Huu-Tri
Huynh
a,
Olivier
Jeannin
a,
Emmanuel
Aubert
b,
Enrique
Espinosa
b and
Marc
Fourmigué
*a
aUniv Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) – UMR 6226, F-35000 Rennes, France. E-mail: marc.fourmigue@univ-rennes1.fr
bUniversité de Lorraine, CNRS, CRM2, F-54000 Nancy, France
First published on 24th November 2020
Introduction of methyl substituents on the achiral 1,2-bis(selenocyanatomethyl)benzene leads to a novel chelating ChB donor, namely 1,2-bis(1-selenocyanatoethyl)benzene (1), as a mixture of three diastereomers, the two anti enantiomers and the syn (meso) form. Structure determinations show the recurrent formation of short Se⋯NC ChB interactions in both the anti (racemic mixture) and syn isomers. Co-crystallization of anti-1 with 4,4′-bipyridine affords a 2
:
1 adduct, (anti-1)2(bipy), with one very short Se⋯NPy ChB (RR = 0.87). Co-crystallization of anti-1 with tetraphenylphosphonium halides (Cl−, Br−, I−) provides 1
:
1 adducts while a 2
:
1 adduct is isolated between syn-1 and Et4NCl, formulated as Et4N+[(syn-1)2Cl−]. Comparison of chloride chelation with anti-1 and syn-1 shows much shorter (NC)Se⋯Cl− ChB interactions with the syn isomer, tentatively rationalized on the basis of theoretical calculations of (i) the electrostatic surface potential of neutral ChB donors and (ii) the chloride BSSE complexation energy.
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Scheme 1 (a) Structures of reported ChB donors chelating halides. (b) Systems with strongly asymmetric selenium activation (c) Organic selenocyanates: Sigma-holes and solid-state association. |
Along these lines, one attempt to restore in ChB systems the strong directionality offered by halogens consists in functionalizing the chalcogen atom with only one strong electron-withdrawing substituent, as for example in rotaxanes incorporating selenomethyltriazolium moieties,19 in selenomethyl- or telluromethyl-acetylenes,20 in icosahedral ortho-carboranes substituted with two methylseleno or methyltelluro groups (Scheme 1b),21 or in selenocyanate derivatives R–SeCN (Scheme 1c).22,23 In such compounds, the electron-withdrawing character of the nitrile substituents strongly activates one of the two σ-holes, in the prolongation of the NC–Se bond, allowing to recover the predictability of interaction with Lewis bases. Indeed, selenocyanates themselves most often crystalize into chains ⋯NC–(R)Se⋯NC–(R)Se⋯ where the lone pair of the nitrogen atom of the nitrile interacts through ChB with the selenium atom showing Se⋯N distances around 3.0 Å, that is a reduction ratio RR (defined as the actual interatomic distance over the sum of the van der Waals radii) around 0.86. Recently, we reported several bis- or tetrakis-substituted selenocyanate derivatives such as A–D in Scheme 2a,23 easily prepared from the corresponding benzylic bromides and KSeCN, which were shown to organize in the solid state with these recurrent chain-like motifs. These ChB interactions are notably enhanced when the selenocyanate is faced with stronger Lewis bases such as pyridines24 or halide anions.25 A remarkable feature of the ortho derivative A (or the 1,2,4,5-tetrakis derivative D) is its ability to chelate one single atom through the two selenium atoms, giving rise to seven-membered rings with either neutral (DMF) or anionic (halides) Lewis bases (Scheme 2b). These interactions were also confirmed in solution by 13C and 77Se NMR.26 With the ChB donors A and D, two specific conformations of the ortho-selenocyanatomethyl arms were observed on the solid state upon halide chelation, namely syn or anti, depending on the halide anion, its coordination number and the