Diruthenium alkynyl compounds with phosphonate capping groups

Article abstract of DOI:10.1021/om301247w

The reaction between Ru₂(ap)₄Cl (ap = 2-anilinopyridinate) and LiC≡C-4-Ph-P(O)(O^tBu)₂ afforded Ru₂(ap)₄-C≡C-4-Ph-P(O)(O ^tBu)₂ (1), which was further reacted with LiC≡C

Full text of DOI:10.1021/om301247w

Note
pubs.acs.org/Organometallics
Diruthenium Alkynyl Compounds with Phosphonate Capping
Groups
Steven P. Cummings, Julia Savchenko, Phillip E. Fanwick, Anastasia Kharlamova, and Tong Ren*
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
S
* Supporting Information
ABSTRACT: The reaction between Ru2(ap)4Cl (ap = 2-
anilinopyridinate) and LiCC-4-Ph-P(O)(OtBu)2 afforded
Ru2(ap)4-CC-4-Ph-P(O)(OtBu)2 (1), which was further
reacted with LiCC-4-Ph-S(CH2)2SiMe3 to yield trans-(4-
Me3Si(CH2)2S-Ph-CC)-Ru2(ap)4-CC-4-Ph-P(O)(OtBu)2
(2). trans-Ru2(DMBA)4(CC-4-Ph-P(O)(OtBu)2)2 (3;
DMBA = N,N′-dimethylbenzamidinate) was obtained from
the reaction between Ru2(DMBA)4(NO3)2 and either HC
C-4-Ph-P(O)(OtBu)2 in the presence of Et3N or LiCC-4-
Ph-P(O)(OtBu)2. Compounds 1−3 were characterized by HR-MS, X-ray diffraction, and voltammetric techniques, which
revealed the retention of essential characteristics in molecular and electronic structures upon the phosphonate capping.
ransition-metal alkynyl compounds have been studied as
RESULTS AND DISCUSSION
■
1,2
Tpromising electronic and optoelectronic materials.
Similar to the earlier studies of other Ru2(ap)4 monoalkynyl
compounds,2,6 Ru2(ap)4-CC-4-Ph-P(O)(OtBu)2 (1) was
prepared via an anion metathesis reaction between Ru2(ap)4Cl
and LiCC-4-Ph-P(O)(OtBu)2 in 46% yield. trans-(4-(TMSE-
S)-Ph-CC)-Ru2(ap)4-CC-4-Ph-P(O)(OtBu)2 (2; TMSE =
Me3Si(CH2)2−) was synthesized in 68% yield from treatment
of compound 1 with 7 equiv of LiCC-Ph-4-S-TMSE. trans-
Ru2(DMBA)4(CC-4-Ph-P(O)(OtBu)2)2 (3) was obtained in
a low yield of 19% from the reaction between
Ru2(DMBA)4(NO3)2 and 2.5 equiv of HCC-4-Ph-P(O)-
(OtBu)2 under weak-base conditions. However, the reaction
between Ru2(DMBA)4(NO3)2 and LiCC-4-Ph-P(O)(OtBu)2
resulted in compound 3 with a much improved yield of 70%.
All three compounds were characterized with HR-MS,
voltammetric, and IR/UV−vis spectroscopic techniques. In
addition, the diamagnetic compounds 2 and 3 were also
characterized with 1H NMR. Compound 1 has a room-
temperature effective magnetic moment of 3.92 μB, consistent
There has been intense interest in modifying conjugated
metal alkynyls with capping groups of high affinity for metal,
semiconductor, or metal oxide surfaces, which is an important
prerequisite for testing their electrical characteristics in devices.
