Dynamics of a cis-dihydrogen/hydride complex of iridium

Article abstract of DOI:10.1021/ic050266c

Insertion of CS₂ into one of the Ir-H bonds of [Ir(H) ₅(PCy₃)₂] takes place to afford the dihydrido dithioformate complex cis-[Ir(H)₂(η²-S ₂CH)(PCy₃)₂] accompanied by the elimination of H₂. Protonation of the dithioformate complex using HBF ₄·Et₂O gives cis-[Ir(H)(η²-H ₂)(η²-S₂CH)(PCy₃) ₂][BF₄] wherein the H atom undergoes site exchange between the dihydrogen and the hydride ligands. The dynamics was found to be so extremely rapid with respect to the NMR time scale that the barrier to exchange could not be measured. Partial deuteration of the hydride ligands resulted in a J(H,D) of 6.5 and 7.7 Hz for the H₂D and the HD₂ isotopomers of cis-[Ir(H)(η²-H₂)(η²- S₂CH)(PCy₃)₂]-[BF₄], respectively. The H-H distance (d_HH) for this complex has been calculated to be 1.05 A, which can be categorized under the class of elongated dihydrogen complexes. The cis-[Ir(H)(η²-H₂)(η²- S₂CH)(PCy₃)₂][BF₄] complex undergoes substitution of the bound H₂ moiety with CH₃CN and CO resulting in new hydride derivatives, cis-[Ir(H)-(L)(η²-S ₂CH)(PCy₃)₂][BF₄] (L = CH ₃CN, CO). Reaction of cis-[Ir(H)₂(η²-S ₂CH)(PCy₃)₂] with electrophilic reagents such as MeOTf and Me₃SiOTf afforded a new hydride aquo complex cis-[Ir(H)(H₂O)(η²-S₂CH)(PCy ₃)₂][OTf] via the elimination of CH₄ and Me₃SiH, respectively, followed by the binding of a water molecule (present in trace quantities in the solvent) to the iridium center. The X-ray crystal structures of cis-[Ir(H)₂(η²-S ₂CH)(PCy₃)₂] and cis-[Ir(H)-(H ₂O)(η²-S₂CH)(PCy₃) ₂][OTf] have been determined.

Full text of DOI:10.1021/ic050266c

Inorg. Chem. 2005, 44, 6203−6210
Dynamics of a cis-Dihydrogen/Hydride Complex of Iridium
H. V. Nanishankar, Saikat Dutta, Munirathinam Nethaji, and Balaji R. Jagirdar*
Department of Inorganic & Physical Chemistry, Indian Institute of Science,
Bangalore 560 012, India
Received February 21, 2005
Insertion of CS2 into one of the Ir
complex cis-[Ir(H)2(
2-S2CH)(PCy3)2] accompanied by the elimination of H2. Protonation of the dithioformate complex
using HBF4 Et2O gives cis-[Ir(H)(
2-H2)( 2-S2CH)(PCy3)2][BF4] wherein the H atom undergoes site exchange between
−H bonds of [Ir(H)5(PCy3)2] takes place to afford the dihydrido dithioformate
η
‚
η
η
the dihydrogen and the hydride ligands. The dynamics was found to be so extremely rapid with respect to the
NMR time scale that the barrier to exchange could not be measured. Partial deuteration of the hydride ligands
resulted in a J(H,D) of 6.5 and 7.7 Hz for the H2D and the HD2 isotopomers of cis-[Ir(H)(
[BF4], respectively. The H H distance (dHH) for this complex has been calculated to be 1.05 Å, which can be
categorized under the class of elongated dihydrogen complexes. The cis-[Ir(H)(
2-H2)( 2-S2CH)(PCy3)2][BF4] complex
undergoes substitution of the bound H2 moiety with CH3CN and CO resulting in new hydride derivatives, cis-[Ir(H)-
(L)( CH3CN, CO). Reaction of cis-[Ir(H)2(
2-S2CH)(PCy3)2][BF4] (L 2-S2CH)(PCy3)2] with electrophilic reagents
such as MeOTf and Me3SiOTf afforded a new hydride aquo complex cis-[Ir(H)(H2O)(
2-S2CH)(PCy3)2][OTf] via the
elimination of CH4 and Me3SiH, respectively, followed by the binding of a water molecule (present in trace quantities
in the solvent) to the iridium center. The X-ray crystal structures of cis-[Ir(H)2(
2-S2CH)(PCy3)2] and cis-[Ir(H)-
(H2O)(
2-S2CH)(PCy3)2][OTf] have been determined.
η η
2-H2)( 2-S2CH)(PCy3)2]-
−
η
η
η
)
η
η
η
η
Introduction
activation barriers for the processes, typically 9-10 kcal/
mol. Heinekey et al. reported that highly symmetric cis
dihydrogen hydride complexes involving no heavy atom
rearrangement show dynamics with much smaller barriers.5a-d
Systems exhibiting such low barriers have been much less
Ever since the first transition metal dihydrogen complex
was reported,1 intensive research activity has taken place to
unravel the possibility of the existence of dihydrogen
complexes as intermediates in the fluxional processes involv-
ing polyhydride complexes.2 The study of the dynamic
processes in complexes containing a hydride and a dihydro-
gen ligand has also attracted enormous attention since such
complexes can be viewed as prototypical examples of
polyhydrides. There have been a large number of reports on
the dynamics of trans3 and the cis dihydrogen hydride
complexes.4 Most of the cis complexes exhibit interesting
dynamics of the H atom site exchange between the dihy-
drogen and the hydride ligands involving significant rear-
rangements of the coligands resulting in relatively higher
(4) (a) Taw, F. L.; Mellows, H.; White, P. S.; Hollander, F. J.; Bergman,
R. G.; Brookhart, M.; Heinekey, D. M. J. Am. Chem. Soc. 2002, 124,
5100-5108. (b) Guari, Y.; Sabo-Etienne, S.; Chaudret, B. J. Am.
Chem. Soc. 1998, 120, 4228-4229. (c) Kranenburg, M.; Kamer, P.
C. J.; van Leeuwen, W. N. M.; Chaudret, B. Chem. Commun. 1997,
373-374. (d) Gusev, D. G.; Hu¨bener, R.; Burger, P.; Orama, O.;
Berke, H. J. Am. Chem. Soc. 1997, 119, 3716-3731. (e) Jia, G.;
Drouin, S. D.; Jessop, P. G.; Lough, A. J.; Morris, R. H. Organome-
tallics 1993, 12, 906-916. (f) Bianchini, C.; Perez, P. J.; Peruzzini,
M.; Zanobini, F.; Vacca, A. Inorg. Chem. 1991, 30, 279-287. (g)
Bampos, N.; Field, L. D. Inorg. Chem. 1990, 29, 587-588. (h)
Bianchini, C.; Peruzzini, M.; Zanobini, F. J. Organomet. Chem. 1988,
354, C19-C22. (i) Crabtree, R. H.; Lavin, M.; Bonneviot, L. J. Am.
Chem. Soc. 1986, 108, 4032-4037.
(5) (a) Pons, V.; Conway, S. L. J.; Green, M. L. H.; Green, J. C.; Herbert,
B. J.; Heinekey, D. M. Inorg. Chem. 2004, 43, 3475-3483. (b)
Oldham, W. J., Jr.; Hinkle, A. S.; Heinekey, D. M. J. Am. Chem. Soc.
