New Iron(III) bis(acetylide) compounds based on the iron cyclam motif

Article abstract of DOI:10.1021/ic200836v

New trans-[Fe(cyclam)(C≡CR)₂]OTf compounds 2a/2b [cyclam = 1,4,8,11-tetraazacyclotetradecane, R = SiⁱPr₃ (a) or Ph (b), and OTf = trifluoromethanesulfonate] were prepared from the reaction between trans-[Fe(cyclam)(OTf)

Full text of DOI:10.1021/ic200836v

COMMUNICATION
pubs.acs.org/IC
New Iron(III) Bis(acetylide) Compounds Based on the Iron Cyclam
Motif
Zhi Cao, William P. Forrest, Yang Gao, Phillip E. Fanwick, Yang Zhang, and Tong Ren*
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
S Supporting Information
b
Chart 1. Common Iron Acetylide Structures
ABSTRACT: New trans-[Fe(cyclam)(CtCR)2]OTf com-
pounds 2a/2b [cyclam = 1,4,8,11-tetraazacyclotetradecane,
R = SiiPr3 (a) or Ph (b), and OTf = trifluoromethanesulfonate]
were prepared from the reaction between trans-[Fe(cyclam)-
(OTf)2]OTf (1) and LiCtCR. The trans arrangement of the
acetylide ligands in 2 was established from the X-ray diffrac-
tion study of 2a, and thedensityfunctionaltheorycalculations
revealed significant dπꢀπ(CtC) interactions.
Scheme 1. Synthesis of Iron(III) Cyclam Bis(acetylide)
Complexes
ransition-metal acetylide compounds have received intense
interest as potential molecular wires and other electronic
T
and optoelectronic materials since the pioneering work on the
linear [M]-acetylidepolymersbyNastandHagihara.1 Facile charge
transfer along the [M](CtC)mꢀ 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 monoacetylide compounds as molecular wires have been
reported by the laboratories of Lapinte and Akita,5 where the
piano-stool-type FeII centers, either CpFe(PꢀP)ꢀ or CpFe-
(CO)2ꢀ, are prevalent (type I in Chart 1). In comparison, the
iron bis(acetylide) compounds, developed mostly by the labora-
tory of Field, are rare and limited to FeII centers with acetylides in
a trans geometry and bidentate chelating phosphines as the
auxiliary ligands (type II in Chart 1).6
Conditions: (i) CF3SO3H (excess), 40 h, room temperature; (ii) 4 equiv
of LiC2R, THF, 1 h.
(theoretical value for S = 5/2: 5.92 μB), indicating that the FeIII
center is high spin. On the other hand, the room temperature
effective moments of compounds 2a and 2b are 1.94 and 1.89 μB,
1
respectively, which are consistent with a S = /2 low-spin FeIII
center. Additionally, these compounds have been further char-
acterized with UVꢀvis, HR-nESI-MS, and electrochemical tech-
niques. The synthetic details and data for compounds 1 and 2 are
provided in the Supporting Information.
During the past decade, efforts from our and other laboratories
have led to an extensive array of diruthenium acetylide com-
pounds and a demonstration of the electronic delocalization
therein.7 While the development of diruthenium compounds
containing exotic acetylide ligands such as geminal diethynyl-
ethenes remains a priority for us,8 we are vigorously seeking
novel structural motifs that would also favor the trans arrange-
ment of two acetylides. Of particular interest to us is the report of
a series of chromium(III) cyclam bis(arylacetylides) by Wagen-
knecht et al., where the acetylide ligands adopt a trans-coordina-
tion geometry.9 Reported in this contribution are the prepara-
tion of iron(III) cyclam bis(acetylide) complexes (2a and 2b in
Scheme 1) and the elucidation of their electronic structures.
As shown in Scheme 1, the synthesis of bis(acetylide) compounds
was preceded by the preparation of trans-[Fe(cyclam)(OTf)2]OTf
(1) from the reaction between trifluoromethanesulfonic acid
(CF3SO3H) and known compound cis-[Fe(cyclam)Cl2]Cl.10
The reaction between compound 1 and 4 equiv of LiCtCR
resulted in trans-[Fe(cyclam)(CtCR)2]OTf as a dark-orange
powder for R as SiiPr3 (2a, 72%) and a dark-purple powder for
R as Ph (2b, 64%), respectively. The trans-triflato compound 1
has a room temperature effective magnetic moment of 5.32 μB
The molecular structure of 2a was determined by single-crystal
X-ray diffraction. The asymmetric unit of crystal 2a contains the
halves of two independent cations. The structural plot and metric
parameters of one of them are presented here, while the complete
listing is given in the Supporting Information. It is clear from
Figure 1 that the coordination sphere around the FeIII center is
pseudooctahedral with the C1ꢀFe1ꢀC1A vector approximately
orthogonal to the plane defined by the four N centers. The FeꢀC
bond length in 2a+ [1.961(3) Å] is fairly close to the FeIIꢀC bond
lengths determined for trans-Fe(PꢀP)2(CtCR)2-type com-
pounds (1.92ꢀ1.97 Å).6
The redox activity of compounds 2a/2b has been examined
carefully using both cyclic voltammetric (CV) and differential
pulse voltammetric (DPV) techniques. The CVs recorded are
shown in Figure 2, while the DPVs are provided in Figure S2.
Received: April 24, 2011
Published: July 18, 2011
r
2011 American Chemical Society
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|

