Chalcogen bonding interactions in organic selenocyanates: from cooperativity to chelation

Olivier Jeannin a, Huu-Tri Huynh a, Asia Marie S. Riel ab and Marc Fourmigué *a
aUniv Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) – UMR 6226, 35000 Rennes, France. E-mail: marc.fourmigue@univ-rennes1.fr
bDepartment of Chemistry, University of Montana, 32 Campus Dr., Missoula, USA

Received 31st January 2018 , Accepted 4th March 2018

First published on 6th March 2018


Intermolecular chalcogen bonding interactions are identified in crystalline organic selenocyanates where a linear Se⋯N[triple bond, length as m-dash]C interaction takes place, leading to the recurrent formation of chain-like motifs ⋯Se(R)–CN⋯Se(R)–CN⋯, stabilized by cooperativity. Analysis of 15 reported structures of such selenocyanates is complemented by the structural determinations of three other novel polytopic selenocyanates, namely 1,3,5-tris(selenocyanatomethyl)benzene (1a), 1,3,5-tris(selenocyanatomethyl)-2,4,6-trimethylbenzene (1b) and 1,2,4,5-tetrakis(selenocyanatomethyl)benzene (2). While the recurrent chain-like motifs with short and linear Se⋯N contacts are indeed observed in the pure compounds, solvates with DMF and AcOEt also demonstrate that the nitrile N atom can be easily displaced from the chalcogen bond by stronger Lewis bases such as carbonyl oxygen atoms, leading in the case of (2)·(DMF)2 to a chelating motif where two neighboring CH2–SeCN groups link to the same oxygen atom through Se⋯O interactions.


Introduction

The rediscovery of halogen bonding (XB) interactions in the last twenty years1 has recently evolved toward the identification of similar properties in the chalcogen, pnictogen or even tetrel series.2,3 Recent illustrations of chalcogen bonding are found for example in the solid state structures of benzo-2,1,3-selenadiazoles,4 benzo-1,3-tellurazoles,5 iso-tellurazole N-oxides,6 or selenophtalic anhydride,7 as well as in the use of chelating bis-tellurophene8 or bis(benzimidazolium-selenomethyl) derivatives9 in catalytic reactions. One striking difference with the halogen bond donors is the distribution of the electrostatic surface potential. While one single charge-depleted area, the σ-hole, is identified in the prolongation of the covalent C–Hal bond, theoretical and experimental investigations of molecules with activated chalcogen atoms have unambiguously demonstrated the presence of two such charge-depleted areas, each of them in the prolongation of the two carbon–chalcogen bonds.2,3,10 The simplest and most convincing example was already identified years ago in Se(CN)2,11 and revisited recently.12 Its X-ray crystal structure (Fig. 1a) shows indeed two nitrogen atoms of neighboring molecules pointing toward the σ-holes of the selenium atom, with Se⋯N intermolecular distances, 2.813(9) and 2.835(7) Å, well below the sum of the van der Waals radii (3.45 Å), and C–Se⋯N angles close to linearity (166–170°). Cocrystallization of Se(CN)2 with 18-crown-6 is also reported to afford a co-crystal (Fig. 1b),13 where again the two σ-holes of the selenium atoms are clearly interacting with two oxygen atoms of the 18-crown-6 in a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 adduct. Selenophtalic anhydride provides another example where both charge-depleted areas were identified from an experimental high resolution X-ray data collection.7
image file: c8nj00554k-f1.tif
Fig. 1 Detail of the solid state structure of (a) Se(CN)2, and (b) Se(CN)2·(18-crown-6) adduct in Se(CN)2·(18-crown-6)1.5. Chalcogen bond interactions are indicated as orange dotted lines.

