Synthesis and reactivity of [(PhB(CH2PPh2) 3-Κ3 P)Ru(NCMe)3]PF6 and its potential as a transfer hydrogenation catalyst

Article abstract of DOI:10.1021/om100552h

The acetonitrile ligands in [(PhB(CH₂PPh₂) ₃-Κ³P)Ru(NCMe)₃]PF₆ (1) (which contains a facially coordinated anionic tripodal phosphine ligand) are labile and are rapidly replaced by other ligands, although the steric profile of the phosphine ligand limits the size and the number of the incoming ligands. Complex 1 and some of its derivatives proved to be very efficient and rapid transfer hydrogenation precatalysts and yielded turnover frequencies (TOFs) greater than those typically observed for most other ruthenium-based catalysts.

Full text of DOI:10.1021/om100552h

Organometallics 2010, 29, 6121–6124 6121
DOI: 10.1021/om100552h
Synthesis and Reactivity of [(PhB(CH2PPh2)3-K3P)Ru(NCMe)3]PF6 and
Its Potential as a Transfer Hydrogenation Catalyst
Jesse M. Walker,† Alex M. Cox,† Ruiyao Wang,‡ and Gregory J. Spivak*,†
†Department of Chemistry, Lakehead University, Thunder Bay, Ontario P7B 5E1, Canada, and
‡Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada
Received June 4, 2010
Summary: The acetonitrile ligands in [(PhB(CH2PPh2)3-κ3P)-
Scheme 1
Ru(NCMe)3]PF6 (1) (which contains a facially coordinated
anionic tripodal phosphine ligand) are labile and are rapidly
replaced by other ligands, although the steric profile of the
phosphine ligand limits the size and the number of the incoming
ligands. Complex 1 and some of its derivatives proved tobe very
efficient and rapid transfer hydrogenation precatalysts and
yielded turnover frequencies(TOFs) greater thanthose typically
observed for most other ruthenium-based catalysts.
The success of the piano-stool complex [(η5-Cp)Ru-
(NCMe)3]PF6 as a versatile ruthenium precatalyst likely
1
can be traced to the substitutional lability of the acetonitrile
ligands,2 which allow it to serve as a convenient source of the
coordinatively unsaturated fragment {(η5-Cp)Ru}þ. Accord-
ingly, [(η5-Cp)Ru(NCMe)3]PF6 functions as a convenient
precursor to an arene activating group3 and has also proven
to be exceptionally useful in a diverse range of organic trans-
formations.4 Indeed, the importance of this compound might
also be gauged by its commercial availability, as well as by
the efforts of several research groups that have endeavored to
devise alternative synthetic strategies5 which are perhaps
more convenient than the original procedure.6
We report here the synthesis, chemistry, and catalytic activ-
ity of a structural counterpart of [(η5-Cp)Ru(NCMe)3]PF6,
which contains, in place of the cyclopentadienyl ligand, the
*To whom correspondence should be addressed. E-mail: greg.spivak@
lakeheadu.ca.
€
(1) Slugovc, C.; Ruba, E.; Schmid, R.; Kirchner, K.; Mereiter, K.
anionic tris(phosphino)borate ligand [PhB(CH2PPh2)3]- (herein
abbreviated as PhBP3).7 The PhBP3 ligand retains the essen-
tial properties of the ubiquitous cyclopentadienyl ligand (i.e.,
isoelectronic, anionic, and face-capping) yet also possesses
characteristics that are quite different (e.g., steric and elec-
tronic properties), and thus it is expected to impart different
chemical properties onto the metal center.7,8
Monatsh. Chem. 2000, 131, 1241.
€
€
(2) Luginbuhl, W.; Zbinden, P.; Pittet, P. A.; Armbruster, T.; Burgi,
H.-B.; Merbach, A. E.; Ludi, A. Inorg. Chem. 1991, 30, 2350.
(3) For example: (a) Semmelhack, M. F.; Chlenov, A. Top. Organo-
met. Chem. 2004, 7, 43. (b) Leone-Stumpf, D.; Lindel, T. Chem. Eur. J.
2001, 7, 3961. (c) Pearson, A. J.; Heo, J.-N. Tetrahedron Lett. 2000, 41,
5991. (d) Pearson, A. J.; Hwag, J.-J. Tetrahedron 2001, 57, 1489. (e) West,
C. W.; Rich, D. H. Org. Lett. 1999, 1, 1819. (f) Pearson, A. J.; Zhang, P.;
Bigan, G. J. Org. Chem. 1997, 62, 4536. (g) Pearson, A. J.; Zhang, P.; Lee, K.
