Unexpected structural and electronic effects of internal rotation in diruthenium paddlewheel complexes containing bulky carboxylate ligands

Article abstract of DOI:10.1016/j.ica.2010.07.024

A series of Ru2II,III complexes containing the bulky carboxylate ligand 2,4,6-triisopropylbenzoate (TiPB) of type trans-[Ru₂(TiPB) ₂(O₂CCH₃)₂X] [X = Cl (1), PF ₆ (2)] and [Ru₂(TiPB)₄X] [X = Cl (3), PF ₆ (4)] have been synthesised. The corresponding Ru2II,II complexes trans-[Ru₂(TiPB)₂(O₂CCH₃) ₂] (5) and [Ru₂(TiPB)₄] (6) were also isolated. Magnetic susceptibility measurements indicate that the diruthenium cores have the expected three (1-4) or two (5 and 6) unpaired electrons consistent with σ²π⁴δ²(δ¹π ¹)³ and σ²π⁴δ ²δ²π² electronic configurations. Compounds 1-4 and 6 were structurally characterised by X-ray crystallography, and show the expected paddlewheel arrangement of carboxylate ligands around the diruthenium core. The diruthenium cores of complexes 3, 4 and 6 are all distorted to minimise steric interactions between the bulky carboxylate ligands. The Ru-Ru bond length in the Ru2II,II complex 6 [2.2425(6) ] is the shortest observed for a diruthenium tetracarboxylate and, surprisingly, is 0.014 shorter than in the analogous Ru2II,III complex 4, despite an increase in the formal Ru-Ru bond order from 2.0 (6) to 2.5 (4). This is rationalised in terms of the extent of internal rotation, or distortion, about the diruthenium core. This was supported by density functional theory calculations on the model complexes [Ru₂(O₂CH)₄] and [Ru₂(O ₂CH)₄]⁺, that demonstrate the relationship between Ru-Ru bond length and internal rotation. Electrochemical and electronic absorption data were recorded for all complexes in solution. Comparison of the data for the 'bis-bis' (1, 2 and 5) and tetra-substituted (3, 4 and 6) complexes indicates that the shortening of the Ru-Ru bond length results in a small increase in energy of the near-degenerate δ¹ and π¹ orbitals.

Full text of DOI:10.1016/j.ica.2010.07.024

Inorganica Chimica Acta 363 (2010) 3856–3864
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Unexpected structural and electronic effects of internal rotation in diruthenium
paddlewheel complexes containing bulky carboxylate ligands
*
Raquel Gracia, Harry Adams, Nathan J. Patmore
Department of Chemistry, University of Sheffield, Sheffield S3 7HF, UK
a r t i c l e i n f o
a b s t r a c t
II;III
Article history:
A series of Ru2
complexes containing the bulky carboxylate ligand 2,4,6-triisopropylbenzoate (TiPB) of
Received 26 November 2009
Received in revised form 2 July 2010
Accepted 12 July 2010
type trans-[Ru2(TiPB)2(O2CCH3)2X] [X = Cl (1), PF6 (2)] and [Ru2(TiPB)4X] [X = Cl (3), PF6 (4)] have been
synthesised. The corresponding Ru2
II;II
complexes trans-[Ru2(TiPB)2(O2CCH3)2] (5) and [Ru2(TiPB)4] (6)
were also isolated. Magnetic susceptibility measurements indicate that the diruthenium cores have the
Available online 16 July 2010
2
2
expected three (1–4) or two (5 and 6) unpaired electrons consistent with r2p4d2(d*p* 3 and r2p4d2d* p*
)
electronic configurations. Compounds 1–4 and 6 were structurally characterised by X-ray crystallogra-
phy, and show the expected paddlewheel arrangement of carboxylate ligands around the diruthenium
Keywords:
Ruthenium
DFT calculations
Carboxylate ligands
Metal–metal interactions
X-ray crystallography
core. The diruthenium cores of complexes 3, 4 and 6 are all distorted to minimise steric interactions
II;II
between the bulky carboxylate ligands. The Ru–Ru bond length in the Ru2
complex 6 [2.2425(6) Å]
is the shortest observed for a diruthenium tetracarboxylate and, surprisingly, is 0.014 Å shorter than in
II;III
the analogous Ru2
complex 4, despite an increase in the formal Ru–Ru bond order from 2.0 (6) to
2.5 (4). This is rationalised in terms of the extent of internal rotation, or distortion, about the diruthenium
core. This was supported by density functional theory calculations on the model complexes [Ru2(O2CH)4]
and [Ru2(O2CH)4]+, that demonstrate the relationship between Ru–Ru bond length and internal rotation.
Electrochemical and electronic absorption data were recorded for all complexes in solution. Comparison
of the data for the ‘bis–bis’ (1, 2 and 5) and tetra-substituted (3, 4 and 6) complexes indicates that the
shortening of the Ru–Ru bond length results in a small increase in energy of the near-degenerate d*
and p* orbitals.
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
Diruthenium tetracarboxylates of form [Ru2(O2CR)4(L)2]n+
(L = solvent or anion) have been isolated in two forms; the mixed
II;III
II;II
Metal–metal multiply bonded tetracarboxylates of form
[M2(O2CR)4(L)x]n+ (L = solvent or anion, x = 0, 1, 2) typically adopt
a ‘‘paddlewheel” geometry, in which each carboxylate ligand
bridges the two metal atoms in equatorial positions, and solvent
molecules or anions (L) occupy the axial positions [1]. Diruthenium
complexes have a unique electronic structure and interesting
physical properties that are responsible for the current popularity
and continued expansion of this field. Potential applications for
these complexes have been found in a wide variety of areas such
as catalysts [2–4] and antitumour metallopharmaceuticals [5–7],
as well their incorporation into supramolecular arrays [8–12] that
can have interesting magnetic [13–15] and optoelectronic proper-
ties [16–18]. Recent studies have also highlighted the potential use
of diruthenium compounds as molecular wires [19,20], and there is
often a strong relationship between the structure of these com-
plexes and their reactivity or electronic structure [21].
valence Ru2
valence Ru2
form and homovalent Ru2
form was first synthesised in 1966, and found to
have three unpaired electrons [22]. The electronic structure was
elucidated with the aid of SCF-X -SW calculations [23] and reso-
nance Raman spectra [24] that indicated a r2p4d2(d*p*
form. The mixed
II;III
a
3
)
electron
II;II
configuration, and a formal Ru–Ru bond order of 2.5. The Ru2
tetracarboxylates are more air-sensitive than their Ru2
II;III
counter-
parts, and were first isolated in 1984 by Wilkinson and co-workers
[25]. These compounds have two unpaired electrons and
a
2
r2p4d2d* p*2 electron configuration, with a formal Ru–Ru bond or-
der of 2.0 [26]. The Ru–Ru bond lengths in these compounds show
only a small dependence on the nature of the carboxylate or axial
ligands. However, in agreement with the general trend for multiply
bonded compounds in which bond length is inversely proportional
II;III
to bond order, the Ru–Ru bond lengths for Ru2
tetracarboxylates
II;II
are slightly shorter than observed for their analogous Ru2 coun-
terparts, consistent with a reduction in the formal Ru–Ru bond or-
der [27].
