C-H bond cleavage in thiophenes by [P(CH2CH2PPh2)3Ru]. UV flash kinetic spectroscopy discloses the ruthenium-thiophene adduct which precedes C-H insertion

Article abstract of DOI:10.1021/om9703898

Photolysis of [(PP₃)RuH₂] (PP₃ = P(CH₂CH₂PPh₂)₃) (1) in THF at 23 °C under an inert atmosphere (N₂, Ar, or He) in the presence of thiophene (T) or ethyl 2-thiophenecarboxylate (2-CO₂EtT) gives the (hydride)2-thienyl complexes [(PP₃)Ru(H)(2-Tyl)] (5) (Tyl = C₄H₃S) and [(PP₃)Ru(H)(2-CO₂EtTyl)] (7) (CO₂EtTyl = C₄H₂(CO₂Et)S), respectively. The C-H insertion products 5 and 7 are also selectively obtained by the thermal reaction of the Ru(0) dinitrogen complex [(PP₃)Ru(N₂)] (4) in THF with T or 2-CO₂EtT. Complexes 5 and 7 are both photochemically and thermally stable. Under comparable conditions, the photolysis of the Os derivative [(PP₃)OsH₂] in THF in the presence of T exclusively yields the C-H insertion product [(PP₃)Os(H)(2-Tyl)]. Photolysis of 1 at 23 °C under nitrogen and in the presence of 2,5-dimethylthiophene (2,5-Me₂T) gives the complex [(PP₃)Ru(η¹-S-2,5-Me₂T)] (8) containing an S-bound thiophene molecule. Complex 8 is also obtained by reaction of isolated 4 with 2,5-Me₂T. The Ru(0) transient, [(PP₃)Ru], generated by laser flash photolysis in either cyclohexane or THF solution, has been detected and characterized by UV-vis spectroscopy. [(PP₃)Ru] reacts with T (k₂ = (1.4 ± 0.2) × 10⁶ dm³ mol^-1 s^-1), yielding an adduct [(PP₃)-Ru(T)]. In cyclohexane, [(PP₃)Ru(T)] decays to the C-H insertion product 5 with first-order kinetics (k = 20 ± 1 s^-1 at 296 K). In THF, the 16 e^- Ru(0) transient forms with THF a short-lived adduct, probably bound through oxygen, [(PP₃)Ru(THF)], which reacts with T to give [(PP₃)Ru(T)] (k_obs = 680 s^-1, [T] = 0.3 mol dm^-3 under Ar). This T adduct decays to [(PP₃)Ru(2-Tyl)H] with first-order kinetics (k = 18 ± 1 s^-1). Similar measurements with 2,5-Me₂T yield a rate constant for [(PP₃)Ru] + 2,5-Me₂T in cyclohexane of (1.0 ± 0.3) × 10⁶ dm³ mol^-1 s^-1. The resulting [(PP₃)Ru(2,5-Me₂T)] is kinetically stable. Flash photolysis of [(PP₃)OsH₂] in cyclohexane with added T yields rate constants for [(PP₃)Os] + T and for isomerization of [(PP₃)Os(T)] of (3.7 ± 0.1) × 10⁵ dm³ mol^-1 s^-1 and 8 ± 1 s^-1, respectively.

Full text of DOI:10.1021/om9703898

Organometallics 1997, 16, 4611-4619
4611
C-H Bon d Clea va ge in Th iop h en es by
[P (CH2CH2P P h 2)3Ru ]. UV F la sh Kin etic Sp ectr oscop y
Discloses th e Ru th en iu m -Th iop h en e Ad d u ct Wh ich
P r eced es C-H In ser tion
Claudio Bianchini,*,1a J uan A. Casares,1a,b Robert Osman,1c David I. Pattison,1c
Maurizio Peruzzini,1a Robin N. Perutz,*,1c and Fabrizio Zanobini1a
Istituto per lo Studio della Stereochimica ed Energetica dei Composti di Coordinazione,
ISSECC-CNR, via J . Nardi 39, 50132 Firenze, Italy, and Department of Chemistry,
University of York, Heslington, York YO1 5DD, U.K.
Received May 12, 1997X
Photolysis of [(PP3)RuH2] (PP3 ) P(CH2CH2PPh2)3) (1) in THF at 23 °C under an inert
atmosphere (N2, Ar, or He) in the presence of thiophene (T) or ethyl 2-thiophenecarboxylate
(2-CO2EtT) gives the (hydride)2-thienyl complexes [(PP3)Ru(H)(2-Tyl)] (5) (Tyl ) C4H3S) and
[(PP3)Ru(H)(2-CO2EtTyl)] (7) (CO2EtTyl ) C4H2(CO2Et)S), respectively. The C-H insertion
products 5 and 7 are also selectively obtained by the thermal reaction of the Ru(0) dinitrogen
complex [(PP3)Ru(N2)] (4) in THF with T or 2-CO2EtT. Complexes 5 and 7 are both
photochemically and thermally stable. Under comparable conditions, the photolysis of the
Os derivative [(PP3)OsH2] in THF in the presence of T exclusively yields the C-H insertion
product [(PP3)Os(H)(2-Tyl)]. Photolysis of 1 at 23 °C under nitrogen and in the presence of
2,5-dimethylthiophene (2,5-Me2T) gives the complex [(PP3)Ru(η1-S-2,5-Me2T)] (8) containing
an S-bound thiophene molecule. Complex 8 is also obtained by reaction of isolated 4 with
2,5-Me2T. The Ru(0) transient, [(PP3)Ru], generated by laser flash photolysis in either
cyclohexane or THF solution, has been detected and characterized by UV-vis spectroscopy.
[(PP3)Ru] reacts with T (k2 ) (1.4 ( 0.2) × 106 dm3 mol-1 s-1), yielding an adduct [(PP3)-
Ru(T)]. In cyclohexane, [(PP3)Ru(T)] decays to the C-H insertion product 5 with first-order
kinetics (k ) 20 ( 1 s-1 at 296 K). In THF, the 16 e- Ru(0) transient forms with THF a
short-lived adduct, probably bound through oxygen, [(PP3)Ru(THF)], which reacts with T to
give [(PP3)Ru(T)] (kobs ) 680 s-1, [T] ) 0.3 mol dm-3 under Ar). This T adduct decays to
[(PP3)Ru(2-Tyl)H] with first-order kinetics (k ) 18 ( 1 s-1). Similar measurements with
2,5-Me2T yield a rate constant for [(PP3)Ru] + 2,5-Me2T in cyclohexane of (1.0 ( 0.3) × 106
dm3 mol-1 s-1. The resulting [(PP3)Ru(2,5-Me2T)] is kinetically stable. Flash photolysis of
[(PP3)OsH2] in cyclohexane with added T yields rate constants for [(PP3)Os] + T and for
isomerization of [(PP3)Os(T)] of (3.7 ( 0.1) × 105 dm3 mol-1 s-1 and 8 ( 1 s-1, respectively.