1997, 119, 11028-11036. (c) Heinekey, D. M.; Oldham, W. J., Jr. J.
Am. Chem. Soc. 1994, 116, 3137-3138. (d) Heinekey, D. M.;
Mellows, H.; Pratum, T. J. Am. Chem. Soc. 2000, 122, 6498-6499.
(e) Wisniewski, L. L.; Mediati, M.; Jensen, C. M.; Zilm, K. W. J.
Am. Chem. Soc. 1993, 115, 7533-7534. (f) Li, S.; Hall, M. B.; Eckert,
J.; Jensen, C. M.; Albinati, A. J. Am. Chem. Soc. 2000, 122, 2903-
2910.
* To whom correspondence should be addressed. E-mail: jagirdar@
ipc.iisc.ernet.in.
(1) (a) Kubas, G. J. Ryan, R. R.; Swanson, B. I.; Vergamini, P. J.;
Wasserman, H. J. J. Am. Chem. Soc. 1984, 106, 451-452. (b) Kubas,
G. J. Metal Dihydrogen and σ Bond Complexes; Kluwer Academic/
Plenum Publishers: New York, 2001.
(2) Gusev, D. G.; Kuhlman, R. L.; Renkema, K. B.; Eisenstein, O.;
Caulton, K. G. Inorg. Chem. 1996, 35, 6775-6783.
(3) Jessop, P. G.; Morris, R. H. Coord. Chem. ReV. 1992, 121, 155-284.
10.1021/ic050266c CCC: $30.25
Published on Web 08/06/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 18, 2005 6203

Nanishankar et al.
(C6D6): δ 14.1-38.5 (m, P(C6H11)3), 231.5 (s, (η2-S2CH)). 1H NMR
(CD2Cl2): δ -22.24 (t, 2H, Ir-H, J(H,P) ) 17.0 Hz), 1.21-2.03
(m, 66H, P(C6H11)3), 13.48 (br s, 1H, (η2-S2CH)). 31P{1H} NMR
(CD2Cl2): δ 27.6 (s, P(C6H11)3).
studied.5e,f The factors that determine the structure and
dynamics of these types of molecules are subtle and not
clearly understood.
We have been interested in realizing cis-dihydride com-
plexes wherein the hydride ligands are related by a mirror
plane and study the protonation of such complexes in an
effort to understand the factors that determine their structures
and dynamics. During the course of our studies on the
insertion reactions of heterocumulenes such as CO2, CS2,
and COS into M-H bonds,6 we found that CS2 inserts into
one of the Ir-H bonds of [Ir(H)5(PCy3)2]7 to afford a
dihydrido dithioformate derivative cis-[Ir(H)2(η2-S2CH)-
(PCy3)2] (1). The two hydride ligands in this complex are in
cis conformation. The protonation of 1 using HBF4‚Et2O
resulted in the cis-[Ir(H)(η2-H2)(η2-S2CH)(PCy3)2][BF4] (2)
derivative, which shows rapid dynamics for the H atom site
exchange between the dihydrogen and the hydride ligands.
In this paper, we report these studies along with the reactivity
of 1 with certain electrophilic reagents.
Protonation Reaction of [Ir(H)2(η2-S2CH)(PCy3)2] 1 Using
HBF4‚Et2O. A 5 mm NMR tube charged with cis-[Ir(H)2(η2-S2CH)-
(PCy3)2] (15 mg) was evacuated and filled with H2 in three cycles.
The dihydride complex was then dissolved in CD2Cl2 (0.6 mL),
and to this solution was added HBF4‚Et2O (5 equiv, 12 µL). The
1H and 31P NMR spectra revealed the complete conversion of the
dihydride into the cis-[Ir(H)(η2-H2)(η2-S2CH)(PCy3)2][BF4] (2)
complex (excess acid is not required for the complete conversion
of 1 to 2; even with 1 equiv of acid, complete conversion can be
1
achieved). H NMR (CD2Cl2, rt (room temperature)): δ -11.63
1
(br t (br s in the H{31P} NMR), 3H, Ir-H3, J(H3,P) ) 5.9 Hz),
1.36-2.08 (m, 66H, P(C6H11)3), 13.38 (br s, 1H, (η2-S2CH)).
31P{1H} NMR (CD2Cl2, rt): δ 18.4 (s, P(C6H11)3). For the variable-
temperature NMR spectroscopic studies, the protonation was carried
out in a similar manner in CDF2Cl/CDFCl2 solvent mixture at 273
K and the tube was inserted into the NMR probe, precooled to 273
K.
Observation of the H-D Isotopomers of cis-[Ir(H)(η2-H2)-
(η2-S2CH)(PCy3)2][BF4]. The dihydrogen hydride complex cis-
[Ir(H)(η2-H2)(η2-S2CH)(PCy3)2][BF4] (2) was prepared as described
above. Through this solution HD gas (generated from NaH and
D2O) was purged at a steady rate for ca. 10 min. The HD
isotopomers formed were observed by 1H NMR spectroscopy (room
temperature). By this method, the extent of deuteration was found
to be very low.
To obtain higher degree of deuteration, freshly prepared dihy-
drogen hydride samples were purged with D2 gas for 30 min and
then they were analyzed by NMR spectroscopy. By this method,
in addition to the achievement of a greater degree of deuteration,
the corresponding H2D and the HD2 isotopomers were also
observed.
Experimental Section
General Procedures. All reactions were carried out under a N2
or Ar atmosphere at room temperature using standard Schlenk8 and
inert-atmosphere techniques unless otherwise stated. Solvents used
for the preparation of the dihydrogen complexes were thoroughly
saturated with either H2 or Ar just before use. The 1H and 31P NMR
spectral data were acquired using an Avance Bruker 400 and 500
MHz spectrometers. Variable-temperature proton T1 measurements
were carried out at 400 MHz using the inversion recovery method.9
The T1 data have been deposited in the Supporting Information.
31P NMR chemical shifts have been measured relative to 85%
H3PO4 (aqueous solution) as an external standard. Elemental
analysis for 1 was carried out at the Department of Organic
Chemistry, IISc (Thermo Finnigan Flash EA1112 instrument), and
for the other complexes at the Regional Sophisticated Instrumenta-
tion Center, Central Drug Research Institute, Lucknow, India. The
[Ir(H)5(PCy3)2] complex and CDF2Cl/CDFCl2 were prepared by
literature methods.7,10
Preparation of cis-[Ir(H)(CH3CN)(η2-S2CH)(PCy3)2][BF4] (3).
The dihydrogen hydride complex cis-[Ir(H)(η2-H2)(η2-S2CH)-
(PCy3)2][BF4] (2) was prepared as described above by starting from
[Ir(H)2(η2-S2CH)(PCy3)2] (50 mg, 0.06 mmol) and HBF4‚Et2O (5
equiv, 40 µL) in CH2Cl2 (10 mL). Without isolation of the
dihydrogen complex, CH3CN (10 equiv, 30 µL) was added and
the reaction mixture was stirred for 2 h. The volatiles were removed
in vacuo resulting in a sticky residue that was washed several times
with petroleum ether. The reddish-brown product of cis-[Ir(H)(CH3-
CN)(η2-S2CH)(PCy3)2][BF4] (3) was obtained in a yield of 41 mg
(71%). Anal. Calcd for C39H71BF4IrNP2S2‚0.5C7H8: C, 50.78; H,
7.42. Found: C, 50.40; H, 6.80 (the presence of toluene was
Preparation of cis-[Ir(H)2(η2-S2CH)(PCy3)2] (1). To a toluene
solution (10 mL) of [Ir(H)5(PCy3)2] (50 mg, 0.06 mmol) under an
atmosphere of Ar was added CS2 (20 equiv, 80 µL) using a syringe.
