Neutral, cationic, and zwitterionic ruthenium(II) atom transfer radical addition catalysts supported by P,N-substituted indene or indenide ligands

Article abstract of DOI:10.1021/om700914k

A series of Ru-based complexes supported by P,N-substituted indene or indenide ligands have been prepared and tested for activity in the atom transfer radical addition of CCl₄ to alkenes. The catalytic performance was found to be influenced by the charge and substitution pattern of the P,N ligand and other coligands, as well as the nature of the counteranion, with the best of these catalysts being effective at low catalyst loadings and under mild conditions.

Full text of DOI:10.1021/om700914k

254
Organometallics 2008, 27, 254–258
Neutral, Cationic, and Zwitterionic Ruthenium(II) Atom Transfer
Radical Addition Catalysts Supported by P,N-Substituted Indene or
Indenide Ligands
Rylan J. Lundgren,† Matthew A. Rankin,† Robert McDonald,‡ and Mark Stradiotto*,†
Department of Chemistry, Dalhousie UniVersity, Halifax, NoVa Scotia, Canada B3H 4J3, and X-Ray
Crystallography Laboratory, Department of Chemistry, UniVersity of Alberta,
Edmonton, Alberta, Canada T6G 2G2
ReceiVed September 12, 2007
A series of Ru-based complexes supported by P,N-substituted indene or indenide ligands have been
prepared and tested for activity in the atom transfer radical addition of CCl4 to alkenes. The catalytic
performance was found to be influenced by the charge and substitution pattern of the P,N ligand and
other coligands, as well as the nature of the counteranion, with the best of these catalysts being effective
at low catalyst loadings and under mild conditions.
Scheme 1. Generic Representation of the Catalysed ATRA of
CCl4 to Monosubstituted Olefins
The metal-catalyzed atom transfer radical addition (ATRA)
of halocarbons to olefins, often referred to as the Kharasch
reaction, represents an efficient and atom-economical method
of C-C bond formation (Scheme 1).1 Although several classes
of metal complexes act as ATRA catalysts,1,2 Ru-based species
are among the most active for this transformation.3 The most
extensively employed Ru-based precatalyst, RuCl2(PPh3)3,4 has
been used to promote the addition of halocarbons to various
olefinic substrates and has found application in related atom
transfer radical cyclizations (ATRC).5 However, major draw-
backs associated with this precatalyst include the relatively high
catalyst loading (g0.5 mol %) and temperatures (g80 °C) that
are often needed to achieve acceptable yields. In the pursuit of
more effective Ru-based ATRA and ATRC catalysts, a number
of ancillary ligation strategies have been evaluated, including
(but not restricted to):3 alkylidene, vinylidene, and N-hetero-
cyclic carbene ligated structures;6 dimeric and multinuclear
phosphine/arene-type systems;7 carborane-supported com-
plexes;8 η5-cyclopentadienyl-type complexes featuring amidi-
nate,9 alkoxide,10 or other coligands. In recent studies by
Demonceau and co-workers,11 divergent catalytic abilities were
noted for half-sandwich Ru(phosphine) complexes supported
by η6-arene, η5-indenyl, η5-C5H5 (Cp), or η5-C5Me5 (Cp*)
ligands, with Cp*Ru(PPh3)2Cl proving capable of mediating the
addition of CCl4 to a variety of olefins at low catalyst loadings
(0.33 mol % Ru) and under mild conditions. More recently,
Severin and co-workers12 observed that [Cp*Ru(PPh3)2-
(MeCN)]+SO3CF3- displayed increased catalytic activity versus
Cp*Ru(PPh3)2Cl in ATRA chemistry, thereby suggesting that
such formally cationic Cp*Ru species may offer advantages over
related neutral precatalysts; indeed, this cationic complex is one
of the most active Ru-based ATRA precatalysts known.
Notwithstanding the diverse set of Ru complexes that have been
(6) (a) Seigal, B. A.; Fajardo, C.; Snapper, M. L. J. Am. Chem. Soc.
2005, 127, 16329. (b) De Clercq, B.; Verpoort, F. J. Organomet. Chem.
2003, 672, 11. (c) Opstal, T.; Verpoort, F. New J. Chem. 2003, 27, 257. (d)
Opstal, T.; Verpoort, F. Tetrahedron Lett. 2002, 43, 9259. (e) De Clercq,
B.; Verpoort, F. Catal. Lett. 2002, 83, 9. (f) De Clercq, B.; Verpoort, F.
Tetrahedron Lett. 2002, 43, 4687. (g) Simal, F.; Demonceau, A.; Noels,
A. F. Tetrahedron Lett. 1999, 40, 5689. (h) Tallarico, J. A.; Malnick, L. M.;
Snapper, M. L. J. Org. Chem. 1999, 64, 344. (i) Richel, A.; Delfosse, S.;
Cremasco, C.; Delaude, L.; Demonceau, A.; Noels, A. F. Tetrahedron Lett.