Considering that the predictability of halogen bonding in crystal engineering strategies is essentially due to the presence of one single σ-hole in the prolongation of the C–X bond, we postulated that the ability of chalcogen atoms to provide a similar predictability despite the presence of two σ-holes can be strongly enhanced if one is able to favor one σ-hole over the other. For that purpose, the use of unsymmetrically substituted chalcogen atoms, with one electron-withdrawing (EWG) group and one electron-releasing (ERG) group should favor the presence of a stronger σ-hole in the prolongation of the EWG–Se bond.9 Based on this assumption, organic selenocyanates appear as ideal candidates, as the selenium atom is simultaneously linked to the strongly electron-withdrawing nitrile substituent and to a more electron-releasing alkyl, benzyl or aryl group. Following earlier conclusions by Bauza et al.,3a investigations of the crystal structures of organic selenocyanates reported in CSD14 confirmed this assumption, as detailed below. We also recently showed that molecules bearing two such selenocyanate moieties,15 as the ortho-, meta- and para-bis(selenocyanato)xylene (Chart 1) are indeed able to act as ditopic chalcogen bond donors when faced with ditopic Lewis bases such as 4,4′-bipyridine to afford one-dimensional structures. We want here to extend this pool of benzylic selenocyanates to the tris and tetrakis-substituted derivatives (Chart 1), as described in a second part below, where their preparation and solid state structures will be reported and analyzed.


image file: c8nj00554k-c1.tif
Chart 1 Reported (top) and novel (1a, 1b, 2) benzylic selenocyanates.

Results

Analysis of reported solid state structures of organic selenocyanates

Most of the organic selenocyanates found in CSD show evidence of a chalcogen bonding interaction involving the selenium atom as chalcogen bond donor. In the following, we will described successively: (i) systems with intramolecular chalcogen bond, (ii) aromatic selenocyanates, (iii) benzylic and allylic selenocyanates, (iv) aliphatic selenocyanates. In the following, the evaluation of the strength of the chalcogen bond in all these systems will be based primarily on the reduction ration of the actual Se⋯Y distance, relative to the sum of the van der Waals radii of the interacting atoms, with Se: 1.90 Å, N: 1.55 Å, O: 1.52 Å, that is dvdW(Se⋯N) = 3.45 Å, dvdW(Se⋯O) = 3.42 Å, dvdW(Se⋯Se) = 3.80 Å.

Intramolecular chalcogen bonding is found in three examples (Fig. 2), namely methyl 2-selenocyanatobenzoate,16 phenacyl selenocyanate17 and 8-(dimethylamino)-1-naphthyl selenocyanate.18 The planarity of the three systems despite the strong steric constraints demonstrates the stabilization brought by the chalcogen bond interaction. The chalcogen bond distances are accordingly very short, with reduction ratio down to 0.72. The interaction takes place in the prolongation of the Se–CN bond, demonstrating also the selective activation of one σ-hole site on the selenium atom by the opposite nitrile group. It already confirms the interest of organic selenocyanates derivatives to activate one single strong σ-hole on the selenium atom.


image file: c8nj00554k-f2.tif
Fig. 2 Details on intramolecular chalcogen bond interactions in three reported examples (with CCDC REFCODE).

Aromatic selenocyanates represent the largest reported group, with an interesting series provided by phenyl selenocyanate,19 pentafluorophenyl selenocyanate,20 1,4-bis(selenocyanato)benzene21 and 2,4,6-tris(trifluoromethyl)phenyl selenocyanate.22 As shown in Fig. 3, a linear Se⋯N interaction involving the nitrogen atom of the C[triple bond, length as m-dash]N group takes place, leading to the recurrent formation of chain-like motifs ⋯Se(R)–CN⋯Se(R)–CN⋯. We also observe a strengthening of the interaction with the most-electron-withdrawing aromatic cores, most probably attributable to an enhancement of the σ-hole on the selenium atom. We believe that such one-dimensional systems are also stabilized by cooperativity, as indeed theoretically demonstrated in model systems.23 Structural characteristics are gathered in Table 1. A similar Se⋯N interaction is also observed in 1,4-bis(selenocyanato)benzene (CSD: SECNBZ) where it develops in two dimensions.


