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Palladium(II) complex formation by indole-3-acetate. Mixed ligand complexes involving a unique spiro-ring formed by cyclopalladation
ELSEVIER
Inorganica Chimica Acta 235 (1995) 367-374
Palladium(II) complex formation by indole-3-acetate. Mixed ligand complexes involving a unique spiro-ring formed by cyclopalladation Masako Takani a, Hideki Masuda b, O s a m u Yamauchi c,, "Faculty of Pharmaceutical Sciences, Kanazawa University, Takara-mach~ Kanazawa 920, Japan b Department of Applied Chemistry, Nagoya Institute of Technology, Showa-ku, Nagoya 466, Japan cDepartraent of Chemistry, Faculty of Science, Nagoya University, Chikusa-ku, Nagoya 464-01, Japan Received 18 November 1994; revised 2 February 1995
Abstract
Formation of mixed ligand palladium(II) complexes involving indole-3-acetate (IA) has been studied by synthetic, spectroscopic and X-ray crystallographic methods. Reaction of IA with Na2PdCI4 in methanol gave NaPd(IAH_1)CI (1) (IAH_I=IA deprotonated from the indole ring), which reacted with pyridine (py) to give [Pd(IAH_~)(py)] (2) as orange crystals. Similar reactions carried out in the presence of water gave Pd(IAH_I). 1.5H20 (1'), which further gave Pd(IAH_I)(py)-0.5H20 (2') by the reaction with py. X-ray crystal structure analysis of 2 revealed a unique dimeric structure, where IAH_ ~ coordinates to Pd(II) through the carboxylate oxygen atom and the tetrahedral C3 atom of the indole nucleus, forming a unique spiroring. The two complex units are bridged by the indole nitrogens in the 3H-indole form, and there is a close contact (2.75 /~,) around the nitrogen-C2 bonds of the five-membered rings of the indole nuclei positioned in parallel with each other. IA in the neutral form was liberated upon refluxing 1 in methanol containing 10% acetic acid, showing that IAH_~ and IA are interconvertible under proper conditions. The ~H and ~3C NMR spectra in CDCI3-CD3OD indicated that the C3 atom of IA in 1 and 2 is tetrahedral. Large shift differences of the C2 proton signals were observed between 1 and 1' and between 2 and 2', which indicates that 1 and 2 are dimers in solution whereas 1' and 2' are monomers and that the differences are due to the close contact between the two indole rings in 2 as detected in the solid state and probably in 1. Keywords: Palladium complexes; Indole-3-acetate complexes; Cyclopalladation; Spiro-ring formation; 3H-Indole structure; Ternary complexes; Crystal structures
1. Introduction The indole ring is an aromatic ring with a high electron density. It is known to play essential roles in the form of the tryptophyl residue in biological systems, such as stabilization of the protein structure by stacking with other aromatic groups [1], formation of a hydrophobic environment due to the highest hydrophobicity among amino acid side chains [2], and formation of an electron transfer pathway in some enzymes [3]. The indole ring located close to the active site of galactose oxidase is inferred to be involved in the stabilization Abbreviations: IA, indole-3-acetate; IA/-I_ i, IA deprotonated from the indole ring; phen, 1,10-phenanthroline; Trp, tryptophan; bpy, 2,2'-bipyridine; DMSO-d~,, dimethyl sulfoxide-d6; IR, infrared; C"D, circular diehroisrn; TMS, tetramethylsilane; py, pyridine. ¢"This paper is dedicated to the memory of Professor Ugo Croatto. * Corresponding author. 0020-1693/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved SSDI 0 0 2 0 - 1 6 9 3 ( 9 5 ) 0 4 4 8 5 - R
of the tyrosine radical formed in the enzymatic reaction [4]. Natural and synthetic indole derivatives often have strong physiological activities as shown by ergot alkaloids and some hallucinogens [5]. Intercalation of the indole ring of tryptophan-containing oligopeptides between the base pairs of D N A is also widely known [6]. Complex formation with the indole nitrogen normally takes place with alkali metals and Grignard derivatives to form ionic metal-nitrogen bonds [7], but transition metal ions readily form chelates with tryptophan (Trp) through its amino and carboxylate groups as revealed for [Cu(phen)(L-Trp)] [8] and [Cu(bpy)(L-Trp)] [9]. Our recent N M R and X-ray diffraction studies on palladium(II)-alkylindole interactions have established that the indole nitrogen can bind with Pd(II) in the form of the 3H-indole with its N H hydrogen at the C3 position (Fig. 1) [10]. On the other hand, cyclopalladation has been reported to occur with the Pd(II)
M. Takani et al. / Inorganica Chimica Acta 235 (1995) 367-374
368
.CH2COO
IH-indole
H. CH2COO
3H-indole
Fig. 1. Structures of 1H- a n d 3H-indoles with the atomic n u m b e r i n g scheme.
complexes involving nitrogen ligands such as pyridine [11]. For indole-Pd(II) complexes, both the cyclopalladated 3H-indole structure [12] and the 1H-indole structure without cyclopalladation [13] are known. The above Pd(II)-alkylindole complexes are formed through the Pd(II)-N1 bond but without cyclopalladation. As an extension of our studies on the properties of indoles such as aromatic ring stacking with coordinated aromatic rings and interactions with metal ions, we have now studied the Pd(II) complex formation of an indole derivative with a coordinating side chain, indole-3acetate (IA), which is known as a plant hormone (auxin). We here report the synthesis, X-ray structure determination, and properties of novel cyclopalladated complexes of deprotonated IA (IAH_I) with a spiro-ring involving the coordinated tetrahedral atom.
2. Experimental 2.1. Materials Potassium indole-3-acetate (IA) and Na2PdCI4 were purchased from Nacalai Tesque and used without further purification. Deuterated solvents, dimethyl sulfoxide-d6 (DMSO-d6), CDCI 3 and CD3OD were from Aldrich. All other chemicals used were of analytical grade.
2.2. Measurements IR spectra were measured in KBr disks with a Shimadzu IR-460 spectrophotometer. Electronic absorption spectra were recorded on a Union Giken SM401 or a Hitachi 330 recording spectrophotometer. Sample concentrations for these spectral measurements were 0.25-0.3 mM ( = m m o l dm -3) in methanol with respect to Pd(II). 1H and 13C NMR spectra were measured in DMSO-d6, CDC13, CDaOD, or their mixtures with a JEOL JNX-EX 270 spectrometer. Chemical shifts are shown in 6values (ppm) with tetramethylsilane (TMS) as an internal reference.
2.3. Syntheses 2.3.1. NaPd(IAH_~)CI (1) A solution of IA (2.0 g, 9.4 rnmol) and Na2PdCL (1.38 g, 4.7 mmol) in methanol was stirred overnight
at room temperature, and the precipitate was filtered (750 mg) and recrystallized twice from methanol to give a red powder (275 mg; yield 8%). Anal. Calc. for CloH7NO2C1NaPd.0.5H20: C, 34.61; H, 2.32; N, 4.04. Found: C, 34.86; H, 2.60; N, 4.04%. A similar reaction carried out in 50% methanol-water gave Pd(IAH_ 1)" 1.51-120 (1') as an insoluble crude product (yield 40%). Anal. Calc. for caoa7NO2Pd" 1.5H20: C, 39.17; H, 3.29; N, 4.57. Found: C, 39.24; H, 2.92; N, 4.61%.
2.3.2. [ ea(IAn_#(py)l (2) A solution of 1 (150 mg, 0.22 mmol) and pyridine (py) (60 mg, 0.76 mmol) in methanol was stirred for 3 h at room temperature and evaporated, and the residue was recrystallized from CHCI3 to give orange needles (110 mg; yield 55%). Anal. Calc. for C15H12N202Pd •CHCI3: C, 40.20; H, 2.74; N, 5.86. Found: C, 39.95; H, 2.57; N, 5.92%. The reaction of 1' (50 mg, 0.17 mmol) and py (20 mg, 0.25 mmol) carried out in methanol gave 2 (yield 12%) and Pd(IAH_ 1)(PY)"0.5H20 (2'), which was more soluble than 2 and was recrystallized from methanol (yield 36%). Anal. Calc. for C15H12N202Pd" 0.5H20: C, 49.00; H, 3.56; N, 7.62. Found: C, 49.32; H, 3.25; N, 7.64%.
