Reduction of dichloro(diaza-phospha)stibanes – isolation of a donor-stabilized distibenium dication†‡

ion led to the formation of a cyclic diazastibaphosphenium cation [P(μ-NTer)2SbCl] . Upon reduction of [Ter2N2PSbCl2], the transient existence of the novel mixed biradicaloid [P(μ-NTer)2Sb] was proven by a trapping experiment with an alkyne, while reduction in the absence of trapping agents afforded the eight-membered heterocycle [Sb2-{μ-(TerN)2P}2]. This constitutional isomer of a dimerized biradicaloid features a bonding situation that indicates the presence of a donor-stabilized [Sb2] 2+ ion.

The redissolved compound 2 exhibits a very broad 31 P NMR resonance (Fig. 4, δ( 31 P, 300 K, 500 MHz) = 332 ppm, ν 1/2 = 900 Hz) under ambient conditions indicating intramolecular dynamics.To shed some light into this dynamics, density functional theory at the pbe1pbe level (for details see the ESI ‡) was applied to study the potential energy surface (PES) of 2M (Fig. 5, M = model with the terphenyl group substituted by phenyl). 39,40In the gas phase, initially three isomers of 2M were considered: 2a, as well as the cyclic 1,3-dichloro-1-stiba-3-phospha-2,4-diazanes cis-and trans-[ClP(µ-NPh) 2 SbCl] 2b.These three isomers were found to be very similar in energy (relative to 2a: cis-2b +2.6, trans-2b +0.1 kJ mol −1 ).In addition, a higher-lying cyclic isomer 2c (30.3 kJ mol −1 ) bearing a SbCl 2 group was located at the PES when studying the intrinsic reaction for the different isomerization processes as depicted in Fig. 5. Isomer 2c is a typical intermediate lying in a very shallow energy valley, therefore cannot be observed in the 31 P NMR spectra.At low temperatures, two singlets were observed in the 31 P NMR spectra of a sample of redissolved crystalline 2a (193 K, 337.1 and 261.2 ppm), which were assigned to 2a and trans-2b (computed δ( 31 P): cis-2b 242, trans-2b 262, 2a 333 ppm).At −80 °C, a ratio of 18 : 1 was found for both species, so the coalesced signal is expected to occur at 331.2 ppm (observed at 100 °C: 327.2 ppm).We do want to stress that the assignment of trans-2b to the NMR resonance at 261.2 ppm is solely based on comparison with computational data.The 31 P NMR data with a coalescence temperature of 313 K indicate a Gibbs activation energy for the rearrangement reaction 2a → trans-2b of 49.5 kJ mol −1 .From a van't Hoff plot, the difference in the energy of both isomers was estimated to be 6.2 kJ mol −1 (cf.ΔE(0 K) = 0.1 kJ mol −1 for the gas phase).Interestingly, single crystal X-ray studies revealed the exclusive    presence of thermodynamically more stable open-chain species 2a in the solid state (yield: 72%, Fig. 6 left).The thermodynamical preference of the open chain species 2a over 2b and 2c is a clear difference to the lighter homologues.For example, the reaction of 1 with arsenic(III) chloride led to the formation of the cyclic arsa-phospha-diazane [ClP-(µ-NTer) 2 AsCl]. 38he structural motif observed for 2a resembles the recently studied NCN-substituted LSbCl 2 -compounds (e.g.L = amidinate donors) of Ragogna et al., 41,42 Jones et al., 43 Dehnicke et al., 44 and S. Schulz et al. 45 which are described as base stabilized dihalopnictanes.In accord with this concept and the fact, that only the open-chain species 2a was observed in the solid state, a short N-Sb bond length (N1-Sb1 2.133(4) Å, Fig. 6), clearly corresponding to a single bond (∑r cov (N-Sb) 2.11 Å), and a significantly longer Sb-N distance, indicating only a secondary interaction (N3-Sb1 2.392(4) Å), are observed.Furthermore, the in-plane Sb-Cl (Sb1-Cl2 2.451(2) Å) bond is elongated compared to the out-of-plane Sb-Cl bond (Sb1-Cl1 2.358(2) Å), as it is found for all these base stabilized pnictogen halides.NBO analyses (NBO = natural bond orbital) 46,47 indicate the donation of electron density from the lone pair (LP) located at the imino-N2 atom into the in-plane antibonding ∑*(Sb-Cl2) orbital. 40In addition, Menshutkin type (dispersion) interactions between one mesityl group of the terphenyl substituent with the Sb atom is observed, a common feature of such complexes.

