Organoselenium( II ) halides containing the pincer 2,6-(Me 2 NCH 2 ) 2 C 6 H 3 ligand – an experimental and theoretical investigation

New organoselenium( II ) halides of the type [RSe] + X − [R = 2,6-(Me 2 NCH 2 ) 2 C 6 H 3 ; X = Cl ( 2 ), Br ( 3 ), I ( 4 )] were prepared by cleavage of the Se – Se bond in R 2 Se 2 ( 1 ) with SO 2 Cl 2 followed by halogen exchange when organo-selenium chloride was treated with NaBr or KI. The reaction between 2 and R ’ 2 MCl n resulted in new ionic [RSe] + [R ’ 2 MCl n +1 ] − [R ’ = 2-(Me 2 NCH 2 )C 6 H 4 , 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 ( 1 H, 13 C, 77 Se, 2D experiments) and mass spectrometry. The ionic nature of 2 and the antimonates species was con ﬁ rmed by conductivity studies. The molecular structures of [{2,6-(Me 2 NCH 2 ) 2 C 6 H 3 }Se] + Cl − · n H 2 O ( 2 ·H 2 O and 2 ·2H 2 O) and [{2,6-(Me 2 NCH 2 ) 2 C 6 H 3 }Se] + [Ph 2 SbCl 4 ] − ( 8 ), respectively, were established by single-crystal X-ray di ﬀ raction, pointing out that the ionic nature of these compounds is also preserved in the solid state, with both nitrogen atoms strongly trans Q5 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 e ﬀ ect of the nature of the halogen species X on the bonding within the 3c-4e N – Se – N moiety.


