Mechanism of Ligand Photodissociation in Photoactivable [Ru(bpy)₂L₂]²⁺ Complexes: A Density Functional Theory Study

A series of four photodissociable Ru polypyridyl complexes of general formula [Ru(bpy)2L2]2+, where bpy = 2,2′-bipyridine and L = 4-aminopyridine (1), pyridine (2), butylamine (3), and γ-aminobutyric acid (4), was studied by density functional theory (DFT) and time-dependent density functional theory (TDDFT). DFT calculations (B3LYP/LanL2DZ) were able to predict and elucidate singlet and triplet excited-state properties of 1−4 and describe the photodissociation mechanism of one monodentate ligand. All derivatives display a Ru → bpy metal-to-ligand charge transfer (MLCT) absorption band in the visible spectrum and a corresponding emitting triplet 3MLCT state (Ru → bpy). 1−4 have three singlet metal-centered (MC) states 0.4 eV above the major 1MLCT states. The energy gap between the MC states and lower-energy MLCT states is significantly diminished by intersystem crossing and consequent triplet formation. Relaxed potential energy surface scans along the Ru−L stretching coordinate were performed on singlet and triplet excited states for all derivatives employing DFT and TDDFT. Excited-state evolution along the reaction coordinate allowed identification and characterization of the triplet state responsible for the photodissociation process in 1−4; moreover, calculation showed that no singlet state is able to cause dissociation of monodentate ligands. Two antibonding MC orbitals contribute to the 3MC state responsible for the release of one of the two monodentate ligands in each complex. Comparison of theoretical triplet excited-state energy diagrams from TDDFT and unrestricted Kohn−Sham data reveals the experimental photodissociation yields as well as other structural and spectroscopic features

The Role of the Amino Protecting Group during Parahydrogenation of Protected Dehydroamino Acids

A series of dehydroamino acids endowed with different protective groups at the amino and carboxylate moieties and with different substituents at the double bond have been reacted with parahydrogen. The observed ParaHydrogen Induced Polarization (PHIP) effects in the ¹H NMR spectra are strongly dependent on the amino protecting group. DFT calculations allowed us to establish a relationship between the structures of the reaction intermediates (whose energies depend on the amido substitution) and the observed PHIP patterns

Coupling Solid-State NMR with GIPAW ab Initio Calculations in Metal Hydrides and Borohydrides

An integrated experimental–theoretical approach for the solid-state NMR investigation of a series of hydrogen-storage materials is illustrated. Seven experimental room-temperature structures of groups I and II metal hydrides and borohydrides, namely, NaH, LiH, NaBH₄, MgH₂, CaH₂, Ca­(BH₄)₂, and LiBH₄, were computationally optimized. Periodic lattice calculations were performed by means of the plane-wave method adopting the density functional theory (DFT) generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional as implemented in the Quantum ESPRESSO package. Projector augmented wave (PAW), including the gauge-including projected augmented-wave (GIPAW), methods for solid-state NMR calculations were used adopting both Rappe–Rabe–Kaxiras–Joannopoulos (RRKJ) ultrasoft pseudopotentials and new developed pseudopotentials. Computed GIPAW chemical shifts were critically compared with the experimental ones. A good agreement between experimental and computed multinuclear chemical shifts was obtained

X-ray Structures and Complete NMR Assignment by DFT Calculations of [Os(bpy)₂(CO)Cl]PF₆ and [Os(bpy)₂(CO)H]PF₆ Complexes

The X-ray structures of [Os(bpy)2(CO)(Cl)][(PF6)] (1) and [Os(bpy)2(CO)(H)][(PF6)] (2) (where bpy = 2,2‘-bipyridine) have been solved. In complex 1, belonging to the C2/cspace group, the Cl- and CO ligands are statically disordered along two almost orthogonal directions, and this disorder may be explained by the steric similarity of the CO and Cl- groups. Conversely, in complex 2, the CO and hydride ligands are rather different and the [Os(bpy)2(CO)(H)] moiety does not show any disorder. A more accurate model of the disordered structure of complex 1 and the hydride position in complex 2 were obtained by DFT calculations. Complete 1H NMR chemical shift assignments were made, using 1D and 2D NMR experiments combined with theoretical calculations. The experimental 1H NMR data have been fully interpreted with the aid of magnetic shielding constant calculations, by means of the GIAO (gauge-including atomic orbitals) method, carried out at the B3LYP level. Proton nuclear shielding constants have been calculated with the 6-311G++(2d) basis set, and geometry optimizations have been carried out employing the LanL2Dz basis set for osmium and the 3-21G or 6-31G(d) basis sets for the other atoms. Calculated and experimental results have been compared with a satisfactory level of agreement. The complete assignment of the proton spectrum of 2, in good agreement with the theoretical data, was confirmed by the 1H−1H NOESY results. By using this mixed experimental and theoretical approach it was also possible to obtain a calculated structure and the 1H NMR assignment of [Os(bpy)2(CO)(CF3SO3)][(CF3SO3)] (3), for which no suitable crystal could be obtained