image file: c8nj00554k-f3.tif
Fig. 3 The chain-like structures formed by chalcogen bonding in (a) phenyl selenocyanate, (b) pentafluorophenyl selenocyanate, and (c) 2,4,6-tris(trifluoromethyl)phenyl selenocyanate.
Table 1 Details of the structural characteristics of the chalcogen bond interactions in reported compounds. RR stands for reduction ratio and is given by the ratio of the observed interatomic distance over the sum of the van der Waals radii of interacting atoms
Compound REFCODE Se⋯N(O,Se) (Å) RR C–Se⋯Y (°) Ref.
a Two crystallographically independent chains. b Two independent molecules alternating in one chain.
Se(CN)2 UQAXUH 2.813(9) 0.815 166.3(2) 12
2.835(7) 0.822 170.4(2)
Se(CN)2·1.5 (18-crown-6) QUHYAV 2.873(2) (O) 0.840 167.2(1) 13
2.938(10) (O) 0.856 161.2(1)
Intramolecular:
Methyl 2-selenocyanatobenzoate AHEQUIN 2.561(1) (O) 0.749 170.34(6) 16
Phenacyl selenocyanate FAGGAV 2.723(6) (O) 0.796 152.0(1) 17
8-(NMe2)-1-Naphthyl selenocyanate GIYJUV 2.456(2) 0.712 172.65(7) 18
Aromatic selenocyanates:
PhSeCN CIBFUP 3.023(3)a 0.876 172.9(1) 19
3.065(4) 0.888 166.1(1)
p-(SeCN)2C6H4 SECNBZ 3.06(2) 0.887 162.32(8) 21
C6F5SeCN BATDIJ 2.958(10)a 0.857 175.6(8) 20
2.964(9) 0.859 172.1(4)
C6H2(CF3)3SeCN KABTEN 2.883(6)b 0.836 173.1(2) 22
2.968(5) 0.860 166.4(2)
3-(SeCN)Pyridine WERYAT 2.843(10) 0.824 174.0(2) 24
Benzylic selenocyanates:
Benzylselenocyanate CIGGOO 2.997(18) 0.869 167.1 25
ortho-Bis(SeCN)xylene NARBIR 2.985(8) 0.865 172.9(3) 26
2.969(9) 0.860 172.6(3)
meta-Bis(SeCN)xylene 2.965(24)a 0.859 175.9(7) 15
3.017(24) 0.874 175.7(7)
3.010(24) 0.872 172.9(7)
3.015(24) 0.874 174.1(7)
para-Bis(SeCN)xylene POXYEH 2.997(18)a 0.860 174.4(8) 27
3.022(18) 0.876 171.8(5)
4-Nitrobenzyl-selenocyanate CIGGEE 3.005(7) (O) 0.879 166.2 25
3.174(9) (O) 0.928 163.9
2-(MeSe)Benzylselenocyanate YUNSIK 3.467(1) (Se) 0.912 163.4(1) 28
Aliphatic selenocyanates:
1,1-Bis(selenocyanatoethyl) cyclohexane GOHMEW 3.199(2) 0.927 145.7(8) 29
Cholesterol derivative ZUTTAL 3.36(2) 0.974 157.9(7) 30


Another interesting example is provided by 3-selenocyanatopyridine24 (Fig. 4) where the pyridinyl nitrogen atom is now engaged in the chalcogen bond, with a short Se⋯NPy distance (RR = 0.824), demonstrating that the nitrile N atom can be easily displaced from the chalcogen bond by stronger Lewis bases, as already illustrated above in the Introduction with the adduct of Se(CN)2 with 16-crown-6 (Fig. 1b).


image file: c8nj00554k-f4.tif
Fig. 4 Chalcogen bonding in 3-selenocyanatopyridine.

Benzylic selenocyanates are easily prepared from benzyl halide and potassium selenocyanates. Many examples were therefore reported and some of them were structurally characterized. Benzylselenocyanate25 itself and the three ortho-,26meta-,15 and para-bis(selenocyanatato)xylene27 have been reported. As shown in Fig. 5, they all exhibit the recurrent ⋯Se(R)–CN⋯Se(R)–CN⋯ chain-like motif. The Se⋯N intermolecular distances are comparable to those reported above in aromatic selenocyanates.


image file: c8nj00554k-f5.tif
Fig. 5 Recurrent ⋯Se(R)–CN⋯Se(R)–CN⋯ chain-like motif in benzylic selenocyanates such as (a) benzylselenocyanate (hydrogen atoms were omitted), (b) ortho-bis(selenocyanatato)xylene, (c) meta-bis(selenocyanatato)xylene, and (d) para-bis(selenocyanatato)xylene.

