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DOI: 10.1039/C5DT01145K (Paper) Dalton Trans., 2015, 44, 11542-11550

Utilization of mixtures of aromatic N-donor ligands of different coordination ability for the solvothermal synthesis of thiostannate containing molecules†

J. Hilbert , C. Näther and W. Bensch *
Institute of Inorganic Chemistry, Christian-Albrechts-University of Kiel, Max-Eyth-Str. 2, 24118 Kiel, Germany. E-mail: wbensch@ac.uni-kiel.de; Fax: +49 431 880–1520; Tel: +49 431 880–2419

Received 23rd March 2015 , Accepted 22nd May 2015

First published on 26th May 2015

Abstract

Utilization of mixtures of differently coordinating aromatic N-donor ligands leads to the formation of the two new compounds {[Ni(phen)2]2Sn2S6}·4,4′-bipy·½H2O I and {[Ni(phen)2]2Sn2S6}·2,2′-bipy II that could be prepared under solvothermal conditions (4,4′-bipy = 4,4′-bipyridine, C10H8N2; phen = 1,10-phenanthroline, C12H8N2; 2,2′-bipy = 2,2′-bipyridine, C10H8N2). In the structures of both compounds Ni–S bond formation is observed which is highly unusual when only bidentate N-donor ligands are applied in the reaction mixture. The detailed analysis of the crystal structure indicates that the presence of 4,4′-bipy and 2,2′-bipy molecules are essential for the stabilization of the arrangement of the constituents. The main structural motif {[Ni(phen)2]2Sn2S6} is arranged generating off center parallel stacking of the phen ligands. The empty spaces between the {[Ni(phen)2]2Sn2S6} moieties are occupied by either 2,2′-bipy (I) or 4,4′-bipy (II) molecules which are oriented towards the phen ligands to form intermolecular π–π interactions.

1 Introduction

Compounds containing thiostannate building blocks may be classified according to different criteria. One group of thiostannates are pure inorganic compounds containing e.g. metal cations and thiostannate anions like in Na4Sn2S6·14H2O,1 Rb6Sn2S7,2 Cs2Sn4S9,3 Cs2Sn2S6 or K2Sn2S5,4 (Rb4(H2O)4)[SnS4],5 K10M4Sn4S17 (M = Mn, Fe, Co, Zn)6 and (NH4)4[Sn2S6]·3H2O7 to mention just a few. In these compounds the cations and anions are held together by mainly electrostatic interactions. The examples demonstrate the structural variability with thiostannate ions displaying different Sn[thin space (1/6-em)]:[thin space (1/6-em)]S ratios and therefore different structural motifs. Another group of compounds is composed of organic cations and thiostannate anions, like e.g. (TMA)[Sn3S7]·2H2O (TMA = tetramethylammonium),8 (DABCOH)2[Sn3S7]·H2O (DABCO = 1,4-diazabicyclo[2.2.2]octane),9 (enH)4[Sn2S6] (en = ethylenediamine),10 or (tmdpH2)[Sn3S7] (tmdp = 4,4′-trimethylenedipiperidine).11 Charge compensation may be also achieved by transition metal or lanthanoide cation centered complexes like in (M(en)3)2[Sn2S6] (M = Mn, Co, Ni, Zn),12–14 (M(dien)2)2[Sn2S6] (M = Mn, Co, Ni; dien = diethylentriamine),14–16 (Nd(dien)3)2[Sn2S6]Cl2, (Nd(dien)3)2[Sn2S6](SH)2,17 (Y2(dien)2(OH)2)[Sn2S6],18 (Ni(1,2-dap)3)2[Sn2S6]·2H2O (1,2-dap = 1,2-diaminopropane),12 (Ni(1,2-dach)3)2[Sn2S6]·4H2O (1,2-dach = 1,2-diaminocyclohexane), (Ni(peha))2[Sn2S6]·H2O (peha = pentaethylenehexamine) or (Ni(aepa)2)2[Sn2S6] (aepa = N-2-aminoethyl-1,3-propandiamine).16

