The modulating possibilities of dicarbollide clusters: Optimizing the Kharasch catalysts

Article abstract of DOI:10.1021/ja036342x

exo-Cluster dicarbollides substitution has allowed tuning of the E° (Ru(^II)/Ru(^III)) potential to obtain the best-performing Kharasch catalyst. We postulate that this is possible through the to-and-fro electron movement between the boron cluster and the sulfonium moieties. Copyright

Full text of DOI:10.1021/ja036342x

Published on Web 09/09/2003
The Modulating Possibilities of Dicarbollide Clusters: Optimizing the
Kharasch Catalysts
Oscar Tutusaus,†,‡ Clara Vin˜as,† Rosario Nu´n˜ez,† Francesc Teixidor,*,† Albert Demonceau,§
Se´bastien Delfosse,§ Alfred F. Noels,§ Ignasi Mata,† and Elies Molins†
Institut de Cie`ncia de Materials de Barcelona (CSIC), Campus de la U.A.B, 08193 Bellaterra, Spain, Laboratory of
Macromolecular Chemistry and Organic Catalysis, UniVersity of Lie`ge, Sart-Tilman (B.6a), B-4000 Lie`ge, Belgium
Received May 26, 2003; E-mail: teixidor@icmab.es
The structurally and coordinatively similar dicarbollide [7,8-
C2B9H11]2- (Dcb)1 and cyclopentadienyl ligands [C5R5]- (Cp#)1
differ in their capacity to stabilize high oxidation states and the
out-of-plane disposition of the open-face substituents.1,2 These
differences offer new alternatives in fields where Cp is a basic
participant and in which association/dissociation of ligands3 and
electron capture/release are key steps. In the Kharasch reaction,
also known as atom transfer radical Addition (ATRA),4 the above-
mentioned steps are fundamental. This is why ATRA reaction was
chosen to compare Dcb and Cp options. To date some of the more
active ATRA catalysts contain Cp-like ligands, e.g., [RuClCp#-
(PPh3)2] (Cp# ) Cp (1), Cp* (2), and indenyl (3)).4b,5 Substitution
of one open-face hydrogen by a SR2 in Dcb leads to the new
monoanionic dicarbollide, [R2S-7,8-C2B9H10]-. We have chosen
the most symmetrical position to locate the R2S group, although
other positions are also possible,6 and [10-R1R2S-7-R-7,8-C2B9H9]-
(4a-f) derivatives were synthesized.7 Reaction of these ligands with
[RuCl2(PPh3)3] in ethanol led to complexes with the [3-H-3,3-
(PPh3)2-8-SR1R2-1-R-3,1,2-RuC2B9H9] (5a-f) general stoichiom-
etry (Scheme 1). All complexes 5a-f, were characterized by NMR
techniques, and the structure of 5a (Figure 1) was determined by
X-ray structure analysis. Crystals of 5a were grown from a solution
of CH2Cl2/hexane.
Figure 1. Molecular structure of 5a‚CH2Cl2 showing the atom-labeling
scheme.
Scheme 1. Preparation of Complexes 5a-f
The 11B NMR spectra of complexes with R ) H and R1 ) R2
(5a, 5b, 5d) display a 1:1:1:3:3 pattern in agreement with Cs
symmetry. The crystal structure determination of 5a [3-H-3,3-
(PPh3)2-8-SMe2-3,1,2-RuC2B9H10] unambiguously indicates that the
metal hydride points toward sulfur (Figure 1). Bond distances of
5a are very similar to those of the noncompensated anion [3-H-
3,3-(PPh3)2-3,1,2-RuC2B9H11]- (6-),8 Ru-P (2.321(3) and 2.298-
(3) Å vs 2.322 and 2.294 Å), and the P-Ru-P angle has the same
value 96.1(1)°. The Ru distance to the C2B3 open face is 1.735(5)
Å for 5a and 1.771 Å for 6. The Ru-B and Ru-C distances in 5a
are very similar, ranging from 2.203(11) to 2.307(10) Å. Therefore
the pendant R2S+ group has not produced significant geometric
changes in the complex. In support of this is the similar angle
between the B(8)-S and the C2B3 plane for 5a (14.2°) and 4a
(17.1°).7 This implies that in the event one PPh3 should dissociate,
the long 3.435(3) Å S-Ru distance in 5a, larger than the van der
Waals radii, should prevent a S-Ru contact.
Table 1. Cyclic Voltammetrya Data and Kharasch Addition of
Carbon Tetrachloride to Styrene and Methyl Methacrylateb
c
c
conversion [%]/yield [%]
styrene MMA
62/5
°
∆E [mV]
E [mV]
catalyst
86
94
94
76
82
98
80
80
88
+133
-83
-10
-266
-266
-287
-266
-366
-368
1
2
3
5a
5b
5c
5d
5e
5f
24/10
86/69
79/79
83/83
74/69
100/99
100/99
100/99
100/99
100/99
100/99
98/94
98/94
100/100
99/99
92/84
92/84
a Sample, 1 mM; Bu4NPF6 (0.1 M) in CH2Cl2; ν ) 50 mV s-1; potentials
are reported in millivolt versus ferrocene as an internal standard. b Reaction
conditions: olefin (9 mmol), carbon tetrachloride (13 mmol), catalyst (0.03
mmol), toluene (4 mL), dodecane (0.25 mL), under nitrogen atmosphere.
The reactions were carried out at 40 °C and stopped after 6 h. c Conversions
and yields are based on the olefin, and determined by GC using dodecane
as internal standard. The Kharasch adducts were characterized by com-
parison with literature data.4b
The Kharasch addition of CCl4 to methyl methacrylate (MMA)
and styrene (Sty) double bonds has been examined to compare the
catalytic activity of 5a-f with 2, which is considered to be the
