Nickel(II) thiolate complex with carbon monoxide and its Fe(II) analog: Synthetic models for CO adducts of nickel-iron-containing enzymes

Full text of DOI:10.1021/ja961968r

J. Am. Chem. Soc. 1996, 118, 8963-8964
8963
Nickel(II) Thiolate Complex with Carbon Monoxide
and Its Fe(II) Analog: Synthetic Models for CO
Adducts of Nickel-Iron-Containing Enzymes
Dao Hinh Nguyen, Hua-Fen Hsu, Michelle Millar,* and
Stephen A. Koch*
Department of Chemistry
State UniVersity of New York at Stony Brook
Stony Brook, New York 11794-3400
Catalina Achim, Emile L. Bominaar, and Eckard Mu¨nck*
Department of Chemistry
Carnegie Mellon UniVersity
Pittsburgh, PennsylVania 15213
Figure 1. Structural diagram of [Ni(PS3*)(CO)]1- 1. Selected bond
distances (Å) and angles (deg): Ni-S1, 2.352(6); Ni-S2, 2.274(5);
Ni-S3, 2.312(6); Ni-C37, 1.75(3); Ni-P1, 2.089(6); C37-O1, 1.15-
(2); S1-Ni-S2, 122.6(2); S1-Ni-S3, 109.8(2); S2-Ni-S3, 124.2-
(2); S1-Ni-P1, 82.6(2); S2-Ni-P1, 83.8(2); S3-Ni-P1, 85.1(2);
S1-Ni-C37, 98.1(9); S2-Ni-C37, 92.8(8); S3-Ni-C37, 97.9(8);
P1-Ni-C37, 176.4(8).
ReceiVed June 11, 1996
Carbon monoxide is a substrate for two distinct reactions
catalyzed by the iron-nickel-containing enzyme carbon mon-
oxide dehydrogenase (CODH).1,2 The oxidation of CO to CO2
occurs at one site (center C) while CO combines with a methyl
group to form an acetyl group at the second site (center A).
Both of these reactions occur at reaction centers that involve
closely coupled Ni and Fe centers. Recent vibrational studies
have suggested that the CO is bound to iron and not to nickel
at center A and that nickel is involved in the CO oxidation at
center C.2 CO has also been used as an inhibitor and a
spectroscopic marker for the nickel-iron-containing hydroge-
nase enzymes.3 Two distinct CO adducts of the nickel-iron
hydrogenases have been spectroscopically detected: (1) an EPR
active form which displays hyperfine couplings indicating direct
Ni-CO coordination4 and (2) a postulated Ni(II)-CO form
Subsequent addition of [(n-Bu)4N]Br provides crystalline [(n-
Bu)4N][Ni(PS3*)(CO)] (1) in good yield.9 [(n-Bu)4N][Ni-
(PS3*)(CO)] was shown to be diamagnetic and to exhibit an
IR stretch at 2029 cm-1 assigned to the Ni(II)-CO center.
The X-ray structure of [(n-Bu)4N][Ni(PS3*)(CO)] (Figure 1)
reveals a distorted trigonal bipyramidal structure with the CO
in the axial position trans to the phosphorus atom.10 The three
equatorial thiolate ligands show a distortion from rigorous C3
symmetry with S-Ni-S angles of 122.6(2), 109.8(2), and
124.2(2)° and Ni-S distances of 2.352(6), 2.274(5), and 2.312-
(6) Å. The Ni center is situated 0.3 Å out of the plane defined
by the three S atoms and toward the CO ligand. The Ni(II)-S
distance is 0.07 Å longer than the corresponding distances in
[NiIII(PS3*)(N-methylimidazole)], which is consistent with the
anticipated difference in metal-sulfur distances of Ni(II)/Ni-
(III) oxidation levels.8b
[(n-Bu)4N][Ni(PS3*)(CO)] is the first example of a structur-
ally characterized Ni-CO complex with thiolate ligands.
