Linear Metal Chain Research Articles

Ni(III) Mott-Hubbard-like State Containing High-Spin Ni(II) in a Semiconductive Bromide-Bridged Ni-Chain Compound.

Halogen-bridged linear chain metal complexes (MX-Chains) are fascinating compounds that have a quasi-one-dimensional (1D) electronic system. In this study, we synthesized the first Ni-based MX-Chain compound having hydroxy groups, i.e., [Ni(dabdOH)2Br]Br2·[Ni(dabdOHx)2Br]0.5·(2-PrOH)0.25·(MeOH)0.25 (1·solvent, x = ∼0.6, dabdOH = (2S,3S)-2,3-diaminobutane-1,4-diol). Single-crystal X-ray diffraction revealed that the MX-Chains in 1·solvent formed sheets and single-chain structures in the superlattice. It suggested an MH-like state, whereas the polarized reflection and Raman spectra suggested a CDW-like state. Magnetic and electron spin resonance measurements revealed that both high-spin Ni(II) (∼15%) and low-spin Ni(III) (∼85%) sites are present in the chain structures, i.e., the metal sites show mixed valency. Therefore, we concluded that 1·solvent adopts an intermediate state between the MH and CDW states. Moreover, a single crystal of 1·solvent exhibited semiconductive characteristics along the chain direction. This finding represents a new structural and electronic state of 1D electronic systems as well as MX-Chains.

X-ray Transient Absorption Studies of Exciton Self-Trapping

We present optical pump/X-ray absorption probe studies to probe changes in electronic distribution and local structure upon formation of a self-trapped exciton. The experiments were carried out on the mixed-valence halide-bridged transition metal linear chain charge density wave material [Pt(en2)][Pt(en2)Cl2]·(ClO4)4. Photoinduced changes in the Pt LIII XANES spectrum were measured on a picosecond time scale following generation of excitons via optical excitation of the intervalence charge transfer transition. Ab initio FEFF9 modeling of the spectra and the associated angular momentum projected density of states was used to interpret the response. We find photoinduced changes reflecting electronic redistribution and local lattice distortions that correspond to strong localization of the self-trapped state.

Long chain defective metal string complex with modulated oligo-α-pyridylamino ligand: Synthesis, crystal structure and properties

Through a pyrimidine and naphthyridine-containing triamine ligand, N2-(pyrimidin-2-yl)-N7-(2-(pyrimidin-2-ylamino)-1,8-naphthyridin-7-yl)-1,8-naphthyridine-2,7-diamine (H3N9-2pm), defective metal string complex [Ni8.33(N9-2pm)4(NCS)2](PF6)2 (1) was successfully synthesized and structurally characterized. The complex consists of a linear metal chain and four deprotonated supporting ligands which are helically wrapped around the metal score. X-Ray crystallographic studies revealed that the complex (1) crystallizes in space group P21/c of the monoclinic system. The small J value (J = −2.90 cm−1) suggests quite a weak magnetic interaction throughout the molecule of complex (1). The weak magnetic interaction in the defective complex indicates that the spin exchange in metal string occurs through the metal core rather than the bridging ligands. The electrochemical study on complex 1 shows abundant redox properties and facility to the reduction in its cyclic voltammogram by displaying three reversible redox couples at E1/2 = - 0.35, - 0.69 and – 0.88 V.

Structures, metallophilic interactions and electronic excitation energy of linear metal chain complexes PdmPtn[PH2(CH2PH)m+n-2CH2PH2]3, a theoretical investigation

