Discontinuum between a thiolate and a thiol ligand

Article abstract of DOI:10.1021/ic011096g

The effect of H-bond donation to the thiolate ligand of (η⁵-C₅H₅)Fe(CO)₂SR (1) to give H-bond adducts (1·HX) and eventually protonation to give [(η⁵-C₅H₅ )Fe(CO)₂(HSR)]⁺ (1H⁺) has been investigated experimentally and computationally. The electronic structures of 1(R = Me), several derivatives of 1(R = Me)·HX, and 1(R = Me)H⁺ have been investigated using DFT (density functional theory) computational methods. As previously suggested, these calculations indicate the HOMO of 1 is FedπSpπ antibonding and largely sulfur in character. The calculations indicate the electronic structure of 1 is not altered markedly by H-bond donation to the S center, but protonation results in a reorganization of the electronic structure of 1H⁺ and a HOMO that is largely metal in character. The reduction of Fe-S distances upon protonation of 1(R = Ph) to give 1(R = Ph)H⁺·BF₄^- (2.282(2) and 2.258(2) A, respectively), as determined by single-crystal X-ray crystallography, also indicates diminished Fedπ-Spπ antibonding. Using the carbonyl stretching frequencies as a gauge of the donor ability of the thiolate ligand, we conclude that H-bonding has a continuous effect on the donor properties of the thiolate ligand of 1 (i.e., is a function of the pK_a of the H-bond donor). A discontinuous effect results when the pK_b of 1 is reached and the complex is protonated. For our study of 1, the maximal effect of H-bonding is about 30% of protonation. Because the position of acid-base equilibrium depends on the relative basicities of the thiolate ligand and the conjugate base of the H-bond donor (and the relative heats of solvation of the acids and their conjugate bases), a true continuum of effects can be anticipated only for systems that are pK-matched in their given environments. Thus, when the conjugate base of the H-bond donor is a stronger base than the thiolate ligand (as in the present case), H-bond donation has a relatively small effect, but protonation triggers a large, discontinuous effect on the electronic structure of 1.

Full text of DOI:10.1021/ic011096g

Inorg. Chem. 2002, 41, 2202−2208
Discontinuum between a Thiolate and a Thiol Ligand
Danny G. McGuire, Masood A. Khan, and Michael T. Ashby*
Department of Chemistry and Biochemistry, The UniVersity of Oklahoma, 620 Parrington OVal,
Room 208, Norman, Oklahoma 73019
Received October 22, 2001
The effect of H-bond donation to the thiolate ligand of (η5-C H )Fe(CO)2SR (1) to give H-bond adducts (1‚HX) and
5
5
eventually protonation to give [(η5-C H )Fe(CO)2(HSR)]+ (1H+) has been investigated experimentally and
5
5
computationally. The electronic structures of 1(R ) Me), several derivatives of 1(R ) Me)‚HX, and 1(R ) Me)H+
have been investigated using DFT (density functional theory) computational methods. As previously suggested,
these calculations indicate the HOMO of 1 is Fedπ−Spπ antibonding and largely sulfur in character. The calculations
indicate the electronic structure of 1 is not altered markedly by H-bond donation to the S center, but protonation
results in a reorganization of the electronic structure of 1H+ and a HOMO that is largely metal in character. The
reduction of Fe−S distances upon protonation of 1(R ) Ph) to give 1(R ) Ph)H+‚BF - (2.282(2) and 2.258(2) Å,
4
respectively), as determined by single-crystal X-ray crystallography, also indicates diminished Fedπ−Spπ antibonding.
Using the carbonyl stretching frequencies as a gauge of the donor ability of the thiolate ligand, we conclude that
H-bonding has a continuous effect on the donor properties of the thiolate ligand of 1 (i.e., is a function of the pK
a
of the H-bond donor). A discontinuous effect results when the pK of 1 is reached and the complex is protonated.
b
For our study of 1, the maximal effect of H-bonding is about 30% of protonation. Because the position of acid−
base equilibrium depends on the relative basicities of the thiolate ligand and the conjugate base of the H-bond
donor (and the relative heats of solvation of the acids and their conjugate bases), a true continuum of effects can
be anticipated only for systems that are pK-matched in their given environments. Thus, when the conjugate base
of the H-bond donor is a stronger base than the thiolate ligand (as in the present case), H-bond donation has a
relatively small effect, but protonation triggers a large, discontinuous effect on the electronic structure of 1.
Introduction
properties of thiolate ligands,13 and thiolate ligands that are
protonated presumably experience a limiting effect.14-18
The observation of H-bonding to cysteinate ligands in
metalloproteins1 has fueled speculation that such dipole
interaction may play a regulatory role. This has prompted
the development of many small molecule models that
incorporate intramolecular NH‚‚‚S hydrogen bonds.2-12 There
is a consensus that dipole interactions decrease the donor
(5) Chung, W. P.; Dewan, J. C.; Walters, M. A. J. Am. Chem. Soc. 1991,
113, 525.
(6) Ueno, T.; Kousumi, Y.; Yoshizawakumagaye, K.; Nakajima, K.;
Ueyama, N.; Okamura, T.; Nakamura, A. J. Am. Chem. Soc. 1998,
120, 12264.
(7) Okamura, T.; Ueyama, N.; Nakamura, A.; Ainscough, E. W.; Brodie,
A. M.; Waters, J. M. J. Chem. Soc., Chem. Commun. 1993, 1658.
(8) Ueyama, N.; Nishikawa, N.; Yamada, Y.; Okamura, T.; Oka, S.;
Sakurai, H.; Nakamura, A. Inorg. Chem. 1998, 37, 2415.
(9) Ueyama, N.; Yamada, Y.; Okamura, T.; Kimura, S.; Nakamura, A.
Inorg. Chem. 1996, 35, 6473.
