Addition of H2 on (sulfur, phosphorus, sulfur)-pincer-based rhodium(I), iridium(I), palladium(II), and platinum(II) complexes: Reactivity and regioselectivity

Article abstract of DOI:10.1021/om050900u

The addition of H₂ on (sulfur, phosphorus, sulfur)-pincer-based 16-VE complexes was studied from both experimental and theoretical points of view. The reaction of H₂ with [M(SPS)(PPh3)] (M = Rh, Ir) led to the formation of dihydride M(III) complexes, which was fully characterized with indium as metal. No reaction occurred with the isoelectronic 16-VE Pt(II) [Pt(SPS)Cl] complex. This reaction was also studied by means of DFT/B3PW91 calculations on the model complexes [M(SPS)(PH3)] (M = Rh, Ir) and by B3PW91//ONIOM(B3PW91:UFF) calculations on real systems. The regioselectivity observed and the greater reactivity of the Ir compound were properly reproduced and related through a thermodynamic cycle to the electronic properties of the metal complex and to the energetics of the M-H bonds under formation. For both Rh and Ir complexes, the transition state for the formation of the dihydride product is a molecular dihydrogen complex located about 9 kcal mol^-1 above the reactants. The reactivity of Pd(II) and Pt(II) analogues ([M(SPS)(X)], X = Cl, PH₃⁺) was also studied, and the formation of M(IV) dihydrides was found to be much less favorable, especially for M = Pd. Non-oxidative-addition processes leading to molecular dihydrogen complexes or to an heterolytic cleavage of H₂ were also explored, but none of them were found to be favored on ΔG grounds.

Full text of DOI:10.1021/om050900u

Organometallics 2006, 25, 1101-1111
1101
Addition of H2 on (Sulfur, Phosphorus, Sulfur)-Pincer-Based
Rhodium(I), Iridium(I), Palladium(II), and Platinum(II) Complexes:
Reactivity and Regioselectivity
Marjolaine Doux, Louis Ricard, Pascal Le Floch,* and Yves Jean*
Laboratoire “He´te´roe´le´ments et Coordination”, UMR CNRS 7653, De´partement de Chimie,
EÄ cole polytechnique, 91128 Palaiseau Ce´dex, France
ReceiVed October 18, 2005
The addition of H2 on (sulfur, phosphorus, sulfur)-pincer-based 16-VE complexes was studied from
both experimental and theoretical points of view. The reaction of H2 with [M(SPS)(PPh3)] (M ) Rh, Ir)
led to the formation of dihydride M(III) complexes, which was fully characterized with iridium as metal.
No reaction occurred with the isoelectronic 16-VE Pt(II) [Pt(SPS)Cl] complex. This reaction was also
studied by means of DFT/B3PW91 calculations on the model complexes [M(SPS)(PH3)] (M ) Rh, Ir)
and by B3PW91//ONIOM(B3PW91:UFF) calculations on real systems. The regioselectivity observed
and the greater reactivity of the Ir compound were properly reproduced and related through a
thermodynamic cycle to the electronic properties of the metal complex and to the energetics of the M-H
bonds under formation. For both Rh and Ir complexes, the transition state for the formation of the dihydride
product is a molecular dihydrogen complex located about 9 kcal mol-1 above the reactants. The reactivity
+
of Pd(II) and Pt(II) analogues ([M(SPS)(X)], X ) Cl, PH3 ) was also studied, and the formation of
M(IV) dihydrides was found to be much less favorable, especially for M ) Pd. Non-oxidative-addition
processes leading to molecular dihydrogen complexes or to an heterolytic cleavage of H2 were also
explored, but none of them were found to be favored on ∆G grounds.
Chart 1
Introduction
Since the pioneering work by Moulton and Shaw,1 there has
been a considerable and continuing interest in the synthesis and
use of pincer ligands in coordination chemistry and catalysis.2
This is largely due to their particular backbone that not only
allows them to meridionally encapsulate transition metals but
also permits a fine-tuning of their coordination behavior and
their electronic properties. Recently, we reported on the synthesis
and the coordination chemistry of SPS-based pincer ligands
featuring two ancillary PdS ligands and a phosphorus atom as
the central moiety.3,4 Among these complexes, the 16-VE
square-planar Rh(I) complex [Rh(SPS)(PPh3)] (1) (Chart 1)
proved to be particularly reactive toward small molecules such
as O2 and CS2 and the corresponding 18-VE Rh(III) complexes
[Rh(η2-O2)(SPS)(PPh3)] and [Rh(η2-CS2)(SPS)(PPh3)] were
structurally characterized (Scheme 1).5 Additionally, complex
1 also reacted with CO and SO2 to yield the expected
η1-coordinated species. Later it was found that the iridium
counterpart of 1, [Ir(SPS)(PPh3)] (2), could also activate O2.6
Scheme 1
In all these cases, coordination of a new molecule occurred
without ligand displacement and induced a complete change of
the geometry of the complex from square planar (complexes 1
and 2) to octahedral (for O2 and CS2), trigonal bipyramidal (for
CO), or square pyramidal (for SO2). Moreover, in all cases, the
attack of the incoming ligand occurred regioselectively, the
coordination taking place syn to the PMe moiety. This surprising
planar discrimination was recently rationalized through DFT
calculations in the case of O2 and CO.6,7
* To whom correspondence should be addressed. E-mail:
yves.jean@poly.polytechnique.fr (Y.J.); lefloch@poly.polytechnique.fr (P.L.F.)
(1) Moulton, C. J.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1976,
1020-1024.
(2) (a) Albrecht, M.; Van Koten, G. Angew. Chem., Int. Ed. 2001, 40,
3750-3781. (b) van der Boom, M. E.; Milstein, D. Chem. ReV. 2003, 103,
1759-1792. (c) Singleton, J. T. Tetrahedron Lett. 2003, 59, 1837-1857.
(3) (a) Doux, M.; Me´zailles, N.; Melaimi, M.; Ricard, L.; Le Floch, P.
Chem. Commun. 2002, 1566-1567. (b) Doux, M.; Ricard, L.; Le Floch,
P.; Me´zailles, N. Dalton Trans. 2004, 2593-2600.
(4) Doux, M.; Me´zailles, N.; Ricard, L.; Le Floch, P. Eur. J. Inorg. Chem.
2003, 3878-3894.
The activation of dihydrogen by transition-metal complexes
has been studied extensively for the two past decades and is of
(5) Doux, M.; Me´zailles, N.; Ricard, L.; Le Floch, P. Organometallics
2003, 22, 4624-4626.
(6) Doux, M.; Ricard, L.; Le Floch, P.; Jean, Y. Organometallics 2005,
24, 1608-1613.
(7) Doux, M.; Le Floch, P.; Jean, Y. J. Mol. Struct. (THEOCHEM) 2005,
724, 73-79.
10.1021/om050900u CCC: $33.50 © 2006 American Chemical Society
Publication on Web 01/31/2006

1102 Organometallics, Vol. 25, No. 5, 2006
Doux et al.
major importance in chemistry8 and biology.9 As part of our
program on the activation of small molecules by square-planar
SPS-based group 9 metal complexes, we recently explored the
reactivity of complexes 1 and 2 toward H2 from both an
experimental and theoretical point of view. These results are
presented herein, as well as a theoretical investigation on the
reactivity of H2 toward other d8 metal complexes of group 10
metals featuring the same SPS ligand such as [M(SPS)X] (M
) Pd, Pt, X ) Cl, PH3+) complexes.
