Study of small oligomers based on Ru2(DMBA)4 and meta-phenylene diethynylene

Article abstract of DOI:10.1016/j.jorganchem.2017.03.011

The reactions between Ru₂ (DMBA)₄(NO₃)₂ (DMBA = N,N’-dimethylbenzamidinate) and meta-phenylene diethynylenes bearing 5-ester substituents (-CO₂ⁱPr, L1; -CO₂Bn, L2) in the presenc

Full text of DOI:10.1016/j.jorganchem.2017.03.011

Journal of Organometallic Chemistry xxx (2017) 1e6
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Study of small oligomers based on Ru2(DMBA)4 and meta-phenylene
diethynylene
Jie-Wen Ying, Carl W. Liskey, Sean N. Natoli, Stella K. Betancourt, Li Liu, Phillip E. Fanwick,
Tong Ren*
Departments of Chemistry, Purdue University, West Lafayette, IN 47907, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 November 2016
Received in revised form
26 February 2017
Accepted 4 March 2017
The reactions between Ru2 (DMBA)4(NO3)2 (DMBA ¼ N,N’-dimethylbenzamidinate) and meta-phenylene
diethynylenes bearing 5-ester substituents (-COi2Pr, L1; -CO2Bn, L2) in the presence of Et2NH afforded a
series of oligomeric compounds with meta-phenylene diethynylene bridge, namely L-[Ru2 (DMBA)4L]m
with m as integers. With L1, the compounds with m ¼ 1e3 (1a, 2a and 3a) were separated and fully
characterized. With L2, only the compound with m ¼ 1 (1b) was successfully isolated. In addition to
routine spectroscopic characterizations, the structures of both compounds 1b and 2a were determined
using single crystal X-ray diffraction. For the series of 1a, 2a and 3a, both the voltammetric and ab-
sorption spectroscopic characteristics bear close resemblance to those of simple Ru2 (DMBA)4(C2R)2
compounds, indicating the absence of significant inter-unit electronic couplings in the oligomers.
© 2017 Elsevier B.V. All rights reserved.
Available online xxx
Dedicated to Rick Adams, a dear friend and
connoisseur of carbon rich organometallics,
on the occasion of his 70th birthday.
Keywords:
Diruthenium
DMBA
Alkynyls
Oligomers
1. Introduction
much of the aforementioned successes on metal-
based on 4d and 5d metals, chemistry of -alkynyls based on 3d
metal complexes of polyaza-macrocycles has received considerable
interest [22e27]. Besides -alkynyl species, another interesting
avenue of metal-alkynyl chemistry is the formation of the Pauson-
Khand type adducts (
2-C≡C) between oligoynes and mono-/oligo-
metallic species [28e32].
A major focus area of our laboratory is the diruthenium alkynyl
chemistry, where the charge mobility along the Ru2-s-alkynyl
backbone has been demonstrated in both bulk solution [33e38]
and nano-scale devices [39e43]. It is worth noting that the devel-
opment of diruthenium and triruthenium alkynyl chemistry has
also benefited from key contributions from the laboratories of
Cotton [44], Bear and Kadish [45e47], Lehn [48] and Peng [49].
Among several families of diruthenium alkynyl compounds
developed in our laboratory, those based on the DMBA (DMBA is
N,N’-dimethylbenzamidinate) supporting ligand are unique in the
formation of Ru2-alkynyl bond under weak base conditions
(Scheme 1) [36,50e52]. In addition to affording symmetric bisal-
kynyl species under mild conditions such as ambient atmosphere
and room temperature, the weak base protocol enables the one-pot
synthesis of novel unsymmetric bisalkynyl species exhibiting vol-
tammetric characteristic consistent with a molecular diode [53]. In
s-alkynyls are
s
Preparation of metal-s-alkynyl compounds has been an active
area of research since the 1950s [1,2]. Extensive efforts in the recent
years revealed many intriguing and useful attributes of metal-
alkynyl species, which include interesting electronic structures
and novel topologies [3], their applications as nonlinear optical
materials [4e6], OLED and photovoltaic materials [7e9], and active
species for molecular electronics [10e13]. The possibility of form-
ing [M-C≡C-Y-C≡C-]n type oligomers/polymers was recognized
from the beginning of metal-alkynyl chemistry [1,14], where M is
one of either coinage metals or platinum group metals and Y is an
aromatic group. During the ensuing decades, the Osaka group led
by Hagihara and Takahashi reported many Pd/Pt based polymers
based on the original [M-C≡C-Y-C≡C-]n motif [14], while the
Cambridge group led by Lewis produced similar polymers based on
mono-nuclear Pt and Ru species [15,16]. More recently, Wong and
coworkers have developed many Pt-based metal-alkynyl polymers
with an emphasis on optoelectronic applications [17e21]. Though
s
h
* Corresponding author.
