Interaction of Cd and Zn with biologically important ligands characterized using solid-state NMR and ab initio calculations

For the first time, coordination geometry and structure of metal binding sites in biologically relevant systems are studied using chemical shift parameters obtained from solid-state NMR experiments and quantum chemical calculations. It is also the first extensive report looking at metal-imidazole interaction in the solid state. The principal values of the (113)Cd chemical shift anisotropy (CSA) tensor in crystalline cadmium histidinate and two different cadmium formates (hydrate and anhydrate) were experimentally measured to understand the effect of coordination number and geometry on (113)Cd CSA. Further, (13)C and (15)N chemical shifts have also been experimentally determined to examine the influence of cadmium on the chemical shifts of (15)N and (13)C nuclei present near the metal site in the cadmium-histidine complex. These values were then compared with the chemical shift values obtained from the isostructural bis(histidinato)zinc(II) complex as well as from the unbound histidine. The results show that the isotropic chemical shift values of the carboxyl carbons shift downfield and those of amino and imidazolic nitrogens shift upfield in the metal (Zn,Cd)-histidine complexes relative to the values of the unbound histidine sample. These shifts are in correspondence with the anticipated values based on the crystal structure. Ab initio calculations on the cadmium histidinate molecule show good agreement with the (113)Cd CSA tensors determined from solid-state NMR experiments on powder samples. (15)N chemical shifts for other model complexes, namely, zinc glycinate and zinc hexaimidazole chloride, are also considered to comprehend the effect of zinc binding on (15)N chemical shifts.     

Cadmium-113 and carbon-13 nuclear magnetic resonance spectrometry of cadmium peptide complexes

The cadmium complexes of glycylglycine, glycylglycylglycine, glycyl-gamma-aminobutyric acid and beta-alanylglycine (beta-Ala-Gly) have been investigated by (113) Cd and (13)C nuclear magnetic resonance spectrometry. The minima observed in plots of the (113)Cd chemical shift vs. pH are consistent with cadmium binding first at the carboxylic site and then at the amino-group site. The chemical shift vs. pH profile for the beta-Ala-Gly system is different from that for the other peptides, and is interpreted as suggesting formation of a five-membered chelate between Cd(II) and glycyl residues at the N-terminal groups and not at the C-terminal groups. The data further indicate that the NH groups of the peptide linkages are probably not involved in complexation with cadmium. Finally, a reported pH-dependent (113)Cd resonance for parvalbumin has been reassigned.     

Dynamics of mononuclear cadmium beta-lactamase revealed by the combination of NMR and PAC spectroscopy

The two metal sites in cadmium substituted beta-lactamase from Bacillus cereus 569/H/9 have been studied by NMR spectroscopy ((1)H, (15)N, and (113)Cd) and PAC spectroscopy ((111m)Cd). Distinct NMR signals from the backbone amides are identified for the apoenzyme and the mononuclear and binuclear cadmium enzymes. For the binuclear cadmium enzyme, two (113)Cd NMR signals (142 and 262 ppm) and two (111m)Cd PAC nuclear quadrupole interactions are observed. Two nuclear quadrupole interactions are also observed, with approximately equal occupancy, in the PAC spectra at cadmium/enzyme ratios < 1; these are different from those derived for the binuclear cadmium enzyme, demonstrating interaction between the two metal ion binding sites. In contrast to the observation from PAC spectroscopy, only one (113)Cd NMR signal (176 ppm) is observed at cadmium/enzyme ratios < 1. The titration of the metal site imidazole (N)H proton signals as a function of cadmium ion-to-enzyme ratio shows that signals characteristic for the binuclear cadmium enzyme appear when the cadmium ion-to-enzyme ratio is between 1 and 2, whereas no signals are observed at stoichiometries less than 1. The simplest explanation consistent with all data is that, at cadmium/enzyme ratios < 1, the single Cd(II) is undergoing exchange between the two metal sites on the enzyme. This exchange must be fast on the (113)Cd NMR time scale and slow on the (111m)Cd PAC time scale and must thus occur in a time regime between 0.1 and 10 micros.     