associated counter ion, without possibility to evaluate the relative stability of both conformations. In order to clarify this point and possibly favor one over the other conformation in these halide recognition processes, we designed an ortho bis(selenocyanate) derivative 1 analogous to A, but bearing an extra methyl group on each benzylic bridge (Scheme 2c). Compound 1 does thus exist as three diastereomers, the two anti enantiomers and the syn (meso) form. We describe here its synthesis, separation of the anti and syn forms and their association with neutral (4,4′-bipyridine) and charged (halide anion) Lewis bases. This novel ChB donor provides also a rare example of introduction of chirality in chalcogen-bonded systems.27 Indeed, only a few examples of selenylation reagents were reported where an intramolecular Se⋯N or Se⋯O interaction rigidifies the chiral reagent and thus allows for stereochemical control,6c,28 while recently a planar chiral ferrocenyl plateform was functionalized with iodomethyl- and selenomethyl-ethynyl moieties29 for evaluation in the Ritter reaction.30
We have performed the bromination reaction of 1,2-diethylbenzene in the same conditions and isolated indeed a 70:
30 anti-syn mixture. Recrystallization from hexane afforded the pure anti isomer whose stereochemistry was confirmed here by single crystal X-ray diffraction. Concentration of the mother liquors gives a 23
:
77 anti-syn mixture based on NMR. The dibromo derivative anti-2 crystallizes in the monoclinic system, S. G. P21/n, with one molecule in general position (Fig. 1a). No short intermolecular Br⋯Br interactions are identified. Nucleophilic substitution with KSeCN was not stereochemically conservative and afforded a mixture of anti- and syn-1, in 3
:
1 ratio when performed from pure anti-2. Recrystallization from acetone allowed to isolate anti-1 in a pure form. It crystallizes in the monoclinic system, S. G. P21/a, with one molecule in general position (Fig. 1b) showing one of the SeCN moieties disordered over two positions with 60
:
40 refined occupancy. From the concentrated mother liquors, some crystals of syn-1 were isolated with difficulties after hexane diffusion. The syn isomer was found to crystallize in the monoclinic system, S. G. P21/c, with one molecule in general position (Fig. 1c).
The solid-state organization of anti-1 exhibits characteristic ChB interactions found in organic selenocyanates. As shown in Fig. 2, selenium atom Se1 acts as a twofold ChB donor, through its two σ-holes, with one very short interaction (RR = 0.86) in the prolongation of the NC–Se(1) bond, toward N2A(−x, −y, −z), and one longer contact (RR = 0.93) in the prolongation of the CH2–Se(1) bond toward N1(−0.5 + x, 0.5 − y, z). On the other hand, selenium atom Se2 in not engaged at all in such a Se⋯N ChB but makes an inversion-centered Se(2A)⋯Se(2A) motif (3.391(6) Å, RR = 0.89) analogous to the so-called type I halogen-bonded motifs found in halogenated molecules. This complex behavior contrasts with that found for the less-substituted achiral derivatives such as the bis(selenocyanato) o-, m-, p-xylylenes where ⋯NC–Se(R)⋯⋯NC–Se(R)⋯ chains are systematically observed. It probably comes as a consequence of the steric constraint brought by the two extra methyl substituents in anti-1. The situation is different in syn-1. As shown in Fig. 3, each of the two selenium atoms acts here as ChB donor toward the nitrogen atom of the selenocyanate of neighboring molecules, giving the recurrent chains mentioned above, with however larger intermolecular distances (RR = 0.89, 0.95) and poorer linearity (the NC–Se⋯N angles here are only 162 and 127°).
