Redox-switchable ring-closing metathesis: Catalyst design, synthesis, and study

Article abstract of DOI:10.1002/chem.201301247

High yielding syntheses of 1-(ferrocenylmethyl)-3-mesitylimidazolium iodide (1) and 1-(ferrocenylmethyl)-3-mesitylimidazol-2-ylidene (2) were developed. Complexation of 2 to [{Ir(cod)Cl}₂] (cod=cis,cis-1,5-cyclooctadiene) or [Ru(PCy₃)Cl₂(=CH-o-O-iPrC₆H₄)] (Cy=cyclohexyl) afforded 3 ([Ir(2)(cod)Cl]) and 5 ([Ru(2)Cl₂(=CH-o-O- iPrC₆H₄)]), respectively. Complex 4 ([Ir(2)(CO) ₂Cl]) was obtained by bubbling carbon monoxide through a solution of 3 in CH₂Cl₂. Spectroelectrochemical IR analysis of 4 revealed that the oxidation of the ferrocene moiety in 2 significantly reduced the electron-donating ability of the N-heterocyclic carbene ligand (ΔTEP=9 cm^-1; TEP=Tolman electronic parameter). The oxidation of 5 with [Fe(η⁵-C₅H₄COMe)Cp][BF₄] as well as the subsequent reduction of the corresponding product [5][BF₄] with decamethylferrocene (Fc*) each proceeded in greater than 95 % yield. Moessbauer, UV/Vis and EPR spectroscopy analysis confirmed that [5][BF ₄] contained a ferrocenium species, indicating that the iron center was selectively oxidized over the ruthenium center. Complexes 5 and [5][BF ₄] were found to catalyze the ring-closing metathesis (RCM) of diethyl diallylmalonate with observed pseudo-first-order rate constants (k _obs) of 3.1×10^-4 and 1.2×10^-5 s^-1, respectively. By adding suitable oxidants or reductants over the course of a RCM reaction, complex 5 was switched between different states of catalytic activity. A second-generation N-heterocyclic carbene that featured a 1',2',3',4',5′- pentamethylferrocenyl moiety (10) was also prepared and metal complexes containing this ligand were found to undergo iron-centered oxidations at lower potentials than analogous complexes supported by 2 (0.30-0.36 V vs. 0.56-0.62 V, respectively). Redox switching experiments using [Ru(10)Cl₂(=CH-o-O-iPrC₆H₄)] revealed that greater than 94 % of the initial catalytic activity was restored after an oxidation-reduction cycle.

Full text of DOI:10.1002/chem.201301247

FULL PAPER
A Redox-Switcheroo: Oxidizing a
series of olefin metathesis catalysts
outfitted with ferrocene-containing N-
heterocyclic carbene ligands was found
to reduce their activities in ring-closing
metathesis reactions; subsequent
reduction restored the catalytic activi-
ties, even over multiple cycles (see
figure).
Olefin Metathesis
K. Arumugam, C. D. Varnado, Jr.,
S. Sproules, V. M. Lynch,
C. W. Bielawski* ............. &&&& — &&&&
Redox-Switchable Ring-Closing Meta-
thesis: Catalyst Design, Synthesis, and
Study
Jolting electrons…
…out of catalysts bearing redox-
active ferrocene groups reduces
their activities in ring-closing meta-
thesis reactions; subsequent reduc-
tion restores catalytic performance.
For more details see the Full Paper
by C. W. Bielawski et al. on page
&&. The authors are indebted to
Dr. Angel Syrett for her kind assis-
tance with the graphics.
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DOI: 10.1002/chem.201301247
Redox-Switchable Ring-Closing Metathesis:
Catalyst Design, Synthesis, and Study
Kuppuswamy Arumugam,[a] C. Daniel Varnado, Jr.,[a] Stephen Sproules,[b]
Vincent M. Lynch,[a] and Christopher W. Bielawski*[a]
Abstract: High yielding syntheses of 1-
(ferrocenylmethyl)-3-mesitylimidazoli-
um iodide (1) and 1-(ferrocenylmeth-
[Fe(h5-C5H4COMe)Cp]
the subsequent reduction of the corre-
sponding product [5][BF4] with deca-
methylferrocene (Fc*) each proceeded
in greater than 95% yield. Mçssbauer,
UV/Vis and EPR spectroscopy analysis
[BF4] as well as
of 3.1ꢂ10ꢀ4 and 1.2ꢂ10ꢀ5 sꢀ1, respec-
tively. By adding suitable oxidants or
reductants over the course of a RCM
reaction, complex 5 was switched be-
tween different states of catalytic activ-
ity. A second-generation N-heterocyclic
carbene that featured a 1’,2’,3’,4’,5’-
pentamethylferrocenyl moiety (10) was
also prepared and metal complexes
containing this ligand were found to
undergo iron-centered oxidations at
lower potentials than analogous
complexes supported by 2 (0.30–0.36 V
vs. 0.56–0.62 V, respectively). Redox
switching experiments using [Ru(10)-
Cl2(=CH-o-O-iPrC6H4)] revealed that
greater than 94% of the initial catalytic
activity was restored after an oxida-
tion–reduction cycle.
