Coupling H2 to Electron Transfer with a 17-Electron Heterobimetallic Hydride: A Redox Switch Model for the H2-Activating Center of Hydrogenase

Article abstract of DOI:10.1021/ja952206j

A meta-stable heterobimetallic mixed-valence ion, , is formed by the one-electron oxidation of Cp^*(dppf)RuH <1, dppf = 1,1'-bis(diphenylphosphino)ferrocene, Cp^* = pentamethylcyclopentadienide>.A remarkable stability toward one-electron oxidation is revealed by the cyclic voltammetry of 1 which contains two reversible oxidations at +0.073 and +0.541 V and a quasireversible oxidation at +0.975 V (vs NHE assigned to Ru(III/II), Ru(IV/III), and Fe(III/II), respectively.The isolable Ru(III) metal hydride, *(dppf)RuH>PF6 (1⁺), is characterized by a NIR absorption at 912 nm (ε = 486 M^-1A cm^-1) assigned to an intervalence transfer band and a series of atom transfer reactions yielding the even electron derivatives *(dppf)RuXH>PF6 (X = H, Cl, Br, I).A crystallographically determined Fe-Ru distance of 4.383(1) Angstroem in 1 is consonant with the classification of 1⁺ as a weakly coupled.Type II mixed-valence ion (H_ab = 627 cm^-1, α² = 3.3x10^-3).This is the first reported example of a mixed-valence bimetallic complex containing the widely used dppf ligand.The ability of 1 to serve as a heterobimetallic catalyst for the reduction of methyl viologen with H2 makes it a unique functional model of hydrogenase enzymes.

Full text of DOI:10.1021/ja952206j

798
J. Am. Chem. Soc. 1996, 118, 798-803
Coupling H2 to Electron Transfer with a 17-Electron
Heterobimetallic Hydride: A “Redox Switch” Model for the
H2-Activating Center of Hydrogenase
Robert T. Hembre,* J. Scott McQueen, and Victor W. Day
Contribution from the Department of Chemistry, UniVersity of NebraskasLincoln,
Lincoln, Nebraska 68588-0304
ReceiVed July 5, 1995. ReVised Manuscript ReceiVed December 20, 1995X
Abstract: A meta-stable heterobimetallic mixed-valence ion, [Fe(II),Ru(III)], is formed by the one-electron oxidation
of Cp*(dppf)RuH {1, dppf ) 1,1′-bis(diphenylphosphino)ferrocene, Cp* ) pentamethylcyclopentadienide}. A
remarkable stability toward one-electron oxidation is revealed by the cyclic voltammetry of 1 which contains two
reversible oxidations at +0.073 and +0.541 V and a quasireversible oxidation at +0.975 V (vs NHE) assigned to
Ru(III/II), Ru(IV/III), and Fe(III/II), respectively. The isolable Ru(III) metal hydride, [Cp*(dppf)RuH]PF6 (1+), is
characterized by a NIR absorption at 912 nm (ꢀ ) 486 M-1 cm-1) assigned to an intervalence transfer band and a
series of atom transfer reactions yielding the even electron derivatives [Cp*(dppf)RuXH]PF6 (X ) H, Cl, Br, I). A
crystallographically determined Fe-Ru distance of 4.383(1) Å in 1 is consonant with the classification of 1+ as a
weakly coupled, Type II mixed-valence ion (Hab ) 627 cm-1, R2 ) 3.3 × 10-3). This is the first reported example
of a mixed-valence bimetallic complex containing the widely used dppf ligand. The ability of 1 to serve as a
heterobimetallic catalyst for the reduction of methyl viologen with H2 makes it a unique functional model of [NiFe]
hydrogenase enzymes.
Introduction
preparation of the Ru(II) hydride, 1, its ability to catalyze the
one-electron reduction of MV2+ with H2 (TMP ) tetrameth-
ylpiperidine, eq 2), and the isolation and characterization of its
17-electron Ru(III) counterpart, 1+, as a delocalized mixed-
valence metal hydride.6
The capacity of hydrogenase enzymes to couple H2 with
electron transfer pathways that reduce one-electron acceptors,
such as cytochrome c3 or methyl viologen (MV2+), poses a
considerable challenge to the development of functional hy-
drogenase models (eq 1).1 In general, reductions in which H2
is used solely as a source of electrons are not well studied
1 (0.1%)
H2 + 2MV2+ + 2TMP acetone 8 2MV+ + 2HTMP+ (2)
beyond the early work of Halpern and his co-workers.2
A
credible hydrogenase reactivity model not only must use H2 to
perform one-electron reductions but must do so with high
thermodynamic efficiency. For instance, in the hydrogenase
of D. gigas an Fe4S4 cluster proximal to the putative H2-
activating site, the Ni center, is reduced at a midpoint redox
potential of -445 mV (vs NHE),3 only 35 mV positive of the
thermodynamic reduction potential of H2 at pH ) 8.
Experimental Section
General Procedures. Al1 manipulations were performed under an
atmosphere of nitrogen (high purity, 99.995%) or argon prepurified
by passage through BASF-BTS catalyst and molecular sieves (3Å-
Linde). Gasses were measured using a mercury manometer against
atmospheric pressure so an “atmosphere of gas” indicates about 735
Torr. Prepurified hydrogen (99.995%) was passed through a column
containing a BASF-BTS catalyst and molecular sieves (3Å-Linde).
Degassed solutions were prepared by freezing in liquid nitrogen,
evacuating, and thawing three times. NMR tube reactions were
performed in resealable 5-mm NMR tubes (Brunfeldt Glass).
H2 H2ase8 2e- + 2H+
(1)
A further challenge to understanding and modeling the
activation of H2 by [NiFe]H2ase is raised by recent structural
studies which show that the nickel resides in a heterobimetallic
site linked to a second metal, believed to be iron, by cysteinate
bridges.4 Speculation that interaction of these two metals is
involved in the activation of H2 has prompted our interest in
the study of a heterobimetallic complex, Cp*(dppf)RuH (1, dppf
) 1,1′-bis(diphenylphosphino)ferrocene, Cp* ) pentamethyl-
cyclopentadienide, Figure 1), to examine the influence of a
proximal Fe(II) on the Cp*L2RuH catalyzed reduction of NAD
model compounds by H2.5 In this paper we report the
Benzene, hexane, THF, and pentane were distilled under nitrogen
or vacuum from sodium or potassium benzophenone. Methylene
chloride and acetonitrile were distilled from P4O10. Methanol was
distilled under nitrogen from freshly activated magnesium turnings.
