Synthesis, crystal structures, and redox behavior of some pentamethylcyclopentadienyl arene ruthenium salts

Article abstract of DOI:10.1016/j.jorganchem.2013.07.023

Hexafluorophosphates of (η⁵-pentamethylcyclopentadienyl)(h6- arene)ruthenium cations (arene = 1,3,5-triethylbenzene, 1,3,5- tris(trimethylsilylmethyl)benzene, and pentamethylbenzene) have been obtained from the reaction of [Ru(η⁵-C

Full text of DOI:10.1016/j.jorganchem.2013.07.023

Journal of Organometallic Chemistry xxx (2013) 1e7
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis, crystal structures, and redox behavior of some
pentamethylcyclopentadienyl arene ruthenium salts
,
,
*
Swagat K. Mohapatra a b, Alexander Romanovc, Tatiana V. Timofeeva c, Seth R. Marder a
,
,
Stephen Barlowa
*
a School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, GA 30332-0400, USA
b KIIT University, Bhubaneswar, Orissa 751024, India
c Department of Chemistry, New Mexico Highlands University, Las Vegas, NM 87701, USA
a r t i c l e i n f o
a b s t r a c t
Hexafluorophosphates of (h h
5-pentamethylcyclopentadienyl)(
6-arene)ruthenium cations (arene ¼ 1,3,5-
Article history:
Received 18 March 2013
Received in revised form
9 July 2013
triethylbenzene, 1,3,5-tris(trimethylsilylmethyl)benzene, and pentamethylbenzene) have been obtained
from the reaction of ½Ruðh5ꢀC5Me5ÞðNCCH3Þ3ꢁþPF6 with the appropriate arene; their X-ray single-
ꢀ
crystal structures have been determined and compared to those of related compounds. The electro-
Accepted 9 July 2013
chemistry of these cations is compared to that of other [Ru(
h
5-C5Me5)(
h
6-arene)]þ and [Fe(h5
-
C5Me5)(
h
6-arene)]þ species; the 1,3,5-C6H3(CH2SiMe3)3 derivative is reduced at the most cathodic po-
Keywords:
Ruthenium
Pentamethylcyclopentadienyl
Arene
Crystal structure
Redox potential
tential (Epc ¼ ꢀ2.96 V vs. ferrocenium/ferrocene). Reduction with sodium amalgam gives a dimer, [Ru(h5
-
C5Me5)]2[
m
-
h5
:
h
5-(arene)2], in the case of arene ¼ 1,3,5-C6H3Et3, analogous to what has been previously
reported for the mesitylene complex, and a mixture of products when arene ¼ C6HMe5. The 1,3,5-
C6H3(CH2SiMe3)3 derivative is comparatively unreactive to sodium amalgam, although long reaction
times lead to desilylation and formation of [Ru(
epotassium alloy, on the other hand, leads to formation of a 18-electron anionic Ru0 complex: Kþ[Ru(h5
C5Me5){
4-1,3,5-C6H3(CH2SiMe3)3}]e.
h m- :h
5-C5Me5)]2[ h5 5-C6H3Me3C6H3Me3]. Use of sodium
-
h
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
dopants of this general class we synthesized hexfluorophosphate
salts of the new 18-electron cations [RuCp*(
6-1,3,5-C6H3(CH2Si
h
We have recently found that the “2 ꢂ 18-electron” dimers formed
Me3)3)]þ, 3D, and [RuCp*( 6-C6HMe5)]þ, 4D. Here we report their
h
by certain 19-electron sandwich compounds, namely [RhCp2]2 [1,2],
synthesis and crystal structures, along with that of ½RuCp ꢃ ðh6
ꢀ
[FeCp*(C6H6)]2 (Cp* ¼
h
5-C5Me5) [3], and [RuCp*(1,3,5-C6H3R3)]2
1; 3; 5 ꢀ C6H3Et3ÞꢁþPF6ꢀ; 2þPF6ꢀ. We also compare the electro-
(R ¼ Me, Et; 12 and 22 respectively) [4,5], can act as highly reducing,
yet air-stable, reductants, capable of n-doping electron-transport
materials relevant to organic electronics, including even compounds
with relatively low-electron affinities such as copper phthalocyanine
and 6,13-bis(tri(isopropyl)silylethynyl)pentacene [5e9]. These di-
mers are synthesized by electrochemical or alkali-metal one-electron
reduction of the corresponding monomeric 18-electron cations.
However, this reaction is by no means general; for example, while the
above-mentioned iron and ruthenium species form dimers,
Feðh5ꢀC5R5Þðh6ꢀC6R06Þ (R ¼ H, Me; R0 ¼ Me, Et) are stable (in the
absence of air) as neutral 19-electron monomers [3,10], and one-
chemistry and alkali-metal reductions of 2De4D and [RuCp*(h6
-
1,3,5-C6H3Me3)]þ, 1D
.