Another class of compounds is characterized by M–S bonds between the thiostannate ion and transition metal cations which are further surrounded by N donor atoms from amine ligands. In such compounds the [Sn2S6]4− and [SnS4]4− anion, respectively, acts as bidentate ligand like in {[Mn(1,2-dach)2(H2O)]2[Sn2S6]},19 {[Mn(en)2]2[Sn2S6]}n,20 {[Mn(1,2-dap)2]2[Sn2S6]}n,21 {[M(tepa)]2[Sn2S6]} (M = Mn, Fe, Co, Ni; tepa = tetraethylenepentamine),22,23 {[Mn(trien)]2[SnS4]}n·4nH2O (trien = triethylenetetramine),24 {[M(tren)]2[Sn2S6]}12,25 (M = Mn, Co; tren = tris(2-aminoethyl-)amine), {[Co(cyclam)]2[Sn2S6]}n·2nH2O26 (cyclam = 1,4,8,11-tetraaza-cyclotetradecane), o-{[Ni(tepa)]2[Sn2S6]}, {[Mn(trien)]2[SnS4]}n16 or in a tetradentate fashion as observed for e.g. {[Mn(phen)2]2[Sn2S6]},27 {[TM(phen)2]2[Sn2S6]}28 (M = Fe, Co) or {[Mn(phen)]2[SnS4]}n·nH2O.29

Finally metal cations may be integrated into the thiostannate unit as observed for (1,4-dabH)2MnSnS4[thin space (1/6-em)]30 (1,4-dab = 1,4-diaminobutane), (DBUH)CuSnS3 (DBU = 1,8-diaza-bicyclo[5.4.0]undec-7-ene) and (1,4-dabH2)Cu2SnS4,31 (1,4-dabH2)Ag2SnS4,32 (enH)3Cu7Sn4S12,33 (DBNH)2Cu6Sn2S8[thin space (1/6-em)]34 (DBN = 1,5-diazabicyclo[4.3.0]non-5-ene), (dienH2)Cu2Sn2S6,35 (NH4)2Ag6Sn3S10,36 (enH2)Ag2SnS4,37 (enH2)HgSnS4,38 (enH2)2Cu8Sn3S12[thin space (1/6-em)]39 and [Mn(dien)2]MnSnS4.21

Many of the above-mentioned compounds were obtained under solvothermal conditions and state-of-art of the chemistry of thiostannates and other thiometallates was reviewed in several articles highlighting the synthetic approaches, the structural variability and flexibility of the anions.40 Analyzing the structures of thiometallate compounds with transition metal cations having bonds to S atoms of the anions, the Mn2+ ion can be easily integrated in the anions independent from the amine applied, whereas Fe2+, Co2+, Ni2+, or Zn2+ prefer bond formation to the N atoms of the amine molecules and M–S bond formation must be forced by applying multidentate amine molecules (tetra- or pentadentate) in the synthesis mixture which do not satisfy the coordination requirements of the cations. Mn2+ seems to have a comparable affinity to both S and N atoms as can be seen from the thiostannate examples presented above but a similar observation was also made for e.g. thioantimonates.41

During a systematic study of the Mn/phen/Sn/S system we prepared {[Mn(phen)2]2(μ2-Sn2S6)}·phen and {[Mn(phen)2]2(μ2-Sn2S6)}·phen·H2O containing co-crystallized phen molecules which are arranged with respect to phen ligands of the Mn2+ centered complexes to achieve attractive so-called π–π interactions.27 The energy involved in these interactions is between ca. 10 and 13 kcal mol−1.42–45 While the interaction energy between stacked phen molecules is low compared to that of covalent or ionic bonds, it seems to be large enough to stabilize frequently observed arrangements of aromatic molecules with respect to each other, like e.g. face-to-face, off-center, slipped or edge-to-face. Modifying the synthesis conditions originally applied for the preparation of {[Mn(phen)2]2(μ2-Sn2S6)}·phen and {[Mn(phen)2]2(μ2-Sn2S6)}·phen·H2O and using Co, Fe instead of Mn we were able to crystallize {[TM(phen)2]2Sn2S6}·phen·H2O (TM = Co, Fe) with similar arrangements of the phen molecules found for the analogous Mn compound.28 This was a surprising result because Co2+/Fe2+ have a strong preference for N donor atoms yielding normally isolated [TM(L)n]2+ (L = bidentate or tridentate ligands) complexes and thiometallate anions.13,16,46–49 All thiometallate compounds displaying a Co–S/Fe–S bond were obtained in the presence of a tetradentate or pentadentate ligand like tren,12,48–50 tepa23 and cyclam,26 with one exception where 1,2-dach (1,2-dach = 1,2 diaminocyclohexane) was used as amine.51

