Filling the Gap in Extended Metal Atom Chains: Ferromagnetic Interactions in a Tetrairon(II) String Supported by Oligo-α-pyridylamido Ligands

Abstract

The stringlike complex [Fe(tpda)Cl] (2; Htpda = N, N-bis(pyridin-2-yl)pyridine-2,6-diamine) was obtained as the first homometallic extended metal atom chain based on iron(II) and oligo-α-pyridylamido ligands. The synthesis was performed under strictly anaerobic and anhydrous conditions using dimesityliron, [Fe(Mes)] (1; HMes = mesitylene), as both an iron source and a deprotonating agent for Htpda. The four lined-up iron(II) ions in the structure of 2 (Fe···Fe = 2.94-2.99 Å, Fe···Fe···Fe = 171.7-168.8°) are wrapped by three doubly deprotonated twisted ligands, and the chain is capped at its termini by two chloride ions. The spectroscopic and electronic properties of 2 were investigated in dichloromethane by UV-vis-NIR absorption spectroscopy, H NMR spectroscopy, and cyclic voltammetry. The electrochemical measurements showed four fully resolved, quasi-reversible one-electron-redox processes, implying that 2 can adopt five oxidation states in a potential window of only 0.8 V. Direct current (dc) magnetic measurements indicate dominant ferromagnetic coupling at room temperature, although the ground state is only weakly magnetic. On the basis of density functional theory and angular overlap model calculations, this magnetic behavior was explained as being due to two pairs of ferromagnetically coupled iron(II) ions ( J = -21 cm using JŜ ·Ŝ convention) weakly antiferromagnetically coupled with each other. Alternating-current susceptibility data in the presence of a 2 kOe dc field and at frequencies up to 1.5 kHz revealed the onset of slow magnetic relaxation below 2.8 K, with the estimated energy barrier U/ k = 10.1(1.3) K.

Strong Electronic and Magnetic Coupling in M4 (M = Ni, Cu) Clusters via Direct Orbital Interactions between Low-Coordinate Metal Centers

“…7,10,11 With decreasing temperature, χ M T for 2 increases initially to a value of 5.80 cm 3 K/mol at 130 K, similar to what has been previously observed in other metal−metal bonded systems. 10,26 This increase is attributed to the depopulation of a low-lying S = 3 / 2 excited state with a smaller magnetic moment (see below). A fit of the magnetic susceptibility data from 150 to 300 K using the van Vleck equation and assuming an S = 5 / 2 ground state and an S = 3 / 2 excited state yielded an energy separation of 221 cm −1 (Figure S8).…”

The Origin of Magnetic Anisotropy and Single-Molecule Magnet Behavior in Chromium(II)-Based Extended Metal Atom Chains

“…The current record spin value is S = 11 for a mixed-valent hexairon complex with an octahedral metal topology . Similar features are encountered in the so-called extended metal atom chains (EMACs), which have attracted attention as molecular analogues of macroscopic wires and benchmark systems for understanding metal–metal interactions. − EMACs consist of three or more metal centers forming a linear array supported by three or four deprotonated oligo-α-pyridylamine (or related) ligands, most often arranged in a helical fashion. − Their molecular wire-like structure has either rigorous or idealized axial symmetry and makes high-spin EMACs potential SMMs. In fact, the tri- and pentachromium­(II) compounds [Cr 3 (dpa) 4 Cl 2 ]·CH 2 Cl 2 ( 1a ·CH 2 Cl 2 ) and [Cr 5 (tpda) 4 Cl 2 ]·4CHCl 3 ·2Et 2 O ( 2a ·4CHCl 3 ·2Et 2 O) have a well-isolated S = 2 state, display an easy-axis anisotropy of similar magnitude, and behave as SMMs with energy barriers of 10.6(6) and 8.6(5) K, respectively [Hdpa = dipyridin-2-yl-amine; H 2 tpda = N 2 , N 6 -di­(pyridin-2-yl)­pyridine-2,6-diamine; see Scheme ].…”

Paramagnetic Metal–Metal Bonded Heterometallic Complexes

“…Extensive work focusing on extending the length of metal chain compounds beyond three metal atoms has been performed on homometallic extended metal atom chains (EMACs), which have been synthesized from 5 to 11 metal atoms in length. ,,,− A similar body of work is currently being developed for heterometallic chains. ,− ,− , A goal of this research area is to explore the attractive optical, magnetic, and conductance properties of the compounds. ,,− ,,,,,,,− ,,− The primary means of electron communication in the longer chains like in the shorter bimetallic and trimetallic series is via σ symmetry d z 2 overlap. , Within this section, both discrete molecular chain and several quasi-1D or “infinite” polymeric chain compounds are described.…”