p-carborane: A new cage spacer for photoactive metal polypyridine dyads

Article abstract of DOI:10.1021/ic060327m

The first example of a binuclear ruthenium complex involving the p-carborane framework in the bridging ligand is reported. The bridging ligand is a symmetric linear array comprising a central p-carborane unit, two p-phenylene spacers, and two 5-yl-2,2′-bipyridine coordinating units. A homobinuclear Ru^II complex, with 2,2′-bipyridine as peripheral ligands, was synthesized and characterized. The Ru^II-Ru^III mixed-valence species, obtained by partial oxidation, has been investigated with steady-state and time-resolved techniques in CH₃CN. The rate of photoinduced electron transfer is 2.3 × 10⁸ s^-1.

Full text of DOI:10.1021/ic060327m

Inorg. Chem. 2006, 45, 4331−4333
p-Carborane: A New Cage Spacer for Photoactive Metal Polypyridine
Dyads
Marco Ghirotti,† Peter F. H. Schwab,† M. Teresa Indelli,† Claudio Chiorboli,‡,§ and
Franco Scandola*,†,‡,§
Dipartimento di Chimica, UniVersita` di Ferrara, INSTM Sezione di Ferrara, and
ISOF-CNR Sezione di Ferrara, 44100 Ferrara, Italy
Received February 27, 2006
The first example of a binuclear ruthenium complex involving the
p-carborane framework in the bridging ligand is reported. The
bridging ligand is a symmetric linear array comprising a central
of novel systems containing p-carborane as a structural motif.
Notably, Hawthorne and co-workers synthesized a variety
of supramolecular species containing carborane derivatives
assembled through appropriate metal centers.6,7
p-carborane unit, two p-phenylene spacers, and two 5-yl-2,2
bipyridine coordinating units. A homobinuclear RuII complex, with
2,2 -bipyridine as peripheral ligands, was synthesized and char-
acterized. The RuII RuIII mixed-valence species, obtained by partial
′-
The p-carborane unit has recently attracted special attention
in relation to electron-transfer processes. Wade et al. studied
the transmission of the electronic effect through p-carborane
substituted with aryl electron-donor and -acceptor moieties,
monitoring the 13C NMR chemical shifts and the UV-vis
spectra.8 With electrochemical experiments on a series of
η-CpFe(CO)2-substituited p-carborane derivatives, Haw-
thorne measured the extent of electronic communication
through the carborane bridge.9,10 Analogous electrochemical
results were obtained by Low and co-workers on p-carborane
functionalized at the carbon vertexes by cobalt clusters.11 It
has recently been suggested that σ-bonded carbon cage
structures may be used as electron tunnel barriers in
molecular electronic circuits.12-14 In this context, the matrix
element relevant to electron tunneling through p-carborane
has been calculated.15
′
−
oxidation, has been investigated with steady-state and time-resolved
techniques in CH3CN. The rate of photoinduced electron transfer
1
is 2.3
×
108 s-
.
Oligonuclear complexes based on ruthenium(II) and
osmium(II) polypyridine components are interesting systems
for photochemical energy conversion and photonic devices.1
In the design of these systems, the bridging ligands are
crucial not only for their structural role but also for providing
electronic coupling between the linked units, thus permitting
intercomponent processes. Rigid rodlike molecules2 are
attractive building blocks for the construction of supra-
molecular arrays. Among these units, 1,12-dicarba-closo-
dodecaborane (1,12-C2B10H12, known as p-carborane) is
particularly interesting.3 The thermal stability, general chemi-
cal inertness, and UV transparency of these units, together
with their ability to form extended rodlike structures (car-
borods),4 have raised considerable interest in the synthesis2,4,5
We have focused our effort in the construction of poly-
nuclear complexes in which photoactive and redox-active
(4) (a) Yang, X.; Jiang, W.; Knobler, C. B.; Hawthorne, M. F. J. Am.
