Zdeněk Trávníček
email: zdenek.travnicek@upol.cz

Richard Pastorek

Pavel Štarha

Igor Popa

Václav Slovák

as precursors for the preparation of graphite oxidation accelerators

Keywords: Nickel(II)complex, Dithiocarbamato ligand, Crystal structure, Graphite oxidation accelerator Nickel(II)complex, Dithiocarbamato ligand, Crystal structure, Graphite oxidation accelerator

Nickel(II) N-benzyl-N-methyldithiocarbamato complexes

 (5), [Ni(BzMedtc)(PPh 3 ) 2 ]BPh 4 (6) and [Ni(BzMedtc)(PPh 3 ) 2 ]PF 6 (7). The obtained complexes 1-7 were characterized by elemental analysis, thermal analysis and spectroscopic methods (IR, UV-Vis, 31 P{ 1 H}-NMR). The results of the magnetochemical and molar conductivity measurements proved the complexes as diamagnetic non-electrolytes (1-4) or 1:1 electrolytes (5-7).

The molecular structures of 4 and 5•H 2 O were determined by a single crystal X-ray analysis. In all cases, the Ni(II) atom is four-coordinated in a distorted square-planar arrangement with the S 2 PX, and S 2 P 2 donor set, respectively. The catalytic influence of selected complexes 1, 3, 5, and 6 on graphite oxidation was studied. The results clearly indicated that the presence of the products of thermal degradation processes of the mentioned complexes have impact on the course of graphite oxidation. A decrease in the oxidation start temperatures by about 60-100 °C was observed in the cases of all the tested complexes in comparison with pure graphite.

____________

Introduction

Many compounds involving the dithiocarbamate (dtc) moiety were reported up to now in connection with their applications. These compounds have been found to be useful as fungicides [1] or accelerators of vulcanization [2]. However, the spectrum of potential use of these compounds may be also broadened on the field of in vitro anticancer activity as can be demonstrated by the following examples. The 4(3H)-quinazolinone derivatives with dithiocarbamate side chains, such as (3,4-dihydro-2-methyl-4-oxoquinazolin-6-yl)-methyl-[4-(4-fluorophenyl)piperazine]-1-carbodithioate, showed promising in vitro anticancer activity (IC 50 = 0.5 µM) against human myelogenous leukaemia cell line (K562) [3]. Another dithiocarbamate derivative, 4-methylsulfinyl-1-(S-methyldithiocarbamyl)-butane (sulforamate), serves as a bearer of cancer chemopreventive effect [4]. The efficient protection of N-benzyl-D-glucamine-dithiocarbamate against cisplatin-induced nephrotoxicity (it is able to chelate platinum and thus to decrease the possibility of its reactions with sulphur-containing renal proteins or enzymes) has also been reported recently [5].

Several thousands of transition metal complexes involving the dithiocarbamate moiety have been prepared and reported to date and some of them are very promising from the biological activity point of view. The Pt(II) complex of the [Pt(esdt)(py)Cl] composition, where esdt = ethylsarcosinedithiocarbamate and py = pyridine, is highly cytotoxic against several human cancer cell lines, and moreover, this substance was not found as nephrotoxic, contrary to cisplatin [6]. [Au(dmdt)X 2 ] and [Au(esdt)X 2 ] are the representatives of non-platinum complex with the promising anticancer activity, which is even higher compared to cisplatin; dmdt = N,N´dimethyldithiocarbamate, X = Cl or Br [7]. The antifungal activity, well-known for dithiocarbamate derivatives, was proved for tin(IV) complexes with pyrrolidinedithiocarbamate [8].

Nickel is known as a suitable transition metal for the preparation of complexes and it is not surprising that over three thousand compounds (SciFinder, 2010) and 163 X-ray structures (Crystallographic Structural Database, CSD, ver. 5.30, September 2009 update [9]) involving an NiS 2 CN motif have been reported to date. Among these compounds, the nickel(II) complexes involving the NiPN(S 2 CN) or NiP 2 (S 2 CN) moiety, such as [Ni(4-MePzdtc)(PPh 3 )(NCS)] [10], and [Ni(MeSdtc)(dppe)] [11], respectively, have been described in the literature; 4-MePzdtc = 4-methylpiperazinedithicarbamate anion, PPh 3 = triphenylphosphine, MeSdtc = N-methyl-N-sulfonyldithiocarbamate, dppe = 1,2-bis(diphenylphosphine)ethane. However, no nickel complexes containing the N-benzyl-N-methyldithiocarbamate anion (BzMedtc) within the mentioned moieties, have been reported to date.

The presented series of compounds follows a great number of nickel dithicarbamato complexes previously prepared at our department [see ref. 12,13 and the references cited therein]. In this paper, we deal with seven nickel(II) complexes, whose structures, according to the literature research, for the first time involves a combination of the BzMedtc and PPh 3 ligands.

The complexes [Ni(BzMedtc)(PPh 3 )X] (1-4) were synthesized from [Ni(BzMedtc) 2 ], using its reaction with [NiX 2 (PPh 3 ) 2 ], where X stands for Cl -(for the complex 1), Br -(2), I -(3) and NCS - (4). The [Ni(BzMedtc)(PPh 3 ) 2 ]Y complexes (Y = ClO 4 -for 5, BPh 4 -for 6 and PF 6 -for 7) were prepared by the reaction of complex 1 with the appropriate alkaline salt and triphenylphosphine (see Scheme 1). The products were characterized by various physical methods including 31 P{ 1 H}-NMR spectroscopy. The X-ray structures of the representatives of both types of complexes, i.e.

[Ni(BzMedtc)(PPh 3 )(NCS)] ( 4) and [Ni(BzMedtc)-(PPh 3 ) 2 ]ClO 4 •H 2 O (5•H 2 O), were determined by a single crystal X-ray analysis. The geometry of these compounds is square-planar with the NiS 2 PN (4) and NiS 2 P 2 (5•H 2 O) donor sets. This work also describes the effect of the nickel(II) dithiocarbamates on the oxidation of graphite as a model study of the coal combustion. As it is commonly known, the ignition temperature of the coal combustion is one of the most important properties influencing many industrial processes. This stage can be affected by the presence of both natural and artificial impurities [14,15]. Formerly, several papers reported simple nickel(II) compounds, such as NiO (mixed with KOH) or Ni(NO 3 ) 2 (mixed with K 2 SO 4 ), as the substances accelerating graphite oxidation [16,17]. Few years ago, we decided to study the influence of the nickel(II, IV) dithiocarbamato complexes [see 13,18,19 and references cited therein] on the course of graphite oxidation. It was found that the presence of these complexes causes the ignition temperature decrease. Similar results were obtained also for the tested complexes 1, 3, 5, and 6, which were chosen as representatives of the presented compounds, as it is discussed in more detail within the framework of this paper.

Results and Discussion

The nickel(II) complexes 1-7 were prepared according to the synthetic strategies depicted in Scheme 1. The obtained molar conductivity values (Table 1) indicate that complexes 1-4 dissolved in acetone behave as non-electrolytes [20], which indirectly proved the coordination of the Cl -(1), Br - (2), I -(3) and NCS -(4) anions to the Ni(II) atom. On the other hand, the molar conductivity values of the remaining complexes proved the ionic nature (1:1 electrolyte type) in acetone (5) or DMF (6, 7) solutions, which means that the ClO 4 -( 5), BPh 4 -( 6) and PF 6 - (7) anions are situated outside of the inner coordination sphere within the structure of the discussed complexes. The results of the room temperature magnetochemical measurements determined the prepared compounds to be diamagnetic, and thus, suggested on square-planar geometries in the vicinity of the central atom.

Table 1:

The results of molar conductivity measurements, IR, UV-Vis and 31 P{H}-NMR spectroscopy of the complexes 1-7

IR (cm -1 ) b λ M a ν(C … S) ν(C … N) other UV-Vis (cm -1 )

Spectroscopic Characterization

The PPh 3 signals in the 31 P{ 1 H}-NMR spectra of the CDCl 3 solutions of 1-7 were found in the range of 21.1-32.1 ppm (Table 1). These values differ significantly from that of -4.52 ppm determined for the free PPh 3), which agrees with the results reported for the nickel(II) complexes with N-benzyl-N-butyldithiocarbamate [12].

The maxima observed in the IR spectra of the nickel(II) complexes 1-7 at 990-998 cm -1 , and 1510-1542 cm -1 (see Table 1) are typical for the ν(C … S), and ν(C … N) vibrations of the dithiocarbamate moiety, respectively [21,22]. The peak of the ν(C … S) stretching vibration is not split, which supports the bidentate coordination of the BzMedtc anion. The peaks of the ν(C≡N) and ν(C-S) vibrations of the NCS - anion were observed in the IR spectrum of 4. The ionic nature of the perchlorate anion in the complex 5, can be supported by two undivided peaks at 622 and 1088 cm -1 [23]. The same conclusion can be made for the complex 7, whose IR spectrum contains the non-split peak at 836 cm -1 assignable to ν(PF 6 -), which again indicates its ionic nature [24]. Diffuse-reflectance electronic spectra of the prepared complexes 1-7 support the assumption of the square-planar geometry in the vicinity of the central Ni(II) atom. Absorption maxima at the 18 400-20 700 cm -1 region may be attributed to the 1 A 1g → 1 B 1g transition of the squareplanar nickel(II) complexes [25,26]. The next maxima, also assignable to the d-d transitions, were found for the complexes 1 (25 200 cm -1 ) and 2 (25 000 cm -1 ). The maxima above 29 000 cm -1 are probably connected with the intraligand charge-transfer transitions in the S 2 CN moiety [27].

X-ray Structure of [Ni(NCS)(BzMedtc)(PPh 3 )] (4) and [Ni(BzMedtc)(PPh 3 ) 2 ]ClO 4 •H 2 O (5•H 2 O)

The X-ray structure of the title complexes 4 (Figure 1) and 5•H 2 O (Figure 2) was determined by a single crystal X-ray analysis. The crystal data and structure refinements are given in Table 2, while the selected bond lengths and angles are summarized in Table 3.

The central Ni(II) ion of the complex 4 (Figure 1) is fourcoordinated by PPh 3 , the isothiocyanate anion, and the bidentate-coordinated N-benzyl-N-methyldithiocarbamate anion (BzMedtc). The atoms of a NiS 2 NP chromophore are arranged in the distorted square-planar geometry around the metal centre (Table 3). The Ni-S bond lengths of the title complex correlate well with the mean value of 2.204 Å (2.155-2.258 Å interval) determined for this band for the 116 square-planar nickel complexes involving bidentate-coordinated dithiocarbamato S-donor ligands, which have been up to now deposited in the Cambridge Structural Database (CSD) [9].

The well-known delocalization of π-electron density within the S 2 CN moiety of the dithiocarbamate anion was observed for 4, since the C-N and C-S bond lengths of the studied compound (Table 2) were found to be significantly shorter compared to the single C-N (1.47 Å) and C-S (1.81 Å) σ-bond lengths [28]. Further, the C-S and C-N bond length values of BzMedtc are comparable with those of the transition metal (TM) complexes involving the (TM)S 2 CNR 2 moiety and deposited within the CSD, which equal 1.717 Å, and 1.326 Å, respectively. The S1 atom is the most deviated one [0.0454(5) Å] from the plane created through the Ni1, S1, S2, P1 and N2 atoms of the chromophore. Further, the described plane forms the dihedral angle of 83.10(5)° with the benzene ring of the BzMedtc molecule and dihedral angles of 67.90(5)°, 69.63(5)° and 47.85(4)° with the benzene rings of PPh 3 containing the C30, C40, and C50 atoms, respectively. Mutual orientation of the phenyl groups of PPh 3 within the molecular structure of 4 can be described by the 67.64(6)° (between benzene rings containing C30 and C40), 66.06(6)° (between benzene rings containing C30 and C50) and 76.53 (6)° (between benzene rings containing C40 and C50) dihedral angles. The C-H•••C, C-H•••S and C•••C types of the intramolecular non-bonding interactions were detected to stabilize the crystal structure of 4 (see Table 4). No typical hydrogen bond is present within the structure of discussed complex. 3.391(3) Symmetry codes: (i) x + 0.5, 0.5 -y, z -0.5; (ii) 1 -x, 1 -y, 1 -z; (iii) 1.5 -x, y + 0.5, 1.5 -z; (iv) -x, -y, -z; (v) -x, 1 -y, -z; (vi) 1 -x, 1 -y, -z; (vii) -x, 1 -y, 1 -z

O D-H•••A d(D-H) d(H•••A) d(D•••A) < (DHA) 4 C3-H3A•••S3 i 0.
As it has been already mentioned in the case of complex 4, the bond lengths within the NiS 2 CN moiety of both structures show on electron density redistribution in connection with π-electron density delocalization (see the C-S and C-N bond lengths in Table 3). As for the Ni-P bonds of 5, their lengths are in good agreement with the mean value of 2.208 Å (2.175-2.252 Å interval) as determined from 36 X-ray structures of tetra-coordinated nickel dithiocarbamato complexes involving at least one PPh 3 ligand and deposited within the CSD.

The S2 atom is the most deviated one [0.0903(6) Å] from the plane created through the atoms of the chromophore. The dihedral angles between this plane and benzene rings involving C10, C20, C30, C40, C50, C60, and C70 are 86.39(5)°, 58.49(7)°, 87.97(5)°, 70.52(5)°, 71.64(5)°, 63.60(6)°, and 71.98(5)°, respectively. Non-bonding interactions of the C-H

•••C and C•••C types (C-H•••O
interactions with the distorted perchlorate anion and water molecule of crystallization are disregarded) were found in the crystal structure of the discussed nickel(II) complex (Table 4).

