Synthesis and characterization of 2-phenylaniline cyclopalladatedcomplexes

Kazem Karami • Corrado Rizzoli • Naser Rahimi

Received: 14 August 2011 / Accepted: 9 September 2011 / Published online: 22 September 2011

� Springer Science+Business Media B.V. 2011

Abstract Reaction of the dinuclear complex [Pd{j2-N20,C1-2-(20-NH2C6H4)C6H4}Cl]2 (1) with ligands (L =4-picoline, sym-collidine) gave the six-membered palla-

dacycles [Pd{j2-N20,C1-2-(20-NH2C6H4)C6H4}Cl(L)] (2).The complex 1 reacted with AgX (X = CF3SO3, BF4)

and bidentate ligands [L–L = phen (phenanthroline), dppe

(bis(diphenylphosphino)ethane), bipy(2,20-bipyridine) anddppp (bis(diphenylphosphino)propane)] giving the mono-

nuclear orthopalladated complexes [Pd{j2-N20,C1-2-(20-NH2C6H4)C6H4}(L–L)] (3) [L–L = phen, dppe, bipy and

dppp]. These compounds were characterized by physico-

chemical methods, and the structure of [Pd{j2-N20,C1-2-(20-NH2C6H4)C6H4}Cl(L)] (L = sym-collidine) was determined

by single-crystal X-ray analysis.

Introduction

The cyclopalladation reaction has been profusely investi-

gated in view of the rich chemistry it renders and is well

documented for a great variety of metal centers and ligands

[1–3]. Cyclopalladated complexes have generated consid-

erable interest due to their applications in organic and

organometallic synthesis [4–6], photochemistry [7, 8],

optical resolution process [9, 10], as biologically active

compounds [11], liquid crystals [12, 13] and mainly in

homogeneous catalysis for the design of new materials

with outstanding properties [4–6, 14–18]. These thermally

and air-stable complexes are easy to handle and their

synthesis is often straightforward.

During the two last decades, the interest in cyclopalladated

compounds derived from N-donor ligands has noticeably

increased [19, 20].

A large variety of N-containing ligands has been cy-

clometalated, including tertiary amines. Nevertheless, only

very recently, a general method to orthopalladate primary

aryl-alkyl amines has been reported.

The orthopalladation of aliphatic amines was initially

reported to fail for primary and secondary amines [21].

In 1968, Cope and Friedrich reported that cyclopallada-

tion of amine complexes needs three essential conditions

including the use of substrate that forms five-membered

rings, use of aromatic rings without electron-withdrawing

substituents and also restrictively tertiary amines [21]. Sev-

eral reports have been published by Fuchita et al. [22, 23],

Vicente et al. [24], Albert et al. [25] and our laboratory [26]

that infringe the Cope’s rules. For the first time, Vicente et al.

[27] reported the synthesis of six-membered palladacycles

from primary amines with electron-withdrawing substitu-

ents that break down all conditions of Cope’s rules.

In this paper, in extension of Albert’s work on primary

amines [28], we have used 2-phenylaniline, which infringes

the conditions of Cope’s rules.

Results and discussion

The complex 1 was prepared following the method by

Albert et al. [28]. In addition, cleavage of the halogeno-

bridge of the complex 1 via nucleophilic attack of some

neutral ligands such as 4-picoline, sym-collidine, phen,

dppe, bipy and dppp was investigated and is shown in

K. Karami (&) � N. RahimiDepartment of Chemistry, Isfahan University of Technology,

84156/83111 Isfahan, Iran

e-mail: karami@cc.iut.ac.ir

C. Rizzoli

Department of General and Inorganic Chemistry,

University of Parma, Parma, Italy

123

Transition Met Chem (2011) 36:841–846

DOI 10.1007/s11243-011-9538-3

Scheme 1. The corresponding complexes [Pd{j2-N20,C1-2-(20-NH2C6H4)C6H4}Cl(L)] (2a, 2b) and [Pd{j

2-N20,C1-2-(20-NH2C6H4)C6H4} (L–L)] (3a–d) were obtained inmoderate to high yields. Also, the protons of the cyclo-

palladated biphenyl unit are assigned by letters in Fig. 1 for

the following discussion.

