User:Mriu/sandbox



Diphosphagermylenes is a class of compounds containing a divalent germanium atom bound to two phosphorous atoms. While these compounds resemble diamidocarbenes, such as N-heterocyclic carbenes (NHC), diphosphagermylenes display bonding characteristics distinct from those of diamidocarbenes. In contrast to NHC compounds, in which there is effective N-C p(π)-p(π) overlap between the lone pair of planar nitrogens and an empty p-orbital of a carbene, systems containing P-Ge p(π)-p(π) overlap are rare. Until 2014, the geometry of phosphorous atoms in all previously reported diphosphatetrylenes are pyramidal, with minimal P-Ge pπ-pπ interaction. It has been suggested that the lack of pπ-pπ in Ge-P bonds is due to the high energetic barrier associated with achieving a planar configuration at phosphorous, which would allow for efficient pπ-pπ  overlap between the phosphorous lone pair and the empty P orbital of Ge. The resulting lack of pi stabilization contributes to the difficulty associated with isolating diphosphagermylene and the Ge-P double bonds. However, utilization of sterically encumbering phosphorous centers has allowed for the isolation of diphosphagermylenes with a planar phosphorous center with a significant P-Ge pπ-pπ interaction.

Synthesis of P-Ge σ-Only Diphosphagermylenes
Reactivity of sterically demanding lithium (fluorosilyl)silylphosphanides with GeI 2 yields green, cubic crystals in moderate yield. The identity of this species was investigated using only multinuclear NMR, elemental analysis, and UV-vis. Computational calculations (at the CIS level with the ab initio Los Alamos pseudopotential method (LAN L 1 DZ)) of the diphosphagermylene electronic structure was in agreement experimentally-derived electronic transition values. Due to disorder, the crystal structure of the diphosphagermylene could not be investigated.

Synthesis of P-Ge π-Stabilized Diphosphagermylenes
The sterically encumbered germylene ligand (Dipp)2PH, where Dipp=2,6-iPr2C6H3, is synthesized by the addition of PCl3 to DippLi-(OEt2), followed by the addition of LiAlH4. (Dipp)2PH is added to PhCH2K, which is combined with GeCl2 to provide (Dipp2P)2Ge. The synthesis results in dark red crystals suitable for x-ray crystallography. The identity of the compound is confirmed by elemental analysis, multinuclear NMR, and x-ray crystallography. This compound is stable in the absence of air and water.

P-Ge σ-only system: Driess's diphosphagermylene
While crystals were formed of Driess's diphosphagermylene complex, the X-ray structure diphosphagermylene could not be analyzed due to disordering. It has been suggested that the three lone pairs in Driess's diphosphagermylene system are composed of Ge(4s, 4p) and P(3s, 3p) valence orbitals. Driess calculated (MP2/DZ+POL//RHF/DZ+ZPE) the reaction profile for the isomerization of E(PH2)2 (E = Si, Ge, Sn, Pb) from a sigma-only, carbene-like system to a tautomer containing trivalent E with a pi bond between E and P. The authors observed that the carbene-like form is preferred over its tautomer for silicon, germanium, tin, and lead analogues.

P-Ge pπ-pπ Stabilized Systems: ((Dipp)2P)2Ge and ((Tripp)2P)2Ge
P-substituted heavier group 14 analogues (Si, Ge, Sn, Pb) of diaminocarbenes are less established. It has been suggested this is due to a high energetic barrier associated with achieving a planar configuration at phosphorous, which would enable p(π)-p(π) overlap between the P lone pair and the empty p orbital of the group 14 center. Differences in donation ability of phosphorous versus nitrogen likely do not play a role in achieving p(π)-p(π) overlap because calculations indicate that the pi donor capacity of phosphorous is similar to that of nitrogen. Consequently, all P atoms in reports on diphosphatetrylenes previous to ((Dipp)2P)2Ge contain pyramidal P with Ge-P bonds of exclusively sigma character. By utilizing sterically encumbered (Dipp)P ligands, p(π)-p(π) in diphosphagermylene was achieved. This compound crystallizes as discrete monomers and is the first crystallographically characterized diphosphagermylene with a two-coordinate Ge center.

