Catalysis of H2/D2 scrambling and other H/D exchange processes by [Fe]-hydrogenase model complexes

Article abstract of DOI:10.1021/ic020237r

Protonation of the [Fe]-hydrogenase model complex (μ-pdt)[Fe(CO)₂(PMe₃)]₂ (pdt = SCH₂CH₂CH₂S) produces a species with a high field ¹H NMR resonance, isolated as the stable {(μ-H)(μ-pdt)[Fe(CO)₂(PMe₃)]₂} ⁺[PF₆]^- salt. Structural characterization found little difference in the 2Fe2S butterfly cores, with Fe···Fe distances of 2.555(2) and 2.578(1) A for the Fe-Fe bonded neutral species and the bridging hydride species, respectively (Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Yarbrough, J. C.; Darensbourg, M. Y. J. Am. Chem. Soc. 2001, 123, 9710). Both are similar to the average Fe···Fe distance found in structures of three Fe-only hydrogenase active site 2Fe2S clusters: 2.6 A. A series of similar complexes (μ-edt)-, (μ-o-xyldt)-, and (μ-SEt)₂[Fe(CO)₂(PMe₃)]₂ (edt = SCH₂-CH₂S; o-xyldt = SCH₂C₆H₄CH₂S), (μ-pdt)[Fe(CO)₂(PMe₂Ph)]₂, and their protonated derivatives likewise show uniformity in the Fe-Fe bond lengths of the neutral complexes and Fe···Fe distances in the cationic bridging hydrides. The positions of the PMe₃ and PMe₂Ph ligands are dictated by the orientation of the S-C bonds in the (μ-SRS) or (μ-SR)₂ bridges and the subsequent steric hindrance of R. The Fe^II(μ-H)Fe^II complexes were compared for their ability to facilitate H/D exchange reactions, as have been used as assays of H₂ase activity. In a reaction that is promoted by light but inhibited by CO, the {(μ-H)(μ-pdt)[Fe(CO)₂(PMe₃)]₂} ⁺ complex shows H/D exchange activity with D₂, producing {(μ-D)(μ-pdt)[Fe(CO)₂(PMe₃)]₂} ⁺ in CH₂Cl₂ and in acetone, but not in CH₃CN. In the presence of light, H/D scrambling between D₂O and H₂ is also promoted by the Fe^II(μ-H)Fe^II catalyst. The requirement of an open site suggests that the key step in the reactions involves D₂ or H₂ binding to Fe^II followed by deprotonation by the internal hydride base, or by external water. As indicated by similar catalytic efficiencies of members of the series, the nature of the bridging thiolates has little influence on the reactions. Comparison to [Fe]H₂ase enzyme active site redox levels suggests that at least one Fe^II must be available for H₂ uptake while a reduced or an electron-rich Fe^IFe^I metal-metal bonded redox level is required for proton uptake.

Full text of DOI:10.1021/ic020237r

Inorg. Chem. 2002, 41, 3917−3928
Catalysis of H /D Scrambling and Other H/D Exchange Processes by
2
2
[Fe]-Hydrogenase Model Complexes
Xuan Zhao, Irene P. Georgakaki, Matthew L. Miller, Rosario Mejia-Rodriguez, Chao-Yi Chiang, and
Marcetta Y. Darensbourg*
Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77843
Received March 28, 2002
Protonation of the [Fe]-hydrogenase model complex (µ-pdt)[Fe(CO)2(PMe3)]2 (pdt ) SCH CH CH S) produces a
2
2
2
species with a high field 1H NMR resonance, isolated as the stable {(µ-H)(µ-pdt)[Fe(CO)2(PMe3)]2}+[PF ]- salt.
6
Structural characterization found little difference in the 2Fe2S butterfly cores, with Fe‚‚‚Fe distances of 2.555(2)
and 2.578(1) Å for the Fe−Fe bonded neutral species and the bridging hydride species, respectively (Zhao, X.;
Georgakaki, I. P.; Miller, M. L.; Yarbrough, J. C.; Darensbourg, M. Y. J. Am. Chem. Soc. 2001, 123, 9710). Both
are similar to the average Fe‚‚‚Fe distance found in structures of three Fe-only hydrogenase active site 2Fe2S
clusters: 2.6 Å. A series of similar complexes (µ-edt)-, (µ-o-xyldt)-, and (µ-SEt)2[Fe(CO)2(PMe3)]2 (edt ) SCH -
2
CH S; o-xyldt ) SCH C H CH S), (µ-pdt)[Fe(CO)2(PMe2Ph)]2, and their protonated derivatives likewise show
2
2
6
4
2
uniformity in the Fe−Fe bond lengths of the neutral complexes and Fe‚‚‚Fe distances in the cationic bridging
hydrides. The positions of the PMe3 and PMe2Ph ligands are dictated by the orientation of the S−C bonds in the
II
II
(µ-SRS) or (µ-SR)2 bridges and the subsequent steric hindrance of R. The Fe (µ-H)Fe complexes were compared
for their ability to facilitate H/D exchange reactions, as have been used as assays of H ase activity. In a reaction
2
that is promoted by light but inhibited by CO, the {(µ-H)(µ-pdt)[Fe(CO)2(PMe3)]2}+ complex shows H/D exchange
activity with D , producing {(µ-D)(µ-pdt)[Fe(CO)2(PMe3)]2}+ in CH Cl2 and in acetone, but not in CH CN. In the
2
2
3
II
II
presence of light, H/D scrambling between D O and H is also promoted by the Fe (µ-H)Fe catalyst. The requirement
2
2
II
of an open site suggests that the key step in the reactions involves D or H binding to Fe followed by deprotonation
2
2
by the internal hydride base, or by external water. As indicated by similar catalytic efficiencies of members of the
series, the nature of the bridging thiolates has little influence on the reactions. Comparison to [Fe]H ase enzyme
2
II
active site redox levels suggests that at least one Fe must be available for H uptake while a reduced or an
2
I
I
electron-rich FeFe metal−metal bonded redox level is required for proton uptake.
Introduction
The simplicity of the binuclear active site of [Fe]-H2ase,
Figure 1a,1-3 the limited involvement of protein residues in
its first coordination sphere construction, and the distinct
possibility that the site emerged billions of years ago and
evolved in a preoxidizing terrestrial environment has inspired
synthetic chemists to pursue organoiron model complexes
as potential catalysts of H2 production or uptake. Disulfidodi-
ironhexacarbonyl, (µ-S2)Fe2(CO)6, a compound that could
Figure 1. (a) Two-iron unit of the H-cluster, active site of Fe-
2- 9
hydrogenase.1-3 (b) Structure of (µ-SCH2CH2CH2S)Fe2(CO)4(CN)2
.
reasonably represent the mobilization of iron sulfide mineral
by carbon monoxide in the genesis of a primordial catalyst,4
serves as synthetic entry into low valent dinuclear iron
complexes. We and others have shown that transformation
of it into a dithiolate bridged FeIFeI complex is easy to
accomplish,5-8 as is the subsequent displacement of two CO
ligands by better donor ligands such as cyanide,6-10 Figure
* To whom correspondence should be addressed. E-mail: marcetta@
mail.chem.tamu.edu.
(1) Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C. Science
1998, 282, 1853.
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J. C. Structure 1999, 7, 13.
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Hatchikian, E. C.; Fontecilla-Camps, J. C. J. Am. Chem. Soc. 2001,
123, 1596.
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Sharma, A.; Yoder, H. S., Jr. Science 2000, 289, 1337.
10.1021/ic020237r CCC: $22.00 © 2002 American Chemical Society
Published on Web 07/02/2002
Inorganic Chemistry, Vol. 41, No. 15, 2002 3917