2003, 44, 6011.
* To whom correspondence should be addressed. Fax: 1-902-494-1310.
Tel: 1-902-494-7190. E-mail: mark.stradiotto@dal.ca.
† Dalhousie University.
‡ University of Alberta.
(1) For selected reviews see: (a) Clark, A. J. Chem. Soc. ReV. 2002, 31,
1. (b) Gossage, R. A.; van de Kuil, L. A.; van Koten, G. A. Acc. Chem.
Res. 1998, 31, 423. (c) Iqbal, J.; Bhatia, B.; Nayyar, N. K. Chem. ReV.
1994, 94, 519.
(2) For selected examples see the following. (a) Ni: Pandarus, V.;
Zargarian, D. Organometallics 2007, 26, 4321. (b) Cu: Muñoz-Molina, J. M.;
Caballero, A.; Díaz-Requejo, M. M.; Trofimenko, S.; Belderraín, T. R.;
Pérez, P. J. Inorg. Chem. 2007, 46, 7725. (c) Zazybin, A. G.; Khusnutdinova,
Yu. R.; Osipova, O. L.; Solomonov, B. N. Russ. J. Gen. Chem. 2005, 75,
734. (d) Pd: Dneprovskii, A. S.; Ermoshkin, A. A.; Kasatochkin, A. N.;
Boyarskii, V. P. Russ. J. Org. Chem. 2003, 39, 933. (e) Cu, Fe: de Campo,
F.; Lastécouères, D.; Verlhac, J.-B. Chem. Commun. 1998, 2117.
(3) (a) Severin, K. Curr. Org. Chem. 2006, 10, 217. (b) Delaude, L.;
Demonceau, A.; Noels, A. F. Top. Organomet. Chem. 2004, 11, 155.
(4) Matsumoto, H.; Nakano, T.; Nagai, Y. Tetrahedron Lett. 1973, 51,
5147.
(7) (a) Quebatte, L.; Haas, M.; Solari, E.; Scopelliti, R.; Nguyen, Q. T.;
Severin, K. Angew. Chem., Int. Ed. 2005, 44, 1084. (b) Quebatte, L.; Solari,
E.; Scopelliti, R.; Severin, K. Organometallics 2005, 24, 1404. (c) Quebatte,
L.; Scopelliti, R.; Severin, K. Angew. Chem., Int. Ed. 2004, 43, 1520.
(8) (a) Tutusaus, O.; Viñas, C.; Nuñez, R.; Teixidor, F.; Demonceau,
A.; Delfosse, S.; Noels, A. F.; Mata, I.; Molins, E. J. Am. Chem. Soc. 2003,
125, 11830. (b) Tutusaus, O.; Delfosse, S.; Demonceau, A.; Noels, A. F.;
Viñas, C.; Teixidor, F. Tetrahedron Lett. 2003, 44, 8421.
(9) (a) Motoyama, Y.; Gondo, M.; Masuda, S.; Iwashita, Y.; Nagashima,
H. Chem. Lett. 2004, 33, 442. (b) Nagashima, H.; Gondo, M.; Masuda, S.;
Kondo, H.; Yamaguchi, Y.; Matsubara, K. Chem. Commun. 2003, 442.
(10) Motoyama, Y.; Hanada, S.; Shimamoto, K.; Nagashima, H.
Tetrahedron 2006, 62, 2779.
(5) Selected examples: (a) Nagashima, H.; Ozaki, N.; Ishii, M.; Seki,
K.; Washiyama, M.; Itoh, K. J. Org. Chem. 1993, 58, 464. (b) Nagashima,
H.; Ozaki, N.; Seki, K.; Ishii, M.; Itoh, K. J. Org. Chem. 1989, 54, 4497.
(c) Hayes, T. K.; Villani, R.; Weinreb, S. M. J. Am. Chem. Soc. 1988, 110,
5533. (d) Nagashima, H.; Wakamatsu, H.; Itoh, K. J. Chem. Soc., Chem
Commun. 1984, 652.
(11) (a) Simal, F.; Wlodarczak, L.; Demonceau, A.; Noels, A. F.
Tetrahedron Lett. 2000, 41, 6071. (b) Simal, F.; Wlodarczak, L.; Demon-
ceau, A.; Noels, A. F. Eur. J. Org. Chem. 2001, 2689.
(12) Quebatte, L.; Scopelliti, R.; Severin, K. Eur. J. Inorg. Chem. 2005,
3353.