Synthesis
The diselenide [2,6-(Me 2 NCH 2 ) 2 C 6 H 3 ] 2 Se 2 (1) was prepared by a slight modification of the procedure previously reported by Poleschner and Seppelt, 18 using n BuLi instead of t BuLi for the ortho lithiation of 1,3-bis(dimethylaminomethyl)benzene, followed 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 SO 2 Cl 2 , at room temperature, in CCl 4 to give the ionic species [{2,6-(Me 2 NCH 2 ) 2 C 6 H 3 }Se] + Cl − (2) as a pale yellow solid.The corresponding bromide 3 and iodide 4 were obtained from the chloride 2 and excess amounts of KBr and KI, respectively, in acetone at room temperature (Scheme 1).
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 ), its structure having been already reported. 25It is interesting to note that during attempts to purify the diselenide 1 from a dichloromethane-n-hexane mixture orange crystals of 2•2H 2 O were isolated, probably formed as a result of the reaction with the halogenated solvent.
The ionic nature of 2 is reflected by its reactivity towards chlorides of the type R′ 2 MCl n (M = Sb, Bi), the chloride ion being transferred to the pnictogen atom in acetonitrile, at room temperature.This results in new ionic species 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 crystallization as hydrates with a different number of co-crystallized   25 The resulting five-membered SeNC 3 rings in the [{2,6-(Me 2 NCH 2 ) 2 C 6 H 3 }Se] + cation are not planar, with the nitrogen atom lying above the best plane defined by the residual SeC 3 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 R N and S N , respectively) 29 and compounds 2•nH 2 O (n = 1, 2) and 8 crystallize as racemates, i.e. 1 : 1 mixtures of (R N1 ,R N2 ) and (S N1 ,S N2 ) isomers (with respect to the two chelate rings in a cation unit).
For 2•2H 2 O, substitutional disorder of halogen and oxygen atoms (50% occupancy for Cl1/O1 and 0.50 for Cl2/O2, respectively) prevents any consideration concerning possible associations 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•2H 2 O can be discounted.By contrast, the monohydrate 2•H 2 O features a dimeric association (Fig. 1), with two isomer cations of the same chirality [either (R N1 ,R N2 )/(R N1 ,R N2 ) or (S N1 ,S N2 )/(S N1 ,S N2 )] bridged through one halide anion [Se(1)⋯Cl( 2 25 Chain polymers of alternating (R N1 ,R N2 )/(R N1 ,R N2 ) and (S N1 ,S N2 )/ (S N1 ,S N2 ) dimer units are formed through further intercation C-H⋯π (Aryl centroid ) contacts (i.e.H⋯Ar centroid 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 Ar centroid smaller than 30°), 30b C(7)-H(7Ah) methylene ⋯Ar centroid {C(1)-C(6)}    The organoselenium compounds 1-8 were investigated in solution by multinuclear NMR spectroscopy ( 1 H, 13 C, 77 Se) and the 1 H and 13 C resonances were assigned based on 2D experiments, according to the numbering scheme illustrated in Scheme 3.
The room temperature NMR spectra of 1-8 exhibit only one set of resonances for the [2,6-(Me 2 NCH 2 ) 2 C 6 H 3 ] group attached to the selenium atom suggesting that only one species is present in solution.Singlet 1 H and 13 C signals were observed both for the methylene and methyl protons and carbons, respectively, indicating equivalence of the two pendant arms and a high symmetry of the cation on the NMR time scale due to a fast conformational change of the chelate five-membered SeC 3 N rings.For compounds 5-8, typical 1 H (sometimes overlapped) and 13 C resonances were observed for the organic groups in the anion.In the case of 5 the spectra show broad 1 H resonances for the methyl and methylene protons in anion − .This pattern of the NMR spectra at room temperature suggests a fluxional behavior corresponding to the dissociation/re-coordination between the nitrogen atoms and the metal atom, with inversion at a threecoordinated nitrogen atom and rotation around the (H 2 )C-N bond.A similar process was previously described for the starting material [2-(Me 2 NCH 2 )C 6 H 4 ] 2 SbCl. 32e 77 Se NMR spectra of compounds 2-8 are consistent with the formation of the ionic species.One singlet resonance is observed around δ 1200 ppm for these compounds, consistent with the presence of only one Se species in solution.The 77 Se resonances in 2-8 are strongly low-field shifted compared to the diorganodiselenide 1 (δ 395.6 ppm) used as the starting material to prepare the organoselenium(II) halides (see Fig. S7 in the ESI ‡).These values are in agreement with the values for analogous compounds described in the literature.8a,15 Theoretical studies Recently, we have reported on the synthesis and structural properties of organoselenium(II) halides of the type [2(Et 2 NCH 2 )-C 6 H 4 ]SeX (X = Cl, Br, I) featuring a three body system N→Se-X.16d In particular, DFT calculations in the gas phase allowed to confirm a polarized 3c-4e nature for it with the entity of the N-Se intramolecular interaction decreasing on passing from X = Cl to X = I and the Se-X interaction correspondingly increasing.In 2-4, the presence of a second N→Se intramolecular interaction to give a T-shaped (N,C,N)Se core results in a complete detachment of the halogen atom from the chalcogen centre with the formation of ionic organoselenium(II) compounds.In order to better establish the nature of the bond in 2-4 and the role played by the formation of the N→Se(C)←N hypervalent core in the ionicity of the title compounds we performed DFT calculations on the free cation [{2,6-(Me 2 NCH 2 ) 2 C 6 H 3 }Se] + (2a) and on 2-4.Furthermore, we also performed on the same systems a topological electron density analysis based on the quantum theory of atomsin-molecules (QTAIM) approach and already applied with success to other types of hypervalent systems. 33he geometries of 2a and 2-4 were first optimized at the DFT level by using the LanL2DZ basis set with relativistic effective core potential, polarisation and diffuse functions for Se and halogen atoms (see the Experimental section and ESI ‡).An overall good match is observed between the calculated structural parameters for 2 and 3 and the corresponding experimental ones determined for 2•H 2 O (see above) and 3•H 2 O. 25 The highest discrepancy is observed for the Se-X distance.Furthermore, while the calculated Se-N2 distance in 2 and 3 is slightly overestimated, the calculated Se-N1 and Se-C distances are very close to the corresponding structural values (Scheme 4), the N1-Se-N2, X-Se-C and N2-Se-C-C angles being also fairly well estimated.Interestingly, for each optimized halide 2-4 the two calculated Se-N distances are slightly different from each other and the difference becomes smaller on passing from X = Cl (2) to X = I (4).It is also important to point out that, in agreement with structural data of While in 2a the HOMO is mainly formed by the 4p z AO on the Se atom and 2p z AOs from the C atoms of the aromatic ring (see Fig. 3), in 2-4 the HOMO is mainly localized on the halogen atoms consisting of a combination of mainly np x and np z AOs, although there is a small contribution from the Se atom mainly with the 4p z AO and decreasing from X = Cl (6.2%) to X = I (2.7%).In all cases the HOMO is antibonding in character [see Table S5 ‡ for the contribution of main Atomic Orbitals (AOs) to the HOMOs and LUMOs in 2a and 2-4].
For all compounds, the LUMO shows a σ character along the N-Se-N axis due to the antibonding combination of 4p x and 2p x AOs from Se and N atoms, respectively. 34Notably, the composition of this MO in terms of the constituent AOs corresponds to the antibonding pσ combination of a 3c-4e system as described by the Rundle-Pimentel model (Fig. 3). 35Also the 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 remarkable 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 between the Se atom and the halogen is calculated [see the trend in the Se-X WBIs on going from X = Cl (2) to X = I (4)], and therefore the degree of ionicity of these organoselenium(II) halides increases on going from Cl to I, which is accompanied by a balancing of the two N-Se interactions.
Overall, we can draw the same conclusions about the nature of the bond involving the Se atom in the compounds considered and the effects on the N→Se(C)←N system due to the presence of the halide species, by taking into account the electron population in the natural molecular orbitals involved in the chemical bonding of the hypervalent N→Se(C)←N moiety and the energy for the n N →σ * Se-N delocalization interaction in compounds 2a and 2-4 evaluated by the second order perturbation theory analysis of the Fock matrix in the NBO basis (Table 3).In particular, it is important to consider the increase in the amount of charge transferred from n N MO to the antibonding MO (σ * Se-N ) on passing from 2 to 4, and the parallel increase in the ΔE (2) n N →σ * Se-N from 39.3 (2) to 46.99 (4), which reach the maximum value (59.45 kcal mol −1 ) in the isolated