TG/DTA Thermal Analysis

The thermal properties of the prepared nickel(II) complexes were studied by TG and DTA methods. The TG and DTA curves of 1 and 6 are depicted in Figure 3. Although the chemical composition of these complexes is different, their thermal decomposition proceeded in a similar manner as indicated by analogical shapes of the TG curves. In cases of 2, 3 and 6, the sharp exo-effects with maxima at 188 °C, 193 °C, and 188 °C, respectively, were accompanied by mass increase on the TG curves, which can be attributed to the oxygen atom insertion into the Ni-P bond [29]. This effect was not registered on the TG curves of the remaining complexes, where the oxygen atom insertion is most likely combined with the decomposition of the organic parts of the studied complexes.

The final products of the TG studies (900 °C) were studied by X-ray powder diffraction (XRD). NiO (PDF-4 No. 01-089-7130) was determined as the final product of the thermal decomposition of 1-4. In the cases of the complexes 6 and 7, the mixtures with significant predominance of NiO were detected. The final product of the TG thermal study of 6 also contains NiS (PDF-4 No. 4-004-5027) and Ni 3 B 2 O 6 (PDF-4 No. 4-008-3203), and some peaks could not be assigned. As for the complex 7, Ni 2 P 2 O 7 (PDF-4 No. 01-074-1604) and Ni 3 (PO 4 ) 2 (PDF-4 No. 01-072-3977) were detected by a XRD, while some peaks were not assigned.

Catalytic Influence of the Complexes on Graphite Oxidation

The influence of the presence of the representative complexes 1, 3, 5 and 6 on graphite oxidation were studied. The results, summarized in Figure 4 (obtained DTG curves) and Table 5 (obtained data), showed an positive influence on graphite oxidation, because the ignition temperature significantly decreased (by about 64-99 °C) after the addition of the tested nickel(II) complexes to a pure graphite.

Pure graphite oxidation proceeds in one step, while in the cases of its mixtures with the tested complexes, two steps in a larger temperature interval were detected on the appropriate DTG curves (Table 5). The first step takes place at lower temperature than pure graphite oxidation, whereas the oxidation rate maxima (T m ) of the second step approximately correspond to pure graphite. In addition, the oxidation mechanisms of both steps are different, as it can be seen from the activation energy (E) and frequency factor (A) values given in Table 5. More graphite was oxidized during the second step with higher E and A values.

At this point, we would like to discuss the described results of the influence on the graphite oxidation in connection with the results of TG/DTA studies. Figure 3 clearly shows that the thermal decomposition of the nickel(II) complexes is almost completely finished at the temperature of ca. 700 °C and the weight loss above this temperature is insignificant. Based on this statement it may be concluded that not directly the discussed nickel(II) Nbenzyl-N-methyldithiocarbamato complexes, but the products of their thermal decomposition processes, affected graphite oxidation. In connection with the above-mentioned facts, the tested complexes (except of 5) were heated in situ to the temperatures corresponding to the beginning and end of graphite oxidation (see Table 5). The products of thermal decomposition relating to the mentioned temperatures were analysed by XRD. As for 1-4, pure NiO was identified at both temperatures (Figure 5). The slight differences in the 2θ values were caused by a dilatation of the sample due to different measurement temperature. On the other hand, the mixtures of NiO, NiS and Ni 3 B 2 O 6 (for 6) and Ni 2 P 2 O 7 and Ni 3 (PO 4 ) 2 (for 7) were detected by an XRD (Figure 5). Their quantitative composition was similar at both temperatures. Some peaks could not be assigned again. (5). This paper also describes the significant catalytic influence of the thermal degradation products of the selected complexes 1, 3, 5, and 6 on graphite oxidation.

Experimental Section

Materials and General Methods

Chemicals and solvents were purchased from Sigma-Aldrich Co., Acros Organics Co., Lachema Co. and Fluka Co. and they were used as received. The starting compounds

[Ni(BzMedtc) 2 ], [Ni(PPh 3 )2Cl 2 ], [Ni(PPh 3 ) 2 Br 2 ], [Ni(PPh 3 ) 2 I 2 ]
and [Ni(PPh 3 ) 2 (NCS) 2 ] were prepared according to the formerly reported synthetic pathways [13,30]. Elemental analyses (C, H, N) were performed on a Fisons EA-1108 CHNS-O Elemental Analyzer (Thermo Scientific). The content of nickel was determined using the chelatometric titration with murexide as an indicator. Chlorine, bromine and iodine contents were determined by the Schöniger method. Diffuse-reflectance spectra were recorded on a Perkin-Elmer Lamda35 UV/Vis spectrometer using a nujol technique. IR spectra (KBr pellets) were recorded on a Perkin-Elmer Spectrum one FT-IR spectrometer in the 450-4000 cm -1 region. The molar conductivity values of 10 -3 M acetone solutions of 1-5 and 5 × 10 -4 M N,N'-dimethylformamide (DMF) solutions of 6 and 7 were determined by an LF 330/SET conductometer (WTW GmbH) at 25 ºC. The room temperature magnetic susceptibilities were measured using the Faraday method with a laboratory designed instrument with a Sartorius 4434 MP-8 microbalance; Co[Hg(NCS) 4 ] was used as a calibrant and the correction for diamagnetism was performed using Pascal constants. Simultaneous thermogravimetric (TG) and differential thermal (DTA) analyses were performed by an Exstar TG/DTA 6200 (Seiko Instruments Inc.) in a platinum crucible and dynamic air atmosphere (100 mL min -1 ) from the laboratory temperature to 1050 °C with a 2.5 °C min -1 temperature gradient. The weights of the studied complexes 1-4, 6 and 7 (5 was not studied for safety reasons, since it contains the perchlorate anion) were ca. 10 mg. 31 P{ 1 H}-NMR spectra of the CDCl 3 solutions were measured on a Bruker Avance 300 spectrometer at 300 K and the spectra were calibrated against 85% H 3 PO 4 used as an external reference. X-ray powder diffraction experiments were performed with a PANalytical X'Pert PRO instrument (Co-Kα radiation) equipped with an X'Celerator detector. Samples were placed on a zero-background Si slide and scanned in the 2θ range of 5-90° in the steps of 0.017°. In situ measurements were realized in the reaction cell XRK900 (Anton Paar). Evaluation was made using HighScore Plus software and PDF-4 database.

Single Crystal X-ray Analysis of the Complexes 4 and 5•H 2 O

A single crystal X-ray measurement was performed on an Xcalibur TM The same software was used for data correction for an absorption effect by the empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm. Both structures were solved by direct methods using SHELXS-97 software [31] and refined on F 2 using the full-matrix least-squares procedure (SHELXL-97). Non-hydrogen atoms were refined anisotropically and all hydrogen atoms were located in difference Fourier maps and refined by using the riding model with C-H = 0.95, 0.98 and 0.99 Å, and U iso (H) = 1.2U eq (CH, CH 2 ) or 1.5U eq (CH 3 ), except for those belonging to the crystal water molecule in 5. The perchlorate anion as well as crystal water molecule was refined as disordered over two positions with the occupancy factors 75% and 25%, and 57% and 43%, respectively. The crystal data and structure refinements of 4 and 5 are given in Table 1. The molecular graphics were drawn and the additional structural calculations were interpreted using DIAMOND [32]. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK (Fax: +44-1223-336033; E-Mail: deposit@ccdc.cam.ac.uk).

Copies of the data can be obtained free of charge on application to CCDC, as the depository numbers CCDC-766048 (4) and CCDC-766049 (5•H 2 O).

The Catalytic Study

Graphite (0.6 g, diameter of particles less than 0.1 mm, ash residue max. 0.2%, mass drying loss max. 0.2%) was mixed with an 5.

acetone solution (2 mL) of the nickel(II) complex 1, 3, 5 or 6 (2, 4, and 7 were not studied because of their insufficient solubility in acetone) to give the final nickel concentration of 2.5 mmol L -1 . The mixtures were homogenized and dried at room temperature for 24 h. The study of the catalytic influence of these samples on graphite was performed on a Netzsch STA 449C device with an α-Al 2 O 3 crucible without a standard (10 °C min -1 heating rate, sample weight of 5.0 mg, 100 mL min -1 dynamic air atmosphere). The intersection point of the DTG curve tangents represents the initial temperatures of the graphite oxidation. The kinetic parameters were calculated by a direct non-linear regression method [33]. [Ni(BzMedtc)(PPh 3 ) 2 ]PF 6 (7): The solutions of PPh 3 (2 mmol; 5 mL of acetone) and K[PF 6 ] (1 mmol; 7 mL of acetone) were separately added to a solution of 1 (1 mmol; 5 mL of chloroform). The reaction mixture was refluxed for a period of 4 h and after that it was filtered through activated carbon. Diethyl ether was added to the filtrate, which caused the formation of the precipitate. The solid was filtered off, washed with diethyl ether and dried at 40 °C under an infrared lamp. Yield: 54%. Colour: red. C 45 H 40 NS 2 P 3 F 6 Ni (M r = 924.5); C 58.5 (calc. 58.8); H 4.4 (4.6); N 1.6 (1.9); S 6.9 (6.5); Ni 6.3 (6.7)%.

Syntheses of the

Entry for the Table of Contents

Seven nickel(II) N-benzyl-N-methyldithiocarbamato (BzMedtc) complexes were prepared and characterized. The structures of [Ni(BzMedtc)(PPh 3 )(NCS)] ( 4) and [Ni(BzMedtc)(PPh 3 ) 2 ]ClO 4 •H 2 O (5•H 2 O) were determined by a single crystal X-ray analysis, which showed the Ni(II) centre as four-coordinated in a distorted square-planar geometry. The positive catalytic effect of the complexes 1, 3, 5, and 6 on the graphite oxidation was determined. The start temperatures of the oxidation of the complexes were by about 60-100 °C lower in comparison with pure graphite. Zdeněk Trávníček,* [a] Richard Pastorek, [a] Pavel Štarha, [a] Igor Popa, [a] and Václav Slovák [b] The nickel(II) N-benzyl-N-methyldithiocarbamato (BzMedtc 6) and [Ni(BzMedtc)(PPh 3 ) 2 ]PF 6 (7). The obtained complexes 1-7 were characterized by elemental analysis, thermal analysis and spectroscopic methods (IR, UV-Vis, 31 P{ 1 H}-NMR). The results of the magnetochemical and molar conductivity measurements proved the complexes as diamagnetic non-electrolytes (1-4) or 1:1 electrolytes (5-7).

The molecular structures of 4 and 5•H 2 O were determined by a single crystal X-ray analysis. In all cases, the Ni(II) atom is four-coordinated in a distorted square-planar arrangement with the S 2 PX, and S 2 P 2 donor set, respectively. The catalytic influence of selected complexes 1, 3, 5, and 6 on graphite oxidation was studied. The results clearly indicated that the presence of the products of thermal degradation processes of the mentioned complexes have impact on the course of graphite oxidation. A decrease in the oxidation start temperatures by about 60-100 °C was observed in the cases of all the tested complexes in comparison with pure graphite.

Introduction

Many compounds involving the dithiocarbamate (dtc) moiety were reported up to now in connection with their applications. These compounds have been found to be useful as fungicides [1] or accelerators of vulcanization [2]. However, the spectrum of potential use of these compounds may be also broadened on the field of in vitro anticancer activity as can be demonstrated by the following examples. The 4(3H)-quinazolinone derivatives with dithiocarbamate side chains, such as (3,4-dihydro-2-methyl-4-oxoquinazolin-6-yl)-methyl-[4-(4-fluorophenyl)piperazine]-1-carbodithioate, showed promising in vitro anticancer activity (IC 50 = 0.5 µM) against human myelogenous leukaemia cell line (K562) [3]. Another dithiocarbamate derivative, 4-methylsulfinyl-1-(S-methyldithiocarbamyl)-butane (sulforamate), serves as a bearer of cancer chemopreventive effect [4]. The efficient protection of N-benzyl-D-glucamine-dithiocarbamate against cisplatin-induced nephrotoxicity (it is able to chelate platinum and thus to decrease the possibility of its reactions with sulphur-containing renal proteins or enzymes) has also been reported recently [5].

Several thousands of transition metal complexes involving the dithiocarbamate moiety have been prepared and reported to date and some of them are very promising from the biological activity point of view. The Pt(II) complex of the [Pt(esdt)(py)Cl] composition, where esdt = ethylsarcosinedithiocarbamate and py = pyridine, is highly cytotoxic against several human cancer cell lines, and moreover, this substance was not found as nephrotoxic, contrary to cisplatin [6]. [Au(dmdt)X 2 ] and [Au(esdt)X 2 ] are the representatives of non-platinum complex with the promising anticancer activity, which is even higher compared to cisplatin; dmdt = N,N´dimethyldithiocarbamate, X = Cl or Br [7]. The antifungal activity, well-known for dithiocarbamate derivatives, was proved for tin(IV) complexes with pyrrolidinedithiocarbamate [8].

