Trichlorostannyl complexes of iridium with both P-donor and N-donor ligands: Preparation and activity as hydrogenation catalysts

Article abstract of DOI:10.1016/j.jorganchem.2009.05.019

Bis(trichlorostannyl) complex IrH(SnCl₃)₂(PPh₃)₂ (1) was prepared by allowing the chloro-derivative IrHCl₂(PPh₃)₃ to react with SnCl₂·2H₂O in ethanol. Instead, treatment of phosphite complexes IrHCl₂P₃ [P = P(OEt)₃ and PPh(OEt)₂] with SnCl₂·2H₂O gave stannyl derivatives IrCl₂(SnCl₃)P₃ (2). Pyrazole-trichlorostannyl complexes IrHCl(SnCl₃)(HRpz)P₂ (3, 4) (R = H, 3-Me; P = PPh₃, PⁱPr₃) were prepared by allowing chloro-derivatives IrHCl₂(HRpz)P₂ to react with SnCl₂·2H₂O. 1,2-Bipyridine-trichlorostannyl complexes IrHCl(SnCl₃)(bpy)P (5) (P = PPh₃, PⁱPr₃) were also prepared. Complexes 1-5 were characterised spectroscopically (IR, ¹H, ³¹P, ¹¹⁹Sn NMR) and a geometry in solution was also established. The trichlorostannyl iridium complexes were evaluated as catalyst precursors for the hydrogenation of 2-cyclohexen-1-one and cinnamaldehyde. The influence of the stannyl group, as well as the steric hindrance of both N-donor and P-donor ligands in the catalytic activity of the complexes is discussed.

Full text of DOI:10.1016/j.jorganchem.2009.05.019

Journal of Organometallic Chemistry 694 (2009) 3142–3148
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Trichlorostannyl complexes of iridium with both P-donor and N-donor ligands:
Preparation and activity as hydrogenation catalysts
*
Gabriele Albertin , Stefano Antoniutti, Stefano Paganelli
Dipartimento di Chimica, Università Ca’ Foscari Venezia, Dorsoduro 2137, 30123 Venezia, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 April 2009
Received in revised form 14 May 2009
Accepted 16 May 2009
Available online 21 May 2009
Bis(trichlorostannyl) complex IrH(SnCl3)2(PPh3)2 (1) was prepared by allowing the chloro-derivative
IrHCl2(PPh3)3 to react with SnCl2Á2H2O in ethanol. Instead, treatment of phosphite complexes IrHCl2P3
[P = P(OEt)3 and PPh(OEt)2] with SnCl2Á2H2O gave stannyl derivatives IrCl2(SnCl3)P3 (2). Pyrazole–trichlo-
rostannyl complexes IrHCl(SnCl3)(HRpz)P2 (3, 4) (R = H, 3-Me; P = PPh3, PiPr3) were prepared by allowing
chloro-derivatives IrHCl2(HRpz)P2 to react with SnCl2Á2H2O. 1,2-Bipyridine-trichlorostannyl complexes
IrHCl(SnCl3)(bpy)P (5) (P = PPh3, PiPr3) were also prepared. Complexes 1–5 were characterised spectro-
scopically (IR, 1H, 31P, 119Sn NMR) and a geometry in solution was also established. The trichlorostannyl
iridium complexes were evaluated as catalyst precursors for the hydrogenation of 2-cyclohexen-1-one
and cinnamaldehyde. The influence of the stannyl group, as well as the steric hindrance of both N-donor
and P-donor ligands in the catalytic activity of the complexes is discussed.
Keywords:
Iridium complexes
Trichlorostannyl ligand
Hydrogenation
Pyrazole and 1,2-bipyridine
Phosphines and phosphites
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Stannyl complexes [M]–SnX3 and [M]–SnR3 (X = halogen, R = al-
kyl or aryl group) of transition metals have been extensively stud-
ied in recent years [1–3], both for the variety of reactions they may
undergo, including ligand-substitution at both metal and tin cen-
tres, and because the introduction of a stannyl ligand may modify
the properties of complexes and often improve their catalytic
activity [4].
However, despite the large number of stannyl complexes pre-
pared, relatively few involve iridium as a metal centre [5], although
several complexes of this metal have shown interesting catalytic
properties.
We are interested in the chemistry of stannyl complexes of
transition metals and have recently reported [6] the synthesis
and reactivity of chloro-, hydride- and organo-stannyl complexes
of Mn(I), Re(I), Ru(II) and Os(II) of the type M(SnR3)(CO)nL5-n
(M = Mn, Re; n = 2, 3), M(SnR3)(Tp)L(PPh3) and M(SnR3)(Cp)L(PPh3)
(M = Ru, Os; R = Cl, H, Me, ArC„C; L = phosphite). We have now ex-
tended these studies to iridium(III) in order to test whether new
stannyl complexes can be prepared and how the introduction of
a stannyl group can change their catalytic properties. The synthesis
of mixed-ligands stannyl complexes of Ir(III) and some studies on
their catalytic activity in the hydrogenation of 2-cyclohexen-1-
one and cinnamaldehyde are reported in this paper.