In our ongoing synthetic work all attempts to synthesize the analogous Ni2+ containing compounds failed despite varying the reaction conditions reported in.27–29 One possible reason is the high stability of the [Ni(phen)3]2+ complex, which is in situ formed under the solvothermal reaction conditions. Hence, we developed a new synthetic strategy applying mixtures of aromatic N-donor ligands which are either strong (phen), medium (2,2′-bipyridine), weak/monodentate coordinating (4,4′-bipyridine) which should lead to the solely coordination of Ni2+ by phen and the other molecule-donor ligands might act as structural stabilizers via intermolecular π–π-interactions.

During the experiments we were able to prepare and characterize the two new compounds {[Ni(phen)2]2Sn2S6}·4,4′-bipy·½H2O (I) and {[Ni(phen)2]2Sn2S6}·2,2′-bipy (II). Here we report the syntheses, crystal structures and spectroscopic data of these compounds.

2 Results and discussion

Synthetic aspects

For the synthesis of thiometallates applying elemental Sn and S a very weak coordinating amine like methylamine is necessary which generates the basic conditions for production of polysulfide anions which attack Sn to form the thiostannate ion.

In our previous investigations of the TM/Sn/S/phen systems (TM = Mn, Fe, Co, Ni) we demonstrated that Mn2+ easily forms bonds to thiostannate anions and five compounds with compositions {[Mn(phen)2]2Sn2S6}, {[Mn(phen)2]2Sn2S6}·phen, {[Mn(phen)2]2Sn2S6}·phen·H2O and {[Mn(phen)2]2[SnS4]2[Mn(phen)]2}·H2O could be prepared of which four were accessible even under stirring conditions.27 The syntheses with Fe and Co yielded only two compounds ({[M(phen)2]2Sn2S6} and {[M(phen)2]2Sn2S6}·phen·H2O, M = Fe, Co) and only under static conditions.28 Applying the synthesis conditions used for TM = Mn, Fe, and Co no compounds were accessible with Ni and the reaction products only contained the crystallized [Ni(phen)3]2+ complex (counter ion: Cl−) and X-ray amorphous sulfides. One reason may be the differing stabilities of the [TM(phen)3]2+ complexes with log[thin space (1/6-em)] β (TM): Mn: 10.5, Fe: 21.2, Co: 19.9, Ni: 24.3.52 To force bond formation between Ni2+ and the thiostannate anion syntheses were performed with mixtures of aromatic bidentate N-donor ligands which have different coordination abilities. The bidentate amine phen is a strong coordinating ligand,52 while 2,2′-bipy has a weak/medium coordination tendency52 and 4,4′-bipy can only act as a monodentate ligand to one Ni2+ center. Both 4,4′-bipy and 2,2′-bipy may act as stabilizing molecules via π–π interactions and both are not strong competitors for phen. During the explorative synthetic work we observed that the ratio phen[thin space (1/6-em)]:[thin space (1/6-em)]bipy is an important parameter. Compound I containing co-crystallized 4,4′-bipy was only obtained when the amount of 4,4′-bipy was between 20–65% of the total amount of amine. The highest yield was observed for a Ni[thin space (1/6-em)]:[thin space (1/6-em)]phen[thin space (1/6-em)]:[thin space (1/6-em)]4,4′-bipy molar ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1. Applying mixtures of phen and 2,2′-bipy compound II crystallized only applying 20–50% 2,2′-bipy of the total amount of amine. For compound II the highest yield was obtained at a molar ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]1 for Ni[thin space (1/6-em)]:[thin space (1/6-em)]phen[thin space (1/6-em)]:[thin space (1/6-em)]2.2′-bipy. When the syntheses were carried out using the analogue cobalt and iron salts (CoCl2·6H2O or FeCl2·4H2O) under the conditions mentioned above, the known compounds {[Co(phen)2]2Sn2S6}28 and {[Fe(phen)2]2Sn2S6},28 were obtained.

