Diorganoruthenium complexes incorporating noninnocent [C6H 2(CH2ER2)2-3,5]2 2-- (E = N, P) bis-pincer bridging ligands: Synthesis, spectroelectrochemistry, and DFT studies

Article abstract of DOI:10.1021/ic701501q

The dinuclear complex [(tpy)Ru^II(PCP-PCP)Ru^II(tpy)] Cl₂ (bridging PCP-PCP = 3,3′,5,5′- tetrakis(diphenylphosphinomethyl)biphenyl, [C₆H₂(CH ₂PPh₂)₂-3,5]₂^2-) was prepared via a transcyclometalation reaction of the bis-pincer ligand [PC(H)P-PC(H)P] and the Ru(II) precursor [Ru(NCN)(tpy)]Cl (NCN = [C ₆H₃(CH₂NMe₂)₂-2,6] ^-) followed by a reaction with 2,2′:6′,2″- terpyridine (tpy). Electrochemical and spectroscopic properties of [(tpy)Ru ^II(PCP-PCP)Ru^II-(tpy)]Cl₂ are compared with those of the closely related [(tpy)Ru^II(NCN-NCN)Ru ^II(tpy)](PF₆)₂ (NCN-NCN = [C₆H ₂(CH₂-NMe₂)₂-3,5]₂ ^2-) obtained by two-electron reduction of [(tpy)Ru ^III(NCN-NCN)Ru^III(tpy)](PF₆)₄. The molecular structure of the latter complex has been determined by single-crystal X-ray structure determination. One-electron reduction of [(tpy)Ru ^III(NCN-NCN)Ru^III(tpy)](PF₆)₄ and one-electron oxidation of [(tpy)Ru^II(PCP-PCP)Ru^II(tpy)] Cl₂ yielded the mixed-valence species [(tpy)Ru^III(NCN-NCN) Ru^II(tpy)]³⁺ and [(tpy)Ru^III(PCP-PCP)Ru ^II(tpy)]³⁺, respectively. The comproportionation equilibrium constants K_c (900 and 748 for [(tpy)Ru ^III(NCN-NCN)Ru^III(tpy)]⁴⁺ and [(tpy)Ru ^II(PCP-PCP)Ru^II(tpy)]²⁺, respectively) determined from cyclic voltammetric data reveal comparable stability of the [Ru^III-Ru^II] state of both complexes. Spectroelectrochemical measurements and near-infrared (NIR) spectroscopy were employed to further characterize the different redox states with special focus on the mixed-valence species and their NIR bands. Analysis of these bands in the framework of Hush theory indicates that the mixed-valence complexes [(tpy)Ru^III(PCP-PCP)Ru^II(tpy)]³⁺ and [(tpy)Ru^III(NCN-NCN)Ru^II(tpy)]³⁺ belong to strongly coupled borderline Class II/Class III and intrinsically coupled Class III systems, respectively. Preliminary DFT calculations suggest that extensive derealization of the spin density over the metal centers and the bridging ligand exists. TD-DFT calculations then suggested a substantial MLCT character of the NIR electronic transitions. The results obtained in this study point to a decreased metal-metal electronic interaction accommodated by the double-cyclometalated bis-pincer bridge when strong σ-donor NMe ₂ groups are replaced by weak σ-donor, π-acceptor PPh ₂ groups.

Full text of DOI:10.1021/ic701501q

Inorg. Chem. 2007, 46, 11133−11144
Diorganoruthenium Complexes Incorporating Noninnocent
2-
[C6H2(CH2ER2)2-3,5]2 (E
) N, P) Bis-Pincer Bridging Ligands:
Synthesis, Spectroelectrochemistry, and DFT Studies
Marcella Gagliardo,† Catelijne H. M. Amijs,† Martin Lutz,‡ Anthony L. Spek,‡,§
,
X
Remco W. A. Havenith,| Frantisˇek Hartl,*,# Gerard P. M. van Klink,†, and Gerard van Koten*,†
Organic Chemistry and Catalysis, Faculty of Science, Utrecht UniVersity, Padualaan 8, 3584 CH
Utrecht, The Netherlands, Crystal and Structural Chemistry, Faculty of Science, Utrecht
UniVersity, Padualaan 8, 3584 CH Utrecht, The Netherlands, Theoretical Chemistry Group,
Faculty of Science, Utrecht UniVersity, Padualaan 8, 3584 CH Utrecht, The Netherlands, and
Van’t Hoff Institute for Molecular Sciences, UniVersity of Amsterdam, Nieuwe Achtergracht 166,
1018 WV Amsterdam, The Netherlands
Received July 27, 2007
The dinuclear complex [(tpy)RuII(PCP
−
PCP)RuII(tpy)]Cl2 (bridging PCP
−
PCP
)
3,3
′
,5,5
′
-tetrakis(diphenylphosphi-
2
nomethyl)biphenyl, [C6H2(CH2PPh2)2-3,5]2 -) was prepared via a transcyclometalation reaction of the bis-pincer
ligand [PC(H)P-PC(H)P] and the Ru(II) precursor [Ru(NCN)(tpy)]Cl (NCN
)
[C6H3(CH2NMe2)2-2,6]-) followed by a
PCP)RuII-
[C6H2(CH2-
−
NCN)RuIII(tpy)](PF6)4. The molecular structure
reaction with 2,2′:6′ −
,2′′-terpyridine (tpy). Electrochemical and spectroscopic properties of [(tpy)RuII(PCP
(tpy)]Cl2 are compared with those of the closely related [(tpy)RuII(NCN
−
NCN)RuII(tpy)](PF6)2 (NCN
−NCN
)
2
NMe2)2-3,5]2 -) obtained by two-electron reduction of [(tpy)RuIII(NCN
of the latter complex has been determined by single-crystal X-ray structure determination. One-electron reduction
of [(tpy)RuIII(NCN
−
NCN)RuIII(tpy)](PF6)4 and one-electron oxidation of [(tpy)RuII(PCP
−
PCP)RuII(tpy)]Cl2 yielded the
+
mixed-valence species [(tpy)RuIII(NCN
−
NCN)RuII(tpy)]3 and [(tpy)RuIII(PCP
−
PCP)RuII(tpy)]3+, respectively. The
comproportionation equilibrium constants Kc (900 and 748 for [(tpy)RuIII(NCN
PCP)RuII(tpy)]2+, respectively) determined from cyclic voltammetric data reveal comparable stability of the [RuIII
−
NCN)RuIII(tpy)]4+ and [(tpy)RuII(PCP
−
−
RuII] state of both complexes. Spectroelectrochemical measurements and near-infrared (NIR) spectroscopy were
employed to further characterize the different redox states with special focus on the mixed-valence species and
their NIR bands. Analysis of these bands in the framework of Hush theory indicates that the mixed-valence complexes
+
+
[(tpy)RuIII(PCP PCP)RuII(tpy)]3 and [(tpy)RuIII(NCN NCN)RuII(tpy)]3 belong to strongly coupled borderline Class
− −
II/Class III and intrinsically coupled Class III systems, respectively. Preliminary DFT calculations suggest that extensive
delocalization of the spin density over the metal centers and the bridging ligand exists. TD-DFT calculations then
suggested a substantial MLCT character of the NIR electronic transitions. The results obtained in this study point
to a decreased metal−metal electronic interaction accommodated by the double-cyclometalated bis-pincer bridge
when strong -donor NMe2 groups are replaced by weak
σ
σ
-donor, -acceptor PPh2 groups.
π
Introduction
covalently linked by π-conjugated organic spacer groups are
a topic of active research.1 Such systems are exploited to
study the degree of electronic communication between
Experimental and theoretical studies of organometallic
compounds containing two or more multivalent metal centers
| Theoretical Chemistry Group, Utrecht University. Present address:
Electronic Structure of Materials, Institute for Molecules and Materials,
Radboud University Nijmegen, Toernooiveld 1, 6525 ED Nijmegen
To whom correspondence pertaining to theoretical calculations should
be addressed. E-mail: r.havenith@science.ru.nl.
* To whom correspondence should be addressed. F.H.: phone, +31-
20-525-6450; fax, +31-20-525-6456; e-mail, f.hartl@uva.nl. G.v.K.: phone,
+31-30-253-3120; fax: +31-30-252-3615; e-mail, g.vankoten@uu.nl.
† Organic Chemistry and Catalysis, Utrecht University.
‡ Crystal and Structural Chemistry, Utrecht University.
§ To whom crystallographic inquiries should be directed. E-mail:
a.l.spek@uu.nl.
# University of Amsterdam.
X Present address: DelftChemTech, Faculty of Applied Sciences, Delft
University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands.
10.1021/ic701501q CCC: $37.00
Published on Web 12/01/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 26, 2007 11133

