Journal of the American Chemical Society, Dec 1, 2008
The dithiolate cofactor for the [FeFe]-hydrogenase models, Fe(2)(xdt)(CO)(2)(dppv)(2) (where xdt ... more The dithiolate cofactor for the [FeFe]-hydrogenase models, Fe(2)(xdt)(CO)(2)(dppv)(2) (where xdt = 1,3-propanedithiolate (pdt), azadithiolate (adt), (SCH(2))(2)NH, and oxadithiolate (odt), (SCH(2))(2)O; dppv = cis-1,2-bis(diphenylphosphino)ethylene) have been probed for their functionality as proton relays enabling formation and deprotonation of terminal hydrides. Compared to the propanedithiolate derivative, the azadithiolate and oxaditiholate show enhanced rates of proton transfer between solution and the terminal site on one Fe center. The results are consistent with the heteroatom of the dithiolate serving a gating role for both protonation and deprotonation. The pK(a) of the transiently formed ammonium (pK(CD(2))(Cl(2)) 5.7-8.2) or oxonium (pK(CD(2))(Cl(2)) -4.7-1.6) regulates the proton transfer. As a consequence, only the azadithiolate is capable of yielding the terminal hydride from weak acids. The aza- and oxadithiolates manifested the advantages of proton relays: the odt derivative proved to be a faster catalyst for hydrogen evolution than the pdt derivative as indicated from cyclic voltammetry plots of i(c)/i(p) vs [H(+)]. The adt derivative was capable of proton reduction from the weak acid [HPMe(2)Ph]BF(4) (pK(CD(2))(Cl(2)) = 5.7). The proton relay function does not apply to the isomeric bridged-hydrides [Fe(2)(xdt)(mu-H)(CO)(2)(dppv)(2)](+), where the hydride is too distant and too basic to interact to be affected by the heteroatomic relay site. None of these mu-H species can be deprotonated.
Treatment of Fe(2)(pdt)(CO)(4)(dppv) (1) with aryldiazonium salts affords the 34e(-) adducts [Fe(... more Treatment of Fe(2)(pdt)(CO)(4)(dppv) (1) with aryldiazonium salts affords the 34e(-) adducts [Fe(2)(pdt)(μ-N(2)Ar)(CO)(4)(dppv)](+) (pdt(2-) = 1,3-propanedithiolate, dppv = cis-C(2)H(2)(PPh(2))(2)). Under some conditions, the same reaction gave substantial amounts of [1](+), the product of electron-transfer. Consistent with the influence of electron transfer in the reactions of some electrophiles with Fe(I)Fe(I) dithiolates, the reaction of [Me(3)S(2)](+) and Fe(2)(pdt)(CO)(4)(dppbz) was found to give [Fe(2)(pdt)(CO)(4)(dppbz)](+) as well as Me(2)S and Me(2)S(2) (dppbz = 1,2-bis(diphenylphosphino)benzene).
The dithiolate cofactor for the [FeFe]-hydrogenase models, Fe(2)(xdt)(CO)(2)(dppv)(2) (where xdt ... more The dithiolate cofactor for the [FeFe]-hydrogenase models, Fe(2)(xdt)(CO)(2)(dppv)(2) (where xdt = 1,3-propanedithiolate (pdt), azadithiolate (adt), (SCH(2))(2)NH, and oxadithiolate (odt), (SCH(2))(2)O; dppv = cis-1,2-bis(diphenylphosphino)ethylene) have been probed for their functionality as proton relays enabling formation and deprotonation of terminal hydrides. Compared to the propanedithiolate derivative, the azadithiolate and oxaditiholate show enhanced rates of proton transfer between solution and the terminal site on one Fe center. The results are consistent with the heteroatom of the dithiolate serving a gating role for both protonation and deprotonation. The pK(a) of the transiently formed ammonium (pK(CD(2))(Cl(2)) 5.7-8.2) or oxonium (pK(CD(2))(Cl(2)) -4.7-1.6) regulates the proton transfer. As a consequence, only the azadithiolate is capable of yielding the terminal hydride from weak acids. The aza- and oxadithiolates manifested the advantages of proton relays: the odt derivative proved to be a faster catalyst for hydrogen evolution than the pdt derivative as indicated from cyclic voltammetry plots of i(c)/i(p) vs [H(+)]. The adt derivative was capable of proton reduction from the weak acid [HPMe(2)Ph]BF(4) (pK(CD(2))(Cl(2)) = 5.7). The proton relay function does not apply to the isomeric bridged-hydrides [Fe(2)(xdt)(mu-H)(CO)(2)(dppv)(2)](+), where the hydride is too distant and too basic to interact to be affected by the heteroatomic relay site. None of these mu-H species can be deprotonated.
