Reactivity of the ruthenium hydride complex [Ru{K3-H,S,S-HB(mt) 3}H(PPh3)2] (HB(mt)3 = hydrotris(methimazolyl)borate): Catalytic dimerization of 1-alkynes and proton transfer reactions

Article abstract of DOI:10.1021/om900157b

The complex [Ru{κ3-H,S,S-HB(mt)₃}H(PPh₃) ₂] (1, HB(mt)₃ = hydrotris(methimazolyl)borate; mt = N-methyl-2-mercaptoimidazolyl) containing a three-center two-electron Ru-H-B bonding interaction has been prepared by

Full text of DOI:10.1021/om900157b

Organometallics 2009, 28, 2787–2798
2787
Reactivity of the Ruthenium Hydride Complex
[Ru{K3-H,S,S-HB(mt)3}H(PPh3)2] (HB(mt)3 )
hydrotris(methimazolyl)borate): Catalytic Dimerization of 1-Alkynes
and Proton Transfer Reactions
Manuel Jime´nez-Tenorio, M. Carmen Puerta,* and Pedro Valerga*
Departamento de Ciencia de Materiales e Ingenier´ıa Metalu´rgica y Qu´ımica Inorga´nica, Facultad de
Ciencias, UniVersidad de Ca´diz, 11510 Puerto Real, Ca´diz, Spain
ReceiVed February 27, 2009
The complex [Ru{κ3-H,S,S-HB(mt)3}H(PPh3)2] (1, HB(mt)3 ) hydrotris(methimazolyl)borate; mt )
N-methyl-2-mercaptoimidazolyl) containing a three-center two-electron Ru-H-B bonding interaction
has been prepared by reaction of [RuHCl(PPh3)3] with Na[HB(mt)3] in MeOH at 40 °C. This compound,
which occurs in solution as a mixture of two stereoisomers (1a and 1b), has been spectroscopically and
structurally characterized. 1 reacts over a two-day period with CH2Cl2/MeOH at room temperature,
furnishing the methimazolate complex [RuH(mt)(PPh3)3] (2). The latter compound exists in solution as
a 55:45 mixture of stereoisomers, and they were structurally characterized. Both 1 and 2 are catalyst
precursors for the dimerization of 1-alkynes (toluene, 85 °C, 2% catalyst load) to mixtures of Z, E, and
gem-stereoisomers of the corresponding enynes. Compound 1 is much more active and selective than 2.
In the presence of benzoic acid, the activity of the catalyst decreases, but the stereoselectivity toward the
(Z)-enyne isomer increases. NMR monitoring of the reaction of 1 with PhCOOH in CD2Cl2 in the
temperature range -60 to 0 °C has allowed the identification of what seems to be a dihydrogen-bonded
complex between the stereoisomer 1b and PhCOOH. This species exhibits a very short minimum
longitudinal relaxation time of 5.9 ms. Protonation of 1 with HBF4 · OEt2 in CD2Cl2 at -60 °C yields the
metastable dihydrogen complex [Ru{κ3-H,S,S-HB(mt)3}(H2)(PPh3)2][BF4] (3a), which loses H2 as the
temperature increases, furnishing the complex [Ru{κ3-H,S,S-HB(mt)3}(PPh3)2][BF4], which is most likely
coordinatively unsaturated. Likewise, the protonation of 2 with HBF4 · OEt2 in CD2Cl2 at -60 °C generates
the dihydrogen complex [Ru(mt)(H2)(PPh3)3][BF4] (4) as a 5:1 mixture of two stereoisomers.
capabilities of the S- or N-donor atoms but also due to the
different conformational nature of the rings in the HB(mt)3-
metal or Tp-metal cages.2-12 Thus, complexation by the three
nitrogen atoms of the Tp ligand (κ3-N,N,N) provides a bicy-
Introduction
Since its introduction by Reglinski and Spicer, the hydrot-
ris(methimazolyl)borate anion ([HB(mt)3]-, mt ) N-methyl-2-
mercaptoimidazolyl) has been regarded as the soft donor atom
equivalent of the N-donor anion hydrotris(pyrazolyl)borate
(Tp).1
(2) Bailey, P. J.; Lorono-Gonzales, D. J.; McCormack, C.; Parsons, S.;
Price, M. Inorg. Chim. Acta 2003, 354, 61–67.
(3) Kuan, S. L.; Leong, W. K.; Goh, L. Y.; Webster, R. D. Organo-
metallics 2005, 24, 4639–4648.
(4) Kuan, S. L.; Leong, W. K.; Goh, L. Y.; Webster, R. D. J. Organomet.
Chem. 2006, 691, 907–915.
(5) Bailey, P. J.; Dawson, A.; McCormack, C.; Moggach, S. A.; Oswald,
I. D. H.; Parsons, S.; Rankin, D. W. H.; Turner, A. Inorg. Chem. 2005, 44,
8884–8898.
(6) Bailey, P. J.; McCormack, C.; Parsons, S.; Rudolphi, F.; Sa´nchez
Perucha, A.; Wood, P. Dalton Trans. 2007, 476–480.
(7) Hill, A. F.; Owen, G. R.; White, A. J. P.; Williams, D. J. Angew.
Chem., Int. Ed. 1999, 38, 2759–2761.
(8) Foreman, M. R.; St, J.; Hill, A. F.; Owen, G. R.; White, A. J. P.;
Williams, D. J. Organometallics 2003, 22, 4446–4450.
(9) Abernethy, R. J.; Hill, A. F.; Tshabang, N.; Willis, A. C.; Young,
R. D. Organometallics 2009, 28, 488–492.
(10) (a) Buccella, D.; Shultz, A.; Melnick, J. G.; Konopka, F.; Parkin,
G. Organometallics 2006, 25, 5496–5499. (b) Bridgewater, B. M.; Parkin,
G. J. Am. Chem. Soc. 2000, 122, 7140–7141. (c) Kimblin, C.; Bridgewater,
B. M.; Hascall, T.; Parkin, G. J. Chem. Soc., Dalton Trans. 2000, 891–
897.
(11) (a) Garner, M.; Lehmann, M.; Reglinski, J.; Spicer, M. D.
Organometallics 2001, 20, 5233–5236. (b) Ojo, J. F.; Slavin, P. A.;
Reglinski, J.; Spicer, M. D.; Garner, M.; Kennedy, A. R.; Teat, S. J. Inorg.
Chim. Acta 2001, 313, 15–20.
Formally, both HB(mt)3 and Tp present analogies, which
anticipate similar stoichiometries for their complexes with
transition metals. However, there are also noticeable differences
in the chemical behavior, not only resulting from the electronic
* To whom correspondence should be addressed. E-mail: carmen.puerta@
uca.es; pedro.valerga@uca.es. Tel: +34 956 016350. Fax: +34 956 016288.
(1) Reglinski, J.; Garner, M.; Cassidy, I. D.; Slavin, P. A.; Spicer, M. D.;
Armstrong, D. R. J. Chem. Soc., Dalton Trans. 1999, 2119–2126.
(12) Patel, D. V.; Mihalcik, D. J.; Kreisel, K. A.; Yap, G. P. A.;
Zakharov, L. N.; Kassel, W. S.; Rheingold, A. L.; Rabinovich, D. J. Chem.
Soc., Dalton Trans. 2005, 2410–2416.
10.1021/om900157b CCC: $40.75
2009 American Chemical Society
Publication on Web 04/17/2009