2.3.3. Recovery of IA from 1 A methanol solution of 1 (200 mg, 0.29 mmol) was refluxed with 10% acetic acid (3 ml) for 3 h and evaporated to dryness. The ether soluble fraction was recrystallized from ether-hexane to give pale yellow needles (15.6 mg; yield 15%), m.p. 171 °C (uncorr.), which were identified to be IA (neutral form) by the IR spectrum. 2.4. X-ray structure determination of 2 Single crystals with the composition [Pd(IAH_ 1)(PY)]2" 4CHC13 suitable for X-ray analysis were obtained from a chloroform solution. Crystal data and experimental details are summarized in Table 1. Diffraction data were collected with an Enraf-Nonius CAD4-EXPRESS four-circle automated diffractometer. The crystal was sealed in a glass capillary tube to avoid loss of chloroform from the crystal. The reflection intensities were monitored by three standard reflections for every 2 h, and the decay of intensities was within 2%. The crystal was a very weak diffractor, which resulted in a lack of high angle diffraction data. The intensities of 9959 (9737 symmetrically independent) reflections were measured with the use of the to--2# scan technique in the 20 range 2-54", of which 2809 reflections with lo>3o(lo) were used for structure determination. Reflection data were corrected for Lorentz and polarization effects. An empirical absorption correction, based on ~Oscans, was applied.
369
M. Takani et al. / Inorganica Chimica Acta 235 (1995) 367-374 Table 1 Crystal data and experimental details for [Pd(IAH_I)(py)]2-4CHCI 3 (2)
C3on24N404Pd2-4CHCI3 1194.86
Formula Formula weight Crystal system Space group
monoclinic P2Jc
a (/~) b (~) c (/~) /3 (0) V (.~3) Z p (gcm -3) (cm- i) F(000) Crystal size (ram) A(Mo Ka) (/~) 20 Limit (*) No. reflections measured No. unique reflections No. reflections used (Io> 3o(lo)) R Rw
24.780(2) 9.5077(7) 20.682(2) 111.773(6) 4525.2 4 1.754 15.40 2352 0.4 × 0.2 × 0.2 0.71069 54 9959 9737 2809 0.0595 0.0616
The structure was solved by the heavy-atom method and refined anisotropically for non-hydrogen atoms by full-matrix least-squares calculations. Refinement was continued until all shifts were smaller than one-third of the standard deviations of the parameters involved. Atomic scattering factors and anomalous dispersion terms were taken from the literature [14]. The hydrogen atoms were included as a fixed contribution in the last cycle; their positions were located at the positions ideally calculated (C-H = 0.95/1,), and their temperature factors were assumed to be isotropic (B = 8.00/~2). The final R and Rw values were 0.0595 and 0.0616, respectively. The weighting scheme w - 1= (o.2(Fo) + (0.015Fo)2) was employed for the crystal. The final difference Fourier map did not show any significant features. The calculations were performed on a Micro VAX-3100 computer by using the program system SDP-MolEN [15]. The final atomic parameters for non-hydrogen atoms are given in Table 2, and the selected bond lengths and angles are listed in Table 3.
3. Results and discussion
3.1. Synthesis and spectra The reaction of Pd(II) with IA in the ratio of 1:2 gave complex 1, which reacted with py to give 2. The elemental analyses show that IA in 1 is dinegative (IAH_I) and that 2 is formed by substitution of the coordinated chloride ion of 1 with py. From comparison of the isolated complexes the four Pd(II) coordination sites do not seem to be occupied in both 1 and 2,
Table 2 Atomic
coordinates
and
thermal
parameters
for
[Pd(IAH_ l)(py)]: •4CHC13 (2) Atom
x
y
z
neq a (/~)
Pd(1) Pd(2) C1(1) C1(2) C1(3) C(1C) C1(11) C1(12) C1(13) C(2C) C1(21) C1(22) CI(23) C(3C) C1(31) b C1(32) b C1(33) b C1(34) b C1(35) b C1(36) b C(4C) O(1I) O(2I) N(II) C(2I) C(3I) C(3AI) C(3BI) C(4I) C(5I) C(6I) C(7I) C(8I) C(9I) O(llI) O(12I) N(llI) C(12I) C(13I) C(13AI) C(13B1) C(14I) C(15I) C(16I) C(17I) C(18I) C(19I) N(1P) C(2P) C(3P) C(4P) C(5P) C(6P) N(llP) C(12P) C(13P) C(14P) C(15P) C(16P)
0.25241(5) 0.24688(5) 0.8760(2) 0.8135(3) 0.9204(2) 0.5372(9) 0.5407(4) 0.5066(4) 0.4885(3) 0.9484(8) 0.9589(3) 0.9975(3) 0.9574(3) 0.3542(6) 0.4059(5) 0.3922(5) 0.3019(5) 0.3228(5) 0.3728(6) 0.4156(4) 0.8577(7) 0.2133(4) 0.1695(5) 0.2103(5) 0.2342(5) 0.2047(6) 0.2035(6) 0.1945(5) 0.1501(6) 0.0974(7) 0.0540(6) 0.0614(6) 0.1140(6) 0.1557(6) 0.2813(4) 0.3239(5) 0.2907(5) 0.2661(6) 0.2959(7) 0.2964(6) 0.3015(6) 0.3496(6) 0.4007(7) 0.4453(6) 0.4363(7) 0.3866(6) 0.3429(5) 0.2972(5) 0.3217(6) 0.3495(8) 0.3555(7) 0.3307(6) 0.3047(6) 0.2022(5) 0.1814(6) 0.1504(7) 0.1390(7) 0.1624(7) 0.1919(7)
0.1367(1) -0.2657(1) 0.0075(7) 0.0918(6) - 0.0567(8) 0.326(3) 0.4971(9) 0.305(1) 0.244(1) 0.032(2) 0.1483(7) -0.1067(8) 0.1160(8) 0.089(2) 0.111(i) 0.159(1) 0.217(1) 0.226(1) 0.116(2) 0.023(1) -0.035(2) 0.208(1) 0.140(1) -0.187(1) -0.149(1) - 0.046(1) - 0.036(2) 0.115(2) -0.039(1) 0.034(2) 0.019(2) - 0.061(2) -0.134(2) -0.119(1) -0.334(1) -0.271(1) 0.060(1) 0.021(2) - 0.082(2) -0.090(2) -0.244(2) -0.098(1) -0.169(2) -0.158(2) -0.079(2) -0.003(2) -0.015(1) 0.327(1) 0.372(2) 0.500(2) 0.580(2) 0.535(2) 0.410(2) -0.455(1) -0.499(2) -0.618(2) -0.702(2) -0.662(2) -0.537(2)
0.44160(6) 0.58440(5) 0.3444(2) 0.2040(3) 0.2394(3) 0.629(1) 0.6529(5) 0.5416(4) 0.6637(4) 0.8511(9) 0.7937(3) 0.8668(4) 0.9289(3) 0.2738(7) 0.2362(6) 0.3645(5) 0.2436(6) 0.2170(6) 0.3573(7) 0.2582(5) 0.2568(8) 0.3438(4) 0.2347(5) 0.4887(5) 0.4454(6) 0.3943(7) 0.3207(7) 0.2966(6) 0.4078(6) 0.3743(7) 0.3997(8) 0.4585(8) 0.4933(7) 0.4651(6) 0.6827(4) 0.7933(4) 0.5381(5) 0.5824(7) 0.6321(7) 0.7045(7) 0.7306(7) 0.6173(7) 0.6492(8) 0.6236(8) 0.5651(8) 0.5333(8) 0.5611(7) 0.4688(5) 0.5350(7) 0.5520(8) 0.5004(8) 0.4338(7) 0.4205(7) 0.5521(5) 0.4855(7) 0.4653(7) 0.5113(8) 0.5802(7) 0.5980(7)
2.78(3) 2.68(3) 8.1(2) 8.7(2) 9.8(2) 9.3(8) 15.7(4) 16.6(3) 13.7(3) 7.3(6) 8.7(2) 12.6(3) 11.2(2) 4.9(5) 9.5(4) 7.2(3) 8.4(3) 9.8(4) 11.0(4) 5.8(2) 5.1(5) 3.6(3) 5.0(3) 2.8(3) 2.2(3) 2.7(4) 3.7(4) 2.7(4) 2.6(4) 3.5(4) 4.2(5) 4.0(4) 4.2(4) 2.5(4) 3.2(3) 4.9(3) 3.3(3) 3.1(4) 3.4(4) 3.3(4) 3.6(4) 3.4(4) 4.2(5) 4.9(5) 4.6(5) 4.1(5) 2.4(4) 2.8(3) 4.2(4) 5.4(5) 4.7(5) 4.2(4) 3.6(4) 3.2(3) 3.6(4) 5.2(5) 5.1(5) 4.8(4) 4.0(5)
"Anisotropically refined atoms are given in the form of the isotropic equivalent displacement parameter defined as Beq= (4/3)[a2/3(1,1)+
b2/3(2,2) +c2/3(3,3)+ab(cos 3,)/3(1,2)+ac(cos /3)/3(1,3)+be(cos a)/3 (2,3)]. b The CI atoms of the chloroform molecule disordered around C(4C) were isotropically refined with 0.5 occupancy.
