Metal Atom Chain Complexes Research Articles

Synthesis and Characterization of a Linear Triiron(II) Extended Metal Atom Chain Complex with Fe-Fe Bonds.

Extended metal atom chain (EMAC) complexes of first-row transition metals with metal-metal bonds have the potential to elicit unique magnetic properties and reactivities. Until now, the library of EMAC complexes with late-first-row transition metals was incomplete because of the omission of a triiron species with Fe-Fe bonding. Herein we report the synthesis and preliminary investigation of the first linear, triiron(II) complex containing close Fe-Fe interactions. The complex is supported by three dianionic 2,6-bis[(trimethylsilyl)amido]pyridine ligands (L), with an overall composition of Fe3L3, and pseudohelical ligand coordination stabilizing the local trigonal-planar geometry at each iron. Fe3L3 was characterized by X-ray diffraction, 1H NMR, cyclic voltammetry, electronic absorption, and Mössbauer spectroscopies. Evans method analysis indicated a large uncompensated spin and an S = 6 ground state, suggesting ferromagnetic coupling in the triiron chain, likely due to direct exchange.

The first defective extended chromium atom chain complex with amine ligand containing naphthyridine and pyrazine

Through a pyrazine and naphthyridine-containing diamino ligand, N2,N7-di(pyrazin-2-yl)-1,8-naphthyridine-2,7-diamine (H2dpznda), defective extended metal atom chain complexes with one chromium(II) metal absent in centre, [Cr5(μ5-dpznda)4Cl2] (1) and [Cr5(μ5-dpznda)4(NCS)2] (2) were obtained. An electrochemistry research showed that the pentachromium(II) complexes were quite resistant to reduction although accessible to oxidation, with two reversible redox couples at E1/2 = +0.59 and +0.30 V.

Excited state dynamics of symmetric and asymmetric Cr3(dpa)4Cl2 measured using femtosecond transient absorption spectroscopy.

Herein, the excited-state dynamics of an extended metal atom chain complex, Cr3(dpa)4Cl2 (dpa = dipyridylamide), in tetrahydrofuran solution were investigated using femtosecond transient absorption spectroscopy. Upon excitation at a wavelength of 330 nm, two distinct excited-state absorption species with varied dynamics were identified and assigned to the symmetric (s-) and unsymmetric (u-) Cr3(dpa)4Cl2. The major species is s-Cr3(dpa)4Cl2 that undergoes rapid conversion at less than 100 fs from the ligand-centred π-π* state, which is the initially accessed state, to the metal-centred d-d state and then vibrational cooling accompanying the structural relaxation at a time constant ∼2.2 ps. Most of the s-form is recovered to the ground state at ∼200 ps. For u-Cr3(dpa)4Cl2, a similar rapid conversion to d-d states is observed, and the geometric/vibrational relaxation is ∼0.8 ps. The second recovery of the ground state with approximately equal amplitude is observed at a time constant of ∼5 ns. This might be because many d-d states exist and about half of them inefficiently couple with the ground state surface.

Magnetic blocking in extended metal atom chains: a pentachromium(II) complex behaving as a single-molecule magnet.

Compound [Cr5(tpda)4Cl2] (H2tpda = N(2),N(6)-di(pyridin-2-yl)pyridine-2,6-diamine), an Extended Metal Atom Chain complex featuring two quadruply-bonded {Cr2} units, exhibits field-induced slow relaxation of its magnetization arising from the terminal chromium(II) ion and provides the first example of a chromium(II)-based Single-Molecule Magnet.

