Complexes of substituted derivatives of 2-(2-pyridyl)benzimidazole with Re(I), Ru(II) and Pt(II): Structures, redox and luminescence properties

Article abstract of DOI:10.1039/b411341a

N,N′-Chelating ligands based on the 2-(2-pyridyl)benzimidazole (PB) core have been prepared with a range of substituents (phenyl, pentafluorophenyl, naphthyl, anthracenyl, pyrenyl) connected to the periphery via alkylation of the benzimidazolyl unit at one of the N atoms. These PB ligands have been used to prepare a series of complexes of the type [Re(PB)(CO)₃Cl], [Pt(PB)(CCR)₂] (where -CCR is an acetylide ligand) and [Ru(bpy) ₂(PB)][PF₆]₂ (bpy = 2,2′-bipyridine). Six of the complexes have been structurally characterised. Electrochemical and luminescence studies show that all three series of complexes behave in a similar manner to the analogous complexes with 2,2′-bipyridine in place of PB. In particular, all three series of complexes show luminescence in the range 553-605 nm (Pt series), 620-640 nm (Re series) and 626-645 nm (Ru series) arising from the ³MLCT state, with members of the Pt(II) series being the most strongly emissive with lifetimes of up to 500 ns and quantum yields of up to 6% in air-saturated CH₂Cl₂ at room temperature. In the Re and Ru series there was clear evidence for inter-component energy-transfer processes in both directions between the ³MLCT state of the metal centre and the singlet and triplet states of the pendant organic luminophores (naphthalene, pyrene, anthracene). For example the pyrene singlet is almost completely quenched by energy transfer to a Re-based MLCT excited state, which in turn is completely quenched by energy transfer to the lower-lying pyrene triplet state. For the analogous Ru(II) complexes the inter-component energy transfer is less effective, with ¹anthracene → Ru(³MLCT) energy transfer being absent, and Ru( ³MLCT) → ³anthracene energy transfer being incomplete. This is rationalised on the basis of a greater effective distance for energy transfer in the Ru(II) series, because the MLCT excited states are localised on the bpy ligands which are remote from the pendant aromatic group; in the Re series in contrast, the MLCT excited states involve the PB ligand to which the pendant aromatic group is directly attached, giving more efficient energy transfer.

Full text of DOI:10.1039/b411341a

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F U L L P A P E R
Complexes of substituted derivatives of 2-(2-pyridyl)benzimidazole
with Re(I), Ru(II) and Pt(II): structures, redox and luminescence
properties
Nail M. Shavaleev,a Zöe R. Bell,a Timothy L. Easun,b Ramune Rutkaite,b Linda Swansonb and
Michael D. Ward*a,b
a
School of Chemistry, University of Bristol, Cantock’s Close, Bristol, UK BS8 1TS
Department of Chemistry, University of Sheffield, Sheffield, UK S3 7HF.
b
E-mail: m.d.ward@sheffield.ac.uk; Fax: +0114 2229346; Tel: +0114 2229484
Received 23rd July 2004, Accepted 22nd September 2004
First published as an Advance Article on the web 6th October 2004
N,N-Chelating ligands based on the 2-(2-pyridyl)benzimidazole (PB) core have been prepared with a range of
substituents (phenyl, pentafluorophenyl, naphthyl, anthracenyl, pyrenyl) connected to the periphery via alkylation
of the benzimidazolyl unit at one of the N atoms. These PB ligands have been used to prepare a series of complexes
of the type [Re(PB)(CO)3Cl], [Pt(PB)(CCR)2] (where –CCR is an acetylide ligand) and [Ru(bpy)2(PB)][PF6]2
(bpy = 2,2-bipyridine). Six of the complexes have been structurally characterised. Electrochemical and luminescence
studies show that all three series of complexes behave in a similar manner to the analogous complexes with 2,2-
bipyridine in place of PB. In particular, all three series of complexes show luminescence in the range 553–605 nm
(Pt series), 620–640 nm (Re series) and 626–645 nm (Ru series) arising from the 3MLCT state, with members of
the Pt(II) series being the most strongly emissive with lifetimes of up to 500 ns and quantum yields of up to 6% in
air-saturated CH2Cl2 at room temperature. In the Re and Ru series there was clear evidence for inter-component
energy-transfer processes in both directions between the 3MLCT state of the metal centre and the singlet and triplet
states of the pendant organic luminophores (naphthalene, pyrene, anthracene). For example the pyrene singlet is
almost completely quenched by energy transfer to a Re-based MLCT excited state, which in turn is completely
quenched by energy transfer to the lower-lying pyrene triplet state. For the analogous Ru(II) complexes the inter-
component energy transfer is less effective, with 1anthracene → Ru(3MLCT) energy transfer being absent, and
Ru(3MLCT) → 3anthracene energy transfer being incomplete. This is rationalised on the basis of a greater effective
distance for energy transfer in the Ru(II) series, because the MLCT excited states are localised on the bpy ligands
which are remote from the pendant aromatic group; in the Re series in contrast, the MLCT excited states involve the
PB ligand to which the pendant aromatic group is directly attached, giving more efficient energy transfer.
In particular, functionalisation at the externally directed NH
Introduction
position of PB allows simple incorporation of PB units into
The enormous popularity of metal–bipyridyl complexes show-
multinucleating bridging ligands which contain two or more
ing luminescence from charge-transfer excited states has resulted
redox- and photo-active components displaying either ground-
in the study of many bidentate diimine-type ligands which may
state electronic interactions or photoinduced electron- or
show analogous behaviour in their complexes.1 The opportuni-
energy-transfer between sites.8
ties for tuning the steric and electronic properties of the lumines-
Facile alkylation of the benzimidazole NH group can be used
cent metal centres using such ‘bpy analogues’ are huge and the
to attach a wide variety of substituents to the metal–diimine core
topic of considerable current research.
in a way that is not so synthetically convenient for metal–bpy
complexes. In this paper accordingly we describe three series of
TheN,N-diiminechelatingligand2-(2-pyridyl)benzimidazole
(PB) has a venerable history in coordination chemistry.2,3 Many
complexes, of Re(I), Ru(II) and Pt(II), with alkylated PB deriva-
of the reported complexes of PB have been of interest because of
tives as ligands; the substituents include aromatic luminophores
thepossibilityof deprotonationof theNHgroupof theimidazole
such as naphthalene, pyrene and anthracene. These complexes
unit, converting the ligand from neutral to anionic forms with
are analogues of known luminescent metal–bpy complexes, and
different properties.2,4 Haga showed in [Ru(bpy)2(PB)]2+ how the
we report here their syntheses, some crystal structures, and their
redox and spectroscopic properties of the complex were strongly
luminescence and redox properties.
dependent on pH (protonation/deprotonation of the peripheral
NH unit)4 or, more subtly, on the existence of hydrogen-bonding
interactions involving the NH group as donor and N-hetero-
cyclic bases as acceptors.5 The same group also demonstrated
that [Ru(bpy)2(PB)]2+ shows 3MLCT luminescence comparable
to that of [Ru(bpy)3]2+, although slightly red-shifted and with a
slightly reduced lifetime.4
Ligands based on PB units have more recently been popular
in two distinct contexts. First, PB units have been used in the
field of self-assembly as components of multinucleating com-
partmental ligands whose complexes form elaborate molecular
architectures such as triple helicates.6 Secondly, following Haga’s
initial observation of the luminescence of [Ru(bpy)2(PB)]2+,4 PB
derivatives have been used as ‘bipyridine analogues’ in a variety
of complexes with metal ions such as Ru(II), Os(II) and Re(I).7,8
Experimental
General details
Organic reagents and metal salts were purchased from Aldrich
or Avocado and used as received. 2-(2-Pyridyl)benzimidazole,9
N-ethyl-2-(2-pyridyl)benzimidazole10 (PBE) and [Ru(bpy)2-
Cl2]·2H2O 11 were prepared according to the literature methods.
