Synthesis and ligand exchange reactions of P2Pd(II) and P 2Pt(II) salicylaldimates

Article abstract of DOI:10.1039/b501827g

Reaction of (dppe)MCl₂ (dppe = 1,2-bis(diphenylphosphino)ethane) with 2-(N-phenyliminomethyl)phenol leads to air-stable (dppe)M(N,O) chelates (M = Pd, 1a M = Pt, 1b). The N-4-methylphenyl derivative of la has been characterized by X-ray analysis. The N,O ligands are kinetically labile and exchange occurs in solution in the presence of other salicylaldimines. In the presence of anilines, a metal-mediated imine exchange process occurs. Hammett analysis reveals that the platinum complexes are sensitive to the electronics at N but not at O. Electron donating groups on the N-aryl ring stabilize the metal complex. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b501827g

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F U L L P A P E R
Synthesis and ligand exchange reactions of P2Pd(II) and P2Pt(II)
salicylaldimates
William D. Kerber, D. Luke Nelsen, Peter S. White and Michel R. Gagne´*
Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North
Carolina, 27599-3290, USA. E-mail: mgagne@unc.edu; Fax: (919)962-6341;
Tel: (919)962-6342
Received 9th February 2005, Accepted 15th March 2005
First published as an Advance Article on the web 29th April 2005
Reaction of (dppe)MCl2 (dppe = 1,2-bis(diphenylphosphino)ethane) with 2-(N-phenyliminomethyl)phenol leads to
air-stable (dppe)M(N,O) chelates (M = Pd, 1a; M = Pt, 1b). The N-4-methylphenyl derivative of 1a has been
characterized by X-ray analysis. The N,O ligands are kinetically labile and exchange occurs in solution in the
presence of other salicylaldimines. In the presence of anilines, a metal-mediated imine exchange process occurs.
Hammett analysis reveals that the platinum complexes are sensitive to the electronics at N but not at O. Electron
donating groups on the N-aryl ring stabilize the metal complex.
Introduction
As part of our continuing interest in developing coordination
compounds that provide properties favorable to their molecular
imprinting, we recently began investigating a class of previously
unreported P2M(N,O-salicylaldimine)-cations (M = Pd, Pt).
Prior to our work a number of examples of this ligand on Pd/Pt
were known, but none had the properties we desired. Some
known monomeric complexes for M = Pd and Pt include (but
are not limited to): bis(salicylaldimine)M(II);1 (salen)M(II);2 (N-
(2-diphenylphosphinophenyl)salicylaldimate)M(II)(L), (L
=
Cl, CH3, CH3CN);3 (N-(2-hydroxyaryl)salicylaldimate)M(II)-
(amine);4 (salicylaldimate)M(II)(diamine);5 (salicylaldimate)-
Pd(II)(p-allyl);6 and (salicylaldimate)Pd(PR3)(CH3).7 Addition-
ally, a number of Ni(Ph)(PPh3)(salicylaldimine) complexes have
been reported for olefin polymerization catalysis;8 isoelectronic
square planar (salicylaldimate)RhI(bis-phosphine) complexes
are also known.9
Fig. 1 ORTEP representation of 2. Hydrogen atoms and counte-
◦
˚
rion omitted for clarity. Selected bond distances (A) and angles ( ):
Pd1–O42 = 2.027(3), Pd1–N27 = 2.104(4), Pd1–P1 = 2.2555(14),
Pd1–P2 = 2.2558(14); P1–Pd1–P2 = 84.29(5), P1–Pd1–O42 = 85.79(11),
P2–Pd1–N27 = 100.57(12), N27–Pd1–O42 = 89.28(16).
Results and discussion
N-Phenylsalicylaldimine chelates of the metal fragment
(dppe)M (M = Pt, Pd) were conveniently prepared by stirring
(dppe)MCl2 with salicylaldehyde and aniline in a 1 : 1 mixture
of CH2Cl2 and MeOH with NaBF4 and Cs2CO3 (eqn. (1)).
is 85.79(11)◦. The complex shows almost no deviation from
planarity at Pd (R bond angles = 359.9◦).
