fac-[Fe11(CN)3(CO)3]- and cis-[FE11(CN)4(CO)2]2-: New members of the class of [Fe11(CN)x(CO)y] compounds

Article abstract of DOI:10.1021/ic015604y

The reaction of Fe(CO)₄I₂ with NaCN or KCN in methanol gives M[Fe¹¹(CN)₃(CO)₃] (M = Na, K), which can be converted to [Ph₄E][Fe¹¹(CN)₃(CO)₃] (E = P, As). Th

Full text of DOI:10.1021/ic015604y

Inorg. Chem. 2002, 41, 158−160
II
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fac-[Fe (CN)3(CO)3]- and cis-[Fe (CN)4(CO)2]2-: New Members of the
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Class of [Fe (CN)x(CO)y] Compounds
Jianfeng Jiang and Stephen A. Koch*
Department of Chemistry, State UniVersity of New York at Stony Brook,
Stony Brook, New York 11794-3400
Received October 16, 2001
The reaction of Fe(CO)4I2 with NaCN or KCN in methanol gives
thesized in the eighteenth century, and [FeII(CN)5(CO)]3-,
which was first reported in the nineteenth century.5,6 [FeII-
(CO)6]2+ was structurally characterized in 1999,7 and we8
and others9 have just reported the synthesis of trans-[FeII-
(CN)4(CO)2]2-. Herein we report the synthesis and charac-
terization of two additional members of this series: fac-
[FeII(CN)3(CO)3]1- (1) and cis-[FeII(CN)4(CO)2]2- (2).
II
M[Fe (CN)3(CO)3] (M) Na, K), which can be converted to [Ph E]-
4
II
II
[Fe (CN)3(CO)3] (E ) P, As). The structure of the fac-[Fe (CN)3-
(CO)3]- (1) anion was established by the X-ray crystal structures
of K-1 and [Ph4As]-1‚H O. Compound 1 has also been character-
2
ized by IR, Raman, and 13C NMR. The coupling of the CO and
CN stretching modes in the vibrational spectra of 1 have been
The reactions of cis-Fe(CO)4I2 with 3 equiv of NaCN or
KCN in refluxing methanol give M[FeII(CN)3(CO)3] (M )
Na, K), which can be converted to [Ph4E][FeII(CN)3(CO)3]
(E ) P, As).10 The facial isomeric structure of the
[FeII(CN)3(CO)3]- anion was established by a combination
of structural and spectroscopic studies. The FAB negative
ion mass spectrum of Ph4P-1 shows the parent anion and
sequential loss of the three CO ligands. The 13C NMR spec-
trum of Ph4P-1 in DMSO shows a single resonance for CO
at 201.47 ppm and a single resonance for CN at 126.5 ppm.
K-1 and [Ph4As]-1‚H2O were the subjects of X-ray
crystallographic investigations.11,12 In both complexes the
[FeII(CN)3(CO)3]- anions are structurally equivalent with
Fe-C(N)av and Fe-C(O)av of 1.925(3) and 1.828(4) Å in
II
analyzed. The reaction of Na-1 with NaCN gives Na2 cis-[Fe -
(CN)4(CO)2] (Na-2), which has been structurally characterized as
[Ph4P][Et4N]-2. Compounds 1 and 2 are additional members of
II
the series of compounds of general formula [Fe (CN)(6-x)(CO)x](x-4)
.
[Fe(CN)x(CO)y] compounds are of interest due to the occurrence
of such coordination modes in hydrogenase enzymes.
Carbon monoxide and cyanide are two of the most
fundamental ligands in inorganic chemistry. The discovery
of [Fe(CN)(CO)] coordination centers in NiFe and Fe-only
hydrogenase enzymes1 has caused us to focus on the series
of compounds with the formula [FeII(CN)(6-x)(CO)x](x-4). In
addition to their basic interest,2 these complexes may be
useful in the synthesis of realistic analogues for the active
sites in these enzymes and as starting materials for the
synthesis of extended solid-state structures3 or supramolecular
assemblies.4 Until recently only two members of this series
of compounds were known: [FeII(CN)6]4-, which was syn-
(4) (a) Contakes, S. M.; Rauchfuss, T. B. Chem. Commun. (Cambridge)
2001, 553-554. (b) Contakes, S. M.; Klausmeyer, K. K.; Milberg, R.
