Synthesis and Conformational Dynamics of Selenanthrene (Oxides): Establishing an Energetic Flexibility Index for Scaffolds.

The use of new dynamic scaffolds for constructing inorganic and organometallic complexes with enhanced reactivities is an important new research direction. Toward this fundamental aim, an improved synthesis of the dynamic scaffold selenanthrene, along with its monoxide, trans-dioxide and the previously unknown trioxide, is reported. A discussion of the potential reaction mechanism for selenanthrene is provided, and all products were characterized using 1H, 13C, and 77Se nuclear magnetic resonance (NMR) spectroscopy and single-crystal X-ray crystallography. The dynamic ring inversion processes (i.e., "butterfly motion") for selenanthrene and its oxides were investigated using variable-temperature 1H NMR and density functional theory calculations. The findings suggest that selenanthrene possesses a roughly equal barrier to inversion as its sulfur analogue, thianthrene. However, selenanthrene oxides evidently possess larger inversion barriers as compared to their sulfur analogues due to the enhanced electrostatic intramolecular interactions inherent between the highly polar selenium-oxygen bond and adjacent C-H moieties. Finally, we propose a quantitative "flexibility index" in deg/(kcal/mol) for various tricyclic scaffolds to provide researchers with a comparative scale of dynamic motion across many different systems.

Distal scaffold flexibility accelerates ligand substitution kinetics in manganese(I) tricarbonyls: flexible thianthrene versus rigid anthracene scaffolds.

This work investigates the effect of molecular flexibility on fundamental ligand substitution kinetics in a pair of manganese(I) carbonyls supported by scaffold-based ligands. In previous work, we reported that the planar and rigid, anthracene-based scaffold with two pyridine 'arms' (Anth-py2, 2) serves as a bidentate, cis donor set, akin to a strained bipyridine (bpy). In the present work, we have installed a more flexible and dynamic scaffold in the form of thianthrene (Thianth-py2, 1), wherein the scaffold in the free ligand exhibits a ∼130° dihedral angle in the solid state. Thianth-py2 also exhibits greater flexibility (molecular motion) in solution compared with Anth-py2, as evidenced by longer 1H NMR T1 times Thianthy-py2 (T1 = 2.97 s) versusAnth-py2 (T1 = 1.91 s). Despite the exchange of rigid Anth-py2 for flexible Thianth-py2 in the complexes [(Anth-py2)Mn(CO)3Br] (4) and [(Thianth-py2)Mn(CO)3Br] (3), respectively, nearly identical electronic structures and electron densities were observed at the Mn center: the IR of 3 exhibits features at 2026, 1938 and 1900 cm-1, nearly identical to the features of the anthracene-based congener (4) at 2027, 1936 and 1888 cm-1. Most importantly, we assessed the effect of ligand-scaffold flexibility on reactivity and measured the rates of an elementary ligand substitution reaction. For ease of IR study, the corresponding halide-abstracted, nitrile-bound (PhCN) cations [(Thianth-py2)Mn(CO)3(PhCN)](BF4) (6) and [(Anth-py2)Mn(CO)3(PhCN)](BF4) (8) were generated in situ, and the PhCN → Br- back-reaction was monitored. The more flexible 3 (thianth-based) exhibited ∼3-4× faster ligand substitution kinetics (k25 C = 22 × 10-2 min-1, k0 C = 43 × 10-3 min-1) than the rigid analogue 4 (anth-based: (k25 C = 6.0 × 10-2 min-1, k0 C = 9.0 × 10-3 min-1) on all counts. Constrained angle DFT calculations revealed that despite large changes in the thianthrene scaffold dihedral angle, the bond metrics of 3 about the metal center remain unchanged; i.e. the 'flapping' motion is strictly a second coordination sphere effect. These results suggest that the local environment of molecular flexibility plays a key role in determining reactivity at the metal center, which has essential implications for understanding the reactivity of organometallic catalysts and metalloenzyme active sites. We propose that this molecular flexibility component of reactivity can be considered a thematic 'third coordination sphere' that dictates metal structure and function.

Crystal and electronic structure of a hexacarbonyldiiron cluster tethered to naphthalene-2-thiolate ligands.

The structure of the previously reported complex bis(µ-naphthalene-2-thiolato-κ2S:S)bis(tricarbonyliron)(Fe-Fe), [Fe2(C10H7S)2(CO)6], has been characterized by X-ray diffraction. In the solid state, the dinuclear complex adopts a butterfly-like shape, with an equatorial-axial spatial orientation of the naphthalene groups covalently coupled to the [S2Fe2(CO)6] unit. The asymmetric unit contains three independent [(µ-naphthalene-2-thiolato)2Fe2(CO)6] molecules. These molecules show intermolecular π-π stacking interactions between the naphthalene rings, which was confirmed by Hirshfield surface analysis. The electronic spectrum of the complex recorded in acetonitrile shows a band centered at 350 nm (ℇ = 4.6 × 103 M-1 cm-1) and tailing into the visible region. This absorption can be attributed to a π→π* electronic transition within the naphthalene moiety and a metal-based d→d transition.