Experimental Results
The reactivity of complexes 1 and 2 toward H2 was studied
by bubbling dihydrogen (1 atm) at -78 °C into freshly prepared
solution of the complexes. Both reactions resulted in a rapid
color change of the solution from brown to orange. Significant
changes were also observed in the 31P NMR spectra of the crude
mixtures (eq 1). For the rhodium complex 3, a second-order
Figure 1. ORTEP view of complex 4. Atoms are drawn as 50%
thermal ellipsoids. The numbering is arbitrary and different from
that used in NMR data. Selected bond lengths (Å) and angles (deg)
are given in Table 3.
Table 1. Crystal Data and Structural Refinement
Details for 4
formula
Mr
C60H51IrP4S2‚2CH2Cl2
1322.06
T (K)
150.0(1)
cryst syst
space group
a (Å)
b (Å)
c (Å)
triclinic
P1h
11.3410(10)
14.4510(10)
18.6570(10)
85.2690(10)
77.1100(10)
73.9350(10)
2863.5(4)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
31P NMR spectrum was observed which did not give structural
information about the structure (AB2C spin pattern with PA )
PMe, PB ) PPh2, PC ) PPh3). On the other hand, the AB2Y
spin system 31P NMR spectrum of the iridium complex 4
indicated a symmetrical structure with two magnetically equiva-
lent Ph2PB(S) ancillary sidearms (δ(THF-d8) 50.3 ppm, 2J(PB-
PA) ) 107.0 Hz, and 3J(PB-PY) ) 27.7 Hz with PA ) PMe, PB
) PPh2, and PY ) PPh3). Moreover, the coupling constant
2J(PA-PY) of 336.0 Hz indicates a trans relation between the
PMe and the PPh3 moieties. The dihydride nature of complexes
2
F (g cm-3
)
1.533
2.742
µ (cm-1
)
cryst size (mm)
F(000)
index ranges
0.21 × 0.20 × 0.12
1328
-14 e h e 15; -20 e k e 20;
-24 e l e 26
30.03/>2σ(I)
673; 21
40 104
16 646
14 664
0.0664
0.0281
1.018
2θmax (deg)/criterion
no. of params refined; data/param
no. of rflns collected
no. of indep rflns
no. of rflns used
wR2
1
3 and 4 was evidenced by H NMR spectroscopy. In each
spectrum, a characteristic chemical shift at -14.64 ppm for 3
and -18.39 ppm for 4 integrating for two protons was observed.
The hydride signals appeared as a virtual triplet for 4 (2J(H-
R1
goodness of fit
2
PA) ) J(H-PY) ) 15.5 Hz with PA ) PMe and PY ) PPh3)
largest diff peak/hole (e Å-3
)
1.216 (0.115)/-1.185 (0.115)
2
and as a doublet of virtual triplets for 3 (2J(H-PA) ) J(H-
PC) ) 12.3 Hz, 1J(H-Rh) ) 24.5 Hz with PA ) P-Me and PC
) PPh3). Apart from the dihydride signals, the 1H NMR spectra
of complexes 3 and 4 are very similar. The stabilities of
complexes 3 and 4 appeared to be very different. The solution
containing the rhodium complex 3 evolved at room temperature
under a N2 atmosphere to yield back the square-planar complex
1 within several minutes, and despite many efforts, complex 3
could not be isolated or crystallized, evaporation of the solvent
leading to the loss of the H2 ligand. Therefore, complex 3 was
only characterized by means of 31P and 1H NMR spectroscopy.
The iridium complex 4 proved to be much more stable and was
isolated and fully characterized by 1H, 13C, and 31P spectroscopy
and elemental analysis.
On the basis of the extreme similarity between the 1H NMR
spectra of 3 and 4, the same geometries for these complexes
can be proposed with a high level of confidence. However, to
fully ascertain the structure of the dihydride complex 3, we
attempted to get structural information from DFT calculations
(vide supra).8c The formulation of 4 was definitively established
through an X-ray crystal structure analysis. An ORTEP view
of one molecule of 4 is presented in Figure 1, crystal data and
structural refinement details are given in Table 1, and relevant
bond distances and angles are listed in Table 3. As can be seen
in Figure 1, complex 4 adopts an octahedral geometry (S1-
Ir-S2 ) 90.14(2)°, S1-Ir-H1 ) 92.6(8)°, H1-Ir-H2 )
86(1)°). Interestingly, in this case again, the addition of H2
occurs syn to the PMe moiety, as in O2, CS2, CO, and SO2
(8) (a) Kubas, G. J. Metal Dihydrogen and Sigma-Bonded Complexes:
Structure, Theory and ReactiVity, 1st ed.; Kluwer Academic/Plenum:
London, 2001. (b) Recent AdVances in Hydride Chemistry; Peruzzini, M.
Poli, R., Eds.; Elsevier: Amsterdam, 2001. (c) Maseras, F.; Lledos, A.;
Clot, E.; Eisenstein, O. Chem. ReV. 2000, 100, 601-636.
(9) Adams, M. W. W.; Stiefel, E. I. Curr. Opin. Chem. Biol. 2000, 4,
214-220.
1
complexes. As suggested by the H NMR, an examination of
the Ir-H (1.57(2) and 1.54(2) Å) and H-H distances (∼2.116
Å) revealed that 4 is an iridium(III) dihydride complex. Other

Addition of H2 on SPS-Pincer-Based Complexes
Organometallics, Vol. 25, No. 5, 2006 1103
Scheme 2
Table 2. Energy Decomposition (according to Scheme 3) for
the Formation of the syn-SS (I), anti-PP (II), syn-PP (III),
and anti-SS (IV) Dihydride Isomers by Addition of H2 on the
[M(SPS)(PH3)] Complexa
was used (note that no NMR experiments under pressure have
been carried out).
(a) M ) Rh
syn-SS
(Ia)
anti-PP
(IIa)
syn-PP
(IIIa)
anti-SS
(IVa)
∆Edist(ML4)
∆ES/T
∑ ) ∆ET,dist
-∆El(H-H)
2[∆El(M-H)]
∆ER
+27.5
-4.0
+23.5
+107.3
-144.8
-14.0
+30.7
-4.0
+26.8
+107.3
-130.6
+3.5
+32.1
+1.4
+33.5
+107.3
-132.1
+8.7
+30.4
+1.1
+31.5
+107.3
-141.5
-2.7
Theoretical Approach
The addition of H2 on the SPS-type pincer-based Rh(I), Ir-
(I), Pd(II), and Pt(II) complexes was studied by means of DFT
calculations. Four modes of attack were considered (syn-SS,
anti-SS, syn-PP, and anti-PP) for the cis addition of H2,
depending whether the incoming H2 molecule is located (i) syn
or anti to the P-Me bond and (ii) coplanar to the M-S bonds
(SS) or to the M-P bonds (PP) (Scheme 2).
Group 9 (M ) Rh, Ir). Calculations were performed on the
model complexes (quoted as [M(SPS)(PH3)]), in which all
phenyl groups were replaced by H atoms, and on the real
complexes (quoted as [M(SPS)(PPh3)]). The regioselectivity of
the addition and the influence of the nature of the metal center
on both the transition state energy and the exothermicity of the
addition reaction are related through a thermodynamic cycle to
the electronic properties of the metal complex and to the energies
of the M-H bonds formed.
(b) M ) Ir
syn-SS
(Ib)
anti-PP
(IIb)
syn-PP
(IIIb)
anti-SS
(IVb)
∆Edist(ML4)
∆ES/T
∑ ) DET,dist
-∆El(H-H)
2[∆El(M-H)]
∆ER
+36.4
-8.0
+28.4
+107.3
-170.9
-35.2
+36.9
-1.2
+35.7
+107.3
-161.0
-18.0
+40.8
+1.7
+42.5
+107.3
-162.7
-12.9
38.9
-3.2
+35.7
+107.3
-162.7
-19.7
a Energies are given in kcal mol-1
.
