Ni(II), Cu(II) and Zn(II) complexes of a bifunctional bis(picolyl)amine (bpa) ligand derived from glycine

Metal-Templated, Tight Loop Conformation of a Cys-X-Cys Biomimetic Assembles a Dimanganese Complex

With the goal of generating anionic analogues to MN₂ S₂ ⋅Mn(CO)₃ Br we introduced metallodithiolate ligands, MN₂ S₂ ^2- prepared from the Cys-X-Cys biomimetic, ema^4- ligand (ema=N,N'-ethylenebis(mercaptoacetamide); M=Ni^II , [V^IV ≡O]²⁺ and Fe^III ) to Mn(CO)₅ Br. An unexpected, remarkably stable dimanganese product, (H₂ N₂ (CH₂ C=O(μ-S))₂ )[Mn(CO)₃ ]₂ resulted from loss of M originally residing in the N₂ S₂ ^4- pocket, replaced by protonation at the amido nitrogens, generating H₂ ema^2- . Accordingly, the ema ligand has switched its coordination mode from an N₂ S₂ ^4- cavity holding a single metal, to a binucleating H₂ ema^2- with bridging sulfurs and carboxamide oxygens within Mn-μ-S-CH₂ -C-O, 5-membered rings. In situ metal-templating by zinc ions gives quantitative yields of the Mn₂ product. By computational studies we compared the conformations of "linear" ema^4- to ema^4- frozen in the "tight-loop" around single metals, and to the "looser" fold possible for H₂ ema^2- that is the optimal arrangement for binucleation. XRD molecular structures show extensive H-bonding at the amido-nitrogen protons in the solid state.

Spectroscopic Studies of Aminoacids Complexes with Biometals

The [Cu(L)2]·H2O, [Co(L)2]·2H2O, [Zn(L)2]·H2O complexes with methionine (L) as ligand, were synthesized in water solution and analyzed by means of: elemental analysis, atomic absorption spectroscopy, thermogravimetry, FT-IR, UV-VIS and EPR spectroscopies. The atomic absorption spectroscopy and elemental measurements confi rm the ratio 1:2 metal ion: methionine composition for the synthesised compounds.The IR spectra show that amino acids act as bidentate ligands with coordination involving the carboxylic oxygen and the nitrogen atom of the amino group. Spectral UV-VIS data confi rmed the covalent metal-ligand bonds, the pseudotetrahedral symmetry around the copper and zinc ions and the octahedral environment for the cobalt ion. Powder ESR spectra at room temperature are typically for monomeric species.

[CuCl3(H2O)](-) complexes aggregated to form hydrate columns in methyl-substituted pyridinium or piperidinium salts

1,2,3-Trimethylpyridinium aquatrichloridocuprate(II), (C(8)H(12)N)[CuCl(3)(H(2)O)], (I), 3,4-dimethylpyridinium aquatrichloridocuprate(II), (C(7)H(10)N)[CuCl(3)(H(2)O)], (II), and 2,3-dimethylpyridinium aquatrichloridocuprate(II), (C(7)H(10)N)[CuCl(3)(H(2)O)], (III), exhibit the same fundamental structure, with (I) and (II) isomorphous and with the unit-cell constants of (III) similar to the reduced unit-cell constants of (I) and (II). The distorted square-planar [CuCl(3)(H(2)O)](-) complex [mirror symmetric in (I) and (II)] forms two semicoordinate Cu···Cl bonds to a neighboring complex to produce a dimer with 2/m symmetry [only inversion symmetry in (III)]. The semicoordinate Cu...Cl bond length of the dimer shows significant elongation at 295 K compared with that at 100 K, while the coordinate Cu-Cl bond lengths are slightly contracted at 295 K compared with those at 100 K. The inorganic dimers are linked by eight hydrogen bonds to four neighboring dimers to establish a checkerboard network layer in the ab plane, with voids between the dimers that accommodate, on both sides, inversion-related organic cation pairs. The organic cations are required by mirror-plane symmetry to be disordered in (I) and (II). The organic cations and [CuCl(3)(H(2)O)](-) complexes are nearly coplanar and tilted out of the layer plane to establish a hybrid organic-inorganic layer structure parallel to (202) [(11-2) in (III)], with hydrate columns (defined by water molecules) and hydrophobic columns (defined by methyl groups) parallel to each other [and along the 2(1) axes in (I) and (II)]. In 1,1-dimethylpiperidinium aquatrichloridocuprate(II), (C(7)H(16)N)[CuCl(3)(H(2)O)], (IV), the bulkier organic cation prevents semicoordinate bonding between complexes, which are hydrogen bonded side-to-side in zigzag chains that place water molecules in columns along half of the 2(1) axes.