2.1. General comments
All synthetic work was carried out in an appropriate
atmosphere (Ar) using standard Schlenk techniques or an inert
atmosphere dry-box. Once isolated, the complexes were found to
be relatively stable in air, but were stored under nitrogen at
À25 °C. All solvents were dried over appropriate drying agents, de-
gassed on a vacuum line, and distilled into vacuum-tight storage
flasks. IrCl3Á3H2O was a Pressure Chemical Co. (USA) product, used
as received. Phosphite PPh(OEt)2 was prepared by the method of
Rabinowitz and Pellon [7]. Other reagents were purchased from
commercial sources in the highest available purity and used as
received. Infrared spectra were recorded on Nicolet Magna 750
or Perkin–Elmer Spectrum-One FT-IR spectrophotometers. NMR
spectra (1H, 31P, 119Sn) were obtained on AC200 or AVANCE 300
Bruker spectrometers at temperatures between À90 and +30 °C,
unless otherwise noted. 1H spectra are referred to internal tetra-
methylsilane; 31P{1H} chemical shifts are reported with respect
to 85% H3PO4, and 119Sn with respect to Sn(CH3)4, and in both cases
downfield shifts are considered positive. COSY, HMQC and HMBC
NMR experiments were performed with standard programs. The
SWAN-MR and INMR software packages [8] were used to treat NMR
data. The conductivity of 10À3 mol dmÀ3 solutions of the com-
plexes in CH3NO2 at 25 °C was measured on a Radiometer CDM
83. Elemental analyses were determined in the Microanalytical
Laboratory of the Dipartimento di Scienze Farmaceutiche, Univer-
sity of Padova (Italy).
* Corresponding author. Fax: +39 041 234 8917.
E-mail address: albertin@unive.it (G. Albertin).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.05.019

G. Albertin et al. / Journal of Organometallic Chemistry 694 (2009) 3142–3148
3143
2.2. Synthesis of precursor compounds
2.3.3. IrHCl(SnCl3)(HRpz)P2 (3, 4) [R = H (a), 3-Me (b); P = PPh3 (3),
PiPr3 (4)]
The chlorocomplex precursors mer- and fac-IrHCl2(PPh3)3,
In a 50-mL three-necked round-bottomed flask were placed
0.5 mmol of the appropriate chlorocomplex IrHCl2(HRpz)P2, an ex-
cess of SnCl2Á2H2O (5 mmol, 1.13 g) and 25 mL of ethanol. The
resulting suspension was refluxed for 3 h and then the volume
was reduced to about 10 mL by evaporation of the solvent under
reduced pressure. The pale yellow solid formed was filtered and
crystallised from CH2Cl2 and ethanol; yield P75%. Anal. Calc. for
C39H35Cl4IrN2P2Sn (3a): C, 44.76; H, 3.37; Cl, 13.55; N, 2.68. Found:
IrHCl2(PiPr3)2, IrHCl2(HRpz)P2 (P = PPh3, PiPr3; R = H, 3-Me),
IrHCl2P3 [P = P(OEt)3, PPh(OEt)2] and the aryltriazenide IrHCl(g2
-
1,3-PhNNNPh)(PPh3)2 were prepared following previously
reported methods [9–13].
2.2.1. IrHCl2(bpy)P (P = PPh3, PiPr3)
In a 50-mL three-necked round-bottomed flask were placed
0.68 mmol of the appropriate precursor IrHCl2P3, an excess of
2,2’-bipyridine (1.0 mmol, 0.16 g) and 15 mL of 1,2-dichloroethane.
The resulting solution was refluxed for 3 h and then the solvent re-
moved under reduced pressure. The oil obtained was triturated
with ethanol (3 mL) giving a yellow solid which was filtered and
crystallised from CH2Cl2 and ethanol; yield P30% for P = PPh3,
P75% for P = PiPr3. IrHCl2(bpy)(PPh3): Anal. Calc. for
C28H24Cl2IrN2P: C, 49.27; H, 3.54; Cl, 10.39; N, 4.10. Found: C,
49.49; H, 3.63; Cl, 10.12; N, 4.05%. 1H NMR (CD2Cl2, 25 °C) d
(ppm): 9.45–7.35 (m, 23H, Ph+bpy), À19.73 (d, 1H, J1HP31 = 18 Hz,
IrH). 31P{1H} NMR (CD2Cl2, 25 °C) d (ppm): 6.7 (s). IrHCl2(bpy)(PiPr3):
Anal. Calc. for C19H30Cl2IrN2P: C, 39.31; H, 5.21; Cl, 12.21; N, 4.83.
Found: C, 39.54; H, 5.13; Cl, 12.45; N, 4.70%. IR (KBr, cm-1): 2229
C, 44.54; H, 3.23; Cl, 13.77; N, 2.55%. IR (KBr, cmÀ1): 3265 (m) mNH
2308 (w) mIrH
1H NMR (CD2Cl2, 25 °C) d (ppm): 10.70 (s, br, 1H,
NH), 7.70–7.31 (m, 30H, Ph), 7.15 (s, br, 1H, H5 Hpz), 6.48 (s, br,
1H, H3 Hpz), 5.92 (t, 1H, H4 Hpz), À20.58 (t, 1H, J = 11.1,
,
.
1H31
P
J
= 197.1,
J
= 189.1 Hz, IrH). 31P{1H} NMR (CD2Cl2,
1H119Sn
1H117Sn
25 °C) d (ppm): A2, À0.14 (s, J
= 228.0). 119Sn{1H} NMR
31H117Sn
(CD2Cl2, 25 °C) d (ppm): A2M, dM -651.9, JAM = 239.5. Anal. Calc.
for C40H37Cl4IrN2P2Sn (3b): C, 45.31; H, 3.52; Cl, 13.37; N, 2.64.
Found: C, 45.49; H, 3.42; Cl, 13.22; N, 2.76%. IR (KBr, cmÀ1): 3270
(m)
br, 1H, NH), 7.70–7.27 (m, 30H, Ph), 6.97 (s, br, 1H, H5 Hpz), 5.68
(s, br, 1H, H4 Hpz), 1.55 (s, 3H, CH3), À20.52 (t, 1H, J = 11.1,
mNH, 2153 (m) mIrH.
