Ligand dependence of binding to three-coordinate Fe(11) complexes

Article abstract of DOI:10.1021/ic802440h

A series of three- and four-coordinate iron(11) complexes with nitrogen, chlorine, oxygen, and sulfur ligands is presented. The electronic variation is explored by measuring the association constant of the neutral ligands and the reduction potential of the iron(11) complexes. Varying the neutral ligand gives large changes in K_eq, which decrease in the order CN^tBu > pyridine >2-picoline > DMF > MeCN > THF > PPh₃. These differences can be attributed to a mixture of steric effects and electronic effects (both σ-donation and π-backbonding). The binding constants and the reduction potentials are surprisingly insensitive to changes in an anionic spectator ligand. This suggests that three-coordinate iron(11) complexes may have similar binding trends as proposed three-coordinate iron(11) intermediates in the FeMoco of nitrogenase, even though the anionic spectator ligands in the synthetic complexes differ from the sulfides in the FeMoco.

Full text of DOI:10.1021/ic802440h

5106 Inorg. Chem. 2009, 48, 5106–5116
DOI: 10.1021/ic802440h
Ligand Dependence of Binding to Three-Coordinate Fe(II) Complexes
Karen P. Chiang,† Pamela M. Barrett,† Feizhi Ding,‡ Jeremy M. Smith,‡ Savariraj Kingsley,†
William W. Brennessel,† Meghan M. Clark,† Rene J. Lachicotte,† and Patrick L. Holland*,†
†Department of Chemistry, University of Rochester, Rochester, New York 14627, and ‡Department of
Chemistry and Biochemistry, New Mexico State University, Las Cruces, New Mexico 88003
Received December 24, 2008
A series of three- and four-coordinate iron(II) complexes with nitrogen, chlorine, oxygen, and sulfur ligands is
presented. The electronic variation is explored by measuring the association constant of the neutral ligands and the
reduction potential of the iron(II) complexes. Varying the neutral ligand gives large changes in Keq, which decrease in
the order CNtBu > pyridine >2-picoline > DMF > MeCN > THF > PPh3. These differences can be attributed to a mixture
of steric effects and electronic effects (both σ-donation and π-backbonding). The binding constants and the reduction
potentials are surprisingly insensitive to changes in an anionic spectator ligand. This suggests that three-coordinate
iron(II) complexes may have similar binding trends as proposed three-coordinate iron(II) intermediates in the FeMoco
of nitrogenase, even though the anionic spectator ligands in the synthetic complexes differ from the sulfides in the
FeMoco.
Introduction
dependence of nitrogenase mutants.7,8 Even though the belt
iron atoms are four-coordinate in the crystallographically
characterized resting state, they could become three-
coordinate and coordinatively unsaturated in the reduced
form that binds substrates.9 This idea has motivated the study
of isolable three-coordinate iron(II) complexes that mimic the
“activated” three-coordinate sites on the FeMoco.6,10-12
Though it might be most desirable to study iron complexes
with three sulfur donors, the available S-coordinated three-
coordinate Fe complexes have a highly congested iron
environment, and are prone to ligand dissociation.13,14 We
have focused instead on three-coordinate complexes in which
two of the donors come from a bulky β-diketiminate
ligand (LtBu,iPr2, Scheme 1), because these complexes are easy
to prepare, highly crystalline, and suitable for detailed
spectroscopic and mechanistic analysis.15 One potential point
Nitrogenases are fascinating iron metalloenzymes that
have the ability to convert dinitrogen to ammonia at ambient
temperature and pressure.1,2 They also reduce a variety of
organic substrates such as alkynes, nitriles, cyanide, isocya-
nide, CO2, azide, nitrous oxide, and cyclopropene.3 So far,
scientists have not unambiguously determined the binding
location of any substrate in a nitrogenase enzyme.
The current “best guesses” for substrate binding sites are
based on a 1.16 A resolution structure of the resting state of the
˚
iron-molybdenum nitrogenase from A. vinelandii.4 The active
site of the iron-molybdenum enzyme is a cluster called
the FeMo cofactor (FeMoco), identified as MoFe7S9X(homo-
citrate), where X represents C4-, N3-, or O2- (Figure 1).
Other nitrogenases lack molybdenum but have a similarly
shaped cluster, based on Extended X-ray Absorption Fine
Structure (EXAFS) evidence.5
(7) Seefeldt, L. C.; Dance, I. G.; Dean, D. R. Biochemistry 2004, 43,
1401–1409.
(8) Dos Santos, P. C.; Igarashi, R. Y.; Lee, H.-I.; Hoffman, B. M.;
One reasonable hypothesis for the location of substrate
binding on the FeMoco is the iron atoms in the “belt” of the
enzyme, and this idea has been supported by the substrate
Seefeldt, L. C.; Dean, D. R. Acc. Chem. Res. 2005, 38, 208–214.
::
(9) Schimpl, J.; Petrilli, H. M.; Blochl, P. E. J. Am. Chem. Soc. 2003, 125,
15772–15778.
(10) Smith, J. M.; Sadique, A. R.; Cundari, T. R.; Rodgers, K. R.;
Lukat-Rodgers, G.; Lachicotte, R. J.; Flaschenriem, C. J.; Vela, J.; Holland,
P. L. J. Am. Chem. Soc. 2006, 128, 756–769.
(11) Lees, N. S.; McNaughton, R. L.; Gregory, W. V.; Holland, P. L.;
Hoffman, B. M. J. Am. Chem. Soc. 2008, 130, 546–555.
(12) Yu, Y.; Sadique, A. R.; Smith, J. M.; Dugan, T. R.; Cowley, R. E.;
Brennessel, W. W.; Flaschenriem, C. J.; Bill, E.; Cundari, T. R.; Holland,
P. L. J. Am. Chem. Soc. 2008, 130, 6624–6638.
*To whom correspondence should be addressed. E-mail: holland@chem.
rochester.edu.
(1) Holland, P. L. In Comprehensive Coordination Chemistry II;
McCleverty, J., Meyer, T. J., Eds.; Elsevier: Oxford, 2004; Vol. 8, p 569-599.
(2) Barney, B. M.; Lee, H.-I.; Dos Santos, P. C.; Hoffman, B. M.; Dean,
D. R.; Seefeldt, L. C. Dalton Trans. 2006, 2277–2284.
(3) Burgess, B. K.; Lowe, D. J. Chem. Rev. 1996, 96, 2983–3011.
(4) Einsle, O.; Tezcan, F. A.; Andrade, S. L. A.; Schmid, B.; Yoshida, M.;
Howard, J. B.; Rees, D. C. Science 2002, 297, 1696–1700.
(13) MacDonnell, F. M.; Ruhlandt-Senge, K.; Ellison, J. J.; Holm, R. H.;
Power, P. P. Inorg. Chem. 1995, 34, 1815–1822.
(14) Lee, S. C.; Holm, R. H. Proc. Natl. Acad. Sci. U.S.A. 2003, 100,
3595–3600.
::
(5) Krahn, E.; Weiss, B. J. R.; Krockel, M.; Groppe, J.; Henkel, G.;
::
Cramer, S. P.; Trautwein, A. X.; Schneider, K.; Muller, A. J. Biol. Inorg.
Chem. 2002, 7, 37–45.
(6) Holland, P. L. Can. J. Chem. 2005, 83, 296–301.
(15) Holland, P. L. Acc. Chem. Res. 2008, 41, 905–914.
r
2009 American Chemical Society
pubs.acs.org/IC
Published on Web 05/12/2009

Article
Inorganic Chemistry, Vol. 48, No. 12, 2009 5107
4
˚
Figure 1. FeMoco of iron-molybdenum nitrogenase in the resting state. X is a light atom (C, N, or O) that lies 2.0 A from the six “belt” iron atoms.
The ironatoms along the “belt” are coordinated to three bridging sulfides. We hypothesize that reductionof the cofactor transiently gives a three-coordinate
iron site that can bind substrate.6
compounds.15 In previous studies, LtBu,iPr2FeCl was synthe-
Scheme 1
sized from reaction of FeCl2(THF)1.5 and the lithium salt of
the diketiminate ligand, producing LiCl as a byproduct.
Unfortunately, the solubilities of LiCl and LtBu,iPr2FeCl are
not different enough to enable easy separation of the com-
pounds without Soxhlet extraction, which is inconvenient on
a large scale. Here, this difficulty is avoided by using the
potassium salt of the bulky β-diketiminate ligand (KLtBu,iPr2).