In tr od u ction
conversion to either C-S or C-H insertion products
have hardly been studied. In part, the scarcity of
information on this primary step is due to the stability
of the detected or isolated metal-thiophene adducts
with respect to their conversion to C-S or C-H inser-
tion products.4 On the other hand, the known C-S and
C-H insertion reactions5 occur so rapidly that the
intermediacy of metal-thiophene adducts has only been
postulated on the basis of theoretical6 or indirect
experimental evidence.5n
Among the authenticated bonding modes of thiophene
(or substituted thiophenes) to metal centers (Chart 1),
the η1-S (I)4a-k and η2-C,C (II)4l-o ones are believed to
feature the immediate precursors to C-S and C-H bond
cleavage, respectively.5c,n The η5 (III) and η4 (IV)
coordination modes may also lead to C-S insertion by
metal complexes,3 but subsequent intra- or intermo-
Hydrodesulfurization (HDS) is a stepwise reaction of
paramount industrial and environmental relevance
whose mechanism(s), however, is still poorly under-
stood.2 In this process, sulfur, contained in various
organic compounds such as thiols, sulfides, disulfides,
and the more refractory thiophenic molecules, is re-
moved from fossil materials upon treatment with H2 in
the presence of heterogeneous catalysts.
Homogeneous modeling studies have recently con-
tributed several mechanistic breakthroughs regarding
most of the steps which may be involved in the metal-
assisted HDS of thiophenes to H2S and hydrocarbons.3
However, the structure of the initial metal-intact
thiophene adduct and the factors which control its
X Abstract published in Advance ACS Abstracts, September 15, 1997.
(1) (a) ISSECC-CNR, Firenze. (b) Current address: Departamento
de Qu´ımica Inorganica, Universidad de Valladolid, Spain. (c) University
of York.
(2) (a) Gates, B. C. Catalytic Chemistry; J ohn Wiley: New York,
1992; Chapter 5, p 390. (b) Startsev, A. N. Rev.-Sci. Engl. 1995, 37,
353. (c) Leffler, W. L. Petroleum Refining; Penn Well Publishing
Company: Tulsa, OK, 1985.
(3) (a) Bianchini, C.; Meli, A. J . Chem. Soc., Dalton Trans. 1996,
801. (b) Angelici, R. J . Bull. Soc. Chim. Belg. 1995, 104, 265. (c)
Angelici, R. J . In Encyclopedia of Inorganic Chemistry; King, R. B.,
Ed.; Wiley: New York, 1994; Vol. III, p 1433. (d) Sa´nchez-Delgado, R.
A. J . Mol. Catal. 1994, 86, 287. (e) Rauchfuss, T. B. Prog. Inorg. Chem.
1991, 570. (f) Reynolds, J . G. Chem. Ind. 1991, 570. (g) Angelici, R. J .
Acc. Chem. Res. 1988, 21, 387.
S0276-7333(97)00389-0 CCC: $14.00 © 1997 American Chemical Society

4612 Organometallics, Vol. 16, No. 21, 1997
Bianchini et al.
Ch a r t 1
dissociation of H2 and formation of products of the types
[(PP3)RuL] (L ) CO, N2, C2H4) and [(PP3)Ru(X)H] (X )
Ph, Et3Si) according to the substrate provided (PP3
)
P(CH2CH2PPh2)3). The reaction intermediate, [(PP3)-
Ru], was detected by laser flash photolysis, and the
kinetics of its reactions with these substrates were
measured.9 The photochemistry of [(PP3)OsH2]10 pro-
ceeds analogously, but oxidative addition occurs with
THF and with alkanes.9
In this work, photolysis of [(PP3)RuH2]8 is employed
to generate the transient [(PP3)Ru] species in solution
for reactions with selected thiophenes such as thio-
phene (T), ethyl 2-thiophenecarboxylate (2-CO2EtT),
and 2,5-dimethylthiophene (2,5-Me2T). Monitoring
these reactions by NMR spectroscopy shows that C-H
insertion occurs for T and 2-CO2EtT to give (hydride)2-
thienyl complexes with no detectable intermediates.
In contrast, a Ru-T adduct is seen by laser flash
photolysis with UV-vis detection, which also allows
the quantification of reaction kinetics. We also include
brief reports of the chemistry of the osmium ana-
logues.
lecular attack by either nucleophilic or electrophilic
reagents is required.7
If it is taken for granted that C-S and C-H inser-
tions, when they occur, are preceded by thiophene
coordination, then the nondetection of intermediates is
a kinetic question exclusively determined by the short
lifetime of these species. Accordingly, fast kinetic
techniques such as UV or IR flash kinetic spectroscopy,
never applied to HDS modeling studies, might be the
methods of choice for detecting the intermediates prior
to C-S or C-H bond scission, as well as providing
structural and kinetic information on the intermediate
itself and its products.
Following the synthesis of [(PP3)RuH2] (1),8 we have
recently demonstrated that photolysis of 1 results in
(4) (a) Robertson, M. J .; White, C. J .; Angelici, R. J . J . Am. Chem.
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Masi, D.; Mealli, C. Inorg. Chem. 1993, 32, 3766. (d) Benson, J . W.;
Angelici, R. J . Organometallics 1993, 12, 680. (e) Benson, J . W.;
Angelici, R. J . Organometallics 1992, 11, 922. (f) Choi, M.-G.; Angelici,
R. J . Inorg. Chem. 1991, 30, 1417. (g) Choi, M.-G.; Angelici, R. J .
Organometallics 1991, 10, 2436. (h) Wasserman, H. J .; Kubas, G. J .;
Ryan, R. R. J . Am. Chem. Soc. 1986, 108, 2294. (i) Guerchais, V.;
Astruc, D. J . Organomet. Chem. 1986, 316, 335. (j) Kuhn, N.; Schu-
mann, H. J . Organomet. Chem. 1984, 276, 55. (k) Kuehn, C. G.; Taube,
H. J . Am. Chem. Soc. 1976, 98, 689. (l) Spera, M. L.; Harman, W. D.
Organometallics 1995, 14, 1559. (m) Cardone, R.; Harman, W. D.;
Taube, H. J . Am. Chem. Soc. 1989, 111, 5969. (n) Choi, M.-G.; Angelici,
R. J . Organometallics 1992, 11, 3328. (o) Choi, M.-G.; Angelici, R. J .
J . Am. Chem. Soc. 1991, 113, 5651.
(5) (a) Paneque, M.; Taboada, S.; Carmona, E. Organometallics 1996,
15, 2678. (b) Bianchini, C.; J ime´nez, M. V.; Meli, A.; Vizza, F.
Organometallics 1995, 14, 3196. (c) Bianchini, C.; J ime´nez, M. V.; Meli,
A.; Moneti, S.; Vizza, F.; Herrera, V; Sa´nchez-Delgado, R. A. Organo-
metallics 1995, 14, 2342. (d) Bianchini, C.; Frediani, P.; Herrera, V;
J ime´nez, M. V.; Meli, A.; Rinco´n, L.; Sa´nchez-Delgado, R. A.; Vizza, F.
J . Am. Chem. Soc. 1995, 117, 4333. (e) Bianchini, C.; J ime´nez, M. V.;
Meli, A.; Moneti, S.; Vizza, F. J . Organomet. Chem. 1995, 504, 27. (f)
Bacchi, A.; Bianchini, C.; Herrera, V.; J ime´nez, M. V.; Mealli, C.; Meli,
A.; Moneti, S.; Peruzzini, M.; Sa´nchez-Delgado, R. A.; Vizza, F. J .
Chem. Soc., Chem. Commun. 1995, 921. (g) Garcia, J . J .; Mann, B. E.;
Adams, H.; Bailey, N. A.; Maitlis, P. M. J . Am. Chem. Soc. 1995, 117,
2179. (h) Bianchini, C.; Meli, A.; Peruzzini, M.; Vizza, F.; Moneti, S.;
Herrera, V.; Sa´nchez-Delgado, R. A. J . Am. Chem. Soc. 1994, 116, 4370.
