Coordinating tectons: Bimetallic complexes from bipyridyl terminated group 8 alkynyl complexes

Article abstract of DOI:10.1021/om500172r

Bipyridyl appended ruthenium alkynyl complexes have been used to prepare a range of binuclear homometallic ruthenium and heterometallic ruthenium - rhenium complexes. The two metal centers are only weakly coupled, as evinced by IR and UV - vis - near NIR spectroelectrochemical experiments and supported by quantum chemical calculations. The alkynyl complexes of the type [Ru(C= Cbpy){L_n}] ({L_n} = {(PPh₃)₂Cp}, {(dppe)Cp}, {Cl(dppm)₂}) undergo reversible one-electron oxidations centered largely on the alkynyl ligands, as has been observed previously for closely related complexes. The homometallic binuclear complexes, exemplified by [Ru(C₂bpy-K²-N′N-RuClCp)(PPh₃)₂Cp] undergo two essentially reversible oxidations, the first centered on the (C2bpy-κ²-N′N-RuClCp) moiety and the second on the Ru(C≡Cbpy)(PPh₃)₂Cp fragment, leading to radical cations that can be described as Class II mixed-valence complexes. The heterometallic binuclear complexes [Ru(C₂bpy-κ²-N′N-ReCl(CO)₃){L_n}] display similar behavior, with initial oxidation on the ruthenium fragment giving rise to a new optical absorption band with Re → Ru(C≡Cbpy) charge transfer character. The heterometallic complexes also exhibit irreversible reductions associated with the Re hetereocycle moiety. (Figure Presented)

Full text of DOI:10.1021/om500172r

Article
pubs.acs.org/Organometallics
Coordinating Tectons: Bimetallic Complexes from Bipyridyl
Terminated Group 8 Alkynyl Complexes
George A. Koutsantonis,*,† Paul J. Low,*,†,‡ Campbell F. R. Mackenzie,† Brian W. Skelton,†,§
and Dmitry S. Yufit‡
†Chemistry, School of Chemistry and Biochemistry and §Centre for Microscopy, Characterisation and Analysis, University of Western
Australia, Crawley, Western Australia 6009, Australia
‡Department of Chemistry, Durham University, South Road, Durham DH1 3LE, U.K.
S
* Supporting Information
ABSTRACT: Bipyridyl appended ruthenium alkynyl complexes have
been used to prepare a range of binuclear homometallic ruthenium and
heterometallic ruthenium−rhenium complexes. The two metal centers
are only weakly coupled, as evinced by IR and UV−vis−near NIR
spectroelectrochemical experiments and supported by quantum
chemical calculations. The alkynyl complexes of the type [Ru(C
Cbpy){Ln}] ({Ln} = {(PPh3)2Cp}, {(dppe)Cp*}, {Cl(dppm)2})
undergo reversible one-electron oxidations centered largely on the
alkynyl ligands, as has been observed previously for closely related
complexes. The homometallic binuclear complexes, exemplified by
[Ru(C2bpy-κ2-N′N-RuClCp)(PPh3)2Cp] undergo two essentially rever-
sible oxidations, the first centered on the (C2bpy-κ2-N′N-RuClCp)
moiety and the second on the Ru(CCbpy)(PPh3)2Cp fragment,
leading to radical cations that can be described as Class II mixed-valence complexes. The heterometallic binuclear complexes
[Ru(C2bpy-κ2-N′N-ReCl(CO)3){Ln}] display similar behavior, with initial oxidation on the ruthenium fragment giving rise to a
new optical absorption band with Re → Ru(CCbpy) charge transfer character. The heterometallic complexes also exhibit
irreversible reductions associated with the Re hetereocycle moiety.
proposed organometallic-coordination polymer approach to
molecular electronic components involves the preparation of
modular organometallic “coordinating tectons” that will be used
as the basic repeating unit to form polymetallic complexes of
well-defined length.
Initially, our interest lies in complexes where spectroscopic,
including spectroelectrochemical, and computational methods
will be used to explore molecular electronic structure as a
function of conformation and redox state in these systems and
assess the influence of conformation on intramolecular charge
transfer process.
INTRODUCTION
■
Molecular electronics involves the use of individual molecules,
or groups of molecules, as functional moieties that may replace
or augment conventional solid-state (usually silicon) electronic
components, and is widely regarded as the ultimate solution to
the growing difficulties facing “top-down” design strategies.1−7
In the construction of hybrid molecular/solid-state electronic
devices, the key challenge lies in the realization of the potential
of single-electron phenomena within hybrid device struc-
tures.8,9 There needs elementary science to be developed that
investigates the fundamental properties of molecules, including
electronic coupling effects, to realize molecule-based electronics
technology.
Many prototypical bimetallic systems have been investigated
to define intramolecular, solution-phase electron transfer
characteristics and identify promising candidate wire-like
molecular moieties,10−12 such as polyynes13−16 and oligo-
(phenylene)ethynylene based structures17 and which have
successfully been translated into designs of organic and
organometallic molecules for study as components in molecular
junctions.18−23
Herein we describe further steps toward this goal and detail
the preparation of a series of complexes whose role as putative
tectons and molecular electronic structure is investigated
through the formation of bimetallic complexes.
EXPERIMENTAL SECTION
■
General Considerations. All reactions were performed under an
atmosphere of high purity argon or nitrogen using standard Schlenk
techniques. Reaction solvents either were purified and dried using an
In this regard, the use of organometallic coordinating tectons
in the assembly of large heterometallic complexes, albeit not
with wire-like geometries, by Lang provides conceptual basis for
further development of these synthetic strategies.24,25 Our
Special Issue: Organometallic Electrochemistry
Received: February 16, 2014
Published: August 6, 2014
© 2014 American Chemical Society
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Innovative Technology SPS-400 (THF, ether, hexane, and toluene)
and degassed prior to use or were purified and dried by appropriate26
means prior to distillation and storage under Argon. No special
precautions were taken to exclude air or moisture during workup. The
compounds [trans-Ru(CCbpy)Cl(dppm)2] (3),27 [RuCl(COD)-
Cp],28 [RuCl(PPh3)2Cp],29 [RuCl(dppe)Cp*],30 [Re(κ2-N′N-
HC2bpy)Cl(CO)3],31 HCCbpy,32,33 and PhCCbpy34 were
synthesized according to literature procedures (HCCbpy = 5-
ethynyl-2,2′-bipyridine; PhCCbpy = 5-(phenylethynyl)-2,2′-bipyr-
idine). All other materials were obtained from commercial suppliers
and used as received.
Chart 1. Complexes Prepared and Studied in This Work
The NMR spectra were recorded on 400 MHz Varian, Bruker AV-
1
500, or Bruker AV-600 spectrometers. H and 13C{1H} spectra were
referenced to residual solvent signals, whereas 31P{1H} spectra were
referenced to external phosphoric acid. IR spectra were recorded using
a Thermo Scientific Nicolet 6700 spectrometer as CH2Cl2 solutions in
a cell fitted with CaF2 windows. UV−vis spectra were recorded on a
PerkinElmer Lambda 25 UV−vis spectrophotometer as CH2Cl2
solutions in a quartz cell, or on a Thermo Array UV−vis
spectrophotometer as CH2Cl2 solutions in a cell with CaF2 windows.
MALDI-mass spectra were recorded using an Autoflex II TOF/TOF
mass spectrometer with a 337 nm laser. Samples in CH2Cl2 (1 mg/
mL) were mixed with a matrix solution of trans-2-[3-(4-tert-
butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) in a
1:9 ratio, with 1 μL of mixture spotted onto a metal target prior to
exposure to the MALDI ionization source. Electrospray mass spectra
were recorded on a Waters LCT Premier spectrometer. Micro-
analytical Services, Research School of Chemistry, Australian National
University, Canberra, Australia, or London Metropolitan University,
London, United Kingdom, performed elemental analyses.
Electrochemical analyses were carried out using either an
EcoChemie Autolab PG-STAT 30 or a Palm Instruments EmStat
potentiostat. A platinum disk electrode with platinum counter and
platinum pseudoreference electrodes were used in CH2Cl2 solutions
containing 0.1 M nBu4NPF6 electrolyte. The decamethylferrocene/
decamethylferrocenium (FeCp*2/[FeCp*2]+) couple was used as an
internal reference, with all potentials reported relative to the
ferrocene/ferrocenium couple (FeCp2/[FeCp2]+) (FeCp*2/
[FeCp*2]+ = −0.48 V in CH2Cl2). Spectroelectrochemical measure-
ments were made in an OTTLE cell of Hartl design,35 from CH2Cl2
n
solutions containing 0.1 M Bu4NPF6 electrolyte. The cell was fitted
into the sample compartment of the Thermo 6700 FTIR or Cary 5000
UV−vis−near NIR spectrophotometer, and electrolysis in the cell was
performed with either a PGSTAT-30 or EmStat potentiostat.
ideal values. Non-H atoms of the disordered atoms were refined with
isotropic displacement parameters only.