1H NMR (CD2Cl2, 25 °C) d (ppm): 10.23 (s,
1H31
P
J
= 195.3, J
= 187.2, IrH). 31P{1H} NMR (CD2Cl2, 25 °C,) d
1H119Sn
1H117Sn
(s) mIrH
2.70 (m, 3H, CH), 1.41, 1.37 (d, 18H, CH3), À25.25 (d, 1H,
= 18 Hz, IrH). 31P{1H} NMR (CD2Cl2, 25 °C) d (ppm): 1.20 (s).
.
1H NMR (CD2Cl2, 25 °C) d (ppm): 9.47–7.48 (m, 8H, bpy),
(ppm): A2, À0.89 (s, J
= 227.5). 119Sn{1H} NMR (CD2Cl2,
31H117Sn
25 °C) d (ppm): A2M, dM À645.6, JAM = 237.0. Anal. Calc. for
C21H47Cl4IrN2P2Sn (4a): C, 29.94; H, 5.62; Cl, 16.84; N, 3.33. Found:
J
1H31
P
C, 29.83; H, 5.75; Cl, 16.70; N, 3.26%. IR (KBr, cmÀ1): 3285 (m) mNH
,
2.3. Synthesis of complexes
2071 (m) mIrH
NH), 7.12 (s, br, 1H, H5 Hpz), 6.50 (m, 2H, H4+H3 Hpz), 2.75 (m,
6H, CH phos), 1.54, 1.48 (d, 36H, CH3), À14.17 (t, 1H, J = 16.0,
.
1H NMR (CD2Cl2, 25 °C) d (ppm): 10.20 (s, br, 1H,
2.3.1. IrH(SnCl3)2(PPh3)2 (1)
1H31
P
In a 25-mL three-necked round-bottomed flask were placed
0.30 g (0.29 mmol) of IrHCl2(PPh3)3, 0.65 g of SnCl2Á2H2O
(2.9 mmol) and 25 mL of ethanol. The resulting suspension was
refluxed for 3 h and then the volume was reduced to about
10 mL by evaporation of the solvent under reduced pressure. The
pale yellow solid formed was filtered and crystallised from CH2Cl2
and ethanol; yield P75%. Anal. Calc. for C36H31Cl6IrP2Sn2: C, 37.02;
H, 2.68; Cl, 18.21. Found: C, 37.24; H, 2.80; Cl, 18.46%. IR (KBr,
J
= 49.0, J
= 47.5, IrH). 31P{1H} NMR (CD2Cl2, 25 °C,) d
1H119Sn
1H117Sn
(ppm): A2, 31.8 (s, J
= 173.5). 119Sn{1H} NMR (CD2Cl2, 25 °C)
31P117Sn
d (ppm): A2M, dM À400.6, JAM = 182.0.
2.3.4. IrHCl(SnCl3)(bpy)P (5) [P = PPh3 (a), PiPr3 (b)]
In a 50-mL three-necked round-bottomed flask were placed
0.42 mmol of the appropriate complex precursor IrHCl2(bpy)P
(P = PPh3, PiPr3), an excess of SnCl2Á2H2O (6.0 mmol, 1.35 g) and
40 mL of ethanol. The resulting suspension was refluxed for 4 h
and then the volume reduced to about 15 mL by evaporation of
the solvent under reduced pressure. An orange solid separated
out which was filtered and crystallised from CH2Cl2 and ethanol;
yield P70%. Anal. Calc. for C28H24Cl4IrN2PSn (5a): C, 38.56; H,
2.77; Cl, 16.26; N, 3.21. Found: C, 38.73; H, 2.65; Cl, 16.07; N,
cmÀ1): 2126 (m) mIrH
(m, 30H, Ph), À12.86 (t, 1H, IrH); (À70 °C) ABX spin syst, dM
À12.64, JAX = 9.5, JBX = 10.0, = 97.1, Sn2 = 960.8,
= 92.0, J
= 917.7 Hz. 31P{1H} NMR (CD2Cl2, 25 °C,) d
.
1H NMR (CD2Cl2, 25 °C) d (ppm): 7.44–6.95
J
J
1H119
1H119Sn1
J
1H117Sn
1H117Sn
(ppm): À3.29 s, br; (À70 °C) AB, dA À1.39, dB -5.66, JAB = 13.2.
119Sn{1H} NMR (CD2Cl2, -70 °C) d (ppm): ABM1, dM1 -395.8,
JAM1 = 2564.5, JBM1 = 295.0; ABM2, dM2 À421.7, JAM2 = 183.0,
JBM2 = 2526.0.
3.32%. IR (KBr, cmÀ1): 2186 (m) mIrH
(ppm): 9.42–7.21 (m, 23H, Ph+bpy), -19.45 (d, 1H, J
.