(i) J ones, W. D.; Chin, R. M.; Crane, T. W.; Baruch, D. M. Organome-
tallics 1994, 13, 4448. (j) Buys, I. E.; Field, L. D.; Hambley, T. W.;
McQueen, A. E. D. J . Chem. Soc., Chem. Commun. 1994, 557. (k)
Bianchini, C.; Meli, A.; Peruzzini, M.; Vizza, F.; Frediani, P.; Herrera,
V.; Sa´nchez-Delgado, R. A. J . Am. Chem. Soc. 1993, 115, 2731. (l)
Selnau, H. E.; Merola, J . S. Organometallics 1993, 12, 1583. (m) Rosini,
G. P.; J ones, W. D. J . Am. Chem. Soc. 1992, 114, 10767. (n) Dong, L.;
Duckett, S. B.; Ohman, K. F.; J ones, W. D. J . Am. Chem. Soc. 1992,
114, 151. (o) J ones, W. D.; Dong, L. J . Am. Chem. Soc. 1991, 113, 559.
(p) Ogilvy, A. E.; Draganjac, M.; Rauchfuss, T. B.; Wilson, S. R.
Organometallics 1988, 7, 1171. (r) Bianchini, C.; Herrera, V.; J ime´nez,
M. V.; Meli, A.; Sa´nchez-Delgado, R. A.; Vizza, F. J . Am. Chem. Soc.
1995, 117, 8567.
Exp er im en ta l Section
Gen er a l In for m a tion . Tetrahydrofuran (THF) was puri-
fied by distillation under nitrogen over LiAlH4, and n-heptane
was distilled over sodium. The solvents were stored over
molecular sieves. Commercial thiophene (Aldrich, 99%) was
purified as described in the literature.11 All of the other
reagents and chemicals, including ethyl 2-thiophenecarboxy-
late, were reagent grade and, unless otherwise stated, were
used as received by commercial suppliers. All reactions and
manipulations were routinely performed under a dry nitrogen,
argon, or helium atmosphere using Schlenk tube techniques.
The solid complexes were collected on sintered-glass frits and
washed with ethanol and petroleum ether (bp 40-70 °C) before
being dried in a stream of nitrogen. Literature methods were
used for the preparation of [(PP3)RuH2] (1),8 [(PP3)RuCl2] (2),8
[(PP3)OsH2] (3),10 and [(PP3)Ru(N2)] (4).9 Photochemical reac-
tions were performed by using a Helios Italquartz UV 13F
apparatus. The photolysis source was a 135 W (principal
emission wavelength at 366 nm) high-pressure mercury vapor
immersion lamp equipped with a water filter to remove excess
heat. Photolysis NMR experiments were carried out in 5 mm
quartz NMR tubes (Wilmad 507-PP-QTZ). Deuterated sol-
vents for NMR measurements (Merck) were dried over mo-
lecular sieves (4 Å). 1H and 13C{1H} NMR spectra were
recorded on a Varian VXR 300, Bruker AC 200P, or Bruker
AVANCE DRX 500 spectrometer operating at 299.94, 200.13,
or 500.13 MHz (1H) and 75.42, 50.32, or 125.80 MHz (13C),
respectively. Peak positions are quoted relative to tetra-
methylsilane and were calibrated against the residual solvent
resonance (1H) or the deuterated solvent multiplet (13C). 13C-
DEPT experiments were run on the Bruker AC 200P spec-
trometer. 1H,13C-2D HETCOR NMR experiments were re-
corded on either the Bruker AC 200P spectrometer using the
XHCORR pulse program or the Bruker AVANCE DRX 500
spectrometer equipped with a 5 mm triple-resonance probe
head for 1H detection and inverse detection of the hetero-
nucleus (inverse correlation mode, HMQC experiment) with
(6) Zonnevylle, M. C.; Hoffmann, R. Surf. Sci. 1988, 199, 320.
(7) (a) Feng, Q.; Rauchfuss, T. B.; Wilson, S. R. Organometallics
1995, 14, 2923. (b) Dailey, K. M. K.; Rauchfuss, T. B.; Rheingold, A.
L.; Yap, G. P. A. J . Am. Chem. Soc. 1995, 117, 6396. (c) Luo, S.;
Rauchfuss, T. B.; Gan, Z. J . Am. Chem. Soc. 1993, 115, 4943. (d)
Krautscheid, H.; Feng, Q.; Rauchfuss, T. B. Organometallics 1993, 12,
3273. (e) Luo, S.; Skaugset, A. E.; Rauchfuss, T. B.; Wilson, S. R. J .
Am. Chem. Soc. 1992, 114, 1732. (f) Skaugset, A. E.; Rauchfuss, T. B.;
Wilson, S. R. J . Am. Chem. Soc. 1992, 114, 8521. (g) Luo, S.; Ogilvy,
A. E.; Rauchfuss, T. B.; Rheingold, A. L.; Wilson, S. R. Organometallics
1991, 10, 1002. (h) Chen, J .; Daniels, L. M.; Angelici, R. J . J . Am. Chem.
Soc. 1990, 112, 199. (i) Hachgenei, J . W.; Angelici, R. J . J . Organomet.
Chem. 1988, 355, 359.
1
no sample spinning. The H,1H-2D COSY NMR experiments
(9) Pattison, D.; Osman, R.; Perutz, R. N.; Bianchini, C.; Casares,
J . A.; Peruzzini, M. J . Am. Chem. Soc. 1997, 119, 8459.
(10) Bianchini, C.; Linn, K.; Masi, D.; Peruzzini, M.; Polo, A.; Vacca,
A.; Zanobini, F. Inorg. Chem. 1993, 32, 2366.
(11) Huckett, S. C.; Sauer, N. N.; Angelici, R. J . Organometallics
1987, 6, 591.
(8) Bianchini, C.; Pe´rez, P.; Peruzzini, M.; Zanobini, F.; Vacca, A.
Inorg. Chem. 1991, 30, 279.
(12) Hall, C.; J ones, W. D.; Mawby, R. J .; Osman, R.; Perutz, R. N.;
Whittlesey, M. K. J . Am. Chem. Soc. 1992, 114, 7425.

C-H Bond Cleavage in Thiophenes
Organometallics, Vol. 16, No. 21, 1997 4613
were routinely conducted on the Bruker AC 200P instrument
in the absolute magnitude mode using a 45° or 90° pulse after
the incremental delay or were acquired on the AVANCE DRX
500 Bruker spectrometer using the gradient-accelerated ver-
sion of the conventional COSY sequence (COSYPS). 1H,1H-
2D NOESY NMR experiments were conducted on the same
instrument in the phase-sensitive TPPI mode in order to
discriminate between positive and negative cross peaks. T1
measurements for the optimization of the proper mixing time
in the NOESY experiments were determined at 25 °C using
the standard π-τ-π/2 inversion-recovery sequence. 31P{1H}
NMR spectra were acquired on the Varian VXR 300 or Bruker
AC 200P instruments operating at 121.42 and 81.01, respec-
tively. Chemical shifts were measured relative to external
85% H3PO4 with downfield values taken as positive. The
proton NMR spectra with broad-band phosphorus decoupling
were recorded on the Bruker AC 200P instrument equipped
with a 5 mm inverse probe and a BFX-5 amplifier device using
the wide-band phosphorus decoupling sequence GARP. In-
frared spectra were recorded as Nujol mulls on a Perkin-Elmer
1600 series FT-IR spectrophotometer between KBr plates. UV
spectra were recorded at room temperature on a Shimadzu
UV-2100 spectrophotometer using quartz cells. Elemental
analyses (C, H, N) were performed using a Carlo Erba model
1106 elemental analyzer.