The robustness of these chains is however questioned when a stronger chalcogen bond acceptor is present. This is indeed the case in 4-nitrobenzyl-selenocyanate25 and 2-(methylselanyl)benzyl selenocyanate28 where either oxygen atoms from a –NO2 group or selenium atom of a –SeMe group act as chalcogen bond acceptors (Fig. 6). The structure of 4-nitrobenzyl-selenocyanate is particularly interesting (Fig. 6a) as it demonstrates that the two σ-holes on the selenium atom are here involved in a chalcogen bonding interaction. The strongest one, at 180° from the CN group gives rise to the shortest Se⋯O contact (3.005 Å), while a weaker one, at 180° from the para-nitrobenzyl group, gives a Se⋯O distance at 3.174 Å, notably shorter than the sum of the van der Waals radii (3.42 Å)


image file: c8nj00554k-f6.tif
Fig. 6 Detail of the supramolecular motifs developed by (a) 4-nitrobenzyl-selenocyanate, and (b) 2-(methylselanyl)benzyl selenocyanate (hydrogen atoms were omitted).

Two examples involving aliphatic selenocyanates involve 1,1-bis(selenocyanatoethyl) cyclohexane29 (CSD: GIHMEW) and a cholesterol derivative (CSD: ZUTTAL).30 They also exhibit this chain motif but with rather weaker interactions (Table 1).

Novel tris- and tetrakis organic selenocyanates: a recurrent chain motif

Following our preliminary experiments aimed to unravel the ability of bis(selenocyanates) derivatives such as the three ortho-,26meta-,15 and para-bis(selenocyanatato)xylene27 to form one-dimensional structures upon co-crystallization with neutral ditopic Lewis bases (4,4′-bipyridine), we turned our attention to the corresponding tris- and tetrakis-substituted derivatives, namely 1,3,5-tris(selenocyanatomethyl)benzene (1a), 1,3,5-tris(selenocyanato methyl)-2,4,6-trimethylbenzene (1b) and 1,2,4,5-tetrakis(selenocyanatomethyl)benzene (2). We describe in the following their synthesis and analyze in details their crystal structures, where the chain-like motif ⋯Se(R)–CN⋯Se(R)–CN⋯ mentioned above is also acting as a powerful supramolecular motif.

Both 1a, 1b and 2 were prepared from the reaction of the corresponding tris(bromomethyl) and tetrakis-(bromomethyl)benzene derivatives with KSeCN (see Experimental section). In the presence of AcOEt or DMF, 1b and 2 also crystallize as solvates, formulated as 1b·AcOEt, 1b·DMF and 2·(DMF)2. We will first describe the crystal structures of 1a, 1b, 1b·DMF and 2 where only Se⋯NC interactions are found, while the structures of 1b·AcOEt and 2·(DMF)2 will be described in a second part, as they also involve Se⋯O chelating interactions with the solvent molecules (see below).

1a crystallizes in the monoclinic system, space group P21/n, 1b in the monoclinic system, space group C2/c, and 1b·DMF in the triclinic system, space group P[1 with combining macron] with in the three structures one molecule in general position. On the other hand, tetra-substituted compound 2 crystallizes on an inversion center, in the monoclinic P21/c space group. As shown in Fig. 7a and c, in the tritopic derivatives 1a and 1b, we find two SeCN groups above the benzene ring with one below. A similar geometry for 1b is found in its DMF solvate (Fig. 7d). Note also the positional disorder of one selenocyanate group in 1b, with a 29[thin space (1/6-em)]:[thin space (1/6-em)]71 distribution for Se31 and Se32.


image file: c8nj00554k-f7.tif
Fig. 7 Molecular structures of (a) 1a, (b) 2, (c) 1b and (d) 1b in 1b·(DMF).

