Dinuclear ortho-metalated palladium(II) compounds with N,N- and N,O-donor bridging ligands. synthesis of new palladium(III) complexes

Article abstract of DOI:10.1021/om100740q

New dinuclear ortho-metalated palladium(II) compounds with N,N′-diarylformamidinates, Pd₂[(C₆H ₄)PPh₂]₂[R′NC(H)NR′]₂ (R′ = C₆H₅, 3a; R′ = p-CH₃C ₆

Full text of DOI:10.1021/om100740q

ARTICLE
pubs.acs.org/Organometallics
Dinuclear Ortho-Metalated Palladium(II) Compounds with N,N- and N,
O-Donor Bridging Ligands. Synthesis of New Palladium(III) Complexes
Dirk Penno,† Francisco Estevan,† Elena Fernꢀandez,§ Pipsa Hirva,‡ Pascual Lahuerta,† Mercedes Sanaꢀu,† and
a
,†
ꢀ
M Angeles Ubeda*
†Departament de Química Inorgꢁanica, Universitat de Valꢁencia, Dr. Moliner 50, 46100 Burjassot, Valencia, Spain
‡Department of Chemistry, University of Eastern Finland, Joensuu Campus, P.O.Box 111, FI-80101 Joensuu, Finland
§Departament de Química Física i Inorgꢁanica, Universitat Rovira i Virgili, C/Marcel.lí Domingo s/n, 43007 Tarragona, Spain
S Supporting Information
b
ABSTRACT:
New dinuclear ortho-metalated palladium(II) compounds with N,N0-diarylformamidinates, Pd2[(C6H4)PPh2]2[R0NC(H)NR0]2
(R0 = C6H5, 3a; R0 = p-CH3C6H4, 3b; R0 = p-CH3OC6H4, 3c) and N,O-donor ligands, Pd2[(C6H4)PPh2]2[N,O]2 (N,O =
succinimidate (5), phtalimidate (6), 2-hydroxypyridinate (7), acetanilidate (8)) have been synthesized and characterized by NMR
6þ
spectroscopy and X-ray diffraction methods. The oxidation with iodobenzene dichloride gave new and rare Pd2 compounds,
Pd2[(C6H4)PPh2]2[R0NC(H)NR0]2Cl2 (R0 = C6H5, 4a; R0 = p-CH3C6H4, 4b). DFT calculations on the Pd24þ f Pd26þ oxidation
reaction show that the substituents on the amidinate N atoms have a greater effect on the reaction energy than the substituents on
the C atom. DFT calculations also confirm that for Pd2[(C6H4)PPh2]2[N,O]2 compounds the symmetric isomers with N atoms
trans to O atoms are the most stable complexes. The palladium dimers were tested as precursors of catalysts in tandem diboration/
arylation/oxidation reactions.
’ INTRODUCTION
compounds of type B exhibit, by cyclic voltammetry, two
reversible oxidation processes indicative of two consecutive
one-electron oxidations to produce species with a Pd26þ core.29
Considerable efforts have been dedicated to prepare paddle-wheel
compounds with a Pd26þ core, without much success.20ꢀ22,30,31 The
first compound of this type, Pd2(hpp)4Cl2 (hpp = anion of
1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine), was synthe-
sized in low yields and was structurally characterized in 1998 by
Cotton and co-workers.25 The molecule contains a PdꢀPd single
bond as well as an axial chloride ligand on each palladium center.
Relatively stable in the solid state, it decomposes rapidly in solution.
We succeeded in preparing compounds of type C (Scheme 1)
in good yield by reaction of compounds of type B with
iodobenzene dichloride.32 This was the first time in which
paddle-wheel dinuclear palladium(III) compounds have been
Palladium complexes have been shown to be good catalysts in
a wide spectrum of catalytic reactions.1ꢀ5 A large family of
dinuclear palladium(II) compounds with the metal atoms at a
variable distance has been reported.6 A representative group of
compounds shows two bridging ligands, frequently carboxylate
ligands, and two cyclometalated ligands in a chelating coordina-
tion mode.7 Compounds with bridging metalated ligands are also
known.8ꢀ19 The typical paddle-wheel structure, common for
other transition metals, is only observed in a limited number of
palladium compounds, these being the two palladium atoms
connected by four bridging N,N-donor (amidinates, guanidi-
nates, and triazenates),20ꢀ25 or N,O-donor ligands (6-methyl-2-
hydroxypyridinate).26
In the last few years we have investigated the synthesis,
characterization, and reactivity of di- and tetranuclear palladium
compounds with ortho-metalated triphenylphosphines acting
as bridging P,C-donor ligands (Scheme 1).27ꢀ29 Some of the
Received: July 27, 2010
Published: March 30, 2011
r
2011 American Chemical Society
2083
dx.doi.org/10.1021/om100740q Organometallics 2011, 30, 2083–2094
|

Organometallics
ARTICLE
Scheme 1. Tetra- and Dinuclear Ortho-Metalated Palladium(II) and -(III) Compounds
4þ
Scheme 2. Tandem Diboration/SuzukiꢀMiyaura Cross-
N,O-donor ligands. Paddle-wheel Pd2 compounds with four
bridging N,N0-diarylformamidinate ligands have been shown
to be promising candidates for chemical oxidation.24 In the
search for new, stable metalated Pd26þ compounds, the nitrogen
donor atoms were expected to increase the electronic charge
on the metal atoms. The complexes have been tested in the
catalytic reaction: tandem diboration/arylation/oxidation of
alkenes.
Coupling Reaction/Oxidation
obtained pure in high yields. Compounds with the general
formula Pd2[(C6H4)PPh2]2(O2CR)2Cl2 were structurally simi-
lar to their Pd24þ precursors, but they contain a PdꢀPd single
bond and an axial chloride ligand on each palladium center
(just as in the case of Pd2(hpp)4Cl2). Some of them were
stable in solution of chlorinated solvents for over 48 h. Crystals
could be stored at room temperature and in air for weeks with-
out apparent decomposition. The carboxylate substituents are
an important factor in the stabilization of the products. Com-
pounds are diamagnetic with shortened PdꢀPd distances.
The presence of a PdꢀPd single bond in these complexes
was supported by hybrid HF/DFT calculations on a simplified
model.32
’ RESULTS AND DISCUSSION
Dinuclear Palladium Compounds with Bridging Forma-
midinate Ligands. Synthesis. The reactions of Pd4[(C6H4)-
PPh2]4Br4 (2) with the different N,N0-diarylformamidinates39
4þ
1aꢀc produced three new Pd2 complexes with the general
formula Pd2[(C6H4)PPh2]2[R0NC(H)NR0]2 (R0 = C6H5, 3a; R0
= p-CH3C6H4, 3b; R0 = p-CH3OC6H4, 3c) in good yields
(78ꢀ87%) (Scheme 3).
All three compounds were structurally characterized by single-
crystal X-ray diffraction methods (Figure 1). In the case of 3c,
due to the poor quality of the data, some of the atoms had to be
refined isotropically. Selected bond distances and angles for the
three compounds are given in Table 1.
Palladium complexes did catalyze the diboration of alkenes,33
despite the unfavorable oxidative addition of a diboron reagent to
Pd(0).34,35 Compounds of types AꢀC have been shown to be
efficient precursors of catalytic diboration of alkenes, this being
the first time that the dinuclear palladium(III) compound has
been tested in catalysis.36 Interestingly, Pd2[(C6H4)PPh2]2-
(O2CH3)2Cl2 and Pd4[(C6H4)PPh2]4Br4 were highly active
for the catalytic tandem diboration/arylation/oxidation of
alkenes, highlighting that the same palladium species catalyzed
the two catalytic cycles for diboration and arylation sequences
with total conversion from the substrate (Scheme 2). This was
the first example of multifaceted properties of a palladium
complex participating in different cycles with identical success.36
Recently Powers and Ritter showed that some reactions of
CꢀCl, CꢀBr, and CꢀO bond formation take place through
dinuclear palladium(III) compounds.37
The three compounds have very similar structures. In com-
4þ
parison to the Pd2 carboxylate compounds Pd2[(C6H4)-
PPh2]2(O2CR)2, the most striking differences are the signifi-
cantly larger PꢀPdꢀPdꢀP torsion angles of 3aꢀc, very similar
6þ
to those of the Pd2
carboxylate compounds Pd2[-
(C6H4)PPh2]2(O2CR)2Cl2.33Only the stereoisomers RP and
SM have been obtained from the four different possible arrange-
ments generated by the configurational chirality and torsion
angles PꢀPdꢀPdꢀP (Scheme 4).29
4þ
Electrochemical Studies. The new Pd2 complexes 3aꢀc
were investigated by cyclic voltammetry in CH2Cl2 as solvent
(Figure 2). The initial anodic scan shows two couples A1/C1 and
A2/C2 at potential values between þ0.75 and þ1.25 V (3a),
þ0.60 and þ1.10 V (3b), and þ0.50 and þ0.85 V (3c). When
the voltammograms were recorded at low scan rates and the
potential was switched at 0.15ꢀ0.20 V past the peak A1, the
anodic-to-cathodic peak separation for the first couple (A1/C1)
was close to 0.059 V, suggesting a reversible one-electron
transfer.
New, stable dinuclear palladium(II) complexes with diphenyl-
(3-methyl-2-indolyl)phosphine (C9H8NPPh2) in a P,N-bridging
coordinated mode have been also synthesized and character-
ized.38 The compound Pd2[(C6H4)PPh2]2[C9H7NPh2][O2C-
CH3] induced a highly successful β-boration/arylation reaction
of electron-deficient alkenes (99% conversion, 89% isolated yield
when dimethylacrylamine was the substrate).38
The formal electrode potentials vs AgCl/Ag for the A1/C1 couples
of 3aꢀc, calculated as the half-sum of the anodic and cathodic peak
potentials, are þ0.792, þ0.664 and þ0.524 V, respectively, decreas-
ing with the electron-donating ability of the aryl substituents at the
formamidinate ligands. The same result is found for the formal
4þ
6þ
Looking for Pd2 precursors of Pd2 compounds, we
present in this paper two new families of paddle-wheel ortho-
metalated palladium(II) compounds with bridging N,N- and
2084
dx.doi.org/10.1021/om100740q |Organometallics 2011, 30, 2083–2094

