Low valent palladium benzoquinone complexes bearing different spectator ligands. The versatile coordinative capability of benzoquinone

Journal of Organometallic Chemistry 749 (2014) 379e386

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Low valent palladium benzoquinone complexes bearing different spectator ligands. The versatile coordinative capability of benzoquinone Luciano Canovese a, *, Fabiano Visentin a, Claudio Santo a, Valerio Bertolasi b a b

Dipartimento di Scienze Molecolari e Nanosistemi, Università Ca’ Foscari, 30129 Venice, Italy Dipartimento di Chimica e Centro di Strutturistica Diffrattometrica, Università di Ferrara, Ferrara, Italy

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 September 2013 Received in revised form 10 October 2013 Accepted 11 October 2013

The synthesis of some benzoquinone palladium complexes bearing different spectator ligands was carried out and the hapticity of the coordinated olefin was inferred from the features of their 1H and 13C NMR spectra. It was shown that benzoquinone coordinates either h2 and h4 and that its coordinative choice is not easily predictable, although the h2 coordination seems to be predominant in the presence of rigid ancillary ligands. The coordinative capability of benzoquinone was tested by means of thermodynamic and kinetic reference reactions and its slightly enhanced inertness with respect to the isofunctional naphthoquinone was assessed. Finally, the re-crystallization by slow diffusion at low temperature of diethylether in a dichloromethane solution of the complex [Pd(h2-bq)(TTbQ-Me)] (bq ¼ benzoquinone, TTbQMe ¼ 8-t-Butylsulfanyl-2-methyl-quinoline) allows the separation of the dimer [Pd2(h2-bq)(TTbQ-Me)2] whose solid state structure was resolved. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: Palladium olefin complexes Benzoquinone hapticity Olefins exchange equilibria

1. Introduction The Pd(0) olefin derivatives are of paramount importance among all the palladium catalysts since they represent or are often identified as the active catalytic species in a variety of crosscoupling reactions [1]. On the basis of the original work of Dewar and Chatt and Duncanson [2] the palladium(0)eolefin bond was extensively studied from theoretical, structural [3], and synthetic points of view [4]. In particular, our group have in some cases measured the degree of stability imparted to the palladium(0) complexes by different olefins. The evaluation of the stability was based on the equilibrium constant of the direct exchange between olefins according to the following reaction [5a,b]:

h

   i h  i Pd h2 ol1 ðLeL0 Þ þ ol2 # Pd h2 ol2 ðLeL0 Þ þ ol1

(1)

A summary of the general results suggesting a comprehensive order of coordinative properties of the most used electron-poor olefins based on the complex [Pd(h2-nq)(Neocup)] (nq ¼ naphthoquinone; Neocup ¼ neocuproine) together with an indication of the dependence of such an order on the nature of the ancillary ligands, was recently reported [6]. Finally a general

* Corresponding author. E-mail address: [email protected] (L. Canovese). 0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jorganchem.2013.10.021

overview on the features of the Pd(0) olefin derivatives bearing labile or hemilabile spectator ligands was reviewed [7]. However, among the number of studied complexes the benzoquinone derivatives are not very abundant [8] and no quantitative data on its coordinative capability can be found in the literature. This fact probably depends on the nature of benzoquinone whose hapticity might be either h2 or h4 and consequently, formation of dimeric [8c] or oligomeric species via the unengaged olefinic vinylic bond cannot be excluded a priori [8e]. Moreover a further complication can arise from the fact that benzoquinone and its isofunctional naphthoquinone can give dimeric oxygen bridged species [8d]. We therefore decided to prepare some palladium(0) complexes with benzoquinone with the aim of setting its coordinative capability in the rank of the olefin stability order so far assessed [6] and understanding the structural features of the synthesized derivatives. The benzo- and naphthoquinone palladium complexes we have prepared are reported in the following Scheme 1.

2. Results and discussion 2.1. General considerations The palladium(0) olefin complexes were obtained according to protocol (a) of Ref. [7] by the concomitant addition of the appropriate alkene (ol) and ligand (LeL0 ) to Pd2DBA3 [9] in anhydrous

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Scheme 1. Ligands and complexes synthesized and studied.

acetone under inert atmosphere (Ar), in the dark. The formation of the complexes was unequivocally apparent by comparison of their 1 H and 13C NMR spectra with those of the free olefins and from their IR spectra. As a consequence of the marked metaleolefin back donation the chemical shifts of the protons and carbons of the coordinated olefins resonate at 1e2.5 and 40e80 ppm upfield with respect to the uncoordinated alkenes, respectively. Moreover, in the IR spectra of the Pd(0) derivatives the nC]O stretching are shifted at lower frequencies than those of the uncoordinated quinones (DnC] 1 O ¼ 25 O 30 cm ). 2.2. Complex [Pd(h4-bq)(NeSt-Bu)] We have firstly investigated the reaction between the flexible ligand NeSt-Bu, the complex Pd2DBA3 and benzoquinone. The CDCl3 1H NMR spectrum at 298 K of the isolated red compound

indicated an equimolecular ratio between the NeSt-Bu ligand and benzoquinone with the signal of the CH2-S system and of the four olefinic protons resonating as a singlet at 4.12 ppm and as an AB system centered at 5.63 ppm, respectively (Fig. 1a). Consistently, the olefinic carbons in the RT 13C NMR spectrum resonate as a couple of signals at 100.0 and 103.8 ppm (Fig. 1b). Both NMR spectra can be explained by invoking the coordination of the olefin and the rapid inversion of the sulfur configuration of the pyridylthioether ligand. It is well known that the rapid inversion of the sulfur configuration can often be frozen [10] and therefore it might be possible to obtain some Supplementary information about the structure of the complexes by lowering the temperature. In Fig. 1c and d the low temperature (193 K) 1H and 13C NMR spectra of the complex are reported and in particular in Fig. 1c an AB system related to the diastereotopic CH2-S thioetheric protons can be detected at 4.12 ppm. This suggests that at 193 K the position of the

L. Canovese et al. / Journal of Organometallic Chemistry 749 (2014) 379e386

Fig. 1. 1H and

C NMR spectra of the complex [Pd(h4-bq)(NeSt-Bu)] recorded at 298 (a, b) and 193 K (c, d).

