Base-Controlled Directed Synthesis of Metal–Methyleneimidazoline (MIz) and Metal–Mesoionic Carbene (MIC) Compounds

Abstract

Reactions of a host of metal precursors with pyridyl-(benzamide)-functionalized C 2 -methyl-protected imidazolium salts [L 1 H 2 ]I and [L 2 H]I afforded the metal−methyleneimidazoline (MIz) compounds [Ru(L 1 -κC 1 )(p-cymene)]I (1), [Mn(L 1 -κC 1 )(CO) 3 ] (2), [Ru(L 2 -κC 1 )(pcymene)Cl]PF 6 (3), and [Ir(L 2 -κC 1 )(Cp*)Cl]PF 6 (4) in the presence of different external bases, such as LiHMDS, Na 2 CO 3 , t BuOK, and NaH. However, the use of NaOAc led to the selective formation of the metal− mesoionic carbene (MIC) compounds [Ru(L 2 -κC 5 )(p-cymene)Cl]PF 6 (5), [Ir(L 2 -κC 5 )(Cp*)Cl]PF 6 (6), [Ir 2 (L 1 -κC 5 )(Cp*) 2 I]PF 6 (8), and the orthometalated compound [Ir(L 1 )(Cp*)I] (7). All compounds have been characterized by spectroscopic techniques and X-ray crystallography. Being more acidic, the C 2 -methyl is readily deprotonated by the external base to give the metal−MIz products. A metal-bound acetate, in contrast, interacts selectively with the imidazolium C 5 −H and drives the reaction toward the metal−MIC formation. DFT calculations support a concerted metalation−deprotonation pathway for selective C−H activation and metalation.

Formation of CNHC∧Calkyl and CNHC∧Caryl κ2-Chelate Complexes through Competitive sp3- and sp2-CH Activations: An Experimental and Computational Study

“…The expected four resonances in the typical range were observed for the aromatic carbon atoms of the benzyl group, confirming that this group did not participate in the cyclometalation. The resonance for the carbene carbon atom C2 was observed at δ = 168.9 ppm and falls in the typical range for cyclometalated Ir III NHC complexes. − , …”

“…In case cyclometalation occurs, it would lead to a six-membered C NHC ∧ C chelate ring involving either sp 3 carbon atom C10 6a or sp 2 carbon atom C14 ( Scheme 2 ). 4 Thus, the ring size may not be essential to determine which CH bond in the intermediate NHC complex is activated.…”

“…The resonance for the carbene carbon atom C2 was observed at δ = 168.9 ppm and falls in the typical range for cyclometalated Ir III NHC complexes. 3b − 3d , 4 …”

“…A variety of sp 3 - and sp 2 -CH activations have been described. While the sp 2 -CH activation has been mainly used to create five-membered chelate rings from N -aryl or six-membered chelate rings from N -alkyl­(aryl) NHCs, the sp 3 -CH activation has been observed for NHCs featuring N -alkyl substituents (Scheme , top). Orthometalation involving a phenylphosphine donor has also been observed in titanium alkylidene complexes…”