A trinickel dipyridylamido complex with metal-metal bonding interaction: prelude to polynickel molecular wires and devices?

Metal-Backboned Polymers with Well-Defined Lengths

Constructing the backbones of polymers with metal atoms is an attractive strategy to develop new functional polymeric materials, but it has yet to be studied due to synthetic challenges. Here, metal atoms are interconnected as the backbones of polymers to yield metal-backboned polymers (MBPs). Rational design of multidentate ligands synthesized via an efficient iterative approach leads to the successful construction of a series of nickel-backboned polymers (NBPs) with well-defined lengths and up to 21 nickel atoms, whose structures are systematically confirmed. These NBPs exhibit strong and length-depended absorption with narrow band gaps, offering promising applications in optoelectronic devices and semiconductors. We also demonstrate the high thermal stability and solution processsability of such nickel-backboned polymers. Our results represent a new opportunity to design and synthesize a variety of new metal-backboned polymers for promising applications in the future.

Transition Metal Chain Complexes Supported by Soft Donor Assembling Ligands

The chemistry of discrete molecular chains constituted by metals in low oxidation states, displaying metal-metal proximity and stabilized by suitable metal-bridging, assembling ligands comprising at least one soft donor atom is comprehensively reviewed; complexes with a single (hard or soft) bridging atom (e.g., μ-halide, μ-sulfide, or μ-PR₂ etc.) as well as "closed" metal arrays (that fall in the realm of cluster chemistry) are excluded. The focus is on transition metal-based systems, with few excursions to cases combining transition and post-transition elements. Most relevant supporting ligands have neutral C, P, O, or S donor (mainly, N-heterocyclic carbene, phosphine, ether, thioether) or anionic donor (mainly phenyl, ylide, silyl, phosphide, thiolate) groups. A supporting-ligand-based classification of the metal chains is introduced, using as the classifying parameter the number of "bites" (i.e., ligand bridges) subtending each intermetallic separation. The ligands are further grouped according to the number of donor atoms interacting with the metal chain (called denticity in the following) and the column of the Periodic Table to which the set of donor atoms belongs (in ascending order). A complementary metal-based compilation of the complexes discussed is also provided in a concise tabular form.

Ligand architecture for triangular metal complexes: a high oxidation state Ni3 cluster with proximal metal arrangement

A new multidentate tetraanionic ligand platform for supporting trinuclear transition metal clusters has been developed. Two trisphenoxide phosphinimide ligands bind three Ni centers in a triangular arrangement. The phosphinimide donors bridge in μ3 fashion and the phenoxides complete a pseudo-square planar coordination sphere around each metal center. Electrochemical studies reveal two pseudo-reversible oxidation events at notably low potentials (-0.80 V and +0.05 V). The one electron oxidized species was characterized structurally, and it is assigned as a NiIII-containing cluster.

A tetranuclear nickel cluster isolated in multiple high-valent states

We report a series of high-valent tetranuclear nickel clusters isolated from the chemical oxidation of an all Ni(ii) ([Ni₄]) neutral cluster. Electrochemical analysis of [Ni₄] reveals three reversible sequential oxidations at 0.248 V (1e^-), 0.678 V (1e^-), and 0.991 V (2e^-) vs. Fc/Fc⁺ corresponding to mono-, di-, and tetra-oxidized species, [Ni₄]⁺, [Ni₄]²⁺, [Ni₄]⁴⁺, respectively. Using spectroscopic, crystallographic, magnetometric, and computational techniques, we assign the primary loci of oxidations to the Ni centers in each case, thus resulting in the isolation of the first tetranuclear all-Ni(iii) cluster, [Ni₄]⁴⁺.

