C-H activation: oxidative addition to an iridium(I) center and reactivity of the resulting iridium(III) species

Thermal reactions of the iridium complex, [Ir(COD)(PMe₃)₃]Cl (COD=1, 5 Cyclooctadiene), with benzene, benzaldehyde, pyridine, and furan yield the C-H addition products of mer-(Me₃P)₃Ir(Phenyl)(H)C! (1), mer-(Me₃P)₃Ir(COC₆H₅)(H)Cl (II), mer-(Me₃P)₃Ir(Pyridyl)(H)Cl (IID, and mer-(Me₃P)₃Ir(furyl)Cl (IV), respectively. Each of these complexes was characterized by 1H, 13C, 31P NMR spectroscopy and single crystal X-ray diffraction. Thermal reaction of the [Ir(COD)(PMe₃)₃]CI with thiophene and benzothiophene produced the C-S addition products forming thiometallacycles of (Me₃P)₃Ir-(CH=CHCH=CHS)CI(V), and (Me₃P)₃Ir(CH=CH-C₆H₄S)Cl (VI).

The C-H addition products are unreactive and require the aid of TI[PF6] to remove the chloride ligand to provide an open coordination site. Complex (I) and 3,3-dimethyl-1-butyne, produced a 1,4-di-t-butyldienyl complex (VIII) by a double insertion of the alkyne with one t-butyl group forming an agostic interaction with iridium. Deuterium labeling experiments revealed the mechanism to involve: initial acetylene coordination after the chloride ligand was removed by TI⁺ followed by a hydride migration forming a vinyl complex and allowing a second alkyne to coordinate, rearrangement of the second alkyne into a vinylidene and finally a migratory-insertion of the vinyl group unto the vinylidene to form (VIII).

Complex (III) and two equivalence of 3,3-dimethyl-1-butyne produced a trans diacetylide complex (XII). The pyridyl ligand was able to remove acidic protons via the nitrogen lone pair, thus altering the course of the reaction. Initial investigations have led to a possible mechanism.

Complex (IV) and 3,3-dimethyl-1-butyne gave an interesting hydrido metallo-vinyl complex (XIV) with the t-butyl group forming an agostic CH-Ir interaction. A deuterium labeling experiment revealed the alkyne to initially coordinate and rearrange into a vinylidene, followed by migratory-insertion of the furan group unto the vinylidene to give (XIV). The furyl group is believed to stabilize the initial intermediate by forming a five coordinate square pyramidal complex through the donation of a pair of electrons from the oxygen atom. Reductive elimination of (XIV) using elevated temperatures gave exclusively the trans 3,3-dimethyl-1-(2 furanyl) -1-butyne (XV). The resonance structure found in (IV) is the most important feature of this work. While most insertions occur between the M-H bond, the resonance structure found in (IV) provides the necessary means for insertion of unsaturates to occur between the M-C bond, leaving the M-H bond intact.

In conclusion, it appears that the reactivity of the phenyl complex (I) is dependent on the alkyne used. While the reactivity of the pyridyl complex (III) is governed by the nitrogen lone pair; the reactivity of the furanyl complex (IV) follows a different pathway due to resonance stability of the furanyl ligand.