Structural isomerism in (p-XC6H4)SeCl3 and (p-XC6H4)SeBr3 (X = F, Cl) compounds. Co-crystallisation of cis- and trans-dimeric forms of (p-ClC6H 4)SeCl2(μ-Cl)2(p-ClC6H4)SeCl2. A new structural modification for the "phSeBr" reagent, Ph2Se2Br2, containing an elongated Se-Se bond

Article abstract of DOI:10.1039/c2dt31539d

A series of di(para-halophenyl)diselenides, (p-XC₆H ₄)₂Se₂ (X = F, Cl) have been reacted with three equivalents of SO₂Cl₂ or Br₂, leading to the formation of selenium(iv) RSeX₃ compounds. The structures of (p-FC₆H₄)SeX₃ (X = Cl, Br) have been determined, and both exhibit a dimeric RSeX₂(μ-X) ₂RSeX₂ structure consisting of two "saw-horse" (p-FC₆H₄)SeX₃ units linked by two halide bridges, with an overall square pyramidal geometry at selenium. In both structures all the selenium and halogen atoms are planar, with both aryl rings located on the same side of the Se₂X₆ plane (cis-isomer). The structure of (p-ClC₆H₄)SeCl₃ also adopts a planar dimeric structure, however both cis- and trans-dimeric molecules are co-crystallised in the unit cell. In contrast, the structure of (p-ClC ₆H₄)SeBr₃ adopts a folded cis-dimeric structure due to steric constraints. Secondary Se...X interactions to the "vacant" sixth coordination site at selenium are a feature of most of these structures, but are most prominent in the folded structure of (p-ClC ₆H₄)SeBr₃. A re-examination of the PhSeBr/PhSeBr₃ system resulted in the isolation of crystals of a second structural form of "PhSeBr". The structure of Ph ₂Se₂Br₂ consists of two PhSeBr units linked by an elongated Se-Se bond of 2.832(4) A, and longer secondary Se...Br interactions of 3.333(4) A to form a chain structure. Further weak Se...Br and Br...Br interactions are present, which form loosely linked rippled sheets of selenium and bromine atoms, similar to the sheets observed for the tetrameric form, Ph₄Se₄Br₄.

