Trans-[Fe(cyclam)(C2R)2]+: A new family of iron(III) bis-alkynyl compounds

Article abstract of DOI:10.1021/om300515r

We previously communicated the preparation and characterization of two trans-[Fe(cyclam)(C≡CR)₂]OTf compounds, 2b and 2c (where cyclam = 1,4,8,11-tetraazacyclotetradecane, R = -SiⁱPr₃ (2b) or -Ph (2c), and OTf = trifluoromethanesulfonate), which were the first examples of Fe(III) bis-alkynyl complexes. In this work, the series has been expanded to include R = -H (2a), -C₂SiMe₃ (2d), -C ₄SiMe₃ (2e), and -Fc (2f), which were prepared from the reaction between cis/trans-[Fe(cyclam)(OTf)₂]OTf (1) and LiC≡CR (NaC≡CH for 2a). Compounds 2a-2f were characterized by spectroscopic/voltammetric techniques as well as high-resolution mass spectrometry (HR-MS). The trans-orientation of the alkynyl ligands were established from the single-crystal X-ray diffraction studies of 2b-2d. Furthermore, the electronic structures of the model compounds 2a′⁺, 2d′⁺, and 2e′⁺ were analyzed with density-functional calculations, which revealed significant d-π(C≡C) interactions.

Full text of DOI:10.1021/om300515r

Article
pubs.acs.org/Organometallics
trans-[Fe(cyclam)(C2R)2]+: A New Family of Iron(III) Bis-Alkynyl
Compounds
Zhi Cao, William P. Forrest, Yang Gao, Phillip E. Fanwick, and Tong Ren*
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
S
* Supporting Information
ABSTRACT: We previously communicated the preparation and
characterization of two trans-[Fe(cyclam)(CCR)2]OTf compounds,
2b and 2c (where cyclam = 1,4,8,11-tetraazacyclotetradecane, R =
−SiiPr3 (2b) or −Ph (2c), and OTf = trifluoromethanesulfonate), which
were the first examples of Fe(III) bis-alkynyl complexes. In this work,
the series has been expanded to include R = −H (2a), −C2SiMe3 (2d),
−C4SiMe3 (2e), and −Fc (2f), which were prepared from the reaction
between cis/trans-[Fe(cyclam)(OTf)2]OTf (1) and LiCCR (NaC
CH for 2a). Compounds 2a−2f were characterized by spectroscopic/
voltammetric techniques as well as high-resolution mass spectrometry
(HR-MS). The trans-orientation of the alkynyl ligands were established
from the single-crystal X-ray diffraction studies of 2b−2d. Furthermore,
+
+
+
the electronic structures of the model compounds 2a′ , 2d′ , and 2e′
were analyzed with density-functional calculations, which revealed
significant dπ−π(CC) interactions.
ransition-metal alkynyl compounds have received intense
interest as potential molecular wires and other electronic
flexibility.9−12 While these Ru2 species are still being
T
investigated with a focus on derivatives containing cross-
conjugated enediyne ligands, such as geminal-diethynylethene
(gem-DEE),13 and their utility in the molecular passivation of
semiconductor surfaces,14 we are seeking other structural motifs
that would also favor the trans-orientation of two alkynyls. Of
particular interest to us is the pseudo-octahedral motif based on
3d metal complexes of cyclam (1,4,8,11-tetraazacyclotetrade-
cane). A number of Cr/Rh(III)-cyclam bis-arylethynyls (cis or
trans) were reported by the laboratory of Wagenknecht, where
the metal-centered emissions are noteworthy.15 Nishi and co-
workers reported Cr(III)-cyclam compounds based on
ethynyltetrathiafulvalenes and the weak ferromagnetism there-
in.16 Examples of M(cyclam) monoalkynyl complexes are
limited to the cobalt(III) ethynylbenzene series by the
laboratory of Shores.17 We recently communicated the
preparation and characterization of two trans-[Fe(cyclam)-
(CCR)2]OTf-type compounds with R = −SiiPr3 (2b) and
−Ph (2c), which were the first examples of bis-alkynyl
compounds based on an Fe(III) core.18 In this contribution,
the scope of the study has been expanded to include both the
synthesis of new compounds, 2a and 2d−2f (Scheme 1), and
the detailed characterizations of type 2 compounds via X-ray
diffraction, DFT calculations, and voltammetric/spectroscopic
studies.