C. Bu lk P h otolysis in THF a t Reflu x Tem p er a tu r e.
A solution of 1 (200 mg, 0.26 mmol) in THF (10 mL) containing
a large excess of T (5 mL, 62.40 mmol) was irradiated under
either nitrogen or helium with UV light without water cooling
in a quartz flask connected to a reflux condenser. Within a
few minutes, the solution reached the reflux temperature.
After 30 min, the UV lamp was turned off and the yellowish
reaction mixture was cooled to room temperature and then
transferred into a Schlenk-flask under nitrogen. Addition of
n-heptane (30 mL) and concentration of the resulting solution
under a brisk current of nitrogen gave cream-colored micro-
crystals of 5. Yield: 87%. IR: ν(Ru-H) 1879 (m) cm-1. Anal.
Calcd for C46H46P4RuS: C, 64.55; H, 5.42. Found: C, 64.27;
H, 5.57. 31P{1H} NMR (22 °C, C6D6, 81.01 MHz), AMQ2 spin
system: δA 146.09 (td, J AM ) 6.8 Hz, J AQ ) 13.1 Hz); δM 49.66
(td, J MQ ) 12.4 Hz,), δQ ) 57.73 (dd). 1H NMR (20 °C, C6D6,
500.13 MHz): δRuH -7.94 (dtd, J HP ) 81.0 Hz, J HP ) 15.2
M
A
Hz, J HP ) 26.4 Hz); δH 7.70 (brt, J H A ) 2.7 Hz, J H
) 3.0
P
H
Q
A
A
Hz, J H
) 2.8 Hz); δHA 7.60 (dd, J H
) 4.8 Hz); δHB 7.89
H
A
H
C
B
B
C
C
(brd). 13C{1H} NMR (20 °C, THF-d8, 50.32 MHz): δC 143.42
2
(brd, J CP ) 21.9 Hz; δC 140.02 (t, J CP ) 6.0 Hz); δC and δC
3
4
5
not observed (masked by the aromatic carbon resonances of
PP3). Compound 5 is photochemically stable in THF solution
up to reflux temperature.
The laser flash photolysis apparatus in York has been
described elsewhere.12 In brief, a XeCl laser (308 nm) excites
the sample, which is monitored by UV-vis absorption of light
from a xenon arc lamp. The arc is pulsed for time scales up
to 250 µs. Samples were prepared in a 10 mm quartz cuvette,
fitted with a PTFE stopcock and degassing bulb. Solid 1 or 3
was loaded into the cuvette in an argon-filled glovebox.
Solvents were dried over CaH2 at reflux and transferred to
the flash cells via cannula on a Schlenk line fitted with a
diffusion pump. The concentration was adjusted to obtain an
absorbance of 0.6-1.0 at 308 nm before degassing the contents
with three freeze-pump-thaw cycles and filling the cell with
the required atmosphere. Measurements were made at 296
( 1 K.
Th er m a l R ea ct ion of [(P P 3)R u H 2] w it h eit h er Th io-
p h en e or E t h yl 2-Th iop h en eca r b oxyla t e. A solution of
1 (200 mg, 0.26 mmol) in neat thiophene (T) or ethyl 2-thio-
phenecarboxylate (2-CO2EtT) was gently heated under nitro-
gen to reflux. After 3 h of reflux, a 0.6 mL sample of the
resulting orange solution was withdrawn and, after dilu-
tion with 0.4 mL of THF-d8, was analyzed by 31P NMR
spectroscopy. This showed no transformation of the starting
dihydride. Identical results were obtained when 1 in THF was
refluxed in the presence of variable amounts of either T or
2-CO2EtT.
Rea ction of [(P P 3)Ru (N2)] w ith Th iop h en e. A 5 mm
screw-cap NMR tube was charged with a ca. 3 × 10-2
M
solution of 4 (0.5 mL), freshly prepared by reduction of 2 with
sodium naphthalenide in THF,9 and with 0.5 mL of THF-d8
under nitrogen. Then ca. 1.2 equiv of T was syringed into the
solution through the serum cap, and a 31P{1H} NMR spectrum
of the solution was immediately acquired, which showed the
partial conversion of 4 to 5 (30% after 1 h, based on 31P NMR
integration). Increasing the temperature of the spectrometer
probe head to ca. 50 °C caused a fast fading of the red color of
4, and the NMR analysis showed the complete transformation
of 4 into 5.
Th er m a l Beh a vior of [(P P 3)Ru (H)(2-Tyl)]. A solution
of 5 (100 mg, 0.12 mmol) in o-xylene (bp 143-145 °C) was
refluxed overnight under nitrogen. A 31P{1H} NMR analysis
showed no transformation of the starting material, which was
recovered quantitatively.
P h ot olysis of [(P P 3)R u H 2] in t h e P r esen ce of E t h yl
2-Th iop h en eca r b oxyla t e. Substitution of 2-CO2Et for T
in all the experiments described above gave identical results.
From the preparative experiment C, the (hydride)2-thienyl
complex [(PP3)Ru(H)(2-CO2EtTyl)] (7) was obtained in 90%
yield (2-CO2EtTyl ) C4H2(CO2Et)S). Compound 7 is therm-
ally (up to 145 °C in o-xylene) and photochemically stable
P h ot olysis of [(P P 3)R u H 2] in t h e P r esen ce of Th io-
p h en e. A. NMR Exp er im en t in THF -d 8. A solution of 1
(20 mg, 0.026 mmol) in THF-d8 (0.6 mL) containing 100 equiv
of T (0.21 mL, 2.60 mmol) prepared either under nitrogen or
helium in a 5 mm quartz NMR tube was irradiated with UV
light at 23 °C. Monitoring the photolysis reaction by 31P{1H}
NMR spectroscopy every 10 min showed the exclusive forma-
tion of the (hydride)2-thienyl complex [(PP3)Ru(H)(2-Tyl)] (5)
(Tyl ) C4H3S). Complex 5 was the only product after 1 h of
UV irradiation. At 60 °C, the photolysis of 1 with T led to the
quantitative formation of 5 in a few minutes. Replacing THF-
d8 with benzene-d6 did not change the reactivity.
in solution. IR: ν(Ru-H) 1902 (m) cm-1
49H50O2P4RuS: C, 63.42; H, 5.43. Found: C, 63.18; H, 5.50.
31P{1H} NMR (20 °C, C6D6, 81.01 MHz), AMQ2 spin system:
A 145.40 (td, J AM ) 7.2 Hz, J AQ ) 13.5 Hz); δM 50.53 (td, J MQ
) 12.1 Hz; this resonance becomes a broad doublet in the
. Anal. Calcd for
C
δ
proton-coupled 31P NMR spectrum, J PH
) ca. 91 Hz); δQ
trans
57.82 (dd). 1H NMR (25 °C, C6D6, 500.13 MHz): δRuH -7.99
(dtd, J HP ) 90.6 Hz, J HP ) 15.4 Hz, J HP ) 26.0 Hz); δH
M
A
Q
A
3
7.69 (dd, J H A ) 2.1 Hz, J H B ) 3.6 Hz); δH 8.41 (d); δOCH
P
A
H
A
CH
B
2
4.36 (q, J HH ) 7.0 Hz); δOCH 3 1.21 (t). 13C{1H} NMR (20 °C,
B. Bu lk P h otolysis in THF a t Room Tem p er a tu r e. A
solution of 1 (200 mg, 0.26 mmol) in THF (10 mL) containing
a large excess of T (5 mL, 62.40 mmol) was irradiated with
UV light at 23 °C under nitrogen or helium. After 1 h, a 0.6
mL sample of the resulting orange solution was withdrawn
and, after dilution with 0.4 mL of THF-d8, was analyzed by
31P NMR spectroscopy. This showed that complete transfor-
mation of the starting dihydride into 5 had occurred.
