Wolfgang Lutz
email: lutz@btu-lc.fta-berlin.de

Btu Cottbus

Luftchemie Kurzhals

Detlef ; Heidemann

R Kurzhals

G Kryukova

D Enke

M Weber

Formation of Mesopores in USY zeolites: a Case Revisited

Keywords: zeolite, dealumination, faujasite, mesopores zeolite, dealumination, faujasite, mesopores

Steaming of NH 4 Y zeolite at 723 K and 873 K is accompanied by the formation of extraframework amorphous aluminosilicate and silica gel in addition to earlier observed extraframework aluminium species. Their occurrence is directly associated with the formation of mesopores. Bulk (intracrystalline) mesopores occur inside the crystallite nuclei and surface (intercrystalline) mesopores are located nearby the crystallite surface. Corrosion of the zeolite framework results in a loss of crystallinity and, consequently, decreased catalytic activity of the USY catalysts synthesized. Analysis of the reasons of mesopore formation may help to reduce these disadvantages.

Introduction

Much work has been done to characterize mesopores in ultra-stable Y zeolites (USY) [1][2][3] since the first reports on their dealumination by steam treatment which have been published almost four decades ago [START_REF] Mcdaniel | Molecular Sieves[END_REF]5]. However, debates over the origin of these mesopores are still continuing. It was generally assumed that this process results from the dealumination of the zeolite carcass followed by the appearance of extra framework aluminium species [6]. The secondary pore system was discussed to improve the molecular transport of educts and products in catalytic conversion processes.

Using texture analysis and 3D-TEM measurements Jansen et al. [7] were able to visualize in this material bulk mesopores surrounded by micropores in addition to cylindrical mesopores connected to the crystallite surface. Similar zeolite morphology was observed earlier by Lynch et al. who reported on a secondary pore system without direct connection to the exterior of the crystals in the steamed and, particularly, in acid leached USY samples [8]. The scheme given in Fig. 1 could elucidate the morphology of a USY particle with the two types of mesopores observed.

Kortunov et al. [9] have reported on molecular transport in the intracrystalline (bulk) mesopores of USY samples. These diffusion experiments have been performed using PFG NMR technique with n-octane and 1,3,5-triisopropylbenzene as probe molecules. Both molecules have critical molecular diameters smaller and larger than the inlet of the zeolite micropores. It was found that the mesopores inside the bulk do not affect diffusion of 1,3,5triisopropylbenzene, therefore one could suggest that these mesopores are not connected with external crystallite surface and not able to contribute to the molecular transport.

Complicating the issue, the formation of siliceous extra-framework species in addition to the aluminium extra-framework admixture in USY has been recently reported [10]. Their occurrence was identified by 29 Si MAS NMR spectroscopic measurements as extra Si(4Al)

and Si(0Al) signals with shapes different from those typical for the zeolite framework. These signals have been assigned to amorphous aluminosilicate and silica gel phases. Al-and Si-rich parts of the non-homogeneously crystallized Y zeolites of generally small particle size [11,12] were suggested to contribute to the appearance of the extra siliceous species. It is highly probable that these newly formed admixtures play important role in changing the total pore architecture of Y zeolites after the steaming.

This paper discusses, therefore, relationship between Si, Al extra-framework species and formation of mesopores in USY. A review on synthesis of NaY, dealumination procedure, appearance of extra-framework ions after the steaming process takes into consideration published works of the pioneers of zeolite chemistry. Along with experimental data reported in this paper, this overview could shed some light onto this well-known topic, being at the moment of renewed interest which relates to the chemistry of modern hierarchical porous zeolites. With this new knowledge on the reasons of mesopore formation in USY zeolites, one can hinder or at least minimize this effect during the synthesis of the initial NaY with a hope to obtain well crystalline zeolite Y frameworks.

Results and Discussion

Sample characterization

According to results of X-ray diffraction analysis (Fig. 2) the initial NaY sample is a pure FAU zeolite with the unit cell parameter a 0 = 2.4665 nm. Corresponding Si/Al ratio has been calculated using empirical relation (1) given by Rüscher et al. [13] x=5.348a 0 -12.898, a 0 in nm (1) (with Si/Al = (1-x)/x and x = Al molar fraction by use of the cell parameter a 0. ).

With an average value of Si/Al = 2.6 the sample is suitable to be dealuminated without total collapse of the zeolite framework [12]. Owing to the removal of framework aluminium by steam treatment, the Si/Al ratio increases to 3.0 and 3.9 at 450°C and 600°C, respectively (Tab. 1). In this case 6% (USY, 450°C) and 16% (USY, 600°C) of an amorphous phase caused by the formation of extra-framework species [10] does appear. Whereas the particle size seems to remain the same as seen from SEM images given in Fig. 3, the diameter of the crystallites calculated from X-ray diffraction data decreases by 18% (450°C) and 19% (600°C), respectively (Tab.1).

The 27 Al MAS NMR spectrum of the initial NaY sample in Fig. 4 (A) shows a peak at 60 ppm usually assigned to aluminium atoms located in the tetrahedra inside the zeolite framework.

The spectra of the USY samples (B, C) characterize extra-framework aluminium by signals lying between 0 and 3 ppm [START_REF] Engelhardt | High Resolution Solid State NMR of Silicates and Zeolites[END_REF]. The broad shoulder centred at 30-35 ppm is assigned to pentahedrally coordinated Al species. This coordination state refers to the structural transition between tetrahedrally and octahedrally coordinated Al ions and gives a hint to the presence of specifically structured aluminium species [START_REF] Gore | [END_REF]. A drastic change of the shape and intensity of NMR signals can be observed after the steaming of parent zeolite at 450°C (B, I) and 600°C (C, I). As evident from Fig. 5, peak positions are generally shifted to higher values. Intensity of peaks assigned to Si(4Al), Si(3Al), and Si(2Al) structure units decrease with simultaneous increase in intensity of signals related to Si(1Al) and Si(0Al) units due to dealumination effect. The line shape of the spectrum is changed in analogy to earlier results on a series of differently synthesized NaY samples [10], especially in the range of the low and high values of chemical shift.

Furthermore, signal broadening and pronounced shoulders are observed in the region between -80 and -90 ppm and at about -112 ppm (B, II and C, II).

After deconvolution procedure, one can see additional signals of extra-framework species (B, III and C, III). These species are thought to be enriched in aluminium (Si(4Al)) and silicon (Si(0Al)), respectively. The signal height of Si(4Al) component, in opposite to the signal width with chemical shifts of -80 to -90 ppm (450°C) and -75 to -90 ppm (600°C), looks rather low but still visible after 4-and 8-fold multiplying of the signal intensity (dotted lines).

It seems likely that the Al-rich material is strongly distorted.

By analogy with data given in ref. [10], both additional signals can be attributed to aluminosilicate (a) and silica gel (s) structures. High-alumina aluminosilicate can be detected directly using Molybdate measurements [START_REF] Iler | The chemistry of Silica[END_REF]. This aluminosilicate has numerous monomeric silicate units. But we were not able to find such short-chained silicates in our sample, probably they are encapsulated by the microporous USY matrix. But when we steamed highalumina FAU zeolite of NH 4 -LSX type at 450°C, we got more than 95% of monomeric silicate units with the help of Molybdate method. After steam treatment of same sample at 600°C, the Molybdate activity decreased significantly since the generated aluminosilicate have been transferred to Molybdate inactive species though we could not exclude formation of silicon and aluminium oxides too.

Contributions of silica gel superimpose strongly the 29 Si MAS NMR spectrum, particularly in the positions of Si(0Al) and Si(1Al) peaks [START_REF] Lutz | [END_REF]. Due to this fact, calculated Si/Al NMR ratios Si/Al = Σ n I n /(I 4 +0.75I 3 + 0.5 I 2 + 0.25 I 1 ),

(I n = intensity of Q 4 /nAl signals in 29 Si MAS NMR spectra, n = 0-4 [START_REF] Engelhardt | High Resolution Solid State NMR of Silicates and Zeolites[END_REF]) are generally higher than those calculated by use of the framework constant a 0 . In our case, there are 4.2 (450°C) and 4.6 (600°C) against lower 3.0 and 3.9 values obtained from X-ray data (Si/Al XRD, see Tab. 1). This means, observed difference can speak in favour of the presence of silica gel admixture.

Partial phase transition observed by 29 Si MAS NMR spectroscopy is confirmed also by IR spectroscopic study. In Fig. 6 the asymmetrical TOT valence vibration (T=Si, Al) [18] changes from 1025 cm -1 (initial NaY) to 1047 cm -1 (450°C) and 1053 cm -1 (600°C) because of a strong superposition of signals of dealuminated zeolite Y matrix and newly formed silica gel. At the same time, double-ring vibration bands [19,[START_REF] Flanigan | Zeolite Chemistry and Catalysis[END_REF] lying at 580 cm -1 (NaY), 586 cm -1 (450°C) and 592 cm -1 (600°C) become less resolved, particularly at the lower steaming temperature, due to the framework amorphization.

TEM observed some differences in the morphology of initial and treated zeolite crystallites, especially at the nanoscale level (Fig. 7). By rising steaming temperature, particle surface changes from the flat (NaY) into rough ( USY, 450°, 600°C), even a thin outer surface layer appeared after steaming at 600 o C which should be attributed to amorphous extra-framework material formed during sample treatment. Lynch et al. [8] have discussed this phenomenon as a capsulation of the solid crystalline part by an amorphous surface layer.

It was practically impossible to perform EDX analysis from the surface layers and bulk of the zeolites to compare Si and Al contents in different particle regions, since the signal coupling needs more than 5 min, a period sufficient for destroying the zeolite structure while irradiated with electron beam. However, TEM gives direct evidence that, though the composition is unknown, this surface layer or film is rather thin, being 2-3 nm in thickness. It is transparent but not smooth. Therefore, due to the critical size, the film could not be detected by XRD, with a further complication that this coverage looks like amorphous rather than crystalline.

From the results obtained one can see that neither XRD, NMR and IR analyses nor TEM give any evidence to the presence of any admixtures in the initial NaY zeolite. Therefore, formation of the siliceous extra-framework species in USY should result from a corrosion of the zeolite crystals themselves. Moreover, it seems probable that observed phase transition could be related to the generation of mesopores in the corroded USY framework.

Pore architecture

Nitrogen adsorption and mercury intrusion experiments show a significant change in pore structure due to the steaming procedure, in analogy to the results of Jansen [7] and Kortunov [9].

The nitrogen isotherm of the initial NaY sample (Fig. 8) shows mainly pure micropore adsorption. A high sorption level is reached already at the beginning of the nitrogen uptake and is finished at p/p 0 close to 0.1. The corresponding micropore surface and volume given in Tab. 2 have typical values of 989 m 2 /g and 0.29 ml/g, respectively. The low contribution of mesopores to the total pore volume here results from the incomplete condensation of nitrogen in the inter-crystalline and inter-particle space of the NaY particles.

To make values of surface area and pore volume of NaY comparable to USY samples, they were extrapolated to theoretical HNaY stoichiometry because such modification cannot be prepared from NH 4 NaY in practice without any dealumination. The stoichiometry of NaY differs in the elemental cell weight by 7.2% which should be taken into account if comparing the adsorption data (Tab. 2).

