Intervalence charge transfer in a "chain-like" ruthenium trinuclear assembly based on the bridging ligand 4,7-phenanthrolino-5,6: 5′,6′-pyrazine (ppz)

Article abstract of DOI:10.1039/b512625h

The IVCT characteristics of the mixed valence forms of the trinuclear complex [{Δ-Ru(bpy)₂}₂{Δ^t-Ru(bpy)(- ppz)₂}]ⁿ⁺ (n = 7, 8; t = trans), and the diastereoisomers (meso and rac) of the dinuclear complex [{Ru(bpy)₂} ₂(-ppz)]⁵⁺, display a marked dependence on the nuclearity and extent of oxidation of the assemblies. The dinuclear species are classified as borderline localised-delocalised mixed valence species while the two mixed valence states of the trinuclear complex exhibit localised behaviour. One-electron oxidation of a terminal Ru centre in the trinuclear case gives rise to a broad, low intensity IVCT band for the +7 mixed valence species which is composed of two underlying Gaussian-shaped bands. The transitions are identified as adjacent and remote IVCT transitions, with the former dominating the intensity of the IVCT manifold. The +8 mixed valence species exhibits a single dominant IVCT band arising from the equivalent IVCT transitions from the central Ruⁱⁱ to peripheral Ruⁱⁱⁱ centres. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b512625h

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Intervalence charge transfer in a “chain-like” ruthenium trinuclear assembly
based on the bridging ligand 4,7-phenanthrolino-5,6:5ꢀ,6ꢀ-pyrazine (ppz)†
Deanna M. D’Alessandro and F. Richard Keene*
Received 7th September 2005, Accepted 18th October 2005
First published as an Advance Article on the web 17th November 2005
DOI: 10.1039/b512625h
The IVCT characteristics of the mixed valence forms of the trinuclear complex [{D-Ru(bpy)2}2{Dt-
Ru(bpy)(l-ppz)2}]n+ (n = 7, 8; t = trans), and the diastereoisomers (meso and rac) of the dinuclear
complex [{Ru(bpy)2}2(l-ppz)]5+, display a marked dependence on the nuclearity and extent of oxidation
of the assemblies. The dinuclear species are classified as borderline localised–delocalised mixed valence
species while the two mixed valence states of the trinuclear complex exhibit localised behaviour.
One-electron oxidation of a terminal Ru centre in the trinuclear case gives rise to a broad, low intensity
IVCT band for the +7 mixed valence species which is composed of two underlying Gaussian-shaped
bands. The transitions are identified as adjacent and remote IVCT transitions, with the former
dominating the intensity of the IVCT manifold. The +8 mixed valence species exhibits a single
II
dominant IVCT band arising from the equivalent IVCT transitions from the central Ru to peripheral
III
Ru centres.
squares and clusters,16,17 polyferrocenes18–21 and alkynyl-bridged
Introduction
molecular wires22,23 have also appeared. As an illustrative ex-
The analysis of intervalence charge transfer (IVCT) transitions in
ample, Bignozzi and coworkers have extensively investigated the
mixed valence complexes provides a window to the elucidation of
IVCT properties of a series of cyano-bridged systems such as
[(NH3)5Ruterm–NC–Mcent(bpy)2–CN–Ruterm(NH3)5]4+ (M = Ru, Os;
the factors that govern electronic delocalisation and the activation
barrier to intramolecular electron transfer.1,2 These studies have fo-
bpy = 2,2ꢀ-bipyridine)24–27 and their analogues [(py)(NH3)4Ruterm
–
cused predominantly on dinuclear complexes;3–7 experimental and
theoretical studies on higher nuclearity polymetallic assemblies
are relatively scarce, and the degree to which the IVCT process is
influenced by the nuclearity, oxidation state and overall geometry
of the systems is not clear.
26,27
NC–Rucent(bpy)2–CN–Ruterm(NH3)4(X)]4+
(X = NH3 or py;
cent and term denote the central and terminal metals, respec-
tively). These studies revealed intriguing differences between
the mixed valence forms of the trinuclear complexes and their
dinuclear counterparts, including multiple IVCT bands due to
The elucidation of fundamental electron transfer processes in
trinuclear complexes provides the link between the understanding
of such processes in dinuclear systems, and in extended arrays
and metallosupramolecular systems which form the basis of
novel molecular devices.8–12 To date, IVCT studies of trinuclear
complexes have focused predominantly on complexes incorpo-
rating polypyridyl iron, ruthenium and osmium components,
linked by bridging ligands such as cyanide and pyrazine13,14
and polypyridyl bridging ligands including 2,3-dpp {2,3-bis(2-
pyridyl)pyrazine},15 however, a few reports of extended struc-
tures such as cyclic and star-burst cyano-bridged molecular
the presence of IVCT transitions between adjacent (Ruterm
→
Rucent) and remote (Ruterm → Ruterm) metal centres. However, in
these “chain-like” systems, the bridging cyano units are angularly
disposed by virtue of the central cis-Ru(bpy)2 chromophore.24–32
The influence of the cis or trans configuration of trinuclear
assemblies was elucidated in IVCT studies of the isomers of
III
II
the trinuclear complexes [{Fe (dppe)Cp}–NC–{Pt (L)2}–CN–
II
{Ru (PPh3)2Cp}]n+ (Cp = cyclopentadienyl anion; dppe = 1,2-
bis(diphenylphosphine)ethane; L = py, CN−), which differ only
in the cis or trans arrangement of the cyano bridging ligands
II
III
at the central metal.33 Remote IVCT transitions (Ru → Fe )
were observed for the trans-configured complexes only, so that
the metal–metal interaction in the trinuclear species is facilitated
by a polynuclear backbone in which the metals and intervening
bridging ligands provide a linear (rather than angular) conduit for
electron transfer.
School of Pharmacy and Molecular Sciences, James Cook Univer-
sity, Townsville, Queensland, 4811, Australia. E-mail: Richard.Keene@
jcu.edu.au; Fax: +61-(0)7-4781-6078
† Electronic supplementary information (ESI) available: 1H NMR chem-
ical shifts and spectra for K-[Ru(bpy)(HAT)2]2+, Kt-[Ru(bpy)(ppz)2]2+
and DDtD-[{Ru(bpy)2}2{Ru(bpy)(l-ppz)2}]6+ (Tables S1 and S2, Fig. S1
and S2); ligand-based reduction potentials and Kc values for the
di- and trinuclear complexes (Table S3); CD spectra of D- and K-
[Ru(bpy)(HAT)2]2+ and Dt- and Kt-[Ru(bpy)(ppz)2]2+ (Fig. S1); a differ-
ential pulse voltammogram of DDtD- [{Ru(bpy)2}2{Ru(bpy)(l-ppz)2}]6+
(Fig. S4); UV/Vis/NIR spectra of [Ru(bpy)(ppz)2]2+, meso-[{Ru(bpy)2}2-
(l-ppz)]4+ and DDtD- [{Ru(bpy)2}2{Ru(bpy)(l-ppz)2}]6+ at +25 ◦C
(Fig. S5); and UV/Vis/NIR spectra of meso-[{Ru(bpy)2}2(l-ppz)]n+ at
−35 ◦C (Fig. S6). See DOI: 10.1039/b512625h
While a number of important aspects relating to the influence
of the increasing nuclearity and overall oxidation state of the
assembly on the IVCT properties have been elucidated, the
majority of studies on the physical characteristics of polymetallic
assemblies have been conducted without regard for the inher-
ent stereochemical complexities in such systems.34,35 Keene and
coworkers provided the first examples of differences in the spectral,
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Fig. 1 The bridging ligands HAT and ppz, and Chem 3D representations of (a) “cluster-like” D3-[{Ru(bpy)2}3(l-HAT)]6+ and (b) “chain-like”
DDtD-[{Ru(bpy)2}2{Ru(bpy)(l-ppz)2}]6+. Hydrogen atoms are omitted for clarity.
electrochemical and photophysical properties of stereoisomers
Experimental
in mono-, di- and trinuclear systems.36–39 Stereochemical effects
General
have also been shown to influence the IVCT properties of a
1D and 2D 1H NMR spectra were performed on a Varian Mercury
300 MHz spectrometer. Chemical shifts for all complexes are
reported relative to 99.9% d3-acetonitrile (CD3CN; Cambridge
range of dinuclear and trinuclear systems: e.g. the diastereoiso-
mers meso(DK)- and rac(DD/KK)-[{Ru(bpy)2}2(l-HAT)]5+,40 and
homochiral (D3/K3)- and heterochiral (D2K/K2D)-[{Ru(bpy)2}3(l-
HAT)]6+ (n = 7, 8),41 which are based on the bridging ligand HAT
(1,4,5,8,9,12-hexaazatriphenylene; Fig. 1(a)). The characteristics
of the IVCT bands in the trinuclear case were markedly different
from those of their dinuclear analogues due to extensive electronic
communication between the Ru(bpy)22+ chromophores, and varied
significantly depending on the extent of oxidation and the overall
geometry of the assembly.