Zhao et al.
1b,9 or by phosphine.11 Not only do the dicyano derivatives
provide a formulation highly similar to that of the active
site but also their central [(µ-SRS)Fe2] cores are structurally
identical. Nevertheless, the enzyme has stabilized the active
site structure in such a conformation that is more similar to
the transition state of FeL3 unit rotation rather than to the
ground-state structure of the model complexes.12 The metal-
metal bonds of the cyanide- or phosphine-substituted bi-
nuclear complexes are sufficiently electron-rich to serve as
bases toward protons, leading to binuclear oxidative addition
and the formation of bridging hydride complexes, [FeII(µ-
H)FeII]+.13,14 The significance of this chemical access to the
FeII oxidation state makes it possible for H2 to bind,
consistent with known η2-H2 in d6 FeII complexes.15
Chart 1
activation of dihydrogen or dideuterium. It should be noted
that, as of now, definitive proof of a metal hydride in biology
is missing; however, such functional model studies as these
provide a “smoking gun” for their presence.
The oxidation states of iron in the binuclear active site of
[Fe]-H2ase have not been unambiguously established. The
ν(CO) and ν(CN) stretching frequencies in FT-IR spectra
of various redox levels of the enzyme are compatible with a
range of oxidation states including FeIIFeII, FeIIFeI, and FeI-
FeI.3,16 Such redox states are not inconsistent with Mo¨ssbauer
spectral results.17 Hence, the organometallic [FeIFeI] and [FeII-
(µ-H)FeII]+ complexes have been explored as functional
models for H2 binding, and activation.18 The latter model
studies have used H/D exchange reactivity, similarly to
assays used for enzyme activity, according to eqs 1-4.19-23
In addition to H/D scrambling in H2/D2 mixtures, and HD
production in the H2/D2O mixtures, the incorporation of
deuterium into the bridging hydride complexes, represented
by D[M] in eqs 1 and 2, is also indicative of the heterolytic
H[M] + D2 f D[M] + HD
(1)
(2)
H[M] + D2O f D[M] + HOD
H2 + D2
9
cat8 HD
(3)
(4)
H2 + D2O
9
cat8 HOD + HD
The presence of substituent ligands with donor ability
better than CO is crucial to the binuclear oxidative addition
of H+ to yield the [FeII(µ-H)FeII]+ complexes from (µ-SR)2-
[Fe (CO)2L]2. While PMe3 is not as good a donor as is CN-
toward FeI, it is sufficiently stabilizing of FeII to permit the
isolation of {(µ-H)(µ-pdt)[Fe(CO)2(PMe3)]2}+, complex 3-H+
(pdt ) SCH2CH2CH2S), without the complications of cyanide
nitrogen protonation competing with protonation of the iron-
iron bond.24 Complex 3-H+ was found to undergo H/D
exchange in the presence of D2 under photolysis to produce
3-D+, as well as to catalyze the formation of HD in H2/D2
mixtures.18 These preliminary results led to the broader study
described later of a series of (µ-SRS)[Fe(CO)2(PR3)]2 and
their conjugate acids, for factors which might affect their
ability to serve as H/D exchange assay models of hydroge-
nase active sites.
(5) Seyferth, D.; Womack, G. B.; Gallagher, M. K.; Cowie, M.; Hames,
B. W.; Fackler, J. P., Jr.; Mazany, A. M. Organometallics 1987, 6,
283.
(6) Lawrence, J. D.; Li, H.; Rauchfuss, T. B.; Be´nard, M.; Rohmer, M.-
M. Angew. Chem., Int. Ed. 2001, 40, 1768.
(7) Lyon, E. J.; Georgakaki, I. P.; Reibenspies, J. H.; Darensbourg, M.
Y. Angew. Chem., Int. Ed. 1999, 38, 3178.
(8) Le Cloirec, A.; Best, S. P.; Borg, S.; Davies, S. C.; Evans, D. J.;
Hughes, D. L.; Pickett, C. J. Chem. Commun. 1999, 2285.
(9) Schmidt, M.; Contakes, S. M.; Rauchfuss, T. B. J. Am. Chem. Soc.
1999, 121, 9736.
Binuclear complexes of formulation (µ-SR)2[Fe(CO)2L]2,
and the isomeric possibilities resulting from SR- and L
orientations, are well-known. In 1973, Ellgen and Gerlach
interpreted kinetic parameters of CO/PR′3 substitution ac-
cording to the steric requirements of both SR and PR′3.25
Even in the absence of X-ray crystal structures, their insight
helped formulate the possibilities for structural isomers which
derive from (a) positioning of the carbons R to the bridging
sulfurs, in syn, syn′, and anti conformations,26 as well as (b)
placing the phosphines in positions trans or cis to the metal-
metal bond or bridging hydride, see Chart 1. Because the
iron atoms of the neutral species are in pseudo-square-
pyramidal coordination geometries, the positions of the
(10) Razavet, M.; Davies, S. C.; Hughes, D. L.; Pickett, C. J. Chem.
Commun. 2001, 847.
(11) De Beer, J. A.; Haines, R. J. J. Organomet. Chem. 1972, 36, 297.
(12) Lyon, E. J.; Georgakaki, I. P.; Reibenspies, J. H.; Darensbourg, M.
Y. J. Am. Chem. Soc. 2001, 123, 3268.
(13) Fauvel, K.; Mathieu, R.; Poilblanc, R. Inorg. Chem. 1976, 15, 976.
(14) Savariault, J.-M.; Bonnet, J.-J.; Mathieu, R.; Galy, J. C. R. Acad. Sci.
1977, 284, C663.
(15) (a) Kubas, G. J. Acc. Chem. Res. 1988, 21, 120. (b) Kubas, G. J. Metal
Dihydrogen and σ-Bond Complexes; Kluwer Academic/Plenum
Press: New York, 2001.
(16) (a) De Lacey, A. L.; Stadler, C.; Cavazza, C.; Hatchikian, E. C.;
Fernandez, V. M. J. Am. Chem. Soc. 2000, 122, 11232. (b) Chen, Z.;
Lemon, B. J.; Huang, S.; Swartz, D. J.; Peters, J. W.; Bagley, K. A.
Biochemistry 2002, 41, 2036.
(17) (a) Popescu, C. V.; Mu¨nck, E. J. Am. Chem. Soc. 1999, 121, 7877.
(b) Adams, M. W. Biochim. Biophys. Acta 1990, 1020, 115. (c) Bennet,
B.; Lemon, B. J.; Peters, J. W. Biochemistry 2000, 39, 7455.
(18) Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Yarbrough, J. C.;
Darensbourg, M. Y. J. Am. Chem. Soc. 2001, 123, 9710.
(19) Sellmann, D.; Geipel, F.; Moll, M. Angew. Chem., Int. Ed. 2000, 39,
561.
(24) Rocchini, E.; Rigo P.; Mezzetti, A.; Stephan, T.; Morris R. H.; Lough,
A. J.; Forde, C. E.; Fong, T. P.; Drouin S. D. J. Chem. Soc., Dalton
Trans. 2000, 3591.
(20) Sellmann, D.; Fu¨rsattel, A. Angew. Chem., Int. Ed. 1999, 38, 2023.
(21) Collman, J. P.; Wagenknecht, P. S.; Hembre, R. T.; Lewis, N. S. J.
Am. Chem. Soc. 1990, 112, 1294.
(22) Albeniz, A. C.; Heinekey, D. M.; Crabtree, R. H. Inorg. Chem. 1991,
30, 3632.
(25) Ellgen, P. C.; Gerlach, J. N. Inorg. Chem. 1973, 12, 2526.
(26) (a) Maresca, L.; Greggio, F.; Sbrignadello, G.; Bor, G. Inorg. Chim.
Acta 1971, 5, 667. (b) De Beer, J. A.; Haines, R. J.; Greatrex, R.;
Greenwood, N. N. J. Organomet. Chem. 1971, 27, C33. (c) Le Borgne,
G.; Grandjean, D.; Mathieu, R.; Poilblanc, R. J. Organomet. Chem.
1977, 131, 429.
(23) Chinn, M. S.; Heinekey, D. M. J. Am. Chem. Soc. 1987, 109, 5865.
3918 Inorganic Chemistry, Vol. 41, No. 15, 2002

