Five-Membered Heterocycles as Linking Units in Strongly Coupled Homobimetallic Group 8 Metal Half-Sandwich Complexes

Article abstract of DOI:10.1021/acs.organomet.5b00104

The synthesis and characterization of a series of alkynyl half-sandwich complexes of type 2,5-((η⁵-C₅H₅)(dppe)MC≡C)₂-^cC₄H₂E (E = O (11), S (12); M = Fe (a), Ru (b); dppe = 1,2-bis(diphenylphosphino)ethane) are reported. The molecular structures of 11a and 12a,b in the solid state have been determined by single-crystal X-ray diffraction. The influence of different metals and the variation of the heterocyclic bridge on the electronic interactions between the terminal redox-active units in 11a,b and 12a,b was studied using electrochemical (cyclic and square wave voltammetry) and spectroelectrochemical (in situ UV-vis/NIR, ESR, and IR spectroscopy) methods and DFT calculations. Electrochemical studies demonstrated that mixed-valent species 11a,b⁺ and 12a,b⁺ exhibit high thermodynamic stabilities with respect to disproportionation (K_C values from 6.87 × 10⁴ to 9.33 x 10⁵). In situ spectroelectrochemical ESR and IR measurements display delocalization of the single electron between the metal centers M/M⁺, revealing that within this setup five-membered heterocycles are well suited to promote intramolecular metal-metal interactions. Furthermore, the UV-vis/NIR spectra of mixed-valent 11a,b⁺ and 12a,b⁺ show intense, narrow, and nonsolvatochromic π(dπ) → π(dπ) absorptions in the NIR region with a high-energy shoulder. Both experimental and computational results suggest that at least two thermally accessible rotation conformers of the organometallic termini of 11a,b and 12a,b contribute to the electronic spectra of these compounds. One of the conformers can clearly be characterized as a delocalized class III system, while the other shows a more localized behavior. (Figure Presented).

Full text of DOI:10.1021/acs.organomet.5b00104

Article
pubs.acs.org/Organometallics
Five-Membered Heterocycles as Linking Units in Strongly Coupled
Homobimetallic Group 8 Metal Half-Sandwich Complexes
Ulrike Pfaff,† Alexander Hildebrandt,† Marcus Korb,† Dieter Schaarschmidt,† Marco Rosenkranz,‡
Alexey Popov,‡ and Heinrich Lang*,†
†Institute of Chemistry, Inorganic Chemistry, Faculty of Natural Sciences, Technische Universitat Chemnitz, D-09107 Chemnitz,
̈
Germany
‡Institute for Solid State Research, Department of Electrochemistry and Conducting Polymers, IFW Dresden, Helmholtzstraße 20,
D-01069 Dresden, Germany
S
* Supporting Information
ABSTRACT: The synthesis and characterization of a series of alkynyl half-
sandwich complexes of type 2,5-((η5-C5H5)(dppe)MCC)2-cC4H2E (E =
O (11), S (12); M = Fe (a), Ru (b); dppe = 1,2-bis(diphenylphosphino)-
ethane) are reported. The molecular structures of 11a and 12a,b in the
solid state have been determined by single-crystal X-ray diffraction. The
influence of different metals and the variation of the heterocyclic bridge on
the electronic interactions between the terminal redox-active units in 11a,b
and 12a,b was studied using electrochemical (cyclic and square wave
voltammetry) and spectroelectrochemical (in situ UV−vis/NIR, ESR, and
IR spectroscopy) methods and DFT calculations. Electrochemical studies
demonstrated that mixed-valent species 11a,b+ and 12a,b+ exhibit high
thermodynamic stabilities with respect to disproportionation (KC values
from 6.87 × 104 to 9.33 × 105). In situ spectroelectrochemical ESR and IR
measurements display delocalization of the single electron between the
metal centers M/M+, revealing that within this setup five-membered heterocycles are well suited to promote intramolecular
metal−metal interactions. Furthermore, the UV−vis/NIR spectra of mixed-valent 11a,b+ and 12a,b+ show intense, narrow, and
nonsolvatochromic π(dπ) → π*(dπ) absorptions in the NIR region with a high-energy shoulder. Both experimental and
computational results suggest that at least two thermally accessible rotation conformers of the organometallic termini of 11a,b
and 12a,b contribute to the electronic spectra of these compounds. One of the conformers can clearly be characterized as a
delocalized class III system, while the other shows a more localized behavior.
sition metals in homo- and heterobimetallic compounds were
investigated. It could be shown that, as expected, larger alkyne
bridges lead to a decrease of the electronic interaction between
the metal centers. Organometallic half-sandwich building
blocks, e.g., ML(PP) (L = Cp (= η5-C5H5) or Cp* (= η5-
C5Me5); M = Fe, Ru, Os; PP = dppe, dppm (= 1,2-
bis(diphenylphosphino)methane), (PPh3)2),25,26,29,35 Mn(η5-
C5H4Me)(dmpe) (dmpe = 1,2-bis(dimethylphosphino)-
ethane),38 Mo(η7-C7H7)(dppe),29 Pt(PR3)2(Ar),25 and ReCp*-
(NO)(PPh3),39,40 are primarily used as redox-active termini,
whereby a direct bond between the metal and the “all-carbon
bridge” is present.
Recently, our research group focused on the electrochemical
behavior of ferrocenyl-functionalized heterocycles41−44 includ-
ing furan,45 thiophene,45,46 pyrrole,45,47,48 phosphole,49 silole,50
titanacyclopenta-2,3,4-triene,51,52 and zirconacyclopentadiene.51
On the example of a series of aromatic diferrocenyl
heterocycles of type 2,5-Fc2-cC4H2E (Fc = Fe(η5-C5H5)(η5-
INTRODUCTION
■
The investigation of organometallic compounds bearing π-
conjugated organic bridging ligands between transition metal
atoms has increased significantly during the last 30 years.1−3 In
particular, alkynyl transition metal complexes are well studied,
due to their high stability, π-conjugation, and rigid structure.
They are well suited, for example, as models for “molecular
wires”4−7 and for the construction of nanoscale electronic
devices.8−12 Besides σ-complexes with end-on bonded alkynes,
tweezer-type complexes with both σ- and π-coordination have
been investigated, as they allow cooperative and synergistic
effects between the metal centers.13−15 Examples of alkynyl
transition metal complexes are given by the works of, for
example, Lapinte,16−19 Low,20−22 and Bruce.23,24 Besides
bridges solely built of alkyne chains,25−28 other organic linkers
such as benzene,1,21,29,30 naphthalene,1,31 anthracene,1,17,32
bipyridine,16 1,12-carbaborane,33 cyanoacetylide,24 dithia[3.3]-
paracyclophane,34 and biferrocene19,35−37 have been introduced
in alkyne-based “all-carbon” units. Hence, the influence of the
chain length and the nature of the organic unit on the
intramolecular electronic communication between the tran-
Received: February 6, 2015
© XXXX American Chemical Society
A
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a
Scheme 1. Synthesis of 5 and 6, 9a,b, 10a,b, 11a,b and 12a,b
a
Conditions: (i) 50 °C, 12 h, 6 mol % CuI, 0.5 mol % [PdCl2(PPh3)2], 6 mol % PPh3, 2.1 equiv trimethylsilyl acetylene. 2,5-Dibromothiophene (4)
was purchased from chemical suppliers.
C5H4); E = O, S, NR; R = CH3, C6H5, C6H4-4-NMe2, C6H4-4-
OMe, C6H4-4-Me, C6H4-3-F, C6H4-4-CO2Et) a direct
correlation between electrochemical and spectroscopic proper-
ties could be shown, due to similar molecular geometries (Fe−
Fe distance, heterocycle) of the respective species.45,47,48,53 The
strength of the electronic coupling of the ferrocenyl/
ferrocenium termini through the heterocyclic unit allowed the
classification of these compounds as class II systems according
to Robin and Day.54
MCCH)2-cC4H2E][PF6]2 (M = Fe, E = O (9a), S (10a);
M = Ru, E = O (9b), S (10b)) (Scheme 1). In situ
deprotonation of 9a,b and 10a,b with KOtBu produced 11a,b
and 12a,b, respectively (Scheme 1, Experimental Section).
After appropriate workup, compounds 11a,b and 12a,b were
obtained as red (11a and 12a) or pale green (11b and 12b)
solids in moderate yields (Experimental Section).
While the synthesis of 11a,b and 12a,b proceeded applying a
straightforward synthesis protocol (Scheme 1), the use of
OsBr(dppe)Cp did not afford the expected isostructural
compounds 2,5-((η5-C5H5)(dppe)OsCC)2-cC4H2E (E = O,
S). Modification of the reaction conditions (different solvents,
temperatures, reaction times, and bases) also did not result in
the desired products.
Compounds 11a,b and 12a,b are less soluble in hexane,
toluene, and diethyl ether; however, they show excellent
solubility in dichloromethane and tetrahydrofuran. They are
stable toward air and moisture in the solid state, while in
solution they readily decompose when exposed to air, but are
stable under an inert gas atmosphere. Characterization details
for 11a,b and 12a,b (1H, 13C{1H}, 31P{1H} NMR, IR
spectroscopy, ESI-TOF mass spectrometry, and elemental
analysis) are given in the Experimental Section and in the
Supporting Information. The structures of 11a and 12a,b in the
solid state are reported. In addition, (spectro)electrochemical
measurements (in situ IR, UV−vis/NIR, ESR) were carried out
to investigate the electronic properties of the mixed-valent
species.
The groups of Lapinte55 and Lui56 found for mixed-valent
[2,5-(η5-C5Me5)(dppe)MCC)2-cC4H2S]+ (M = Fe, Ru)
solvent-independent and narrow IVCT bands in NIR
spectroelectrochemical studies. On the Mossbauer time scale
̈
the charge is delocalized on both metal centers M. IR
spectroelectrochemistry revealed two distinct CC vibrations,
characterizing them as class II/III borderline systems.54 These
investigations prompted us to combine the well-studied and
electron-rich half-sandwich moieties M(dppe)Cp (M = Fe, Ru,
Os; Cp = (η5-C5H5)) with the five-membered heterocycles
(vide supra) with a series of complexes of type 2,5-((η5-
C5H5)(dppe)MCC)2-cC4H2E (E = O, S). (Spectro)-
Electrochemical methods and DFT calculations have been
used to investigate the effect of various group 8 metals and
heteroatoms on the electronic properties and intramolecular
interactions of the latter type of compounds. Furthermore, the
IR-active alkyne unit allowed to study the intramolecular
electron transfer interactions on the IR time scale.