Nickel is known as a suitable transition metal for the preparation of complexes and it is not surprising that over three thousand compounds (SciFinder, 2010) and 163 X-ray structures (Crystallographic Structural Database, CSD, ver. 5.30, September 2009 update [9]) involving an NiS 2 CN motif have been reported to date. Among these compounds, the nickel(II) complexes involving the NiPN(S 2 CN) or This work also describes the effect of the nickel(II) dithiocarbamates on the oxidation of graphite as a model study of the coal combustion. As it is commonly known, the ignition temperature of the coal combustion is one of the most important properties influencing many industrial processes. This stage can be affected by the presence of both natural and artificial impurities [14,15]. Formerly, several papers reported simple nickel(II) compounds, such as NiO (mixed with KOH) or Ni(NO 3 ) 2 (mixed with K 2 SO 4 ), as the substances accelerating graphite oxidation [16,17]. Few years ago, we decided to study the influence of the nickel(II, IV) dithiocarbamato complexes [see 13,18,19 and references cited therein] on the course of graphite oxidation. It was found that the presence of these complexes causes the ignition temperature decrease. Similar results were obtained also for the tested complexes 1, 3, 5, and 6, which were chosen as representatives of the presented compounds, as it is discussed in more detail within the framework of this paper.

Results and Discussion

The nickel(II) complexes 1-7 were prepared according to the synthetic strategies depicted in Scheme 1. The obtained molar conductivity values (Table 1) indicate that complexes 1-4 dissolved in acetone behave as non-electrolytes [20], which indirectly proved the coordination of the Cl -(1), Br - (2), I -(3) and NCS -(4) anions to the Ni(II) atom. On the other hand, the molar conductivity values of the remaining complexes proved the ionic nature (1:1 electrolyte type) in acetone (5) or DMF (6, 7) solutions, which means that the ClO 4 -( 5), BPh 4 -( 6) and PF 6 -( 7) anions are situated outside of the inner coordination sphere within the structure of the discussed complexes. The results of the room temperature magnetochemical measurements determined the prepared compounds to be diamagnetic, and thus, suggested on square-planar geometries in the vicinity of the central atom.

Table 1:

The results of molar conductivity measurements, IR, UV-Vis and 31 P{H}-NMR spectroscopy of the complexes 1-7

IR (cm -1 ) b λ M a ν(C … S) ν(C … N) other UV-Vis (cm -1 )

Spectroscopic Characterization

The PPh 3 signals in the 31 P{ 1 H}-NMR spectra of the CDCl 3 solutions of 1-7 were found in the range of 21.1-32.1 ppm (Table 1). These values differ significantly from that of -4.52 ppm determined for the free PPh 3), which agrees with the results reported for the nickel(II) complexes with N-benzyl-N-butyldithiocarbamate [12].

The maxima observed in the IR spectra of the nickel(II) complexes 1-7 at 990-998 cm -1 , and 1510-1542 cm -1 (see Table 1) are typical for the ν(C … S), and ν(C … N) vibrations of the dithiocarbamate moiety, respectively [21,22]. The peak of the ν(C … S) stretching vibration is not split, which supports the bidentate coordination of the BzMedtc anion. The peaks of the ν(C≡N) and ν(C-S) vibrations of the NCS - anion were observed in the IR spectrum of 4. The ionic nature of the perchlorate anion in the complex 5, can be supported by two undivided peaks at 622 and 1088 cm -1 [23]. The same conclusion can be made for the complex 7, whose IR spectrum contains the non-split peak at 836 cm -1 assignable to ν(PF 6 -), which again indicates its ionic nature [24]. Diffuse-reflectance electronic spectra of the prepared complexes 1-7 support the assumption of the square-planar geometry in the vicinity of the central Ni(II) atom. Absorption maxima at the 18 400-20 700 cm -1 region may be attributed to the 1 A 1g → 1 B 1g transition of the squareplanar nickel(II) complexes [25,26]. The next maxima, also assignable to the d-d transitions, were found for the complexes 1 (25 200 cm -1 ) and 2 (25 000 cm -1 ). The maxima above 29 000 cm -1 are probably connected with the intraligand charge-transfer transitions in the S 2 CN moiety [27].

X-ray Structure of [Ni(NCS)(BzMedtc)(PPh 3 )] (4) and [Ni(BzMedtc)(PPh 3 ) 2 ]ClO 4 •H 2 O (5•H 2 O)

The X-ray structure of the title complexes 4 (Figure 1) and 5•H 2 O (Figure 2) was determined by a single crystal X-ray analysis. The crystal data and structure refinements are given in Table 2, while the selected bond lengths and angles are summarized in Table 3.

The central Ni(II) ion of the complex 4 (Figure 1) is fourcoordinated by PPh 3 , the isothiocyanate anion, and the bidentate-coordinated N-benzyl-N-methyldithiocarbamate anion (BzMedtc). The atoms of a NiS 2 NP chromophore are arranged in the distorted square-planar geometry around the metal centre (Table 3). The Ni-S bond lengths of the title complex correlate well with the mean value of 2.204 Å (2.155-2.258 Å interval) determined for this band for the 116 square-planar nickel complexes involving bidentate-coordinated dithiocarbamato S-donor ligands, which have been up to now deposited in the Cambridge Structural Database (CSD) [9].

The well-known delocalization of π-electron density within the S 2 CN moiety of the dithiocarbamate anion was observed for 4, since the C-N and C-S bond lengths of the studied compound (Table 2) were found to be significantly shorter compared to the single C-N (1.47 Å) and C-S (1.81 Å) σ-bond lengths [28]. Further, the C-S and C-N bond length values of BzMedtc are comparable with those of the transition metal (TM) complexes involving the (TM)S 2 CNR 2 moiety and deposited within the CSD, which equal 1.717 Å, and 1.326 Å, respectively. The S1 atom is the most deviated one [0.0454(5) Å] from the plane created through the Ni1, S1, S2, P1 and N2 atoms of the chromophore. Further, the described plane forms the dihedral angle of 83.10(5)° with the benzene ring of the BzMedtc anion and dihedral angles of 67.90(5)°, 69.63(5)° and 47.85(4)° with the benzene rings of PPh 3 containing the C30, C40, and C50 atoms, respectively. Mutual orientation of the phenyl groups of PPh 3 within the molecular structure of 4 can be described by the 67.64( 6)° (between benzene rings containing C30 and C40), 66.06( 6)° (between benzene rings containing C30 and C50) and 76.53( 6)° (between benzene rings containing C40 and C50) dihedral angles. The

C-H•••C, C-H•••S and C•••C
types of the intermolecular non-bonding van der Waals contacts were detected to stabilize the crystal structure of 4 (see Table 4). No typical hydrogen bond is present within the structure of discussed complex. In the case of 5•H 2 O, the Ni(II) ion is four-coordinated in the distorted square-planar geometry (Figure 2), similarly to the above described structure of complex 4, by the bidentate-coordinated S-donor BzMedtc ligand and by two P-donor PPh 3 molecules. Selected bond lengths and angles are given in Table 3. The positive charge of the [Ni(BzMedtc)(PPh 3 ) 2 ] + cation is compensated by the perchlorate anion; the shortest Ni1•••Cl1 distance equals 7.131(2) Å.

Table 3: Selected bond lengths (Å) and angles (°) for the complexes [Ni

(NCS)(BzMedtc)(PPh 3 )] (4) and [Ni(BzMedtc) (PPh 3 ) 2 ]ClO 4 •H 2 O (5•H 2 O) Compound 4 5•H 2 O Bond Lengths Ni1-S1 2.2245 (5) 2.2154(6) Ni1-S2 2.1758(5) 2.2257(6)

Table 4

Selected intermolecular non-bonding contacts and their parameters (Å, °) determined for the complexes 4 and 5

•H 2 O D-H•••A d(D-H) d(H•••A) d(D•••A) < (DHA) 4 C3-H3A•••S3 i 0.
C13•••C71 v 3.262(4) C70•••C70 iv 3.391(3) Symmetry codes: (i) x + 0.5, 0.5 -y, z -0.5; (ii) 1 -x, 1 -y, 1 -z; (iii) 1.5 -x, y + 0.5, 1.5 -z; (iv) -x, -y, -z; (v) -x, 1 -y, -z; (vi) 1 -x, 1 -y, -z; (vii) -x, 1 -y, 1 -z
As it has been already mentioned in the case of complex 4, the bond lengths within the NiS 2 CN moiety of both structures show on electron density redistribution in connection with π-electron density delocalization (see the C-S and C-N bond lengths in Table 3). As for the Ni-P bonds of 5, their lengths are in good agreement with the mean value of 2.208 Å (2.175-2.252 Å interval) as determined from 36 X-ray structures of tetra-coordinated nickel dithiocarbamato complexes involving at least one PPh 3 ligand and deposited within the CSD.

The S2 atom is the most deviated one [0.0903(6) Å] from the plane created through the atoms of the chromophore. The dihedral angles between this plane and benzene rings involving C10, C20, C30, C40, C50, C60, and C70 are 86.39(5)°, 58.49(7)°, 87.97(5)°, 70.52(5)°, 71.64(5)°, 63.60(6)°, and 71.98(5)°, respectively. Non-bonding interactions of the C-H

•••C and C•••C types (C-H•••O
interactions with the distorted perchlorate anion and water molecule of crystallization are disregarded) were found in the crystal structure of the discussed nickel(II) complex (Table 4).

TG/DTA Thermal Analysis

The thermal properties of the prepared nickel(II) complexes were studied by TG and DTA methods. The TG and DTA curves of 1 and 6 are depicted in Figure 3. Although the chemical composition of these complexes is different, their thermal decomposition proceeded in a similar manner as indicated by analogical shapes of the TG curves. In cases of 2, 3 and 6, the sharp exo-effects with maxima at 188 °C, 193 °C, and 188 °C, respectively, were accompanied by mass increase on the TG curves, which can be attributed to the oxygen atom insertion into the Ni-P bond [29]. This effect was not registered on the TG curves of the remaining complexes, where the oxygen atom insertion is most likely combined with the decomposition of the organic parts of the studied complexes.

The final products of the TG studies (900 °C) were studied by X-ray powder diffraction (XRD). NiO (PDF-4 No. 01-089-7130) was determined as the final product of the thermal decomposition of 1-4. In the cases of the complexes 6 and 7, the mixtures with significant predominance of NiO were detected. The final product of the TG thermal study of 6 also contains NiS (PDF-4 No. 4-004-5027) and Ni 3 B 2 O 6 (PDF-4 No. 4-008-3203), and some peaks could not be assigned. As for the complex 7, Ni 2 P 2 O 7 (PDF-4 No. 01-074-1604) and Ni 3 (PO 4 ) 2 (PDF-4 No. 01-072-3977) were detected by a XRD, while some peaks were not assigned.

Catalytic Influence of the Complexes on Graphite Oxidation

The influence of the presence of the representative complexes 1, 3, 5 and 6 on graphite oxidation were studied. The results, summarized in Figure 4 (obtained DTG curves) and Table 5 (obtained data), showed an positive influence on graphite oxidation, because the ignition temperature significantly decreased (by about 64-99 °C) after the addition of the tested nickel(II) complexes to a pure graphite.

Pure graphite oxidation proceeds in one step, while in the cases of its mixtures with the tested complexes, two steps in a larger temperature interval were detected on the appropriate DTG curves (Table 5). The first step takes place at lower temperature than pure graphite oxidation, whereas the oxidation rate maxima (T m ) of the second step approximately correspond to pure graphite. In addition, the oxidation mechanisms of both steps are different, as it can be seen from the activation energy (E) and frequency factor (A) values given in Table 5. More graphite was oxidized during the second step with higher E and A values.

At this point, we would like to discuss the described results of the influence on the graphite oxidation in connection with the results of TG/DTA studies. Figure 3 clearly shows that the thermal decomposition of the nickel(II) complexes is almost completely finished at the temperature of ca. 700 °C and the weight loss above this temperature is insignificant. Based on this statement it may be concluded that not directly the discussed nickel(II) Nbenzyl-N-methyldithiocarbamato complexes, but the products of their thermal decomposition processes, affected graphite oxidation. In connection with the above-mentioned facts, the tested complexes (except of 5) were heated in situ to the temperatures corresponding to the beginning and end of graphite oxidation (see Table 5). The products of thermal decomposition relating to the mentioned temperatures were analysed by XRD. As for 1-4, pure NiO was identified at both temperatures (Figure 5). The slight differences in the 2θ values were caused by a dilatation of the sample due to different measurement temperature. On the other hand, the mixtures of NiO, NiS and Ni and its mixture with the complexes 1 (black dashed lines), 3 (dark gray line), 5 (gray line) and 6 (light gray line); the tangent method was employed to determine the initial temperatures of the graphite oxidation Ni 3 (PO 4 ) 2 (for 7) were detected by an XRD (Figure 5). Their quantitative composition was similar at both temperatures. Some peaks could not be assigned again.