and optoelectronic materials since the pioneering work on the
linear [M]-alkynyl polymers by Nast and Hagihara.1 Facile
charge transfer along the [M]−(CC)m− linkage has been
demonstrated for a number of metal centers in both bulk
solution studies2,3 and nanojunction measurements.4 Many
inspiring examples of iron monoalkynyl compounds as
molecular wires have been reported by the laboratories of
Lapinte3 and Akita,5 where the piano-stool-type Fe(II) centers,
either CpFe(P−P)− or CpFe(CO)2−, are prevalent (type I in
Chart 1). In comparison, iron bis-akynyl compounds,
Chart 1. Common Fe Alkynyl Structures
developed primarily in the laboratory of Field, are rare and
limited to Fe(II) centers with the alkynyl ligands in a trans-
geometry and bidentate chelating phosphines as the auxiliary
ligands (type II in Chart 1).6,7 Recently, the electronic and
magnetic interactions among Fe(III)(P−P)2 units connected by
a 1,3,5-triethynylbenzene bridge were elegantly elaborated by
the laboratory of Shores.8
Work from several laboratories including ours revealed that
Ru2(II,III) and Ru2(III,III) alkynyl compounds are excellent
building blocks for molecular wires due to their unrivaled redox
Received: June 9, 2012
Published: August 13, 2012
© 2012 American Chemical Society
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Scheme 1. Synthesis of Bis-alkynyl Fe(III) Cyclam Complexes
RESULTS AND DISCUSSION
■
As shown in Scheme 1, the synthesis of trans-[Fe(cyclam)(C
CR)2]OTf-type compounds began with the preparation of cis/
trans-[Fe(cyclam)Cl2]Cl using the literature method,19 afford-
ing the cis- (yellow) and trans-isomers (brown) in yields of 86%
and 8%, respectively. Subsequently, the reaction between cis-
[Fe(cyclam)Cl2]Cl and trifluoromethanesulfonic acid (HOTf)
in large excess resulted in a cis/trans-[Fe(cyclam)(OTf)2]OTf
(1) mixture (with the cis- isomer being predominant) in
excellent yield (89%). The reaction between compound 1 and
2.2 equiv of NaCCH yielded compound 2a (45%) in 10 h,
while those between 1 and 2.2 equiv of LiCCR led to
compounds 2b (R = −SiiPr3), 2c (R = −Ph), 2d (R =
−C2SiMe3), 2e (R = −C4SiMe3), and 2f (R = −Fc) in yields of
57%, 65%, 33%, 11%, and 63%, respectively.
Figure 1. ORTEP plot of [2b]+ at the 30% probability level. H atoms
were omitted for clarity.
Both the type 1 and 2 compounds are paramagnetic and
cannot be unambiguously characterized with 1H NMR
spectroscopy. Instead, these compounds were authenticated
by HR-nESI-MS and single-crystal X-ray diffraction studies (2b,
2c, and 2d). The room-temperature magnetic susceptibility
measurement of 1 revealed an effective magnetic moment of
5.32 μB, which is slightly below the spin-only value for an S = 5/
2 species (5.92 μB). The room-temperature effective magnetic
moments of compounds 2a, 2b, 2c, 2d, 2e, and 2f are 1.90,
1.94, 1.89, 1.96, 1.93, and 1.92 μB, respectively, which are in
agreement with an S = 1/2 ground state (1.73 μB). The contrast
in the magnetic properties between 1 and type 2 compounds is
attributed to the fact that alkynyls are strong-field ligands and
stabilize a low-spin Fe(III) center, while −OTf ligands are
weak-field ligands giving rise to a high-spin Fe(III) center.
Single-crystal X-ray diffraction studies were performed for
compounds 2b, 2c, and 2d, and the ORTEP plots of the cations
are shown in Figures 1−3, respectively, with selected bond
lengths and angles listed in Table 1. Compounds 2b, 2c, and 2d
were crystallized in triclinic, monoclinic, and orthorhombic
settings, respectively. The asymmetric unit of crystal 2b
contains the halves of two independent cations. Since the
two independent cations are very similar in geometries, the
structural plot and selected key metric parameters are presented
for only one of them herein. Each of the asymmetric units of
crystals 2c and 2d contains one-half of the compound, which is
related to the other half through an inversion center situated at
the Fe(III) site. Similar to [Cr(cyclam)(CCAr)2]+-type
compounds, the pseudo-octahedral coordination mode is
adopted by all [Fe(cyclam)(CCR)2]+ species with the 
Figure 2. ORTEP plot of [2c]+ at the 30% probability level. H atoms
were omitted for clarity.
Figure 3. ORTEP plot of [2d]+ at the 30% probability level. H atoms
were omitted for clarity.