The present study broadens this previous investigation of the
IVCT properties of “cluster-like” homo-dinuclear and homo-
trinuclear ruthenium systems41 and hetero-dinuclear and hetero-
trinuclear ruthenium and osmium systems42 based on HAT
{Fig. 1(a)}, to the “chain-like” trinuclear complex DDtD-
[{Ru(bpy)2}2{Ru(bpy)(l-ppz)2}]6+, shown in Fig. 1(b). The
mononuclear “core” [Ru(bpy)(ppz)2]2+ possesses two geometric
isomers, trans and cis, in addition to the K and D enantiomeric
forms of each, and the “t” denotes the trans arrangement of the
ppz ligands at the central metal.
The bridging ligand ppz (4,7-phenanthrolino-5,6:5ꢀ,6ꢀ-pyrazine)
is closely related electronically and structurally to HAT and the
bridging ligand 2,3-dpp which have been utilised extensively in the
construction of tri-, tetra- and higher nuclearity assemblies.43–45
The planarity of the ligand overcomes the problem of conforma-
tional lability in complexes incorporating 2,3-dpp,35 and permits
the assessment of the influence of the overall “chain-like” geometry
of the assembly by comparison with the IVCT properties for the
closely related trinuclear “cluster-like” systems based on HAT.41
1
Isotope Laboratories (CIL)) at d = 1.93 ppm. H NMR assign-
ments were performed with the assistance of COSY experiments to
identify each pyridine ring system. Elemental microanalyses were
performed at the Microanalytical Unit in the Research School of
Chemistry, Australian National University.
Circular dichroism (CD) spectra were recorded in acetonitrile
solution at concentrations of ca. 2–3 × 10−5 M in a 1.0 cm path
length cell, using a JASCO J-715 spectropolarimeter. CD spectra
are presented as De (M−1 cm−1) vs. wavelength k (nm).
Electrochemical measurements were performed under argon
using a Bioanalytical Systems (BAS) 100A electrochemical anal-
yser. Cyclic (CV) and differential pulse (DPV) voltammograms
were recorded in a standard three-electrode cell using a glassy
carbon or platinum button working electrode, a platinum wire
auxiliary electrode and an Ag/AgCl reference electrode (0.1 M
[(n-C4H9)4N]PF6 in CH3CN). Ferrocene was added as an in-
ternal standard on completion of each experiment {the fer-
rocene/ferrocenium couple (Fc+/Fc0) occurred at +550 mV vs.
Ag/AgCl}. Solutions contained 0.1 M [(n-C4H9)4N]PF6 as the
electrolyte. Cyclic voltammetry was performed with a sweep rate
of 100 mV s−1; differential pulse voltammetry was conducted with
a sweep rate of 4 mV s−1 and a pulse amplitude, width and period
of 50 mV, 60 ms and 1 s, respectively. All potentials are reported
3 mV.
UV/visible/near-infrared (UV/Vis/NIR) spectroelectrochem-
istry was performed using a CARY 5E spectrophotometer
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interfaced to Varian WinUV software. The absorption spectra
of the electrogenerated mixed valence species were obtained in
situ by the use of a cryostatted optically semi-transparent thin
layer electrosynthetic (OSTLE) cell mounted in the path of the
spectrophotometer.46 An account of the procedure employed
in the spectroelectrochemical measurements has been detailed
previously.41
diamine (Aldrich, 99%) was freshly distilled from KOH.49 Tri-
fluoromethanesulfonic acid (3 M) was distilled in an all-glass
apparatus under vacuum. 1,2-Dichlorobenzene (Aldrich, 99%)
was distilled under vacuum (bp 180–184 ◦C at ∼30 mmHg). 2-
Methoxyethanol (Fluka) was distilled at 59 ◦C (300 mmHg) under
vacuum.
2,2,2-Trifluoroethanol (TFE; Aldrich, 98%) and 1-methyl-2-
pyrrolidinone (Aldrich, 99.8%) were used as received. All other
solvents were of laboratory grade unless otherwise specified.
Fuming nitric acid (> 90%) was prepared in an all-glass apparatus
by the distillation of a m◦ixture of 70% HNO3 (100 cm3) and 98%
H2SO4 (100 cm3) at 125 C.50 Trimethylamine N-oxide dihydrate
(TMNO, Fluka) was purified by sublimation at 120 ◦C under
vacuum and stored under argon. Aqueous solutions of the sodium
salts of (−)-O,Oꢀ-dibenzoyl-L-tartrate and (+)-di-O,Oꢀ-4-toluoyl-
D-tartrate were prepared by neutralisation of the corresponding
acids (Fluka, 98+%) using sodium hydroxide (NaOH).
Solutions for the spectroelectrochemial experiments contained
0.1 M [(n-C4H9)4N]PF6 supporting electrolyte in CH3CN and
the complex (ca. 1 × 10−3 M). All solutions were purged with
N2 prior to transference (via a syringe) into the OSTLE cell.
◦
The temperature was stabilised to 0.3 C prior to commencing
electrolysis. The dinuclear systems required approximately 6 h for
data collection at −35 ◦C, while the trinuclear systems required
approximately 10 h and were conducted at −15 ◦C due to
limitations on the continuous supply of coolant gas. The integrity
of the spectral data was checked by the observation of stable
isosbestic points, and the complete regeneration of the starting
HAT was supplied by Dr Nicholas Fletcher.51
spectrum following each stage of oxidation.
The IVCT spectra were scaled as e(m)/m dm
ꢀ
1,47
and decon-
Ligand synthesis
volution of the NIR transitions was performed using the curve-
fitting subroutine implemented within the GRAMS32 commercial
software package, as described previously.41 Based on the repro-
ducibility of the parameters obtained from the deconvolutions,
the uncertainties in the energies (mmax), intensities {(e/m)max} and
bandwidths (Dm1/2) were estimated as 10 cm−1, 0.001 M−1 and
10 cm−1, respectively.
The synthesis of ppz involved a three-stage procedure:
(a) 5-Methoxy-4,7-phenanthroline hydrogen sulfate. This
compound was prepared via a modification of the literature
procedure.52
A two-necked 3 dm3 round bottom flask was fitted with a
mechanical stirrer and condenser, and 3-nitrobenzenesulfonic acid
(Na+ salt; 100 g, 0.444 mol), 2-methoxy-1,4-pheneylenediamine-
sulfate (50 g, 0.212 mol) and glycerol (934.7 g, 9.53 mol) were added
to the flask. A solution of 98% H2SO4 (510 cm3) in H2O (270 cm3)
was prepared, allowed to cool to room temperature, and added in a
single portion. The mixture was heated at reflux (∼140 ◦C) for 6 h,
during which time the mixture attained a dark red colouration.
The solution was cooled to room temperature, poured over ice
(540 g) and basified to pH 10 with 50% NaOH (ca. 1200 cm3).
The aqueous solution was extracted with chloroform until the
chloroform extracts were pale yellow. A large amount of solid
Na2SO4 precipitated during the extraction: the solids were filtered
off and washed with chloroform until the organic extracts were
pale yellow. The extracts were combined, dried over anhydrous
Na2SO4 and the solvent was removed in vacuo. The resulting dark
red oil was evaporated to a paste in the fumehood.
Materials
Hydrated ruthenium trichloride (RuCl3·3H2O; Strem, 99%),
2,2ꢀ-bipyridine (bpy; Aldrich, 99+%), 2,3-diaminonaphthalene
(Fluka), 4,7-phenanthroline (Aldrich), 3-nitrobenzenesulfonic
acid (Na+ salt, Aldrich, 98%), 2-methoxy-1,4-phenylenediamine
hydrate (Aldrich, 95%), glycerol (APS Finechem), sulfuric acid
(H2SO4; Ajax, 98%), ammonium hydrogen carbonate (NH4HCO3;
May and Baker), stannous chloride (SnCl2.2H2O, Ajax), lithium
chloride (LiCl; Aldrich, 99+%), ammonium hexafluorophosphate
(NH4PF6; Aldrich, 99.99%), potassium hexafluorophosphate
(KPF6; Aldrich, 98%), zinc chloride (ZnCl2·xH2O; Fluka, 98%),
hydrazine hydrate (Aldrich), ethylene glycol (Ajax 95%), sodium
R
toluene-4-sulfonate (Aldrich, 98%), DOWEXꢀ 1 × 8, 50–100 mesh
R
(Aldrich) and Amberliteꢀ IRA-400 (Aldrich) anion exchange
The isolation of the solid as the hydrogen sulfate salt was
conducted according to the method of Bonhoˆte.53 The residue
was dissolved in a minimum volume of methanol (120 cm3) and
98% H2SO4 was added to precipitate a brown solid. The precipitate
was isolated by vacuum filtration, washed with ethanol (50 cm3)
followed by diethyl ether (20 cm3) and dried in vacuo. Yield: 40.6 g
resins, Celite (Aldrich) and laboratory reagent solvents were used
as received. Tetra-n-butylammonium hexafluorophosphate ([n-
C4H9)4N]PF6; Fluka, 99+%) was dried in vacuo at 60 ◦C prior
to use and ferrocene (Fc; BDH) was purified by sublimation prior
to use. SP Sephadex C-25 and Sephadex LH-20 (Amersham Phar-
˚
macia Biotech), and silica gel (200–400 mesh, 60 A, Aldrich) were
1
(60%). The H NMR spectrum and melting point were identical
employed for the chromatographic separation and purification of
ruthenium complexes. Silica gel (Kieselgel 60H; 5–40 lm) was used
for liquid vacuum chromatography.48
to those reported previously.53
(b) 4,7-Phenanthroline-5,6-dione. This compound was pre-
pared according to a modification of the literature method.53 5-
Methoxy-4,7-phenanthroline hydrogen sulfate (40.63 g; 0.132 mol)
was dissolved in 98% H2SO4 (150 cm3). With ice bath cooling,
fuming HNO3 (60 cm3) was added dropwise with stirring. The
mixture was refluxed at 96 ◦C for 10 h, then cooled to room
temperature, poured onto ice (ca. 900 g), and neutralised to
pH 6.8–7.0 (pH meter) by the stepwise addition of ammonium
Acetonitrile (CH3CN; Aldrich, 99.9+ %), and propionitrile
(PN; Aldrich) were distilled over CaH2, while acetone (BDH,
HPLC grade) was distilled over K2CO3 and dichloromethane
over CaCl2 prior to use. Pyridine was distilled from KOH,
N,N-dimethylformamide (DMF; AR, Ajax) was stirred with
˚
4 A molecular sieves and distilled under reduced pressure
(76 ◦C at 39 mmHg) immediately prior to use, and ethylene-
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bicarbonate, maintaining the temperature below 10 ◦C throughout
the procedure. After 2 h, the pale yellow precipitate was isolated
by vacuum filtration, washed with chilled H2O (1500 cm3), chilled
acetone (250 cm3) and chilled diethyl ether (250 cm3), and dried in
exchanger (96 × 1.6 cm) and chromatographed using 0.10 M (+)-
di-O,Oꢀ-4-toluoyl-D-tartrate solution as eluent. Resolution of the
enantiomeric forms was achieved with ∼50 cm of travel down
the column. The products from the two bands were precipitated
on addition of aqueous KPF6; the mixtures were refrigerated
overnight and the solids isolated by filtration, washed with chilled
water (3 cm3), diethyl ether (3 × 10 cm3) and dried in vacuo.
