Ion pairing effects in intramolecular heterolytic H2 activation in an Ir(III) complex: A combined theoretical/experimental study

Article abstract of DOI:10.1039/b207339k

A free pendant 2-amino group in a benzoquinolinate ligand bound to Ir(III) can cause heterolytic dissociation of an adjacent Ir-H₂, depending on the nature of the phosphine, L. For L = PMePh₂, heterolysis does not occur and an H₂ complex, [IrH(H₂)(bq-NH₂)L₂]BF₄, is seen, but if L = PPh₃ or PCy₃, heterolysis does occur and a hydride product, [IrH₂(bq-NH₃)L₂]BF₄, is formed by proton transfer from the bound H₂ to the pendant NH₂ group. The electronic effect of L is not dominant. Theoretical studies (DFT calculations) show that the H₂ complex is predicted to be more stable for all the phosphines used, if the anion is ignored. We propose that the hydride isomer, formed when L is bulky, depends on ion pairing effects for its stability. The calculated electrostatic potentials for the two isomers suggest that the counter anion has to be located much closer to the metal in the H₂ complex than in the hydride where the anion is much farther from the metal. The bulky phosphines PPh₃ and PCy₃ favor remote ion pairing and therefore favor the hydride isomer because steric effects disfavor close ion pairing as confirmed by ONIOM (B3PW91/UFF) calculations of the ion pair geometry. An improved synthesis of [IrH₅(PCy₃)₂] is reported.

Full text of DOI:10.1039/b207339k

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Ion pairing effects in intramolecular heterolytic H2 activation in an
Ir(III) complex: a combined theoretical/experimental study
Karin Gruet,a Eric Clot,*b Odile Eisenstein,*b Dong Heon Lee,ad Ben Patel,a
Alceo Macchioni*c and Robert H. Crabtree*a
a
Department of Chemistry, Yale University, New Haven, CT 06520-8107, USA.
E-mail: robert.crabtree@yale.edu
´
LSDSMS (UMR 5636), Case courrier 14, Universite Montpellier 2, 34095, Montpellier cedex 5,
France. E-mail: odile.eisenstein@univ-montp2.frclot@univ-montp2.fr
b
c
`
Dipartimento di Chimica, Universita di Perugia, Via Elce di Sotto 8, 06123, Perugia, Italy.
E-mail: alceo@unipg.it
Department of Chemistry, Chonbuk National University, Chonju 561-756, Korea
d
Received (in Montpellier, France) 25th July 2002, Accepted 10th September 2002
First published as an Advance Article on the web 31st October 2002
A free pendant 2-amino group in a benzoquinolinate ligand bound to Ir(III) can cause heterolytic dissociation of
an adjacent Ir–H2 , depending on the nature of the phosphine, L. For L ¼ PMePh2 , heterolysis does not occur
and an H2 complex, [IrH(H2)(bq-NH2)L2]BF4 , is seen, but if L ¼ PPh3 or PCy3 , heterolysis does occur and a
hydride product, [IrH2(bq-NH3)L2]BF4 , is formed by proton transfer from the bound H2 to the pendant NH2
group. The electronic effect of L is not dominant. Theoretical studies (DFT calculations) show that the H2
complex is predicted to be more stable for all the phosphines used, if the anion is ignored. We propose that the
hydride isomer, formed when L is bulky, depends on ion pairing effects for its stability. The calculated
electrostatic potentials for the two isomers suggest that the counter anion has to be located much closer to the
metal in the H2 complex than in the hydride where the anion is much farther from the metal. The bulky
phosphines PPh3 and PCy3 favor remote ion pairing and therefore favor the hydride isomer because steric
effects disfavor close ion pairing as confirmed by ONIOM (B3PW91/UFF) calculations of the ion pair
geometry. An improved synthesis of [IrH5(PCy3)2] is reported.
the hydride heterolysis product is more remote from the metal.
Ion pairing has been implicated in a few other organometallic
systems.7
Introduction
Heterolytic H2 activation1 (eqn. 1), currently attracting
increasing attention, may often go via an acidic H2 complex.2
For example, an all-Fe hydrogenase has recently been pro-
posed to contain a basic amino group immediately adjacent
to the H2 binding site capable of acting as a base for deproto-
nating coordinated H2 .3
Results and discussion
Choice of system
In order to separate out the effect of the pendant amino group,
we needed to compare the 2-aminobenzoquinolinate (bq-NH2)
system with the benzoquinolinate system, studied previously,8
lacking a pendant amino group (bq-H). In this early work with
bq-H (eqn. 2), we showed that H2 replaces a labile ligand such
as water or acetone in the starting material, 1, to give the H2
complex, 2. Bases such as excess NEt3 cause deprotonation
and H2 heterolysis to give the hydride, 3.