Chem. Soc. 1992, 114, 9719. (b) Herzog, A.; Jalisatgi, S. S.; Knobler,
C. B.; Wedge, T. J.; Hawthorne, M. F. Chem.sEur. J. 2005, 11, 7155.
(5) Muller, J.; Base, K.; Magnera, T.; Michl, J. J. Am. Chem. Soc. 1992,
114, 9721.
(6) Zheng, Z.; Knobler, C. B.; Mortimer, M. D.; Kong, G.; Hawthorne,
M. F. Inorg. Chem. 1996, 35, 1235.
(7) Jude, H.; Disteldorf, H.; Fischer, S.; Wedge, T.; Hawkridge, A. M.;
Arif, A. M.; Hawthorne, M. F.; Muddiman, D. C.; Stang, P. J. Am.
Chem. Soc. 2005, 127, 12131.
* To whom correspondence should be addressed. E-mail: snf@unife.it.
† Universita´ di Ferrara.
‡ ISOF-CNR Sezione di Ferrara.
§ INSTM Sezione di Ferrara.
(1) (a) Balzani, V.; Juris, A.; Venturi, M.; Campagna, S.; Serroni, S. Chem.
ReV. 1996, 96, 759. (b) Harriman, A.; Ziessel, R. Chem. Commun.
1996, 15, 1707. (c) Barigelletti, F.; Flamigni, L. Chem. Soc. ReV. 2000,
29, 1. (d) Scandola, F.; Chiorboli, C.; Indelli, M. T.; Rampi, M. A. In
Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH:
Weinheim, Germany, 2001; Vol. III, Chapter 2.1, p 337. (e) De Cola,
L.; Belser, P. In Electron Transfer in Chemistry; Balzani, V., Ed.;
Wiley-VCH: Weinheim, Germany, 2001; Vol. V, Chapter 3, p 97.
(f) Huynh, M. H. V.; Dattelbaum, D. M.; Meyer, T. J. Coord. Chem.
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Top. Curr. Chem., 2005, 257, 63.
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(9) Bitner, T. W.; Wedge, T. J.; Hawthorne, M. F.; Zirk, J. I. Inorg. Chem.
2001, 40, 5428.
(10) Wedge, T. J.; Herzog, A.; Huertas, R.; Lee, M. W.; Knobler, C. B.;
Hawthorne, M. F. Organometallics 2004, 23, 482.
(11) Fox, M. A.; Paterson, M. A. J.; Nervi, C.; Galeotti, F.; Puschmann,
H.; Howard, J. A. K.; Low, P. J. Chem. Commun. 2001, 1610.
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(13) Berlin, Y. A.; Hutchison, G. R.; Rempala, P.; Ratner, M. A.; Michl,
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(14) Pati, R.; Karna, S. P. Phys. ReV. B 2004, 69, 155419.
(15) Pati, R.; Pineta, A. C.; Pandey, R.; Karna, S. P. Chem. Phys. Lett.
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and references cited therein.
(3) Bregadze, V. Chem. ReV. 1992, 92, 209.
10.1021/ic060327m CCC: $33.50
Published on Web 05/06/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 11, 2006 4331

COMMUNICATION
Scheme 1
metal polypyridine units are linked by bridges based on
p-carborane units. For this purpose, we have made use of
the known reactivity of carboranes,3,7,16,17 which can be
functionalized at the C-H vertexes via metalation. Herein,
we report the synthesis of the homonuclear RuII dimer, RuII‚
3‚RuII (Scheme 1). The p-carborane reacts with 2.2 equiv
of 4-bromoiodobenzene,18 giving the bis-substitution product
119 (80% yield). The ligands 2 and 3 were synthesized by a
Pd(PPh3)4-catalyzed Stille cross-coupling protocol: the reac-
tion between the dibromo intermediate 1 and 2.2 equiv of
the trimethyltin derivative of the 5-bromobipyridine20 in
m-xylene gave a mixture of the bridging ligand 3 (yield ca.
