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)

Abstract

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)₅CoNHC(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) ų, Z = 2, and R_F (R_wF) = 0.038 (0.047) for 3262 observed reflections (I > 3.0 σ(I)). Crystals of [(dien)PtNHC(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) ų, 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 °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.