E-mail address: tren@purdue.edu (T. Ren).
http://dx.doi.org/10.1016/j.jorganchem.2017.03.011
0022-328X/© 2017 Elsevier B.V. All rights reserved.
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2
J.-W. Ying et al. / Journal of Organometallic Chemistry xxx (2017) 1e6
Scheme 1. Ru2-alkylnylation under Weak Base Conditions.
the aforementioned studies, ligation of mono-ethynes was the
focus. We became intrigued by the meta-phenylene diethynylene
type ligands, based on which hexagon and helical supramolecules
were realized in the laboratories of Moore and others [54e56].
Reported herein are the reactions between Ru2 (DMBA)4(NO3)2 and
meta-phenylene diethynylene ligands under aerobic weak base
conditions, and the identification of resultant oligomers.
Scheme 3. Reaction between Ru2-(DMDA)4(NO3)2 and m-phenylene diethynylene.
2. Results and discussion
ether solution (2a). The structural plots of 1b and 2a are shown in
Figs. 1 and 2, respectively, with selected bond lengths and angles
listed in the captions. The asymmetric unit of the crystal of 1b
contains only one half of the diruthenium molecule, which is
related to the other half via a crystallographic 2-fold axis passing
through the midpoint of Ru1- Ru10 bond. The Ru1- Ru10 bond
length in 1b (2.4675 (7) Å) is comparable to those previously
determined for the Ru2 (DMBA)4(C2Ar)2 type compounds
(2.45e2.46 Å) [51e53] and consistent with the existence of a Ru-Ru
single bond and a ground state configuration of p4d2p*4. The Ru1-
C1 bond is short (1.998 (6) Å), reflecting the formation of a strong
2.1. Syntheses
Syntheses of meta-arylene diethynylene ligands were based on
the Sonogashira type cross coupling reactions as outlined in
Scheme 2 [57,58], and the experimental details are provided in the
supplementary content. Trimethylsilyl (TMS) was retained as the
protecting group of terminal ethyne owing to its chemical stability
over the free ethyne. In order to optimize the solubility of the
resultant L-[Ru2 (DMBA)4-L]m type compounds, two side chains,
isopropyl (iPr) and benzyl (Bn), on the aromatic ring of the organic
spacer were introduced through esterification, as shown in Scheme
2. Typical desilylation of L1TMS2 and L2 TMS2 using K2CO3 in MeOH
would lead to transesterification of the ester group [59]. Hence, the
desilylation of L1TMS2 and L2 TMS2 was achieved in excellent yield
with TBAF (containing 5% of water) in THF.
s(Ru-C) bond at the expense of s(Ru-Ru) bond [50]. It is clear from
the structural parameters that there is a significant deviation from
an idealized lantern structure in the first coordination sphere of the
While the preparation of the symmetric Ru2 (DMBA)4(C2R)2
type compounds using LiC2R was successful and expedient [50], the
presence of an ester group in both L1 and L2 prevents the use of
lithiation method. Hence, the weak base assisted reaction is the
ideal alternative for the Ru2 (DMBA)4 compounds based on L1 and
L2, as shown in Scheme 3. Generally, the reactions between Ru2
(DMBA)4(NO3)2 and excess L1/L2 in the presence of diethylamine
(Et2NH) afforded a mixture of Ln-[Ru2 (DMBA)4-Ln]m as monitored
by TLC. Typically, overnight reaction gave the Ru2 based monomer
(m ¼ 1) as the major product when three equiv of Ln was used.