113Cd Solid‐State NMR for Probing the Coordination Sphere in Metal–Organic Frameworks

Spectroscopic techniques are a powerful tool for structure determination, especially if single‐crystal material is unavailable. ¹¹³Cd solid‐state NMR is easy to measure and is a highly sensitive probe because the coordination number, the nature of coordinating groups, and the geometry around the metal ion is reflected by the isotropic chemical shift and the chemical‐shift anisotropy. Here, a detailed investigation of a series of 27 cadmium coordination polymers by ¹¹³Cd solid‐state NMR is reported. The results obtained demonstrate that ¹¹³Cd NMR is a very sensitive tool to characterize the cadmium environment, also in non‐single‐crystal materials. Furthermore, this method allows the observation of guest‐induced phase transitions supporting understanding of the structural flexibility of coordination frameworks.     

A solid-state NMR investigation of single-source precursors for group 12 metal selenides; M[N(iPr2PSe)2]2 (M = Zn, Cd, Hg)

The first solid-state NMR investigation of dichalcogenoimidodiphosphinato complexes, M[N(R(2)PE)(2)](n), is presented. The single-source precursors for metal-selenide materials, M[N((i)Pr(2)PSe)(2)](2) (M = Zn, Cd, Hg), were studied by solid-state (31)P, (77)Se, (113)Cd, and (199)Hg NMR at 4.7, 7.0, and 11.7 T, representing the only (77)Se NMR measurements, and in the case of Cd[N((i)Pr(2)PSe)(2)](2)(113)Cd NMR measurements, to have been performed on these complexes. Residual dipolar coupling between (14)N and (31)P was observed in solid-state (31)P NMR spectra at 4.7 and 7.0 T yielding average values of R((31)P,(14)N)(eff) = 880 Hz, C(Q)((14)N) = 3.0 MHz, (1)J((31)P,(14)N)(iso) = 15 Hz, alpha = 90 degrees , beta = 26 degrees . The solid-state NMR spectra obtained were used to determine the respective phosphorus, selenium, cadmium, and mercury chemical shift tensors along with the indirect spin-spin coupling constants: (1)J((77)Se,(31)P)(iso), (1)J((111/113)Cd,(77)Se)(iso), (1)J((199)Hg,(77)Se)(iso), and (2)J((199)Hg,(31)P)(iso). Density functional theory magnetic shielding tensor calculations were performed yielding the orientations of the corresponding chemical shift tensors. For this series of complexes the phosphorus magnetic shielding tensors are essentially identical, the selenium magnetic shielding tensors are also very similar with respect to each other, and the magnetic shielding tensors of the central metals, cadmium and mercury, display near axial symmetry demonstrating an expected deviation from local S(4) symmetry.     

Comparison of the binding of cadmium(II), mercury(II), and arsenic(III) to the de novo designed peptides TRI L12C and TRI L16C

Designed alpha-helical peptides of the TRI family with a general sequence Ac-G(LKALEEK)(4)G-CONH(2) were used as model systems for the study of metal-protein interactions. Variants containing cysteine residues in positions 12 (TRI L12C) and 16 (TRI L16C) were used for the metal binding studies. Cd(II) binding was investigated, and the results were compared with previous and current work on Hg(II) and As(III) binding. The metal peptide assemblies were studied with the use of UV, CD, EXAFS, (113)Cd NMR, and (111m)Cd perturbed angular correlation spectroscopy. The metalated peptide aggregates exhibited pH-dependent behavior. At high pH values, Cd(II) was bound to the three sulfurs of the three-stranded alpha-helical coiled coils. A mixture of two species was observed, including Cd(II) in a trigonal planar geometry. The complexes have UV bands at 231 nm (20 600 M(-1) cm(-1)) for TRI L12C and 232 nm (22 600 M(-1) cm(-1)) for TRI L16C, an average Cd-S bond length of 2.49 A for both cases, and a (113)Cd NMR chemical shift at 619 ppm (Cd(II)(TRI L12C)(3)(-)) or 625 ppm (Cd(II)(TRI-L16C)(3)(-)). Nuclear quadrupole interactions show that two different Cd species are present for both peptides. One species with omega(0) = 0.45 rad/ns and low eta is attributed to a trigonal planar Cd-(Cys)(3) site. The other, with a smaller omega(0), is attributed to a four-coordinate Cd(Cys)(3)(H(2)O) species. At low pH, no metal binding was observed. Hg(II) binding to TRI L12C was also found to be pH dependent, and a 3:1 sulfur-to-mercury(II) species was observed at pH 9.4. These metal peptide complexes provide insight into heavy metal binding and metalloregulatory proteins such as MerR or CadC.     