J. Org. Chem. 1996, 61, 6581. (h) Dembek, A. A.; Fagan, P. J. Organome-
tallics 1996, 15, 1319. (i) Dembek, A. A.; Fagan, P. J. Organometallics 1995,
14, 3741. (j) Pearson, A. J.; Park, J. G.; Yang, S. H.; Chuang, Y.-H. J. Chem.
Soc., Chem. Commun. 1989, 1363. (k) Moriarty, R. M.; Ku, Y.-Y.; Gill, U. S.
Organometallics 1988, 7, 660.
The complex [(PhBP3-κ3P)Ru(NCMe)3]PF6 (1) was read-
ily prepared by reacting dimeric [(PhBP3-κ3P)RuCl]28a with
2 equiv of AgPF6 in a 1:1 mixture of acetonitrile and THF
and isolated as analytically pure, colorless crystals in moderate
yields (53%) after recrystallization (Scheme 1). The solid
form is air-stable; however, solutions of 1 prepared in the
(4) For example: (a) Machin, B. P.; Howell, J.; Mandel, J.; Blanchard,
N.; Tam, W. Org. Lett. 2009, 11, 2077. (b) Czekelius, C. Indian J. Chem. B
2009, 48, 1704. (c) Trost, B. M.; Gutierrer, A. C. Org. Lett. 2007, 9, 1473.
(d) Trost, B. M.; Huang, X. Chem. Asian J. 2006, 1, 469. (e) Tanaka, S.;
ꢀ
Saburi, H.; Kitamura, M. Adv. Synth. Catal. 2006, 348, 375. (f) Fernandez,
(7) (a) Peters, J. C.; Feldman, J. D.; Tilley, T. D. Organometallics
2001, 21, 4050. (b) Barney, A. A.; Heyduk, A. F.; Nocera, D. G. Chem.
Commun. 1999, 2379. (c) Peters, J. C.; Feldman, J. D.; Tilley, T. D. J. Am.
Chem. Soc. 1999, 121, 9871.
(8) (a) Betley, T. A.; Peters, J. C. Inorg. Chem. 2003, 42, 5074.
(b) Brown, S. D.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003,
125, 322. (c) Jenkins, D. M.; Di Bilio, A. M.; Allen, M. J.; Betley, T. A.;
Peters, J. C. J. Am. Chem. Soc. 2002, 124, 15336. (d) Jenkins, D. M.; Betley,
T. A.; Peters, J. C. J. Am. Chem. Soc. 2002, 124, 11238. (e) Peters, J. C.;
Feldman, J. D.; Tilley, T. D. Organometallics 2001, 21, 4065. (f) Shapiro,
I. R.; Jenkins, D. M.; Thomas, J. C.; Day, M. W.; Peters, J. C. Chem.
Commun. 2001, 2152.
I.; Hermatschweiler, R.; Pregosin, P. S. Organometallics 2006, 25, 323.
(g) Trost, B. M.; Shen, H. C.; Horne, D. B.; Toste, F. D.; Steinmetz, B. G.;
Koradin, C. Chem. Eur. J. 2005, 11, 2577. (h) Trost, B. M.; Rudd, M. T. J.
Am. Chem. Soc. 2005, 127, 4763. (i) Trost, B. M.; Rudd, M. T. J. Am. Chem.
Soc. 2002, 124, 4178. (j) Trost, B. M.; Toste, F. D. J. Am. Chem. Soc. 2002,
124, 5025. (k) Trost, B. M.; Rudd, M. T. J. Am. Chem. Soc. 2001, 123, 8862.
(l) Trost, B. M.; Toste, F. D. Tetrahedron Lett. 1999, 40, 7739.
€
(5) (a) Kundig, E. P.; Monnier, F. R. Adv. Synth. Catal. 2004, 346,
901. (b) Trost, B. M.; Older, C. M. Organometallics 2002, 21, 2544.
(6) Schrenk, J. L.; McNair, A. M.; McCormick, F. B.; Mann, K. R.
Inorg. Chem. 1986, 25, 3501.
r
2010 American Chemical Society
Published on Web 10/25/2010
pubs.acs.org/Organometallics

6122 Organometallics, Vol. 29, No. 22, 2010
Walker et al.
open air slowly turn green over a period of hours. Unfortu-
nately, attempts to prepare 1 in reasonable yields via a more
direct route (e.g., from RuCl2(NCMe)4) have so far failed.