The 1H NMR spectra of 11a,b and 12a,b show a
characteristic singlet for the η5-bonded cyclopentadienyl groups
in the range 4.2−4.7 ppm, whereby the heavier ruthenium atom
causes a shift to lower field. For the heteroaromatic moiety
cC4H2E a singlet (11a, 5.75 ppm; 11b, 5.70 ppm; 12a, 6.29
ppm; 12b, 6.26 ppm) for the protons in 3,4-position was
detected. Due to the different electron density in the
heterocycles, the signals of 12a,b are shifted to lower field,
when compared with the signals of the isostructural furan
RESULTS AND DISCUSSION
■
Synthesis and Characterization. 2,5-Bis-
(trimethylsilylethynyl)-functionalized heterocycles 2,5-
(Me3SiCC)2-cC4H2E (E = O (5), S (6)) were synthesized
by the reaction of Me3SiCCH with 2,5-Br2-cC4H2E (E = O
(3), S (4)) under typical Sonogashira C,C cross-coupling
conditions in diisopropylamine (Scheme 1). Compounds 2,5-
((η5-C5H5)(dppe)MCC)2-cC4H2E (E = O (11), S (12); M =
Fe (a), Ru (b)) were obtained in a three-step “one-pot”
reaction55 based on the deprotection of 5 and 6 with K2CO3 in
methanol, followed by treatment of the thus obtained 2,5-
(HCC)2-cC4H2E species with MCl(dppe)Cp (M = Fe (7),
Ru (8); Cp = (η5-C5H5)), whereby the dissociation of the M−
Cl bond is facilitated by the addition of 2.3 equiv of [NH4]PF6
to afford bis(vinylidene) compounds [2,5-((η5-C5H5)(dppe)-
complexes.57 In the H NMR the aromatic phenyl protons of
1
the dppe group are observed as triplets.58
Characteristic in the 13C{1H} NMR spectra of all complexes
are the signals for the ethynyl units, which resonate at ca. 117
ppm (CC-M) and ca. 135 ppm (CC-cC4H2E), respectively
(Experimental Section). Noteworthy in the 13C{1H} NMR
B
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Figure 1. ORTEP diagram (50% probability level) of the molecular structure of 11a with the atom-numbering scheme. Hydrogen atoms and solvent
molecules have been omitted for clarity. Selected bond distances (Å), angles (deg), and torsion angles (deg): average D−Fe = 1.723, C34−C35 =
1.363(4), C35−C36 = 1.413(4), C36−C37 = 1.358(4), C34−O1 = 1.390(3), C37−O1 = 1.386(3), C33−C34 = 1.409(4), C32−C33 = 1.219(4),
C32−Fe1 = 1.903(3), C37−C38 = 1.414(4), C38−C39 = 1.219(4), C39−Fe2 = 1.903(3), P1−Fe1 = 2.1564(7), P2−Fe1 = 2.1721(7), P3−Fe2 =
2.1620(7), P4−Fe2 = 2.1609(7); C35−C34−C33 = 131.7(2), C37−O1−C34 = 106.69(19), O1−C37−C38 = 120.3(2), C32−Fe1−P1 = 87.92(8),
C32−Fe1−P2 = 84.77(8), C39−Fe2−P3 = 89.88(8), C39−Fe2−P4 = 86.31 (8), P1−Fe1−P2 = 85.83(3), P4−Fe2−P3 = 85.25(3), C33−C32−Fe1
= 178.0(2), C38−C39−Ru2 = 176.0(2); C34−C35−C36−C37 = −0.5(3), C38−C37−O1−C34 = 179.6(2), P1−C18−C19−P2 = −36.1(2), P3−
C57−C58−P4 = −39.3(2), Fe1−C32−C33−C34 = −15(8), C37−C38−C39−Fe2 = −92(3), C18−P1−Fe1−C32 = −71.23(12), C58−P4−Fe2−
C39 = 58.52(12) (D denotes the centroid of C5H5).
Figure 2. ORTEP diagram (50% probability level) of the molecular structures of 12a (left) and 12b (right) with the atom-numbering scheme.
Hydrogen atoms and solvent molecules have been omitted for clarity. Selected bond distances (Å), angles (deg), and torsion angles (deg):
Compound 12a: average D−Fe = 1.713, C1−S1 = 1.748(3), C4−S1 = 1.749(4), C1−C5 = 1.418(5), C5−C6 = 1.229(5), C6−Fe1 = 1.883(3), P1−
Fe1 = 2.1580(11), P2−Fe1 = 2.1600(11), P3−Fe2 = 2.1712(11), P4−Fe2 = 2.1590(10); C2−C1−C5 = 129.1(3), C2−C1−S1 = 109.9(2), C6−
Fe1−P1 = 89.41(12), C6−Fe1−P2 = 85.52(11), C39−Fe2−P3 = 84.26(11), C39−Fe2−P4 = 88.30(10), P1−Fe1−P2 = 85.53(4), P4−Fe2−P3 =
85.77(4), C5−C6−Fe1 = 175.1(3), C38−C39−Fe2 = 177.6(3); C1−C2−C3−C4 = 0.6(5), C38−C4−S1−C1 = 179.7(3), P1−C24−C25−P2 =
39.8(3), P3−C57−C58−P4 = 32.3(3), C1−C5−C6−Fe1 = 66(8), C4−C38−C39−Fe2 = −32(13), C24−P1−Fe1−C6 = 74.93(16), C58−P4−
Fe2−C39 = 68.97(16) (D = denotes the centroid of C5H5). Compound 12b: average D−Ru = 1.896, C1−S1 = 1.742(4), C4−S1 = 1.752(4), C1−
C5 = 1.433(5), C5−C6 = 1.200(5), C6−Ru1 = 2.020(4), P1−Ru1 = 2.2486(10), P2−Ru1 = 2.2595(9), P3−Ru2 = 2.2525(10), P4−Ru2 =
2.2472(10); C2−C1−C5 = 108.3(3), C2−C1−S1 = 108.3(3), P1−Ru1−C6 = 82.38(10), P2−Ru1−C6 = 92.96(10), C39−Ru2−P3 = 87.13(10),
C39−Ru2−P4 = 84.54 (11), P2−Ru1−P1 = 82.91(3), P4−Ru2−P3 = 83.28(4), C5−C6−Ru1 = 174.2(3), C38−C39−Ru2 = 173.6(3); C1−C2−
C3−C4 = 0.6(5), C38−C4−S1−C1 = −18.8(3), P1−C24−C25−P2 = −51.3(3), P3−C57−C58−P4 = 41.6(3), C1−C5−C6−Ru1 = −27(5), C4−
C38−C39−Ru2 = −63(5), C24−P1−Ru1−C6 = 67.77(16), C58−P4−Ru2−C39 = −58.58(1) (D = denotes the centroid of C5H5).
spectra of 11a,b and 12a,b is that the phenyl carbon atoms
close to the phosphorus nuclei show resonance signals with
triplet and/or doublet-of-triplet multiplicities (Experimental
Section). Metzinger has shown within a study of symmetric
diphosphanes that triplet multiplicities for protons occur, when
the P,P coupling constant JPP exceeds the 10-fold value of JHP
(JPP ≥ 10JHP), exposing such a signal pattern as deceptively
simple.58,59 Furthermore, due to the prochirality, two sets of
signals were detected for the phenyl substituents at each
phosphorus atom. The 31P{1H} NMR spectra of 11a,b and
12a,b display one resonance signal for the dppe ligands at 120
ppm for the iron complexes and at 100 ppm for the ruthenium
C
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Figure 3. Voltammograms of dichloromethane solutions containing 1.0 mmol·L−1 of 11a,b or 12a,b at 25 °C. Supporting electrolyte
[nBu4N][B(C6F5)4] (0.1 mol·L−1). Left: Cyclic voltammograms (scan rate: 100 mV·s−1). Right: Square-wave voltammograms (scan rate: 1 mV·s−1).
Dotted lines: CVs of 11b and 12b in a low potential range including only the redox events of the half-sandwich termini.
species (Experimental Section). A characteristic νCC vibration
(11a, 2049 cm−1; 11b, 2062 cm−1; 12a, 2044 cm−1; 12b, 2053
cm−1) is observed for the alkynyl unit in 11a,b and 12a,b in the
IR spectra, which is in accordance with metal-coordinated
acetylides.13−15,55,56,60
The molecular structures of 11a (Figure 1) and 12a,b
(Figure 2) in the solid state were determined by single-crystal
X-ray diffraction analysis. The respective organometallic
compounds were crystallized by diffusion of hexane into a
tetrahydrofuran solution containing 11a or diffusion of toluene
into a dichloromethane solution containing 12a or 12b at
ambient temperature. Important bond distances (Å), bond
angles (deg), and torsion angles (deg) are summarized in the
captions of Figures 1 and 2. For crystal and structure
refinement data see the Experimental Section.
(Fe−P: 2.1564(7) to 2.1721(7); Ru−P: 2.2472(10) to
2.2595(9) Å) are not that sensitive (Figures 1 and 2). The
P−M−P angle decreases somewhat from M = Fe (11a and 12a,
85.25(3)° to 85.83(3)°) to Ru (12b, 82.91(3)° and 83.28(4)°).
The metallocenyl substituents slightly bend out of the
heterocyclic plane, with the maximum reached for the metal
ions (11a, 0.351(8); 12a, 0.403(11); 12b, 0.538(10) and
−0.500(10) Å). This results in a somewhat C-shaped bending
for 11a and 12a and an S-shape for 12b (Figure SI1). The
rotation of the C5H5 ring of the (η5-C5H5)(P2)MCC
fragments (M = Fe, Ru; P2 see Table SI1) toward each other
reveals an anti-arrangement for 11a and 12a,b, which is in
accordance with the only literature-reported example for M =
Fe,37 whereas for M = Ru also deviating conformations are
reported (Table SI1).36,37,66−73
Compounds 11a and 12a,b crystallize in the monoclinic
space group P21/c with one molecule in the asymmetric unit.
Comparing the central five-membered heterocyclic cores,
significant differences between the C,C single (C2−C3) and
C,C double (C1−C2 and C3−C4) bonds57,61 are present in
11a and 12a (C−C: 11a, 1.413(4); 12a, 1.411(5); longest
CC bond: 11a, 1.363(4); 12a, 1.359(5) Å) in contrast to
12b (C−C: 1.387(5); longest CC bond: 1.369(5) Å), which
indicates a higher delocalization in the Ru-containing
thiophene. To the best of our knowledge, the only reported
and crystallographically characterized examples of end-on
transition metal bonded MCC−cC4H2E−CCM hetero-
cyclic compounds contain Au, Pt, and Hg metal fragments (E =
S), and the CC bond lengths are similar to the ones observed
for 11a, 12a, and 12b.62−65 The M−M (M = Fe, Ru) distances
in 11a and 12a,b increase, as expected, by increasing the size of
the transition metal atoms (Fe: 11a, 10.9056(5); 12a,
11.3397(7); Ru: 12b, 11.6880(5) Å) and, furthermore, by
changing the heteroelement of the five-membered ring from
oxygen (11a, 10.9056(5) Å) to sulfur (12a, 11.3397(7) Å).45
In 11a and 12a,b the size of the metal determines the M−
centroid cyclopentadienyl distances (= D of C5H5) (average
Fe−D: 1.72; average Ru−D: 1.90 Å), while the M−P distances
Electrochemistry. The electrochemical properties of 11a,b
and 12a,b have been determined by cyclic voltammetry (CV)
and square-wave voltammetry (SWV) (Figure 3) (dichloro-
methane solutions containing the respective analyte (1.0 mM)
and [nBu4N][B(C6F5)4] (0.1 M) as supporting electro-
lyte).46,74−80 The data of the CV measurements have been
recorded at a scan rate of 100 mV·s−1 at 25 °C and are
summarized in Table 1. All redox potentials are referenced to
the FcH/FcH+ redox couple (E°′ = 0 mV, FcH = Fe(η5-
C5H5)2).81
The metal atoms M (= Fe, Ru) of 11a,b and 12a,b could be
oxidized separately, showing two reversible redox events
(Figure 3, Table 1). In addition to the redox events of the
half-sandwich moieties, a reversible redox process of the
heterocyclic backbone was observed at higher potential, for 11a
at E°3′ = 835 mV and 12a at E°3′ = 775 mV (Table 1).
