A Four-Coordinate Pr4+ Imidophosphorane Complex.

The imidophosphorane ligand, [NPtBu3]- (tBu = tert-butyl), enables isolation of a pseudo-tetrahedral, tetravalent praseodymium complex, [Pr4+(NPtBu3)4] (1-Pr), which is characterized by a suite of physical characterization methods including single-crystal X-ray diffraction, electron paramagnetic resonance, and L3-edge X-ray near-edge spectroscopies. Variable-temperature direct-current magnetic susceptibility data, supported by multiconfigurational quantum chemical calculations, demonstrate that the electronic structure diverges from the isoelectronic Ce3+ analogue, driven by increased crystal field. The four-coordinate environment around Pr4+ in 1-Pr, which is unparalleled in reported extended solid systems, provides a unique opportunity to study the interplay between crystal field splitting and spin-orbit coupling in a molecular tetravalent lanthanide within a pseudo-tetrahedral coordination geometry.

Modulation of Fe-Fe distance and spin in diiron complexes using tetradentate ligands with different flanking donors.

Here we report the synthesis and characterization of diiron complexes containing triaryl N4 and N2S2 ligands derived from o-phenylenediamine. The complexes display significant differences in Fe-Fe distances and magnetic properties that depend on the identity of the flanking NMe2 and SMe donor groups.

What is the nature of the uranium(iii)-arene bond?

Complexes of the form [U(η6-arene)(BH4)3] where arene = C6H6; C6H5Me; C6H3-1,3,5-R3 (R = Et, iPr, tBu, Ph); C6Me6; and triphenylene (C6H4)3 were investigated towards an understanding of the nature of the uranium-arene interaction. Density functional theory (DFT) shows the interaction energy reflects the interplay between higher energy electron rich π-systems which drive electrostatic contributions, and lower energy electron poor π-systems which give rise to larger orbital contributions. The interaction is weak in all cases, which is consistent with the picture that emerges from a topological analysis of the electron density where metrics indicative of covalency show limited dependence on the nature of the ligand - the interaction is predominantly electrostatic in nature. Complete active space natural orbital analyses reveal low occupancy U-arene π-bonding interactions dominate in all cases, while δ-bonding interactions are only found with high-symmetry and electron-rich C6Me6. Finally, both DFT and multireference calculations on a reduced, formally U(ii), congener, [U(C6Me6)(BH4)3]-, suggests the electronic structure (S = 1 or 2), and hence metal oxidation state, of such a species cannot be deduced from structural features such as arene distortion alone. We show that arene geometry strongly depends on the spin-state of the complex, but that in both spin-states the complex is best described as U(iii) with an arene-centred radical.

CASPT2 molecular geometries of Fe(II) spin-crossover complexes.

Using fully internally contracted (FIC)-CASPT2 analytical gradients, geometry optimizations of spin-crossover complexes are reported. This approach is tested on a series of Fe(II) complexes with different sizes, ranging from 13 to 61 atoms. A combination of active space and basis set choices are employed to investigate their role in determining reliable molecular geometries. The reported strategy demonstrates that a wave function-based level of theory can be used to optimize the geometries of metal complexes in reasonable times and enables one to treat the molecular geometry and electronic structure of the complexes using the same level of theory. For a series of smaller Fe(II) SCO complexes, strong field ligands in the LS state result in geometries with the largest differences between DFT and CASPT2; however, good agreement overall is observed between DFT and CASPT2. For the larger complexes, moderate sized basis sets yield geometries that compare well with DFT and available experimental data. We recommend using the (10e,12o) active space since convergence to a minimum structure was more efficient than with truncated active spaces despite having similar Fe-ligand bond distances.

Role of Substituents at 3-position of Thienylethynyl Spacer on Electronic Properties in Diruthenium(II) Organometallic Wire-like Complexes.

A series of organometallic complexes [Cl(dppe)2 Ru-C≡C-(3-R-C4 H2 S)-C≡C-Ru(dppe)2 Cl] (3-R-C4 H2 S=3-substituted thienyl moiety; R=-H, -C2 H5 , -C3 H7 , -C4 H9 , -C6 H13 , -OMe, -CN in 5 a-5 g respectively) have been synthesized by systematic variation of 3-substituents at the thienylethynyl bridging unit. The diruthenum(II) wire-like complexes (5 a-5 g) have been achieved by the reaction of thienylethynyl bridging units, HC≡C-(3-R-C4 H2 S)-C≡CH (4 a-4 g) with cis-[Ru(dppe)2 Cl2 ]. The wire-like diruthenium(II) complexes undergo two consecutive electrochemical oxidation processes in the potential range of 0.0 - 0.8 V. Interestingly, the wave separation between the two redox waves is greatly influenced by the substituents at the 3-position of the thienylethynyl. Thus, the substitution on 3-position of the thienylethynyl bridging unit plays a pivotal role for tuning the electronic properties. To understand the electronic behavior, density functional theory (DFT) calculations of the selected diruthenium wire-like complexes (5 a-5 e) with different alkyl appendages are performed. The theoretical data demonstrate that incorporation of alkyl groups to the thienylethynyl entity leaves unsymmetrical spin densities, thus affecting the electronic properties. The voltammetric features of the other two Ru(II) alkynyl complexes 5 f and 5 g (with -OMe and -CN group respectively) show an apparent dependence on the electronic properties. The electronic properties in the redox conjugate, (5 a+ ) with Kc of 3.9×106 are further examined by UV-Vis-NIR and FTIR studies, showing optical responses in NIR region along with changes in "-Ru-C≡C-" vibrational stretching frequency. The origin of the observed electronic transition has been assigned based on time-dependent DFT (TDDFT) calculations.

Heavy ligand atom induced large magnetic anisotropy in Mn(ii) complexes.

In the search for single molecule magnets, metal ions are considered pivotal towards achieving large magnetic anisotropy barriers. In this context, the influence of ligands with heavy elements, showing large spin-orbit coupling, on magnetic anisotropy barriers was investigated using a series of Mn(ii)-based complexes, in which the metal ion did not have any orbital contribution. The mixing of metal and ligand orbitals was achieved by explicitly correlating the metal and ligand valence electrons with CASSCF calculations. The CASSCF wave functions were further used for evaluating spin-orbit coupling and zero-field splitting parameters for these complexes. For Mn(ii) complexes with heavy ligand atoms, such as Br and I, several interesting inter-state mixings occur via the spin-orbit operator, which results in large magnetic anisotropy in these Mn(ii) complexes.