Oxygen- versus carbon-coordination of the alpha-stabilized phosphorus ylide Ph3P=C(H)R in palladacycles bearing secondary amines

Article abstract of DOI:10.1007/s11243-011-9478-y

The ortho-metalated complex [Pd(x){κ ² (C,N)-[C ₆H₄CH₂NRR′ (Y)}] (2a-4a and 2b-3b) was prepared by refluxing in benzene equimolecular amounts of Pd(OAc)₂ and secondary benzylamine [a, EtNHCH₂

Full text of DOI:10.1007/s11243-011-9478-y

Transition Met Chem (2011) 36:363–367
DOI 10.1007/s11243-011-9478-y
Oxygen- versus carbon-coordination of the alpha-stabilized
phosphorus ylide Ph3P=C(H)R in palladacycles bearing
secondary amines
•
Kazem Karami Mina Mohamadi Salah
Received: 10 February 2011 / Accepted: 2 March 2011 / Published online: 18 March 2011
Ó Springer Science+Business Media B.V. 2011
Abstract The ortho-metalated complex [Pd(x){j2(C,N)-
[C6H4CH2NRR0 (Y)}] (2a–4a and 2b–3b) was prepared by
refluxing in benzene equimolecular amounts of Pd(OAc)2
and secondary benzylamine [a, EtNHCH2Ph; b, t-BuN-
HCH2Ph followed by addition of excess NaCl. The reac-
tion of the complexes [Pd(x){j2(C,N)-[C6H4CH2NRR0
(Y)}] (2a–4a and 2b–3b) with a stoichiometric amount of
Ph3P=C(H)COC6H4-4-Z (Z = Br, Ph) (ZBPPY) (1:1 molar
ratio), in THF at low temperature, gives the cationic
derivatives [Pd(OC(Z-4-C6H4C=CHPPh3){j2(C,N)-[C6H4-
CH2NRR0(Y)}] (5a–9a, 4b–6b, and 4b0–6b0), in which the
ylide ligand is O-coordinated to the Pd(II) center and trans
to the ortho-metalated C(6)H(4) group, in an ‘‘end-on
carbonyl’’. Ortho-metallation, ylide O-coordination, and
C-coordination in complexes (5a–9a, 4b–6b, and 4b0–6b0)
were characterized by elemental analysis as well as various
spectroscopic techniques.
the isomeric form d (Scheme 1). Form b would account for
the C-coordination, while isomers c and d would explain
O-coordination. This ambidentate character facilitates the
preparation of stable metal complexes (Scheme 2) in which
the ylide can be O- (both cisoid and transoid forms) [3] or
C-coordinated [4–9].
The activation of C–H bonds in organic compounds
promoted by transition metals is an important research
topic nowadays [10]. Synthesis of cyclo-palladated com-
plexes has attracted considerable attention from a number
of research groups, due to its implication, in the funda-
mental steps of several catalytic cycles, in the functional-
ization of simple substrates through ortho-metallation, and
in other relevant chemical processes [11–16].
Experimental
All chemicals were purchased from Fluka and Merck
companies. 1H NMR (500 MHz) and 31P NMR (202 MHz)
spectra were recorded in CDCl3 solutions at room tem-
perature (TMS was used as an internal standard) on a
Bruker Avance spectrometer.
Introduction
Phosphorus ylides constitute an important class ofcompound
in the field of organometallic chemistry, particularly due to
their interesting applications in metal-promoted organic
syntheses [1]. Carbonyl stabilized phosphorus ylides are
interesting ligands because they can behave as C or O donors
owing to the delocalization of the ylidic electron pair [2].