Transesterification and amide cis–trans isomerization in Zn and Cd complexes of the chelating amino acid ligand Boc–Asp(Dpa)–OBzl

The amino acid derivative Boc-Asp-OBzl (Boc=N-butyloxycarbonyl; Asp=aspartic acid; Bzl=benzyl) was functionalized by coupling its carboxylate side chain to dipicolylamine. This yielded the tridentate nitrogen donor ligand Boc-Asp(Dpa)-OBzl (-OBzl). The compound -OBzl contains three different carbonyl groups: a tertiary amide linkage between Asp and Dpa, a C-terminal benzyl ester function, and an N-terminal urethane protecting group. NMR spectra were used to compare the reactivity of these moieties. The Boc protecting group gives rise to two isomers, (E, 9%) and (Z, 91%). Coordination of Cd(NO3)2 and Zn(NO3)2 yielded the complexes and. These compounds have significantly reduced barriers to rotation about the tertiary amide C-N bond compared with the free ligand (-OBzl:18.5 kcal mol-1 in CDBr3;: 12.9 kcal mol-1 in (CD3)2CO;: 13.8 kcal mol-1 in (CD3)2CO). Both complexes readily undergo transesterification in methanol or CD3OD. Experimental pseudo-first order rate constants were determined in CD3OD and (CD3)2CO:CD3OD (3:1;). It was found that the zinc complex (k=(2.28+/-0.02)x10(-4) s-1) is significantly more reactive than the cadmium complex (k=(1.41+/-0.03)x10(-6) s-1). In order to study their tertiary amide cis-trans isomerization, the cadmium complex [(-OCH3)Cd(NO3)2] was synthesized, and the zinc complex [(-OCD3)Zn(NO3)2] was generated in situ in (CD3)2CO:CD3OD (3:1). The barriers to rotation were determined (:14.1 kcal mol-1 in CD3OD;: 13.4 kcal mol-1 in (CD3)2CO:CD3OD (3:1)). Our results show that the stronger Lewis-acid zinc(II) is significantly more active than cadmium(II) in the acceleration of the transesterification. This is in marked contrast to the tertiary amide bond rotation which is comparably fast with both metal ions.

Aromatic Interactions in Unusual Backbone Nitrogen-Coordinated Zinc Peptide Complexes: A Crystallographic and Spectroscopic Study