While exploring the reactivity of these new complexes, we
discovered that two distinct modes of ligand exchange were
operative; substitution of the entire salicylaldimine fragment
(eqn. (2))
(1)
Aqueous workup afforded the yellow crystalline solids 1a and 1b
in good yields. Both compounds could be stored on the benchtop
for months without decomposition.
X-Ray quality crystals of an analog of 1a, (dppe)Pd(N-
(4-methylphenyl)salicyaldimate) (2), were obtained by vapor
diffusion of n-pentane into a saturated CH2Cl2 solution. An
ORTEP representation is shown in Fig. 1 along with selected
(2)
and substitution of the aniline fragment (eqn. (3)).
˚
bond lengths and angles. The Pd–O bond (2.027(3) A) is shorter
˚
˚
than the Pd–N bond (2.104(4) A) by 0.077 A, likely the result
of a slightly stronger ionic component to the Pd–O bonding.10
˚
The Pd–P bonds are nearly equivalent; Pd–P1 is 2.2555(14) A
and Pd–P2 is 2.2558(14) A. The larger steric demand of the
˚
N-aryl imine versus the phenoxide is apparent from the cisoid
angles P–Pd–(N/O); P2–Pd–N is 100.57(12)◦ while P1–Pd–O
(3)
1 9 4 8
D a l t o n T r a n s . , 2 0 0 5 , 1 9 4 8 – 1 9 5 1
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
©

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The former is almost certainly associative in nature11 and
proceeds through a mixed salicylaldimine complex. The latter
reactivity can be readily rationalized as a Lewis acid-promoted
imine exchange reaction (Fig. 2).12–16
Fig. 3 Hammett plots of Krel vs. rp. (᭹) Entries 1–3, Table 1. (᭜) Entries
5–7, Table 1. Linear regressions forced through (0,0).
Fig. 2 Possible mechanism for metal-mediated imine exchange.
Complexes 1a and 1b undergo both ligand exchange processes
in a wide variety of solvents, both protic (MeOH), and aprotic
(CH2Cl2, PhCl, MeNO2). The palladium complexes reach equi-
librium more rapidly than their platinum analogs, however they
slowly abstract chloride from chlorinated solvents at elevated
temperature to form (dppe)PdCl2. On the other hand, 1b was
stable in refluxing chlorobenzene for several days.
The multiple ligand exchange reactions displayed by these
complexes allowed the electronic influence of the salicylaldimine
to be easily probed. Complex 1b was equilibrated with either
substituted anilines or salicylaldimines to assess the impact
of electron density changes at both the N and O positions
(Table 1). Hammett plots17 using the rp substituent parameters18
indicated a strong correlation between the relative free energy
of 1b and the electronic influence of X on the one hand, and
little correlation with the electronic influence of Y on the other
(Fig. 3). A large negative slope (q = −3.07) was observed with
X, clearly indicating that the complex is stabilized by increased
electron density at N while a relative insensitivity to electronic
perturbations was observed at O.
strong electrong withdrawing groups (NO2), but not to good
donors like –OMe.8b Computationally, these substituents had
only a small effect on the relative energetics of catalyst activation,
propagation, and chain transfer.19 Much more significant were
steric effects on the rates of catalysis, which confirmed the
experimental observations that catalyst activation, productivity,
and especially deactiviation, were sensitive to the size of both
halves of the ligand.20
In contrast to our late metal case, which has minimal p-
donor M–L character, oxophilic early metals have properties
that show significantly larger electronic sensitivities to positions
para to the phenolate oxygen, for example van Koten’s NO-
chelated V(IV) complexes.21 This situation can reasonably be
ascribed to strengthened p-donor interactions (and hence p-
communication) with the electron deficient metals. Similarly,
high valent intermediates like those achieved in Jacobsen’s
Mn(salen)-catalyzed olefin epoxidations are sensitive to elec-
tronic perturbations para to the salicylaldimine oxygen, which
provided a mechanism for tuning of enantioselectivity.22 Given
the magnitude of the effects with strong binding aryloxides
and the attenuated affects in the present case, we reason that
removing p-donation from the bonding also removes an efficient
mechanism for electronic communication.