13

t-Bu substituent of sulfur is frozen with respect to the coordination plane of the complex. Consistently, the four protons of the benzoquinone resonate as a couple of AB systems centered at 5.42 and 5.51 ppm (DdH ¼ 0.09 ppm), respectively, while the original RT signals of the benzoquinone carbons at 100.0 and 103.8 ppm (Fig. 1b) split into four different peaks at 94.5, 99.5 and 104.5, 108.2 ppm, respectively (DdC ¼ 13.7 ppm) (Fig. 1d). The observed NMR features suggest the h4 hapticity of the benzoquinone in the [Pd(h4-bq)(NeSt-Bu)] complex. As a matter of fact, the position and closeness of the olefinic protons (DdH ¼ 0.09 ppm) and carbons (DdC ¼ 13.7 ppm) in the low temperature spectra indicate that the olefinic atoms are all involved in the bonds with the metal, their differences in chemical shifts being only dependent on the asymmetry of the NeSt-Bu ligand. 2.3. Complex [Pd(h2-bq)(TTbQ-Me)] For comparison with the above reported derivative of the flexible NeSt-Bu ligand, we have synthesized the complex [Pd(h2bq)(TTbQ-Me)] bearing the rigid TTbQ-Me as NitrogeneSulfur spectator ligand. We thought that different fluxional behavior could have impart different structural behaviour to its derivatives. As can be seen in Fig. 2a and b the CDCl3 1H and 13C NMR spectra of the complex [Pd(h2-bq)(TTbQ-Me)] recorded at RT do not seemingly differ from those of the above described NeSt-Bu derivative. The olefinic protons and carbons resonate as an AB system centered at 5.62 and a broad singlet at 100.5 ppm, respectively,

Fig. 2. 1H and

381

13

whereas the low temperature (193 K) spectra are remarkably different from those described before (Fig. 2c and d). In such a case the olefinic protons split into a couple of well separated AB systems centered at 4.82 and 6.18 ppm (DdH ¼ 1.36 ppm) while the olefinic carbons are detected as two widely separated couples of four distinct signals at 135.3, 134.5 and 67.6, 63.5 ppm (DdC ¼ 71.8 ppm). Apparently, the reported NMR spectra describe a quite different coordinative situation of the olefinic fragment which in this latter case might coordinate h2, the signals at 193 K at 63.4 and 67.6 and the AB system at 4.71 and 4.92 ppm being related to the atoms of the coordinating double bond while the protons and the carbons at 6.10, 6.22 and 134.5, 135.3 ppm, respectively, can be traced back to the uncoordinated olefinic atoms laying out of the coordination plane of the complex. Notably, other authors reached a similar conclusion in a detailed study of benzoquinone derivatives of Pt(0) [11]. Moreover, we know that naphthoquinone, thanks to its peculiar structure, coordinates h2 [5b,12]. Thus, we have synthesized on purpose and for comparative aims the complex [Pd(nq)(TTbQ-Me)]. The palladium coordinated olefinic carbons of such a complex resonate at RT at 59.5 and 62.7 ppm while the protons were observed as an AB system at 4.83 and 4.96 ppm. These signals are almost coincident with those related to the coordinating part of the olefin fragment observed in the low temperature 1H and 13 C NMR spectra of the complex [Pd(bq)(TTbQ-Me)]. As a countercheck we have re-synthesized the complex [Pd(h2-nq)(NeStBu)] [5b] and its low temperature 1H and 13C spectra (at variance with the case of the related species [Pd(h4-bq)(NeSt-Bu)] described

C NMR spectra of the complex [Pd(h2-bq)(TTbQ-Me)] recorded at 298 (a, b) and 193 K (c, d).

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above) unequivocally suggests an h2 coordinative mode for the naphthoquinone olefin with its olefin protons and carbons resonating at 4.9 ca. and within 60 O 65 ppm, respectively. However, the freezing of the sulfur inversion of the complex [Pd(bq)(TTbQ-Me)] would generate a couple of diastereoisomers (endo and exo [5b]) and consequently eight different groups of signals for the protons and carbons of the benzoquinone system. Since at the attained temperature only four groups of signals are detected, we think that only the fluxional behaviour of the benzoquinone involving the rapid interchange of the two double bonds of the olefin is frozen while sulfur inversion is still operative this phenomenon being strongly influenced by the nature of the ancillary ligands and therefore not always reproducible within a narrow temperature interval [5b,6,10,13]. The most evident difference between the NeSt-Bu and TTbQ-Me ligands arises from the different rigidity of the chelating ring and at first sight it seems reasonable to think that, ceteris paribus, the more flexible structure of the spectator ligand NeSt-Bu would support the h4 hapticity which requires a tetrahedral distortion around the palladium center [8d]. We have therefore synthesized the complexes [Pd(h2bq)(Neocup)], [Pd(h4-bq)(Dic)2] and [Pd(h4-bq)(DPPQ-Me)] in order to verify the coordinative tendency of benzoquinone on the basis of NMR spectral analogies with the derivatives described above. The initial impression that the rigidity or flexibility of the scaffold of the complexes respectively influence the h2 or h4 coordination mode of the olefin, although somehow confirmed by the complexes [Pd(h2-bq)(Neocup)] (1H NMR at 193 K; one signal at 4.60 for the protons of the coordinated double bond and at 6.12 ppm for those of the uncoordinated one) and [Pd(h4bq)(Dic)2] (1H NMR at 193 K: only one signal at 5.58 ppm, 13C NMR at 193 K: only one signal at 101.9 ppm), failed in the case of the rigid DPPQ-Me ligand whose derivative unequivocally displays a h4 coordinated benzoquinone. As a matter of fact, the 1H NMR spectrum at 193 K displays an AA0 BB0 X multiplet at 5.6 ppm ca. due to the concomitant coupling among the protons and phosphorus (see Supplementary material), whereas the olefin carbons resonate as a couple of signals within 100 and 105 ppm with the signal of the carbon trans to phosphorus resonating as a doublet centered at 104.2 ppm with JCP ¼ 8.7 Hz. Apparently, the DPPQ-Me ligand rigidity is somehow offset by its steric and/or electronic characteristics which play complementary roles that are hard to disentangle [14].