By crystal structure analysis, the bond lengths of the two germanium phosphorous bonds are 2.2337 Å (P1-Ge) and 2.3823 Å (P2-Ge). While the phosphorous center of P1-Ge is pyramidal, the P2-Ge phosphorous is trigonal planar. Moreover, the planes of P1-Ge-P2 and C-P1-Ge are nearly in coincident. These results are consistent with multiple bond character between a trigonal planar phosphorous (P1) and Ge. It has been suggested that only one P of the diphosphagermylene is planar because there is competition between the two phosphorous lone pairs and the empty P orbitals at the Ge center if both phosphorous atoms are planar. This would result in a weaker P-Ge interaction that would not be sufficient to overcome the energy of planarizing both P atoms.

In addition, ((Dipp)2P)2Ge was modified such that a iPr groups was added to the para position of (Dipp)2P, to make (Tripp)2P. The donating effect of an additional iPr group has little effect on the the bonding and structure of the diphosphagermylene.

Solution and Solid State NMR
A single, broad singlet is observed at 3.2 ppm at room temperature in the solution state PNMR of ((Dipp)2P)2Ge. This signal is consistent with rapid exchange between the planar and pyramidal phosphorous centers. As the temperature is reduced to -80 C, the signal becomes two broad, equal intensity singlets at -42.0 ppm and 8.0.

Two peaks with isotropic chemical shifts of 81.9 and -61.6 ppm, in a 1:1 ratio are observed in the solid state PNMR. No other signals are observed in the PNMR. In general diphosphagermylenes with pyramidal phosphorous centers exhibit a chemical shift close to those of free phosphine. In addition, planar phosphorous centers of GeIV=P compounds generally have a downfield PNMR shift. Consequently, peaks at 81.9 and -61.6 ppm have been assigned as planar and pyramidal phosphorous centers of ((Dipp)2P)2Ge, respectively. This has been supported by DFT calculations that predict PNMR shifts of planar and pyramidal phosphorous centers of ((Dipp)2P)2Ge are at 100 and -61 ppm.

Natural Bond Orbital (NBO) Analysis
Natural bond orbital analysis has been carried out on the P-Ge pπ-pπ system, ((Dipp)2P)2Ge. Inspection of the MOs reveals that the HOMO-1 consists of a π orbital, resulting from donation from the planar P lone pair into the empty P orbital of the germylene center. Natural bond orbital analysis reveals that this bond is 77% P-based and 0.3 eV higher in energy than the P-Ge σ bond. In contrast, the lone pair of the pyramidal phosphorous center is essentially sp hybridized and directed away from the germanium center. The germanium lone pair is predominantly s-character. Wiberg bond indices for Ge-P1 and Ge-P2 bonds are 1.33 and 0.89, respectively, which is consistent with a double bond between Ge-P1 and a single bond between Ge-P2.

Atoms in Molecules (AIM) Analysis
AIM analysis of ((Dipp)2P)2Ge suggests that there is a double bond between P1-Ge. Bond order can be assessed by measuring the ellipticity, a measure of anisotropic electron density, at the bond critical point. For example, butane, ethylene, and ethyne have a bond ellipticity of 0.01, 0.30, and 0.00 respectively, which correspond to a single, double, and a triple bond. A bond critical point between P1-Ge with ρ=0.091 and ellipticity 0.297 was observed in ((Dipp)2P)2Ge, consistent with a double bond. . This contrasts ρ=0.083 and ellipticity 0.064 at the bond critical point of P2-Gein ((Dipp)2P)2Ge. Delocalization index (DI) was also used to predict the bond order of ((Dipp)2P)2Ge. DI values for P1-Ge and P2-Ge were determined to be 1.275 and 0.843, consistent with a P1-Ge double bond and Wiberg bond indices calculated.