Zwitterionic amidinates as effective ligands for platinum nanoparticle hydrogenation catalysts.

Ligand control of metal nanoparticles (MNPs) is rapidly gaining importance as ligands can stabilize the MNPs and regulate their catalytic properties. Herein we report the first example of Pt NPs ligated by imidazolium-amidinate ligands that bind strongly through the amidinate anion to the platinum surface atoms. The binding was established by 15N NMR spectroscopy, a precedent for nitrogen ligands on MNPs, and XPS. Both monodentate and bidentate coordination modes were found. DFT showed a high bonding energy of up to -48 kcal mol-1 for bidentate bonding to two adjacent metal atoms, which decreased to -28 ± 4 kcal mol-1 for monodentate bonding in the absence of impediments by other ligands. While the surface is densely covered with ligands, both IR and 13C MAS NMR spectra proved the adsorption of CO on the surface and thus the availability of sites for catalysis. A particle size dependent Knight shift was observed in the 13C MAS NMR spectra for the atoms that coordinate to the surface, but for small particles, ∼1.2 nm, it almost vanished, as theory for MNPs predicts; this had not been experimentally verified before. The Pt NPs were found to be catalysts for the hydrogenation of ketones and a notable ligand effect was observed in the hydrogenation of electron-poor carbonyl groups. The catalytic activity is influenced by remote electron donor/acceptor groups introduced in the aryl-N-substituents of the amidinates; p-anisyl groups on the ligand gave catalysts several times faster the ligand containing p-chlorophenyl groups.

DFT calculations of 29Si-NMR chemical shifts in Ru(II) silyl complexes: searching for trends and accurate values.

The (29)Si chemical shifts in a series of closely related Ru(II) silyl complexes have been calculated by DFT methods and compared to the experimental values. The factors that lead to possible discrepancies between experimental and calculated values have been identified. It is shown that it is necessary to include the spin-orbit coupling associated with the relativistic effects of the heavy atoms for quantitative agreement with observed chemical shifts but trends are reasonably reproduced when the calculations do not include this correction. An NBO analysis of the NMR contributions from the bonds to Si and the Si core shows the greater importance of the former and a fine tuning originating from the latter.

Ligand effect on the NMR, vibrational and structural properties of tetra- and hexanuclear ruthenium hydrido clusters: a theoretical investigation.

Structural and spectroscopic properties of tetranuclear ruthenium hydrido clusters, and to a less extent, of hexanuclear ruthenium hydrido clusters, are investigated theoretically. Some of these (H)(n)Ru(k)(L)(m) (k = 4, 6) clusters were experimentally synthesized and characterized. Non-existing structures are also considered in order to examine the role of ligands on their structure, vibrational spectra and (1)H NMR chemical shifts. The calculated properties are found in very good agreement with experimental data, when available. Beyond the intrinsic interest elicited by transition metal clusters, these compounds are also considered in this paper as relevant to diamagnetic ruthenium nanoparticles as well as building blocks of hcp surfaces, which is the ruthenium nanoparticle lattice. On the basis of the very good agreement between experiments and theory, the structural and spectroscopic properties of several model clusters are also predicted in order to bring additional data which may help to analyze the spectral signature of ruthenium nanoparticles. A particular emphasis is put on (1)H NMR, which is of high practical importance for characterizing the presence of hydrides in ruthenium clusters and nanoparticles. Several topics are discussed: the structural preference of surface hydrides for terminal-, edge-bridging or face-capping coordination modes, hydrides adsorption energies, the possible presence of interstitial hydrogen atoms, the dependence of (1)H chemical shifts on ligands and on electron counting.

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.

The electronic spectrum of AgCl2: ab initio benchmark versus density-functional theory calculations on the lowest ligand-field states including spin-orbit effects.