Compared to compounds 11a and 12a the redox events of the
ruthenium derivatives 11b and 12b are shifted to anodic
potentials (Table 1), indicating that the ruthenium metal center
is more difficult to oxidize. This fact is supported by, for
17,32
example, 9,10-((η5-C5Me5)(dppe)MCC)2-cC10H8
(di-
chloromethane, [N(nBu)4][PF6] as supporting electrolyte; M
= Fe, E°1′ = −400 mV; M = Ru, E°1′ = −170 mV) and 2,5-
D
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Table 1. Cyclic Voltammetry Data (Potentials vs FcH/FcH+)
with a Scan Rate of 100 mV·s−1 at a Glassy Carbon Electrode
of 1.0 mmol·L−1 Solutions of the Analytes in Anhydrous
Dichloromethane Containing 0.1 mol·L−1 of
The redox splitting ΔE°′ between the MCp(dppe) moieties
in 11a,b and 12a,b decreases from the furan (11a, 350 mV;
11b, 340 mV) to the thiophene species (12a, 315 mV; 12b, 285
mV) (Table 1). A further decreasing trend is found from the
iron- (11a, 12a) to the ruthenium-containing compounds (11b,
12b). A comparison of 11a and 12a with 2,5-diferrocenyl
heterocycles (furan, ΔE°′ = 290 mV; thiophene, ΔE°′ = 260
mV)77 shows a similar decreasing trend of the redox separation.
This progress can be explained with the stronger electronic
coupling of the furan to the ferrocenyls or half-sandwich
moieties, compared to thiophene, caused by the better overlap
of the participating orbitals.77 Furthermore, the stronger
electron donor capabilities of the FeCp(dppe) moiety84 as
well as the direct metal−carbon bond are beneficial concerning
electronic coupling to the heterocyclic bridge, when compared
with the ferrocenyl analogues, and thus higher redox
separations can be achieved. In contrast to the iron complexes,
RuCp(dppe) termini are mixing more efficiently with the
CC π-system, giving a higher ligand-based radical character
of the frontier orbitals after oxidation,7,85 which in turn leads to
a destabilization of the aromatic bridging system. The observed
redox separations are in the same range as the Cp*(dppe)-
MCC-substituted thiophenes (M = Fe, ΔE°′ = 340 mV;55 M
= Ru, ΔE°′ = 320 mV),56 furan (M = Fe, ΔE°′ = 440 mV),86
and N-methyl pyrrole (M = Fe, ΔE°′ = 355 mV).86 The high
ΔE°′ values indicate intramolecular electronic interactions
between the metal centers M (M = Fe, Ru) through the 2,5-
bis(ethynyl)heterocyclic core. The large KC values (Table 1)
indicate sufficient thermodynamic stability of the mixed-valent
species, and therefore, these molecules are well suited for
spectroelectrochemical investigations (vide infra).
[nBu4N][B(C6F5)4] as Supporting Electrolyte at 25 °C
a
a
a
E°1′ (mV)
E°2′ (mV)
(ΔE
E°3′ (mV)
(ΔE
(ΔE
ΔE°′
c
pb
(mV))
pb
(mV))
pb
(mV))
d
compd
(mV)
KC
11a
11b
12a
12b
−750 (60)
−595 (72)
−710 (64)
−520 (64)
−400 (66)
−255 (76)
−395 (65)
−235 (76)
b
835 (84)
350
340
315
285
9.33 × 105
5.20 × 105
2.21 × 105
6.87 × 104
e
1015
775 (83)
e
895
a
E°′ = formal potential. ΔEp = difference between oxidation and
c
reduction potential. ΔE°′ = potential difference between the two
d
metal-based redox processes. KC = comproportionation constant (RT
e
ln KC = ΔE°′F). Oxidation potential Epa.
diferrocenyl/ruthenocenyl thiophene (E°1′ = −94 mV,
dichloromethane, [N(nBu)4][B(C6F5)4];77 E°1′ = 281 mV,
dichloromethane, [N(nBu)4][BF4]),82 which show a similar
trend of the potential values in accordance with the different
ionization potentials of Fe2+ and Ru2+, respectively.83 In the
cyclic voltammogram of 11b and 12b, however, an irreversible
oxidation for the heterocyclic core and further oxidation (11b,
1300 mV; 12b, 1215 mV) and reduction processes (between
200 and 1000 mV) occur (Figure 3). As a consequence of these
irreversible events, the follow-up products most likely
decompose, and hence the concentration of 11b and 12b in
solution is reduced. Therefore, measurements in which a higher
reversal potential is applied showed a less reversible behavior
for E°1′ and E°2′ of the half-sandwich moieties (Figure 3).
Figure 4. UV−vis/NIR spectra of 11a at rising potentials vs Ag/AgCl: −200 to 250 mV (left top), 250 to 1000 mV (bottom). Right top:
Deconvolution of the NIR absorptions at 250 mV of in situ generated 11a+ using three Gaussian-shaped graphs. Measurement conditions: 25 °C,
dichloromethane, 0.1 mol·L−1 [nBu4N][B(C6F5)4] as supporting electrolyte.
E
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Spectroelectrochemisry. 1. UV−vis/NIR and IR Spec-
troelectrochemistry. The spectroelectrochemical UV−vis/NIR
and IR measurements were performed in an OTTLE (optically
transparent thin-layer electrochemistry) cell (quartz windows
for UV/vis−NIR, CaF2 windows for IR).87 Dry dichloro-
methane solutions containing 0.1 mol·L−1 of [nBu4N][B-
(C6F5)4] as the supporting electrolyte and 2.0 mmol·L−1 of
11a,b or 12a,b were used at 25 °C. The potentials have been
increased stepwisely using a step height of 25, 50, or 100 mV,
whereby the neutral binuclear complexes 11a,b and 12a,b were
oxidized to mixed-valent 11a,b+ and 12a,b+, resepectively, and
finally to dicationic 11a,b2+ and 12a,b2+ (11an+, Figure 4; 11bn+,
12a,bn+ Supporting Information, Figures SI2−SI4; n = 0, 1, 2).
Neutral 11a,b and 12a,b showed ligand-centered π → π*
transitions57 at 350 to 400 nm in the visible region of the
spectrum, while, as expected, no absorptions could be detected
in the NIR region (1000−3000 nm). Upon potential increase,
intense absorptions between 1000 and 2000 nm were observed,
demonstrating the formation of mixed-valent species 11a,b+
and 12a,b+. With further oxidation to dicationic 11a,b2+ and
12a,b2+ the absorptions disappear. Simultaneously, a new band
at ca. 750 nm occurs, which can be assigned to an LMCT
transition (ligand-to-metal charge transfer).55,56 Deconvolution
of the spectra of 11a,b+ and 12a,b+ was applied to determine
the physical parameters (wavenumber (νmax), extinction (εmax),
full-width at half-maximum (fwhm) (Δν1/2)) of the excitations
in the NIR region. The sum of the three overlapping Gaussian-
shaped functions reflects the experimental graph. While the
small and broad Gaussian function at around 10 000 cm−1
represents an LMCT transition (Table 2, Figures 4 and SI2−
SI4), the band shape reveals two low-energy π(dπ) → π*(dπ)
transitions, involving the metal−ligand−metal setup.88
absorptions in the NIR range of the spectrum (Figure 5),
indicating strong intramolecular electronic interactions between
Figure 5. Solvent-dependency of 11b+ (2.0 mmol·L−1) in dichloro-
methane (red), acetonitrile (blue), and propylene carbonate (green).
Measurement conditions: 25 °C, 0.1 mol·L−1 [nBu4N][B(C6F5)4] as
supporting electrolyte.
the metal centers M/M+ (11a and 12a, Fe(II)/Fe(III); 11b and
12b, Ru(II)/Ru(III)) through the bis(alkynyl) heterocyclic
core. The MCp*(dppe)-substituted thiophene (M = Fe,55 M =
Ru56) and furan analogues (M = Fe)86 show absorptions similar
to the NIR bands of 11a and 12a,b. Different ancillary ligands
(Cp*), solvent effects, and applied electrolytes are responsible
for a variation of the intensity and band shape. Nevertheless,
Lapinte55 and Liu56 found solvent-independent excitations with
a high-energy shoulder, revealing two superimposed transitions
by deconvolution.
A possible reason for the complex excitation phenomena in
the NIR region of mixed-valent 11a,b+ and 12a,b+ is
presumably related to the presence of a couple of thermally
accessible conformers with varying excitation energies (Figure
6). Kaupp and Low demonstrated this behavior on the
a
Table 2. UV−vis/NIR Data of 11a,b+ and 12a,b+
νmax
Δν1/2
compd
transition
(cm−1
)
εmax (L·mol−1·cm−1
)
(cm−1
)
11a+ π(dπ) → π*(dπ)
π(dπ) → π*(dπ)
LMCT
11b+ π(dπ) → π*(dπ)
π(dπ) → π*(dπ)
LMCT
12a+ π(dπ) → π*(dπ)
π(dπ) → π*(dπ)
LMCT
12b+ π(dπ) → π*(dπ)
π(dπ) → π*(dπ)
LMCT
5980
7260
21 190
16 135
3250
1190
2810
2890
1170
2850
5900
1180
3090
1860
1580
1395
2630
10 040
8200
7530
9660
6250
11 180
5515
2550
17 900
16 460
2170
Figure 6. Different conformations of the half-sandwich unit in 11 and
12 with respect to the bis(alkynyl) heterocyclic plane.
6770
9500
7675
12 880
7460
examples of [{Ru(dppe)Cp*}2(μ-CCC6H4CC)]+ and
[trans-{Ru(dppe)2Cl}2(μ-CCC6H4CC)]+.93 The latter
complex is less likely to form rotational conformers, due to
its more symmetric coordination sphere, but [{Ru(dppe)-
Cp*}2(μ-CCC6H4CC)]+ can build conformers in which
the orbitals of the redox-active unit are overlapping with
different orbitals of the bridging units’ π-system. Time-
dependent DFT calculations showed that conformers with a
lower P−Ru−Ru−P dihedral angle showed a more delocalized
electronic situation.93
9205
10 590
5550
a
In dichloromethane containing 0.1 mol·L−1 of [NnBu4][B(C6F5)4] as
supporting electrolyte at 25 °C.