In the IR spectra of the complexes, the t(N–H) band that issensitive to complexation appeared from 3,171 to 3,417 cm-1

and 3,097 to 3,234 cm-1 for asymmetric and symmetric

stretching, respectively, whereas for N-coordination, a low-

ering of the t(N–H) band is expected [28]. Also, the t(P–C)band frequencies occur at 741 and 740 cm-1 in the parent

ligands dppe and dppp, respectively, and are shifted to lower

frequencies for 3b and 3d, suggesting some removal of the

electron density of the P-C bands. In the IR spectra of 3b and

3d, two signals appeared for P-C bonds due to the different

trans effects of aminic nitrogen and arylic carbon.

The 1H NMR signals for the a proton of complexes 2a,

2b, 3a, 3c and 3d resonated at lower frequencies (6.56,

6.39, 7.08, 6.37, 6.93 and 6.26 ppm, respectively) than the

parent ligand 2-phenylaniline and appeared as a doublet

due to the coupling of the b proton with the a proton. It

should be noted that the double signals for the a proton of

complexes 3b and 3d are slightly broad. It may be due to

the effectiveness of the phosphorus nucleus with the

a proton. Also, the 13C NMR signals for the C2 shifted to

upfield and occurred at 120.6(2a), 120.4(2b), 120.3(3a),

126.4(3b), 120(3c) and 120.8 ppm(3d). The chemical shift

of the proton and C2 is due to the anisotropic effect of the

aromatic ligand rings. Thus, it can be concluded that

4-picoline and sym-collidine, in 2a and 2b, have cis

arrangement with the palladated phenyl ring.

The 31P NMR of 3b shows two singlets at 33.15 and

41.15 ppm. The phosphorus nucleus signals of 3b are slightly

broad, which may be due to a fast equilibrium of conformers dand k (d $ k). The 31P NMR of 3d also shows two differentsignals at 11.8 and 17.1 ppm. According to the 31P NMR of

free ligand dppe and dppp [29], when the saturated five-

membered chelate ligand dppe coordinates to Pd(II) in theorthopalladated complex 3b, the coordination chemical

shifts, D = d (coordinate) - d (free), are about 45 and54 ppm, whereas the values for the saturated six-membered

chelate ligand dppp in complex 3d are 29 and 34.3 ppm.

Moreover, in the 31P NMR of 3d are also shown two other

different signals at -0.8 and -1.1 ppm that may be due to

release of the metal center by a phosphorus atom and revol-

ving around the Pd-P bond. In this suggested mechanism due

to the sensation of different electronic spaces by phosphorus

nucleus there are two more signals in the 31P NMR of 3d.

The X-ray crystal structure of compound 2b was

determined. Selected bond distances (Å) and angles (�) arelisted in Table 1; crystallographic data and parameters

concerning data collection and structure solution and

refinement are summarized in Table 2. Figures 2 and 3

show an ORTEP view of the compound 2b and H-bond

between acetone and the amine hydrogen of the complex,

respectively. 2b crystallized in the monoclinic space group

P21/n. Thus, two pairs of enantiomeric molecules are

related by an inversion center (Z = 4).

The palladium atom adopts a distorted square planar

coordination geometry (maximum displacement 0.107(2) Å

for atom C1), as suggested by the angles subtended by the

Scheme 1 (i) X = 4-picoline,sym-collidine(molar ratio

1/4-picoline or sym-collidine = 1:2), (THF:

acetone = 15:5), room

temperature, 6 h. (ii)

(A) L = AgBF4 or Ag(OTf)

(molar ratio 1/AgBF4 = 1:2),(THF: acetone = 15:5), room

temperature, 30 min,

(B) X = phen, dppe, bipy, dppp(molar ratio solution/X = 1:1),room temperature, 50 min

Fig. 1 Assignedcyclopalladated biphenyl unit

protons by letters

842 Transition Met Chem (2011) 36:841–846

123

ligands at the Pd(II) varying from 86.20(7) to 91.93(6) Å andfrom 175.34(5) to 176.44(6) Å. The summation of the bond

angles around the palladium is 360.2 �C. It should be notedthat the main cause of chirality is the non-planar structure of

the six-membered palladacycles in this kind of compounds.