We have recent been interested in synthesising diruthe-
nium complexes containing the bulky carboxylate ligand
* Corresponding author. Tel.: +44 114 2229418.
E-mail address: n.patmore@sheffield.ac.uk (N.J. Patmore).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.07.024

R. Gracia et al. / Inorganica Chimica Acta 363 (2010) 3856–3864
3857
furan was purified by distillation over sodium wire, and all other
solvents obtained from a ‘‘Grubbs” solvent purification system.
The sodium salt of 2,4,6-triisopropylbenzoic acid (HTiPB) was syn-
thesised by reacting a freshly prepared NaOMe solution in MeOH
with the acid. The compounds [Ru2(OAc)4Cl] [35] and [Ru2(OAc)4]
[36] were synthesised according to literature procedures, and all
other chemicals were acquired from commercial sources.
HO
O
"HTiPB"
2.3. Synthesis of [Ru2(OAc)2(TiPD)2Cl] (1)
Scheme 1
A solution of Ru2(OAc)4Cl (0.275 g, 0.58 mmol) and TiPBH
(0.360 g, 1.45 mmol) in MeOH (20 mL) was refluxed under an ar-
gon atmosphere for 16 h. The solution was allowed to cool and
the solvent removed in vacuo. The solid was redissolved in THF
(10 mL) and precipitated with hexane (20 mL). The light brown
product was isolated by filtration and dried in vacuo. Yield = 0.34 g
(71%). Crystals of [Ru2(OAc)2(TiPD)2Cl]ꢁ2MeCN suitable for X-ray
diffraction were grown from slow evaporation of an acetonitrile
solution containing 1. MALDI-TOF-MS: calcd. monoisotopic MW
for Ru2C36O8H52Cl 849.59, found m/z 816.27 (M+ꢀCl, 100%).
IR(cmꢀ1): 2960m, 2930w, 2868w, 1604m, 1570w, 1472w, 1463w,
1440m, 1405s, 1382w, 1359m, 1348w, 1316w, 1154m, 1112m,
1088w, 1015w, 874m, 858w, 813m, 796w, 770w, 689s, 663m,
641s. Anal. Calc. for Ru2C36O8H52Cl: C, 50.9; H, 6.16. Found: C,
2,4,6-triisopropyl benzoic acid (HTiPB, Scheme 1). This ligand has
been successfully employed by Cotton, Murillo and co-workers to
4þ
4þ
synthesise quadruply bonded Cr2 and Mo2 species, as well as
4þ
Rh2 compounds [21,28,29], in which the steric bulk of the ligand
prevent intermolecular association in the solid-state, and permit-
ted structural characterisation without axial coordination of exog-
enous ligands. Diruthenium complexes with large ligands such as
adamantly carboxylate [30] and metallocene carboxylates [31,32]
have been previously reported. In this paper, we report the synthe-
II;III
II;II
sis and characterisation of a series of Ru2
and Ru2 complexes
containing TiPB. The use of the bulky TiPB ligand imparts unusual
structural distortions that result in the shortening of the metal–
metal bond. This effect was rationalised with the aid of theoretical
calculations and the impact on the electronic structure and spec-
troscopic properties are also discussed. Some aspects of this work
have been previously reported as a communication [33].
49.9; H, 6.47%. UV–Vis (THF) [kmax, nm (e
, Mꢀ1 cmꢀ1)]: 466
(1400), 622 (110), 1130 (30).
2.4. Synthesis of [Ru2(OAc)2(TiPB)2(PF6)] (2)
2. Experimental
A solution of [Ru2(OAc)2(TiPB)2Cl] (0.100 g, 0.12 mmol) and
AgPF6 (0.100 g, 0.40 mmol) in MeOH (20 mL) was stirred at room
temperature for 18 h. The solvent was removed in vacuo, and the
product was redissolved in THF (10 mL) and filtered through celite.
Hexane was added to the filtrate to precipitate the product as an
orange solid, which was isolated by filtration and dried in vacuo.
Yield = 0.084 g (73%). Orange crystals of [Ru2(TiPB)2(OAc)2(-
THF)2](PF6)ꢁTHF were grown by carefully layering a THF solution
of 2 with hexane. MALDI-TOF-MS: calcd. monoisotopic MW for
Ru2C36O8H52PF6 961.14, found m/z 816.27 (M+ꢀPF6, 100%).
IR(cmꢀ1): 2961m, 2930w, 1867w, 1570w, 1441w, 1412w, 1403w,
1400w, 1366w, 1270s, 1236m, 1191m, 1149m, 1126m, 1088m,
1031s, 982w, 890w, 845s, 813m, 782m, 688m, 642m. UV–Vis
2.1. Physical measurements
Matrix assisted laser desorption/ionisation time-of-flight (MAL-
DI-TOF) mass spectrometry was performed on a Bruker Reflex III
(Bruker, Breman, Germany) mass spectrometer operated in linear,
positive ion mode with a N2 laser. Laser power was used at the
threshold level required to generate signal. DCTB [trans-2-(3-(4-t-
butyl-phenyl)-2-methyl-2-propenylidene)malononitrile] was used
as the matrix and prepared as a saturated solution in THF.
Allotments of matrix and sample were thoroughly mixed together;
0.5 mL of this was spotted on the target plate and allowed to dry. IR
spectra were recorded as solid samples with a Perkin–Elmer Spec-
trum RX I FT-IR spectrometer equipped with a DuraSamplIR II dia-
mond ATR probe and universal press. Magnetic moments were
determined at room temperature using a Sherwood Scientific Mag-
way MSB Mk1 magnetic susceptibility balance. Molar diamagnetic
corrections were applied to the magnetic susceptibility data on the
basis of Pascal’s constants [34]. Electronic absorption spectra were
recorded in THF solutions at room temperature using a Varian Cary
5000 UV–Vis–NIR spectrophotometer. Elemental analyses were
carried out by the Microanalytical Service of the Department of
Chemistry at Sheffield with a Perkin–Elmer 2400 analyzer. Electro-
chemical studies were carried out in N2-purged methanol solutions
with 0.1 M [nBu4N](PF6) as supporting electrolyte. A standard
three-electrode system was used with a Pt microdisc and a large
surface area Pt wire as the working and counter electrodes, respec-
tively. Potentials were measured in reference to a Ag/AgCl refer-
ence, with all potentials quoted for a scan rate of 0.10 V sꢀ1. At
the end of every experiment, ferrocene was added as an internal
standard, with the Fc/Fc+ couple consistently observed at +0.43 V
versus Ag/AgCl.