370
M. Takani et aL / Inorganica Chimica Acta 235 (1995) 367-374
Table 3 Selected bond lengths (,~,) and angles (°) for [Pd(IAH_t)(py)]2.4CHCI3 (2) Bond length Pd(1)-O(lI) Pd(1)-N(1P) Pd(1)-N(11I) Pd(1)-C(3I)
2.008(8) 2.084(10) 2.002(10) 2.124(12)
Pd(2)-O(11I) Pd(2)-N(llP) Pd(2)-N(II) Pd(2)-C(laI)
2.006(7) 2.093(11) 1.992(10) 2.145(14)
Bond angle O(II)-Pd(1)-N(1P) O(II)-Pd(1)-N(llI) O(II)-Pd(1)-C(3I) N(1P)-Pd(1)-N(1 lI) N(1P)-Pd(1)-C(3I) N(llI)-Pd(1)--C(3I) Pd(1 )--C(3I)-C(2I) Pd(1)--C(3I)-C(aAI) Pd(1)-C(3I)--C(4I) C(2I)--C(3I)-C(3AI) C(2I)--C(3I)--C(4I) C(3AI)-C(3I)--C(4I)
89.4(4) 178.2(4) 80.4(4) 92.3(4) 169.1(5) 97.9(5) 100.1(8) 102.3(9) 105.4(9) 127.0(13) 98.4(12) 120.5(10)
O(llI)-Pd(E)-N(II) O(llI)-Pd(2)-N(llP) O(11I)-Pd(2)--C(13I) N(lI)-Pd(E)-N(11P) N(lI)-Pd(2)-C(13I) N(llP)-Pd(2)-C(13I) Pd(2)--C( 13I)--C(121) Pd(2)--C(13I)-C(13AI) Pd(2)-C(13I)-C(14I) C( 12I)-C(13I)-C(13AI) c(121)-c( 13I)-.c(14I) C(13AI)-C(13I)-C(14I)
176.2(4) 91.1(4) 81.0(4) 90.9(4) 97.0(5) 172.0(5) 100.0(8) 102.4(10) 103.0(9) 124.7(14) 100.7(13) 121.9(11)
suggesting that the complexes may be dimeric. Since 1 gave IA upon refluxing with 10% acetic acid in methanol, coordinated IA (IAH_I) appeared to retain its indole structure in 1. However, the three characteristic peaks for IA at 266, 280 and 289 nm observed for the 1:2 Pd(II)-IA system prepared by mixing Pd(II) and IA were not detected in the spectra for 1 and 2 (Table 4). The spectra in the region 300-500 nm exhibited two unusually strong peaks at 336 and 440 nm ( e - 2900-3800 M -1 crn -1) and 326 and 420 nm (e= 4500-5900 M -1 cm-~), respectively, where Na2PdC14 has two bands with lower intensities (E< 1000 M -~ cm- 1). The IR and ~H NMR spectra of the isolated complexes 1 and 2 showed no NH signals expected for the indole ring (Table 5). The 1H and 13C NMR spectra were assigned by two-dimensional correlation spectroscopy (XHJH and ~H-~3C COSY). The 1H NMR spectra revealed that the doublet peaks (J= 1 Hz) of the C H 2 protons of IAH_ ~ are split into AB-type quartet peaks (J= 17.2-17.3 Hz) in 1 and 2 showing that the two protons are under different conditions probably due to
the neighboring groups. One of the CH2 protons of coordinated IA is shifted upfield relative to IA by 0.85 and 0.81 ppm in 1 and 2, respectively. The spectral data in Table 5 further indicate that the C2 proton (H-2) signal of 1 and 2 is shifted upfield ( A S = - 0 . 6 9 and -0.47 ppm, respectively) relative to that of IA, whereas the C7 proton (H-7) signal is shifted downfield (AS= 1.15 and 1.05 ppm, respectively). The a protons of py (py-aH) in 2 suffer an upfield shift (AS= -0.47 ppm) relative to those of Pd(py)C12. The 13C NMR spectrum of 2 indicates some characteristic signals, from which we see that: (i) C3 is tetrahedral with a considerably low chemical shift (8 61.6 ppm); (ii) C2 (8 163.5 ppm) is more like an imine carbon [10,12,16] rather than an aromatic carbon [17]; (iii) CTa and the carbons of the carboxylate and methylene groups are shifted downfield (AS= 13.0, 5.6 and 2.4 ppm, respectively) (Table 6). Our previous study revealed that Pd(II) coordinates the indole ring at the nitrogen atom with the NH proton shifted to the C3 atom (3H-indole structure) [10]. Because the 1H NMR spectra of I and 2 exhibited no H-3 signals and elemental analyses were
Table 4 Absorption spectral data System
Jr,..:, (nm) (e)
Pd-IA (1:2)
219 289 210 336 210 420 218 326
NaPd(IA)CI (1) Pd(IA)(py) (2) Pd(IA)(py((2')
(28000) (5400) (end, 27000) (3800) (end, 33200) (4500) (28200) (4100)
266 (850o) 333 (4100) 242 (sh, 12200) 440 (2900) 267 (18300)
280 (740o) 438 (290o) 266 (12200)
260 (16200) (39o0)
288 (8500)
4o4
326 (5900)
M. Takani et aL / lnorganica Chimica Acta 235 (1995) 367-374
371
Table 5
t H NMR spectral data (270 MHz)" Proton
NaPd(IA)CI (1)b
Pd(IA) (1')b
Pd(IA)(py) (2)c
Pd(IA)(py) (2')c
IA b
IA c
CH2
2.66(d, 1H) J = 17.3 3.56(d,lH) J = 17.3
2.80(d, 1H) J = 17.5 3.53(d, 1H) J = 17.5
2.81(d, 1H) J = 17.2 3.68(d, 1H) J = 17.2
2.89(d, 1H) J = 17.8 3.54(d, 1H) J = 17.8
3.51(s, 2H)
3.62(d, 2H) J = 1.0
H-2
6.45(s)
8.36(s)
6.65(s)
8.41(s)
7.14(s)
7.12(s)
H-4
7.74(d) J=7.3 7.39(0 J=7.4
7.62(d) J=7.6 7.10(t) J=7.5
7.88(d) J=7.5 7.47(t,d) J=7.5, <1.0
7.76(d) J=7.5 7.14(m)
7.55(d) J=7.9 6.93(t,d) J=7.7, <1.0
7.61(d,m) J=7.5, < 1.0 6.99(t,d) J=7.5, <1.0
7.55(t) J=7.4
6.71(t) J=7.6
7.55(t,d) J=7.5, 1.5
7.02(t,d) J=7.7, <1.0
7.07(t,d) J=7.5, 1.3
8.45(br.s)
6.65(d) J=7.6
8.37(d) J=7.6
7.30(d) J=7.9
7.32(d,t) J=7.5, <1.0
H-5 H-6 H-7 py-a
6.75(m, 2H)
8.34(d,d)
8.42(m)
Pd(py)Cl2,
8.81(d,m)
J = 6.4, 1.3
J ~ 5.0, 1.0
py-/3
7.36(m)
7.38(m)
7.38(m)
py-~/
7.86(t,t) J = 7.6, 1.3
7.88(t,t) J = 7.5, 1.5
7.82(t,t) J = 7.6, 1.5
NH
10.78(br.s)
" Chemical shifts 8 in ppm and coupling constants J in Hz. b In DMSO-d6---CDCI3. c In CDCIa-CDaOD. Table 6
13C NMR spectral data (67.8 MI-Iz) ° Carbon
Pd(IA) (1') ~
Pd(IA)(py) (2) °
Pd(IA)(py) (2') °
CH2 C2 C3 C3a CA C5 C6 C7 C7a COO
35.5 166.6 63.9 138.7 120.0 123.9 124.8 117.0 146.6 186.0
37.9 163.5 61.6 136.1 121.7 125.7 126.1 120.1 150.5 186.6 150.6 125.8 139.5
36.6 168.9 59.0 140.2 120.8 125.6 124.7 117.5 147.1 188.2 151.1 126.0 139.7
py-ot
py-fl PY-3'
Pd(py)Cl2 ~
IA ~
35.5 123.7 112.0 128.7 119.5 119.1 121.8 111.7 137.5 181.0 153.3 125.2 138.7
" Chemical shifts 8 in ppm.
b In DMSO-d6-CDCI3. c In CDCI3-CD3OD.
consistent with IAH_ a, H-3 is concluded to be lost due to palladation. On the other hand, the amount of Pd(IAH_~) in the solutions, prepared by mixing Pd(II) and IA in the ratio of 1:2 in methanol and kept standing for one day, was estimated from the NMR spectra to be only about 9% of the total Pd(II) used. The IR spectra exhibited the C = O stretching band for IA (neutral form), 1, 2 and IA (ionized form) at 1730, 1610, 1630 and 1560 cm-1, respectively, which indicates
that the carboxylate group is coordinated to Pd(II) in 1 and 2. From the above spectral data we conclude that complexes 1 and 2 involve IAH_ 1 in the 3H-indole structure coordinated through the nitrogen, the C3(sp 3) carbon and the carboxylate oxygen.
