Low‐Valent Group 14 Phosphinidenide Complexes [({SIDipp}P)2M] Exhibit P–M pπ–pπ Interaction (M=Ge, Sn, Pb)

Abstract Herein, the synthesis of new low‐valent Group 14 phosphinidenide complexes [({SIDipp}P)2 m] exhibiting P–M pπ–pπ interactions (SIDipp=1,3‐bis(2,6‐diisopropylphenyl)‐imidazolidin‐2‐ylidene, M=Ge, Sn, Pb), is presented. These compounds were investigated by means of structural, spectroscopic, and quantum‐chemical methods. Furthermore, the monosubstituted compounds [(SIDippP)MX]2 (M=Sn, X=Cl; M=Pb, X=Br) are presented, which show dimeric structures instead of multiple bonding interaction.

Subsequentreactions of (SIDipp)PK with (SIMes)MX 2 (M = Ge, Sn, Pb;X = Cl or Br) at low temperatures in toluene in a1 :2 molar ratio led to deep purple colored suspensions. After removal of the formed KCl and exchange of the solvents torage of the saturated solutionsi np entane at low temperatures (6 8C for 3 and 5 or À32 8Cf or 4)y ieldeds ingle crystalso f[ (SI-Dipp)P] 2 M( M= Ge 3,S n4,P b5)a sd ark violet crystals in moderate yield (Scheme 2).
All three compounds crystallize isotypically in the monoclinic space group P2 1 /c with one molecule of pentane in the asymmetric unit. All compounds were characterized by 1 HNMR, 13 CNMR, 31 PNMR, IR spectroscopy,a nd elemental analysis. It is worth mentioning that all compounds, especially compound 5, are sensitivet owards light, particularlyi ns olution,s hown by a color change from deep purple towards pale yellow,associated with precipitation of the respective metallic powder.T he 31 PNMR spectrum of 3 shows as ingleta t1 45.2 ppm, which is ad ramatic lowfield shift compared with other germanium substituted phosphinidenides (K[(SIMesP) 3 Ge] À11.4; [2] (IDipp)PÀ GePh 3 À145.1; [18] (SIDipp)PGePh 3 À114.7 ppm [18] ). This is in line with the trends of calculated partial charges at the Ma tom (see Ta bles S11a nd S12, Supporting Information), but one should be aware,t hat the main effect usually comes from the differences in the response of the density to the magnetic field. This presumption is supported by the fact that this kind of lowfield shift in the 31 PNMR spectrah as also been observed for planarly coordinated phosphorus atoms with Ge=Pmultiple bonds( e.g. Mes 2 GePAr", 175.4 ppm, Ar" = 2,4,6-tri-tert-butylphenyl). [33] Another indication of ap p-pp interaction is the intensivec olor of the compounds. In contrast to the compounds [(Dipp) 2 P] 2 Ea nd [(Tripp) 2 P] 2 E( E= Ge, Sn) of Izod and co-workers, only one signal and no line broadening are observedi n the NMR spectra indicating that the phosphorus atoms are chemically andm agnetically equivalenta nd both are involved in the p-p interaction. [29,34] The NCNg roup shows ap seudotriplets plitting in the 13 C{ 1 H} NMR spectrum.
To verify the presumption of pp-pp interactions between the tetrel atom and both phosphorus atoms, we performed quantum chemical calculations with the scalar-relativistic local exact two-component (DLU-X2C) Hamiltonian [41][42][43] employing all-electron triple-zeta basis sets. [44,45] Several common density functionals (see the Supporting Information) were selected together with fine grids for numerical integration [45] and the multipole-accelerated resolution of the identitya pproximation for the Coulomb term [46] as implemented in the latest version of the TURBOMOLE program package. [47] Based on the analytical data, the TPSSh [48] functional performsb est (see the Support-    ing Information for the resultsofa ll functionals). Thus,only the results with TPSSh will be discussed herein. The PÀMa nd CÀP bond lengthsa re overestimatedb y1and 2pm, respectively. The trend of the 31 PNMR shifts from Ge to Sn is in reasonable agreement with the experimental findings whereas the individual shifts are overestimated by about 20 ppm. Thisd oes not hold for compound 5 as for lead spin-orbit effects are important for the magnetic properties. [49,50] As imilar behavior is observed for the 13 CNMR shifts. As expected, the Wiberg bond index (WBI) for the PÀMb onds is greater than one for both bonds in all three compounds, especially for compound 3 ( Table 2). This is ac lear indicator for the multiple-bondc haracter.W en otei np assing that for the recently reported compound K[(SIMesP) 3 M] [2] (M = Ge, Sn, Pb), in whicha ll MÀP bonds have single-bond character,t he WBI resulting from our calculations is even somewhat smaller than one (between 0.78 for M = Pb and 0.85 for M = Ge). The WBI for the PÀMb onds of 3 to 5 decreasesw ith rising atom number.I nt he same manner the WBI for the CÀPb onds rises, indicating that these p-bonds (C=Pv s. P=M) are contrary effects. Moreover,t he psystem is delocalized over the CÀPÀMÀPÀCb onds. The effect on the CÀPb ond is observable in the slight shortening of d(CÀP) going from 3 to 5.Spin-orbit effects do not significantly alter the WBI (see the Supporting Information for details on the quantum chemical calculations). In all three compounds, the HOMO is represented particularly by the lone pairs at the tetrel atoms. Furthermore, there is a considerable amount of electron density at the phosphorous atoms ( Figure 3). The HOMOÀ1i st he p-CÀPb ond, the HOMOÀ2( see Figure 3) is the p-bonding combination of po rbitals of the metal atom and the phosphorus atoms, slightly deformed due to the twisting of the NHC ligands. The corresponding p*-orbital is the LUMO orbital (see Figure 3). According to time-dependent (TD)-DFT [51][52][53][54] calculations with the DLU-X2CH amiltonian, the UV/Vis absorption maxima( see Ta ble 2) correspondt os inglet excitations from the HOMOÀ1 to the LUMO, that is, mainly from the p-orbitals of the phosphorusa toms to the p-orbitals of the metal center. We note that the HOMOÀ1a nd the HOMO are close in energy (energy differences: 3:0 .1, 4:0 .1, 5:0 .0 5eV) but differ in shape and symmetry.T he redshift observed in the UV/Vis spectra from 3 to 5 with rising atomicn umber is explained by the energetic position of the p-orbital of the metal atom, which decreases from Ge to Sn to Pb, resulting in decreasing LUMO energies from 3 to 4 to 5 and thusind ecreasing excitation energies.
To investigate the necessity of two (SIDipp)P ligandsf or the formation of compounds showing pp-pp interaction, reactions of 2 with (SIMes)MX 2 (M = Ge, Sn, Pb;X = Cl or Br) in a1 :1 molar ratio were performed. In all cases mixtures of (SI-DippP) 2  Compound 6 crystallizesi nt he monoclinic space group P2 1 /n with three molecules of toluene. In solid state, the compoundf orms ad imer with ac entral bended P 2 Sn 2 cycle (see Figure 4). The central P 2 Sn 2 cycle shows ab utterfly conformation. Both phosphorus atoms are pyramidally coordinated by the NHC ligand and two tin atoms (sum of angles at P1:3 09.8 and P2:3 03.68). The orientation of the ligandsw ith respectt o the central cycle is unusual, because two stericallyd emanding substituents and one chlorine ligand are situated at the same side of the ring, which leads to smaller P-Sn2-Cl2 angles (93.1(1) and 91.8(1)8)i nc omparison with the P-Sn1-Cl1 angles (96.0(1) and 96.6(1)8). This kind of P 2 Sn 2 cycles are already known in the literature,b ut usually the tin atoms are in oxidation state Sn(+ +IV) (e.g. [Tripp2SnPH] [55] or [tBu2SnPH] [56] ). With Sn II higher aggregates like heterocubanes were formed.  The PÀSn bond lengths in 6 (259.6(1)-266.3(2) pm) are in good accordance with literature known PÀSn II compounds, whereby however, P1ÀSn2 is slightly shorter. [23] Compound 6 was characterized by 1 HNMR, 13 CNMR, 31 PNMR, 119 Sn NMR, IR spectroscopy and elementala nalysis (Table 3). In the 31 PNMR spectrum, compound 6 shows as inglet signal at À66.2 ppm ( 1 J SnÀP % 1000 Hz), which is quite au sual chemical shift for tin substituted phosphinidenides. [2] In the 1 HNMR spectrum two signals for the isopropyl substituents are observed, which results from an inhibited rotationa long the PÀCb ond in solution. The 119 Sn NMR spectrum of 6 shows only one triplet signal at 235.8 ppm ( 1 J PÀSn = 1027Hz), showingt hat the tin atoms are equivalent on the NMR time scale and that compound 6 is ad imericcompound also in solution.
The reactiono f2 with (SIMes)PbBr 2 ,i na1:1m olar ratio, yielded the heavierc ongener [(SIDippP)PbBr] 2 (7,s ee Figure 5). Compound 7 exhibitst he same butterfly shaped central P 2 Pb 2 cycle. Even the bromine and NHC ligands are arranged in the same manner,b ut the crystal structure of 7 is not isotypic, due to the lack of lattice solvent.