One of the most investigated capping groups is thiol for its
facile attachment to gold. There are many interesting examples
of the synthesis of sulfur-capped inorganic/organometallic
compounds and their incorporation into junctions/devices in
the literature.3 Terminal olefins and alkynes have been
investigated for attachment onto H-passivated silicon.4 Silica
surfaces are also highly relevant to electronic devices because of
the omnipresence of native silica on untreated silicon wafers,
for which a proven capping group is organophosphonate.5
However, metal alkynyl compounds with one or more capping
organophosphonate groups have not yet been reported to the
best of our knowledge. In this contribution, we describe the
synthesis and characterization of diruthenium alkynyl com-
pounds containing one (1 and 2) and two capping
phosphonates (3).
2
3
with a S = /2 ground state. On the basis of the longest λmax
from vis−near-IR spectra, the optical gaps (Eop) are 1.65, 1.20,
and 1.42 eV for compounds 1−3, respectively, which are
comparable to the optical gaps measured for analogous
compounds without phosphonate caps (see Table 4 of ref 2).
Further confirmation of the identity of compounds 2 and 3
came from X-ray diffraction studies, and the structural plots of
2 and 3 are given in Figures 1 and 2, respectively. The
coordination geometry around the diruthenium core in
compound 2 is consistent with that of the previously studied
trans-Ru2(ap)4(C2R)2 type compounds. The Ru1−Ru2 bond
length is 2.4681(8) Å in 2 and is within the range of 2.441−
Chart 1. Diruthenium Alkynyls with Phosphonate Capping
Groups
Received: December 21, 2012
Published: February 5, 2013
© 2013 American Chemical Society
1129
dx.doi.org/10.1021/om301247w | Organometallics 2013, 32, 1129−1132

Organometallics
Note
Figure 1. Molecular structure of 2. Hydrogens are omitted for clarity.
Selected bond lengths (Å) and angles (deg): Ru1−Ru2, 2.4681(8);
Ru2−C14, 1.972(9); Ru1−C1, 1.997(8); C1−C2, 1.175(9); C14−
C15, 1.225(10); Ru1−N(av), 2.042[6]; Ru2−N(av), 2.069[6]; Ru1−
Ru2−C14, 165.3(2); Ru2−Ru1−C1, 163.8(2).
Figure 3. Cyclic and differential pulse voltammograms of compounds
1−3.
CONCLUSION
■
Three diruthenium alkynyl compounds with organophospho-
nate capping groups have been prepared, and these compounds
retain the key features of the electronic structure of the
unsubstituted precursors, especially the rich redox activity and
small energy gaps. Compounds 2 and 3 are attractive
candidates for device fabrication because of their bicapping
nature.7 Compound 3 can be sandwiched between two layers of
dielectric oxide upon the cleavage of tert-butyls using Me3SiCl/
Et3N,8 while compound 2 can be incorporated into a metal−
molecule−insulator junction upon stepwise removal of tert-
butyl and TMSE groups. These aspects are being pursued in
our laboratory through collaboration with device engineers.
Figure 2. Molecular structure of compound 3. There is a
crystallographic inversion center bisecting the Ru−Ru bond. Hydro-
gens are omitted for clarity. Selected bond lengths (Å) and angles
(deg): Ru1−Ru1′, 2.4484(5); Ru1−C1, 1.971(4); C1−C2, 1.208(5);
Ru1−N(av), 2.047[5]; Ru1−Ru1′−C1, 164.6(1).
2.476 Å noted for trans-Ru2(ap)4(C2R)2.2 In comparison to
Ru2(ap)4(C2R) type compounds, the Ru−C bond lengths of 2
are shortened, which is attributed to both the increase of the
oxidation state of the Ru2 core and an enhanced Ru−C bond
strength. The geometric parameters around the Ru2 core in 3
are typical of Ru2(DMBA)4(C2R)2 type compounds. The Ru−
Ru bond length in 3 (2.4484(5) Å) is consistent with the
assignment of a Ru−Ru single bond. The Ru−C bond length
(1.971(4) Å) is in agreement with those determined for other
Ru2(DMBA)4(C2R)2 compounds (1.95−2.00 Å).2 In addition,
compound 3 displays significant structural distortion around
the first coordination sphere of the Ru2 core that is the hallmark
of a second-order Jahn−Teller effect found in many Ru2(III,III)
compounds.2 Structural data of both compounds 2 and 3
indicated that the introduction of a tert-butylphosphonate
substituent does not cause significant structural alteration in
comparison to the unsubstituted analogues, and a minimal
perturbation of the electronic structure can be inferred.