In summary, we have described the synthesis and structure
of a new class of monocationic (dppe)M(II)(salicylaldimate)
complexes for M = Pd, Pt. The electronic influence of the (N,O)
ligand was probed through a series of exchange equilibria, and it
was found that 1a is stabilized by electron rich imine donors but it
is insensitive to the electronics of phenoxide donors. The relative
insensitivity of the oxygen position was rationalized by noting
that in contrast to early and/or high valent metal complexes,
minimal p-donor interactions are present in square-planar d8-
complexes, and that this appears to attenuate the electronic
communication between the ligand and the metal.
The electronic effect observed at N is intuitively reasonable,
and presumably reflects bonding between the neutral N-ligand
and a cationic metal complex that is reliant on the N-basicity
for stability. More unexpected, however, was the lack of a strong
trend in the position para to the phenolate oxygen. In the neutral
Ni(II) salicylaldimine catalysts mentioned in the introduction,
the olefin polymerization activity was moderately sensitive to
Table 1 Salicylaldimine exchange equilibria
Experimental
Entrya
X
Y
Keq
DG/kcal mol−1
b
General
1
2
3
4
5
6
7
OMe
Me
Cl
H
H
H
H
H
H
H
H
OMe
Me
Cl
7.20
2.82
0.188
1c
0.710
1.18
0.452
−1.31
−0.685
1.11
All reagents were used as received without further purification.
In all cases, solvents were used without drying or distillation.
(dppe)PtCl2, (dppe)PdCl2, and 5-substituted salicylaldimines
were prepared from procedures modified from the literature.23
Compounds 1a and 1b were synthesized under dinitrogen and
ligand exchange reactions were performed under air. NMR
spectra were recorded on a Bruker Avance 400 or 300 MHz
spectrometer; chemical shifts are given in ppm and are referenced
to residual solvent resonances (1H, 13C) or an 85% H3PO4
external standard (31P). Elemental analysis was performed by
Complete Analysis Laboratories, Inc.
0
0.226
−0.109
0.525
a Equimolar amounts of 1b and the appropriate salicylaldimine or
aniline were heated at 60 ◦C in MeNO2 until equilibrium was reached
as judged by 31P NMR. [1b] = 0.0250 M. b Relative concentrations
determined by 31P NMR. Average of three measurements. c By definition.
D a l t o n T r a n s . , 2 0 0 5 , 1 9 4 8 – 1 9 5 1
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Syntheses
Crystallographic data. Empirical formula = PdP2C40H36-
NOBF4; fw (g mol−1) = 801.87; space group = P21/n;
(dppe)Pd(2-(N-phenyliminomethyl)phenolate)(BF4) (1a). To
a flask containing 20 mg aniline (0.21 mmol) and 26 mg salicy-
laldehyde (0.21 mmol) in 24 mL 1 : 1 CH2Cl2/MeOH was added
123 mg (dppe)PdCl2 (0.214 mmol), 56 mg NaBF4 (0.51 mmol),
and 76 mg Cs2CO3 (0.233 mmol). The reaction was heated
to reflux for four hours, then cooled to ambient temperature
and poured into 25 mL water. The biphase was extracted with
CH2Cl2 (3×) and the combined extracts were washed with
water (2×), dried over MgSO4, and evaporated in vacuo. The
crude yellow solid was recrystallized from CH2Cl2/tBuOMe
to yield 140 mg (84%) of 1a containing 0.04 equiv. CH2Cl2.
1H NMR (400 MHz, CDCl3) d 7.87 (m, 5H), 7.61 (m, 2H),
7.54 (m, 6H), 7.41 (m, 8H), 7.32 (t, 1H, J = 7.1 Hz), 7.19 (d,
1H J = 8.0 Hz), 6.84 (t, 1H, J = 7.1 Hz), 6.66 (t, 3H, J =
˚
a = 11.8713(10); b = 15.5493(13); c = 20.1471(17) A; b =
◦
−3
103.517(1) ; V = 3615.9(5) A ; Z = 4; T = −100 ◦C; Dc =
3
˚
−1
˚
1.473 g cm ; k = 0.71073 A; l = 0.66 mm ; Rf = 0.057; Rw =
0.052.