1H NMR spectra were recorded on a Jeol GX-400 spectrometer,
and all mass spectra (FAB and EI) on a VG-Autospec instru-
ment. IR spectra were recorded on a Perkin-Elmer Spectrum
One instrument; UV/Vis absorption spectra were recorded
on Perkin-Elmer Lambda 2 or Cary 50 spectrometers using
CH2Cl2 solutions. Steady-state luminescence and excitation
3 6 7 8
D a l t o n T r a n s . , 2 0 0 4 , 3 6 7 8 – 3 6 8 8
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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spectra were recorded in aerated CH2Cl2 solutions, diluted to
give an absorbance of 0.1 or less at the excitation wavelength,
on a Perkin-Elmer LS-50 luminescence spectrometer. Lumine-
scence lifetimes were acquired on an Edinburgh Instruments
199 spectrometer operating under single photon counting
conditions. The MHz repetition-rate excitation source was
either an IBH NanoLED-05 (for 450 nm excitation, used for the
Ru complexes) or an IBH NanoLED-03 (for 370 nm excitation,
used for the Re and Pt complexes). Fluorescence emission was
isolated via the use of appropriate narrow band (±10 nm) inter-
ference filters (550 or 600 nm, as appropriate). Electrochemical
measurements were performed with an EcoChimie Autolab 100
potentiostat, using a conventional three-electrode cell with Pt-
wire working and counter electrodes and an Ag/AgCl reference
electrode. Ferrocene was used in each experiment as an inter-
nal standard and all potentials are quoted with respect to the
ferrocene–ferrocenium couple.
(2H, m; 2 anthacenyl CH), 7.96 (1H, td, J 8, 1.6; pyridyl H4),
7.64 (1H, d, J 8; pyridyl H3), 7.4–7.5 (5H, m; 4 benzimidazolyl
CH and anthracenyl CH); 7.25 (2H, s; CH2), 7.01 (1H, t, J 8;
anthracenyl CH), 6.64 (1H, t, J 8; anthracenyl CH), 6.27 (1H, d,
J 8; anthracenyl CH).
Data for PBPyr. Alkylating agent: bromomethylpyrene
(2.33 g, 7.9 mmol); yield 1.73 g, 55%). Anal. Calc. for C29H19N3:
C, 85.1; H, 4.7; N, 10.3. Found: C, 84.5; H, 4.7; N, 10.0%. EIMS:
m/z 409 (85%, M +). kmax/nm (e/M−1 cm−1): 346 (45000), 329
(42400), 314 (33900), 278 (47700), 267 (29500), 244 (70300). 1H
NMR (CD2Cl2): d 8.48–8.55 (2H, m; 2 × pyrene CH), 8.42 (1H,
d, J 5; pyridyl H6), 8.2–8.3 (3H, m; pyridyl H3 and 2 × pyrene
CH), 8.04–8.1 (2H, m; 2 × pyrene CH), 7.92–8.0 (2H, m; 2 ×
pyrene CH), 7.87 (1H, d, J 8; pyrene CH), 7.83 (1H, td, J 8, 1.5;
pyridyl H4), 7.15–7.35 (5H, m; 4 benzimidazolyl CH and pyridyl
H5), 6.99 (2H, s; CH2).
Preparations of Re(I) complexes
Procedure for preparation of ligands
A mixture of Re(CO)5Cl and the appropriate ligand in a 1:1
molar ratio was heated to reflux in dry, degassed toluene
(30 cm3) under N2 for 24 h and then cooled down to room
temperature. The resulting yellow/orange precipitate was
filtered off, and washed with hexane and ether. The precipitate
was dissolved in CH2Cl2, hexane was added to the solution, the
CH2Cl2 was removed under reduced pressure. The resulting sus-
pension of the target complex in hexane was filtered off, washed
with hexane and ether and dried under vacuum. All complexes
are air- and moisture-stable yellow solids generally soluble in
chlorinated solvents, THF, and acetone, but insoluble in hexane
and ether. It should be noted that attempted column chromato-
graphy of the complexes on alumina eluting with CH2Cl2 was
not successful.
Dry 2-(2-pyridyl)benzimidazole (1.5 g, 7.7 mmol) was added to
a suspension of NaH (0.62 g, 26 mmol) in dry DMF (30 cm3)
under N2 and stirred for 30 min at room temperature. NaH was
used as a 60% suspension in mineral oil and was washed with
dry degassed hexane before the reaction to remove the oil. A
slight excess of the appropriate bromo- or chloromethyl deriva-
tive (8–9 mmol) was added to the resulting suspension and the
mixture was heated at 100 °C for 24 h. The reaction mixture
was cooled down and poured into water (200 cm3). The pre-
cipitate formed was collected by filtration, dissolved in CH2Cl2,
dried with MgSO4, reduced in volume and purified by column
chromatography on silica eluting with MeOH–CH2Cl2 (0/100 to
2/98) or on alumina eluting with CH2Cl2. The blue-luminescent
fraction containing the product was reduced in volume and
ethanol was added to it. The CH2Cl2 was evaporated to give
a suspension of the product in ethanol that was filtered off,
washed with ethanol and hexane, and dried to give pure product
in reasonable yield.
Data for Re–PBPh. Yield: 86%. Anal. Calc. for C22H15ClN3-
O3Re: C, 44.7; H, 2.6; N, 7.1. Found: C, 44.7; H, 2.3; N, 7.1%.
FABMS: m/z 614 (14% {M + Na}+), 591 (60% {M}+), 563 (23%
{M − CO}+), 556 (100% {M − Cl}+), 535 (16% {M − 2CO}+).
IR (CH2Cl2/cm−1): 2022, 1917, 1895. X-Ray quality crystals were
grown by slow evaporation of a CH2Cl2/heptane solution of the
complex.
Data for PBPh. Alkylating agent: benzyl bromide (1.1 cm3,
1.58 g, 9.2 mmol); yield 1.46 g (67%). Anal. Calc. for C19H15N3:
C, 80.0; H, 5.3; N, 14.7. Found: C, 80.2; H, 5.5; N, 14.8%. EIMS:
m/z 284 (100% {M}+). kmax/nm (e/M−1 cm−1): 311 (21600).
Data for Re–PBN. Yield: 90%. Anal. Calc. for C26H17ClN3-
O3Re: C, 48.7; H, 2.7; N, 6.6. Found: C, 48.5; H, 2.6; N, 6.5%.
FABMS: m/z 641 (12%, {M}+), 613 (6% {M − CO}+), 606 (22%
{M − Cl}+). IR (CH2Cl2/cm−1): 2022, 1917, 1895.
Data for PBF. Alkylating agent: 2,3,4,5,6-pentafluorobenzyl
bromide (1.3 cm3, 2.25 g, 8.6 mmol); yield 2.05 g (71%). Anal.
Calc. for C19H10N3F5: C, 60.8; H, 2.7; N, 11.2. Found: C, 61.3;
H, 2.7; N, 11.4%. EIMS: m/z 375 (90% {M}+), 356 (100%
{M − F}+). kmax/nm (e/M−1 cm−1): 310 (22200). 1H NMR
(CD2Cl2): d 8.64 (1H, d, J 4; pyridyl H6), 8.42 (1H, d, J 8;
pyridyl H3), 7.89 (1H, td, J 8, 1.6; pyridyl H4), 7.78 (1H, m;
pyridyl H5), 7.35–7.44 (2H, m; benzimidazolyl C6H4), 7.31 (2H,
m; benzimidazolyl C6H4), 6.44 (2H, s; CH2).
Data for Re–PBA. Yield: 72%. Anal. Calc. for C30H19ClN3-
O3Re: C, 52.1; H, 2.8; N, 6.1. Found: C, 51.5; H, 2.0; N,
6.2%. FABMS: m/z 691 (6% {M}+), 656 (7% {M − Cl}+). IR
(CH2Cl2/cm−1): 2022, 1917, 1895.
Data for Re–PBF. Yield: 70%. Anal. Calc. for C22H10ClF5N3-
O3Re: C, 38.8; H, 1.5; N, 6.2. Found: C, 39.1; H, 1.3; N, 6.2%.
FABMS: m/z 704 (15% {M + Na}+), 681 (60%, M+), 646 (100%
{M − Cl}+), 625 (20% {M − 2CO}+), 597 (10% {M − 3CO}+).
IR (CH2Cl2/cm−1): 2023, 1918, 1897. X-Ray quality crystals were
grown by slow evaporation of a THF–heptane solution of the
complex.
Data for PBN. Alkylating agent: 2-bromomethylnaphthalene
(1.80 g, 8.1 mmol); yield 0.85 g (33%). Anal. Calc. for C23H17N3:
C, 82.4; H, 5.1; N, 12.5. Found: C, 82.3; H, 5.2; N, 12.7%.
EIMS: m/z 335 (100%, M+). kmax/nm (e/M−1 cm−1): 311 (22600).
1H NMR (CD2Cl2): d 8.62 (1H, d, J 5; pyridyl H6), 8.46 (1H, d,
J 8; pyridyl H3), 7.86 (1H, td, J 8, 1.6; pyridyl H4), 7.72–7.82
(3H, m; pyridyl H5 and 2 naphthyl), 7.68 (1H, m; naphthyl), 7.56
(1H, s; naphthyl), 7.2–7.45 (7H, m; 3 naphthyl CH and 4 benzi-
midazolyl CH), 6.36 (2H, s; CH2).