Gagliardo et al.
Scheme 1. Synthesis of the Dinuclear Complexes Containing the Double-Cyclometalated Bis-Pincer Bridging Ligand [NCN-NCN]a
a Reagents and experimental conditions: (i) 2.5 equiv of CuCl2, MeOH, reflux, 5 h; (ii) NH4PF6, H2O, rt; (iii) N2H4‚H2O, H2O/MeCN, rt; (iv) [Cp2Fe]PF6,
H2O/MeCN, rt.
metallic redox units and identify potential candidates for
diverse materials applications, for example, in the emerging
field of molecular electronics, telecommunications, and
aerospace.2
tempts to address the issue of the extensive mixing of metal
and ligand orbitals and electronic coupling in the mixed-
valence redox state require a thorough analysis of experi-
mental (electrochemical, spectral, and magnetic) data and
support from quantum chemical calculations being aware of
their limitations (DFT).
There are several factors affecting the degree of electronic
coupling and thereby the metal-metal interactions and
electron-transfer properties in such species. The nature of
the metallic termini, the bridging and ancillary ligands, and
the solvent play a crucial role. In the past decades, a plethora
of diverse diruthenium systems [RuLx](µ-bridge)[RuLx]
were synthesized and studied.3 The fundamental question
about the role of the π-conjugated bridging ligand in the
electronic communication between the metal centers has
evoked an increasing number of spectroelectrochemical
studies aimed at spectroscopic characterization of these
homometallic species in several electrochemically accessible
redox states with special focus on the so-called mixed-
valence (Ru(II)/Ru(III)) state. Among these systems, ex-
amples of organometallic dinuclear complexes featuring
unsaturated polyyndiyl (-CtC-)n, polyenediyl (-CHd
CH-)n, aryldiethynyl (-CtC(Ar)CtC-), aryldiethenyl
(-CHdCH(Ar)CHdCH-), and cyanoacetylide (-CtC-
CtN‚‚‚) ligands are of special interest as their oxidation is
often significantly localized also on the bridging ligand.1b,4
Given the substantial contribution of the noninnocent bridg-
ing ligand to the complex HOMO (the target orbital for the
oxidation), the interpretation of the mixed-valence state on
grounds of the classical Hush theory and the Robin and Day
classification scheme becomes ambiguous. Obviously, at-
Apart from the unsaturated carbon chains as bridging
ligands, the complexes [(ttpy)RuII(tpbp)2RuII(ttpy)](PF6)2
(ttpy ) 4′-tolyl-2,2′:6′,2′′-terpyridine; tpbpH2 ) 3,3′,5,5′-
tetrapyridylbiphenyl)5 and [(tpy)RuIII(NCN-NCN)RuIII(tpy)]-
(PF6)4 (4, Scheme 1)6 featuring strong σ-donor, dianionic
carbon-ligating bridging ligands (Chart 1), have also been
reported in the literature as precursors of mixed-valence
complexes absorbing strongly in the NIR region: Kc ) 600,
NIR band at 1620 nm, and ꢀ ) 26 000 M-1 cm-1 and Kc )
1100, NIR band at 1875 nm, and ꢀ ) 33 000 M-1 cm-1. On
the basis of these data, the electronic coupling between the
ruthenium centers in [(tpy)RuIII(NCN-NCN)RuII(tpy)]3+ is
stronger compared to that in [(ttpy)RuIII(tpbp)RuII(ttpy)]3+.
Thus, the extent of electronic communication between the
ruthenium centers in such systems can be tuned by varying
the electron-donating ability of the double-cyclometalated
bis-pincer bridge. With the aim to address the role of the
electronic properties of this bridging ligand in the metal-
metal interaction and the associated redox and spectroscopic
behavior we focused our efforts on the properties of the
analogous mixed-valence complex [(tpy)RuIII(PCP-PCP)-
RuII(tpy)]3+ (17 in Scheme 3, vide infra) obtained by one-
electron oxidation of the novel dinuclear precursor [(tpy)-
RuII(PCP-PCP)RuII(tpy)]Cl2 (16). The latter complex was
synthesized by a selective metalation of the bis-pincer ligand
[PC(H)P-PC(H)P] via a transcyclometalation procedure
followed by reaction with tpy. As part of the spectroelec-
trochemical study, the near-infrared absorption spectrum of
the mixed-valence species [(tpy)RuIII(PCP-PCP)RuII(tpy)]3+
(17) was recorded and compared to that of [(tpy)RuIII(NCN-
NCN)RuII(tpy)]3+ (5). The extent to which replacement of
(1) (a) Molecular Switches; Feringa, B. L., Ed.; Wiley-VCH: Weinheim,
2001. (b) Ren, T. Organometallics 2005, 24, 4854-4870.
(2) (a) Infrared Optical Circuits and Components: Design and Applica-
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Ganguly, B.; Das, A.; Ramakrishna, G.; Palit, D. K.; Ghosh, H. N.
Inorg. Chem. 2005, 44, 2414-2425. (f) Fabrizi De Biani, F.; Dei, A.;
Sangregorio C.; Sorace L. J. Chem. Soc., Dalton Trans. 2005, 3868-
3873. (g) D’Alessandro, D. M.; Topley, A. C.; Davies, M. S.; Keene,
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(4) (a) Cordiner, R. L.; Smith, M. E.; Batsanov, A. S.; Albesa-Jove´, D.;
Hartl, F.; Howard, J. A. K.; Low, P. J. Inorg. Chim. Acta 2006, 359,
946-961. (b) Gao, L.-B.; Liu, S.-H.; Zhang, L.-Y.; Shi, L.-X.; Chen,
Z.-N. Organometallics 2006, 25, 506-512. (c) Klein, A.; Lavastre,
O.; Fiedler, J. Organometallics 2006, 25, 635-643. (d) Roberts, R.
L.; LeGuedennic, B.; Halet, J.-F.; Hartl, F.; Howard, J. A. K.; Low,
P. J. Manuscript in preparation. (e) Maurer, J.; Winter, R. F.; Sarkar,
B.; Za´lisˇ, S. J. Solid State Chem. 2005, 9, 738-749.
11134 Inorganic Chemistry, Vol. 46, No. 26, 2007

Diorganoruthenium Complexes
Scheme 2. Synthesis of the Bis-Pincer Ligand 13a
a Reagents and conditions: (i) (a) 2 equiv of Mg, THF, rt, 3 h; (b) reflux, ca. 15 h; (ii) B(OMe)3, pinacol, AcOH, rt, 1 h; (iii) 7, Na2CO3, [PdCl2(dppf)],
DMF/THF/H2O (1:1:1 v/v), 50 °C, 15 h; (iv) BF3‚Et2O, AcBr, CH2Cl2, reflux, 18 h; (v) (a) HPPh2‚Et2O, nBuLi, THF, -78 °C; (b) rt, 18 h; (vi) HBF4‚Et2O,
rt, 18 h.
Unlike complex 2, in which two anions [CuCl2]- are
Chart 1
packed above and below the planar biphenylene bridge (Ar-
Ar dihedral angle ) 0(4)°) at a distance of ca. 3.5 Å,6a the
molecular structure of 4 displays a torsion angle of 24.2(4)°
between the two phenyl rings (C51-C41-C42-C52). The
torsion angle in 4 is slightly smaller than that found in the
diruthenium(II) derivative 6 (Scheme 1) (36.0(2)°)6b and in
a majority of 4,4′-disubstituted (organic) biphenyls (average
value of 37°).7 The other geometrical parameters of 4 and
the two-electron-reduced 6 are almost identical. Thus, in both
the ‘hard’ NMe2 donor groups by the ‘soft’ PPh2 groups in
the dianionic bridging ligand affects the electron-transfer
[RuIII-RuIII] and [RuII-RuII] species the preferred low-
energy solid-state structure has the phenyl rings twisted with
process in both mixed-valence species has been rationalized
respect to each other. These data indicate that the solid-state
both in the framework of Hush theory and at a quantum
chemical level by DFT and TD-DFT calculations.
planarity of the central biphenylene systems in the [RuIII-
RuIII] complex 2 is not a result of a favored electronic
situation but rather an effect of the crystal packing of the
[CuCl2]- anions.
Results and Discussion
Structural Characterization of [(tpy)RuIII(NCN-NCN)-
RuIII(tpy)](PF6)4 (4). Copper(II)-mediated oxidative coupling
of the readily accessible 18-electron mononuclear building
block [RuII(NCN)(tpy)]Cl (1, Scheme 1) gives the dinuclear
34-electron complex [(tpy)RuIII(NCN-NCN)RuIII(tpy)]-
(CuCl2)4 (2) in good yield (85%) via a modified literature
procedure. The paramagnetic mononuclear para-chlorinated
compound [RuIII{C6H2(CH2NMe2)2-2,6-Cl-4}(tpy)](CuCl2)2
(3) is formed as a minor product (7%).
1
The sharp resonances in the aromatic region of the H
NMR spectrum, associated with the tpy and bridging pincer
ligand, reveal that 4 is diamagnetic. Complex 4 is EPR silent
both at room temperature and at 77 K in the solid state as
well as in MeCN. Its diamagnetic nature has also been
confirmed by magnetic susceptibility measurements (SQUID)
performed in the temperature range 5-300 K. All these data
are indicative of very strong antiferromagnetic coupling8
between the two Ru(III) centers mediated by the bridging
bis(carbon-ligating) biphenylene moiety, which can be
ascribed to a significant interaction between the half-filled
dπ orbital of the Ru(III) centers with occupied high-lying π
orbitals localized on the bridging ligand.9
-
Replacement of the two counterions [CuCl2]- in 2 by PF6
ions yields complex 4, the structure of which has been
determined by single-crystal X-ray structure determination
(Figure 1; selected bond lengths and angles are listed in Table
1). The molecular structure of 4 in the crystal is very similar
to that of 2.6 The two identical ruthenium(III) centers are
embedded in a distorted octahedral environment that com-
prises the terdentate NCN-pincer and tpy ligands coordinated
in a meridional arrangement.
Synthesis of [(tpy)RuII(PCP-PCP)RuII(tpy)]Cl2 (16).
The synthetic protocol employed to obtain the dinuclear
[RuII-RuII] complex 16 in a high yield (ca. 70%) involved
preparation of the novel ligand 13 (Scheme 2) followed by
(7) (a) Cassalone, G.; Mariani, C.; Mugnoli, A.; Simonetta, M. Acta
Crystallogr., Sect. B 1969, 25, 1741-1750. (b) Iwamoto, T.; Miyoshi,
T.; Sasaki, Y. Acta Crystallogr., Sect. B 1974, 30, 292-295. (c) Brock,
C. P.; Kuo, M.-S.; Levy, H. A. Acta Crystallogr., Sect. B 1978, 34,
981-985. (d) Naae, D. G. Acta Crystallogr., Sect. B 1979, 35, 2765-
2768.
(8) Carlin, R. L. Magneto Chemistry; Springer-Verlag: Berlin, 1986.
(9) Kahn, O. In Magneto-Structural Correlations in Exchange Coupled
Systems; Willet, R. D., Gatteschi, D., Kahn, O., Eds.; NATO ASI
Series 37; D. Riedel Publishing Co.: Dordrecht, 1984.
(5) (a) Beley, M.; Collin, J.-P.; Sauvage, J.-P. Inorg. Chem. 1993, 32,
4539-4543. (b) Patoux, C.; Launay, J.-P.; Beley, M.; Chodorowski-
Kimmes, S.; Collin, J.-P.; James, S.; Sauvage, J. -P. J. Am. Chem.
Soc. 1998, 120, 3717-3725.
(6) (a) Sutter, J.-P.; Grove, D. M.; Beley, M.; Collin, J.-P.; Veldman, N.;
Spek, A. L.; Sauvage, J.-P.; van Koten, G. Angew. Chem., Int. Ed.
Engl. 1994, 33, 1282-1285. (b) Steenwinkel, P.; Grove, D. M.;
Veldman, N.; Spek, A. L.; van Koten, G. Organometallics 1998, 17,
5647-5655.
Inorganic Chemistry, Vol. 46, No. 26, 2007 11135