KE ¼ ½Pdðol2 ÞðLeL0 Þ$½ol1 =½Pdðol1 ÞðLeL0 Þ$½ol2  ½Pd0 ¼ ½Pdðol2 ÞðLeL0 Þ þ ½Pdðol1 ÞðLeL0 Þ ½ol1  þ ½ol2  ¼ ½ol2 0 Dl ¼ ε1 $½Pdðol1 ÞðLeL0 Þ þ ε2 $½Pdðol2 ÞðLeL0 Þ where ε1 and ε2 are the extinction coefficients of the complexes [Pd(ol1)(LeL0 )] and [Pd(ol2)(LeL0 )], respectively with KE and ε2 as the parameters to be optimized. The extinction coefficient ε2 turned out to be coincident with that directly determined from the LamberteBeer analysis carried out at the same wavelength for the independently synthesized final product. The non-linear regression analysis of the spectrophotometric titration of the complex [Pd(h4nq)(NeS-tBu)] with benzoquinone (bq) is reported as an example in Fig. 3. The determined equilibrium constants are collected in Table 1 whereas all the related regression data can be found in the Supplementary material. As an internal check of consistency the reaction between the complex [Pd(h4-bq)(NeSt-Bu)] and naphthoquinone was also carried out and the ensuing equilibrium constant (KE1 ¼ 0.51  0.03) is in full agreement with the result of the reverse reaction reported in Table 1 (KE ¼ 2.2  0.1). The data in Table 1 clearly indicate that irrespectively of the hapticity of the final derivative, benzoquinone is slightly more coordinating than naphthoquinone, probably thanks to its reduced steric hindrance. Exploiting the remarkable bulkiness of the olefin tetramethylethylenetetracarboxylate (tmetc) we have also determined the reaction rates involved in the equilibrium reaction:

h   i Pd h4  bq ðNeSt  BuÞ   i kf h þ tmetc # Pd h4  tmetc ðNeSt  BuÞ þ bq kr

First of all we have measured the rate of the forward reaction kf by reacting the starting complex [Pd(h4-bq)(NeSt-Bu)] ([Pd]0 z 1  104 mol dm3) with a constant excess of tmetc ([tmetc]0 ¼ 1  102 O 4  102 mol dm3) in order to drive the equilibrium to completion (z96%), minimize the contribution of the reverse reaction (kr) and provide pseudo-first order conditions. Under these conditions the absorbance obeys the monoexponential law Dt ¼ DN þ (D0  DN)exp(kobst) where D0, DN and kobs are the initial, the final absorbance and the observed rate constant, respectively and represent the parameters to be

2.4. Determination of olefin substitution equilibrium and rate constants We undertook this investigation in order to assess the coordination efficiency of h2 or h4 coordinated benzoquinone and compare the results with the stability sequence we have so far established for a number of palladium(0) alkene derivatives. The mutual coordinative capability of two olefins and the related stability order were interpreted on the basis of the magnitude of the equilibrium constant of olefins exchange [5b,6,7]: KE

½Pdðol1 ÞðLeL0 Þ þ ol2 #½Pdðol2 ÞðLeL0 Þ þ ol1

(2)

After a preliminarily investigation of the fast exchange reaction by 1H NMR technique in CDCl3 or CD2Cl2 (see Experimental) we have measured the equilibrium constant by means of a spectrophotometric titration of the complex under study with microaliquots of concentrated benzoquinone solutions in CHCl3 at 298 K. The rapidly established spectral changes were recorded in the 300e 600 nm wavelength interval and analyzed by a non linear leastsquares program based on the following model:

Fig. 3. Fit of absorbance data versus [bq] at 440 nm in CHCl3 for the reaction: ½Pdðh2  nqÞðNeS  tBuÞ þ bq ¼ ½Pdðh4  bqÞðNeSt BuÞ þ nqðKE ¼ 2:2  0:1Þ.

L. Canovese et al. / Journal of Organometallic Chemistry 749 (2014) 379e386 Table 1 KE values determined by spectrophotometric titration in CHCl3 at 298 K for the reaction: ½Pdðh2  nqÞðLeL0 Þ þ bq ¼ ½Pdðhx  bqÞðLeL0 Þ þ nq x ¼ 2; 4. Complex

KE

[Pd(h -nq)(TTbQ-Me)] [Pd(h2-nq)(NeS-tBu)] [Pd(h2-nq)(Neocup)] [Pd(h2-nq)(Dic)2]

2.6 2.2 1.2 1.3

2

   

0.2 0.1 0.1 0.2

optimized by non linear regression analysis of the absorbance versus time data. The calculated values of kobs fit the expression kobs ¼ kf [tmetc] and its linear regression analysis allows the determination of the rate constant of the forward reaction (kf ¼ (8.6  0.3)  103 mol1 dm3 s1; see Supplementary material). The determined kf can be traced back to the kinetic stability of the h4 coordinated benzoquinone and might be compared with the value of the rate constant determined so far [5b] related to the displacement of naphthoquinone from the complex [Pd(h2-nq)( NeSt-Bu)] with tmetc ðk0f ¼ ð2:95  0:04Þ  102 mol1 dm3 s1 Þ. Comparison of the slightly differing rate constants is in agreement with the thermodynamic measurements and appears to confirm the enhanced inertness of benzoquinone also from the kinetic point of view [15]. 2.5. X-ray structure determination 2.5.1. Complex [Pd2(m-bq)(TTbQ-Me)2] An attempt at re-crystallizing the complex [Pd(h2-bq)(TTbQMe)] by slow diffusion of diethylether in dichloromethane solution at low temperature yields the co-precipitation of two compounds with different crystal habits. The 1H NMR spectrum of the CDCl3 solution of the mixture shows the presence in solution of two different species, namely the largely preponderant complex [Pd(h2-bq) (TTbQ-Me)] and a new species displaying an 1:2 ratio of benzoquinone to TTbQ-Me ligand (see Experimental). Only the crystals of the latter were suitable for an X-ray analysis and the ORTEP [16] view of the ensuing structure is reported in Fig. 4 whereas a selection of bond distances and angles is given in Table 2. The compound is a