(10) Ueno, T.; Nishikawa, N.; Moriyama, S.; Adachi, S.; Lee, K.; Okamura,
T.; Ueyama, N.; Nakamura, A. Inorg. Chem. 1999, 38, 1199.
(11) Kapteijn, G. M.; Grove, D. M.; Smeets, W. J. J.; Kooijman, H.; Spek,
A. L.; Vankoten, G. Inorg. Chem. 1996, 35, 534.
(12) Ueno, T.; Ueyama, N.; Nakamura, A. J. Chem. Soc., Dalton Trans.
1996, 3859.
* To whom correspondence should be addressed. E-mail: mtashby@
chemdept.chem.ou.edu.
(1) Hydrogen bonds to cysteinate ligands is indicated for copper proteins
(e.g., plastocyanin: Guss, J. M.; Freeman, H. C. J. Mol. Biol. 1983,
169, 521), heme iron proteins (e.g., cytochrome P-450: Poulos, T.
L.; Finzel, B. C.; Howard, A. J. J. Mol. Biol. 1987, 195, 687) and
iron-sulfur proteins (e.g., rubredoxin: Day, M. W.; Hsu, B. T.;
Joshuator, L.; Park, J. B.; Zhou, Z. H.; Adams, M. W. W.; Rees, D.
C. Protein Sci. 1992, 1, 1494).
(2) Ueyama, N.; Taniuchi, K.; Okamura, T.; Nakamura, A.; Maeda, H.;
Emura, S. Inorg. Chem. 1996, 35, 1945.
(3) Huang, J. O.; Ostrander, R. L.; Rheingold, A. L.; Walters, M. A. Inorg.
Chem. 1995, 34, 1090.
(13) Caulton, K. G. New J. Chem. 1994, 18, 25.
(14) Sellmann, D.; Sutter, J. ACS Symp. Ser. 1996, 653, 101.
(15) Wander, S. A.; Reibenspies, J. H.; Kim, J. S.; Darensbourg, M. Y.
Inorg. Chem. 1994, 33, 1421.
(4) Walters, M. A.; Dewan, J. C.; Min, C.; Pinto, S. Inorg. Chem. 1991,
30, 2656.
(16) Treichel, P. M.; Schmidt, M. S.; Crane, R. A. Inorg. Chem. 1991, 30,
379.
2202 Inorganic Chemistry, Vol. 41, No. 8, 2002
10.1021/ic011096g CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/26/2002

Discontinuum between Thiolate and Thiol
Given the weakened donor properties of thiolate ligands that
bear H-bonds, the redox potentials of metals are influenced,
and this is of profound significance with respect to redox-
active metalloproteins. Most of our understanding of the
effects of H-bonding to thiolates has been garnered from
electrochemical measurements on the aforementioned model
compounds.3,18-21 Because such electrochemical experiments
are carried out at high ionic strength, interpretation of the
effects of H-bonding are complicated by the composite
influence of the medium. This has prompted us to explore
the donor properties of dipole-perturbed thiolate ligands using
spectroscopic probes.22 The carbonyl ligand is a sensitive
reporter of electronic changes that take place at metal centers.
Indeed, carbonyl ligands in metalloproteins and their model
compounds have offered considerable insight into the
electronic structures of their metal centers.23 Other studies
have employed carbonyl ligands to explore the effect
H-bonding to chloride ligands has on their ability to π-sta-
bilize unsaturated metal centers.24 In the present study, we
employ the electron-rich d6 iron-thiolate complex 1 as a
model for exploring the effects of Fe-S(R)‚‚‚H-X interac-
tions on the donor properties of the thiolate ligand in low-
dielectric solvents. Hydrogen-bond donors (HX) yield ad-
ducts of 1 in solution (1‚HX) that may be observed under
equilibrium conditions. Sufficiently acidic compounds pro-
tonate 1 to give a thiol complex (1H+). The effect of H-bond
donation and eventual protonation of 1 (as reported by
carbonyl stretching frequencies and modeled by density
functional theory (DFT) calculation) is reported herein.
Experimental Section
Chemicals, Solvents, and General Procedures. All operations
were carried out using Schlenk or glovebox techniques under argon
or nitrogen unless stated otherwise. Hydrocarbon solvents were
distilled from sodium/benzophenone, and CH2Cl2 was distilled from
CaH2. All solvents were degassed by three freeze-pump-thaw
cycles before use. CpFe(CO)2SPh, CpFe(CO)2Cl, and CpFe(CO)2I
were prepared by literature methods.25-27 CH3COOH, CClH2-
COOH, CCl2HCOOH, CCl3COOH, CF3COOH, and HBF4‚O(CH3)2
were purchased from Aldrich Chemical Co. and used as received.
1
Instruments and References. H NMR spectra were recorded
on a Varian Inova 400 spectrometer. The NMR samples were
prepared in tubes that had been glass-blown onto Schlenk adapters.
The solutions were freeze-pump-thawed before flame-sealing
1
under vacuum. H NMR spectra were referenced to the residual
solvent peak of CDHCl2 (5.32 ppm). UV-vis spectra were recorded
using a Hewlett-Packard HP8453 diode array spectrophotometer.
Infrared spectra were recorded using a Bruker IFS 66/S FTIR with
a DTGS detector.
Preparation of [CpFe(CO)2(HSPh)]BF4, 1(R ) Ph)‚HBF4. A
procedure different than that previously reported was employed to
synthesize 1(R ) Ph)‚HBF4.28 To a solution of 1(R ) Ph) (150
mg, 0.53 mmol) in CH2Cl2 (50 mL) was added HBF4‚O(CH3)2 (64
µL, 0.53 mmol). The color of the solution changed immediately
from red to yellow. After the solution was stirred for 5 min, pentane
was added, and the product precipitated. The yellow powder was
recovered using a Schlenk frit, and it was dried in vacuo for 4 h to
give 1(R ) Ph)‚HBF4 (170 mg, 0.46 mmol, 87% yield). The
product was characterized by comparison of its NMR and IR spectra
with those of an authentic sample. IR(CH2Cl2, cm-1): νCO 2067,
2025. 1H NMR (CH2Cl2, 400 MHz, 20 °C): δ 7.58-7.40 (m, Ph),
5.38 (s, SH), 5.34 (s, Cp).