The micropore adsorption of USY steamed at 450°C (Fig. 8) is finished at p/p 0 = 0.1, too. At higher relative pressure of 0.83, a mesopore filling takes place and this process ends at 0.95.

One can see a remarkable broad hysteresis which is typical for mesopores with restricted pore openings. The isotherm of the 600°C sample (Fig. 8) shows similar characteristics but the nitrogen volume consistent with hysteresis is higher, and a 2-step hysteresis is clearly visible.

This behaviour is due to a significantly higher amount of mesopores which are formed inside the nuclei and nearby the crystallite surface.

With the increase of the mesopore surface from 10 m 2 /g (450°C) to 24 m 2 /g (600°C) the area of micropores decreases from 1060 m 2 /g (HNaY) to 995 m 2 /g (450°C) and 898 m 2 /g (600°C).

The mesopore volume of 0.021 ml/g and 0.064 ml/g increases in the same tendency whereas total and micropore volume decreases. Comparing to HNaY, one can see nearly two-fold increase of mesopore volume for USY (450°C) and its progressive growth for USY (600°C) due to higher effect of dealumination (Tab. 2).

According to mercury intrusion, the samples contain macropores of 100 to 10000 nm in diameter. These macropores characterize the space between the crystallites of the polycrystalline zeolite particles and their inter-particle space.

The macroporous structure of NaY with an average pore diameter of about 1700 nm appears to be similar to that of the USY (600°C) sample (Figs. 9). In contrast, the pore diameter of USY (450°C) is reduced to 1400 nm. It seems likely that at 450°C the space between the individual crystallites is filled out by a bigger quantity of extra-framework gel-type species.

Mesopores occur in a diameter range below 20 nm. They contribute to the total pore formation only to less than 1% (450°C) and 1.7 % (600°C). The corresponding pore volume amounts to <0.015 cm 3 /g and 0.029 cm 3 /g. Thus the pore volumes are smaller than those determined by nitrogen adsorption (right columns in Tabs 2 and 3). Due to the detection limit of mercury intrusion (minimum 3.8 nm pore opening), the Hg cannot penetrate all mesopores so that a part of them seems to be closed (bulk mesopores) and the other open (surface mesopores) as reported in refs. [7] and [9]. The volume of the open mesopores amounts to about the half of the total mesopore volume (Tabs. 2 and 3) so that the other half characterizes the closed mesopores surrounded by the microporous zeolite framework.

Formation of mesopores

Now we can get our experimental results and reference data by formulating the reason what is the driving force for the mesopore development during NaY steaming. Dealumination itself cannot be responsible for this effect because silicate ions migrate quickly into the newly created Al vacancies inside the USY framework [START_REF] Steel | [END_REF]22]. Some observations in the present work and in previous studies by means of 29 Si MAS NMR, XRD and IR spectroscopy [10] as well as energy-dispersive X-ray diffraction (EDXRD) measurements [23] give evidence to a partial amorphization of the zeolite Y crystals under action of steam.

Different behaviours in structural transformation of FAU crystals which depends on their chemical composition were found for series of X-and Y-type zeolites. Whereas X zeolites (Si/Al = 1.0-1.5) [12] and high-silica Y samples (Si/Al > 100) [START_REF] Ehrhardt | Catalysis by Microporous Materials[END_REF] were completely decomposed under hydrothermal stress, the structure of Y zeolites with medium composition (Si/Al = 2.3-2.7) has been changed only partially [10,12]. In our case, the crystals of Y zeolite with Si/Al ratio equal to 2.6 might be a mixture composed of X zeolite and high-silica Y phase. But the presence of such a mixture seems to be rather problematic due to observed corrosion effects, i.e. formation of mesopores inside the crystallites and on their surface [7,9].

Therefore, specific behaviour of our sample upon thermal action of steam could be explained by a strong Si/Al gradient in its crystallites.

The semi-empirical approaches which describe the Si/Al ratio of FAU type zeolites by use of 29

Si MAS NMR [25] or X-ray data [26] did not take into account possible effect of such Si/Al gradient [27]. Due to fine size of the zeolite crystallites, it is difficult to get a direct proof of the gradient presence, e.g. using EDX analysis. According to our observation and data found here and in ref. [10], the crystallite size of all differently synthesised NaY samples amounts only to 85 -225 nm. The crystallites form polycrystalline particles which diameter does not exceed 1000 nm (Tab. 1). The Si/Al gradient equalizes thus over the volume of the separate NaY particle. But to explain the phenomenon of inhomogeneous structural state of NaY zeolite, we may suggest the following scheme based on the NaY crystallization mechanism.

The synthesis of NaY begins usually with a fast soluble source for aluminate and a slowly soluble source for silicate molecules given in excess [START_REF] Barrer | Hydrothermal Chemistry of Zeolites[END_REF][START_REF] Breck | Zeolite Molecular Sieves[END_REF][START_REF] Zhdanov | Synthetic Zeolites[END_REF]. As a result of mixing them, a compact SiO 2 gel including the complete amount of dissolved aluminate ions precipitates at first. Owing to successive dissolution of the SiO 2 gel, short-chained silicate ions are formed.

Of about 10% of the total amount of SiO 2 involved was found to be assembled in the shortchained aluminosilicate molecules with a ratio of Si/Al close to one [START_REF] Fahlke | [END_REF]. They are relatively stable in the mother liquor at room temperature [START_REF] Zhdanov | Chimija Zeolitov, Isd. Nauka[END_REF][START_REF] Antonić | [END_REF]. However, at the elevated synthetic temperature (e.g. 90°C) these molecules re-arrange and form seeds of the X-type zeolite that initiate a crystal growth. Frequently, X-type seeds are added purposely into synthesis batch in order to direct and accelerate the growth of zeolite Y crystal [START_REF] Zhdanov | Synthetic Zeolites[END_REF][34][35].

With increasing synthesis time, such growing crystals change their Si/Al distribution at the surface with a ratio greatly exceeding one due to the uptake of the dissolved silicate ions being meanwhile in excess [36]. Thereby, it is a common observation that the batch in the final state still contains big excess of non-reacted silicate. Because of the non-adequate Alconsumption at the beginning of crystallization, the concentration of the aluminate ions gets finally too low to promote further a formation of sodalite cages as building units of the FAU structure [START_REF] Barrer | Hydrothermal Chemistry of Zeolites[END_REF][START_REF] Breck | Zeolite Molecular Sieves[END_REF][START_REF] Zhdanov | Synthetic Zeolites[END_REF]. This fact explains why NaY samples with Si/Al ratios lying above 2.2 have the limited crystal size [12].

The Al-rich nucleus of zeolite Y is decomposed under action of water molecules by "acid" hydrolysis of the framework [10]. The formed silicate and aluminate ions react to produce aluminosilicate below 450 o C [37] and additionally Si and Al oxides at 600°C [START_REF] Iler | The chemistry of Silica[END_REF].

But formation of aluminosilicate requires sodium ions which are present mainly in the nucleus of NaY crystallite. Indeed, our samples contain residual sodium ions in a concentration as high as 20%. But these extra ions seem to be distributed over the whole crystallite volume of the actual Y sample as a result of homogeneous NH 4 + /Na + ion exchange at the first step of the steaming procedure so that the formation of aluminosilicate is reduced. However, the strong signal of pentahedrally coordinated aluminium in the 27 Al MAS NMR spectrum in Fig. 4 (B) gives a hint to the decomposition of the nuclei in the USY crystals. The shape of Si(nAl) signals in the 29 Si MAS NMR spectra provides further evidence for the possible existence of Si/Al gradient. While similar signals obtained from zeolite NaX and, particularly zeolite NaA are sharp because of their homogeneous environments, the Si(nAl) spectral peak in NaY noticeably broadens that may indicate to Si/Al gradient in zeolite particles [START_REF] Engelhardt | High Resolution Solid State NMR of Silicates and Zeolites[END_REF].

At the same time, the Si-rich surface of Y crystals decomposes according to the "alkaline" mechanism [10] that leads to the formation of silica gel. The appearance of new hydroxyl groups [38] indicates particularly this process. TEM micrographs given in Fig. 7 show the result of this process as a significantly changed surface morphology with a thin extra surface layer. The observed shell is called "silica-alumina" [39] because it involves also the octahedrally coordinated extra-framework Al species which presence was confirmed by XPS measurements [40]. The corrosion of the crystallite surface [1] proceeds simultaneously with the shrinking of the crystallite diameter up to 20% due to the removal of framework aluminium (Tab. 1). Obviously, XRD particle sizes given in Table 1 could not be considered as absolute values calculated with Scherrer equation since the determination error in the case of well crystalline samples is usually big. But using our XRD diffractometer and software package along with internal Si standard we were able to register the broadening of the characteristic XRD peaks in USY comparing to that of the initial zeolite. Therefore, rather than provide the absolute crystallite values, we show here the tendency in the change of crystallite diameter in the initial and steamed samples. earlier observation [42] to 79%.

The sample treatment was performed in steam under pressure of 1bar at 450°C or 600°C for 7

hours. The heating rate of the process was equal to 10°C per minute. The sample was contacted with steam after reaching the choosing temperature.

The obtained USY zeolite was characterized in the hydrated state by X-ray diffraction, 29 Si MAS NMR measurement, IR spectroscopy, SEM and TEM, nitrogen adsorption, mercury porosity and Molybdate measurements. Si MAS experiments, respectively. The 27 Al MAS spectra were obtained using single pulse excitation with pulse durations of 1 µs (π/12 pulses). Up to 1024 scans were recorded with a recycle delay of 1s. The 29 Si MAS spectra were obtained using single pulse excitation consisting of 4 µs pulses (π/2 pulses) and recycle delays of 120 s to exclude saturation effects.

Up to 700 FIDs were accumulated to obtain reliable signal-to-noise ratio. The 27 Al and 29 Si spectra were externally referenced to 1 M aqueous AlCl 3 solution and to liquid Me 4 Si at 0 ppm, respectively. The quantitative line shape analysis of the 29 Si MAS NMR spectra has been performed by use of the dmfit software package [43]. Every resolved component of the experimental NMR spectra was described by only one single line with a pure Gaussian line shape. The position (chemical shift), the line width and the altitude (height) of the corresponding lines were optimized by a least square fit using this program. The relative intensities of the components of every simulated spectrum were obtained by integrating over the component line compared to the total integral over all component lines of the simulated spectrum.

IR absorption spectra were taken on a Shimadzu FTIR 8400S spectrometer with a resolution of ± 1 cm -1 by use of 30 scans. For analysis, 0.5 mg of the sample was pressed with 400 mg KBr into a pellet while spectra were registered in the IR range from 400 to 4000 cm -1 .

SEM analysis was conducted with JSM-6460 LV JEOL scanning electron microscope at the resolution power of 3 nm in low vacuum (10 Pa), without sputtering the conducting layer on the surface of zeolite particles.