2788 Organometallics, Vol. 28, No. 9, 2009
Jime´nez-Tenorio et al.
clo[2.2.2] cage containing six-membered rings, whereas the
HB(mt)3 ligand forms eight-membered rings when bound
through its three sulfur atoms to the metal (κ3-S,S,S), resulting
in a bicyclo[3.3.3] cage.5 One important consequence is that
κ3-S,S,S-HB(mt)3 complexes potentially display a novel form
of atropisomerism in which rotation is restricted about a three-
dimensional cage rather than about a single bond.6 Several
ruthenium complexes containing the HB(mt)3 ligand bound in
a κ3-S,S,S fashion have been reported.2–6 One important feature
of the HB(mt)3 ligand observed in some ruthenium complexes
is the ability of binding by means of a B-H-Ru three-center
two-electron bonding.3,4,7–9 This results in a κ3-H,S,S coordina-
tion mode that involves the formation of a bicyclo[1.3.3] cage,
as in the case of the complex [Ru{κ3-H,S,S-HB(mt)3}H(CO)-
(PPh3)].8 This more compact arrangement seems to be preferred
under certain circumstances to the geometrical constraints
imposed by the bicyclo[3.3.3] unit in the κ3-S,S,S coordination
mode. When bound in this fashion, the HB(mt)3 ligand in
[Ru{κ3-H,S,S-HB(mt)3}H(CO)(PPh3)] undergoes a thermal de-
hydrogenation, which involves intramolecular BH activation.
This process leads to the remarkably stable ruthenaboratrane
complex [Ru{κ3-S,S,S-B(mt)3}(CO)(PPh3)], which contains a
ruthenium to boron dative bond.7 The homologous osmabo-
ratrane complex [Os{κ3-S,S,S-B(mt)3}(CO)(PPh3)] has been also
reported.13 The ruthenaboratrane complex [Ru(κ3-S,S,S-
B(mt)3)(CO)(PPh3)] is also readily generated by elimination of
RH from [Ru{κ3-H,S,S-HB(mt)3}R(CO)(PPh3)] (R ) CHdCH2,
CHdCHCPh2(OH), CHdCH(4-MeC6H4), C6H5) upon BH ac-
tivation. [Ru{κ3-S,S,S-B(mt)3}(CO)(PPh3)] is rather inert once
formed and does not react with H2 at ambient temperatures and
pressures to re-form [Ru{κ3-H,S,S-HB(mt)3}H(CO)(PPh3)].7,8
Furthermore, the reaction of [Ru{κ3-H,S,S-HB(mt)3}H(CO)-
(PPh3)] with PhCtCH in dichloromethane leads to the quantita-
tive formation of styrene and [Ru{κ3-S,S,S-B(mt)3}(CO)(PPh3)]
over a period of 23 h at room temperature.8 No alkyne coupling
products have been observed in this process, most likely due to
the robust nature of the ruthenaboratrane complex.
Our research group has been studying for some time the
chemistry of ruthenium complexes bearing hydrotris(pyra-
zolyl)borate and phosphine ligands.14-18 We have developed a
number of catalytic application with these complexes to
processes such as alkyne coupling reactions,15 synthesis of enol-
lactone,16 or enantioselective solvent-free cycloaddition reac-
tions.18 Given the versatility of the coordination modes for the
HB(mt)3 ligand, we became interested in testing the ability of
ruthenium complexes bearing this ligand as catalyst precursors
in alkyne coupling reactions. Given the fact that no catalytic
activity toward PhCtCH has been reported for [Ru{κ3-H,S,S-
HB(mt)3}H(CO)(PPh3)],8 we decided to avoid the use of CO,
replacing it with PPh3. Rather unexpectedly, we have found that
the resulting complex [Ru{κ3-H,S,S-HB(mt)3}H(PPh3)2] (1) is
catalytically active for the dimerization of 1-alkynes. This is
only the second example of a catalytic process effected by
methimazolylborate complexes.10a The ruthenaboratrane com-
plex [Ru{κ3-S,S,S-B(mt)3}(PPh3)2] has not been observed in our
system. It seems as if the CO ligand strongly contributes to the
stabilization of the ruthenaboratrane species. In this work we
describe the preparation, structural characterization, and chemi-
cal properties of [Ru{κ3-H,S,S-HB(mt)3}H(PPh3)2], its catalytic
activity toward the dimerization of 1-alkynes, and a NMR study
of its protonation reactions.
Experimental Section
All synthetic operations were performed under a dry dinitrogen
or argon atmosphere following conventional Schlenk techniques.
Tetrahydrofuran, diethyl ether, and petroleum ether (boiling point
range 40-60 °C) were obtained oxygen- and water-free from an
Innovative Technology, Inc. solvent purification apparatus. Other
solvents (toluene, dichloromethane, methanol) were of anhydrous
quality and used as received. All solvents were deoxygenated
immediately before use. The ligand sodium tris(methimazolyl)borate
(Na[HB(mt)3]) was prepared following suitable adaptations of
published procedures.1,2 [RuHCl(PPh3)3] was prepared by a modi-
fied procedure developed in our laboratory.19 Reaction of
HBF4 · OEt2 and benzoic acid were used as supplied by Aldrich.
IR spectra were taken in Nujol mulls on a Perkin-Elmer Spectrum
1000 FTIR spectrophotometer. NMR spectra were taken on a Varian
Unity 400 MHz or Varian Gemini 300 MHz equipment. Chemical
shifts are given in ppm from SiMe4 (1H and 13C{1H}), 85% H3PO4
(31P{1H}), or BF3 · OEt2 (11B). Longitudinal relaxation times (T1)
measurements were made by the inversion-recovery method.
Microanalysis was performed on an elemental analyzer model
LECO CHNS-932 at the Servicio Central de Ciencia y Tecnolog´ıa,
Universidad de Ca´diz.
[Ru{K3-H,S,S-HB(mt)3} H(PPh3)2], 1. To a slurry of [RuH-
Cl(PPh3)3] (0.47 g. 0.5 mmol) in methanol (10 mL) was added
sodium hydrotris(methimazolyl)borate (Na[HB(mt)3], 0.2 g, ca. 0.5
mmol). The mixture was stirred in a water bath at 45 °C for 30
min. The color of the mixture changed gradually from purple to
yellow, and a microcrystalline yellow precipitate was formed. The
precipitate was filtered off, washed with ethanol and petroleum
ether, and dried in vacuo. The resulting material is a mixture of
two diastereoisomers. It was recrystallized by dissolving the crude
product in hot toluene, filtering the solution, and layering with
diethyl ether. Greenish-yellow crystals were obtained, containing
one toluene molecule of crystallization. Yield: 0.4 g, 75%. Anal.
Calcd for C48H47N6BP2RuS3 · C7H8: C, 61.7; H, 5.18; N, 7.9. Found:
C, 61.4; H, 5.01, N, 8.1. IR: ν(RuH) 2012, ν(BH) 2170 cm-1. The
product exists in solution as a mixture of two stereoisomers, 1a
and 1b, present in variable amounts depending on the solvent. NMR
data for isomer 1a in CD2Cl2 (relative amount 64%): 1H NMR (400
2
MHz, CD2Cl2, 298 K): δ -17.68 (t, JHP ) 27.7 Hz, 1 H, RuH),
-3.89 (m br, 1 H, RuHB), 3.04 (s, 6 H, NCH3 of coordinated
methimazolate rings), 3.65 (s, 3 H, NCH3 of uncoordinated
methimazolate ring), 6.38, 6.77 (d, 3JHH ) 2.2 Hz, 1 H each, CHCH
3
of uncoordinated methimazolate ring), 6.48, 6.68 (d, JHH ) 1.9
Hz, 2 H each, CHCH of coordinated methimazolate rings),
6.91-7.49 (m, 30 H, P(C6H5)3). 31P{1H} NMR (161.89 MHz,
CD2Cl2, 298 K): δ 63.8 (s). 13C{1H} NMR (101 MHz, CD2Cl2,
298 K): δ 34.26 (NCH3 of coordinated methimazolate rings); 35.04
(s, NCH3 of uncoordinated methimazolate ring); 118.30, 120.26
(s, CHCH of uncoordinated methimazolate ring); 119.15, 119.66
(s, CHCH of coordinated methimazolate rings); 126.82, 127.92,
129.3, 134.86 (s, P(C6H5)3); 166.1, 166.5 (s, CdS). 11B NMR (128
MHz, CD2Cl2, 298 K): 1.6 (br). NMR data for isomer 1b in CD2Cl2
(13) Foreman, M. R. St.-J.; Hill, A. F.; White, A. J. P.; Williams, D. J.
Organometallics 2004, 23, 913–916.
(14) Jime´nez-Tenorio, M. A.; Jime´nez-Tenorio, M.; Puerta, M. C.;
Valerga, P. Inorg. Chim. Acta 1997, 259, 77–84.
(15) Jime´nez-Tenorio, M. A.; Jime´nez-Tenorio, M.; Puerta, M. C.;
Valerga, P. Organometallics 2000, 19, 1333–1342.
(16) Jime´nez-Tenorio, M.; Puerta, M. C.; Valerga, P.; Moreno-Dorado,
F. J.; Guerra, F. M.; Massanet, G. M. Chem. Commun. 2001, 2324–2325.
(17) Jime´nez-Tenorio, M.; Palacios, M. D.; Puerta, M. C.; Valerga, P.
Organometallics 2005, 24, 3088–3098.
1
(relative amount 36%): H NMR (400 MHz, CD2Cl2, 298 K): δ
-12.35 (t, 2JHP ) 24.3 Hz, 1 H, RuH), -8.11 (m br, 1 H, RuHB),
(18) Jime´nez-Tenorio, M.; Palacios, M. D.; Puerta, M. C.; Valerga, P.
J. Mol. Catal. A: Chem. 2007, 261, 64–72.
(19) Jime´nez-Tenorio, M.; Puerta, M. C.; Valerga, P. Inorg. Chem. 1994,
33, 3515–3520.