3.2. Molecular structure of 2 revealed by X-ray analysis The crystal structure revealed by X-ray analysis consists of four discrete molecules [Pd(IAH_l)(py)]2 and sixteen chloroform molecules in a unit cell. Fig. 2 shows the molecular structure of 2 with the atomic numbering scheme which is different from that in Fig. 1 used for the spectral assignment. It has a pseudo-Ci symmetry for the center of the P d . . . P d vector and forms a dimeric structure bridged by the indole rings. Pd(II) binds with the carboxylate oxygen and the C3 atom of the indole ring (numbered C3I and C13I in Fig. 2) to form a novel five-membered spiro chelate ring as a result of cyclopaUadation. The nitrogen atoms of two py molecules occupy positions in a square-planar geometry with a tilting angle of 20.8 and 21.6 °, and the indole nitrogen atom ( N I = N I I and N l l I in Fig. 2) coordinates to a neighboring Pd(II), forming a dimeric structure. The N1--C2 bonds of the two bridging indole rings are involved in unusually short contact with each other with the distance of 2.75/k. The distance between the two Pd(II) ions is 4.865(2)/~. The Pd-N1 bonds,
372
M. Takani et al. / Inorganica Chimica Acta 235 (1995) 367-374
O(21)
C(61)
~
C
(
2
P
~C(14P)
)
C
(
I
~
~
C(16I) O(12I) C(4P) C(3P) Fig. 2. Molecularstructure of 2 with the atomicnumberingschemewhichis used solelyfor Tables 2 and 3. 2.002(10) and 1.992(10) /~, are slightly shorter than those detected for [Pd(2-methylindole)2Cl2] (2.024(7) A) and [Pd(2,5-dimethylindole)2C12] (2.02(3) /~) [10], and the Pd-carboxylate oxygen bonds, 2.008(8) and 2.000(8)/~, are at the short end of the values previously reported (2.006-2.029 A) [18] within e.s.d. The Pd-C3 bonds (2.124(12) and 2.145(14)/~) and the Pd-pyridine nitrogen bonds (2.084(10) and 2.093(11)/~) are within the normal ranges 2.003-2.144 and 2.033-2.149/~, respectively, reported for other cyclopalladated complexes [lla,e,18,19].
3.3. Structural transformation of [ PdCI,] z- to cyclopalladated dimeric complexes in solution Na2PdCL reacted with IA (potassium salt) in methanol to give 1 as a crystalline powder, which was converted to 2 by the reaction with py in methanol. When 1 was dissolved in DMSO-d6-CDCI3, it gave the 1H NMR spectrum with a singlet peak of H-2 at 6.45 ppm corresponding to an imine proton as expected from the 3H-indole structure, but upon standing for one day the original spectrum changed to give a spectrum which was similar to that of 1' (Table 5). The result and the procedure of isolation of 1' indicate that the Pd(II)-N1 bond is cleaved in the presence of water or DMSO. A large chemical shift difference (1.65 ppm) was observed between the H-2 signals of 2 and 2' (Fig. 3), and a similar shift (1.91 ppm) was also observed between 1 and 1' (Table 5). On the basis of the NMR spectra and the dimeric structure of 2 shown in Fig. 2, we conclude that 1' and 2' are monomers which are devoid of the Pd(II)-N1 bonds and that the spectral changes are due to formation of the monomers which causes the downfield and upfield shifts of the H-2 and H-7 signals, respectively, due to the cleavage of the Pd(II)-N1 bonds; the ring current effect resulting from
the close contact between the five-membered rings of the indole nuclei disappears upon bond cleavage to cause the downfield shift of H-2. The 8 values for H-2 of 1' and 2' (8.26--8.41 ppm) correspond with the value of ~ 8.0 ppm reported for 3H-indole [20]. Complex 1 which has the same 1:1:1 ratio of Pd(II) and two ligands as in 2 is concluded to have a similar dimeric structure, the chloride ion of which is most probably replaced by py to give 2 as shown in Scheme 1. Refluxing 1 in methanol with added acetic acid gave IA in the neutral form, which indicates that 1H- and 3H-indole structures are interconvertible under the conditions employed.
4. Conclusions
Reactions of [PdCl4] 2- with IA in methanol gave a dimeric complex 1, which further reacted with py to give complex 2 as crystals. X-ray analysis revealed the coordination structure with a unique spiro-ring formed as a result of cyclopaUadation at the C3 atom of the 3H-indole ring. One of the chelate rings is a fivemembered ring composed of C3 and the side chain CH2COO- moiety, and the other ring is a macrocyclic (eight-membered) ring formed by the two bridging N1 atoms. In solution the complexes gradually undergo structural changes probably due to the reaction with water or DMSO molecules leading to cleavage of the Pd-N1 bridge. Since Pd-C bond formation easily occurs under mild conditions, similar reactions may be possible in Pd(II)-catalyzed reactions and reactions involving Pd(II) and indole derivatives. The cyclopalladation in the present case may give rise to stereoisomers due to the asymmetric (23 atom, but 1 and 2 are optically inactive, indicating that they are racemic mixtures. It is interesting to note in this
M. Takani et al. / Inorganica Chimica Acta 235 (1995) 367-374
373
5. Supplementary material (a) H-7
py-olt
H-2
Tables of positional parameters and their estimated standard deviations, refined displacement parameter expressions (beta), general displacement parameter expressions (U), bond lengths, bond angles, torsion angles, and observed and calculated structure factors are available from the authors upon request.
py-~-I
py-TH
H-4
m
H-6
Acknowledgements ..,
The authors express their sincere thanks to the Ministry of Education, Science and Culture of Japan for the Grant-in-Aid for Scientific Research (No. 06640719) to M.T. and the Grant-in-Aid for Scientific Research on Priority Areas (Bioinorganic Chemistry) (No. 04225102) to O.Y. (b)
References H-2 I"1-7 H4
py~ H-4
py-i~H
py-yH
f
~
f
H-$
&ppm 8.5
8.0
7.5
7.0
6.5
Fig. 3. tH NMR spectra of Pd(IA)(py) (2) (a) and Pd(IA)(py) (2') (b) in CDCI3.
PY
-
2
1 Scheme 1.
connection that the side chain CH2 protons are in different environments as indicated by the AB-type coupling in the 1H NMR spectrum (Table 5). The present findings will add to our knowledge of versatile bonding modes, reactivities and electronic properties of the indole ring in metal complexes and possibly in biological systems.
[1] S.K. Burley and G.A. Petsko, Science, 229 (1985) 23. [2] Y. Nozaki and C. Tanford, J. Biol. Chem., 246 (1971) 2211. [3] (a) J.M. Mauro, L.A. Fishel, J.T. Hazzard, T.E. Meyer, G. Tollin, M.A. Cusanovich and J. Kraut, Biochemistry, 27 (1988) 6243; (b) H. Pelletier and J. Kraut, Science, 258 (1992) 1748; (c) P. Nordlund and H. Eklund, Z Mol. Biol., 232 (1993) 123. [4] (a) N. Ito, S.E.V. Phillips, C. Stevens, Z.B. Ogel, M.J. McPherson, J.N. Keen, FLD.S. Yadav and P.F. Knowles, Nature, 350 (1991) 87; (b) N. Ito, S.E.V. Phillips, K.D.S. Yadav and P.F. Knowles, J. Mol. Biol., 238 (1994) 794. [5] (a) W.J. Houlihan (ed.), Indoles, Parts 1, 2 and 3, Wiley, New York, 1972 and 1979; (b) A.R. Katritzky and C.W. Rees (eds.), Comprehensive Heterocyclic Chemistry, Vol. 4, Pergamon, Oxford, 1984; (c) L.S. Hegedus, Angew. Chem., Int. Ed. Engl., 27 (1988) 1113. [6] C. H~l~ne and J.-C. Maurizot, CRC Crit. Rev. Biochem., (1981) 213. [7] (a) Y. Omote, N. Fukada and N. Sugiyama, Nippon Kagaku Zasshi, 90 (1969) 1283; (b) M.G. Reinecke, J.F. Sebastian, H.W. Johnson, Jr., and C. Pyun, J. Org. Chem., 36 (1971) 3091; 37 (1972) 3066, and Refs. therein. [8] (a) K. Aoki and H. Yamazaki, J. Chem. Sot., Dalton Trans., (1987) 2017; (b) H. Masuda, O. Matsumoto, A. Odani and O. Yamauchi, Nippon Kagaku Kaishi, (1988) 783. [9] H. Masuda, T. Sugimori, A. Odani and O. Yamauchi, lnorg. Chim. Acta, 180 (1991) 73. [10] O. Yamauchi, M. Takani, K. Toyoda and H. Masuda, Inorg. Chem., 29 (1990) 1856. [11] (a) G.R. Newkome, W.E. Puckett, V.K. Gupta and G.E. K i e f e r , Chem. Rev., 86 (1986) 451; (b) D.W. Evans, G.R. Baker and G.R. Newkome, Coord. Chem. Rev., 93 (1989) 155; (c) G.R. Newkome, K.J. Theriot, B.K. Cheskin, D.W. Evans and G.R. Baker, Organometallics, 9 (1990) 1375; (d) A. Yoneda, M. Ouchi, T. Hakushi, G.R. Newkome and F.R. Fronczek, Chem. Lett., (1993) 709; (e) A. Yoneda, T. Hakushi, G.R. Newkome, Y. Morimoto and N. Yasuoka, Chem. Lett., (1994) 175. [12] R. Robson, Inorg. Chim. Acta, 57 (1982) 71. [13] T.A.K. AI-Allaf, M.T. Ayoub and L.J. Rashan,J. Inorg. Biochem., 38 (1990) 47. [14] J.A. Ibers and W.C. Hamilton (eds.), International Tables for X-ray Crystallography, Vol. IV, Kynoch, Birmingham, UK, 1974.