Acetone was dried and vacuum transferred from MgSO4. THF-d8 was
stored over sodium benzophenone ketyl under vacuum in a flask with
a Teflon valve and transferred at reduced pressure to reaction samples.
Cp*(PPh3)2RuCl,8 Cp(dppf)RuH,9 Cp*(Ph2PMe)2RuH,10 [MV][I]2,11
12
and [Fc]PF6 were prepared as described in the literature. The
hexafluorophosphate salts were generated by metathesis with NH4PF6.
TMP was purchased from Aldrich and distilled from P4O10. Cobal-
tocene was purchased from Strem Chemicals Inc., Bu3SnH from
Aldrich, and ruthenium trichloride hydrate from Colonial Metals, Inc.
Analyses. Elemental analyses were performed by Midwest Micro-
labs, Indianapolis, IN. Crystallographic data was collected at Crysta-
(1) Albracht, S. P. J. Biochim. Biophys. Acta 1994, 1188, 167-204.
(2) (a) Halpern, J. Q. ReV. Chem. Soc. 1956, 10, 463-79. (b) Harrod,
J. F.; Ciccone, S.; Halpern, J. Can. J. Chem. 1961, 39, 1372-76.
(3) Roberts, L. M.; Lindahl, P. A. J. Am. Chem. Soc. 1995, 117, 2565-
72.
(5) Hembre, R. T.; McQueen, J. S. J. Am. Chem. Soc. 1994, 116, 2141-
42.
(4) The distance from Ni to the second metal (X) is 2.7 Å in the D.
gigas [NiFe] hydrogenase: Volbeda, A.; Charon, M.-H.; Piras, C.; Hatch-
ikian, E. C.; Frey, M.; Fontecilla-Camps, J. C. Nature 1995, 373, 580-87.
(6) Taube and co-workers have reported a localized mixed-valence metal
hydride of Os(IV)/Fe(II).7
(7) Li, Z. W.; Yeh, A.; Taube, H. Inorg. Chem. 1994, 33, 2874-81.
0002-7863/96/1518-0798$12.00/0 © 1996 American Chemical Society

Coupling H2 to Electron Transfer
J. Am. Chem. Soc., Vol. 118, No. 4, 1996 799
Figure 1. (a) Perspective drawing of the solid-state structure of Cp*(dppf)RuH, 1. For clarity, Ru, Fe, and P atoms are represented by large
cross-hatched, striped, and shaded spheres, respectively; carbon and hydrogen atoms are represented by medium-sized and small open spheres. (b)
Perspective drawing of the solid-state structure of Cp(dppf)RuH, 2, viewed, labeled, and represented as in part a (the hydride was not
crystallographically located).9 (c) Perspective drawing of 1 showing staggered conformation of the ferrocene group in the dppf ligand. For clarity
all hydrogen atoms except the hydride ligand have been omitted. (d) Schematic of the Fe-P2-RuH core of 1.
lytics Co., Lincoln, NE. IR spectra were recorded on an Analect RFX-
65 FT-IR spectrometer using a CaF2 cell for liquid samples. NMR
mmol), and zinc (455 mg, 7.0 mmol) was stirred for 48 h under nitrogen.
A methanolic solution (15 mL) containing Na (130 mg, 5.6 mmol)
was added via cannula and heated at reflux for 4 h. The solvent was
removed at reduced pressure and the residue was extracted with
benzene. The benzene was removed at reduced pressure and the
resulting residue washed with methanol and dried in vacuo yielding
553 mg of an orange powder (87% yield).
1
spectra were recorded on GE Omega 300 and 500 (13C, H, and 31P)
spectrometers. Solvent adsorptions were used as internal standards {δ
1H: THF-d7, δ ) 3.58, 1.73; CHD2CN, δ ) 1.93; C6D5H, δ ) 7.15;
CDHCl2, δ ) 5.32 ppm}. A 3% aqueous H3PO4 solution was used as
an external standard for 31P spectra. Near-IR and UV-vis measure-
ments were made on a Cary 14 UV-vis apparatus with an Olis upgrade
for NIR measurements.
Cyclic voltammograms were obtained on a Cypress Systems
Potentiostat Model CYSY-1R equipped with a Model CS-1090 com-
puter-controlled electroanalytical system. A glassy carbon or 0.15 mm
diameter platinum wire electrode was used as the working electrode, a
silver wire as the reference electrode, and a platinum wire as the counter
electrode. All solutions were prepared and measurements were
performed in a drybox under a nitrogen atmosphere. Samples were
prepared in THF with the concentration of electrolyte, [Bu4N][PF6], at
0.1 M and that of the analyte and reference, [Cp2Co][PF6], at 0.4 mM.
Cp*(dppf)RuH, 1. Method 1. A benzene solution (25 mL)
containing [Cp*RuCl2]n (247 mg, 0.81 mmol), dppf (520 mg, 0.95
Method 2. A benzene solution (30 mL) of Cp*Ru(PPh3)2Cl (500
mg, 0.63 mmol) and dppf (378 mg, 0.68 mmol) was heated at reflux
under nitrogen for 10 h. A dark yellow solid formed and when cooled
was filtered, washed with benzene twice, and dried in vacuo.
A
methanolic solution containing Na (120 mg, 5.2 mmol) was added via
cannula to a suspension of the collected solid, Cp*Ru(dppf)Cl, in
methanol and heated at reflux for 48 h. Allowing the suspension to
cool resulted in the precipitation of a yellow solid which was filtered
under nitrogen, washed three times with methanol, and dried in vacuo
yielding 385 mg of an orange powder (78% yield). A crystalline solid
suitable for X-ray analysis was obtained by layering methanol on a
saturated benzene solution in a tube sealed with a Teflon valve. 1H
NMR (C6D6) δ -12.31 (t, J ) 35.8 Hz, 1H), 1.40 (s, 15 H), 3.70 (s,
2 H), 3.82 (s, 2 H), 4.09 (s, 2 H), 4.65 (s, 2 H), 7.19 (m, Ph), 8.03 (br
s, Ph), 8.19 (br s, Ph). 31P NMR (C6D6) δ 64.8 (s). IR (THF) νRu-H
) 1946 cm-1 (br, w). Anal. Calcd for (C44H44P2RuFe): C, 66.7; H,
5.6. Found: C, 66.3; H, 5.5.
(8) Treichel, P. M.; Komar, D. A.; Vincenti, P. J. Synth. React. Inorg.