This potassium salt was synthesized by treating HLtBu,iPr2
with benzylpotassium22 in diethyl ether.23 Reaction of
KLtBu,iPr2 with FeCl2(THF)1.5 in toluene reproducibly gave
a clean product from which KCl could be removed by
filtration. LtBu,iPr2FeCl was isolated as red crystals in very
high yield (97%).
Double-metathesis reactions of LtBu,iPr2FeCl led to
of concern is that the identity of the donor atoms to iron
three-coordinate iron-thiolate (LtBu,iPr2FeSPh, LtBu,iPr2
FeSPhCF3, LtBu,iPr2FeSTol) and phenoxide (LtBu,iPr2
-
-
is different in the β-diketiminate complexes (N donors)
than in the FeMoco (S donors). It is reasonable to suspect
that this difference in spectator ligands might influence
the binding constants and redox potentials, invalidating the
β-diketiminate complexes as informative nitrogenase mimics.
Therefore, it is of interest to learn the influence of spectator
ligands on binding to three-coordinate iron complexes. In
this paper, we vary one of the three donors in three-coordi-
nate iron complexes, and measure the binding constant
(Kassoc) of a number of neutral ligands to gain systematic
knowledge about the trends in these complexes.
There are more general implications as well. First, the
binding preferences of trigonal-planar iron complexes are not
well understood.16,17 Also, the influence of spectator ligands
on binding constants of neutral ligands is of general interest
in coordination chemistry and catalysis and has rarely been
studiedquantitativelyfor a range of different types ofligands.
Binding of neutral ligands to porphyrin-iron complexes has
been studied in some detail, but can be complicated by spin
state changes upon binding.18-20 In contrast, the iron(II)
complexes studied here are always high-spin as three- or four-
coordinate complexes.
FeOPh, LtBu,iPr2FeOTol)24 complexes. In some of these
compounds, para-substituents on the aryl rings were used
to enable isolation of single crystals, or to introduce an
electroniceffect(seebelow). Reactionsof LtBu,iPr2FeCl and
1 equiv of the appropriate sodium salt were carried out in
diethyl ether at room temperature. The complexes were
isolated as red-orange crystals by cooling pentane solu-
tions, in yields of 70-85%. The purity of these compounds
was typically ascertained using UV-vis spectroscopy,
which showed a characteristic visible band in each complex
between 500-600 nm, and using 1H NMR spectroscopy.
As with other three-coordinate iron(II) diketiminate
complexes,15 the 1H NMR spectra had peaks over a range
from roughly +120 to -120 ppm. The signals could be
integrated to give tentative assignments. In cases where
this left ambiguity, resonances were assigned based on
proximity to the paramagnetic iron(II) center (protons
closer to the metal center are shifted further from
0 ppm).15 The number of resonances in the spectra are
as expected for averaged C2v symmetry, indicating rapid
rotation around the Fe-S, Fe-O, S-C, and O-C bonds
on the NMR time scale. The Evans method25,26 was used
to calculate the solution magnetic moments, which were
5.5 ( 0.4 Bohr magnetons for all compounds. This
indicates a ground state of S = 2, consistent with the
expectations for high-spin Fe(II) with substantial orbital
Results
Synthesis of Three-Coordinate FeII Complexes. We have
shown that LtBu,iPr2FeCl21 is a versatile starting material
for the synthesis of a variety of three-coordinate iron(II)
(16) Cummins, C. C. Prog. Inorg. Chem. 1998, 47, 685–836.
(17) Alvarez, S. Coord. Chem. Rev. 1999, 193-195, 13–41.
(18) Sanders, J. K. M.; Bampos, N.; Clyde-Watson, Z.; Darling, S. L.;
Hawley, J. C.; Kim, H.-J.; Mak, C. C.; Webb, S. J. Porphyrin Handbook
2000, 3, 1–48.
(19) Ellis, P. E.; Jones, R. D., Jr.; Basolo, F. J. Chem. Soc., Chem.
Commun. 1980, 54–55.
(22) Bailey, P. J. C., R. A.; Dick, C. M.; Fabre, S.; Henderson, L. C.;
Herber, C.; Liddle, S. T.; Lorono-Gonzalez, D.; Parkin, A.; Parsons, S.
Chem. Eur. J. 2003, 9, 4820–4828.
(23) Adhikari, D.; Tran, B. L.; Zuno-Cruz, F. J.; Sanchez Cabrera, G.;
Mindiola, D. J.; Chiang, K. P.; Cowley, R. E.; Dugan, T. R.; Holland, P. L.
Inorg. Synth., in press.
(20) Ellis, P. E.; Linard, J. E.; Szymanski, T.; Jones, R. D.; Budge, J. R.;
Basolo, F. J. Am. Chem. Soc. 1980, 102, 1889–1896.
(21) Smith, J. M.; Lachicotte, R. J.; Holland, P. L. Chem. Commun. 2001,
1542–1543.
(24) Eckert, N. A.; Smith, J. M.; Lachicotte, R. J.; Holland, P. L. Inorg.
Chem. 2004, 43, 3306–3321.
(25) Schubert, E. M. J. Chem. Educ. 1992, 69, 62.
(26) Evans, D. F. J. Chem. Soc. 1959, 2003–2005.

5108 Inorganic Chemistry, Vol. 48, No. 12, 2009
Chiang et al.
Table 1. Relevant Bond Lengths and Anglesa
bond/angle
LtBu,iPr2Fe(SPh)
LtBu,iPr2Fe(STol)
LtBu,iPr2Fe(SPhCF3)
LtBu,iPr2Fe(OTol)
˚
Fe-N (A)
1.959(2)
1.969(2)
2.2523(6)
95.20(6)
144.69(5)
119.98(5)
110.06(7)
1.969(1)
1.971(1)
2.2584(5)
94.49(5)
146.28(4)
118.56(4)
114.37(5)
1.960(1)
1.968(1)
2.2642(5)
94.94(5)
146.97(4)
117.13(4)
114.70(5)
1.965(2)
1.972(2)
1.833(2)
94.76(8)
140.20(8)
125.01(9)
138.8(2)
˚
Fe-E (A)
N-Fe-N (deg)
N-Fe-E (deg)
Fe-E-C (deg)
a E represents S or O.
angles is greater than 359°.21,24,28 Interestingly, the sulfur
atoms lie off the C2 axis of the diketiminate-iron group,
giving N-Fe-S angles that differ by 25°, 28°, and 30° in
the three thiolate complexes. The cause for this “T” shape
may be that the sulfur atom prefers a relatively small
angle. The Fe-S-C angles in our complexes range from
110° to 115°, which fall close or within a standard devia-
tion of the average Fe-S-C angle, 110(4)°, in the Cam-
bridge Structural Database (CSD). To accommodate a
thiolate ligand with this Fe-S-C angle between the
bulky aryl rings of the diketiminate, the iron coordination
must distort. In the aryloxide complex LtBu,iPr2FeOTol,
the angle at oxygen is somewhat larger at 138.8(2)°, also
within a standard deviation of the average Fe-O-C
angle in the CSD, 133(8)°. Because the larger angle at
oxygen causes less steric pressure on the iron coordina-
tion geometry, the N-Fe-O angles in the aryloxide
complex are more similar (Δ ∼ 15°) than in the more
distorted thiolates (Δ ∼ 25-30°).
Cyclic Voltammetry Studies of Iron(II) Complexes. The
redox chemistry of the three-coordinate complexes was
examined using cyclic voltammetry in diethyl ether
(Et2O) with 0.1 M NBu4BArF as electrolyte (BArF =
tetrakis[3,5-bis(trifluoromethyl)phenyl]borate). The use
of this unusual solvent/electrolyte combination is neces-
sary because more polar solvents like tetrahydrofuran
(THF) and acetonitrile (MeCN) coordinate to the iron
atom, as shown below.
The complexes show reversible reductions at the low
potential of -2.4 to -2.6 V vs ferrocene, which we ascribe
to an iron(II)/iron(I) couple (Figure 3). Peak separation
(Epa - Epc) values for the iron(II)/iron(I) couple are
between 100 mV and 200 mV, which is typical for solvents
of relatively low polarity.29 Additionally, an irreversi-
ble oxidation (Epa) occurs between +0.3 to +0.5 V (for
example, see Figure 3a). We have not explored the
oxidation products.
The cyclic voltammetry of the complexes was also
measured in acetonitrile (MeCN) with 0.1 M NBu4PF6
electrolyte. As discussed below, MeCN binds to these
three-coordinate complexes to give a four-coordinate
solvent adduct at high concentrations of MeCN. There-
fore, results obtained in MeCN reflect the redox behavior
of the four-coordinate complex LtBu,iPrFe(Y)(NCMe).
