Theory of NMR shifts and endor spectroscopy in paramagnetic complexes

A theory of NMR and ENDOR spectroscopy in paramagnetic complexes is presented. The relevant matrix elements of both the contact and pseudo-contact interaction within the eigenstates of the magnetic electrons are treated exactly. It is shown, that the interactions depend upon the azimuthal angle ? of the resonant nucleus, even for high point symmetries of the paramagnetic center. In the case of NMR shift measurements, where the spin-lattice relaxation time is short, the interaction of the nuclear spin with the electron's spin and orbital motion can be described by a temperature-dependent induced magnetic field which is not parallel to the applied field. The results differ even in lowest order of the multipole expansion from previous theories. The pseudo-contact shift for rare-earth complexes is correlated to susceptibility in a modified form. In iron-series complexes, such a correlation is generally not possible. Here, the familiar point-dipole approximation used in earlier theories is valid only, if the orbital angular momentum is quenched or if the resonant nucleus lies on the p-fold rotation axis and p ≥ 2.     

Ligand conformation in lanthanide complexes by NMR paramagnetic shifts: L-proline and L-valine

A fitting procedure is descdribede to establish the conformation and molecular dynamics of flexible molecules co-ordinated to lanthanide cations from NMR paramagnetic shift data of ¹H and ¹³C resonances. The results from L -proline and L -valine in aqueous solution at low pH values are consistent with peak assignments given by independent methods. A free rotation of the pyrrolidine ring is proposed in the case of L -proline, and a rigid lock in the case of L -valine, and angle and distance values are given for the cation and ligand positions in the complex.     

Use of proton NMR to study low symmetry bonding effects in transition metal complexes

The 300 MHZ proton NMR chemical shifts of the mixed complex (salicylaldehyde)(acetyl-acetone)ethylenediimino nickel(II) and the two corresponding non-mixed complexes were studied. The differences in chemical shifts were very small, ∼ 0.05 ppm, but by looking at both the mixed and non-mixed species in the same solution relative values could be determined. The differences in chemical shifts were attributed to both a geometry effect and a basicity effect.     

Sulfur K-edge X-ray absorption spectroscopy as a probe of ligand-metal bond covalency: metal vs ligand oxidation in copper and nickel dithiolene complexes

A combination of Cu L-edge and S K-edge X-ray absorption data and density functional theory (DFT) calculations has been correlated with 33S electron paramagnetic resonance superhyperfine results to obtain the dipole integral (Is) for the S 1s-->3p transition for the dithiolene ligand maleonitriledithiolate (MNT) in (TBA)2[Cu(MNT)2] (TBA= tetra-n-butylammonium). The results have been combined with the Is of sulfide derived from XPS studies to experimentally obtain a relation between the S 1s-->4p transition energy (which reflects the charge on the S atom, QSmol) and the dipole integral over a large range of QSmol. The results show that, for high charges on S, Is can vary from the previously reported Is values, calculated using data over a limited range of QSmol. A combination of S K-edge and Cu K- and L-edge X-ray absorption data and DFT calculations has been used to investigate the one-electron oxidation of [Cu(MNT)2]2- and [Ni(MNT)2]2-. The conversion of [Cu(MNT)2]2- to [Cu(MNT)2]- results in a large change in the charge on the Cu atom in the molecule (QCumol) and is consistent with a metal-based oxidation. This is accompanied by extensive charge donation from the ligands to compensate the high charge on the Cu in [Cu(MNT)2]- based on the increased S K-edge and decreased Cu L-edge intensity, respectively. In contrast, the oxidation of [Ni(MNT)2]2- to [Ni(MNT)2]- results in a small change in QNimol, indicating a ligand-based oxidation consistent with oxidation of a molecular orbital, psiSOMO (singly occupied molecular orbital), with predominant ligand character.     