TEM analysis was performed with JEM-2010 JEOL transmission electron microscope at 100 kV accelerating potential and 1 nm resolution limit. After ultrasonic treatment of the sample in alcohol, the drop of this suspension was placed onto a holey-carbon copper grid for TEM observation. To prevent possible collapse of the zeolite structure which often occurs during observation of this material at the electron microscope column, we used special low dose regime with a decreased intensity of the incident electron beam. This enabled us to focus the image and to take a micrograph in a short time (less than 2 min), without any observable changes in the zeolite structure or the hole drilling.

Nitrogen sorption measurements were performed by use of a Sorptomatic 1990 apparatus by ThermoFinnigan. All samples were degassed at 623 K before measurement for at least 24 hours at 10 -5 mbar. Adsorption and desorption isotherms were measured over a range of relative pressures (p/p 0 ) from 0 to 1.0. The surface area of the micropores O S, micro and the micropore volume V P, micro were determined according to Dubinin-Radushkevich-Kaganer [START_REF] Dubinin | Proc. Acad. Sci[END_REF]. The total pore volume V P was estimated from the amount of gas adsorbed at the relative Page 14 of 32

Wiley-VCH ZAAC pressure p/p 0 = 0.996 assuming that pores were filled subsequently with condensed adsorptive in the normal liquid state. The t-plot method [START_REF] Lippens | [END_REF] was used to determine the surface area of the mesopores O S, meso . Finally, the mesopore volume V P, meso was calculated according to V P, meso = V P -V P, micro .

(

The mercury intrusion measurements were carried out on a Pascal 440 apparatus by ThermoFinnigan. The cumulative pore volume at a given pressure represents the total volume of mercury taken up by the sample at that pressure. A contact angle of 141.3° for Hg and a cylindrical pore model were used to interpret the results using the Washburn equation [START_REF] Washburn | Proc. Nat. Acad. Sci[END_REF].

For Molybdate measurements 5 mg of the samples were dissolved in 100 ml of a 10 -2 M hydrochloric acid at 273 K in which the -Si-O-Al-bonds are hydrolysed immediately whereas the -Si-O-Si-polymer building units are stable. By the following treatment at 298 K the silicate units are decomposed in the presence of 2 ml of a 10% solution of molybdic acid. The molybdic acid only reacts with monomeric silicate ions giving rise to the yellow βdodecamolybdato silicic acid complex. Its formation depends on the degradation velocity of the polymeric silicate units. The process is followed spectroscopically at a wavelength of 400 nm with a Shimadzu UV-1601 spectrometer. The slope of the resulting curves is a direct function of the degree of condensation of the formed silicate units. It may be noted that the degree of condensation of the silicate units in aluminosilicates generally decreases with increasing substitution of silicon by aluminium atoms.

Captions to the Tables

Tab. 2) difference of total to micropore volume

Fig. 5 (

 5 Fig.5 (A, I) shows the initial29 Si MAS NNR spectrum of the parent NaY samples. After the deconvolution procedure (II), five signals typical of the NaY framework with Si(nAl) building units (n = 0-4) in the range of chemical shifts between -84 and -108 ppm become recognisable. Simulation of the signals was performed by use of a set of single lines with a Gaussian line shape (see Experimental section). The simulation procedure (III) did not reveal the presence of non-zeolitic siliceous admixtures in this sample.

  us to propose a generalised structural model of the micropores formation based on the concept of the siliceous extra-framework species in the steamed USY zeolites. During steaming parts of the zeolite Y are decomposed. This process is accompanied by the formation of silica gel on the crystal surface and amorphous aluminosilicate inside the zeolite framework. Both phases possess higher density compared to that of the zeolite framework which leads inevitably to the generation of mesopores inside the zeolite bulk, particularly in the nuclei of the USY crystallites[7,9]. Important to this concept is a suggestion that initial NaY zeolite crystallites have a systematic Si/Al gradient with a high portion of silicon at the crystal surface and increased content of aluminium in the centre of crystallites. This gradient, being structural feature of the Y zeolite, reflects on specific synthesis mechanism of this zeolite type.The crystallites of initial NaY zeolite seem to have a systematic Si/Al gradient. Because they are generally small in size[41] and form polycrystalline particles it is difficult to get the solid experimental evidence for the detection of this phenomenon. But it can be characterized indirectly by evaluation of the hydrothermal stability of the framework and analysis of decomposition products of the transformations of NH 4 NaY (ion exchange) into USY modification (steam treatment). Steaming results in formation of two types of mesopores: closed bulk (intracrystalline) mesopores surrounded by the micropores of the zeolite framework and open surface (intercrystalline) mesopores at the crystallites surface. Former are generated by decomposition of the Al-rich nuclei while latter appears as a result of the partial destruction of the Si-rich surface. The open surface mesopores are in direct contact to the internal macropores of the polycrystalline zeolite particles. Because of partial decomposition of the resulting USY zeolite, further extra-framework siliceous species are formed, in addition to the well-known pentahedrally and octahedrally coordinated extraframework aluminium. There are silica gel at the crystallite surface and aluminosilicate inside the framework. Thus, the generation of mesopores is directly associated with the formation of the siliceous extra-framework species. Their occurrence leads to a loss of crystallinity and decrease of the catalytic active sites of Brǿnsted type. More homogeneous Si and Al distributions in the parent NaY zeolites which could be possible to achieve via new synthesis pathways can reduce the crystal corrosion. Experimental section Commercial NaY zeolite of the GRACE Division was investigated. Ammonium form of the samples with a particle size distribution of 0.4 -1.0 µm obtained by a three-fold ion exchange in 0.1 M aqueous solution of NH 4 NO 3 at room temperature for 1 hour in each step by use of a liquid/solid mass ratio of 100 was used. The obtained NH 4 NaY zeolite was washed with deionised water and dried at 120°C for 1 hour. The ion exchange was limited in analogy to

The

  XRD data were recorded on a Philips PW 1800 powder diffractometer equipped with a graphite monochromator and Cu Kα radiation at 40 kV and 40 mA. Measurements were performed in the 2θ range between 5 ° and 85 ° with a step of 0.03 ° and a coupling time of 1 s count per step. The powder patterns were measured with SuperQ software from Philips Analytical B. V., Almelo. XRD patterns were evaluated by using of the Profit-software (Philips Electronics N. V.) as well as a self-developed program for calculation of lattice constants, relative crystallinity and crystallite sizes. The crystallite size calculations were performed using Scherrer equation with internal standard of well crystalline Si powder.The solid-state NMR spectra were recorded at room temperature on a Bruker Avance 400 spectrometer, operating at frequencies of 104.3 MHz for27 Al and 79.5 MHz for29 Si. A 4 mm double tuned ( 1 H-X) MAS probe (Bruker Biospin) was used to perform MAS NMR measurements at spinning rates of 14 kHz in the27 Al MAS experiments and 12 kHz in the[START_REF] Breck | Zeolite Molecular Sieves[END_REF]

Fig. 2 XFig. 5 29Fig. 8 Fig. 8 Fig. 9

 25889 Fig.2X-ray diffraction pattern of the initial NaY (A) and the USY zeolites steamed

  1 Characteristic data of the initial NaY and the steamed USY zeolites

	Tab. 1 Tab. 2	Characteristic data of the initial NaY and the steamed USY zeolites Pore volume and surface area of the initial NaY and the steamed USY zeolites
		determined by nitrogen sorption uptake
	Zeolite		a o	Si/Al	SEM size of	XRD size of	degree of
	Sample		nm micropore	primary par-ticles, nm total XRD NMR ratio 1) mesopore micropore crystallites, nm mesopore 2) crystallinity, % mesopore 3)
	Tab. 2 USY, 450°C Pore volume and surface area of the initial NaY and the steamed USY zeolites determined by nitrogen sorption uptake measurements NaY 2.465 2.6 2.6 400 -1000 225 surface surface volume volume volume volume 100 2.458 3.0 4.2 400 -1000 184 m 2 /g m 2 /g ml/g ml/g ml/g ∆ ml/g 94 USY, 600°C 2.441 3.9 4.6 400 -1000 182 NaY 989 -0.31 0.29 0.007 -84 HNaY 1) 1060 -0.32 0.31 0.007 0
	USY, 450°C	995	10	0.31	0.28	0.021	0.014
	Tab. 3 1) values are determined by use of the lattice constant a 0 and 29 Si MAS NMR signal intensities Pore characteristic of the initial NaY and the steamed USY zeolites determined USY, 600°C 898 24 0.31 0.24 0.064 0.057
	by mercury intrusion measurements according to eqs. (1) and (2), respectively
	1) NaY values extrapolated to theoretical HNaY (20% sodium ions) modification according to the mass
	difference of 7,2%

Wiley-VCH ZAAC


Thomas ; Klapoetke

Joerg ; Lmu Stierstorfer

Cheistry Lmu

Niko Fischer

Lmu Piercey

Davin ; Lmu

Cheistry Scheutzow

Susanne ; Lmu

Cheistry

Prof. Dr Thomas M Klapötke

Davin Piercey

Susanne Scheutzow

Jörg Stierstorfer

Herbert Schumann

Diaminouronium Nitriminotetrazolates -Thermally Stable Explosives

Keywords: Crystal Structure, Energetic Material, Sensitivities, Tetrazoles Crystal Structure, Detonation Parameters, Energetic Materials, Sensitivities, Tetrazoles, Crystal Structure, Energetic Materials, Sensitivities, Tetrazoles

Diaminouronium 5-nitriminotetrazolate (1) and diaminouronium 1-methyl-5-nitriminotetrazolate (2) were synthesized by the reaction of diaminourea with 5-nitriminotetrazole (3) and 1-methyl-5-nitriminotetrazole (4), respectively. The energetic compounds 1 and 2 were fully characterized by single crystal X-ray diffraction, NMR spectroscopy, IR-and Raman spectroscopy as well as DSC measurements. The sensitivities towards impact, friction and electrical discharge were determined. Several detonation parameters (e.g. heat of explosion, detonation velocity) were computed by the EXPLO5 computer code based on calculated (CBS-4M) heats of formation and X-ray densities. In addition, the detonation velocity of 1 was experimentally determined and compared to the computed values.