Gagliardo et al.
Figure 1. Displacement ellipsoid plot of complex 4 (50% probability level). Hydrogen atoms, the PF6- counterions, and disordered solvent molecules have
been omitted for clarity.
Scheme 3. Synthesis of the Dinuclear Complexes Containing the Biscyclometalated Bis-Pincer Bridging Ligand [PCP-PCP]a
a Reagents and conditions: (i) C6H6, reflux, 2 d; (ii) tpy, MeOH; (iii) [Ce(OTf)4], MeCN, rt.
Table 1. Selected Bond Lengths and Angles and Torsion Angles in
Complex 4
iodobenzene (7) was reacted with 2 equiv of Mg in THF.
The Grignard reagent 8 was then subsequently treated in THF
bond lengths [Å]
with B(OMe)3 and pinacol to form 9, which is highly soluble
Ru1-C11
Ru1-N11
Ru1-N21
Ru1-N31
Ru1-N41
Ru1-N51
C41-C42
1.914(3)
2.185(2)
2.208(2)
2.084(2)
2.087(2)
2.087(2)
1.459(4)
Ru2-C12
Ru2-N12
Ru2-N22
Ru2-N32
Ru2-N42
Ru2-N52
1.913(3)
2.198(2)
2.194(2)
2.087(2)
2.086(2)
2.095(2)
in organic solvents and, in contrast to the majority of boronic
acids, insoluble in water. A Pd-catalyzed Suzuki cross-
coupling between the pinacol-protected boronic acid 9 and
7 gave 10 in high yield. This sequence was especially
convenient because of the high yields and mild reaction
conditions applied. The ease of this preparation is another
advantage as none of these steps requires tedious purification
procedures. In addition, unreacted 7 can be easily recovered
and reused. Replacement of the MeO groups in 10 by Br
substituents was carried out, following the procedure reported
by Dijkstra et al.11 Thus, reaction of 10 with the Lewis acid
BF3‚Et2O and acetyl bromide in CH2Cl2 at reflux gave 11
in a good yield. Ligand 13 was obtained applying a two-
step procedure previously reported for the preparation of
multi-PCP pincer ligand systems.11 Treatment of 11 in situ
with Li-PPh2(BH3) gave 12, which was subsequently
deprotected with HBF4‚OEt2, yielded bis-pincer ligand 13
as a sticky white solid.
bond angles [deg]
C11-Ru1-
N41
N11-Ru1-
N21
N31-Ru1-
N51
177.34(10)
C12-Ru2-N42
N12-Ru2-N22
N32-Ru2-N52
177.65(10)
160.78(8)
152.96(9)
160.21(8)
152.61(9)
torsion angles [deg]
Ru1-N11-
C71-C21
-34.8(2)
-36.7(2)
24.7(4)
Ru2-N12-C72-
-32.6(2)
-36.1(3)
24.2(4)
C22
Ru1-N21-
C101-C61
C31-C41-
C42-C32
Ru2-N22-C102-
C62
C51-C41-C42-
C52
C31-C41-
C42-C52
-156.0(3)
C51-C41-C42-
C32
-155.1(3)
Mixing 13 with two equivalents of ruthenium precursor
14 ([RuCl(NCN)(PPh3)],12 NCN ) [C6H3(CH2NMe2)2-2,6]-)
resulted in selective transcyclometalation of the two PCP
introduction of ruthenium metal centers via a transcyclom-
etalation (TCM) reaction10 to give the dinuclear complex 15
and its subsequent reaction with tpy to give 16 (Scheme 3).
In the synthesis of 13 (Scheme 2), 3,5-bis(methoxymethyl)-
(11) (a) Dijkstra, H. P.; Steenwinkel, P.; Grove, D. M.; Lutz, M.; Spek, A.
L.; van Koten, G. Angew. Chem., Int. Ed. 1999, 38, 2186-2188. (b)
Dijkstra, H. P.; Meijer, M. D.; Patel, R.; Kreiter, R.; van Klink, G. P.
M.; Lutz, M.; Spek, A. L.; Canty, A. J.; van Koten, G. Organometallics
2001, 20, 3159-3168.
(12) Sutter, J.-P.; James, S. L.; Steenwinkel, P.; Karlen, T.; Grove, D. M.;
Veldman, N.; Smeets, W. J. J.; Spek, A. L.; van Koten, G.
Organometallics 1996, 15, 941-948.
(10) (a) Dani, P.; Karlen, T.; Gossage, R. A.; Smeets, W. J. J.; Spek, A.
L.; van Koten, G. J. Am. Chem. Soc. 1997, 119, 11317-11318. (b)
Albrecht, M.; Dani, P.; Lutz, M.; Spek, A. L.; van Koten, G. J. Am.
Chem. Soc. 2000, 122, 11822-11833. (c) Dani, P.; Albrecht, M.; van
Klink, G. P. M.; van Koten, G. Organometallics 2000, 19, 4468-
4476.
11136 Inorganic Chemistry, Vol. 46, No. 26, 2007

Diorganoruthenium Complexes
Scheme 4. Synthesis of the Ligand 21 and Dinuclear Complexes 23 and 6a
a Reagents and conditions: (i) HNMe2, CH2Cl2, rt, 3 h; (ii) (a) 2.3 equiv of tBuLi, Et2O, -78 °C; (b) rt, 14 h; (iii) [RuCl2(PPh3)], THF, rt, 5 h; (iv) (a)
tpy, MeOH; (b) excess NH4PF6.
sites of ligand 13 and concomitant formation of two
equivalents of the bis(aminoarene) NC(H)N-pincer ligand
(Scheme 3). The 1H NMR resonances revealed two equiva-
lent {(PCP)RuCl(PPh3)} units, which may be attributed to
free rotation about the central C-C bond in the PCP-PCP
ligand. In solution, each ruthenium center shows structural
features similar to that reported for the mononuclear species
[RuCl(PCP)(PPh3)]13 (PCP ) [C6H3(CH2PPh2)2-2,6]-), i.e.,
a distorted square-pyramidal geometry of the ruthenium
coordination sphere, with the PPh3 ligand in the apical
position and the two phosphorus centers of the PCP-pincer
units together with the Cipso atom forming the base of the
square-pyramidal arrangement. The diastereotopic benzylic
protons appear as a set of doublets of virtual triplets.10c The
31P{1H} NMR spectrum consists of a low-field triplet due
to PPh3 and a doublet due to the two equivalent phosphorus
atoms of the PCP-pincer units. Subsequent reaction of the
resulting air- and moisture-sensitive dark green complex 15
with two equivalents of tpy gave 16 as an air-stable orange
of the para-position with respect to the metal center, which
reacts further with copper to give complex cluster aggregates
involving combinations of Cu(I), Cu(II), and halide ions.15
Subsequently, as proposed for the arylcopper aggregates,15
these inner-sphere-activated complexes react with CuCl2 to
give unstable {(NCN)RuIII(tpy)}-containing radicals that
convert to dinuclear species or para-chlorinated species such
as 3 and CuCl via a halide ion transfer oxidation pathway.
The reactivity of 19 toward CuCl2 suggests that the
enhanced π-accepting character of the phosphorus atoms of
the PCP-pincer ligand (∆E1/2 ) 0.16 V vs Fc/Fc+) with
respect to the ‘hard’ σ-donor character of the amine-nitrogen
atom of the NCN-pincer ligand in 1 (∆E1/2 ) -0.08 V vs
Fc/Fc+) is the main factor controlling the activation of the
para position and, in turn, formation of dinuclear complexes
in the presence of CuCl2. However, formation of the chloro-
functionalized complex 20 indicates that the aryl para
position in 19 is somehow activated toward substitution, as
observed for a number of cyclometalated Ru(II) com-
pounds.16
Replacement of the bromine atoms in 11 by dimethyl-
amino groups11 gave the bis-pincer ligand precursor 3,3′,5,5′-
tetrakis[(dimethylamino)methyl]biphenyl 21 (Scheme 4). The
latter compound was already prepared earlier by Lagunas et
al. but in a lower yield through a different synthetic
procedure.17 With compound 21 in hand we also attempted
the alternative synthesis of the dinuclear complex [(tpy)-
RuII(NCN-NCN)RuII(tpy)](PF6)2 (6) via transmetalation of
the dilithium complex 22 with [RuCl2(PPh3)3] to give 23
followed by reaction with tpy (Scheme 4).12 However, this
1
solid in good yield. The H and 31P{1H} NMR data of 16,
similar to those recorded for mononuclear [RuII(PCP)(tpy)]-
Cl14 (19), reveal a high degree of symmetry in solution. The
31P{1H} NMR spectrum exhibits a single resonance at 43.1
ppm for the equivalent phosphorus centers of each {(tpy)-
Ru(PCP)} fragment.
Attempts to synthesize the complex [(tpy)RuIII(PCP-
PCP)RuIII(tpy)](PF6)4 (18) by reacting [Ru(PCP)(tpy)]Cl14
(19) with CuCl2 resulted in formation of the mononuclear,
para-chlorinated complex [RuII{C6H2(CH2PPh2)2-2,6-Cl-4}-
(tpy)](PF6) (20) in ca. 40% yield and desired dinuclear 18
in low yield (15%) only. The yield of 18 remained low even
when a large excess of CuCl2 was used and/or the reaction
time was prolonged.
The mechanism of the Cu(II)-mediated oxidative C-C
coupling, which gave rise to the smooth formation of
dinuclear [RuIII-RuIII] species 4, was not studied in detail,
and the intermediates involved in the process have not been
isolated or identified so far.6b In this proposed mechanism,
the first reaction step must involve oxidation of the ruthenium
center in mononuclear complex 1 and formation of Cu(I)
ions.6 Oxidation of the ruthenium center results in activation
(13) (a) Jia, G.; Lee, H. M.; Williams, I. D. J. Organomet. Chem. 1997,
534, 173-180. (b) Dani, P.; van Klink, G. P. M.; van Koten, G. Eur.
J. Inorg. Chem. 2000, 7, 1465-1470.
(14) Gagliardo, M.; Dijkstra, H. P.; Coppo, P.; De Cola, L.; Lutz, M.; Spek,
A. L.; van Klink, G. P. M.; van Koten, G. Organometallics 2004, 23,
5833-5840.
(15) (a) van Koten, G.; Leusink, A. J.; Noltes, J. G. J. Organomet. Chem.
1975, 84, 117-127. (b) van Koten, G.; Noltes, J. G. J. Organomet.
Chem. 1975, 84, 419-429.
(16) Gagliardo, M.; Snelders D. J. M.; Chase, P. A.; Klein Gebbink, R. J.
M.; van Klink, G. P. M.; van Koten, G. Angew. Chem., Int. Ed. 2007,
46, 8558-8573.
(17) Lagunas, M.-C.; Gossage, R. A.; Spek, A. L.; van Koten, G.
Organometallics 1998, 17, 731-741.
Inorganic Chemistry, Vol. 46, No. 26, 2007 11137

