A hemilabile manganese(i)–phenol complex and its coordination induced O–H bond weakening

Abstract

The known compound K[(PO)₂Mn(CO)₂] (PO = 2-((diphenylphosphino)methyl)-4,6-dimethylphenolate) (K[1]) was protonated to form the new Mn(I) complex (HPO)(PO)Mn(CO)₂ (^H1) and was determined to have a pK_a approximately equal to tetramethylguanidine (TMG). The reduction potential of K[1] was determined to be −0.58 V vs. Fc/Fc⁺ in MeCN and allowed for an estimation of an experimental O–H bond dissociation free energy (BDFE_O–H) of 73 kcal mol⁻¹ according to the Bordwell equation. This value is in good agreement with a corrected DFT computed BDFE_O–H of 68.0 kcal mol⁻¹ (70.3 kcal mol⁻¹ for intramolecular H-bonded isomer). The coordination of the protonated O-atom in the solid-state ^H1 was confirmed using FTIR spectroscopy and X-ray crystallography. The phenol moiety is hemilabile as evident from computation and experimental results. For instance, dissociation of the protonated O-atom in ^H1 is endergonic by only a few kcal mol⁻¹ (DFT). Furthermore, [1]⁻ and other Mn(I) compounds coordinated to PO and/or HPO do not react with MeCN, but ^H1 reacts with MeCN to form ^H1+MeCN. Experimental evidence for the solution-bound O-atoms of ^H1 was obtained from ¹H NMR and UV-vis spectroscopy and by comparing the electronic spectra of bona fide 16-e⁻ Mn(I) complexes such as [{PNP}Mn(CO)₂] (PNP = ⁻N{CH₂CH₂(PⁱPr₂)}₂) and [(Me₃SiOP)(PO)Mn(CO)₂] (^Me3Si1). Compound ^H1 is only meta-stable (t_1/2 0.5–1 day) and decomposes into products consistent with homolytic O–H bond cleavage. For instance, treatment of ^H1 with TEMPO resulted in formation of TEMPOH, free ligand, and [Mn^II{(PO)₂Mn(CO)₂}₂]. Together with the experimental and calculated weakened BDFE_O–H, these data provide strong evidence for the coordination and hemilability of the protonated O-atom in ^H1 and represents the first example of the phenolic Mn(I)–O linkage and a rare example of a “soft-homolysis” intermediate in the bond-weakening catalysis paradigm.