Organoselenium(ii) halides containing the pincer 2,6-(Me 2NCH2)2C6H3 ligand-an experimental and theoretical investigation

Article abstract of DOI:10.1039/c3dt52886c

New organoselenium(ii) halides of the type [RSe]⁺X^- [R = 2,6-(Me₂NCH₂)₂C₆H₃; X = Cl (2), Br (3), I (4)] were prepared by cleavage of the Se-Se bond in R ₂Se₂

Full text of DOI:10.1039/c3dt52886c

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Organoselenium(II) halides containing the pincer
2,6-(Me2NCH2)2C6H3 ligand – an experimental and
theoretical investigation†‡
Cite this: Dalton Trans., 2014, 43,
2221
Alexandra Pop,a Anca Silvestru,a Emilio José Juárez-Pérez,b Massimiliano Arca,c
Vito Lippolis*c and Cristian Silvestru*a
New organoselenium(II) halides of the type [RSe]+X− [R = 2,6-(Me2NCH2)2C6H3; X = Cl (2), Br (3), I (4)] were
prepared by cleavage of the Se–Se bond in R2Se2 (1) with SO2Cl2 followed by halogen exchange when organo-
selenium chloride was treated with NaBr or KI. The reaction between 2 and R’2MCln resulted in new ionic
−
[RSe]+[R’2MCln+1
]
[R’ = 2-(Me2NCH2)C6H4, n = 1, M = Sb (5), Bi (6); R’ = Ph, M = Sb, n = 1 (7) or n = 3 (8)]
species. All new compounds were investigated in solution by multinuclear NMR spectroscopy (1H, 13C, 77Se,
2D experiments) and mass spectrometry. The ionic nature of 2 and the antimonates species was confirmed
by conductivity studies. The molecular structures of [{2,6-(Me2NCH2)2C6H3}Se]+Cl−·nH2O (2·H2O and
2·2H2O) and [{2,6-(Me2NCH2)2C6H3}Se]+[Ph2SbCl4]− (8), respectively, were established by single-crystal X-ray
diffraction, pointing out that the ionic nature of these compounds is also preserved in the solid state,
with both nitrogen atoms strongly trans coordinated to the selenium atom of the cation. Theoretical
calculations carried out at the DFT level were exploited to investigate the nature of the bonding in compounds
2–4 and the free cation [RSe]+ (2a). A topological analysis based on the theory of Atoms-In-Molecules (AIM)
and Electron Localization Function (ELF) jointly to a Natural Bond Orbital (NBO) approach was used to shed
light on the effect of the nature of the halogen species X on the bonding within the 3c-4e N–Se–N moiety.
Received 13th October 2013,
Accepted 13th November 2013
DOI: 10.1039/c3dt52886c
www.rsc.org/dalton
aDepartamentul de Chimie, Centrul de Chimie Supramoleculară Organică şi
Organometalică (CCSOOM), Facultatea de Chimie şi Inginerie Chimică,
Universitatea Babeş-Bolyai, RO-400028, Cluj-Napoca, Romania.
Introduction
The chemistry of organoselenium compounds based on aro-
matic ligands bearing substituents which contain donor atoms
able to provide intramolecular N–Se coordination raised con-
siderable interest in recent years, mainly due to their appli-
cations in biology, asymmetric synthesis, catalysis or
microelectronics.1–16 While one pendant arm ligands such as
E-mail: cristian.silvestru@ubbcluj.ro; Fax: (+40) 264-590818; Tel: (+40) 264-593833
bDepartment of Physics, Photovoltaic and Optoelectronic Devices Group, Universitat
Jaume I, 12071 Castelló de la Plana, Spain
cDipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Cagliari,
S.S. 554 Bivio per Sestu, 09042 Monserrato (CA), Italy. E-mail: lippolis@unica.it
†Dedicated to Professor Antonio Laguna (Universidad de Zaragoza, Spain) on
the occasion of his 65th birthday.
‡Electronic supplementary information (ESI) available: Synthetic details for the 2-(Me2NCH2)C6H4, 2-[E(CH2CH2)2NCH2]C6H4 (E = O, NMe),
preparation of 1; crystal data and structure refinement for 2·H2O, 2·2H2O and 8
2-(2′-oxazolinyl)phenyl, 2-(RNvCH)C6H4 or related ones were
quite often used in organoselenium chemistry, only a few
compounds containing “pincer” aromatic groups have been
(Table S1); hydrogen bonds lengths and angles in the dimer association of
2·H2O (Table S2); packing diagrams for 2·H2O (Fig. S1–S3); selected bond dis-
tances and angles for the anion in 8 (Table S3); packing diagrams for 8 (Fig. S4–
reported.1,6 Examples include mainly organic groups with
oxygen as a potential donor atom in the pendant arm, e.g. the
diselenides such as [2,6-{EtOCH(Me)}2C6H3]2Se2,17 [2,6-(MeOCH2)2-
C6H3]2Se2,18 [4-tBu-2,6-R2C6H3]2Se2 [R = (CH2O)2CH, (O)CH,
S6); stacked 77Se NMR spectra (CDCl3 r.t.) for 2, 3, and 4 (Fig. S7); Z-matrices of
the optimized structures of 2a, 2–4, 2a′, and 2′–4′; selected optimized structural
parameters for 2a and 2–4 (Table S4); contribution of the main atomic orbital
(AOs) to HOMOs and LUMOs calculated for 2a and 2–4 (Table S5); isosurfaces of
selected Kohn–Sham MOs calculated for 2a and 2 (Fig. S8); calculated QTAIM
parameters at the BCPs of the Se–C, Se–N, and Se–X bonds in 2a and 2–4
(Table S6); core and valence shell population (number of electrons) from ELF of
i
PhNH(O)C,19 MeO(O)C, PrO(O)C20], or related mononuclear
species as, for example, the selenides [2,6-{MeOCH(Me)}2C6H3]-
Se atom in compounds 2a and 2–4 (Table S7); BCPs in the structure corres- SeCH(Me)CH(Ph)[C6H2(OMe)3-2′,4′,6′]21 or [4-tBu-2,6-{MeO(O)C}2-
ponding to the global and local minima from PES scan for 2 (Fig. S9) and corres-
C6H3]SePh,22 the halide [4-tBu-2,6-{MeO(O)C}2C6H3]SeBr20 or
[2,6-{MeOCH(Me)}2C6H3]SeOTf.21 Compounds based on the
ponding relative electronic energy differences and energy interaction for the
H⋯Cl bonds (Table S8); sum of the calculated electronic and thermal enthalpies
and zero-point correction energies for 2a, 2a′, 2–4 and 2′–4′, and relative enthal-
asymmetric ligand 2-NO2-6-(PhNvCH)C6H3 were also reported
recently.23 Organoselenium compounds containing (N,C,N)
pies ΔHf (Table S9). CCDC 952118–952120. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c3dt52886c
“pincer” ligands are quite rare and limited to species bearing
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the 2,6-(Me2NCH2)2C6H3 on selenium. Furukawa and co-
workers24 reported the structural characterization of [{2,6-
(Me2NCH2)2C6H3}Se]+[PF6]−, the first ionic species containing
an organoselenium cation stabilized through two intramolecu-
lar N–Se interactions.3 This compound was obtained by react-
ing the selenides [2,6-(Me2NCH2)2C6H3]SeR (R = Me, n-octyl)
t
with BuOCl followed by treatment with K[PF6]. The dealkyla-
tion reaction leading to the same cation was also achieved by
treatment of the above selenides with bromine. Later on, Pole-
schner and Seppelt18 reported that reaction of [2,6-
(Me2NCH2)2C6H3]2Se2 with XeF2 in CH2Cl2 produced [{2,6-
(Me2NCH2)2C6H3}Se]+[HF2]− which could be converted by
K[PF6] into the same ionic species described by Furukawa.