By comparison with similar complexes of known configuration,38
band 1 was assigned as D-(−)-[Ru(bpy)(HAT)2]2+ and band 2 as K-
(+)-[Ru(bpy)(HAT)2]2+. UV/Vis in CH3CN, k/nm {emax/M−1 cm−1
(band 1 = band 2)}: 210 (52 089), 275 (54 956), 408 (11 326), 466 (11
089). CD spectra in CH3CN, k/nm {De/M−1 cm−1 (band 1, band
2)}: 206 (−18.0, + 19.6), 224 (+8.85, −8.51), 259 (+7.58, −8.02),
297 (−49.4, +47.2), 318 (−53.0, +52.0), 392 (−15.8, +15.6), 482
(−9.73, +10.1).
1
vacuo overnight. Yield: 24.0 g (86%). The H NMR and melting
point were identical to those reported previously.53
(c) 4,7-Phenanthrolino-5,6:5ꢀ,6ꢀ-pyrazine (ppz). This com-
pound was prepared according to a modification of the lit-
erature procedures.52,53 4,7-Phenanthroline-5,6-dione (21.95 g,
0.104 ◦mol) was dissolved in freshly distilled methanol (380 cm3)
at 30 C. Freshly distilled ethylenediamine (7.3 cm3, 0.110 mol)
was added in a single portion, at which point the mixture attained
a red colouration. The reaction was stirred at 30 ◦C for 2 h, and
the solvent removed by rotary evaporation to yield an orange
solid. The residue was dissolved in H2O (500 cm3) and extracted
with chloroform (3 × 100 cm3). The solution volume was reduced
in vacuo and chromatographed on silica gel via gradient elution
using 10% methanol–1% NH4OH in dichloromethane. The ppz
was isolated as the light yellow band which eluted prior to a
deep purple band (which presumably contained the non-aromatic
tetrahydroquinoxaline analogue).54 The solvent was removed in
vacuo and the beige solid was washed with acetone. Yield: 15.7 g
[{Ru(bpy)2}2(l-ppz)](PF6)4. The synthesis and purification
were performed under similar conditions to those de-
38
scribed previously for [{Ru(bpy)2}2(l-HAT)](PF6)4 using cis-
[Ru(bpy)2Cl2]·2H2O (111 mg, 0.213 mmol) and ppz (24 mg,
0.103 mmol). The dinuclear species was isolated as a purple solid.
Yield: 140 mg (83%). Anal.: Calcd for C54H40F24N12P4Ru2: C, 39.6;
H, 2.46; N, 10.3%. Found: C, 39.4; H, 2.44; N, 10.1%. Further
characterisation was performed following the diastereoisomer
separation.
1
(65%). The H NMR and melting point were identical to those
reported previously.52,53
Separation of the diastereoisomers was achieved as described
above using aqueous 0.25 M sodium toluene-4-sulfonate solution
as the eluent. Bands 1 and 2 were determined to be the meso
and rac diastereoisomers, respectively, as established by NMR
characterisation. 1H NMR (d ppm; CD3CN): (band 1; meso) 7.25
(H5d, 2H, J = 8, 5 Hz, dd), 7.39 (H5b, 2H, J = 8, 5 Hz, dd), 7.42
(H5c, 2H, J = 8, 5 Hz, dd), 7.50 (H5a, 2H, J = 8, 5 Hz, dd), 7.54
(H6d, 2H, J = 5, 1.5 Hz, dd), 7.73 (H6a, 2H, J = 5, 1.5 Hz, dd),
7.75 (H6c, 2H, J = 5, 1.5 Hz, dd), 7.85 (H6b, 2H, J = 5, 1.5 Hz,
dd), 7.93 (H2/3 ppz, 2H, s), 7.99 (H4d, 2H, J = 8, 8 Hz, dd), 8.01
(H7/10 ppz, J = 10, 8 Hz, dd), 8.06 (H4c, 2H, J = 8, 8 Hz, dd),
8.09 (H4b, 2H, J = 8, 8 Hz, dd), 8.15 (H4a, 2H, J = 8, 8 Hz,
dd), 8.23 (H6/11 ppz, J = 5, 1.5 Hz, dd), 8.39 (H3d, 2H, J = 8,
1.5 Hz, dd), 8.43 (H3c, 2H, J = 8, 1.5 Hz, dd), 8.52 (H3b, 2H,
J = 8, 1.5 Hz, dd), 8.58 (H3a, 2H, J = 8, 1.5 Hz, dd), 9.32 (H8/9
ppz, J = 8, 1.5 Hz, dd); (band 2; rac) 7.02 (H5b, 2H, J = 8, 5 Hz,
dd), 7.36 (H5d, 2H, J = 8, 5 Hz, dd), 7.38 (H5c, 2H, J = 8, 5 Hz,
dd), 7.42 (H6d, 2H, J = 5, 1.5 Hz, dd), 7.51 (H5a, 2H, J = 8,
5 Hz, dd), 7.61 (H6b, 2H, J = 5, 1.5 Hz, dd), 7.66 (H6c, 2H, J =
5, 1.5 Hz, dd), 7.76 (H6a, 2H, J = 5, 1.5 Hz, dd), 7.96 (H2/3 ppz,
2H, s), 8.00 (H4d, 2H, J = 8, 8 Hz, dd), 8.01 (H7/10 ppz, J = 10,
8 Hz, dd), 8.07 (H4b, 2H, J = 8, 8 Hz, dd), 8.13 (H4a, 2H, J = 8,
8 Hz, dd), 8.15 (H4c, 2H, J = 8, 8 Hz, dd), 8.23 (H6/11 ppz, J =
5, 1.5 Hz, dd), 8.48 (H3d, 2H, J = 8, 1.5 Hz, dd), 8.51 (H3b, 2H,
J = 8, 1.5 Hz, dd), 8.53 (H3a, 2H, J = 8, 1.5 Hz, dd), 8.55 (H3c,
2H, J = 8, 1.5 Hz, dd), 9.31 (H8/9 ppz, J = 8, 1.5 Hz, dd).
Complex syntheses and diastereoisomer separations
The mononuclear ruthenium complexes [Ru(DMSO)4Cl2],55 cis-
[Ru(bpy)2Cl2].2H2O,56 and the carbonyl complexes [Ru(CO)2Cl2]n,
[Ru(bpy)(CO)2Cl2] and [Ru(bpy)(CO)2(CF3SO3)2] 57 were pre-
pared according to the literature procedures. [Ru(bpy)Cl4] 58 was
supplied by Dr Bradley Patterson.
[Ru(bpy)(HAT)2](PF6)2. This complex was synthesised via a
procedure analogous to that reported previously for [Ru(bpy)(2,3-
dpp)2](PF6)2.59 A suspension of HAT (200 mg, 0.854 mmol) was
heated in ethylene glycol (10 cm3) for 5 min via microwave heating
to complete dissolution. [Ru(bpy)Cl4] (45 mg, 0.113 mmol) was
added in four portions over 10 min with heating, during which
time the solution attained an orange colouration. The mixture was
heated for a further 5 min, then quenched with distilled water (ca.
50 cm3) and filtered to recover the excess HAT ligand. One major
band was collected from the SP Sephadex C-25 cation exchange
column (20 × 2 cm) using 0.15 M NaCl solution as eluent.
Following the addition of a saturated solution of aqueous KPF6
to the eluate the mixture was chilled by refrigeration overnight.
The orange precipitate was isolated by filtration, washed with
chilled water (3 cm3), diethyl ether (3 × 10 cm3) and dried in
vacuo. Yield: 70.2 mg (61%). Anal.: Calcd for C34H20F12N14P2Ru:
C, 40.2; H, 1.98; N, 19.3%. Found: C, 40.0; H, 2.05; N, 19.2%.