Dalton Transactions Paper
This 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) surrounded by two halogen or chalcogen A species. 36The introduction 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 Resonance 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. 37According 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 Lewistype 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 featuring, 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 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 resonance hybrid in all cases.
][40][41][42] 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). 33s expected, the H 38 value is negative at the BCPs of the Se-N bonds; however, the positive values for the calculated ∇ 2 ρ 38 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's 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 ∇ 2 ρ, indicate an ionic or very polar nature for this interaction.Notably, the values of ρ and ∇ 2 ρ 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 molecules 2a and 2-4 by performing a topological analysis of the electron localization function (ELF). 43Two 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 bipyramidal (tbp) disposition of the V(Se) and V(Se,B) (B = N, C) disynaptic 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 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 bonds 35 does not require octet expansion.44d Examination of the N v (Se) values (number of electrons in the valence shell, shared and unshared) from ELF analysis for compounds 2a  A number of valence shell electrons, N v (Se), higher than 8 have been found in "T-shaped" compounds featuring 3c-4e I-Se(C)-X moieties (X = I, Br, Cl).In those cases, N v (Se) ranges between 8.5 and 10.0 electrons, thus revealing a high donor capacity of the N atoms in the present N→Se(C)←N hypervalent systems and a high tendency to share their lone pair with the selenium centre in these compounds. 33he fraction of electrons calculated within the disynaptic valence basins V(Se,X) (X = Cl, Br, I) is very low in agreement with an ionic nature of this bond, while the 3c-4e nature for the N→Se(C)←N system is confirmed by the number of electrons very close to 4 within the disynaptic valence basins V(Se,V) (Table S7 ‡).
An intriguing structural aspect revealed by our calculations in the gas phase is the position of the halogenide anion with respect to the cation 2a; in particular, the halogenide does not 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 structures of the hydrated species 2•H 2 O, 2•2H 2 O, and 3•H 2 O confirm the capability of the halides to be differently oriented with respect to the 2a unit, their relative positions being also determined by the extended network of hydrogen bonding interactions.
In order to understand the origin of the disposition of the halogenide anions with respect to the cation 2a in the gas phase, a potential energy surface (PES) scan was therefore carried out by calculating the total electronic energy E for 2, optimized for the C-Se-Cl angle and the Se-N-N-Cl and C-C-Se-Cl torsion angles in the range 150/210°, −140/120°, −180/ 180°, respectively.These three scans explore all possible chloride positions in the hemisphere above the cation 2a and the 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 Fig. 5a and 5c) symmetrically disposed with respect to the 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 features two minima, one of which (yellow point in Fig. 5b) corresponds 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 considered 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 imaginary harmonic frequency calculated at −31 cm −1 and associated with the C-Se-Cl bending vibration and, therefore, it can be considered a transition state in the movement of the chloride ion from one side of the aromatic ring to the other side (green points in Fig. 5a and 5d) as confirmed by QST3 (Gaussian09, Synchronous Transit-Guided Quasi-Newton Method ) analysis (see the Experimental section).
On the grounds of the results obtained from the PES scans we have considered the geometry of 2 in correspondence to the global and local minima found (yellow points in Fig. 5a-c and grey point in Fig. 5b) and in correspondence to the saddle 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 typical of hydrogen bonding interactions.In particular, we have evaluated the energy interaction of each H⋯Cl bond using the Espinosa-Molins-Lacomte (EML) equation (Table S8 ‡). 45The 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 of the chloride ion with respect to the cation 2a in the ground state in the gas phase (see ESI ‡ for a more detailed discussion).
16d In these compounds a strong internal N→Se interaction is established in the solid state, thus resulting in a distorted T-shaped (C,N)SeX core with a polarized 3c-4e nature for the chemical bonding in the N→Se-X moiety.In the gas phase a progressive decrease of the N→Se interaction is calculated on going from X = Cl to X = I.In principle, neutral organoselenium(II) compounds of this kind featuring an N→Se-X moiety can also be formed starting from cation 2a via substitution of one Me 2 NCH 2 pendant arm at the selenium atom and coordination with a halide.
In order to quantify the relative stability of compounds 2-4 featuring the N→Se(C)←N hypervalent system (slightly unbalanced by the presence of the X − anion) with respect to compounds 2′-4′ featuring N→Se(C)-X moieties, the formation enthalpy of the species X − , 2a, 2a′, 2-4 and 2′-4′ have been calculated.Hence, we have optimized the geometry of 2′-4′  S9 (ESI ‡).Interestingly, compounds 2′-4′, featuring only one N→Se interaction and the N→Se-X framework, are thermodynamically more stable than the corresponding compounds 2-4 featuring 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 demonstrated by NMR measurements.This interconversion in fact would require an initial high energy cost to break one of the two N-Se interactions.In this context, an in-depth theoretical investigation at the DFT level was carried out.The geometries of compounds 2-4 have been optimized and the potential energy surface (PES) has been explored in order to ascertain the possible position  Overall, DFT calculations show that the N-Se-N system can be envisaged as a strongly polarized 3c-4e three-body system.Accordingly, the KS-LUMO is clearly represented by the antibonding σ-combination of atomic orbitals as described by the Rundle-Pimentel bonding scheme.The nature of the X − halide is reflected in an unbalancing of the N-Se-N system towards a charge-transfer N→Se-N system on passing from 4 to 2. This conclusion is supported by the calculation of Wiberg bond indexes and NBO calculations.Further calculations, carried out within the framework of Bader's quantum theory of atoms-in-molecules (QTAIM), confirm the polarized nature of N-Se bonds in 2-4.The topological analysis of the electron localized function (ELF) supports a hypervalent nature of the Se atom.Notably, a comparison of the AIM parameters calculated for the N-Se-N system of 2a/2-4 and I-Se(C)-X systems (X = Cl, Br, I) featured by T-shaped dihalogen adducts of selenocarbonyl donors shows similar values for both three-body systems, providing a further example of how apparently different chemical systems can share similar bonding nature.This confirms the suggestive conclusions derived from a recent structural survey on A-B-C systems containing different combinations of halogen and chalcogen species embedded within charge-transfer and T-shaped hypervalent systems, 36 which share a common correlation between the A-B and B-C bond distances accountable by means of the bond-valence model.