Data for Re–PBPyr. Yield: 84%. Anal. Calc. for C32H19ClN3-
O3Re: C, 53.7; H, 2.7; N, 5.9. Found: C, 54.0; H, 2.6; N,
6.1%. FABMS: m/z 715 (8%, M+), 680 (7% {M − Cl}+). IR
(CH2Cl2/cm−1): 2022, 1917, 1895. X-Ray quality crystals were
grown by slow evaporation of THF/heptane solution of the
complex.
Data for PBA. Alkylating agent: 9-chloromethylanthracene
(1.74 g, 7.7 mmol); yield 1.61 g (54%). Anal. Calc. for C27H19N3:
C, 84.1; H, 5.0; N, 10.9. Found: C, 83.2; H, 4.7; N, 10.3%.
+
EIMS: m/z 385 (80%, M ). kmax/nm (e/M−1 cm−1): 390 (9500),
Preparations of Ru(II) complexes
1
370 (10100), 351 (6700), 308 (19500), 258 (166000). H NMR
(CD2Cl2): d 8.74 (1H, d, J 4; pyridyl H6), 8.54 (1H, s; anthracenyl
A mixture of [Ru(bpy)2Cl2]·2H2O (100 mg, 0.19 mmol) and the
appropriate ligand (0.2 mmol) was heated to reflux in degassed
H10), 8.42–8.5 (3H, m; pyridyl H3 and 2 anthracenyl CH), 8.07
D a l t o n T r a n s . , 2 0 0 4 , 3 6 7 8 – 3 6 8 8
3 6 7 9

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ethanol (30 cm3) under N2 for 24 h to give a clear red solution.
A few drops of a saturated aqueous solution of KNO3 were
added to the solution and the resulting mixture was evapo-
rated; the solid residue was dissolved in CH3CN, loaded on to
a silica column and eluted with KNO3 (saturated aqueous solu-
tion)–H2O–CH3CN starting with the ratio 1.5:4:200 and slowly
increasing the polarity by increasing the proportions of water
and saturated aqueous KNO3 relative to MeCN. The main red
fraction was collected, evaporated to dryness and re-dissolved
in a small amount of CH3CN. The excess of KNO3 was filtered
off and the filtrate was evaporated to dryness, dissolved in water
(adding a few drops of CH3CN if necessary) and the product
was precipitated by the addition of an aqueous solution of
KPF6. The product was filtered off, washed with water and ether,
and dried under vacuum.
for 10 min, followed by addition of the appropriate acetylene
(10-fold excess). The resulting suspension was stirred under N2
at room temperature protected from light for 2–5 days. The reac-
tion mixture could be sonicated in a small ultrasound cleaning
bath to facilitate the reaction. By the end of reaction the sus-
pension had dissolved to give a yellow/red solution from which
the solvent was evaporated. The solid residue was dried under
vacuum to remove traces of iPr2NH and the product purified
by column chromatography on alumina, eluting with CH2Cl2
unless stated otherwise. The fraction containing the product
was collected and reduced in volume to 5 cm3. The complex
was then precipitated by addition of hexane, filtered off, washed
with hexane and ether, and dried under vacuum. All of the
Pt(II)–diacetylide complexes are air- and moisture-stable solids
of yellow/orange colour which are soluble in CH2Cl2, acetone
and tetrahydrofuran, and insoluble in ether and hexane.
Data for Ru–PBN. Yield: 42%. Anal. Calc. for C43H33F12N7-
P2Ru: C, 49.7; H, 3.2; N, 9.4. Found: C, 49.2; H, 3.1; N, 9.1%.
FABMS: m/z 894 (100% {M − PF6}+), 608 (85% {M − 2PF6 −
CH2naphthyl}+).
Data for Pt–PBE–Ph. Yield: 76%. Anal. Calc. for C30H23N3Pt:
C, 58.1; H, 3.7; N, 6.8. Found: C, 57.9; H, 3.5; N, 6.6%. FABMS:
m/z 621 (32% {M}+). X-Ray quality crystals were grown by slow
evaporation of a CH2Cl2–heptane solution of the complex.
Data for Ru–PBF. Yield: 46%. Anal. Calc. for C39H26F17N7-
P2Ru: C, 43.4; H, 2.4; N, 9.1. Found: C, 42.8; H, 2.3; N,
8.9%. FABMS: m/z 934 (100% {M − PF6}+), 608 (75%
{M − 2PF6 − CH2C6F5}+).
Data for Pt–PBE–CF3. Yield: 82%. Anal. Calc. for C32H21F6-
N3Pt: C, 50.8; H, 2.8; N, 5.6. Found: C, 51.0; H, 2.5; N, 5.3%.
FABMS: m/z 757 (16% {M}+).
Data for Ru–PBA. Yield: 62%. Anal. Calc. for C47H35F12N7-
P2Ru: C, 51.9; H, 3.2; N, 9.0. Found: C, 51.4; H, 3.2; N, 8.9%.
FABMS: m/z 1112 (2% {M + Na}+), 944 (40% {M − PF6}+), 608
(100% {M − 2PF6 − CH2anthracenyl}+). X-Ray quality crystals
were grown by slow evaporation of an acetone–heptane solution
of the complex.
Data for Pt–PBPh–Ph. Yield: 47%. Anal. Calc. for C35H25-
N3Pt: C, 61.6; H, 3.7; N, 6.2. Found: C, 61.5; H, 3.7; N, 6.4%.
FABMS: m/z 682 (14% {M}+).
Data for Pt–PBPh–CF3. Yield: 78%. Anal. Calc. for C37H23F6-
N3Pt: C, 54.3; H, 2.8; N, 5.1. Found: C, 54.9; H, 2.9; N, 5.3%.
FABMS: m/z 841 (4% {M + Na}+), 819 (40% {M}+).
Data for Ru–PBPyr. Yield: 81%. Anal. Calc. for C49H35F12N7-
P2Ru: C, 52.9; H, 3.2; N, 8.8. Found: C, 52.7; H, 3.1; N, 8.5%.
FABMS: m/z 968 (40% {M − PF6}+), 608 (100% {M − 2PF6 −
CH2pyrenyl}+).
Data for Pt–PBPh–Py. Yield: 60%. Anal. Calc. for C33H23-
N5Pt: C, 57.9; H, 3.4; N, 10.2. Found: C, 58.2; H, 3.1; N, 10.2%.
FABMS: m/z 707 (42% {M + Na}+), 685 (100% {M}+). X-Ray
quality crystals were grown by slow evaporation of THF–
heptane solution of the complex.
Preparations of Pt(II) complexes
These were prepared in two steps: first the ligand (L) is reacted
with Pt(DMSO)2Cl2 to give PtLCl2; the dichloride is then
converted to the diacetylide PtL(CCR)2 in a separate step.
Data for Pt–PBF–Ph. Yield: 64%. Anal. Calc. for C35H20F5-
N3Pt: C, 54.4; H, 2.6; N, 5.4. Found: C, 54.6; H, 2.8; N, 5.6%.
FABMS: m/z 771 (27% {M}+).
Preparation of the dichloride intermediates
Data for Pt–PBN–Ph. Yield: 83%. Anal. Calc. for C39H27N3Pt:
C, 63.9; H, 3.7; N, 5.7. Found: C, 63.8; H, 3.5; N, 5.6%. FABMS:
m/z 755 (15% {M + Na}+), 733 (70% {M}+).
A mixture of Pt(DMSO)2Cl2 (0.21 g, 0.5 mmol) and the
appropriate ligand (1 equivalent) in degassed acetonitrile
(30 cm3) was refluxed under N2 for 24 h. The reaction mixture
was then cooled to 0 °C and the resulting yellow precipitate was
filtered off, and washed with hexane and ether. All complexes
are air- and moisture-stable yellow solids which are generally
insoluble in organic solvents. The complexes show yellow/
orange luminescence in the solid state.
X-Ray crystallography
For each complex a suitable crystal was coated with hydrocarbon
oil and attached to the tip of a glass fibre, which was then trans-
ferred to a Bruker-AXS SMART diffractometer under a stream
of cold N2. Details of the crystal parameters, data collection
and refinement for each of the structures are collected in
Table 1. After data collection, in each case an empirical absorp-
tion correction (SADABS) was applied,12 and the structures
were then solved by conventional direct methods and refined
on all F2 data using the SHELX suite of programs.13 In all cases
non-hydrogen atoms were refined with anisotropic thermal para-
meters; hydrogen atoms were included in calculated positions
and refined with isotropic thermal parameters.