Further structural characterisation was performed following the
separation and chiral resolution of the geometrical isomers and
their enantiomeric forms.
[Ru(bpy)2(py)2](PF6)2. This complex was prepared ac-
cording to a minor variation on the literature procedure.60
[Ru(bpy)2Cl2].2H2O (0.164 g, 0.315 mmol) and pyridine
(0.655 cm3, 8.10 mmol) were combined in aqueous methanol
(13 cm3, 1 : 1 v/v) and the mixture heated at reflux for 4 h during
which time the solution attained an orange-red colouration. The
mixture was cooled to room temperature and the solvent was
removed by rotary evaporation. The resultant red residue was
Resolution of [Ru(bpy)(HAT)2](PF6)2. The mononuclear com-
plex was resolved via an analogous procedure to that described
−
previously for [Ru(bpy)2(HAT)]2+.38 The PF6 salt was converted
R
to the corresponding aqueous Cl− form by stirring with DOWEXꢀ
1 × 8 anion exchange resin (50–100 mesh, Cl− form). The
aqueous solution was sorbed onto SP Sephadex C-25 cation
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dissolved in a minimum volume of methanol (ca. 1 cm3) and the
complex was precipitated by the addition of diethyl ether. The
precipitate was collected by vacuum filtration and washed with
diethyl ether (3 × 10 cm3).
vacuo. Yield: 140 mg (46%). Anal.: Calcd for C38H24F12N10P2Ru:
C, 45.1; H, 2.39; N, 13.8%. Found: C, 45.0; H, 2.20; N, 13.5%.
Further structural characterisation was performed following the
separation and chiral resolution of the geometrical isomers and
their enantiomeric forms.
The complex was purified by cation-exchange chromatography
(SP Sephadex C-25). The solid was converted to the chloride salt
R
by stirring an aqueous suspension with DOWEXꢀ 1 × 8 anion
Separation and resolution of trans-[Ru(bpy)(ppz)2](PF6)2. The
separation of the geometric isomers and chiral resolution of
their enantiomeric forms was achieved during a single column
chromatographic procedure.
exchange resin (50–100 mesh, Cl− form), and the solution was
loaded onto the Sephadex column (20 × 2 cm) and eluted with
0.2 M NaCl solution. The orange band was precipitated as the
PF6 salt by the addition of aqueous KPF6. The complex was
The complex (ca. 90 mg) as the Cl− form (obtained by stirring
−
R
isolated by filtration, washed with copious amounts of diethyl
ether and dried in vacuo. Yield: 0.290 g (80%). The 1H NMR was
identical to that reported previously.60
an aqueous suspension with DOWEXꢀ 1 × 8 anion exchange
resin; 50–100 mesh, Cl− form) was absorbed onto a column of SP
Sephadex C-25 cation exchanger and eluted using aqueous 0.10 M
sodium toluene-4-sulfonate solution. The column was sealed at the
top and bottom to permit recycling of the broadening band and
small portions of the front and back of the band were collected
on each pass down the column. The complex was extracted from
each fraction using dichloromethane following the addition of
a saturated solution of aqueous KPF6. The organic layers were
dried over Na2SO4, filtered and the solvent removed by rotary
evaporation. Purification of the solids was achieved on a short
column of silica gel (3 × 2 cm). A solution of the complex in
acetone was loaded onto the column and washed alternately with
acetone, water and acetone and eluted with acetone containing
5% NH4PF6. Addition of water and removal of the acetone under
reduced pressure afforded an orange solid which was collected
by filtration through Celite and washed with diethyl ether (3 ×
5 cm3). The precipitate was washed off the Celite into a test
tube with acetone (ca. 2 cm3), evaporated un◦der a stream of
Resolution of [Ru(bpy)2(py)2](PF6)2. [Ru(bpy)2(py)2](PF6)2 was
R
converted to the Cl− salt by stirring the complex with DOWEXꢀ
1 × 8 anion exchange resin (50–100 mesh, Cl− form), and the
solution applied to a cation-exchange column (SP Sephadex C-
25; 96 × 1.6 cm). The two enantiomers were resolved on elution
with an aqueous solution of sodium (−)-O,Oꢀ-dibenzoyl-L-tartrate
(0.10 M). The complex in each band was precipitated using a
saturated solution of KPF6 and collected by filtration.
Purification was achieved on silica gel as described below. The
complex was washed with water (3 × 5 cm3) then copious amounts
of diethyl ether and dried in vacuo. Bands 1 and 2 were assigned
as D-(−)- and K-(+)- , respectively, based on exciton analysis of
their CD spectra61,62 and comparisons with related compounds
with known absolute configurations.63,64
[Ru(bpy)(ppz)(CO)2](PF6)2. This complex was prepared ac-
cording to an adaption of the literature method.57 [Ru(bpy)(CO)2-
(CF3SO3)2] (0.180 g, 0.294 mmol) and ppz (136.7 mg, 0.589 mmol)
were dissolved in 95% ethanol (30 cm3) under N2. The solution
was refluxed for 30 min during which time the mixture attained
a yellow colouration. The mixture was refluxed for a further
60 min and evaporated to dryness by rotary evaporation. The
beige residue was dissolved in boiling water and filtered to remove
unreacted ligand (ppz). A saturated aqueous solution of NH4PF6
(5 cm3) was added to yield a white precipitate which was isolated
by filtration and washed with copious amounts of chilled water
followed by diethyl ether. Yield: 240 mg (98%) Anal.: Calcd for
C40H24F12N10O4P2Ru: C, 55.4; H, 2.77; N, 16.2%. Found: C, 55.2;
H, 2.83; N, 16.3%.
1
dry nitrogen and dried in vacuo for 3 h at 50 C. By using H
NMR and CD spectroscopies, as well as by comparison with
similar complexes of known configurations,38,65 the identities of
the front and back fractions of the eluting band (bands 1 and
4, respectively) were established as the Dt-[Ru(bpy)(ppz)2]2+ and
Kt-[Ru(bpy)(ppz)2]2+ isomers, respectively (t = trans). The other
fractions contained various admixtures of the Dc-[Ru(bpy)(ppz)2]2+
and Kc-[Ru(bpy)(ppz)2]2+ isomers (c = cis).
By comparison with similar complexes of known con-
figurations,38 band 1 was assigned as Dt-[Ru(bpy)(ppz)2]2+ and
band 4 as Kt-[Ru(bpy)(ppz)2]2+. UV/Vis in CH3CN, m/cm−1
{emax/M−1 cm−1 (band 1 = band 4)}: 464 (12 850), 426 (13 110),
279 (53 333), 264 (60 520), 257 (60 580), 243 (54 710). CD spectra
in CH3CN, k/nm {De/M−1 cm−1 (band 1, band 4)}: 250 (10.0,
−8.00), 270 (29.4, −26.8), 294 (−81.5, 80.5), 380 (11.8, −10.20),
420 (17.7, −16.6), 470 (−13.9, 14.5).
[Ru(bpy)(ppz)2](PF6)2. The bridging ligand ppz (225 mg,
0.970 mmol) was dissolved in 2-methoxyethanol (40 cm3) and the
solution purged with N2 for 20 min. [Ru(bpy)(ppz)(CO)2](PF6)2
(270 mg, 0.323 mmol) and a three-fold excess of TMNO (31 mg,
0.413 mmol) were added and the mixture heated to reflux for
3 h in subdued light. During this time the solution attained a deep
red colouration with the concurrent production of trimethylamine
which was trapped using a bubbler containing HCl solution.
The crude reaction mixture was dissolved in water and purified
via a gradient elution procedure on SP Sephadex C-25 cation
exchanger with aqueous NaCl (0.1–0.15 M) eluent. Precipitation
of the desired product from the major orange band was achieved
by addition of a saturated aqueous solution of NH4PF6. The
orange solid was isolated by vacuum filtration, washed with
chilled water (2 × 5 cm3) and copious diethyl ether and dried in
Stereoselective synthesis of [{D-Ru(bpy)2}2{Dt-Ru(bpy)(l-
ppz)2}](PF6)6. The complexes Dt-[Ru(bpy)(ppz)2](PF6)2 (7.0 mg,
6.92 lmol) and D-[Ru(bpy)2(py)2](PF6)2 (17.4 mg, 15.2 lmol)
were combined in ethylene glycol (1 cm3 containing 10% water)
and the mixture was heated at 120 ◦C for 5 h in the absence
of light. During this time the initially orange solution attained
a deep purple colouration. The reaction mixture was cooled to
room temperature, followed by the addition of water (ca. 5 cm3).
The isolation of the desired trinuclear product from the crude
mixture was achieved via a gradient elution procedure with SP
Sephadex C-25 cation exchanger (20 × 2 cm) using aqueous NaCl
solution (0.2–0.8 M) in the absence of light. Two bands were
1064 | Dalton Trans., 2006, 1060–1072
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observed: a faint orange band (presumably containing unreacted
excess [Ru(bpy)2(py)2]2+) which eluted with 0.2 M NaCl, followed
by a major purple band which eluted with 0.5 M NaCl. The
product from the latter was precipitated by the addition of
saturated aqueous KPF6, and chilled by refrigeration overnight.
