Protonation studies of the new iron carbonyl cyanide trans-[Fe(CO) 3(CN)2]2-: Implications with respect to hydrogenases

Article abstract of DOI:10.1021/ic034455b

The new iron carbonyl cyanide trans-[Fe(CN)₂(CO) ₃]^2-, [2]^2-, forms in high yield via photosubstitution of Fe(CO)₅ with 2 equiv of Et₄NCN. Protonation of [2]^2- generated [HFe(CN)

Full text of DOI:10.1021/ic034455b

Inorg. Chem. 2003, 42, 5046−5048
Protonation Studies of the New Iron Carbonyl Cyanide
trans-[Fe(CO)3(CN)2]2-: Implications with Respect to Hydrogenases
Ajay Kayal and Thomas B. Rauchfuss*
School of Chemical Sciences, UniVersity of Illinois at Urbana-Champaign, Urbana, Illinois 61801
Received April 30, 2003
Scheme 1. Synthesis of trans-[Fe(CO)3(CN)2]2-
The new iron carbonyl cyanide trans-[Fe(CN)2(CO)3]2-, [2]2-, forms
in high yield via photosubstitution of Fe(CO)5 with 2 equiv of
Et4NCN. Protonation of [2]2- generated [HFe(CN)2(CO)3]-, [2H]-,
the first H−Fe−CN−CO species. Further protonation gives dihy-
drogen. This simple system provides insights into hydrogen
evolution by the hydrogenase enzymes, which also feature H−Fe−
CN−CO centers.
Iron carbonyl cyanide complexes1-7 have assumed special
importance in light of the discovery of Fe-CO-CN motifs
in the active sites of both Fe-only and [Ni-Fe] hydrogenases,
the enzymes responsible for virtually all bioprocessing of
dihydrogen.8-10 Thus, Fe-CO-CN-H species may be
considered as minimalist biomimetic models of Fe-only and
possibly the [Ni-Fe] hydrogenases. Although several Fe-
CO-CN species have been described, none features hydro-
genic ligands, the sine qua non of hydrogenases.11 Herein,
we describe the synthesis of the first hydrides of iron
carbonyl cyanides and the conversion of these hydrides into
dihydrogen. The development of functional analogues12,13 of
the hydrogenase enzymes could lay the foundation for new
fuel cell and related clean-energy applications.14,15
Initial studies examined the protonation of the well-known
Fe(0) complex K[Fe(CO)4CN] (K[1]).5,6 Treatment of K[1]
with 1 equiv of HCl (2.0 M solution in Et2O) in MeCN
solution (-30 °C) leads to an 1:8 mixture of HFe(CO)4CN
(1H, δ -9.67) and Fe(CO)4CNH (broad, δ 8.50). This
mixture readily decomposes to, inter alia, Fe(CO)5. The
instability of 1H can be attributed to the facile elimination
and lability of HCN, which is facilitated by the four electron-
withdrawing CO groups. The Fe sites of both families of
hydrogenases feature donor ligands (thiolates) in addition
to CO and CN-; furthermore, the Fe site in [NiFe] hydro-
genase features two CN- ligands.8 This logic suggested that
the replacement of a further CO by CN- might stabilize the
Fe-H bond.
The Et4N+ salt of [Fe(CN)2(CO)3]2- ([22-]) can be
synthesized on a gram scale from Fe(CO)5 by photosubsti-
tution with 2 equiv of Et4NCN (Scheme 1). The product was
isolated as colorless crystals from Et2O-MeCN (70% yield).
The same salt can also be generated by stepwise photosub-
stitution from [1]- as well as by thermal displacement of
bda from [Fe(CO)3(bda)]16 (bda ) PhCHdCHCOCH3). The
salt (Et4N)2[Fe(CN)2(CO)3] air oxidizes to give a mixture
of [Fe(CN)5CO]3- and trans-[Fe(CN)4(CO)2]2-.2,3,7
Crystallographic analysis of (Et4N)2[2] reveals the expected
trigonal bipyramidal (D3h) arrangement of the CN- and CO
ligands (Figure 1). The Fe-CO (1.779(2) Å) and Fe-CN
(1.925(2) Å) distances indicate extensive π-back-bonding in
the Fe-CO bond and the predominantly σ-bond character
* Author to whom correspondence should be addressed. E-mail:
rauchfuz@uiuc.edu.
(1) Jiang, J.; Koch, S. A. Inorg. Chem. 2002, 41, 158-160.
(2) Jiang, J. F.; Acunzo, A.; Koch, S. A. J. Am. Chem. Soc. 2001, 123,
12109-12110.
(3) Jiang, J. F.; Koch, S. A. Angew. Chem., Int. Ed. 2001, 40, 2629-
2631.
(4) Rauchfuss, T. B.; Contakes, S. M.; Hsu, S. C. N.; Reynolds, M. A.;
Wilson, S. R. J. Am. Chem. Soc. 2001, 123, 6933-6934.
(5) Goldfield, S. A.; Raymond, K. N. Inorg. Chem. 1974, 13, 770-775.
(6) Ruff, J. K. Inorg. Chem. 1969, 8, 86-89.
(7) Contakes, S. M.; Hsu, S. C. N.; Rauchfuss, T. B.; Wilson, S. R. Inorg.
Chem. 2002, 41, 1670-1678.
(8) Frey, M. ChemBioChem 2002, 3, 153-160.
(9) Darensbourg, M. Y.; Lyon, E. J.; Smee, J. J. Coord. Chem. ReV. 2000,
206-207, 533-561.
(10) Fontecilla-Camps, J. C.; Ragsdale, S. W. AdV. Inorg. Chem. 1999,
47, 283-333.
(11) Huhmann-Vincent, J.; Scott, B. L.; Kubas, G. J. Inorg. Chim. Acta
1999, 294, 240-254.
(12) Gloaguen, F.; Lawrence, J. D.; Rauchfuss, T. B. J. Am. Chem. Soc.
2001, 123, 9476-9477.
(15) Hydrogen as a Fuel: Learning from Nature; Cammack, R., Ed.; Taylor
and Francis Inc.: New York, 2001.
(16) Domingos, A. J. P.; Howell, J. A. S.; Johnson, B. F. G.; Lewis, J.
Inorg. Synth. 1976, 16, 103-107.
(13) Darensbourg, M. Y.; Lyon, E. J.; Zhao, X.; Georgakaki, I. P. Proc.
Natl. Acad. Sci. U.S.A. 2003, 100, 3683-3688.
(14) Cammack, R. Nature 1999, 397, 214-215.
5046 Inorganic Chemistry, Vol. 42, No. 17, 2003
10.1021/ic034455b CCC: $25.00 © 2003 American Chemical Society
Published on Web 07/31/2003