Crystallography. The crystal data for 1−3, 5−7, 9, 10, 12−15 are
summarized in Table S2 (Supporting Information) with the complexes
depicted in Chart 1 and in the figures below, where ellipsoids have
been drawn at the 50% probability level, unless otherwise stated.
Crystallographic data for the structures were collected at 100(2) K
(180 K for 3b) on an Oxford Diffraction Gemini diffractometer fitted
with Cu Kα radiation (for 2, 3.2Tol, 3b, 12) or Mo Kα (1,10, and 13).
Data for 9, 14, and 15 were collected on an Oxford Diffraction
XCalibur diffractometer fitted with Mo Kα radiation Following
analytical absorption corrections and solution by direct methods, the
structures were refined against F2 with full-matrix least-squares using
the program SHELXL-97.36 Ligand nitrogen atoms were distinguished
from carbon atoms on the basis of geometries and refinement. All
hydrogen atoms were added at calculated positions and refined by use
of riding models with isotropic displacement parameters based on
those of the parent atoms. Except were mentioned below, anisotropic
displacement parameters were employed throughout for the non-
hydrogen atoms. All H atoms were added at calculated positions and
refined by use of a riding model with isotropic displacement
parameters based on the isotropic displacement parameter of the
parent atom. For complex 3, two structures were obtained and, in the
one containing disordered toluene solvent, one of the two solvent
toluene molecules was modeled as being disordered over two sets of
sites each with site occupancies set at 0.5 after trial refinement. In the
other, 3b, one of the bipyridyl rings was similarly modeled as
disordered. Geometries of the disordered atoms were restrained to
In the structure of 9, three of the dichloromethane solvent
molecules were modeled as being disordered. Their geometries were
restrained to ideal values.
The data for compounds 5 and 7 were collected at 120 and 100 K,
respectively, on a Rigaku Saturn 724+ diffractometer at Station I19 of
the Diamond Light Source synchrotron (undulator, l = 0.6889 Å, w-
scan, 1.0°/frame). The data for compound 6 were collected on a
Bruker SMART CCD 6000 diffractometer (graphite monochromator,
λMo Kα, λ = 0.71073 Å) at 100 K. The structures were solved by direct
methods and refined by full-matrix least-squares on F2 for all data
using SHELXTL36 and OLEX2.37
Synthesis of the Complexes. [Ru(CCbpy)(dppe)Cp*] (1).
[RuCl(dppe)Cp*] (200 mg, 0.298 mmol), HCCbpy (65 mg, 0.359
mmol), and NH4PF6 (100 mg, 0.613 mmol) were suspended in dry
MeOH (40 mL) and heated to reflux for 2 h, during which a yellow
suspension became a deep red solution. The solution was cooled to
room temperature before the addition of DBU, followed by 30 min of
stirring, resulting in the formation of an orange precipitate. The
mixture was cooled to 0 °C, and the reaction product was collected on
a glass frit and washed with MeOH (2 × 3 mL) and Et2O (3 mL) to
give the product as an orange powder (214 mg, 88%). Crystals suitable
for X-ray analysis were grown through the vapor diffusion of n-pentane
into a CH2Cl2 solution of the complex. Anal. Calcd for
C48H46N2P2Ru11: C, 70.83; H, 5.70; N, 3.44. Found: C, 70.77; H,
5.64; N, 3.52. H NMR (CDCl3, 400 MHz): 1.57 (s, 15H, Cp*),
2.01−2.15 (m, 2H, PCH2CH2P), 2.63−2.73 (m, 2H, PCH2CH2P),
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7.05 (d, 1H, H5′), 7.19 (m, 1H, H4′), 7.22−7.26 (m, 8H, Hmeta),
7.29−7.40 (m, 9H, Hortho and H3′), 7.73−7.76 (m, 4H, Hpara), 8.02 (s,
Cp*), 2.11−2.21 (m, 2H, PCH2CH2P), 2.61−2.69 (m, 2H,
PCH2CH2P), 7.06 (dd, 1H, H3′), 7.15 (ddd, 1H, H5′), 7.18 (d, 1H,
H4), 7.22−7.41 (m, 12H, Hortho and Hpara), 7.41−7.46 (m, 4H, Hmeta),
7.67−7.73 (m, 4H, Hmeta), 7.89−7.94 (m, 2H, H4′ and H3), 8.18 (s,
1H H6), 8.88 (dd, 1H, H6′). 13C{1H} NMR (CD2Cl2, 151 MHz): δ
10.0 (s, Me5Cp), 29.7 (m, PCH2CH2P), 93.9 (s, Me5Cp), 110.2 (s,
Cβ), 122.1 (s, C3), 122.7 (s, C3), 125.2 (s, C5′), 128.1 (m, Cortho),
129.7 (m, Cpara), 131.4 (s, C5), 133.5 (s, Cmeta), 136.3 (m, Cipso), 138.5
(s, C4′), 139.0 (s, C4), 147.0 (s, C2), 152.9 (s, C6′), 154.3 (s, C2′),
156.9 (s, C2′), 160.0 (m, Cα), 190.7 (s, CO), 198.4 (s, CO). 31P{1H}
3
3
1H, H6), 8.08 (d, 1H, H4, JHH = 8 Hz), 8.25 (d, 1H, H3, JHH = 8
Hz), 8.61 (dd, 1H, H6′). 13C{1H} NMR (CDCl3, 100 MHz): 10.0 (s,
Me5Cp), 29.4 (m, PCH2CH2P), 92.8 (s, Me5Cp), 107.7 (s, Cβ),
120.1(s, C3), 120.6 (s, C3′), 122.5 (s, C5′), 127.4 (t, Phmeta), 127.6 (t,
Phmeta), 129.2 (s, Phpara), 133.3 (t, Phortho), 136.4 (s, C5), 136.8 (s,
1
4
C4′) 137.4 (s, C4), 138.7 (dd, Cipso, JCP = 39.7 Hz, JCP = 6.4 Hz),
2
141.0 (t, Cα, JCP = 24.4 Hz), 149.0 (s, C2), 149.1 (s, C6′), 151.0 (s,
C6), 157.0 (s, C2′). 31P{1H} NMR (CDCl3, 162 MHz): 79.72 ppm
(s). IR (CH2Cl2 solution): νCC 2067 cm−1 (2044 cm−1 shoulder).
MS (MALDI): m/z 814 ([M]+, 100%), 663 ([Ru(CO)(dppe)Cp*]+,
90%), 635 ([Ru(dppe)Cp*]+, 30%). UV−vis (CH2Cl2) λ (nm) [ε ×
104 M−1 cm−1]: 399 [2.37].
3
3
NMR (CD2Cl2, 243 MHz): 80.86 (d, JPP = 16 Hz), 80.62 (d, JPP
=
16 Hz). IR (CH2Cl2 solution): νCC 2038 cm−1, νCO 2018 cm−1,
νCO 1915 cm−1, and νCO 1893 cm−1. MS (MALDI): m/z 663
([Ru(CO)(dppe)Cp*]+, 100%), 635 ([Ru(dppe)Cp*]+, 70%), 1120
([M]+, 5%). UV−vis (CH2Cl2) λ (nm) [ε × 104 M−1 cm−1]: 387
[1.21], 506 [1.72].
[Ru(CCbpy)(PPh3)2Cp] (2). [RuCl(PPh3)2Cp] (400 mg, 0.551
mmol), HCCbpy (150 mg, 0.826 mmol), and NH4PF6 (200 mg,
1.23 mmol) were refluxed in MeOH for 2 h, during which the yellow
suspension became a deep red solution. The solution was cooled to
room temperature before the addition of DBU (0.5 mL), followed by
30 min of stirring, resulting in the formation of a yellow precipitate.