Gagliardo et al.
of π-back-donation compared to PCP-PCP.14 The oxidation
potential of 16 is virtually identical to that reported for
mononuclear reference 19, indicating that the electronic
properties of the metal centers in both complexes are very
similar. The electronic communication between the metal
centers in [(tpy)RuIII(PCP-PCP)RuII(tpy)]3+ shifts the sec-
ond oxidation less positively. Despite the π-acceptor char-
acter of the PCP-PCP ligand, the anodic wave separation
for complex 17 (∆E1/2 ) 170 mV) is slightly smaller
compared to 5 (∆E1/2 ) 180 mV), corresponding to com-
proportionation equilibrium constants Kc ) 748 for 17 and
900 for 5, respectively. The Kc values reflect the stability of
the [RuIII-RuII] state with respect to disproportionation under
the experimental conditions of the voltammetric scan. It is a
matter of discussion whether they may also be used to
estimate the degree of electronic interaction in the mixed-
valence species across the bridging ligand.18 A cautionary
warning on the use of the electrochemical data is required
since the voltammetric wave separations are influenced also
by other, sometimes even dominant factors (e.g., differences
in solvation, supporting electrolyte, repulsion between the
two similarly charged metal centers, synergistic factors due
to metal-ligand back-bonding) which can modify electro-
static interaction in dinuclear cations.4e,19 The comparably
low values of Kc for 5 and 17 (within the 5% experimental
error) are not consistent with a strong interaction between
the metal redox sites, in contrast to the large electronic
coupling parameters VAB derived from the NIR absorption
of the complexes (vide infra). This discrepancy may reflect
strong participation of the bridging ligands in the oxidation,
as confirmed by the DFT calculations (vide infra).
Table 2. Electrochemical Data for Dinuclear Complexes 6 and 16 and
Reference Mononuclear Complexes 1 and 19
E
1/2 (V)a
Ox.2
d
compound
Ox.1
Red.
Kc
[RuII(NCN)] (1)b
-0.08
-0.07
0.16
-1.94
-1.97
-1.94
-1.95
[RuII(NCN-NCN)RuII] (6)c
[RuII(PCP)] (19)b
-0.25
900
748
[RuII(PCP-PCP)RuII] (16)
0.13
-0.04 (0.68)
a Measured at 298 K in butyronitrile containing 10-1 M TBAH. Potentials
are reported in V vs Fc/Fc+. b Reference 14. c Reference 6b. d Compropor-
tionation equilibrium constant Kc ) exp(∆E1/2/25.69) at 298 K.20 Ox.1 )
first formally RuII/III in mononuclear and dinuclear complexes. Ox.2 )
second formally RuII/III in dinuclear complexes. Red. ) tpy/tpy-.
Figure 2. Anodic part of the cyclic voltammograms of 6 (a) and 16 (b).
Conditions: Pt disk microelectrode, MeCN/10-1 M TBAH, T ) 293 K, V
The cathodic CV scans of 5 and 17 show well-defined
reversible waves at -1.97 (∆Ep ) 0.60 mV) and -1.95 V
(∆Ep ) 0.58 mV), respectively. On the basis of the DPV
peak current intensities, these largely tpy-localized reduction
processes are consistent with single-step transfers of two
electrons. The reduction potentials of the dinuclear complexes
and reference mononuclear compounds 1 and 19 are almost
identical. This means that the π* systems of the ancillary
tpy ligands are not affected by the electronic interaction
between the metal centers. No cathodic waves were observed
for the bridging cyclometalated pincer ligands in 5 and 17
before the solvent discharge.
) 100 mV s-1
.
protocol gave 6 only in a low yield (15%). The inherently
high solubility of 23 and 6 in organic solvents caused a
considerable loss of material during the workup procedure.
Thus, the Cu(II)-mediated oxidative C-C coupling (Scheme
1) proved to be a more convenient procedure for the
preparation of dinuclear organoruthenium-terpyridine com-
plexes containing the bridging NCN-NCN ligand.
Electrochemistry. The dinuclear complex [(tpy)RuII-
(PCP-PCP)RuII(tpy)]2+ (16) was studied by cyclic vol-
tammetry (CV) and differential pulse voltammetry (DPV)
in MeCN at T ) 298 K. The electrochemical data are
summarized in Table 2. Dinuclear complex 46 (i.e., 2e-
oxidized 6) and mononuclear precursors 114 and 1914 are also
presented for reference purposes.
Similar to 6, the anodic region of the cyclic voltammogram
of 16 (Figure 2b) shows two well-separated one-electron
oxidation steps. The current for both peaks is linearly
dependent on ν1/2 for the scan rates studied (0.05-1 V s-1)
indicating completely reversible, diffusion-controlled pro-
cesses. Complex 16 is first oxidized to [(tpy)RuIII(PCP-
PCP)RuII(tpy)]3+ (17) at 0.13 V vs Fc/Fc+ (∆Ep ) 0.70 mV).
In the second step, at -0.04 V (∆Ep ) 0.68 mV), the mixed-
valence complex is further oxidized to [(tpy)RuIII(PCP-
PCP)RuIII(tpy)]4+ (18). The oxidation potentials of 16 are
significantly higher than those of 6, reflecting the absence
Electronic Absorption Spectra and UV-vis Spectro-
electrochemistry. Table 3 lists the absorption maxima, λmax
,
and molar absorption coefficients, ꢀmax, for dinuclear com-
plexes 46, 6,6 and 16 and mononuclear species 114 and 19.14
The absorption spectrum of 16 (Figure 3), recorded in MeCN
at room temperature, presents features characteristic for the
(18) (a) Hush, N. S. Electrochim. Acta 1968, 13, 1005-1023. (b)
Richardson, D. E.; Taube, H. Coord. Chem. ReV. 1984, 60, 107-
129. (c) Robin, M. B.; Day, P. AdV. Inorg. Chem. Radiochem. 1967,
10, 247-422. (d) Demadis, K. D.; Hartshorn, C. M.; Meyer, T. J.
Chem. ReV. 2001, 101, 2655-2685. (e) D’Alessandro, D. M.; Keene,
F. R. Chem. ReV. 2006, 106, 2270-2298.
(19) (a) Barrie´re, F. W. E.; Geiger, W. E. J. Am. Chem. Soc. 2006, 128,
3980-3989. (b) D’Alessandro, D. M.; Keene, F. R. Dalton Trans.
2004, 3950-3954.
(20) Sutton, J. E.; Sutton, P. M.; Taube, H. Inorg. Chem. 1979, 18, 1017-
1021.
11138 Inorganic Chemistry, Vol. 46, No. 26, 2007

Diorganoruthenium Complexes
Figure 5. UV-vis spectra recorded before and after complete one-electron
oxidation of the mononuclear complex [RuII(PCP)(tpy)]+ (19) to [RuIII-
(PCP)(tpy)]2+ (19+). Conditions: OTTLE cell, T ) 293 K, MeCN/3 ×
10-1 M TBAH.
nm) compared to 6 (λmax ) 657 nm) is qualitatively consistent
with the electrochemical results, i.e., higher stabilization of
the HOMO in 16 due to dπ(Ru) f π*(PCP-PCP) back-
donation (HOMO-LUMO gap, ∆E ) 2.08 eV) compared
to 6 (HOMO-LUMO gap, ∆E ) 1.90 eV).
Figure 3. UV-vis spectra recorded before and after the complete two-
electron oxidation of complex [(tpy)RuII(PCP-PCP)RuII(tpy)]2+ (16) to
[(tpy)RuIII(PCP-PCP)RuIII(tpy)]4+ (18). Conditions: OTTLE cell, T ) 293
K, MeCN/3 × 10-1 M TBAH.
UV-vis spectroelectrochemical measurements with com-
plex 16 were performed in MeCN at room temperature using
an optically transparent thin-layer electrochemical (OTTLE)
cell22 to obtain accurate reference electronic absorption
spectra of the oxidized species and probe the stability of the
complex in several redox states. One-electron oxidation of
16 to 17 at 0.13 V caused partially decreased intensity of
the composed band at 485 nm, consistent with the predomi-
nant MLCT character of the electronic transitions involved.
The initial UV-vis spectrum of 16 could be restored by
back-reduction of 17, demonstrating the reversibility of the
anodic process. A full description of the UV-vis-NIR
absorption spectrum of mixed-valence 17 (Figure 4) is given
in the next section.
Further oxidation of mixed-valence species 17 to 18
(Figure 3) at -0.04 V caused complete disappearance of the
band at 485 nm and appearance of two weak bands at 401
and 460 nm. A weak unresolved absorption in this region is
also present in the UV-vis spectrum of the one-electron
oxidized mononuclear Ru(III) complex 19+ (see Figure 5).
Similarly to the [RuIII-RuIII] complex 4,6 the electronic
absorption spectrum of green isoelectronic complex 18
displays a very intense, composed low-energy band centered
at 747 nm having most likely a predominant ligand-to-metal
charge-transfer (LMCT) character.5a In the course of elec-
trolysis, sharp isosbestic points at 297, 324, 378, and 550
nm are maintained, and the stepwise reverse reduction of
18 fully recovers the spectra of 17 and 16. The results
obtained by the spectroelectrochemical study confirm the
reversible nature of the anodic processes on a longer time
scale than that accessible by cyclic voltammetry.
Figure 4. UV-vis-NIR spectrum of the mixed-valence complex [(tpy)-
RuIII(PCP-PCP)RuII(tpy)]3+ (17) obtained by chemical oxidation of 16
with [Ce(OTf)4] in MeCN at 293 K. Inset: the deconvoluted NIR band
envelope of 17.
Table 3. UV-vis Absorption Data for the Mononuclear Complexes 1
and 19 and the Dinuclear Complexes 4, 6, 16, and 18
compound
λ
max, nma (ꢀmax × 104, M-1 cm-1
)
[RuII(NCN)] (1)b
632 (0.51), 593 (0.53), 524 (0.73),
352 (1.02), 318 (2.09), 280 (3.31)
657(11.70), 306 (3.84), 275 (4.66)
618(1.58), 593 (1.67), 528 (1.81),
377 (3.72), 324 (7.30), 280 (5.95)
479 (0.51), 307 (2.48), 273 (1.95)
485 (0.90), 363 (1.90), 311 (4.00),
279 (3.56)
[RuII(NCN-NCN)RuII] (6)c
[RuIII(NCN-NCN)RuIII] (4)c
[RuII(PCP)] (19)b
[RuII(PCP-PCP)RuII] (16)
[RuIII(PCP-PCP)RuIII] (18)
896 (1.2), 747 (3.12), 417 (1.4),
317 (3.11), 214 (5.3)
a Measured at 298 K in MeCN. b Reference 14. c Reference 6
class of ruthenium(II)-terpyridine complexes. The UV
region is dominated by intense π f π* intraligand (IL)
transitions within the bridging pincer and ancillary terpyridine
ligands. The broad absorption bands in the visible region
belong to electronic transitions that are assigned a predomi-
nant metal-to-ligand charge-transfer (MLCT) character due
to spin-allowed dπ(Ru) f π*(tpy).21
(21) (a) Sauvage, J.-P.; Collin, J.-P.; Chambron, J.-C.; Guillerez, S.;
Coudret, C.; Balzani, V.; Barigelletti, F.; De Cola, L.; Flamigni, L.
Chem. ReV. 1994, 94, 993-1019. (b) Zhou, X.; Ren, A.-M.; Feng,
J.-K. J. Organomet. Chem. 2005, 690, 338-347.
(22) (a) Krejcˇ´ık, M.; Daneˇk, M.; Hartl, F. J. Electroanal. Chem., Interfacial
Electrochem. 1991, 317, 179-187. (b) Hartl, F.; Luyen, H.; Nieu-
wenhuis, H. A.; Schoemaker, G. C. Appl. Spectrosc. 1994, 48, 1522-
1528.
The considerable shift to higher energy of the MLCT
bands in the UV-vis spectrum of complex 16 (λmax ) 485
Inorganic Chemistry, Vol. 46, No. 26, 2007 11139