Organometallics
ARTICLE
Scheme 3. Synthesis of Pd2[(C6H4)PPh2]2[R0NC(H)NR0]2 Compounds 3aꢀc
Figure 1. ORTEP views of 3aꢀc with ellipsoids representing 30% probability and H atoms omitted for clarity.
electrode potentials for the A2/C2 couples, which are þ1.179,
þ1.043, and þ0.803 V, respectively.
reduction processes of the parent Pd complexes were detected
in the available potential region (down to ꢀ1.8 V).
In the case of 3a, the peak A2 at 1.179 V might correspond
to the oxidation of Pd25þ in the core or to the oxidation of the
N,N0-diphenylformamidinate ligand. The cyclic voltammetry of
N,N0-diphenylformamidine and silver N,N0-diphenylformamidi-
nate showed in each case an irreversible peak at a value close to
1.2 V, supporting the idea that a ligand oxidation takes place.
When the scan was initiated in the negative direction, no
The observed electrochemical behavior can be described as
two consecutive one-electron-transfer processes, starting from
4þ
6þ 2þ
5þ þ
the Pd2
complexes to successively yield [Pd2
]
and
[Pd2
]
cationic species. The relatively large separation
between the two associated A1/C1 and A2/C2 couples indicates
a certain PdꢀPd interaction. In spite of the observed behavior for
6þ
3a, its oxidation with Cl2 afforded the corresponding Pd2
2085
dx.doi.org/10.1021/om100740q |Organometallics 2011, 30, 2083–2094

Organometallics
ARTICLE
Table 1. Selected Distances (Å) and Angles (deg) for Com-
pounds 3aꢀc
3a
3b
3c
Pd(1)ꢀPd(2)
2.715(3)
2.284(7)
2.11(2)
2.717(2)
2.711(3)
2.282(10)
2.07(3)
2.15(2)
2.03(3)
86.8(3)
103.3(3)
Pd(1)ꢀP(1)
2.263(5)
Pd(1)ꢀNtrans-P
2.102(13)
2.155(12)
2.010(15)
86.20(13)
101.46(17)
Pd(1)ꢀNtrans-C
2.138(18)
2.02(2)
Pd(1)ꢀC(42)
P(1)ꢀPd(1)ꢀPd(2)
P(1)ꢀPd(1)ꢀPd(2)ꢀP(2)
86.49(17)
100.0(3)
Scheme 4. Four Possible Stereoisomers of the
{Pd2[(C6X4)PPh2]2}2þ Building Block
compound that was well-characterized by NMR spectroscopy.
All the results coincide with those observed for the carboxylate
complexes Pd2[(C6H4)PPh2]2[O2CR]2, although these show
generally higher electrode potentials for the A1/C1 and A2/C2
couples (between þ0.840 and þ1.490 V).29
Computational analysis of the frontier molecular orbitals
shows that the HOMO orbitals consist of a mixture of the
palladium d orbitals and the formamidinate ligands for all
compounds 3aꢀc (Figure S1 in the Supporting Information).
In all cases, similar σ* orbitals are formed with only
slightly differing composition. With the aryl substitution of the
nitrogen ligands, the HOMO orbitals were destabilized (EHOMO
= ꢀ4.70, ꢀ4.59, and ꢀ4.42 eV for 3aꢀc, respectively), which
can explain the observed shift in the formal electrode potentials.
Furthermore, the energy of the HOMO-1, which consists mainly
of the formamidinate ligands, was very close to the energy of the
HOMO with slightly decreasing differences of 0.20, 0.16, and
0.13 eV for 3aꢀc, respectively. Destabilization of the HOMO-1
energy similar to that of the HOMO energy was also predicted by
the calculations, indicating that the one-electron-oxidation pro-
cess originates from both the metal-based and the formamidi-
nate-based orbitals.
Figure 2. Cyclic voltammograms of 3aꢀc.
(R0 = C6H5, 3a; R0 = p-CH3C6H4, 3b; R0 = p-CH3OC6H4, 3c)
(Scheme 5).
However, in the case of the p-anisyl-substituted formamidinate
compound 3c the appearance of a brown precipitate indicated a
decomposition of the starting compound. 3c is easily and
reversibly oxidized in cyclic voltammetry but does not give a
stable Pd26þ complex by oxidation with iodobenzene dichloride.
The oxidation with bromine led to decomposition in the case of
all three compounds 3aꢀc.
The Pd26þ compounds 4a,b, however, were also not as stable as
4þ
Chemical Oxidation of Formamidinate Compounds, Pd2
f Pd26þ. The oxidation of compounds 3aꢀc with iodobenzene
dichloride showed different results for 3a,b on one hand and 3c
on the other. In the case of the phenyl- and p-tolyl-substituted
formamidinate compounds 3a,b, the typical color change from
yellow to red was observed after addition of the oxidizing
agent and clean products could be isolated as red solids. They
was hoped for. They decompose in solution even more quickly than
6þ
the Pd2
trifluoroacetate complex Pd2[(C6H4)PPh2]2(O2C-
CF3)2Cl2. The 13C NMR spectra had to be recorded at ꢀ20 °C
in order to slow down the decomposition and obtain clean results.
Single crystals of X-ray diffraction quality could not be grown. The
main decomposition product proved to be the tetranuclear Pd(II)
compound Pd4[(C6H4)PPh2]4Cl4, which was identified by NMR
spectroscopy and single-crystal X-ray diffraction.
6þ
were characterized by NMR spectroscopy as the Pd2 com-
plexes with general formula Pd2[(C6H4)PPh2]2[R0NC(H)NR0]2
2086
dx.doi.org/10.1021/om100740q |Organometallics 2011, 30, 2083–2094

