Heterobimetallics of nickel-iron dinitrosyl: Electronic control by chelate and diatomic ligands

Article abstract of DOI:10.1021/ic990631y

Reaction of [PPN][Fe(NO)₂(SePh)₂] (1) with dimeric [Ni(μ-SCH₂CH₂SCH₂CH₂S)]₂ in the presence of additional NO₂^- produced the neutral heterobimetallic [(ON)Ni{μ-SCH₂CH₂)₂S}Fe(NO)₂] complex (2). The X-ray crystal structures of 1 and 2 show distorted tetrahedral iron dinitrosyl groups, assigned according to the Feltham-Enemark notation as {Fe(NO)₂}⁹. The Fe-NO bonds are off linearity by an average of ? 10°for compounds 1 and 2, while a more linear Ni-NO coordination with a Ni-NO distance of 1.644(2) A was found in 2. The ν(NO) value of complex 2 is consistent with an assignment for {Ni(NO)}⁹ of Ni⁰(NO)⁺ as is known for analogous phosphine derivatives, P₃Ni⁰(NO)⁺. EPR signals of g values = 2.02-2.03 confirmed the existence of the odd electron in the chalcogenated {Fe(NO)₂}⁹ compounds. Two {Fe(NO)₂}¹⁰ complexes coordinated by the nickel(II) dithiolate, (bismercaptoethanediazacyclooctane)nickel(II), (Ni-1), (Ni-1)Fe(CO)(NO)₂ and (Ni-1)Fe(NO)₂, were prepared for comparison to the Ni⁰(NO)⁺ derivative and other monomeric and homodimetallic derivatives of the Fe(NO)₂ fragment. While the oxidation level of Fe(NO)₂ is the primary determinant of ν(NO) values, they are also highly sensitive to ancillary ligands and, thereby, the distal metal influence through the bridging thiolate donor.

Full text of DOI:10.1021/ic990631y

480
Inorg. Chem. 2000, 39, 480-484
Heterobimetallics of Nickel-Iron Dinitrosyl: Electronic Control by Chelate and Diatomic
Ligands
Wen-Feng Liaw,*,† Chao-Yi Chiang,† Gene-Hsiang Lee,‡ Shie-Ming Peng,‡
Chia-Huei Lai,§ and Marcetta Y. Darensbourg*,§
Department of Chemistry, National Changhua University of Education, Changhua 50058, Taiwan,
Department of Chemistry & Instrumentation Center, National Taiwan University, Taipei 10764, Taiwan,
and Department of Chemistry, Texas A&M University, College Station, Texas 77843
ReceiVed June 2, 1999
Reaction of [PPN][Fe(NO)2(SePh)2] (1) with dimeric [Ni(µ-SCH2CH2SCH2CH2S)]2 in the presence of additional
NO2- produced the neutral heterobimetallic [(ON)Ni{(µ-SCH2CH2)2S}Fe(NO)2] complex (2). The X-ray crystal
structures of 1 and 2 show distorted tetrahedral iron dinitrosyl groups, assigned according to the Feltham-Enemark
notation as {Fe(NO)2}9. The Fe-NO bonds are off linearity by an average of ≈10° for compounds 1 and 2, while
a more linear Ni-NO coordination with a Ni-NO distance of 1.644(2) Å was found in 2. The ν(NO) value of
complex 2 is consistent with an assignment for {Ni(NO)}9 of Ni0(NO)+ as is known for analogous phosphine
derivatives, P3Ni0(NO)+. EPR signals of g values ) 2.02-2.03 confirmed the existence of the odd electron in the
chalcogenated {Fe(NO)2}9 compounds. Two {Fe(NO)2}10 complexes coordinated by the nickel(II) dithiolate,
(bismercaptoethanediazacyclooctane)nickel(II), (Ni-1), (Ni-1)Fe(CO)(NO)2 and (Ni-1)Fe(NO)2, were prepared for
comparison to the Ni0(NO)+ derivative and other monomeric and homodimetallic derivatives of the Fe(NO)2
fragment. While the oxidation level of Fe(NO)2 is the primary determinant of ν(NO) values, they are also highly
sensitive to ancillary ligands and, thereby, the distal metal influence through the bridging thiolate donor.
Introduction
ligands as spectroscopic probes if well-characterized model
complexes are made available.