Magnesium-stabilised transition metal formyl complexes: structures,bonding, and ethenediolate formation

Herein we report the first comprehensive series of crystallographically characterised transition metal formyl complexes. In these complexes, the formyl ligand is trapped as part of a chelating structure between a transition metal (Cr, Mn, Fe, Co, Rh, W, and Ir) and a magnesium (Mg) cation. Calculations suggest that this bonding mode results in significant oxycarbene-character of the formyl ligand. Further reaction of a heterometallic Cr–Mg formyl complex results in a rare example of C–C coupling and formation of an ethenediolate complex. DFT calculations support a key role for the formyl-intermediate in ethenediolate formation. These results show that well-defined transition metal formyl complexes are potential intermediates in the homologation of carbon monoxide.

Herein we report a comprehensive series of crystallographically characterised transition metal formyl complexes.     

Theoretical investigation on multinuclear NMR chemical shifts of some tris(trifluoromethyl)boron complexes

Tris(trifluoromethyl)boron complexes have unusual properties and may find applications in many fields of chemistry, biology, and physics. To gain insight into their NMR properties, the isotropic ¹¹B, ¹³C, and ¹⁹F NMR chemical shifts of a series of tris(trifluoromethyl)boron complexes were systematically studied using the gauge‐included atomic orbitals (GIAO) method at the levels of B3LYP/6‐31 + G(d,p)//B3LYP/6‐31G* and B3LYP/6‐311 + G(d,p)//B3LYP/6‐311 + G(d,p). Solvent effects were taken into account by polarizable continuum models (PCM). The calculated results were compared with the experimental values. The reason that the structurally inequivalent fluorine atoms in a specific species give a same chemical shift in experimental measurements is attributed to the fast rotation of CF₃ group around the B? C(F₃) bond because of the low energy barrier. The calculated ¹¹B, ¹³C(F₃), and ¹⁹F chemical shifts are in good agreement with the experimental measurements, while the deviations of calculated ¹³C(X, X = O, N) chemical shifts are slightly large. For the latter, the average absolute deviations of the results from B3LYP/6‐311 + G(d,p)//B3LYP/6‐311 + G(d,p) are smaller than those from B3LYP/6‐31 + G(d,p)//B3LYP/6‐31G*, and the inclusion of PCM reduces the deviation values. The calculated ¹⁹F and ¹¹B chemical shieldings of (CF₃)₃BCO are greatly dependent on the optimized structures, while the influence of structural parameters on the calculated ¹³C chemical shieldings is minor. Copyright © 2009 John Wiley & Sons, Ltd.     

Transition metal NMR chemical shifts and polarizability effect in organometallic complexes

The literature data on substituent influence on the ⁵¹V, ⁵⁵Mn, ⁵⁷Fe, ⁵⁹Co, ⁶¹Ni, ⁹⁵Mo, ¹⁰³Rh, ¹⁸³W, ¹⁸⁷Os and ¹⁹⁵Pt NMR chemical shifts (δ) and on J (M, P; M = Mn, Fe, Mo, Rh, W, Os) coupling constants have been analyzed for 30 series of the organometallic complexes. It has been established for the first time that the δ and J values depend on the inductive, resonance and polarizability effects of substituents. The polarizability effect is caused by the partial charge on the central M atom. The contribution of this effect ranges from 3 to 86%. Copyright © 2009 John Wiley & Sons, Ltd.     

A proton NMR investigation of metalmetal bonding in trimethyltin-aluminium, -gallium, -indium and -thallium organometallic compounds

Proton NMR data for the Group III methyl derivatives, MMe₃ and LiMMe₄ are compared with NMR data for the novel tin—Group III-metal bonded species, Li[Me₃SnMMe₃] (M  Al, Ga, In and Tl) and for Li[(Me₃Sn)_n-TlMe_4?n] (n = 0 to 4), reported here for the first time.The presence of tinmetal bonding in these derivatives is established by the observed tin-across-metal coupling constants and for the thallium derivatives by the additional observation of thallium-across-tin coupling.The variation in the magnitudes of ²J(SnCH), ²J(TlCH), ³J(SnMCH) and ³J(TlSnCH) are reported as a function of M and as a function of the number of Me₃Sn groups bond to thallium in the [(Me₃Sn)_nTIME₄?_n]^?anions. Proposals concerning the factors governing the changes in these coupling constants and the chemical shifts are presented.     

