Impact of high π-density on the coordination properties of π-excess aromatic neutral σ 2 P ligands – P ( π )-donor bonds to Ag + and HgCl 2 †

Trivalent phosphorus compounds are widely used ligands in coordination chemistry and homogenous late transition metal catalysis, but studies on ligands with dicoordinated (σP) phosphorus are restricted almost completely to phosphabenzenes and some phosphaalkenes. Complexes with π-excess-aromatic σP-azaphosphole ligands, apart from metal(0) complexes, are extremely sparsely documented, and only one, a 1,2-azaphosphole allylnickel chloride complex, has been characterized by crystal structure analysis. Complexes of the π-rich 1,3-azaphospholes with phosphorus in conjugation to nitrogen (N–CvP ↔ NvC–P) are still unknown for non-zerovalent transition metals. We report here the first examples with d metal cations; these 1,3-benzazaphosphole Ag(I) and HgCl2 complexes display a hitherto unprecedented coordination pattern. Structurally characterized σP–Ag(I) complexes are very rare, and we are not aware of any Hg(II) complexes with σP-ligands. Addition of toluene and NH3 to an equimolar mixture of 1 9 and AgCl at −60 °C and warming to r.t. led to the formation of an inversion-symmetric benzazaphosphole silver chloride complex with a central Ag2Cl2 ring (Scheme 1). Colourless crystals of 2 were obtained on overlayering a concentrated solution in THF with hexane. The crystal structure analysis (Fig. 1) displays distorted tetrahedral coordination of Ag with an interplanar angle of 80.8° between the Ag2Cl2 and the P3–Ag–P3′ planes. The P–Ag bonds do not lie within the extended ligand ring plane, but are tilted away by 32.4 and 12.7°. The bond Scheme 1 Formation of silver complexes of 1 (Np = neopentyl) and 3 (schematic structure presentation of 3).

