Synthesis and spectroscopic (¹H NMR, ESR) characterization of new aryloxy-Mn(II) complexes: steric control over <i>O</i>- vs. phenyl-π-coordination of ArO^- ligands (ArO^- = C₆H₅O^-, 4-methyl-C₆H₄O^-, 3,5-dimethyl-C₆H₃O^-, 2,6-di-<i>tert</i>-butyl-C₆H₃O^-, 2,6-dimethyl-C₆H₃O^-) to the "Mn(II)Cp" moiety, and their reactivity with carbon dioxide

Abstract

The coordination chemistry of phenoxide ligands, such as C 6 H 5 O – , 4-(CH 3 )-C 6 H 4 O – , 3,5-(CH 3 ) 2 -C 6 H 3 O – , 2,6-(tert-butyl) 2 -C 6 H 3 O – , 2,6-(CH 3 ) 2 -C 6 H 3 O – , to Mn(II) has been investigated because of the possible implication of Mn(II)–phenoxide complexes as intermediates in the phenylphosphate carboxylase enzyme, a protein which catalyses the selective carboxylation of phenylphosphate to 4-OH-benzoic acid using CO 2 . We report here the synthesis and characterization of [CpMn(µ-OAr)(THF)] 2 (ArO = C 6 H 5 O – , 4-(CH 3 )-C 6 H 4 O – , 3,5-(CH 3 ) 2 -C 6 H 3 O – , 2,6-(CH 3 ) 2 -C 6 H 3 O – ) and [CpMn(η 5 -ArO)] (ArO = 2,6-(tert-butyl) 2 -C 6 H 3 O – and 2,6-(CH 3 ) 2 -C 6 H 3 O – ) complexes, the first examples of mixed-sandwich complexes with Cp and phenate as π-ligands. The latter bear the 2,6-substituted phenoxide π-coordinated to the [Mn(Cp)] + moiety. The different mode of bonding of the phenoxide ligands to Mn(II), substantiated by 1 H NMR and electron spin resonance (ESR) spectroscopy, is controlled by the steric hindrance of substituents at the 2- and 6-position. The reactivity of the π-bonded ligand towards CO 2 is also reported as a quite rare example of nucleophilic attack at the cumulene by the ring-carbon of the phenoxide, which is driven by electron density localization at the 4-position generated upon π-coordination to Mn(II).Key words: Mn(II)-complexes, phenoxide ligands, 1 H NMR spectroscopy, ESR spectroscopy, reaction with carbon dioxide.