AHCTUNGTRENNUNG
yl)-3-mesitylimidazol-2-ylidene
were developed. Complexation of 2 to
[{Ir(cod)Cl}2] (cod=cis,cis-1,5-cyclooc-
tadiene) or [Ru(PCy3)Cl2(=CH-o-O-
iPrC6H4)] (Cy=cyclohexyl) afforded 3
([Ir(2)(cod)Cl]) and 5 ([Ru(2)Cl2(=CH-
o-O-iPrC6H4)]), respectively. Complex
([Ir(2)(CO)2Cl]) was obtained by
(2)
ACHTUNGTRENNUNG
N
confirmed that [5]ACTHUNGTRENNU[G BF4] contained a
ferrocenium species, indicating that the
iron center was selectively oxidized
over the ruthenium center. Complexes
5 and [5]ACTHUGNTERN[NUG BF4] were found to catalyze
the ring-closing metathesis (RCM) of
diethyl diallylmalonate with observed
ACHTUNGTRENNUNG
4
bubbling carbon monoxide through a
solution of 3 in CH2Cl2. Spectroelectro-
chemical IR analysis of 4 revealed
that the oxidation of the ferrocene
moiety in 2 significantly reduced the
electron-donating ability of the N-het-
erocyclic carbene ligand (DTEP=
9 cmꢀ1; TEP=Tolman electronic pa-
pseudo-first-order rate constants (kobs
)
Keywords: ferrocene · homogene-
ous catalysis · olefin metathesis · re-
dox-switchable catalysis · rutheni-
um · spectroelectrochemistry
rameter). The oxidation of
5 with
Introduction
Redox-switchable catalysis (RSC) is a growing field of study
that utilizes redox active ligands to influence the catalytic
activities displayed by coordinated metals.[1] In a typical re-
action (Scheme 1), a redox-switchable catalyst facilitates a
given transformation (e.g., A!B) at a given rate, k1, when
the ligand on the catalyst is in a neutral state. However,
upon oxidation or reduction, the activity or selectivity of the
catalyst may change (i.e., k1 ¼k2), or the catalyst may facili-
tate a new transformation altogether (e.g., C!D). Such
redox-switchable events frequently utilize a metallocene
moiety to influence the ligandꢁs electron-donating ability
Scheme 1. General scheme for RSC. RAG=redox active group, [O]=ox-
idation, [R]=reduction, A and C=reactants, B and D=products and k1,
k2 =rate constants.
and, as a result, the catalytic activity displayed by the metal
complex.[2] For example, in a seminal report, Wrighton ele-
gantly demonstrated that a diphenylphosphinocobaltocene-
ligated rhodium complex (A; see Figure 1) catalyzed the hy-
drogenation of cyclohexene faster than its oxidized cobalto-
cenium analogue; conversely, the oxidized complex was
found to catalyze hydrosilylations faster than its neutral pre-
cursor.[3] Gibson and Long later reported a ferrocenyl-sub-
stituted titanium-based ring-opening polymerization catalyst
(B) that showed a significantly decreased activity toward
rac-lactide upon oxidation.[4] More recently, Diaconescu
demonstrated that redox-switchable ferrocene-containing
alkoxide (M=Y or Ce, R=tBu) or aryloxide phosfen (M=
In, R=Ph) complexes (C) may be used to control the ring-
[a] Dr. K. Arumugam, C. D. Varnado, Jr., Dr. V. M. Lynch,
Prof. Dr. C. W. Bielawski
Department of Chemistry and Biochemistry
The University of Texas at Austin, Austin, TX 78712 (USA)
Fax : (+1)512-471-8696
E-mail: bielawski@cm.utexas.edu
[b] Dr. S. Sproules
WestCHEM, School of Chemistry
University of Glasgow, Glasgow G12 8QQ (UK)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201301247.
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Redox-Switchable Ring-Closing Metathesis
FULL PAPER
second-generation derivative, which showed improved
redox-switchable functions.
Following a modified literature procedure,[15] N-mesityl
imidazole was alkylated with (ferrocenylmethyl)trimethyl-
ammonium iodide to afford 1 in 83% yield (Scheme 2). The
Figure 1. Representative examples of redox-switchable catalysts.
opening polymerization of l-lactide and trimethylene carbo-
nate, respectively.[5]
Each of the aforementioned redox active ligands are mul-
tidentate, which contributes to the high stability of the cor-
responding complexes, but also imposes restrictions on the
types of catalysts that may be synthesized with redox-switch-
able functionalities.[6] A general solution to this drawback
may lie in the area of N-heterocyclic carbenes (NHCs),
which are strongly coordinating, monodentate ligands[7] that
have received widespread attention in broad range of appli-
cations,[8] particularly in organometallic catalysis.[9] Excellent
examples of transformations that have benefitted from
NHC-supported complexes include olefin metatheses,[10] hy-
drogenations,[11] and a sundry of coupling reactions.[6,12] An
important subclass of NHCs are those that feature N-ferro-
cenyl groups, which are relatively straightforward to pre-
pare, often display reversible electrochemistry, and coordi-
nate to a wide range of transition metals.[13] Moreover, we
and others have shown that the electron density at the metal
centers ligated to N-ferrocenyl-substituted NHCs correlate
with the oxidation state of the ferrocenyl unit.[13s,t,v,w] Since
catalytic activity is often intimately tied to the electronic
characteristics of ligated metals, RSC may provide new ave-
nues for modulating the instrinsic chemo- and regioselectivi-
ties displayed by catalysts supported by redox-active NHCs.