Conclusions

Seven nickel(II) complexes of the [Ni(BzMedtc)(PPh 3 )X] (1-4) and [Ni(BzMedtc)(PPh 3 ) 2 ]Y (5-7) compositions were synthesized; X = Cl -for 1, Br -for 2, I -for 3 and NCS -for 4, Y = ClO 4 -for 5, BPh 4 -for 6 and PF 6 -for 7). The obtained powder products were fully characterized by elemental analysis, magnetochemical and molar conductivity measurements, thermal analysis and spectroscopic methods (IR, UV-Vis, 31 P{ 1 H}-NMR). The molecular structures of the complexes [Ni(BzMedtc)(PPh 3 )(NCS)] ( 4) and [Ni(BzMedtc)(PPh 3 ) 2 ]ClO 4 •H 2 O (5•H 2 O) were determined by a single crystal X-ray analysis, which proved the distorted square-planar geometry with the bidentatecoordinated S-donor BzMedtc anion accompanied by PPh 3 and NCS -(4) or by two PPh 3 (5). This paper also describes the significant catalytic influence of the thermal degradation products of the selected complexes 1, 3, 5, and 6 on graphite oxidation. [Ni(PPh 3 ) 2 I 2 ] and [Ni(PPh 3 ) 2 (NCS) 2 ] were prepared according to the formerly reported synthetic pathways [13,30]. Elemental analyses (C, H, N) were performed on a Fisons EA-1108 CHNS-O Elemental Analyzer (Thermo Scientific). The content of nickel was determined using the chelatometric titration with murexide as an indicator. Chlorine, bromine and iodine contents were determined by the Schöniger method. Diffuse-reflectance spectra were recorded on a Perkin-Elmer Lamda35 UV/Vis spectrometer using a nujol technique. IR spectra (KBr pellets) were recorded on a Perkin-Elmer Spectrum one FT-IR spectrometer in the 450-4000 cm -1 region. The molar conductivity values of 10 -3 M acetone solutions of 1-5 and 5 × 10 -4 M N,N'-dimethylformamide (DMF) solutions of 6 and 7 were determined by an LF 330/SET conductometer (WTW GmbH) at 25 ºC. The room temperature magnetic susceptibilities were measured using the Faraday method with a laboratory designed instrument with a Sartorius 4434 MP-8 microbalance; Co[Hg(NCS) 4 ] was used as a calibrant and the correction for diamagnetism was performed using Pascal constants. Simultaneous thermogravimetric (TG) and differential thermal (DTA) analyses were performed by an Exstar TG/DTA 6200 (Seiko Instruments Inc.) in a platinum crucible and dynamic air atmosphere (100 mL min -1 ) from the laboratory temperature to 1050 °C with a 2.5 °C min -1 temperature gradient. The weights of the studied complexes 1-4, 6 and 7 (5 was not studied for safety reasons, since it contains the perchlorate anion) were ca. 10 mg. 31 P{ 1 H}-NMR spectra of the CDCl 3 solutions were measured on a Bruker Avance 300 spectrometer at 300 K and the spectra were calibrated against 85% H 3 PO 4 used as an external reference. X-ray powder diffraction experiments were performed with a PANalytical X'Pert PRO instrument (Co-Kα radiation) equipped with an X'Celerator detector. Samples were placed on a zero-background Si slide and scanned in the 2θ range of 5-90° in the steps of 0.017°. In situ measurements were realized in the reaction cell XRK900 (Anton Paar). Evaluation was made using HighScore Plus software and PDF-4 database.

Experimental Section

Materials and General Methods

Single Crystal X-ray Analysis of the Complexes 4 and 5•H 2 O

A single crystal X-ray measurement was performed on an Xcalibur TM The same software was used for data correction for an absorption effect by the empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm. Both structures were solved by direct methods using SHELXS-97 software [31] and refined on F 2 using the full-matrix least-squares procedure (SHELXL-97). Non-hydrogen atoms were refined anisotropically and all hydrogen atoms were located in difference Fourier maps and refined by using the riding model with C-H = 0.95, 0.98 and 0.99 Å, and U iso (H) = 1.2U eq (CH, CH 2 ) or 1.5U eq (CH 3 ), except for those belonging to the crystal water molecule in 5. The perchlorate anion as well as crystal water molecule was refined as disordered over two positions with the occupancy factors 75% and 25%, and 57% and 43%, respectively. The crystal data and structure refinements of 4 and 5 are given in Table 1. The molecular graphics were drawn and the additional structural calculations were interpreted using DIAMOND [32]. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK (Fax: +44-1223-336033; E-Mail: deposit@ccdc.cam.ac.uk).

Copies of the data can be obtained free of charge on application to CCDC, as the depository numbers CCDC-766048 (4) and CCDC-766049 (5•H 2 O). 5.

The Catalytic Study

Graphite (0.6 g, diameter of particles less than 0.1 mm, ash residue max. 0.2%, mass drying loss max. 0.2%) was mixed with an acetone solution (2 mL) of the nickel(II) complex 1, 3, 5 or 6 (2, 4, and 7 were not studied because of their insufficient solubility in acetone) to give the final nickel concentration of 2.5 mmol L -1 . The mixtures were homogenized and dried at room temperature for 24 h. The study of the catalytic influence of these samples on graphite was performed on a Netzsch STA 449C device with an α-Al 2 O 3 crucible without a standard (10 °C min -1 heating rate, sample weight of 5.0 mg, 100 mL min -1 dynamic air atmosphere). The intersection point of the DTG curve tangents represents the initial temperatures of the graphite oxidation. The kinetic parameters were calculated by a direct non-linear regression method [33]. [Ni(BzMedtc)(PPh 3 ) 2 ]ClO 4 (5): LiClO 4 •3H 2 O (2 mmol) was poured to a suspension of the complex 1 (1 mmol) and PPh 3 (2 mmol) in methanol (25 mL). The reaction mixtures were refluxed for 3 h. The resulting solution was filtered through activated carbon and left to stand at room temperature. The crystals of 5•H 2 O suitable for a single crystal X-ray analysis formed from the mother liquor in a few days. The complex was filtered off, washed with methanol and diethyl ether and dried at 40 °C under an infrared lamp. Yield: 47%. Colour: pink. [Ni(BzMedtc)(PPh 3 ) 2 ]PF 6 (7): The solutions of PPh 3 (2 mmol; 5 mL of acetone) and K[PF 6 ] (1 mmol; 7 mL of acetone) were separately added to a solution of 1 (1 mmol; 5 mL of chloroform). The reaction mixture was refluxed for a period of 4 h and after that it was filtered through activated carbon. Diethyl ether was added to the filtrate, which caused the formation of the precipitate. The solid was filtered off, washed with diethyl ether and dried at 40 °C under an infrared lamp. Yield: 54%. Colour: red. C 45 H 40 NS 2 P 3 F 6 Ni (M r = 924.5); C 58.5 (calc. 58.8); H 4.4 (4.6); N 1.6 (1.9); S 6.9 (6.5); Ni 6.3 (6.7)%. TG/DTA data: the decomposition began at 99 °C and finished at 782 °C, exothermic peaks with the maxima at 188, 207, 310 and 472 °C.

Syntheses of the Nickel(II)

  The nickel(II) N-benzyl-N-methyldithiocarbamato (BzMedtc) complexes [Ni(BzMedtc)(PPh 3 )Cl] (1), [Ni(BzMedtc)(PPh 3 )Br] (2), [Ni(BzMedtc)(PPh 3 )I] (3) and [Ni(BzMedtc)(PPh 3 )(NCS)] (4) were synthesized using the reaction of [Ni(BzMedtc) 2 ] and [NiX 2 (PPh 3 ) 2 ], where X = Cl, Br, I and NCS. Subsequently, the complex 1 was used for the preparation of [Ni(BzMedtc)(PPh 3 ) 2 ]ClO 4

  2 mol -1 (10 -3 M acetone solutions for 1-5 and 5 × 10 -4 M DMF solutions for 6 and 7); [b] KBr pellets (w = weak, m = middle, s = strong); [c] nujol technique; [d] CDCl 3 solutions (s = singlet, sp = septuplet)

  3 molecule, which is caused by electron density redistribution from phosphorus towards the Ni(II) centre as a consequence of PPh 3 coordination. The spectrum of 7 contains a septuplet at -143.6 ppm assignable to the PF 6 -anion. It should be noted that the chemical shift values of the complexes 1-4 increased in the order [Ni(Cl)(BzMedtc)(PPh 3 )] (1) < [Ni(NCS)(BzMedtc)(PPh 3 )] (4) < [Ni(Br)(BzMedtc)(PPh 3 )] (2) < [Ni(I)(BzMedtc)(PPh 3 )] (

Scheme 1 .

 1 Scheme 1. Schematic representation of the synthetic procedures applied for the preparation of the nickel(II) complexes 1-7; X = Cl -(1), Br -(2), I -(3) and NCS -(4), MY = LiClO 4 •3H 2 O (5), Na[BPh 4 ] (6) and K[PF 6 ] (7).

  Final R indices [I>2σ(I)] R 1 = 0.0282, wR 2 = 0.0673 R 1 = 0.0342, wR 2 = 0.1009 R indices (all data) R 1 = 0.0366, wR 2 = 0.0692 R 1 = 0.0388, wR 2 = 0.1031 Largest peak/hole (e Å -3 ) 0.344/-0.212 1.343 and -0.611

Figure 1 .

 1 Figure 1. The molecular structure of [Ni(BzMedtc)(PPh 3 )(NCS)] complex (4) with non-hydrogen atoms drawn as thermal ellipsoids at the 50% probability level.

  Figure 2. The molecular structure of [Ni(BzMedtc)(PPh 3 ) 2 ]ClO 4 •H 2 O (5•H 2 O) with non-hydrogen atoms drawn as thermal ellipsoids at the 50% probability level. The hydrogen atoms, perchlorate anion and water molecule of crystallization omitted for clarity.

Table 5 :

 5 Characteristic temperatures and kinetic parameters of catalytic influence on graphite given for pure graphite and its mixtures with nickel(II) complexes 1, 3, 5 and 6 Sample T (°C) T m (°C) Step n A (s -1 ) T = temperature range of oxidation; T m = oxidation rate maximum; n = reaction order; A = frequency factor; E = activation energy (kJ mol -1 ); w = mass of sample oxidized

Figure 3 .

 3 Figure 3. TG/DTA curves for complexes 1 and 6.

Figure 4 .

 4 Figure 4. DTG curves obtained for pure graphite (black lines) and its mixture with the complexes 1 (black dashed lines), 3 (dark gray line), 5 (gray line) and 6 (light gray line); the tangent method was employed to determine the initial temperatures of the graphite oxidation

Figure 5 .

 5 Figure 5. XRD patterns of the thermal decomposition products of the complexes 3 and 6. The detected diffraction lines belong to NiO for 3, and NiO, NiS and Ni 3 B 2 O 6 for 6; the start and end temperatures are given in Table5.

  The complex 1 was synthesized using the reaction of [Ni(BzMedtc) 2 ] (1 mmol) and [NiCl 2 (PPh 3 ) 2 ] (1 mmol). Both starting compounds were suspended in 25 mL of chloroform (CHCl 3 ). The mixture was stirred at room temperature until the reaction components were completely dissolved. The resulting solution was filtered through activated carbon. Diethyl ether was added to the filtrate which was left to stand at laboratory temperature. The precipitate of 1 formed in a few days. The product was filtered off, washed with diethyl ether and dried at 40 °C under an infrared lamp. The complexes 2-4 were prepared as described for 1, but [NiBr 2 (PPh 3 ) 2 ] (for 2), [NiI 2 (PPh 3 ) 2 ] (for 3) and [Ni(NCS) 2 (PPh 3 ) 2 ] (for 4) were used instead of [NiCl 2 (PPh 3 ) 2 ]. In the case of the complex 4, the crystals suitable for a single crystal X-ray analysis were obtained from the mother liquor in two days. [Ni(BzMedtc)(PPh 3 )Cl] (1): Yield: 78%. Colour: violet. C 27 H 25 NS 2 PClNi (M r = 552.8); C 58.7 (calc. 58.4); H 4.6 (4.6); N 2.5 (2.4); S 11.6 (11.3); Cl 6.4 (6.5); Ni 10.6 (10.6)%. [Ni(BzMedtc)(PPh 3 )Br] (2): Yield: 90%. Colour: violet. C 27 H 25 NS 2 PBrNi (M r = 597.2); C 54.3 (calc. 54.2); H 4.2 (4.1); N 2.3 (2.0); S 10.7 (10.3); Br 13.4 (13.1); Ni 9.8 (9.6)%. [Ni(BzMedtc)(PPh 3 )] (3): Yield: 70%. Colour: dark violet. C 27 H 25 NS 2 PINi (M r = 644.2); C 50.3 (calc. 49.9); H 3.9 (3.9); N 2.2 (2.2); S 10.0 (10.0); I 19.7 (19.3); Ni 9.1 (9.2)%. [Ni(BzMedtc)(PPh 3 )(NCS)] (4): Yield: 82%. Colour: red. C 28 H 25 N 2 S 3 PNi (M r = 575.4); C 58.5 (calc. 58.9); H 4.4 (4.1); N 4.9 (4.7); S 16.7 (16.6); Ni 10.2 (10.3)%. [Ni(BzMedtc)(PPh 3 ) 2 ]ClO 4 (5): LiClO 4 •3H 2 O (2 mmol) was poured to a suspension of the complex 1 (1 mmol) and PPh 3 (2 mmol) in methanol (25 mL). The reaction mixtures were refluxed for 3 h. The resulting solution was filtered through activated carbon and left to stand at room temperature. The crystals of 5•H 2 O suitable for a single crystal X-ray analysis formed from the mother liquor in a few days. The complex was filtered off, washed with methanol and diethyl ether and dried at 40 °C under an infrared lamp. Yield: 47%. Colour: pink. C 45 H 40 NS 2 P 2 ClO 4 Ni (M r = 879.0); C 61.5 (calc. 61.8); H 4.6 (4.3); N 1.6 (1.5); S 7.3 (6.9); Cl 4.2 (4.6); Ni 6.7 (6.8)%. [Ni(BzMedtc)(PPh 3 ) 2 ]BPh 4 (6): 2 mmol of Na[BPh 4 ] was added to a suspension of 1 (1 mmol) and PPh 3 (2 mmol) in 25 mL of methanol. The pink solid appeared during 3 h of the reaction mixture refluxing. The product was removed by filtration, washed (methanol and diethyl ether) and dried at 40 °C under an infrared lamp. Yield: 69%. Colour: pink. C 69 H 60 NS 2 P 2 BNi (M r = 1098.8); C 75.4 (calc. 75.1); H 5.5 (5.2); N 1.3 (1.5); S 5.8 (5.6); Ni 5.3 (5.7)%.