Metal-Org. Chem. 1984, 14, 383-400.
(9) Bruce, M. I.; Butler, I. R.; Cullen, W. R.; Koutsantonis, G. A.; Snow,
M. R.; Tiekink, E. R. T. Aust. J. Chem. 1988, 41, 963-69.
(10) Jia, G.; Morris, R. H. J. Am. Chem. Soc. 1991, 113, 875-83.
(11) Weidel, H.; Russo, M. Monatsh. Chem. 1882, 3, 850-56.
(12) Lyatifov, I. R.; Solodovnikov, S. P.; Babin, V. N.; Materikova, R.
B. Z. Naturforsch. 1979, 34B, 863-66.
This hydride is not soluble in acetonitrile or methanol but is soluble
in THF, benzene, methylene chloride, and acetone. Dissolution of 1
in chloroform or dichloroethane results in the formation of Cp*(dppf)-
RuCl within an hour. In methylene chloride 1 is stable for days at
ambient temperature but slowly decomposes at 80 °C. A low-
temperature NMR study showed no broadening of the cyclopentadienyl
(13) (a) Day, V. W.; Eberspacher, T. A.; Klemperer, W. G.; Planalp, R.
P.; Schiller, P. W.; Yagasaki, A.; Zhong, B. Inorg. Chem. 1993, 32, 1629-
37. (b) Besecker, C. J.; Day, V. W.; Klemperer, W. G.; Thompson, M. R.
J. Am. Chem. Soc. 1984, 106, 4125-36.

800 J. Am. Chem. Soc., Vol. 118, No. 4, 1996
Hembre et al.
Table 1. Crystallographic Data for Cp*(dppf)RuH (2-H)a
protons of the ferrocene ligand at -80 °C. The staggered conformation
of the dppf ligand present in the solid state must, therefore, undergo a
rapid “ring flip”.
C44H44P2RuFe 791.71 amu
P21/n {an alternate setting of P21/c-C2h5 (No. 14)}
density ) 1.557 g cm-3
monoclinic (T ) 20 °C)
V ) 3616(1) Å3
[Cp*(dppf)Ru(H)]PF6, [1]PF6. To a THF (20 mL) solution of 1
(256 mg, 0.32 mmol) was added [Fc]PF6 (100 mg, 0.30 mmol) at -78
°C under nitrogen. The solution was stirred for 4 h in which time it
became a dark maroon color. Cold pentane was added via cannula
causing the precipitation of the product which was filtered, washed
three times with cold pentane, and dried in vacuo yielding 278 mg
Z ) 4
a ) 12.913(2) Å
b ) 15.041(3) Å
c ) 18.831(4) Å
1
(98% yield) of a light maroon solid. The broadened peaks of the H
R ) 90.0
NMR of the solid in CD2Cl2 was consistent with a paramagnetic
product. No hydride resonance or 31P signal was observed. Allowing
the NMR tube to sit for several hours resulted in a lighter colored
solution and very faint 31P signals at 44 and 38 ppm. After 12 h, NMR
indicated the formation of [Cp*(dppf)Ru(H)2]+ and [Cp*(dppf)Ru-
(THF)]+.
â ) 98.62(2)°
γ ) 90.00°
R(unweighted) ) 0.019
(8305 independent absorption corrected
reflections having 2θ < 55°) GOF ) 1.02
a The complete data set is available from the Crystalytics Co.,
Lincoln, NE.
[Cp*(dppf)Ru(H)(Cl)]PF6. To a 25-mL flask containing [1]PF6
(27 mg, 30 µmol) was added carbon tetrachloride (1.5 mL) and THF
(4 mL). After an hour of stirring the solvent was removed at reduced
pressure and the residue dried for 2 h under vacuum. 1H NMR (CD2-
Cl2) δ -7.45 (t, J ) 32 Hz, 1H), 1.22 (s, 15H), 4.15 (s, 2H), 4.18 (s,
2H), 4.39 (s, 2H), 4.97 (s, 2H), 7.40-7.75 (m, 20H). {1H}31P NMR
(CD2Cl2) δ 38.3. Anal. Calcd for C44H44ClF6FeP3Ru: C, 54.3; H,
4.5. Found: C, 54.1; H, 4.7.
with acetone (10 mL) as the solvent. After 5 h at 23 °C the initially
light yellow solution became very dark blue.
Crystallographic Study of 1. At 20 ( 1 °C, single crystals of 1
are monoclinic, space group P21/n [an alternate setting of P21/c-C5
2h
(No. 14)] with a ) 12.913(2) Å, b ) 15.041(3) Å, c ) 18.831(4) Å,
â ) 98.62(2)°, V ) 3616(1) Å3, and Z ) 4 {dcalcd ) 1.557 g cm-3
;
[Cp*(dppf)Ru(H)(Br)]PF6. To a foil-wrapped resealable NMR tube
was added [1]PF6 (8 mg, 9 µmol), BrMn(CO)5 (6 mg, 22 µmol), and
THF. The dark gold solution was analyzed by 31P NMR indicating
the formation of one major product at δ 37.6 ppm which was assumed
to be the correct product from the similarity in chemical shift to the
chloride and iodide derivatives. 1H NMR (CD3CN) δ -7.50 (t, J )
32 Hz, 1H), 1.40 (t, J ) 1.5 Hz, 15H), 4.18 (s, 2H), 4.24 (s, 2H), 4.43
(s, 2H), 5.09 (s, 2H), 7.46 (m, 20H). {1H}31P NMR (CD3CN) δ 37.6.