Full text of DOI:10.1039/c2dt31539d

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ARTICLE TYPE
Structural isomerism in (p–XC6H4)SeCl3 and (p–XC6H4)SeBr3 (X = F,
Cl) compounds. Co–crystallisation of cis– and trans– dimeric forms of
(p–ClC6H4)SeCl2(ꢀ–Cl)2(p–ClC6H4)SeCl2. A new structural modification
for the “PhSeBr” reagent, Ph2Se2Br2, containing an elongated Se–Se
bond.‡
5
Nicholas A. Barnes,* Stephen M. Godfrey, Ruth T. A. Ollerenshaw, Rana Z. Khan and Robin G.
Pritchard
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
10 A series of di(para–halophenyl)diselenides, (p–XC6H4)2Se2 (X = F, Cl) have been reacted with three
equivalents of SO2Cl2 or Br2, leading to the formation of selenium(IV) RSeX3 compounds. The structures
of (p–FC6H4)SeX3 (X = Cl, Br) have been determined, and both exhibit a dimeric RSeX2(ꢀ–X)2RSeX2
structure consisting of two “saw–horse” (p–FC6H4)SeX3 units linked by two halide bridges, with an
overall square pyramidal geometry at selenium. In both structures all the selenium and halogen atoms are
15 planar, with both aryl rings located on the same side of the Se2X6 plane (cis– isomer). The structure of (p–
ClC6H4)SeCl3 also adopts a planar dimeric structure, however both cis– and trans–dimeric molecules are
co–crystallised in the unit cell. In contrast, the structure of (p–ClC6H4)SeBr3 adopts a folded cis–dimeric
structure due to steric constraints. Secondary Se⋅⋅⋅X interactions to the “vacant” sixth coordination site at
selenium are a feature of most of these structures, but are most prominent in the folded structure of (p–
20 ClC6H4)SeBr3. A re–examination of the PhSeBr/PhSeBr3 system resulted in the isolation of crystals of a
second structural form of “PhSeBr”. The structure of Ph2Se2Br2 consists of two PhSeBr units linked by
an elongated Se–Se bond of 2.832(4) Å, and longer secondary Se⋅⋅⋅Br interactions of 3.333(4) Å to form a
chain structure. Further weak Se⋅⋅⋅Br and Br⋅⋅⋅Br interactions are present, which form loosely linked
rippled sheets of selenium and bromine atoms, similar to the sheets observed for the tetrameric form,
25 Ph4Se4Br4.
2,6–[C6H3–3,5–{2,6–(iPr)2C6H3}2]2 (Bpq).9 The majority of
Introduction
ArSeI compounds (and many ArSeCl and ArSeBr compounds),
are stabilised by short NꢁꢁꢁSe–I or OꢁꢁꢁSe–I intermolecular
interactions from heteroatoms present on pendant arms on the
50 aryl rings,10 or by hypervalent coordination from non–bonded
halide ions,11 or halogen substituents on the aryl ring.12 In this
way a linear N/O/XꢁꢁꢁSe–X system is set up, which stabilises the
Se–X bond. Recently, the structure of “MeSeI” was reported,13
which exists in the solid–state as a CT compound, Me2Se2I2,
55 where the Se–Se bond is intact, and the I2 molecule interacts with
just one selenium atom. The same authors have also reported the
structure of Me2Se2.2I2 where I2 molecules interact with both of
the selenium atoms.13
The surprising structural diversity of RSeX compounds is
60 illustrated by the “PhSeX” series (X = Cl, Br, I).14ꢀ18 When X =
Cl or Br tetrameric species, Ph4Se4X4, are formed in the solid–
state,14,15 which consist of four PhSeX units linked together into a
square by weak Se–Se bonds [2.993(3) to 3.051(2) Å], as shown
Organoselenium halides have been known for many years and are
versatile electrophilic reagents used in a range of different
organic transformations.1ꢀ3 Organoselenium halides are found in
30 both the +2 (RSeX) and +4 (RSeX3 / R2SeX2 / R3SeX) oxidation
states, and are usually synthesised via the reactions of halogens or
halogenating agents with selenoethers or diselenides.2
The reactions of diselenides (R2Se2) with halogens produce
both RSeX selenium(II) and RSeX3 selenium(IV) species,
35 depending on the ratio and oxidising power of the halogen, e.g.
RSeF compounds are highly unstable with respect to
disproportionation to RSeF3 and R2Se2,4 and reactions of R2Se2
with fluorinating agents such as XeF2 usually result in the
exclusive formation of RSeF3 compounds.4,5 Whilst both RSeX
40 and RSeX3 species can usually be formed when X = Cl and Br,
there are no reports of RSeI3 compounds. Reactions of R2Se2 with
I2 usually leads to products with an “RSeI” stoichiometry,
although examples containing genuine covalent selenium–iodine
bonds are rare.6 The only examples known are with very bulky R
45 groups such as 2,4,6–(tBu)3C6H2,7 2,4,6–(Me)3C6H2,8 and C6H3–
in Fig. 1(a) for Ph4Se4Br4. By contrast, PhSeI is
a
65 centrosymmetric dimer, (Ph2Se2I2)2,16 which is a CT species
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where one selenium atom acts as a donor towards iodine [Se–I:
2.992 Å], whilst the other behaves as a weak acceptor [Se–I:
3.588 Å].
considerably more soluble, and orange–red solids were isolated
either by concentration (or complete removal) of the mother
liquor to 10mL, followed by addition of pentane to precipitate the
There have been fewer structural studies reported for RSeX3 60 product. The compounds obtained were characterised by
5
systems. A small number of RSeCl3 compounds have been
structurally characterised and again show variations. The
structure of PhSeCl3 is polymeric, and consists of individual
“saw–horse” shaped PhSeCl3 units linked into a chain polymer by
bridging chlorines, the overall geometry at selenium being square
elemental analysis, multinuclear NMR spectroscopy, Raman
spectroscopy and single crystal X–ray diffraction.
Structural studies of (p–FC6H4)SeX3 (X = Cl, Br)
10 pyramidal, see Fig. 1(b).19 Dimeric structures with two bridging
chlorine atoms, [RSeCl2( –Cl)2RSeCl2], have been established
65
Crystals of (p–FC6H4)SeCl3 were obtained directly from the
reaction liquor (diethyl ether) on cooling, whilst crystals of (p–
FC6H4)SeBr3 were obtained from a chloroform solution of the
compound layered with pentane. The structures of (p–
70 FC6H4)SeCl3 and (p–FC6H4)SeBr3 are isomorphous (monoclinic,
C2/c), and both exhibit a dimeric structure consisting of two
RSeX3 units of “saw–horse” geometry, linked together via two
ꢀ
for CF3SeCl3,20 1,2–(SeCl3)2C6H4,21 and 4–(Me3Si)2N–3,5–
(iPr)2C6H2SeCl3.22 In the first two examples the R groups are on
the same side of the Se2Cl6 unit (cis isomer), whilst in the latter
15 the bulky substituted aryl rings are on opposite sides (trans
isomer). The only other reported RSeCl3 structure is {o–
(Me2NCH2)C6H4}SeCl3, where the selenium atom also bonds to
the nitrogen atom on the pendant arm, resulting in a square
pyramidal geometry. The monomeric units form a weak dimer
20 linked by long SeꢁꢁꢁCl interactions.23
halide bridges, RSeX2(ꢀ–X)2RSeX2. This type of dimeric
structure differs considerably from the previously reported
75 PhSeX3 (X = Cl, Br) series, where the trichloride has a polymeric
structure, and the tribromide forms a network structure.19 These
findings illustrate that the structure of ArSeX3 (X = Cl, Br)
compounds are dependent on the nature of the aryl group. A
similar diversity in structure has been observed for ArTeCl3 and
80 ArTeBr3 systems, where both PhTeCl3 and PhTeBr3 have chain
polymeric structures,26ꢀ27 but substituted aryl derivatives have
been shown to exhibit other structural motifs. For example, (p–
OPhC6H4)TeCl3 is dimeric with trans–(p–OPhC6H4) groups,28
whilst a number of other ArTeCl3 structures are polymeric but
85 only have a single chloride bridge.29ꢀ30 Other ArTeBr3 structures
usually have dimeric structures, such as (p–EtOC6H4)TeBr3.30
The molecular structures of (p–FC6H4)SeCl3 and (p–
FC6H4)SeBr3 are shown in Figs. 2 and 3 respectively, along with
selected bond lengths and angles. As a result of dimerisation, the
90 selenium atoms in both (p-FC6H4)SeCl3 and (p–FC6H4)SeBr3 are
in a square pyramidal geometry, with two terminal and two
bridging halogen atoms, and the p-FC6H4 rings in the apical
position. The p-FC6H4 rings bound to each of the selenium atoms
are located on the same side of the Se2X6 plane, therefore both
95 structures exhibit a cis–configuration. The essentially co–planar
arrangement of all the selenium and halogen atoms in the both
structures contrasts with the previously reported cis–dimeric
structure of CF3SeCl3, which is a folded dimer where each
selenium atom is co–planar to the four chlorine atoms bound to it,
100 but is not co–planar to the second selenium atom and two
terminal chlorine atoms at the other side of the dimer.20 This
folding is likely to be due to a steric clash between the two bulky
CF3 groups. In the structure of (p-FC6H4)SeCl3, the two p-FC6H4
rings bend slightly towards one another and stack directly over
105 each other with a C(ipso)–SeꢁꢁꢁSe–C(ipso) torsion angle of 0.0°.
The structure of (p-FC6H4)SeBr3 also shows the two aryl rings
bending towards each other, although here the rings are slightly
offset, with a C(ipso)–SeꢁꢁꢁSe–C(ipso) torsion angle of 3.44°.
The only reported structure of an RSeBr3 compound is that of
PhSeBr3, which exhibits a very different structure from PhSeCl3,
see Fig. 1(c).19 The geometry around the selenium atom is again
square pyramidal, but the structure consists of a primary
25 “PhSeBr2” unit, with two additional weaker Se–Br bonds to
bromide ions. These bromide ions (which are in a distorted
tetrahedral environment) each interact with four different
“PhSeBr2” units. To complete the structure each of the two
bromine atoms bound strongly to selenium (which are in a
30 pyramidally distorted square planar environment) interact via
BrꢁꢁꢁBr contacts to four “PhSeBr2” units. The novel network
structure of PhSeBr3 is thus built up by extensive SeꢁꢁꢁBr and
BrꢁꢁꢁBr contacts, and exhibits molecular, ionic and CT aspects.
35 Insert Fig. 1 near here
In view of the paucity of structural data reported for RSeX3
systems, we decided to study the reactions of the para–
substituted diaryl diselenides (p–XC6H4)2Se2 (X = F, Cl) with
40 SO2Cl2 and Br2. The compounds (p–ClC6H4)SeCl3 and (p–
ClC6H4)SeBr3 are known,24 but no structural details have
emerged. The only solid–state investigation has been an IR /
Raman spectroscopic study of (p–ClC6H4)SeBr3 by Clark and Al–
Turahi.25 We now report a detailed structural and spectroscopic
45 study of (p–FC6H4)SeX3 (X = Cl, Br) and (p–ClC6H4)SeX3 (X =
Cl, Br).
Results and discussion
Synthesis of ArSeX3 (X = Cl, Br) compounds
50
The ArSeX3 (X = Cl, Br) compounds were prepared by the
reaction of the diselenides (p–FC6H4)2Se2 and (p–ClC6H4)2Se2
with three molar equivalents of SO2Cl2, or Br2, at ambient
temperature in anhydrous diethyl ether. In the case of the
55 trichlorides the products precipitated as cream coloured solids
after several hours stirring. In contrast, the tribromides were
110 Insert Figs. 2 and 3 near here