Gagliardo et al.
Table 4. NIR Spectral Data for the Mixed-Valence Species
[(tpy)RuIII(PCP-PCP)RuII(tpy)]3+ (17) and
intense charge transfer absorptions (LMCT) arising in the
visible region, clearly demonstrates its association with the
mixed-valence state.
[(tpy)RuIII(NCN-NCN)RuII(tpy)]3+ (5) Recorded at 298 K in MeCN
17
5
The experimental parameters of the three separate NIR
electronic transitions of 17 are presented in Table 4. Similarly
to 5, the observed ∆ν1/2 value for each band is considerably
lower than that predicted by Hush theory for Class II systems.
Lower bandwidths are usually taken as evidence for a Class
III system. However, the fairly low intensity of the bands,
with their maxima virtually independent of the solvent,
classify 17 as a borderline, nearly delocalized Class II/Class
III mixed-valence complex with considerable coupling
between the ruthenium centers.24
complex
max (nm)
ꢀmax (cm-1 M-1
max (cm-1
∆νobsb (cm-1
∆νcalcdc (cm-1
AB (eV)
Γg
band 1
band 2
band 3
λ
1290
555
1609
1984
6215
1879
3789
0.035d
0.50
2066
3982
4840
1267
3343
0.036d
0.62
1875a
33 000
5333
1616
3509
0.33e,f
0.70
)
ν
)
7751
2424
4231
0.023d
0.43
)
)
V
a Identical with the main component of the composed NIR band.
b Observed half-height bandwidth (∆ν1/2) of the NIR band. c Half-height
bandwidth calculated from eq 1 in the main text. d Coupling parameter
calculated from eq 2 in the main text. e Coupling parameter calculated from
eq 3 in the main text. [RuIII-RuII] charge-transfer distance d ) rAB ) 11
Å (average value derived from the crystallographic data obtained6 for [RuII-
RuII] complexes 4 and 6 and [RuIII-RuIII] complex 8. f Much smaller value
VAB ) 0.11 eV obtained from the Hush formula (eq 2). g Delocalization
parameter calculated from eq 4 in the main text.
The experimental ∆ν1/2 and νmax values (Table 4) allow
for the estimation of the electronic coupling parameter (VAB
)
using eq 2 for 5 and eq 3 for 17, which hold for Class III
and Class II/Class III systems, respectively.24
VAB ) νmax/2
(2)
(3)
NIR Spectra of Mixed-Valence Species. Hush Theory.
The NIR spectrum of the mixed-valence complex [(tpy)RuIII-
(NCN-NCN)RuII(tpy)]3+ (5) displays an intense asymmetric
absorption band (Table 4) originating from two closely
spaced electronic transitions (λmax ) 1875 nm with a shoulder
at 1650 nm) attributed originally to intervalence charge
transfer (IVCT) absorption.6a The IVCT term corresponds
to [RuIII(bridge)RuII] T [RuII(bridge)RuIII] isomerization in
a valence-localized Class II compound. For complex 5, the
main component of the NIR absorption, with a maximum
coinciding with that of the band envelope (λmax ) 1875 nm),
is fairly narrow, the experimental half-width ∆ν1/2 ) 1616
cm-1 being much smaller than the half-width of 3509 cm-1
calculated from eq 1 introduced by Hush for Class II mixed-
valence systems. This difference in the ∆ν1/2 values indicates
a much stronger electronic delocalization in the complex.
In view of its redox properties, the high intensity of the NIR
band (ꢀmax ) 3.3 × 104 M-1 cm-1), and negligible solvato-
chromism, it can be concluded that complex 5 belongs to
Class III systems with extensively delocalized frontier
orbitals.2d,e,23 Therefore, the NIR bands are more correctly
assigned as charge-resonance bands rather than IVCT transi-
tion.
VAB ) [2.06 × 10-2 (ꢀmax∆ν1/2ν )1/2/rAB
]
max
For a more conclusive assignment of 5 to Class III and 17
to borderline Class II/Class III, we employed the delocal-
ization parameter Γ (eq 4), introduced by Brunschwig et al.24a
as a useful criterion for determining whether a mixed-valence
system is weakly, moderately, or strongly coupled.
Γ ) (1 - θ) ) [1 - (∆ν1/2/(2310νmax)1/2)]
(4)
The value of Γ ≈ 0.70 for complex 5 (Table 4) indeed
complies with its assignment to intrinsically delocalized Class
III, which agrees with a stronger electronic delocalization
compared to 17.
The results of the Hush analysis point to a decreased
electronic coupling between the metal centers in the mixed-
valence species by introducing the π-acceptor PPh2 moieties
in the pincer arms of the bridging ligand as a result of a
larger delocalization of the unpaired electron in the SOMO
of 5 compared to that of 17. For dinuclear systems featuring
frontier orbitals extensively delocalized over the metal centers
and the bridging ligand, the reasoning whether the NIR
absorption corresponds to charge resonance25 in Class III and
borderline Class II/Class III systems rather than to IVCT
transitions,24a characteristic of valence-localized (Class II)
systems, has a limited value, and quantum mechanical
calculations are required to clarify the exact nature of these
transitions. As recent studies showed that DFT calculations
are valuable in the description of the electronic structure of
analogous mixed-valence organometallic systems,26 we per-
formed an initial DFT and TD-DFT study of the mixed-
valence complexes as described in the following section.
1/2
∆ν1/2 ) (2310νmax
)
(1)
One-electron oxidation of 16 in MeCN with one equivalent
of [Ce(OTf)4] generates the mixed-valence species [(tpy)-
RuIII(PCP-PCP)RuII(tpy)]3+ (17), which displays a broad
band in the NIR region (Figure 4). The absorption envelope
has been deconvoluted into three Gaussian-shaped sub-bands
(Figure 4, inset). Further oxidation of 17 by the second
equivalent of Ce(IV) resulted in formation of the deep green
colored [RuIII-RuIII] complex 18, its UV-vis spectrum well
reproducing the spectroelectrochemical result (Figure 3). The
observed complete disappearance of the composed NIR band
on the one-electron oxidation of 17 to 18, accompanied by
(24) (a) Brunschwig, B. S.; Creutz, C.; Sutin, N. Chem. Soc. ReV. 2002,
31, 168-184. (b) D’Alessandro, D. M.; Keene, F. R. Chem. Soc. ReV.
2006, 35, 424-440. (c) Kaim, W.; Lahiri, G. K. Angew. Chem. Int.
Ed. 2007, 46, 1778-1796.
(25) Szeghalmi, A. V.; Erdmann, M.; Engel, V.; Schmitt, M.; Amthor, S.;
Kriegisch, V.; No¨ll, G.; Stahl, R.; Lambert, C.; Leusser, D.; Stalke,
D.; Zabel, M.; Popp, J. J. Am. Chem. Soc. 2004, 126, 7834-7845.
(23) Demadis, K. D.; Hartshorn, C. M.; Meyer, T. J. Chem. ReV. 2001,
101, 2655-2686.
11140 Inorganic Chemistry, Vol. 46, No. 26, 2007

