Phenyl-Bridged Ferrocene/Ruthenium Alkynyl Heterobimetallic Complexes: Syntheses, Characterization, and Electrochemical, Spectroscopic, and Computational Investigation

Article abstract of DOI:10.1002/ejic.202000042

Two isomeric phenyl-bridged ferrocenyl and ruthenium alkynyl complexes 1 and 2 are synthesized and characterized through NMR, elemental analysis, and X-ray single-crystal diffraction. The electronic properties of the complexes are investigated via electro

Full text of DOI:10.1002/ejic.202000042

DOI: 10.1002/ejic.202000042
Full Paper
Heterobimetallic Complexes
Phenyl-Bridged Ferrocene/Ruthenium Alkynyl Heterobimetallic
Complexes: Syntheses, Characterization, and Electrochemical,
Spectroscopic, and Computational Investigation
Ya-Ping Ou,*[a] Aihui Wang,[a] Ande Yuan,[a] Chuang Yin,[a] and Fang Hu[b]
Abstract: Two isomeric phenyl-bridged ferrocenyl and ruth- by the dominant spin–spin density distribution in RuCp*(dppe)
enium alkynyl complexes 1 and 2 are synthesized and charac- terminals. The large potential difference (ΔE1/2) and compropor-
terized through NMR, elemental analysis, and X-ray single-crys- tionation constants (Kc) observed in complex 1 suggest that 1+
tal diffraction. The electronic properties of the complexes are has better chemical stability than 2+. Strong electronic commu-
investigated via electrochemical studies, UV/Vis-NIR and IR nication from 1+ are detected through the observed NIR ab-
spectroelectrochemistry, and theoretical calculations. Cyclic vol- sorption band in 1+ and large Δν(C≡ C) value of 1→1+. The
tammetry and square-wave voltammetry technologies show broad NIR absorption in 1+ is reproduced and assigned to ferro-
two successive redox behaviors from Ru(II) and ferrocenyl cen- cenyl to the Ru(III) center charge transfer transition with major
ters in 1 and 2, respectively. These behaviors are supported contributions from the ꢀ-HOSO→ꢀ-LUSO transition.
Introduction
tronic communication between two redox-active centers. In-
creasing attention has been paid to asymmetrical heterobime-
Conjugative binuclear and polynuclear organometallic com-
plexes have received tremendous interest due to their potential
applications as dye-sensitized solar cells,[1] electrochromic ma-
terials,[2] redox switchable NLOs,[3] and luminescent materials.[4]
Interestingly, electron transfer properties are evaluated on the
basis of mixed-valence systems generated by redox-active bi-
metallic complexes[5] that are similar to the classical inorganic
Creutz–Taube ion.[6] This approach provides a well-defined
method for studying charge delocalization and exploring novel
optical transition properties.
Recently, a large number of conjugated ligand-linked mono-
nuclear and binuclear metal complexes and their redox and
mixed valence properties have been investigated, and consider-
able redox-active end-groups, such as ferrocene,[7] M(dppe)Cp*
or M(dppe)2Cl (M = Ru, Fe, Os, Mo; dppe:1,2-bis[diphenylphos-
tallic mixed-valence models [M1–bridge–M2]n+ [14]
,
which in-
clude two different mixed-valent isomers (valence tautomers),
red n+
ox n+
namely, [M1ox–bridge–M2
]
and [M1red–bridge–M2
] .
These isomers exhibit ground-state energy difference (ΔG0),
which contributes to the energy of optical transition νmax but
˜
not to total reorganization energy (λ).[15] Therefore, considering
this point, Hush's theory is equally applicable to the interva-
lence charge transfer analysis of such asymmetrical systems.[16]
Ferrocene and Ru units with reversible redox couple nature
and good chemical stability often serve as end-groups and are
applied to construct heterobimetallic complexes.[17] As early as
1994, the groups of Sato[17b] and Long[17a] successively reported
ferrocenylacetylene–ruthenium complexes, wherein the Ru
termini are Ru(PP)(η-C5H5) (PP = 2PPh3, dppe [1,2-bis{diphenyl-
phosphino}ethane] or dppf (1,l′-bis[diphenylphosphino]ferro-
phino]ethane; Cp*
=
pentamethylcyclopentadiene),[8] and
cene) and Ru(dppm)2Cl (dppm
=
1,2-bis[diphenylphos-
RuCl(CO)(P ligand) (P ligand = [PiPr3]2 or [PMe3]3),[9] cyclometal-
ated Ru,[10] and polynuclear terminals,[11] and others,[5a,12] are
applied and feature effective stepwise redox processes. In addi-
tion, different carbon-rich bridge cores, including polyene,[9e]
polyacetylene,[11c] and polycyclic aromatic hydrocarbons[8f,13]
have been introduced into mixed-valence systems to tune elec-
phino]methane). Strong electronic coupling is discovered in this
type of asymmetrical redox system through electrochemical
and spectroscopic technologies. Subsequently, several Fe–Ru
systems bridged by rich-carbon chains are constructed by
Long[17c] and Bruce[17e] et al. These complexes have revealed
lower oxidation potential than ferrocenylacetylide and exhib-
ited electronic communication from Fc to RuIII in FeIIRuIII mixed-
valence systems. By contrast, the groups of Jia,[17d] Winter,[17f]
Zhong,[17g] Chen,[17h] and Patra[17i] demonstrated that high
redox potentials occur at the Ru termini rather than at Fc. This
phenomenon indicates that the redox potential of Ru terminals
is regulated on a large scale by the different auxiliary ligands
and the coordination forms of the Ru center. In addition, varia-
ble bridge cores can play a significant role in tuning electronic
interactions between two heteronuclear end-caps.[18] Recently,
[a] College of Chemistry and Material Science, Hengyang Normal University,
Key Laboratory of Functional Metal-Organic Compounds of Hunan
Province, Key Laboratory of Functional Organometallic Materials of Hunan
Province College,
Hengyang, Hunan 421008, P.R. China
E-mail: ouyaping@hynu.edu.cn
[b] Faculty of Materials Science and Chemical Engineering, Ningbo University,
Ningbo 315211, China
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.202000042.
