Wen-Zhen Wang

Two Linear Undecanickel Mixed-Valence Complexes: Increasing the Size and the Scope of the Electronic Properties of Nickel Metal Strings**

The importance of one-dimensional (1D) transition-metal complexes stems from their ability to provide a fundamental understanding of metal–metal interactions and electron transport along an extended metal-atom chain (EMAC), and from the perspective of taking advantage of their specific properties for potential applications, such as molecular metal wires and switches. A series of string complexes of oligo-apyridylamino ligands ranging from 3 to 9 core metal atoms has been synthesized and characterized by Cotton s group and our group. Attempts to characterize such very long EMACs with high electron conductivity were hindered by the synthetic difficulties rapidly increasing with the size of the metal chain. We synthesized [Ni9(m9-peptea)4Cl2] ten years ago, but all attempts to characterize a longer chain of Ni atoms have, to date, been unsuccessful, owing to very low yields and to the instability of the target compound, probably because of the high flexibility of large pyridylamino ligands. Recently we developed a new family of ligands by substituting rigid and potentially redox active naphthyridine (na) groups for the pyridine (py) rings. Naphthyridinemodulated ligands stabilize nickel ions in a low oxidation state, giving rise to mixed-valent [Ni2(napy)4] 3+ units (napy= naphthyridine). Using this strategy, we obtained a series of stable, low-oxidation-state-nickel string complexes combining mixed-valency, a property important in the development of novel electronic materials, with an enhanced electron mobility, which is able to increase the conductance of molecular metal wires. We report a new tetranaphthyridyltriamine ligand, N-(2(1,8-naphthyridin-7-ylamino)-1,8-naphthyridin-7-yl)-N-(1,8naphthy-ridin-2-yl)-1,8-naphthyridine-2,7-diamine (H3tentra) and two undecanickel complexes of the deprotonated tentra trianion, [Ni11(tentra)4Cl2](PF6)4 (1) and [Ni11(tentra)4(NCS)2](PF6)4 (2). The ligand H3tentra was synthesized on the basis of Buchwald s palladium-catalyzed procedures by the crosscoupling of bis(2-chloro-1,8-naphthyridin-7-yl)amine and 2amino-1,8-naphthyridine. Undecametallic complex [Ni11(tentra)4Cl2](PF6)4 (1) was obtained by the reaction of anhydrous NiCl2 with the H3tentra ligand in an argon atmosphere employing naphthalene as solvent and tBuOK as a base to deprotonate the amine groups. The thiocyanate species (2) was obtained from 1 by an axial ligand exchange reaction. The crystal structures of 1 and 2 are shown in Figure 1 and the Supporting Information Figure 1S, respectively. Both 1 and 2 are tetracationic molecules associated each with four PF6 counterions. They crystallize in unusually large cells, with one dimension exceeding 50 . The Ni11 chain of 1 and 2 is linear and wrapped in a helical manner by four tentra trianions. In both complexes, the atoms of the axial ligands are collinear with the Ni11 axis; the molecular lengths are 27.7 and 32.4 for 1 and 2, respectively. These are the longest EMAC complexes reported to date. The nature of the axial ligand does not significantly affect the metal–metal bond length, and no obvious structural change is observed for compound 2 with respect to 1. Therefore, we will only analyze the structure of 1 in detail. Selected bond lengths for 1 are displayed in Figure 1c together with the corresponding values obtained from geometry optimization at the DFT/B3LYP level. Molecule 1 consists of eleven nickel atoms in a linear chain with the Ni-Ni-Ni bond angles in the range of 179–1808. The N-Ni-Ni-N torsion angles for adjacent nickel are between 13.0 and 18.78, much smaller than those in oligo-a-pyridylamino ligand EMAC complexes (ca. 22.58). Metal–metal distances usually decrease from the end to the center of the chain in both nickel and cobalt EMACs of oligo-a-pyridyl[*] Dr. R. H. Ismayilov, Dr. W.-Z. Wang, Dr. G.-H. Lee, S.-A. Hua, Prof. Dr. S.-M. Peng Department of Chemistry, National Taiwan University 1, Sec. 4, Roosevelt Road, Taipei, 106 (Taiwan, ROC) Fax: (+886)2-8369-3765 E-mail: smpeng@ntu.edu.tw

Four quadruple metal-metal bonds lined up: linear nonachromium(II) metal string complexes.