H2/D2 Scrambling by [Fe]-Hydrogenase Models
Table 1. Infrared Data of the Fe2S2 Derivatives in the Carbonyl Stretching Frequency Region (CH3CN Solutions unless Otherwise Indicated)
compounda
ν(CO), cm-1
[(µ-SEt)Fe(CO)2(PMe3)]2 (1)
1977(s), 1931(m), 1908(s)
1982(s), 1944(s), 1908(s), 1896(m, br)
1979(m), 1942(s), 1898(s)
1983(m), 1948(s), 1903(s)
1982(ms), 1946(s), 1910(ms)
1985(m), 1960(s), 1910(s), 1903(sh)b
1981(w), 1960(s), 1912(s), 1904(sh)b
2046(s), 2025(ms), 1990(s)
2034(s), 1994(s)
(µ-edt)[Fe(CO)2(PMe3)]2 (2)
(µ-pdt)[Fe(CO)2(PMe3)]2 (3)
(µ-o-xyldt)[Fe(CO)2(PMe3)]2 (4)
(µ-pdt)[Fe(CO)2(PMe2Ph)]2 (5)
syn-(µ-SCH2CH2PPh2)2Fe2(CO)4 (6)
anti-(µ-SCH2CH2PPh2)2Fe2(CO)4 (7)
{(µ-H)[(µ-SEt)Fe(CO)2(PMe3)]2}+[PF6]- (1-H+)
{(µ-H)(µ-edt)[Fe(CO)2(PMe3)]2}+[PF6]- (2-H+)
{(µ-H)(µ-pdt)[Fe(CO)2(PMe3)]2}+[PF6]- (3-H+)
{(µ-H)(µ-o-xyldt)[Fe(CO)2(PMe3)]2}+[PF6]- (4-H+)
{(µ-H)(µ-pdt)[Fe(CO)2(PMe2Ph)]2}+[PF6]- (5-H+)
[(µ-H)(syn-(µ-SCH2CH2PPh2)2Fe2(CO)4)]+[CF3SO3]- (6-H+)
[(µ-H)(anti-(µ-SCH2CH2PPh2)2Fe2(CO)4)]+[CF3SO3]- (7-H+)
2029(s), 1989(s)
2034(s), 1992(m)
2034(s), 1992(s)
2049(sh), 2039(s), 1995(m, br)b
2053(m), 2033(s), 2000(m, br)b
a edt ) SCH2CH2S; o-xyldt ) SCH2C6H4CH2S. b THF solution.
{(µ-H)(µ-SRS)[Fe(CO)2(PMe3)]2}+[PF6]-. According to the
literature,13 a typical preparation is presented here. To a methanol
solution of (µ-SRS)[Fe(CO)2(PMe3)]2 was added excess concen-
trated HCl forming a red-orange solution. A red or yellow-orange
solid precipitated upon addition of a saturated aqueous solution of
NH4PF6 to the solution. The solid was filtered and washed first
with water and then ether to remove any starting material; yields
were on the order of 50-60%. Crystals suitable for X-ray studies
were obtained by slow diffusion of hexanes into CH2Cl2 or MeOH
solutions of the hydrides.
substituent P-donor ligands are designated as apical and
basal, resulting in the four possible conformations shown.
In this manuscript, the ap/ba designations are retained for
the protonated complexes, n-H+, even though the iron atoms
in the resultant bridging hydrides are in octahedral coordina-
tion environments.
Experimental Section
Materials and Techniques. All manipulations were performed
using standard Schlenk-line and syringe/rubber septa techniques
under N2 or in an argon atmosphere glovebox. Solvents were of
reagent grade and purified as follows: Dichloromethane was
distilled over P2O5 under N2. Acetonitrile was distilled once from
CaH2 and once from P2O5, and then freshly distilled from CaH2
immediately before use. Diethyl ether, toluene, THF, and hexane
were distilled from sodium/benzophenone under N2. The following
materials were of reagent grade and used as received: Fe3(CO)12,
1,3-propanedithiol, 1,2-ethanedithiol, ethanethiol, 1,2-benzene-
dimethanethiol, PMe3, PMe2Ph, concentrated HCl and NH4PF6
(Aldrich Chemical Co.), and deuterated solvents and D2 (Cambridge
Isotope Laboratories).
H/D Exchange Reactions. The H/D exchange reactions of all
hydrides were carried out as ∼0.10 M solutions in medium-pressure
-
NMR tubes (Wilmad, 528-PV-7) as PF6 or -SO3CF3 salts. The
hydrides were dissolved in CH2Cl2 and then transferred to the
degassed, N2-filled NMR tubes. Before pressurizing with D2 or H2,
the tubes were gently degassed under vacuum. The tubes were then
irradiated either by exposure to sunlight by placing on the window
sill, to ambient laboratory light, or to irradiation from a sunlamp
(65 W, 120 V, PLANT GRON SHOW 65BR30/PL). The growth
+
+
2
of (µ-D)Fe2 and decrease of (µ-H)Fe2 were monitored by H
1
NMR or H NMR, respectively.
Competition Reactions of H/D Exchange. Equal moles of dif-
ferent hydrides (50-60 mg, ca. 0.1 mmol) were dissolved together
in ∼0.8 mL CH2Cl2 and transferred to an NMR tube. Following
Infrared spectra were recorded on an IBM IR/32 using a 0.1
mm NaCl cell or on a ReactIR 1000 system from Applied Systems
Inc., equipped with an MCT detector and 30 bounce SiCOMP in
situ probe. 1H, 13C, and 31P NMR (85% H3PO4 was used as external
reference) spectra were recorded on a Unity+ 300 MHz supercon-
ducting NMR instrument operating at 299.9, 75.43, and 121.43
+
the procedure described previously, the growth of (µ-D)Fe2 for
each hydride was monitored by 2H NMR. Integration of the high-
field signal intensities established relative H/D exchange rates.
Catalytic Formation of HD from H2 and D2. A 10 mg (0.016
mmol) portion of {(µ-H)(µ-pdt)[Fe(CO)2(PMe3)]2}+[PF6]- was
dissolved in 1.0 g of CD2Cl2 and transferred to the NMR tube.
This tube was first pressurized with 6 bar H2 and then with D2 to
a total of 12 bar (ca. 0.5 mmol H2 and D2). The tube was placed in
2
MHz, respectively. H NMR spectra were recorded on a Unity
Inova-400 NMR instrument with a 5 mm autoswitchable probe
operating at 61.35 MHz.
Preparations. Full descriptions of syntheses and characteriza-
tions are presented in Supporting Information.
1
the sunlight, and H NMR spectra were taken at time intervals to
(µ-SRS)[Fe(CO)2(PMe3)]2. The dinuclear iron hexacarbonyl
compounds, (µ-SRS)Fe2(CO)6, were synthesized according to
literature methods.5 The PMe3 derivatives of these compounds were
prepared by refluxing excess PMe3 with the corresponding (µ-SRS)-
Fe2(CO)6 in hexane until the starting compound was consumed as
indicated by IR. The solution was then filtered through Celite, and
analytically pure compounds were obtained after removal of solvent
and excess PMe3 by vacuum. The (µ-pdt)[Fe(CO)2(PMe2Ph)]2 was
prepared in similar fashion, with yields on the order of 60-90%.
Crystals suitable for X-ray analysis were obtained by evaporative
concentration of hexane solutions. The ν(CO) IR positions of the
series of neutral complexes and the hydrides in THF or CH3CN
are given in Table 1. Positions in other solvents are listed in
Supporting Information.
follow the growth of HD from H2 and D2.
H/D Exchange between D2O and 3-H+. To a CH2Cl2 solution
of 3-H+ (60 mg, 0.094 mmol) in an NMR tube was added 1, 2, 3,
and 4 µL of D2O; 2H NMR spectra of the sample were taken after
each addition of D2O. The tube was then left in the dark overnight
and the spectrum taken again before and after exposure to sunlight.
H/D Exchange between H2O and D2. To an NMR tube
containing a CH2Cl2 solution of 3-H+ (60 mg, 0.094 mmol) was
added 2 µL (0.11 mmol) of H2O. The NMR tube was then
2
pressurized with 10 bar D2 and kept in the dark for 12 h. The H
NMR spectra of this sample were recorded before and after
exposure to sunlight for 2 h and for 10 h.
Comparisons of Stability of Hydrides in CH2Cl2 Solutions.
Hydride salts were weighed out (ca. 0.1 mmol) in an Ar-filled
Inorganic Chemistry, Vol. 41, No. 15, 2002 3919

