Palladium(I) phosphido-bridged dinuclear derivatives with both phosphine and amine ligands

Article abstract of DOI:10.1016/S0020-1693(98)00295-3

A large excess of pyridine reacts with the dinuclear derivative of palladium(I) [Pd₂(μ-PBu^t₂)(PPh₃) ₃]BF₄ ((2)BF₄) yielding firstly [Pd₂(μ-PBu^t₂)(Py)(PPh₃) ₃]BF₄, ((4)BF₄) and then [Pd₂(μ-PBu^t₂)(Py)₂(PPh ₃)₂]BF₄, ((5)BF₄) as a mixture of two isomers. The reactions of (5)BF₄ with CO, isoprene and CS₂ were investigated and compared to the corresponding reactions of its analogues containing only phosphines or phosphites as terminally bonded ligands; the higher lability of the pyridine molecules renders complex (5b)BF₄ a promising reagent which easily furnishes the 26 e^- fragment Pd₂(μ-PBu^t₂)(PPh₃)₂, with two adjacent unsaturated palladium centers.

Full text of DOI:10.1016/S0020-1693(98)00295-3

Inorganica Chimica Acta 284 (1999) 246±251
Palladium(I) phosphido-bridged dinuclear derivatives
with both phosphine and amine ligands
Piero Leonia,b,*, Stefano Papuccia, Marco Pasqualia
a
Á
Dipartimento di Chimica e Chimica Industriale, Universita di Pisa, I-56126 Pisa, Italy
bScuola Normale Superiore, Piazza dei Cavalieri, I-56126 Pisa, Italy
Received 10 April 1998; accepted 3 July 1998
Abstract
A large excess of pyridine reacts with the dinuclear derivative of palladium(I) [Pd2(m-PBut2)(PPh3)3]BF4 ((2)BF4) yielding ®rstly [Pd2(m-
PBut2)(Py)(PPh3)3]BF4, ((4)BF4) and then [Pd2(m-PBut2)(Py)2(PPh3)2]BF4, ((5)BF4) as a mixture of two isomers. The reactions of (5)BF4
with CO, isoprene and CS2 were investigated and compared to the corresponding reactions of its analogues containing only phosphines or
phosphites as terminally bonded ligands; the higher lability of the pyridine molecules renders complex (5b)BF4 a promising reagent which
easily furnishes the 26 e fragment Pd2(m-PBut2)(PPh3)2, with two adjacent unsaturated palladium centers. # 1999 Elsevier Science S.A.
All rights reserved.
Keywords: Palladium(I) complexes; Dinuclear complexes; Phosphide complexes; Amine complexes
1. Introduction
(L ˆ 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane)
[16]. Scheme 1 shows some examples of dinuclear com-
pounds containing at least one nitrogen donor [17±22], but
details concerning their reactivity are rather scarce, with the
exception of the class of derivatives containing 2-diphenyl-
phosphino pyridine [1,21,22].
The chemistry of palladium derivatives with nitrogen
donor ligands has been known for a long time and is still
being developed [1]. It deals mainly with compounds of
palladium(II) and, more recently, of palladium(IV) [2±7],
while it is less developed with regard to lower oxidation
states. Ligands bearing nitrogen donors linked to a conjugate
p system, although less used than CO, phosphines or iso-
nitriles for the stabilization of low oxidation states, can
delocalize an excess of electron density and behave as
suitable ligands to electron-rich palladium complexes [1].
Actually the series of palladium(0) derivatives (LL)Pd(L0),
in which L0 is a quinone [8,9] or an activated ole®n [10] and
LL is bipy, o-phen, tmeda or (Py)2, was reported as early as
1974, but its recognition was obscured by the contemporary
remarkable success enjoyed by palladium(0) phosphine
derivatives. The ®eld is, however, destined to grow in the
future, since zerovalent palladium complexes with diimine
ligands have been shown to catalyze C±C bond-forming
reactions [11±15].
Our group is investigating the reactivity of phosphido-
bridged palladium(I) cationic systems of general formula
‡
[Pd2(m-PBut2)(L)n] (n ˆ 3, 4; L ˆ PR2H, PR3, P(OR)3, see
As concerns the oxidation state ‡ 1, only a few deriva-
tives have been reported until now [16±22], among which a
rare example of mononuclear Pd(I) complex [Pd(L)]PF6
*Corresponding author. E-mail: leoni@dcci.unipi.it
Scheme 1.