2. Experimental section
2.1. General details
All operations were performed under an atmosphere of nitro-
gen, either using standard Schlenk techniques or in a glove box.
Tetrahydrofuran (THF) was distilled from sodium benzophenone,
while toluene was dried over CaH2. Benzene-d6 was dried over Nae
electron reduction of [Ru(
h
5-C5R5)(
h
6-C6H6)]þ (R ¼ H, Me) leads to
K
and stored in the glove þbox. ½RuCp*ðNCMeÞ3ꢁþPF6 [11],
ꢀ
formation of the 18-electron monomeric Ru(h h
5-C5R5)( 5-C6H7)
þ
ꢀ
½RuCp*ðh6ꢀ1; 3; 5 ꢀ C6H3Me3Þꢁ PF6ꢀ; 1 PF6 [12], ½RuCp*ꢀðh6
ꢀ
rather than to dimerization [4]. In an effort to develop new dimeric
1; 3; 5 ꢀ C6H3Et3ÞꢁþPF6ꢀ; 2 PF6 [5], ½FeCp*ðh6ꢀC6H6Þꢁ PF6 [3],
þ
ꢀ
þ
þ
ꢀ
and ½FeCp*ðh6ꢀ1; 3; 5 ꢀ C6H3Me3Þꢁ PF6 [13] were synthesized as
previously described. All other reagents were purchased from Alfae
Aesar and used without further purification. 1H and 13C{1H} NMR
* Corresponding authors.
E-mail address: stephen.barlow@chemistry.gatech.edu (S. Barlow).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.07.023
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S.K. Mohapatra et al. / Journal of Organometallic Chemistry xxx (2013) 1e7
spectra were recorded on a Bruker AMX 400 MHz spectrometer. 1H
and 13C chemical shifts were referenced to tetramethylsilane using
the residual proton signal of the solvent and the carbon resonances
of the deuterated solvent, respectively. Unless stated otherwise,
carbon signals were observed as singlets. CHN analyses were car-
ried out by Atlantic Microlabs (Norcross, GA) using a LECO 932
CHNS elemental analyzer, while the alkali metal content of Kþ3e
was determined by ALS Environmental (Tucson, AZ) using ICP-AES.
Mass spectra were measured on an Applied Biosystems 4700 Pro-
teomics Analyzer. The electrochemical data were acquired using
cyclic voltammetry in 0.1 M nBu4NPF6 in dry THF under nitrogen,
using a CH Instruments 620D potentiostat, a glassy carbon working
electrode, a platinum wire auxiliary electrode, and, as a pseudo-
reference electrode, a silver wire anodized in 1 M aqueous potas-
sium chloride solution, at a scan rate of 50 mV sꢀ1. Ferrocene was
used as an internal reference.
(s, 1H, arene CH), 2.18 (s, 6H, arene CH3), 2.15 (s, 3H, arene 3-CH3),
2.12 (s, 6H, arene CH3), 1.78 (s, 15H, Cp* CH3). 13C{1H} NMR
(100 MHz, acetone-d6): d 99.8 (arene quat.), 99.7 (arene quat.), 99.4
(arene quat.), 93.5 (Cp* quat.), 91.9 (arene CH), 17.7 (arene CH3), 14.5
(arene CH3), 14.0 (arene CH3), 9.1 (Cp* CH3). Anal. calcd for
C
21H31F6PRu: C 47.63, H 5.90. Found: C 47.77, H 6.04. ESI-MS: m/z
385 (cation Mþ).
2.5. Synthesis of Kþ3e
NaeK alloy (3:1,ꢀ0.50 g, CAUTION, pyrophoric in air) was added to
a solution of 3þPF6 (0.47 g, 0.65 mmol) in THF; the reaction was
stirred for 1 h at room temperature, after which time the THF so-
lution was decanted off and evaporated under reduced pressure.
