Electrochemically induced C-Br and C-I bond activation by the Pd3(dppm)3Co2+ cluster, and characterization of the reactive Pd3(dppm)3Co+ intermediate: The first confidently identified param

Full text of DOI:10.1021/ja002417a

4340
J. Am. Chem. Soc. 2001, 123, 4340-4341
Table 1. Consumed Q for RX Activationa
Electrochemically Induced C-Br and C-I Bond
Activation by the Pd3(dppm)3CO2 Cluster, and
+
b
Q mol of electron/
+
Characterization of the Reactive Pd3(dppm)3CO
Intermediate: The First Confidently Identified
Paramagnetic Pd Cluster
molar ratio
mol of
3 3 3 2 3 3
RX/Pd (dppm) (CO)(CF CO (dppm)
)+ Pd (CO)2+
RX
n-BuBr
1.1
1.1
1.5
1.1
1.1
0
no reactivity
no reactivity
0.86
0.80
0.39
Me
Me
2
CHBr
CBr
Br
David Brevet,1a Dominique Lucas, H e´ l e` ne Cattey,
1a
1a
3
1a,b
,1a
PhCH
MeI
2
Fr e´ d e´ ric Lema ˆı tre,, Yves Mugnier,* and
1b
Pierre D. Harvey*
1
0.18
EtI
PhCH
n-BuI
CH
Me
1.1
1.2
1.1
1.1
1.1
0.72
0.70
0.81
0.45
Laboratoire de Synth e` se et d’ EÄ lectrosynth e` se
Organom e´ talliques, CNRS UMR 5632, Facult e´ des
Sciences Gabriel, UniVersit e´ de Bourgogne
2
CH
2
I
2
I
2
6
BouleVard Gabriel, 21000 Dijon, France
3
CI
0.16
D e´ partement de Chimie, UniVersit e´ de Sherbrooke
a
)+
3 3 3 2
Only the data for the starting material Pd (dppm) (CO)(CF CO
Sherbrooke, Qu e´ bec, Canada J1K 2R1
-
-
-
are summarized. For L ) Cl or Br , reactivity with R-X is also
observed, but not quantified. b Q is determined for systems at comple-
tion by using the same electrochemical cell and electrodes. Note that
the common R-Cl molecules are not activated.
ReceiVed July 5, 2000
ReVised Manuscript ReceiVed December 26, 2000
Simple ligand substitution processes represent the very basis
of many important organometallic reactivities and catalyses.
However, these processes can be very slow or impossible for a
given metal at a given oxidation state. Upon a one-electron
reduction or oxidation, the substitution may become very fast.
4 6
containing 0.2 M NBu PF as supporting electrolyte. The
electrochemically induced reactions proceed according to eq 1,
+
Pd (dppm) (CO)(L) + RX h
3
3
These well-known catalytic processes are called “zero electron”
+
+
-
Pd (dppm) (CO)(X) + “R ” + L (1)
processes2,3 and have recently been described for organometallic
3
3
systems by Amatore et al.4
where L- ) CF
CO
-
and X ) Br , I . This reaction also
-
-
-
3
2
We now wish to report an unprecedented electron-transfer chain
catalyzed ligand substitution reactivity for a Pd cluster, specifically
applied for C-Br and C-I bond activation. The reductive
cleavage of the C-X bonds (X ) halogen) represents a very
important topic of research, particularly for polyhalogenated
-
-
-
applies for L ) Cl and Br .
The cluster products are readily identified by the comparison
of the cyclic voltammograms and P NMR spectra of authentic
samples, while evidence for “R ” has been provided by GCMS.
31
7a
+
7b
5
Coulometric measurements indicate that less than one electron/
mole of cluster (Q) is necessary to complete the electrolysis (Table
compounds. During the course of this work, the formal identi-
+
fication of the reactive key intermediate, Pd
3
(dppm)
3
CO , is made.
1). When the quantity of RX substrates is increased, Q repro-
This complex is the first confidently characterized paramagnetic
Pd cluster.
ducibly decreases, consistent with the increased rate of reactivity.
By injecting a small amount of electricity (0.04 equiv/cluster for
1.1 equiv of substrate MeI for instance), and stopping the
electrolysis before the current reaches zero, the substitution
reactions go to completion after 1 h. Doing so, the turnover
number for this specific example becomes 24 ((1 - 0.04)/0.04)
vs 1.5 ((1 - 0.39)/0.39) for the exhaustive electrolysis (Table 1,
entry no. 5).