1H NMR (CD2Cl2, 25 °C) d
= 21.0,
1H31
P
J
= 102.7, J
= 98.6 Hz, IrH). 31P{1H} NMR (CD2Cl2, 25 °C)
1H119Sn
1H117Sn
2.3.2. IrCl2(SnCl3)P3 (2) [P = P(OEt)3 (a), PPh(OEt)2 (b)]
d (ppm): A, À2.20 (s, J
= 198.0). 119Sn{1H} NMR (CD2Cl2,
31H117Sn
In a 25-mL three-necked round-bottomed flask were placed
0.2 mmol of the appropriate hydride IrHCl2P3, an excess of
SnCl2Á2H2O (2.0 mmol, 0.45 g) and 10 mL of ethanol. The resulting
solution was stirred at room temperature for 3 h and the yellow so-
lid formed was filtered and crystallised from CH2Cl2 and ethanol;
yield P90%. Anal. Calc. for C18H45Cl5IrO9P3Sn (2a): C, 21.91; H,
4.60; Cl, 17.97. Found: C, 21.76; H, 4.52; Cl, 17.75%. IR (polyethyl-
25 °C)
d
(ppm): AM, dM À328.4, JAM = 206.0. Anal. Calc. for
C19H30Cl4IrN2PSn (5b): C, 29.63; H, 3.93; Cl, 18.41; N, 3.64. Found:
C, 29.45; H, 3.89; Cl, 18.64; N, 3.53%. IR (KBr, cmÀ1): 2148 (m) mIrH
.
1H NMR (CD2Cl2, 25 °C) d (ppm): 9.74–7.31 (m, 8H, bpy), 2.28 (m,
3H, CH phos), 1.22, 1.17, 0.94 0.90 (d, 18H, CH3), À18.44 (d, 1H,
J
P = 15.8,
J
= 114.6,
J
= 110.7, IrH). 31P{1H} NMR
1H117Sn
1H31
1H119Sn
(CD2Cl2, 25 °C,) d (ppm): A, 14.3 (s, J
= 3475). 119Sn{1H}
31P117Sn
ene, cmÀ1): 333, 310 (m)
(m, 18H, CH2), 1.34, 1.33 (t, 27H, CH3). 31P{1H} NMR (CD2Cl2, 25 °C)
d (ppm): A2B spin syst, dA 56.6, dB 45.9, JAB = 34.3, J = 360.1,
m
IrCl. 1H NMR (CD2Cl2, 25 °C) d (ppm): 4.33
NMR (CD2Cl2, 25 °C) d (ppm): AM, dM À280.3, JAM = 3637.
2.3.5. General procedure for hydrogenation experiments
31H119Sn
J
= 342.6, J
= 6107.0, J = 5839.2 Hz. Anal. Calc.
31H117Sn
A 150-mL stainless steel reaction vessel was charged, under a
nitrogen purge, with 5.2 mmol of substrate, 0.0104 mmol of the
catalytic precursor and 5 mL of solvent. The reactor was then pres-
surised with 50 atm of hydrogen, heated at 80–100 °C for the due
time (see Tables 1 and 2), cooled to room temperature and the
gas allowed to vent off. For analytical purposes, the target products
were recovered from the reaction mixture by flash silica gel chro-
matography (n-hexane/ether, 8/2).
31H117Sn
31H119Sn
for C30H45Cl5IrO6P3Sn (2b): C, 33.28; H, 4.19; Cl, 16.37. Found: C,
33.40; H, 4.33; Cl, 16.55%. 1H NMR (CD2Cl2, 25 °C) d (ppm): 7.92–
6.87 (m, 15H, Ph), 4.32–3.96, 3.73–3.43 (m, 12H, CH2), 1.33, 1.14
(t, 18H, CH3). 31P{1H} NMR (CD2Cl2, 25 °C) d (ppm): A2B, dA 83.9,
dB 74.2, JAB = 24.8,
J
= 280.8,
J
= 4948.0. 119Sn{1H}
31H117Sn
31H117
NMR (CD2Cl2, 25 °C) d (ppm): A2BM, dMSn À535.0, JAM = 293.8,
JBM = 5177.6.

3144
G. Albertin et al. / Journal of Organometallic Chemistry 694 (2009) 3142–3148
Table 1
P
Hydrogenation of 2-cyclohexen-1-one catalysed by iridium complexes.
H
exc. SnCl2•2H2O
– PPh3
IrHCl2(PPh3)3
P
Ir
Run
Cat.
T (°C)
Conv. (%)
B yield (%)
SnCl3
1
2
3
4
5
6
7
8
9
10
IrHCl2(Hpz)(PPh3)2
80
80
100
80
80
100
80
80
100
80
100
69.0
94.6
5.6
–
1.0
–
–
100
69.0
94.6
5.6
–
1.0
–
–
SnCl3
IrHCl(SnCl3)(Hpz)(PPh3)2 (3a)
IrHCl(SnCl3)(Hpz)(PPh3)2 (3a)
IrHCl2(Hpz)(PiPr3)2
1 ( I )
IrHCl(SnCl3)(Hpz)(PiPr3)2 (4a)
IrHCl(SnCl3)(Hpz)(PiPr3)2 (4a)
IrHCl2(H-3-Mepz)(PPh3)2
Scheme 1.
IrHCl(SnCl3)(H-3-Mepz)(PPh3)2 (3b)
IrHCl(SnCl3)(H-3-Mepz)(PPh3)2 (3b)
IrCl2(SnCl3)[P(OEt3)]3 (2a)
4.4
57.5
4.4
57.5
SnCl3
2-Cyclohexen-1-one = 5.2 mmol; catalyst = 0.0104 mmol; substrate/catalyst (molar
ratio) = 500/1; toluene = 5 mL; p(H2) = 50 atm; reaction time = 22 h.
Cl
Cl
P
exc. SnCl2•2H2O
IrHCl2P3
Ir
P
P
3. Results and discussion
2 ( II )
P = P(OEt)3 (a), PPh(OEt)2 (b)
3.1. Preparation of complexes
Scheme 2.