383

dinuclear Pd(0) complex containing a pair of neutral TTbQ-Me molecules and one p-benzoquinone as ligands. Each palladium is bonded to the heterodienic ligand through the quinoline N and the S of the t-buthylsulphanyl substituent, and h2-coordinated to the p-benzoquinone ligand. The long C1]C2 double bond length of 1.422(9)  A in the p-benzoquinone ligand clearly shows a back donation from the metal centers to the ligand, which is almost perpendicular to the Pd coordination planes, forming angles of 80.6 . The PdePd and PdeC distances are similar to those reported in another dinuclear Pd(0) complex h2coordinated to p-benzoquinone, [Pd2(bq)2(nbe)2] [8d], where at variance with our complex two benzoquinones bridge the two palladium centers. The structure of the complex [Pd2(mbq)(TTbQ-Me)2] is somehow in agreement with our observation about the h2 coordinative tendency of the benzoquinone in the presence of rigid spectator ligands. 3. Conclusions We have synthesized some Pd(0) benzoquinone derivative bearing different spectator ligands. In any case, on the basis of well characterizing 1H and 13C NMR features it was possible to assign the coordinative hapticity of the olefin which in the case of NeS ligands seems to be somehow influenced by the rigidity of the latter. In order to rate the coordinative strength of benzoquinone within the olefin stability order proposed so far, we have measured the equilibrium constant for the displacement of naphthoquinone from its palladium complexes by benzoquinone and shown that benzoquinone is slightly more coordinating. The slightly enhanced thermodynamic stability parallels the slightly higher kinetic inertness of the benzoquinone derivatives which was also measured. Finally the structure of a dimeric complex [Pd2(TTbQ-Me)2(mbq)] co-precipitating with the monomeric complex [Pd(h2bq)(TTbQ-Me)] was determined. 4. Experimental All solvents were purified by standard procedures and distilled under argon immediately before use [17]. 1D- and 2D-NMR spectra were recorded using a Bruker 300 Avance spectrometer. Chemical shifts (ppm) are given related to TMS (1H and 13C NMR). UVeVis spectra were recorded on a PerkineElmer Lambda 40 spectrophotometer equipped with a PerkineElmer PTP 6 (Peltier temperature programmer) apparatus. IR spectra were recorded on a Perkine Elmer Spectrum One spectrophotometer.

Table 2 Selected bond distances and angles ( A and degrees) for [Pd2(m-bq)(TTbQ-Me)2].

Fig. 4. ORTEP view of the dinuclear Pd(0) complex ([Pd2(m-bq)(TTbQ-Me)2]) showing the thermal ellipsoids at 30% probability level.

Distances Pd1ePd10 Pd1eC1 Pd1eS1 C1eC2 C2eC30

3.0023(9) 2.069(6) 2.367(2) 1.422(9) 1.475(9)

Pd1eC2 Pd1eN1 C1eC3 C3eO1

2.140(6) 2.188(5) 1.452(9) 1.246(8)

Angles C1ePd1eS1 C1ePd1eC2 C2ePd1eS1 C2ePd1ePd10 S1ePd1ePd10

112.8(2) 39.4(3) 152.1(2) 81.1(2) 102.20(5)

C1ePd1eN1 C1ePd1ePd10 C2ePd1eN1 S1ePd1eN1 N1ePd1ePd10

163.7(2) 84.9(2) 124.3(2) 83.4(1) 93.9(1)

Dihedral angle Pd1,N1,S1,C1,C2, C1,C2,C3,C10 ,C20 ,C30

80.6(4)

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4.1. Synthesis of the palladium precursor and ligands The palladium precursor Pd2(DBA)3$ CH3Cl [9] and the ligands NeSt-Bu [18], DPPQ-Me [19], Neocup and DIC, were synthesized according to published procedures or are commercial grade products. 4.2. 2-Methyl-8-thioterbutylquinoline (TTbQ-Me) To a solution of KOH (8 g; 142.6 mmol) in DMSO (20 ml), sodium terbutylate (3.20 g; 26.4 mmol) and 8-bromoquinoline (2 g; 9.0 mmol) were added under inert atmosphere (Ar) and the mixture was stirred at R.T. for 5 h and overnight at 80  C. 50 ml of H2O and 50 ml of diethylether were added to the resulting solution and the organic phase was separated and dried on Na2SO4. The suspension was filtered off (G3) and the resulting clear solution was dried under vacuum. The crude product was purified by flash chromatography on silica column with a mixture of CH2Cl2/Et2O (70/30 v/v). 0.95 g (46% yield) of the title product was obtained upon evaporation of the solvent. 1 H NMR (CDCl3, T ¼ 298 K, ppm) d: 1.40 (s, 9H, tBu), 2.81 (s, 3H, CH3 quinoline), 7.31 (d, 1H, J ¼ 8.4 Hz, H3), 7.44 (dd, 1H, J ¼ 8.1, 7.2 Hz, H6), 7.77 (dd, 1H, J ¼ 8.1, 1.4 Hz, H5), 8.00 (dd, 1H, J ¼ 7.2, 1.4 Hz, H7), 8.04 (d, 1H, J ¼ 8.40 Hz, H4). 13 1 C{ H} NMR (CDCl3, T ¼ 298 K, ppm) d: 25.6 (CH3, eCH3 quino3 6 line), 31.3 (CH3, CMe3), 46.9 (C, CMe3), 122.0 (CH, C ), 125.0 (CH, C ), 10 5 8 4 127.0 (C, C ), 128.4 (CH, C ), 133.2 (C, C ), 136.4 (CH, C ), 137.9 (CH, C7), 148.8 (C, C9), 159.2 (C, C2). Anal calc. for C14H17NS: C, 72.68; H, 7.41; N, 6.05. Found C, 72.71; H, 7.29; N, 5.93%. 4.3. [Pd(h2-bq)(TTbQ-Me)]

