Electrosynthesis of binuclear ruthenium complexes from [RuCl3(dppb)(L)] precursors [L = pyridine, 4-methylpyridine or dimethyl sulfoxide; dppb = l,4-bis(diphenylphosphino)butane]

Article abstract of DOI:10.1039/b001422m

Electrolysis has been examined as a method of synthesis for [(L)(dppb)Ru((i-Cl)3RuCl(dppb)] complexes, where dppb = l,4-bis(diphenylphosphino)butane and L = pyridine (py), 4-methylpyridine (4-pic) or dimethyl sulfoxide (DMSO), by using [RuCl3(dppb)(L)] as precursors. The products of the electrolysis were characterized by 31P-{'H} NMR, cyclic voltammetry and near infrared spectroscopy. The presence of the [Ru2Cl5(dppb)2] complex in the electrochemical cell suggests a mechanism by which the starting original species from the bulk solution reacts with the reduced form [RuCl2(dppb)(L)] generated at the surface of the electrode. The crystal structure of the precursor ;ner-[RuCl,(dppb)(4-pic)l was determined by X-ray diffraction; The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b001422m

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Electrosynthesis of binuclear ruthenium complexes from
[RuCl3(dppb)(L)] precursors [L ؍ pyridine, 4-methylpyridine or
dimethyl sulfoxide; dppb ؍ 1,4-bis(diphenylphosphino)butane]
Karen Wohnrath,a Márcio Peres de Araujo,a Luis Rogério Dinelli,b Alzir Azevedo Batista,*b
Icaro de Sousa Moreira,c Eduardo Ernesto Castellanod and Javier Ellenad
a Instituto de Química, UNESP, CP 355, 14800-900, Araraquara, SP, Brazil
b Departamento de Química, Universidade Federal de São Carlos, CP 676, 13565-905,
São Carlos, SP, Brazil
c Departamento de Química Orgânica e Inorgânica, Universidade Federal do Ceará,
CP 2200, Fortaleza, CE, Brazil
d Departamento de Física e Informatica, Instituto de Física de São Carlos,
Universidade de São Paulo, CP 369, 13560-970, São Carlos, SP, Brazil
Received 21st February 2000, Accepted 2nd August 2000
First published as an Advance Article on the web 7th September 2000
Electrolysis has been examined as a method of synthesis for [(L)(dppb)Ru(µ-Cl)3RuCl(dppb)] complexes, where
dppb = 1,4-bis(diphenylphosphino)butane and L = pyridine (py), 4-methylpyridine (4-pic) or dimethyl sulfoxide
(DMSO), by using [RuCl3(dppb)(L)] as precursors. The products of the electrolysis were characterized by 31P-{1H}
NMR, cyclic voltammetry and near infrared spectroscopy. The presence of the [Ru2Cl5(dppb)2] complex in the
electrochemical cell suggests a mechanism by which the starting original species from the bulk solution reacts with
the reduced form [RuCl2(dppb)(L)] generated at the surface of the electrode. The crystal structure of the precursor
mer-[RuCl3(dppb)(4-pic)] was determined by X-ray diffraction.
oxygen. IR spectra were recorded from CsI pellets on a Bomen-
Introduction
Michelson 102 instrument, UV/Vis spectra in CH2Cl2 with a
HP 8452 A spectrophotometer and 31P-{1H} NMR spectra in
CH2Cl2 solution at room temperature with a Bruker 400 MHz
spectrometer (161 MHz) with the chemical shifts reported
relative to H3PO4 (85%). Cyclic voltammetric measurements
were carried out at room temperature in freshly distilled CH2Cl2
containing 0.1 mol lϪ1 Bu4NϩClO4Ϫ, using an EG&G/PARC
electrochemical system consisting of a 273A potentiostat or
BAS 100B Electrochemical Analyzer. A three-electrode system
with resistance compensation was used throughout. The work-
ing and auxiliary electrodes were a stationary platinum foil and
a wire, respectively. The reference electrode was Ag/AgCl in a
Luggin capillary, 0.1 mol LϪ1 NBu4ClO4 in CH2Cl2, a medium
in which ferrocene is oxidized at 0.43 V vs. Fcϩ/Fc; all poten-
tials are referred to this electrode. In controlled-potential
electrolysis a platinum mesh was used as working electrode and
the auxiliary electrode was separated from the solution by a
sintered-glass disk. Elemental analyses were performed at the
Institute of Chemistry of the University of São Paulo, São
Paulo.