CH
2
C6D6, 50.32 MHz): δC 166.90 (s), δC 143.41 (td, J CP ) 22.4,
6
2
6.2 Hz, assigned by a DEPT-NMR experiment), δC 140.11 (t,
3
J CP ) 12.5 Hz, assigned by a 2D-1H,13C-HMQC NMR experi-
ment), δC ca. 132.8 (masked by the aromatic carbon reso-
4
nances of PP3, assigned by a 2D-1H,13C-HMQC NMR experi-
ment), δC not observed (masked by the aromatic carbon
5
resonances of PP3), δOCH
59.82 (s), δOCH
15.41 (s).
CH
3
CH
2
3
2

4614 Organometallics, Vol. 16, No. 21, 1997
Bianchini et al.
Sch em e 1
P h otolysis of [(P P 3)Ru H2] in th e P r esen ce of 2,5-
Dim eth ylth iop h en e. A. NMR Exp er im en t in THF -d 8
u n d er Nitr ogen . A 5 mm quartz NMR tube was charged
under nitrogen with a mixture of 1 (20 mg, 0.026 mmol) and
2,5-dimethylthiophene (2-Me2T) (0.5 mL) in 0.5 mL of THF-
Sch em e 2
d8
.
The tube was irradiated with UV light at 23 °C.
Monitoring the photolysis reaction by 31P{1H} NMR spectros-
copy showed the partial conversion of 1 to several products.
After 3 h, the following product composition was determined
by 31P{1H} NMR spectroscopy: 1 (12%), 4 (26%), [{(Ph2PCH2-
CH2)2PCH2CH2PPh(C6H4)}RuH] (6)9 (10%), [(PP3)Ru(C4H2-
Me2S)] (8) (40%), unidentified products (12%). 31P{1H} NMR
of 8 (20 °C, THF-d8, 81.01 MHz): AM3 spin system: δA 162.25
(q, J AM ) 23.6 Hz); δM 65.26 (d).
B. NMR Exp er im en t in THF -d 8 u n d er Heliu m .
A
solution of 1 (20 mg, 0.026 mmol) in a THF-d8/2,5-Me2T
mixture (1.0 mL, 1:1 v/v) prepared under helium in a 5 mm
quartz NMR tube was irradiated with UV light at 23 °C. After
1 h of photolysis, NMR analysis gave the following product
composition: 1 (17%), 6 (20%), 8 (48%), unidentified products
(15%).
of nitrogen gave pale yellow microcrystals of the (hydride)2-
thienyl complex 9. Yield: 80%. IR: ν(Os-H) 2092 (m) cm-1
.
Anal. Calcd for C46H46OsP4S: C, 58.46; H, 4.91. Found: C,
58.21; H, 5.06. 31P{1H} NMR (22 °C, C6D6, 121.42 MHz) AMQ2
spin system: δA 120.96 (d, J AM ) 6.4 Hz, J AQ ≈ 0 Hz); δM 21.41
(d, J MQ ) 4.9 Hz); δQ 22.33 (dt). 1H NMR (20 °C, C6D6, 200.13
C. NMR Exp er im en t in Nea t 2,5-Dim eth ylth iop h en e.
A solution of 1 (20 mg, 0.026 mmol) in neat 2,5-Me2T (0.7 mL)
prepared under helium in a 5 mm quartz NMR tube containing
a sealed capillary with acetone-d6 for the internal lock was
irradiated with UV light at 23 °C. After 1 h, a 31P{1H} NMR
spectrum showed the complete disappearance of 1 with forma-
tion of 6 (12%) and 8 (48%) and other unidentified products.
Reaction of [(P P 3)Ru (N2)] with 2,5-Dim eth ylth ioph en e.
A 5 mm NMR tube was charged under nitrogen with 30 mg of
4 (0.043 mmol) and 0.7 mL of 2,5-Me2T (6.15 mmol) at room
temperature. Monitoring the reaction by 31P{1H} NMR spec-
troscopy showed the slow transformation of 4 into 8. After 12
h, an overall transformation of ca. 65% was observed. Pro-
longed reaction times (48 h) did not change the ratio between
8 and 4. Any attempt to precipitate 8 from this dark red
solution resulted in extensive decomposition and gave intrac-
table sticky materials. Extensive decomposition was analo-
gously observed when the solvent was removed in vacuo at
room temperature. The resulting oily residue, however, still
MHz): δOsH -8.98 (dtd, J HP ) 76.2 Hz, J HP ) 9.5 Hz, J HP
)
)
M
A
Q
27.4 Hz); δH 7.14 (brd, J H
) 4.6 Hz); δH 6.74 (dd, J H
H
H
A
A
B
B
B
C
3.2 Hz); δHC ca. 7.0 (masked by the aromatic proton resonances
of PP3, assigned by a 1H,1H-2D COSY NMR experiment).
Resu lts
Syn th esis a n d Ch a r a cter iza tion of [(P P 3)M(H)-
(2-th ien yl)] (M ) Ru , th ien yl ) C4H3S (5); M ) Ru ,
th ien yl ) C4H2(CO2Et)S (7); M ) Os, th ien yl )
C4H3S (9)). Photolysis (290 nm < λ < 376 nm) of [(PP3)-
RuH2] (1) in THF under an atmosphere of nitrogen has
recently been reported to give the trigonal-bipyramidal
dinitrogen adduct [(PP3)Ru(N2)] (4),9 occasionally con-
taminated by the ortho-metalated complex [{(Ph2PCH2-
CH2)2PCH2CH2PPh(C6H4)}RuH] (6) (vide infra). The
latter compound is selectively formed when the photo-
chemical reaction is carried out under He or Ar (Scheme
1).9
Neither compound 4 nor 6 forms when the photolysis
of 1 is carried out at room temperature in the presence
of either T or 2-CO2EtT either under N2 or He. In
contrast, these photochemical reactions exclusively yield
the (hydride)2-thienyl complexes [(PP3)Ru(H)(2-Tyl)] (5)
(Tyl ) C4H3S) and [(PP3)Ru(H)(2-CO2EtTyl)] (7) (CO2-
EtTyl ) C4H2(CO2Et)S), resulting from the regioselec-
tive insertion of Ru into the R-C-H bond of the
thiophenes (Scheme 2). Monitoring the photolysis reac-
tions by NMR spectroscopy in quartz tubes shows that
the conversion of 1 to the 2-thienyl products proceeds
1
contained 8 (ca. 15%). A H NMR spectrum in THF-d8 was
acquired, which showed the absence of resonances in the region
from 3 to 5 ppm where the pseudo-olefinic hydrogens of η2-
C,C-thiophenes are known to resonate.4l,m,o We thus assign 8
as [(PP3)Ru(η1-S-C4H2Me2S)] (δMe 2.20). In an independent
experiment, the reaction mixture was transferred under
nitrogen into a quartz cuvette. The UV spectrum showed an
absorption maximum centered at 400 nm.
P h ot olysis of [(P P 3)OsH 2] (3) in t h e P r esen ce of
Th iop h en e. A. NMR Exp er im en t in THF -d 8. A solution
of 310 (20 mg, 0.023 mmol) in THF-d8 (0.6 mL) containing 100
equiv of T (0.21 mL, 2.60 mmol) prepared under nitrogen in a
5 mm quartz NMR tube was irradiated with UV light at 23
°C. Monitoring the photolysis reaction by 31P{1H} NMR
spectroscopy showed the quantitative formation of the (hy-
dride)2-thienyl complex [(PP3)Os(H)(2-Tyl)] (9) after 30 min.