10.1021/om700914k CCC: $40.75
2008 American Chemical Society
Publication on Web 12/27/2007

Ru(II) Atom Transfer Radical Addition Catalysts
Organometallics, Vol. 27, No. 2, 2008 255
Chart 1
Figure 1. ORTEP diagram for 5f, shown with 50% displacement
ellipsoids and with the atomic numbering scheme depicted. Selected
H atoms and the triflate counteranion have been omitted for clarity.
Only one of the two crystallographically independent molecules
of 5f is shown. Selected bond lengths (Å) and angles (deg) data
for 5f: Ru-P, 2.2956(8); Ru-N1, 2.302(2); Ru-N2, 2.067(3);
P-C3, 1.796(3); N1-C2, 1.454(4); C1-C2, 1.516(4); C2-C3,
1.338(4); Ru-N2-C8, 170.7(3); N2-C8-C9, 178.8(4).
the charge and substitution pattern of the coordinated P,N and
other coligands, as well as the nature of the accompanying
counteranion. The most active of the Ru complexes explored
herein (5b) proved capable of mediating the ATRA of CCl4 to
a variety of olefins under mild conditions. Also described are
the results of catalytic ATRA experiments employing AIBN
(AIBN ) azobis(isobutyronitrile)) as a cocatalyst, which enabled
Ru loadings as low as 0.02 mol % to be utilized effectively.
shown to catalyze ATRA processes, mechanistic details of this
reaction remain elusive, and definitive structure–reactivity
relationships that are needed in order to guide the future
development of increasingly effective catalysts have not yet been
established. Consequently, “universal” Ru-based ATRA catalysts
that are highly active, are long-lived, and exhibit a wide substrate
scope are unknown.
Despite the established utility of P,N ligands in engendering
desirable reactivity properties to platinum-group metal cata-
lysts,13 the application of P,N ligands in Ru-mediated ATRA
has yet to be reported in the literature. In this context, and as
part of our ongoing research program comparing the reactivity
of structurally related neutral, cationic, and formally zwitteri-
onic14 metal complexes supported by P,N-substituted indene and
indenide ligands,15 we became interested in evaluating the utility
of such Ru complexes in promoting the ATRA of chlorocarbons
to olefins, especially in light of the interesting stoichiometric
and catalytic reactivity exhibited by Ru derivatives of 1-iPr2P-
2-Me2N-indene (1a).15a,e We disclose herein the application of
previously reported neutral, cationic, and zwitterionic Ru
complexes derived from 1a, as well as several new Ru
derivatives of 1-Ph2P-2-Me2N-indene (1b16), as precatalysts for
ATRA chemistry (Chart 1). Notably, the catalytic performance
exhibited by such Ru species was found to be influenced by
Results and Discussion
In order to survey a broad set of Ru-based complexes for
ATRA reactions, we sought to expand our library of catalysts
based on P,N-substituted indene or indenide ligands by preparing
a new set of complexes derived from 1-Ph2P-2-Me2N-indene
(1b). The addition of 0.25 equiv of [Cp*RuCl]4 to 1b afforded
Cp*Ru(Cl)(κ2-1-Ph2P-2-Me2N-indene) in 97% isolated yield.
Isomerization of this complex to 4b (86% isolated yield, Chart
1) was achieved upon exposure to NEt3 in THF. Treatment of
4b with an appropriate chloride-abstracting agent in aceto-
nitrile afforded the corresponding cationic complexes [Cp*Ru-
(MeCN)(κ2-3-Ph2P-2-Me2N-indene)]+X- (X ) BF4, 5b, 94%;
X ) B(C6F5)4, 5d, 91%; X ) SO3CF3, 5f, 90%). Preparation
of the zwitterionic complex 6b was achieved in 89% isolated
yield by the addition of K2CO3 to an acetonitrile solution of
4b. The structures of these new complexes were assigned on
the basis of NMR data and, in the case of 5f, X-ray diffraction
data (Figure 1).17 As expected, 5f exhibits a piano-stool
geometry reminiscent of the crystallographically characterized
complex 5a.15e
(13) For selected reviews, see: (a) Källström, K.; Munslow, I.; Ander-
sson, P. G. Chem. Eur. J. 2006, 12, 3194. (b) Guiry, P. J.; Saunders, C. P.
AdV. Synth. Catal. 2004, 346, 497. (c) Chelucci, G.; Orrù, G.; Pinna, G. A.
Tetrahedron 2003, 59, 9471.
The Ru-mediated ATRA of CCl4 to styrene at 60 °C was
employed as a preliminary test reaction with which to survey
the catalytic abilities of the P,N-ligated complexes depicted in
Chart 1. Both cationic (2a-c) and zwitterionic (3) compounds
supported by η6-arene ligands proved to be catalytically inactive,
yielding negligible amounts of the desired addition product.
