Dbf-ruthenocenes: Towards chiral halfsandwich Lewis acidic Dbf complexes

Article abstract of DOI:10.1016/j.ica.2011.03.038

In order to develop a new class of chiral metallocene catalysts, halfsandwich ruthenocenes of dibenzo[c,g]fluorenide (Dbf^-1) have been prepared. Structural and spectroscopic characterization of (η⁵- Dbf)Ru(PPh₃)₂

Full text of DOI:10.1016/j.ica.2011.03.038

Inorganica Chimica Acta 374 (2011) 205–210
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Inorganica Chimica Acta
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Dbf–ruthenocenes: Towards chiral halfsandwich Lewis acidic Dbf complexes
⇑
Frank Pammer, Yu Sun, Werner R. Thiel
FB Chemie, Technische Universität Kaiserslautern, Erwin-Schrödinger-Strasse, Geb. 54, 67633 Kaiserslautern, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 21 March 2011
In order to develop
a new class of chiral metallocene catalysts, halfsandwich ruthenocenes of
dibenzo[c,g]fluorenide (Dbfꢀ1) have been prepared. Structural and spectroscopic characterization of
(g g
5-Dbf)Ru(PPh3)2Cl and [( 5-Dbf)Ru(PPh3)2(NCMe]+[SbF6]ꢀ provide evidence for strong metal–ligand
binding in these sterically crowded complexes. In the solid state the complexes assume conformations
ideal for transfer of the stereo information unto a prochiral substrate.
Ó 2011 Elsevier B.V. All rights reserved.
Dedicated to Prof. Dr. Wolfgang Kaim on the
occasion of his 60th birthday.
Keywords:
Ruthenocenes
Dibenzofluorenide
X-ray structure
1. Introduction
tion of cationic halfsandwich complexes of the group VIII-metals
iron and ruthenium in asymmetric catalysis. In the past two dec-
Due to its molecular structure, allowing the ‘‘localization’’ of
aromaticity in the five-membered and in two of the four six-mem-
bered rings, the dibenzo[c,g]fluorenide ligand (Dbfꢀ1) is capable of
stabilizing transition metal centers almost as efficiently as the
cyclopentadienide ligand (Cpꢀ1) does. In our investigations on this
ligand system we were able to show the high stability of Dbf com-
plexes with a series of examples and could elucidate the electronic
structure of these compounds by studies on their reactivity and by
means of theoretical calculations [1]. In addition to its electronic
features, the dibenzo[c,g]fluorenide ligand is an intrinsically chiral
Cp derivative, since it possesses a binaphthyl backbone. Although it
readily racemizes at low temperature, studies on the catalytic
activity of Dbf complexes are beneficial due to the fact that substi-
tuting the hydrogen atoms in the 8- and 80-positions results in con-
formationally stable Dbf analogs. Since ruthenium catalyzes a
series of (enantioselective) transformations we therefore were
interested in the synthesis and characterization of Dbf complexes
of this metal.
ades a number of Lewis-acid catalysts with chiral phosphane li-
gands have been synthesised that facilitate enantioselective
Diels–Alder-reactions [9]. Herein we report the synthesis and
structural characterization of ruthenium halfsandwich complexes
bearing the dibenzo[c,g]fluorenide ligand (Dbfꢀ1), which can be
considered as precursors for the development of chiral organome-
tallic ruthenium based Lewis acid-catalysts.
2. Experimental
2.1. General remarks
Elemental analyses were carried out at the Department of
Chemistry (TU Kaiserslautern). Infrared spectra were recorded
with a Perkin–Elmer FT-IR 1000 spectrometer. NMR spectra were
recorded with a Bruker Avance 400 spectrometer. The assignment
of the NMR data is according to Scheme 1. All reactions were car-
ried out under an atmosphere of dinitrogen.
Fluorenyl (Flu) complexes of ruthenium have only scarcely been
described in literature. Aside from
g
1-fluorenide complexes [2],
2.2. Chloro(
triphenylphosphineruthenium(II) (1)
g
5-dibenzo[c,g]fluorenide)
and the sandwich compounds (Cp⁄)Ru(Flu) [3,4] (Cp⁄: 1,2,3,4,5-
pentamethylcyclopentadienyl) and (Cp⁄)Ru(Flu⁄) [5], (Flu⁄:
1,2,3,4,5,6,7,8,9-nonamethylfluorenyl) and a (
g g
5-Flu)( 6-cymene)
66.6 mg (250
ene in a Schlenk vessel. The solution was cooled to ꢀ15 °C and
172 l of n-butyllithium (275 mol, 1.6 M in hexane) was added
lmol) of DbfH were dissolved in 5 ml of dry tolu-
complex [6] only few
g
5-fluorenide ruthenium complexes have
been reported [7], including an ansa-carborane compound [8].