Ru Hydride Complexes with Hydrotris(methimazolyl)borate
Organometallics, Vol. 28, No. 9, 2009 2789
3.26, 3.31, 3.53 (s, 3 H each, NCH3 of methimazolate rings), 5.94,
6.20, 6.52, 6.55, 6.58, 6.66 (s br, 1 H each, CH of methimazolate
rings), 6.91-7.49 (m, 30 H, P(C6H5)3). 31P{1H} NMR (161.89 MHz,
CD2Cl2, 298 K): δ 61.3 (d, 2JPP ) 36 Hz), 73.6 (br). 13C{1H} NMR
(101 MHz, CD2Cl2, 298 K): δ 34.37, 34.57, 34.92 (NCH3); 117.60,
118.46, 118.64, 119.05, 119.82, 120.1 (s, CH of methimazolate
ring); 127.03, 127.11, 127.75, 128.55, 128.48, 134.28 (P(C6H5)3);
165.5, 166.0, 166.5 (s, CdS). 11B NMR (128 MHz, CD2Cl2, 298
K): 0.7 (br). NMR data for isomer 1a in C2D2Cl4 (relative amount
NMR tubes, were frozen by immersion into liquid nitrogen. The
corresponding amount of PhCOOH or HBF4 · OEt2 (via micropipet)
was added. The solvent was allowed to melt. Then, the tubes were
shaken, to mix the reagents, and stored in an ethanol/liquid nitrogen
bath. The samples prepared in this way were studied by NMR at
low temperatures. The sample was removed from the bath and
inserted into the precooled probe of the Varian UNITY-400
spectrometer at 223 K. Once shims were adjusted, the probe was
warmed to the desired temperature. The NMR temperature control-
ler was previously calibrated against a methanol sample, with the
reproducibility being (0.5 °C.
100%): 1H NMR (400 MHz, C2D2Cl4, 298 K): δ -17.51 (t, 2JHP
)
27.6 Hz, 1 H, RuH), -4.50 (br, 1 H, RuHB), 2.63 (s, 6 H, NCH3
of coordinated methimazolate rings), 3.23 (s, 3 H, NCH3 of
Selected Spectral Data for [Ru{K3-H,S,S-HB(mt)3}(H2)(PPh3)2]-
[BF4], 3a. 1H NMR (400 MHz, CD2Cl2, 233 K): δ -8.78 (m br, 1
H, RuHB), -5.94 (br, 2 H, Ru(H2), (T1)min ) 19.8 ms), 3.38 (s, 6
H, NCH3 of coordinated methimazolate rings), 3.71 (s, 3 H, NCH3
of uncoordinated methimazolate ring), 6.249 (s br, 1 H, CHCH of
uncoordinated methimazolate rings; the other CHCH signals of
uncoordinated methimazolate ring is obscured by aromatic peaks),
6.692 (s br, 2 H, CHCH of coordinated methimazolate rings; the
other CHCH signal of coordinated methimazolate ring is obscured
by aromatic peaks), 7.22, 7.24, 7.31 (m, 30 H, P(C6H5)3). 31P{1H}
3
uncoordinated methimazolate ring), 5.84, 6.26 (d, JHH ) 2.2 Hz,
1 H each, CHCH of uncoordinated methimazolate ring), 6.06, 6.31
(d, 3JHH ) 1.9 Hz, 2 H each, CHCH of coordinated methimazolate
rings), 6.55, 6.68, 6.97 (m, 30 H, P(C6H5)3). 31P{1H} NMR (161.89
MHz, C2D2Cl4, 298 K): δ 63.0 (s). 13C{1H} NMR (101 MHz,
C2D2Cl4, 298 K): δ 32.86 (NCH3 of coordinated methimazolate
rings); 33.86 (s, NCH3 of uncoordinated methimazolate ring);
116.73, 118.98 (s, CHCH of uncoordinated methimazolate ring);
117.69, 118.15 (s, CHCH of coordinated methimazolate rings);
126.33, 127.15, 127.96, 133.42 (P(C6H5)3); 164.37, 164.53 (s, CdS).
[RuH{K2-N,S-mt}(PPh3)3], 2. To a solution of 2-mercapto-1-
methylimidazole (methymazole, 0.23 g, ca. 2 mmol) in THF (15
mL) was added LiBun (1.3 mL of a 1.6 M solution in hexanes, ca.
2.1 mmol) at room temperature. The mixture was stirred at room
temperature for 5 min. Then, it was added via cannula to a slurry
of [RuHCl(PPh3)3] (1.8 g. ca. 2 mmol) in THF (20 mL). The
resulting mixture was stirred for 4 h at 60 °C. A thick yellow
suspension was obtained. The yellow precipitate was filtered off.
It was washed with two portions of ethanol and with one portion
of petroleum ether and dried in vacuo. This product is obtained as
a 1:1 mixture of diastereoisomers. No separation was attempted.
The crude product can be recrystallized from dichloromethane/
petroleum ether or dichloromethane/ethanol mixtures. Yield: 1.6 g,
80%. Anal. Calcd for C58H51N2P3RuS: C, 69.5; H, 5.13; N, 2.8.
Found: C, 69.2; H, 5.33, N. 2.6. IR: ν(RuH) 1946 cm-1. NMR
data for 2 (1:1 mixture of stereoisomers): 1H NMR (400 MHz, C6D6,
2
NMR (161.89 MHz, CD2Cl2, 233 K): δ 43.6 (d, JPP ) 27 Hz),
45.5 (br).
Selected Spectral Data for [Ru{K3-H,S,S-HB(mt)3}(PPh3)2][BF4],
1
3b. H NMR (400 MHz, CD2Cl2, 273 K): δ -3.62 (m br, 1 H,
RuHB), 3.31 (s, 6 H, NCH3 of coordinated methimazolate rings),
3.65 (s, 3 H, NCH3 of uncoordinated methimazolate ring), 6.34,
7.51 (s br, 1 H each, CHCH of uncoordinated methimazolate ring),
6.87, 7.31 (s br, 2 H each, CHCH of coordinated methimazolate
rings), 6.95, 7.09, 7.35 (m, 30 H, P(C6H5)3). 31P{1H} NMR (161.89,
2
CD2Cl2, 273 K): δ 35.9 (br), 76.6 (d, JPP ) 35.9 Hz).
Selected Spectral Data for [Ru{K2-N,S-mt}(H2)(PPh3)3]-
[BF4], 4. NMR data for 4 (5:1 mixture of stereoisomers): NMR
1
data for the major isomer (relative amount 84%): H NMR (400
MHz, CD2Cl2, 213 K): δ -10.09 (br, 2 H, Ru(H2), (T1)min ) 26.1
ms), 2.34 (s, 3 H, NCH3), 5.29, 5.88 (s, 1 H each, CHCH);
6.40-7.80 (m, 30 H, P(C6H5)3). 31P{1H} NMR (161.89 MHz,
2
2
CD2Cl2, 213 K): δ 38.7 (d, JPP′ ) 19.2 Hz), 48.5 (t, JPP′ ) 19.2
1
2
2
Hz). NMR data for the minor isomer (relative amount 16%): H
298 K): δ -16.00 (dt, JHP ) 27.4, JHP′ ) 20.6 Hz, 1 H, RuH),
3
NMR (400 MHz, CD2Cl2, 213 K): -6.54 (br, 2 H, Ru(H2), (T1)min
) 24.6 ms), 2.43 (s, 3 H, NCH3), 5.59, 6.03 (s br, 1 H each, CHCH);
6.40-7.80 (m, 30 H, P(C6H5)3). 31P{1H} NMR (161.89 MHz,
2.35 (s, 3 H, NCH3), 5.03, 7.00 (d, JHH ) 1.3 Hz, 1 H each,
2
2
CHCH); -13.65 (dt, JHP ) 27.4, JHP′ ) 20.6 Hz, 1 H, RuH),
2.61 (s, 3 H, NCH3), 5.84, 5.50 (s br, 1 H each, CHCH); 6.80-7.10,
7.49, 7.68 (m, 90 H, P(C6H5)3). 31P{1H} NMR (161.89 MHz, C6D6,
298 K): δ 48.0 (d, 2JPP′ ) 25.7 Hz), 70.4 (t, 2JPP′ ) 25.7 Hz); 48.3
(d, 2JPP′ ) 29.5 Hz), 71.8 (t, 2JPP′ ) 29.5 Hz). 13C{1H} NMR (101
MHz, C6D6, 298 K): δ 29.99, 30.27 (NCH3); 115.41, 115.52,
125.68, 126.12 (s, CHCH); 126.92, 127.04, 127.12, 127.89, 134.91,
135.11, 135.29, 138.36, 138.53, 138.71 (s, P(C6H5)3); CdS not
observed.
2
2
CD2Cl2, 213 K): 37.4 (d, JPP′ ) 23.1 Hz), 51.3 (t, JPP′ ) 23.1
Hz).
X-ray Structure Determinations. Single crystals of compounds
1 and 2 were obtained and mounted on a glass fiber to carry out
the crystallographic study. Crystal data and experimental details
are given in Table 1. X-ray diffraction data collection was measured
at 100 K on a Bruker Smart APEX CCD 3-circle diffractometer
using a sealed tube source and graphite-monochromated Mo KR
radiation (λ ) 0.71073 Å) at the Servicio Central de Ciencia y
Tecnolog´ıa de la Universidad de Ca´diz. In each case four sets of
frames were recorded over a hemisphere of the reciprocal space
by omega scans with δ(ω) 0.30 degrees and exposure of 10 s per
frame. Correction for absorption was applied by scans of equivalents
using the program SADABS.20 An insignificant crystal decay
correction was also applied. The structure was solved by direct
methods, completed by subsequent difference Fourier syntheses,
and refined on F2 by full matrix least-squares procedures using the
programs contained in the SHELXTL package.21 Non-hydrogen
atoms were refined with anisotropic displacement parameters. For
compound 2 the ligand 2-mercapto-1-methylimidazolate was found
disordered. The disorder was modeled using two orientations
exchanging coordination positions of nitrogen and sulfur atoms and
Catalytic Alkyne Dimerization Reactions. General procedure:
A Schlenk tube was loaded with 0.02 mmol of the catalyst 1, 2, or
1 plus benzoic acid (5 mg, ca. 0.04 mmol) and toluene (4 mL).
Then, 1 mmol of the corresponding 1-alkyne was added. The
reaction mixture was heated at 85 °C using a thermostated shaking
bath for 18 h. At the end of this period, the solvent was removed
in vacuo. The residue was extracted with petroleum ether and
filtered through a plug of silica gel. The silica gel was washed with
several portions of petroleum ether, and the washings were
combined with the filtrate. Removal of the solvent using a rotary
evaporator first and a vacuum pump afterward afforded the
corresponding alkyne dimers as a mixture of stereoisomers Z/E/
gem. The ratios of the different stereoisomers present in the mixture
1
were established by integration of the relevant signals in the H
NMR spectra.
Solution NMR Study of the Protonation Reactions of 1
and 2. Solutions of the respective hydride complex 1 or 2 in CD2Cl2
unless otherwise stated, prepared under an argon atmosphere in
(20) Sheldrick, G. M. SADABS; University of Go¨ttingen: Germany, 2001.
(21) Sheldrick, G. M. SHELXTL Version 6.10, Crystal Structure Analysis
Package; Bruker AXS: Madison, WI, 2000.