To explore the potential of ferrocene-containing NHCs in
RSC, we describe herein a series of redox-switchable olefin
metathesis catalysts and demonstrate their utility in control-
ling ring-closing metathesis (RCM) reactions.[14]
Scheme 2. Synthesis of 2 and various metal complexes (Fc=ferrocenyl,
Mes=mesityl, cod=cis,cis-1,5-cyclooctadiene): a) CH3CN, 808C, 24 h,
83% yield. b) NaHMDS (1 equiv), toluene, 258C, 1 h, 96% yield. c) i)
NaHMDS (1 equiv), toluene, 258C, 1 h; ii) [{Ir
258C, 12 h, 85% yield. d) CO (1 atm), CH2Cl2, 258C, 30 min, 92% yield.
e) i) NaHMDS (1 equiv), toluene, 258C, 1 h; ii) [Ru(PCy3)Cl2(=CH-o-O-
iPrC6H4)] (0.9 equiv), 258C, 12 h, 62% yield f) [Fe(h5-C5H4COMe)Cp]-
AHCTNUGTREN[GUNN BF4] (1 equiv), CH2Cl2, 258C, 2 h, 90% yield. g) Decamethylferrocene
ACHTUNGTRNE(NUGN cod)Cl}2] (0.5 equiv),
AHCTUNGTRENNUNG
(1 equiv), CH2Cl2, 258C, 1 h, 93% yield.
salient spectroscopic characteristics displayed by the isolated
salt were consistent with the values reported for analogous
compounds (e.g., the 1H NMR chemical shift of the
ꢀ
Cimidazolium
H group was recorded at d = 9.48 ppm;
[D6]DMSO).[13g] Treatment of 1 with sodium hexamethyldisi-
lazide (NaHMDS) in toluene afforded N-ferrocenylmethyl-
N’-mesitylimidazol-2-ylidene (2), as evidenced by the ab-
sence of the imidazolium 1H NMR signal in conjunction
with the appearance of a diagnostic 13C NMR signal at d=
217.1 ppm (C6D6).[16] X-ray diffraction quality single crystals
of 2 were grown from a saturated solution in toluene at
ꢀ358C.[17] The solid-state structure of the free carbene
ꢀ
(Figure 2) exhibited a longer average C1 N bond length
and a contracted N1-C1-N2 bond angle than its imidazolium
precursor (Figure S2 in the Supporting Information). More-
over, the pendant N-ferrocenylmethyl group was oriented
ꢀ
Results and Discussion
toward the carbenoid nucleus (C1 Fe1 =4.298 ꢃ) in 2 yet
ꢀ
away in 1 (C1 Fe1 =5.246 ꢃ), presumably due to differen-
Our efforts began with the synthesis of a ferrocene-contain-
ing NHC in conjunction with a detailed study of the elec-
tron-donating ability of the ligand as a function of the oxida-
tion state of the redox-active group. To facilitate key elec-
trochemical and spectroscopic measurements, a series of Ir-
based complexes that were supported by the aforemen-
tioned NHC ligand were prepared and evaluated. Ultimate-
ly, the results from these studies helped to rationalize the ac-
tivity displayed by a first-generation redox-switchable olefin
tial packing effects. As free NHCs are known to ligate to
metallocenes,[18] the isolation and solid-state elucidation of
NHCs bearing N-ferrocenyl and N-ferrocenylmethyl groups
is rare.[13w] Indeed, to the best of our knowledge, 2 is the
first crystalline NHC bearing a pendant N-ferrocenyl group.
The introduction of free NHC 2 (generated in situ) to
0.5 equivalents of [{IrACHTNUGTRNEUNG(cod)Cl}2] in toluene led to the forma-
tion of 3 (Scheme 2), which was subsequently isolated in
85% yield by filtration. Bubbling CO through a solution of
3 in CH2Cl2 for 30 min, followed by purification of the resi-
metathesis catalyst and guided the development of
a
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C. W. Bielawski et al.
Figure 4. ORTEP diagram of 4 rendered using POV-Ray. Thermal ellip-
soid plots were drawn at the 50% probability level. Hydrogen atoms are
ꢀ
omitted for clarity. Selected bond lengths [ꢃ] and angles [8]: C1 N1
Figure 2. ORTEP diagram of 2 rendered using POV-Ray. Thermal ellip-
soid plots were drawn at the 50% probability level. Hydrogen atoms and
solvent molecules are omitted for clarity. Data are shown for one of two
independent molecules in the asymmetric unit. Selected bond lengths [ꢃ]
ꢀ
ꢀ
ꢀ
ꢀ
1.350(5), C1 N2 1.363(5), C2 N1 1.372(5), C3 N2 1.382(5), C2 C3
ꢀ
ꢀ
ꢀ
ꢀ
1.336(6), C4 N2 1.444(5), C13 N1 1.483(5), C1 Ir1 2.070(4), Cl1 Ir1
ꢀ
ꢀ
2.3456(11), C24 Ir1 1.821(5), C25 Ir1 1.887(5), N1-C1-N2 104.5(3), N1-
C13-C14 113.0(3).
ꢀ
ꢀ
ꢀ
ꢀ
and angles [8]: C1 N1 1.368(4), C1 N2 1.369(4), C2 N1 1.391(4), C3 N2
ꢀ
ꢀ
ꢀ
1.393(4), C2 C3 1.342(4), C4 N2 1.443(4), C13 N1 1.460(4), N1-C1-N2
101.9(2), N1-C13-C14 115.6(3).
([5]ACTHNURTGNENU[G BF4]) was also prepared for comparative purposes.
Treatment of 1 with NaHMDS in toluene followed by filtra-
due obtained upon evaporation using a series of n-pentane
washes, afforded 4 in 93% yield. X-ray quality crystals of 3
and 4 were independently grown by slowly diffusing n-pen-
tane into concentrated CH2Cl2 solutions of the complexes.