As shown by their cyclic (CV) and differential pulse (DPV)
voltammograms in Figure 3, compounds 1−3 display several
one-electron-redox processes. In each case, there is a reversible
oxidation (B in 1, Ru2(III,III)/Ru2(II,III); A in 2 and 3,
Ru2(III,IV)/Ru2(III,III)), and a reversible reduction (C in 1,
Ru2(II,III)/Ru2(II,II); B in 2 and 3, Ru2(III,III)/Ru2(II,III)).
An irreversible one-electron reduction was also observed for
compound 2 (C, Ru2(II,III)/Ru2(II,II)) in the far cathodic
region. The electrode potentials of these couples are
comparable to those of the corresponding compounds with
unsubstituted arylacetylide ligands. Electrochemical HOMO−
LUMO gaps (Eg = e[E(1+/0) − E(0/1−)]) are 1.28, 1.13, and
1.59 eV for compounds 1−3, which are also very close to those
of the unsubstituted analogues.2
EXPERIMENTAL SECTION
■
General Considerations. n-BuLi (2.5 M in hexanes) was
purchased from Sigma-Aldrich. Di-tert-butyl phosphite was purchased
from Alfa Aesar. THF was distilled over Na/benzophenone under an
N2 atmosphere. Ru2(ap)4Cl6 and Ru2(DMBA)4(NO3)2 were
9
prepared according to literature procedures. All reactions were
performed using standard Schlenk techniques. Vis−near-IR spectra
were obtained with a JASCO V-670 UV−vis−near-IR spectropho-
tometer. Infrared spectra were obtained on a JASCO FT-IR 6300
spectrometer via ATR on a ZnSe crystal. Magnetic susceptibility data
were measured at 293 K with a Johnson Matthey Mark-1 magnetic
susceptibility balance. Cyclic voltammograms were recorded in 0.2 M
n-Bu4NPF6 and 1.0 mM diruthenium species solution (THF, N2
degassed) on a CHI620A voltammetric analyzer with a glassy-carbon
working electrode (diameter 2 mm), Pt-wire counter electrode, and an
Ag/AgCl reference electrode with ferrocene used as an internal
reference (0.570 V).
Synthesis of Di-tert-butyl(4-(ethynyl)phenyl)phosphonate.
((4-Iodophenyl)ethynyl)trimethylsilane (285 mg, 0.95 mmol) and di-
tert-butyl phosphite (250 μL, 0.95 mmol) were refluxed in toluene for
18 h in the presence of Cs2CO3 (1.14 mmol), Pd(OAc)2 (0.095
mmol), and triphenylphosphine (0.57 mmol). The product was
extracted via EtOAc and washed with NH4Cl; after solvent removal,
the red residue was washed with hexanes. The remaining residue was
dissolved in 1/3 EtOAc/hexanes and filtered through a silica pad to
yield a pale yellow oil after solvent removal. Desilylation with K2CO3
resulted in the desired ligand (250 mg, 89%) as a sticky yellow solid.
1
Data: IR (cm−1) 3178 (H−CC), 2100 (CC); H NMR δ 1.422
(s, 18H, tBu), 3.150 (d, 1H, HCC), 7.46−7.50 (m, 2H, Ar), 7.67−
7.75 (m, 2H, Ar).