20%) and the ligand 2 (yield ca. 10%).21 The RuII complexes
were prepared by reacting 2 and 3 with 1.1 and 2.2 equiv,
respectively, of Ru(bpy)2Cl2 in refluxing methanol/chloro-
form (1:1). The mononuclear complex RuII‚2 and the
homonuclear dyad RuII‚3‚RuII were purified by size-
exclusion chromatography (Sephadex LH20, methanol as the
eluent).22
2+
Figure 1. Absorption spectra of RuII‚3‚RuII (straight line), Ru(bpy)3
(dotted line), and the bridging ligand 3 (dashed line) in acetonitrile.
and two irreversible24 reductive waves with Ep,c ) -1.45
and -1.82 V were detected. The oxidative wave involves
metal center oxidation and is bielectronic in nature.25 In
analogy with the assignments for similar compounds,26 the
first reduction is thought to be based on the bridging ligand
whereas the following irreversible reduction waves are based
on the peripheral bipyridine ligands. The bridging ligand
reduction has the same wave amplitude as the metal oxidation
and is thus a two-electron process as well. Altogether, the
electrochemical results are consistent with a very weak
degree of electronic communication between the ruthenium
centers and between the two halves of the bridging ligand
through the p-carborane group.27-29
Cyclic voltammetry of RuII‚3‚RuII in acetonitrile showed
a single reversible oxidative wave with E1/2 ) 1.28 V vs
SCE (∆Ep ) 140 mV).23 In the reductive region, a single
reversible wave E1/2 ) -1.24 V vs SCE (∆Ep ) 90 mV)
(16) Coult, R.; Fox, M. A.; Gill, W. R.; Herbertson, P. L.; Hugh, J. A.;
MacBride, W. K. J. Organomet. Chem. 1993, 462, 19.
(17) Schoberl, U.; Magnera, T. F.; Harrison, R. M.; Fleischer, F.; Pflug, J.
L.; Schwab, P. F. H.; Meng, X.; Lipiak, D.; Noll, B. C.; Allured, V.
S.; Rudalevige, T.; Lee, S.; Michl, J. J. Am. Chem. Soc. 1997, 119,
3907.
(18) We have tested two synthetic ways: without catalyst16 and with Pd
catalyst.17 The two methods gave equivalent results. We choose the
way without the catalyst for practical reasons.
The absorption spectra of ligand 3 and RuII‚3‚RuII are
shown in Figure 1, together with that of Ru(bpy)3 (for
purposes of comparison). The absorption spectrum of RuII‚
3‚RuII is typical for a ruthenium(II) polypyridine complex.
2+
(19) The product is purified by sublimation from the p-carborane unreacted
1
and monosubstituted product. 1: H NMR (CDCl3) δ 7.30 (dd, 4H),
7.10 (dd, 4H), 1.2-3 (10H, BHCb); 11B NMR (CDCl3) δ 12.320 (s,
10B).
(20) This reagent was made with a stoichiometric amount of BuLi and a
slight excess of Me3SnCl in dichloromethane at -78 °C, as described
by: Pugliesi, A.; et al. Eur. J. Org. Chem. 2003, 1552.
(23) Cyclic voltammetry was performed in argon-deaerated acetonitrile at
298 K with TBAPF6 as the electrolyte (0.1 M), scan rate 100 mV s-1
,
SCE as the reference electrode, and glassy carbon as the working
electrode. Identical results were obtained with Pt as the working
electrode.
(21) Bridging ligand 3 precipitated as a white solid. The ligand 2 was
isolated from the reaction mixture by preparative thin-layer chroma-
tography on aluminium oxide (100:2:1 cyclohexane/ethyl acetate/
triethylamine as the eluent). 2: 1H NMR (CDCl3) δ 8.85 (s, H6′), 8.70
(d, H6), 8.40 (t, H3, H3′), 7.95 (d, H4′), 7.85 (t, H4), 7.50 (d, 2Hph),
7.35 (m, 4Hph, H5), 7.10 (d, 2Hph); 11B NMR (CDCl3) δ 12.170 (s,
10B). 3: 1H NMR (CDCl3) δ 8.90 (s, 2H6′), 8.70 (d, 2H6), 8.50 (m,
2H3, 2H3′), 8.00 (d, 2H4′), 7.85 (t, 2H4), 7.55 (d, 4Hph), 7.35 (d, 4Hph),
7.30 (m, 2H5); 11B NMR (CDCl3) δ 12.170 (s, 10B).