Diorganoruthenium Complexes
Figure 6. Highest R-occupied MOs of 5 (116b) and 17-H (108b).
Table 5. Mulliken28 Spin Population for 5 and 17-H
group
5
17-H
Ru(II)/Ru(III)
biphenylene (C atoms)
0.627 (0.314/0.314)
0.395
0.552 (0.276/0.276)
0.463
Table 6. Lowest-Lying Excitations (∆E) in the NIR Spectrum (with f
> 0.001) for 5 and 17-H (c2 > 0.3)
5
17-H
∆E (nm)
∆E (nm)
band
1
(eV)
f
assignment
(eV)
f
assignment
1488
0.421 0.52 â-115b f 1509
â-116b
0.644 0.77 â-107b f
â-108b
(0.83)
1407
0.36 â-114b f
â-116b
(0.82)
2
0.297 0.64 â-114b f 1051
â-116b
0.019 0.98 â-105b f
â-108b
Figure 7. Contour plots of the frontier orbitals of 5 and 17-H involved in
the NIR electronic transitions presented in Table 6.
(0.88)
0.29 â-115b f
â-116b
(1.18)
result of alteration of the relative energetic positions of the
Ru d orbitals by the donating ability of the bridging ligand
can be rationalized in terms of a simple orbital correlation
diagram argument. The highest occupied R orbitals (116b
for 5, 108b for 17-H, Figure 6) in both compounds can be
described as the ‘antibonding’ combination of the π-type d
orbitals on Ru and the HOMO of an unperturbed biphenylene
system. Let us consider the two limiting cases where the Ru
d orbitals are much lower or higher in energy than the
HOMO of an unperturbed biphenyl. In the former case, the
doubly occupied, ‘bonding’ combination resembles the Ru
d orbitals while the singly occupied, ‘antibonding’ combina-
tion is similar to the unperturbed biphenyl HOMO; in the
latter case, the situation is reversed. Thus, rising the Ru
d-orbital energy leads to more pronounced Ru d character
of the singly occupied orbital and consequently to a higher
spin population on the metals.
Density Functional Theory. DFT calculations were
performed on mixed-valence complex 5 and on the model
compound 17-H containing the bridging ligand [C6H2(CH2-
PH2)2-3,5]22- instead of [C6H2(CH2PPh2)2-3,5]22- to reduce
the computational efforts. According to the computed DFT
results, the unpaired electron in the mixed-valence species
is strongly delocalized. Being aware that these results may
arise from an artificial preference of DFT methods for
delocalization,27 we performed additional UHF calculations,
which have also suggested that oxidation of the biphenylene
bridging ligand occurs. The ground-state electronic configu-
rations of the pincer compounds were calculated to be for 5
(NCN) (C2 symmetry) ((1a)2(1b)2...(125a)2(113b)2(114b)2-
(115b)2(116b)1) and for 17-H (PCP) (C2 symmetry) ((1a)2-
(1b)2...(117a)2(105b)2(106b)2(107b)2(108b)1). The Mulliken28
spin population (Table 5) for both compounds shows an equal
spin population on both Ru centers and a substantial
involvement of the carbon atoms of the bridging biphenylene
unit. The fact that the spin population on the Ru centers is
higher for 5 than for 17-H reflects the larger stabilization of
the Ru d orbitals in 17 due to the poorer σ-donating ability
of the PCP-PCP bridging ligand compared to that of
NCN-NCN, in line with the obtained electrochemical results
(i.e., the different anodic electrode potentials of complexes
6 and 16, see Table 2). The change in spin population as a
The near equality of the ruthenium centers thwarts the
assignment of the observed near-infrared bands as IVCT
transitions. Consequently, these bands were assigned on the
basis of TD-DFT calculations. Table 6 presents the characters
of the lowest-lying calculated NIR transitions with oscillator
strength (f) larger than 0.001. The calculated excitation
energies are in reasonable agreement with the experimental
NIR absorption maxima of 5 and 17 (Table 3), although
overestimated.
(26) (a) Ro¨der, J. C.; Meyer, F.; Hyla-Kryspin, I.; Winter, R. F.; Kaifer,
E. Chem. Eur. J. 2003, 9, 2636-2648. (b) van Slageren, J.; Winter,
R. F.; Hartmann, S. J. Organomet. Chem. 2003, 670, 137-143. (c)
Mauer, J.; Winter, R. F.; Sarkar, B.; Fiedler, J.; Za´lisˇ, S. Chem.
Commun. 2004, 1900-1901. (d) Bu¨hl, M.; Thiel, W. Inorg. Chem.
2004, 43, 6377-6382.
For both compounds, the lowest-lying bands correspond
to excitation of a â electron to the singly occupied orbital.
The experimental band observed at 1875 nm for 5 may be
attributed to the two bands calculated at 1488 and 1407 nm
and those at 2066 and 1609 nm in the NIR spectrum of 17
to the transitions calculated for 17-H at 1509 and 1051 nm.
The TD-DFT calculations do not predict the band at 1290
nm obtained by deconvolution of the NIR band of 17.
(27) (a) Sodupe, M.; Bertran, J.; Rodr´ıguez-Santiago, L.; Baerends, E. J.
J. Phys. Chem. A 1999, 103, 166-170. (b) Gru¨ning, M; Gritsenko,
O. V.; van Gisbergen, S. J. A.; Baerends, E. J. J. Phys. Chem. A 2001,
105, 9211-9218.
(28) Mulliken, R. S. J. Chem. Phys. 1955, 23, 1833-1840.
Inorganic Chemistry, Vol. 46, No. 26, 2007 11141

Gagliardo et al.
Therefore, we surmise that the broadness of the NIR band
in 17 is probably due to an unresolved vibrational progres-
sion.27
not important in this regard as their cathodic responses
remain unaffected in the studied series of mono- and
dinuclear complexes (Table 2). The facile preparation of this
class of complexes and their stability to air, moisture, and
high temperatures (decomposition above 350 °C) represent
their additional advantageous characteristics together with
the fast and selective interconversion between the [RuIII-
RuIII], formally [RuIII-RuII], and [RuII-RuII] redox forms.
The results obtained are encouraging for future development
of these species directed toward photochromic devices.
Moreover, the possibility to introduce substituents at the 4′-
position of the terminal terpyridine ligands opens the
possibility to explore the anchoring of these complexes on
materials such as polymers, dendrimers, and semiconductor
surfaces to form larger supramolecular architectures.
Plots of the frontier orbitals of 5 and 17-H involved in
the NIR transitions (Figure 7) give an indication of the charge
transfer upon excitation. The â-HOMO (115b), HOMO-1
(114b), and â-LUMO (116b) of 5 reside on the metal and
the π orbitals of the bridging ligand. The â-HOMO (107b)
of 17-H is mainly localized on the metal with tails on the
bridging ligand, while the â-HOMO-2 (105b) is mainly metal
in character. The NIR absorption bands for both complexes
are thus not originated by a single IVCT transition, as
previously proposed,5,6 but are composed of transitions with
a predominant MLCT and some minor LMCT character. A
more in-depth theoretical study is currently in progress to
resolve the discrepancies between theory and experiment and
the effect of the preference of DFT for a delocalized state
on the calculated transition energies and assignments.
Experimental Section
General. All experiments were carried out under an atmosphere
of dry nitrogen using standard Schlenk techniques. Benzene,
toluene, pentane, hexane, and Et2O were distilled from sodium/
benzophenone. CH2Cl2 was dried over CaH2, MeOH over magne-
sium, and MeCN over P2O5. All solvents were freshly distilled under
nitrogen prior to use. NMR spectra [1H (300.1 MHz), 13C (75.5
MHz), and 31P{1H} (121.4 MHz)] were recorded on a Varian Inova
300 MHz spectrometer. Chemical shifts are given in ppm relative
to the residual solvent signal (1H and 13C NMR spectra) or 85%
H3PO4 external reference (31P{1H} NMR spectra). All reagents were
purchased from Acros Chemicals or Aldrich and used as received.
Compounds 1,12 14,12 and 1914 were synthesized according to
literature procedures. Elemental analyses were performed by Dornis
and Kolbe, Mikroanalytisches Laboratorium, Mu¨lheim a. d. Ruhr,
Germany. High-resolution electrospray ionization (ESI) mass
spectra were recorded with a Micromass LC-TOF mass spectrom-
eter at the Biomolecular Mass Spectrometry group, Utrecht
University.
Electrochemical Measurements. Cyclic and square-wave vol-
tammetric scans were performed with a gastight single-compartment
cell under an atmosphere of dry nitrogen. The cell was equipped
with Pt microdisk working (apparent surface area of 0.42 mm2), Pt
wire auxiliary, and Ag wire pseudoreference electrodes. The
working electrode was carefully polished with a 0.25 µm grain
diamond paste between scans. The potential control was achieved
with a PAR Model 283 potentiostat. All redox potentials are
reported against the ferrocene/ferrocenium (Fc/Fc+) redox couple
used as an internal standard (E1/2 ) +0.63 V vs NHE).30,31 Bu4-
NPF6 (3 × 10-1 M; TBAH) was used as supporting electrolyte.
All electrochemical samples were 5 × 10-4 M in the studied
complexes.
Conclusions
In contrast to the mononuclear complex [RuII(NCN)(tpy)]-
Cl, reaction of [RuII(PCP)(tpy)]Cl with an excess of CuCl2
does not result in smooth formation of a dinuclear Ru(III)
species. Therefore, synthesis of the [(tpy)RuII(PCP-PCP)-
RuII(tpy)]Cl2 complex was carried out by a selective double
transcyclometalation of the pincer ligand [C6H3(CH2PPh2)2-
3,5]2, abbreviated as [PC(H)P-PC(H)P], with the ruthenium
precursor [RuCl(NCN)(PPh3)] followed by coordination of
ancillary terpyridine ligands. The electrochemical and spec-
troscopic properties of [(tpy)RuII(PCP-PCP)RuII(tpy)]Cl2
were investigated and compared with those of the related
complex [(tpy)RuII(NCN-NCN)RuII(tpy)](PF6)2. One-elec-
tron reduction of [(tpy)RuIII(NCN-NCN)RuIII(tpy)](PF6)4
and one-electron oxidation of [(tpy)RuII(PCP-PCP)RuII-
(tpy)]Cl2 lead to formation of the mixed-valence species
[(tpy)RuIII(NCN-NCN)RuII(tpy)]3+ and [(tpy)RuIII(PCP-
PCP)RuII(tpy)]3+, respectively. Both products absorb strongly
in the NIR spectral region. Analysis of the features of the
NIR absorption bands by Hush theory shows that the mixed-
valence species [(tpy)Ru(PCP-PCP)Ru(tpy)]3+ belongs to
borderline Class II/Class III (nearly delocalized) systems (Γ
≈ 0.5). The complex [(tpy)Ru(NCN-NCN)Ru(tpy)]3+ is an
intrinsically delocalized Class III compound (Γ ≈ 0.7).
Clearly, for both mixed-valence species the extensively
delocalized bonding situation excludes the assignment of the
NIR bands to IVCT transitions, as suggested for [(tpy)RuIII-
(NCN-NCN)RuII(tpy)]3+ in the literature. Delocalization in
both complexes has been predicted by DFT calculations,
which document an equal spin population of both Ru centers.
TD-DFT calculations have suggested that the low-lying NIR
bands can be assigned as predominantly MLCT in character.
The results of this study show that the ‘soft’ donor and
π-acceptor PPh2 moieties in the pincer arms of the PCP-
PCP ligand bridge attenuate the delocalization of the
unpaired electron in the mixed-valence species. In general,
fine-tuning of the electronic coupling can be achieved by
modification of the donor characteristics of the pincer arms
of the bridging ligand. The ancillary terpyridine ligands are
Electronic Absorption Spectroscopy. UV-vis spectra were
obtained on a Varian Cary 1 spectrophotometer using matched 1
cm quartz cells (Hellma) and operating with 0.5 nm spectral
resolution.
UV-vis Spectroelectrochemistry. All experiments were per-
formed with an optically transparent thin-layer electrochemical
(OTTLE) cell22 equipped with a Pt minigrid working electrode and
quartz optical windows. The controlled-potential electrolyses were
(29) Adams, S. J.; Fey, N.; Parfitt, M.; Pope, S. J. A.; Weinstein, J. A.
Dalton Trans. 2007, 4446-4456.
(30) Pavlishchuk, V. V.; Addison, A. W. Inorg. Chim. Acta 2002, 298,
97-102.
(31) Gritzner, G.; Ku˚ta, J. Pure Appl. Chem. 1984, 56, 461-466.
11142 Inorganic Chemistry, Vol. 46, No. 26, 2007