In both structures the bridging Se–X bonds are significantly
longer than the terminal Se–X bonds, and there is asymmetry in
2
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radii (for Se & Cl: 3.65 Å, Se & Br: 3.73 Å).42 In each of the two
terminal Se–X bonds. This results in the shorter terminal Se–X 60 structures pairs of dimers stack diagonally down the cell parallel
both the magnitude of the bridging Se–X bonds, and of the
bond being trans to the longer bridging Se–X bond, and vice
versa. This type of trans–effect has long been established for
main group halogen compounds,31,32 and is particularly
pronounced for bridged systems where a lone pair is present, as is
to the c–axis. In (p-FC6H4)SeCl3 these pairs are linked by a weak
non–classical hydrogen bond, Cl(2)⋅⋅⋅H(8) of 2.91 Å , see Fig. 4,
whilst in (p–FC6H4)SeBr3 the most significant contacts between
pairs are weak Br⋅⋅⋅Br contacts of 3.541(2) Å. The extended
5
the case for selenium(IV) halide species. The structures are thus 65 structures are shown in the supplementary data, and feature
closely related to anionic systems such as [Se2X10]2– (X = Cl,
Br).32,33 The overall distances across the four linear X–Se–X
extensive π–stacking of the p–FC6H4 rings, and aggregation of
the fluorine atoms into fluorophillic regions.
10 angles in each of the (p-FC6H4)SeX3 structures are fairly
consistent, 4.83 to 4.97 Å in (p-FC6H4)SeCl3, and 5.17 to 5.27 Å
in (p–FC6H4)SeBr3. Without the trans–effect this would result in
an average Se–Cl bond of 2.41 to 2.48 Å, and an average Se–Br
bond of 2.58 to 2.63 Å. The terminal Se–X bonds (see Figs. 2 and
15 3 and Table 1) are similar to the sum of the covalent radii for the
relevant elements (Se & Cl: 2.15 to 2.22 Å, Se & Br: 2.30 to 2.40
Å),34ꢀ35 with considerably longer bridging Se–X bonds. The
bonding at selenium can be interpreted as containing two
orthogonal three–centered four–electron (3c–4e) X–Se–X
20 fragments crossing at selenium, with a more conventional 2c–2e
bond to the aryl ring. The 3c–4e systems result in X–Se–X angles
which are somewhat deviated from linear, due to the asymmetry
in the Se–X bonds. The trans X–Se–X angles in (p-FC6H4)SeCl3
vary between 172.15(9)° and 175.29(9)°, and those in (p–
25 FC6H4)SeBr3 between 169.33(9)° and 173.09(8)°.
Insert Fig. 4 near here
70
Structural studies of (p–ClC6H4)SeX3 (X = Cl, Br)
Small crystals of (p–ClC6H4)SeCl3 were obtained by warming an
acetonitrile solution of the compound, followed by refrigeration
75 for two weeks. Crystals of (p–ClC6H4)SeBr3 were obtained from
a dichloromethane solution of the compound layered with diethyl
ether. In contrast to the (p-FC6H4)SeX3 (X = Cl, Br) series the
two compounds are not directly isostructural. The structure of (p–
ClC6H4)SeCl3 (monoclinic, C2/c) contains two independent
80 molecules in the asymmetric unit. Both of these molecules are
dimeric but whilst one dimer exhibits a cis–arrangement of the
(p-ClC6H4) rings, the other has a trans– arrangement, with the
aryl rings on opposite sides of the Se2Cl6 plane. Other ArEX3
structures are known where both the cis– and trans–isomers can
85 be crystallised, such as PhTeI3,43 but to the best of our knowledge
this is the first example where both isomers are co–crystallised in
the same unit cell. The molecular structure of (p–ClC6H4)SeCl3 is
shown in Fig. 5, along with selected bond lengths and angles.
Insert Table 1 near here
The structures of (p-FC6H4)SeCl3 and (p–FC6H4)SeBr3 have
30 been compared with the small number of previously reported
RSeCl3 and RSeBr3 structures, and with examples of the related
[Se2X10]2– (X = Cl, Br) anions,33,36ꢀ40 see Table 1. The magnitude
of the terminal and bridging Se–Cl bonds in (p-FC6H4)SeCl3 are
consistent with those observed for other dimeric RSeCl3 species
35 and [Se2Cl10]2– anions, though the latter show a wider spread for
the bridging Se–Cl bonds.
Similarly, the structure of (p–FC6H4)SeBr3 displays terminal
Se–Br bonds comparable in length to those in PhSeBr3 despite the
very different structures. However, the additional secondary
40 Se⋅⋅⋅Br interactions in PhSeBr3 are considerably longer than the
bridging Se–Br bonds in (p–FC6H4)SeBr3, and those in
[Se2Br10]2– anions.
The structures of (p-FC6H4)SeCl3 and (p–FC6H4)SeBr3 both
exhibit a secondary SeꢁꢁꢁX interaction to the vacant sixth
45 coordination site, resulting in a weak pairing of dimers, as shown
for (p-FC6H4)SeCl3 in Fig. 4. This expansion of the coordination
environment to six is a common feature of organo–selenium(IV)
halide structures, such as CF3SeCl3,20 and Me2SeX2 (X = Cl,
Br).41 However, it should be noted that no secondary Se⋅⋅⋅Cl
90 Insert Fig. 5 near here
All the selenium and chlorine atoms are co–planar in both the
cis– and trans–dimers, with the (p–ClC6H4) rings again
occupying the apical position of the square pyramid at each
95 selenium atom. The increase in steric bulk upon going from (p–
FC6H4) to (p–ClC6H4) means that the two rings do not directly
overlap as was the case for (p-FC6H4)SeCl3, and the C(ipso)–
SeꢁꢁꢁSe–C(ipso) torsion angle for the cis–dimer of (p–
ClC6H4)SeCl3 is 5.60°. The same torsion angle for the trans–
100 dimer is 180.0°, due to the presence of a crystallographic
inversion centre which was also previously observed for the
trans–dimeric structure of 4–(Me3Si)2N–3,5–(iPr)2C6H2SeCl3.22
The magnitude of the Se–Cl bonds in the structure can be
compared to (p-FC6H4)SeCl3 and other RSeCl3 structures, see
105 Table 1. The trans effect of the Se–Cl bonds is apparent in both
dimers, but whilst the asymmetry of the terminal Se–Cl bonds is
similar in each of the dimers, the asymmetry of the bridging Se–
Cl bonds is significantly more pronounced in the trans–dimer
than in the cis. The result is that whilst the overall linear Cl–Se–
110 Cl distance is fairly consistent in the cis–dimer (between 4.86 and
4.94 Å), in the trans–dimer there is a wider disparity (4.85 and
5.01 Å), and the trans form is thus more loosely linked into
dimers than the cis. A comparison of the angles around the
selenium atoms in both structures shows that they are close to the
115 expected angles for a square–based pyramidal geometry. The
50 interactions
were
observed
for
4–(Me3Si)2N–3,5–
(iPr)2C6H2SeCl3, (planar dimer with trans aryl groups),
presumably due to the high steric bulk of the aryl groups.22 Both
of the (p-FC6H4)SeX3 structures display two secondary SeꢁꢁꢁX
interactions, Se(1)ꢁꢁꢁCl(4): 3.390(2) Å / Se(2)ꢁꢁꢁCl(3): 3.402(3) Å
55 in (p-FC6H4)SeCl3, and Se(2)ꢁꢁꢁBr(6): 3.506(15) Å / Se(1)ꢁꢁꢁBr(3):
3.549(2) Å in (p–FC6H4)SeBr3. These distances are considerably
longer than the primary Se–X bonds in the dimers, but are still
significantly shorter than the sum of the relevant van der Waals
trans–dimer of (p-ClC6H4)SeCl3 shows
a more distorted
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dimension linked to neighbouring pairs by long Br⋅⋅⋅Br contacts,
Cl angles varying between 167.94(7)° and 172.31(8)°, compared 60 Br(1)⋅⋅⋅Br(5): 3.5329(15) Å and Br(2)⋅⋅⋅Br(6): 3.5536(16) Å. The
geometry at selenium than the cis–dimer, with the linear Cl–Se–
with 172.30(7)° and 176.72(7)° in the cis–dimer.
extended structure again features extensive π–stacking, and is
In the extended structure of (p-ClC6H4)SeCl3 the presence of a
secondary Se⋅⋅⋅Cl contact to extend the geometry at selenium to
six is not consistently observed. The Se(1) atom in the cis–dimer
exhibits an interaction to one of the bridging chlorine atoms of
the trans–dimer, Se(1)ꢁꢁꢁCl(11): 3.3542(19) Å, but no such
secondary Se⋅⋅⋅Cl contacts are observed either at the other
10 selenium atom in the cis–dimer, or the selenium atoms of the
trans–dimer. Both cis– and trans–dimers stack down the
crystallographic b–axis, with extensive π–stacking of the p–
ClC6H4 rings, as shown in Fig. 6. The crystal packing also
features a number of ClꢁꢁꢁCl interactions which act in concert with
15 the Se⋅⋅⋅Cl interactions to link the dimeric units, see
supplementary data.
discussed in more detail in the supplementary information.
5
Raman spectroscopic studies
65
The Raman spectrum of (p–FC6H4)SeCl3 displays three strong
bands between 368 and 303 cmꢀ1 and five weaker intensity bands
between 272 and 168 cmꢀ1, whilst the spectrum of (p–
ClC6H4)SeCl3 displays two strong bands between 357 and 307
70 cmꢀ1 and five weaker bands between 249 and 118 cmꢀ1. The
intense peaks above 300 cmꢀ1 are likely to be due to ν(Se–Cl)
stretching bands, by comparison with the data observed for
PhSeCl3.19 The weaker intensity bands may be due to a
combination of ν(Se–Ph) modes, bridging ν(Se–Cl) bands, or Se–
75 Cl bending modes.
Clark and Al–Turahi have previously reported far IR and
Raman spectroscopic data for a series of ArSeBr3 compounds,
including (p–ClC6H4)SeBr3.25 They concluded that the spectra
were consistent with a distorted octahedral environment with
80 approximate C3v symmetry, where there are three short bonds
(two Se–Br and one Se–C) and three long (Se–Br) bonds, and
therefore interpreted the structure as ionic, consisting of
[ArSeBr2]+ cations and [Br]– anions, and probably linked into a
tetramer, as in the related structure of SeBr4.44 However, this
85 interpretation also fits the ArSeBr3 dimeric structures reported
herein for (p–XC6H4)SeBr3 (X = F, Cl), when considering the
nature of the bonds, as previously discussed. We observe four
strong bands between 260 and 187 cmꢀ1 for (p–FC6H4)SeBr3 and
three bands between between 271 and 182 cmꢀ1 for (p–
90 ClC6H4)SeBr3 which are assigned as ν(Se–Br) stretching bands.
Insert Fig. 6 near here
20
Crystals of (p–ClC6H4)SeBr3 were solved in the monoclinic
space group P21/n. The compound has a cis–dimeric structure,
but in contrast to (p–FC6H4)SeBr3 it is a folded dimer rather than
planar. The molecular structure of (p–ClC6H4)SeBr3 is shown in
Fig. 7 along with selected bond lengths and angles.
25
Insert Fig. 7 near here
The dimeric structure of (p–ClC6H4)SeBr3 is considerably
more bent at the bridging bromine atoms than (p–FC6H4)SeBr3,
30 leading to a highly folded dimeric structure. The Se–Br–Se angles
at the bridging bromine atoms are 80.52(4)° and 81.44(4)°,
compared to 94.27(6)° and 96.87(6)° in the planar dimer of (p–
FC6H4)SeBr3. The magnitude of the terminal Se–Br bonds in (p–
ClC6H4)SeBr3 are consistent with those observed for (p–
35 FC6H4)SeBr3, whereas the bridging Se–Br bonds cover a much
wider range, see Table 1. The much greater asymmetry in the
bridging Se–Br bonds compared with those in (p–FC6H4)SeBr3 is
likely to be a consequence of the folded nature of the dimer. This
increase in asymmetry also results in a small increase in