This class of ligand shows an ambidentate character (C-
versus O-coordination) that can be rationalized in terms of
the potential resonance forms a–c (Scheme 1), together with
Preparation of PhBPPY (1)
To a solution of 2-bromo-4-phenylacetophenon (0.825 g,
3 mmol) in CHCl3 (20 mL), a solution of PPh3 (0.786 g,
3 mmol) in CHCl3 (5 mL) was added dropwise. The
mixture was stirred at room temperature for 4 h, and then
it was evaporated to dryness. The residue was reacted
with NaOH (2 g, 0.5 mmol) in EtOH/H2O (20 mL), giving
Ph3PCHC(O)C6H5Ph as a yellow solid. Yield: 1.87 g,
81.9%. M.p. 230–231 °C. IR (KBr disk, cm-1): t 1507, 1H
NMR (500 MHz, CDCl3, ppm): d = 4.5 (d, 1H, CHP,
K. Karami (&) ꢀ M. M. Salah
Department of Chemistry, Isfahan University of Technology,
Isfahan, Iran
e-mail: karami@cc.iut.ac.ir
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Transition Met Chem (2011) 36:363–367
General procedure for the synthesis of cyclo-palladated
complexes {Pd(C6H4CH2NRR0)Y(ZBPPY)}
(R = H, R0 = Et, t-Bu; Y = 4-Picoline,
Me3Py, PPh3; Z = Ph, Br)
To a solution of Pd(OAc)2 (0.116 g, 0.52 mmol) in ben-
zene (15 mL), secondary benzylamine [a, EtNHCH2Ph;
b, t-BuNHCH2Ph; 1 mmol] was added. The mixture was
refluxed for 24 h at 50 °C. The mixture was then evapo-
rated to dryness, MeOH was added to the solid, and the
green suspension was treated with NaCl or NaBr
(1.118 mmol) at room temperature.
A yellow suspension was immediately produced, and
stirring was maintained for 12 h at room temperature, after
which the precipitate was filtered off, then addition of
4-picoline, Me3Py, or PPh3 (1.0 mmol) in CH2Cl2 (15 cm3)
gave a clear solution immediately. After stirring for 8 h at
room temperature, the solution was filtered through a plug
of Celite or MgSO4. Crude complexes resulted that pre-
cipitated as solids.
Scheme 1 Resonance forms of keto-phosphorus ylides (free)
These solids were dissolved in CH2Cl2 at room tem-
perature, and n-hexane were added to give a powder, which
was filtered off and then air-dried to give complexes 2a, 3a,
4a, 2b, and 3b in Scheme 1. Then, to a suspension of (2a,
3a, 4a, 2b, and 3b) (0.1194 g, 0.1 mmol) in THF (12 mL)
was added AgTfO (0.032 g, 0.126 mmol).
The resulting mixture was stirred for 30 min at room
temperature with exclusion of light and then filtered over
magnesium sulfate. Phosphorus ylide (0.237 g, 0.52 mmol)
(1 or 2) was added, and the resulting solution was stirred
for 45 min after the reaction time. The solvent was evap-
orated to dryness, and the residue was treated with n-hex-
ane (10 mL) to give (5a, 6a, 7a, 8a, 9a, 4b, 4b0, 5b, 5b0,
6b, and 6b0) in Scheme 1.
Scheme 2 Resonance forms of keto-phosphorus ylides (complex)
2JHH = 24 Hz), 7.4 (t, 1H, Hp, C6H5, 3JHH = 7 Hz), 7.5 (t,
2H, Hm, C6H5, 3JHH = 10.5 Hz), 7.5 (t, 2H, Hm, C6H4), 7.5
3
(m, 6H, Hm, PPh3), 7.6 (m, 2H, Ho, C6H5, JHH = 6 Hz),
7.7 (t, 3H, Hp, PPh3, 3JHH = 6 Hz), 7.8 (m, 6H, Ho, PPh3),
8.1 (d, 2H, Ho, C6H4). 31P {1H} NMR (CDCl3, ppm):
d = 17.2 (s, 1P, CHP) [2, 9, 17].