[Cp*(dppf)Ru(H)(I)]PF6. To a THF solution (2 mL) containing
[1]PF6 (28 mg, 3 µmol) was added an iodine/THF (0.1 M) solution
(0.5 mL) with stirring for 75 min. A dark red solid was isolated by
precipitation with hexanes. Recrystallization from MeOH/benzene
afforded crystals. 1H NMR (CD3CN) δ -7.50 (t, J ) 32 Hz, 1H),
1.40 (t, J ) 1.5 Hz, 15H), 4.18 (s, 2H), 4.24 (s, 2H), 4.43 (s, 2H), 5.09
(s, 2H), 7.46 (m, 20H). {1H}31P NMR (CD3CN) δ 37.6. Anal. Calcd
for C44H44F6FeIP3Ru: C, 49.6; H. 4.2. Found: C, 50.0; H, 4.4.
µa(Mo KR) ) 1.32 mm-1}. A total of 8305 independent absorption
corrected reflections having 2θ(Mo KR) < 55.0° (the equivalent of
1.0 limiting Cu KR spheres) were collected on a computer-controlled
Nicolet autodiffractometer using full (0.90° wide) ω scans and graphite-
monochromated Mo KR radiation. The computer programs and
procedures used for data collection, data reduction, structure solution,
and refinement of 1 have been reported elsewhere;13 a summary of the
crystallographic data for the present study is given in Table 1.
The structure was solved using “heavy atom” techniques with the
Siemens SHELXTL-PC software package as modified at Crystalytics
Company. The resulting structural parameters have been refined to
convergence {R1(unweighted, based on F) ) 0.036 for 5419 indepen-
dent absorption-corrected reflections having 2θ(Mo KR) < 55.0° and
I > 3σ(I)} using counter-weighted full-matrix least-squares techniques
and a structural model which incorporated anisotropic thermal param-
eters for all non-hydrogen atoms and isotropic thermal parameters for
all hydrogen atoms.
[Cp*(dppf)Ru(H)2]PF6. To a solution (0.5 mL) containing [1]PF6
(9 mg, 9 µmol) in THF-d8 was added Bu3SnH (18 mg, 62 µmol). After
30 min 1H and 31P NMR spectra indicated the formation of [Cp*(dppf)-
RuH2]PF6. 1H NMR (THF) δ -7.76 (t, J ) 25.7 Hz, 1H), 1.26 (s,
15H), 4.21 (s, 4H), 4.22 (s, 4H), 7.4 (m), 7.62 (br s), and 7.92 (m).
{1H}31P NMR (THF) δ 59.2. Anal. Calcd for C44H45F6FeP3Ru: C,
56.2; H, 4.8. Found: C, 55.9; H, 4.9.
The hydride hydrogen atom (H1) was located from a difference
Fourier map and refined as an independent isotropic atom. The five
methyl groups (C1m, C2m, C3m, C4m, C5m and their hydrogens) were
refined as rigid rotors with sp3-hybridized geometry and a C-H bond
length of 0.96 Å. The initial orientation of each methyl group was
determined from difference Fourier positions for the hydrogen atoms.
The final orientation of each methyl group was determined by three
rotational parameters. The refined positions for the rigid rotor methyl
groups gave C-C-H angles which ranged from 102° to 114°. The
remaining hydrogen atoms were included in the structure factor
calculations as idealized atoms (assuming sp2 hybridization of the
carbon atoms and a C-H bond length of 0.96 Å) “riding” on their
respective carbon atoms. The isotropic thermal parameter for H1 refined
to a final value of 3(1) Å2. The isotropic thermal parameter of each
remaining hydrogen atom was fixed at 1.2 times the equivalent isotropic
thermal parameter of the carbon atom to which it is covalently bonded.
Disproportionation of [1]PF6 in Solution. Into a flask with a
Teflon valve and a quartz cuvette fused to a side-arm was placed 12
mg of [1]PF6 (13 µmol), and 4 mL of THF was placed in the sidearm.
To begin the reaction the flask was tilted upright allowing the THF to
run into the bulb. The reaction was followed by the decreasing
absorbance at 912 nm for 570 min at 23 °C. Extrapolation based on
the rate of disproportionation yielded an extinction coefficient of 468.
Analysis of a plot of the reciprocal of [1+] vs time yielded a second-
order rate constant of 3.5 × 10-2 M-1 s-1
.
A retardation in the decomposition rate of [1-H]PF6 was measured
by following a reaction prepared in the same manner as above, but
with 44 mg of 1 (0.056 mmol) added prior to the addition of THF.
After 10 h 50% of [1]PF6 remained.
Results and Discussion
Synthesis and Characterization of Cp*(dppf)RuH, 1.
Established procedures, with minor adaptation, work well for
the synthesis of 1. Its chloride counterpart, Cp*(dppf)RuCl, is
prepared in good yield by either the Bruce or Suzuki methods.14
In the former dppf displaces triphenylphosphine from Cp*(Ph3P)2-
RuCl and in the latter dppf reacts with a Ru(III) polymer,
[Cp*RuCl2]n, in the presence of zinc. Treatment of Cp*(dppf)-
RuCl with NaOMe yields the hydride, 1, as an orange powder,
Reduction with H2 Catalyzed by 1. 1 (6.2 mg, 0.01 mmol) and
[Cp2Fe][PF6] (337 mg, 1.0 mmol) were placed in a flask with a Teflon
valve and acetone (10 mL) was added by vacuum transfer. TMP (200.0
µL, 1.2 mmol) was added by syringe under a flow of argon, and the
solution was degassed and then placed under an atmosphere of H2.
Progress of the reaction was followed by the dissolution of the insoluble
[Cp2Fe]PF6. After 2 h the solution was concentrated to a residue and
extracted with hexanes (3 × 10 mL). Following the removal of the
hexanes 176 mg of ferrocene (94% yield) was obtained.
(14) (a) Ashby, G. S.; Bruce, M. I.; Tomkins, I. B.; Wallis, R. C. Aust.
J. Chem. 1979, 32, 1003-16. (b) Oshima, N.; Suzuki, H.; Moro-Oka, Y.
Chem. Lett. 1984, 1161-64.