0020-1693/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(98)00295-3

P. Leoni et al. / Inorganica Chimica Acta 284 (1999) 246±251
247
sieves under an inert atmosphere. [Pd2(m-PBut2)-
(PPh3)3]BF4 ((2)BF4) was prepared as previously described
[24]. IR spectra were registered with a Perkin±Elmer FT-IR
1725X spectrophotometer and NMR spectra with a Varian
Gemini 200BB spectrometer, frequencies were referenced
to the residual signal of the deuterated solvent (1H) and to
external 85% H3PO4 (31P).
2.2. Preparation of [Pd2(ꢀ±PBut2)(PPh3)2(Py)2]BF4
((5b)BF4)
Complex (2)BF4 (323 mg, 0.26 mmol) was dissolved in
pyridine (5 ml) giving a red solution which turns orange±
yellow in ca 15 min. After 30 min n-hexane (15 ml) was
added, causing the precipitation of an orange oil which
solidi®ed under vigorous stirring. The yellow solid was
®ltered off, washed with n-hexane and vacuum-dried
(241 mg, 82%). Anal. Calc. for C54H58BF4N2P3Pd2: C,
57.5; H, 5.18; N, 2.48. Found: C, 56.9; H, 5.25; N,
Scheme 2.
Scheme 2) [23±29]; the central [Pd2(m-PBut2)] core of
these complexes proved to be stable under various reaction
conditions, ensuring the conservation of the dinuclear nature
of its derivatives, while some of the terminal phosphorus
ligands were displaced by reversible reactions with other
monodentate (CO) [23,25] or bidentate (isoprene [25,26],
CS2 [24,27]) ligands. Since phosphorus ligands have a good
af®nity for the dipalladium core, substitution equilibria are
sometimes so unfavorable as to be hardly perceptible or the
reactions are easily reverted in vacuo [23±27]. The ability of
other ligands to bind the complex can only be inferred
2.44%. H NMR (CDCl3, 298 K): ꢁ (ppm) 7.25 (m, 40H,
C6H5P ‡ C5H5N), 1.1 [d, 3J(HP) ˆ 15 Hz, 18H,
‡
1
1
PC(CH3)3]. IR (Nujol, KBr): 1055 s, br (ꢂBF) cm
.
2.3. Reaction of [Pd2(ꢀ-PBut2)(PPh3)2(Py)2]BF4 with CS2
A red CDCl3 solution of (5b)BF4 (20 mg, 0.09 mmol in
0.5 ml) turns orange a few seconds after the addition of CS2
1
(20 ml, 0.332 mmol). The 31Pf Hg NMR spectrum of the
solution shows the signals of [Pd2(m-PBut2)(PPh3)2(m,h2,h2-
CS2)]BF4 as the unique P-containing product (ꢁ (ppm): 379
(t, m-P), 25.7 [d, PPh3, 2J(PP) ˆ21 Hz] [24]). The reaction
is also quantitative with equimolar amounts of the reagents.
‡
indirectly, for example, the reaction of (1) with ethylene
ends up with the insertion of the ole®n into the P±H bond of
the secondary phosphines, but the possible intermediate
ole®n complex was neither isolated nor observed as a
transient species [26]. The substitution of at least some
of the phosphorus ligands in structures (1) ±(3) with easily
displaceable ligands is therefore one of our strategies toward
more reactive systems.
2.4. Reaction of [Pd2(ꢀ-PBut2)(PPh3)2(Py)2]BF4 with
isoprene
‡
‡
A red CDCl3 solution of (5b)BF4 (20 mg, 0.09 mmol, in
0.5 ml) turns yellow a few seconds after the addition of
1
The reaction of [Pd2(m-PBut2)(PPh3)3]BF4 ((2)BF4) with
pyridine, giving high yields of [Pd2(m-PBut2)(Py)2-
(PPh3)2]BF4 ((5)BF4) with both phosphine and amine
ligands ®ts this plan and is described here. The reactions
of (5)BF4 with CO, isoprene and CS2 are also reported and
compared to the corresponding reactions of the phosphine or
isoprene (10 ml, 0.1 mmol). The 31Pf Hg NMR spectrum of
the
solution
shows
the
signals
of
[Pd2(m-
PBut2)(PPh3)2(m,h2,h2-isoprene)]BF4 as the unique P-con-
2
taining product (ꢁ (ppm): 356.7 [t, J(PP) ˆ 34 Hz, m-P],
21.7 [dd, 2J(PP) ˆ 34 Hz, 3J(PP) ˆ 88 Hz, PPh3], 20.9 [dd,
2J(PP) ˆ 34 Hz, J(PP) ˆ 88 Hz, PPh3)] [28]).