The mixture was cooled to 0 °C, and the reaction product was
collected on a glass frit and washed with MeOH (2 × 5 mL) to give
the product as a yellow powder (348 mg, 73%). Crystals suitable for X-
ray analysis were obtained through the slow diffusion of hexane into a
CDCl3 solution of the complex. This complex matches the
spectroscopic data presented previously,31 with the following revised
13C{1H} assignments; 13C{1H} (CDCl3, 151 MHz): δ 85.5 (s, Cp),
112.2 (s, Cβ), 120.3 (s, C3), 120.8 (s, C3′), 122.7 (s, C5′), 127.2 (s,
[Ru(CCbpy-κ2-N′N-RuClCp)(PPh3)2Cp] (6). [Ru(CCbpy)-
(PPh3)2Cp] (2) (100 mg, 0.115 mmol) and [RuCl(COD)Cp] (38
mg, 0.123 mmol) were dissolved in acetone (30 mL), and the solution
was stirred at room temperature overnight. The solvent volume was
reduced to ca. 10 mL in vacuo, followed by the addition of diethyl
ether (30 mL) and cooling to −15 °C. The precipitate that formed was
collected and washed with diethyl ether (2 × 5 mL) to give the
product as purple crystals (92 mg, 75%). Crystals suitable for X-ray
analysis were obtained through layer diffusion of hexane into a CH2Cl2
solution of the complex. Anal. Calcd for C58H47ClN2P2Ru2: C, 65.01;
H, 4.42; N, 2.61. Found: C, 64.89; H, 4.51; N, 2.53. 1H NMR
(CD2Cl2, 500 MHz): δ 4.18 (s, 5H, Cpbpy), 4.42 (s, 5H, CpPP), 7.13−
7.21 (m, 13H, Hmeta and H5′), 7.26−7.30 (m, 7H, Hpara and H3′),
7.43−7.52 (m, 12H, Hortho), 7.68 (dd, (1H, H4′), 7.75 (d, 1H, H3),
7.84, (d, 1H, H4), 9.29, (d, 1H, H6), 9.55 (d, 1H, H6′). 13C{1H}
NMR (CD2Cl2, 126 MHz): δ 69.3 (s, Cpbpy), 86.1 (s, CpPP), 112.6 (s,
Cβ), 121.0 (s, C3), 121.3 (s, C3′), 123.6 (s, C5′), 127.8 (m, Cmeta),
128.5 (s, C5), 129.2 (s, Cpara), 134.6 (m, Cortho and C4′), 135.7 (s, C4),
137.0 (t, Cα, 2JCP = 24.3 Hz), 138.9 (m, Cipso), 149.3 (s, C2), 155.2 (s,
C6′), 156.7 (m, C6 and C2′). 31P{1H} NMR (CDCl3, 162 MHz): δ
49.29 (s), 49.26 (s). IR (CH2Cl2 solution): νCC 2045 cm−1. MS
(MALDI): m/z 719 ([Ru(CO)(PPh3)2Cp]+, 100%), 1036 ([M −
Cl]+, 15%), 774 ([M − PPh3 − Cl]+, 12%), 1072 ([M + H]+, 5%).
UV−vis (CH2Cl2) λ (nm) [ε × 104 M−1 cm−1]: 295 [2.43], 374
[1.31], 434 [2.36].
2
2
C5), 127.3 (t, JCP = 4.4 Hz, Cortho), 128.4 (t, JCP = 25.7 Hz, Cα),
128.7 (s, Cpara), 133.9 (t, 3JCP = 4.8 Hz, Cmeta), 136.8 (s, C4′), 138.0 (s,
1
C4), 138.8 (t, JCP = 21.0 Hz, Cipso), 149.2 (s, C6′), 149.7 (s, C2),
151.2 (s, C6), 156.9 (s, C2′).
[Ru(CCbpy-κ2-N′N-RuClCp)(dppe)Cp*] (4). [Ru(CCbpy)-
(dppe)Cp*](1) (50 mg, 0.061 mmol) and [RuCl(COD)Cp] (20
mg, 0.065 mmol) were dissolved in acetone (30 mL) and stirred at
room temperature overnight. The solvent volume was reduced to ca.
10 mL in vacuo, followed by the addition of diethyl ether (30 mL) and
cooling to −15 °C. The precipitate that formed was collected and
washed with diethyl ether (2 × 5 mL) to give the product as purple
crystals (43 mg, 69%). Anal. Calcd for C53H51ClN2P2Ru2: C, 62.68; H,
1
5.06; N, 2.76. Found: C, 62.55; H, 4.95; N, 2.86. H NMR (CD2Cl2,
[Ru(CCbpy-κ2-N′N-ReCl(CO)3)(PPh3)2Cp] (7). [Ru(CCbpy)-
(PPh3)2Cp] (2) (55 mg, 0.063 mmol) and [ReCl(CO)5] (26 mg,
0.072 mmol) were dissolved in toluene (30 mL), and the solution was
refluxed for 2 h, during which a bright yellow solution became deep
red. The solvent volume was reduced to ca. 5 mL in vacuo, followed by
the addition of hexane (20 mL) and cooling to −15 °C overnight. The
precipitate that formed was collected and washed with hexane (3 × 5
mL) to give the product as a red powder (38 mg, 51%). Crystals
suitable for X-ray crystallography were grown through vapor diffusion
of n-pentane into a CD2Cl2 solution of the compound. Anal. Calcd for
C56H42ClN2O3P2ReRu·1.5CH2Cl2: C, 53.00; H, 3.48; N, 2.15. Found:
600 MHz): 1.59 (s, 15H, Cp*), 2.16 (m, 2H, PCH2CH2P), 2.68 (m,
2H, PCH2CH2P), 4.11 (s, 5H, Cp), 6.92 (d, 1H, H3′), 7.18 (ddd, 1H,
H5′), 7.21−7.29 (m, 4H, Hmeta), 7.33−7.43 (m, 8H, Hortho and Hmeta),
7.44−7.48 (m, 4H, Hpara), 7.60 (d, 1H, H4, 3JHH = 8.4 Hz), 7.65 (ddd,
1H, H4′), 7.72−7.80 (m, 5H, Hortho and H3), 8.79 (s, 1H, H6), 9.50
(d, 1H, H6′). 13C{1H} NMR (CD2Cl2, 151 MHz): δ 10.2 (s, Me5Cp),
29.8 (m, PCH2CH2P), 69.2 (s, Cp), 93.5 (s, Me5Cp), 108.6 (s, Cβ),
120.8 (s, C3), 121.1 (s, C3′), 123.4 (s, C5′), 127.9 (m, Cmeta), 129.1
(s, C5), 129.6 (m, Cpa1ra), 133.7 (m, Cortho), 134.6 (s, C4′), 135.8 (s,
4
C4), 138.8 (dd, Cipso, JCP = 72.9 Hz, JCP = 34.0 Hz), 148.7 (s, C2),
2
150.1 (t, Cα, JCP = 24.0 Hz), 155.1 (s, C6′), 156.0 (s, C6), 156.8 (s,
1
3
C2′). 31P{1H} NMR (CD2Cl2, 243 MHz): 81.17 (d, JPP = 16 Hz),
C, 53.40; H, 3.25; N, 2.14. H NMR: δ 4.43 (s, 5H, Cp), 7.10 (ddd,
3
80.90 (d, JPP = 16 Hz). IR (CH2Cl2 solution): νCC 2042 cm−1. MS
1H, H5′), 7.15−7.18 (m, 12H, Hortho), 7.26−7.29 (m, 6H, Hpara),
7.38−7.45 (m, 13H, Hmeta and H3′), 7.89 (d, 1H, H4), 8.00 (m, 2H,
H3 and H4′), 8.63 (d, 1H, H6), 8.94 (d, 1H, H6′). 13C{1H} NMR
(CD2Cl2, 151 MHz): δ 86.4 (s, Cp), 113.7 (s, Cβ), 122.3 and 122.9 (s,
C3 and C3′), 125.5 (s, C5′), 127.9 (s, Cortho), 129.3 (s, Cpara), 130.8 (s,
C4′), 131.6 (s, C4), 132.3 (s, C5), 134.0 (m, Cmeta), 138.8 (m, Cipso),
146.6 (m, Cα), 147.8 (s, C2), 153.0 (s, C6′), 154.6 (s, C6), 156.8 (s,
C2′), 190.6 (s, CO), 198.4 (s, CO). 31P{1H} NMR (CDCl3, 162
MHz): δ 49.1 (s). IR (CH2Cl2 solution): νCC 2043 cm−1, νCO 2019
cm−1, νCO 1916 cm−1 and νCO 1894 cm−1. MS (MALDI): m/z 719
([Ru(CO)(PPh3)2Cp]+, 100%), 691 ([Ru(PPh3)2Cp]+, 20%), 1140
([M − Cl]+, 5%). UV−vis (CH2Cl2) λ (nm) [ε × 104 M−1 cm−1]: 302
[1.98], 481 [1.79].
(MALDI): m/z 981 ([M − Cl]+, 100%), 663 ([Ru(CO)(dppe)Cp*]+,
50%), 635 ([Ru(dppe)Cp*]+, 30%), 1015 ([M]+, 10%), 814 ([M −
RuClCp]+, 10%). UV−vis (CH2Cl2) λ (nm) [ε × 104 M−1 cm−1]: 299
[1.75], 367 [1.42], 450 [1.84].
[Ru(CCbpy-κ2-N′N-ReCl(CO)3)(dppe)Cp*] (5). [Ru(CCbpy)-
(dppe)Cp*](1) (70 mg, 0.086 mmol) and [ReCl(CO)5] (33 mg,
0.091 mmol) were dissolved in toluene (40 mL), and the solution was
refluxed for 2 h, during which a bright yellow solution became deep
purple. The solvent volume was reduced to ca. 10 mL in vacuo,
followed by the addition of hexane (20 mL) and cooling to 0 °C. The
precipitate was collected and washed with hexane (3 × 3 mL) to give
the product as purple crystals (77 mg, 80%). Crystals suitable for X-ray
analysis were obtained through the slow diffusion of hexane into a
CDCl3 solution of the complex. Anal. Calcd for
C51H46ClN2O3P2Re1Ru1: C, 54.71; H, 4.14; N, 2.50. Found: C,
54.78; H, 4.12; N, 2.60. 1H NMR (CD2Cl2, 600 MHz): δ 1.60 (s, 15H
[Ru(CCbpy-κ2-N′N-RuClCp)Cl(dppm)2](8). [Ru(CCbpy)Cl-
(dppm)2] (85 mg, 0.078 mmol) and [RuCl(COD)Cp] (30 mg,
0.097 mmol) were dissolved in acetone (15 mL), and the solution was
stirred at room temperature overnight. The solvent volume was
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reduced to ca. 5 mL in vacuo, and the precipitate was collected and
washed with diethyl ether (3 × 3 mL) to give the product as red
crystals (75 mg, 69%). Anal. Calcd for C67H56Cl2N2P4Ru2 C, 62.57; H,
C17H13ClN2Ru: C, 53.48; H, 3.43; N, 7.34. Found: C, 53.33; H, 3.50;
1
N, 7.24. H NMR (CDCl3, 500 MHz): δ 3.38 (s, 1H, CCH), 4.31
(s, 5H, Cp), 7.31 (m, 1H, H5′), 7.71−7.76 (m, 2H, H3′ and H4′),
7.92−7.97 (m, 2H, H4 and H3), 9.64 (dd, 1H H6′), 9.73 (s, 1H, H6).
