The transition metal behavior of Mn affords a highly effective mechanism to generate stimuli responsive MRI contrast. The distinct structural preferences and paramagnetic properties of each Mn oxidation state enable rational design of complexes where the MRI contrast properties can be effectively switched “off” and “on” by redox mediated processes. We are developing a new class of Mn complexes that can be converted from poorly contrasting Mn(IV) to highly contrasting Mn(II) in response to biochemical reactions. The Mn(IV) complexes (Mn(IV)-L) are engineered to undergo enzymatically triggered ligand-metal auto-redox to generate the corresponding complex of Mn(II) and the 2 e-oxidized ligand (Mn(II)-Lox). In order to understand the structure-activity relationships dictating Mn(IV)-L vs. Mn(II)-Lox formation we developed the bis-N,N’-(2-hydroxybenzyl)-N,N’-ethylenediaminedi-2-hydroxyethylene (HBEH) ligand to stabilize water soluble complexes of Mn(IV) and then systematically modulated the ligand electronics by varying the 5 position of the 2-hydroxybenzyl donor. The Mn complexes were analyzed by NMR, HPLC, MS, UV-vis, X-ray diffraction, and cyclic voltammetry. We also synthesized and characterized the corresponding Ti(IV) complexes as redox innocent controls. We found that 5-NHR functionalized HBEH supports Mn(IV)-L when R is more electron withdrawing (-COPh, -COCF3), but spontaneously reacts to yield Mn(II)-Lox when R is less electron withdrawing (-COCH3, -Fmoc). The Ti(IV)-L complex is stable regardless of ligand electronics. We will discuss our preliminary efforts to generate highly contrasting Mn(II)-Lox from Mn(IV)-L via enzymatic action on the 5-NHR substituent.

Mn(IV) complexes for biochemically activated MRI contrast

255th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.