3
‡
‡
phosphite derivatives (1) ±(3) .
2.5. Reaction of [Pd2(ꢀ-PBut2)(PPh3)2(Py)2]BF4 with CO
2. Experimental
Complex (5b)BF4 (275 mg, 0.24 mmol) was dissolved in
THF (15 ml) under a CO atmosphere. The red solution
quickly turned yellow, after 15 min most of the solvent
was vacuum-evaporated, with frequent restoration of the CO
atmosphere to avoid decarbonylation. An oily residue pre-
cipitated out after addition of Et2O (15 ml); the oil was
redissolved in 2 ml of THF and Et2O (15 ml) was added,
causing the precipitation of a yellow solid which was ®ltered
off and vacuum-dried (150 mg, 60% yield). Anal. Calc. for
2.1. General Data
All manipulations were performed under an atmosphere
of puri®ed nitrogen or carbon monoxide by using standard
Schlenck techniques. Solvents were dried by standard pro-
cedures and freshly distilled under nitrogen. Deuterated
solvents were used as purchased and stored on molecular

248
P. Leoni et al. / Inorganica Chimica Acta 284 (1999) 246±251
Table 1
1
31Pf Hg NMR chemical shifts (ꢁ, ppm) and coupling constants (J, Hz)
Cation
L2
L3
L4
L5
ꢁP1
ꢁP2
ꢁP4
ꢁP5
2J12
2J14
2J15
3J24
3J25
2J45
‡
‡
(2)
(4)
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
Py
PPh3
PPh3
PPh3
Py
324.0
317.0
296.2
309.2
318.0
348.4
370.2
357.3
8.02
25.3
27.6
11.7
10.2
10.1
37
46
52
50
13
25
25
70
179
180
44
21
52
22
21
198
215
19
21
Py
15.1
10.1
10.9
11.8b
17.7
14.8
25.8
‡
a
(5a)
Py
‡
(5b)
Py
PPh3
CO
22.5
247
24
‡
‡
‡
(8a)
(9a)
Py
PPh3
PPh3
CO
25.0b
17.7
49
25
156
a
CO
CO
CO
(9b)
PPh3
PPh3
27
27
161
245
9
‡
(10)
CO
28
a Not detected due to magnetic equivalence.
b Assignment of P2 and P5 may be reversed.
C46H48BF4O2P3Pd2: C, 53.8; H, 4.72. Found: C, 53.3; H,
1
at 317.0 ppm [2J(PPtrans) ˆ 180 Hz] assigned to the brid-
ging P nucleus. Three well-resolved groups of resonances
were observed in the region of metal-bonded phosphines: a
doublet [2J(PPtrans) ˆ 180 Hz] of triplets [2J(PPcis)
3J(PP) ˆ 21 Hz] at 27.6 ppm for the phosphine pseudo-
trans to the bridging phosphide, and an AB system at
15.1 [ddd, J(PP) ˆ 21, 46, 215 Hz] and 10.2 ppm [dt,
J(PP) ˆ 215, 21 Hz] for the two pseudo-cis phosphines.
These resonances are slowly replaced by new signals
assigned to the two isomers of [Pd2(m-PBut2)(Py)2-
(PPh3)2]BF4 ((5)BF4) arising from the substitution of a
PPh3 with a pyridine molecule. The symmetrical kinetic
isomer (5a)BF4 [ꢁˆ296.2 (t) and 10.1 (d) ppm], with two
phosphines pseudo-cis [2J(PP) ˆ 52 Hz] to the bridging
phosphide, slowly and quantitatively isomerizes to the
thermodynamically stable isomer (5b)BF4 [ꢁ ˆ 309.2 (dd,
J(PP) ˆ 50, 247 Hz), 22.5 (dd, J(PP) ˆ 24, 247 Hz) and
10.9 ppm (dd, J(PP) ˆ 24, 50 Hz)] (Scheme 3). Complex
(5b)BF4 was isolated as a yellow powder in good yields (see
Section 2).