Co-crystal formation with anti-1 was investigated with both neutral Lewis bases such as 4,4′-bipyridine and anionic halide salts such as Ph4PX (X = Cl−, Br−, I−). With 4,4′-bipyridine, co-crystallization afforded a chalcogen-bonded structure of 2:
1 stoichiometry formulated as (anti-1)2(bipy). It crystallizes in the monoclinic system, S. G. P21/a, with the 4,4′-bipyridine lying on an inversion center while linking the inversion-related enantiomers of anti-1 (Fig. 4). Both selenium atoms act here as ChB donors toward the pyridinic nitrogen atom, with one very short ChB, for Se(1)⋯N(3) (RR = 0.87) whereas the Se(2)⋯N(3) interaction is essentially a van der Waals contact distance, with a marked directionality for both interactions as the NC–Se⋯N(3) angles amount to 173.62(13) and 168.66(11)° respectively. Besides, the Se2 selenium atom acts as a ChB donor in a Se(2)⋯N(2) contact through its second weaker σ-hole located in the prolongation of the CH2–Se bond, while Se(1) forms an inversion-centered Type I Se⋯Se motif. Note that the co-crystals formed from 4,4’-bipyridine and the unsubstituted meta- or para- bis(selenocyanato)xylylenes are characterized with notably stronger Se⋯N ChB interactions, with RR values in the range 0.82–0.84.24a
Co-crystals of anti-1 with the three tetraphenyl phosphonium halides, i.e. Ph4PCl, Ph4PBr and Ph4PI, all crystallize in a 1:
1 stoichiometry with the halide in a μ2 environment, chelated by the ditopic ChB donor. The chloride and bromide salts are isostructural, and crystallize with Et2O solvate in the triclinic system, space group P1, with the anionic complex in general position. As shown in Fig. 5a and b, the environment of the Cl− and Br− anions is asymmetric with one (NC)–Se⋯X− ChB slightly shorter than the other. In both cases, the overall Se⋯X− distance corresponds to a reduction ratio (considering the ionic radius of the halide rather than the van der Waals radius of the neutral atom, i.e. Cl−: 1.81 Å, Br−: 1.96 Å, I−: 2.20 Å) in the range 0.88–0.90. These values are notably larger than those reported earlier in the unsubstituted achiral analogs chelating Cl− or Br− anions, which exhibit RR values as small as 0.84.25
The situation is more complex in the iodide adduct (Fig. 5c). It crystallizes without solvent in the orthorhombic system, space group P212121, with the anionic complex in general position. The Se⋯I− distances are here associated with RR values of 0.88–0.89 and one (N)C–Se⋯I− angle deviates notably from 180° (167°).
These three structures and the relatively long Se⋯X− distances seem to indicate that the introduction of the methyl substituents on the benzylic positions induces, in the anti-isomer at least, an unfavorable effect on the ChB donor ability of this chelating system. At first sight, this effect might have two origins, (i) the electron-donating effect of the methyl groups might decrease the overall ChB donor ability of the selenium atoms, and/or (ii) the steric constraints brought by the methyl groups do not favor the optimal “coordination” of the halide anion, at least in the anti conformation of the chelate.
One first element of answer can be already found in the crystal structure obtained from the association of Et4NCl and the syn isomer, using a syn-enriched sample. A 2:
1 association formulated as (Et4N+)[(syn-1)2Cl−] is indeed isolated, which crystallizes in the monoclinic system, space group P21/a, with the Cl− anion lying on an inversion center in a μ4 square-planar environment (Fig. 6). The Se⋯Cl− distances are very short (3.11–3.16 Å, RR = 0.84-0.85), even shorter than those reported in the chloride adduct of the achiral chelating ortho-bis(selenocyanate)xylylene A (3.17–3.20 Å, RR = 0.85-0.86) or the 1,2,4,5-tetrakis(selenocyanoatomethyl)benzene E (3.16–3.22 Å, RR = 0.85-0.87).25
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Fig. 6 Detail of the anionic moiety [(syn-1)2Cl−] in its Et4N+ salt. Relevant bond distances and angles: Se(1)⋯Cl: 3.108(3) Å, C–Se(1)⋯Cl: 176.2(2)°; Se(2)⋯Cl: 3.160(1) Å, C–Se(2)⋯Cl: 176.0(3)°. |
In order to rationalize these differences, theoretical calculations of the relative energies of the molecules (anti vs. syn) and their Cl− adducts were performed. The total energy was calculated for the syn and anti forms of 1 [B3LYP 6-311++G(d,p)] after geometry optimization, keeping the C2 geometry for the anti form, and the Cs geometry for the syn form (Fig. 7). Under these conditions, the anti form is more stable by 2.31 kcal mol−1 (9.7 kJ mol−1) than the syn form. Without symmetry constraints, the syn form is able to find a more stable conformation through an intramolecular Se⋯Se ChB interaction (3.958 Å), reducing the difference to 0.68 kcal mol−1 (2.8 kJ mol−1).