(THF) [kmax, nm (e
, Mꢀ1 cmꢀ1)]: 458 (499).
2.5. Synthesis of [Ru2(TiPB)4Cl] (3)
A Schlenk tube charged with a stir bar and a mixture of
Ru2(OAc)4Cl (0.330 g, 0.70 mmol) and excess 2,4,6-triisopropyl-
benzoic acid (HTiPB) (1.5 g, 6.04 mmol) was heated at 220 °C for
6 h. Every 2 h, the mixture was cooled to room temperature, and
the acetic acid that condensed on the upper portion of the Schlenk
tube removed in vacuo. After 6 h of heating the mixture was cooled,
and excess 2,4,6-triisopropylbenzoic acid (HTiPB) removed by sub-
limation at 130 °C, 10ꢀ3 Torr, to afford 3 as a pink powder.
Yield = 0.839 g (98%). Excess HTiPB ligand was recovered from
the sublimation finger and used in subsequent reactions. Crystals
of [Ru2(TiPB)4Cl(MeOH)]ꢁMeOH suitable for X-ray diffraction were
grown by slow evaporation of a MeOH solution containing 3. MAL-
DI-TOF-MS: calcd. monoisotopic MW for Ru2O8C64H92Cl 1227.46,
found m/z 1192.22 (MꢀCl+, 100%). IR (cmꢀ1): 2955m, 2922w,
2868w, 1605m, 1440m, 1410m, 1384s, 1361w, 1320w, 1258w,
1158w, 1109m, 940w, 876w, 811w, 778w, 734w, 642m. Anal. Calc.
for Ru2O8C64H92Cl: C, 62.65; H, 7.56. Found: C, 62.45; H, 7.35%. UV–
2.2. Materials
All experimental manipulations were performed under an inert
atmosphere using standard Schlenk line techniques. Tetrahydro-
Vis (THF) [kmax, nm (e
, Mꢀ1 cmꢀ1)]: 470 (1460), 616 sh (262), 1148
(60).

3858
R. Gracia et al. / Inorganica Chimica Acta 363 (2010) 3856–3864
2.6. Synthesis of [Ru2(TiPB)4(PF6)] (4)
in every case. The internal rotation angle (v) was constrained from
0° to 20° in 2° increments with the rest of the geometry allowed to
fully optimise in D4 symmetry.
A suspension of Ru2(TiPB)4Cl (0.200 g, 0.16 mmol) and AgPF6
(0.100 g, 0.40 mmol) in MeOH (20 mL) was stirred for 18 h. The
solvent was removed in vacuo from the resulting orange solution
with grey precipitate. The mixture was redissolved in THF
(10 mL) and filtered through celite. Red-orange crystals of [Ru2(-
TiPB)4(THF)2](PF6) were grown by layering this solution with hex-
ane. The axially coordinated THF molecules were removed by
heating the crystals at 70 °C for 5 h in vacuo to give Ru2(TiPB)4(PF6)
as an orange powder. Yield = 0.203 g (93%). MALDI-TOF-MS: calcd.
monoisotopic MW for Ru2O8C64H92PF6 1336.51, found m/z 1192.39
(MꢀPF6+, 100%). IR (cmꢀ1): 2962m, 2928w, 2872w, 1605m, 1458m,
1389s, 1362w, 1320w, 1157w, 1109w, 1028w, 835vs, 738w, 645m.
Anal. Calc. for Ru2O8C64H92PF6: C, 57.51; H, 6.94. Found: C, 56.88;
2.10. X-ray crystallography
Data were collected were measured on a Bruker Smart CCD area
detector with Oxford Cryosystems low temperature system. After
integration of the raw data and merging of equivalent reflections,
an empirical absorption correction was applied (SADABS) based on
comparison of multiple symmetry-equivalent measurements
[44]. The structures were solved by direct methods (SHELXS-97)
and refined by full-matrix least squares on weighted F2 values
for all reflections using the SHELX suite of programs [45]. All hydro-
gens were included in the models at calculated positions using a
riding model with U(H) = 1.5 ꢂ Ueq (bonded carbon atom) for
methyl and hydrogens and U(H) = 1.2 ꢂ Ueq (bonded carbon atom)
for methine, methylene and aromatic hydrogens.
H, 7.25%. UV–Vis (THF) [kmax, nm (e
, Mꢀ1 cmꢀ1)]: 439 (367), 1080
(10).
2.7. Synthesis of [Ru2(OAc)2(TiPB)2] (5)
The isopropyl groups and solvent molecules present in the
structures caused disorder problems in all of the structures. Two
of the TiPB isopropyl groups in 1 were disordered over two sites
with occupancies of 0.80/0.20 and 0.75/0.25. This disorder is
responsible for the short intermolecular contact between the cal-
culated hydrogens H17C and H19D. A THF solvent molecule in
2(THF)2 was disordered over two positions about a centre of inver-
sion with equal occupancy. One of the TiPB groups in 3(MeOH) was
disordered over two positions (0.47/0.53 site occupancy), along
with the methyl group of the axially coordinated MeOH molecule
(0.58/0.32 site occupancy), and atoms associated with the both dis-
ordered components refined isotropically. The full experimental
details and CIF files for complexes 4(THF)2 and 6(THF)2 have been
communicated previously [33].
A
solution of Ru2(OAc)4 (0.250 g, 0.57 mmol) and TiPBH
(0.360 g, 1.45 mmol) in MeOH (20 mL) was refluxed under an ar-
gon atmosphere for 14 h. The solvent was removed under vacuum,
and the solid redissolved in THF (10 mL) and filtered. Solvent was
removed from the filtrate, and the light brown solid was washed
with hexanes before drying in vacuo. Yield = 0.32 g (69%). Numer-
ous attempts to grow crystals of 5 from a variety of solvents were
unsuccessful. MALDI-TOF-MS: calcd. monoisotopic MW for Ru2-
C36O8H52 816.17, found m/z 816.19 (M, 100%). IR(cmꢀ1): 2961m,
2930w, 2866w, 1605w, 1528m, 1481w, 1437m, 1408s, 1363m,
1343vw, 1320w, 1260m, 1155w, 1089w, 1021m, 942w, 872w,
814w, 690s, 663w, 643s. UV–Vis (THF) [kmax, nm (e
, Mꢀ1 cmꢀ1)]:
448 (1176).