These four-coordinate complexes showed a quasireversi-
ble reduction at low potential and an irreversible oxida-
tion (Epa) at 0 to +0.3 V. The only difference between
the different complexes is that LtBu,iPr2FeOPh possesses
Figure 2. ORTEP drawings of the molecular structures of
(a) LtBu,iPr2FeSTol and (b) LtBu,iPr2Fe(OTol). Analogous drawings of
L
tBu,iPr2Fe(SPh) and LtBu,iPr2Fe(SPhCF3) are given in the Supporting
Information. Thermal ellipsoids are shown at50% probability. Hydrogen
atoms have been omitted for clarity.
angular momentum, as studied in detail for LtBu,iPr2FeCl
and LtBu,iPr2FeCH3.27
The molecular structures of LtBu,iPr2FeSPh, LtBu,iPr2
-
FeSTol, LtBu,iPr2FeSPhCF3, and LtBu,iPr2FeOTol were
determined by X-ray crystallography. Thermal-ellipsoid
plots of thecrystallographicmodels areshown inFigure 2,
and relevant bond lengths and angles are given in Table 1.
The iron centers are planar, and the sum of the three
(27) Andres, H.; Bominaar, E.; Smith, J. M.; Eckert, N. A.; Holland,
::
P. L.; Munck, E. J. Am. Chem. Soc. 2002, 124, 3012–3025.
(28) Vela, J.; Smith, J. M.; Yu, Y.; Ketterer, N. A.; Flaschenriem, C. J.;
Lachicotte, R. J.; Holland, P. L. J. Am. Chem. Soc. 2005, 127, 7857–7870.
(29) Barriere, F.; LeSuer, R. J.; Geiger, W. E. Trends Mol. Electrochem.
2004, 413–444.

Article
Inorganic Chemistry, Vol. 48, No. 12, 2009 5109
Figure 3. Voltammograms of LtBu,iPr2FeCl in Et2O with 0.1 M NBu4BArF electrolyte. (a) Full scan at 500 mV/s. (b) The Fe2+/1+ wave at 100 mV/s. Other
voltammograms are given in the Supporting Information.
Table 2. E1/2 Values for Reduction of Complexes LtBu,iPr2FeY in Et2O/
observed previously for four-coordinate iron(II) diketi-
minate complexes.32
NBu4BArF and LtBu,iPr2FeY(NCMe) in MeCN/NBu4PF6
a
To verify the geometry of the adducts, we determined
the solid-state structures of LtBu,iPr2Fe(SPh)(L) (L = tert-
butyl isocyanide, CNtBu; 1-methylimidazole, MeIm; N,
N-dimethylformamide, DMF) using X-ray crystallogra-
phy. Thermal-ellipsoid plots are shown in Figure 4, and
important bond lengths and angles are listed in Table 3.
The fourth donor binds from the direction normal to the
plane of the three-coordinate complexes: the angles be-
tween the N2S plane and the Fe-L bond were 77°, 86°,
and 83° in the three structures. This is consistent with the
trigonal-pyramidal geometry observed previously in
some ligand adducts of LtBu,iPr2FeCl.24,28
Y
Et2O
MeCN
Cl
OPh
SPh
SPhCF3
-2.46(1)
-2.56(3)
-2.41(2)
-2.40(2)
-2.32(2)
-2.27(6)
-2.30(6)
-2.33(1)
a Potentials are given in volts relative to the ferrocenium/ferrocene
couple at 0 V.30,31
an additional quasireversible response corresponding to
an oxidation at E1/2 = -93 mV.
The E1/2 values for the iron(II)/iron(I) couple in both
solvents are shown in Table 2. These values are averages
from scans with sweep rates between 100-1000 mV/s and are
referenced to the ferrocene couple (+0.64 V vs NHE).30,31
The greater coordination number in the ligand adducts
increased the lengths of the bonds to the spectator
ligands. The Fe-S bond length on average increased
A precipitate formed during the measurement of LtBu,iPr2
-
˚
˚
0.035(2) A while the Fe-N bonds increased 0.0526(4) A
from the three-coordinate thiolate complex LtBu,iPr2Fe
(SPh). The difference between the two N-Fe-S angles
within the same complex ranged from 12°-18°, as in the
three-coordinate thiolate complexes. The M-S-C angles
are typical of those found in the CSD (109 ( 4°).
Fe(OPh)(NCMe), which caused the peak to shift in the
negative direction. This change is reflected in the larger
error ((60 mV) associated with the LtBu,iPr2Fe(OPh)
complex. Although these reduction potentials do not strictly
represent thermodynamic potentials, they can be used for
comparison of the complexes as described below in the
Discussion section.
Binding Constants of Neutral Donors to LtBu,iPr2FeCl,
L
tBu,iPr2Fe(SPh), LtBu,iPr2Fe(SPhCF3), and LtBu,iPr2Fe
Characterization of Four-Coordinate Fe(II) Complexes.
Each of the three-coordinate iron(II) complexes was cap-
able of binding an additional neutral ligand. The amount
of added ligand necessary to ensure binding was different
depending on the ligand (see below), but was always
accompanied by a change from red to an orange color.
The four-coordinate iron(II) complexes have broader
1H NMR spectra than the three-coordinate iron(II) com-
plexes, and fall in a narrower chemical shift range (see
Supporting Information). This was observed for every
added ligand, suggesting that the broader peaks arise not
from ligand exchange but rather from slower electronic
relaxation in the four-coordinate complexes. Consis-
tent with this idea, 1H NMR spectra like these have been
(OPh). We determined the trends in the binding constants
for added neutral ligands, as a function of the added
ligand (L) and of the anionic spectator ligand (Y). To
carry out the binding studies we monitored the visible
absorbance peak of the three-coordinate iron complexes
using UV-vis spectrophotometry.
For these experiments, 10 mM stock solutions of the
three-coordinate complexes in toluene were sequentially
diluted to give samples that were 1.0 mM in LtBu,iPr2FeY.
The donor ligand, L, was present in amounts varying
from 0.25 mM to 1.0 M. A color change from red to
orange is observed depending on the particular ligand and
amount added. This corresponds to the disappearance of
the absorbance peak in the 500-600 nm range. An
isosbestic point is observed near 500 nm because of the
(30) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877–910.
(31) The potential of ferrocene in MeCN/NBu4PF6 is +0.40 V vs SCE,
and SCE is at +0.24 V vs NHE.
(32) Vela, J.; Cirera, J.; Smith, J. M.; Lachicotte, R. J.; Flaschenriem, C.
J.; Alvarez, S.; Holland, P. L. Inorg. Chem. 2007, 46, 60–71.

5110 Inorganic Chemistry, Vol. 48, No. 12, 2009
Chiang et al.
complex (M) and the four-coordinate complex (ML)
in solution (eq 1) upon addition of L. The equili-
brium constant (Keq) can be represented as shown in
eqs 1 and 2.
Keq
M þ L hML
ð1Þ
ð2Þ
½MLꢀ
Keq
¼
½Mꢀ½Lꢀ
While no clear absorbance band is associated with the
four-coordinate complex, its concentration can be calcu-
lated from the loss of absorbance (ΔAλmax) of the distinct
peak of the three-coordinate complex near 550 nm with
addition of L. The value of ΔAλmax is plotted against the
initial concentration of ligand added, [L]0, to produce a
binding curve (Figure 5). Data were only used if an
isosbestic point was observed.
The equilibrium constant (Keq) is determined by fit-
ting the binding curves to one of two equations depend-
ing on whether the ligands bind weakly or strongly to
L
tBu,iPr2FeY. A ligand that binds most of the iron at 1
equiv of L is defined as being “strong binding.” Ligands
that show less than one-half binding of iron at 1 equiv are
considered “weak binding.”
In the weak binding scenario, the concentration of ML
at equilibrium can be represented by taking the difference
of initial and final concentration of M ([ML] = [M]0 -
[M]f). This can be incorporated into an equation to fit a
binding curve (eq 3).
½MLꢀ
½Mꢀ0
Keq½Lꢀ0
¼
ð3Þ
Keq½Lꢀ0 þ 1
In eq 3, [L]0 is substituted for [L] because in the
weak binding situation the concentration of free ligand
at equilibrium [L] is virtually identical to the initial
concentration of ligand [L]0. However, this assum-
ption is not valid under strong binding conditions. There-
fore, eq 4 was formulated taking into account this
difference.