Conversely, all of the Cp*Ru species surveyed exhibited
(14) Formally zwitterionic complexes are defined herein as those species
lacking conventional resonance structures that place an anionic charge onto
one of the pnictogen donor atoms of the ancillary P,N ligand.
(15) Representative examples: (a) Lundgren, R. J.; Rankin, M. A.;
McDonald, R.; Schatte, G.; Stradiotto, M. Angew. Chem., Int. Ed. 2007,
46, 4732. (b) Wile, B. M.; McDonald, R.; Ferguson, M. J.; Stradiotto, M.
Organometallics 2007, 26, 1069. (c) Cipot, J.; McDonald, R.; Ferguson,
M. J.; Schatte, G.; Stradiotto, M. Organometallics 2007, 26, 594. (d) Wile,
B. M.; Burford, R. J.; McDonald, R.; Ferguson, M. J.; Stradiotto, M.
Organometallics 2006, 25, 1028. (e) Rankin, M. A.; McDonald, R.;
Ferguson, M. J.; Stradiotto, M. Organometallics 2005, 24, 4981.
(16) Cipot, J.; Wechsler, D.; Stradiotto, M.; McDonald, R.; Ferguson,
M. J. Organometallics 2003, 22, 5185.
(17) Selected crystal data for 5f: monoclinic (P21/n); a ) 18.3091(12)
Å; b ) 19.9442(13) Å; c ) 19.2586(12) Å; ꢀ ) 91.1091(9)°; V ) 7031.2
(8) Å3; Z ) 8; GOF ) 1.042; R1 ) 0.0414; wR2 ) 0.1050.

256 Organometallics, Vol. 27, No. 2, 2008
Lundgren et al.
Table 1. Screening of Complexes for ATRA of CCl4 to Styrenea
Table 3. ATRA of CCl4 to Olefins Catalyzed by Ru Complexes in
the Presence of 5 mol % AIBNa
entry
catalyst
conversn (%)b
yield (%)c
entry catalyst
olefin
S:Cb time (h) yield (%)c
1
2
3
4
5
6
7
8
4a
4b
5a
5b
5c
5d
5e
5f
84
91
71
>99
87
>99
69
>99
93
78
78
66
97
75
95
43
90
78
82
55
1
2
3
4
5
6
7
8
4b
5d
5f
5b
5b
5b
5b
5b
styrene
styrene
styrene
styrene
p-chlorostyrene
R-methylstyrene
1-hexene
5000
5000
5000
5000
5000
3750
2000
24
24
24
24
28
30
24
24
89
49
88
87
90
85
99
66
9
10
11
6a
6b
7
methyl methacrylate 2000
94
79
a Conditions: reactions performed at 60 °C in toluene; olefin:CCl4
)
1:4; 5 mol % AIBN relative to olefin; [olefin] ) 1.4 M. b Substrate
(olefin) to Ru catalyst ratio. c Yield based on formation of the ATRA
product as determined by GC using dodecane as an internal standard.
See the Experimental Section for complete experimental details.
a Conditions: reactions performed in toluene for 4 h at 60 °C;
Ru:olefin:CCl4 ) 1:300:600; [olefin] ) 1.4 M. b Conversion determined
on the basis of the styrene consumed. c Yield based on formation of the
ATRA product as determined by GC using dodecane as an internal
standard. See the Experimental Section for complete experimental
details.
conducted at 60 °C, the related cationic complexes 5b,f remained
effective at lower temperatures, providing 95% yield of the
desired addition product after 5 h (Table 2, entries 3 and 5).
Other styrene derivatives were also employed successfully as
substrates in combination with 5b, affording the desired ATRA
product in high yields (Table 2, entries 6 and 7). However,
whereas the catalytic performance of 5b with styrene-type
substrates is comparable to that of [Cp*Ru(PPh3)2(MeCN)]+
Table 2. ATRA of CCl4 to Olefins Catalyzed by Ru Complexesa
entry catalyst
olefin
temp (°C) time (h) yield (%)b
1
2
3
4
5
6
7
8
9
4b
5d
5f
6b
5b
5b
5b
5b
5b
styrene
styrene
styrene
styrene
styrene
p-chlorostyrene
R-methylstyrene
1-hexene
24
24
24
24
24
24
40
40
40
5
5
5
5
5
5
4
6
4
50
48
95
62
95
83
91
37
34
–
SO3CF3 reported by Severin and co-workers,12 5b performed
less effectively in the ATRA of CCl4 to 1-hexene (Table 2, entry
8) and methyl methacrylate (Table 2, entry 9) under the standard
conditions of 0.33 mol % 5b at 40 °C.
