Phosphine-ligated dinitrosyl iron complexes for redox-controlled NO release

Abstract

Here we present the syntheses and structural, spectroscopic, as well as electrochemical properties of four dinitrosyl iron complexes (DNICs) based on silicon- and carbon-derived di- and tripodal phosphines. Whereas CH3C(CH2PPh2)3 and Ph2Si(CH2PPh2)2 coordinate iron in a η(2) - binding mode, CH3Si(CH2PPh2)3 undergoes cleavage of one Si-C bond to afford [Fe(NO)2(P(CH3)Ph2)2] at elevated temperatures. The complexes were characterized by IR spectroelectrochemistry as well as UV-vis measurements. The oxidized {Fe(NO)2}(9) compounds were obtained by oxidation with (NH4)2[Ce(NO3)6] and their properties evaluated with Mössbauer and IR spectroscopy. Stability experiments on the complexes suggest that they are capable of releasing their NO-ligands in the oxidized {Fe(NO)2}(9) but not in the reduced {Fe(NO)2}(10) form. A detailed DFT analysis is provided in order to understand the electronic configurations and the complexes' ability to release NO.

Carbon/Silicon Exchange at the Apex of Diphos‐ and Triphos‐Derived Ligands – More Than Just a Substitute?

“…As expected, similar results were not observed for the carbon derivative that yields [κ 2 ‐MeC(CH 2 PPh 2 ) 3 Ni(MeCN) 3 ] 2+ ( 7 ) . Another example of silicon–carbon bond cleavage is observed for the reaction of Fe(NO) 2 (CO) 2 with MeSi(CH 2 PPh 2 ) 3 . While affording complex [κ 2 ‐MeSi(CH 2 PPh 2 ) 3 Fe(NO) 2 ] ( 8b ) at lower temperature, an immediate Si–C bond cleavage and formation of [(PhPCH 3 ) 2 Fe(NO) 2 ] ( 9 ) was observed at higher temperatures (Figure ).…”

“…In contrast, 12a is inactive under the same experimental conditions. While theoretical calculations show no influence of the C/Si exchange on the donor capability of the phosphorus atoms and both complexes have a tetrahedral structure, the C–E–C bond angles in the periphery of the metal center are larger in complex 12a (115.57° vs. 111.32° in 12b ) and the P–Fe–P bond angles decrease from 97.82° in 12b to 91.08° in 12a . On the basis of experimental as well as theoretical findings, it was suggested that the difference in activity is due to modulation of the steric constraints.…”

“…Irreversible decomposition of Triphos Si ligands as well as their stable congeners obtained under varied reaction conditions , …”

“…While the exchange of carbon and silicon is a straightforward method to modulate the reactivity of metal complexes, the differences are not always as distinct. For example, Fe(CO) 2 (NO) 2 reacts with MeE(CH 2 PPh 2 ) 3 to afford complexes [κ 2 ‐MeE(CH 2 PPh 2 ) 3 Fe(NO) 2 ] {E = C ( 8a ) or Si ( 8b )} . Both complexes adopt a tetrahedral geometry with similar Fe–N and Fe–P bond lengths, while the P–Fe–P, N–Fe–N, and N–Fe–P bond angles are vastly different.…”

“…The N–O stretching frequencies then show a distinct difference: complex 8a has a larger IR shift of the N–O stretching frequencies to 1806 and 1733 cm –1 , compared to 1742 and about 1649 (broad) cm –1 for the oxidized complex 8b + . It was suggested that NO release from the oxidized complex 8b + might be less favorable as a result of the slightly altered NO bond situation …”