The crude solid was extracted in toluene, filtered through Celite,
and evaporated to give a red-orange solid, which was washed with
cold pentane (3 mL) and dried under high vacuum (0.18 g,
0.29 mmol, 45%; CAUTION, pyrophoric in air). 1H NMR (400 MHz,
2.2. Synthesis of 1,3,5-tris(trimethylsilylmethyl)benzene
benzene-d6):
d 4.54 (s, 1H, Hb), 4.33 (s, 1H, Hc), 2.46 (d, JH-
This compound was obtained by a modification of a literature
procedure [14]. nBuLi (75 mL of a 2.87 M solution in hexanes,
215 mmol) and TMEDA (16 mL, 110 mmol) were stirred together at
room temperature for 10 min and then added dropwise to mesi-
tylene (5 mL, 35 mmol) cooled to ꢀ78 ꢄC. After 1 h at ꢀ78 ꢄC, the
reaction vessel was brought slowly to room temperature, stirred for
another 24 h, and cooled again to ꢀ78 ꢄC; chlorotrimethylsilane
(22.8 mL, 180 mmol) was then added dropwise (CAUTION: vigorous
reaction takes place) via a dropping funnel. The reaction mixture
was stirred at room temperature overnight and filtered through
Celite; the residue was washed with hexane and the washings were
combined with the filtrate. This combined filtrate was washed with
5% aq. NaHCO3, then 5% aq. HCl, and finally with 5% aq. NaHCO3
again. The organic layer was dried over Na2SO4. The volatiles were
removed by rotary evaporation and the resulting light yellow liquid
was fractionally distilled (175e180 ꢄC, 0.07 mbar) to give the
¼ 13.6 Hz, 1H, Hg), 1.86 (s, 15H, Cp*), 1.36 (m, 3H, overlap of Ha, Hd,
H
Hf), 1.09 (d, JH-H ¼ 16 Hz, 1H, He), 0.80 (m, 2H, Hh), 0.38 (s, 9H, Hj),
0.31 (s, 9H, Hk), 0.05 (s, 9H, Hi). 13C{1H} NMR (100 MHz, benzene-
d6):
d 143.9 (arene quat. Cl), 126.4 (arene CH, Cc), 83.5 (arene quat.
Cm), 79.9 (Cp* quat.), 77.9 (arene CH, Cb), 55.6 (arene CH, Ca), 53.2
(arene quat., Cn), 28.8 (CH2, Ch), 25.3 (CH2, Cfg), 23.5 (CH2, CHde),
12.5 (Cp* CH3), 0.7 (TMSk), e0.1 (TMSi), e0.5 (TMSj). Alkali metal
analysis for C28H51KRuSi3: K 6.39. Found: K 6.03 (Na < 0.9%). The
resonances were mostly assigned using HSQC and NOE experi-
ments; the quaternary 13C arene resonances, however, were
assigned on the basis of chemical shifts with reference to values
reported for other h
4-arenes in the literature and to the arene CH
assignments. The labels used in the assignment of 1H and 13C res-
onances are shown below in Fig. 6.
2.6. Single crystal X-ray structure determinations
product (2.50 g, 21%). 1H NMR (400 MHz, chloroform-d):
3H, arene CH),1.97 (s, 6H, CH2), 0.00 (s, 27H, TMS CH3). 13C{1H} NMR
d 6.38 (s,
Crystallographic data and important refinement parameters for
crystals of the hexfluorophosphates of 2D, 3D, and 4D are pre-
sented in Table 1. The X-ray diffraction experiments were carried
(100 MHz, chloroform-d):
d
139.9 (arene quat.), 123.7 (arene CH),
26.8 (CH2), e1.6 (TMS CH3).
out with a Bruker Apex II CCD area detector diffractometer, using
2.3. Synthesis of 3þPF6
ꢀ
ꢀ
graphite-monochromated Mo Ka radiation (
l
¼ 0.71073 A) at 100(2)
K. Absorption corrections were applied semi-empirically using the
Freshly prepared ½RuCp*ðNCMeÞ3ꢁþPF6 (2.00 g, 3.96 mmol)
was added to a thoroughly N2-deoxygenated solution of 1,3,5-
tris(trimethylsilylmethyl)benzene (2.80 g, 8.5 mmol) and 1,2-
dichloroethane (25 mL). The mixture was stirred for 12 h at room
temperature and then at reflux for another 12 h. Evaporation of the
solvent yielded a brown oily residue, which was extracted with
acetone. The extracts were passed through a neutral alumina col-
umn, removing some brown material, and then evaporated to give
a light yellow solid, which was recrystallized from dichloro-
methane and diethyl ether to give an off-white crystalline solid
ꢀ
(1.44 g, 50%). 1H NMR (400 MHz, acetone-d6):
3H), 1.90 (s, CH2, 6H), 1.87 (s, Cp*, 15H), 0.07 (s, TMS, 27H). 13C{1H}
NMR (100 MHz, acetone-d6): 105.6 (arene quat.), 94.6 (Cp* quat),
d 5.5 (s, arene CH,
d
85.9 (arene CH), 23.5 (CH2), 9.8 (Cp* CH3), e1.6 (TMS CH3). Anal.
calcd for C28H51F6PRuSi3: C 46.84, H 7.15. Found: 45.96, H 6.86. ESI-
MS: m/z 573 (cation Mþ).