Recently, an X-ray diffraction study revealed the ionic nature
of the organoselenium(II) bromide [{2,6-(Me2NCH2)2C6H3}Se]+-
Br−·H2O. The crystal contains dimers featuring two bromide
anions, two cations, and two water molecules connected
through Br⋯Se and Br⋯H interactions.25
The importance of electrophilic selenium reagents,
[ArSe]+X−, in the functionalization of olefins/acetylenes is well
documented and several ionic species containing organosele-
nium cations, stabilized through intramolecular coordinating
groups including “pincer” ligands, and noncoordinating
anions were used.3,23
Scheme 1 Preparation of compounds 2–4.
It should be mentioned here that cleavage of the Se–Se
bond in diselenide 1 could also be achieved with elemental
bromine, and recrystallization in open atmosphere yielded the
hydrate [{2,6-(Me2NCH2)2C6H3}Se]+Br−·H2O (3·H2O), its struc-
ture having been already reported.25 It is interesting to note
that during attempts to purify the diselenide 1 from a dichloro-
methane–n-hexane mixture orange crystals of 2·2H2O were iso-
lated, probably formed as a result of the reaction with the
halogenated solvent.
As a part of our interest in organometallic compounds with
(N,C,N) “pincer” ligands, herein we report on the synthesis
and spectroscopic (NMR, MS) characterization of several
The ionic nature of 2 is reflected by its reactivity towards
chlorides of the type R′2MCln (M = Sb, Bi), the chloride ion
being transferred to the pnictogen atom in acetonitrile, at
ionic species of the type [RSe]+X− [R = 2,6-(Me2NCH2)2C6H3;
−
X
=
Cl (2), Br (3), I (4)] and [RSe]+[R′2MCln+1
]
[R′ =
room temperature. This results in new ionic species [RSe]+-
2-(Me2NCH2)C6H4, n = 1, M = Sb (5), Bi (6); R′ = Ph, M = Sb, n =
1 (7) or n = 3 (8)] as well as on the crystal and molecular struc-
tures of [{2,6-(Me2NCH2)2C6H3}Se]+Cl−·nH2O (2·H2O and
2·2H2O) and [{2,6-(Me2NCH2)2C6H3}Se]+[Ph2SbCl4]− (8). In order
to better understand the N–Se–N three-body system in the free
cation [RSe]+ (2a) and the effect of the halogenide X− in com-
pounds 2–4, DFT theoretical investigations exploiting a topolo-
gical analysis based on the theory of Atoms-In-Molecules (AIM)
and Electron Localization Function (ELF) combined with a
Natural Bond Orbital (NBO) approach were carried out.
−
[R′2MCln+1
]
[R′ = 2-(Me2NCH2)C6H4, n = 1, M = Sb (5), Bi (6);
R′ = Ph, M = Sb, n = 1 (7) or n = 3 (8)] (Scheme 2). Such reac-
tions between an onium halide and a diorganopnictogen(III)
halide26 or diorganoantimony(V) trihalide27 are well
documented.
All compounds were obtained as dioxygen stable, crystalline
solids, which melt without decomposition. The chloride 2 is
highly hygroscopic, a behaviour also suggested by its crystalli-
zation as hydrates with a different number of co-crystallized
Results and discussion
Synthesis
The diselenide [2,6-(Me2NCH2)2C6H3]2Se2 (1) was prepared by
a slight modification of the procedure previously reported by
Poleschner and Seppelt,18 using BuLi instead of BuLi for the
ortho lithiation of 1,3-bis(dimethylaminomethyl)benzene, fol-
lowed by insertion of elemental selenium into the newly
formed carbon–lithium bond (see ESI‡).8a The cleavage of the
Se–Se bond in 1 was achieved by treatment with SO2Cl2, at
room temperature, in CCl4 to give the ionic species [{2,6-
(Me2NCH2)2C6H3}Se]+Cl− (2) as a pale yellow solid. The corres-
ponding bromide 3 and iodide 4 were obtained from the chlor-
ide 2 and excess amounts of KBr and KI, respectively, in
acetone at room temperature (Scheme 1).
n
t
Scheme 2 Preparation of compounds 5–8.
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water molecules (see the subsequent discussion). The com-
pounds are soluble in organic solvents, such as chloroform,
methylene dichloride, acetonitrile or DMSO. MS and NMR
data as well as elemental analytical data are consistent with
the expected formulas. For all compounds 2–8 the ESI+ mass
spectra constantly contain [2,6-(Me2NCH2)2C6H3Se+] as the
base peak. The conductivity of the chloride 2 and the antimo-
nates 5, 7 and 8 in MeCN solutions is consistent with their
ionic nature and formulation.
Table 1 Selected interatomic distances (Å) and angles (°) for the cation
in 2·H2O, 2·2H2O and 8
2·H2O
2·2H2O
8
C(1)–Se(1)
N(1)–Se(1)
N(2)–Se(1)
N(1)–Se(1)–N(2)
C(1)–Se(1)–N(1)
C(1)–Se(1)–N(2)
1.889(2)
2.183(2)
2.184(2)
161.31(8)
80.41(9)
81.04(9)
1.883(3)
2.175(3)
2.177(3)
162.00(12)
81.09(15)
80.94(15)
1.886(3)
2.178(3)
2.160(3)
161.95(11)
81.06(13)
81.02(13)
Solid state and solution behavior
A common feature of compounds 2·nH2O (n = 1, 2) and 8 is
that the crystals contain [{2,6-(Me2NCH2)2C6H3}Se]+ cations in
which strong N–Se intramolecular interactions (see Table 1)
[N–Se 2.183(2)/2.184(2) Å in 2·H2O, 2.175(3)/2.177(3) Å in
2·2H2O, and 2.160(3)/2.178(3) Å in 8] are established by both N
atoms of the pendant arms, in trans positions to each other
[N–Se–N 161.31(8)° in 2·H2O, 162.00(12)° in 2·2H2O, and
161.95(11)° in 8]. This results in a T-shaped (N,C,N)Se core
(hypervalent 10-Se-3 species)28 similar to what was observed in
the cation of [{2,6-(Me2NCH2)2C6H3}Se]+[PF6]− (N–Se 2.154/
2.1840 Å; N–Se–N 161.9°)24 or the related organoselenium(II)
bromide [{2,6-(Me2NCH2)2C6H3}Se]+Br−·H2O [N–Se 2.181(3)/
2.185(3) Å; N–Se–N 161.6(1)°].25
Single-crystals of 2·H2O and 8 were obtained by slow diffusion
of n-hexane into a dichloromethane solution of the com-
pounds, and their structures, as well as that of the dihydrate
2·2H2O, were established by X-ray diffraction. The structures of
2·H2O and 8 are shown in Fig. 1 and 2 and selected inter-
atomic distances and bond angles in the [{2,6-
(Me2NCH2)2C6H3}Se]+ cation for all compounds are listed in
Table 1.
The resulting five-membered SeNC3 rings in the [{2,6-
(Me2NCH2)2C6H3}Se]+ cation are not planar, with the nitrogen
atom lying above the best plane defined by the residual SeC3
system. This induces planar chirality (with the aromatic ring
and the nitrogen atom as the chiral plane and the pilot atom,
respectively; for one five-membered ring the isomers are given
as RN and SN, respectively)29 and compounds 2·nH2O (n = 1, 2)
and 8 crystallize as racemates, i.e. 1 : 1 mixtures of (RN1,RN2
)
and (SN1,SN2) isomers (with respect to the two chelate rings in
a cation unit).