40 distortion of the angles around selenium. The linear Br–Se–Br
angles around the selenium atoms vary between 166.32(5)° and
170.45(6)°, compared to values of 169.33(9)° to 173.09(8)° for
(p–FC6H4)SeBr3.
The extended structure again shows secondary Se⋅⋅⋅Br
45 interactions to the vacant sixth coordination site on both selenium
atoms which results in pairing of dimers. However, in contrast to
(p–FC6H4)SeBr3 both selenium atoms interact with the same
bromine atom of a neighbouring dimer, Se(1)⋅⋅⋅Br(3): 3.4107(14)
Å and Se(2)⋅⋅⋅Br(3): 3.3976(14) Å, as shown in Fig. 8. This
50 change in the “pairing motif” is likely to be due to the folded
nature of the dimer, although it allows closer approach of pairing
dimers, as the secondary interactions are shorter than those of
3.506(15) and 3.549(2) Å observed for the planar dimer (p–
FC6H4)SeBr3.
NMR spectroscopic studies of the ArSeX3 (X = Cl, Br)
compounds
1
95 The H, 13C{1H} and 77Se{1H} NMR spectra of (p–XC6H4)SeCl3
and (p–XC6H4)SeBr3 (X = F, Cl) were recorded in CDCl3
solution, with the anticipated peaks of the substituted aryl rings
being observed in the 1H and 13C{1H} NMR spectra. The
77Se{1H} NMR spectra of the ArSeCl3 compounds display singlet
100 resonances at 618 ppm for (p–FC6H4)SeCl3 and 622 ppm for (p–
ClC6H4)SeCl3. To the best of our knowledge the only reported
77Se{1H} NMR data for an RSeCl3 compound is for 4–
(Me3Si)2N–3,5–(iPr)2C6H2SeCl3, (δSe: 940 ppm).22 The
significantly lower frequencies observed here suggest that the
105 nature of the aryl group has a significant impact of the 77Se
chemical shift. This dependency on the R group has also been
observed for related ArSeCl species, which commonly display δSe
between 900 and 1050 ppm,45ꢀ48 for example δSe: 1045 ppm for
PhSeCl,15 although the shift for the fully fluorinated (C6F5)SeCl
110 is observed at 801 ppm.46 The NMR data is suggestive that the
ArSeCl3 species reported here are monomeric in solution (with
saw–horse geometry at selenium), as only a single peak was
observed for (p–ClC6H4)SeCl3 rather than two, which would be
expected if both cis– and trans–dimeric forms were retained in
115 solution.
55
Insert Fig. 8 near here
In contrast, the 77Se{1H} NMR spectra of the (p–XC6H4)SeBr3
compounds exhibit peaks at 857 ppm (X = F), and 847 ppm (X =
Each of these “paired” dimers then stack down the a–cell
4
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Cl). Whilst there is no reported 77Se{1H} NMR data for RSeBr3 60 formula Ph2E2Br2 (with an intact E–E bond), has a very different
species, the peaks observed here for the (p–XC6H4)SeBr3
compounds are close to the shift reported (δSe: 867) for the
selenium(II) species PhSeBr.14 Wynne and Pearson have
previously reported that PhSeBr3 is approx. 80% dissociated in
solution to PhSeBr and Br2, based on cryoscopic molecular
weight measurements.49 We therefore recorded the 77Se{1H}
NMR spectrum of a sample of PhSeBr3 in CDCl3, which
displayed only one peak at 861 ppm, which is close to the
structure, where bromine has added across only one of the two
tellurium atoms, resulting in one oxidised tellurium atom in a
“saw–horse” geometry, and one unoxidised tellurium atom in a
bent geometry.54
There is considerable variation in the two Se–Br bonds in
Ph2Se2Br2, where the Se(1)–Br(1) bond is 2.449(4) Å, and the
Se(2)–Br(2) bond is 2.352(5) Å. The Se–Br bonds in Ph4Se4Br4
are intermediate between these values, ranging between 2.381(2)
and 2.391(2) Å.14 The elongated Se(1)–Br(1) bond may be
5
65
10 literature value for PhSeBr,14 and therefore agrees with the
observations of Wynne and Pearson. Indeed, the dissociation of 70 considered to be a bridging Se–Br bond, as atom Br(1) weakly
selenium(IV) bromide species such as SeBr4 and Ph2SeBr2 has
been long known,50ꢀ52 with the order of degree of dissociation
being SeBr4 > PhSeBr3 > Ph2Se2Br2.49,50
It therefore appears from the solution–phase NMR
spectroscopic data that both (p–FC6H4)SeBr3 and (p–
ClC6H4)SeBr3 exhibit similar behaviour to PhSeBr3, and both
dissociate in CDCl3 to give “ArSeBr” species and Br2, in contrast
to the (p–XC6H4)SeCl3 (X = F, Cl) compounds which do not
bridges to the Se(2) atom of a neighbouring Ph2Se2Br2 molecule,
see Fig. 10. This Br(1)⋅⋅⋅Se(2) contact of 3.333(4) Å is much
longer than either of the two Se–Br bonds, but is well within the
sum of the van der Waals radii of Se and Br, 3.73 Å.42
15
75
Insert Fig. 10 near here
The structure thus propagates in a chain polymeric fashion,
with atom Se(1) in a T–shaped geometry, and atom Se(2) in a
20 appear to dissociate in solution.
80 “saw–horse” geometry as a result of the additional contact. The
structure further extends by very weak contacts from the
“terminal” Br(2) atom to both atoms Br(1) and Se(1) of a
neighbouring chain, Se(1)ꢁꢁꢁBr(2): 3.691(5) Å, and Br(1)ꢁꢁꢁBr(2):
3.665(4) Å. A sheet of selenium and bromine atoms is therefore
85 formed which is similar to that observed for Ph4Se4Br4,14 where
the extended solid–state structure consists of planar sheets made
up of Se4 and Br4 squares, with the phenyl rings stacking between
the sheets. The sheet of selenium and bromine atoms in
Ph2Se2Br2 is more rippled, but still essentially planar, and is made
90 up of 5–membered Se2Br3 rings, and 7–membered Se3Br4 rings,
see Fig. 11. The phenyl rings then stack between the sheets, see
Fig 12.
Crystal structure of Ph2Se2Br2
In view of the similar solution phase behaviour of the two (p–
25 XC6H4)SeBr3 compounds and PhSeBr3 we decided to investigate
whether a dimeric form of PhSeBr3 could also be obtained by
varying the re–crystallisation solvent. The previously reported
network structure of PhSeBr3 was obtained from cooling of a
warmed diethyl ether solution of the compound.19 We
30 recrystallised a sample of PhSeBr3 (from dichloromethane
layered with diethyl ether), resulting in the formation of a mixture
of dark orange and pinkish purple crystals. An examination of the
unit cell of the orange crystals showed that they were the
previously reported PhSeBr3. However, the purple crystals were
35 crystallographically characterised and shown to be a selenium(II)
species, Ph2Se2Br2, which consists of two “PhSeBr” units linked
together by a Se–Se bond. This structure contrasts with the
established structure of the commercially available “PhSeBr”
reagent which forms deep purple crystals of the tetrameric
40 Ph4Se4Br4 compound, which consists of four “PhSeBr” units
linked into a square motif by weak Se⋅⋅⋅Se interactions.14 The
structure of Ph2Se2Br2 is shown in Fig. 9, along with selected
bond lengths and angles.
Insert Fig. 11 and 12 near here
95 Conclusions
The series of (p–XC6H4)SeCl3 and (p–XC6H4)SeBr3 compounds
(X = F, Cl) exhibit doubly bridged ArSeX2(ꢀ–X)2ArSeX2 dimeric
structures, in contrast to PhSeCl3 (chain polymer) and PhSeBr3
100 (network structure). (p–FC6H4)SeCl3 and (p–FC6H4)SeBr3 are
planar dimers with the aryl rings on the same side of the plane
(cis–dimers), whilst (p–ClC6H4)SeCl3 co–crystallises as a mixture
of cis– and trans–planar dimers, and (p–ClC6H4)SeBr3 adopts a
folded cis–dimeric structure. These dimers feature the selenium
105 atom in a square pyramidal geometry with an additional weak
Se⋅⋅⋅X interaction usually observed at the vacant sixth
coordination site. The exception is in the structure of (p–
ClC6H4)SeCl3 where only one of the selenium atoms in the cis–
45 Insert Fig. 9 near here
Ph2Se2Br2 crystallises in the monoclinic space group P21 and
contains an elongated Se–Se bond of 2.832(4) Å which links two
PhSeBr fragments. This Se–Se bond is considerably longer than
50 that of diphenyl diselenide, 2.29(1) Å,53 but significantly shorter
than the very weak Se–Se bonds in the tetrameric form
(Ph4Se4Br4), where the distances vary between 3.004(2) and
3.051(2) Å.14 The structure of Ph2Se2Br2 is reminiscent of half of
the structure of (Ph2Se2I2)2, which is a CT dimer.16 The Se–Se
55 bond in (Ph2Se2I2)2 is much shorter (2.347 Å) than the Se–Se
bond in Ph2Se2Br2, which reflects the nature of (Ph2Se2I2)2 where
the Ph2Se2 group is much less perturbed and dimerises via weaker
CT interactions.16 The structure of Ph2Se2Br2 contrasts with that
of “PhTeBr” which although also exhibiting a structure of
dimer has
a secondary Se⋅⋅⋅Cl interaction, with no such
110 interactions observed in the trans–dimer.
An NMR spectroscopic study of the ArSeX3 species suggests
that in solution the ArSeCl3 compounds exist as monomers,
whilst the ArSeBr3 compounds extensively dissociate to ArSeBr
and Br2.
115
A
second structural isomer of “PhSeBr” has been
crystallographically characterised. In contrast to the previously
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reported “Ph4Se4Br4” form, Ph2Se2Br2 exists as a dimer with an
elongated Se–Se bond of 2.832(4) Å. This form of “PhSeBr” was
ellipsoid plots were generated using ORTEP–3 for Windows,58
and other pictures generated using the MERCURY program.59
Crystallographic data has been deposited at the CCDC, reference
numbers XXX–XXX. These data can be obtained free of charge
isolated as crystals from
a solution of PhSeBr3, where
dissociation to PhSeBr and Br2 had occurred. The extended
5
structure forms a chain polymer via linking of Ph2Se2Br2 60 from The Cambridge Crystallographic Data Centre via
molecules by Se⋅⋅⋅Br interactions, and then further into sheets of
selenium and bromine atoms by weaker Se⋅⋅⋅Br and Br⋅⋅⋅Br
interactions. The isolation of a second structural form of
“PhSeBr” may be a consequence of this form being obtained via
www.ccdc.cam.ac.uk/data_request/cif.
Insert Table 2 near here
10 dissociation of PhSeBr3 in solution.
65 Synthetic details
The structures reported herein further illustrate the wide
structural diversity of ArSeX and ArSeX3 species, where small
changes in the nature of either the aryl group or the halide may
result in the formation of different structural motifs.
15
Synthesis of (p–FC6H4)2Se2: 3.159 g of magnesium turnings
(0.130 moles) were added under a stream of dry nitrogen to a
three necked round bottomed flask containing 250 mL of
70 sodium–dried diethyl ether. 20.0 g (12.55 mL, 0.114 moles) of 1–
bromo–4–fluorobenzene in 100 mL of dry ether was added
dropwise by means of a pressure equalizing dropping funnel. The
rate of addition was controlled so as to maintain a gentle reflux of
the solution (initiation of the Grignard with a drop of 1,2–
75 dibromoethane may be necessary). Once all of the 1–bromo–4–
fluorobenzene was added the solution was left to stir at room
temperature until all of the magnesium turnings had been
consumed (around one hour). After this time 9.79 g (0.124 moles)
of selenium powder was added to the solution in approx. 0.5 g