UV-Vis and IR Measurements. Infrared spectroscopy mea-
surements were made using a solution cell with NaCl plates and a
0.5 mm Teflon spacer. A quartz cell with a 0.2 mm path length
was utilized in the UV-vis experiments. These path lengths
permitted the same solutions to be employed for both measurements.
All IR spectra were analyzed by taking the second derivative of
each spectrum and then by fitting using the Gaussian amplitude
method in the computer program Peakfit.29 The spectra were in
general fitted with the minimum number of Gaussian functions
necessary to reproduce the line shapes. However, in a few cases,
additional functions were employed when it was known by
independent experiments that peaks overlapped to produce a
symmetrical experimental peak. In such cases, the positions of the
overlapping functions were fixed.
Intermolecular Interaction of 1 and 1H+. Individual CH2Cl2
solutions of 1(R ) Ph) and 1(R ) Ph)‚HBF4 (10 mM each) were
prepared. The spectra that were obtained for these solutions were
compared with the spectra of CH2Cl2 solutions of 1(R ) Ph) (10
mmol) to which less than 1 equiv of HBF4‚O(CH3)2 was added.
The spectrum of Figure S1, which resulted when 0.5 equiv of HBF4‚
O(CH3)2 was added, was satisfactorily fit with the parameters that
(17) Henderson, R. A.; Oglieve, K. E. J. Chem. Soc., Dalton Trans. 1998,
1731.
(18) Allan, C. B.; Davidson, G.; Choudhury, S. B.; Gu, Z. J.; Bose, K.;
Day, R. O.; Maroney, M. J. Inorg. Chem. 1998, 37, 4166.
(19) Sellmann, D.; Becker, T.; Knoch, F. Chem. Eur. J. 1996, 2, 1092.
(20) Farmer, P. J.; Reibenspies, J. H.; Lindahl, P. A.; Darensbourg, M. Y.
J. Am. Chem. Soc. 1993, 115, 4665.
(21) Huang, J.; Ostrander, R. L.; Rheingold, A. L.; Leung, Y. C.; Walters,
M. A. J. Am. Chem. Soc. 1994, 116, 6769.
(22) Rose, K.; Shadle, S. E.; Eidsness, M. K.; Kurtz, D. M.; Scott, R. A.;
Hedman, B.; Hodgson, K. O.; Solomon, E. I. J. Am. Chem. Soc. 1998,
120, 10743.
(23) See, for example, infrared studied of hemoglobin (Dong, A. C.;
Caughey, W. S. Methods Enzymol. 1994, 232, 139; Friedman, J. M.
Methods Enzymol. 1994, 232, 205) and [NiFe]hydrogenases (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 and
references therein).
(24) Yandulov, D. V.; Caulton, K. G.; Belkova, N. V.; Shubina, E. S.;
Epstein, L. M.; Khoroshun, D. V.; Musaev, D. G.; Morokuma, K. J.
Am. Chem. Soc. 1998, 120, 12553.
(25) Ahmad, M.; Bruce, R.; Knox, G. R. J. Organomet. Chem. 1966, 6, 1.
(26) Piper, T. S.; Cotton, F. A.; Wilkinson, G. Inorg. Nucl. Chem. 1955,
1, 165.
(27) Piper, T. S.; Cotton, F. A.; Wilkinson, G. Inorg. Nucl. Chem. 1956,
3, 104.
(28) Treichel, P. M.; Rosenhein, L. D. Inorg. Chem. 1981, 20, 942.
(29) PeakFit, Version 4.06; AISN Software Inc.: Saugus, MA, 1991.
Inorganic Chemistry, Vol. 41, No. 8, 2002 2203

McGuire et al.
Table 1. Crystal Structure Refinement Data for CpFe(CO)2SPh (1(R )
Ph)) and [CpFe(CO)2(HSPh)](BF4) (1(R ) Ph)‚HBF4)
1(R ) Ph)
1(R ) Ph)‚HBF4
chemical formula
fw
Cl3H10FeO2S
286.12
Cl3H11BF4FeO2S
373.94
cryst syst
space group
a, Å
b, Å
c, Å
monoclinic
P21/n
monoclinic
P21/c
6.147(2)
15.933(3)
12.711(3)
97.92(3)
1233.1(5)
4
6.3987(10)
7.4616(14)
31.177(5)
95.206(12)
1482.4(4)
4
â, deg
V Å3
Figure 1. Thermal ellipsoid drawing of the crystal structure of 1(R )
Ph) at the 50% level with the labeling scheme.
Z
temp, K
λ, Å
188(2)
0.71073
1.541
188(2)
0.71073
1.675
1.202
0.0543, 0.1330
0.0785, 0.1637
1.092
F
calcd, g cm-3
µ, mm-1
1.376
R1, wR2 [I > 2σ(I)]a
R1, wR2
0.0536, 0.1437
0.0657, 0.1558
1.122
GOF on F2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) [∑[w(Fo2 - Fc )]/∑[w(Fo )2]]1/2; w
2
2
2
) 1/∑[σ2(Fo)2+(aP)2+(bP)2+(cP)2], P ) (Fo)2[1/3 + [2(Fo)2/3]] for Fo
2
g 0 (otherwise zero). GOF ) [∑[w(Fo2 - Fc )]/(n - m)]1/2, where n ) no.
of reflections observed and m ) no. parameters.
were derived from individual spectra of the authentic samples of
1(R ) Ph) and 1(R ) Ph)‚HBF4.
Solutions of 1(R ) Ph) with Incremental Concentrations of
CCl3COOH. Six CH2Cl2 solutions of 1(R ) Ph) (10 mM) with
CCl3COOH (0, 10, 50, 100, 500, 1000 mM) were prepared under
nitrogen (Figure 3).
