Binuclear Alkyne Manganese Carbonyls: Their Rearrangements to Allene, Allyl, and Vinylcarbene Derivatives by Hydrogen Migration from Methyl Substituents. Issue 4 (30th December 2021) - Hu, Yucheng
Abstract: Binuclear alkyne manganese carbonyls of the type (RC≡CR')Mn2 (CO) n (R and R'=methyl or dimethylamino; n =8, 7, 6) and their isomers related to the experimentally known (MeC2 NEt2 )Mn2 (CO) n ( n =8, 7) structures have been investigated by density functional theory. The alkyne ligand remains intact in the only low energy (Me2 N)2 C2 Mn2 (CO)8 isomer, which has a central Mn2 C2 tetrahedrane unit and is otherwise analogous to the well‐known (alkyne)Co2 (CO)6 derivatives except for one more CO group per metal atom. The low‐energy structures of the unsaturated (Me2 N)2 C2 Mn2 (CO) n ( n =7, 6) systems include isomers in which the nitrogen atom of one of the dimethylamino groups as well as the C≡C triple bond of the alkyne is coordinated to the central Mn2 unit. In other low‐energy (Me2 N)2 C2 Mn2 (CO) n ( n =7, 6) isomers the alkyne C≡C triple bond has broken completely to form two separate bridging dimethylaminocarbyne Me2 NC ligands analogous to the experimentally known iron carbonyl complex (Et2 NC)2 Fe2 (CO)6 . The (alkyne)Mn2 (CO) n ( n =8, 7, 6) systems of the alkynes MeC≡CMe and Me2 NC≡CMe with methyl substituents have significantly more complicated potential surfaces. In these systems the lowest energy isomers have bridging ligands derived from the alkyne in which one or two hydrogen atoms have migrated from a methyl group to one or both of the alkyne carbon atoms. These bridging ligands include allene, manganallyl, and vinylcarbene ligands, the first two of which have been realized experimentally in research by Adams and coworkers. Theoretical studies suggest that the mechanism for the conversion of the simple alkyne octacarbonyl (MeC2 NMe2 )Mn2 (CO)8 to the dimethylaminomanganaallyl complex Mn2 (CO)7 [μ‐η 4 ‐C3 H3 Me2 ] involves decarbonylation to the heptacarbonyl and the hexacarbonyl complexes. Subsequent hydrogen migrations then occur through intermediates with C−H−Mn agostic interactions to give the final product. Eight transition states for this mechanistic sequence have been identified with activation energies of ∼20 kcal/mol for the first hydrogen migration and ∼14 kcal/mol for the second hydrogen migration. Abstract : The binuclear alkyne manganese carbonyls (RC≡CR')Mn2 (CO) n (R and R'=methyl or dimethylamino; n =8, 7, 6) and their isomers related to experimentally known (MeC2 NEt2 )Mn2 (CO) n ( n =8, 7) structures have been investigated by density functional theory. Low‐energy (Me2 N)2 C2 Mn2 (CO) n structures have central Mn2 C2 tetrahedrane units or separate bridging Me2 NC carbyne units. However, low‐energy (Me2 NC2 Me)Mn2 (CO)6 and Me2 C2 Mn2 (CO)6 structures exhibit hydrogen migration from the methyl substituent to give species with bridging allene, manganaallyl, and vinylcarbene ligands.