Experimental data relating to the structure determinations of
all complexes are displayed in Table 5.
2.8. Synthesis of [Ru2(TiPB)4] (6)
Hydrogen gas was bubbled through a MeOH solution (50 mL)
containing RuCl3ꢁ3H2O (0.300 g, 1.15 mmol) and catalytic
3. Results and discussion
a
amount of platinum black (10 mg) for 4 h. The resulting ruthenium
‘‘blue” solution was filtered through celite into a Schlenk flask
charged with NaTiPB (0.650 g, 2.41 mmol). A reflux condensor
was attached to the Schlenk flask, and the solution refluxed for
16 h to give a brown-yellow solution. The solution was cooled to
room temperature and filtered through celite. The filtrate was
dried in vacuo, and the product redissolved in hexane and filtered
again before removing the solvent in vacuo to afford 6 as a
brown-red powder. Yield = 0.683 g (50%). Crystals of [Ru2(-
TiPB)4(THF)2] suitable for X-ray diffraction were grown by slow
evaporation of a THF solution. MALDI-TOF-MS: calcd. monoiso-
topic MW for Ru2O8C64H92PF6 1192.49, found m/z 1192.43 (M+,
100%). IR (cmꢀ1): 2958m, 2927w, 2871w, 1604m, 1517m,
1460m, 1403s, 1361w, 1317w, 1257w, 1156w, 1108m, 1005w,
938w, 875m, 801w, 775w, 640w. Anal. Calc. for Ru2O8C64H92: C,
64.51; H, 7.78. Found: C, 64.16; H, 8.06%.
3.1. Synthesis and characterisation
We first attempted to synthesise the diruthenium(II,III) com-
plex [Ru2(TiPB)4Cl] (TiPB = 2,4,6-triisopropylbenzoate) by tradi-
tional metathesis reactions. The reaction of [Ru2(O2CCH3)4Cl]
with 4 equiv. of HTiPB in refluxing methanol solutions did not pro-
duce the desired tetra-substituted complex. Instead, the ‘bis–bis’
complex trans-[Ru2(TiPB)2(O2CCH3)2Cl] (1) was formed. The trans
arrangement of the bulky ligands was determined by X-ray crystal-
lography, vide infra, and is observed in related ‘bis–bis’ TiPB com-
plexes of dimolybdenum [46] and dirhodium [47] carboxylates as
it minimises steric interactions between the ligands. Synthesis of
diruthenium ‘bis–bis’ complexes have been previously reported;
refluxing 2 equiv. of HO2CCMe3 with [Ru2(O2CCH3)4Cl] in
a
MeOH/H2O mixture generates [Ru2(O2CCH3)2(O2CCMe3)2Cl] [48].
Despite the presence of excess HTiPB in the synthesis of 1, it does
not undergo further substitution to tri- and tetra-substituted spe-
cies under these conditions. The mechanism for substitution in
[M2(O2CR)4] complexes involves initial coordination of the incom-
ing carboxylate ligand to the axial position [49], and it is likely that
blocking this position with two bulky carboxylate ligands provides
a kinetic barrier to further substitution.
A different synthetic route was used for the synthesis of the
mixed-valence compound [Ru2(TiPB)4Cl]. Excess of the carboxylic
acid (HTiPB) and [Ru2(OAc)4Cl] were heated at 210 °C in a melt
reaction. Every 2 h, the reaction was allowed to cool and acetic acid
removed in vacuo, to drive the reaction to completion. Excess li-
gand was recovered by sublimation, and [Ru2(TiPB)4Cl] (3) isolated
2.9. DFT calculations
Molecular structure calculations on the model complexes
[Ru2(O2CH)4]+ and [Ru2(O2CH)4] were performed using density
functional theory as implemented in the GAUSSIAN03 suite of pro-
*
grams [37]. The B3LYP functional [38–40] and the 6-31G (5d) basis
set [41] were used for H, C, and O, along with the SDD energy con-
sistent pseudopotentials for ruthenium [42]. This functional and
basis set combination was chosen as it has been found to be the
most effective in benchmark studies on related [M2(O2CR)4] com-
plexes [43]. Unrestricted open-shell calculations were performed

R. Gracia et al. / Inorganica Chimica Acta 363 (2010) 3856–3864
3859
2
2
in essentially quantitative yield. In order to more accurately com-
pare structural parameters between related species, complexes of
form [Ru2(O2CR)4(THF)2]0/+ were targeted, that would have iso-
structural diruthenium cores. The complexes [Ru2(TiP-
B)2(OAc)2(PF6)] (2) and [Ru2(TiPB)4(PF6)] (4), which could be
crystallised as THF diadducts from THF solutions, were prepared
by reaction of the corresponding chloride complexes 1 and 3 with
AgPF6.
indicate the presence of two unpaired electrons and a r2p4d2d* p*
electron configuration. The higher magnetic susceptibility values
II;III
for Ru2
complexes 1–4, ranging from 4.0 to 4.5 BM, are consis-
3
tent with three unpaired electrons and a r2p4d2(d*p*
) electron
configuration. Further confirmation of the oxidation state of the
complexes comes from examination of the separation between
the symmetric and asymmetric carboxylate stretching frequencies
(DmCO ) in their IR spectra. In agreement with previous studies [27],
2
Incomplete substitution was also observed from the reaction of
the diruthenium(II,II) complex [Ru2(O2CCH3)4] and HTiPB in reflux-
ing methanol solutions, and the complex trans-[Ru2(TiP-
B)2(O2CCH3)2] (5) was isolated. The homoleptic complex
[Ru2(TiPB)4] (6) was synthesised in good yield using a slight mod-
ification of the original preparation described by Wilkinson and co-
workers; a ‘‘ruthenium blue” solution, generated from reduction of
[RuCl3ꢁ3H2O] with H2 in methanol, was refluxed with NaTiPB. The
synthesis of compounds 1–6 is summarised in Scheme 2.
All complexes are significantly more soluble than the parent
[Ru2(OAc)4]0/+ complexes, and can be dissolved in non-donor sol-
vents such as dichloromethane. Complex 6 is also soluble in hex-
ane solutions. Signals corresponding to the [Ru2(O2CR)4]+ parent
ion were observed in the MALDI-TOF mass spectra of complexes
the Ru2II;III complexes show a much smaller separation between the
symmetric and asymmetric CO2 stretching frequencies (38–
69 cmꢀ1) than the Ru2 complexes (91–120 cmꢀ1). The magnetic
II;II
susceptibilities, and asymmetric and symmetric carboxylate
stretching frequencies in the IR spectra of complexes 1–6 are
shown in Table 1. The stretching frequencies for the TiPB and
OAc carboxylate groups in the ‘bis,bis’ complexes are sufficiently
different to allow them both to be resolved, and both sets are re-
ported in Table 1.