In the same fashion, 1 (3 µmol), [MV][PF6]2 (45.0 mg, 0.10 mmol),
and TMP (50 µL, 0.30 mmol) were combined and placed under H2

Coupling H2 to Electron Transfer
J. Am. Chem. Soc., Vol. 118, No. 4, 1996 801
insoluble in methanol or acetonitrile, but readily dissolved in
THF, benzene, and acetone. Reaction with chlorinated solvents
such as chloroform or dichloroethane limits their usage, but
methylene chloride solutions are stable for a couple of days at
ambient temperature.
in 1 (Table 2) is similar to that in the other Ru(II) hydrides
(96-101°). Likewise, 1 is typical of this group in the angles
at which the metal hydride bond intersects the P1-Ru-P2 plane
and the H1-Ru-Cg1 plane intersects the P1-Ru-P2 plane:
76.9° vs 73.6-85.4° and 89.6° vs 87.4-88.8°, respectively.22
The angle of intersection of the P1-Ru-P2 plane with the plane
defined by the cyclopentadienyl carbons is a bit higher in 1
(73.5°) than in the C5H5 derivatives (65.6-68.9°) which may
reflect the influence of the methyl groups of Cp*. For instance,
the same angle in [Cp*(PPh3)2RhH]PF6 is increased to 79.2°.23
X-ray analysis of a crystal of 1 revealed the molecular
structure of the heterobimetallic hydride, Cp*(dppf)RuH, as
shown in Figure 1 with a Ru-H bond length of 1.43(4) Å. Both
d6 metals in 1 are pseudooctahedral 18-electron centers with
nine of their twelve formal coordination sites occupied by three
substituted cyclopentadienyl rings.15 The steric properties of
the chelating dppf ligand16 require a nearly linear C1g-Ru‚‚‚
Fe17 arrangement with the ferrocenyl group situated opposite
the C5Me5 bound to ruthenium ( C1g-Ru‚‚‚Fe ) 175.6°). Thus
the hydride ligand, both phosphorus atoms, and the four phenyl
rings of the dppf ligand occupy positions in a band perpendicular
to this nearly linear C1g-Ru‚‚‚Fe grouping and between the Ru-
bonded cyclopentadienyl ligand and the ferrocenyl group.18
The Cp rings in the dppf ligand of 1 are within 3° of perfectly
staggered (Figure 1c). For instance, the dihedral angle defined
by P1-C11-C21-P2 is 38.3°. In solution rapid flipping between
pseudoenvelope conformations renders these phosphorus atoms
symmetry equivalent, even at -80 °C. The dimensions of the
heterobimetallic core, of interest in the redox properties of 1,
are defined by the nonbonded distances of ∼3.40 Å between
the iron and the two phosphorus atoms, 4.38 Å between the
iron and ruthenium atoms, and 4.09 Å between the iron and
the metal hydride ligand (Figure 1d). Along with a P-Ru-P
angle of 98° the coordination of dppf to Ru(II) in 1 is as expec-
ted for an 18-electron complex. It is interesting to note, how-
ever, that the structure of [(dppf)Pd(PPh3)][BF4]2, with a dative
interaction between the iron of dppf and Pd(II), has been repor-
ted by Sato.20 The Fe-Pd distance of 2.88 Å and a P-Pd-P
angle of 156° in this complex demonstrates that the geometry
of the heterobimetallic core of 1 is surprisingly flexible.
The steric crowding imposed in a half-sandwich derivative
of C5Me5 is nicely demonstrated by comparison of 1 with Cp-
(dppf)RuH, 2 (Figure 1b).24 The metric parameters of their Ru
coordination spheres and heterobimetallic cores, Fe-P2-RuH,
are essentially identical (Figure 1d).25 The only striking
structural difference between 1 and 2 is that two of the four
dppf phenyl rings have rotated about their P-C bonds to adopt
orientations more nearly parallel to the C5Me5 plane. Replacing
the hydrogens of a cyclopentadienyl ligand with methyls
dramatically increases its “cone angle”.26 This added “radial
bulk” allows the C5Me5 to act as a van der Waals “cap” for the
ruthenium octahedron which prevents the ortho hydrogens on
the phenyl rings from approaching this portion of the coordina-
tion sphere. As a result the orientation of the phenyl rings for
the dppf ligands is more restricted in 1 than in 2. In essence,
the substitution of methyl groups for hydrogen on the Ru-bonded
cyclopentadienyl ring of 2 has compressed the phenyl groups
of dppf and introduced a preferred orientation in which the
phenyl rings are “flattened”, i.e., perpendicular to the Ru-C1g
vector. One of these phenyl rings is adjacent to the hydride
ligand and produces a smaller hydrophobic pocket for this ligand
in 1 than 2.29 As discussed below, we attribute the unusual
stability of the Ru(III) derivative of 1, at least in part, to this
(21) (a) Lemke, F. R.; Brammer, L. Organometallics 1985, 14, 3980-
87. (b) Lister, S. A.; Redhouse, A. D.; Simpson, S. J. Acta Crystallogr.
1992, C48, 1661-63. (c) Smith, K.-T.; Rømming, C.; Tilset, M. J. Am.
Chem. Soc. 1993, 115, 8681-89.
(22) The RuH bond of Cp(PPh3)2RuH is anomolous in this group with
an interplanar angle between its centroid-Ru-H and P-Ru-P planes of
76.5° (see Table 5 in ref 21a).
(23) Mingos, D. M. P.; Minshall, P. C.; Hursthouse, M. B.; Malik, K.
M. A.; Willoughby, S. D. J. Organomet. Chem. 1979, 181, 169-82.
(24) In the solid state 2 consists of two crystallographically-independent
molecules.9 There are no significant differences in their structures relevant
to a comparison with 1, so we present only one of the structures in Figure
1b. It has been renumbered to coincide with 1. Unfortunately, the hydride
ligand on Ru could not be crystallographically located for either molecule
of 2.
(25) The close structural similarity for the immediate metal coordination
spheres of 1 and 2 would seem to indicate that the differences in their
chemical behavior must be due to electronic effects produced by replacing
the five hydrogens on the Cp ligand in 2 by methyl groups and/or the steric
effects of such substitutions on ligand atoms not bonded directly to a metal.
While the electronic effects of substituting methyl for hydrogen on a
cyclopentadienyl ligand are well documented and can be probed electro-
chemically, the steric effects of such a substitution cannot be so easily
assessed.26
(26) Both ligands are disc-shaped with a central 2.42 Å diameter core
which is 3.40 Å thick (the van der Waals diameter27 of an aromatic carbon
atom). Surrounding this central 2.42 Å disc is a ring of five hydrogens for
C5H5 or five methyl groups for C5Me5. A ring of five hydrogens extends
the disc radially another 2.29 Å and this annulus is 2.40 Å thick (the van
der Waals diameter27 of a hydrogen atom) while a ring of five methyls
extends the disc radially another 3.54 Å and this annulus is 4.00 Å thick
(the van der Waals diameter27 of a methyl group). In terms of cone angles
a value of 131° for C5H5 can be compared with 182° for C5Me5.28
(27) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell
University Press: Ithaca, NY, 1960; p 260.
