Synthesis, characterization, and reactivity of cationic hydride [HPd(diphosphine)2]+CF3 SO3-, the missing member of the family [HM(dppe)2]+X- (M = Ni, Pd, Pt). DFT QM/MM structural predictions for the [HPd(dppe)2]+ moiety

Article abstract of DOI:10.1021/ic020631p

The synthesis, characterization, and properties of the cationic hydride [HPd(dppe)₂]⁺CF₃SO₃ ^-·1/8THF, the missing member of the family [HM(dppe)₂]⁺X^- (M = Ni, Pd, Pt), are described. The Pd hydride is not stable in solution and may react as either a proton or a hydride donor. DFT QM/MM calculations of the [HPd(dppe)₂]⁺ moiety have allowed us to predict its structure and reactivity.

Full text of DOI:10.1021/ic020631p

Inorg. Chem. 2002, 41, 6550−6552
Synthesis, Characterization, and Reactivity of Cationic Hydride
[HPd(diphosphine)2]+CF3SO -, the Missing Member of the Family
3
[HM(dppe)2]+X- (M ) Ni, Pd, Pt). DFT QM/MM Structural Predictions for
the [HPd(dppe)2]+ Moiety
,†
Michele Aresta,* Angela Dibenedetto,† Eliana Amodio,† Imre Pa´pai,‡ and Ga´bor Schubert‡
Department of Chemistry and METEA Research Center, UniVersity of Bari, Via Celso Ulpiani 27,
70126 Bari, Italy, and Chemical Research Centre, Hungarian Academy of Sciences,
H-1525 Budapest, P.O.B. 17, Hungary
Received October 16, 2002
-
The synthesis, characterization, and properties of the cationic
that analyzes for [HPd(dppe)2]+CF3SO3 ‚1/8THF (1),5 stable
hydride [HPd(dppe)2]+CF SO -‚1/8THF, the missing member of
for days under dinitrogen at 253 K (eq 1) in the solid state.
3
3
the family [HM(dppe)2]+X- (M ) Ni, Pd, Pt), are described. The
Pd hydride is not stable in solution and may react as either a
proton or a hydride donor. DFT QM/MM calculations of the [HPd-
(dppe)2]+ moiety have allowed us to predict its structure and
reactivity.
+
-
Pd(dppe) + CF SO H )
(1)
[HPd(dppe)2] CF3SO3
2
3
3
1
The infrared spectrum of solid 1 (CsI disks, Nujol) shows
a band at 1916 cm-1 (νPd-H) that is at lower energy than
that found for the corresponding Ni (1964 and 1943 cm-1)
and Pt (2070 cm-1) hydrido complexes.2 Other bands are at
1268 (νas SO3), 1223 (νs CF3), 1143 (νas CF3), 1030 (νs SO3),
637, 571, and 518 (SO3 deformations) cm-1. The IR
absorptions suggest that the CF3SO3- anion is not coordinated
to palladium.6 The 900-800 cm-1 region of the IR spectrum
shows the presence of bands of medium intensity at 873,
864, 821, and 812 cm-1 and a weak band at 845 cm-1, due
to CH2 rocking vibrations of the diphosphine ligands. We
have previously shown7 that a square planar geometry of
two dppe ligands around a metal center generates two bands
of medium intensity and a weak band in the 900-800 cm-1
region, while a regular or distorted tetrahedral arrangement
gives rise to four medium intensity bands and a weak band
in the same region. Therefore, the IR data suggest that the
[HPd(dppe)2]+ moiety of the solid Pd-hydride does not have
a square pyramidal geometry (Figure 1a), with four P atoms
in a square planar arrangement, and support a capped
tetrahedron (Figure 1b) or a distorted trigonal bipyramid
(Figure 1c) as the most probable structure in the solid state.
In this paper, we describe the synthesis, characterization,
and properties of the cationic hydride [HPd(dppe)2]+CF3-
-
SO3 ‚1/8THF (1), the missing member of the family
[HM(dppe)2]+X- (M ) Ni, Pd, Pt). While the Ni1 and Pt2
-
analogues of [HPd(dppe)2]+CF3SO3 ‚1/8THF are known for
some time, 1 could not be isolated2 by using the same
procedures as for Ni and Pt. Recently, using an independent
synthetic methodology, we were able to observe in solution
a nonisolable species that contained cation [HPd(dppe)2]+
with BPh4- as a noncoordinating counterion.3 Now we report
the correct conditions for the isolation of 1 as a yellow solid
that has made possible the study of its reactivity. We also
describe DFT QM/MM calculations carried out for the
cationic [HPd(dppe)2]+ moiety.