1130
dx.doi.org/10.1021/om301247w | Organometallics 2013, 32, 1129−1132

Organometallics
Note
Synthesis of Ru2(ap)4-CC-4-Ph-P(O)(OtBu)2 (1). Ru2(ap)4Cl
(55 mg, 0.060 mmol), previously dried under vacuum for 30 h at 45
°C, was dissolved in 7 mL of THF. In a Schlenk tube containing di-
tert-butyl (4-((trimethylsilyl)ethynyl)phenyl)phosphonate (25 mg,
0.085 mmol) in 2 mL of THF was added 35 μL of n-BuLi in hexanes
(0.085 mmol) at 0 °C. After 40 min at 0 °C, the lithiated alkyne was
added to the Ru2(ap)4Cl solution. The reaction mixture turned brown
and was stirred overnight. The reaction mixture was filtered through a
silica plug and the residue after solvent removal was recrystallized from
1/4 EtOAc/hexanes to yield 36 mg of 1 as a green powder (46% based
on Ru). Data for 1: Rf = 0.08 (1/3 THF/hexanes); nESI-HR-MS [M +
H]+ 1173.229 (calcd 1173.248); visible spectrum λmax (nm, ε (M−1
cm−1)) 753 (6100), 477 (8800); IR (cm−1) 2048 (m) (CC). Cyclic
voltammetry (E1/2/V, ΔEp/V, ibackward/iforward): 0.476, 0.10, 0.94;
−0.798, 0.079, 0.83; μeff = 3.92 μB.
104.457(7)°, γ = 90.476(6)°, V = 3713.4(3) Å3, Z = 2, Dcalcd
=
1.257 g cm−3, R1 = 0.063, wR2 = 0.185. Crystal data of 3·(hexane):
C74H102N8O6P2Ru2, fw =1463.78, monoclinic, C2/c, a = 35.077(2) Å,
b = 13.4364(7) Å, c = 19.0626(7) Å, β = 121.914(3)°, V = 7626.2(6)
Å3, Z = 4, Dcalcd = 1.275 g cm−3, R1 = 0.050, wR2 = 0.099.
ASSOCIATED CONTENT
* Supporting Information
■
S
Figures, a table, and CIF files giving high-resolution mass
spectra and vis−near-IR spectra of compounds 1−3, X-ray
crystallographic details of 2 and 3, and crystal data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Synthesis of trans-(4-(TMSE-S)-Ph-CC)-Ru2(ap)4-CC-4-
Ph-P(O)(OtBu)2 (2). To a THF solution of TMSE-S-4-Ph-CCH
chilled at 0 °C was added 320 μL of n-BuLi, and the mixture was
stirred at 0 °C for 40 min. The lithiated ligand was added to a Schlenk
flask containing 107 mg of 1 (0.091 mmol), which yielded a red
solution after stirring for 36 h. Upon exposure to an oxygen
atmosphere for 15 min, the solution became blue, and the reaction
mixture was filtered through a silica pad. After removal of the solvent,
the residue was purified through a deactivated silica column beginning
with hexanes/EtOAc, followed by recrystallization from THF/hexanes
to yield 88 mg of a blue solid (0.067 mmol, 68%). Data for 2: Rf = 0.25
in 1/1/3 TEA/THF/hexanes; nESI-HR-MS [M + H]+ 1406.323
(calcd 1406.330); vis−near-IR spectrum λmax (nm, ε (M−1 cm−1)):
1034 (5900), 627 (10100); IR (cm−1) 2069 and 2073(s) (CC); 1H
NMR δ 0.054 (s, 9H), 0.95 (d, 2H), 1.508 (s, 18H), 3.05 (d, 2H);
5.837 (s, 8H), 6.11 (d, 2H), 6.29 (t, 4H); 6.37 (d, 4H), 6.95 (t, 12H),
7.15 (t, 6H), 7.33 (t, 4H), 9.26 (d, 4H). Cyclic voltammetry (E1/2/V,
ΔEp/V, ibackward/iforward): 0.746, 0.035, 0.65; −0.385, 0.037, 0.96;
−1.540, 0.031, 0.75.
AUTHOR INFORMATION
Corresponding Author
*E-mail: tren@purdue.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the financial support from both the
National Science Foundation (CHE 1057621) and the USAF
Asian Office of Aerospace Research & Development (Grant
FA2386-12-1-4006). T.R. acknowledges the IR/D support from
the National Science Foundation during the preparation of this
paper.