2790 Organometallics, Vol. 28, No. 9, 2009
Jime´nez-Tenorio et al.
Table 1. Summary of Crystallographic Data for 1 and 2
1
2
formula
fw
C55H55BN6P2RuS3
1070.05
C58H51N2P3RuS
1002.05
T (K)
100(2)
100(2)
cryst size (mm)
cryst syst
space group
cell params
0.41 × 0.27 × 0.16
0.55 × 0.39 × 0.09
triclinic
monoclinic
j
P1 (no. 2)
P21/m (no. 14)
a ) 11.8849(8) Å
b ) 33.952(2) Å
c ) 12.1312(9) Å
a ) 11.549(2) Å
b ) 12.335(3) Å
c ) 17.962(4) Å
R ) 87.68(3)°
ꢀ ) 82.99(3)°
γ ) 82.60(3)°
2517.7(10)
2
ꢀ ) 92.019(2)°
volume (Å3)
4892.1(6)
4
1.361
0.502
2072
Z
Fcalc (g cm-3
)
1.411
0.544
1108
µ(Mo KR) (mm-1
F(000)
)
max. and min.
transmn factors
θ range for
data collection
reflns collected
unique reflns
no. of obsd
0.919, 0.840
1.000, 0.829
1
Figure 1. High-field region of the H NMR spectra (400 MHz,
25 °C) of 1 in CD2Cl2 (A), C2D2Cl4 (B), and C6D6 (C).
1.67 < θ < 25.0
0.99 < θ < 25.03
16 622
33 651
8719 (Rint ) 0.0282) 8597 (Rint ) 0.0986)
7675
6734
reflns (I > 2σI)
no. of params
final R1, wR2
values (I > 2σI)
final R1, wR2
values (all data)
residual electron
623
615
0.0668, 0.1384
0.0506, 0.1087
0.0781, 0.1925
0.0735, 0.1188
+2.447, -2.184
+1.257, -0.514
-3
density peaks (e Å
)
the thermal parameters of involved non-hydrogen atoms constrained
by EADP instructions. The site occupation factors for the two
orientations refined to 0.57 and 0.43, respectively. The coordinates
of hydride atoms in both cases and, for compound 1, the hydrogen
bonded to the metal and the boron of hydrotris(methimazolyl)borate
ligand were refined with thermal parameters linked to those of
ruthenium. The remaining hydrogen atoms were geometrically
positioned and isotropically refined using the riding model. The
program ORTEP-322 was used for plotting.
Results and Discussion
Preparation of the Complexes. The purple five-coordinate
complex [RuHCl(PPh3)3] reacts cleanly with a stoichiometric
amount of Na[HB(mt)3] in MeOH at 40 °C, affording the
hydride complex [Ru{κ3-H,S,S-HB(mt)3}H(PPh3)2] (1) as a
yellow microcrystalline material rather than the [Ru{κ3-S,S,S-
HB(mt)3}H(PPh3)2] isomer. This synthetic procedure is similar
to the one used for the preparation of the hydrido complex
[TpRuH(PPh3)2], modified by using Na[HB(mt)3] instead of
KTp.10 Shortly before submission of the present work, the
preparation of the related bis(methimazolyl)borate complex
[Ru{κ3-H,S,S-H2B(mt)2}H(PPh3)2] by reaction of [RuHCl
(PPh3)3] with Na[H2B(mt)2] in dichloromethane was reported.9
Compound 1 is insoluble in MeOH, petroleum ether, or diethyl
ether and very poorly soluble in acetone. Its IR spectrum (Nujol)
shows a medium ν(RuH) band at 2012 cm-1 and another weaker
band at 2170 cm-1 attributable to ν(BHRu) in the three-centered
Ru-H-B bond. The values reported for ν(BHRu) in the related
complexes [Ru{κ3-H,S,S-HB(mt)3}H(PPh3)(CO)]8 and [Ru{κ3-
H,S,S-H2B(mt)2}H(PPh3)2]9 are 2213 and 2120 cm-1, respec-
tively. The NMR spectra of 1 are solvent dependent. Compound
1 exists in solution in the form of two diastereoisomers, 1a and
Figure 2. 31P{1H} NMR spectra (161.89 MHz, 25 °C) of 1 in
CD2Cl2 (A), C2D2Cl4 (B), and C6D6 (C).
1b, respectively. The relative amount of each of these stereoi-
somers depends on the solvent used. Figures 1 and 2 show
1
respectively the high-field region of the H NMR spectra of 1
in CD2Cl2 (A), C2D2Cl4 (B), and C6D6 (C) and the corresponding
31P{1H} NMR spectra in the same solvents.
The 1H NMR spectra of 1 in CD2Cl2 (Figure 1A) or in C6D6
(Figure 1C) show two sets of hydride resonances of different
intensity, which appear as triplets with typical cis-phosphine
coupling constants, along with two broad multiplet resonances
ascribed to the RuHB protons of each of the stereoisomers. The
intensity ratio for the two sets of resonances is 64:36 in CD2Cl2
and 83:17 in C6D6. The corresponding 31P{1H} NMR spectra
shown in Figures 2A and 2C show one intense singlet
attributable to one of the stereoisomers (1a), for which
(22) Farruggia, L. J. ORTEP-3 for Windows, version 1.076. J. Appl.
Crystallogr. 1997, 30, 565.