fastest and highest Kharasch conversion capacity Ru complex.4b
All 5a-f complexes are superior in all aspects to 2, as shown in
Table 1.
In particular for 5c and with regard to MMA and Sty, total
turnover numbers (TTN) of 4200 and 9000, and initial turnover
frequencies (TOF) of 1880 and 1500 h-1 were obtained at 40 °C,
† Institut de Ciencia de Materials de Barcelona, CSIC.
‡ O. Tutusaus is enrolled in the UAB PhD Program.
§ University of Lie`ge.
9
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J. AM. CHEM. SOC. 2003, 125, 11830-11831
10.1021/ja036342x CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Consequently, catalyst precursors with E°’s near -370 mV are not
as efficient as these with E°’s near -270 mV, but all of them are
more efficient than 2, 3, and 1 with E°’s between -83 mV and
+133 mV. The fact that the highest catalytic activity is found neither
at one extreme of E°’s values nor the other, but between, implies
that both species, RuII and RuIII, must be equally stabilized by the
same ligand system and that for a maximum efficiency of the
catalytic conversion process E° must be in a narrow range of
potentials. In this way 2 and 3 are more efficient than 1, confirming
that the closer to -270 mV is E°, the best for maximum catalytic
performance. This proves that a direct relationship between ATRA
catalyst efficiency and E° does exist for these complexes.
Charge-compensated carborane ligands have thus allowed the
adequate tuning of the E° values of the Cp# ligands, permitting the
necessary potential to be reached through exo-cluster substitution.
It is our interpretation that this has been possible through a to-and-
fro electron density movement, facilitated by the uniqueness of the
boron cluster-sulfonium bridge. We believe that the capacity to
donate and to retrieve electron density from the metal makes the
[10-R2S-7,8-C2B9H10]- system very adequate when two oxidation
states are required to be stabilized by the same ligand system in
different steps of a catalytic process.
Figure 2. Styrene (9,b,2,[) and adduct (0,O,4,]) vs time for the
Kharasch addition of CCl4 to styrene at 40 °C catalyzed by complexes 5a,
5c, and 2 without PPh3 and in the presence of 12 equiv of PPh3. (a) Complex
5a without PPh3 (9,0) and with 11.9 equiv of PPh3 (b,O); complex 5c
without PPh3 (2,4) and with 12.1 equiv of PPh3 ([,]). (b) 2 without PPh3
(9,0), with 12.05 (2,4) and 12.4 equiv of PPh3 (b,O).
Acknowledgment. This work has been supported by CICYT,
MAT01-1575 and Generalitat de Catalunya, 2001/SGR/00337.
respectively, as opposed to maximum TTN of 1600-1700 and TOF
of 400 h-1 observed for 2.4b In addition, the TTN for 5c is even
higher than the one obtained for the pincer N,C,N-chelating
aryldiaminonickel complex, to this moment the most efficient
ATRA catalyst reported, with TTN of 1730 and TOF of 400 h-1
for MMA.9 With the superior reactivity of 5c for this type of
reaction, it remained to be determined whether it is due to: (i) the
capacity of R2S+ to donate two electrons to the metal after
dissociation of one phosphine; (ii) the fine-tuning to optimal
potential in the Ru catalyst made by [10-R2S-7,8-C2B9H10]-, or
(iii) a combination of both.
Point (i) was addressed by adding free PPh3 to the reaction
mixture, reasoning that addition of phosphine would decrease the
rate dramatically, as observed in reactions in which a phosphine
dissociative pathway is operative.4a,10,11 Addition of up to 12 equiv
of PPh3 per equiv of catalyst in reactions catalyzed by 5a and 5c
slowed the reaction rate (Figure 2a) in a manner parallel to that
with 2 (Figure 2b). Complex 2 does not have the capacity to
internally satisfy the electronic demands of a complex having lost
one ancillary ligand. With the behavior of 5a/5c parallel to that of
2, we considered that the stabilizing capacity of the R2S+ group
on the ligand’s dissociation could be discounted, in agreement with
the structural data discussed earlier.
Supporting Information Available: Crystallographic data (CIF)
and X-ray structure details (PDF) of 5a‚CH2Cl2. This material is
available free of charge via the Internet http://pubs.acs.org.
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Possibility (ii) was addressed correlating the E° values for the
RuII f RuIII process with the catalytic Kharasch activity toward a
common substrate. Cyclic voltammetry data are displayed in Table
1. On the basis of these data, the catalyst activity order is 5a-d >
5e-f > 2,3 > 1. Interestingly, groupings of catalysts can be made,
both from the catalytic activity side, and from the E° point of view.
In this way, 5a-d have E°’s near -270 mV and are the most active
catalysts; close in activity are 5e and 5f with E°’s near -370 mV.
JA036342X
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