13{1H} NMR (CDCl3, 126 MHz): δ 70.3 (Cp), 79.3 (Cβ), 83.1 (Cα),
120.5 (C5), 121.2 (C3), 122.3 (C3′), 125.1 (C5′), 134.6 (C4′), 137.2
(C4), 155.3 (3 signals, C2, C2′, and C6′), 157.8 (C6). IR (KBr disk):
νCCH 3133 cm−1, νCC 2098 cm−1. Mp ≥ 300 °C (dec). MS
(MeCN, ES (+)): m/z 388 ([Ru(NCMe) (HC2bpy)Cp]+, 100%, 382
([M]+, 15%). UV−vis (CH2Cl2) λ (nm) [ε × 104 M−1 cm−1]: 259
[1.74], 313 [3.88], 367 [0.62], 538 [0.43].
1
4.39; N, 2.18. Found: C, 56.79; H, 3.87; N, 1.92. H NMR (CD2Cl2,
600 MHz): δ 4.11 (s, 5H, Cp), 4.93−5.03 (m, 4H, PCH2P), 6.21 (d,
1H, H4), 7.17−7.26 (m, 16H, Hortho), 7.31−7.38 (m, 8H, Hpara), 7.40−
7.55 (m, 18H, Hmeta, H3′ and H5′), 7.67 (ddd, 1H, H4′), 7.77 (d, 1H,
H3), 8.25 (s, 1H, H6), 9.53 (d, 1H, H6′). 13C{1H} NMR (CD2Cl2,
151 MHz): δ 49.8 (m, PCH2P), 68.8 (s, Cp), 109.7 (s, Cβ), 120.3 and
120.4 (s, C3 and C3′), 123.1 (s, C5′), 127.2 (m, Cortho), 129.7 (m,
Cpara), 133.3 (m, Cmeta), 134.2 (m, Cipso and C5), 134.8 (s, C4′), 135.3
(s, C4), 144.0 (m, Cα), 148.3 (s, C6), 154.8 (s, C2), 156.0 (s, C6′),
156.3 (s, C2′). 31P{1H} NMR (CDCl3, 243 MHz): −6.32 ppm (s). IR
(CH2Cl2 solution): νCC 2048 cm−1. MS (MALDI): m/z 1251 ([M −
Cl]+, 100%), 933 ([RuCl (CO)(dppm)2]+, 55%) 1283 ([M]+, 10%),
910 ([RuCl(dppm)2]+, 10%). UV−vis (CH2Cl2) λ (nm) [ε × 104 M−1
cm−1]: 267 [4.09], 448 [1.72].
[ReCl(CO)3(κ2-N′N-PhC2bpy)] (12). PhCCbpy (200 mg, 0.78
mmol) and [ReCl(CO)5] (235 mg, 0.65 mmol) were dissolved in
toluene (100 mL), and the solution was heated to reflux for 2 h, during
which a colorless solution became bright yellow. The solvent volume
was reduced to 20 mL in vacuo, and the mixture was cooled to 0 °C.
The precipitate was collected on a filter and washed with EtOH (3 × 5
mL) to give the product as a yellow powder (285 mg, 78%). Anal.
Calcd for C21H12ClN2O3Re: C, 44.88; H, 2.15; N, 4.98. Found: C,
[Ru{CCbpy-κ2-N′N-ReCl(CO)3}Cl(dppm)2] (9). [Ru(CCbpy)-
Cl(dppm)2] (50 mg, 0.046 mmol) and ReCl(CO)5 (22 mg, 0.061
mmol) were dissolved in toluene (50 mL), and the solution was
refluxed for 1 h, during which a bright yellow solution became deep
red. The solvent volume was reduced to ca. 5 mL in vacuo, followed by
the addition of hexane (25 mL) and cooling to 0 °C. The precipitate
that formed was collected and washed with hexane (3 × 5 mL) to give
the product as red crystals (59 mg, 90%). Crystals suitable for X-ray
analysis were obtained through the slow evaporation of a CH2Cl2/
hexane (1:1) solution of the complex under an inert atmosphere. Anal.
Calcd for C65H51Cl2N2O3P4ReRu·3CH2Cl2: C, 49.65; H, 3.49; N,
1.70. Found: C, 49.45; H, 3.41; N, 2.13. 1H NMR (CD2Cl2, 600
MHz): δ 4.98 (m, 4H, PCH2P), 6.47 (dd, 1H, H4), 7.18−7.32 (m,
16H, Hmeta), 7.35−7.46 (m, 8H, Hpara), 7.48−7.54 (m, 16H, Hortho),
7.65 (ddd, 1H, H5′), 7.67 (ddd, 1H, H4′), 7.95 (ddd, 1H, H3′), 7.98
(dd, 1H, H3), 8.01 (dd, 1H, H6′), 8.93 (dd, 1H, H6). 13C{1H} NMR
(CD2Cl2, 151 MHz): δ 49.6 (PCH2P), 111.5 (s, Cβ), 121.6 and 121.7
(C3 and C3′), 124.8 and 125.1 (C5 and C5′), 127.7 (Cmeta), 129.6
(Cpara), 132.8 (Cortho), 133.5 (Cipso), 138.2 and 138.4 (C4 and C4′),
1
45.12; H, 2.27; N, 4.81. H NMR (CD2Cl2, 500 MHz): δ 7.41−7.47
(m, 3H, Hortho and H5′), 7.56 (m, 1H, Hpara), 7.62 (m, 2H, Hmeta),
8.08−8.12 (m, 2H, H3′, and H4′), 8.15−8.19 (m, 2H, H3 and H4),
9.08 (dd, 1H, H6′), 9.16 (s, 1H H6). 13C{1H} NMR (CD2Cl2, 126
MHz): δ 84.1 (Cβ), 97.8 (Cα), 121.8 (Cipso), 123.1 and 123.9 (C3 and
C3′), 124.7 (C5), 127.6 (C5′), 129.1 (Cortho), 130.3 (Cpara), 132.4
(Cmeta), 139.6 (C4), 141.3 (C4′), 153.5 (C6′), 154.3 (C2), 155.5
(C6), 155.6 (C2′), 189.8 (CO), 197.8 (CO). IR (KBr disk): νCC
2222 cm−1, νCO 2025 cm−1, νCO 1914 cm−1, and νCO 1894 cm−1.
Mp = 281−284 °C. MS (MeCN, ES (+)): m/z 644 ([M + 2 MeCN]+,
20%), 601 ([M + MeCN]+, 20%). UV−vis (CH2Cl2) λ (nm) [ε × 104
M−1 cm−1]: 255 [7.27], 344 [10.13].
[ReCl(CO)3(κ2-N′N-HC2bpy)] (13).38 [ReCl(CO)5] (102 mg, 0.282
mmol) and HCCbpy (60 mg, 0.33 mmol) were dissolved in toluene
(40 mL). The solution was heated to reflux for 30 min during which
the colorless solution turned deep red and then developed a yellow
color along with the formation of a yellow precipitate. The yellow
powder was collected and recrystallized from CH2Cl2/toluene to yield
the product (98 mg, 0.20 mmol, 72%). Anal. Calcd for
C15H8ClN2O3Re·0.75CH2Cl2: C, 34.42; H, 1.74; N, 5.10%. Found:
C, 34.26; H, 1.65; N, 5.47%. 1H NMR (CDCl3, 500 MHz): δ 3.53 (s,
1H, CCH), 7.57 (ddd, 1H, H5′), 8.08 (m, 2H, H3′ and H4′), 8.14
(dd, 1H, H4), 8.18 (dd, 1H, H4), 9.08 (ddd, 1H, H6′), 9.12 (dd, 1H,
H6). 13C{1H} NMR (CDCl3, 125.7 MHz): δ 77.7 (Cβ), 85.7 (Cα),
122.4 (C3), 123.2 and 123.4 (C3′ and C5′), 127.4 (C5), 138.9 (C4′),
141.5 (C4), 153.4 (C2), 154.7 and 155.0 (C2′ and C6′), 155.8 (C6).
