Solid-State 13C NMR Evidence for Long Multicenter Intradimer Bonding in Zwitterion-like Structures

The principal values of the ¹³C chemical shift tensor for the β and δ polymorphs of π-[TTF⋅⋅⋅TCNE] (TTF=tetrathiafulvalene; TCNE=tetracyanoethylene) have been analyzed to understand the abnormally long intra-dimer bonding of singlet π-[TTF^δ+⋅⋅⋅TCNE^δ−]. These structures possess 12 intradimer contacts <3.40 Å, with the shortest intra π-[TTF⋅⋅⋅TCNE] separations involving 2-center (2c) C−S and 3c C−C−C orbital overlap contributions between the [TTF]^δ+ and [TCNE]^δ−. This solid-state NMR study compares the [TTF⋅⋅⋅TCNE] ¹³C tensor data against previously reported π-[TTF]₂²⁺ and π-[TCNE]₂²⁻ homo-dimers to determine how the tensor principal values change as a function of electronic structure for both TTF and TCNE moieties. In the β and δ phases of [TTF⋅⋅⋅TCNE], the TCNE ethylenic ¹³C shift tensors predict TCNE oxidation states of −0.46 and −0.73, respectively. The TTF sites are less similar to benchmark ¹³C data with the β-phase differing primarily in the ethylenic π-electrons. The δ form differs significantly from the homo-dimer data at all principal values at both the ethylenic and CH sites, indicating changes to both the π-electrons and σ-bonds. In both hetero-dimer phases, the NMR changes supports long bond formation at nitrile and CH sites not observed in homo-dimers.     

Structure and stability of the [TCNE](2)(2-) dimers in dichloromethane solution: a computational study

The solution behavior of [TCNE](.-), which forms long-living pi-[TCNE]22- dimers, is computationally studied by B3LYP and MCQDPT/CASSCF(2,2) calculations (a multiconfigurational quasi-degenerate perturbative calculation using a CASSCF(2,2) wavefunction, which properly accounts for the dispersion interaction). B3LYP calculations indicate minimum-energy [TCNE](2)(2-)(dichloromethane)(4) aggregates, a solvent where pi-[TCNE](2)(2-) dimers are spectroscopically observed. Their existence is attributed to [TCNE](.-)...solvent interactions that exceed the [TCNE](.-)...[TCNE](.-) repulsion. The lowest energy minimum at the B3LYP level corresponds to an open-shell singlet electronic structure, a metastable minimum where the shortest interanion C...C distance is 5.23 A. A slightly less stable minimum is also found for the closed-shell singlet when double-occupancy of the orbitals is imposed, but it converts into the open-shell singlet minimum when the double occupancy is relaxed. At the MCQDPT/CASSCF(2,2) level, the only minimum is for the closed-shell singlet (24.0 kcal/mol (101 kJ/mol) more stable than the dissociation products), consistent with experimental enthalpy of dimerization of [TCNE](.-) in dichloromethane solutions. It has an interanion C...C distance of 2.75 A and is in accord with the UV-vis experimental properties of the [TCNE](.-) solutions.     

A solid-state nitrogen-15 NMR and ab initio study of nitrobenzenes

Insight into the unexpectedly small range of isotropic nitrogen chemical shifts in nitrobenzene derivatives is gained through measurements of the chemical shift (CS) tensor by solid-state NMR experiments and ab initio molecular orbital (MO) and density functional theory (DFT) calculations. The principal components, delta(ii), of the (15)N CS tensors have been measured for nitrobenzene, 4-nitroaniline, 4-nitrotoluene, 4-nitroanisole, 4-nitroacetophenone, nitromesitylene, and 2,4,6-tri-tert-butylnitrobenzene. No obvious correlations of the delta(ii) values with traditional reactivity parameters were observed. The CS tensor components change significantly for the para-substituted nitrobenzenes, but these variations nearly cancel to yield isotropic shifts that fall in a range of only 3 ppm. Ab initio calculations of the delta(ii) values at the HF level are in poor agreement with the experimental values, whereas MP2 calculations and DFT calculations employing the B3LYP functional are in better agreement with experiment. The calculated (B3LYP/6-311G) delta(ii) values follow a trend in which delta(11) and delta(33) increase while delta(22) decreases with the accepted electron withdrawing ability of the para substituent. These changes tend to cancel yielding a variation in delta(iso) of only 4 ppm. These calculations indicate that the CS tensor has the same orientation as the carbon CS tensor in the isoelectronic benzoate anion: delta(11) bisects the O-N-O angle, delta(33) is perpendicular to the NO(2) plane, and delta(22) is in the NO(2) plane and perpendicular to delta(11).     