Ru Hydride Complexes with Hydrotris(methimazolyl)borate
Organometallics, Vol. 28, No. 9, 2009 2791
Chart 1
and phosphine ligands may exchange their positions by a Berry
pseudorotation-type mechanism. A similar dissociation of the
BHRu linkage has been proposed to explain the observed
reactivity of [Ru{κ3-H,S,S-HB(mt)3}H(CO)(PPh3)] toward phe-
nylacetylene.8 However, the occurrence of a solvent-dependent
equilibrium between two possible stereoisomers has not been
mentioned in the case of [Ru{κ3-H,S,S-HB(mt)3}H(CO)(PPh3)]8
or the recently reported [Ru{κ3-H,S,S-H2B(mt)2}H(PPh3)2].9
The structure of the isomer 1a was confirmed by X-ray crystal
structure analysis of the solvate 1 · CH3C6H5. An ORTEP view
of the complex [Ru{κ3-H,S,S-HB(mt)3}H(PPh3)2] is shown in
Figure 1, together with the most relevant bond distances and
angles.
The structure of 1 (isomer 1a) consists of a packing of
distorted octahedral molecules of the complex and of toluene
solvate molecules. As inferred spectroscopically for 1a, the
hydride and phosphine ligands are in mutually cis positions.
There is one uncoordinated methimazolato ring pointing away
from the ruthenium atom and one three-center Ru-H-B
interactioninapositiontranstothehydrideligand(H1-Ru1-H1b
159(3)°). Ru-H separations and the general dimensions com-
pare well with the values reported for the related compounds
[Ru{κ3-H,S,S-HB(mt)3}H(CO)(PPh3)]8 and [Ru{κ3-H,S,S-
H2B(mt)2}H(PPh3)2].9 It is interesting to note the occurrence
of short contacts between the hydride atom H1 and two ortho
hydrogen atoms of phenyl groups of PPh3 ligands. The separa-
tions H1 · · · H42 and H1 · · · H18 are respectively 1.96 and 2.17
Å. A short contact of 2.28 Å between the hydride atom and
one ortho hydrogen of a phenyl group has been observed in
the complex [Ru{κ3-H,S,S-HB(mt)3}H(CO)(PPh3)], indicative
of a possible hydrogen-bonding interaction.8 In our case, the
contacts are shorter and suggest stronger interactions, which
contribute to fix the orientations of the PPh3 ligands relative to
the hydride ligand. This phenomenon was recognized and
discussed previously by Junk and Steed for a number of hydride
complexes.23 These H · · · H interactions might account for the
relatively short values of the minimum longitudinal relaxation
time (T1)min measured for the hydride proton in 1. In CD2Cl2
solution the hydride proton in the major isomer 1a has a (T1)min
of 377 ms, whereas in the minor isomer 1b the value for (T1)min
of 164 ms is even shorter.
equivalent phosphorus atoms are expected. Along with this
singlet, two resonances for the minor stereoisomer (1b) are
observed. This is particularly evident in the spectrum in CD2Cl2
solution (Figure 2A), in which one of the resonances appears
as one doublet and the other as a broad peak. This pattern
suggests that the two phosphorus atoms are not equivalent in
the stereoisomer 1b present in minor quantity. The NMR spectra
in CDCl3 are very similar to those recorded in CD2Cl2, the ratio
of stereoisomers 1a:1b being 77:23. At variance with this, the
1H NMR spectrum of 1 in C2D2Cl4 shows only one triplet
hydride resonance at -17.51 ppm and one broad resonance
centered at -4.50 ppm attributable to the RuHB proton (Figure
1B). In this solvent, the 31P{1H} NMR spectrum of 1 shows
one singlet, indicative of the equivalence of the phosphorus
1
atoms (Figure 2B), and both H and 13C(1H) NMR spectra
suggest two different chemical environments for the methima-
zolate rings in the ratio 2:1 as inferred from the integration of
1
the relevant signals in the H NMR spectrum. These spectral
data suggest that in C2D2Cl4 solution only the isomer 1a is
present. Hence, the structure proposed for 1a is octahedral, with
one hydride ligand and two equivalent cis-phosphorus atoms.
This requires that the Ru-H-B interaction occurs in position
trans to the hydride ligand, and the remaining positions are
occupied by two sulfur atoms from the coordinated methima-
zolato rings. One of the methimazolato rings of the HB(mt)3
ligands remains uncoordinated. This structure is similar to that
found for the complexes [Ru{κ3-H,S,S-HB(mt)3}H(CO)(PPh3)]
(considering that in 1 there is one PPh3 ligand in the position
occupied by CO)8 and [Ru{κ3-H,S,S-H2B(mt)2}H(PPh3)2].9 The
stereoisomer 1b, always present in minor amounts in solution,
has inequivalent phosphorus atoms in a relative cis-disposition,
Attempts to recrystallize compound 1 from CH2Cl2/MeOH
solutions over a two-day period afforded yellow crystals of a
compound other than 1. X-ray crystal structure analysis showed
it to be the methimazolate complex [RuH{κ2-N,S-mt}(PPh3)3]
(2). This compound consists of a 57:43 mixture of two
stereoisomers, and both stereoisomers were found present in
the same crystal cell. ORTEP views of each of the stereoisomers
of complex 2 are shown respectively in Figures 2 and 3, together
with the most relevant bond distances and angles.
The structure of compound 2 consists of discrete distorted
octahedral molecules, with a meridional disposition of the PPh3
ligands. The methimazolate ligand appears disordered in two
possible orientations with relative occupations of 57% and 43%,
respectively. Each orientation corresponds to one of the two
stereoisomers present in the crystal. In one of them (isomer 2a,
Figure 5) the hydride ligand appears in a position trans to the
coordinated nitrogen, whereas the sulfur atom is trans to the
P(2) atom. In the other isomer (2b, Figure 3) the hydride ligand
is in a position trans to the sulfur atom, and the coordinated
2
2
as inferred from the magnitude of the JHP and JPP′ coupling
constants. One of the phosphorus resonances appears broad, with
unresolved coupling (Figure 2A), suggesting that this atom is
most likely trans to the RuHB interaction. For this stereoisomer,
the 1H and 13C{1H} NMR spectra suggest three different
chemical environments for the methimazolate rings. The
structures proposed for isomers 1a and 1b are shown in Chart
1.The two isomers 1a and 1b presumably interconvert by
dissociation of the BHRu interaction to yield the coordinatively
unsaturated intermediate [Ru{κ2-S,S-HB(mt)3}H(PPh3)2], as
shown in Chart 1. In this five-coordinate species, the hydride
(23) Junk, P.-C.; Steed, J. W. J. Organomet. Chem. 1999, 587, 191–
194.

2792 Organometallics, Vol. 28, No. 9, 2009
Jime´nez-Tenorio et al.
Figure 5. ORTEP drawing (50% thermal ellipsoids) of the isomer
2b (43%) of [RuH{κ2-N,S-mt}(PPh3)3] (2). Hydrogen atoms except
hydride have been omitted. Selected bond lengths (Å) and angles
(deg) with estimated standard deviations in parentheses: Ru-N(1B)
2.28(2),Ru-S(1B)2.586(5),C(55B)-S(1B)1.777(15),C(55B)-N(1B)
1.21(3), P(1)-Ru-S(1B) 161.6(6), N(1B)-Ru-H(1) 100.4(15),
N(1B)-Ru-S(1B) 66.2(6), N(1B)-C(55B)-S(1B) 125.5(19). All
the other bond distances and angles are identical to those in 2a.
Figure 3. ORTEP drawing (50% thermal ellipsoids) of [Ru{κ3-
H,S,S-HB(mt)3}H(PPh3)2] (1). Hydrogen atoms except hydride and
BH have been omitted. Selected bond lengths (Å) and angles (deg)
with estimated standard deviations in parentheses: Ru1-P1
2.2714(17), Ru1-P2 2.3211(16), Ru1-S2 2.3958(17), Ru1-S1
2.4570(17), Ru1-H1 1.51(7), Ru1-H1b 1.90(7), B1-H1b 1.20(7),
H1 · · · H421.96,H1 · · · H182.17,P1-Ru1-P295.69(6),S1-Ru1-S2
88.88(6), P1-Ru1-S2 89.65(6), P2-Ru1-S1 84.54(6), N4-B1-N1
105.5(5), H1-Ru1-H1b 159(3); B1-H1b-Ru1 141.46(5).
compare well with the values reported in the literature for
ruthenium methimazolate complexes, such as [Ru{κ2-N,S-mt}2-
(PPh3)2]24 or [Ru{κ2-N,S-mt}(R)(CO)(PPh3)2] (R ) CHdCHC6-
H4Me, CPhdCHPh, CHdCHC6H9, C6H5; CtCC6H5).25
These results suggest that the HB(mt)3 ligand in 1 undergoes
a slow degradation process in CH2Cl2/MeOH solution with
involves cleavage of the B-N bond of at least one of the
methimazolate rings, leading to 2a/b as the only identified
isolable products, with yields close to 50% based upon the initial
amount of ruthenium complex. The cleavage of methimazolyl
groups from either Na[H2B(mt)2] or Na[HB(mt)3] to furnish
methimazolatecomplexeshasprecedentsintherecentliterature.24,26
Complex 2 can be more adequately prepared in high yield by
reaction of [RuHCl(PPh3)3] with Li(mt) in THF. In this fashion
a mixture of stereoisomers was also obtained. Isomer separation
was not attempted. The 1H, 31P{1H}, and 13C(1H) NMR spectra
of 2 in C6D6 solution are consistent with the presence of two
stereoisomers in a 55:45 ratio (by integration of the relevant
hydride resonances in the 1H NMR spectrum). Thus, two
separate high-field hydride resonances (doublet of triplets, X
1
Figure 4. ORTEP drawing (50% thermal ellipsoids) of the isomer
2a (57%) of [RuH{κ2-N,S-mt}(PPh3)3] (2). Hydrogen atoms except
hydride have been omitted. Selected bond lengths (Å) and angles
(deg) with estimated standard deviations in parentheses: Ru-N(1A)
2.240(14), Ru-S(1A) 2.480(3), Ru-H(1) 1.55(4), Ru-P(1)
2.2686(11),Ru-P(2)2.3454(11),Ru-P(3)2.3493(10),C(55A)-S(1A)
1.661(14), C(55A)-N(1A) 1.191(16), P(2)-Ru-P(3) 155.68(4),
P(2)-Ru-P(1) 100.15(4), P(3)-Ru-P(1) 98.10(4), P(1)-Ru-H(1)
98.0(14), P(1)-Ru-S(1A) 164.63(13), N(1A)-Ru-H(1) 155.3(14),
N(1A)-Ru-S(1A) 61.1(4), N(1A)-C(55A)-S(1A) 114.0(11).
part of an AM2X spin system) are observed in the H NMR
spectrum, whereas the 31P{1H} NMR spectrum shows two sets
of phosphorus resonances, which are consistent with the
presence of two separate A2M spin systems as expected.
Catalytic Dimerization of 1-Alkynes. The catalytic dimer-
ization of 1-alkynes is atom-economical and a straightforward
method for the preparation of conjugated enynes, which are
important building blocks in organic synthesis.27-31 Com-
plexes 1 and 2 are catalyst precursors for the dimerization
of 1-alkynes to give the corresponding enynes as mixtures
of Z, E, and in some instances also gem-stereoisomers. The
reactions were carried out in toluene at 85 °C for a period
nitrogen atom is trans to P(2). All bond lengths and angles in
both stereoisomers 2a/b are identical except those affecting
directly the atoms of the methimazolate ring. The hydride ligand
was located in a difference Fourier map and refined, leading to
a Ru-H separation of 1.55(4) Å. The Ru-N and Ru-S bond
lengths as well as the dimensions of the methimazolate ligand
(26) Hill, A. F.; Smith, M. K. Dalton Trans. 2006, 28–30.
(27) (a) Trost, B. M.; Chan, C.; Ruhter, G. J. Am. Chem. Soc. 1987,
109, 3486–3487. (b) Ohshita, J.; Furumori, K.; Matsuguchi, A.; Ishikawa,
M. J. Org. Chem. 1990, 55, 3277–3280. (c) Horton, A. D. J. Chem. Soc.,
Chem. Commun. 1992, 185–187. (d) Barluenga, J.; Gonza´lez, J. M.;
Llorente, I.; Campos, P. J. Angew. Chem., Int. Ed. Engl. 1993, 32, 893–
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(24) Dewhurst, R. D.; Hansen, A. R.; Hill, A. F.; Smith, M. K.
Organometallics 2006, 25, 5843–5846.
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Slawin, A. M. Z. Dalton Trans. 2005, 1930–1939.

