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.

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The barrier to rotation around the N-alkenyl bond of 38 N-alkenyl-N-alkylacetamide derivatives was measured (ΔG(⧧) rotation varied between <8.0 and 31.0 kcal mol(-1)). The most important factor in controlling the rate of rotation was the level of alkene substitution, followed by the size of the nitrogen substituent and, finally, the size of the acyl substituent. Tertiary enamides with four alkenyl substituents exhibited half-lives for rotation between 5.5 days and 99 years at 298 K, sufficient to isolate enantiomerically enriched atropisomers. The radical cyclizations of a subset of N-alkenyl-N-benzyl-α-haloacetamides exhibiting relatively high barriers to rotation round the N-alkenyl bond (ΔG(⧧) rotation >20 kcal mol(-1)) were studied to determine the regiochemistry of cyclization. Those with high barriers (>27 kcal mol(-1)) did not lead to cyclization, but those with lower values produced highly functionalized γ-lactams via a 5-endo-trig radical-polar crossover process that was terminated by reduction, an unusual cyclopropanation sequence, or trapping with H2O, depending upon the reaction conditions. Because elevated temperatures were necessary for cyclization, this precluded study of the asymmetric transfer in the reaction of individual atropisomers. However, enantiomerically enriched atropsiomeric enamides should be regarded as potential asymmetric building blocks for reactions that can be accomplished at room temperature.

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.