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Scheme 1. Synthetic path for phenyl-bridged asymmetrical dinuclear Fe–Ru complexes 1–2.
explorations on the mechanism of charge transfer in the form mately 86.5 ppm, and the low-field carbon shift of approxi-
through-bond or through-space shape have widely focused on mately 109.6 ppm was attributed to the coordination of the
symmetrical organic[19] or inorganic[20] mixed-valence systems. metal Ru atom with the C≡ C bond. The single peak at about
To date, examples of ferrocenyl and Ru-ethynyl linked by two 80 ppm of 31P NMR further characterized these structures.
bridge isomers (1,4- and 1,3-phenyl) have not been reported.
Thus, this paper reports the syntheses of two isomeric heterobi-
metallic complexes (ferrocenyl-1,4-benzene-C≡ C-RuCp*dppe) 1
The molecular structure of complex 1 (CCDC number:
1977229) was determined through X-ray crystallography to gain
insight into its bonding properties and solid structure and com-
pare with the DFT-optimized structures. A deep red-colored sin-
gle crystal of 1 suitable for single-crystal X-ray diffraction was
grown by layering hexanes on CH2Cl2 solution. The molecular
structure of 1 and its packing viewing along the axis a, b, and
c directions are displayed in Figure 1 and Figure 2. Table S1
presents the details of data collection and refinement. The se-
lected bond lengths and angles of the crystal structure 1 and
the DFT-optimized structures 1n+–2n+(n = 0, 1) obtained using
the (U)B3LYP/6-31G* theory (Lanl2DZ for the Ru, Fe atom) func-
tion are provided in Table 1 and Table S2, respectively. As
shown in Figure 1, the Ru(dppe)Cp* terminal group from the
molecular structure of 1 exhibited pseudo-octahedral geome-
try. The lengths of the two Ru–P bonds were 2.270 and 2.2257 Å
and (ferrocenyl-1,3-benzene-C≡ C-RuCp*dppe)
2 (Cp* = η-
C5Me5; dppe = 1,2-bis[diphenylphosphino]ethane) (Scheme 1).
The bonding properties and electronic coupling of the two
complexes are investigated through X-ray single-crystal diffrac-
tion, (spectro)electrochemistry, and density functional theory
(DFT) calculations.
Results and Discussion
Syntheses, Characterization, and Crystallographic Analysis
The synthetic route toward heterobimetallic 1–2 is outlined in
Scheme 1. First, intermediates 1a and 2a were prepared via the
diazotization reactions of ferrocene and bromoaniline in accord-
ance with a similar procedure in the literature.[21] These inter-
mediates acted as the starting materials for the syntheses of
precursors 1b and 2b. Finally, the TMS-protected compounds
1b and 2b were deprotected using KF and reacted in situ with
(RuCl[dppe]Cp*) under reflux conditions.[22] The target com-
plexes 1–2 were obtained through filtration and characterized
using conventional spectroscopic methods. The characteristic
1H NMR and 13C NMR signals from the RuCp*(dppe) of the two
isomeric complexes were similar to those of previously reported
monoruthenium ethynyl complexes.[23] The 1H NMR spectra ex-
hibited the characteristic CH3 proton signal from η-C5Me5 in
approximately 1.56 ppm and the two –CH2– proton signals of
dppe between 2.04–2.75 ppm. The three types of protons in
ferrocenyl resonated at 4.02–4.26 ppm. The corresponding 13C
NMR spectrum exhibited the intense carbon signals of η-C5Me5
at 10.0 and 92.5 ppm and those of CH2 from dppe at 29.4 ppm.
In addition, two C≡ C carbon peaks were located in approxi-
Figure 1. Molecular structure of complex 1 showing the atom labeling
scheme.
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and those of the Ru ethynyl fragments, namely, Ru(1)–C(37),
C(37)–C(38), and C(38)–C(39), were 1.999, 1.211, and 1.433 Å,
respectively. The bond lengths of the geometry-optimized
structure 1cal were negligibly different from those of the crystal
structure, and an elongation of the triple bond (C[37]–C[38])
and a shortening of the single bond (Ru[1]–C[37] and C[38]–
C[39]) were observed. Given the oxidation of the full-optimized
structure from the neutral state 1–2 to monocations 1+–2+, the
bonds of Ru(1)–C(37) and C(38)–C(39) for 1 and Ru(1)–C(33) and
C(34)–C(26) were gradually shortened, whereas those of C(37)–
C(38) for 1 and the triple bond C(33)–C(34) for 2 were extended.