Crystal structures

The compound {[Ni(phen)2]2Sn2S6}·4,4′-bipy·½H2O (I) crystallizes in the monoclinic space group C2/c with four formula units in the unit cell with all atoms on general positions except the two unique Ni and Sn atoms. {[Ni(phen)2]2Sn2S6}·2,2′-bipy (II) crystallizes in the monoclinic space group P21/n with two formula units in the unit cell and all atoms are located on general sites.

Both structures feature the [Sn2S6]4− anion and charge compensating Ni2+ centered complexes. The Ni2+ cations of the [Ni(phen)2]2+ complexes have two Ni–S bonds leading to the formation of discrete molecules (Fig. 1).


image file: c5dt01145k-f1.tif
Fig. 1 Structure of the {[Ni[phen)2]2Sn2S6} moieties in I as a representative. Labelling of the atoms for II are given in parentheses. The hydrogen atoms are omitted for clarity.

The Ni–S bond lengths (2.4960 Å–2.5047 Å) as well as the Ni–N bonds (2.083 Å–2.124 Å) are within the range reported in literature.12–14,16,23 (Table 1).

Table 1 Selected bond lengths (Å) and angles (°) of compounds I and IIa
  I   II
a Symmetry transformations used to generate equivalent atoms: a: −x + 1, y, −z + 1/2; b: −x + 1, −y + 1, −z.
Sn1–S1 2.3366(10) Sn–S3 2.3296(9)
Sn1–S1a 2.3382(10) Sn–S2 2.3475(9)
Sn1–S2 2.4532(9) Sn–S1b 2.4488(9)
Sn1–S2a 2.4630(9) Sn–S1 2.4524(9)
S1–Sn–S1a 100.59(5) S2–Sn–S3 100.43(3)
S2–Sn–S2a 92.78(4) S1–Sn–S1b 92.46(3)
S1–Sn–S2a 117.27(4) S1–Sn–S3 120.84(3)
S1a–Sn–S2a 115.02(3) S1–Sn–S2 113.00(3)
S2–Sn–S3 117.28(4) S2–Sn–S1b 118.10(3)
S2a–Sn–S3 114.57(3) S3–Sn–S1b 113.30(3)
Sn1–S1–Sn2 87.46(3) Sn–S1–Snb 87.54(3)
 
Ni1–N1 2.103(3) Ni1–N1 2.120(3)
Ni1–N2 2.122(3) Ni1–N2 2.161(3)
Ni1–N21 2.083(3) Ni1–N21 2.106(3)
Ni1–N22 2.124(3) Ni1–N22 2.112(3)
Ni1–S1 2.4960(11) Ni1–S2 2.4946(10)
Ni1–S3 2.5047(11) Ni1–S3 2.4984(10)

The angles around Ni2+ scatter between 78.64(13)° and 175.86(9)° (I) and between 77.38(11)° and 172.28(9)° (II), respectively, indicative for a severe distortion of the octahedra, but still in the range of literature data.16 The distortion is caused by the fixed position of the N-atoms in the phen-molecule leading to acute angles around the Ni2+ cations (I: N1–Ni1–N2: 78.64(13), N21–Ni1–N22: 79.03(13); II: N1–Ni1–N2: 77.38(11), N21–Ni1–N22: 78.25(12)) (Table S1†). The values of the dihedral angles between the phen ligands are around 90° ± 10° (Table S2†) and they are comparable with literature data.27,28 The [Sn2S6]4− ion which is generated by edge-sharing of two SnS4 tetrahedra exhibits the typical Sn–S bonding pattern of short Sn–St (t = terminal) and longer Sn–Sb (b = bridging) bonds. Comparing the geometric parameters of the thiostannate ion of the title compounds with those of the discrete anion [Sn2S6]4−(ref. 1,12–16,46) the Sn–S bond lengths do not differ significantly (Table 1). Different, however, are the bond angles which cover a wide range from 87.46 to 117.27° in I and they are between 87.54° and 120.84° in II evidencing a strong deviation from ideal tetrahedral geometry, a phenomenon also observed previously.12,23,26–28,53

Besides the {[Ni(phen)2]2Sn2S6} moiety, the structure of I contains an additional 4,4′-bipy and a water molecule, which is disordered over two half occupied positions. The {[Ni(phen)2]2Sn2S6} units are arranged as rods in all three crystallographic directions (Fig. 2). The 4,4′-bipy molecules are assembled in a chain-like fashion along [100] and [001] (Fig. 2). The water molecule is located between adjacent 4,4′-bipy molecules and the N⋯O separation of 2.922 Å indicates hydrogen bonding interaction.


image file: c5dt01145k-f2.tif
Fig. 2 Parallel arrangement of the molecules in I within the ab-plane (top) and bc-plane (bottom), respectively. Hydrogen atoms are omitted for clarity.