Fig. 1 Dimer association of (RN1,RN2)-cations in 2·H2O, showing the
atom numbering scheme. The atoms are drawn with 50% probability
ellipsoids [symmetry equivalent atoms (1 − x, y, 1.5 − z), (−0.5 + x, 0.5 +
y, z), (−0.5 + x, −0.5 + y, z) and (1.5 − x, −0.5 + y, 1.5 − z) are given by
“prime”, “a”, “b” and “c”, respectively].
For 2·2H2O, substitutional disorder of halogen and oxygen
atoms (50% occupancy for Cl1/O1 and 0.50 for Cl2/O2, respect-
ively) prevents any consideration concerning possible associ-
ations in the crystal based on hydrogen bonding or contacts
with heavy atoms. However, the distance between selenium
and a heavy atom is larger than 5.0 Å so that any interactions
between cations and anions in the crystal of 2·2H2O can be dis-
counted. By contrast, the monohydrate 2·H2O features a
dimeric association (Fig. 1), with two isomer cations of the
same chirality [either (RN1,RN2)/(RN1,RN2) or (SN1,SN2)/(SN1,SN2)]
bridged through one halide anion [Se(1)⋯Cl(2) 3.558(1) Å; Se(1)⋯
Cl(2)⋯Se(1′) 146.06(1)°; cf. ∑rcov(Se,Cl) 2.16 Å, ∑rvdW(Se,Cl)
3.81 Å30a] of the (Cl−)2(H2O)2 unit built on Cl⋯H inter-
actions [Cl(2a)⋯H(13b) 2.24 Å; Cl(1b)⋯H(14b) 2.33 Å; cf.
∑rvdW(Cl,H) 3.01 Å30a]. A similar dimer association was shown
to be present in the crystal of the analogous bromide 3·H2O.25
Chain polymers of alternating (RN1,RN2)/(RN1,RN2) and (SN1,SN2)/
(SN1,SN2) dimer units are formed through further inter-
cation C–H⋯π (Arylcentroid) contacts (i.e. H⋯Arcentroid contacts
shorter than 3.1 Å, with an angle γ between the normal to the
aromatic ring and the line defined by the H atom and Arcentroid
smaller than 30°),30b C(7)–H(7Ah)methylene⋯Arcentroid{C(1)–C(6)}
Fig. 2 Structure of ionic species 8, showing both the anion and the
(RN1,RN2)-cation with the atom numbering scheme (hydrogen atoms are
omitted for clarity). The atoms are drawn with 50% probability ellipsoids.
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2.92 Å, γ = 11.4°. Additional weak Cl⋯H and O⋯H contacts
The 77Se NMR spectra of compounds 2–8 are consistent
result in a 3D architecture in the crystal of 2·H2O (see for with the formation of the ionic species. One singlet resonance
details Table S2, Fig. S1, S2a–c, S3a–c in the ESI‡). is observed around δ 1200 ppm for these compounds, consist-
The coordination geometry around the antimony atom in ent with the presence of only one Se species in solution. The
the anion of 8 deserves no special comments. It is octahedral 77Se resonances in 2–8 are strongly low-field shifted compared
with the phenyl groups, placed in trans positions, basically to the diorganodiselenide 1 (δ 395.6 ppm) used as the starting
bisecting the Cl–Sb–Cl angles of the square-planar SbCl4 material to prepare the organoselenium(II) halides (see Fig. S7
plane. The Sb–Cl bond distances fall within the range 2.4323(8)– in the ESI‡). These values are in agreement with the values for
2.4932(8) Å (see Table S3 in ESI‡), as observed for other analogous compounds described in the literature.8a,15
related ionic compounds.27g They are of intermediate length
Theoretical studies
between terminal and bridging antimony–halogen bonds in
the dimer (Ph2SbCl3)2 [2.346(4), 2.388(4)
Å vs. 2.620(4), Recently, we have reported on the synthesis and structural pro-
2.839(4) Å].31 Weak Se⋯Cl contacts [3.680(1) Å] are established perties of organoselenium(II) halides of the type [2(Et2NCH2)-
between cations and anions (Fig. 2). Dimer associations are C6H4]SeX (X = Cl, Br, I) featuring a three body system
formed between pairs of (RN1,RN2)-cation/Ph2SbCl4 and (SN1,SN2)- N→Se–X.16d In particular, DFT calculations in the gas phase
cation/Ph2SbCl4 based on C–H⋯π (Phcentroid
)
contacts allowed to confirm a polarized 3c-4e nature for it with the
which involve one phenyl group of the anion [C(12)– entity of the N–Se intramolecular interaction decreasing on
H(12A)methyl⋯Phcentroid{C(13′a)–C(18′a)} 2.68 Å, γ = 7.5°]. A passing from X = Cl to X = I and the Se–X interaction corres-
layer of dimer cation–anion associations is built through anion– pondingly increasing. In 2–4, the presence of a second N→Se
anion Cl⋯H interactions [Cl(3′)⋯H(17c)phenyl 2.87 Å] and intramolecular interaction to give a T-shaped (N,C,N)Se core
C–H⋯π (Arylcentroid) contacts between (RN1,RN2)- and (SN1,SN2)- results in a complete detachment of the halogen atom from
cations [C(10)–H(10A)methylene⋯Arcentroid{C(1′b)–C(6′b)} 2.78 Å, the chalcogen centre with the formation of ionic organosele-
γ = 3.1°]. In the crystal of 8, additional weak Cl⋯H and C–H⋯π nium(II) compounds. In order to better establish the nature of
(Arylcentroid) contacts between layers result in 3D architectures the bond in 2–4 and the role played by the formation of the
(see for details Fig. S4, S5a,b, S6a–c in the ESI‡).
N→Se(C)←N hypervalent core in the ionicity of the title com-
The organoselenium compounds 1–8 were investigated in solu- pounds we performed DFT calculations on the free cation
tion by multinuclear NMR spectroscopy (1H, 13C, 77Se) and the [{2,6-(Me2NCH2)2C6H3}Se]+ (2a) and on 2–4. Furthermore, we
1H and 13C resonances were assigned based on 2D experiments, also performed on the same systems a topological electron
according to the numbering scheme illustrated in Scheme 3.
density analysis based on the quantum theory of atoms-
The room temperature NMR spectra of 1–8 exhibit only one in-molecules (QTAIM) approach and already applied with
set of resonances for the [2,6-(Me2NCH2)2C6H3] group attached success to other types of hypervalent systems.33
to the selenium atom suggesting that only one species is
The geometries of 2a and 2–4 were first optimized at the
present in solution. Singlet H and 13C signals were observed DFT level by using the LanL2DZ basis set with relativistic
both for the methylene and methyl protons and carbons, effective core potential, polarisation and diffuse functions for
respectively, indicating equivalence of the two pendant arms Se and halogen atoms (see the Experimental section and ESI‡).
