Mn complexes with amino acid derived PN ligands were used in the catalytic transfer hydrogenation (TH) of ketone and chalcone substrates in 2-propanol with mild heating. Moreover, chalcones are reduced selectively to the saturated ketone at short times and can be fully converted to the alcohol when reactions are prolonged. The mechanism of chalcone reduction was briefly considered. Allylic alcohols are not reactive in 2-propanol, but quantitative isomerization occurs in toluene. Thus, we suspect that the allylic alcohols are dehydrogenated and the resulting ketone is formed through a direct 1,4-hydrogenation of the chalcone. Finally, several other related ligands that have been used in Mn-based TH reactions were explored to test the viability of ligand design in favoring chemoselectivity. The β-amino phosphine ligands proved most effective in this regard.

Dehydrohalogenation of fac-Mn­(κ2-N,P-PicP)­(CO)3Br (1 H ) and fac-Mn­(κ2-N,P-LutP)­(CO)3Br (1 Me ) with equimolar K­[N­(SiMe3)]2 afforded the reactive, aromatized 18-electron complexes fac-Mn­(κ3-N,C,P-PicP)­(CO)3 (2 H ) and fac-Mn­(κ3-N,C,P-LutP)­(CO)3 (2 Me ), respectively, with atypical binding modes. 2 H and 2 Me activate H2 across the Mn­(I)–C bond to furnish hydride complexes fac-Mn­(κ2-N,P-PicP)­(CO)3H (4 H ) and fac-Mn­(κ2-N,P-LutP)­(CO)3H (4 Me ), respectively. Both 2 H and 2 Me were observed to produce dearomatized 18-electron complexes Mn­(κ2-N,P-PicP*)­(CO)4 (3 H ) and Mn­(κ2-N,P-LutP*)­(CO)4 (3 Me ), respectively, when reacted with CO. The reactive Mn­(I)–C moiety of 2 H and 2 Me reacts with a variety of electrophiles: benzyl cyanide to form fac-Mn­(κ3-N,N′,P-(LutP-BnCN))­(CO)3 (6), E-chalcone to form fac-Mn­(κ3-N,C,P-LutP-chalcone)­(CO)3 (7), and AlCl3 to form fac-Mn­(κ3-N,Cl,P-PicP-AlCl3)­(CO)3 (8 H ) and fac-Mn­(κ3 -N,Cl,P-LutP-AlCl3)­(CO)3 (8 Me ) featuring a rare intramolecular Mn-(μ-Cl)-Al moiety. Mn­(I)-catalyzed Michael addition was explored. Dehydrohalogenation of 1 H with alkoxides afforded the substituted complex fac-Mn­(κ2-N,P-PicP)­(OR)­(CO)3 (10 H ) that was also reactive toward H2 forming 4 H . Under identical conditions with alkoxide bases, 1 Me affords 2 Me demonstrating dichotomous behavior. We also explored the iPr-substituted analogues of 1 H and 1 Me (11 H and 11 Me , respectively) and found them to be essentially identical in behavior. Complexes 1 R and 11 R were found to be excellent catalysts for styrene hydrogenation. The relevance of the κ3-N,C,P binding mode discovered herein to catalysis is briefly discussed.

Dehydrohalogenation of pyridine-derived pincer ruthenium complexes often leads to dearomatized moieties, such as in Milstein’s PNN-Ru­(CO)­(Cl)­(H) (1Py) catalyst. Thus, we were surprised to find an aromatized κ3-N,C,P binding mode in the lutidine-derived bidentate analogue [{LutP′}­Ru­(CO)­(H)­(PPh3)] (2), instead of a dearomatized compound, upon dehydrohalogenation of [{LutP}­Ru­(CO)­(Cl)­(H)­(PPh3)] (1). The reaction of 2 with H2 results in the formation of the cis-dihydride [{LutP}­Ru­(CO)­(H)2(PPh3)] (3), and labeling studies confirm cooperative metal–ligand activation. 3 exhibits reversible photoisomerization, forming another cis-dihydride isomer (4) upon irradiation. The lability of 4 toward ligand substitution was leveraged to demonstrate photoenhanced H2 production via acceptorless alcohol dehydrogenation. Labeling studies implicate metal–ligand cooperative (MLC) processes during the photocatalytic reaction, but they appear to be off-path processes on the basis of our mechanistic study of the system. The latter emphasizes that aromatization/dearomatization may not be necessary for acceptorless transformations, which is generally consistent with several contemporary studies on analogous Ru catalysts.