IR (CH2Cl2, cm−1): 2121 (w) ν(CC), 2024, 1922, and 1900 (vs)
ν(CO). IR (Nujol): ν(HCC) 3185 (w) cm−1. FAB+ MS: m/z 486
(45%, [M]+), 450 (100%, [M-Cl]+).
2
146.6 (C2), 152.4 (s, C6′), 153.5 (s, JCP = 25.7 Hz, Cα), 154.0 (s,
C6), 156.4 (C2′), 197.6 and 197.8 (CO). 31P{1H} NMR (CD2Cl2, 243
MHz): δ −6.99 (m). IR (CH2Cl2 solution): νCC 2049 cm−1, νCO
2019 cm−1, νCO 1915 cm−1, and νCO 1894 cm−1. MS (MALDI): m/
z 933 ([RuCl(CO)(dppm)2]+, 100%), 910 ([RuCl(dppm)2]+, 75%),
1383 ([M]+, 20%). UV−vis (CH2Cl2) λ (nm) [ε × 104 M−1 cm−1]:
261 [4.93], 325 [1.64], 480 [2.35].
[RuCl(κ2-N′N PhC2bpy)Cp] (10). PhCCbpy (100 mg, 0.39 mmol)
and [RuCl(COD)(Cp)] (115 mg, 0.37 mmol) were dissolved in
acetone (20 mL), and the solution was stirred at room temperature for
20 h. The solvent volume was reduced to 10 mL in vacuo, and Et2O
(10 mL) was added before the mixture was cooled to 0 °C for 1 h. The
precipitate that formed was collected and washed with Et2O (2 × 3
mL) to give the product as a bright purple powder (128 mg, 75%).
Crystals suitable for X-ray analysis were obtained through the vapor
diffusion of n-pentane into a CH2Cl2 solution of the complex. Anal.
Calcd for C23H17ClN2Ru: C, 60.33; H, 3.74; N, 6.12. Found: C, 60.24;
RESULTS AND DISCUSSION
■
Alkynyl Complexes. The cyclopentadienyl ruthenium
alkynyl complexes 1 and 2 were prepared using well-trodden
methodologies.39,40 The syntheses of complexes 2,41 3,27 and
1338 have been reported, but in the case of complex 2, a more
concise synthesis is reported here and a reinterpretation of the
13C{1H} NMR assignments is presented and supported with
2D NMR experiments, similarly with complex 13. The X-ray
crystal structures of 2 and 3 are presented here for the first
time.
1
H, 3.96; N, 6.12. H NMR (CD2Cl2, 500 MHz): δ 4.31 (s, 5H Cp),
7.28 (m, 1H, H5′), 7.18 (d, 1H, H4), 7.39−7.42 (m, 3H, Hortho and
Hpara), 7.58 (m, 2H, H3 and H3′), 7.67 (ddd, 1H, H4′), 7.73 (dd, 1H,
H4), 9.63 (dd, 1H, H6′), 9.77 (d, 1H, H6). 13C{1H} NMR (CD2Cl2,
126 MHz): δ 69.1 (Cp), 84.1 (Cβ), 94.3 (Cα), 120.4 and 120.8 (C3
and C3′), 121.1 (C5), 121.3 (Cipso), 123.9 (C5′), 127.7 (Cortho), 128.5
(Cpara), 131.0 (Cmeta), 133.6 (C4′), 135.6 (C4), 153.5 (C2), 154.2
(C6′), 154.5 (C2′), 156.2 (C6). IR (KBr disk): νCC 2220 cm−1. Mp
≥ 300 °C. MS (MeCN, ES (+)): m/z 464 ([Ru(NCMe)(PhC2bpy)-
Cp]+, 100%), 458 ([M]+, 15%). UV−vis (CH2Cl2) λ (nm) [ε × 104
M−1 cm−1]: 263 [2.98], 330 [7.97], 546 [0.69].
The bimetallic complexes were accessed through ligand
substitution reactions on [RuCl(COD)Cp], for 4, 6 and, 8 or
[ReCl(CO)5], for 5, 7, and 9, in good yield. Similar reactions
provided access to 10−13, which are cogent examples for the
comparison of physical and spectroscopic properties of 4−9.
In the infrared spectra of the complexes the weak ν(CC)
band of the free ligand (2097 cm−1) shifted to a strong alkynyl
stretch in the range 2038−2222 cm−1, the majority lying in the
[RuCl(κ2-N′N-HC2bpy)Cp] (11). HCCbpy (63 mg, 0.35 mmol)
and [RuCl(COD)Cp] (101 mg, 0.33 mmol) were dissolved in acetone
(15 mL), and the solution was stirred at room temperature for 22 h.
The solvent volume was reduced to 5 mL in vacuo and cooled to 0 °C.
The precipitate was collected and washed with Et2O (2 × 10 mL) to
give the product as a purple powder (102 mg, 82%). Anal. Calcd for
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range 2038−2070, for 1−9, with bands for 10−13 between
2098 and 2222 cm−1. Complexes 11 and 13 also displayed weak
ν(CH) bands for at 3122 and 3184 cm−1, respectively.
The binuclear complexes 4−9 had ν(CC) bands that were
some ca. 30 cm−1 lower than those for the mononuclear 1−3,
the lower frequency attributed to a decrease of triple-bond
character on account of withdrawal of electron density to the
coordinated metal moiety at the bipyridyl nitrogens, and
amplified by the increase in mass associated with the addition
of the second metal unit. The ReCl(CO)3 appended
complexes, 5, 7, 9, 12, and 13, all exhibit IR ν(CO) spectra
characteristic of fac coordination of the of the CO ligands.
The proton NMR spectra contained the expected resonances
for the cyclopentadienyl moieties with the methyl groups of the
Cp* ligands at ca. 1.6 ppm. In the spectra of 2, 4, 6−8, 10, and
11, signals for Cp were consistent with those normally observed
for neutral ruthenium complexes and at ca. 4.4 ppm for Cp
ligands attached to Ru(bisphospine) moieties and at ca. 4.1, for
4, 6 and 8, and at ca. 4.3 for 10 and 11 for those Cp ligands
attached to Ru(bpy), notably the complex [RuCl(κ2-N′N-
1
bpy)Cp] has a resonance at 4.35 ppm for Cp in its H NMR
spectrum.28,42 The alkynyl protons of 11 and 13 were observed
as singlets at 3.38 and 3.53 ppm, respectively.
The 13C{1H} NMR spectra obtained for the complexes
contained resonances that were diagnostic of the presence of an
alkyne in all complexes. The revised assignment of the alkynyl
carbons was achieved with the aid of 2-D HMBC experiments
and the observation of coupling to the phosphorus nuclei,
typically ca. 20 Hz. The Cα resonances were observed to be
uniformly downfield of the Cβ peaks for the alkynyl complexes
were all identified with the Cα resonances all uniformly
downfield of Cβ, with the latter at ca. 110 ppm and the former
in the range ca. 150−130 ppm. The 31P{1H}-spectra contained
singlets characteristic of the respective Ru(phosphine) moieties.
Other resonances associated with the respective ligands were
observed in the expected regions.
In an attempt to acquire microanalytical data for complex 8
we analyzed five different samples prepared from three different
repeat reactions and have yet to get acceptable results. For this
reason we have added copies of the characterization data (1H,
13C, and 31P NMR and MALDI-TOF MS) to the Supporting
Information that establish the absence of detectable contam-
inants.
Figure 1. Molecular representations of the structures of (a) 1, (b) 2
(one of the molecules), and (c) 3. Hydrogen atoms omitted to aid in
clarity. Atomic displacement envelopes shown at the 50% probability
level.
still appeared to still be crystalline. A second data set on these
crystals, 3b (Supporting Information) showed that, although
they were still crystalline, the b cell length had decreased by
about 12% with the other cell dimensions remaining almost
unaltered. The decrease in the length of the crystal cell b
parameter was clearly due to loss of the toluene solvent. The
geometries of the molecules of the two complexes did not show
any significant differences. The bimetallic complexes 5−7 and 9
also crystallized as solvates.
The chloroform solvate molecule of 5·CHCl3 is closely
associated with the Cp ring; the Me5C5 centroid···H−CCl3
distance is 2.38 Å (Figure 2). These halocarbon H-bonds have
been observed previously, notably in the supramolecular
interactions of sarcophagine45 complexes and metal calixarene
complexes.46−48 Hirshfeld surfaces, calculated using Crystal-
Explorer 3.149,50 provide a convenient method through which to
investigate the nature of intermolecular interactions for many
classes of complexes51−55 and is well-suited to the analysis of
the interactions in solvated structures. Using this approach, we
find that the structure of 9 contains four CH2Cl2 molecules of
solvation.