(24) The irreversible behavior is likely caused by electrode adsorption.
(25) Comparative experiments with, e.g., (bpy)2Ru(bbpe)(bpy)24+ (bbpe )
1,2-bis[4-(4′-methyl-2,2′-bipyridinyl)]ethane), yield identical wave
amplitudes.
(26) In a related binuclear complex, where the bipyridine moieties of the
bridging ligand also carry a phenyl substituent in the 5 position, the
first reduction wave occurs at -1.19 V (Warnmark, K.; Baxter, P. N.
W.; Lehn, J.-M. Chem. Commun. 1998, 993).
(27) The separation of oxidation waves is usually taken as a measure
(among other factors) of the degree of electronic communication
between the redox centers.28 It has been pointed out, however, that
for some strongly coupled systems such a correlation may not be
valid.29
(22) RuII‚2: 1H NMR (CD3OD) δ 8.70 (m, 6H3,3′), 8.40 (d, 1H4′), 8.20
(m, 5H4), 7.90-7.70 (m, 6H6,6′), 7.60-7.20 (m, 5H5,5′ 8Hph); 11B NMR
(CD3OD) δ 8.37 (s, 10B); ESI-MS m/z 472 (M - 2PF6)2+ (main peak).
RuII‚3‚RuII: 1H NMR (CD3OD) δ 8.70 (m, 12H3,3′), 8.34 (d, 2H4′),
8.12 (m, 10H4), 7.90-7.70 (m, 12H6,6′), 7.50 (m, 10H5,5′), 7.32 (m,
8Hph); 11B NMR (CD3OD) δ 8.37 (s, 10B); ESI-MS m/z 358 (M -
4PF6)4+ (main peak).
(28) Creutz, C. Prog. Inorg. Chem. 1983, 30, 1.
4332 Inorganic Chemistry, Vol. 45, No. 11, 2006

COMMUNICATION
red-shifted and somewhat intensified, so as to almost hide
the ground-state bleach. These differences confirm the notion
that the lowest MLCT state monitored in transient absorption
involves the bridging ligand rather than the terminal ones.
Very similar results are obtained for the mononuclear
compound, RuII‚2. In both cases, the transient absorption
decayed with the same lifetime as emission.
Finally, the homonuclear RuII‚3‚RuII was partially oxi-
dized to obtain the mixed-valence compound RuII‚3‚RuIII.
This reaction was performed using CeIV as the oxidant31 in
acidified acetonitrile (0.1 M trifluoroacetic acid). The oxida-
tion was monitored by the decrease in the MLCT band
intensity at 450 nm and was stopped when the RuII
concentration was reduced to half of the initial value. Because
the two metal units have the same oxidation potential, the
process yields a 1:2:1 distribution of the RuII‚3‚RuII, RuIII‚
3‚RuII, and RuIII‚3‚RuIII species. Of these species, RuIII‚
3‚RuIII is nonluminescent, and RuII‚3‚RuII should exhibit
long-lived emission (see above), while for RuIII‚3‚RuII the
emission of the RuII unit is expected to be quenched by
electron transfer to the RuIII center.31 Single-photon-counting
experiments carried out on the semioxidized solutions show,
besides the expected long-lived emission, an intense com-
ponent of 4.4-ns lifetime. Thus, the rate constant of photo-
induced electron transfer across the carborane-based bridging
ligand is 2.3 × 108 s-1. This rate constant can be compared
with the value 1.1 × 109 s-1 obtained by De Cola and co-
workers31 on a related RuII-RuIII binuclear complex, where
the conjugation is effectively interrupted by the rigid
saturated bicyclo[2.2.2]octane spacer. The comparison sug-
gests an analogous insulating role for the two cage spacers,
although a precise evaluation is prevented by other differ-
ences (vinylene instead of p-phenylene additional spacers)
between the two bridges. According to the calculations of
Pati et al., the electron-transfer coupling matrix element is
larger for p-carborane15 than for bicyclo[2.2.2]octane.12 In
the comparison between the two systems, the difference in
electronic coupling through the cage spacer is offset by the
difference in the extent of conjugation through the intermedi-
ate groups.