ꢀ
2.4. Synthesis of 4þPF6
4þPF6 was obtained as a white crystalline solid (1.26 g, 50%) iꢀn
ꢀ
an analogous fashion to 3þPF6 from ½RuCp*ðNCMeÞ3ꢁþPF6
ꢀ
Fig. 1. ORTEP view (50% ellipsoids) of the cation in the crystal structure of 2þPF6
Hydrogen atoms and counter ions are omitted for clarity.
.
ꢀ
(2.40 g, 4.8 mmol) and 1,2,3,4,5-pentamethylbenzene (5.00 g,
33.7 mmol) in dichloroethane (20 mL). 1H NMR (acetone-d6):
d
5.81
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S.K. Mohapatra et al. / Journal of Organometallic Chemistry xxx (2013) 1e7
3
Fig. 2. ORTEP view (50% ellipsoids) of the cation in the crystal structure of 3þPF6
.
ꢀ
Hydrogen atoms and counter ions are omitted for clarity.
Fig. 4. Variation of average RueCarene (circles) and RueCCp* (triangles) bond lengths (r)
in crystal structures of RuCp*(h
6-arene) salts with the number of arene alkyl sub-
stituents (n). Error bars are the estimated standard deviations for the average bond
lengths. The lines are least-squares fits to the data and are described by the equations
r(RueCarene) ¼ 0.00495n þ 2.201 A and r(RueCCp*) ¼ 0.00230n þ 2.174 A.
APEX2 program [15]. The structures were solved by direct methods
and refined by the full-matrix least-squares against F2 in an
anisotropic (for non-hydrogen atoms) approximation. All hydrogen
atom positions were refined in an isotropic approximation using a
“riding” model with the Uiso(H) parameters equal to 1.2 Ueq(Ci), or,
for methyl groups, equal to 1.5 Ueq(Cii), where U(Ci) and U(Cii) are
respectively the equivalent thermal parameters of the carbon
atoms to which the corꢀresponding H atoms are bonded. The
asymmetric unit of 2þPF6 contains one complete cation and two
half anions, with the phosphorus atoms of the anions, P1 and P2,
ꢀ
ꢀ
dimers can be potentially increased by either a cathodic shift of the
reduction potential of the corresponding monomeric 18-electron
cation or by a weakening of the central CeC bond of the dimer
[5,7]. Additional alkyl groups or more strongly electron-donating
substituents might, therefore, afford more strongly reducing sys-
tems. However, as noted in the introduction, not all RuCp*(arene)
complexes dimerize when formed by reduction of the corre-
sponding [RuCp*(arene)]þ cations; in many cases, including that
where arene ¼ C6Me6, hydrogen-atom abstraction to give species of
located on inversion centers. The asymmetric units of 3þPF6 and
ꢀ
4þPF6ꢀconsist of a complete cation and a complete anion, but in
the latter structure the anion is disordered, with two orientations of
the anion present with occupancies of 0.66 and 0.34. All calcula-
tions were performed using the SHELXTL software [16].
the form RuCp*(h
5-areneH) is preferred. In the case of 3, trime-
thylsilylmethyl substituents were chosen since they are known to
be significantly more electron-donating than simple alkyl sub-
stituents (for example, values of the Hammett coefficient, sp, of
0.00, e0.17, and ꢀ0.21 have been obtained for H, Me, and CH2SiMe3
substituents, respectively [17]) and so would be expected to
cathodically shift the RuCp*(arene)]þ/0 potential, whereas the
greater bulk of the substituents might be expected to contribute to
a weaker CeC bond in the dimer than that in 12. We were also
interested in whether a dimer could be formed following reduction
3. Results and discussion
3.1. Synthesis
The dimer 12 has previously been reported by Gusev and co-
workers [4], and we have previously reported 22, which we syn-
thesized in order to obtain a more soluble analog of 12 for solution
doping studies and for mechanistic crossover experiments with 12
[5]. As we have discussed elsewhere, the reducing strength of these
Fig. 5. Reductive cyclic voltammograms of 1Dꢀ PF6 (top, broken line) and its iron
0.1 M nBu4NþPF6 at a scan rate of 50 mV sꢀ1
.
ꢀ
analogue, ½FeCp ꢃ ðh6ꢀ1; 3; 5 ꢀ C6H3Me3ÞꢁþPF6 (bottom, solid line), recorded in THF/
ꢀ
Fig. 3. ORTEP view (50% ellipsoids) of the cation in the crystal structure of 4þPF6
Hydrogen atoms and counter ions are omitted for clarity.
.
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S.K. Mohapatra et al. / Journal of Organometallic Chemistry xxx (2013) 1e7
Fig. 6. Assignment of 1H (left) and 13C (right) NMR resonances for Kþ3e showing chemical shifts in ppm (see experimental section).
of RuCp*(C6Me5H)]þ, 4D, and whether the additional methyl
groups would ꢀlead to a moꢀre reducing dimer than 12.
