Breaking the cycle: Impact of sterically-tailored tetra(pyrazolyl)lutidines on the self-assembly of silver(I) complexes

A improved preparation of the pentadentate ligand α,α, α′,α′-tetra(pyrazolyl)lutidine, pz₄lut, and the syntheses of three new alkyl-substituted pyrazolyl derivatives pz ^4′₄lut (pz^4′ = 4-methylpyrazolyl), pz₄lut (pz* = 3,5-dimethylpyrazolyl), and pz^DIP ₄lut (pz^DIP = 3,5-diisopropylpyrazolyl) are described. The silver(I) complexes of these ligands were studied to ascertain the impact of pyrazolyl substitution, if any, on their binding modes and on solubility issues. In the solid state, [Ag(pz₄lut)](BF₄) (1), [Ag(pz ^4′₄lut)](BF₄) (2), and [Ag(pz ₄lut)](BF₄) (3) give cyclic dications as a result of two ligands sandwiching two silver centers where each ligand binds the metals through only pyrazolyl nitrogen donors. This cyclic motif is similar to those observed in the silver complexes of tetra(pyridyl)lutidine PY5-R derivatives (where the central pyridyl does not bind) and in related tetra(pyrazolyl)-m- xylene complexes. While suitable single crystals of [Ag(pz^DIP ₄lut)](BF₄) (4) could not be obtained, those of [Ag(pz^DIP₄lut)](OTf) (5) showed infinite polymeric chains secured via silver-bound μ-κ²N_pz, κ²N_pz- ligands. The different binding mode of the latter ligand versus the former three is likely due to unfavorable steric interactions between the bulky iso-propyl (pyrazolyl) substituents and the central pyridyl rings of hypothetical cyclic architectures. The combined electrospray ionization mass spectrometry (ESI(+)-MS), variable-temperature NMR (VT NMR), and diffusion pulsed field-gradient spin-echo (PFGSE) NMR data indicate that the solid state structures of each 1-5 are neither retained nor static in CD₃CN, rather the cations are monomeric in solution.