Significant O–H Bond Weakening in CpMn(CO)₂(CH₃OH): Evidence for the Generation of the CpMn(CO)₂(CH₃O) Radical upon H Atom Abstraction by O₂

Abstract

The UV−visible photolysis of CpMn(CO) 3 in methanol generates CpMn(CO) 2 (CH 3 OH), which upon H atom abstraction using either O 2 , dpph, or H 2 O 2 gives the CpMn(CO) 2 (CH 3 O) radical complex. The radical with a lifetime of 10 to 20 min has been mainly characterized using FTIR and ESR spectroscopy and chemical oxidation studies. Together with density functional theory calculations, it is shown that the O−H bond is significantly weakened in the CpMn(CO) 2 (CH 3 OH) complex compared to free methanol. NBO spin density analysis suggests that the bond cleavage is facilitated by the formation of the CpMn(CO) 2 (CH 3 O) radical in which the electron spin is localized at the metal center rather than at the oxygen atom.

CO-Photolysis-Induced H-Atom Transfer from MnIO–H Bonds

“…The lack of reactivity in the dark is consistent with the high O–H bond dissociation free energy (BDFE O–H ) of 1 that we estimate using density functional theory (DFT) for this report. Therefore, we posit that the OH ligands in 1 undergo O–H bond weakening as a result of photolysis of a CO ligand, similar to findings with CpMn­(CO) 2 (MeOH) by Fan and co-workers . The findings with TEMPO are briefly contextualized within the framework of developing new strategies to produce H 2 from water and light.…”

“…Using light to liberate a CO ligand provides an alternative path to an otherwise inaccessible thermal ligand dissociation. The multitude of O–H bond-weakening reactions induced by the photolysis of Mn I –CO bonds ,,, on platforms that also reduce water to H 2 provides a possible link between CO-photolysis-induced O–H bond weakening and H 2 formation.…”

“…4 Fan and coworkers demonstrated substantial O−H bond weakening in a methanol (MeOH) adduct coordinated to CpMn(CO) 2 . 8 It follows that similar O−H bond weakening may also occur in the CpMn(CO) 2 (OH 2 ) species. In a related study, Beweries and co-workers showed that an ansa-titanocene-aqua complex can facilitate cyclic "water splitting", where the O−H bond in the aqua ligand is cleaved homolytically and transferred to (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO).…”

“…For instance, the first two steps in a water-splitting reaction with CpMn­(CO) 3 is carbon monoxide (CO) photolysis and subsequent water coordination to CpMn­(CO) 2 . Fan and co-workers demonstrated substantial O–H bond weakening in a methanol (MeOH) adduct coordinated to CpMn­(CO) 2 . It follows that similar O–H bond weakening may also occur in the CpMn­(CO) 2 (OH 2 ) species.…”

“…Therefore, we posit that the OH ligands in 1 undergo O−H bond weakening as a result of photolysis of a CO ligand, similar to findings with CpMn(CO) 2 (MeOH) by Fan and coworkers. 8 The findings with TEMPO are briefly contextualized within the framework of developing new strategies to produce H 2 from water and light.…”