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

Abstract

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 (k_{25 C} = 22 × 10⁻² min⁻¹, k_{0 C} = 43 × 10⁻³ min⁻¹) than the rigid analogue 4 (anth-based: (k_{25 C} = 6.0 × 10⁻² min⁻¹, k_{0 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.