Introduction

Nitrogen rich materials play an important role in the design of new energetic materials for use in propellants, explosives, and pyrotechnics [1]. For a nitrogen rich material to find practical application as a high explosive it needs to posses high thermal and mechanical stabilities, while at the same time satisfying the increasing demand for higher performing (high detonation velocity, pressure and heat of explosion) materials. Unfortunately, in many cases high performance and low sensitivity appear to be mutually exclusive; many high performing materials are not stable enough to find practical use [2] and many materials with the desired sensitivity do not posses the performance requirements of a material to replace a commonlyused explosive [3]. This trend is exemplified in the range of 5-membered azoles from pyrazole to pentazole, where pyrazole is not used in energetics due to low performance, and the few pentazole derivatives known are highly unstable [4]. One of the most promising heterocyclic backbones for the preparation of high-performing energetics is the tetrazole ring. Possessing high heats of formation resulting from the nitrogen-nitrogen bonds, ring strain, and high density, the tetrazole ring has allowed the preparation of high performing primary [START_REF] Huynh | Proc. Natl. Acad. Sci[END_REF] and secondary [START_REF] Fischer | [END_REF] explosives, illustrating the dynamic nature of the tetrazole ring; depending on the ring substitutents and anion/cation pairing, tetrazole based energetics can span the spectrum of sensitivity from insensitive to highly sensitive (primary explosives). Additionally, due to their aromatic ring, tetrazoles are generally thermally stable. Of the tetrazoles suitable for use in secondary explosives, nitriminotetrazoles are one of the most promising due to high thermal stability [7]. Apart from the high heats of formation, for a molecule or salt to be a high-performing energetic, a high oxygen balance is required. The oxygen balance is given in percent and represents the amount of oxygen with respect to the molar mass of the molecule, which is necessary to convert all of its non-oxidizing content to their respective oxides and is easily calculated by the equation Ω (%) =(wO -2xC -1/2yH -2zS) . 1600/M (w: number of oxygen atoms, x: number of carbon atoms, y: number of hydrogen atoms, z: number of sulfur atoms). Previous work with nitriminotetrazoles and especially alkylated nitriminotetrazoles [7], has led to compounds with high performance despite negative oxygen balances. The use of oxygen-containing cations with nitriminotetrazoles to improve the oxygen balance of the final nitriminotetrazolate salts is a previously unexplored field. In this work we describe the synthesis and properties of two new energetic nitriminotetrazoles containing the diaminouronium cation.

Results and Discussion

Synthesis

The synthetic routes to compounds 1 and 2 are depicted in Scheme 1. As they are both ionic species, they are accessible by facile Brønstead acid base chemistry. An aqueous solution of diaminourea, which is readily soluble in water, is reacted with an equimolar amount of 5nitriminotetrazole and 1-methyl-5-nitriminotetrazole respectively, which also were dissolved in warm water prior to addition.

N H N H NH 2 H 2 N O N N N H H N N NO 2 N H N H NH 3 H 2 N O N N N H N N NO 2 Me N H N H NH 3 H 2 N O N N N H N N NO 2 N N N N N NO 2 Me 1 2
Scheme 1. Synthesis of 1 and 2 via Brønstead acid base chemistry Diaminourea as well as 5-nitriminotetrazole and 1-methyl-5-nitriminotetrazole were synthesized according to literature procedure [8,9] which can be seen in Scheme 2. Hydrazinolysis of dimethylcarbonate in aqueous solution leads to diaminourea, whereas the acidic methylated and unmethylated nitriminotetrazoles are obtained by methylation of 5aminotetrazole using sodium hydroxide and dimethyl sulphate and following nitration with fuming nitric acid or direct nitration of 5-aminotetrazole respectively.

MeO OMe O N 2 H 4 . H 2 O (2eq) N H N H NH 2 H 2 N O N N N H N NH 2 2) (MeO) 2 SO 2 3) HNO 3 (100%) 1) NaOH N N N H N N NO 2 Me N N N H N NH 2 HNO 3 (100%) N N N H H N N NO 2
Scheme 2. Synthesis of the starting materials.

Crystal Structures

The single crystal X-ray diffraction data was collected using an Oxford Xcalibur3 diffractometer with a Spellman generator (voltage 50 kV, current 40 mA) and a KappaCCD detector. The data collection was undertaken using the CRYSALIS CCD software [10] and the data reduction was performed with the CRYSALIS RED software [10]. The structure was solved with SIR-92 [11] and refined with SHELXL-97 [START_REF] Sheldrick | Shelxl-97, Program for the Refinement of Crystal Structures[END_REF] implemented in the program package WinGX [START_REF] Farrugia | [END_REF] and finally checked using PLATON [START_REF] Spek | A Multipurpose Crystallographic Tool[END_REF]. Due to the chiral space groups in both structure, the Friedel pairs were merged. In all structures potential hydrogen atoms have been observed and freely refined. Selected data and parameters from the X-ray data collection and refinement are given in Table 1. Further information regarding the crystalstructure determination have been deposited with the Cambridge Crystallographic Data Centre [START_REF]Crystallographic data for the structure(s) have been deposited with the Cambridge Crystallographic Data Centre. Copies of the data can be obtained free of charge on application to The Director[END_REF] as supplementary publication Nos XXX (1) and XXX (2). -3 is quite high in comparison with other tetrazolate salts [START_REF] Stierstorfer | Dissertation, Energetic Materials based on 5-Aminotetrazole[END_REF]. The molecular unit is shown in Figure 1. The structure of the 5-nitriminotetrazolate anion is comparable to that found in corresponding copper(II) salts [START_REF] Klapötke | [END_REF]. Bond lengths are given in Table 2. The diaminouronium cation follows the structure described e.g. for diaminouronium 5-(4-amino-1,2,5-oxadiazol-3-yl)tetrazolate [18]. 6)) as well as of selected intermolecular H-bonds is shown in Figure 2.

Figure 2.

View on selected hydrogen bonds in the structure of 1. Symmetry codes (i) -0.5+x, 1-y, 0.5+z; (ii) 0.5+x, 1-y, -0.5+z Diaminouronium 1-methyl-5-nitriminotetrazolate (2) crystallizes in the orthorhombic chiral space group P2 1 2 1 2 1 . Its density of 1.585 g cm -3 is significantly lower than that of 1, however, it is in the range of other 1-methyl-5-nitriminotetrazolate salts with nitrogen-rich counter cations [7]. The anions are planar (<(O1-N6-C1-N4 = 1.22(2)°) and show comparable bond lengths and angles to copper(II) 1-methyl-5-nitriminotetrazolates in the literature [20]. The structure of 2 also shows an intense hydrogen-bond network. All of the hydrogen atoms of the diaminouronium cations are involved in hydrogen bridges, which are shown in Figure 4.

Figure 4.

View on the hydrogen bridges of one diaminouronium cation in the structure of 2. Symmetry codes (i) 2-x, 0.5+y, 0.5-z; (ii) 1-x, 0.5+y, 0.5-z; (iii) -1+x, y, z; (iv) 0.5-x, -y, 0.5+z; (v) -1+x, y, z; (vi) 1.5-x, -y, 0.5+z (vii) 1+x, y, z Table 2. Bond lengths and bond angles of 1 and 2.

atoms 1 2 O1-N6 1.240(2) 1.249(2) O2-N6 1.269(2) 1.276(2) N5-N6 1.313(2) 1.315(2) N5-C1 1.377(3) 1.372(3) N1-C1 1.338(3) 1.346(3) N4-C1 1.330(3) 1.341(3) N1-N2 1.347(3) 1.347(2) N2-N3 1.298(2) 1.297(3) N3-N4 1.355(3) 1.370(2) N1-C2 1.459(3) O3-C2(3) 1.244(2) 1.235(3) N7-N8 1.419(2) 1.407(3) N7-C2(3) 1.386(2) 1.346(3) N9-N10 1.417(3) 1.422(3) N9-C2(3) 1.331(3) 1.338 (3)

Spectroscopy

Both compounds described were investigated using 1 H and 13 C NMR spectroscopy. For better comparison with literature values, all spectra were measured using d 6 -DMSO as solvent and all chemical shifts are given with respect to TMS ( 1 H, 13 C).

To identify the specified compounds NMR spectroscopy, especially carbon NMR spectra are a valuable method since the spectra are dominated by only two or three signals respectively. The signals for the ring carbon atoms in 1 and 2 can be found at 158.0 ppm (1) and 157.6 ppm (2). Hence, no shift of this signal in the anionic species is observed due to the methylation of the tetrazole ring. The signal of the carbon atom of the carbonyl moiety of the cation is observed at around 159.5 ppm. An additional methyl signal at 33.0 ppm for the methylated nitriminotetrazolate can be assigned and its position is in accordance with values found in literature [7]. IR and Raman spectroscopy is also suitable for the identification of the described nitriminotetrazolate salts. IR and Raman spectra of both compounds were measured and the absorptions were assigned according to commonly observed values found in literature [7,[21][22][START_REF] Hesse | Spektroskopische Methoden in der organischen Chemie[END_REF]. The range between 3550 and 3230 cm -1 is mainly determined by the N-H stretching vibrations of the diaminouronium cation. For the methylated compound 2 the symmetric C-H stretching vibration of the methyl group can nicely be observed in the Raman spectrum at 2957 cm -1 . Another characteristic IR absorption is the C=O-stretching vibration of the cation at 1692 cm -1 in both cases. The tetrazole ring system itself exhibits stretching and deformation vibrations that can be detected as a set of signals lying in the range from 1031 cm -1 to 1637 cm -1 with a C=N-stretching vibration between 1617 and 1637 cm -1 .

Sensitivities and thermal behavior

The impact sensitivity tests were carried out according to STANAG 4489 [START_REF]NATO standardization agreement (STANAG) on explosives, impact sensitivity tests[END_REF] modified instruction [START_REF]WIWEB-Standardarbeitsanweisung 4-5.1.02, Ermittlung der Explosionsgefährlichkeit, hier der Schlagempfindlichkeit mit dem Fallhammer[END_REF] using a BAM (Bundesanstalt für Materialforschung) drophammer [26]. The friction sensitivity tests were carried out according to STANAG 4487 [START_REF]NATO standardization agreement (STANAG) on explosive, friction sensitivity tests[END_REF] modified instruction [START_REF]Ermittlung der Explosionsgefährlichkeit oder der Reibeempfindlichkeit mit dem Reibeapparat[END_REF] using the BAM friction tester. The classification of the tested compounds results from the "UN Recommendations on the Transport of Dangerous Goods" [START_REF]Impact: Insensitive > 40 J, less sensitive ≥ 35 J, sensitive ≥ 4 J, very sensitive ≤ 3 J; friction: Insensitive > 360 N, less sensitive = 360[END_REF]. All compounds were tested upon the sensitivity towards electrical discharge using the Electric Spark Tester ESD 2010 EN [30]. The values for the impact sensitivities could be determined to 5 J (2) and 3 J (3) and therefore both compounds have to be classified as sensitive and very sensitive towards impact. The same classification applies to the friction sensitivity. With values of 108 N (1) and 160 N (2) both nitriminotetrazolate salts are sensitive towards friction. Compared to the triaminoguanidinium salt of 1-methyl-5-nitriminotetrazole, which was formerly investigated by our research group [7] and exhibits values of 6 J and 240 N, the diaminouronium salt is more sensitive especially towards friction. The sensitivities towards electrical discharge were determined to be 0.20 J (2) and 0.15 J (3). Differential scanning calorimetry (DSC) measurements to determine the melt-and decomposition temperatures of 1 and 2 (about 1.5 mg of each energetic material) were performed in covered Al-containers with a hole (0.1 mm) in the lid for gas release and a nitrogen flow of 20 mL per minute on a Linseis PT 10 DSC [31] calibrated by standard pure indium and zinc at a heating rate of 5°C min -1 . The decomposition temperatures are given as absolute onset temperatures and both compounds decompose at around 200°C with a slightly higher value for methylated 2 (204°C) compared to 198°C for unmethylated 1. Compared to triaminoguanidinium (210°C) and diaminoguanidinium 1-methyl-5-nitriminotetrazolate (208°C), where the oxo (=O) moiety of the cation is exchanged by a hydrazo (=NH-NH 2 ) and an imino (=NH) moiety respectively, there is not a siginficant difference in thermal stability.