3.2. Molecular structure
Both 3þPF6 and 4þPF6 were obtained in good yield as more-
or-less colorless crystalline solids from the reactioꢀn of an excess of
the appropriate arene with ½RuCp*ðNCMeÞ3ꢁþPF6 [11] in dichlo-
roethane, in close analogy to the previously reported syntheses of
Single crystals of 2þPF6ꢀ, 3þPF6ꢀ, and 4þPF6ꢀ suitable for single
crystal X-ray structure determination were obtained by slow
evaporation of acetone solutions at room temperature. ORTEP
representations of the cations in the structures are shown in
Figs. 1e3, cell parameters and details of the data collection, solu-
tion, and refinement are given in Table 1, while selected bond
lengths and angles are given in Table 2, along with comparative
1þPF6 and 2þPF6 (Scheme 1) [5,12]. The sandwich compounds
were fully characterized by 1H and 13C NMR spectroscopy,
elemental analysis, and electrospray mass spectrometry. In the case
of 3D, the arene ligand was not commercially available, but was
obtained by a modification of a literature procedure [14] in which
mesitylene is deprotonated by a large excess of n-butyllithium/
N,N,N0,N0-tetramethylethylenediamine and the resulting trianion is
quenched with chlorotrimethylsilane.
ꢀ
ꢀ
data for some other structures of [RuCp*(h
6-arene)]þ salts from the
literature [18e20]. In the structures of the 2D and 3D cations, the
terminal methyl and SiMe3 groups, respectively, of the arene sub-
stituents are bent away from the metal, thus meaning that the
bulky substituents in the latter cation have little effect on the co-
ordination geometry around the metal. The cations in all three
structures have approximate axial symmetry, having essentially
parallel Cp* and arene rings (the dihedral angles are 1.06ꢄ, 0.66ꢄ,
and 0.76ꢄ, for 2þPF6ꢀ, 3þPF6ꢀ, and 4þPF6ꢀ, respectively) and
centroidemetalecentroid angles very close to 180ꢄ, consistent with
what is seen for other ruthenium cyclopentadienyl arene cation
structures in the literature [18e24]. RueC bond distances are
similar to those seen in related structures, with the RueCCp* bonds
being shorter than the RueCarene bonds. Interestingly, as shown in
Fig. 4, arene alkylation among the structures summarized in Table 2
appears to be correlated with a slight increase of the average Rue
Table 1
Crystal and structural refinement data.
2þPF6
3þPF6
ꢀ
ꢀ
ꢀ
4þPF6
Empirical
C22H33F6PRu
C28 H51F6PRuSi3
C21H31F6PRu
formula
fw
543.52
718.00
529.50
Cryst color, habit
Crystal size, mm
Colorless prism
0.34 ꢂ 0.29 ꢂ
0.17
Colorless prism
0.39 ꢂ 0.35 ꢂ
0.18
Colorless prism
0.39 ꢂ 0.36 ꢂ 0.23
Cryst syst
Monoclinic
P21/c
Monoclinic
P21/n
Monoclinic
P21/n
ꢀ
Space group
Carene bond length (variation of 0.035 A) and, to a lesser extent, of
the average RueCCp* distance (variation of 0.018 A) [25].
ꢀ
ꢀ
a, A
17.8785(15)
9.0536(8)
15.3516(13)
105.744(2)
2391.7(4)
4
1.509
0.775
1112
2.37e28.00
ꢀ23 ꢅ h ꢅ 23
ꢀ11 ꢅ k ꢅ 11
ꢀ20 ꢅ l ꢅ 20
26,200
10.9578(6)
17.0020(10)
18.9233(11)
92.9430(10)
3520.8(3)
4
1.355
0.641
1496
2.10e28.00
ꢀ14 ꢅ h ꢅ 14
ꢀ22 ꢅ k ꢅ 22
ꢀ24 ꢅ l ꢅ 24
39,149
9.1215(4)
17.9170(8)
13.8564(6)
104.5580(10)
2191.84(17)
4
1.605
0.843
1080
1.90e28.00
ꢀ12 ꢅ h ꢅ 12
ꢀ23 ꢅ k ꢅ 23
ꢀ18 ꢅ l ꢅ 18
24,497
ꢀ
b, A
ꢀ
c, A
3.3. Electrochemistry
b
, deg
3
ꢀ
V, A
The potentials at which the RuII compounds 1De4D are reduced
to the corresponding 19-electron neutral RuI compounds 1e4 were
measured using cyclic voltammetry in THF/nBu4NþPF6ꢀ. All four
compounds show qualitatively similar voltammograms, a repre-
sentative example of which is shown in Fig. 5. As previously
Z
rcalc, g cmꢀ3
m
(Mo K
F (000)
Range, deg
a
), mmꢀ1
q
Index ranges
No. of reflns
collected
No. of indep
reflns
5774
8507
5291
(Rint ¼ 0.0353)
(Rint ¼ 0.0437)
(Rint ¼ 0.0336)
No. of data/
restraints/
params
4897/0/282
6818/0/366
4448/15/278
GOF (F2)
1.035
1.008
1.037
R1 (F) [I > 2
s
(I)]
0.0362
0.1000
0.0323
0.0855
0.0453
0.1309
wR2 (F2)
(all data)
Peaks min/max
e A
0.817/ꢀ0.693
0.572/ꢀ0.397
1.153/ꢀ1.260
ꢀꢀ3
Scheme 1.