Cadmium-113 NMR studies on homoleptic complexes containing thioether ligands: the crystal structures of [Cd([12]aneS4)2](ClO4)2, [Cd([18]aneS4N2)](PF6)2 and [Cd([9]aneS3)2](PF6)2

We report the measurement of 113Cd NMR chemical shift data for homoleptic thioether and related aza and mixed aza/thiacrown complexes. In a series of Cd(II) complexes containing trithioether to hexathioether ligands, we observe solution 113Cd NMR chemical shifts in the range of 225 to 731 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, or (c) the size of the macrocycle ring increases without a change in the nature or number of the donor atoms. Changes in the identity of non-coordinating anions such as perchlorate or hexafluorophosphate have little effect upon the 113Cd NMR chemical shift in solution. We report the X-ray structure of the complex [Cd([12]aneS4)2](ClO4)2 ([12]aneS4 = 1,4,7,10-tetrathiacyclododecane) (1) which shows the first example of octakis(thioether) coordination of a metal ion, forming an unusual eight-coordinate square antiprismatic structure. We report the X-ray structure of the complex [Cd([9]aneS3)2](PF6)2 ([9]aneS3 = 1,4,7-trithiacyclononane) (3a) which shows hexakis(thioether) coordination to form a distorted octahedral structure. We have also prepared and characterized the Cd(II) complex of a mixed azathiacrown, [Cd([18]aneS4N2)](PF6)2 ([18]aneS4N2 = 1,4,10,13-tetrathia-7,16-diazacyclooctadecane) (6). Its X-ray structure shows a distorted octahedral S4N2 environment around the Cd(II) with the ligand coordinated in the rac fashion. We observe a solvent- and temperature-dependent 14N-1H coupling in the 1H NMR spectrum of the complex which is not present in analogous complexes with this ligand.     

Experimental and theoretical evaluation of multisite cadmium(II) exchange in designed three-stranded coiled-coil peptides

An important factor that defines the toxicity of elements such as cadmium(II), mercury(II), and lead(II) with biological macromolecules is metal ion exchange dynamics. Intriguingly, little is known about the fundamental rates and mechanisms of metal ion exchange into proteins, especially helical bundles. Herein, we investigate the exchange kinetics of Cd(II) using de novo designed three-stranded coiled-coil peptides that contain metal complexing cysteine thiolates as a model for the incorporation of this ion into trimeric, parallel coiled coils. Peptides were designed containing both a single Cd(II) binding site, GrandL12AL16C [Grand = AcG-(LKALEEK)(5)-GNH(2)], GrandL26AL30C, and GrandL26AE28QL30C, as well as GrandL12AL16CL26AL30C with two Cd(II) binding sites. The binding of Cd(II) to any of these sites is of high affinity (K(A) > 3 × 10(7) M(-1)). Using (113)Cd NMR spectroscopy, Cd(II) binding to these designed peptides was monitored. While the Cd(II) binding is in extreme slow exchange regime without showing any chemical shift changes, incremental line broadening for the bound (113)Cd(II) signal is observed when excess (113)Cd(II) is titrated into the peptides. Most dramatically, for one site, L26AL30C, all (113)Cd(II) NMR signals disappear once a 1.7:1 ratio of Cd(II)/(peptide)(3) is reached. The observed processes are not compatible with a simple "free-bound" two-site exchange kinetics at any time regime. The experimental results can, however, be simulated in detail with a multisite binding model, which features additional Cd(II) binding site(s) which, once occupied, perturb the primary binding site. This model is expanded into differential equations for five-site NMR chemical exchange. The numerical integration of these equations exhibits progressive loss of the primary site NMR signal without a chemical shift change and with limited line broadening, in good agreement with the observed experimental data. The mathematical model is interpreted in molecular terms as representing binding of excess Cd(II) to surface Glu residues located at the helical interfaces. In the absence of Cd(II), the Glu residues stabilize the three-helical structure though salt bridge interactions with surface Lys residues. We hypothesize that Cd(II) interferes with these surface ion pairs, destabilizing the helical structure, and perturbing the primary Cd(II) binding site. This hypothesis is supported by the observation that the Cd(II)-excess line broadening is attenuated in GrandL26AE28QL30C, where a surface Glu(28), close to the metal binding site, was changed to Gln. The external binding site may function as an entry pathway for Cd(II) to find its internal binding site following a molecular rearrangement which may serve as a basis for our understanding of metal complexation, transport, and exchange in complex native systems containing α-helical bundles.     