Inorganic Chemistry
COMMUNICATION
Scheme 2. Assignments of Reductions in Compound 2 Based
on the ECE Mechanism
Figure 1. ORTEP plot of 2a+ at the 30% probability level. The triflate
anion and H atoms were omitted for clarity. Selected bond lengths (Å)
and angles (deg): Fe1ꢀC1, 1.961(3); C1ꢀC2, 1.210(4); Fe1ꢀN1,
2.013(2); Fe1ꢀN2, 2.007(2); C1ꢀFe1ꢀN1, 91.63(11); C1ꢀFe1ꢀN2,
89.69(11); N1ꢀFe1ꢀN2, 94.57(9).
Figure 2. CVs recorded for compounds 2a and 2b in a 0.20 M THF
solution of Bu4NPF6. The inset in the CV of 2a is the cathodic scan up to
ꢀ0.6 V with the current scaled by a factor of 0.85 for clarity.
Figure 3. Absorption spectra of compounds 1 (dashed), 2a (solid), and
2b (dotted) recorded in acetonitrile.
There is no detectable oxidation couple up to +1.0 V for both
compounds 2a and 2b, which reflect the electron deficiency of
the FeIII species. Both compounds undergo a reversible 1eꢀ
reduction (A), which is attributed to the reduction of FeIII to FeII.
The formal potential of 2b is cathodically shifted from that of 2a,
indicating that the FeIII center is substantially stabilized by an
extended delocalization involving both phenyl rings. Interest-
ingly, the reversible reduction A in 2a, shown as an inset in
Figure 2, becomes irreversible when the cathodic sweep is
extended beyond ꢀ1.0 V, and a second reduction B is observed.
Such behavior can be explained by an electrochemicalꢀchem-
icalꢀelectrochemical (ECE) mechanism shown in Scheme 2:
one of the CtCSiiPr3 ligands in 2a dissociates under more
cathodic bias to yield a new species 2a00, which undergoes the
second reduction reversibly. The oxidation of 2a00 on the return
sweep results in peak C. Previously, Field et al. examined the
redox activity of trans-Fe(DMPE)(CtCPh)2 [DMPE = bis-
(dimethylphosphine)ethane] and found two consecutive 1eꢀ oxi-
dation couples, FeIII/II and FeIV/III, at +0.01 and 0.99 V in THF
versus SCE.11 In comparison, the FeIII/II couple in 2b appears
at ꢀ0.525 V versus Ag/AgCl, which is about 0.57 V more
cathodic (after the correction for reference electrodes) than that
of Fe(DMPE)(CtCPh)2. This clearly demonstrates that the Fe
center can be selectively stabilized at formal oxidation state II or
III by employing soft (P) or hard (N) auxiliary ligands,
respectively.
suspected that the weaker bands were the vibronic sidebands of
the most intense band (470 nm for 2a and 555 nm for 2b).
However, a closer examination of the spectra plotted versus the
wavenumber (Figure S3) revealed a large variation in the
energy spacing between bands, which precludes the vibronic
coupling interpretation. Nevertheless, a plausible assignment
of the transitions can be achieved using the strong field limit of
the TanabeꢀSugano diagram for a d5 configuration:12 the spin-
allowed excitations from the 2T2g ground state to 2T1g, 2A2g, 2Eg,
2
and A1g excited states results in peaks at 470/550, 430/501,
410/443, and 370/407 nm in 2a/2b, respectively. Highly struc-
tured dꢀd transitions were also observed for a series of arylethy-
nyl trans-[Cr(cyclam)(CtCAr)2]+ (Ar = C6H5, C7H7, and
C7H4F3), albeit with extinction coefficients about an order of
magnitude smaller than those of compounds 2a/2b.9 As dis-
cussed in the density functional theory (DFT) section below,
strong Fe—CtC π interactions are prevalent in 2a/2b, which
enables high molar absorption coefficients through intensity
stealing.
Compounds 2a and 2b are quite different from the previously
studied iron bis(acetylide) compounds6 in both the oxidation
state (III vs II) and the auxiliary ligands (cyclam vs bidentate
phosphines). Yet, the unusually intense dꢀd transitions ob-
served for both 2a and 2b imply significant dπ(Fe)ꢀπ(CtC)
interactions that typify the CpFeL2(CtCR)-type compounds.2
These observations prompted us to examine the electronic
