ic7b00176_si_003.cif (1.52 MB)
Imidazole-Appended Macrocyclic Complexes of Fe(II), Co(II), and Ni(II) as ParaCEST Agents
dataset
posted on 2017-03-30, 14:05 authored by Patrick J. Burns, Jordan M. Cox, Janet R. MorrowThe solution chemistry
and solid state structures of the Co(II), Fe(II), and Ni(II) complexes
of N,N′-bis(imidazole-2-ylmethyl)-4,10-diaza-15-crown-5
(HINO) are reported. The Co(II) and Ni(II) complexes
of HINO are the first examples of paraCEST agents (paramagnetic
chemical exchange saturation transfer) that feature exchangeable imidazole
NH protons. The crystal structures of [Co(HINO)]CoCl4·H2O and [Fe(HINO)](CF3SO3)2 have the metal ions coordinated
to four nitrogen and three oxygen donor atoms of the macrocyclic ligand
in a distorted pentagonal bipyramidal geometry. In [Ni(HINO)](CF3SO3)2, the nickel ion
is bound to only two of the three ether oxygens and three nitrogens
to produce a complex with a distorted octahedral geometry. The 1H NMR spectra of the three paramagnetic complexes show resonances
characteristic of effective C2 symmetry
in solution. CEST peaks attributed to the imidazole NH proton of the
pendent group are observed at 32 and 55 ppm away from bulk water for
[Co(HINO)]2+ and [Ni(HINO)]2+, respectively, on a 11.7 or 9.4 T NMR spectrometer. For
both complexes, an optimal CEST effect was observed at pH 7.2, and
the rate constant for proton exchange under these conditions was 1.0
× 103 s–1. [Fe(HINO)]2+ did not produce a CEST peak due to oxidation of the
complex in water at neutral pH. The CEST peak of [Co(HINO)]2+ or [Ni(HINO)]2+ is observed
in the presence of human serum albumin (HSA). These complexes show
enhanced kinetic inertness toward dissociation in acid or in the presence
of HSA in comparison to analogous complexes with amide pendent groups.