Hydrogenation of benzonitrile to benzylamine catalyzed by ruthenium hydride complexes with P-NH-NH-P tetradentate ligands: Evidence for a hydridic-protonic outer sphere mechanism

Article abstract of DOI:10.1021/om700783e

The reaction of RuHCl(PPh₃)₃ with the tetradentate ligand [PPh₂((ortho-C₆H₄)CH ₂NHCH₂^-)]₂ {ethP₂ ^- (NH)₂} in THF produces the new complex trans-RuHCl{ethP₂(NH)₂} (1) as a mixture of two isomers. The complex RuHCl{ethP₂(NH)₂} (1) when activated with KO^tBu/KH is a very active catalyst for the hydrogenation of benzonitrile to benzylamine in toluene, more active than the known catalyst Ru(H₂)₂H₂^- (PCy₃) ₂ (2). A mixture of 1 and 2 and base also results in efficient conversion of benzonitrile to benzylamine. The complex RuHCl{tmeP ₂(NH)₂} (3) where tmeP₂(NH)₂ is [PPh₂((ortho-C₆H₄)CH₂^- NHCMe₂^-)]₂ is a less active catalyst for this reaction. These catalyst systems are air sensitive and extremely moisture sensitive. Experimental and theoretical (DFT) evidence is presented for a new mechanism for nitrile hydrogenation: the successive hydrogenation of the CN triple bond and then the CN double bond of the intermediate imine by H ⁺/H^- transfer from a trans dihydride active catalyst. The amido complex RuH{tmeP₂N(NH)} (4) has similar activity to 3/base for the base-free hydrogenation of benzonitrile and is moderately active for the catalytic hydration of benzonitrile. The new complex RuH(BH₄) {ethP₂^- (NH)₂} (7) was prepared by reacting 1 with NaBH₄ but is found to be a poor catalyst for nitrile hydration.

Full text of DOI:10.1021/om700783e

5940
Organometallics 2007, 26, 5940-5949
Hydrogenation of Benzonitrile to Benzylamine Catalyzed by
Ruthenium Hydride Complexes with P-NH-NH-P Tetradentate
Ligands: Evidence for a Hydridic-Protonic Outer
Sphere Mechanism
Tianshu Li, Ines Bergner, F. Nipa Haque, Marco Zimmer-De Iuliis, Datong Song, and
Robert H. Morris*
Department of Chemistry, UniVersity of Toronto, 80 St George Street, Toronto, Ontario M5S 3H6, Canada
ReceiVed August 1, 2007
-
The reaction of RuHCl(PPh3)3 with the tetradentate ligand [PPh2((ortho-C6H4)CH2NHCH2 )]2 {ethP2-
(NH)2} in THF produces the new complex trans-RuHCl{ethP2(NH)2} (1) as a mixture of two isomers.
The complex RuHCl{ethP2(NH)2} (1) when activated with KOtBu/KH is a very active catalyst for the
hydrogenation of benzonitrile to benzylamine in toluene, more active than the known catalyst Ru(H2)2H2-
(PCy3)2 (2). A mixture of 1 and 2 and base also results in efficient conversion of benzonitrile to
benzylamine. The complex RuHCl{tmeP2(NH)2} (3) where tmeP2(NH)2 is [PPh2((ortho-C6H4)CH2-
-
NHCMe2 )]2 is a less active catalyst for this reaction. These catalyst systems are air sensitive and extremely
moisture sensitive. Experimental and theoretical (DFT) evidence is presented for a new mechanism for
nitrile hydrogenation: the successive hydrogenation of the CN triple bond and then the CN double bond
of the intermediate imine by H+/H- transfer from a trans dihydride active catalyst. The amido complex
RuH{tmeP2N(NH)} (4) has similar activity to 3/base for the base-free hydrogenation of benzonitrile and
is moderately active for the catalytic hydration of benzonitrile. The new complex RuH(BH4){ethP2-
(NH)2} (7) was prepared by reacting 1 with NaBH4 but is found to be a poor catalyst for nitrile hydration.
Scheme 1. Hydrogenation of Acetophenone Catalyzed by
Complexes 4 and 5
Introduction
The hydrogenation of nitriles to primary amines whether
catalyzed by heterogeneous1-4 or homogeneous5-8 catalysts can
result in the formation of side products consisting of imines
and secondary and tertiary amines. The side products are the
result of reactions of imine intermediates with the product amine.
The selectivity to the primary amine can be low for heteroge-
neous catalysts unless special conditions such as pressures of
ammonia2 or protecting agents are added;1 nevertheless nitriles
are hydrogenated to primary amines on a large scale in industry.8
In certain cases homogeneous catalysts are very selective
(>90%) in producing the primary amine.1,9-14 The complex
RhH(PiPr3)3 catalyzes the hydrogenation of benzonitrile exclu-
sively to benzylamine at 23 °C and 5 bar H2 in THF13 although
the conversion is only 45% due to poisoning of the catalyst by
the product unless excess CO2 is present.1 The dihydrogen
complex Ru(H2)2H2(PCy3)2 (2)15 was also found to be a catalyst
for the hydrogenation of nitriles under mild conditions (see also
below).10 An anionic ruthenium complex11,12 and a ruthenium
amido complex9 are good catalysts at higher temperatures (80-
90 °C) and pressures (6-60 atm). The latter system was
proposed to transfer H+/H- to the coordinated nitrile (in the
inner coordination sphere) to give a coordinated imine that is
subsequently hydrogenated to the amine.
(1) Xie, X.; Liotta, C. L.; Eckert, C. A. Ind. Eng. Chem. Res. 2004, 43,
7907-7911.
(2) Kukula, P.; Studer, M.; Blaser, H.-U. AdV. Synth. Catal. 2004, 346,
1487-1493.
(3) Blaser, H. U.; Malan, C.; Pugin, B.; Spindler, F.; Steiner, H.; Studer,
M. AdV. Synth. Catal. 2003, 345, 103-151.
(4) Gomez, S.; Peters, J. A.; Maschmeyer, T. AdV. Synth. Catal. 2002,
344, 1037.
(5) Toti, A.; Frediani, P.; Salvini, A.; Rosi, L.; Giolli, C.; Giannelli, C.
C. R. Chim. 2004, 7, 769-778.
(6) Bianchini, C.; Dal Santo, V.; Meli, A.; Oberhauser, W.; Psaro, R.;
Vizza, F. Organometallics 2000, 19, 2433-2444.
(7) Chin, C. S.; Lee, B. N. Catal. Lett. 1992, 14, 135-140.
(8) Nishimura, S. Handbook of Heterogeneous Catalytic Hydrogenation
for Organic Synthesis; John Wiley & Sons: New York, 2001; Chapter 7,
p 254-285.
The question of inner sphere versus outer sphere reactivity
is a topic of great interest.16,17 The Noyori transfer hydrogenation
(9) Takemoto, S.; Kawamura, H.; Yamada, Y.; Okada, T.; Ono, A.;
Yoshikawa, E.; Mizobe, Y.; Hidai, M. Organometallics 2002, 21, 3897-
3904.
(10) Beatty, R. P.; Paciello, R. A. U.S. Patent 5,726,334, 1998.
(11) Grey, R. A.; Pez, G. P.; Wallo, A. J. Am. Chem. Soc. 1981, 103,
7536-7542.