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M. Takani et aL / Inorganica Chimica Acta 235 (1995) 367-374
[15] MolEN, an interactive structure solution procedure, EnrafNonius, Delft, Netherlands. [16] (a) J.A.R. Rodrigues and L.I. Verardo, J. HeterocycL Chem., 27 (1990) 855; (b) L. Stetaniak, I. Ando, H. Yoshimizu, J. Lipkowski and G.A. Webb, J. Crystallogr. Spectrosc. Res., 21 (1991) 51. [17] (a) S.P. Singh, S.S. Parmar, V.I. Stenberg and S.A. Farnum, J. Heterocycl. Cherry, 15 (1978) 13; (b) M.S. Marales-Rios, J. Espineira and P. Joseph-Nathan, Magn~ Reson. Chem., 25 (1987)
377; (c) P. Joseph-Nathan, R.E. del Rio and M.S. MaralesRios, Heterocycles, 27 (1988) 377. [18] A.G. Orpen, L. Brammer, F.H. Allen, O. Kennard, D.G. Watson and R. Taylor, J. Cher~ Soc., Dalton Trans., (1989) S1. [19] G.R. Newkome, W.E. Puckett, G.E. Kiefer, V.IC Gupta, F.R. Fronczek, D.C. Pantaleo, G.L. McClure, J.B. Simpson and W.A. Deutsch, lnorg. Chem., 24 (1985) 811. [20] A.H. Jackson and A.E. Smith, Tetrahedron, 21 (1965) 989.
Inorganica Chimica Acta 235 (1995) 367-374
Palladium(II) complex formation by indole-3-acetate. Mixed ligand complexes involving a unique spiro-ring formed by cyclopalladation Masako Takani a, Hideki Masuda b, O s a m u Yamauchi c,, "Faculty of Pharmaceutical Sciences, Kanazawa University, Takara-mach~ Kanazawa 920, Japan b Department of Applied Chemistry, Nagoya Institute of Technology, Showa-ku, Nagoya 466, Japan cDepartraent of Chemistry, Faculty of Science, Nagoya University, Chikusa-ku, Nagoya 464-01, Japan Received 18 November 1994; revised 2 February 1995
Abstract
Formation of mixed ligand palladium(II) complexes involving indole-3-acetate (IA) has been studied by synthetic, spectroscopic and X-ray crystallographic methods. Reaction of IA with Na2PdCI4 in methanol gave NaPd(IAH_1)CI (1) (IAH_I=IA deprotonated from the indole ring), which reacted with pyridine (py) to give [Pd(IAH_~)(py)] (2) as orange crystals. Similar reactions carried out in the presence of water gave Pd(IAH_I). 1.5H20 (1'), which further gave Pd(IAH_I)(py)-0.5H20 (2') by the reaction with py. X-ray crystal structure analysis of 2 revealed a unique dimeric structure, where IAH_ ~ coordinates to Pd(II) through the carboxylate oxygen atom and the tetrahedral C3 atom of the indole nucleus, forming a unique spiroring. The two complex units are bridged by the indole nitrogens in the 3H-indole form, and there is a close contact (2.75 /~,) around the nitrogen-C2 bonds of the five-membered rings of the indole nuclei positioned in parallel with each other. IA in the neutral form was liberated upon refluxing 1 in methanol containing 10% acetic acid, showing that IAH_~ and IA are interconvertible under proper conditions. The ~H and ~3C NMR spectra in CDCI3-CD3OD indicated that the C3 atom of IA in 1 and 2 is tetrahedral. Large shift differences of the C2 proton signals were observed between 1 and 1' and between 2 and 2', which indicates that 1 and 2 are dimers in solution whereas 1' and 2' are monomers and that the differences are due to the close contact between the two indole rings in 2 as detected in the solid state and probably in 1. Keywords: Palladium complexes; Indole-3-acetate complexes; Cyclopalladation; Spiro-ring formation; 3H-Indole structure; Ternary complexes; Crystal structures
1. Introduction The indole ring is an aromatic ring with a high electron density. It is known to play essential roles in the form of the tryptophyl residue in biological systems, such as stabilization of the protein structure by stacking with other aromatic groups [1], formation of a hydrophobic environment due to the highest hydrophobicity among amino acid side chains [2], and formation of an electron transfer pathway in some enzymes [3]. The indole ring located close to the active site of galactose oxidase is inferred to be involved in the stabilization Abbreviations: IA, indole-3-acetate; IA/-I_ i, IA deprotonated from the indole ring; phen, 1,10-phenanthroline; Trp, tryptophan; bpy, 2,2'-bipyridine; DMSO-d~,, dimethyl sulfoxide-d6; IR, infrared; C"D, circular diehroisrn; TMS, tetramethylsilane; py, pyridine. ¢"This paper is dedicated to the memory of Professor Ugo Croatto. * Corresponding author. 0020-1693/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved SSDI 0 0 2 0 - 1 6 9 3 ( 9 5 ) 0 4 4 8 5 - R
of the tyrosine radical formed in the enzymatic reaction [4]. Natural and synthetic indole derivatives often have strong physiological activities as shown by ergot alkaloids and some hallucinogens [5]. Intercalation of the indole ring of tryptophan-containing oligopeptides between the base pairs of D N A is also widely known [6]. Complex formation with the indole nitrogen normally takes place with alkali metals and Grignard derivatives to form ionic metal-nitrogen bonds [7], but transition metal ions readily form chelates with tryptophan (Trp) through its amino and carboxylate groups as revealed for [Cu(phen)(L-Trp)] [8] and [Cu(bpy)(L-Trp)] [9]. Our recent N M R and X-ray diffraction studies on palladium(II)-alkylindole interactions have established that the indole nitrogen can bind with Pd(II) in the form of the 3H-indole with its N H hydrogen at the C3 position (Fig. 1) [10]. On the other hand, cyclopalladation has been reported to occur with the Pd(II)
M. Takani et al. / Inorganica Chimica Acta 235 (1995) 367-374
368
.CH2COO
IH-indole
H. CH2COO
3H-indole
Fig. 1. Structures of 1H- a n d 3H-indoles with the atomic n u m b e r i n g scheme.
complexes involving nitrogen ligands such as pyridine [11]. For indole-Pd(II) complexes, both the cyclopalladated 3H-indole structure [12] and the 1H-indole structure without cyclopalladation [13] are known. The above Pd(II)-alkylindole complexes are formed through the Pd(II)-N1 bond but without cyclopalladation. As an extension of our studies on the properties of indoles such as aromatic ring stacking with coordinated aromatic rings and interactions with metal ions, we have now studied the Pd(II) complex formation of an indole derivative with a coordinating side chain, indole-3acetate (IA), which is known as a plant hormone (auxin). We here report the synthesis, X-ray structure determination, and properties of novel cyclopalladated complexes of deprotonated IA (IAH_I) with a spiro-ring involving the coordinated tetrahedral atom.
2. Experimental 2.1. Materials Potassium indole-3-acetate (IA) and Na2PdCI4 were purchased from Nacalai Tesque and used without further purification. Deuterated solvents, dimethyl sulfoxide-d6 (DMSO-d6), CDCI 3 and CD3OD were from Aldrich. All other chemicals used were of analytical grade.
2.2. Measurements IR spectra were measured in KBr disks with a Shimadzu IR-460 spectrophotometer. Electronic absorption spectra were recorded on a Union Giken SM401 or a Hitachi 330 recording spectrophotometer. Sample concentrations for these spectral measurements were 0.25-0.3 mM ( = m m o l dm -3) in methanol with respect to Pd(II). 1H and 13C NMR spectra were measured in DMSO-d6, CDC13, CDaOD, or their mixtures with a JEOL JNX-EX 270 spectrometer. Chemical shifts are shown in 6values (ppm) with tetramethylsilane (TMS) as an internal reference.