5a) Yield: 0.10 g (80%), m.p: 173–175 °C. FT-IR
(cm-1, KBr); m (N–H) 3200, m (C=O) 1481. 1H NMR
2
(500 MHz, CDCl3, ppm); d = 0.9 (t, 3H, CH3, JHH
=
5 Hz), 1.3 (m, 1H, CH2), 2.4 (m, 1H, CH2), 3.0 (s, 3H,
CH3), 3.1 (s, 1H, NH), 3.8 (dbr, 1H, CH2), 4.5 (m, 1H,
CH2), 4.5 (s, 1H, CHP), 6.7 (s, 1H, Ph), 7–8. (m, ?3H, Ph,
?9H, Ph, ?4H, Py, ?6Hm, PPh3, ?3Hp, PPh3, ?6Ho,
PPh3). 31P {1H} NMR (CDCl3): d = 16.4 (s, 1P, PPh3).
Elemental analysis Calc.: C, 61.4; H, 4.6; N, 3.0. Found: C,
61.4; H, 4.7; N, 3.1%. K: 112 X-1 mol-1 cm2.
Preparation of BrBPPY (2)
To a solution of 2,4-dibromoacetophenone (0.834 g, 3 mmol)
in CHCl3 (20 mL), a solution of PPh3 (0.786 g, 3 mmol) in
CHCl3 (5 mL) was added dropwise. The mixture was stirred
at room temperature for 4 h, and then it was evaporated to
dryness. The residue was reacted with NaOH (2 g, 0.5 mmol)
in MeOH/H2O (20 mL), giving PPh3CHC(O)C6H5Br as a
white solid. Yield: 0.56 g, 67.9%. m.p. 183 °C, IR (KBr,
6a) Yield: 0.08 g (75%), m.p: 170–174 °C. FT-IR
(cm-1, KBr); m (N–H) 3216, m (C=O) 1493. 1H NMR
2
(500 MHz, CDCl3, ppm); d = 0.9 (t, 3H, CH3, JHH
5 Hz), 1.32 (m, 1H, CH2), 2.4 (m, 1H, CH2), 2.4 (s, 3H,
=
1
cm-1): m 1519, H NMR (500 MHz, ppm, CDCl3): d = 4.4
2
CH3), 2.9 (s, 1H, NH), 3.8 (d, 1H, CH2, JHH = 15 Hz),
2
4.6 (d, 1H, CH2, JHH = 15 Hz), 4.4 (dbr, 1H, CHP), 6.0
2
(d, CHP, JPH = 23.9), 7.0 (m, 2H, C6H4CO), 7.5 (m, 6H,
Hm, 3C6H5), 7.6 (m, 3H, Hp, 3C6H5), 7.7 (m, 6H, Ho, 3C6H5),
8 (m, 2H, C6H4CO). 31P {1H} NMR (CDCl3, ppm):
d = 16.00 ppm [2, 9, 17].
(sbr, 1H, Ph), 6.7–7.1 (m, 3H, Ph), 7.5–7.8 (m, 4H, Ph,
?2Hm, Py, ?2Ho, Py, ?6Hm, PPh3, ?3Hp, PPh3, ?6Ho,
PPh3). 31P {1H} NMR (CDCl3): d = 16.6 (s, 1P, PPh3).
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Transition Met Chem (2011) 36:363–367
365
Elemental analysis Calc.: C, 53.6; H, 4.1; N, 3.0. Found: C,
53.6; H, 4.1; N, 3%. K: 122 X-1 mol-1 cm2.
4b, 4b0) Yield: 0.052 g (61%), m.p: 188–190 °C. FT-
1
IR (cm-1, KBr); m(N–H) 3217, m (C=O) 1478, 1619. H
7a) Yield: 0.11 g (81%), m.p: 189–192 °C. FT-IR
(cm-1, KBr); m (N–H) 3221, m (C=O) 1481. 1H NMR
(500 MHz, CDCl3, ppm); d = 1.63 (t, 3H, CH3), 2.4 (s,
3H, CH3), 3.0 (s, 3H, CH3), 3.2 (s, 3H, CH3), 3.8 (d, 1H,
CH2, 2JHH = 20 Hz), 4.5 (d, 1H, CH2, 2JHH = 20 Hz), 4.8
NMR (300 MHz, CDCl3, ppm); d = 0.9–1.3 (m, 9H,
CH3), 2.4 (s, 3H, CH3), 3.9 (sbr, 1H, CHP), 4.0 (d, 1H,
2
2
CH2, JHH = 12 Hz), 4.5 (d, 1H, CH2, JHH = 12 Hz),
3.8 (sbr, 1H, NH), 5.4 (sbr, 1H, CHP), 6.0–7.0 (m, 4H,
Ph),7.2–9.2 (m, 9H, Ph, ?6Hm, PPh3, ?3Hp, PPh3, ?6Ho,
PPh3, ?2Hm, Py, ?2Ho, Py, both isomer). 31P {1H} NMR
(CDCl3): d = 14.2 (s, 1P, PPh3), 17.2 (s, 1P, PPh3), 22.5
(s.1P, PPh3). Elemental analysis Calc.: C, 62.1; H, 4.90;
N, 2.9. Found: C, 62.20; H, 5.0; N, 3.0%. K:
120 X-1 mol-1 cm2.
5b, 5b0) Yield: 0.08 g (92%), m.p: 183–185 °C. FT-IR
(cm-1, KBr); m(N–H) 3219, m (C=O) 1491, 1620. 1H NMR
(300 MHz, CDCl3, ppm); d = 0.9–1.3 (m, 9H, CH3), 2.4
(s, 3H, CH3), 3.9 (dbr, 1H, CHP), 4.4–4.5 (d, 2H, CH2), 3.8
(sbr, 1H, NH), 5.4 (sbr, 1H, CHP), 6.1 (sbr, 1H, Ph),
6.5–6.9 (m, 3H, Ph),7.2–9.2 (m, 9H, Ph, ?6Hm, PPh3,
?3Hp, PPh3, ?6Ho, PPh3, ?2Hm, Py, ?2Ho, Py, both
isomer). 31P {1H} NMR (CDCl3): d = 14.0 (s, 1P, PPh3),
17.1 (s. 1P, PPh3), 24.2(s, 1P, PPh3). Elemental analysis
Calc.: C, 54.5; H, 4.4; N, 2.9. Found: C, 54.6; H, 4.5; N,
3.0%. K: 125 X-1 mol-1 cm2.
2
(s, 1H, NH), 4.1 (d, 1H, CHP, JPH = 8 Hz), 5.7 (s, 1H,
Ph), 6.7–7 (m, 3H, Ph), 7.1–8.0 (m, ?9H, Ph, ?2H, Pym,
?6Hm, PPh3, ?3Hp, PPh3, ?6Ho, PPh3). 31P {1H} NMR
(CDCl3): d = 17.2 (s, 1P, PPh3). Elemental analysis Calc.:
C, 62.1; H, 4.90; N, 2.9. Found: C, 62.1; H, 5.0; N, 3.0%.
K: 139 X-1 mol-1 cm2.
8a) Yield: 0.11 g (81%), m.p: 178–180 °C. FT-IR
(cm-1, KBr); m (N–H) 3226, m (C=O) 1492. 1H NMR
(500 MHz, CDCl3, ppm); d = 0.9 (t, 3H, CH3,
2JHH = 7 Hz), 1.3–1.5 (m, 2H, CH2), 2.4 (s, 3H, CH3), 3.0
(s, 3H, CH3), 3.2 (s, 3H, CH3), 3.8 (d, 1H, CH2,
2
2JHH = 10 Hz), 4.5 (d, 1H, CH2, JHH = 10 Hz), 4.8 (s,
1H, NH), 4.4 (d, 1H, CHP, 2JPH = 21 Hz), 5.7 (s, 1H, Ph),
6.7–7.0 (m, 3H, Ph), 7.1–7.8 (m, ?4H, Ph, ?2H, Py,
?6Hm, PPh3, ?3Hp, PPh3, ?6Ho, PPh3). 31P {1H} NMR
(CDCl3): d = 17.2 (s, 1P, PPh3). Elemental analysis Calc.:
C, 54.5; H, 4.4; N, 2.9. Found: C, 54.6; H, 4.6; N, 3.1%. K:
