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)

Dimethylcyanamide (N=CNMe(2)) has been coordinated to both hard and soft electrophiles ((NH3)(5)Co3+, (NH3)(5)Os3+, (dien)Pt2+) which activate (similar to x 10(6)) 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. [(NH3)(5)CoNH=C(NH2)NMe(2)](S2O6)(3/2). H2O, CoC3H26N8O10S3, were triclinic, space group <P(1)over bar>, a = 11.565(2) Angstrom, b = 10.629(5) Angstrom, c = 8.026(1) Angstrom, alpha = 84.93(3)degrees, beta = 76.01(1)degrees, gamma = 73.82(3)degrees, V = 919.2(5) Angstrom(3), Z = 2, and R(F) (R(wF)) = 0.038 (0.047) for 3262 observed reflections (I > 3.0 sigma(I). Crystals of [(dien)PtNH=C(NH2)NMe(2)](CF3SO3)(2), PtC9H22N6O6 S2F6, are monoclinic, space group P2(1)/c, a = 13.857(4), b = 14.748(4) Angstrom, c = 22.092(4) Angstrom, beta = 105.38(2)degrees, V = 4353(2) Angstrom(3), Z = 8, and R(F) (R(wF)) = 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 degrees C), in each case producing coordinated N,N-dimethylurea ([dienPtNHCONMe(2)](+), [(NH3)(5)CoNHCONMe(2)](2+), [(NH3)(5)OsNHCONMe(2)](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.