Figure 4. ORTEP drawings of the molecular structures of (a) LtBu,iPr2Fe(SPh)
(CNtBu), (b) LtBu,iPr2Fe(SPh)(MeIm) and, (c) LtBu,iPr2Fe(SPh)(DMF). Thermal
ellipsoids are shown at 50% probability. Co-crystallized solvent in the LtBu,iPr2Fe
(SPh)(MeIm) structure, and all hydrogen atoms, are omitted for clarity.
½MLꢀ ¼
ꢂ
ꢃ
1=2
ꢀ
ꢁ
ꢀ
ꢁ
2 -4½Lꢀ0½Mꢀ0
1
Keq
1
Keq
½Mꢀ0 þ ½Lꢀ0 þ
-
½Mꢀ0 þ ½Lꢀ0 þ
2
Table 3. Important Bond Distances and Angles for the Four-Coordinate Fe(II)
Complexesa
ð4Þ
bond/angle
CNtBu
MeIm
DMF
For these experiments, we were interested in learning
about the dependence of the binding constant on (a) the
type of neutral donor ligand L, and (b) the nature of the
spectator ligand Y. To address point (a), a number of
representative ligands L were chosen with different donor
atoms, steric size, and electronic properties. The values of
Keq were determined for binding to both LtBu,iPr2FeCl and
to LtBu,iPr2Fe(SPh), to ensure that any trends seen were
not peculiar to a single choice of Y. To address point (b),
two of the ligands (zMeCN and DMF) were evaluated
across a broader range of supporting ligands Y, including
the phenolate complex and the CF3-substituted thiophe-
nolate complex.
˚
Fe-N (A)
2.000(2),
2.013(2)
2.2776(15)
2.089(2)
96.19(5)
118.04(7)
104.36(6)
2.021(2),
2.0262(19)
2.2955(11)
2.094(2)
96.85(8)
125.58(6)
107.05(8)
2.0181(11),
2.0212(11)
2.2877(4)
2.0832(10)
95.44(4)
˚
Fe-S (A)
Fe-L (A)
N-Fe-N (deg)
S-Fe-L (deg)
Fe-S-L (deg)
˚
110.31(3)
112.94(5)
a L represents the fourth donor atom (C, N, or O).
growth of a shoulder in the four-coordinate complex
around 400 nm (see Supporting Information). Equilibrium
is immediately established between the three-coordinate

Article
Inorganic Chemistry, Vol. 48, No. 12, 2009 5111
Figure 5. Binding curves for (a) LtBu,iPr2FeSPh to acetonitrile, an example of weak binding, and (b) LtBu,iPr2FeSPh to CNtBu, an example of strong
binding.
Table 4. Calculated Binding Constants for Neutral Ligands to LtBu,iPr2FeCl, LtBu,iPr2Fe(SPh), LtBu,iPr2Fe(SPhCF3), and LtBu,iPr2Fe(OPh)
ligand
PPh3
THF
MeCN
DMF
2-picoline
Pyridine
CNtBu
LtBu,iPr2FeCl Keq LtBu,iPr2Fe(SPh) Keq
Keq(Cl)/Keq(SPh)
LtBu,iPr2Fe(SPhCF3) Keq
LtBu,iPr2Fe(OPh) Keq
donor atom
0.70 ( 0.08
0.80 ( 0.01
9.8 ( 0.5
1.0 ( 0.5
0.15 ( 0.01
5.2 ( 0.2
0.7 ( 0.5
5.3 ( 0.1
1.9 ( 0.1
2.4 ( 0.1
2.2 ( 0.1
3.7 ( 0.3
1.8 ( 0.7
P
O
N
O
N
N
C
11.7 ( 0.9
230 ( 10
4.7 ( 0.4
160 ( 10
390 ( 20
160 ( 5
660 ( 50
300 ( 30
41000 ( 6000
73000 ( 27000
11000 ( 2000
41000 ( 7000
Irrespective of Y, added ligands PPh3, THF, MeCN,
DMF, and 2-picoline were appropriate for use of the “weak
binding” equation (eq 3) and exhibited Keq values bet-
ween 10-1 to 103. Pyridine and CNtBu were fitted with
the “strong binding” equation (eq 4) and were associated
with Keq values above 104. These data are summarized in
Table 4.
thiolates have extremely bulky ligands that enable them
to resist the tendency of thiolates to bridge between
metals.13,28,36,39,40 The SR group in most such literature
compounds is a substituted thiophenolate with bulky tert-
butyl groups in the ortho- and para-positions. Here, on the
other hand, the use of a bulky β-diketiminate ligand
enables the use of smaller thiolates.
The values of Keq are highly dependent on the neutral
ligand L. However, the Keq values are similar with the
same added ligand L and different anionic ligands Y.
Equivalently, the trends of L binding are similar no
matter which Y is used. The similarity in the trends is
most evident when comparing Keq values of LtBu,iPr2FeCl
Binding of Neutral Ligands to Low-Coordinate Iron. In
this paper we report the binding constants for a range of
ligands to low-coordinate Fe(II) complexes, determined
using UV-vis spectroscopy. Binding of the added donor
ligand is instantaneous as observed by the immediate
color change of the solution. Crystal structures show that
the ligand binds to LtBu,iPr2FeY in a 1:1 stoichiometry.
The 1:1 binding is further supported by the observation of
an isosbestic point when comparing samples at different
concentrations of added ligand (see Supporting Informa-
tion, Figures S3-S6 for examples).
The measured values of Keq varied greatly with varia-
tion of the neutral ligand but were independent of the
donor atom and hybridization of the donor atom in
the neutral ligand. In an effort to discern what factors
were responsible for the observed trend, we explored
both steric and electronic explanations in a systematic
fashion.
and LtBu,iPr2Fe(SPh), where the Keq values for LtBu,iPr2
-
FeCl are approximately twice that of LtBu,iPr2Fe(SPh).
Discussion
Three-Coordinate Iron(II) Thiolate Complexes. The
three LtBu,iPr2FeSAr complexes described here add to
the few examples of three-coordinate iron thiolate
complexes. The most common S-coordinated iron
complexes with a coordination number of three are
Fe2( μ-SR)2 dimers.33-38 Previous three-coordinate iron
(33) Ohki, Y.; Ikagawa, Y.; Tatsumi, K. J. Am. Chem. Soc. 2007, 129,
10457–10465.
(34) Ohta, S.; Ohki, Y.; Ikagawa, Y.; Suizu, R.; Tatsumi, K. J. Organo-
Steric Effect of Ligands. Steric contributions were
surveyed by calculating the cone and solid angles of the
fourth donor ligands. The more familiar cone angle (θ), as
met. Chem. 2007, 692, 4792–4799.
(35) Power, P. P.; Shoner, S. C. Angew. Chem. 1991, 103, 308-309 (See
also Angew. Chem., Int. Ed. Engl. 1991, 30(3), 330-332).
(36) Hauptmann, R.; Kliss, R.; Schneider, J.; Henkel, G. Z. Anorg. Allg.
Chem. 1998, 624, 1927–1936.
(37) Ruhlandt-Senge, K.; Power, P. P. Bull. Soc. Chim. Fr. 1992, 129, 594.
(38) Sydora, O. L.; Henry, T. P.; Wolczanski, P. T.; Lobkovsky, E. B.;
Rumberger, E.; Hendrickson, N. H. Inorg. Chem. 2006, 45, 609–626.
(39) Lee, H. K.; Luo, B.-S.; Mak, T. C. W.; Leung, W.-P. J. Organomet.
Chem. 1995, 489, C71–C73.
(40) Groysman, S.; Wang, J.-J.; Tagore, R.; Lee, S. C.; Holm, R. H. J.
Am. Chem. Soc. 2008, 130, 12794–12807.

5112 Inorganic Chemistry, Vol. 48, No. 12, 2009
Chiang et al.
described by Tolman,41 describes the amount of space a
ligand occupies by measuring the angle created at the
apex of a cone that completely contains the ligand
(Figure 6a). The concept of a solid angle (Ω) is less
familiar, but is perhaps a more accurate way to represent
the steric congestion of a ligand by taking the ligand’s
shape into account. It is measured in steradians and
represented by eq 5 where A is the area of a circle created
on a sphere if a light is shone on the ligand from a metal
located at the center of the sphere (Figure 6b).
Figure 6. Graphical representations of a cone angle (a) and solid
angle (b).
Table 5. Calculated Solid and Cone Angles of Ligands Used in This Study
A
r2
Ω ¼
ð5Þ
ligand
PPh3
THF
solid ang (Ω)
cone ang (θ)
Keq(Cl)
The cone angles and solid angles for ligands of interest
were calculated using the program SolidG, provided by
Ilia Guzei.42 For each ligand, we chose 10 representative
structures (listed in the Supporting Information) contain-
ing the ligand of interest in the CSD. The values for each
ligand were calculated and averaged to give the values
shown in Table 5.