Figure 2. Transient absorption spectra in acetonitrile of RuII‚3‚RuII (b)
2+
and Ru(bpy)3 (2) taken immediately after the laser pulse (λexc ) 355
nm; half-width ) 10 ns).
Intense ligand-centered bands dominate the UV region. The
π-π* transitions corresponding to the peripheral bipyridine
ligands appear at 288 nm, while an additional shoulder is
around 310 nm (see, for comparison, the spectrum of 3).
The visible region is dominated by moderately intense metal-
to-ligand charge-transfer (MLCT) bands, with a maximum
at 450 nm. The similarity between the MLCT energies of
RuII‚3‚RuII and Ru(bpy)3 confirms again the absence of
2+
extensive delocalization through the bridging ligand. The
spectrum of RuII‚2 is virtually identical with that of RuII‚
3‚RuII, except for the smaller (by a factor of ca. 2) molar
absorptivities.
The complexes RuII‚3‚RuII and RuII‚2 are photolumines-
cent upon excitation at 450 nm in a methanol solution at
em
max
room temperature (λ
) 625 nm) as well as in a 1:1
em
ethanol/methanol rigid matrix at 77 K (λ ) 580 nm). In
max
both cases, the emission is slightly but significantly red-
shifted (ca. 20 and 10 nm, respectively) with respect to that
of Ru(bpy)32+. This suggests that the lowest, emitting MLCT
state involves the bridging ligand rather than the terminal
ones. In both cases, the emission decayed exponentially with
a lifetime of ca. 1 µs in deaerated acetonitrile.
The transient spectrum obtained upon nanosecond laser
flash photolysis of RuII‚3‚RuII (methanol, 10 ns after 355-
nm excitation) is compared in Figure 2 with that of
Ru(bpy)32+. As usual for ruthenium(II) polypyridine com-
plexes,30 the positive absorption bands in the transient spectra
are principally due to π-π* transitions of the ligand radical
anion (formally present in the MLCT excited state). The
transient bleaching at ca. 450 nm results from loss of ground-
state absorption. The apparent bleach in the red portion of
the spectrum is due to MLCT emission. When compared to
Ru(bpy)32+, the main difference is that the high-energy
positive absorption bands of RuII‚3‚RuII are substantially
In conclusion, the electrochemical and spectroscopic
behavior of the new p-carborane-bridged RuII binuclear
species indicate little delocalization through this bridge. The
relatively slow rate obtained for photoinduced electron
transfer in the mixed-valence species confirms the low
“conducting” ability of the p-carborane spacer. Further work
will be directed, in particular, toward the synthesis of
modular bridges and of heteronuclear complexes.
Acknowledgment. The authors are grateful to E. Spettoli
for experimental assistance and to MIUR (Projects PRIN 03
and FIRB-RBNE019H9K) and EC (Grant G5RD-CT-2002-
00776, MWFM) for financial support.
(29) (a) Arnold, D. P.; Heath, G. A.; James, D. A. J. Porphyrins
Phthalocyanines 1999, 3, 5. (b) Yeomans, B. D.; Humphrey, D. G.;
Heath, G. A. J. Chem. Soc., Dalton Trans. 1997, 4153.
IC060327M
(30) Braterman, P. S.; Harriman, A.; Heath, G. A.; Yellowlees, L. J. J.
Chem. Soc., Dalton Trans. 1983, 1801.
(31) De Cola, L.; Balzani, V.; Barigelletti, F.; Flamighi, L.; Belser, P.;
von Zelewsky, A.; Frank, M.; Vogtle, F. Inorg. Chem. 1993, 32, 5228.
Inorganic Chemistry, Vol. 45, No. 11, 2006 4333