The relatively fast reactions between [Pd
3
(dppm)
3 3
CO](CF -
6
CO
voltammetry, coulometry, and P NMR spectroscopy in THF
2 2
) and various RX substrates have been monitored by cyclic
3
1
*
Authors to whom correspondence should be addressed: Harvey: Phone:
(
819) 821-7092. Fax: (819) 821-8017. E-mail: pharvey@courrier.usherb.ca.
Mugnier: Phone and Fax: (33) 3 80 39 60 91. E-mail: Yves.Mugnier@
u-bourgogne.fr.
(
(
(
1) (a) Universit e´ de Bourgogne. (b) Universit e´ de Sherbrooke.
This electron-transfer chain catalyzed process occurs via the
generation of the 45-electron Pd (dppm) (CO)(L) species, which
3 3
2) Sav e´ ant, J. M. Acc. Chem. Res. 1980, 13, 323 and the references therein.
3) (a) Chanon, M. Acc. Chem. Res. 1987, 20, 214. (b) Astruc, D. Angew.
is found to be stable at the electrochemical time scale, and
Chem., Int. Ed. Engl. 1988, 27, 643.
-
(
4) Amatore, C.; Jutand, A.; Thouin, L.; Verpeaux, J. N. L’actualit e´ chim.
provides an interpretable EPR spectrum (see below). For L )
1
998, 43.
-
CF
CO
These intermediates are reactive toward R-X species (X ) Br,
I) to form the Pd (dppm) (CO)(X) complex, and are also stable
on the electrochemical time scale. In the presence of Pd
3
CO
2
2
, a complete adduct dissociation (Pd
3
(dppm)
3 3
(CO)(CF -
(
(
5) Fry, A. J.; Singh, A. H. J. Org. Chem. 1994, 59, 8172.
6) (a) X-ray crystallographic data and host-guest binding measurements
+
-
6
) f Pd
3
(dppm)
3
(CO) + CF CO ) is readily expected.
3 2
-
indicate that one of the CF
3
CO
2
anions is located inside the cavity described
6
b,c
by the dppm-phenyl groups above the unsaturated Pd -face. The binding
3
2+
3
3
constants are weak and the cluster is best formulated as Pd
3
(dppm)
, the larger counteranion
is not found in the cavity. The corresponding halide adducts (X ) Cl,
3
(CO) ‚‚‚
-
3 2 3 3 6 2
CF CO ) . For the analogue [Pd (dppm) (CO)](PF )
- 6b
3
(dppm)
3
-
-
(
PF
+
6
(CO)(L) as starting material, the electron transfer between Pd
3
6c
Br, I) exhibit much stronger binding constants, and are appropriately referred
+
2+
7c
to as Pd
in the presence of PF
3
(dppm)
3
(CO)(X) . An exhaustive study shows that Pd
3
(dppm)
3
(CO)
(7) (a) The complexes have been prepared according to literature methods
-
-
-
31
6
, CF
3
CO
2
, or X exhibits either a single two-electron,
and the P NMR and electrochemical data are as follow: δ(acetone-d
6
) )
-
-
-
-
or two one-electron reduction waves. The occurrence of one or the other
-1.26 (PF
6
), -7.03 (CF
3
CO
) -0.77V (Cl ), -0.68V (Br ), and -0.77V (I ), in THF
solutions containing 0.2 M n-Bu NPF
. (b) The presence of the generated R+
fragment has been demonstrated by performing the electrocatalysis of Me CI
(see Table 1, entry 11) in the presence of phenol (in the same molar quantity
of Me CI). At the end of the reaction, the solvent has been evaporated and
the residue was extracted in Et O. The Ph-O-CMe ether coupling product
2
), -6.53 (Cl ), -6.14 (Br ), and -6.40 ppm
depends on temperature, solvent, and anion concentration.6d,e While the reactive
(I ), and E1/2
-
2+/0
- - -
intermediate is issued from a one-electron reduction of Pd
3
(dppm)
3
(CO)(CF
3
-
4
6
+
CO
2
) , the bulk two-electron electrolysis experiments show no reactivity
3
-
toward these halocarbons. Selected electrochemical data for the PF
6
and
-
2+
-
2+/+
CF
)
3
CO
2
salts are the following: Pd
3
(dppm)
3
(CO) (as PF
6
salt), E1/2
3
+/0
+
2
-0.29V, E1/2
) -0.66V vs SCE; Pd
3
(dppm) (CO)(CF
3
3
CO
)
(as
2
3
-
2+/0
CF
3
CO
2
salt), E1/2
NPF
) -0.48V vs SCE, both in THF solutions containing
is readily detected by GCMS, which results from an electrophilic attack of
the carbocation onto the phenol substrate. The low yield of 25% is due to
inefficient trapping for this 1:1 stoichiometric reaction. No R-R coupling or
R-H products were observed, indicating that radical-type reaction does not
occur. In the absence of cluster, the blank tests show no reactivity. (c)
Manojlovic-Muir, L.; Muir, K. W.; Lloyd, B. R.; Puddephatt, R. J. J. Chem.
