60th Annual Report on Research 2015

C-H activation is an integral step in the functionalization of inert small molecules and is essential for the utilization of hydrocarbon feedstocks as high-value materials such as fuels or fine chemical precursors. Understanding how the ligand environment of a metal center affects the selectivity and efficacy of C-H activation is crucial in the top-down development of new catalytic systems that rely on such processes.

Our approach involved the synthesis of pyridine-substituted pincers, PC_pyP, to generate several iridium complexes (Scheme 1) starting from ortho-bromo-diphenylphosphine (1). Compound 1 was oxidized to the corresponding phosphine oxide (2a) or sulfide (2b) prior to the reduction of the alcohol group to generate 3, which was reduced to the desired pincer 4.

Scheme 1. Synthesis of iridium hydride complexes [(PC_pyP)IrH₂] and [(PC_pyP)IrH₃Na(THF)]₂.

Reaction of 4 with [(COD)IrCl]₂ initially affords a κ² adduct, 5, which was isolated and structurally characterized. Compound 5 is stable as a solid, but when heated in toluene, two iridium chloride complexes, 6-fac and 6-mer, formed, with the facial isomer as the major product. Different reactivity was observed when the more labile [(COE)₂IrCl]₂ reacted with 4: only the 6-fac formed and no other intermediates were observed by NMR spectroscopy.

Complex 6-fac reacted with NaH in THF to give the iridium dihydride complex 7 (Scheme 1). Interestingly, when a large excess of NaH reacted with 6-fac, a dimeric iridium trihydride complex 8 was formed that contains two Na ions bridged by hydrides. The crystal structure of 8 showed that one Na ion was coordinated by two pyridine groups of the ligands, while the other Na ion was coordinated with two THF molecules.

Scheme 2. Activation of COD and synthesis of [(PC_pyP)Ir(COD)].

Iridium(I) complexes are usually good catalysts, therefore, we also attempted to synthesize the corresponding iridium(I) complexes bearing 4 (Scheme 2). Treatment of 5 with KO^tBu at room temperature afforded the iridium(III) complex 9, which was isolated and structurally characterized. Compound 9 features a dianionic COD ligand, which was likely formed by the activation of the neutral COD ligand at the allylic position by an in situ generated iridium(I) species followed by migratory insertion of the other C=C bond of the COD ligand into the Ir-H bond. Upon heating of 9 in toluene, the expected iridium(I) complex 10 was formed as the major product. Complex 10 can also be synthesized by the deprotonation of 4, followed by a salt metathesis reaction with [(COD)IrCl]₂.

Scheme 3. Activation of ethers by iridium hydride complex [(PC_pyP)IrH₂].

Initial reactivity studies showed that neither the iridium chloride 6-fac nor the dihydride 7 are good catalysts for alkane dehydrogenation under various conditions, probably due to the relatively strong coordination of pyridine group in nonpolar reaction media such as cyclooctane and t-butylethylene. However, we have found that pyridine dissociates in polar solvents, thus facilitating a series of C-H bond activation reactions. Therefore, upon treatment of 7 with norbornene (NBE), both tetrahydrofuran and methyl-t-butylether can be activated selectively at their α-positions at room temperature to give the corresponding hydrides 11 and 12 (Scheme 3). Both complexes were further characterized by X-ray diffraction studies, which showed the coordination of pyridine to the iridium center.

Scheme 4. Reaction of 7 and 6-fac with phenyl acetylene.

Considering the dissociation of pyridine in the presence of polar reagents and its basic nature, we attempted to functionalize the THF group in 11. Interestingly, treatment of 11 with phenylacetylene (Scheme 4) afforded a zwitterionic compound 13, which was isolated and characterized by X-ray diffraction studies. In 13, the original THF group and the hydride are still intact, but an alkenyl carbon atom is now coordinated to iridium with the pyridine nitrogen atom also attached to the same alkenyl carbon (Scheme 4). Therefore, neither the insertion of the alkyne into the Ir-H bond nor the expected C-C bond formation between the alkyne and THF group occurred. The same experiment performed by using phenylacetylene-d₁ indicated the presence of a vinylidene intermediate, which was likely attacked by the proximal basic pyridine group. The isolation of 13 demonstrated the basic and nucleophilic role of the pyridine group. However, the reaction of 6-fac with phenylacetylene afforded the expected Ir-H bond insertion product, 14, in which the fac configuration was retained.

Scheme 5. Synthesis of iridium silyl and internal base-stabilized silylene complexes

We also realized that the iridium(I) complex 10 may be a good starting materials for a wide range of oxidative addition reactions. Unfortunately, 10 does not react with alcohols such as ethanol and iso-propanol even at elevated temperature. Also, no reaction was observed with phenylacetylene. However, 10 slowly reacted with Ph₂SiH₂ to give an interesting iridium silyl dihydride complex, 15, in high yield (Scheme 5). The most striking feature of 15 was showed by its ²⁹Si NMR spectrum at 47.6 (dd) ppm from coupling with both cis (9.8 Hz) and trans (172.1 Hz) phosphines; the silicon value is significantly shifted toward low field compared to the corresponding values in base free iridium silylene complexes (~250 ppm). Based on this spectroscopic data, we propose that 15 is an internal base stabilized iridium silylene complex. Compound 10 also reacted with PhSiH₃, but only a mixture was formed and no clean compound could be isolated from that mixture. The dihydride complex 7 also reacts with PhSiH₃ and Ph₂SiH₂ under mild conditions to give the corresponding iridium silyl hydride complexes 16 and 17 in high yield (Scheme 5). Interestingly, upon heating, 16 converted to 15 in quantitative yield based on NMR spectroscopy.

       The ACS PRF grant helped the Iluc group to establish a new project. The support provided by the grant made possible hiring a postdoctoral fellow with expertise in C-H activation reactions, expertise that the group lacked at the time. The funding provided was so timely and had such a great impact that the group published 8 high profile publications directly acknowledging the ACS grant and allowed the PI to submit several proposals to federal agencies.