Phosphorus chemical shift tensors of phosphido ligands in ruthenium carbonyl compounds: (31)P NMR spectroscopy of single-crystal and powder samples and ab initio calculations

The phosphorus chemical shift (CS) tensors of several ruthenium carbonyl compounds containing a phosphido ligand, micro), bridging a Ru [bond] Ru bond were characterized by solid-state (31)P NMR spectroscopy. As well, an analogous osmium compound was examined. The structures of most of the clusters investigated have approximate local C(2v) symmetry about the phosphorus atom. Compared to the "isolated" PH(2)(-) anion, the phosphorus nucleus of a bridging phosphido ligand exhibits considerable deshielding. The phosphorus CS tensors of most of the compounds have spans ranging from 230 to 350 ppm and skews of approximately zero. Single-crystal NMR was used to investigate the orientation of the phosphorus CS tensors for two of the compounds, Ru(2)(CO)(6)(mu(2)-C [triple bond] C [bond] Ph)(mu(2)-PPh(2)) and Ru(3)(CO)(9)(mu(2)-H)(mu(2)-PPh(2)). The intermediate component of the phosphorus CS tensor, delta(22), lies along the local C(2) axis in both compounds. The least shielded component, delta(11), lies perpendicular to the Ru [bond] P [bond] Ru plane while the most shielded component, delta(33), lies perpendicular to the C [bond]P [bond] C plane. The orientation of the phosphorus CS tensor for a third compound, Ru(2)(CO)(6)(mu(2)-PPh(2))(2), was investigated by the dipolar-chemical shift NMR technique and was found to be analogous, suggesting it to be the same in all compounds. Ab initio calculations of phosphorus magnetic shielding tensors have been carried out and reproduce the orientations found experimentally. The orientation of the CS tensor has been rationalized using simple frontier MO theory. Splittings due to (99,101)Ru [bond] (31)P spin-spin coupling have been observed for several of the complexes. A rare example of (189)Os [bond] (31)P spin-spin splittings is observed in the (31)P MAS NMR spectrum of the osmium cluster, where (1)J((189)Os, (31)P) is 367 Hz. For this complex, the (189)Os nuclear quadrupolar coupling constant is on the order of several hundred megahertz.     

Phosphorus Chemical Shift Tensors of Phosphole Derivatives Determined by (31)P NMR Spectroscopy of Powder Samples

The results of a systematic solid-state (31)P NMR study of 5-phenyldibenzophosphole, DBP, its chalcogenides, and some of its transition metal complexes are reported. Phosphorus chemical shift tensors have been obtained from (31)P NMR spectra of stationary samples and of samples spinning about the magic angle. The spans of the phosphorus chemical shift tensors for DBP and its chalcogenides are comparable to those of the corresponding compounds of triphenylphosphine; however, the asymmetry of the tensors for the DBP series reflects the reduced local symmetry at phosphorus. For the complexes (DBP)M(CO)(5) and cis-(DBP)(2)M(CO)(4), where M is a group 6 transition metal (Cr, Mo, W), the most shielded component of the phosphorus shift tensor is found to be relatively independent of the metal or complex, delta(33) = -41 +/- 8 ppm, and is thought to lie along or close to the P-M bond axis direction. In contrast, delta(11) and delta(22) show considerable variation but decrease systematically on descending the group from Cr to W. Group 10 metal complexes, (DBP)(2)MX(2), have also been investigated, including several trans geometric isomers of nickel, cis and trans isomers of palladium, and cis isomers of platinum. The phosphorus shift tensors are nonaxially symmetric with spans in the range 50 -150 ppm. The phosphorus shift tensors of the two nonequivalent DBP ligands of (DBP)(2)PtX(2) (X = Cl, Br) exhibit quite different principal components. The intermediate component of the shift tensor is thought to lie along the Pt-P bond in these complexes. Some of the complexes exhibit interesting MAS-frequency-dependent (31)P NMR spectra.     