Zhao et al.
Chart 2
glovebox, and each was dissolved in 2 mL of CH2Cl2. The hydride
solutions were transferred to a medium-pressure NMR tube. A
reference ν(CO) IR spectrum of each sample was taken prior to
pressurizing the NMR tubes to 7.5 bar H2 and placing side by side
in the sunlight. The pressure was released, and samples for IR
measurements were withdrawn after 1, 2, 3, and 5 days. Repres-
surization to 7.5 bar was required before continuation of the
irradiation experiment.
In Situ Monitor of Stability of {(µ-H)(µ-pdt)[Fe(CO)2-
(PMe3)]2}+[PF6]- in CH3CN and in CH2Cl2. A 0.20 g sample of
{(µ-H)(µ-pdt)[Fe(CO)2(PMe3)]2}+[PF6]- was dissolved in 30 mL
of CH3CN or CH2Cl2 and transferred to the reaction vessel of the
ReactIR. IR spectra were taken every 1 min over the course of 5
h (CH3CN) or every 2 min for 10 h (CH2Cl2) while the solution
was being magnetically stirred and irradiated by a sunlamp.
Computational Details. The structures of examined complexes
were optimized using density functional theory (DFT)27 with the
Becke3 parameter hybrid exchange functional28 and LYP correlation
functional29 (B3LYP). All calculations were performed using
Gaussian 98 programs,30 using a Dunning basis set of double-ú
quality for C, H, and O atoms.31 A modified version of the Hay
and Wadt double-ú basis set with effective core potentials (ECP)32
was used for Fe atoms taken from the work of Couty and Hall.33
A polarization function optimized for the double-ú basis set and
ECPs of Hay and Wadt32 were used for S and the P atoms taken
from the work of Ho¨llwarth et al.34
X-ray Structure Determination. The X-ray data were collected
on a Bruker Smart 1000 CCD diffractometer or a Bruker Apex
CCD diffractometer and covered a hemisphere of reciprocal space
by a combination of three sets of exposures. The space groups were
determined on the basis of systematic absences and intensity
statistics. The structures were solved by direct methods. Anisotropic
displacement parameters were determined for all non-hydrogen
atoms. Hydrogen atoms were placed at idealized positions and
refined with fixed isotropic displacement parameters. The following
is a list of programs used: for data collection and cell refinement,
SMART;35 data reduction, SAINTPLUS;36 structure solution,
(27) Parr, R. G.; Yang, W. Density -functional theory of atoms and
molecules; Oxford University Press: Oxford, 1989.
(28) (a) Beck, A. D. Phys. ReV. A 1988, 38, 3098. (b) Beck, A. D. J. Chem.
Phys. 1993, 98, 1372. (c) Beck, A. D. J. Chem. Phys. 1993, 98, 5648.
(29) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (b)
Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989,
157, 200.
(30) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone,
V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.;
Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R.
L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,
A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.;
Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle,
E. S.; Pople, J. A. Gaussian 98, revision A.7; Gaussian, Inc.:
Pittsburgh, PA, 1998.
SHELXS-86 (Sheldrick);37 structure refinement, SHELXL-97 (Sheld-
rick);38 and molecular graphics and preparation of material for
publication, SHELXTL-Plus, version 5.1 or later (Bruker).39
Results and Discussion
Characterization of the Series: The Solid State. Shown
in Chart 2 are the compounds used in this study along with
designations adopted throughout. The stick drawings repre-
sent the conformers found in the solid-state structures, as
determined by X-ray crystallography for all members of the
series excepting compounds 5, 6-H+, and 4-H+. Crystals of
(31) (a) Dunning, T. H., Jr. J. Chem. Phys. 1970, 53, 2823. (b) Dunning,
T. H., Jr.; Hay, P. J. Methods of Electronic Structure Theory; Plenum
Press: New York, 1977, Vol. 3.
(32) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Hay, P.
J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 284. (c) Hay, P. J.; Wadt,
W. R. J. Chem. Phys 1985, 82, 299.
(33) Couty, M.; Hall, M. B. J. Comput. Chem. 1996, 17, 1359.
(34) Ho¨llwarth, A.; Bo¨hme, M.; Dapprich, S.; Ehlers, A. W.; Gobbi, A.;
Jonas, V.; Ko¨hler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G.
Chem. Phys. Lett. 1993, 208, 237.
(36) SAINT-Plus, version 6.02; Bruker: Madison, WI, 1999.
(37) Sheldrick, G. SHELXS-86: Program for Crystal Structure Solution;
Institu¨t fu¨r Anorganische Chemie der Universita¨t: Gottingen, Ger-
many, 1986.
(38) Sheldrick, G. SHELXL-97: Program for Crystal Structure Refinement;
Institu¨t fu¨r Anorganische Chemie der Universita¨t: Gottingen, Ger-
many, 1997.
(35) SMART 1000 CCD; Bruker Analytical X-ray Systems: Madison, WI,
1999.
(39) SHELXTL, version 5.1 or later; Bruker: Madison, WI, 1998.
3920 Inorganic Chemistry, Vol. 41, No. 15, 2002

H2/D2 Scrambling by [Fe]-Hydrogenase Models
Table 2. Crystallographic Dataa for Complexes 1, 1-H+, 2, 2-H+, 4, and 5-H+
1
1-H+
2
2-H+
4
5-H+
empirical formula
fw
cryst syst
space group
unit cell
a (Å)
C14H28Fe2O4P2S2
498.12
triclinic
C14H29F6Fe2O4P3S2
644.1
monoclinic
P21/c
C10H22Fe2O4P2S2
468.06
monoclinic
P21/c
C10H23F6Fe2O4P3S2
614.03
monoclinic
C2/c
C18H26Fe2O4P2S2
544.15
monoclinic
P21/n
C23H29F6Fe2O4P3S2
752.19
triclinic
P1h
P1h
10.474(2)
14.233(3)
15.932(3)
75.656(4)
89.180(4)
86.004(4)
2295.6(8)
4
10.3731(9)
12.5853(11)
20.2228(17)
14.560(2)
10.3348(15)
13.2183(19)
25.837(3)
17.3690(18)
22.502(2)
10.6231(11)
18.8448(19)
12.3312(13)
9.9108(12)
10.0961(12)
15.8252(18)
98.283(2)
107.219(2)
96.810(2)
1474.8(3)
2
b (Å)
c (Å)
R (deg)
â (deg)
98.0290(10)
99.743(3)
116.039(2)
111.464(2)
γ (deg)
V (Å3)
2614.2(4)
4
1.637
0.0278
0.071
1960.3(5)
4
1.586
0.0529
0.1391
9073.2(16)
16
1.798
0.0751
0.1743
2297.4(4)
4
1.573
0.0366
0.0951
Z
d(calcd), g/cm3
R1b [I > 2σ(I)]
wR2c
1.441
0.0453
0.1159
1.694
0.0679
0.1716
a Obtained using graphite-monochromatized Mo KR radiation (λ ) 0.71073 Å) at 110(2) K for all structures. b R1 ) ∑||Fo| - |Fc||/∑Fo. c wR2 )
[∑[w(Fo - Fc )2]/∑w(Fo )2]]1/2
.
2
2
2
Table 3. Selected Metric Data for Binuclear Iron Phosphino Carbonyls Bridged by Thiolates
1
1-H+
2
2-H+
3
3-H+
4
5-H+
6
7
7-H+
Fe‚‚‚Fe
2.5097(7) 2.5708(4) 2.5159(6) 2.5742(13) 2.555(2) 2.5784(8) 2.5825(7) 2.5859(7) 2.603(2) 2.5621(7) 2.580(1)
Fe-H
N/A
1.66(2)
N/A
1.69(6)
N/A
N/A
1.710(14) N/A
N/A N/A
1.58(6)
N/A
N/A
N/A
N/A
1.664(6)
Fe-Pap
Fe-Pba
2.219(1)a 2.2366(6)a 2.2075(8) N/A
2.2153(8) 2.236(1)
N/A
N/A
N/A
N/A
2.2212(8) 2.2392(16) 2.234(3) 2.2523(12) 2.2207(10) 2.2543(11) 2.209(2) 2.2142(8) 2.223(1)
Fe-CCO,ap
Fe-CCO,ba
1.769(3) 1.774(6)
1.772(9) 1.779(4)
1.742(10) 1.778(4)
1.771(4)
1.760(4)
1.776(4)
1.800(4)
1.782(8) 1.778(2) 1.789(5)
1.753(8) 1.762(3) 1.793(5)
1.7552(3)d 1.778(3)d 1.759(3)b 1.770(6)
c
Fe-Sµ-SR
2.280(1)
0.315
2.798
2.2793(7) 2.2521(8) 2.2562(16) 2.254(2) 2.2717(11) 2.252(1)
2.2678(11) 2.265(2) 2.2581(8) 2.266(1)
Fe dspe
0.180
0.365
2.897
101.7
0.197
2.924
104.2
0.376
3.026
109.2
0.231
3.064
109.9
0.360
0.209
0.303
2.826
99.3
0.329
2.945
104.7
0.180
2.969
105.6
S‚‚‚S
2.808
3.107
3.064
dihedralf (deg)
Fe-S-Fe (deg)
S-Fe-S (deg)
94.6
96.5
114.7
110.6
66.78(3)
75.70(3)
68.66(2)
76.04(2)
67.91(2) 69.57(5)
80.06(3) 80.78(6)
69.06(8) 69.15(4)
84.34(11) 84.77(4)
69.98(3)
87.22(3)
69.52(3)
85.00(4)
70.15(7) 69.12(3) 69.39(8)
77.18(8) 86.54(3) 81.84(10)
Lap-Fe-Lba (deg) 98.99(11) 93.86(7)
Lap-Fe-S (deg) 99.45(3) 96.82(2)
97.2(1)
94.8(4)
96.8(4)
93.91(16) 96.93(14) 93.06(15) 98.15(3) 100.32(9) 96.42(4)
103.70(6) 95.9(2)
104.0(3) 98.70(13) 102.94(12) 98.36(13) 99.22(3) 114.93(7) 93.34(20)
a Average over two Fe-Pap bonds. b Average over three Fe-CCO,ba bonds. c Average over four Fe-Sµ-SR bonds. d Average over four Fe-CCO,ba bonds.
e Fe dsp ) Displacement of Fe from best plane of S2L(ba)2 toward Lap. f Defined by the intersection of the two SFe2 planes.
the latter hydride were twinned, and while metric data are
not reliable, the positions of the phosphine ligands are certain.
Furthermore, analyses of solution NMR data, vide infra,
corroborate the structural assignments of all as given in Chart
2. Crystallographic data are given in Table 2, listings of
selected metric data are given in Table 3, and full structure
reports are available in Supporting Information. The molec-
ular structures of complexes 1 and 1-H+, 2 and 2-H+, 4,
and 5-H+ are shown as ball-and-stick representations in
Figures 2-5, respectively. Details of the structures of 3,
3-H+, 6, 7, and 7-H+ were presented elsewhere.18,40
links and a minimum of steric hindrance at the bridge, finds
the PMe3 ligands in the ap/ba arrangement. Nevertheless,
on protonation, the resulting 2-H+ hydride conforms to the
ba/ba (transoid) orientation adopted by all other n-H+
complexes in this class. The 1 and 1-H+ pair of complexes
shows a marked contrast. In agreement with an earlier study
on SMe bridged complexes,26c the S-C bond is in the syn
conformation which exacts steric hindrance on the basal
coordination positions about iron and directs the PMe3
ligands into apical sites, trans to the FeI-FeI bond or FeII-
(µ-H)FeII units.
The S-C orientations of all compounds containing the S
to S linked bridges are necessarily of the syn′ orientation,
and with one exception, all these compounds have the PMe3
or PMe2Ph ligands in the ba/ba (transoid) conformation and
are cis to the FeI-FeI bond or FeII(µ-H)FeII units. That
exception is complex 2, which, with the simplest of S to S
Each bridging thiolate in complexes 6, 7, and 7-H+ is
connected to the P-donor site via an ethylene unit.40 Complex
6 has a syn conformation at the S-C bonds, which directs
the chelated PPh2 donors in the ba/ba (transoid) positions.
For 7 and 7-H+, the anti arrangement of the S-C bonds en-
sures that one P-donor goes to the apical and one to the basal
positions in the Fe(CO)2P termini. Thus, the arrangement of
P-donors in complex 7 is the same as that in complex 2.
(40) Zhao, X.; Hsiao, Y.-M.; Lai, C.-H.; Reibenspies, J. H.; Darensbourg,
M. Y. Inorg. Chem. 2002, 41, 699.
Inorganic Chemistry, Vol. 41, No. 15, 2002 3921