Halide abstraction in 2
Ring closure in 2a was easily achieved by addition of a Lewis acid such as GaCl 3 via halide abstraction (Fig. 7), affording the cyclic red cationic species (3, δ( 31 P) 374 ppm, Fig. 6

Reduction of 2
Reduction of the yellow dichlorostibane 2 was studied using different reducing agents such as Mg in thf (Fig. 8 and 9) and KC 8 (Fig. 10 and 11) in benzene at ambient temperature.Both reactions are rather slow and need ambient temperature.Treatment of 2 with magnesium turnings in thf led to a species, which was first characterized by a singlet 31 P NMR resonance at 351 ppm (4, Fig. 9) indicating a di-coordinated phosphorus atom.Interestingly, only a minuscule change of the yellow  colour was observed.Single crystal X-ray structure elucidation unequivocally revealed the presence of a magnesium salt of the bis(terphenylimino)-phosphide anion (4).Remarkably, there is quite a significant difference in the 31  .This effect can be rationalized by consideration of the hardness (in the sense of the hard-and-soft-acid and base concept) and different cation radii (Li + : 0.73, Mg 2+ : 0.71 and K + : 1.51 Å for the coordination number four).Li + and Mg(Cl) + feature a small but very similar ion radius and thus a stronger downfield shift.The K + ion is considerably larger, hence the cation-anion separation is larger and the charge transfer is of lesser extent as confirmed by NBO analyses (NBO charge K 0.90675; Li 0.82680; second order perturbation theory analysis, maximal contribution: N-K 3, N-Li 13 kcal mol −1 ). 40From these data it can be concluded, that strongly interacting ion pairs (with a larger charge transfer) display a 31 P NMR shift for [(TerN) 2 P] − around 350 ppm, while for weakly interacting ion pairs this value is shifted to higher field (320-300 ppm).
The solvent clearly influences the reduction process, since the obtained product 4 contains coordinated solvent molecules.To avoid the formation of decomposition product 4, the reduction was carried out with KC 8 in benzene, which is a less coordinating solvent (Fig. 10).2][53][54][55][56][57][58] Finally, within the course of three hours the color of the reaction mixture turned orange.After filtration and concentration orange block-shaped crystals could be isolated in moderate yield (22%). Surprisingly, instead of the expected biradicaloid [P(µ-NTer    E 4 N 4 cage compound as illustrated in Fig. 1 52,[55][56][57][58][59][60] displaying a considerable degree of π-bonding (cf.∑r cov (Sb-Sb) = 2.8 vs. ∑r cov (SbvSb) = 2.66 Å). 59 Hence, the eight-membered heterocycle can also be considered as two condensed five-membered rings.The Sb-N bond lengths in 6 amount to 2.372(2) Å, which is considerably larger than the sum of covalent radii (∑r cov (Sb-N) = 2.11 Å). 59 Moreover, the short P-N distances of 1.613(2) Å are in the typical range of PN double bonds (∑r cov (PvN) = 1.62 Å). 59 Therefore, it can be deduced that the symmetrically bridging NPN ligand retains its anionic nature in the bridging mode, as indicated by these phosphorus-nitrogen bond lengths, which is similar to analogous structures with μ-N,N′ bridging complexes (cf.averaged P-N values 1.601(2) in 1, 1.611 Å in 4).The molecular structure of heterocycle 6 is reminiscent to the (NCN) 2 P 2 scaffold of [P 2 {µ-(MesN) 2 CNMes} 2 ]. 42,61 However, unlike the planar N 4 P 2 Sb 2 bicyclic fragment in 6, the (NCN) 2 P 2 scaffold has a puckered arrangement of the two five-membered P 2 N 2 C rings, where the two planes are 100.4°toeach other (Fig. 4). 