B. Bu lk P h otolysis in THF -d 8 a t Room Tem p er a tu r e.
A solution of [(PP3)OsH2] (3) (200 mg, 0.23 mmol) in THF (10
mL) containing a large excess of T (5 mL, 62.40 mmol) was
photolyzed in a quartz flask at room temperature. Within a
few minutes, the solution became pale orange. After 1 h of
photolysis, the orange solution was transferred into a Schlenk-
flask under nitrogen. Addition of n-heptane (30 mL) and
concentration of the resulting solution under a brisk current

C-H Bond Cleavage in Thiophenes
Organometallics, Vol. 16, No. 21, 1997 4615
Sch em e 3
Ch a r t 2
of T occurs at the carbon atom adjacent to, and activated
by, the sulfur atom.4a,5k,n From the 1H{31P} NMR
spectrum of 7 one can exclude the formation of regio-
isomer VIII which should contain two singlets for the
with no detectable intermediates. Experiments at
significantly lower temperatures (<0 °C) showed re-
duced conversion to 1 but no intermediate species.
Consistent with the photochemical reactions under
N2, the dinitrogen adduct 4 in THF is not stable in the
presence of T or 2-CO2EtT and transforms into the
corresponding (hydride)2-thienyl compounds (Scheme
3). Monitoring the thermal reactions between 4 and the
thiophenes by 31P NMR spectroscopy shows slow con-
version at room temperature and rapid reaction at 50
°C. No intermediates are observed prior to C-H bond
cleavage.
Once formed, the thienyl complexes 5 and 7 are
photochemically and thermally (up to 145 °C) stable
under an inert atmosphere. Cleavage of the C-S bond
to give metallathiacycles does not occur even for 7 in
which the electron-withdrawing carboalkoxy substituent
in the thienyl ligand might stabilize the transition state
to C-S insertion.5b
1
thienyl C-H protons. The observation in the H NMR
spectrum of a doublet at 8.41 ppm and of a doublet of
doublets at 7.69 ppm (this collapses to a doublet with
J H
) 3.6 Hz in the 1H{31P} spectrum) does not
AHB
discriminate, however, between structures VII and IX
resulting from insertion at the 2- and 4-positions. These
two regioisomers would exhibit similar 1H NMR pat-
terns for the thienyl protons, while the electron-
withdrawing character of the CO2Et substituent might
balance the sulfur atom for the activation of the
proximal C-H group.
Successful discrimination between regioisomers VII
and IX has been achieved by looking at the spatial
relationships between the hydride ligand and the thie-
nyl protons with 2D-1H,1H phase-sensitive NOESY
spectroscopy (τmix ) 1100 ms) after the complete proton
network in the complex had been assigned through a
combination of 1D- and 2D-NMR experiments (see
Experimental Section). The essential information ob-
tained from the NOESY phase-sensitive correlation
experiment is summarized in Figure 1.
The most diagnostic NOE contacts are those between
the methyl protons of the CO2Et group and only one of
the two thienyl protons, HB (cross peak A), and between
the hydride atom and the other thienyl proton, HA (cross
peak B). Consistently, the other thienyl proton, HB,
does not show any NOE contact to the terminal hydride.
Additional evidence in favor of structure VII is provided
by the absence of any dipolar interaction between the
hydride resonance and the protons of the CO2Et group.
Identical regioselectivity of C-H insertion, as well as
overall complex structure, is observed when transient
[(PP3)Os], photolytically generated in THF from [(PP3)-
OsH2],10 is reacted with T (Scheme 4). The resulting
(hydride)2-thienyl complex [(PP3)Os(H)(2-Tyl)] (9) ex-
hibits spectroscopic characteristics (NMR, IR) which are
fully comparable with those of the Ru derivative 5 (see
Experimental Section).
The identification of 5 and 7 as cis-(hydride)2-thienyl
complexes has been achieved unambiguously by NMR
spectroscopy. In particular, the 31P and the hydride
regions of the 1H NMR spectra are fully comparable
with those of related cis-(hydride)σ-organyl complexes
such as [(PP3)RuH(CtCPh)] and [(PP3)RuH{(MeO2C)Cd
CH(CO2Me)}], which exhibit octahedral coordination
geometries (where the terminal hydride and the organyl
ligand, trans to the bridgehead phosphorus donor, are
mutually cis).13 For this reason, a detailed account of
these spectra is not given here. In contrast, it is worth
commenting on the regiochemistry of the C-H insertion,
1
which has been determined unequivocally by plain H
NMR spectroscopy for 5 and by 2D-1H,1H phase-sensi-
tive NOESY spectroscopy for 7.
For T, C-H insertion may occur at the 2- or 3-position
to give regioisomers V and VI,5k,n while three regioiso-
mers (VII-IX) are possible for 2-CO2EtT due to inser-
tion into three chemically inequivalent C-H bonds
(Chart 2). The pattern observed for the three thienyl
1
proton resonances in the H{31P} NMR spectrum of 5
(two doublets and a doublet of doublets) is exclusively
consistent with the assignment as a 2-thienyl complex.
Indeed, although examples of C-H cleavage at the
3-carbon atom are known,5k,n the predominant reactivity
Syn t h esis a n d Ch a r a ct er iza t ion of t h e η1-S-
2,5-Dim et h ylt h iop h en e Com p lex [(P P 3)R u (η1-S-
C4H2Me2S)]. Photolysis of 1 in THF in the presence of
a large excess of 2,5-Me2T (100 equiv) for 3 h under a
nitrogen atmosphere gives partial conversion of the
dihydride complex to a mixture of products among which
the most abundant (40%) is a new species that we assign
(13) Bianchini, C.; Frediani, P.; Masi, D.; Peruzzini, M.; Zanobini,
F. Organometallics 1994, 13, 4616.
(14) Myers, A. W.; J ones, W. D.; McClements, S. M. J . Am. Chem.
Soc. 1995, 117, 11704.

4616 Organometallics, Vol. 16, No. 21, 1997
Bianchini et al.
F igu r e 1. (a) Section of the phase-sensitive 2D-1H NOESY spectrum of 7 (500.13 MHz, C6D6, 25 °C, τmix ) 1100 ms)
showing the negative cross peak (A) between the methyl protons of the CO2Et group and the proton HB of the thienyl
group. (b) Contour plot of a section of the same spectrum showing the close contact (B) between the hydride at δ -7.99
and the proton HA of the thienyl group.
Sch em e 4
8, was obtained by removing the excess amount of 2,5-
Me2T under reduced pressure. The H NMR spectrum
1
in THF-d8 of the residue does not contain signals in the
region from 3 to 5 ppm where the pseudo-olefinic
protons of η2-C,C-thiophenes, including η2-2,5-Me2T, are
known to resonate.4l,m,o The UV-vis spectrum of the
35:65 mixture of 4 and 8 shows an absorption maximum
at ca. 400 nm. Since isolated 4 absorbs at this wave-
length,9 one cannot assign the observed UV band
exclusively to 8, but the spectrum suggests that the
predominant product 8 also absorbs at 400 nm. On the
basis of the overall experimental evidence, we thus
assign 8 as a trigonal-bipyramidal Ru(0) complex in
which the axial position trans to the bridgehead P atom
is occupied by an S-bound 2,5-Me2T molecule.
