Transition Met. Chem.
, 21, 11-15 (1996) Ru-catalysed hydrogenation 11
Regioselective homogeneous hydrogenation of heteroaromatic
nitrogen compounds by use of [RuH(CO)(NCMe)2(PPh3)2]BF 4
as the precatalyst
Merlin Rosales*, Janeth Navarro, Ligbel S~inchez, Angel Gonz~ilez, Ysaias Alvarado, Rafil Rubio, Carlos De La Cruz~and
Tamara Rajmankina t
Inorganic Chemistry Laboratory and ~Molecular and Atomic Spectroscopy Laboratory, Facultad de Ciencias,
La Universidad del Zulia (LUZ), Apdo. 526, Maracaibo, Venezuela
Summary Experimental
The complex [RuH(CO)(NCMe)2(PPh3)2]BF ~ (1) is All manipulations were carried out with rigorous exclu-
an efficient and regioselective catalyst precursor for the sion of air, which is strictly required for successful hydro-
hydrogenation of polyaromatic nitrogen compounds genation. Q and iQ were purified by distillation at
such as quinoline (Q), isoquinoline (iQ), indole (ln), 5,6- reduced pressure, while 5,6-BQ, 7,8-BQ and A were re-
and 7,8-benzoquinoline (BQ) and acridine (A) under rela- crystallized from benzene-EtOH. Solvents were dried by
tively mild reaction conditions (125 ~ 4 atm H2). The known procedures and distilled under an inert atmos-
order of individual initial rates was: A > Q > 5,6- phere prior to use. H 2 was dried by passing through
BQ > 7,8-BQ > in > iQ, reflecting both steric and elec- a column containing CaSO4. Complex (1) was prepared
tronic effects. For the regioselective homogeneous hydro- by the procedure recently punished by us (s), The i.r.
genation of A to 9,10-dihydroacridine (DHA) catalysed by spectra of the complexes (in KBr) were recorded on
complex (I), a kinetic study was carried out; the experi- a Perkin-Elmer 1725-X instrument with an MCT detector
mentally determined rate law was r = kl [Ru] [H2]. These using the diffuse reflectance technique. ~H- and 3~p{~H}-
findings are consistent with a mechanism involving the n.m.r, spectra were recorded on a Bruker AM-300 spec-
hydrogenation of [RuH(CO)(A)(NCMe)(PPh3)2]BF 4 to trometer; chemical shifts are expressed in p.p.m, upfield
yield DHA and the unsaturated species [RuH(CO)- fl'om M%Si and H3PO4, respectively.
(NCMe)(PPh3)2]BF ~ in the rate-determining step.
Catalytic hydrogenations
Introduction The systems and methods used for the catalytic reactions
The hydrogenation of polynuclear heteroaromatic nitro- at low(7'9) and high hydrogen pressures (7'1~ were similar
gen compounds is an area of considerable interest because to those reported previously. For the kinetic study of the
it is related to the industrially important hydrodenit- A reduction, the ratio of substrate:catalyst was varied
rogenation (HDN) process, in which the nitrogen content between ca. 40 and 150 (see Table 2).
of the various fuel products derived from coal and petro- Each reaction was repeated at least twice in order to
leum is minimized. Thermodynamic studies of the HDN ensure reproducibility of the results. The percentage of
process indicate that selective hydrogenation of the nitro- hydrogenation of polyaromatic nitrogen compounds in
gen-containing ring occurs prior to any carbon-nitrogen the catalytic reactions was restricted to 5-10% in order to
bond cleavage reaction (1). Nevertheless, there are relative- ensure that the error introduced by the initial-rate method
ly few examples of homogeneous catalysts for the hydro- was kept well within the accepted limits for a kinetic
genation of this type of compound. Several homogeneous study(11). The data were plotted as molar concentration of
Rh, Ir, Ru and Os systems have been published by Fish the product versus time, yielding straight lines; initial
and co-workers (2-~) and S~mchez-Delgado and co- hydrogenation rates were obtained from the correspond-
workers (s'6) for the homogeneous reduction of quinoline ing slopes. All the straight lines were fitted by conven-
(Q) and related compounds using molecular hydrogen; tional linear regression programs to r 2 > 0.98.
the Ru and Rh complexes were found to be the most The composition of the catalytic mixtures was analysed
efficient. by means of a 610 Series UNICAM gas chromatograph
Recently we have reported that the regioselective fitted with a thermal conductivity detector and a 3 m 10%
reduction of Q to 1,2,3,4-tetrahydroquinoline (THQ) SE-30 on a Supelcoport glass column using He as carrier
may be carried out using the cationic complex gas; the results were quantified with a UNICAM 4815
[RuH(CO)(NCMe)2(PPh3)2]BF ~ (1) as the catalyst computing integrator.
precursor; a kinetic and mechanistic study was also re- The H2 concentrations in solution of toluene(12) and
ported (7). In this paper, we present new results on the xylene~13) were calculated from the data reported in the
hydrogenation of quinoline, isoquinoline (iQ), indole (In), literature.