The structures, metallophilic interactions and electronic excitation energy of linear metal chain complexes PdmPtn[PH2(CH2PH)m+n-2CH2PH2]3 (m + n = 2–5) (1–18) have been investigated by using MP2 and density functional theory (DFT) methods. Calculated results indicated that geometries of PdmPtn(PH2CH2PH2)3 (m + n = 2) (1–3) optimized by SVWN5 method are well consistent with those by MP2 method and experiment. Then, SVWN5 functional was used to the investigation of the large system of PdmPtn[PH2(CH2PH)m+n-2CH2PH2]3 complexes. The lowest energy structures of complexes 1–18 optimized by SVWN5 method are singlet. The lengths of Pd∙∙∙Pd and Pt∙∙∙Pt of Pdm[PH2(CH2PH)m-2CH2PH2]3 (m > 2) and Ptn[PH2(CH2PH)n-2CH2PH2]3 (n > 2) complexes at SVWN5 level are shorter than those of Pd2(PH2CH2PH2)3 and Pt2(PH2CH2PH2)3, indicating the interactions of Pd∙∙∙Pd and Pt∙∙∙Pt increase with the increase of the size of Pdm[PH2(CH2PH)m-2CH2PH2]3 and Ptn[PH2(CH2PH)n-2CH2PH2]3 complexes. Pd atoms in the mixed PdmPtn[PH2(CH2PH)m+n-2CH2PH2]3 complexes are favorable to locate at the ends of the metal chain. The lengths of Pd∙∙∙Pd and Pt∙∙∙Pt bonds in the mixed metal chain of PdmPtn[PH2(CH2PH)m+n-2CH2PH2]3 complexes have no significantly variation compared with those of the Pdm+n[PH2(CH2PH)m+n-2CH2PH2]3 and Ptm+n[PH2(CH2PH)m+n-2CH2PH2]3. But, the distances of Pd∙∙∙Pt of the mixed metal chain of PdmPtn[PH2(CH2PH)m+n-2CH2PH2]3 (m + n > 2) complexes are shorter than those of PdPt(PH2CH2PH2)3, showing the interaction between Pd and Pt increases as the increase of m + n. The WBIs of Pd∙∙∙Pd, Pd∙∙∙Pt and Pt∙∙∙Pt of complexes 1–18 are in the range of 0.22–0.25, 0.27–0.29 and 0.31–0.35, respectively, thus, there are weak interactions between adjacent transition metals in complexes 1–18. QTAIM analysis indicates that the interactions of Pd∙∙∙Pd, Pd∙∙∙Pt and Pt∙∙∙Pt of complexes 1–18 are not covalent, ionic or charge-shift bond. The gradient isosurfaces of complexes 1–18 show that the interaction between the adjacent transition metals is metallophilic interactions. The first electronic excitation energy of complexes 1–18 associated with HOMO → LUMO transition nonlinearly increases with m+n. In addition, the first electronic excitation energies of PdmPtn[PH2(CH2PH)m+n-2CH2PH2]3 complexes follow the order of Pdm+n[PH2(CH2PH)m+n-2CH2PH2]3 < PdmPtn[PH2(CH2PH)m+n-2CH2PH2]3 < Ptm+n[PH2(CH2PH) m+n-2CH2PH2]3.

Metallo‐Scorpionates: First Generation of Trimetallic, Homoleptic [Ru]–M–[Ru] Complexes (M = Fe, Co, Ni, Cu)

The first metallo‐scorpionate ligands, closely related to Trofimenko's scorpionates, are obtained by formal replacement of the hydrido‐boron moiety of hydrido‐tris(pyrazolyl)borate by an (arene)RuII fragment. Coordination to divalent and trivalent 3d‐transition metals (Fe, Co, Ni, Cu) gives access to a series of homoleptic, heterotrimetallic complexes containing linear metal chains bridged by pyrazole ligands. Synthetic aspects, spectroscopic, structural and electrochemical properties are reported and compared to those of standard scorpionate complexes.