Study of Electronic and Vibrational Structures of Reduced, Neutral, and Oxidized Ni3(dpa)4X2 Using Density Functional Theory and Raman Spectroscopy

The electronic and vibrational structures of trinickel metal string complexes [Ni₃(dpa)₄X₂]^1-,0,1+ (X = Cl, NCS) were investigated using both theoretical calculations and spectroscopic methods. We used the density functional theory (DFT) method B3LYP*-D3, including less exact exchange energy and the van der Waals interaction of metal ions, to obtain the geometries and vibrational structures, which were found to be in excellent agreement with the experimental data. The ground state of Ni₃(dpa)₄X₂ is an antiferromagnetic (AF) singlet state, and the next state is a quintet state, which was detected using temperature-dependent Raman spectroscopy under a magnetic field. The vibrational structure of the quintet state is nearly identical to that of the AF state, according to the measured Raman spectra, except that the stretching of Ni-Cl is blue-shifted from 282.5 cm^-1 in the AF state to 283.8 cm^-1 in the quintet state. Two oxidized Ni₃ complexes were found to have [Ni₃]⁷⁺ cores, the doublet [Ni₃(dpa)₄]³⁺ without axial ligands and the quartet [Ni₃(dpa)₄X₂]⁺. Complex [Ni₃(dpa)₄X₂]^-, which was produced from a reduction reaction by gold nanoparticles at room temperature, consists of a quartet state as the ground state and a doublet state lying nearby.

Paramagnetic Metal-Metal Bonded Heterometallic Complexes

Significant progress has been made in the past 10-15 years on the design, synthesis, and properties of multimetallic coordination complexes with heterometallic metal-metal bonds that are paramagnetic. Several general classes have been explored including heterobimetallic compounds, heterotrimetallic compounds of either linear or triangular geometry, discrete molecular compounds containing a linear array of more than three metal atoms, and coordination polymers with a heterometallic metal-metal bonded backbone. We focus in this Review on the synthetic methods employed to access these compounds, their structural features, magnetic properties, and electronic structure. Regarding the metal-metal bond distances, we make use of the formal shortness ratio (FSR) for comparison of bond distances between a broad range of metal atoms of different sizes. The magnetic properties of these compounds can be described using an extension of the Goodenough-Kanamori rules to cases where two magnetic ions interact via a third metal atom. In describing the electronic structure, we focus on the ability (or not) of electrons to be delocalized across heterometallic bonds, allowing for rationalizations and predictions of single-molecule conductance measurements in paramagnetic heterometallic molecular wires.

Facile Axial Ligand Substitution in Linear Mo≣Mo-Ni Complexes

Clean axial ligand substitution reactions of heterometallic extended metal atom chains (HEMACs) supported by the dpa ligand (dpa = 2,2'-dipyridylamine) have been synthetically challenging due to side reactions that alter the trimetallic core. Following the hypothesis that a heterometallic core containing second-row transition metals would be more robust toward ligand substitution, we report the synthesis of three new heterotrimetallic compounds, Mo₂Ni(dpa)₄(OTf)₂ (1), Mo₂Ni(dpa)₄(NCS)₂ (2), and Mo₂Ni(dpa)₄(NCSe)₂ (3) that are obtained cleanly and in good yield. Compound 1 may be synthesized either directly by reaction of Ni(OTf)₂ with Mo₂(dpa)₄ (4) or indirectly, by reaction of Mo₂Ni(dpa)₄Cl₂ (5) with 2 equiv of TlOTf. Axial ligand substitution on 1 via solutions containing NaNCS or KNCSe afford 2 or 3, respectively. X-ray crystal structures of 1, 2, and 3 present short Mo-Ni distances of 2.458(8)Å /2.47(1) Å, 2.548(1), and 2.546(1), respectively. Density functional theory (DFT) calculations indicate a 3-center 3-electron σ bonding interaction between the Mo₂ quadruply bonded core and the Ni in both 1 and 2. These complexes were analyzed by SQUID magnetometry, supporting the presence of a high spin Ni²⁺ center with S = 1.