Organometallics
ARTICLE
Scheme 5. Synthesis of Pd26þ Compounds 4a,b
Scheme 7. N,O-Donor Ligands Employed in the Synthesis of
New Pd24þ Compounds
Scheme 8. Possible Isomers for Pd24þ Compounds with N,
O-Donor Ligands
Scheme 6. Amidinate Ligands in Pd24þ Complexes
Table 2. Effect of Different Amidinate Ligands on the Reac-
tion Energy and Gibbs Free Energy (kJ/mol) of the Oxidation
Reaction:
Scheme 9. Synthesis of Pd24þ Compounds with N,O-Donor
Ligands
Pd2½ðC6H4ÞPPh2ꢁ2½R0NCðRÞNR0ꢁ2 þ Cl2 f
Pd2½ðC6H4ÞPPh2ꢁ2½R0NCðRÞNR0ꢁ2Cl2
R0 = H
R0 = Ph
R
ΔE
ΔG
ΔE
ΔG
Ph
ꢀ206
ꢀ211
ꢀ200
ꢀ189
ꢀ146
ꢀ149
ꢀ138
ꢀ129
ꢀ78
ꢀ81
ꢀ17
ꢀ19
ꢀ19
ꢀ42
CH3
H
ꢀ84
Mixtures of products were always obtained which decomposed
rapidly.
CF3
ꢀ106
Dinuclear Palladium Compounds with Bridging N,O-Do-
nor Ligands. Four different N,O ligands have been used in the
synthesis of a new family of Pd24þ complexes (Scheme 7).
These ligands coordinate to the Pd24þ core in a nonsymmetric
way which can lead to up to three different isomers (Scheme 8):
(a) The N,O-donor ligands are in a head-to-tail (H-T)
arrangement, with the N atoms trans to P atoms. The
molecule is C2 symmetric.
(b) The N,O-donor ligands are in a head-to-tail (H-T)
arrangement, with the N atoms trans to the metalated
C atoms. The molecule is C2 symmetric.
(c) The N,O-donor ligands are in a head-to-head (H-H)
arrangement. The molecule retains no symmetry.
4þ
6þ
Computational Studies on the Pd2 f Pd2 Oxidation
Reaction. Density functional theory (DFT) calculations on the
4þ
6þ
Pd2 f Pd2 oxidation reaction were performed to gain
insights into the influence of the different ligands and their
substituents on the oxidation reaction and to find different
6þ
amidinate ligands that would give more stable Pd2 com-
pounds. As this type of ligand can have different substituents at
the N as well as at the C atom (Scheme 6), different combina-
tions of substituents were calculated. The results are shown in
Table 2.
The computational results show that the substituents on
the amidinate N atoms have a greater effect on the reaction
6þ
Synthesis. When the N,O-donor ligand was succinimidate,
phthalimidate, or 2-hydroxypyridinate, Pd2[(C6H4)PPh2]2[N,
O]2 complexes (5ꢀ7) were obtained by adding a mixture of the
H(N,O) ligand and potassium hydroxide in methanol to a
suspension of 2 in dichloromethane (Scheme 9). The reactions
were followed by 31P NMR spectroscopy. The products were
obtained in good yields (77ꢀ88%), the reactions always giving
mixtures of two different isomers: one symmetric (a or b in
Scheme 8) and one asymmetric (c).
energy than do the substituents on the central C atom. Pd2
complexes with R0 = H are more stabilized. This great difference
seems to be caused by steric rather than electronic effects. For
the substituents on the C atom, an electron-donating group
has a stabilizing effect for R0 = H and a destabilizing effect for
R0 = Ph.
We tried to synthesize the benzamidinate complex Pd2-
[(C6H4)PPh2]2[HNC(Ph)NH]2 in order to prove the compu-
tational results. Benzamidine was readily obtained from its
hydrochloride salt.40 The complex could not be obtained either by
reaction of benzamidine with 2 or Pd2[(C6H4)PPh2]2[O2CCH3)]2
inthepresenceofabaseorbyreactionof2with silver benzamidinate.
A typical 31P NMR spectrum is depicted in Figure 3. The
singlet peak at 17.4 ppm corresponds to two equal P atoms in a
symmetric Pd2 compound (a or b), whereas the two doublets at
2087
dx.doi.org/10.1021/om100740q |Organometallics 2011, 30, 2083–2094

Organometallics
ARTICLE
Figure 3. 31P NMR spectrum of the product mixture of the reaction of 2 and succinimidate.
Thus, the product 8 was obtained in good yield (72%) and
could be identified by NMR spectroscopy as a cleanly symmetric
isomer (a or b in Scheme 8). There was no trace of the
asymmetric isomer in this case. The absolute configuration of
the product could not be resolved by X-ray diffraction, as no
suitable single crystals could be grown.
Table 3. a,b:c Ratio for Pd2[(C6H4)PPh2]2[N,O]2
Compounds
Pd2[(C6H4)PPh2]2[N,O]2
a,b:c ratio
5
6
7
3:1
10:1
5:1
5ꢀ8 were reacted with iodobenzene dichloride, but no clean
Pd26þ products could be obtained.
Isomerization of Pd2 Compounds. When 31P NMR spectra
were recorded directly of single crystals that had been shown to
contain only symmetric compounds of type a, different results
were observed. For the 2-hydroxypyridinate compound 7a, the
spectrum showed only the expected singlet, whereas in the case
of the imidate products 5a and 6a, the spectra confirmed the
presence in solution of different isomers, proved to be identical
with those of the isomer mixtures of the synthesis reaction, a
singlet and two doublets. This indicated that these products in
solution exist in a dynamic equilibrium between the two isomers
a and c.
As equilibria of this type usually depend on the nature of the
solvent, a series of 31P NMR spectra of the originally obtained
mixtures of 5ꢀ7 was recorded in different deuterated solvents.
Table 5 gives the symmetric/asymmetric isomer ratios found,
depending on the solvent.
In the case of compound 7a, the solvent has no effect on the
symmetric/asymmetric isomer ratio, and no dynamic exchange
between the two isomers was observed in solution. However, the
solvent has an important effect on the symmetric/asymmetric
isomer ratio for compounds 5a and 6a, displacing the equilibrium
in favor of one isomer or the other. Toluene-d8, as the least
polar solvent of those employed, favors the symmetric and
less polar configuration a, whereas the more polar solvents
21.5 and 15.1 ppm are assigned to the two different P atoms in
the asymmetric compound (c). The a,b:c ratio for the different
compounds are displayed in Table 3.
All attempts to separate the isomers by column chromatogra-
phy were unsuccessful.
Products 5ꢀ7 crystallized well, and single crystals suitable for
X-ray diffraction could be obtained. They were shown to be
crystals of pure symmetric compounds of type a (N atoms trans
to P atoms). The molecular structures are depicted in Figure 4,
and selected bond lengths and angles can be found in Table 4.
Crystals containing pure isomers of the isomers b and c could not
be found.
In the three structures the palladium units are in very similar
coordination cores. 7a, with 2-hydroxypyridinate ligands, has a
shorter PdꢀPd distance than the imidate compounds 5a and 6a,
while the PꢀPdꢀPdꢀP torsion angle is smaller for 5a than for
6a and 7a. Bond lengths and angles around the metal core are
4þ
very similar to those found for the ortho-metalated Pd2
compounds with carboxylate ligands,29 and again only molecules
with RP and SM conformations were observed.
A single isomer as reaction product was obtained by reacting
Pd2[(C6H4)PPh2]2[O2CCH3]2 and the acetanilidate ligand,
using sodium hydride as a base (Scheme 10).
2088
dx.doi.org/10.1021/om100740q |Organometallics 2011, 30, 2083–2094

