Preferential N–H⋯:C hydrogen bonding involving ditopic NH-containing systems and N-heterocyclic carbenes. Issue 69 (20th November 2020)

• Record Type: Journal Article
• Authors: Kinney, Zacharias J.; Rheingold, Arnold L.; Protasiewicz, John D.
• Is Part Of: RSC advances. Volume 10:Issue 69(2020)
• Page Start: 42164
• Page End: 42171
• Publication Date: 2020-11-20
• Subjects: Chemistry -- Periodicals; 540.5
• DOI: 10.1039/d0ra08490e
• Languages: English
• ISSNs: 2046-2069

Abstract

Abstract : Non-traditional hydrogen bonds between a singlet carbene and a series of ditopic secondary amines is detailed. Both the solid- and solution-state metrics reveal the strength of these associations are dependent on the p K a of the NH -containing molecule. Abstract : Hydrogen bonding plays a critical role in maintaining order and structure in complex biological and synthetic systems. N -heterocyclic carbenes (NHCs) represent one of the most versatile tools in the synthetic chemistry toolbox, yet their potential as neutral carbon hydrogen bond acceptors remains underexplored. This report investigates this capability in a strategic manner, wherein carbene-based hydrogen bonding can be assessed by use of ditopic NH -containing molecules. N–H bonds are unique as there are three established reaction modes with carbenes: non-traditional hydrogen bonding adducts (X–H⋯:C), salts arising from proton transfer ([H–C] + [X] − ), or amines from insertion of the carbene into the N–H bond. Yet, there are no established rules to predict product distributions or the strength of these associations. Here we seek to correlate the hydrogen bond strength of symmetric and asymmetric ditopic secondary amines with 1, 3-bis(2, 6-diisopropylphenyl)imidazol-2-ylidene (IPr, a representative NHC). In symmetric and asymmetric ditopic amine adducts both the solid-state (hydrogen bond lengths, NHC interior angles) and solution-state ( 1 H Δ δ of NH signals, 13 C signals of carbenic carbon) can be related to the p K a of the parent amine.