42,61Additionally, the molecular geometry about the phosphorus atoms is trigonal pyramidal and the P-P distances are in the range of typical P-P single bonds.Recently, Jones et al. reported on base-stabilized diarsenes [As 2 -{µ-(ArN) 2 -CR} 2 ] (Ar = 2,6-diisopropyl-phenyl, R = N( c Hex) 2 , N( i Pr) 2 , t Bu) 43 displaying also a planar structure of the central structural motif along with a AsvAs double bond similar to the situation found in 6 (Fig. 10).It is interesting to note that the isovalence-electronic triazenide species, [N(µ-NTer) 2 Sb], 62 exists as a monomeric compound and does not dimerize to form the distibenium bicyclic species.
According to DFT calculations, 40 compound 6 possesses a closed shell singlet ground state, hence it does not exhibit the typical high reactivity of biradicaloids, e.g. it does not react with small molecules as alkynes, phosphaalkynes, isonitriles or diphenyldiazene.In the 31 P NMR spectrum, a singlet resonance at 325.7 ppm is observed for 6, which is very similar to the value found for the "free" solvated [(TerN) 2 P] − ion as in the potassium salt 1 (322-324 ppm, vide supra), also indicating the ionic nature of the interaction between the [(TerN) 2 P] − and [Sb 2 ] 2+ moieties.To gain further insight into the bonding situation of 6, MO (MO = molecular orbital), NBO and ELF (electron localization function) [63][64][65] computations were carried out.Analysis of the frontier orbitals supports the existence of a highly localized SbvSb double bond, as illustrated by the HOMO−1 (Fig. 12, HOMO−1 π bond, HOMO−2 σ bond, HOMO = highest occupied molecular orbital).The LUMO (lowest unoccupied molecular orbital) represents the antibonding counterpart, thus a π bond order of one can be considered.While the HOMO is NPN ligand centered, HOMO−12 features large coefficients for the lone pairs at the Sb atoms, which possess considerable s character.
In accord with this MO picture, NBO analyses find as the best Lewis representation a [Sb 2 ] 2+ ion bearing a double bond that interacts with two [(TerN) 2 P] − ions (Fig. 10).[74][75][76] In 6, the SbvSb double bond domain is characterized by a dumb-bell shaped region of localized electrons (ELF = 0.75) in accord with the description of a classical double bond in a planar environment (e.g. in ethylene C 2 H 4 ) and in contrast to the situation of non-classical double bonds (slipped double bonds) usually found between heavy main group elements. 63,74However, as pointed out by Grützmacher and Fässler dipnictogens of the type REvER (E = N-Bi) fall within the category of classical double bonds displaying a strong resemblance to the situation in ethylene. 74inally, we want to address the question why dimerization occurred via Sb-Sb linkage and ring opening in the biradicaloid intermediate [P(µ-NTer) 2 Sb] rather than cage formation (vide supra).For this reason, we computed the biradical character β (β = c 2 2 /(c 1 2 + c 2 2 ) utilizing the method reported by Miliordios et al. 77 The coefficients c 1 and c 2 were obtained from the complete active space self-consistent field (CASSCF(2,2)) level of theory.According to these computations, the biradical character of [P(µ-NTer) 2 Sb] amounts to only 8%, which is rather small for dipnictadiazanediyls (cf.25-33% for [E(µ-NTer)] 2 , E = P, As, cf.ESI 4.4.‡). 62Nevertheless, biradicaloid reactivity was experimentally observed for [P(µ-NTer) 2 Sb] ("monomeric" 6).