COMMUNICATION
Figure 2. 1H NMR (500 MHz, CD3CN) spectrum (hydride region) (left)
and 13C NMR (125 MHz, CD3CN) spectrum (CN region) (right) of [Et4N]-
[HFe(CO)3(CN)2].
Figure 1. Structure of the dianion ([2]2-) in (Et4N)2[Fe(CO)3(CN)2] with
thermal ellipsoids (50% probability) and selected atom labels; hydrogen
atoms are not shown. Selected bond distances (Å) and angles (deg): Fe-
(1)-C(1), 1.785(2); Fe(1)-C(2), 1.780(2); Fe(1)-C(3), 1.773(2); Fe(1)-
C(4), 1.921(2); Fe(1)-C(5), 1.929(2); C-O(av), 1.161(2); C-N(av),
1.160(2); C(4)-Fe(1)-C(5), 177.7(1); Fe(1)-C-O(av), 178.9(2); Fe(1)-
C-N(av), 178.4(2); OC-Fe-CO(av), 120.0(1); NC-Fe-CO(av), 90.0-
(1).
Scheme 2. Isomers of [HFe(CO)3(CN)2]-, [2H]-
Figure 3. IR spectra of (Et4N)2[2] (a) and (Et4N)[2H] (b) (CH3CN).
The most notable feature of the IR spectrum of [2H]- is
the ca. 175 cm-1 shift of the (νCO av
stretching frequency
upon protonation (Figure 3), indicative of substantial elec-
tronic rearrangement associated with conversion of the five-
coordinate zerovalent metal center to the divalent octahedral
)
of the Fe-CN bond. In octahedral and square planar cyano-
carbonyls, CO is typically trans to the strong donor CN-.
The IR spectrum of (Et4N)2[2] in MeCN, displaying νCN
and νCO bands at 2059 and 1844 cm-1, respectively, is
consistent with the solid state structure as is the 13C NMR
spectrum. The cyclic voltammogram of (Et4N)2[2] shows an
irreversible oxidation at -400 mV; in contrast, [1]- oxidizes,
again irreversibly, at +150 mV (all potentials vs Ag/AgCl
in MeCN). The 550 mV shift in the Eox and the ca. 175 cm-1
shift in νCO both indicate that [2]2- is substantially more
electron-rich than [1]-.
derivative. The shift is comparable to the ∆(νCO)av shift of
166 cm-1 previously recorded for the protonation of
[Co(CH3CN)6][Co(CO)4]2 in MeCN.17 Although splitting of
the νCO bands is not observed, νCO and νCN bands in the IR
spectra of [2H]- in MeCN and [Fe(CO)3(CN)2Br]- in THF,
which adopts isomer B (in Scheme 2), are very similar.18
(Et4N)2[2] is protonated by NH4PF6 (pKa ) 16 in MeCN),
whereas 1H is fully deprotonated by C6H5NH2 (pKa ) 11).
Thus, 1H is >105 more acidic than [2H]-. These data further
establish that the protonation in [2]2- occurs at the Fe(0)
center and not at the CN- to yield the hydride, [2H]-,
although protonation at the metal center in cyanometalates
is rare.19,20
Protonation of [2]2- with 1 equiv of p-TsOH (or NH4-
PF6) gave the hydride [HFe(CO)3(CN)2]- (Et4N[2H]), three
isomers of which are possible (Scheme 2).
Low temperature (-30 to 0 °C, MeCN solution) pro-
tonations revealed the formation of two major isomers (δ
-7.90, -8.33) with a third minor isomer (δ -11.2).
Protonation is complete at 10 °C with the three isomers at δ
-7.90, -8.30, -11.2 in the ratio of 10:6:1, regardless of