One recent development in metallocene chemistry is the applica-
l
l
drop-wise. While slowly warming to ambient temperature, the
solution was stirred for 3 h. A colorless precipitate of (Dbf)Li
formed during this time. This suspension was stirred at ambient
⇑
Corresponding author. Tel.: +49 631 2052752; fax: +49 631 2054676.
E-mail address: thiel@chemie.uni-kl.de (W.R. Thiel).
temperature while 239.7 mg (250 lmol) of finely powdered
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.03.038

206
F. Pammer et al. / Inorganica Chimica Acta 374 (2011) 205–210
tion to ꢀ40 °C to yield 37.1 mg of the complex as orange red crys-
9
3
tals suitable for X-ray diffraction. The crystalline material contains
half an equivalent of CH2Cl2 and an additional 1.5 equivalents of
acetonitrile. Taking into account the solvents of crystallization, this
4
5
amounted to a yield of 29.2 lmol (81%). Alternatively 2 can be
8
6
purified by layering a solution of the complex in CH2Cl2 with
Et2O. The crystalline material obtained in this fashion is bright
red in color and contains half an equivalent of CH2Cl2 and Et2O
7
Scheme 1. Numbering of the NMR spectra.
each. Yield: 37.1 mg (29.2
l
mol, 81%). 1H NMR (400 MHz, CDCl3,
3
293.5 K): d [ppm] = 9.07 (d, 2H, JHH = 8.3 Hz, 8,80-H), 7.88 (t, 2H,
3JHH = 7.7 Hz, 7,70-H), 7.61 (t, 2H, JHH = 7.5 Hz, 6,60-H), 7.42 (d,
3
Ru(PPh3)3Cl2 [15] and another 15 ml of dry toluene were added.
The initially formed brown suspension turned to red within
20 min. The reaction mixture was stirred for another 20 h, was fil-
tered and the residue was extracted with another 5 ml of dry tolu-
ene. The obtained dark red solution was concentrated and brought
to crystallize at ꢀ40 °C. The mother liquor was removed via a can-
nula and the remaining crystals were washed with a mixture of
toluene/pentane (1:5, v:v). Three crops of dark red crystals were
gathered by repeated concentration of the mother liquor and cool-
ing to ꢀ40 °C. The product obtained in this fashion contained two
equivalents of solvent ((Dbf)Ru(PPh3)2Clꢁ2 toluene). A total of
110 mg of product were isolated. Taking the solvent into account
3
3
2H, JHH = 7.8 Hz, 5,50-H), 7.21 (t, 6H, JHH = 7.3 Hz, PPh3: p-H),
3
3
6.98 (t, 12H, JHH = 7.6 Hz, PPh3: m-H), 6.86 (d, 2H, JHH = 8.9 Hz,
3,30/4,40-H), 6.76–6.84 (m, 12H, PPh3: o-H), 6.16 (d, 2H,
3JHH = 8.9 Hz, 3,30/4,40-H), 4.21 (s, 1H, 9-H), 2.25 (s, 3H, NCMe).
13C NMR (100 MHz, CDCl3, 293.5 K): d [ppm] = 107.9 ( 5-C5),
g
93.4 (g
5-C5), 60.4 (9-CH), 4.9 (NCMe), the 13C-resonanzes in the
aromatic region could not be interpreted due to PC-coupling and
signal broadening and slow decomposition of the complex. 31P
NMR (162 MHz, CDCl3, 293.6 K): d [ppm] = 49.84. IR (KBr):
m
[cmꢀ1] = 3141 (w), 3447 (m), 2973 (w), 2925 (w), 2865 (w), 2265
(w), 1967 (br w), 1903 (br w), 1827 (br w), 1777 (br w), 1615
(w), 1586 (w), 1574 (w), 1542 (w), 1512 (w), 1499 (w), 1480 (s),
1435 (s), 1313 (m), 1266 (w), 1247 (w), 1210 (w), 1184 (m),
1160 (w), 1118 (w), 1089 (s), 1027 (m), 1000 (m), 936 (w), 920
(w), 858 (m), 806 (m), 744 (s), 697 (s), 658 (s), 625 (w), 581 (w),
530 (s), 521 (s), 496 (s), 467 (w), 427 (m). MALDI-TOF MS: m/
z = 629.256; calc. (C39H38PRu) = [(Dbf)Ru(PPh3)]+ = 629.10. Mp =
this amounts to a yield of 99 lmol, 40%. Single crystals for charac-
terization by X-ray diffraction were obtained from the third crys-
tallization fraction at ꢀ40 °C. The complex could also be purified
by layering a solution in CH2Cl2 with pentane. The microcrystalline
material obtained in this fashion did not contain solvent, but was
not suitable for X-ray diffraction. 1H NMR (400 MHz, CDCl3,
3
293.5 K): d [ppm] = 9.27 (d, 2H, JHH = 8.3 Hz, 8,80-H), 7.69 (t, 2H,
3JHH = 7.3 Hz, 7,70-H), 7.47 (t, 2H, JHH = 7.4 Hz, 6,60-H), 7.32 (d,
3
3
2H, JHH = 7.2 Hz, 5,50-H), 7.00-7.22 (br m, 18H, PPh3: o-H/p-H),
3
6.70–6.92 (br, 12 H, PPh3: m-H), 6.68 (d, 2H, JHH = 8.9 Hz, 3,30/
Table 1
Crystallographic data for 1 and 2.