These results suggested that the first-step oxidation can origi-
nate from the Ru center. In addition, three packing views along
the axis a, b, and c directions all revealed a lamellar crystal
packing model, the end groups (ferrocenyl and RuCp*(dppe))
of molecules overlapped with each other, and displayed head-
to-tail form (Figure 2).
Table 1. Selected bond lengths [Å] and angles [deg] from the crystal structure
of 1 and the full DFT-optimized structures 1n+(n = 0, 1).
1exp
1cal
1+
Ru(1)-C(37)
Ru(1)-P(1, 2)
C(37)-C(38)
C(38)-C(39)
C(39)-C(40)
C(40)-C(41)
C(41)-C(42)
C(42)-C(43)
C(43)-C(44)
C(44)-C(39)
C(42)-C(49)
C(49)-C(48)
C(49)-Fe(1)
1.999
2.270, 2.257
1.211
1.433
1.384
1.380
1.387
1.394
1.382
1.406
1.476
1.423
2.052
2.022
2.340, 2.336
1.235
1.426
1.416
1.390
1.408
1.407
1.391
1.414
1.475
1.438
2.103
1.951
2.378, 2.395
1.247
1.409
1.419
1.385
1.412
1.413
1.389
1.420
1.467
1.440
2.094
P(1)-Ru(1)-P(2)
Ru(1)-C(37)-C(38)
C(37)-C(38)-C(39)
83.21
176.10
174.18
83.62
176.00
179.59
82.30
174.76
178.36
phene-ethynyl ruthenium complex,[8j] in the same experimental
condition, and the E1/2 value is 0.011 V. Based on the above
data, we can conclude that ethynylbenzene-RuCp*dppe has
lower half-wave potential than ferrocenyl site, and the two
redox processes are consequently assigned. This assignment to
redox reaction order was in contrast with the reported heterobi-
metallic [(tppz)(PPh3)2RuC≡ C-C6H4-C≡ CFc](ClO4) complex due to
the higher redox potential from the Ru center than that from
ferrocenyl,[17h] which was contributed to different auxiliary li-
gands from ruthenium center. In addition, additional C≡ C bond
insertion between phenyl and ferrocenyl caused the distance
Figure 2. Molecular packing viewing of complex 1 (a) along axis a, (b) along
axis b, (c) along axis c.
Electrochemical Studies
The electrochemical behaviors were investigated via cyclic vol-
tammetry and square-wave voltammetry methods by using
Figure 3. CVs (a) and SWVs (b) of intermediates 1a–2a and heterobimetallic
complexes 1–2 in CH2Cl2 solution (scan rate: 0.1 V s–1, f = 10 Hz).
nBu4NPF6 (0.1
M) as the supporting electrolyte in dry CH2Cl2
solution to understand further the redox properties of com-
plexes 1a–2a and 1–2. As shown in Figure 3 and Table 2, the
mononuclear ferrocenyl complexes 1a and 2a exhibited well-
defined redox processes with half-wave potential (E1/2) values of
0.522 and 0.530 V, respectively. Subsequently, two continuous
single-electron redox behaviors were found in heterobimetallic
complexes 1–2, and the two separated reversible redox peaks
of complex 1 (0.212 and 0.574 V) and 2 (0.283 and 0.564 V)
corresponded to the Ru(II) and ferrocene end groups, respec-
tively. Ethynylbenzene-RuCp*dppe has been investigated by
many groups.[8h,8i] The electrochemical data exhibited E1/2 val-
ues between 0–0.23 V, depending on measurement condition
and reference. In addition, we have reported the α-benzothio-
Table 2. Electrochemical data for intermediates 1a–2a and heterobinuclear
iron-Ru complexes 1 and 2.[a]
[c]
Complexes
E
1/2(1)[V]
E
1/2(2) [V]
ΔE [mV][b]
Kc
1a
2a
1
–
–
0.522
0.530
0.574
0.564
–
–
362
281
–
–
0.212
0.283
1.32 × 106
5.62 × 104
2
[a] Potential data in volts vs. Fc+/Fc are from single scan cyclic voltammo-
grams recorded at 298 K in 0.1 dichloromethane solution of (Bu4N)(PF6).
M
[b] ΔE = E1/2(2) – E1/2(1) denotes the potential difference between two redox
processes. [c] The comproportionation constants, Kc, were calculated by the
formula Kc = exp(ΔE/25.69) at 298 K.