Electronic mechanism in cadmium chemical shift

Cadmium chemical shifts are studied theoretically by the ab initio molecular orbital method. The compounds studied are CdMe₂, CdMeEt, CdEt₂, CdMe(OMe), Cd(OMe)₂, CdMe(SMe) and Cd(SMe)₂. The calculated values of the Cd chemical shifts agree excellently with the experimental values, showing quantitative reliability of the theoretical method used in this paper. The cadmium chemical shift is mainly due to the p mechanism in the paramagnetic term. The contribution of the d mechanism is small. Therefore, the metal chemical shift is proportional to the π-electron donating ability of the ligands. The diamagnetic contribution, which is determined solely by a structural factor, is small for the chemical shift. The difference between the methyl and ethyl ligands is attributed partly to the p mechanism (paramagnetic) and partly to the structural factor (diamagnetic). The outer d orbitals of the sulfur in the SMe ligand are unimportant for the Cd chemical shift.     

Solution and solid-state NMR studies of some cadmium-selenone complexes

Cadmium(II) complexes of Imidazolidine-2-selenone (ImSe) and its derivatives have been prepared with the general formula Cd(RImSe)2Cl2 (where R=Me, Et, Pr, etc.). These complexes are characterized by elemental analysis, IR and NMR (1H, 13C, 77Se and 113Cd) spectroscopy. An upfield shift in C=Se resonance of selenones in 13C NMR and in 77Se and high-frequency shifts in N-H resonances in 1H are consistent with the selenium coordination to Cd(II). The 77Se nucleus in Cd(ImSe)2Cl2 is shielded by 38 ppm on coordination, relative to the free ligand. The principal components of the 77Se, 113Cd and 13C shielding tensors for the complexes were determined from solid-state NMR data. Large selenium chemical shift anisotropies were observed for these complexes.     

Relationship between Solid State NMR Parameters and X-ray Structural Data in Tricadmium Phosphates

31P and (113)Cd MAS NMR spectra of solid beta'-tricadmium phosphate (beta'-TCdP) show a number of highly resolved resonances that agree well with the number of independent crystallographic sites indicated by the results of X-ray diffraction studies. A correlation of the (31)P chemical shifts with the crystallographic sites for the six different PO(4)(3)(-) groups in the unit cell of beta'-TCdP has been obtained by a method based on the computation of bond strength at oxygen atoms in phosphate moieties. The assignment of the (113)Cd resonances has been carried out on the basis of the relationship between the asymmetry of the chemical shift tensor (evaluated by analysis of the spinning side bands intensities in the MAS spectrum) and a geometric parameter related to the distortion from the bipyramidal trigonal coordination at each cadmium center. Samples of tricadmium phosphate with different degrees of magnesium substitution for cadmium were investigated by (31)P MAS NMR, (113)Cd MAS NMR, and X-ray diffraction. The results of these investigations showed that the magnesiums distribute randomly in the cadmium sites, inducing a marked decrease in the order of the structure.     