The reaction mixture was stirred overnight after which time the
volatiles were stripped leaving behind a yellow solid. The product
of cis-[Ir(H)2(η2-S2CH)(PCy3)2] was crystallized from a toluene
solution via slow evaporation of solvent at room temperature.
Yellow crystals of 1 were obtained in a yield of 42 mg (76%).
Anal. Calcd for C37H69IrP2S2: C, 53.39; H, 8.35. Found: C, 53.94;
1
1
confirmed using H NMR spectroscopy). H NMR (CDCl3): δ
-19.83 (t, 1H, Ir-H, J(H,P) ) 13.0 Hz), 1.20-2.21 (m, 66H,
P(C6H11)3), 2.72 (s, 3H, CH3CN), 12.88 (br s, 1H, (η2-S2CH)).
31P{1H} NMR (CDCl3): δ 11.9 (s, P(C6H11)3). ES-MS: m/z ) 832
[M+ - (CH3CN + BF4-)].
1
H, 8.23. H NMR (C6D6): δ -21.64 (t, 2H, Ir-H, J(H,P) ) 17.0
Hz), 1.30-2.28 (m, 66H, P(C6H11)3), 13.89 (br s, 1H, (η2-S2CH)).
31P{1H} NMR (C6D6): δ 28.1 (s, P(C6H11)3). 13C{1H} NMR
Preparation of cis-[Ir(H)(CO)(η2-S2CH)(PCy3)2][BF4] (4). The
dihydrogen hydride complex cis-[Ir(H)(η2-H2)(η2-S2CH)(PCy3)2]-
[BF4] (2) was prepared as described above starting from [Ir(H)2-
(η2-S2CH)(PCy3)2] (100 mg, 0.12 mmol) and HBF4‚Et2O (4 equiv,
65 µL) in CH2Cl2 (10 mL). Without isolation of the dihydrogen
complex, CO gas (1 atm) was purged through this solution for 5
min during which time the color of the solution turned from reddish
yellow to brown. The reaction mixture was stirred for an additional
1 h, and then the solvent was stripped under vacuum and the
reddish-brown solid of cis-[Ir(H)(CO)(η2-S2CH)(PCy3)2][BF4] (4)
(6) (a) Gandhi, T.; Jagirdar, B. R. Inorg. Chem. 2005, 44, 1118-1124.
(b) Gandhi, T.; Nethaji, M.; Jagirdar, B. R. Inorg. Chem. 2003, 42,
4798-4800. (c) Gandhi, T.; Nethaji, M.; Jagirdar, B. R. Inorg. Chem.
2003, 42, 667-669.
(7) Brinkmann, S.; Morris, R. H.; Ramachandran, R.; Park, S.-H. Inorg.
Synth. 1998, 32, 303-308.
(8) (a) Shriver, D. F.; Drezdon, M. A. The Manipulation of Air-SensitiVe
Compounds, 2nd ed.; Wiley: New York, 1986. (b) Herzog, S.;
Dehnert, J.; Luhder, K. In Techniques of Inorganic Chemistry;
Johnassen, H. B., Ed.; Interscience: New York, 1969; Vol. VII.
(9) Hamilton, D. G.; Crabtree, R. H. J. Am. Chem. Soc. 1988, 110, 4126-
4133.
1
(10) Siegel, J. S.; Anet, F. A. L. J. Org. Chem. 1988, 53, 2629-2630.
was dried under vacuum. Yield: 90 mg (85%). H NMR (CD2-
6204 Inorganic Chemistry, Vol. 44, No. 18, 2005

Dynamics of a cis-Dihydrogen/Hydride Complex of Ir
Table 1. Crystallographic Data for cis-[Ir(H)2(η2-S2CH)(PCy3)2] (1)
and cis-[Ir(H)(H2O)(η2-S2CH)(PCy3)2][OTf] (5)
Cl2): δ -18.70 (t, 1H, Ir-H, J(H,P) ) 12.0 Hz), 1.20-2.20 (m,
66H, P(C6H11)3), 12.40 (br s, 1H, (η2-S2CH)). 31P{1H} NMR
(CD2Cl2): δ 17.5 (s, P(C6H11)3). 13C{1H} NMR (CD2Cl2): δ
18.10-29.60 (m, PCy3), 164.00 (t, Ir-CO, J(C,P) ) 26.8 Hz),
238.09 (s, (η2-S2CH)). IR (KBr): ν(CO) 2026 cm-1. ES-MS: m/z
) 860 [M+ - BF4-]. (Satisfactory elemental analysis data for this
compound could not be obtained because of the formation of
noncombustible metal fluorides during combustion.)
param
1
5
formula
fw
cryst syst
space group
a, Å
C37H69IrP2S2
832.18
monoclinic
P21/c
12.534(9)
16.756(11)
19.027(13)
90.00
107.660(11)
90.00
C52H86F3IrO4P2S3
1182.53
triclinic
P1h
14.486(7)
14.576(7)
16.653(9)
84.049(8)
67.300(7)
61.362(7)
2832(3)
2
b, Å
c, Å
Reaction of [Ir(H)2(η2-S2CH)(PCy3)2] with MeOTf/Isolation
of cis-[Ir(H)(H2O)(η2-S2CH)(PCy3)2][OTf] (5). To a toluene
solution (10 mL) of [Ir(H)2(η2-S2CH)(PCy3)2] (50 mg, 0.06 mmol)
was added MeOTf (3 equiv, 20 µL, 0.18 mmol) using a syringe,
and the reaction mixture was stirred for 1 h. Upon stripping of the
volatiles under vacuo, a thick red sticky residue was obtained which
was washed several times with petroleum ether. The product was
crystallized from its concentrated toluene solution via slow evapora-
tion of the solvent at room temperature. Yield: 30 mg (50%). The
product was identified as cis-[Ir(H)(H2O)(η2-S2CH)(PCy3)2][OTf]
(5) by NMR spectroscopy. Anal. Calcd for C38H70F3IrO4P2S2: C,
R, deg
â, deg
γ, deg
V, Å3
Z
3808(4)
4
1.452
D
calcd, g/cm3
1.385
T, K
293(2)
293(2)
λ, Å
0.710 73
3.724
0.0206
0.710 73
2.574
0.0359
µ, mm-1
Ra
a
Rw
0.0480
0.0848
2
a R ) ∑(|Fo| - |Fc|)/∑|Fo|, Rw ) [∑w(|Fo| - |Fc|)2/∑w|Fo| ]1/2 (based
on reflections with I > 2σ(I)).
1
45.67; H, 7.06. Found: C, 46.02; H, 6.59. H NMR (C6D6): δ
-19.03 (t, 1H, Ir-H, J(H,P) ) 13.9 Hz), 1.11-2.21 (m, 66H,
P(C6H11)3), 3.40 (s, 2H, H2O), 11.92 (br s, 1H, (η2-S2CH)). 31P{1H}
NMR (C6D6): δ 16.5 (s, P(C6H11)3).
were refined isotropically. The hydride ligands in 1 were located
from the difference Fourier map and refined isotropically. All the
other hydrogen atoms in 1 and 5 were generated in idealized
positions and refined in a riding model. The crystallographic data
are summarized in Table 1.