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S.K. Mohapatra et al. / Journal of Organometallic Chemistry xxx (2013) 1e7
5
Table 2
Comparison of key geometric characteristics (A, ꢄ) of RuCp*(
6-arene) salts.
ꢀ
h
a
a
Arene, anion
RueCCp*
RueCarene
RueCtCp*
RueCtarene*
CtCp*eRueCtarene
Range
Average
Range
Average
C6H6, [RuCp*Br3]e [18]
C6H6, [RhCp*Cl3]e [19]
C6H5Me, [RuCp*Cl3]e [24]
2.170 (5)e2.182 (5)
2.180 (5)e2.192 (5)
2.148 (8)e2.179 (8)
2.177 (4)e2.194 (3)
2.179 (2)e2.185 (2)
2.169 (4)e2.192 (4)
2.180 (2)e2.22 (2)
2.175 (2)
2.185 (3)
2.162 (6)
2.184 (3)
2.181 (1)
2.181 (4)
2.193 (6)
2.181 (6)e2.208 (6)
2.192 (6)e2.219 (5)
2.181 (8)e2.219 (9)
2.210 (3)e2.235 (3)
2.209 (2)e2.241 (2)
2.214 (3)e2.245 (4)
2.211 (6)e2.250 (7)
2.192 (6)
2.206 (8)
2.207 (5)
2.219 (4)
2.221 (5)
2.226 (4)
2.227 (5)
1.811
1.810
1.794
1.813 (1)
1.811 (1)
1.811 (1)
1.80
1.711
1.703
1.714
1.708 (1)
1.709 (1)
1.719 (1)
1.753
177.8
178.4
178.9
179.44 (6)
179.52 (4)
179.36 (8)
178.8
ꢀ
C6H3Et3 (2D), PF6
C6H3(CH2SiMe3)3 (3D), PF6
ꢀ
C6HMe5 (4D), PF6
ꢀ
C6Me6, TCNQꢆe [20,21]
a
Ct denotes the centroid of the ring.
reported for other [RuCp(arene)]þ and [RuCp*(arene)]þ arene de-
rivatives [4,26] and many other 18-electron sandwich complexes of
the 4d- and 5d-metals [2,27e29], these are not reversible (with no
observable reoxidation peak), consistent with the expected chem-
ical reactivity of the 19-electron complexes. As shown in Table 3,
replacement of the methyl substituents of 1D by ethyl groups in 2D
lead to only a very small cathodic shift in the peak reduction po-
tential, Epc, whereas the CH2SiMe3 groups of 3D lead to a cathodic
shift of almost 0.3 V, consistent with Hammett coefficients,
sp(Me) w sp(Et) > sp(CH2SiMe3) [17]. The small cathodic shift of ca.
0.1 V seen on addition of two methyl groups to 1D to give 4D is
similar to the difference seen between the peak reduction poten-
slow that the 19-electron monomer can be observed in solution
[31], whereas ruthenium examples rapidly give either dimers (e.g.,
12) and/or hydrogen atom abstraction products (e.g., RuCp*(h5
-
C6H7)) [4].
3.4. Chemical reduction
As noted above, Gusev et al. have previously reported the
chemical and electrochemical reduction reactions of a variety of
cyclopentadienyl arene ruthenium cationic complexes [4,26];
depending on the substituents in question and the reduction con-
ditions, a variety of products can be formed including dimers,
hydrogen-reduced species, and, in some case, ligand redistribution
products (for example, RuCp2 is formed among the reduction
tials of 1D and [RuCp*( 6-C6Me6)]þ in acetonitrile [4].