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between two redox centers to increase and featured that re- band indicated that the intervalence charge transfer transition
ported [(tppz)(PPh3)2RuC≡ C-C6H4-C≡ CFc](ClO4) complex exhibit occurred in 1+ from Fc to the Ru(III) center. This phenomenon
smaller ΔE1/2 (320 mV)[17h] than that of complex 1 (362 mV). was proven by the following TDDFT calculations. Upon continu-
However, for [(tppz)(PPh3)2RuC≡ CFc](ClO4) complex with shorter ous oxidation to the dication 12+, two obvious peaks at 743 and
redox centers distance, which revealed larger ΔE1/2 1396 nm weakened and almost disappeared, and a weak band
(780 mV).[17h] Above comparative results indicated that distance was found at 1055 nm. By contrast, as the meta-phenyl-bridged
dependence of ΔE1/2 value. Compared with those of 1a and 2a, rutheniumethynyl ferrocenyl complex 2 was oxidized to 2+
the redox potentials of ferrocenyl units (Fc/Fc+) of 1 and 2 were through electrolysis, the NIR absorption became barely detecta-
positive because of the electron delocalization effect from the ble, and only the π→π* absorption collapse was observed.
oxidation of the low-potential Ru center from Ru(II) to Ru(III). In Moreover, weak peaks appeared at 410 and 711 nm. Finally, the
addition, the redox potential of Ru center in complex 1 shifted absorption of the monocation 2+ almost remained unchanged
to a lower potential than that in complex 2, accounting for after the potentials were continuously increased. These results
the large difference in the ΔE1/2 and the comproportionation revealed that the isomeric mixed-valence systems had different
constant (Kc) in complex 1. ΔE1/2 and Kc are crucial parameters spectral features and indicated that the mixed-valence system
for evaluating thermodynamic stability. Thus, these results im- 1+ may exhibit better charge delocalization and interaction be-
plied that the mixed-valence complex 1+ had higher chemical tween Ru-Fc end groups than 2+.
stability than 2+. In addition, the ΔE splitting depends on sev-
eral factors,[24] among which the electrostatic repulsion, the sol-
vent and the supporting electrolyte, so ΔE value should not
be taken as a parameter for quantitative measurement of the
electronic coupling. Therefore, the electronic coupling degree
will be explored further by the following spectroelectrochemi-
cal studies and spin density distribution.
Table 3. Electron absorption spectroscopic data of 1n+ and 2n+ (n = 0, 1, 2).
Complex
UV/Vis/NIR absorption λmax [nm] (εmax (dm3 mol–1 cm–1))
1
330 (13440)
1+
12+
2
428 (5599), 743 (3251), 1396 (2042)
382 (5557), 477 (3678), 749 (1497), 1055 (1510)
228 (19108), 317 (13195)
273 (24325), 410 (2196), 711 (487)
709 (352)
2+
22+
UV/Vis-NIR Spectroelectrochemical Studies
UV/Vis-NIR spectroscopy was monitored by potential-controlled
In order to quantify the magnitude of the electronic coupling
electrochemical oxidation within the OTTLE cell in and determine the class in which the MV complex 1+ belongs,
CH2Cl2/10–1
M
NBu4PF6 at 298 K to explore the electronic transi- we analyzed the NIR IVCT absorption (Figure S2) of 1+ by virtue
tion of complexes 1–2 and their various oxidation states. The of Husy's theory.[25] According to NIR absorption with almost
gradual spectral changes accompanying the one- and two-elec- symmetrical peak shape in 1+ and the following TDDFT predic-
tron oxidation of complex 1 are displayed in Figure 4. The UV/ tion, we concluded that the NIR absorption can mainly be as-
Vis-NIR spectra of 2n+ (n = 0, 1, 2) generated through electro- signed to inter-valence charge transfer(IVCT) transition.
lytic experiments are shown in Figure S1. The detailed elec- Therefore, we obtained related parameters by analyzing this
tronic absorption spectroscopic data are listed in Table 3. As band (νmax = 7110 cm–1, Δν1/2 = 4051 cm–1, and εmax
shown in Figure 4 and Figure S1, an evident intense band at 2005
–1 cm–1), and according to the Hush formula,[26] elec-
330 nm for complex 1 and two weak peaks at 228 and 317 nm tronic coupling parameter Hab = 2.06 × 10–2(εmax
=
M
1/2
ν
maxΔν1/2
)
/
for complex 2 exhibited the π→π* transition character.[17i,18] (rab), where rab is the linear distance between metal ruthenium
The gradual disappearance of the strong π→π* absorption and iron atoms in the crystal structure (10.481 Å), and the calcu-
upon gradual one-electron oxidation to singly oxidized states lated Hab was 472 cm–1, which was much lower than νmax/2
for complex 1 was accompanied by the emergence of three (3555 cm–1). Above results indicated that MV complex 1+ may
new absorption bands at 428, 743, and 1396 nm. The broad NIR be classified as a class II MV system.
Figure 4. UV/Vis-NIR spectral changes recorded during the oxidation 1→1+ (a) and 1+→12+ (b) in CH2Cl2/0.1
M nBu4NPF6 at 298 K within an OTTLE cell.
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IR Spectroelectrochemical Studies
Table 4. Infrared ν(C≡ C) vibrational frequencies (cm–1) of the rutheniumethy-
nyl-ferrocene complexes 1–2 in their various oxidation states.