Using the thermally stable salts of [Fe(2)(SR)(2)(CO)(3)(PMe(3))(dppv)]BAr(F)(4), we found that t... more Using the thermally stable salts of [Fe(2)(SR)(2)(CO)(3)(PMe(3))(dppv)]BAr(F)(4), we found that the azadithiolates [Fe(2)(adtR)(CO)(3)(PMe(3))(dppv)](+) react with high pressures of H(2) to give the hydride [Fe(2)(mu-H)(adtR)(CO)(3)(dppv)(PMe(3))]BAr(F)(4). The related oxadithiolate and propanedithiolate complexes are unreactive toward H(2). Molecular hydrogen is proposed to undergo heterolysis assisted by the amine followed by isomerization of an initially formed terminal hydride. Use of H(2) and D(2)O gave the deuteride as well as the hydride, implicating protic intermediates.
Understanding the catalytic process of the heterolytic splitting and formation of molecular hydro... more Understanding the catalytic process of the heterolytic splitting and formation of molecular hydrogen is one of the key topics for the development of a future hydrogen economy. With an interest in elucidating the enzymatic mechanism of the [Fe(2)(S(2)C(2)H(4)NH)(CN)(2)(CO)(2)(μ-CO)] active center uniquely found in [FeFe]hydrogenases, we present a detailed spectroscopic and theoretical analysis of its inorganic model [Fe(2)(S(2)X)(CO)(3)(dppv)(PMe(3))](+) [dppv = cis-1,2-bis(diphenylphosphino)ethylene] in two forms with S(2)X = ethanedithiolate (1edt) and azadithiolate (1adt). These complexes represent models for the oxidized mixed-valent Fe(I)Fe(II) state analogous to the active oxidized "H(ox)" state of the native H-cluster. For both complexes, the (31)P hyperfine interactions were determined by pulse electron paramagnetic resonance and electron nuclear double resonance (ENDOR) methods. For 1edt, the (57)Fe parameters were measured by electron spin-echo envelope modulation and Mössbauer spectroscopy, while for 1adt, (14)N and selected (1)H couplings could be obtained by ENDOR and hyperfine sublevel correlation spectroscopy. The spin density was found to be predominantly localized on the Fe(dppv) site. This spin distribution is different from that of the H-cluster, where both the spin and charge densities are delocalized over the two Fe centers. This difference is attributed to the influence of the "native" cubane subcluster that is lacking in the inorganic models. The degree and character of the unpaired spin delocalization was found to vary from 1edt, with an abiological dithiolate, to 1adt, which features the authentic cofactor. For 1adt, we find two (14)N signals, which are indicative for two possible isomers of the azadithiolate, demonstrating its high flexibility. All interaction parameters were also evaluated through density functional theory calculations at various levels.
Decades of biophysical study on the hydrogenase (H(2)ase) enzymes have yielded sufficient informa... more Decades of biophysical study on the hydrogenase (H(2)ase) enzymes have yielded sufficient information to guide the synthesis of analogs of their active sites. Three families of enzymes serve as inspiration for this work: the [FeFe]-H(2)ases, [NiFe]-H(2)ases, and [Fe]-H(2)ases, all of which feature iron centers bound to both CO and thiolate. Artificial H(2)ases affect the oxidation of H(2) and the reverse reaction, the reduction of protons. These reactions occur via the intermediacy of metal hydrides. The inclusion of amine bases within the catalysts is an important design feature that is emulated in related bioinspired catalysts. Continuing challenges are the low reactivity of H(2) toward biomimetic H(2)ases.
Experimental and computational experiments show that the electrophile MeS(+) attacks a single Fe ... more Experimental and computational experiments show that the electrophile MeS(+) attacks a single Fe center in Fe(2)(propanedithiolate)(CO)(4)(PMe(3))(2) followed by isomerization of this terminal thiolato complex to the corresponding μ-SMe derivative.
