Heterolytic dihydrogen activation in an iridium complex with a pendant basic group

Article abstract of DOI:10.1039/a808601j

Experimental and theoretical studies show that H₂ reacts with an Ir phosphine complex having a basic pendant amino group to give either an H₂ or an Ir-H^...H-N hydrogen-bonded hydride complex, depending on the basicity of the phosphine ligands.

Full text of DOI:10.1039/a808601j

Heterolytic dihydrogen activation in an iridium complex with a pendant basic
group
Dong-Heon Lee,a Ben P. Patel,a Eric Clot,b Odile Eisenstein*b 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, Université de Montpellier 2, 34095 Montpellier cedex 5, France. E-mail:
b
eisenst@gauss.lsd.univ-montp2.fr
Received (in Bloomington, IN, USA) 4th November 1998, Accepted 22nd December 1998
+
Experimental and theoretical studies show that H2 reacts
+
with an Ir phosphine complex having a basic pendant amino
NH2
group to give either an H2 or an Ir–H···H–N hydrogen-
bonded hydride complex, depending on the basicity of the
phosphine ligands.
NH2
N
Ir
H2
N
Ir
L
L
– H2O
OH2
H2
L
L
H
H
Heterolytic hydrogen activation by a transition metal complex
can occur by deprotonation of an intermediate dihydrogen
complex by an external base, as shown in several studies.1–6 We
have shown that appending an NH2 group at the 2-position of a
cyclometalated 7,8-benzoquinolinate ligand (bq-NH2) can lead
to ligand binding via both M–L bonds and –N–H···L hydrogen
bonds.7 Since the pendant NH2 group is basic, we have now
examined the possibility that the pendant group can act as an
internal base to deprotonate an adjacent H2 ligand.
We have compared the reactivity of a series of bq-NH2
complexes with that of the corresponding unsubstituted benzo-
quinolinate analogue (bq-H) that lacks the pendant group. In the
comparison bq-H system, the known cationic aqua complex,
[IrH(bq-H)(OH2)(PPh3)2]BF4 1, reacts with H2 to give a
cationic H2 complex, 2, previously characterized in detail.8
Addition of ButLi as base leads to deprotonation of the H2
ligand to form the known neutral dihydride, [IrH2(bq-
H)(PPh3)2] 3 (Scheme 1).
4
5
+
+
NH3
N
L
Ir
H
L
H
6
Scheme 2 L = PPh3, a; PMePh2, b; PEt2Ph, c or PBun3, d.
This conversion is completely reversible: removing the dihy-
drogen gas from the solution with a stream of dinitrogen causes
the resonances at d 223.2 and 225.7 to disappear, and the
signal at d 216.1 to reappear in the 1H NMR spectrum.
Complex 6a readily loses H2 in the solid state, and as a result we
were unable to isolate it for elemental analysis or IR studies.
Theoretical work (DFT-B3PW91)† on the model systems
[IrH(bq-NH2)(H2)(PH3)2]+ 7a and [IrH2(bq-NH3)(PH3)2]+ 8a
gave the unexpected result that the H2 complex is the more
stable form by 12 kcal mol21, contrary to the result in the
experimental system. In considering why there might be a
difference between the experimental pair 5/6 and the theoretical
model pair 7/8 (Scheme 3), we considered the possibility that
the change from PPh3 to PH3 was responsible. If so, we felt that
the more basic alkyl phosphines would be a better experimental
comparison.
+
+
H2
N
Ir
N
Ir
L
– H2O
L
OH2
H2
L
L
H
H
1
2
ButLi
Accordingly, we studied the species, [IrH(bq-NH2)(OH2)(P-
3 2
Bun ) ]BF4 4d, made via the standard route,7,8 where the aryl
N
L
groups have been replaced by alkyls. Hydrogen was passed for
1 min at 280 °C in the presence of anhydrous MgSO4 to
facilitate removal of water. Even with such a small change, we
find that the H2 complex, 5d, is now formed on reaction with
H2, not the dihydride 6d. The identity of the H2 complex
Ir
H
L
H
3
Scheme 1
1
followed from the H NMR data, which show two separate
resonances below 230 K, a broad one at d 24.7 for the
The aqua complex, [IrH(bq-NH2)(OH2)(PPh3)2]BF4 4a, in
the new pendant amine system also reacts with H2 to give, not
the corresponding H2 complex, 5a, but via heterolytic activation
to give the dihydride, 6a, instead (Scheme 2). Complex 6a was
fully characterized as [IrH2(bq-NH3)(PPh3)2]BF4 and its iden-
tity is shown, for example, by the spectral data. The reaction is
coordinated H2 molecule and a sharp one at d 217.2 for the
+
+
+
NH2
NH3
N
Ir
N
Ir
1
accompanied by the loss of both the H NMR triplet hydride
L
L
H2
resonance at d 216.4 and the amino resonance at d 6.1
H
L
L
characteristic of 4a, and the appearance of two sharp, mutually
H
H
2
2
coupled (dt, JHHA 8.5; JPH 14.6 Hz) hydride resonances at d
223.2 and 225.7, and a broad resonance at d 3.8, which are
respectively assigned to the cis-hydrides and the NH3+ protons.
8
7
Scheme 3 L = PH3, a; PH2F, b; PHF2, c; PF3, d.
Chem. Commun., 1999, 297–298
297