The following compounds were synthesized in an analogous way using the appropriate ligand and alkene. 4.4. [Pd(h4-bq)(NeS-tBu)] Red microcrystals. Yield 45%. 1 H NMR (CDCl3, T ¼ 298 K, ppm) d: 1.42 (s, 9H, tBu), 4.13 (s, 2H, CH2S), 5.63 (broad AB system, 4H, CH]CH), 7.34 (dd, 1H, J ¼ 7.8, 5.0 Hz, H5), 7.47 (d, 1H, J ¼ 7.8 Hz, H3), 7.81 (td, 1H, J ¼ 7.8, 1.5 Hz, H4), 8.40 (d, 1H, J ¼ 5.0, H6). 13 1 C{ H} NMR (CDCl3, T ¼ 298 K, ppm) d: 30.2 (CH3, CMe3), 38.8 (CH2, eCH2S), 48.6 (C, CMe3), 100.1 (CH, CH]CH), 103.4 (CH, CH] CH), 122.7 (CH, C3), 124.1 (CH, C5), 138.2 (CH, C4), 149.7 (C, C2), 158.7 (CH, C6), 180.8 (C, CO), 186.8 (C, CO). 1 H NMR (CD2Cl2, T ¼ 298 K, ppm) d: 1.31 (s, 9H, tBu), 4.12 (broad AB systems, 2H, CH2S), 5.43 (broad AB system, 2H, CH]CH), 5.60 (broad AB system, 2H, CH]CH), 7.34 (dd, 1H, J ¼ 7.8, 5.0 Hz, H5), 7.52 (d, 1H, J ¼ 7.8 Hz, H3), 7.82 (t, 1H, J ¼ 7.8, Hz, H4), 8.15 (d, 1H, J ¼ 5.0, H6). 13 1 C{ H} NMR (CD2Cl2, T ¼ 298 K, ppm) d: 29.9 (CH3, CMe3), 38.8 (CH2, eCH2S), 48.9 (C, CMe3), 95.7 (CH, CH]CH), 99.3 (CH, CH]CH), 104.2 (CH, CH]CH),107.5 (CH, CH]CH),123.4 (CH, C3),124.4 (CH, C5), 138.9 (CH, C4), 148.6 (C, C2), 159.0 (CH, C6), 180.3 (C, CO), 186.9 (C, CO). IR (KBr pellets): nCN 1570, nCO 1600; 1633 cm1. Anal calc. for C18H19NO2PdS: C, 48.55; H, 4.84; N, 3.54. Found: C, 48.71; H, 4.91; N, 3.37%. 4.5. [Pd(h4-bq)(DPPQ-Me)] Brown microcrystals. Yield 60%. H NMR (CDCl3, T ¼ 298 K, ppm) d: 3.07 (s, 3H, CH3 quinoline), 5.70, 5.60 (AA0 BB0 X system, 4H, CH]CH), 7.41e7.50 (m.10H, PPh), 7.57 (d, 1H, J ¼ 8.4 Hz, H3), 7.61 (dd, 1H, J ¼ 8.0, 7.2 Hz, H6), 7.85 (ddd, 1H, J ¼ 8.0, 7.2, 1.4 Hz, H7), 7.97 (d, 1H, J ¼ 8.0 Hz, H5), 8.24 (d, 1H, J ¼ 8.4 Hz, H4). 13 1 C{ H} NMR (CDCl3, T ¼ 298 K, ppm) d: 30.1 (CH3, eCH3 quinoline), 100.9 (CH, CH]CH trans-N), 103.9 (d, CH, JCP ¼ 8.7 Hz, CH]CH transP), 124.0 (CH, C3), 126.5 (CH, C6), 127.6 (C, C10), 131.2 (C, C5), 131.3 (CH, C8), 137.8 (CH, C7), 138.4 (CH, C4), 151.6 (C, C9), 165.9 (C, C2), 185.2 (C, CO), 186.7 (C, CO). 31P{1H} NMR (CDCl3, T ¼ 298 K, ppm) d: 29.8. IR (KBr pellets): nCN 1564, nCO 1612 cm1. Anal calc. for C28H22NO2PPd: C, 62.06; H, 4.09; N, 2.58. Found: C, 61.97; H, 4.18; N, 2.43%. No remarkable changes are detectable in the NMR spectra recorded at low temperature. 1

To 64.3 mg (0.278 mmol) of TTbQ-Me dissolved in anhydrous acetone (20 ml) in a two necked flask, 30 mg (0.278 mmol) of pbenzoquinone and 120 mg (0.116 mmol) of Pd2DBA3$CHCl3 were added in sequence under inert atmosphere (Ar). The resulting mixture was stirred in the dark for 30 min, filtered on a celite filter and evaporated under vacuum to a small volume. Addition of Et2O induces the precipitation of the complex which was filtered off and dried in a desiccator for 5 h. 82.2 mg of the title compound as a red solid were obtained (yield 80%). 1 H NMR (CDCl3, T ¼ 298 K, ppm) d: 1.37 (s, 9H, tBu), 2.99 (s, 3H, CH3 quinoline), 5.62 (broad AB system, 4H, CH]CH), 7.51 (d, 1H, J ¼ 8.4 Hz, H3), 7.58 (dd, 1H, J ¼ 8.1, 7.3 Hz, H6), 7.93 (dd, 1H, J ¼ 8.1, 1.3 Hz, H5), 8.02 (dd, 1H, J ¼ 7.3, 1.3 Hz, H7), 8.22 (d, 1H, J ¼ 8.4 Hz, H4). 13 1 C{ H} NMR (CDCl3, T ¼ 298 K, ppm) d: 29.6 (CH3, eCH3 quinoline), 30.9 (CH3, CMe3), 54.6 (C, CMe3), 100.5 (bs, CH, CH]CH), 123.8 (CH, C3), 125.9 (CH, C6), 128.0 (C, C10), 130.3 (C, C8), 130.6 (CH, C5), 138.3 (CH, C4), 138.8 (CH, C7), 149.4 (C, C9), 165.0 (C, C2), 186.9 (C, CO), 188.4 (C, CO). 1 H NMR (CD2Cl2, T ¼ 193 K, ppm) d: 1.26 (s, 9H, tBu), 2.87 (s, 3H, CH3 quinoline), 4.71 (d, 1H, J ¼ 5.8 Hz, CH]CH), 4.92 (d, 1H, J ¼ 5.8 Hz, CH]CH), 6.10 (d, 1H, J ¼ 9.8 Hz, CH]CH), 6.22 (d, 1H, J ¼ 9.8 Hz, CH]CH), 7.51 (d, 1H, J ¼ 8.4 Hz, H3), 7.58 (dd, 1H, J ¼ 8.1, 7.3 Hz, H6), 7.96 (dd, 1H, J ¼ 8.1, 1.3 Hz, H5), 8.01 (dd, 1H, J ¼ 7.3, 1.3 Hz, H7), 8.26 (d, 1H, J ¼ 8.4 Hz, H4). 13 1 C{ H} NMR (CD2Cl2, T ¼ 193 K, ppm) d: 29.0 (CH3, eCH3 quinoline), 30.3 (CH3, CMe3), 54.9 (C, CMe3), 63.5 (s, CH, CH]CH), 67.6 (s, CH, CH]CH), 124.2 (CH, C3), 126.2 (CH, C6), 127.9 (C, C10), 128.9 (C, C8), 131.1 (CH, C5), 134.5 (s, CH, CH]CH), 135.3 (s, CH, CH]CH), 138.8 (CH, C4), 139.2 (CH, C7), 149.2 (C, C9), 165.0 (C, C2), 186.6 (C, CO), 188.3 (C, CO). IR (KBr pellets): nCN 1575, nCO 1613; 1636 cm1. Anal calc. for C20H21NO2PdS: C, 53.88; H, 4.75; N, 3.14. Found C, 53.71; H, 4.79; N, 3.01%.