Complexes with general formula [RuCl3(dppb)(L)] were all
obtained by using the same procedure. Thus [RuCl3(dppb)-
(4-pic)] was prepared from [RuCl3(dppb)]ؒ[H2O]14 (0.100 g, 1.53
mmol) and 4-methylpyridine (31 µl, 3.21 mmol) in dichloro-
methane (10 ml) with stirring at room temperature for 12 h.
The volume of the resulting red–orange solution was reduced to
ca. 1 ml and diethyl ether added to precipitate a red–orange
solid which was filtered off, washed well with ether and dried
under vacuum. Suitable crystals for X-ray analysis were grown
by slow evaporation of a dichloromethane–diethyl ether solu-
tion. Yield: 0.087 g (78%). Calc. for C34H35Cl3NP2Ru: C, 56.17;
H, 4.85; N, 1.93%. Found: C, 56.21; H, 5.06; N, 1.99%.
The chemistry of ruthenium() complexes containing a single
bis(phosphine) ligand per metal centre has been of interest
because of its importance in catalytic hydrogenation reac-
tions.1–7 To develop new methodologies for the preparation of
compounds containing the “RuCl2(dppb)” core, we described
recently the synthesis and characterization of [RuCl2(dppb)-
(L)2] complexes from either [RuCl2(dppb)(PPh3)] or [{RuCl2-
(dppb)}2(µ-dppb)] where dppb = Ph2P(CH2)4PPh2 and L = N-
donor ligands.8
The ruthenium complex [Ru2Cl4(BINAP)2(NEt3)] [BI-
NAP = 2,2Ј-bis(diphenylphosphino)-1,1Ј-binaphthyl] has been
successively used as a catalyst for the asymmetric hydrogen-
ation of several prochiral substrates.9–12 This type of complex,
with 1,4-bis(diphenylphosphino)butane, exhibits the general
formula [Ru2Cl4(dppb)2(L)] or [(L)(dppb)Ru(µ-Cl)3RuCl-
(dppb)], where L = NEt3, pyridine, acetone, acetophenone,
dimethyl sulfoxide, dimethyl sulfide, tetramethylene sulfoxide
or tetrahydrothiophene. They form from [RuCl2(dppb)(PPh3)]
or [Ru2Cl4(dppb)2] on reaction with the respective ligands.4,7
In this work we present a new and effective route for the
synthesis of triply bridged binuclear complexes of the type
[(L)(dppb)Ru(µ-Cl)3RuCl(dppb)], by reductive electrolysis of
[RuCl3(dppb)(L)], where L = pyridine (py), 4-methylpyridine
(4-pic) or dimethyl sulfoxide (DMSO).
Experimental
Chemicals employed in this work were of reagent-grade quality
(Aldrich). Tetrabutylammonium perchlorate (Fluka purum)
was recrystallized from ethanol–water and dried overnight,
under vacuum, at 100 ЊC. Reagent-grade solvents (Merck) were
appropriately distilled, dried and stored over Linde 4 Å molec-
ular sieves. Purified argon was used for the removal of dissolved
For the [RuCl3(dppb)(py)] complex: Yield 0.10 g (91.43%).
Calc. for C33H33Cl3NP2Ru: C, 55.59; H, 4.66; N, 1.96%. Found:
DOI: 10.1039/b001422m
J. Chem. Soc., Dalton Trans., 2000, 3383–3386
This journal is © The Royal Society of Chemistry 2000
3383

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Table 1 Crystal data and structure refinement of the [RuCl3(dppb)-
(4-pic)] complex
C, 55.63; H, 4.71; N, 1.91%. The synthesis of [RuCl3(dppb)-
(DMSO)] is reported elsewhere.13
Color/shape
Empirical formula
Formula weight
T/K
Crystal system
Space group
a/Å
b/Å
Red/prism
C34H35Cl3NP2Ruؒ0.5CH2Cl2
768.95
293(2)
Orthorhombic
Pbca
14.785(10)
17.559(5)
27.448(5)
7125.8(54)
8
7.356
X-Ray diffraction analysis of [RuCl3(dppb)(4-pic)]
A red prismatic crystal of the complex was used in the single-
crystal X-ray diffraction experiment. All measurements were
made on a Enraf-Nonius CAD-4 diffractometer with graphite
monochromated Cu-Kα (λ = 1.54184 Å) radiation. Unit cell
parameters and orientation matrices for data collection were
obtained from a least-squares refinement using the setting
angles of 25 reflections in the θ range 11–30Њ. The data were
collected in the ω–2θ scan mode. One standard reflection was
measured every 30 min and used to apply a decay correction.