The selective synthesis of heterobimetallic complexes presents
challenges, particularly in the area of thiolate-bridged M(µ-
SR)2M′ systems where ideal precursors are apt to exist as stable
polynuclear species and product speciation into homopoly-
nuclear compounds is also prevalent. Interesting targets of this
type are in biomimicry of the (CysS)2Ni(µ-SCys)2Fe(CN)2(CO)
active site of [NiFe]H2ase, intriguing for the heterobimetallic
site and also for the initial observation of nature’s use of
diatomic ligands in catalytically active site construction.1 Since
then, the Fe-only H2ase has likewise been established to contain
diatomic ligands in its binuclear active site.2,3 While the origin
and role of the diatomic ligands are as much a question as is
the requirement for two metals in the active site, two different
metals in the case of [NiFe]H2ase, advances in understanding
are expected to result from the advantages offered by diatomic
Recently, Osterloh and co-workers reported that a Ni(µ-
SR)2Fe(NO)2 heterobimetallic was produced upon displacement
of two carbonyls of Fe(CO)2(NO)2 by a square planar N2S2
complex of Ni(II).4 The resultant iron dinitrosyl unit maintained
quite low ν(NO) values of <1700 cm-1, as in its precursor Fe-
(CO)2(NO)2, where the iron nitrosyl unit has the formal electron
assignment, {Fe(NO)2}10. This formalism invokes the En-
emark-Feltham notation which stresses the well-known cova-
lency and delocalization in the electronically amorphous
Fe(NO)2 unit, without committing to a formal oxidation state
on Fe, that is, assignments to FeII(NO-)2, Fe0(NO)2, Fe-II(NO+)2,
or some mixture thereof.5 By application of our stepwise ligand
exchange route, and with the use of a flexible S3 precursor of
Ni(II) in the presence of nitrite anion, we have prepared a second
Ni(µ-SR)2Fe(NO)2 unit, complex 2 below, and report it herein
as an example of formal oxidation state assignments of Ni0-
{Fe(NO)2}9.
Examples of nitric oxide coordination to iron and the
spectroscopic signals of dinitrosyl iron complexes (DNIC) are
of much interest, particularly in light of their role(s) in sulfur-
rich protein uptake and degradation.6 Furthermore, DNICs have
been suggested as possible forms for stabilization and transport
of NO in biological systems.7 Thus, the precursor to the
heterobimetallic complex 2, (PhSe)2Fe(NO)2-, (complex 1), was
also isolated and characterized by X-ray crystallography and
infrared spectroscopy. To address the heterometal influence on
* To whom correspondence should be addressed.
† National Changhua University of Education.
‡ National Taiwan University.
§ Texas A&M University.
(1) (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) de Lacey, A. L.; Hatchikian, E. C.; Volbeda, A.; Frey,
M.; Fontecilla-Camps, J. C.; Fernandez, V. M. J. Am. Chem. Soc.
1997, 119, 7181. (d) Garcin, E.; Vernede, X.; Hatchikian, E. C.;
Volbeda, A.; Frey, M.; Fontecilla-Camps, J. C. Structure 1999, 7, 557.
(2) (a) Pierik, A. J.; Hulstein, M.; Hagen, W. R.; Albracht, S. P. J. Eur.
J. Biochem. 1998, 258, 572. (b) Van Dam, P. J.; Reijerse, E. J.; Hagen,
W. R. Eur. J. Biochem. 1997, 248, 355. (c) Van Der Spek, T. M.;
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J.; Hagen, W. R.; Albracht, S. P. J. Eur. J. Biochem. 1996, 237, 629.
(3) (a) Nicolet, Y.; Piras, C.; Legrand, P.; Hatchikian, C. E.; Fontecilla-
Camp, J. C. Structure 1999, 7, 13. (b) Peters, J. W.; Lanzilotta, W.
N.; Lemon, B. J.; Seefeldt, L. C. Science 1998, 282, 1853.
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10.1021/ic990631y CCC: $19.00 © 2000 American Chemical Society
Published on Web 01/13/2000

Heterobimetallics of Nickel-Iron Dinitrosyl
Inorganic Chemistry, Vol. 39, No. 3, 2000 481
Table 1. Summary of Crystallographic Data for
[PPN][Fe(NO)2(SePh)2] (1) and
[(ON)Ni(µ-S(CH2)2S(CH2)2S)Fe(NO)2] (2)
Syntheses. Complexes [Ni(µ-S(CH2)2S(CH2)2S)]2,10 [PPN][[Fe(CO)3-
(SePh)3],11 Fe(NO)2(CO)2,12 and (bme-daco)Ni(II)13 were synthesized
by published procedures. (PPN ) bis triphenylphosphineiminium
cation).