3.2. Solid-state structures
Crystals suitable for X-ray crystallography were obtained from
THF solutions in the case of 2, 4 and 6, an acetonitrile solution in
the case of 1, and a methanol solution in the case of 3.
The structures of the ‘bis–bis’ complexes 1 (Fig. 1) and 2 (Fig. 2)
show the expected ‘paddlewheel’ arrangement of ligands about the
diruthenium core, with bulky carboxylate ligands orientated trans
II;II
1–6. The same molecular ions are observed for analogous Ru2
and Ru2
II;III
complexes as the chloride ion was removed upon ioni-
sation, as observed in previous studies [50]. The magnetic suscep-
II;II
tibilities of 3.3 and 2.9 BM for the Ru2
complexes 5 and 6
MeOH/H2O
trans-Ru2(O2CCH3)2(TiPB)2Cl
Ru2(O2CCH3)4Cl + 2HTiPB
Δ, 16h
1
MeOH
16h
trans-Ru2(O2CCH3)2(TiPB)2Cl + AgPF6
trans-Ru2(O2CCH3)2(TiPB)2(PF6)
2
HTiPB
Ru2(O2CCH3)4Cl + 4HTiPB
210°C, 6h
Ru2(TiPB)4Cl
3
MeOH
16h
Ru2(TiPB)4(PF6)
Ru2(TiPB)4Cl + AgPF6
4
MeOH/H2O
Δ, 16h
trans-Ru2(O2CCH3)2(TiPB)2
Ru2(O2CCH3)4 + 2HTiPB
5
1. Pt black (cat.), H2, MeOH
RuCl3⋅3H2O
Ru2(TiPB)4
2. NaTiPB, Δ MeOH, 16h
6
Scheme 2
Table 1
Infrared carboxylate stretching frequencies and magnetic susceptibility data for complexes 1–6. All data were obtained from solid samples at 298 K.
Compound
‘‘TiPB”
sym(CO2) (cmꢀ1
‘‘OAc”
sym(CO2) (cmꢀ1
leff (BM)
m
)
m
asym(CO2) (cmꢀ1
)
D
m
(CO2) (cmꢀ1
)
m
)
m
asym(CO2) (cmꢀ1
)
Dm
(CO2) cmꢀ1
Ru2(II,III)
[Ru2(OAc)4Cl]a
Ru2(OAc)2(TiPD)2Cl (1)
Ru2(OAc)2(TiPD)2(PF6) (2)
Ru2(TiPD)4Cl (3)
1394
1405
1412
1453
1463
1450
59
58
38
4.1
4.5
4.5
4.2
4.0
1382
1403
1384
1389
1440
1441
1440
1458
58
38
56
69
Ru2(TiPD)4(PF6) (4)
Ru2(II,II)
Ru2(OAc)4
b
1440
1437
1560
120
91
2.8
3.3
2.9
[Ru2(OAc)2(TiPD)2] (5)
[Ru2(TiPD)4] (6)
1408
1403
1528c
1517
120
114
1528c
a
b
c
Data obtained from Refs. [51,22].
Data obtained from Ref. [25].
Coincident peaks.

3860
R. Gracia et al. / Inorganica Chimica Acta 363 (2010) 3856–3864
in both of these species shows little distortion and essentially has
the expected pseudo-D4h symmetry.
The crystal structure of 3(MeOH), is shown in Fig. 3. The bond
lengths associated with this complex (Table 2) are unremarkable,
although it can be seen that the diruthenium core of this molecule
is distorted to minimise steric interactions. This distortion, or
internal rotation (v), can be measured by examining the OCO –
2
Ru–Ru–OCO torsion angle. The internal rotation for 3(MeOH) is
2
similar to that observed in the other diruthenium(II,III) homoleptic
complex 4(THF)2, vave ꢃ 8.0°.
The identical coordination environment about the diruthenium
cores in complexes 4(THF)2 and 6(THF)2 allows direct comparison
of their structural parameters, summarised in Table 2. The average
Fig. 1. Extended packing structure of 1, highlighting the ‘zig-zag’ one-dimensional
chain that is propagated along the crystallographic c-axis. Solvent molecules,
hydrogen atoms and some minor disorder components have been omitted for
clarity.
Ru–OCO bond lengths in 4(THF)2 (2.018 Å) and 6(THF)2 (2.058 Å)
2
II;III
II;II
are in the range expected for Ru2
and Ru2 tetracarboxylates,
with the small increase in the bond lengths a reflection of the
5þ
reduction in electrostatic attraction from the respective Ru2
and Ru2 cores [27].
4þ
The Ru–Ru bond lengths for diruthenium tetracarboxylates
II;II
II;III
show a small decrease upon going from related Ru2
to Ru2
complexes. For example, the Ru–Ru bond length of 2.262(3) Å in
[Ru2(O2CCH3)4(OH2)2] decreases to 2.248(1) Å in [Ru2(O2C-
CH3)4(OH2)2][BF4] [36,52]. This reduction reflects the increase in
the formal bond order from 2.0 (Ru2II;II) to 2.5 (Ru2II;III). It is a rela-
tively small change because the bond being formed is the most
weakly-bonding d-bond, and there is an increase in oxidation state
which leads to d-orbital contraction that weakens the strength of
the
r
- and
p
-bonds. The Ru–Ru bond length of 2.2567(3) Å for
II;III
4(THF)2 is at the shorter end of the range expected for Ru2
tet-
II;II
racarboxylates but, remarkably, the bond length for the Ru2
complex 6(THF)2 is even shorter, 2.2425(6) Å. This is the shortest
reported Ru–Ru bond length for a diruthenium tetracarboxylate;
the previous shortest being 2.248(1) Å found in [Ru2(O2C-
CH3)4(OH2)2][BF4] [53]. The observed decrease in metal–metal
bond length concomitant with a decrease in formal bond order is
a rare phenomenon in metal–metal multiply bonded compounds.