Linear pentanuclear nickel(II) and tetranuclear copper(II) complexes with pyrazine-modulated tripyridyldiamine ligand: Synthesis, structure and properties

By using tripyridyldiamine ligand, N,N’-di(pyrazin-2-yl)pyridine-2,6-diamine (H2dpzpda), linear pentanuclear nickel(II) [Ni5(µ5-dpzpda)4Cl2] (1) and tetranuclear copper(II) [Cu4(Hdpzpda)2(CH3COO)6] (2) complexes were first synthesized and structurally characterized. This pentanickel linear metal chain is helically wrapped by four syn-syn-syn-syn type dpzpda2- ligands. There are two types of NiNi distances existing in the complex. The terminal NiNi distances bonded to the axial ligand are longer (2.3877(8) Å) affected by the axial ligands. The inner NiNi distances are very short and remain constant (2.3071(6) Å). Two terminal Ni(II) ions bonded to the axial ligands are in a square-pyramidal (NiN4Cl) environment and exhibit long NiN bonds (2.097(4) Å), which are consistent with a high-spin Ni(II) configuration. The inner three Ni(II) ions display short NiN (1.8915(8)–1.903(4) Å) bond distances, which are consistent with a square planar (NiN4), the diamagnetic arrangement of a low-spin Ni(II) configuration. Temperature-dependent magnetic research revealed an antiferromagnetic interaction between the two terminal atoms through a superexchange pathway along metal cores with a parameter of about −51.0 cm–1. Observation of the first oxidation peak in the cyclic voltammograms of 1 at +1.206 V revealed that this complex is the most resistant to oxidation among all known pentanuclear nickel strings with unmodulated and modulated oligo-α-pyridylamido ligands. The structure of 2 consists of four copper atoms linearly placed with a copper(II) acetate dimer in the centre. In complex 2 ligand H2dpzpda coordinates to the Cu(II) atoms as a tetradentate ligand in an all anti conformation. Copper complex 2 is built to a 1-D supramolecular chain structure through intermolecular hydrogen bond and strong π-π interaction (3.19 Å) between the pyridine and pyrazine rings. From the magnetic susceptibility data, 2 shows strong antiferromagnetic coupling, which was satisfactorily fitted using Hamiltonian H = J1(S1S2 + S3S4) + J2S2S3. The best fitting result were obtained with the parameters J1 = 10.64 cm−1, J2 = −175.36 cm−1, g = 2.11.

Linear CuI2 Pd0 , CuI Pd02 , and AgI2 Pd0 Metal Chains Supported by Rigid N,N'-Diphosphanyl N-Heterocyclic Carbene Ligands and Metallophilic Interactions.

Selective copper(I) to palladium(0) transmetallation of P-donors from the rigid N,N'-diphosphanyl-imidazol-2-ylidene C3 H2 [NP(tBu)2 ]2 (PCNHC P) was observed when known [Cu3 (μ3 -PCNHC P,κP,κCNHC ,κP)2 ](OTf)3 was reacted with [Pd(PPh3 )4 ]. When 1.2 equivalents of [Pd(PPh3 )4 ] was used, the product [Cu2 Pd(μ3 -PCNHC P,κP,κCNHC ,κP)2 ](OTf)2 (2(OTf)2 ) was obtained, which features a CuI -CuI -Pd0 chain and appears to be the first linear heterotrinuclear complex with d10 -d10 interactions between Pd0 and CuI . When the Cu3 precursor was reacted with 3.0 equivalents of [Pd(PPh3 )4 ], the complex [CuPd2 (μ3 -PCNHC P,κP,κCNHC ,κP)2 ](OTf)2 (3(OTf)2 ) was obtained, which, on the basis of magnetic measurements, DFT calculations, and computed nuclear shieldings, was formulated as containing a Pd0 -CuI -Pd0 chain with an electron hole delocalized over the whole cation, including the metal chain. Similarly, selective transmetallation of the P-donors in [Ag3 (μ3 -PCNHC P,κP,κCNHC ,κP)2 ](OTf)3 from silver to palladium (originating from [Pd(PPh3 )4 ]) gave the linear chain [Ag2 Pd(μ3 -PCNHC P,κP,κCNHC ,κP)2 ](OTf)2 (5(OTf)2 ), which on the basis of NMR spectroscopy comprises an AgI -AgI -Pd0 metal core. However, X-ray diffraction data collected on various samples of 5(OTf)2 were modeled with 50:50 metal disorder at the terminal positions, corresponding to a (AgI /Pd0 )-AgI -(AgI /Pd0 ) formulation. Upon standing in solution, 5(OTf)2 transformed to 6(OTf)2 , the regioisomer of 5(OTf)2 in which the Pd center has migrated to the central position of an AgI -Pd0 -AgI chain. Prolonged standing in CH2 Cl2 or by reaction with [PtCl2 (NCMe)2 ] converts complex 6(OTf)2 to the AgI /PdII complex [Ag2 PdCl2 (μ3 -PCNHC P,κP,κCNHC ,κP)2 ](OTf)2 (7(OTf)2 ). The structural data of 2(OTf)2 , 3(OTf)2 , and 7(OTf)2 establish significant heterometallophilic interactions.