Grand canonical Monte Carlo simulations of the 129Xe NMR line shapes of xenon adsorbed in (+/-)-[Co(en)3]Cl3

The 129Xe NMR line shapes of xenon adsorbed in the nanochannels of the (+/-)-[Co(en)3]Cl3 ionic crystal have been calculated by grand canonical Monte Carlo (GCMC) simulations. The results of our GCMC simulations illustrate their utility in predicting 129Xe NMR chemical shifts in systems containing a transition metal. In particular, the nanochannels of (+/-)-[Co(en)3]Cl3 provide a simple, yet interesting, model system that serves as a building block toward understanding xenon chemical shifts in more complex porous materials containing transition metals. Using only the Xe-C and Xe-H potentials and shielding response functions derived from the Xe@CH4 van der Waals complex to model the interior of the channel, the GCMC simulations correctly predict the 129Xe NMR line shapes observed experimentally (Ueda, T.; Eguchi, T.; Nakamura, N.; Wasylishen, R. E. J. Phys. Chem. B 2003, 107, 180-185). At low xenon loading, the simulated 129Xe NMR line shape is axially symmetric with chemical-shift tensor components delta(parallel) = 379 ppm and delta(perpendicular) = 274 ppm. Although the simulated isotropic chemical shift, delta(iso) = 309 ppm, is overestimated, the anisotropy of the chemical-shift tensor is correctly predicted. The simulations provide an explanation for the observed trend in the 129Xe NMR line shapes as a function of the overhead xenon pressure: delta(perpendicular) increased from 274 to 292 ppm, while delta(parallel) changed by only 3 ppm over the entire xenon loading range. The overestimation of the isotropic chemical shifts is explained based upon the results of quantum mechanical 129Xe shielding calculations of xenon interacting with an isolated (+/-)-[Co(en)3]Cl3 molecule. The xenon chemical shift is shown to be reduced by about 12% going from the Xe@[Co(en)3]Cl3 van der Waals complex to the Xe@C2H6 fragment.     

Synthesis and reactivity of the Tl1 salts of

Synthons Tl1[TCNE]*- (1) and Tl12[TCNE]2- (2), for [TCNE]*- and [TCNE]2-, respectively, in metathesis reactions have been quantitatively prepared and characterized. The structure of 1 was solved and refined in a monoclinic unit cell at 27 degrees C [C2/c, a = 12.6966 (12) angstroms, b=7.7599 (7) angstroms, c=15.5041 (15) angstroms, beta = 96.610 (5) degrees , V= 1517.4 (2) angstroms3, Dcalcd = 2.911 gcm-3, Z=8, R1 = 0.0575, omegaR2=0.0701] and exhibits nuCN absorptions at 2,191 (s) and 2,162 (s) cm-1 consistent with metal-bound [TCNE]*-. The structure of 1 consists of a distorted square antiprismatic octacoordinate Tl1 bound to six monodentate [TCNE]*-s with TlN separations ranging from 2.901 to 3.171 angstroms averaging 3.020 angstroms, and one bidentate [TCNE]*- with TlN separations averaging 3.279 angstroms. The TlN bonding is attributed to electrostatic bonding. The [TCNE]*-s form dimerized zigzag chains with intra- and interdimer separations of 2.87 and 3.29 angstroms, respectively. The tight pi-[TCNE](2)2- dimer is diamagnetic and has the shortest intradimer [TCNE]*- distance reported. These synthons for [TCNE]*- and [TCNE]2- in metathesis reactions lead to the precipitation of, for example, TlIX (X = Cl, Br, OAc). Reaction of 1 with MnIII(porphyrin)X (X = Cl, OAc) forms the molecule-based magnets of [MnIII(porphyrin)][TCNE] composition, while the reaction of [CrI(C6H6)2]Br and (Me2N)2CC(NMe2)2Cl2, [TDAE]Cl2, with 1 forms [CrI(C6H6)2] [TCNE] and [TDAE][TCNE]2, respectively. The structure of [TDAE][TCNE]2.MeCN was solved and refined in an orthorhombic unit cell at 21 degrees C [I222, a = 10.2332(15), b = 13.341(6), c = 19.907(8) angstroms, V= 2717.7 angstroms3, Z = 4; Dcalcd = 1.216 gcm-3, R=0.083, Romega = 0.104] and exhibits upsilonCN absorptions at 2,193 (m), 2,174 (s), and 2,163 (s) cm-1 consistent with isolated [TCNE](2)2- , in contrast to the aforementioned TlI bound [TCNE](2)2-. The reaction of 2 with [TDAE]Cl2 forms [TDAE]2+[TCNE]2-.     