and a high symmetry of the cation on the NMR time scale due An overall good match is observed between the calculated
to a fast conformational change of the chelate five-membered structural parameters for 2 and 3 and the corresponding
1
1
SeC3N rings. For compounds 5–8, typical H (sometimes over- experimental ones determined for 2·H2O (see above) and
lapped) and 13C resonances were observed for the organic 3·H2O.25 The highest discrepancy is observed for the Se–X dis-
groups in the anion. In the case of 5 the spectra show broad tance. Furthermore, while the calculated Se–N2 distance in 2
1H resonances for the methyl and methylene protons in anion and 3 is slightly overestimated, the calculated Se–N1 and Se–C
[2-{(Me2NCH2)C6H4}2SbCl2]−. This pattern of the NMR spectra distances are very close to the corresponding structural values
at room temperature suggests a fluxional behavior corres- (Scheme 4), the N1–Se–N2, X–Se–C and N2–Se–C–C angles
ponding to the dissociation/re-coordination between the nitro- being also fairly well estimated. Interestingly, for each opti-
gen atoms and the metal atom, with inversion at a three- mized halide 2–4 the two calculated Se–N distances are slightly
coordinated nitrogen atom and rotation around the (H2)C–N different from each other and the difference becomes smaller
bond. A similar process was previously described for the start- on passing from X = Cl (2) to X = I (4). It is also important to
ing material [2-(Me2NCH2)C6H4]2SbCl.32
point out that, in agreement with structural data of 2·H2O,
2·2H2O and 3, in the optimized geometries of 2–4 the halide
ions adopt a similar disposition with respect to the [RSe]+
cation as shown in Scheme 4. Selected optimized structural
parameters for 2a and 2–4 (Table S4‡), and the Z-matrices of
optimized structures have been deposited as ESI.‡
An examination of the nature of Kohn–Sham (KS) HOMOs
and LUMOs (Fig. 3 for 2a and 2) is in agreement with a slightly
unbalanced 3c-4e nature for the N→Se(C)–N three-body system
Scheme 3 Numbering scheme for NMR assignments.
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Table 2 Selected NBO charges and Wiberg bond indexes calculated
for 2a and 2–4
2a
0.615
−0.532
−0.532
−0.288
2
3
4
NBO
Se
N2
N1
C
0.671
0.660
0.655
−0.535
−0.521
−0.306
−0.834
0.304
−0.534
−0.523
−0.306
−0.835
0.319
−0.533
−0.526
−0.306
−0.845
0.339
X
Wiberg index
Se–N2
Se–N1
Se–C
Se–X
0.408
0.409
0.997
—
0.448
0.960
0.1481
0.438
0.956
0.1480
0.425
0.955
0.1385
Scheme 4 Optimized geometries for the [RSe]+X− [R
(Me2NCH2)2C6H3] species [free cation (2a); X = Cl (2), Br (3), I (4)].
=
2,6-
bonding and non-bonding MOs corresponding to the Rundle–
Pimentel σ-bonding scheme primarily localized on the N–Se–N
fragment can be envisaged among the calculated Kohn–Sham
MOs (see Fig. S8‡ for 2a and 2).
On passing from X = Cl (2) to X = I (4), NBO analysis clearly
indicates a progressive increase in the electronic population
on the chalcogen atom that bears
a positive charge,
accompanied by a slight increase in the negative charge on N1
and the halogen centre. An examination of calculated Wiberg
indexes for the Se–N bonds (Table 2) shows that, as deduced
from the corresponding calculated bond lengths (Table S4‡),
on passing from X = Cl (2) to X = I (4) the entity of the Se–N2
interaction systematically increases while that of the Se–N1
interaction slightly decreases. The sum of the bond indexes
calculated for the two Se–N bonds remains roughly constant,
with a slight increase on passing from 2 to 4, in agreement
with a progressive strengthening of the N–Se–N three-body
system. In the free cation 2a the Wiberg indexes for the two
Se–N bonds are equal. These trends are determined by the
change in the halogen species, which are engaged in a remark-
able ionic interaction with the Se center as supported by the
trends in the calculated NBO negative charge on the halogens
on passing from X = Cl (2) to X = I (4). However, on increasing
the electronegativity of the halogen, some electron sharing
Fig. 3 Kohn–Sham HOMO and LUMO isosurfaces calculated for 2a
(left) and 2 (right). Contour value = 0.05 e.
in 2–4, with the N atom of one pendant group acting as a between the Se atom and the halogen is calculated [see the
donor towards the Se–N acceptor system formed by the N atom trend in the Se–X WBIs on going from X = Cl (2) to X = I (4)],
of the other pendant arm of the pincer ligand.
and therefore the degree of ionicity of these organoselenium(II)
While in 2a the HOMO is mainly formed by the 4pz AO on halides increases on going from Cl to I, which is accompanied
the Se atom and 2pz AOs from the C atoms of the aromatic by a balancing of the two N–Se interactions.
ring (see Fig. 3), in 2–4 the HOMO is mainly localized on the
Overall, we can draw the same conclusions about the nature
halogen atoms consisting of a combination of mainly npx and of the bond involving the Se atom in the compounds con-
npz AOs, although there is a small contribution from the Se sidered and the effects on the N→Se(C)←N system due to the
atom mainly with the 4pz AO and decreasing from X = Cl presence of the halide species, by taking into account the elec-
(6.2%) to X = I (2.7%). In all cases the HOMO is antibond- tron population in the natural molecular orbitals involved in
ing in character [see Table S5‡ for the contribution of the chemical bonding of the hypervalent N→Se(C)←N moiety
*
main Atomic Orbitals (AOs) to the HOMOs and LUMOs in 2a and the energy for the nN→σSe–N delocalization interaction in
and 2–4].
compounds 2a and 2–4 evaluated by the second order pertur-
For all compounds, the LUMO shows a σ character along bation theory analysis of the Fock matrix in the NBO basis
the N–Se–N axis due to the antibonding combination of 4px (Table 3). In particular, it is important to consider the increase
and 2px AOs from Se and N atoms, respectively.34 Notably, the in the amount of charge transferred from nN MO to the anti-
*
composition of this MO in terms of the constituent AOs corres- bonding MO (σSe–N) on passing from 2 to 4, and the parallel
ponds to the antibonding pσ combination of a 3c-4e system increase in the ΔE(2)nN→σSe–N from 39.3 (2) to 46.99 (4), which
*
as described by the Rundle–Pimentel model (Fig. 3).35 Also the reach the maximum value (59.45 kcal mol−1) in the isolated
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*
observed from 1.6% to 1.2% on going from 2 to 4. The limiting
structure having no Se–N bonds has no contribution to the reso-
nance hybrid in all cases.
Table 3 Electron occupancy of the three valence NBOs σSe–N, σSe–N
,
and nN, participating in the chemical bonding of the hypervalent
N–Se(C)–N moiety in 2a, 2–4, and stabilization energy, ΔE(2)nN→σSe–N
,
*
due to electron delocalization from the donor to the acceptor NBOs
With the aim of further investigating the nature of the
chemical bonding in [{2,6-(Me2NCH2)2C6H3}Se]+X− we per-
formed a topological electron density analysis on 2a and 2–4
based on the quantum theory of atoms-in-molecules (QTAIM)
approach developed by Bader (the calculated QTAIM properties
at the bond critical points (BCPs) of the Se–C, Se–N and Se–X
bonds for 2a and 2–4 have been deposited in the ESI,
Table S6‡).38–42
ΔE(2)nN→σSe–N
*
)
Compound σSe–N
σSe–N
nN
Totala
(kcal mol−1
*
2a
2
3
0.34302 1.94844 1.64290 3.9344 59.45
0.30640 1.95118 1.70214 3.9597 39.53
0.31335 1.95034 1.69367 3.9574 42.65
0.32136 1.94899 1.68278 3.9534 46.99
4
a Total electron occupancy: σSe–N + σSe–N + nN.
*
The QTAIM properties at the BCPs of the Se–C bonds are
very similar in 2a and 2–4 and in agreement with a covalent
Se–C single bond picture as found for Se–C bonds in I–Se(C)–X
hypervalent systems (X = I, Br, Cl).33
RSe+ cation 2a. The total occupancy of the three centers in the
hypervalent N→Se(C)←N moiety is close to 4 electrons in all
cases.
As expected, the H38 value is negative at the BCPs of the Se–
N bonds; however, the positive values for the calculated ∇2ρ38
indicate an overall high polar covalent character for these
bonds, in agreement with the very different NBO and QTAIM
charges (Tables 2 and S6‡) calculated on the Se and N atoms.