As shown in Figures 3 and 4, the aforementioned Ir com-
tion and subsequent addition of 0.9 equivalents [Ru-
AHCTNUGERTNG(NNU PCy3)Cl2(=CH-o-O-iPrC6H4)] to the filtrate lead to the for-
mation of 5 (Scheme 2). The complex was isolated in 62%
yield through precipitation from cold n-pentane (ꢀ358C)
followed by filtration. Reflective of a more electron-rich en-
vironment at the metal center due to the incorporation of
the strongly donating NHC ligand,[22a] the diagnostic alkyli-
dene signal (Ru=CHa) displayed by [RuACHTUNGRTNEUNG(PCy3)Cl2(=CH-o-
O-iPrC6H4) at d=17.45 ppm shifted to d=16.35 ppm in 5
(CD2Cl2). X-ray diffraction quality single crystals of 5 were
grown by slow vapor diffusion of n-pentane into a saturated
solution of the complex in CH2Cl2. As shown in Figure 5,
the structure of the complex adopted a distorted square-pyr-
Figure 3. ORTEP diagram of 3 rendered using POV-Ray. Thermal ellip-
soid plots were drawn at the 30% probability level. Hydrogen atoms are
ꢀ
omitted for clarity. Selected bond lengths [ꢃ] and angles [8]: C1 N1
ꢀ
ꢀ
ꢀ
ꢀ
1.363(11), C1 N2 1.356(12), C2 N1 1.382(13), C3 N2 1.396(12), C2 C3
ꢀ
ꢀ
ꢀ
ꢀ
1.354(15), C4 N2 1.461(13), C13 N1 1.441(12), C1 Ir1 2.041(9), Cl1 Ir1
2.358(3), N1-C1-N2 103.9(8), N1-C13-C14 112.2(8).
plexes adopted square-planar coordination geometries as ex-
pected for d8 metals in strong ligand field environments. The
ꢀ
N1-C1-N2 bond angles and the Ir1 C1 bond lengths meas-
ured in the solid-state structures of 3 and 4 are in good
agreement with data reported for analogous com-
pounds.[13v,19]
Next, efforts were directed toward the synthesis of Ru-
based complexes bearing 2.[10g–i,20] Due to their high stability
and high catalytic activities in RCM reactions,[21] efforts
were directed toward the synthesis of a Hoveyda–Grubbs
type[22] (HG-type) analogue (i.e., 5); an oxidized derivative
Figure 5. ORTEP diagram of 5 rendered using POV-Ray. Thermal ellip-
soid plots were drawn at the 50% probability level. Hydrogen atoms are
ꢀ
omitted for clarity. Selected bond lengths [ꢃ] and angles [8]: C1 N1
ꢀ
ꢀ
ꢀ
ꢀ
1.353(7), C1 N2 1.369(7), C2 N1 1.382(7), C3 N2 1.384(7), C2 C3
ꢀ
ꢀ
ꢀ
ꢀ
1.333(8), C4 N2 1.456(7), C13 N1 1.482(7), Ru1 C1 1.980(6), Ru1 C24
ꢀ
ꢀ
ꢀ
1.822(6), Ru1 O1 2.261(4), Ru1 Cl1 2.3435(17), Ru1 Cl2 2.3571(17),
N1-C1-N2 103.1(5), N1-C13-C14 113.3(5), Cl1-Ru1-Cl2 151.43(6), N2-C1-
Ru1 135.8(5).
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Redox-Switchable Ring-Closing Metathesis
FULL PAPER
amidal geometry with the alkylidene unit oriented perpen-
dicular to the C1-Ru1-Cl1-Cl2-O1 mean plane. The Cl1-Ru-
Cl2 (151.42(6)8) and O1-Ru1-C1 (179.3(2)8) bond angles are
in accord with data reported for other HG-type com-
plexes.[22] Using a series of nuclear overhauser enhancement
spectroscopy (NOESY) experiments, the solution-state
structure (CD2Cl2) of the complex was determined to be
similar to the structure observed in the solid state (see the
Supporting Information). Treatment of 5 with acetylferroce-
the change in electron density at the Ir center in 4 upon oxi-
dation of the ferrocene-containing ligand (i.e., 2).[24] Com-
pound 4 exhibited nCO bands at 2064 and 1980 cmꢀ1, which
are intermediate of other NHC-supported [Ir(CO)2Cl] com-
plexes (trans: 2055–2072 cmꢀ1, cis: 1971–1989 cmꢀ1) reported
in the literature.[13v,19f,g,25] The corresponding Tolman elec-
tronic parameter[26] (TEP) of 2 was calculated from the
aforementioned nCO frequencies using an equation devel-
oped by Crabtree[27] and later modified by Nolan[19g] (TEP=
0.847ꢀnav +336 cmꢀ1) to be similar (2049 cmꢀ1) to the TEPs
reported for 1,3-diadamantylimidazolylidene (2049.5 cmꢀ1)
nium tetrafluoroborate ([Fe(h5-C5H4COMe)Cp]
ACTHNUTRGENNU[G BF4]) in
CH2Cl2 followed by the addition of Et2O afforded a dark
green precipitate (90% yield), which was subsequently iden-
and
1,3-di(tert-butyl)-imidazolylidene
(2050.1 cmꢀ1).[19g]
tified as [5]
netic resonance (EPR) and Mçssbauer spectroscopy (see
below). The complex [5][BF4] was successfully reduced to 5
G
Upon the bulk oxidation of 4 at 0.75 V (vs. SCE), an in-
crease in the nCO frequencies was observed (Figure 6), which
resulted in a significant increase in the calculated TEP
in CH2Cl2 using decamethylferrocene (Fc*) and the product
was isolated in 95% yield, thus demonstrating that the two
complexes were chemically interconvertable.
To measure the electronic properties of the aforemen-
tioned compounds, a series of electrochemical measure-
ments were conducted in CH2Cl2 with [NACHTNUGTRENN(UG nBu4)]CAHTUNGTRENN[GUN PF6] as the
electrolyte; key data are summarized in Table 1 (see Fig-
Table 1. Summary of the electrochemical data.[a]
[b]
Compound
E1/2 [V]
DE1/2 [mV]
2
3
4
5
6
0.67 (qr)[c]
0.56 (r), 0.96 (ir)
0.59 (r)
0.62 (r), 1.01 (r)
0.66 (r)
–
400
–
370
–
[a] Data obtained by CV and DPV in CH2Cl2 with 0.10m [NACHTNURTGENN(UG nBu4)]ACHTUTGNREN[NGUN PF6]
electrolyte and referenced vs. SCE. [b] r=reversible, ir=irreversible,
qr=quasi-reversible. [c] In THF (decomposition of 2 was observed in
CH2Cl2).
