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

Deprotonation of aminophosphaalkenes (RMe 2 Si) 2 C= PN(H)(R') (R=Me, iPr; R'=tBu, 1-adamantyl (1-Ada), 2,4,6-tBu 3 C 6 H 2 (Mes * )) followed by reactions of the corresponding Li salts Li[(RMe 2 Si) 2 C= P(M)(R')] with one equivalent of the corresponding P-chlorophosphaalkenes (RMe 2 Si) 2 C=PCl provides bisphosphaalkenes (2,4-diphospha-3-azapentadienes) [(RMe 2 Si) 2 C=P] 2 NR'. The thermally unstable tert-butyliminobisphosphaalkene [(Me 3 Si) 2 C=P] 2 NtBu (4a) 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 C=PCl, 4a furnishes an azatriphosphabicyclohexene C 3 (SiMe 3 ) 5 P 3 NtBu (5) that gave red single crystals. Compound 5 contains a diphosphirane ring condensed with an azatriphospholene system that exhibits an endocylic P=C double bond and an exocyclic ylidic P (+) C (-) (SiMe 3 ) 2 unit. Using the bulkier iPrMe 2 Si substituents at three-coordinated carbon leads to slightly enhanced thermal stability of 2,4-diphospha-3-azapentadienes [(iPr-Me 2 Si) 2 C=P] 2 NR' (R' = tBu: 4b; R' = 1-Ada: 8). According to a low-temperature crystal-structure determination, 8 adopts a non-planar structure with two distinctly differently oriented P=C 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-tBu 3 C 6 H 2 ) at the central imino function lead to mixtures of symmetric and unsymmetric rotamers, thus implying hindered rotation around the P—N bonds in persistent compounds [(RMe 2 Si) 2 C=P] 2 NMes* (11a, 11b). DFT calculations for the parent molecule [(H 3 Si) 2 C=P] 2 NCH 3 suggest that the non-planar distortion of compound 8 will have steric grounds.