High-Resolution 113Cd MAS NMR Investigation of Structure and Bonding in Cadmium Thiocyanate Coordination Compounds. Distance Dependence of Cadmium-Nitrogen Indirect Spin-Spin Coupling Constants

The H-D-isotope effect of heavy water affecting ligand-mediated nanoparticle formation in SANS and NMR experiments

An isotopic effect of normal (H2O) vs. heavy water (D2O) is well known to fundamentally affect the structure and chemical properties of proteins, for instance. Here, we correlate the results from small angle X-ray and neutron scattering (SAXS, SANS) with high-resolution scanning transmission electron microscopy to track the evolution of CdS nanoparticle size and crystallinity from aqueous solution in the presence of the organic ligand ethylenediaminetetraacetate (EDTA) at room temperature in both H2O and D2O. We provide evidence via SANS experiments that exchanging H2O with D2O impacts nanoparticle formation by changing the equilibria and dynamics of EDTA clusters in solution as investigated by nuclear magnetic resonance analysis. The colloidal stability of the CdS nanoparticles, covered by a layer of [Cd(EDTA)]2- complexes, is significantly reduced in D2O despite the strong stabilizing effect of EDTA in suspensions of normal water. Hence, conclusions about nanoparticle formation mechanisms from D2O solutions reveal limited transferability to reactions in normal water due to isotopic effects, which thus need to be discussed for contrast match experiments.

Quasi-aromatic Möbius chelates of Cadmium(II) nitrite and/or nitrate

We report the design, structural, spectroscopic and computational characterizations of the two new quasi-aromatic Möbius chelate coordination compounds fabricated from Cd(NO3)2·4H2O and a bulky helical organic ligand derived from benzildihydrazone...

Quantitative Solid-State NMR Study on Ligand-Surface Interaction in Cysteine-Capped CdSe Magic-Sized Clusters

Ligand-surface interaction of semiconductor nanoparticles (NPs) controls their optoelectronic properties, and thus examination of the interaction is essential for the nanoelectronic applications of NPs. Herein, solid-state nuclear magnetic resonance (NMR) is performed to unravel the ligand-surface interaction in cysteine-capped CdSe magic-sized clusters. ¹⁵N-¹¹³Cd through-bond J-filtered NMR directly shows the presence of the nitrogen-cadmium chemical bond for the first time and indicates that ∼43% of the amines form the chemical bond. ¹⁵N-¹¹³Cd through-space dipolar-correlated NMR reveals that ∼54% of the amines locate nearby the surface cadmium with the average nitrogen-cadmium distance of 0.247 nm. The average distance is comparable with that estimated by J-filtered NMR. The difference of the two ratios (∼11%) proposes that some amines locate on the surface without forming the chemical bond, and these amines affect the relatively long observed distance in the dipolar-based experiment. Our study shows effectiveness of solid-state NMR for investigation of the ligand-surface interactions of NPs.

From discrete molecule, to polymer, to MOF: mapping the coordination chemistry of Cd(II) using (113)Cd solid-state NMR

Studies of three related Cd(II) systems (a discrete [Cd(II)2] unit, a one-dimensional [Cd(II)2]n coordination polymer and a Cd(II)-based MOF) all derived from the ligand 2,4,6-tris(2-pyrimidyl)-1,3,5-triazine, reveal an exceptionally rare example of (113)Cd-(113)Cd J coupling in the polymer that is detectable by solid-state NMR ((2)JCd-Cd = ∼65 Hz).

A comparison of the amorphization of zeolitic imidazolate frameworks (ZIFs) and aluminosilicate zeolites by ball-milling

Amorphization of zeolitic imidazolate frameworks during ball-milling is much more rapid than that of aluminosilicate zeolites.