Ru Hydride Complexes with Hydrotris(methimazolyl)borate
Organometallics, Vol. 28, No. 9, 2009 2793
Table 2. Product Distribution for Alkyne Dimerization Reactions Catalyzed by Compounds 1 and 2a
a Reaction conditions: toluene, 85 °C, 18-20 h, 2% catalyst.
of 18-20 h with catalyst loads of 2%. The enynes were
isolated at the end of the reaction period upon solvent
removal, extraction with petroleum ether, and flash chroma-
tography on silica gel. The yields of isolated enynes and the
stereoisomer composition of the mixtures for each of the
catalytic runs are shown in Table 2.
The yields of isolated enynes were in the range 45-98%.
When 2 was used as catalyst precursor, mixtures of Z, E, and
gem-stereoisomers were obtained in all cases. The only instance
in which a gem-stereoisomer was observed when 1 was used
as catalyst was the reaction of Me3SiCtCH. The Z/E stereo-
isomer ratio was dependent on the R group in the alkyne. We
were interested in testing the ability of 1 and 2 to act as catalyst
precursors for the addition of benzoic acid to 1-alkynes to yield
the corresponding enol esters. Thus, a catalytic reaction test was
conducted with benzoic acid and phenylacetylene (2:1 ratio)
using a 2% load of 1 in toluene at 85 °C. No enol ester resulted
from this test. Instead, the corresponding enyne was recovered
quantitatively. However, the Z/E stereoisomer ratio 88:12 was
very different from the value of 53:47 for the Z/E ratio obtained
in the direct dimerization run (i.e., with no added PhCOOH).
The effect was not so marked in the reaction catalyzed by 2. In
this case the Z/E stereoisomer ratio changed from 55:33 to 76:
10 in the presence of PhCOOH, with a content of gem-isomer
that remained essentially unaltered. Clearly, the addition of
PhCOOH has an effect on these catalytic systems, modifying
the stereoselectivity. Thus we carried out a series of dimerization
reactions with a 2% catalyst load and catalytic amount (4-5%)
of PhCOOH. Results are summarized in Table 3.
(28) (a) Bianchini, C.; Peruzzini, M.; Zanobini, F.; Frediani, P.; Albinati,
A. J. Am. Chem. Soc. 1991, 113, 5453–5454. (b) Bianchini, C.; Innocenti,
P.; Peruzzini, M.; Romerosa, A.; Zanobini, F.; Frediani, P. Organometallics
1996, 15, 272–285. (c) Duchateau, R.; van Wee, C. T.; Meetsma, A.;
Teuben, J. H. J. Am. Chem. Soc. 1993, 115, 4931–4932. (d) Jun, C.-H.;
Lu, Z.; Crabtree, R. H. Tetrahedron Lett. 1992, 33, 7119–7120. (e)
Matsuzaka, H.; Takagi, Y.; Ishii, Y.; Nishio, M.; Hidai, M. Organometallics
1995, 14, 2153–2155.
(29) (a) Bianchini, C.; Frediani, P.; Masi, D.; Peruzzini, M.; Zanobini,
F. Organometallics 1994, 13, 4616–4632. (b) Slugovc, C.; Mereiter, K.;
Zobetz, E.; Schmid, R.; Kirchner, K. Organometallics 1996, 15, 5275–
5277. (c) Katayama, H.; Nakayama, M.; Nakano, T.; Wada, C.; Akamatsu,
K.; Ozawa, F. Macromolecules 2004, 37, 13–17. (d) Wakatsuki, Y.;
Yamazaki, H.; Kumegawa, N.; Satoh, T.; Satoh, Y. J. Am. Chem. Soc. 1991,
113, 9604–9610. (e) Wakatsuki, Y.; Koga, N.; Yamazaki, H.; Morokuma,
K. J. Am. Chem. Soc. 1994, 116, 8105–8111. (f) Echavarren, A. M.; Lo´pez,
J.; Santos, A.; Montoya, J. J. Organomet. Chem. 1991, 414, 393–400. (g)
Trost, B. M.; Dean Toste, F. D.; Pinkerton, A. B. Chem. ReV. 2001, 101,
2067–2096. (h) Le Paih, J.; Monnier, F.; Derien, S.; Dixneuf, P. H.; Clot,
E.; Eisenstein, O. J. Am. Chem. Soc. 2003, 125, 11964–11975. (i) Naota,
T.; Takaya, H.; Murahashi, S.-I. Chem. ReV. 1998, 98, 2599–2660.
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1725. (b) Bruce, M. I.; Hall, B. C.; Zaitseva, N. N.; Skelton, B. W.; White,
A. H. J. Organomet. Chem. 1996, 522, 307–310. (c) Yang, S.-M.; Chan,
C.-W.; Cheung, K.-K.; Che, C.-M.; Peng, S.-M. Organometallics 1997, 16,
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Ianelli, S.; Pelizzi, G. Organometallics 1995, 14, 4114–4125.
Figure 6 shows a bar diagram that allows a direct comparison
of the differences in percentages of Z stereoisomer in the enyne
mixtures for the reactions catalyzed by 1 and 1+PhCOOH,
respectively.
A very significant increase in the relative amounts of the Z
stereoisomers is produced in all cases. The addition of PhCOOH
not only induces a significant change in the stereoselectivity of
these reactions but also modifies the catalytic reaction rate and
hence the turnover frequency (TOF ) (moles of converted
alkyne)/[(moles of catalyst)(time)]). We monitored by 1H NMR
in toluene-d8 at 85 °C the catalytic conversion of phenylacety-
lene into the corresponding enynes by following the disappear-
ance of the acetylenic proton resonance of phenylacetylene at
ca. 3 ppm. The resulting plot is shown in Figure 7.
(31) (a) Yi, Ch. S.; Liu, N.; Rheingold, A. L.; Liable-Sands, L. M.;
Guzei, I. A. Organometallics 1997, 16, 3729–3731. (b) Yi, Ch. S.; Liu, N.
Organometallics 1998, 17, 3158–3160. (c) Yi, Ch. S.; Liu, N. Organome-
tallics 1996, 15, 3968–3971.
The dimerization reaction catalyzed by 1 is very fast and
reaches quantitative conversion within the initial 5 min (TOF

