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

Abstract

The [(NH 3) 5CoNCCH 2CONH 2] 3+ ion hydrates in aqueous base to form [(NH 3) 5CoNHCOCH 2CONH 2] 2+ (33%) and a cobalt(II) species. The kinetics have been measured: k obsd=(( k OH+ Kk e − )[OH −])/(1+ K[OH −]) where K=7.9×10 3 (p K a=9.9), k OH=50 M −1 s −1 and k e − =1.4×10 −2 s −1 . In liquid ammonia the complex is unreactive while in aqueous acid it solvolyzes slowly. The [(NH 3) 5CoNCC 6H 42-CONH 2] 3+ complex cyclizes rapidly in aqueous base, liquid ammonia and dimethyl sulfoxide/Cs 2CO 3 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= k 0+ k 1/[H +], where k 0=1.1×10 −5 s −1 and k 1= k OH/ K w=4.3×10 −8 M s −1 (25 °C, I=1 M, LiClO 4), whence k OH=2.5×10 6 M −1 s −1. These data remove an anomaly in the literature. In Me 2SO, but not water, the reaction is acid catalyzed, a reaction pathway new in coordinated nitrile chemistry. The aliphatic analogue [(NH 3) 5CoNC(CH 2) 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 [(NH 3) 5CoNHCO(CH 2) 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 10 4-fold slower than the o-phenylene analogue. The [(NH 3) 5CoNC(CH 2) 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.