Efficient Catalysis of Transfer Hydrogenation of Ketones and Oxidation of Alcohols with Newly Designed Half-Sandwich Rhodium(III) and Iridium(III) Complexes of Half-Pincer Chalcogenated Pyridines

The in situ generated ArE¯ (E =
S or Se)
reacts with (2-chloromethyl)­pyridine in N2 atmosphere,
resulting in half-pincer (N, S/Se) ligands (2-arylchalcogenomethyl)­pyridine
(L1–L3; aryl = Ph/2-pyridyl for S,
Ph for Se). Half-sandwich complexes [(η5-Cp*)­Rh­(L)­Cl]­[PF6] (1, 2), [(η5-Cp*)­Rh­(L2)­CH3CN]­[PF6]2 (3), and [(η5-Cp*)­Ir­(L)­Cl]­[PF6] (4–6), where L = L1–L3,
have been synthesized by reacting L with [(η5-Cp*)­RhCl­(μ-Cl)]2 and [(η5-Cp*)­IrCl­(μ-Cl)]2 respectively. The ligands and
complexes have been characterized by IR, HR-MS, and 1H, 13C­{1H}, and 77Se­{1H} NMR
spectra. The single-crystal structures of all the complexes (1–6) have been determined with X-ray crystallography
[Rh–S, Rh–Se, Ir–S, Ir–Se bond distances:
2.383(2)/2.353(2), 2.4867(9), 2.343(2)/2.341(3), 2.453(1) Å,
respectively]. Each metal has pseudo-octahedral
half-sandwich “piano-stool” geometry. The PF6– ions in crystals are packed between half-sandwich
complex molecules, resulting in various H···F secondary
interactions. Complexes 1–6 have
been explored for catalysis of oxidation of secondary alcohols with N-methylmorpholine-N-oxide and transfer
hydrogenation reaction of ketones with 2-propanol and found very efficient,
as revealed by TON values, which are up to 9.6 × 103 and 9.8 × 103 respectively for the two catalytic
reactions. The formation of the M–H bond has been noticed in
the intermediates of both the catalytic reactions. DFT calculations
indicate a reactivity order for the complexes of Rh > Ir, which
is
also found for the two catalytic reactions experimentally. The calculated
(DFT) and observed bond distances and angles of 1–6 are reasonably close.