Tuning coordination modes of pyridine/thioether Schiff base (NNS) ligands to mononuclear manganese carbonyls

Abstract

We have investigated the coordination modes of NNS Schiff base, thioether ligands to manganese(I) carbonyls. The ligands contain ortho substituted pyridines (H, CH₃, OCH₃, fluorophenyl) and varying substituents (H, CH₃) at the Schiff base linkage. In general, reaction of [Mn(CO)₅Br] with a tridentate NNS ligand in CH₂Cl₂ affords species in which the thioether-S may be bound or unbound to the manganese center, depending on the steric and electronic substitution in the ligand framework; as a result, the complexes exhibit two or three carbonyl ligands, respectively. Aldehyde-derived ligand frames (_R1N_HNS) generally afford complexes of type [(_RNNS)Mn(CO)₃Br] (1_CO, 2_CO, 3_CO; R = H, OCH₃, CH₃) that exhibit incomplete ligation of the chelate (S not bound) in X-ray structures. In contrast, use of the iminomethyl ligand (N_MeNS) affords a complex of formula [(N_MeNS)Mn(CO)₂Br] (4_CO), in which the mixed N/thioether-S stabilizes the {Mn(CO)₂}⁺ fragment. In solid state IR spectra, complexes of type [(_RNNS)Mn(CO)₃Br] (1_CO through 3_CO) afford three ν(CO) in the range ∼2060–1865 cm⁻¹; the dicarbonyl complex [(N_MeNS)Mn(CO)₂Br] (4_CO) exhibits two carbonyl stretches in the range ∼1920–1845 cm⁻¹. Prolonged storage of the tricarbonyl [(_MeNNS)Mn(CO)₃Br] (3_CO) in presence of trace dioxygen affords the dibromide species [(_MeNNS)Mn(Br)₂] (3_Br), in which the thioether S reliably binds to the Mn(II) center. Complexes 1_CO–3_CO exhibit simple, diamagnetic ¹H NMR spectra in CD₂Cl₂. The S-ligated complex 4_CO exhibits spectra consistent with a mixture of an S-bound (6-coordinate) and S-unbound (5-coordinate) species as represented by [(N_MeNS)Mn(CO)₂Br] ↔ [(N_MeNS)Mn(CO)₂Br]. Lastly, we obtained crystal structures of the S-bound and S-unbound conformers derived from the same ligand – the fluorophenyl derived _FPhNNS, namely [(_FPhNNS)Mn(CO)₃Br] (5_CO-a) and [(_FPhNNS)Mn(CO)₂Br] (5_CO-b). This report represents several examples of a thioether-stabilized {Mn(CO)₂}⁺ fragment, a deviation from the usual ‘piano stool’ Mn(I) tricarbonyl motif. We highlight that coordination of these NNS ligands to Mn(I) carbonyls occurs on a soft conformational landscape, and that ligand substituents can be rationally employed to favor the desired coordination mode.