Effects of cis/trans-Configuration and Ligand Substitution of the Cyanidometal Bridge on Metal to Metal Charge Transfer Properties in Mixed Valence Complexes

To investigate the effects of cis/trans-configuration of the cyanidometal bridge and the electron donating ability of the auxiliary ligand on the cyanidometal bridge on metal to metal charge transfer (MMCT) in cyanidometal-bridged mixed valence compounds, two groups of trinuclear cyanidometal-bridged compounds cis/trans-[Cp(dppe)Fe(μ-NC)Ru(4,4'-dmbpy)₂ (μ-CN)Fe(dppe)Cp][PF₆ ]_n (n=2 (cis/trans-1[PF₆ ]₂ ), 3 (cis/trans-1[PF₆ ]₃ ), 4 (cis/trans-1[PF₆ ]₄ )) and cis/trans-[Cp(dppe)Fe(μ-NC)Ru(bpy)₂ (μ-CN)Fe(dppe)Cp][PF₆ ]₃ (cis/trans-2[PF₆ ]₃ ) were synthesized and fully characterized. The experimental results indicate that for these one- and two-electron oxidation mixed valence compounds, the trans-configuration compounds are more beneficial for MMCT than the cis-configuration compounds, and increasing the electron donating ability of the auxiliary ligand on the cyanidometal bridge is also conductive to MMCT. Moreover, compounds cis/trans-1[PF₆ ]_n (n=3, 4) and cis/trans-2[PF₆ ]₃ belong to localized compounds by analyzing the experimental characterization results, supported by the TDDFT calculations.

The MMCT excited state of a localized mixed valence cyanido-bridged RuII-Ru-RuII complex

To investigate MMCT excited states of MV complexes, two symmetrical tetranuclear cyanido-bridged localized MV complexes RuIICNRuIII,III2NCRuII have been designed and synthesized. The ultrafast time-resolved transient absorption (TA) spectroscopy experiment reveals that the MMCT rate of 1 and 2 is 0.18 × 1014 s-1 (τ = 5.7 × 10-14 s) and 0.29 × 1013 s-1 (τ = 3.46 × 10-13 s), respectively, which suggests that the MMCT rate or the lifetime of the MMCT excited state may be controlled by a slight change of the substituent group on the metal center.

A New Generation of Metal String Complexes: Structure, Magnetism, Spectroscopy, Theoretical Analysis, and Single Molecular Conductance of an Unusual Mixed-Valence Linear [Ni5]8+ Complex

Two new linear pentanickel complexes [Ni5(bna)4(Cl)2][PF6]2 (1) and [Ni5(bna)4(Cl)2][PF6]4 (2; bna=binaphthyridylamide), were synthesized and structurally characterized. A derivative of 1, [Ni5(bna)4(NCS)2][NCS]2 (3), was also isolated for the purpose of the conductance experiments carried out in comparison with [Ni5(tpda)4(NCS)2] (4; tpda=tripyridyldiamide). The metal framework of complex 2 is a standard [Ni5]10+ core, isoelectronic with that of [Ni5(tpda)4Cl2] (5). Also as in 5, complex 2 has an antiferromagnetic ground state (J=-15.86 cm(-1)) resulting from a coupling between the terminal nickel atoms, both in high-spin sate (S=1). Complex 1 displays the first characterized linear nickel framework in which the usual sequence of NiII atoms has been reduced by two electrons. Each dinickel unit attached to the naphthyridyl moieties is assumed to undergo a one-electron reduction, whereas the central nickel formally remains NiII. DFT calculations suggest that the metal framework of the mixed-valence complex 1 should be described as intermediate between a localized picture corresponding to NiII-NiI-NiII-NiI-NiII and a fully delocalized model represented as (Ni2)3+-NiII-(Ni2)3+. Assuming the latter model, the ground state of 1 results from an antiferromagnetic coupling (J=-34.03 cm(-1)) between the two (Ni2)3+ fragments, considered each as a single magnetic centre (S=3/2). An intervalence charge-transfer band is observed in the NIR spectrum of 1 at 1186 nm, suggesting, in accordance with DFT calculations, that 1 should be assigned to Robin-Day class II of mixed-valent complexes. Scanning tunnelling microscopy (STM) methodology was used to assess the conductance of single molecules of 3 and 4. Compound 3 was found approximately 40% more conductive than 4, a result that could be assigned to the electron mobility induced by mixed-valency in the naphthyridyl fragments.

Influence of Electron-Attractor Substituents on the Magnetic Properties of Ni(II) String Complexes

Density functional theory calculations carried out on the anionic Ni(II) chain complexes {trinickeltetrakis[2,6-bis(amido)pyridine]}2- and {trinickeltetrakis[N,N'-bis(sulfonyl)pyridyldiamido]}2- show that the electron-attractor character of the sulfonyl substituent modifies the basicity of the amido ligand ends so as to raise the electronic state of the underlying metal atom from low spin (S = 0) to high spin (S = 1). The computed elongation of the outermost Ni-N bonds with the grafting of SO2H substituents is in agreement with this interpretation.

