Group 10 Bis(iminosemiquinone) Complexes and Their Propensity to Perform Hydrogen Atom Transfer Reactions

Non-innocent ligands can be versatile tools in providing catalytic function to organometallic redox catalysts. In the case of the redox-active ^tBuClipH₄ ligand (N,N´-bis(2-hydroxy-3,5-di-tert-butylphenyl)-2,2´-diamino-4,4´-di-tert-butylbiphenyl), the ligand can undergo 2H⁺/2e⁻ redox changes, shuttling between an oxidized bis(iminosemiquinone) and a reduced bis(aminophenoxide), when coordinated to group 10 metals. The frontier orbitals of bis(iminosemiquinone) complexes consist of a ligand-centered orbital, and a higher-lying orbital that is largely ligand-centered but is metal-ligand π antibonding. These two orbitals are collectively occupied by two electrons. The complexes are always ground-state singlets, but many of the studied complexes possess low-lying triplet states that can be thermally populated. Measurement of the singlet-triplet energy gap by variable-temperature NMR, in combination with UV-Vis spectroscopic data of the cationic, neutral, and anionic forms of the complex, have provided a comprehensive view of the electronic nature of the complexes.

Group 10 bis(iminosemiquinone) metal complexes of cis geometry were hypothesized to be able to accomplish a concerted transfer of a pair of hydrogen atoms from an organic substrate. What has been observed is that the mechanism of hydrogen atom transfer is stepwise in all investigated reactions. In regards to 2H⁺/2e⁻ reactivity, the palladium complex (^tBuClip)Pd dehydrogenates hydrazines and hydroxylamines to generate the (^tBuClipH₂)Pd complex. In comparison, the platinum analogue is much less oxidizing, and dehydrogenates hydrazobenzene reversibly (∆G° = 1.85 kcal/mol at 22 °C). Not only are the complexes different in terms of their thermodynamic reducing power, but also in how the initial hydrogen atom transfer affects the transfer of the second hydrogen atom. The (^tBuClipH₂)Pd complex shows practically no coupling between the two hydrogen atom transfer events, both thermodynamically and kinetically. The platinum complex displays much stronger coupling between the two hydrogen atom transfer events and favors two electron chemistry more than the palladium complex. While the extent of coupling in the analogous nickel complex remains unknown, it is estimated to be more like the platinum complex in this regard.

In the reactions involving the bis(iminosemiquinone) complexes with hydrazobenzene, all three complexes display exceedingly large primary kinetic isotope effects that are beyond the theoretical limit for a classical hydrogen atom transfer. Such large KIEs are interpreted to arise from hydrogen atom tunneling. The reactivity with the palladium complex also has a non-linear dependence of rate on mole fraction protium. The proposed explanation is that a change in the fate of the intermediate hydrazyl radical (disproportionation vs. further reactivity with palladium), depending on its isotopic composition, alters the stoichiometric factor of the hydrogen atom transfer events, and therefore the observed rates of reaction.

In exploring the hydrogen atom transfer with alkynes, only the bis(aminophenoxide)platinum complex was observed to have reactivity. However, rather than the stepwise hydrogen atom transfer seen between these complexes and nitroso/azo substrates, (^tBuClipH₂)Pt engages in ene-type reactivity with cyclooctyne. The evident stereoselectivity and regioselectivity of the product, along with results from radical trapping experiments, strongly suggest a concerted addition reaction. (^tBuClipH₂)Pt was also observed to have unique reactivity with other electron withdrawing alkynes.