3.32(6)
1.81(6)
1.60(0)
1.63(4)
2.36(3)
1.90(2)
1.65(1)
124(1)
89(2)
0.70
0.80
MeCN
DMF
2-Picoline
Pyridine
CNtBu
83.69(3)
85(1)
102.8(6)
91.6(5)
85.0(3)
9.80
390
660
41000
90000
Despite a variety of spectator ligands in the reference
complexes (see Supporting Information for full lists of
reference complexes), there was little variation in the
derived cone angle and solid angle values for the same
target ligand. A further non-systematic survey confirmed
that the coordination number and metal type do not
significantly affect the cone and solid angle calculated for
a given ligand. In addition, the cone angle and solid angle
correlate quite well with one another. So, these values are
likely to be accurate representations of the relative amount
of steric hindrance caused by each of these ligands in the
four-coordinate complexes LtBu,iPr2Fe(Y)(L).
The size of most ligands was similar: aside from
2-picoline and triphenylphosphine, the solid angles varied
in the narrow range 1.60 to 1.90. Given the wide variation
in Keq between the ligands of similar size, it is clear that
steric effects are not dominant in this system. However,
the two largest ligands are insightful. 2-Picoline was
chosen as a ligand that is electronically similar to pyridine
but is substantially larger (solid angle of 2.36 vs 1.90).
The added size reduces the binding constant by a factor of
30-60. In addition, the bulkiest ligand, PPh3, was the
most weakly binding. Overall, steric effects have an
influence on Keq for 2-picoline and PPh3 but do not
explain the trends with the other neutral ligands.
Electronic Properties of Ligands: σ-Effects. To rationa-
lize the σ-bonding capacity of different ligands, we com-
pared the binding constants to the gas-phase proton
affinities of ligands, as well as the pKa of their conjugate
acids. We were not able to find a literature value for the
pKa of protonated t-butyl isocyanide in MeCN, and so it
was calculated using a recently reported semiempirical
density functional theory (DFT) method.43 The relevant
values are shown in Table 6.
No correlation was found between Keq and the proton
affinities. There was a weak correlation of Keq with the
pKa values if CNtBu and the bulky ligands are left out
(Figure 7). Therefore, the ligands that bind most tightly
to a proton also bind most tightly to the iron atom.
However, the large deviation of the isocyanide indicates
that σ-effects are not the sole explanation for the electro-
nic effects on Keq.
Electronic Properties of Ligands: π-Effects. t-Butyl
isocyanide is the strongest π-acceptor ligand studied here
and hasthe largest equilibrium constant, suggesting thatπ-
backbonding might account for the remaining deviation
from the trends discussed above. Previous binding studies
of low-coordinate iron(I) to alkynes and alkenes showed
that π-backbonding was the dominant factor determining
the affinity of different ligands.44 A very recent computa-
tional study has found that the binding constants in
iridium systems can be related to backbonding. 45
Out of the ligands studied, the ability to accept electron
density is greatest for CNtBu.46 Low-lying unoccupied
molecular orbitals (LUMOs) are also present in pyridine
and PPh3, but in the last case the great steric hindrance of
the phenyl groups on the phosphine prevents this ligand
from binding strongly. In attempts to quantitatively
compare the π-backbonding ability of these ligands, the
carbonyl stretch values of tungsten pentacarbonyl com-
plexes (W(CO)5L) containing the ligand of interest were
collected (see Table 7). Unfortunately, there was no
evident correlation between this measure of backbonding
ability and the equilibrium constant for binding to three-
coordinate iron.
To further test the importance of π-backbonding,
L
tBu,iPr2FeCl and LtBu,iPr2FeSPh were each treated with
1 atm of CO under a range of conditions. However, these
reactions yielded starting material, decomposition pro-
ducts, or diamagnetic products as observed by 1H NMR.
Infrared spectroscopy of these reactions showed no
absorbances from 1700-2600 cm-1 where C-O stretches
of β-diketiminate iron(I) and iron(II) carbonyl com-
pounds have been observed.10,47,48 Therefore, if CO binds
(44) Yu, Y.; Smith, J. M.; Flaschenriem, C. J.; Holland, P. L. Inorg. Chem.
2006, 45, 5742–5751.
(45) Gusev, D. G. Organometallics 2009, 28, 763–770.
(46) Barbeau, C.; Turcotte, J. Can. J. Chem. 1976, 54, 1603–1611.
(47) Smith, J. M.; Lachicotte, R. J.; Holland, P. L. Organometallics 2002,
21, 4808–4814.
(41) Tolman, C. A. Chem. Rev. 1977, 77, 313–348.
(42) Guzei, I. A.; Wendt, M. Dalton Trans. 2006, 3991–3999.
(43) Ding, F.; Smith, J. M.; Wang, H. J. J. Org. Chem. 2009, 74, 2679–
2691.
(48) Sadique, A. R.; Brennessel, W. W.; Holland, P. L. Inorg. Chem. 2008,
47, 784–786.

Article
Table 6. H+ Affinity Values and pKa Values in MeCN for the Ligands Studied
Inorganic Chemistry, Vol. 48, No. 12, 2009 5113
Table 7. Collected CO Stretch Values (cm-1) of W(CO)5L
ligand
PPh3
THF
MeCN
DMF
2-picoline
Pyridine
CNtBu
H+ affinitya (kJ/mol)
pKa (MeCN)
Keq(Cl)
W(CO)5L
A1
A1
E
Keq(Cl)
7.61b
1.1c
0.7
0.8
9.8
390
660
41000
90000
PPh3
THFb
2072
2074
2083
2067
2076
2065
1942
1912
1931
1847
1980
1922
1939
1933
1948
1917
0.7
0.8
9.8
390
41000
90000
a
972.8
822.1
779.2
887.5
949.1
930
0
MeCNc
DMFc
6.1c
13.32c
12.53b
2.71d
Pyridined
CNtBue
1952
870.7
a Bancroft, G. M.; Dignard-Bailey, L.; Puddephatt, R. J. Inorg.
Chem. 1986, 25, 3675-3680. b We substituted the value known for
dihydrofuran: Paur-Afshari, R.; Lin, J.; Schultz, R. H. Organometallics
2000, 19, 1682-1691. c Stolz, I. W.; Dobson, G. R.; Sheline, R. K. Inorg.
Chem. 1963, 2, 323-326. d Kraihanzel, C. S.; Cotton, F. A. Inorg. Chem.
1963, 2, 533-540. e King, R. B.; Saran, M. S. Inorg. Chem. 1974, 13,
74-78.
a Hunter, E. P.; J. Phys. Chem. 1998, 27, 3991-3999. b Kaljurand, I.;
::
::
::
Kutt, A.; Soovali, L.; Rodima, T.; Maemets, V.; Leito, I.; Koppel, I. A.
J. Org. Chem. 2005, 70, 1019-1028. c Izutsu, K. Acid-Base Dissociation
Constants in Dipolar Aprotic Solvents; Blackwell Scientific: Oxford,
1990, pp 17-35. d Calculated: see ref 43 and Experimental Section.
Because the binding constants are most precise with
weak-binding ligands, we used MeCN and DMF as
added neutral donors across the range of spectator
ligands Y. The binding curves using LtBu,iPr2Fe(OPh)
and LtBu,iPr2Fe(SPhCF3) yielded Keq values that were
similar to the chloride and thiophenolate complexes
(see Table 4 above). No clear trends are discernible,
casting doubt on an electronic explanation for the small
difference between binding to thiolate and to chloride
complexes.
We also attempted to evaluate the electronic effect of Y
on the complex using cyclic voltammetry (CV) to deter-
mine the reduction potential (the oxidations were irrever-
sible). These studies were performed in both Et2O, to
evaluate the three-coordinate species, and MeCN, to
evaluate the four-coordinate species. The Fe2+/1+ waves
were more reversible in Et2O than MeCN. This may
indicate greater stability of the three-coordinate iron(I)
species than the four-coordinate iron(I) species. Only
small variations in potential (< 200 mV) are observed
upon changing the anionic ligand Y. In the two solvents,
these small shifts follow opposite trends. For example,
LtBu,iPr2Fe(OPh) has the most negative reduction poten-
tial in Et2O, while LtBu,iPr2Fe(OPh)(NCMe) has the least
negative reduction potential value in MeCN. The reduc-
tion potentials appear to be roughly 150 mV less negative
in MeCN than in Et2O. However, because the waves in
MeCN are quasireversible, it would be dangerous to draw
any conclusion from this trend. Overall, the similarity of
the potentials with different Y suggests that the electro-
negativity of the third donor does not significantly influ-
ence the electronic properties of the metal center.