The structure of 1 (Figure 1a) may be compared with that of
four related Ru(II) hydrides of the CpL2RuH family {L2 )
(PMe3)2,21a (PPh3)2,21b dppp,21c and dppf9}, but none of these
contain the pentamethylcyclopentadienide ligand. In a careful
analysis of the structures of this class of metal hydrides Lemke
and Bremmer compare several structural features of the “three-
legged piano stool” geometry.21a First, the 98° P1-Ru-P2 angle
(15) Each of the five-membered rings for these cyclopentadienyl ligands
are coplanar to within 0.006 Å. The least-squares mean planes for the two
cyclopentadienyls of the dppf ligand are within 7.2° of being parallel to
each other with the Fe(II) atom between them, 1.64 Å from each mean
plane. Phosphorus atoms P1 and P2 are displaced by 0.04 and 0.09 Å from
these mean planes toward the Fe atom. The Ru(II) atom is 1.91 Å from
the C5 ring of the C5Me5 ligand and the five methyl carbons are displaced
by 0.17-0.25 Å on the opposite side.
(16) Gan, K.-S.; Hor, T. S. A. In Ferrocenes; Togni, A., Hayashi, T.,
Eds.; VCH: Weinheim, 1995; pp 3-104.
(17) The symbols C1g, C2g, and C3g are used to represent the centers-
of-gravity for the five-membered rings of the Cp* and each of the two
cyclopentadienyl rings of the dppf ligand, respectively.
(18) The average19 Ru-C, Ru-P, and Fe-C bond lengths are 2.263(4,
18, 30, 5), 2.265(1, 6, 6, 2), and 2.045(4, 16, 27, 10) Å. The P1-Ru-P2,
H1-Ru-C1g,17 and C2g-Fe-C3g17 angles are 97.9(1)°, 121.0°, and 176.0°.
The average P-Ru-H1 and P-Ru-C1g angles are 82(2, 1, 1, 2)° and (-,
3, 3, 2)°. The average P-C and dppf cyclopentadienyl and phenyl C-C
bond lengths are 1.848(4, 9, 23, 6), 1.427(6, 10, 27, 10), and 1.387(7, 7,
21, 24) Å. The average ring C-C and ring-to-methyl C-C bond lengths
for the Cp* ligand are 1.427(6, 8, 16, 5) and 1.503(6, 3, 7, 5) Å.
(19) The first number in parentheses following an average value of a
bond length or angle is the root-mean-square estimated standard deviation
of an individual datum. The second and third numbers are the average
and maximum deviations from the average value, respectively. The fourth
number represents the number of individual measurements which are
included in the average value.
(28) White, D.; Coville, N. J. AdV. Organomet. Chem. 1994, 36, 95-
158.
(29) This pocket contains four short H‚‚‚H contacts between the hydride
ligand and hydrogens of the surrounding ligands. These contacts are shown
with dashed lines in Figure 1a and have the following separations: H1‚‚‚H25,
2.29 Å; H1‚‚‚H32, 2.06 Å; H1‚‚‚H66, 2.84 Å; and H1‚‚‚H1ma, 2.90 Å.
(20) Sato, M.; Shigeta, H.; Sekino, M.; Akabori, S. J. Organomet. Chem.
1993, 458, 199-204.

802 J. Am. Chem. Soc., Vol. 118, No. 4, 1996
Hembre et al.
Table 2. Selected Bond Lengths and Bond Angles in Crystalline
Cp*(dppf)RuH (1)a
typeb
parameter
typeb
parameter
Bond Lengths, Å
Ru-P1
Ru-H1
Fe-C2g
P1-C11
P1-C31
P1-C51
2.271(1)
1.43(4)
Ru-P2
Ru-C1g
Fe-C3g
P2-C21
P2-C41
P2-C61
2.259(1)
1.910(-)c
1.649(-)c
1.825(4)
1.846(4)
1.859(4)
1.643(-)c
1.847(4)
1.847(4)
1.862(4)
Bond Angle, deg
P1RuP2
H1RuP1
97.9(1)
82(2)
H1RuC1g
H1RuP2
P2RuC1g
121.0(-)c
81(2)
P1RuC1g
C2gFeC3g
RuP1C11
RuP1C31
RuP1C51
C11P1C31
C11P1C51
C31P1C51
P1C11C12
P1C11C15
P1C31C32
P1C31C36
P1C51C52
P1C51C56
128.4(-)c
176.0(-)c
123.8(1)
115.4(1)
116.6(1)
99.7(2)
129.0(-)c
RuP2C21
RuP2C41
RuP2C61
C21P2C41
C21P2C61
C41P2C61
P2C21C22
P2C21C25
P2C41C42
P2C41C46
P2C61C62
P2C61C66
118.6(1)
118.3(1)
113.3(1)
103.7(2)
97.8(2)
102.1(2)
129.6(3)
123.5(3)
117.7(3)
124.3(3)
123.9(3)
118.0(3)
97.6(2)
99.6(2)
Figure 2. Cyclic voltammogram of 1 (0.4 mM in THF) and
cobaltocene (0.4 mM, -750 mV vs NHE) with 0.1 M TBAF at a Pt-
wire electrode; scan rate ) 200 mV/s. A plot of ia against the square
root of the scan speed is linear across a range of 20-1000 mV/s (ia/ic
) 0.9-1.1; correlation coefficient >0.99) for E1 and E2, as expected
for a diffusion-controlled electrode process.
128.6(3)
124.8(3)
120.0(3)
122.2(3)
123.4(3)
119.0(3)
imposed structure around the hydride ligand which must retard
inner-sphere reactions of its metal-hydrogen bond.
a The numbers in parentheses are the estimated standard deviations
in the last significant digit. b Atoms are labeled in agreement with Figure
1. c See footnote 17.