methyl methacrylate
It has been demonstrated recently that the addition of
cocatalysts such as the radical initiator AIBN can result in
significantly enhanced efficiency in metal-mediated ATRA,18
presumably by facilitating the regeneration of catalytically active
metal species.19 In applying this protocol to the more effective
precatalysts under scrutiny herein, we observed that the use of
5 mol % AIBN (relative to olefin) in reactions conducted at
60 °C allowed for the efficient ATRA of CCl4 to styrene at Ru
loadings of 0.02 mol % (Table 3). Under these conditions, the
neutral precatalyst 4b performed on a par with cations 5f and
5b in providing the desired ATRA product in ca. 88% yield
after 24 h (Table 3, entries 1, 3, and 4); surprisingly, the
a Conditions: reactions performed in toluene; Ru:olefin:CCl4
)
1:300:600; [olefin] ) 1.4 M. b Yield based on formation of the ATRA
product as determined by GC using dodecane as an internal standard.
See the Experimental Section for complete experimental details.
moderate (43%) to excellent (97%) yields of the addition product
after 4 h when employing a catalyst-to-substrate ratio of 1:300
(Table 1). The neutral complexes 4a,b and 6a,b (zwitterionic)
showed very similar ATRA activity, affording 78–80% of the
addition product after 4 h at 60 °C. In contrast, the catalytic
efficiency of the salts 5a-f differed significantly, with precata-
lysts featuring PPh2-based ligands consistently outperforming
analogous complexes supported by PiPr2-based ligands. While
under the conditions surveyed 5b proved to be the most effective
precatalyst in mediating the addition of CCl4 to styrene (>99%
conversion, 97% yield; Table 1, entry 4), the related acetonitrile-
stabilized cations supported by 3-Ph2P-2-Me2N-indene (5d,f)
also provided excellent results (Table 1, entries 6 and 8). Given
the postulate that ATRA catalysis could be facilitated by
coordinative unsaturation at Ru,1,3 we investigated the activity
of 7, which in solution has been shown to provide access to the
coordinatively unsaturated species [Cp*Ru(κ2-3-Ph2P-2-Me2N-
indene)]+B(C6F5)4- by way of reversible intramolecular C-H
activation.15e However, the observation that 7 afforded only 55%
yield after 4 h (Table 1, entry 11) and proved to be one of the
least effective Cp*Ru precatalysts examined in our reactivity
survey suggests that coordinated MeCN might play an impor-
-
structurally related cationic complex 5d featuring the B(C6F5)4
counteranion proved inferior to the aforementioned precatalysts
(Table 3, entry 2). A catalyst mixture comprised of 5 mol %
AIBN and 0.02–0.05 mol % 5b also allowed for the efficient
ATRA of CCl4 to substituted styrenes (Table 3, entries 5 and
6), 1-hexene (Table 3, entry 7), and methyl methacrylate (Table
3, entry 8); notably, the yields achieved for the last two
substrates exceeded greatly those obtained by use of AIBN-
free conditions (Table 2, entries 8 and 9).
Despite the desirable catalytic performance exhibited by the
Cp*Ru complexes herein for the ATRA of CCl4 to olefins, we
have not been able to extend this catalysis to the use of CHCl3
as a substrate. Essentially no conversion was observed for the
ATRA of CHCl3 to styrene when employing the precatalysts
studied in this work at the 1 mol % Ru catalyst loading level.20
Furthermore, efforts to use 0.1 mol % 5b as a precatalyst for
this transformation in the presence of 5 mol % AIBN also
tant role in engendering desirable catalytic properties to
- 12
[Cp*Ru(PPh3)2(MeCN)]+SO3CF3
,
as well as to 5a-f.
Having established the catalytic utility of Cp*Ru complexes
supported by P,N-substituted indene or indenide ligands for the
ATRA of CCl4 to styrene and at 60 °C, the ability of such
complexes to mediate this transformation at 24 °C was tested
(Table 2); given the results of our preliminary catalytic survey
(Vide supra), only Cp*Ru complexes supported by κ2-3-Ph2P-
2-Me2N-indene were examined. While the catalytic productivity
observed for the neutral species 4b and 6b, as well as the salt
5d, at 24 °C was diminished significantly relative to experiments
(18) (a) Eckenhoff, W. T.; Pintauer, T. Inorg. Chem. 2007, 46, 5844.
(b) Quebette, L.; Thommes, K.; Severin, K. J. Am. Chem. Soc. 2006, 128,
7440.
(19) The use of Mg in a three-fold excess relative to the olefin substrate
has also been shown to enhance Ru-catalyzed ATRA: Thommes, K.; Içli,
B.; Scopelliti, R.; Severin, K. Chem. Eur. J. 2007, 13, 6899.