80 portions over the course of 30 minutes. The olive–green solution
was then stirred at ambient temperature for two hours. The
system was then left stirring overnight with a stream of air
bubbling through the solution. The product was extracted by
shaking with 3 x 50 mL portions of 3.0 M dilute HCl
85 (CAUTION: exothermic), followed by separation of the diethyl
ether phase. The organic phase was dried over magnesium
sulfate, filtered, and the solvent removed in vacuo to yield
analytically pure (p–FC6H4)2Se2 in the form of an orange oil
(16.35 g, 82.2% yield). Characterising data: Calculated for
Experimental
Reagents and physical measurements
20 The compounds synthesised herein are sensitive to moisture,
therefore standard Schlenk techniques were employed
throughout, with all additions being performed in an argon–filled
glove box. Diethyl ether was purchased commercially (BDH) and
freshly distilled from sodium/benzophenone ketyl before use.
25 Deuterated chloroform (Goss Scientific, dried over molecular
sieves) was obtained commercially. (p–FC6H4)2Se2 was prepared
via a modification of the literature method of Pappalardo and co–
workers,55 as outlined below. (p–ClC6H4)2Se2, magnesium
turnings, selenium powder, sulfuryl chloride and bromine were
30 all purchased commercially (Aldrich), whilst 1–bromo–4–
fluorobenzene was purchased from Fluorochem. All reagents
were used as received. Elemental analyses were performed by the
University of Manchester Departmental Microanalytical service.
Raman spectra were collected as solid samples on a Nicolet–
35 Nexus combined FT–IR/FT–Raman spectrometer, using the
OMNIC E.S.P 5.1 software package. Spectra were recorded to
1
90 C12H8F2Se2: C, 41.4; H, 2.3; Found: C, 41.6; H, 2.5%. H NMR
(CDCl3): 6.92–7.08 [m, 4H], 7.44–7.68 [m, 4H]. 13C{1H} NMR
(CDCl3): 116.4 [d, Cm, 2J(CF) = 21.2], 125.7 [s, Ci], 134.8 [d, Co,
1
3J(CF) = 8.2], 163.0 [d, Cp, J(CF) = 248.3]. 19F NMR (CDCl3):
the low frequency cut–off point of 100 cm–1. H, 13C{1H}, and
1
112.8 [s, br]. 77Se{1H} NMR (CDCl3): 491.4 [s].
77Se{1H} NMR spectra were recorded on a Bruker AVANCE III
400 spectrometer operating at 400.1, 100.6 and 76.3 MHz
40 respectively. Peak positions are quoted relative to external TMS
(1H/13C) or Me2Se (77Se), using the high frequency positive
convention throughout.
95
Synthesis of ArSeX3 (X = Cl, Br) adducts
(p–FC6H4)SeCl3: 0.740 g (2.1 x 10–3 moles) of (p–FC6H4)2Se2
was dissolved in 30 mL of anhydrous Et2O. Under a stream of
100 dry nitrogen 0.52 mL (0.861 g / 6.4 x 10–3 moles) of SO2Cl2 was
added dropwise. The yellow solution turns an orange–brown
colour, with gradual precipitation of a yellowish solid over two
hours. The reaction was left to stir for ca. 5 days, after which time
the solid was isolated using standard Schlenk techniques, and
105 dried in vacuo to yield the product as a cream coloured solid in
71.5 % yield. Calculated for C6H4FCl3Se: C, 25.7; H, 1.4; Cl,
38.0; Found: C, 25.9; H, 1.5; Cl, 37.7%. 1H NMR (CDCl3): 6.92–
7.18 [m, 2H], 7.66–7.78 [m, 2H]. 13C{1H} NMR (CDCl3): 117.0
Crystallographic details
45
Details of the structural analyses are summarised in Table 2.
Diffraction data were recorded on a Nonius κ–CCD four–circle
diffractometer using graphite–monochromated Mo–Kα radiation
(λ = 0.71073 Å), at 100(2) K. Structural data were solved by
50 direct methods, with full–matrix least squares refinement on F2
using the SHELX–97 program.56 Absorption corrections were
applied by the multi–scan method using the SORTAV program.57
Non–hydrogen atoms were refined with anisotropic thermal
parameters, whilst all hydrogen atoms were modelled in ideal
55 positions. Flack parameter for Ph2Se2Br2 is 0.18(14). All thermal
2
[d, Cm, J(CF) = 22.2], 133.1 [s, Ci], 138.1 [s, Co], 164.3 [d, Cp,
110 1J(CF) = 253.6]. 77Se{1H} NMR (CDCl3): 618 [s]. Raman: 3072,
1583, 1483, 1294, 1232, 1163, 1039, 1005, 808, 731, 688, 627,
368, 345, 303, 272, 249, 206, 187, 168.
6
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(p–ClC6H4)SeCl3: Was synthesised as (p–FC6H4)SeCl3 above
except using 0.632 g (1.7 x 10ꢀ3 moles) of (p–ClC6H4)2Se2 and
studentship (RTAO), and for providing funding for the IR–
Raman facility (Grant No: GR/M30135), NMR facility (Grant
0.40 mL (0.671 g / 5.0 x 10ꢀ3 moles) of SO2Cl2 in anhydrous 60 No: GR/L52246), and X–ray facility (Research Initiative Grant).
Et2O. The product was formed as a cream coloured solid in 66.2
5
%
yield. Mpt. 175–179°C (172°C lit.)24 Calculated for
References
C6H4Cl4Se: C, 24.2; H, 1.3; Cl, 47.8; Found: C, 24.4; H, 1.3; Cl,
47.6%. 1H NMR (CDCl3): 7.20 [s, 2H, 3J(HH) = 8.4 Hz], 7.54 [s,
2H, 3J(HH) = 8.4 Hz]. 13C{1H} NMR (CDCl3): 124.8 [s, Ci],
129.8 [s, Cm], 135.8 [s, Co], 137.0 [s, Cp]. 77Se{1H} NMR
10 (CDCl3): 622 [s]. Raman: 3114, 3056, 1564, 1180, 1094, 1036,
993, 722, 623, 357, 307, 249, 199m, 174, 137, 118.
School of Chemistry, University of Manchester, Oxford Road,
Manchester, UK M13 9PL. E-mail: nick_barnes28@hotmail.com
† Electronic Supplementary Information (ESI) available: [Details of the
65 crystal packing features for (p–FC6H4)SeX3 (X = Cl, Br) and (p–
ClC6H4)SeX3 (X
=
Cl, Br); Figures S1 to S5]. See
DOI: 10.1039/b000000x/
‡ This paper is dedicated to the memory of Dr Steve Godfrey (1966ꢀ
2011).
(p–FC6H4)SeBr3: 0.585 g (1.7 x 10–3 moles) of (p–FC6H4)2Se2
was dissolved in 30 mL of anhydrous Et2O. Under a stream of
15 argon 0.260 mL (0.809 g / 5.0 x 10–3 moles) of Br2 was added
dropwise. The deep orange–red solution was left to stir for ca. 12
hours, after which time the solvent was removed in vacuo to
leave a sticky orange solid. The residue was washed with 2 x 5
mL of anhydrous pentane and dried for two hours to leave an
20 orange solid (53.6 % yield). Calculated for C6H4FBr3Se: C, 17.4;
70
75
1
2
3
Selenium Reagents and Intermediates in Organic Synthesis,
ed. C. Paulmier, Pergamon Press, Oxford, UK, 1986.
Organoselenium Chemistry, ed. D. Liotta, John Wiley, New
York, 1987.
Organoselenium Chemistry: Modern Developments in
Organic Synthesis, ed. T. Wirth, SpringerꢀVerlag, Berlin,
2000.
H. Poleschner and K. Seppelt, Chem. Eur. J., 2004, 10, 6565.
T. M. Klapötke, B. Krumm and M. Scherr, Inorg. Chem.,
2008, 47, 4712.
See, WꢀW. du Mont, AꢀM. von Salzen, F. Ruthe, E. Seppälä,
G. Mugesh, F. A. Devillanova, V. Lippolis and N. Kuhn, J.
Organomet. Chem., 2001, 623, 14, and references therein.
WꢀW. du Mont, S. Kubiniok, K. Peters and HꢀG. von
Schnering, Angew. Chem. Int. Ed., 1987, 26, 780.
J. Jeske, P. G. Jones, AꢀM. von Salzen and WꢀW. du Mont,
Acta Crystallogr., Sect. E, 2002, 58, o350.
4
5
1
H, 1.0; Br, 58.0; Found: C, 17.2; H, 1.1; Br, 58.2%. H NMR
80
6
(CDCl3): 6.93–7.01 [m, 2H], 7.72–7.77 [m, 2H]. 13C{1H} NMR
(CDCl3): 116.9 [d, Cm, 2J(CF) = 21.2], 124.2 [s, Ci], 138.5 [d, Co,
3J(CF) = 9.2], 164.3 [d, Cp, 1J(CF) = 252.6]. 77Se{1H} NMR
25 (CDCl3): 857 [s]. Raman: 3064, 1583, 1479, 1398, 1298, 1236,
1163, 1063, 1039, 1005, 808, 627, 337, 260, 245, 202, 187.
7
8
9
85
K. Goto, D. Sonoda, K. Simada, S. Sase and T. Kawashima,
Angew. Chem. Int. Ed., 2010, 49, 545.
(p–ClC6H4)SeBr3: Was synthesised as (p–FC6H4)SeBr3 above,
except using 0.635 g (1.7 x 10–3 moles) of (p–ClC6H4)2Se2 and
30 0.256 mL (0.799 g / 5.0 x 10–3 moles) of Br2 in anhydrous Et2O.
The deep orange–red solution was left to stir for ca. 12 hours,
concentrated in volume to ca. 5 mL, and 10 mL of anhydrous
pentane was added. The product precipitated as an orange–red
10 See, A. J. Mukherjee, S. S. Zade, H. B. Singh and R. B. Sunoj,
Chem. Rev., 2010, 110, 4357, and references therein.
11 M. Iwaoka, H. Komatsu, T. Katsuda and S. Tomoda, J.
Organomet. Chem., 2000, 611, 164.
12 M. Iwaoka, H. Komatsu, T. Katsuda and S. Tomoda, J. Am.
Chem. Soc., 2002, 124, 1902; M. Iwaoka, H. Komatsu, T.
Katsuda and S. Tomoda, J. Org. Chem., 2005, 70, 321.
13 B. Mueller, T. T. Takaluoma, R. S. Laitinen and K. Seppelt,
Eur. J. Inorg. Chem., 2011, 4970.
14 N. A. Barnes, S. M. Godfrey, R. T. A. Halton, I. Mushtaq, R.
G. Pritchard and S. Sarwar, Dalton Trans., 2006, 1517.
15 N. A. Barnes, S. M. Godfrey, R. T. A. Halton, I. Mushtaq, S.
Parsons, R. G. Pritchard and M. Sadler, Polyhedron, 2007, 26,
1053.
16 S. Kubiniok, WꢀW. du Mont, S. Pohl and W. Saak, Angew.
Chem. Int. Ed., 1988, 27, 431.
17 N. M. Zaripov, M. V. Popik, L. V. Vilkov and T. G.
Mannafov, Zh. Strukt. Chim., 1980, 21, 151.
18 N. Beatriz Okulik, A. H. Jubert and E. A. Castro, J. Mol.
Model., 2012, 18, 913.
19 N. A. Barnes, S. M. Godfrey, R. T. A. Halton and R. G.
Pritchard, Dalton Trans., 2005, 1759.
20 C. J. Marsden, G. M. Sheldrick and R. Taylor, Acta
Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem., 1977,
33, 139.
21 G. Heckmann and G. Wolmershäuser, Chem. Ber., 1993, 126,
1071.
22 A. Maaninen, R. T. Boeré, T. Chivers and M. Parvez, Z.
Naturforsch., Teil B, 1999, 54, 1170.
23 M. Kulcsar, A. Silvestru, C. Silvestru, J. E. Drake, C. L. B.
Macdonald, M. E. Hursthouse and M. E. Light, J. Organomet.
Chem., 2005, 690, 3217.
90
%
yield). Mpt. 120–123°C (123–124°C lit.)24
95
solid (49.4
35 Calculated for C6H4ClBr3Se: C, 16.7; H, 0.9; Cl, 8.2; Br, 55.7;
1
Found: C, 16.9; H, 0.8; Cl, 7.9; Br, 55.8%. H NMR (CDCl3):
3
3
7.28 [s, 2H, J(HH) = 8.5 Hz], 7.69 [s, 2H, J(HH) = 8.5 Hz].
13C{1H} NMR (CDCl3): 127.4 [s, Ci], 129.8 [s, Cm], 136.5 [s, Co],
137.1 [s, Cp]. 77Se{1H} NMR (CDCl3): 847 [s]. Raman: 3118,
40 3060, 1556, 1182, 1094, 1040, 990, 719, 623, 345, 310, 271, 209,
182, 154, 112.
100
105
110
115
120
Formation of dimeric form of “PhSeBr”: Ph2Se2Br2
45 A sample (ca. 0.20 g) of PhSeBr3 (formed via the method
outlined previously),19 was dissolved in 10 mL of dry
dichloromethane in a rotaflo tube, and 20 mL of dry diethyl ether
was carefully syringed on top to form two separate layers. The
tube was sealed and the layers allowed to slowly mix. A mixture
50 of orange crystals (PhSeBr3) and pink–purple crystals
(Ph2Se2Br2) were formed. A sample of the crystals of Ph2Se2Br2
were removed from the tube and characterised. Calculated for
C12H10Br2Se2: C, 30.5; H, 2.1; Br, 33.9; Found: C, 30.9; H, 2.2;
1
Br, 33.8%. H NMR (CDCl3): 7.32–7.44 [m, 3H], 7.70–7.86 [m,
55 2H].
24 O. Behaghel and H. Siebert, Chem. Ber., 1933, 66, 708; D. G.
Foster, Recl. Trav. Chim. Pays-Bas, 1934, 53, 405.
25 E. R. Clark and M. A. AlꢀTurahi, J. Organomet. Chem., 1977,
124, 391.
Acknowledgements
The authors would like to thank EPSRC for a research
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26 N. W. Alcock and W. D. Harrison, J. Chem. Soc., Dalton
Trans., 1982, 251; F. W. B. Einstein and T. Jones, Acta
Crystallogr., Sect. B, 1982, 38, 617.
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| Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