Organometallics
ARTICLE
Figure 4. ORTEP views of 5aꢀ7a with ellipsoids representing 30% probability and H atoms omitted for clarity.
Table 4. Selected Distances (Å) and Angles (deg) for Com-
Scheme 10. Synthesis of Pd24þ Compound 8
pounds 5aꢀ7a
5a
6a
7a
Pd(1)ꢀPd(2)
2.7487(4)
2.2636(11)
1.984(4)
2.090(3)
2.176(3)
85.98(3)
96.26(4)
2.7480(9)
2.268(2)
1.993(8)
2.090(7)
2.170(5)
85.29(6)
98.49(8)
2.6860(9)
2.258(2)
1.992(9)
2.103(8)
2.130(7)
84.88(7)
100.49(8)
Pd(1)ꢀP(1)
Pd(1)ꢀC(42)
Pd(1)ꢀN(1)
Pd(1)ꢀO(1)
P(1)ꢀPd(1)ꢀPd(2)
P(1)ꢀPd(1)ꢀPd(2)ꢀP(2)
Table 5. Solvent Effect on the Symmetric/Asymmetric Ratio
of Pd2[(C6H4)PPh2]2(N,O)2 Compounds
chloroform and dichloromethane favor the more polar asym-
metric configuration c.
The fact that isomerization is observed for compounds 5 and 6
but not for compounds 7 and 8 can be explained by the latter’s
lack of a second oxygen atom. For 5 and 6 the isomerization can
take place by a simple “sliding” movement of one of the imidate
ligands (Scheme 11). A reaction like this has been described for
an Rh2 analogue of 5, Rh2[(C6H4)PPh2]2(OC4NH4O)2.41 Iso-
merization of 7 or 8 would have to include an additional
“rotation” step of the moving ligand, the energy barrier for which
seems to be too high at room temperature.
Density Functional Theory (DFT) Calculations on the Iso-
mers of Compounds 5, 7, and 8. Density functional theory
(DFT) calculations on the different isomers of compounds 5, 7,
and 8 were performed, and the results of these calculations
coincide with the experimental findings (Table 6).
CDCl3
CD2Cl2
toluene-d8
5
6
7
3:1
10:1
5:1
1.5:1
9:1
8:1
26:1
5:1
5:1
asymmetric isomer c. The difference in relative energies is
significantly higher for compound 8 than for the other two
compounds. All these results agree with the experimental find-
ings, where 8 was obtained as a pure isomer (due to the
calculations it can be concluded to be isomer a) and 5 and 7
were obtained as isomer mixtures with more isomer a than c,
while isomer b was never observed.
For all three compounds the symmetric isomer a was found to
be the most stable one by difference. The second symmetric
isomer b showed to have even higher relative energy than the
Ortho-Metalated Palladium Compounds with Bridging N,
N- and N,O-Donor Ligands as Precursors in Catalysis. The
catalytic properties of compounds 3a, 4a, and 5, as examples of
2089
dx.doi.org/10.1021/om100740q |Organometallics 2011, 30, 2083–2094

Organometallics
ARTICLE
Scheme 11. Possible Isomerization Mechanism for Compound 5
Table 6. Relative Energies and Relative Gibbs Free Energies
(kJ/mol) for the Different Isomers of 5, 7, and 8 (Referenced
to Respective Isomers a):
Scheme 12. Palladium Complexes with N,O- and N,N-Do-
nor Bridging Ligands Mediate the Catalytic Diboration/
Monoarylation/Oxidation of Alkenes
5
7
8
isomer
ΔE
ΔG
ΔE
ΔG
ΔE
ΔG
a
b
c
0
0
0
0
0
0
14.2
11.1
19.9
18.1
15.7
10.0
21.5
14.4
37.4
28.2
45.9
33.8
Table 7. Palladium Complexes with N,O- and N,N-Donor
Bridging Ligands Mediate the Catalytic Diboration of
Alkenesa
optimized reaction conditions described in the Experimental
Section (Scheme 2).
The diboration reactions with styrene as substrate yielded the
diborated product in moderate conversions (47ꢀ69% conver-
sion; Table 7, entries 1ꢀ3). When the reactions were carried out
at 78 °C, almost complete conversion was observed (94ꢀ97%,
Table 7, entries 4ꢀ6). The subsequent SuzukiꢀMiyaura reac-
tions with 4-nitrobromobenzene as reagent did not yield the
desired arylated product. This was not surprising for an aryldi-
boronate intermediate, despite the recent successful palladium-
catalyzed cross-coupling reaction from aryl-substituted second-
ary boronic esters reported by Crudden et al.42
The palladium-catalyzed diboration reaction of 3,3-dimethyl-
1-butene yielded moderate conversions at room temperature for
the three precursor catalysts (57ꢀ66%; Table 7, entries 7ꢀ9).
No reactions at higher temperatures were carried out, due to the
low boiling point of the substrate. However, the subsequent
palladium-catalyzed cross-coupling reaction/oxidation allowed
the formation of the carbohydroxylated adduct in quantitative
conversion (Scheme 12).
Palladium dimers 3a, 4a, and 5, with N,N- and N,O-donor
bridging ligands, were shown to be less active than complex 2 and
Pd2[(C6H4)PPh2]2(O2CCH3)2Cl2, for the catalytic diboration
reaction. This difference in catalytic behavior was especially
6þ
surprising in the case of 4a, a Pd2 complex very similar to
Pd2[(C6H4)PPh2]2(O2CCH3)2Cl2. It was expected that it
would be reduced to Pd4[(C6H4)PPh2]4Cl4 when reacted
with B2(cat)2. However, instead of the expected color change
from red to yellow, the reaction mixture turned dark brown
and the appearance of a very fine precipitate could be observed,
a Standard conditions: substrate/Pd = 1/0.05, B2cat2 (2 equiv), NaOAc
(1 equiv), solvent THF, t = 4 h. b Determined by 1H NMR spectroscopy.
4þ
very similar to the reactions with Pd2 compounds Pd2-
[(C6H4)PPh2]2(O2CR)2 and also compounds 3a and 5. It seems
that in all these cases reduction to Pd(0) and the formation of
phosphine-stabilized Pd nanoparticles could be involved.36
palladium dimers with N,N- and N,O-bridging ligands, were
explored in the tandem diboration/SuzukiꢀMiyaura cross-cou-
pling reactions followed by an oxidation pathway, employing the
2090
dx.doi.org/10.1021/om100740q |Organometallics 2011, 30, 2083–2094