The X 2pi(g), 2sigma(g)+, and 2delta(g) states of AgCl2 have been studied through benchmark ab initio complete active space self-consistent field plus second-order complete active space multireference Moller-Plesset algorithm (CASSCF+CASPT2) and complete active space self-consistent field plus averaged coupled pair functional (CASSCF+ACPF) and density-functional theory (DFT) calculations using especially developed basis sets to study the transition energies, geometries, vibrational frequencies, Mulliken charges, and spin densities. The spin-orbit (SO) effects were included through the effective Hamiltonian formalism using the LambdaSSigma ACPF energies as diagonal elements. At the ACPF level, the ground state is 2pi(g) in contradiction with ligand-field theory, SCF, and large CASSCF; the adiabatic excitation energies for the 2sigma(g)+ and 2delta(g) states are 1640 and 18,230 cm(-1), respectively. The inclusion of the SO effects leads to a pure omega = 32(2pi(g)) ground state, a omega = 12 (66%2pi(g) and 34%2sigma(g)+) A state, a omega = 12 (34%2pi(g) and 66%2sigma(g)+) B state, a omega = 52(2delta(g))C state, and a omega = 32(99%2delta(g))D state. The X-A, X-B, X-C, and X-D transition energies are 485, 3715, 17 246, and 20 110 cm(-1), respectively. The B97-2, B3LYP, and PBE0 functionals overestimate by approximately 100% the X 2pi(g)-2sigma(g)+T(e) but provide a qualitative energetic ordering in good agreement with ACPF results. B3LYP with variable exchange leads to a 42% optimal Hartree-Fock exchange for transition energies but all equilibrium geometries get worsened. Asymptotic corrections to B3LYP do not provide improved values. The nature of the bonding in the X 2pi(g) state is very different from that of CuCl2 since the Mulliken charge on the metal is 1.1 while the spin density is only 0.35. DFT strongly delocalizes the spin density providing even smaller values of around 0.18 on Ag not only for the ground state, but also for the 2sigma(g)+ state.

Comparative studies of the spectroscopy of CuCl2: DFT versus standard ab initio approaches.

The X2Pi g-2Sigma g+, X2Pi g-2Delta g, X2Pi g-2Sigma u+, X2Pi g-2Pi u transitions on CuCl2 have been studied using several exchange-correlation functionals from the various types of density functional theory (DFT) approaches like local density approximation (LDA), generalized gradient approximation (GGA), hybrid and meta-GGA. The results are compared with the experience and with those coming from the most sophisticated nondynamic and dynamic electronic correlation treatments using the same relativistic effective core potentials and especially developed basis sets to study the electronic structure of the five lowest states and the corresponding vertical and adiabatic transition energies. The calculated transition energies for three of the hybrid functionals (B3LYP, B97-2, and PBE0) are in very good agreement with the benchmark ab initio results and experimental figures. All of the other functionals largely overestimate the X2Pi g-2Sigma g+ and X2Pi g-2Delta g transition energies, many of them even placing the 2Delta g ligand field state above the charge transfer 2Pi u and 2Sigma u+ states. The relative weight of the Hartree-Fock exchange in the definition of the functional used appears to play a key role in the accurate description of the LambdaSSigma density defined by the orientation of the 3d hole (sigma, pi, or delta) on Cu in the field of both chlorine atoms, but no simple connection of this weight with the quality of the spectra has been found. Mulliken charges and spin densities are carefully analyzed; a possible link between the extent of spin density on the metal for the X2Pi g state and the performance of the various functionals was observed, suggesting that those that lead to the largest values (close to 0.65) are the ones that best reproduce these four transitions. Most functionals lead to a remarkably low ionicity for the three ligand field states even for the best performing functionals, compared to the complete active space (SCF) (21, 14) ab initio values. These findings show that not only large variational ab initio calculations can produce reliable spectroscopic results for extremely complex systems where delicate electronic correlation effects have to be carefully dealt with. However, those functionals that were recently shown to perform best for a series of molecular properties [J. Chem. Phys. 121 3405 (2004)] are not the ones that produce the best transition energies for this complex case.

Theoretical study of Ln(III) complexes with polyaza-aromatic ligands: geometries of [LnL(H2O)n]3+ complexes and successes and failures of TD-DFT.

The accuracy and the usefulness of density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations for the theoretical study of Ln (La, Eu, Lu) complexes have been investigated. The geometries calculated at the DFT level for [Ln(H2O)nL]3+ complexes have been successfully compared with crystallographic data. TD-DFT is able to offer valuable insights into VUV spectra of lanthanide complexes. However, the results obtained on the largest ligand (i.e., 2,4,6-tri-(pyridin-2-yl)-1,3,5-triazine (Tptz)) have to be considered as a failure of TD-DFT.

Recent progress in atomic and chemical group effective potentials.

Recent progress on atomic and chemical group effective potentials is presented. The reviewed effective potentials follow a shape-consistent extraction technique from ab initio data, within a scalar relativistic approximation. Two types of averaged relativistic effective core potentials are considered: the correlated ones where a part of the correlation energy is included in the effective potential, and the polarized ones for which only the core polarization effects are taken into account. In addition spin-orbit polarized pseudopotentials have been extracted, and the effects of the core polarization are tested on the atomic spectroscopy of iodine. Finally a very recent chemical group effective methodology is presented, reducing the number of both electrons and nuclei explicitly treated. Chemical transferability is investigated, and test calculations on a cyclopentadienyl effective group potential are presented.