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Figures, Tables and captions
Br
Se
Se
Se
Se
Br
Br
Cl
Cl
Cl
Cl
Se
Cl
Se
Br
Se
Se
Cl
Cl
(b)
(a)
Br
Br
Br
Br
Br
Se
Br
(c)
Figure 1 (a) Structure of Ph4Se4Br4 showing “square” motif, (b) Part of the polymeric chain structure of PhSeCl3, (c)
Repeat unit of the network structure of PhSeBr3.

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Figure 2 ORTEP representation of the molecular structure of (p-FC6H4)SeCl3. Thermal ellipsoids are shown at the
30% probability level. Selected bond lengths [Å] and angles [°]: Se(1)–Cl(1): 2.556(2), Se(1)–Cl(2): 2.201(3), Se(1)–
Cl(3): 2.277(3), Se(1)–Cl(4): 2.766(2), Se(2)–Cl(1): 2.766(3), Se(2)–Cl(4): 2.639(2), Se(2)–Cl(5): 2.189(3), Se(2)–
Cl(6): 2.231(3), Se(1)–C(1): 1.961(8), Se(2)–C(7): 1.954(9), C(4)–F(4): 1.362(10), C(10)–F(10): 1.356(10), Cl(1)–
Se(1)–Cl(3): 175.29(9), Cl(2)–Se(1)–Cl(4): 172.15(9), Cl(1)–Se(1)–Cl(2): 89.46(9), Cl(2)–Se(1)–Cl(3): 93.56(10),
Cl(3)–Se(1)–Cl(4): 91.85(8), Cl(1)–Se(1)–Cl(4): 84.80(7), Cl(1)–Se(2)–Cl(5): 172.60(9), Cl(4)–Se(2)–Cl(6):
172.41(9), Cl(1)–Se(2)–Cl(4): 83.24(7), Cl(1)–Se(2)–Cl(6): 90.51(9), Cl(5)–Se(2)–Cl(6): 94.31(10), Cl(4)–Se(2)–
Cl(5): 91.51(9), Se(1)–Cl(1)–Se(2): 96.85(7), Se(1)–Cl(4)–Se(2): 94.92(7).