Significantly, as observed for Wiberg bond indexes, the
difference between the values calculated for δ(Se,N1) and
δ(Se,N2)38 decreases on going from 2 to 4 reaching the same
value in 2a, where the two Se–N bonds are balanced and are not
affected by the presence of the halogenide ion. Furthermore,
the sum of the δ(Se,N1) and δ(Se,N2) correlation indexes in 2a
and 2–4 is very close to the values found for δ(Se,C). Finally,
the QTAIM properties at the BCPs of the Se–X bonds, especially
the positive values calculated for H and the low positive values
calculated for ∇2ρ, indicate an ionic or very polar nature for
this interaction. Notably, the values of ρ and ∇2ρ at the BCPs
of the Se–X bond decrease on passing from 2 to 4 indicating,
at least in the gas-phase, an increase in the polarity of this
bond on passing from X = Cl to I.
Fig. 4 shows the localization basins calculated for the mole-
cules 2a and 2–4 by performing a topological analysis of the
electron localization function (ELF).43 Two well-separated
monosynaptic valence V(Se) basins containing unshared
valence electrons can be recognized around the selenium atom
in each compound. In all cases, a distorted trigonal bipyrami-
dal (tbp) disposition of the V(Se) and V(Se,B) (B = N, C) di-
synaptic valence basins containing shared valence electrons
around the selenium atom can be recognized. No disynaptic
valence basins V(Se,X) (X = Cl, Br, I) are found, indicating a
basically ionic interaction between the halogenide ions and
the 2a cation.
The theoretical data discussed so far clearly indicate that in
the free cation 2a a symmetric balanced 3c-4e system is
present, which recalls other roughly linear different A–E–A
systems featuring a central chalcogen species (E = S, Se) sur-
rounded by two halogen or chalcogen A species.36 The intro-
duction of the halide X− in 2–4 results in a progressive
unbalancing of the N–Se–N moiety towards a N→Se–N system,
which can be therefore envisaged as a charge-transfer (CT)
system.
It is interesting to apply the recently developed Natural Reso-
nance Theory (NRT) method to the hypervalent N–Se(C)–N
moiety in 2a and 2–4, which offers a convenient and effective
ab initio procedure to calculate candidate resonance structures
and the corresponding weights.37 According to the results
reported in Table 4, the N–Se(C)–N moiety in 2a and 2–4 can
be described as roughly a 1 : 1 mixture of two resonance Lewis-
type structures featuring each a single Se–N bond. In 2a the
contributions of the two structures (N2:Se–N1 and N2–Se:N1)
to the resonance hybrid are identical and equal to 50%. In 2–4
the greatest contribution is calculated from the structure fea-
turing, according to previous DFT calculations, the shortest
Se–N bond (Table 4). The difference in the weight between the
two N2:Se–N1 and N2–Se:N1 resonance structures decreases
on passing from 2 to 4 and a decrease of the contribution
from the limiting structure featuring two single Se–N bonds is
Table 4 Percentage weights of principal resonance structures calcu-
lated for the N–Se(C)–N moiety in 2a and 2–4
The ELF analysis allows discriminating between core and
valence basins and calculating the corresponding electron
population, which is reported in Table S7.‡
Whether or not the octet rule applies to hypervalent atoms
in a three-body system is a matter of current debate,44a and in
this context the term hypercoordination has been proposed
instead.44b,c Notably, the Rundle–Pimentel view of 3c-4e
bonds35 does not require octet expansion.44d Examination of
the Nv(Se) values (number of electrons in the valence shell,
shared and unshared) from ELF analysis for compounds 2a
Compound
N2:Se–N1
N2–Se:N1
N2–Se–N1
N2:Se:N1
2a
2
3
50.0
57.7
56.3
54.5
49.7
40.7
42.2
44.3
0.3
1.6
1.5
1.2
0.0
0.0
0.0
0.0
4
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plane defined by the aromatic ring and, therefore, equivalent.
The saddle points feature the chloride anion lying in the plane
defined by the aromatic ring and forming a C–Se–Cl angle of
180° in the case of the C–Se–Cl scan. The Se–N–N–Cl scan fea-
tures two minima, one of which (yellow point in Fig. 5b) corres-
ponds to one of the two minima shared by the C–Se–Cl and
C–C–Se–Cl scans for the position of the anion in the optimized
structure. The other minimum (grey point in Fig. 5b) is
0.75 kcal mol−1 higher in energy and, therefore, should be con-
sidered a local minimum in the PES scan.
The geometry of 2 corresponding to the saddle point in the
C–Se–Cl PES scan (C–Se–Cl = 180°) is characterized by an ima-
ginary harmonic frequency calculated at −31 cm−1 and associ-
ated with the C–Se–Cl bending vibration and, therefore, it can
be considered a transition state in the movement of the chlor-
ide ion from one side of the aromatic ring to the other side
Fig. 4 Electron localization function (ELF) for 2a and 2–4 (the isosur-
faces include points at which the localization domain for the ELF is set
equal to 0.82 for all molecules). C(A): core basins containing the core
shell of electrons for each atom A; V(A,B): disynaptic valence basins con-
taining valence electrons shared by two interacting atoms A and B and
connecting the corresponding core basins; V(A): monosynaptic valence
basins containing the unshared valence electrons around a given core
basin C(A); the proton has no core electrons.
and 2–4 reveals that in all cases Nv(Se) > 8 and very close to 11, (green points in Fig. 5a and 5d) as confirmed by QST3
thus suggesting
a
hypervalent nature for the Se atom (Gaussian09, Synchronous Transit-Guided Quasi-Newton Method)
(Table S7‡).
analysis (see the Experimental section).
A number of valence shell electrons, Nv(Se), higher than
8 have been found in “T-shaped” compounds featuring 3c-4e we have considered the geometry of 2 in correspondence to the
I–Se(C)–X moieties (X = I, Br, Cl). In those cases, Nv(Se) ranges
between 8.5 and 10.0 electrons, thus revealing a high donor grey point in Fig. 5b) and in correspondence to the saddle
On the grounds of the results obtained from the PES scans
global and local minima found (yellow points in Fig. 5a–c and
capacity of the N atoms in the present N→Se(C)←N hyper-
valent systems and a high tendency to share their lone pair
with the selenium centre in these compounds.33
The fraction of electrons calculated within the disynaptic
valence basins V(Se,X) (X = Cl, Br, I) is very low in agreement
point in Fig. 5a. In these structures, the chloride ion is
involved in hydrogen bonds with the methyl groups from the
amino pendant arms of the pincer ligand (Fig. S8‡). We have
evaluated the QTAIM parameters at the BCPs of all hydrogen
bonds involving the Cl atom in these structures, which are
with an ionic nature of this bond, while the 3c-4e nature for typical of hydrogen bonding interactions. In particular, we
the N→Se(C)←N system is confirmed by the number of
electrons very close to 4 within the disynaptic valence basins
V(Se,N) (Table S7‡).