X-ray Structures and Complete NMR Assignment by DFT Calculations of [Os(bpy)₂(CO)Cl]PF₆ and [Os(bpy)₂(CO)H]PF₆ Complexes

The X-ray structures of [Os(bpy)2(CO)(Cl)][(PF6)] (1) and [Os(bpy)2(CO)(H)][(PF6)] (2) (where bpy = 2,2‘-bipyridine) have been solved. In complex 1, belonging to the C2/cspace group, the Cl- and CO ligands are statically disordered along two almost orthogonal directions, and this disorder may be explained by the steric similarity of the CO and Cl- groups. Conversely, in complex 2, the CO and hydride ligands are rather different and the [Os(bpy)2(CO)(H)] moiety does not show any disorder. A more accurate model of the disordered structure of complex 1 and the hydride position in complex 2 were obtained by DFT calculations. Complete 1H NMR chemical shift assignments were made, using 1D and 2D NMR experiments combined with theoretical calculations. The experimental 1H NMR data have been fully interpreted with the aid of magnetic shielding constant calculations, by means of the GIAO (gauge-including atomic orbitals) method, carried out at the B3LYP level. Proton nuclear shielding constants have been calculated with the 6-311G++(2d) basis set, and geometry optimizations have been carried out employing the LanL2Dz basis set for osmium and the 3-21G or 6-31G(d) basis sets for the other atoms. Calculated and experimental results have been compared with a satisfactory level of agreement. The complete assignment of the proton spectrum of 2, in good agreement with the theoretical data, was confirmed by the 1H−1H NOESY results. By using this mixed experimental and theoretical approach it was also possible to obtain a calculated structure and the 1H NMR assignment of [Os(bpy)2(CO)(CF3SO3)][(CF3SO3)] (3), for which no suitable crystal could be obtained

Computational and Spectroscopic Studies of New Rhenium(I) Complexes Containing Pyridylimidazo[1,5-<i>a</i>]pyridine Ligands: Charge Transfer and Dual Emission by Fine-Tuning of Excited States

Three new Re(CO)3Cl complexes (ReL1−ReL3) containing the N,N-bidentate ligands 1-(2-pyridyl)-3-phenylimidazo[1,5-a]pyridine (L1), 1-(2-pyridyl)-3-(4-tert-butylphenyl)imidazo[1,5-a]pyridine (L2), and 1-(2-pyridyl)-3-(4-dimethylaminophenyl)imidazo[1,5-a]pyridine (L3) were synthetized and fully characterized. Photophysical properties of L1−L3 and ReL1−ReL3 were studied with absorption and emission spectroscopy. The X-ray structure of ReL3 was determined. Time-dependent DFT (TDDFT) calculations were performed in order to elucidate the electronic structures and the excited states of ligands and complexes. Ligands L1 and L2 show 1π−π* emission with limited charge-transfer character (CT), while L3 emits from an excited state with higher CT character due to the presence of a dimethylamino group. No emissive metal-to-ligand charge-transfer (MLCT) states are found for the rhenium complexes. ReL1 and ReL2, although similar to their ligands, display a ligand-centered 1π−π*/3π−π* dual emission; singlet emissions fall at 21.8 × 103 (458 nm) and 22.6 × 103 cm−1 (443 nm), respectively, and the structured triplet emissions have two peaks at 17.9 × 103 (558 nm) and 16.3 × 103 cm−1 (613 nm) in both complexes. ReL3 emits from a ligand-centered CT state at 18.9 × 103 cm−1 (530 nm). Finally, the complex [Re(L1)(CO)3py]PF6 (ReL1py) (where py = pyridine) was prepared and studied by spectroscopy and computational methods. The complex has high-energy emission centered at 22.9 × 103 cm−1 (437 nm). DFT calculations show that dual fluorescence almost disappears due to the reduced spin–orbit coupling. Finally, electrochemical properties of ligands and rhenium complexes have been investigated

Computational and Spectroscopic Studies of New Rhenium(I) Complexes Containing Pyridylimidazo[1,5-<i>a</i>]pyridine Ligands: Charge Transfer and Dual Emission by Fine-Tuning of Excited States