ð1Þ
We have briefly reported4 on a 2-aminobenzoquinolinate
(bq-NH2) Ir(III) complex, designed to favor heterolysis of
H2 . The free –NH2 , located on the bq framework near the
H2 binding site, can act as an intramolecular base causing pro-
ton abstraction from H2 . The rigidity of the bq framework
prevents the amino group from directly binding to the metal,
which would block the H2 binding site. Broadly similar com-
plexes with an H2 binding site and a basic group on a ligand
have been reported by Morris, Sabo-Etienne and Chaudret.5,6
We reported4 that moving from PPh3 to more basic ligands
such as PMePh2 led to the formation of an H2 complex with-
out heterolysis, so we ascribed the difference to an electronic
effect. The work described here shows that this proposal is
invalid since the very basic PCy3 also gives heterolysis. We
have now looked for the origin of this effect by a combined
experimental/theoretical study on this system. This leads us
to propose that ion pairing is the key factor that favors hetero-
lysis for bulky phosphines because the anion binding site for
In the prior communication4 we reported that, depending on
the phosphine, the aqua complex [IrH(bq-NH2)(OH2)-
(L)2]BF4 , 4, reacts with H2 to afford either the dihydrogen
compound [IrH(H2)(bq-NH2)(L)2]BF4 , 5, or the hydride
[IrH2(bq-NH3)(L)2]BF4 , 6 (eqn. 3). Only dihydrogen com-
pounds 5 were obtained upon hydrogenation of the parent sys-
tem [IrH(bq-H)(OH2)(PPh3)2]BF4 , 1, bearing no adjacent
pendant group.8 Since the dihydrogen compounds of type 5
were originally4 obtained only with the more basic alkyl phos-
phines and the hydride 6 with the less basic PPh3 , we ascribed
this behavior to the electronic effect of the phosphines. We
now report the hydride 6 also forms with the very basic PCy3
which has now led us to carry out a combined theoretical/
experimental study that identifies a steric effect acting in the
80
New J. Chem., 2003, 27, 80–87
DOI: 10.1039/b207339k
This journal is # The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2003

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[IrH(bq-NH2)(OH2)L2]BF4 (4: a, L ¼ PPh3 ; b, L ¼ P(p-
C6H4CH3)3 ; c, L ¼ P(p-C6H4OCH3)3 ; d, L ¼ PMePh2) are
known (4a)10 or could be synthesized by the known route10
or variants described in the experimental section. All were fully
characterized as detailed in the experimental section or in prior
papers. These syntheses (Scheme 1) involve cyclometalation
10
of 2-aminobenzoquinoline with [(cod)IrL2]X/H2 or using
preformed [IrH2(OH2)2L2]X in moist CH2Cl2 at room tem-
perature to give the aqua complex [IrH(bq-NH2)(OH2)L2]X
(4, bq-NH2 ¼ 2-benzoquinolinato; X ¼ BF4 or PF6) or using
preformed [IrH2(Me2CO)2L2]X in acetone at room temperature
to give the acetone complex [IrH(bq-NH2)(Me2CO)L2]BF4
(40, the prime means acetone is the labile ligand).
ð2Þ
The required PCy3 acetone complex [IrH(bq-NH2)(OC-
Me2)(PCy3)2]BF4 (40e) had not previously been reported and
some synthetic effort proved to be necessary to obtain it
because the standard route10 from [(cod)Ir(PR3)2]BF4/H2 ,
used above, was not available as [(cod)Ir(PCy3)2]X is
unknown. Instead we have prepared it from the known11
[IrH5(PCy3)2] by protonation in acetone with HBF4 to give
[Ir(PCy3)2(OCMe2)2(H)2]BF4 in situ, followed by reaction with
2-aminobenzoquinoline to give the desired 40e. Surprisingly, it
was not possible to synthesize the aqua species for L ¼ PCy3
but the acetone complex proved well behaved. Otherwise, the
results were not dependent on the nature of the leaving grouꢀp,
alternate ion pairs formed from 5 and 6 as the proposed origin
of the effect.
acetone or water, or on the nature of the anion, BF4ꢀ or PF6
.
We also report an improved synthesis of [IrH5(PCy3)2] by a
one step route from commercial [Ir(cod)Cl]2 , treatment of
which in CH2Cl2 with NaOMe and PCy3 under H2 gives the
product in 59% yield. The isolation of the analytically pure
pentahydride only requires filtration and washing. The spec-
troscopic characterization, 1H NMR: ꢀ11.27 ppm (t,
2JHP ¼ 12.21 Hz), 31P NMR: 33.42 ppm (s), IR: n(Ir–
H) ¼ 1929.1 cmꢀ1, is in full agreement with the published
data.11
ð3Þ
Reaction of 4a–d with H2 : experiment
At ꢀ70 ꢁC in CD2Cl2 , the aqua complex, [IrH(bq-NH2)-
(OH2)(PPh3)2]BF4 , 4a, reacts with H2 to give the hydride,
[IrH2(bq-NH3)(PPh3)2]BF4 , 6a, via H2 heterolysis. The 1H
NMR spectrum shows two sharp hydride resonances in the
terminal Ir–H region at ꢀ23.24 ppm and ꢀ25.63 ppm that
are coupled to the cis-phosphines and to each other (dt,
Synthesis
The required ligand precursor, 2-amino-7,8-benzoquinoline,
was readily prepared from 7,8-benzoquinoline and
NaNH2 .9,10 Most of the required starting aqua complexes
2
2
0
JHH ¼ 8.50 Hz; JPH ¼ 14.65 Hz). These are clearly normal
Scheme 1
New J. Chem., 2003, 27, 80–87
81

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hydrides with the normal 2JHH and 2JPH values usually seen10
in these systems. A broad resonance at 4.56 ppm of intensity
with the appearance of two broad signals for the cis-hydrides
at ꢀ25.06 (1H) and ꢀ21.04 ppm (1H), as well as a broad signal
at 4.29 ppm (3H) assigned to the ammonium protons. Had the
electronic effect been dominant, the basic PCy3 ligand should
have afforded the H2 complex, but NMR resonances for this
species were completely absent and the spectra of the two
forms are so different that no doubt is possible.
To eliminate the very remote possibility that the ligands L in
this series do not exert their usual electronic effects, we made
the carbonyl derivatives of 4a–d and 40e by reaction with CO
at 1 atm in CH2Cl2 (eqn. 4). As expected, these showed
n(CO) IR frequencies entirely consistent with the usual increas-
ing Tolman12 basicity order (values in parentheses): PPh3 ,
2026 (2068.9); P(p-C6H4CH3)3 , 2024 (2066.7); P(p-
C6H4OCH3)3 , 2023 (2066.1); PMePh2 , 2021 (2067.0); PCy3 ,
1995 (2056.4) cmꢀ1. Our system is more sensitive to variation
in the phosphine than is Tolman’s classic LNi(CO)3 , because
we have a PR3/CO ratio of 2:1 versus 1:3 for Tolman. The
increase in basicity on moving to PCy3 was very large, as
expected, and so we can rule out an unexpected inversion of
basicity in this case.
0
+
3H is assigned to the bq-NH3 protons for compound 6a.
The triplet hydride resonance at ꢀ16.22 ppm (2JPH ¼ 14.65
Hz) and the amino resonance at 6.23 ppm characteristic of
the starting material 4a reappears on passing a stream of N2 ,
showing this conversion is reversible. The presence of anhy-
drous MgSO4 in the NMR tube is helpful to remove water
and drive the equilibrium over to 6.
In an effort to see if an electronic effect operates in the PAr3
system, we looked at the phosphines, P(p-C6H4CH3)3 and P(p-
C6H4OCH3)3 . Hydride complexes were also obtained both
with P(p-C6H4CH3)3 , 6b, and P(p-C6H4OCH3)3 , 6c. Warming
the samples to room temperature led to loss of H2 below
ꢀ20 ꢁC, with regeneration of the starting aqua complex, so
none of 6a–c could be isolated as solids.