3
4,40-H), 6.37 (d, 2H, JHH = 8.9 Hz, 3,30/4,40-H), 3.88 (s, 1H, 9-H).
1H NMR (400 MHz, CDCl3, 325.4 K):
d
[ppm] = 9.30 (d, 2H,
Compound
1
2
3JHH = 8.3 Hz, 8,80-H), 7.69 (t, 2H, JHH = 7.9 Hz, 7,70-H), 7.48 (t,
3
Formula
Formula weight
T (K)
C
67.5H51Cl2P2Ru
C62.5H51.5ClF6N2.5P2RuSb
1271.77
150(2)
3
3
2H, JHH = 7.4 Hz, 6,60-H), 7.34 (d, 2H, JHH = 7.7 Hz, 5,50-H), 7.11-
1060.54
150(2)
3
7.20 (m, 12H, PPh3: o-H), 7.05 (t, 6H, JHH = 7.2 Hz, PPh3: p-H),
3
3
Wavelength (Å)
1.54184 (Cu K
a)
0.71073 (Mo Ka)
6.84 (t, 12H, JHH = 7.3 Hz, PPh3: m-H), 6.70 (d, 2H, JHH = 8.9 Hz,
Crystal size (mm) 0.20 ꢂ 0.11 ꢂ 0.08
0.58 ꢂ 0.26 ꢂ 0.17
3
3,30/4,40-H), 6.34 (d, 2H, JHH = 8.9 Hz, 3,30/4,40-H), 3.90 (s, 1H, 9-
Crystal system
Space group
a (Å)
monoclinic
P21/c
triclinic
H). 13C NMR (100 MHz, CDCl3, 293.5 K): d [ppm] = 134.4 (br),
133.3, 132.1 (d, JPC = 9.9 Hz), 131.7, 131.6, 129.3, 126.6 (d,
JPC = 12.2 Hz), 128.4 (br), 126.9 (t, JPC = 4.4 Hz), 126.1, 124.8,
ꢀ
P1
20.5204(2)
11.95810(10)
20.9918(2)
90
11.0368(2)
21.8684(5)
23.8217(5)
69.657(2)
88.7380(10)
88.354(2)
5388.26(19)
4
b (Å)
c (Å)
120.1, 108.4 (
NMR (162 MHz, CDCl3, 293.5 K):
(162 MHz, CD2Cl2, 293.5 K):
g
5-C), 94.2 (t, JPC = 3.1 Hz,
g
5-C), 56.2 (9-CH). 31P
[ppm] = 48.40. 31P NMR
[ppm] = 47.86. IR (KBr):
a
(°)
d
b (°)
93.5910(10)
90
5140.96(8)
4
1.370
3.859
d
m
c
(°)
[cmꢀ1] = 3051 (m), 2916 (w), 1615 (br, m), 1585 (w), 1571 (w),
1495 (w), 1481 (m), 1456 (w), 1433 (s), 1313 (w), 1262 (w),
1247 (w), 1189 (w), 1157 (w), 1118 (w), 1086 (m), 1049 (w),
1029 (w), 999 (w), 937 (w), 853 (w), 827 (w), 799 (m), 758 (w),
740 (m), 730 (w), 695 (s), 621 (w), 530 (w), 521 (s), 499 (w), 490
(w), 465 (m), 433 (w), 424 (w). MALDI-TOF MS: m/z = 629.256.
Calcd.: (C39H38PRu) = [(Dbf)Ru(PPh3)]+ = 629.10. Mp: decomposi-
tion above 110 °C. Elemental Anal. Calc. for: C57H43ClP2Ru: C,
73.90; H, 4.68, exp.: C 73.65, H 5.13.