Structure, energetics, and bonding of first row transition metal pentazolato complexes: a DFT study

Quantum chemical calculations with gradient-corrected (B3LYP) density functional theory for the mono- and bispentazolato complexes of the first row transition metals (V, Cr, Mn, Fe, Co, and Ni), the all-nitrogen counterparts of metallocenes, were performed, and their stability was investigated. All possible bonding modes (e.g. eta1, eta2, eta3, and eta5) of the pentazolato ligand to the transition metals have been examined. The transition metal pentazolato complexes are predicted to be strongly bound molecules. The computed total bond dissociation enthalpies that yield free transition metal atoms in their ground states and the free pentazolato ligands were found in the range of 122.0-201.9 (3.7-102.3) kcal mol(-1) for the bispentazolato (monopentazolato) complexes, while those yielding M2+ and anionic pentazolato ligands were found in the range of 473.2-516.7 (273.6-353.5) kcal mol(-1). The electronic ground states of azametallocenes along with their spectroscopic properties (IR, NMR, and UV-vis) obtained in a consistent manner across the first transition metal series provide means for discussion of their electronic and bonding properties, the identification of the respective azametallocenes, and future laboratory studies. Finally, exploring synthetic routes to azametallocenes it was found that a [2 + 3] cycloaddition of dinitrogen to a coordinated azide ligand with nickel(II) does not seem to provide a promising synthetic route for transition metal pentazolato complexes while the oxidative addition of phenylpentazole and fluoropentazole to Ni(0) bisphosphane complexes merits attention for the experimentalists.     

NMR spectroscopy on heavy nuclei of transition metal complexes. The role of polarization effect

Chemical shifts (δ) and spin-spin coupling constants (J) in the NMR spectra on heavy nuclei M (⁵¹V, ⁵⁵Mn, ⁵⁷Fe, ⁹⁵Mo, ¹⁰³Rh, ¹⁸⁷Os, ¹⁹⁵Pt) for 27 series of transition metal complexes have been analyzed. In general case in the absence of steric factors the values of δ and J depend on three effects of substituents X: inductive, resonance, and polarization. The latter effect increases with the decrease in the distance between M and X and with growing charge on atom M. The contribution of the polarization effect varies from 0 to 80% depending on the type of the series.     

Theoretical investigation of electron transfer transition in tetracyanoethylene-contained organic complexes

In this work, the authors use complete active space self-consistent field method to investigate the photoinduced charge-separated states and the electron transfer transition in complexes ethylene-tetracyanoethylene and tetramethylethylene-tetracyanoethylene. Geometries of isolated tetracyanoethylene, ethylene, and tetramethylethylene have been optimized. The ground state and the low-lying excited states of ethylene and tetracyanoethylene have been optimized. The state energies in the gas phase have been obtained and compared with the experimentally observed values. The torsion barrier of tetracyanoethylene has been investigated through the state energy calculation at different conformations. Attention has been particularly paid to the charge-separated states and the electron transfer transition of complexes. The stacked conformations of the donor-acceptor complexes have been chosen for the optimization of the ground and low-lying excited states. Equilibrium solvation has been considered by means of conductor-like screening model both in water and in dichloromethane. It has been found that the donor and tetracyanoethylene remain neutral in complexes in ground state (1)A(1) and in lowest triplet state (3)B(1), but charge separation appears in excited singlet state (1)B(1). Through the correction of nonequilibrium solvation energy based on the spherical cavity approximation, pi-->pi* electron transfer transition energies have been obtained. Compared with the experimental measurements in dichloromethane, the theoretical results in the same solvent are found higher by about 0.5 eV.     