The solid state organization of 1a (Fig. 8) is characterized by the occurrence of two strong Se⋯N chalcogen bonds (see Table 2) leading to the formation of infinite chains ⋯Se1–C1[triple bond, length as m-dash]N1⋯Se1–C1[triple bond, length as m-dash]N1⋯ and ⋯Se2–C2[triple bond, length as m-dash]N2⋯Se2–C2[triple bond, length as m-dash]N2⋯ running along the a direction (Fig. 8a). The third selenocyanate group connects those chains along b through a weaker ⋯Se3–C3[triple bond, length as m-dash]N3⋯ Se3–C3[triple bond, length as m-dash]N3⋯ chalcogen bond.


image file: c8nj00554k-f8.tif
Fig. 8 Solid state organization of 1a showing (a) the chalcogen-bonded chains running along a, and (b) the lateral Se3⋯N3 interactions.
Table 2 Structural characteristics of chalcogen bonds in the crystal structures of 1a, 1b, 1b·(DMF) and 2
Compound Interaction Se⋯Y (Å) RR C–Se⋯Y (°)
a (1 + x, y, z). b (0.5 + x, 1.5 − y, 0.5 + z). c (0.5 − x, 0.5 − y, 1 − z). d (−x, 1 − y, 1 − z). e (−x, y, 1.5 − z). f (−x, −y, 1 − z). g (−1 + x, y, z). h (2 − x, −y, −z). i (x, 1.5 − y, −0.5 + z). j (1 + x, 1.5 − y, 0.5 + z). k (2 − x, 2 − y, 1 − z).
1a
Intrachain Se1⋯N1a 3.023(5) 0.876 168.5(2)
Se2⋯N2a 3.011(5) 0.873 169.3(2)
Interchain Se3⋯N3b 3.243(8) 0.941 134.8(2)
1b
Intrachain Se1⋯N31c 2.986(18) 0.865 171.4(4)
Se1⋯N32c 3.292(10) 0.954 155.8(2)
Se2⋯N1d 3.188(4) 0.924 177.4(2)
Interchain Se31⋯Se31e 3.078(5) 0.810 178.6(5)
Se32⋯N1f 3.336(6) 0.967 155.8(3)
1b·(DMF)
Intrachain Se1⋯N1a 2.960(4) 0.858 175.4(1)
Se2⋯N2a 2.964(2) 0.859 176.1(1)
With DMF Se3⋯N3g 2.972(3) 0.861 173.8(1)
Se1⋯O1h 3.284(6) 0.960 163.8(1)
2
Intra layer Se1⋯N1i 3.085(5) 0.894 164.0(2)
Inter layer Se1⋯N2j 3.202(20) 0.928 167.6(2)
Se2⋯N1k 3.370(14) 0.977 170.0(2)


The structure of 1b is more complex (Fig. 9). Two selenium atoms (Se1, Se2) interact with nitrogen atoms to generate a chain running along a [1 −1 0] direction. A third disordered selenium atom (Se31) makes a very short Se⋯Se contact with a neighboring chain running along the [1 1 0] direction. As a consequence of this interchain Se⋯Se interaction, one nitrogen atom (N2) is not engaged in a short contact.


image file: c8nj00554k-f9.tif
Fig. 9 Solid state organization of 1b showing the chalcogen-bonded chains running along a.

The structure of the DMF solvate of 1b is shown in Fig. 10 and it strongly differs from that of the pure compound. Indeed, the three selenocyanate groups are now engaged in short and directional Se⋯N interactions running parallel to each other to form a chain, while one of the selenium atoms also interacts weakly (RR = 0.96) with the carbonyl oxygen atom of DMF.


image file: c8nj00554k-f10.tif
Fig. 10 Solid state organization of 1b·DMF.

The structure of 2 (Fig. 11) is characterized by a strong Se1⋯N1 interaction leading to the formation of layers. These layers are interconnected along a through two weaker interactions, one involving again Se1 as chalcogen bond donor, the other involving Se2 (see Table 2).


image file: c8nj00554k-f11.tif
Fig. 11 Projection view along a of one layer in 2, built out the strongest Se1⋯N1 chalcogen bond.