Theoretical Calculations

All calculations were carried out using the Gaussian G03W (revision B.03) program package [START_REF] Frisch | Gaussian 03, Revision B04[END_REF]. The enthalpies (H) and free energies (G) were calculated using the complete basis set (CBS) method of Petersson and coworkers in order to obtain very accurate energies. The CBS models use the known asymptotic convergence of pair natural orbital expressions to extrapolate from calculations using a finite basis set to the estimated complete basis set limit. CBS-4 begins with a HF/3-21G(d) geometry optimization; the zero point energy is computed at the same level. It then uses a large basis set SCF calculation as a base energy, and a MP2/6-31+G calculation with a CBS extrapolation to correct the energy through second order. A MP4(SDQ)/6-31+(d,p) calculation is used to approximate higher order contributions. In this study we applied the modified CBS-4M method (M referring to the use of minimal population localization) which is a re-parametrized version of the original CBS-4 method and also includes some additional empirical corrections [33]. The enthalpies of the gas-phase species M were computed according to the atomization energy method (eq. 1) (Tables 345 The solid state energy of formations (Table 8) of 1 and 2, the lattice energy (U L ) and lattice enthalpy (∆H L ) (Table 7) were calculated from the corresponding molecular volumes (Table 6) according to the equations provided by Jenkins [36]. With the calculated lattice enthalpy (Table 7) the gas-phase enthalpy of formation (Table 5) was converted into the solid state (standard conditions) enthalpy of formation (Table 8). These molar standard enthalpies of formation (∆H m ) were used to calculate the molar solid state energies of formation (∆U m ) according to equation (2) (Table 8).

∆U m = ∆H m -∆n RT (2) (∆n being the change of moles of gaseous components)

pV / RT = 1 + xe βx x = (κ Σ X i k i ) / [V (T + θ)
] α α = 0.5, β = 0.176, κ = 14.71, θ = 6620.

The detonation parameters calculated with the EXPLO5 program (V5.04) of compounds 1 and 2 using the experimentally determined densities (X-ray) are summarized in Table 9 and compared to the values calculated for the triaminoguanidinium 1-methyl-5nitriminotetrazolate and commonly used RDX.

Table 9.

Detonation parameters

1 2 TAG-1- MeATNO 2 RDX ρ / g cm -3 [a]
1.81 1.59 1.57 1.80 Ω / % [b] -36.3 -54.7 -64.5 -21.6 ∆ Ex U° / kJ kg -1 [c] -4848 -4850 -4781 -6125 T det / K [d] 3335 3246 3091 4236 P CJ / kbar [e] 337 252 255 349 V Det. / m s -1 [f] 8979 8172 8309 8748 V o / L kg -1 [g] 848 832 847 739

[a] X-ray density. [b] Oxygen balance. [c] Energy of explosion. [d] Detonation temperature. [e] Detonation pressure.

[f] Detonation velocity. [g] Volume of explosion gases assuming only gaseous products.

Due to the much higher crystal density, the value for the detonation velocity for the parent compound 1 (8979 m s -1 ) exceeds the one of methylated 2 (8127 m s -1 ) by far and is even slightly higher when compared to RDX. The detonation pressures, however, only differ slightly. Again, a comparison of 2 to the triaminoguanidinium salt of 1-methyl-5nitriminotetrazole seems to be appropriate since both compounds only differ by one isolobal moiety which is the hydrazo (=NH-NH 2 ) moiety in the case of the triaminoguanidinium salt and the oxo (=O) moiety in 2. There is only one value, which is in the same range for both compounds, which is the explosion energy (-4895 kJ kg -1 (2), -4888 kJ kg -1 (TAG-1-MeATNO 2 )). Not only the detonation velocity, but also the detonation pressure of the triaminoguanidinium salt is substantially higher than for the diaminouronium salt 2, even though having a slightly lower crystal density and a lower oxygen balance. In summary, the 5nitriminotetrazolate salt 1 shows much better performance than its methylated sister compound 2. Further, the exchange of the triaminoguanidinium cation by the diaminouronium cation does not improve the detonation performance in case of the 1-methyl-5nitriminotetrazolate anion, although one would expect a higher performance for oxygen containing cations such as the diaminouronium cation. The reason, why we observe inverse behaviour might be the connectivity of the oxygen in the cation. In the case of diaminourea, it is connected to a carbon atom, which therefore is already partially oxidized and does not release as much energy as a carbon atom with a lower oxidation state. Probably, oxygen containing cations involving a nitro functionality such as the aminonitroguanidinium cation lead to a better performance of the materials containing the respective anions investigated in this work.

Velocity of detonation test

In order to evaluate the detonation velocity of 1 experimentally, compound 1 was prepared on a 50 g scale. The detonation velocity tests were performed in an OZM laboratory detonation chamber (model KV-250). The measurement of the detonation velocity was performed using the OZM detonating velocity measuring system EXPLOMET-FO-2000. The use of the fiber optic technique ensures excellent electrical noise immunity. For the detonation velocity measurement a PE tube with an inner diameter of 14 mm was used, which was equipped with three optical fibers with a distance of 17 mm between them. The amount of compound 1 used for the test was 11.6 g resulting in a density of 0.699 g cm -3 . The compound was loaded into the PE tube and manually compressed with a force of ca. 30 N. As a booster charge 0.30 g of nitropenta (PETN) were added on top and carefully compressed manually using ca. 20 N force. Initiation was achieved with an electrically ignited detonator containing PETN and RDX (ORICA, DYNADET C2-25MS). To evaluate the used method, the same setup was employed for measuring the detonation velocity of RDX under the same conditions. 14.4 g of RDX were filled into the tube (ρ = 0.869 g cm -3 ) and for best comparison of the measured values, also 0.30 g of PETN were added on top of the RDX. The results of the measurement can be seen in Table 10. The measured values between fiber 1 and 2 and between 2 and 3 were averaged and compared to the values computed with the EXPLO5 (version 5.04) software [START_REF] Sućeska | EXPLO5.04 program[END_REF]. The recorded values for both measurements (RDX as well as compound 1) are in surprisingly good agreement with the computed values and differ by less than 200 m s -1 for compound 1 and only 41 m s -1 for RDX.

Conclusion

From this combined theoretical and experimental study the following conclusions can be drawn:

• Diaminouronium nitriminotetrazolate (1) and diaminouronium 1-methyl nitriminotetrazolate (2) were prepared in high yield and purity from the corresponding nitriminotetrazole and diaminourea.

• The crystal structures of 1 and 2 were determined by low-temperature singlecrystal X-ray diffraction. The compounds crystallize in the space groups Pn and P2 1 2 1 2 1 with densities of 1.806 and 1.585 gcm -3 respectively. Additionally both compounds were fully characterized by vibrational spectroscopy (IR and Raman) 1 H and 13 C NMR, mass spectroscopy, and elemental analysis.

• Thermal stabilities of 1 and 2 were investigated by DSC and decompose at 198 and 204 °C respectively.

• The sensitivities of towards friction, impact and electrostatic discharge were investigated by BAM methods. 1 and 2 were found to have impact sensitivities of 5 J and 3 J respectively, friction sensitivities of 108 N and 160 N respectively, and ESD sensitivities of 0.2 J and 0.15 J respectively. These are within the range of commonly-used high explosives such as RDX.

• Using calculated heats of formation and experimentally obtained crystal densities the detonation parameters (heat of explosion, explosion temperature, detonation pressure and velocity) were calculated. 1 has a detonation velocity of 8979 m s -1 and a detonation pressure of 337 kbar. 2 has a detonation velocity of 8172 m s -1 and a detonation pressure of 252 kbar.

Experimental Part

Caution! Methylated and unmethylated 5-nitriminotetrazole and their salts are energetic materials with increased sensitivities towards shock and friction. Therefore, proper safety precautions (safety glass, face shield, earthened equipment and shoes, Kevlar ® gloves and ear plugs) have to applied while synthesizing and handling the described compounds. C spectra were recorded using a JEOL Eclipse 270, JEOL EX 400 or a JEOL Eclipse 400 instrument. The chemical shifts quoted in ppm in the text refer to typical standards such as tetramethylsilane ( 1 H, 13 C). To determine the melting and decomposition temperatures of the described compounds a Linseis PT 10 DSC (heating rate 5°C min -1 ) was used. Infrared spectra were measured using a Perkin Elmer Spectrum One FT-IR spectrometer as KBr pellets. Raman spectra were recorded on a Bruker MultiRAM Raman Sample Compartment D418 equipped with a Nd-YAG-Laser (1064 nm) and a LN-Ge diode as detector. Mass spectra of the described compounds were measured at a JEOL MStation JMS 700 using FAB technique. To measure elemental analyses a Netsch STA 429 simultaneous thermal analyzer was employed.

Diaminouronium 5-nitriminotetrazolate (1)

5-Nitriminotetrazole (0.89 g, 6.85 mmol) is dissolved in a few milliliters of water and added to a solution of diaminourea (0.62 g, 6.85 mmol). The mixture was warmed, stirred at elevated temperature for ten minutes and the solvent was removed under vacuum. The white solid residue was recrystallized from ethanol to yield 1.96 g (6.10 mmol, 89%) of 2 as a white crystalline material. [START_REF] Frisch | Gaussian 03, Revision B04[END_REF], 1232 (4), 1150 (9), 1112 (3), 1081 (7), 1030 (42), 1017 (60), 962 (2), 921 [START_REF] Huynh | Proc. Natl. Acad. Sci[END_REF], 875 [START_REF] Fischer | [END_REF], 748 (2), 741 [START_REF] Spek | A Multipurpose Crystallographic Tool[END_REF], 578 [START_REF] Huynh | Proc. Natl. Acad. Sci[END_REF], 486 (7), 430 (2), 420 [START_REF] Spek | A Multipurpose Crystallographic Tool[END_REF], 384 (9), 252 (10); 1

Diaminouronium 1-methyl-5-nitriminotetrazolate (2)