1
2
Preparation of [PPN][Fe(NO)2(SePh)2] (1). The compound [PPN]-
[[Fe(CO)3(SePh)3] (0.2 mmol, 0.115 g) was loaded into a 100-mL
Schlenk flask with 0.028 g (0.4 mmol) of NaNO2, and 30 mL of THF
was added. The reaction mixture was stirred at 50 °C for 24 h, after
which 15 mL of hexane was added. The resulting mixture was filtered
to separate the yellow precipitate, NaSePh. Upon solvent removal in
vacuo from the filtrate, [PPN][Fe(NO)2(SePh)2] was obtained as a dark
red brown solid (0.075 g, 39%). Recrystallization from concentrated
THF solution with ether diffusion gave dark red crystals used in the
X-ray diffraction study. IR (νNo): 1735 s, 1694 vs cm-1 (THF); 1741
empirical formula
fw (g/mol)
temp, K
wavelength, Å
cryst syst
space group
C48H40O2N3P2FeSe2
966.57
298
0.7107
monoclinic
C2/c
C4H8O3N3S3NiFe
356.86
298
0.7107
triclinic
P1h
unit cell dimensions
a ) 16.769(4) Å
a ) 7.0616(12) Å;
R ) 96.809(10)°
b ) 7.4228(11) Å;
â ) 93.367(11)°
c ) 11.4142(12) Å;
γ ) 95.630(12)°
589.71(15)
2
b ) 14.972(4) Å;
â ) 100.36(3)°
c ) 18.019(5) Å
s, 1697 vs cm-1 (CH2Cl2). UV-vis absorption spectrum (THF) [λmax
,
nm (ꢀ, M-1 cm-1)]: 452(1544), 360(3943), 342(5095). Anal. Calcd
for C48H40O2N3P2FeSe2: N, 1.28; C, 60.4; H, 4.15. Found: N, 1.34;
C, 60.2; H, 4.13.
vol, Å3
4450.2(20)
4
1.443
3.7
3.2
Z
F
calc, Mg/m3
2.010
2.6
3.8
Preparation of [(ON)Ni(µ-S(CH2)2S(CH2)2S)Fe(NO)2] (2). Into a
100-mL Schlenk flask were loaded the starting materials [PPN][Fe-
(NO)2(SePh)2] (0.40 mmol, 0.388 g), NaNO2 (0.40 mmol, 0.028 g),
and [Ni(µ-S(CH2)2S(CH2)2S)]2 (0.20 mmol, 0.086 g). A 30-mL portion
of CH2Cl2 was added to give a dark purple solution. The solution
mixture was stirred at 50 °C overnight, after which 15 mL of hexane
was added to precipitate the side products [PPN][Fe(NO)Cl3] and
NaSePh. The resulting mixture was filtered and solvent was removed
from the filtrate under vacuum. One milliliter of CH2Cl2 was added to
redissolve the products and hexane (12 mL) was added to obtain a
dark green solid isolated by filtration (0.061 g, 43%). Recrystallization
from ether gave green brown crystals which were soluble in hexane,
THF, and methylene dichloride. IR (νNo): 1805 m, 1767 s, 1725 s cm-1
(CH2Cl2); 1798 m, 1763 s, 1723 s cm-1 (THF). Absorption spectrum
(CH2Cl2) [λmax, nm (ꢀ, M-1 cm-1)]: 491(519), 385(919). Anal. Calcd
for C4H8O3N3S3NiFe: N, 11.78; C, 13.46; H, 2.26. Found: N, 11.68;
C, 13.54; H, 2.21.
R(F) [I > 2σ(I)],a %
wR(F 2) all data,a %
a Residuals: R(F) ) ∑|Fo - Fc|/∑Fo; wR(F 2) ) {∑w(|Fo | - |Fc2|)2/
2
[∑w(Fo )2]}1/2
.
2
adjacent metal diatomics, analogues of the Osterloh et al.
complex4 based on (bismercaptoethanediazacyclooctane)nickel-
(II), (bme-daco)Ni(II), Ni-1, as the metallothiolate ligand to {Fe-
(NO)2(CO)} and {Fe(NO)2} were prepared and their spectro-
scopic signals compared with a host of monomeric and
homodimetallic derivatives of the Fe(NO)2 fragment. The ability
to reference the Fe(NO)2 moiety within a consistent framework
permits conclusions regarding the electronic character of the
metal-modified ancillary ligands.
Preparation of (Ni-1)Fe(CO)(NO)2 and (Ni-1)Fe(CO)2(NO)2. To
a stirred solution of (bme-daco)Ni(II), Ni-1 (0.044 g, 0.15 mmol), in
acetonitrile (30 mL) was added Fe(CO)2(NO)2 (16 µL, 0.16 mmol).
The reaction was monitored immediately by IR to confirm the loss of
bands for the starting material with formation of (Ni-1)Fe(CO)(NO)2,
THF solution spectra (cm-1): (νCO) 2006 br; (νNO): 1733 s, 1691 vs.
