Breaking the Cycle: Impact of Sterically-Tailored Tetra(pyrazolyl)lutidines on the Self-Assembly of Silver(I) Complexes

Abstract

A improved preparation of the pentadentate ligand alpha,alpha,alpha',alpha'-tetra(pyrazolyl)lutidine, pz(4)lut, and the syntheses of three new alkyl-substituted pyrazolyl derivatives pz(4')(4)lut (pz(4') = 4-methylpyrazolyl), pz*(4)lut (pz* = 3,5-dimethylpyrazolyl), and pz(DIP)(4)lut (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(pz(4)lut)](BF(4)) (1), [Ag(pz(4')(4)lut)](BF(4)) (2), and [Ag(pz*(4)lut)](BF(4)) (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)(4)lut)](BF(4)) (4) could not be obtained, those of [Ag(pz(DIP)(4)lut)](OTf) (5) showed infinite polymeric chains secured via silver-bound mu-kappa(2)N(pz),kappa(2)N(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 CD(3)CN, rather the cations are monomeric in solution.