Thermal effects and vibrational corrections to transition metal NMR chemical shifts

Both zero-point and classical thermal effects on the chemical shift of transition metals have been calculated at appropriate levels of density functional theory for a number of complexes of titanium, vanadium, manganese and iron. The zero-point effects were computed by applying a perturbational approach, whereas classical thermal effects were probed by Car-Parrinello molecular dynamics simulations. The systematic investigation shows that both procedures lead to a deshielding of the magnetic shielding constants evaluated at the GIAO-B3 LYP level, which in general also leads to a downfield shift in the relative chemical shifts, delta. The effect is small for the titanium and vanadium complexes, where it is typically on the order of a few dozen ppm, and is larger for the manganese and iron complexes, where it can amount to several hundred ppm. Zero-point corrections are usually smaller than the classical thermal effect. The pronounced downfield shift is due to the sensitivity of the shielding of the metal centre with regard to the metal-ligand bond length, which increase upon vibrational averaging. Both applied methods improve the accuracy of the chemical shifts in some cases, but not in general.     

Haloacyl complexes of boron, [(CF3)3BC(O)Hal]- (Hal=F, Cl, Br, I)

The haloacyltris(trifluoromethyl)borate anions [(CF3)3BC(O)Hal]- (Hal=F, Cl, Br, I) have been synthesized by reacting (CF3)3BCO with either MHal (M=K, Cs; Hal=F) in SO2 or MHal (M=[nBu4N]+, [Et4N]+, [Ph4P]+; Hal=Cl, Br, I) in dichloromethane. Metathesis reactions of the fluoroacyl complex with Me3SiHal (Hal=Cl, Br, I) led to the formation of its higher homologues. The thermal stabilities of the haloacyltris(trifluoromethyl)borates decrease from the fluorine to the iodine derivative. The chemical reactivities decrease in the same order as demonstrated by a series of selected reactions. The new [(CF3)3BC(O)Hal]- (Hal=F, Cl, Br) salts are used as starting materials in the syntheses of novel compounds that contain the (CF3)3B-C fragment. All borate anions [(CF3)3BC(O)Hal]- (Hal=F, Cl, Br, I) have been characterized by multinuclear NMR spectroscopy (11B, 13C, 17O, 19F) and vibrational spectroscopy. [PPh4][(CF3)3BC(O)Br] crystallizes in the monoclinic space group P2/c (no. 13) and the bond parameters are compared with those of (CF3)3BCO and K[(CF3)3BC(O)F]. The interpretation of the spectroscopic and structural data are supported by DFT calculations [B3LYP/6-311+G(d)].     

Computational modeling of polyoxotungstates by relativistic DFT calculations of (183)W NMR chemical shifts

The (183)W nuclear shielding in a variety of tungsten polyoxometalates (POM) (Lindqvist, Anderson, decatungstates, Keggin) of different shapes and charges has been modeled by DFT calculations that take into account relativistic effects, by means of the zero-order regular approximation (ZORA), and solvent effects, by the conductor-like screening model (COSMO) continuum method. The charge/surface area ratio (q/A) is proposed as an indicator of the charge density to which the solvation energies of all POMs are correlated in a satisfactory way. Among the various theoretical levels tested (ZORA scalar or spin-orbit, frozen-core or all-electron basis set, geometry optimization in the gas phase or in the continuum solvent, etc.), the best results are obtained when both geometry optimization in solvent and spin-orbit shielding are included (mean absolute error of delta=35 ppm). The quality of the computed chemical shifts depends systematically on the charge density as expressed by q/A; thus, POMs with low q/A ratios display the best agreement with experimental data. The performance of the method is such that computed values can aid the assignment of the (183)W NMR spectra of polyoxotungstates, as shown by the case of alpha-[PW(11)TiO(40)](5-), whose six signals are ranked computationally so as to almost reproduce the experimental ordering even though the signals are spaced by as little as 5 ppm.     

103Rh NMR chemical shifts in organometallic complexes: a combined experimental and density functional study

Experimental 103Rh NMR chemical shifts of mono- and binuclear rhodium(I) complexes containing s- or as-hydroindacenide and indacenediide bridging ligands with different ancillary ligands (1,5-cyclooctadiene, ethylene, carbonyl) are presented. A protocol, based on density functional theory calculations, was established to determine 103Rh NMR shielding constants in order to rationalise the effects of electronic and structural variations on the spectroscopic signal, and to gain insight into the efficiency of this computational method when applied to organometallic systems. Scalar and spin-orbit relativistic effects based on the ZORA (zeroth order regular approximation) level have been taken into account and discussed. A good agreement was found for model compounds over a wide range of chemical shifts of rhodium (approximately 10,000 ppm). This allowed us to discuss the experimental and calculated delta(103Rh) in larger complexes and to relate it to their electronic structure.     