After 1.5 h of stirring, the resulting red-brown solution had two νNO
bands at 1677 (ms) and 1630 (s) cm-1, as expected for (Ni-1)Fe(NO)2,
based on the spectroscopy of the Osterloh et al. complex.4 Both
compounds are thermally unstable and decomposed under vacuum; they
dissolve in polar solvents such as THF and acetonitrile.
Experimental Section
General Methods and Materials. Solvents were reagent grade and
were purged by nitrogen prior to use. All syntheses and product
isolations were carried out under N2 using standard Schlenk and
glovebox techniques.
Physical Measurements. Infrared spectra were recorded on a Bio-
Rad FTS-185 instrument using a 0.1-mm-spaced KBr sealed cell.
Elemental analyses were carried out by a CHN analyzer (Heraeus).
EPR spectra were measured on a Bruker ESP 300 spectrometer
equipped with an Oxford ER910A cryostat at 100 K. The spin
concentration for each EPR sample was determined by the comparison
of the double integral of the entire EPR spectrum of the sample with
that of a standard, that is, 1 mM Cu(II) in 10 mM EDTA.8
Electrochemical measurements were performed by a BAS-100A
electroanalyzer utilizing glassy carbon working, Ag/AgNO3 reference,
and platinum auxiliary electrodes. Cyclic voltammograms were obtained
from 2.5 mM analyte concentration in CH2Cl2 using 0.1 M [n-Bu4N]-
[PF6] as a supporting electrolyte. Potentials were scaled to NHE using
ferrocene as an internal standard.9
Results and Discussion
Syntheses and Molecular Structures. Presented in Scheme
1 is the reaction which led to dark red, crystalline [PPN+][Fe-
(NO)2(SePh)2-]. The molecular structure of the anion, Figure
1, is that of a distorted tetrahedron, elongated along the Se-
Fe-Se and N-Fe-N edges. The nitrosyls are slightly bent
( Fe-N-O ) 169°) and flared toward each other.6 The Fe-
SePh average distance of 2.395(1) Å is significantly shorter than
that of the six-coordinate fac-Fe(CO)3(SePh)3- (Fe-Se average
) 2.459(2) Å)11a and tetrahedral [Fe(SePh)4]2- (Fe-Se average
) 2.460(12) Å).14 This is rationalized by the higher coordination
The X-ray crystal structures were solved at the Department of
Chemistry and Instrumentation Center at National Taiwan University,
Taipei, Taiwan. X-ray crystallographic data were obtained on a Nonius
CAD 4 diffractometer with graphite-monochromated Mo KR radiation
employing the θ/2θ scan mode. A æ scan absorption correction was
made. Structural determinations were made using the NRCC-SDP-VAX
package of programs. Cell parameter and data collection summaries
for [PPN][Fe(NO)2(SePh)2] (1) and [(ON)Ni(µ-S(CH2)2S(CH2)2S)Fe-
(NO)2] (2) are given in Table 1.
(10) (a) Harley-Mason, J. J. Chem. Soc. 1952, 146. (b) Baker, D. J.;
Goodall, D. C.; Moss, D. S. Chem. Commun. 1969, 325.
(11) (a) Liaw, W.-F.; Lai, C.-H.; Chiang, M.-H.; Lee, C.-K.; Lee, G.-H.;
Peng, S.-M. J. Chem. Soc., Dalton Trans. 1993, 2421. (b) Liaw, W.-
F.; Chiang, M.-H.; Liu, C.-J.; Harn, P.-J.; Liu, L.-K. Inorg. Chem.
1993, 32, 1536. (c) Liaw, W.-F.; Chen, C.-H.; Lee, C.-M.; Lin, G.-
Y.; Ching, C.-Y.; Lee, G.-H.; Peng, S.-M. J. Chem. Soc., Dalton Trans.
1998, 353.
(8) Orme-Johnson, N. R.; Orme-Johnson, W. H. Methods Enzymol. 1978,
(12) Hieber, W.; Beutner, H. Z. Anorg. Allg Chem. 1963, 320, 101.
(13) Mills, D. K.; Reibenspies, J. H.; Darensbourg, M. Y. Inorg. Chem.
1990, 29, 4364.
52, 252.
(9) Gagne, R. R.; Koval, C. A.; Lisensky, G. C. Inorg. Chem. 1980, 19,
2854.
(14) McConnachie, J. M.; Ibers, J. A. Inorg. Chem. 1991, 30, 1770.

482 Inorganic Chemistry, Vol. 39, No. 3, 2000
Liaw et al.