Zhao et al.
Figure 4. Thermal ellipsoid representations (50% probability) of the
molecular structure of (µ-oxyl)[Fe(CO)2PMe3]2, 4.
Figure 2. Thermal ellipsoid representations (50% probability) of the
molecular structures of (a) [(µ-SEt)Fe(CO)2PMe3]2, 1, and (b) (µ-H)[(µ-
SEt)Fe(CO)2PMe3]2+, 1-H+.
Figure 5. Thermal ellipsoid representations (50% probability) of the
molecular structure of (µ-H)(µ-pdt)[Fe(CO)2PMe2Ph]2+, 5-H+.
The Fe-P and Fe-C bond distances show very slight
increases following protonation while the Fe-S distances
are statistically indistinguishable. Because in the protonated
derivatives the formal oxidation state of iron increases to
+2, the consistency in Fe-P or Fe-S distances is evidence
for the stereochemical integrity of the hydride ligand. That
is, the increase in coordination number about the iron (which
should increase the M-L distance) compensates for the
increased charge on iron (which should decrease the M-L
distance).
While still minor, the largest metric differences within the
series are in the S to S “wing-tip” distances. The SEt deriv-
atives 1 and 1-H+ are ca. 0.1 Å closer together than are the
S to S linked complexes which spread the wing-tips accord-
ing to the organic linker size. Complex 6 which has the S-P
chelated S-CH2 in syn arrangement is similar to complex
1. In each pair of structurally characterized n and n-H+
compounds, there is seen a decrease in the displacement of
Fe from the S2(CO)L planes (L ) CO or PR3) toward the
apical ligand from an average of 0.35 Å in the neutral, Fe-
Fe bonded complexes to ca. 0.20 Å in the bridging hydrides.
Computational Results. The molecular structures of 3
and 3-H+ determined by X-ray crystallography are supported
by DFT computations. As indicated in Figure 6a, the HOMO
of 3 is characterized by a large lobe underneath the Fe2S2
frame representing the electron density of the metal-metal
bond, symmetrically distributed as indicated by Mulliken
charges on both iron atoms of -1.07. The optimized Fe-
Fe distance of 2.548 Å is in good agreement with that from
Figure 3. Thermal ellipsoid representations (50% probability) of the
molecular structures of (a) (µ-edt)[Fe(CO)2PMe3]2, 2, and (b) (µ-H)(µ-edt)-
[Fe(CO)2PMe3]2+, 2-H+.
The Fe‚‚‚Fe distances, Table 3, range from 2.51 to 2.60
Å in the neutral complexes in which there is an Fe-Fe bond;
they show slight increases on the order of 0.02-0.05 Å on
protonation to produce the bridging hydride complexes. The
µ-SEt bridged species 1 and 1-H+ show the greatest differ-
ence in Fe-Fe distance; nevertheless, they are approximately
an order of magnitude less than changes observed on
protonation of nonsupported metal-metal bonds.41 The small
increase in Fe-Fe distance reflects the minor structural
rearrangement resulting from changing an edge-bridged bi-
square-pyramid into a face-bridged bi-octahedron.
(41) Nataro, C.; Angelici, R. J. Inorg. Chem. 1998, 37, 2975.
3922 Inorganic Chemistry, Vol. 41, No. 15, 2002

H2/D2 Scrambling by [Fe]-Hydrogenase Models
Infrared spectra of the protonated, cationic complexes in the
ν(CO) region show an increase in average ν(CO) values of
ca. 70-90 cm-1 over their neutral conjugate bases. Also
notable is the simplification of the ν(CO) IR spectra on going
from the neutral to the protonated or bridging hydride
complexes; the latter typically show only two adsorptions,
indicating electronic isolation or lack of electronic coupling
of the discrete FeII units in the n-H+ complexes. There are
minor, but complicated, ν(CO) wavenumber differences
which depend on the P-donor ligand as well as on the S-R
orientation.26a
NMR Data and Variable Temperature Studies. Listed
in Table 4 are chemical shift values for the bridging hydride
complexes. A notable feature of the JH-P values is the small,
3-5 Hz, P to H coupling constant for the trans position of
hydride and phosphorus seen in complexes 1-H+ and 7-H+.
All other JH-P values are in the range 19-24 Hz and
characterize the cis orientation of the hydride and the
phosphines. The JP-D values are about 1/7 of the JP-H values
as predicted from the ratio of the magnetogyric ratios (γ) of
H (26.75 rad T-1 s-1) and D (4.106 rad T-1 s-1).
Variable temperature (VT) NMR spectroscopy revealed
the intramolecular site exchanges that are associated with
(µ-pdt)[Fe(CO)2(PMe3)]2 and {(µ-H)(µ-pdt)[Fe(CO)2(PMe3)]2}+.
1
Figure 6. HOMOs of complexes 3 and 3-H+: (a) HOMO (orbital 94) of
complex 3; (b) HOMO (orbital 94) of complex 3-H+; (c) and (d) orbitals
83 and 82 of 3-H+ illustrating primarily hydride-based electron density.
The H NMR spectra showed that the fluxionality of the
propanedithiolate bridge that was observed in (µ-pdt)[Fe-
(CO)3]212,42 holds in the neutral disubstituted PMe3 derivative
as well as in the protonated complex (Figure 7a), with a
similar coalescence temperature, approximately -60 °C.
Variable temperature 13C NMR spectra in the CO region
show that the CO site exchange is localized in the individual
Fe(CO)2(PMe3) units in the neutral compound, again analo-
gous to the CO site exchange of (µ-pdt)[Fe(CO)3]2.12 The
lack of this pyramidal rotor feature in the protonated species
is consistent with higher barriers in octahedral coordination
environments of the FeII centers, Figure 7b, as compared to
the pentacoordinate precursor.
For complex 2, the temperature-dependent 31P NMR
spectra show a single resonance at 22.0 ppm at room
temperature, in agreement with the known fluxionality of
the Fe(CO)2L rotors in such complexes, vida supra.12 On
lowering the temperature, coalescence is observed at ca. -60
°C, and two new signals appear at 28.6 and 17.6 ppm,
consistent with the ap/ba conformer in the solid-state
structure of 2. For complex 3, the single 31P resonance is
due to the equivalence of the P-donor sites according to their
position and to the rapid interconversion of the irondithia-
cyclohexane rings. On achieving the stopped-exchange region
for the latter, two 31P resonances are observed, at 29.6 and
25.7 ppm, resulting from the slightly different chemical
environments imposed by the fixed S to S bridge (Figure
7b). An isomer of complex 3 was noted by low intensity
resonances which coalesced and reappeared as two small
broad peaks as the temperature was lowered.
the crystal structure, 2.555(2) Å. For the protonated complex,
DFT computations find the Fe‚‚‚Fe distance to be 2.648 Å
(experimental ) 2.578(1) Å), and the HOMO (orbital 94,
Figure 6b) has a prominent S-component. The electron
density identified as hydride-based is stabilized in orbitals
83 and 82, Figure 6c and 6d, respectively.
Solution Structure and Fluxional Characteristics. Space
filling models indicate that the syn-ap/ap combination seen
in complexes 1 and 1-H+ minimizes steric interactions
between SEt and PMe3, while such an ap/ap positioning of
PMe3 ligands in 3, 4, and 6 would exact greater steric
repulsions between substituent ligands and the S to S linked
dithiolate. Hence, the observed structures represent sterically
favored forms. The role of electronics in conformer stability,
that is, whether there is an electronic preference for the better
donor ligand to be cis or trans to the Fe-Fe bond density or
the Fe(µ-H)Fe, is not clear. The observation of the ap/ba
arrangement of the P-donor ligands of complex 2 suggests
the thermodynamic preference is minimal and perhaps within
the dictates of crystal packing forces. Indeed, variable
temperature NMR data confirm ligand site exchange and
rotation of the Fe(CO)2PMe3 units, vide infra.
Infrared Data. While oxidative addition of a proton to
the Fe-Fe bond barely perturbs the structures of the n and
n-H+ series, the shift in metal-metal bond density to the
hydride ligand and the development of positive charge on
iron is most clearly indicated by the vibrational spectra of
the CO reporter ligands. The solution infrared spectra of the
neutral complexes in the ν(CO) region show minor differ-
ences that are consistent with the symmetry of the complexes
found in the solid-state X-ray crystal structures, Table 1.
H/D Exchange Studies: H/D Exchange Reaction be-
tween n-H+ and D2. The experimental protocol established
(42) Winter, A.; Zsolnai, L.; Huttner, G. Z. Naturforsch. 1982, 37b, 1430.
Inorganic Chemistry, Vol. 41, No. 15, 2002 3923