A theoretical study of the NMR spin-spin coupling constants of the complexes [(NC)(5)Pt-Tl(CN)(n)](n-) (n = 0-3) and [(NC)(5)Pt-Tl-Pt(CN)(5)](3-): a lesson on environmental effects

The molecular geometries and the nuclear spin-spin coupling constants of the complexes [(NC)(5)Pt-Tl(CN)(n)](n-), n = 0-3, and the related system [(NC)(5)Pt-Tl-Pt(CN)(5)](3-) are studied. These complexes have received considerable interest since the first characterization of the n = 1 system by Glaser and co-workers in 1995 [J. Am. Chem. Soc. 1995, 117, 7550-7551]. For instance, these systems exhibit outstanding NMR properties, such as extremely large Pt-Tl spin-spin coupling constants. For the present work, all nuclear spin-spin coupling constants J(Pt-Tl), J(Pt-C), and J(Tl-C) have been computed by means of a two-component relativistic density functional approach. It is demonstrated by the application of increasingly accurate computational models that both the huge J(Pt-Tl) for the complex (NC)(5)Pt-Tl and the whole experimental trend among the series are entirely due to solvent effects. An approximate inclusion of the bulk solvent effects by means of a continuum model, in addition to the direct coordination, proves to be crucial. Similarly drastic effects are reported for the coupling constants between the heavy atoms and the carbon nuclei. A computational model employing the statistical average of orbital-dependent model potentials (SAOP) in addition to the solvent effects allows to accurately reproduce the experimental coupling constants within reasonable limits.     

Simulation of 59Co NMR Chemical Shifts in Aqueous Solution

⁵⁹Co chemical shifts were computed at the GIAO‐B3LYP level for [Co(CN)₆]^3?, [Co(H₂O)₆]³⁺, [Co(NH₃)₆]³⁺, and [Co(CO)₄]^? in water. The aqueous solutions were modeled by Car–Parrinello molecular dynamics (CPMD) simulations, or by propagation on a hybrid quantum‐mechanical/molecular‐mechanical Born–Oppenheimer surface (QM/MM‐BOMD). Mean absolute deviations from experiment obtained with these methods are on the order of 400 and 600 ppm, respectively, over a total δ(⁵⁹Co) range of about 18 000 ppm. The effect of the solvent on δ(⁵⁹Co) is mostly indirect, resulting primarily from substantial metal–ligand bond contractions on going from the gas phase to the bulk. The simulated solvent effects on geometries and δ(⁵⁹Co) values are well reproduced by using a polarizable continuum model (PCM), based on optimization and perturbational evaluation of quantum‐mechanical zero‐point corrections.     

The "invisible" 13C NMR chemical shift of the central carbon atom in [(Ph3PAu)6C]2+: a theoretical investigation

The experimental 13C NMR chemical shift of the central carbon atom in the octahedral [(Ph3PAu)6C]2+ cluster was investigated on the basis of relativistic density functional calculations. In order to arrive at independent model conclusions regarding the value of the chemical shift, a systematic study of the dependence of the cluster structure on the phosphine ligands, the chosen density functionals, and the basis set size was conducted. The best structures obtained were then used in the NMR calculations. Because of the cage-like cluster structure a pronounced deshielding of the central carbon nucleus could have been expected. However, upon comparison with the 13C NMR properties of the related complex [C{Au[P(C6H5)2(p-C6H4NMe2)]}6]2+, Schmidbaur et al. have assigned a signal at delta=135.2 ppm to the interstitial carbon atom. Our calculations confirm this value in the region of the aromatic carbon atoms of the triphenylphosphine ligands. The close-lying signals of the 108 phenyl carbon atoms can explain the difficulties of assigning them experimentally.     

Characterising lone-pair activity of lead(II) by 207Pb solid-state NMR spectroscopy: coordination polymers of [N(CN)2]- and [Au(CN)2]- with terpyridine ancillary ligands