Reducing the equivalents of Ln to two, the reaction under similar
conditions resulted in significant amounts of dimer, trimer and
higher oligomers in addition to the monomer. These oligomers
were readily identified on TLC as their Rf values decrease with the
increasing degree of oligomerization. Using flash column chroma-
tography, we were able to obtain usable quantities of compounds
1a-3a and 1b, which were characterized by ESI-MS (Figs. S2e S5),
1H NMR and elemental analysis. All compounds in solid state are
indefinitely stable under ambient conditions.
2.2. Molecular structures
Fig. 1. Molecular structure of 1b. The half of the molecule is related to the other half by
a crystallographic axis passing through the Ru1-Ru10 bond. Selected bond lengths (Å)
and angles (deg): Ru1-Ru10, 2.4675 (7); Ru1-C1, 1.998 (6); Ru1-N1, 2.001 (4); Ru1-N2,
2.119 (4); Ru1-N3, 1.995 (4); Ru1-N4, 2.073 (4); Ru10-Ru1-C1, 166.0 (2).
Single crystals of X-ray quality were successfully grown for
compounds 1b and 2a from THF/hexanes solution (1b) and THF/
Scheme 2. Preparation of m-diethylnylbenzene ligands.
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j.jorganchem.2017.03.011

J.-W. Ying et al. / Journal of Organometallic Chemistry xxx (2017) 1e6
3
Fig. 2. Molecular structure of 2a. Acetylenic carbons are labeled and shown as ellipsoids. Selected bond lengths (Å) and angles (deg): Ru1-Ru2, 2.462 (1); Ru3-Ru4, 2.456 (1); Ru1-
C14, 1.958 (11); Ru2-C51, 2.019 (11); Ru3-C64, 1.963 (10); Ru4-C101, 1.953 (11); (Ru-Nlong)av.2.005 [9], (Ru-Nshort)av., 2.091 [9]; Ru2-Ru1-C14, 165.5 (3); Ru1-Ru2-C51, 164.6 (3); Ru4-
Ru3-C64, 169.0 (3); Ru3-Ru4-C101, 167.6 (3).
Ru2 core: the two short (Ru1-N1 and Ru1-N3) and two long (Ru1-
N2 and Ru1-N4) Ru-N bonds, and the non-linear Ru10-Ru1-C1
angle (166.0 (2)). As discussed in detail initially for the Ru2
(DArF)4(C2Ar)2 type compounds [60], the structural distortion
observed herein is attributed to a second order Jahn-Teller effect.
The structure of compound 2a is shown in Fig. 2, where the main
feature is the formation of the dimer of dimer through simulta-
neous axial ligation of L1 to two Ru2 (DMBA)4 units. There are many
structural examples of two mononuclear units ([M]) bridged by 1,3-
[52], consistent with the electron deficient nature of L1 compared
to phenylacetylide. The appearance of successive reductions B and
C was not observed previously for the Ru2 (DMBA)4(C2Ar)2 type
compounds [52]. It is possible that one of the two L1 ligands dis-
sociates upon the first reduction (B) to yield a neutral Ru2 (DMBA)4
(L1), and further reduction of the latter yields couple C. The CV of 2a
is very similar to that of 1a, other than slight broadening of the
couple A. Clearly, the two Ru2 (DMBA)4 units in 2a do not interact
across the m-L1 bridge, unlike the cases of two Ru2 units bridged by
diethynylbenzene, such as [M]
¼
RuCl(dppm)2 [61], Pt
linear oligoyn-diyls [33,34,36e38]. Finally, the CV of 3a consists of
two quasi-reversible couples A and B that are somewhat broader
than those of 1a and 2a. Yet, there is no clear evidence for inter-unit
couplings in 3a. Curiously, there is only one reduction for 3a,
implying that the trimer molecule remains intact on reduction.
Presently, we do not have a good rationale for the contrast in the
voltammetric behavior of 3a with those of 1a and 2a. Similar
electronic properties were observed for the benzyl series.
(PiPr3)2(C2Ph) [62], Cp*Fe (dppe) [63], heterometallic Ru/Re [64], Pt
(PEt3)2I [65] and Co(cyclam)Cl [25]. Compound 2a is the only
example of two dinuclear units linked by 1,3-diethynylbenzene to
the best of our knowledge. It is worth noting that the Re2 (DMBA)4
type compounds are also apt in forming linear coordination poly-
mers with either ditopic oxoanions or di-carboxylates as the linker
[66,67]. Clearly, the M2 (DMBA)4 type compounds are excellent
synthons for metallo olgomers/polymers through axial ligation.