Oxidation of 1. Electrochemical oxidation of metal hydrides
is rarely reversible,30 but analysis of 1 by cyclic voltammetry
reveals two reVersible oxidations at +0.073 and +0.541 V and
a quasireversible oxidation at +0.97 V vs NHE (E1, E2, and
E3, respectively; Figure 2). Under the same conditions Cp-
(dppf)RuH shows only a quasireversible oxidation at +0.28 V,
an irreversible oxidation at +.69 V, and another quasireversible
redox couple at +0.97 V (Table 3). Assignment of E1 to Ru-
(III/II) and E2 to Ru(IV/III) in 1 is supported by comparison
with the oxidations of Cp*(Ph2MeP)2RuH (3) which contains
no iron. For 3 both a reversible redox couple {E1 ) +0.050
V; Ru(III/II)} and an irreversible oxidation {E2 ) +0.496 V;
Ru(IV/III)} at potentials similar to those of 1 must be ruthenium
centered. Oxidation of iron in dppf occurs irreversibly at +0.87
V and is typically shifted positive by 0.1-0.2 V upon coordina-
tion of its phosphines in a chelate complex.33 Comparison of
the chloride derivatives reveals redox potentials for E1 that are
shifted by about 400 mV while the second oxidations all occur
at ∼1.0 V, characteristic of a coordinated dppf. Because all of
the dppf derivatives in Table 3 exhibit an oxidation at ∼0.97 V
we assign E3 of the hydrides and E2 of the chlorides (Figure 2)
to Fe(III/II) of the dppf ligand.
Table 3. Redox Potentials of Cp*L2RuH Complexesa,b
X ) H
X ) Cl
c
c
E1
E2
E3
E1
E2
Cp*(dppf)RuX
Cp*(Ph2PMe)RuX +0.050 (+0.496)
Cp(dppf)RuX (+0.284) (+0.695) (+0.972) +0.675 +0.967
+0.073 +0.541 (+0.975) +0.523 +1.033
+0.480
a All potentials are measured in THF solution with 0.1 M [Bu4N]PF6
vs cobaltocene (-0.750) and are reported vs NHE. b Values in
parentheses indicate anodic peak potentials from irreversible redox
couples. c E3 of the hydrides and E2 of the chlorides are assigned to
the Fe(III/II) couple of the dppf ligand.
ferrocinium ion. This species may be reduced by cobaltocene
to its parent hydride (eq 4) or precipitated by addition of pentane
at low temperature, isolated, and dissolved in THF to give a
purified solution of the Ru(III) hydride.34
[Fc]PF6
[Cp*(dppf)RuH]PF
1 {
}
(4)
6
Cp2Co
1+
The optical spectrum of 1+ in methylene chloride contains
an absorption in the NIR at 912 nm (ꢀ ) 486 M-1 cm-1, Figure
3). As predicted by Hush for a weakly delocalized intervalence
transition, the energy of the NIR band is sensitive to the
dielectric properties of the solvent (νmax ∼ 1/n2 - 1/Ds)36 and
its bandwidth (3.38 × 103 cm-1; Figure 3) is within 15% of
that predicted by the analysis of Taube for an asymmetric mixed-
valence ion.37 The Ru(III) hydride, 1+, is thus characterized
These assignments dictate an equilibrium constant for electron-
transfer disproportionation of 1+ (eq 3) and predict a novel
stability for this 17-electron metal hydride.
-8
}
2[1][PF6] {K ) 1.8 × 10
[Cp*(dppf)RuH][PF6]2 + Cp*(dppf)RuH (3)
In fact, a red solution of Cp*(dppf)RuH+ in CH2Cl2 is
(34) This procedure is analogous to that reported by Lapinte for the
preparation of [Cp*(dppe)FeH]PF6.35
(35) Hamon, P.; Toupet, L.; Hamon, J. R.; Lapinte, C. Organometallics
1992, 11, 1429-31.
(36) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391-444.
(37) Taube approximates an upper limit for the internal energy difference
between two states, νjo, as the difference in the corresponding redox
potentials, E1 - E2: νjop - νjo ) (∆νj1/2)2/2.31.38 For 1+ we use E3 assigned
to the [Fe(III/II), Ru(IV)] couple as an upper limit for the necessary [Fe-
(III/II), Ru(III)] potential (E1 - E3 ) 902 mV). This yields a calculated
bandwidth of 2.92 × 103 cm-1 slightly less than that observed (obsvd/
calcd ) 1.15).
generated by the stoichiometric oxidation of 1 with the
(30) Tilset has reported two closely related examples of near-reversibility
in CpL2RuH complexes,31 but in general electrochemical oxidation of
hydrides of this family are irreversible.10 Only two examples of fully
reversible electrochemical one-electron oxidations have been reported.32
(31) Smith, K. T.; Roemming, C.; Tilset, M. J. Am. Chem. Soc. 1993,
115, 8681-89.
(32) (a) Sharp, P.; Frank, K. G. Inorg. Chem. 1985, 24, 1808-13. (b)
Detty, M. R.; Jones, W. D. J. Am. Chem. Soc. 1987, 109, 5666-73.
(33) Zanello, P. In Ferrocenes; Togni, A., Hayashi, T., Eds.; VCH:
Weinheim, 1995; pp 317-430.

Coupling H2 to Electron Transfer
J. Am. Chem. Soc., Vol. 118, No. 4, 1996 803
proceeds at a modest rate (∼8.5 turnovers/min) at ambient
temperature and pressure and is poisoned by stoichiometric
amounts of halide or acetonitrile. As mentioned above (eq 2)
the same method may be employed in the reduction of MV2+
to MV+. With the exception of hydrogenase enzymes,45 1 is
the first example of a homogeneous catalyst for reduction of
MV2+ with H2. Because 1 and the H2-activating center in
hydrogenase act as templates for the conversion of a two-elec-
tron reducing agent, H2, into one-electron equivalents (2Cp2Fe
or 2 MV+), we refer to them as “redox switch” catalysts.47
H2 + 2 TMP + [Cp2Fe]PF6 1 (1%)8 Cp2Fe + 2 [TMPH]PF6
THF
(7)
Conclusions
Figure 3. NIR spectrum of [1]PF6 (0.6 × 10-3 M) in CH2Cl2. νmax
)
We report herein the discovery of Cp*(dppf)RuH (1), an
active and well-defined catalyst for performing one-electron
reductions with H2. The unprecedented electrochemical char-
acterization of two reversible one-electron oxidations of 1 and
the reactivity of its Ru(III) hydride derivative, 1+, suggest that
1+ is a key to the performance of 1 as a “redox switch” catalyst.
