Macrocyclic zinc complexes for stabilisation of non-linear NH⋯S bonds in solution and solid state

The structural and spectroscopic properties of well-defined NH⋯S hydrogen bonding interactions is scarce in the literature, only few suitable model compounds have been described. Herein, we describe a set of macrocyclic Zn amino-thiophenolato complexes and their deuterated derivatives, in which secondary amino groups are brought into close proximity to Zn-bound S (thiophenolate) atoms, thereby stabilizing intramolecular R₂NH⋯S bonding interactions. Metrical data, spectroscopic signatures, strength, type, and dynamic properties of these NH⋯S bonding interactions were determined for the liquid and solution state, and referenced to an integrated, non-H bonded NH donor serving as "internal standard". Structural and spectroscopic properties for the NH⋯S bonds are as follows: (i) N⋯S bond and H⋯S bond distances ∼3.14 Å and ∼2.36 Å, respectively (ii) frequency shift Δν ∼ 130 cm^-1 (for ν(NH)), (iii) no intensity enhancement of the ν(NH) stretching band, (iv) relative line broadening factor of the ν(NH) bands (FWHM(ν(NH⋯S))/FWHM(ν(NH)) ∼ 2.0), (v) wavenumber ratio ν(NH⋯S)/ν(ND⋯S) = 1.34, (vi) downfield shift of 1H NMR signals Δδ ∼ 5 ppm (for δ(NH)), (vii) bond critical point of type (3, -1) (i.e. attractive H⋯S interaction, QTAIM analysis), (viii) dominant electrostatic interaction with a binding energy of ∼ 9 kcal mol^-1 (at B3LYP-D3/def2-TZVP level of theory), (ix) according to NBO analysis, the NH⋯S bond is described by the type lone pair(S) → σ*(N-H) interaction with stabilization energies of ∼ 7.5 kcal mol^-1, and (x) NH/ND exchange rate slower than NMR timescale. These findings offer benchmarks for theoretical explorations and molecular design of specific, intramolecular R₂NH⋯S bonding interactions within constrained environments, that may also be relevant for understanding sulfur-containing active sites in biological systems.