Imino-Bridged Bisphosphaalkenes (2,4-Diphospha-3-azapentadienes)

摘要

Deprotonation of aminophosphaalkenes (RMe₂Si)₂CPN(H)(R′) (R=Me, iPr; R′=tBu, 1-adamantyl (1-Ada), 2,4,6-tBu₃C₆H₂ (Mes*)) followed by reactions of the corresponding Li salts Li[(RMe₂Si)₂CP(M)(R′)] with one equivalent of the corresponding P-chlorophosphaalkenes (RMe₂Si)₂CPCl provides bisphosphaalkenes (2,4-diphospha-3-azapentadienes) [(RMe₂Si)₂CP]₂NR′. The thermally unstable tert-butyliminobisphosphaalkene [(Me₃Si)₂CP]₂NtBu (4 a) undergoes isomerisation reactions by Me₃Si-group migration that lead to mixtures of four-membered heterocyles, but in the presence of an excess amount of (Me₃Si)₂CPCl, 4 a furnishes an azatriphosphabicyclohexene C₃(SiMe₃)₅P₃NtBu (5) that gave red single crystals. Compound 5 contains a diphosphirane ring condensed with an azatriphospholene system that exhibits an endocylic PC double bond and an exocyclic ylidic P(+)C(−)(SiMe₃)₂ unit. Using the bulkier iPrMe₂Si substituents at three-coordinated carbon leads to slightly enhanced thermal stability of 2,4-diphospha-3-azapentadienes [(iPrMe₂Si)₂CP]₂NR′ (R′=tBu: 4 b; R′=1-Ada: 8). According to a low-temperature crystal-structure determination, 8 adopts a non-planar structure with two distinctly differently oriented PC sites, but 31P NMR spectra in solution exhibit singlet signals. 31P NMR spectra also reveal that bulky Mes* groups (Mes*=2,4,6-tBu₃C₆H₂) at the central imino function lead to mixtures of symmetric and unsymmetric rotamers, thus implying hindered rotation around the PN bonds in persistent compounds [(RMe₂Si)₂CP]₂NMes* (11 a, 11 b). DFT calculations for the parent molecule [(H₃Si)₂CP]₂NCH₃ suggest that the non-planar distortion of compound 8 will have steric grounds.