Neighbouring group participation reactions in nitrile- and amido-N-bonded pentaamminecobalt(III) complexes in which the unique ligand contains a remote amide group

The [(NH₃)₅CoNCCH₂CONH₂]³⁺ ion hydrates in aqueous base to form [(NH₃)₅CoNHCOCH₂CONH₂] ²⁺ (33%) and a cobalt(II) species. The kinetics have been measured: k_obsd=((k_OH+Kk_e-)[OH^-])/(1+K[OH ^-]) where K=7.9×10³ (pK_a=9.9), k_OH=50 M^-1 s^-1 and k_e-=1.4×10^-2s^-1. In liquid ammonia the complex is unreactive while in aqueous acid it solvolyzes slowly. The [(NH₃)₅CoNCC₆H₄-2-CONH ₂]³⁺ complex cyclizes rapidly in aqueous base, liquid ammonia and dimethyl sulfoxide/Cs₂CO₃ to form only pentaammine(3-imino-1-oxo-isoindolino-exo-N)cobalt(III). The reaction also occurs in aqueous acid, with a spontaneous path identified: k_obsd=k₀+k₁/[H⁺], where k₀=1.1×10^-5 s^-1 and k₁=k_OH/K_w=4.3×10^-8 M s^-1 (25°C, I=1 M, LiClO₄), whence k_OH=2.5×10⁶ M^-1 s^-1. These data remove an anomaly in the literature. In Me₂SO, but not water, the reaction is acid catalyzed, a reaction pathway new in coordinated nitrile chemistry. The aliphatic analogue [(NH₃)₅CoNC(CH₂)₂CONH ₂]³⁺ also cyclizes in aqueous base to form pentaammine(5-imino-2-oxo-pyrrolidino-exo-N)cobalt(III) (90%), but here OH^- addition to the bound nitrile is competitive, yielding [(NH₃)₅CoNHCO(CH₂)₂CONH ₂]²⁺ (10%). The kinetics of the reactions were measured: k_obsd=[OH^-]{k_OH+Kk_cyc}, [OH^-]=(1-40)×10^-5 M, where k_OH=40 M^-1 s^-1, approximately 10⁴-fold slower than the o-phenylene analogue. The [(NH₃)₅CoNC(CH₂)₂CONH ₂]³⁺ ion solvolyzes slowly in aqueous acid, while in liquid ammonia it cyclizes, and only pentaammine(2-oxo-5-iminopyrrolidino-exo-N)cobalt(III) is formed. Amido-N-bonded pentaamminecobalt(III) complexes of malonamide, succinamide and glutaramide were synthesized from their free ligands. The malonamido-N and succinamido-N complexes react slowly in aqueous base and the products are the amido-N-bonded complexes of malonamic and succinamic acids. The intermediate imide complexes, one of which has been synthesized independently, were not detected. The glutaramido-N complex undergoes only slow solvolysis. Attempts to synthesize the phthalamido-N complex from the free ligand yielded only pentaammine (phthalimido-N)cobalt(III); the intermediate diamide complex cyclizes rapidly as soon as it is formed. In acid solution all diamide complexes are protonated on the coordinated amide and undergo slow rearrangement to their amide-O forms with parallel and subsequent solvolysis in both water and dimethylsulfoxide.