Rare Organometallic Complex of Divalent, Four-Coordinate Iridium: Synthesis, Structural Characterization, and First Insights into Reactivity^†

An application of the new sterically bulky P∧O bidentate phosphine tBu2PC(CF3)2OH (1) in the cyclometalation of 2-(3,5-bis(trifluoromethyl)phenyl)-4-methylpyridine (4) by IrCl3 resulted in the stabilization of unusual coordination states of iridium: from the rare example of a mononuclear, paramagnetic, square-planar IrII complex, trans-Ir[η2-OC(CF3)2PtBu2]2 (2), to a coordinatively unsaturated, five-coordinated monohydride, trans-Ir(H)[η2-OC(CF3)2PtBu2]2 (3), and a mono-cyclometalated complex, C∧NIr(H)(CO)[η2-OC(CF3)2PtBu2] (6). The complex 2 can be synthesized by two alternative methods: by the aerial oxidation of 3 in the catalytic presence of a base, such as Et4NOH, or by the sublimation of 3 under vacuum. The IrII complex 2 undergoes hydrogenation to the IrIII−H complex 3. The reaction between carbon monoxide and a THF solution of complex 2 led to the reduction to the IrI complex trans-Ir(CO)[η2-OC(CF3)2PtBu2][η1-tBu2PC(CF3)2OH] (11). The C-cyclometalated complex Ir[η2-OC(CF3)2PtBu2][η3-CH2CMe2P(tBu)C(CF3)2O] (12) is formed upon heating of aprotic solutions of complex 2. The square-pyramidal five-coordinate IrIII hydride 3 reacts with sterically small ligands to form coordinately saturated octahedral species. For example, carbon monoxide forms trans-Ir(H)(CO)[η2-OC(CF3)2PtBu2]2 (9) and 4-pyrrolidin-1-yl-pyridine (L) gives trans-Ir(H)(L)[η2-OC(CF3)2PtBu2]2 (10). There is a significant difference in bond lengths between the oxygen and iridium atoms in the divalent state versus the trivalent state for equivalent ligands according to X-ray analysis. Divalent iridium has an average IrII−O distance of 1.970(2) Å, while trivalent iridium has an average IrIII−O distance of 1.996(2) Å. The reinvestigation of the original claim in the 1970s on the isolation of a square-planar IrII complex in the reaction between the phosphine tBu2PC6H3(OMe)2-2,6 (15) and IrCl3 in 2-propanol resulted in the isolation of two square-pyramidal, five-coordinate IrIII hydrides: trans-Ir(H)[η2-OC6H3(iPr-6)(OMe-3)PtBu2]2 (16) and trans-Ir(H)[η2-OC6H2(OMe-3)PtBu2]2 (17). The X-ray analysis of 17 reveals that the triclinic unit cell parameters are in agreement with the published room-temperature unit cell for the IrII complex. Bond lengths between the iridium and oxygen atoms in 17 and in the published IrII complex are 2.012(3) and 2.00 Å, respectively. This falls into our range for red IrIII complexes. The 1H NMR spectrum of 17 shows a characteristic upfield triplet for IrIII−H at δ −48.0 (t, 2JP,H = 11.9 Hz). The aerial oxidation of 16 in the catalytic presence of base led to the paramagnetic IrIV complex Ir[η3-OCH2CMe2P(tBu)C6H3(iPr-6)(OMe-3)-O]2 (20).