The dark purple solid was isolated by filtration and washed
copiously with diethyl ether. Yield: 12.4 mg (74%) Anal.: Calcd for
C78H56F36N18P4Ru3: C, 38.7; H, 2.33; N, 10.4%. Found: C, 38.6; H,
2.37; N, 10.2%. The 1H NMR spectra are reported in the Results
and Discussion.
viously for D-(−)- and K-(+)-[Ru(HAT)3]2+ and was attributed to
the weak coupling of the electronic transition dipole moments
localised on the ligands.65 An interpretation of the CD spectra on
the basis of exciton theory61,62,69,70 was thus avoided, and the assign-
ment of the absolute configurations for the bands obtained from
resolution of [Ru(bpy)(BL)2]2+ (BL = HAT, ppz) was achieved by
comparison with similar complexes of known absolute configu-
ration; i.e. [Ru(bpy)2(HAT)]2+38 and [Ru(HAT)3]2+.65 The band 1
and 2 eluates obtained from the resolution of [Ru(bpy)(BL)2]2+
were assigned as the D-(−) and K-(+) enantiomers, respectively,
while bands 1 and 4 for trans-[Ru(bpy)(ppz)2]2+ were assigned as
the D-(−) and K-(+) enantiomers, respectively.
Results and discussion
1H NMR studies. The proton labelling schemes employed in
the assignment of the 1H NMR spectra of [Ru(bpy)(HAT)2]2+
and trans-[Ru(bpy)(ppz)2]2+ are shown in Fig. 2. The coordinated
bpy ligands exhibit the expected coupling constant values71 and
coupling patterns based on the symmetry requirements of the
complexes.
Synthesis and structural characterisation
Mononuclear precursors for a stereochemically pure trinuclear
assembly. The synthesis of [Ru(bpy)(ppz)2]2+ was based on the
previously reported synthetic methodology for tris(heteroleptic)
complexes of ruthenium(II), [Ru(pp)(ppꢀ)(ppꢀꢀ)]2+ via the dicar-
bonylation of [Ru(pp)(ppꢀ)(CO)2]2+.57,66 The H NMR spectrum
1
The 1H NMR chemical shifts of the mononuclear
[Ru(bpy)(BL)2]2+ (BL = HAT, ppz) complexes (Fig. S2 and
Table S1 of the ESI†) were determined by 1H COSY spec-
of [Ru(bpy)(ppz)2]2+ revealed a 1 : 1 ratio of the cis and trans
geometric isomers. The isolation of the trans-[Ru(bpy)(ppz)2]2+
geometric isomer, and chiral resolution of the corresponding D
and K enantiomeric forms, was achieved in a single step by cation-
exchange chromatography (SP Sephadex C-25) using 0.10 M
sodium toluene-4-sulfonate solution as eluent.67,68 Since the anion
is achiral, the concurrent resolution of the D and K enantiomeric
forms with geometric isomer separation arises from the inherent
chirality of the polydextran Sephadex support.
tra and assigned by comparisons with the structurally-related
38
complex [Ru(bpy)2(HAT)]2+
and consideration of the rela-
tive degree of diamagnetic anisotropic interactions between the
stereochemically-related ligands.
[Ru(bpy)(HAT)2]2+. As a consequence of the C2 point group
symmetry of [Ru(bpy)(HAT)2]2+, the proton resonances for each
py ring of the bpy ligand are equivalent, as well as for both
HAT ligands. The bpy H5 and H6 protons are oriented over the
plane of the HAT ligand, and are assigned as the most upfield
resonances at 7.39 and 7.83 ppm, respectively, due to the shielding
influence of HAT. The H2 proton of HAT was assigned to the
resonance at 8.39 ppm (J = 3 Hz, d), while the H11 proton of
HAT was assigned to the resonance at 8.29 ppm (J = 3 Hz, d).
The latter experiences a relatively greater shielding due to the
increased anisotropic interaction with the bpy ligand compared
with the HAT ligand, which is relatively p electron deficient. The
most downfield resonances are attributed to overlapping singlet
resonances from the H6 and H7 protons of HAT, which are
oriented away from the shielding influence of the adjacent bpy and
HAT ligands. The remaining assignments were based on coupling
[Ru(bpy)(HAT)2](PF6)2 was synthesised by the reaction of
[Ru(bpy)Cl4] with excess HAT under microwave conditions ac-
cording to the previously reported procedure for the synthesis
of [Ru(bpy)(2,3-dpp)2](PF6)2.59 Chiral resolution of the D and K
enantiomeric forms of [Ru(bpy)(HAT)2](PF6)2 was achieved by
cation-exchange chromatography (SP Sephadex C-25; aqueous
0.10 M (+)-di-O,Oꢀ-4-toluoyl-D-tartrate solution as eluent) in an
analogous manner to that described previously for the resolution
of [Ru(bpy)2(HAT)](PF6)2.38
The CD spectra of D- and K-[Ru(bpy)(HAT)2]2+, and Dt- and Kt-
[Ru(bpy)(ppz)2]2+ are shown in Fig. S1 (Supporting Information)†,
and exhibit relatively weak circular dichroism for the p → p*
transitions in comparison with other polypyridylruthenium(II)
complexes.61–63,69,70 A similar observation has been reported pre-
Fig. 2 Proton numbering scheme for (a) K-[Ru(bpy)(HAT)2]2+ and (b) Kt-[Ru(bpy)(ppz)2]2+
.
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(DD/KK)-[{Ru(bpy)2}2(l-ppz)]4+ (Fig. S3(b)†). The complexes
trans-[Ru(bpy)(ppz)2]2+ and rac-(DD/KK)-[{Ru(bpy)2}2(l-ppz)]4+
also possess a C2 symmetry, and the spectrum of the trinuclear
complex may be interpreted as the summation of the spectra for the
mono- and dinuclear analogues. Ring b is oriented over the plane
of the ppz and adjacent bpy ring e (rather than bpy (ring d) for the
dinuclear case) so that the H5 proton is assigned as the most upfield
resonance at 7.17 ppm (cf. 7.08 ppm in the dinuclear complex).
constant values and comparisons with the previously assigned
related complex [Ru(bpy)2(HAT)]2+.38
trans-[Ru(bpy)(ppz)2]2+. The C2 point group symmetry of
trans-[Ru(bpy)(ppz)2]2+ is expected to give rise to 12 non-
equivalent proton resonances, while the C1 point group symmetry
for the corresponding cis geometric isomers would give rise to 24
1
non-equivalent proton resonances. As shown in Fig. S2,† the H
NMR spectrum of Dt-[Ru(bpy)(ppz)2]2+ is similar to that for D-
[Ru(bpy)(HAT)2]2+, with two additional resonances due to the H8
and H9 protons of ppz. The chemical shifts for the bpy resonances
are similar to those in [Ru(bpy)(HAT)2]2+, with the H5 and H4
resonances experiencing upfield shifts of 0.05 ppm due to the
increased shielding influence of ppz relative to HAT. The H3
protons of ppz are oriented over the adjacent bpy ligand, and
occur slightly upfield (8.11 ppm) of the resonance at 8.22 ppm
for the H6 ppz which is oriented over the plane of the adjacent
ppz ligand. The ppz H8 and H9 protons, which are oriented away
from the shielding influence of the adjacent bpy and ppz ligands,
are assigned as the most downfield resonances (cf. H6 and H7 HAT
resonances in [Ru(bpy)(HAT)2]2+). The remaining assignments
were based on coupling constant values and comparisons with
the assignments for [Ru(bpy)(HAT)2]2+.38
1
The H chemical shifts for pyridyl rings a and c are comparable
to those in the dinuclear analogue as they are oriented away from
the bridging ligand, while ring d exhibits slight shifts relative to
the dinuclear analogue due to the influence of the ppz (rather
than bpy) ligand across the bridge. The H5 and H6 resonances of
ring e are oriented over the plane of the ppz ligand in a similar
environment to the corresponding protons of ring d. Accordingly,
1
the protons of the two rings possess similar H chemical shifts;
however, the ring e protons experience a slightly greater shielding
influence due to the adjacent bpy (rather than ppz) ligand across
the bridge, and thus lie slightly upfield of the ring d resonances.
The ppz resonances for H2, H9, H10 and H11 experience the
most pronounced shifts relative to the mononuclear analogue, as
these protons are now oriented towards the terminal Ru centre,
and therefore experience the shielding influence of the terminal bpy
rings. For example, the H11 resonance occurs at 9.26 ppm in the
mononuclear complex compared with 8.09 ppm in the trinuclear
complex since the proton is now situated over the shielding cone
of ring a. In the dinuclear complex, the H2/3, H6/11, H7/10 and
H8/9 protons are related by a C2 axis. In the trinuclear complex,
distinct resonances are observed for each proton of the ppz ligand.
For instance, the H8 and H9 protons (dd, J = 8, 1.5 Hz), previously
observed as overlapping resonances at 9.31 ppm in the dinuclear
complex, are now observed as two separate resonances at 9.33
and 9.36 ppm with the H8 proton assigned as the most downfield
resonance.
Stereoselective synthesis of a trinuclear assembly. The synthesis
of DDtD-[{Ru(bpy)2}2{Ru(bpy)(l-ppz)2}]6+ was performed via a
modification of the stereoretentive synthetic procedure reported
by Patterson67 involving the reaction of 2.2 equivalents of D-
[Ru(bpy)2(py)2]2+ with Dt-[Ru(bpy)(ppz)2]2+.