Both mer- and fac-iridium(III) complexes IrHCl2(PPh3)3 [9] react
with SnCl2Á2H2O to give hydride-bis(trichlorostannyl) derivative
IrH(SnCl3)2(PPh3)2 (1), which was isolated in good yield and char-
acterised (Scheme 1).
The reaction proceeds with the insertion of SnCl2 into both Ir–Cl
bonds of the starting complex and concurrent loss of one triphen-
ylphosphine, to yield pentacoordinate final derivative 1.
Related hydride complexes IrHCl2P3 [12] [P = P(OEt)3 and
PPh(OEt)2] also react with SnCl2Á2H2O, but in this case the
reaction gives exclusively dichloro(trichlorostannyl) complexes
IrCl2(SnCl3)P3 (2), as shown in Scheme 2.
two Ir–Cl bonds, yielding pentacoordinate bis(stannyl) complex 1.
Instead, with the less bulky phosphites, no dissociation of ligand
was observed and octahedral monostannyl complex 2 resulted.
However, the reaction with SnCl2 probably involves several steps
which lead to substitution of the hydride ligand.
Hydride complexes IrHCl2(HRpz)P2 [11] (R = H, 3-Me; P = PPh3,
PiPr3), containing pyrazole as a ligand, also react with SnCl2Á2H2O
to give the trichlorostannyl derivatives IrHCl(SnCl3)(HRpz)P2 (3,
4), as shown in Scheme 3.
No loss of phosphite was observed, and the reaction seemed to
proceed only with substitution of the hydride by the SnCl3 group,
The reaction proceeds with the insertion of SnCl2 into only one
Ir–Cl bond to give hydride–chlorostannyl complexes 3, 4, which
were isolated in good yield and characterised.
À
giving final dichloro complex 2. Its formation was somewhat unex-
pected, in view of the results obtained with IrHCl2(PPh3)3 (Scheme
1), but was explained on the basis of the initial insertion of SnCl2
into one Ir–Cl bond to give the intermediate IrHCl(SnCl3)P3, in
which the SnCl3 group may be trans to the hydride. The well-
known trans labilising effect [14] of SnCl3, in the presence of excess
SnCl2, should favour the substitution of hydride by one ClÀ to yield
the final complex IrCl2(SnCl3)P3 (2). However, the insertion of SnCl2
into the Ir–H bond, giving IrCl2(SnHCl2)P3, followed by chloride ex-
change of the tin–hydride group SnHCl2 yielding 2, cannot be
excluded.
In every case, the nature of the phosphine ligand (P) is impor-
tant in determining the nature of the reaction product of hydrides
IrHCl2P3 with SnCl2Á2H2O. Although triphenylphosphine yielded
pentacoordinate hydridebis(trichlorostannyl) complex 1, octahe-
dral complex 2 was exclusively formed with phosphites P(OEt)3
and PPh(OEt)2. In the first case, the dissociation of the bulky tri-
phenylphosphine probably favours the insertion of two SnCl2 into
The easy formation of stannyl complexes stabilised by pyrazole
ligands prompted us to extend reactions with SnCl2 to other irid-
ium complexes containing N-donor ligands. Our attention focused
on both 1,2-bipyridine (bpy) complexes IrHCl2(bpy)P and 1,3-
diaryltriazenide derivatives IrHCl(1,3-PhNNNPh)P2 (P = PPh3,
PiPr3), and results showed that, whereas the 1,2-bipyridine com-
plexes react quickly with SnCl2Á2H2O to give trichlorostannyl
derivatives IrHCl(SnCl3)(bpy)P (5) (Scheme 4), triazenide com-
pounds do not react with SnCl2Á2H2O and the starting complexes
were recovered unchanged, even after long reaction times in
refluxing ethanol (Scheme 5).
The lack of reactivity of the 1,3-triaryldiazenide complexes to-
wards SnCl2 was quite unexpected, and was attributed to the stoi-
chiometry of the complexes, which contain only one chloride
ligand. The precursors giving trichlorostannyl complexes do con-
tain the dichloro-hydride moiety [IrHCl2], which undergoes
insertion by SnCl2 to afford [Ir]–SnCl3 species. The anionic nature
Table 2
Hydrogenation of cinnamaldehyde catalysed by iridium complexes
Run
Cat.
T (°C)
Conv. (%)
F yield (%)
G yield (%)
H yield (%)
1
2
3
4
5
6
7
8
9
10
IrHCl2(Hpz)(PPh3)2
IrHCl(SnCl3)(Hpz)(PPh3)2 (3a)
IrHCl2(Hpz)(PPh3)2
80
80
33.7
17.7
91.4
60.9
52.1
13.1
6.9
2.0
43.1
71.2
33.7
17.7
75.4
48.3
43.4
12.1
4.0
2.0
20.9
36.7
–
–
–
–
–
8.8
2.3
–
–
–
100
100
100
100
100
100
100
100
16.0
3.8
6.4
1.0
2.9
–
IrHCl(SnCl3)(Hpz)(PPh3)2 (3a)
IrHCl2(Hpz)(PiPr3)2
IrHCl(SnCl3)(Hpz)(PiPr3)2 (4a)
IrHCl2(H-3-Mepz)(PPh3)2
IrHCl(SnCl3)(H-3-Mepz)(PPh3)2 (3b)
IrCl2(SnCl3)[P(OEt3)]3 (2a)
IrH(SnCl3)2(PPh3)2 (1)
19.3
7.5
2.9
26.9
Cinnamaldehyde = 5.2 mmol; catalyst = 0.0104 mmol; substrate/catalyst (molar ratio) = 500/1; toluene = 5 mL; p(H2) = 50 atm; reaction time = 17 h.