The Pd1–C1 bond distance (2.0014(17) Å) is similar to those

found in other orthopalladated complexes [30, 31], and the

Pd1–N2 distance falls also in the usual range found for this

bond [32]. The Pd1–Cl1 (2.4212(5) Å) bond distance is not

significantly different from those found in related complexes

[28, 31]. Also, the distance of Pd1–N1 (2.0545(15) Å) is

close to such bonds in other complexes [27].

The six-membered palladacycle assumes a twist-boat

conformation, with puckering parameters QT, h and u of0.761(2) Å, 112.44(12)� and 149.26(14)�, respectively[33]. The dihedral angle between the aromatic rings of the

2-phenylaniline ligand is 35.33(6)�. In the crystal structure,centrosymmetrically related complex molecules are linked

into dimers by pairs of N—H…Cl hydrogen bonds (N1—H1N, 0.84(2) Å, H1N…Cl1i, 2.68(2) Å; N1…Cl1i,3.435(2) Å; N1—H1N…Cl1i, 152(2)�; symmetry code:(i) 1 - x, 1 - y, 1 - z), forming rings of R2

2 (8) graph set

motif (Fig. 3). The structure is further stabilized by inter-

molecular N—H…O hydrogen bonds involving the ace-tone solvent molecule (N1—H2N, 0.89(2) Å, H2 N…O1,2.07(2) Å; N1…O1, 2.928(2) Å; N1—H2N…O1,164.2(17)�).

Experimental

All chemicals and solvents were purchased from Merck

and Aldrich. Infrared spectra were recorded on a JASCO

Fig. 3 Hydrogen bonding between acetone and hydrogen amine ofthe complex 2b

Table 1 Selected bond distances (Å) and angles (�) for complex 2b

Pd1–N1 2.0545(15)

Pd1–N2 2.0599(15)

Pd1–C1 2.0014(17)

Pd1–Cl1 2.4212(5)

C1–Pd1–Cl1 175.34(5)

N1–Pd1–N2 176.44(6)

C1–Pd1–N1 86.20(7)

N1–Pd1–Cl1 91.56(5)

Cl1–Pd1–N2 90.52(4)

N2–Pd1–C1 91.93(6)

Table 2 Crystal data and structure refinement details for complex 2b

Empirical formula C20H21ClN2Pd�C3H6OFormula weight 489.32

Temperature (K) 294

Radiation (k, Å) Mo Ka (0.71073)

Crystal system Monoclinic

Space group P21/n

a (Å) 9.3129(17)

b (Å) 15.029(3)

c (Å) 16.459(3)

b (�) 95.102(3)V (Å3) 2,294.5(7)

Z 4

Dcal (Mg/m3) 1.416

l (mm-1) 0.94

Crystal size (mm3) 0.28 9 0.22 9 0.17

No. of reflections measured 30,760

No. of independent reflections 5,544

No. of data/restraints/parameters 5,544/24/266

R[F2 [ 2r(F2)] 0.024wR(F2) 0.064

Rint 0.021

S 1.04

Fig. 2 X-ray molecular structure of compound 2b

Transition Met Chem (2011) 36:841–846 843

123

680 Plus spectrophotometer using pressed disks of dis-

persed samples of the compounds in KBr. 1H NMR spectra

in CDCl3 and DMSO were recorded at 500 MHz on a

Bruker Avance instrument. The 13C NMR spectra in CDCl3and DMSO were recorded at 75 MHz on a Bruker Avance

instrument. The 31P{1H} NMR spectra in CHCl3 were

recorded at 121.4 MHz in a Bruker Avance instrument.

Chemical shifts are reported in d values (ppm) [relative toSiMe4 for

1H and 85% H3PO4 for31P].