V (Å3)
Z
qcalc (g/cm3)
1.568
0.955
l
(Cu K
a
) (mmꢀ1
)
h range (°)
Index ranges
4.26–62.67
ꢀ23 6 h 6 22,
ꢀ13 6 k 6 13,
ꢀ23 6 l 6 24
26 450
4.02–32.41
ꢀ15 6 h 6 16,
ꢀ28 6 k 6 32,
ꢀ35 6 l 6 34
65 278
Reflection
collected
Unique
8119 (0.0248)
33 835 (0.0282)
reflections
(Rint
Absorption
correction
)
2.3. (Acetonitrile)(
triphenylphosphineruthenium(II) hexafluoroantimonate (2)
g
5-dibenzo[c,g]fluorenide)
semi-empirical from equivalents (multiscan)
Data/restraints/
parameters
8119/30/659
33 835/21/1374
1.097
Forty milligrams of (Dbf)Ru(PPh3)2Clꢁ(C7H8)2 (36
lmol) were
Goodness-of-fit
(GOF) on F2
Final R indices
1.072
dissolved in 10 ml of dry CH2Cl2 in a Schlenk tube. Dry acetonitrile
(0.1 ml) was added, the reaction mixture was cooled to 0 °C and
12.4 mg (36.3 mmol) AgSbF6 were added as a solid. While slowly
warming to ambient temperature the red solution was stirred
overnight. Subsequently the precipitate of AgCl was filtered off
and the orange-red solution was concentrated until crystallization
set in. The product was isolated by cooling the concentrated solu-
R1 = 0.0246, wR2 = 0.0678
R1 = 0.0288, wR2 = 0.0691
R1 = 0.0577, wR2 = 0.1727
R1 = 0.0841, wR2 = 0.1991
6.819/ꢀ4.797
[I > 2r
(I)]a
R indicesa (all
data)
D
qmax/min (e Åꢀ3
)
0.373/ꢀ0.508
[w(Fo ꢀ Fc2)2]/
R .
[w(Fo2)2]}1/2
a
2
R1
=
R
||Fo| ꢀ |Fc||/
R
|Fo|; wR2 = {
R

F. Pammer et al. / Inorganica Chimica Acta 374 (2011) 205–210
207
161–162 °C. Elemental Anal. Calc. for C59H46F6NP2RuSbꢁ(1/2CH2Cl2)ꢁ
(2/3CH3CN): C, 60.63; H, 4.05; N, 1.95. Found: C, 59.97; H, 3.82; N,
2.02%.
Dbf ligand is observed at 3.90 ppm. Compound 1 is thus the only
5-Dbf complex up to now that causes an upfield shift of the 9-H
g
proton when compared to the free ligand DbfH (4.14 ppm in CDCl3).
The signals of the 8,80-protons appear distinctly separated from the
3
other aromatic resonances at 9.30 ppm (d, JHH = 8.3 Hz). Via HH-
2.4. X-ray structure analyses
COSY NMR the 7,70- and 6,60-CH groups can be unambiguously as-
signed to triplets at 7.69 ppm (3JHH = 7.9 Hz) and 7.48 ppm
(3JHH = 7.4 Hz), while the 5,50-protons appear at 7.34 ppm (d,
3JHH = 7.7 Hz). The 3,30- and 4,40-protons yield doublets a 6.70 and
6.34 ppm. The phenyl groups of the phosphine ligands give three
signal groups in the 1H NMR spectrum. The para-protons yield a
Single crystals of compounds 1 and 2 were carefully affixed
with an adhesive onto the tips of glass fibers. The glass fibers were
subsequently mounted on the goniometer head in a nitrogen
stream at 150 K. Table 1 summarizes the cell parameters as well
as the experimental details of the data collection and the structure
refinements of both crystals. The structures were solved using di-
rect method (SIR92) [10] and completed by subsequent difference
Fourier syntheses, and refined by full-matrix least-squares proce-
dures [11]. Semi-empirical absorption corrections from equiva-
lents (Multiscan) were carried out [12]. All non-hydrogen atoms
were refined with anisotropic displacement parameters. The qual-
ity of the crystals of compound 2 which were available for X-ray
structure analysis was limited. The insufficient absorption correc-
tion left a high residual electron density around Sb1, the heaviest
atom in the system. Additionally the counter-anion SbF6ꢀ, the
un-coordinated co-solvents CH3CN and CH2Cl2 were partially dis-
ordered. The disorder treatment of all these positions resulted in
unstable refinements. However, in the structure of the cation
[(Dbf)Ru(PPh3)2(NCMe]+ no obvious abnormality was observed.
All hydrogen atoms were placed in calculated positions and refined
by using a riding model.
3
triplet at 7.05 ppm (6H, JHH = 7.2 Hz), the signals of the meta-
and ortho-hydrogen atoms can then be can be assigned by HH-COSY
NMR at 6.84 ppm (t, 3JHH = 7.7 Hz) and 7.11–7.20 ppm (m), respec-
tively. In the 13C NMR spectrum only the resonances of the carbon
atoms of the
g
5-coordinated ring can be identified at 56.2 ppm for
the 9-CH group and at 91.2 and 108.4 ppm for the quaternary
carbon atoms. The remaining aromatic resonances could not be
interpreted due to line broadening, superposition and PC-coupling.
