Molecular Shape and Solvation of the Lacunar, Saddle-Shaped, and Planar Metal Cyclidene Complexes: Molecular Dynamics...

Molecular dynamics simulations have been used to study the three-dimensional distribution of methanol solvent molecules around three cobalt(II) cyclidene complexes differing in details of their ligand structures. The ligands are a planar unbridged 14-membered macrocycle in Co([14]Cyc), a saddle-shaped unbridged 16-membered macrocycle in Co([16]Cyc), and a lacunar bridged 16-membered macrocycle in Co(C6[16]Cyc). All three complexes contain five-coordinate cobalt(II) with the metal ion bound to four nitrogen donor atoms from the macrocycle and one nitrogen donor from an axial methylimidazole. Distinctly different solvation patterns are exemplified for the three complexes by the positions of the maximum in the Co-O pair distribution function (at r(Co-O) = 2.5, 3.7, and 4.5 Å for Co([14]Cyc), Co([16]Cyc), and Co(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 Å in the "cavity", respectively). Analysis of the anisotropic solvent structure reveals the presence of a methanol molecule directly above the cobalt(II) center, at a distance of ca. 2.5 Å, 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) shrinks by 0.3 Å in methanol solution, as compared to vacuum simulations. These results confirm the experimentally based (decrease in absolute value of enthalpies and entropies of dioxygen binding) suggestion that extensive solvation of the cobalt(II) center reduces its accessibility to incoming small molecules.