Synthesis of compound (1) [28]

A suspension formed by 0.250 g (1.41 mmol) of PdCl2,

0.239 g (1.41 mmol) of 2-phenylaniline, 0.115 g (1.41 mmol)

of NaOAc, 0.164 g (2.82 mmol) of NaCl and 20 mL of MeOH

was stirred at room temperature for 1 week. The precipitate

was filtered off, washed with 5 mL of water and 5 mL of

diethyl ether and dried under vacuum. Yield: 75%. Charac-

terization data: Anal. Calc. for C24H20N2Cl2Pd2: C, 46.48; H,

3.25; N, 4.52. Found: C, 46.3; H, 3.4; N, 4.4%. IR (cm-1):

ta(NH2) = 3,283, ts(NH2) = 3,234.

Synthesis of compounds (2)

2a: A suspension formed by 0.3339 g (0.5388 mmol) of 1,

0.1002 g (1.0776 mmol) of 4-picoline and 15 mL of acetone

was stirred at room temperature for 6 h, and the solution was

concentrated under vacuum; 5 mL of diethyl ether was

added and stirred for 5 h, then filtered off and dried under

vacuum. Yield: 67.4%. IR (cm-1): ta(NH2) = 3,171,ts(NH2) = 3,097.

1H NMR (500 MHz, CDCl3): 2.39 (s, 3H,

CH3), 5.09 (br s, 2H, NH2), 6.56 (d, 1H, Ha,3JHH = 7.5 Hz),

6.91 (t, 2H, Hb,c,3JHH = 6.8 Hz), 7.14–7.25 (m, 5H, Hf,g,h

and Hm 4-picoline), 7.48 (d, 1H, Hd,3JHH = 7.4 Hz), 7.62

(d, 1H, He,3JHH = 7.6 Hz), 8.5 (d, 2H, Ho 4-picoline,

3JHH = 5.2 Hz).13C NMR (75.5 MHz, CDCl3): 21.1, 120.6,

125.1, 125.5, 125.6, 125.8, 126.3, 127.3, 127.7, 128.3, 134.6,

136.2, 137.3, 139.2, 148.8, 149.8, 152.5.

2b: A suspension formed by 0.3296 g (0.5319 mmol) of 1,

0.1287 g (1.0637 mmol) of sym-collidine and 15 mL of

acetone was stirred at room temperature for 5 h and the solvent

was removed by vacuum; 5 mL of diethyl ether was added and

stirred for 5 h, then filtered off and dried under vacuum. Yield:

82.95%. Characterization data: IR (cm-1): ta(NH2) = 3,306,ts(NH2) = 3,245.

1H NMR (500 MHz, CDCl3): 2.3(s, 3H,

CH3 p), 3.04(s, 6H, CH3 o), 5(br s, 2H, NH2), 6.39(d, 1H, Ha,3JHH = 7.5 Hz), 6.83(t, 1H, Hb,

3JHH = 7.2 Hz), 6.93(s, 2H,

Hm sym-collidine), 7.07(t, 1H, Hc,3JHH = 7.2 Hz),

7.25–7.32(m, 3H, Hf,g,h), 7.43(d, 1H, Hd,3JHH = 7.4 Hz),

7.58(d, 1H, He,3JHH = 7.3 Hz).

13C NMR (75.5 MHz,

CDCl3): 20.6, 27.7, 120.4, 123.6, 125.4, 126.3, 126.9, 127.6,

128.3, 134.2, 134.6, 137.6, 139.3, 144.7, 149.6, 158.7.