C−Fe−C vector approximately orthogonal to the plane
defined by the four N centers of cyclam, as shown in Figures
1−3. The Fe−C bond lengths in [2b]+ (1.961(3) Å), [2c]+
(1.967(5) Å), and [2d]+ (1.935(8) Å) are in agreement with
the Fe(II)−C bond lengths determined for the trans-Fe(P−
P)2(CCR)-type compounds (1.92−1.97 Å).6,20,21 It is
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Table 1. Selected Bond Lengths (Å) and Angles (deg) for
Compounds [2b]+, [2c]+, and [2d]+
[2b]+
[2c]+
[2d]+
Fe1−C1
Fe1−N1
Fe1−N2
C1−C2
C2−Si1
1.961(3)
2.013(2)
2.007(2)
1.210(4)
1.833(3)
Fe1−C1
Fe1−N1
Fe1−N2
C1−C2
C2−C3
1.967(5)
1.999(4)
2.008(4)
1.211(7)
1.437(7)
Fe1−C1
Fe1−N1
Fe1−N2
C1−C2
C2−C3
C3−C4
1.935(8)
2.002(6)
2.011(6)
1.230(8)
1.38(1)
1.223(9)
C1−Fe1−
180.0(2)
C1−Fe1−
179.99(1) C1−Fe1−
179.99(1)
89.7(3)
89.5(3)
86.0(3)
177.5(6)
C1A
C1A
C1A
C1−Fe1−
N1
91.63(1) C1−Fe1−
88.23(2) C1−Fe1−
N1
N1
C1−Fe1−
N2
89.69(1) C1−Fe1−
90.86(2) C1−Fe1−
N2
N2
N1−Fe1−
N2
94.57(9) N1−Fe1−
84.84(2) N1−Fe1−
N2
N2
Fe1−C1−
C2
174.1(2)
Fe1−C1−
C2
172.8(4)
Fe1−C1−
C2
noteworthy that the shortening of the Fe−C bond in [2d]+ is
accompanied by the elongation of the Fe-bound CC bond
(1.230 Å, the longest among the three structurally characterized
compounds). Such a structural feature is indicative of an
enhanced contribution of the cumulenic resonance structure in
addition to the nominal acetylenic resonance structure.10,22
The redox activity of compounds 2a−2f is very relevant to
the general interest in using trans-bis-alkynyl metal complexes
as the building blocks for molecular wires and was examined
carefully using cyclic voltammetric (CV) and differential pulse
voltammetric (DPV) techniques. The CVs recorded are shown
in Figure 4, while the DPVs are provided in the Supporting
Information (Figure S3). There is no detectable oxidation
couple up to +1.0 V for compounds 2a−2e, which reflects the
electron deficiency of the Fe(III) species. All compounds
undergo a reversible 1e− reduction (A), which is attributed to
the reduction of Fe(III) to Fe(II). The formal potential of 2c is
anodically shifted from those of 2a and 2b, indicating that the
reduction of the Fe(III) center becomes easier with arylethynyl
ligands due to an extended π-delocalization. Compounds 2d
and 2e exhibit voltammagrams very similar to those of 2a−2c
albeit with the reduction potentials anodically shifted, namely,
−0.364 V for −(CC)2TMS (2d) and −0.215 V for −(C
C)3TMS (2e).12 This drastic anodic shift is due to the
increased electron-withdrawing nature of the alkynyl ligand
with each additional acetylene unit.23
Figure 4. Cyclic voltammograms recorded for compounds 2a−2f in a
0.20 M THF solution of Bu4NPF6.
fragments with M as Ru,24 [Cu3(dppm)3]3+,25 Ru3(dpa)4
(dpa = dipyridylamidate),26 and Ru2(DMBA)4 (DMBA =
N,N′-dimethylbenzamidinate).11,12 For the latter compounds, a
remarkably large ΔE1/2 of ca. 300 mV was observed, and the
mixed-valent [Fc−Fc]+ ion was identified as a class III Robin−
Day ion on the basis of spectroelectrochemical analysis. In
contrast, the oxidation waves of Fc (B) are apparently not
stepwise in both the CV (Figure 4) and DPV (Figure 5)
Previously, the redox activity of trans-Fe(DMPE)(CCPh)2
(DMPE = bis(dimethylphosphine)ethane) was investigated by
Field and co-workers using cyclic voltammetry, and two
consecutive 1e− oxidation couples, Fe(+3/+2) and Fe(+4/
+3), were observed at +0.01 and +0.99 V, respectively, versus
SCE.20 In comparison, the Fe(+3/+2) couple in 2a−2c and 2f
appears between −0.525 and −0.594 V versus Ag/AgCl, which
is ca. 0.60 V more cathodic (after the correction for the
difference in reference electrodes) than that of trans-Fe-
(DMPE)(CCPh)2. This clearly demonstrates that the Fe
center can be selectively stabilized at the formal oxidation state
of +2 or +3 by employing soft (P) or hard (N) auxiliary ligands,
respectively.
The presence of the two ferrocenyl (−Fc) termini in
compound 2f offers a unique opportunity to assess the
efficiency of electronic delocalization across the −CC−Fe−
CC− fragment. Previously, the stepwise Fc oxidations with
substantial ΔE1/2 were observed for −CC−M−CC−
Figure 5. DPVs of the Fc oxidation region measured for 2f under both
the standard conditions (dashed line) and conditions according to the
Richardson−Taube method (solid line).
recorded under standard conditions, but broad due to the
coalescence of two 1e− waves. To estimate the separation of the
two 1e− waves, the Richardson−Taube method was em-
ployed.27 The DPV of 2f, recorded under the conditions
stipulated by Richardson and Taube, yielded a half-width of 146
mV (Figure 5), which corresponds to a ΔE1/2 of 74 mV.