A two-dimensional cadmium(II) coordination polymer based on 1-cyanomethyl-4-aza-1-azoniabicyclo[2.2.2]octane and thiocyanate ligands

A cadmium-thiocyanate complex, poly[(1-cyanomethyl-4-aza-1-azoniabicyclo[2.2.2]octane-κ(4)N)octakis-μ2-thiocyanato-κ(8)N:S;κ(8)S:N-tricadmium(II)], [Cd3(C8H14N3)2(NCS)8]n, was synthesized by the reaction of 1-cyanomethyl-4-aza-1-azoniabicyclo[2.2.2]octane chloride, cadmium nitrate tetrahydrate and potassium thiocyanide in aqueous solution. In the crystal structure, there are two independent types of Cd(II) cation (one on a centre of inversion and one in a general position) and both are in distorted octahedral coordination environments, coordinated by N and S atoms from different ligands. Neighbouring Cd(II) cations are linked together by thiocyanate bridges to form a two-dimensional network. Hydrogen-bonding interactions are involved in the formation of a three-dimensional supramolecular network.

A two-dimensional cadmium(II) polymer based on nicotinic acid and thiocyanate ligands

A cadmium–thiocyanate complex, poly[[bis(nicotinic acid-κN)di-μ-thiocyanato-κ2N:S;κ2S:N-cadmium(II)] monohydrate], {[Cd(NCS)2(C6H5NO2)2]·H2O}n, was synthesized by the reaction of nicotinic acid, cadmium nitrate tetrahydrate and potassium thiocyanide in aqueous solution. In the crystal structure, each CdIIcation is in a distorted octahedral coordination environment, coordinated by the N and S atoms of nicotinic acid and thiocyanate ligands. Neighbouring CdIIcations are linked together by thiocyanate bridges to form a two-dimensional network. Hydrogen-bond interactions between the uncoordinated solvent water molecules and the organic ligands result in the formation of the three-dimensional supramolecular network.

A one-dimensional CdII coordination polymer constructed from 4-(dimethylamino)pyridinium-1-acetate ligands and thiocyanate coordination bridges

A new cadmium-thiocyanate complex, namely catena-poly[1-carboxymethyl-4-(dimethylamino)pyridinium [cadmium(II)-tri-μ-thiocyanato-κ(4)N:S;κ(2)S:N] [[[4-(dimethylamino)pyridinium-1-acetate-κ(2)O,O']cadmium(II)]-di-μ-thiocyanato-κ(2)N:S;κ(2)S:N]], {(C9H13N2O2)[Cd(NCS)3][Cd(NCS)2(C9H12N2O2)]}n, was synthesized by the reaction of 4-(dimethylamino)pyridinium-1-acetate, cadmium nitrate tetrahydrate and potassium thiocyanide in aqueous solution. In the crystal structure, two types of Cd(II) atoms are observed in distorted octahedral coordination environments. One type of Cd(II) atom is coordinated by two O atoms from the carboxylate group of the 4-(dimethylamino)pyridinium-1-acetate ligand and by two N atoms and two S atoms from four different thiocyanate ligands, while the second type of Cd(II) atom is coordinated by three N atoms and three S atoms from six different thiocyanate ligands. Neighbouring Cd(II) atoms are linked by thiocyanate bridges to form a one-dimensional zigzag chain and a one-dimensional coordination polymer. Hydrogen-bond interactions are involved in the formation of the supramolecular network.

A new two-dimensional anionic cadmium(II) polymer constructed through thiocyanate coordination bridges

A new cadmium-thiocyanate complex, poly[4-(dimethylamino)pyridin-1-ium [di-μ-thiocyanato-κ(2)N:S;κ(2)S:N-thiocyanato-κN-cadmium(II)]], {(C7H11N2)[Cd(NCS)3]}n, was synthesized by the reaction of cadmium thiocyanate and 4-(dimethylamino)pyridine hydrochloride in aqueous solution. In the crystal structure, each Cd(II) ion is square-pyramidally coordinated by three N and two S atoms from five different thiocyanate ligands, four of which are bridging. The thiocyanate ligands play different roles in the build up of the structure; one role results in the formation of [Cd2(NCS)2] building blocks, while the other links the building blocks and cations via N-H···S hydrogen bonds. The N-H···S hydrogen bonds and weak π-π stacking interactions are involved in the formation of both a two-dimensional network structure and the supramolecular network.