Synthesis of compounds (3)

3a: A suspension formed by 0.333 g (0.5374 mmol) of 1,

0.2761 g (1.0748 mmol) of AgOTf in THF/acetone (15:5)

was stirred at room temperature for 30 min under light

protection. The solution was filtered through MgSO4 and

addition of 0.194 g (1.0748 mmol) of phenanthroline turned

red-brown color into beige immediately. It was stirred for

50 min and then filtered off. Yield: 63%. IR (cm-1):

ta(NH2) = 3,417, ts(NH2) = 3,270.1H NMR (500 MHz,

CDCl3): 5.72(d, 2H, NH2,2JHH = 10.1 Hz), 7.08(d, 1H, Ha),

7.22(t, 2H, Hb,c,3JHH = 3.6 Hz), 7.31-7.35(m, 3H, Hf,g,h),

7.5(d, 1H, phen, 3JHH = 6.4 Hz), 7.6(d, 1H, phen,3JHH = 7.5 Hz), 7.74–7.79(m, 2H, phen), 7.85(br s, 1H,

phen), 8.11(t, 1H, phen, 3JHH = 4.7 Hz), 8.31(d, 1H, Hd,3JHH = 7.3 Hz), 8.91(d, 1H, He,

3JHH = 4.6 Hz), 9.17(br s,

1H, phen), 9.82(br s, 1H, phen). 13C NMR (75.5 MHz,

CDCl3): 120.3, 125, 126.2, 126.6, 126.7, 127.1, 127.3, 127.8,

128.4, 128.9, 135.7, 138.4, 139, 151.8.

3b: A suspension formed by 0.333 g (0.5374 mmol) of

1, 0.209 g (1.0748 mmol) of AgBF4 in THF/acetone

(15:5) was stirred at room temperature for 30 min under

light protection. The solution was filtered through MgSO4and addition of 0.428 g (1.0748 mmol) of dppe turned

red-brown color into light green–light beige immediately.

It was stirred for 50 min and then filtered off. Yield:

64.3%. IR (cm-1): (NH2) = 3,372.1H NMR (500 MHz,

CDCl3): 1.67 (br s, 2H, CH2), 2.33 (br s, 2H, CH2), 6.37(d,

1H, Ha,3JHH = 8 Hz), 6.72(br s, 1H, Hb), 6.89–7.6 (m,

24H, Hc,f,g,h and 2PPh2), 7.66(d, 1H, Hd,3JHH = 6.5 Hz),

7.71(t, 1H, He,3JHH = 6.3 Hz).

13C NMR (75.5 MHz,

CDCl3): 115.6, 126.4, 127.1, 127.5, 127.8, 128.5, 128.7,

128.8, 129, 129.2, 129.3, 129.4, 129.8, 130, 130.4,

130.8, 131.7, 132, 132.5, 132.6, 133.6, 134.6, 134.8,

136.5. 31P NMR (202.4 MHz, DMSO): 33.15 (br s), 41.15

(br s).

3c: A suspension formed by 0.291 g (0.4696 mmol) of 1,

0.183 g (0.9392 mmol) of AgBF4 in THF/acetone (15:5) was

stirred at room temperature for 30 min under light protection.

The solution was filtered through MgSO4 and addition of

0.1466 g (0.9392 mmol) of bipy turned red-brown color into

light immediately. It was stirred for 50 min and then filtered

off. Yield: 73.3%. IR (cm-1): ta(NH2) = 3,303, ts(NH2) =3,234. 1H NMR (500 MHz, CDCl3): 3.31(br s, 2H, NH2), 6.93

(d, 1H, Ha,3JHH = 11.2 Hz), 7.23–7.32(m, 4H, Hb,f,g,h),

7.51(dt, 2H, bipy, 3JHH = 7.6 Hz4JHH = 1.4 Hz), 7.66(dt,

2H, bipy, 3JHH = 7.2 Hz4JHH = 1.9 Hz), 7.73 (t, 1H, Hc,

3JHH = 6.9 Hz), 7.89 (t, 1H, Hd,3JHH = 6.7 Hz), 8.29 (t, 1H,

bipy, 3JHH = 7.7 Hz), 8.35 (t, 1H, bipy,3JHH = 7.6 Hz), 8.48

(d, 1H, He,3JHH = 5.2 Hz), 8.61 (d, 1H, Ho bipy,

3JHH =

8 Hz), 8.66 (d, 1H, Ho bipy,3JHH = 8.1 Hz).

13C NMR

(75.5 MHz, DMSO): 120, 123.8, 124.1, 124.3, 125.8, 126.4,

844 Transition Met Chem (2011) 36:841–846

123

127.7, 127.9, 128.2, 128.9, 136.6, 137.8, 138.7, 139, 141,

141.2, 149.8, 151.6, 153.6, 153.8, 156.1.