2794 Organometallics, Vol. 28, No. 9, 2009
Jime´nez-Tenorio et al.
Table 3. Product Distribution for Alkyne Dimerization Reactions Catalyzed by Compounds 1 and 2 in the Presence of Catalytic Amounts of
PhCOOHa
a Reaction conditions: toluene, 85 °C, 18-20 h, 2% catalyst, 4-5% PhCOOH.
Therefore, the increase in stereoselectivity observed for the
reactions carried out with added PhCOOH simultaneously
involves a decrease in the activity of the catalyst. This suggests
that the nature of the catalytically active species is modified to
a great extent when the reactions are carried out with 1 as
catalyst precursor either in the absence or in the presence of
benzoic acid. We carried out stoichiometric reactions of 1 with
2 equiv of PhCOOH in toluene at 85 °C in an attempt to
understand the true nature of the modified complex. It is
reasonable to consider the formation of [Ru(κ3-H,S,S-
HB(mt)3)(PhCOO)(PPh3)] under these conditions. An orange
solid was isolated from this reaction. The resulting material
showed IR bands attributable to coordinated benzoate, but no
1
bands for RuH or BH bands were present. However, the H
and 31P{1H} NMR spectra of this substance indicated that it is
actually a complex mixture of products (multiple phosphorus
resonances in the 31P{1H} NMR spectrum, including free PPh3)
Figure 6. Differences in percentages of Z stereoisomer in the enyne
from which no conclusions could be drawn. Monitoring of the
reaction of 1 with 2 equiv of PhCOOH in C6D6 at room
temperature showed the slow formation of free PPh3 and the
concomitant appearance of one singlet resonance at 31.47 ppm
corresponding to an unidentified species. When the temperature
was raised above 50 °C, many new phosphorus resonances
appeared, and the only peak that could be recognized was the
one corresponding to free PPh3.
mixtures for the reactions catalyzed by 1 (white bars) and
1+PhCOOH (black bars).
The dimerization of 1-alkynes to enynes is known to occur
via either alkynyl-vinylidene or alkynyl-alkyne coupling.15,28–31
We can propose a mechanism for the catalytic dimerization of
1-alkynes mediated by 1 as shown in Scheme 1.
As it has been proposed by Hill and co-workers for [Ru{κ3-
H,S,S-HB(mt)3}H(CO)(PPh3)], the dissociation of the BHRu
linkage provides a vacant coordination site for the phenylacety-
lene.8 This is followed by hydroruthenation to provide the
intermediate [Ru{κ3-H,S,S-HB(mt)3}(CHdCHPh)(CO)(PPh3)],
which undergoes BH activation and styrene reductive elimina-
tion, furnishing the ruthenaboratrane complex [Ru(κ3-S,S,S-
B(mt)3)(CO)(PPh3)] as the final product. In our case, the catalytic
cycle starts in a similar fashion, but the corresponding ruthen-
aboratrane [Ru{κ3-S,S,S-B(mt)3}(PPh3)2] has never been ob-
served in our system. The catalytically active species is most
likely a coordinatively unsaturated alkynyl complex of the type
[Ru{κ2-S,S-HB(mt)3}(CtCR)(PPh3)2], as it has been postulated
Figure 7. Plot of conversion as a function of time for the
dimerization reaction of phenylacetylene in toluene-d8 catalyzed
by 1 (∆), 1+PhCOOH (0), or 2 (O) at 85 °C. Catalyst load 2%
(PhCOOH 4%).
600 h-1). In the presence of benzoic acid, the profile of the
conversion versus time curve changes, and the reaction rate is
slower. At 20 min the conversion is ca. 50% (TOF 92 h-1). At
this time, the dimerization reaction catalyzed by 2 has a
conversion below 20% (TOF 31 h-1), suggesting that compound
2 is a much less active catalyst than 1 or even 1+PhCOOH.

Ru Hydride Complexes with Hydrotris(methimazolyl)borate
Organometallics, Vol. 28, No. 9, 2009 2795
Scheme 1. Proposed Reaction Sequence for the Catalytic
Dimerization of 1-Alkynes Mediated by Compound 1
Scheme 2
The 1-alkyne dimerization reactions mediated by 1 in the
presence of PhCOOH seem to follow a different pathway, which
involves the modification of the catalyst precursor. For this
reason, we carried out further spectroscopic studies of the
interaction of 1 with benzoic acid in an attempt to identify the
chemical changes in the structure of the catalyst precursors. The
effect of a stronger acid such as HBF4 was also studied.
Protonation Reactions. The reaction of 1 with PhCOOH in
the complex to acid ratio of 1:2 in CD2Cl2 was monitored in
the temperature range -60 to +25 °C. The VT 1H and 31P{1H}
NMR spectra of the mixture are shown in Figure 8.
Apparently, no significative changes were observed in the
hydride region of the 1H NMR spectrum or in the 31P(1H) NMR
in other instances for the ruthenium-mediated dimerization of
1-alkynes.15,29–31 This species might be generated directly from
the π-alkyne intermediate upon H2 elimination, or alternatively
from the vinyl species [Ru{κ3-H,S,S-HB(mt)3}(CHdCHR)-
(PPh3)2] upon reaction with one alkyne molecule and release
of styrene. Coordination of another alkyne molecule to the
alkynyl intermediate is followed by alkyne-alkynyl coupling.
The reaction with further alkyne releases the final enynyl product
and regenerates the catalytically active alkynyl complex.
The catalytic cycle shown in Scheme 1 can be adapted to
account for the 1-alkyne dimerization reactions catalyzed by
compound 2. In this case, the initial step would be a phosphine
dissociation to provide a vacant coordination site. However, this
vacant site might be also generated considering a hemilabile
behavior for the methimazolate ligand, changing its coordination
from κ2-N,S to either κ1-N or κ1-S, as shown in Scheme 2.
In each case, the reaction with 1-alkyne followed by elimina-
tion of either hydrogen or styrene would lead to a coordinatively
unsaturated σ-alkynyl complex, which might act as the catalyti-
cally active species. The fact that several pathways are feasible
accounts for the low stereoselectivity observed in the enynes,
resulting from the reactions using 2 as catalyst precursor.
1
Figure 8. VT H NMR (left, hydride region, 400 MHz) and VT
31P{1H} NMR (right, 161.89 MHz) of a sample made up by addition
of 2 equiv of PhCOOH to a CD2Cl2 solution of 1 at -80 °C,
followed by subsequent warming to the indicated temperatures.

2796 Organometallics, Vol. 28, No. 9, 2009
Jime´nez-Tenorio et al.
Upon addition of an excess of HBF4 · OEt2 at -80 °C, both
the 1H and 31P{1H} NMR spectra are completely different from
those of the parent hydride complex 1. At -60 °C, the 1H NMR
spectrum shows in the hydride region two broad multiplet
resonances ascribed to RuHB protons, plus one broad resonance
centered at -5.95 ppm, for which the shortest T1 measured was
19.8 ms at -30 °C. The shortest T1 for the broad RuHB
resonances is 111 ms for the signal at -8.6 ppm and 136 ms
for the signal -3.6 ppm. The 31P{1H} NMR spectrum at -60
°C shows two sets of resonances corresponding to two AM spin
systems. Each of the sets consists of one doublet and one broad
resonance with unresolved coupling. As the temperature rises,
the intensity of one of the sets increases, whereas the other one
1
decreases. In analogous fashion, in the H NMR spectra the
broad signal at -5.95 ppm and the RuHB resonance at -8.6
ppm decrease as the temperature is raised. At 25 °C, only one
species is observed, having inequivalent phosphorus atoms and
maintaining the RuHB interaction. These observations are
consistent with the formation of a metastable cationic dihydro-
gen complex [Ru{κ3-H,S,S-HB(mt)3}(H2)(PPh3)2]+ (3a) at low
temperature, having inequivalent phosphorus atoms as inferred
from the 31P{1H} NMR spectra. We have observed no spectral
evidence for the protonation at a site other than the hydride,
i.e., the kinetic S-protonation of the pendant methimazolyl group.
If this indeed happens, the proton is rapidly transferred to the
hydride site, furnishing the dihydrogen complex 3a. As tem-
perature rises, the dihydrogen ligand is lost and a new species
having also inequivalent phosphorus atoms and an intact RuHB
interaction is generated. This species has an intense green color,
and it is already present at -60 °C. Attempts to isolate this
compound as a solid were unsuccessful. The fact that the
protonation experiments were performed under an argon atmo-
sphere rules out the formation of a dinitrogen complex by
substitution of the coordinated dihydrogen molecule. The
spectral data are consistent with the presence of a coordinatively
unsaturated complex of the type [Ru{κ3-H,S,S-HB(mt)3}-
(PPh3)2]+ (3b), generated upon dihydrogen loss, as shown in
Scheme 3, although the formation of a 18-electron complex with
a coordinated [BF4]- anion cannot be ruled out.
Although this assignment must be regarded as tentative, it is
consistent with spectral data and it is also reasonable from the
chemical point of view. There are many examples in the
literature of labile dihydrogen complexes that furnish 16-electron
species upon release of the coordinated H2 molecule. In some
instances the resulting coordinatively unsaturated species have
been isolated and characterized in full.19,33 Summing up, the
protonation reaction of 1 with HBF4 leads to the formation of
the metastable cationic dihydrogen complex 3a, which upon
H2 loss generates either the 16-electron complex 3b or its labile
adduct with [BF4]-. These results are completely different from
those obtained in the reaction of 1 with PhCOOH. Hence, the
resonance having the very short (T1)min of 5.9 ms in the 1H NMR
spectrum cannot be attributed to the formation of the dihydrogen
complex 3a, for which the shortest T1 measured was 19.8 ms.
Furthermore, the value of 5.9 ms for (T1)min would imply an
unrealistically short H-H bond distance, shorter than 0.74 Å.34
Our research group17,35 and other researchers36,37 have recently
1
Figure 9. VT H NMR (left, hydride region, 400 MHz) and VT
31P{1H} NMR (right, 161.89 MHz) of a sample made up by addition
of an excess of HBF4 · OEt2 to a CD2Cl2 solution of 1 at -80 °C
followed by subsequent warming to the indicated temperatures.
spectrum, apart from a broadening of the hydride resonance at
-12.15 ppm attributed to isomer 1b. The longitudinal relaxation
time T1 measured for this resonance at -60 °C was 23 ms. This
is considerably shorter than the value of 164 ms for the T1 of
the hydride resonance of 1b measured in the absence of
PhCOOH at the same temperature. On the other hand, the value
of T1 for the resonance at -17.74 ppm is 454 ms, being very
similar to the T1 of 490 ms measured for the hydride resonance
of 1a in the absence of PhCOOH at -60 °C. As the temperature
rises, the T1 of the hydride resonance at ca. -12.2 ppm gets
gradually shorter, reaching a minimum value, (T1)min, of 5.9 ms
at -20 °C. The value of (T1)min for the resonance at ca. -17.7
ppm is 384 ms at +25 °C, a value very similar to that found
for the hydride resonance of 1a in the absence of PhCOOH
(356 ms at +25 °C). The resonances of the proton of the RuHB
fragment for each of the species are present in the entire range
of temperature, suggesting that the RuHB linkage is not affected
by the presence of PhCOOH. These observations suggest that
only the isomer 1b interacts to a great extent with PhCOOH.
Under these conditions, the formation of a species containing a
dihydrogen bond, namely, [Ru{κ3-H,S,S-HB(mt)3}(H · · · HOOC-
Ph)(PPh3)2], should be expected. A shortening of the T1 for the
hydride resonance with respect to the value of the parent hydride
complex is a characteristic of dihydrogen-bonded complexes,32
as it is also a minimum shift in the position hydride resonance
and the fact that the 31P{1H} NMR remains essentially un-
changed. However, the observed (T1)min of 5.9 ms is very short
even considering the formation of a cationic dihydrogen complex
of the type [Ru{κ3-H,S,S-HB(mt)3}(H2)(PPh3)2]+, for a which
a (T1)min in the range 15 to 30 ms should be expected. In order
to explore the possibility of the formation of an intermediate
dihydrogen complex in the system, we carried out the proton-
ation with an excess of HBF4 · OEt2 in CD2Cl2. The VT 1H and
31P{1H} NMR spectra of the mixture are shown in Figure 9.
(33) (a) de los R´ıos, I.; Jime´nez-Tenorio, M.; Puerta, M. C.; Salcedo,
I.; Valerga, P. J. Chem. Soc., Dalton Trans. 1997, 4619–4624. (b) Barthazy,
P.; Stoop, R. M.; Wo¨rle, M.; Togni, A.; Mezzetti, A. Organometallics 2000,
19, 2844–2852. (c) Chin, B.; Lough, A. J.; Morris, R. H.; Schweitzer, C. T.;
D’Agostino, C. Inorg. Chem. 1994, 33, 6278–6288.
(34) Morris, R. H. Coord. Chem. ReV. 2008, 252, 2381–2394.
(35) Jimenez-Tenorio, M.; Palacios, M. D.; Puerta, M. C.; Valerga, P.
Inorg. Chem. 2007, 46, 1001–1012.
(32) (a) Custelcean, R.; Jackson, J. S. Chem. ReV. 2001, 101, 1963–
1980. (b) Epstein, L. M.; Belkova, N. V.; Shubina, E. S. In Recent AdVances
in Hydride Chemistry; Peruzzini, M., Poli, R. Eds.; Elsevier: Amsterdam,
The Netherlands, 2001; Chapter 14, pp 391-418,and references therein.

