That main group-centered biradicals can be used to activate small molecules has been shown several times.^1-7 In particular, formal cyclo-butadienyl derivatives of the Niecke biradical, [⋅CCl(μ-Mes*P)₂C(Cl)⋅] (1,3-diphospha-cyclo-butane-2,4-diyle, Mes*=2,4,6-tri-tert-butylphenyl=“super-mesityl”),^{5, 8-14} such as [⋅P(μ-N−R)₂P⋅],¹⁵ are particularly well suited, and so it is not surprising that they can activate molecules with single and multiple bonds, often leading to [2.1.1] bicycles (Scheme 1).^{6, 16} In recent years, we have focused primarily on four-membered pnictogen-centered biradicals of the type [⋅P(μ-N−R)₂P⋅] (6R) with R=Ter (=2,6-Mes₂C₆H₃, Mes=2,4,6-Me₃C₆H₂), Hyp (=tris(trimethylsilyl)-silyl),¹⁷ as well as Bbp (=2,6-bis[bis(trimethyl-silyl)methyl]phenyl)¹⁸ and have shown that these phosphorus-centered biradicals are particularly suitable for molecular activation applications.¹⁹ These biradicals (6R) can be prepared easily starting from the 1,3-dichloro-cyclo-diphosphadiazanes (5, Scheme 2) by reduction with magnesium turnings in almost quantitative reactions. However, to prevent oligomerization (especially dimerization), the substituent on the nitrogen must be sterically very demanding.^{6, 20} For this purpose, the Ter substituent on nitrogen has been mainly used, but also the Hyp or Bbp substituents. Since we are also interested in getting rid of these sterically demanding substituents for follow-up chemistry, we looked at trityl-based substituents, hoping to split them off by strong nucleophiles. In this context, we have worked out a new synthetic route for the preparation of a trityl-substituted biradical, [⋅P(μ-N-^tBuPh₃C)₂P⋅] (6T), bearing the tris(4-tert-butyl-phenyl)methyl-substituent (^tBuPh₃C=T, Scheme 2). The synthesis as well as the use of the biradical 6T in the reaction with tolan, which led to a very unusual result, is reported here.

We started this project by developing a synthesis route for the unknown biradical [⋅P(μ-N-^tBuPh₃C)₂P⋅] (6T). Starting from 1-bromo-4-tert-butylbenzene, we succeeded in isolating 6T in a six-steps synthesis route as displayed in Scheme 2. The first three steps represented high-yielding processes (yields: 99 (1), 83 (2), and 97 % for 3), while the synthesis of tris(4-tert-butylphenyl)amino(dichloro)phosphane (4) gave only a yield of 46 % due to frequent washing of the reaction mixture to remove LiCl (see ESI). Addition of a strong base such as Et₃N to aminodichlorophosphane 4 yields 1,3-dichloro-2,4-tris(4-tert-butylphenyl)methyl-cyclo-1,3-diphospha-2,4-diazane (5) in a straight forward reaction (yield: 79 %). The final step, the reduction with magnesium turnings afforded the formation of biradical 6T in an almost quantitative reaction. Unfortunately, we faced a separation problem, as biradical 6T displayed a low solubility like MgCl₂ in organic solvents such as THF, benzene, fluorobenzene, CH₂Cl₂, DME or toluene. To prove the existence of 6T, we reacted the mixture biradical 6T / MgCl₂ with tolan because it is known that biradicals of type 6R (R=Ter, Hyp) react to give [2.1.1.] bicyclic compounds such as 7. Indeed, after completion of the reduction with magnesium as indicated by ³¹P NMR reaction control, addition of tolan led to a soluble product which was identified by its characteristic chemical shift in the NMR spectrum (δ[³¹P{¹H}]=231, cf. 246 ppm 6Ter⋅tolan).¹⁹ To generate more tolan addition product 7, we performed the reaction a second time, but this time we added tolan along with the Mg turnings in a one-pot synthesis to save time (Scheme 2). To our surprise, this resulted in the formation of a completely different reaction product, namely a magnesium double salt with the formal composition [Mg(THF)₂]²⁺[(^tBuPh₃C−N)₂P]⁻[N(P₂C₄Ph₄)]⁻. Single crystals suitable for X-ray structure analysis could be obtained from a saturated THF solution at ambient temperatures, so that the molecular structure of the magnesium salt could be unequivocally determined (Figure 1). Besides single crystal structure elucidation, magnesium salt 8 was fully characterized by means of elemental analysis, NMR and vibrational spectroscope (see analytical data). For example, the ³¹P NMR data in C₆D₆ solution clearly show the presence of a separated NPN (389.9) and PNP moiety (158 ppm); both signals appeared as singlets. Compound 8 can be prepared in bulk, is stable under argon atmosphere and decomposes above 130 °C.