Single crystals of 1 for examination by X-ray diffraction were
obtained by diffusion of pentane into a solution of the complex
in toluene. Compound 1 crystallises as dark red prisms in the
monoclinic space group P21/c with four complex molecules and
two molecules of toluene in the unit cell (Fig. 1). As the molecular
structure shows, the bulky phosphine ligands impose a consider-
able steric strain on the Dbf moiety. The chiral ligand forms a
relatively large torsion angle C10–C1–C11–C20 of 13.684(2)°,
as compared to other
Mn(CO)3: 10.75(5)° [1a], maximum (
g
5-Dbf complexes (minimum (
g
5-Dbf)
g
5-Dbf)( 3-allyl)Mo(CO)3:
g
3. Results and discussion
ꢀ15.6(13)° [1b]). This large torsion angle however does not corre-
late with the steric pressure of the PPh3-groups, which causes a
massive deformation of the Dbf framework and results in the clos-
est approach of the 8,80-protons yet observed (H9, H19 in Fig. 1,
1.9233 Å). The phosphine ligands are positioned ‘‘in front’’ of the
Dbf ligand, thus evading the binaphthyl system. Still, two phenyl
groups protrude beneath the naphthyl arms. The phenyl group
The halfsandwich complex (
by heating RuCl3 with PPh3 in ethanol in the presence of an excess
of cyclopentadiene (CpH) [13]. The formation of
5-Ind)
g
5-Cp)Ru(PPh3)2Cl can be obtained
(
g
Ru(PPh3)2Cl [14] can be achieved in the same fashion, although
the use of Ru(PPh3)3Cl2 [15] as the starting material and the addi-
tion of KOH is necessary. By reaction of DbfLi with Ru(PPh3)3Cl2 in
dry toluene, the halfsandwich complex (g
5-Dbf)Ru(PPh3)2Cl (1)
can be isolated in satisfactory yield. It has to be pointed out that,
due to the good solubility of Ru(PPh3)3Cl2 in toluene, the reaction
occurs in the neat solvent, without additional donors like THF or
Et2O (Scheme 2).
C6
C7
C8
C10
C18
C4
C17
C16
C5
C19
The complex can be isolated by crystallization either from a
concentrated solution in toluene at ꢀ40 °C or by overlaying a solu-
tion of 1 in CH2Cl2 with pentane. The MALDI-TOF mass spectrum of
the complex shows an intense signal around m/z = 629 that corre-
sponds to the [(Dbf)Ru(PPh3)]+-fragment. The isotope pattern
proves the presence of the presumed mononuclear ruthenium spe-
cies (see Fig. 8 in Supplementary material). The [(Dbf)Ru(PPh3)2]+
cation, which initially formed upon ionization by splitting off of
the chloro ligand, is obviously not sufficiently persistent to be
detected.
C9
C3
C20
C1
C15
C11
C12
C2
C21
C14
C13
Ru1
Cl1
P1
The PPh3 ligands of 1 yield one signal in the 31P NMR at 48.7 ppm
(CDCl3), which provides strong evidence for a rapid racemisation of
P2
the
g
5-coordinated Dbf ligand. At ambient temperature signal
broadening occurs in the 1H NMR. Therefore 1H NMR spectra have
been recorded at 40 °C. The resonance of the 9-H proton of the
Fig. 1. Molecular structure of (g
5-Dbf)Ru(PPh3)2Cl (1) in the solid state. The crystal
Ru(PPh3)3Cl2
+ DbfLi
Ru
Toluol
PPh3
PPh3
structure contains two molecules of toluene, which have been omitted for clarity.
Characteristic bond lengths (Å) and angles (°): Ru1–C1 2.317(2), Ru1–C2 2.339(2),
Ru1–C21 2.199(2), Ru1–C12 2.265(2), Ru1–C11 2.331(2), Ru1–Ct1 1.9356, Ru1–Cl1
2.4330(5), Ru1–P1 2.3084(6) Ru1–P2 2.3074(5), C3–C4 1.341(3), C13–C14 1.343(3),
H9–H19 1.9233 C10–C1–C11–C20 13.684(2).
R.T., 20 h
Cl
40 %
g
5-Dbf)Ru(PPh3)2Cl starting from Ru(PPh3)3Cl2.