Figure 6. Normalized IR difference spectra showing the shift in the nCO
frequencies upon oxidation (Eapp = +0.75 V) of 4 ([4]0 =1 mm) in CH2Cl2
with 0.1m [NACHTNUGTRENNGU(nBu4)]ACHTUNGTREN[NGUN PF6] as the supporting electrolyte. The arrows indi-
ures 7 and S5–S9 in the Supporting Information for the cor-
responding cyclic voltammograms (CVs) and differential
pulse voltammograms (DPVs)). Since the iodide counterion
in 1 was found to undergo oxidation along with the ferro-
cenyl substituent, the analogous hexafluorophosphate salt 6
cate the direction of the spectral changes over time.
(2058 cmꢀ1). As a point of reference, the TEP measured for
oxidized 2 (i.e., 2+) was greater than the analogous value
calculated for tricyclohexylphosphine (2056.4 cmꢀ1).[19g] Re-
gardless, the result suggested to us that 2 may be switched
between two different states of ligand-donating ability by
changing the oxidation state of the ferrocenyl unit.
Building on the aforementioned results, the electrochem-
istry of the HG-type complex 5 was probed and found to ex-
hibit two reversible oxidation processes at 0.62 V and 1.01 V
(vs. SCE, Figure 7). The first oxidation was assigned to the
Fe2+/Fe3+ couple, while the second oxidation was attributed
to a Ru2+/Ru3+ oxidation process (vide infra). To gain addi-
tional insight into the electronic structure of 5+, controlled-
potential coulometry at 0.75 V (vs. SCE) with concomitant
UV/Vis absorption spectroscopy was performed. As shown
in Figure 8, new absorption maxima near 239 nm and about
600 nm were observed for the first oxidation process. The
band at around 600 nm has been previously ascribed to the
formation of a ferrocenium (Fc+) species.[28] After the for-
([2H]
ACHTUNGTRENNUNG
[CH3CH2)3O]
ACHTUNGTRENNUNG
single, reversible Fe2+!Fe3+ oxidation process at 0.66 V (vs.
saturated calomel electrode, SCE), which was comparable
to the values reported in the literature for other N-ferroce-
nylimidazolium salts.[13g] In contrast, complex 3 exhibited
two oxidation processes at 0.56 V and 0.96 V (vs. SCE),
which were attributed to the Fe2+/Fe3+ and Ir1+/Ir2+ redox
couples, respectively. While one reversible Fe2+/Fe3+ couple
was measured at 0.59 V (vs. SCE) for 4, the corresponding
Ir1+/Ir2+ couple was not observed within the solvent
window, likely due to the electron-withdrawing carbonyl
groups present in the complex.
Considering that the electron-donating abilities of NHCs
may be conveniently monitored by measuring the carbonyl
stretching frequencies (nCO) of NHC-supported [Ir(CO)2Cl]
complexes, subsequent efforts were directed toward probing
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C. W. Bielawski et al.
Figure 7. Cyclic voltammogram of 5 in CH2Cl2 with 1 mm of analyte and
0.1m [N(nBu4)]
[PF6], scan rate 100 mVsꢀ1. Inset: the differential pulse
A
ACHTUNGTRENNUNG
voltammogram of 5 under identical conditions (50 mV pulse amplitude).
Figure 9. X-band EPR spectrum of 5+ (black line) in a CH2Cl2/toluene
solution at 5 K and corresponding simulation (red line). Experimental
conditions: frequency, 9.4361 GHz; power, 0.63 mW; modulation, 3.0 mT.
Simulation parameters: g=4.24, 1.29, 1.29; W=110, 500, 500ꢂ10ꢀ4 cmꢀ1
;
g-strain, sx =0.42. The asterisk denotes a Ru3+ impurity that accounted
for <1% of the total signal intensity.
heterogeneity in the frozen glass. The line broadening was
believed to arise from the bulky pendent arm of the Fe-con-
taining group, which challenged an ability to observe the g?
in the EPR spectrum of 5+. However, the formation of a
ferrocenium species was supported by the large g anisotropy
(Dg=gj jꢀg? =2.95), which stems from the unquenched or-
2
bital angular momentum within the degenerate E2g ground
state. Since this g anisotropy cannot be generated by a Ru3+
d5 ion (S=1/2) in this ligand field nor to a quartet (S=3/2)
ground state, the signals were assigned as described above.
To further support the notion that the ferrocenyl unit in 5
underwent oxidation upon the addition of [Fe(h5-
Figure 8. Electronic absorption spectra recorded during the bulk oxida-
C5H4COMe)Cp]ACTHNUGTRNEUNG[BF4], 5 and [5]BF4 were independently an-
tion of 5!51+ (top) (Eapp = +0.75 V) and 51+!52+ (bottom) (Eapp = +
alyzed using Mçssbauer spectroscopy. As shown in
Figure 10,[31] complex 5 is characterized by an isomer shift at
d=0.50 mmsꢀ1 and a large quadrupole splitting at DEQ =
2.31 mmsꢀ1, which is consistent with the low-spin Fe2+ d6
center of ferrocene. The oxidized product 5+ exhibits the
same isomer shift, but the quadrupole splitting collapses to
DEQ =0.13 mmsꢀ1, a value characteristic of a ferrocenium
species.[31] Collectively, the electronic absorption, EPR, and
Mçssbauer spectra revealed that the first oxidation process
is ferrocene-based.