Cadmium(II) complex formation with selenourea and thiourea in solution: an XAS and 113Cd NMR study

The complexes formed in methanol solutions of Cd(CF(3)SO(3))(2) with selenourea (SeU) or thiourea (TU), for thiourea also in aqueous solution, were studied by combining (113)Cd NMR and X-ray absorption spectroscopy. At low temperature (~200 K), distinct (113)Cd NMR signals were observed, corresponding to CdL(n)(2+) species (n = 0-4, L = TU or SeU) in slow ligand exchange. Peak integrals were used to obtain the speciation in the methanol solutions, allowing stability constants to be estimated. For cadmium(II) complexes with thione (C═S) or selone (C═Se) groups coordinated in Cd(S/Se)O(5) or Cd(S/Se)(2)O(4) (O from MeOH or CF(3)SO(3)(-)) environments, the (113)Cd chemical shifts were quite similar, within 93-97 ppm and 189-193 ppm, respectively. However, the difference in the chemical shift for the Cd(SeU)(4)(2+) (578 pm) and Cd(TU)(4)(2+) (526 ppm) species, with CdSe(4) and CdS(4) coordination, respectively, shows less chemical shielding for the coordinated Se atoms than for S, in contrast to the common trend with increasing shielding in the following order: O > N > Se > S. In solutions dominated by mono- and tetra-thiourea/selenourea complexes, their coordination and bond distances could be evaluated by Cd K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. At ~200 K and high excess of thiourea, a minor amount (up to ~30%) of [Cd(TU)(5-6)](2+) species was detected by an upfield shift of the (113)Cd NMR signal (up to 423 ppm) and an amplitude reduction of the EXAFS oscillation. The amount was estimated by fitting linear combinations of simulated EXAFS spectra for [Cd(TU)(4)](2+) and [Cd(TU)(6)](2+) complexes. At room temperature, [Cd(TU)(4)](2+) was the highest complex formed, also in aqueous solution. Cd L(3)-edge X-ray absorption near edge structure (XANES) spectra of cadmium(II) thiourea solutions in methanol were used to follow changes in the CdS(x)O(y) coordination. The correlations found from the current and previous studies between (113)Cd NMR chemical shifts and different Cd(II) coordination environments are generally useful for evaluating cadmium coordination to thione-containing or Se-donor ligands in biochemical systems or for monitoring speciation in solution.

Cadmium(II) complex formation with cysteine and penicillamine

The complex formation between cadmium(II) and the ligands cysteine (H(2)Cys) and penicillamine (H(2)Pen = 3,3'-dimethylcysteine) in aqueous solutions, having C(Cd(II)) approximately 0.1 mol dm(-3) and C(H(2)L) = 0.2-2 mol dm(-3), was studied at pH = 7.5 and 11.0 by means of (113)Cd NMR and Cd K- and L(3)-edge X-ray absorption spectroscopy. For all cadmium(II)-cysteine molar ratios, the mean Cd-S and Cd-(N/O) bond distances were found in the ranges 2.52-2.54 and 2.27-2.35 A, respectively. The corresponding cadmium(II)-penicillamine complexes showed slightly shorter Cd-S bonds, 2.50-2.53 A, but with the Cd-(N/O) bond distances in a similar wide range, 2.28-2.33 A. For the molar ratio C(H(2)L)/C(Cd(II)) = 2, the (113)Cd chemical shifts, in the range 509-527 ppm at both pH values, indicated complexes with distorted tetrahedral CdS(2)N(N/O) coordination geometry. With a large excess of cysteine (molar ratios C(H(2)Cys)/C(Cd(II)) >or= 10), complexes with CdS(4) coordination geometry dominate, consistent with the (113)Cd NMR chemical shifts, delta approximately 680 ppm at pH 7.5 and 636-658 ppm at pH 11.0, and their mean Cd-S distances were 2.53 +/- 0.02 A. At pH 7.5, the complexes are almost exclusively sulfur-coordinated as [Cd(S-cysteinate)(4)](n-), while at higher pH, the deprotonation of the amine groups promotes chelate formation. At pH 11.0, a minor amount of the [Cd(Cys)(3)](4-) complex with CdS(3)N coordination is formed. For the corresponding penicillamine solutions with molar ratios C(H(2)Pen)/C(Cd(II)) >or= 10, the (113)Cd NMR chemical shifts, delta approximately 600 ppm at pH 7.5 and 578 ppm at pH 11.0, together with the average bond distances, Cd-S 2.53 +/- 0.02 A and Cd-(N/O) 2.30-2.33 A, indicate that [Cd(penicillaminate)(3)](n-) complexes with chelating CdS(3)(N/O) coordination dominate already at pH 7.5 and become mixed with CdS(2)N(N/O) complexes at pH 11.0. The present study reveals differences between cysteine and penicillamine as ligands to the cadmium(II) ion that can explain why cysteine-rich metallothionines are capable of capturing cadmium(II) ions, while penicillamine, clinically useful for treating the toxic effects of mercury(II) and lead(II) exposure, is not efficient against cadmium(II) poisoning.