(12) Pez, G. P.; Grey, R. A.; Corsi, J. J. Am. Chem. Soc. 1981, 103,
7528-7535.
(13) Yoshida, T.; Okano, T.; Otsuka, S. J. Chem. Soc., Chem. Commun.
1979, 870-871.
(14) Joshi, A. M.; MacFarlane, K. S.; James, B. R.; Frediani, P. Stud.
Surf. Sci. Catal. 1992, 73, 143-146.
(15) Arliguie, T.; Chaudret, B.; Morris, R. H.; Sella, A. Inorg. Chem.
1988, 27, 598-599.
(16) Samec, J. S. M.; Ba¨ckvall, J.-E.; Andersson, P. G.; Brandt, P. Chem.
Soc. ReV. 2006, 35, 237-248.
10.1021/om700783e CCC: $37.00 © 2007 American Chemical Society
Publication on Web 10/25/2007

Benzonitrile Hydrogenation
Table 1. Crystallographic Data for
Organometallics, Vol. 26, No. 24, 2007 5941
Scheme 2
RuH(NHCOPh){tmeP2(NH)2} (6) and
RuH(BH4){ethP2(NH)2} (7)
6 (H2O)
7
formula
fw
space group
T, K
C51H55N3O2P2Ru
904.99
P21/n
C40H43BN2P2Ru
725.59
P1h
150(2)
150(2)
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
17.693(4)
12.837(3)
20.962(4)
90
112.64(3)
90
10.163(2)
15.129(3)
24.273(5)
99.54(3)
98.42(3)
104.56(3)
3492.6(12)
4
4394.1(15)
4
The complex RuHCl{cyP2(NH)2}, where cyP2(NH)2 is the
tetradentate P-NH-NH-P ligand (R,R)-PPh2(ortho-C6H4)CH2-
NH-C6H10-NHCH2(ortho-C6H4)PPh2, is a precursor to a
Z
wavelength, Å
0.71073
1.368
0.1087
0.1469
0.71073
1.380
0.1808
F
calcd, mg/m3
highly active asymmetric ketone hydrogenation catalyst.24
A
R1 (all data)
wR2
0.1328
trans dihydride RuH2{cyP2(NH)2} is proposed to transfer
hydride from ruthenium and proton from nitrogen to the CdO
bond of acetophenone in the second coordination sphere.24,25
We wondered whether nitriles could be hydrogenated in a
similar fashion. Since an enantiopure ligand is not required, we
investigated the use of the tetradentate P-NH-NH-P ligands
ethP2(NH2) ([PPh2(ortho-C6H4)CH2NHCH2-]2)24 and tmeP2-
(NH)2 ([PPh2(ortho-C6H4)CH2NHCMe2-]2).25 The second ligand
allows the synthesis of well-defined catalysts for ketone
hydrogenation: the crystallographically characterized hydrido
amido complex trans-RuH{tmeP2N(NH)} (4) and the dihydride
complex trans-RuH2{tmeP2(NH)2} (5) (Scheme 1). These
provide additional information on the nitrile hydrogenation
reaction and resulted in the discovery of a nitrile hydration
catalyst.
Table 2. Selected Bond Distances and Angles for
RuH(NHCOPh){tmeP2(NH)2} (6)
Distances, Å
Ru(1)-N(3)
Ru(1)-N(1)
Ru(1)-P(2)
N(2)-C(10)
N(1)-C(7)
C(45)-O(1)
2.178(3)
2.182(4)
2.255(1)
1.481(6)
1.488(6)
1.269(5)
Ru(1)-N(2)
Ru(1)-P(1)
N(3)-C(45)
N(2)-C(9)
N(1)-C(8)
Ru(1)-H(0)
2.179(4)
2.252(1)
1.304(5)
1.526(6)
1.517(6)
1.64(4)
Angles, deg
N(3)-Ru(1)-N(2)
N(2)-Ru(1)-N(1)
N(2)-Ru(1)-P(1)
N(3)-Ru(1)-P(2)
N(1)-Ru(1)-P(2)
C(45)-N(3)-Ru(1)
O(1)-C(45)-C(46)
90.2(1)
78.3(2)
N(3)-Ru(1)-N(1)
N(3)-Ru(1)-P(1)
N(1)-Ru(1)-P(1)
N(2)-Ru(1)-P(2)
P(1)-Ru(1)-P(2)
O(1)-C(45)-N(3)
N(3)-C(45)-C(46)
91.1(1)
94.34(9)
92.1(1)
169.5(1)
91.86(9)
168.4(1)
134.4(3)
117.8(3)
90.5(1)
98.89(4)
122.8(4)
119.4(4)
Experimental Section
Table 3. Selected Bond Distances and Angles for
General. All preparations and manipulations were carried out
under hydrogen, nitrogen, or argon atmospheres with the use of
standard Schlenk, vacuum line, and glove box techniques in dry,
oxygen-free solvents. Tetrahydrofuran (THF), toluene, diethyl ether,
and hexanes were dried and distilled from sodium benzophenone
ketyl. Deuterated solvents were degassed and dried over activated
molecular sieves. Potassium t-butoxide and 2-(diphenylphosphino)
benzaldehyde were supplied by the Aldrich Chemical Co. Ben-
zonitrile was purchased in 99% pure, anhydrous form from Aldrich,
vacuum distilled under argon, and stored over 4 Å molecular sieves
under Ar. Cyclohexene was vacuum distilled under argon and stored
over 4 Å molecular sieves under Ar. The complexes RuHCl{tmeP2-
(NH)2},25 RuH{tmeP2N(NH)},25 Ru(COD)(COT),26 Ru(H2)2H2-
RuH(BH4){ethP2(NH)2} (7), Molecule 1
Distances, Å
Ru(1)-N(2)
2.158(4)
2.248(2)
1.59(4)
2.0
Ru(1)-N(1)
Ru(1)-P(1)
Ru(1)-H(1B)
2.170(4)
2.249(2)
1.95(5)
Ru(1)-P(2)
Ru(1)-H(1RU)
H(3B)‚‚‚H(2C)
Angles, deg
N(2)-Ru(1)-N(1)
N(1)-Ru(1)-P(2)
N(1)-Ru(1)-P(1)
Ru(1)-H(1B)-B(1)
80.9(2)
169.7(1)
89.8(1)
148(4)
N(2)-Ru(1)-P(2)
N(2)-Ru(1)-P(1)
P(2)-Ru(1)-P(1)
90.2(1)
170.2(1)
98.75(6)
catalyst Ru(η6-arene)H(NH2CHPhCHPhNHTs) was proposed to
add H+/H- to CdN bonds of imines by the metal-ligand
bifunctional outer sphere mechanism.18 However, the actual
hydride acceptor has recently been revealed to be in some cases
an iminium CdN bond.19 Our research group used the outer
sphere concept and the RuH‚‚‚HN motif to find very active
ketone and imine H2-hydrogenation catalysts,20-23 and now one
for benzonitrile.