Table 3. Selected Theoretical Parameters Optimized for the
Complex [Ir(SPS)(PR3)(H)2] at the DFT-B3PW91 (R ) H,
Model Complex Ib) and ONIOM(B3PW91:UFF) (R ) Ph,
Real Complex Ib-rc) Levels of Calculation and Experimental
Values for the Complex [Ir(SPS)(PPh3)(H)2] (4)a
DFT-B3PW91 ONIOM(B3PW91:UFF)
X-ray
Different geometries of the [Rh(SPS)(PH3)(H)2] complexes
were optimized by considering four modes of attack, as defined
in Scheme 2. In each case a minimum-energy structure was
characterized. In agreement with the planar discrimination
reported previously for this complex, the syn-SS isomer was
found to be the most stable, the energy ordering being as follows
(in kcal mol-1): syn-SS (0) (Ia) < anti-SS (+11.3) (IVa) <
anti-PP (+17.5) (IIa) < syn-PP (+22.7) (IIIa). Furthermore,
the syn-SS isomer is the only isomer which is significantly more
stable than the isolated reactants (∆ER ) -14.0 kcal mol-1).
The structures of the four minima (Ia-IVa) are given in Figure
2, together with selected geometrical parameters. In the four
isomers, both the H‚‚‚H and the Rh-H distances are consistent
with a cis-dihydride structure (in the most stable syn-SS isomer
(Ia) H‚‚‚H ) 2.085 Å and Rh-H ) 1.550 Å). The syn-SS (Ia),
anti-PP (IIa), and syn-PP (IIIa) isomers can be described as
Rh(III) octahedral complexes, while the anti-SS attack induced
the decoordination of a sulfur ligand, leading to a Rh(III) center
in a distorted square base pyramidal environment with a hydride
at the apical position (IVa).
The electronic factors responsible for the regioselectivity of
the cis oxidative addition of H2 can be analyzed by means of a
thermodynamic cycle which connects the separate reactants
(1[Rh(SPS)(PH3)] + H2) to the dihydride species (Scheme 3).
The reaction is decomposed in the following steps: (i) distortion
of the 1[Rh(SPS)(PH3)] complex from its equilibrium geometry
to its geometry in the product (∆Edist(ML4)); (ii) formation of
the triplet state of this distorted metal fragment (∆ES/T) with
(Ib, R ) H)
(Ib-rc, R ) Ph)
(4, R ) Ph)
Ir-P1
2.289
2.295
1.583
1.583
2.558
2.557
2.175
1.834
1.795
1.795
1.761
1.761
2.017
2.017
100.4
313.6
91.5
2.272
2.334
1.583
1.585
2.526
2.524
2.133
1.835
1.802
1.803
1.784
1.784
2.022
2.024
101.4
312.1
85.7
2.2635(6)
2.2920(6)
1.57(2)
Ir-P4
Ir-H1
Ir-H2
1.54(2)
Ir-S1
2.4717(6)
2.4514(5)
2.116
Ir-S2
H1-H2
P1-Me
1.824(2)
1.785(2)
1.799(2)
1.771(2)
1.762(2)
2.0140(8)
2.0127(7)
101.8(1)
310.9
90.14(2)
176.70(2)
92.6(8)
86(1)
111.7(1)
111.3(1)
P1-C2
P1-C6
P2-C2
P3-C6
P2-S1
P3-S2
C2-P1-C6
∑(C-P1-C)
S1-Ir-S2
P1-Ir-P4
S1-Ir-H1
H1-Ir-H2
P1-C2-P2
P1-C6-P3
173.2
90.8
86.8
111.7
111.7
171.0
93.6
84.7
101.0
101.5
a Distances are given in Å and angles in deg. The numbering of atoms
is shown in Figure 4.
geometricalparameters are quite similar to those for other SPS-
based group 9 complexes.5,6
Similar experiments were carried out with the isoelectronic
Pt(II) complex 5 (eq 2), but no other compound than the starting
material could be observed even when a high pressure of H2

1104 Organometallics, Vol. 25, No. 5, 2006
Doux et al.
Figure 2. Optimized geometries of unsubstituted rhodium complexes Ia-VIa. Hydrogen atoms are omitted for clarity, except for those
bound to the metal center. Selected geometrical parameters (bond distances in Å and angles in deg) are as follows. Ia: Rh-H ) 1.550;
S1-Rh-S2 ) 93.9, P1-Rh-P4 ) 171.3. IIa: Rh-H1 ) 1.574, Rh-H2 ) 1.565; S1-Rh-S2 ) 172.2, P1-Rh-P4 ) 101.7. IIIa:
Rh-H1 ) 1.603, Rh-H2 ) 1.554; S1-Rh-S2 ) 172.0, P1-Rh-P4 ) 106.2. IVa: Rh-H1 ) 1.510, Rh-H2 ) 1.547, Rh-S1 ) 2.480,
Rh-S2 ) 4.736; S1-Rh-H2 ) 124.6, S1-Rh-H1 ) 165.3, P1-Rh-P4 ) 172.1. Va: Rh-H1 ) 1.638, Rh-H2 ) 1.684, Rh-S1 )
2.426, Rh-S2 ) 2.426. VIa: Rh-H1 ) 1.562, S2-H2 ) 1.358, Rh-S1 ) 2.530, Rh-S2 ) 3.252; S1-Rh-H1 ) 168.0. E is the relative
energy, given in kcal mol-1
.
Scheme 3
two unpaired electrons (the metal fragment is now in the optimal
situation to bind two additional ligands); (iii) breaking of the
H-H bond (-∆El(H2)); (iv) formation of the two Rh-H bonds
(2[∆El(M-H)]).6,7,10 The energy variation associated with the
addition reaction can thus be expressed as ∆ER ) ∆Edist(ML4)
+ ∆ES/T - ∆El(H-H) + 2[∆El(M-H)]. In the following
discussion, the first two terms will be most often considered
together, since their sum (quoted as ∑ ) ∆ET,dist) represents
the energy required to “prepare” the metal fragment for the
oxidative addition reaction. This energy decomposition scheme
clearly shows that the addition process of H2 is favored by a
low ∆ET,dist value and a large M-H bonding energy.
The results of this energy decomposition are given in Table
2a. As has been shown previously,6,7,10 the triplet state energy
of a metal fragment turns out to be an important factor in
predicting its reactivity for an oxidative addition reaction. As a
matter of fact, the most stable isomer (syn-SS, Ia) has the
(10) (a) Lesnard, H.; Demachy, I.; Jean, Y.; Lledos, A. Chem. Commun.
2003, 850-851. (b) Tomas, J.; Lledos, A.; Jean, Y. Organometallics 1998,
17, 4932-4939. (c) Su, M.-D.; Chu, S.-Y. J. Am. Chem. Soc. 1997, 119,
5373-5383.