Isotropic Non-Heisenberg Behavior in M3(dpa)4Cl2 Extended Metal Atom Chains

Isotropic deviations to the standard Heisenberg Hamiltonian have been extracted for a series of trinuclear extended metal atom chain complexes, namely, [Ni(3)(dpa)(4)Cl(2)], and the hypothetical [NiPdNi(dpa)(4)Cl(2)] and [Pd(3)(dpa)(4)Cl(2)], following a scheme recently proposed by Labéguerie and co-workers (J. Chem. Phys 2008, 129, 154110) within the density functional theory framework. Energy calculations of broken symmetry monodeterminantal solutions of intermediate M(s,tot.) values can provide an estimate of the magnitude of the biquadratic exchange interaction (λ) that accounts for these deviations in systems with S = 1 magnetic sites. With the B3LYP functional, we obtain λ = 1.37, 13.8, and 498 cm(-1) for the three molecules, respectively, meaning that a simple Heisenberg Hamiltonian is enough for describing the magnetic behavior of the Ni(3) complex but definitely not for Pd(3). In the latter case, the origin of such extreme deviation arises from (i) an energetically affordable local non-Hund state (small intrasite exchange integral, K ∼ 1960 cm(-1)) and (ii) a very effective overlap between Pd-4d orbitals and a large J. Furthermore, this procedure enables us to determine the relative weights of the two types of magnetic interactions, σ- and δ-like, that contribute to the total magnetic exchange (J = J(σ) + J(δ)). In all of the systems, J is governed by the σ interaction by 95-98%.

Analysis of the Magnetic Coupling in M3(dpa)4Cl2Systems (M = Ni, Pd, Cu, Ag) by Ab Initio Calculations

The magnetic exchange parameter (J) of a series of neutral and oxidized trinuclear extended metal atom chain complexes [M(3)(dpa)(4)Cl(2)](0/1+), where dpa is the anion of di(2-pyridyl)amine, has been extracted employing the complete active space second-order perturbation theory (CASPT2). The computed magnetic coupling constant for Ni(3)(dpa)(4)Cl(2) (-98 cm(-1)) and the mono-oxidized Cu(3) complex (-35 cm(-1)) are in excellent agreement with the experimental estimates. This consistence is though not achieved for Cu(3)(dpa)(4)Cl(2), with three S = 1/2 magnetic centers, for which we surprisingly obtain 1/3 of the experimental value only. This worrying result and the possible sources for such large discrepancy are analyzed. The magnetic coupling has been also analyzed for the hypothetical complexes with Pd(3) and Ag(3) metal chains. Much larger couplings are obtained, which is ascribed to the larger spatial extent of the magnetic orbitals. Furthermore, we analyze the relative importance of the magnetic coupling paths through the sigma-system (through metal coupling) and the delta-system (through ligand coupling).

Enantioseparation of extended metal atom chain complexes: Unique compounds of extraordinarily high specific rotation

Extended metal atom chains (EMACs) contain a linear metal chain wrapped by various ligands. Most complexes are of the form M(3)(dpa)(4)X(2), where M = metal, dpa = 2,2'-dipyridylamide, and X = various anions. The ligands form helical coils about the metal chain, which results in chiral EMAC complexes. The EMACs containing the metals Co and Cu were partially separated in polar organic mode using a vancomycin-based chiral stationary phase. Under similar conditions, two EMACs with Ni metal and varying anions could be baseline separated. The polar organic mode was used because of the instability of the compounds in aqueous mobile phases. Also, these conditions are more conducive to preparative separations. Polarimetric measurements on the resolved enantiomers of Ni(3)(dpa)(4)Cl(2) indicate that they have extraordinarily high specific rotations (on the order of 5000 deg cc/g dm).

Linear Metal Atom Chain on GaN(0001) by Chemical Vapor Deposition

The adsorption, reaction, growth, and the bonding configuration of a trinuclear linear metal atom chain complex, tetrakis(2,2‘-dipyridylamino)chromium(VI) chloride (DPCC, Cr3(μ3-dpa)4Cl2), on the GaN(0001) surface at 110 K were studied using synchrotron-induced X-ray photoelectron spectroscopy (XPS) and static secondary ion mass spectrometry (SIMS). XPS studies revealed three Cr chemical states of equal surface population present on the sample surface exposed to DPCC of low doses. The SIMS spectra showed main signals contributed from the sputter desorption of pyridine and the dipyridylamino ligand. Studies on small model molecules, such as pyridine and dipyridylamine (Hdpa), exposed to the sample surface showed that upon adsorption at 110 K, these small molecules were present on the sample surface in the molecular form only. They yielded the molecule-Ga adducts formed during SIMS detection. The absence of these adduct signals in the SIMS spectra taken from the DPCC-exposed surface indicated that DPCC chel...