  ) complexes [Ni(BzMedtc)(PPh 3 )Cl] (1), [Ni(BzMedtc)(PPh 3 )Br] (2), [Ni(BzMedtc)(PPh 3 )I] (3) and [Ni(BzMedtc)(PPh 3 )(NCS)] (4) were synthesized using the reaction of [Ni(BzMedtc) 2 ] and [NiX 2 (PPh 3 ) 2 ], where X = Cl, Br, I and NCS. Subsequently, the complex 1 was used for the preparation of [Ni(BzMedtc)(PPh 3 ) 2 ]ClO 4 (5), [Ni(BzMedtc)(PPh 3 ) 2 ]BPh 4 (

  NiP 2 (S 2 CN) moiety, such as [Ni(4-MePzdtc)(PPh 3 )(NCS)][10], and [Ni(MeSdtc)(dppe)][11], respectively, have been described in the literature; 4-MePzdtc = 4-methylpiperazinedithicarbamate anion, PPh 3 = triphenylphosphine, MeSdtc = N-methyl-N-sulfonyldithiocarbamate, dppe = 1,2-bis(diphenylphosphine)ethane. However, no nickel complexes containing the N-benzyl-N-methyldithiocarbamate anion (BzMedtc) within the mentioned moieties, have been reported to date.The presented series of compounds follows a great number of nickel dithicarbamato complexes previously prepared at our department [see ref. 12,13 and the references cited therein]. In this paper, we deal with seven nickel(II) complexes, whose structures, according to the literature research, for the first time involves a combination of the BzMedtc BzMedtc)(PPh 3 )X] (1-4) were synthesized from [Ni(BzMedtc) 2 ], using its reaction with [NiX 2 (PPh 3 ) 2 ], where X stands for Cl -(for the complex 1), Br -(2), I -(3) and NCS -(4). The [Ni(BzMedtc)(PPh 3 ) 2 ]Y complexes (Y = ClO 4 -for 5, BPh 4 -for 6 and PF 6 -for 7) were prepared by the reaction of complex 1 with the appropriate alkaline salt and triphenylphosphine (see Scheme 1). The products were characterized by various physical methods including 31 P{ 1 H}-NMR spectroscopy. The X-ray structures of the representatives of both types of complexes, i.e. [Ni(BzMedtc)(PPh 3 )(NCS)] (4) and [Ni(BzMedtc)-(PPh 3 ) 2 ]ClO 4 •H 2 O (5•H 2 O), were determined by a single crystal X-ray analysis. The geometry of these compounds is square-planar with the NiS 2 PN (4) and NiS 2 P 2 (5•H 2 O) donor sets.

  2 mol -1 (10 -3 M acetone solutions for 1-5 and 5 × 10 -4 M DMF solutions for 6 and 7); [b] KBr pellets (w = weak, m = middle, s = strong); [c] nujol technique; [d] CDCl 3 solutions (s = singlet, sp = septuplet)

  3 molecule, which is caused by electron density redistribution from phosphorus towards the Ni(II) centre as a consequence of PPh 3 coordination. The spectrum of 7 contains a septuplet at -143.6 ppm assignable to the PF 6 -anion. It should be noted that the chemical shift values of the complexes 1-4 increased in the order [Ni(Cl)(BzMedtc)(PPh 3 )] (1) < [Ni(NCS)(BzMedtc)(PPh 3 )] (4) < [Ni(Br)(BzMedtc)(PPh 3 )] (2) < [Ni(I)(BzMedtc)(PPh 3 )] (

Scheme 1 .

 1 Scheme 1. Schematic representation of the synthetic procedures applied for the preparation of the nickel(II) complexes 1-7; X = Cl -(1), Br -(2), I -(3) and NCS -(4), MY = LiClO 4 •3H 2 O (5), Na[BPh 4 ] (6) and K[PF 6 ] (7).

  data collection (°) 2.90≤θ≤25.00 3.03≤θ≤25.00 Index ranges (h, k, l) Final R indices [I>2σ(I)] R 1 = 0.0282, wR 2 = 0.0673 R 1 = 0.0342, wR 2 = 0.1009 R indices (all data) R 1 = 0.0366, wR 2 = 0.0692 R 1 = 0.0388, wR 2 = 0.1031 Largest peak/hole (e Å -3 ) 0.344/-0.212 1.343 and -0.611

Figure 1 .

 1 Figure 1. The molecular structure of [Ni(BzMedtc)(PPh 3 )(NCS)] complex (4) with non-hydrogen atoms drawn as thermal ellipsoids at the 50% probability level.

  Figure 2. The molecular structure of [Ni(BzMedtc)(PPh 3 ) 2 ]ClO 4 •H 2 O (5•H 2 O) with non-hydrogen atoms drawn as thermal ellipsoids at the 50% probability level. The hydrogen atoms, perchlorate anion and water molecule of crystallization were omitted for clarity.

  T = temperature range of oxidation; T m = oxidation rate maximum; n = reaction order; A = frequency factor; E = activation energy (kJ mol -1 ); w = mass of sample oxidized

Figure 3 .

 3 Figure 3. TG/DTA curves for complexes 1 and 6.

Figure 4 .

 4 Figure 4. DTG curves obtained for pure graphite (black lines) and its mixture with the complexes 1 (black dashed lines), 3 (dark gray line), 5 (gray line) and 6 (light gray line); the tangent method was employed to determine the initial temperatures of the graphite oxidation

  Chemicals and solvents were purchased from Sigma-Aldrich Co., Acros Organics Co., Lachema Co. and Fluka Co. and they were used as received. The starting compounds [Ni(BzMedtc) 2 ], [Ni(PPh 3 )2Cl 2 ], [Ni(PPh 3 ) 2 Br 2 ],

Figure 5 .

 5 Figure 5. XRD patterns of the thermal decomposition products of the complexes 3 and 6. The detected diffraction lines belong to NiO for 3, and NiO, NiS and Ni 3 B 2 O 6 for 6; the start and end temperatures are given in Table5.

Complexes 1- 7 [

 7 Ni(BzMedtc)(PPh 3 )Cl] (1), [Ni(BzMedtc)(PPh 3 )Br] (2), [Ni(BzMedtc)(PPh 3 )] (3) and [Ni(BzMedtc)(PPh 3 )(NCS)] (4):The complex 1 was synthesized using the reaction of [Ni(BzMedtc) 2 ] (1 mmol) and [NiCl 2 (PPh 3 ) 2 ] (1 mmol). Both starting compounds were suspended in 25 mL of chloroform (CHCl 3 ). The mixture was stirred at room temperature until the reaction components were completely dissolved. The resulting solution was filtered through activated carbon. Diethyl ether was added to the filtrate which was left to stand at laboratory temperature. The precipitate of 1 formed in a few days. The product was filtered off, washed with diethyl ether and dried at 40 °C under an infrared lamp. The complexes 2-4 were prepared as described for 1, but [NiBr 2 (PPh 3 ) 2 ] (for 2), [NiI 2 (PPh 3 ) 2 ] (for 3) and [Ni(NCS) 2 (PPh 3 ) 2 ] (for 4) were used instead of [NiCl 2 (PPh 3 ) 2 ]. In the case of the complex 4, the crystals suitable for a single crystal X-ray analysis were obtained from the mother liquor in two days. [Ni(BzMedtc)(PPh 3 )Cl] (1): Yield: 78%. Colour: violet. C 27 H 25 NS 2 PClNi (M r = 552.8); C 58.7 (calc. 58.4); H 4.6 (4.6); N 2.5 (2.4); S 11.6 (11.3); Cl 6.4 (6.5); Ni 10.6 (10.6)%. TG/DTA data: the decomposition began at 177 °C and finished at 704 °C with the experimental weight loss of 84.5% (calc. to NiO residue: 86.5%), exothermic peaks with the maxima at 231, 302, 420 and 463 °C. [Ni(BzMedtc)(PPh 3 )Br] (2): Yield: 90%. Colour: violet. C 27 H 25 NS 2 PBrNi (M r = 597.2); C 54.3 (calc. 54.2); H 4.2 (4.1); N 2.3 (2.0); S 10.7 (10.3); Br 13.4 (13.1); Ni 9.8 (9.6)%. TG/DTA data: the decomposition began at 156 °C and finished at 1000 °C with the weight loss of 86.0% (calc. to NiO residue: 87.5%), exothermic peaks with the maxima at 188, 282, 398 and 490 °C. [Ni(BzMedtc)(PPh 3 )] (3): Yield: 70%. Colour: dark violet. C 27 H 25 NS 2 PINi (M r = 644.2); C 50.3 (calc. 49.9); H 3.9 (3.9); N 2.2 (2.2); S 10.0 (10.0); I 19.7 (19.3); Ni 9.1 (9.2)%. TG/DTA data: the decomposition began at 180 °C and finished at 829 °C with the weight loss of 86.6% (calc. to NiO residue: 88.4%), endothermic peak with the maximum at 269 °C and exothermic peaks with the maxima at 193, 246, 301, 457 and 544 °C. [Ni(BzMedtc)(PPh 3 )(NCS)] (4): Yield: 82%. Colour: red. C 28 H 25 N 2 S 3 PNi (M r = 575.4); C 58.5 (calc. 58.9); H 4.4 (4.1); N 4.9 (4.7); S 16.7 (16.6); Ni 10.2 (10.3)%. TG/DTA data: the decomposition began at 155 °C and finished at 1047 °C with the weight loss of 84.7% (calc. to NiO residue: 87.0%), endothermic peak with the maximum at 198 °C and exothermic peaks with the maxima at 227, 304, 336, 442 and 541 °C.

  C 45 H 40 NS 2 P 2 ClO 4 Ni (M r = 879.0); C 61.5 (calc. 61.8); H 4.6 (4.3); N 1.6 (1.5); S 7.3 (6.9); Cl 4.2 (4.6); Ni 6.7 (6.8)%. [Ni(BzMedtc)(PPh 3 ) 2 ]BPh 4 (6): 2 mmol of Na[BPh 4 ] was added to a suspension of 1 (1 mmol) and PPh 3 (2 mmol) in 25 mL of methanol. The pink solid appeared during 3 h of the reaction mixture refluxing. The product was removed by filtration, washed (methanol and diethyl ether) and dried at 40 °C under an infrared lamp. Yield: 69%. Colour: pink. C 69 H 60 NS 2 P 2 BNi (M r = 1098.8); C 75.4 (calc. 75.1); H 5.5 (5.2); N 1.3 (1.5); S 5.8 (5.6); Ni 5.3 (5.7)%. TG/DTA data: the decomposition began at 124 °C and finished at 764 °C, exothermic peaks with the maxima at 128, 154, 178, 524 and 607 °C.

  The molecular structure of [Ni(BzMedtc)(PPh3)(NCS)] complex(4) with non hydrogen atoms drawn as thermal ellipsoids at the 50% probability level. 1216x926mm (96 x 96 DPI) of [Ni(BzMedtc)(PPh3)2]ClO4•H2O (5•H2O) with non hydrogen atoms drawn as thermal ellipsoids at the 50% probability level. The hydrogen atoms, perchlorate anion and water molecule of crystallization were omitted for clarity. 1216x926mm (96 x 96 DPI) for pure graphite (black lines) and its mixture with the complexes 1 (black dashed lines), 3 (dark gray line), 5 (gray line) and 6 (light gray line); the tangent method was employed to determine the initial temperatures of the graphite oxidation 269x192mm (600 x 600 DPI) the thermal decomposition products of the complexes 3 and 6. The detected diffraction lines belong to NiO for 3, and NiO, NiS and Ni3B2O6 for 6; the start and end temperatures are given in Table 5. 269x192mm (600 x 600 DPI) the synthetic procedures applied for the preparation of the nickel(II) complexes 1-7; X = Cl-(1), Br-(2), I-(3) and NCS-(4), MY = LiClO4•3H2O (5), Na[BPh4] (6) and K[PF6] (7). 109x58mm (600 x 600 DPI)

Table 2 .