Solid-State Structures. The structures of the alkynyl
complexes 1−3 are depicted in Figure 1, with selected
interatomic parameters collected in the Supporting Information
(Tables S1, S3−S8). These tables also collect the data
associated with the previously reported 241 and 3,27 but for
which no structures have been previously determined. We have
previously published the structure of the dppe analogue of 3,
viz. [Ru(CCbpy)Cl(dppe)2].27
The bond lengths and angles about the {Ru(PPh3)2Cp} (1),
{Ru(dppe)Cp*} (2), or {trans-RuCl(dppm)2} (3) cores are
unremarkable and consistent with other alkynyl complexes of
this type, and the metal−C2 parameters are also consistent with
other metal−ligand systems. Here also the uncoordinated C
Cbpy units in 1−3 are strictly comparable to the analogous
units in other complexes containing this uncoordinated
ligand.27,38,43,44
Crystals of 3·toluene were mounted on a fiber under an
atmosphere of cold CO2 and transferred quickly to the cold
stream to avoid any solvent loss. Some crystals were allowed to
remain at room temperature for several days, after which they
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Figure 2. Molecular representations of the structure of complex 5 (a)
showing the proximity of the solvate CHCl3 to the Cp ring. Atomic
displacement envelopes shown at 50% probability. (b) Hirshfeld
surface of the solvate CHCl3 in relation to 5. The majority of hydrogen
atoms omitted to aid in clarity in (a).
The structures of the bimetallic complexes 6, 7, and 9 are
depicted in Figure 3. It is clear from the data in Tables S1 and
S3−S8 (Supporting Information) that the coordination of a
metal to the bipyridyl unit has had no significant effect on the
length of the CC triple bond in any of the bimetallic
complexes. Similarly, the structurally characterized, bipyridyl
coordinated monometallic complexes 10 and 12 (Figure 4) and
previously published 13 prepared for structural and spectro-
scopic comparisons show no appreciable differences in
analogous distances. Clearly, the presence of the bpy ligated
metal does not lead to any structurally significant differences in
the formal valence bond representations, nor the evolution of
any substantive degree of cumulenic character in the alkynyl
linker.
To the best of our knowledge, there are no structurally
characterized derivatives of the {Ru(bpy)Cp} moiety to allow
comparison with 6 and 10. Therefore, the known compound
15 was prepared and its structure determined crystallo-
graphically. In this model system, 15, the distance of the Cp
ring from the metal (as measured by the Ru−centroid distance,
Ru−Cp′) is 1.78 Å (1.777−1.781 Å), which is about 0.1 Å
shorter than that found in the {Ru(PPh3)2Cp} core of 6, and
0.6 Å shorter than that for [RuCl(PPh3)2Cp].56 The Ru−Cl
distance was around 2.45 Å (2.4475(3) to 2.4676(12) Å),
which is consistent with other complexes. The binding of the
bpy ligand was consistent across the three complexes 6, 10, and
15, with Ru−N distances around 2.08 Å (2.0751(10) to
2.109(3) Å) and N−Ru−N bond angles of 76° (76.09 to
76.58). These bond lengths are slightly longer, with a narrower
bond angle than seen in [Ru(bpy)3](PF6)2 with a bond length
of 2.056 Å and an angle of 78.7°.57
Figure 3. Molecular representations of the structures of (a) 6, (b) 7,
and (c) 9. Hydrogen atoms have been omitted to aid in clarity. Atomic
displacement envelopes shown at 20% probability level.
There are two independent molecules of 12 in the
asymmetric unit although of opposite chirality in the space
group P212121. Otherwise, the two molecules are similar, the
difference being confined to minor orientations of the phenyl
and bipyridyl rings. The dihedral angles between two pyridyl
rings are 7.2(5) and 0.5(4)° for molecules 1 and 2 respectively.
The dihedral angles between the phenyl ring and each of the
bpyridyl rings are 2.3(5), 5.5(6)° for molecule 1 and 8.3(6) and
7.8(6)° for molecule 2.
The structure of ligand 14 was obtained, with three
independent molecules in the structure. All three molecules
are linear along the axis of the CC bond and have almost
coplanar pyridine rings in the bpy moiety, with the nitrogen
atoms in a transoid configuration and dihedral angles between
the rings of 5.40(8), 4.85(8), and 8.42(8)°. In molecules 1 and
2, the phenyl rings are almost coplanar with the bipyridine
moiety, with dihedral angles of 5.39(8) and 3.37(8)°,
respectively, whereas in molecule 3, the phenyl ring is
considerably rotated form the plane of the bpy moiety, with a
dihedral angle of 40.35(8)°.
Electrochemistry. The redox properties of the mono-
metallic complexes (1−3, 10, 11), bimetallic complexes (4−9),
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on the ruthenium center and the alkynyl substituent.17,59,60 The
oxidations are apparently reversible or quasi-reversible with the
ratio of peak currents being close to unity and anodic/cathodic
wave separations (ΔEp) between 70 and 130 mV, which
compare with ΔEp = 70 mV for the internal standards
(ferrocene, FeCp2, or decamethylferrocene, FeCp*2). In the
case of 3 there is an additional anodic feature at ca. 0.35 V
observed on the reverse scan that also exhibits a cathodic return
wave at 0.30 V, which indicates an initial EC process.
Expectedly, the electron rich, Cp* derivative 1 is oxidized at
the most negative potential of the monometallic alkynyl species
1−3, whereas E1/2 for the Ru(PPh3)2Cp analogue 2 is some 260
mV more positive. In contrast to the variation in redox
potentials of 1−3, the mononuclear Ru(bpy)Cp complexes 10,
11, and 15 all oxidize at ca. 0 V. Cyclic voltammograms of the
ruthenium, binuclear complexes, 4, 6, 8 all exhibit two
oxidation events the first of which fall near −0.2 V.
Consideration of the relative potentials in Table 1 suggests
the two redox processes in the bimetallic complexes 4, 6, and 8
can be approximated in terms of sequential oxidation of the
{RuCl(bpy)Cp} and {Ru(CCbpy)L2Cp} fragments.
In contrast, the heterometallic binuclear complexes 5, 7, and
9 display greater variation in the first oxidation process, with
E1/2 falling between 0.14 for the {Ru(dppe)Cp*} complex 5
and 0.40 V for the {Ru(PPh3)2Cp} analogue 7. In this case it is
likely that the oxidation is centered on the metal−alkynyl
moiety and shifted to more positive potentials by the effect of
the electron withdrawing {ReCl(CO)3(bpy)} fragment. These
bimetallic complexes also contain an irreversible reduction
event at ca. −2 V associated with the {ReCl(CO)3(bpy)}.
Spectroelectrochemical Studies. IR and UV−vis−near
IR spectroelectrochemical (SEC) investigations were under-
taken to shed further illumination on the nature of these
complexes, their redox chemistry and electronic structures.
The IR spectra of 1−3 each display a ν(CC) band near
2070 cm−1, split by either the Fermi resonance61,62 or the
presence of different rotamers in solution.63 Oxidation of
complexes 1−3 results in the shift ν(CC) from ca. 2070 to
ca. 1920 cm−1 (Figure 5) consistent with the formation of the
ruthenium alkynyl radical cations [1−3]+,17,59,60 together with a
second, presumably ν(CC) band near 2050 cm−1, which was
subsequently consumed on further oxidation. On reduction of
each of [1−3]+ the ν(CC) band associated with the neutral
Figure 4. Molecular representations of the structures of (a) 10 and (b)
12 (one of the molecules). Hydrogen atoms omitted to aid in clarity.
Atomic displacement envelopes shown at 50% probability.
and [RuCl(κ2-N′N-bpy)Cp] (15) were measured by cyclic
voltammetry, and relevant data are collected in Table 1.
Typically, ruthenium alkynyl complexes undergo one-
electron oxidations to give the corresponding radical cations
in which the unpaired electron is delocalized over the metal and
alkynyl ligand. As a consequence, the redox potentials of
ruthenium alkynyl complexes are a function of both the ligands
Table 1. Electrochemical Data for the Complexes 1−11 and
a
15
first oxid
second oxid
red.
E1/2
(V)
ΔEp
E1/2
(V)
ΔEp
compd
(mV)
Ia/Ic
(mV)
Ia/Ic Ep (V)
1
2
0.00
77
1.09
1.24
1.09
1.10
1.42
1.08
2.55
1.04
1.07
0.99
0.90
1.08
0.26
0.14
79
73
3
4
−0.16
0.14
76
0.09
74
93
1.58
5
73
−1.91
2.08
6
−0.19
0.40
76
0.07
0.26
7
79
−1.87
1.21
8
−0.16
0.32
83
119
9
127
113
85
−1.95
10
11
15
0.07
0.05
−0.02
85
a
CH2Cl2 solutions containing 0.1 M nBu4NPF6 electrolyte. The
decamethylferrocene/decamethylferricenium (FeCp*2/[FeCp*2]+)
couple was used as an internal reference, with all potentials reported
relative to the ferrocene/ferricenium couple (FeCp2/[FeCp2]+ = 0 V,
such that FeCp*2/[FeCp*2]+ = −0.48 V in CH2Cl2).58 Scan rate of
100 mV/s at room temperature.
Figure 5. IR spectral changes accompanying the oxidation of complex
n
3 in an OTTLE cell, CH2Cl2 /0.1 M Bu4NPF6 electrolyte.