G. Albertin et al. / Journal of Organometallic Chemistry 694 (2009) 3142–3148
3145
P
Cl
H
exc. SnCl2•2H2O
IrHCl2(HRpz)P2
Ir
HRpz
SnCl3
P
P = PPh3; R = H (a), 3-Me (b)
3 ( III )
P
Ir
P
Ir
H
H
Cl
Hpz
Cl
exc. SnCl2•2H2O
IrHCl2(Hpz)P2
P = PiPr3
or
4
Hpz
SnCl3
SnCl3
P
P
( IV )
( V )
Scheme 3.
SnCl3
SnCl3
P
H
Cl
N
N
exc. SnCl2•2H2O
IrHCl2(bpy)P
Ir
or
Ir
N
N
H
Cl
P
5a ( VI )
5b ( VII )
P = PPh3 (a), PiPr3 (b)
Scheme 4.
Ph
N
P
H
exc. SnCl2•2H2O
N
Ir
P
Δ
Cl
N
Fig. 1. Observed (bottom) and calculated (top) 1H NMR spectra, in the hydride
region, of complex 1 in CD2Cl2 at À70 °C. The simulated spectrum was obtained
using the parameters reported in Section 2.
Ph
P = PPh3, PiPr3
Scheme 5.
those of the hydride ligand which, at À80 °C, appears as an ABX
multiplet (X = 1H) at À12.64 ppm (Fig. 1).
of 1,3-triazenide yields monochloro complexes IrHCl(1,3-
PhNNNPh)P2, which turn out to be unreactive towards SnCl2 inser-
tion. This lack of reactivity may be due to the trans influence of 1,3-
triazenide itself, which makes both HÀ and ClÀ ligands trans to the
PhNNNPh unreactive towards insertion of the SnCl2 group.
The new trichlorostannyl complexes 1–5 were isolated as
yellow or orange solids, stable in air and in solution of common
organic solvents, in which they behave as non-electrolytes. Analyt-
ical and spectroscopic data (IR and 1H, 31P, 119Sn NMR) support the
proposed formulations.
Variable-temperature NMR spectra indicate that pentacoordi-
nate complex IrH(SnCl3)2(PPh3)2 (1) is fluxional. The singlet
appearing at 20 °C in the 31P spectra changes as the temperature
is lowered and, at À80 °C, results in a sharp quartet with the char-
acteristic 119Sn and 117Sn satellites, due to coupling with the two
tin nuclei. The spectra can be simulated with an AB model with
the parameters reported in Section 2, and indicate the magnetic
non-equivalence of the two phosphine ligands. The two stannyl
groups are also magnetically non-equivalent, matching the pres-
ence at À80 °C of two well-separated multiplets at À421.7 and at
À395.8 ppm in the 119Sn NMR spectrum. The multiplicity of the
signals is due to coupling with the phosphorus nuclei of the two
PPh3 and the spectra can be simulated with two ABM models
(M = 119Sn) with the parameters reported in the Section 2. The 1H
NMR spectra also support the proposed formulation for 1, showing
the signals of the phenyl protons of the triphenylphosphine and
For a pentacoordinate complex, trigonal–bipyramidal (TBP)
geometry may be reasonably proposed and, in the case of complex
1, the NMR data fit one of type I (Scheme 1). Although square–
pyramidal geometry cannot be completely excluded, literature
data [15] lead us to propose a TBP structure of type I for our bis(tri-
chlorostannyl) complex 1.
In the far region, the IR spectrum of phosphite complex
IrCl2(SnCl3)[P(OEt)3]3 2a shows two bands at 333 and 310 cm-1
attributed to the mIrCl of the two chloride ligands in a mutually cis
position. In the temperature range between +20 and À80 °C, the
31P NMR spectra of both compounds 2a and 2b appear as A2B mul-
tiplets, with the characteristic satellites due to coupling with 117Sn
and 119Sn nuclei. However, further support for the presence of the
stannyl ligand comes from the 119Sn NMR spectra of 2b, which
show one multiplet at À535 ppm due to coupling with 31P nuclei.
The spectra may be simulated with an A2BM model (M = 119Sn)
with the parameters reported in Section 2. The values of J
119Sn31
P
also indicate that the SnCl3 group is probably in trans position with
respect to one phosphite ligand. On the basis of these data,
fac geometry II (Scheme 2) is proposed for trichlorostannyl
compound 2.
The IR spectra of pyrazole complexes IrHCl(SnCl3)(HRpz)(PPh3)2
3 show a weak band at 2308–2153 cmÀ1, attributed to the mIrH of
the hydride and the medium-intensity absorption at 3265–
3270 cmÀ1 characteristic of mNH of the pyrazole ligand. The 1H
NMR spectra show not only the signals of the phenyl protons of

3146
G. Albertin et al. / Journal of Organometallic Chemistry 694 (2009) 3142–3148
PPh3, but also a relatively broad signal at 10.70–10.23 ppm, attrib-
uted to the NH proton of the pyrazole. Further support for the pres-
ence of this ligand comes from the signals observed at 7.15, 6.48,
5.92 (3a) and 6.97, 5.68 ppm (3b) which were correlated in a COSY
experiment and attributed to the CH proton of Hpz and H-3-Mepz,
respectively. A sharp triplet also appears in the low frequency re-
gion at À20.58 (3a) (Fig. 2) and À20.52 ppm (3b), with the charac-
teristic satellites due to coupling with 117Sn and 119Sn nuclei,
attributed to the hydride ligand. The high values observed for the
troscopic data do not unambiguously assign a geometry to com-
pound 4a, and only X-ray studies will allow us to decide
between geometries IV and V (Scheme 3).