Electron-transfer salts of 1,2,3,4,5-pentamethylferrocene, Fe(II)(C5Me5)(C5H5). Structure and magnetic properties of two 1:1 and two 2:3 Fe(C5Me5)(C5H5) electron-transfer salts of tetracyanoethylene

The reaction of Fe(II)(C5Me5)(C5H5), FeCpCp, with percyano acceptors, A [A = C4(CN)6 (hexacyanobutadiene), TCNQF4 (perfluoro-7,7,8,8-tetracyano-p-quinodimethane), and DDQ (2,3-dichloro-5,6-dicyanobenzoquinone)], results in formation of 1:1 charge-transfer salts of [Fe(III)CpCp]*]*+[A]*- composition. With A = TCNQ (7,7,8,8-tetracyano-p-quinodimethane) a 1:2 electron-transfer salt with FeCpCp forms. With A = TCNE (tetracyanoethylene) a pair of 1:1 salts as well as a pair of 2:3 salts of [FeCpCp]2[TCNE]3.S (S = CH2Cl2, THF) have been isolated and characterized by single-crystal X-ray diffraction. [FeCpCp][TCNE] consists of parallel 1-D.D(*+)A(*-)D(*+)A(*-)D(*+)A(*-). chains, while [FeCpCp][TCNE].MeCN has a herringbone array of D(*+)A2(2-)D(*+) dimers separated by solvent molecules. Although each [TCNE](-) is disordered, the diamagnetic [TCNE]2(2-) dimer is structurally different from those observed earlier with an intradimer separation of 2.79 A. The [TCNE](-) in the 2:3 [FeCpCp]2[TCNE]3.S exists as an eclipsed diamagnetic [TCNE]2(2-) dimer with an intradimer ethylene C.C separation of 2.833 and 2.903 A for the CH2Cl2- and THF-containing materials, respectively. The bond distances and angles for all the cations are essentially equivalent, and the distances are essentially equivalent to those previously reported for [FeCp2](*+) and [FeCp2](*+) cations. The average Fe-C5H5-ring and Fe-C5Me5-ring centroid distances are 1.71 and 1.69 A, respectively, which are 0.05 A longer than reported for Fe(II)CpCp. The one-electron reduction potential for Fe(II)CpCp is 0.11 V (vs SCE). The 5 K EPR of [FeCpCp](*+)[BF4](-) exhibits an axially symmetric powder pattern with g(parallel) = 4.36 and g(perpendicular) = 1.24, and the EPR parameters are essentially identical to those reported for ferrocenium and decamethylferrocenium. The high-temperature magnetic susceptibility for polycrystalline samples of these complexes can be fit by the Curie-Weiss law, chi = C/(T - theta), with low theta values and mu(eff) values from 2.08 to 3.43 mu(B), suggesting that the polycrystalline samples measured had varying degrees of orientation. [FeCpCp][TCNE] exhibits the highest effective moment of 3.43 mu(B)/Fe and weak ferromagnetic coupling, as evidenced from the theta of 3.3 K; however, unexpectedly, it does not magnetically order above 2 K. The formation of the four phases comprising FeCpCp and TCNE emphasizes the diversity of materials that may form and the present inability to predict neither solid-state compositions nor structure types.     

Control of two-electron four-center (2e-/4c) C-C bond formation observed for tetracyanoethenide dimerization, [TCNE]2(2-)

Cu(PPh3)3(TCNE) (TCNE = tetracyanoethylene) and 14 other examples form [TCNE]22- dimers possessing a long 2.89 +/- 0.05 A two-electron four-center (2e-/4c) C-C bond in the solid state. This bond arises from the overlap of the b2g pi* singly occupied molecular orbital (SOMO) on each [TCNE]*- fragment, forming a filled bonding orbital of b2u symmetry, and the stabilizing effect of the cation...anion interactions in the crystal that exceed the anionic repulsion. In contrast, Mn(C5H5)(CO)2(TCNE) exhibits a related, but different, [TCNE]*-...TCNE]*- motif in the solid state that lacks the 2e-/4c C-C bonding. To better understand the unusual nature of 2e-/4c C-C bonding, the genesis of the differences between their respective pi-[TCNE]*-...TCNE]*- interactions was sought. The lack of 2e-/4c C-C bond formation is attributed to the weaker radical character of the [TCNE]*- ligand, which has a total spin population of only 0.5 electron, half of that required for two S = 1/2 [TCNE]*- moieties to form a [TCNE]22- dimer. Hence, the antiferromagnetic MnII-[TCNE]*- intramolecular interaction (between the formally S = 1/2 Mn-bound [TCNE]*- and the paramagnetic Mn(II)) dominates over the intermolecular pi-[TCNE]*--[TCNE]*- spin coupling (between two S = 1/2 [TCNE]*- needed to form [TCNE]22-). Therefore, by selecting specific metal ions that can interact with sigma-[TCNE]*-, dimerization forming [TCNE]22- can be favored or disfavored.     