h
Table 3 and Fig. 5 also shows that the reduction potential of 1D
is substantially cathodically shifted relative to that of its iron
products of [RuCp(h
6-C6H6)]þ). Consistent with one of these re-
analog, [FeCp*(h
6-1,3,5-C6H3Me3)]þ, consistent with the literature
ports [4], we found that Na/Hg reduction of 1D gives the dimer 12
(Scheme 2), and that 2D is reduced to an analogous dimer 22 under
for other group 8 mixed-ring cations (for example, potentials
previously reported for [MCp*(
h
6-C6H6)]þ {M ¼ Fe, Ru} in aceto-
ꢀ
the same conditions [5]. We attempted the reduction of 4þPF6
nitrile [4,30]) and with the general tendency for the heavier
transition elements to prefer higher oxidation states more strongly
than their lighter congeners. Moreover, the electrochemistry of the
iron compounds is considerably more reversible (Iox/Ired ¼ 1.0),
indicating that the reduced iron species are all clearly less reactive
than their ruthenium analogs. This is consistent with the greater
tendency of the heavy transition metals to comply with the 18-
electron rule, specifically with the fact that some iron species,
using Na/Hg in THF; MALDI mass spectra (m/z ¼ 385, consistent
with the monomer cation, as is seen in the MALDI spectra of 12 and
22) and elemental analysis are consistent with formation of a dimer,
but NMR spectra were complicated, suggesting the possibility of an
isomer mixture, perhaps accompanied by one or more isomers of a
hydrogen reduction product.
While 1D, 2D, and 4D can all be reduced using 1% Na amalgam,
3þPF6 is largely unaffected by this reducing agent, at least on a
ꢀ
such as FeCp*(h
6-C6Me6), are stable as monomers, and that the
dimerization of others, such as FeCp*(h
6-C6H6), is sufficiently
Table 3
Electrochemical potentials vs. ferrocenium/ferrocene for the reduction of 18-
electron group 8 mixed-ring cations to the corresponding 19-electron species.
Cation
THF/0.1 M ꢀ
nBu4NþPF6
MeCN/0.1 M
nBu4NþBFe4 c
a
E1/2/Vd
Epc/Ve
b
E1/2/V
Epc
[FeCp*(
[FeCp*(
h
h
6-C6H6)]þ
6-1,3,
ꢀ2.06
ꢀ2.13
e
e
ꢀ2.07
e
e
e
5-C6H3Me3)]þ
[RuCp*(
[RuCp*(
h
h
6-C6H6)]þ
6-1,3,
e
e
e
e
e
ꢀ2.72
ꢀ2.81
ꢀ2.67
ꢀ2.70
ꢀ2.96
5-C6H3Me3)]þ, 1D
[RuCp*(
5-C6H3Et3)]þ, 2D
[RuCp*(
6-1,3,5-C6H3
h
6-1,3,
e
e
e
e
e
e
h
(CH2SiMe3)3)]þ, 3D
[RuCp*(
[RuCp*(
h
h
6-C6Me5H)]þ, 4D
e
e
ꢀ2.78
e
e
e
6-C6Me6)]þ
e
ꢀ2.91
a
This work.
Peak potentials recorded at a scan rate of 50 mV sꢀ1
Converted from data originally reported vs. SCE using a value of þ0.40 V for
b
c
.
FeCp2þ/0 vs. SCE in MeCN/0.1 M nBu4NþPF6 taken from Ref. [32].
ꢀ
d
From Ref. [30].
e
Peak potentials at 100 mV sꢀ1 from Ref. [4].
Scheme 2.
Please cite this article in press as: S.K. Mohapatra, et al., Journal of Organometallic Chemistry (2013), http://dx.doi.org/10.1016/
j.jorganchem.2013.07.023

6
S.K. Mohapatra et al. / Journal of Organometallic Chemistry xxx (2013) 1e7
relatively short timescale (2 h), presumably due to the slightly more
cathodic potential required for its reduction (see above). Evidently
the reduction of RuCp*(arene) cations with Na/Hg is of borderline
feasibility. Indeed, the electrochemical potential for sodium
amalgam has been estimated as ca. e2.2 to ꢀ2.4 V vs. FeCpþ2 /0 [32].
Longer reaction times still do not afford any direct reduction
products of 3D and the majority of the material remains un-
changed; however, 1H and 13C NMR spectroscopy indicated for-
mation of the RuCp*(mesitylene) dimer, 12, in 15e20% yield.