(20) The observation of desirable performance in Ru-mediated ATRA
chemistry employing CCl4 but not CHCl3 (or vice versa) is well-estab-
lished.3a,7a

Ru(II) Atom Transfer Radical Addition Catalysts
Organometallics, Vol. 27, No. 2, 2008 257
J ) 1.0 Hz, 1H, C5-H or C6-H), 7.02 (t, 3JHH ) 7.5 Hz, 1H, C6-H
or C5-H), 6.96 (m, 1H, C7-H or C4-H), 3.47–3.35 (m, 2H,
C(Ha)(Hb)), 3.22 (s, 6H, NMea and NMeb), 1.52 (d, J ) 1.5 Hz,
proved unproductive. The complete lack of reactivity observed
when using CHCl3 as a substrate cannot be explained exclusively
in terms of a deactivating effect of this substrate on the Ru
precatalysts employed, given that the ATRA of CCl4 to styrene
using CHCl3 as the solvent still provided good yields of the
desired CCl4 addition product (5b:olefin ) 1:300; 24 °C, 5 h,
87% yield).
2
15H, C5Me5). 13C{1H} NMR (CD2Cl2): δ 176.4 (d, JPC ) 22.8
Hz, C2), 144.0 (d, JPC ) 4.8 Hz, C3a or C7a), 140.8 (d, JPC ) 3.7
Hz, C7a or C3a), 136.2–135.6 (m, P-aryl-C and C3), 134.1 (d, JPC
) 12.6 Hz, P-aryl-CH’s), 133.9 (d, JPC ) 10.1 Hz, P-aryl-CH’s),
131.9 (d, 1JPC ) 45.8 Hz, P-aryl-C), 130.1 (P-aryl-CH), 129.1 (P-
aryl-CH), 128.6 (d, JPC ) 9.4 Hz, P-aryl-CH’s), 127.5 (d, JPC
)
Summary and Conclusions
9.6 Hz, P-aryl-CH’s), 126.7 (C5 or C6), 125.5 (C6 or C5), 125.1
(C4 or C7), 122.9 (C7 or C4), 81.1 (d, J ) 2.4 Hz, C5Me5), 31.4
(d, 3JPC ) 8.9 Hz, C1), 10.6 (C5Me5). 31P{1H} NMR (CD2Cl2): δ
43.3.
The exploration of a diverse set of neutral, cationic, and
zwitterionic Ru species derived from P,N-substituted indenes
has established that, for this family of precatalysts, ATRA
activity is influenced by a number of structural factors. While
(p-cymene)Ru complexes of this type were found to be
ineffective catalysts, related Cp*Ru complexes, in particular
cationic species supported by 3-Ph2P-2-Me2N-indene, displayed
high activity for the ATRA of CCl4 to styrene under mild
conditions and at low catalyst loadings. When structurally
analogous salts of the type [Cp*Ru(MeCN)(κ2-3-Ph2P-2-Me2N-
indene)]+X- were employed as ATRA precatalysts at 24 °C, a
considerable counteranion effect on catalyst performance was
observed. While the complexes explored herein proved inactive
for the ATRA of CHCl3, the use of AIBN as a cocatalyst
provided enhanced catalytic performance for the ATRA of CCl4,
with moderate to excellent yields obtained for a series of
unsaturated substrates at low Ru loadings (0.05–0.02 mol %).
Given that the oxidation of Ru(II) to Ru(III) upon abstraction
of a chlorine atom from CCl4 represents a viable mechanistic
step in such ATRA catalysis,3 the superior performance noted
for the Cp*Ru complexes in our study may be attributable to
the greater ability of the anionic η5-C5Me5- ligand to stabilize
such Ru(III) intermediates. However, given that precise details
regarding the mechanism of Ru-mediated ATRA are lacking,3
we are unable to provide a rationale for the consistently superior
performance of complexes supported by κ2-3-Ph2P-2-Me2N-
indene versus those featuring κ2-3-iPr2P-2-Me2N-indene ligation,
nor are we able to comment definitively regarding the origins
of the divergent catalytic performance of structurally related
neutral, cationic, and zwitterionic complexes. Nonetheless, it
is our hope that these findings will contribute to the rational
discovery of new, more robust, and effective Ru-based ATRA
catalysts.
Synthesis of 5f. To a glass vial containing a magnetically stirred
orange solution of 4b (0.21 g, 0.34 mmol) in MeCN (4 mL) was
added solid AgSO3CF3 (0.090 g, 0.35 mmol) all at once. The
addition caused an immediate formation of a precipitate. The vial
was then sealed with a PTFE-lined cap, and the solution was stirred
magnetically for 2 h. 31P NMR data collected on an aliquot of this
crude reaction mixture indicated the quantitative formation of 5f.
The reaction mixture was then filtered through Celite, yielding a
yellow solution. The solvent and other volatile materials were
subsequently removed in vacuo, affording a waxy yellow solid.