Simple stoichiometric consideration led to the assumption that four equivalents of magnesium and tolan reacted with three equivalents of dichloro-cyclo-diazadiphosphane 5 to afford two equivalents of magnesium salt 8 along with two equivalents of MgCl₂ and ^tBuPh₃C−Cl (Scheme 3). The formation of ^tBuPh₃C-Cl was proven by NMR spectroscopy (see ESI). It should be noted that this formal stoichiometric reaction does not represent the reaction mechanism. It can be assumed that this heterogenous reaction includes donor-stabilized magnesium(I) species²¹ as well as oligomers of the type [P₂N₂R_xCl_y ⋅ (tolan)_z]ⁿ⁻ stabilized by complexation at the magnesium centre.

Due to complex reaction, we are not able to propose a reaction mechanism. However, the formal [R−N−P−N−R]⁻ anion has been observed and isolated in the reaction of 6Ter with NHCs (N-Heterocyclic Carbene) yielding [P(NHC)₂]⁺[Ter−N−P−N−Ter]⁻.²² The latter reaction means that a formal P⁺ ion was excised from the biradical 6Ter and stabilized by two NHCs, which, however, is impossible in the course of the formation of 8. Hence, we assume that both anions in 8 are formed from a formal “N₃P₃” trimeric precursor stabilized as ligand at the magnesium cation to split into [NPN]⁻ (=[(^tBuPh₃C−N)₂P]⁻) as well as a [PNP]⁻ (=[N(P₂C₄Ph₄)]⁻) ions upon reaction with the tolan. Especially the latter anion, [N(P₂C₂Ph₄)]⁻, is very unusual as it represents a formal 1,4-diphospha-benzene (also known as 1,4-diphosphinines)^23-27 that is bridged by a N⁻ ion. Furthermore, we would like to point out that the reaction can also proceed without in-situ biradical formation (6Ter), for example, by first reducing the tolan, which can then react with the dichlorophosphane 7.