 2 Crystal data and structure refinements for [Ni(NCS)(BzMedtc)(PPh 3 )] (4) and [Ni(BzMedtc)(PPh 3 ) 2 ]ClO 4 •H 2 O (5•H 2 O)

	Compound	4	5•H 2 O
	Empirical formula	C 28 H 25 N 2 NiPS 3 C 45 H 42 ClNNiO 5 P 2 S 2
	Formula weight	575.36	897.02
	Temperature (K)	113(2)	110(2)
	Wavelength (Å)	0.71073	0.71073
	Crystal system	Monoclinic	Triclinic
	Space group	P2 1 /n	P-1
	a (Å)	13.6177(3)	11.47995(13)
	b (Å)	10.3556(2)	13.24049(16)
	c (Å)	19.1287(4)	15.02884(19)
	α (°)	90	70.3319(11)
	β (°)	92.4015(18)	84.5683(10)
	γ (°)	90	87.3995(9)
	V (Å 3 )	2695.6(9)	2141.23(4)
	Z, D -16≤h≤16	-13≤h≤13
		-9≤k≤12	-15≤k≤15
		-22≤l≤22	-17≤l≤15
	Reflections collected/unique	22656/4743	18531/7526
	(R int )	(0.0246)	(0.0108)
	Max./min. transmission	0.7825/0.7471 0.8099/0.7578
	Data/restraints/parameters	4743/0/317	7526/0/553
	Goodness-of-fit on

calc (g cm -3 ) 4, 1.418 2, 1.391 Absorption coefficient 1.032 mm -1 0.734 mm -1 Crystal size (mm) 0.30×0.25×0.25 0.40×0.35×0.30 F (000) 1192 932 θ range for data collection (°) 2.90≤θ≤25.00 3.03≤θ≤25.00 Index ranges (h, k, l)

Table 3 :

 3 Selected

		bond lengths (Å) and angles (°) for the
	complexes [Ni(NCS)(BzMedtc)(PPh 3 )] (4) and [Ni(BzMedtc) (PPh 3 ) 2 ]ClO 4 •H 2 O (5•H 2 O)
	Compound	4	5•H 2 O
	Bond Lengths
	Ni1-S1	2.2245(5)	2.2154(6)
	Ni1-S2	2.1758(5)	2.2257(6)
	Ni1-P1	2.2162(5)	2.2165(6)
	Ni1-P2	-	2.2316(6)
	Ni1-N2	1.8564(17)	-
	C1-S1	1.7125(19)	1.731(2)
	C1-S2	1.7214(19)	1.721(2)
	C1-N1	1.306(2)	1.307(3)
	N2-C4	1.163(2)	-
	C4-S3	1.621(2)	-
	Bond Angles
	S1-Ni1-S2	78.643(19)	78.29(2)
	S1-Ni1-P1	171.63(2)	91.28(2)
	S1-Ni1-N2	92.06(5)	-
	S1-Ni1-P2	-	167.08(2)
	S2-Ni1-P1	93.384(19)	167.76(2)
	S2-Ni1-P2	-	88.91(2)
	S2-Ni1-N2	170.70(5)	-
	P1-Ni1-N2	95.91(5)	-
	P1-Ni1-P2	-	101.24(2)
	Ni1-S1-C1	85.71(7)	86.52(8)
	Ni1-S2-C1	87.04(7)	86.42(8)
	S1-C1-S2	108.60(11)	108.61(12)
	S1-C1-N1	125.67(15)	126.50(17)
	S2-C1-N1	125.73(15)	124.89(17)
	Ni1-N2-C4	167.29(16)	-
	N2-C4-S3	178.52(19)	-
	In the case of 5•H 2 O, the Ni(II) ion is four-coordinated in
	the distorted square-planar geometry (Figure 2), similarly to
	the above described structure of complex 4, by the
	bidentate-coordinated S-donor BzMedtc ligand and by two
	P-donor PPh 3 molecules. Selected bond lengths and angles

Table 4

 4 Selected intramolecular non-bonding interactions and their parameters (Å, °) determined for the complexes 4

and 5•H 2

  2 diffractometer (Oxford Diffraction Ltd.) with Sapphire2 CCD detector, and with Mo Kα (Monochromator Enhance, Oxford Diffraction Ltd.) and at 113 K (4) and 110 K (5•H 2 O). Data collection and reduction were performed by a CrysAlis software [CrysAlis CCD and CrysAlis RED, Version 1.171.33.52, Oxford Diffraction Ltd., Abingdon, England, 2009].

Table 2 .

 2 Crystal data and structure refinements for [Ni(NCS)(BzMedtc)(PPh 3 )] (4) and [Ni(BzMedtc)(PPh 3 ) 2 ]ClO 4 •H 2 O (5•H 2 O)

	Compound	4	5•H 2 O
	Empirical formula	C 28 H 25 N 2 NiPS 3 C 45 H 42 ClNNiO 5 P 2 S 2
	Formula weight	575.36	897.02
	Temperature (K)	113(2)	110(2)
	Wavelength (Å)	0.71073	0.71073
	Crystal system	Monoclinic	Triclinic
	Space group	P2 1 /n	P-1
	a (Å)	13.6177(3)	11.47995(13)
	b (Å)	10.3556(2)	13.24049(16)
	c (Å)	19.1287(4)	15.02884(19)
	α (°)	90	70.3319(11)
	β (°)	92.4015(18)	84.5683(10)
	γ (°)	90	87.3995(9)
	V (Å 3

Table 5 :

 5 Characteristic temperatures and kinetic parameters of catalytic influence on graphite given for pure graphite and its mixtures with nickel(II) complexes 1, 3, 5 and 6

	Sample T (°C)	T m (°C) Step n	A (s -1 )	E	w (%)
	Graphite 778-859 832		0.7 1.97×10 9 247
	1	679-838 792	I	1.3	3.62×10 7	190	43.58
				II	0.9	1.09×10 9	235	56.42
	3	696-879 838	I	0.9	4.08×10 6	174	34.48
				II	0.9	9.10×10 7	220	65.52
	5	693-860 815	I	1.0	1.47×10 7	183	36.06
				II	1.0	4.08×10 9	250	63.94
	6	714-861 812	I	0.8	5.88×10 6
				II	1.1	1.17×10 9

Acknowledgement

This work was financially supported by The Ministry of Education, Youth and Sports of the Czech Republic (a grant no. MSM6198959218). The authors also thank Assoc. Prof. Zdeněk Šindelář for magnetic susceptibility measurements, Dr. Jan Filip for X-Ray powder diffraction experiments.

Acknowledgement

This work was financially supported by The Ministry of Education, Youth and Sports of the Czech Republic (a grant no. MSM6198959218). The authors also thank Assoc. Prof. Zdeněk Šindelář for magnetic susceptibility measurements, and Dr. Jan Filip for X-Ray powder diffraction experiments.

Entry for the Table of Contents

Seven nickel(II) N-benzyl-N-methyldithiocarbamato (BzMedtc) complexes were prepared and characterized. The structures of [Ni(BzMedtc)(PPh 3 )(NCS)] ( 4) and [Ni(BzMedtc)(PPh 3 ) 2 ]ClO 4 •H 2 O (5•H 2 O) were determined by a single crystal X-ray analysis, which showed the Ni(II) centre as four-coordinated in a distorted square-planar geometry. The positive catalytic effect of the complexes 1, 3, 5, and 6 on the graphite oxidation was determined. The start temperatures of the oxidation of the complexes were by about 60-100 °C lower in comparison with pure graphite. Zdeněk Trávníček,* [a] Richard Pastorek, [a] Pavel Štarha, [a] Igor Popa, [a] and Václav Slovák [b]


Uwe Ruschewitz
email: uwe.ruschewitz@uni-koeln.de[

Heinrich Billetter

Tim Wallraff

Ronald I Smith

Ulrich Schwarz

Prof Bernd Harbrecht

Ternary Transition Metal Acetylides A(I

Keywords: Acetylide , Alkali Metal, Neutron Diffraction, Palladium, Platinum Acetylide, Alkali Metal, Band Structure, Neutron Diffraction, Palladium, Platinum

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The crystal structures of K 2 PdC 2 , K 2 PtC 2 , and Rb 2 PdC 2 ( 1 m 3 P , Z = 1) were refined from time-of-flight powder neutron diffraction data (Polaris, ISIS/RAL) at room temperature and 4.5 K. The resulting C-C distances within the C 2 2-anion are in the range 124.4 -124.7 pm (295 K). This is distinctively longer than the expected distance for a C-C triple bond (120 pm). At low temperatures slightly longer C-C distances are found (125.0 -126.7 pm), which is due to an increasing wobbling of the C 2 dumbbells around their centre of gravity with increasing temperature. This is also reflected in the temperature dependence of the lattice parameters a and c. Lattice parameter c, which is parallel to the C 2 dumbbell, decreases with increasing temperature, whereas a shows a typical positive thermal expansion. Differing to previous work ternary palladium acetylides A 2 PdC 2 were obtained as red (A = Na) and yellow powders (A = K, Rb) by using a slightly improved synthesis. By means of diffuse reflectivity direct band gaps were determined to 2.09 eV (A = Na), 2.55 eV (A = K), and 2.77 eV (A = Rb). This is in good agreement with direct band gaps obtained from band structure calculations. These band structure calculations indicate very small indirect band gaps of only a few tenth of an eV.

____________

Introduction

In the last decade we have been able to synthesize and characterize numerous ternary alkali metal transition metal acetylides [1]. Na 2 PdC 2 and Na 2 PtC 2 were the first examples of this new class of compounds [2]. Their crystal structures

consist of ] ) C ( M [ 2 2 / 2 2 0 1 - ∞
chains, which are separated by sodium cations. In a more detailed analysis it was shown that the C-C distances are elongated compared to a C-C triple bond [3]. Based on band structure calculations this finding was attributed to a strong backbonding from palladium and platinum into the antibonding states of the C 2 units. This was confirmed by Raman spectroscopic data. Measurements of the electrical conductivity and band structure calculations revealed that these compounds are semiconductors with very small indirect band gaps (approx. 0.2 eV). In these investigations Na 2 PdC 2 and Na 2 PtC 2 were always obtained as black powders. In subsequent work also the analogous potassium [3], rubidium and cesium compounds [4] were prepared, again as black powders.

] ) C ( M [ 2 / 2 2 0 1 - ∞
chains were observed in compounds of composition AM I C 2 (A = Li-Cs, M I = Cu [5], Ag [6], Au [7]). These compounds are insulators and colorless (M I = Cu, Ag) or yellow (M I = Au). Only slightly elongated C-C distances with respect to a C-C triple bond were found in these compounds indicative for a weaker backbonding from the metal M + into the antibonding states of the C 2 units. But the different electronic properties of compounds of composition A 2 M 0 C 2 and AM I C 2 despite their similar crystal structures are intriguing and still not fully understood, although some explanations based on band structure calculations have been given [3].

In the following we will present a detailed structural analysis of some compounds of composition A 2 M 0 C 2 as well as new measurements and calculations of the electronic properties of these compounds. The former have been feasible as by a slightly improved synthesis samples of ternary palladium acetylides A 2 PdC 2 have been accessible, which show a red (A = Na) and yellow colour (A = K, Rb), resp.

Results and Discussion

The crystal structures of K 2 PdC 2 , K 2 PtC 2 , and Rb 2 PdC 2 were refined in space group 1 m 3 P using time-of-flight powder neutron diffraction data. These refinements confirm the crystal structures already obtained from X-ray powder diffraction data [3,4]. All compounds crystallize isostructural to Na 2 PdC 2 [2]. As an example the crystal structure of K 2 PdC 2 is shown in Figure 1 chains are formed, which run parallel to each other along the c axis. These chains are separated by the K + cations. Each C 2 dumbbell is coordinated by six K + cations that form a distorted octahedral coordination. The K + cations are surrounded very asymmetrically by three Pd atoms and three C 2 anions in a more or less side-on fashion. Details of the structural investigations on K 2 PdC 2 at 295 K and 4.5 K as well as selected distances and angles are given in Table 1. At low temperatures (4.5 K) the C-C distance within the C 2 anion was determined to 125.6 pm, which is distinctly larger than the value expected for a C-C triple bond (120 pm) [START_REF] Pauling | The Nature of the Chemical Bond[END_REF]. This is in agreement with the results obtained for Na 2 PdC 2 and Na 2 PtC 2 [3]. It was shown that a strong backbonding from Pd/Pt into the antibonding states of the C 2 unit is responsible for these elongated C-C bonds [3]. This is confirmed by the Pd-C distances (201.9 pm at 4.5 K), which are shorter than the values calculated from theoretical radii [START_REF] Pauling | The Nature of the Chemical Bond[END_REF]. Therefore the following mesomeric Lewis structures can be formulated:

…-Pd-C≡C-Pd-… ↔ …=Pd=C=C=Pd=… (1) (2)
K 2 PdC 2 is best described by a mixture of both structures, (2) takes the backbonding from palladium into the antibonding orbitals of the C 2 unit into account. The K-C distances are similar to those found in K 2 C 2 [START_REF] Hemmersbach | [END_REF], and the K-Pd distances (328.1 pm) are short, but not unprecedented in the literature (e.g. K 3 PdD 3 : K-Pd = 302.6 pm [10]). [a] Bank 1/Bank 2/Bank 3 (see Experimental Section)

In Table 1 structural parameters obtained at 295 K and 4.5 K are given. Surprisingly, some distances and lattice parameter c increase with decreasing temperature. This is also shown in Figure 2, where the temperature dependence of lattice parameters of K 2 PdC 2 is shown, as investigated in the temperature range 295 -4.5 K. This negative thermal expansion of c is due to the fact that the C 2 dumbbells, which are aligned parallel to c, wobble around their centre of gravity. As this wobbling increases with increasing temperature, c decreases, whereas a increases with increasing temperature. This behavior is also reflected in the C-C and Pd-C distances. The C-C distance obtained at 4.5 K can be assumed as being the "real" distance, whereas at 295 K only a projection of the vibrating C 2 dumbbell is obtained, which necessarily leads to a shorter C-C distance. The same effect can be found -in a less pronounced form -for the Pd-C distance. 0.033/0.027/0.036 [a] 0.027/0.020/0.041 [a] 0.015/0.013/0.015 [a] 0.010/0.008/0.021 [a] No. of refined parameters 59 59 K -K / pm K -Pt / pm 377.4(2) 3x 328.9(1) 3x 376.9(2) 3x 327.26 [START_REF] Hemmersbach | [END_REF] 2 and3 Comparing some structural features of K 2 PdC 2 and K 2 PtC 2 it is intriguing that the unit cell volume and lattice parameter c are distinctively smaller in the platinum compound. Furthermore the Pd-C distance is 3 pm longer than the Pt-C distance. All these effects are due to relativistic effects, which should peak at the pair Ag/Au, but are still significant for Pd/Pt [13].