CoII/MnII MOFs containing the characteristic double metal chains: Synthesis, structure and magnetic property

Two isostructural CoII and MnII metal-organic frameworks (MOFs), {[M2(tztr)2(H2O)]·CH3CN·H2O}n (M=CoII for 1 and MnII for 2), have been successfully synthesized based on a semi-rigid 3-(tetrazol-5-yl)triazole (H2tztr) ligand. Crystal structure analyses reveal that the complexes possess three-dimensional (3D) frameworks featuring two types of interesting 1D zig-zag and linear metal chains. Magnetic measurements reveal that the special bridge within the double chains of complex 1 provides unexpected spin canting, long-range antiferromagnetic ordering and relaxation dynamics. Whereas the isostructural 2 displays normal antiferromagnetic coupling.

Synthesis, crystal structure and properties of a pyrimidine modulated tripyridyldiamino ligand and its complexes

A new pyrimidine-modulated ligand, N2-(pyridin-2-yl)-N6-(pyrimidin-2-yl)pyridine-2,6-diamine (H2pppmda) (1), its mononuclear copper(II) complex [Cu(H2pppmda)(CH3OH)](ClO4)2 (2) and pentanickel metal string complex [Ni5(μ5-pppmda)4Cl2] (3) have been synthesized and structurally characterized. X-ray analysis showed that the free ligand exhibits the syn–anti–syn–syn conformation and intermolecular hydrogen bonds link the ligand molecules to a 1-D network along the c axis. H2pppmda coordinates to the metal atom with all-anti mode in the copper mononuclear complex 2 and the coordination geometry of the Cu(II) ion is an elongated octahedron. The [Ni5(μ5-pppmda)4Cl2] complex involves a Ni5 linear chain unit where all the Ni–Ni–Ni angles are nearly 179° and which is terminated by the two axial ligands. The pentanuclear linear metal chain is helically wrapped by four syn–syn–syn–syn type pppmda2− ligands. There are two types of Ni–Ni distance in this complex. Terminal Ni–Ni distances bonded with the axial ligand are longer (2.3669(8)Å); the inner Ni–Ni distances are short at 2.2987(6)Å. The terminal Ni(II) ions bonded with the axial ligands are square-pyramidal (NiN4Cl) with long Ni–Nav bonds (2.096Å), consistent with a high-spin Ni(II) configuration. The inner three Ni(II) ions have short Ni–Nav bond distances (1.902(4)–1.888(4)Å), consistent with a square planar (NiN4), diamagnetic arrangement of a low-spin Ni(II) configuration. This compound exhibits magnetic behavior similar to [Ni5(μ5-tpda)4Cl2], indicating an antiferromagnetic interaction of the two terminal high-spin Ni(II) ions. Electrochemical studies showed that the pentanickel(II) complex can undergo one reversible oxidation at +0.76V. The introduction of an electron-withdrawing pyrimidine ring to the spacer ligand retarded the oxidation of the pentanickel metal string and stabilized the complex.