Clearly, the {Fc---Fc}+ species in 2f is a class II Robin−Day
mixed-valent ion at best,28 indicating that the −CC−
Fe(cyclam)−CC− fragment is not as efficient as the
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aforementioned Ru and Ru2 species in mediating electronic
delocalization.
As shown in Figure 6 for 2a, 2d, 2e, and 2f, type 2
compounds generally display structured d−d bands in their
Figure 7. Energies of the lowest energy absorption maxima of 2a, 2d,
and 2e in CH3CN solutions and their reciprocal fit function with n as
the number of (CC) units.
organic polyynes (expressed in the form of the power law E ≈
n−x).32
Compounds 2a−2f are quite different from the previously
studied iron bis-alkynyl compounds in both the oxidation state
(III versus II) and the auxiliary ligands (cyclam versus bidentate
phosphines). In addition, the unusually intense d−d transitions
observed for 2a−2f imply significant dπ−π(CC) interactions
that typify CpFeL2(CCR) compounds.3,33 These observa-
tions prompted us to explore the electronic structure of type 2
compounds using density-functional theory calculations at the
B3LYP/LanL2DZ level (Gaussian03 program).34 The calcu-
Figure 6. UV−vis absorption spectra of compounds 2a, 2d, 2e, and 2f
recorded in CH3CN.
+
+
UV−vis spectra that have extinction coefficients higher than
values expected for centrosymmetric species. Analysis pre-
sented in the prior communication precluded vibronic
sidebands as the origin of the observed fine structure.18
Instead, the observed transitions can be assigned using the
strong-field limit of the Tanabe−Sugano diagram for a d5 ion:29
lations were performed on the model cations 2a′ , 2d′ , and
+
2e′ , where the number of −CC− units in the alkynyl ligand
increases from 1 to 3. The optimized bond lengths and angles
+
for 2d′ are in good agreement with the crystal structural data
of 2d+. The optimized bond lengths and angles for the first
+
+
coordination sphere of 2a′ and 2e′ are comparable to those
determined for 2b+, 2c+, and 2d+. The computed energies and
counter plots of the most relevant MOs for the model
2
the spin-allowed excitations from the T2g ground state to the
2
2
2
2T1g, A2g, Eg, and A1g excited states result in peaks at 425/
559/637, 399/503/563, 374/457/503, and 333/414/454 nm in
2a/2d/2e, respectively. Structured d−d bands were also
observed for the series of trans-[Cr(cyclam)(CCAr)2]+
arylethynyl complexes (where Ar = −C6H5, −C6H4CH3,
−C6H4CF3, and −C6H5F), albeit with extinction coefficients
about an order of magnitude smaller than those of compounds
2a−2f.15 The spectrum of 2f is also structured with absorptions
at 444, 509, and 570 nm, with distinctly broadened peaks.
While the exact cause of the broadening is undetermined, it is
plausible that the presence of Fc reduces the effective symmetry
and hence the further splitting of ligand field terms.
+
+
+
compounds 2a′ , 2d′ , and 2e′ are given in Figure 8, while a
detailed list of the optimized geometries and MOs are given in
the Supporting Information.
+
The optimized Fe−C bond lengths of 2a′ (1.939 Å) and
+
2d′ (1.915 Å) are slightly shorter than the X-ray experimental
values of 2b+ (1.961 Å) and 2d+ (1.935 Å). The C1C2 bond
+
length of 2a′ is elongated to 1.266 Å (versus 1.210 Å, X-ray
value for 2b+), and the C1C2 and C3C4 bond lengths of
+
2d′ are elongated to 1.270 and 1.254 Å (versus 1.230 and
1.223 Å, X-ray values for 2d+), respectively, due to the tendency
of overestimation of electronic delocalization by the B3LYP
method.35 With the additional −CC− units in the backbone,
The availability of trans-[Fe(cyclam)(C2nR)2]+-type com-
plexes with n = 1, 2, and 3 allows for an examination of the
energy dependence on the number of −CC− units and
possible inference with the degree of π-delocalization.
Specifically, the optical transition energy corresponding to the
lowest energy absorption maxima in 2a, 2d, and 2e was plotted
versus 1/n (Figure 7), and a linear relationship is apparent. It
could be inferred by extrapolation that the lowest electron
excitation energy (E(n)) for trans-[Fe(cyclam)(C2nR)2]+ would
be about 1.44 eV as n approaches infinity. However, the π-
conjugation tends to reach saturation long before n approaches
infinity in practice, and the saturation point n is often called the
effective conjugation length.30 With a limited range of n, the π-
conjugation in trans-[Fe(cyclam)(C2nR)2]+-type complexes is
far from saturation. It is worth noting that similar correlations
were established for both the Re capped polyyn-diyls31 and
+
the optimized Fe−C bond length in 2e′ is shortened to 1.905
Å due to the extended electronic delocalization.