Conclusions
Hereinw ep resented the new twofold phosphinidenide-substituted tetrylenes( SIDippP) 2 M( M= Ge 3,S n4,P b5)e xhibiting unique pp-pp interaction, which resembles with the stabilization of the singlet state found in NHC ligands. As far as we know,c ompound 5 is the first example for this kind of interaction between phosphorousa nd lead atoms.F or the lighter congener,o nly very few examples are described in literature. The character of the multiple bond between the tetrel atom and the NHC stabilized phosphinidenide wass hown by means of structural, spectroscopica nd quantum-chemical methods. Furthermore, we werea ble to show that the twofold coordination with phosphinidenides at the tetrel is necessary,b ecause the monosubstituted compounds [(SIDippP)MX] 2 (M = Sn, X = Cl;M= Pb, X = Br) tend to dimerize in solution as well as in the solid state ands how no sign of pp-pp interaction. Moreover, these compounds show the influence of the NHC ligand,s ince the SIDipp ligand is necessary to obtain the low valent compounds 2-4.W ith the slightly smaller NHC substituent SIMes the ate-complexes [(SIMesP) 3 M] À are formed. [2]    Chem. Eur.J.2020, 26,192 -197 www.chemeurj.org 2020 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Future investigations will focus on the reactivity of the lowvalent tetrylenes towards multiple bondsi ns mall molecules (e.g. CO, CO 2 ,o rN O) as well as the coordinationt owards Lewis acids. Also, reductivec luster formation startingf rom the compound 6 and 7 will be ak ey issue. This provides access to exclusively NHC coordinated binary cage compounds.