Implications for Modeling Nitrogenase. As described in
the Introduction, our group has used diketiminate com-
plexes as mimics of one- and two-iron “belt” sites on
nitrogenase.6 This has naturally raised questions about
the use of nitrogen donors to mimic a sulfur-coordinated
iron site. In this study, the diketiminate was not varied,
but one of the ligands was systematically changed over a
range from “hard” oxygen and chlorine ligands to “soft”
thiolate. This enabled us to experimentally gauge the
influence of changing one spectator ligand on the binding
ability and other properties of a three-coordinate iron
complex.
Figure 7. Plot of log(Keq) vs pKa (MeCN) for different ligands L. As
discussed above, PPh3 and 2-picoline fall from the trend because of steric
congestion in PPh3 and 2-picoline (Table 5). The deviation of CNtBu may
be attributable to π-backbonding (see below).
at all to the three-coordinate iron complexes, it is a very
weak interaction. Note that related three-coordinate
iron-alkyl complexes react with CO to give a low-spin
iron-acyl product from CO insertion, so weak CO inter-
actions must be possible in some three-coordinate iron(II)
compounds.47
Therefore, despite a likely influence of π-backbonding
in isocyanide complexes, it is not the dominant factor
either. Overall, the trends in the binding strength of
neutral ligands are determined by a combination of steric
and electronic effects, none of which dominates.
Steric and Electronic Effects from the Spectator
Ligand Y. As noted above, there was a small increase in
binding constants of neutral ligands to LtBu,iPr2FeCl as
compared to the binding constants to LtBu,iPr2FeSPh. One
could reasonably attribute this difference to electronic
effects (the greater electronegativity of chlorine vs sulfur),
or to steric effects (the smaller size of chlorine vs sulfur).
Even though the effect was small, we took this opportu-
nity to compare the binding constants with two additional
complexes: one with an electron-withdrawing substituent
on the arylthiolate, LtBu,iPr2Fe(SC6H4CF3), and one with
oxygen in place of sulfur, LtBu,iPr2Fe(OPh). Both of these
ligands are nearly isosteric with the arylthiolate ligand in
In this context, it is interesting that the binding con-
stants for a variety of neutral ligands are similar
with oxygen, sulfur, or chloride spectator ligands in one
L
tBu,iPr2FeSPh.

5114 Inorganic Chemistry, Vol. 48, No. 12, 2009
Chiang et al.
KLtBu,iPr2 was synthesized by adding benzylpotassium22 to
position. This similarity was evident with neutral ligands
that are π-donors (e.g., THF) or π-acceptors (e.g.,
CNtBu). The reduction potentials of the iron(II) com-
plexes were also similar over the range of spectator
ligands. This insensitivity to the spectator ligand suggests
that changes in donor, such as from diketiminates to
thiolates or sulfides, may give only a small influence on
binding energies and redox potentials. Of course, in the
broader field of coordination chemistry, the identity of
spectator ligands often influences binding energies and
mechanisms;49,50 however, in the three-coordinate sys-
tems studied here, the effects of changing one ligand are
surprisingly small.
HLtBu,iPr2 in diethyl ether.23
CV measurements were obtained using a Cypress Systems
3100 potentiostat. The working electrode was glassy carbon
with a 1 mm diameter working area, and Ag wires were used
as auxiliary and reference electrodes. All measurements were
referenced with an internal ferrocene standard, and reported
relative to the Cp2Fe+/Cp2Fe couple. Acetonitrile used for
CV experiments was vacuum transferred after alumina
treatment.
X-ray Crystallography. Single crystals were mounted on a
glass capillary tube or fiber and mounted on a Siemens SMART
CCD or Bruker SMART APEX II CCD Platform diffract-
ometer for data collection under a cold N2 stream at 193(2) K or
100.0(1) K. Full data collection was carried out using Mo KR
radiation (graphite monocromator) with appropriate frame
times ranging from 20 to 90 s and typical detector distance
around 5.09 cm. Total data collection time was generally
between 12 and 24 h. Structures were refined using SIR97 or
SHELXS-97 and refined using SHELXL-97 (Table 8). Space
groups were determined based on systematic absences and
intensity statistics. A direct-methods solution was calculated
which provided most non-hydrogen atoms from the E-map.
Full-matrix least-squares/difference Fourier cycles were per-
formed which located the remaining non-hydrogen atoms. All
non-hydrogen atoms were refined with anisotropic displace-
ment parameters. All hydrogen atoms were placed in ideal
positions and refined as riding atoms with relative isotropic
displacement parameters.
In recent publications, Holland15 and Peters51 have
explored geometric influences on the unusual electronic
structures and reactivity of high-spin, late-metal com-
plexes with three or four ligands, respectively. These
works show that the geometry of the metal has a pro-
nounced effect on its electronic structure, and it is likely
that the geometric influences are much more important
than the precise electronics of the spectator ligand in
determining the properties of the resulting complex.
Further systematic studies are needed to evaluate the
influence of geometry on measurable chemical para-
meters like binding constants.
Modified Procedure for LtBu,iPr2FeCl21. Under an atmo-
sphere of N2, KLtBu,iPr2 (0.50 g, 0.92 mmol), FeCl2(THF)1.5
(0.202 g, 0.919 mmol), and toluene (25 mL) were added to a
150 mL resealable tube. The mixture was heated in an oil bath at
100 °C for 18 h and turned very dark red in color. The solu-
tion was cooled, filtered through Celite, and concentrated to
7 mL. The solution was warmed to dissolve a small amount of
solid formed during concentration, and cooled to -40 °C over-
night to afford dark red crystals. The supernatant was de-
canted and the red solid was washed with 1 mL of pentane to
remove trace LtBu,iPr2H, leaving LtBu,iPr2FeCl (0.531 g, 97%). 1H
NMR (C6D6, 500 MHz): δ 104 (s, 1H, R-H), 42 (s, 18H, tBu), 2.6
(s, 4H, m-aryl), -27 (s, 12H, iPr methyl), -108 (s, 4H, iPr
methine), -111 (s, 12H, iPr methyl), -115 (s, 2H, p-aryl).
Occasionally, trace water gives up to 5% of LtBu,iPrH, which is
Experimental Section
Synthetic Methods. All manipulations were performed
under a nitrogen atmosphere by standard Schlenk techniques
or in an M. Braun glovebox maintained at or below 1 ppm of O2
and H2O. Glassware was dried at 150 °C overnight. H NMR
1
data were recorded on a Bruker Avance 500 spectrometer
(500 MHz) at 22 °C and referenced internally to residual
protiated solvent (C6HD5 at δ 7.16 ppm). Peaks of these
paramagnetic complexes are all singlets. Relative integrations
of peaks and assignments are given except for the four-coordi-
nate complexes, where overlap of peaks is too great to integrate
accurately. Solution magnetic susceptibilities were determined
at 294 K using Evans’ method.25,26 Electronic spectra were
recorded between 400 and 800 nm on a Cary 50 UV-visible
spectrophotometer, using screw-cap quartz cuvettes of 1 cm
optical path length. Elemental analyses were performed by the
Microanalysis Laboratory at the University of Illinois
(Urbana, Illinois) or Columbia Analytical Services (Tucson,
AZ). Infrared spectra (600-4000 cm-1) were recorded on
KBr pellets in a Shimadzu FTIR spectrophotometer (FTIR-
8400S).
1
observed as a white solid, or as peaks at 1-2 ppm of the H
NMR spectrum. The most reliable way to gauge purity is
UV-visible spectroscopy in toluene (λmax = 559 nm; ε =
63(3) M-1 cm-1).
LtBu,iPr2Fe(Y) (Y = SPh, STol, SPhCF3, OPh, OTol).
The appropriate thiophenol or phenol was treated with a slight
excess of NaH (1.05 equiv), and stirred in THF for 2 h. The
solution was filtered through Celite, and solvent was removed
under vacuum.
Acetonitrile, pentane, diethyl ether, THF, and toluene were
purified by passage through activated alumina and “deoxygen-
izer” columns obtained from Glass Contour Co. DMF was
To a stirring solution of LtBu,iPr2FeCl (0.0593 g, 0.100 mmol)
in Et2O (5-10 mL) was added 1.0 equiv of the appropriate
sodium salt. The resulting orange colored solution and suspen-
sion was stirred for 2 h. The solvent was evaporated under
vacuum, and the residue was extracted with pentane (10 mL)
and filtered through Celite to give a clear red-orange solution.
