title: 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 identical withCNMe(2)) has been coordinated to both hard and soft electrophiles ((NH(3))(5)Co(3+), (NH(3))(5)Os(3+), (dien)Pt(2+)) which activate ( approximately x10(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. Crystals of [(NH(3))(5)CoNH=C(NH(2))NMe(2)](S(2)O(6))(3/2).H(2)O, CoC(3)H(26)N(8)O(10)S(3), were triclinic, space group P&onemacr;, a = 11.565(2) Å, b = 10.629(5) Å, c = 8.026(1) Å, alpha = 84.93(3) degrees, beta = 76.01(1) degrees, gamma = 73.82(3) degrees, V = 919.2(5) Å(3), Z = 2, and R(F)() (R(w)(F)()) = 0.038 (0.047) for 3262 observed reflections (I > 3.0 sigma(I)). Crystals of [(dien)PtNH=C(NH(2))NMe(2)](CF(3)SO(3))(2), PtC(9)H(22)N(6)O(6) S(2)F(6), are monoclinic, space group P2(1)/c, a = 13.857(4), b = 14.748(4) Å, c = 22.092(4) Å, beta = 105.38(2) degrees, V = 4353(2) Å(3), 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 degrees C), in each case producing coordinated N,N-dimethylurea ([dienPtNHCONMe(2)](+), [(NH(3))(5)CoNHCONMe(2)](2+), [(NH(3))(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.
authors.affiliation: The 3D Centre, University of Queensland, Brisbane, Qld 4072, Australia; Department of Chemistry, Stanford University, Stanford, California 94305-5080; Department of Chemistry, University College (NSW), Australian Defence Force Academy, Canberra, ACT 2600, Australia; and Department of Chemistry, University of Western Australia, Perth, WA 6907, Australia.
authors.forename: David P.
authors.initials: DP
authors.lastname: Fairlie
authors.forename: W. Gregory
authors.initials: WG
authors.lastname: Jackson
authors.forename: Brian W.
authors.initials: BW
authors.lastname: Skelton
authors.forename: Huo
authors.initials: H
authors.lastname: Wen
authors.forename: Allan H.
authors.initials: AH
authors.lastname: White
authors.forename: Wasantha A.
authors.initials: WA
authors.lastname: Wickramasinghe
authors.forename: Tai Chin
authors.initials: TC
authors.lastname: Woon
authors.forename: Henry
authors.initials: H
authors.lastname: Taube