Thiophenes can bind 16-electron metal fragments
through either the S atom or a CdC bond.4 Equilibrium
mixtures of η1-S and η2-C,C isomers have also been
observed.4n,o It is generally agreed that alkyl substit-
uents in the thiophene favor η1-S coordination by a
synergic effect: the alkyl substituents enhance both the
basicity of the S atom and the steric hindrance at the
CdC bonds.4d,e In the specific case of 2,5-Me2T, only
one example of η2-C,C coordination has been reported,
i.e., [Os(NH3)5(2,5-Me2T)]2+ where the supporting metal
fragment is not sterically demanding at all,4l while η1-S
complexes are much more numerous.4
as [(PP3)Ru(η1-S-C4H2Me2S)] (8) (vide infra). The other
products are the dinitrogen complex 4 (26%), the o-
metalated complex 6 (10%), and some unidentified
products (12%). Under a helium atmosphere, the
photochemical reaction is faster and produces 8 and 6
together with some unidentified products. After 1 h,
the following product distribution was determined by
31P NMR integration: 6, 20%; 8, 48%; unidentified
products, 15%. A similar product distribution was also
observed in neat 2,5-Me2T as the solvent.
A more selective and cleaner reaction occurs when
isolated 4 is dissolved in neat 2,5-Me2T under nitrogen
at room temperature (NMR experiment), as only 8 is
formed in ca. 65% yield after 12 h. Interestingly, the
ratio between 4 and 8 does not change with time within
48 h (Scheme 3), which suggests the occurrence of an
equilibrium mixture.
The 31P{1H} NMR spectrum of 8 consists of a tem-
perature-invariant AM3 spin system with chemical
shifts and coupling constants (δA 162.25 (q, J AM ) 23.6
Hz); δM 65.26 (d)) which are typical of trigonal-bipyra-
midal (PP3)Ru(0) complexes where the axial position
trans to the bridgehead phosphorus is occupied by a
neutral σ-donor (e.g., 4 or [(PP3)Ru(CO)]).9 Although 8
could not be isolated in the solid state due to its
instability, an oily residue, still containing ca. 15% of
In conclusion, both electronic and steric factors may
contribute to explain why 2,5-Me2T forms a stable η1-S
adduct with [(PP3)Ru] while T or 2-CO2EtT do not: the
two methyl substituents, in fact, enhance the donor
ability of the sulfur atom and sterically congest the C-C
double bonds.

C-H Bond Cleavage in Thiophenes
Organometallics, Vol. 16, No. 21, 1997 4617
20 s-1, which varies by only ca. 5% over the range of T
concentrations used (Figure 2b and Chart 3a). Neither
fast nor slow rate constants were affected by replace-
ment of the Ar atmosphere by H2 or N2.
Flash photolysis of 1 in THF shows that transient
[(PP3)Ru] rapidly forms a short-lived solvento adduct
in which THF probably binds the metal center through
oxygen.9 In the absence of quenchers, [(PP3)Ru(THF)]
decays to form the cyclometalated complex 6 with kobs
) 5 s-1,9 while under N2 it converts to the dinitrogen
complex 4 with a rate constant of 50 s-1. Repetition of
the experiment in THF with ca. 0.3 mol dm-3 T under
Ar results in the conversion of [(PP3)Ru(THF)] to [(PP3)-
Ru(T)] (kobs ) 680 s-1, k2 ≈ 2.2 × 103 dm3 mol-1 s-1),
which subsequently decays with a first-order rate
constant of 18 s-1 (Chart 3b). The reactions were
repeated under atmospheres of H2 and N2, but neither
fast nor slow decay rates were affected.
Similar laser flash photolysis experiments were car-
ried out with 1 in a cyclohexane solution in the presence
of 2,5-Me2T (0.1-0.34 mol dm-3). The observed rate
constant varied linearly with [2,5-Me2T], yielding k2 )
(1.0 ( 0.3) × 106 dm3 mol-1 s-1. As with T, a consider-
able change in absorbance remained after the decay was
complete. Unlike the thiophene case, however, no
further decay was observed up to 1 s (the upper
instrumental limit).
F igu r e 2. (a) Transient decay of absorption monitored at
400 nm following laser flash photolysis (308 nm) of 1 in a
cyclohexane solution in the presence of T ([T] ) 0.19 mol
dm-3). Notice the residual absorption of the product [(PP3)-
Ru(T)]. (b) Decay in absorption of [(PP3)Ru(T)] monitored
on a millisecond time scale after generation as in a.
Laser flash photolysis measurements were also car-
ried out with [(PP3)OsH2], 3, in cyclohexane in the
presence of thiophene (0.39-0.93 mol dm-3) under
argon. The results paralleled those with 1. Transient
[(PP3)Os], monitored at 400 nm, reacted with thiophene
in two sequential steps: (a) a fast decay process which
was linearly dependent on thiophene concentration
yielded k2 ) (3.7 ( 0.1) × 105 dm3 mol-1 s-1, (b) a slower
decay which was independent of [T] with kobs ) 8 ( 1
s-1 (Chart 3a). The wavelength of maximum absor-
bance for the long-lived species was ca. 440 nm.
Identification of the product resulting from the reac-
tion of transient [(PP3)Ru] with T is possible by com-
parison with absorption spectra of related complexes
and by consideration of the evidence from preparative
experiments. Since the latter have shown that the C-H
insertion product 5 is thermally stable, we can exclude
this from consideration. Also, the absorption band at
410 ( 10 nm is close to those of several trigonal-
bipyramidal (PP3)Ru(0) complexes containing a neutral
ligand in the apical position trans to the bridgehead
phosphorus (vide supra).9 We therefore assign this
species as [(PP3)Ru(T)] (10), where T binds the metal
center through either the sulfur atom or a CdC bond.4
Indeed, the first-order decay of 10 fits isomerization to
the C-H insertion product, 5, the product observed in
steady-state experiments. The alternative pathway of
reversible dissociation of T, followed by cyclometalation
of the [(PP3)Ru] fragment to give 6 (rate constant 350
s-1),9 should yield an inverse dependence on [T]. J ust
such behavior was observed in the reaction of [(PP3)-
Ru] with THF in cyclohexane.9 The observations made
in the presence of 2,5-Me2T are also consistent with the
steady-state measurements. The initial product, as-
signed as [(PP3Ru(2,5-Me2T)], shows no further decay
in keeping with the NMR observation that this is a
stable complex. The rate constant for the reaction of
[(PP3)Ru] + 2,5-Me2T is similar to that with T. We have
F igu r e 3. Plot of the pseudo-first-order rate constant for
reaction of Ru(PP3) with T versus [T].
La ser F la sh P h otolysis Exp er im en ts. Extensive
experiments have recently shown that a transient
species with λmax 395 nm is formed when 1, dissolved
in cyclohexane, is subjected to a laser flash at 308 nm.9
In the absence of added reagents, the transient decays
by first-order kinetics in ca. 20 ms. The transient,
which is identified as the 16-electron Ru(0) complex
[(PP3)Ru], reacts with a variety of reagents including
N2, H2, CO, C6H6, C2H4, and THF. The resulting
second-order rate constants are all close to 106 dm3
mol-1 s-1
. The trigonal-bipyramidal Ru(0) products
[(PP3)RuL] (L ) N2, THF, CO, C2H4) have absorption
maxima in the region 400-440 nm.9
Laser flash photolysis experiments were carried out
on solutions of 1 (ca. 10-4 mol dm-3) in cyclohexane
containing T (0.02-0.15 mol dm-3). The resulting
[(PP3)Ru] transient may be monitored at 400 nm and
decays with pseudo-first-order kinetics with kobs be-
tween 1.77 × 104 and 20.9 × 104 s-1 to yield [(PP3)Ru-
(T)] (Figure 2a, see below). A plot of kobs vs [T] is linear
(Figure 3) and yields a second-order rate constant of (1.4
( 0.2) × 106 dm3 mol-1 s-1. The species [(PP3)Ru(T)]
has an absorption maximum at 410 ( 10 nm, and itself
decays with first-order kinetics with a rate constant of

4618 Organometallics, Vol. 16, No. 21, 1997
Bianchini et al.