the solvent used for the protonation (MeNO2, CH2Cl2,
MeCN).
Further protonation of [2H]- in principle should afford
Fe(CN)2(CO)3(H2), which would be of obvious relevance to
the H2-evolution function of the hydrogenases. Treatment
of [2H]- with excess p-TsOH (pKa ) 8) at room temperature
completely consumed the hydride and gave 67% yield H2,
analyzed by 1H NMR spectroscopy and gas chromatography.
The evolution of H2 is associated with an equimolar evolution
of CO (analyzed by gas chromatography) and the formation
of a yet unidentified Fe-containing product (νCN and νCO
The 1H-coupled 13C NMR spectrum (Figure 2) of [2H*]-
at 10 °C consists of two intense doublets in the CN region
(δ 135.2 (J ) 16 Hz), 132.9 (J ) 13 Hz)) consistent with
the predominance of isomers A and B (Scheme 2), both of
which have similar coupling to 13CN as observed in the 13
C
NMR spectra of enriched samples. It proved difficult to
separate a pure sample of [2H]- from the co-formed
conjugate salt; however, a solid sample, analytically found
to be a 1:1 mixture of (K)[2H] and co-formed [K(18-c-6)]-
Cl was obtained by the protonation of [K(18-c-6)2][2] with
anhydrous HCl in MeCN at 0 °C followed by precipitation
with diethyl ether.
(17) Moore, E. J.; Sullivan, J. M.; Norton, J. R. J. Am. Chem. Soc. 1986,
108, 2257-2263.
(18) Liaw, W. F.; Lee, J. H.; Gau, H. B.; Chen, C. H.; Jung, S. J.; Hung,
C. H.; Chen, W. Y.; Hu, C. H.; Lee, G. H. J. Am. Chem. Soc. 2002,
124, 1680-1688.
(19) Gloaguen, F.; Lawrence, J. D.; Rauchfuss, T. B.; Be´nard, M.; Rohmer,
M.-M. Inorg. Chem. 2002, 41, 6573-6582.
(20) Bianchini, C.; Laschi, F.; Ottaviani, M. F.; Peruzzini, M.; Zanello,
P.; Zanobini, F. Organometallics 1989, 8, 893-899.
Inorganic Chemistry, Vol. 42, No. 17, 2003 5047

COMMUNICATION
Scheme 3
In summary, a minimalist, purely inorganic representation
of the Fe site in the metallo hydrogenases has been prepared
via the synthon [Fe(CN)2(CO)3]2-, the newest and most
reactive member of the growing iron carbonyl cyanide
family.1-7 The new dianion gives rise to the first Fe-CN-
H-CO species and further connects to the formation of
dihydrogen via a pathway that implicates an FeII-CN-CO-
H2 intermediate.
Acknowledgment. This research was supported by the
NIH and DOE.
bands at 2108 and 2069 cm-1 in MeCN). Interestingly,
treatment of [2H]- with excess p-TsOD gave mainly D2,
2
Supporting Information Available: Crystallographic informa-
tion file (CIF) for [Et4N]2[2] and experimental details. This material
is available free of charge via the Internet at http://pubs.acs.org.
identified by H NMR spectroscopy. This finding can be
rationalized by the occurrence of a rapid protonation-
deprotonation equilibrium of the proposed intermediate,
Fe(CN)2(CO)3(H2-xDx) (Scheme 3).
IC034455B
5048 Inorganic Chemistry, Vol. 42, No. 17, 2003