A series of zinc complexes with dipeptide ligands of the type Dpg-Xaa was synthesized, where Dpg is dipicolylglycine and Xaa is phenylalanine (Phe), tyrosine (Tyr), tryptophan (Trp), 2-naphthylalanine (Nal), or glycine (Gly). It was shown that aromatic interactions promote the unusual coordination of an anionic peptide backbone nitrogen atom to zinc. This binding mode was, for the first time, characterized by X-ray structure analyses of the electrically neutral complexes [(Dpg-Phe)(-H)Zn], [(Dpg-Tyr)(-H)Zn], [(Dpg-Trp)(-H)Zn], and [(Dpg-Nal)(-H)Zn]. The pKa values for amide nitrogen deprotonation were determined by 1H NMR titrations {[(Dpg-Phe)Zn], 7.17; [(Dpg-Tyr)Zn], 6.85; [(Dpg-Trp)Zn], 6.85; [(Dpg-Nal)Zn], 6.64; [(Dpg-Gly)Zn], 8.54}. It was calculated that aromatic interactions contribute ca. -8 to -11 kJ/mol of stabilizing free enthalpy changes in the derivatives with aromatic amino acid side chains. These are the first quantitative data obtained for crystallographically characterized metal complexes. A comparison with the literature shows that it is difficult to distinguish between pi-cation attraction and pi-pi stacking. However, it is evident that modification of small peptides with synthetic pyridine ligands enhances their ability to stabilize secondary structures by noncovalent interactions. This is an important consideration for the design of biomimetic metallopeptides.

The Activation of Tertiary Carboxamides in Metal Complexes: An Experimental and Theoretical Study on the Methanolysis of Acylated Bispicolylamine Copper(II) Complexes

It is a well-established concept that the C-N bond cleavage of carboxamide functions is facilitated by the coordination of a metal ion to the carbonyl oxygen atom. In contrast, the alternative C-N bond activation by coordination of a neutral tertiary carboxamide nitrogen atom has not been studied. We present the first results on the effect of nitrogen pyramidalization in N-coordinated metal complexes on the methanolysis of tertiary carboxamide groups. An analysis of the reactions products obtained from the methanol cleavage of [(N-Acyl-bpa)Cu]2+ (bpa = N,N-bispicolylamine) complexes is presented together with experimental and high-level theoretically calculated structures. The strong effect of different anions on the amide pyramidalization and subsequent C-N-bond cleavage is evaluated. We show that dichloro complexes [(N-Acyl-bpa)CuCl2] have much less activated amide groups than the corresponding triflate species. They should therefore be less reactive. However, [(N-Acyl-bpa)CuCl2] complexes dissociate in solution to give cationic monochloro complexes [(N-Acyl-bpa)Cu(S)Cl]+ (S = solvent molecule). Theoretical calculations show that the amide pyramidalization in the monochloro complexes is equal to that in the corresponding CF3SO3- salts. Consequently, chloro and triflato complexes are cleaved with similar rates and efficiencies. Parallels to and differences in the reactivity of purely organic distorted amides are discussed.

LZnX complexes of tripodal ligands with intramolecular RN–H hydrogen bonding groups: structural implications of a hydrogen bonding cavity, and of X/R in the hydrogen bonding geometry/strength

Tripodal ligands N(CH2Py)3-n(CH2Py-6-NHR)n(R=H, n=1-3 L1-3, n=0 tpa; R=CH2tBu, n=1-3 L'1-3) are used to investigate the effect of different hydrogen bonding microenvironments on structural features of their LZnX complexes (X=Cl-, NO3-, OH-). The X-ray structures of [(L2)Zn(Cl)](BPh4)2.0.5(H2O.CH3CN), [(L3)Zn(Cl)](BPh4)3.CH3CN, [(L'1)Zn(Cl)](BPh4) 1', [(L'2)Zn(Cl)](BPh4)2'.CH3OH, and [(L'3)Zn(Cl)](BPh4)3' have been determined and exhibit trigonal bipyramidal geometries with intramolecular (internal) N-HCl-Zn hydrogen bonds. The structure of [(L'2)Zn(ONO2)]NO3 4'.H2O with two internal N-HO-Zn hydrogen bonds has also been determined. The axial Zn-Cl distance lengthens from 2.275 A in [(tpa)Zn(Cl)](BPh4) to 2.280-2.347 A in 1-3, 1'-3'. Notably, the average Zn-N(py) distance is also progressively lengthened from 2.069 A in [(tpa)Zn(Cl)](BPh4) to 2.159 and 2.182 A in the triply hydrogen bonding cavity of 3 and 3', respectively. Lengthening of the Zn-Cl and Zn-N(py) bonds is accompanied by a progressive shortening of the trans Zn-N bond from 2.271 A in [(tpa)Zn(Cl)](BPh4) to 2.115 A in 3 (2.113 A in 3'). As a result of the triply hydrogen bonding microenvironment the Zn-Cl and Zn-N(py) distances of 3 are at the upper end of the range observed for axial Zn-Cl bonds, whereas the axial Zn-N distance is one of shortest among N4 ligands that induce a trigonal bipyramidal geometry. Despite the rigidity of these tripodal ligands, the geometry of the intramolecular RN-HX-Zn hydrogen bonds (X=Cl-, OH-, NO3-) is strongly dependent on the nature of X, however, on average, similar for R=H, CH2tBu.