An investigation of lanthanum coordination compounds by using solid-state 139La NMR spectroscopy and relativistic density functional theory

Lanthanum-139 NMR spectra of stationary samples of several solid La(III) coordination compounds have been obtained at applied magnetic fields of 11.75 and 17.60 T. The breadth and shape of the 139La NMR spectra of the central transition are dominated by the interaction between the 139La nuclear quadrupole moment and the electric field gradient (EFG) at that nucleus; however, the influence of chemical-shift anisotropy on the NMR spectra is non-negligible for the majority of the compounds investigated. Analysis of the experimental NMR spectra reveals that the 139La quadrupolar coupling constants (C(Q)) range from 10.0 to 35.6 MHz, the spans of the chemical-shift tensor (Omega) range from 50 to 260 ppm, and the isotropic chemical shifts (delta(iso)) range from -80 to 178 ppm. In general, there is a correlation between the magnitudes of C(Q) and Omega, and delta(iso) is shown to depend on the La coordination number. Magnetic-shielding tensors, calculated by using relativistic zeroth-order regular approximation density functional theory (ZORA-DFT) and incorporating scalar only or scalar plus spin-orbit relativistic effects, qualitatively reproduce the experimental chemical-shift tensors. In general, the inclusion of spin-orbit coupling yields results that are in better agreement with those from the experiment. The magnetic-shielding calculations and experimentally determined Euler angles can be used to predict the orientation of the chemical-shift and EFG tensors in the molecular frame. This study demonstrates that solid-state 139La NMR spectroscopy is a useful characterization method and can provide insight into the molecular structure of lanthanum coordination compounds.     

Experimental and computational characterization of the 17O quadrupole coupling and magnetic shielding tensors for p-nitrobenzaldehyde and formaldehyde

We have used solid-state 17O NMR experiments to determine the 17O quadrupole coupling (QC) tensor and chemical shift (CS) tensor for the carbonyl oxygen in p-nitro-[1-(17)O]benzaldehyde. Analyses of solid-state 17O NMR spectra obtained at 11.75 and 21.15 T under both magic-angle spinning (MAS) and stationary conditions yield the magnitude and relative orientation of these two tensors: CQ = 10.7 +/- 0.2 MHz, etaQ = 0.45 +/- 0.10, delta11 = 1050 +/- 10, delta22 = 620 +/- 10, delta33 = -35 +/- 10, alpha = 90 +/- 10, beta = 90 +/- 2, gamma = 90 +/- 10 degrees. The principal component of the 17O CS tensor with the most shielding, delta33, is perpendicular to the H-C=O plane, and the tensor component with the least shielding, delta11, lies along the C=O bond. For the 17O QC tensor, the largest (chi(zz)) and smallest (chi(xx)) components are both in the H-C=O plane being perpendicular and parallel to the C=O bond, respectively. This study represents the first time that these two fundamental 17O NMR tensors have been simultaneously determined for the carbonyl oxygen of an aldehyde functional group by solid-state 17O NMR. The reported experimental solid-state 17O NMR results provide the first set of reliable data to allow evaluation of the effect of electron correlation on individual CS tensor components. We found that the electron correlation effect exhibits significant influence on 17O chemical shielding in directions within the H-C=O plane. We have also carefully re-examined the existing experimental data on the 17O spin-rotation tensor for formaldehyde and proposed a new set of best "experimental" 17O chemical shielding tensor components: sigma11 = -1139 +/- 80, sigma22 = -533 +/- 80, sigma33 = 431 +/- 5, and sigma(iso) = -414 +/- 60 ppm. Using this new set of data, we have evaluated the accuracy of quantum chemical calculations of the 17O CS tensors for formaldehyde at the Hartree-Fock (HF), density-functional theory (DFT), M?ller-Plesset second-order perturbation (MP2), and coupled-cluster singles and doubles (CCSD) levels of theory. The conclusion is that, while results from HF and DFT tend to underestimate the electron correlation effect, the MP2 method overestimates its contribution. The CCSD results are in good agreement with the experimental data.     

Proton shielding tensors in potassium hydrogen maleate

The proton NMR in single crystals of potassium hydrogen maleate has been sttudied by means of multiple-pulse line-narrowing techniques. The magnetic shielding tensors of all magnetically inequivalent protons in the unit cell could be determined independently. Two of these protons are carboxylic, forming hydrogen bonds. The orientations of the shift tensors are consistent with the position of the hydrogens at the midpoints of the 0–0 intervals. The range of anisotropy of 32 ppm, found for the shift tensor of the caboxylic hydrogen, is larger than that found for hydrogen bonds in acids and seems to be characteristics of acidic salts.The other protons in the unit cell are olefinic. Two features distinguish this type of protons from those studied so far: (1) The magnetic shielding tensor is not even approximately axially symmetric, the principal values being ?2.4, ?5.1, ?7.3 ± 05 ppm (from adamantane); and (2) the principal directions reflect all characteristic directions of the carboncarbon double bond (while the CH direction is of no importance). The principal value in the direction perpendicular to the sp² system is the least shielded one.     