CCDC reference number 269568.
See http://www.rsc.org/suppdata/dt/b5/b501827g/ for cry-
stallographic data in CIF or other electronic format.
Acknowledgements
We gratefully acknowledge the NIGMS (GM60578) for funding.
W.D.K. thanks Glaxo Smith Kline for a graduate fellow-
ship. M.R.G. is a Camille-Dreyfus Teacher-Scholar.
7.6 Hz), 6.55 (m, 3H), 2.71 (m, 2H), 2.52 (m, 2H); 31P{ H} NMR
1
References and notes
(162 MHz, CDCl3) d 63.6 (d, JP–P = 28.2 Hz), 59.6 (d, JP–P
=
28.2 Hz); 13C{ H}{ P} (75 MHz, CDCl3)24 d 167.1, 164.4, 153.7,
136.8, 136.0, 133.2, 133.0, 132.7, 132.6, 129.6, 129.5, 128.7,
126.5, 124.9, 122.5, 121.5, 119.1, 33.4, 23.8. Anal. Calcd. for
C39H34BF4NOP2Pd·(0.04 CH2Cl2): C, 59.26; H, 4.34; N, 1.77.
Found: C, 59.50; H, 4.37; N, 1.76%.
1
31
1 (a) B. K. Sadashiva and A. Ghode, Liq. Cryst., 1994, 16, 33; (b) A.
Syamal and B. K. Gupta, Indian J. Chem., Sect. A., 1984, 23, 260;
(c) J. Barbera, R. Gimenez, N. Gimeno, M. Marcos, M. Pina and
J. L. Serrano, Liq. Cryst., 2003, 30, 651; (d) J. M. Kerr, C. J. Suckling
and B. Peter, J. Chem. Soc., Perkin Trans.1, 1990, 887.
2 (a) G. A. Shagisultanova, Teor. Eksp Khim., 1991, 27, 330; (b) X.
Zhou, J. Huang, X. Yu, Z. Zhou and C. Che, J. Chem. Soc., Dalton
Trans., 2000, 1075; (c) K. J. Miller, J. H. Baag and M. M. Abu-Omar,
Inorg. Chem., 1999, 38, 4510.
3 (a) P. Bhattacharyya, J. Parr and A. M. Slawin, J. Chem. Soc.,
Dalton Trans., 1998, 3609; (b) P. Shi, Y. Liu, S. Peng and S. Liu,
Organometallics, 2002, 21, 3203; (c) J. Parr and A. M. Slawin, Inorg.
Chim. Acta, 2000, 116.
4 (a) A. J. Goshe, I. M. Steele and B. Bosnich, J. Am. Chem. Soc., 2003,
125, 444; (b) G. D. Cuny, K. D. Landgrebe and T. P. Smith, Bioorg.
Med. Chem. Lett., 1999, 9, 237; (c) S. Dagaonkar and B. H. Mehta,
Asian J. Chem., 1995, 7, 611.
5 (a) B. A. Howell, L. G. Beholz and B. B. S. Sastry, J. Therm. Anal.,
1993, 40, 395; (b) G. Sanchez, J. L. Serrano, J. Garcia, G. Lopez, J.
Perez and E. Molins, Inorg. Chim. Acta, 1999, 287, 37.
6 (a) M. Ghedini, B. Panunzi and A. Roviello, Liq. Cryst., 1998, 25,
225; (b) I. D. Rae, B. E. Reichert and B. O. West, J. Organomet.
Chem., 1974, 81, 227.