Soc., Chem. Commun. 1995, 536.
0
.2 M n-Bu
4
6
. (b) Provencher, R.; Aye, K. T.; Drouin, M.: Gagnon, J.;
Boudreault, N.; Harvey, P. D. Inorg. Chem. 1994, 33, 368. (c) Harvey, P. D.;
Hierso, K.; Braunstein, P.; Morise, X. Inorg. Chim. Acta 1996, 250, 337. (d)
Gauthron, I.; Mugnier, Y.; Hierso, K.; Harvey, P. D. Can. J. Chem. 1997, 75,
1
182. (e) Lema ˆı tre, F.; Brevet, D.; Vallat, A.; Lucas, D.; Mugnier, Y. In
preparation.
1
0.1021/ja002417a CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/12/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 18, 2001 4341
Scheme 1. Reaction Mechanism for the Electrochemically
-
-
-
-
-
-
-
3 2
Induced X /L Exchange (L ) CF CO , Cl , Br ; X )
-
-
Br , I )
+
(
3 3 3
dppm) (CO)(X) and Pd (dppm) (CO)(L) is thermodynamically
favorable (potentials provided in refs 6a and 7a), hence driving
the catalytic cycle (Scheme 1). To demonstrate this, the ligand
+
exchange between Pd
3
(dppm)
3
(CO)(Br) and BuI was attempted,
and Q was found to be also <1 equiv/mol. The reverse reaction,
however, does not occur. We find no reactivity for common
chlorocarbons and the data included in Table 1 indicate that the
rate of reactivity varies as I (smaller Q values) > Br (larger Q
values) > Cl (no reactivity), illustrating the expected trend in
+
C-X bond strength. Stabilized carbocations such as Me
3
C
+
3 3
Figure 1. Top: Experimental EPR spectrum of Pd (dppm) (CO) in THF
at 293 K Bottom: Calculated EPR spectrum.
+
provide lower Q, reducing the probability of back reactions (R
-
+
X f RX). Interestingly no reactivity is observed for i-PrBr
and n-BuBr, further illustrating the importance of the carbocation
stability. This stability appears to be fine-tuned between i-PrBr
and t-BuBr, where no and slower reactivities are observed,
respectively. Finally, Q for MeI is unexpectedly significantly
respectively. The larger EPR signals precluded spectral resolution
for Pd coupling in this case. The relative magnitude of the A
31
105
constants (A( P) . A( Pd)) is also consistent with literature
12
data for other polynuclear compounds.
smaller than Q for RCH
being similar. Speculatively, the smaller MeI substrate may
interact more efficiently with the Pd
dppm cavity than that of the longer RCH
Attempts to chemically prepare the related and important 43-
electron cluster intermediate Pd
this cluster is cleanly prepared with [Pd
starting material and BPh
2 2
CH I (R ) H, Et, Ph), the latter values
Attempts to chemically prepare the corresponding X-adducts,
Pd
using BPh
behavior of Pd
dppm) (CO)I is observed upon high sweep rates, or in the
3 3
(dppm) (CO)X (X ) Cl, Br, I), uniformly failed, including
+
center inside its phenyl-
- 13
3
.
Voltammetric studies on the electrochemical
(dppm) (CO)I show that the intermediate Pd -
3 3
4
2
CH
2
I substrates.6b
+
3
(
3
+
(dppm)
3
CO were made. Indeed
(dppm) (CO)](PF as
(as reducing agent). The IR
6e
3
presence of an excess of iodide salt.
3
3
6 2
)
Paramagnetic compounds of palladium are scarce and this series
is limited primarily to mononuclear Pd(III) species with some
rare examples of mononuclear Pd(I) and binuclear Pd(II)-Pd(III)
-
8,9
4
-
1
spectrum exhibits an IR ν(CO) absorption at 1785 cm , which
is expectedly red-shifted in comparison with the more oxidized
14-16
+
complexes.
3 3
To our knowledge, the Pd (dppm) (CO) is the
Pd
cm
3
(dppm)
3
(CO)2 species due to back-bonding (ν(CO) ) 1835
+
first example of a confidently identified paramagnetic Pd cluster.