When strictly anhydrous CF3SO3H is added in stoichio-
4
metric amount to a solution of Pd(dppe)2 in anhydrous
tetrahydrofuran (THF) at 273 K, a yellow solid precipitates
* To whom correspondence should be addressed. E-mail: aresta@
metea.uniba.it.
(5) To a yellow solution of Pd(dppe)2 (0.219 g, 0.242 mmol) in anhydrous
THF (25 mL), prepared under dinitrogen at 293 K, was added strictly
anhydrous CF3SO3H (21.4 µL, 0.242 mmol), and the resulting mixture
was stirred at 273 K for 3 h, during which a yellow solid was formed.
The solution was concentrated and the obtained yellow solid filtered
under dinitrogen, washed with anhydrous THF (3 × 2 mL), dried in
vacuo, and characterized as [HPd(dppe)2]+CF3SO3-‚1/8THF (1).
Yield: 0.180 g, 0.170 mmol, 70.0%. Anal. Calcd for C53H49F3O3P4-
SPd ‚1/8THF, 1: C, 60.49; H, 4.74; P, 11.66; Pd, 10.02; S, 3.02%.
Found: C, 60.87; H, 5.07; P, 11.49; Pd, 10.07; S, 2.84%.
† University of Bari.
‡ Hungarian Academy of Sciences.
(1) Schunn, R. A. Inorg. Chem. 1970, 9, 394-395.
(2) Berning, D. E.; Noll, B. C.; DuBois, D. L. J. Am. Chem. Soc. 1999,
121, 11432-11447.
(3) (a) Aresta, M.; Quaranta, E.; D’Andola, G. Book of Abstracts, EURO-
Hydrides 2000, Universite´ de Bourgogne, Dijon, France, September
6-9 2000; OC7. (b) Aresta, M.; Dibenedetto, A.; Amodio, E.;
Tommasi, I. Eur. J. Inorg. Chem. 2002, 8, 2188-2193. (c) Aresta,
M.; Quaranta, E. J. Organomet. Chem., in press.
(4) Mason, M. R.; Verkade, J. G. Organometallics 1992, 11, 2212-2220.
(6) Aresta, M.; Quaranta, E. Organometallics 1993, 12, 2032-2043.
(7) Aresta, M.; Sacco, A. Gazz. Chim. Ital. 1972, 102, 755-780.
6550 Inorganic Chemistry, Vol. 41, No. 25, 2002
10.1021/ic020631p CCC: $22.00 © 2002 American Chemical Society
Published on Web 11/19/2002

COMMUNICATION
-
[Pd(dppe)2]2+2(CF3SO3 ). When 1 is added to CH3CN at
298 K, it promptly reacts to afford a yellow solid that
analyses for Pd(dppe)2, and molecular hydrogen. From the
yellowish mother solution, a white precipitate is obtained,
-
characterized as [Pd(dppe)2]2+2(CF3SO3 ) (see eq 2). This
reaction demonstrates that 1 can transfer a hydride or a
proton. The mechanism of formation of H2 and the role of
the solvent are still under investigation.
Figure 1. Possible structures of the cationic [HPd(dppe)2]+ moiety.
2[HPd(dppe)2]+CF3SO3
)
-
-
Pd(dppe)2 + [Pd(dppe)2]2+2(CF3SO3 ) + H2 (2)
Reaction 2 is a new unprecedented reduction of a Pd-
(II)-H complex to Pd(0) through a proton transfer. A second
molecule undergoes H- abstraction, affording a Pd2+ moiety
and H2. The nature of the products has been confirmed by
comparison with authentic samples and by 1H and 31P NMR
spectra of the isolated species. We have monitored the
Figure 2. (a) IMOMM-optimized structure of the [HPd(dppe)2]+ cation
with selected bond distances (in Å). The phenyl groups of the dppe ligands
are represented by open spheres; the hydrogens of the ethane groups have
been omitted for clarity. (b) Schematic structure of the [HM(PR3)4]+ species.
1
reaction of 1 with CH3CN in CD2Cl2 by H and 31P NMR.