This solution was concentrated to 5 mL, warmed briefly to
achieve a homogeneous solution, cooled and placed in a freezer
(-35 °C) to afford red-orange crystals. Specific yields are given
below.
˚
dried over 3 A molecular sieves. Deuterated benzene was dried
over activated alumina in a bomb flask, and the alumina was
filtered off into a storage container prior to use. Before use, an
aliquot of each solvent (except acetonitrile) was tested with a
drop of sodium benzophenone ketyl in THF solution. Celite and
alumina were dried overnight at 200 °C under vacuum.
FeCl2(THF)1.5 was synthesized by the method of Kern.52
(49) Bryndza, H. E.; Domaille, P. J.; Paciello, R. A.; Bercaw, J. E.
Organometallics 1989, 8, 379–385.
(50) Henderson, R. A. Chem. Rev. 2005, 105, 2365-2437. In [4Fe-4S]
clusters, changing from terminal chloride to thiolate ligands has been
reported to change the substitution mechanism from associative to disso-
ciative.
LtBu,iPr2Fe(SPh). Yield 80%. 1H NMR (C6D6, 500 MHz):
δ 87 (1H, R-H), 56 (2H, o-SPh), 50 (2H, m-SPh), 40 (18H, tBu),
6.1 (4H, m-Ar), -7.6 (1H, p-SPh), -24 (12H, iPr methyl),
-99 (4H, iPr methine), -112 (12H, iPr methyl), -114 (2H,
p-Ar) ppm. μeff (Evans, C6D6): 5.1(5) μB. UV-vis (Tol) λmax
:
(51) Jenkins, D. M.; Peters, J. C. J. Am. Chem. Soc. 2005, 127, 7148–7165.
(52) Kern, R. J. J. Inorg. Nucl. Chem. 1962, 24, 1105–1109.
550 nm (51(3) M-1 cm-1). IR (KBr): 2960 (s), 2870 (m),

Article
Inorganic Chemistry, Vol. 48, No. 12, 2009 5115
Table 8. X-ray Structure Parameters for the Three- And Four-Coordinate Complexes Discussed in This Work
LtBu,iPr2Fe
(SPh)
LtBu,iPr2Fe
(SPhCF3)
LtBu,iPr2Fe
(STol)
LtBu,iPr2Fe
(OTol)
LtBu,iPr2Fe(SPh)
(CNtBu)
LtBu,iPr2Fe(SPh)
(MeIm) C6H6
LtBu,iPr2Fe(SPh)
(DMF)
3
empirical formula C41H58N2SFe C42H57N2F3SFe
fw
C42H60N2SFe C42H60N2OFe
680.83
193(2)
C46H67N3SSFe
749.94
100.0(1)
monoclinic
P21/n
10.693(9)
19.719(16)
21.014(17)
90
96.683(12)
90
4401(6)
4
1.132
C51H70N4SFe
827.02
100.0(1)
triclinic
P1
9.575(5)
12.487(5)
20.562(5)
79.982(5)
77.463(5)
88.120(5)
2363.2(17)
2
C44H65N3OSFe
739.9
100.0(1)
triclinic
P1
10.3921(12)
12.4143(14)
17.392(2)
102.907(2)
99.220(2)
94.130(2)
2145.2(4)
2
666.8
193(2)
734.81
193(2)
664.77
193(2)
temperature (K)
cryst system
space group
monoclinic
P21/n
10.8405(15)
16.368(2)
21.579(3)
monoclinic
P21/c
22.1226(10)
9.6586(4)
20.9138(9)
monoclinic
P21/c
monoclinic
P21/c
˚
a (A)
22.1137(11)
9.6346(5)
20.4372(10)
22.0892(14)
9.6154(6)
20.5038(13)
˚
b (A)
˚
c (A)
β (deg)
91.007(2)
114.980(1)
114.858(1)
115.819(1)
3
V (A )
˚
3828.3(9)
4
4050.7(3)
4
3950.9(3)
4
3920.2(4)
4
Z
F (g/cm3)
1.157
0.477
0.0420
0.1012
1.017
1.205
0.468
0.0344
0.0946
1.087
1.145
0.464
0.0377
0.0897
1.039
1.126
0.147
0.0534
0.1114
1.058
1.162
0.400
0.0562
0.1016
0.975
1.145
0.434
0.0431
0.0949
1.010
μ (mm-1
R1
)
0.423
0.0489
0.1113
1.018
wR2 [I >2σ(I)]
GOF
1624(w), 1577 (w), 1502 (m), 1473 (w), 1459 (w), 1430 (s),
1375 (m), 1354 (s), 1317 (m), 1217 (w), 1096 (w), 1024 (w),
780 (w), 736 (m), 697 (w) cm-1 Anal. Calcd for C41H58-
N2SFe: C, 73.84; H, 8.76; N, 4.20%. Found: C, 74.20; H, 9.01;
N, 4.27%.
LtBu,iPr2Fe(STol). Yield 75%. 1H NMR (C6D6, 500 MHz):
δ 87 (1H, R-H), 54 (2H, o-STol), 50 (2H, m-STol), 45
(3H, CH3-STol), 40 (18H, tBu), 5.9 (4H, m-Ar), -24 (12H,
iPr methyl), -99 (4H, iPr methine), -111 (12H, iPr methyl),
-114 (2H, p-Ar) ppm. UV-vis (Tol.) λmax: 553 nm (45(2) M-1
cm-1). IR (KBr, cm-1): 2963 (s), 2867 (m), 1624 (m), 1535 (w),
1503 (m), 1485 (m), 1458 (m), 1430 (m), 1375 (m), 1355 (s), 1316
(m), 1218 (w), 1108 (w), 1087 (m), 932 (w), 805 (m), 762 (m).
Anal. Calcd for C42H60N2SFe: C, 74.09; H, 8.88; N, 4.11%.
Found: C, 73.78; H, 8.84; N, 4.04%.
LtBu,iPr2Fe(SPhCF3). Yield 85%. 1H NMR (C6D6,
500 MHz): δ 84 (1H, R-H), 54 (2H, o-SPh), 49 (2H, m-SPh),
40 (18H, tBu), 7.9 (4H, m-Ar), -24 (12H, iPr methyl), -98 (4H,
iPr methine), -111 (12H, iPr methyl), -118 (2H, p-Ar) ppm. μeff
(Evans, C6D6): 4.9(5) μB. UV-vis (Tol) λmax: 563 nm (42(2) M-1
cm-1). IR (KBr cm-1): 2964 (s), 2873 (w), 1600 (m), 1503 (m),
1353 (s), 1323 (vs), 1158 (m), 1116 (m), 1091 (m), 1062 (w),
1014 (w), 827 (w), 800 (w), 781 (w). Anal. Calcd for
C42H57N2SFe: C, 68.64; H, 7.82; N, 3.81%. Found: C, 68.37;
H, 8.09; N, 3.80%.
LtBu,iPr2Fe(OPh). Yield 78%. 1H NMR (C6D6, 500 MHz):
δ 119 (1H, R-H), 94 (2H, o-OPh), 60 (2H, m-OPh), 42 (18H,
tBu), -4.4 (4H, m-Ar), -7.2 (1H, p-OPh), -30 (12H, iPr
methyl), -103 (2H, p-Ar), -118 (4H, iPr methine, 12H, iPr
LtBu,iPr2Fe(SPh)(CNtBu). In a 20 mL scintillation vial,
CNtBu (70 μL, 0.619 mmol) was added via syringe to a red-
orange solution of LtBu,iPr2Fe(SPh) (81.0 mg, 0.122 mmol) in
Et2O (8 mL). The solution was cooled to -35 °C overnight to
give orange crystals (67.8 mg, 75%). 1H NMR (C6D6, 500
MHz): δ 33, 27, 25, 20, 13, 8.1, 4.0, 2.1, 1.2, 0.29, -4.4, -4.9,
-17, -50 ppm. IR (KBr): 3057 (w), 2962 (vs), 2868 (m), 2168 (s),
2127 (m), 1618 (m), 1491 (m), 1473 (m), 1465 (w), 1431 (m), 1352
(s), 1315 (s), 1254 (w), 1215 (m), 1153 (w), 1105 (w), 1024 (w),
933 (w), 887 (w), 795 (w), 762 (w), 739 (w), 692 (w) cm-1. Anal.
Calcd for C46H67N3SFe: C, 73.67; H, 9.00; N, 5.60%. Found:
C, 72.87; H, 9.01; N, 5.58%.
