A non-heme iron(II) complex bearing a ligand with secondary sphere hydrogen bond (H-bond) donors, (tris(6-phenylaminopyridylmethyl)amine, TPANHPh, rapidly reduces nitrite (NO2-) to nitric oxide (NO) in the absence of exogenous additives, affording a Fe(III)2(μ-O)2 diamond core. An electronically analogous complex containing a ligand without H-bonds (tris(6-methylpyridylmethylamine), TPAMe, also reduces NO2- to NO and forms an Fe(III)2(μ-O)2 core, but is four orders of magnitude slower, highlighting the impact of H-bonds to promote NO2- reduction. We compare the structural and spectroscopic differences of the two Fe(III)2(μ-O)2 complexes and show that H-bonding interactions weaken the Fe-O bonds, perturb the electronic structure of the Fe2O2 cores, and thereby engender distinct reductive stability profiles.

The intermolecular N-N coupling of NO in a dinitrosyl iron complex (DNIC) induced by hydrogen bond donors in the secondary coordination sphere to form N2O is reported. A family of complexes containing pendant anilines in the secondary coordination sphere were synthesized and characterized. Reduction of the {Fe(NO)2}9 complex [Fe(PhNHPDI)(NO)2][BPh4] (3) to the {Fe(NO)2}10 Fe(PhNHPDI)(NO)2 (4) results in intermolecular N-N coupling to form N2O. Similar reactions of the control {Fe(NO)2}9 complex [Fe(PhNMePDI)(NO)2][BPh4] (7), which does not have H-bonding groups in the secondary coordination sphere, do not result in N2O formation. The hydrogen bonding capabilities of the complexes were explored spectroscopically and computationally.

The deoxygenation of environmental pollutants CO2 and NO2- to form value-added products is reported. CO2 reduction with subsequent CO release and NO2- conversion to NO are achieved via the starting complex Fe(PPhPDI)Cl2 (1). 1 contains the redox-active pyridinediimine (PDI) ligand with a hemilabile phosphine located in the secondary coordination sphere. 1 was reduced with SmI2 under a CO2 atmosphere to form the direduced monocarbonyl Fe(PPhPDI)(CO) (2). Subsequent CO release was achieved via oxidation of 2 using the NOx- source, NO2-. The resulting [Fe(PPhPDI)(NO)]+ (3) mononitrosyl iron complex (MNIC) is formed as the exclusive reduction product due to the hemilabile phosphine. 3 was investigated computationally to be characterized as {FeNO}7, an unusual intermediate-spin Fe(III) coupled to triplet NO- and a singly reduced PDI ligand.

Selective coupling of NO by a nonclassical dinuclear dinitrosyliron complex (D-DNIC) to form N2O is reported. The coupling is facilitated by the pyridinediimine (PDI) ligand scaffold, which enables the necessary denticity changes to produce mixed-valent, electron-deficient tethered DNICs. One-electron oxidation of the [{Fe(NO)2}]210/10 complex Fe2(PyrrPDI)(NO)4 (4) results in NO coupling to form N2O via the mixed-valent {[Fe(NO)2]2}9/10 species, which possesses an electron-deficient four-coordinate {Fe(NO)2}10 site, crucial in N-N bond formation. The hemilability of the PDI scaffold dictates the selectivity in N-N bond formation because stabilization of the five-coordinate {Fe(NO)2}9 site in the mixed-valent [{Fe(NO)2}]29/10 species, [Fe2(Pyr2PDI)(NO)4][PF6] (6), does not result in an electron-deficient, four-coordinate {Fe(NO)2}10 site, and hence no N-N coupling is observed.

The reduction of nitrogen oxides (NxOyn-) to dinitrogen gas by samarium(ii) iodide is reported. The polyoxoanions nitrate (NO3-) and nitrite (NO2-), as well as nitrous oxide (N2O) and nitric oxide (NO) were all shown to react with stoichiometric amounts of SmI2 in THF for the complete denitrification to N2.

Metalloenzymes catalyze many important reactions by managing the proton and electron flux at the enzyme active site. The motifs utilized to facilitate these transformations include hemilabile, redox-active, and so called proton responsive sites. Given the importance of incorporating and understanding these motifs in the area of coordination chemistry and catalysis, we highlight recent milestones in the field. Work incorporating the triad of hemilability, redox-activity, and proton responsivity into single ligand scaffolds will be described.

Incorporation of the triad of redox activity, hemilability, and proton responsivity into a single ligand scaffold is reported. Due to this triad, the complexes Fe(PyrrPDI)(CO)2 (3) and Fe(MorPDI)(CO)2 (4) display 40-fold enhancements in the initial rate of NO2- reduction, with respect to Fe(MeOPDI)(CO)2 (7). Utilizing the proper sterics and p Ka of the pendant base(s) to introduce hemilability into our ligand scaffolds, we report unusual {FeNO} x mononitrosyl iron complexes (MNICs) as intermediates in the NO2- reduction reaction. The {FeNO} x species behave spectroscopically and computationally similar to {FeNO}7, an unusual intermediate-spin Fe(III) coupled to triplet NO- and a singly reduced PDI ligand. These {FeNO} x MNICs facilitate enhancements in the initial rate.

Metal complexes composed of redox-active pyridinediimine (PDI) ligands are capable of forming ligand-centered radicals. In this Forum article, we demonstrate that integration of these types of redox-active sites with bioinspired secondary coordination sphere motifs produce direduced complexes, where the reduction potential of the ligand-based redox sites is uncoupled from the secondary coordination sphere. The utility of such ligand design was explored by encapsulating redox-inactive Lewis acidic cations via installation of a pendant benzo-15-crown-5 in the secondary coordination sphere of a series of Fe(PDI) complexes. Fe(15bz5PDI)(CO)2 was shown to encapsulate the redox-inactive alkali ion, Na+, causing only modest (31 mV) anodic shifts in the ligand-based redox-active sites. By uncoupling the Lewis acidic sites from the ligand-based redox sites, the pendant redox-inactive ion, Na+, can entice the corresponding counterion, NO2-, for reduction to NO. The subsequent initial rate analysis reveals an acceleration in anion reduction, confirming this hypothesis.

Utilizing the proton-responsive pyridinediimine ligand [(2,6-iPrC6H3)(NCMe)(N(iPr)2C2H4)(NCMe)C5H3N] (didpa), the ligand-based reduction of nitrate (NO3−) to nitric oxide (NO) was achieved.

A series of pyridinediimine (PDI) iron complexes that contain a pendant 15-crown-5 located in the secondary coordination sphere were synthesized and characterized. The complex Fe((15c5)PDI)(CO)2 (2) was shown in both the solid state and solution to encapsulate redox-inactive metal ions. Modest shifts in the reduction potential of the metal-ligand scaffold were observed upon encapsulation of either Na(+) or Li(+).

The reduction of NO2− to NO is achieved with a FePDI complex containing a proton-responsive secondary coordination sphere coupled with redox-active sites.

A set of distorted square planar Cu(i) complexes were synthesized and characterized utilizing the sterically encumbering pyridinediimine ligand, iPrPDI (where iPrPDI = 2,6-(2,6-iPr2C6H3NCMe)2C5H3N).