Issue 45, 2020

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 pKa 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 (BDFEO–H) of 73 kcal mol−1 according to the Bordwell equation. This value is in good agreement with a corrected DFT computed BDFEO–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 (t1/2 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 [MnII{(PO)2Mn(CO)2}2]. Together with the experimental and calculated weakened BDFEO–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.

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

Supplementary files

Article information

Article type
Paper
Submitted
14 mar 2020
Accepted
22 apr 2020
First published
23 apr 2020

Dalton Trans., 2020,49, 16217-16225

Author version available

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

K. J. Kadassery, M. R. Crawley, S. N. MacMillan and D. C. Lacy, Dalton Trans., 2020, 49, 16217 DOI: 10.1039/D0DT00973C

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