The arrangement of the constituents for optimal intermolecular interactions in I becomes clear viewing along [10−1]: The {[Ni(phen)2]2Sn2S6} moieties are interlaced along [101] and the 4,4′-bipy molecules are located in the thus generated voids (Fig. 3).


image file: c5dt01145k-f3.tif
Fig. 3 Arrangement of the molecules in I viewed along [10−1]. The 4,4′-bipy molecules are located in the free spaces between the {[Ni(phen)2]2Sn2S6} moieties. Hydrogen atoms are omitted for clarity.

The phen ligands of neighbored complexes exhibit a short intermolecular distance of 3.489 Å and two even shorter separations are observed between phen and co-crystallized 4,4′-bipy molecules at 3.343–3.455 Å. Such short distances between off-centered parallel stacked aromatic molecules indicate π–π interactions (Fig. 4 and 5).42–45,54,55


image file: c5dt01145k-f4.tif
Fig. 4 Off-center parallel orientation of the phen ligands in I within the ab-plane. The purple dashed lines indicate the shortest intermolecular distances. Hydrogen atoms are omitted for clarity.

image file: c5dt01145k-f5.tif
Fig. 5 Off-center parallel orientation of the phen ligands as well as 4,4′-bipy molecules in I along [101]. The purple dashed lines indicate the shortest intermolecular distances. Hydrogen atoms are omitted for clarity.

In the structure of II the {[Ni(phen)2]2Sn2S6} moieties are lined-up along a and c, respectively (Fig. 6). The 2,2′-bipy molecules are located between two adjacent {[Ni(phen)2]2Sn2S6} units yielding a sequence AAB along [100] where A stands for the phen Ligand and B for the 2,2′-bipy molecule.


image file: c5dt01145k-f6.tif
Fig. 6 Arrangement of the molecules in II within the ac-plane. The 2,2′-bipy molecules are disordered over two positions between two pairs of phen ligands of the {[Ni(phen)2]2Sn2S6} moieties. Hydrogen atoms are omitted for clarity.

Like in compound I the aromatic components adopt orientations which allow attractive intermolecular interactions. Along [010] all molecules are arranged in rows, while along [101] the phen ligands are staggered. The 2,2′-bipy molecules can now be aligned parallel to the upper and the lower phen ligand of adjacent rows (Fig. 7, top) leading either to a parallel arrangement or a staggered arrangement (Fig. 7, bottom). The intermolecular distances for these orientations between the 2,2′-bipy molecule and the phen ligands are almost identical with 3.279–3.826 Å for the parallel and 3.335–3.921 Å and 3.396–4.059 Å for the staggered orientations. As discussed above and in agreement with literature data such distances can be regarded as π–π interactions.54,55


image file: c5dt01145k-f7.tif
Fig. 7 Arrangement of the 2,2′-bipy molecules between the {[Ni(phen)2]2Sn2S6} moieties (top) in the structure of II. The shortest intermolecular distances are shown (purple dashed lines, distances in Å, bottom). For reasons of clarity only one arrangement of the 2,2′-bipy molecule is shown. Hydrogen atoms and part of the {[Ni(phen)2]2Sn2S6} moieties as well as hydrogen atoms are omitted.

Physcial properties

The Sn–S modes in the Raman spectra of thiostannates are located in the region of 400 to 100 cm−1. The assignment of the modes for compounds I and II was done on the basis of the data documented for the [Sn2S6]4− anion1,56,57 and NiS2[thin space (1/6-em)]58 (Table 2).
Table 2 Data of the signals in the Raman spectra of Na4Sn2S6·14H2O1 and compounds I and II compared to the [Sn2S6]4− anion1,56,57 and NiS2[thin space (1/6-em)]58 in cm−1
[Sn2S6]4− NiS2 I II
391 394 399
341 336 340
301 298 298
281 283 273 276
222 219 212
190 181 180