1.2 V) in CH2Cl2 with 0.1m [NACTHNUTGRENNUG(nBu)4]ACHTUNTGNER[NUNG PF6] as the supporting electrolyte
at ꢀ258C. The arrows indicate the direction of the spectral changes over
time.
mation of 5+ from 5 was complete, controlled-potential
coulometry was performed at 1.2 V (vs. SCE) to generate
52+. A new band at 426 nm (e=1.03ꢂ104 mꢀ1 cmꢀ1), consis-
tent with the oxidation of Ru2+!Ru3+, was observed.[29]
To support the electrochemical and spectroelectrochemi-
cal UV/Vis assignments, a series of electron paramagnetic
resonance (EPR) and Mçssbauer spectroscopy experiments
were performed. After oxidizing 5 with acetylferrocenium
Having evaluated the ligand-donating ability of 2 as a
function of its oxidation state, efforts were directed toward
exploring the utility of the latter in RSC. Using a modified
literature procedure,[32] the ring-closing metathesis (RCM)
of diethyl diallylmalonate (7) ([7]0 =0.1m) to its respective
tetrafluoroborate ([Fe(h5-C5H4COMe)Cp]
spectrum of the corresponding product [5]ACHTUNGTRENNNUG
ACHTUNGTRENNUNG
ed in a dichloromethane/toluene glass at 5 K. As shown in
Figure 9, the EPR spectrum revealed a sharp feature at gj j
=4.24, a result consistent with the presence of a ferroceni-
um species.[28b,30] Moreover, the broadening of the g? fea-
ture was successfully modeled by including a large g-strain
value (s=0.42) in the simulation to account for the micro-
1
cyclic product (8) was monitored by H NMR spectroscopy
at 308C in CD2Cl2 using 1 mol% 5 or [5]ACTHUNGRTNEUNG[BF4] as the cata-
lyst; see Equation (1). As shown in Figure 11, an 80% con-
version of starting material to product was observed within
1 h when 5 was employed as the catalyst and the corre-
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Figure 10. Zero-field Mçssbauer spectra of 5 (blue, top) and [5]ACTHNUTRGNE[UNG BF4]
(red, bottom) recorded using solid samples at 80 K. The open circles re-
flect the experimental data; the solid lines are the corresponding spectral
fits.
Figure 12. Plots of the percent conversion of 7 to 8 versus time as cata-
lyzed by 1 mol% of 5; conditions: [7]0 =0.1m, CD2Cl2, 308C. The arrows
indicate the time at which one equivalent of said reagent with respect to
5 was added. The corresponding rate constants over the given periods of
time are indicated.
Figure 11. Plots of the percent conversion of 7 to 8 versus time as cata-
lyzed by 1 mol% of 5 (triangles) or [5]
[7]0 =0.1m, CD2Cl2, 308C.
ACHTUNGRTEN[NUNG BF4] (diamonds). Conditions:
scopy (see Figure 12). After 10 min (ca. 16% conversion of
7 to 8; kobs =3.0ꢂ10ꢀ4 sꢀ1), one equivalent (with respect to
sponding pseudo-first-order rate constant (kobs) was calculat-
5) of a chemical oxidant, [Fe(h5-C5H4COCH3)Cp]
ACTHNGUTRENUN[G BF4], was
ed to be 3.1ꢂ10ꢀ4 sꢀ1. In contrast, the analogous reaction in-
added to the reaction mixture. The addition resulted in the
solution changing color from yellow to brown and was ac-
companied by a significant decrease in catalytic activity
(kobs =0.98ꢂ10ꢀ5 sꢀ1), consistent with the in situ conversion
of 5!5+. Subsequent addition of 1.1 equivalents (with re-
spect to 5) of a chemical reductant, decamethylferrocene
(Fc*), to the mixture restored the initial yellow color and
the relatively high catalytic activity (kobs =3.8ꢂ10ꢀ5 sꢀ1). As
shown in Figure 12 (bottom), the aforementioned switching
cycle was successfully repeated multiple times over the
course of a single reaction.[33,34]
volving [5]ACHTUNGTRENNUNG[BF4] was measured to be significantly slower
(kobs =1.2ꢂ10ꢀ5 sꢀ1) and the catalyst converted only 7% of 7
to 8 after 1 h. We hypothesized that the oxidation of the fer-
rocene moiety (Fe2+!Fe3+) in complex 5 reduced the elec-
tron density at the ligated metal center and attenuated the
catalytic activity accordingly.[10i]
Building on the observation that 5 and [5]ACHTNUTGRNEUNG[BF4] catalyzed
the RCM of 7 with different rate constants, subsequent ef-
Close inspection of the redox-switching data summarized
in Figure 12 revealed that only about 13% of the initial cat-
alytic activity, as determined by the respective rate con-
stants, was successfully restored.[35] Considering that the oxi-
dation potentials of the iron and ruthenium centers in 5 dif-
fered by only 380 mV, we attributed the attenuated activity
to partial ruthenium oxidation, which led to premature cata-
lyst decomposition. To develop a more robust catalyst, ef-
forts were directed towards incorporating a 1’,2’,3’,4’,5’-pen-
forts were directed toward modulating the activity of the
catalyst over the course of a reaction. Following the addition
of 5 (1 mol%) to a CD2Cl2 solution of 7 ([7]0 =0.1m), the re-
1
sulting mixture was monitored at 308C by H NMR spectro-
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C. W. Bielawski et al.
tamethylferrocene (Me5Fc) moiety into an NHC scaffold,
which was expected[36] to undergo oxidation at about
300 mV lower potential than the parent ferrocene analogue.
As summarized in Scheme 3, treatment of 1’,2’,3’,4’,5’-pen-
tamethylferrocenylmethanol[37] with HBF4 in the presence of
mesitylimidazole led to formation of the imidazolium salt 9
as a crystalline product.[13u,36] In order to evaluate and com-
pare electron-donating ability of the corresponding NHC
(i.e., 10) to 2, the iridium complex 11 was prepared in a sim-
ilar manner to that described for the synthesis of 3 and sub-
jected to electrochemical analysis.
buried volumes, a measure of steric bulk, displaced by li-
gands 2 and 10 were calculated to be similar (29.4% vs.