A solid-state 31P NMR study of 1:1 silver–triphenylphosphine complexes — Interpretation of 1J(107,109Ag,31P) values,

High-resolution solid-state 31P NMR spectroscopy was used to investigate a series of 1:1 silver–triphenylphosphine complexes, [Ph3PAgX]n, where X is a monovalent anion and n = 1, 2, 3, 4, or ∞. The 31P CP MAS NMR spectra reveal the number of distinct phosphorus sites in these complexes as well as the |1J(109Ag,31P)| values, which range from 401 ± 10 Hz (X = N3–) to 869 ± 10 Hz (X = SO3CF3–). The data obtained here and in earlier investigations indicate that |1J(109Ag,31P)| values for silver–tertiary phosphine complexes decrease as Ag–P bond lengths increase. This experimental conclusion is supported by DFT calculations, which also indicate that the Fermi-contact mechanism is the only important spin–spin coupling mechanism for 1J(109Ag,31P) in these complexes. In addition, the crystal structure of a silver–triphenylphosphine trifluoroacetate tetramer was determined using X-ray crystallography, and the structure of a silver–triphenylphosphine chloride tetramer was reinvestigated.

A Chelate-Stabilized Ruthenium(σ-pyrrolato) Complex: Resolving Ambiguities in Nuclearity and Coordination Geometry through 1H PGSE and 31P Solid-State NMR Studies

Reaction of RuCl2(PPh3)3 with LiNN' (NN' = 2-[(2,6-diisopropylphenyl)imino]pyrrolide) affords a single product, with the empirical formula RuCl[(2,6-iPr2C6H3)N=CHC4H3N](PPh3)2. We identify this species as a sigma-pyrrolato complex, [Ru(NN')(PPh3)2]2(mu-Cl)2 (3b), rather than mononuclear RuCl(NN')(PPh3)2 (3a), on the basis of detailed 1D and 2D NMR characterization in solution and in the solid state. Retention of the chelating, sigma-bound iminopyrrolato unit within 3b, despite the presence of labile (dative) chloride and PPh3 donors, indicates that the chelate effect is sufficient to inhibit sigma --> pi isomerization of 3b to a piano-stool, pi-pyrrolato structure. 2D COSY, SECSY, and J-resolved solid-state 31P NMR experiments confirm that the PPh3 ligands on each metal center are magnetically and crystallographically inequivalent, and 31P CP/MAS NMR experiments reveal the largest 99Ru-31P spin-spin coupling constant (1J(99Ru,31P) = 244 +/- 20 Hz) yet measured. Finally, 31P dipolar-chemical shift spectroscopy is applied to determine benchmark phosphorus chemical shift tensors for phosphine ligands in hexacoordinate ruthenium complexes.

Solid-state NMR spectroscopy of the quadrupolar halogens: chlorine-35/37, bromine-79/81, and iodine-127

A thorough review of 35/37Cl, 79/81Br, and 127I solid-state nuclear magnetic resonance (SSNMR) data is presented. Isotropic chemical shifts (CS), quadrupolar coupling constants, and other available information on the magnitude and orientation of the CS and electric field gradient (EFG) tensors for chlorine, bromine, and iodine in diverse chemical compounds is tabulated on the basis of over 200 references. Our coverage is through July 2005. Special emphasis is placed on the information available from the study of powdered diamagnetic solids in high magnetic fields. Our survey indicates a recent notable increase in the number of applications of solid-state quadrupolar halogen NMR, particularly 35Cl NMR, as high magnetic fields have become more widely available to solid-state NMR spectroscopists. We conclude with an assessment of possible future directions for research involving 35/37Cl, 79/81Br, and 127I solid-state NMR spectroscopy.