As noted in Table 2 and 3 K 2 PtC 2 and Rb 2 PdC 2 contain some unreacted Pt and Pd, resp., whereas K 2 PdC 2 was obtained as a single-phase product. This result of the refinements is somewhat surprising, as a surplus of the respective alkali metal acetylide was used in all syntheses, which was not detected in the neutron diffraction patterns. K 2 PdC 2 and Rb 2 PdC 2 were obtained as yellowish powders with no visible amounts of black palladium powder. 0.034/0.027/0.030 [a] 0.024/0.018/0.032 [a] 0.021/0.019/0.020 [a] 0.013/0.009/0.022 [a] No Based on the crystal structures obtained at 4.5 K band structures were calculated for K 2 PdC 2 , K 2 PtC 2 , and Rb 2 PdC 2 . They all look similar and are comparable to the one published for Na 2 PdC 2 [3]. As an example the band structure obtained for K 2 PdC 2 is shown in Figure 3. As a characteristic feature a very small indirect band gap (A-L) of approx. 0.2 eV and a direct band gap (A-A) of more than 2 eV is found. The small indirect band gap is mainly caused by a large dispersion of the conduction band between A and L [3]. The direct band gaps obtained from the band structure calculations of K 2 PdC 2 and Rb 2 PdC 2 are given in Table 4. They range from 2.30 eV to 2.37 eV and should lead to absorptions in the visible region of the electromagnetic spectrum. In order to probe the optical band gap, diffuse reflectivity spectra of several ternary palladium and platinum acetylides were recorded. Meaningful spectra were obtained only for compounds, whose powders show a non-black color (Na 2 PdC 2 : red; K 2 PdC 2 , Rb 2 PdC 2 : yellow). The results are shown in Figure 4. The band gaps were taken as the inflection points of these spectra. They are listed in Table 4. K 2 PtC 2 and Cs 2 PdC 2 [4] could only be obtained as black powders. Their diffuse reflectivity spectra did not give any significant inflection points to determine the band gaps of these compounds. In Table 4 theoretically and experimentally obtained direct band gaps of Na 2 PdC 2 , K 2 PdC 2 , and Rb 2 PdC 2 are summarized. The experimental data fit nicely to the colors of the compounds under investigation. The size of the direct band gaps obtained from band structure calculations is in good agreement with the experimental band gaps. But the trend -increasing band gaps from Na 2 PdC 2 to Rb 2 PdC 2 -is not reproduced. On the other hand it is surprising at all that such a good agreement between experimental and theoretical band gaps is obtained, as theoretical calculations based on the LMTO method usually tend to underestimate band gaps by up to 50 %.

Conclusions

Neutron diffraction studies on K 2 PdC 2 , K 2 PtC 2 , and Rb 2 PdC 2 confirm the structural trends found for Na 2 PdC 2 and Na 2 PtC 2 [3]. C-C bond distances are elongated compared to a C-C triple bond. For Na 2 PdC 2 and Na 2 PtC 2 it was shown that this is due to a backbonding from the metal (Pd, Pt) into the antibonding states of the C 2 anion [3]. Similar effects must be assumed for the ternary acetylides of the heavier alkali metals.

By a slightly modified synthesis, which mainly consists of a repeated grinding and heating of the resulting samples, ternary palladium acetylides A 2 PdC 2 (A = Na, K, Rb) were obtained as colored powders (A = Na: red; A = K, Rb: yellow). Using these samples the direct band gaps could be determined. Na 2 PdC 2 and K 2 PdC 2 were obtained as samples free of unreacted palladium. Rb 2 PdC 2 however contains 14 wt. % palladium. But the amount seems to be small enough so that a determination of the direct band gap was still possible. Despite several attempts to optimize their syntheses Cs 2 PdC 2 as well as all ternary alkali metal platinum acetylides could only be obtained as black powders so that a determination of their optical (direct) band gaps was not possible. From neutron diffraction studies it was determined that the sample of K 2 PtC 2 contains 27 wt. % unreacted platinum. It must be assumed that this amount is too high to obtain a colored powder suitable for diffuse reflectivity measurements.

The optical band gap of A 2 PdC 2 (A = Na, K, Rb) is shifted from 594 nm ≡ 2.09 eV (A = Na) to 448 nm ≡ 2.77 eV (A = Rb). Unfortunately, samples of Cs 2 PdC 2 could not be measured, but following the trend a larger band gap can be expected for this compound. One could even think of a "band gap tuning" by using quaternary acetylides with a mixed occupancy of the alkali metal sites. Actually, we have been able to synthesize compounds of composition AA'PdC 2 (A, A' = alkali metal) [14], but up to now all these compounds could only be obtained as black powders. The optimization of their syntheses is part of our current research efforts in this field as well as the synthesis of the "missing" members Li 2 M 0 C 2 (M 0 = Pd, Pt). But their synthesis seems to be difficult. It can be speculated that Li 2 C 2 , which is the only thermodynamically stable binary alkali metal acetylide [15], is less reactive than the binary acetylides of the heavier alkali metals. Possibly for the same reasons all our attempts to synthesize ternary alkaline earth metal acetylides of composition A II M 0 C 2 (A II = Mg -Ba; M 0 = Pd, Pt) have also failed up to now. CaC 2 , SrC 2 , and BaC 2 are thermodynamically stable compounds, whereas MgC 2 shows a low thermal stability and decomposes at approx. 770 K [16]. Therefore it seems to be necessary to develop a completely new synthetic strategy for the synthesis of these compounds.

Experimental Section

Synthesis. Na 2 PdC 2 , K 2 PdC 2 , K 2 PtC 2 , and Rb 2 PdC 2 were synthesized by reacting the respective alkali metal acetylide in a slight surplus and fine Pd/Pt sponge [START_REF] Brauer | Handbuch der Präparativen Anorganischen Chemie[END_REF] in an inert Argon atmosphere. Na 2 C 2 , K 2 C 2 , and Rb 2 C 2 were obtained as described in the literature [START_REF] Hemmersbach | [END_REF]11]. Prior to the reaction the purity of these alkali metal acetylides was checked by X-ray powder diffraction (Huber G670, CuKα 1 radiation, capillary Ø 0.3 mm). K 2 PdC 2 : In a typical reaction 613 mg (6 mmol) K 2 C 2 and 530 mg (5 mmol) Pd powder were mixed, carefully ground and sealed in a glass ampoule in an argon atmosphere (1 bar). The glass ampoule was heated in a furnace at 553 K over night. The resulting powder was ground again and the whole procedure was repeated up to three times, until a yellow powder was obtained. Na 2 PdC 2 , K 2 PtC 2 , and Rb 2 PdC 2 were obtained in analogous reactions. The resulting samples are red (Na 2 PdC 2 ) and yellow (Rb 2 PdC 2 ), only K 2 PtC 2 remained as a black powder after this treatment. All sample handling was carried out in a glovebox in an argon atmosphere. Sample purity was checked by X-ray powder diffraction (Huber G670, CuKα 1 radiation, capillary Ø 0.3 mm). K 2 PtC 2 and Rb 2 PdC 2 contain minor amounts of unreacted Pt and Pd, resp. This was confirmed by neutron powder diffraction data (see Table 2 and3). Neutron powder diffraction. Time-of-flight neutron powder diffraction data were collected on the Polaris diffractometer [START_REF] Hull | [END_REF] at the ISIS spallation source (Rutherford Appleton Laboratory, UK). Approx. 3 g of each sample (K 2 PdC 2 , K 2 PtC 2 , Rb 2 PdC 2 ) were loaded into thin-walled cylindrical vanadium sample cans (Ø 6 mm, height ≈ 50 mm, wall thickness ≈ 0.15 mm), which were sealed air-tight with indium wire. All sample handling was carried out in a glovebox (argon atmosphere). For low temperature measurements an AS Scientific "orange" helium cryostat was used, which allowed measurements at temperatures down to 4.5 K. Data collected in three detector banks centred at 2θ = 90° (d max = 4.2Å, Bank 1), 2θ = 145° (d max = 3.2Å, Bank 2), and 2θ = 35° (d max = 8.2Å, Bank 3) were used in structure refinement. All refinements were carried out with the GSAS suite of programs [19]. Further details of the crystal-structure investigations may be obtained from the Fachinformationzentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen, Germany, on quoting the depository numbers CSD-421489 (K 2 PdC 2 , RT), CSD-421490 (K 2 PdC 2 , 4.5 K), CSD-421491 (K 2 PtC 2 , RT), CSD-421492 (K 2 PtC 2 , 4.5 K), CSD-421493 (Rb 2 PdC 2 , RT), CSD-421494 (Rb 2 PdC 2 , 4.5 K) (http://www.fiz-karlsruhe.de/). Diffuse reflectivity. Optical band gaps were determined from diffuse reflection (Varian Cary 500, white standard BaSO 4 , ratio 10 : 1) under inert conditions at room temperature. The absorption energy was taken as the inflection point of the step-like decrease of the diffuse reflectivity. Band-structure calculations. For electronic band-structure calculations, the LMTO method [START_REF] Andersen | Tight-Binding Approach to Computational Materials Science[END_REF]21] within the local density approximation (LDA), taking the von Barth-Hedin approximation, was used [22]. Atomic-sphere radii were chosen by an automated procedure for the TB-LMTO-ASA calculations. Scalar relativistic corrections were included and the calculations were checked for k- point convergence.

Figure 1 .

 1 Figure 1. Crystal structure of K 2 PdC 2 with K (large grey balls), Pd (large dark balls) and C (small white balls). C-C bonds are drawn as white solid lines, Pd-C (dark), K-C (grey), and K-Pd (grey) contacts are drawn as dotted lines. The K 6 polyhedron around one C 2 unit is emphasized.

Figure 2 .

 2 Figure 2. Temperature dependence of lattice parameters of K 2 PdC 2 as obtained from neutron powder diffraction data (Polaris, ISIS/RAL): a (empty squares), c (filled circles).

  , resp. Again C-C distances of approx. 125.0 pm and 126.7 pm are found comparable to that of K 2 PdC 2 . Similar effects for the temperature dependence of lattice parameters, C-C, and Pd/Pt-C distances are observed for these compounds. It should be mentioned that despite the negative thermal expansion of lattice parameter c the unit cell volume shows the usual positive thermal expansion. The resulting distances are also comparable to those of known compounds (e.g. Rb 2 C 2 : Rb-C = 308.6 pm[11]; Rb 3 PdD 3 : Rb-Pd = 323.6 pm[12]) so that K 2 PtC 2 and Rb 2 PdC 2 also fit nicely into the trend of ternary palladium and platinum acetylides.

Figure 3 .

 3 Figure 3. Band structure of K 2 PdC 2 . The Fermi level is at E = 0 eV, the symmetry points are given in the standard notation for the hexagonal cell.

Figure 4 .

 4 Figure 4. Diffuse reflectivity of Na 2 PdC 2 , K 2 PdC 2 , and Rb 2 PdC 2 depending on the radiation wavelength.

Table 1 .

 1 K 2 PdC 2 : summary of neutron powder diffraction investigations (time-of-flight method, 3 detector banks, Polaris at ISIS, Rutherford Appleton Laboratory, UK).

		295 K		4.5 K
	Space group, Z	P	3	1 m	(no. 164), 1	P	3	1 m	(no. 164), 1
	a / pm	510.575(4)	507.646(5)
	c / pm	528.211(7)	529.392(8)
	V /nm 3	0.119249(2)	0.118149(2)
	K on 2 d	z = 0.2788(2)	z = 0.2787(2)
	U iso / pm 2	239(3)		31(2)
	Pd on 1 a
	U iso / pm 2	143(2)		14(1)
	C on 2 c	z = 0.38207(8)	z = 0.38135(7)
	U iso / pm 2	134(1)		25(1)

Bragg 0.129/0.086/0.042

[a]

0.030/0.019/0.039

[a]

R P 0.032/0.032/0.034

[a]

0.018/0.015/0.017

[a]

wR P 0.029/0.026/0.035

[a]

0.012/0.009/0.018

[a]

Table 2 .

 2 K 2 PtC 2 : summary of neutron powder diffraction investigations (time-of-flight method, 3 detector banks, Polaris at ISIS, Rutherford Appleton Laboratory, UK). The sample contains 27(1) wt. % platinum.

		295 K		4.5 K
	Space group, Z	P	3	1 m	(no. 164), 1	P	3	1 m	(no. 164), 1
	a / pm	512.322(7)	509.41(1)
	c / pm	521.81(1)	522.69(2)
	V /nm 3	0.118612(3)	0.117467(5)
	K on 2 d	z = 0.2754(5)	z = 0.2746(4)
	U iso / pm 2	224(7)		58(5)

  Structural details of K 2 PtC 2 and Rb 2 PdC 2 are summarized in Table

			3x
	K -C / pm	300.83(5) 3x	299.26(4) 3x
		346.0(2) 3x	345.0(1) 3x
	Pt -C / pm	198.6(1) 2x	198.8(1) 2x
	C -C / pm	124.7(2)	125.0(2)
	[a] Bank 1/Bank 2/Bank 3 (see Experimental Section)

Table 3 .

 3 Rb 2 PdC 2 : summary of neutron powder diffraction investigations (time-of-flight method, 3 detector banks, Polaris at ISIS, Rutherford Appleton Laboratory, UK). The sample contains 14(1) wt. % palladium.

		295 K		4.5 K
	Space group, Z	P	3	1 m	(no. 164), 1	P	3	1 m	(no. 164), 1
	a / pm	535.675(4)	532.288(5)
	c / pm	528.888(6)	529.932(9)
	V /nm 3	0.131431(2)	0.130030(2)
	Rb on 2 d	z = 0.2786(1)	z = 0.2792(1)
	U iso / pm 2	232(2)		26(1)
	Pd on 1 a
	U iso / pm 2	141(2)		8(1)
	C on 2 c	z = 0.3824(1)	z = 0.3805(1)

Table 4 .