The band shapes fulfill the criteria for class III systems based
on the two-state Hush model89,90 for symmetric mixed-valent
species, since the excitations are very intense (εmax ≥ 5000 L·
mol−1·cm−1). While the fwhm of the second excitation fits
rather to a class II IVCT band, the small fwhm of the lower
The electronic coupling parameter Hab,89−91 which indicates
the strength of the electronic interaction between two redox-
active termini, has been calculated according to the equation
Hab = 1/2 νmax and is between 2750 and 4830 cm−1 for 11a,b
and 12a,b (2,5-(Cp*(dppe)FeCC)2-cC4H2O: Hab = 2665
cm−1;86 2,5-(Cp*(dppe)FeCC)2-cC4H2S: Hab = 2515
cm−1;55 2,5-(Cp*(dppe)RuCC)2-cC4H2S: Hab = 3523
energy absorption complies with the class III criterion (Δν1/2
≤
2000 cm−1) (Table 2).91 The solvatochromic behavior of these
absorptions was studied exemplary with 11b using solvents with
different dipole moments μ (dichloromethane (μ = 1.60 D),
acetonitrile (μ = 3.93 D), propylene cabonate (μ = 4.90 D)).92
The measurements proved solvent independency for both
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cm−1).56 The coupling parameters of 11a,b and 12a,b are in a
coupling parameters contribute to ΔGr and consequently to
ΔE°′, but also the influence of λ becomes noticeable. As a result
of this, the ruthenium-containing molecules 11b and 12b
showed smaller ΔE°′ values despite possessing higher coupling
parameters Hab. It can be concluded that the diadiabatic
reorganization energy λ for the softer ruthenium atom,
compared to iron, is smaller and therefore overcompensates
Hab.
comparable range, for example, the Creutz−Taube ion (Hab
=
/
3205 cm−1).94 Calculation of the Γ parameter (Γ = 1 − (Δν1/2
(Δν1/2)
theo))91 of mixed-valent 11a,b+ and 12a,b+, a classi-
fication criterion of Brunschwig et al.95 for mixed-valent
compounds, gives values of 0.22 and 0.73 for the two NIR
excitations (Tables 2 and 3). With Γ values ≥ 0.5 a further
Table 3. Hab and Γ Parameters of 11a,b+ and 12a,b+ for the
π(dπ) → π*(dπ) Transition
Δν1/2(theo)
a
d
compd
transition
Hab (cm−1
)
(cm−1
)
Γ
11a+ π(dπ) → π*(dπ)
π(dπ) → π*(dπ)/IVCT 3630/1083
11b+ π(dπ) → π*(dπ)
2990
2628
4095
4352
4724
3569
16460
4210
4611
0.55
0.31
0.73
0.39
0.67
0.22
0.63
0.69
b
4100
bc
,
π(dπ) → π*(dπ)/IVCT 4830/760
12a+ π(dπ) → π*(dπ)
2758
π(dπ) → π*(dπ)/IVCT 3385/1065
b
12b+ π(dπ) → π*(dπ)
3838
b
π(dπ) → π*(dπ)/IVCT 4603/545
a
b
1
Calculated with the equation Hab = /2νmax
.
Calculated with the
1/2/rab). As rab
equation Hab = 2.06 × 10−2 × ((νmaxεmaxΔν1/2
)
Figure 7. Comparison of the UV−vis/NIR spectra of 11a+,b+ and
12a+,b+. Measurement conditions: 2.0 mM analyte, 25 °C, dichloro-
methane, 0.1 mol·L−1 [nBu4N][B(C6F5)4] as supporting electrolyte.
geometrical metal−metal distances derived from X-ray crystallographic
c
data were used, see X-ray crystallography part. rab (11.25 Å) has been
d
estimated from the other values for 11a+ and 12a,b+. Calculated with
the equation Γ = 1 − (Δν1/2/(Δν1/2)theo).
indication for strongly coupled systems is given. However, Γ
values ≤ 0.5 of the second π(dπ) → π*(dπ) transitions rather
classify 11a,b+ and 12a,b+ as class II species.95 The
deconvolution methodology described above utilizes Gaus-
sian-shaped functions to simulate the experimental spectra. Part
The IR-active alkynyl unit enables us to investigate the
degree of charge delocalization of the mixed-valent species on
the IR time scale.97,98 During the in situ IR spectroelec-
trochemical measurements the characteristic νCC vibrations at
ca. 2050 cm−1 for the neutral complexes 11 and 12,
respectively, disappear (Figures 8 and SI5−SI7). Simulta-
neously a new band at lower energy (1980−1960 cm−1)
appears owing to the formation of a mixed-valent species and
therefore to a partially cumulenic character of the original (η5-
C5H5)(dppe)MCC unit (Figures 8 and SI5−SI7, Table 4).
The detection of one narrow stretching vibration for 11a,b+ and
12a,b+ indicates that the single electron is delocalized between
the two metal centers. Hence, complexes 11a,b and 12a,b are
strongly coupled systems on the IR time scale. In comparison,
for compounds with similar structural motifs, the appearance of
of the reason for the small bandwidths within class III systems
88
is a low-energy cutoff of the bands at νmax
,
and thus a
Gaussian-shaped spectral simulation overestimates the bands’
slope at the high-energy side. Since the second NIR absorption
overlaps with the high-energy side of the main absorption, the
parameters derived from this band should be handled with care.
In this respect the high-energy NIR transition for all
compounds cannot be assigned to class II or III unambiguously.
While this band showed solvent-independence, a typical class
III feature, the physical parameters derived from deconvolution
(especially the fwhm value and the Γ criterion), however, argue
for a class II assignment.
The electronic coupling for the furan derivatives 11a,b is
higher than that of isostructural thiophenes 12a,b (Table 3,
Figure 7). This difference most probably causes the individual
differences in the respective ΔE°′ values as contribution to the
resonance stabilization (11a, ΔE°′ = 350 mV, Hab = 2990/3630
cm−1 → 12a, ΔE°′ = 315 mV, Hab = 2758/3385 cm−1; 11b,
ΔE°′ = 340 mV, Hab = 4100/4830 cm−1 → 12b, ΔE°′ = 285
mV, Hab = 3838/4603 cm−1). The resonance stabilization term
(ΔGr), as a contribution to the free energy of comproportio-
nation, can be calculated for class III systems according to eq 1,
whereas λ is the diadiabatic reorganization energy.91,95,96
two distinctive vCC stretching vibrations is observed within
‐‐‐
class II systems.19,35,36 Upon close inspection of the IR
characteristics of 11a,b+ and 12a,b+ small shoulders at ca.
1945 cm−1 can be found, which may indicate the presence of a
thermally accessible conformer with a more localized electronic
structure. Further oxidation to dicationic 11a,b2+ and 12a,b2+
and the increase of the cumulenic character of the vibration led
to a formal hypsochromical shift of the absorption band to
1920 cm−1.
To support the interpretations of the UV−vis/NIR and ESR
spectra, exemplarily mononuclear complex 2-(RuCp(dppe)-
CC)-thiophene (15) was synthesized. The cyclic voltammo-
gram of 15 showed only one irreversible oxidation process of
the Ru(dppe)Cp group at Epa = −170 mV (see Supporting
Information, Figure SI8). It is suspected that upon oxidation a
reactive radical cation is formed and polymerization as well as
other side reactions can occur through the unsubstituted 5
position. Therefore, further spectroelectrochemical investiga-
tions were excluded. To suppress such processes, 2-(RuCp-
(dppe)-CC)-5-methyl thiophene (16) was synthesized.
However, the CV also showed an irreversible oxidation at Epa
ΔGr = −2(Hab − λ/4)
(1)
It is reasonable that varying the heterocyclic unit (11 → 12)
has no significant effect on the value of λ. Therefore, changes in
ΔGr are predominantly caused by changes in the coupling
parameter Hab. However, changing the metal from iron to
ruthenium (a → b) has a significant impact on the diadiabatic
reorganization energy, and thus not only do the different
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Figure 8. IR spectroelectrochemical spectra of 11a at rising potentials vs Ag/AgCl. Left: −200 mV to 510 mV (11a → 11a+). Right: 510 mV to 1000
mV (11a+ → 11a2+). The blue line belongs to the mixed-valent species 11a+ and is labeled with vCC. Measurement conditions: 25 °C,
‐‐‐
dichloromethane solution containing 10.0 mmol·L−1 of 11a, 0.1 mol·L−1 of [NnBu4][B(C6F5)4] as supporting electrolyte.
Table 4. Spectroelectrochemical IR Data for 11a,bn+ and
whose intensity evolution during the electrochemical cycles
correlated well with the transferred charge. The spectra of 11a+,
12a+, and 12b+ are featureless lines with the effective peak
width of 6, 11, and 22 G, respectively. For 11b+ a hyperfine
structure is characteristic (albeit not fully resolved), the origin
of which is discussed below.100 The isotropic g-factor values for
all radical-cations are close to 2.03−2.04 (Table 5); broad-field
12a,bn+ (n = 0, 1, 2)
compd νC,C (cm−1) (n = 0) νC,C (cm−1) (n = 1) νC,C (cm−1) (n = 2)
11an+
11bn+
12an+
12bn+
2052
2065
2048
2058
1986
1973
1982
1961
1930
1932
1919
1921
Table 5. Experimental and DFT-Computed Calculated g
Values for 11a,b+ and 12a,b+
= −190 mV (Supporting Information, Figure SI8). Addition-
ally, on increasing the potential range to 900 mV a second more
quasi-reversible redox event (Epa = 600 mV) was observed,
which can be assigned to the oxidation of the thiophene core.
In contrast to the electrochemically stable benzene-Ru(dppe)-
Cp from Low,99 the thiophenes are susceptible to diverse side
reactions when oxidized.
2. ESR Spectroelectrochemistry. Further confirmation of
the delocalized nature of the radical cations 11a,b+ and 12a,b+
was obtained by in situ ESR spectroelectrochemistry studies and
DFT calculations. Figure 9 shows the X-band ESR spectra
measured in situ during electrochemical oxidation of the
compounds at their first oxidation potentials. All species readily
formed ESR-detectable signals at corresponding potentials
a
compd
g1
g2
g3
Δg
giso
11a+
expt
1.9997
2.0007
2.0316
2.0284
2.0366
2.0463
2.0345
2.0395
n/a
2.0650
2.0331
2.1283
2.0728
2.0437
2.1409
n/a
0.0653
0.0324
0.1216
0.0670
0.0421
0.1337
n/a
2.0347
2.0207
2.0572
2.0423
2.0266
2.0626
2.0322
2.0281
2.0268
2.0389
2.0330
2.0332
calcd (0,0)
calcd (0,90) 2.0067
12a+
11b+
12b+
exp.
2.0058
2.0017
calcd (0,0)
calcd (0,90) 2.0072
expt
n/a
calcd (0,0)
1.9991
2.0331
2.0235
n/a
2.0522
2.0569
n/a
0.0531
0.0570
n/a
calcd (0,90) 1.9999
expt
n/a
calcd (0,0)
1.9992
2.0396
2.0286
2.0603
2.0708
0.0610
0.0704
calcd (0,90) 2.0003
a
Experimental giso values are for the room-temperature spectra; giso
values obtained by averaging g-tensor components determined in
frozen solution are 2.0321 for 11a+ and 2.0416 for 12a+.
scans did not reveal the presence of any other ESR signals. The
g-factors are noticeably higher than the free electron value
(2.0032) and point to a significant metal contribution to the
spin density. At the same time, deviations from the free electron
value can be considered as relatively small, when compared to
Fe- and Ru-based radicals with predominant localization of the
spin density on the metal centers.