The characteristic C≡ C stretching frequency[27] of complexes 1
and 2 provided powerful spectroscopic probes for investigating
the charge delocalization properties of their respective mixed-
valence states 1+ and 2+. Therefore, IR spectroelectrochemical
experiments were conducted on complexes 1 and 2, and differ-
ent valence states, namely, 1n+ and 2n+ (n = 0, 1, 2), were gener-
Complex
Freq.
n = 0
n = 1
n = 2
1
ν(C≡ C)
2066(m)
1973(w),
1927(s)
1972(w),
1933(m)
1970(w),
1928(w)
1938(w)
2
2065(m)
ated in the electrolytic experiments in CH2Cl2/NBu4PF6 (0.1
M)
at 298 K for IR spectroscopy. The corresponding IR spectra in
the ν(C≡ C) region are displayed in Figure 5 and Figure S3, and
the ν(C≡ C) vibrational frequencies (cm–1) are listed in Table 4.
The ν(C≡ C) bands of neutral complexes 1 and 2 appeared at
2066 and 2065 cm–1, respectively. Upon the first oxidation to
1+ and 2+, the ν(C≡ C) absorptions from neutral molecules were
replaced by two medium intense low-energy absorptions (1+:
1973 and 1927 cm–1; 2+: 1972 and 1933 cm–1) and revealed
essential properties based on the redox reaction of rutheniume-
thynyl units. The calculated maximum ν(C≡ C) shift (Δν[C≡ C])
from neutral molecules to monocations was 139 cm–1 for 1→1+,
which was slightly larger than that calculated for 2→2+
(132 cm–1). The comparison of the Δν(C≡ C) data similarly re-
vealed that the singly oxidized state 1+ (1,4-phenyl-bridged ru-
theniumethynyl–ferrocenyl) had stronger charge delocalization
properties than 2+ because the 1,4-benzene bridge in 1+ can
provide effective electron transfer path.[28] Slight shifts in
ν(C≡ C) were found after full oxidization to dications 12+ and
orbital energies of neutral compounds [1] and [2] are shown in
Figure 6. The selected frontier molecular orbitals from [1]+–[2]+
and [1]2+–[2]2+ are displayed in Figures S4–S5. The spin density
distribution with the Mulliken segmental analysis of [1]+ and
[2]+ is depicted in Figure 7. The molecular orbitals LUMO+1,
LUMO, HOMO, and HOMO-1 of neutral [1] and [2] featured simi-
lar energy values but different orbital compositions. The HO-
MOs from [1] were delocalized over the entire Ru–Fc system
with large contributions from the Ru–ethynyl terminal, in which
the contribution from ferrocenyl to HOMOs was almost nonex-
istent for [2]. A large difference can be observed in the LUMO+1
orbital.
2
2+, and only two weak absorptions at 1970 and 1928 cm–1 and
a broad peak at 1928 cm–1 for 22+ that can originate from the
minor contributions of the second-step oxidation of the iron
center were found for 12+
.
Figure 6. Selected frontier molecular orbital energies and profiles for com-
pounds [1]-[2]. Contour values: 0.02 (e/bohr3)1/2
.
Figure 5. IR spectral absorptions of the Ru(C≡ C) stretching vibration for 1n+
(n = 0, 1, 2) generated by electrolysis experiments in CH2Cl2/NBu4PF6 (0.1
M)
at 298 K (neutral state: black line; monocation: red line; dication: blue line).
Figure 7. Spin-density distributions in [1]+ and [2]+ with the corresponding
compositions (ferrocenyl/phenyl/C≡ C/RuCp*dppe). Contour values: 0.02 (e/
DFT and TDDFT Calculations
bohr3)1/2
.
Complexes [1]n+ and [2]n+ (n = 0, 1, 2) were optimized using
DFT calculations on the basis of (U)B3LYP/6-31G* theory
The DFT-calculated spin densities of [1]+ and [2]+ showed
(Lanl2DZ for the Ru and Fe atom) to further understand the similar Mulliken composition difference with the HOMOs of [1]
electronic properties of heterobimetallic Ru–Fe structures. The and [2]. The spin density distribution from [1]+ was delocalized
solvent effects were considered on the basis of the conductor over the rutheniumethynyl–phenyl–Fc skeleton with predomi-
polarizable continuum model (CPCM) in CH2Cl2 (see details in nant contributions from the RuCp*(dppe) moiety (56 %) and a
the Experimental Section). The representative frontier molecular minor contribution from the Fc metal center (5 %). For [2]+, a
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Figure 8. Isosurface plots of molecular orbitals involved in the major electronic excitations for [1] and [1]+. Contour values: 0.02 (e/bohr3)1/2
.
Table 5. Major electronic excitations for neutral molecules [1]-[2] and singly oxidized species [1]+-[2]+ determined by TD-DFT methods.
