…”