The idea of electron donation from sulfur to selenium is not contradicted by the crystal structure of compound 3.5. A CDS (version 5.30, updated May 2009) search of complexes containing an assigned Se-S single bond resulted in 19
compounds having Se-S bond lengths that range from 2.158(4) Å to 2.291(1) Å.17,
18 When Se and S are in the
peri-positions of a rigid naphthalene ring, an S-Se
bond has a distance of 2.2442(1) Å.19 In 3.5, the Se(1)…S(1) distance is 2.721(2) Å, which is slightly shorter than the non-bonded Se(1)…S(1) distance in precursor 3.2 (3.063(2) Å), but much longer than any of the formal Se-S single bond distances found in the CSD search. This leaves room for the possibility of a positive interaction weaker than a formal single bond.
Based on the crystal structure, there does not appear to be any other reason for the sulfur and selenium to be forced closer toward each other besides a favorable interaction between them. Table 3-3 contains selected bond distances and angles for 3.2 and for the mono-brominated product, 3.5. The largest structural difference in the backbone lies in the inner peri-angle, Se(1)-C(1)-C(10)
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Table 3-3. Selected bond lengths (Å) and angles (°) for 3.5 and 3.2. 3.5 3.2 S(1)…Se(1) 2.721(2) 3.063(2) Se(1) - C(1) 1.955(8) 1.907(9) S(1) - C(9) 1.796(9) 1.813(8) C(1)-Se(1) -C(11) 101.9(3) 98.1(3) Se(1)-C(1)-C(2) 119.2(7) 119.9(6) Se(1) -C(1)-C(10) 119.5(6) 122.3(6) C(9)-S(1)-C(17) 102.7(4) 102.1(4) S(1)-C(9)-C(8) 118.3(7) 113.8(7) S(1)-C(9)-C(10) 119.8(7) 122.3(6) C(2)-C(1)-C(10) 121.2(8) 117.7(8) C(10)-C(9)-C(8) 121.7(8) 123.5(8) C(1)-C(10)-C(9) 126.0(8) 127.8(7) C(4)-C(5)-C(10)-C(1) 0.0(11) 7.4(13) C(6)-C(5)-C(10)-C(9) 0.1(10) 2.9(13) C(4)-C(5)-C(10)-C(9) -179.2(8) -174.5(8) C(6)-C(5)-C(10)-C(1) 179.3(8) -175.2(8)
Mean Plane Deviations
S(1) 0.139(11) -0.320(11)
and the outer peri-angle S(1)-C(9)-C(8). These angles suggest that both the Se(1)
substituent and the S(1) substituent lean in toward each other in 3.5. Another difference lies in the torsion angles, where 3.5 is quite planar compared to 3.2. The planarity of the entire naphthalene-substituent system in 3.5 suggests the possibility of system-wide resonance in the molecule, including all of the carbon atoms, both of the peri-substituents, and the bromine atom bound to the selenium.
The two phenyl rings in 3.5 are not in the plane of the rest of the atoms and therefore not part of such a system.
In hypervalent interactions involving three centers, four electrons are shared between the three centers- two bonding and two non-bonding. Hypervalency is suggested when the central atom in the set bears more electrons than an octet within its valence shell in a Lewis-dot structure (a classic example of this is the Br3- ion also present in 3.5). In order for hypervalency to occur, a linear
orientation between the three centers is required.20 Correspondingly, the structure of 3.5 shows a very linear arrangement of the bromine, selenium, and sulfur atoms (176.33(6)°). In fact, the peri-positions in naphthalene seem to provide a
particularly good environment for three atoms to precisely align in close proximity and form a three center four electron (3c-4e) system, as shown in Figure 3-11. Some possible resonance contributors to 3.5 are shown in Figure 3- 12. Structures A and B are examples of hypervalency; however, due to the long
51
X E
E E
X
Figure 3-11. Examples of linear, weak hypervalent 3c-4e type interactions, X-E…E (left) and X…E-C (right).
Se(1)…S(1) distance, we believe structure C, with no Se-S bond interaction, is a greater contributor than the other two.
Se S Br Se S Br Se S Br A B C
Figure 3-12. Possible resonance structures of 3.5.
3.3.3. Compound 3.6
The third peri-substituted naphthalene, a tellurium analog, is 3.3. Reaction
of Br2 with this compound results in the trans-dibrominated product 3.6.
Compound 3.6 crystallizes in the C2/c space group with R1 = 6.96% (Figure 3-
13). Table 3-4 lists selected bond distances and angles for 3.6 and 3.3.