It has been observed for [Tc2Cl8]2ꢀ/3ꢀ ions, for which a ꢃ0.05 Å in-
crease in bond length was observed despite a 0.5 increase in the
Fig. 2. Molecular structure of the diruthenium core of trans-[Ru2(OAc)2(-
TiPB)2(THF)2](PF6), [2(THF2)] with anisotropic displacement parameters drawn at
the 50% level. Hydrogen atoms have been omitted for clarity, and symmetry-
equivalent atoms generated using the symmetry operation ꢀx, ꢀy, ꢀz.
to one another. This trans arrangement of the ligands is a reflection
of the steric factors involved, analogous to the behaviour observed
in both the solid-state and solution for other [M2(TiPB)2(O2CR)2]
(M = Mo [46], W [46], Rh [47]) complexes. Complex 1 contains
two crystallographically independent {Ru2(O2CR)4} units that are
linked by chloride ions [Ru1–Cl1–Ru2 = 125.96(3)°] propagating a
formal Tc–Tc bond order, ascribed to changes in the
r, p and d
bond-strengths upon changing the oxidation states [54]. However,
the oxidation state of the metals does not fully explain why
6(THF)2 should have the shortest Ru–Ru bond length found to date
given that this trend is not observed in other isostructural
[Ru2(O2CR)4]0/+ pairs. Inspection of the structures of 4(THF)2 and
6(THF)2 reveals unusually large internal rotation about the diru-
thenium core to minimise steric interactions between the bulky li-
gands, as highlighted in Fig. 4. This distortion can be measured by
zig-zag polymeric chain structure in the solid-state, as observed
II;III
in other Ru2
tetracarboxylate chlorides [27]. Complex 2
crystallises as the axially coordinated THF adduct, [Ru2(TiP-
B)2(OAc)2(THF)2](PF6) [2(THF)2], with a typical Ru–OTHF bond dis-
tance of 2.269(2) Å. Other bond lengths and angles associated
with the diruthenium core in these complexes are presented in Ta-
ble 2. The Ru–Ru bond lengths of 2.2827(6) Å and 2.2858(5) Å in 1
examining the OCO –Ru–Ru–OCO torsion angle (
v), which is greater
2
2
for 6(THF)2 (vave = 13.8°) than 4(THF)2 (vave = 8.0°). Only the d-
and 2.2688(6) Å in 2 are within the narrow range of 2.248–2.310 Å
bond overlap and strength is dependent on internal rotation, and
II;III
II;II
found for Ru2
tetracarboxylates [1]. The slightly shorter Ru–Ru
v
is greater for 6(THF)2 as Ru2
complexes have no net d-bond
bond length of the diadduct 2 by comparison to the polymeric 1
is also consistent with previous studies [27]. The {Ru2(O2C)4} core
to restrict the rotation. We previously proposed [33] that this
internal rotation is responsible for the unusually short Ru–Ru
Table 2
Selected bond lengths (Å), angles (°) and torsion angles (°) for complexes 1, 2, 3, 4 and 6. Average values are given for Ru–OCO bond lengths and OCO –Ru–Ru–OCO torsion angles.
2
2
2
Complex
[Ru2(TiPB)2(OAc)2Cl]
[Ru2(TiPB)2(OAc)2(THF)2](PF6)
[Ru2(TiPB)4Cl(MeOH)]
[Ru2(TiPB)4(THF)2](PF6)
[Ru2(TiPB)4(THF)2]
Abbreviation
Ru–Ru
1
2(THF)2
2.2688(6)
2.021a
3(MeOH)
2.2797(5)
2.026a
2.261(3)
2.4880(12)
176.22(10)
178.18(3)
7.7a
4(THF)2
2.2567(3)
2.018a
6(THF)2
2.2425(6)
2.058a
2.284a
2.020a
Ru–OCO
2
Ru–LAx
Ru–Cl
Ru–Ru–LAx
Ru–Ru–Cl
2.269(2)
2.259a
2.308(3)
2.539a
174.91(7)
0.3a
178.3a
8.0a
177.19(9)
13.8a
174.9a
1.3a
OCO –Ru–Ru–OCO
(vave
)
2
2
a
Average values.

R. Gracia et al. / Inorganica Chimica Acta 363 (2010) 3856–3864
3861
Scheme 3
Fig. 3. Crystal structure of 3(MeOH) with anisotropic displacement parameters
drawn at the 50% level. Hydrogen atoms and disordered components have been
omitted for clarity.
Fig. 5. Calculated Ru–Ru bond lengths with increasing internal rotation for
[Ru2(O2CH)4]+ (blue) and [Ru2(O2CH)4] (red). (For interpretation of the references
in color in this figure legend, the reader is referred to the web version of this article.)
II;III
complex by comparison to its Ru2
analogue, reflecting the in-
crease in formal Ru–Ru bond order from 2.0 to 2.5. As the internal
rotation increases, the metal–metal bond length decreases for both
[Ru2(O2CH)4] and [Ru2(O2CH)4]+. The calculations for these model
II;II
complexes suggest that when
v
> 10° for a Ru2 tetracarboxylate
II;III
it will have a shorter bond length than an analogous Ru2
tetra-
carboxylate with D4h symmetry (
v
= 0°), highlighting that rela-
tively small distortions can have a significant influence. The
calculations also support that the proposed relationship between
internal rotation and metal–metal bond length. The unusual exper-
imental results, in which 6(THF)2 (vave = 13.8°) has a shorter Ru–Ru
bond length than 4(THF)2 ave = 8.0°) is reflected in the calcula-
(
v
Fig. 4. Comparison of the distorted {Ru2(O2CR)4} cores in the crystal structures of
4(THF)2, (a), and 6(THF)2, (b). The structures are viewed down the Ru–Ru bond, with
axial THF ligands and the phenyl and iPr groups of the TiPB ligands removed for
clarity.
tions; the calculated Ru–Ru bond length for [Ru2(O2CH)4] at
v
= 13.8° (ꢃ2.260 Å) is shorter than for [Ru2(O2CH)4]+ at
v = 8.0°
(ꢃ2.264 Å).
Fig. 5 also show that internal rotation does not affect the Ru–Ru
bond length in [Ru2(O2CH)4]+ to as greater extent as [Ru2(O2CH)4],
which is likely to be due to increased electrostatic repulsion be-
tween the Ru atoms in the higher oxidation state. The d-bond over-
lap will also be reduced by twisting the molecule, which will in
turn reduce Ru–Ru bond strength for [Ru2(O2CH)4]+. However, d-
bond overlap is related to internal rotation by the value of cos2v
distance in 6(THF)2 because it reduces the effective bridging dis-
tance of the carboxylate, as demonstrated in Scheme 3.
3.3. Computational studies
[57], so at
v = 8° there is only a 4% reduction in overlap meaning
In order to support the proposed relationship between Ru–Ru
bond length and internal rotation, calculations on the model com-
pounds [Ru2(O2CH)4]+ and [Ru2(O2CH)4] were performed using
density functional theory (DFT) as implemented in the GAUSSIAN 03
program. Geometry optimisation was performed on the model
complexes in D4 symmetry, whilst varying the internal rotation an-
gle from 0° to 20° in 2° increments.
this is likely to only have a small effect, especially given that this
molecule only has a net d-bond order of 0.5.