Theoretical studies on hydrogen bonding, NMR chemical shifts and electron density topography in alpha, beta and gamma-cyclodextrin conformers

Hydrogen-bonded interactions in alpha-, beta-, and gamma-CD conformers are investigated from the molecular electron density topography and chemical shift in the nuclear magnetic resonance (NMR) spectra calculated by using the Gauge Invariant Atomic Orbital (GIAO) method within the framework of density functional theory. For the lowest-energy CD conformers in the gas phase, the O3-H...O2' hydrogen-bonding interactions are present. Calculated 1H NMR chemical shifts (delta H) correlate well with the hydrogen-bond distance as well as electron density at the bond critical point in the molecular electron density (MED) topography. The conformers of beta- and gamma-CD comprised of relatively strong secondary hydroxyl interactions are stabilized by solvation from polar solvents.     

An NMR, IR and theoretical investigation of (1)H chemical shifts and hydrogen bonding in phenols

The change in (1)H NMR chemical shifts upon hydrogen bonding was investigated using both experimental and theoretical methods. The (1)H NMR spectra of a number of phenols were recorded in CDCl(3) and DMSO solvents. For phenol, 2- and 4-cyanophenol and 2-nitrophenol the OH chemical shifts were measured as a function of concentration in CDCl(3). The plots were all linear with concentration, the gradients varying from 0.940 (phenol) to 7.85 (4-cyanophenol) ppm/M because of competing inter- and intramolecular hydrogen bonding. Ab initio calculations of a model acetone/phenol system showed that the OH shielding was linear with the H...O=C distance (R) for R < 2.1 A with a shielding coefficient of - 7.8 ppm/A and proportional to cos(2)phi where phi is the H...O=C--C dihedral angle. Other geometrical parameters had little effect. It was also found that the nuclear shielding profile is unrelated to the hydrogen bonding energy profile. The dependence of the OH chemical shift on the pi density on the oxygen atom was determined as ca 40 ppm/pi electron. This factor is similar to that for NH but four times the value for sp(2) hybridized carbon atoms. The introduction of these effects into the CHARGE programme allowed the calculation of the (1)H chemical shifts of the compounds studied. The CHARGE calculations were compared with those from the ACD database and from GIAO calculations. The CHARGE calculations were more accurate than other calculations both when all the shifts were considered and also when the OH shifts were excluded. The calculations from the ACD and GIAO approaches were reasonable when the OH shifts were excluded but not as good when all the shifts were considered. The poor treatment of the OH shifts in the GIAO calculations is very likely due to the lack of explicit solvent effects in these calculations.     

Theoretical study of first-row transition metal porphyrins and their carbonyl complexes