Modeling NMR chemical shift: A survey of density functional theory approaches for calculating tensor properties

The NMR chemical shift, a six-parameter tensor property, is highly sensitive to the position of the atoms in a molecule. To extract structural parameters from chemical shifts, one must rely on theoretical models. Therefore, a high quality group of shift tensors that serve as benchmarks to test the validity of these models is warranted and necessary to highlight existing computational limitations. Here, a set of 102 13C chemical-shift tensors measured in single crystals, from a series of aromatic and saccharide molecules for which neutron diffraction data are available, is used to survey models based on the density functional (DFT) and Hartree-Fock (HF) theories. The quality of the models is assessed by their least-squares linear regression parameters. It is observed that in general DFT outperforms restricted HF theory. For instance, Becke's three-parameter exchange method and mpw1pw91 generally provide the best predicted shieldings for this group of tensors. However, this performance is not universal, as none of the DFT functionals can predict the saccharide tensors better than HF theory. Both the orientations of the principal axis system and the magnitude of the shielding were compared using the chemical-shift distance to evaluate the quality of the calculated individual tensor components in units of ppm. Systematic shortcomings in the prediction of the principal components were observed, but the theory predicts the corresponding isotropic value more accurately. This is because these systematic errors cancel, thereby indicating that the theoretical assessment of shielding predictions based on the isotropic shift should be avoided.     

13C and (15)N chemical shift tensors in adenosine,guanosine dihydrate, 2'-deoxythymidine,and cytidine

The (13)C and (15)N chemical shift tensor principal values for adenosine, guanosine dihydrate, 2'-deoxythymidine, and cytidine are measured on natural abundance samples. Additionally, the (13)C and (15)N chemical shielding tensor principal values in these four nucleosides are calculated utilizing various theoretical approaches. Embedded ion method (EIM) calculations improve significantly the precision with which the experimental principal values are reproduced over calculations on the corresponding isolated molecules with proton-optimized geometries. The (13)C and (15)N chemical shift tensor orientations are reliably assigned in the molecular frames of the nucleosides based upon chemical shielding tensor calculations employing the EIM. The differences between principal values obtained in EIM calculations and in calculations on isolated molecules with proton positions optimized inside a point charge array are used to estimate the contributions to chemical shielding arising from intermolecular interactions. Moreover, the (13)C and (15)N chemical shift tensor orientations and principal values correlate with the molecular structure and the crystallographic environment for the nucleosides and agree with data obtained previously for related compounds. The effects of variations in certain EIM parameters on the accuracy of the shielding tensor calculations are investigated.     

Iron pentacarbonyl as a precursor for molecule-based magnets: formation of Fe[TCNE](2) (T(c) = 100 K) and Fe[TCNQ](2) (T(c) = 35 K) magnets

The reaction of tetracyanoethylene (TCNE) and 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) with Fe(CO)(5) leads to formation of magnetically ordered materials of Fe[TCNE](2) (T(c) = 100 K) and Fe[TCNQ](2) (T(c) = 35 K) composition, respectively. In contrast, the reaction with 1,2-dichloro-5,6-dicyanobenzoquinone (DDQ) leads to a paramagnetic material.     

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.     

Synthesis and characterization of reduced heme and heme/copper carbonmonoxy species