As mentioned above, both 1b and 2 were found to also co-crystallize with solvent molecules, affording two different solvates, namely 1b·AcOEt and 2·(DMF)2. Their molecular structures are detailed in Fig. 12, their structural characteristics in Table 3.


image file: c8nj00554k-f12.tif
Fig. 12 Details of the supramolecular organization in (a) 1b·AcOEt, and (b) 2·(DMF)2.
Table 3 Structural characteristics of chalcogen bonds in the crystal structures of the solvates 1a, 1b, 2
Compound Interaction Se⋯Y (Å) RR C–Se⋯Y (°)
a (0.5 − x, 0.5 − y, 1 − z). b (−0.5 + x, −0.5 + y, z). c (−x, −y, 1 − z). d (1 − x, −y, 1 − z).
1b·(AcOEt) Se1⋯N3a 3.174(4) 0.92 177.0(1)
Se2⋯O11b 2.925(6) 0.855 166.5(1)
Se2⋯O12b 2.871(7) 0.839 168.8(2)
Se3⋯N2c 2.965(3) 0.859 174.9(1)
2·(DMF)2 Se1⋯O1d 2.946(12) 0.861 171.9(2)
Se2⋯O1d 2.937(49) 0.859 175.2(2)


In 1b·AcOEt, the AcOEt molecule is disordered on two equivalent (50[thin space (1/6-em)]:[thin space (1/6-em)]50) positions. In the solid state (Fig. 12a), the carbonyl oxygen atom competes now with the nitrile groups to engage in a chalcogen bond with one of the three selenium atoms of the 1b molecule. As a consequence, the infinite chain motifs observed above in the structure of 1b (Fig. 9) are now cut into a defined segment of three chalcogen bonds in the order (O11, O12)⋯Se2–C[triple bond, length as m-dash]N2⋯Se3–C[triple bond, length as m-dash]N3⋯Se1–C[triple bond, length as m-dash]N1, with the N1 nitrogen atom not engaged in chalcogen bonding. The structure of the DMF solvate, 2·(DMF)2, reveals another facet of these benzylic selenocyanate derivatives. Indeed, as shown in Fig. 12b, the molecule, located on inversion center, actually binds with the carbonyl oxygen atoms of the two DMF molecules, with two neighboring selenium atoms interacting with the same oxygen atom, providing a very attractive chelating system.

Conclusions

We have demonstrated here that organic selenocyanates provide a tutorial example of chalcogen bond donors. Compared with symmetrical selenides where experimental and theoretical evidences confirm the presence of two equivalent σ-holes located in the C–Se–C plane in the prolongation of the C–Se bonds, the unsymmetrical character of R–Se–CN derivatives strongly favor the σ-hole in the prolongation of the NC–Se bond. Recurrent features are found from this extensive set of crystal structures based on organic selenocyanates. The most striking one is the formation of one-dimensional superstructures derived from the complementary nature of the R–SeCN molecules, with the lone pair of the nitrogen interacting with the selenium σ-hole through a linear C–Se⋯NC interaction and a reduction ration (RR) around 0.86–0.87. This interaction is however relatively weak since it is displaced by pyridinyl nitrogen atom or with carbonyl (or nitro) oxygen atom, leading then to slightly stronger chalcogen bonds, with RR values down to 0.82. The observed chain-like structures are also a probable consequence of an extra stabilization brought by cooperativity. Also, the structure of 2·(DMF)2 with two ortho CH2SeCN groups bonding to the carbonyl oxygen atom of the DMF (Fig. 12b) demonstrates that such ortho-substituted derivatives can adapt their geometry to act as chelating systems. Such ditopic or tritopic chelate structures have been recently considered in halogen bonded systems,31 either for anion recognition purposes32 or for catalytic applications.33 In these reported examples however, complex structural motifs have to be elaborated to orient two (or three) iodine atoms in a convergent interaction. The example of this selenocyanate derivative 2 in its DMF solvate demonstrates that such a goal can most probably be reached in much simpler molecules than these complex poly-iodinated ones. Work is also underway to test these assumptions.