4.6. [Pd(h2-nq)(TTbQ-Me)] Brown microcrystals. Yield 88%. H NMR (CDCl3, T ¼ 298 K, ppm) d: 1.33 (s, 9H, tBu), 3.05 (s, 3H, CH3 quinoline), 4.83, 4.96 (AB system, J ¼ 7.1 Hz, CH]CH), 7.41e7.52 (m, 4H, H3,H6,CH aryl naphthoquinone), 7.84 (dd, 1H, J ¼ 8.0, 1.3 Hz, H5), 7.92 (dd, 1H, J ¼ 7.3, 1.3 Hz, H7), 7.98 (m,1H, CH aryl naphthoquinone), 8.07 (m, 1H, CH aryl naphthoquinone), 8.26 (d, 1H, J ¼ 8.4 Hz, H4). 13 1 C{ H} NMR (CDCl3, T ¼ 298 K, ppm) d: 29.7 (CH3, eCH3 quinoline), 30.8 (CH3, CMe3), 54.6 (C, CMe3), 59.5 (CH, CH]CH), 62.7 (CH, CH]CH), 123.7 (CH, C3), 125.2 (CH, aryl naphthoquinone), 125.3 (CH, aryl naphthoquinone), 125.7 (CH, C6), 127.9 (C, C10), 130.2 (C, C8), 130.4 (CH, C5), 131.1 (CH, aryl naphthoquinone), 131.2 (CH, aryl naphthoquinone), 135.8 18 (C, aryl naphthoquinone), 135.9 (C, aryl naphthoquinone),138.2 (CH, C4) 138.8 (CH, C7), 149.3 (C, C9), 164.9 (C, C2), 185.3 (C, CO), 186.2 (C, CO). IR (KBr pellets): nCO 1623 cm1. Anal calc. for C24H23NO2PdS: C, 58.12; H, 4.67; N, 2.82. Found: C, 58.25; H, 4.41; N, 2.71%. 1

L. Canovese et al. / Journal of Organometallic Chemistry 749 (2014) 379e386

4.7. [Pd(h4-bq)(Neocup)] Purple microcrystals. Yield 77%. 1H NMR (CD2Cl2, T ¼ 298 K, ppm) d: 2.99 (s, 6H, CH3), 5.45 (bs, 4H, CH]CH), 7.72 (d, 2H, J ¼ 8.3 Hz, H3), 7.84 (s, 2H, H5), 8.32 (d, 2H, J ¼ 8.3 Hz, H4). 1 H NMR (CD2Cl2, T ¼ 193 K, ppm) d: 2.84 (s, 6H, CH3), 4.60 (bs, 2H, CH]CH), 6.12 (bs, 2H, CH]CH), 7.64 (d, 2H, J ¼ 8.3 Hz, H3), 7.70 (s, 2H, H5), 8.27 (d, 2H, J ¼ 8.3 Hz, H4). The 13C NMR spectrum was not recorded owing to the low solubility of the complex. IR (KBr pellets): nCO 1619 cm1. Anal calc. for C20H16N2O2Pd: C, 56.82; H, 3.81; N, 6.63. Found: C, 57.01; H, 3.79; N, 6.45%. 4.8. [Pd(h4-bq)(Dic)2] Yellow microcrystals. Yield 95%. 1 H NMR (CD2Cl2, T ¼ 298 K, ppm) d: 2.45 (s, 12H, aryleCH3), 5.59 (bs, 4H, CH]CH), 7.17 (d, J ¼ 7.5 Hz, 4H, aryl m-H), 7.27 (t, J ¼ 7.3 Hz, 2H, aryl p-H). 1 H NMR (CD2Cl2, T ¼ 193 K, ppm) d: 2.36 (s, 12H, aryleCH3), 5.58 (s, 4H, CH]CH), 7.14 (d, J ¼ 7.5 Hz, 4H, aryl m-H), 7.25 (t, J ¼ 7.3 Hz, 2H, aryl p-H). 13 1 C{ H} NMR (CD2Cl2, T ¼ 298 K, ppm) d: 18.9 (CH3, aryleCH3), 101.9 (CH, CH]CH), 126.7 (C, aryl i-C), 127.7 (CH, aryl m-C), 129.1 (CH, aryl p-C), 135.3 (C, aryl o-C), 156.5 (C, NC), 187.3 (C, CO). 13 1 C{ H} NMR (CD2Cl2, T ¼ 203 K, ppm) d: 18.4 (CH3, aryleCH3), 101.9 (CH, CH]CH), 126.4 (C, aryl i-C), 127.9 (CH, aryl m-C), 129.5 (CH, aryl p-C), 135.6 (C, aryl o-C), 154.7 (C, NC), 187.9 (C, CO). IR (KBr pellets): nCO 1623, nCN 2129, 2152 cm1. Anal calc. for C24H22N2O2Pd: C, 60.45; H, 4.65; N, 5.87. Found: C, 60.61; H, 4.78; N, 5.77%. 4.9. [Pd(h2-nq)(Dic)2] Yellow microcrystals. Yield 84%. H NMR (CD2Cl2, T ¼ 298 K, ppm) d: 2.42 (s, 12H, aryleCH3), 4.91 (s, 2H, CH]CH), 7.16 (d, J ¼ 7.3 Hz, 4H, aryl m-H), 7.26 (t, J ¼ 7.3 Hz, 2H, aryl p-H), 7.59 (m, 2H, CH nq), 8.03 (m, 2H, CH nq). 13 1 C{ H} NMR (CD2Cl2, T ¼ 233 K, ppm) d: 18.7 (CH3, aryleCH3), 64.7 (CH, CH]CH), 125.3 (CH, CH nq), 126.5 (C, aryl i-C), 127.8 (CH, aryl m-C), 129.3 (CH, aryl p-C), 132.0 (CH, CH nq), 135.4 (C, aryl o-C), 135.5 (C, C nq), 155.1 (C, NC), 185.6 (C, CO). IR (KBr pellets): nCO 1642, nCN 2145, 2161 cm1. Anal calc. for C28H24N2O2Pd: C, 63.82; H, 4.59; N, 5.32. Found: C, 63.71; H, 4.67; N, 5.19%. The complexes [Pd(h2-nq)(NeS-tBu)] [5b], and [Pd(h2nq)(Neocup)] [6] were synthesized according to published procedures. 1