The equilibrium geometry, relative energies, normal mode frequencies, and electron and spin density distributions for first-row transition metal porphyrins M(P) (M is a transition metal in the oxidation state +2, P = C₂₀H₁₂N₄) and their five-and six-coordinate carbonyl complexes M(P)CO and M(P)(CO)(AB) (AB = CO, CN^?, CS) in different spin states have been calculated by the density functional theory B3LYP method with the 6-31G and 6-31G* basis sets. The energies of binding of the CO group to M(P) molecules D(M-CO) have been estimated. The calculated properties change as a function of the metal, the number of carbonyl groups (shown for Fe(P) as an example), and the multiplicity. Calculations show that, for five-coordinate complexes M(P)CO with M = Ti and V, high-spin states and significant D(M-CO) energies are typical. For Fe(P)CO, a singlet with a small D(M-CO) energy is preferable. For Cr(P)CO and Mn(P)CO (which also have small D(M-CO) energies), the states with different spins, which strongly differ in geometry and electronic structure, are close in energy, within 0.1–02. eV. The energy of binding of CO to M(P)CO (M = Cr, Mn, Fe) is considerably higher than the energy of binding of CO to M(P), which is evidence that the transformation of five-coordinate metalloporphyrins into six-coordinate ones is energetically favorable. The behavior of the D(M-CO) energies is interpreted using a qualitative model that considers not only the effects of participation (or nonparticipation) of “active” $ d_{x^2 - y^2 } The equilibrium geometry, relative energies, normal mode frequencies, and electron and spin density distributions for first-row transition metal porphyrins M(P) (M is a transition metal in the oxidation state +2, P = C₂₀H₁₂N₄) and their five-and six-coordinate carbonyl complexes M(P)CO and M(P)(CO)(AB) (AB = CO, CN⁻, CS) in different spin states have been calculated by the density functional theory B3LYP method with the 6-31G and 6-31G* basis sets. The energies of binding of the CO group to M(P) molecules D(M-CO) have been estimated. The calculated properties change as a function of the metal, the number of carbonyl groups (shown for Fe(P) as an example), and the multiplicity. Calculations show that, for five-coordinate complexes M(P)CO with M = Ti and V, high-spin states and significant D(M-CO) energies are typical. For Fe(P)CO, a singlet with a small D(M-CO) energy is preferable. For Cr(P)CO and Mn(P)CO (which also have small D(M-CO) energies), the states with different spins, which strongly differ in geometry and electronic structure, are close in energy, within 0.1–02. eV. The energy of binding of CO to M(P)CO (M = Cr, Mn, Fe) is considerably higher than the energy of binding of CO to M(P), which is evidence that the transformation of five-coordinate metalloporphyrins into six-coordinate ones is energetically favorable. The behavior of the D(M-CO) energies is interpreted using a qualitative model that considers not only the effects of participation (or nonparticipation) of “active” , and , d _xz , and d _yz AO in bonding of M to the P ring and axial ligands, but also the fraction of the total bond energy consumed for the preparation (promotion) of those “valence states” of the M(P) molecules that are realized in M(P)CO and M(P)(CO)(AB) complexes. For the series of compounds Fe(P)(CO)₂ − Fe(P)(CO)(CS) − Fe(P)(CS)₂ − Fe(P)(CO)(CN⁻) in the singlet, triplet, and ionized states, the trans influence of axial ligands in low-spin metalloporphyrins is shown to follow the same qualitative scheme as is typical of octahedral transition metal complexes: in mixed-ligand complexes (as compared to the symmetric ones), the stronger bond becomes shorter and even stronger, while the weaker bond becomes longer and even weaker. It is assumed that the same scheme will persist for more complicated low-spin six-coordinate metalloporphyrins in the states with the vacant AO and occupied d _xz and d _xz AOs involved in bonding with both axial ligands with the filled shell. Original Russian Text ? O.P. Charkin, A.V. Makarov, and N.M. Klimenko, 2008, published in Zhurnal Neorganicheskoi Khimii, 2008, Vol. 53, No. 5, pp. 781–794.     

Constitutional,configurational and conformational analysis of transition metal coordination complexes

Summary A computational approach to conformational analysis is applied to the study of transition metal coordination complexes. The method provides a means of rapidly exploring conformational space without any inherent reliance on energy calculations and is therefore applicable to a wide variety of systems. It has been incorporated into an algorithm which explores the constitutional, configurational and conformational degrees of freedom for a metal ion and a number of potential ligands. The program determines which of the possible coordination complex products could form stable conformations and can therefore be used to rationalise the products obtained from the mixture. The method is illustrated using two cases: the cobalt(III)-triethylenetetramine-glycine system and complexes of diindolopyridine derivatives.Abbreviations en ethylenediamine - trien triethylenetetramine - gly glycine - RMS root mean square - 3D three dimensional     