Linear hexanuclear nickel complexes with rich electrochemical features and facility to reduction

Two novel linear hexanuclear nickel complexes [Ni6(μ6-dpznda)4Cl2](PF6)2(1) and [Ni6(μ6-dpznda)4(NCS)2](PF6)2(2) (H2dpznda = N2,N7-di(pyrazin-2-yl)-1,8-naphthyridine-2,7-diamine) were synthesized and structurally characterized. Both the two complexes consist of a linear metal chain and four supporting ligands which are helically wrapped around the metal core. The single crystal X-ray structural analysis showed that the complex 1 belonged to rhombohedral system, space group R-3 with a = b = 34.2051(8), c = 20.7751(5) Å, V = 21050.2(9) Å3 and Z = 9. Direct-current magnetic susceptibility measurements showed weak antiferromagnetic interactions with coupling parameters of g = 2.04 and J = −8.27 cm−1 for 1 and g = 2.02 and J = −12.62 cm−1 for 2, respectively (Ĥ = –JŜ1Ŝ2, S1 = S2 = 1). The decrease of magnetic moments at low temperature was partly attributed to ZFS. The electrochemical study on complex 1 shows rich features and facility to reduction in its cyclic voltammogram by displaying four reversible redox couples at E1/2 = +0.01, −0.29, −0.64 and −0.73 V (vs. Ag/AgCl).

An approach to the electronic structure of molecular junctions with metal clusters of atomic thickness.

TD-DFT calculations predict a linear dependence of the energies of charge transfer states of Agn-pyrazine-Agn molecular junctions on the inverse of the size (1/n) of the linear metal chains. The density of charge (qeff = q/n) in the metal-to-metal charge transfer excited states (CTMM: Agnq-pyrazine-Agn-q) smoothly tunes the electronic structure of the junction, especially the metal-to-molecule charge transfer states (CT0 and CT1) and the first excited singlet of pyrazine (S1,Pz). In enlarged junctions, pyrazine bonds preferably to one of the Agn clusters and this weak adsorption produces a significant unexpected asymmetry for forward and reverse charge transfer processes.

ChemInform Abstract: Linear Metal Chains in Ca2M2X (M: Pd, Pt; X: Al, Ge): Origin of the Pairwise Distortion and Its Role in the Structure Stability.

AbstractThe new compounds Ca2Pd2Ge (I), Ca2Pt2Ge (II), Ca2Pd2Al (III), and Ca2Pt2Al (IV) are prepared by melting and annealing mixtures of the elements (Ta ampules in evacuated silica tubes, 1.

Linear Metal Chains in Ca2M2X (M = Pd, Pt; X = Al, Ge): Origin of the Pairwise Distortion and Its Role in the Structure Stability

A series of four new analogue phases Ca2M2X (M = Pd, Pt and X = Al, Ge) were prepared by direct combination of the respective elements in stoichiometric mixtures at high temperature in order to analyze the impact of valence electron count (vec) and electronegativity differences (Δχ) on the structure selection and stability. Their crystal structures, as determined from single-crystal X-ray diffraction data, correspond to two different but closely related structure types. The first compound, Ca2Pd2Ge (I), is an unprecedented ternary ordered variant of the Zr2Al3-type (orthorhombic, Fdd2). The three other phases, Ca2Pt2Ge (II), Ca2Pd2Al (III) and Ca2Pt2Al (IV), adopt the Gd2Ge2Al-type structure (monoclinic, C2/c). All title structures feature linear chains of the noble metals (Pd or Pt). The Pd linear chains in I are undistorted with equidistant Pd···Pd atoms, whereas the metal chains in II–IV are pairwise distorted, resulting in short connected {Pd2} or {Pt2} dumbbells that are separated by longer M···M contacts. The occurrence and magnitude of the pairing distortion in these chains are controlled by the vec and the Δχ between the constituent elements, a result which is supported by analysis of the calculated Bader effective charges. The metal chains act as charge modulation units, critical for the stability and the electronic flexibility of the structures by an adequate adjustment of the metal–metal bond order to both the vec and the degree of charge transfer. Thus, Ca2Pd2Ge (28 ve/f.u) is a Zintl-like, charge optimized phase with formally zerovalent Pd atoms forming the undistorted metal chains; semimetallic properties are predicted by TB-LMTO calculations. In contrast, the isoelectronic Ca2Pt2Ge is predicted to be a good metal with the Fermi level located at a local maximum of the DOS, a fingerprint of potential electronic instability. This is due to greater charge transfer to the more electronegative Pt atoms forming the metal chains and probably to packing frustration in the well packed structure that may prevent a larger distortion of the Pt chains. However, the instability is suppressed in the aliovalent but isostructural phases Ca2M2Al (27 ve/f.u) with an enhancement of the pairing distortion within the metal chains but lower M–M bond order. Further reduction of the vec as in Ca2M2Cd (26 ve/f.u) may induce a transition toward the more geometrically flexible W2CoB2-type with a low dimensional structure, to create more room for a larger distortion of the metal chain as dictated by the shortage of valence electrons.