An intriguing structural aspect revealed by our calculations
in the gas phase is the position of the halogenide anion with
have evaluated the energy interaction of each H⋯Cl bond
using the Espinosa–Molins–Lacomte (EML) equation
(Table S8‡).45 The sum of the H⋯Cl interaction energies for
the considered structures indicates that intramolecular H⋯Cl
interactions play a major role in determining the disposition
respect to the cation 2a; in particular, the halogenide does not of the chloride ion with respect to the cation 2a in the ground
lie in the plane defined by the aromatic ring in the direction of
the C–Se bond, but it is located closer to one of the N atoms
(N1) forming a C–Se–X (X = Cl, Br, I) angle ranging from 159.0 (2)
to 163.4° (4) and a C–C–Se–X dihedral angle ranging from
52.2 (2) to 64.8° (4) (Table 2). On the other hand, the crystal
state in the gas phase (see ESI‡ for
discussion).
a more detailed
In previous studies, the synthesis, characterization and elec-
tronic properties of organoselenium(II) compounds of the type
[2-(R2NCH2)C6H4]SeX (R = Me, Et; X = Cl, Br, I) have been
structures of the hydrated species 2·H2O, 2·2H2O, and 3·H2O reported.15,16d In these compounds a strong internal N→Se
confirm the capability of the halides to be differently oriented
with respect to the 2a unit, their relative positions being also
interaction is established in the solid state, thus resulting in a
distorted T-shaped (C,N)SeX core with a polarized 3c-4e nature
determined by the extended network of hydrogen bonding for the chemical bonding in the N→Se–X moiety. In the gas
interactions.
phase a progressive decrease of the N→Se interaction is calcu-
In order to understand the origin of the disposition of the
lated on going from X = Cl to X = I. In principle, neutral orga-
halogenide anions with respect to the cation 2a in the gas noselenium(II) compounds of this kind featuring an N→Se–X
phase, a potential energy surface (PES) scan was therefore
moiety can also be formed starting from cation 2a via substi-
carried out by calculating the total electronic energy E for 2,
tution of one Me2NCH2 pendant arm at the selenium atom
optimized for the C–Se–Cl angle and the Se–N–N–Cl and C–C– and coordination with a halide.
Se–Cl torsion angles in the range 150/210°, −140/120°, −180/
In order to quantify the relative stability of compounds 2–4
featuring the N→Se(C)←N hypervalent system (slightly un-
180°, respectively. These three scans explore all possible chlor-
ide positions in the hemisphere above the cation 2a and the balanced by the presence of the X− anion) with respect to
results are shown in Fig. 5. The C–Se–Cl and C–C–Se–Cl scans
give rise to two common global minima (yellow points in
compounds 2′–4′ featuring N→Se(C)–X moieties, the forma-
tion enthalpy of the species X−, 2a, 2a′, 2–4 and 2′–4′ have been
Fig. 5a and 5c) symmetrically disposed with respect to the calculated. Hence, we have optimized the geometry of 2′–4′
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Fig. 5 PES scans calculated for 2 at the DFT level on varying (a) the C–Se–Cl angle, (b) the Se–N–N–Cl, and (c) C–C–Se–Cl dihedral angles in the
range 150/210°, −140/120°, −180/180°, respectively; (d) top view of the three performed PES scans together.
(see ESI‡) and calculated the formation enthalpy of the cation
species 2a and 2a′ by considering the equation [RSe]+ + X−
→
RSeX [ΔHf(2a/2a′) = ΔHf(2–4/2′–4′) − ΔHf(X−)]. The results are
summarized in Table S9 (ESI‡).
Interestingly, compounds 2′–4′, featuring only one N→Se
interaction and the N→Se–X framework, are thermodynami-
cally more stable than the corresponding compounds 2–4 fea-
turing the N–Se(C)–N hypervalent moiety by about 4 kcal
mol−1. In contrast, the 2a cation is much more stable than the
2a′ cation by 25.5 kcal mol−1 (see Fig. 6). This justifies the
absence of 2–4 to 2′–4′ inter-conversions in solution as demon-
strated by NMR measurements. This interconversion in fact
would require an initial high energy cost to break one of the
two N–Se interactions.
Conclusions
Fig. 6 Enthalpy of formation (ΔHf298, kcal mol−1) for compounds 2–4
starting from 2a, and for compounds 2’–4’ starting from 2a via the inter-
mediate cation 2a’.
Compounds of the type [{2,6-(Me2NCH2)2C6H3}Se]+ X− [X = Cl
(2), Br (3), I (4)] have been synthesized starting from the disele-
nide [2,6-(Me2NCH2)2C6H3]2Se2 (1). The novel compounds have
been fully characterized by spectroscopic means and com-
pounds 2·H2O, 2·2H2O, and [{2,6-(Me2NCH2)2C6H3}Se]+-
In this context, an in-depth theoretical investigation at the
[Ph2SbCl4]− (8) have been characterized by single-crystal X-ray DFT level was carried out. The geometries of compounds 2–4
diffraction analysis. The structural and spectroscopic charac- have been optimized and the potential energy surface (PES)
terization suggests the N–Se–N moiety as a 3c-4e system.
has been explored in order to ascertain the possible position
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of the anion X− with respect to the cation [{2,6- measurements of 10−4 M solutions in acetonitrile (HPLC
(Me2NCH2)2C6H3}Se]+ (2a). Overall, DFT calculations show that grade) were performed at 25 °C using a Bench Conductivity/
the N–Se–N system can be envisaged as a strongly polarized TDS Meter CON 510 instrument containing 2-ring stainless
3c-4e three-body system. Accordingly, the KS-LUMO is clearly steel Ultem-body Conductivity/TDS electrode (cell constant K =
represented by the antibonding σ-combination of atomic 1.0) with a built-in temperature sensor (EC-CONSEN 91W/
orbitals as described by the Rundle–Pimentel bonding 35608-50). Melting points were measured with an Electro-
scheme. The nature of the X− halide is reflected in an unbalan- thermal 9200 apparatus and are not corrected. Elemental ana-
cing of the N–Se–N system towards a charge-transfer N→Se–N lyses were carried out with a CHN-Analysator Type FlashAE
system on passing from 4 to 2. This conclusion is supported 1112 (Co. Thermo) instrument. Solvents were dried and freshly
by the calculation of Wiberg bond indexes and NBO calcu- distilled under argon prior to use. The starting materials, 1,3-
lations. Further calculations, carried out within the framework (Me2NCH2)2C6H4,48 Ph2SbCl,49 Ph2SbCl3·H2O,50 [2-(Me2NCH2)-
of Bader’s quantum theory of atoms-in-molecules (QTAIM), C6H4]2SbCl,32 and [2-(Me2NCH2)C6H4]2BiCl,51 were prepared
confirm the polarized nature of N–Se bonds in 2–4. The topo- according to literature methods. The reagents SO2Cl2, KBr,
logical analysis of the electron localized function (ELF) sup- KI and nBuLi were purchased from Sigma Aldrich and used
ports a hypervalent nature of the Se atom. Notably, a without further purification.
comparison of the AIM parameters calculated for the N–Se–N
Synthesis of [{2,6-(Me2NCH2)2C6H3}Se]+Cl− (2). A solution
system of 2a/2–4 and I–Se(C)–X systems (X = Cl, Br, I) featured of 1 (1.76 g, 3.26 mmol) in CCl4 (30 mL) was treated dropwise
by T-shaped dihalogen adducts of selenocarbonyl donors with a solution of SO2Cl2 (0.44 g, 3.26 mmol) in CCl4 (20 mL).
shows similar values for both three-body systems, providing a The reaction mixture was stirred at room temperature for 24 h.
further example of how apparently different chemical systems Then the resulting solid product was filtered off, dissolved in
can share similar bonding nature. This confirms the sugges- CH2Cl2 and the organic solution was washed with a saturated
tive conclusions derived from a recent structural survey on solution of KOH. The organic phase was separated, and dried
A–B–C systems containing different combinations of halogen over Na2SO4, and removal of the solvent in vacuo gave 2 (1.45 g,
and chalcogen species embedded within charge-transfer and 73%) as a pale yellow solid, m.p. 106 °C. Anal. calcd for
T-shaped hypervalent systems,36 which share a common corre- C12H19ClN2Se (305.71): C 47.15, H 6.26, N 9.16; Found:
lation between the A–B and B–C bond distances accountable C 47.35, H 6.28, N 9.10%. ΛM 344 Ω−1 cm2 mol−1
.
