A Donor‐Stabilized Zwitterionic “Half‐Parent” Phosphasilene and Its Unusual Reactivity towards Small Molecules

Abstract

The stabilization of the labile, zwitterionic "half-parent" phosphasilene 4 L'Si=PH (L'=CH[(C=CH2)CMe(NAr)2]; Ar=2,6-iPr2C6H3) could now be accomplished by coordination with two different donor ligands (4-dimethylaminopyridine (DMAP) and 1,3,4,5-tetramethylimidazol-2-ylidene), affording the adducts 8 and 9, respectively. The DMAP-stabilized zwitterionic "half-parent" phosphasilene 8 is capable of transferring the elusive parent phosphinidene moiety (:PH) to an unsaturated organic substrate, in analogy to the "free" phosphasilene 4. Furthermore, compounds 4 and 8 show an unusual reactivity of the Si=P moiety towards small molecules. They are capable of adding dimethylzinc and of activating the S-H bonds in H2S and the N-H bonds in ammonia and several organoamines. Interestingly, the DMAP donor ligand of 8 has the propensity to act as a leaving group at the phosphasilene during the reaction. Accordingly, treatment of 8 with H2S affords, under liberation of DMAP, the unprecedented thiosilanoic phosphane LSi=S(PH2) 16 (L=HC(CMe[2,6-iPr2C6H3N])2). Compounds 4 and 8 react with ammonia both affording L'Si(NH2)PH2 17, respectively. In addition, the reaction of 8 with isoproylamine, p-toluidine, and pentafluorophenylhydrazine lead to the corresponding phosphanylsilanes L'Si(PH2)NHR (R=iPr 18 a; R=C6H5-CH3 18 b, R=NH(C6F5) 18 c), respectively.

A Persistent 1,2‐Dihydrophosphasilene Adduct

“…Computations show that the Si-P p-bond strength in the parent P-silyl-phosphasilene is increased and the Si À Pb ond length is shortened by hyperconjugation effects due to the silyl substituent at phosphorus. [12,30] Thestronger s-donor ability of the silyl group is also demonstrated by the upfield-shifted 31 Presonance signal in 5 (d( 31 P) = À331.7 ppm) compared to that in 2 (d( 31 P) = À301.4 ppm).…”

“…The 31 PNMR spectrum of 2 shows adoublet of doublets with 29 Si satellites at d( 31 P) = À301.4 ppm ( 1 J(P,H) = 132.0 Hz, 2 J(P,H) = 11.3 Hz, 1 J(Si,P) = 120.7 Hz). The 1 J(P,H) coupling constant is 12 Hz smaller than that of the Me 4 -NHC-stabilized form of II (NHC-II), [30] indicating lower 3s character of the PÀ Hbond. The 31 Psignal of 2 is shifted to higher field compared with the resonance of NHC-II (d( 31 P) = À259.8 ppm) and is even more shielded than signals of existing "half-parent" phosphasilenes.T he upfield-shifted 29 Si signal (d( 29 Si) = À25.6 ppm, 1 J(Si,P) = 120.7 Hz) is similar to the 29 Si resonance signal of III (d( 29 Si) = À21.1 ppm).…”

“…These species and related low-valent Group 14 compounds are stabilized either by taking advantage of steric congestion through the presence of bulky substituents and/or by pushpull electronic effects. Only recently,t he first isolable "half-parent" phosphasilenes I, [28] II, [29,30] and III [31] of the type R 2 Si = PH or R(H)Si = PR could be realized (Scheme 1). However,a1,2-dihydrophosphasilene IV with hydrogen atoms at silicon and phosphorus R(H)Si=PH, which are closer to the parent system, has not been reported to date.…”

Ein beständiges 1,2‐Dihydrophosphasilen‐Addukt

“…Rechnungen zeigen, dass die Si-P-p-Bindungsstärke in dem P-Silyl-Phosphasilen-Stammsystem zunimmt und die Si-P-Bindung durch Hyperkonjugation der Silylgruppen am Phosphor verkürzt wird. [12,30] …”

“…Die Gründe dafür sind unter anderem die ausgeprägte Polarität der Si=P-Bindung,d ie durch die hçhere Elektronegativität des Phosphors im Vergleich zum Silicium induziert wird, und das Fehlen eines großen Substituenten am Phosphorzentrum. [10,12] Wirb erichten hier über die Synthese des ersten 1,2-Dihydrophosphasilen-Addukts und die Isolierung und strukturelle Charakterisierung seines R(H)Si=P(H)!W(CO) 5 [30] was darauf hindeutet, dass der 3s-Charakter der P-H-Bindung Einkristallrçntgenstrukturanalyse geeignet waren, konnten in einer konzentrierten n-Hexan-Lçsung der Zersetzungsmischung bei 7 8 8Ce rhalten werden, jedoch kann seine Struktur wegen der mangelnden Kristallqualität nicht diskutiert werden. [35] Hochauflçsende Massenspektrometrie mit chemischer Ionisation bei Atmosphärendruck (HR-APCI-MS) beweist eindeutig die Zusammensetzung von 3.…”