M.; Wilson, S. R.; Rauchfuss, T. B. Organometallics 1998, 17, 3633-
3635. (c) Larionova, J.; Gross, M.; Pilkington, M.; Andres, H.;
Stoeckli-Evans, H.; Gudel, H. U.; Decurtins, S. Angew. Chem., Int.
Ed. 2000, 39, 1605. (d) Sokol, J. J.; Shores, M. P.; Long, J. R. Angew.
Chem., Int. Ed. 2001, 40, 236-239.
(5) Muller, J. A. C. R. Hebd. Seances Acad. Sci. 1887, 104, 992.
(6) Jiang, J.; Acunzo, A.; Koch, S. A. J. Am. Chem. Soc. 2001, 123,
12109-12120.
(7) (a) Bley, B.; Willner, H.; Aubke, F. Inorg. Chem. 1997, 36, 158-
160. (b) Bernhardt, E.; Bley, B.; Wartchow, R.; Willner, H.; Bill, E.;
Kuhn, P.; Sham, I. H. T.; Bodenbinder, M.; Brochler, R.; Aubke, F.
J. Am. Chem. Soc. 1999, 121, 7188-7200.
(8) Jiang, J.; Koch, S. A. Angew. Chem., Int. Ed. 2001, 40, 2629-2631.
(9) Rauchfuss, T. B.; Contakes, S. M.; Hsu, S. C. N.; Reynolds, M. A.;
Wilson, S. R. J. Am. Chem. Soc. 2001, 123, 6933-6934.
(10) Preparation of [Ph4P]-fac-[FeII(CN)3(CO)3]: Fe(CO)4I2 (0.889 g, 2.11
mmol) and NaCN (0.320 g, 6.53 mmol) were refluxed in 35 mL of
MeOH for 20 min. The MeOH was removed, and the residue was
washed with CH3CN. The white powder was dissolved in H2O, and
an aqueous solution of PPh4Cl (0.80 g) was added. The product
crystallized in 60% yield.
* Author to whom correspondence should be addressed. E-mail:
stephen.koch@sunysb.edu.
(1) (a) Nicolet, Y.; de Lacey, A. L.; Vernede, X.; Fernandez, V. M.;
Hatchikian, E. C.; Fontecilla-Camps, J. C. J. Am. Chem. Soc. 2001,
123, 1596-1601. (b) De Lacey, A. L.; Stadler, C.; Cavazza, C.;
Hatchikian, E. C.; Fernandez, V. M. J. Am. Chem. Soc. 2000, 122
(45), 11232-11233. (c) Bennett, B.; Lemon, B. J.; Peters, J. W.
Biochemistry 2000, 39, 7455-7460. (d) Nicolet, Y.; Lemon, B. J.;
Fontecilla-Camps, J. C.; Peters, J. W. Trends Biochem. Sci. 2000, 25,
138-143. (e) Pierik, A. J.; Roseboom, W.; Happe, R. P.; Bagley, K.
A.; Albracht, S. P. J. J. Biol. Chem. 1999, 274, 3331-3337. (f)
Maroney, M. J.; Bryngelson, P. A. J. Biol. Inorg. Chem. 2001, 6, 453-
459. (g) Fontecilla-Camps, J. C.; Ragsdale, S. W. AdV. Inorg. Chem.
1999, 47, 283-333. (h) Adams, M. W. W.; Stiefel, E. I. Curr. Opin.
Chem. Biol. 2000, 4, 214-220.
(2) (a) Sharpe, A. G. The Chemistry of Cyano Complexes of the Transition
Metals; Academic Press: New York, 1976. (b) Dunbar, K. R.; Heintz,
R. A. Prog. Inorg. Chem. 1997, 45, 283 and references therein.
(3) Iwamoto, T.; Nishikiori, S.; Kitazawa, T.; Yuge, H. J. Chem. Soc.,
Dalton Trans. 1997, 4127-4136.
(11) Crystal data for K[Fe(CN)3(CO)3]: rhombohedral space group R3h. a
) b ) 6.9564(13) Å, c ) 37.063(1) Å, R ) â ) 90°, γ ) 120°,
volume ) 1553.2(6) Å3, Z ) 6. Full-matrix least-squares on F2 with
R1 ) 0.0328, wR2 ) 0.0821 for 509 independent reflections.