28
(PCy3)2,27 and RuHCl(PPh3)3 were prepared by the literature
methods. The tetradentate ligand ethP2(NH)2 was prepared by the
literature method.29 NMR spectra were recorded on a Varian Unity-
1
1
500 (500 MHz for H), a Varian Unity-400 (400 MHz for H), or
a Varian Gemini 300 MHz spectrometer (300 MHz for 1H and 121.5
for 31P). All 31P chemical shifts were measured relative to 85%
H3PO4 as an external reference. 1H chemical shifts were measured
relative to partially deuterated solvent peaks but are reported relative
to tetramethylsilane. All infrared spectra were obtained on a Nicolet
(17) Casey, C. P.; Bikzhanova, G. A.; Cui, Q.; Guzei, I. A. J. Am. Chem.
Soc. 2005, 127, 14062-14071.
(18) Noyori, R.; Hashiguchi, S. Acc. Chem. Res. 1997, 30, 97-102.
(19) Åberg, J. B.; Samec, J. S. M.; Ba¨ckvall, J.-E. Chem. Commun. 2006,
2771-2773.
(24) Rautenstrauch, V.; Hoang-Cong, X.; Churlaud, R.; Abdur-Rashid,
K.; Morris, R. H. Chem. Eur. J. 2003, 9, 4954-4967.
(25) Li, T.; Churlaud, R.; Lough, A. J.; Abdur-Rashid, K.; Morris, R.
H. Organometallics 2004, 23, 6239-6247.
(20) Abdur-Rashid, K.; Lough, A. J.; Morris, R. H. Organometallics
2001, 20, 1047-1049.
(26) Pertici, P.; Vitulli, G. Inorg. Synth. 1983, 22, 176-181.
(27) Chaudret, B.; Devillers, J.; Poilblanc, R. Organometallics 1985, 4,
1727-1732.
(21) Abdur-Rashid, K.; Lough, A. J.; Morris, R. H. Organometallics
2000, 19, 2655-2657.
(22) Abdur-Rashid, K.; Guo, R.; Lough, A. J.; Morris, R. H.; Song, D.
AdV. Synth. Catal. 2005, 347, 571-579.
(28) Hallman, P. S.; McGarvey, B. R.; Wilkinson, G. J. Chem. Soc. A
1968, 3143-3150.
(23) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem. ReV.
2004, 248, 2201-2237.
(29) Gao, J.-X.; Wan, H.-L.; Wong, W.-K.; Tse, M.-C.; Wong, W.-T.
Polyhedron 1996, 15, 1241-1251.

5942 Organometallics, Vol. 26, No. 24, 2007
Li et al.
2
Table 4. Catalytic Hydrogenation of Benzonitrile to
Benzylaminea
65.66 (AB), 64.81 (AB, JHP ) 28.8 Hz). IR (Nujol): 3373 (w,
NH), 3284 (w, NH), 1869 (m, RuH), 1590 (m, CdO), 1554 (m,
CdN).
entry
catalyst
baseb
time (h)
conv (%)
Reaction of RuHCl{tmeP2(NH)2} and Ru(H2)2H2(PCy3)2 with
KOtBu, H2, and Benzonitrile. RuHCl{tmeP2(NH)2}25 (8 mg, 1 ×
10-2 mmol), Ru(H2)2H2(PCy3)227 (7 mg, 1 × 10-2 mmol), and KOt-
Bu (2 mg, 2 × 10-2 mmol) were mixed in a pressure NMR tube
under Ar. C6D6 (1 mL) was added to make a yellow-brown solution.
1H NMR (C6D6) hydride region: δ -5.2 (m, trans-Ru(H)2{tmeP2-
(NH)2}25), -7.9 (br s, Ru(H2)2H2(PCy3)2), -9.2 (br s, unknown),
1
2
3
4
5
6
7
8
9
1
1
1
1
none
18
<18
18
3-6
3
3
50
3
0
100
0
100
31
KOtBuc
KOtBu/H2Od
KOtBu/KHc,e
KOtBu/KHc,e,f
KOtBuc
1
KHd
0
2
none
95.7
99.5
10
99
9
1 + 2g
KOtBuc
-12.5 (br s, Ru2H6(PCy3)415), -23.9 (dd, JHP 24.9 Hz, RuH-
2
3
KOtBu
24
22
3
10
11
3 + 2
4
KOtBu
{tmeP2N(NH)}25). 31P{1H} NMR (C6D6): δ 58.4 (s, Ru2H6(PCy3)4),
61.1 (d, 2JPP 24.3 Hz, RuH{tmeP2N(NH)}25), 65.4 (d, RuH{tmeP2N-
(NH)}), 76.2 (s, Ru(H2)2H2(PCy3)2), 77.7 (s, Ru(H)2{tmeP2(NH)2}).
It was then frozen by cooling in liquid N2, H2 gas was added, and
the mixture was warmed up to room temperature to give a yellow
a 20 °C, 14 atm H2, dry toluene (5 mL), nitrile:Ru ) 180:1, [substrate]
) 0.18 M, [catalyst] ) 1.0 mM. b [KOtBu] ) 9 mM. c The base is not
completely dissolved. d 50 mg H2O added. e KH (3 mg, 15 mM). f With
nitrile:cyclohexene:Ru ) 180:180:1, only the nitrile is hydrogenated. g 0.5
mM of each catalyst, 7 atm H2.
1
2
brown solution. H NMR (C6D6) hydride region: δ -5.3 (t, JHP
17.7 Hz, Ru(H)2{tmeP2(NH)2}), -7.9 (br s, Ru(H2)2H2(PCy3)2).
31P{1H} NMR (C6D6): δ 9.7 (s, PCy3), 76.3 (s, Ru(H2)2H2(PCy3)2),
77.7 (s, Ru(H)2{tmeP2(NH)2}). Benzonitrile (25 mg, 0.24 mmol)
was added under Ar. After freezing and adding H2 gas, the mixture
was warmed up to room temperature. A red solution was obtained
after shaking. The mixture was examined by NMR immediately.
Scheme 3
2
1H NMR (C6D6) hydride region: δ -10.9 (t, JHP 27.5, Ru-
2
(PhCN)2H2(PCy3)2), -13.66 (dd, 1H, RuH, JHP 20.8, 23.6 Hz,
RuH(PhCONH){tmeP2(NH)2}). 31P{1H} NMR (C6D6) δ: 9.7 (s,
PCy3), 42.9 (s, Ru(PhCN)2H2(PCy3)2), 65.66 (AB, RuH(PhCONH)-
{tmeP2(NH)2}), 64.81 (AB, 2JHP 28.8 Hz, RuH(PhCONH){tmeP2-
(NH)2}).
Synthesis of RuH(BH4){ethP2(NH)2} (7). RuHCl{ethP2(NH)2}
(1) (75 mg, 0.1 mmol) was suspended in benzene (10 mL). NaBH4
(100 mg, 2.7 mmol) in ethanol (10 mL) was added. A yellow clear
solution was formed. The mixture was stirred for 30 min at 65 °C
and one additional hour at room temperature. The solvent was
evaporated to give a white residue. Benzene (10 mL) was added,
and the mixture was stirred for 30 min. A yellow solution was
obtained after filtering the mixture through Celite to remove sodium
chloride. After evaporation of most of the solvent, hexane (10 mL)
was added to give a white precipitate. The mixture was filtered
and washed with hexane (1 mL × 3) and dried in vacuum to give
a light yellow solid (70 mg, 96.5%). Two isomers 7a (35%) and
7b (65%) were observed by NMR. A yellow crystal of RuH(BH4)-
{ethP2(NH)2} was obtained by the vapor diffusion of hexane into
a C6D6 solution of 7 under Ar after 3 days. Anal. Calcd for
C51H53N3OP2Ru: C, 69.06; H, 6.02; N, 4.74. Found: C, 69.21; H,
550 Magna-IR spectrometer. Microanalyses were performed at the
University of Toronto.