General procedures
The 1 H, 13 13 C resonances were assigned using 2D NMR experiments (COSY, HSQC and HMBC) which were performed using standard pulse sequences in the version with z-gradients, as delivered by Bruker with TopSpin 1.3 PL6 spectrometer control and processing software.The 77 Se spectra were obtained using diphenyldiselenide as an external standard.Chemical shifts are reported relative to dimethylselenide (δ 0 ppm) by assuming that the resonance of Ph 2 Se 2 is at δ 461 ppm. 46The NMR spectra were processed using the MestReC and MestReNova software. 47The ESI + mass spectra were recorded using an Esquire 3000 ion-trap mass spectrometer (Bruker Daltonic GmbH) and a Thermo Scientific LTQ Orbitrap XL mass spectrometer.Samples were introduced by direct infusion with a syringe pump.Nitrogen served both as the nebulizer gas and the dry gas.Helium served as a cooling gas for the ion trap and a collision gas for MS n experiments.Conductivity measurements of 10

Crystal structure
The crystals were mounted with epoxy glue on cryoloops and the data were collected with a Bruker SMART APEX diffractometer 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 hydrogen atoms were refined with a riding model and a mutual isotropic thermal parameter.For structure solving and refinement the software package SHELX-97 was used. 52Compound 2•2H 2 O 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. 53Additional crystallographic data are summarized in Table S1.‡