The solid was then treated with CH2Cl2 (5 mL), and the resulting
mixture was filtered through Celite. After removal of CH2Cl2 in
vacuo, the residual solid was then washed with pentane (5 × 1.5
mL) and the product was then dried in vacuo to yield 5f as a yellow
powder (0.24 g, 0.31 mmol, 90%). 1H NMR (CD2Cl2): δ 7.72–7.66
(m, 2H, P-aryl-H’s), 7.52–7.42 (m, 9H, 8 P-aryl-H’s and either
C4-H or C7-H), 7.19 (apparent d of t, J ) 7.5 Hz, J ) 1.0 Hz, 1H,
C5-H or C6-H), 7.06 (t, 3JHH ) 8.0 Hz, 1H, C6-H or C5-H), 6.86
3
(d, JHH ) 8.0 Hz, 1H, C7-H or C4-H), 3.71–3.59 (m, 2H,
C(Ha)(Hb)), 3.29 (s, 3H, NMea), 3.09 (s, 3H, NMeb), 1.89 (d, 5JPH
) 2.0 Hz, 3H, CH3CN), 1.53 (d, J ) 2.0 Hz, 15H, C5Me5). 13C{1H}
NMR (CD2Cl2): δ 177.5 (d, 2JPC ) 22.4 Hz, C2), 144.0 (d, JPC
)
1
5.3 Hz, C3a or C7a), 139.0 (C7a or C3a), 135.1 (d, JPC ) 32.3
Hz, C3), 133.4 (d, JPC ) 12.9 Hz, P-aryl-CH’s), 132.1 (d, JPC
)
10.2 Hz, P-aryl-CH’s), 131.5–131.2 (m, P-aryl-C and P-aryl-CH),
130.6–130.2 (m, P-aryl-C and P-aryl-CH), 129.4 (d, JPC ) 10.4
Hz, P-aryl-CH’s), 129.1 (d, JPC ) 9.6 Hz, P-aryl-CH’s), 127.3 (C5
or C6), 126.7 (C6 or C5), 125.9 (CH3CN), 125.6 (C4 or C7), 123.1
(C7 or C4), 85.6 (C5Me5), 60.7 (NMea), 53.6 (NMeb), 31.3 (d, 3JPC
) 9.4 Hz, C1), 10.3 (C5Me5), 4.3 (CH3CN). 31P{1H} NMR
(CD2Cl2): δ 43.1. A crystal of 5f suitable for single-crystal X-ray
diffraction analysis was grown from a mixture of diethyl ether and
CH2Cl2 at -37 °C.
Experimental Section
Synthesis of 6b. To a glass vial containing a magnetically stirred
orange solution of 4b (0.23 g, 0.37 mmol) in MeCN (7 mL) was
added solid anhydrous K2CO3 (0.10 g, 0.74 mmol) all at once. The
vial was then sealed with a PTFE-lined cap, and the solution was
stirred magnetically for 48 h. During this time period, the reaction
mixture gradually lightened from an orange suspension into a
yellow-orange suspension. The reaction mixture was filtered through
Celite to yield a yellow-orange solution, and 31P NMR data
collected on an aliquot of this crude reaction mixture indicated the
quantitative formation of 6b. The mixture was dried in vacuo, and
the residue was then triturated with pentane (2 × 1.5 mL). The
remaining product was then dried in vacuo to yield 6b as an
analytically pure yellow powder (0.20 g, 0.33 mmol, 89%). Anal.
Calcd for C35H39PN2Ru: C, 67.81; H, 6.35; N, 4.52. Found: C,
67.92; H, 6.66; N, 4.05. 1H NMR (CD3CN): δ 7.76–7.29 (m, 10H,
Representative synthetic procedures (for 4b, 5f, and 6b) and
catalytic protocols are provided below. Other experimental details
pertaining to the synthesis of Cp*Ru(Cl)(κ2-1-Ph2P-2-Me2N-
indene), 5b, and 5d and the crystallographic solution and refinement
of 5f are given in the Supporting Information.
