I wish you the best of luck in Germany (you lucky dog!) and hope to hear about your great scientific achievements in the future. This thesis focuses on the study of multifunctional and non-innocent ligands in the context of small molecule activation in a variety of mono- and multinuclear complexes.
Synthesis of ligand para-terphenyl diphosphine ligands
The development of new dinucleation ligand frameworks and complexes capable of both redox processes and enabling new chemical transformations is currently a topic of active research.6 Our group recently used bis- and tris(phosphinoaryl)benzene ligands as multinuclear frameworks for Ni and Pd complexes , where the central arene participates as a flexible donor ligand.5g, 7. Here we report the rational synthesis of a series of isostructural low oxidation state M2M'-type heterometallic complexes supported by two para-terphenyl diphosphine ligands with different central arene electronics.
Synthesis of heterometallic trinuclear complexes
The reduction of 3-(OMe)2 is ∼200 mV more negative than 3, a consequence of the more electron-rich supporting arene. Recalling the electrochemical behavior of 4 and 8 , a pair of nearly reversible reduction events V) is observed for complex 6 .
Synthesis of homometallic multinuclear carbonyl complexes of Fe, Co, and Ni
P bonding
Aerobic oxidations catalyzed by Pd, which additionally use the reversible proton-two-electron pair of 1,4-benzoquinone, have important applications in organic methodology. 6 The use of 1,4-benzoquinone and its derivatives as stoichiometric or catalytic oxidants alone has also been recently reviewed.7 Recent mechanistic studies have highlighted the complex reaction pathways available in the combination of a redox-active metal (Pd), a redox-active organic additive that can also act as a ligand (1,4-benzoquinone), and O2.6a , 8 Although Pd catalysis uses 1,4-benzoquinone as a direct oxidant, we are particularly interested in the reactivity of the reduced counterpart as a site to provide protons and electrons for substrate activation in the presence of a reactive metal site. At the same time, the effect of the nature of the quinonoid fragment (reduced vs. oxidized) on the chemistry of Pd with O2 is particularly important for organic methodology. Efforts to directly incorporate quinonoid moieties into ligand frameworks for Pd have been reported.9 Although interconversion of the hydroquinone and quinone forms of these ligands has been observed in some cases, oxygen reduction with these Pd systems has not been reported.
Our group reported the use of bis- and trisphosphinoarylbenzene ligands as scaffolds for π-bonded mono- and polynuclear transition metal complexes.11 In cationic Ni–H complexes, the harmlessness of the central arene moiety with respect to H-migration was observed, as well as with dinuclear Fe and Co carbonyl complexes according to the partial reduction of the ring.11e, 11g Incorporation of a catechol moiety into the ligand framework capable of transferring more electrons and protons or other electrophiles during oxygen reduction was also recently reported for the Mo complex.11c In this case, there is no evidence , that the metal center undergoes inner-sphere chemistry with O2, although electronic coupling with the quinonoid moiety and electrophile transfer from catechol is key to reactivity. Similar to the dimethoxy version presented in Chapter 2 (Section 2.1 1-(OMe)2), hindered rotation results in multiple 31P{1H} ppm) resonances at room temperature as well as a broadened 1H spectrum.
Synthesis of 1-H
Synthesis of reported metal complexes
Both complex 2 and 3 bind the quinone moiety in a η4 manner, similar to previously reported metal-quinone complexes.13a-c, 15 Although η4 bonding has a precedent for Pd, it should be noted that most Pd quinone structures exhibit η2 coordination . 13c-e, 15b, 16 Both complexes 2 and 3 exhibit a distorted tetrahedral geometry with similar τ4' values of 0.54 and 0.55, respectively, using the two phosphine donors and the centroids of the coordinated C-C double bonds as ligand contacts . Free quinone in DMSO shows two reductions of the corresponding radical anion and the closed shield anion at -0.91 V and -1.71 V versus Fc/Fc+.4c Complex 2 shows a quasi-reversible reduction event centered at -1.59 V, possibly a two -electron process and an irreversible oxidation event at 0.77 V versus Fc/Fc+ (Figure 2). However, additional oxidation events are observed at -1.09 and -0.4 V, corresponding to the reduction events indicating multi-species formation after a two-electron reduction of 3.
Electrochemical data from the literature for Pd complexes with quinonoid units incorporated into the ligand scaffold show reductions at much milder potentials, suggesting that direct coordination of Ni or Pd with the quinone π system results in significant changes in the electronic properties of the ligand.9b, 9d , 9e. Attempts were made to regenerate the hydroquinone form of the central arene by directly reducing the quinone in 3.
Summary of reduction attempts of 3
The formation of this species suggests that the decomposition of 4-H in solution may occur via a metal-mediated isomerization of the hydroquinone moiety. It is therefore possible that initial activation of the dioxygen may occur at the Pd0 center prior to subsequent activation of the hydroquinone moiety. This indicates that direct oxidation of the hydroquinone moiety of the ligand by dioxygen is not a facile reaction pathway and likely does not occur in the 4-H reaction.
To test the direct oxidation of the hydroquinone moiety to 6-H by dioxygen, 5-H was treated with O2, since, to the best of our knowledge, dioxygen activation via the PdII-Cl bond has not been reported. These data indicate that at lower temperatures oxygen binding and activation occurs only by Pd without any participation of hydroquinone.
Summary of room temperature reactivity
Me was found to be stable for over four weeks at -78°C in THF and for several hours at -40°C without any sign of decomposition. This information further supports the assignment of direct dioxygen bonding at the Pd0 center without involvement of the hydroquinone group. Furthermore, the increased stability of 8-Me suggests that the hydroquinone group, when present, facilitates further reactivity with the Pd-coordinated O group.