Figure 2. Labeling scheme for [(ON)Ni(µ-S(CH2)2S(CH2)2S)Fe(NO)2]
(2) with thermal ellipsoids drawn at the 50% probability level. Fes
S(1), 2.2907(9); FesS(3), 2.3039(9); FesN(1), 1.670(2); FesN(2),
1.674(3); N(1)sO(1), 1.163(3); N(2)sO(2), 1.159(3); NisS(1), 2.2947-
(9); NisS(2), 2.2701(9); NisS(3), 2.2931(9); NisN(3), 1.644(2);
N(3)sO(3), 1.119(3); Ni‚‚‚Fe, 2.8001(6). S(1)sFesS(3), 103.73(3);
S(1)sFesN(1), 114.88(9); S(1)sFesN(2), 106.02(9); N(1)sFesN(2),
115.06(12); S(1)sNisS(2), 91.67(3); S(1)sNisS(3), 103.94(3); S(1)
NisN(3), 120.53(10); S(2)sNisS(3), 92.31(4); S(2)sNisN(3), 126.69-
(9); S(3)sNisN(3), 115.83(10); FesS(1)sNi, 75.28(3); FesS(3)s
Ni, 75.05(3); Fe N(1)sO(1), 166.21(23); FesN(2)sO(2), 167.3(3);
NisN(3)sO(3), 175.1(3).
Figure 1. Labeling scheme for [Fe(NO)2(SePh)2]- (1) with thermal
ellipsoids drawn at the 50% probability level. FesSe, 2.395(1); Fes
N(1), 1.669(4); N(1)sO(1), 1.162(5). SesFesSe(a), 114.08(5); Ses
FesN(1), 107.88(13); SesFesN(1a), 105.06(13); N(1)sFesN(1a),
117.20(19); FesN(1)sO(1), 169.2(4).
Scheme 1
cally differ from the analogous features in complex 4, 81.46(6)°
and 74.10(6)°, respectively. Further appropriate comparisons lie
in the FesN and NsO parameters; the FesN bond distances
(average 1.67 Å) are not disturbed by the change of donor sets
in complexes 1 versus 2 which are very slightly longer than
the 1.65-Å average seen for 4. The Fe-NO bonds are off
linearity by an average of ≈10° and, as previously noted for
complex 1, they bend toward each other.
The Ni-NO distance (1.644(2) Å) in complex 2 is compa-
rable with the terminal Ni-NO of 1.653(4) Å in [Ni(NO)-
(PTA)3]+ (PTA ) 1,3,5-triaza-7-phosphaadamantane);17 the
nitrosyl ligand is coordinated in a nearly linear mode with the
Ni-N(3)-O(3) ) 175.1(3)°. Together with similar ν(NO)
values found for complexes 2 and [Ni(NO)(PTA)3]+, vide infra,
we conclude the site is best described as NO+ coordinated to
nickel(0).
number of the former and higher anionic charge of the latter.
Structural and spectroscopic features are the same for 1 and
[Fe(NO)2(SPh)2]-.15
Subsequent reaction of the [Fe(NO)2(SePh)2]- anion with
dimeric [Ni(µ-SCH2CH2SCH2CH2S)]2 in the presence of ad-
-
ditional NO2 produced the neutral heterobimetallic [(ON)Ni-
{(µ-SCH2CH2)2S}Fe(NO)2] complex 2, as a thermally stable,
dark green-brown solid isolated in ≈70% yield. Complex 2 is
soluble in hexane and THF and forms slightly air-sensitive
solutions in CH2Cl2. Crystals suitable for X-ray crystallography
were grown from concentrated hexane solution and were found
to be in the triclinic P1h space group.
Figure 2 displays a thermal ellipsoid plot of the neutral Ni-
Fe bimetallic complex 2 and selected distances and angles are
given in the caption. The constraints of the bismercapto-
ethylenesulfide ligand generates ≈90° Sthioether-Ni-Sthiolate
angles,16 enforcing a severe distortion from a tetrahedral at the
four-coordinate nickel site. Nevertheless, the NiS2Fe core is
minimally hinged (the angle between NiS2 and FeS2 planes )
162.8°).
All M-Sbr thiolate distances of complex 2 are substantially the
same, 2.29 Å, while the Ni-Sthioether distance is 0.02-Å shorter.
The average Fe-S-Ni angle of 75.2° and the Fe-Ni distance
of 2.8001(6) Å are similar to those of complex 4, 76.11(4)°
and 2.797(1) Å, respectively. (A stick drawing of complex 4 is
found in Table 2.) The large Sbr-Ni-Sbr of 103.94(3)° and
S-Fe-S of 103.73(3)° found in complex 2 however dramati-
Spectroscopic and Electrochemical Measurements. Com-
plexes 1, 2, and 3, the {Fe(NO)2}9 species, in Table 2, have an
odd number of electrons and were found to have similar EPR
properties: a single isotropic signal with no hyperfine coupling
to the NO ligands. The g values of 2.021, 2.021, and 2.028, for
1, 2, and 3, respectively, are consistent with halogenonitrosyl
iron complexes, [X2Fe(NO)2]- (X ) Cl, Br, I) which also have
tetrahedral geometry and an electron assignment of {Fe(NO)2}9
1
with S ) /2.18 Weaker EPR signals, assumed to be due to
impurities, were observed for solutions of complexes 4, 5, and
6 (spin quantitations of 0.07-0.10 spin per molecule as
compared to ≈1.0 for 1, 2, and 3).