Theoretical calculations
Quantum chemical DFT calculations were performed on 2a, 2a′, 2-4 and 2′-4′ structures using the commercial suite Gaussian 09 (Rev.B.01), 54 with Adamo and Barone's mPW1PW functional. 55Although the use of all-electron basis sets provides better accuracy, pseudo-potential techniques are useful when relativistic effects must be taken into account.Thus, the double-ζ plus polarisation all-electron ( pVDZ) basis set by Horn, Schafer and Ahlrichs for C, H, and N, 56 and the LANL2DZdp basis sets with relativistic core potentials, polarisation and diffuse functions for Se and halogen atoms were adopted. 57,58Vibrational analyses were used to check the nature of the stationary points and none of the optimised geometries presented imaginary frequencies at the DFT level unless otherwise specified in the text.From these frequency calculations, theoretical gas-phase enthalpy changes were obtained for the above compounds and Cl − , Br − and I − ions at the same level of theory to evaluate the relative stability of cations 2a and 2a′.Mulliken charges were obtained from Gaussian 09 calculations, NPA charges were calculated using NBO Version 5.0 59 and QTAIM (Bader) charges from the software package AIMAll. 60Wiberg 2-center indexes were calculated using NBO Version 5.0, 2-center delocalisation indexes in the framework of the QTAIM [δ(A,B)] are obtained from AIMAll, 60 and 3-center indexes [δ(A,B,C)] from WBader. 61Analysis of the atomic orbital (AOs) contributions to the HOMO and LUMO from the "Mulliken routine" of Multiwfn 2.5. 62Second Q6 -order analysis of the Fock matrix energies and NRT results were obtained using NBO Version 5.0.Topological analysis in the framework of the QTAIM was done using AIMAll at the same level of theory. 63The analysis of the ELF function was carried out with ToPMoD 64,65 using all-electron densities and the ELF isosurfaces were visualised with Molekel. 66HOMO and LUMO isosurfaces were obtained using Gabedit 2.4.0 67 6-(Me 2 NCH 2 ) 2 C 6 H 3 on selenium.Furukawa and coworkers 2), Br (3), I (4)] and [RSe] + [R′ 2 MCl n+1 ] − [R′ = 2-(Me 2 NCH 2 )C 6 H 4 , 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 structures of [{2,6-(Me 2 NCH 2 ) 2 C 6 H 3 }Se] + Cl − •nH 2 O (2•H 2 O and 2•2H 2 O) and [{2,6-(Me 2 NCH 2 ) 2 C 6 H 3 }Se] + [Ph 2 SbCl 4 ] − (8).In order to better understand the N-Se-N three-body system in the free cation [RSe] + (2a) and the effect of the halogen X in compounds 2-4, DFT theoretical investigations exploiting a topological 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.

Fig. 2
Fig.2Structure of ionic species 8, showing both the anion and the (R N1 ,R N2 )-cation with the atom numbering scheme (hydrogen atoms are omitted for clarity).The atoms are drawn with 50% probability ellipsoids.
journal is © The Royal Society of Chemistry 2014 Dalton Trans., 2014, 00, 1-13 | 5 2a.The total occupancy of the three centers in the hypervalent N→Se(C)←N moiety is close to 4 electrons in all cases.

Fig. 4
Fig. 4 Electron localization function (ELF) for 2a and 2-4 (the isosurfaces 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 containing 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.

Table 1 .
The compounds 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-(Me 2 NCH 2 ) 2 C 6 H 3 Se + ] as the base peak.The conductivity of the chloride 2 and the antimonates 5, 7 and 8 in MeCN solutions is consistent with their ionic nature and formulation.Single-crystals of 2•H 2 O and 8 were obtained by slow diffusion of n-hexane into a dichloromethane solution of the compounds, and their structures, as well as that of the dihydrate 2•2H 2 O, were established by X-ray diffraction.The structures of 2•H 2 O and 8 are shown in Fig. 1 and 2 and selected interatomic distances and bond angles in the [{2,6-(Me 2 NCH 2 ) 2 C 6 H 3 }Se] + cation for all compounds are listed in

Table 4
Percentage weights of principal resonance structures calculated for the N-Se(C)-N moiety in 2a and 2-4

Table 3
(2)ctron occupancy of the three valence NBOs σ Se-N , σ* Se-N , and n N , participating in the chemical bonding of the hypervalent N-Se(C)-N moiety in 2a, 2-4, and stabilization energy, ΔE(2)n N →σ * Se-N , due to electron delocalization from the donor to the acceptor NBOs Se-N + σ * Se-N + n N .