158 Inorganic Chemistry, Vol. 41, No. 2, 2002
10.1021/ic015604y CCC: $22.00 © 2002 American Chemical Society
Published on Web 12/22/2001

COMMUNICATION
Figure 1. Structural diagram of a fragment of the solid-state structure of
K[FeII(CN)3(CO)3]. Selected distances (Å) and angles (deg): Fe-C2 1.828-
(4), Fe-C1 1.925(3), K-N1 2.813(3), N1-C1 1.133(4), C2-O1 1.124-
(4), C2-Fe-C2′ 96.12(13), C1-Fe-C1′ 87.43(13), C1-Fe-C2 88.34(13),
K1-N1-K1′ 90.05(9), K1-N1-C1 130.0(2).
Figure 2. Vibrational spectra of Na-fac-[FeII(CN)3(CO)3] in H2O: (a) IR
spectrum; (b) Raman spectrum; (c) Raman spectrum of 13CO-substituted
compound.
K-1 and 1.935(4) and 1.810(9) Å in [Ph4As]-1‚H2O. In
[Ph4As]-1‚H2O, two of the cyano groups are engaged in a
hydrogen-bonding interaction with a water molecule. K-1
crystallized in the rhombohedral space group R3h with the
result that the [FeII(CN)3(CO)3]- anions have crystallographic
C3 symmetry (Figure 1). The CN- ligands interact with the
K cations and provide structural evidence for the ordering
of the CN- and CO ligands in the anions. The K and the N
atoms of the cyano groups are arranged in a 2-dimensional
CdI2 type structure with each K interacting with six µ2-N
atoms in a distorted octahedral arrangement. The Fe(CO)3
groups decorate the top and bottom of the layers with no
additional molecules between the layers. These compounds
are the first structurally characterized [M(CN)3(CO)3]n-
complexes, but fac-[Mo(CN)3(CO)3]3- has been reported.13
As can be anticipated from the now classic theory of M-CO
bonding,14 there is a direct relationship between the Fe-
C(O) distance and the CO stretching frequency (in DMF)15
in a series of [Fe(CN)(CO)] complexes: [(PS3)FeII(CN)(CO)]-
(1.71 Å, 1904 cm-1);16 [FeII(CN)5(CO)]3- (1.75 Å, 1931
cm-1);6 trans-[Fe(CN)4(CO)2]2- (1.80 Å, 2029 cm-1);8 and
1 (1.82 Å, 2085 cm-1).
compared to the CO stretches. As predicted by group theory,
the Raman transitions are coincident with the IR bands. Since
the compound is a light yellow, the intensities of the bands
in the Raman spectra are not affected by resonance enhance-
ment. The low intensity of totally symmetric stretching
normal modes in the Raman spectra of metal-carbonyl
compounds is well precedented.17
Isotopic substitution with 13CO (Figure 2) results not only
in the expected large shifts of the bands assigned to CO but
also significant shifts in the bands assigned as CN stretches.18
This is an indication of coupling between the (predominately)
CO stretching frequencies and the (predominately) CN
stretching frequencies of like symmetries. A Cotton-
Kraihanzel analysis19 of the vibrational spectra including data
from the 13CO and 13CN labeled species18 (assuming the
interactions of just the CO and CN stretching modes)
quantifies the interaction: FCO 18.02 mdyn/Å, FCN 17.42,
FCO-CO 0.26, FCN-CN, 0.02, FCO-CN (cis) 0.05, FCO-CN (trans)
0.19. The observation of CN-CO coupling in 1 results from
the relatively small separation in the frequencies of the CO
and CN stretching modes.20,21
In the [FeII(CN)(6-x)(CO)x](x-4) series, both the CO and CN
stretching frequencies are shifted to higher energy with
increasing CN- by CO substitution.8 The 13C NMR reso-