Close examination of metric parameters listed in the captions of
Figs. 1 and 2 reveals a strong resemblance in the geometry of the
first coordination sphere between 1b and 2a.
2.4. Electronic absorption spectra
The Vis-NIR spectra of compounds 1a, 2a and 3a are shown in
Fig. 4, and all three compounds display two intense absorptions at
ca. 500 and 880 nm. The overall pattern of the Vis-NIR spectra is
very similar to those of Ru2 (DMBA)4(C≡CAr)2 type compounds
reported in literature [11], which all display two major absorptions
at 500e503 nm and 874e881 nm. According to an early TD-DFT
2.3. Electrochemistry
Rich redox activity has been the hallmark of diruthenium
alkynyl compounds, and the trans-Ru2 (DMBA)4(C2R)2 type com-
pounds often undergo two reversible 1eꢀ processes: an oxidation
and a reduction [11]. These characteristics are retained in the
compounds reported here (Table 1), and the presence of at least
two 1eꢀ couples is clear in the cyclic voltammograms (CV) of
compounds 1a e3a shown in Fig. 3. The CV of 1a constitutes of a
reversible oxidation (A) and two successive quasi-reversible re-
ductions (B and C). The E1/2(A) of 1a is about 0.09 V more positive
than that reported for Ru2 (DMBA)4(C2Ph)2 (0.52 V vs. Ag/AgCl)
Table 1
Electrode potentials for compounds 1a, 2a and 3a.
Compd
E1/2(A)/V
E1/2(B)/V
E1/2(C)/V
1a
2a
3a
0.61
0.57
0.62
ꢀ0.88
ꢀ0.89
ꢀ0.90
ꢀ1.01
ꢀ1.04
n/a
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J.-W. Ying et al. / Journal of Organometallic Chemistry xxx (2017) 1e6
3. Conclusions
We presented a successful strategy in preparing a unique series
of Ru2-based oligomers, and fully characterized lower members
(n ¼ 1e3) of the series based on L1. The structural features of the
first coordination sphere of Ru2 center in compounds 1b and 2a are
very similar to those of previously studied Ru2 (DMBA)4(C2R)2 type
compounds. The electrochemical and spectroscopic features of
oligomers with n ¼ 2 and 3 are very similar to those of n ¼ 1,
indicating a minimal interunit electronic coupling.
4. Experimental
4.1. General procedure and materials
3-Bromo-5-iodobenzoic acid was purchased from Alfa Aesar,
potassium carbonate, benzyl bromide, and isopropyl bromide from
Aldrich, diethylamine, triethylamine, tetrabutylammonium fluo-
ride, and 3-bromo-5-iodobenzoic acid from ACROS, trimethylsilyl
acetylene from GFS, and silica gel from Merck. Ru2 (DMBA)4(NO3)2
was prepared according to literature procedure [68]. THF was ob-
tained either from the distillation over Na/benzophenone under an
N2 atmosphere prior to use or obtained from a solvent purification
system (Innovative Technology Inc.). Elemental analysis was per-
formed by Atlantic Microlab, Norcross, GA. 1H NMR spectra were
recorded on a Varian Mercury 200 NMR spectrometer with
Fig. 3. Cyclic voltammograms of compounds 1a, 2a and 3a recorded in 0.20 M (n-
Bu)4NPF6 THF solution at a scan rate of 0.10 V/s. The concentration of compounds is
such that [Ru2] is always 1.0 mM.
chemical shifts (d) referenced to the residual CHCl3 in CDCl3,
40000
respectively. UVevis spectra were obtained with
a
Cary 50
UVeViseNIR spectrophotometer. Mass spectra were recorded on a
Bruker Autoflex® MALDI-TOF Mass Spectrometer. Mass spectra
were collected on a TripleTOF 5600 System (Sciex, Concord,
Ontario, Canada) using nano-electrospray ionization. Both cyclic
voltammograms (CVs) were recorded in 0.2 M (n-Bu)4NPF6 solution
(THF, N2-degassed) on a CHI620A voltammetric analyzer with a
glassy carbon working electrode (diameter ¼ 2 mm), a Pt-wire
auxiliary electrode and a Ag/AgCl reference electrode. The con-
centration of diruthenium species was always 1.0 mM.