Ch a r t 3
previously observed that rate constants for the reaction
of [(PP3)Ru] show only small variation with substrate.9
The reactivity of [(PP3)Os] toward thiophene in cyclo-
hexane is very similar to that of [(PP3)Ru], with rate
constants for the initial attack and for the insertion step
reduced by factors of ca. 4 and 2, respectively.
in equilibrium with η1-S-isomers4n,o (these are precur-
sors to C-S bond cleavage).5n,14 The selective formation
of C-H insertion products when [(PP3)Ru] reacts with
T or 2-CO2EtT thus suggests the exclusive intermediacy
of an η2-C,C adduct, although one may not exclude that
the two isomers are in equilibrium and that C-S
insertion does not occur for steric and electronic reasons
(vide infra). From the laser flash photolysis experi-
ments, it is not possible to distinguish which intermedi-
ate is formed or whether both are present at equilibri-
um. In fact, the time scale of decay of the T adduct is
relatively long, so during this period there could be a
rapid equilibrium between η2-C,C and η1-S isomers.
From the clean formation of the C-H activation prod-
uct, one can deduce that the η2-C,C adduct must be
present. On the other hand, the formation of an η1-S
complex with Me2T and the detection of [(PP3)Ru(η1-O-
THF)] (when [(PP3)Ru] is quenched with only THF)9
make the initial formation of an S-bound intermediate
equally plausible.
Discu ssion
As shown by deuterium labeling experiments,15 C-H
bond cleavage of thiophenes is a reaction occurring on
heterogeneous HDS catalysts. Moreover, though there
is no evidence which correlates C-H activation to C-S
activation under actual reactor conditions, homogeneous
modeling studies show that thienyl ligands may be entry
points to ring-opening reactions.16 It is thus surprising
that the mechanism of C-H bond cleavage as well as
the factors (electronic and steric properties of the metal
environment) that control C-H and C-S insertions
have not yet been fully elucidated.
It is generally agreed that the C-H bond cleavage of
thiophenes by 16-electron metal fragments to give
(hydride)thienyl complexes proceeds through the inter-
mediacy of η2-C,C-thiophene species,3,5n which may be
In the large majority of thermal reactions between
unsaturated metal fragments and thiophenes, C-H
insertion is kinetically favored over C-S insertion, but
the C-S oxidative-addition products are thermodynami-
cally preferred.5c,e,h,n,k,o An exception to the rule is
provided by [Tp*Rh(PR3)] (Tp* ) hydrotris(3,5-dim-
ethyl-1-pyrazolyl)borate; R ) Me, Et), which brings
about the activation of both C-H and C-S bonds from
(15) (a) Smith, G. V.; Hickley, C. C.; Behbahany, F. J . Catal. 1973,
30, 218. (b) Benson, J . W.; Schrader, G. L.; Angelici, R. J . J . Catal.
1995, 96, 283.
(16) (a) Chisholm, M. H.; Haubrich, S. C.; Martin, J . D.; Streib, W.
E. J . Chem. Soc., Chem. Commun. 1994, 683. (b) Erker, G.; Petrenz,
R.; Kru¨ger, C.; Lutz, F.; Weiss, A.; Werner, S. Organometallics 1992,
11, 1646.
T, reversing the order of thermodynamic stability.5a
A

C-H Bond Cleavage in Thiophenes
Organometallics, Vol. 16, No. 21, 1997 4619
Ch a r t 4
particular case is also represented by [Cp2Mo] and
[Cp2W], generated by photolysis of dihydride precursors.5i
The Mo fragment inserts exclusively into an R-C-H
bond of T to give a photochemically stable 2-thienyl
complex, whereas the W fragment initially forms a C-S
insertion product that photochemically converts to the
(hydride)2-thienyl isomer by an intramolecular mech-
anism. Intramolecular mechanisms have also been
proposed for the thermal rearrangement of the thienyl
and benzo[b]thienyl complexes [(triphos)IrH2(thienyl)]
to the corresponding metallathiacycles,5e whereas a
dissociative mechanism has been reported for the
conversion of the dibenzo[b,d]thienyl complex [(triphos)-
IrH2(4-DBTyl)] to the C-S insertion product [(triphos)-
IrH(η2-C,S-C12H8S)] (triphos ) MeC(CH2PPh2)3].5c Fi-
nally, irradiation of [Fe(dmpe)2H2] at 0 °C in the
presence of T, 2-methylthiophene, or 3-methylthiophene
leads to the formation of both C-H and C-S insertion
products (ratio not indicated) which are not photolabile
(dmpe ) 1,2-bis(dimethylphosphino)ethane).5j
In summary, from a perusal of the metal fragments
which are capable of activating C-H bonds in thiophenes
(Chart 4), one may infer that C-H insertion is favored
by large steric crowding at the metal center while C-S
insertion is favored by a high electron density on the
metal.
The reactivity of [(PP3)Ru] toward T, 2-CO2EtT, and
2,5-Me2T nicely fits this scenario. Among the metal
fragments shown in Chart 4, [(PP3)Ru] is certainly the
most encumbered (at least three phenyl substituents
point to the incoming thiophene) and one of the least
electron-rich together with [Cp2Mo] and [Tp*Rh(PR3)]
which favor C-H activation. Only when the thiophene
bears two methyl substituents in the 2,5-positions does
η1-S-coordination prevail over η2-C,C, but the resulting
S-bound adduct does not evolve to the C-S insertion
product because the metal slippage from η1-S to η2-C,S
is hindered by the substituents. In T and 2-CO2EtT,
the sulfur atom is a poorer σ-donor and η2-C,C coordina-
tion is thus preferred, leading to selective C-H scission.
The ability of the Ru(0) fragment [(PP3)Ru] to coordinate
double bonds has precedent in the synthesis of [(PP3)-
Ru(C2H4)] by treatment of transient [(PP3)Ru] with
ethene.9
an η1-S complex is formed with 2,5-dimethylthiophene
(Schemes 2 and 3). Laser flash photolysis combined
with UV-vis detection has shown that an intact Ru(0)
thiophene adduct is formed initially, which decays to
the C-H activation product with first-order kinetics.
The adduct [(PP3)Ru(T)] is formed from Ru(PP3) + T
much more rapidly in cyclohexane than in THF. How-
ever, the rate constant for conversion of [(PP3)Ru(T)]
to [(PP3)Ru(2-Tyl)H] is ca. 20 s-1 in both solvents at 295
K (Chart 3). The selectivity toward C-H insertion
exhibited by [(PP3)Ru] has been interpreted in terms of
prevailing steric effects.
Ack n ow led gm en t. Thanks are due to the Progetto
strategico “Tecnologie Chimiche Innovative”, CNR, Rome,
Italy, and to the EC (contract ERB IC15CT960746) for
financial support. The York group acknowledges the
support of EPSRC and British Gas.
Con clu sion s
The interaction of transient [(PP3)Ru] with selected
thiophenes has been studied. This 16-electron metal
fragment reacts with thiophene and ethyl 2-thiophene-
carboxylate to give (hydride)2-thienyl complexes, whereas
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