1H NMR studies. The proton labelling for the ligands
employed in the assignment of the 1H NMR spectrum is
shown in Fig. 3. The 1H NMR chemical shifts of DDtD-
[{Ru(bpy)2}2{Ru(bpy)(l-ppz)2}]6+ (Table S2 and Fig. S3(c) of the
ESI†) were assigned by comparison with the structurally-related
complexes [Ru(bpy)2(ppz)]2+ and meso- and rac-[{Ru(bpy)2}2(l-
ppz)]4+. The 1H NMR assignments of the latter are shown in
Fig. S3(a) and (b).†
Electrochemistry and electronic spectroscopy
Electrochemical data for trans-[Ru(bpy)(ppz)2]2+, meso- and
rac-[{Ru(bpy)2}2(l-ppz)]n+ and DDtD-[{Ru(bpy)2}2{Ru(bpy)(l-
ppz)2}]6+ were obtained by cyclic and differential pulse voltam-
metry. The potentials of the metal-based oxidation processes and
the ligand-based reduction processes are provided in Tables 1 and
S3 of the ESI,† respectively.
The mono- and dinuclear systems exhibit one and two reversible
one-electron redox processes, respectively, corresponding to suc-
cessive oxidation of the metal centres. The trinuclear complex
DDtD-[{Ru(bpy)2}2{Ru(bpy)(l-ppz)2}]6+ is characterised by two
closely-spaced one-electron redox processes (Eox1 ∼1137 mV,
II
DEox(2−1) < 100 mV) corresponding to oxidation of the terminal Ru
III
centres to Ru , followed by one-electron oxidation of the central
Ru . The relative peak currents and areas are clearly discernible
II
Proton numbering scheme for DDtD-[{Ru(bpy)2}2{Ru(bpy)-
from the differential pulse voltammogram (Fig. S4, Supporting
Information†) are consistent with this assignment. The approxi-
mately simultaneous oxidation of the terminal centres contrasts to
the well-separated (DEox(2−1) ≈ 250 mV) potentials in the trinuclear
[{Ru(bpy)2}3(l-HAT)]6+ system (Table 1), and suggests that the
terminal centres in the DDtD-[{Ru(bpy)2}2{Ru(bpy)(l-ppz)2}]6+
complex are not significantly coupled electronically through
the chain-like framework. The potential of the metal-based
Fig.
3
(l-ppz)2}]6+
.
The C2 point group symmetry of DDtD-[{Ru(bpy)2}2-
{Ru(bpy)(l-ppz)2}]6+ gives rise to five non-equivalent bpy pyridyl
rings in addition to eight non-equivalent ppz proton resonances,
resulting in 36 non-equivalent proton environments. As expected,
the spectrum bears a striking resemblance to that for rac-
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Table 1 Electrochemicala data (in mV), and Kc valuesb for the di- and trinuclear complexes in 0.1 M [(n-C4H9)4N]PF6–CH3CN at +25 ◦C. The redox
potentials for [Ru(bpy)3]2+ are included for comparison
b
b
Complex
DEox(3−2) (Kc × 10−3
)
DEox(2−1) (Kc × 10−3
)
Eox3
Eox2
Eox1
880
1154
1056
[Ru(bpy)3]2+
trans-[Ru(bpy)(ppz) 2]n+
c
meso-[{Ru(bpy)2}2(l-ppz)]n+
224(6.26)
216(4.58)
<100(<0.05)
1280
1278
rac-[{Ru(bpy)2}2(l-ppz)]n+
1062
DDtD -[{Ru(bpy)2}2{Ru(bpy)(l-ppz)2}]n+
250(17.3)
1388
∼1137(2e−)
a All potentials ( 3 mV) in 0.1 M [(n-C4H9)4N]PF6–CH3CN at +25 ◦C vs. Fc+/Fc0. b Kc values determined from Kc = exp{DEoxF/RT} where F/RT =
38.92 V−1 at 298 K.3 c The redox properties of the D and K enantiomers were identical.
Table 2 UV/Vis/NIR spectrala data for the di- and trinuclear complexes
redox process is more anodic in the trinuclear complex vs. the
in 0.1 M [(n-C4H9)4N]PF6–CH3CN at −35 and −15 ◦C, respectively. The
mononuclear analogue.
All complexes exhibited multiple reversible ligand-based reduc-
NIR spectral data are indicated in bold type
10/cm−1
tions in the cathodic region (Table S3, Supporting Information†).
In the dinuclear complexes, the first two reductions are assigned
to sequential one-electron reduction of the ppz bridging ligand,
due to the greater stabilisation of its p* level relative to the
peripheral bpy ligands, followed by sequential one-electron re-
duction processes associated with the terminal bpy ligands. The
assignments are consistent with those reported for meso- and rac-
[{Ru(bpy)2}2(l-HAT)]4+.38
m
Complex
n+ {(e/m)max 0.001/M−1
trans-[Ru(bpy)(ppz)2]n+
meso-[{Ru(bpy)2}2(l-ppz)]n+
2
4
5
21570 (0.5958)
53450 (0.5590)
17640 (1.4682)
23440 (0.9021)
5370 (0.8721)
11670 (0.1566)
13320 (0.1400)
17370 (0.9003)
24900 (0.3950)
11630 (1.9560)
16950 (0.2601)
17630 (1.4715)
23420 (0.8987)
5390 (0.8494)
11720 (0.1488)
13090 (0.2145)
17340 (0.9043)
24980 (0.3862)
11640 (1.9510)
16930 (0.2600)
17210 (1.9525)
18600 (1.7859)
23710 (1.4173)
5340 (0.2078)
17270 (1.6056)
sh 18750 (1.3558)
23850 (1.0362)
8230 (0.3920)
18220 (1.5478)
21860 (0.7723)
10970 (0.7441)
18210 (0.6440)
23050 (0.5867)
In the trinuclear complex, the first two one-electron reduction
processes correspond to the reduction of the two equivalent
(and slightly interacting) ppz bridging ligands. The assignments
for the subsequent reduction processes are ambiguous, however
comparison with the related mono- and dinuclear complexes
suggests that the third two-electron process consists of overlapping
contributions from a second ppz reduction and a bpy-based re-
duction process for the bpy coordinated to the central [Ru(bpy)(l-
ppz)2]2+ chromophore. These assignments are consistent with
those reported previously in extensive electrochemical studies of
the related trinuclear complexes [{Ru(bpy)2}2{Ru(bpy)(l-BL)2}]6+
(BL = 2,3- or 2,5-dpp; as stereoisomeric mixtures) by Denti and
coworkers.72
The first two one-electron reduction processes in [Ru(bpy)-
(ppz)2]2+ are assigned to the sequential reduction of the ppz ligands,
while the third one-electron process is bpy-based (by comparison
with the bpy-based reduction potentials for [Ru(bpy)3]2+). The
splitting of 246 mV between the ppz reductions indicates elec-
tronic communication between the ppz ligands. In the trinuclear
complex, the lesser splitting between the two ppz-based reduction
processes (168 mV) implies decreased coupling between the ppz
ligands relative to the mononuclear species, as there is diminished
electron density at the central Ru due to the terminal Ru centres.
This contrasts to the larger splittings of 592 and 596 mV in meso
and rac diastereoisomers of the dinuclear complex [{Ru(bpy)2}2(l-
ppz)]n+ which arise as the two electrons are localised on the
same ppz bridging ligand. The two ppz-based reduction processes
are shifted anodically in the trinuclear complex relative to
[Ru(bpy)(ppz)2]2+ due to the decrease in the electron density of the
ppz ligands through backbonding interactions with the terminal
Ru centres.
6
4
5
rac-[{Ru(bpy)2}2(l-ppz)]n+
6
6
DDtD − [{Ru(bpy)2}2{Ru(bpy)(l-ppz)2}]n+
7
8
9
a sh = shoulder of band. b The corresponding molar absorption coefficients
(emax/M−1 cm−1) are given by the product of (e/m)max and mmax at a given
wavelength.
complexes are presented in Fig. 4 and S5, respectively.† Low
temperature spectroelectrochemical generation of the complexes
was required in each case as the electrogenerated species were
unstable at room temperature.
The spectra of meso- and rac-[{Ru(bpy)2}2(l-ppz)]4+ exhibit
comparable features to the diastereoisomers of [{Ru(bpy)2}2(l-
HAT)]4+ described previously.41 The spectra over the region
15 000–30 000 cm−1 are characterised by a combination of over-
The complete UV/Vis/NIR spectral data for the mono-, di- and
trinuclear systems (for the range 3050–30 000 cm−1) are provided
in Table 2, and an overlay of the spectra for the unoxidised forms
of the complexes is shown in Fig. S5 of the ESI.† The spectral
characteristics for the mixed valence states of the di- and trinuclear
II
II
lapping dp(Ru ) → p*(ppz) and dp(Ru ) → p*(bpy) singlet metal-
to-ligand (1MLCT) transitions, with the former occurring at lower
energy. The spectrum of the diastereoisomeric mixture has been
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III
III
assigned as p(bpy) → dp(Ru ) and p(ppz) → dp(Ru ) LMCT
transitions. The energies of these bands are consistent with the
III
previously assigned p(bpy) → dp(Ru ) LMCT transitions at
III
14 815 and 17 160 cm−1 in [Ru (bpy)3]3+,74 and with the p(bpy) →
III
III
dp(Ru ) and p(HAT) → dp(Ru ) LMCT absorption bands in
the region 10 000–16 000 cm−1 for meso- and rac-[{Ru(bpy)2}2(l-
HAT)]4+.38
The spectrum of DDtD-[{Ru(bpy)2}2{Ru(bpy)(l-ppz)2}]6+ ex-
hibits a broad absorption band between 15 000–20 000 cm−1,
as shown in Fig. 4. The band receives contributions from three
underlying transitions of approximately equal intensities at 17 560,
16 150 and 19 930 cm−1 (obtained by Gaussian deconvolution)
II
1
which are assigned as dp(Ru ) → p*(ppz) MLCT transitions.