Orange block-shaped crystals of magnesium salt 8 crystallize in the triclinic space group P with two formula units per unit cell (Figure 1). In the crystal, magnesium salt 8 forms a molecular complex of the type [Mg(THF)₂]²⁺[NPN]⁻[PNP]⁻ (with [NPN]⁻=[(^tBuPh₃C−N)₂P]⁻) and [PNP]⁻=[N(P₂C₄Ph₄)]⁻, Scheme 2). Between these molecular complexes in the solid no significant short intermolecular distances, which are indicative for significant interactions, were found as the inorganic core Mg[NPN][PNP] sits in a pocket formed by two ^tBuPh₃C substituents and two C₂Ph₂ substituents. The coordination sphere around the Mg²⁺ ion is best described as distorted trigonal bipyramidal with the two oxygen of the two coordinating THF molecules in the axial position (∢(O1−Mg−O2)=175.7(1)°, d(Mg−O1)=2.172(2) Å, d(Mg−O2)=2.177(2) Å; Σr_cov(Mg−O)=2.02 Å).²⁸ The formal trigonal plane is formed by the two N atoms of the [NPN]⁻ ligand (∢(N1−Mg−N2)=70.1(1)°; d(Mg−N1)=2.134(3) Å, d(Mg−N2)=2.133(3) Å; Σr_cov(Mg−N)=2.10 Å)²⁸ and the apical N atom of the [PNP]⁻ ligand (d(Mg−N1)=2.129(3) Å). As expected, compared to the rather acute N1−Mg−N2 angle, the N3−Mg−N1 and N3−Mg−N2 angles are strongly increased (145.0(1)°, 144.9(1)°). The N1−P1−N2 angle is considerably smaller than the P2−N3−P3 angle with 109.2(2)°. Both N−P bonds of the [NPN]⁻ ion are significantly shorter with 1.606(3) and 1.610(3) Å compared to the N−P bonds of the [PNP]⁻ ion (1.653(2) and 1.638(3) Å). Therefore, partial double bonding can be discussed for the P−N bonds in [NPN]⁻, while the P−N bonds in [PNP]⁻ represent highly polarized single bonds with a significantly smaller degree of π bonding (cf. Σr_cov(P−N)=1.82 Å, Σr_cov(P=N)=1.62 Å,²⁹ including polarization effects according to Schomaker and Stevenson: Σr_cov(P−N)=1.76 Å, Σr_cov(P=N)=1.52 Å).³⁰ It should be noted that the small degree of π bonding within the [PNP]⁻ ion (with tri-coordinated P atoms) can be attributed to hyperconjugative effects^{31, 32} between the lone pair (LP) of the N atom (p-type atomic orbital=p-AO) and the four antibonding P−C bonds (p-AO-LP(N)→σ(P−C) bonds, E²=40.6 kcal mol⁻¹) according to natural bond analysis (NBO, see below).^{33, 34} The P−C bonds between 1.89 and 1.90 Å are in the range of a typical single bond (Σr_cov(P−C)=1.86 Å).²⁸ As expected, the C−C bonds within the six-membered P₂C₄ ring are in the range of a classic double bond (1.344(5) and 1.356(5) Å; Σr_cov(C=C)=1.34 Å).²⁸ The C1−P2−P3−C4 torsion angle is 98.8(2)°, thus featuring a puckered six-membered P₂C₄ heterocycle, while both P₂C₂ moieties are almost planar with a deviation from planarity smaller than 2°.

NBO and MO calculations were performed (NBO=Natural Bond Orbital, MO=Molecular Orbital, PBE0/def2svp+gd3bj) to get insight into the charge transfer and bonding within coordination compound 8. The energetically most favoured Lewis formula for 8 according to NBO analyses is Lewis formula A in Scheme 4, which features formally separated Mg²⁺, [NPN]⁻, [PNP]⁻ ions and two neutral THF molecules. However, as indicated by the donor-acceptor energies as well computed NPA charges (Natural population analysis), there are donor-acceptor interactions between the ligands and the central Mg²⁺ ion, which is also illustrated by the partial charge of magnesium with +1.77e corresponding to a charge transfer of ca. 0.23e (Figure 2). Interestingly, amongst the donors are also all P atoms of the two ligands, which interact via their lone pair with the empty s-orbital localized at the Mg²⁺ ion. The donor-acceptor energies involved in these interactions are in the range between 5–15 kcal mol⁻¹. A closer look at the [NPN]⁻ ion reveals the delocalization of the π bond as depicted in the resonance B ↔ C (Scheme 4). Delocalization of one of the two lone pairs (which sits in a p-AO) at the formally negatively charged nitrogen atom in the [PNP]⁻ ion into the four σ*(P−C) bonds also introduces some π character along the P−N−P unit, while weakening the P−C bonds (resonance D ↔ H). Delocalization of the p-type LP at the apical N is also evident in the MO picture as illustrated by the HOMO (Figure 3). The P−N single as well as double bonds in both anions are also strongly polarized towards the N atom ([NPN]⁻: σ(P−N) 26/74 %, π(P−N) 23/77 %; [PNP]⁻: σ(P−N) 27/73 %).

In conclusion, we have been able to show that depending on the reaction sequence biradical 6T, generated from Mg and [Cl−P(μ-N-^tBuPh₃C)₂P−Cl] (5), can either form a [2.1.1] bicyclic compound in the reaction with tolan, or an ion complex of the type [Mg(THF)₂]²⁺[NPN]⁻[PNP]⁻ featuring a novel [NPN]=[N(P₂C₂Ph₄]⁻ ion, when the reduction process is carried out in the presence of tolan. This novel anion can also be regarded as an anionic [2.2.1] bicycle with a [N]⁻ ion trans-annular bridging a 1.4-diphosphabenzene.