Presumably the formation of 12 occurs by desilylation of 3D by
either hydroxide (traces of which may have been introduced on the
surface of the sodium) or fluoride (originating from PF6ꢀ), both of
which are known to cleave Me3Siebenzyl bonds [33e36], followed
by reduction and dimerization of the more easily reduced 1D cation
resulting from this cleavage. Use of a more powerful reducing agent
e Na/K alloy e led to the formation of a benzene and toluene-
soluble red-orange solid (in some cases accompanied by variable
amounts of the desilylated dimer 12) that, in contrast to dimers
such as 12 and 22, was pyrophoric in air, although the solid could be
stored for several weeks without decomposition under inert at-
mosphere. The 1H (13C) NMR spectrum indicates that all three arene
protons (CH carbon atoms), all three silyl groups, and all three
benzyl methylene groups are inequivalent. Moreover, the protons
of each methylene are noticeably diastereotopic; two of the CH2
groups are seen as pairs of well-separated doublets (i.e., each
constitutes an AX spin system), while another is observed as an AB
multiplet. The protons of two of the arene CH groups and two of the
CH2 groups show strong inter-ring NOEs with the Cp* protons,
while a CH signal at 4.33 ppm and the protons of one of the CH2 AX
systems do not. These observations are consistent with the arene
adopting an h4 coordination mode. Additional NOE experiments
allow all the proton resonances to be assigned, as shown in Fig. 6.
The NOE experiments allow us to assign the nominally sp2 CH
group seen at the unusually high-field chemical shift of 1.39 ppm to
cyclopentadienyl/arene complexes of a group 8 metal. Moreover,
FeCp(arene) anions are likely to be 20-electron
6-arene species
[50,51], consistent with the general tendency for sandwich com-
pounds of the 3d metals to retain axial symmetry, even when this
leads to deviation from the 18-electron rule.
h
It is unclear to us exactly why 3e is isolated, rather than the
corresponding dimer or hydrogen abstraction products [53]. It is
worth noting that Na/K reduction of 1D and 2D still leads to the
dimer, suggesting that formation of the two-electron reduction
product is a not only a consequence of the stronger reducing agent.
Perhaps the greater bulk of the substituents reduces the rate of
these reactions sufficiently to allow time for a second electron to be
transferred to 3ꢆ from the highly reducing Na/K.
4. Conclusion
The X-ray structures of three [RuCp*(arene)]þPF6 salts, where
ꢀ
arene ¼ 1,3,5-triethylbenzene, 1,3,5-tris(trimethylsilylmethyl)ben-
zene, and 1,2,3,4,5-pentamethylbenzene, have been determined.
Their electrochemical and chemical reduction has been examined. All
three undergo one-electron reduction processes at highly cathodic
potentials. Whereas reduction of the 1,3,5-triethylbenzene example
gives an air-stable dimer, (RuIICp*)2( h5 5-C6H3Et3C6H3Et3), anal-
m- :h
ogous to that previously reported for the mesitylene derivative, Na/K
reduction of [RuIICp*{ 6-C6H3(CH2SiMe3)3}]þ led to the formation of
Kþ[Ru0Cp*{ 4-C6H3(CH2SiMe3)3}]e, which was extensively charac-
h
h
terized by NMR methods and represents the first example of a group
8 [MCp(arene)]ꢀ derivative to be isolated.
Acknowledgments
We thank Solvay SA, the National Science Foundation (through
the STC Program, DMR-0120967, and the PREM program, DMR-
0934212), and the Office of Naval Research (N00014-11-1-0313)
for support, Les Gelbaum for help with NMR measurements, and
John Bacsa with his efforts to determine the structure of Kþ3e from
poor quality crystals.
the CH group at one end of the
in Fig. 6). Similarly upfield shifts have been reported for the cor-
responding resonances of other
4-arene complexes; for example,
Os(
4-C6H6)( 6-C6H6) (3.00 pm) [37,38], [Cr( 4-naph)(CO)3]2e
(2.00 ppm) [39], [Mn(
4-naph)(CO)3]e (2.44 ppm) [40], TaMe(h4
naph)(dmpe)2 (1.51 ppm) [41], and Fe(
4-C6H6)tmps (1.69 ppm)
[42,43] {naph naphthalene, dmpe Me2PCH2CH2PMe2;
h
4 portion of the arene (labeled “a”
h
h
h
h
h
-
Appendix A. Supplementary material
h
¼
¼
CCDC 926155, 926156 and 926157 contain the supplementary
crystallographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
tmps ¼ MeSi(CH2PMe2)3}. The high-field shift is consistent with the
expected distortion of the geometry of the Ca atom from planarity,
as revealed in the crystal structures of various other
h
4-arene
complexes. An HSQC experiment allows most of the 13C resonances
to be assigned; the Ca resonance is also seen at characteristically
high field (55.6 ppm), similar to what is seen for Os(
C6H6) (47.2 ppm) [37,38], [Cr(
4-naph)(CO)3]2e (56.2 ppm) [39],
[Mn(
4-naph)(CO)3]e (57.6 ppm) [40], and Fe( 4-C6H6)tmps
(48.5 ppm) [42,43]. The
4-arene and the pyrophoric nature of the
h -
4-C6H6)(h6
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Please cite this article in press as: S.K. Mohapatra, et al., Journal of Organometallic Chemistry (2013), http://dx.doi.org/10.1016/
j.jorganchem.2013.07.023