Diorganoruthenium Complexes
carried out with a PA4 potentiostat (EKOM, Polna´, Czech
Republic). All electrochemical samples were 5 × 10-4 M in the
studied complex and contained 3 × 10-1 M Bu4NPF6. In situ UV-
vis spectra were recorded on a Hewlett-Packard 8453 diode-array
spectrophotometer.
immediately after addition of CuCl2 (0.8 g, 0.6 mmol). The mixture
was stirred at reflux for 5 h, after which time the solvent was
removed in vacuo. The green residue was redissolved in MeCN (3
mL), and the solution was treated with an excess of aqueous NH4-
PF6. The obtained precipitate was washed with water and subjected
to silica gel column chromatography (elution with MeCN/H2O/
satd aq. KNO3 (6:3:1 v/v)). Removal of the solvents from the first
and second eluted fractions gave 3 (7%) and 4 (yield: 0.34 g, 85%)
as red and green solids, respectively. Single copper-colored crystals
of 4 suitable for X-ray diffraction analysis were obtained by slow
evaporation of a solution of 4 in MeCN (2 mL) containing toluene
(5 mL). 1H NMR (300 MHz, CD3CN): δ 8.92 (d, 1JHH ) 8.11 Hz,
X-ray Crystal Structure Determination of Complex 4.
[C54H58N10Ru2](PF6)4 + disordered solvent, Mw ) 1629.12 [derived
values do not contain the contribution from the disordered solvent
molecules], orange plate, 0.24 × 0.24 × 0.06 mm, triclinic, P1h
(no. 2), a ) 16.5095(1) Å, b ) 17.6890(1) Å, c ) 18.5598(2) Å,
R ) 97.2109(4)°, â ) 110.7621(4)°, γ ) 116.6773(5)°, V )
4257.69(7) Å3, Z ) 2, Dx ) 1.271 g cm-3 [derived values do not
contain the contribution from the disordered solvent molecules], µ
) 0.52 mm-1 [derived values do not contain the contribution from
the disordered solvent molecules]. There were 64 494 reflections
measured on a Nonius Kappa CCD diffractometer with rotating
anode (graphite monochromator, λ ) 0.71073 Å) up to a resolution
of (sin θ/λ)max ) 0.60 Å-1 at T ) 150 K. An absorption correction
based on multiple measured reflections was applied (0.92-0.98
correction range). A total of 15 366 reflections were unique (Rint
) 0.0657). The structure was solved with automated Patterson
methods32 and refined with SHELXL-9733 against F2 of all
reflections. Non-hydrogen atoms were refined with anisotropic
displacement parameters. All hydrogen atoms were introduced in
geometrically idealized positions and refined with a riding model.
The crystal structure contains large voids (1449.8 Å3/unit cell) filled
with disordered solvent molecules. Their contribution to the
structure factors was secured by back-Fourier transformation using
the SQUEEZE routine of the PLATON program,34 resulting in 431
electrons/unit cell. A total of 855 parameters were refined with no
restraints. R1/wR2 [I > 2σ(I)]: 0.0398/0.0903. R1/wR2 [all reflns]:
0.0573/0.0959. S ) 0.958. Residual electron density between
-0.67 and 1.44 e Å-3. Geometry calculations and checking for
higher symmetry were performed with the PLATON program.34
Computational Details. Geometries of 5 (C2) and 1-H (C2) were
optimized with GAMESS-UK35 at the UB3LYP level of theory
using the Stuttgart 1997 ECP for Ru and the 6-31G** basis set for
all other atoms. TD-DFT calculations were performed with
GAUSSIAN 0336 using the same basis set.
1
4H, tpy(3′, 5′)), 8.77 (d, JHH ) 6.6 Hz, 2H, tpy(6, 6′′)), 8.76 (s,
1
2
4H, Ar), 8.59 (t, JHH ) 8.1 Hz, 2H, tpy(4′)), 8.22 (d, JHH ) 7.0
Hz, 4H, tpy(3, 3′′)), 8.21 (t, 1JHH ) 8.39 Hz, 4H, tpy(4, 4′′)), 7.62
(t, JHH ) 6.45 Hz, 4H, Pyr(5, 5′′)), 4.01 (s, 8H, CH2), 1.28 (s,
1
24H, NCH3). 13C NMR (75 MHz, CDCl3): δ 158.1, 157.9, 153.8,
148.8, 142.6, 142.5, 142.0, 129.9, 129.3, 127.1, 124.3, 114.0, 74.8
(NCH3), 53.83 (CH2). Anal. Calcd for C54H58F24N10P4Ru2: C,
39.81; H, 3.59; N, 8.60. Found: C, 39.75; H, 3.68; P, 8.55.
[3,5-Bis(methoxymethyl)pinacolboranebenzene] (9). Magne-
sium lumbs (1.56 g, 64.2 mmol) were stirred vigorously under a
nitrogen atmosphere in a three-necked round-bottom flask for 1 h
followed by addition of THF (10 mL). A few drops of 7 (or
alternatively 1,2-dichloroethane) were added to start the reaction.
A solution of 7 (9.38 g, 32.11 mmol, 0.5 equiv) in THF (90 mL)
was then added dropwise. During the addition the solution turned
dark gray. After the addition was complete, the mixture was stirred
at room temperature for 3 h and subsequently heated (ca. 15 h) to
reflux until full conversion of 7 to 8 was reached. The conversion
of 7 was determined by 1H NMR spectroscopy by analyzing aliquots
of the mixture quenched with S2Me2. The obtained solution was
filtered to remove the excess Mg and diluted by adding THF (100
mL). After addition of B(OMe)3 (7.5 mL, 65.7 mmol, 1.0 equiv),
the obtained mixture was heated at 50 °C for 15 h. Next, pinacol
(4.15 g, 0.55 equiv) was added at room temperature, followed by
AcOH (0.5 equiv) after 10 min. The reaction mixture was stirred
at room temperature for 1 h and then evaporated to dryness, and
Et2O (200 mL) was added. The precipitated salts were removed
by filtration. Removal of the solvent in vacuo gave 9 as yellow
[(tpy)RuIII(NCN-NCN)RuIII(tpy)](PF6)4 (4). A dark blue solu-
tion of 1 (0.13 g, 0.25 mmol) in dry MeOH (10 mL) turned red
1
oil. Yield: 9.16 g, 98%. H NMR (300 MHz, CDCl3): δ 7.69 (s,
2H, Ar), 7.45 (s, 1H, Ar), 4.49 (s, 4H, CH2), 3.38 (s, 6H, OCH3),
1.38 (s, 12H, CCH3). 13C NMR (75 MHz, CDCl3): δ 137.9, 133.7,
130.2, 127.12, 83.9 (OCH3), 74.7 (CH2), 58.3 (CCH3), 25.0 (CH3).
Anal. Calcd for C16H25BO4: C, 65.77; H, 8.62. Found: C, 65.85;
H, 8.56.
(32) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.; Garcia-
Granda, S.; Gould, R. O.; Smits, J. M. M.; Smykalla C. The DIRDIF99
program system; Technical Report of the Crystallography Laboratory;
University of Nijmegen: The Netherlands, 1999.
(33) Sheldrick, G.M. SHELXL-97, Program for crystal structure refinement;
University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(34) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7-13.
[C6H3(CH2OMe)2-3,5]2 (10). A mixture of 7 (2.56 g, 8.8 mmol),
9 (2.56 g, 8.8 mmol), Na2CO3 (2.23 g, 21.0 mmol), and [PdCl2-
(dppf)] (0.16 g, 0.23 mmol) in degassed DMF/THF/H2O (1:1:1 v/v)
(120 mL) was heated at 50 °C for 15 h under a nitrogen atmosphere.
The organic products formed were extracted with Et2O (2 × 70
mL). Evaporation of the solvent gave crude 10 as a brown oil
(containing 10% of 7) that was purified by column chromatography
(silica, pentane/Et2O (3:7 v/v)). Yield: 1.97 g, 68%. 1H NMR (300
MHz, CDCl3): δ 7.51 (s, 4H, Ar), 7.29 (s, 2H, Ar), 4.52 (s, 8H,
CH2), 3.42 (s, 12H, OCH3). 13C NMR (75 MHz, CDCl3): δ 141.4,
139.2, 126.2, 126.0, 74.8, 58.4 (CH2). Anal. Calcd for C20H26O4:
C, 72.70; H, 7.93. Found: C, 72.87; H, 7.95.
(35) Guest, M. F.; Bush, I. J.; van Dam, H. J. J.; Sherwood, P.; Thomas,
J. M. H.; van Lenthe, J. H.; Havenith, R. W. A.; Kendrick, J. Mol.
Phys. 2005, 103, 719-747.
(36) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, Jr., T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G.
A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P.
Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J.
B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision
B.05; Gaussian, Inc.: Pittsburgh, PA, 2003.
[C6H3(CH2Br)2-3,5]2 (11). A mixture of 10 (5.29 g, 1.6 mmol),
BF3‚Et2O (30 g, 2.17 mmol), and AcBr (31.5 g, 2.56 mmol) in
CH2Cl2 (150 mL) was stirred at reflux for 18 h. The mixture was
then cooled to 0 °C by an ice bath. Next, saturated aqueous K2CO3
solution was added slowly until no evolution of gas was observed.
Inorganic Chemistry, Vol. 46, No. 26, 2007 11143

