Manganese(II), iron(II), cobalt(II), and copper(II) complexes of an extended inherently chiral tris-bipyridyl cage

doi:10.1073/pnas.0508539103

Manganese(II), iron(II), cobalt(II), and copper(II) complexes of an extended inherently chiral tris-bipyridyl cage

^*Centre for Heavy Metals Research, School of Chemistry, University of Sydney, Sydney NSW 2006, Australia; and ^§School of Pharmacy and Molecular Sciences, James Cook University, Queensland 4811, Australia

Edited by Jack Halpern, University of Chicago, Chicago, IL, and approved November 28, 2005 (received for review September 29, 2005)

Abstract

Manganese(II), iron(II), cobalt(II), and copper(II) derivatives of two inherently chiral, Tris(bipyridyl) cages (L and L′) of type [ML]-(PF₆)₂(solvent)_n and [FeL′](ClO₄)₂ are reported, where L is the hexa-tertiary butyl-substituted derivative of L′. These products were obtained by using the free cage and metal template procedures; the latter involved the reductive amination of the respective Tris-dialdehyde precursor complexes of iron(II), cobalt(II), or nickel(II). Electrochemical, EPR, and NMR studies have been used to probe the nature of the individual complexes. X-ray structures of the manganese(II), iron(II), and copper(II) complexes of L and the iron(II) complex of L′ are presented; these are compared with the previously reported structures of the corresponding nickel(II) complex and metal-free cage (L). In each complex the metal cation occupies the cage's central cavity and is coordinated to six nitrogens from the three bipyridyl groups. The cations [MnL]²⁺ and [FeL]²⁺ are isostructural but both exhibit a different arrangement of the bound cage to that observed in the corresponding nickel(II) and copper(II) complexes. The latter have an exo-exo arrangement of the bridgehead nitrogen lone pairs, with the metal inducing a triple helical twist that extends ≈22 Å along the axial length of each complex. In contrast, [MnL]²⁺ and [FeL]²⁺ have their terminal nitrogen lone pairs directed endo, causing a significant change in the configuration of the bound ligand. In [FeL′]²⁺, the cage has both bridgehead nitrogen lone pairs orientated exo. Semiempirical calculations indicate that the observed endo-endo and exo-exo arrangements are of comparable energy.

Footnotes

↵ † To whom correspondence should be addressed. E-mail: lindoy{at}chem.usyd.edu.au.
↵ ‡ Permanent address: Department of Chemistry, University of Victoria, Victoria, BC, Canada V8W 3P6.
Conflict of interest statement: No conflicts declared.
This paper was submitted directly (Track II) to the PNAS office.
Abbreviations: t-Bu, tertiary butyl; dmb, 5,5′-dimethyl-2,2′-bipyridyl.
Data deposition: The atomic coordinates have been deposited in the Cambridge Structural Database, Cambridge Crystallographic Data Centre, Cambridge CB2 1EZ, United Kingdom (CCDC reference nos. 284933–284938).