Ru Hydride Complexes with Hydrotris(methimazolyl)borate
Organometallics, Vol. 28, No. 9, 2009 2797
Scheme 3
2 exists in solution as a 55:45 mixture of two diastereoisomers
1a and 1b, the protonation reaction leads to two isomeric
cationic dihydrogen complexes [Ru(mt)(H2)(PPh3)3][BF4] (4a
and 4b), as shown.
The resulting mixture of dihydrogen complexes 4a/4b is
not equimolar. An isomer ratio of ca. 5:1 was observed. This
allowed the accurate determination of the NMR resonances
due to each of the isomers. However, the spectral data do
not tell unambiguously which of the isomers, 4a or 4b, is
respectively the major and the minor species in the mixture.
1
For each of the dihydrogen complexes, the H NMR spectra
show one broad high-field resonance corresponding to the
coordinated H2. These resonances exhibit (T1)min values of
24.6 ms (minor stereoisomer) and 26.1 ms (major stereoiso-
mer), respectively, fully consistent with the formulation of
4a and 4b as dihydrogen complexes. The 31P{1H} NMR
spectra of 4a/4b consist of two sets of resonances in the
approximate ratio 5:1, corresponding to A2M spin systems,
indicative of the mer-disposition of the PPh3 ligands in these
derivatives, as in the parent complex 2. The dihydrogen
complexes 4a/4b decompose to a mixture of uncharacterized
species when the temperature is raised above 0 °C.
reported several examples of hydride-proton transfer reactions
for which very short T1 values have been measured, in some
cases shorter than 1 ms. Although the presence of paramagnetic
impurities undergoing degenerate spin exchange with the parent
hydride complex has been invoked to justify such observations
in certain instances,37 there is actually no satisfactory explanation
for the anomalous shortening of T1. Although everything seems
to point to the formation of a dihydrogen-bonded species
between 1 and PhCOOH, this represents another example of a
system having an ultrafast hydridic proton relaxation. The
reasons why this occurs remain as yet unknown. It is important
to remark on the fact that only the isomer 1b seems to interact
with PhCOOH, whereas the isomer 1a is not visibly affected.
Clearly, the protonation reactions of 1 are not simple processes,
as recently remarked by Lledo´s and co-workers referring in
general to the protonation of transition metal hydrides.38 The
different behavior of isomers 1a and 1b toward PhCOOH might
be related to the observed increase in the stereoselectivity of
the dimerization of 1-alkynes catalyzed by 1 in the presence of
PhCOOH, although at this stage it is not possible to rationalize
the exact cause for this fact.
Conclusions
The hydrotris(methimazolyl)borate complex [Ru{κ3-H,S,S-
HB(mt)3}H(PPh3)2] (1) was prepared and characterized. 1 exists
in solution as a mixture of two stereoisomers, namely, 1a and
1b, in proportions that are solvent-dependent. The reaction with
CH2Cl2/MeOH leads to the hydridomethimazolato complex
[RuH{κ2-N,S-mt}(PPh3)3] (2), which exists both in solution and
in the solid state as a mixture of stereoisomers 2a/2b. Both 1
and 2 are catalysts for the dimerization of 1-alkynes to the
corresponding enynes, which are obtained as mixtures of
stereoisomers. The addition of PhCOOH to 1 produces an
increase in the stereoselectivity of the catalytic dimerization
reaction, directing the reaction toward the formation of the
Z-stereoisomers. A NMR study of the interaction of 1 with
PhCOOH in CD2Cl2 has revealed the formation of what appears
to be a dihydrogen-bonded complex between isomer 1b and
PhCOOH, but the resulting species exhibits an anomalously
short (T1)min of 5.9 ms, for which no satisfactory explanation
We also studied the protonation reaction of the hydridome-
thimazolato complex 2 using HBF4 in CD2Cl2. Since compound
(36) Castellanos, A.; Ayllo´n, J. A.; Sabo-Ettienne, S.; Donnadieu, B.;
Chaudret, B.; Yao, W.; Kavallieratos, K.; Crabtree, R. H. C. R. Acad. Sci.
Paris, Ser. IIc 1999, 2, 359–368.
(37) Belkova, N. V.; Collange, E.; Dub, P.; Epstein, L. M.; Lemenovskii,
D. A.; Lledo´s, A.; Maresca, O.; Maseras, F.; Poli, R.; Revin, P. O.; Shubina,
E. S.; Vorontsov, E. V. Chem.sEur. J. 2005, 11, 873–888.
(38) Besora, M.; Lledo´s, A.; Maseras, F. Chem. Soc. ReV. 2009, 38,
957-966.

2798 Organometallics, Vol. 28, No. 9, 2009
Jime´nez-Tenorio et al.
has been found. The protonation of 1 with HBF4 leads to the
metastable dihydrogen complex 3a, which upon gradual H2 loss
generates what seems to be a coordinatively unsaturated species
3b. In a similar fashion, the protonation of 2 with HBF4 yields
the isomeric cationic dihydrogen complexes 4a and 4b, which
decompose at temperatures above 0 °C.
financial support, and Johnson Matthey plc for generous loans
of ruthenium trichloride.
Supporting Information Available: Tables of X-ray structural
data, including data collection parameters, positional and thermal
parameters, and bond distances and angles for complexes 1 and
2a/b. This material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. We thank the Ministerio de Ciencia y
Tecnolog´ıa (DGICYT, Project CTQ2007 60137/BQU) and
Junta de Andaluc´ıa (PAI FQM188, P08-FQM-03538) for
OM900157B