Trivalent phosphorus compounds are widely used ligands in coordination chemistry and homogenous late transition metal catalysis, but studies on ligands with dicoordinated (σ 2 P) phosphorus are restricted almost completely to phosphabenzenes 1 and some phosphaalkenes. 2Complexes with π-excess-aromatic σ 2 P-azaphosphole ligands, apart from metal(0) complexes, 3,4 are extremely sparsely documented, 5,6 and only one, a 1,2-azaphosphole allylnickel chloride complex, 6 has been characterized by crystal structure analysis.Complexes of the π-rich 1,3-azaphospholes with phosphorus in conjugation to nitrogen (N-CvP ↔ N + vC-P − ) are still unknown for non-zerovalent transition metals.We report here the first examples with d 10 metal cations; these 1,3-benzazaphosphole Ag(I) and HgCl 2 complexes display a hitherto unprecedented coordination pattern.Structurally characterized σ 2 P-Ag(I) complexes are very rare, 7,8 and we are not aware of any Hg(II) complexes with σ 2 P-ligands.
Addition of toluene and NH 3 to an equimolar mixture of 1 9 and AgCl at −60 °C and warming to r.t.led to the formation of an inversion-symmetric benzazaphosphole silver chloride complex with a central Ag 2 Cl 2 ring (Scheme 1).Colourless crystals of 2 were obtained on overlayering a concentrated solution in THF with hexane.The crystal structure analysis (Fig. 1) displays distorted tetrahedral coordination of Ag + with an interplanar angle of 80.8°between the Ag 2 Cl 2 and the P3-Ag-P3′ planes.The P-Ag bonds do not lie within the extended ligand ring plane, but are tilted away by 32.4 and 12.7°.The bond Scheme 1 Formation of silver complexes of 1 (Np = neopentyl) and 3 (schematic structure presentation of 3).lengths and angles within the ligand are only marginally changed (P-C3a slightly shortened) compared to 1. 9 A cationic silver complex of a different coordination type was formed in THF at room temperature from a mixture of 1 and AgSbF 6 .Crystals of 3 were grown by slow concentration of a solution of the pale brown crude product in CD 3 OD.They proved to be less stable than 2 and decomposed within a few days, but crystallographic analysis of freshly prepared crystals of 3 clearly revealed two [Ag 4 (1) 4 THF 4 (SbF 6 ) 2 ] 2+ clusters with μ-P coordination and four uncoordinated SbF To find out if divalent mercury is also able to form complexes with 1,3-benzazaphospholes, the reaction of 1 with HgCl 2 in THF was explored.Oxidation of phosphorus to a λ 5 P heterocycle as reported for the reaction of 2,4,6-triphenylphosphabenzene with Hg(OAc) 2 10 was not observed.The NMR spectra of the yellow solid, with a marked upfield 31 P coordination chemical shift in C 6 D 6 (Δδ 31 = −84.8ppm), indicated complex formation, but attempts to grow single crystals for a detailed structural characterization failed.A crystalline 1,3benzazaphosphole-HgCl 2 complex was finally obtained from the recently reported σ 3 P,σ 2 P-hybrid benzazaphosphole 4 11 and HgCl 2 in THF (Scheme 2).XRD analysis of the yellow single crystals of 5 (Fig. 2), grown from concentrated THF solution, showed a P,P′-chelate structure as expected.However, whereas in the recently reported Mo(0)(CO) 4 (P,P′)-chelate complex with 4 the zero-valent metal is only slightly (by 14.4°) tilted out of the ring plane with shorter ring-P-Mo than Ph 2 P-Mo(0) distances and marginal changes within the ligand, 11  .0ppm, similar to that of (Ph 3 P) 2 HgCl 2 (Δδ = 39 ppm 12 ), and upfield shift for the ring-P atom, which is however much smaller than for the μ-bridging coordination in 3 and even smaller than for the two terminal benzazaphosphole ligands in 2. The coordination at the ring-P atom is labile, displaying a rather sharp doublet by two-bond P-P-coupling but no 1 J ( 119 Hg 31 P) satellites, whereas the coordinated phosphanyl group exhibits a broad signal associated with the dynamic behaviour at the other P-atom.The coordination of two benzazaphosphole ligands at each Ag(I) atom, the tilting of the P-Ag bond out of the ring plane in 2, the μ-coordination mode in 3 and the unprecedented side-on coordination in the chelate complex 5 distinguish the complexes of the π-excess-type PvC-N ligands 1 and 4 from phosphabenzene complexes, which usually coordinate nonzero-valent transition metals within the ring plane. 1 To shed light on the background of these differences and to understand the unusual bonding in more detail, density functional calculations were carried out at the ωB97D/6-31G* level. 13First a second order perturbation analysis of the donor-acceptor interaction between NBOs at the crystal structures of 2 and 5 was performed.While in the case of 2 both P lone pairs at each atom exhibit about 60.5 kcal mol −1 stabilization donating to Ag, an additional 6.4 kcal mol −1 stabilization interaction was obtained for the (PvC bond)-Ag interaction in the case of the 32.4°tilted azaphosphole ring, showing some donor contribution of the π electron pair to the dative bonding.In the case of 5 the interaction between the σ 3 P lone pair and mercury amounts to 129.0 kcal mol −1 , while the interaction Scheme 2 Synthesis of the chelate complex 5. between the σ 2 P and mercury is only 12.1 kcal mol −1 .The small interaction with the mainly s-type 14 σ 2 P lone pair is in agreement with the near perpendicular position of the Hg atom with respect to the benzazaphosphole ring.The apparently lost (P lone pair)-Hg interaction, however, is amply compensated by the 40.0 kcal mol −1 stabilization arising from the interaction between the PvC π-orbital and mercury.This interaction is also clearly seen on the HOMO of 5 with the involvement of both Hg and the mainly PvC bonded delocalised π orbital (the HOMO of the benzazaphosphole ligand) as is shown in Fig. 3.The involvement of the (largely) PvC bonded orbital in the dative bonding is in agreement with the lengthening of the PvC distance (see above).It is noteworthy that no backbonding from mercury to π* type orbitals was detected.Optimization of the structure of 5 at the ωB97D/6-31G* level resulted in a shift of the Hg atom from phosphorus towards the ring nitrogen, together with the formation of an intramolecular Cl⋯H bond (see Fig. S3.3 in the ESI †).This behaviour is in agreement with the conclusions drawn from the solution NMR data discussed above.To understand why mercury prefers to interact with the PvC bond rather than the P lone pair, further model calculations were carried out.In order to exclude the effect of any steric constraint caused by the interlocking of the two phosphorus atoms of 4, the structure of various L(PMe 3 )HgCl 2 complexes was investigated, where L contained σ 2 P in different bonding environments.
During the geometry optimization of the benzazaphosphole the HgCl 2 unit occupies an out-of-plane position similar to that in 5 (albeit with larger P-Hg distances), even if the geometry optimization is started from a structure where Hg lies in the plane of the benzazaphosphole 15 ring.The complexes with the other ligands exhibit similar structural characteristics, with the exception of the in-plane complexed phosphabenzene ligand.The optimized structures can be seen in Fig. S3.4 of the ESI.† It is noteworthy that the σ 2 P-Hg distance (between 2.57 and 3.49 Å) is related to the interaction energy between the π-system and mercury.Strongest interactions are obtained with ligands (L) having high energy (and localised) HOMO (Table 1) as can easily be explained by simple perturbation theory arguments.
Accordingly, while the phosphabenzene ligand with stabilized PvC π-orbitals (in agreement with the aromaticity) has a limited π-interaction and thus prefers the "classical" n P → Hg bonding, the electron-rich PvC-N units (high energy HOMO) prefer the π interaction.The aromatic stabilization on the PvC π-orbital 16 of the 1,3-azaphosphole (compared to the 1,3-azaphospholine), together with the delocalization, weakens the interaction with mercury.It should be noted that a phenyl mercury compound with intermolecular phenyl Hg π-interactions is known; 17 however, its linear geometry is not distorted by the weak interaction.
In conclusion, we have presented the first complexes of π-rich σ 2 P-aromatic 1,3-azaphosphole ligands with non-zerovalent late transition metals and revealed unprecedented significant π-donor contributions in the coordinative bond.Our findings suggest to search for related P(π)-donor complexes which open up new tuning possibilities in the highly important field of phosphorus-based late-transition metal catalysis.
This work was supported by the Deutsche Forschungsgemeinschaft.We thank Dr M. K. Kindermann, G. Thede and M. Steinich for NMR, mass spectra and elemental analyses.Support from OTKA K 105417 and the Jedlik fellowship (L.K.) is also gratefully acknowledged.

6 −
counter-ions per unit cell (in space group P2 1 /c).Full refinement was not possible, with the THF molecules and SbF 6 − ions being disordered (for details see ESI †).The sharp singlets lacking coupling with 107/109 Ag in the CD 3 OD solution of silver complexes 2 and 3 indicate a rapid ligand exchange and thus kinetic lability.Complex formation is indicated mainly by upfield coordinated 31 P chemical shifts, much stronger for the μ-P bridged 3 (Δδ = −76.7 ppm) than the terminally coordinated 2 (Δδ = −18.9ppm).Characteristic features in the 13 C NMR spectra are decreasing 1 J PC coupling constants of C2 and C3a and downfield shifts of the C-2 signals, whereas proton NMR data are scarcely altered.

Fig. 3
Fig. 3 The HOMO of 5 showing the involvement of one Hg d-orbital and the delocalised PvC π-orbital in the bonding.