Organometallics
ARTICLE
’ CONCLUSIONS
density functional B3PW91 was selected for the quantum chemical
studies.49,50 The basis set was comprised of a StuttgartꢀDresden effective
small core potential augmented with an extra p-polarization function for
palladium (SDD(p)) and a standard 6-31G* all-electron basis set for other
atoms. This method was found to yield reliable geometries in comparison
with the available experimental structures (see Tables S1 and S2 in the
Supporting Information). Frequency analysis with no scaling was performed
to ensure minima for the optimized structures. Thermodynamic values were
calculated at 298 K.
The electrochemical studies were carried out at room temperature
with a 273A PAR potentiostat in a three-electrode cell. The working
electrode was a Pt electrode with a surface of 3.1 mm2, the counter
electrode a Pt wire, and the reference electrode Ag/AgCl (saturated
KCl). The concentration was 0.5 mM for compounds 3a,b but in the
case of 3c was 0.1 mM because of its poor solubility. The solvent was
0.2 M Bu4NPF6ꢀCH2Cl2, and the scan rate was 50 mV/s. Under these
conditions Ea for the couple Fc/Fcþ was 0.550 V.
Preparation of Silver N,N0-Diarylformamidinates 1aꢀc.
These compounds were prepared by variation of a literature
procedure.39 An aqueous solution of ammonium hydroxide (20%,
0.91 mL, 10 mmol) was added to a solution of the corresponding N,
N0-diarylformamidine (8.89 mmol) in methanol (40 mL). Upon addi-
tion of a solution of silver nitrate (1.51 g, 8.89 mmol) in water (5 mL)
with vigorous stirring, a white voluminous precipitate formed, which
was collected by filtration, briefly washed with methanol, and vacuum-
dried in the absence of light. It was used in the following synthesis
without further purification. Yield: 1a, 1.45 g, 54%; 1b, 2.00 g, 68%; 1c,
2.26 g, 70%.
New dinuclear ortho-metalated palladium(II) compounds
with N,N0-diarylformamidinates, Pd2[(C6H4)PPh2]2[R0NC-
(H)NR0]2 (R0 = C6H5, 3a; R0 = p-CH3C6H4, 3b; R0 = p-CH3-
OC6H4, 3c), and N,O-donor ligands, Pd2[(C6H4)PPh2]2[N,O]2
(N,O = succinimidate (5), phthalimidate (6), 2-hydroxypyridi-
nate (7), acetanilidate (8)) have been synthesized and character-
ized by NMR spectroscopy and X-ray diffraction methods. The
6þ
oxidation with iodobenzene dichloride gave Pd2 compounds
only for the formamidinate complexes Pd2[(C6H4)PPh2]2[R0NC-
(H)NR0]2Cl2 (R0 = C6H5, 4a; R0 = p-CH3C6H4, 4b), which were
not as stable as the counterpart carboxylate derivatives and evolved
to Pd4[(C6H4)PPh2]4Cl4 as the main decomposition product.
DFT calculations on the Pd24þ f Pd26þ oxidation reaction show
that the substituents on the amidinate N atoms have a greater
effect on the reaction energy than the substituents on the C atom.
Pd2[(C6H4)PPh2]2[N,O]2 (N,O = succinimidate, phthalimi-
date, 2-hydroxypyridinate) compounds have been obtained as
a mixture of isomers with symmetric and asymmetric configura-
tions, but only the symmetric isomer was synthesized when the
N,O-donor ligand was acetanilidate. Compounds whose ligands
have a second oxygen atom show a dynamic equilibrium between
the isomers; this isomerization can take place by a simple
“sliding” movement of one of the imidate ligands. DFT calcula-
tions confirm that the symmetric isomers with N atoms trans to
O atoms are the most stable complexes; only these isomers have
been crystallized and characterized by X-ray diffraction methods.
The palladium dimers were tested as precursors of catalysts in
tandem diboration/arylation/oxidation reactions, providing high
activity in the organodiboronate formation, under reflux condi-
tions. The catalytic tandem reaction has been designed to trans-
form the diboronate intermediates into the monoarylated
product, which after oxidative workup provides the carbohy-
droxylated adduct in high yields.
Preparation of Pd2[(C6H4)PPh2]2[R0NC(H)NR0]2 (R0 = C6H5,
3a; R0 = p-CH3C6H4, 3b; R0 = p-CH3OC6H4, 3c). The correspond-
ing silver diarylformamidinates 1aꢀc (0.492 mmol) were added to
suspensions of 2 (200 mg, 0.112 mmol) in dichloromethane (20 mL),
and the mixtures were stirred in the absence of light for 3 h. The resulting
yellow solutions were separated from the white precipitates by filtration
and then evaporated to dryness. The yellow crude products were
dissolved in dichloromethane, filtered over a short plug of silica, and
precipitated by addition of hexane to give yellow microcrystalline
powders, which were collected by filtration and washed with hexane.
Single crystals for X-ray diffraction were grown by slow diffusion
of diethyl ether into a dichloromethane solution of the product. Yield:
3a, 205 mg, 81%; 3b, 206 mg, 78%; 3c, 242 mg, 87%.
’ EXPERIMENTAL SECTION
All reactions were carried out under a dry argon (synthesis) or
nitrogen (catalysis) atmosphere using Schlenk techniques. Solvents
were purified according to standard procedures.43 Commercially avail-
able reagents were used as purchased. Di-p-tolylformamidine and di-p-
Characterization data for 3a: 1H NMR (CDCl3, 400 MHz)
δ 6.52ꢀ6.75 (m, 16H, ar), 6.82ꢀ6.90 (m, 6H, ar), 6.98ꢀ6.99 (m,
2H, ꢀNCHNꢀ), 7.02ꢀ7.16 (m, 12H, ar), 7.20ꢀ7 0.30 (m, 10H, ar),
anisylformamidine,44 iodobenzene dichloride (PhI Cl2),45 and Pd-
3
46
(dba)2 were synthesized according to literature procedures. Solvent
13
7.35ꢀ7.49 (m, 4H, ar); 31P NMR (CDCl3, 162 MHz) δ 16.2 (s);
mixtures are v/v mixtures. Column chromatography was performed on
silica gel (35ꢀ70 mesh).
C NMR (CDCl3, 101 MHz) δ 121ꢀ151 (ar), 160.9 (s, ꢀNCHNꢀ),
165.5 (m, metalated). Anal. Calcd for C62H50N4P2Pd2: C, 66.14; H,
4.48; N, 4.98. Found: C, 66.68; H, 4.86; N, 4.85.
NMR spectra were recorded on Varian Gemini 300 MHz, Varian
Mercury 400 MHz, and Bruker Avance 400 MHz spectrometers as
solutions in deuterated chloroform at 25 °C, unless specified otherwise.
Chemical shifts are reported in ppm, using TMS (1H, 13C) and 85%
H3PO4 (31P) as references. The coupling constants (J) are in hertz (Hz).
Elemental analyses were provided by the analytical laboratory of the
University of Joensuu, Joensuu, Finland.
X-ray crystal structure data for 3a: formula C62H50N4P2Pd2, triclinic
system, space group P1, a = 13.142(3) Å, b = 20.177(4) Å, c = 22.774(5)
Å, R = 102.44(3)°, β = 97.16(3)°, γ = 91.35(3)°, V = 5843(2) Å3, Z = 4,
crystal dimensions 0.18 ꢂ 0.21 ꢂ 0.26 mm3, Mo KR radiation, 293(2)
K, 16 968 reflections, 9121 independent reflection (μ = 0.709 mmꢀ1),
refinement (on F2) with SHELXTL (version 6.1), 1261 parameters,
960 restraints, R1 = 0.0931 (I > 2σ) and wR2 (all data) = 0.3133,
GOF = 1.053, maximum/minimum residual electron density 1.126/
X-ray structure determinations were carried out on a Bruker-Nonius
Kappa CCD diffractometer (Mo KR radiation, λ = 0.710 73 Å). The
structures were solved by direct methods using the SHELXTL soft-
ware package.47 The correct positions for the heavy atoms were deduced
from an E map. Subsequent least-squares refinement and difference
Fourier calculations revealed the positions of the remaining non-
hydrogen atoms. Hydrogen atoms were placed in geometrically
generated positions and refined riding on the atom to which they are
attached.
ꢀ0.730 e Åꢀ3
.
1
Characterization data for 3b: H NMR (CDCl3, 400 MHz) δ 2.13
(s, 6H, ꢀCH3), 2.22 (s, 6H, ꢀCH3), 6.36ꢀ6.38 (m, 4H, ar), 6.54ꢀ6.72
(m, 14H, ar), 6.88ꢀ6.90 (m, 2H, ꢀNCHNꢀ), 6.91ꢀ6.93 (m, 4H, ar),
7.01ꢀ7.05 (m, 4H, ar), 7.08ꢀ7.15 (m, 8H, ar), 7.18ꢀ7.24 (m, 6H, ar),
7.34ꢀ7.39 (m, 4H, ar); 31P NMR (CDCl3, 162 MHz) δ 15.8 (s); 13
C
All computational calculations were carried out with the Gaussian03
NMR (CDCl3, 101 MHz) δ 20.6 (s, ꢀCH3), 20.7 (s, ꢀCH3), 121ꢀ149
program package.48 The DFT level of theory with the nonlocal hybrid
(ar), 160.6 (s, ꢀNCHNꢀ), 166.0 (m, metalated). Anal. Calcd for
2091
dx.doi.org/10.1021/om100740q |Organometallics 2011, 30, 2083–2094