Gagliardo et al.
Subsequently, the collected organic layer was dried over MgSO4,
filtered, and concentrated to 5 mL. Hexane was added slowly,
resulting in precipitation of pure 11 as white crystals, which were
collected by filtration and dried in vacuo. Yield: 5.48 g, 65.0%.
1H NMR (300 MHz, CDCl3): δ 7.52 (s, 4H, Ar), 7.44 (s, 2H, Ar),
4.55 (s, 8H, CH2). 13C NMR (75 MHz, CDCl3): δ 141.4, 139.4,
129.2, 128.0, 32.8 (CH2). Due to its toxicity, elemental and ESI
analyses of 11 were not carried out. Caution: benzylic bromides
can act as powerful lachrymators and should be used with adequate
Ventilation and precaution against skin contact or ingestion.
[C6H3(CH2PPh2)2-3,5]2 ([PC(H)P-PC(H)P]) (13). nBuLi (3.5
mL, 5.5 mmol, 1.6 M in hexane) was added under stirring to a
solution of HPPh2‚BH3 (1.1 g, 5.5 mmol) in THF (80 mL) cooled
to -78 °C. The temperature was then allowed to rise to 293 K,
and stirring was continued for another 15 h. Subsequently, a solution
of 11 (0.73 g, 1.4 mmol) in THF (30 mL) was added at -78 °C.
The mixture was then stirred for another 18 h at room temperature.
All volatiles were evaporated, and the obtained residue was
dissolved in Et2O. The organic layer was washed with H2O, dried
over MgSO4, and concentrated until only a few milliliters remained.
The precipitated white solid was collected, redissolved in a few
milliliters of CH2Cl2, and filtered. Addition of EtOH caused
precipitation of 11 (Scheme 4) as a white solid that was collected
by filtration, washed with hexane, and dried in vacuo. Yield: 0.93
g, 67.4%. 1H NMR (300 MHz, CDCl3): δ 7.59-7.56 (m, 22H, Ar
+ PAr2), 7.49-7.25 (m, 24H, PAr2), 3.41 (d, 2JHP ) 11.7 Hz, 8H,
CH2), 0.90 (br s, 12H, BH3). 13C NMR (75 MHz, CDCl3) δ 139.8-
127.94 (Ar + PAr2), 34.07 (d, 2JCP ) 32.9 Hz, CH2). 31P{1H} NMR
(121.4 MHz, CDCl3): δ 18.9 (s). Anal. Calcd for C64H66B4P4: C,
76.69; H, 6.64; P, 12.36. Found: C, 76.78; H, 6.53; P, 12.26.
Removal of BH3. HBF4‚Et2O (5 mL, 13.2 mmol) was added
under a nitrogen atmosphere to a stirred solution of 12 (0.2 g, 0.2
mmol) in CH2Cl2 at 0 °C. The temperature was then allowed to
rise to 293 K, and stirring was continued for another 18 h. A
saturated aqueous NaHCO3 solution was added dropwise at 0 °C,
resulting in a considerable gas evolution. When the addition was
complete, the mixture was stirred at room temperature for another
1 h. The organic layer was collected and dried over MgSO4. After
evaporation of all the volatiles, 13 was obtained as an air- and a
moisture-sensitive white sticky solid. Yield: 0.140 g, 80%. 1H NMR
(300 MHz, CDCl3): δ 7.43-7.32 (m, 40H, PAr2), 6.84 (s, 4H,
Ar), 6.60 (s, 2H, Ar), 3.34 (s, 8H, CH2). 31P{1H} NMR (121.4 MHz,
CDCl3): δ -8.6 (s).
residual amount remained. Addition of Et2O resulted in precipitation
of 16 as an orange air-stable solid. The latter was collected by
centrifugation, washed with hexane, and dried in vacuo. The crude
product was further purified by column chromatography (silica,
MeCN/H2O (8.5:1.5 v/v)). Yield: 0.112 g, 66.6%. H NMR (300
MHz, CD3OCD3): δ 9.08 (d, 1JHH ) 7.8 Hz, 4H, tpy(3′, 5′)), 8.63
1
1
1
(d, JHH ) 8.6 Hz, 2H, tpy(6, 6′′)), 8.47 (t, JHH ) 7.95 Hz, 2H,
tpy(4′)), 8.20 (s, 4H, Ar), 7.65 (t, 1JHH ) 7.65 Hz, 4H, tpy(4, 4′′)),
2
1
7.56 (d, JHH ) 5.0 Hz, 4H, tpy(3, 3′′)), 7.16 (vt, JHH ) 8.2 Hz,
8H, para-H PAr2), 7.02 (vt, 1JHH ) 7.30 Hz, 16H, ortho-H PPh2),
6.83 (t, 1JHH ) 6.0 Hz, 4H, tpy(5, 5′′)), 6.74 (m, 8H, meta-H PAr2),
4.00 (vt, 8H, 1JHP ) 3.0 Hz, CH2-P). 31P{1H} NMR (81.91 MHz,
CD3OCD3): δ ) 43.1 (s). Anal. Calcd for C94H74Cl2N6P4Ru2: C,
67.02; H, 4.43; N, 4.99; P, 7.35. Found: C, 66.75; H, 4.36; N,
5.10; P, 7.22.
[C6H3(CH2NMe2)2-3,5]2 ([NC(H)N-NC(H)N]) (21). Neat
NHMe2 (20 mL, 280 mmol) was added under stirring to a solution
of 11 (2.69 g, 5.1 mmol) in CH2Cl2 (40 mL) cooled to 0 °C. The
temperature of the reaction mixture was allowed to rise to 293 K,
and stirring was continued for 15 h, after which aqueous NaOH
(50 mL, 4 M) was added. The CH2Cl2 fraction was collected,
washed with saturated aqueous NaCl (50 mL), dried over MgSO4,
and filtered. Evaporation of the filtrate gave product 21 as a yellow
oil, which was further purified by column chromatography (alumina,
CH2Cl2, 3% NEt3). Yield: 1.35 g, 70%. NMR data of the obtained
compound were consistent with values reported elsewhere.17
[C6H2Li-4-(CH2NMe2)2-3,5]2 (22). tBuLi (2.76 mL, 4.14 mmol,
1.5 M in pentane) was added under stirring to a solution of 21
(0.63 g, 1.8 mmol) in Et2O at -78 °C. The resulting orange
suspension was allowed to warm to room temperature and stirred
for another 14 h. Off-white crystals of 22, obtained upon cooling
to -30 °C, were separated by the pale yellow Et2O layer. They
were collected, washed with cold Et2O, and dried in vacuo. Yield:
0.4 g, 55.0%. Due to its high flammability, compound 22 was used
directly in the following reaction without further purification.
[(tpy)RuII(NCN-NCN)RuII(tpy)](PF6)2 (6). A solution of
[RuCl2(PPh3)3] (0.76 g, 0.8 mmol) in THF (20 mL) was slowly
added at room temperature under vigorous stirring to a solution of
22 (0.16 g, 0.4 mmol) in THF (20 mL). The color of the mixture
turned rapidly dark blue, consistent with formation of the dinuclear
complex 23 (Scheme 6). After stirring for another 5 h, the volatiles
were removed in vacuo and a solution of 2,2′:6′,2′′-terpyridine (0.19
g, 0.8 mmol) was added in one portion. The resulting mixture was
stirred at reflux for 6 h. Next, the solvent was removed in vacuo,
and the blue residue was washed with pentane and redissolved in
MeCN (5 mL). Addition of an excess of aqueous NH4PF6 caused
precipitation of 6 as a dark blue solid that was collected, washed
with water, and dried in vacuo. Yield: 0.16 g, 15%. NMR data of
product 6 are consistent with the previously reported values for 6
prepared according to the synthesis route presented in Scheme 1.6b
[{RuCl(PPh3)}2(4,4′-{C6H2(CH2PPh2)2-2,6}2] (15). A solution
of 13 (0.140 g, 0.15 mmol) in C6H6 (20 mL) was added to a solution
of 14 (0.17 g, 0.15 mmol) in C6H6 (10 mL). The reaction mixture
was stirred at room temperature for 2 days. The solvent was
removed in vacuo till only a small residual amount remained.
Addition of hexane caused precipitation of an air- and a moisture-
sensitive deep green solid that was collected by filtration on a glass
filter and washed with cold hexane. Yield: 0.21 g, 70%. 1H NMR
(300 MHz, CDCl3): δ 7.77-7.37 (m, 18H, Ar), 7.07-7.66 (m,
Acknowledgment. The authors kindly acknowledge Dr.
S. Gracea-Tanase and Dr. E. Bouwman from the University
of Leiden for having performed EPR and magnetic suscep-
tibility measurements on complex 4. NWO/NCF is thanked
for the supercomputer time on TERAS/ASTER, SARA (The
Netherlands, Project No. SG-032). This work was supported
by the Council for Chemical Sciences of the Dutch Orga-
nization for Scientific Research (NWO). R.W.A.H. acknowl-
edges financial support from the NWO Grant 700.53.401.
2
2
64H, Ar + PAr2), 3.46 (dvt, JHH ) 15.6 Hz, JHP ) 6.0 Hz, 4H,
CH2), 2.49 (br d, 2JHH ) 16.5 Hz, 4H, CH2). 31P{1H} NMR (80.91
MHz, CDCl3): δ 82.6 (t, JPP ) 30.0 Hz, PPh3), 37.5 (d, JPP
2
2
)
31.5 Hz, PCP). Anal. Calcd for C100H82Cl2P6Ru2‚2CH2Cl2 : C,
64.06; H, 4.53; P, 9.72. Found: C, 63.90; H, 4.64; P, 9.22.
[(tpy)RuII(PCP-PCP)RuII(tpy)]Cl2 (16). A solution of 2,2′:
6′,2′′-terpyridine (0.05 g, 0.2 mmol) was added in one portion to
deep green 15 (0.21 g, 0.1 mmol). The mixture was stirred at reflux
temperature for 3 days, during which time the color of the solution
turned dark orange. The solvent was removed in vacuo until a small
IC701501Q
11144 Inorganic Chemistry, Vol. 46, No. 26, 2007