Complex
Wavelength [nm]
Osc. str (f)
Major contributions
Main assignment
[1]
383
364
1298
926
778
727
364
359
1018
711
0.0936
0.0319
0.0854
0.0705
0.0766
0.1971
0.0211
0.0834
0.0160
0.1431
HOMO-LUMO+1 (52 %)
HOMO-LUMO+4 (44 %)
π-π*
ILCT
MM′CT
MLCT
MLCT
MLCT/ILCT
ILCT
π-π*
MM′CT
MLCT/ILCT
[1]+
ꢀ-HOSO→ꢀ-LUSO (78 %)
α-HOSO-1→α-LUSO+9 (47 %)
α-HOSO-1→α-LUSO+9 (32 %)
ꢀ-HOSO-4→ꢀ-LUSO (52 %)
HOMO-1-LUMO+1 (40 %)
HOMO-LUMO+3 (40 %)
[2]
[2]+
ꢀ-HOSO→ꢀ-LUSO (95 %)
ꢀ-HOSO-3→ꢀ-LUSO (62 %)
large spin density residue was present on the RuCp*(dppe) cen- of [1]+, such that that the NIR transition signal cannot be de-
ter (62 %) and received almost no contribution from the Fc unit. tected in the spectroelectrochemical experiments. In addition,
These results indicated that the first one-electron oxidation 700–800 nm absorption bands were considered as MLCT/ILCT,
processes (complex 1: E1/2(1) = 0.212 V; complex 2: E1/2(1) = which had major contributions from the α-HOSO-1→α-LUSO+9
0.283 V) were associated with Ru-dominated initial oxidation and the ꢀ-HOSO-4→ꢀ-LUSO transitions for [1]+ and the
steps and further confirmed the greater electronic communica- ꢀ-HOSO-3→ꢀ-LUSO for [2]+.
tion between the Ru–iron center in [1]+ than in [2]+, which was
previously suggested by the electrochemical and spectroelec-
trochemical results. In addition, 38 %–39 % of the contribution
Conclusion
to the spin density arising from the bridge linker (benzene
ethynyl) revealed that bridge ligands partially participated in
redox processes.
In this report, we described the syntheses and structural charac-
terization of the isomeric phenyl-bridged rutheniumethynyl fer-
rocenyl heterobimetallic complexes 1 and 2 and illustrated their
TDDFT calculations for neutral [1]–[2] and singly oxidized electronic and spectral properties in different redox states. Elec-
states [1]+ and [2]+ were performed on the above DFT-opti- trochemical studies indicated that the two complexes exhibited
mized structure on the basis of CPCM in CH2Cl2 to understand two well-separated redox processes based on Ru(II)/Ru(III) and
their spectral absorptions. Selected frontier orbitals that were Fc/Fc+ couples. The first redox behavior from the Ru(II)/Ru(III)
involved in major transitions for [1]n+ and [2]n+ (n = 0, 1) are couple was confirmed by the spin density distribution calcula-
shown in Figure 8 and Figure S6, and the corresponding param- tions of [1]+ and [2]+. The large ΔE1/2 and Kc values indicated
eters are listed in Table 5. First, for neutral [1] and [2], the ab- that the chemical stability of 1+ was better than that of 2+. The
sorptions in the UV region can be assigned mainly to π–π* broad NIR absorption observed in 1+ was assigned to Fc to Ru
transitions and intraligand (IL)-CT by TDDFT predictions (Table 4 (MM′CT) by TDDFT calculations. 2+ almost lacked absorption in
and Figure 8 and Figure S6). For [1]+, the NIR absorption at the NIR region. The comparison of Δν(C≡ C) generated by 1→1+
1396 nm from UV–Vis–NIR spectroscopy was well reproduced and 2→2+ revealed that considerable charge delocalization can
by the TDDFT calculations. As shown in Table 4, the calculated occur in 1+. Thus, the theoretical and experimental results indi-
NIR electronic transitions were found at 1298 nm, which was cated that the 1,4-phenyl-bridged rutheniumethynyl and ferro-
close to the experimental results, and dominated by the contri- cenyl complex 1 featured better electronic communication be-
bution from ꢀ-HOSO→ꢀ-LUSO (78 %) with 0.0854 of Osc. str (f) tween the heterobinuclear centers than the metaisomer 2. This
that embodied the evident ferrocenyl→Ru unit charge transfer work will provide a novel path for the design of complicated
(MM′CT) character. TDDFT revealed that [2]+ displayed MM′CT, mixed-valence systems with RuCp*(dppe) and ferrocenyl as end
but its intensity was weaker with 0.0160 of Osc. str (f) than that groups though bridge unit modification.
Eur. J. Inorg. Chem. 2020, 859–867
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nitrogen atmosphere for 24 h. The crude product was collected by
filtration, washed with methanol and hexane, The solid was dis-
solved in dichloromethane and precipitated from slow diffusion
with hexane. The solid was filtered and dried to give 1 as a red
Experimental Section
General Materials: All manipulations were carried out at room tem-
perature under a nitrogen atmosphere using standard Schlenk tech-
niques, unless otherwise stated. Solvents were predried, distilled,
and degassed prior to use, except those for spectroscopic measure-
ments, which were of spectroscopic grade. The reagents 4-bro-
moaniline, 3-bromoaniline, ferrocene, trimethylsilylacetylene, KF,
1
brown powder (140 mg, 52 %). H NMR (500 MHz, CDCl3): δ = 1.56
(s, 15H, CH3), 2.06 (br, 2H), 2.70 (br, 2H), 4.03 (s, 5H, Fc), 4.23 (t,
J(HH) = 5.0 Hz, 2H, Fc), 4.53 (t, J(HH) = 5.0 Hz, 2H, Fc), 6.73 (d,
J(HH) = 10.0 Hz, 2H), 7.16 (d, J(HH) = 10.0 Hz, 2H), 7.20- 7.37 (m,
16H, dppe-Ph), 7.78–7.81 (m, 4H, dppe-Ph). 13C NMR (125 MHz,
CDCl3): δ = –10.04 (CH3), 29.42 (CH2), 66.03 (Fc), 68.28 (Fc), 69.40
(Fc), 86.78 (Fc-C≡ C), 92.49 (C5H5), 109.84 (Ru-C≡ C), 125.45, 127.12,
127.16, 127.19, 127.33, 127.37, 127.40, 128.79, 129.11, 130.04,
132.71, 133.13, 133.18, 133.22, 133.71, 133.75, 133.79, 136.75,
137.13, 138.73, 138.99. 31P NMR (200 MHz, CDCl3): δ = 80.84. FTIR
(KBr, υ, cm–1 ): 2067 (C≡ C). Elemental analysis calcd. (%) for
Pd(PPh3)4 and
NaNO2
were
commercially
available.