Zhao et al.
2
+
-
Table 4. 1H and H NMR Spectral Data for (µ-H)(µ-SR)2[Fe(CO)2P]2 Complexes in CD2Cl2 as PF6 Salts unless Otherwise Noted
complex
µ-Ha (δ, ppm)
JP-H (Hz)
µ-D (δ, ppm)
JP-D (Hz)
(µ-H)(µ-SEt)2[Fe(CO)2PMe3]2+, 1-H+
(µ-H)(µ-edt)[Fe(CO)2PMe3]2+, 2-H+
-15.79
-17.32
-15.2
-15.06
-15.0
-17.5
-16.9
3.6
22.8
22
21.2
21.9
21.4
24/4.6 b
-15.9 (-15.65, -15.7)
br
-17.45
-15.3
-15.17
-15.1
-17.6
-17.0
3.54
3.31
3.07
3.31
3.25
3.56/n.o.b
(µ-H)(µ-pdt)[Fe(CO)2PMe3]2+, 3-H+
(µ-H)(µ-o-xyldt)Fe(CO)2PMe3]2+, 4-H+
(µ-H)(µ-pdt)[Fe(CO)2PMe2Ph]2+, 5-H+
(µ-H)(syn-µ-SCH2CH2PPh2)[Fe(CO)2]2+, 6-H+ c
(µ-H)(anti-µ-SCH2CH2PPh2)[Fe(CO)2]2+, 7-H+ c
a All split as triplet, except for 7-H+. b The larger coupling constant assigned to the cis Ph2PCH2-; the smaller to the trans. c As OTf- salts.
exchange reaction was corroborated by the H-decoupled 31
P
NMR spectrum, Figure 8b, which showed a small triplet at
21.54 ppm resulting from the coupling between P and (µ-
+
D)Fe2 with the same coupling constant (3.32 Hz) as that
2
obtained from the H NMR. Another single peak at 21.46
+
ppm is due to the H-decoupled 31P signal from (µ-H)Fe2 ,
Figure 8b.
The H/D exchange reaction between (µ-H)Fe2+ and D2 is
affected by several factors, including light, solvent, added
ligands, and counterion. The following control experiments
are described for 3-H+ [PF6-]; however, all complexes
behaved similarly.
(1) Light. When a CH2Cl2 solution of 3-H+ under the
presence of D2 (10 bar) was left in the dark at 22 °C for
4-6 days, there was no indication of the formation of 3-D+.
Both natural and artificial light sources promoted the reaction,
and the rate or extent of the reaction over time depended on
the strength of the light source.
Figure 7. Fluxional characteristics of (µ-pdt)[Fe(CO)2(PMe3)]2 (3) and
{(µ-H)(µ-pdt)[Fe(CO)2(PMe3)]2}+ (3-H+).
(2) Solvent. The optimal solvent for H/D exchange studies
in terms of solubility, stability, and activity was CH2Cl2.
Nevertheless, a small amount of solid formed at the bottom
of NMR tubes during the photoassisted H/D exchange
reaction. The solution IR spectra of the photolyzed samples
in the ν(CO) region were identical to those of the original
sample; the residue showed no ν(CO) bands. In acetonitrile,
the decomposition process was obvious and extensive on
exposure to sunlight for 0.5 h, showing color changes from
red-orange to red-brown. There was no H/D exchange
activity in the decolorized sample in acetonitrile, and the 1H
and 2H NMR spectra showed the formation of other hydride
species in the range -6 to -10 ppm. The high field
resonances are attributed to CH3CN/CO substitution products,
{(µ-H)(µ-pdt)[Fe2(CO)4-x(CH3CN)x(PMe3)2]}+[PF6]-, one of
which has been isolated and characterized. The loss of
activity in acetonitrile is attributed to the good ligating ability
of acetonitrile which blocks the coordination of D2 to FeII,
as well as to the rapid decomposition of the hydride in
photolyzed CH3CN solution. Less decomposition is observed
in acetone, and H/D exchange activity is retained.
(3) Added Ligands. In the presence of CO, the H/D
exchange reaction was very slow, indicating that CO is an
inhibitor of this reaction by competing for the same
coordination site as D2. Whereas added PMe3 might also
compete for the photoinduced open site, it can also act as a
base, deprotonate the bridging hydride, and generate the
inactive neutral binuclear compounds, which was the obser-
vation. In the presence of 5 equiv of PMe3, H/D exchange
activity completely ceased within 4 h. Added alkenes also
Figure 8. (a) 2H NMR and (b) 31P NMR spectra of D2-enriched 3-H+, 8
bar D2 in CH2Cl2.
by Sellmann and others was followed.19-23 Typically, CH2-
Cl2 solutions of each hydride, 1-H+ through 7-H+, were
placed in medium-pressure NMR tubes which were pres-
surized with D2 (7-12 bar) and exposed to sunlight. H/D
exchange into the bridging hydride position of n-H+ and D2
occurred in all as indicated by 2H NMR. For example, H/D
exchange between 3-H+ and D2 in CH2Cl2 solution resulted
in the formation of 3-D+, which showed up as a triplet at
2
-15.28 ppm in the high field region of the H NMR with
JP-D ) 3.32 Hz, Figure 8a. After long reaction periods, a
minor resonance (<5%), a doublet centered at 6.29 ppm with
coupling constant of 72.9 Hz, was also observed and assigned
+
to DPMe3 . A complete listing of the bridging hydride and
deuteride chemical shifts is given in Table 4. The H/D
3924 Inorganic Chemistry, Vol. 41, No. 15, 2002