Synthesis of RuHCl{ethP2(NH)2} (1). RuHCl(PPh3)3 (673 mg,
0.729 mmol) and ethP2(NH)2 (470 mg, 0.772 mmol) were dissolved
in THF (12 mL) under a nitrogen atmosphere. The purple solution
was refluxed for 1.5 h to give a yellow precipitate. The mixture
was filtered and washed with diethyl ether and dried under vacuum
to give a yellow powder. The yellow powder was composed of
two isomers 1a and 1b with different configurations of the nitrogens
(yield: 370 mg, 68.1%). Anal. Calcd for C40H39ClN2P2Ru: C,
1
64.38; H, 5.27; N, 3.75. Found: C, 63.82; H, 5.58; N, 3.65. H
2
NMR (C6D6): δ -17.62 (t, 1H, RuH, JHP 23 Hz, 1a), -18.50
(dd, 1H, RuH, 2JHP 23, 28 Hz, 1b), 0.88-2.16 (m, 8H, CH2, 1a +
1b), 2.92-3.34 (m, 8H, HNCH2Ph, 1a + 1b), 3.51 (m, 1H, NH,
1b), 4.02 (m, 1H, NH, 1a), 4.35 (m, 1H, NH, 1b), 5.49 (m, 1H,
NH, 1a), 6.76-7.68 (m, 28H, ArH, 1a), 6.00-8.25 (m, 28H, ArH,
1b). 31P{1H} NMR (C6D6): δ 65.7 (br s, 1a), 64.26 (d, 1b), 60.29
1
2
6.35; N, 4.39. H NMR (C6D6): δ -14.26 (t, 7a, RuH, JHP 25.8
Hz,), -0.97 (br, BH4, 7a+7b), 0.88 (m, 2H, 7a, CH2), 1.95 (m,
2H, 7a CH2), 3.30-3.60 (m, 2H, 7a HNCH2Ph), 3.90 (m, 1H, 7a
NH), 4.38 (m, 1H, 7a NH), 5.08 (m, 2H, 7a HNCH2Ph), 6.69-
2
(d, JPP 26.5 Hz).
2
Synthesis of RuH(NHCOPh){tmeP2(NH)2} (6). RuH{tmeP2N-
(NH)} (50 mg, 0.068 mmol), degassed water (50 mg, 3.0 mmol),
and benzonitrile (20 mg, 0.20 mmol) were dissolved in THF (3
mL) and refluxed for 3 h under H2. The solvent was removed by
vacuum to give a brown residue. Et2O (5 mL) was added to the
mixture, and this was stirred overnight to give a yellow precipitate.
The mixture was filtered and washed with Et2O (2 mL × 3) and
hexane (2 mL × 3) and dried in vacuum to give a yellow solid (41
mg, 68%). A red crystal of RuH(NHCOPh){tmeP2(NH)2} was
obtained by the vapor diffusion of hexane into a C6D6 solution of
RuH(NHCOPh){tmeP2(NH)2} under Ar after a week. Anal. Calcd
for C51H53N3OP2Ru: C, 69.06; H, 6.02; N, 4.74. Found: C, 69.21;
H, 6.35; N, 4.39. 1H NMR (C6D6): δ 10.71 (m, 1H, NH...O), 8.60
(t, 3JHH 7 Hz, 2H, ArH), 8.0-6.8 (m, 29H, ArH), 6.03 (t, 3JHH 7.5
Hz, 2H, ArH), 4.81 (s, 1H, NHCO), 4.28 (m, 1H, CH2), 4.11 (m,
2H, CH2), 3.90 (m, 1H, CH2), 3.48 (m, 1H, NH), 1.28 (s, 3H, CH3),
1.20 (s, 3H, CH3), 1.10 (s, 3H, CH3), 0.83 (s, 3H, CH3), -13.66
8.15 (m, 56H, ArH, 7a+7b), -15.08 (dd, 7b RuH, JHP 21.8 Hz,
25.6 Hz), 1.22 (m, 1H, 7b CH2), 1.43 (m, 1H, 7b CH2), 2.25 (m,
1H, 7b CH2), 2.38 (m, 1H, 7b CH2), 2.70 (br, 1H, 7b NH), 4.10
(br s, 1H, 7b NH). 31P NMR (C6D6): δ 65.43 (s, 7a), 66.4 (d, 2JPP
2
32 Hz, 7b), 64.7 (d, JPP 32 Hz, 7b).
Catalysis. The toluene utilized for the hydrogenation reactions
was dried and distilled over sodium and further stirred with KH
under argon for 18 h immediately prior to use. The KH/toluene
mixture was then filtered through a Celite plug to remove
undissolved solids. The substrates were passed through a plug of
alumina immediately prior to use. All catalyst and substrate
solutions were prepared in an argon drybox. The atmosphere was
tested immediately prior to preparation of catalyst solutions and
was found to contain minimal amounts of oxygen and water.
Hydrogenation reactions were done at a constant pressure of 14
atm of H2 gas using a 50 mL Parr hydrogenation reactor. The
temperature was maintained at 20 °C using a constant temperature
water bath. The Parr reactor was flushed several times with
2
(dd, 1H, RuH, JHP 20.8, 23.6 Hz). 31P{1H} NMR (C6D6): δ

Benzonitrile Hydrogenation
Organometallics, Vol. 26, No. 24, 2007 5943
Scheme 4. Proposed Hydrogenation Mechanisma
Figure 1. Hydrogenation of benzonitrile in toluene catalyzed by
(a) 1/KOtBu (entry 2, Table 4); (b) 1/KOtBu/KH (entry 4); (c) 2
(entry 7). The error bars reflect the variation in several experiments.
a Only the drawing of 1 shows the complete ligand structure.
mmol), and benzonitrile (100 mg, 1.0 mmol) were mixed in acetone-
d6 (1 mL). The mixtures were heated at 80 °C. The conversion to
1
benzamide was determined by H NMR spectroscopy.
Computation. All calculations were performed using Gaussi-
an03.30 The calculations were done on a workstation with two 2.8
GHz Opteron X2 with 4 GB of RAM and Red Hat Linux Enterprise
Edition. All calculations used the rMPW1PW91 density functional
method. Ruthenium was treated with the SDD basis set to include
relativistic effects and an effective core potential, and H, C, N,
and P were treated with the triple-ú basis set 6-311++G** which
includes diffuse functionals and additional p orbitals on H as well
as additional d orbitals on C, N, and P. The structures were
optimized in the gas phase at 1 atm of pressure and 298 K. Full
vibrational analyses were performed on the optimized structure to
obtain values of ∆H, ∆G, and ∆S. The QST2 or QST3 method
was utilized to locate transition states. All transition states were
found to have one imaginary frequency. IRC calculations were used
to show that the reactants led to the products via these transition
states.
Solvation by toluene was modeled using the polarized continuum
model (PCM). Single point energy calculations were performed on
the gas-phase optimized structures. The following modifications
to the PCM model were used: the bondi radii were used to specify
explicit hydrogen atoms, and “scfvac” was added to calculate
∆Gsolvation. The default solvent parameters included in Gaussian03
were used for toluene.