4.10. [Pd2(m-bq)(TTbQ-Me)2] As reported elsewhere (see text) the title complex coprecipitates with the complex [Pd(h2-bq)(TTbQ-Me)] as a consequence of a re-crystallization attempt. The 1H NMR data are summarized in sequence. 1 H NMR (CDCl3, T ¼ 298 K, ppm) d: 1.17 (s, 18H, tBu), 3.25 (s, 6H, CH3 quinoline), 4.12, 4.26 (AA0 BB0 system, 2H, J ¼ 7.9, 3.0, 1.0 Hz,. J CH]CH), 7.33 (d, 2H, J ¼ 7.5 Hz, H3), 7.47 (dd, 2H, J ¼ 8.3, 7.4 Hz, H6), 7.81 (d, 2H, J ¼ 8.3, Hz, H5), 7.92 (d, 2H, J ¼ 7.4, Hz, H7), 7.99 (d, 2H, J ¼ 7.5 Hz, H4). 4.11. Preliminary studies and equilibrium measurements All the equilibrium reactions were preliminarily followed by 1H NMR technique by dissolving the complex under study in 0.6 ml of

385

CDCl3 ([Pd(ol1)(LeL0 )] ¼ 1  102 O 3  102 mol dm3) and adding microaliquots of a concentrated solution of the entering olefin (ol2) in CDCl3 according to reaction 1. The reaction progress was followed by monitoring the signal for the disappearance of the starting complex and the contemporary appearance of the final product ([Pd(ol2)(LeL0 )]). The UVeVis preliminary study was carried out by placing 3 ml of freshly prepared solution of the complex [Pd(ol1)(Le L0 )] ([[Pd(ol1)(LeL0 )]]0 ¼ 1  104 mol dm3) in the thermostatted (298 K) cell compartment of the UVeVis spectrophotometer. Microaliquots of solutions containing the exchanging olefin ol2 at appropriate concentrations were added and the absorbance changes were monitored in the 250e400 nm wavelength interval or at fixed wavelength. 4.12. Crystal structure determinations The crystal data of compound [Pd2(m-bq)(TTbQ-Me)2] were collected at room temperature using a Nonius Kappa CCD diffractometer with graphite monochromated Mo-Ka radiation. The data sets were integrated with the Denzo-SMN package [20] and corrected for Lorentz, polarization and absorption effects (SORTAV) [21]. The structure was solved by direct methods using SIR97 [22] system of programs and refined using full-matrix least-squares with all non-hydrogen atoms anisotropically and hydrogens included on calculated positions, riding on their carrier atoms. The dinuclear Pd(0) complex is situated on a twofold axis passing between the Pd atoms and through the center of the benzoquinone ligand. The crystal packing includes also a molecule of solvent CH2Cl2 per molecule of Pd(0) complex placed on a two foldaxis passing through the carbon atom. All calculations were performed using SHELXL-97 [23] and PARST [24] implemented in WINGX [25] system of programs. The crystal data are given in Table 1SI. Appendix A. Supplementary material CCDC 959194 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam. ac.uk/products/csd/request/. Appendix B. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jorganchem.2013.10.021. References [1] (a) F. Diederich, P.J. Stang, in: F. Diederich, P.J. Stang (Eds.), Metal-catalyzed Cross-coupling Reactions, Wiley-VCH, Weinheim, 1998; (b) M.J. Calhorda, J.M. Brown, N.A. Cooley, Organometallics 10 (1991) 1431; (c) K. Selvakumar, M. Valentini, P.S. Pregosin, A. Albinati, Organometallics 18 (1999) 4591; (d) R.F. Heck, Acc. Chem. Res. 12 (1979) 146; (e) R.F. Heck, Comprehensive Organic Synthesis, vol. 4, Pergamon, Oxford, 1991; (f) M.J. Brown, K.K. Hii, Angew. Chem. Int. Ed. Engl. 108 (1996) 679; (g) M. Tschoerner, P.S. Pregosin, A. Albinati, Organometallics 18 (1999) 670; (h) A. de Meijere, F.E. Meyer, Angew. Chem. Int. Ed. Engl. 106 (1994) 2473; (i) J.K. Stille, Angew. Chem. Int. Ed. Engl. 25 (1986) 508; (j) V. Farina, in: E.W. Abel, F.G.A. Stone, G. Wilkinson (Eds.), Comprehensive Organometallic Chemistry II, Pergamon, Oxford, 1995, p. 12 (Chapter 3.4); (k) V. Farina, G.P. Roth, Adv. Metal Org. Chem. 5 (1996) 1; (l) B. Crociani, S. Antonaroli, L. Canovese, P. Uguagliati, F. Visentin, Eur. J. Inorg. Chem. (2004) 732; (m) A. Pfaltz, Acta Chem. Scand. 50 (1996) 189; (n) O. Reiser, Angew. Chem. Int. Ed. Engl. 105 (1993) 576. [2] (a) H. Hagelin, M. Svensson, B. Akermark, P.-O. Norby, Organometallics 18 (1999) 4574;