Refinement of the structure of Pt–PBE–Ph proceeded
without any problems. For Re–PBF, Pt–PBPh–Py and
Re–PBPyr·thf, the ‘squeeze’ command was used to eliminate
residual electron density peaks which presumably corresponded
to highly disordered solvent molecules which could not be
properly identified or refined; in Re–PBPyr·thf the thermal
parameters for the lattice thf molecule that could be located are
rather large. In Re–PBPh·CH2Cl2 the atoms C(51), C(61) and
C(16) were refined with isotropic thermal parameters to keep the
Data for Pt(PBE)Cl2. Yield: 85%. Anal. Calc. for C14H13Cl2-
N3Pt: C, 34.4; H, 2.7; N, 8.6. Found: C, 34.8; H, 2.4; N, 8.3%.
Data for Pt(PBPh)Cl2. Yield: 87%. Anal. Calc. for C19H15Cl2-
N3Pt: C, 41.4; H, 2.7; N, 7.6. Found: C, 41.7; H, 2.3; N, 7.1%.
FABMS: m/z 574 (4% {M + Na}+); 516 (10% {M − Cl}+).
Data for Pt(PBF)Cl2. Yield: 91%. Anal. Calc. for C19H10F5Cl2-
N3Pt: C, 35.6; H, 1.6; N, 6.6. Found: C, 36.1; H, 1.5; N, 6.6%.
Data for Pt(PBN)Cl2. Yield: 98%. Anal. Calc. for C23H17Cl2-
N3Pt: C, 45.9; H, 2.9; N, 7.0. Found: C, 46.4; H, 3.1; N, 7.4%.
Preparation of the diacetylide complexes
A mixture of the appropriate platinum(II) chloride complex (0.2
mmol), anhydrous CuI (7 mg, catalyst), and dry iPr2NH (2 cm3)
in dry, degassed dichloromethane (30 cm3) under N2 was stirred
3 6 8 0
D a l t o n T r a n s . , 2 0 0 4 , 3 6 7 8 – 3 6 8 8

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Table 1 Crystal, data collection and refinement details for the crystal structuresa
Complex
Ru–PBA·Me2CO
Re–PBPh·CH2Cl2
Re–PBF
Re–PBPyr·thf
Pt–PBPh–py
Pt–PBE–Ph
Formula
Mr
Crystal system
Space group
C50H41F12N7OP2Ru
1146.91
Monoclinic
P21/n
C23H17Cl3N3O3Re
675.95
Orthorhombic
Pca21
C22H10ClF5N3O3Re
680.98
Triclinic
P1
C36H27ClN3O4Re
787.26
Triclinic
P1
C33H23N5Pt
684.65
Monoclinic
P21/c
C30H23N3Pt
620.60
Monoclinic
P21/n
T/K
a/Å
b/Å
c/Å
a/°
b/°
173
100
173
173
173
173
22.253(7)
8.776(3)
26.835(13)
90
113.88(2)
90
4792(3)
4
1.590
8.908(3)
12.523(4)
20.764(5)
90
90
90
2316.3(11)
4
1.938
0.5 × 0.15 × 0.15
5.624
9.3838(15)
12.105(2)
12.532(2)
99.044(15)
111.359(17)
91.924(17)
1302.8(4)
2
12.055(2)
12.376(3)
13.047(2)
108.288(14)
100.199(15)
99.195(13)
1770.0(6)
2
12.2261(14)
12.0913(19)
22.271(3)
90
99.635(12)
90
3245.8(8)
4
1.401
13.435(4)
7.5867(13)
23.436(5)
90
105.215(14)
90
2305.0(10)
4
1.788
c/°
V/Å3
Z
Dc/g cm−3
Crystal size/mm
l/mm−1
1.736
0.5 × 0.2 × 0.2
4.828
1.477
0.4 × 0.2 × 0.2
3.548
0.3 × 0.1 × 0.05
0.488
0.5 × 0.5 × 0.4
4.349
0.4 × 0.2 × 0.05
6.112
Data, restraints,
parameters
Final R1, wR2
8428, 84, 660
5208, 1, 283
5904, 0, 316
8044, 0, 406
7426, 0, 352
5140, 0, 308
0.0721, 0.1930
0.0403, 0.0876
0.0223, 0.0514
0.0284, 0.0668
0.0290, 0.0744
0.0355, 0.0763
a All data sets were collected on a Bruker SMART diffractometer with Mo-Ka radiation (k = 0.71073 Å, 2h limit 55°).
compounds in DMF using NaH as base. Yields were respectable
and all ligands were fully characterised by the usual techniques
(see Experimental section for details).
refinement stable as the data was weak. For Ru–PBA·Me2CO the
thermal parameters of the F atoms of the hexafluorophosphate
anions were refined with isotropic restraints, apart from atoms
F(11) and F(12) which needed to be refined with isotropic
thermal parameters.
CCDC reference numbers 245715–245720.
See http://www.rsc.org/suppdata/dt/b4/b411341a/ for crystal-
lographic data in CIF or other electronic format.
The metal complexes (Scheme 1) were prepared using stan-
dard methods, as for related metal-diimine complexes. Thus,
reaction of a pyridylbenzimidazole ligand L with Re(CO)5Cl
in toluene at reflux afforded Re(CO)3ClL; reaction with
Ru(bpy)2Cl2 in ethanol at reflux, followed by anion metathesis,
afforded [Ru(bpy)2L][PF6]2; and reaction with Pt(dmso)2Cl2
afforded intermediates PtLCl2, which were further reacted with
an acetylene RCCH in the presence of CuI and diisopropyl-
amine to give the ultimate products PtL(CCR)2. All complexes
provided satisfactory analytical and mass spectroscopic data,
and several have been structurally characterised (Figs. 1–6; see
also Tables 1 and 2, for crystallographic data and selected struc-
tural parameters, respectively).
Results and discussion
Syntheses and structures of complexes
The ligands (Scheme 1) were readily prepared by alkylation
of the parent compound PB with appropriate halomethyl
Fig. 1 Molecular structure of Re–PBPh.
The Re(I) complexes Re–PBPh, Re–PBF and Re–PBPyr
(Figs. 1–3) all have very similar coordination geometries about
the metal centre, with a fac-tricarbonyl disposition of ligands
and a near-octahedral geometry typical of Re(I)–diimine–
tricarbonyl complexes of this type.14 The Re–N distance to the
benzimidazole ring is slightly shorter than that to the pyridyl
donor, as was observed recently for Re–PBE.7b The bite angles
of the chelating PB ligands are close to 74° in every case; the
ligands are not completely planar, with small twists between the
pyridyl and benzimidazolyl components, of ca. 2.5° in Re–PBF;
10° in Re–PBPyr; and 5° in Re–PBPh. The C–O distances lie
Scheme 1
D a l t o n T r a n s . , 2 0 0 4 , 3 6 7 8 – 3 6 8 8
3 6 8 1

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Fig. 2 Molecular structure of Re–PBF.
Fig. 5 Molecular structure of Pt–PBE–Ph.
Fig. 3 Molecular structure of Re–PBPyr.
Fig. 4 Molecular structure of the complex cation of Ru–PBA.
in the range 1.11–1.17 Å. Partial localisation of single and
double bonds in the imidazole unit is clear, as the structure of
Re–PBF illustrates: the N(21)–C(22) distance of 1.325(4) Å,
formally the CN bond, is the shortest distance within the
five-membered imidazole ring. All other bond distances within
this ring, and also within the pyridyl ring, lie in the range
1.35–1.40 Å; in contrast, the formally single bond between the
rings, C(12)–C(22), has a distance of 1.471 Å. The same general
pattern occurs in the structures of Re–PBPh and Re–PBPyr. In
all three cases, the aromatic unit pendant from the benzimidazole
group is almost perpendicular to the benzimidazole mean plane,
with the mean plane of the substituent making an angle of 74.6,
87.6 and 88.4° with the benzimidazole plane for Re–PBPh,
Re–PBF and Re–PBPyr, respectively.
Fig. 6 Molecular structure of Pt–PBPh–Py.
slightly shorter than the Ru–N(pyridyl) distance [2.084(7) Å];
the chelating bite angle of the PBA ligand is 77°, and the twist
between the two components of this ligand is 12°. The partial
localisation of bonds within the five-membered imidazole ring
is apparent, with the double bond N(21)–C(22) being the short-
est of all bonds in the ligand, at 1.32(1) Å. The mean plane of
the anthracene unit is inclined at 87.5° to the mean plane of the
benzimidazole unit.
