Secondary coordination sphere interactions within the biomimetic iron azadithiolate complexes related to Fe-only hydrogenase: dynamic measure of electron density about the Fe sites.
A series of iron azadithiolate complexes possessing an intramolecular secondary coordination sphere interaction and an ability to reduce HOAc at the potential near the first electron-transfer process are reported. A unique structural feature in which the aza nitrogen has its lone pair point toward the apical carbonyl carbon is observed in [Fe(2)(mu-S(CH(2))(2)NR(CH(2))(2)S)(CO)(6-x)L(x)](2) (R = (n)Pr, x = 0, 1a; R = (i)Pr, x = 0, 1b; R = (n)Pr, L = PPh(3), x = 1, 2; R = (n)Pr, L = P(n)Bu(3), x = 1, 3) as biomimetic models of the active site of Fe-only hydrogenase. The presence of this weak N...C(CO(ap)) interaction provides electronic perturbation at the Fe center. The distance of the N...C(CO(ap)) contact is 3.497 A in 1a. It increases by 0.455 A in 2 when electronic density of the Fe site is slightly enriched by a weak sigma-donating ligand, PPh(3). A longer distance (4.040 A) is observed for the P(n)Bu(3) derivative, 3. This N...C(CO(ap)) distance is thus a dynamic measure of electronic nature of the Fe(2) core. Variation of electronic richness within the Fe(2) moiety among the complexes reflects on their electrochemical response. Reduction of 2 is recorded at the potential of -2.17 V, which is 270 mV more negative than that of 1. Complex 3 requires additional 150 mV for the same reduction. Such cathodic shift results from CO substitution by phosphines. Electrocatalytic hydrogen production from HOAc by both kinds of complexes (all-CO and phosphine-substituted species) requires the potential close to that for reduction of the parent molecules in the absence of acids. The catalytic mechanism of 1a is proposed to involve proton uptake at the Fe(0)Fe(I) redox level instead of the Fe(0)Fe(0) level. This result is the first observation among the all-CO complexes with respect to electrocatalysis of HOAc.
Liu
Yu-Chiao
YC
Institute of Chemistry, Academia Sinica, Nankang, Taipei 115, Taiwan.
Tu
Ling-Kuang
LK
Yen
Tao-Hung
TH
Lee
Gene-Hsiang
GH
Yang
Shu-Ting
ST
Chiang
Ming-Hsi
MH
D019342:Acetic Acid / Q000737:chemistry; D001372:Aza Compounds / Q000737:chemistry*; D001672:Biocompatible Materials / Q000737:chemistry*; D004563:Electrochemistry; D006864:Hydrogenase / Q000737:chemistry* / Q000378:metabolism; D007501:Iron / Q000737:chemistry*; D007506:Iron-Sulfur Proteins / Q000737:chemistry* / Q000378:metabolism; D008958:Models, Molecular; D011789:Quantum Theory
D001372:Aza Compounds; D001672:Biocompatible Materials; D007506:Iron-Sulfur Proteins; D007501:Iron; C118830:iron hydrogenase; D006864:Hydrogenase; D019342:Acetic Acid