The evolution of the σ-hole amplitude has been determined for three molecules, bearing either a methyl group on each benzylic carbon (that is in anti-1 and syn-1) or a hydrogen atom (that is the unsubstituted molecule A in Scheme 1) or, for comparison purposes, a fluorine atom (instead of a methyl group) in the fluorine-substituted derivatives of A (that is anti-3 and syn-3). As shown in Fig. 8a, we note a very small difference between anti-1 and syn-1, with Vs,max values of 46.26 and 47.05 kcal mol−1 respectively, i.e. a slightly larger Vs,max value for syn-1. Such a small difference cannot explain the much shorter ChB interaction experienced with the Cl− anion with syn-1 (see above). The evolution in the series of substituents Me2/H2/F2 follows the expected trend, with a strengthening of the Vs,max in the order F2 > H2 > Me2 associated with the electron withdrawing effect of the fluorine atoms. Furthermore, an even stronger electropositive area (54.3 kcal mol−1) is identified in the fluorine substituted syn-3 in-between the two activated benzylic hydrogen atoms located in α position with respect to the fluorine atoms. These calculations demonstrate that, whatever the substituents, the syn form exhibits systematically a slightly stronger σ-hole than the anti form. However, they are unable to explain the much shorter ChB interaction with Cl−.
Calculations performed on 1:
1 adducts with the syn and anti forms of 1 and a chloride anion give geometries very close to those observed in the crystal structures of Ph4P+[(anti-1)Cl−]·(Et2O)0.5 and Et4N+[(syn-1)2Cl−] (Fig. 9). The notably shorter Se⋯Cl− distances with syn-1 (3.11-3.16 Å, vs. 3.26-3.33 Å with anti-1) is very well reproduced by the calculations, which give a Se⋯Cl− distance of 3.139 Å in the syn-1·Cl− adduct (Cs geometry) that is notably smaller than the 3.210 Å distance calculated in anti-1·Cl− adduct (C2 geometry). The overall energy is very similar in both adducts ΔG(syn-1·Cl− − anti-1·Cl−) = 1.29 kcal mol−1, being slightly more stable with anti- adducr 1. On the other hand, the BSSE complexation energy (−35.36 kcal mol−1 for anti-1·Cl−, −35.55 kcal mol−1 for syn-1·Cl−) gives a small advantage to the syn adduct. It appears therefore that the shorter Se⋯Cl− ChB interaction experienced with the syn-1 ChB donor is not associated with a sizeable stabilization of this chloride adduct, when compared with its anti analog. As shown in Fig. 8, the electropositive area (in blue) interacting with Cl− exceeds that delineated with the selenium atoms only and include also the two benzylic hydrogen atoms. The overall interactions in both adducts thus implies also CBz–H⋯Cl− contacts, whose geometrical features (for both experimental and calculated structures) are collected in Table 1. These contacts are quite short (2.79-2.86 Å vs. a van der Waals contact distance of 3.01 Å) even if they deviate notably from linearity (C–H⋯Cl− angles in the range 123–128°). Their contribution to the overall stabilization of the adducts can probably not be omitted.
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Fig. 9 Optimized geometries of the anti-1·Cl− and syn-1·Cl− adducts with indicated symmetry constraints. |
H⋯Cl− dist (Å) | C–H⋯Cl– ang. (°) | ||
---|---|---|---|
(anti-1)Cl− | X-ray | 2.863(6) | 128.1(2) |
2.800(4) | 127.0(2) | ||
Calcd. | 2.522 | 138.1 | |
(syn-1)Cl− | X-ray | 2.794(2) | 125.2(3) |
2.811(2) | 123.0(4) | ||
Calcd. | 2.573 | 134.9 |
anti-2 m.p. 89 °C. 1 H NMR (300 MHz, CDCl3) δ 2.15 (6H, d, J3 = 6.84 Hz); 5.71 (2H, q, J3 = 6.84); 7.34–7.40 (2H, m); 7.58–7.63 (2H, m)
syn-21 H NMR (300 MHz, CDCl3) δ 2.10 (6H, d, J3 =6.90 Hz); 5.61 (2H, q, J3 = 6.90); 7.32-7.38 (2H, m); 7.58-7.63 (2H, m).