Addition of H2 on SPS-Pincer-Based Complexes
Organometallics, Vol. 25, No. 5, 2006 1105
Figure 3. Optimized geometries of unsubstituted iridium complexes Ib-VIb. Hydrogen atoms are omitted for clarity, except for those
bound to the metal center. Selected geometrical parameters (bond distances in Å and angles in deg) are as follows. Ib: Ir-H ) 1.583;
S1-Ir-S2 ) 91.5, P1-Ir-P4 ) 171.2. IIb: Ir-H1 ) 1.611, Ir-H2 ) 1.607; S1-Ir-S2 ) 173.0, P1-Ir-H1 ) 168.9, P4-Ir-H2 )
171.6. IIIb: Ir-H1 ) 1.632, Ir-H2 ) 1.596; S1-Ir-S2 ) 172.5, P1-Ir-P4 ) 106.0, P1-Ir-H1 ) 172.1. IVb: Ir-H1 ) 1.551, Ir-H2
) 1.567, Ir-S1 ) 2.410, Ir-S2 ) 4.735; S1-Ir-H1 ) 137.5, S1-Ir-H2 ) 152.6, P1-Ir-P4 ) 1723.0. Vb: Ir-H1 ) 1.650, Ir-H2 )
1.696; H1-Ir-H2 ) 180.0. VIb: Ir-H1 ) 1.588, S2-H2 ) 1.356, Ir-S1 ) 2.475, Ir-S2 ) 3.260; S1-Ir-H1 ) 166.3. E is the relative
energy, given in kcal mol-1
.
smallest ∆ET,dist value.11 Furthermore, formation of this isomer
is also favored by the largest Rh-H bond energy. The planar
discrimination between the syn-SS and anti-SS attacks (∆(∆ER)
) 11.3 kcal mol-1 in favor of the former) and between the syn-
PP and anti-PP isomers (∆(∆ER) ) 5.2 kcal mol-1 in favor of
the latter) can be mainly traced to the triplet state energy of the
metal fragment: ∆(∆ET,dist) ) +8.0 kcal mol-1 for the SS
isomers and +6.7 kcal mol-1 for the PP isomers.
Although the syn-SS isomer was found to be more stable than
the isolated reactants by 14.0 kcal mol-1, it must be noted that
the entropy term does not favor this addition process. The
computed ∆G value is actually close to zero (∆GR ) -2.2 kcal
mol-1), a result which is in agreement with the reversibility of
the addition observed in solution. Note that the cis-trans
isomerization is not expected to occur in the dihydride complex,
the energy of the isomer with the hydrides trans to each other
(Figure 2, Va) being located 24.1 kcal mol-1 above that of the
cis (syn-SS) isomer (Ia).12,13 Finally, a second pathway has been
envisioned for the activation of dihydrogen by the square-planar
Rh(I) complex [Rh(SPS)(PH3)]. The complex resulting from an
heterolytic activation of H2 (H- attacking the metal center and
H+ reacting with a lone pair of a decoordinated S ancillary
ligand) has been studied.14 Heterolytic activation is classical
for metal sulfide compounds.15 The resulting nearly square-
planar Rh(I) complex (quoted as [Rh(H)(SP‚‚‚SH)(PH3)])
(Figure 2, VIa) was found to be located 34.9 kcal mol-1 above
Ia, so that its formation can be excluded on energetics grounds.
Note that our attempts to find a stable molecular dihydrogen
complex failed, the starting complexes evolving toward a
previously optimized dihydride structure.
Similar calculations were performed on the Ir counterpart.
The same energy ordering was found for the four isomers
depicted in Scheme 2 (kcal mol-1): syn-SS (0) (Ib) < anti-SS
(+15.5) (IVb) < anti-PP (+17.2) (IIb) < syn-PP (+22.3)
(IIIb). Structures of the four minima (Ib-IVb) are presented in
Figure 3 together with selected geometrical parameters. The
lowest energy structure (syn-SS, Ib) is in agreement with the
experimental structure (Figure 1), and the main geometrical
parameters are satisfactorily reproduced (Table 3): Ir-H )
1.583 Å (exptl 1.55(2) Å average), Ir-P1 ) 2.289 Å (exptl
2.2635(6) Å), Ir-P4 ) 2.295 Å (exptl 2.2920(6) Å), Ir-S1 )
2.558 Å (exptl 2.4717(6) Å), Ir-S2 ) 2.557 Å (exptl 2.4514-
(6) Å). The structures of the three other isomers resemble those
(14) (a) Gruet, K.; Clot, E.; Eisenstein, O.; Lee, D. H.; Patel, B.;
Macchioni, A.; Crabtree, R. H. New J. Chem. 2003, 27, 80-87. (b) Lee,
D. H.; Patel, B. P.; Clot, E.; Eisenstein, O.; Crabtree, R. H. Chem. Commun.
1999, 297-298. (c) Collman, J. P. Nat. Struct. Biol. 1996, 3, 213-217. (d)
Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and
Applications of Organotransition Metal Chemistry; University Science
Books: Mill Valley, CA, 1987. (e) Clapham, S. E.; Hadzovic, A.; Morris,
R. H. Coord. Chem. ReV. 2004, 248, 2201-2237. (f) Abdur-Rashid, K.;
Clapham, S. E.; Hadzovic, A.; Harvey, J. N.; Lough, A. J.; Morris, R. H.
J. Am. Chem. Soc. 2002, 124, 15104-15118.
(15) (a) Linck, R. C.; Pafford, R. J.; Rauchfuss, T. B. J. Am. Chem. Soc.
2001, 123, 8856-8857. (b) Ienco, A.; Calhorda, M. J.; Reinhold: J.; Reineri,
F.; Bianchini, C.; Peruzzini, M.; Vizza, F.; Mealli, C. J. Am. Chem. Soc.
2004, 126, 11954-11965. (c) Sellmann, D.; Prakash, R.; Heinemann, F.
W.; Moll, M.; Klimowicz, M. Angew. Chem., Int. Ed. 2004, 43, 1877-
1880.
(11) The triplet state is found to be more stable than the singlet state in
some of the distorted metal fragments. This result should be considered
with some caution, since DFT calculations are often considered to
overstabilize the high spin states.
(12) The addition of H2 on Vaska’s complexes also leads to octahedral
complexes with cis dihydrides.
(13) Deutsch, P. P.; Eisenberg, R. Chem. ReV. 1988, 88, 1147-1161.

1106 Organometallics, Vol. 25, No. 5, 2006
Doux et al.
described above for the Rh analogue. The energy decomposition,
according to the thermodynamic cycle given in Scheme 3, also
reveals similar trends (Table 2b). In particular, the formation
of the syn-SS isomer is favored both by the lowest value for
∆ET,dist and by the largest Ir-H bond energy.16 No cis-trans
isomerization is expected to occur in the syn-SS isomer, since
the trans isomer (Vb) was located 21.3 kcal mol-1 higher in
energy. Finally, the nearly square-planar monohydride complex
[Ir(H)(SP‚‚‚SH)(PH3)] (VIb), resulting from an heterolytic
activation of H2, was also studied. It was found to be 45.3 kcal
mol-1 higher in energy than Ib; thus, its formation can be
excluded.
The major change on going from the Rh to the Ir complex is
the large increase of the exothermicity of the addition reaction,
from -14.0 to -35.2 kcal mol-1. The use of the thermodynamic
cycle described in Scheme 3 offers a simple explanation for
this result. Comparison of the energy decomposition for the Rh
(Ia) and Ir (Ib) syn-SS isomers (Table 2) shows that the
energetic term associated with the distortion of the metal
fragment (∆ET,dist) works in favor of the Rh complex, by 4.9
kcal mol-1. The factor which makes the reaction with the Ir
complex more exothermic is the M-H bonding energy, which
is larger for Ir (-85.45 kcal mol-1 per bond) than for Rh (-72.4
kcal mol-1 per bond).17 This result exemplifies another important
factor in predicting the reactivity of a complex for an oxidative
addition reaction: the energetics of the metal-ligand bonds
under formation. The larger exothermicity found for the iridium
complex nicely rationalizes the different behavior of two
complexes with respect to the addition of H2. As a matter of
fact, ∆GR is now largely negative (-22.2 kcal mol-1 for Ir
instead of -2.2 kcal mol-1 for Rh), in agreement with the
experimental observation of an irreversible formation of a
dihydride upon H2 addition on the Ir complex.