113Cd Shielding Tensors of Monomeric Cadmium Compounds Containing Nitrogen Donor Atoms. 3. CP/MAS Studies on Five-Coordinate Cadmium Complexes Having N(3)X(2) (X = H, N, O, and S) Donor Atoms

The principal elements of the (113)Cd shielding tensor for a set of five- coordinate compounds having mixed donor atoms coordinating to the cadmium were determined via CP/MAS NMR experiments. The first complex, [HB(3,5-Me(2)pz)(3)]CdBH(4) (where pz = pyrazolyl), has a CdN(3)H(2) inner coordination sphere. The isotropic chemical shift in the solid state is 355.1 ppm, and its chemical shift anisotropy (CSA, Deltasigma) is -596 ppm with an asymmetry parameter (eta) of 0.64. The second complex, [HB(3,5-Me(2)pz)(3)]Cd[H(2)B(pz)(2)], has five nitrogen donor atoms bonded to the cadmium. This N(5) or N(3)N(2) compound was the only material of this study to manifest dipolar splitting of the cadmium resonance from the quadrupolar (14)N. The isotropic chemical shift, CSA, and the value of eta for this material were therefore determined at higher field where the dipolar splitting was less than the linewidth, yielding values of 226.6 ppm, -247 ppm, and 0.32, respectively. A second N(5) material, [HB(3-Phpz)(3)]Cd[H(2)B(3,5-Me(2)pz)(2)], was also investigated and has an isotropic shift of 190.2 ppm, a CSA of 254 ppm, and an eta of 0.86. Also studied was [HB(3-Phpz)(3)]Cd[(Bu(t)CO)(2)CH], which has an CdN(3)O(2) inner core. The isotropic chemical shift of this complex is 173.6 ppm, and the values of Deltasigma and eta were determined to be -258 ppm and 0.38, respectively. The final compound, [HB(3,5-Me(2)pz)(3)]Cd[S(2)CNEt(2)], with N(3)S(2) donor atoms, has an isotropic shift of 275.8 ppm, an eta of 0.51, and a CSA of +375 ppm. Utilizing previous assignments, the most shielded tensor element was determined to be oriented normal to the plane of the tridentate ligand. The shielding tensor information is used to speculate on the coordination geometry of the CdN(3)O(2) inner core complex.     

Solution and solid-state structural studies of epoxide adducts of cadmium phenoxides. Chemistry relevant to epoxide activation for ring-opening reactions

The reaction of Cd[N(SiMe(3))(2)](2) with 2 equiv of the corresponding phenol in toluene has led to the isolation of [Cd(O-2,6-R(2)C(6)H(3))(2)](2) derivatives, where R represents the sterically bulky (t)Bu and Ph substituents. The dimeric nature of these complexes in the solid state has been established via X-ray crystallography, i.e., trigonal geometry around cadmium is observed in 1 (R = (t)Bu) where the two cadmium centers are bridged by two phenoxides with each metal containing a terminal phenoxide. Complex 2 (R = Ph) contains an additional interaction of the metal centers with carbon atoms of the aromatic substituents on the phenoxide ligands. These dimeric structures are maintained in weakly coordinating solvents as revealed by (113)Cd NMR in d(2)-methylene chloride, which displays (111)Cd-(113)Cd coupling. Nevertheless, because of the excessive steric requirements of these phenoxide ligands, these dimers are easily disrupted in solution by weak donor ligands such as epoxides. Three bisepoxide adducts have been isolated as crystalline solids and characterized by X-ray crystallography. As previously observed in other Cd(O-2,6-(t)Bu(2)C(6)H(3))(2) x L(2) complexes, these epoxide adducts adopt a crystallographically imposed square-planar geometry about the cadmium centers, with the exception of the exo-2,3-epoxynorbornane derivative, which displays a distorted tetrahedral geometry. Temperature-dependent (113)Cd NMR studies have established that there is little difference in the binding abilities of these epoxides with either complex 1 or complex 2. Importantly, it is concluded from these studies that the lack of reactivity of alpha-pinene oxide and exo-2,3-epoxynorbornane toward copolymerization reactions with carbon dioxide, in the presence of zinc bisphenoxide catalysts, is not due to differences in epoxide metal binding. This is further affirmed by the isolation and crystallographic characterization of the very stable Zn(O-2,6-(t)Bu(2)C(6)H(3))(2) x (exo-2,3-epoxynorbornane)(2) derivative.     