5,6- and 7,8-benzoquinoline (BQ) and acridine (A), includ-
ing a kinetic and mechanistic study of the hydrogenation
of the last compound. Preparation of [RuH(CO) (tll-N-iQ)2(PPh3)2]BF4
(2)
To a solution of complex (I) (300rag, 0.36mmol) in
CHC13 (15 cm 3) was added iQ (1 cm 3, 8.47 mmol), and the
* Author to whom all correspondence should be directed. mixture was stirred vigorously under reflux tbr 3 h, giving
0340-4285 9 1996 Chapman & Hall
12 Rosales et al. Transition Met. Chem., 21, 11-15 (1996)
a yellow solution, which was evaporated under vacuum to 0.08
about one-third of its original volume. A pale yellow
precipitate was obtained by addition of n-pentane, which
was washed with several portions of Et20 and n-pentane, 0.06 A Q
and then dried in vacuo. Yield 280mg, 78%. (Found: C,
65.4; H, 4.6; N, 2.9; RuC55H45N2P2OBF~ calcd.: C, 66.1;
H, 4.5; N, 2.8%.) I.r.: 2010(m) [v(RuH)], 1937(s) [v(CO)]
and 1080(vs)cm 1 [v(BF)]. ~H-n.m.r. (CDC13, 298K): 0.04
9.5-7.1p.p.m. (series of multiplets, iQ and PPh3);
- 12.8 p.p.m. It, R u - - H , J(HP) = 18 Hz]. 3~p{1H}-n.m.r.
(CDC13, 298 K): 45.5 p.p.m. (s).
0.02
In
Preparation of [ R u H ( C O ) (rll-N-THiQ)2(PPh3)2]BF4
(3)
0.00
A solution of complex (1) (400rag, 0.49mmol) and iQ 0 5000 10 000 15 000
(1 cm 3, 8.47 retool) in benzene (10 cm3), and a stirring bar
Time (s)
were introduced into a glass-lined stainless steel autoclave
(125 cm3). Air was removed by flushing three times with Figure 1. Representative examples of the hydrogenation of
H2; the reactor was then heated in a silicone-oil bath polyaromatic nitrogen compounds by complex (I). Conditions
which was thermostatted to the reaction temperature as in Table 1.
(125 ~ and subsequently charged to the desired pressure
(4 atm H2). After 2 h the reaction mixture reached - 10 ~ Table 1. Hydrogenation data for polyaromatic nitrogen com-
where it was kept overnight. A light yellow solid was pounds catalysed by [RuH(CO)(CNMe)2(PPha)2]BF4a
obtained, which was washed with portions of Et20 and
n-pentane and dried in vacuo. Yield 360 rag, 74%. (Found: Substrate Product 106 rib TN c
C, 64.5; H, 5.2; N, 2.9; RuCssH53NzPzOBF4, calcd.: C,
65.6; H, 5.3; N, 2.8%.) I.r.: 1980(m) [v(RuH)]. 1930(s)
[v(CO)] and 1080(vs)cm- 1 [v(BF)], 1H-n.m.r. (CDC13, @ (7.67 _+0.01) 16
298K): 9.5-7.1p.p.m. (series of multiplets, THiQ and H
PPh3); 2.6p.p.m. (m, H 1, THiQ), 2.2 (m, H3, THiQ), 1.6
(m, H4, THiQ); - 12.9 p.p.m. (t, R u - - H , J(HP) = 18 Hz).
3tp{1H}-n.m.r. (CDC13, 298 K): 101.6 p.p.m. (s).
~---~N ~-~INH (0.25 _+ 0.01) <1
Results and discussion % (2.71+0.05) 6
Catalytic hydrogenation H
Complex (1) was chosen as the catalyst precursor for the ,,36+00 3
hydrogenation of polyaromatic nitrogen compounds on
the basis of its known catalytic activity (7'9'1~ Q, iQ, ln,
5,6-BQ and 7,8-BQ and A were used as substrates since
they are considered to be simple models of petroleum and
coal compounds. ,0 +001, 1
Although the reactions can be carried out under condi- H H
tions somewhat milder than those used here, the reaction
rates were more conveniently measured at 125 ~ and
~ ~ (20.10_+0.01) 43
4 atm of H 2 in hydrocarbon solvents (toluene or xylene).
H
Under these conditions, (1) was an efficient and re-
gioselective precatalyst for the reduction of all six hetero- "Conditions: [ R u ] = 1.7 x 10 3M; [-substrate] =0.17M; P = 4 a t m H2;
cycles used. Q, iQ, 5,6-BQ and 7,8-BQ were hydrogenated T = 125 ~ solvent = xylene; br i = initial rate; CTN = t u r n o v e r n u m b e r
to their corresponding 1,2,3,4-tetrahydro derivatives, (1 h).
while In and A were reduced to 2,3-dihydroindole (DHIn)
and 9,10-dihydroacridine (DHA), respectively. ca. 15 times more active than for In. A similar difference
Selected examples of the reaction behaviour are shown for the Q and iQ reduction was found by Fish et al. (3)
in Figure 1. Table 1 contains the initial rates and the using the complex [Rh(r/5-Cp*)(p-xylene)] 2 +. These find-
turnover numbers for the reduction of the heterocycles. ings may be explained by the higher basicity of the
The substrate to catalyst ratio was 100 in all cases, with nitrogen atom in iQ compared to Q.
turnover number per hour varying from 1 to 43. The For the reduction of Q and its two benzo derivatives,
order of individual rates was A > Q > 5,6-BQ > 7,8- steric effects are clearly important. For example, com-
BQ > In > iQ, reflecting both steric and electronic effects. pound 7,8-BQ, which is the most sterically hindered at
The higher resonance stabilization energy of the nitro- nitrogen, is reduced at the slowest rate, while the relatively
gen ring for Q over that of A could be the main reason why unhindered Q is reduced at the fastest rate.
reduction of A is about three times faster than that of Q. All the reaction mixtures were homogeneous solutions
On the other hand, (1) is ca. 30 times toore active for the with no evidence of metallic components. The homogene-
hydrogenation of Q than for its structural isomer iQ, and ity of the reactions was further established by the well