The reaction of [Ir(H)2(η2-S2CH)(PCy3)2] with Me3SiOTf under
similar reaction conditions as the above reaction with MeOTf also
afforded cis-[Ir(H)(H2O)(η2-S2CH)(PCy3)2][OTf] (5). Yield: 53%.
Determination of the pKa of cis-[Ir(H)(η2-H2)(η2-S2CH)-
(PCy3)2][BF4] (2). A 20 mg (0.02 mmol) sample of [Ir(H)2(η2-S2-
CH)(PCy3)2] was dissolved in 0.5 mL of CD2Cl2. The solution was
freeze-pump-thaw degassed in three cycles and then saturated
with argon. Then HBF4‚Et2O (4 µL, 1 equiv) was added to convert
the starting dihydride into the dihydrogen/hydride complex, cis-
[Ir(H)(η2-H2)(η2-S2CH)(PCy3)2][BF4] (2). To this solution, distilled
water (saturated with argon) (0.4 µL, 1 equiv) was added at room
temperature. Within minutes, an equilibrium was established as
shown in eq 6. The concentrations of cis-[Ir(H)(η2-H2)(η2-S2CH)-
(PCy3)2][BF4] (2), [Ir(H)2(η2-S2CH)(PCy3)2] (1), and H3O+ were
obtained by integration of the respective signals in the hydride
region (for the iridium complexes) and the signal due to H3O+ (δ
Results and Discussion
Synthesis and Characterization of cis-[Ir(H)2(η2-S2CH)-
(PCy3)2] (1). The insertion of CS2 into the Ir-H bond of
[Ir(H)5(PCy3)2] afforded the cis-[Ir(H)2(η2-S2CH)(PCy3)2]
complex accompanied by the elimination of H2 (eq 1). Upon
workup and crystallization, yellow crystals of the complex
were obtained in 76% yield.
1
6.64) in the H NMR spectrum. The concentration of H2O which
is needed to compute the Keq of reaction 6 was back-calculated by
subtracting the concentration of H3O+ (formed due to protonation
of H2O, observable in the 1H NMR spectrum) from the initial
concentration of H2O (known quantity) and multiplying it by the
ratio of molecular weight of H3O+ and H2O. This was done since
the signal due to H2O could not be observed in the presence of
H3O+ in the 1H NMR spectrum. The pKa value of 2 was calculated
from the equilibrium depicted in eq 6 using the expression given
in eq 7.
X-ray Structure Determinations of cis-[Ir(H)2(η2-S2CH)-
(PCy3)2] (1) and cis-[Ir(H)(H2O)(η2-S2CH)(PCy3)2][OTf] (5).
Suitable crystals of 1 and 5 were chosen after examination under
a microscope. The unit cell parameters and the intensity data were
collected on a Bruker SMART APEX CCD diffractometer equipped
with a fine-focus Mo KR X-ray source. The SMART software was
used for data acquisition and the SAINT software for data
reduction.11a Absorption corrections were made using SADABS
program11b and Multiscan method11c for 1 and 5, respectively. The
structures were solved and refined using the SHELX programs.11d
The iridium atom position in both the complexes was observed by
the Patterson method, and the non-hydrogen atoms were located
by successive difference Fourier maps and were refined anisotro-
pically. The two toluene molecules observed in the unit cell of 5
1
The H NMR spectrum of 1 shows a triplet for the two
hydrides at δ -21.64 due to coupling with the two equivalent
cis phosphorus nuclei with a J(H,Pcis) of 17.0 Hz and a broad
singlet at δ 13.89 for the dithioformate hydrogen. The PPh3
analogue, cis-[Ir(H)2(η2-S2CH)(PPh3)2], reported by Robinson
and Sahajpal12 has very similar NMR spectral features. The
31P{1H} NMR spectrum is composed of only one singlet at
δ 28.1, indicating the trans disposition of the two phosphines.
The 13C{1H} NMR spectrum shows a singlet at δ 231.5 for
the dithioformate carbon.
An X-ray crystallographic study of cis-[Ir(H)2(η2-S2CH)-
(PCy3)2] (1) has been carried out, and the ORTEP diagram
is shown in Figure 1. The structure consists of a severely
(11) (a) Bruker SMART and SAINT, versions 6.22a; Bruker AXS: Madison,
WI, 1999. (b) Blessing, R. H. Acta Crystallogr., Sect. A 1995, 51,
33-38. (c) Sheldrick, G. M. SADABS, Version 2, Multiscan Absorption
Correction Program; University of Go¨ttingen: Go¨ttingen, Germany,
2001. (d) Sheldrick, G. M. SHELXL-97 Program for the Solution of
Crystal Structures; University of Go¨ttingen: Go¨ttingen, Germany,
1997.
(12) Robinson, S. D.; Sahajpal, A. Inorg. Chem. 1977, 2718-2722.
Inorganic Chemistry, Vol. 44, No. 18, 2005 6205

Nanishankar et al.
[Ir(H)2(η2-S2CH)(PCy3)2] (1) has five available sites for
protonation: two sulfurs and one carbon of the dithioformate
moiety and the two hydride ligands. Upon protonation of 1
using HBF4‚Et2O, we obtained the corresponding dihydrogen
hydride complex which we believe is cis-[Ir(H)(η2-H2)(η2-
S2CH)(PCy3)2][BF4] (2) (eq 2). We found no evidence of
protonation on any other site even in the presence of excess
acid. Recent work in our laboratories showed that the site
of protonation in a ruthenium hydride dithioformate complex
of the type trans-[Ru(H)(SC(S)H)(dppe)2] (dppe ) Ph2PCH2-
CH2PPh2) is the sulfur resulting in a hydride dithioformic
acid derivative.6a On the other hand, the protonation of
[Os(H)(CO)(S2CH)(PiPr3)2] complex in CD2Cl2 gave the
corresponding dihydrogen complex and, in Et2O solvent, the
site of protonation was the dithioformate resulting in a
methane dithiolate derivative.14 Use of HOTf (pKa ) -14
estimated by the Hammett H0 method; in pure water, its
acidity is leveled to that of H3O+, pKa ) -1.75),15 a stronger
acid than HBF4‚Et2O, also afforded only the dihydrogen
hydride complex. Our attempts to isolate 2 in the solid state
failed, and we could only recover certain unidentifiable
species as confirmed by NMR spectroscopy; therefore, we
characterized 2 using only NMR spectroscopy. The 1H NMR
spectrum of 2 at room temperature shows only one broad
triplet at δ -11.63 which appears as a broad singlet in the
Figure 1. ORTEP view of cis-[Ir(H)2(η2-S2CH)(PCy3)2] (1) at the 50%
probability level.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
cis-[Ir(H)2(η2-S2CH)(PCy3)2] (1) and
cis-[Ir(H)(H2O)(η2-S2CH)(PCy3)2][OTf] (5)
1
5
Ir(1)-H(38)
Ir(1)-H(39)
Ir(1)-P(1)
Ir(1)-P(2)
Ir(1)-S(1)
Ir(1)-S(2)
Ir(1)-O(4)
C(37)-S(1)
C(37)-S(2)
O(3)-O(4)a
O(1)-O(4)b
1.5528(315)
1.5460(237)
2.3218(12)
2.3217(16)
2.4677(14)
2.4510(16)
2.3903(12)
2.3731(11)
2.3032(12)
2.4789(15)
2.144(2)
1.689(4)
1.661(3)
2.592(4)
2.712(3)
1.657(3)
1.671(3)
1
hydride region of the H{31P} NMR spectrum. The triplet
pattern is a result of coupling of the two cis P atoms with
the three equivalent hydride ligands, and the broadness of
the signals is suggestive of a rearrangement process that is
rapid on the NMR time scale. The 31P{1H} NMR spectrum
is composed of only one singlet at δ 18.4, suggesting that
the trans disposition of the phosphine ligands is retained upon
protonation. In an attempt to get an insight into the dynamic
processes of the hydride ligands, we studied the low-
temperature NMR behavior of 2 down to 185 K in toluene-
d8. While extensive line broadening of the hydride signal
took place at 185 K, no limiting spectrum was apparent. Even
at 158 K in CDF2Cl/CDFCl2 solvent, only a single hydride
P(1)-Ir(1)-P(2)
P(1)-Ir(1)-S(1)
P(1)-Ir(1)-S(2)
P(2)-Ir(1)-S(1)
P(2)-Ir(1)-S(2)
S(1)-Ir(1)-S(2)
S(1)-C(37)-S(2)
P(1)-Ir(1)-O(4)
P(2)-Ir(1)-O(4)
157.76(2)
98.79(5)
98.51(4)
99.21(4)
99.84(3)
70.02(4)
116.01(16)
158.96(2)
94.30(5)
98.81(3)
92.47(5)
102.23(4)
71.34(3)
112.9(2)
88.78(8)
88.90(8)
a Intramolecular hydrogen bond. b Intermolecular hydrogen bond.