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species at 2070 cm−1 is observed to grow back into the
spectrum, together with the band at 2050 cm−1 of
approximately equal intensity. The identity of these new
species responsible for the persistent ν(CC) band at 2050
cm−1 is still undetermined. Initial theses revolved around the
possibility of contamination but careful attention to detail in
the collection of new data using rigorously purified materials
discounted that possibility. Similarly, the possible ionization of
the remaining halide in 3, in the electrolyte used, giving rise to
closely related alkynyl stretches in the IR spectrum was also
discounted. The appearance of a similarly putative alkynyl band
on oxidation was also observed in the IR SEC study of
[Ru(CCbpy)(dppe)Cp*] (1) and [Ru(CCbpy)-
(PPh3)2Cp] (2) where the formation of ionized products or
solvent stabilized cations can be discounted.
It was posited that the new band is a result of a chemical
process following the initial oxidation of the alkynyl complex on
the relatively long time scale associated with the spectroelec-
trochemical study compared with the voltammetric measure-
ments. Therefore, chemical oxidation of 1 using the
acetylferrocenium ion in CH2Cl2 was performed to test the
hypothesis. The reaction was monitored by IR spectroscopy
and showed the formation of the radical alkynyl cation after
some hours, evinced by the observation of a band at 1920 cm−1
and accompanied by a band for the unknown species at 2030
cm−1. After 16 h, the radical alkynyl cation had decomposed to
the carbonyl cation,64,65 [Ru(CO)(dppe)Cp*]+, giving a new
band at 1972 cm−1, with the band attributed to the unknown
complex unchanged.
The nature of the species present in the solution was probed
by mass spectrometry and analysis of the spectra obtained was
inconclusive, apart from expected daughter ions related to the
[Ru(dppe)Cp*]+ core and ions related to the carbonyl cation.
However, there was a doubly charged ion observed at 1249 m/
2z, which displayed a characteristic Ru2 isotope pattern that led
us to suggest that the oxidation of 3 led to some
oligomerization of the alkynyl radical cation complex, most
likely dimerization. Oxidation of alkynyl complexes leading to
oligiomerisation was demonstrated in many systems,66−69 and
the noninnocent nature of ligands in redox reactions was
highlighted by one of us recently.60
Figure 6. IR spectral changes accompanying the oxidation of complex
(a) 8 to [8]+ and (b) [8]+ to [8]2+ in an OTTLE cell, CH2Cl2 /0.1 M
nBu4NPF6 electrolyte.
The IR SEC study of the ruthenium, binuclear complexes, 4,
6, and 8, (illustrated for 8 in Figure 6) shows that there is a
slight shift (Δν(CC): 4/[4]+ −9 ; [6]/[6]+ −9; 8/[8]+ −5
cm−1) to lower wavenumber of the ν(CC) band on the first
oxidation of the complex, consistent with initial oxidation
localized on the {RuCl(bpy)Cp} moiety and therefore not
significantly affecting the triple bond character of the alkynyl
complex. However, on further oxidation to the dication the
ν(CC) band loses intensity, becoming almost indistinguish-
able from the baseline in the case of [4]2+ and [6]2+, which
suggests a limited dipole over the alkynyl moiety and shifts to
lower frequency; both observations are consistent with the
second oxidation being more associated with the {Ru-
(PPh3)2Cp} fragment. However, in the case of 8, the ν(C
C) band has enough intensity to be observed at 1947 cm−1 but
is significantly less intense than the neutral species. This feature
is found to be reversible on reduction under the conditions of
the spectroelectrochemical measurements.
Figure 7. IR spectral changes accompanying the oxidation of complex
9 in an OTTLE cell, CH2Cl2 /0.1 M Bu4NPF6 electrolyte.
n
to lower energy and causing it to be largely obscured by the
ν(CO) bands associated with the {Re(CO)3(bpy)} fragment,
which change little in intensity or position. The shoulder at
1940 cm−1 in [9]+ likely arises from the incomplete obstruction
of the ν(CC) stretch.
The heterometallic binuclear complexes, 5, 7, and 9 all
displayed consistent IR SEC behavior, illustrated here by
complex 9 (Figure 7). Oxidation occurs at the Ru alkynyl
substituent, resulting in the typical shift in ν(CC) frequency
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Figure 8. Plots of selected molecular orbitals (isocontour value 0.04 (e/bohr3)1/2): (a) the LUMO (top) and HOMO (bottom) of 2′; (b) the
HOMO and HOMO−1 of 6′; (c) the LUMO (top) and HOMO (bottom) of 7′.
The bimetallic complexes 4−9 were further investigated by
UV−vis−NIR spectroelectrochemical studies, seeking any
additional evidence for interactions between the metallic
centers mediated by the ethynyl bipyridyl bridging moiety.
The homobimetallic ruthenium complexes 4, 6, and 8 all
exhibit similar spectroscopic profiles, with the MLCT
transitions associated with the ruthenium−alkynyl fragment
observed near ca. 450 nm. On oxidation, these bands are only
slightly red-shifted to ca. 500 nm, together with a very low
intensity IVCT band near 1200 nm (8300 cm−1). These trends
are consistent with the initial oxidation of the {RuCl(bpy)Cp}
fragment, which leads to an inductive stabilization of the bpy
ligand π*-system and hence lowering in the MLCT transition
energy. Further oxidation to the dications [4]2+, [6]2+, and
[8]2+ causes a collapse in the lowest energy feature, consistent
with the IVCT assignment, and the 500 nm band, with new
bands at ca. 700−800 nm and a characteristic blue shift in the
MLCT band consistent with the greater degree of RuCC
character in the second oxidation event. In addition, in the case
of [8]2+ a Cl → {RuCC}+ LMCT transition, which is
significantly narrower and more intense than the IVCT band, is
also observed at 1280 nm (7800 cm−1).63
Further investigation of the heterobimetallic Ru−Re
complexes using UV−vis−NIR spectroelectrochemical meth-
ods also supports these assignments. In the case of 5, 7, and 9,
the only accessible oxidation process causes a blue shift in the
ruthenium−alkynyl MLCT band. For [5]+, the dπ−dπ
transition often associated with [Ru(CCR)(L2)Cp′]+ radical
cations was observed clearly near 1500 nm (6650 cm−1),59 with
a Re → {Ru(CCR)}+ CT band observed near 730 nm
(13,700 cm−1), similar processes being known for related Re/
Fe complexes.70,71
Computational Studies. To aid in the interpretation of
the electrochemical and spectroelectrochemical results, and to
arrive at a more comprehensive description of the electronic
structure of the complexes described here, a series of DFT
calculations (B3LYP/LANL2DZ Ru and Re/6-31G** all other
atoms/CPCM−dichloromethane solvent model) were carried
out on the representative series [2′]n+ (n = 0, 1), [6′]n+ (n = 0,
1, 2), and [7′]n+ (n = 0, 1), where the prime (′) notation is used
to distinguish the computational systems from the physical
samples.
Optimized bond lengths and angles from 2′, 6′, and 7′ were
in good agreement with those of the crystallographically
determined structures (Tables S3−8, Supporting Information)
with overestimation of the Ru−P, Ru−Cl, and Re−Cl, and Ru−
Cp′ bond lengths by ca. 4%. Calculated vibrational frequencies
(scaled here by 0.95),72 particularly the ν(CC) and ν(CO)
modes, provide an excellent point for comparison of the model
and physical systems in the various electrochemically accessible
redox states (Table 1), and the close agreement between these
data give confidence in the conclusions drawn from the
computational work.
Compound 2′ is another member in the now well-studied
family of complexes Ru(CCR)(PR3)2Cp′,59,73−75 and the
electronic structure is, not surprisingly, similar to the many
other examples of this family described earlier at various levels
of theory. Of the various minima of 2′ that can be identified,
differing in the relative orientations of the N atoms relative to
the Ru(PPh3)2Cp fragment and each other, the one represented
by the structure depicted in Chart 1, with the bpy plane
approximately bisecting the P−Ru−P angle, is the global
minimum. The analogous structure with the bpy fragment
oriented as found in the crystallographically determined
structure lies some 2.3 kJ mol−1 eV higher in energy.
Similar transitions are apparent on close inspection of the
spectrum of the Ru(PPh3)2Cp derivative [7]+, although these
features are rather less pronounced. The spectrum of [9]+ also
features low energy transitions at 1230 and 860 nm, assigned to
Cl → {RuCC}+ LMCT and Re → {Ru(CCR)}+ CT
bands. As oxidation proceeds, other features at 990, 640, 383,
and 356 nm grow in, accompanied by a loss of isosbestic points
and indicating a degree of decomposition of the sample over
the time scale of the spectroelectrochemical experiment.
The HOMO is distributed over the RuCCbpy
backbone (Ru 33%, CC 29%, bpy 24%) whereas at the
level of theory employed the LUMO is essentially (88%)
composed of the bpy π* system, Figure 8. In comparison with
2′, the 17-e compound [2′]+ offers the usual pattern of
elongated RuP and RuCp′ bond lengths, which reflects
decreases in metalligand π-back-bonding. The shorter Ru
C1 and C2C3 bond lengths and longer C1C2 bond in
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[2′]+ are consistent with the composition and nodal structure
of the β-LUSO (Ru 38%, CC 26%, bpy 21%), which has
obvious similarities with the HOMO of 2. The calculated
ν(CC) frequencies in 2′ and [2′]+ track the geometric
changes and orbital structure, decreasing from 2034 cm−1 (2′)
to 1922 cm−1 ([2′]+) as the CC character decreases on
oxidation, in very good agreement with the IR spectroelec-
trochemical results. The substantial ethynylbipyridine ligand
character in the radical cation [2′]+ also supports the notion of
interligand coupling proposed above to explain the chemical
behavior of the closely related complexes [1−3]+.
sequential oxidation of the RuCl(bpy)Cp and RuCCbpy
moieties.