Three new Re(CO)3Cl complexes (ReL1−ReL3) containing the N,N-bidentate ligands 1-(2-pyridyl)-3-phenylimidazo[1,5-a]pyridine (L1), 1-(2-pyridyl)-3-(4-tert-butylphenyl)imidazo[1,5-a]pyridine (L2), and 1-(2-pyridyl)-3-(4-dimethylaminophenyl)imidazo[1,5-a]pyridine (L3) were synthetized and fully characterized. Photophysical properties of L1−L3 and ReL1−ReL3 were studied with absorption and emission spectroscopy. The X-ray structure of ReL3 was determined. Time-dependent DFT (TDDFT) calculations were performed in order to elucidate the electronic structures and the excited states of ligands and complexes. Ligands L1 and L2 show 1π−π* emission with limited charge-transfer character (CT), while L3 emits from an excited state with higher CT character due to the presence of a dimethylamino group. No emissive metal-to-ligand charge-transfer (MLCT) states are found for the rhenium complexes. ReL1 and ReL2, although similar to their ligands, display a ligand-centered 1π−π*/3π−π* dual emission; singlet emissions fall at 21.8 × 103 (458 nm) and 22.6 × 103 cm−1 (443 nm), respectively, and the structured triplet emissions have two peaks at 17.9 × 103 (558 nm) and 16.3 × 103 cm−1 (613 nm) in both complexes. ReL3 emits from a ligand-centered CT state at 18.9 × 103 cm−1 (530 nm). Finally, the complex [Re(L1)(CO)3py]PF6 (ReL1py) (where py = pyridine) was prepared and studied by spectroscopy and computational methods. The complex has high-energy emission centered at 22.9 × 103 cm−1 (437 nm). DFT calculations show that dual fluorescence almost disappears due to the reduced spin–orbit coupling. Finally, electrochemical properties of ligands and rhenium complexes have been investigated

Probing Hydrogen Bond Networks in Half-Sandwich Ru(II) Building Blocks by a Combined ¹H DQ CRAMPS Solid-State NMR, XRPD, and DFT Approach

The hydrogen bond network of three polymorphs (<b>1α</b>, <b>1β</b>, and <b>1γ</b>) and one solvate form (<b>1·H</b>_<b>2</b><b>O</b>) arising from the hydration–dehydration process of the Ru­(II) complex [(<i>p</i>-cymene)­Ru­(κN-INA)­Cl₂] (where INA is isonicotinic acid), has been ascertained by means of one-dimensional (1D) and two-dimensional (2D) double quantum ¹H CRAMPS (Combined Rotation and Multiple Pulses Sequences) and ¹³C CPMAS solid-state NMR experiments. The resolution improvement provided by homonuclear decoupling pulse sequences, with respect to fast MAS experiments, has been highlighted. The solid-state structure of <b>1γ</b> has been fully characterized by combining X-ray powder diffraction (XRPD), solid-state NMR, and periodic plane-wave first-principles calculations. None of the forms show the expected supramolecular cyclic dimerization of the carboxylic functions of INA, because of the presence of Cl atoms as strong hydrogen bond (HB) acceptors. The hydration–dehydration process of the complex has been discussed in terms of structure and HB rearrangements

Ligand-Selective Photodissociation from [Ru(bpy)(4AP)₄]²⁺: a Spectroscopic and Computational Study

The new complex [Ru(bpy)(4AP)4]2+ (1), where bpy = 2,2′-bipyridine and 4AP = 4-aminopyridine, undergoes selective photodissociation of two 4APs upon light excitation of the metal−ligand-to-ligand charge-transfer (MLLCT) band at 510 nm. The photoproducts of the reaction are mer-[Ru(bpy)(4AP)3(H2O)]2+ (2a) and trans-(4AP)[Ru(bpy)(4AP)2(H2O)2]2+ (3a). Photodissociation occurs in two consecutive steps with quantum yields of φ1 = (6.1 ± 1.0) × 10−3 and φ2 = (1.7 ± 0.1) × 10−4, respectively. Complex 1 was characterized by combined spectroscopic and theoretical techniques. EXAFS experiments at the Ru K-edge (22 117 eV) of 1 in an aqueous solution gave a Ru−N distance of 2.09 ± 0.01 Å. Photoproducts were characterized by electronic spectroscopy, 1D and 2D NMR, and mass spectrometry. Singlet and triplet excited states of 1 were studied by density functional theory (DFT) and time-dependent DFT for characterizing the optical properties of the complex. In the singlet state, 1MC (metal-centered) dissociative states lie 0.65 eV above the main 1MLLCT transition in the visible region of the UV−vis absorption spectrum. In the triplet state, the energy difference between these states is not reduced. However, potential energy curves of singlet and triplet excited states of 1 along the Ru−N(axial 4AP) and Ru−N(equatorial 4AP) stretching coordinates show that the release of the first 4AP may occur from the triplet state by mixing of 3MLLCT and 3MC dissociative states. This mixing is favored when the Ru−N(equatorial 4AP) bond is elongated, explaining the formation of the photoproduct 2a