A structural study of a three-membered linear metal chain compound at elevated pressure

Single crystal X-ray diffraction studies of the symmetrical molecular wire compound Co3(dpa)4Cl2·(dcm) have been performed up to a pressure of 3.6 GPa using both synchrotron and conventional sources. It is found that the terminal Co-Cl bond distance initially increases by 0.013(4) Å at 0.32 GPa after which it continuously decreases. Extensive theoretical calculations show that population of a thermally excited state containing increased Co-Cl anti-bonding character is a possibility at 0.32 GPa. The relative occupancy of the disordered dcm solvent molecule changes significantly with pressure and this is explained by the analysis of void spaces and Hirshfeld surfaces at different pressures. At 3.2 GPa, fingerprint plots derived from Hirshfeld surfaces indicate that neighbouring metal chain compounds approach each other such that short H···H interactions appear.

Femtosecond dynamics of exciton localization: self-trapping from the small to the large polaron limit

We use femtosecond vibrational wavepacket techniques to time-resolve the coupled electronic and vibrational dynamics of exciton self-trapping in a series of materials in which the relative strength of the electron–phonon coupling can be compositionally tuned from the small to the large polaron limit. Transient absorption experiments are carried out in the quasi-one-dimensional halide-bridged mixed-valence transition metal linear chain complexes [Pt(en)2][Pt(en)2X2]⋅(ClO4)4 (en=ethylenediamine, C2H8N2) with X=Cl, Br and I. In each complex, we detect the formation of the self-trapped exciton through the appearance of its characteristic red-shifted optical absorption, and find that self-trapping occurs on a time scale of the order of a single vibrational period of the optical phonon mode that dominates the self-trapping dynamics. The associated optical phonon response, detected as wavepacket oscillations that modulate the exciton absorption, shows a significant softening of the optical phonon frequency compared to that of the unexcited system. The degree of softening is found to vary significantly with coupling strength, ranging from more than 40% in the strongly coupled chloride-bridged complex to less than 20% in the weakly coupled iodide-bridged complex. We relate these results to the extent of electronic delocalization by comparison with the electronic properties of the ground states of the materials and with the properties of their equilibrated self-trapped electronic states predicted by theoretical modeling.