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L. Canovese et al. / Journal of Organometallic Chemistry 749 (2014) 379e386 (b) M.J.S. Dewar, Bull. Soc. Chim. Fr. 18 (1951) C71eC79; (c) J. Chatt, L.A. Duncanson, J. Chem. Soc. (1953) 2939. P. Espinet, A.C. Albeniz, in: R. Crabtree, M. Mingos (Eds.), PalladiumeCarbon p Bonded Complexes, Comprehensive Organometallic Chemistry III, vol. 8, Elsevier, 2007. (a) R. Van Asselt, C.J. Elsevier, W.J.J. Smeets, A.L. Speck, Inorg. Chem. 33 (1994) 1521e1531; (b) P.T. Cheng, C.D. Cook, S.C. Nyburg, K.Y. Wan, Inorg. Chem. 10 (1971) 2210e 2213; (c) F. Ozawa, T. Ito, Y. Nakamura, A. Yamamoto, J. Organomet. Chem. 168 (1979) 375e391; (d) R. Van Asselt, C.J. Elsevier, Tetrahedron 50 (1994) 323e334; (e) F. Gomez-de la Torre, F.A. Jalon, A. Lopez-Agenjo, B.R. Manzano, A. Rodriguez, T. Sturm, W. Weissensteiner, M. Martinez-Ripoll, Organometallics 17 (1998) 4634e4644; (f) S. Antonaroli, B. Crociani, J. Organomet. Chem. 560 (1998) 137e146; (g) M. Tschoerner, G. Trabesinger, A. Albinati, P.S. Pregosin, Organometallics 16 (1997) 3447e3453. (a) L. Canovese, F. Visentin, P. Uguagliati, B. Crociani, J. Chem. Soc. Dalton Trans. (1996) 1921e1926; (b) L. Canovese, F. Visentin, G. Chessa, P. Uguagliati, A. Dolmella, J. Organomet. Chem. 601 (2000) 1e15. L. Canovese, F. Visentin, C. Santo, A. Dolmella, J. Organomet. Chem. 694 (2009) 411e419. L. Canovese, F. Visentin, Inorg. Chim. Acta 363 (2010) 2375e2386. (a) M. Hiramatsu, K. Shiozaki, T. Fujinami, S. Sakai, J. Organomet. Chem. 218 (1981) 409e416; (b) M. Hiramatsu, K. Shiozaki, T. Fujinami, S. Sakai, J. Organomet. Chem. 246 (1983) 203e211; (c) R.A. Klein, P. Witte, R. van Belzen, J. Fraanje, K. Goubitz, M. Numan, H. Schenk, J.M. Ernsting, C.J. Elsevier, Eur. J. Inorg. Chem. (1998) 319e330; (d) K. Selvakumar, A. Zapf, A. Spannenberg, M. Beller, Chem. Eur. J. 8 (2002) 3901e3906; (e) Y. Yamamoto, T. Ohno, K. Itoh, Organometallics 22 (2003) 2267e2272. T. Hukai, H. Kawazura, Y. Ishii, J.J. Bonnet, J.A. Ibers, J. Organomet. Chem. 65 (1974) 253e266. L. Canovese, G. Chessa, F. Visentin, P. Uguagliati, Coord. Chem. Rev. 248 (2004) 945e954 and references therein. M.J. Chetcuti, J.A.K. Howard, M. Pfeffer, J.L. Spencer, F.G.A. Stone, J. Chem. Soc. Dalton Trans. (1981) 276e283. We have so far synthesized some palladium(0) naphthoquinone pyridylthioether derivatives characterized by an equimolecular ratio between the spectator ligand and the h2 coordinated olefin. The palladium coordinated

[13]

[14]

[15]

[16] [17] [18] [19] [20] [21] [22]

[23] [24] [25]

olefinic carbons of all those complexes resonated at RT within 61 and 65 ppm while their protons were observed in the narrow interval between 4.87 and 4.96 ppm, which is almost coincident with those observed in the low temperature 1H and 13C NMR spectra of the complexes [Pd(bq)(TTbQ-Me)] and [Pd(nq)(TTbQ-Me)]. The inversion of the absolute configuration of sulfur in pyridyl or quinolyl thioether derivatives of palladium depends on many factors. Thus, the rigidity of the ancillary ligands and their steric requirements and basicity can remarkably influence the freezing temperature (see Ref. [10]). We have tried to optimize the geometry of the compounds under study at DFT PBE level using a double polarized basis set of the DMol3 program but despite the experimental results the calculations point to a h2 hapto olefin coordination mode for all the reported complexes. On the basis of the relationships KE1 ¼ [Pd(h2-bq)(NeSt-Bu)][nq]/[Pd(h2nq)(NeSt-Bu)][bq] ¼ 0.51  0.03 (see text) and the previously determined KE2 ¼ [Pd(h2-tmetc)(NeSt-Bu)][nq]/[Pd(h2-nq)(NeSt-Bu)][tmetc] ¼ 0.6  0.4 [Ref. [5b]], exploiting the expression KE3 ¼ KE1  KE2 we have calculated the equilibrium constant KE3 ¼ [Pd(h2-tmetc)(NeSt-Bu)][bq]/[Pd(h2-bq)(NeStBu)][tmetc] ¼ 0.3  0.2. Finally from the expression KE3 ¼ kf/kr we have also roughly estimated kr ¼ (2.8  2)  102 mol1 dm3 s1. The contemporary determination of both kf and kr was not possible due to the very long reaction time required in the case of second order conditions. The determination of the reverse reaction constant kr under pseudo first order conditions (benzoquinone as entering olefin) was also impossible because of the unfavorably high extinction coefficient of benzoquinone. M.N. Burnett, C.K. Johnson, ORTEP III, Report ORNL-6895, Oak Ridge National Laboratory, Oak Ridge, TN, 1996. W.L.F. Amarego, D.D. Perrin, Purification of Laboratory Chemicals, third ed., Pergamon, New York, 1988. L. Canovese, F. Visentin, P. Uguagliati, G. Chessa, A. Pesce, J. Organomet. Chem. 566 (1998) 61e71. L. Canovese, F. Visentin, G. Chessa, P. Uguagliati, C. Santo, A. Dolmella, Organometallics 24 (2005) 3297e3308. Z. Otwinowski, W. Minor, in: C.W. Carter, R.M. Sweet (Eds.), Methods in Enzymology, vol. 276, Academic Press, London, 1997, pp. 307e326. Part A. R.H. Blessing, Acta Crystallogr. Sect. A 51 (1995) 33e38. A. Altomare, M.C. Burla, M. Camalli, G.L. Cascarano, C. Giacovazzo, A. Guagliardi, A.G. Moliterni, G. Polidori, R. Spagna, J. Appl. Crystallogr. 32 (1999) 115e119. G.M. Sheldrick, SHELX-97, Program for Crystal Structure Refinement, University of Gottingen, Germany, 1997. M. Nardelli, J. Appl. Crystallogr. 28 (1995) 659. L.J. Farrugia, J. Appl. Crystallogr. 32 (1999) 837e838.