distorted octahedron. The two PCy3 phosphines are trans to
one another, and the dithioformate moiety is bound to the
metal in an η2-fashion. The two hydride ligands ap-
proximately trans to the dithioformate ligand were located
from the successive difference Fourier maps. The geometry
around the carbon of the S2CH moiety is slightly deviated
from an ideal sp2 carbon atom (116.01(16)°). The two C-S
distances (S(1)-C(37) and S(2)-C(37)) are 1.657(3) and
1.671(3) Å, respectively. These intermediate distances
between C-S single and double bonds, which signify
delocalization of electron density over the S-CH-S frag-
ment, have been observed previously in certain ruthenium
complexes by us6b and others.13 The dithioformate bite angle
S(1)-Ir(1)-S(2) is 70.02(4)°. Pertinent bond lengths and
angles have been summarized in Table 2.
1
signal was observed in the H NMR spectra, which could
be due to a highly fluxional trihydride or a rapidly exchang-
ing dihydrogen/hydride structure. The variable-temperature
31P{1H} NMR spectrum showed no change in the spectral
characteristics except that the phosphine resonances under-
went an upfield shift and broadened with decrease in the
temperature.
Preparation and Characterization of cis-[Ir(H)(η2-H2)-
(η2-S2CH)(PCy3)2][BF4] (2). It is interesting to note that cis-
To unequivocally establish the presence of both hydride
and dihydrogen ligands, we reacted cis-[Ir(H)(η2-H2)(η2-S2-
CH)(PCy3)2][BF4] (2) with CH3CN and CO to obtain the
products of displacement of H2, cis-[Ir(H)(L)(η2-S2CH)-
(13) (a) Gervasio, G.; Vastag, S.; Szalontai, G.; Marko´, L. J. Organomet.
Chem. 1997, 533, 187-191 and references therein. (b) Gao, Y.; Holah,
D. G.; Hughes, A. N.; Spivak, G. J.; Havighurst, M. D.; Magnuson,
V. R.; Polyakov, V. Polyhedron 1997, 16, 2797-2807. (c) Gopinathan,
S.; Unni, I. R.; Gopinathan, C.; Puranik, V. G.; Tavele, S. S.; Guru
Row, T. N. Polyhedron 1987, 6, 1859-1861.
(14) Albeniz, M. J.; Buil, M. L.; Esteruelas, M. A.; Lo´pez, A. M.; Oro, L.
A.; Zeier, B. Organometallics 1994, 13, 3746-3748.
(15) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456-463.
6206 Inorganic Chemistry, Vol. 44, No. 18, 2005

Dynamics of a cis-Dihydrogen/Hydride Complex of Ir
Figure 3. Plot of T1 (400 MHz) versus temperature for cis-[Ir(H)2(η2-S2-
CH)(PCy3)2] (1).
Figure 2. Plot of T1 (400 MHz) versus temperature for cis-[Ir(H)(η2-
H2)(η2-S2CH)(PCy3)2][BF4] (2).
1:1:1 triplets in the 1H{31P} NMR spectrum and exhibit small
downfield isotope shifts of ca. 40 ppb. Several other
complexes reported in the literature show downfield isotope
shifts.5b,16 Highly deuterated samples of 2 were obtained by
purging the solution containing the corresponding dihydrogen
complex with D2 gas for a considerable period of time. The
resulting samples contained H3, H2D, and HD2 isotopomers.
The resonances due to H2D and HD2 isotopomers are
downfield shifted with respect to 2 by ∆δ ) 97 and 49 ppb
(273 K) and 153 and 71 ppb (193 K), respectively, suggesting
the operation of isotopic perturbation arising from nonstatis-
tical distribution of deuterium. In addition, the three reso-
nances show temperature-dependent chemical shifts: upon
lowering of the temperature, the signals moved more
downfield. The chemical shifts of H3, H2D, and the HD2
isotopomers have been plotted as a function of the observa-
tion temperature, and the figure showing the plot has been
deposited in the Supporting Information. However, in the
observed temperature region, decoalescence of the signals
into separate resonances for the H2 and the hydride moieties
could not be achieved.
(PCy3)2][BF4] (L ) CH3CN (3), CO (4)) (eq 3). The free H2
liberated was detected using 1H NMR spectroscopy (singlet
1
at δ 4.60). The H NMR spectrum of cis-[Ir(H)(CH3CN)-
(η2-S2CH)(PCy3)2][BF4] (3) shows a triplet at δ -19.83 for
the hydride coupled to the two cis P nuclei and a singlet at
δ 2.72 for the bound nitrile moiety. The 31P{1H} NMR
spectrum is composed of only one singlet which is suggestive
of the trans disposition of the two phosphine ligands. The
1H and the 31P NMR spectral characteristics of cis-[Ir(H)-
(CO)(η2-S2CH)(PCy3)2][BF4] (4) are similar to those of the
nitrile complex. ν(CO) of this complex is 2026 cm-1.