Synthesis of 4b. To a glass vial containing a magnetically stirred
solution of freshly prepared Cp*Ru(Cl)(κ2-1-Ph2P-2-Me2N-indene)
(0.30 g, 0.49 mmol) in THF (8 mL) was added NEt3 (3 mL). The
vial was then sealed with a PTFE-lined cap, and the solution was
stirred magnetically for 72 h. 31P NMR data collected on an aliquot
of this solution indicated clean conversion to 4b. The THF solvent
and other volatile materials were then removed in vacuo, yielding
a dark red oily solid. The solid was then triturated with pentane (2
× 1.5 mL), followed by pentane washes (2 × 1.5 mL), and the
product was then dried in vacuo to yield 4b as an analytically pure
orange powder (0.26 g, 0.42 mmol, 86%). Anal. Calcd for
C33H37PNRuCl: C, 64.41; H, 6.07; N, 2.28. Found: C, 64.44; H,
6.00; N, 2.24. 1H NMR (CD2Cl2): δ 7.87–7.81 (m, 2H, P-aryl-
H’s), 7.63–7.56 (m, 2H, P-aryl-H’s), 7.42–7.31 (m, 7H, 6 P-aryl-
H’s and either C4-H or C7-H), 7.09 (apparent d of t, J ) 7.0 Hz,
3
P-aryl-H’s), 7.17 (d, JHH ) 8.0 Hz, 1H, C4-H or C7-H), 6.75 (d,
3JHH ) 8.0 Hz, 1H, C7-H or C4-H), 6.52 (m, 1H, C5-H or C6-H),
6.38 (m, 1H, C6-H or C5-H), 5.99 (d, J ) 4.5 Hz, 1H, C1-H),
3.34 (broad s, 3H, NMea), 3.22 (broad s, 3H, NMeb), 1.51 (d, J )
1.5 Hz, 15H, C5Me5). 13C{1H} NMR (CD3CN): δ 138.5 (m, C3a
or C7a), 127.9 (C7a or C3a), 133.4–133.6 (m, P-aryl-CH’s),

258 Organometallics, Vol. 27, No. 2, 2008
Lundgren et al.
129.8–129.6 (m, P-aryl-CH’s), 129.0–128.4 (m, P-aryl-CH’s), 120.3
(C4 or C7), 119.1 (C7 or C4), 116.2 (C5 or C6), 115.5 (C6 or C5),
The Schlenk flask was then placed in a temperature-controlled oil
bath set at 60 °C. The progress of the reaction was monitored using
GC-FID methods as described above. For all experiments, the
identity of the ATRA product was confirmed by comparison of 1H
NMR data obtained from the isolated product with values reported
in the literature. All reported catalytic results represent the average
of a minimum of two trials. Published control experiments confirm
the inability of 5.0 mol % AIBN (in the absence of Ru) to mediate
the ATRA of CCl4 to styrene under the conditions employed
herein.18b
3
87.3 (d, JPC ) 11.3 Hz, C1), 84.2 (C5Me5), 66.0 (NMea), 55.3
(NMeb), 10.6 (C5Me5). 31P{1H} NMR (CD3CN): δ 33.7.
Representative Protocol for Atom Transfer Radical
Addition Experiments. In a typical Ru-catalyzed atom transfer
radical addition (ATRA) experiment (Table 2, entry 6), a stock
solution of 5b was prepared in THF (8.9 mg; 0.0126 mmol in 3.000
mL) and the required amount (1097 µL; 0.0046 mmol) was added
to a glass reaction vial. The solvent within the vial was removed
in vacuo, and to the residue was added 1.000 mL of a substrate
stock solution containing p-chlorostyrene (371 µL), CCl4 (590 µL),
and dodecane (188 µL) in toluene (1050 µL) (Ru:olefin:CCl4 )
1:300:600; [olefin] ) 1.4 M). The mixture was stirred at 24 °C,
and the progress of the reaction was monitored using GC-FID
methods by passing a small aliquot of the reaction mixture solution
through a short plug of silica. Yields of the desired ATRA product
were determined from GC-FID data by using dodecane as an
internal standard. In a representative experiment employing AIBN
as a cocatalyst (Table 3, entry 5), a stock solution of 5b was
prepared in THF (8.9 mg; 0.0126 mmol in 3.000 mL); the required
amount (132 µL; 0.00055 mmol) was added to a Schlenk flask,
and the solvent was removed in vacuo. In a separate glass container,
AIBN (93 mg; 0.57 mmol) was added to a 8.200 mL toluene stock
solution of the substrates (1382 µL of p-chlorostyrene, 4395 µL of
CCl4) with dodecane (643 µL) added as an internal standard; 2.000
mL of this solution was then added to the Schlenk flask containing
5b (Ru:AIBN:olefin:CCl4 ) 1:250:5000:2000; [olefin] ) 1.4 M).
Acknowledgment is made to the Natural Sciences and
Engineering Research Council (NSERC) of Canada (includ-
ing a Discovery Grant for M.S. and Postgraduate Scholar-
ships for R.J.L. and M.A.R.), Dalhousie University, the
Canada Foundation for Innovation, and the Nova Scotia
Research and Innovation Trust Fund for their generous
support of this work. We also thank Drs. Michael Lumsden
and Katherine Robertson (Atlantic Region Magnetic Reso-
nance Center, Dalhousie University) for their assistance in
the acquisition of NMR data.
Supporting Information Available: Text giving experimental
details as well as a CIF file giving single-crystal X-ray diffraction
data for 5f. This material is available free of charge via the Internet
at http://pubs.acs.org.
OM700914K