Ru–PBA (Fig. 4) likewise has a typical pseudo-octahedral
geometry with bond distances and angles in the normal range
for Ru(II)–diimine complexes; the Ru–N separations lie in
the range 2.041(7)–2.084(7) Å, cf. 2.06 Å for [Ru(bpy)3]2+.15
Again, the Ru–N(benzimidazole) distance [2.057(7) Å] is
3 6 8 2
D a l t o n T r a n s . , 2 0 0 4 , 3 6 7 8 – 3 6 8 8

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Table 2 Selected bond distances (Å) and angles (°) for the six
structures
Table 2 (Contd.)
Pt–PBE–Ph
Ru–PBA·Me2CO
C(31)–Pt(1)–C(41)
C(31)–Pt(1)–N(21)
C(41)–Pt(1)–N(21)
C(31)–Pt(1)–N(11)
C(41)–Pt(1)–N(11)
88.6(2)
N(21)–Pt(1)–N(11)
C(32)–C(31)–Pt(1)
C(31)–C(32)–C(33)
C(42)–C(41)–Pt(1)
C(41)–C(42)–C(43)
77.77(16)
177.4(5)
174.2(6)
176.0(5)
178.1(6)
Ru(1)–N(51)
Ru(1)–N(41)
Ru(1)–N(31)
2.040(8)
2.056(8)
2.059(9)
Ru(1)–N(61)
Ru(1)–N(21)
Ru(1)–N(11)
2.061(8)
2.061(7)
2.082(7)
97.88(18)
172.69(18)
175.0(2)
95.61(19)
N(51)–Ru(1)–N(41)
97.1(3)
N(31)–Ru(1)–N(21)
98.0(3)
N(51)–Ru(1)–N(31) 173.1(3)
N(41)–Ru(1)–N(31)
N(51)–Ru(1)–N(61)
N(41)–Ru(1)–N(61)
N(31)–Ru(1)–N(61)
N(51)–Ru(1)–N(21)
78.6(4)
78.3(3)
85.7(3)
95.9(3)
86.9(3)
The Pt(II) complexes Pt–PBE–Ph and Pt–PBPh–Py (Figs. 5
and 6) have essentially square planar coordination geometries,
with the biggest distortion from an ideal geometry arising
from the limited bite angle of the chelating PB ligand (ca.
78° in each case). In these complexes the shortening of the
Pt–N(benzimidazole) distance compared to the Pt–N(pyridyl)
distance is barely significant. The Pt–C distances are likewise
equivalent within the uncertainty limits so there is no obvious
trans effect arising from the electronic inequivalence between
the pyridyl and benzimidazolyl donors. The CC bonds have
distances of 1.21–1.22 Å, in obvious contrast to the immedi-
ately following C–C single bonds (1.43–1.45 Å). The torsion
angle between the pyridyl and benzimidazolyl units is 3° for
Pt–PBE–Ph and 1° for Pt–PBPh–Py. As in the above cases,
the formally double CN bond within the five-membered ring
of the benzimidazolyl unit is the shortest bond length in the
PB ligand, at 1.331(4) Å for Pt–PBPh–Py and 1.341(6) Å for
Pt–PBE–Ph, although the effect is not as marked as it is in the
Re(I) and Ru(II) complexes described above. In Pt–PBPh–Py,
the pendant phenyl ring makes an angle of 89.9° with the
benzimidazole mean plane. In Pt–PBE–Ph adjacent near-planar
units stack in pairs such that there is an axial PtPt contact
of 3.374 Å [Fig. 5(b)]; in Pt–PBPh–Py the adjacent pairs are
stacked in an ‘offset’ manner so that there are no obvious axial
PtPt interactions [Fig. 6(b)].
N(41)–Ru(1)–N(21) 172.6(3)
Re–PBPh·CH2Cl2
Re(1)–C(41)
Re(1)–C(51)
Re(1)–C(61)
1.910(7)
1.919(7)
1.930(8)
Re(1)–N(21)
Re(1)–N(11)
Re(1)–Cl(1)
2.159(6)
2.218(7)
2.4786(19)
C(41)–Re(1)–C(51)
C(41)–Re(1)–C(61)
C(51)-Re(1)–C(61)
87.2(3)
87.8(3)
87.9(3)
C(61)–Re(1)–N(11)
N(21)–Re(1)–N(11)
C(41)–Re(1)–Cl(1)
C(51)–Re(1)–Cl(1)
C(61)–Re(1)–Cl(1)
N(21)–Re(1)–Cl(1)
N(11)–Re(1)–Cl(1)
96.0(3)
74.4(2)
93.0(2)
94.48(19)
177.5(2)
83.31(16)
82.91(18)
C(41)–Re(1)–N(21) 100.3(3)
C(51)–Re(1)–N(21) 172.2(2)
C(61)–Re(1)–N(21)
94.2(3)
C(41)–Re(1)–N(11) 173.7(3)
C(51)–Re(1)–N(11)
97.9(2)
Re–PBF
Re(1)–C(41)
Re(1)–C(61)
Re(1)–C(51)
1.901(3)
1.910(3)
1.926(3)
Re(1)–N(21)
Re(1)–N(11)
Re(1)–Cl(1)
2.161(2)
2.189(3)
2.4960(8)
C(41)–Re(1)–C(61)
C(41)–Re(1)–C(51)
C(61)–Re(1)–C(51)
C(41)–Re(1)–N(21)
88.23(13)
90.22(13)
88.90(13)
95.05(11)
C(51)–Re(1)–N(11)
N(21)–Re(1)–N(11)
C(41)–Re(1)–Cl(1)
C(61)–Re(1)–Cl(1)
C(51)–Re(1)–Cl(1)
N(21)–Re(1)–Cl(1)
N(11)–Re(1)–Cl(1)
174.70(11)
73.82(9)
176.26(9)
94.06(9)
92.78(9)
82.18(6)
85.00(7)
C(61)–Re(1)–N(21) 169.41(10)
C(51)–Re(1)–N(21) 101.13(12)
Absorption and luminescence properties
Electronic spectra of the complexes are summarised in
Table 3. In general, the lowest-energy absorption in each case
is the metal-to-ligand charge transfer (MLCT) transition which
is characteristic of these Re(I),16 Ru(II)17 and Pt(II)18 chromo-
phores with diimine ligands. For the Re(I) complexes these occur
at 400 nm and are of relatively low intensity, being obscured
in some cases by the more intense and sharper transitions
associated with aromatic pendant groups such as anthracene
and pyrene. For the Pt(II) series of complexes these MLCT
transitions are more intense and clearly defined, and lie between
396 and 411 nm. For the Ru(II) series, the MLCT transition is at
lower energy (459 nm). In all cases, the energies of these MLCT
transitions are very similar to those observed for the analogous
2,2-bipyridine complexes.16–18
C(41)–Re(1)–N(11)
C(61)–Re(1)–N(11)
91.82(11)
96.06(11)
Re–PBPyr·thf
Re(1)–C(31)
Re(1)–C(51)
Re(1)–C(41)
1.913(4)
1.920(4)
1.925(4)
Re(1)–N(21)
Re(1)–N(11)
Re(1)–Cl(1)
2.166(3)
2.201(3)
2.4868(9)
C(31)–Re(1)–C(51)
C(31)–Re(1)–C(41)
C(51)–Re(1)–C(41)
C(31)–Re(1)–N(21)
87.52(16)
88.52(15)
87.48(17)
94.77(13)
C(41)–Re(1)–N(11)
N(21)–Re(1)–N(11)
C(31)–Re(1)–Cl(1)
C(51)–Re(1)–Cl(1)
C(41)–Re(1)–Cl(1)
N(21)–Re(1)–Cl(1)
N(11)–Re(1)–Cl(1)
98.31(15)
73.82(10)
177.53(12)
94.37(11)
93.15(11)
83.33(7)
C(51)–Re(1)–N(21) 100.32(13)
C(41)–Re(1)–N(21) 171.64(14)
C(31)–Re(1)–N(11)
93.93(14)
84.03(7)
C(51)–Re(1)–N(11) 174.06(12)
Luminescence spectra of all complexes were recorded in
air-equilibrated CH2Cl2 at ambient temperature (see Table 4).
