Synthesis of a Molecule with Five Different Adjacent Pnictogens

Abstract The first molecular compound with all five pnictogens was obtained by a multi‐step reaction. Lithiation of the (bisamido)diazadiarsetidine (tBuNAs)2(tBuNH)2 in aliphatic solvents leads to the dimeric metallated species [(tBuNAs)2(tBuNLi)2]2 (12). Upon reactions with AsCl3, SbCl3 and BiCl3 the polycyclic compounds [(tBuNAs)2(tBuN)2]PnCl (Pn=As (2), Sb (3), Bi (4)) can be obtained. Conversion of 2–4 with [tBu2SbP(tBu)Li(OEt2)]2 leads to the remarkable interpnictogens [(tBuNAs)2(tBuN)2]PnP(tBu)SbtBu2 (Pn=As (5), Sb (6), Bi (7)), whereby 7 is the first example of a molecule containing all five Group 15 elements. The compound with adjacent AsNBiPSb‐chains is surprisingly stable and does not show high sensibility against light as the labile Bi−P bond might suggest.

Crystallizationo f1 from n-pentaneo rD ME (DME = 1,2-dimethoxyethane) yields different molecular structures:W hen using n-pentane, 1 crystallizes in the monoclinic space group C2/c as ad imer ([(tBuNAs) 2 (tBuNLi) 2 ] 2 (1 2 )). The molecule consists of three stacked distorted cubes so that each lithium ion is coordinated by four nitrogen atoms (Figure 1l eft). Furthermore, agostici nteractions of the methyl groupso ft he tert-butyl moieties to Li can be observed leadingt oadistorted trigonal bipyramidal coordination of each lithium atom. As imilar structure motiveh as been observed within [(cyNAs) 2 (cyNLi) 2 ] 2 ,b ut the segments deviate more from ideal cube-like shapes leading to adistorted tetrahedral conformationofNa nd Li. [34] When crystallizing from DME, the structure consists of infinite chains of monomeric units of 1 being bridged with one molecule of DME on each Li atom (1·dme,F igure 1r ight). It crystallizes in the monoclinic space group P2 1 /n with four formula units per unit cell.
When adding an equimolars olution of PnCl 3 (Pn = As, Sb, Bi) to 1 dissolved in DME at À60 8Ct he tricyclic compounds 2-4 can be obtained in good to excellent yields (62-95 %). 3 and 4 have already been prepared in the past, but for 4 no analytical data were provided. [32,33] NMR spectroscopy in C 6 D 6 of the compounds 2-4 was conducted at room temperature and for the 1 Hn ucleus (300 MHz) the resultsu nexpectedly differ (see Supporting Information ford etails). For 2 the spectrum exhibits two singlets:O ne located at 1.24 ppm with an integral of 9 and the other one at 1.46 ppm with an integral of 27 hydrogen atoms. The spectrum for 3 shows three singletsa t1 .10, 1.31 and 1.64 ppm with integrals of 9, 9a nd 18 protons and for 4 two singlets can be observed at 1.18 and 1.51 ppm with integrals of 18 protons each. To resolve these discrepancies, temperatured ependent 1 HNMR spectra for 2-4 were conducted in [D 8 ]toluene( 500 MHz) in ar ange from 190 Kt o3 50 K( see Figures S18-S20). Therein it can be seen that at 190 Kf or all compounds (2-4)t he signals for the tert-butyl moieties located at the (AsN) 2 ring split into two signals with an integral of 9 protons each. Coalescence can be observed for 4 at av alue of about 250K.F or 2 and 3 the same trend is observable, whereby an exact temperature of coalescencecannotbedetermined. Due to increasingb roadening of the signals of 2 it can be assumed that coalescence probably occurs at around3 60 K. The counterintuitive pattern of the signals of 2 at room temperature can be accountedt oac rossingo ft he signals randomly leadingt oas harp singlet with the integral of 27 protons and as econd signal with an integral of 9. The coalescence in 2 can be explain by swinging of the As-Cl entityf rom one side to the other. 3 and 4 show additional coordination sites of the Sb/Bi atom in the solid-state structure (see below) and most likely ap yramidal inversiont akes place by changing the coordinating nitrogen atom. Such an inversionw as also described within the analogous (bisamido)diazadiphosphetidine by L. Stahl and co-workers. [22] Calculation of the activation energy Scheme1.ReactionSchemef or the preparation of the (bisamido)diazadiarsetidine, [32] its lithiation to 1 2 ,aswellasthe furtherconversion with the pnictogent richlorides to 2-4 and finallythe reaction with thel ithiated di-tertbutyl-stibino-tert-butyl-phosphane to 5-7. for the inversion of 2-4 using the line shape analysisy ields an activation energy of 59(2) kJ mol À1 for 2,6 2(3) kJ mol À1 for 3 and 47(3)kJmol À1 for 4.T he reaction of 1 with PCl 3 was also carried out ac ouple of times, but as can be seen in the solidstate structure ( Figure S1) ar eorganization of the bicyclus takes place and am ixed AsN 2 Pr ing is formed.
