Using Substituted [Fe4N(CO)12]− as a Platform To Probe the Effect of Cation and Lewis Acid Location on Redox Potential

The impact of cationic and Lewis acidic functional groups installed in the primary or secondary coordination sphere (PCS or SCS) of an (electro)­catalyst is known to vary depending on the precise positioning of those groups. However, it is difficult to systematically probe the effect of that position. In this report, we probe the effect of the functional group position and identity on the observed reduction potentials (E p,c) using substituted iron clusters, [Fe4N­(CO)11R] n , where R = NO+, PPh2-CH2CH2-9BBN, (MePTA+)2, (MePTA+)4, and H+ and n = 0, −1, +1, or +3 (9-BBN is 9-borabicyclo(3.3.1)­nonane; MePTA+ is 1-methyl-1-azonia-3,5-diaza-7-phosphaadamantane). The cationic NO+ and H+ ligands cause anodic shifts of 700 and 320 mV, respectively, in E p,c relative to unsubstituted [Fe4N­(CO)12]−. Infrared absorption band data, νCO, suggests that some of the 700 mV shift by NO+ results from electronic changes to the cluster core. This contrasts with the effects of cationic MePTA+ and H+ which cause primarily electrostatic effects on E p,c. Lewis acidic 9-BBN in the SCS had almost no effect on E p,c.