Experimental section

Synthesis

1,3,5-Tris(bromomethyl)benzene, 1,3,5-tris(bromomethyl)-2,4,6-trimethylbenzene, 1,2,4,5-tetrakis(bromomethyl)benzene were obtained from Aldrich or Across and used as received.
Synthesis of 1a. A solution of potassium selenocyanate (0.32 g, 2.24 mmol, 4 equiv.) in acetone (5 mL) is added dropwise over a period of 10 minutes on a solution of 1,3,5-tris(bromomethyl)benzene (0.2 g, 0.56 mmol, 1 equiv.) in acetone (5 mL). The solution gets cloudy and a solid appear. The reaction is monitored by TLC (eluent petroleum ether/ether 1/2). After completion of the reaction (typically 25 minutes), the mixture was filtered. The filtrate was evaporated under reduced pressure. The white solid was washed twice with 15 mL of warm water (40 °C) in ultrasonic bath during 5 minutes. The solid was filtered and dried overnight at 80 °C. M.p. 168 °C, yield 45%. 1H NMR (300 MHz, d6-acetone) δ 7.51 (s, 3H), 4.48 (s, 6H). 13C NMR (300 MHz, d6-acetone): 138.9 (C[double bond, length as m-dash]C), 129.3 ([double bond, length as m-dash]CH), 102.1 (CN), 31.6 (CH2). 77Se (d6-DMSO, 25 °C): 314.26 ppm (3 Se, s). Elem. anal. calculated for C12H9N3Se3: C, 33.36; H, 2.10; N, 9.72%. Found: C, 33.97; H, 2.46; N, 9.31%.
Synthesis of 1b. Synthesis of 1b. Procedure adapted from Lari et al.34 1,3,5-Tris(bromomethyl)-2,4,6-trimethylbenzene (0.098 g, 0.25 mmol) was added to a 50 mL round bottom flask, dissolved in 20 mL of acetone, and purged with Ar for 10 minutes. Potassium selenocyanate (0.209 g, 1.503 mmol) was dissolved in acetone (7 mL). KSeCN solution was added dropwise and stirred for 1 hour under inert atmosphere. The reaction was filtered and condensed under vacuum to leave a crude off-white, beige powder. The crude powder was dissolved in the minimal amount of DMF, precipitated out with H2O and filtered to give a white powder that was recrystallized by vapor diffusion of ether into a solution of 1b in ethyl acetate, leaving clear, colorless crystals (0.094 g, 81%). Using DMF instead of AcOEt afforded the DMF solvate. 1H (300 MHz, d6-acetone 25 °C): 8.6234–8.6114 (6H, S), 7.5059–74938 (9H, s). 13C (300 MHz, d6-acetone, 25 °C): 138.8764, 132.7166, 102.1961, 29.7677, 17.0772. 77Se (d6-DMSO, 25 °C): 250.45 (3 Se, s). Elem. anal. calcd for C15H15N3Se3: C, 37.99; H, 3.19; N, 8.86%. Found: C, 37.82; H, 3.34; N, 8.75%.
Synthesis of 2. A solution of potassium selenocyanate (0.38 g, 2.6 mmol, 6 equiv.) in DMF (5 mL) is added dropwise over a period of 10 min in a solution of 1,2,4,5-tetrakis(bromomethyl)benzene (0.2 g, 0.4 mmol, 1 equiv.). An orange colour appears quickly and the solution gets cloudy. The reaction is monitored by TLC (eluent petroleum ether/ether 1/2). After completion of the reaction (typically 25 minutes), addition of 15 mL of warm water (40 °C) precipitated a solid. The solid was washed twice with 15 mL of warm water (40 °C) in ultrasonic bath during 5 minutes. The solid was filtered and dried overnight at 80 °C. White solid, dec T > 160 °C (yield 94%). Recrystallization was performed from PhCN by vapor diffusion of Et2O. When DMF is used rather than PhCN, a DMF solvate is obtained instead formulated as 2·(DMF)2. 1H NMR (300 MHz, DMSO-d6) δ 7.38 (s, 2H), 4.45 (s, 8H). 13C NMR (300 MHz, DMSO-d6): 136.8 (C[double bond, length as m-dash]C), 133.8 ([double bond, length as m-dash]CH), 104.9 (CN), 29.6 (CH2). 77Se NMR (d6-DMSO, 25 °C): 311.21 (4Se, s). Elem. anal. calcd for C15H15N3Se3: C, 30.57; H, 1.83; N, 10.19%. Found: C, 31.31; H, 2.19; N, 9.82%.