Cation-Anion Arrangement Patterns in Self-Assembled Pd2L4 and Pd4L8 Coordination Cages

Compounds featuring one-dimensional regular arrangements of stacked metal complexes and alternating [cation-anion]_∞ sequences have raised considerable interest owing to their peculiar electronic and optical properties as well as guest inclusion capabilities. While traditional ways to realize these structural motifs rely on crystalline compounds, exclusively existing in the solid state, recent progress in the area of metal-mediated supramolecular self-assembly allows for the rational synthesis of structurally well-defined short stretches of stacked metal complexes and cation-anion arrangements. Therefore, metal cations, counteranions, and suitably designed organic bridges are allowed to self-assemble in solution. While the bridges can be designed as cross-linkers to yield extended two- or three-dimensional networks such as layered materials, metal-organic frameworks (MOFs), or porous coordination polymers (PCPs), they can also be tailored to lead to discrete nanoscopic objects. Supramolecular helicates, grids, and knots belong to this class of compounds, and a particularly interesting subfamily are coordination cages and capsules, which possess nanosized cavities with the ability to encapsulate guest molecules. Here, we focus on coordination cages consisting of two or more square-planar Pd(II) or Pt(II) metal cations, bridged by banana-shaped bis-monodentate pyridyl ligands that encapsulate various guest molecules, usually anions, in their cavities. Monoanions as well as dianions with localized or delocalized charges can be bound with remarkable complementarity between cage and guest in terms of size and shape. We show how dimerization of the prototypical [Pd₂L₄] cages into their interpenetrated dimers [Pd₄L₈] leads to an increase in cavity number from one to three while the cavity volume decreases. Usually, all three pockets of these double cages are filled with monoanions such as BF₄^- or Cl^-, thus leading to well-defined linear [Pd-anion]₃Pd stacks, as observed by X-ray studies. The ligand-based mechanical coupling of the linearly aligned cavities leads to interesting effects concerning guest encapsulation cooperativity, such as allosteric binding and triggered sequential uptake. While most of the so far reported coordination cages consist of only a single type of ligand, recent advances in rational assembly strategies allow for high-yielding syntheses of structurally defined multicomponent architectures by integrative self-sorting mechanisms. One family of heteroleptic [Pd₂L₂L'₂] cages whose formation is based on shape-complementarity between two different ligands, L and L', is introduced. Furthermore, the implementation of ligand-based functions such as redox activity, photochromic behavior, specific binding sites, chirality, and catalytic activity allows us to study systems with properties far beyond basic structural features. We showcase selected examples of self-assembled cages whose guest uptake or even overall structural integrity is reversibly switched by light or small molecules with potential application in stimuli responsive materials (e.g., for sequestration of pollutants or stabilization of reactive compounds) up to functional nanosystems (e.g., diagnostic devices or supramolecular catalysts) and molecular machines.

Planar PtPd3 Complexes Stabilized by Three Bridging Silylene Ligands

A heterobimetallic PtPd₃ complex supported by three bridging diphenylsilylene ligands, [Pt{Pd(dmpe)}₃ (μ₃ -SiPh₂ )₃ ] (1, dmpe=1,2-bis(dimethylphosphino)ethane), has been synthesized from mononuclear Pd and Pt complexes. The hexagonal core composed of Pt, Pd, and Si atoms is slightly larger than that of the tetrapalladium complex, [Pd{Pd(dmpe)}₃ (μ₃ -SiPh₂ )₃ ] (2). Reaction of PhSiH₃ with complex 1 in the presence and absence of Ph₂ SiH₂ results in the formation of a tetranuclear complex with silyl and hydride ligands at the Pt center, [PtH(SiPh₂ H){Pd(dmpe)}₃ (μ₃ -SiHPh)₃ ] (3), and an octanuclear complex, [{Pt{Pd(dmpe)}₃ (μ₃ -SiHPh)₃ }₂ (κ² -dmpe)] (5), respectively. Both M-Si (M=Pt, Pd) bond lengths and the ²⁹ Si NMR chemical shifts of 1 and 2 are located between those of mononuclear late transition-metal complexes with a silylene ligand and complexes with donor-stabilized silylene ligands. CuI and AgI adducts of 1 and 2, formulated as [M(μ-M'I){Pd(dmpe)}₃ (μ₃ -SiPh₂ )₃ ] (M=Pt, Pd; M'=Cu, Ag), undergo elimination of CuI (AgI) and regenerate the tetrametallic complexes upon heating or addition of a chelating diphosphine. Elimination of AgI from 2-AgI occurs more rapidly than elimination of CuI from 2-CuI, as determined from the results of kinetics experiments.