Effects of Electric Field on the Structures of Metal String Complexes M&lt;sub&gt;3&lt;/sub&gt;(dpa)&lt;sub&gt;4&lt;/sub&gt;Cl&lt;sub&gt;2&lt;/sub&gt; (M=Co, Rh, Ir; dpa=dipyridylamide)

As potential molecular wire species, the geometrical and electronic structures of metal string complexes M 3 (dpa) 4 Cl 2 (1: M=Co, 2: M=Rh, 3: M=Ir; dpa=dipyridylamide) were investigated theoretically using density functional theory with the PBE0 functional by considering the interaction of an external electric field along the M 3 6+ linear metal chain. The results show that the ground states of the complexes are all doublets. There is a 3-center-3-electron σ bond delocalized over the M 3 6+ chain for 1 and 2, while there isa 3-center-4-electron σ bond and a weak δ bond among the Ir 3 6+ chain in 3. Moving down the column of Co, Rh, and Ir elements in the periodic table, the complexes with the corresponding metals showed some regular trends, such as stronger M - M bonds, smaller LUMO-HOMO gaps, weaker anti-ferromagnetic spin coupling among the M 3 6 + chains, and stronger spin delocalization from M 3 6 + to ligands. In the external electric field along the Cl4→Cl5 direction, the M3 ― Cl5 bonds at the low potential side tend to be shortened, while the M2―Cl4 distances at the high potential side increase. With the increase of electric field, the average M―M distances slightly decrease, which is beneficial for electron transport. When the electric field increases, the molecular energy decreases and the dipole moment linearly increases. Moreover, the negative charge moves from Cl5 at the low potential end towards Cl4 at the high potential end, and the spin electron moves from M3 at the low potential end to M1 and M2 at the high potential end, while the positive charges transfer in the opposite direction along the M 3 6 + chain of 3. However, there is no charge transfer between dpa - ligands and M 3 6+ chain or Cl - ligands. The LUMO-HOMO gaps decrease with increasing electric field, which is beneficial for electron transfer. The sensitivity of the frontier orbitals to the electric field is different, which leads to the orbital level crossing for LUMO or HOMO. Moving down the column of metal elements in the periodic table, the complexes with the corresponding metals showed weaker orbital level crossing for LUMO or HOMO and smaller deviation of average M―M distances due to the effect of the electric field.

Fine tuning of pentachromium(ii) metal string complexes through elaborate design of ligand

New pentachromoium metal string complexes [Cr5(μ5-L)4X2] (X = Cl−, L = dppzda2− (1), dpzpda2− (2); X = NCS−, L = dppzda2− (3), dpzpda2− (4)) were designed and synthesized through pyrazine-modulation of tripyridyldiamine ligand. X-Ray crystallographic studies revealed a linear metal chain structure consisting of two quadruple Cr–Cr bonds and a separated high spin Cr(II) at an end in crystallized form. A quintet ground state was observed for all pentachromium(II) molecules by magnetic study with g values of 2.04–2.18. While the electronic structure remained unchanged after the modification of ligands, electrochemistry showed a significant change in the molecular orbital energy levels of metal string molecules. Observation of the first oxidation peak of 1 at +0.57 V and of 2 at +0.73 V revealed that these complexes are quite resistant to oxidation. Single molecular conductance measurements showed that the complex exhibited good electronic conductance.

Metal−Metal Interactions in Stacked Mononuclear and Dinuclear Rhodium 2,2′-Biimidazole Carbonyl Complexes

Noncovalent metal−metal interactions in the solid state were studied by investigating the stacking of the cationic mononuclear [Rh(H2bim)(CO)2]+ (H2bim = 2,2′-biimidazole) and neutral dinuclear [Rh2(R2bim)Cl2(CO)4] (R = Me, methyl or Bn, benzyl) complexes. In both cases, the d8 rhodium atoms possessed square planar coordination geometries. The cationic [Rh(H2bim)(CO)2]+ complexes with a chelating H2bim ligand all formed polymeric stacks with variable Rh···Rh distances (3.2743(3)−3.430(6) Å), depending on the counter-anions. In the solid state, the neutral dinuclear complexes with bridging Bn2bim formed isolated complexes with no metal−metal interactions between the dinuclear [Rh2(Bn2bim)Cl2(CO)4] motifs. The use of the Me2bim ligand led to a tetranuclear complex [Rh2(Me2bim)Cl2(CO)4]2 with an intermolecular Rh···Rh distance of 3.4104(7) Å. A neutral polymeric stack of [Rh2(Me2bim)Cl2(CO)4]n was obtained only in crystals with ethanol of crystallization. The effect of the intermolecular metal−metal interactions on the absorption properties of the compounds was studied using time-dependent density functional theory (TD-DFT) methods. A bathochromic shift was observed in the λmax metal-to-ligand charge transfer (MLCT) absorption with decreasing Rh···Rh distance. The shift was also observed spectroscopically. In the case of [Rh(H2bim)(CO)2][BF4], a clear dependence of color on temperature was also observed in the solid state due to the changes in Rh···Rh distance. Furthermore, the formation of an infinite linear metal chain generated metallic luster on the crystals.