Nickel(I) thiolate-CO complexes have been generated in
solution but have not been isolated.11 It is noted that Ni(II)-
CO complexes with any type of ligand are rare.12 The axial
coordination of the CO in 1 contradicts the molecular orbital
prediction of a preference for equatorial coordination of π-acid
ligands in trigonal bipyramidal d8 compounds.13 Examples are
known for both axial and equatorial coordination of CO in
trigonal bipyramidal Ni(II) compounds. In NiX2(CO)(PMe3)2
(X ) Cl, I)12a,b the CO occupies an equatorial position, while
in NiI2(CO)(1,1′-bis(dimethylarsine)ferrocene)12c the CO coor-
dinates in the axial position. The axial coordination of CO in
the latter compound was rationalized to result from the constraint
of the bidentate arsine ligand. Likewise, the coordination of
which displays a CO stretch at 2060 cm-1 5
. The recent X-ray
structure determination of the nickel-iron-containing hydro-
genase revealed that the nickel is bridged by two cysteines to
a second metal which is presumably iron.6 In both the
hydrogenase and CODH enzymes substantial cysteine coordina-
tion to the metal centers has been established or suggested.6,7
We report the synthesis and characterization of the first example
of a nickel(II) thiolate-CO complex and its Fe(II) analog, which
is an unprecedented paramagnetic Fe(II)-CO complex.
Nickel complexes of the tripod phosphine trithiolate ligand
tris(2-thiophenyl)phosphine (PS3) and its sterically hindered
analog tris(3-phenyl-2-thiophenyl)phosphine (PS3*) have been
recently investigated.8 The reaction of Ni(acac)2 with Li3[PS3*]
in MeOH generates a yellow-brown solution which changes to
a deep green color upon the addition of carbon monoxide.
(1) (a) Ragsdale, S. W. Crit. ReV. Biochem. Mol. Biol. 1991, 26, 261-
300. (b) Xia, J.; Lindahl, P. A. J. Am. Chem. Soc. 1996, 118, 483-484,
and reference therein.
(2) (a) Qiu, D.; Kumar, M.; Ragsdale, S. W.; Spiro, T. G. Science
(Washington, D. C.) 1994, 264, 817-819. (b) Kumar, M.; Qiu, D.; Spiro,
T. G.; Ragsdale, S. W. Science (Washington, D. C.) 1995, 270, 628-630.
(c) Qiu, D.; Kumar, M.; Ragsdale, S. W.; Spiro, T. G. J. Am. Chem. Soc.
1995, 117, 2653-2654. (d) Hu, Z.; Spangler, N. J.; Anderson, M. E.; Xia,
J.; Ludden, P. W.; Lindahl, P. A.; Mu¨nck, E. J. Am. Chem. Soc. 1996, 118,
830-845.
(3) Albracht, S. P. J. Biochim. Biophys. Acta 1994, 1188, 167-204.
(4) van der Zwaan, J. W.; Coremans, J. M. C. C.; Bouwens, E. C. M.;
Albracht, S. P. J. Biochim. Biophys. Acta 1990, 1041, 101-110.
(5) Bagley, K. A.; Van Garderen, C. J.; Chen, M.; Duin, E. C.; Albracht,
S. P. J.; Woodruff, W. H. Biochemistry 1994, 33, 9229-9236. Bagley, K.
A.; Duin, E. C.; Roseboom, W.; Albracht, S. P. J.; Woodruff, W. H.
Biochemistry 1995, 34, 5527-5535.
(6) Volbeda, A.; Charon, M.-H.; Piras, C.; Hatchikian, E. C.; Frey, M.;
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Soc. 1995, 117, 7065-7070.
(8) (a) Franolic, J. D.; Wang, W. Y.; Millar, M. J. Am. Chem. Soc. 1992,
114, 6588. (b) Nguyen, D. H.; Millar, M. Submitted for publication.
(9) Electronic spectrum of 1 in DMF (λ, nm (ꢀM)): 297 (10 300 M-1
cm-1), 368 (6250), 722 (913).
(10) Crystal data for 1 (NiPS3NOC53H60): monoclinic, P21/c (no. 14),
a ) 16.816(4) Å, b ) 16.179(2) Å, c ) 17.901(4) Å, â ) 97.94(1)°,
V)4823(1) Å3, Z ) 4. A total of 2470 unique reflections with I > 3σ(I)
were refined to R ) 0.063 and Rw ) 0.094.
(11) Yamamura, T.; Sakurai, S.; Arai, H.; Miyamae, H. J. Chem. Soc.,
Chem. Commun. 1993, 1656. Marganian, C. A.; Vazir, H.; Baidya, N.;
Olmstead, M. M.; Mascharak, P. K. J. Am. Chem. Soc. 1995, 117, 1584-
1594.