Freezing solutions of 11a+ and 12a+ after their electrolysis
afforded powder-like ESR spectra with rhombic symmetry. The
experimentally determined diagonal elements of the g-tensor
span the range from 1.9997 to 2.0321 in 11a+ and from 2.0058
to 2.0728 in 12a+. Anisotropy of the g-tensor, Δg, calculated as
the difference between the largest and the smallest diagonal
elements, is 0.0653 in 11a+ and 0.0670 in 12a+. Dong and
Hendrickson101 showed that anisotropy of the g-factor in
radicals of mixed-valence complexes correlates with the electron
transfer rate. In particular, the Δg values are smaller than 1.1
due to the delocalized spin density character. The values
Figure 9. ESR spectra of electrochemically generated cation radicals
measured at room temperature (RT) and in frozen solution (105 K).
Note the change of the sweep width for the frozen solutions.
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determined in this work are much smaller than this threshold
criterion, and thus the ESR study also confirms the delocalized
nature of the complexes.
parentheses, which correspond to rotation angles of each half-
sandwich unit with respect to the heterocycle plane. In this
notation, the rotamer shown in Figure 6, left, is described as
(0,0), whereas the rotamer shown in Figure 6, right, with one
unit rotated by 90° is denoted as (0,90). The studies of the
potential energy surface at the PBE/TZ2P level showed that
the structures with (0,0), (0,180), and (180,180) are almost
isoenergetic and correspond to the energy minima, whereas the
structures with the rotation angles of 90° are energy maxima
(see Supporting Information for further details). At the
B(35)LYP level, the rotamers (0,90) are only a few kJ·mol−1
less stable than rotamers (0,0). Relative energies of (0,90)
rotamers of 11a+ and 12a+ with respect to (0,0) rotamers are
only 4 and 1 kJ/·mol−1, respectively. For 11b+ and 12b+ the
energies of the (0,90) rotamers are somewhat higher, 11 and 10
kJ·mol−1, respectively, but still remain rather small. Thus, free
rotation of the half-sandwich units is expected at room
temperature, and therefore the spectral characteristics measured
experimentally are averaged responses of many rotamers.
Despite the similar energies, electronic structures of the
rotamers can be substantially different. Figure 10c,d shows spin
density distributions in (0,0) and (0,90) rotamers of 11a+ and
11b+ (similar surfaces are obtained for 12a+ and 12b+). In the
(0,0) rotamers the spin density is equally distributed over the
metal centers and the bridge (Figure 10) and the shape of the
lobes roughly resembles that of the HOMO. Mulliken spin
populations of the metal atoms are 0.23/0.26 in 11a+ and 0.16/
0.16 in 11b+. The rotation of a half-sandwich unit by 90° results
in significant changes of the spin density distribution, especially
in 11a+. Spin populations of the metal atoms in the units at 0°
are increased to 0.91 and 0.21 in 11a+ and 11b+, respectively,
whereas those of the metal atoms in the rotated units are
decreased to 0.02 in 11a+ and 0.10 in 11b+ (note that 11a+ and
12a+ suffer from rather large spin contaminations, and hence
the values are probably too high). Thus, whereas (0,0)
conformers are typical delocalized systems, (0,90) conformers
exhibit a substantial degree of localization.
DFT Calculations. The electronic structures of 11a,b and
12a,b in the neutral and cationic states were studied by DFT
calculations. First, atomic coordinates were optimized at the
PBE/TZ2P level. Then, the frontier MO analysis, spin density
distribution, time-dependent DFT calculations, and calculations
of the ESR parameters were performed using a modified
version of the B1LYP functional (denoted hereafter as
B(35)LYP) in which the fraction of exact exchange was
increased to 35% following the work of Kaupp et al.93,102 They
showed that this functional provides a balanced description of
localization/delocalization phenomena in mixed-valence com-
plexes. In the B(35)LYP calculations, also ZORA scalar-
relativistic corrections and COSMO solvation corrections
(dichloromethane, ε = 9.08) were considered. The basis set
was a SARC-modified version of the def2-TZVP basis set
specially tailored for calculations with ZORA correction (see
the Supporting Information for further details of calculations).
Figure 10a shows the HOMO and HOMO−1 isosurfaces of
11a, which are well representative also for the other complexes.
Coexistence of rotamers with localized and delocalized spin
density distribution may be the reason for the two-band
absorption pattern in the NIR range (Figures 4, 5, 7, and
Supporting Information Figures SI2−SI4) as proposed earlier
by Kaupp, Low, and co-workers.93 Indeed, TD-DFT
computations show that the intense NIR excitation in (0,0)
rotamers has lower energy than in (0,90) rotamers (Figure 11;
note that TD-DFT systematically overestimates excitation
energies). In Figure 11 also the difference electronic densities
for the excitations with the highest oscillator strength are
shown. Similar to the spin density distribution described in
Figure 10, the different excitation density in (0,0) rotamers is
equally delocalized over the two metals, whereas in (0,90)
rotamers one of the metals has a higher contribution.
Variation of the spin density distribution with rotation of the
half-sandwich entities also manifests itself in the EPR
parameters, especially in the g-tensor diagonal elements and
isotropic g value. Table 5 compares experimental values to the
computed ones. Neither (0,0) nor (0,90) rotamers alone are
able to describe the experimental data for 11a+ and 12a+. The
DFT-predicted g-tensor anisotropy and the isotropic g-factor
are too small in (0,0) rotamers and too large in (0,90) ones;
however, their combination gives a reasonable match to the
measured values.
Figure 10. (a) HOMO and HOMO−1 in 11a; (b) schematic
description of the changes in MO population induced by oxidation and
appearance of the new optical excitation (denoted by red arrow); (c)
spin density isosurfaces in two rotamers of 11a+; (d) spin density
isosurfaces in two rotamers of 11b+. In MO isosurfaces, yellow is “+”
and cyan is “−”; in the spin density isosurface, green is “+” and orange
is “−”. Phenyl groups and hydrogen atoms are not shown for clarity.
The HOMO is delocalized over the bridging unit with a large
contribution of the central heterocycle. Metal contributions are
rather small but not negligible either. On the contrary, in the
HOMO−1 the metal contribution is enhanced, whereas the
heterocycle contribution is almost negligible. A large part of the
HOMO−1 is also delocalized over the acetylide fragments. The
relatively large gap between the two orbitals indicates
delocalization of the cation radical. When compound 11a is
oxidized, the HOMO is depopulated and a new optical
excitation appears as shown in Figure 10b, which is responsible
for NIR bands in the absorption spectra of the cations.
Since rotation of the half-sandwich units may result in a
significant variation of the electronic structure of the complexes
and even change a delocalized system into a localized one,
special attention was devoted to this point in the cationic
structures. To define a rotamer, we used two numbers in
Rotational flexibility significantly affects hyperfine coupling
(hfc) constants. Average 31P hfc constants in 11a+-(0,0) are 6.9
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determined using single-crystal X-ray diffraction analysis
showing anti-arrangements of the metal half-sandwich moieties
toward each other. The electronic properties of 11a,b and
12a,b have been studied by electrochemical measurements,
whereby the respective compounds show well-separated
reversible one-electron redox events for the two redox-active
termini. The calculated large KC values (6.87 × 104 to 9.33 ×
105) validate their high thermodynamic stability. Further
spectroelectrochemical studies indicated that the thiophene
and furan units as bridging ligands in 11a,b and 12a,b,
respectively, are beneficial for intramolecular electron transfer
studies in the mixed-valent species. Mixed-valent 11a,b+ and
12a,b+ reveal charge delocalization within the UV−vis/NIR and
ESR and on the IR time scale. In UV−vis/NIR studies 11a,b+
and 12a,b+ show intense, narrow, and solvent-independent
π(dπ) → π*(dπ) absorptions. Due to the presence of a couple
of thermally accessible conformers with varying excitation
energies, two NIR absorptions are observed. The IR spectra
give one narrow bathochromic shifted vCC stretching vibration
for 11a,b+ and 12a,b+, due to the ‐‐i‐ncreasing cumulenic
character of the (η5-C5H5)(dppe)MCC moiety. For all
compounds, the formation of cation radicals at the first
reduction step was detected by ESR spectroscopy. The low
anisotropy of the g-factor in frozen solution is in accordance
with the delocalized nature of the species. These results
characterize 11a,b and 12a,b as strongly coupled compounds
according to the classification of Robin and Day54 and enlarge
the family of organometallic class III systems.88,103,104 However,
both spectroscopic (NIR, IR, ESR) and computational (DFT/
TD-DFT) results are best described by a mixture of least two
thermally accessible rotation conformers of the organometallic
termini of 11a,b and 12a,b. While for one of the conformers
(0,0) a delocalized structure is found, rotation of one of the
redox-active moieties of 90° (0,90) results in a more localized
behavior.
Figure 11. TD-DFT-predicted excitation spectra of 11a+ and 11b+
rotamers in the NIR range and difference densities for the most
intense excitations (red is “+” and blue is “−”). Phenyl groups and
hydrogen atoms are not shown for clarity.
1
G, and those of two H nuclei in the heterocycle are −2.2 G.
Preferential localization of the spin density near a half-sandwich
unit in the (0,90) rotamer leads to a large asymmetry of hfc
values: phosphorus atoms are now divided into two pairs with
1
31P hfc constants of −10.7 and 1.2 G. H constants in 11a+-
(0,90) are 0.4 and −1.6 G. A similar situation is predicted for
the rotamers of 12a+: 31P hfc constants are 5.4 G in (0,0) and
1
−12.1/1.1 G in the (0,90) rotamers, whereas H values are
−1.7 and −0.5 G. Thus, 31P values in 11a+ and 12a+ have
similar absolute values but an opposite sign in (0,0) and (0,90)
rotamers and therefore upon thermal averaging compensate
each other.
A less profound effect has the internal rotation on the hfc
constants in 11b+ and 12b+. Here the 31P hfc constants are near
10 G in (0,0) and 10.9/2.6 G in (0,90) rotamers, and hence
thermal averaging is expected to produce values near 7−8 G.
1H hfc constants are close to −2 G in both compounds. For the
two isotopes of Ru with the nuclear spin 5/2 (99Ru (12.7%),
101Ru (17.1%)) theory predicts hfc values of about 6 G (see the
Supporting Information for full details). Thus, in contrast to
11a+ and 12a+, 11b+ and 12b+ are predicted to have relatively
large 31P hfc constants and also non-negligible hyperfine
coupling with the two Ru isotopes. This explains why the EPR
signals of 11b+ and 12b+ are significantly broader at room
temperature than their analogues 11a+ and 12a+. The hyperfine
structure partially resolved in the radical cation of 11b+ can be
assigned predominantly to the 31P nuclei.
EXPERIMENTAL SECTION
■
General Conditions. All reactions were carried out under an
atmosphere of argon using standard Schlenk techniques. Drying of
hexane, diethyl ether, and dichloromethane was performed with a
double column solvent filtration system, working pressure 0.5 bar.
Tetrahydrofuran was purified by distillation from sodium/benzophe-
none ketyl, and methanol was purified by distillation from magnesium.
Diisopropylamine was purified by distillation from calcium hydride.