1-Methyl-5-nitriminotetrazole (1.44 g, 10.0 mmol) is dissolved in a few milliliters of warm water and added to a solution of diaminourea (0.90 g, 10.0 mmol). The mixture was heated to reflux for one minute and the solvent was removed under vacuum afterwards. The white solid residue was recrystallized from an ethanol/water mixture. Yield: 2.15 g (9.20 mmol, 92%). DSC (5 °C min -1 , °C): 163°C (m.p.), 204°C (dec.); IR (KBr, cm -1 ): ν ~ = 3415 (s), 3371 (s), 3355 (s), 3326 (s), 3306 (s), 3230 (m), 3139 (m), 2956 (m), 2666 (m), 2019 (w), 1692 (s), 1619 (s), 1515 (s), 1466 (s), 1425 (m), 1411 (m), 1383 (s), 1324 (vs), 1295 (m), 1279 (m), 1240 (m), 1177 (m), 1163 (m), 1117 (m), 1108 (m), 1052 (w), 1032 (m), 989 (w), 960 (w), 928 (w), 881 (w), 774 (w), 752 (w), 740 (w), 691 (w), 636 (w), 581 (w); Raman (1064 nm, 400 mW, 25 °C, cm -1 ): ν ~ = 3326 (5), 3140 (3), 3008 (2), 2957 [START_REF] Farrugia | [END_REF], 1690 (7), 1636 (2), 1504 (91), 1465 [START_REF]NATO standardization agreement (STANAG) on explosive, friction sensitivity tests[END_REF], 1411 [START_REF] Sheldrick | Shelxl-97, Program for the Refinement of Crystal Structures[END_REF], 1371 [START_REF] Huynh | Proc. Natl. Acad. Sci[END_REF], 1322 (8), 1301 (36), 1240 (3), 1179 (9), 1109 [START_REF]Impact: Insensitive > 40 J, less sensitive ≥ 35 J, sensitive ≥ 4 J, very sensitive ≤ 3 J; friction: Insensitive > 360 N, less sensitive = 360[END_REF], 1052 [START_REF] Huynh | Proc. Natl. Acad. Sci[END_REF], 1033 (100), 992 [START_REF] Huynh | Proc. Natl. Acad. Sci[END_REF], 961 (10), 883 (11), 776 (2), 754 (9), 693 [START_REF] Klapötke | [END_REF], 585 (7), 498 (9), 454 (4), 377 (8), 294 (20), 240 (7), 216 (5); 1 H NMR (DMSO-d 6 , 25 °C, ppm) δ: 7.79 (s, 7H, -NH -NH 2 , -NH 3 + ), 3.67 (s, 3H, -CH 3 ); 13

Figure 1 . 2 -

 12 Figure 1. Molecular unit of diamouronium 5-nitriminotetrazolate (1). Ellipsoids of nonhydrogen atoms are drawn at the 50 % probability level.

Figure 3 .

 3 Figure 3. Molecular unit of diamouronium 1-methyl-5-nitriminotetrazolate (2). Ellipsoids of non-hydrogen atoms are drawn at the 50 % probability level.

DSC ( 5

 5 °C min -1 , °C): 180°C (m.p.), 198°C (dec.); IR (KBr, cm -1 ): ν ~ = 3550 (s), 3414 (vs), 3234 (s), 2321 (w), 2024 (w), 1692 (s), 1637 (s), 1617 (vs), 1543 (m), 1512 (s), 1441 (m), 1385 (m), 1326 (s), 1229 (m), 1136 (s), 1110 (m), 1089 (m), 1062 (m), 1031 (m), 1006 (w), 963 (m), 921 (w), 870 (w), 773 (m), 746 (m), 696 (m), 623 (s), 570 (m), 482 (m); Raman (1064 nm, 250 mW, 25 °C, cm -1 ): ν ~ = 1542 (100), 1447 (4), 1390 (8), 1340 (2), 1327

  H NMR (DMSO-d 6 , 25 °C, ppm) δ: 8.56 (s, 8H, -NH -NH 2 , -NH 3 + ); 13 C NMR (DMSO-d 6 , 25 °C, ppm) δ: 159.3 (C(O)(NHNH 2 )(NHNH 3 + )), 158.0 (CN 4 ); m/z (FAB + ): 91.1 [C(O)(N 2 H 3 ) 2 +H + ]; m/z (FAB -): 129.0 [ATNO 2 -]; EA (C 2 H 8 N 10 O 3, 220.15): calc.: C 10.91, H 3.66, N 63.62 %; found: C 11.19, H 3.78, N 63.42 %; BAM drophammer: 5 J; friction tester: 108 N; ESD: 0.20 J (at grain size 500 -1000µm).

Table 1

 1

	. X-ray data and parameters
		1	2
	Formula	C 2 H 8 N 10 O 3	C 3 H 10 N 10 O 3
	Form. weight [g mol -1 ]	220.18	234.12
	Crystal system	Monoclinic	Orthorhombic
	Space group	Pn (No. 7)	P2 1 2 1 2 1 (No. 19)
	Color / Habit	Colorless rods	Colorless plates

Table 3 .

 3

			CBS-4M results
			point group	-H 298 / a.u.	NIMAG
	DAU +		C s	335.795706	0
	HAtNO 2	-	C s	516.973495	0
	1MeAtNO 2	-C 1	556.194636	0
	H			0.500991	0
	C			37.786156	0
	N			54.522462	0
	O			74.991202	0

)

[34]

.

∆ f H°( g, M, 298) = H (Molecule, 298) -∑H°( Atoms, 298) + ∑∆ f H°( Atoms, 298)

(1)

Table 4 .

 4 Literature

		values for atomic ∆H°f 298 / kcal mol -1
		NIST [35]
	H	52.1
	C	171.3
	N	113.0
	O	59.6

Table 5 .

 5 Enthalpies of the gas-phase species M.

	M			Formula	∆ f H°(g,M) / kcal mol -1
	DAU +			CH 7 N 4 O	155.6
	HAtNO 2	-		CHN 6 O 2	35.9
	1MeAtNO 2	-	C 2 H 3 N 6 O 2	166.3

Table 6 .

 6

				Molecular volumes.
				V M / Å 3	V M / nm 3
	DAU +			81	0.081
	HAtNO 2	-		121	0.121
	1MeAtNO 2	-	164	0.164

Table 7 .

 7 Lattice

	energies and lattice enthalpies.
	V M / nm 3	U L / kJ mol -1	∆H L / kJ mol -1	∆H L / kcal mol -1

Table 8 .

 8 Solid state enthalpies and energies of formation (∆ f U°) The program is based on the chemical equilibrium, steady-state model of detonation. It uses the Becker-Kistiakowsky-Wilson's equation of state (BKW EOS) for gaseous detonation products and Cowan-Fickett's equation of state for solid carbon. The calculation of the equilibrium composition of the detonation products is done by applying modified White, Johnson and Dantzig's free energy minimization technique. The program is designed to enable the calculation of detonation parameters at the CJ point. The BKW

		∆ f H°(s) /	∆ f H°(s)/	∆n	∆ f U°(s)/	M /	∆ f U°(s) /
		kcal mol -1	kJ mol -1		kJ mol -1	g mol -1	kJ kg -1
	1	70.4	294.6	10.5	320.7	220.20	1456.3
	2	80.1	335.4	11.5	363.9	234.23	1553.7
	Detonation parameters
	The calculation of the detonation parameters was performed with the program package
	EXPLO5 (version 5.04) [37].

equation in the following form was used with the BKWN set of parameters (α, β, κ, θ) as stated below the equations and X i being the mol fraction of i-th gaseous product, k i is the molar covolume of the i-th gaseous product [38]:

Table 10 .

 10

	Calculated and measured detonation velocities of 1 and RDX
		1	RDX
	mass (g)	11.6	14.4
	density (g cm -3 )	0.699	0.869
	V det, exp. (m s -1 )	4384	5339
	V det, theor. (m s -1 )	4568	5370

Page 12 of 16 Wiley-VCH ZAAC All

  chemicals and solvents were employed as received (Sigma-Aldrich, Fluka, Acros).1 H and 13

  C NMR (DMSO-d 6 , 25 °C, ppm) δ: 159.6 (C(O)(NHNH 2 )(NHNH 3 + )), 157.6 (CN 4 ), 33.1 (CH 3 ); m/z (FAB + ): 91.1 [C(O)(N 2 H 3 ) 2 +H + ]; m/z (FAB -): 143.0 [1-MeATNO 2 -]; EA (C 3 H 10 N 10 O 3, 234.18): calc.: C 15.39, H 4.30, N 59.81 %; found: C 15.30, H 4.07, N 59.98 %; BAM drophammer: 3 J; friction tester: 160 N; ESD: 0.15 J (at grain size 100 -500µm).

Wiley-VCH ZAAC

Acknowledgment

Financial support of this work by the Ludwig-Maximilian University of Munich (LMU), the U.S. Army Research Laboratory (ARL), the Armament Research, Development and Engineering Center (ARDEC), the Strategic Environmental Research and Development Program (SERDP) and the Office of Naval Research (ONR Global, title: "Synthesis and Characterization of New High Energy Dense Oxidizers (HEDO) -NICOP Effort ") under contract nos. W911NF-09-2-0018 (ARL), W911NF-09-1-0120 (ARDEC), W011NF-09-1-0056 (ARDEC) and 10 WPSEED01-002 / WP-1765 (SERDP) is gratefully acknowledged. The authors acknowledge collaborations with Dr. Mila Krupka (OZM Research, Czech Republic) in the development of new testing and evaluation methods for energetic materials and with Dr. Muhamed Sucesca (Brodarski Institute, Croatia) in the development of new computational codes to predict the detonation and propulsion parameters of novel explosives. We are indebted to and thank Drs. Betsy M. Rice and Brad Forch (ARL, Aberdeen, Proving Ground, MD) and Mr. Gary Chen (ARDEC, Picatinny Arsenal, NJ) for many helpful and inspired discussions and support of our work.


Matthias ; Tamm
email: m.tamm@tu-bs.de[

Thomas Glöge

Dejan Petrovic

Cristian G Hrib

Constantin Daniliuc

Eberhardt Herdtweck

Peter G Jones

Dr Herbert Schumann

Synthesis and Structural Characterization of an Isomorphous Series of Bis(imidazolin-2-imine) Metal Dichlorides Containing the First Row Transition Metals Mn, Fe, Co, Ni , Cu, and Zn

Keywords: N ligands, Chelates, Transition Metals, Imidazolin-2-imines N Ligands, Chelates, Transition metals, Imidazolin-2-imines

The bis(imidazolin-2-imine) ligand N,N '-bis(1,3-diisopropyl-4,5dimethylimidazolin-2-ylidene)-1,2-ethanediamine (BL iPr ) has been used to synthesize a complete series of transition metal dichloride complexes of the type [(BL iPr )MCl 2 ] with M = Mn, Fe, Co, Ni, Cu, Zn. The molecular structure of each complex was established by Xray diffraction analysis; the complexes are isomorphous and crystallize in the monoclinic space group C2/c, allowing a comparison of their structural parameters in a constant packing environment. The complexes exhibit distorted tetrahedral geometries around the metal atoms with the dihedral angles between the MN 2 and MCl 2 planes ranging from 53.9° (M = Cu) to 75.5° (M = Zn).

