The Largest 15N–15N Coupling Constant Across an NHN Hydrogen Bond

摘要: Since the discovery of NMR coupling constants across hydrogen bonds A–H···B containing nuclei with spin 1/2, such as A, B =19F, 15N,[1–3] it has been established that these parameters can not only be used to detect bridges in biomolecules[4] but also determine geometries strong solution.[5] It shown experimentally and by ab initio calculations[1] scalar two-bond 2JAB attain maximum values when A···B distances are at a minimum. Whereas have for FHF NHF bonds, corresponding maxima still unknown case NHN bridges. To date, 2J15N15N less than 11 Hz detected nucleic acid base pairs,[3] protonated sponges,[6,7] six-[8] seven-membered[9] H-chelates. In contrast, DFT calculations gave 2JNN = 25 Hz,[5d] shortest possible N···N distance about 2.5 A.[10] More reliable high-level coupled-cluster EOM small model systems predicted even larger constants.[11,12] Herein we describe novel class anionic H-chelates 15N–15N more 16 Hz. These anions were obtained deprotonation 2,3-dipyrrol-2-ylquinoxalines (DPQs, Scheme 1). DPQs synthesized studied colorimetric anion receptors charge-dense species, fluoride.[13] However, their unusual geometry makes likely monodeprotonated forms would unusually short thus large constants. This expectation realized, present herein results various spectroscopy experiments characterize intramolecular anions. Scheme 1 Chemical structures substituted (DPQ) deprotonated mono- dianions. 1: 2,3-dipyrrol-2-ylquinoxaline, 2: 6-nitro-2,3-dipyrrol-2-ylquinoxaline, 3: 6,7-dinitro-2,3-dipyrrol-2-ylquinoxaline. The monoanions ... The 1–3 generated treatment solutions DPQ precursors 5:1 CD2Cl2/[D6]DMSO mixtures which amounts solid NaH added. Minute quantities dihydrogen produced, along monoanion major organic product (see Experimental Section). [D6]DMSO was added solvate Na+ counterions, while CD2Cl2 primary solvent because provided reduced viscosity needed carry out spectroscopic analyses lower temperatures (conditions under proton exchange found slow). Figure 1a shows signals pyrrolic protons, labeled H1 H17, [15N2]2 recorded 233 K CD2Cl2/[D6]DMSO. Under conditions, two observed, namely 11.81 11.93 ppm. Each signal is split into doublet, 1J15N1H −97.6 −97.7 Hz, respectively. accord those pyrroles[14] porphyrins.[15] 15N{1H} spectrum (Figure 1b) lines pyrrole nitrogen atoms [15N2]2. No between 15N finding consistent absence bonds. Figure 1 Partial 1H spectra its [15N2]2− dissolved mixture (5:1): a) H17 2 K; b) c) 193 K, ... In single 20.65 ppm observed 1c). pair doublets, JN17H17 −55.2 JN1H1 −24.7 Unfortunately, could establish clearly whether intrinsic or arise from fast nondegenerate tautomerism tautomers b according 1. We favor latter possibility, change slightly temperature.[16] assignment tentative, based on assumption [15N2]2b− energetically favored corroborated described below. average both couplings −40 same value symmetrical 3− 298 natural abundance. Values |−40| bonds.[3,5d] The depicted Figure 1d. Again, observed. ascribe upfield N17 downfield N1. Their intensities equal they experience different nuclear Overhauser effects.[17] appear each constant JNN 16.5 Hz; this largest date. To further mono-deprotonated derived 1–3, effect D-for-H isotope respective chemical shifts. Selected reproduced 2; deuteron broadened usual quadrupole relaxation. neutral species [15N2]2, deuterium produces shift δ(NDN)−δ(NHN) respect original proton-containing material 2a). −0.86 seen 2c) deuterated. Figure 2 a) Partial 2H 0.02M solution (10:1) fraction xD ≈ 0.3 labile (pyrrolic NH) sites. ... Similarly, −0.6 parent compound 2b). [15N2]2−, deuteration shifts −5 N1 +2.5 2d). As previously,[18] effects correspond roughly sum rNH+rHN≅rNN order 2.6 A. The presented 2e f provide support notion 1− 3−, although symmetrical, otherwise analogous 2−. shifts, δ =20.66 20.59 ppm, −1.13 −0.88 respectively, (i.e., upfield) after deuteration. Given expected behavior, did synthesize study isotopologues anions. corroborate estimated hydrogen-bond distances, calculated equilibrium all using methods. Table range, comparison experimental require corrections anharmonic zero-point vibrations. energy tautomer 2a− 5.9 kJmol−1 2b−, difference supports above-mentioned assignments associated JNH. Table 1 Equilibrium 1−, 2−, B3PW91/6–31 +G** level. Several interesting questions arise. One why much related sponges.[6,7] Possible explanations differences overall charge systems, hybridization atoms, specific nature intermolecular interactions. Although required, do think dominant, hydrogen-bonded protons types compounds similar. A second question generally smaller calculations. believe result reflect observation[18] heavy-atom strongest structures. disparity arises space required quantum vibrations,[19] an leads eventually reduction value. In conclusion, exhibiting date. efforts will quantify relation observable systems.