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Figure 3 ORTEP representation of the molecular structure of (p-FC6H4)SeBr3. Thermal ellipsoids are shown at the
30% probability level. Selected bond lengths [Å] and angles [°]: Se(1)–Br(1): 2.401(2), Se(1)–Br(2): 2.374(2), Se(1)–
Br(5): 2.885(2), Se(1)–Br(6): 2.822(2), Se(2)–Br(3): 2.453(2), Se(2)–Br(4): 2.374(2), Se(2)–Br(5): 2.716(2), Se(2)–
Br(6): 2.896(2), Se(1)–C(1): 1.944(15), Se(2)–C(7): 1.956(16), C(4)–F(1): 1.369(19), C(10)–F(2): 1.337(19), Br(1)–
Se(1)–Br(6): 169.33(9), Br(2)–Se(1)–Br(5): 172.95(8), Br(1)–Se(1)–Br(2): 94.32(8), Br(1)–Se(1)–Br(5): 90.42(7),
Br(5)–Se(1)–Br(6): 83.18(6), Br(2)–Se(1)–Br(6): 91.34(7), Br(3)–Se(2)–Br(5): 173.09(8), Br(4)–Se(2)–Br(6):
171.45(9), Br(3)–Se(2)–Br(4): 93.28(8), Br(4)–Se(2)–Br(5): 89.23(7), Br(5)–Se(2)–Br(6): 84.85(6), Br(3)–Se(2)–
Br(6): 91.93(7), Se(1)–Br(5)–Se(2): 96.87(6), Se(1)–Br(6)–Se(2): 94.27(6).

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Figure 4 Pairing of dimers via secondary Se⋅⋅⋅X interactions in the structure of (p-FC6H4)SeCl3, resulting in distorted
octahedral geometry at selenium. These pairs link via C–H⋅⋅⋅Cl non–classical hydrogen bonds, whilst in (p-
FC6H4)SeBr3 the pairs link by weak Br⋅⋅⋅Br contacts. Atom and contacts colour code: Carbon (grey), hydrogen (white),
fluorine (lime green), chlorine (bright green), selenium (pink). Green lines: intramolecular bridging Se–Cl bonds
within dimer, blue dashed lines: weaker Se⋅⋅⋅Cl or H⋅⋅⋅Cl intermolecular contacts.

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Figure 5 ORTEP representation of the molecular structure of (p-ClC6H4)SeCl3. Thermal ellipsoids are shown at the
30% probability level, and hydrogen atoms omitted for clarity. Selected bond lengths [Å] and angles [°]: Se(1)–Cl(1):
2.250(2), Se(1)–Cl(2): 2.205(2), Se(1)–Cl(3): 2.6225(19), Se(1)–Cl(5): 2.7287(19), Se(2)–Cl(3): 2.7310(19), Se(2)–
Cl(5): 2.6201(19), Se(2)–Cl(6): 2.2112(19), Se(2)–Cl(7): 2.240(2), Se(3)–Cl(9): 2.261(2), Se(3)–Cl(10): 2.1949(19),
Se(3)–Cl(11): 2.5899(19), Se(3)–Cl(11i): 2.8184(19), Se(1)–C(1): 1.959(7), Se(2)–C(7): 1.948(7), Se(3)–C(13):
1.951(7), C(4)–Cl(4): 1.737(7), C(10)–Cl(8): 1.753(7), C(16)–Cl(12): 1.736(8), Cl(1)–Se(1)–Cl(3): 172.30(7), Cl(2)–
Se(1)–Cl(5): 176.72(7), Cl(1)–Se(1)–Cl(2): 93.93(7), Cl(1)–Se(1)–Cl(5): 88.50(7), Cl(3)–Se(1)–Cl(5): 85.69(6),
Cl(2)–Se(1)–Cl(3): 91.69(7), Cl(3)–Se(2)–Cl(6): 173.96(7), Cl(5)–Se(2)–Cl(7): 172.76(7), Cl(3)–Se(2)–Cl(5):
85.69(6), Cl(5)–Se(2)–Cl(6): 88.27(7), Cl(6)–Se(2)–Cl(7): 93.32(7), Cl(3)–Se(2)–Cl(7): 92.65(7), Se(1)–Cl(3)–Se(2):
94.25(6), Se(1)–Cl(5)–Se(2): 94.36(6), Cl(9)–Se(3)–Cl(11): 172.31(8), Cl(10)–Se(3)–Cl(11i): 167.94(7), Cl(9)–Se(3)–
Cl(10): 94.48(7), Cl(10)–Se(3)–Cl(11): 90.65(7), Cl(11)–Se(3)–Cl(11i): 80.52(6), Cl(9)–Se(3)–Cl(11i): 93.54(6),
Se(3)–Cl(11)–Se(3i): 99.48(6). Symmetry operation used to generate equivalent atoms: i: ½-x, ½-y, 1-z.

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Figure 6 Unit cell of (p-ClC6H4)SeCl3 viewed down the crystallographic b–axis, showing cis– and trans–dimers
linked by Se⋅⋅⋅Cl and Cl⋅⋅⋅Cl contacts. The crystal packing also features extensive offset π–stacking of the aryl rings.
Atom and contacts colour code: Carbon (grey), hydrogen (white), chlorine (bright green), selenium (pink). Green
lines: intramolecular bridging Se–Cl bonds within dimer, blue dashed lines: weaker Se⋅⋅⋅Cl or Cl⋅⋅⋅Cl intermolecular
contacts.

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Figure 7 ORTEP representation of the molecular structure of (p-ClC6H4)SeBr3. Thermal ellipsoids are shown at the
30% probability level. Selected bond lengths [Å] and angles [°]: Se(1)–Br(1): 2.3436(15), Se(1)–Br(3): 2.4479(15),
Se(1)–Br(2): 2.6886(15), Se(1)–Br(6): 3.0288(15), Se(2)–Br(4): 2.3693(16), Se(2)–Br(5): 2.4900(15), Se(2)–Br(2):
2.9481(15), Se(2)–Br(6): 2.6520(16), Se(1)–C(1): 1.955(12), Se(2)–C(7): 1.953(11), C(4)–Cl(1): 1.752(11), C(10)–
Cl(2): 1.744(12), Br(1)–Se(1)–Br(6): 166.32(5), Br(2)–Se(1)–Br(3): 168.59(6), Br(1)–Se(1)–Br(3): 95.73(5), Br(1)–
Se(1)–Br(2): 93.33(5), Br(2)–Se(1)–Br(6): 82.10(4), Br(3)–Se(1)–Br(6): 87.50(4), Br(2)–Se(2)–Br(4): 167.64(5),
Br(5)–Se(2)–Br(6): 170.45(6), Br(4)–Se(2)–Br(5): 93.88(5), Br(2)–Se(2)–Br(5): 87.76(4), Br(2)–Se(2)–Br(6):
84.27(4), Br(4)–Se(2)–Br(6): 92.80(5), Se(1)–Br(2)–Se(2): 81.44(4), Se(1)–Br(6)–Se(2): 80.52(4).

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Figure 8 Pairing of folded dimers in the structure of (p–ClC6H4)SeBr3, via Se⋅⋅⋅Br interactions. Atom / contacts colour
code: Carbon (grey), hydrogen (white), chlorine (bright green), bromine (brown), selenium (pink). Brown lines:
intramolecular bridging Se–Br bonds within dimer, blue dashed lines: weaker Se⋅⋅⋅Br intermolecular contacts.

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Figure 9 ORTEP representation of the molecular structure of Ph2Se2Br2. Thermal ellipsoids are shown at the 30%
probability level. Selected bond lengths [Å] and angles [°]: Se(1)–Se(2): 2.832(4), Se(1)–Br(1): 2.449(4), Se(2)–Br(2):
2.352(5), Se(1)–C(1): 1.96(3), Se(2)–C(7): 1.93(3), Br(1)–Se(1)–Se(2): 174.83(19), Br(1)–Se(1)–C(1): 96.4(9), Se(2)–
Se(1)–C(1): 85.3(10), Se(1)–Se(2)–Br(2): 93.59(14), Se(1)–Se(2)–C(7): 101.9(10), Br(2)–Se(2)–C(7): 99.3(10).

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Figure 10 Propagation of chain structure in Ph2Se2Br2 via bridging Br(1)···Se(2) interactions of 3.333(4) Å. Hydrogen
atoms omitted for clarity. Symmetry operations used to generate equivalent atoms: ii: -1+x, y, z, iii: 1+x, y, z.
Figure 11 Sheet of selenium and bromine atoms in the extended solid–state structure of Ph2Se2Br2. Phenyl rings
omitted for clarity.

