Biomimetic Model Featuring the NH Proton and Bridging Hydride Related to a Proposed Intermediate in Enzymatic H-2 Production by Fe-Only Hydrogenase
ABSTRACT Iron azadithiolate phosphine-substituted complex and its protonated species featuring the NH proton and/or bridging Fe hydride, [Fe(2)(mu-S(CH(2))(2)N(n)Pr(H)(m)(CH(2))(2)S)(mu-H)(n)(CO)(4)(PMe(3))(2)](2)((2m+2n)+) (1, m = n = 0; [1-2H(N)](2+), m = 1, n = 0; [1-2H(N)2H(Fe)](4+), m = n = 1), are prepared to mimic the active site of Fe-only hydrogenase. X-ray crystallographic analyses of these three complexes reveal that two diiron subunits are linked by two azadiethylenethiolate bridges to construct a dimer-of-dimer structure. (31)P NMR spectroscopy confirms two trimethylphosphine ligands within the diiron moiety are arranged in the apical/basal configuration, which is consistent with the solid-state structural characterization. Deprotonation of the NH proton in [1-2H(N)](2+) and [1-2H(N)2H(Fe)](4+) occurs in the presence of triethanolamine (TEOA), which generates 1 and [1-2H(Fe)](2+), respectively. Deprotonation of the Fe hydride is accomplished by addition of bistriphenylphosphineimminium chloride ([PPN]Cl). It is observed that the Fe hydride species, [1-2H(Fe)](2+), is a kinetic product which converts to its thermodynamically stable tautomer, [1-2H(N)](2+), in solution, as evidenced by IR and NMR spectroscopy. The pK(a) values of the aza nitrogen and the diiron sites are estimated to be 8.9-15.9 and <8.9, respectively. [1-2H(N)2H(Fe)](4+) has been observed to evolve H(2) electrocatalytically at a mild potential (-1.42 V vs Fc/Fc(+)) in CH(3)CN solution. Catalysis of [1-2H(N)2H(Fe)](4+) is found to be as efficient as that of the related diiron azadithiolate complexes. In the absence of a proton source, [1-2H(N)2H(Fe)](4+) undergoes four irreversible reduction processes at -1.26, -1.42, -1.82, and -2.43 V, which are attributed to the reduction events from [1-2H(N)2H(Fe)](4+), [1-2H(Fe)](2+), [1-2H(N)](2+), and 1, respectively, according to bulk electrolysis and voltammetry in combination of titration experiments with acids.
- [Show abstract] [Hide abstract]
ABSTRACT: 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.Inorganic Chemistry 07/2010; 49(14):6409-20. DOI:10.1021/ic100484a · 4.79 Impact Factor
Chapter: Fe–H Complexes in Catalysis[Show abstract] [Hide abstract]
ABSTRACT: Organic syntheses catalyzed by iron complexes have attracted considerable attention because iron is an abundant, inexpensive, and environmentally benign metal. It has been documented that various iron hydride complexes play important roles in catalytic cycles such as hydrogenation, hydrosilylation, hydroboration, hydrogen generation, and element–element bond formation. This chapter summarizes the recent developments, mainly from 2000 to 2009, of iron catalysts involving hydride ligand(s) and the role of Fe–H species in catalytic cycles. KeywordsCatalysis-Electrochemical reduction-Hydroboration-Hydrogenation-Hydrosilylation-Iron hydride complex-Photochemical reduction01/2011: pages 27-81;
- [Show abstract] [Hide abstract]
ABSTRACT: In the pursuit of a "rotated" structure, and exploration of the influence of the aza nitrogen lone pair, the Fe(I)Fe(I) model complexes wherein two Fe(CO)(3-x)P(x) moieties are significantly twisted from the ideal configuration (torsion angle >30°) are reported. [Fe(2)(μ-S(CH(2))(2)N(i)Pr(X)(CH(2))(2)S)(CO)(4)(κ(2)-dppe)](2)(2+) (X = H, 4; Me, 5) prepared from protonation and methylation, respectively, of [Fe(2)(μ-S(CH(2))(2)N(i)Pr(CH(2))(2)S)(CO)(4)(κ(2)-dppe)](2), 1, possess Φ angles of 34.1 and 35.4° (av.), respectively. Such dramatic twist is attributed to asymmetric substitution within the Fe(2) unit in which a dppe ligand is coordinated to one Fe site in the κ(2)-mode. In the presence of the N···C(CO(ap)) interaction, the torsion angle is decreased to 10.8°, suggesting availability of lone pairs of the aza nitrogen sites within 1 is in control of the twist. Backbones of the bridging diphosphine ligands also affect distortion. For a shorter ligand, the more compact structure of [Fe(2)(μ-S(CH(2))(2)N(i)Pr(CH(2))(2)S)(μ-dppm)(CO)(4)](2), 7, is formed. Dppm in a bridging manner allows achievement of the nearly eclipsed configuration. In contrast, dppe in [Fe(2)(μ-S(CH(2))(2)N(i)Pr(CH(2))(2)S)(μ-dppe)(CO)(4)](2), 6, could twist the Fe(CO)(3-x)L(x) fragment to adopt the least strained structure. In addition, the NC(CO(ap)) interaction would direct the twist towards a specific direction for the closer contact. In return, the shorter N···C(CO(ap)) distance of 3.721(7) Å and larger Φ angle of 26.5° are obtained in 6. For comparison, 3.987(7) Å and 3.9° of the corresponding parameters are observed in 7. Conversion of (μ-dppe)[Fe(2)(μ-S(CH(2))(2)N(i)Pr(CH(2))(2)S)(CO)(5)](2), 2, to complex 1 via an associative mechanism is studied.Dalton Transactions 02/2011; 40(11):2528-41. DOI:10.1039/c0dt01332c · 4.10 Impact Factor