dihydrogen bond11 is elongated, especially for PH3 (dNH
=
Table 1 IR data for [IrH(bq-NH2)(CO)(L)2](BF4) which shows the order of
1.135 Å) where the hydride is the most basic and the dihydrogen
bond distance is the shortest (dH···H = 1.412 Å). The compounds
cannot be isolated because they lose hydrogen too readily,
consistent with the presence of a strong N–H···H–Ir interaction.
This 1.4 Å H···H distance is significant because it is con-
siderably shorter than any so far suggested for a dihydrogen
bond, consistent with a particularly strong interaction, and is
even in the range proposed for stretched dihydrogen complexes,
so the species could even be considered as representing an
arrested intermediate stage of heterolytic H2 activation. Going
from PH3 to PF3 leads to an elongation of dH···H to 1.618 Å,
however, beyond the upper limit of 1.6 Å proposed for stretched
H2 complexes, showing the strong sensitivity of the structure to
the nature of the phosphine.
donor power of the phosphines studied
Ligand (L)
nCO/cm21
PPh3 (9a)
2026.5
2021.8
2015.3
2004.6
PMePh2 (9b)
PEt2Ph (9c)
PBun3 (9d)
classical hydride. These chemical shifts are close to those for the
unsubstituted analogue [IrH(bq-H)(H2)(PPh3)2]BF4 2.8 The HD
complex (d1-5b)shows a 1JHD coupling of 29.3 Hz.
Similar results were obtained with other basic phosphines,
PEt2Ph and PMePh2, so the result is general. In no case was an
equilibrium seen between dihydrogen complex 5 and hydride 6.
This implies that moving to a more basic phosphine decreases
the acidity of the coordinated H2 ligand in 5 and correspond-
ingly increases the basicity of the terminal hydride in 6. This
results in the proton moving completely from the NH3+ group of
6 to the terminal hydride to give 5 on changing from PPh3 to any
of the more basic phosphines.
This work shows that the very strong dependence1,2,20 of the
pKa of the H2 ligand has interesting effects on the reactivity of
these complexes toward H2 and on the structure of the resulting
hydrides even with relatively small changes in the ligand
sphere. The pKa of the H2 ligand seems to be unusually sensitive
to back bonding effects, even though the changes in dHH are
relatively modest (Table 2). From a theoretical standpoint, great
care should be taken in modelling such systems.
This large change of structure is rather surprising for such a
relatively small change in ligand, and we wanted to verify that
the basicity of the phosphines was indeed varying in the
expected manner. Reaction of the aqua complex, 4 with CO (1
atm) in CH2Cl2 gave the carbonyl species [IrH(bq-
NH2)(CO)(L)2]BF4 (9, L = PPh3, a; PMePh2, b; PEt2Ph, c;
We thank the NSF (R. H. C.), the University of Montpellier
and CNRS (O. E.) for funding and Bruno Chaudret (Toulouse)
for discussions.
Notes and references
PBun , d). The IR data obtained for these complexes (Table 1)
3
† Computational details: All the calculations were performed with the
Gaussian 94 set of programs12 at the B3PW91 level13,14 level. Iridium was
represented with the Hay–Wadt relativistic core potential (ECP) for the 60
innermost electrons and its associated double-z basis set.15 Phosphorus