Introduction

The coordination chemistry of poly(guanidine) ligands is currently receiving considerable interest, [1] since these ligands generally act as efficient electron-donor ligands towards transition metals and are able to form particularly strong metal-nitrogen bonds. [2,3,4,5] These unique properties are attributable to the ability of the guanidine CN 3 moiety to delocalize a positive charge, affording compounds with enhanced basicity and nucleophilicity. [6] This tendency is even more pronounced in imidazolin-2imines, since the imidazole ring is particularly effective in stabilizing a positive charge. [7] Accordingly, poly(imidazolin-2-imine) ligands such as the bidentate nitrogen donor ligand N,N '-bis(1,3-diisopropyl-4,5dimethylimidazolin-2-ylidene)-1,2-ethanediamine (BL iPr ) can be described by the limiting resonance structures A and B (Scheme 1), with the contribution of the ylidic form B increasing considerably upon metal coordination. The resulting excellent donor properties of the ligands BL R (R = Me, iPr) were documented by their ability to stabilize coordinatively unsaturated 16-electron halfsandwich complexes and by the preparation of highly reactive copper(I) complexes. [8,9,10] In addition, it was demonstrated that the strong polarisation of the exocyclic C=N bond in zinc complexes of the type [(BL Me )ZnX 2 ] (X = Cl, OAc) is beneficial for the observation of high catalytic activity in ring-opening lactide polymerisation. [11] BL iPr , R = iPr In view of the large number of applications that have been reported for 3d-metal complexes of related 1,4diaza-1,3-butadiene (α-diimine) ligands, [12] e. g. the use of iron, cobalt and nickel complexes as polymerization catalysts, [13] we became interested in studying the reactivity of BL iPr toward first row transition metal dichlorides with the metal ranging from manganese to zinc. We assumed that the diimine ligand BL iPr is able to stabilize four-coordinate metal complexes, since it should provide enough electron density and, together with the anionic chloride ligands, sufficient steric bulk to prevent further coordination of a second BL iPr equivalent or additional solvent molecules. Accordingly, the preparation and structural characterization of a complete series of transition metal complexes of the type [(BL iPr )MCl 2 ] (M = Mn, Fe, Co, Ni, Cu, Zn) is presented in this contribution. Because all the complexes crystallized isomorphously, a detailed comparison of their structural parameters can be undertaken without the need to consider different packing effects.

Results and Discussion

The complexes [(BL iPr )MCl 2 ] (M = Mn, 1; M = Fe, 2; M = Co, 3; M = Ni, 4; M = Cu, 5; M = Zn, 6) were prepared by the reaction of the diimine ligand BL iPr with a suitable metal(II) chloride source (Scheme 2, see experimental part). Addition of an excess of the BL iPr ligand leads neither to the coordination of a second BL iPr equivalent nor to the replacement of chloride anions from the inner coordination sphere, as was observed for CoCl 2 complexes containing related guanidine-hybrid ligands. [5b] For all complexes, the composition [(BL iPr )MCl 2 ] was confirmed by elemental analysis. Since complexes 1 -5 are paramagnetic, only the diamagnetic zinc complex 6 was additionally characterized by NMR spectroscopy. Its 1 H NMR spectrum in acetone-d 6 exhibits one septet at 5.00 ppm together with one doublet at 1.35 ppm, indicating that rotation around the exocyclic C-N bonds is fast on the NMR time scale. In addition, two singlets are observed at 3.40 ppm and 2.12 ppm, which can be assigned to the C 2 H 4 -bridge and the methyl groups at the C4/5 positions of the imidazoline rings, respectively. Re-crystallization by diffusion of n-hexane into THF solutions of the complexes afforded colourless (1, 6), beige (2), blue (3) and red (4) single crystals, whereas brown crystals of the copper complex 5 were obtained by diffusion of diethyl ether into an acetonitrile solution. The crystal structures of 1 -6 were determined by X-ray diffraction analyses (Figure 1); 1 -6 are isomorphous and crystallize in the monoclinic space group C2/c with Z = 4. All complexes display crystallographic C 2 -symmetry about an axis passing through the metal atom and the centre of the C1-C1a bond, allowing a detailed comparison of the structures without the need to consider differences in packing. Related studies were published by Leigh and Kubiak for complexes of metal(II) halides with the ligands N,N,N',N'-tetramethylpropyl-1,3-diamine (tmpn) and 6,6'-(2,4,6-triisopropylphenyl)-2,2'bipyridine (tripbipy). [14,15] In both series, however, two different space groups were observed for the complexes [(tmpn)MCl 2 ] (M = Mn -Cu) and [(tripbipy)MCl 2 ] (M = Fe -Zn), respectively.

((Figure 1 here))

In the solid state, the [(BL iPr )MCl 2 ] moieties of the complexes 1 -6 are connected with four neighbouring molecules by intermolecular C7-H7B⋅⋅⋅π contacts between isopropyl methyl groups and the imidazole rings, which is illustrated by the packing of compound 4 shown in Figure 2. All metal atoms are four-coordinate and display distorted tetrahedral geometries with the diimine ligand forming a five-membered chelate ring. The N1-M-N1' bite angles range from 81.28(6)° (in 2) and 84.39( 5)° (in 6) and are thus significantly smaller than expected for an ideal tetrahedral arrangement (109.5°). Accordingly, expanded Cl-M-Cl' angles between 113.95(3)° (in 3) and 118.84(3)° (in 1) are observed (Table 1); in the copper complex 5, however, a relatively small Cl-Cu-Cl' angle of 104.35(3)° is found, which is in agreement with 5 being most strongly distorted toward a square planar geometry (vide infra). Since all complexes display C 2symmetry in the solid state, the degree of distortion from an ideal tetrahedron toward a square can be described by the dihedral angle between the N1-M-N1' and Cl-M-Cl' planes, respectively, which can adopt values between 90° (tetrahedron) and 0° (square). In all cases (Table 1), these angles deviate significantly from 90° and follow the order 6 (75.5°) > 1 (74.5°) > 3 (72.8°) > 2 (72.2°) > 4 (68.6°) > 5 (53.9°).

((Figure 2 here))

Another method for describing the distortion from ideal geometries in four-coordinate complexes was proposed by Houser, [16] who introduced the geometry index τ 4 = [360° -(α + β)]/141°, where α and β represent the largest angles in the four-coordinate species. The values of τ 4 will range from 1.0 for a perfect tetrahedron to zero for a perfect square planar geometry. Intermediate structures, including trigonal pyramidal and seesaw, will afford values 0 < τ 4 < 1.0. In our series 1 -6, the largest angles are N1-M-Cl and N1'-M-Cl', which are identical because of the crystallographic C 2 -symmetry (Table 1). The calculated τ 4 follow almost the same order as the dihedral angles mentioned above with 6 (0.82) ≈ 1 (0.82) > 2 (0.79) > 3 (0.78) > 4 (0.75) > 5 (0.57). Again, the manganese and zinc complexes 1 and 6 show the smallest deviation from a tetrahedral geometry, whereas the copper complex 5 exhibits the strongest distortion. Similar τ 4 values have been established for the complexes [(tripbipy)MCl 2 ] (M = Fe -Zn), [15] and this trend is also in accord with an increasing impact of Jahn-Teller distortion, which should be absent for the d 5 -, d 7 -and d 10complexes 1, 3 and 6 and is expected to be most strongly pronounced for the Cu(II) d 9 -system 5. [17] It should be noted that, in the related copper(II)-hydroxo complex [Cu II (µ-OH)(BL iPr )] 2 [PF 6 ] 2 , the BL iPr ligand has also proved capable of stabilizing a square planar environment. [10a] Partial charge separation and delocalization in the coordinated BL iPr ligand (Scheme 1) can be deduced from the observation of large M-N1-C2-N2 torsion angles of about 60° in all complexes (Table 1), indicating a perturbed π-interaction between the coordinated nitrogen atoms and the imidazole rings. Consequently, the exocyclic C2-N1 bonds are elongated and approach the intra-ring C2-N2 and C2-N3 distances. The degree of charge separation can be illustrated by the parameter ρ = 2a/(b+c), with a, b and c representing the exo-(a) and endocyclic (b, c) distances within the CN 3 guanidine moiety. [5b,5d] The ρ values in all complexes are very similar (ρ ≈ 0.97) and indicate substantial charge delocalization, albeit to a smaller extent than found for cyclopentadienyl-and arene-ruthenium complexes. [8a] Since the imidazole rings in 1 -6 are not fully perpendicular with the N1-M-N1' plane, the resulting C 2symmetric propeller-like conformation of the BL iPr ligand can also be held partly responsible for the observation of distorted tetrahedral geometries (vide supra), which seem to be a compromise between electronic and steric factors. The M-N1 bond lengths follow the expected trend as predicted by the ionic radii of the respective metal atoms (Table 1), [18] except for the copper complex 5, which exhibits the shortest M-N [Cu-N1 = 1.9709( 12) Å] and M-Cl bond lengths [Cu-Cl = 2.2460(5) Å]; this behaviour might be attributed to the fact that 5 exhibits a significantly stronger distortion from a tetrahedral geometry than the other complexes. The M-Cl bonds also follow the same trend, even though the differences are much less pronounced (Figure 3). Comparison with the related series [(tripbipy)MCl 2 ] (M = Fe -Zn) shows a similar trend for the M-N and M-Cl bond lengths; in all cases, however, shorter M-N bonds together with longer M-Cl bonds are observed for complexes 1 -6, indicating that the BL iPr is a significantly stronger N-donor ligand than the bipyridine ligand tripbipy. [15] The metal-nitrogen bonds in 1 -6 are also shorter than those found for guanidine complexes containing a related ethylenebridged bis(imidazolidin-2-imine) ligand, [5b,5c] revealing that the presence of an unsaturated heterocycle in BL iPr affords a ligand with slightly enhanced donor properties, presumably because of a stronger contribution from the resonance structure B (Scheme 1). [7c] In contrast, the Zn-N bonds in 6 [2.0342( 9) Å] are slightly longer than those found for the complex [(BL Me )ZnCl 2 ], for which two different Zn-N distances of 2.019(1) and 2.032(1) Å were determined. [11] Even shorter metal-nitrogen bond distances were observed for a series of tetrahedral complexes [(hppH) 2 MCl 2 ] (M = Mn, Fe, Co, Ni) with hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2a]pyrimidine, in which the guanidine ligands bond to the metal atoms with additional interaction between the secondary NH groups and the metal-bound chlorides. [19]

Conclusions

The ligand BL iPr proved to be a suitable ligand for controlling the coordination number and geometry of six first row transition metals. Its reaction with metal(II) chlorides (M = Mn, Fe, Co, Ni, Cu, Zn) afforded in all cases mononuclear complexes with distorted tetrahedral coordination environments, which seems to be the strongly favoured coordination mode for the BL iPr ligand in comparison with the alternative square planar or octahedral geometries. Particularly with regard to the large number of applications that have been reported for 3d-metal complexes of related α-diimine ligands, [12] the potential use of complexes 1 -6 as homogeneous catalysts, e. g. for olefin polymerisation, can be envisaged.

