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

Dalton Transactions Pub Date: 2014-05-13 DOI: 10.1039/C4DT00600C

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 (R1NHNS) generally afford complexes of type [(RNNS)Mn(CO)3Br] (1CO, 2CO, 3CO; 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 (NMeNS) affords a complex of formula [(NMeNS)Mn(CO)2Br] (4CO), 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] (1CO through 3CO) afford three ν(CO) in the range ～2060–1865 cm?1; the dicarbonyl complex [(NMeNS)Mn(CO)2Br] (4CO) exhibits two carbonyl stretches in the range ～1920–1845 cm?1. Prolonged storage of the tricarbonyl [(MeNNS)Mn(CO)3Br] (3CO) in presence of trace dioxygen affords the dibromide species [(MeNNS)Mn(Br)2] (3Br), in which the thioether S reliably binds to the Mn(II) center. Complexes 1CO–3CO exhibit simple, diamagnetic 1H NMR spectra in CD2Cl2. The S-ligated complex 4CO exhibits spectra consistent with a mixture of an S-bound (6-coordinate) and S-unbound (5-coordinate) species as represented by [(NMeNS)Mn(CO)2Br] ? [(NMeNS)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] (5CO-a) and [(FPhNNS)Mn(CO)2Br] (5CO-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.

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