DFT calculations of 1H and 13C NMR chemical shifts in transition metal hydrides

Transition metal hydrides are of great interest in chemistry because of their reactivity and their potential use as catalysts for hydrogenation. Among other available techniques, structural properties in transition metal (TM) complexes are often probed by NMR spectroscopy. In this paper we will show that it is possible to establish a viable methodological strategy in the context of density functional theory, that allows the determination of 1H NMR chemical shifts of hydride ligands attached to transition metal atoms in mononuclear systems and clusters with good accuracy with respect to experiment. 13C chemical shifts have also been considered in some cases. We have studied mononuclear ruthenium complexes such as Ru(L)(H)(dppm)2 with L = H or Cl, cationic complex [Ru(H)(H2O)(dppm)2]+ and Ru(H)2(dppm)(PPh3)2, in which hydride ligands are characterized by a negative 1H NMR chemical shift. For these complexes all calculations are in relatively good agreement compared to experimental data with errors not exceeding 20% except for the hydrogen atom in Ru(H)2(dppm)(PPh3)2. For this last complex, the relative error increases to 30%, probably owing to the necessity to take into account dynamical effects of phenyl groups. Carbonyl ligands are often encountered in coordination chemistry. Specific issues arise when calculating 1H or 13C NMR chemical shifts in TM carbonyl complexes. Indeed, while errors of 10 to 20% with respect to experiment are often considered good in the framework of density functional theory, this difference in the case of mononuclear carbonyl complexes culminates to 80%: results obtained with all-electron calculations are overall in very satisfactory agreement with experiment, the error in this case does not exceed 11% contrary to effective core potentials (ECPs) calculations which yield errors always larger than 20%. We conclude that for carbonyl groups the use of ECPs is not recommended, although their use could save time for very large systems, for instance in cluster chemistry. The reliance of NMR chemical shielding on dynamical effects, such as intramolecular rearrangements or trigonal twists, is also examined for H2Fe(CO)4, K+[HFe(CO)](-), HMn(CO)5 and HRe(CO)5. The accuracy of the theory is also examined for complexes with two dihydrogen ligands (Tp*RuH(H2)2 and [FeH(H2)(DMPE)2]+) and a ruthenium cluster, [H3Ru4(C6H6)4(CO)]+. It is shown that for all complexes studied in this work, the effect of the ligands on the chemical shielding of hydrogen coordinated to metal is suitably calculated, thus yielding a very good correlation between experimental chemical shifts and theoretical chemical shielding.     

Molecular orbital calculations on transition metal complexes : XXIII. 1H nmr shifts in sandwich,mixed sandwich,and related tricarbonyl complexes

INDO SCF MO calculations have been carried out for a variety of diamagnetic sandwich, mixed sandwich, and related tricarbonyl complexes of the 3d series, and for the free ligand systems of the cyclopentadienyl anion (Cp^?), the neutral benzene molecule (Bz), and the cycloheptatrienyl cation (Ch⁺). The π bond orders for the CC linkages of the ligand rings all show significant, but broadly comparable, reductions on complexation, and the ¹H NMR shift for a given ring proton, relative to that for the appropriate free ligand, Δτ, shows a good linear correlation with the corresponding change in the charge density at that proton, ΔPππ(H). The plot of Δτ against ΔPππ(H) shows a positive (upfield) intercept of about 2.5 ppm on the Δτ axis, and it is concluded that the results provide evidence for an appreciable diminution in the aromatic character of the ligand rings on complex formation.     

A Theoretical study of the interaction of ethylene with transition metal complexes

In order to be able to describe the ethylene molecule bonded to an active site of a Titanium- or Nickel fluoride crystal, we have used the Hartree-Fock-Slater model, implemented by a Discrete Variational Method, as introduced by Ellis and Painter. The study of the ethylene molecule at a crystal surface then gives a clear, easily interpreted picture of the electronic structure. The-back donation from metal to olefin is found to be extremely important, both in the Ti- and in the Ni-complex. This back donation is caused by a strong interaction of ad orbital of the central ion with a ^* as well as the ^* molecular orbital of ethylene. As a result of these interactions, the C-C bond of ethylene is weakened considerably.A comparison is made between the Ti-ethylene and the Ni-ethylene systems.

---

Zusammenfassung Die Elektronenstruktur von Äthylen, das an ein aktives Zentrum eines Titan- bzw. Nickelfluoridkristalls gebunden ist, ist mittels der Hartree-Fock-Slater-Theorie kombiniert mit der sog. Discrete Variational Method theoretisch behandelt worden. Es zeigt sich, daß die-Backdonation sowohl im Nickel- als auch im Titankomplex außerordentlich wichtig ist; sie entsteht durch eine starke Wechselwirkung eines Metall-d-Orbitals sowohl mit einem ^* als auch einem ^*-Orbital des Äthylens. Dies führt zu einer erheblichen Schwächung der Doppelbindung.Außerdem werden die beiden Systeme (Titan und Nickel) miteinander verglichen.

---

---

Alfred P. Sloan Research Fellow