3.4. Electrochemical studies
II;III
Ru2
tetracarboxylates typically show a one electron quasi-
The variation in the calculated Ru–Ru bond length by compari-
son to the internal rotation is shown in Fig. 5. With no internal
reversible reduction, the potential of which can vary by up to
0.5 V depending on the nature of the solvent, electrolyte or axially
coordinated ligand [58]. The redox potentials of 1–6 in MeOH solu-
tions are given in Table 3. All complexes show a single electron
quasi-reversible reduction (Ru2II;III) or oxidation (Ru2II;II) process
between 0.02 and 0.08 V (versus Ag/AgCl), corresponding to the
rotation (v = 0), the calculated Ru–Ru bond lengths of 2.271 Å
and 2.266 Å, for the [Ru2(O2CH)4] and [Ru2(O2CH)4]+ complexes,
respectively, are comparable to those obtained from recent de-
tailed computational studies on [Ru2(O2CR)4]0/+ complexes
employing the similar functionals and basis sets [55,56]. As
4þ
Ru25þ/Ru2 couple. These values are comparable to that observed
II;II
5þ
expected, the Ru–Ru bond length is slightly longer for the Ru2
for the Ru24þ/Ru2
couple in [Ru2(O2CCH3)4] (0.04 V in THF

3862
R. Gracia et al. / Inorganica Chimica Acta 363 (2010) 3856–3864
Table 3
carboxylate ligand set will have the same redox potentials. The
Electrochemical data for complexes I–VI.
II;III
Ru2
complexes 3 and 4 (E1/2 = 0.02) are slightly more difficult
Compound
Epc (V)
Epa (V)
E1/2 (V)a
to reduce than their ‘bis–bis’ counterparts 1 and 2 (E1/2 = 0.08 V).
This trend is consistent with a shorter Ru–Ru bond length for the
tetra-substituted complexes, which would increase the energy of
Ru2(II,III)
[Ru2(OAc)2(TiPD)2Cl] (1)
[Ru2(OAc)2(TiPD)2(PF6)] (2)
[Ru2(TiPD)4Cl] (3)
ꢀ0.50
ꢀ0.50
ꢀ0.46
ꢀ0.46
ꢀ0.25
ꢀ0.26
ꢀ0.37
ꢀ0.37
ꢀ0.38
ꢀ0.38
ꢀ0.41
ꢀ0.41
the antibonding (d*
p
*) orbitals making the complexes more diffi-
II;II
cult to reduce. A similar trend is observed for the Ru2 complexes,
with the increase in energy of these orbitals reflected in the fact
that the homoleptic complex 6 is easier to oxidise than the ‘bis–
bis’ analogue 5. This suggests that the internal rotation, and short
Ru–Ru bond lengths, observed in the solid-state structures of the
homoleptic complexes persist in solution.
[Ru2(TiPD)4(PF6)] (4)
Ru2(II,II)
[Ru2(OAc)2(TiPD)2] (5)
[Ru2(TiPD)4] (6)
[Ru2(O2CCH3)4] [36]b
ꢀ0.47
ꢀ0.45
ꢀ0.32
ꢀ0.36
ꢀ0.39
ꢀ0.41
ꢀ0.39
a
Cyclic voltammograms recorded in 0.1 M [NnBu4](PF6) MeOH solutions. Values
are referenced with respect to the Fc/Fc+ couple.
b
Electrolyte solution 0.2 M [NnBu4](PF6) in THF.
3.5. UV–Vis spectroscopy
ꢀ
solution) [36]. In methanol solutions, any axially coordinated PF6
The electronic absorption spectra of diruthenium tetracarboxy-
or Clꢀ ions will dissociate to give the solvent coordinated complex
lates were first interpreted with the aid of SCF-Xa-SW calculations
of form [Ru2(O2CR)4(MeOH)2]+, hence complexes with the same
by Norman and co-workers [23], and a number of subsequent
Table 4
Electronic absorption data for complexes 1–5 recorded in THF solutions. No
p
(Ru–O, Ru2) ? p*(Ru2) absorption for complex 6 was observed in the visible region.
d(Ru2) ? d*(Ru2)
Complex
p
(Ru–O, Ru2) ? p*(Ru2)
k (nm)
k (cmꢀ1
)
e
k (nm)
k (cmꢀ1
)
e
Ru2(II,III)
[Ru2(OAc)2(TiPD)2Cl] (1)
[Ru2(OAc)2(TiPD)2(PF6)] (2)
[Ru2(TiPD)4Cl] (3)
466
458
470
439
21459
21834
21277
22779
1400
499
1460
367
1130
8850
30
a
a
a
1148
1080
8710
9259
60
10
[Ru2(TiPD)4(PF6)] (4)
Ru2(II,II)
[Ru2(OAc)2(TiPD)2] (5)
448
22321
1176
a
Absorption too weak to be observed.
Table 5
Crystallographic details for complexes [Ru2(TiPB)2(OAc)2Cl] (1), [Ru2(TiPB)2(OAc)2(THF)2](PF6) [2(THF)2], [Ru2(TiPB)4Cl(MeOH)] [3(MeOH)], [Ru2(TiPB)4(THF)2](PF6) [4(THF)2],
[Ru2(TiPB)4(THF)2] [6(THF)2].