2.3. Syntheses 2.3.1. NaPd(IAH_~)CI (1) A solution of IA (2.0 g, 9.4 rnmol) and Na2PdCL (1.38 g, 4.7 mmol) in methanol was stirred overnight
at room temperature, and the precipitate was filtered (750 mg) and recrystallized twice from methanol to give a red powder (275 mg; yield 8%). Anal. Calc. for CloH7NO2C1NaPd.0.5H20: C, 34.61; H, 2.32; N, 4.04. Found: C, 34.86; H, 2.60; N, 4.04%. A similar reaction carried out in 50% methanol-water gave Pd(IAH_ 1)" 1.51-120 (1') as an insoluble crude product (yield 40%). Anal. Calc. for caoa7NO2Pd" 1.5H20: C, 39.17; H, 3.29; N, 4.57. Found: C, 39.24; H, 2.92; N, 4.61%.
2.3.2. [ ea(IAn_#(py)l (2) A solution of 1 (150 mg, 0.22 mmol) and pyridine (py) (60 mg, 0.76 mmol) in methanol was stirred for 3 h at room temperature and evaporated, and the residue was recrystallized from CHCI3 to give orange needles (110 mg; yield 55%). Anal. Calc. for C15H12N202Pd •CHCI3: C, 40.20; H, 2.74; N, 5.86. Found: C, 39.95; H, 2.57; N, 5.92%. The reaction of 1' (50 mg, 0.17 mmol) and py (20 mg, 0.25 mmol) carried out in methanol gave 2 (yield 12%) and Pd(IAH_ 1)(PY)"0.5H20 (2'), which was more soluble than 2 and was recrystallized from methanol (yield 36%). Anal. Calc. for C15H12N202Pd" 0.5H20: C, 49.00; H, 3.56; N, 7.62. Found: C, 49.32; H, 3.25; N, 7.64%.
2.3.3. Recovery of IA from 1 A methanol solution of 1 (200 mg, 0.29 mmol) was refluxed with 10% acetic acid (3 ml) for 3 h and evaporated to dryness. The ether soluble fraction was recrystallized from ether-hexane to give pale yellow needles (15.6 mg; yield 15%), m.p. 171 °C (uncorr.), which were identified to be IA (neutral form) by the IR spectrum. 2.4. X-ray structure determination of 2 Single crystals with the composition [Pd(IAH_ 1)(PY)]2" 4CHC13 suitable for X-ray analysis were obtained from a chloroform solution. Crystal data and experimental details are summarized in Table 1. Diffraction data were collected with an Enraf-Nonius CAD4-EXPRESS four-circle automated diffractometer. The crystal was sealed in a glass capillary tube to avoid loss of chloroform from the crystal. The reflection intensities were monitored by three standard reflections for every 2 h, and the decay of intensities was within 2%. The crystal was a very weak diffractor, which resulted in a lack of high angle diffraction data. The intensities of 9959 (9737 symmetrically independent) reflections were measured with the use of the to--2# scan technique in the 20 range 2-54", of which 2809 reflections with lo>3o(lo) were used for structure determination. Reflection data were corrected for Lorentz and polarization effects. An empirical absorption correction, based on ~Oscans, was applied.
369
M. Takani et al. / Inorganica Chimica Acta 235 (1995) 367-374 Table 1 Crystal data and experimental details for [Pd(IAH_I)(py)]2-4CHCI 3 (2)
C3on24N404Pd2-4CHCI3 1194.86
Formula Formula weight Crystal system Space group
monoclinic P2Jc
a (/~) b (~) c (/~) /3 (0) V (.~3) Z p (gcm -3) (cm- i) F(000) Crystal size (ram) A(Mo Ka) (/~) 20 Limit (*) No. reflections measured No. unique reflections No. reflections used (Io> 3o(lo)) R Rw
24.780(2) 9.5077(7) 20.682(2) 111.773(6) 4525.2 4 1.754 15.40 2352 0.4 × 0.2 × 0.2 0.71069 54 9959 9737 2809 0.0595 0.0616
The structure was solved by the heavy-atom method and refined anisotropically for non-hydrogen atoms by full-matrix least-squares calculations. Refinement was continued until all shifts were smaller than one-third of the standard deviations of the parameters involved. Atomic scattering factors and anomalous dispersion terms were taken from the literature [14]. The hydrogen atoms were included as a fixed contribution in the last cycle; their positions were located at the positions ideally calculated (C-H = 0.95/1,), and their temperature factors were assumed to be isotropic (B = 8.00/~2). The final R and Rw values were 0.0595 and 0.0616, respectively. The weighting scheme w - 1= (o.2(Fo) + (0.015Fo)2) was employed for the crystal. The final difference Fourier map did not show any significant features. The calculations were performed on a Micro VAX-3100 computer by using the program system SDP-MolEN [15]. The final atomic parameters for non-hydrogen atoms are given in Table 2, and the selected bond lengths and angles are listed in Table 3.
3. Results and discussion
3.1. Synthesis and spectra The reaction of Pd(II) with IA in the ratio of 1:2 gave complex 1, which reacted with py to give 2. The elemental analyses show that IA in 1 is dinegative (IAH_I) and that 2 is formed by substitution of the coordinated chloride ion of 1 with py. From comparison of the isolated complexes the four Pd(II) coordination sites do not seem to be occupied in both 1 and 2,
Table 2 Atomic
coordinates
and
thermal
parameters
for
[Pd(IAH_ l)(py)]: •4CHC13 (2) Atom
x
y
z
neq a (/~)
Pd(1) Pd(2) C1(1) C1(2) C1(3) C(1C) C1(11) C1(12) C1(13) C(2C) C1(21) C1(22) CI(23) C(3C) C1(31) b C1(32) b C1(33) b C1(34) b C1(35) b C1(36) b C(4C) O(1I) O(2I) N(II) C(2I) C(3I) C(3AI) C(3BI) C(4I) C(5I) C(6I) C(7I) C(8I) C(9I) O(llI) O(12I) N(llI) C(12I) C(13I) C(13AI) C(13B1) C(14I) C(15I) C(16I) C(17I) C(18I) C(19I) N(1P) C(2P) C(3P) C(4P) C(5P) C(6P) N(llP) C(12P) C(13P) C(14P) C(15P) C(16P)
0.25241(5) 0.24688(5) 0.8760(2) 0.8135(3) 0.9204(2) 0.5372(9) 0.5407(4) 0.5066(4) 0.4885(3) 0.9484(8) 0.9589(3) 0.9975(3) 0.9574(3) 0.3542(6) 0.4059(5) 0.3922(5) 0.3019(5) 0.3228(5) 0.3728(6) 0.4156(4) 0.8577(7) 0.2133(4) 0.1695(5) 0.2103(5) 0.2342(5) 0.2047(6) 0.2035(6) 0.1945(5) 0.1501(6) 0.0974(7) 0.0540(6) 0.0614(6) 0.1140(6) 0.1557(6) 0.2813(4) 0.3239(5) 0.2907(5) 0.2661(6) 0.2959(7) 0.2964(6) 0.3015(6) 0.3496(6) 0.4007(7) 0.4453(6) 0.4363(7) 0.3866(6) 0.3429(5) 0.2972(5) 0.3217(6) 0.3495(8) 0.3555(7) 0.3307(6) 0.3047(6) 0.2022(5) 0.1814(6) 0.1504(7) 0.1390(7) 0.1624(7) 0.1919(7)
0.1367(1) -0.2657(1) 0.0075(7) 0.0918(6) - 0.0567(8) 0.326(3) 0.4971(9) 0.305(1) 0.244(1) 0.032(2) 0.1483(7) -0.1067(8) 0.1160(8) 0.089(2) 0.111(i) 0.159(1) 0.217(1) 0.226(1) 0.116(2) 0.023(1) -0.035(2) 0.208(1) 0.140(1) -0.187(1) -0.149(1) - 0.046(1) - 0.036(2) 0.115(2) -0.039(1) 0.034(2) 0.019(2) - 0.061(2) -0.134(2) -0.119(1) -0.334(1) -0.271(1) 0.060(1) 0.021(2) - 0.082(2) -0.090(2) -0.244(2) -0.098(1) -0.169(2) -0.158(2) -0.079(2) -0.003(2) -0.015(1) 0.327(1) 0.372(2) 0.500(2) 0.580(2) 0.535(2) 0.410(2) -0.455(1) -0.499(2) -0.618(2) -0.702(2) -0.662(2) -0.537(2)
0.44160(6) 0.58440(5) 0.3444(2) 0.2040(3) 0.2394(3) 0.629(1) 0.6529(5) 0.5416(4) 0.6637(4) 0.8511(9) 0.7937(3) 0.8668(4) 0.9289(3) 0.2738(7) 0.2362(6) 0.3645(5) 0.2436(6) 0.2170(6) 0.3573(7) 0.2582(5) 0.2568(8) 0.3438(4) 0.2347(5) 0.4887(5) 0.4454(6) 0.3943(7) 0.3207(7) 0.2966(6) 0.4078(6) 0.3743(7) 0.3997(8) 0.4585(8) 0.4933(7) 0.4651(6) 0.6827(4) 0.7933(4) 0.5381(5) 0.5824(7) 0.6321(7) 0.7045(7) 0.7306(7) 0.6173(7) 0.6492(8) 0.6236(8) 0.5651(8) 0.5333(8) 0.5611(7) 0.4688(5) 0.5350(7) 0.5520(8) 0.5004(8) 0.4338(7) 0.4205(7) 0.5521(5) 0.4855(7) 0.4653(7) 0.5113(8) 0.5802(7) 0.5980(7)
2.78(3) 2.68(3) 8.1(2) 8.7(2) 9.8(2) 9.3(8) 15.7(4) 16.6(3) 13.7(3) 7.3(6) 8.7(2) 12.6(3) 11.2(2) 4.9(5) 9.5(4) 7.2(3) 8.4(3) 9.8(4) 11.0(4) 5.8(2) 5.1(5) 3.6(3) 5.0(3) 2.8(3) 2.2(3) 2.7(4) 3.7(4) 2.7(4) 2.6(4) 3.5(4) 4.2(5) 4.0(4) 4.2(4) 2.5(4) 3.2(3) 4.9(3) 3.3(3) 3.1(4) 3.4(4) 3.3(4) 3.6(4) 3.4(4) 4.2(5) 4.9(5) 4.6(5) 4.1(5) 2.4(4) 2.8(3) 4.2(4) 5.4(5) 4.7(5) 4.2(4) 3.6(4) 3.2(3) 3.6(4) 5.2(5) 5.1(5) 4.8(4) 4.0(5)
"Anisotropically refined atoms are given in the form of the isotropic equivalent displacement parameter defined as Beq= (4/3)[a2/3(1,1)+
b2/3(2,2) +c2/3(3,3)+ab(cos 3,)/3(1,2)+ac(cos /3)/3(1,3)+be(cos a)/3 (2,3)]. b The CI atoms of the chloroform molecule disordered around C(4C) were isotropically refined with 0.5 occupancy.