Figure 2. Hydrogenation in toluene (7 atm) of benzonitrile using
RuHCl{ethP2(NH)2}/KOtBu for the first 180 min and then adding
Ru(H2)2H2(PCy3)2.
hydrogen gas at the pre-set pressure prior to adding the components
of the reaction mixture. The benzonitrile (103 mg, 1.0 mmol), KOt-
Bu (5 mg), and KH (3 mg), suspended in toluene (4 mL), were
added to the reactor first, followed immediately by a suspension
of RuHCl{ethP2(NH)2} (4 mg, 5 × 10-3 mmol) in toluene (1 mL).
The ruthenium complex had somewhat variable solubility, possibly
due to the presence of the two isomers. Aliquots of the reaction
mixture were quickly withdrawn with a syringe under a flow of
hydrogen at timed intervals by venting the Parr reactor. Samples
were added to CHCl3 in air to destroy the catalyst and terminate
the reaction. Concentrations of benzonitrile and benzyl amine were
determined by gas chromatography with a Chrompack capillary
column (Chirasil-Dex CB 25 m*0.25 mm) and H2 as carrier gas at
a column pressure of 6 psi, an oven temperature of 110 °C, an
injector temperature of 250 °C, and an FID at 275 °C. The retention
times for benzonitrile and benzylamine are 4.2 and 5.5 min,
respectively.
The initial DFT studies utilized the simple models for the
catalysts, RuH(NHCH2CH2NH2)(PH3)2 and RuH2(en)(PH3)2. These
results were then extended to a more complete model of the
catalysts. Here tetradentate ligands were employed with PPh2 groups
replaced by PH2 groups, i.e., RuH(PH2C6H4CH2NCH2-CH2-
NHC6H4PH2) A and trans-RuH2({PH2C6H4CH2NHCH2-}2) B.
Acetonitrile was used to model benzonitrile to reduce the size of
the calculation.
Results
X-ray Diffraction Structure Determination of RuH(NHCOPh)-
{tmeP2(NH)2} (6) and RuH(BH4){ethP2(NH)2} (7). Crystals
suitable for X-ray diffraction were obtained by vapor diffusion.
Data were collected on a Nonius Kappa-CCD diffractometer using
Mo K radiation (λ 0.71073 Å). The CCD data were integrated and
scaled using the DENZO-SMN software package, and the structure
was solved and refined using SHELXTL V6.0. The crystallographic
data are listed in Table 1 and selected bond distances and angles
in Tables 2 and 3. The hydrides were located and refined with
isotropic thermal parameters.
Preparation of trans-RuHCl{ethP2(NH)2} (1). This com-
plex is prepared in 68% yield as a yellow powder by reacting
RuHCl(PPh3)3 with the tetradentate ligand ethP2(NH)2,24 by the
same method used for making RuHCl{cyP2(NH)2} (Scheme
2).24
1
The H and 31P NMR spectra of RuHCl{ethP2(NH)2} (1)
signal the presence of two isomers 1a and 1b that are very
similar to those of the reported complex RuHCl{tmeP2(NH)2}
(3a and 3b).25 Isomer 1a exhibits a broad peak at 65.7 ppm in
Hydration of Benzonitrile Catalyzed by RuH{tmeP2N(NH)}
or RuH(BH4){enP2(NH)2}. RuH{tmeP2N(NH)} or RuH(BH4)-
{enP2(NH)2} (4 mg, 0.005 mmol), degassed water (180 mg, 10
(30) Frisch, M. J., et al. Gaussian 03, revision D.01; 2003. See the
Supporting Information for the complete author list.

5944 Organometallics, Vol. 26, No. 24, 2007
Li et al.
Figure 3. Reaction coordinate diagram (free energies at 298 K) for the reaction of the amido complex A with dihydrogen in the gas phase
(s) and with toluene solvation (‚‚‚).
dissolved, and the rate is somewhat variable. The selectivity to
the primary amine is >99% under these conditions. The catalyst
only hydrogenates the nitrile in a nitrile/cyclohexene mixture
(entry 5). Without 1 there is no hydrogenation (entry 6). The
1/base combination is more active and selective9 than the known
catalyst 2 at 20 °C (entry 7). However, the system with 1 does
not hydrogenate acetonitrile, presumably because of the acidity
of its CH bonds.
When a combination of the bis dihydrogen complex Ru-
(H2)2H2(PCy3)2 (2) and complex 1 in 1:1 ratio is used as a
catalyst for the hydrogenation of benzonitrile in benzene at the
lower pressure of 7 atm H2 pressure, the conversion reaches
99.5% in 3 h (Table 4, entry 8). Complex 1 with base is not
Figure 4. Calculated structures of the end-on dihydrogen adduct
C (Ru‚‚‚H 3.48; H-H 0.74, Ru-N(amido) 2.01 Å), the transition
active at 7 atm H2, but when 2 is added, the hydrogenation
proceeds rapidly (Figure 2), at a rate much greater than that of
state for the heterolytic splitting of H2 E* (Ru‚‚‚H 1.80, H‚‚‚H 1.02,
1 or 2 alone. In the absence of 1, complex 2 has an induction
H‚‚‚N 1.40, Ru-N(amido) 2.17 Å), and the dihydride product B
period of about 90 min before the hydrogenation of substrate
(Ru-H 1.69, H‚‚‚H 2.48, H‚‚‚N 1.02, Ru-N 2.18 Å).
starts. This induction period is not affected by the presence of
KOtBu or benzylamine product. Therefore, there seems to be a
the 31P{1H} NMR spectrum and a triplet for the hydride at
-17.6 ppm in the H NMR. This spectrum is very similar to
synergic effect of the combination of 1 and 2 on the rate of
hydrogenation.
1
Catalytic Hydrogenation of Benzonitrile with a Combina-
tion of Ru(H2)2H2(PCy3)2 and RuHCl{tmeP2(NH)2} or RuH-
{tmeP2N(NH)}. Complex 3 with the methylated diamine
backbone is a less active catalyst for nitrile hydrogenation (entry
9, Table 4). The combination of Ru(H2)2H2(PCy3)2 and RuHCl-
{tmeP2(NH)2} with base is more active, with a conversion of
99% after 22 h (entry 10). The rates of these reactions are less
than those involving 1, presumably because the bulky methyl-
ated backbone of 3 interferes with catalysis.
that of the crystallographically characterized isomer 3a which
has two NH hydrogen atoms syn to the hydride ligand. Isomer
1b exhibits two doublets at 64.26 and 60.29 ppm in the 31P
NMR spectrum and a doublet of doublets for the hydride at
-18.50 ppm with P-H coupling constants (2JHP 23, 28 Hz)
appropriate for mutually cis hydride and phosphorus nuclei. This
spectrum is similar to that of 3b, so therefore isomer 1b, like
3b, is proposed to have one NH hydrogen syn to the chloride
and the other NH syn to the hydride.
To eliminate the influence of base and to simplify the system,
the hydridoamido complex RuH{tmeP2N(NH)}25 (4) was also
tested for benzonitrile hydrogenation. There is 9% conversion
after 3 h (Table 4, entry 11), comparable with the result for
complex 3 with base (Table 4, entry 9).
