Imino-bridged bisphosphaalkenes (2,4-diphospha-3-azapentadienes)

Article abstract of DOI:10.1002/chem.200903166

Deprotonation of aminophosphaalkenes (RMe₂Si)₂C= PN(H)(R') (R = Me, iPr; R' = tBu, 1adamantyl (1-Ada), 2,4,6-tBu ₃C₆H₂ (Mes^*)) followed by reactions of the corresponding Li salts Li[(RMe₂Si)₂C= P(M)(R')] with one equivalent of the corresponding P-chlorophosphaalkenes (RMe₂Si)₂C=PCl provides bisphosphaalkenes (2,4-diphospha-3-azapentadienes) [(RMe₂Si)₂C=P] ₂NR'. The thermally unstable tert-butyliminobisphosphaalkene [(Me₃Si)₂C=P]₂NtBu (4a) undergoes isomerisation reactions by Me₃Si-group migration that lead to mixtures of four-membered heterocyles, but in the presence of an excess amount of (Me ₃Si)₂C=PCl, 4a 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 P=C double bond and an exocyclic ylidic P⁽⁺⁾-C^(-),(SiMe₃) ₂unit. Using the bulkier tPrMe₂Si substituents at three-coordinated carbon leads to slightly enhanced thermal stability of 2,4-diphospha-3-azapentadienes [(tPrMe₂Si)₂C=P] ₂NR' (R' = tBu: 4b; R' = l-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 ³¹P NMR spectra in solution exhibit singlet signals. ³¹P NMR spectra also reveal that bulky Mes^* groups (Mes^* = 2,4,6-tBu ₃C₆H₂) 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 [(RMe₂Si) ₂C=P]₂NMes^* (11a, 11b). DFT calculations for the parent molecule [(H₃Si)₂C=P]₂NCH ₃ suggest that the non-planar distortion of compound 8 will have steric grounds.