RuCp*(dppe)Cl[29] and Intermediates 1a, 2a, 1b and 2b were pre-
pared by similar reported literature.[21,22]
General Synthesis of Intermediates 1a and 2a
1a: Under ice bath (5 °C), to a H2O (10 mL) and concentrated HCl
(10 mL) solution of 4-bromoaniline (2.22 g, 0.13 mol), portionwise
addition of H2O (10 mL) solution of NaNO2 (0.89 g, 0.13 mol), after
the reaction mixture was stirred 1 h, urea (0.3 g, 0.005 mol) was
added to the system to remove the rest HNO2, and generated a
yellow diazotized salt solution which was kept under 0 °C, to which
added gradually (about 1–2 h) a diethyl ether solution (50 mL) of
containing ferrocene (1.20 g, 6.45 mmol), C16H33(CH3)3NBr (0.12 g,
0.33 mmol). The reaction mixture was stirred at room temperature
for 0.5 h. After completion of the reaction, the diethyl ether was
removed, and the crude product was purified by steam distillation
to remove the rest ferrocene and recrystallized by petroleum ether.
C57H61FeP2Ru: C 70.95, H 6.37; found C 70.89, H 6.40.
Preparation for 2: The procedure of 2 was similar to that for 1.
Cp*(dppe)RuCl (227 mg, 0.34 mmol), 2b (100 mg, 0.28 mmol), KF
(129 mg, 2.21 mmol), CH3OH (20 mL), THF (4 mL). Yield: 167 mg
(62 %) of a yellow solid. 1H NMR (500 MHz, CDCl3): δ = 1.57 (s, 15H,
CH3), 2.06–2.09 (m, 2H), 2.68–2.73 (m, 2H), 4.03 (s, 5H, Fc), 4.25 (t,
J(HH) = 5.0 Hz, 2H, Fc), 4.52 (t, J(HH) = 5.0 Hz, 2H, Fc), 6.68 (d,
J(HH) = 5.0 Hz, 1H), 6.89 (s, 1H), 6.97 (t, J(HH) = 5.0 Hz, 1H), 7.03 (d,
J(HH) = 5.0 Hz, 1H), 7.21–7.40 (m, 16H, dppe-Ph), 7.80–7.84 (m, 4H,
dppe-Ph). 13C NMR (125 MHz, CDCl3): δ = –10.04 (CH3), 29.42 (CH2),
66.56 (Fc), 68.34 (Fc), 69.48 (Fc), 86.53 (Fc-C≡ C), 92.49 (C5H5), 109.63
(Ru-C≡ C), 120.84, 127.13, 127.17, 127.21, 127.35, 127.38, 127.42,
127.90, 128.44, 128.83, 131.02, 133.13, 133.17, 133.22, 133.77,
133.81, 133.85, 136.76, 137.14, 137.80, 138.76, 139.03. 31P NMR
(200 MHz, CDCl3): δ = 81.00. FTIR (KBr, υ, cm–1 ): 2064 (C≡ C). Elemen-
tal analysis calcd. (%) for C57H61FeP2Ru: C 70.95, H 6.37; found C
70.98, H 6.36.
1
Yield 1.0 g (45 %) of orange red solid. H NMR (500 MHz, CDCl3): δ
(ppm) 4.04 (5H, s, Fc-H), 4.33 (2H, t, J(HH) = 5.0 Hz, Fc-H), 4.61 (2H,
t, J(HH) = 5.0 Hz, Fc-H), 7.34 (2H, d, J(HH) = 10.0 Hz), 7.40 (2H, d,
J(HH) = 10.0 Hz).
2a: The procedure of 2a was similar to that for 1a. 3-bromoanilne
(2.22 g, 0.13 mol), NaNO2 (0.89 g, 0.13 mol), ferrocene (1.2 g,
6.45 mmol), C16H33(CH3)3NBr (0.12 g, 0.33 mmol), urea (0.3 g,
5 mmol), concentrated HCl (10 mL). Yield: 1.5 g (68 %) of an orange
red solid. 1H NMR (500 MHz, CDCl3): δ (ppm) 4.06 (5H, s, Fc-H), 4.33
(2H, t, J(HH) = 5.0 Hz, Fc-H), 4.62 (2H, t, J(HH) = 5.0 Hz, Fc-H), 7.15
(1H, t, J(HH) = 10.0 Hz), 7.31 (1H, d, J(HH) = 10.0 Hz), 7.39 (1H, t,
J(HH) = 10.0 Hz), 7.59 (1H, t, J(HH) = 5.0 Hz).
