Proton transfer and hydrogen bonding in the organic solid state: a combined XRD/XPS/ssNMR study of 17 organic acid–base complexes

The properties of nitrogen centres acting either as hydrogen-bond or Brønsted acceptors in solid molecular acid-base complexes have been probed by N 1s X-ray photoelectron spectroscopy (XPS) as well as 15N solid-state nuclear magnetic resonance (ssNMR) spectroscopy and are interpreted with reference to local crystallographic structure information provided by X-ray diffraction (XRD). We have previously shown that the strong chemical shift of the N 1s binding energy associated with the protonation of nitrogen centres unequivocally distinguishes protonated (salt) from hydrogen-bonded (co-crystal) nitrogen species. This result is further supported by significant ssNMR shifts to low frequency, which occur with proton transfer from the acid to the base component. Generally, only minor chemical shifts occur upon co-crystal formation, unless a strong hydrogen bond is formed. CASTEP density functional theory (DFT) calculations of 15N ssNMR isotropic chemical shifts correlate well with the experimental data, confirming that computational predictions of H-bond strengths and associated ssNMR chemical shifts allow the identification of salt and co-crystal structures (NMR crystallography). The excellent agreement between the conclusions drawn by XPS and the combined CASTEP/ssNMR investigations opens up a reliable avenue for local structure characterization in molecular systems even in the absence of crystal structure information, for example for non-crystalline or amorphous matter. The range of 17 different systems investigated in this study demonstrates the generic nature of this approach, which will be applicable to many other molecular materials in organic, physical, and materials chemistr

Influence of Water Ligands on Structural Diversity: From a One-Dimensional Linear Coordination Polymer to Three-Dimensional Ferrimagnetic Diamondoid Metal-Organic Frameworks

Four three-dimensional (3D) metal-organic frameworks [Mn3(3-Me-sal)4(py)4]n (1), [Mn3(4-Me-sal)4(py)4- (MeOH)]n * n(H2O) (2), [Mn3(5-Me-sal)4(py)4(H2O)2]n * n(MeOH) (3), and [Mn3(3-Me-sal)4(4-Me-py)4]n (4) and the one-dimensional (1D) coordination polymer {[Mn2(4-Me-sal)2(4-Me-py)2(H2O)2(MeOH)2][Mn(4-Me-sal)2(4-Me-py)2]}n (5) have been synthesized, where x-Me-salH2 ) x-methyl salicylic acid (x ) 3, 4, 5), py ) pyridine, and 4-Me-py ) 4-methyl-pyridine. The 3D frameworks of compounds 1-4 can be described as diamondoid networks. Magnetic studies show that weak MnII-MnIII antiferromagnetic interactions (in the range of -0.55 to -0.22 K) mediated by syn-anti carboxylate bridges are present in all compounds. While 5 remains paramagnetic down to 1.8 K, the 3D networks exhibit long-range ferrimagnetic ordering below 7.4 K for 1, 4.6 K for 2, 3.0 K for 3, and 7.7 K for 4. The decrease of the critical temperature reflects the increase of the coordination number around the Mn(II) site from four in 1, five in 2, and six in 3 that lower the bond strength and the magnetic interactions. This result also reinforces the hypothesis that the structures of 1 and 4 are similar as suggested by the X-ray analysis