Activation of Aromatic C−C Bonds of 2,2’‐Bipyridine Ligands

Abstract

4,4'-Disubstituted-2,2'-bipyridine ligands coordinated to Mo and Re cationic fragments become dearomatized by an intramolecular nucleophilic attack from a deprotonated N-alkylimidazole ligand in cis disposition. The subsequent protonation of these neutral complexes takes place on a pyridine carbon atom rather than at nitrogen, weakening an aromatic C-C bond and affording a dihydropyridyl moiety. Computational calculations allowed for the rationalization of the formation of the experimentally obtained products over other plausible alternatives.

Regiochemistry Control by Bipyridine Substituents in the Deprotonation of ReI and MoIIN‐Alkylimidazole Complexes

“…This behavior is reminiscent of that previously found for the nonsubstituted Mo II bipy complexes, whereas for 4,4′‐X 2 bipy (X=Cl, Br) derivatives products of C−C coupling between imidazole C2 and bipy C2 were observed …”

“…A 2D HMBC NMR spectrum of 17 b showed a two‐bond correlation between the bipy hydrogen atom at 5.77 ppm and the 13 C NMR signal at 78.9 ppm, assigned to a bipy quaternary carbon atom (DEPT‐135), demonstrating that the attack occurred at C2 of the bipy ligand, as predicted by DFT calculations (Scheme ). Analogous nucleophilic attack at quaternary C2 atoms of 4,4′‐dihalobipyridines upon deprotonation of N ‐alkylimidazole ligands had been previously found by us at {Mo(η 3 ‐allyl)(CO) 2 } fragments, with similar spectroscopic features …”

“…For the {Mo(η 3 ‐methallyl)(CO) 2 } fragment formation of imidazol‐2‐yl products is preferred over C−C coupling and dearomatization, unless dihalo‐substituted α‐diimines are used to favor the C−C coupling products (with C2 of 4,4′‐Cl 2 bipy, as reported in the preliminary communication, and with C2, i.e., analogous to bipy C6, of 4,7‐Cl 2 phen reported herein).…”

“…The starting structures to develop the potential‐energy surface for the four mechanisms studied ( M in Scheme ) are the neutral species resulting from the deprotonation of the central CH group of the N ‐RIm ligand of [Re(CO) 3 (4,4′‐R′ 2 bipy)( N ‐RIm)]OTf compounds. These possible reaction pathways have been already considered in our previous studies and comprise: 1) C−C coupling reactions between the deprotonated imidazole C atom (C Im ) and an ortho pyridyl C atom of the bipy ligand, that is, bipy C6 or C2 (pathway A1 or A2, respectively, in Scheme ) to afford the corresponding dearomatized products P A1 and P A2 , respectively, and 2) formation of C ‐bonded imidazol‐2‐yl product P B by a two‐step mechanism starting with nucleophilic attack at a cis ‐CO ligand (pathway 1B in Scheme ) or by formation of a κ 2 ‐ C , N ‐imidazolyl transition state (pathway 2B in Scheme ). Table lists the Gibbs energy of each species (transition states, intermediates, and products) relative to the corresponding M of each reactive process.…”

“… Summary schematic view of the types of compounds obtained depending on the metal and the nature of the substituents at imidazole ring or of the bipy/phen backbone. Shaded compounds have been previously reported …”