Reactivity of 2-pyridinecarboxylic esters with cadmium(II) halides: study of (113)Cd NMR solid state spectra and crystal structures of hexacoordinated complexes [CdI(2)(C(5)H(4)NCOOMe)(2)] and [CdI(2)(C(5)H(4)NCOOPr(n))(2)

The series of complexes [CdX(2)(C(5)H(4)NCOOR)] (X = Cl or Br; R = Me, Et, Pr(n)() or Pr(i)()) and [CdX(2)(C(5)H(4)NCOOR)(2)] (X = I; R = Me, Et, Pr(n)(), or Pr(i)()) have been obtained by the addition reaction of esters of 2-pyridinecarboxylic acid to cadmium(II) halides. X-ray crystal structures of two complexes [CdI(2)(C(5)H(4)NCOOR)(2)], R = Me (10) and R = Pr(n)() (12), have been determined. In both cases, the structure consists of discrete neutral monomeric units where the cadmium atom has a distorted octahedral coordination with CdI(2)N(2)O(2) core, two halides being in cis disposition. Structural information is compared with that deduced from (113)Cd CPMAS NMR experiments. Chemical shift anisotropies are discussed in terms of distortions produced in cadmium octahedra. The orientation of the principal axes of (113)Cd shielding tensor is also analyzed and related to the disposition of ligands in the structures of two analyzed compounds.     

A novel cadmium aminophosphonate: X-ray powder diffraction structure, solid-state IR and NMR spectroscopic determination of the fine structure of the organic moieties

A new divalent cadmium phosphonate, Cd2Cl2(H2O)4(H2L), has been synthesized from the ethylenediamine-N,N'-bis(methylenephosphonic acid) (H4L). The obtained microcrystalline compound has been characterized by solid-state IR spectra and 13C, 31P, and 113Cd CP MAS NMR. The static 13P NMR spectra have been also recorded to give the delta11, delta22, and delta33 chemical shift parameters for both compounds. The spectral data, collected for Cd2Cl2(H2O)4(H2L), are in an agreement with its X-ray powder diffraction structure solved with the cell dimensions a = 16.6105(10), b = 7.1572(4), and c = 6.8171(4) A and beta = 98.327(4) degrees. The octahedral coordination sphere of the cadmium atoms consists of two phosphonate oxygen atoms, two water oxygen atoms, and the two chlorine atoms. Cadmium atoms are bridged by the chlorine atoms forming four-membered rings. The phosphorus atoms exhibit a tetrahedral coordination with two oxygen atoms bonded to the cadmium atoms with P-O distances of 1.503(10) and 1.504(10) A. The third oxygen atom, showing a longer P-O distance (1.546(9) A), is not bonded to the metal center, nor is it bonded to a proton. The combined IR and NMR proton-phosphorus cross-polarization kinetic data together with the X-ray data confirm that the cadmium phosphonate has the zwitterionic structure (NH2(+)CH2P(O2Cd2)O-) similar to the initial aminophosphonic acid H4L.     