Experimental Section

General: All operations were performed in an atmosphere of dry argon by using Schlenk and vacuum techniques. All solvents were purified by standard methods and distilled prior to use. 1 H and 13 C NMR spectra were recorded on a Bruker DPX 400 device. The chemical shifts are given in ppm relative to TMS. The spin coupling patterns are indicated as s (singlet), d (doublet) and sept (septet). Elemental analyses (C, H, N) were performed on a Elementar Vario EL III CHNS elemental analyzer. Mass spectrometry was performed with a Finnigan MAT 90 device. CoCl 2 (anhydr.), NiCl 2 (DME) were obtained from Aldrich and CuCl 2 from Riedel-deHaën. FeCl 2 (THF) 1.3 and the BL iPr ligand were synthesized according to published procedures. [20, 8b] Synthesis of [(BL iPr )MnCl 2 ] (1): A solution of BL iPr (200.0 mg, 0.48 mmol) in THF (10 mL) was added to a suspension of MnCl 2 (59.8 mg, 0.475 mmol) in THF (20 mL). The mixture was stirred for 48 h. The volume was reduced to 5 mL and the product was precipitated with n-hexane (30 mL). Filtration, washing with n-hexane (2 × 10 mL) and drying in vacuo afforded the product as a light yellow solid (yield: 213.7 mg, 82.1%). Found: C, 53.28; H, 8.11; N, 15.28. Calc. for C 24 H 44 N 6 MnCl 2 : C, 53.14; H, 8.18; N, 15.49%.

Synthesis of [(BL iPr )FeCl 2 ] (2):

A solution of BL iPr (217 mg, 0.521 mmol) in THF (10 mL) was added to a solution of FeCl 2 (THF) 1.3 (66 mg, 0.521 mmol) in THF (20 mL). The mixture was stirred overnight. A white precipitate formed and the product was fully precipitated with n-hexane (30 mL). Filtration, washing with n-hexane (2 × 10 mL) and drying in vacuo afforded the product as a beige solid (yield: 221 mg, 78%). Found: C, 53.29; H, 8.25; N, 15.32

Synthesis of [(BL iPr )CoCl 2 ] (3):

A solution of BL iPr (166.7 mg, 0.40 mmol) in THF (5 mL) was added to a suspension of CoCl 2 (50.0 mg, 0.39 mmol) in THF (15 mL). The mixture was stirred overnight. The volume was reduced to 5 mL and the product was precipitated with n-hexane (30 mL). Filtration, washing with n-hexane (2 × 5 mL) and drying in vacuo afforded the product as a blue solid (yield: 97.4 mg, 45.6%). Found: C, 53.23; H, 8.38; N, 14.96. Calc. for C 24 H 44 N 6 CoCl 2 : C, 52.75; H, 8.12; N, 15.38%. MS (EI, 70 eV): m/z = 545 (M + ), 208 (BL iPr /2).

Synthesis of [(BL iPr )NiCl 2 ] (4): A solution of BL iPr (263 mg, 0.63 mmol) in THF (10 mL) was added dropwise to a solution of [Ni(DME)Cl 2 ] (132 mg, 0.60 mmol) in THF (10 mL) at room temperature. During addition of the ligand the color changes to deep-red; the solution was stirred for 12 hours followed by reduction of THF volume to 10 mL. The product was fully precipitated upon addition of n-hexane (40 mL). Filtration, washing with n-hexane (2 × 10 mL) and drying in vacuo afforded the product as a deep-red solid (324 mg, 99%). Found: C, 53.16; H, 8.28; N, 14.89. Calc. for C 24 H 44 N 6 NiCl 2 : C, 52.77; H, 8.12; N, 15.38%. MS (EI, 70 eV) m/z = 546 (M + ), 511 (M + -Cl, 20), 416 (BL iPr+ ).

Synthesis of [(BL iPr )CuCl 2 ] (5): A solution of BL iPr (125.4 mg, 0.30 mmol) in acetonitrile (5 mL) was added to a suspension of CuCl 2 (40.0 mg, 0.30 mmol) in acetonitrile (15 mL). The mixture was stirred overnight. The acetonitrile was removed in vacuo and the residue redissolved in THF (5 mL). The product was precipitated with n-hexane (30 mL). Filtration, washing with n-hexane (2 × 5 mL) and drying in vacuo afforded the product as a blue solid (yield: 110.8 mg, 66.7%

Synthesis of [(BL iPr )ZnCl 2 ] (6):

A solution of BL iPr (150.0 mg, 0.36 mmol) in THF (10 mL) was added to a solution of ZnCl 2 (49.1 mg, 0.36 mmol) in THF (15 mL). The mixture was stirred overnight, during which the product precipitated. The volume was reduced to 5 mL and the product was completely precipitated with n-hexane (30 mL). Filtration, washing with nhexane (2 × 5 mL) and drying in vacuo afforded the product as a white solid (yield: 180.3 mg, 91.7%). Found: C, 52.28; H, 8.06; N, 14.503. Calc. for C 24 H 44 N 6 ZnCl 2 : C, 52.13; H, 8.02; N, 15.20%. MS (EI, 70 eV): m/z = 552 (M + ), 208 (BL iPr /2). 1 H-NMR (400 MHz, acetone-d 6 , 25°C): δ [ppm] = 5.00 (sept, 4H, CH(CH 3 ) 2 , 3 J = 7.1 Hz), 3.40 (s, 4H, CH 2 ), 2.12 (s, 12H, CCH 3 ), 1.35 (d, 24H, CH(CH 3 ) 2 , 3 J = 7.1 Hz); 13 C-NMR (100 MHz, acetone-d 6 , 25°C): δ [ppm] = 155.9 (C2), 119.8 (CCH 3 ), 54.5 (CH 2 ), 48.4 (NCH(CH 3 ) 2 ), 22.1 (CCH 3 ), 10.5 (NCH(CH 3 ) 2 ).

X-ray Crystal Structure Determinations. Data were recorded on area detectors (Nonius Diffraction for 2, otherwise Oxford Diffraction) at low temperature. Absorption corrections were performed on the basis of multi-scans. Structures were refined anisotropically using the program SHELXL-97. [21] Hydrogen atoms were included using rigid methyl groups or a riding model. Numerical details are summarized in Table 2. Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-745604-9 for 1-6 respectively. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [fax.: (internat.) + 44 1223/336-033; e-mail: deposit@ccdc.cam.ac.uk]. ) with thermal displacement parameters drawn at 50% probability; selected bond lengths and angles are shown in Table 1. , where α and β represent the largest angles in four-coordinate species; here α = β, because of the crystallographic C2symmetry of complexes 1 -6. [b] Effective ionic radii for tetrahedral complexes, see ref. [18]. Ionic radii in Å [b] 0.66 0.63 0.58 0.55 0.57 0.60

Co

  Scheme 1. Resonance structures A and B for the bis(imidazolin-2-imine) ligands BL R (R = iPr, Me).

  Scheme 2. Synthesis of the metal complexes 1 -6.

Figure 3 .

 3 Figure 3. M-N (▲) and M-Cl (■) bond lengths [Å] in complexes 1 -6.

Figure 1 .

 1 Figure 1. ORTEP diagram of the complexes [(BL iPr )MCl 2 ] (M = Mn, 1; M = Fe, 2; M = Co, 3; M = Ni, 4; M = Cu, 5; M = Zn, 6) with thermal displacement parameters drawn at 50% probability; selected bond lengths and angles are shown in Table1.

Figure 2 .

 2 Figure 2. Packing of compound 4 showing the H7B⋅⋅⋅π contacts (2.69 Å) as thick dashed bonds; The H7B-. The view direction is perpendicular to the ab plane in the region z ca. 1/4.

  ). Found: C, 53.23; H, 8.38; N, 15.32. Calc. for C 24 H 44 N 6 CuCl 2 : C, 52.31; H, 8.05; N, 15.25%.

Table 1 .

 1 Selected bond lengths [Å] and angles[°] for 1, 2, 3, 4, 5 and 6 with estimated standard deviations in parentheses.

		1 (M = Mn)	2 (M = Fe)	3 (M = Co)	4 (M = Ni)	5 (M = Cu)	6 (M = Zn)
	M-N1	2.1224(12)	2.0668(16)	2.0142(14)	1.9951(9)	1.9709(12)	2.0342(9)
	M-Cl	2.3356(5)	2.2795(7)	2.2585(5)	2.2502(4)	2.2460(5)	2.2500(4)
	N1-C2	1.3248(17)	1.330(2)	1.329(2)	1.3308(14)	1.3286(18)	1.3264(14)
	N2-C2 [a]	1.3651(18)	1.364(2)	1.362(2)	1.3680(14)	1.3592(17)	1.3664(14)
	N3-C2 [a]	1.3645(18)	1.367(2)	1.362(2)	1.3614(14)	1.3644(18)	1.3615(13)
	N1-M-N1'	81.46(6)	81.28(6)	84.00(8)	83.58(5)	83.39(7)	84.39(5)
	Cl-M-Cl'	118.64(3)	118.05(3)	113.95(3)	114.64(3)	104.35(3)	115.46(2)
	N1-M-Cl	122.45(3)	124.14(5)	124.83(4)	127.38(3)	140.14(4)	122.54(3)
	N1-M-Cl'	103.70(3)	102.70(5)	103.83(4)	101.43(3)	98.52(4)	104.67(3)
	M-N-C-N	59.84	58.76	60.12	59.16	57.59	61.25
	Angle between MCl2 and MN2 planes	74.5	72.2	72.8	68.6	53.9	75.5
	Angle sum around N1	357.0	357.7	357.3	357.7	358.6	356.7
	ρ	0.971	0.974	0.976	0.975	0.976	0.972
	τ4 [a]	0.82	0.79	0.78	0.75	0.57	0.82
	Angle between imidazole planes	67.1	66.0	66.1	66.6	64.8	66.2

Table 2 .

 2 Crystallographic data for 1, 2, 3, 4, 5 and 6.

		1	2	3	4	5	6
	Empirical formula	C24H44Cl2MnN6	C24H44Cl2FeN6	C24H44Cl2CoN6	C24H44Cl2NiN6	C24H44Cl2CuN6	C24H44Cl2ZnN6
	a (Å)	14.4058(4)	14.4589(2)	14.3819(6)	14.3552(2)	14.2387(4)	14.4222(11)
	b (Å)	11.2178(4)	11.2089(10)	11.1473(4)	11.1407(2)	11.0831(2)	11.1543(8)
	c (Å)	17.4655(6)	17.5226(2)	17.4156(6)	17.4547(4)	17.5153(4)	17.4094(15)
	α (°)	90	90	90	90	90	90
	β (°)	99.302(4)	99.6184(6)	99.463(6)	99.402(2)	98.623(2)	99.567(8)
	γ (°)	90	90	90	90	90	90
	V (Å 3 )	2785.33(16)	2799.94(6)	2754.06(18)	2753.98(9)	2732.83(11)	2761.7(4)
	Z	4	4	4	4	4	4
	formula weight	542.49	543.40	546.48	546.26	551.09	552.92
	space group	C2/c	C2/c	C2/c	C2/c	C2/c	C2/c
	T (°C)	-173	-100	-173	-173	-173	-173
	λ (Å)	0.71073	0.71073	0.71073	0.71073	0.71073	0.71073
	Dcalcd (g cm -3 )	1.294	1.289	1.318	1.317	1.339	1.330
	µ (mm -1 )	0.69	0.75	0.84	0.92	1.02	1.11
	R(Fo)	0.0306	0.0363	0.0341	0.0266	0.0290	0.0223
	Rw(Fo 2 )	0.0563	0.0948	0.0673	0.0744	0.0777	0.0607

Acknowledgments

This study was financially supported by the Deutsche Forschungsgemeinschaft (DFG) through grants Ta 189-6. ____________