Synthesis, structure and comparison of the DNA cleavage ability of metal complexes M(ii)L with the N-(2-ethoxyethanol)-bis(2-picolyl)amine ligand L (M = Co, Ni, Cu and Zn)

Complexes of a N,N-bis(2-picolyl)amine (bpa) derivative with a pendant ethoxyethanol side chain (bpa(CH2)2O(CH2)2OH) (1) with late divalent transition metal ions Co(II), Ni(II), Cu(II) and Zn(II) have been studied. All complexes, [[bpa(CH2)2O(CH2)2OH]Co(NO3)](NO3) (1Co), [[bpa(CH2)2O(CH2)2OH]Ni(NO3)](NO3) (1Ni), [[bpa(CH2)2O(CH2)2OH]Cu(H2O)(NO3)](NO3) (1Cu) and [[bpa(CH2)2O(CH2)2OH]Zn(NO3)](NO3) (1Zn), were comprehensively characterized and their X-ray single crystal structures have been determined. The complexes show hexacoordinated geometries, in which 1 acts as a tetradentate (1Cu) or pentadentate (1Co, 1Ni and 1Zn) ligand. DNA cleavage experiments have been performed on supercoiled double stranded DNA plasmids in order to compare the cleavage efficiency of all four metals in the same ligand environment of 1. In this assay, 1Co and 1Cu showed the highest cleavage efficiency, whereas 1Ni and 1Zn were virtually inactive. Quantification of the gel electrophoresis bands showed that more than 80% of the plasmid has suffered at least one single strand cut in the case of 1Cu, and about 50% of the plasmid was nicked by 1Co. The differential cleavage activity is discussed in relation to the structural findings and a mechanism is proposed for 1Cu.

Peptide nucleic acid-metal complex conjugates: facile modulation of PNA-DNA duplex stability

Conjugates of peptide nucleic acids (PNA) and metal binding ligands were prepared using solid-phase synthesis. Stability of duplexes of bis-picolylamine-PNA conjugates and DNA was found to be modulated by equimolar concentrations of bioavailable metal ions: Ni(2+), Zn(2+)>Cu(2+). Sequence specificity of PNA was not compromised in the presence of these metal ions.

The reactivity of N-coordinated amides in metallopeptide frameworks: molecular events in metal-induced pathogenic pathways?

The amino acid derived tertiary amide ligand tert-butoxycarbonyl-(S)-alanine-N,N-bis(picolyl)amide (Boc-(S)-Ala-bpa, 1) has been synthesized as a model for metal-coordinating peptide frameworks. Its reactions with copper(II) and cadmium(II) salts have been studied. Binding of Cu2+ results in amide bond cleavage and formation of [(bpa)(solvent)Cu]2+ complexes. In contrast, the stable, eight-coordinate complex [(Boc-(S)-Ala-bpa)Cd(NO3)2] (5) has been isolated and characterized by X-ray crystallography. An unusual tertiary amide nitrogen coordination is observed in 5; this gives rise to significantly reduced cis-trans isomerization barriers. Possible implications for metal-induced conformational changes in proteins are discussed.