Dicopper(II) metallacyclophanes as multifunctional magnetic devices: a joint experimental and computational study

Metallosupramolecular complexes constitute an important advance in the emerging fields of molecular spintronics and quantum computation and a useful platform in the development of active components of spintronic circuits and quantum computers for applications in information processing and storage. The external control of chemical reactivity (electro- and photochemical) and physical properties (electronic and magnetic) in metallosupramolecular complexes is a current challenge in supramolecular coordination chemistry, which lies at the interface of several other supramolecular disciplines, including electro-, photo-, and magnetochemistry. The specific control of current flow or spin delocalization through a molecular assembly in response to one or many input signals leads to the concept of developing a molecule-based spintronics that can be viewed as a potential alternative to the classical molecule-based electronics. A great variety of factors can influence over these electronically or magnetically coupled, metallosupramolecular complexes in a reversible manner, electronic or photonic external stimuli being the most promising ones. The response ability of the metal centers and/or the organic bridging ligands to the application of an electric field or light irradiation, together with the geometrical features that allow the precise positioning in space of substituent groups, make these metal-organic systems particularly suitable to build highly integrated molecular spintronic circuits. In this Account, we describe the chemistry and physics of dinuclear copper(II) metallacyclophanes with oxamato-containing dinucleating ligands featuring redox- and photoactive aromatic spacers. Our recent works on dicopper(II) metallacyclophanes and earlier ones on related organic cyclophanes are now compared in a critical manner. Special focus is placed on the ligand design as well as in the combination of experimental and computational methods to demonstrate the multifunctionality nature of these metallosupramolecular complexes. This new class of oxamato-based dicopper(II) metallacyclophanes affords an excellent synthetic and theoretical set of models for both chemical and physical fundamental studies on redox- and photo-triggered, long-distance electron exchange phenomena, which are two major topics in molecular magnetism and molecular electronics. Apart from their use as ground tests for the fundamental research on the relative importance of the spin delocalization and spin polarization mechanisms of the electron exchange interaction through extended π-conjugated aromatic ligands in polymetallic complexes, oxamato-based dicopper(II) metallacyclophanes possessing spin-containing electro- and chromophores at the metal and/or the ligand counterparts emerge as potentially active (magnetic and electronic) molecular components to build a metal-based spintronic circuit. They are thus unique examples of multifunctional magnetic complexes to get single-molecule spintronic devices by controlling and allowing the spin communication, when serving as molecular magnetic couplers and wires, or by exhibiting bistable spin behavior, when acting as molecular magnetic rectifiers and switches. Oxamato-based dicopper(II) metallacyclophanes also emerge as potential candidates for the study of coherent electron transport through single molecules, both experimentally and theoretically. The results presented herein, which are a first step in the metallosupramolecular approach to molecular spintronics, intend to attract the attention of physicists and materials scientists with a large expertice in the manipulation and measurement of single-molecule electron transport properties, as well as in the processing and addressing of molecules on different supports.

