Pyridine-Based Sulfoxide Pincer Complexes of Rhodium and Iridium

The new pyridine-based sulfoxide pincer ligand 1 (2-(diethylaminomethyl)-6-(tert-butylsulfinylmethyl)pyridine = S(O)NN) reacts cleanly with Rh2(COE)4Cl2 to form the neutral water- and air-stable RhI complex [Rh(S(O)NN)(NCCH3)], 1. The cationic complexes [Rh(S(O)NN(NCCH3)][BF4] (2) and [Ir(S(O)NN)(COE)][BF4] (5) were obtained by the reaction of 1 with the appropriate metal precursors. The corresponding carbonyl complexes [Rh(S(O)NN)(CO)][PF6] (4) and [Ir(S(O)NN)(CO)][BF4] (7) exhibit νCO in the IR spectra that shows that ligand 1 is a relatively poor σ-donor ligand compared to the more common PNP-type ligands, resulting in rather electron-poor metal centers. The carbonyl compounds can be deprotonated to the remarkably stable dearomatized complexes Rh(S(O)NN*)(CO) (8) and Ir(S(O)NN*)(CO) (9). DFT studies on 8 revealed a high electron delocalization over the pyridine ring system and the sulfur atom, which explains the high stability of 8 toward reprotonation. Complete protonation of 8 was achieved in acetic acid.