To determine the existence of either a fluxional trihydride
or a fluxional dihydrogen/hydride species, we carried out
the variable-temperature spin-lattice relaxation time (T1)
measurements in CDF2Cl/CDFCl2. Figure 2 shows a plot of
T1 (400 MHz) versus temperature for 2. The data presented
in Figure 2 for complex 2 are the population-weighted
average values since we could not obtain the frozen out
spectra. The short T1 values observed for 2 qualitatively
indicate the presence of a bound H2 ligand in a dihydrogen/
hydride structure. We also determined the variable-temper-
ature spin-lattice relaxation times for the terminal hydride
ligands of the starting dihydride complex 1, and the data
are graphically shown in Figure 3. To obtain the H-H
distances in the fast and the slow rotation regimes of the
dihydrogen ligand in 2, we treated the data presented in
Figures 2 and 3 as has been done by Heinekey and
co-workers for certain rhodium and iridium complexes
earlier.5b The details of the calculations can be found in the
Supporting Information. From the analysis, we obtained a
T1(min) (400 MHz) of 20 ms. Thus, H-H distances of 0.88
and 1.11 Å have been obtained from these calculations for
the fast and the slow spinning regimes, respectively, of the
bound H2 ligand in 2. Solutions containing 2 purged with
HD gas resulted in the partial conversion of H3 to the H2D
isotopomers. The H2D isotopomers appear as approximately
Figure 4 shows the hydride region of the 1H NMR
spectrum of complex 2 at 263 K (500 MHz) with deuterium
incorporated in the hydride ligands. The signal due to H2D
is approximately a 1:1:1 triplet with J(H,D) ) 6.5 Hz
(16) (a) Bianchini, C.; Moneti, S.; Peruzzini, M.; Vizza, F. Inorg. Chem.
1997, 36, 5818-5825. (b) Bullock, R. M.; Song, J.; Szalda, D. J.
Organometallics 1996, 15, 2504-2516. (c) Heinekey, D. M.; Liegeois,
A.; van Roon, M. J. Am. Chem. Soc. 1994, 116, 8388-8389. (d)
Albinati, A.; Bakhmutov, V. I.; Caulton, K. G.; Clot, E.; Eckert, J.;
Eisenstein, O.; Gusev, D. G.; Grushin, V. V.; Hauger, B. E.; Klooster,
W. T.; Koetzle, T. F.; McMullan, R. K.; O’Loughlin, T. J.; Pe´lissier,
M.; Ricci, J. S.; Sigalas, M. P.; Vymenits, A. B. J. Am. Chem. Soc.
1993, 115, 7300-7312. (e) Miller, R. L.; Toreki, R.; LaPointe, R. E.;
Wolczanski, P. T.; Van Duyne, G. D.; Roe, D. C. J. Am. Chem. Soc.
1993, 115, 5570-5588. (f) Bianchini, C.; Linn, K.; Masi, D.; Peruzzini,
M.; Polo, A.; Vacca, A.; Zanobini, F. Inorg. Chem. 1993, 32, 2366-
2376. (g) Collman, J. P.; Wagenknecht, P. S.; Hutchison, J. E.; Lewis,
N. S.; Lopez, M. A.; Guilard, R.; L’Her, M.; Bothner-By, A. A.;
Mishra, P. K. J. Am. Chem. Soc. 1992, 114, 5654-5664. (h) Michos,
D.; Luo, X.; Crabtree, R. H. Inorg. Chem. 1992, 31, 4245-4250. (i)
Nanz, D.; von Philipsborn, W.; Bucher, U. E.; Vananzi, L. M. Magn.
Reson. Chem. 1991, 29, S38-44. (j) Desrosiers, P. J.; Cai, L.; Lin,
Z.; Richards, R.; Halpern, J. J. Am. Chem. Soc. 1991, 113, 4173-
4184. (k) Earl, K. A.; Jia, G.; Maltby, P. A.; Morris, R. H. J. Am.
Chem. Soc. 1991, 113, 3027-3039. (l) Antoniutti, S.; Albertin, G.;
Amendola, P.; Bordignon, E. Chem. Commun. 1989, 229-230. (m)
Heinekey, D. M.; Payne, N. G.; Sofield, C. D. Organometallics 1989,
8, 1824-1826.
Inorganic Chemistry, Vol. 44, No. 18, 2005 6207

Nanishankar et al.
was based on the procedure used by Calvert and Shapley18
for studying fluxional agostic methyl groups. We also carried
out similar analysis of our data using Heinekey’s method5b
and obtained the limiting chemical shifts of the dihydrogen
and the terminal hydride ligands and the energy difference
between deuterium substitution of the H2 site versus the
hydride site. The details of the analysis have been deposited
in the Supporting Information. Thus, δH2 ) -10.58 ppm,
δH ) -13.61 ppm, J(H,D) ) 21.6 Hz, ∆E1 ) -0.151 kcal/
mol, and ∆E2 ) -0.118 kcal/mol.19 For a somewhat related
iridium complex, [Ir(H)Tp(η2-H2) (PMe3)]+ (Tp ) hydrotris-
(1-pyrazolyl)borate), Heinekey et al.5b found these differences
in energy to be -0.130 and -0.107 kcal/mol, respectively.
Consistent with Heinekey’s analysis, the deuterium concen-
trates in the terminal hydride position in 2.
H-H Distance. The T1 data analysis (see Supporting
Information) gave a T1 (min) (400 MHz, CDF2Cl/CDFCl2)
of 20 ms for complex 2 which is consistent with an H-H
distance (dHH) of 0.88 Å (fast spinning) or 1.11 Å (slow
spinning). The J(H,D) values for H2D and the HD2 isoto-
pomers were found to be 6.5 and 7.7 Hz, respectively. Thus,
the H-H distance (dHH) calculated from the inverse relation-
ship between dHH and J(H,D) (21.6 Hz; vide infra) is 1.05
Å.20,21 Complexes of this type belong to the class of elongated
dihydrogen complexes which show H-H distances inter-
mediate between those of dihydrogen complexes (e1 Å) and
dihydride complexes (g1.5 Å). Such structures represent
arrested intermediate states in the very important process of
oxidative addition of H2 to a metal center. There have been
only a few examples of elongated dihydrogen complexes of
iridium reported in the literature.4i,5b,e,22
Figure 4. Hydride region of the 1H NMR spectrum of cis-[Ir(H)(η2-H2)(η2-
S2CH)(PCy3)2][BF4] (2) in CDF2Cl/CDFCl2 at 263 K (500 MHz) with
deuterium incorporation.
whereas the resonance due to HD2 species exhibits a quintet
with J(H,D) ) 7.7 Hz. The observed HD couplings of the
hydride resonance varies from ca. 6.4 to 5.0 Hz for the H2D
and from ca. 7.7 to 7.1 Hz for the HD2 isotopomers over
the temperature range studied, and this observation has been
graphically shown in a figure deposited in the Supporting
Information.
Isotopic Perturbation of Equilibrium. The H3, H2D, and
the HD2 isotopomers experience large temperature-dependent
downfield chemical shifts signaling the isotopic perturbation
of equilibrium.17 The observed isotope shifts may result from
perturbation of an equilibrium within a single dihydrogen/
hydride ground-state structure wherein the deuterium gets
incorporated in a particular site (eq 4).
Hydride Dynamics. Since static low-temperature-limiting
1H NMR spectra could not be obtained even at 158 K, we
presume that the activation energy for the H atom site
exchange, ∆Gq, must be less than 5 kcal/mol. A similar
observation was made earlier by Heinekey and co-workers5b
for an iridium complex; however, the limiting chemical shifts
for the hydride and the dihydrogen ligands were estimated
using the isotope perturbation of resonance effect and the
∆Gq was calculated to be e5 kcal/mol. Two distinct dynamic
processes can be considered: (a) rotation of the H2 ligand
around the M-H2 bond axis since barriers to hydrogen
rotation are quite low, except in d2 systems;23 (b) H atom
The spectrum in Figure 4 which shows downfield shifts
for the H2D and HD2 isotopomers confirms the hydride site
preference for deuterium in a single dihydrogen/hydride
structure. A second possibility (eq 5) wherein perturbation
of an equilibrium between a dihydrogen/hydride complex
and a trihydride complex may be ruled out on the basis of
the T1 and the HD coupling data.