1H NMR
by means of the bond-valence model.
(CDCl3): δ 2.94 (12 H, s, H-8,10, CH3), 4.14 (4 H, s, H-7,9, CH2),
7.32 (3 H, m, H-3,4,5, C6H3). 13C NMR (CDCl3): δ 48.81
(s, C-8,10, CH3), 63.92 (s, C-7,9, CH2), 125.86 (s, C-3,5), 128.40
(s, C-4), 132.29 (s, C-2,6), 132.46 (s, C-1). 77Se NMR (CDCl3):
δ 1202.2 (s). MS (ESI+), m/z (%): 271 [2,6-(Me2NCH2)2C6H3Se+].
Synthesis of [{2,6-(Me2NCH2)2C6H3}Se]+Br− (3). A solution
Experimental section
General procedures
The 1H, 13C and 77Se NMR spectra were recorded in CDCl3 and of 2 (0.500 g, 1.64 mmol) in acetone (30 mL) was treated drop-
DMSO solutions at room temperature on a Bruker AVANCE 300 wise with a solution of KBr (0.380 g, 3.19 mmol) in acetone
instrument operating at 300.11, 75.46 and 57.24 MHz, respect- (20 mL). The reaction mixture was stirred at room temperature
ively. The 1H chemical shifts are reported in δ units (ppm) rela- for 24 h and then filtered and removal of the solvent from the
tive to the residual peak of the deuterated solvent (ref. CHCl3: clear solution gave 3 (0.47 g, 83%) as a pale orange solid,
1H 7.26 ppm; DMSO-d5: 1H 2.50 ppm). The 13C chemical shifts m.p. 143 °C. Anal. calcd for C12H19BrN2Se (350.16): C 41.16,
are reported in δ units (ppm) relative to the peak of the solvent H 5.47, N 8.00; Found: C 41.21, H 5.49, N 7.97%. 1H NMR
1
(ref. CHCl3: 13C 77.0 ppm; DMSO-d6: 13C 39.43 ppm). H and (CDCl3): δ 3.06 (12 H, s, H-8,10, CH3), 4.19 (4 H, s, H-7,9, CH2),
13C resonances were assigned using 2D NMR experiments 7.31 (3 H, m, H-3,4,5, C6H3). 13C NMR (CDCl3): δ 49.33
(COSY, HSQC and HMBC) which were performed using stan- (s, C-8,10, CH3), 64.35 (s, C-7,9, CH2), 125.97 (s, C-3,5), 128.47
dard pulse sequences in the version with z-gradients, as deli- (s, C-4), 132.11 (s, C-2,6), 132.75 (s, C-1). 77Se NMR (CDCl3):
vered by Bruker with TopSpin 1.3 PL6 spectrometer control δ 1211.2 (s). MS (ESI+), m/z (%): 271 [2,6-(Me2NCH2)2C6H3Se+].
and processing software. The 77Se spectra were obtained using
Synthesis of [{2,6-(Me2NCH2)2C6H3}Se]+I− (4). Prepared as
diphenyldiselenide as an external standard. Chemical shifts described for 3 from 2 (0.500 g, 1.64 mmol) and KI (0.540 g,
are reported relative to dimethylselenide (δ 0 ppm) by assum- 3.25 mmol) in acetone (50 mL). Work-up of the reaction
ing that the resonance of Ph2Se2 is at δ 461 ppm.46 The NMR mixture gave
4 (0.55 g, 85%) as a pale yellow solid,
spectra were processed using the MestReC and MestReNova m.p. 216 °C. Anal. calcd for C12H19IN2Se (397.16): C 36.29,
software.47 The ESI+ mass spectra were recorded using an H 4.82, N 7.05; Found: C 36.29, H 4.52, N 6.98%. 1H NMR
Esquire 3000 ion-trap mass spectrometer (Bruker Daltonic (CDCl3): δ 3.03 (12 H, s, H-8,10, CH3), 4.22 (4 H, s, H-7,9, CH2),
GmbH) and a Thermo Scientific LTQ Orbitrap XL mass 7.31 (3 H, m, H-3,4,5, C6H3). 13C NMR (CDCl3): δ 49.10
spectrometer. Samples were introduced by direct infusion with (s, C-8,10, CH3), 64.19 (s, C-7,9, CH2), 125.95 (s, C-3,5),
a syringe pump. Nitrogen served both as the nebulizer gas 128.47 (s, C-4), 132.34 (s, C-2,6), 132.75 (s, C-1). 77Se NMR
and the dry gas. Helium served as a cooling gas for the ion (CDCl3):
δ
1206.5 (s). MS (ESI+), m/z (%): 271 [2,6-
trap and a collision gas for MSn experiments. Conductivity (Me2NCH2)2C6H3Se+].
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Synthesis
of
[{2,6-(Me2NCH2)2C6H3}Se]+[2-{(Me2NCH2)- 13C NMR (DMSO-d6): δ 48.19 (s, C-8,10, CH3), 63.03 (s, C-7,9,
C6H4}2SbCl2]− (5). A solution of [2-(Me2NCH2)C6H4]2SbCl CH2), 125.52 (s, C-3,5), 128.04 (s, C-4), 128.75 (s, C-para),
(0.139 g, 0.33 mmol) in acetonitrile (20 mL) was added drop- 129.89 (s, C-meta), 130.90 (s, C-2,6), 131.87 (s, C-ipso), 133.07
wise to a stirred solution of 2 (0.100 g, 0.33 mmol) in aceto- (s, C-1), 133.59 (s, C-ortho). 77Se NMR (DMSO-d6): δ 1187.6 (s).
nitrile (15 mL). The reaction mixture was stirred for 24 h and MS (ESI+), m/z (%): 271 [2,6-(Me2NCH2)2C6H3Se+].
then the solvent was removed in vacuo to give 5 (0.21 g,
87%) as a yellow powder, m.p. 103 °C. Anal. calcd for
C30H43Cl2N4SbSe (731.32): C 49.27, H 5.93, N 7.66; Found:
Crystal structure
The crystals were mounted with epoxy glue on cryoloops and
the data were collected with a Bruker SMART APEX diffracto-
meter by using graphite-monochromated Mo-Kα radiation
(λ = 0.71073 Å) at room temperature (297 K). The structures
were refined with anisotropic thermal parameters. The hydro-
gen atoms were refined with a riding model and a mutual iso-
tropic thermal parameter. For structure solving and
refinement the software package SHELX-97 was used.52 Com-
pound 2·2H2O was refined as substitutional disorder of
halogen and oxygen atoms with 0.50 occupancy for Cl1/O1 and
0.50 for Cl2/O2. The drawings were created with the Diamond
program.53 Additional crystallographic data are summarized in
Table S1.‡
C 49.25, H 5.90, N 7.58%. ΛM 203 Ω−1 cm2 mol−1 1H NMR
.
(CDCl3): δ 2.26 (12 H, s, br, H-8′, CH3), 2.97 (12 H, s, H-8,10,
CH3), 3.68 (4 H, s, br, H-7′, CH2), 4.17 (4 H, s, H-7,9, CH2),
7.18–7.33 (9 H, m, H-3′,4′,5′ + H-3,4,5, C6H4 + C6H3), 7.84 (2 H,
s, br, H-6′, C6H4). 77Se NMR (CDCl3): δ 1202.4 (s). MS (ESI+),
m/z (%): 271 [2,6-(Me2NCH2)2C6H3Se+].