Crystallography

Data were collected on an APEXII, Bruker-AXS diffractometer at room temperature for 1a, 1b, 2, 2·DMF, and on D8 VENTURE Bruker AXS diffractometer at 150 K for 1b·DMF and 1b·EtOAc. Both diffractometers operate with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The structures were solved by direct methods using the SIR92 program,35 and then refined with full-matrix least-square methods based on F2 (SHELXL-2014/7)36 with the aid of the WINGX program.37 All non-hydrogen atoms were refined with anisotropic atomic displacement parameters. H atoms were finally included in their calculated positions. Crystallographic data on X-ray data collection and structure refinements are given in Table 4. CCDC 1820888–1820893 contains the supplementary crystallographic data for this paper.
Table 4 Crystallographic data
1a 1b 1b·DMF 1b·EtOAc 2 2·(DMF)2
CCDC 1820888 1820891 1820889 1820890 1820892 1820893
Formula C12H9N3Se3 C15H15N3Se3 C18H22N4OSe3 C19H23N3O2Se3 C14H10N4Se4 C20H24N6O2Se4
Formula moiety C12H9N3Se3 C15H15N3Se3 C15H15N3Se3, C3H7NO C15H15N3Se3, C4H8O2 C14H10N4Se4 C14H10N4Se4, 2(C3H7NO)
FW (g mol−1) 432.10 474.18 547.27 562.28 550.10 696.29
System Monoclinic Monoclinic Triclinic Monoclinic Monoclinic Triclinic
Space group P21/n C2/c P[1 with combining macron] C2/c P21/c P[1 with combining macron]
a (Å) 5.9630(2) 18.8209(9) 5.9559(5) 18.5055(17) 5.4173(12) 9.224(5)
b (Å) 23.1856(8) 10.1135(5) 10.0181(8) 10.3314(9) 13.172(3) 9.382(5)
c (Å) 10.0548(4) 17.9834(7) 17.6631(13) 23.327(2) 11.795(2) 9.591(5)
α (deg) 90.00 90.00 79.153(3) 90.00 90.00 108.086(15)
β (deg) 93.046(2) 99.039(2) 82.977(3) 107.551(4) 97.451(14) 116.283(14)
γ (deg) 90.00 90.00 84.524(3) 90.00 90.00 102.687(15)
V3) 1388.17(9) 3380.5(3) 1024.47(14) 4252.2(7) 834.5(3) 642.3(6)
T (K) 296(2) 296(2) 150(2) 150(2) 296(2) 296(2)
Z 4 8 2 8 2 1
D calc (g cm−1) 2.068 1.863 1.774 1.757 2.189 1.800
μ (mm−1) 7.933 6.525 5.400 5.209 8.792 5.74
Total refls 9778 15377 31696 48703 6327 14597
θ max (°) 27.491 27.491 27.505 27.553 27.638 27.506
Abs corr Multi-scan Multi-scan Multi-scan Multi-scan Multi-scan Multi-scan
T min, Tmax 0.627, 0.924 0.243, 0.593 0.495, 0.994 0.575, 0.901 0.810, 0.916 0.184, 0.532
Uniq. refls 3183 3860 4678 4906 1923 2942
R int 0.0346 0.0464 0.0517 0.0604 0.0655 0.0519
Uniq. refls (I > 2σ(I)) 2299 2312 4335 3914 1281 2248
R 1 0.0513 0.0381 0.0316 0.0342 0.0439 0.040
wR2 (all data) 0.1066 0.0991 0.0706 0.0816 0.1185 0.0906
GOF 1.097 0.891 1.198 1.06 0.914 1.118
Res. dens. (e Å−3) 0.765, −0.727 0.505, −0.636 0.865, −1.3 0.535, −1.471 0.745, −0.632 0.891, −0.941


Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial supports from (i) ANR (Paris, France) through contract ANR-17-CE07-0025-02, (ii) Rennes Métropole (Decision A17.612) and (iii) the Chateaubriand Fellowship of the Office for Science & Technology of the Embassy of France in the United States are acknowledged. We also thank CDIFX (Rennes) for access to X-ray diffraction facilities and C. Orione (Scanmat Rennes) for the 77Se NMR experiments.

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Footnote

CCDC 1820888–1820893. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c8nj00554k

This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018