The 1MLCT energy is red-shifted relative to the analogous mono-
and dinuclear complexes due to the increased stabilisation of the
p*(ppz) orbitals in the trinuclear complex. The higher energy
transitions in the region 20 000–30 000 cm−1 involve overlapping
II
II
dp(Ru ) → p*(ppz) and dp(Ru ) → p*(bpy) 1MLCT transitions.
Spectroelectrochemical oxidation of the trinuclear complex
allowed the generation of the mixed valence +7 and +8 forms,
◦
and fully-oxidised +9 forms at −15 C (Fig. 4). Stable isosbestic
points were observed in the spectral progressions accompanying
each stage of oxidation. On one-electron oxidation, the lowest
energy component of the 1MLCT manifold at 17 560 cm−1 (which is
II
assigned as a dp(Ru ) → p*(ppz) transition) decreases in intensity
while the two higher energy components are undiminished. The
band at 23 710 cm−1 decreases in intensity and experiences a slight
II
blue-shift on one-electron oxidation, consistent with its dp(Ru ) →
II
p*(bpy) assignment. Further oxidation of the second terminal Ru
III
centre to Ru generates the +8 mixed valence state and results in a
decrease in the intensity of the lowest energy band at 17 560 cm−1
1
in the MLCT manifold. A relatively narrow absorption band
appears at 21 860 cm−1 which is due predominantly to a dp(Ru ) →
II
p*(ppz) 1MLCT transition associated with the central (unoxidised)
II
Ru centre. This band collapses on further oxidation to the
II
III
+9 state. Oxidation of the formally Ru centres to Ru was
accompanied by an increase in the intensity of the LMCT
transitions in the region 10 000–16 000 cm−1.
Fig. 4 Spectroelectrochemistry for the oxidation of the trinuclear
complex DDtD-[{Ru(bpy)2}2{Ru(bpy)(l-ppz)2}]n+ (n = 6–9) at −15 ◦C.
(a) Spectroelectrochemical changes for the oxidation reaction DDtD-[{Ru-
(bpy)2}2{Ru(bpy)(l-ppz)2}]6+ →DDtD-[{Ru(bpy)2}2{Ru(bpy)(l-ppz)2}]7+
The inset shows IVCT band (—) of the latter in addition to the bands
obtained from Gaussian deconvolution (· · ·). (b) Spectroelectrochemi-
cal changes for the oxidation reaction DDtD-[{Ru(bpy)2}2{Ru(bpy)-
(l-ppz)2}]7+ → DDtD-[{Ru(bpy)2}2{Ru(bpy)(l-ppz)2}]8+. The inset shows
the NIR spectrum (-·-) of the latter in addition to the bands obtained
from Gaussian deconvolution (· · ·). (c) Summary of the spectra of
.
Intervalence charge transfer
The NIR spectra of the di- and trinuclear systems are shown in
Fig. 4 and S6,† respectively. The results for the IVCT band param-
eters derived from the deconvolution procedure are summarised
in Table 3.
Dinuclear systems. The first oxidation process for the di-
astereoisomers of the dinuclear complex [{Ru(bpy)2}2(l-ppz)]4+
was characterised by the appearance of an intense new band in the
region 3500–8000 cm−1 (mmax = 5370 (meso) and 5390 cm−1 (rac)),
which collapsed completely on removal of the second electron: on
this basis, the band was assigned as an IVCT transition (Table 3,
Fig. 4). The bands appear asymmetrical and narrower on the lower
energy side with bandwidths at half-height of 1560 cm−1 for both
diastereoisomers. The IVCT transitions are qualitatively similar
to those observed for [{Ru(bpy)2}2(l-HAT)]5+.41 Quantitative
analysis of the bands according to the classical two-state theory75
suggests that the [{Ru(bpy)2}2(l-ppz)]5+ diastereoisomers may be
similarly described as borderline localised–delocalised systems,
since the experimental bandwidths are significantly narrower than
DDtD-[{Ru(bpy)2}2{Ru(bpy)(l-ppz)2}]n+
.
completely assigned previously on the basis of resonance Raman
studies.73 In the present work, minor differences were observed in
the band energies and intensities between the diastereoisomeric
forms. The 1MLCT energies in meso- and rac-[{Ru(bpy)2}2(l-
ppz)]4+ are red-shifted by ca. 300 cm−1 relative to their related
[{Ru(bpy)2}2(l-HAT)]4+ species, which is consistent with the more
delocalised nature of ppz (and the stabilisation of the lowest
unoccupied p* molecular orbital) compared with HAT. The mixed
valence (+5) species is characterised by an IVCT band in the region
3500–5000 cm−1, as shown in Fig. S5.† The intense absorption
bands in the region 10 000–16 000 cm−1 for the +6 species are
1068 | Dalton Trans., 2006, 1060–1072
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Table 3 Spectral data for the di- and trinuclear complexes at −35 and −15 ◦C, respectively. For the dinuclear species, the parameters for the overall NIR
band envelopes are shown in bold type. Details of the deconvoluted bands are in normal type
◦
a
Complex
Component
mmax 10/cm−1
(e/m)max
0.001/M−1
Dm1/2 10/cm−1
Dm1/2 c/cm−1
meso-[{Ru(bpy)2}2(l-ppz)]5+
5370
3960b
5220
5715
6825
0.8721
0.1037
0.3981
0.4893
0.1885
0.8494
0.1487
0.3683
0.5539
0.1677
1.293
1560
920
775
1410
3055
1560
1110
960
3140
2695
3095
3240
3540
3150
2690
3100
3240
3670
1
2
3
rac-[{Ru(bpy)2}2(l-ppz)]5+
5390
3940b
5250
1
2
3
5730
7330
1490
2560
1510
2070
1460
1470
2090
3980
2530
2730
2215
2415
2680
1860
2765
3390
3650
3865
2710
2010
2410
3075
DDtD-[{Ru(bpy)2}2{Ru(bpy)(l-ppz)2}]6+
DDtD-[{Ru(bpy)2}2{Ru(bpy)(l-ppz)2}]7+
17560
16150
19930
21230
23370
5310*
8255*
10755
12970
15755
3410
1.617
1.497
0.5235
0.6790
0.2099
0.0892
0.0599
0.0435
0.6550
0.0649
0.0747
0.3864
0.0798
0.3318
0.0775
0.1218
0.6995
0.2147
0.2314
d
3250
4050
DDtD-[{Ru(bpy)2}2{Ru(bpy)(l-ppz)2}]8+d
DDtD-[{Ru(bpy)2}2{Ru(bpy)(l-ppz)2}]9+
5590
8230*
12080
16660
5090
4050
8195
10940
12935
15690
a The corresponding◦molar absorption coefficients (emax/M−1 cm−1) are given by the product of (e/m)max and mmax at a given wavelength. b Artifact peak at
detector limit. c Dm1/2 = [16RTln2(k)]1/2 for a weakly-coupled two-state system where 16RTln2 = 1836 cm−1 at −35 ◦C (dinuclear systems) and 1990 cm−1
at −15 ◦C (trinuclear systems).75 d Transitions of IVCT origin are indicated by asterisks.
◦
the predicted bandwidths (Dm1/2 ) of 31◦40 and 3150 cm−1 for the
meso and rac forms, respectively (Dm1/2 = [16RTln2(k)]1/2 for a
weakly-coupled two-state system).75 From eqn (1), the lower limits
for Hab are 610 cm−1 for both diastereoisomers, where rab is approx-
oxidation of the +6 species at −15 ◦C. The NIR spectra of the +7
and +8 mixed valence forms (Fig. 4) exhibit striking differences
from one another (which lie well outside the limits of experimental
error), and from their dinuclear analogues.
˚
imated as the geometrical metal–metal separation of 6.65 A.
The generation of the +7 mixed valence species was accompa-
nied by the appearance of a broad absorption band in the range
3500–12 000 cm−1 (Fig. 4, Tables 2 and 3). Spectral deconvolution
revealed the presence of three underlying transitions at 5310, 8255
and 10 755 cm−1, separated by 2945 and 2500 cm−1, respectively.
