Double proton transfer in crystals of 1,3,4,6,7,8‐hexahydro‐2H‐pyrimido[1,2‐a] pyrimidine (hppH):¹³C and¹⁵N CPMAS NMR study of (hppH)₂

Abstract

In this paper, we report an example of intermolecular solid‐state proton transfer in the bicyclic guanidine, hppH. A combination of X‐ray crystallography, CPMAS NMR (13C and 15N) and theoretical calculations allows us to determine that a double proton transfer takes place in the (hppH)2 dimer with an activation energy of about 50 kJ mol−1. According to the B3LYP/6‐311++G(d,p) calculations, the double proton transfer occurs non‐symmetrically through a zwitterion. Copyright © 2009 John Wiley & Sons, Ltd.

Cover Picture: Laves Phases, γ‐Brass, and 2×2×2 Superstructures: A New Class of Quasicrystal Approximants and the Suggestion of a New Quasicrystal (Chem. Eur. J. 22/2008)

“…The most important geometrical changes induced by protonation is delocalization and “partial protonation” of the second guanidine moiety. Delocalization is clearly shown by the Δ CN values25 [measure of delocalization, defined as d (C–N single ) – d (C–N double )], for which values generally fall in the range of 0 Å (for a fully delocalized system) to <0.14 Å (for localized bonding). The Δ CN values calculated for the protonated guanidino group of 18H + are smaller than that of the neutral precursor 18 (Δ CN = 0.034, and 0.145 Å, respectively), which is in accord with the increase in delocalization upon protonation.…”

Metal‐free Carbon Monoxide (CO) Capture and Utilization: Formylation of Amines

“…Although studies of capturing CO in superacids (HF−SbF 5 or HSO 3 F−SbF 5 ) have been conducted, CO fixation in basic media has not been evaluated . TBD‐CO adducts were converted to TBD‐CHO, which transferred a formyl group to a variety of amines with good yields . Considering the utility of formylated amines in various organic syntheses, the C−N bond formation of amines using a C1 source is a valuable synthetic method .…”

“…The IR spectrum of TBD‐CO adducts revealed a peak at 2279 cm −1 , which was higher than that of free CO (2143 cm −1 ) (Figure b). According to a previous study of the IR frequency of cationic CO, the bond strength of protonated CO is stronger compared to that of free CO. Based on IR and NMR analysis, we believe that TBD adducts involve the partial protonation or interaction of CO molecules with a positively charged species in the dimeric TBD structure (Scheme ) . In contrast to TBD‐CHO, TBD‐CO adducts liberated CO at an ambient pressure, which demonstrated the non‐covalent interaction of CO with TBD (see supporting information, Scheme S1).…”

“…Figure a shows the reaction energy diagram for the formylation of benzyl amine, in which TBD reacted with CO and transferred the formyl group to benzyl amine. The dimeric TBD species d1 is thought to participate in this reaction because two TBD molecules can readily interact with each other though hydrogen bonding, NH ⋅ N (Figure b) . The interaction energy between two TBD molecules was computationally evaluated to be ΔG=−5.34 kcal/mol (all energy values hereafter are presented as ΔG).…”

Exceptional Superbasicity of Bis(guanidine) Proton Sponges Imposed by the Bis(secododecahedrane) Molecular Scaffold: A Computational Study

“…The most important geometrical changes induced by protonation is delocalization and "partial protonation" of the second guanidine moiety. Delocalization is clearly shown by the Δ CN values [25] [measure of delocalization, defined as d (C-N single ) -d(C-N double )], for which values generally fall in the range of 0 Å (for a fully delocalized system) to Ͻ0.14 Å (for localized bonding). The Δ CN values calculated for the protonated guanidino group of 18H + are smaller than that of the neutral precursor 18 (Δ CN = 0.034, and 0.145 Å, respectively), which is in accord with the increase in delocalization upon protonation.…”