H2/D2 Scrambling by [Fe]-Hydrogenase Models
Figure 10. 1H NMR spectra showing the relative ratio of HD/H2 from a
CD2Cl2 solution containing 3-H+, 6 bar H2, and 6 bar D2: (a) before
exposure to light; (b) 14 days exposure (1:1); (c) 40 days exposure (2:1).
procedure, the 5-H+ complex, containing PMe2Ph ligands
but otherwise structurally similar to 3-H+, showed an H/D
exchange capability comparable to that of the 3-H+ analogue.
While these experiments established the relative order of H/D
exchange, they also indicated inexact reproducibilities. Thus,
while a numerical ranking is not allowed by our experimental
design, the overall relative H/D exchange reactivity is given
here. There is approximately an order of magnitude between
2
Figure 9. High field H NMR spectra of 1:1 mixture of 2-H+ and 3-H+
under 12 bar D2 in CH2Cl2 solution at different sunlight exposure times.
impeded the build up of (µ-D)Fe2+ species by blocking sites
needed for D2 binding.
(4) Counterion Effect. When the better coordinating anion
[SO3CF3]- was used as a counterion for 3-H+, the solution
changed color from red-orange to red-brown as H/D ex-
2
change occurred. The H NMR spectrum showed the high
the slowest and the fastest H/D exchange rates: 1-H+
2-H+ > 3-H+ > 4-H+ > 7-H+.
>
field µ-D resonance at -15.3 ppm as well as another hydride
species as a doublet of doublets centered at -6.83 ppm with
coupling constants of 3.50 and 4.72 Hz. Smaller counterions
such as chloride promoted the deprotonation reactivity and
decreased H/D exchange.
All of the described observations are consistent with the
requirement of an open site for D2 or H2 binding prior to
H/D exchange.
Catalysis of HD Formation from H2 and D2. A medium-
pressure NMR tube containing 10 mg (0.019 mmol) of 3-H+
in CD2Cl2 was pressurized with H2 to 6 bar (0.60 mmol),
followed by D2 to 12 bar (0.60 mmol); the formation of HD
1
was monitored by H NMR. As shown in Figure 10a, the
1H NMR spectrum taken before the sample was exposed to
light only showed the presence of H2 as a single peak at 4.6
ppm. After exposure to sunlight over the course of 2 days,
a small triplet centered at 4.57 ppm with coupling constant
of 42.8 Hz emerged, resulting from the formation of HD.
The intensity of this triplet continued to increase with time,
requiring under natural light conditions two weeks until its
intensity became the same as that of H2, Figure 10b. Full
equilibration of H2/HD/D2 occurred over the course of several
weeks, Figure 10c. This experiment clearly showed the
catalytic ability of the hydride 3-H+, as well as its stability
in CH2Cl2. The upfield region of the sample showed that
the hydride triplet at -15.3 ppm remained. When CH2Cl2
Ranking of H/D Exchange Activity. The following
competition experiments were used to establish the relative
order of H/D exchange reactivity within the series of n-H+
compounds. Mixtures of the hydride salts dissolved in CH2-
Cl2 and placed in NMR tubes were pressurized with D2 and
photolyzed by exposure to sunlight. The growth of µ-D
signals was monitored by 2H NMR spectroscopy. For
example, as shown in Figure 9, a sample composed of a 1:1
mixture of 2-H+ and 3-H+ showed the resonance at -17.44
ppm for 2-D+ to be clearly visible in 1.5 h, with only a minor
signal at -15.3 ppm for 3-D+. In 4.5 h, both had gained
intensity with the relative integrated intensities of 2-D+/3-
D+ ) 4.6. With longer time, the µ-D signals both increased
in intensity and equalized, reaching 1:1 within 5 days.
Similarly, complexes 1-H+ and 2-H+ were compared for
exchange with D2, indicating a slightly greater exchange rate
for the monodentate thiolate, SEt. At 1 h of photolysis time,
the signal intensity of 1-D+) 1.7× that of 2-D+. A 1:1:1
mixture of 2-H+, 3-H+, and 4-H+ showed uptake of D2 and
H/D exchange in relative ratio of 7.5:3:1 after an irradiation
time of 1.5 h. A 1:1 mixture of 4-H+ and 7-H+ showed that
the ratio of 4-D+/7-D+ was 1.68 after exposure to sunlight
for 1.5 h under D2. A 1:1 mixture of 1-H+ and 7-H+ showed
that the ratio of the integrated signals 1-D+/7-D+ was 7:1
after 30 min of exposure to sunlight. Following the same
2
was used as solvent, the H NMR spectrum showed the
catalytic formation of HD from H2 and D2 as a single peak
at 4.67 ppm as well as the D2 peak at 4.63 ppm.
2
H/D Exchange between D2O and 3-H+. The H NMR
-
spectrum of a CH2Cl2 solution of 3-H+ as its PF6 salt,
containing 1-4 µL of D2O, displayed signals at 1.67 ppm
varying little with different amounts of D2O. There was no
formation of 3-D+, even after standing for >12 h in the dark.
However, on exposure to sunlight for 5 h, the triplet at -15.3
2
ppm appeared in the H NMR spectrum, indicating H/D
exchange with the bridging hydride. The intensity of this
resonance was 0.27 relative to the solvent peak at 5.32 ppm,
1
and the position of H-enriched water shifted to 1.77 ppm.
The intensity of the µ-D resonance peak increased slightly
Inorganic Chemistry, Vol. 41, No. 15, 2002 3925

Zhao et al.
after further exposure to sunlight. In comparison with the
D2 and 3-H+ reactivity, the H/D exchange in 3-H+/D2O was
very slow. The latter exchange process was accelerated by
the addition of PPN+Cl-, even in the absence of light.
3-H+ Catalyzed H/D Exchange between H2O and D2.
Following standing in the dark for 12 h, a sample containing
3-H+, H2O, and D2 showed a resonance for dissolved
D-enriched water at 1.63 ppm in the 2H NMR spectrum; its
intensity was ca. 0.18 of the resonance for natural abundance
deuterium in the CH2Cl2 solvent at 5.32 ppm. No signal was
found for 3-D+ in the upfield region. When this sample was
exposed to sunlight for 2 h, the characteristic triplet at -15.3
ppm in the 2H NMR indicated formation of 3-D+; its intensity
was ca. 5.4 relative to the solvent peak. The resonance at
1.63 ppm shifted to 1.75 ppm and became very strong; its
intensity was ca. 25 times that of the solvent peak. The other
product, HD, appeared as a small peak at 4.67 ppm in the
2H NMR spectrum. Exposure to sunlight for 10 h resulted
in no further changes in the dissolved water signal; however,
the intensity of the bridging deuteride at -15.3 ppm
increased to 7.6 times that of the solvent peak.
the initial solution. The precipitate dissolved in acetone but
showed no ν(CO) bands. After exposure to sunlight for 5
days, the IR monitor showed ca. 90% of 5-H+ remained in
solution. Under the same conditions, ca. 84% of 3-H+
remained present in solution. Complex 2-H+ is less stable
compared to 3-H+ and 5-H+; 2-H+ degraded by ca. 20%
after only 3 days with exposure to sunlight. Compound 4-H+,
which has SCH2C6H4CH2S as bridging dithiolate, is the most
unstable and decomposed completely in the same time period.
For compounds 1-H+ and 7-H+, there are isomerization
processes which occur concomitantly with the decomposition
process. Before irradiation, the IR spectrum of 1-H+ showed
three CO bands at 2046(s), 2026(s), and 1991(s) cm-1 in
CH2Cl2 solution. After exposure to sunlight for 5 days, the
band pattern changed and shifted slightly to 2040(ms),
2026(s), and 1987(s) cm-1; a small band at 1867 cm-1 also
appeared. The IR monitor showed ca. 70% of 1-H+ present
in the solution. Under H/D exchange conditions, compound
7-H+ isomerized to 6-H+, the more stable isomer of the PS
ligand derivative, over the course of 5 days. There was ca.
67% of the hydride remaining in solution. The order of the
relative stability of all the hydrides studied is the following:
5-H+ > 3-H+ > 2-H+ > 1-H+ ≈ 7-H+ > 4-H+.
Extraneous Peaks. In our previous report18 and in the
2
samples described here, a H NMR resonance found in the
range 1.5-2.8 ppm for the H/D exchange reaction between
3-H+ and D2, and up to 10% of the 3-D+ resonance, was
not assigned. This resonance varied its position in this range,
differing from experiment to experiment; however, its
intensity could not be correlated with the extent of H/D
exchange into the bridging hydride position of the n-H+.
While the position of undissolved, or globules, of D2O in
CH2Cl2 is at 4.8 ppm, dissolved water in trace amounts is in
the higher field region, ca. 1.70 ppm, and is the most
reasonable candidate for this spurious peak. Thus, trace
amounts of H2O in the NMR tube while preparing the
samples formed HDO or D2O after the H/D exchange
between H2O and D2, catalyzed by 3-H+. So, the most
reasonable candidate for the spurious peak is dissolved water.
If not adventitious, dissolved water, present in the NMR tube,
the “spurious peak” is a species that exchanges readily with
D2O. This was established by spiking samples with D2O.
There was no D incorporation to the methyl group of [DP-
(CH3)3]+, which, if so, would have appeared as a doublet at
1.89 ppm with coupling constant of 2.45 Hz. Control
experiments showed that the position of [DP(CD3)3]+ did
not shift during the course of H/D exchange reaction.
The possibility that the spurious peak could be related to
an S-D resonance remains, although SD derived from free
Conclusions
The H/D exchange processes that characterize H2ase
enzyme activity are observed in the dinuclear organometallic
complexes [FeII(µ-H)FeII]+ of Chart 2, complexes which have
structural features in common with the active site of [Fe]-
H2ase. The parent (µ-SRS)[Fe(CO)3]2 reacts with added
ligands such as CN- or PR3 to yield disubstituted complexes
with complete regioselectivity in the product (µ-SRS)[Fe-
(CO)2L]2. In contrast to their hexacarbonyl precursors, the
Fe-Fe bonds of these complexes are sufficiently basic to
be used in reactions with electrophiles (H+, SR+). By this
chemical reaction, the Fe centers achieve a +2 oxidation
state wherein H2 binding and heterolytic activation is
facilitated. Thus, label scrambling in H2/D2 and H2/D2O
mixtures requires protonation of the inactive FeIFeI com-
plexes, producing [FeII(µ-H)FeII]+ where two metals have
shared the two-electron oxidative addition. It should be noted
that such a chemical oxidation is not required to activate
the binuclear enzyme active site for H2 uptake as the redox
activity of the enzyme permits electrochemical oxidation.
However, the presence of a metal hydride is a mechanistic
requirement for an H/D exchange activity pathway in H2/D2
mixtures.
While the remaining lone pair on sulfur of the bridging
thiolate has been proposed as a repository for protons in
theoretical model studies of hydrogenase activity of [Fe]H2-
ase,43 our results both in theory (the HOMO of the FeIFeI
complexes is the Fe-Fe bond density) and in experiment
(the product of protonation is the [FeII(µ-H)FeII]+ complex)
suggest otherwise.
2
HS(CH2)3SH with H resonance at 1.42 ppm showed little
position shift on addition of D2O.
Stability of Hydrides in CH2Cl2 Solutions. Side by side
stability comparisons of the PF6- salts of the hydrides 1-H+,
2-H+, 3-H+, 4-H+, and 5-H+, and 7-H+ as its [SO3CF3]-
salt, in CH2Cl2 solutions were made in the same NMR tubes
as used for H/D exchange studies. Pressurized with H2 (7.5
bar) at room temperature and exposed to sunlight, these were
sampled for IR spectra in the CO region. In all solutions,
small amounts of an insoluble solid formed at the bottom of
NMR tube, and the solution became darker compared with
Both the H2/D2 and the H2/D2O exchange processes require
photolysis, indicating the necessity for creating an open site
(43) (a) Cao, Z.; Hall, M. B. J. Am. Chem. Soc. 2001, 123, 3734. (b)
Bruschi, M.; Fantucci, P.; De Gioia, L. Inorg. Chem. 2002, 41, 1421.
3926 Inorganic Chemistry, Vol. 41, No. 15, 2002