The Re(I) complexes showed only weak luminescence in every
case, but the origin of the luminescence varied between different
complexes. In two cases (Re–PBN, Re–PBPh and Re–PBF) the
broad, unstructured emission band at 620–640 nm, with a quan-
tum yield of ca. 10−3, is typical of the weak luminescence from
Pt–PBPh–Py
Pt(1)–C(41)
Pt(1)–C(51)
Pt(1)–N(21)
Pt(1)–N(11)
1.947(4)
1.958(4)
2.062(3)
2.075(3)
C(41)–C(42)
C(42)–C(43)
C(51)–C(52)
C(52)–C(53)
1.220(5)
1.428(6)
1.205(5)
1.427(5)
3
the MLCT level of the Re–diimine chromophore, with lumi-
C(41)–Pt(1)–C(51)
C(41)–Pt(1)–N(21)
C(51)–Pt(1)–N(21)
C(41)–Pt(1)–N(11)
C(51)–Pt(1)–N(11)
86.08(16)
98.68(14)
173.91(14)
175.08(13)
97.10(14)
N(21)–Pt(1)–N(11)
C(42)–C(41)–Pt(1)
C(41)–C(42)–C(43)
C(52)–C(51)–Pt(1)
C(51)–C(52)–C(53)
77.91(12)
174.0(3)
178.1(4)
174.1(3)
176.6(4)
nescence parameters comparable to those of [Re(bpy)(CO)3Cl]
under the same conditions.16 Similar emission spectra were
obtained at a range of different excitation wavelengths, with
(in the case of Re–PBN) no trace of any naphthalene-based
luminescence appearing at any excitation wavelength, indicat-
ing complete energy transfer from the singlet excited state of
the naphthalene chromophore to the metal centre, resulting in
3MLCT luminescence.
Pt–PBE–Ph
Pt(1)–C(31)
Pt(1)–C(41)
Pt(1)–N(21)
Pt(1)–N(11)
1.942(5)
1.950(5)
2.070(4)
2.079(4)
C(31)–C(32)
C(32)–C(33)
C(41)–C(42)
C(42)–C(43)
1.207(7)
1.450(7)
1.210(7)
1.440(7)
For Re–PBA and Re–PBPyr, however, excitation into the
MLCT transition at 400 nm gives no detectable Re-based
luminescence; in contrast, excitation of the sharp absorption
D a l t o n T r a n s . , 2 0 0 4 , 3 6 7 8 – 3 6 8 8
3 6 8 3

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Table 3 Absorption spectra of the complexes (CH2Cl2, room
temperature)
Table 4 Luminescence properties of the complexes (CH2Cl2, room
temperature)
Complex
k/nm (10−3e/M−1 cm−1
)
Complex
kema/nm
s/ns
b
Pt–PBPh–Py
Pt–PBPh–CF3
Pt–PBE–CF3
Pt–PBE–Ph
396 (7.8),a 347 (17), 332 (19), 305 (40), 246 (34)
402 (8.4),a 348 (20), 330 (20), 294 (43), 245 (29)
400 (8.5),a 348 (21), 332 (21), 293 (43), 244 (29)
409 (7.3),a 349 (19), 331 (21), 295 (sh), 265 (35),
248 (35)
Pt–PBPh–Py
553
560
556
595
605
597
600
623
404
515
398
225
230
274
278
—
—
—
—
—
0.054
0.059
0.055
0.027
0.021
0.030
0.027
10−3
10−3
10−3
10−4
10−4
0.011
0.011
10−3
Pt–PBPh–CF3
Pt–PBE–CF3
Pt–PBE–Ph
Pt–PBF–Ph
Pt–PBN–Ph
Pt–PBPh–Ph
Re–PBN
Pt–PBF–Ph
414 (6.8),a 346 (16), 328 (sh), 295 (sh), 264 (35),
250 (34)
Pt–PBN–Ph
Pt–PBPh–Ph
Re–PBPh
Re–PBN
Re–PBA
Re–PBF
Re–PBPyr
411 (7.2),a 349 (18), 332 (20), 267 (40), 247 (38)
411 (7.3),a 348 (18), 330 (21), 264 (35), 248 (35)
387 (4.2),a 343 (19), 328 (20), 239 (22)
388 (4.1),a 343 (18), 328 (19), 276 (13)
393 (12), 373 (14), 346 (20), 336 (20), 258 (160)
394 (3.9),a 342 (18), 327 (20), 239 (21)
400 (sh),a 347 (58), 331 (48), 317 (29), 278 (48),
267 (31), 244 (87)
Re–PBPh
620
Re–PBF
640
Re–PBA
395, 418, 440c
Re–PBPyr
Ru–PBF
400
645
626
630
276
356
232
4
Ru–PBPyr
Ru–PBA
Ru–PBF
Ru–PBA
459 (15),a 339 (sh), 321 (sh), 289 (67), 244 (33)
459 (14),a 393 (9.2), 372 (10), 323 (sh), 290 (62),
257 (132)
400, 421, 445c
630
Ru–PBN
312
0.015
a Emission maxima are uncorrected. b Quantum yields were calcu-
lated using [Ru(bpy)3]2+ in aerated water ( = 0.028) as standard.
c Anthracene-based luminescence.
Ru–PBPyr
Ru–PBN
459 (16),a 347 (58), 330 (52), 317 (38), 290 (69),
278 (76), 267 (40), 244 (97)
459 (15),a 340 (sh), 320 (sh), 289 (70), 244 (36)
a Lowest-energy MLCT transition.
bands associated with the anthracene or pyrene units in the UV
region gives very weak, structured luminescence (  10−4) from
these aromatic units, centred at 380 nm for pyrene and 418 nm
for anthracene. The contrast between the two types of lumine-
scence behaviour is shown in Fig. 7. The absence of metal-based
luminescence in these complexes can be ascribed to quenching
of the Re-centred 3MLCT excited state by the lower-lying triplet
stateof theanthraceneorpyreneunits. Fromtheonsetof lumine-
scence at the high-energy end of the luminescence spectrum of
Re–PBN and Re–PBP (i.e. the 0–0 transition between electroni-
cally excited and ground states) at 550 nm, the 3MLCT energy
available to the Re chromophore in these complexes is estimated
to be 18200 cm−1; the triplet energies of pyrene and anthracene
are ca. 16600 cm−1 (ref. 19a) and 14500 cm−1 (ref. 16c) respec-
tively, so such energy transfer is thermodynamically favourable,
and has been observed by Schanze et al. in ReI(bpy)/anthracene
dyads.16c The observation of normal Re-based luminescence
in Re–PBN, in which the naphthalene triplet energy is much
higher than that of the metal 3MLCT state and therefore cannot
quench the metal-based luminescence, confirms this. Fig. 8
illustrates the approximate energy levels for the contrasting
cases of Re–PBN and Re–PBA; the energies of the 1MLCT and
3MLCT Re states are taken from the absorption and lumines-
cence spectra of Re–PBN.
Fig. 8 Energy-level diagram for (a) Re–PBA and (b) Re–PBN. For
details see main text.
1
occur either to the MLCT state [spin-allowed; process (i) in
Fig. 8(a)] followed by inter-system crossing to the 3MLCT
state [process (ii)], or directly to the 3MLCT state [process
(iii), a spin-forbidden process facilitated by the high spin–orbit
3
coupling of the Re atom]. The MLCT state then undergoes a
3
second energy-transfer step to give the lower-lying An state
[process (v)], which is efficiently quenched by dissolved oxygen.
An exactly similar double energy-transfer sequence (aromatic
singlet → metal 3MLCT → aromatic triplet) has been proposed
recently to account for the behaviour of [Ru(bpy)3]2+/pyrene19
and [Ru(bpy)3]2+/anthracene20 dyads; the same sequence of
events can also be suggested for Re–PBPyr. The presence of
very weak residual fluorescence from the pyrene and anthracene
units [Fig. 8(a), process (vi)] in Re–PBPyr and Re–PBA is
simply ascribable to incomplete energy transfer for the first step,
possibly due to the low gradient for energy transfer; in Re–PBN,
where the singlet excited state of the naphthalene unit is consi-
derably higher in energy [Fig. 8(b)], no naphthalene-based emis-
1
In Re–PBA, the initially generated An level, which has a
1
similar energy to the MLCT state of the Re centre, transfers
its excitation energy to the metal centre. This may in principle
1
sion was detected: all excitation energy from the Naph state
ends up populating the 3MLCT state, with consequent Re-based
emission [Fig 8(b), process (vi)].
In contrast to the Re(I) complexes, the series of Pt(II)
complexes showed strong, long-lived luminescence with quantum
yields and lifetimes considerably better, in some cases, than
[Ru(bpy)3]2+ under the same conditions; as with the Re(I) com-
plexes, the luminescence behaviour of these Pt(II)–PB complexes
is generally similar to that of the 2,2-bipyridyl analogues.18 It is
noticeable that the presence of electron-withdrawing substituents
on the acetylene units (C6H4CF3; 2-pyridyl) has a substantially
beneficial effect on the photophysical properties of the com-
plexes, with higher energy for the luminescence, higher quantum
yields and longer lifetimes (see Table 4). This is ascribable to the
fact that decreased p-donor/increased p-acceptor for the acetylide
Fig. 7 Luminescence spectra of Re–PBA and Re–PBF (CH2Cl2, room
temperature).
