Correlation of a Solution-State Conformational Change between Mercuric Chloride Complexes of Tris[(2-(6-methylpyridyl))methyl]amine with X-ray Crystallographic Structures

Structures, metal ion affinities, and fluorescence properties of soluble derivatives of tris((6-phenyl-2-pyridyl)methyl)amine

Metal complexes of tris((6-phenyl-2-pyridyl)methyl)amine (2) have hydrophobic cavities that potentially accommodate small molecules. However, the utility of this attractive motif has been hampered by the poor solubility of such complexes in many common solvents. In this study, two tripodal ligands (3, tris-[6-(3,4,5-trimethoxy-phenyl)-pyridin-2-ylmethyl]-amine, and 4, tris((6-(3,4,5-tris(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)phenyl)pyridin-2-yl)methyl)amine) derived from 2 were prepared with enhanced solubility in organic and aqueous solvents. The X-ray crystallographic analyses of selected ligands and complexes revealed that the hydrophobic cavities inside the zinc complexes were retained after derivatization. Fluorescence, nuclear magnetic resonance (NMR), and potentiometric titration studies, which were enabled by the improved solubility, were performed to investigate the binding properties of the soluble ligands (3 and 4) with metal ions such as Zn(2+) and Cu(2+). When saturating quantities of Zn(2+) ions are added to ligand 3 in acetonitrile, the fluorescence emission maximum exhibits a pronounced red shift of approximately 80 nm (from 376 to 457 nm) and is enhanced by a factor of >100 when measured at 520 nm. The fluorescence properties of the Zn(2+) ion-coordinated ligands in the Zn(3) complex are consistent with a charge-transfer character in the excited state, with possible contributions from a planarization of the pyridyl-trimethoxyphenyl groups in the excited state, and from excitonic interactions.

Bis-tridentate Chelates of an Asymmetric Ligand: X-ray Structures and Solution NMR Characterization of Divalent Zinc Triad Metal Ion Complexes of N-(2-pyridylmethyl)-N-(2-(methylthio)ethyl)amine

Divalent zinc triad metal ion complexes of type M(L)(2)(ClO(4))(2) (L = N-(2-pyridylmethyl)-N-(2-(methylthio)ethyl)amine) with N(4)S(2) metal coordination spheres were isolated and characterized by X-ray crystallography and variable temperature proton NMR. Although bis-tridentate chelates have nine geometric isomers, the crystallographically characterized complexes of all three metal ions had trans facial octahedral coordination geometry with C(i) symmetry. Despite the low coordination number and geometric preferences of d(10) metal ions, which facilitate inter- and intramolecular exchange processes, dilute solutions of these bis-tridentate chelates exhibited slow geometric isomerization. Symmetry, sterics and shielding arguments supported specific isomeric assignments for the major and minor chemical shift environments observed at low temperature. At elevated temperature, rapid intramolecular exchange occurred for all three complexes but slow intermolecular exchange on the coupling constant time scale was evidenced through detection of J(HgH) interactions for Hg(L)(2) (2+). These unusual observations are discussed in the context of the zinc triad metal ion coordination chemistry of related bis-tridentate chelates.

Investigation of Group 12 Metal Complexes with a Tridentate SNS Ligand by X-ray Crystallography and1H NMR Spectroscopy

Two series of zinc triad complexes containing the ligand 2,6-bis(methylthiomethyl)pyridine (L1) were synthesized and characterized by X-ray crystallography and solution-state 1H NMR spectroscopy. The distorted meridional octahedral M(L1)2(ClO4)2 series includes the first structurally characterized Zn(II) and Cd(II) complexes with N2(SR2)4 coordination spheres. Coordination of HgCl2 and ZnCl2 with 1 equiv of ligand afforded mononuclear, five-coordinate species Hg(L1)Cl2 and Zn(L1)Cl2, respectively, with distorted square-pyramidal and trigonal-bipyramidal geometries. With CdCl2, the dimeric [Cd(L1)Cl(mu-Cl)]2 complex was obtained. The distorted octahedral coordination geometry of each Cd(II) center in this complex is formed by one tridentate ligand, two bridging chloride ions, and one terminal chloride ion. NMR spectra indicate that the intermolecular ligand-exchange rate of [M(L1)2](2+) decreased in the order Cd(II) > Zn(II) > Hg(II). Slow intermolecular ligand-exchange conditions on the chemical-shift time scale were found for 1:2 metal-to-ligand complexes of L(1) with Hg(II) and Zn(II) but not Cd(II). Slow intermolecular ligand-exchange conditions in acetonitrile-d(3) solutions permitting detection of (3-5)J(199Hg1H) were found for 1:1 and 1:2 Hg(ClO4)2/L1 complexes, but not for the related Cd(ClO4)2) complexes. The magnitudes of J(199Hg1H) for equivalent protons were smaller in [Hg(L1)2](2+) than in [Hg(L1)(NCCH3)x](2+). The relative intermolecular ligand-exchange rates of the zinc triad complexes investigated here suggest that the toxicity of Hg(II) is accentuated by the relative difficulty of displacing it from the coordination sites encountered.