The resonance of the symmetric Sn–St stretching mode is found around 390 cm−1 in the discrete anion. In the title compounds the [Sn2S6]4− unit has bonds to Ni2+ resulting in slightly shorter Sn–S bonds leading to a shift of the signal to higher wave numbers. The Sn–S–Sn vibrations are located at lower wave numbers at about 340 cm−1. The mode at 280 cm−1 could be caused by a Sn2S2 ring vibration, the energetic differences between the resonances of I and II and those reported for Na4Sn2S6·14H2O could be generated by slightly differing bond lengths and angles. But since the Ni–S vibrations are located in this region as well, a detailed assignment is not possible.

In the IR spectra, the absorptions can be assigned to phen59–61, the Ni–N stretching vibration at ≈ 420 cm−1 (ref. 27 and 28) and to one of the bipy molecules62–65 (Table 3). The absorptions of the 4,4′-bipy molecule are mostly overlapping with the absorptions of phen, but for example at ca. 800 cm−1 only 4,4′-bipy causes a signal, which therefore can only be observed in compound I. For 2,2′-bipy (II) this absorption is shifted to slightly higher wavenumbers (818/819 cm−1) due the different position of the N-atoms in the aromatic ring.

Table 3 Absorptions in the IR-spectra of compounds I and II compared to 1,10-phen59–61, 4,4′-bipy62,63 and 2,2′-bipy64,65
1,10-phen 4,4′-bipy 2,2′-bipy I II Assignment
3035 3023 — 3042 3035w ν(C–H)
— — 2982 — 2979w ν(C–H)
— — 2916 — 2922w ν(C–H)
1616 1604 — 1624 1620 ν(C[double bond, length as m-dash]C)
— — 1595 — 1598 ν(C–C) + ν(C–N)
1586 1585 — 1587 — ν(C[double bond, length as m-dash]C)
— — 1579 — 1578 ν(C[double bond, length as m-dash]C)
1560 — 1555 — 1557 ν(C[double bond, length as m-dash]C)
1516 — 1517 1510 1510 ν(C[double bond, length as m-dash]C)
1494 1491 — 1494 1491 ν(C[double bond, length as m-dash]C)
— — 1458 — 1456 δ(C[double bond, length as m-dash]C–H)
1421 1416 — 1419 1416 Combination band
1344 — — 1342 1344 ν(C[double bond, length as m-dash]C)
— — 1253 — 1255 ν(C[double bond, length as m-dash]C)
1216 1218 — 1218 1222 ν(C[double bond, length as m-dash]C)
— — 1210 — 1206 δ(C[double bond, length as m-dash]C–H)
1137 1133 1238 1136 1134 δ(C[double bond, length as m-dash]C–H)
1092 — — 1096 1096 ν (C[double bond, length as m-dash]C)
— 1068 — 1065 — Ring breathing
1035 — 1038 — 1036 ν(C–N)
996 992 — 991 — δ(C[double bond, length as m-dash]C–H)
987 — — — 985 δ(C[double bond, length as m-dash]C–H)
869 — — 866 — ν(ring)
853 — — — 858 δ(C[double bond, length as m-dash]C[double bond, length as m-dash]C)
848 — — 844 846 δ(C[double bond, length as m-dash]C[double bond, length as m-dash]C)
— — 818 — 819 δ(C[double bond, length as m-dash]C–H)
— 800 — 802 — δ(C[double bond, length as m-dash]C–H)
779 — — 780 779 δ(C–H)
— — 758 — 753 δ(C[double bond, length as m-dash]C–H)
721 724 — 724 722 δ(C–H)
643 — — 640 637 ν(ring)
625 — 616 — 620 δ(C–H)
606 607 — 609 — δ(C[double bond, length as m-dash]C[double bond, length as m-dash]C)
478 — — 476 — ν(ring)
— — — 422 422 ν(Ni–N)

In the UV/vis spectra of both compounds three bands are observed (see Fig. S4 and S5†). The first two absorptions (I: ∼2.48 eV and ∼3.32 eV; II: ∼2.55 eV and ∼3.16 eV) can be assigned to the Ni2+ d–d transition 3A2g → 3T(3F) and 3A2g → 3T(3P).66 The third band (I: 4.63 eV; II: 4.59 eV) might probably be traced back to the π → π* transition of the aromatic amines.59