Journal of the American Chemical Society, Dec 1, 2008
The dithiolate cofactor for the [FeFe]-hydrogenase models, Fe(2)(xdt)(CO)(2)(dppv)(2) (where xdt ... more The dithiolate cofactor for the [FeFe]-hydrogenase models, Fe(2)(xdt)(CO)(2)(dppv)(2) (where xdt = 1,3-propanedithiolate (pdt), azadithiolate (adt), (SCH(2))(2)NH, and oxadithiolate (odt), (SCH(2))(2)O; dppv = cis-1,2-bis(diphenylphosphino)ethylene) have been probed for their functionality as proton relays enabling formation and deprotonation of terminal hydrides. Compared to the propanedithiolate derivative, the azadithiolate and oxaditiholate show enhanced rates of proton transfer between solution and the terminal site on one Fe center. The results are consistent with the heteroatom of the dithiolate serving a gating role for both protonation and deprotonation. The pK(a) of the transiently formed ammonium (pK(CD(2))(Cl(2)) 5.7-8.2) or oxonium (pK(CD(2))(Cl(2)) -4.7-1.6) regulates the proton transfer. As a consequence, only the azadithiolate is capable of yielding the terminal hydride from weak acids. The aza- and oxadithiolates manifested the advantages of proton relays: the odt derivative proved to be a faster catalyst for hydrogen evolution than the pdt derivative as indicated from cyclic voltammetry plots of i(c)/i(p) vs [H(+)]. The adt derivative was capable of proton reduction from the weak acid [HPMe(2)Ph]BF(4) (pK(CD(2))(Cl(2)) = 5.7). The proton relay function does not apply to the isomeric bridged-hydrides [Fe(2)(xdt)(mu-H)(CO)(2)(dppv)(2)](+), where the hydride is too distant and too basic to interact to be affected by the heteroatomic relay site. None of these mu-H species can be deprotonated.
Treatment of Fe(2)(pdt)(CO)(4)(dppv) (1) with aryldiazonium salts affords the 34e(-) adducts [Fe(... more Treatment of Fe(2)(pdt)(CO)(4)(dppv) (1) with aryldiazonium salts affords the 34e(-) adducts [Fe(2)(pdt)(μ-N(2)Ar)(CO)(4)(dppv)](+) (pdt(2-) = 1,3-propanedithiolate, dppv = cis-C(2)H(2)(PPh(2))(2)). Under some conditions, the same reaction gave substantial amounts of [1](+), the product of electron-transfer. Consistent with the influence of electron transfer in the reactions of some electrophiles with Fe(I)Fe(I) dithiolates, the reaction of [Me(3)S(2)](+) and Fe(2)(pdt)(CO)(4)(dppbz) was found to give [Fe(2)(pdt)(CO)(4)(dppbz)](+) as well as Me(2)S and Me(2)S(2) (dppbz = 1,2-bis(diphenylphosphino)benzene).
The dithiolate cofactor for the [FeFe]-hydrogenase models, Fe(2)(xdt)(CO)(2)(dppv)(2) (where xdt ... more The dithiolate cofactor for the [FeFe]-hydrogenase models, Fe(2)(xdt)(CO)(2)(dppv)(2) (where xdt = 1,3-propanedithiolate (pdt), azadithiolate (adt), (SCH(2))(2)NH, and oxadithiolate (odt), (SCH(2))(2)O; dppv = cis-1,2-bis(diphenylphosphino)ethylene) have been probed for their functionality as proton relays enabling formation and deprotonation of terminal hydrides. Compared to the propanedithiolate derivative, the azadithiolate and oxaditiholate show enhanced rates of proton transfer between solution and the terminal site on one Fe center. The results are consistent with the heteroatom of the dithiolate serving a gating role for both protonation and deprotonation. The pK(a) of the transiently formed ammonium (pK(CD(2))(Cl(2)) 5.7-8.2) or oxonium (pK(CD(2))(Cl(2)) -4.7-1.6) regulates the proton transfer. As a consequence, only the azadithiolate is capable of yielding the terminal hydride from weak acids. The aza- and oxadithiolates manifested the advantages of proton relays: the odt derivative proved to be a faster catalyst for hydrogen evolution than the pdt derivative as indicated from cyclic voltammetry plots of i(c)/i(p) vs [H(+)]. The adt derivative was capable of proton reduction from the weak acid [HPMe(2)Ph]BF(4) (pK(CD(2))(Cl(2)) = 5.7). The proton relay function does not apply to the isomeric bridged-hydrides [Fe(2)(xdt)(mu-H)(CO)(2)(dppv)(2)](+), where the hydride is too distant and too basic to interact to be affected by the heteroatomic relay site. None of these mu-H species can be deprotonated.