Through a new pyrazine-modulated penta-pyridyl-tetraamine ligand, H(4)N(9)-mpz, linear nonachromium(II) complexes with four quadruple metal-metal bonds were successfully obtained, and their structure, magnetic and electrochemistry properties were studied.

The nano-scale molecule with the longest delocalized metal–metal bonds: linear heptacobalt(II) metal string complexes [Co7(µ7-L)4X2]

A new type of pyrazine-modulated oligo-alpha-pyridylamino ligands, N2-(pyrazin-2-yl)-N6-(6-(pyrazin-2-ylamino)pyridin-2-yl)pyridine-2,6-diamine (H3pzpz) and N2-(pyrazin-2-yl)-N6-(6-(pyridin-2-ylamino)pyridin-2-yl)pyridine-2,6-diamine (H3tpz), were synthesized and characterized by IR, 1H NMR and MS(FAB). Using and , the linear heptacobalt(II) metal string complexes [Co7(micro7-L)4X2] (L=pzpz3-, X=Cl-, NCS-; L=tpz3-, X=Cl-, X=NCS-) were synthesized and structurally characterized. The structures showed the shortest Co-Co distance (2.194 A) and the longest Co chain (13.5 A) obtained to date with direct Co-Co bonds. The Co-Co distances are in the range 2.194-2.309 A. Electrochemical studies showed two reversible oxidations and one reversible reduction, while all the redox reactions of H3pzpz complexes, and , occurred at higher potentials than H3tpz complexes, and . The complexes are fairly stable to oxidation. Temperature-dependent magnetic research on revealed anomalous magnetic behavior with intermediate magnetic moment values between quartet and doublet states, and deviation from the Curie-Weiss law.

Metal-metal bonding and structures of metal string complexes Cr3(dpa)4Cl2, Cr3(dpa)4(NCS)2, and [Cr3(dpa)4Cl2](PF6) from IR, Raman, and surface-enhanced Raman spectra.

We recorded infrared, Raman, and surface-enhanced Raman scattering (SERS) spectra of metal-string complexes Cr(3)(dpa)(4)X(2) (dpa = di(2-pyridyl)amido, X = Cl, NCS) and [Cr(3)(dpa)(4)Cl(2)](PF(6)) and dipyridylamine (Hdpa) to determine their vibrational frequencies and to study their structures. For the SERS measurements these complexes were adsorbed on silver nanoparticles in aqueous solution to eliminate the constraints of a crystal lattice. From the results of analysis of the vibrational normal modes we assign the infrared band at 346 cm(-1) to the Cr(3) asymmetric stretching vibration of the symmetric form and the Raman line at 570 cm(-1) to the Cr-Cr stretching mode for the unsymmetric form of Cr(3)(dpa)(4)Cl(2). Complex Cr(3)(dpa)(4)Cl(2) exhibits both symmetric (s-) and unsymmetric (u-) forms in solution but Cr(3)(dpa)(4)(NCS)(2) only the s-form. The structures for both complexes in their ground states have the s-form. The oxidized complex [Cr(3)(dpa)(4)Cl(2)](PF(6)) has only a u-form for which the Cr-Cr stretching mode is assigned to the band at 570 cm(-1). From the variation with temperature from 23 to 60 degrees C of the intensity of this line, we obtained the proportion of the u-form Cr(3)(dpa)(4)Cl(2); the enthalpy change is thus obtained to be DeltaH = 46.2 +/- 3.3 kJ mol(-1) and the entropy change is DeltaS = 138 +/- 10.3 J K(-1) mol(-1) for the reaction u-Cr(3)(dpa)(4)Cl(2) <--> s-Cr(3)(dpa)(4)Cl(2). From the spectral intensities and band frequencies in SERS spectra, Hdpa is expected to adsorb on a silver nanoparticle with the amido nitrogen and pyridyl rings tilted from the silver surface, whereas the trichromium complex with the chromium ion line is orthogonal to the silver surface normal in aqueous silver solution.

