Size Discrimination in the Coordination Chemistry of an Isoindoline Pincer Ligand with Cd(II) and Zn(II)

The reactions of Cd2+ and Zn2+ with the pyridine-arm isoindoline ligand 4‘-MeLH = 1,3-bis[2-(4-methylpyridyl)imino]isoindoline produced the series of octahedrally coordinated complexes M(4‘-MeL)2, [M(4‘-MeLH)2]2+, and [M(4‘-MeL)(4‘-MeLH)]+. The complexes M(4‘-MeL)2 resulted from reactions of the respective metal perchlorates with
deprotonated ligand, whereas the complexes [M(4‘-MeLH)2](ClO4)2 resulted from reactions with ligand in the absence
of added base. The mixed-ligand complexes [M(4‘-MeL)(4‘-MeLH)]+ were generated in solution by reactions of
equimolar quantities of M(4‘-MeL)2 and [M(4‘-MeLH)2]2+. Whereas [Cd(4‘-MeL)(4‘-MeLH)]+ is stable in solution,
[Zn(4‘-MeL)(4‘-MeLH)]+ converts to and establishes equilibrium with the tetrahedrally coordinated, trinuclear complex
[Zn3(4‘-MeL)4]2+. The complexes Cd(4‘-MeL)2 (1), Zn(4‘-MeL)2 (2), and [Cd(4‘-MeL)(4‘-MeLH)]ClO4 (5) were
characterized by single-crystal X-ray diffraction, with the latter complex being shown to contain 4‘-MeLH coordinated
as a protonated iminium zwitterionic ligand. The [M(4‘-MeLH)2]2+ and [M(4‘-MeL)(4‘-MeLH)]+ complexes are tautomeric
in solution because of the shuttling of the iminium protons between imine N atoms. The rate of prototropic tautomerism
in [Cd(4‘-MeLH)2]+ was followed by 1H NMR spectroscopy. Over the temperature range 276−312 K, a linear Eyring
plot with the activation parameters ΔG⧧ = 16.0 ± 0.1 kcal/mol, ΔH⧧ = 2.9 ± 0.1 kcal/mol, and ΔS⧧ = −44.0 ±
0.3 cal/mol·K was obtained.