Solutions of 1(R ) Ph) with the Same Concentration of
Various Carboxylic Acids. Six CH2Cl2 solutions of 1(R ) Ph)
(10 mM) with 150 mM CH3COOH, CClH2COOH, CCl2HCOOH,
CCl3COOH, CF3COOH, and HBF4‚O(CH3)2 were prepared under
nitrogen (Figure S2).
Solutions of CpFe(CO)2X (X ) Cl,I) with CF3COOH.
Individual solutions of CpFe(CO)2X (X)Cl,I) (10 mM in CH2Cl2)
were prepared. The spectra that were obtained for these solutions
were compared with spectra of individual CH2Cl2 solutions of CpFe-
(CO)2X (X ) Cl,I) (10 mmol) that also contained CF3COOH (150
mM) (Table 4).
Solutions of 1(R ) Ph) and CpFe(CO)2Cl in CH2Cl2 Solutions
with Added Electrolyte. Individual CH2Cl2 solutions of CpFe-
(CO)2X (X ) SPh,Cl) (10 mM each) were prepared. The spectra
that were measured for these solutions were compared with the
spectra of CH2Cl2 solutions of CpFe(CO)2X (X ) Cl,I) (10 mmol)
that also contained (NBu4)(PF6) (1 M) (Table 4).
Figure 2. Thermal ellipsoid drawing of the crystal structure of 1(R )
Ph)‚HBF4 at the 50% level with the labeling scheme.
Lorentzand polarization effects, and empirical absorption corrections
based on ψ scans were applied.31 Both structures were solved by
direct methods and refined by least-squares on F2 using all
reflections using the SHELXTL (Siemens) system.32 For 1(R )
Ph)‚HBF4, the hydrogen atom bound to S was located in a
difference map, and it was refined isotopically. All other hydrogen
atoms were included with idealized parameters. Thermal ellipsoid
drawings of 1(R ) Ph) and 1(R ) Ph)‚HBF4 are illustrated in
Figures 1 and 2, respectively. The hydrogen atom H(1) on the S
-
atom of 1(R ) Ph)‚HBF4 forms a H-bond with F(1) of the BF4
anion as illustrated in Figure 2. Additional crystallographic results
are summarized in Tables S1-S9.
Electronic Structure and Geometry Calculations. The mo-
lecular mechanics and molecular orbital calculations for 1(R ) Me),
1(R ) Ph)‚HOOCF3, 1(R ) Ph)‚HBF4, and 1(R ) Ph)H+ were
performed with SPARTAN 5.0 running on a Silicon Graphics IRIS
Indigo 2 Solid Impact with an R10000 processor. The initial
geometries were optimized using the Merck molecular force field
(MMFF). The resulting geometries were refined using the semiem-
pirical PM3(tm) method. Final geometries were obtained at the pBP/
DF* level by optimization of all metric parameters. The perturbative
Becke-Perdew method (pBP) is analogous to the BP method, but
the gradient correction is introduced only after convergence based
on the local potential alone has been achieved. DN* is a Gaussian
basis set similar to 6-31G*, except five pure d-type functions are
Solutions of 1(R ) Ph) and CCl3COOH with Added Elec-
trolyte. Four CH2Cl2 solutions of 1(R ) Ph) (10 mM) with CCl3-
COOH (0, 50, 100, 250 mM) were prepared under nitrogen.
Sufficient (NBu4)(PF6) was added to each of the solutions to give
[CCl3COOH] + [(NBu4)(PF6)] ) 1 M (1.00, 0.95, 0.90, and 0.75
M, respectively). The IR spectra of the resulting solutions are
illustrated in Figure S3.
X-ray Crystal Structures of 1(R ) Ph) and 1(R ) Ph)‚HBF4.
Crystals of 1(R ) Ph) and 1(R ) Ph)‚HBF4 suitable for analysis
by X-ray diffraction were obtained by evaporation of a saturated
pentane and CH2Cl2 solution, respectively, under nitrogen. The
conditions that were employed to collect and process the X-ray
data are summarized in Table 1. Data for both crystals were
collected at -85 °C using a Siemens P4 diffractometer and Mo
KR (λ ) 0.71075 Å) radiation.30 The data were corrected for
(31) North, A. C. T.; Phillips, D. C.; Mathews, F. S. Acta Crystallogr.
1968, A24, 351.
(32) SHELXTL Software Package for the Determination of Crystal
Structures, Release 5.1; Siemens Analytical X-ray Instruments, Inc.:
Madison, WI, 1995.
(30) Siemens XCSANS: X-ray Single-Crystal Analysis System, Version 2.1;
Siemens Analytical X-ray Instruments, Inc.: Madison, WI, 1994.
2204 Inorganic Chemistry, Vol. 41, No. 8, 2002

Discontinuum between Thiolate and Thiol
Table 2. Selected Bond Distances (Å), Bond Angles (deg), and
Torsional Angles (deg) for CpFe(CO)2SPh (1(R ) Ph)) and
[CpFe(CO)2(HSPh)](BF4) (1(R ) Ph)‚HBF4)
used instead of six second-order Gaussians. The resulting coordi-
nates of the computed structures of 1(R ) Me), 1(R ) Ph)‚
HOOCF3, 1(R ) Ph)‚HBF4, and 1(R ) Ph)H+ are available as
Supporting Information (Tables S10-S13). Computed metric data
for 1(R ) Me) and 1(R ) Me)H+ are compared with experimental
data for 1(R ) Ph) and 1(R ) Ph)‚HBF4 in Table 3. Computed
vibrational frequencies are summarized in Table 4.