Carbon monoxide readily binds to heme and copper proteins, acting as a competitive inhibitor of dioxygen. As such, CO serves as a probe of protein metal active sites. In our ongoing efforts to mimic the active site of cytochrome c oxidase, reactivity toward carbon monoxide offers a unique opportunity to gain insight into the binding and spectroscopic characteristics of synthetic model compounds. In this paper, we report the synthesis and characterization of CO-adducts of ((5/6)L)Fe(II), [((5/6)L)Fe(II)...Cu(I)](B(C(6)F(5))(4)), and [(TMPA)Cu(I)(CH(3)CN)](B(C(6)F(5))(4)), where TMPA = tris(2-pyridylmethyl)amine and (5/6)L = a tetraarylporphyrinate tethered in either the 5-position ((5)L) or 6-position ((6)L) to a TMPA copper binding moiety. Reaction of ((5/6)L)Fe(II) [in THF (293 K): UV-vis 424 (Soret), 543-544 nm; (1)H NMR delta(pyrrole) 52-59 ppm (4 peaks); (2)H NMR (from ((5)L-d(8))Fe(II)) delta(pyrrole) 53.3, 54.5, 55.8, 56.4 ppm] with CO in solution at RT yielded ((5/6)L)Fe(II)-CO [in THF (293 K): UV-vis 413-414 (Soret), 532-533 nm; IR nu(CO)(Fe) 1976-1978 cm(-1); (1)H NMR delta(pyrrole) 8.8 ppm; (2)H NMR (from ((5)L-d(8))Fe(II)-CO) delta(pyrrole) 8.9 ppm; (13)C NMR delta((CO)Fe) 206.8-207.1 ppm (2 peaks)]. Experiments repeated in acetonitrile, acetone, toluene, and dichloromethane showed similar spectroscopic data. Binding of CO resulted in a change from five-coordinate, high-spin Fe(II) to six-coordinate, low-spin Fe(II), as evidenced by the upfield shift of the pyrrole resonances to the diamagnetic region ((1)H and (2)H NMR spectra). Addition of CO to [((5/6)L)Fe(II)...Cu(I)](B(C(6)F(5))(4)) [in THF (293 K): UV-vis ((6)L only) 424 (Soret), 546 nm; (1)H NMR delta(pyrrole) 54-59 ppm (multiple peaks); (2)H NMR (from [((5)L-d(8))Fe(II).Cu(I)](B(C(6)F(5))(4))) delta(pyrrole) 53.4 ppm (br)] gave the bis-carbonyl adduct [((5/6)L)Fe(II)-CO...Cu(I)-CO](B(C(6)F(5))(4)) [in THF (293 K): UV-vis ((6)L only) 413 (Soret), 532 nm; IR nu(CO)(Fe) 1971-1973 cm(-1), nu(CO)(Cu) 2091-2093 cm(-1), approximately 2070(sh) cm(-1); (1)H NMR delta(pyrrole) 8.7-8.9 ppm; (2)H NMR (from [((5)L-d(8))Fe(II)-CO...Cu(I)-CO](B(C(6)F(5))(4))) delta(pyrrole) 8.9 ppm; (13)C NMR delta((CO)Fe) 206.8-208.1 ppm (2 peaks), delta((CO)Cu) 172.4 ((5)L), 178.2 ((6)L) ppm]. Experiments in acetonitrile, acetone, and toluene exhibited spectral features similar to those reported. The [((5/6)L)Fe(II)-CO.Cu(I)-CO](B(C(6)F(5))(4)) compounds yielded (CO)(Fe) spectra analogous to those seen for ((5/6)L)Fe(II)-CO and (CO)(Cu) spectra similar to those seen for [(TMPA)Cu(I)-CO](B(C(6)F(5))(4)) [in THF (293 K): IR nu(CO)(Cu) 2091 cm(-1), approximately 2070(sh) cm(-1); (13)C NMR delta((CO)Cu) 180.3 ppm]. Additional IR studies were performed in which the [((5)L)Fe(II)-CO...Cu(I)-CO](B(C(6)F(5))(4)) in solution was bubbled with argon in an attempt to generate the iron-only mono-carbonyl [((5)L)Fe(II)-CO.Cu(I)](B(C(6)F(5))(4)) species; in coordinating solvent or with axial base present, decreases in characteristic IR-band intensities revealed complete loss of CO from copper and variable loss of CO from the heme.     

Factors affecting the solid-state structure and dimensionality of mercury cyanide/chloride double salts, and NMR characterization of coordination geometries