115 X-1 mol-1 cm2.
6b, 6b0) Yield: 0.08 g (80%), m.p: 173–177 °C. FT-IR
(cm-1, KBr); m (N–H) 3233, m (C=O) 1480, 1601. 1H
NMR (300 MHz, CDCl3, ppm); d = 1.2–1.3 (m, 9H,
CH3), 1.9 (sbr, 1H, NH), 3.9 (d, 1H, CHP,
9a) Yield: 0.11 g (72%), m.p: 174–177 °C. FT-IR
(cm-1, KBr); m (N–H) 3218, m (C=O) 1494. 1H NMR
(500 MHz, CDCl3, ppm); d = 0.9 (t, 3H, CH3), 3.0 (m,
1H, CH2), 3.2 (m, 1H, CH2), 3.9 (dbr, 1H, CH2), 4.8 (dbr,
2
2JPH = 15 Hz), 4.5 (d, 1H, CH2, JHH = 2.4 Hz), 4.7 (d,
2
1H, CH2, JHH = 1.4 Hz), 5.6 (sbr, 1H, CHP), 6.2 (m,
2
1H, CH2), 4.4 (d, 1H, CHP, JPH = 10 Hz), 2.2 (sbr, 1H,
1H, Ph), 6.3 (m, 1H, Ph), 6.80 (m, 1H, Ph), 6.90 (m, 1H,
Ph), 7.2–8.2 (m, 9H, Ph, ?12Hm, PPh3, ?6Hp, PPh3,
?12Ho, PPh3, both isomer). 31P {1H} NMR (CDCl3):
d = 18.4 (s, 1P, PPh3), 23.8 (s, 1P, PPh3) 40.3 (s, 1P, Pd-
PPh3), 41.2 (s, 1P, Pd-PPh3). Elemental analysis Calc.: C,
65.5; H, 4.9; N, 1.2. Found: C, 65.7 H, 3.00; N, 1.40%.
K: 131 X-1 mol-1 cm2 (Scheme 3).
NH), 6.3 (m, 1H, Ph), 6.4 (tbr, 1H, Ph), 6.9 (tbr, 1H, Ph),
7.0 (dbr, 1H, Ph), 7.4–7.8 (m, 4H, Ph, ?12Hm, PPh3,
?6Hp, PPh3, ?12Ho, PPh3). 31P {1H} NMR (202.44 MHz,
CDCl3, ppm): d = 17.2 (s, 1P, PPh3), 40.8 (s, 1P, Pd-
PPh3). Elemental analysis Calc.: C, 58.4; H, 4.2; N, 1.3.
Found: C, 58.5; H, 4.3; N, 1.4%. K: 109 X-1 mol-1 cm2.
Scheme 3 i) Benzene, Reflux 24 h, 60 °C. ii) MeOH, NaX (X = Cl, Br), 12 h. iii) CH2Cl2, Y, 8 h. iv) AgTfO, THF. V) ZBPPY (Z = Ph, Br)
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Result and discussion
Spectroscopic study
The m(CO) bands, which are sensitive to complexation,
occur at 1,507 and 1,519 cm-1 in the parent ylides, as in
the case of other resonance-stabilized ylides [17, 18, 20].
In general, coordination of the ylide through C causes an
increase in m(CO), while for O-coordination, a lowering of
m(CO) is expected. Spectroscopic techniques such as IR
and NMR spectroscopy enable one to distinguish between
O(carbonyl)- and C(methine)-coordination.
Scheme 4 Geometric isomers for O-coordinated complex
The IR absorption bands observed for complexes 5a–9a
show a lowering of m(CO) at 1,481, 1,493, 1,481, 1,492,
and 1,494 cm-1, indicating coordination of the ylide oxy-
gen for these complexes. The IR spectra of the 4b–6b
complexes show an intense absorption at 1,478, 1,491, and
1,480 cm-1 respectively, corresponding to the stretching
m(CO). This absorption has been shifted to lower energies
with respect to the free ylides.
1
The H NMR spectra of the complexes 5a–6b show a
doublet or broad signal attributed to the proton methine CH
at 3.8–4.4 ppm in O-bound complexes [3, 17, 20–22] and
the C-bound complexes 4b0–6b0 observed in 5.4–5.6 ppm.