(12) (a) Saint-Joly, C.; Mari, A.; Gleizes, A.; Dartiguenave, M.;
Dartiguenave, Y.; Galy, J. Inorg. Chem. 1980, 19, 2403-2410. (b) Elbaze,
G.; Dahan, F.; Dartiguenave, M.; Dartiguenave, Y. Inorg. Chim. Acta 1984,
85, L3-L5. (c) Pierpont, C. G.; Eisenberg, R. Inorg. Chem. 1972, 4, 828-
832. (d) Janikowski, S. K.; Radonovich, L. J.; Groshens, T. J.; Klabunde,
K. J. Organometallics 1985, 4, 396-398. (e) Zebrowski, J. P.; Hayashi, R.
K.; Dahl, L. F. J. Am. Chem. Soc. 1993, 115, 1142-1144.
(13) (a) Rossi, A. R.; Hoffmann R. Inorg. Chem. 1975, 14, 365-374.
(b) Macgregor, S. A.; Lu, Z.; Eisenstein, O.; Crabtree, R. H. Inorg. Chem.
1994, 33, 3616-3618.
S0002-7863(96)01968-3 CCC: $12.00 © 1996 American Chemical Society

8964 J. Am. Chem. Soc., Vol. 118, No. 37, 1996
Communications to the Editor
in solution and in the solid state, indicating that the solid state
structure is unchanged in solution. Compound 2 exhibits a
reversible oxidation at +0.083 V (vs SCE). However, bulk
electrolysis indicates the oxidation is associated with loss of
CO.
Carbon monoxide compounds usually have low-spin elec-
tronic states. A diamagnetic ground state is not permitted for
a d6 compound with an undistorted trigonal bipyramidal
structure. Accordingly, [NEt4][Fe(PS3*)(CO)] shows isotro-
1
pically shifted resonances in the H NMR14 and a solid state
magnetic susceptibility that are consistent with a paramagnetic
ground state. A paramagnetic Fe(II)-CO complex has not been
previously reported.17
Figure 2. Structural diagram of [Fe(PS3*)(CO)]1- 2. Selected bond
distances (Å) and angles (deg): Fe-S1, 2.290(4); Fe-P1, 2.165(7);
Fe-C13, 1.88(3); S1-Fe-S1′, 119.35(3); S1-Fe-P1, 85.4(1); S1-
Fe-C13, 94.6(1); P1-Fe-C13, 180.00.
We have recorded Mo¨ssbauer spectra of 2 from 4.2 to 200 K
in applied fields up to 8.0 T. These spectra show that 2 is an
integer spin paramagnet, and they impose axial symmetry on
the spin Hamiltonian used in the simulations. This symmetry
is in accord with the X-ray structure. The 4.2 K zero-field
spectrum of Figure 3 exhibits a sharp quadrupole doublet18 with
∆EQ ) 2.31(2) mm/s and δ ) 0.25(1) mm/s (relative to Fe
metal at 298 K). The low value of δ rules out a high-spin
ferrous site, whereas the observed paramagnetism is in conflict
with a low-spin ferrous site. Thus, the data suggest a ground
state with intermediate spin S ) 1 for 2. The Mo¨ssbauer spectra
were simulated with the S ) 1 spin Hamiltonian
H ) D(Sz2 - 2/3) + gâH‚S + S‚A‚I + 1/6∆EQ(3Iz2 - 15/4)
(1)
for which D ) +30(6) cm-1, gz ) g ) 2,19 A ) -6.5 MHz,
Az ) -48 MHz, and ∆EQ ) -2.31 mm/s. The negative sign
of ∆EQ suggests a concentration of negative charge along z,
i.e., along the CO-Fe-P axis. Electronic structure calculations
aimed at an interpretation of the fine and hyperfine structure
parameters of 2 are in progress.
The lack of precedents for Ni(II)(CO) in model thiolate
compounds has been a justification for discounting their
existence in biological systems. This work suggests that CO
coordination to a biological Ni(II) cysteine center must be
considered as a viable possibility. This work also suggests that
a Fe(II)(CO) center in a non-heme iron protein could be
paramagnetic. The reactivity of these nickel and iron thiolate-
carbonyl complexes are being investigated in an attempt to
model the acetyl-CoA synthase function of CO dehydrogenase.