1(R ) Ph)
1(R ) Ph)‚HBF4
F1-S1
2.283(2)
1.766(6)
1.770(6)
1.724(6)
1.772(5)
2.259(1)
1.819(3)
1.789(3)
1.711(3)
1.776(4)
1.35(7)
Fe1-C12
Fe1-C13
Fe1-Cpa
S1-C6
Results and Discussion
S1-H1
S1-F1
H1-F1
3.363(4)
2.20(8)
Model System. We will begin by explaining our choice
of metal thiolate complex, solvent system, and H-bond
donors that were employed in this study. We have previously
shown that the organometallic complexes CpFe(CO)2SR (1)
are highly electron-rich, as evidenced by the high-lying first
ionization potentials (IPs) observed in their gas-phase pho-
toelectron spectra and the fact that they readily react with
electrophiles such as alkyl halides33 and electron-deficient
alkynes.34 The highest occupied molecular orbitals (HOMOs)
of these complexes are Fedπ-Spπ antibonding and largely
sulfur in character. Free-energy relationships have been
observed between the HOMO IPs and the carbonyl stretching
frequencies with respect to the rates of SN2 electrophilic
attack on sulfur.33,34 Thus, the carbonyl stretching frequencies
are direct reporters of the nucleophilicity of the sulfur. In
the present study, we concern ourselves with the effects of
dipolar H-bonding on the donor properties of the thiolate
ligand. Because the addition of H-bond donors (Brønsted
acids) to 1 can lead to decomposition of the complex, we
employed relatively stable aryl derivative 1(R ) Ph) in this
study. Alkyl derivatives of 1 tend to be less stable that aryl
derivatives. It was desirable to employ nonpolar solvents in
our measurements for two reasons. First, 1 is a relatively
weak H-bond acceptor, and polar solvents tend to be
competitive H-bond acceptors. Second, 1H+ tends to undergo
solvolysis with displacement of the relative weak thiol ligand.
Thus, 1 has the added advantage (as an organometallic) of
being soluble in nonpolar organic solvents. We initially
employed CCl4 as a solvent because it has been extensively
used to study H-bonding effects,35 but we found 1(R ) Ph)
decomposes (via an apparent chain-radical mechanism) in
CCl4 to give the known dimer [(CpFe(CO)2)2(S2Ph2)]2+.36
We found no such problem with CH2Cl2. After investigating
several classes of H-bond donors, we settled on carboxylic
acids because they exhibited the appropriate range of H-bond
donor ability and suitable solvent properties. We note from
the outset that our discussion of the effects of adding
carboxylic acids to CH2Cl2 solutions of 1 is restricted to
analyzing trends. Whereas acid/base titrations in water can
be treated quantitatively, the polar solutes that we employ
have a significant effect on the properties of the solvents
themselves. Furthermore, aggregation of polar solute mol-
ecules in nonpolar solvents can lead to localized effects.
Accordingly, a quantitative analysis of the titration experi-
ments that are described herein is not provided because of
Fe1-S1-C6
Fe1-C12-O1
Fe1-C13-O2
S1-Fe1-C12
S1-Fe1-C13
Cp-Fe1-S1
Cp-Fe1-C12
Cp-Fe1-C13
S1-H1-F1
113.5(2)
178.3(6)
178.2(5)
87.1(2)
111.2(2)
176.4(3)
178.4(3)
96.04(9)
94.19(9)
120.4(1)
122.1(1)
123.5(1)
147(3)
90.4(2)
123.6(3)
124.8(4)
125.9(4)
Cp-Fe1-S1-C6
C12-Fe1-S1-C6
C13-Fe1-S1-C6
Fe1-S1-C6-C7
Fe1-S1-C6-C11
86.4(3)
155.9(3)
-49.6(2)
44.1(2)
93.6(4)
-89.4(3)
-142.8(3)
-48.7(3)
163.4(3)
-17.8(5)
a Cp ) centroid of the η5-C5H5 ligand.
the inherent complexity of modeling dipolar reactions in a
nonpolar environment.
Effect of Protonation on the Molecular Structure of 1.
To determine the effect of protonation on the molecular
structure of 1 and to later calibrate the molecular orbital
calculations, [CpFe(CO)2(HSPh)](BF4) (1(R ) Ph)‚HBF4)
was synthesized, and its molecular structure was determined
by single-crystal X-ray crystallography. Apparently, the
crystal structure of 1(R ) Ph)‚HBF4 is the first of an
Fe(II)-thiol complex to be reported, although there are
several structures of Fe(III)-thiol complexes known, all
porphyrin complexes.37,38 For the sake of comparison, we
also determined the structure of 1(R ) Ph). The structure
of 1(R ) Et) has been reported previously.39 The structures
of 1(R ) Ph) and 1(R ) Ph)‚HBF4 are summarized in
Tables 1 and 2. Thermal ellipsoid drawings of 1(R ) Ph)
and 1(R ) Ph)‚HBF4 are provided in Figures 1 and 2,
respectively. Selected metric data for 1(R ) Ph) and 1(R )
Ph)‚HBF4 are compared with relevant data from the literature
in Table 3.
The crystal structure of Fe(TPP)(SPh)(HSPh) reveals that
the Fe-S bond length for the Fe(III)-benzenethiolate group
(2.27(2) Å) is shorter than that of the Fe(III)-benzenethiol
group (2.43(2) Å).38 This is not unexpected because a thiol
is a weaker Lewis base than a thiolate. Remarkably, the
Fe-S distance of 1(R ) Ph)‚HBF4 is shorter than the
corresponding distance for 1(R ) Ph)! We attribute the
reduction of Fe-S distance from 2.282(2) Å for 1(R ) Ph)
(37) Byrn, M. P.; Katz, B. A.; Keder, N. L.; Levan, K. R.; Magurany, C.
J.; Miller, K. M.; Pritt, J. W.; Strouse, C. E. J. Am. Chem. Soc. 1983,
105, 4916.
(38) Collman, J. P.; Sorrell, T. N.; Hodgson, K. O.; Kulshrestha, A. K.;
Strouse, C. E. J. Am. Chem. Soc. 1977, 99, 5180.