Reagents. 2,5-Dibromothiophene (4), furan, KOtBu, N-bromo-
succinimide, trimethylsilyl acetylene, copper(I) iodide, potassium
carbonate, tetra-n-butylammonium fluoride, and ammonium hexa-
fluorophosphate were purchased from commercial suppliers and used
without further purification. 2,5-Dibromofuran,76 2,5-bis-
(trimethylsilyl)thiophene (6),56 [Fe(η5-C5H5)(η2-dppe)Cl] (7),22,105
[Ru(η5-C5H5)(η2-dppe)Cl] (8),106 [Os(η5-C5H5)(η2-dppe)Br],107
[nBu4N][B(C6F5)4],47,75,76,79,80,108−111 and [PdCl2(PPh3)2]112 were
prepared according to published procedures.
CONCLUSION
■
Within this study the synthesis of a series of iron and
ruthenium half-sandwich alkynyl complexes bearing a thio-
phene or furan π-conjugated connectivity (2,5-((η5-C5H5)-
(dppe)MCC)2-cC4H2E (E = O (11), S (12); M = Fe (a), Ru
(b)) is reported. The appropriate Me3SiCC-protected
heterocycles were synthesized in a Sonogashira C,C cross-
coupling reaction, and the following attachment of the
MCp(dppe) moieties was realized in a three-step consecutive
“one-pot” reaction. The influence of Fe and Ru and the
heterocyclic core on the intramolecular electronic metal−metal
interactions in the mixed-valent species has been investigated.
The structures of 11a and 12a,b in the solid state were
Instruments. 1H NMR (500.3 MHz), 13C{1H} NMR (125.8
MHz), and 31P{1H} NMR (202.5 MHz) spectra were recorded at 298
K in the Fourier transform mode. Chemical shifts are reported in δ
units (parts per million) using undeuterated solvent residues as
internal standard (CDCl3: 1H at 7.26 ppm and 13C{1H} at 77.16 ppm;
1
C6D6: H at 7.16 ppm and 13C{1H} at 128.06 ppm).
Single-Crystal X-ray Diffraction Analysis. Data for 11a and
12a,b were collected with a diffractometer using graphite-mono-
chromatized Mo Kα (λ = 0.71073 Å) (12a and 12b) or Cu Kα
radiation (1.54184 Å) (11a). The molecular structures were solved by
direct methods using SHELXS-97113 and refined by full-matrix least-
squares procedures on F2 using SHELXL-97.114 All non-hydrogen
J
DOI: 10.1021/acs.organomet.5b00104
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
atoms were refined anisotropically, and a riding model was employed
in the treatment of the hydrogen atom positions. Graphics of the
molecular structures have been created by using ORTEP.115
(17s11p7d1f) for Fe, and {811111111111/611111111/51111/1}/
(19s14p9d1f) for Ru.
General Procedure: Synthesis of the 2,5-Bis(trimethylsilyl)-
Electrochemistry. Measurements on 1.0 mmol·L−1 solutions of
the analytes in anhydrous air-free dichloromethane containing 0.1 mol·
L−1 of [nBu4N][B(C6F5)4] as supporting electrolyte were conducted
under a blanket of purified argon at 25 °C. A three-electrode cell,
which includes a Pt auxiliary electrode, a glassy carbon working
electrode (surface area 0.031 cm2), and a Ag/Ag+ (0.01 mol·L−1
AgNO3) reference electrode mounted on a Luggin capillary, was used.
The working electrode was pretreated by polishing on a microcloth
first with a 1 μm and then with a 1/4 μm diamond paste. The
reference electrode was built from a silver wire inserted into a solution
of 0.01 mol·L−1 [AgNO3] and 0.1 mol·L−1 [nBu4N][B(C6F5)4] in
acetonitrile in a Luggin capillary with a Vycor tip. This Luggin capillary
was inserted into a second Luggin capillary with a Vycor tip filled with
ethynyl Heterocycles 5 and 6. [PdCl2(PPh3)2] (0.5 mol %) and
i
[CuI] (6.0 mol %) were added to degassed Pr2NH (20 mL), and the
mixture was stirred at ambient temperature for 10 min. Then, 3 or 4,
2.1 equiv of trimethylsilylacetylene, and PPh3 (6.0 mol %) were added,
whereby the orange solution changed to dark brown. Within 10 min a
precipitate formed and the reaction mixture was stirred overnight at 50
°C. The crude product was worked up by evaporation of the solvent,
and the residue was dissolved in diethyl ether (50 mL) and filtered
through a pad of Celite. The dark organic phase was evaporated, and
the crude product was purified by sublimation (temperature: 50 °C,
pressure: 5 mbar).
2,5-Bis(trimethylsilyl)furan (5). 2,5-Dibromofuran (6.0 g, 26.5
mmol), 0.5 mol % of [PdCl2(PPh3)2] (196 mg, 0.28 mmol), 6.0 mol %
of [CuI] (637 mg, 3.35 mmol), 6.0 mol % of PPh3 (878 mg, 3.35
mmol), and 2.1 equiv of trimethylsilylacetylene (7.88 mL, 55.8 mmol)
were used. Yield: 2.064 g (7.92 mmol, 30% based on 2,5-
dibromofuran); colorless solid, soluble in hexane. Anal. Calcd for
C14H20OSi2 (260.48 g/mol) [%]: C, 64.55; H, 7.74. Found: C, 64.58;
H, 7.83. Mp: 68 °C. 1H NMR [CDCl3, ppm] δ: 0.24 (s, 18 H,
SiC3H9), 6.53 (s, 2 H, H-3/4). 13C{1H} NMR [CDCl3, ppm] δ: −0.26
(SiC3H9), 93.80 (C4NCCSi), 100.56 (C4NCCSi), 116.43 (C-3/
4), 137.48 (C-2/5). IR data [KBr, cm−1] ν: 1502 (s, νCC), 2157 (s,
νCC), 2899 (w, νs‑CH3), 2963 (m, νas‑CH3), 3120, 3150 (w, νC−H).
HR-ESI-MS [m/z]: calcd for C14H20OSi2 283.0945, found 283.0968
[M + Na]+.
a
0.1 mol·L−1 dichloromethane solution of [nBu4N][B-
(C6F5)4].47,75,76,78−80,109−111 Successive experiments under the same
experimental conditions showed that all formal reduction and
oxidation potentials were reproducible within 5 mV. Experimentally
potentials were referenced against an Ag/Ag+ reference electrode, but
results are presented referenced against ferrocene116,117 (FcH/FcH+
couple = 220 mV vs Ag/Ag+, ΔEp = 61 mV; FcH = Fe(η5-C5H5)2) as
an internal standard as required by IUPAC.81 When decamethylferro-
cene (Fc* = Fe(η5-C5Me5)2) was used as an internal standard, the
experimentally measured potential was converted into E vs FcH/FcH+
(under our conditions the Fc*/Fc*+ couple was at −614 mV vs FcH/
FcH+, ΔEp = 60 mV).118 Data were then manipulated on a Microsoft
Excel worksheet to set the formal redox potentials of the FcH/FcH+
couple to E°′ = 0.00 V. The cyclic voltammograms were taken after
two typical scans and are considered to be steady-state cyclic
voltammograms in which the signal pattern does not differ from the
initial sweep.
General Procedure: Synthesis of 2,5-((η5-C5H5)(η2-dppe)M-
(CC)2-cC4H2E (E = O (11), S (12); M = Fe (a), Ru (b)). Compound
5 or 6 was dissolved in 30 mL of degassed methanol, and 2.3 equiv of
K2CO3 was added in a single portion. The thus obtained reaction
mixture was stirred overnight at ambient temperature. Then 2.2 equiv
of [M(η5-C5H5)(η2-dppe)Cl] (7, M = Fe; 8, M = Ru) and 2.3 equiv of
[NH4]PF6 were added in a single portion, and the reaction mixture
was refluxed for 3.5 h, whereby a color change from black to red (7)
and from yellow to pale green (8) was observed. After cooling the
reaction mixture to ambient temperature it was treated with 2.3 equiv
of KOtBu, and stirring was continued for 30 min. The resulting
precipitate was filtered off and washed with methanol (15 mL) and
hexane (20 mL). The product was purified by crystallization from
tetrahydrofuran at ambient temperature.
IR and UV−vis/NIR Spectroelectrochemistry. The spectroelec-
trochemical measurements of 11a,b and 12a,b (2.0 mM) in anhydrous
dichloromethane containing [nBu4N][B(C6F5)4] (0.1 M) as the
supporting electrolyte were performed in an OTTLE cell with quartz
(UV/vis−NIR) or CaF2 windows (IR)87 at 25 °C. Between the
spectroscopic measurements the applied potentials have been
increased stepwisely using a step height of 25, 50, or 100 mV. At
the end of the measurements the analyte was reduced at −500 mV for
15 min, and an additional spectrum was recorded to prove the
reversibility of the oxidations.
ESR Spectroelectrochemical Measurements. For the electro-
chemical ESR investigations a special flat cell119−121 (dimensions 0.5 ×
8 × 40 mm3) with quartz windows was used. The three-electrode cell
utilized a laminated platinum sheet as a working electrode (diameter of
the opening in the lamination foil 4 mm) and a platinum wire as
reference and counter electrode. The measurements were carried out
on 1.0 mmol·L−1 solutions of the analytes in anhydrous dichloro-
methane containing 0.2 mol·L−1 of [nBu4N][B(C6F5)4] as supporting
electrolyte. The ESR spectra were recorded using an EMX X-Band
ESR spectrometer, whereby the ESR spectrometer was triggered by a
potentiostat, and the triggering was performed using the software
package Potmaster v2 × 43.
Computational Part. Optimization of atomic coordinates was
performed using the PBE functional122 in the Priroda code.123,124 The
basis sets were TZ2P-quality {6s,3p,2d}/(11,6,2) for C and O,
{3s,1p}/(5,1) for H, {10s,6p,2d}/(15,11,2) for S, and SBK-type core
effective potentials with {5s,5p,4d}/(9,9,8) valence parts for Fe and
Ru. Following the finding of Kaupp et al. on the suitable fraction of the
exact exchange term for a reliable description of mixed-valence
complexes,93,102 calculations of all electronic properties reported here
were performed using the B1LYP functional with an exact exchange
term increased to 35%. These calculations were performed using the
Orca suite125−127 with ZORA scalar relativistic correction,128 COSMO
solvation correction for dichloromethane, and full-electron SARC-
def2-TZVP129 basis sets, including {311/1}/(5s1p) for H, {611111/
411/11/1}/(11s6p2d1f) for C and O, {71111111/6111/21/1}/
(14s9p3d1f) for S and P, {8111111111/611111/4111/1}/
2,5-((η5-C5H5)(η2-dppe)FeCC)2-cC4H2O (11a). 2,5-Bis-
(trimethylsilyl)ethynylfuran (5) (114 mg, 0.44 mmol), 2.3 equiv of
K2CO3 (139 mg, 1.0 mmol), 2.2 equiv of 7 (534 mg, 0.96 mmol), 2.3
equiv of [NH4]PF6 (164 mg, 1.0 mmol), and 2.3 equiv of KOtBu (113
mg, 1.0 mmol) were used. Yield: 306 mg (0.26 mmol, 61% based on
5); red solid, soluble in dichloromethane and tetrahydrofuran. Anal.
Calcd for C70H60P4Fe2O (1152.81 g/mol) [%]: C, 72.93; H, 5.25.