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M. Takani et aL / Inorganica Chimica Acta 235 (1995) 367-374
Table 3 Selected bond lengths (,~,) and angles (°) for [Pd(IAH_t)(py)]2.4CHCI3 (2) Bond length Pd(1)-O(lI) Pd(1)-N(1P) Pd(1)-N(11I) Pd(1)-C(3I)
2.008(8) 2.084(10) 2.002(10) 2.124(12)
Pd(2)-O(11I) Pd(2)-N(llP) Pd(2)-N(II) Pd(2)-C(laI)
2.006(7) 2.093(11) 1.992(10) 2.145(14)
Bond angle O(II)-Pd(1)-N(1P) O(II)-Pd(1)-N(llI) O(II)-Pd(1)-C(3I) N(1P)-Pd(1)-N(1 lI) N(1P)-Pd(1)-C(3I) N(llI)-Pd(1)--C(3I) Pd(1 )--C(3I)-C(2I) Pd(1)--C(3I)-C(aAI) Pd(1)-C(3I)--C(4I) C(2I)--C(3I)-C(3AI) C(2I)--C(3I)--C(4I) C(3AI)-C(3I)--C(4I)
89.4(4) 178.2(4) 80.4(4) 92.3(4) 169.1(5) 97.9(5) 100.1(8) 102.3(9) 105.4(9) 127.0(13) 98.4(12) 120.5(10)
O(llI)-Pd(E)-N(II) O(llI)-Pd(2)-N(llP) O(11I)-Pd(2)--C(13I) N(lI)-Pd(E)-N(11P) N(lI)-Pd(2)-C(13I) N(llP)-Pd(2)-C(13I) Pd(2)--C( 13I)--C(121) Pd(2)--C(13I)-C(13AI) Pd(2)-C(13I)-C(14I) C( 12I)-C(13I)-C(13AI) c(121)-c( 13I)-.c(14I) C(13AI)-C(13I)-C(14I)
176.2(4) 91.1(4) 81.0(4) 90.9(4) 97.0(5) 172.0(5) 100.0(8) 102.4(10) 103.0(9) 124.7(14) 100.7(13) 121.9(11)
suggesting that the complexes may be dimeric. Since 1 gave IA upon refluxing with 10% acetic acid in methanol, coordinated IA (IAH_I) appeared to retain its indole structure in 1. However, the three characteristic peaks for IA at 266, 280 and 289 nm observed for the 1:2 Pd(II)-IA system prepared by mixing Pd(II) and IA were not detected in the spectra for 1 and 2 (Table 4). The spectra in the region 300-500 nm exhibited two unusually strong peaks at 336 and 440 nm ( e - 2900-3800 M -1 crn -1) and 326 and 420 nm (e= 4500-5900 M -1 cm-~), respectively, where Na2PdC14 has two bands with lower intensities (E< 1000 M -~ cm- 1). The IR and ~H NMR spectra of the isolated complexes 1 and 2 showed no NH signals expected for the indole ring (Table 5). The 1H and 13C NMR spectra were assigned by two-dimensional correlation spectroscopy (XHJH and ~H-~3C COSY). The 1H NMR spectra revealed that the doublet peaks (J= 1 Hz) of the C H 2 protons of IAH_ ~ are split into AB-type quartet peaks (J= 17.2-17.3 Hz) in 1 and 2 showing that the two protons are under different conditions probably due to
the neighboring groups. One of the CH2 protons of coordinated IA is shifted upfield relative to IA by 0.85 and 0.81 ppm in 1 and 2, respectively. The spectral data in Table 5 further indicate that the C2 proton (H-2) signal of 1 and 2 is shifted upfield ( A S = - 0 . 6 9 and -0.47 ppm, respectively) relative to that of IA, whereas the C7 proton (H-7) signal is shifted downfield (AS= 1.15 and 1.05 ppm, respectively). The a protons of py (py-aH) in 2 suffer an upfield shift (AS= -0.47 ppm) relative to those of Pd(py)C12. The 13C NMR spectrum of 2 indicates some characteristic signals, from which we see that: (i) C3 is tetrahedral with a considerably low chemical shift (8 61.6 ppm); (ii) C2 (8 163.5 ppm) is more like an imine carbon [10,12,16] rather than an aromatic carbon [17]; (iii) CTa and the carbons of the carboxylate and methylene groups are shifted downfield (AS= 13.0, 5.6 and 2.4 ppm, respectively) (Table 6). Our previous study revealed that Pd(II) coordinates the indole ring at the nitrogen atom with the NH proton shifted to the C3 atom (3H-indole structure) [10]. Because the 1H NMR spectra of I and 2 exhibited no H-3 signals and elemental analyses were
Table 4 Absorption spectral data System
Jr,..:, (nm) (e)
Pd-IA (1:2)
219 289 210 336 210 420 218 326
NaPd(IA)CI (1) Pd(IA)(py) (2) Pd(IA)(py((2')
(28000) (5400) (end, 27000) (3800) (end, 33200) (4500) (28200) (4100)
266 (850o) 333 (4100) 242 (sh, 12200) 440 (2900) 267 (18300)
280 (740o) 438 (290o) 266 (12200)
260 (16200) (39o0)
288 (8500)
4o4
326 (5900)
M. Takani et aL / lnorganica Chimica Acta 235 (1995) 367-374
371
Table 5
t H NMR spectral data (270 MHz)" Proton
NaPd(IA)CI (1)b
Pd(IA) (1')b
Pd(IA)(py) (2)c
Pd(IA)(py) (2')c
IA b
IA c
CH2
2.66(d, 1H) J = 17.3 3.56(d,lH) J = 17.3
2.80(d, 1H) J = 17.5 3.53(d, 1H) J = 17.5
2.81(d, 1H) J = 17.2 3.68(d, 1H) J = 17.2
2.89(d, 1H) J = 17.8 3.54(d, 1H) J = 17.8
3.51(s, 2H)
3.62(d, 2H) J = 1.0
H-2
6.45(s)
8.36(s)
6.65(s)
8.41(s)
7.14(s)
7.12(s)
H-4
7.74(d) J=7.3 7.39(0 J=7.4
7.62(d) J=7.6 7.10(t) J=7.5
7.88(d) J=7.5 7.47(t,d) J=7.5, <1.0
7.76(d) J=7.5 7.14(m)
7.55(d) J=7.9 6.93(t,d) J=7.7, <1.0
7.61(d,m) J=7.5, < 1.0 6.99(t,d) J=7.5, <1.0
7.55(t) J=7.4
6.71(t) J=7.6
7.55(t,d) J=7.5, 1.5
7.02(t,d) J=7.7, <1.0
7.07(t,d) J=7.5, 1.3
8.45(br.s)
6.65(d) J=7.6
8.37(d) J=7.6
7.30(d) J=7.9
7.32(d,t) J=7.5, <1.0
H-5 H-6 H-7 py-a
6.75(m, 2H)
8.34(d,d)
8.42(m)
Pd(py)Cl2,
8.81(d,m)
J = 6.4, 1.3
J ~ 5.0, 1.0
py-/3
7.36(m)
7.38(m)
7.38(m)
py-~/
7.86(t,t) J = 7.6, 1.3
7.88(t,t) J = 7.5, 1.5
7.82(t,t) J = 7.6, 1.5
NH
10.78(br.s)
" Chemical shifts 8 in ppm and coupling constants J in Hz. b In DMSO-d6---CDCI3. c In CDCIa-CDaOD. Table 6
13C NMR spectral data (67.8 MI-Iz) ° Carbon
Pd(IA) (1') ~
Pd(IA)(py) (2) °
Pd(IA)(py) (2') °
CH2 C2 C3 C3a CA C5 C6 C7 C7a COO
35.5 166.6 63.9 138.7 120.0 123.9 124.8 117.0 146.6 186.0
37.9 163.5 61.6 136.1 121.7 125.7 126.1 120.1 150.5 186.6 150.6 125.8 139.5
36.6 168.9 59.0 140.2 120.8 125.6 124.7 117.5 147.1 188.2 151.1 126.0 139.7
py-ot
py-fl PY-3'
Pd(py)Cl2 ~
IA ~
35.5 123.7 112.0 128.7 119.5 119.1 121.8 111.7 137.5 181.0 153.3 125.2 138.7
" Chemical shifts 8 in ppm.