On treatment of the complex 2 with 1 equiv of CeIV (as
cerium ammonium nitrate), the EPR signal at g ) 2.021 was
lost. Addition of the weak reductant, Cp2Co, produced no change
in the EPR spectrum. However, addition of the LiHBEt3 reagent
to the EPR sample tube produced an axial signal of g ) 2.602
and 2.018. This product has not been identified.
(17) PTA ) phosphotriazaadamantane, a neutral P-donor ligand. Darens-
bourg, D. J.; Decuir, T. J.; Stafford, N. W.; Robertson, J. B.; Draper,
J. D.; Reibenspies, J. H.; Katho´, A.; Joo´, F. Inorg. Chem. 1997, 36,
4218.
(15) Strasdeit, H.; Krebs, B.; Henkel G. Z. Naturforsch. 1986, 41b, 1357.
(16) Kaasjager, V. E.; Henderson, R. K.; Bouwman, E.; Lutz, M.; Spek,
A. L.; Reedijk, J. Angew. Chem., Int. Ed. 1998, 37, 1668.
(18) Connelly, N. G.; Gardner, C. J. Chem. Soc., Dalton Trans. 1976, 1525.

Heterobimetallics of Nickel-Iron Dinitrosyl
Inorganic Chemistry, Vol. 39, No. 3, 2000 483
Table 2. Selected Structure and Infrared Data for Dichalcogenide
Fe(NO)2 Derivatives {Fe(NO)2}9,10 Based on Enemark-Feltham
Notation6
Figure 3. Cyclic voltammogram of a 2.5-mV solution of complex 2
in 0.1 M TBAHFP/CH2Cl2 with a glassy carbon working electrode at
a scan rate of 200 mV/s.
2.4,20 Thus, the charge separation indicated by the formal
oxidation states of the heterobimetallic complexes 2 (Ni0/{Fe-
(NO)2}9) and 4 (NiII/{Fe(NO)2}10) is reflected in the νNO values.
The similarity of νNO values in the Fe(NO)2 units of complexes
1 and 3, despite the negative charge on the former, is rationalized
by the firm localization of H+ on the two nitrogen bases, as
indicated in the structure drawn for 3. The higher νNO values
of 2 versus 3 would then imply that the Ni0(NO+) unit
substantially neutralizes the cis thiolate negative charges, and
the [(ON)Ni0S3]- unit serves as a bidentate donor ligand of -1
charge to the {Fe(NO)2}9 unit.
Figure 3 displays the cyclic voltammogram (CV) of complex
2 with its two quasi-reversible redox events centered at +690
and -970 mV. Because the CV of [PPN][(PhSe)2{Fe(NO)2}9]
has a reduction feature at -1160 mV, the -970-mV event of
complex 2 is assumed to be due to the {Fe(NO)2}9/{Fe(NO)2}10
couple. With this assignment, we conclude that the anionic
(ON)Ni(0)-dithiolate unit donates slightly less electron density
to the {Fe(NO)2}9 unit as compared to the two phenylselenide
donors and stabilizes this event by 190 mV. This result is
consistent with the ν(NO) values in the IR spectra as discussed
above. The quasi-reversible anodic event at +687 mV could
be assigned to the {Fe(NO)2}9/{Fe(NO)2}8 couple, consistent
with the observed loss of the EPR signal of chemically oxidized
2. Alternatively, assignment to a nickel-based oxidation, the Ni0/I
couple, would have to invoke coupling of the {Fe(NO)2}9 and
the NiI (d9) centers to account for the loss of paramagnetism
and the EPR signal.
a Dichloromethane. b CsI. c KBr. d Acetonitrile. e THF. f Cyclohex-
ane. g Tetrachloroethylene. h Shown are average values.
As noted in Table 2, the CH2Cl2 solution spectrum of the
neutral heterobimetallic compound 2 shows three infrared bands
in the νNO region. The highest frequency band at 1805 cm-1 is
identical to the νNO band of the tetracoordinate, pseudotetra-
hedral Ni(0) nitrosyl complex [Ni(NO)(PTA)3]+ (νNO ) 1805
cm-1 in CH2Cl2); thus, we can project an electron donor ability
of the (µ-(SCH2CH2)2S)Fe(NO)2 unit to [NiNO]+ to be the same
as that of the P-donor ligands, assuming an oxidation state of
zero for nickel and the presence of NO as NO+.17 That is, the
three S donors comprised of a {Fe(NO)2}9-modified dithiolate
and the one thioether is equivalent to three P donors of the
phosphotriazaadamantane ligand, PTA.