Synthesis
of
[{2,6-(Me2NCH2)2C6H3}Se]+[2-{(Me2NCH2)-
C6H4}2BiCl2]− (6). Prepared as described for 5 from 2 (0.100 g,
0.33 mmol) and [2-(Me2NCH2)C6H4]2BiCl (0.169 g, 0.33 mmol)
in acetonitrile (30 mL). Work-up of the reaction mixture gave 6
(0.234 g, 87%) as a yellow powder, m.p. 110 °C. Anal. calcd for
C30H43BiCl2N4Se (818.54): C 44.02, H 5.29, N 6.84; Found:
C 44.21, H 5.38, N 6.82%. 1H NMR (CDCl3): δ 2.36 (12 H, s,
H-8′, CH3), 2.84 (12 H, s, H-8,10, CH3), 3.74 (4 H, s, H-7′, CH2),
Theoretical calculations
4.05 (4 H, s, H-7,9, CH2), 7.22 (3 H, m, H-3,4,5, C6H3), 7.38
3
(4 H, m, H-4′,5′, C6H4), 7.49 (2 H, d, H-3′, C6H4, JHH 7.0 Hz), Quantum chemical DFT calculations were performed on 2a,
3
8.52 (2 H, d, H-6′, C6H4, JHH 6.9 Hz). 13C NMR (CDCl3): 2a′, 2–4 and 2′–4′ structures using the commercial suite Gaus-
δ 45.89 (s, C-8′, CH3), 48.78 (s, C-8,10, CH3), 63.88 (s, C-7,9, sian 09 (Rev. B.01),54 with Adamo and Barone’s mPW1PW
CH2), 67.93 (s, C-7′, CH2), 125.81 (s, C-3,5), 128.00 (s, C-4′), functional.55 Although the use of all-electron basis sets pro-
128.16 (s, C-4), 129.67 (s, C-3′), 130.98 (s, C-5′), 139.47 (s, C-6′), vides better accuracy, pseudo-potential techniques are useful
132.32 (s, C-2,6), 132.50 (s, C-1), 145.95 (s, C-2′), 183.95 when relativistic effects must be taken into account. Thus, the
(s, C-1′). 77Se NMR (CDCl3): δ 1202.2 (s). MS (ESI+), m/z (%): double-ζ plus polarisation all-electron (pVDZ) basis set by
271 [2,6-(Me2NCH2)2C6H3Se+].
Horn, Schafer and Ahlrichs for C, H, and N,56 and the
Synthesis of [{2,6-(Me2NCH2)2C6H3}Se]+[Ph2SbCl2]− (7). Pre- LANL2DZdp basis sets with relativistic core potentials, polaris-
pared as described for 5 from 2 (0.100 g, 0.33 32.7 mmol) and ation and diffuse functions for Se and halogen atoms were
Ph2SbCl (0.103 g, 0.33 mmol) in acetonitrile (35 mL). Work-up adopted.57,58 Vibrational analyses were used to check the
of the reaction mixture gave 7 (0.174 g, 85%) as a yellow nature of the stationary points and none of the optimised geo-
powder, m.p. 239 °C. Anal. calcd for C24H29Cl2N2SbSe (617.13): metries presented imaginary frequencies at the DFT level
C 46.71; H 4.74, N 4.54; Found: C 46.80, H 4.78, N 4.50%. ΛM unless otherwise specified in the text. From these frequency
1
312 Ω−1 cm2 mol−1. H NMR (CDCl3): δ 2.66 (12 H, s, H-8,10, calculations, theoretical gas-phase enthalpy changes were
CH3), 3.82 (4 H, s, H-7,9, CH2), 7.08–7.23 (9 H, m, H-3,4,5 + obtained for the above compounds and Cl−, Br− and I− ions at
H-meta + para, C6H3 + C6H5), 8.00 (4 H, s, br, H-ortho, C6H5). the same level of theory to evaluate the relative stability of
13C NMR (CDCl3): δ 45.55 (s, C-8,10, CH3), 63.65 (s, C-7,9, cations 2a and 2a′. Mulliken charges were obtained from Gaus-
CH2), 125.72 (s, C-3,5), 127.65 (s, C-para), 127.79 (s, C-meta), sian 09 calculations, NPA charges were calculated using NBO
128.24 (s, C-4), 131.99 (s, C-2,6), 132.23 (s, C-1), 135.87 Version 5.0 59 and QTAIM (Bader) charges from the software
(s, C-ortho), 149.91 (s, C-ipso). 77Se NMR (CDCl3): δ 1202.7 (s). package AIMAll.60 Wiberg 2-center indexes were calculated
MS (ESI+), m/z (%): 271 [2,6-(Me2NCH2)2C6H3Se+].
using NBO Version 5.0, 2-center delocalisation indexes in the
Synthesis of [{2,6-(Me2NCH2)2C6H3}Se]+[Ph2SbCl4]− (8). Pre- framework of the QTAIM [δ(A,B)] are obtained from AIMAll,60
pared as described for 5 from 2 (0.060 g, 0.20 mmol) and and 3-center indexes [δ(A,B,C)] from WBader.61 Analysis of the
Ph2SbCl3·H2O (0.080 g, 0.20 mmol) in acetonitrile (40 mL). atomic orbital (AOs) contributions to the HOMO and LUMO
Work-up of the reaction mixture gave 8 (0.112 g, 81%) as a was carried out with the “Mulliken routine” of Multiwfn 2.5.62
yellow powder. The title compound was recrystallized from Second-order analysis of the Fock matrix energies and NRT
acetonitrile to give yellow crystals, m.p. 191 °C. Anal. calcd for results were obtained using NBO Version 5.0. Topological ana-
C24H29Cl4N2SbSe (688.04): C 41.90, H 4.25, N 4.07; Found: lysis in the framework of the QTAIM was done using AIMAll at
42.01, H 4.29, N 4.02%. ΛM 157 Ω−1 cm2 mol−1 1H NMR the same level of theory.63 The analysis of the ELF function
.
(DMSO-d6): δ 2.79 (12 H, s, H-8,10, CH3), 4.08 (4 H, s, H-7,9, was carried out with ToPMoD64,65 using all-electron densities
CH2), 7.27–7.32 (3 H, m, H-3,4,5, C6H3), 7.52–7.64 (6 H, m, and the ELF isosurfaces were visualised with Molekel.66
H-meta + para, C6H5), 8.09 (4 H, d, H-ortho, C6H5, 3JHH 7.4 Hz). HOMO and LUMO isosurfaces were obtained using Gabedit
2230 | Dalton Trans., 2014, 43, 2221–2233
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2.4.067 and the figures from scans and QST3 calculations were
obtained using PyMOL.68
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14 Y. Miyake, Y. Nishibayashi and S. Uemura, Bull. Chem. Soc.
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Acknowledgements
Financial support from the National University Research
Council of Romania (research projects PN-II-ID-PCE-2011-3- 15 T. M. Klapötke, B. Krumm and K. Polborn, J. Am. Chem.
0659 and PN-II-ID-PCE-2011-3-0933) is greatly appreciated. We Soc., 2004, 126, 710.
also thank the NATIONAL CENTER FOR X-RAY DIFFRACTION 16 (a) M. Kulcsar, A. Silvestru, C. Silvestru, J. E. Drake,
(Babes-Bolyai University, Cluj-Napoca, Romania) for the
support with the solid state structure determinations and the
University of Cagliari for financial support.
C. L. B. MacDonald, M. E. Hursthouse and M. E. Light,
J. Organomet. Chem., 2005, 690, 3217; (b) M. Kulcsar,
A. Beleaga, C. Silvestru, A. Nicolescu, C. Deleanu,
C. Todasca and A. Silvestru, Dalton Trans., 2007, 2187;
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