ZnCl2·2H2NN(H)But·H2O contains a tetrahedral metal centre with two coordinated hydrazine ligands. Hydrogen bonds between the hydrazine and water, along with weaker intermolecular NH…Cl, OH…Cl and NH…O hydrogen bonds, generate an associated lattice.
There is considerable current effort expended in the search for active absorber materials for photovoltaic applications, with a clear emphasis on the use of earth-abundant elements. Cu2ZnSnS4 (CZTS) figures highly in this search (Ito and Nakazawa, 1988; Ramasamy et al., 2012; Zhou et al., 2013), and cells with efficiencies of over 11% have now been reported (Todorov et al., 2013). One drawback in the production of CZTS in these high-efficiency cells is the necessity to employ hydrazine, which is explosive, hepatotoxic (Choudhary and Hansen, 1998) and carcinogenic (Roe et al., 1967), as a reaction medium. One possible solution to this problem is the use of precursors which embody hydrazine (or its organo-substituted equivalents), so that the reducing environment is introduced in solid form.
Synthesis of hydrazine adducts of simple salts of copper (Nicholls and Swindells, 1969; Srivastava et al., 1980; Dowling and Glass, 1988), zinc (Quang and Novakovskii, 1968; Quang et al. 1969; Rahman et al., 1986, 1988) and tin (Aggarwal and Makhija, 1965) are available from many years ago and generally lack complete characterisation. Crystallographically characterised examples are limited to ZnX2·2H2NNH2 (X=Cl) (Ferrari et al., 1963), NCS (Ferrari et al., 1965a), OAc (Ferrari et al., 1965b)], ZnCl2·2H2NNMe2 (Elsegood and Redshaw, 2008) and [H3NNH2]+[CuCl3]- (Bushuyev et al., 2013), while, as far as we are aware, no examples including Sn(II) have been reported.
As part of our ongoing interest in the synthesis of precursors for CZTS formation, (Kociok-Köhn et al., 2014a,b), we now report the synthesis and structure of ZnCl2·2H2NN(H)But·H2O, which may have application in this general area, where easily synthesised, stable compounds of good solubility are required.
ZnCl2·2H2NN(H)But has been synthesised from the combination of ZnCl2 and H2NN(H)But in toluene. As prepared, microanalysis suggests an anhydrous material, while slow crystallisation from Tetrahydrofuran (THF) under aerobic conditions resulted in crystals of the monohydrate, ZnCl2·2H2NN(H)But·H2O (1) suitable for structural analysis.
The structure of compound 1 (Figure 1) contains a tetrahedral metal centre similar to that of ZnCl2·2H2NNMe2 (Elsegood and Redshaw, 2008). However, 1 differs from both this and the structure of Zn(OAc)2·2H2NNH2 (Ferrari et al., 1965b) in two important respects. Firstly, unlike the structure which involves H2NNH2 which acts as a μ2-NN bridge between metal centres (Ferrari et al., 1965a,b), the t-butyl hydrazine in 1 is monodentate to the metal, as also seen in ZnCl2·2H2NNMe2 and [H3NNH2]+[CuCl3]- (Bushuyev et al., 2013). As a result, the Zn-N bonds in 1 are shorter [2.055(2) Å] than those in Zn(OAc)2·2H2NNH2 [2.179(7), 2.206(7) Å] though the six-coordinate nature of zinc in the latter undoubtedly has an influence. Secondly, while the bond angles at zinc in 1 span either side of the ideal tetrahedral angle of 109.5°, the ∠N-Zn-N is notably narrow [100.60(10)°] as a result of the intramolecular N(2)-H(2)…N(4) hydrogen bond. In comparison, the corresponding angle in ZnCl2·2H2NNMe2 is 104.34°, where the lack of a β-NH precludes such hydrogen bonding. There is no variation in the N-N bond length between the monodentate donors in these two structures [1: 1.447(3), 1.449(3); ZnCl2·2H2NNMe2: 1.441, 1.451 Å], though the Zn-Cl bonds are longer in 1 [2.2363(7), 2.2312(7) vs. 2.247, 2.242 Å].