3 6 8 4
D a l t o n T r a n s . , 2 0 0 4 , 3 6 7 8 – 3 6 8 8

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ligands will strengthen the ligand field at the metal and cause the
d(p) orbitals to drop in energy. Consequently, the luminescence
maxima for these complexes (550–560 nm) are at significantly
higher energy than the others which do not have electron-with-
drawing substituents (emission maxima ca. 600 nm). The higher
energy of the 3MLCT state results in more intense luminescence,
in accordance with the energy-gap law, as higher overtones of
molecular vibrations will be needed to effect non-radiative decay.
For Pt–PBN–Ph, no luminescence from the naphthyl unit was
detected, indicating (as for Re–PBN) efficient energy transfer
from the aromatic antenna unit to the metal centre.
change in the wavelength (and not just the intensity) of emission
when the pyridyl residues interact with a metal cation may allow
use of this complex and others like it as a ratiometric sensor for
quantifying metal ions.
The Ru–PB complexes show in the 630–650 nm region the
characteristic luminescence from the 3MLCT excited state,17
at slightly lower energies than [Ru(bpy)3]2+ (consistent with
the reduced energy of the MLCT absorption maximum, i.e.
459 nm rather than 450 nm) and with reduced quantum yields
(typically, 1%, with the exception of Ru–PBA; see below)
and lifetimes of a few hundred nanoseconds. From the onset
of the emission spectra at the high energy-end (570 nm), the
energy of the 3MLCT state for these Ru–PB chromophores can
be estimated as 17500 cm−1, which is slightly lower than the
The strong beneficial effect of electron-withdrawing groups
attached to the acetylide ligands prompted us to examine
the effect of protonation of the pendant pyridyl residues of
Pt–PBPh–Py on the luminescence. These experiments were
performed in MeCN to avoid the possibility of the protonated
complex precipitating from CH2Cl2; the result is shown in Fig. 9.
In MeCN the MLCT absorption maximum is at 390 nm, slightly
blue-shifted compared to the situation in CH2Cl2. On bubbling
HCl vapour to the solution there is a substantial change in the
absorption spectrum in the 300–400 nm region (Fig. 9(a)), with
the MLCT transition moving to 367 nm and approximately
trebling in intensity, in agreement with expectations based on the
acetylide ligand becoming a better p-acceptor/poorer p-donor.
The transition at 340 nm likewise gains substantially in intensity,
although it remains at the same position. The two spectra have
an iso-absorbing point at 396 nm which was used for excitation
in the luminescence spectra. The luminescence maximum of Pt–
PBPh–Py is at 545 nm, with a shoulder on the low-energy side
arising from vibronic effects. Bubbling HCl vapour through the
solution (Fig. 9(b)) resulted in a blue-shift of the luminescence
to 516 nm, with the low-energy shoulder (now at 548 nm) being
more clearly resolved, and in increase in quantum yield of about
15% relative to the non-protonated form. Clearly, protonation
of the pendant pyridyl groups is increasing their electron-
withdrawing ability and consequently increasing the energy and
intensity of luminescent emission. This offers interesting possi-
bilities for use of the molecules as luminescent sensors for metal
ions, since coordination of the pyridyl residues to metal ions will
have a similar electronic effect to protonation. Importantly, a
3
energy of the Re-based MLCT state, the normal pattern for
Re(I) and Ru(II)–diimine complexes.16b,d
For Ru–PBN and Ru–PBPyr, excitation at higher wave-
lengths—into the UV absorption bands of the pendant aromatic
chromophores (at 263 or 317 nm for naphthalene and pyrene
respectively)—also resulted in Ru-based luminescence but
with a slightly reduced quantum yield (by  20% compared to
direct excitation of the Ru–PB chromophore). Also present was
very weak, almost vestigial, luminescence characteristic of the
naphthalene or pyrene units in the 350–400 nm region (Fig. 10).
Energy transfer from the appended organic chromophore singlet
state to the Ru centre is therefore occurring with less than 100%
efficiency, despite the fact that these pendant organic chromo-
phores have singlet energies that are sufficiently high in energy
to act as energy-donors, and are spatially close to the metal
centre. In fact the driving force for energy transfer from 1Naph
1
and Pyr to the metal centre should be greater than in the Re
series, because the MLCT levels of the Ru chromophore are
lower in energy,16b,d so the incompleteness of the energy transfer
is surprising. In Ru–PBPyr, energy transfer in the other direc-
3
tion, from the Ru MLCT level to the triplet pyrene level, is
clearly not a significant quenching pathway since the quantum
yields and lifetimes for Ru-based luminescence of Ru–PBPyr,
Ru–PBN and Ru–PBF are all comparable. This is also surprising
3
given that the Ru-based MLCT state is expected to be higher
in energy than the 3Pyr state by 1000 cm−1. We note, however,
that this gradient is less than that which occurs in Re–PBPyr,
where 3MLCT(Re)→3Pyr was efficient and resulted in complete
quenching of the Re-based luminescence.
Fig. 10 Luminescence spectrum of Ru–PBPyr showing weak residual
pyrene-based luminescence (*) in addition to the 3MLCT luminescence.
Similar behaviour (incomplete energy transfer between
metal and organic chromophores) occurs when anthracene is
the pendant chromophore, in Ru–PBA. In this complex the
metal-centred luminescence at 630 nm is an order of magnitude
less intense than in the other Ru(II) complexes and, depending
on the excitation wavelength, structured fluorescence from the
anthracene singlet excited state can also be seen between 390 and
450 nm (Fig. 11). Excitation spectra recorded monitoring the Ru-
based emission at 630 nm, and the anthracene-based emission at
420 nm, show quite clearly distinct absorption features which
are mutually independent (Fig. 12). Thus, anthracene-based
Fig. 9 Effect of protonation of the pendant pyridyl groups on the
(a) absorption and (b) luminescence spectra of PtPy–PBPh. In (a) the
solid line is the spectrum of the complex on its own; the dashed line
shows the effect of protonation. In (b) the dashed line is the lumine-
scence spectrum of the complex on its own; the dotted line shows the
effect of protonation.
D a l t o n T r a n s . , 2 0 0 4 , 3 6 7 8 – 3 6 8 8
3 6 8 5

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luminescence occurs only on irradiation into the strong anthra-
cene absorption at 260 nm and the weaker absorptions between
340 and 400 nm. Conversely, Ru-based luminescence only occurs
on excitation into those features of the absorption spectrum at
270–350 nm, and at 459 nm, which are characteristic of the
Ru–tris(diimine) chromophore. The two luminescent states (1An
and 3MLCT) are only weakly interacting, with energy transfer
from the anthracene singlet excited state to the Ru centre being
absent (since no sensitised Ru-based emission occurs follow-
ing excitation of anthracene) despite the fact that it is thermo-
dynamically favourable. The very weak anthracene luminescence
nonetheless means that the 1An state is substantially quenched,
which can be ascribed to direct conversion to the (non-emissive)
low-energy triplet state of anthracene (3An) by inter-system
crossing caused by the proximity of a heavy metal ion. In
contrast, the anomalous weakness of the Ru-based luminescence
(Table 4) can only be ascribed to (incomplete) energy transfer to
the 3An level (ca. 14500 cm−1), which is not luminescent under
the conditions of the experiment. Similar behaviour was also
observed for Re–PBA, although in this case quenching of the
3MLCT state by the 3An state was complete rather than partial,
presumably due in part to the slightly higher thermodynamic
gradient in Re–PBA compared to Ru–PBA. Similar behaviour
has been observed in solution for a [Ru(bpy)3]2+/anthracene dyad
in which luminescence from both components was completely
quenched.20
Time-resolved measurements on Ru–PBA showed that the
Ru-based luminescence, although of low intensity, is still reason-
ably long-lived with s = 232 ns. The very weak anthracene-based
luminescence (monitored at 400 nm) is much shorter-lived with
a lifetime of about 4 ns, much less than the value of ca. 11 ns
which is typical for unquenched anthracene fluorescence.21 This,
together with the very low intensity, confirms a high degree of
quenching of the anthracene excited state.