Even-Numbered Metal Chain Complexes: Synthesis, Characterization, and DFT Analysis of [Ni4(μ4-Tsdpda)4(H2O)2] (Tsdpda2- = N-(p-toluenesulfonyl)dipyridyldiamido), [Ni4(μ4-Tsdpda)4]+, and Related Ni4 String Complexes

The synthesis and the X-ray structure of two complexes exhibiting a linear chain of four nickel atoms is reported, following Ni4(mu4-phdpda)4 (1), which had been characterized previously. [Ni4(mu4-Tsdpda)4(H2O)2], where H2Tsdpda is N-(p-toluenesulfonyl)dipyridyldiamine (2), is axially coordinated to two water molecules, at variance with 1. One-electron oxidation of 2 resulted in the loss of the axial ligands, yielding [Ni4(mu4-Tsdpda)4]+, [3]+, which was also structurally characterized. Finally, we report the structure of Ni4(mu4-DAniDANy)4 (4), a complex synthesized starting from the new ligand N,N'-bis-p-anisyl-2,7-diamino-1,8-naphthyridine. Magnetic measurements concluded that 4 is diamagnetic, like 1, whereas 2 is antiferromagnetic (-2J(14) = 80 cm(-)(1), using the Heisenberg Hamiltonian H = -2J(14) S(1).S(4)), as are other axially coordinated chains with an odd number of nickel atoms. DFT calculations are reported on these complexes in order to rationalize their electronic structure and their magnetic behavior. The magnetic properties of the [Ni4]8+ complexes are governed by the electronic state of the Ni(II) atoms, which may be either low-spin (S = 0), or high-spin (S = 1). DFT calculations show that the promotion to high spin of two Ni atoms in the chain, either external or internal, depends on the interplay between axial and equatorial coordination. The synergy between axial coordination and the presence of electron-withdrawing toluenesulfonyl substituents in 2 favors the promotion to the high-spin state of the terminal Ni atoms, thus yielding an antiferromagnetic ground state for the complex. This is at variance with complexes 1 and 4, for which the lowest quintet state results from the promotion to high spin of the internal nickel atoms, together with an important ligand participation, and is destabilized by 9 to 16 kcal mol(-1) with respect to the diamagnetic ground state.

Discrete iridium pyridonate chains with variable metal valence: nature and energetics of the Ir-Ir bonding from DFT calculations

The structure of the Ir(I) complex [Ir2(mu-OPy)2(CO)4] (Opy = 2-pyridonate) has been fully characterized in its head-to-head (A) configuration as a "dimer of dimers" AA in which two binuclear complexes are connected by means of a weak, but unsupported, iridium-iridium interaction (Ir(2)...Ir(2A) 2.9808(6) A). The head-to-tail isomer, referred to as B, was found in equilibrium with A in solution. It has been shown that this complex can be oxidized by diiodine to give iridium chains with highly selective configurations and general formula I-[Ir2(mu-OPy)2(CO)4]n-I (n = 1-3). The synthesis of IAI (1), of the isomers IAAI (2AA) and IABI (2AB), and of IABAI (3) is reported. DFT calculations have been carried out on A and B and on the known isomers of 1-3, as well as on two isomers of the hypothetic chain of eight Ir(1.25) atoms corresponding to n = 4. The stability of the metal chain is assigned to a 2-electron/2n-center sigma bond delocalized along the metal backbone and supplemented with a weak attractive interaction of the metallophilic type. Calculations confirm that further oxidation of the Ir chains corresponding to n > 1 by iodine, yielding the cleavage of one or two unsupported bond(s), is a highly exothermic process. The formation of the I-[Ir2(mu-OPy)2(CO)4]n-I chains is also computed to be exothermic, either highly for n = 1 or still significantly for n = 2 and 3. At variance with these results, the formation of an octanuclear chain is predicted to be no more than marginally exothermic (DeltaG = 1.7 kcal.mol(-1)), mainly because of interligand strain induced by the steric bulk of the amidate rings.

Electron Delocalization in Nickel Metallic Wires: A DFT Investigation of Ni3(dpa)4Cl2 and [Ni3(dpa)4]3+ (dpa = Dipyridylamide) and Extension to Higher Nuclearity Chains

The electronic structure of Ni(3)(dpa)(4)Cl(2) (1) has been investigated within the framework of the density functional theory (DFT), using two types of exchange-correlation functionals and various basis sets. The "broken-symmetry" approach proposed by Noodleman for the characterization of electronic states displaying an antiferromagnetic coupling has been applied to 1. All calculations lead to the conclusion that the ground state results from an antiferromagnetic coupling between the terminal Ni atoms, both displaying a high-spin electronic configuration. The central Ni atom is in a low-spin configuration, but is involved in a superexchange interaction connecting the two magnetic centers. These results are in agreement with the assignments recently proposed by the group of F. A. Cotton on the basis of magnetic measurements. It is shown that the ground state electronic configuration calculated for 1 provides the trinickel framework with some delocalized sigma bonding character. The observed geometry of 1 is accurately reproduced by the broken-symmetry solution. The doublet ground state assigned to the oxidized species [Ni(3)(dpa)(4)](3+) (2) and the dramatic contraction of the coordination sphere of the terminal metals observed upon oxidation are also confirmed by the calculations. However, the formal Ni-Ni bond order is not expected to increase in the oxidized species. The contraction of the Ni-Ni distance in 2 is shown to result in part from the vanishing of the important trans influence originating in the axial ligands, and for the rest from a more efficient shielding of the metal nuclear charge along the Ni-Ni-Ni axis. The conclusions deduced from the analysis of the bonding in 1 and 2 can be extended to their homologues with higher nuclearity. More specifically, it is predicted that the single occupancy of the most antibonding sigma orbital, extending over the whole metal framework, will provide the (Ni(p))(2)(p)(/(2)(p)(+1)+) chains with some delocalized bonding character and, possibly, with electrical conduction properties.