Organometallics
ARTICLE
C66H58N4P2Pd2: C, 67.07; H, 4.95; N, 4.74. Found: C, 66.42; H, 5.12;
N, 4.94.
to give a yellow microcrystalline powder, which was collected by filtra-
tion and washed with hexane. In solution the products were mixtures of
the symmetric product a and the asymmetric product c. Single crystals
of 5a, 6a, and 7a for X-ray diffraction were grown by slow diffusion of
diethyl ether into a dichloromethane (5a) or chloroform (6a, 7a) solu-
tion of the products. Yield: 5, 92 mg, 88%; 6, 96 mg, 83%; 7, 80 mg, 77%.
Characterization data for 5: 31P NMR (CDCl3, 162 MHz) a, δ 16.8
(s); c, δ 20.8 (d, J = 37), 14.5 (d, J = 37); a:c ratio 3:1. Anal. Calcd for
C44H36N2O4P2Pd2: C, 56.73; H, 3.90; N, 3.01. Calcd for C44H36-
X-ray crystal structure data for 3b: formula C68H62Cl4N4P2Pd2,
triclinic system, space group P1, a = 13.6960(4) Å, b = 14.3380(6) Å,
c = 19.0430(7) Å, R = 90.338(3)°, β = 104.917(3)°, γ = 118.008(18)°,
V = 3154.9(2) Å3, Z = 2, crystal dimensions 0.21 ꢂ 0.23 ꢂ 0.24 mm3,
Mo KR radiation, 293(2) K, 9661 reflections, 5468 independent
reflections (μ = 0.834 mmꢀ1), refinement (on F2) with SHELXTL
(version 6.1), 731 parameters, 0 restraints, R1= 0.0851 (I > 2σ) and wR2
(all data) = 0.2654, GOF = 1.063, maximum/minimum residual electron
N2O4P2Pd2 CH2Cl2: C, 53.17; H, 3.77; N, 2.76. Found: C, 52.72; H,
3
density 1.520/ꢀ1.274 e Åꢀ3
.
3.77; N, 2.75.
Characterization data for 3c: 1H NMR (CD2Cl2, 500 MHz) δ 3.63 (s,
6H, ꢀCH3), 3.70 (s, 6H, ꢀCH3), 6.44ꢀ6.48 (m, 8H, ar), 6.58ꢀ6.61 (m,
4H, ar), 6.66ꢀ6.69 (m, 2H, ar), 6.71ꢀ6.76 (m, 10H, ꢀNCHNꢀ þ ar),
7.03ꢀ7.06 (m, 4H, ar), 7.11ꢀ7.14 (m, 4H, ar), 7.17ꢀ7.25 (m, 8H, ar),
7.27ꢀ7.30 (m, 2H, ar), 7.33ꢀ7.37 (m, 4H, ar); 31P NMR (CD2Cl2, 162
MHz) δ17.0 (s); 13CNMR(CD2Cl2, 101MHz) δ55.48 (s, ꢀCH3), 55.52
(s, ꢀCH3), 112ꢀ154 (ar), 160.1 (s, ꢀNCHNꢀ), 166.3 (m, metalated).
Anal. Calcd for C66H58N4O4P2Pd2: C, 63.62;, H, 4.69; N, 4.50. Found: C,
64.01; H, 4.97; N, 4.39.
X-ray drystal structure data for 5a: formula C45H38Cl2N2O4P2Pd2,
orthorhombic system, space group Pbca, a = 19.2370(2) Å, b =
20.3130(2) Å, c = 21.4700(3) Å, V = 8398.64(17)Å3, Z = 8, crystal
dimensions: 0.23 ꢂ 0.20 ꢂ 0.18 mm3, Mo KR radiation, 293(2) K,
54 202 reflections, 9553 independent reflections (μ = 1.107 mmꢀ1),
refinement (on F2) with SHELXTL (version 6.1), 515 parameters,
0 restraints, R1 = 0.0511 (I > 2σ) and wR2 (all data) = 0.1508,
GOF = 1.063, maximum/minimum residual electron density 0.940/
ꢀ1.392 e Åꢀ3
.
Characterization data for 6: 31P NMR (CDCl3, 162 MHz) a, δ 16.2
(s); c, δ 20.3 (d, J = 37), 14.4 (d, J = 37); a:c ratio 10:1. Anal. Calcd for
C52H36N2O4P2Pd2: C, 60.78; H, 3.53; N, 2.73. Calcd for C52H36-
X-ray crystal structure data for 3c: formula C66H58N4O4P2Pd2, triclinic
system, space group P1, a = 14.564(3) Å, b = 18.997(4) Å, c = 21.815(4) Å,
R= 103.35(3)°, β= 96.20(3)°, γ= 93.82(3)°, V= 5812(2) Å3, Z=2,crystal
dimensions 0.26 ꢂ 0.22 ꢂ 0.21 mm3, Mo KR radiation, 293(2) K, 6173
reflections, 3583 independent reflections (μ= 0.726 mmꢀ1),refinement (on
F2) with SHELXTL (version 6.1), 1406 parameters, 1378 restraints, R1 =
0.0705 (I > 2σ) and wR2 (all data) = 0.2318, GOF = 1.186, maximum/
N2O4P2Pd2 2CHCl3: C, 51.21; H, 3.02; N, 2.21. Found: C, 51.84; H,
3
3.09; N, 2.28.
X-ray crystal structure data for 6a: formula C105H73Cl3N4O11P4Pd4,
monoclinic system, space group P21/c, a = 12.842(3) Å, b = 22.648(5) Å,
c = 33.392(7) Å, β = 90.89(3)°, V = 9711(3) Å3, Z = 2, crystal
dimensions 0.26 ꢂ 0.23 ꢂ 0.19 mm3, Mo KR radiation, 293(2) K,
30 394 reflections, 15 111 independent reflections (μ = 0.939
mmꢀ1), refinement (on F2) with SHELXTL (version 6.1), 1166
parameters, 0 restraints, R1 = 0.0915 (I > 2σ) and wR2 (all data) =
0.2662, GOF = 1.050, maximum/minimum residual electron density
minimum residual electron density 0.933/ꢀ1.025 e Åꢀ3
.
Preparation of Pd2[(C6H4)PPh2]2[R0NC(H)NR0]2Cl2 (R0
=
C6H4, 4a). A solution of iodobenzene dichloride (18 mg, 0.066 mmol)
in acetonitrile (0.5 mL) was added to a suspension of 3a (0.044 mmol) in
acetonitrile (5 mL) at 0 °C. The suspension immediately changed from
