Acyloxydiene–Fe(CO)3 complexes can act as enzyme-triggered CO-releasing molecules (ET-CORMs). Their biological activity strongly depends on the mother compound from which they are derived, i.e. cyclohexenone or cyclohexanedione, and on the position of the ester functionality they harbour. The present study addresses if the latter characteristic affects CO release, if cytotoxicity of ET-CORMs is mediated through iron release or inhibition of cell respiration and to what extent cyclohexenone and cyclohexanedione derived ET-CORMs differ in their ability to counteract TNF-α mediated inflammation. Irrespective of the formulation (DMSO or cyclodextrin), toxicity in HUVEC was significantly higher for ET-CORMs bearing the ester functionality at the outer (rac-4), as compared to the inner (rac-1) position of the cyclohexenone moiety. This was paralleled by an increased CO release from the former ET-CORM. Toxicity was not mediated via iron as EC50 values for rac-4 were significantly lower than for FeCl2 or FeCl3 and were not influenced by iron chelation. ATP depletion preceded toxicity suggesting impaired cell respiration as putative cause for cell death. In long-term HUVEC cultures inhibition of VCAM-1 expression by rac-1 waned in time, while for the cyclohexanedione derived rac-8 inhibition seems to increase. NFκB was inhibited by both rac-1 and rac-8 independent of IκBα degradation. Both ET-CORMs activated Nrf-2 and consequently induced the expression of HO-1.This study further provides a rational framework for designing acyloxydiene–Fe(CO)3 complexes as ET-CORMs with differential CO release and biological activities. We also provide a better understanding of how these complexes affect cell-biology in mechanistic terms.

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Five manganese(i) tricarbonyl complexes of the general formulae [Mn(bpea(N=CHC6H4R))(CO)3]PF6 and [Mn(bpea(NHCH2C6H4R))(CO)3]PF6 based on the tridentate bis(pyrazolyl)ethylamine (bpea) ligand, each containing a pendant 4-substituted phenyl group with R = H, I, and C≡C-H, were synthesized and fully characterized, including X-ray structure analysis for three compounds. All complexes are stable in the dark in aqueous buffer for an extended period of time. However, CO-release could be triggered by illumination at 365 nm, establishing these compounds as novel photoactivatable CO-releasing molecules (PhotoCORMs). The influence of the imine vs. amine group in the ligands on the electronic structure and the photophysical behavior was investigated with the aid of DFT and TDDFT calculations. Solution IR studies on selected compounds allowed identification of intermediates resulting from the photoreaction. Finally, light-induced CO release from a model compound was demonstrated both in PBS buffer and in vitro in human umbilical vein endothelial cells (HUVECs) using COP-1 as a fluorescent switch-on probe.

Novel η4-acyloxy-cyclohexadiene-Fe(CO)3 complexes (with variable length of the acyloxy chain) were synthesized as potential enzyme-triggered carbon monoxide (CO)-releasing molecules (ET-CORMs). The molecular structure of two complexes was additionally confirmed by X-ray crystallography. The enzyme-triggered CO-releasing activity of the compounds was assessed under physiological conditions (37 °C, 0.1 M phosphate buffer, pH = 7.4) by headspace gas chromatography (GC) and additionally by means of a myoglobin assay (UV). The relative rate of CO release and the amount of liberated CO were found to depend on the length of the acyloxy chain and its position at the diene unit (outer or inner position). Some of the new ET-CORMs exhibited very good biological activity as assessed in different cellular assays (cytotoxicity, protective effect against hypothermia-associated cell damage, and inhibition of TNF-α-mediated VCAM-1 expression).

In the course of an attempt to synthesize 2acyloxycyclohexa-1,3-diene-Fe(CO) 3 complexes in nonracemic form, we reinvestigated the "fully diastereoselective" Fe(CO) 3 complexation of (S,S)-2-ketopinoyloxy-1,3-cyclohexadiene, which had been described by Yeh and co-workers (Organometallics 2001, 20, 289−295). However, after cleaving off the chiral auxiliary unit, we only obtained racemic complexes, also for a related substrate. For this reason, we performed control experiments to exclude possible racemization mechanisms and confirmed the configurational integrity of the dienol-Fe(CO) 3 intermediates using stereochemically defined dihydrocarvone-derived complexes. We finally could show that the complexation described by Yeh actually proceeds without any detectable diastereoselectivity. At the end, the virtually inseparable diastereomers of the chiral complexation products could be distinguished by careful NMR and chiral HPLC analyses. Article pubs.acs.org/Organometallics