-
1 6b
). The isotropic EPR spectrum of this cluster in THF at
2
93 K (Figure 1) exhibits a well-defined septet with a relative
Acknowledgment. P.D.H. thanks NSERC (Natural Sciences and
Engineering Research Council) and FCAR (Fonds Concert e´ s pour
l’Avancement de la Recherche) for funding. Y.M. is grateful to CNRS
intensity approaching 1:6:15:20:15:6:1, along with weaker hy-
perfine structures.10 Curve fitting analysis allowed interpretation
(Centre National de la Recherche Scientifique) for funds.
of the spectrum and the results indicate the presence of a single
electron delocalized over the three equivalent Pd atoms: g )
Supporting Information Available: Experimental EPR spectrum of
31
-4
-1
105
-4
-1
2
.065, A( P) ) 75.8 × 10 cm , A( Pd) ) 7.6 × 10 cm
05 5
2
+
Pd
3 3
(dppm) (CO) in a frozen solution and Experimental Section for the
1
(
Pd, S ) /
, 22.2% natural abundance). Evidence for coupling
electrochemistry (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
with both the Pd and P atoms is completely consistent with the
atomic contributions of the SOMO (a ). Previous EHMO calcula-
tions on the dication indicated that this MO is composed of in-
plane metal d orbitals (dxy, dx2-y2 with minor p
and p contribu-
tions) and some phosphorus components (p , p ), forming M-M
, A||( P), and A
values extracted from EPR spectra of frozen solutions (100 K)
2
JA002417A
x
y
(
12) Data for the Rh
(O
CEt)
(PPh
)
+
dimer as an example: A||( P) )
-1 103 -4 -1
31
2
2
4
3
2
-4
-1
31
-4
y
x
205 × 10 cm ; A
⊥
( P) ) 152 × 10 cm ; A||
(
Rh) ) 13 × 10 cm
;
1
03
and M-P antibonding MO’s.11 The g||, g
31
31
⊥
A ( Rh) ) not resolved. Kawamura, T.; Fukamachi, K.; Sowa, T.; Hayashida,
⊥
⊥
( P)
S.; Yonezawa, T. J. Am. Chem. Soc. 1981, 103, 364.
(
13) These paramagnetic species exhibit limited stability after several hours
-
4
-1
-4
-1
31
are 2.059, 2.078, 88.8 × 10 cm , and 85.0 × 10 cm ,
even at -30 °C. Cyclic voltammetry, EPR, P NMR, and IR spectroscopy
indicate evolution towards decomposition where a strong IR absorption at
1669 cm- and P NMR peak at 30.1 ppm (acetone - d
1
31
) are clearly observed
(
8) (a) Strauss, S. H. Chem. ReV. 1993, 93, 927. (b) The biphenyl product
6
is readily identified by GCMS from the borate anion oxidation for the reactions
for these uncharacterized products. Further studies on these species will be
2
+
-
6e
between Pd
3
(dppm)
3
(CO) and BPh
4
.
published in due course.
(
9) (a) A two-step procedure to generate this same paramagnetic species
(14) (a) Gallo, E.; Ragaini, F.; Cenini, S.; Demartin, F. J. Organomet. Chem.
1999, 586, 190. (b) Yeo, J. S. L.; Vittal, J. J.; Hor, T. S. A. Chem. Commun.
1999, 1477. (c) Jasper, S. A., Jr.; Hoffman J. C.; Todd, L. J. Inorg. Chem.
1998, 37, 6060.
(15) (a) Larin, G. M.; Zvereva, G. A. Dokl. Akad. Nauk.1996, 347, 58. (b)
Mincheva, N.; Gencheva, G.; Mitewa, M.; Gochev, G.; Mehandjiev, D. Synth.
React. Inorg. Met.-Org. Chem. 1997, 27, 1191.
(16) DeGray, J. A.; Geiger, W. E.; Lane, G. A.; Rieger, P. H. Inorg. Chem.
1991, 30, 4100.
2
+
is also possible from the two-electron reduction of Pd
3
(dppm)
3
(CO) (i.e. at
+
0/+
-
)
0.62V vs SCE), followed by a one-electron oxidation with Tl (E1/2 (Tl)
9b
-0.582V vs SCE). (b) CRC Handbook of Chemistry and Physics, 64th
ed.; CRC Press Inc.: Boca Raton, FL, 1983; p D-159.
(
10) The EPR spectra were acquired on a Bruker ESP 300 instrument and
spectral analyses were performed with the commercially available software
WinEPR of SimFonia (Version1.25; Bruker Analytisch Messteknic).
(
11) Harvey, P. D.; Provencher, R. Inorg. Chem. 1993, 32, 61.