(18) Calvert, R. B.; Shapley, J. R. J. Am. Chem. Soc. 1978, 100, 7726-
7727.
(19) ∆E1 is the energy difference between Ir(HD)H and Ir(H2)D.; ∆E2 is
the energy difference between Ir(D2)H and Ir(HD)D.
(20) (a) Heinekey, D. M.; Luther, T. A. Inorg. Chem. 1996, 35, 4396-
4399. (b) Maltby, P. A.; Schlaf, M.; Steinbeck, M.; Lough, A. J.;
Morris, R. H.; Klooster, W. T.; Koetzle, T. F.; Srivastava, R. C. J.
Am. Chem. Soc. 1996, 118, 5396-5407. (c) King, W. A.; Luo, X.-L.;
Scott, B. L.; Kubas, G. J.; Zilm, K. W. J. Am. Chem. Soc. 1996, 118,
6782-6783.
Heinekey and co-workers analyzed the chemical shifts and
the J(H,D) data for hydrotris(pyrazolyl)borate dihydrogen/
hydride complexes of Rh and Ir to obtain the limiting
chemical shifts of the H2 and the hydride ligands as well as
the J(H,D) in the bound dihydrogen ligand.5b Their method
(21) dHH (Å) ) -0.0167[J(H, D) (Hz)] + 1.42.
(22) (a) Heinekey, D. M.; Lledo´s, A.; Lluch, J. M. Chem. Soc. ReV. 2004,
33, 175-182. (b) Pons, V.; Heinekey, D. M. J. Am. Chem. Soc. 2003,
125, 8428-8429.
(23) (a) Jalo´n, F. A.; Otero, A.; Manzano, B. R.; Villasen˜or, E.; Chaudret,
B. J. Am. Chem. Soc. 1995, 117, 10123-10124. (b) Sabo-Etienne,
S.; Chaudret, B.; el Makarim, H. A.; Bartlelat, J.; Daudey, J.; Ulrich,
S.; Limbach, H.; Moise, C. J. Am. Chem. Soc. 1995, 117, 11602-
11603.
(17) Saunders, M.; Kates, M. R. J. Am. Chem. Soc. 1977, 99, 8070-8071.
6208 Inorganic Chemistry, Vol. 44, No. 18, 2005

Dynamics of a cis-Dihydrogen/Hydride Complex of Ir
Scheme 1. Reaction of cis-[Ir(H)2(η2-S2CH)(PCy3)2] (1) with MeOTf
exchange between the dihydrogen and the hydride ligands.
The combination of overall positive charge and the presence
of phosphine coligands in complex 2 could be expected to
result in the heterolytic cleavage of the bound H2 followed
by a proton transfer to the hydride moiety rendering the three
hydride ligands equivalent with respect to the NMR time
scale. The pKa of the bound H2 ligand in 2 has been estimated
to be ∼-1.0 (see later). We also cannot rule out a second
possible mechanism: since the H-H bond in 2 is elongated,
a substantial degree of bond formation (between the elon-
gated H2 moiety and the hydride) involving an associatiVe
type of mechanism to generate a trihydrogen intermediate
or transition state could take place in a highly concerted
manner rendering the three hydrogens equivalent.1b Such a
stretching of the H2 toward an adjacent hydride has been
calculated to be a low-energy process that leads to a transition
state with trihydrogen character in [M(H)4(Cp)(η2-H2)(PR3)]+
(M ) Mo; W) complexes.24 The presumed barrier for the
exchange process in our system (e5 kcal/mol), in agreement
1
with the inability to decoalesce the H NMR signals in the
hydride region even at 158 K, is consistent with the proposed
trihydrogen species. Heinekey and co-workers were able to
measure the H2/H exchange rate in a ruthenium dihydrogen/
hydride complex [Ru(η2-H2)(L2)(PCy3)2]+ and found an
form H3O+ (detected using 1H NMR spectroscopy, br s at δ
6.64) and the recovery of the starting dihydride complex [Ir-
(H)2(η2-S2CH)(PCy3)2] (1), indicating that the complex 2 is
quite acidic. It is rather intriguing to note that 5 cannot be
made by any other independent route. These reactions have
been summarized in Scheme 1. It is rather interesting to note
that the dithioformate moiety acts as a spectator under these
reaction conditions. In contrast, we found that the electro-
philic reagents such as the ones employed in the present work
actually functionalize the dithioformate moiety of certain
ruthenium hydride dithioformate complexes, whereas the
hydride ligand is unaffected.6a
We have been able to crystallize the aquo complex cis-
[Ir(H)(H2O)(η2-S2CH)(PCy3)2][OTf] (5) and determine its
structure by X-ray crystallography. The molecular structure
of cis-[Ir(H)(H2O)(η2-S2CH)(PCy3)2][OTf] complex is shown
in Figure 5. The structure consists of a severely distorted
octahedral cation. In addition, the structure solution revealed
the presence of two toluene molecules. The Ir(1)-O(4) bond
distance was found to be 2.144(2) Å; all the other bond
lengths are comparable to those in cis-[Ir(H)2(η2-S2CH)-
(PCy3)2] (1). The geometry around the carbon of the
approximate free energy of activation, ∆Gq ) 5.5 kcal/
120
mol.5d A mechanism that is consistent with a highly concerted
exchange process involving a ruthenium-trihydrogen-like
transition state has been suggested. A combination of
experimental and theoretical studies on [IrX(H)2(η2-H2)-
(PR3)2] revealed exchange barriers of 1.9 (1.5(2) kcal/mol
from inelastic neutron scattering studies), 1.8, and 1.7 kcal/
mol for X ) Cl, Br, and I, respectively.5e,f A mechanism
involving oxidative addition/reductive elimination pathway
through a tetrahydride intermediate has been suggested on
the basis of theoretical calculations.
Reactivity of cis-[Ir(H)2(η2-S2CH)(PCy3)2] toward Elec-
trophilic Reagents. We examined the reactivity behavior
of 1 with electrophilic reagents such as MeOTf and Me3-
SiOTf. In both the reactions, the electrophile presumably
attacks one of the hydride ligand to generate the alkane/
silane that is eliminated. The CH4 and the Me3SiH generated
were observed spectroscopically. The residual water present
in the solvent then binds to the metal in the vacant site thus
created to afford cis-[Ir(H)(H2O)(η2-S2CH)(PCy3)2][OTf] (5).
An even trace amount of moisture in rigorously dried solvents
is enough for the formation of the aquo complex. In addition
to the aquo complex, another unidentifiable iridium hydride
species was also obtained. When these reactions were carried
out in slightly wet solvents, the only product obtained was
cis-[Ir(H)(η2-H2)(η2-S2CH)(PCy3)2][OTf]. The electrophilic
reagent in this case reacts first with the water present in the
solvent to generate HOTf (a broad singlet at δ 13.80 in the
1H NMR spectrum), which causes the protonation of the
dihydride. In addition to HOTf, a signal due to MeOH
formed was also observed in the 1H NMR spectrum. Addition
of a slight excess water to 2 resulted in its protonation to
Figure 5. Molecular structure of cis-[Ir(H)(H2O)(η2-S2CH)(PCy3)2][OTf]
(5) showing the intra- and the intermolecular H-bonds between the iridium-
bound aquo ligand and the OTf counterion.