Scheme 2. Synthesis of (

208
F. Pammer et al. / Inorganica Chimica Acta 374 (2011) 205–210
containing the atoms C52–C53–C54–C55–C56–C57 assumes an al-
most co aligned conformation with respect to the C11–C12–C13–
C14–C15–C20-ring in the upward bent naphthyl arm, as evidenced
by an angle of only 2.484° between the two ring planes. A similar
orientation is not found for the phenyl group interacting with the
downward bent naphthyl arm. Instead, on order to avoid the steric
pressure inflicted by the PPh3 unit, the whole naphthyl system
curves upward and twists, before bending down again. Quantita-
tively this deformation can be demonstrated by the positions of
the protons H4, H6 and H9 relative to the five-membered ring
C1–C2–C21–C12–C11. H4 and H6 are lifted by 1.0592 and
0.9083 Å above the ring plane, while H9 is located ꢀ0.9617 Å
beneath.
procedure by reacting (1) with AgSbF6 in the presence of acetoni-
trile, to give the ionic compound [(Dbf)Ru(PPh3)2(NCMe)]+[SbF6]ꢀ
(2) in over 80% yield (Scheme 3). The complex crystallizes well,
but inevitably incorporates half an equivalent of dichloromethane
as well as Et2O or acetonitrile into the crystal lattice, depending on
the crystallization procedure. Analogous compounds bearing inde-
nyl- and cyclopentadienyl ligands such as ([(
g
5-Ind)Ru(PPh3)2-
g
5-Cp)Ru(PPh3)2(NCMe)]+(BF4ꢀ) [20])
(NCMe)]+(BF4ꢀ) [19] and [(
are also known.
The 1H NMR spectrum in CDCl3 proves the conversion of the
complex. The signal of the 8,80-protons has shifted as compared
to the parent compound 1 (9.07 versus 9.30 ppm). The resonance
of the 9-H group is now observed at 4.21, 0.3 ppm downfield from
where the corresponding signal for 1 is found (3.90 ppm). Curi-
ously, the resonances in the 31P NMR are not affected by the ligand
exchange. The complex exhibits a 31P resonance at 49.8 ppm
The chloro ligand is positioned between the naphthyl groups
2.4330(5) Å from the ruthenium center, forming a dihedral angle
Cl1–Ru1–C21–H21 of 150.494(1)°, thus deviating significantly
from an ideal trans position. Curiously it has to be noted that the
anion is shifted towards the downward pointing naphthyl arm.
Furthermore several Cl–H contacts ranging from 2.6 to 3.2 Å
(H53–Cl1 2.6454 Å, H39–Cl1 2.7728 Å, H27–Cl1 2.7798 Å, H47–
Cl1 3.1714 Å) are found. With a Ru–Ct1 distance of 1.9356 Å the
Dbf ligand is less closely bound to the metal than the indenyl frag-
(CDCl3), ((
proves the rapid racemisation of the
g
5-Dbf)Ru(PPh3)2Cl: 48.7 ppm, CDCl3), which again
5-coordinated Dbf ligand
g
at room temperature. The 13C NMR resonances of the five-mem-
bered ring can be easily assigned to signals at 107.9 and
93.4 ppm for the quaternary carbon atoms and 60.4 for the carbon
atom of the 9-CH group. The coordinated molecule of acetonitrile
can be identified by NMR spectroscopy as well. The signal of the
methyl group is detected in the 1H NMR spectrum at 2.25 ppm,
thus being shifted downfield by 0.15 ppm as compared to the free
molecule in solution. The effects of coordination are even more
prominent in the 13C NMR spectrum: here resonances at
4.9 ppm, for the methyl group and at around 130 ppm for the
nitrile carbon atom were observed (free acetonitrile: 1.9,
116.4 ppm). The nitrile resonance could not be individually
assigned, due to signal broadening and PC-coupling. Nevertheless,
ment in (g
5-Ind)Ru(PPh3)2Cl (1.9175(2) Å) [16]. In the analogous
Cp [17] and Cp⁄ [18] complexes the distance between the metal
center and ring plane is significantly smaller as well (1.8466(8)
and 1.8875(2) Å). The deformation of the ligand backbone results
in an unsymmetrical coordination of the five-membered ring to
the central atom. While the quaternary carbon atoms C1, C2, and
C11 exhibit surprisingly similar carbon–metal bond lengths
(Ru1–C1 2.317(2) Å, Ru1–C2 2.339(2) Å, Ru1–C11 2.331(2) Å), the
bonds to C21 and C12 appear significantly shortened in compari-
son (2.199(2) and 2.265(2) Å). Thus the ruthenium atom is shifted
away from the ring center in direction to the bond C21–C12, due to
the steric interaction of the P1–PPh3 group with the downward
pointing naphthyl moiety. The PPh3 groups are located in almost
identical distances from the metal center (Ru1–P1 2.3084(6) Å,
Ru1–P2 2.3074(5) Å). The corresponding Ru–P bonds in
CpRu(PPh3)Cl and Cp⁄Ru(PPh3)Cl were found to be 2–4 pm longer
(CpRu(PPh3)Cl: Ru–P 2.3366(11), 2.3353(9) Å; Cp⁄Ru(PPh3)Cl: Ru–
a
correlation signal in the HMBC-NMR spectrum of the
complex unambiguously identifies the quaternary carbon atom
connected to the above mentioned methyl group (see Fig. 15 in
the Supplementary material).