The thermal properties of both compounds were investigated by simultaneously differential thermoanalysis and thermogravimetry. On heating compound I at 4 °C min−1 one mass step of 40.8% is observed in the TG curve that is accompanied with an endothermic peak in the DTA curve at about 340 °C (Fig. S6†). On further heating a second much smaller mass step is observed and in the following the samples mass decreases continuously. The experimental mass loss observed in each step are not in agreement with those calculated for a stepwise removal of the organic ligands. Therefore, this reaction seems to be more complex. For compound II several poorly resolved mass steps are observed, all of them accompanied with endothermic events in the DTA curve, indicating that the organic ligands are stepwise removed.

Both compounds exhibit paramagnetic behaviour (see Fig. S8 and S9†). The value for the Weiss constant is near zero demonstrating that the Ni2+ cations are magnetically isolated. The effective magnetic moment for Ni2+ amounts to 2.88 μB which is near the spin only value.

3 Experimental section

Synthesis

General. All chemicals were used as purchased without further purifications. The compounds were prepared under solvothermal conditions in glass tubes (inner volume 11 mL) using NiCl2·6H2O, Sn, sulfur, phen, 4,4′-bipy and 2,2′-bipy, respectively. The reaction products were filtered off after reaction, washed with water and ethanol and dried in vacuo. Afterwards crystals in the products were separated manually. The homogeneity of the separated samples was checked by X-ray powder diffraction and elemental analysis.
Synthesis of {[Ni(C12H8N2)2]2Sn2S6}·C10H8N2·½H2O (I). 59.5 mg (0.25 mmol) NiCl2·6H2O, 29.7 mg (0.25 mmol) Sn, 24.1 mg (0.75 mmol) S, 45.1 mg (0.25 mmol) phen (C12H8N2) and 39.0 mg (0.25 mmol) 4,4′-bipy (C10H8N2) with 1.5 mL methylamine (40%, aqueous solution, abcr) and 0.5 mL water (pH ≈ 14) were reacted at 120 °C for 7 days. The product consisted of dark red-brown crystals of I and black to brown powder and crumbs. The yield of the manually separated crystals was app. 15% (based on tin). According to EDX analysis the brownish-black product contains Ni, Sn and S in varying composition. However, the X-ray powder diffraction pattern did not show reflections and one can only assume that these are X-ray amorphous Ni/Sn sulfides. When the synthesis is carried out under stirring conditions, a brownish powder of I is obtained after five hours with a yield being about more than twice as high as under static conditions (appr. 45%, based on tin). Elemental analysis, results in %: found: C 48.47, H 2.71, N 9.54, calculated: C 48.61, H 2.88, N 9.77.
Synthesis of {[Ni(C12H8N2)2]2Sn2S6}·C10H8N2 (II). 59.5 mg (0.25 mmol) NiCl2·6H2O, 29.7 mg (0.25 mmol) Sn, 24.1 mg (0.75 mmol) S, 90.1 mg (0.5 mmol) phen (C12H8N2) and 39.0 mg (0.25 mmol) 2,2′-bipy (C10H8N2) with 1.5 mL methylamine (40%, aqueous solution, abcr) and 0.5 mL water were reacted in a glass tube (pH ≈ 14). The mixture was heated at 120 °C for 7 days. The product contained deep red crystals of II and greyish-black powder and crumbs. The yield of the crystals was app. 20–25% (based on tin). The yield could be increased significantly under stirring conditions (appr. 65%, based on tin). The byproduct is X-ray amorphous and according to EDX data contains Ni, Sn and S. Elemental analysis, results in %: found: C 48.65, H 2.79, N 9.73, calculated: C 48.91, H 2.83, N 9.83.

Structure determination

The intensity data for the compounds were collected using a STOE IPDS-1 (Imaging Plate Diffraction System) with Mo-Kα radiation. The structures were solved with direct methods using the program SHELXS-97[thin space (1/6-em)]67 and the refinements were done against F2 with SHELXL-97.68 For all non-hydrogen atoms anisotropic displacement parameters were used. The hydrogen atoms were positioned with idealized geometry and were refined using a riding model. In compound I the O–H hydrogen atoms were located in the difference Fourier map, their bond lengths set to ideal values and refined using a riding model. The water molecule is disordered over two half occupied positions and was refined using a split model. The structure contains additional non-coordinating 4,4′-bipy molecules that are located on centres of inversion. In compound II, the non-coordinating 2,2′-bipy molecule is disordered and was refined using a split model with restraints for bond lengths and angles.