Excited-state dynamics of the metal string complex Co3(dpa)4(NCS)2 from femtosecond transient absorption spectra.

Transient absorption spectroscopy is used to study the excited-state dynamics of Co(3)(dpa)(4)(NCS)(2), where dpa is the ligand di(2-pyridyl)amido. The pi pi*, charge-transfer, and d-d transition states are excited upon irradiation at wavelengths of 330, 400 and 600 nm, respectively. Similar transient spectra are observed under the experimental temporal resolution and the transient species show weak absorption. We thus propose that a low-lying metal-centered d-d state is accessed immediately after excitation. Analyses of the experimental kinetic traces reveal rapid conversion from the ligand-centered pi pi* and the charge-transfer states to this metal-centered d-d state within 100 fs. The excited molecule then crosses to a second d-d state within the ligand-field manifold, with a time coefficient of 0.6-1.4 ps. Because the ground-state bleaching band recovers with a time coefficient of 10-23 ps, we propose that an excited molecule crosses from the low-lying d-d state either directly within the same spin system or with spin crossing via the state (2)B to the ground state (2)A(2) (symmetry group C(4)). In this trimetal string complex, relaxation to the ground electronic surface after excitation is thus rapid.

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.

Transition metal complexes of a super rigid anthyridine ligand: structural, magnetic and DFT studies

Transition metal complexes of iron(II), cobalt(II), nickel(II), copper(II) and zinc(II) with a plane, super rigid ligand, 1,13,14-triaza-dibenz[a,j]anthracene (L), [M2L4](ClO4)4 (M = Fe, Co, Ni, Cu, Zn), were synthesized and their magnetic properties were studied. All complexes are dinuclear, tetracationic complexes bridged by four tridentate ligands. In a complex each L coordinates to two metal atoms with two nitrogen atoms chelating to one metal and the third nitrogen atom mono-coordinating to another metal. Crystal structure analysis indicated that the molecular structures of the five complexes are essentially the same despite significant distortion from the Jahn–Teller effect of the copper complex. The Cu–Cu distance is 2.907(1) A, and all other M–M distances are ca. 3.3 A. Antiferromagnetic couplings were observed for the Fe, Co, Ni and Cu complexes. The exchange parameters were J = −6 and −129 cm−1 for Ni and Cu complexes, respectively. The mechanism was discussed by spin-polarization, superexchange coupling and DFT calculations. The J value obtained from the density functional B3LYP calculations is in good agreement with the experimentally determined value.

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.

Excited-State Dynamics of Metal String Complex Ni3(dpa)4X2from Femtosecond Transient Absorption Spectra

The excited-state dynamics of Ni(3)(dpa)(4)X(2), in which dpa is the ligand di(2-pyridyl)amido and X = NCS or Cl, are investigated by transient absorption spectroscopy. The pi pi* and dd states are excited upon irradiation at wavelengths of 330 and 600 nm, respectively. Similar transient spectra are observed under the experimental temporal resolution. The transient species also show weak absorption. It is proposed that a low-lying metal-centered dd state is accessed immediately after excitation. Analyses of the experimental kinetic traces reveal a rapid conversion from a ligand-centered pi pi* state to a metal-centered dd state in 0.1-0.4 ps. Vibrational cooling occurs with a time coefficient of 3.0-15.9 ps. From the spectral shift observed in the transient spectra relative to the steady-state spectra, the dd state is assigned as B(1)/B(2)(Ni(t)). This dd state eventually converts to the electronic ground state, in about 100 ps for the isothiocyanate complex and 200 ps for the chloride. In this trimetal string complex, relaxation to the ground electronic surface after excitation is therefore rapid.

Bis(morpholin-4-ium) tetra-chlorido-cobalt(II).

The title compound, (C4H10NO)2[CoCl4], is an ionic compound consisting of two protonated tetrahydro-1,4-oxazine (morpholine) cations and a [CoCl4]2− dianion. The CoII ion is in a tetrahedral coordination geometry. The cations exhibit chair-shaped conformations. A three-dimensional supramolecular architecture is formed through N—H⋯Cl and C—H⋯Cl hydrogen bonds between the dianions and the cations.