ꢀ
The complex crystallizes in the triclinic space group P1. There
are two crystallographically independent molecules present in
the unit cell. The two structures – in the following referred to as
a
and b – differ only slightly from one another and will thus be
P 2.3449(6), 2.3363(6) Å). In (
Ru–P bond length are found, due to the unequal steric interaction
of the indenyl ring with the two phosphine moieties ((
5-In-
g
5-Ind)Ru(PPh3)2Cl two differing
discussed together. In the solid state the cationic molecules form
ꢀ
layers with the SbF6 counteranions. Further remaining cavities
g
in the crystal lattice are occupied by solvent molecules, amounting
to 0.5 equivalents of CH2Cl2 and 1.5 equivalents of acetonitrile per
complex molecule. The structure of one of the cations
[(Dbf)Ru(PPh3)2(NCMe)]+ in the solid state is presented in Fig. 2.
The solid state structure of 2 is similar to the structure found for
1: the PPh3 ligands assume positions on the side of the complex
that not shielded by the Dbf-system, while the acetonitrile is found
in between the two naphthyl moieties. As has been observed for
the chloro complex, the acetonitrile group is not coordinated in a
symmetrical way. Instead a noticeable shift towards the downward
pointing naphthyl arm is found, leading to a dihedral angle H21–
d)Ru(PPh3)2Cl: Ru–P 2.2681(5), 2.3306(6) Å), one being in the
range of the Cp and Cp⁄ complexes, while the other is even shorter
than the Ru–P distances found for the Dbf complexes discussed
here. The double bonds in position 3–4 and 30–40 appear to be sig-
nificantly shortened, with bond lengths of C3–C4 = 1.341(3) Å and
C13–C14 = 1.343(3) Å, respectively.
Since cationic complexes of the type [Cp(PPh3)2Ru(solv)]X (solv:
coordinating solvent molecule; X: weakly coordinating anion) cat-
alyze Diels–Alder reactions, we were interested in a similar system
bearing the Dbf instead of the Cp unit. A number of such catalyti-
cally active cationic systems have been described [9]. The chloro
C21–Ru1–N1 of ꢀ150.948(6)° (2-
a, 2-b: H81–C81–Ru2–N2
ꢀ146.477(7)°). Accordingly the angles between the ruthenium,
ligand in (
g
5-Dbf)Ru(PPh3)2Cl (1) can be cleaved off via a standard
nitrogen atom and the nitrile carbon atoms C58 deviate signifi-
cantly
– by 9–10° – from the ideal value: 2-a, Ru1–N1–
C58 = 169.841(4)°; 2-b, Ru2–N2–C118 = 170.895(4)°. The distances
of the nitrogen atoms from the ruthenium centers are slightly
longer than in the corresponding Cp complex Ru1–N1 =
2.047(3) Å (2-a, 2-b: Ru2–N2 = 2.050(3) versus 2.0412(40) Å). It
has to be noted, that the triple bonds of the acetonitrile ligands
F
AgSbF6
F
F
F
F
Ru
Sb
F
Ru
CH2Cl2/NCMe
0º - R.T., 20 h
PPh3
PPh3
PPh3
PPh3
Cl
N
are longer than expected for a noncoordinating nitrile (C–C„N:
1.136 Å [21], 2-
while the corresponding bond in the Cp analog [(g5
Cp)Ru(PPh3)2(NCMe)]BF4 was found to be significantly shorter
(1.1283 Å) [20]. The ruthenium-phosphorus bonds in 2- (Ru1–
a: N1–C58 1.150(5) Å, 2-b: N2–C118 1.144(5) Å),
-
81 %
Scheme 3. Synthesis of [(Dbf)Ru(PPh3)NCMe]+(SbF6)ꢀ (2).