Finally, turning attention to the mixed-metal complexes, 7′
and [7′]+ successfully modeled both the geometric properties
of the crystallographically determined structure of 7 (Tables S3
and S7, Supporting Information) and the ν(CC) and ν(CO)
frequencies from the IR spectroelectrochemical experiments
(Table S10, Supporting Information). Importantly, the ν(C
C) band in 7′ (2020 cm−1), which is well removed from the
three ν(CO) bands (1979, 1878, 1864 cm−1) shifts by some
−94 cm−1 in [7′]+ and falls within the ν(CO) bands, which are
only modestly shifted to higher wavenumbers (ν(CC) 1926
cm−1; ν(CO) 1984, 1887, 1872 cm−1). Clearly, oxidation of 7 is
associated with changes in the electron density in the RuC
Cbpy moiety, which is reflected in the orbital structures of 7′
and [7′]+, Figure 8. Unsurprisingly, the HOMO of 7′ displays
the familiar RuCCbpy character and nodal properties (Ru
37%, CC26%, bpy 20%) associated with the HOMO of 2′
and the HOMO−1 of 6′. Other Ru(PPh3)2Cp based orbitals
contribute to the HOMO−1 and HOMO−2. The ReCl-
(CO)3(bpy) fragment contributes mixed ReCl orbitals to the
HOMO−3 and HOMO−4, which lie ca. 1 eV below the
HOMO, whereas the LUMO is heavily bpy π* in character
(84%). In [7′]+ the β-LUSO retains similar RuCCbpy
character (Ru 42%, CC 22%, bpy 19%) consistent with
oxidation at this metal−organic fragment. This leads to a
degree of orbital reordering, with stabilization of the occupied
β-orbitals localized on the Ru(PPh3)2Cp fragment that now lies
below the ReCl based fragment orbitals, supporting the
assignment of the Re → {Ru(CCR)}+ CT bands observed
in the spectroelectrochemical experiments.
The electronic structure of bimetallic 6′ allows some useful
comparisons of phosphine and bpy ligated RuXL2Cp frag-
ments,28,76,77 a comparison that has additional relevance given
the extensive knowledge of RuX(PR3)2Cp′ chemistry, and
recent proposals for water oxidation catalysts based on the less
well explored {Ru(bpy)Cp′} moiety.78 The coordination of the
RuClCp fragment to the bpy moiety in 2′ has no discernible
structural effect on the Ru(CCbpy)(PPh3)2Cp fragment,
whereas the various bond lengths and angles in the RuCl-
(bpy)Cp fragment in 6′ are similar to those observed
crystallographically for 10. Perhaps of greatest interest is the
contraction of the RuCp′ bond length in the bpy
coordinated fragment vs the phosphine coordinated fragments,
which arises from the extremely efficient σ-donor properties of
the bpy ligand and increased RuCp back-bonding. This back-
bonding contribution is reflected in the structure of the
HOMO of 6′, which is localized on the RuCl(bpy)Cp fragment
(Ru2 62%, Cp 14%). The RuCCbpy moiety that
comprises the HOMO of 2′ features heavily in the HOMO−1
of 6′ (Ru1 31%, CC 23%, bpy 16%), which lies some 0.18 eV
lower in energy than the HOMO, Figure 8. The local
coordinates most appropriate for describing the two RuXL2Cp
fragments are approximately orthogonal (Ru1C1Ru2Cl
99.99°), and consequently there is little mixing between the
metal-based orbitals through the ethynyl-bipyridyl bridge.
The cation radical [6′]+ features bond lengths at the Ru(C
Cbpy)(PPh3)2Cp fragment that are essentially unchanged from
those of 6′ and 2′. In contrast, there is a modest (0.01−0.02 Å)
contraction of the Ru2N bond lengths, and a more
significant shortening of the Ru2Cl bond (reflecting a
greater electrostatic attraction between the formally d5, RuIII
center, and the Cl atom) and elongation of the Ru2Cp′
distance as the metalring back-bonding interactions are
diminished. The orthogonal relationship between the HOMO
and HOMO−1 and localization on each of the two RuXL2Cp
fragments is also evident in [6′]+. On the basis of the structural
characteristics and the distribution of the β-HOSO (Ru1 40%,
CC 25%, bpy 17%) and β-LUSO (Ru2 60%, Cp2 13%, Cl
10%) in [6′]+ and the observation of a low intensity IVCT
band in each of [4]+, [6]+, and [8]+ (vide supra) these
bis(ruthenium) radical cations can be accurately described as
weakly coupled, or Class II, d5/d6 mixed-valence complexes.
This localized electronic structure description is entirely
consistent with the IR spectroelectrochemical observations,
with the limited shift in the ν(CC) frequency between 6 and
[6]+ (Δν(CC) = −8 cm−1) mirrored in the models 6′ and
[6′]+ (Δν(CC) = −16 cm−1).
CONCLUSIONS
■
Some new bipyridyl appended ruthenium alkynyl complexes
have been prepared, and these have allowed access to a range of
binuclear homometallic ruthenium and heterometallic ruthe-
nium−rhenium complexes. The coordination of a second metal
center to the alkynyl complexes has little impact on the
structures of these complexes except that imposed by the
packing of these into the crystal lattice.
In the bimetallic complexes, the IR and UV−vis spectroelec-
trochemical experiments showed the two metal centers were
found to be only weakly coupled, as evinced and supported by
quantum chemical calculations. The alkynyl complexes of the
type [Ru(CCbpy){Ln}] ({Ln} = {(PPh3)2Cp}, {(dppe)-
Cp*}, {Cl(dppm)2}) undergo reversible one-electron oxida-
tions centered largely on the alkynyl ligands as has been
observed previously for closely related complexes.
The homometallic binuclear complexes, represented by
[Ru(C2bpy-κ2-N′N-RuClCp)(PPh3)2Cp] undergo two essen-
tially reversible oxidations, the first centered on the (C2bpy-κ2-
N′N-RuClCp) moiety and the second on the Ru(C
Cbpy)(PPh3)2Cp fragment, leading to radical cations that can
be described as Class II mixed-valence complexes. The
heterometallic binuclear complexes [Ru(C2bpy-κ2-N′N-ReCl-
(CO)3){Ln}] display a similar behavior, with initial oxidation
on the ruthenium fragment giving rise to a new optical
absorption band with Re → Ru(CCbpy) charge transfer
character. The heterometallic complexes also exhibit irreversible
reductions associated with the Re hetereocycle moiety.
In summary, we have investigated the electronic structure of
the bimetallic compounds prepared by us.
Although [6]2+ proved to be unstable under the conditions of
the spectroelectrochemical experiment, the localization of the
β-LUSO and β-HOSO on the RuCl(bpy)Cp and RuCCbpy
fragments in [6′]+, respectively, is consistent with the
interpretation of the cyclic voltammogram of 6 in terms of
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ASSOCIATED CONTENT
* Supporting Information
■
S
Cyclic voltammograms, IR spectra, and UV−vis−near-IR
spectra of complexes during oxidation in a spectroelectrochem-
ical cell. Tables of crystallographic data, bond lengths, and bond
1
angles. H, 13C, and 31P NMR and MALDI-TOF MS spectra
for complex 8. Cartesian coordinates of all optimised
geometries as a .mol file. Table of observed and calculated
vibrational frequencies. Cartesian coordinates. Tables of energy
and composition data. CIF and Checkcif files for compounds
1−3, 9, 10, and 12−15 (CCDC 980469−980478) and 5−7
(CCDC 986580−986582). This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
*G. Koutsantonis. E-mail: george.koutsantonis@uwa.edu.au.
*P. J. Low. E-mail: paul.low@uwa.edu.au.
Author Contributions
C.F.R.M. performed all of the synthetic experimental work
reported in this paper. B.W.S. and D.S.Y. are responsible for the
crystallographic content. All authors contributed to the writing
of this manuscript. All authors approve of the final version of
the manuscript.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
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This work was supported in part by the Danish National
Research Foundation (DNRF93) Center for Materials
Crystallography. We acknowledge the Diamond Light Source
for an award of instrument time on the Station I19 (MT 6749)
and the instrument scientists for support. The authors
acknowledge the facilities and the scientific and technical
assistance of the Australian Microscopy and Microanalysis
Research Facility at the Centre for Microscopy, Character-
isation and Analysis, The University of Western Australia, a
facility funded by the University, State and Commonwealth
Governments. P.J.L. gratefully acknowledges support from the
Australian Research Council (FT120100073). C.F.R.M. was the
holder of an Australian Postgraduate Award.
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