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Figure 12 Extended structure of Ph2Se2Br2 viewed down the a–crystallographic axis, showing the arrangement of
stacking of the phenyl rings between essentially planar sheets of selenium and bromine atoms. Atom colour code:
Carbon (grey), hydrogen (white), bromine (brown), selenium (pink).

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Compound
Se–X bond lengths / Å
Structural type(for RSeX3)
Reference
PhSeCl3
2.198(4)–2.251(5) (terminal)
2.616(5)–2.726(5) (bridging)
2.13(2)–2.25(2) (terminal)
2.51(2)–2.75(2) (bridging)
2.193(4)–2.259(4) (terminal)
2.497(3)–2.697(4) (bridging)
2.206(2)–2.257(2) (terminal)
2.587(2)–2.749(2) (bridging)
2.3459(9)–2.4349(9) (terminal)
Chain polymer
19
20
21
22
23
CF3SeCl3
cis–dimer (folded)
1,2–(SeCl3)2C6H4
cis–dimer (imposed by
bridging C6H4 group,
Se2Cl6 unit is folded)
4–(Me3Si)2N–3,5–(iPr)2C6H2SeCl3
trans–dimer (planar)
{o–(Me2NCH2)C6H4}SeCl3
(p–FC6H4)SeCl3
Monomer
2.189(3)–2.277(3) (terminal)
2.556(2)–2.766(2) (bridging)
cis–dimer (planar)
This work
(p–ClC6H4)SeCl3
cis dimer:
Co–crystallized cis– and
trans–dimers (both planar)
This work
2.205(2)–2.250(2) (terminal)
2.6201(19)–2.7310(19) (bridging)
trans dimer:
2.1949(19)–2.261(2) (terminal)
2.5899(19)–2.8184(19) (bridging)
2.3538(14)–2.3590(13) (terminal)
PhSeBr3
Network structure built
19
from [PhSeBr2] fragments
3.0891(13)–3.0976(13) (secondary) and Se⋅⋅⋅Br and Br⋅⋅⋅Br
interactions.
(p–FC6H4)SeBr3
2.374(2)–2.453(2) (terminal)
2.716(2)–2.896(2) (bridging)
2.3436(15)–2.4900(15) (terminal)
2.6520(16)–3.0288(15) (bridging)
2.217(3)–2.306(3) (terminal)
2.519(2)–2.803(3) (bridging)
2.208(2)–2.210(2) (terminal)
2.725(2)–2.733(2) (bridging)
2.205(3)–2.265(3) (terminal)
2.576(3)–2.799(3) (bridging)
2.189(2)–2.222(3) (terminal)
2.688(2)–2.816(3) (bridging)
2.393(3)–2.432(4) (terminal)
2.756(4)–2.877(4) (bridging)
cis–dimer (planar)
cis–dimer (folded)
This work
(p–ClC6H4)SeBr3
This work
[Ph4As]2[Se2Cl10]
36
37
38
39
33
[Ph(NH2)C=NC(Ph)NHCl]2[Se2Cl10]
[Et4N]2[Se2Cl10]
[S4N3]2[Se2Cl10]
[Et4N]2[Se2Br10]

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[C4H10NO]2[Se2Br10]
2.369(1)–2.390(1) (terminal)
2.871(1)–2.923(1) (bridging)
40
Table 1 Comparison of terminal and bridging Se–X bond lengths in structures of RSeX3 compounds, and related [Se2X10]2– anions (X = Cl, Br).
(p–FC6H4)SeCl3
C12H8Cl6F2Se2
(p–FC6H4)SeBr3
C12H8Br6F2Se2
(p–ClC6H4)SeCl3
C18H12Cl12Se3
Empirical formula
Fw
560.80
827.56
890.56
Colour, habit
Crystal system
Space group
Crystal size
Colourless, block
Monoclinic
Orange, block
Monoclinic
Colourless, prism
Monoclinic
C2/c (no. 15)
C2/c (no. 15)
C2/c (no. 15)
0.12 x 0.12 x 0.05 mm3
0.04 x 0.06 x 0.10 mm3
0.12 x 0.12 x 0.06 mm3
Unit cell dimensions
a = 17.4590(6) Å,
α
= 90°
= 100.7710(10)°
= 90°
a = 17.7188(6) Å,
α
= 90°
= 97.236(1)°
= 90°
a = 25.4102(7) Å,
b = 10.7139(3) Å,
c = 20.2732(6) Å,
5512.7(3) Å3
100(2) K
α
β
γ
= 90°
= 92.780(2)°
= 90°
b = 8.7778(3) Å,
c = 23.2622(9) Å,
3502.2(2) Å3
100(2) K
β
b = 9.1035(3) Å,
c = 23.4934(11) Å,
3759.4(3) Å3
100(2) K
β
γ
γ
Volume
T
Z
D calcd.
λ
8
8
8
2.127 mg/m3
0.71073 Å
5.147 mm-1
2144
2.924 mg/m3
0.71073 Å
16.698 mm-1
3008
2.146 mg/m3
0.71073 Å
5.178 mm-1
3408
ꢀ
(Mo-Kα)
F(000)
range
θ
3.23 to 26.00°
12163 (3423 unique)
0.088
2.98 to 25.70°
19016 (3551 unique)
0.117
3.07 to 25.50°
20467 (5108 unique)
0.081
No. of reflections
Rint
R1 / wR2
0.0664 / 0.1543
0.1067 / 0.1773
1.469 and –0.934 eÅ-3
0.0803 / 0.1792
0.1272 / 0.2001
0.0567 / 0.1326
0.0902 / 0.1554
1.982 and –1.103 eÅ-3
R1 / wR2 (all data)
Largest diff. peak
and hole
1.645 and –2.752 eÅ-3
GOF
1.276
1.272
1.037
Table 2 Crystallographic parameters for compounds (p–FC6H4)SeCl3, (p–FC6H4)SeBr3, (p–ClC6H4)SeCl3, (p–ClC6H4)SeBr3 and Ph2Se2Br2.
(p–ClC6H4)SeBr3
C12H8Br6Cl2Se2
Ph2Se2Br2
Empirical formula
Fw
C12H10Br2Se2
471.94
860.46
Colour, habit
Crystal system
Space group
Crystal size
Orange, plate
Monoclinic
Purple, prism
Monoclinic
P21 (no. 4)
P21/c (no. 14)
0.02 x 0.14 x 0.14 mm3
0.05 x 0.05 x 0.12 mm3
Unit cell dimensions
a = 7.2480(1) Å,
α
β
γ
= 90°
= 96.980(1)°
= 90°
a = 6.0108(5) Å,
b = 7.1843(7) Å,
c = 15.6165(18) Å,
674.37(12) Å3
100(2) K
α
β
= 90°
= 90.004(4)°
γ = 90°
b = 25.3497(5) Å,
c = 10.7265(3) Å,
1956.22(7) Å3
100(2) K
Volume
T
Z
D calcd.
λ
4
2
2.921 mg/m3
0.71073 Å
16.301 mm-1
1568
2.324 mg/m3
0.71073 Å
11.378 mm-1
440
ꢀ
(Mo-Kα)
F(000)
range
θ
2.94 to 26.00°
21237 (3805 unique)
0.0696
3.63 to 25.60°
5484 (2061 unique)
0.158
No. of reflections
Rint
R1 / wR2
0.0670 / 0.1660
0.0821 / 0.1756
1.881 and –2.760 eÅ-3
0.0866 / 0.2130
0.1222 / 0.2544
R1 / wR2 (all data)
Largest diff. peak
and hole
2.487 and –2.202 eÅ-3
GOF
1.138
1.068
Table 2 (cont.) Crystallographic parameters for compounds (p–FC6H4)SeCl3, (p–FC6H4)SeBr3, (p–ClC6H4)SeCl3, (p–ClC6H4)SeBr3 and Ph2Se2Br2.

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Structural isomerism in (p–XC6H4)SeCl3 and (p–XC6H4)SeBr3 (X = F, Cl) compounds. Co–
crystallisation of cis– and trans– dimeric forms of (p–ClC6H4)SeCl2(µ–Cl)2(p–ClC6H4)SeCl2. A
new structural modification for the “PhSeBr” reagent, Ph2Se2Br2, containing an elongated Se–Se
bond.
Nicholas A. Barnes, Stephen M. Godfrey, Ruth T. A. Ollerenshaw, Rana Z. Khan and Robin G.
Pritchard
Structural isomerism has been observed for the selenium(IV) compounds (p-XC6H4)SeCl3 and (p-XC6H4)SeBr3 (X = F, Cl),
which may adopt dimeric structures which are either cis–planar, cis–folded, or, in the case of (p-ClC6H4)SeCl3, both cis and
trans–dimers are found to co–crystallise in the unit cell. A new structural modification of “PhSeBr” is also reported, which is
dimeric, Ph2Se2Br2, with an elongated Se–Se bond.