The intensity of the hydride signal at -7.58 ppm decreases
with time, while a new signal appears at ca. -4.97 ppm
(doublet, 2JP-H ) 220.4 Hz), which then vanishes with time.
Simultaneously, H2 is formed that gives a signal at 4.58 ppm.
The 31P{1H} spectrum shows the slow disappearance of the
signal at 34.54 ppm, while three new signals appear at 32.23,
33.38, and 58.35 ppm. The signal at 33.38 disappears with
time, leaving only signals at 32.23 (Pd(dppe)2) and 58.35
([Pd(dppe)2]2+) ppm. Such features agree with the formation
of a short-living intermediate hydride that converts into the
final products reported in eq 2. When 1 is recrystallized from
CH2Cl2/THF, a new hydride is isolated that shows a broad
IR Pd-H stretching at 1900 cm-1. The rest of the IR
spectrum is substantially similar to that of 1, with the
exception of the region 900-800 cm-1 where changes are
evident. Attempts to run the 1H and 31P NMR spectra
afforded the conversion of the hydride into the products
described in reaction 2. Such features show that hydride 1
is not stable in solution and can be isolated in reaction 1
because of its low solubility in THF at 273 K.
To gain more insight into the structure of the cationic hydride
species, we performed hybrid QM/MM calculations for the
[HPd(dppe)2]+ moiety using the IMOMM methodology.8
The optimized structure of the gas phase [HPd(dppe)2]+
moiety indeed displays a distorted tetrahedral arrangement
of the two diphosphine ligands around the metal center with
the hydride atom positioned above a P3 face of the distorted
P4 tetrahedron. The calculated Pd-P distances fall between
2.3 and 2.40 Å (Figure 2a), indicating that the coordinated
dppe ligands are not equivalent.
This is not surprising given that one of the diphosphines
is coplanar with the Pd-H bond (dppe|), whereas the other
lies nearly perpendicular to the hydride bond (dppe ). This
structure can be paralleled with those of [HM(PR3)4]+ type
monodentate phosphine hydride complexes,9 which adopt a
C3V trigonal bipyramidal geometry with the hydride ligand
in the axial position (see Figure 2b). However, the relatively
narrow bite angle (∼86°) of the chelating dppe imposes a
constraint on dppe that coordinates with longer Pd-P bonds
than dppe|. Our structural predictions for [HPd(dppe)2]+ are
consistent with the X-ray structure of an analogue complex,
HCo(dppe)2, recently reported by DuBois et al.10
Solid 1 reacts with CO2 (6 MPa) to afford a white complex
that shows in its IR spectrum in Nujol two bands at 1630
and 1655 cm-1, due to a formate moiety. Conversely, 1 does
not react with carbonyl complexes such as Ni(CO)2(PPh3)2
in CD2Cl2.
1
The H NMR spectrum of 1 in CD2Cl2 at 293 K shows,
in addition to the resonances due to the aromatic and aliphatic
protons,11a a quintet of multiplets at -7.58 ppm (1 H, 2JP-H
) 54.4 Hz) due to the hydride. The 31P{1H} NMR shows a
broad singlet at 34.48 ppm.11b Such features match quite well
the data reported for the Ni and Pt relevant complexes.2,10
When reaction 1 is carried out in CH3CN/THF, the resulting
palladium hydride complex is not stable, so that a white solid
is obtained that corresponds to the dicationic Pd complex
Compound 1 reacts very easily with a variety of H+-donor
or -acceptor species. In particular, it behaves as a hydride
donor toward acids over a wide range of pKa values (-16
< pKa < 10).12 In fact, an excess of CF3SO3H may convert
-
1 into the dicationic Pd complex [Pd(dppe)2]2+2(CF3SO3 ),
with release of H2.13 Much more intriguing is the reaction
of 1 with the iminium tetraphenylborate salt [(PhCH2)HNd
CMe2]BPh4, which affords the Pd(II) dicationic complex and
(8) See Supporting Information.
(9) Lin, W.; Wilson, S. R.; Girolami, G. S. Inorg. Chem. 1997, 36, 2662-
2669.
(12) CF3SO3H, pKa ) -16 (in H2O); [(PhCH2)HNdCMe2]BPh4, pKa )
5.64 (in H2O/EtOH 10%).
(13) The addition of CF3SO3H to a solution of 1 in CD2Cl2 causes the
appearance of a signal at 4.55 ppm in the 1H NMR spectrum because
of H2, while the 31P signal shifts from 34.48 to 58.35 ppm. Such
resonance is characteristic of [Pd(dppe)2]2+2(CF3SO3-) that can be
isolated from the reaction mixture.