4.90%. IR (Nujol, KBr): 2091 s, 2064 s (ꢂCO) cm
.
3. Results and discussion
A red solution of complex (2)BF4 dissolved in neat
pyridine turned yellow in a few minutes and was analyzed
1
by 31Pf Hg NMR spectroscopy (Table 1). The spectrum
showed the complete conversion of (2)BF4 into one species
which, on the basis of its NMR data, has been identi®ed as
[Pd2(m-PBut2)(Py)(PPh3)3]BF4 ((4)BF4); this complex
forms by coordination of a pyridine molecule to the vacant
‡
site of cation (2) (Scheme 3). Actually the spectrum shows
that the framework of phosphorus nuclei has remained
intact: nearly all the broad NMR resonances observed at
room temperature sharpen at 358C, except a broad doublet
3.1. Reactivity of (5b)BF4
As a quick and simple test for a comparison of the relative
lability of nitrogen versus phosphorus ligands in these
systems, we studied the reactions of complex (5b)BF4 with
bidentate ligands (LL) such as CS2 or isoprene.
If, as expected, the pyridine molecules were to have been
preferably substituted, then these reactions would have
furnished the same type of compounds formed in the
‡
‡
corresponding reactions of cations (1) ±(3) , that is,
[Pd2(m-PBut2)(PR3)2(m,h2,h2-LL)]BF4, which were pre-
viously characterized for R ˆ Ph, Me, Et, OMe, OEt and
R3 ˆ But2H (Scheme 4) [24±29].
Actually, complex (5b)BF4 reacts quickly and quantita-
tively with both CS2 and isoprene yielding, respectively,
Scheme 3.
[Pd2(m-PBut2)(PPh3)2(m,h2,h2-CS2)]BF4
and
[Pd2(m-

P. Leoni et al. / Inorganica Chimica Acta 284 (1999) 246±251
249
Scheme 4.
PBut2)(PPh3)2(m,h2,h2-isoprene)]BF4, whose spectral para-
meters were found to be identical to those of samples
prepared independently by known procedures [24,29]. It
is also worth noting that the pyridine molecules are quanti-
tatively substituted by using equimolar reactant ratios,
whereas a large ligand excess was necessary for the sub-
stitution of phosphine or phosphite ligands in the corre-
observed for the chemical shifts of PPh3 molecules cis to
‡
m-P in the tetrasubstituted [Pd2(m-PBut2)(L)4] cations (see
Table 1). This could be an indication that (8)BF4 has a
‡
different structure, and (8b) , with a ®ve-coordinated pal-
‡
ladium center, is a possible alternative to (8a) .
The ®nal spectrum contains only the resonances assigned
to the two isomeric dicarbonyls (9a)BF4 and (9b)BF4. The
symmetrical isomer (9a)BF4 gives a triplet at 348.4 ppm and
a doublet at 17.7 ppm (2J(PP) ˆ 25 Hz), consistent with the
presence of two equivalent phosphines pseudo-cis to the
bridging phosphorus, and the other isomer (9b)BF4 gives
three distinct group of resonances at 370.2 ppm [dd,
2J(PP) ˆ 25, 161 Hz], 27 ppm [dd, 2J(PP) ˆ 161,
3J(PP) ˆ 9 Hz] and 14.8 ppm [dd, 2J(PP) ˆ 25,
3J(PP) ˆ 9 Hz] for the bridging phosphide and the
pseudo-trans and pseudo-cis PPh3 molecules, respectively.
The reaction is easily reversed and, by simply restoring a
dinitrogen atmosphere, complex (5b)BF4 is quantitatively
reformed; however, with some experimental care, the
two isomers can be isolated as a yellow powder in satis-
factory yield. The solid shows strong ꢂCO absorptions
‡
‡
sponding reactions of cations (1) ±(3) .