The maximum decay was 2%. The data collection and reduction
were performed with the programs CAD 414 and XCAD 4,15
respectively. A numerical absorption correction16 implemented
in the program PLATON17 was applied.
The structure was solved by direct methods with SHELXS
86.18 The Fourier map obtained showed all non-hydrogen
atoms. The model was refined by a full-matrix least-squares
procedure on F2 by means of SHELXL 93.19 After inclusion of
the complete complex in the refinement, the Fourier-difference
map showed several peaks near the inversion center at (¹, 0, ¹).
c/Å
V/Å3
Z
µ/mmϪ1
Reflections collected
Independent/observed reflections
6567
6194 [Rint = 0.0829]/2213
[I < 2σ(I)]
6191/1/345
0.0565, 0.1228
0.2352, 0.1589
Data/restraints/parameters
Final R1, wR2 indices [I > 2σ(I)]
(all data)
¯
²
¯
²
They were interpreted as a CH2Cl2 solvent molecule with just
one half per asymmetric unit. The disorder was modeled
including four chlorine atoms and a carbon atom, which could
be interpreted as an independent CH2Cl2 molecule in two
different positions: C(1s), Cl(1s) and Cl(2s); and C(1s), Cl(3s)
and Cl(4s), respectively. The hydrogen atoms of the aromatic
rings and CH2 were set isotropic with a thermal parameter 20%
greater than the equivalent isotropic displacement parameter of
the carbon atom to which each one is bonded; this percentage
was set to 50% for the hydrogen atoms of the methyl group. All
hydrogen atoms were stereochemically positioned and refined
with the riding model.19 The C–H bond lengths in the aromatic
rings, CH2 and CH3 groups were set equal to 0.93, 0.97 and
0.96 Å, respectively. The aromatic rings were treated as rigid
groups and all non-H atoms refined anisotropically, with the
exception of the disordered solvent atoms. Atomic scattering
factors were taken from ref. 20.
Fig. 1 EPR spectrum of [RuCl3(dppb)(4-pic)] (X band frequency) at
Ϫ160 ЊC in the solid state.
The X-ray data collection and experimental details are
summarized in Table 1 and relevant interatomic distances and
angles are listed in Table 2.
CCDC reference number 186/2132.
See http://www.rsc.org/suppdata/dt/b0/b001422m/ for crys-
tallographic files in .cif format.
Results and discussion
IR spectra of the [RuCl3(dppb)(4-pic)] complex show the typ-
ical bands of the coordinated phosphine ligand at 1485, 1096,
1000 and 513 (νP–C), 696 (γC–H), 332 (νP–C) and 274 cmϪ1 (νRu–Cl).8
An absorption at 435 cmϪ1 can tentatively be attributed to
(νRu–N).21 The EPR spectrum shows g values (g1 = 2.487;
g2 = 2.101; g3 = 1.866) (Fig. 1) typical for complexes with strong
rhombic distortions. For the complex with the pyridine ligand
these are g1 = 2.928, g2 = 2.037 and g3 = 1.607.
Fig. 2 An ORTEP view of the asymmetric unit of [RuCl3(dppb)-
(4-pic)], showing the atom labeling and 30% probability ellipsoids.
Fig. 2 is an ORTEP22 view of [RuCl3(dppb)(4-pic)]. The
asymmetric unit consists of one [RuCl3(dppb)(4-pic)] and half
a CH2Cl2 molecule. The ruthenium() ion is in a distorted
octahedral environment; two phosphorus atoms, a chloride
and a nitrogen atom of the 4-methylpyridine group form an
equatorial plane; two other chlorides are in mutually trans
positions, slightly bending toward the 4-methylpyridine group.
The Ru–Cl distances (ca. 2.3 Å) are in the normal range for
ruthenium() complexes.23,24 The Ru–P(2) distance, slightly
longer than the Ru–P(1) in the 4-methylpyridine complex
(Table 2), can be attributed to the better σ-donor properties of
the chloride with regard to the pyridine ring from the ligand
trans to the phosphorus atoms. The distance Ru–P(1) is similar
to the length, 2.326(2) Å,23 found in mer-[RuCl3(PPh3)-
(MeIm)2] (MeIm = 1-methylimidazole) where the PPh3 is trans
to the methylimidazole.