The 31P{1H} NMR spectrum of 18-electron 1 at room
temperature reveals a sharp singlet at 37.5 ppm for the
coordinated ligand, substantially upfield of the parent
16-electron dimer (64.0 ppm).8a The sharpness of the signal
would seem to suggest that the PhBP3 ligand is stereochemi-
1
cally rigid under these conditions. However, the H NMR
spectrum of 1 shows a single, slightly broadened signal at
1.40 ppm assigned to the hydrogen atoms of the methylene
bridges of the PhBP3 ligand in 1, with no resolved coupling to
phosphorus, thus revealing some dynamic behavior under
these conditions. Unfortunately, this signal does not resolve
at low temperatures (-70 °C) and only broadens slightly.
Slow diffusion of diethyl ether into a saturated MeCN solution
of 1 yielded single crystals suitable for X-ray analysis. Two
views of the structure of 1 are displayed in Figure 1.9 Viewing
down the C3 ruthenium-boron vector reveals that the PhB-
(CH2)3 bridging unit of the PhBP3 ligand is staggered slightly
(∼37°) with respect to the RuP3 face of the octahedron and
that the phosphorus phenyl groups of the PhBP3 ligand
adopt a propeller-like conformation. In fact, the facially
coordinating acetonitrile ligands experience slight crowding
(mean N-Ru-N angle of 84.4°), presumably as a result of
the steric constraints imposed by the phenyl groups on the
phosphorus atoms. In comparison, the acetonitrile ligand in
[([9]aneS3-κ3S)Ru(NCMe)3]þ ([9]aneS3 = 1,4,7-trithiacyclo-
nonane),10 [(Tp-κ3N)Ru(NCMe)3]þ (Tp = hydridotris(pyra-
zolyl)borate),11 [(η6-C6H6)Ru(NCMe)3]2þ,2 and [(η5-Cp)Ru-
(NCMe)3]þ 2 experience slightly less crowding (mean N-
Ru-N angles of 87.3, 89.9, 86.0, and 87.3°, respectively),
likely because of the smaller steric profiles of their face-capping
ligands. The average Ru-N distance in 1 (2.108 A) is larger
than in [([9]aneS3-κ3S)Ru(NCMe)3]þ (2.074 A),10 [(Tp-κ3N)-
Ru(NCMe)3]þ (2.045 A),11 [(η6-C6H6)Ru(NCMe)3]2þ (2.055A)2
and [(η5-Cp)Ru(NCMe)3]þ (2.083 A),2 reflecting not only
the steric demands of the PhBP3 ligand but, perhaps more
importantly, the particularly high trans influence typically
exerted by members of this class of phosphine ligand.12
Reactions of 1 with various ligands L (Scheme 1) are gen-
erally fast. The rapidity of these reactions suggests the aceto-
nitrile ligands are relatively labile. For L = PR3, only mono-
substitution is observed, even with excess phosphine or phosphite,
yielding [(PhBP3-κ3P)Ru(PR3)(NCMe)2]PF6 (2a, R = Me;
2b, R = OMe). The 31P{1H} NMR spectra of 2a,b reveal the
spin patterns expected for monosubstitution. For example,
the spectrum of 2a shows a resolved overlapping doublet of
doublets (35.8 ppm) assigned to the two phosphorus atoms
of the PhBP3 ligand trans to the acetonitrile ligands (see
Scheme 1), while a doublet of triplets is observed for each
of the remaining phosphorus atoms of the PhBP3 ligand
Figure 1. Two views of the cation of 1 (the hydrogen atoms,
counterion, and solvent molecules have been omitted for clarity):
(a) down the Ru-B vector; (b) side view. Selected bond lengths
(A) and angles (deg): Ru(1)-P(1) = 2.3187(8), Ru(1)-P(2) =
2.3285(9), Ru(1)-P(3) = 2.3363(8), Ru(1)-N(2) = 2.105(3),
Ru(1)-N(1) = 2.108(3), Ru(1)-N(3) = 2.112(3); N(2)-Ru(1)-
N(1) = 84.08(10), N(2)-Ru(1)-N(3) = 83.24(11), N(1)-Ru(1)-
N(3) = 85.75(11), P(1)-Ru(1)-P(2) = 89.38(3), P(1)-Ru(1)-
P(3) = 86.12(3), P(2)-Ru(1)-P(3) = 88.43(3).