A series of lead(II) coordination polymers containing [N(CN)₂]^? (DCA) or [Au(CN)₂]^? bridging ligands and substituted terpyridine (terpy) ancillary ligands ([Pb(DCA)₂] ( 1 ), [Pb(terpy)(DCA)₂] ( 2 ), [Pb(terpy){Au(CN)₂}₂] ( 3 ), [Pb(4′‐chloro‐terpy){Au(CN)₂}₂] ( 4 ) and [Pb(4′‐bromo‐terpy)(μ‐OH₂)_0.5{Au(CN)₂}₂] ( 5 )) was spectroscopically examined by solid‐state ²⁰⁷Pb MAS NMR spectroscopy in order to characterise the structural and electronic changes associated with lead(II) lone‐pair activity. Two new compounds, 2 and [Pb(4′‐hydroxy‐terpy){Au(CN)₂}₂] ( 6 ), were prepared and structurally characterised. The series displays contrasting coordination environments, bridging ligands with differing basicities and structural and electronic effects that occur with various substitutions on the terpyridine ligand (for the [Au(CN)₂]^? polymers). ²⁰⁷Pb NMR spectra show an increase in both isotropic chemical shift and span (Ω) with increasing ligand basicity (from δ_iso=?3090 ppm and Ω=389 ppm for 1 (the least basic) to δ_iso=?1553 ppm and Ω=2238 ppm for 3 (the most basic)). The trends observed in ²⁰⁷Pb NMR data correlate with the coordination sphere anisotropy through comparison and quantification of the Pb? N bond lengths about the lead centre. Density functional theory calculations confirm that the more basic ligands result in greater p‐orbital character and show a strong correlation to the ²⁰⁷Pb NMR chemical shift parameters. Preliminary trends suggest that ²⁰⁷Pb NMR chemical shift anisotropy relates to the measured birefringence, given the established correlations with structure and lone‐pair activity.     

Density functional computations of 99Ru chemical shifts: relativistic effects, influence of the density functional, and study of solvent effects on fac-[Ru(CO)3I3]-

Solvent effects on the 99Ru NMR chemical shift of the complex fac-[Ru(CO)3I3]- are investigated computationally using density functional theory. Further, benchmark calculations of the 99Ru shift for a set of ten Ru complexes have been performed in order to calibrate the computational model and to determine the importance of relativistic effects on the 99Ru nuclear magnetic shielding and on the chemical shift. A computational model for fac-[Ru(CO)3I3]- that includes both explicit solvent molecules and a continuum model is shown to yield the best agreement with experiment. Relativistic corrections are shown to be of minor importance for determining 99Ru chemical shifts. On the other hand, the nature of the density functional is of importance. In agreement with literature data for ligand trends of 99Ru chemical shifts, the chemical shift range for different solvents is also best reproduced by a hybrid functional.     

Characterization of three members of the electron-transfer series [Fe(pda)2]n (n=2-, 1-, 0) by spectroscopy and density functional theoretical calculations [pda=redox non-innocent derivatives of N,N'-bis(pentafluorophenyl)-o-phenylenediamide(2-, 1.-, 0)

The four-coordinate iron complexes, [Fe(III)(pda(2-))(pda(.-))] (1) and [AsPh(4)](2)[Fe(II)(pda(2-))(2)] (2) were synthesized and fully characterized; pda(2-) is the closed-shell ligand N,N'-bis(pentafluorophenyl)-o-phenylenediamido(2-), and pda(.-) represents its one-electron-oxidized pi-radical anion. Single-crystal X-ray diffraction studies of 1 and 2 performed at 100(2) K reveal a distorted tetrahedral coordination environment at the iron centers, as a result of the intramolecular pi-pi interactions between C(6)F(5) rings. The electronic structures of 1 and 2 were unambiguously determined by a combination of (57)Fe M?ssbauer and electronic spectroscopy, magnetic susceptibility measurements, X-ray crystallography, and DFT calculations. Compound 1 contains an intermediate-spin Fe(III) ion (S(Fe)=3/2) strongly antiferromagnetically coupled to a pi-ligand radical (S(R)=1/2) yielding an S(t)=1 ground state. Complex 2 possesses a high-spin Fe(II) center (S(Fe)=2) with two closed-shell dianionic ligands. Complexes 1 and 2 are members of the redox series [Fe(pda)(2)](n) with n=0 for 1 and n=2- for 2. The anion n=1- has been reported previously in the coordination salt [Fe(dad)(3)][Fe(pda)(2)] (3; dad=N,N'-bis(phenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene). A complicated temperature-dependent electronic structure has been observed for this salt. Here, DFT calculations performed on 3 confirm the previous assignments of spin- and oxidation-states. Thus, [Fe(pda)(2)](n) (n=0, 1-, 2-) constitutes an electron-transfer series, which has also been established by cyclic voltammetry; the mono- and dications (n=1+ and 2+) are also accessible in solution, but have not been further investigated. The (57)Fe M?ssbauer spectra of [Fe(pda)(2)](n) species in 1 and 3 show extremely large quadrupole splitting constants due to addition of the valence and covalence contributions that have been confirmed by DFT calculations.     