The lowest energy component dominates the manifold and is
2.06 × 10−2(mmax
emaxDm1/2)
1/2
Hab
=
(1)
rab
Since electronic coupling decreases the effective electron transfer
distance, Hab is likely to lie closer to the value in the fully-
delocalised limit (i.e. Hab
=
1 mmax75) of 2625 and 2620 cm−1 for the
II
2
assigned as an adjacent IVCT transition between the central Ru
meso and rac diastereoisomers, respectively. On the basis of the
electronic coupling values, the diastereoisomers exhibit a slightly
greater degree of delocalisation compared with the corresponding
diastereoisomer of [{Ru(bpy)2}2(l-HAT)]5+. Spectral deconvolu-
tion of the NIR bands reveals three underlying Gaussian-shaped
components {Fig. 4(b) and Table 3} which may be assigned as
the three underlying spin–orbit coupling components observed
previously for the IVCT bands in [{Ru(bpy)2}2(l-HAT)]5+.41
Alternatively, according to a delocalised description in which
the bridging ligand is included as a third electronic state, the
IVCT transition arises from electron transfer between the bonding
and non-bonding molecular orbitals within the molecular orbital
manifold of the system.76–82
III
and terminal Ru centres. This transition occurs in the same
region as the IVCT band for [{Ru(bpy)2}(l-ppz)]5+. The second
component is assigned as a remote IVCT transition between the
two terminal Ru centres, since the 2945 cm−1 separation between
IVCT(1) and (2) is inconsistent with their assignment as spin–orbit
components (as was the case for [{Ru(bpy)2}3(l-HAT)]7+ 41). The
2500 cm−1 separation between the second and third components
suggests that the latter is an LMCT rather than an IVCT band;
it occurs at approximately the same position as the LMCT bands
at 10 940 cm−1 in the +9 state, and at 11 670 and 13 320 cm−1 in
meso-[{Ru(bpy)2}2(l-ppz)]5+.
A classical analysis of the IVCT bands yields the following
◦
parameters: IVCT(1) − Dm1/2 = 3250 cm−1 and Hab = 480 cm−1
◦
assuming rab = 6.65 A; IVCT(2) − Dm1/2 = 4050 cm−1 and
˚
Trinuclear system. The two mixed valence states of the trinu-
clear diastereoisomers were generated upon one- and two-electron
Hab = 60 cm−1 assuming rab = 13.3 A (i.e. twice the through-bond
˚
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II
III
distance for the adjacent transition). The experimental bandwidths
(Dm1/2) are comparable to those predicted from theory, and the
magnitudes of Hab indicate that both transitions are localised.
For IVCT(1), Dm1/2 > Dm1/2◦, which suggests the presence of
underlying spin–orbit components beneath the Gaussian-shaped
IVCT manifold. As shown in Fig. 5(b), the results are in
marked contrast to the borderline localised–delocalised IVCT in
the dinuclear analogues, and the [{Ru(bpy)2}3(l-HAT)]7+ mixed
valence species {Fig. 5(a)}.41 From the electrochemical studies,
the presence of the third Ru chromophore in the ppz-bridged
trinuclear complex reduces the electron density at the central Ru,
giving rise to decreased coupling of the bridging ligands through
the central metal in the trinuclear compared with the mononuclear
complex. This decreased coupling is also manifested in the IVCT
properties by a significantly reduced coupling between the adjacent
metal centres in the trinuclear relative to the dinuclear species.
IVCT transitions from the central Ru to terminal Ru centres. A
classical analysis yields Hab = 860 cm−1 {eqn (1)} assuming rab is
˚
6.65 A. The transitions are localised and occur at higher energy
than the corresponding adjacent IVCT in the +7 mixed valence
form due to the decreased electron delocalisation in the system,
and hence destabilisation of the acceptor Ru orbitals. This is
III
consistent with previous observations for the +7 and +8 states
for the trinuclear l-HAT complex,41 and with previous literature
reports for IVCT transitions in related di- and trinuclear systems.83
The assignment of the bands in the region at energies lower than
7000 cm−1 is ambiguous due to the presence of comproportiona-
tion equilibria, however these may be reasonably ascribed to the
residual absorptions of the mono-oxidised +7 species.
The transition at 12 080 cm−1 may correspond to an IVCT
transition to the singlet or triplet “exciton” state of the 8230 cm−1
transition, as discussed previously for [{Ru(bpy)2}3(l-HAT)]8+.41
However, an assignment as an LMCT absorption is more probable
as LMCT bands of similar energy are present in the +9 species,
and in the dinuclear analogues. Since the two holes are situated on
III
remote localised Ru centres, the coupling of the spins (parallel or
˚
antiparallel) should be significantly weaker given the ∼13.3 A sep-
aration of the metals, compared with the “cluster” arrangement in
[{Ru(bpy)2}3(l-HAT)]8+.41 Nevertheless the possibility of an IVCT
transition to the second “exciton” state cannot be discounted, and
the relatively weak transition may be obscured by the low-lying
LMCT and MLCT transitions.
Conclusions
The IVCT properties of the mixed valence forms of the trinuclear
complex DDtD-[{Ru(bpy)2}2{Ru(bpy)(l-ppz)2}]n+ (n = 7, 8) and
the diastereoisomers (meso and rac) of the dinuclear complex
[{Ru(bpy)2}2(l-ppz)]5+ display a marked dependence on the nucle-
arity and extent of oxidation of the mixed valence assemblies. The
IVCT properties of the diastereoisomeric forms of [{Ru(bpy)2}2(l-
ppz)]5+ are similar, and are comparable to those in [{Ru(bpy)2}2(l-
HAT)]5+. Both dinuclear complexes are classified as borderline
localised–delocalised mixed valence species.
By comparison, the two mixed valence states of the trin-
uclear complex DDtD-[{Ru(bpy)2}2{Ru(bpy)(l-ppz)2}]6+ exhibit
localised behaviour. One-electron oxidation of a terminal Ru
centre gives rise to a broad, low intensity IVCT band for the
+7 mixed valence species which is composed of two underlying
Gaussian-shaped bands. The transitions are identified as adjacent
and remote IVCT transitions, with the former dominating the
intensity of the IVCT manifold. The +8 mixed valence species
exhibits a single dominant IVCT band arising from the equivalent
Fig. 5 Comparison of the IVCT transitions for the di- and tri-
nuclear mixed valence systems: (a) heterochiral-[{Ru(bpy)2}3(l-HAT)]n+
(n = 7, 8) and meso-[{Ru(bpy)2}2(l-HAT)]5+; (b) DDtD-[{Ru(bpy)2}2-
{Ru(bpy)(l-ppz)2}]n+ (n = 7, 8) and meso-[{Ru(bpy)2}2(l-ppz)]5+
.
The intensity of the IVCT bands decreased on subsequent
oxidation of the second terminal Ru centre in the +8 mixed valence
species, with the appearance of a new band at 8230 cm−1 (Fig. 4 and
5(b), Tables 2 and 3) which is assigned to two degenerate adjacent
II
III
IVCT transitions from the central Ru to peripheral Ru centres,
shown in Fig. 6.
Fig. 6 Schematic illustration of the proposed origins of the IVCT transitions (denoted by arrows) in the +7 and +8 mixed valence forms of
DDtD-[{Ru(bpy)2}2{Ru(bpy)(l-ppz)2}]n+ (n = 6–9).
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Despite the similarity in the IVCT properties of the dinuclear
complexes [{Ru(bpy)2}2(l-ppz)]4+ and [{Ru(bpy)2}2(l-HAT)]5+,
the IVCT characteristics of the mixed valence forms of DDtD-
[{Ru(bpy)2}2{Ru(bpy)(l-ppz)2}]6+ are markedly different from
those of the corresponding mixed valence forms of [{Ru(bpy)2}3(l-
HAT)]6+ reported previously.41 As shown in Fig. 1(a), the three
Ru centres in the latter are equivalently disposed, and share the
available electron density. By comparison, a “chain-like” arrange-
ment of the three metal centres in DDtD-[{Ru(bpy)2}2{Ru(bpy)(l-
ppz)2}]6+ {Fig. 1(b)} gives rise to a decreased coupling through
the central metal. As a result, the [{Ru(bpy)2}3(l-HAT)]6+ com-
plex exhibits a comparable degree of electronic coupling to its
dinuclear analogue, while the degree of electronic coupling in
DDtD-[{Ru(bpy)2}2{Ru(bpy)(l-ppz)2}]6+ is reduced relative to its
dinuclear counterpart.
16 H. Oshio, H. Onodera, O. Tamada, H. Mizutani, T. Hikichi and T. Ito,
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Qualitatively, a localised description based on the geometrical
II/III
properties of the dp(Ru ) orbitals provides a reasonable rationale
for the IVCT behaviour in the localised trinuclear system. How-
ever, treatment of the full vibronic coupling problem is necessary
in order to provide a quantitative analysis of these borderline
localised-to-delocalised dinuclear complexes.
The existence of high quality experimental data that will test
and guide developments in the conceptual theories for IVCT in
higher nuclearity systems, and the extension of the IVCT probe
to stereochemically unambiguous trinuclear species, are extremely
significant to our understanding of fundamental intramolecular
electron transfer phenomena. This represents a key step in the
rational design of higher nuclearity polymetallic assemblies in
which the electron transfer processes may be controlled and
ultimately exploited for novel practical applications.
31 C. A. Bignozzi, R. Argazzi, C. Chiorboli, F. Scandola, R. B. Dyer, J. R.
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37 T. J. Rutherford and F. R. Keene, Inorg. Chem., 1997, 36, 2872.
38 T. J. Rutherford, O. Van Gijte, A. Kirsch-De Mesmaeker and F. R.
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39 J. A. Treadway, P. Chen, T. J. Rutherford, F. R. Keene and T. J. Meyer,
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Acknowledgements
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We are grateful to Dr Rob Gauci for his assistance with the
synthesis of ppz. This work was supported by the Australian
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