Suitable single crystals of compound 2 can be obtained from as olutioni nt oluene at À30 8Ca sc olourless blocks. It crystallizes in the monoclinic space group P2 1 /m with two formula units per unit cell (Figure 2). The structure is isotype to the solid state structures of the analogous diazadipnictetidines (tBuNP) 2 (tBuN) 2 PnCl (Pn = P, As). [19] Single crystals of compound 4 can also be obtained from toluene at À30 8Ca sy ellow blocks. It crystallizesw itho ne formula unit per unit cell in the triclinic space group P1 containing additionally half am olecule of toluene. In the solid state, compound 4 is dimeric due to an additional secondary bonding from achlorine atom to the bismuth atom of the next molecule. Due to af urther coordination of the Bi atom by N2 the coordination number of Bi is five (Figure 3).
When using the compounds 2-4 as educts for further conversion with the lithiatedd i-tert-butyl-stibino-tert-butyl-phosphane, the compounds[ ( tBuNAs) 2 (tBuN) 2 ]PnP(tBu)SbtBu 2 (Pn = As, Sb, Bi) 5-7 can be obtained in moderate to good yields (Scheme1). [8] 5 and 6 represent novel quaternary and 7 the first quinternary interpnictogen molecule ever described. For the preparation it is of crucial importance to work with exact stoichiometriesd ue to ah indered crystallization when byproducts or excessive reactants are present.H owever,w hen trying to synthesize 5,t he reactioni ss imply too unselective to obtain ac lean product. The ratios of the different compounds can be affected by varying solvents and temperature, but the amount of different compounds being formed is too high and af ractional crystallization was not successful.I tw as once possible to obtain single crystalso ft he compound 5 to perform single crystal X-ray diffraction, but due to bad crystal quality the data were of poor quality( Figure S2). One side product was identified as the diarsane[ {(tBuNAs) 2 (tBuN) 2 }As] 2 formed by reduction of 2.Inthe 31 P{ 1 H} NMR spectra of 5-7 the arsenic compound 5 shows as ignificantly more low-field-shifted signal at 61.7 ppm compared to 6 or 7 with 22.5 and 37.6 ppm, respectively.
In contrast to the compounds 1-6 which are colourless or yellow, 7 has an intensivered colour.Int he UV/Vis spectrum of 7,amaximum appearsi nt he UV region at 352.5 nm with molar extinction coefficients of 7.2 10 5 lm ol À1 m À1 (FigureS4). Furthermore there is as trong band in the visible region located at 441.5 nm with am olar extinction coefficient of 2.9 10 5 lmol À1 m À1 ,e xplaining the intensiver ed colouro ft his compound. For comparison we also measured the UV/Vis Spectrum of 6 whicho nly shows minor absorbance in the visible region. For the elucidation of these differences we calculated [41] the electronic excitation energies for both compounds with timedependentd ensity functional theory using the PBE0 functional [42] for optimized structure parameters. We employed the quasi-relativistic (one-electron) exact two-component method, X2C, [43,44] with triple zeta bases [45] and integration grids [46] optimized for this purpose. The experimental resultsa re well reproduced, as shown in Figure S5, apart from ac onsequent blueshift by ca. 30 nm. In particular, like in the measured spectra, in 7 the first excitation peak (at 411nm, marked in red in Figure S5) is well separated from the others, while in 6 it is not (first peak at 348 nm). In both compounds, the first excitation has almost pure HOMO-LUMOc haracter,t he HOMO in both cases is localized dominantly at the Pa tom, the LUMO shows large contributionsf rom Bi1/Sb1. The differencei nt he excitation energies of ca. 0.5 eV is in line with the difference of the HOMO-LUMO gaps (ca. 0.6 eV), which mainly arises from different LUMO energies (7: À1.11eV, 6: À0.622 eV). Responsible for these differences are both the changeo ft he pnictogenea nd the slightly different molecular structure. In order to show this, we optimized two hypotheticals tructurers, 6Bi with Bi at the central position startingf rom the structure parameters of 6, and 7Sb with Sb at the central position starting from the structure parameters of 7.F or both structures, which are about 5kJmol À1 higheri ne nergy than the correspondingo riginal structures 6 and 7,n ol ow-energy peak is found, but the first excitationsa re close to that of compound 6 (350 nm for 6Bi and 366 nm for 7Sb).

Experimental Section
General See the Supporting Information.