atoms were also represented with Los Alamos ECPs and their associated
double-z basis set16 augmented by a polarisation d function.17 A 6-31G(d,p)
basis set18,19 was used for the atoms bound directly to Ir (H, C, and N) and
for the atoms of the amido group (N and H), all the other atoms have been
described with a 6-31G basis set.18 Full geometry optimisations within Cs
symmetry (the bq-NH2 ligand is planar) have been carried out within the
framework of DFT (B3PW91).
verifies that the basicity is indeed higher for the more highly
alkylated phosphines. Having only one CO but two L groups
makes this system much more sensitive to change of phosphine
than Tolman’s LNi(CO)3 system,9 but the trends are essentially
the same.
Returning to the theoretical work, we have a rare case where
the quantum model PH3 is inadequate to reproduce the
experimental observations on a PPh3 complex. In order to
model the more electron-accepting PR3 groups, we have now
moved to PFH2 (7b/8b), PF2H (7c/8c), and PF3 (7d/8d); of
course these are far more electron-withdrawing than the
experimental PPh3 group, but we were only interested in seeing
if the correct trends could be reproduced. Indeed, as shown in
Table 2, the calculated energies for 7 and 8 do alter in the
expected fashion, confirming that the acid/base character of the
Ir–H/Ir–(H2) system shows an unexpectedly strong dependence
on the nature of the phosphine.
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Table 2 The theoretical structural (Å) and energetic parameters (kcal
mol 2 1) of dihydrogen complex 7 and dihydride 8 with change of
phosphine
9 A. Tolman, Chem Rev., 1977, 77, 313.
10 K. Gruet, D.-H. Lee and R. H. Crabtree, unpublished work.
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7
8
Ligand
dHH
dH···H
dNH
dH···H
DEa
12 Gaussian 94, Revision B.3, M. J. Frisch, G. W. Trucks, H. B. Schlegel,
P. M. W. Gill, B. G. Johnson, M. A. Robb, J. R. Cheeseman, T. Kerth,
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Stewart, M. Head-Gordon, C. Gonzalez and J. A. Pople, Gaussian Inc,
Pittsburgh PA, 1995.
PH3
0.861
0.851
0.856
0.847
1.854
1.829
1.873
1.849
1.135
1.090
1.093
1.073
1.412
1.548
1.502
1.618
212
22.5
+0.5
+3.0
PFH2
PF2H
PF3
a E(8) 2 E(7) is reported, so negative values imply the dihydrogen complex
is more stable.
13 A. D. Becke, J. Chem. Phys., 1993, 98, 5648
The structural parameters of 7 and 8 from the theoretical
work (Table 2) are in good agreement with the solid state
structure of [IrH(bq-NH2)(CO)(L)2]PF6 for which experimental
structural data is available.10 For all phosphine ligands, 7 and 8
both correspond to energy minima. In 7, the dihydrogen ligand
is always coplanar with the cis-Ir–H bond and the pendant NH2
group is coplanar and conjugated with the bq ligand. As
expected, the H–H distance in 7 decreases (from 0.861 to 0.847
Å) with increasing F substitution on the phosphine (Table 2)
showing the effects of the diminished back-donation along the
series. In 8, the N–H bond interacting with the hydride via a
14 J. P. Perdew and Y. Wang, Phys. Rev. B, 1992, 45, 13244
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Communication 8/08601J
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