The IR spectra of 2,2’-bipyridine complexes IrHCl(SnCl3)(bpy)P
5 show a medium-intensity band at 2186–2148 cmÀ1 attributed
to the mIrH of the hydride ligand. The 1H NMR spectra confirm its
presence, showing a doublet at À19.45 (5a) and À18.44 ppm
(5b), with the characteristic satellites of 119Sn and 117Sn nuclei,
whose J
values of 102.7 (5a) and 114.6 Hz (5b) suggest a
1H119Sn
J
of 197.1 (3a) and 195.3 Hz (3b) also suggest a mutually
mutually cis position between the hydride and the SnCl3 ligand.
The values of J (21 and 15.8 Hz) also suggest a mutually cis po-
1H119Sn
trans position for the hydride and stannyl groups. At temperatures
between +20 and À80 °C, the 31P NMR spectra show a sharp sin-
glet, matching the magnetic equivalence of the two phosphine li-
gands. The 119Sn NMR spectrum also appears as a sharp triplet at
À651.9 (3a) (Fig. 3) and À645.6 ppm (3b), due to coupling with
the two equivalent phosphorus nuclei, indicating the presence of
the SnCl3 group. On the basis of these data, trans geometry (III)
may reasonably be proposed for pyrazole complexes 3.
1H31
P
sition between the hydride and phosphite ligands.
The 119Sn NMR spectrum of IrHCl(SnCl3)(bpy)(PPh3) 5a appears
as a doublet at À328.4 ppm, with a J
value of 206 Hz, sug-
119Sn31
P
gesting a mutually cis position between the SnCl3 group and the
PPh3 ligand. On the basis of these data, cis–cis geometry VI may
be proposed for triphenylphosphine complex 5a.
The 119Sn NMR spectrum of the related IrHCl(SnCl3)(bpy)(PiPr3)
The 1H NMR spectra of the related triisopropylphosphine
IrHCl(SnCl3)(Hpz)(PiPr3)2 (4a) shows the pyrazole NH proton at
10.2 ppm, whereas the hydride resonance appears as a triplet at
À14.17 ppm, with the satellites of 117Sn and 119Sn. In contrast with
5b appear as a doublet at À280.3 ppm, but with a J
value of
119Sn31
P
3637 Hz, which suggests a mutually trans position of SnCl3 with
respect to phosphine. On the basis of these data, cis–trans geome-
try VII is proposed for triisopropylphosphine complex 5b.
3, the value of J
of 49.0 Hz suggests a mutually cis position of
1H119Sn
the hydride and stannyl SnCl3 ligands. In the temperature range
between +20 and À80 °C, the 31P NMR spectrum shows a sharp sin-
glet, fitting the magnetic equivalence of the two phosphine li-
gands; the 119Sn spectrum appears as a triplet at À400.6 ppm,
fitting the proposed formulation for the complex. However, spec-
3.2. Catalytic activity
It is known that iridium(III) complexes are catalytically active in
the hydrogenation of olefins [16] and, with respect to iridium(I)
compounds, have higher tolerance to air oxidation; however, irid-
ium derivatives with the metal atom in oxidation state +III are not
often employed as catalyst precursors [17]. In the last few years,
some iridium(III) complexes, modified with various phosphino li-
gands such as xyliphos or BINAP, for example, have been success-
fully employed in the enantioselective hydrogenation of imines
[17]. To our knowledge, iridium(III) compounds containing the
SnCl3 moiety have never been used as catalyst precursors for the
hydrogenation of unsaturated substrates. Based on the striking re-
sults obtained in processes catalysed by trichlorostannyl Pt(II)
complexes [1b,18], it was interesting to evaluate the influence of
the SnCl3 moiety on the activity of iridium(III) catalysts. Trichloro-
stannyl iridium complexes were employed in the hydrogenation of
the a,b-unsaturated substrates 2-cyclohexen-1-one (A) and cinna-
*
maldehyde (E), in order to examine the selectivity of our catalytic
systems towards hydrogenation of the carbon–carbon and carbon–
oxygen double bonds, respectively (Schemes 6 and 7).
A set of experiments was carried out on 2-cyclohexen-1-one (A)
at 80 °C and 50 atm of H2 for 22 h, and the results are shown in Ta-
ble 1.
We first evaluated the catalytic activity of IrHCl2(Hpz)(PPh3)2,
the precursor of trichlorostannyl derivative IrHCl(SnCl3)-
(Hpz)(PPh3)2 (3a). The catalytic system was very active and also
selective, affording exclusively cyclohexanone (B) as a reaction
product, in quantitative yield (run 1 of Table 1). Also complex 3a,
like the dichloro precursor, selectively furnished saturated ketone
B but in about 70% yield, showing lower catalytic activity. When
the reaction temperature was increased to 100 °C, cyclohexanone
was selectively obtained to about 95%. Surprisingly, when
IrHCl(SnCl3)(Hpz)(PiPr3)2 (4a) was used as the catalyst, the reactiv-
-20.0 -20.1 -20.2 -20.3 -20.4 -20.5 -20.6 -20.7 -20.8 -20.9 -21.0 -21.1
ppm
Fig. 2. Observed (bottom) and calculated (top) 1H NMR spectra, in the hydride
region, of complex 3a in CD2Cl2 at 25 °C. The asterisk indicates an impurity. The
simulated spectrum was obtained using the parameters reported in Section 2.
*
OH
O
O
OH
Ir cat.