LtBu,iPr2Fe(SPh)(DMF). In a 20 mL scintillation vial,
DMF (94 μL, 1.21 mmol) was added via syringe to a red-orange
solution of LtBu,iPr2Fe(SPh) (81.8 mg, 0.123 mmol) in Et2O
(10 mL). Upon swirling the solution orange crystals began to
form. Cooling to -35 °C gave orange crystals in two crops
(84.5 mg, 93%). Anal. Calcd for C45H67N3OSFe: C, 71.42; H,
8.85; N, 5.68%. Found: C, 70.66; H, 8.71; N, 5.46%. IR (KBr):
3063(w), 2968 (vs), 2868 (m), 1651 (vs), 1578 (m), 1527 (w), 1487
(m), 1460 (s), 1431 (s), 1383 (s), 1360 (s), 1315 (m), 1251 (w), 1217
(w), 1184 (w), 1153 (w), 1109 (m), 1063 (w), 1024 (w), 933 (w),
1
885 (w), 794 (w), 764 (m), 739 (m) 692 (m) cm-1. H NMR
(C6D6, 500 MHz): δ 33, 18.0, 17.6, 5.8, 1.4, 1.2, -3.9, -20, -34,
-67 ppm.
LtBu,iPr2Fe(SPh)(MeIm). In a 20 mL scintillation vial,
N-methylimidazole (20 μL, 0.251 mmol) was added dropwise
via syringe to a red-orange solution of LtBu,iPr2Fe(SPh) (80.8 mg,
0.121 mmol) in Et2O (10 mL). Upon swirling the solution, an
orange solid precipitated immediately. Placing in the freezer
(-35 °C) overnight gave a fluffy orange solid (81.7 mg, 90%).
This complex can be crystallized from benzene. 1H NMR
(C6D6, 500 MHz): δ 49, 27, 23, 22, 12, 1.2, 2.0, 5.6, 20, 28, 48,
89 ppm. IR (KBr): 3110 (w), 2966 (s), 2866 (m), 1578 (m), 1533
(m), 1489 (s), 1466 (m), 1433 (m), 1379 (s), 1356 (vs), 1315 (s),
1284 (w), 1252 (w), 1217 (m), 1186 (w), 1153 (w), 1099 (m), 1022
(w), 939 (w), 779 (w), 742 (m) cm-1. Anal. Calcd for
C45H64N4SFe: C, 72.17; H, 8.61; N, 7.46%. Found: C, 72.82;
H, 8.71; N, 6.93%.
methyl) ppm. μeff (Evans, C6D6): 5.6(5) μB. UV-vis (Tol) λmax
:
533 nm (56(3) M-1 cm-1). IR (KBr cm-1): 2959 (s), 2867 (w),
1588 (m), 1490 (s), 1430 (w), 1376 (m), 1359 (s), 1318 (m), 1294
(m), 1217 (w), 1180 (w), 1096 (w), 871 (w), 802 (w), 755 (m),
691 (w). Anal. Calcd for C41H58N2OFe: C, 75.67; H, 8.98;
N, 4.30%. Found: C, 75.29; H, 8.90; N, 4.08%.
LtBu,iPr2Fe(OTol). Yield 75%. 1H NMR (C6D6, 400 MHz):
δ 117 (1H, R-H), 94 (2H, o-OTol), 83 (3H, CH3-OTol), 55 (2H,
m-OTol), 41 (18H, tBu), -4.1 (4H, m-Ar), -29 (12H, iPr
methyl), -101 (2H, p-Ar), -115 (4H, iPr methine, 12H, iPr
methyl) ppm. UV-vis (Et2O) λmax: 531 nm (79(4) M-1 cm-1).
IR (KBr cm-1): 2963 (s), 2870 (w), 1618 (w), 1502 (s), 1459 (w),
1430 (w), 1360 (s), 1319 (m), 1280 (m), 1105 (w), 934 (w), 873 (w),
821 (m), 779 (w), 762 (m). Anal. Calcd for C42H60N2OFe: C,
75.88; H, 9.09; N, 4.21%. Found: C, 74.83; H, 9.13; N, 4.22%.
This compound is somewhat unstable in solution, depositing a
solid over time. Therefore, its purity is suspect, consistent with
the elemental analysis data.
UV-vis Equilibrium Experiments. Three-coordinate iron
complexes were dissolved in toluene and filtered through a plug
of Celite before adding any reagent(s). Fitting used Kaleida-
Graph 3.6.2 (Synergy Software). For weak binding, the plot of
[L]0 versus ΔAλmax was fit to eq 3 with refinement of the binding
constant Keq and a scaling factor. For strong binding, the plot of
[L]0 versus ΔAλmax was fit to eq 4 with refinement of [M]0, Keq,
and a scaling factor. It was necessary to refine [M]0 because the
quality of the fit was drastically affected by small variations in

5116 Inorganic Chemistry, Vol. 48, No. 12, 2009
Chiang et al.
[M]0, which are unavoidable because of small amounts of
decomposition. The refined value of [M]0 was always between
0.8 mM and 1.1 mM, consistent with the intended 1.0 mM
concentration of iron complex.
free energies:
t
ΔG ¼ ΔGgð BuNCH þÞ -ΔGgðBHþÞ þ
t
ΔGsolvð BuNCÞ þ
Computations. The SolidG program42 was used to calculate
cone and solid angles for the neutral ligand of interest. The
program utilizes the crystallographic information file (CIF) of
a crystal structure to determine spatial orientation of a ligand.
The values in Table 5 reflect the average solid and cone angles
determined from 10 structures. We used our crystallographic
coordinates for DMF and CNtBu complexes (Figure 4). To
supplement these data, we used the Cambridge Structural
Database (CSD) to find other iron complexes containing these
and other ligands of interest.1 Priority was given to four-
coordinate iron(II) complexes. When necessary, complexes
with an increased coordination number and/or strong-field
ligands were also used. Because of the scarcity of iron com-
plexes with 2-picoline as a ligand, cobalt and nickel complexes
were used to determine the cone and solid angles of this
ligand. A full listing of complexes used is in the Supporting
Information.
ΔGsolvðBHþÞ -ΔGsolvð BuNCH þÞ -ΔGsolvðBÞ ð8Þ
t
where ΔGg(tBuNCH+) represents the gas-phase free energy
change of the deprotonation reaction, tBuNCH+ ftBuNC + H+.
The gas-phase geometry optimization and free energy calcu-
lation were performed at the B3LYP/6-311++G(d,p) level of
theory. The solvation free energies in MeCN were obtained
using the Polarizable Continuum Model (PCM) at the B3LYP/
6-31+G(d) level. The gas-phase optimized geometries were
used for all of the solution-phase calculations.
From the gas-phase free energies and solvation free energies,
we can obtain the pKa of tBuNCH+ using eq 7. In our calcula-
tions, we chose several bases as our reference: PhNH2 (experi-
mental pKa = 10.6); Et3N (experimental pKa = 18.8); pyridine
(experimental pKa = 12.5); and, PhNMe2 (experimental pKa =
11.4).53 Using these reference bases gave pKa values for
tBuNCH+ of 2.70, 2.82, 3.11, and 2.23, respectively. We take
the average value of 2.71 as the pKa of tBuNCH+ in MeCN.
The pKa of CNtBu was calculated through means of the acid-
base reactions
Acknowledgment. The authors thank William Jones
and Joseph Dinnocenzo for valuable input and discus-
sions, and thank Eleni Skrombolas for initial experi-
ments. This work was funded by the National Institutes
of Health (Grant GM065313 to P.L.H.) and the Depart-
ment of Education (GAANN fellowship to K.P.C.).
tBuNCH þ þ B f tBuNCH þ BHþ
ð6Þ
where B is a reference base whose pKa is already known
experimentally. This approach 43 bypasses the treatment of the
solvated proton, whose chemical structure is often unknown
and difficult to model theoretically.
The pKa of tBuNCH+ can be determined by calculating the
total free energy change of eq 6:
Supporting Information Available: Details of NMR spectra,
equilibrium measurements, cyclic voltammetry, solid angle
calculation (PDF) and crystallography (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
ΔG
2:303RT
t
pKað BuNCH þÞ ¼ pKaðBHþÞ þ
ð7Þ
The total free energy change (ΔG) can be evaluated as
a combination of gas-phase free energies and solvation
(53) Kaljurand, I.; Kutt, A.; Soovali, L.; Rodima, T.; Maemets, V.; Leito,
I.; Koppel, I. A. J. Org. Chem. 2005, 70, 1019–1028.