Single electron transfer-mediated selective endo- and exocyclic bond cleavage processes in azaphosphiridine chromium(0) complexes: a computational study.

Azaphosphiridines κP pentacarbonylchromium(0) complexes 2a,b (2a: R = H; 2b: R = Me) exhibit an average ring strain ranging from 24.2 to 25.7 kcal mol(-1) as obtained from homodesmotic reactions at the LPNO-NCEPA1/def2-TZVPP//BP86/def2-TZVP level. Parent azaphosphiridine chromium complex 1 is more stable than the ylidic P-iminiumphosphanide chromium complex isomer 6, which is obtained from (formal) endocyclic P-C bond cleavage. Computational evidence is provided for an insertion of carbon monoxide into the P-N bond of 1 to form 1,3-azaphosphetidin-2-one chromium complex 11, as the reaction was exergonic by -15.1 kcal mol(-1). The VBSD (variation of bond strength descriptors) methodology unveiled that SET (single electron transfer) oxidation of trimethyl-azaphosphiridine chromium complex 2b results in selective endocyclic P-C bond cleavage to afford the trimethyl-iminiumphosphanyl radical cation complex 13(•+). SET reduction of a wide variety of differently P-substituted azaphosphiridine complex derivatives (2a: R = H; 2b: R = Me; 2c: R = Cp; 2d: R = Cp*; 2e: R = CHTms(2); 2f: R = CMe(3); 2g: R = CMe(2)Ph; 2h: R = CMePh(2); 2j: R = Ph; 2k: R = C(6)F(5); Cp*: pentamethylcyclopentadienyl; Tms: trimethylsilyl) lead to selective decomplexation and thus to the corresponding unligated azaphosphiridines 14. Only in case of the P-trityl substituted azaphosphiridine complexes 2i does the SET reduction preferably cleave the exocyclic P-C bond thus affording azaphosphiridinide complex 12(-) and the triphenylmethyl radical.