+
+
+
The DFT results of 2a′ , 2d′ , and 2e′ are in agreement
with the ligand-field theory prediction for a d5 species in a
strong field: an empty eg set as LUMO (dx −y ) and LUMO+1
(dz ) and an occupied t2g set as HOMO−2, HOMO−1, and
2
2
2
SOMO. Those closely related MOs, as well as energy levels, are
plotted in Figure 8. The loss of orbital degeneracy for the eg
and t2g sets is due to both the low symmetry of the cyclam
ligand (C2 only) and Jahn−Teller instability (in both Oh and
D4h settings). The two highest occupied MOs, namely,
HOMO−1 and SOMO, consist of the antibonding combina-
tions of dπ and π(CC), which are typical for transition-metal
alkynyls.36 The contribution from the π(CC) orbitals
+
+
becomes more prominent in 2d′ and 2e′ as the π-orbitals
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+
+
+
Figure 8. Molecular orbital diagrams for model compounds 2a′ , 2d′ , and 2e′ (from left to right). The directions pointed toward the N atoms of
the cyclam ring are designated as the x and y axes.
extend over a longer framework, as shown in the HOMO−1
and SOMO. Similar trends were noted for the piano stool type
Fe(II) monoalkynyl compounds3 and diplatinum bridged by
EXPERIMENTAL SECTION
■
General Procedures. cis-[Fe(cyclam)Cl2]Cl was prepared as
previously described.19 1,4,8,11-Tetraazacyclotetradecane was pur-
chased from Strem Chemicals, and anhydrous iron(II) chloride, triflic
acid, sodium acetylide (suspended in mineral oil), and n-BuLi (2.5 M
in hexanes) were from Sigma-Aldrich. The alkynyl ligands,
triisopropylsilylacetylene, 1,4-bis(trimethylsilyl)-2,3-butadiyne, and
phenylacetylene were purchased from GFS Chemicals. Ethynylferro-
cene and 1,6-bis(trimethylsilyl)-1,3,5-hexatriyne were prepared accord-
ing to the literature procedures.38 All reagents were used as received
with no further purification. THF was distilled under a nitrogen
atmosphere over Na/benzophenone. UV−vis spectra were obtained
with a JASCO V-670 spectrophotometer in CH3CN solutions. FT-IR
spectra were measured on a Jasco FT/IR-6300 as neat samples via a
ZnSe ATR. All HR-nESI-MS spectra were performed on a prototype
version of a QqTOF tandem mass spectrometer in CH2Cl2/MeOH
(50/50, v/v) (Q-Star Pulsar XL; Applied Biosystems/Sciex, Concord,
ON, Canada). Masses were calculated by isotopic distribution utilizing
Analyst 1.5 software (Applied Biosystems/Sciex, Concord, ON,
Canada). Magnetic susceptibility was measured at 294 K with a
Johnson Matthey Mark-I magnetic susceptibility balance. Cyclic and
differential voltammograms were recorded in 0.2 M (n-Bu)4NPF6
solution (THF, N2-degassed) on a CHI620A voltammetric analyzer
with a glassy carbon working electrode (diameter = 2 mm), a Pt-wire
auxiliary electrode, and a Ag/AgCl reference electrode. The
concentration of the Fe species is always 1.0 mM. The ferrocenium/
ferrocene couple was observed at 0.570 V (vs Ag/AgCl) at the noted
experimental conditions.
polyyne-diyls.37 The LUMO is dominated by the dx −y and N
lone pairs with no contribution from the acetylenic units due to
the orbital orthogonality. The LUMO+1 is best described as
2
2
2
σ*(Fe−C) with the dominant contribution from the dz orbital.
+
+
The SOMO−LUMO gaps calculated for 2a′ (2.86 eV), 2d′
+
(2.33 eV), and 2e′ (2.05 eV) are fairly close to the d−d
transition optical gaps for 2a (425 nm, or 2.92 eV), 2d (559
nm, or 2.22 eV), and 2e (637 nm, or 1.95 eV), respectively. It is
clear from Figure 8 that the SOMOs become progressively
stabilized with the increasing number of −CC− units. This
trend is in agreement with the observed anodic shift of
reduction potentials from 2a to 2e. In addition, the pronounced
π(CC) contributions to both the SOMO and the HOMO−1
offer the possibility of d−d bands “stealing” intensity. It should
be noted that the data presented here are based on ground-state
calculations, and more rigorous comparison should be based on
TD-DFT-type calculations.