(17.4 ppm) and the PMe3 ligand (-20.7 ppm), both of which
are trans coordinated. The cis and trans 2JPP couplings (30-40
and 238 Hz, respectively) fall within the ranges observed
for other Ru(II) complexes bearing tripodal phosphines.13
Slightly larger phosphines such as PEt3 (cone angle 132°;
compare with PMe3, with a cone angle of 118°, and P(OMe)3,
with a cone angle of 107°)14 showed no reaction with 1,
whereas [(η5-Cp)Ru(NCMe)3]þ easily accommodates more
sterically demanding phosphines such as PCy3, or even up to
three P(OMe)3 ligands,15 further suggesting the face occu-
pied by the three acetonitrile ligands in 1 is comparatively
(9) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.
(10) Landgrafe, C.; Sheldrick, W. S. J. Chem. Soc., Dalton Trans.
1994, 1885.
€
(11) Ruba, E.; Simanko, W.; Mereiter, K.; Schmid, R.; Kirchner, K.
Inorg. Chem. 2000, 39, 382.
(12) (a) Beach, M. T.; Walker, J. M.; Larocque, T. G.; Deagle, J. L.;
Wang, R.; Spivak, G. J. J. Organomet. Chem. 2008, 693, 2921.
(b) Thomas, C. M.; Peters, J. C. Inorg. Chem. 2004, 43, 8.
(13) (a) Chaplin, A. B.; Dyson, P. J. Inorg. Chem. 2008, 47, 381.
(b) S€ul€u, M.; Venanzi, L. M. Inorg. Chim. Acta 1999, 293, 70. (c) Bianchini,
C.; Meli, A.; Moneti, S.; Vizza, F. Organometallics 1998, 17, 2636.
ꢀ
(d) Delgado, G.; Rivera, A. V.; Suarez, T.; Fontal, B. Inorg. Chim. Acta
1995, 233, 145. (e) Michos, D.; Luo, X.-L.; Crabtree, R. H. Inorg. Chem.
1992, 31, 4245.
(14) Tolman, C. A. Chem. Rev. 1977, 77, 313.

Note
Organometallics, Vol. 29, No. 22, 2010 6123
Table 1. Transfer Hydrogenation of Various Ketones by 1, 2a or [(PhBP3-κ3P)RuCl]2 at 86 °C using 2-Propanol/KOH
a 0.2 mol % catalyst load. b Determined after 10 min by GC analysis and averaged over two independent experiments. c Turnover frequencies ((mol of
ketone converted into alcohol)/((mol of catalyst) h)) were determined at 50% conversion and averaged over two independent experiments. d Reaction
was performed at 23 °C. e Determined from a single experiment. f Reaction was performed with no base.
more crowded. Similarly, reactions of 1 with ethylenedia-
mine rapidly yielded [(PhBP3-κ3P)Ru(en)(NCMe)]PF6 (3),
while reactions with tetramethylethylenediamine (TMEDA)
yielded only starting materials, even after extended periods.
In contrast, [(η5-Cp)Ru(NCMe)3]PF6 reacts with TMEDA
to give [(η5-Cp)Ru(TMEDA)(NCMe)]PF6 at room tempera-
ture in quantitative yields.16
In order to assess its catalytic potential, the activity of 1
was screened as a transfer hydrogenation17 catalyst precursor
using several different aromatic and aliphatic ketone substrates
(eq 1). Indeed, [(PhBP3-κ3P)RuCl]2 has already shown excep-
tional activity as a hydrogenation catalyst.18
all cases, the compounds rapidly catalyzed the reduction of
the carbonyl group to the corresponding alcohol with high
yields. Although we did not optimize the reaction conditions
(e.g., catalyst loadings, choice or concentration of base, etc.),
under the conditions we explored, the observed TOFs (4000-
34 000 h-1) are much higher than those typically observed for
many ruthenium catalysts.17c,19 For comparative purposes,
we also examined complex 2a (entry 4) and [(PhBP3-κ3P)-
RuCl]2 (entry 5) using acetophenone as the test substrate.
Compound 1 was the fastest of the three (TOF = 30000 h-1),
followed by 2a (TOF = 18300 h-1), while the dimer [(PhBP3-
κ3P)RuCl]2 proved to be the slowest (TOF = 5930 h-1).