Changing the phosphine to PMePh2 gave a very different
result. The appropriate aqua complex, [IrH(bq-NH2)(OH2)
(PMePh2)2]BF4 , 4d, also reacts with H2 at ꢀ70 ꢁC but to give
the H2 complex [IrH(H2)(bq-NH2)(PMePh2)2]BF4 , 5d. This
assignment follows from the very different 1H NMR spectrum.
The NH2 and hydride peaks characteristic of compound 4d
disappear and are replaced by three new signals: an Ir–H
triplet at ꢀ15.90 ppm (2JPH ¼ 14.65 Hz) of intensity 1H, a
characteristic broad Ir(H2) resonance of intensity 2H at
ꢀ3.75 ppm, and a broad NH2 resonance of intensity 2H at
6.15 ppm appropriate for compound 5d. As noted previously,
an H2 complex was also seen with PEt2Ph and P(n-Bu)3 , but
we report only the PMePh2 case in full here because unlike
the oily PEt2Ph and P(n-Bu)3 species, complex 4d crystallized
well and its characterization was completely convincing. Like
6, 5 also loses H2 on warming above ꢀ20 ꢁC, and so cannot
be isolated as a solid.
ð4Þ
Initial theoretical analysis
The first model systems studied (DFT, B3PW91)13 were dihy-
drogen complex [Ir(bq-NH2)(PH3)2(H2)(H)]+ (7) and hydride
[Ir(bq-NH3)(PH3)2(H)2]+ (8), where the only differences versus
the experimental compounds 5 and 6 are the phosphine ligand
(PH3 vs. PPh3) and the absence of anion. The optimized geo-
metries are shown in Fig. 1 and selected geometrical para-
meters are given in Table 1. The key finding is that
dihydrogen complex 7 is strongly favored theoretically with a
convincingly large energy preference of 14.4 kcal molꢀ1 while
the hydride 6 (L ¼ PPh3) is favored experimentally. The
The nature of the leaving group was not important: upon
exposure to hydrogen, the acetone complexes (40a–d) also
afford the same compounds, H2 complex 5 or hydride 6, with
exactly the same dependence on the phosphines as is obtained
from the analogous aqua complex. However, the hydrogena-
tion is complete only for 40a and 40e, not for 40b–d, no doubt
because the acetone is slightly more strongly bound than
water.
˚
An electronic effect?
H–H distance of the dihydrogen complex, 0.87 A, is normal.
Also notable is the dihydrogen bonded14 N–Hꢂ ꢂ ꢂH–Ir group
Because less basic phosphines led to the hydride complexes,
whereas more basic phosphines led to the dihydrogen com-
plexes, we proposed in the prior communication4 that the
change in the heterolytic H2 activation equilibrium was due
to the electronic effects of the phosphines. We expected the
basicity of the phosphine to influence the pKa of coordinated
H2 . In free H2 , the pKa is ca. 35, but upon binding to the
metal, it becomes very much more acidic (15 to ꢀ5).5,6 This
acidification may be due to the metal fragment stabilizing
the Hꢀ product of the heterolytic dissociation while binding
H2 relatively weakly. A basic phosphine should enhance the
back bonding to the empty s* orbital of H2 , causing tighter
H2 binding and lowered acidity. To check this electronic inter-
pretation, we decided to examine the case of PCy3 , the most
basic, but also one of the most bulky phosphines in common
use. If electronic factors dominate, an H2 complex should be
formed.
˚
of the hydride form 8 with its short Hꢂ ꢂ ꢂH distance of 1.379 A.
A problem with 7 and 8 is the use of PH3 as model ligand,
since this is neither electronically or sterically comparable with
the experimental ligands. To probe the electronic issue, we cal-
culated the situation for the series PH3ꢀxFx (x ¼ 0–3) to look
at the sensitivity of the equilibrium 7/8 as a function of change
of the phosphine electronic effect. The Tolman12 electronic
parameters (TEPs) for these ligands are: PH3 , 2083.2; PH2F,
2090.9; PHF2 , 2100.9; PF3 , 2110.8 cmꢀ1, so they cover
a broad range of values going from the range typical for
Reaction of the PCy3 complex, 40e, with H2 : experiment
In complete disaccord with the electronic effect model, hydro-
genation of the PCy3 acetone complex 40e, at ꢀ80 ꢁC in
CD2Cl2 , led to the hydride compound 6e with heterolytic H2
activation. This follows from the disappearance of the triplet
Ir–H signal at ꢀ14.23 ppm (2JHP ¼ 14.65 Hz, 1H), and of
the broad NH2 signal at 5.83 ppm (2H) of the amino group,
Fig. 1 Optimized geometry (B3PW91) for the dihydrogen complex
IrH(H2)(bq-NH2)(PH3)2+, 7, and the hydride complex Ir(H)2(bq-
NH3)(PH3)2+, 8.
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˚
Table 1 Geometrical parameters (distances in A, angles in degrees)
for the dihydrogen complexes IrH(H2)(bq-NH2)(PHxF3ꢀx)2
+, 7, and
the hydride complexes Ir(H)2(bq-NH3)(PHxF3ꢀx 2
)
+, 8, for x ¼ 0–3.
The relative electronic energy (DE/kcal molꢀ1) and relative free
enthalpy (DG/kcal molꢀ1) correspond to the transformation 8 ! 7.
The numbering of the hydrogen atoms is shown in Fig. 1 for the
PH3 case
PH3
PH2F
PHF2
PF3
Complex 7 H2–H3
H3ꢂ ꢂ ꢂH4
0.870
0.866
0.858
0.850
1.912
1.008
1.698
1.379
1.150
1.963
1.009
1.674
1.479
1.111
2.055
1.010
1.673
1.494
1.095
2.126
1.012
1.682
1.613
1.074
N2–H4
Complex 8 Ir–H2
H2ꢂ ꢂ ꢂH3
N2–H3
cIr–H2ꢂ ꢂ ꢂH3
94.2
94.7
97.0
95.7
cH2ꢂ ꢂ ꢂH3–N2 176.5
178.7
ꢀ6.6
ꢀ9.4
178.5
ꢀ2.1
ꢀ4.0
176.9
+1.0
+0.8
8 ! 7
DE
DG
ꢀ14.4
ꢀ15.6
˚
Fig. 3 Change in the H2ꢂ ꢂ ꢂH3 and N2–H3 bond distances (A) in 8 as
a function of TEP12,15 for the different PHxF3ꢀx considered. See Fig. 1
for the atom numbering in 8.
phosphites to that of CO.15 PH3 is a significantly less strong
donor than PPh3 or P(alkyl)3 , however, so we felt the need
to look at the trend for PH3ꢀxFx and extrapolate it into a
region more appropriate to the experimental phosphines.