An 17O NMR and quantum chemical study of monoclinic and orthorhombic polymorphs of triphenylphosphine oxide

Solid-state (17)O NMR spectroscopy is employed to characterize powdered samples of known monoclinic and orthorhombic modifications of (17)O-enriched triphenylphosphine oxide, Ph(3)PO. Precise data on the orientation-dependent (17)O electric field gradient (EFG) and chemical shift (CS) tensors are obtained for both polymorphs. While the (17)O nuclear quadrupolar coupling constants (C(Q)) are essentially identical for the two polymorphs (C(Q) = -4.59 +/- 0.01 MHz (orthorhombic); C(Q) = -4.57 +/- 0.01 MHz (monoclinic)), the spans (Omega) of the CS tensors are distinctly different (Omega = 135 +/- 3 ppm (orthorhombic); Omega = 155 +/- 5 ppm (monoclinic)). The oxygen CS tensor is discussed in terms of Ramsey's theory and the electronic structure of the phosphorus-oxygen bond. The NMR results favor the hemipolar sigma-bonded R(3)P(+)-O(-) end of the resonance structure continuum over the multiple bond representation. Indirect nuclear spin-spin (J) coupling between (31)P and (17)O is observed directly in (17)O magic-angle-spinning (MAS) NMR spectra as well as in (31)P MAS NMR spectra. Ab initio and density-functional theory calculations of the (17)O EFG, CS, and (1)J((31)P,(17)O) tensors have been performed with a variety of basis sets to complement the experimental data. This work describes an interesting spin system for which the CS, quadrupolar, J, and direct dipolar interactions all contribute significantly to the observed (17)O NMR spectra and demonstrates the wealth of information which is available from NMR studies of solid materials.

Residual 31P, 35,37Cl dipolar coupling in 31P MAS spectra of chlorocyclophosphazenes

In 31P MAS NMR spectra of chlorocyclophosphazenes, characteristic splittings have been observed for PCI or PCl2 groups. At different applied magnetic fields, the fine structure and total width of the patterns change in a characteristic way, demonstrating that the splittings are due to indirect spin-spin and residual dipolar interactions with the chlorine nuclei directly bonded to phosphorus. For trans-nongeminal N3P3Cl3(NMe2)3 and N3P3Cl6 as examples, the spectra have been analyzed to obtain information on chlorine nuclear quadrupole coupling constants and 35,37Cl, 3P indirect spin-spin coupling constants. Neglect of these interactions may result in misinterpretations of the multiplicity in 3P MAS spectra of chlorophosphazenes.

Wide spectral range nonlinear optical crystals of one-dimensional coordination solids [Et4N][Cd(SCN)3] and [Et4N][Cd(SeCN)3] and the general design criteria for [R4N][Cd(XCN)3] (Where R = Alkyl and X = S, Se, Te) as NLO crystals

We report herein two new nonlinear optical (NLO) crystals, [Et4N][Cd(XCN)3], where X = S (1) and Se (2), that are transparent from 220 to 3300 nm, covering the entire near-ultraviolet, the visible, and the near-infrared spectral regions and giving rise to a very wide and continuous optical window, which is useful for many frequency conversion applications. Both 1 and 2 exhibit highly efficient second harmonic generation (SHG) as measured via the Kurtz-Perry method. The corresponding [Me4N]+ salts, [Me4N][Cd(XCN)3 where X = S (3) and Se (4), show no SHG effects. All four structures adopt noncentrosymmetric space groups (Cmc2(1) for 1 and 2 and Pna2(1) for 3 and presumably 4) and are based on one-dimensional anionic [Cd(XCN)3-] infinity zigzag chains. However, a detailed analysis of the structures of [R4N][Cd(XCN)3], where R = Et, Me and X = S, Se revealed that the difference in the second-order nonlinear responses of the Et4N+ salts (1 and 2) and the Me4N+ salts (3 and 4) is attributable to the relative alignment of the [Cd(XCN)3-] infinity zigzag chains, being parallel in the crystals of 1 and 2 but antiparallel in the crystals of 3 and 4. Also reported, for the first time, are the synthesis and crystal structure of [Et4N][Cd(SeCN)3] (2). Compound 2 crystallizes in an orthorhombic unit cell of Cmc21 space group symmetry with lattice parameters of 9.938(1) A, 16.868(2) A, 11.054(1) A, and Z = 4. Other issues related to the molecular and crystal engineering of this class of NLO materials are also discussed.

