LZnX complexes of tripodal ligands with intramolecular RN–H hydrogen bonding groups: structural implications of a hydrogen bonding cavity, and of X/R in the hydrogen bonding geometry/strength

Abstract

Tripodal ligands N(CH₂Py)_3−n(CH₂Py-6-NHR)_n (R = H, n = 1–3 L^1–31–31–3, n = 0 tpa; R = CH₂^tBu, n = 1–3 L^′1–3′1–3′1–3) are used to investigate the effect of different hydrogen bonding microenvironments on structural features of their LZnX complexes (X = Cl⁻, NO₃⁻, OH⁻). The X-ray structures of [(L²22)Zn(Cl)](BPh₄) 2·0.5(H₂O·CH₃CN), [(L³33)Zn(Cl)](BPh₄) 3·CH₃CN, [(L′¹11)Zn(Cl)](BPh₄) 1′, [(L′²22)Zn(Cl)](BPh₄) 2′·CH₃OH, and [(L′³33)Zn(Cl)](BPh₄) 3′ have been determined and exhibit trigonal bipyramidal geometries with intramolecular (internal) N–H⋯Cl–Zn hydrogen bonds. The structure of [(L′²22)Zn(ONO₂)]NO₃4′·H₂O with two internal N–H⋯O–Zn hydrogen bonds has also been determined. The axial Zn–Cl distance lengthens from 2.275 Å in [(tpa)Zn(Cl)](BPh₄) to 2.280–2.347 Å in 1–3, 1′–3′. Notably, the average Zn–N_py distance is also progressively lengthened from 2.069 Å in [(tpa)Zn(Cl)](BPh₄) to 2.159 and 2.182 Å in the triply hydrogen bonding cavity of 3 and 3′, respectively. Lengthening of the Zn–Cl and Zn–N_py bonds is accompanied by a progressive shortening of the trans Zn–N bond from 2.271 Å in [(tpa)Zn(Cl)](BPh₄) to 2.115 Å in 3 (2.113 Å in 3′). As a result of the triply hydrogen bonding microenvironment the Zn–Cl and Zn–N_py distances of 3 are at the upper end of the range observed for axial Zn–Cl bonds, whereas the axial Zn–N distance is one of shortest among N₄ ligands that induce a trigonal bipyramidal geometry. Despite the rigidity of these tripodal ligands, the geometry of the intramolecular RN–H⋯X–Zn hydrogen bonds (X = Cl⁻, OH⁻, NO₃⁻) is strongly dependent on the nature of X, however, on average, similar for R = H, CH₂^tBu.