31.5%, respectively) using Cavalloꢁs method.[38]
With complex 12 in hand, the electron-donating ability of
10 was measured and compared to that of 2. The nCO fre-
quencies displayed by 12 were recorded at 2064 and
1979 cmꢀ1, and were nearly identical to those measured for
4. Likewise, the TEP value of 10 (2048 cmꢀ1) was calculated
to be similar to that of 2 (2049 cmꢀ1). The CV of 12 (Fig-
ure S11 in the Supporting Information) revealed that the
first oxidation process occurs at 0.34 V (vs. SCE; Table 2),
which is 250 mV cathodically shifted when compared to the
Table 2. Summary of the electrochemical data.[a]
[b]
Compound
E1/2 [V]
DE1/2 [mV]
11
12
13
0.30 (r), 0.97 (ir)
0.34 (r)
0.36 (r), 1.02 (r)
670
–
660
[a] Data obtained by CV and DPV in CH2Cl2 with 0.10m [NACHTNURTGENN(UG nBu4)]ACHTUNGTRENNUNG[PF6]
electrolyte and referenced vs. SCE. [b] r=reversible, ir=irreversible.
analogous process displayed by 4. Even though the ligand
oxidation occurred at lower potential, the resulting TEP dif-
ference between 12 and 12+ is nearly identical to that meas-
ured between 2 and 2+ (cf., 8 vs. 9 cmꢀ1, respectively), as de-
Scheme 3. Synthesis of 10 and various metal complexes (Me5Fc=
1’,2’,3’,4’,5’-pentamethylferrocene). a) i) HBF4, CH2Cl2, 1 min; ii) N-mesi-
tylimidazole, 12 h, 41% yield. b) NaHMDS (1 equiv), benzene, 1 h.
c) i) NaHMDS (1 equiv), benzene, 258C, 15 min; ii) [{IrACHTNUGTRENUNG(cod)Cl}2]
(0.5 equiv), 258C, 12 h, 79% yield. d) CO (1 atm), CH2Cl2, 258C, 2 h,
82% yield. e) i) NaHMDS (1 equiv), toluene, 258C, 30 min; ii) [Ru-
termined by
a spectroelectrochemical experiment (Fig-
ure S13 in the Supporting Information).
To test the performance of 10 in RSC, Ru complex 13 was
prepared in an analogous manner to 5. Unfortunately, treat-
ACHTUNGTRENNUNG(PCy3)Cl2(=CH-o-O-iPrC6H4)] (0.5 equiv), 258C, 3 h; iii) S8 (2 equiv),
12 h, 48% yield.
ing 10 with [RuACTHNUTRGNEUNG(PCy3)Cl2(=CH-o-O-iPrC6H4)] yielded a
mixture of products, one of which was a complex in which
the phosphine ligand was coordinated to the Ru center, yet
the isopropyloxy group was dissociated, as evidenced by di-
Cyclic voltammetry of 11 (Figure S10 in the Supporting
1
Information) in CH2Cl2 in the presence of [N
G
[PF6] as
agnostic H NMR (a doublet at d=20.95 ppm and a singlet
the electrolyte revealed that the iron-centered oxidation
process occurred at 0.30 V (vs. SCE), which corresponded to
a 260 mV cathodic shift as compared to the analogous oxi-
dation process exhibited by 3. However, the iridium oxida-
tion processes displayed by 3 and 11 were found to occur at
similar potentials (cf., 0.96 V vs. 0.97 V, respectively), which
suggested to us that even though the Me5Fc-containing
ligand (10) underwent oxidation at a lower overall potential
than its Fc parent (2), there was minimal difference in the
donating ability of the two ligands.
To further characterize the steric and electronic properties
of 10, the complex 12 (i.e., [Ir(10)(CO)2Cl]) was prepared
by stirring a solution of 11 under an atmosphere of CO (g)
and isolated by precipitation. X-ray diffraction analysis of
single crystals grown by the slow diffusion of n-pentane into
a saturated solution of 12 in chloroform were in accord with
the recorded spectroscopic data and confirmed the identity
of this complex (Figure S4 in the Supporting Information).
As expected, 12 exhibited a square-planar geometry with
bond lengths and bond angles comparable to those meas-
ured in the solid-state structure of 4. Likewise, the percent
at d=16.68 ppm) and 31P NMR (d=23.5 and 10.9 ppm) sig-
nals. To maximize the yield of 13, the aforementioned reac-
tion mixture was stirred with two equivalents of elemental
sulfur, an effective phosphine scavenger,[39] followed by
silica column chromatography under an inert atmosphere
which afforded the desired complex as a yellow solid in
48% isolated yield. X-ray quality crystals of 13 were ob-
tained as red needles by vapor diffusion of pentane into a
saturated solution of the complex in benzene. The solid-
state structure of 13 exhibits a distorted square-pyramidal
geometry with bond lengths and angles similar to those
measured for the analogous solid-state structure of
(Figure 13).