Figure 4. Optimized structure of the complex [Ir(SPS)(PPh3)(H2)]
(Ib-rc) at the ONIOM (B3PW91:UFF) level of calculation. Atoms
included in the QM part are shown in ball-and-stick format, and
atoms included in the MM part are represented by tubes.
Table 4. Energy Decomposition (According to Scheme 3) for
the Formation of the syn-SS Complexes by Addition of H2
on the Model Complexes [M(SPS)(PH3)] (M ) Rh (Ia), Ir
(Ib)) and on the Real Complexes [M(SPS)(PPh3)] (M ) Rh
(Ia-rc), Ir (Ib-rc))a
Rh (Ia)
Rh (Ia-rc)
Ir (Ib)
Ir (Ib-rc)
∆Edist(ML4)
∆ES/T
∑ ) ∆ET,dist
-∆El(H-H)
2[∆El(M-H)]
∆ER
+27.5
-4.0
+23.5
+107.3
-144.8
-14.0
+21.1
-4.2
+16.9
+107.3
-144.7
-20.5
+36.4
-8.0
+28.4
+107.3
-170.9
-35.2
+30.3
-9.1
+21.2
+107.3
-170.2
-41.7
To evaluate the influence of the phenyl substituents located
on the central heterocycle and the triphenylphosphine ligands,
additional calculations were performed on the real complexes
(rc) at the B3PW91//B3PW91:UFF level. The geometry of the
reactant [M(SPS)(PPh3)] (M ) Rh, Ir) and the syn-SS species
[M(SPS)(PPh3)(H)2] (Ia-rc, M ) Rh; Ib-rc, M ) Ir) were
optimized. The main theoretical parameters of the Ir compound
Ib-rc (Figure 4) are reported in Table 3 for sake of comparison
with the theoretical parameters of the unsubstituted complex
and with experimental values.
From an energetic point of view, the exothermicity of the
addition reaction increases by 21.2 kcal mol-1 on going from
Rh to Ir (Table 4); the same value was found for the
unsubstituted model complexes. The thermodynamic cycle
depicted in Scheme 3 was recalculated with the real complexes.
The energy decomposition given in Table 4 clearly shows that
a Energies are given in kcal mol-1
.
the M-H bonding energy is still responsible for the change in
the reaction energy ∆ER. Note that the ∆El(M-H) bond energies
computed for the real complexes are close to those found for
the unsubstituted models (-85.1 kcal mol-1 (Ir) and -72.35
kcal mol-1 (Rh) instead of -85.45 and -72.4 kcal mol-1
,
respectively).17
Finally, the mechanism for the addition of H2 to the [M(SPS)-
(PH3)] complexes was elucidated by the optimization and the
characterization of the transition state structure leading to the
most stable syn-SS isomer (Figure 5). For both of them (TS-
Ia, M ) Rh; TS-Ib, M ) Ir), the H-H bond is only slightly
elongated (0.818 and 0.825 Å for TS-Ia and TS-Ib, respectively)
with respect to its value in the isolated H2 molecule (0.744 Å),
and the M-H distances are 0.3-0.4 Å longer than in the
dihydride product. On the other hand, the metal fragment is
distorted from its initial square-planar geometry, with S-M-S
angles of 139.6° (M ) Rh, TS-Ia) and 153.3° (M ) Ir, TS-
Ib). Therefore, the transition state can be described as a five-
coordinated Rh(I) or Ir(I) complex in a distorted-trigonal-
bipyramidal geometry, with a molecular dihydrogen as an
equatorial ligand. Such a slight H-H bond lengthening in the
transition state has been already found for the H2 oxidative
addition to Vaska’s complexes.13 According to the values
optimized for the bending of the M-S bonds, the transition
state for the Ir complex, TS-Ib, is found to be “earlier” than
that for the Rh complex, in agreement with the greater
exothermicity of the addition reaction found for the iridium
compound. In that respect, the trend found for the activation
energies (∆Eq) is rather unexpected, since it is found to be
slightly higher for Ir, TS-Ib (+9.2 kcal mol-1), than for Rh,
(16) In the isomers which adopt a similar pseudo-octahedral geometry
within a given metal, the M-H bond energy is significantly larger for M-H
trans to M-S (syn-SS isomer) than for M-H trans to M-P (syn-PP and
anti-PP isomers). This result may be traced to the weaker trans influence
exerted by the sulfur ligand.
(17) (a) Martinho Simoes, J. A.; Beauchamp, J. L. Chem. ReV. 1990,
90, 629-688. (b) In our thermodynamic cycle, the M-H bond energy is
defined with respect to the distorted (or nonreoganized) metal fragment.
Another possibility is to define this term with respect to the metal fragment
in its optimized geometry. In that case, the distortion energy does not appear
explicitly in the thermodynamic cycle but is included in the bond energy
term. The absolute values of M-H bond energy depend on this choice, but
the general trend (increase of the bond energy in going from Rh to Ir)
remains: El(Rh-H) ) -60.7 kcal mol-1 instead of -71.3 kcal mol-1 and
El(Ir-H) ) -72.4 kcal mol-1 instead of -85.5 kcal mol-1. Note that the
lower values are in good agreement with experimental data for Rh (61.8 (
5 kcal mol-1) and Ir (-74.0 ( 5 kcal mol-11) complexes, which are derived
from the thermodynamic cycle in which the metal fragment is taken in its
ground-state geometry.

Addition of H2 on SPS-Pincer-Based Complexes
Organometallics, Vol. 25, No. 5, 2006 1107
Note that the barrier for the reverse reaction (syn-SS dihydride
f TS) increases from 22.6 kcal mol-1 (Rh) to 44.4 kcal mol-1
(Ir). This result can be analyzed by a thermodynamic cycle,
similar to that of Scheme 3, which connects the dihydride
complexes (Ia or Ib) to the transition state complexes (TS-Ia
or TS-Ib). The step mainly responsible for this trend involves
the breaking of two M-H bonds (+144.8 and +170.9 kcal
mol-1 for Rh and Ir, respectively (see Table 2)).
It is noteworthy that, in both transition states, the interaction
energy between the two fragments is very small, despite rather
short M-H distances (1.887 Å (av) for Rh and 1.920 Å (av)
for Ir). This striking result can be qualitatively rationalized by
analyzing the main molecular orbital interactions at work in
Figure 5. Optimized geometries of the transition states for the
addition of H2 on the unsubstituted rhodium and iridium complexes
(TS-Ia and TS-Ib, respectively). Hydrogen atoms are omitted for
clarity, except for those bound to the metal center. Selected
geometrical parameters (bond distances in Å and angles in deg)
are as follows. Rh (TS-Ia): H-H ) 0.818, Rh-H (av) ) 1.886;
S-Rh-S ) 131.6, P1-Rh-P4 ) 170.7. Ir (TS-Ib): H-H )
0.825, Ir-H (av) ) 1.921; S-Ir-S ) 153.3, P1-Ir-P4 ) 162.3.
the transition state (Scheme 5): The σH MO interacts mainly
2
Scheme 5. Main Orbital Interactions between H2 and the
Metal Fragment in the Transition State Structures TS-Ia
and TS-Ib
Table 5. Energy Decomposition (According to Scheme 4) of
the Activation Energy (∆Eq) for the Addition of H2 Leading
to the syn-SS Dihydride (M ) Rh (TS-Ia), Ir (TS-Ib))a
Rh (TS-Ia)
Ir (TS-Ib)
∆ES,dist
+9.8
+2.0
-3.2
+8.6
+7.8
+2.4
-1.0
+9.2
∆Estretch(H-H)
∆Eint
∆Eq
a Energies are given in kcal mol-1
.