Single-Crystal (31)P NMR and X-ray Diffraction Study of a Molybdenum Phosphine Complex: (5-Methyldibenzophosphole)pentacarbonylmolybdenum(0)

The molecular structure of (5-methyldibenzophosphole)pentacarbonylmolybdenum(0), 1, has been determined by X-ray crystallography. The crystal is monoclinic C2/c, Z = 8, with unit cell dimensions of: a = 31.113(2) Å, b =7.7917(5) Å, c = 17.9522(12) Å, and beta = 122.135(4) degrees. Least-squares refinement converged to R(F) = 0.0245 for 2407 independent reflections. The molecular structure is typical of phosphine-substituted metal carbonyls. It contains an approximate mirror plane which bisects the dibenzophosphole framework. Phosphorus-31 NMR spectra of powder and single-crystal samples of 1 have been obtained with cross-polarization and (1)H high-power decoupling. The (31)P CP/MAS NMR spectra exhibit exceptionally well-resolved satellites due to spin-spin coupling interactions with (95,97)Mo (I = (5)/(2)). Using first-order perturbation theory, the multiplets have been analyzed to yield (1)J((95,97)Mo,(31)P) = 123(2) Hz and estimates of the molybdenum nuclear quadrupolar coupling constants, chi((95)Mo) = -0.87 MHz and chi((97)Mo) = 10.1 MHz. Phosphorus-31 NMR spectra of a large single crystal of 1 have been investigated as a function of orientation about three orthogonal axes in the applied magnetic field. Analysis of the data yields the three principal components of the phosphorus chemical shift tensor, delta(11) = 112 ppm, delta(22) = -23 ppm, and delta(33) = -40 ppm; delta(22) lies close to the Mo-P bond (8 degrees ), while delta(11) lies in the approximate mirror plane. The phosphorus chemical shift tensor determined for 1 is compared with the limited anisotropic phosphorus shift data available in the literature.

Nucleophilic Addition of CH, NH, and OH Bonds to the Phosphadiazonium Cation and Interpretation of (31)P Chemical Shifts at Dicoordinate Phosphorus Centers

The phosphadiazonium cation [MesNP](+) reacts quantitatively with the fluorenylide anion, MesNH(2), and MesOH (Mes = 2,4,6-tri-tert-butylphenyl), resulting in formal insertion of the N-P moiety into the H-Y (Y = C, N, O) bonds. Specifically, reaction of MesNPCl with fluorenyllithium gives the aminofluorenylidenephosphine [crystal data: C(31)H(38)NP, monoclinic, P2(1)/c, a = 9.568(8) Å, b = 24.25(2) Å, c = 11.77(1) Å, beta = 101.38(8) degrees, Z = 4]. Similarly, reaction of [MesNP][GaCl(4)] with MesNH(2) gives the diaminophosphenium salt [MesN(H)PN(H)Mes][GaCl(4)] [crystal data: C(36)H(60)Cl(4)GaN(2)P, monoclinic, C2/c, a = 24.921(2) Å, b = 10.198(4) Å, c = 16.445(2) Å, beta = 93.32(1) degrees, Z = 4], and reaction with MesOH gives the first example of an aminooxyphosphenium salt [MesN(H)POMes][GaCl(4)]. It is proposed that the reactions involve nucleophilic attack at phosphorus followed by a 1,3-hydrogen migration from Y to N. Experimental evidence for the formation of sigma-complex intermediates is provided by the isolation of [MesNP-PPh(3)][SO(3)CF(3)] [crystal data: C(37)H(44)F(3)NO(3)P(2)S, triclinic, P&onemacr;, a = 10.663(1) Å, b = 19.439(1) Å, c = 10.502(1) Å, alpha = 103.100(7) degrees, beta = 113.311(7) degrees, gamma = 93.401(7) degrees, Z = 2]. As part of the unequivocal characterization of the aminooxyphosphenium salt, detailed solid-state (31)P NMR studies and GIAO calculations on the phosphenium cations have been performed. Contrary to popular belief, the phosphorus shielding in dicoordinate cations is not caused by the positive charge but results from efficient mixing between the phosphorus lone pair and pi orbitals.