Selected refinement results are summarized in Table 4. Structural data have been deposited in the Cambridge Crystallographic Data Centre as publication no. CCDC 1054460 (I), CCDC 1054461 (II).

Table 4 Selected details of the data collection and structure refinement results
  {[Ni(phen)2]2Sn2S6}·4,4′-bipy·½H2O {[Ni(phen)2]2Sn2S6}·2,2′-bipy
Crystal system Monoclinic Monoclinic
Space group C2/c P21/n
M (g mol−1) 1442.18 1424.16
a (Å) 18.3431(6) 10.5715(2)
b (Å) 19.4475(6) 9.9086(2)
c (Å) 15.0835(5) 24.9960(4)
α (°) 90 90
β (°) 95.556(2) 92.8000(10)
γ (°) 90 90
V (Å3) 5355.4(3) 2615.17(8)
Temperature (K) 200(2) 200(2)
Z 4 2
D calculated (g cm−3) 1.789 1.809
μ (mm−1) 1.9.03 1.946
Scan range (°) 1.53 ≤ θ ≤ 28.00 1.63 ≤ θ ≤ 27.93
Reflections collected 31[thin space (1/6-em)]723 44[thin space (1/6-em)]107
Independent reflections 6419 6238
Observed reflections 5042 5617
Goodness-of-fit on F2 1.084 1.059
R values (I > 2σ(I)) R 1 = 0.0466 R 1 = 0.0480
wR2 = 0.1038 wR2 = 0.1274
R values (all data) R 1 = 0.0641 R 1 = 0.0520
wR2 = 0.1113 wR2 = 0.1307
Res. elec. dens. (e Å−3) 0.706 and −0.801 1.024 and −0.770

X-ray powder diffractometry

The X-ray powder diffraction patterns were recorded on a STOE Stadi-P powder diffractometer (Cu-Kα1 radiation, λ = 1.540598 Å, Ge monochromator) in transmission geometry.

SEM and EDX

Scanning electron microscopy investigations and energy dispersive X-ray analyses (EDX) were done with a Philips Environmental Scanning Electron Microscope ESEM XL30 equipped with an EDAX detector.

Raman spectroscopy

Raman spectra were recorded with a Bruker IFS 66 Fourier transform Raman spectrometer (wavelength: 541.5 nm) in the region from 100 to 3500 cm−1.

Infrared spectroscopy

MIR spectra (400–4000 cm−1) were recorded with a Bruker Alpha P spectrometer.

UV/visible spectroscopy

UV/vis spectra were recorded with an UV-vis-NIR two channel spectrometer Cary 5 from Varian Techtron Pty., Darnstadt at room temperature of powdered samples with BaSO4 powder as reference material. The absorption data were calculated applying the Kubelka–Munk relation for diffuse reflectance data.

Thermal properties

DTA-TG measurements were performed using a Netzsch STA 409 CD under a nitrogen flow of 75 mL min−1 and at a heating rate of 4 K min−1. The instrument was calibrated using standard reference materials.

Magnetic properties

The magnetic properties were investigated using a physical properties measurement system (PPMS) Model 600 from Quantum Design at H = 100 Oe in the temperature range 1.9–325 K.

4 Conclusions

In the manuscript we presented a new synthesis strategy based on the observation of different coordination abilities of aromatic N-donor ligands towards Ni2+. Applying suitable mixtures of the aromatic amine molecules with the proper ratio between Ni2+ and the strong coordinating phen ligand, products crystallized where the medium/weak or non-resp. monodentate aromatic amine molecule acts as stabilizer of the structures via π–π-interactions. Currently we experimentally investigate whether this is a new general concept, which can be applied to other transition metals opening new opportunities for the generation of hitherto not accessible thiostannates.

Acknowledgements

Financial support by the State of Schleswig-Holstein and the DFG is gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available: PXRD pattern, selected angles of the octahedral Ni2+ enivironment, IR- and Raman spectra. CCDC 1054460 and 1054461. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5dt01145k
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