The resulting trend in calculated DG for 7/8, plotted against
the TEP (Fig. 2), shows that the dihydrogen complex 7 is still
favored for all L more basic than PHF2 ; only for the extremely
electron-withdrawing ligand PF3 , does the system favor
hydride 8. This implies that within the assumptions made in
the calculation, the experimental systems would have been
expected to strongly favor the dihydrogen complex 7 for all
the experimental phosphines, since these are all much more
strongly electron-donating than PHF2 .
Ion pairing effects
The theoretical analysis above indicates that all the phosphines
in the series chosen would be expected to give the dihydrogen
form and heterolysis should never be observed except with
ligands like PF3 that are far less donating than any used experi-
mentally. In searching for possible explanations, we noted that
both of the anomalous cases where heterolysis was observed
involve bulky (PPh3) or very bulky (PCy3) phosphines. It
seemed very unlikely that this bulk could have a direct influ-
ence on the M(H2)(bq-NH2)/MH(bq-NH3) interconversion
because both H2 and H ligands are sterically small and the
energy deficit to be compensated is so large.
Large energies are known to arise by ion pairing: in CH2Cl2 ;
these are typically in the range of 20–30 kcal molꢀ1 for organic
cases.16 Very similar complexes to the present ones such as
[Ir(PRPh2)2(dipy)(H)2]X (R ¼ Ph, Me, X ¼ BF4 , PF6 ,
CF3SO3 , BPh4) have been unambiguously shown in our prior
NMR work to give rise to tight ion pairing in CH2Cl2 .17 In
these stable complexes, the location of the anion, identified
by NOE studies, was near the dipy ligand, on the side remote
from the metal. Our prior theoretical analysis17 using the cal-
culated electrostatic potential of the molecule reproduced the
experimental location of the counterion, lending credence to
this approach. The present bq system, with its extra benzene
ring in the heterocyclic ligand blocking the binding site found
for the dipy complex, is expected to strongly disfavor such
binding. Instead, we thought ion pairing might occur near
the H2 of the H2 complex and the bq-NH3 group of the
hydride complex. In this case, the electronically expected H2
isomer could only be favored in practice if the ion pairing
could occur unhindered by bulky L. If L is bulky, we thought
that remote ion pairing at the bq-NH3 site might be the only
allowed configuration thus favoring the hydride isomer.
The calculated electrostatic potential contour plots in the bq
plane of the two isomers are very different, consistent with our
picture of ion pairing at close versus remote sites. Fig. 4 illus-
trates the situation. The M(H2)(bq-NH2) isomer has a pre-
ferred anion binding region between the (H2) ligand and the
(–NH2) group, close to the metal. In contrast, the MH(bq-
NH3) isomer strongly prefers anion binding in the vicinity of
the (–NH3) group, far from the metal (Scheme 2). This differ-
ence may be the factor that for bulky L favors the MH(bq-
NH3) isomer, with its remote anion binding site, over the
M(H2)(bq-NH2) isomer, with its anion binding site close to
the metal and also to the bulky PR3 ligands.
Dihydrogen bonding
Also of interest in these calculations is the change of the N–H
and Hꢂ ꢂ ꢂH bond lengths as the phosphine is varied (Table 1).
This indicates that the dihydrogen bonding14 is strongly
enhanced by the more donor ligands. Increasing donor power
of the phosphine is expected to enhance the basic character of
the metal–hydride bond and therefore enhance dihydrogen
bonding. As shown in Fig. 3, the N–H bond elongates signifi-
cantly and the Hꢂ ꢂ ꢂH distance dramatically shortens on mov-
˚
ing to more donor phosphines. The Hꢂ ꢂ ꢂH distance of 1.379 A
for PH3 is far shorter than any experimentally observed value;
these are usually14 in the range 1.7–2.0 A.
˚
Fig. 2 Evolution of the free enthalpy DG (kcal molꢀ1) for the trans-
formation 8 ! 7 as a function of the Tolman Electronic Parameter
(TEP)12,15 for the different PHxF3ꢀx considered.
To confirm this analysis, combined quantum mechanical/
molecular mechanics calculations, with the ONIOM
(B3PW91/UFF) method, have been carried out on the full
New J. Chem., 2003, 27, 80–87
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Fig. 5 Optimized geometry (ONIOM(B3PW91/UFF)) for the ion
pair dihydrogen complex [IrH(H2)(bq-NH2)(PPh3)2][BF4], 9, and the
ion pair hydride complex [Ir(H)2(bq-NH3)(PPh3)2][BF4], 10. Atoms
included in the QM part are shown in ball-and-stick format and atoms
included in the MM part are shown in black.
Fig. 4 Contour plot of the electrostatic potential in the plane of the
bq ligand for 7 and 8. Contour lines are shown from 0.1 a.u. to 0.3 a.u.
in 0.02 increments.
˚
˚
differ from those in 10 (Hꢂ ꢂ ꢂH ¼ 1.517 A, N–H ¼ 1.067 A,
˚
˚
10; Hꢂ ꢂ ꢂH ¼ 1.539 A, N–H ¼ 1.060 A, 11) as the N–H proton
systems [IrH(H2)(bq-NH2)(PPh3)2][BF4], 9, and [Ir(H)2(bq-
NH3)(PPh3)2][BF4], 10. Only the phenyl groups of the PPh3
ligands have been treated at the MM level, all the remaining
atoms, and particularly the counter anion BF4ꢀ, were treated
within the DFT framework.
ꢀ
is not directly involved in the interaction with BF4
.