5
The CV of 13 (Figure S12 in the Supporting Information)
revealed an iron-centered oxidation at 0.36 V, corresponding
to a 260 mV cathodic shift relative to the analogous oxida-
tion process displayed by 5. Consequently, the redox
window measured between the Fe and Ru-centered oxida-
tion processes exhibited by 13 was broadened to 660 mV, as
opposed to the 380 mV window displayed by 5. The wider
electrochemical window facilitated the use of a weaker oxi-
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Redox-Switchable Ring-Closing Metathesis
FULL PAPER
Conclusions
In summary, we have developed straightforward syntheses
of NHCs that feature redox-active N-ferrocenyl substituents
and ligated these compounds to a series of iridium and cata-
lytically active ruthenium complexes. By measuring the nCO
frequencies of [Ir(2)(CO)2Cl], the ligand-donating ability of
the NHC 2 was found to decrease (DTEP=9 cmꢀ1) upon se-
lective oxidation of the ferrocene moiety. Electrochemical
analysis of 5 (i.e., [Ru(2)Cl2(=CH-o-O-iPrC6H4)]) revealed
two successive one-electron oxidations, corresponding to
Fe2+!Fe3+ followed by Ru2+!Ru3+, respectively, as con-
firmed using a series of electrochemical, spectroelectro-
chemical UV/Vis absorption Mçssbauer and EPR experi-
Figure 13. ORTEP diagram of 13 rendered using POV-Ray. Thermal el-
lipsoid plots were drawn at the 50% probability level. Hydrogen atoms
ments. Complexes 5 and [5]ACHTNUTRGNEUN[G BF4] were found to catalyze the
ring-closing metathesis (RCM) of diethyl diallylmalonate
(7) at 308C, but at significantly different rates. Moreover,
while the RCM reaction catalyzed by 5 was successfully at-
tenuated through the addition of a chemical oxidant, the
catalytic activity was only partial restored (ca. 13%) upon
subsequent chemical reduction, presumably due to prema-
ture catalyst decomposition. Regardless, these results corro-
borated the hypothesis that the oxidation of the ligand in 5
afforded a complex with diminished electron density at the
metal center and therefore reduced catalytic activity. To in-
crease the robustness of the catalyst and to improve its
redox-switchable functions, an NHC bearing a 1’,2’,3’,4’,5’-
pentamethylferrocene substituent was prepared (10). Com-
plexes supported by 10 were found to oxidize at potentials
that were at least 250 mV lower than the analogues that
contained the parent ferrocene moieties. Moreover, more
than 94% of the initial RCM activity displayed by a Ru-
based olefin metathesis catalyst supported by 10 was re-
stored after a full redox-switching cycle. To the best of our
knowledge, these are the first homogeneous, redox-switcha-
ble, NHC-supported Ru catalysts that have been used to
control RCM through changes in ligand electronics. More
broadly, the results presented herein are expected to guide
the development of other olefin metathesis and other NHC-
supported catalysts, the intrinsic chemo- and regioselectivi-
ties of which are determined by the oxidation state of the
redox-switchable ligand.
ꢀ
are omitted for clarity. Selected bond lengths [ꢃ] and angles [8]: C1 N1
ꢀ
ꢀ
ꢀ
ꢀ
1.364(4), C1 N2 1.368(4), C2 N1 1.382(4), C3 N2 1.393(4), C2 C3
ꢀ
ꢀ
ꢀ
ꢀ
1.340(5), C4 N2 1.443(4), C13 N1 1.479(4), Ru1 C1 1.988(4), Ru1 C29
ꢀ
ꢀ
ꢀ
1.822(4), Ru1 O1 2.277(2), Ru1 Cl1 2.3432(12), Ru1 Cl2 2.3287(11),
N1-C1-N2 103.8(3), N1-C13-C14 111.6(3), Cl1-Ru1-Cl2 149.82(4), N2-C1-
Ru1 134.7(3).
dant (i.e., [Fc]ACHTUNGTRENNUNG
[BF4]; E1/2 =0.475 V in CH2Cl2 vs. SCE)[40] to
oxidize 13.
A
ring-closing reaction was initiated by adding 13
(1 mol%) to a solution of 7 in CD2Cl2 ([7]0 =0.1m) and
monitored at 308C by H NMR spectroscopy. As shown in
1
Figure 14, the neutral complex catalyzed the reaction with a
rate constant (kobs) of 4.5ꢂ10ꢀ5 sꢀ1. However, upon oxida-
tion with [Fc]ACHTUNGTRENNUNG[BF4], the rate constant was reduced to 0.86ꢂ
10ꢀ5 sꢀ1. While the aforementioned rate constants are similar
to those measured for 5 and 5+ respectively, the subsequent
addition of decamethylferrocene to the in situ generated
13+ restored greater than 94% of the catalytic activity dis-
played by its neutral precursor (kobs =4.25ꢂ10ꢀ5 sꢀ1).
Acknowledgements
This material is based upon work supported in part by the U. S. Army
Research Laboratory under grant number W911NF-09-1-0446. Addition-
al support from the National Science Foundation (CHE-1266323 and
CHE-0741973) and the Robert A. Welch Foundation (F-0046) is also ac-
knowledged. We thank Steve Sorey for his assistance with the NMR ex-
periments. Support from the EPSRC National UK EPR Facility and
Service is gratefully acknowledged.
Figure 14. Plots of the percent conversion of 7 to 8 versus time as cata-
lyzed by 1 mol% of 13, conditions: [7]0 =0.1m, CD2Cl2, 308C. The arrows
indicate the time at which one equivalent of said reagent with respect to
13 was added. The corresponding rate constants over the given periods of
time are indicated.
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Redox-Switchable Ring-Closing Metathesis
FULL PAPER
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[29] Spectroelectrochemical
analysis
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[RuACHTUNGETRNNU(G PCy3)Cl2(=CH-o-O-
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identical to those described above. While IMes-HG2 catalyzed the
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subsequent addition of [Fe(h5-C5H4COCH3)Cp]
ACHTNUTRGENNG[U BF4] reduced the
measured rate constant (kobs =6.8ꢂ10ꢀ5 sꢀ1), catalytic activity spon-
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Received: April 2, 2013
Published online: && &&, 0000
Chem. Eur. J. 2013, 00, 0 – 0
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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