CONCLUSION
■
A new family of Fe(III) bis-alkynyl compounds 2a−2f was
prepared under mild conditions. Strong π-interactions between
the occupied dπ and π(CC) orbitals are evident from both
the absorption spectra and the DFT calculations. The formal
potential of the Fe(+3/+2) couple is highly dependent on the
number of acetylenic (−CC−) units in the −(CC)n−R
ligand. Type 2 compounds generally display intense and
structured bands in the visible absorption spectra, which are
assigned as spin-allowed d−d transitions. Ongoing efforts in our
laboratory focus on the preparation of both trans-[Fe(cyclam)-
(C2nR)2]+ (n > 3) and trans-[Fe(cyclam)(gem-DEE)2]+-type
compounds and the elaboration of the π-conjugation therein.
Preparation of cis/trans-[Fe(cyclam)OTf2]OTf (1). A flask fitted
with a nitrogen bubbler was charged with cis-[Fe(cyclam)Cl2]Cl
(0.453 g, 1.17 mmol) and 10 g (66.6 mmol) of triflic acid. Nitrogen
was bubbled through the green solution for 40 h. The resulting light
yellow solution was transferred to a large beaker, to which was added
150 mL of anhydrous ether. Upon stirring and scratching, a light
yellow powder formed, which was collected via filtration. Yield: 0.70 g
(0.995 mmol), 89% based on Fe. HR-nESI-MS (m/e, based on 56Fe):
+
554.041, corresponding to [M]+ (C12H24F6S2O6N4Fe1 , calcd.
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Table 2. Crystal Data for Compounds 2b, 2c, and 2d
2b·C2H6O
2c
2d
molecular formula
fw
C35H72F3FeN4O4SSi2
814.07
C27H34F3FeN4O3S
607.50
C25H42F3FeN4O3SSi2
647.72
space group
a, Å
P1(No. 2)
P21/c (No. 14)
9.5167(6)
Pbca (No. 61)
12.0900(11)
11.6199(11)
24.842(3)
̅
9.2957(5)
16.0961(6)
17.2103(9)
106.581(5)
105.399(4)
104.772(4)
2221.8(1)
2
b, Å
11.8798(6)
12.2650(9)
c, Å
α, deg
β, deg
90.108(5)
γ, deg
V, Å3
1386.6(1)
3489.9(6)
4
Z
2
ρcalcd, g cm−3
μ, mm−1
T, K
1.217
1.455
1.233
5.076
150
4.096
5.556
150
150
no. of rflns collected
no. of indep rflns
final R indices
(I > 2σ(I))
GOF on F2
40 150
13 139
15 312
6834 (R(Int) = 0.038)
R1 = 0.051
wR2 = 0.140
1.038
1940 (R(Int) = 0.069)
R1 = 0.076
wR2 = 0.216
1.038
2166 (R(Int) = 0.103)
R1 = 0.095
wR2 = 0.252
1.039
554.039). UV−vis, λmax (nm, ε (M−1 cm−1)): 511 (640), 498 (770),
458 (800), 372 (4100), 334 (9900). FT-IR (ν, cm−1): 887 (N−H
bending and CH2 rocking region for trans) and 858 (N−H bending
and CH2 rocking region for cis). μeff = 5.32 μB. Anal. Found (calcd) for
C13H24N4Fe1S3O9F9·CF3SO3 (1·CF3SO3H): C, 19.78 (19.63); H, 3.02
(2.94); N, 6.91 (6.54).
mixture was stirred for 1 h to yield a dark red solution, and then the
reaction was quenched with the addition of a few drops of water. The
solution was filtered through silica, eluting with 30% CH3CN in
CH2Cl2. Upon removal of the solvent, the oily residue was added into
cold Et2O (ca. 100 mL), yielding a magenta precipitate. Yield: 0.139 g
(0.229 mmol), 65% based on Fe. HR-nESI-MS (m/e, based on 56Fe):
+
Preparation of trans-[Fe(cyclam)(C2H)2]OTf (2a). To an 80 mL
THF solution of cis/trans-[Fe(cyclam)(OTf)2]OTf (0.250 g, 0.355
mmol) was added 2.2 equiv of NaC2H (0.781 mmol in THF) at room
temperature. The reaction mixture was stirred for 10 h to yield a red-
orange solution, and then the reaction was quenched with the addition
of a few drops of water. The solution was filtered through silica eluting
with 30% CH3CN in CH2Cl2. Upon removal of the solvent, the oily
residue was added into cold Et2O (ca. 100 mL), yielding a pink
precipitate. Yield: 0.072 g (0.158 mmol), 45% based on Fe. HR-nESI-
MS (m/e, based on 56Fe): 306.152, corresponding to [M]+
458.211, corresponding to [M]+ (C26H34N4Fe1 , calcd 458.213). UV−
vis, λmax (nm, ε (M−1 cm−1)): 555 (4300), 500 (3200), 443 (2000),
407 (1500). FT-IR (ν(CC), cm−1): 2077 and 889 (N−H bending
and CH2 rocking region for trans). Cyclic voltammogram [E1/2/V,
ΔEp/V, ibackward/iforward]: A, −0.525, 0.035, 0.930. μeff = 1.89 μB. Anal.