Compound
1
2(THF)2
3(MeOH)
4(THF)2
6(THF)2
Empirical formula
Formula weight
T (K)
C40H58Cl1N2O8Ru2
932.47
100(2)
C48H76F6O11P1Ru2
1176.20
150(2)
C66H99Cl1O10Ru2
1290.04
100(2)
C72H108F6O10P1Ru2
1480.69
120(2)
C72H108O10Ru2
1335.72
100(2)
k (Å)
0.71073
0.71073
0.71073
0.71073
0.71073
Crystal system
Space group
a (Å)
b (Å)
c (Å)
monoclinic
C2/c
26.875(4)
13.725(2)
23.720(4)
90
94.706(9)
90
8720(2)
triclinic
P1
monoclinic
Cc
monoclinic
P21/n
17.8308(9)
25.3963(13)
18.4580(9)
90
117.629(2)
90
7405.3(7)
4
monoclinic
C2/c
17.4405(19)
25.876(3)
17.9577(19)
90
116.524(2)
90
7251.1(14)
4
ꢀ
8.1076(14)
12.585(2)
14.136(2)
73.449(3)
76.284(3)
83.977(3)
1342.1(4)
1
25.7872(17)
16.5393(12)
20.4714(14)
90
127.926(2)
90
6887.1(8)
4
1.244
a
(°)
b (°)
c
(°)
V (Å3)
Z
8
dcalc (Mg mꢀ3
)
1.421
0.803
1.455
0.667
1.328
0.498
1.224
0.469
l
(mmꢀ1
)
0.529
F(0 0 0)
3848
609
2720
3108
2832
h range (°)
Index ranges
1.52–27.67
ꢀ30 6 h 6 35
ꢀ17 6 k 6 17
ꢀ30 6 l 6 30
64 730
1.54–26.46
ꢀ10 6 h 6 9
ꢀ15 6 k 6 15
ꢀ17 6 l 6 17
11 100
1.59–28.18
ꢀ34 6 h 6 33
ꢀ21 6 k 6 21
ꢀ27 6 l 6 26
50 120
1.31–27.59
ꢀ23 6 h 6 22
ꢀ33 6 k 6 29
ꢀ24 6 l 6 23
102 554
1.52–27.56
ꢀ22 6 h 6 22
ꢀ15 6 k 6 33
ꢀ23 6 l 6 21
36 056
Reflections collected
Independent reflections [R(int)
Observed reflections [I > 2r(I)]
]
10 083 [0.0476]
7930
5502 [0.0356]
4397
15 315 [0.0347]
13277
17 065 [0.0372]
12943
8372 [0.0376]
6175
Data completeness to hmax
0.989
0.992
0.986
0.994
0.998
Data/restraints/parameters
10 083/96/520
1.025
5502/0/313
1.015
15 315/12/709
1.072
17 065/115/811
1.116
8372/447/440
1.050
Goodness-of-fit (GOF) on F2
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0350,
wR2 = 0.0837
R1 = 0.0538,
wR2 = 0.0953
1.127/ꢀ0.723
R1 = 0.0440,
wR2 = 0.1015
R1 = 0.0598,
wR2 = 0.1175
0.859/ꢀ0.574
R1 = 0.0415,
wR2 = 0.0954
R1 = 0.0576,
wR2 = 0.1072
0.984/ꢀ0.877
R1 = 0.0435,
wR2 = 0.1064
R1 = 0.0642,
wR2 = 0.1175
2.850/ꢀ0.761
R1 = 0.0538,
wR2 = 0.1391
R1 = 0.0783,
wR2 = 0.1567
1.033/ꢀ1.462
Largest difference peak and hole
(e Åꢀ3
)

R. Gracia et al. / Inorganica Chimica Acta 363 (2010) 3856–3864
3863
ligands, and is greater for 6 as is has no net d-bond. The increased
internal rotation was proposed to account for the unusually short
Ru–Ru bond length in 6, which was supported by DFT calculations
that show a decrease in the Ru–Ru bond length as the internal rota-
tion increases. Spectroscopic and electrochemical studies on all the
complexes showed that the shortening of the Ru–Ru bond also
influences the relative energies of the Ru2 d* and p* orbitals. This
relationship between metal–metal bond length and internal rota-
tion is important in relation to metal–metal multiply bonded pad-
dlewheel complexes in general, as metal–metal bond lengths are
also likely to be related to the physical properties or reactivity of
such species.
Acknowledgements
The Royal Society and the University of Sheffield are gratefully
acknowledged for funding. We also thank the Ohio Supercomput-
ing Center and Prof. Malcolm H. Chisholm for computational time.
Fig. 6. UV–Vis spectrum of 1 in THF at room temperature. The marked absorptions
are assigned to the following transitions:
p r(Ru-axial
(Ru–O, Ru2) ? p*(Ru2), I;
ligand) ? p*(Ru2), II; d(Ru2) ? d*(Ru2), III.
Appendix A. Supplementary material
experimental studies were used to correctly assign the transitions
observed in the electronic spectra [59–61]. The electronic absorp-
tion spectra for complexes 1–6 were recorded in THF, and the data
CCDC 743819, 743820, 743821, 706412 and 706413 contain the
supplementary crystallographic data for 1, [2(THF)2], [3(MeOH)],
[4(THF)2] and [6(THF)2]. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif. Supplementary data associated with
this article can be found, in the online version, at doi:10.1016/
j.ica.2010.07.024.
is summarised in Table 4. The most intense band in the visible re-
II;III
gion of Ru2
tetracarboxylates in solution is typically observed at
ꢃ450 nm and has been assigned to a
p(Ru–O, Ru2) ?
p
*(Ru2) tran-
sition. This transition is observed at a similar energy for the Ru2
complexes 1, 2 and 3, but is shifted to higher energy for complex
4, as shown in Table 4. The shift of the
p(Ru–O, Ru2) ? p
*(Ru2)
transition to higher energy for 4 is consistent with a shorter Ru–
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oxylates, a weak low energy band at ꢃ1100 nm is observed in
solution corresponding to the d(Ru2) ? d*(Ru2) transition. A similar
trend is observed in the energy of this transition, with the highest
energy absorption observed for complex 4. A weak absorption near
620 nm also appears in the spectra of the chloride containing com-
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is assigned to a
The electronic absorption spectra of Ru2
have not been studied as extensively as their Ru2
r
(Ru-axial ligand) ?
p
*(Ru2) transition.
II;II
tetracarboxylates
II;III
counterparts.
However, the peak observed at 448 nm in the electronic absorption
spectrum of 5 can be assigned to the p(Ru–O, Ru2) ? p
*(Ru2) tran-
II;III
sition observed in related Ru2
complexes [63]. Surprisingly, the
p(Ru–O, Ru2) ? p
*(Ru2) transition for complex 6 is not observed.
Given that this complex has the shortest Ru–Ru bond length, the
Ru2 p* orbitals are likely to be destabilised the most, and the
p
(Ru–O, Ru2) ? p
*(Ru2) transition shifted to even higher energy
than observed for 4. Therefore, it is possible that this absorption
in 6 is now obscured by intense absorptions in the UV region.
4. Conclusions
Diruthenium tetracarboxylate ‘bis–bis’ complexes of form trans-
[Ru2(OAc)2(TiPB)2]0/+ have been synthesised via carboxylate ex-
change reactions between HTiPB and the corresponding diruthe-
nium tetracetate complex in refluxing methanol solutions, with
the bulky carboxylate ligands orientated trans to one another to
minimise steric interactions. Structural studies on the tetra-substi-
tuted complexes 4 and 6 show an unusual reduction in the Ru–Ru
bond length despite a decrease in the formal Ru–Ru bond order
from 2.5 to 2.0, respectively. These studies also revealed unusual
structural distortion, or internal rotation, about the diruthenium
cores that serves to minimise steric interactions between the bulky

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