(39) English, R. B.; Nassimbeni, L. R.; Haines, R. J. J. Chem. Soc., Dalton
Trans. 1978, 1379.
(33) Ashby, M. T.; Enemark, J. H.; Lichtenberger, D. L. Inorg. Chem. 1988,
27, 191.
(34) Ashby, M. T.; Enemark, J. H. Organometallics 1987, 6, 1318.
(35) Raevsky, O. A. J. Phys. Org. Chem. 1997, 10, 405.
(36) Treichel, P. M.; Rosenhein, L. D. Inorg. Chem. 1984, 23, 4018.
Inorganic Chemistry, Vol. 41, No. 8, 2002 2205

McGuire et al.
Table 3. Experimental and Computed Metric Data for CpFe(CO)2SR
(1) and [CpFe(CO)2(HSR)]+ (1H+)
cmpd
Fe-S Fe-COavg Fe-Cp Fe-S-C ref
Experimental
CpFe(CO)2SEt
CpFe(CO)2SPh
[CpFe(CO)2(HSPh)](BF4) 2.259(1) 1.80(2)
2.296(2) 1.751(5) 1.76(1) 107.0(2) 39
2.283(2) 1.768(2) 1.724(6) 113.5(2)
a
a
1.711(3) 111.2(2)
Computed
CpFe(CO)2SMe
2.330
2.284
1.762(4) 1.757
1.784(1) 1.752
106.1
112.5
a
a
[CpFe(CO)2(HSMe)]+
a This study.
to 2.258(2) Å for 1(R ) Ph)‚HBF4 to relief of the four-
electron destabilization that results in Fedπ-Spπ antibonding
in 1. This interpretation is also supported by molecular orbital
calculations that reproduce the observed trend in bond lengths
(vide infra). We note a similar trend has been observed for
(t-BuSH)Cr(CO)5 versus its deprotonated analogue.40 Other
metric data of interest include the position of the H atom
bound to the S atom of 1(R ) Ph)‚HBF4, which was located
in a difference map and refined. The H atom is 1.35(7) Å
-
from the S atom and 2.20(8) Å from a F atom of the BF4
counterion. The resulting S-H‚‚‚F angle of 147(3)° suggests
a nearly optimal dipolar interaction.41
Effect of Protonation on the Spectra of 1. Addition of
1 equiv of strong acids such as HBF4‚OMe2 to solutions of
1(R ) Ph) in nonpolar solvents such as CH2Cl2 results in
an immediate change in color from red to yellow, which
corresponds to loss of the Fe-S LMCT band in the visible
spectrum (cf. Figure 3, top). We have characterized the
product of this reaction as the thiol complex [CpFe(CO)2-
(HSPh)](BF4) (1(R ) Ph)‚HBF4).28 There is a corresponding
increase in the carbonyl stretching frequencies upon proto-
nation of 1, which is consistent with a reduction in the donor
ability of the thiol ligand with respect to the thiolate (Figure
3, middle). To interpret the experiments that will be described
in the next section, it was important to establish whether
the presence of 1H+ influences the spectra of 1. The addition
of less than 1 equiv of HBF4‚OMe2 produces a mixture of 1
and 1H+ (Figure S1). The carbonyl stretching frequencies
of 1(R ) Ph) are unperturbed under such conditions, which
indicates no association of 1 and 1H+.
Effect of H-Bonding on the Spectra of 1. The addition
of relatively weak acids of different effective acidities results
in changes in the electronic and vibrational spectra that can
be attributed to the equilibria of eq 1. Thus, an increase in
the concentration of CCl3COOH results in gradual loss of
the Fe-S LMCT band in the electronic spectrum and a shift
of the carbonyl vibrational modes to higher frequency. Rather
than a discrete species, we intend 1‚HX to represent the
continuum of change that takes place when the S-H-X
H-bond is polarized from X to S. The absence of an isosbestic
point at relatively low concentrations of carboxylic acid
(before the appearance of 1H+) and the fact that the band-
Figure 3. Top: changes in the UV-visible spectra of 1(R ) Ph) in
CH2Cl2 upon addition of trichloroacetic acid. Note the lack of isosbestic
points. The spectrum of 1(R ) Ph)‚HBF4 is shown for comparison.
Middle: corresponding changes in the IR spectra of the same solutions
that were used to collect the UV-vis spectra. Bottom: normalized IR data
that illustrate a shift in frequency without significant change in line width.
width of the asymmetric carbonyl vibrational mode remains
constant while its maximum shifts to higher frequency (Fig-
ure 3, bottom) provide evidence for a continuum of change
rather than a shift in equilibrium between discrete species 1
and 1‚HX. Such a polarization is expected to depend on
several factors, including the relative polarities of the SH
and XH bonds and the abilities of the medium to stabilize
the species involved. Therefore, one might expect the
equilibrium to shift to the right with the effective acidity of
HX.41,42 Indeed, for a given set of concentrations, we observe
such a relationship experimentally (Table S2).
Maximum Effect of H-Bonding on the Donor Ability
of a Thiolate Ligand. Although the range of acidities of
the H-bond donors that were employed in this study in
CH2Cl2 was immense, the acidities of H-bond donors in an
aqueous environment are more restricted (by the autopro-
tolysis of water). We note that amide NH bonds, which are
the conventional donors to cysteinate ligands in metallopro-
teins, are considerably less acidic than carboxylic acids.
(40) Darensbourg, M. Y.; Longridge, E. M.; Payne, V.; Reibenspies, J.;
Riordan, C. G.; Springs, J. J.; Calabrese, J. C. Inorg. Chem. 1990, 29,
2721.
(41) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University
Press: New York, 1997.
(42) Jeffrey, G. A.; Saenger, W. Hydrogen Bonding in Biological Structures;
Springer-Verlag: New York, 1991.