To summarise this section, we can say two things. Firstly, the
metal–PB chromophores show luminescence properties from
3
their MLCT excited states which are comparable in energy,
intensity and lifetime to those of the better-known metal–
bipyridyl chromophores. Secondly, the complexes with pendant
aromatic chromophores, particularly anthracene and pyrene,
show complex photophysical behaviour due to inter-component
energy-transfer processes between the organic and metal-PB
chromophores, which are more efficient (in both directions) for
the Re series than the Ru series. The slightly lower energies of
the MLCT states for the Ru series compared to the Re series may
explain why energy transfer from the metal 3MLCT state to the
3Pyr or 3An state is incomplete or absent in the Ru series, because
the gradient is reduced. However, this ordering of energy levels
should result in faster energy transfer in the other direction,
1
1
from Pyr or An to the metal centre, which is clearly not the
case; this energy transfer is also poorer for the Ru series, being
incomplete in Ru–BPPyr and absent in Ru–PBA.
A plausible explanation for this is the different spatial locali-
sation of the MLCT states in the Ru and Re series. For the Re
series there is only one diimine ligand, the PB unit; the 1MLCT
and 3MLCT excited states must necessarily involve this ligand.
Accordingly the effective distance for energy transfer between
the MLCT states and organic chromophores is small, since the
pyrene, anthracene and naphthalene units are directly attached
to the PB ligand with only a single atom (methylene) spacer. In
the Ru series, however, there are two different types of diimine
ligand; the PB unit and the bipyridyl ligands. If the MLCT states
involve the bipyridyl ligands and not the PB ligand, then they
will be localised in a region of space much further away from
the appended organic chromophore, giving a greater effective
distance for energy transfer between the components. In the
original paper describing [Ru(bpy)2(PB)]2+ complexes,4 Haga
pointed out that benzimidazole is a slightly better p-donor/
poorer p-acceptor than bipyridine, and our electrochemical
data (described below) confirm that bpy is a slightly better
electron-acceptor than PB in a homologous pair of complexes.
This means that, in the mixed-ligand series of Ru complexes, the
lowest-energy 3MLCT state should indeed be localised on one of
the bipyridyl ligands rather than the PB unit, and the effective
energy-transfer distance to or from the organic chromophores
will be much larger than in the Re series. An exactly similar effect
was described recently by Constable et al., in which the rate of
energy transfer between {Ru(terpy)2}2+ and {Os(terpy)2}2+
Fig. 11 Luminescence spectra for Ru–PBA, measured using excitation
at 370 nm (solid line) and 269 nm (dashed line).
3
units depended on whether the Ru-based MLCT excited state
involved the terpyridine ligand closer to the Os(II) centre or the
one which was more remote.22
Redox properties
The redox properties of representative members of each series
were measured by cyclic voltammetry in CH2Cl2 solution using a
scan rate of 0.5 V s−1 (Fig. 13). Ru–PBN showed a reversible one-
electron process at +0.86 V vs. Fc/Fc+ which we ascribe to the
Ru(II)/Ru(III) couple (cf. +0.89 V for [Ru(bpy)3]2+ under the same
conditions). There are also two reversible one-electron processes
at −1.72 and −1.98 V, clearly ligand-based; the first two reduc-
tions of [Ru(bpy)3]2+ occur at −1.72 and −1.92 V vs. Fc/Fc+.
Given the fact that the PB unit is a slightly poorer p-acceptor
than bpy,4 it is likely that these two reductions are both bpy-
centred. Similar results were found for Ru–PBA (metal-based
Fig. 12 (a) UV/Vis absorption spectrum of Ru–PBA; (b) excitation
spectra of Ru–PBA, recorded using the anthracene-based emission at
400 nm (solid line) and the ruthenium-based emission at 630 nm (dashed
line).
3 6 8 6
D a l t o n T r a n s . , 2 0 0 4 , 3 6 7 8 – 3 6 8 8

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process, +0.83 V; ligand-based processes, −1.76 and −1.95 V vs.
Fc/Fc+). In this case however the ligand-based processes showed
weak return waves and are hence irreversible. In addition, for
Ru–PBA a second anodic process was present at +1.03 V, which
was the same intensity as the others, i.e. a one-electron process.
It was not however fully reversible, with the return wave being
smaller than the outward wave. This process can be assigned to
oxidation of the pendant anthracene unit to its radical cation
(irreversible oxidation of anthracene to its radical cation in
CH2Cl2 has been reported to occur at +1.04 V vs. Fc/Fc+).23 In
Ru–PBPyr the Ru(II)/Ru(III) couple is obscured by an electrode
absorption/desorption process, but two ligand-based couples
are at −1.70 and −1.97 V, with the first being reversible but the
second being broad (peak-peak separation,  200 mV).
on the outward scan being −1.94 V. The irreversible anthracene-
based oxidation was also present, with the peak potential of the
outward wave being at +1.33 V.
Importantly, the PB-based redox couple of members of
the Re(I) series occur at slightly more negative potentials (by
ca. 100 mV) than the bpy-based redox couple of [Re(bpy)-
(CO)3Cl],16a which allows a direct comparison between the
electronic properties of coordinated PB vs. bpy ligands. These
electrochemical data confirm that PB is a poorer p-acceptor
than bpy (cf. Haga’s initial paper).4 This supports the sugges-
tion that in the Ru-based complexes the MLCT excited states
are localised on the bpy ligands rather than the PB ligand, which
accounts for the poor energy transfer between the MLCT states
and the pendant aromatic chromophores, as described above.24
Conclusions
The main general conclusion to arise from this work is that
PB-based ligands provide redox and photophysical properties
in their complexes which are generally comparable to those
obtained with the much better studied 2,2-bipyridyl ligands,
although with slightly lower 3MLCT energies. The ease of func-
tionalisation of the PB units compared to bipyridine provides
a considerable advantage for synthesis of multi-chromophoric
complexes. Two further points of interest also arose. Firstly, the
efficiency of photoinduced energy transfer between the metal
centre and the pendant organic luminophore appears to depend
3
on the spatial localisation of the MLCT excited state of the
metal centre. If the 3MLCT excited state involves the PB unit to
which the organic luminophore is connected (as in the Re series),
energy transfer occurs over a short distance and is virtually
quantitative; if however the 3MLCT excited state is localised on
an ancillary (bpy) ligand away from the PB unit and its pendant
luminophore, the inter-component energy transfer occurs over a
larger distance and is incomplete. Secondly, the Pt series showed
particularly intense and long-lived luminescence, especially with
electron-withdrawing substituents on the acetylide ligands. With
2-pyridyl groups on the acetylides, protonation/deprotonation of
these pyridyl units provides a reversible way of increasing or de-
creasing the electron-withdrawing effect of the substituents and
hence modulating the luminescence properties of the complex.
Acknowledgements
We thank the Royal Society/NATO for a post-doctoral fellow-
ship (N. M. S.) and EPSRC for a post-doctoral fellowship
(Z. R. B.) and a PhD studentship (T. L. E.).
Fig. 13 Cyclic voltammograms of (a) Pt–PBE–Ph; (b) Re–PBPh;
(c) Ru–PBN in CH2Cl2 at a scan rate of 500 mV s−1 with a Pt disk work-
ing electrode. * denotes the redox couple of ferrocene which was added
as an internal reference.
Members of the Pt(II) series all showed a reversible one-
electron couple at negative potential characteristic of a ligand-
based reduction; the potentials are −1.83, −1.80 and −1.77 V
vs. Fc/Fc+ for Pt–PBPh–Py, Pt–PBE–Ph and Pt–PBE–CF3,
respectively. Wholly irreversible oxidations are present
between +0.8 and +1 V vs. Fc/Fc+. For comparison, members
of the Pt(diimine)(CCR)2 series show a ligand-based reduction
on the bipyridyl or phenanthroline unit at ca. −1.7 to −1.8 V
vs. Fc/Fc+, and (sometimes) an irreversible oxidation at around
+0.8 V vs. Fc/Fc+.18a It is clear from this, as from the measure-
ments on the Ru series, that the PB ligands confer very similar
electronic properties on their complexes to bipyridine ligands.
Members of the Re(I) series showed irreversible Re(I)/Re(II)
processes at positive potentials, and ligand-based processes
at negative potentials which are reversible at high scan rates
(500 mV s−1). This behaviour is characteristic of related Re(I)–
bipyridyl complexes.16a For Re–PBPh the metal-based couple is
completely irreversible (no return wave); the peak of the outward
wave occurs at +1.96 V vs. Fc/Fc+. The reversible ligand-centred
couple occurs at −1.84 V. Re–PBPyr behaves similarly, with an
irreversible oxidation showing evidence of electrode adsorp-
tion, but a ligand-centred couple at −1.82 V. For Re–PBA the
metal-based irreversible process is at +0.96 V, and this time the
ligand-based process is also irreversible with the peak potential
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