a

F. Pammer et al. / Inorganica Chimica Acta 374 (2011) 205–210
209
In both complexes discussed above, the sterically demanding
PPh3 ligands assume positions opposite to the binaphthyl system,
leaving only the coordination position beneath the binaphthyl sys-
tem for the remaining fourth ligand. In this position the substrate
of a to-be-synthesised catalytically active complex must be coordi-
nated (e.g. acroleine). Due to the torsion of the binaphthyl system,
one enantiofacial side of the substrate would always be shielded
more effectively than the other. This should allow the catalyst to
differentiate between the two enantiotopic sides. Therefore com-
plexes like 2 can be used as Lewis-acid-catalysts directly. However,
it has been shown, that yields and enantioselectivities achieved
with acetonitrile complexes are often unsatisfactory. Much better
results are obtained when the adduct of the complex with an alde-
hyde is used [9a]. The substitution of the chloro ligand or the ace-
tonitrile group by acroleine was not successful for 1 and 2 up to
now.1
C15
C16
C12
C17
C18
C8
C9
C14
C13
C7
C19
C20
C10
C11
C1
C2
C6
C5
C21
C4
C3
N1
Ru1
P1
C59
C58
P2
Compound 2 showed no catalytic activity in [4 + 2]-cycloaddi-
tion of acroleine to cyclopentadiene, which is a standard reaction
for Diels–Alder catalysts. This may be attributed to the massive
steric crowding of the active center, which inhibits the displace-
ment of the acetonitrile ligand and the approach of reaction part-
ners to the substrate. It is also possible, that the PPh3 ligands are
too basic, thus lower the Lewis acidity of the metal center beneath
the activity threshold. As mentioned by Kündig [9h,i] electron rich
phosphines can inhibit catalytic activity. Much better results might
therefore be achieved with perfluorinated phosphites.
Fig. 2. Molecular structure of one of the cations [(Dbf)Ru(PPh3)2(NCMe)]+ in the
solid state. The crystal structure contains additional solvent molecules and anions,
which have been omitted for clarity. Characteristic bond lengths (Å) and angles (°):
Ru1–C1 2.344(4), Ru1–C12 2.344(4), Ru1–C2 2.278(4), Ru1–C11 2.300(4), Ru1–N1
2.047(3), Ru1–P2 2.3233(10), Ru1–P1 2.3323(10), Ru1–C21 2.214(4), C3–C4
1.353(6), C13–C14 1.343(6), P1–Ru1–P2 95.72(4), P1–Ru1–N1 92.70(9), P2–Ru1–
N1 92.50(9) Ru1–N1–C58 169.8(3).
4. Conclusion
P1 2.3323(9) Å, Ru1–P2 2.3233(10) Å) and 2-b (Ru2–P3
2.3235(9) Å, Ru2–P4 2.3302(9) Å) are elongated by 1.5–2.5 pm as
compared to (Dbf)Ru(PPh3)2Cl (Ru1–P1 2.3084(6) Å, Ru1–P2
2.3074(5) Å). The torsion angles of the binaphthyl systems C10–
C1–C11–C20 and C70–C61–C71–C80 yield different values for the
Treatment of (PPh3)3RuCl2 with LiDbf results in the formation of
(g
5-Dbf)Ru(PPh3)2Cl. The chloro ligand can be exchanged against
an acetonitrile ligand by reacting this compound with AgSbF6 in
the presence of acetonitrile leading to the ionic compound
[(Dbf)Ru(PPh3)2(NCMe)]+[SbF6]ꢀ. The
g
5-coordinated Dbf ligands
two conformers (2-
a
: ꢀ14.654(9)°, 2-b: ꢀ17.696(9)°) and in conse-
quence lead to different distances between 8,80-protons H9–H19
(1.9534 Å) and H69–H79 (2.0063 Å). In spite of the larger torsion
angle the deformation of the ligand backbone appears to be less
pronounced in 2 than in (Dbf)Ru(PPh3)2Cl. For example in the
undergo rapid racemization in both cases. However, in both solid
state structures, the chiral conformation of the Dbf ligand is frozen.
We are presently working on stereochemically stable Dbf deriva-
tives which may open up a new concept for enantioselective
catalysis.
downward pointing naphthyl arms the H14 and H16 (2-a, 2-b:
H74, H76) are lifted by 0.8703 and 0.5105 Å (0.9141 and
0.5507 Å) above the corresponding planes of the five-membered
rings ((Dbf)Ru(PPh3)2Cl: H4 1.0592 Å, H6 0.9083 Å). The twist of
the terminal phenylene rings also is less pronounced. The distance
Appendix A. Supplementary material
CCDC 812240 and 812241 contain the supplementary crystallo-
graphic data for (1) and (2). These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.ica.2011.03.038.
of the
g a
5-C5 rings from the metal centers is almost identical in 2-
and 2-b (Ru1–Ct1 1.9402 Å and Ru2–Ct2 1.9449 Å), but slightly lar-
ger than in the starting material (Ru1–Ct1 1.9356 Å). The metal-
centroid distance surpasses the corresponding value in [(g5
-
Cp)Ru(PPh3)2(NCMe)]BF4 by almost 10 pm (1.8480(3) Å). We attri-
bute this to the far smaller steric interaction of the Cp ligand with
the PPh3 groups, as compared to Dbf. This assumption is corrobo-
rated by the
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