1
Found: C, 72.80; H, 5.33. Mp: 170 °C (dec). H NMR [C6D6, ppm]
δ: 1.92 (m, 4 H, CH2-dppe), 2.60 (m, 4 H, CH2-dppe), 4.24 (s, 10 H,
C5H5), 5.75 (bs, 2 H, H-3/4), 6.95 (m, 8 H, C6H5/o-H), 7.01 (m, 4 H,
C6H5/p-H), 7.13 (m, 8 H, C6H5/m-H), 7.16 (m, 4 H, C6H5/p-H),
7.21 (m, 8 H, C6H5/o-H), 8.01 (m, 8 H, C6H5/m-H). 13C{1H} NMR
[C6D6, ppm] δ: 28.51−28.87 (dd, 1/2JC−P ≈ 23 Hz, CH2-dppe), 79.78
2
(s, C5H5), 108.44 (s, C-3/4), 111.80 (s, C-2/5), 117.57 (pt, JC−P
=
25.33 Hz, FeCC), 127.85−127.92 (m, C6H5-m/m′), 128.71, 129.32
(s, C6H5-p/p′), 132.49, 134.06 (pt, 2JC−P ≈ 4 Hz, C6H5-o/o′), 137.52 (s,
1
4
FeCC), 138.91 (dt, JC−P = 24.8 Hz, JC−P = 5.8 Hz, Ci-C6H5),
142.72 (dt, Ci′-C6H5). 31P{1H} NMR [C6D6, ppm] δ: 119.58 (dppe).
IR data [KBr, cm−1] ν: 1093 (m, νC−O), 1432, 1482 (m, δC−H), 1547
(m, νCC), 2049 (s, νCC), 3045 (w, νC−H). HR-ESI-MS [m/z]:
calcd for C70H60P4Fe2O 1152.2292, found 1152.2290 [M]+.
Crystal Data for 11a. Single crystals of 11a were obtained by
diffusion of hexane into a tetrahydrofuran solution containing 11a at
25 °C. C70H60P4Fe2O, Mr = 1152.76 g·mol−1, crystal dimensions 0.35
× 0.25 × 0.25 mm, monoclinic, P21/c, λ = 0.541 84 Å, a = 9.6703(2)
Å, b = 25.2992(5) Å, c = 22.6350(5) Å, β = 94.250(2)°, V = 5522.4(2)
Å3, Z = 4, ρcalcd = 1.386 g·cm−3, μ = 5.661 mm−1, T = 105 K, θ range =
K
DOI: 10.1021/acs.organomet.5b00104
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
3.49−67.50°, reflections collected 20 438, independent 9772, R1
0.0477, wR2 = 0.1215 [I ≥ 2σ(I)].
=
ppm] δ: 98.74 (dppe). IR data [KBr, cm−1] ν: 1433, 1478 (m, δC−H),
1500 (m, νCC), 2053 (s, νCC), 3047 (w, νC−H). HR-ESI-MS [m/
z]: calcd for C70H60P4Ru2S 1260.1452, found 1260.1410 [M]+.
Crystal Data for 12b. Single crystals of 12b were obtained by
diffusion of toluene into a dichloromethane solution containing 12b at
25 °C. C70H60P4Ru2S, Mr = 1259.26 g·mol−1, crystal dimensions 0.38
× 0.30 × 0.20 mm, monoclinic, P21/c, λ = 0.710 73 Å, a = 16.8002(5)
Å, b = 15.7240(4) Å, c = 22.0975(7) Å, β = 104.717(3)°, V =
5645.9(3) Å3, Z = 4, ρcalcd = 1.481 g·cm−3, μ = 0.729 mm−1, T = 293 K,
θ range = 2.85−25.68°, reflections collected 39 855, independent
10 684, R1 = 0.0495, wR2 = 0.1306 [I ≥ 2σ(I)].
2,5-((η5-C5H5)(η2-dppe)RuCC)2-cC4H2O (11b). 2,5-Bis-
(trimethylsilyl)ethynylfuran (5) (107 mg, 0.41 mmol), 2.3 equiv of
K2CO3 (130 mg, 0.94 mmol), 2.2 equiv of 8 (542 mg, 0.90 mmol), 2.3
equiv of [NH4]PF6 (154 mg, 0.94 mmol), and 2.3 equiv of KOtBu
(106 mg, 0.94 mmol) were used. Yield: 258 mg (0.21 mmol, 50%
based on 5); pale green solid, soluble in dichloromethane and
tetrahydrofuran. Anal. Calcd for C70H60P4Ru2O (1243.26 g/mol) [%]:
1
C, 67.62; H, 4.86. Found: C, 67.37; H, 4.86. Mp: 145 °C (dec). H
NMR [C6D6, ppm] δ: 1.96 (m, 4 H, CH2-dppe), 2.61 (m, 4 H, CH2-
dppe), 4.69 (s, 10 H, C5H5), 5.70 (s, 2 H, H-3/4), 6.93−7.00 (m, 16
General Procedure: Synthesis of 15 and 16. 2-(Trimethylsilyl)-
ethynylthiophene (13) or 2-(trimethylsilyl)ethynyl-5-methylthiophene
(14) was dissolved in 30 mL of degassed methanol, and 2.3 equiv of
K2CO3 was added in a single portion. The thus obtained reaction
mixture was stirred overnight at ambient temperature. Then 2.2 equiv
of [Ru(η5-C5H5)(η2-dppe)Cl] (8) and 2.3 equiv of [NH4]PF6 were
added in a single portion, and the reaction mixture was refluxed for 3.5
h. After cooling the reaction mixture to ambient temperature it was
treated with 2.3 equiv of KOtBu, and stirring was continued for 2 h.
The resulting precipitate was filtered off and washed with methanol
(15 mL) and hexane (20 mL). The product was dried in an oil pump
vacuum.
H, C6H5), 7.18−7.24 (m, 16 H, C6H5), 7.99 (m, 8 H, C6H5/m-H).
1/2
13C{1H} NMR [C6D6, ppm] δ: 28.30−28.67 (dd,
J
≈ 22.95
C−P
Hz,CH2-dppe), 83.06 (s, C5H5), 103.36 (s, C-2/5), 108.77 (s, C-3/4),
2
117.65 (pt, JC−P = 25.37 Hz, RuCC), 127.87−127.96 (m,
2
C6H5-m/m′), 128.82, 129.45 (s, C6H5-p/p′), 132.26, 134.26 (pt, JC−P
≈ 5.2 Hz, C6H5-o/o′), 138.23 (dt, JC−P = 27.3 Hz, 4JC−P = 3.6 Hz, Ci-
1
1
4
C6H5), 139.27 (s, RuCC), 142.93 (dt, JC−P = 25.8 Hz, JC−P = 9.0
Hz, Ci′-C6H5). 31P{1H} NMR [C6D6, ppm] δ: 98.76 (dppe). IR data
[KBr, cm−1] ν: 1095 (m, νC−O), 1431, 1482 (m, δC−H), 1549 (m,
νCC), 2062 (s, νCC), 3044 (w, νC−H). HR-ESI-MS [m/z]: calcd
for C70H60P4Ru2O 1244.1680, found 1244.1682 [M]+.
2,5-((η5-C5H5)(η2-dppe)FeCC)2-cC4H2S (12a). 2,5-Bis-
(trimethylsilyl)ethynylthiophene (6) (50 mg, 0.18 mmol), 2.3 equiv
of K2CO3 (57 mg, 0.42 mmol), 2.2 equiv of 7 (220 mg, 0.40 mmol),
2.3 equiv of [NH4]PF6 (68 mg, 0.42 mmol), and 2.3 equiv of KOtBu
(47 mg, 0.24 mmol) were used. Yield: 113 mg (0.09 mmol, 54% based
on 6); red solid, soluble in dichloromethane and tetrahydrofuran. Anal.
Calcd for C70H60P4Fe2S (1168.88 g/mol) [%]: C, 71.93; H, 5.17.
2-((η5-C5H5)(η2-dppe)RuCC)-cC4H3S (15). 2-(Trimethylsilyl)-
ethynylthiophene (13) (60 mg, 0.33 mmol), 1.3 equiv of K2CO3 (60
mg, 0.43 mmol), 1 equiv of 8 (200 mg, 0.33 mmol), 1.3 equiv of
[NH4]PF6 (71 mg, 0.43 mmol), and 1.3 equiv of KOtBu (48 mg, 0.43
mmol) were used. Yield for C37H32P2RuS (671.73 g/mol): 115 mg
(0.17 mmol, 51% based on 13); yellow solid, soluble in dichloro-
1
methane and tetrahydrofuran. Mp: 218 °C. H NMR [C6D6, ppm] δ:
1
Found: C, 71.49; H, 5.23. Mp: 210 °C (dec). H NMR [C6D6, ppm]
1.92−2.03 (m, 2 H, CH2-dppe), 2.49−2.60 (m, 2 H, CH2-dppe), 4.70
(s, 5 H, C5H5), 6.50−6.51 (m, 2 H, H-3/5), 6.63 (dd, 1 H, 3JH−H = 5.0
Hz, 4JH−H = 3.7 Hz, H-4), 6.96−7.00 (m, 6 H, C6H5), 7.19−7.23 (m, 6
δ: 1.88 (m, 4 H, CH2-dppe), 2.52 (m, 4 H, CH2-dppe), 4.21 (s, 10 H,
C5H5), 6.29 (bs, 2 H, H-3/4), 6.96 (m, 8 H, C6H5/o-H), 7.01 (m, 4 H,
C6H5/p-H), 7.16 (m, 8 H, C6H5/m-H), 7.20 (m, 4 H, C6H5/p-H),
H, C6H5), 7.30 (m, 4 H, C6H5/p-H), 7.98 (m, 4 H, C6H5/m-H).
7.26 (m, 8 H, C6H5/o-H), 7.97 (m, 8 H, C6H5/m-H). 13C{1H} NMR
1/2
13C{1H} NMR [C6D6, ppm] δ: 28.25−28.62 (dd,
J
≈ 23
1/2
C−P
[C6D6, ppm] δ: 28.49−28.86 (dd,
J
≈ 23 Hz,CH2-dppe), 79.61
C−P
Hz,CH2-dppe), 83.00 (s, C5H5), 104.30 (s, C-2), 120.22 (s, C-5),
122.25 (pt, 2JC−P = 25.5 Hz, RuCC), 125.39 (s, C-3), 126.19 (s, C-
4), 127.96−128.04 (m, C6H5-o/m), 128.94, 129.60 (s, C6H5-p/p′),
(s, C5H5), 114.31 (s, C-2/5), the signal for FeCC could not be
detected, 124.85 (s, C-3/4), 127.92 (m, C6H5-m/m′), 128.74, 129.32 (s,
C6H5-p/p′), 132.46, 134.01 (pt, 2JC−P ≈ 4.2 Hz, C6H5-o/o′), 138.97 (dt,
1JC−P = 23.7 Hz, 4JC−P = 4.3 Hz, Ci-C6H5), 139.64 (s, FeCC), 142.84
(dt, Ci′-C6H5). 31P{1H} NMR [C6D6, ppm] δ: 119.72 (dppe). IR data
[KBr, cm−1] ν: 1431, 1480 (m, δC−H), 1500 (m, νCC), 2044 (s,
νCC), 3046 (w, νC−H). HR-ESI-MS [m/z]: calcd for C70H60P4Fe2S
1168.2064, found 1168.2019 [M]+.