Cyclic voltammetric measurements of [RuCl3(dppb)(4-pic)]
in CH2Cl2 (Fig. 3) reveal an irreversible RuIII → RuII process
[Epc = Ϫ0.08 V]. Our electrochemical data suggest that binu-
clear [Ru2Cl4(dppb)2(4-pic)] and [Ru2Cl5(dppb)2] are formed in
solution according to reactions (2) and (3), which compete with
each other and with (4).
3384
J. Chem. Soc., Dalton Trans., 2000, 3383–3386

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Table 2 Selected bond lengths [Å] and angles [Њ] for the [RuCl3(dppb)-
(4-pic)] complex, e.s.d.s in parentheses
Ru(1)–P(1)
Ru(1)–P(2)
Ru(1)–N(1)
Ru(1)–Cl(1)
Ru(1)–Cl(2)
Ru(1)–Cl(3)
P(1)–C(1)
2.329(3)
2.413(3)
2.209(9)
2.327(3)
2.372(3)
2.344(3)
1.843(7)
1.840(6)
P(1)–C(13)
P(2)–C(16)
P(2)–C(17)
P(2)–C(23)
N(1)–C(33)
N(1)–C(29)
C(29)–C(30)
1.838(11)
1.828(11)
1.839(6)
1.825(6)
1.331(12)
1.367(12)
1.379(14)
P(1)–C(7)
P(1)–Ru(1)–P(2)
P(1)–Ru(1)–Cl(2)
P(1)–Ru(1)–Cl(3)
N(1)–Ru(1)–P(1)
N(1)–Ru(1)–P(2)
N(1)–Ru(1)–Cl(1)
N(1)–Ru(1)–Cl(2)
N(1)–Ru(1)–Cl(3)
Cl(1)–Ru(1)–P(1)
Cl(1)–Ru(1)–P(2)
Cl(1)–Ru(1)–Cl(2)
C(1)–P(1)–Ru(1)
C(23)–P(2)–Ru(1)
C(16)–P(2)–Ru(1)
C(17)–P(2)–Ru(1)
92.59(11)
88.80(11)
92.97(11)
172.9(2)
94.4(2)
86.1(2)
84.2(2)
Cl(1)–Ru(1)–Cl(3)
Cl(2)–Ru(1)–P(2)
Cl(3)–Ru(1)–Cl(2)
Cl(3)–Ru(1)–P(2)
C(33)–N(1)–C(29)
C(33)–N(1)–Ru(1)
C(29)–N(1)–Ru(1)
C(13)–P(1)–C(7)
C(13)–P(1)–C(1)
C(13)–P(1)–Ru(1)
C(7)–P(1)–Ru(1)
170.84(11)
177.94(10)
93.14(10)
88.30(10)
116.7(10)
124.7(8)
118.6(7)
103.9(5)
99.4(4)
Fig.
4
Cyclic voltammograms of the electrolysis product of
86.5(2)
[RuCl3(dppb)(4-pic)] at Ϫ0.30 V in CH2Cl2 in NBu4ClO4, scan rate 100
mV sϪ1, 50% of the content, (——) of [Ru2Cl4(dppb)2(4-pic)] (–––) and
of trans-[RuCl2(dppb)2(4-pic)2] monomer (–·–).