Found (calcd) for C27H34N4Fe1SO3F3 (2c): C, 53.24 (53.12); H, 5.44
(5.61); N, 9.16 (9.18).
Preparation of trans-[Fe(cyclam)(C4TMS)2]OTf (2d). To an 80
mL THF solution of cis/trans-[Fe(cyclam)(OTf)2]OTf (0.250 g, 0.355
mmol) was added 2.2 equiv of LiC4TMS (0.781 mmol in THF,
prepared in situ from n-BuLi and Me3SiC4SiMe3) at room temperature.
The reaction mixture was stirred for 1 h to yield a dark red solution,
and then the reaction was quenched with the addition of a few drops
of water. The solution was filtered through silica eluting with 30%
CH3CN in CH2Cl2. Upon removal of the solvent, the oily residue was
added into cold Et2O (ca. 100 mL), yielding a magenta precipitate.
Yield: 0.075 g (0.116 mmol), 33% based on Fe. HR-nESI-MS (m/e,
+
(C14H26N4Fe1 , calcd. 306.150). UV−vis, λmax (nm, ε (M−1 cm−1)):
425 (4800), 399 (4200), 374 (2800), 333 (2900). FT-IR (ν(CC),
cm−1): 2038 and 889 (N−H bending and CH2 rocking region for
trans). Cyclic voltammogram [E1/2/V, ΔEp/V, ibackward/iforward]: A,
−0.594, 0.052, 0.85. μeff = 1.90 μB. Anal. Found (calcd) for
C15H26N4Fe1SO3F3 (2a): C, 39.51 (39.31); H, 5.69 (5.72); N, 12.35
(12.22).
+
Preparation of trans-[Fe(cyclam)(C2SiiPr3)2]OTf (2b). To an 80
mL THF solution of cis/trans-[Fe(cyclam)(OTf)2]OTf (0.250 g, 0.355
mmol) was added 2.2 equiv of LiC2SiiPr3 (0.781 mmol in THF,
prepared in situ from n-BuLi and HC2SiiPr3) at room temperature. The
reaction mixture was stirred for 1 h to yield a dark red solution, and
then the reaction was quenched with the addition of a few drops of
water. The solution was filtered through silica eluting with 30%
CH3CN in CH2Cl2. Upon removal of the solvent, the oily residue was
added into cold Et2O (ca. 100 mL), yielding a dark yellow precipitate.
Yield: 0.156 g (0.203 mmol), 57% based on Fe. HR-nESI-MS (m/e,
based on 56Fe): 498.229, corresponding to [M]+ (C24H42N4Si2Fe1 ,
calcd 498.229). UV−vis, λmax (nm, ε (M−1 cm−1)): 559 (4000), 503
(3300), 457 (2100), 414 (1700). FT-IR (ν(CC), cm−1): 2162,
2125, 2080, 2010, and 889 (N−H bending and CH2 rocking region for
trans). Cyclic voltammogram [E1/2/V, ΔEp/V, ibackward/iforward]: A,
−0.364, 0.035, 0.98. μeff = 1.96 μB. Anal. Found (calcd) for
C25H42N4Fe1Si2SO3F3 (2d): C, 46.18 (46.15); H, 6.61 (6.51); N,
8.61 (8.61).
Preparation of trans-[Fe(cyclam)(C6TMS)2]OTf (2e). To an 80
mL THF solution of cis/trans-[Fe(cyclam)(OTf)2]OTf (0.250 g, 0.355
mmol) was added 2.2 equiv of LiC6TMS (0.781 mmol in THF,
prepared in situ from n-BuLi and Me3SiC6SiMe3) at room temperature.
The reaction mixture was stirred for 1 h to yield a dark red solution,
and then the reaction was quenched with the addition of a few drops
of water. The solution was filtered through silica eluting with 30%
CH3CN in CH2Cl2. Upon removal of the solvent, the oily residue was
added into cold Et2O (ca. 100 mL), yielding a dark green precipitate.
Yield: 0.027 g (0.039 mmol), 11% based on Fe. HR-nESI-MS (m/e,
+
based on 56Fe): 618.421, corresponding to [M]+ (C32H66N4Si2Fe1 ,
calcd 618.417). UV−vis, λmax (nm, ε (M−1 cm−1)): 470 (4800), 433
(3000), 404 (1300), 364 (650). FT-IR (ν(CC), cm−1): 2015 and
883 (N−H bending and CH2 rocking region for trans). Cyclic
voltammogram [E1/2/V, ΔEp/V, ibackward/iforward]: A, −0.585, 0.062,
0 . 9 6 . μ e f f
= 1 . 9 4 μ B . A n a l . F o u n d ( c a l c d ) f o r
C33H66N4Si2Fe1SO3F3·H2O (2b·H2O): C, 50.04 (50.24); H, 7.93
(8.69); N, 7.41 (7.10).