TS-Ia (+8.6 kcal mol-1). The same holds for the ∆Gq values
(19.1 and 19.6 kcal mol-1 for M ) Rh, Ir, respectively). These
values are fully consistent with a reaction experimentally
conducted at -78 °C.
This result was analyzed by means of a thermodynamic cycle
in which, according to the results given above, the transition
state is described as a slightly elongated molecular dihydrogen
molecule in interaction with the distorted [M(SPS)(PH3)] metal
fragment in its singlet state (Scheme 4). The activation energy
(∆Eq) is thus equal to the sum of (i) the distortion energy of
the metal fragment in its singlet state (∆Edist(ML4), (ii) the
stretching energy of H2 (∆Estretch(H-H)), and (iii) the interaction
energy of the two fragments (∆Eint).
The computed ∆Edist(ML4) term, which represents most of
the activation energy (Table 5), is actually smaller for the Ir
metal fragment, which is less distorted than its Rh analogue
(+7.8 kcal mol-1 instead of +9.8 kcal mol-1). The interaction
energy term (∆Eint) found to be more favorable for M ) Rh
(-3.2 kcal mol-1) than for M ) Ir (-1.0 kcal mol-1) is thus
responsible for the slight increase of the activation energy in
going from M ) Rh to M ) Ir.
2
with the occupied dz orbital and, to a lesser extent, with the
LUMO of the d8 metal fragment in a butterfly geometry. On
the other hand, σH * interacts with the occupied nonbonding
dyz orbital. The important point is that a destabilizing four-
electron interaction (σH T dz ) adds to the two stabilizing two-
2
2
2
electron interactions (σH T LUMO donation and dyz T σH *
2
2
back-donation interactions). The existence of this repulsive
interaction leads to an antibonding M-H2 character of the HO
in the transition state and gives a qualitative understanding of
the nearly zero total interaction energy computed between the
metal and dihydrogen fragments.
This qualitative analysis is confirmed by the shape of the
two highest occupied MOs pictured in Figure 6: the HOMO-1
is bonding between M and H2 and characterizes the back-
donation interaction (dyz T σH *), while the HOMO is anti-
bonding between M and H2 (dz (polarized) T σH ).
2
2
2
Scheme 4

1108 Organometallics, Vol. 25, No. 5, 2006
Doux et al.
Table 6. Energy Decomposition (According to Scheme 3) for
the Formation of the syn-SS Complexes by Addition of H2
on the Complexes [Pd(SPS)(X)] (X ) Cl (Ic), (PH3)+ (Id))
and [Pt(SPS)(X)] (X ) Cl (Ie), (PH3)+ (If))a
PdCl (Ic)
PdPH3+ (Id)
PtCl (Ie)
PtPH3+ (If)
∆Edist(ML4)
∆ES/T
∑ ) ∆ET,dist
-∆El(H-H)
2[∆El(M-H)]
∆ER
25.0
+9.8
+32.2
+11.0
+43.2
107.3
-125.9
+24.6
+35.0
+4.9
+40.1
+5.5
+34.8
107.3
+39.9
107.3
+45.6
107.3
-119.8
+22.3
-147.5
-0.3
-151.7
+1.2
a Energies are given in kcal mol-1
.
Moreover, the ∆GR values are now positive for the four
complexes (+33.7, +36.5, +11.5, and +13.4 kcal mol-1 for
Ic-f, respectively), in agreement with the experimental obser-
vation. As a matter of fact, the addition of H2 on [Pt(SPS)(Cl)]
(5) has been carried out in MeOH and CH2Cl2 at 20 bar and no
reaction occurred. These results reflect the difficulty of forming
a group 10 M(IV) complex, in particular in the Pd case,
compared to the formation of Rh(III) or Ir(III) complexes. The
energetic decompositions given in Table 2 (M ) Rh (Ia) and
M ) Ir (Ib)) and Table 6 (M ) Pd (Ic,d)) and M ) Pt (Ie,f))
allow us to trace the electronic factors responsible for such a
change. Let us first note that the energy required to distort the
reactant in its singlet state (∆Edist) is similar for Rh and Pd
complexes (27.5 (av) and 28.6 (av) kcal mol-1, respectively)
Figure 6. Drawing of the two highest occupied MOs in the
transition state structure TS-Ia for the addition of H2 on the [Rh-
(SPS)(PH3)] complex.
In conclusion, the regioselectivity of the addition of H2 to
the [Ir(SPS)(PH3)] complex is well reproduced by DFT calcula-
tions and a similar structure is predicted for the Rh analogue.
The planar discrimination in favor of the syn-SS isomer is
governed by a metal fragment property, i.e. the energy required
to form the distorted complex in its lowest triplet state, and by
the strength of the M-H bonds under formation. On the other
hand, the increase of the exothermicity of the reaction on going
from Rh to Ir is traced by the increase of the M-H bonding
energy. Finally, the transition state structure for this oxidative
addition reaction is depicted as a molecular dihydrogen complex
located only 8-9 kcal mol-1 above the reactants.
Group 10 (M ) Pd, Pt). The addition of H2 was studied on
the SPS-type pincer-based Pd(II) and Pt(II) unsubstituted model
complexes [M(SPS)(X)] (X ) Cl, (PH3)+). Among the various
dihydride structures optimized and characterized as minima, the
syn-SS isomer was always found to be the most stable (Ic-f,
Figure 7). The addition reaction was found to be much easier
for M ) Pt (∆ER ) -0.3 and +1.2 kcal mol-1 for X ) Cl and
(PH3)+, respectively) than for M ) Pd (+22.3 and +24.6 kcal
mol-1 for X ) Cl and (PH3)+, respectively).
and for Ir and Pt (36.4 (av) and 37.6 (av) kcal mol-1
,
respectively). Two factors were found to work against the
reactivity of group 10 metal complexes: (i) the singlet/triplet
energy separation in the distorted reactant (∆ES/T for the syn-
SS isomer), which is found to be larger by 13-15 kcal mol-1
;
(ii) the energy gain associated with the formation of the two
M-H bonds, decreasing by about 22 kcal mol-1 on going from
Rh to Pd or from Ir to Pt. These two factors thus contribute to
a similar extent to the different behavior of group 9 and group
10 metal complexes [M(SPS)(X)] toward the oxidative addition
of H2.
These results led us to search for H2 addition products which
kept unchanged the oxidation number of the metal center.
Molecular dihydrogen complexes were actually optimized and
characterized as minima for the two Pd complexes (VIIc,d) and
for the Pt (VIIe) complex with X ) Cl (Figure 8). In these
structures, a sulfur center is decoordinated and a nearly square-
planar arrangement is found at the Pd(II) or Pt(II) metal center.
As was found for group 9 complexes, the M-H bonding
energy increases on going from the second to the third transition
metal series (61.4 and 74.8 kcal mol-1 per bond for Pd and Pt,
respectively (mean values)). However, in marked contrast with
the group 9 metal complexes, for which the addition of H2 was
found to be very exothermic, the computed reaction energy
(∆ER) is either close to zero (Pt) or largely positive (Pd).