yellow to red. After 10 min of stirring the red microcrystalline precipitate was
collected by filtration and washed with cold acetonitrile and diethyl ether.
Yield: 48 mg, 91%. 1H NMR (CDCl3, 400 MHz): δ 6.06ꢀ6.08 (m, 4H, ar),
6.72ꢀ6.81 (m, 10H, ar), 6.91ꢀ6.94 (m, 2H, ar), 6.98ꢀ7.10 (m, 16H, ar),
7.14ꢀ7.16 (m, 2H, ꢀNCHNꢀ), 7.30ꢀ7.34 (m, 8H, ar), 7.44ꢀ7.47
(m, 2H, ar), 7.93ꢀ7.96 (m, 2H, ar), 8.26ꢀ8.30 (m, 4H, ar). 31P NMR
(CDCl3, 162 MHz): δ ꢀ16.3 (s). 13C NMR (CDCl3, 75 MHz, 253 K): δ
123ꢀ151 (ar), 159.1 (m, metalated), 168.8 (s, ꢀNCHNꢀ).
2.722/ꢀ2.119 e Åꢀ3
.
Characterization data for 7: 31P NMR (CDCl3, 162 MHz) a, δ 17.9
(s); c, δ 20.3 (d, J = 28), 16.9 (d, J = 28); a:c ratio 5:1. Anal. Calcd for
C46H36N2O2P2Pd2: C, 59.82; H, 3.93; N, 3.03. Calcd for C46H36-
N2O2P2Pd2 2CHCl3: C, 49.60; H, 3.30; N, 2.41. Found: C, 49.91; H,
3
3.37; N, 2.46.
X-ray crystal structure data for 7a: formula C97H81Cl9N4O5P4Pd4,
triclinic system, space group P1, a = 13.7810(2) Å, b = 18.3740(2) Å, c =
19.3370 Å, R = 92.7680(5)°, β = 90.2160(6)°, γ = 91.0030(5)°, V =
4889.85 Å3, Z = 2, crystal dimensions 0.27 ꢂ 0.25 ꢂ 0.20 mm3, Mo KR
radiation, 293(2) K, 11 939 reflections, 11 939 independent reflections
(μ = 1.088 mmꢀ1), refinement (on F2) with SHELXTL (version 6.1),
1100 parameters, 0 restraints, R1 = 0.0914 (I > 2σ) and wR2 (all data) =
0.2607, GOF=1.148, maximum/minimum residual electron density
Preparation of Pd2[(C6H4)PPh2]2[R0NC(H)NR0]2Cl2 (R0
=
p-CH3C6H4, 4b). A solution of iodobenzene dichloride (17 mg, 0.063
mmol) in acetonitrile (0.5 mL) was added to a suspension of 3b (0.042
mmol) in acetonitrile (5 mL) at 0 °C. Immediately, a red solution was
obtained. After 5 min of stirring the solution was evaporated to dryness and
the resulting red solid was dissolved in the minimum amount of diethyl ether.
Addition of hexane produced a red microcrystalline precipitate, which was
collected by filtration and washed with hexane. Yield: 40 mg, 76%. 1H NMR
(CDCl3, 400 MHz): δ 2.12 (s, 6H, ꢀCH3), 2.25 (s, 6H, CH3), 5.93ꢀ5.95
(m, 4H, ar), 6.56ꢀ6.58 (m, 4H, ar), 6.73ꢀ6.77 (m, 4H, ar), 6.85ꢀ6.93 (m,
10H, ar), 7.05ꢀ7.23 (m, 10H, ꢀNCHNꢀ þ ar), 7.29ꢀ7.33 (m, 6H, ar),
7.41ꢀ7.45 (m, 2H, ar), 7.90ꢀ7.93 (m, 2H, ar), 8.25ꢀ8.30 (m, 4H, ar). 31P
NMR (CDCl3, 162 MHz): δ ꢀ16.4 (s). 13C NMR (CDCl3, 75 MHz,
253 K): δ 20.9 (s, ꢀCH3), 21.0 (s, ꢀCH3), 123ꢀ149 (ar), 159.4
(m, metalated), 168.6 (s, ꢀNCHNꢀ).
2.147/ꢀ2.369 e Åꢀ3
.
Preparation of Pd2[(C6H4)PPh2]2(N,O)2 (8, N,O = Acet-
anilidate). Acetanilide (38 mg, 0.280 mmol) was added to a suspen-
sion of sodium hydride (60% in mineral oil, 12 mg, 0.300 mmol) in dry
tetrahydrofuran under an argon atmosphere. When the emission of
hydrogen ceased, a solution of 2 (60 mg, 0.070 mmol) in dry
dichloromethane was added. After it was stirred for 15 min, the solution
was evaporated to dryness. The yellow crude product obtained was
extracted with dichloromethane, filtered over a short plug of silica, and
precipitated by addition of hexane to give a yellow microcrystalline
powder, which was collected by filtration and washed with hexane. Yield:
Preparation of Pd2[(C6H4)PPh2]2(N,O)2 (5, N,O = Succini-
midate; 6, N,O = Phthalimidate; 7, N,O = 2-Hydroxypy-
ridinate). The protonated N,O-donor ligand (0.270 mmol) and
potassium hydroxide (0.270 mmol) were dissolved in the minimum
amount of methanol. This solution was added at 0 °C to a solution of 2
(100 mg, 0.056 mmol) in dichloromethane (10 mL). After it was stirred
for 1 h at the same temperature, the solution was evaporated to dryness.
The yellow crude product obtained was extracted with dichloromethane,
filtered over a short plug of silica, and precipitated by addition of hexane
1
51 mg, 72%. H NMR (CDCl3, 400 MHz): δ 0.96 (s, 6H, ꢀCH3),
6.47ꢀ6.52 (m, 6H, ar), 6.56ꢀ6.64 (m, 6H, ar), 6.70ꢀ6.73 (m, 4H, ar),
6.86ꢀ6.95 (m, 6H, ar), 7.03ꢀ7.07 (m, 4H, ar), 7.10ꢀ7.14 (m, 2H, ar),
7.40ꢀ7.48 (m, 6H, ar), 7.70ꢀ7.75 (m, 4H, ar). 31P NMR (CDCl3, 162
MHz): δ 17.5 (s). 13C NMR (CDCl3, 101 MHz): δ 21.2 (s, ꢀCH3),
120ꢀ150 (ar), 164.4 (m, metalated), 174.4 (m, OC(CH3)N).
2092
dx.doi.org/10.1021/om100740q |Organometallics 2011, 30, 2083–2094