Hydrogenation of Benzonitrile. Table 4 outlines the devel-
opment of the catalytic system with 1 (Scheme 3) and a
comparison to the known homogeneous catalyst 2. Complex 1
in toluene under 14 atm H2 at 20 °C is not a catalyst for PhCN
hydrogenation (entry 1, Table 4). However, when activated with
KOtBu under these conditions, it becomes an active catalyst
for the selective hydrogenation of benzonitrile to benzylamine
(Table 4, entry 2; Figure 1). There is no conversion when wet
benzonitrile (entry 3) or wet toluene solvent is used or when
isopropanol is used as the solvent. The addition of potassium
hydride to remove traces of water improved the rate of
conversion (entry 4; Figure 1), making this a very active catalyst
for benzonitrile hydrogenation. The base is not completely
In order to better understand the mechanism of the combined
catalysts for the hydrogenation of benzonitrile, a reaction was
carried out in a pressure NMR tube. The known compounds
RuHCl{tmeP2(NH)2},25 Ru(H2)2H2(PCy3)2,27 and KOtBu in a
molar ratio of 1:1.1:1.8 were mixed under Ar in C6D6 to produce
a red solution. The known compounds RuH{tmeP2(N(NH)} (4),
trans-RuH2{tmeP2(NH)2} (5),25 the dimeric species Ru2H6-
1
(PCy3)4,15 and Ru(H2)2H2(PCy3)2 are detected by H and 31P

Benzonitrile Hydrogenation
Organometallics, Vol. 26, No. 24, 2007 5945
Figure 5. Reaction of acetonitrile with the trans-dihydride complex B via the hydrogen-bonded nitrile adduct F and the transition state G*
to give the amido complex A plus imine via the hydrogen-bonded imine adduct H. The solid line refers to free energies at 298 K for the
system in the gas phase, and the dotted line, for the system with toluene solvation.
NMR. This observation is explained by the transfer of dihy-
drogen from the dihydrogen complex 2 to the amido complex
4 to produce the dihydride 5 and the dimer. Then, this red
solution was reacted with hydrogen to give a yellow-brown
solution containing only 5 and 2 and some free PCy3. Therefore,
the synergetic effect might, in part, result from complex 2
supplying dihydrogen rapidly to 4 so that the active dihydride
5 can form. However, theory predicts that the activation of
dihydrogen by 4 is not the turnover limiting step, and so there
may be another, as yet unidentified, explanation for this synergic
effect. The addition of benzonitrile led to a dark red solution
with benzylamine formation and the formation of the complex
RuH(PhCONH){tmeP2(NH)2} (6), resulting from the reaction
of the complex 4 or 5 with benzonitrile and traces of water
(see below).
Mechanism of Hydrogenation. Conventional mechanisms
of nitrile hydrogenation involve hydride migration to a κ1-N
σ-bonded or κ2-NC π-bonded NCPh ligand.5,9 The high activity
and selectivity of the catalyst system 1/base point to a different
mechanism: the successive hydrogenation of the CN triple bond
and then the CN double bond in outer sphere reactions (Scheme
4). The cycle is entered via the reaction of complex 1 with base
to produce the hydridoamido complex I. Heterolytic splitting
of dihydrogen across the ruthenium amido bond generates
catalyst II as in the ketone hydrogenation mechanism. A
possible, new step is the outer sphere transfer of hydride from
ruthenium and proton from nitrogen to the nitrile CN bond on
going from II to the unstable imine intermediate PhCHdNH
and I via transition state III*. The imine intermediate must then
be immediately hydrogenated in a similar H-/H+ transfer via
transition state IV* (an HOL mechanism23) to account for the
high selectivity of the catalyst. The high hydrogenation activity
of the dihydride species with a coordinatively saturated and 18
electron ruthenium(II) center is more consistent with an outer
sphere mechanism than an inner sphere one where a donor of
the tetradentate ligand would have to dissociate to allow the
nitrile to coordinate to the metal and be attacked by the hydride.
In keeping with this idea, the CdC bond of cyclohexene is not
hydrogenated under these conditions.
Figure 6. Calculated structures of the hydrogen-bonded imine
adduct (F, C-N(nitrile) 1.15, N(nitrile)‚‚‚H 2.31, H-N(amine)
1.01; Ru-N(amine) 2.19, Ru-H 1.70, H(hydride)‚‚‚C(nitrile) 3.15
Å, C-C-N(nitrile) 178°), the transition state for outer sphere
transfer of hydride from Ru and proton from amine (G*; dimensions
of the 6-membered RuHCNHN ring: Ru-H 1.78, H‚‚‚C 1.50, C-N
1.19, N(nitrile)‚‚‚H 1.78, H-N(amine) 1.05, N(amine)-Ru 2.14
Å, C-C-N 142°), and the adduct of the newly born imine with
the amido complex A (H, Ru‚‚‚HC 3.1, H-C 1.10, C-N 1.27,
N-H 1.02, H(imine)‚‚‚N(amido) 2.24, N(amido)-Ru 2.02 Å,
C-C-N 123°).
tmeP2(NH)2 system. The complexes 4 and 5 with the latter
ligand are also nitrile hydrogenation catalysts, although they
are much less active than 1/base, giving 9% conversion after 3
h (entry 11, Table 4) and 16% after 18 h. They are also much
less active ketone hydrogenation catalysts than ones with the
cyP2(NH)2 ligands, presumably because of steric interference
in the catalytic process by the methyl groups in the backbone
of the tmeP2(NH)2 ligand.25 An alternative mechanism might
involve anionic hydride complexes as has been proposed for
ruthenium triphenylphosphine systems.11,12 However, we have
no NMR evidence for such species.
DFT Calculations. Scheme 4 is supported by DFT calcula-
tions30 using the MPW1PW91 functional on the simplified
complexes RuH(PH2C6H4CH2NCH2-CH2NHC6H4PH2) A and
trans-RuH2({PH2C6H4CH2NHCH2-}2) B as models for the
Catalysts I and II have not yet been directly observed for
the ethP2(NH)2 ligand system but are well characterized for the

5946 Organometallics, Vol. 26, No. 24, 2007
Li et al.
Figure 7. Reaction of the imine MeCHdNH with the trans-dihydride complex B via the hydrogen-bonded imine adduct J and the transition
state K* to give the amido complex A plus the amine MeCH2NH2 via the hydrogen-bonded amine adduct L. The solid line refers to free
energies at 298 K for the system in the gas phase, and the dotted line, for the system with toluene solvation.
Scheme 5
Scheme 6
Figure 8. Calculated structures of the hydrogen-bonded imine
adduct (J, C-N(imine) 1.27, N(imine)‚‚‚H(amine) 2.10, H-N(amine)
1.02; Ru-N(amine) 2.19, Ru-H 1.69, H(hydride)‚‚‚C(imine) 4.0
Å), the transition state for outer sphere transfer of hydride from
Ru and proton from amine (K*; dimensions of the 6-membered
RuHCNHN ring: Ru-H 1.77, H..C 1.57, C-N 1.32, N‚‚‚H 1.74,
H-N 1.06, N-Ru 2.14 Å), and the adduct of the newly born amine
with the amido complex A (L, Ru‚‚‚N(amine product) 2.32,
N(amido)-Ru 2.12 Å).
well the observed structure of RuH{tmeP2N(NH)}25 (Ru-
amido catalyst I and the dihydride catalyst II, respectively, of
Scheme 4 and CH3CN as a substitute for benzonitrile. It is
unlikely that this substitition will greatly change the energetics
of polar bond hydrogenation. The reason that acetonitrile is not
hydrogenated by our catalyst system is due to the reactivity of
the C-H bonds of the acetonitrile (work in progress) in a side
reaction that poisons the catalyst, not because of a much higher
hydrogenation activation energy for acetonitrile than benzoni-
trile.