(dppe)Pt(2-(N-phenyliminomethyl)phenolate)(BF4) (1b). To
a flask containing 52 mg aniline (0.56 mmol) and 68 mg salicy-
laldehyde (0.56 mmol) in 50 mL 1 : 1 CH2Cl2/MeOH was added
332 mg (dppe)PtCl2 (0.500 mmol), 126 mg NaBF4 (1.15 mmol),
and 227 mg Cs2CO3 (0.697 mmol). The reaction was heated
to reflux for two hours, then cooled to ambient temperature
and poured into 50 mL water. The biphase was extracted with
CH2Cl2 (3×) and the combined extracts were washed with water
(2×), dried over MgSO4, and evaporated in vacuo. The crude
yellow solid was recrystallized from CH2Cl2/tBuOMe to yield
1
31
361 mg (82%) of 1b containing 0.03 equiv. CH2Cl2. H{ P}
NMR (400 MHz, CDCl3) d 8.04 (br s, 1H, JH–Pt = 460 Hz), 7.86
(d, 4H, J = 7.2 Hz), 7.47 (m, 16H), 7.26 (m, 2H), 6.84 (t, 1H, J =
7.2 Hz), 6.73 (m, 2H), 6.65 (m, 2H), 6.53 (d, 2H, J = 7.6 Hz),
2.54 (m, 2H), 2.32 (m, 2H); 31P{ H} NMR (121 MHz, CDCl3)
1
7 (a) H. Liang, J. Liu, X. Li and Y. Li, Polyhedron, 2004, 23, 1619;
(b) D. J. Darensbourg, C. G. Ortiz and J. C. Yarbrough, Inorg. Chem.,
2003, 42, 6915.
d 35.9 (d, JP–P = 11.3 Hz, JPt–P = 3314 Hz), 31.9 (d, JP–P
=
3
1
31
11.4 Hz, JPt–P = 3710 Hz); 13C{ H}{ P} (75 MHz, CDCl3)24
d 165.2, 162.4, 153.9, 137.2, 135.5, 133.1, 133.0, 132.5, 129.4,
128.6, 127.1, 126.1, 124.1, 123.0, 121.1, 119.1, 117.6, 34.5, 23.8.
Anal. Calcd. for C39H34BF4NOP2Pt·(0.03 CH2Cl2): C, 53.33; H,
3.91; N, 1.59. Found: C, 53.01; H, 3.72; N, 1.55%.
8 See for example: (a) E. F. Connor, T. D. Younkin, J. I. Henderson,
A. W. Waltman and R. H. Grubbs, Chem. Commun., 2003, 2272;
(b) C. Wang, S. Friedrich, T. R. Younkin, R. T. Li, R. H. Grubbs,
D. A. Bansleben and M. W. Day, Organometallics, 1998, 17, 3149.
9 (a) V. Alteparmakian and S. D. Robinson, Inorg. Chim. Acta, 1986,
116, 37; (b) J. T. Mague and M. O. Nutt, J. Organomet. Chem., 1979,
166, 63.
Equilibrium measurements (typical procedure)
˚
10 The covalent radius of oxygen is only 0.04 A shorter than that of
A solution of 22.0 mg 1a (0.0251 mmol) in 1.00 mL of a
0.0250 M stock solution of 4-methylaniline in nitromethane
(0.025 mmol) was sealed in a J-Young NMR tube and heated to
60 ◦C. The reaction was monitored by 31P NMR until no changes
were observed in the relative peak areas of the compounds
(48–72 hours) at which time three separate 31P spectra were
collected.25 Equilibrium constants were calculated from the
average molar ratios of the two P2Pt(N,O) complexes.
nitrogen, see L. Pauling, The Nature of the Chemical Bond, Cornell
University Press, Ithaca, NY, 1960.
11 (a) R. J. Cross, Adv. Inorg. Chem., 1989, 34, 219; (b) R. J. Murinik,
Rev. Inorg. Chem., 1979, 1, 1.
12 For approaches to imine metathesis, see: (a) M. C. Burland, T. Y.
Meyer and M. H. Baik, J. Org. Chem., 2004, 69, 6173; (b) M. C.
Burland and T. Y. Meyer, Inorg. Chem., 2003, 42, 3438; (c) M. C.
Burland, T. W. Pontz and T. Y. Meyer, Organometallics, 2002, 21,
1933; (d) S. A. Bell, T. Y. Meyer and S. J. Geib, J. Am. Chem. Soc.,
2002, 124, 10698; (e) R. L. Zuckerman, S. W. Krska and R. G.