structure of type 2 compounds using the spin-unrestricted
DFT calculations at the B3LYP/BP86/LanL2DZ level (Gaussian03
program).13 The calculations were based on the model cations
The UVꢀvis absorption spectra of compounds 1 and 2a/2b
display rich features (Figure 3). Compound 1 has a very broad
low-intensity absorption between 400 and 500 nm due to dꢀd
transitions and an intense band at 334 nm due to ligand-to-
metal charge transfer (OTf to FeIII). More intriguingly, both
compounds 2a and 2b display highly structured bands in the
visible region: namely, those at 370, 410, 430, and 470 nm for
2a and at 407, 443, 501, and 555 nm for 2b. Initially, it was
+
+
+
2a0 /2b0 . The model cation 2a0 was optimized based on the
crystal structure of 2a+ without truncation, and 2b0 was opti-
+
mized from 2a0 , with SiiPr3 groups being replaced by Ph groups.
+
The most relevant valence molecular orbitals (MOs) are pro-
vided in Figure 4, while a detailed listing of the optimized
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Inorganic Chemistry
COMMUNICATION
’ ASSOCIATED CONTENT
S
Supporting Information. Synthesis and characterization
b
of compounds 1, 2a, and 2b, DFT calculations for model com-
pounds 2a0 and 2b0, and an X-ray crystallographic file in CIF format
for the structure determination of compound 2a. This material
is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: tren@purdue.edu.
’ ACKNOWLEDGMENT
We gratefully acknowledge financial support from the National
Science Foundation (Grant CHE 1057621) and Purdue University.
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Figure 4. MO diagram for model compounds 2a0 (left) and 2b0
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(Figures S4ꢀS6 and Tables S1ꢀS4).
The DFT results of 2a0 and 2b0+ are in agreement with the
+
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empty eg set as the lowest unoccupied MO (LUMO; dx2ꢀy2) and
LUMO+1 (dz2) and an occupied t2g set as highest occupied MO
(HOMO)ꢀ2, HOMOꢀ1, and singly occupied MO (SOMO).
The loss of orbital degeneracy is caused by both the low
symmetry of the cyclam ligand (C2 only) and the JahnꢀTeller
instability (in both Oh and D4h settings). The two HOMOs,
namely, HOMOꢀ1 and SOMO, consist of the antibonding
combinations of dπ and π(CtC), and the contribution from
the latter is very significant. Similar interactions were noted for
the piano-stool-type iron(III) monoacetylide compounds by
Paul et al.14 The computed SOMOꢀLUMO gaps are qualita-
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or 2.64 eV) and 2b (550 nm or 2.25 eV). The SOMO and
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+
HOMOꢀ1 of 2b0 are of higher energy in comparison to those of
+
2a0 because of stronger antibonding character through the
introduction of phenyl groups. On the other hand, the LUMO
levels of both 2a0 and 2b0+ are approximately the same because
+
of the absence of the contribution from the acetylide ligands. The
reduction of the SOMOꢀLUMO optical gap in compound 2b
should be attributed to destabilization of the occupied dπ
orbital(s). In addition, the pronounced π(CtC) contributions
to both SOMO and HOMOꢀ1 offer the possibility of intensity
“stealing” in dꢀd bands.
In conclusion, we offered the first examples of iron(III)
bis(acetylide) compounds 2a/2b based on a iron cyclam synthon
(1). Strong π interactions between the occupied dπ and
π(CtC) orbitals are evident from both the absorption spectra
and DFT calculations. Further syntheses of type 2 derivative
compounds with donor/acceptor-substituted alkynes, isolation
of their 1eꢀ reduction derivative (FeII), and the photophysical
properties of these unique species are being investigated in our
laboratory.
(14) Paul, F.; Toupet, L.; Thepot, J. Y.; Costuas, K.; Halet, J. F.;
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