2
3
132.05 (pt, JC−P ≈ 5.4 Hz, C6H5-o′), 132.56 (pt, JC−P = 1.6 Hz,
2
1
RuCC), 134.29 (pt, JC−P ≈ 5.2 Hz, C6H5-m′), 137.80 (dt, JC−P
=
=
28.0 Hz, 4JC−P = 4.5 Hz, Ci-C6H5), 142.91 (dt, 1JC−P = 26.5 Hz, 4JC−P
9.7 Hz, Ci′-C6H5). 31P{1H} NMR [C6D6, ppm] δ: 98.95 (dppe). IR
data [KBr, cm−1] ν: 1091 (s, νC−C), 1432, 1481 (m, δC−H), 1571,
1582 (w, νCC), 2069 (s, νCC), 2903, 2923 (w, νs‑CH3), 2984 (w,
νas‑CH3), 3048 (w, νC−H). HR-ESI-MS [m/z]: calcd for C37H32P2RuS
672.0747, found 672.0743 [M]+; calcd 695.0645, found 695.0599 [M
+ nNa]+.
Crystal Data for 12a. Single crystals of 12a were obtained by
diffusion of toluene into a dichloromethane solution containing 12a at
25 °C. C70H60P4Fe2S, Mr = 1168.82 g·mol−1, crystal dimensions 0.30 ×
0.20 × 0.09 mm, monoclinic, P21/c, λ = 0.710 73 Å, a = 9.8267(2) Å, b
= 25.4997(5) Å, c = 22.3763(6) Å, β = 92.369(2)°, V = 5602.2(2) Å3,
Z = 4, ρcalcd = 1.386 g·cm−3, μ = 0.714 mm−1, T = 107 K, θ range =
2.93−26.00°, reflections collected 27 913, independent 10 947, R1 =
0.0548, wR2 = 0.1217 [I ≥ 2σ(I)].
2-((η5-C5H5)(η2-dppe)RuCC)-5-methyl-cC4H2S (16). 2-
(Trimethylsilyl)ethynyl-5-methyl thiophene (14) (63 mg, 0.32
mmol), 1.3 equiv of K2CO3 (58 mg, 0.42 mmol), 1 equiv of 8 (194
mg, 0.32 mmol), 1.3 equiv of [NH4]PF6 (68 mg, 0.42 mmol), and 1.3
equiv of KOtBu (47 mg, 0.42 mmol) were used. Yield for
C38H34P2RuS (685.76 g/mol): 110 mg (0.16 mmol, 49% based on
13); yellow solid, soluble in dichloromethane and tetrahydrofuran.
2,5-((η5-C5H5)(η2-dppe)RuCC)2-cC4H2S (12b). 2,5-Bis-
(trimethylsilyl)ethynylthiophene (6) (113 mg, 0.41 mmol), 2.3
equiv of K2CO3 (130 mg, 0.94 mmol), 2.2 equiv of 8 (539 mg, 0.90
mmol), 2.3 equiv of [NH4]PF6 (153 mg, 0.94 mmol), and 2.3 equiv of
KOtBu (105 mg, 0.94 mmol) were used. Yield: 360 mg (0.29 mmol,
70% based on 6); pale green solid, soluble in dichloromethane and
tetrahydrofuran. Anal. Calcd for C70H60P4Ru2S (1259.33 g/mol) [%]:
1
Mp: 206 °C. H NMR [C6D6, ppm] δ: 1.96−2.03 (m, 2 H, CH2-
dppe), 2.07 (d, 3 H, 4JH−H = 0.8 Hz, CH3), 2.55−2.62 (m, 2 H, CH2-
dppe), 4.71 (s, 5 H, C5H5), 6.34 (dd, 1 H, 3JH−H = 3.4 Hz, 4JH−H = 1.1
3
Hz, H-4), 6.39 (d, 1 H, JH−H = 3.4 Hz, H-3), 6.95−6.98 (m, 6 H,
1
C6H5), 7.20−7.23 (m, 6 H, C6H5), 7.31 (m, 4 H, C6H5/p-H), 8.00 (m,
C, 66.76; H, 4.80. Found: C, 66.77; H, 4.90. Mp: 210 °C (dec). H
4 H, C6H5/m-H). 13C{1H} NMR [C6D6, ppm] δ: 15.36 (s, CH3),
NMR [C6D6, ppm] δ: 1.93 (m, 4 H, CH2-dppe), 2.55 (m, 4 H, CH2-
dppe), 4.67 (s, 10 H, C5H5), 6.26 (s, 2 H, H-3/4), 6.95−7.00 (m, 16
H, C6H5), 7.20−7.29 (m, 16 H, C6H5), 7.96 (m, 8 H, C6H5/m-H).
1/2
28.26−28.63 (dd,
J
≈ 23 Hz,CH2-dppe), 82.98 (s, C5H5), 104.78
C−P
2
(s, C-2), 120.09 (pt, JC−P = 25.6 Hz, RuCC), 124.34 (s, C-4),
13C{1H} NMR [C6D6, ppm] δ: 28.20−28.57 (dd, 1/2JC−P ≈ 22.95 Hz,
125.49 (s, C-3), 127.95−128.05 (m, C6H5-o/m), 128.90, 129.58 (s,
3
2
2
C6H5-p/p′), 130.67 (pt, JC−P = 1.6 Hz, RuCC), 132.08 (pt, JC−P
=
CH2-dppe), 82.89 (s, C5H5), 105.52 (s, C-2/5), 117.63 (pt, JC−P
=
5.3 Hz, C6H5-o′), 133.94 (s, C-5), 134.32 (pt, 3JC−P = 5.1 Hz,C6H5-m′),
25.62 Hz, RuCC), 125.27 (s, C-3/4), 127.50−127.75 (m,
1
4
2
137.95 (dt, JC−P = 28.2 Hz, JC−P = 4.6 Hz, Ci-C6H5), 143.00 (dt,
C6H5-m/m′), 128.83, 129.45 (s, C6H5-p/p′), 132.26 (pt, JC−P ≈ 5.0
4
1JC−P = 26.4 Hz, JC−P = 9.6 Hz, Ci′-C6H5). 31P{1H} NMR [C6D6,
2
Hz, C6H5-o), 133.10 (s, RuCC), 134.19 (pt, JC−P ≈ 5.3 Hz,
ppm] δ: 99.03 (dppe). IR data [KBr, cm−1] ν: 1091 (s, νC−C), 1433,
1480 (m, δC−H), 1585 (w, νCC), 2065 (s, νCC), 2848 (w, νs‑CH3),
C6H5-o′), 138.20 (dt, 1JC−P = 28.3 Hz, 4JC−P = 4.4 Hz, Ci-C6H5), 143.00
(dt, 1JC−P = 25.3 Hz, 4JC−P = 8.6 Hz, Ci′-C6H5). 31P{1H} NMR [C6D6,
L
DOI: 10.1021/acs.organomet.5b00104
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
2913 (w, νas‑CH3), 3049 (w, νC−H). HR-ESI-MS [m/z]: calcd for
(19) Lohan, M.; Justaud, F.; Lang, H.; Lapinte, C. Organometallics
C38H34P2RuS 686.0904, found 686.0892 [M]+.
2012, 31, 3565−3574.
(20) Schauer, P. A.; Low, P. J. Eur. J. Inorg. Chem. 2012, 2012, 390−
411.
ASSOCIATED CONTENT
* Supporting Information
■
́
(21) Khairul, W. M.; Fox, M. A.; Schauer, P. A.; Albesa-Jove, D.;
Yufit, D. S.; Howard, J. A. K.; Low, P. J. Inorg. Chim. Acta 2011, 374,
461−471.
(22) Long, E. M.; Brown, N. J.; Man, W. Y.; Fox, M. A.; Yufit, D. S.;
Howard, J. A. K.; Low, P. J. Inorg. Chim. Acta 2012, 380, 358−371.
(23) Bruce, M. I.; Kramarczuk, K. A.; Skelton, B. W.; White, A. H. J.
Organomet. Chem. 2010, 695, 469−473.
(24) Bruce, M. I.; Fox, M. A.; Low, P. J.; Nicholson, B. K.; Parker, C.
R.; Patalinghug, W. C.; Skelton, B. W.; White, A. H. Organometallics
2012, 31, 2639−2657.
(25) Bruce, M. I.; Costuas, K.; Davin, T.; Halet, J.-F.; Kramarczuk, K.
A.; Low, P. J.; Nicholson, B. K.; Perkins, G. J.; Roberts, R. L.; Skelton,
B. W.; Smith, M. E.; White, A. H. Dalton Trans. 2007, 5387−5399.
(26) Bruce, M. I.; Kramarczuk, K. A.; Skelton, B. W.; White, A. H. J.
Organomet. Chem. 2010, 695, 469−473.
(27) Fitzgerald, E. C.; Brown, N. J.; Edge, R.; Helliwell, M.; Roberts,
H. N.; Tuna, F.; Beeby, A.; Collison, D.; Low, P. J.; Whiteley, M. W.
Organometallics 2012, 31, 157−169.
(28) Coat, F.; Paul, F.; Lapinte, C.; Toupet, L.; Costuas, K.; Halet, J.-
F. J. Organomet. Chem. 2003, 683, 368−378.
S
ORTEP diagrams of 11a and 12a,b; UV−vis/NIR spectra and
their deconvolution (11b and 12a,b); IR spectra (spectroelec-
trochemistry) of 11b and 12a,b; data for DFT and spin density.
Crystallographic data of 11a and 12a,b are also available from
the Cambridge Crystallographic Database as file numbers
CCDC 1023501 (11a), 1023360 (12a), and 1023358 (12b).
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00104.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: heinrich.lang@chemie.tu-chemnitz.de. Phone: +49 (0)
371-531-21210. Fax: +49 (0)371-531-21219.
Notes
The authors declare no competing financial interest.
(29) Fitzgerald, E. C.; Ladjarafi, A.; Brown, N. J.; Collison, D.;
Costuas, K.; Edge, R.; Halet, J.-F.; Justaud, F.; Low, P. J.; Meghezzi, H.;
Roisnel, T.; Whiteley, M. W.; Lapinte, C. Organometallics 2011, 30,
4180−4195.
(30) Quardokus, R. C.; Lu, Y.; Wasio, N. A.; Lent, C. S.; Justaud, F.;
Lapinte, C.; Kandel, S. A. J. Am. Chem. Soc. 2012, 134, 1710−1714.
(31) Tanaka, Y.; Shaw-Taberlet, J. A.; Justaud, F.; Cador, O.; Roisnel,
T.; Akita, M.; Hamon, J.-R.; Lapinte, C. Organometallics 2009, 28,
4656−4669.
(32) Fox, M. A.; Le Guennic, B.; Roberts, R. L.; Brue, D. A.; Yufit, D.
S.; Howard, J. A. K.; Manca, G.; Halet, J.-F.; Hartl, F.; Low, P. J. J. Am.
Chem. Soc. 2011, 133, 18433−18446.
ACKNOWLEDGMENTS
■
We are grateful to the Fonds der Chemischen Industrie (FCI)
for generous financial support. M.K. and D.S. thank the FCI for
a Ph.D. fellowship. We thank Dipl.-Chem. J. M. Speck for
fruitful discussions. Dedicated to Prof. Dr. Stefan Spange on the
occasion of his 65th birthday.
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