Complex 11 is marginally less stable than 9 but by only 0.4
kcal molꢀ1; the QM parts differ by only 1.9 kcal molꢀ1 as a
result of an optimal arrangement for anion–cation specific
interactions in 11.
In conclusion the ONIOM calculations strongly suggest that
ion pairing is responsible for the unexpected experimental
observation of 6 in the case of L ¼ PPh3 . With close contacts
between acidic protons and BF4ꢀ, strong electronic preference
for the dihydrogen complex may be strongly reduced as illu-
strated by 9 and 11. Electronic and steric components of the
ion pairing energy contribute to the effect with a more efficient
interaction and a weaker steric repulsion in the hydride
isomers.
For both systems, the position of the anion was in agree-
ment with the analysis based on the electrostatic potential
and the optimized geometries are shown in Fig. 5. BF4ꢀ is clo-
ser to the metal in 9 than in 10 (as illustrated by the Irꢂ ꢂ ꢂB dis-
˚
˚
tances: 4.208 A, 9; 5.241 A, 10). In 9 there are two short Hꢂ ꢂ ꢂF
contacts between cation and anion, one with the H2 ligand
˚
(Hꢂ ꢂ ꢂF ¼ 2.068 A) and the other with the NH2 group
˚
(Hꢂ ꢂ ꢂF ¼ 1.781A). In 10, three short contacts are present,
+
but only with the NH3 group (Hꢂ ꢂ ꢂF ¼ 1.668, 1.687, and
˚
2.434 A). As a consequence of the ion pair formation, the dihy-
˚
drogen bond is longer in 10 than in 8 (Hꢂ ꢂ ꢂH ¼ 1.379 A, 8;
˚
Hꢂ ꢂ ꢂH ¼ 1.517 A, 10) and the N–H bond is shorter (N–
Experimental probes of ion pairing
˚
˚
H ¼ 1.150 A, 8; N–H ¼ 1.067 A, 10).
ꢀ
Unfortunately, experimental attempts to exchange the BF4
The inclusion of the counter anion reduces the relative
energy between the two isomers with 9 (dihydrogen complex)
now being only 3 kcal molꢀ1 more stable than 10 (hydride
complex). When both components of the ONIOM energy are
considered individually, the assumption of increased steric
repulsion within 9 is confirmed. The energy difference between
the QM parts of 9 versus 10 is 6 kcal molꢀ1 in favor of dihydro-
gen complex 9, illustrating the stabilizing electronic contribu-
tion of ion pairing in 10. This value should be compared
with the 14.4 kcal molꢀ1 energy difference found between dihy-
drogen 7 and hydride 8, implying that the anion stabilizes the
hydride form by ca. 8 kcal molꢀ1. For the MM parts, 9 is 3
kcal molꢀ1 less stable than 10, confirming that ion pair forma-
tion close to the H2 ligand introduces larger steric repulsions.
In our attempt to model ion pairing with NH3+ we expected
counter-anion of 6a and 6e in a CD2Cl2 solution at ꢀ80 ꢁC
with a smaller one, like Fꢀ, with the goal of switching the iso-
mer to the unobserved dihydrogen complex 5a,e, all failed.
Instead, we always obtained the neutral dihydride [Ir(bq-
+
NH2)(PR3)2)(H)2] formed by deprotonation of the NH3
group by Fꢀ, acting as a base. Even using the weaker base,
Clꢀ, led to the same deprotonation. This deprotonation pro-
duct was known because it had been already synthesized by
another route, the action of NaBH4 on the aqua or acetone
complexes.
Polar solvents can separate tight ion pairs, but all attempts
in our system led to displacement of the coordinated H2 and
formation of the solvent complex where the labile site is occu-
pied by the solvent.
19F,1H-HOESY NMR spectra have previously7 given
invaluable information on ion paiꢀr structure by revealing con-
+
to see Hꢂ ꢂ ꢂF contacts with all the NH3 protons as in 10.
However, the electrostatic potential for 8 (Fig. 4) reveals that
ion pair formation is not restricted to the NH3+ group. In par-
ticular, H3 of bq is a viable candidate for such Hꢂ ꢂ ꢂF short
contacts. Indeed we optimized another minimum (Fig. 6), 11,
ꢀ
tacts between the BF4 or PF6 anions and the complex
+
also with three short Hꢂ ꢂ ꢂF contacts: two are with NH3
˚
(Hꢂ ꢂ ꢂF ¼ 1.652 and 1.661 A) and one is with the H3
˚
(Hꢂ ꢂ ꢂF ¼ 2.219 A). The dihydrogen bond parameters in 11
Fig. 6 Alternate ion pair geometry for the hydride complex
[Ir(H)2(bq-NH3)(PPh3)2][BF4], 11. The H3 proton bq is explicitly
shown to illustrate the close contact with BF4ꢀ. Atoms included in
the QM part are shown in ball-and-stick format and atoms included
in the MM part are shown in black.
Scheme 2
84
New J. Chem., 2003, 27, 80–87

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cation. Some of the hydride complexes 5 and 6 studied here
proved unsuitable, however, because of thermal instability
and solubility problems. Two related stable and soluble
complexes, [Ir(bq-H)(PPh3)2H(CO)]PF6 ,18 and [Ir(bq-NH2)-
(PPh3)2H(CO)]PF6 , did give good data. The anion–cation
contacts observed in the NMR spectrum (CD2Cl2 , 208 K,
376.63 MHz)19 showed the anions in the expected locations,
next to the bq-H 2- and 3-positions in the former and next
to the amino group in the latter. Contacts with ortho and meta
PPh3 protons were also detected. For the key case of [Ir(bq-
NH3)(PPh3)2(H)2]PF6 , 6a, formed in equilibrium from 40a
and H2 , data were obtained showing strong contacts for the
30 min at 0 ꢁC (ice bath). The intermediate is then precipitated
with hexanes and redissolved in moist CH2Cl2 (10 ml). Amino-
benzoquinoline (0.25 mmol) is then added and the mixture stir-
red for 30 min. The final product is precipitated with hexanes
and recrystallized from CH2Cl2/Et2O.