In the reaction of organic monocationic chlorides or coordinatively saturated metal-ligand complex chlorides with linear, neutral Hg(CN)(2) building blocks, the Lewis-acidic Hg(CN)(2) moieties accept the chloride ligands to form mercury cyanide/chloride double salt anions that in several cases form infinite 1-D and 2-D arrays. Thus, [PPN][Hg(CN)(2)Cl].H(2)O (1), [(n)Bu(4)N][Hg(CN)(2)Cl].0.5 H(2)O (2), and [Ni(terpy)(2)][Hg(CN)(2)Cl](2) (4) contain [Hg(CN)(2)Cl](2)(2-) anionic dimers ([PPN]Cl = bis(triphenylphosphoranylidene)ammonium chloride, [(n)Bu(4)N]Cl = tetrabutylammonium chloride, terpy = 2,2':6',6' '-terpyridine). [Cu(en)(2)][Hg(CN)(2)Cl](2) (5) is composed of alternating 1-D chloride-bridged [Hg(CN)(2)Cl](n)(n-) ladders and cationic columns of [Cu(en)(2)](2+) (en = ethylenediamine). When [Co(en)(3)]Cl(3) is reacted with 3 equiv of Hg(CN)(2), 1-D [[Hg(CN)(2)](2)Cl](n)(n-) ribbons and [Hg(CN)(2)Cl(2)](2-) moieties are formed; both form hydrogen bonds to [Co(en)(3)](3+) cations, yielding [Co(en)(3)][Hg(CN)(2)Cl(2)][[Hg(CN)(2)](2)Cl] (6). In [Co(NH(3))(6)](2)[Hg(CN)(2)](5)Cl(6).2H(2)O (7), [Co(NH(3))(6)](3+) cations and water molecules are sandwiched between chloride-bridged 2-D anionic [[Hg(CN)(2)](5)Cl(6)](n)(6n-) layers, which contain square cavities. The presence (or absence), number, and profile of hydrogen bond donor sites of the transition metal amine ligands were observed to strongly influence the structural motif and dimensionality adopted by the anionic double salt complex anions, while cation shape and cation charge had little effect. (199)Hg chemical shift tensors and (1)J((13)C,(199)Hg) values measured in selected compounds reveal that the NMR properties are dominated by the Hg(CN)(2) moiety, with little influence from the chloride bonding characteristics. delta(iso)((13)CN) values in the isolated dimers are remarkably sensitive to the local geometry.     

Probing hydrogen bonding and ion-carbonyl interactions by solid-state 17O NMR spectroscopy: G-ribbon and G-quartet

We report solid-state 17O NMR determination of the 17O NMR tensors for the keto carbonyl oxygen (O6) of guanine in two 17O-enriched guanosine derivatives: [6-17O]guanosine (G1) and 2',3',5'-O-triacetyl-[6-17O]guanosine (G2). In G1.2H2O, guanosine molecules form hydrogen-bonded G-ribbons where the guanine bases are linked by O6...H-N2 and N7...H-N7 hydrogen bonds in a zigzag fashion. In addition, the keto carbonyl oxygen O6 is also weakly hydrogen-bonded to two water molecules of hydration. The experimental 17O NMR tensors determined for the two independent molecules in the asymmetric unit of G1.2H2O are: Molecule A, CQ=7.8+/-0.1 MHz, etaQ=0.45+/-0.05, deltaiso=263+/-2, delta11=460+/-5, delta22=360+/-5, delta33=-30+/-5 ppm; Molecule B, CQ=7.7+/-0.1 MHz, etaQ=0.55+/-0.05, deltaiso=250+/-2, delta11=440+/-5, delta22=340+/-5, delta33=-30+/-5 ppm. In G1/K+ gel, guanosine molecules form extensively stacking G-quartets. In each G-quartet, four guanine bases are linked together by four pairs of O6...H-N1 and N7...H-N2 hydrogen bonds in a cyclic fashion. In addition, each O6 atom is simultaneously coordinated to two K+ ions. For G1/K+ gel, the experimental 17O NMR tensors are: CQ=7.2+/-0.1 MHz, etaQ=0.68+/-0.05, deltaiso=232+/-2, delta11=400+/-5, delta22=300+/-5, delta33=-20+/-5 ppm. In the presence of divalent cations such as Sr2+, Ba2+, and Pb2+, G2 molecules form discrete octamers containing two stacking G-quartets and a central metal ion, that is, (G2)4-M2+-(G2)4. In this case, each O6 atom of the G-quartet is coordinated to only one metal ion. For G2/M2+ octamers, the experimental 17O NMR parameters are: Sr2+, CQ=6.8+/-0.1 MHz, etaQ=1.00+/-0.05, deltaiso=232+/-2 ppm; Ba2+, CQ=7.0+/-0.1 MHz, etaQ=0.68+/-0.05, deltaiso=232+/-2 ppm; Pb2+, CQ=7.2+/-0.1 MHz, etaQ=1.00+/-0.05, deltaiso=232+/-2 ppm. We also perform extensive quantum chemical calculations for the 17O NMR tensors in both G-ribbons and G-quartets. Our results demonstrate that the 17O chemical shift tensor and quadrupole coupling tensor are very sensitive to the presence of hydrogen bonding and ion-carbonyl interactions. Furthermore, the effect from ion-carbonyl interactions is several times stronger than that from hydrogen-bonding interactions. Our results establish a basis for using solid-state 17O NMR as a probe in the study of ion binding in G-quadruplex DNA and ion channel proteins.