Experimental section
All manipulations were carried out under oxygen-and moisture-free conditions under argon using standard Schlenk or

Dalton Transactions Paper
This journal is © The Royal Society of Chemistry 2016

Synthesis of 3
To a solution of [Ter 2 N 2 PSbCl 2 ] (191 mg, 0.217 mmol) in dichloromethane (4 ml), a solution of GaCl 3 (39 mg, 0.221 mmol) is added dropwise at −80 °C.The initially yellow solution immediately turns red and is stirred for further 15 minutes at the same temperature before being warmed to 20 °C.The solution is then concentrated until crystallization commences (0.5 ml) and left undisturbed overnight, which leads to the formation of red needle-shaped crystals (168 mg, 0.159 mmol, 73%).Crystals suitable for X-ray structure elucidation were obtained by repeated recrystallization from dichloromethane between 25

Synthesis of 4
[Ter 2 N 2 PSbCl 2 ] (200 mg, 0.228 mmol) and magnesium turnings (80 mg) were combined in a flask.To this mixture, 10 ml THF was added and the suspension was stirred overnight at ambient temperature with a glass stirring bar.Out of the initially yellow solution a black precipitate was formed.Volatiles were removed in vacuo and the residue was extracted with 10 ml benzene and washed with an additional 3 ml of benzene.The combined filtrate was concentrated to incipient crystallization (approx. 2 ml) and left undisturbed overnight, resulting in the deposition of light yellow crystals.The supernatant was removed via syringe and the crystals were dried in vacuo (130 mg, 0.159 mmol, 70%).

Synthesis of 5
[Ter 2 N 2 PSbCl 2 ] (180 mg, 0.205 mmol), diphenylacetylene (53 mg) and magnesium turnings (80 mg) were combined in a flask.To the mixture, 10 ml thf was added.After stirring with a glass-covered stirring bar, the initially yellow solution turned orange and a black precipitate formed.Volatiles were removed in vacuo and the residue was extracted with 5 ml benzene and then washed with another portion of 3 ml of benzene.The extract was then concentrated to incipient crystallization and left undisturbed overnight, affording orange crystals.The mother liquor was removed via syringe and the crystals were dried in vacuo (82 mg, 0.083 mmol, 40%).

Synthesis of 6
[Ter 2 N 2 PSbCl 2 ] (215 mg, 0.245 mmol) was dissolved in 5 ml of benzene while stirring with a glass stirring bar.To the solution, KC 8 (70 mg, 0.518 mmol) was added.The solution adopted a dark green colour after 5 minutes.To ensure completion of the reaction, the suspension was stirred for 3 hours at ambient temperature.The suspension was filtered over a sinter padded with kieselguhr (Celite) and the residue was washed with another portion of 2 ml of benzene.The combined filtrate was concentrated to incipient crystallization (approx. 1 ml) and left undisturbed overnight.Orange blockshaped crystals were obtained.The supernatant was removed via syringe and the crystals were dried in vacuo (43

Fig. 1
Fig. 1 Reduction of cyclodichlorodiphosphadiazane A yielding biradicloid B or its dimers C and D depending on the steric strain.

Fig. 3
Fig. 3 Preparation of NPN-substituted dichlorostibane 2a and its equilibrium with trans-2b as indicated by 31 P NMR studies.