(10) Ciancanelli, R.; Noll, B. C.; DuBois, D. L.; Rakowski DuBois, M. J.
Am. Chem. Soc. 2002, 124, 2984-2992.
(11) (a) 1H NMR (CD2Cl2, 500 MHz, 293 K): δ 2.28 (s broad, 8 H, Ph2-
PCH2CH2PPh2), 7.24 (m, 32 H, o- and m-dppe), 7.40 (t, 8 H, p-dppe).
(b) 31P NMR experiments were run in CD2Cl2 at 202.48 MHz and
293 K.
Inorganic Chemistry, Vol. 41, No. 25, 2002 6551

COMMUNICATION
the amine that is the result of an attack by the hydride onto
the electrophilic carbon of the imine moiety (eq 3). The latter
reaction is much slower than the former. Reaction 3 affords
the same products observed when Pd(dppe)2 is reacted with
the iminium tetraphenylborate salt.3c
general formula EX4 with either a nucleophile (eq 4, where
E is the carbon atom involved in a SN process) or an
electrophile (eq 5, where E is a metal center involved in
protonation or alkylation processes).
2
EX4 + Y- a EX4Y- a EX3Y + X-
(4)
[HPd(dppe)2]+CF3SO3- + [(PhCH2)HNdCMe2]BPh4 )
[Pd(dppe)2]2+(CF3SO3 )(BPh4 ) + (PhCH2)HNsCHMe2
EX4 + A+ ) EX4(A)+
(5)
-
-
The nature of the pentacoordinated adduct that is a
common feature of the two processes and its geometry are
still matter of investigation by theoretical groups for detailing
the energy of the reaction and the barriers of interconverting
limit structures.15
(3)
We have compared the reactivity of the [HPd(dppe)2]+
species with those of corresponding Ni and Pt analogues by
calculating the energy of the gas phase [HM(dppe)2]+ + BH+
) [M(dppe)2]2+ + H2 + B reaction with various base (B)
molecules and obtained the following trend for the hydride
donor abilities: Pd > Pt > Ni.8 We have also estimated the
deprotonation energies of the [HM(dppe)2]+ complexes and
found that the most acidic species corresponds to M ) Pd.8
Our results thus suggest that the [HPd(dppe)2]+ complex is
more reactive and less stable than its first and third row
analogues which is in line with experimental evidence
available for the hydride of the Co and Ni triads.14 They
also provide a reasonable rationale for previous difficulties
in synthesizing the Pd hydride.2
Acknowledgment. The Italian authors thank the Ministry
of the University for financial support (Project MM03027791)
and a grant to Dr. E. Amodio. The OTKA F037345 grant
and computer time from NIIF Supercomputer Center are
acknowledged by the Hungarian authors. All authors ac-
knowledge COST support.
Supporting Information Available: Computational details,
Cartesian coordinates of the IMOMM-optimized structure of the
[HPd(dppe)2]+ moiety, deprotonation energies calculated for the
HM(dppe)2+ + B ) M(dppe)2 + BH+ reactions (M ) Ni, Pd, Pt;
B ) NH3, NEt3 and CF3SO3-), energies of the gas phase [HM-
(dppe)2]+ + BH+ ) [M(dppe)2]2+ + H2 + B reactions (M ) Ni,
Pd, Pt; B ) NH3, NEt3, CF3SO3-). This material is available free
of charge via the Internet at http://pubs.acs.org.
The reaction of the 18e- species Pd(dppe)2 with a proton
donor is of general interest as it can be considered a paradigm
of the class of reactions of covalent tetrahedral species of
IC020631P
(14) (a) Curtis, C.; Miedaner, A.; Ellis, W. W.; DuBois, D. L.. J. Am. Chem.
Soc. 2002, 124, 1918-1925, (b) Price, A. J.; Ciancanelli, R.; Noll, B.
C.; Curtis, C. J.; DuBois, D. L.; Rakowski DuBois, M. Organometallics
2002, 21, 4833-4839.
(15) Final EValuation Workshop of the COST D9 Action; Advanced
Computational Chemistry of Increasingly Complex Systems, Smo-
lenice, Slovakia, 24-26 May 2002.
6552 Inorganic Chemistry, Vol. 41, No. 25, 2002