Theoretical studies of electronic structures, magnetic properties and electron conductivities of one-dimensional Ni(n) (n = 3, 5, 7) complexes

Electronic structures, magnetic properties and electron conductivities of linearly aligned one-dimensional (1-D) Ni(II)3, Ni(II)5 and Ni(II)7 complexes, i.e. [Ni3(dpa)4NCS2], [Ni5(tpda)4X2] (X = Cl, CN, N3, NCS) and [Ni7(teptra)4Cl2], are systematically investigated by the broken-symmetry B3LYP calculations and simulations based on an elastic scattering Green's function theory. Calculated spin densities appear only at terminal Ni ions, while inner Ni ions are the closed-shell. The calculated effective exchange integrals (J(ab)) values reproduce well the experimental results that indicate anti-ferromagnetic (AF) interactions between two terminal Ni ions. Natural orbitals and their occupation numbers show that a change in the weak AF couplings by axial ligands in penta-nickel complexes originates in σ-type orbitals. Simulated electron conductivities of [Ni3(dpa)4NCS2] and [Ni5(tpda)4NCS2] semi-quantitatively correspond to the experimental results. By the analyses, it is elucidated that electrons are mainly transmitted by σ-type orbitals, but the bonds between Au and axial ligands are also dominant factors for conductivity.

Towards a low-spin configuration in extended metal atom chains. Theoretical study of trimetallic systems with 22 metal electrons

Different electronic configurations of a series of trinuclear heterometallic chains with 22 metallic electrons, MM'M(dpa)(4)X(2) (M = Co, Rh; M' = Ni, Pd; X = Cl, NCS), have been modelled in search of new systems with novel electrical properties. For this purpose, we explore the possibility of obtaining low-spin (extensively closed-shell) states by introducing chemical changes to the reference compound CoPdCo(dpa)(4)Cl(2) (1), isoelectronic to the herein studied systems, but possessing magnetically coupled localized electrons. The discussion is based on the orbital energies obtained by the DFT methodology. Among the systems herein analysed, CoNiCo(dpa)(4)(NCS)(2) has only two unpaired electrons vs. six in the case of 1, its closed-shell configuration appearing at high energies. For Rh(2)M-based chains, changes go a step further and the RhPdRh(dpa)(4)Cl(2) and RhPdRh(dpa)(4)(NCS)(2) molecules present a closed-shell ground state in close competition with the broken symmetry solution with S = ½ on each Rh(II). One-electron reduction of the latter compounds has been computed with marked structural changes. Our calculations show that the two lowest 23-electron states are separated by 7-8 kcal mol(-1) in favour of the state with an unpaired localized electron on the δ(Pd-N)* orbital instead of the delocalized one (σ(nb))(2)(σ*)(1).

A reduced β-diketiminato-ligated Ni3H4 unit catalyzing H/D exchange

An investigation concerning the stepwise reduction of the β-diketiminato nickel(II) hydride dimer [LNi(μ-H)(2)NiL], 1 (L = [HC(CMeNC(6)H(3)(iPr)(2))(2)](-)), has been carried out. While the reaction with one equivalent of potassium graphite, KC(8), led to the mixed valent Ni(I)/Ni(II) complex K[LNi(μ-H)(2)NiL], 3, treatment of 1 with two equivalents of KC(8) surprisingly yielded in the trinuclear complex K(2)[LNi(μ-H)(2)Ni(μ-H)(2)NiL], 4, in good yields. The Ni(3)H(4) core contains one Ni(II) and two Ni(I) centers, which are antiferromagnetically coupled so that a singlet ground state results. 4 represents the first structurally characterized molecular compound with three nickel atoms bridged by hydride ligands, and it shows a very interesting chemical behavior: Single-electron oxidation yields in the Ni(II)(2)Ni(I) compound K[LNi(μ-H)(2)Ni(μ-H)(2)NiL], 5, and treatment with CO leads to the elimination of H(2) with formation of the carbonyl complex K(2)[LNi(CO)](2), 6. Beyond that, it could be shown that 4 undergoes H/D exchange with deuterated solvents and the deuteride-compound 4-D(4) reacts with H(2) to give back 4. The crystal structures of the novel compounds 3-6 have been determined, and their electronic structures have been investigated by EPR and NMR spectroscopy, magnetic measurements, and DFT calculations.