Fig. 5
Fig. 5 Potential energy surface of 2M (M = model, terphenyl group substituted by phenyl; the isomerisation process, leading to cis-2b, was omitted for clarity, see the ESI; ‡ TS = transition state).
right) and the GaCl 4 − ion.The formal charge in 3 is located on the dicoordinated P atom, hence it will subsequently be denoted as the phosphenium cation, even though the NBO charges indicate stronger charge localization on Sb than on P (Sb +1.61, P +1.36e).The large positive charge at the Sb center mainly arises from strongly polarized Sb-N σ-bonds.Interest-ingly, upon halide abstraction the phosphenium cation is formed rather than the stibenium cation.Also there is no exchange of the Cl − ion between the P and Sb atoms of the four-membered ring in solution, which can be observed on the NMR time scale, in accord with computation indicating that the P + centred species is considerably favoured over the Sb + centred ion by 59.3 kJ mol −1 .The hypothetical stibenium cation [ClP(μ-NTer)Sb] + , that could result from Cl shifting from Sb to P, would have been expected to exhibit green colour 48,49 (see computations in the ESI ‡).The P + center in [ClSb(μ-NTer)-P] + can be better stabilized by delocalization of the nitrogen lone pairs (LP) into the empty p atomic orbitals (AO) of the P + ion than the Sb + center in hypothetical [ClP(μ-NTer)Sb] + due to the relative orbital mismatch for the donation of an N lone pair into a 3p (P) or 5p (Sb) acceptor orbital in accordance with NBO analysis data.This formal p-LP(N) → p-AO(P) hyperconjugation accounts for a significant π-bond character along the N-P-N unit (see below).Thus both P-N distances are rather short and display a double bond character (P1-N2 1.625(3), P1-N1 1.643(4) Å, cf.∑r cov (N-P) 1.82 Å), 38,49 while the Sb-N distances correspond well with single bonds (Sb1-N1 2.136(3), Sb1-N2 2.171(3), ∑r cov (N-Sb) 2.11 Å).
) 2 E] (E = P, As, Sb) do not activate small molecules, e.g.alkynes, to form [2.1.1]bicycles,but possess similar β values compared to [P(µ-NTer) 2 Sb] ([N(µ-NTer) 2 E], β: E = P 15%, As 10%, Sb 6%, Bi 0%).Thus the success of the trapping reaction of [P(µ-NTer) 2 Sb] with alkynes to form [2.1.1]bicycle5 was unexpected (see Fig.9), as well as the formal dimerization to 6 if no trapping reagent was available.We had expected a kinetically stabilized [P(µ-NTer) 2 Sb] biradical as it was the case for [E(µ-NTer) 2 E] (E = P, As) or [N(µ-NTer) 2 E] (E = P-Sb).However, compared to the lighter congeners [P(µ-NTer) 2 Sb] features rather long N-Sb bonds and acute N-Sb-N angles due to predominantly ionic bonding thus providing less steric protection around the antimony and allowing dimerization.Interestingly, for 6 the computed biradical character amounts to zero and therefore, a conventional closed-shell species is anticipated.This confirms the observation, that no activation of small molecules bearing multiple bonds was achieved, even though steric congestion contributes to the inertness of 6 as well.ConclusionsIn conclusion, the initially targeted biradicaloid [P(µ-NTer) 2 Sb] remains elusive, its existence as transient species, however, could be corroborated by trapping with diphenylacetylene forming a [2.1.1]bicycle.In the absence of a trapping reagent, [P(µ-NTer) 2 Sb] dimerizes via Sb-Sb double bond formation affording a novel planar eight-membered N 4 P 2 Sb 2 heterocycle (6).Due to the highly polar Sb-N bond situation, the formal dimer 6 might also be regarded as a donor stabilized [SbvSb] 2+ ion in accord with MO, NBO and ELF considerations.
if smaller substituents are utilized.The result is that compound 6 is devoid of biradicaloid character.Notably, for the phenyl substituted model compound 6M, all possible cage compounds are thermodynamically more favoured compared to the eightmembered heterocyclic isomer (ca.35-79 kJ mol −1 , see the ESI ‡).This situation changes dramatically when the terphenyl substituted experimentally observed compound 6 is considered.In this case, the cage compounds do not represent a minimum on the PES anymore.Obviously, the folding about the Sb-Sb axis, which would lead to the formation of α-cage compound 6, is hindered for steric reasons.Bulkier substituents may cause stabilization of the monomeric stiba-phosphadiazanediyl and prevent dimerization.It is worth mentioning that in an attempt to obtain the putative intermediate [(Ter 2 N 2 PSbCl) 2 ] by reduction with one equivalent of KC 8 , only the unreacted starting material and 6 were isolated.