Compound | anti-1 | syn-1 | anti-2 | (anti-1)2·bipy |
---|---|---|---|---|
Formula | C12H12N2Se2 | C12H12N2Se2 | C10H12Br2 | C17H16N3Se2 |
FW (g mol−1) | 342.16 | 342.16 | 292.02 | 420.25 |
Crystal system | Monoclinic | Monoclinic | Monoclinic | Monoclinic |
Space group | P21/a | P21/n | P21/n | P21/a |
a (Å) | 7.5686(4) | 8.5502(6) | 8.2203(8) | 7.1221(4) |
b (Å) | 21.7486(11) | 7.0090(4) | 14.6643(16) | 28.6165(13) |
c (Å) | 8.2075(4) | 22.6261(15) | 9.4380(9) | 8.6600(5) |
α (°) | 90.00 | 90.00 | 90.00 | 90.00 |
β (°) | 105.783(2) | 90.121(3) | 110.312(3) | 105.763(2) |
γ (°) | 90.00 | 90.00 | 90.00 | 90.00 |
V (Å3) | 1300.07(11) | 1355.94(15) | 1066.96(19) | 1698.62(16) |
T (K) | 150(2) | 296(2) | 150(2) | 296(2) |
Cryst. dim. (mm3) | 0.16 × 0.12 × 0.05 | 0.23 × 0.17 × 0.02 | 0.12 × 0.09 × 0.02 | 0.26 × 0.21 × 0.18 |
Z | 4 | 4 | 4 | 4 |
D calc (g cm−3) | 1.748 | 1.676 | 1.818 | 1.643 |
μ (mm−1) | 5.663 | 5.429 | 7.544 | 4.353 |
Abs. corr. | Multi-scan | Multi-scan | Multi-scan | Multi-scan |
T min, Tmax | 0.447, 0.753 | 0.344, 0.897 | 0.447, 0.860 | 0.347, 0.457 |
Total refls. | 20013 | 22207 | 19035 | 12513 |
Uniq. refls. (Rint) | 2993 (0.0572) | 3062 (0.0753) | 2439 (0.0571) | 3912 (0.0291) |
Unique refls. | 2631 | 1862 | 1943 | 3041 |
(I > 2s(I))R1 | 0.027 | 0.0495 | 0.0220 | 0.0367 |
wR2 (all data) | 0.066 | 0.1128 | 0.0513 | 0.0717 |
GoF | 1.067 | 1.076 | 1.052 | 1.085 |
Res. dens (e Å−3) | 0.50, −0.65 | 0.41, −0.52 | 0.43, −0.78 | 0.39, −0.64 |
Compound | Ph4PCl(anti-1) ·0.5 Et2O | Ph4PBr(anti-1) ·0.5 Et2O | Ph4PI(anti-1) | Et4NCl(syn-1)2 |
---|---|---|---|---|
CCDC | ||||
Formula | C38H37ClN2O0.50PSe2 | C38H37BrN2O0.50PSe2 | C36H32IN2PSe2 | C16H22Cl0.50N2.50Se2 |
FW (g mol−1) | 754.03 | 798.49 | 808.42 | 425 |
Crystal system | Triclinic | Triclinic | Orthorhombic | Monoclinic |
Space group |
P![]() |
P![]() |
P212121 | P21/c |
a (Å) | 9.2775(11) | 9.3752(5) | 9.2747(2) | 9.0534(5) |
b (Å) | 13.5469(16) | 13.5967(6) | 10.6749(2) | 13.1176(6) |
c (Å) | 14.3091(15) | 14.3500(7) | 34.3002(6) | 16.4261(7) |
α (°) | 82.498(4) | 82.051(2) | 90.00 | 90.00 |
β (°) | 88.218(4) | 87.549(2) | 90.00 | 112.672(2) |
γ (°) | 83.497(4) | 83.173(2) | 90.00 | 90.00 |
V (Å3) | 1771.3(4) | 1798.15(15) | 3395.94(11) | 1800.00(15) |
T (K) | 296(2) | 296(2) | 296(2) | 296(2) |
Cryst. dim. (mm3) | 0.23 × 0.11 × 0.10 | 0.28 × 0.16 × 0.13 | 0.22 × 0.10 × 0.08 | 0.25 × 0.21 × 0.01 |
Z | 2 | 2 | 4 | 4 |
D calc (g·cm−3) | 1.414 | 1.475 | 1.581 | 1.568 |
μ (mm−1) | 2.238 | 3.242 | 3.161 | 4.179 |