The hydrated nature of 1 leads to a complex pattern of hydrogen bonds (Figure 2 and Table 1). In addition to the intramolecular N-H…N hydrogen bond, the water is hydrogen bonded to one of the ligated hydrazines [O(1)-H(10A)…N(2)] also as part of the asymmetric unit. However, it also forms additional intermolecular hydrogen bonds to different, symmetry-related species [O(1)-H(10B)…Cl(1); N(1)-H(1A)…O(1)], as both a donor and an acceptor. Furthermore, each chlorine forms two bifurcated intermolecular hydrogen bonds [O(1)-H(10B)..Cl(1), N(3)-H(3a)…Cl(1); N(3)-H(3B)…Cl(2), N(1)-H(1B)…Cl(2)]. In fact, only sterically crowded H(4) is not involved in the hydrogen bonding network. Interestingly, no hydrogen bonding is seen in the only closely related species, ZnCl2·2H2NNMe2 (Elsegood and Redshaw, 2008).
|O1–H10A…N2||0.86 (4)||2.02 (4)||2.866 (3)||167 (4)|
|O1–H10B…Cl1a||0.93 (6)||2.54 (6)||3.237 (2)||132 (4)|
|N1–H1A…O1b||0.80 (4)||2.21 (4)||2.953 (3)||157 (3)|
|N1–H1B…Cl2c||0.94 (3)||2.49 (3)||3.373 (2)||156 (3)|
|N2–H2…N4||0.82 (3)||2.58 (3)||3.320 (3)||152 (3)|
|N3–H3A…Cl1d||0.76 (4)||2.77 (4)||3.470 (3)||153 (4)|
|N3–H3B…Cl2c||0.84 (3)||2.58 (3)||3.347 (3)||154 (3)|
Symmetry codes: (a) -x+1, -y+1, -z+2; (b) -x+2, -y+1, -z+2; (c) -x+2, -y, -z+2; (d) -x+1, -y, -z+2.
ZnCl2 (0.5 g, 3.74 mmol) was stirred in toluene (10 mL) and tBu(H)NNH2 (0.66 g, 7.49 mmol) was added dropwise and left to stir for a further 1 h. The fine white precipitate which formed dissolved when THF (10 mL) was added. On cooling to -20°C colourless crystals of 1 formed (0.99 g, 85%, 76–78°C). Analysis found (calculated for C8H24Cl2N4Zn): C 30.6 (30.7), H 7.85 (7.74), N 17.8 (17.9)%. 1H NMR (300 MHz, CDCl3) δ ppm: 1.22 (s, 9H, CH3) 4.49 (s, 3H, NH, NH2), 13C NMR (75 MHz, CDCl3) δ ppm: 26.0 (CCH3), 55.8 (CCH3).
Experimental details relating to the single-crystal X-ray crystallographic study are summarised in Table 2. Data were collected on a Nonius Kappa CCD diffractometer (Enraf-Nonius B.V., Rotterdam, The Netherlands) at 150(2) K using Mo-Kα radiation (λ=0.71073 Å). Structure solution followed by full-matrix least squares refinement was performed using the WinGX-1.70 suite of programmes (Farrugia, 1999). An absorption correction (semi-empirical from equivalents) was applied.
|Space group||P1̅ (no. 2)|
|a (Å)||7.5136 (5)|
|b (Å)||9.1439 (5)|
|c (Å)||12.6830 (7)|
|α (°)||109.689 (4)|
|β (°)||93.255 (3)|
|γ (°)||95.099 (4)|
|V (Å3)||813.67 (9)|
|ρcalc (Mg m-3)||1.349|
|Crystal size (mm)||0.30×0.25×0.15|
|Theta range (°)||5.47–27.52|
|Independent reflections [R(int)]||3688 [0.069]|
|Reflections observed [>2σ(I)]||3129|
|Data Completeness (%)||98.7|
|Max. and min. transmission||0.772, 0.520|
|Goodness-of-fit on F2||1.06|
|Final R1, wR2 indices [I>2σ(I)]||0.0427, 0.1061|
|R1, wR2 indices (all data)||0.0522, 0.1136|
|Largest diff. peak, hole (eÅ3)||1.16, -0.86|
Crystallographic data for the structural analysis (in CIF format) has been deposited with the Cambridge Crystallographic Data Centre, CCDC no. 1050666. Copies of this information may be obtained from the Director, CCDC, 12 Union Road, Cambridge, CB21EZ, UK (Fax: +44-1233-336033; e-mail: firstname.lastname@example.org or www.ccdc.cam.ac.uk).
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