Models for Arginine−Metal Binding. Synthesis of Guanidine and Urea Ligands through Amination and Hydration of a Cyanamide Ligand Bound to Platinum(II), Osmium(III), and Cobalt(III)

• Published in: Inorganic chemistry 1997-03, Vol.36 (6), p.1020-1028
• Main Authors: Fairlie, David P, Jackson, W. Gregory, Skelton, Brian W, Wen, Huo, White, Allan H, Wickramasinghe, Wasantha A, Woon, Tai Chin, Taube, Henry
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic961138e

Dimethylcyanamide (N≡CNMe2) has been coordinated to both hard and soft electrophiles ((NH3)5Co3+, (NH3)5Os3+, (dien)Pt2+) which activate (∼×106) the nitrile toward attack by nucleophiles such as ammonia and hydroxide. Amination with liquid ammonia gave a rare coordinated guanidine (N,N-dimethylguanidine) ligand, which NMR spectra and X-ray crystal structures show to be charge neutral rather than anionic. Crystals of [(NH3)5CoNHC(NH2)NMe2](S2O6)3/2·H2O, CoC3H26N8O10S3, were triclinic, space group P1̄, a = 11.565(2) Å, b = 10.629(5) Å, c = 8.026(1) Å, α = 84.93(3)°, β = 76.01(1)°, γ = 73.82(3)°, V = 919.2(5) Å3, Z = 2, and R F (R w F ) = 0.038 (0.047) for 3262 observed reflections (I > 3.0 σ(I)). Crystals of [(dien)PtNHC(NH2)NMe2](CF3SO3)2, PtC9H22N6O6 S2F6, are monoclinic, space group P21/c, a = 13.857(4), b = 14.748(4) Å, c = 22.092(4) Å, β = 105.38(2)°, V = 4353(2) Å3, Z = 8, and R F (R w F ) = 0.034 (0.038) for 6778 reflections. Coordination geometries around the metals are octahedral and square planar, respectively, the guanidine skeletons being planar with bond angles and lengths characteristic of the metal−imino (rather than metal−amino) tautomer. The complexes are very stable in coordinating solvents (DMSO; water, pH 3−11) indicating high affinity of guanidine ligands for metal ions. Hydration of the dimethylcyanamide ligand is base-catalyzed, and first-order in [OH-] (0.05−0.5 M NaOH; k = k s + k OH[OH-], k OH = 2−5 M-1 s-1, 25 °C), in each case producing coordinated N,N-dimethylurea ([dienPtNHCONMe2]+, [(NH3)5CoNHCONMe2]2+, [(NH3)5OsNHCONMe2]2+). Hydration rates are surprizingly similar despite differing radial extensions of the metal d-orbitals, a finding consistent with their comparable polarizing powers but contrary to expectation from other work. The relevance of metal activation of nitriles to biological systems is discussed.