Mercury-199 NMR Studies of Thiacrown and Related Macrocyclic Complexes: The Crystal Structures of [Hg(18S6)](PF6)2 and [Hg(9N3)2](ClO4)2

We wish to report the first measurements of (199)Hg NMR chemical shift data for a series of homoleptic Hg(II) complexes with thiacrown ligands and related aza and mixed thia/aza macrocycles. In mercury(II) complexes containing trithiacrown through hexathiacrown ligands, we observed (199)Hg NMR chemical shifts in the range of -298 to -1400 ppm. Upfield chemical shifts in these NMR spectra are seen whenever (a) the number of thioether sulfur donors in the complex is decreased, (b) a thioether sulfur donor is replaced by a secondary nitrogen donor, and (c) the size of the macrocycle ring increases without a change in the nature or number of the donor atoms. Changes in noncoordinating anions, such as hexafluorophosphate and perchlorate, have little effect on the (199)Hg chemical shift. For several complexes, we observed (3)J((199)Hg-(1)H) coupling in the range of 50-100 Hz, the first example of proton-mercury coupling through a C-S thioether bond. Also, we obtained unusual upfield (13)C NMR chemical shifts for methylene resonances in several of the thiacrown complexes which correspond to distortions within the five- and six-membered chelate rings bound to the mercury ion. We report the X-ray crystal structure of the complex [Hg(18S6)](PF(6))(2) (18S6 = 1,4,7,10,13,16-hexathiacyclooctadecane). The molecule crystallizes in the rare trigonal space group Pm1 with hexakis(thioether) coordination around the Hg(II) center confirming previous X-ray photoemission spectroscopic data on the compound. The lack of an observable (199)Hg NMR signal for the complex is the result of the identical length (2.689(2) Angstroms) of all six Hg-S bonds. We additionally report the X-ray structure of the complex [Hg(9N3)(2)](ClO(4))(2) (9N3 = 1,4,7-triazacyclononane) which shows hexakis(amine) coordination of the 9N3 to form a distorted trigonal prismatic structure. Solution dissociation of the one of the 9N3 ligands from the mercury ion is confirmed by multinuclear NMR experiments. For six-coordinate macrocyclic Hg(II) complexes, N6 donor sets have a preference for trigonal prisms while S6 donor sets favor octahedral structures.

Solid-State and Solution-State Coordination Chemistry of the Zinc Triad with the Mixed N,S Donor Ligand Bis(2-methylpyridyl) Sulfide

The binding of group 12 metal ions to bis(2-methylpyridyl) sulfide (1) was investigated by X-ray crystallography and NMR. Seven structures of the chloride and perchlorate salts of Hg(II), Cd(II), and Zn(II) with 1 are reported. Hg(1)(2)(ClO(4))(2), Cd(1)(2)(ClO(4))(2), and Zn(1)(2)(ClO(4))(2).CH(3)CN form mononuclear, six-coordinate species in the solid state with 1 binding in a tridentate coordination mode. Hg(1)(2)(ClO(4))(2) has a distorted trigonal prismatic coordination geometry while Cd(1)(2)(ClO(4))(2) and Zn(1)(2)(ClO(4))(2).CH(3)CN have distorted octahedral geometries. With chloride anions, the 1:1 metal to ligand complexes Hg(1)Cl(2), [Cd(1)Cl(2)](2), and Zn(1)Cl(2) are formed. A bidentate binding mode that lacks thioether coordination is observed for 1 in the four-coordinate, distorted tetrahedral complexes Zn(1)Cl(2) and Hg(1)Cl(2). [Cd(1)Cl(2)](2) is dimeric with a distorted octahedral coordination geometry and a tridentate 1. Hg(1)Cl(2) is comprised of pairs of loosely associated monomers and Zn(1)Cl(2) is monomeric. In addition, Hg(2)(1)Cl(4) is formed with alternating chloride and thioether bridges. The distorted square pyramidal Hg(II) centers result in a supramolecular zigzagging chain in the solid state. The solution (1)H NMR spectra of [Hg(1)(2)](2+) and [Hg(1)(NCCH(3))(x)()](2+) reveal (3)(-)(5)J((199)Hg(1)H) due to slow ligand exchange found in these thioether complexes. Implications for use of Hg(II) as a metallobioprobe are discussed.

Sterically demanding multidentate ligand tris[(2-(6-methylpyridyl))methyl]amine slows exchange and enhances solution state ligand proton NMR coupling to (199)Hg(II)

The solution state coordination chemistry of Hg(ClO(4))(2) with tris[(2-(6-methylpyridyl))methyl]amine (TLA) was investigated in acetonitrile-d(3) by proton NMR. Although Hg(II) is a d(10) metal ion commonly associated with notoriously rapid exchange between coordination environments, as many as six ligand environments were observed to be in slow exchange on the chemical shift time scale at select metal-to-ligand ratios. One of these ligand environments was associated with extensive heteronuclear coupling between protons and (199)Hg and was assigned to the complex [Hg(TLA)](2+). The (5)J((1)H(199)Hg) = 8 Hz associated with this complex is the first example of five-bond coupling in a nitrogen coordination compound of Hg(II). The spectral complexity of related studies conducted in acetone-d(6) precluded analysis of coordination equilibria. Crystallographic characterization of the T-shaped complex [Hg(TLAH)(CH(2)COCH(3))](ClO(4))(2) (1) in which two pyridyl rings are pendant suggested that the acidity of acetone combined with the poor coordinating abilities of the neutral solvent adds additional complexity to solution equilibria. The complex crystallizes in the triclinic space group P1 macro with a = 9.352(2) A, b = 12.956(2) A, c = 14.199(2) A, alpha = 115.458(10) degrees, beta = 90.286(11) degrees, gamma = 108.445(11) degrees, and Z = 2. The Hg-N(amine), Hg-N(pyridyl), and Hg-C bond lengths in the complex are 2.614(4), 2.159(4), and 2.080(6) A, respectively. Relevance to development of (199)Hg NMR as a metallobioprobe is discussed.