Using the thermally stable salts of [Fe(2)(SR)(2)(CO)(3)(PMe(3))(dppv)]BAr(F)(4), we found that t... more Using the thermally stable salts of [Fe(2)(SR)(2)(CO)(3)(PMe(3))(dppv)]BAr(F)(4), we found that the azadithiolates [Fe(2)(adtR)(CO)(3)(PMe(3))(dppv)](+) react with high pressures of H(2) to give the hydride [Fe(2)(mu-H)(adtR)(CO)(3)(dppv)(PMe(3))]BAr(F)(4). The related oxadithiolate and propanedithiolate complexes are unreactive toward H(2). Molecular hydrogen is proposed to undergo heterolysis assisted by the amine followed by isomerization of an initially formed terminal hydride. Use of H(2) and D(2)O gave the deuteride as well as the hydride, implicating protic intermediates.
Understanding the catalytic process of the heterolytic splitting and formation of molecular hydro... more Understanding the catalytic process of the heterolytic splitting and formation of molecular hydrogen is one of the key topics for the development of a future hydrogen economy. With an interest in elucidating the enzymatic mechanism of the [Fe(2)(S(2)C(2)H(4)NH)(CN)(2)(CO)(2)(μ-CO)] active center uniquely found in [FeFe]hydrogenases, we present a detailed spectroscopic and theoretical analysis of its inorganic model [Fe(2)(S(2)X)(CO)(3)(dppv)(PMe(3))](+) [dppv = cis-1,2-bis(diphenylphosphino)ethylene] in two forms with S(2)X = ethanedithiolate (1edt) and azadithiolate (1adt). These complexes represent models for the oxidized mixed-valent Fe(I)Fe(II) state analogous to the active oxidized "H(ox)" state of the native H-cluster. For both complexes, the (31)P hyperfine interactions were determined by pulse electron paramagnetic resonance and electron nuclear double resonance (ENDOR) methods. For 1edt, the (57)Fe parameters were measured by electron spin-echo envelope modulation and Mössbauer spectroscopy, while for 1adt, (14)N and selected (1)H couplings could be obtained by ENDOR and hyperfine sublevel correlation spectroscopy. The spin density was found to be predominantly localized on the Fe(dppv) site. This spin distribution is different from that of the H-cluster, where both the spin and charge densities are delocalized over the two Fe centers. This difference is attributed to the influence of the "native" cubane subcluster that is lacking in the inorganic models. The degree and character of the unpaired spin delocalization was found to vary from 1edt, with an abiological dithiolate, to 1adt, which features the authentic cofactor. For 1adt, we find two (14)N signals, which are indicative for two possible isomers of the azadithiolate, demonstrating its high flexibility. All interaction parameters were also evaluated through density functional theory calculations at various levels.
Decades of biophysical study on the hydrogenase (H(2)ase) enzymes have yielded sufficient informa... more Decades of biophysical study on the hydrogenase (H(2)ase) enzymes have yielded sufficient information to guide the synthesis of analogs of their active sites. Three families of enzymes serve as inspiration for this work: the [FeFe]-H(2)ases, [NiFe]-H(2)ases, and [Fe]-H(2)ases, all of which feature iron centers bound to both CO and thiolate. Artificial H(2)ases affect the oxidation of H(2) and the reverse reaction, the reduction of protons. These reactions occur via the intermediacy of metal hydrides. The inclusion of amine bases within the catalysts is an important design feature that is emulated in related bioinspired catalysts. Continuing challenges are the low reactivity of H(2) toward biomimetic H(2)ases.
Experimental and computational experiments show that the electrophile MeS(+) attacks a single Fe ... more Experimental and computational experiments show that the electrophile MeS(+) attacks a single Fe center in Fe(2)(propanedithiolate)(CO)(4)(PMe(3))(2) followed by isomerization of this terminal thiolato complex to the corresponding μ-SMe derivative.
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