The gas-phase and toluene-phase free energies and structures
of the species in each step of the reaction are plotted in Figures
3-8. Calculations utilizing the even more simplified models
RuH(PH3)2(HNCH2CH2NH2) and trans-RuH2(PH3)2(en) give
very similar results. The geometry calculated for the structure
A (Ru-N(amido) 2.01, Ru-N(amine) 2.18, Ru-H 1.57, Ru-
P(trans to amido) 2.24, Ru-P(trans to amine) 2.22 Å) matches
N(amido) 2.001(2), Ru-N(amine) 2.164(2), Ru-H 1.54(3),
Ru-P(trans to amido) 2.2495(7), Ru-P(trans to amine) 2.2303-
(7) Å). Similarly, that of B (Ru-N(amine) 2.18, Ru-H 1.69,
Ru-P 2.21 Å) agrees with the observed structure trans-RuH2-
{tmeP2(NH)2}25 (Ru-N(amine) 2.196(2) and 2.182(2), Ru-H
1.6, Ru-P 2.2265(6) and 2.2205(6) Å).
Each of the three steps in the catalytic cycle of Scheme 4,
dihydrogen splitting, nitrile hydrogenation, and imine hydro-
genation, have similar free energies of activation (15-17 kcal/
mol) in the gas phase. When solvation is included by use of
the PCM model, the nitrile and imine hydrogenation steps have
activation free energies of 21-22 kcal/mol, higher than that of
the H2 splitting reaction at 16 kcal/mol. Therefore, the substrate
hydrogenation steps are predicted to be the rate-limiting steps.
This contrasts with the hydrogenation of ketones where the H2

Benzonitrile Hydrogenation
Organometallics, Vol. 26, No. 24, 2007 5947
Figure 10. Molecular structure of RuH(BH4){enP2(NH)2} (7,
isomer b).
Scheme 7
Figure 9. Molecular structure of RuH(NHCOPh){tmeP2(NH)2} (6).
splitting step is turn-over limiting.31 Overall, the hydrogenation
of acetonitrile to the ethylamine is calculated to be exothermic
by 20 kcal/mol in the gas phase and 19 kcal/mol in solution.
In the H2 splitting step of Scheme 4 (Figures 3 and 4), the
amido complex A reacts with dihydrogen to give successively
more unstable adducts, first end-on (C) (Figure 4) and then side-
on (D, Ru-H 1.83, H-H 0.81, Ru-N(amido) 2.12 Å). The
transition state E* lies 16 kcal/mol (∆Gq) above the reactants
with or without solvation by toluene. The ∆Hq value is 7.0 kcal/
mol. This agrees with the experimental values of 14.5 and 7.6,
respectively, for the complex RuH(NHCMe2CMe2NH2)(PPh3)2.31
This transition state has an imaginary vibrational mode (1248.43i
cm-1) that involves the heterolytic splitting of the H-H bond
toward producing the aminedihydride complex B. The hetero-
lytic splitting of dihydrogen using the simplified models RuH-
(PH3)2(HNCH2CH2NH2) and trans-RuH2(PH3)2(en) has already
been thoroughly investigated using the B3LYP functional with
comparable results.31,32
The reaction of acetonitrile with the dihydride catalyst B
involves three steps (Figures 5 and 6). First, a hydrogen-bonded
nitrile adduct is formed (F). Then in a novel, concerted process,
the hydride from ruthenium and proton from an amine hydrogen
of the ligand are transferred to the nitrile triple bond in the outer
coordination sphere of the metal via transition state G* (this
corresponds to III* in Scheme 4). The transition state G* lies
15 (∆Gq, gas phase) or 21 (∆Gq, toluene) kcal/mol above the
reactants. An animation of the vibrational mode with the
imaginary frequency (290i cm-1) shows the carbon and nitrogen
of the CN bond moving toward the hydride and proton,
respectively, while the carbon of the CN bond moves to
accept the hydride. This produces the hydrogen-bonded imine
adduct H.
(∆Gq gas phase) or 22 kcal/mol (∆Gq toluene) above the
reactants. This corresponds to transition state IV* in Scheme
4. Yamakawa and co-workers proposed a similar transition state
for the hydrogenation of CH2dNH by Ru(C6H6)H(NH2CH2-
CH2O)33 at 9.8 kcal/mol above reactants, but we are not aware
of a report about the outer sphere hydrogenation of a nitrile.
Poisoning of the Catalyst by Water. As indicated above,
an NMR scale reaction of dihydrido complex 5 with hydrogen
and benzonitrile led to the formation of the amidato complex 6
along with benzylamine (Scheme 5). Complex 6 can also be
prepared by the reaction of the amido complex 4 with water
and benzonitrile.
Complex 6 was isolated as a yellow powder, crystallized,
and characterized by X-ray crystallography (Figure 9). The
ruthenium is octahedral with the hydride ligand situated trans
to a benzamidate ligand. There is a strong intramolecular
hydrogen bond (H(1)-O(1) 2.1 Å) between the amide and the
amine group of the tetradentate ligand defining a six member
ring of Ru(1)-N(3)-C(45)-O(1)-H(1)-N(1). The short C(45)-
N(3) distance of 1.304(5) Å is consistent with some double bond
character. A preliminary crystal structure of the complex RuH-
(NHCOPh){ethP2(NH)2} showed the same similar features.
Neither this complex nor complex 6 is an active catalyst for
nitrile hydrogenation, and so they represent poisoned catalyst
structures. The complex RuTp(PPh3)(H2O)(NHC(O)CH3)34 has
an acetoamido ligand in a six member Ru-N-C-O-H-O ring
involving a cis aqua ligand; the bond distances are very similar
to those of 6. The former complex is prepared by the reaction
of the hydrido nitrile complex RuTp(PPh3)H(NCCH3) with
water. The hydride that is cis to the nitrile is thought to assist
in activating the water.
In a set of steps similar to that of nitrile hydrogenation, the
imine MeCHdNH reacts with dihydride B to produce the amine
product MeCH2NH2 and regenerate the amido catalyst A
(Figures 7 and 8). The transition state K* (437i cm-1) lies 16
Complex 6 exhibits a doublet of doublets at 10.71 ppm in
1
the H NMR spectrum for the NH involved in the hydrogen
(33) Yamakawa, M.; Ito, H.; Noyori, R. J. Am. Chem. Soc. 2000, 122,
(31) Abdur-Rashid, K.; Clapham, S. E.; Hadzovic, A.; Harvey, J. N.;
Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2002, 124, 15104-15118.
(32) Hedberg, C.; Ka¨llstro¨m, K.; Arvidsson, P. I.; Andersson, P. G.;
Brandt, P. J. Am. Chem. Soc. 2005, 127, 15083-15090.
1466-1478.
(34) Leung, C. W.; Zheng, W. X.; Wang, D. X.; Ng, S. M.; Yeung, C.
