On, off and intermediate coordination of a bridgehead triarylamine donor in tripodal complexes: Towards the tuning of coordinative bond distance

Abstract

A series of first row transition metal complexes of the tripodal ligand 2,2′,2″-nitrilotribenzoic acid H₃333L has been prepared and characterised by X-ray crystallography: Mononuclear [M(L)]⁻ species [Cu(H₂O)₄]₃[Cu(L)(H₂O)]₆·25H₂O (2), [Co(H₂O)₆][Co(L)(H₂O)]·8H₂O (4), [Zn(H₂O)₆] [Zn(L)(H₂O)]·8H₂O (5) and a neutral [M(L)] complex [Fe^III(L)(H₂O)₃]·5H₂O (8) are formed as well as dimeric [M(L)]₂²⁻ species (HNEt₃)₂[Cu(L)]₂·2CH₃CN (1), (HNEt₃)₃[Ni(L)]₂(ClO₄)·H₂O (3), (HNEt₃)₂[Fe^II(L)]₂·2CH₃CN (6) and (HNEt₃)₂[Fe^III₂(L)₂(μ-O)] (7). The complexes display a unique variation in the M–N distance (2.09 Å for Cu^II to 3.29 Å for Fe^III) to the bridgehead triphenylamine donor and are classified into compounds with “On”,”Off” and “Intermediate” N-coordination. The trigonal-bipyramidal coordination polyhedron changes towards tetrahedral in the intermediate and octahedral in the Off-state. The M–N distance of individual complexes is reversibly tuned by external chemical input such as changes of metal ion oxidation state (Fe^II/Fe^III) or variation of the axial coligand as a consequence of solvent or pH variation. Possible reasons for the exceptional tolerance of the M–N bond to distance variations are discussed under consideration of gas phase DFT calculations of [Zn(L)]⁻.