3d: A suspension formed by 0.3385 g (0.5462 mmol) of

1, 0.2126 g (1.0924 mmol) of AgBF4 in THF/acetone (15:5)

was stirred at room temperature for 30 min under light

protection. The solution was filtered through MgSO4 and was

added 0.45 g (1.0924 mmol) of dppp that red-brown color

into beige immediately. It was stirred for 50 min and then

filtered off. The final precipitate was light green. Yield:

90.1%. IR (cm-1): t(NH2) = 3,383.1H NMR (500 MHz,

CDCl3): 1.3(t, 2H, CH2,3JHH = 9.8 Hz), 1.59(br s, 1H,

CH2), 1.72(br s, 2H, CH2), 5.3(br s, 2H, NH2), 6.26(d, 1H,

Ha,3JHH = 4.6 Hz), 6.36(d, 1H, Hb,

3JHH = 7.7 Hz), 6.87(t,

1H, Hc,3JHH = 7.2 Hz), 7.03–7.84(m, 25H, Hd,e,f,g,h and

2PPh2).13C NMR (75.5 MHz, CDCl3): 15.2, 17.8, 21.8,

120.8, 125.1, 125.2, 125.3, 127.3, 127.6, 127.7, 127.9, 128.3,

128.4, 128.6, 128.68, 128.76, 128.8, 128.9, 129.3, 129.6,

130.3, 130.6, 131.2, 131.4, 132, 132.2, 132.4, 132.6, 133.7,

133.8, 134, 134.8, 135, 137, 137.2, 139.1, 140.4, 141.4. 31P

NMR (121.5 MHz, CDCl3): 17.1(br s), 11.8(br s), -0.8(s),

-1.1(s).

Crystal structure

Crystals suitable for the X-ray molecular structure determi-

nation of 2b were obtained by interface method of solution of

2b in acetone/hexane (1:3 v/v). Intensity data were collected

at ambient temperature (294(2) K) using graphite-mono-

chromated Mo Ka radiation (k = 0.71073 Å) on a BrukerAPEX-II CCD diffractometer. Data were corrected for

absorption using the SABABS program [34]. All non-

hydrogen atoms were refined anisotropically. The amine

H atoms were located in a difference Fourier map and

refined freely. All other H atoms were calculated geometri-

cally and refined using a riding/rotating model, with

C–H = 0.93–0.96 Å and with Uiso(H) = 1.2 U/eq(C) or 1.5

Ueq(C) for methyl H atoms. The methyl C atoms of the

acetone molecule show rather high displacements parame-

ters suggesting the presence of disorder. Any attempt to

refine these atoms using a disordered model was, however,

unsuccessful. The displacement parameters of the C22 and

C23 carbon atoms therefore restrained with the use of the

ISOR/SIMU instructions of SHELXL-97 [35].

Supplementary material

Crystallographic data for the structural analysis of 2b have

been deposited at the Cambridge Crystallographic Data

Centre, CCDC, No. 827638 for 2b. Copy of this information

can be obtained from The Director, CCDC, 12 Union Road,

Cambridge CB2 1EZ, UK (Fax: 44-1233-336033; e-mail:

deposit@ccdc.ac.uk or http://www.ccdc.cam.ac.uk).

Acknowledgments We are grateful to the Department of Chemis-try, Isfahan University of Technology, Isfahan, Iran, and Department

of General and Inorganic Chemistry, University of Parma, Parma,

Italy.

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Synthesis and characterization of 2-phenylaniline cyclopalladated complexesAbstractIntroductionResults and discussionExperimentalSynthesis of compound (1) [28]Synthesis of compounds (2)Synthesis of compounds (3)Crystal structure

Supplementary materialAcknowledgmentsReferences