S2 1. Synthesis General. Unless otherwise noted, reagents and solvents were of commercial origin and used without further purification. Naphthalene was resublimed and NaSCN was recrystallized from acetone before use. Dichloromethane and diethyl ether for the synthesis of 1a•Et2O were purified using an Inert Technologies solvent purification system. For the synthesis of 2a4CHCl32Et2O and 2b anhydrous dichloromethane (Sigma-Aldrich) was directly used, while chloroform was washed with water, stirred over CaCl2 overnight and distilled under N2; diethyl ether was pre-dried over CaCl2 overnight and distilled from its sodium benzophenone ketyl solution under N2; n-hexane was dried over 4A molecular sieves; all these solvents were degassed by three freeze-pump-thaw cycles. 1 The H2tpda ligand was prepared as described elsewhere, 2,3 recrystallized from boiling methanol or 2-propanol and checked by 1 H NMR spectroscopy and melting point measurement. All reactions involving chromium(II) complexes were carried out under Ar or N2 atmosphere using Schlenk techniques or glovebox methods. Elemental analysis was carried out by Microlab Kolbe (Oberhausen, Germany, 1a•Et2O) or on a Carlo Erba EA1110 CHNS-O automatic analyzer (2a4CHCl32Et2O and 2b). The IR spectra were measured on a Nicolet 6700 FT-IR spectrometer using a Smart iTR accessory between 600 and 4000 cm-1 with 4 cm-1 resolution (1a•Et2O) or on a JASCO 4700 FT-IR spectrometer between 400 and 4000 cm-1 with 2 cm −1 resolution (2a4CHCl32Et2O and 2b). Electrospray Ionization Mass Spectrometry (ESI-MS) was performed on an Agilent Technologies 6310A Ion Trap LC-MS(n) spectrometer. Synthesis of [Cr3(dpa)4Cl2]•Et2O (1a•Et2O). Complex 1a was prepared following a literature procedure 4,5 and isolated as the monodiethylether solvate 6,7 by layering a dichloromethane solution with diethyl ether, yielding a crop of dark green crystals in 60% yield after a week of diffusion. Anal. Calcd for

The structure of pentachromium(ii) extended metal atom chain [Cr(tpda)Cl] (2), which behaves as a single molecule magnet at low temperature, was investigated by Density Functional Theory (DFT) calculations and spectroscopic studies without the constraints of a crystal lattice (Htpda = N,N-bis(pyridin-2-yl)pyridine-2,6-diamine). DFT studies both in the gas phase and including CHCl solvent effects indicate that an unsymmetric structure (C point group), with pairs of formally quadruply-bonded metal ions and one terminal metal center, is slightly more stable (2.9 and 3.9 kcal mol) than a symmetric structure (D point group). Isotopically-labelled samples (2-d and 2-d) have then been prepared to aid in molecular symmetry determination by combined H andH NMR studies in dichloromethane solution. The spectra are strongly suggestive of a symmetric (D) framework, indicating fast shuttling between the two unsymmetric forms over the timescale of NMR experiments. Procedures for a high-yield Pd-free synthesis of Htpda and for site-selective post-synthetic H/D exchange of aromatic Htpda hydrogens are also reported.

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.