Preparation of trans-[Fe(cyclam)(C2Ph)2]OTf (2c). To an 80
mL THF solution of cis/trans-[Fe(cyclam)(OTf)2]OTf (0.250 g, 0.355
mmol) was added 2.2 equiv of LiC2Ph (0.781 mmol in THF, prepared
in situ from n-BuLi and HC2Ph) at room temperature. The reaction
+
based on 56Fe): 546.228, corresponding to [M]+ (C28H42N4Si2Fe1 ,
calcd 546.229). UV−vis, λmax (nm, ε (M−1 cm−1)): 637 (3700), 563
(1800), 503 (870), 444 (720). FT-IR (ν(CC), cm−1): 2142, 2117,
2034, 2000, and 889 (N−H bending and CH2 rocking region for
6204
dx.doi.org/10.1021/om300515r | Organometallics 2012, 31, 6199−6206

Organometallics
Article
trans). Cyclic voltammogram [E1/2/V, ΔEp/V, ibackward/iforward]: A,
−0.215, 0.040, 0.78. μeff = 1.93 μB. Anal. Found (calcd) for
C29H42N4Si2Fe1SO3F3·2CF3SO3H·CH2Cl2 (2e·2CF3SO3H·CH2Cl2):
C, 35.68 (35.46); H, 4.00 (4.28); N, 5.40 (5.17).
ACKNOWLEDGMENTS
■
We gratefully acknowledge the financial support from both the
National Science Foundation (CHE 1057621) and the Purdue
Research Fund. T.R. acknowledges the IR/D support from the
National Science Foundation during the preparation of the
manuscript.
Preparation of trans-[Fe(cyclam)(C2Fc)2]OTf (2f). To an 80 mL
THF solution of cis/trans-[Fe(cyclam)(OTf)2]OTf (0.250 g, 0.355
mmol) was added 2.2 equiv of LiC2Fc (0.781 mmol in THF, prepared
in situ from n-BuLi and HC2Fc) at room temperature. The reaction
mixture was stirred for 1 h to yield a dark red solution, and then the
reaction was quenched with the addition of a few drops of water. The
solution was filtered through silica eluting with 30% CH3CN in
CH2Cl2. Upon removal of the solvent, the oily residue was added into
cold Et2O (ca. 100 mL), yielding a dark purple precipitate. Yield: 0.185
g (0.225 mmol), 63% based on Fe. HR-nESI-MS (m/e, based on
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+
56Fe): 674.145, corresponding to [M]+ (C34H42N4Fe3 , calcd
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X-ray Data Collection, Processing, Structure Analysis, and
Refinement for Crystals 2b, 2c, and 2d. Single crystals of
compounds 2b, 2c, and 2d were grown via slow diffusion of a saturated
CH3CN solution of the desired compound into an Et2O/hexanes (9:1,
v/v) solution. X-ray diffraction data were collected on a Rigaku
RAPID-II image plate diffractometer using Cu Kα radiation (λ =
1.54184 Å) at 150 K, and the structures were solved using the
structure solution program DIRDIF200839 and refined using SHELX-
TL.40 Relevant information on the data collection and figures of merit
of final refinement are listed in Table 2.
COMPUTATIONAL DETAILS
■
By using density functional theory method, the calculations were based
+
+
+
+
on the model cations 2a′ , 2d′ , and 2e′ . The model compound 2d′
was optimized on the basis of the crystal structure of cation 2d+
+
without truncation. The model 2a′ was built on the basis of the
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crystal structure of 2b+ with the −SiiPr3 groups of 2b+ being replaced
+
with −H. The model 2e′ was optimized from the crystal structure of
2d+ with the insertion of one −CC− unit between the −SiMe3 and
Fe−CC−CC− fragments. Both B3LYP and BP8641 were applied
as exchange−correlation functionals, and the former produced more
reliable results compared with experimental data. In the calculations,
quasi-relativistic pseudo-potentials of the Fe valence electrons were
employed and the LanL2DZ basis sets associated with the pseudo-
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Gaussian03 program package.34 No negative frequency was observed
in the vibrational frequency analysis, which indicates that these Fe(III)
bis-alkynyl complexes are stable equilibrium structures.
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ASSOCIATED CONTENT
■
S
* Supporting Information
High-resolution mass spectra of all compounds, UV−vis spectra
of compounds 1, 2b, and 2c, DFT calculations for model
+
+
+
compounds 2a′ /2d′ /2e′ , and DPVs for compounds 2a−2e;
X-ray crystallographic details (CIF) of 2b, 2c, and 2d. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
(14) Cummings, S. P.; Cao, Z.; Liskey, C. W.; Geanes, A. R.;
Fanwick, P. E.; Hassell, K. M.; Ren, T. Organometallics 2010, 29,
2783−2788. Cummings, S. P.; Geanes, A. R.; Fanwick, P. E.;
Kharlamova, A.; Ren, T. J. Organomet. Chem. 2011, 696, 3955−
*E-mail: tren@purdue.edu.
Notes
The authors declare no competing financial interest.
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