Organometallics
ARTICLE
Typical Catalytic β-Diboration, Subsequent Suzuki Reac-
tion, and Oxidative Workup. Bis(catecholato)diboron (0.4 mmol)
was added to a solution of the catalyst (5 mol %, 0.01 mmol Pd) and
sodium acetate (0.2 mmol) in tetrahydrofuran (2 mL) under nitrogen.
The solution was stirred for 5 min, the substrate (0.2 mmol) was added,
and stirring was continued for 4 h at room temperature. The diboration
(16) Takani, M.; Masuda, M.; Yamauchi, O. Inorg. Chim. Acta 1995,
235, 367.
(17) Lenarda, M.; Nardin, G.; Pellizer, G.; Braye, E.; Graziani, M.
J. Chem. Soc., Chem. Commun. 1985, 1536.
(18) Canty, A. J.; Minchin, N. J.; Engelhardt, L. M.; White, A. H.
Inorg. Chim. Acta 1985, 102, L29.
(19) Cera, M.; Cerrada, E.; Laguna, M.; Mata, J. A.; Teruel, H.
Organometallics 2002, 21, 121.
1
conversions were determined by H NMR spectroscopy of 0.1 mL
aliquots of the reaction mixtures. After the reaction mixture was heated
to reflux, cesium carbonate (0.6 mmol), 4-nitrobromobenzene (0.4
mmol), and water (degassed, 0.2 mL) were added and the reaction
mixture was stirred for 15 h. After the mixture was cooled to room
temperature, NaOH(aq) (3 M, 1 mL) and H2O2 (30%, 1 mL) were
added carefully and stirring was continued for 2 h. The oxidation was
quenched by adding a saturated aqueous solution of sodium thiosulfate
(1 mL) and NaOH(aq) (1 M, 10 mL). Then the reaction mixture was
extracted with ethyl acetate (3 ꢂ 20 mL) and the united organic phases
were washed with brine (20 mL), dried over magnesium sulfate, and
dried under reduced pressure. The products obtained were analyzed by
1H NMR spectroscopy51 to determine the degree of conversion and the
nature of the reaction products.
(20) Cotton, F. A.; Matusz, M.; Poli, R. Inorg. Chem. 1987, 26, 1472.
(21) Cotton, F. A.; Matusz, M.; Poli, R.; Feng, X. J. Am. Chem. Soc.
1988, 110, 1144.
(22) Yao, C. L.; He, L. P.; Korp, J. D.; Bear, J. L. Inorg. Chem. 1988,
27, 4389.
(23) Abul-Haj, M.; Quiros, M.; Salas, J. M. Polyhedron 2004, 23, 2373.
(24) Berry, J. F.; Cotton, F. A.; Ibragimov, S. A.; Murillo, C. A.;
Wang, X. Inorg. Chem. 2005, 44, 6129.
(25) Cotton, F. A.; Gu, J.; Murillo, C. A.; Timmons, D. J. J. Am. Chem.
Soc. 1998, 120, 13280.
(26) Bancroft, D. P.; Cotton, F. A.; Falvello, L. R.; Schwotzer, W. Inorg.
Chem. 1986, 25, 1015.
(27) Aarif, A. M.; Estevan, F.; García-Bernabꢀe, A.; Lahuerta, P.; Sanaꢀu,
ꢀ
M.; Ubeda, M. A. Inorg. Chem. 1997, 36, 6472.
(28) Estevan, F.; García-Bernabꢀe, A.; Lahuerta, P.; Sanaꢀu, M.;
’ ASSOCIATED CONTENT
ꢀ
Ubeda, M. A.; Ramirez de Arellano, M. C. Inorg. Chem. 2000, 39, 5964.
ꢀ
(29) Koshevoy, I. O.; Lahuerta, P.; Sanaꢀu, M.; Ubeda, M. A.;
S
Supporting Information. Text, tables, and figures giving
b
Domꢀenech, A. Dalton Trans. 2006, 5536.
(30) Umakoshi, K.; Ichimura, A.; Kinoshita, I.; Ooi, S. Inorg. Chem.
1990, 29, 4005.
computational details and CIF files giving full crystallographic
data for 3aꢀc and 5aꢀ7a. This material is available free of charge
via the Internet at http://pubs.acs.org.
(31) Bond, A. M.; Canty, A. J.; Cooper, J. B.; Tedesco, V.; Traill,
P. R.; Way, D. M. Inorg. Chim. Acta 1996, 251, 185.
(32) Cotton, F. A.; Koshevoy, I. O.; Lahuerta, P.; Murillo, C. A.; Sanaꢀu,
’ AUTHOR INFORMATION
ꢀ
M.; Ubeda, M. A.; Zhao, Q. J. Am. Chem. Soc. 2006, 128, 13674.
Corresponding Author
*Tel: 00 34 963543147. Fax: 00 34 963543929. E-mail: angeles.
ubeda@uv.es.
(33) Lillo, V.; Mas-Marzꢀa, E.; Segarra, A. M.; Carbꢀo, J. J.; Bo, C.;
Fernꢀandez, E. Chem. Commun. 2007, 338.
(34) Cui, Q.; Musaev, D. G.; Morokuma, K. Organometallics 1998,
17, 742.
(35) Sakaki, S.; Kikuno, T. Inorg. Chem. 1997, 36, 22.
(36) Penno, D.; Lillo, V.; Koshevoy, I. O.; Sanaꢀu, M.; Ubeda, M. A.;
’ ACKNOWLEDGMENT
ꢀ
Lahuerta, P.; Fernꢀandez, E. Chem. Eur. J. 2008, 14, 10648.
(37) (a) Powers, D. C.; Ritter, T. Nat. Chem. 2009, 1, 302.
(b) Powers, D. C.; Geibel, M. A. L.; Klein, E. M. N.; Ritter, T. J. Am. Chem.
Soc. 2009, 131, 17050.
We are grateful for financial support from the MEC of Spain
(CTQ2010-16226/BQU, CTQ2008-06466) and from the Acad-
emy of Finland (P.H., Grant 129772).
(38) Bonet, A.; Gulyꢀas, H.; Koshevoy, I. O.; Estevan, F.; Sanaꢀu, M.;
Ubeda, M. A.; Fernꢀandez, E. Chem. Eur. J. 2010, 16, 6382.
’ REFERENCES
ꢀ
(39) Archibald, S. J.; Alcock, N. W.; Busch, D. H.; Whitcomb, D. R.
Inorg. Chem. 1999, 38, 5571.
(40) Green, N. J.; Xiang, J.; Chen, J.; Chen, L.; Davies, A. M.; Erbe,
D.; Tam, S.; Tobin, J. F. Bioorg. Med. Chem. 2003, 11, 2991.
(41) Esteban, J.; Hirva, P.; Lahuerta, P.; Martínez, M. Inorg. Chem.
2006, 45, 8776.
(42) Imao, D.; Glasspoole, B. W.; Laberge, V. S.; Crudden, C. M.
J. Am. Chem. Soc. 2009, 131, 5024.
(43) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of
Laboratory Chemicals, 2nd ed.; Pergamon: Oxford, U.K., 1980.
(44) Lin, C.; Protasiewicz, J. D.; Smith, E. T.; Ren, T. Inorg. Chem.
1996, 35, 6422.
(1) Farina, V. Adv. Synth. Catal. 2004, 346, 1553.
(2) Soderberg, B. C. G. Coord. Chem. Rev. 2004, 248, 1085.
(3) Stahl, S. S. Angew. Chem., Int. Ed. 2004, 43, 3400.
(4) Mueller, J. A.; Goller, C. P.; Sigman, M. S. J. Am. Chem. Soc. 2004,
126, 9724.
(5) Nicolaou, K. C.; Bulger, P. G.; Sarlah, D. Angew. Chem., Int. Ed.
2005, 44, 4442.
(6) Jain, V. K.; Jain, L. Coord. Chem. Rev. 2005, 249, 3075.
(7) Ryabov, A. D. Chem. Rev. 1990, 90, 403.
(8) Garcia-Ruano, J. L.; Gonzalez, A. M.; Lopez-Solera, I.; Masaguer,
J. R.; Navarro-Ranninger, C.; Raithby, P. R.; Rodríguez, J. H. Angew. Chem.,
Int. Ed. 1995, 34, 1351.
(45) Lucas, H. J.; Kennedy, E. R. Org. Synth. 1942, 22, 69.
(46) Rettig, M. F.; Maitlis, P. M. Inorg. Synth. 1977, 17, 134.
(47) SHELXTL (Version 6.10); Bruker, 2000.
(9) Masciocchi, N.; Ragaini, F.; Sironi, A. Organometallics 2002,
21, 3489.
(10) Tian, G.;Boyle, P. D.;Novak, B. M.Organometallics 2002, 21, 1462.
(11) Vicente, J.; Arcas, A.; Fernandez-Hernandez, J. M.; Bautista, D.
Organometallics 2001, 20, 2767.
(12) Ruíz, J.; Rodríguez, V.; Cutillas, N.; Pardo, M.; Pꢀerez, J.; Lꢀopez,
G.; Chaloner, P.; Hitchcock, P. B. Organometallics 2001, 20, 1973.
(13) Nakatsu, K.; Kafuku, K.; Yamaoka, H.; Isobe, K.; Nakamura, Y.;
Kawaguchi, S. Inorg. Chim. Acta 1981, 54, L69.
(14) Culkin, D. A.; Hartwig, J. F. J. Am. Chem. Soc. 2002, 124, 9330.
(15) Ruiz, J.; Rodriguez, V.; Lopez, G.; Casabo, J.; Molins, E.;
Miravitlles, C. Organometallics 1999, 18, 1177.
(48) 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.;
2093
dx.doi.org/10.1021/om100740q |Organometallics 2011, 30, 2083–2094

Organometallics
ARTICLE
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.
(49) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(50) Perdew, J. P.; Wang, Y. Phys. Rev. B 1992, 45, 13244.
(51) Miller, S. P.; Morgan, J. B.; Nepveux, F. J.; Morken, J. P. Org.
Lett. 2004, 6, 131.
2094
dx.doi.org/10.1021/om100740q |Organometallics 2011, 30, 2083–2094