High-Spin, Five-Coordinate Complexes of Cobalt(II), Nickel(II), and Copper(I1) with Linear, Pentadentate Keto Iminato Ligands.

Chen, Yuying; Chu, Doris C.; McKinney, Billy; Willis, Lawrence; Cummings, Sue

doi:10.1021/ic50224a090

Cited by 3 publications

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“…Similar results have been obtained for low-spin Co(II) complexes with Schiff bases derived from salicylaldehyde, acetylacetone, or related β-diketones. [61][62][63][64] In the case of Co(II) porphyrins, five-coordinate species usually dominate in solutions, but six-coordinate adducts can be obtained at high concentrations of axial bases. [65][66][67] Interestingly, pyridine derivatives form six-coordinate complexes with cobalt(II) porphyrins relatively easily, while imidazole derivatives preferentially form five-coordinate adducts.…”

Section: Resultsmentioning

confidence: 99%

Ligand Binding inside the Cavities of Lacunar and Saddle-Shaped Cyclidene Complexes: Molecular Mechanics and Molecular Dynamics Studies

Rybak‐Akimova

Kuczera

2000

Inorg. Chem.

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Cobalt(II) complexes with tetradentate macrocyclic cyclidene ligands are known to coordinate one additional axial base molecule, leaving the sixth vacant coordination site at the metal ion available for small ligand (e.g., O2) binding. Molecular mechanics and molecular dynamics simulations provide a microscopic view of 1-methylimidazole (MeIm) binding within the cavities of several lacunar (bridged) and saddle-shaped (unbridged) cyclidenes and uncover the roles of the bridges and the walls of the clefts in steric protection of the cobalt(II) coordination site. Short bridges (C3 and C6) prevent inside-the-cavity MeIm binding because of severe ligand distortions leading to high-energy penalties (58 and 25 kcal/mol, respectively), while long bridges (C8 and C12) flip away from the MeIm binding site, allowing for penalty-free MeIm inclusion. In the unbridged saddle-shaped complex, there is no energy difference between inside- and outside-the-cavity MeIm binding. The preferential existence of the coordinatively unsaturated, five-coordinate species Co(unbrCyc)(MeIm)2+ should therefore be explained by electronic, rather than steric, factors. Molecular dynamics and free energy simulations reveal the presence of a weak (ca. 4 kcal/mol in the gas phase and ca. 2 kcal/mol in methanol solution) noncovalent MeIm binding site at the entrance of the cleft of cobalt(II) unbridged cyclidene, at a distance of about 4 A from the metal ion. The macrocycle geometry remains undistorted at such large Co-N(MeIm) separations, while the cavity opens up by 0.9 A upon covalent MeIm binding (Co-N(MeIm) distance of 2 A). An increase in macrocycle strain energy upon MeIm inclusion is compensated by favorable nonbonded interactions between the incoming base and the walls of the unbridged cyclidene.

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