 4 Theoretical and experimental direct band gaps of ternary acetylides A I 2 Pd 0 C 2 (A I = Na, K, Rb) as determined by band structure calculations and measurements of diffuse reflectivity.

		Colour	Experimental	Theoretical direct
			direct band gap	band gap
	Na 2 PdC 2	Red	594 nm ≡ 2.09 eV	2.78 eV
	K 2 PdC 2	Yellow	486 nm ≡ 2.55 eV	2.30 eV
	Rb 2 PdC 2	Yellow	448 nm ≡ 2.77 eV	2.37 eV

Acknowledgments

The help of Dr. Winfried Kockelmann with collecting neutron powder diffraction data is greatly acknowledged.

Wiley


Bernhard Neumüller

Dr Kurt Dehnicke
email: dehnicke@chemie.uni-marburg.de

3 CN) 2 ]

Keywords: Berylliumbromide-acetonitrile complex, X-ray diffraction, IR spectroscopy

Direktsynthese und Kristallstruktur von [BeBr 2 (CH 3 CN) 2 ] One-step synthesis and crystal structure of [BeBr 2 (CH

a one-step synthesis from beryllium powder and bromine in acetonitrile at room temperature to give colourless single crystals, which were characterized by X-ray diffraction and IR spectroscopy. Compound krebserregender Arbeitsstoffe; siehe auch die Literatur in der Übersicht [3]). Es wirkt zudem auch mutagen [START_REF] Kaim | Bioanorganische Chemie[END_REF].

1 2 Berylliumhalogenide) 1 Bei 1

 1211 crystallizes orthorhombically in the space group Pnma with four formular units per unit cell. Lattice dimensions at 100(2) K: a = 1253.1(1), b = 1003.7(1), c = 650.8(1) pm, R 1 = 0.0217. The structure of 1 consists of monomeric molecules with site symmetry m and distances Be-Br of 214.7(5) pm and Be-N of 171.3(4) pm. ___________________________________________________________________________ Die Synthese solvatisierter Berylliumhalogenide [BeX 2 (D) 2 ], vor allem mit X = Cl, Br, die wegen ihrer guten Löslichkeit in den entsprechenden Donorsolventien D häufig für Folgereaktionen Anwendung finden [1-4], setzt die aufwendige Synthese der BeX 2 voraus. Man erhält sie zweckmäßig aus den Elementen bei Temperaturen oberhalb von 500 °C. Über ein verbessertes Eintopfverfahren zur Herstellung von [BeBr 2 (SMe 2 ) 2 ] wurde unlängst berichtet [5], jedoch wurde auch hier zunächst BeBr 2 in einem ersten Schritt synthetisiert. Nach unseren Erfahrungen der bereits bei Raumtemperatur verlaufenden Umsetzungen von Berylliumpulver mit Iod in Gegenwart von Solvatmolekülen D (D = Pyridin, N-Methylimidazol, Di-isopropylcarbodiimid, Tetramethylharnstoff), die zu Verbindungen mit den Dikationen [Be(D) 4 ] 2+ und Iodid-oder Polyiodid als Gegenionen führen [6], haben wir nach demselben Muster den Molekülkomplex [BeBr 2 (CH 3 CN) 2 ] herstellen können. Wir berichten hierüber im folgenden. Ergebnisse Eine Suspension von Berylliumpulver in trockenem Acetonitril reagiert beim Zutropfen der äquivalenten Menge Brom spontan exotherm gemäß Gleichung (1) unter Bildung von Berylliumdibromid-di-Acetonitril: Be + Br 2 + 2 CH 3 CN → [BeBr 2 (CH 3 CN) 2 ] (1Anwendung von überschüssigem Acetonitril entsteht eine gelbe Lösung, aus der sich das Rohprodukt 1 nach dem Evakuieren in vollständiger Ausbeute als gelber Kristallbrei abscheidet. Umkristallisation aus gesättigter Lösung in Acetonitril durch langsames Verdampfen des Lösungsmittels führt zu farblosen Einkristallen, die wir kristallographisch charakterisiert haben (siehe unten). Es sei bemerkt, daß Reaktion (1) bei Anwendung eines Überschusses an Brom oder durch eine zu hohe Reaktionstemperatur von der Nebenreaktion der bekannten cyclo-Trimerisierung von Acetonitril [7] unter Bildung von 2,4,6-Trimethyl-1,3,5-triazin begleitet werden kann. Hierzu ist bereits die im Vergleich zu Verbindung 1 schwächere Lewis-Säure LiCl in siedendem Acetonitril befähigt [8]. kristallisiert orthorhombisch in der Raumgruppe Pnma mit vier Formeleinheiten pro Elementarzelle. Sie kristallisiert damit isotyp mit dem Chloridokomplex [BeCl 2 (CH 3 CN) 2 ] [10]. In Abbildung 1 ist die Molekülstruktur von 1 wiedergegeben. Einziges Symmetrieelement ist eine Symmetrieebene, die von den Atomen Be(1), Br(1) und Br(2) besetzt ist, so daß C s -Symmetrie resultiert, wenngleich die beiden Be-Br-Bindungslängen nur sehr wenig verschieden sind, so daß C 2v -Symmetrie nahezu erreicht wird. > Abb. 1 < Die Be-Br-Abstände sind mit 214.7(5) pm etwas länger als in dem Thioetherkomplex [BeBr 2 (SMe 2 ) 2 ] [5] mit 212.1(3) pm, jedoch kürzer als in dem Diethyletherkomplex [BeBr 2 (OEt 2 ) 2 ] [1] mit 217.3(8) pm, was vor allem eine Folge der unterschiedlich langen Beryllium-Donoratomabstände ist. In entsprechender Weise reagieren darauf die Br-Be-Br-Bindungswinkel mit 115.5(2)° in [BeBr 2 (CH 3 CN) 2 ] (1), mit 117.6(2)° in [BeBr 2 (SMe 2 ) 2 ] [5] und mit 111.6(4)° in [BeBr 2 (OEt 2 ) 2 ] [1]. Analog hierzu sind die Be-N-Bindungen in Verbindung 1 mit 171.3(4) pm deutlich kürzer als in den Chlorido-Nitrilkomplexen [BeCl 2 (CH 3 CN) 2 ] [10] mit 172.9(3) pm und [BeCl 2 (PhCN) 2 ] [2] mit 174.0(3) pm. Die Koordination der Acetonitrilmoleküle an die Berylliumatome erfolgt in beiden genannten Beispielen nahezu linear mit Be-NC-Bindungswinkeln von 171.7(2)° im Chloridokomplex [10] und mit 171.6(3)° im isotypen Bromidokomplex 1. Zugleich führt die Koordination der Acetonitrilmoleküle in beiden Fällen zu einer Verkürzung des C≡N-Abstandes um etwa 3 pm gegenüber dem nichtkoordinierten Molekül [11], was auch mit den IR-spektroskopischen Befunden im Einklang ist.Infrarot-SpektrumIm IR-Spektrum von [BeBr 2 (CH 3 CN) 2 ] (1) sind für Symmetrie C s nach den irreduziblen Darstellungen[START_REF] Weidlein | Schwingungsspektroskopie[END_REF] insgesamt 39 Grundschwingungen N i der Rassen A' und A'' die nach den Auswahlregeln sämtlich IR-aktiv sind[START_REF] Weidlein | Schwingungsspektroskopie[END_REF]. In Tabelle 2 sind die Bandenmaxima im Bereich von 100 bis 4000 cm -1 enthalten, deren Anzahl nicht annähernd dieser Erwartung entspricht, wobei zudem einige sehr schwache Banden als Ober-oder Kombinationsschwingungen zu beschreiben sind. Dies hängt damit zusammen, daß die inneren Schwingungen der Acetonitrilliganden wegen ihrer nahezu orthogonalen Anordnung zueinander mit einem N-Be-N-Bindungswinkel von 102.1(3)° nicht nennenswert miteinander koppeln können. Somit ist die Anzahl dieser Schwingungsmoden reduziert. Es ist daher sinnvoll, das Erwartungsspektrum der Verbindung 1 in zwei Fragmente zu gliedern, nämlich in N 2 BeBr 2 mit Symmetrie C s und in eine Einheit CH 3 CN mit C 3v -Symmetrie. Tatsächlich entspricht dieses Modell sehr gut dem experimentellen Befund (Tabelle 2) mit N i = 6A' + 3A'' für N 2 BeBr 2 und N i = 4A 1 + 4E für CH 3 CN, mithin insgesamt 17 IR-aktive Grundschwingungen. Die in Tabelle 1 im einzelnen getroffenen Zuordnungen stützen sich neben dem Schwingungsspektrum des flüssigen Acetonitrils [13] vor allem auf die Spektren zahlreicher Acetonitrilkomplexe [14] sowie auf das Raman-Spektrum von [BeBr 2 (SMe 2 ) 2 ] [5]. So beobachten wir auch im IR-Spektrum von Verbindung 1 die typische, durch Fermi-Resonanz bedingte Aufspaltung von νCN in das Dublett 2333/2303 cm -1 und ihre kurzwellige Verschiebung gegenüber dem Dublett im freien Acetonitril bei 2293/2253 cm -1 [13]. Auch die CC-Valenzschwingung im Spektrum von 1 wird kurzwellig von 917 cm -1 [13] nach 1021 cm -1 verschoben. Beide Effekte lassen sich auf die Verkürzung der CC-und der CN-Bindungen durch die Koordination am Berylliumion sowie auf die stark polarisierende Wirkung des kleinen Be 2+ -Ions (41 pm bei C.N. 4 [15]) zurückführen. Andererseits ist die antisymmetrische BeBr 2 -Valenzschwingung im Spektrum von 1 mit 582 cm -1 merklich längerwellig als im Raman-Spektrum von [BeBr 2 (SMe 2 ) 2 ] mit 613 cm -1 [5], was durch die etwa 2.5 pm längeren BeBr 2 -Abstände in Verbindung 1 bedingt ist. Schließlich lassen sich die beiden Be-N-Valenzschwingungen (Rassen A' und A'') für das N 2 BeBr 2 -Fragment von 1 mit den relativ kurzwelligen Bandenlagen von 923 und 962 cm -1 mit den vergleichsweise kurzen Be-N-Abständen [3] von 171.3(4) pm begründen. und seine Verbindungen sind stark toxisch (MAK-Liste A2

  CCDC 769235 beim Cambridge Crystallographic Data Centre hinterlegt.Kopien der Daten können kostenlos bei CCDC, 12 Union Road, Cambridge CB2 1EZ (Fax: +44-1223-336-033; E-mail: deposit@ccdc.cam.ac.uk), Großbritannien, angefordert werden.

	Der Umgang mit Berylliumverbindungen erfordert daher die Anwendung besonderer Arbeitsbedingungen. Siehe hierzu die aktuelle Übersicht aus arbeitsmedizinischer Sicht [17]. Die Versuche erfordern Ausschluß von Feuchtigkeit. Acetonitril (Sigma-Aldrich) wurde in der Aldra SORB-Qualität ohne weitere Reinigung eingesetzt. Berylliumpulver (99%ig) wurde von der Firma ACROS bezogen. Brom wurde über Phosphorpentoxid aufbewahrt und mittels einer Injektionsspritze entnommen. Das IR-Spektrum (siehe Tabelle 2) wurde mittels des Bruker-Gerätes IFS-88 registriert, Silberchloridscheiben, kapillare Schicht. [BeBr 2 (CH 3 CN) 2 ] (1): Berylliumpulver (44 mg, 0.49 mmol) wurde in 5.0 mL Acetonitril suspendiert und unter Rühren tropfenweise bei 20 °C innerhalb von 20 min mit einer Lösung von Brom (0.78 g, 0.49 mmol) in 2.0 mL Acetonitril versetzt. Es wurde noch 3 h weitergerührt und anschließend von wenig Niederschlag abfiltriert. Einengen des orangeroten Filtrats unter vermindertem Druck bis auf 3 mL führte zu reichlich gelbem kristallinem Niederschlag, der durch zweitägiges Ruhigstellen unter der Mutterlauge zu blaßgelben Einkristallen führt, die in dünner Schicht farblos erscheinen. Ausbeute (nicht optimiert): 0.58 g = 49%, bezogen auf Beryllium. Zum vollständigen IR-Spektrum siehe Textteil. Die kristallographischen Daten (ohne Strukturfaktoren) wurden als "supplementary Tabelle 2 IR-Spektrum von [BeBr 2 (CH 3 CN) 2 ] (1) cm -1 Intensität Zuordnung 2936 sh ν s CH 3 2921 m ν as CH 3 2333 m * 2303 m νCN 2259 vw 1475 m δ as CH 3 1308 w δ s CH 3 1233 vw 1151 m ρCH 3 1021 m νCC 962 st ν as BeN 2 923 w ν s BeN 2 878 vw 772 vw 746 st δCCN 582 vst ν as BeBr 2 445 w ν s BeBr 2 379 w δBeN 2 352 w 329 vst ρBeN 2 publication no. Page 5 of 11 ZAAC Page 9 of 11 ZAAC
	255	vst	ωBeN 2
	205	w	τN 2 BeBr 2
	180	vst	δBeBr 2
	*) Fermi-Resonanz
			Wiley-VCH Wiley-VCH

Wiley-VCH ZAAC