The solution structure of (Me3Si)3CSiBr--an ab initio/NMR study

The first halosilylene stable in solution was investigated by ab initio/NMR calculations (IGLO SOS-DFPT PW91/B2//B3LYP/6-31+G(d)). The delta (29)Si(calc) of (Me(3)Si)(3)CSiBr (446 ppm) does not agree with the measured NMR signal at 106 ppm assigned to the free halosilylene. From the possible silylene complexes in the reaction solution, two structures agree with the observed NMR signal: the (Me(3)Si)(3)CSiBr(2) anion (delta (29)Si(calc)=124 ppm) and the unsolvated and solvated complex of the anion with two Li(+) (delta (29)Si(calc)=117 and estimated 134 ppm). Additionally the delta (29)Si(calc) of alkylsilylenes, R-Si-X, ranging from 200 to 900 ppm are presented to guide NMR identification in future silylene synthesis.     

Reactions with Oleum under Harsh Conditions: Characterization of the Unique [M(S2O7)3]2− Ions (M=Si,Ge, Sn) in A2[M(S2O7)3] (A=NH4, Ag)

The reactions of group 14 tetrachlorides MCl₄ (M=Si, Ge, Sn) with oleum (65 % SO₃) at elevated temperatures lead to the unique complex ions [M(S₂O₇)₃]^2?, which show the central M atoms in coordination with three chelating S₂O₇^2? groups. The mean distances M? O within the anions increase from 175.6(2)–177.5(2) pm (M=Si) to 186.4(4)–187.7(4) pm (M=Ge) to 201.9(2)–203.5(2) pm (M=Sn). These distances are reproduced well by DFT calculations. The same calculations show an increasing positive charge for the central M atom in the row Si, Ge, Sn, which can be interpreted as the decreasing covalency of the M? O bonds. For the silicon compound (NH₄)₂[Si(S₂O₇)₃], ²⁹Si solid‐state NMR measurements have been performed, with the results showing a signal at ?215.5 ppm for (NH₄)₂[Si(S₂O₇)₃], which is in very good agreement with theoretical estimations. In addition, the vibrational modes within the [MO₆] skeleton have been monitored by Raman spectroscopy for selected examples, and are well reproduced by theory. The charge balance for the [M(S₂O₇)₃]^2? ions is achieved by monovalent A⁺ counter ions (A=NH₄, Ag), which are implemented in the syntheses in the form of their sulfates. The sizes of the A⁺ ions, that is, their coordination requirements, cause the crystallographic differences in the crystal structures, although the complex [M(S₂O₇)₃]^2? ions remain essentially unaffected with the different A⁺ ions. Furthermore, the nature of the A⁺ ions influences the thermal behavior of the compounds, which has been monitored for selected examples by thermogravimetric differential thermal analysis (DTA/TG) and XRD measurements.     

Selectively Measuring π Back‐Donation in Gold(I) Complexes by NMR Spectroscopy

Even though the Dewar–Chatt–Duncanson model has been successfully used by chemists since the 1950s, no experimental methodology is yet known to unambiguously estimate the constituents (donation and back‐donation) of a metal–ligand interaction. It is demonstrated here that one of these components, the metal‐to‐ligand π back‐donation, can be effectively probed by NMR measurements aimed at determining the rotational barrier of a C?N bond (ΔH_r^≠) of a nitrogen acyclic carbene ligand. A large series of gold(I) complexes have been synthesized and analyzed, and it was found that the above experimental observables show an accurate correlation with back‐donation, as defined theoretically by the appropriate charge displacement originated upon bond formation. The proposed method is potentially of wide applicability for analyzing the ligand effect in metal catalysts and guiding their design.     

Transition metal-boron complexes BnM: from bowls (n = 8-14) to tires (n = 14)

Transition metal-boron complexes BnM have been predicted at density functional theory level to be molecular bowls (n = 8-14) hosting a transition metal atom (M) inside or molecular tires (n = 14) centered with a transition metal atom. Small Bn clusters prove to be effective inorganic ligands to all the VB-VIIIB transition metal elements in the periodic table. Density functional evidences obtained in this work strongly suggest that bowl-shaped fullerene analogues of Bn units exist in small BnM complexes and the bowl-to-tire structural transition occur to the first-row transition metal complexes BnM (M = Mn, Fe, Co) at n = 14, a size obviously smaller than n = 20 where the 2D-3D structural transition occurs to bare Bn. The half-sandwich-type B12Cr (C3v), full sandwich-type (B12)2Cr (D3d), bowl-shaped B14Fe (C2), and tire-shaped B14Fe (D7d) and B14Fe- (C7v) are the most interesting prototypes to be targeted in future experiments. These BnM complexes may serve as building blocks to form extended boron-rich BnMm tubes or cages (m > or = 2) or as structural units to be placed inside carbon nanotubes with suitable diameters.