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. 2018 Dec 12;140(49):17040-17050.
doi: 10.1021/jacs.8b08520. Epub 2018 Nov 30.

Hemilabile Proton Relays and Redox Activity Lead to {FeNO} x and Significant Rate Enhancements in NO2- Reduction

Pui Man Cheung, Kyle T Burns, Yubin M Kwon, Megan Y Deshaye, Kristopher J Aguayo, Victoria F Oswald  1 , Takele Seda, Lev N Zakharov  2 , Tim Kowalczyk, John D Gilbertson
Affiliations

Hemilabile Proton Relays and Redox Activity Lead to {FeNO} x and Significant Rate Enhancements in NO2- Reduction

Pui Man Cheung et al. J Am Chem Soc. .

Abstract

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.

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Conflict of interest statement

The authors declare no competing financial interests.

Figures

Figure 1.
Figure 1.
Solid-state structures (30% probability) of Fe(PyrrPDI)(CO)2 (3, left) and Fe(MorPDI)(CO)2 (4, right). The H atoms have been omitted for clarity. Selected bond lengths (Å) and angles (°) for 1: Fe(1)-C(28) 1.784(1), Fe(1)-C(29) 1.782(2), Fe(1)-N(1) 1.944(1), Fe(1)-N(2) 1.849(1), Fe(1)-N(3) 1.948(2), C(2)-N(1) 1.332(2), C(8)-N(3) 1.324(2), and C(28)Fe(1)C(29) 95.65(7), N(2)Fe(1)C(28) 153.81(7), N(1)Fe(1)N(3) 154.16(6). Selected bond lengths (Å) and angles (°) for 2: Fe(1)-C(28) 1.780(2), Fe(1)-C(29) 1.769(2), Fe(1)-N(1) 1.954(1), Fe(1)-N(2) 1.851(1), Fe(1)-N(3) 1.953(1), C(2)-N(1) 1.332(2), C(8)-N(3) 1.332(2), and C(28)Fe(1)C(29) 97.62(9), N(2)Fe(1)C(29) 156.22(7), N(1)Fe(1)N(3) 153.42(5).
Figure 2.
Figure 2.
Solid state structures (30% probability) of [Fe(PyrrPDI)(NO)2]+ (5, left) and [Fe(MorPDI)(NO)2]+ (6, right). The H atoms and the BPh4− counterion have been omitted for clarity. Selected bond lengths (Å) and angles (°) for 5: Fe(1)-N(1) 2.216(2), Fe(1)-N(2) 2.080(1), Fe(1)-N(3) 2.138(2), Fe(1)-N(5) 1.693(2), Fe(1)-N(6) 1.696(2), N(1)-C(2) 1.285(2), N(3)-C(8) 1.283(2), and N(5)-Fe(1)-N(6) 108.7(1), N(2)-Fe(1)-N(6) 132.85(8), N(1)-Fe(1)-N(3) 147.38(6). Selected bond lengths (Å) and angles (°) for 6: Fe(1)-N(1) 2.233(3), Fe(1)-N(2) 2.065(2), Fe(1)-N(3) 2.171(3), Fe(1)-N(4) 1.694(2), Fe(1)-N(5) 1.696(3), N(1)-C(2) 1.280(3), N(3)-C(8) 1.290(3), and N(4)-Fe(1)-N(5) 108.6(1), N(2)-Fe(1)-N(4) 128.9(1), N(1)-Fe(1)-N(3) 148.23(9).
Figure 3.
Figure 3.
Initial rate plots for the reduction of TBANO2 by selected Fe(PDI)(CO) species and 4 equiv [HNEt3]+ (top) and [HLut]+ (bottom). Black circle = 7, red triangle = 8, blue diamond = 3, and green square = 4.
Figure 4.
Figure 4.
Solid state structures (30% probability) of [Fe(PyrrPDI)(NO)]+ (9, left) and [Fe(MorPDI)(NO)]+ (10, right). The H atoms and the BPh4− counterion have been omitted for clarity. Selected bond lengths (Å) and angles (°) for 9: Fe(1)-N(1) 2.031(1), Fe(1)-N(2) 1.830(2), Fe(1)-N(3) 1.894(1), Fe(1)-N(4) 2.066(1), Fe(1)-N(5) 1.677(2), N(1)-C(2) 1.315(2), N(3)-C(8) 1.313(2), and N(5)-Fe(1)-N(4) 104.09(7), N(2)-Fe(1)-N(4) 152.92(7), N(1)-Fe(1)-N(3) 147.66(6). Selected bond lengths (Å) and angles (°) for 10: Fe(1)-N(1) 2.035(1), Fe(1)-N(2) 1.836(1), Fe(1)-N(3) 1.894(1), Fe(1)-N(4) 2.076(1), Fe(1)-N(5) 1.679(1), N(1)-C(2) 1.314(1), N(3)-C(8) 1.313(2), and N(4)-Fe(1)-N(5) 104.59(5), N(2)-Fe(1)-N(4) 144.87(4), N(1)-Fe(1)-N(3) 151.92(4).
Figure 5.
Figure 5.
Mulliken spin populations (left) and spin density (right) of MNIC 9.
Figure 6.
Figure 6.
Highest-occupied corresponding orbitals (COs) and αβ overlaps from the BS(2,2) solution for 9.
Scheme 1.
Scheme 1.
Reduction of NO2−, illustrating the role of the hemila-bile pendant base.
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