4a (BF4 salt). The complex was previously prepared by
method A.10
4b (PF6 salt). The complex was best prepared by method
1
C. H NMR (CD2Cl2 , 500 MHz, 293K, ppm) d ꢀ16.61 (br.
+
bq-NH3 proton, as predicted by the theoretical study. Con-
s, Ir–H), 1.60 (br. s, 2H, H2O), 2.22 (s, 18H, Me), 6.00–8.10
(m, 39H, bq + NH2 + Ph). 1H NMR (CD2Cl2 , 500 MHz,
183 K, ppm) d ꢀ16.61 (br. s, Ir–H), 1.60 (br. s, 2H, H2O),
2.22 (s, 18H, Me), 6.00–8.10 (m, 39H, bq + NH2 + Ph).
31P{1H} NMR (CD2Cl2 , 125 MHz, 293 K, ppm) d 18.83 (s).
IR (thin film, cmꢀ1): n 2186.7 (IrH), 3361.1 (NH), 3464.2
(NH). Anal. Calcd. for C55H54F6IrN2OP3 : C, 57.04, H, 4.70,
N, 2.42%. Found: C, 56.89, H, 4.66, N, 2.28%. Yield: 80%.
tacts with the ortho and meta PPh3 protons as well as the 3-
protons of bq were seen but no contacts with the Ir–H protons.
The theoretical prediction was therefore in excellent agreement
with the subsequent experimental confirmation by NMR spec-
troscopy.
Conclusion
The equilibrium between the M(H2)(bq-NH2) and MH(bq-
NH3) isomers seems to be decided by ion pairing effects. The-
oretical studies in which no anion is included show that the
M(H2)(bq-NH2) form is predicted to be more stable for all
the phosphines used. The observation of the MH(bq-NH3) iso-
mer for bulky phosphines is therefore anomalous. The calcu-
lated electrostatic potentials for the two isomers suggest that
the counter anion should be paired much closer to the metal
in the M(H2)(bq-NH2) case but that in the MH(bq-NH3) iso-
mer it can be much farther from the metal. Bulky phosphines
seem to favor remote ion pairing and therefore the MH(bq-
NH3) isomer where steric effects disfavor close ion pairing.
4c (PF6 salt). The complex was best prepared by method C.
1H NMR (CD2Cl2 , 500 MHz, 293 K, ppm) d ꢀ16.76 (br. s,
1H, Ir–H), 1.89 (br. s, 2H, H2O), 3.72 (s, 18H, OMe), 6.20–
8.00 (m, 33H, bq + NH2 + Ph). 1H NMR (CD2Cl2 , 500
MHz, 183 K, ppm) d ꢀ16.61 (br. s, Ir–H), 1.60 (br. s, 2H,
H2O), 2.22 (s, 18H, Me), 6.00–8.10 (m, 39H, bq + NH2 + Ph).
31P{1H} NMR (CD2Cl2 , 125 MHz, 293 K, ppm) d 16.69 (s).
IR (thin film, cmꢀ1): n 2212.3 (IrH), 3361.1 (NH), 3469.2
(NH). Anal. Calcd. for C55H54F6IrN2O7P3 : C, 52.57, H,
4.34, N, 2.23%. Found: C, 52.48, H, 4.22, N, 2.25%. Yield:
81%.
4d (BF4 salt). The complex was best prepared by method C.
1H NMR (CD2Cl2 , 500 MHz, 293 K, ppm) d ꢀ16.77 (t, 1H,
2JH–P ¼ 14.65 Hz, Ir–H), 1.50 (br. s, 2H, H2O), 1.60 (s, 6H,
Experimental section
All operations were carried out under argon atmosphere using
standard Schlenk techniques. Solvents were dried over calcium
hydride (CH2Cl2) or sodium/benzophenone (Et2O). Solvents
were degassed prior use. IR spectra were recorded on a Midac
M1200 FT-IR spectrometer. Microanalyses were carried out
by Robertson Microlit Laboratories. 1H NMR and 31P{1H}
NMR spectra were either recorded on GE Omega 300 or GE
Omega 500 spectrometers, chemical shifts were measured with
reference to the residual solvent resonance. The starting
[Ir(cod)L2]X (X ¼ BF4 or PF6) complexes were made as pre-
viously reported.11a,b
1
Me), 6.24 (br. s, 2H, NH2), 6.41–7.63 (m, 27H, bq + Ph). H
NMR (CD2Cl2 , 500 MHz, 183 K, ppm) d ꢀ16.34 (t, 1H,
2JH–P ¼ 14.65 Hz, Ir–H), 1.70 (s, 2H, Me), 6.10 (s, 1H,
NHa) 6.39–7.89 (m, 28H, bq + NHb + Ph). 31P{1H} NMR
(CD2Cl2 , 125 MHz, 293 K, ppm) d 5.08 (s). IR (thin film,
cmꢀ1): n 2186.7 (IrH), 3319.8 (NH), 3412.7 (NH). Anal. Calcd.
for C39H38F4IrN2OP2B: C, 52.53, H, 4.29, N, 3.14%. Found:
C, 52.42, H, 4.19, N, 2.94%. Yield: 42%.
(2-Amino-7,8-benzoquinolinato)(acetone)bis(tricyclohexylphos-
phine)iridium(III) hexafluorophosphate (40e)
(2-Amino-7,8-benzoquinolinato)(solvento)bis(tri(p-tolyl)phos-
phine)iridium(III) salts (4, solvent ¼ water; 40 solvent ¼ acetone)
These compounds can be prepared by one of three related
The complex was best prepared by a variant of method B.