A further useful comparison comes from the reactions
with carbon monoxide. A red solution of (5b)BF4 turns
1
quickly orange under CO. The 31Pf Hg NMR analysis
( 808C) shows complete disappearance of the starting
material and appearance of resonances from three new
complexes. One of them disappears after a few minutes
and can be formulated as the intermediate [Pd2(m-
PBut2)(PPh3)2(CO)(Py)]BF4 [(8)BF4, AMX, ꢁ ˆ 318.0
ppm, dd, 2J(PP) ˆ 49, 13 Hz; ꢁˆ25.0 ppm, dd,
2J(PP) ˆ 49, 3J(PP) ˆ 156 Hz;
ꢁ ˆ 11.8 ppm,
dd,
2J(PP) ˆ 13, 3J(PP) ˆ 156 Hz]; the small values of
3
2J(PP) and the large value of J(PP) make the structure
shown in Scheme 5 for (8a)BF4, with two PPh3 molecules
pseudo-cis to the bridging phosphide and nearly aligned to
the Pd±Pd vector, more preferable than other possible
isomers. One of the reviewers noted that one of the values
of ꢁp (25.0 ppm) falls out of the range (10.1±17.7 ppm)
1
in the IR (nujol, KBr) 2064 and 2091 cm and, when
redissolved under CO in acetone-d6, gives the same 31P
NMR resonances discussed above; the ratio of the two
‡
‡
dicarbonyl isomers [(9a) :(9b) ] is 1:2 at 298 K and 1:1
at 238 K.
By recording the spectrum of solid (9a)BF4 dissolved in
acetone-d6 at 298 K under a dinitrogen atmosphere, we
observed the formation of a small amount of another com-
plex, (10)BF4, containing
a
bridging phosphide
2
[ꢁ ˆ 357.3 ppm, dd, J(PP) ˆ 245, 70 Hz], a pseudo-trans
[ꢁ ˆ 27 ppm, dd, 2J(PP) ˆ 245, 3J(PP) ˆ 28 Hz] and a
pseudo-cis phosphine [ꢁ ˆ25.8 ppm, dd, 2J(PP) ˆ70,
3J(PP) ˆ28 Hz]. The new signals, which disappear by
restoring the CO atmosphere, may be explained by assum-
ing the dissociation of one of the carbonyls. This can be
followed, in principle, by one of at least six possibilities:
(a) the remaining CO molecule bridges the two metals, or
the vacant coordination site is occupied by: (b) a dini-
trogen molecule, (c) a solvent molecule, (d) a C±H or
a C=C bond of a phosphine aryl group, (e) a ¯uorine atom
Scheme 5.

250
P. Leoni et al. / Inorganica Chimica Acta 284 (1999) 246±251
of the counterion, (f) by nothing, the complex remain-
‡
ing unsaturated as cation (2) itself [23]. The unsymme-
trical disposition of the PPh3 molecules makes the ®rst
hypothesis unlikely and points (b) and (c) are ruled out
by the observation that the same resonances appear under an
argon atmosphere and in different solvents (acetone-d6 or
CDCl3). Points (d) and (e) are less readily addressed,
especially if the equilibria are shifted toward the uncoor-
dinated species, due to possible NMR line broadening.
However, we had no signi®cant evidences for aryl or
(2)
1
BF4 coordination even by running 19F, H, or 31P NMR
spectra at 808C and, for the time being, we propose that
(10)BF4 is a monocarbonyl bisphosphine species with a
structure, as illustrated, similar to that of complex 2
(Scheme 2).
In conclusion, we have reported here the ®rst phosphido-
bridged dinuclear complexes containing nitrogen donors as
terminal ligands. Compared to secondary or tertiary alkyl
and aryl phosphines, or alkyl phosphites, the pyridine
molecules exhibit a higher lability, rendering complex
(5b)BF4 a promising reagent which easily furnishes the
26 e fragment Pd2(m-PBut2)(PPh3)2, with two adjacent
unsaturated palladium centers.
Attempted isolation of pure (10)BF4 as a solid failed and
always yielded complex mixtures.
Acknowledgements
The preparation of mono-phosphido-bridged dipalla-
dium(I) dicarbonyls, like complex (9)BF4 is not straightfor-
ward and deserves a brief comment.
Financial support from the Ministero della Ricerca
Scienti®ca e Tecnologica (MURST) and the Consiglio
Nazionale delle Ricerche (CNR, Rome) is gratefully
acknowledged.
We have previously shown that the carbonylation of type
‡
‡
(3) (P ˆ PCy2H, PMe3, P(OMe)3, P(OEt)3) or (2)
(P ˆ PPh3) cations invariably yields a monocarbonyl deri-
vative still containing three terminal P-ligands as shown in
Eq. (1).
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