2206 Inorganic Chemistry, Vol. 41, No. 8, 2002

Discontinuum between Thiolate and Thiol
Table 4. Experimentala (R ) Ph) and Computedb (R ) Me) Carbonyl
Stretching Frequencies for (η5-C5H5)Fe(CO)2(SR), 1, Its
Hydrogen-Bonded Adducts, and Related Compounds
However, acidity is defined not only by the chemical nature
of the species but also by its environment. The acidities (and
basicities) of particular functional groups are known to vary
several orders of magnitude within proteins, which is due in
large part to the dielectric environment.43 For example, the
presence of a metal can lower the pKa of the sulfhydryl group
of cysteine by several pK units.44 For the present case,
evolution from a nonpolar to a polar dielectric environment
is simulated by simply varying the concentration of a given
H-bond donor (because the H-bond donors are more polar
than the solvent itself). Thus, discrete species such as 1‚HX
are not observed when [HX] is varied. Instead, a continuum
of change is observed that reflects the combined influences
of H-bond donation to the thiolate ligand and the polarity of
the solvent environment (Figure 3). Eventually, the solvent
environment becomes sufficiently polar to stabilize the
conjugate acids and bases, and ionization occurs (i.e.,
protonation of the thiolate takes place). Equilibrium mixtures
of 1‚HX and 1H+ (Figure 3, middle) reveal the upper limit
of the effect of H-bonding on the donor properties of the
thiolate ligand of 1. Further differentiation of the relative
basicities of S and X simply results in proton transfer to the
S. The effects of H-bonding are continuous only to the “end
point” of acid titration, at which point protonation occurs
and a discrete change in the donor properties of the thiolate
ligand takes place. Interestingly, the upper limit of the effect
of H-bonding does not appear to be sensitive to the nature
of X-.
pKa (in obsd νCO
obsd FCO (ki)
calcd νCO
(cm-1
cmpd
1H+
acid
HBF4
water)
(cm-1
)
(mdyn/Å)
)
<0
2069 2027 16.939 (0.347) 2068 2031c
1‚TFA
1‚TCA
1‚DCA
1‚CAA
1‚AcOH
1
F3CCO2H
Cl3CCO2H
Cl2HCCO2H
ClH2CCO2H
H3CCO2H
none
<0.5 2040 1994 16.431 (0.374) 2035 1993
0.7 2039 1993 16.414 (0.374)
1.5 2035 1990 16.357 (0.365)
2.9 2033 1986 16.309 (0.381)
4.8 2032 1985 16.292 (0.381) 2032 1990
2032 1985 16.292 (0.381) 2025 1980
2053 2007 16.643 (0.377)
FpCld
none
FpCl‚TFA F3CCO2H
<0.5 2058 2014 16.741 (0.361)
FpCl
FpI
(NBu4)(PF6)
none
2053 2007 16.643 (0.377)
2041 1996 16.455 (0.366)
FpI
F3CCO2H
<0.5 2041 1996 16.455 (0.366)
a Concentrations of 10 mM 1 and 175 mM acid in CH2Cl2. b Calculated
at the pDF/BP* level for the Brønsted acid adducts of CpFe(CO)2SMe using
Spartan 5.0. The results of these calculations are available in the Supporting
Information. c These are the vibrational frequencies for the naked 1H+ ion
in the gas phase. The values of 2043 and 1998 were calculated for the tight
ion pair 1‚HBF4. d Fp ) (η5-C5H5)Fe(CO)2.
Fe19 and Ni18 complexes upon protonation. While the ab-
solute values of ∆E1/2 predicted by eq 3 may not be accurate,
we are more concerned here with the relative importance of
H-bonding versus protonation. It has been reported that the
redox potentials of model NiI-thiolate complexes are shifted
by +250 mV in polar solvents that are capable of H-bonding
versus nonpolar solvents.20 Unfortunately, the corresponding
effects of protonation are not known for these systems.
Furthermore, similar dielectric effects are observed for the
redox potentials of molecules that are not expected to be
significant H-bond acceptors. Using the value of FCO ) 16.46
mdyn/Å that is observed for 1‚TFA, we estimate a maximal
effect of 300 e ∆E1/2 e 400 mV for H-bonding to 1. This
corresponds to an effect for H-bonding that is about 30% of
that observed upon protonation.
It is noteworthy that the observed interaction between 1
and H-bond donors is not simply a dielectric effect in that
(NBu4)(PF6) does not affect νco. Furthermore, the corre-
sponding iodide complex is not influenced by acid (Table
4). There is an effect of acid on νco of the chloride derivative
(consistent with the harder character of Cl- vs I-), albeit
less significant than the effect on 1 under similar conditions.
The series FpX may offer the opportunity to systematically
investigate the effect of H-bond donation as a function of
X. Although the addition of an “innocent” electrolyte such
as (NBu4)(PF6) does not influence its spectra (Table 4),
solvatochromism is observed for 1 (Table 5). There is an
approximate linear dependence of λmax on the dielectric
constant of the solvent (Figure 5). This trend may be
attributed to stabilization of the ground state (dipole stabi-
lization of the antibonding HOMO of 1, Figure 4) or Franck-
Condon destabilization of the photoexcited state.50 However,
H-Bonding/Protonation versus Redox Potentials. It
would be useful to understand the relative impact of
H-bonding versus protonation on the redox potential of 1.
Unfortunately, 1+ rapidly dimerizes upon oxidation,36 and
irreversible cyclic voltammograms are obtained. Also, the
thiol ligand of 1H+ is rapidly displaced by even weakly
donating electrolytes. Furthermore, the aforementioned sol-
vent dielectric effects complicate the interpretation. We have
previously established a relationship between the first IP and
FCO of 1:33
IP ) 2.68FCO - 36.6
(1)
Correlation between electrochemical and photoionization data
generally yields empirical relationships of the form45-49
E1/2 ) (0.6 to 0.8)IP - (3.5 to 4.5)
Using the empirical formula
∆E1/2 ) λ∆FCO (i.e., for 1.6 e λ e 2.1
(2)
and the observed ∆FCO ) 0.65 mdyn/Å) (3)
we predict a shift of the oxidation potential of 1 by 1.0-1.3
V upon protonation. This range may be compared with the
500-700 mV positive shift that has been observed for model
(46) Gower, M.; Kane-Maguire, L. A.; Maier, J. P.; Sweigart, D. A. J.