The lower energy NO bands of 2 are shifted by ≈+30 cm-1
from those of the anionic monomeric precursor, Fe(NO)2(SePh)2-.
Known to serve as a reporter of electron density at iron similarly
to CO,19 the shifts in NO frequencies reflect a variation in
negative charge on the Fe(NO)2 unit as follows: 4 < 3 ∼ 1 <
Further Comments
The assignment of formal oxidation levels to the Fe(NO)2
units compared in this study was done in an effort to describe
the electron-donating abilities of the ancillary ligands within a
similar framework. With this approach, we are able to reference
the nickel(0)dithiolate, compound 2, and nickel(II)dithiolate
ligands, compounds 4, 5, and 6, with respect to mononuclear
complexes with rather unambiguous additional ligands to Fe-
(NO)2.21,22 The following conclusions are made:
(20) Baltusis, L. M.; Karlin, K. D.; Rabinowitz, H. N.; Dewan, J. C.;
Lippard, S. J. Inorg. Chem. 1980, 19, 2627.
(21) (a) Brockway, L. O.; Anderson, J. S. Trans. Faraday Soc. 1937, 38,
1233. (b) Hedberg, L.; Hedberg, K.; Satija, S. K. Swanson, B. I. Inorg.
Chem. 1985, 24, 2766.
(22) Albano, V. G.; Araneo, A.; Bellon, P. L.; Ciani, G.; Manassero, M. J.
Organomet. Chem. 1974, 67, 413.
(19) Horrocks, W. D.; Taylor, R. C. Inorg. Chem. 1963, 2, 723.

484 Inorganic Chemistry, Vol. 39, No. 3, 2000
Liaw et al.
(1) Two oxidation levels, {Fe(NO)2}9 and {Fe(NO)2},10 are
observed in the heterobimetallic molecular units examined in
-
this work. The paradigms for the former are (PhS)2Fe(NO)2
and (PhSe)2Fe(NO)2- while that of the latter is (CO)2Fe(NO)2.
The π-donor ability of thiolate (or selenoate) stabilizes the
former {Fe(NO)2}9 or oxidized form, while the π-accepting
ability of CO stabilizes the latter, reduced form.
(2) The oxidation level of Fe(NO)2 is the primary determinant
of the ν(NO) position. Within a series of the same oxidation
level, the ν(NO) reporter shows a sensitivity to ancillary ligands
to {Fe(NO)2} similarly to ν(CO) as described in classic studies
such as that of Horrocks and Taylor in a homologous series of
Co(NO)(CO)(PR3) complexes.19 In an earlier study, we refer-
enced the nickel(II) dithiolate, (bme-daco)Ni as a S-donor ligand
to Fe0(CO)4 relative to (Ph3P)Fe(CO)4 using the noncontroversial
CO ligand as a spectroscopic probe.23 There, we found the
monodentate metallothiolate ligand to be a better donor, shifting
the ν(CO) values negatively on average by ≈30 cm-1 from
(Ph3P)Fe(CO)4.
(3) The formation and the ν(NO) values of the NiFe bimetallic
compound 2 is consistent with a dichalcogenide derivative of
{Fe(NO)2}9 that is partially charge-neutralized by interaction
of a single cationic charge, that is, [Ni(NO)]+. This gives rise
to the highest ν(NO) frequency of the {Fe(NO)2}9 series.
Alternatively, the heterometallic may be described as a
[S′S2Ni(NO+)]- moiety bound by an interacting {Fe(NO)2}9
unit which effectively softens the thiolate donors of S(CH2CH2
S-)2.
(4) The ν(NO) positions of compounds 4, 5, and 6, those
containing the {Fe(NO)2}10 moiety, Table 2, are consistent with
those of compounds 7, 8, and 9 given the established donor
ability of NiSR versus PPh3 discussed above. That is, the nickel
thiolate is a better donor than phosphine, producing the lowest
observed ν(NO) in the entire series for compounds 4 and 6.
Thus, for the purpose of interpreting the IR spectra, the NiII(µ-
SR)2Fe(NO)2 and the Ni0(µ-SR)2Fe(NO)2 complexes may be
viewed as mononuclear Fe(NO)2 centers with ligands (i.e.,
thiolates) modified by distal metal interactions.24 The oxidation
level of the Fe(NO)2 unit may be rationalized according to the
ability of the chalcogenide (or metal-modified chalcogenide)
ligand to stabilize the oxidized, {Fe(NO)2},9 or reduced, {Fe-
(NO)2},10 forms. A 2+ charge on the distal metal, as in the
NiII-bound dichalcogenide, creates neutral sulfur donor sites and
stabilizes the electron-rich or reduced form, as does CO or PR3.