Synthesis, structure, magnetism, and electrochemical properties of a linear pentanuclear Ni5 compound derived from an oligo-α-pyridylamino ligand: Ni5(μ-dmpdda)4(NCS)2

The synthesis, crystal structures, electrochemical, and magnetic properties of a linear pentanuclear Ni5 compound derived from an oligo-α-pyridylamino ligand, [Ni5(μ-dmpdda)4(NCS)2] [dmpdda-H2 = N,N′-di(4-methylpyridin-2-yl)pyridine-2,6-diamine], are reported. Ni5(μ-dmpdda)4(NCS)2 involve a Ni5 linear chain unit with all of the Ni–Ni–Ni angles being nearly 180°, terminated by two axial ligands. The pentanuclear linear metal chain is helically wrapped by four syn–syn–syn–syn type dmpdda2− ligands. There are two types of Ni–Ni distances in this complex. Terminal Ni–Ni distances bonded with the axial ligand are longer (2.377 Å); the inner Ni–Ni distances are short at 2.2968 Å. Terminal Ni(II) ions bonded with the axial ligands are square-pyramidal (NiN4NCS) with long Ni–N bonds (2.092 Å), consistent with a high-spin Ni(II) configuration. The inner three Ni(II) ions have short Ni–N (1.901–1.925 Å) bond distances, consistent with a square planar (NiN4), diamagnetic arrangement of a low-spin Ni(II) configuration. This compound exhibits magnetic behavior similar to [Ni5(μ-tpda)4(NCS)2], indicating an antiferromagnetic interaction of two terminal high-spin Ni(II) ions.

Synthesis, Structure, Magnetism and Electrochemical Properties of Linear Pentanuclear Complex: Ni5(µ‐dmpzda)4(NCS)2

Abstract The synthesis, crystal structure, electrochemical and magnetic properties of a type of pyridine‐modulated linear pentanuclear complex, [Ni5(µ‐dmpzda)4(NCS)2] [dmpzda‐H2=N,N′‐di(4‐methyl pyridin‐2‐yl)pyrazine‐2,6‐diamine], were studied. The complex involves a Ni5 linear chain unit with all of the Ni‐Ni‐Ni being nearly 180°, terminated by the two axial ligands. This pentanuclear linear metal chain is helically wrapped by four syn‐syn‐syn‐syn type dmpzda ligands. There are two types of Ni–Ni distances existing in the complex. The terminal Ni–Ni distances bonded with the axial ligand are longer (2.3821 Å) affected by the axial ligands. The inner Ni–Ni distances are very short and remain constant (2.2959 Å). Two terminal Ni(II) ions bonded with the axial ligands are in a square‐pyramidal (NiN4NCS) environment and exhibit long Ni–N bonds (2.103 Å), which are consistent with a high‐spin Ni(II) configuration. The inner three Ni(II) ions display short Ni–N (1.886–1.906 Å) bond distances, which are consistent with a squareplanar (NiN4), diamagnetic arrangement of a low‐spin Ni(II) configuration. This compound exhibits similar [Ni5(µ‐tpda)4(NCS)2] magnetic behavior, indicating an antiferromagnetic interaction of two terminal high‐spin Ni(II) ions in these complexes.