H.; Zhou, Z. Y.; Lin, Z. Y.; Lau, C. P. Organometallics 2007, 26, 1924-
1933.

5948 Organometallics, Vol. 26, No. 24, 2007
Li et al.
Figure 11. Reaction coordinate diagram for the hydrogenation of acetonitrile to ethylamine catalyzed by the model complex RuH(PH2C6H4-
CH2NCH2CH2NHC6H4PH2). The solid line refers to free energies of the system at 298 K while the dotted line refers to that with toluene
solvation.
Table 5. Hydration of Benzonitrile in Acetone-d6
catalyst
[cat.] (M)
[sub] (M)
[H2O] (M)
T (°C)
t (h)
conv %
RuH{tmeP2N(NH)}
RuHBH4{ethP2(NH)2}
KOH
5 × 10-3
5 × 10-3
5 × 10-3
1.1
1.0
1.1
10
10
10
80
80
80
40
44
44
25
7
7
bond with the amide, a singlet at 4.81 ppm for the proton of
the NH group of the amide, a multiplet at 3.48 ppm for the
other NH, and a doublet of doublets at -13.66 ppm for the
Ru-H. There is an AB pattern at 65.66 and 64.81 ppm in the
31P{1H} NMR spectrum. The IR absorptions at 1590 cm-1 (m)
and 1554 cm-1 (m) are similar to those of a Pt amide complex
reported by Maresca et al.35 while the complex RuTp(PPh3)-
(H2O)(NHC(O)CH3) displays a carbonyl absorption at 1540
a dihydrogen bond39,40 from one NH with an H‚‚‚H distance of
about 2.0 Å.
The two isomers in benzene-d6 solution are in a ratio of 7a:
7b ) 1:2 according to the NMR spectra. The NMR patterns
are very similar to those of isomers 1a and 1b. Therefore, we
assign isomer 7a to a structure in which the N-H hydrogens
are both syn to the hydride. This has a triplet at -14.26 ppm
2
with a coupling constant JHP ) 25.8 Hz in the 1H NMR
spectrum and a singlet at 65.43 ppm in the 31P{1H} NMR
spectrum. Isomer 7b gives a doublet of doublets at -15.08 ppm
cm-1 34
.
Preparation and Structure of RuH(BH4){ethP2(NH)2} (7).
The identification of the structure of complex 6 suggested to
us the possibility that such ruthenium complexes could serve
as catalysts for the hydration of benzonitrile to benzamide. Since
KOH or KOtBu/H2O will catalyze the hydration of benzonitrile,
we sought ruthenium catalysts that would operate under base-
free conditions. RuH(BH4)(binap)(diamine) was prepared by
Ohkuma et al. as a catalyst for hydrogenation of simple ketones
under base-free conditions,36 and we have used similar com-
plexes RuH(BH4)(binap)(P-nor) for base-free tandem asym-
metric Michael addition-hydrogenation37 and complexes RuH-
(BH4)(binop)(diamine) for this base-free tandem reaction as well
as for asymmetric transfer hydrogenation reactions.38 A suspen-
sion of complex 1 in benzene was heated at 65 °C with an excess
of NaBH4 in ethanol. The light yellow complex trans-RuH-
(BH4){ethP2(NH)2} (7) is obtained in 96% yield as a mixture
of isomers (Scheme 6). The structure of one enantiomer of 7b
as determined by X-ray crystallography is shown in Figure 10.
There are two enantiomers in the unit cell. One has the R, R
configuration at the stereogenic nitrogen atoms while the other
(Figure 10) has the S, S configuration. The boron hydride accepts
1
(2JHP ) 21.8 Hz, 25.6 Hz) in the H NMR spectrum and two
doublets at 66.4 and 64.7 ppm (2JPP ) 32 Hz) in the 31P{1H}
NMR spectrum. The BH4- moiety gives a broad signal at -0.97
1
ppm in the H NMR, very similar to the resonance of RuH-
(BH4)(binap)(1,2-diamine) complexes.36
Nitrile Hydration. Complexes 4 and RuH(BH4){ethP2(NH)2}
were tested as nitrile hydration catalysts (Scheme 7, Table 5).
Complex 4 is a moderately active catalyst at 80 °C in
comparison to others reported in the literature.41-49 It produced
more benzamide than that observed for the control reaction using
a similar concentration of KOH as the catalyst. However, it
deactivated over time so that the complete conversion was not
obtained. The complex RuH(BH4){ethP2(NH)2} gave a similar
conversion to KOH and is a poor catalyst.
Conclusions
In the presence of base, the complexes RuHCl{ethP2(NH)2}
(1) and RuHCl{tmeP2(NH)2} (3) are active in the catalytic
hydrogenation at 14 atm H2 of benzonitrile to benzylamine, with
1 being more active than 3, probably for steric reasons. The
dihydrogen complex Ru(H2)2H2(PCy3)2, 2, has a synergic effect
when mixed with 1 or 3 and base, making an even more active
system. The complexes with tetradentate ligand systems are
extremely moisture sensitive in the presence of KOtBu and
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J. Am. Chem. Soc. 1995, 117, 12875-12876.

Benzonitrile Hydrogenation
Organometallics, Vol. 26, No. 24, 2007 5949
PhCN and form stable benzamide complexes such as RuH-
(NHCOPh){tmeP2(NH)2}. When KH is used to remove the trace
amount of water in the catalytic hydrogenation of benzonitrile
with RuHCl{ethP2(NH)2} and base, the hydrogenation is
particularly rapid and selective. A mechanism is proposed that
involves the successive outer sphere transfer of dihydrogen first
to the CN triple bond and then to the CN double bond. This is
the first demonstration that nitrile hydrogenation can proceed
via an outer sphere mechanism. The overall theoretical reaction
coordinate diagram (Figure 11) predicts that the activation
energy for the dihydrogen splitting will be similar to that of
nitrile hydrogenation and imine hydrogenation in the gas phase
while the predicted activation energies for the outer sphere
hydrogenation steps are greater than H2 splitting when solvation
is taken into account. There are difficulties in conducting kinetic
runs that prevent the testing of these conclusions for the present
system. Future experimental and theoretical work will probe
reactions of the amido complex 4 with the acidic C-H bonds
of acetonitrile in order to explain the catalyst poisoning by this
substrate.
The complex RuH{tmeP2N(NH)} (4) is a poorly active base-
free nitrile hydrogenation catalyst and moderately active hydra-
tion catalyst while the borohydride complex is not active.
Limitations of our catalyst systems for nitrile hydrogenation to
be overcome are their high water and oxygen sensitivity and
their intolerance to nitriles with acidic groups.
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K.; Takai, K. Organometallics 2005, 24, 6287-6290.
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Acknowledgment. R.H.M. thanks NSERC Canada for a
discovery grant and Johnson Matthey for a loan of ruthenium
chloride. We thank Professor Dmitri Goussev for help with the
DFT calculations.
(46) Breno, K. L.; Pluth, M. D.; Tyler, D. R. Organometallics 2003, 22,
1203-1211.
Supporting Information Available: X-ray crystallographic files
for complexes 6 and 7. Details of the DFT calculations and the
complete ref 30. This material is available free of charge via the
Internet at http://pubs.acs.org.
(47) Breno, K. L.; Pluth, M. D.; Landorf, C. W.; Tyler, D. R.
Organometallics 2004, 23, 1738-1746.
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