95.00(10)
86.84(9)
91.54(9)
119.2(3)
111.1(3)
115.3(4)
121.6(3)
116.7(4)
114.6(3)
C(32)–C(31)–C(30) 116.0(11)
C(32)–C(31)–C(34) 122.8(14)
C(30)–C(31)–C(34) 121.2(14)
C(33)–C(32)–C(31) 120.6(11)
The 31P-{1H} NMR spectrum of the yellow solid in CH2Cl2
shows a singlet at δ 40.5 and two independent AB quartets
centred at δA 54.80 and δB 45.93 (JAB = 37), δC 52.38 and δD
51.94 (JCD = 43 Hz). The singlet signal was attributed to
[RuCl2(dppb)(4-pic)2] confirmed by independent synthesis.8
The two AB quartet patterns were attributed to the presence of
[Ru2Cl4(dppb)2(4-pic)]. To confirm this the complex was
chemically generated from [RuCl2(dppb)(PPh3)] as described
for other ligands such as amine, dimethyl sulfoxide or carbon
monoxide.4 It was characterized by microanalysis, 31P-{1H}
NMR and cyclic voltammetry. The data obtained were the
same as for the [Ru2Cl4(dppb)2(4-pic)] complex obtained
electrochemically as described above. The cyclic voltammogram
of the electrolysis product is shown in Fig. 4. In this cyclic
voltammogram it is possible to detect the presence of the
mixed valence complex [Ru2Cl5(dppb)2] (peaks 1-1Ј,2-2Ј)
[Epa(1) = 0.13 and Epc(1Ј) = Ϫ0.01; Epa(2) = 0.65 and Epc(2Ј) =
0.60 V].13 There are peaks that certainly belong to binuclear
[Ru2Cl4(dppb)2(4-pic)] (peaks 3-3Ј,5-5Ј) [Epa(3) = 0.48 and
Epc(3Ј) = 0.36; Epa(5) = 1.47 and Epc(5Ј) = 1.28 V]. These values
are in agreement with those found for the respective species
isolated chemically (Fig. 4). The cyclic voltammogram of the
monomer [RuCl2(dppb)(4-pic)2] (Fig. 4) shows an electro-
chemical process (peaks 6-6Ј) coincident with one of those
mentioned above (Epa = 0.51 and Epc = 0.36 V).8
Exhaustive electrolysis of [RuCl3(dppb)(4-pic)] at Ϫ0.3 V
produces only a triply bridged complex, whose identity has
previously been confirmed as [Ru2Cl4(dppb)2(4-pic)] (0.170 g,
94%) (Fig. 4). The same results were achieved for [RuCl3-
(dppb)(py)] and [RuCl3(dppb)(DMSO)]. The electrochemical
process at about 1.18 V in the electrolysed mixture is due to the
oxidation of chloride.25
The compounds [Ru2Cl4(dppb)2(DMSO)] and [Ru2Cl4-
(dppb)2(py)] were obtained similarly by electrolysis from the
corresponding [RuCl3(dppb)(L)] complexes and authenticated
by their known 31P-{1H} NMR data.4
C(29)–C(30)–C(31) 120.5(12)
N(1)–C(33)–C(32)
123.7(12)
Fig. 3 Cyclic voltammograms of [RuCl3(dppb)(4-pic)], 1.0 × 10Ϫ3 mol
Ϫ1, in CH2Cl2 with 0.1 mol lϪ1 NBu4ClO4, scan rate 100 mV sϪ1
l
,
measured at a platinum electrode.
[RuCl3(dppb)(4-pic)] ϩ eϪ →
“RuCl2(dppb)(4-pic)” ϩ ClϪ (1)
[RuCl3(dppb)(4-pic)] ϩ “RuCl2(dppb)(4-pic)” →
Ru2Cl5(dppb)2 ϩ 2 (4-pic) (2)
2 “RuCl2(dppb)(4-pic)” →
Ru2Cl4(dppb)2(4-pic) ϩ 4-pic (3)
“RuCl2(dppb)(4-pic)” ϩ 4-pic →
RuCl2(dppb)(4-pic)2 (4)
To pursue this hypothesis further, we electrolysed (at Ϫ0.30
V) 50% (based on current passing) of the content (0.02 g in 10
ml of CH2Cl2 0.1 mol lϪ1 NBu4ClO4) of [RuCl3(dppb)(4-pic)]
complex. The electrolysed solution was evaporated and the
solid formed extracted with CCl4 (10 ml), resulting in a red
solution and a pale yellow solid (9.7 mg; mixture of mono and
binuclear ruthenium() complexes). This solid was separated by
filtration and the red solution collected and concentrated to
1 ml; addition of hexane (10 ml) yielded a bright red solid (0.6
mg, 3.47%; based on Ru). The red solid was characterized as a
mixed valence complex [Ru2Cl5(dppb)2] by UV/Vis/Near IR,
cyclic voltammetry and EPR as described elsewhere.13,24
Acknowledgements
We thank CNPq, CAPES, FINEP and FAPESP for financial
support. We are also indebted to Dr Graham A. Heath for
reading the manuscript and for discussions.
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J. Chem. Soc., Dalton Trans., 2000, 3383–3386