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It is interesting that even with the addition of two bulky bromine atoms, the structural features of 3.6 and 3.3 are very similar. The torsion angles C(6)- C(5)-C(10)-C(9) and C(6)-C(5)-C(10)-C(1) are slightly more distorted and the S(1) atom lies further from the naphthalene plane in 3.6, but both differences are very small. The sulfur deviation, at least, is likely due to the need to make room for one of the bromine atoms. The Br-Te-Br angle in 3.6 of 176.67(4)° is similar to those in previously reported R2TeBr2 complexes.21-23 The Te(1)-Br(1)
(2.702(11) Å) and Te(1)-Br(2) (2.6688(12) Å) distances are quite similar, with the Br-Te-Br axis turned so that one Br atom sits closer to the sulfur than does the other Br atom. In the solid state, an inversion center between two molecules of 3.6 creates a Te(1)…Br(1)’ distance and Te(1)’…Br(1) distance of 3.69 Å (Figure 3-
Table 3-4. Selected bond lengths (Å) and angles (°) for 3.6, 3.3a, and 3.3b. 3.6 3.3a 3.3b1 S(1)…Te(1) 3.075(2) 3.0684(13) 3.0984(11) Te(1) - C(1) 2.124(10) 2.141(5) 2.100(5) S(1) - C(9) 1.782(11) 1.770(5) 1.771(5) C(1)-Te(1)-C(11) 97.2(3) 95.1(2) 94.7(2) Te(1)-C(1)-C(2) 116.0(7) 117.2(4) 117.2(3) Te(1) -C(1)-C(10) 123.1(7) 122.9(3) 123.2(4) C(9)-S(1)-C(17) 102.6(4) 103.2(2) 101.0(3) S(1)-C(9)-C(8) 116.6(8) 116.4(3) 116.0(4) S(1)-C(9)-C(10) 121.3(7) 122.8(4) 122.9(3) C(2)-C(1)-C(10) 120.9(19) 119.6(4) 119.5(4) C(10)-C(9)-C(8) 121.8(10) 120.7(4) 120.8(5) C(1)-C(10)-C(9) 127.6(9) 126.1(4) 126.1(5) C(4)-C(5)-C(10)-C(1) 4.3(13) 4.3(9) -5.2(9) C(6)-C(5)-C(10)-C(9) 2.3(12) 4.3(9) -4.3(9) C(4)-C(5)-C(10)-C(9) -174.3(8) -174.6(6) 173.8(5) C(6)-C(5)-C(10)-C(1) -179.1(8) -176.9(6) 176.7(6)
Mean Plane Deviations
S(1) 0.250(12) 0.146(7) 0.449(7)
Te(1) -0.401(12) -0.565(7) -0.406(7)
1The analogous numbering scheme.
14). From the molecular structure, it certainly seems that 3.6 is simply the product of the complete oxidative addition of one molecule of elemental bromine to the tellurium atom (e in Scheme 3-3).
Figure 3-14. Packing interactions in 3.6.
3.4. Conclusion
Three peri-substituted naphthalene compounds containing -SPh and -EPh
(where E = S (3.1), Se (3.2), and Te (3.3)) as substituents have been crystallized and their crystal structures reported. Despite the size difference in the chalcogen atom at the peri-position, these three compounds are structurally very similar.
However, when these compounds are reacted with bromine, they form quite different products.
In 3.1, the sulfur atoms act to activate the various aromatic rings for oxidative addition of the bromine at the para- and ortho-positions to form 3.4.
This may be because disubstituted sulfur simply holds its electron density too close to interact with the dihalogen molecule, or perhaps the sulfur atom is too small and the naphthalene and phenyl rings too bulky, to fit one or more additional substituents on the sulfur.
Of the compounds presented in this chapter, the selenium-diiodine compound 3.7, the monobrominated selenium compound 3.5, and the dibrominated tellurium compound 3.6 are relevant to the mechanism of oxidative addition of dihalogen to organochalcogen atoms presented by Detty et al. and
reproduced in Scheme 3-3.8 While compound 3.6 is the product of dihalogen
addition no matter which mechanism in Scheme 3-3 is taken, 3.7 and 3.5 are believed to potentially be trapped intermediates. These structures (and ones from the literature) cast doubt as to whether “side-on” association of a dihalogen to a chalcogen atom occurs and also on formation of the cis-disubstituted product (a or
c in Scheme 3-3). Quite simply, there are not any η2 dihalogen-chalcogen complexes known, and also, no cis disubstituted chalcogens. Compound 3.5, in
particular, is potentially a trapped intermediate in the oxidation pathway, analogous to structure d in Scheme 3-3. This is in part evidenced by its uniqueness in the literature.
Given this evidence, we can suggest that the oxidation of diorganochalcogen compounds proceeds through a pathway such as that shown in Scheme 3-5. In this pathway, there is no need for a cis-dihalogen complex (c in
Scheme 3-3) to form, as it will be less stable than the trans-substituted product.
55 R2E + Br2 f ast R E R Br Br R E+ R Br Br- R E R Br Br