Bergman, J. Am. Chem. Soc., 2000, 122, 751; (f) G. To´th, I. Pinter
and A. Messmer, Tetrahedron Lett., 1974, 9, 735.
Crystallography
13 For approaches to transimination see: (a) G. Reddelien, Chem. Ber.,
1920, 53B, 355; (b) C. R. Hauser and D. S. Hoffenberg, J. Am. Chem.
Soc., 1955, 77, 4885; (c) W. Lopatin, P. R. Young and T. C. Owen,
J. Am. Chem. Soc., 1979, 101, 960.
14 Although no free salicylaldimine was detected, we cannot rigorously
exclude the possibility of a salicylaldimine exchange reaction (eqn.
(2)) coupled with imine exchange, since controls show that salicylim-
ines easily exchange anilines under the reaction conditions.
15 Cu(II) activation of salicylaldehydes to nucleophilic addition by cy-
clohexylamine has been demonstrated: B. O. West, in New Pathways
in Inorganic Chemistry, ed. E. A. V. Ebsworth, A. G. Maddock and
A. G. Sharpe, Cambridge University Press, London, 1968, pp. 303–
325.
Crystals suitable for X-ray analysis of (dppe)Pd(N-(4-methyl-
phenyl)salicyaldimate), prepared analogously to 1b, were grown
at room temperature from a saturated CH2Cl2 solution with
slow diffusion of pentane. Single crystals were mounted in
oil on the end of a fiber. Intensity data were collected on a
Siemens SMART diffractometer with CCD detection using Mo-
˚
Ka radiation of wavelength 0.710 73 A (x scan mode). The
structure was solved by direct methods and refined by least
squares techniques on F using structure solution programs from
the NARCVAX System.26 All non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were placed in calculated
˚
positions (C–H = 0.96 A) and allowed to ride on the atoms
16 Gibson has recently reported addition of amide to the imine carbon
of a stannous salicylaldimate: N. Nimitsiriwat, E. L. Marshall,
to which they were bonded.
1 9 5 0
D a l t o n T r a n s . , 2 0 0 5 , 1 9 4 8 – 1 9 5 1

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V. C. Gibson, M. R. J. Elsegood and S. H. Dale, J. Am. Chem.
Soc., 2004, 126, 13598.
17 L. P. Hammett, J. Am. Chem. Soc., 1937, 59, 96.
18 C. Hansch, A. Leo, S. H. Unger, K. H. Kim, D. Nikaitani and E. J.
Lien, J. Med. Chem., 1973, 16, 1207.
19 M. S. W. Chan, L. Deng and T. Ziegler, Organometallics, 2000, 19,
2741.
20 E. F. Connor, T. R. Younkin, J. I. Henderson, A. W. Waltman and
R. H. Grubbs, Chem. Commun., 1993, 2272.
22 E. N. Jacobsen, E. Zhang and M. L. Gu¨ler, J. Am. Chem. Soc., 1991,
113, 6703.
23 (a) P. Gugger, S. O. Limmer, A. A. Watson, A. C. Willis and S. B.
Wild, Inorg. Chem., 1993, 32, 5692; (b) A. K. Sharma, B. Khera
and N. K. Kaushik, Synth. React. Inorg. Met.-Org. Chem., 1983, 13,
491.
24 Not all aromatic carbons were observed in the 13C NMR spectrum.
1
25 Spectra were collected at 162 MHz using a standard 31P{ H}
parameter set with a 3 s delay between scans. Enough scans (typically
128) were collected to give a signal-to-noise ratio of >100 : 1.
26 E. J. Gabe, Y. Le Page, J. P. Charland, F. L. Lee and P. S. White,
J. Appl. Crystallogr., 1989, 22, 384.
21 See for example: H. Hagen, A. Barbon, E. E. van Faassen, B. T. G.
Lutz, J. Boersma, A. L. Spek and G. van Koten, Inorg. Chem., 1999,
38, 4079.
D a l t o n T r a n s . , 2 0 0 5 , 1 9 4 8 – 1 9 5 1
1 9 5 1