1
The H NMR spectra of the complexes showed only one
2
signal for the CH methine with a coupling constant J(P–H)
near to that of the free ylides and close to those observed in
other O-bonded complexes, this coupling is usually smaller
in C-linked ylides [4–9, 20, 23, 24].
Moreover, the IR spectra of complexes 4b0–6b0 show
a sharp, very strong absorption at 1,619, 1,620, and
1,601 cm-1 respectively, corresponding to the carbonyl
absorption. This absorption is shifted to higher energies
Moreover, the 31P{1H} NMR spectra of the 5a–6a
complexes show a broad signal attributed to the PCH group
at 16.3–16.6 ppm, the presence of broad signal can be
explained by the presence of two isomers, namely transoid
and cisoid in these complexes (Scheme 4), and the 7a–9a
complexes show a sharp singlet at 17.2–17.8 ppm.
The 31P{1H} NMR spectrum of the 4b complex shows two
sharp signals attributed to the PCH group at 14.2, 17.2 ppm,
and the 5b complex shows these at 14.0, 17.1 ppm. The
presence of two lines of different intensities can be explained
by the presence of two isomers in these complexes. The
31P{1H} NMR spectrum of the 6b complex shows a sharp
signal attributed to the PCH group at 18.4 ppm, shifted
slightly upfield relative to the free ylides and is again in
agreement with O-coordination [3, 17, 20–22].
with respect to that in the free ylides (1507 and 1519 cm-1
)
[10]. The observation of this positive shift for m(CO) in the
complexes indicates that the ylides are C-coordinated to
Pd(II).
The m(P?–C-) band which is also diagnostic of the
coordination occurs at 845, 829 cm-1 in (C6H5)3P?–
CH2COC6H4-Ph and (C6H5)3P?–CH2COC6H4-Br (phos-
phonium salt), respectively, and at 861, 846 cm-1 in
(C6H5)3PCHCOC6H4-Ph and (C6H5)3PCHCOC6H4-Br,
respectively. In this study, the m(P?–C-) values for 5a, 6a,
7a, 8a, 9a, 4b, 4b0, 5b, 5b0, 6b, and 6b0 complexes (811,
810, 805, 800, 803, 807, 808, and 802 cm-1) were shifted
to lower frequency, suggesting some removal of electron
density from the P–C bond.
31
The P{1H} NMR spectra of the 4b0–6b0 complexes
1
show a sharp signal attributed to the PCH group at 22.5,
24.1, and 23.8 ppm, respectively, which are shifted to
downfield with respect to the parent ylides, in good
agreement with the C-bonding of the ylides. The 31P-{1H}
NMR spectrum of the 9a complex shows a sharp signal
attributed to the PPh3 group coordinated to the Pd metal at
40.8 ppm, and the 6b, 6b0 complexes show two sharp
signals attributed to the PPh3 group coordinated to the Pd
metal at 40.3 and 41.2 ppm, the presence of two lines of
different intensities can be explained by the steric effects
of tertiarybutylbenzylamine and PPh3, causing the presence
of two isomers in these complexes.
Analysis of the aromatic region of the H NMR spectra
of the complexes 5a–6b0 confirmed the metallation. In
1
the H NMR spectra of the complexes 5a–6b0 containing
unsubstituted phenyl rings, a set of four different signals
(sometimes described as a set of three different signals or a
multiplet) in the aromatic region corresponding to the four
protons in the ortho-palladated ring is found. Moreover, for
PPh3 and pyridine derivatives, the aromatic proton H6
appears at a considerably lower frequency relative to the
free amines due to the anisotropic shielding by the adjacent
pyridine, that is, coordinated nearly perpendicular to the
square-planar palladium(II) plane, or the P-phenyl rings.
This effect supports the mutually trans position of the
pyridine or phosphine ligand and the N(RR0) group in
solution [19].
These complexes are stable in the solid state or in acetone
solution. The molar conductivity of complexes corresponds
to univalent electrolyte (100–135 X-1 mol-1 cm2 [25]).
123

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367
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