[IrH5(PCy3)2] (0.25 mmol) was dissolved in acetone (10 ml)
to which 1.5 equiv. HPF6 was added. The pale yellow inter-
mediate, [IrH2(OCMe2)2(PCy3)2]PF6 , was precipitated with
Et2O, dried in vacuo (240 mg, 0.22 mmol) and immediately car-
ried forward by redissolving in acetone (10 ml). Addition of 2-
amino-7,8-benzoquinoline (43.6 mg, 0.22 mmol) in acetone (10
ml) is carried out with stirring, followed by precipitation of the
product after 15 min with hexane (100 ml). Recryst: CH2Cl2/
Et2O. 1H NMR (CD2Cl2 , 500 MHz, 293 K, ppm) d ꢀ14.34 (t,
1H, 2JH–P ¼ 14.65 Hz, Ir–H), 0.60–2.00 (m, 72H,
Cy + CH3COCH3), 5.83 (s, 2H, NH2), 6.55–8.30 (m, 7H,
methods. In method
A
(for 4), reported previously,10
[Ir(cod)L2]X (X ¼ BF4 or PF6 ; 0.25 mmol) and 2-aminoben-
zoquinoline (0.25 mmol) in CH2Cl2 (10 ml) are treated with
H2 (1 atm) for 30 min at 0 ꢁC (ice bath). The solvent volume
is reduced by 50% on the vacuum line and the product is pre-
cipitated with hexanes–Et2O (1:1, 10 ml) and recrystallized
from CH2Cl2/Et2O. Method B (for 40) goes through an inter-
mediate acetone complex [IrH2(Me2CO)2L2]X.20 [Ir(cod)L2]X
(0.25 mmol) in acetone (10 ml) is first treated with H2 (1
atm) for 30 min at 0 ꢁC (ice bath). The intermediate is then pre-
cipitated with Et2O then redissolved in acetone (10 ml) to
which is added 2-aminobenzoquinoline (0.25 mmol) in acetone
(10 ml). The mixture is stirred for 30 min and the final product
precipitated with hexanes and recrystallized from CH2Cl2/
Et2O. Method C (for 4), goes through an intermediate aqua
complex [IrH2(H2O)2L2]X.20 [Ir(cod)L2]X (0.25 mmol) in
moist CH2Cl2 (10 ml) is first treated with H2 (1 atm) for
1
bq). H NMR (CD2Cl2 , 500 MHz, 183 K, ppm) d ꢀ14.23 (t,
1H, 2JH–P ¼ 14.65 Hz, Ir–H), 0.60–2.00 (m, 72H,
Cy + CH3COCH3), 6.05 (s, 1H, NHa) 6.70–8.50 (m, 7H, bq),
7.55 (s, 1H, NHb). 31P{1H} NMR (CD2Cl2 , 125 MHz, 293
K, ppm) d 14.86 (s). IR (thin film, cmꢀ1): n 1564 (C O),
=
2176 (IrH), 3364 (NH), 3427 (NH). Anal. Calcd. for
C58H58F6IrN2O7P3 : C, 53.83, H, 4.52, N, 2.16%. Found: C,
53.60, H, 4.43, N, 2.27%. Yield: 82%.
New J. Chem., 2003, 27, 80–87
85

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Observation of reaction with H2
L ¼ PMePh2. 1H NMR (CD2Cl2 , 500 MHz, 293 K, ppm) d
2
ꢀ15.52 (t, 1H, JHP ¼ 12.48 Hz, Ir–H), 1.72 (s, 6H, Me), 5.96
The appropriate salt, 4, (10 mmol) was dissolved in CD2Cl2 (0.5
ml) in an NMR tube with anhydrous MgSO4 (5 mg) also pre-
sent, cooled to ꢀ70 ꢁC (EtOH bath cooled with liq. N2) and H2
passed for 30 min. The NMR tube was transferred to a pre-
cooled probe (ꢀ80 ꢁC) and the spectrum observed.
(s, 2H, NH2), 6.80–8.84 (m, 27H, bq + Ph). IR (thin film,
=
cmꢀ1): n 2021.7 (C O), 2197.0 (Ir–H), 3391.9 (N–H), 3479.5
(N–H). Not obtained analytically pure.
L ¼ PCy3. 1H NMR (CD2Cl2 , 500 MHz, 293 K, ppm) d
2
ꢀ16.63 (t, 1H, JHP ¼ 14.65 Hz, Ir–H), 1.00–2.00 (m, 60H,
Spectrum of 6a. 1H NMR (CD2Cl2 , 500 MHz, 203 K, ppm)
Cy), 6.06 (s, 2H, NH2), 7.0–8.3 (m, 7H, bq). 31P{1H} NMR
d ꢀ25.63 (dt, 1H, 2JH–H ¼ 8.50 Hz, 2JH–P ¼ 14.65 Hz, Ir–H),
0
(CD2Cl2 , 125 MHz, 293 K, ppm) d 2.26 (s). IR (thin film,
2
2
cmꢀ1): n 1994.5 (C O), 2121.5 (Ir–H), 3353.0 (N–H), 3508.2
(N–H). Not obtained analytically pure.
0
ꢀ23.24 (dt, 1H, JH–H ¼ 8.50 Hz, JH–P ¼ 14.65 Hz, Ir–H),
4.56 (br. s, 3H, NH3+), 6.80–8.00 (m, 37H, bq + Ph). 1H
NMR (CD2Cl2 , 500 MHz, 293 K, ppm) d ꢀ25.83 (dt, 1H,
=
2
2
2
0
JH–H ¼ 8.50 Hz, JH–P ¼ 14.65 Hz, Ir–H), ꢀ23.19 (dt, 1H,
Improved synthesis of pentahydridobis(tricyclohexylphosphine)-
iridium(V)
2
0
JH–H ¼ 8.50 Hz, JH–P ¼ 14.65 Hz, Ir–H), 1.70 (br. s, 2H,
free H2O), 4.69 (br. s, 3H, NH3+), 6.80–8.00 (m, 37H,
bq + Ph).
Hydrogen gas was bubbled overnight at room temperature in
an orange suspension of [Ir(cod)Cl]2 (1 eq, 685 mg, 0.98
mmol), PCy3 (4 eq, 1100 mg, 3.92 mmol) and NaOMe (2 eq,
106 mg, 1.96 mmol) in degassed dichloromethane (40 mL),
under stirring. A white solid was obtained by filtration
in vacuo, washed with H2O, then Et2O, and dried in vacuo
(877 mg, 59%). 1H NMR (CD2Cl2 , 500 MHz, 293 K, ppm)
6b. 1H NMR (CD2Cl2 , 500 MHz, 203 K, ppm) d ꢀ25.91 (br.
s, 1H, Ir–H), ꢀ23.38 (br. s, 1H, Ir–H), 2.14 (s, 3H, Me), 4.15
(br. s, 3H, NH3+), 6.56–8.14 (m, 31H, bq + Ph).
6c. 1H NMR (CD2Cl2 , 500 MHz, 203 K, ppm) d ꢀ26.10 (br.
s, 1H, Ir–H), ꢀ23.44 (br. s, 1H, Ir–H), 3.66 (s, 3H, OMe), 4.29
(br. s, 3H, NH3+), 6.42–8.24 (m, 31H, bq + Ph).