REFERENCES
■
(1) Paul, F.; Lapinte, C. Coord. Chem. Rev. 1998, 178−180, 431−509.
Costuas, K.; Rigaut, S. Dalton Trans. 2011, 40, 5643−5658. Wong, W.-
Y.; Ho, C.-L. Coord. Chem. Rev. 2006, 250, 2627−2690. Wong, W.-Y.;
Ho, C.-L. Acc. Chem. Res. 2010, 43, 1246−1256.
Synthesis of trans-Ru2(DMBA)4(CC-4-Ph-P(O)(OtBu)2)2 (3).
Preparation a. Ru2(DMBA)4(NO3)2 (63 mg, 0.069 mmol) and di-
tert-butyl (4-((trimethylsilyl)ethynyl)phenyl)phosphonate (50 mg and
0.17 mmol) were stirred in 20 mL of THF and 2 mL of Et3N for 30 h
in open air. The reaction mixture was filtered through a Celite plug,
and the crude product was recrystallized from THF/hexanes to yield 3
as a red crystalline solid (18 mg, 0.13 mmol, 19%).
(2) Ren, T. Organometallics 2005, 24, 4854−4870.
(3) Schull, T. L.; Kushmerick, J. G.; Patterson, C. H.; George, C.;
Moore, M. H.; Pollack, S. K.; Shashidhar, R. J. Am. Chem. Soc. 2003,
125, 3202−3203. Blum, A. S.; Ren, T.; Parish, D. A.; Trammell, S. A.;
Moore, M. H.; Kushmerick, J. G.; Xu, G.-L.; Deschamps, J. R.; Pollack,
S. K.; Shashidhar, R. J. Am. Chem. Soc. 2005, 127, 10010−10011. Kim,
B.; Beebe, J. M.; Olivier, C.; Rigaut, S.; Touchard, D.; Kushmerick, J.
G.; Zhu, X.-Y.; Frisbie, C. D. J. Phys. Chem. C 2007, 111, 7521−7526.
Liu, K.; Wang, X. H.; Wang, F. S. ACS Nano 2008, 2, 2315−2323.
Mahapatro, A. K.; Ying, J.; Ren, T.; Janes, D. B. Nano Lett. 2008, 8,
2131−2136. Lu, Y.; Quardokus, R.; Lent, C. S.; Justaud, F.; Lapinte,
C.; Kandel, S. A. J. Am. Chem. Soc. 2010, 132, 13519−13524. Chen, I.-
W. P.; Fu, M.-D.; Tseng, W.-H.; Yu, J.-Y.; Wu, S.-H.; Ku, C.-J.; Chen,
C.-h.; Peng, S.-M. Angew. Chem., Int. Ed. 2006, 45, 5814−5818. Yin,
C.; Huang, G.-C.; Kuo, C.-K.; Fu, M.-D.; Lu, H.-C.; Ke, J.-H.; Shih, K.-
N.; Huang, Y.-L.; Lee, G.-H.; Yeh, C.-Y.; Chen, C.-h.; Peng, S.-M. J.
Am. Chem. Soc. 2008, 130, 10090−10092. Luo, L.; Benameur, A.;
Brignou, P.; Choi, S. H.; Rigaut, S.; Frisbie, C. D. J. Phys. Chem. C
2011, 115, 19955−19961. Ballmann, S.; Hieringer, W.; Secker, D.;
Preparation b. Di-tert-butyl (4-((trimethylsilyl)ethynyl)phenyl)-
phosphonate (71 mg, 0.242 mmol) in 5 mL of THF was treated with
120 μL of n-BuLi in hexanes at −78 °C for 50 min. The lithiated
ligand was transferred into
a Schlenk tube containing
Ru2(DMBA)4(NO3)2 (100 mg, 0.110 mmol) in 15 mL of THF.
The reaction mixture quickly turned wine red and was stirred
overnight under N2. The reaction mixture was filtered through a silica
plug, and the residue was recrystallized in 1/3 THF/hexanes to yield
105 mg of 3 as a red powder (70% based on Ru).