nances for the CN- ligands shift to higher frequency as the
CO/CN- ratio increases.8
The solvent dependence (H-bonding vs aprotic solvents)
of the energy of CO vibrational bands in 122 are smaller than
those in trans-[FeII(CN)4(CO)2]2- and [FeII(CN)5(CO)]3-.6,8
The solvent induces shifts in the CO stretching frequencies,
The vibrational spectra of 1 (Figure 2) provide a textbook
example of a C3V symmetric complex. The IR spectrum of
Na-1 in H2O shows two IR-allowed CO stretches at 2096
and 2121 cm-1 with the degenerate asymmetric E stretch at
lower energy and greater intensity than the A1 symmetric
stretch. The two IR-allowed CN stretches, which occur at
2140 and 2162 cm-1, are markedly reduced in intensity
(12) Crystal data for [Ph4As][Fe(CN)3(CO)3]‚H2O: triclinic space group
P1h, a ) 9.1789(10) Å, b ) 12.6561(14) Å, c ) 13.9666(15) Å, R )
116.693(2)°, â ) 102.184(2)°, γ ) 90.213(2)°, volume ) 1407.9(3)
Å3, Z ) 2. Full-matrix least-squares on F2 with R1 ) 0.0511, wR2 )
0.1305 for 4047 independent reflections. Selected bond distances (Å)
and angles (deg): Fe-C1 1.820(8), Fe1-C2 1.804(8), Fe1-C3 1.805-
(8), Fe1-C(4) 1.937(7), Fe1-C(5) 1.931(8), Fe1-C6 1.938(8), C2-
Fe1-C3 95.2(3), C2-Fe1-C1 94.1(3), C3-Fe1-C1 95.1(3).
(13) Contakes, S. M.; Rauchfuss, T. B. Angew. Chem., Int. Ed. 2000, 39,
1984-1986.
(14) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. AdVanced
Inorganic Chemistry; John Wiley & Sons: New York, 1999.
(15) Where appropriate the average of the symmetric and asymmetric νCO
is given.
(16) Hsu, H.-F.; Koch, S. A.; Popescu, C. V.; Mu¨nck, E. J. Am. Chem.
Soc. 1997, 119, 8371-8372.
(17) (a) Kettle, S. F. A.; Paul, I.; Stamper, P. J. J. Chem. Soc., Chem.
Commun. 1971, 235-236. (b) Kettle, S. F. A.; Paul, I.; Stamper, P. J.
J. Chem. Soc., Dalton Trans. 1972, 2413-2418.
(18) IR spectrum (H2O) of Na[Fe(CN)3(13CO)3]: 2138 cm-1 (w), 2084
(m), 2048 (s); Na[Fe(13CN)3(CO)3] 2147 cm-1 (m), 2101 (m) (sh),
2087 (s).
(19) Cotton, F. A.; Kraihanzel, C. S. J. Am. Chem. Soc. 1962, 84, 4432.
(20) Darensbourg, D. J. Inorg. Chem. 1972, 11, 1606.
(21) Lai, C. H.; Lee, W. Z.; Miller, M. L.; Reibenspies, J. H.; Darensbourg,
D. J.; Darensbourg, M. Y. J. Am. Chem. Soc. 1998, 120, 10103-
10114.
(22) The IR spectrum of Ph4P-1 in DMF (2062 (s), 2108 (s), 2136 (w),
2148 (w) cm-1) shows shifts of 34 cm-1 (E(CO)) and 13 cm-1 (A1(CO)
to lower energy compared to those in H2O.
)
Inorganic Chemistry, Vol. 41, No. 2, 2002 159

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symmetry with the Fe-C(O) distance of 1.758(6) Å and the
two independent Fe-C(N) distances of 1.935(5) and 1.952-
(6) Å. The shorter Fe-C(O) distance in the cis vs trans
isomers (1.758(6) vs 1.800(5) Å) is consistent with the
greater π-back-bonding by two cis carbonyl groups. This is
the first example of a cis-[Fe(CN)4L2]n- compound with two
monodentate ligands. There are only a handful of structurally
characterized examples of such compounds for any transition
metal.24 The cis isomer is considerably more reactive than
the trans isomer. Aqueous solutions of 2 decay on the order
of minutes into a mixture of products including 1 and
[FeII(CN)5(CO)]3-. As monitored by IR spectroscopy, added
NaCN inhibits this reaction.