Physical Measurements: 1H, 13C, and 31P NMR spectra were col-
lected on a Varian Mercury Plus 500 spectrometer (500 MHz). 1H
and 13C NMR chemical shifts are relative to TMS, and 31P NMR
chemical shifts to 85 % H3PO4. Elemental analyses (C, H, N) were
performed with a Vario ElIII Chnso instrument. Solid IR spectra was
recorded on a Nicolet Avatar spectrometer from Nujol mulls that
were suspended between KBr discs. Electrochemical measurements
were conducted with a CHI 660C potentiostat. A single-compart-
ment electrochemical cell contained a pre-polished platinum disk
working electrode (d = 0.5 mm), a platinum wire counter electrode,
and a silver wire pseudo-reference electrode. Spectroelectrochemi-
cal experiments at room temperature were performed with an air-
tight optically transparent thin-layer electrochemical (OTTLE) cell
(optical path length of ca. 200 μm) equipped with a Pt minigrid
working electrode and CaF2 windows.[30] The cell was positioned in
the sample compartment of a Bruker Tensor FT-IR spectrometer
(1 cm–1 spectral resolution, 8 scans) or a Shimadzu UV-3600 UV/Vis-
NIR spectrophotometer. The controlled-potential electrolyses were
carried out with a CHI 660C potentiostat. Dry CH2Cl2 degassed by
bubbling with argon for 10 min was used to prepare solutions of
General Synthesis of Intermediates 1b and 2b
1b: Trimethylsilylacetylene (0.70 mL, 5.00 mmol) was added to a
stirred solution of 1a (341 mg, 1.00 mmol), CuI (19 mg, 0.10 mmol),
and [Pd(PPh3)4] (116 mg, 0.10 mmol) in Et3N (10 mL) and THF
(20 mL) under an argon atmosphere, the mixture was heated to
reflux for 24 h. The solution was then cooled down and filtered
through a bed of Celite. The filtrate was evaporated under reduced
pressure and purified by silica gel column chromatography (hexane)
to give 1a of a red brown solid. Yield: 180 mg, 50 %. 1H NMR
(500 MHz, CDCl3): δ = 0.27 (s, 9H, SiMe3), 4.02 (s, 5H, Fc-H), 4.35 (2H,
t, J(HH) = 5.0 Hz, Fc-H), 4.65 (2H, t, J(HH) = 5.0 Hz, Fc-H), 7.31–7.36
(br, 2H, Ph-H), 7.40 (s, 2H, Ph-H).
2b: The procedure of 2b was similar to that for 1b. 2a (341 mg,
1.00 mmol), CuI (19 mg, 0.10 mmol), Pd(PPh3)4 (116 mg, 0.10 mmol),
triethylamine (10 mL) and THF (20 mL), (trimethylsilyl)acetylene
(0.70 mL, 5.00 mmol). Yield: 200 mg (56 %) of a red brown solid. 1H
NMR (500 MHz, CDCl3): δ = 0.26 (s, 9H, SiMe3), 4.04 (5H, s, Fc-H),
4.30 (2H, t, J(HH) = 5.0 Hz, Fc-H), 4.63 (2H, t, J(HH) = 5.0 Hz, Fc-H),
7.14 (1H, d, J(HH) = 5.0 Hz), 7.29 (1H, d, J(HH) = 5.0 Hz), 7.41 (1H,
s), 7.60 (1H, d, J(HH) = 5.0 Hz).
10–3
M M Bu4NPF6 (dry, recrystallized) added as
complexes and 10–1
the supporting electrolyte.
Computational Details: Density functional theory (DFT) calcula-
tions were performed using the Gaussian09 software[31] at the
B3LYP/6-31G* levels of theory. The basis set employed was 6-31G*
(Lanl2DZ for Ru, Fe atom). Geometry optimization was performed
without any symmetry constraints. Electronic transitions were calcu-
lated by the time-dependent DFT (TD-DFT) method. The solvation
effects in dichloromethane are included for a part of the calcula-
General Synthesis of Heterobimetallic Rutheniumethynyl Ferro-
cenyl Complexes.[8f]
Preparation for 1:
A
solution of Cp*(dppe)RuCl (227 mg,
0.34 mmol), 1b (100 mg, 0.28 mmol), and KF (129 mg, 2.21 mmol) tions with the conductor-like polarizable continuum model
in 20 mL of CH3OH and 4 mL of THF was heated to reflux under
(CPCM).[32]
Eur. J. Inorg. Chem. 2020, 859–867
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Full Paper
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Acknowledgments
The authors acknowledge the financial support from the
National Natural Science Foundation of China (21602049), the
Natural Science Foundation of Hunan Province, China (Nos.
2017JJ3004), the Opening Subjects of Hunan Province Key Lab-
oratory of Functional Metal-Organic Compounds (No.
MO19K04), the Aid Programs for Technology Innovative Team
and Key Discipline in Education Department of Hunan Province,
and the Support Plan for Talents in Hengyang Normal Univer-
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Keywords: Iron · Ruthenium · Ferrocenes ·
Electrochemistry · Mixed-valent compounds
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