The 17-electron metal hydride 1+ is characterized as a weakly
delocalized mixed valence ion. It is the first example of such
a mixed-valence ion in a dppf complex and it inspires specula-
tion on the possible role of similar odd-electron states in the
novel chemistry of dppf catalysts of Pd and Ni.16 Likewise, as
a heterobimetallic catalyst for one-electron reductions with H2
it is a unique model for the H2-reaction center of [NiFe]
hydrogenases which may access similar delocalized oxidation
states to facilitate the activation of H2.
10.96 × 103 cm-1 (ꢀ ) 486), ν1/2 ) 3.38 × 103 cm-1
.
as a weakly delocalized [Ru(III)/Fe(II)] mixed-valence ion (R2
) 3.3 × 10-3, Hab ) 627 cm-1).39,41
Reactivity of 1+. Atom transfer reactions readily convert
1+ to 18-electron Ru(IV) derivatives. For instance, reaction
with Bu3SnH yields [Cp*(dppf)RuH2]+ and reactions with CCl4,
BrMn(CO)5, or I2 yield [Cp*(dppf)RuHX]PF6 derivatives (eq
5). This class of reaction is well-known for M(I) 17-electron
(C5R5)M(CO)3 complexes (M ) Cr, Mo, W), but is not for their
M(III) CpL2MX counterparts.43
[1]PF6 + XY
XY ) HSnBu3, CCl4, BrMn(CO)5, I2
f
[Cp*(dppf)RuX(H)]PF6 (5)
Acknowledgment. We are grateful to the Nebraska NSF-
EPSCoR Program (OSR-9255225) for their support of this
research, to Jody Redepenning and Mike Anderson for elec-
trochemical measurements and expertise, to Steve DiMagno for
use of a Cary 14 with an Olis UV/VIS/NIR spectrophotometer
upgrade, to the Crystalytics Co. (Lincoln, NE) for the use of
their facilities, and Boulder Scientific Co. (Mead, CO) for a
generous gift of C5Me5H.
X ) H, Cl, Br, I
Related to these reactions is a slow process of H-atom transfer
disproportionation in which 1+ is converted to [Cp*(dppf)Ru-
(THF)]PF6 and [Cp*(dppf)RuH2]PF6 (eq 6). This reaction mode
is a common obstacle to the isolation of Ru(III) hydrides related
to 1+.30 The disproportionation of 1+ (1-3 mM) in THF has
a second-order dependence of the concentration of 1+ (k ) 3.5
× 10-2 M-1 s-1 at 23 °C) and is retarded over 10-fold by the
addition of 4 equiv of 1. The latter observation is inconsistent
with a concerted H-atom transfer or a proton transfer initiated
process, but may be explained by a reversible electron-transfer
disproportionation of 1+ (eq 3) followed by a proton transfer
(a kinetic study of this reaction is in progress).44
Supporting Information Available: A complete report of
the X-ray crystallographic study of 1 (21 pages). This material
is contained in many libraries on microfiche, immediately
follows this article in the microfilm version of the journal, can
be ordered from the ACS, and can be downloaded from the
Internet; see any current masthead page for ordering information
and Internet access instructions.
2[1]PF6 THF8 [Cp*(dppf)Ru(THF)]PF6 +
JA952206J
[Cp*(dppf)Ru(H)2]PF6 (6)
(42) (a) Sato, M.; Shintate, H.; Kawata, Y.; Sekino, M.; Katada, M.;
Kawata, S. Organometallics 1994, 13, 1956-62. (b) Sato, M.; Mogi, E.;
Katada, M. Organometallics 1995, 14, 4837-43. (c) Dowling, N.; Henry,
P. Inorg. Chem. 1982, 21, 4088-95. (d) Kotz, J. C.; Getty, E. E.; Lin, L.
Organometallics 1985, 4, 610-12.
“Redox Switch” Catalysis. In our synthesis of 1+ the
ferrocinium ion is reduced by 1 in a stoichiometric reaction (eq
4), but it can also be reduced by H2 in the presence of a base
(TMP) and a catalytic amount of 1 (eq 7). This reduction
(43) Astruc, D. Electron Transfer and Radical Processes in Transition-
Metal Chemistry; VCH: Weinheim, 1995; Chapter 5.
(38) Dowling, N.; Henry, P. M.; Lewis, N. A.; Taube, H. Inorg. Chem.
1981, 20, 2345-48.
(44) Evidence reported by Tilset supports the proposal that Ru(III)
hydrides, generated by the electrochemical oxidation of CpL2RuH, rapidly
protonate their Ru(II) parents.30 The electrochemical stability of 1+, along
with the above noted effect of [1] on its disproportionation, rules out proton
transfer from 1+ to 1 as a kinetically significant reaction mode. Tilset also
reports that small amounts of acetonitrile accelerate the disproportionation
of [CpL2RuH]+ in THF.30 We observe the same effect on 1+ and are
pursuing a study of these H-atom disproportionation mechanisms.
(45) Woo, G.-J.; Wasserfallen, A.; Wolfe, R. S. J. Bacteriol. 1993, 175,
5970-77.
(39) Hab and R2 values and the contribution of the mixed-valence
resonance to the ground state stability of 1+ (∆GR ) Hab2/νmax ) 0.1 kcal/
mol) are calculated40 using the Fe-Ru distance determined for 1 (4.383 Å,
Figure 1).
(40) Creutz, C. Prog. Inorg. Chem. 1983, 30, 1-74.
(41) Few examples of delocalized heterobimetallic mixed-valence com-
plexes exist, but a similar CpRu(II) case with an acetylene bridge between
ferrocene and Ru fragments has been reported (R2 ) 2.8 × 10-2).42a In
addition, four [(NH3)5Ru]2+-cyanoferrocene complexes (R2 ∼ 2 ×
(46) Further studies of the one-electron reduction of organic substrates
10-3 38,42c and a series of complexes with Pd(II) directly bonded to the Cp
)
with H2 catalyzed by 1 are in progress.47
(47) Hembre, R. T.; McQueen, J. S. In submission.
ring of a substituted ferrocene are known.42d