1a
2a
3a
30000
ε
20000
10000
0
4.2. Preparation of compounds 1a, 2a and 3a
In
a
100 mL round bottom flask, isopropyl-3,5-
(0.438 mmol,
bis(trimethylsilylethynlene)benzoate
0.156 g) (L1TMS2) was desilylated with TBAF in 20 mL THF solution.
The reaction was stirred at room temperature for 20 min and then
was filtered through a 2 cm silica gel pad. The filtrate was added
into a suspension of (0.218 mmol, 0.200 g) of Ru2 (DMBA)4(NO3)2 in
50 mL THF and 10 mL of Et2NH. The reaction mixture was stirred at
room temperature overnight and filtered through a celite pad. The
solvents were removed on rotovap and the residue was dried under
vacuum. The residue was purified by column chromatography and
eluted with THF/hexanes (1/5, v/v). Several fractions were
collected. The first fraction was identified as monomer 1a (yield:
28% based on Ru). Data for 1a: 1H NMR: 7.72 (d, 4H, aromatic),
7.67e7.38 (m, 14H, aromatic), 7.27e7.01 (m, 8H, aromatic), 5.19 (m,
2H, -COOCH-), 3.29 (s, 24H, CH3N-), 3.03 (s, 2H, HC≡C-), 1.30 (m,
12H, CH3-). MS-FAB (m/z, based on 101Ru): 1213 [Mþ ꢀ H]. ToF-MS
400
600
800
1000
λ (nm)
Fig. 4. Visible-NIR absorption spectra of compounds 1a, 2a and 3a recorded in THF
solution.
study of the Ru2 (DMBA)4(C2nFc)2 type compounds [35], the lower
energy absorption can be attributed to the
transition, where the axial alkynyl ligand contributes through the
antibonding overlap between (C≡C) and *(Ru2). The absorption
around 510 nm is due to a mixture of both the L(N) / *(Ru2) and
(Ru2) / *(Ru2) transitions. It is clear from Fig. 4 that the intensity
of both bands increase linearly with the increasing degree of olig-
omerization, and there is no discernible new feature associated
with the dimer (2a) or trimer (3a) when compared with the
monomer (1a). Hence, the interunit electronic couplings are
insignificant, consistent with the conclusion drawn from voltam-
metric data.
p*(Ru2) / d*(Ru2)
(m/z, based on 101Ru) 1214 [Mþ]. Electrochemistry, E1/2/V,
DEp/V,
p
p
d
ibackward/iforward: A, 0.613, 0.058, 0.926; B, ꢀ0.876, 0.185, 0.927;
Anal. Found (calcd) for C64H66N8O4Ru2$0.5hexane: C, 64.10 (64.04);
H, 5.74 (5.86); N, 8.83 (8.92).
The second fraction was identified as the dimeric compound 2a
(yield 12% based on Ru). Data for 2a: 1H NMR: 7.63 (d, 4H, aromatic),
7.41e7.27 (m, 27H, aromatic), 6.96 (m, 18H, aromatic), 5.12 (m, 3H,
-COOCH-), 3.21 (s, 30H, CH3N-), 3.18 (s, 18H, CH3N-), 2.95 (s, 2H,
d
d
HC≡C-), 1.33e1.20
(m,
18H,
CH3-);
HRMS
(MALDI)
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J.-W. Ying et al. / Journal of Organometallic Chemistry xxx (2017) 1e6
5
(C114H120N16O6Ru4) calcd 2216.5890 [Mþ], found 2216.5913 [Mþ];
MALDI (m/z, based on 101Ru): 2216 [Mþ]; ToF-MS (m/z, based on
101Ru) 2214 [Mþ]. Electrochemistry, E1/2/V,
Ep/V, ibackward/iforward
A, 0.572, 0.100, 0.978; B, ꢀ0.885, 0.163, 0.394.
Acknowledgement
D
:
We gratefully acknowledge the financial support from the Na-
tional Science Foundation (CHE 0715404 and CHE 1362214). Li Liu
thanks the financial support from China Sponsorship Council (CSC
No. 201408420170).