The [Ni0NO+] unit effectively generates a monoanionic dithi-
olate ligand donor set, which stabilizes the oxidized form.
Interestingly, both have the same Ni-Fe distances of 2.8 Å,
but distinctly different hinge angles of the Ni(µ-SR)2Fe unit,
104.6° for complex 4 and 162.8° for compound 2. The
maintenance of the same Ni-Fe distance is possible because
the S-Fe-S and S-Ni-S in the core NiS2Fe unit open by
≈30° on going from the butterfly structure of 4 to the roughly
planar structure of 2, with a concomitant shift of the Fe(NO) 2
unit relative to the NiS2 plane.
matching the geometrical requirement of the Ni(µ-SR)2Fe unit
as well as maintaining the preferred coordination number of
the nickel. One would expect the protein’s cysteinyl sulfurs
which bridge the Ni-Fe active site of [NiFe]H2ases to be
capable of such flexibility. In fact, as obserVed in the protein
crystal structures, the irregular orientation of the two terminal
and two bridging CysS about nickel (pseudo-tetrahedral, or
distorted, open-site square pyramid) could indicate a balance
of geometrical requirements for different redox leVels of nickel
in different states of the enzyme. In small molecule model
chemistry, two different ligands are required to mimic the effect.
[It should be noted that the usefulness of the (µ-SCH2CH2)2S
ligand, was previously recognized in the preparation of a
binuclear structural model for Fe only hydrogenase.16]
Electron coupling through the 2.8-Å Ni-Fe distances found
in the heterometallics with the Fe(NO)2 unit is expected, but
not yet established. The Ni-Fe distances in NiII-FeII hetero-
metallics that do not contain the Fe(NO)2 unit are ≈3.0 Å or
larger.22,25,26 Interestingly, the natural heterobimetallic found in
the [NiFe]H2ase enzyme active site has a Ni-Fe distance of
2.9 Å.1 The much shorter distance of 2.6 Å in compound 10,27
listed in Table 2 as NiII{Fe(NO)2},10 probably implies a Ni-
Fe bond character and an assignment of NiI{Fe(NO)2}9 would
be equally, if not more, realistic.
Finally, it is noted that the usual exchange of two NO donor
ligands for three two-electron donors such as CO and CN-
produces a fragment, Fe(NO)2, that could be interchangeable
with isolobal Fe(CO)(CN)2, recently established as important
in the hydrogenases. Because all three diatomic ligands have
infrared stretches in the same region, isotopic labeling should
be a requirement for confidence in spectroscopic assignments
and vibrational studies.
Acknowledgment. The authors acknowledge the financial
support of the National Science Council (Taiwan to W.F.L. and
S.M.P.) and the National Science Foundation (U.S.A., CHE-
9812355 to M.Y.D.). We are grateful for the assistance of Huay-
Keng Loke in the EPR measurements, to Jason Smee for
information retrieval and computer display, and to Dr. Frank
Osterloh for his helpful critique of the manuscript.
Supporting Information Available: Tables of crystal data and
experimental conditions for the X-ray studies, atomic coordinates and
Beq values, complete listings of bond lengths and bond angles, and
anisotropic temperature factors for [PPN][Fe(NO)2(SePh)2] and [(ON)-
Ni(µ-S(CH2)2SCH2)2S)Fe(NO)2]. This material is available free of
charge via the Internet at http://pubs.acs.org.
Such geometric differences as determined by the nickel
oxidation state in 4 versus that in 2 demand ligands capable of
IC990631Y
(25) Mills, D. K.; Hsiao, Y. M.; Farmer, P. J.; Atnip, E. V.; Reibenspies,
J. H.; Darensbourg, M. Y. J. Am. Chem. Soc. 1991, 113, 1421.
(26) Colpas, G. J.; Day, R. O.; Maroney, M. J. Inorg. Chem. 1992, 31,
5053.
(23) Lai, C.-H.; Reibenspies, J. H.; Darensbourg, M. Y. Angew. Chem.,
Int. Ed. Engl. 1996, 35, 2390.
(24) Musie, G.; Farmer, P. J.; Tuntulani, T.; Reibenspies, J. H.; Darens-
bourg, M. Y. Inorg. Chem. 1996, 35, 2176.
(27) Chau, C. N.; Wojcicki, A.; Calligaris, M.; Nardin, G. Inorg. Chem.
Acta 1990, 168, 105.