H2/D2 Scrambling by [Fe]-Hydrogenase Models
Scheme 1
Scheme 2
effects of adventitious water would indeed be to promote
such H/D exchange, numerous repetitions of the reactions
under strenuously dry conditions yield the same results.
Relationships to the [Fe]H2ase Enzyme Active Site. The
minor structural differences between the neutral FeIFeI and
the protonated FeII(µ-H)FeII compounds suggest that the
formation of such FeIIFeII species could be a low activation
barrier step in the catalytic mechanism of H+ uptake by the
enzyme. Although the bridging thiolates have remarkable
steric control of the position of substituent phosphines, there
is no significant difference between the H/D exchange
reactivities of the monodentate bridging thiolates and the
bidentate analogues. From this, we conclude that the three
light atom S to S linker cofactor is not required for at least
some of the reactivity expected for the enzyme active site;
two cysteines could have just as easily accomplished the
bridging dithiolate structural features as indeed they do in
the [NiFe]H2ases.47 Therefore, reasons for the presence of
the pdt or µ-SCH2NHCH2S should lie in biosynthesis, active
site import mechanisms, or the need to facilitate heterolytic
H2 activation by an internal amine base.3
for H2 or D2 binding on the hydride-containing catalyst. As
expressed earlier,18 this may result from CO loss or a hydride
shift; if the former precedes η2-H2 binding, a subsequent
rotation and hydride shift (from bridging to terminal) is
required for an internal heterolytic exchange process, Scheme
1. A stable form of such a species was reported for the
complex (P-N)(η2-H2)Ru(µ-Cl)2(µ-H)Ru(H)(PPh3)2 (P-N
) Fe(η-C5H3(CHMeNMe2)P(i-Pr)2-1,2)(η-C5H5).44 As shown
in Scheme 1, positioning of the η2-H2 in close proximity to
the hydride gives opportunity for a trihydrogen transition
state, formulated as a binuclear binding. The suggestion for
such a species has precedence in the RuRu complex.44
Nevertheless, a mononuclear trihydride or trihydrogen species
located on a single metal is an alternate possibility, also with
precedence.45
Whereas H/D exchange into the µ-H position from D2O
is very slow, the presence of H2 greatly accelerates the
process. This exchange as well as the catalysis of H/D
scrambling in an H2/D2O mixture is also explained on the
basis of η2-H2 binding to FeII and the greatly increased acidity
resulting therefrom.46 Scheme 2 displays a likely reaction
scenario of events that follow, ultimately directing H/D
exchange into the µ-H position.
The protonation of the FeI-FeI bond shifts the ν(CO)
values ca. 80 cm-1 higher than those of the neutral
compounds and places them in the range of the as-isolated
enzyme (1996 cm-1).3 Only at the FeIIFeII redox level do
we see H/D exchange activity involving gaseous H2 or D2.
We do not achieve mixed valent oxidation states in our
studies. We do not see H2 production from the [FeII(µ-H)-
FeII]+ model complexes, as further protonation is impossible
even with strong acids. Thus, H2 formation must be preceded
by reduction, at least to the FeIIFeI, and possibly lower, redox
states, as indicated by preliminary electrocatalytic generation
of H2 in our laboratories and others.48 That H2 uptake is at
FeII centers is fully supported by our studies of the described
homovalent, FeIIFeII models.
Acknowledgment. We acknowledge financial support
from the National Science Foundation (Grants CHE-
9812355, CHE-0111629 for this work and CHE 85-13273
for the X-ray diffractometer and crystallographic computing
system) and contributions from the R. A. Welch Foundation.
Appreciation is expressed to Prof. Dieter Sellmann for
encouraging this study. We would also like to thank Lisa
Thomson for assistance with the computations and the
Supercomputing Facility at Texas A&M University for
In view of the promotional effect of water on the D2/n-
H+ H/D exchange process, one might question the validity
of the intramolecular mechanism of Scheme 1. While the
(44) (a) Hampton, C.; Cullen, W. R.; James, B. R. J. Am. Chem. Soc. 1988,
110, 6918. (b) Jackson, S. A.; Eisenstein, O. Inorg. Chem. 1990, 29,
3910.
(45) (a) Luo, X.-L.; Crabtree, R. H. J. Am. Chem. Soc. 1990, 112, 6912.
(b) Maseras, F.; Lledo´s, A.; Clot, E.; Eisenstein, O. Chem. ReV. 2000,
100, 601.
(46) (a) Fong, T. P.; Lough, A. J.; Morris, R. H.; Mezzetti, A.; Rocchini.
E.; Rigo, P. J. Chem. Soc., Dalton Trans. 1998, 2111. (b) Abdur-
Rashid, K.; Fong, T. P.; Greaves, B.; Gusev, D. G.; Himman, J. G.;
Landau, S. E.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2000,
122, 9155.
(47) (a) Volbeda, A.; Charon, M.-H.; Piras, C.; Hatchikian, E. C.; Frey,
M.; Fontecilla-Camps, J. C. Nature 1995, 373, 580. (b) Volbeda, A.;
Garcin, E.; Piras, C.; De Lacey, A. L.; Fernandez, V. M.; Hatchikian,
E. C.; Frey, M.; Fontecilla-Camps, J. C. J. Am. Chem. Soc. 1996,
118, 12989. (c) Garcin, E.; Vernede, X.; Hatchikian, E. C.; Volbeda,
A.; Frey, M.; Fontecilla-Camps, J. C. Structure 1999, 7, 557. (d)
Higuchi, Y.; Yagi, T.; Yasuoka, N. Structure 1997, 5, 1671. (e)
Higuchi, Y.; Ogata, H.; Miki, K.; Yasuoka, N.; Yagi, T. Structure
1999, 7, 549.
(48) Gloaguen, F.; Lawrence, J. D.; Rauchfuss, T. B. J. Am. Chem. Soc.
2001, 123, 9476.
Inorganic Chemistry, Vol. 41, No. 15, 2002 3927

Zhao et al.
crystallographic tables for complexes 1, 1-H+, 2, 2-H+, 4, 5-H+;
13CO exchange experiments. This material is available free of charge
via the Internet at http://pubs.acs.org.
computer time. R.M.-R. gratefully acknowledges the Uni-
versidad Autonoma de Queretaro and PROMEP for their
support.
Supporting Information Available: Synthesis and character-
ization of compounds n and n-H+; molecular structure and X-ray
IC020237R
3928 Inorganic Chemistry, Vol. 41, No. 15, 2002