In the reaction mixtures of Fe(CO)4I2 with NaCN under
the conditions we investigated, there is no indication of the
IR bands anticipated for the mer-[Fe(CN)3(CO)3]- isomer
or those found for the trans-[FeII(CN)4(CO)2]2- isomer.8,9
We have recently reported that trans-[FeII(CN)4(CO)2]2- and
[FeII(CN)5(CO)]3- can be synthesized by the simple reaction
of FeCl2 with NaCN in H2O or MeOH under an atmosphere
of CO.6,8 The characteristic IR bands for neither 1 nor 2 have
ever been observed in these reactions. The combined results
suggest that the products in both types of reactions are
produced under kinetic control.
Figure 3. Structural diagram of cis-[FeII(CN)4(CO)2]2-. Selected distances
(Å) and angles (deg): Fe-C1 1.758(6), Fe-C2 1.952(6), Fe-C3 1.935-
(2), C1-O1 1.145(6), C2-N2 1.117(7), C3-N3 1.126(6), C1-Fe-C1′
92.5(4), C1-Fe-C2 91.5(2), C1-Fe-C3 88.7(2), C1-Fe-C3′ 178.5(3),
C3-Fe-C3′ 90.0(3), C2-Fe-C3 88.7(2).
and the associated shifts in the redox potential (including
[FeII(CN)6]4-) are dependent on the number of CN- ligands
and the associated increase in the negative charge in the
[FeII(CN)(6-x)(CO)x](x-4) series.6,8 The electrochemical studies
of 1 show no oxidation or reduction processes from -1.5 to
+ 2.8 V (vs SCE) in CH3CN. An extrapolation from our
previous observations for [FeII(CN)(6-x)(CO)x](x-4) (x ) 0,
1, 2) would estimate that the Fe3+/Fe2+ couple should be
>+2.4 V (vs SCE) in aprotic solvents.8 The CO ligands
destabilize oxidative processes while the CN- ligands
destabilize reductive processes.
The reaction of Na-1 with NaCN in MeOH generated cis-
Na2[FeII(CN)4(CO)2], Na2-2, which can also be made directly
from the reaction of cis-Fe(CO)4I2 with 4 equiv of NaCN in
MeOH. The IR spectrum of 2 in DMF, which is consistent
with its C2V structure, shows two equally intense CO stretches
at 1967 and 2022 cm-1 and two weaker CN stretches at 2080
and 2106 cm-1. The X-ray structure of [PPh4][NEt4]-2
confirms the cis arrangement of the CO ligands (Figure 3).23
The cis-[FeII(CN)4(CO)2]2- anion has crystallographic C2
Although metal-cyanide chemistry is nearly 300 years
old, the recent resurgence in this field suggests that it will
continue to provide important contributions to basic inorganic
chemistry with implications to metalloenzymes and materials
chemistry.
Acknowledgment. This work was supported by a Na-
tional Institutes of Health grant (GM 58000). We acknowl-
edge Dr. Alasdair Bell and Prof. Peter Tonge for help
obtaining the Raman spectra and Prof. Michelle Millar and
Albert Haim for discussion.
Supporting Information Available: X-ray crystallographic files
(CIF) for K[Fe(CN)3(CO)3], [Ph4As][Fe(CN)3(CO)3]‚H2O, and
[Ph4P][Et4N]-cis-[Fe(CN)4(CO)2]. This material is available free of
charge via the Internet at http://pubs.acs.org.
(23) The crystals were obtained from the reaction of Ph4P-1 with Et4NCN
in CH2Cl2. Crystal data for [Ph4P][Et4N][Fe(CN)4(CO)2]: monoclinic
space group C2/c, a ) 11.476(2) Å, b ) 29.561(6) Å, c ) 11.246(2)
Å, â ) 109.23(3)°, volume ) 3602.2(12) Å3, Z ) 4. Full-matrix least-
squares on F2 with R1 ) 0.0589, wR2 ) 0.1605 for 2609 independent
reflections.
IC015604Y
(24) (a) Weigand, W.; Nagel, U.; Beck, W. Z. Naturforsch. 1988, B43,
328. (b) Smith, J.; Purcell, W.; Lamprecht, G. J.; Roodt, A. Polyhedron
1996, 15, 1389.
160 Inorganic Chemistry, Vol. 41, No. 2, 2002