Figure 7. Optimized geometries of unsubstituted palladium and platinum complexes Ic-f (syn-SS isomers). Hydrogen atoms are omitted
for clarity, except for those bound to the metal center. Selected geometrical parameters (bond distances in Å and angles in deg) are as
follows. Ic: Pd-H1 ) 1.521; S-Pd-S ) 96.6, P1-Pd-Cl ) 173.2. Id: Pd-H1 ) 1.540; S-Pd-S ) 97.0, P1-Pd-P4 ) 172.3. Ie:
Pd-H1 ) 1.544; S-Pt-S ) 94.2, P1-Pt-Cl ) 177.5. If: Pt-H1 ) 1.560; S-Pt-S ) 93.8, P1-Pt-P4 ) 176.1. E, given in kcal mol-1
is the energy compared to that of the starting material.
,

Addition of H2 on SPS-Pincer-Based Complexes
Organometallics, Vol. 25, No. 5, 2006 1109
Figure 8. Optimized geometries of unsubstituted palladium and platinum molecular dihydrogen complexes VIIc-e. Hydrogen atoms are
omitted for clarity, except for those bound to the metal center. Selected geometrical parameters (bond distances in Å and angles in deg) are
as follows. VIIc: H1-H2 ) 0.814, Pd-H1 ) 1.792, Pd-S1 ) 2.346, Pd-S2 ) 4.493; S1-Pd-H1 ) 168.7. VIId: H1-H2 ) 0.834,
Pd-H1 ) 1.864, Pd-S1 ) 2.406, Pd-S2 ) 3.953; S1-Pd-H1 ) 163.2. VIIe: H1-H2 ) 0.931, Pd-H1 ) 1.642, Pd-S1 ) 2.355,
Pd-S2 ) 4.145; S1-Pd-H1 ) 161.6. Energies are given in kcal mol-1 with respect to the starting materials.
Table 7. Energy Variation (∆ER, in kcal mol-1) Associated
with the Formation of the syn-SS Dihydride Complex I, the
Experimental Section
General Considerations. All reactions were routinely performed
under an inert atmosphere of argon or nitrogen by using Schlenk
and glovebox techniques and dry deoxygenated solvents. Dry THF
and hexanes were obtained by distillation from Na/benzophenone.
Dry ether was obtained by distillation from CaCl2 and then NaH
and dry CH2Cl2 from P2O5. CD2Cl2 and THF-d8 were stored on 4
Å Linde molecular sieves in the glovebox. Nuclear magnetic
resonance spectra were recorded on a Bru¨ker Advance 300
Molecular Dihydrogen Complex VII, and the Monohydride
MH‚‚‚SH Complex VI by Addition of H2 on the Pd and Pt
Starting Materials
PdCl (c)
PdPH3+ (d)
PtCl (e)
PtPH3+ (f)
syn-SS (I)
Mol. H2 (VII)
MH‚‚‚SH (VI)
+22.3
+11.7
+9.0
+24.6
+20.0
+8.2
-0.3
+8.1
+1.9
+1.2
+0.2
1
spectrometer operating at 300.0 MHz for H, 75.5 MHz for 13C,
For the Pd compounds, these molecular dihydrogen complexes
(VIIc and VIId) were found to be more stable than the
corresponding syn-SS dihydride isomers (Ic and Id), by 10.6
and 4.6 kcal mol-1, respectively. On the other hand, the Pt(IV)
syn-SS dihydride (Ie) remains more stable than the Pt(II)
molecular dihydrogen complex (VIIe) by 8.4 kcal mol-1 (Table
7).
and 121.5 MHz for 31P. Solvent peaks are used as internal reference
relative to Me4Si for 1H and 13C chemical shifts (ppm); 31P chemical
shifts are relative to a 85% H3PO4 external reference. Coupling
constants are given in hertz. The following abbreviations are used:
s, singlet; d, doublet; t, triplet; q, quadruplet; p, pentuplet; m,
multiplet; v, virtual. Elemental analyses were performed by the
“Service d’analyse du CNRS”, at Gif sur Yvette, France. Complexes
1,5 2,6 and 54 were prepared according to reported procedures.
Synthesis of Complex 3. H2 (1 atm) was bubbled for 30 s into
a freshly prepared solution of 1 in THF-d8 (0.5 mL) at room
temperature. The solution immediately turned from brown to deep
orange. The lifetime of complex 3 was approximately several
minutes in solution (THF) at room temperature under a N2
atmosphere. Due to its intrinsic instability, complex 3 was only
characterized by 31P and 1H NMR spectroscopy. 31P NMR and 1H
NMR of the crude mixture revealed the formation of a single
Other minima with a M(II) metal center were found for the
four starting complexes. They can be described as resulting from
a heterolytic activation of H2: a monohydride complex is formed
by the addition of H- on the metal, while H+ adds to a lone
pair of a decoordinated sulfur center (Figure 9).14 These
complexes, quoted as [MH‚‚‚SH], were found to be the most
stable addition product for the Pd complexes, located 9.0 and
8.2 kcal mol-1 above the reactants for X ) Cl (VIc) and X )
(PH3)+ (VId), respectively (Table 7). On the other hand, they
turn out to be competitive with the syn-SS dihydride of the Pt
complexes (Ie,f), the computed reaction energy ∆ER being close
to zero (+1.9 and +0.2 kcal mol-1 for VIe,f, respectively)
(Table 6). Note, however, that the ∆GR values associated with
these [MH‚‚‚SH] complexes are all positive (19.4, 17.5, 11.4,
and 10.3 kcal mol-1 for VIc-f, respectively).
In conclusion, the oxidative addition of H2 on group 10
[M(SPS)(X)] complexes (X ) Cl, (PH3)+) leading to M(IV)
dihydrides is found to be easier for Pt than for Pd complexes.
For the latter, molecular dihydrogen and monohydride M(II)
adducts are significantly more stable, while the monohydride
complex is competitive in energy with the dihydride in the case
of platinum. In marked contrast with the results obtained for
group 9 metals, the energy variations ∆ER associated with these
addition reactions are either positive or close to zero and the
associated ∆GR values are always positive. These results are in
agreement with the experimental observation, since no reaction
occurs, even under high hydrogen pressure, between H2 and
group 10 d8 [M(SPS)(X)] complexes.
1
1
product. H NMR (THF-d8): δ -14.64 (dt, J(H-Rh) ) 24.5,
2
2J(H-PA) ) J(H-PC) ) 12.3, 2H, Rh-H), 1.83 (m, 3H, CH3),
4
4
5.60 (vq, J(H-PB) ) J(H-PA) ) 3.7, 1H, H4), 6.80-7.84 (m,
45H, H of Ph). 31P NMR (THF-d8; second-order spectrum which
could not be modeled): δ 36.5-47.9 (AB2C, m, PAMe and PC-
PH3), 49.6-50.8 (AB2C, m, ∑J ) 145.8, PBPh2).