Chem. Soc., Dalton Trans. 1977, 316.
(43) Fersht, A. Enzyme Structure and Mechanism; W. H. Freeman: New
York, 1985.
(44) Hightower, K. E.; Huang, C. C.; Casey, P. J.; Fierke, C. A.
Biochemistry 1998, 37, 15555.
(47) Byers, B. P.; Hall, M. B. Organometallics 1987, 6, 2319.
(48) Hubbard, J. L.; Lichtenberger, D. L. J. Chem. Phys. 1981, 75, 2560.
(49) Lichtenberger, D. L.; Elkadi, Y.; Gruhn, N. E.; Hughes, R. P.; Curnow,
O. J.; Zheng, X. Organometallics 1997, 16, 5209.
(45) Lichtenberger, D. L.; Sellmann, D.; Fenske, R. F. J. Organomet. Chem.
1976, 117, 253.
(50) Drago, R. S. Physical Methods for Chemists, 2nd ed.; Saunders College
Publishing: Ft. Worth, TX, 1992; pp 135-137.
Inorganic Chemistry, Vol. 41, No. 8, 2002 2207

McGuire et al.
Figure 4. Highest occupied molecular orbitals (HOMOs) of various Brønsted acid adducts of 1(R ) Me) as calculated by the DFT method. Left to right:
1(R ) Me), 1(R ) Me)‚TFA, 1(R ) Me)‚HBF4, and 1(R ) Me)H+. Note that the HOMO is largely S in character until the H-bond is polarized toward
S, at which time the HOMO becomes largely Fe in character (with concomitant increase in Cp character).
Table 5. Spectroscopic Data for CpFe(CO)2SPh (1(R ) Ph)) in
hydrogen bonding are capable of modifying the redox
properties of metal centers.
Various Solvents
solvent polarity
UV-vis
(λmax, nm)
IRa
solvent
pentane
EΝ
(νCO, cm-1
)
Τ
Conclusion
0.009
0.111
0.207
0.309
0.355
0.460
0.568
0.602
0.762
0.898
582
572
564
560
558
552
550
550
542
2035, 1992
2029, 1985
2028, 1988
2032, 1985
2029, 1980
2031, 1983
2032, 1987
2033, 1990
2022, 1985
2045, 2005
We conclude that when the conjugate base of the H-bond
donor is a stronger base than the thiolate ligand (as in the
present case), H-bond donation has a relatively small effect,
but protonation results in a large, discontinuous effect on
the electronic structure of 1. Because protonation depends
on the relative basicities of the thiolate ligand and the con-
jugate base of the H-bond donor, a true continuum of effects
can be anticipated only for systems that are pK-matched.52-54
Such a match should permit low-barrier transformation of a
π-donor thiolate ligand55 into a π-acceptor thiol ligand.56,57
We are currently exploring this hypothesis.
benzene
tetrahydrofuran
dichloromethane
acetone
acetonitrile
1-pentanol
1-butanol
methanol
2,2,2-trifluoroethanol
<542 sh
a Italicized vibrational frequencies lie below significant solvent features,
and their values are therefore suspect.
Acknowledgment. The financial support of the National
Science Foundation (CHE-9612869), Oklahoma Center for
the Advancement of Science and Technology (OCAST
HR98-078), and the Petroleum Research Fund (ACS-PRF
35088-AC) is gratefully acknowledged. D.G.M. was the
recipient of a Department of Education G.A.A.N.N. Fellow-
ship.
Supporting Information Available: Tables of crystallographic
data, fractional coordinates of the non-hydrogen atoms, anisotropic
thermal parameters, hydrogen atom parameters, and selected
interatomic distances and angles for 1(R ) Ph) and 1(R ) Ph)‚
HBF4; DFT computed coordinates of 1(R ) Me), 1(R ) Me)H+,
1(R ) Me)‚TFA, and 1(R ) Me)‚AcOH; three figures (S1-S3).
This material is available free of charge via the Internet at
http://pubs.acs.org.
Figure 5. Relationship between the dielectric constant of nonprotic solvents
and λmax of the LMCT band of 1(R ) Ph) and 1(R ) C6H4-p-CF3) (O and
×, respectively).
IC011096G
the latter rationalization is suggested by the corresponding
values of νco that indicate specific solute-solvent interactions
in the ground state that are not proportional to ENT. It is
noteworthy that all rubredoxins (FeII/III(cys)4) feature a con-
served tyrosine with an arene group that is in close proximity
to one of the cysteinate ligands, and deletion of this resi-
due affects the redox potential of the iron center.51 Thus,
specific dipolar solute-solvent dipolar interactions other than
(51) Low, D. W.; Hill, M. G. J. Am. Chem. Soc. 1998, 120, 11536.
(52) Chen, J. G.; McAllister, M. A.; Lee, J. K.; Houk, K. N. J. Org. Chem.
1998, 63, 4611.
(53) Kumar, G. A.; McAllister, M. A. J. Am. Chem. Soc. 1998, 120, 3159.
(54) Cleland, W. W.; Kreevoy, M. M. Science 1994, 264, 1887.
(55) Ashby, M. T. Comments Inorg. Chem. 1990, 10, 297.
(56) Kraatz, H. B.; Jacobsen, H.; Ziegler, T.; Boorman, P. M. Organome-
tallics 1993, 12, 76.
(57) Jacobsen, H.; Kraatz, H. B.; Ziegler, T.; Boorman, P. M. J. Am. Chem.
Soc. 1992, 114, 7851.
2208 Inorganic Chemistry, Vol. 41, No. 8, 2002