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

Deprotonation of aminophosphaalkenes (RMe 2 Si) 2 CPN(H)(R′) (R=Me, i Pr; R′= t Bu, 1‐adamantyl (1‐Ada), 2,4,6‐ t Bu 3 C 6 H 2 (Mes*)) followed by reactions of the corresponding Li salts Li[(RMe 2 Si) 2 CP(M)(R′)] with one equivalent of the corresponding P ‐chlorophosphaalkenes (RMe 2 Si) 2 CPCl provides bisphosphaalkenes (2,4‐diphospha‐3‐azapentadienes) [(RMe 2 Si) 2 CP] 2 NR′. The thermally unstable tert ‐butyliminobisphosphaalkene [(Me 3 Si) 2 CP] 2 N t Bu ( 4 a ) undergoes isomerisation reactions by Me 3 Si‐group migration that lead to mixtures of four‐membered heterocyles, but in the presence of an excess amount of (Me 3 Si) 2 CPCl, 4 a furnishes an azatriphosphabicyclohexene C 3 (SiMe 3 ) 5 P 3 N t Bu ( 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 (−) (SiMe 3 ) 2 unit. Using the bulkier i PrMe 2 Si substituents at three‐coordinated carbon leads to slightly enhanced thermal stability of 2,4‐diphospha‐3‐azapentadienes [( i PrMe 2 Si) 2 CP] 2 NR′ (R′= t Bu: 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 31 P NMR spectra in solution exhibit singlet signals. 31 P NMR spectra also reveal that bulky Mes* groups (Mes*=2,4,6‐ t Bu 3 C 6 H 2 ) at the central imino function lead to mixtures of symmetric and unsymmetric rotamers, thus implying hindered rotation around the PN bonds in persistent compounds [(RMe 2 Si) 2 CP] 2 NMes* ( 11 a , 11 b ). DFT calculations for the parent molecule [(H 3 Si) 2 CP] 2 NCH 3 suggest that the non‐planar distortion of compound 8 will have steric grounds.