Synthesis of Complex 4. H2 (1 atm) was bubbled for 3 min
into a freshly prepared solution of 2 (C ) 0.05 M, 0.15 mmol) in
THF (3 mL) at room temperature. The solution immediately turned
from brown to deep orange. After the solvent was removed, the
solid was washed first with hexanes (3 × 2 mL) and then with
diethyl ether (3 × 2 mL). The resulting solid was dissolved in CH2-
Cl2 and the solution filtered through Celite. After drying, 4 was
recovered as a yellow solid. Yield: 130 mg (76%). Anal. Calcd
for C60H51IrP4S2 (1152.2): C, 62.54; H, 4.46. Found: C, 62.24;
1
2
2
H, 4.17. H NMR (CD2Cl2): δ -18.39 (vt, J(H-PA) ) J(H-
PY) ) 15.5), 1.87 (d, 2 J(H-PA) ) 8.7, 3H, CH3), 5.60 (t, 4 J(H-
PB) ) 5.6, 1H, H4), 6.73-8.12 (m, 45H, H of Ph). 13C NMR
(CD2Cl2): δ 20.4 (m, CH3), 71.4 (m, C2,6), 116.4 (m, C4H), 127.2-
134.5 (m, CH and C of Ph), 137.4 (dd, J(H-P) ) 44.6, J(H-P) )
4.0, C of Ph), 141.7 (bs, C3,5), 154.2 (bs, C of Ph). 31P NMR (CD2-
2
2
Cl2): δ 9.7 (AB2Y, td, J(PA-PY) ) 336.0, J(PB-PA) ) 107.0,

1110 Organometallics, Vol. 25, No. 5, 2006
Doux et al.
Figure 9. Optimized geometries of unsubstituted palladium and platinum complexes VIc-f. Hydrogen atoms are omitted for clarity,
except for those bound to the metal and to the decoordinated S centers. Selected geometrical parameters (bond distances in Å and angles
in deg) are as follows. VIc: Pd-H1 ) 1.535, Pd-S1 ) 2.538, S2-H2 ) 1.403, H2-Cl ) 2.143; S1-Pd-H1 ) 176.2, P1-Pd-Cl )
156.7. VId: Pd-H1 ) 1.555, Pd-H2 ) 2.745, Pd-S1 ) 2.458, S2-H2 ) 1.359; S1-Pd-H1 ) 178.9. VIe: Pt-H1 ) 1.553, Pt-H2
) 1.931, Pt-S1 ) 2.520, S2-H2 ) 1.491; S1-Pt-H1 ) 175.5. VIf: Pt-H1 ) 1.571, Pt-H2 ) 2.265, Pt-S1 ) 2.461, S2-H2 ) 1.394;
S1-Pt-H1 ) 178.7. Energies (in kcal mol-1) are given with respect to the starting materials.
2
3
PAMe), 16.8 (dt, J(PY-PA) ) 336.0, J(PY-PB) ) 27.7, PYPh3),
were found to be lower than 1.0 kcal mol-1 on going from basis
set A to basis set B. The ∆G values, given at 298 K, are derived
from calculations with the basis set A, since they require the
frequency calculations. QM/MM optimizations of real complexes
were performed at the ONIOM(B3PW91:UFF) level with the
phenyl substituents in the MM part.24 The QM part was treated at
the DFT-B3PW91 level with basis set A (see above), and the UFF
force field was used for the MM part.25 Finally, DFT-B3PW91
single-point calculations were performed on the optimized structures
(B3PW91//B3PW91:UFF calculations) using the 6-31G basis set
for the C and H atoms of the phenyl substituents and the basis set
B for the other atoms.
X-ray Crystallographic Study. Crystals of compound 4 suitable
for X-ray diffraction were obtained by slow evaporation of CD2-
Cl2. Data were collected at 150.0(1) K on a Nonius Kappa CCD
diffractometer using a Mo KR (λ ) 0.710 70 Å) X-ray source and
a graphite monochromator. All data were measured using ψ and ω
scans. Experimental details are described in Table 1. The crystal
structures were solved using SIR 9726 and Shelxl-97.27 ORTEP
drawings were made using ORTEP III for Windows.28 CCDC-
286775 contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge at www.ccdc.
cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystal-
lographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, U.K.;
fax (internat.) +44-1223/336-033; e-mail deposit@ccdc.cam.ac.uk.
2
3
50.3 (dd, J(PB-PA) ) 107.0, J(PB-PY) ) 27.7, PBPh2).
Computational Details. Calculations were performed with the
GAUSSIAN 03 series of programs.18 Density functional theory
(DFT)19 was applied for the unsubstituted complexes (in which the
phenyl substituents were replaced by H atoms) with the B3PW91
functional.20 A quasi-relativistic effective core potential operator
was used to represent the 28 innermost electrons of the rhodium
and palladium atoms and the 60 innermost electrons of iridium and
platinum atoms.21 The basis set for the metal was that associated
with the pseudopotential, with a standard double-ú LANL2DZ
contraction21 completed by a set of polarization f functions.22
Geometry optimizations on the model complexes were performed
with the 6-31+G* basis for P, S, and Cl atoms, 6-31G* for the
carbon atoms, 6-31++G** for the hydrogen atoms of H2, and
6-31G for the other hydrogen atoms (basis set A).23 The stationary
points were characterized by full vibration frequency calculations.
The energies were then recomputed using the same basis set on
the metal center and the 6-31++G** basis set for all the other
atoms (basis set B).23 Note that the changes in relative energies
(18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K.
N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A.
D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A.
G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian
03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
Acknowledgment. We thank the CNRS, the DGA, and the
EÄ cole polytechnique for supporting this work. IDRIS (Project
No. 061616) is also acknowledged for the allowance of
computer time.
Supporting Information Available: Figures and tables giving
the optimized geometry, energy, and frequencies of [M(SPS)(PR3)]
(M ) Rh, Ir; R ) PH3, PPh3), Ia-VIa, Ib-VIb, Ia-rc, Ib-rc,
(19) (a) Ziegler, T. Chem. ReV. 1991, 91, 651-667. (b) Parr, R. G.; Yang,
W. DFT; Oxford University Press: Oxford, U.K., 1989.
(24) Svensson, M.; Humbel, S.; Froese, R. D. J.; Matsubara, T.; Sieber,
S.; Morokuma, K. J. J. Phys. Chem. 1996, 100, 19357-19363.
(25) Rappe´, A. K.; Casewitt, C. J.; Colwell, K. S.; Goddard, W. A.; Skiff,
W. M. J. Am. Chem. Soc. 1992, 114, 10024-10035.
(26) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.; Giacovazzo,
C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. SIR97, an
integrated package of computer programs for the solution and refinement
of crystal structures using single-crystal data.
(27) Sheldrick, G. M. SHELXL-97; Universita¨t Go¨ttingen, Go¨ttingen,
Germany, 1997.
(28) Farrugia, L. J. ORTEP-3; Department of Chemistry, University of
Glasgow, Glasgow, Scotland.
(20) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.
(21) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299-310.
(22) Ehlers, A.; Bo¨hme, M.; Dapprich, S.; Gobbi, A.; Ho¨llwarth, A.;
Jonas, V.; Ko¨hler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G. Chem.
Phys. Lett. 1993, 208, 111-114.
(23) (a) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. v.
R. J. Comput. Chem. 1983, 4, 294-301. (b) Francl, M. M.; Pietro, W. J.;
Hehre, W. J.; Binkley, J. S.; Gordon, M. S.; DeFrees, D. J.; Pople, J. A. J.
Chem. Phys. 1982, 77, 3654-3665. (c) Hehre, W. J.; Ditchfield, R.; Pople,
J. A. J. Chem. Phys. 1972, 56, 2257-2272. (d) Hariharan, P. C.; Pople, J.
A. Theor. Chim. Acta 1973, 28, 213-222.

Addition of H2 on SPS-Pincer-Based Complexes
Organometallics, Vol. 25, No. 5, 2006 1111
TS-Ia, TS-Ib, [M(SPS)(X)] (M ) Pd, Pt, X) Cl, PH3+), Ic-f,
VIc-f, and VIIc-e, the entropy and free energy for [M(SPS)-
(PH3)] (M ) Rh, Ir), Ia-b, TS-Ia, and TS-Ib, and crystal data for
4. This material is available free of charge via the Internet at
http://pubs.acs.org.
OM050900U