2
d ꢀ11.27 (t, 5H, JHP ¼ 12.21 Hz, Ir–H), 1.10–2.25 (m, 66H,
Cy). 31P{1H} NMR (CD2Cl2 , 125 MHz, 293 K, ppm) d
33.42 (s). IR (thin film, cmꢀ1): n(Ir–H) 1929.1. Anal. Calcd.
for C36H68IrP2Cl.0.33CH2Cl2 : C, 55.51, H, 9.19%. Found:
C, 55.65, H, 8.69%. Yield: 59%.
6e. 1H NMR (CD2Cl2 , 500 MHz, 203 K, ppm) d ꢀ25.03 (br.
s, 1H, Ir–H), ꢀ21.06 (br. s, 1H, Ir–H), 0.60–2.40 (m, 60H, Cy),
4.29 (br. s, 3H, NH3+), 6.28–8.62 (m, 7H, bq).
19F,1H-HOESY NMR spectra
5d. 1H NMR (CD2Cl2 , 500 MHz, 208 K, ppm) d ꢀ15.90 (br.
s, 1H, Ir–H), ꢀ3.75 (br. s, 2H, Ir–H2), 1.70 (s, Me), 5.52 (s,
NH2) 6.3–7.8 (m, bq + Ph).
These were obtained following the procedure of ref. 17.
Computational details
All calculations were performed with the Gaussian 98 set
of programs21 within the framework of hybrid DFT
(B3PW91)22 for complexes 7 and 8 and with the ONIOM
method23 for complexes 9, 10, and 11. These three complexes
were optimized at the ONIOM(B3PW91/UFF) level, where
the QM part was treated within the framework of density func-
tional theory at the B3PW91 level22 and the UFF force field24
was used for the molecular mechanics calculations. In all cal-
culations (QM and QM/MM) the iridium atom was repre-
sented by the relativistic effective core potential (RECP)
from the Stuttgart group (17 valence electrons) and its asso-
ciated (8s7p5d)/[6s5p3d] basis set,25 augmented by an f polar-
ization function (a ¼ 0.95). The phosphorus atoms were also
treated with Stuttgart’s RECPs and the associated basis
set,26 augmented by a polarization d function (a ¼ 0.387). A
6-31G(d,p) basis set was used for the atoms directly bound
to Ir (N,C, and H) and the atoms of the NH2 (or NH3) group.
The remaining atoms were treated by a 6-31G basis set. Full
optimizations of geometry without any constraint were
performed for both types of calculations (B3PW91 and
ONIOM(B3PW91/UFF)), followed by analytical computa-
tion of the Hessian matrix to confirm the nature of the located
extrema as minima on the potential energy surface. DG values
were calculated at 298 K within the harmonic frequency
approximation.
(2-Amino-7,8-benzoquinolinato)(carbonyl)bis(tricyclohexylphos-
phine)iridium(III) hexafluorophosphate
For the case of the benzoquinolinate with L ¼ PPh3 , the com-
plex has been reported and structurally characterized.17 For
the present complexes, the same method, treating 4 or 40
(0.15 mmol) in CH2Cl2 (10 ml) with CO (1 atm) for 30 min,
isolating with hexanes, then recrystallizing from CH2Cl2/
hexanes, was used.
L ¼ PPh3. 1H NMR (CD2Cl2 , 500 MHz, 293 K, ppm) d
2
ꢀ14.98 (t, 1H, JHP ¼ 12.21 Hz, Ir–H), 5.66 (s, 2H, NH2),
6.87–7.60 (m, 37H, bq + Ph). 31P{1H} NMR (CD2Cl2 , 125
MHz, 293 K, ppm) d 7.16 (s). IR (thin film, cmꢀ1): n 2026.0
=
(C O), 2222.6 (Ir–H), 3397.1 (N–H), 3510.5 (N–H). Anal.
Calcd. for C50H40F6IrN2OP3 : C, 55.40, H, 3.72, N, 2.58%.
Found: C, 55.07, H, 3.72, N, 2.48%. Yield: 83%. Yellow solid.
L ¼ P(p-C6H4CH3)3. 1H NMR (CD2Cl2 , 500 MHz, 293 K,
2
ppm) d ꢀ15.07 (t, 1H, JHP ¼ 12.21 Hz, Ir–H), 2.24 (s, 18H,
Me), 5.61 (s, 2H, NH2), 6.87–7.62 (m, 31H, bq + Ph).
31P{1H} NMR (CD2Cl2 , 125 MHz, 293 K, ppm) d 5.36 (s).
IR (thin film, cmꢀ1): n 2024.5 (C O), 2227.7 (Ir–H), 3392.0
=
(N–H), 3510.5 (N–H). Anal. Calcd. for C56H52F6IrN2OP3–
CH2Cl2 : C, 54.63, H, 4.34, N, 2.23%. Found: C, 54.94, H,
4.39, N, 2.09%. Yield: 83%. Pale pink solid.
Acknowledgements
1
L ¼ P(p-C6H4OCH3)3. H NMR (CD2Cl2 , 500 MHz, 293
2
K, ppm) d ꢀ15.14 (t, 1H, JHP ¼ 12.21 Hz, Ir–H), 3.74 (s,
This work was supported by grants from the Ministero
´
dell’Universita e della Ricerca Scientifica e Tecnologica
(MURST, Rome, Italy), Programma di Rilevante Interesse
Nazionale, Cofinanziamento 2000-1 (A. M.), the CNRS and
18H, OMe), 5.79 (s, 2H, NH2), 6.57–7.66 (m, 31H, bq + Ph).
31P{1H} NMR (CD2Cl2 , 125 MHz, 293 K, ppm) d 2.85 (s).
IR (thin film, cmꢀ1): n 2023.4 (C O), 2226.9 (Ir–H), 3397.4
=
´
the Universite Montpellier II (E. C. and O. E.), and the
´
DOE (R. H. C., K. G.). R. H. C. thanks the Universite
Montpellier II for a position of invited professor.
(N–H), 3505.8 (N–H). Anal. Calcd. for C56H52F6IrN2O7P3–
CH2Cl2 : C, 50.75, H, 4.03, N, 2.08%. Found: C, 50.31, H,
4.09, N, 1.81%. Yield: 82%. Pale pink solid.
86
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