Figure 3. Mo¨ssbauer spectra of polycrystalline 2 recorded in zero field
at 4.2 K (A) and in a parallel 8.0 T field at 10 K (B). Solid lines are
spectral simulations based on eq 1 assuming fast spin relaxation.
the CO in the axial position in 1 may be the result of the
structural constraint exerted by the tripod ligand.
The coordination of CO to nickel in 1 is robust. The
application of a continuous vacuum to a solution of 1 in DMF
does not result in the loss of the CO ligand. This is in contrast
to the lability of the CO ligand in some of the previously
reported Ni(II)-CO complexes.12,13b However, attempts to
oxidize 1 to a Ni(III)-CO species result in loss of CO.
Electrochemical studies of 1 indicate an irreversible oxidation
at +270mV (vs SCE) and an irreversible reduction at -400
mV.
The iron analog of 1 is synthesized by the reaction of
FeCl2‚4H2O with Li3(PS3*) in CH3CN under a CO atmo-
sphere.14 [Et4N][Fe(PS3*)(CO)] (2) crystallizes in a cubic space
group such that the anion has a crystallographic C3 axis (Figure
2).15 The iron is situated 0.19 Å out of the S3 plane with an
Fe-S distance of 2.290(4) Å. The Fe-C distance (1.88(3) Å)
is 0.1 Å longer than the corresponding Ni-C distance in 1.
There are numerous examples of Fe(II)-CO complexes includ-
ing examples with thiolate ligands.16 The CO stretching
frequency (1940 cm-1) of [Et4N][Fe(PS3*)(CO)] is the same
Acknowledgment. This research was supported by NIH Grant GM
31849 (S.A.K.), NIH Grant GM 36308 (M.M.), and NSF Grant MCB
9406274 (E.M.).
Supporting Information Available: Tables of crystallographic
parameters, atomic coordinates, thermal parameters, and bond distances
and angles for 1 and 2 (14 pages). See any current masthead page for
ordering and Internet access instructions.
JA961968R
(17) Another example of a paramagnetic Fe(II)(CO) compound was
published after this paper was submitted for publication. Ray, M.; Golombek,
A, P.; Hendrich, M. P.; Young, V. G., Jr.; Borovik, A. S. J. Am. Chem.
Soc. 1996, 118, 6084.
(18) The spectrum of Figure 3 reveals a paramagnetic contaminant,
amounting to 5% of total Fe.
(19) A magnetic susceptibility measurement of 2 at 298 K (sample of
Figure 3) yielded a magnetic moment of 2.81 BM (different synthetic
samples gave values of 2.81-2.85 BM). Taking into account the observed,
but unidentified, impurity, a magnetic moment between 2.74 and 2.86 BM
was obtained for 2. This result confirms the S ) 1 assignment for compound
2 and indicates that g ) 2. For a given value of D, the low-temperature
magnetic hyperfine interactions in the x-y plane are proportional to g A /
D, while those along the z axis depend essentially on gzAze-D/kT; i.e., A, D,
and g are strongly correlated.
(14) For 2, UV-vis in DMF (λ, nm (ꢀM)) 332 (1.24 × 104), 357 (1.18
× 104), 524(4.00 × 103), 744(5.01 × 102); 1H NMR (DMSO-d6) δ -29.68
(3H), -5.88 (3H), 1.13 (12H, NCH2CH3), 3.15(8H, NCH2), 7.73-7.55
(12H), 8.85 (3H), 10.26 (3H); CV (1 mM in CH2Cl2 with 0.1 M [NBu4]-
[BF4], Pt electrode, vs SCE) E1/2(∆Ep) ) 0.083 V (67 mV).
(15) Crystal data for 2 (FePS3ONC45H44): cubic, P213, a ) 15.8730(8)
Å, V ) 3999.2(2) Å3, Z ) 4. A total of 648 unique reflections with I >
3σ(I) were refined to R ) 0.052 and Rw ) 0.055. The NEt4 cation was
located on a 3-fold axis and was disordered.
(16) Sellmann, D.; Weiss, R.; Knoch, F.; Dengler, J. Inorg. Chem. 1990,
29, 4107-4114. Sellmann, D.; Brinker, G.; Moll, M.; Herdtweck, E. J.
Organomet. Chem. 1988, 327, 403-418. Sellmann, D.; Becker, T.; Knoch,
F.; Moll, M. Inorg. Chim. Acta 1994, 219, 75-84.