Data for 3: 1H NMR δ 1.40 (s, 36H, tBu), 3.23 (s, 24H, CH3), 7.03
(d, 8H, aromatic), 7.12 (d, 4H, aromatic), 7.45 (d, 12 H, aromatic),
7.67 (d, 4H, aromatic); nESI-HR-MS [M + H]+ 1378.437 (calcd
1378.442); vis−near-IR spectrum λmax (nm, ε (M−1 cm−1): 874
(1900), 503 (11500); IR (cm−1) 2069(s), (CC). Cyclic
voltammetry (E1/2/V, ΔEp/V, ibackward/iforward): 0.630, 0.099, 0.96;
−0.993, 0.10, 0.87.
Zheng, Q.; Gladysz, J. A.; Gorling, A.; Weber, H. B. ChemPhysChem
̈
2010, 11, 2256−2260.
X-ray Data Collection, Processing, and Structure Analysis
and Refinement. Single crystals of compounds 2 and 3 were grown
via slow cooling of THF/benzene/hexanes and THF/hexanes
solutions, respectively. X-ray diffraction data of 2 and 3 were collected
on a Rigaku RAPID-II image plate diffractometer (Cu Kα) and a
NONIUS Kappa CCD diffractometer (Mo Kα), respectively. The
(4) Gauthier, N.; Argouarch, G.; Paul, F.; Humphrey, M. G.; Toupet,
L.; Ababou-Girard, S.; Sabbah, H.; Hapiot, P.; Fabre, B. Adv. Mater.
2008, 20, 1952−1956. Cummings, S. P.; Savchenko, J.; Ren, T. Coord.
Chem. Rev. 2011, 255, 1587−1602.
(5) Loewe, R. S.; Ambroise, A.; Muthukumaran, K.; Padmaja, K.;
Lysenko, A. B.; Mathur, G.; Li, Q. L.; Bocian, D. F.; Misra, V.; Lindsey,
J. S. J. Org. Chem. 2004, 69, 1453−1460. Li, Q. L.; Surthi, S.; Mathur,
G.; Gowda, S.; Zhao, Q.; Sorenson, T. A.; Tenent, R. C.;
Muthukumaran, K.; Lindsey, J. S.; Misra, V. Appl. Phys. Lett. 2004,
85, 1829−1831.
small size of the crystal used limited the data collection of 2 to 2θmax
<
133°. The structures were solved using the structure solution program
DIRDIF200810 and refined using SHELX-TL.11 While the structural
refinement of 3 was routine, the SQUEEZE routine in PLATON was
used to remove the residual solvent in 2. Crystal data of 2:
C H N O PRu SSi, fw =1405.73, triclinic, P1, a = 10.6343(2) Å, b
(6) Zou, G.; Alvarez, J. C.; Ren, T. J. Organomet. Chem. 2000, 596,
152−158.
̅
73 75
8
3
2
= 19.3018(3) Å, c = 20.5439(14) Å, α = 113.617(8)°, β =
1131
dx.doi.org/10.1021/om301247w | Organometallics 2013, 32, 1129−1132

Organometallics
Note
(7) Li, Q. L. Mod. Phys. Lett. B 2008, 22, 1183−1202.
(8) Sekine, M.; Iimura, S.; Nakanishi, T. Tetrahedron Lett. 1991, 32,
395−398.
(9) Xu, G.-L.; Jablonski, C. G.; Ren, T. Inorg. Chim. Acta 2003, 343,
387−390.
(10) Beurskens, P. T.; Beurskens, G.; de Gelder, R.; Garcia-Granda,
S.; Gould, R. O.; Smits, J. M. M. The DIRDIF2008 Program System;
Crystallography Laboratory, University of Nijmegen, Nijmegen, The
Netherlands, 2008.
(11) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112−122.
1132
dx.doi.org/10.1021/om301247w | Organometallics 2013, 32, 1129−1132