The transfer hydrogenation reaction still proceeds at room
temperature for complex 1 (entry 2), but much more slowly
(TOF = 1050 h-1), and required ca. 2 h to reach ∼75%
completion, as determined by GC analysis. Furthermore, the
presence of a base is also important; less than 2% conversion
after about 12 h was observed with complex 1 when the
2-propanolic KOH solution was eliminated from the reac-
tion (entry 3). High concentrations of KOH alone are known
to catalyze the hydrogenation of ketones in 2-propanol at
elevated temperatures.20 However, we observed no product
formation when catalyst 1 was eliminated from the reaction
under the conditions we employed in our catalytic investiga-
tions (i.e., 20:1 acetophenone/KOH, 10 mL of 2-propanol).
The catalytic experiments (see Table 1, entries 1 and 6-9)
were performed by first pretreating complex 1 (0.2 mol %)
with the ketone substrate (4 mmol) in refluxing 2-propanol
(86 °C). Upon addition of a base (0.1 M KOH in 2-propanol,
25-fold excess vs 1) at the desired temperature, the reactions
began almost immediately, as indicated by GC analysis. In
€
(15) (a) Ruba, E.; Simanko, W.; Mauthner, K.; Soldouzi, K. M.;
Slugovc, C.; Mereiter, K.; Schmid, R.; Kirchner, K. Organometallics
1999, 18, 3843. (b) Gill, T. P.; Mann, K. R. Organometallics 1982, 1, 485.
(16) Gemel, C.; Huffman, J. C.; Caulton, K. G.; Mauthner, K.;
Kirchner, K. J. Organomet. Chem. 2000, 593-594, 342.
(19) Exceptionally high TOFs have been observed for a number of
ruthenium catalysts, for example: (a) Dani, P.; Karlen, T.; Gossage,
R. A.; Gladiali, S.; van Koten, G. Angew. Chem., Int. Ed. 2000, 39, 743.
(b) Baratta, W.; Da Ros, P.; Del Zotto, A.; Sechi, A.; Zangrando, E.; Rigo, P.
Angew. Chem., Int. Ed. 2004, 43, 3584. (c) Lundgrun, R. J.; Rankin, M. A.;
McDonald, R.; Schatte, G.; Stradiotto, M. Angew. Chem., Int. Ed. 2007, 46,
4732.
(17) (a) Gladiali, S.; Alberico, E. Chem. Soc. Rev. 2006, 35, 226.
€
(b) Samec, J. S. M.; Backvall, J.-E.; Andersson, P. G.; Brandt, P. Chem. Soc.
Rev. 2006, 35, 237. (c) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord.
Chem. Rev. 2004, 248, 2201. (d) Noyori, R; Ohkuma, T. Angew. Chem., Int.
Ed. 2001, 40, 40. (e) Noyori, R; Hashiguchi, S. Acc. Chem. Res. 1997, 30,
97.
ꢀ
ꢀ
ꢀ
ꢀ
(18) Jimenez, S.; Lopez, J. A.; Ciriano, M. A.; Tejel, C.; Martı
´
nez, A.;
(20) Carrion, M. C.; Jalon, F. A.; Manzano, B. R.; Rodrı
´
guez, A. M.;
ꢀ
ꢀ
Sanchez-Delgado, R. A. Organometallics 2009, 28, 3193.
Sepulveda, F.; Maestro, M. Eur. J. Inorg. Chem. 2007, 3961.

6124 Organometallics, Vol. 29, No. 22, 2010
Walker et al.
Finally, when the transfer hydrogenation of acetophenone
was performed on a gram scale using 1 as a catalyst, relatively
pure (when compared to an authentic sample) 1-phenyletha-
nol was isolated in essentially quantitative yield.
precatalyst. Current efforts in our laboratory are directed
toward expanding upon the work presented herein.
Acknowledgment. We gratefully acknowledge finan-
cial support from the Natural Sciences and Engineering
Research Council of Canada (NSERC).
In summary, similar to its structural counterpart [(η5-Cp)-
Ru(NCMe)3]PF6, the acetonitrile ligands in complex 1 are
labile and are readily replaced by other ligands. However, the
sterics of the PhBP3 ligand appear to control the extent of
substitution, as well as the size of the incoming ligands.
Exploiting the lability of the acetonitrile ligands, complex 1
proved to be an effective and promising transfer hydrogenation
Supporting Information Available: Text describing full syn-
thetic procedures and spectroscopic data for all compounds and a
CIF file giving crystallographic details for complex 1. This material
is available free of charge via the Internet at http://pubs.acs.org.