b In DMSO-d6-CDCI3. c In CDCI3-CD3OD.
consistent with IAH_ a, H-3 is concluded to be lost due to palladation. On the other hand, the amount of Pd(IAH_~) in the solutions, prepared by mixing Pd(II) and IA in the ratio of 1:2 in methanol and kept standing for one day, was estimated from the NMR spectra to be only about 9% of the total Pd(II) used. The IR spectra exhibited the C = O stretching band for IA (neutral form), 1, 2 and IA (ionized form) at 1730, 1610, 1630 and 1560 cm-1, respectively, which indicates
that the carboxylate group is coordinated to Pd(II) in 1 and 2. From the above spectral data we conclude that complexes 1 and 2 involve IAH_ 1 in the 3H-indole structure coordinated through the nitrogen, the C3(sp 3) carbon and the carboxylate oxygen.
3.2. Molecular structure of 2 revealed by X-ray analysis The crystal structure revealed by X-ray analysis consists of four discrete molecules [Pd(IAH_l)(py)]2 and sixteen chloroform molecules in a unit cell. Fig. 2 shows the molecular structure of 2 with the atomic numbering scheme which is different from that in Fig. 1 used for the spectral assignment. It has a pseudo-Ci symmetry for the center of the P d . . . P d vector and forms a dimeric structure bridged by the indole rings. Pd(II) binds with the carboxylate oxygen and the C3 atom of the indole ring (numbered C3I and C13I in Fig. 2) to form a novel five-membered spiro chelate ring as a result of cyclopaUadation. The nitrogen atoms of two py molecules occupy positions in a square-planar geometry with a tilting angle of 20.8 and 21.6 °, and the indole nitrogen atom ( N I = N I I and N l l I in Fig. 2) coordinates to a neighboring Pd(II), forming a dimeric structure. The N1--C2 bonds of the two bridging indole rings are involved in unusually short contact with each other with the distance of 2.75/k. The distance between the two Pd(II) ions is 4.865(2)/~. The Pd-N1 bonds,
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M. Takani et al. / Inorganica Chimica Acta 235 (1995) 367-374
O(21)
C(61)
~
C
(
2
P
~C(14P)
)
C
(
I
~
~
C(16I) O(12I) C(4P) C(3P) Fig. 2. Molecularstructure of 2 with the atomicnumberingschemewhichis used solelyfor Tables 2 and 3. 2.002(10) and 1.992(10) /~, are slightly shorter than those detected for [Pd(2-methylindole)2Cl2] (2.024(7) A) and [Pd(2,5-dimethylindole)2C12] (2.02(3) /~) [10], and the Pd-carboxylate oxygen bonds, 2.008(8) and 2.000(8)/~, are at the short end of the values previously reported (2.006-2.029 A) [18] within e.s.d. The Pd-C3 bonds (2.124(12) and 2.145(14)/~) and the Pd-pyridine nitrogen bonds (2.084(10) and 2.093(11)/~) are within the normal ranges 2.003-2.144 and 2.033-2.149/~, respectively, reported for other cyclopalladated complexes [lla,e,18,19].
3.3. Structural transformation of [ PdCI,] z- to cyclopalladated dimeric complexes in solution Na2PdCL reacted with IA (potassium salt) in methanol to give 1 as a crystalline powder, which was converted to 2 by the reaction with py in methanol. When 1 was dissolved in DMSO-d6-CDCI3, it gave the 1H NMR spectrum with a singlet peak of H-2 at 6.45 ppm corresponding to an imine proton as expected from the 3H-indole structure, but upon standing for one day the original spectrum changed to give a spectrum which was similar to that of 1' (Table 5). The result and the procedure of isolation of 1' indicate that the Pd(II)-N1 bond is cleaved in the presence of water or DMSO. A large chemical shift difference (1.65 ppm) was observed between the H-2 signals of 2 and 2' (Fig. 3), and a similar shift (1.91 ppm) was also observed between 1 and 1' (Table 5). On the basis of the NMR spectra and the dimeric structure of 2 shown in Fig. 2, we conclude that 1' and 2' are monomers which are devoid of the Pd(II)-N1 bonds and that the spectral changes are due to formation of the monomers which causes the downfield and upfield shifts of the H-2 and H-7 signals, respectively, due to the cleavage of the Pd(II)-N1 bonds; the ring current effect resulting from
the close contact between the five-membered rings of the indole nuclei disappears upon bond cleavage to cause the downfield shift of H-2. The 8 values for H-2 of 1' and 2' (8.26--8.41 ppm) correspond with the value of ~ 8.0 ppm reported for 3H-indole [20]. Complex 1 which has the same 1:1:1 ratio of Pd(II) and two ligands as in 2 is concluded to have a similar dimeric structure, the chloride ion of which is most probably replaced by py to give 2 as shown in Scheme 1. Refluxing 1 in methanol with added acetic acid gave IA in the neutral form, which indicates that 1H- and 3H-indole structures are interconvertible under the conditions employed.
4. Conclusions
Reactions of [PdCl4] 2- with IA in methanol gave a dimeric complex 1, which further reacted with py to give complex 2 as crystals. X-ray analysis revealed the coordination structure with a unique spiro-ring formed as a result of cyclopaUadation at the C3 atom of the 3H-indole ring. One of the chelate rings is a fivemembered ring composed of C3 and the side chain CH2COO- moiety, and the other ring is a macrocyclic (eight-membered) ring formed by the two bridging N1 atoms. In solution the complexes gradually undergo structural changes probably due to the reaction with water or DMSO molecules leading to cleavage of the Pd-N1 bridge. Since Pd-C bond formation easily occurs under mild conditions, similar reactions may be possible in Pd(II)-catalyzed reactions and reactions involving Pd(II) and indole derivatives. The cyclopalladation in the present case may give rise to stereoisomers due to the asymmetric (23 atom, but 1 and 2 are optically inactive, indicating that they are racemic mixtures. It is interesting to note in this
M. Takani et al. / Inorganica Chimica Acta 235 (1995) 367-374
373
5. Supplementary material (a) H-7
py-olt
H-2
Tables of positional parameters and their estimated standard deviations, refined displacement parameter expressions (beta), general displacement parameter expressions (U), bond lengths, bond angles, torsion angles, and observed and calculated structure factors are available from the authors upon request.
py-~-I
py-TH
H-4
m
H-6
Acknowledgements ..,
The authors express their sincere thanks to the Ministry of Education, Science and Culture of Japan for the Grant-in-Aid for Scientific Research (No. 06640719) to M.T. and the Grant-in-Aid for Scientific Research on Priority Areas (Bioinorganic Chemistry) (No. 04225102) to O.Y. (b)
References H-2 I"1-7 H4
py~ H-4
py-i~H
py-yH
f
~
f
H-$
&ppm 8.5
8.0
7.5
7.0
6.5
Fig. 3. tH NMR spectra of Pd(IA)(py) (2) (a) and Pd(IA)(py) (2') (b) in CDCI3.
PY
-
2
1 Scheme 1.
connection that the side chain CH2 protons are in different environments as indicated by the AB-type coupling in the 1H NMR spectrum (Table 5). The present findings will add to our knowledge of versatile bonding modes, reactivities and electronic properties of the indole ring in metal complexes and possibly in biological systems.
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