Cadmium(II) N-acetylcysteine complex formation in aqueous solution

The complex formation between Cd(II) ions and N-acetylcysteine (H(2)NAC) in aqueous solution was investigated using Cd K- and L(3)-edge X-ray absorption and (113)Cd NMR spectroscopic techniques. Two series of 0.1 M Cd(II) solutions with the total N-acetylcysteine concentration c(H2NAC) varied between 0.2-2 M were studied at pH 7.5 and 11.0, respectively. At pH = 11 a novel mononuclear [Cd(NAC)(4)](6-) complex with the average Cd-S distance 2.53(2) ? and the chemical shift δ((113)Cd) = 677 ppm was found to dominate at a concentration of the free deprotonated ligand [NAC(2-)] > 0.1 M, consistent with our previous reports on cadmium tetrathiolate complex formation with cysteine and glutathione. At pH 7.5 much higher ligand excess ([HNAC(-)] > 0.6 M) is required to make this tetrathiolate complex the major species. The (113)Cd NMR spectrum of a solution containing c(Cd(II)) = 0.5 M and c(H2NAC) = 1.0 M measured at 288 K showed three broad signals at 421, 583 and 642 ppm, which can be attributed to CdS(3)O(3), CdS(3)O and CdS(4) coordination sites, respectively, in oligomeric Cd(II)-NAC species with single thiolate bridges between the cadmium ions.     

Cadmium(II) and zinc(II) complexes of S-confused thiaporphyrin

The synthesis of 5,10,15,20-tetraphenyl-2-thia-21-carbaporphyrin [S-confused thiaporphyrin, (SCPH)H] was optimized. The formation of the phlorin was detected, which was saturated at the meso carbon adjacent to thiophene. Phlorin converted readily to (SCPH)H in the final oxidation process. Insertion of cadmium(II) and zinc(II) into S-confused thiaporphyrin yielded (SCPH)Cd(II)Cl and (SCPH)Zn(II)Cl complexes. The macrocycle acted as a monoanionic ligand. Three nitrogen atoms and the C(21)H fragment of the inverted thiophene occupied equatorial positions. The compensation of the metal charge required the apical chloride coordination. The characteristic C(21)H resonances of the inverted thiophene ring were located at 1.71 and 1.86 ppm in the 1H NMR spectra of (SCPH)Cd(II)Cl and (SCPH)Zn(II)Cl, respectively. The proximity of the thiophene fragment to the metal ion induced direct scalar couplings between the spin-active nucleus of the metal (111/113Cd) and the adjacent 1H nucleus (J(CdH) = 8.97 Hz). The interaction of the metal ion and C(21)H also was reflected by significant changes of C(21) chemical shifts: (SCPH)Zn(II)Cl, 92.9 ppm and (SCPH)Cd(II)Cl, 88.2 ppm (free ligand (SCPH)H, 123.7 ppm). The X-ray analysis performed for (SCPH)Cd(II)Cl confirmed the side-on cadmium-thiophene interaction. The Cd...C(21) distance (2.615(7) A) exceeded the typical Cd-C bond lengths, but was much shorter than the corresponding van der Waals contact. The density functional theory (DFT) was applied to model the molecular structures of zinc(II) and cadmium(II) complexes of S-confused thiaporphyrin. Subsequent AIM analysis demonstrated that the accumulation of electron density between the metal and thiophene, which is necessary to induce these couplings, was fairly small. A bond path linked the cadmium(II) ion to the proximate C(22) carbon of the thiophene.     

Complexation of Zn(II), Cd(II) and Hg(II) with thiourea and selenourea: A 1H, 13C, 15N, 77Se and 113Cd solution and solid-state NMR study

Zinc(II), cadmium(II) and mercury(II) complexes of thiourea (TU) and selenourea (SeU) of general formula M(TU)₂Cl₂ or M(SeU)₂Cl₂ have been prepared. The complexes were characterized by elemental analysis and NMR (¹H, ¹³C, ¹⁵N, ⁷⁷Se and ¹¹³Cd) spectroscopy. A low-frequency shift of the C=S resonance of thiones in ¹³C NMR and high-frequency shifts of N–H resonances in ¹H and ¹⁵N NMR are consistent with sulfur or selenium coordination to the metal ions. The Se nucleus in Cd(SeU)₂Cl₂ in ⁷⁷Se NMR is deshielded by 87?ppm on coordination, relative to the free ligand. In comparison, the analogous Zn(II) and Hg(II) complexes show deshielding by 33 and 50?ppm, respectively, indicating that the orbital overlap of Se with Cd is better. Principal components of ⁷⁷Se and ¹¹³Cd shielding tensors were determined from solid-state NMR data.     

