Molecular shape and solvation of the lacunar, saddle-shaped, and planar metal cyclidene complexes: Molecular dynamics studies

Molecular dynamics simulations have been used to study the three-dimensiona l distribution of methanol solvent molecules around three cobalt(II) cyclid ene complexes differing in details of their ligand structures. The ligands are a planar unbridged 14-membered macrocycle in Co([14]Cyc), a saddle-shap ed unbridged 16-membered macrocycle in Co([16]Cyc), and a lacunar bridged 1 6-membered macrocycle in Co(C6[16]Cyc). All three complexes contain five-co ordinate cobalt(II) with the metal ion bound to four nitrogen donor atoms f rom the macrocycle and one nitrogen donor from an axial methylimidazole. Di stinctly different solvation patterns are exemplified for the three complex es by the positions of the maximum in the Co-O pair distribution function ( at r(Co-O) = 2.5, 3.7, and 4.5 Angstrom for Co([14]Cyc), Co([16]Cyc), and C o(C6[16]Cyc), respectively) and by the number of methanol molecules within the macrocyclic cleft (1.5, 0.7, and 0.4 molecules at a Co-O distance of 5. 25 Angstrom in the "cavity", respectively). Analysis of the anisotropic sol vent structure reveals the presence of a methanol molecule directly above t he cobalt(II) center, at a distance of ca. 2.5 Angstrom, for planar Co([14] Cyc), and the absence of solvent from such close proximity to the metal ion for the remaining complexes. The bridge further protects the sides of the cavity from the solvent. The width of an empty cavity of Co(C6[16]Cyc) shri nks by 0.3 Angstrom in methanol solution, as compared to vacuum simulations . These results confirm the experimentally based (decrease in absolute valu e of enthalpies and entropies of dioxygen binding) suggestion that extensiv e solvation of the cobalt(II) center reduces its accessibility to incoming small molecules.