Metal-ligand cooperativity (MLC) involving reversible aromatization/dearomatization of pyridine-derived pincer ligands is considered important in acceptorless (de)hydrogenation for dihydrogen generation and storage. Dehydrohalogenation of pyridine-derived pincer ruthenium complexes often leads to dearomatized moieties. Thus, we were surprised to find an aromatized kappa-3-NCP binding mode in [{LutP`}Ru(CO)(H)(PPh3)] (2) upon dehydrohalogenation of the lutidine-derived PN/P complex [{LutP}Ru(CO)(Cl)(H)(PPh3)] (1) with KOtBu. The reaction of H2 with 2 results in formation of a cis-dihydride [{LutP}Ru(CO)(H)2(PPh3)] (3) and labeling studies confirm cooperative metal-ligand activation. 3 exhibits reversible photochemistry that we leveraged to demonstrate a unique strategy for unsensitized single-component photocatalytic H2 production via acceptorless alcohol dehydrogenation. Although labeling studies implicate MLC processes during the photocatalytic reaction, they may be off-path intermediates, emphasizing that aromatization/dearomatization may not be necessary for acceptorless transformations.

No abstract

<p>Beta-amino phosphines derived from amino-acids are excellent supporting ligands for Mn(I) transfer hydrogenation catalysts. Namely, a variety of ketones and chalcones are efficiently reduced under mild conditions (2-propanol, 3 h, 60 °C). Moreover, chalcones are selectively reduced to the saturated ketone in 2-propanol and allylic alcohols are quantitatively converted into the same product in toluene.</p>

Metal-ligand cooperativity (MLC) involving reversible aromatization/dearomatization of pyridine-derived pincer ligands is considered important in acceptorless (de)hydrogenation for dihydrogen generation and storage. Dehydrohalogenation of pyridine-derived pincer ruthenium complexes often leads to dearomatized moieties. Thus, we were surprised to find an aromatized kappa-3-NCP binding mode in [{LutP`}Ru(CO)(H)(PPh3)] (2) upon dehydrohalogenation of the lutidine-derived PN/P complex [{LutP}Ru(CO)(Cl)(H)(PPh3)] (1) with KOtBu. The reaction of H2 with 2 results in formation of a cis-dihydride [{LutP}Ru(CO)(H)2(PPh3)] (3) and labeling studies confirm cooperative metal-ligand activation. 3 exhibits reversible photochemistry that we leveraged to demonstrate a unique strategy for unsensitized single-component photocatalytic H2 production via acceptorless alcohol dehydrogenation. Although labeling studies implicate MLC processes during the photocatalytic reaction, they may be off-path intermediates, emphasizing that aromatization/dearomatization may not be necessary for acceptorless transformations.

Dehydrohalogenation of pyridine-derived pincer ruthenium complexes often lead to dearomatized moieties, such as in Mil-stein’s PNN-Ru(CO)(Cl)(H) (1Py) catalyst. Thus, we were surprised to find an aromatized 3-N,C,P binding mode in the lutidine-derived bidentate analog [{LutP`}Ru(CO)(H)(PPh3)] (2), instead of a dearomatized compound, upon dehydrohalo-genation of [{LutP}Ru(CO)(Cl)(H)(PPh3)] (1). The reaction of 2 with H2 results in formation of a cis-dihydride [{LutP}Ru(CO)(H)2(PPh3)] (3) and labeling studies confirm cooperative metal-ligand activation. 3 exhibits reversible photo-chemistry, forming another cis-dihydride isomer (4). The lability of 4 toward ligand substitution was leveraged to demon-strate a unique example of photoswitchable H2 production via acceptorless alcohol dehydrogenation. Labeling studies impli-cate metal-ligand cooperative (MLC) processes during the photocatalytic reaction, but they appear to be off-path processes based on our mechanistic study of the system. The latter emphasizes that aromatization/dearomatization may not be neces-sary for acceptorless transformations, which is generally consistent with several contemporary studies on analogous Ru cata-lysts.