Synthesis and spectroscopic and structural studies of a new cadmium(II)-citrate aqueous complex. Potential relevance to cadmium(II)-citrate speciation and links to cadmium toxicity

The presence of cadmium in the environment undoubtedly contributes to an increased risk of exposure and ultimate toxic influence on humans. In an effort to comprehend the chemical and biological interactions of Cd(II) with physiological ligands, like citric acid, we explored the requisite aqueous chemistry, which afforded the first aqueous Cd(II)-citrate complex [Cd(C(6)H(6)O(7))(H(2)O)](n)() (1). Compound 1 was characterized by elemental analysis, and spectroscopically by FT-IR and (113)Cd MAS NMR. Compound 1 crystallizes in the orthorhombic space group P2(1)2(1)2(1), with a = 6.166(2) A, b = 10.508(3) A, c = 13.599(5) A, V = 881.2(5) A(3), and Z = 4. The X-ray structure of 1 reveals the presence of octahedral Cd(II) ions bound to citrate ligands in a molecular crystal lattice. Citrate acts as a tridentate binder promoting coordination to one Cd(II) through the central alcoholic moiety, one terminal carboxylate group, and the central carboxylate group. In addition, the central carboxylate binds to three Cd(II) ions. Specifically, one of the oxygens of the central carboxylate serves as a bridge to two neighboring Cd(II) ions, while the other oxygen binds to a third Cd(II). A bound water molecule completes the coordination requirements of Cd(II). (113)Cd MAS NMR studies project the spectroscopic signature of the nature of the coordination environment around Cd(II) in 1, thus corroborating the X-ray findings. Collectively, the data at hand are in line with past solution studies. The latter predict that other similar low molecular mass Cd(II)-citrate complexes may exist in the acidic pH region, thus influencing the uptake of cadmium by living (micro)organisms, their ability to metabolize organic substrates, and possibly Cd(II) toxicity.     

Cadmium(II), nickel(II), and zinc(II) complexes of vacataporphyrin: a variable annulene conformation inside a standard porphyrin frame

5,10,15,20-Tetraaryl-21-vacataporphyrin, 1 (butadieneporphyrin, annulene-porphyrin hybrid), which contains a vacant space instead of heteroatomic bridge, gives diamagnetic zinc(II) 1-ZnCl and cadmium(II) 1-CdCl and paramagnetic nickel(II) 1-NiCl complexes. A metal ion is bound in the macrocyclic cavity by three pyrrolic nitrogens. Coordination imposes a steric constraint on the geometry of the ligand and leads to two stereoisomers with a butadiene fragment oriented toward 1-MCl-i or outward 1-MCl-o of the macrocyclic center. 1-CdCl-o, 1-ZnCl-o, and the free base share a common 1H NMR spectral pattern as the basic structural features of 1 are preserved after metal ion insertion. The 1H NMR spectra of 1-CdCl-i and 1-ZnCl-i reflect a decrease of aromaticity accounted for by the inverted butadiene geometry. The proximity of the butadiene fragment to the metal ion induces direct couplings between the spin-active nucleus of the metal ((111/113)Cd) and the adjacent 1H nuclei of butadiene. The pattern of chemical shifts detected for isomeric 1-NiCl-i and 1-NiCl-o is typical for high-spin nickel(II) complexes of porphyrin analogues. Resonances 2,3-H of 1-NiCl-o or 1-NiCl-i present the chemical shift typical for the beta-H pyrrolic position despite the vacancy in the location of nitrogen-21. Coordination of imidazole, methanol-d4, acetonitrile-d3, or chloride converts 1-NiCl-i and 1-NiCl-o into distinct species which contain two axial ligands: 1-Ni(Im)2+; 1-Ni(CD3OD)2+; 1-Ni(CD3CN)2+; 1-Ni(Cl)2-. The density functional theory (DFT) has been applied to model the molecular and electronic structure of feasible 1-CdCl stereoisomers. The total energies calculated using the B3LYP/LANL2DZ approach demonstrate a very small energy difference (2.3 kcal/mol) between 1-CdCl-o and 1-CdCl-i stereoisomers consistent with their concurrent formation.