The third fraction was identified as the trimeric compound 3a
(yield 5% based on Ru). Data for 3a: 1H NMR: 7.67 (d, 2H, aromatic),
7.42e7.28 (m, 39H, aromatic), 6.97 (s, 31H, aromatic), 5.21e5.04 (m,
4H, -COOCH-), 3.27 (m, 72H, CH3N-), 3.02 (s, 2H, HC≡C-), 1.34e1.25
(m, 24H, CH3-); MALDI (m/z, based on 101Ru): 3216 [Mþ]; HRMS
(MALDI) (C164H174N24O8Ru6) calcd 3216.8380 [Mþ], found
3216.8389 [Mþ]; ToF-MS (m/z, based on 101Ru) 3219 [Mþ].
Appendix A. Supplementary data
Synthesis and characterizations of ligands L1TMS2 and L2TMS2
(PDF); ESI-MS, 1H NMR and FT-IR spectra for compounds 1a/1b/1c.
Crystallographic data for the structural analysis have been depos-
ited with the Cambridge Crystallographic Data Center, CCDC
1518596 and 1518597 for compounds 1b and 2a, respectively.
Copies of this information may be obtained free of charge from, The
Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, (Fax: þ44-
1233-336033; email: deposit@ccdc.cam.ac.uk or www: http://ccdc.
cam.ac.uk).
4.3. Preparation of compounds 1b
In
a
100
mL
round
bottom
(L2TMS2)
flask,
benzyl-3,5-
mmol,
bis(trimethylsilylethynyl)benzoate
(0.329
0.133 g) was desilylated with TBAF in 20 mL THF solution. The re-
action was stirred at room temperature for 20 min and then was
filtered through a 2 cm silica gel pad. The filtrate was added into a
suspension of Ru2 (DMBA)4(NO3)2 (0.164 mmol, 0.150 g) in 50 mL
THF and 30 mL of Et2NH, and the reaction mixture was stirred at
room temperature overnight. The progress of the reaction was
monitored by TLC. Upon the complete consumption of Ru2
(DMBA)4(NO3)2, several more polar but less abundant products
were present besides the dominant product 1b. The reaction
mixture was filtered through a celite pad, and the volatiles were
removed on rotovap. The residue was purified by column chro-
matography with THF/hexanes (1/5, v/v). While clean separation of
more polar fractions failed, compound 1b was obtained in a yield of
25%. Data for 1b: ESI (m/z, based on 101Ru): 1260 [Mþ-2. C≡CH]; 1H
NMR: 7.67 (d, 4H, aromatic), 7.48e7.39 (m, 24H, aromatic), 7.00 (d,
10H, aromatic), 5.21e5.15 (m, 2H, -COOCH2-), 3.29 (s, 24H, MeN-),
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2017.03.011.
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Single crystals of 1b were obtained by slow evaporation of a
hexanes-THF solution, and those of 2a from an ether-THF solution.
X-ray intensity data were measured on a Nonius-Kappa CCD X-ray
diffractometer using MoK
a
(l
¼ 0.71073 Å). Structures were solved
and refined using the Bruker SHELXTL© (Version 5.1) software
package. Crystallographic data are given in Table 2.
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Table 2
Crystallographic parameters for compounds 1b and 2a.
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1b
2a
Formula
Fw
Space group
C72H66N8O4Ru2
1309.52
C2/c
C122H140N16O8Ru4
2362.86
P1
a (Å)
b (Å)
c (Å)
18.3996 (3)
14.0424 (2)
28.8534 (6)
90
101.097 (7)
90
7315.6 (2)
4
1.189
10.5615 (3)
19.9650 (5)
31.7100 (10)
76.224 (12)
81.988 (12)
77.64 (2)
6316.1 (3)
2
1.242
0.515
Mo K
150 (1)
0.092, 0.212
a
b
g
(ꢁ)
(ꢁ)
(ꢁ)
Volume (Å3)
Z
dcalc. (gcmꢀ3
)
m
(mmꢀ1
)
0.451
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Radiation
T (K)
R1, wR2 (I > 2s(I))
Mo K
150 (1)
0.062, 0.178
a
(0.71073)
a (0.71073)
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6
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