H2
-610
-620
-630
-640
-650
ppm
-660
-670
-680
-690
+
+
Fig. 3. Observed (bottom) and calculated (top) 119Sn{1H} NMR spectra of complex
3a in CD2Cl2 at 25 °C. The asterisk indicates an impurity. The simulated spectrum
was obtained using the parameters reported in Section 2.
C
A
B
D
Scheme 6.

G. Albertin et al. / Journal of Organometallic Chemistry 694 (2009) 3142–3148
3147
O
OH
O
OH
Ir cat.
H2
F
E
G
H
Scheme 7.
ity fell dramatically: at best, when the reaction was carried out at
100 °C, cyclohexanone was only obtained in traces (run 6 of Table
1). The dichloro complex not containing the SnCl3 moiety also
turned out to be practically inactive, affording cyclohexanone in
amounts of less than 6%. Owing to the quite good results obtained
with trichlorostannyl complex 3a, containing the triphenylphosph-
ino ligand, we tested the activity of the analogous complex
IrHCl(SnCl3)(H-3-Mepz)(PPh3)2 (3b) with a methyl group on posi-
tion 3 of the pyrazole moiety. Disappointingly, this catalyst was
not able to hydrogenate 2-cyclohexen-1-one and, even at 100 °C,
its catalytic activity was practically negligible (runs 8 and 9 of
Table 1); moreover, its direct precursor IrHCl2(H-3-Mepz)(PPh3)2
did not show any catalytic activity either. Lastly, the reaction
was carried out in the presence of phosphite complex IrCl2(SnCl3)-
[P(OEt3)]3 (2a): this catalyst showed fairly good activity, affording
exclusively cyclohexanone in about 58% yield (run 10 of Table 1).
Interesting results were obtained in the hydrogenation of cinna-
containing the (H-3-Mepz) moiety, being similar to the data
achieved in 2-cyclohexen-1-one hydrogenation. Lastly, when phos-
phite complex IrCl2(SnCl3)[P(OEt3)]3 (2a) was used as catalyst, cin-
namaldehyde hydrogenation afforded a mixture containing the
saturated aldehyde F and both alcohols G and H. Unlike the case
of 2-cyclohexen-1-one hydrogenation, this catalyst precursor also
proved quite good at reducing the carbon–oxygen double bond,
thus furnishing almost equimolecular amounts of 3-phenylprop-
anal (F) and 3-phenylpropanol (G) (run 9 of Table 2).
These iridium(III) complexes show interesting catalytic activity
in the hydrogenation of a,b-unsaturated substrates but, contrary to
our expectations, the tricholorostannyl moiety negatively affected
their catalytic performance. The steric hindrance created by this
bulky group certainly played a detrimental role in the catalytic
cycle, lowering the yield of hydrogenation products with respect
to dichloro catalyst precursors not containing the SnCl3 unit. In
addition, when the triphenylphosphine ligand was replaced by
more hindered phosphino groups such as triisopropylphosphine,
catalytic activity decreased greatly. The detrimental effect due to
steric factors was also highlighted by the dramatic fall in activity
when iridium triphenylphosphino complexes containing the
H-3-Mepz moiety instead of the simple Hpz were used in the
hydrogenation process. As expected, all these iridium complexes
maldehyde. The hydrogenation of a,b-unsaturated aldehydes, par-
ticularly cinnamaldehyde, is an important process in the
manufacture of some useful fine chemicals as intermediates for
the synthesis of pharmaceuticals, additives for food flavours, and
valuable building blocks for fragrances [19]. Many rhodium-,
ruthenium- and iridium-based catalytic systems have been
employed in the selective hydrogenation of
a
,b-unsaturated alde-
mainly reduce the carbon–carbon double bond of the a,b-unsatu-
hydes [20]: iridium complexes generally show lower activity but
higher selectivity. Very recently, cinnamaldehyde was hydroge-
nated in the presence of iridium(I) complexes with tris(ortho-ani-
syl)phosphine and other bulky phosphine ligands: in all cases,
these catalysts prevalently reduced the carbon–carbon double
bond [20].
rated substrates: in particular, in the case of 2-cyclohexen-1-one
hydrogenation, this peculiarity can be exploited for selective
production of the corresponding saturated ketone.
Acknowledgements
An initial experiment was carried out with IrHCl2(Hpz)(PPh3)2
as the catalytic precursor: after 17 h at 80 °C and 50 atm of H2,
the conversion of substrate E was less than 34% and the saturated
aldehyde 3-phenylpropanal (F) was the only reaction product (run
1 of Table 2). When the reaction temperature was increased to
100 °C, the activity of this catalyst increased greatly, reaching
about 92% of substrate conversion, but at the expense of selectiv-
ity: about 75% of 3-phenylpropanal (F) and 16% of 3-phenylpropa-
nol (G) were also found in the reaction mixture (run 3 of Table 2).
Like the results observed in the hydrogenation of 2-cyclohexen-1-
one, trichlorostannyl derivative IrHCl(SnCl3)(Hpz)(PPh3)2 (3a)
showed lower catalytic activity, 3-phenylpropanal only being pro-
duced in about 18% yield (run 2 of Table 2); when the reaction was
carried out at 100 °C, 60.9% of substrate conversion was achieved.
Also in this case, the reaction temperature also affected selectivity:
besides 3-phenylpropanal (F), the prevailing reaction products, 3-
phenylpropanol (G) and cinnamyl alcohol (H), were formed (run
4 of Table 2).
The financial support of MIUR (Rome) – PRIN 2008 is gratefully
acknowledged. We thank Daniela Baldan for her technical
assistance.
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