(24) Bayse, C. A.; Hall, M. B.; Pleune, B.; Poli, R. Organometallics 1998,
17, 4309-4315.
Inorganic Chemistry, Vol. 44, No. 18, 2005 6209

Nanishankar et al.
dithioformate moiety (S(1)-C(37)-S(2) angle of 112.9(2)°)
is quite strained from an ideal sp2 carbon. The water
coordinated to the iridium atom forms one intra- and one
intermolecular hydrogen bonds with the OTf counterion
making a dimer at the inversion center. The intramolecular
O(3)-O(4) distance was found to be 2.592(4) Å, shorter in
comparison to the 2.712(3) Å for the corresponding inter-
molecular O(1)-O(4) distance. The important bond lengths
and angles have been summarized in Table 2.
CN)(η2-S2CH)(PCy3)2][BF4] using HBF4‚Et2O (up to 70
equiv) or HOTf (up to 50 equiv) did not afford the
corresponding dihydrogen complex cis-[Ir(η2-H2)(CH3CN)-
(η2-S2CH)(PCy3)2]2+, a dicationic species that could be
expected to be quite acidic compared to a monocationic
dihydrogen complex. Similar observations were made in the
attempted protonation reactions of cis-[Ir(H)(CO)(η2-S2CH)-
(PCy3)2][BF4]. In both the cases, we recovered the starting
hydride complexes.
Acidity Measurement. The acidities of the dihydrogen
complexes reported in the literature have been measured on
the pseudoaqueous pKa scale.20b,25 The equilibrium constant
Keq of reaction 6
Conclusions
The protonation reactions of the dihydride complex cis-
[Ir(H)2(η2-S2CH)(PCy3)2] with HBF4‚Et2O resulted in a
highly dynamic dihydrogen hydride derivative cis-[Ir(H)-
(η2-H2)(η2-S2CH)(PCy3)2][BF4]. Even low-temperature NMR
spectroscopy did not yield limiting spectra with separate
resonances due to the dihydrogen and the hydride moieties
indicating that the ∆Gq for the H atom site exchange process
must be quite small (e5 kcal/mol). It can be concluded that
the dynamic process involves minimal movement of the
coligands rendering the three hydrogens equivalent. The
J(H,D) for the partially deuterated isotopomers of cis-[Ir-
(H)(η2-H2)(η2-S2CH)(PCy3)2][BF4] complex is 21.6 Hz,
which corresponds to a dHH of 1.05 Å. Such species belong
to the category of elongated dihydrogen complexes.
The protonation of the monocationic hydride complexes
cis-[Ir(H)(L)(η2-S2CH)(PCy3)2]+ (L ) CH3CN, CO) using
either excess HBF4‚Et2O or excess HOTf did not afford the
corresponding dicationic dihydrogen complexes suggesting
that a much stronger acid is required and that such deriva-
tives, if realized, could be expected to be highly acidic and
extremely unstable.
has been obtained from the 1H NMR spectral integrations
(converted into concentrations; see Experimental Section)
of the hydride signals of the iridium complexes and the H3O+
species and the calculated concentration of H2O. As sug-
gested earlier by Kristja´nsdo´ttir and Norton,26 the pKa value
(in CD2Cl2) of the dihydrogen/hydride complex 2 has been
estimated from the equilibrium constant Keq and the pKa of
H3O+ (-1.74, aqueous)15 using eq 7.
pKa{[Ir(H)(η2-H2)(η2-S2CH)(PCy3)2]+} )
pKa{[H3O]+} + pKeq (7)
A Keq of 0.194 was obtained for the reaction 6. Thus, using
the above expression, a pKa of -1.0 was obtained for
complex 2 indicating that it is quite acidic. The high acidity
of complex 2 supports our proposed mechanism involving
the heterolytic cleavage of the bound H2 ligand followed by
the proton transfer to the hydride rendering the three
hydrogens equivalent.
Acknowledgment. We are grateful to the Council of
Scientific and Industrial Research and the Department of
Science and Technology of India for the financial support
and to the Department of Science and Technology of India
for funding the procurement of a 400 MHz NMR spectrom-
eter under the “FIST” program and an X-ray diffractometer.
We also thank Prof. Siddhartha Sarma for some useful
suggestions.
Protonation Reactions of cis-[Ir(H)(CH3CN)(η2-S2CH)-
(PCy3)2][BF4]. The attempted protonation of cis-[Ir(H)(CH3-
(25) (a) Ng, S. M.; Fang, Y. Q.; Lau, C. P.; Wong, W. T.; Jia, G.
Organometallics 1998, 17, 2052-2059. (b) Rocchini, E.; Mezzetti,
A.; Ru¨egger, H.; Burckhardt, U.; Gramlich, V.; Del Zotto, A. D.;
Martinuzzi, P.; Rigo, P. Inorg. Chem. 1997, 36, 711-720. (c) Schlaf,
M.; Lough, A. J.; Maltby, P. A.; Morris, R. H. Organometallics 1996,
15, 2270-2278. (d) Cappellani, E. P.; Drouin, S. D.; Jia, G.; Maltby,
P. A.; Morris, R. H.; Schweitzer, J. Am. Chem. Soc. 1994, 116, 3375-
3388. (e) Chin, B.; Lough, A. J.; Morris, R. H.; Schweitzer, C. T.;
D’Agostino, C. Inorg. Chem. 1994, 33, 6278-6288. (f) Jia, G.; Lough,
A. J.; Morris, R. H. Organometallics 1992, 11, 161-171. (g) Jia, G.;
Morris, R. H. J. Am. Chem. Soc. 1991, 113, 875-883. (h) Jia, G.;
Morris, R. H.; Schweitzer, C. T. Inorg. Chem. 1991, 30, 593-594.
(i) Jia, G.; Morris, R. H.; Inorg. Chem. 1990, 29, 581-582.
(26) Kristja´nsdo´ttir, S. S.; Norton, J. R. In Transition Metal Hydrides,
Dedieu, A., Ed.; VCH: Weinheim, Germany, 1992; Chapter 9, pp
324-334.
Supporting Information Available: X-ray crystallographic data
for cis-[Ir(H)2(η2-S2CH)(PCy3)2] (1) and cis-[Ir(H)(H2O)(η2-S2CH)-
(PCy3)2][OTf] (5) in CIF format, a structure of complex 5 showing
the hydrogen bonds and a list of hydrogen bonds in complex 5, T1
data (400 MHz) for cis-[Ir(H)2(η2-S2CH)(PCy3)2] (1) and cis-[Ir-
(H)(η2-H2)(η2-S2CH)(PCy3)2][BF4] (2), plots of the 1H NMR
chemical shifts versus temperature and the H, D coupling constants
versus temperature for the HD isotopomers, and details of the
analysis of NMR spectral data. This material is available free of
charge via the Internet at http://pubs.acs.org.
IC050266C
6210 Inorganic Chemistry, Vol. 44, No. 18, 2005