Abs. corr. | Multi-scan | Multi-scan | Multi-scan | Multi-scan |
T min, Tmax | 0.744, 0.799 | 0.542, 0.656 | 0.691, 0.777 | 0.366, 0.959 |
Total refls. | 43521 | 29306 | 24731 | 4122 |
Uniq. refls. (Rint) | 8141 (0.0375) | 8249 (0.0298) | 7781 (0.0247) | 4121 |
Unique refls. (I > 2s(I)) | 5829 | 5578 | 6512 | 2069 |
R 1 | 0.0434 | 0.0371 | 0.0327 | 0.0624 |
wR 2 (all data) | 0.1222 | 0.091 | 0.070 | 0.1591 |
GoF | 1.142 | 1.059 | 1.02 | 1.027 |
Res. dens (e Å−3) | 0.652, −0.951 | 0.898, −0.596 | 0.557, −0.391 | 0.568, −0.561 |
Flack param. | — | — | 0.64(1) | — |
Whereas anti-1 optimized under C2 symmetry toward a true energy minimum, syn-1 optimized toward a Cs molecular structure associated with a small imaginary frequency (−20 cm−1). Further optimization of that structure without symmetry constrain led to a true energy minimum characterized by an intramolecular Se⋯Se interaction [d(Se⋯Se) = 3.958 Å, RR = 1.04; α(C–Se⋯Se) = 154.6°)], with the (NC)Se σ-hole of one of the two selenium atoms pointing toward one lone pair of the second chalcogen atom. The Cs constrained syn-1 molecular structure is then reported for comparison purposes, since in that structure the two chalcogen (NC)Se σ-holes are oriented as in the halide adducts. The same situation occurs for syn-A which was calculated under constrained Cs symmetry (imaginary frequency = −12 cm−1; the two σ-holes pointing outward the molecule) and also under C1 symmetry [intramolecular Se⋯Se interaction d(Se⋯Se) = 3.692 Å, RR = 0.97; α(C–Se⋯Se) = 169.4°]. For fluorine-substituted syn-3 molecule, the Cs symmetry constrained structure corresponds also to a saddle point (imaginary frequency = −9 cm−1; the two σ-holes pointing outward the molecule) but the unconstrained C1 symmetry structure does not present the previously observed intramolecular Se⋯Se interaction due to an unfavorable relative NCSe orientations [d(Se⋯Se) = 4.044 Å, RR = 1.06; α(C–Se⋯Se) = 128.4°].
Molecular structures of the (anti-1)Cl− and (syn-1)Cl− were also optimized using the same calculation conditions and true energy minima were obtained. Basis Set Superposition Error corrected structures and complexation energies were obtained through the counterpoise method of Boys & Bernard.39 More details are given in the Supporting Information file (Cartesian coordinates of optimized molecular structures).
Footnote |
† Electronic supplementary information (ESI) available: Computational details. CCDC 2040289–2040296. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0nj05293k |
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