Proposed dioxygen activation mechanism for 4-H and 4-Me
No reaction of 6-H or 6-Me with dioxygen was observed at -30 °C by 31P{1H} NMR (CD3CN), suggesting that the activation of dioxygen with 4-H is unlikely to occur via a cationic Pd -H species. The similarity between Int1 and 8-Me by NMR further suggests that the activation of dioxygen by 4-H does not occur via a Pd-H species, given that the precursor to 8-Me does not have acidic protons. Gas quantification experiments using a Toepler pump were used to determine the equivalents of dioxygen consumed by each reaction (Table 1).
The binding of a single equivalent of dioxygen to 4-Me at low temperatures precludes the formation of a bis-(η1-superoxo) (Scheme 6, a). Finally, cooperative activation of dioxygen between the Pd center and the hydroquinone ligand is also ruled out, as no evidence for hydroquinone oxidation was observed by IR for Int1.
Possible Dioxygen Activation Mechanisms
Cooling the solution of Int2 to −78 °C could not stop the conversion to 3 , although the reaction was slower. If the oxygen release has already occurred in the formation of Int2, the proposed anionic oxygenic fragment may be a hydroxide ligand. However, if dioxygen release occurs in the conversion of Int2 to 3, then the bound oxygenic fragment may be a hydroperoxo species.
Since intermediate Int2 is not observed at the higher concentrations of the NMR experiments, the steps after the formation of Int2 must be slower than its generation. Kinetics analysis of the conversion of Int2 to 3 was run to determine the reaction order and gain insight into the reaction mechanism.
Proposed Dioxygen Activation Mechanism of 6-H
The reaction mixture is then dried under reduced pressure and returned to the glovebox with an inert atmosphere. The reaction mixture was allowed to stir for 16 h before the volatiles were removed under reduced pressure to give a purple residue. The reaction mixture was allowed to stir for 2 hours, during which time the solution became homogeneously orange.
The reaction mixture was allowed to stir for 3 hours to give a pale pink solution before being dried under reduced pressure. The reaction mixture was allowed to stir for 2 hours during which time the solution became a homogeneous yellow/orange. After stirring for 16 h, the reaction mixture was dried under reduced pressure to give the product as a foamy yellow/orange solid in quantitative yield.
The reaction mixture was then degassed under dynamic vacuum at -78 °C with vigorous stirring for 8 min.
Synthesis of ligand 1
Synthesis of nickel and palladium complexes
This is confirmed by the 1H NMR, where broadened resonances for the central pyridine protons are observed at 7.82 and 4.35 ppm for the orthopyridyl (C3 and C4 protons) and para-pyridyl (C1 proton) signals, respectively . The proton signals are significantly shifted up relative to those of the free ligand at 8.96 and 7.89 ppm. This indicates a significant disruption of the electronic environment of pyridine upon binding of the nickel center, as seen in Ni complexes on cognate ligand platforms.5a, 11 The H-C1 shows the largest shift upward, consistent with the fact that proton is directly bonded to the carbon atom, showing that the strongest interactions are with the nickel center.
Interestingly, H-C1 does not show any upshift and instead H-C3/C4 show a larger upshift from 1. Attempts to selectively functionalize the pyridine nitrogen of 1 with Lewis acids prior to metalation have been challenged by the relative lack of selectivity for binding to the phosphine.
Electrophile functionalization of pyridine nitrogen
This shift is probably due to the electronic effect caused by binding of the Lewis acid. Consequently, strong metal-pyridine π-system interactions lead to an even larger upward shift of the H-C1 compared to 2Ni. This probably arises from the cationic nature of the electrophile bound to the pyridine nitrogen (B(C6F5)3 vs Me+), resulting in a significantly more electron-deficient central pyridine.
The above structural parameters demonstrate the plasticity of pyridine π-system and its potential to rearrange as a function of Lewis acid functionalization of the pyridine nitrogen. Because of the metal center's larger size, the coordination angle of the diphosphine is larger with palladium.
Synthesis of small molecule-coordinated Ni complexes
The chemical shift of C1 (7.55 ppm) and C3/C4 protons (9.01 ppm) appears in the aromatic region, suggesting weaker metal-pyridine π-system interactions than either of the CO complexes. Overall, the effect of a change in the π-acidity of the donor ligand shifts the C1-H chemical shift by approximately half of that resulting from the bonding of B(C6F5)3 to the pyridine nitrogen (H -C1 δ(ppm): 4Ni(CO)-B(C6F5)3 4.61). Similarly, NMR comparisons across the series of 4Ni(L)-Me (L = CN-, N3-, MeCN, CNtBu, CO) and 5Ni allow an assessment of the strength of Ni-pyridine π -system's interaction with a large number of small molecules, while keeping the pyridine substitution constant (Figure 8).
Additionally, asymmetric signals are observed for the central pyridine protons that are consistent with hydroboration or hydrosilylation of a pyridine C-N bond rather than a 1,4-selective reaction or a formal oxidative addition to the nickel center. Complete assignments of the central pyridine carbons were also made (See Experimental Section), which support the assignment of the 1,2 substitution model.
Reactivity at pyridine beyond Lewis acid binding: synthesis of dearomatized pyridine complexes
The filtrate volatiles were removed under reduced pressure to give the product as a dark brown lumpy solid. The solution was then filtered through a pad of Celite and the resulting filtrate volatiles were removed under reduced pressure to give the pure product as an orange solid without further purification. The filtrate volatiles were removed under reduced pressure to give the product as a dark brown solid. circle. 5 mL) and transferred to a 20 mL scintillation vial equipped with a magnetic stirrer.
The filtrate volatiles were then removed under reduced pressure to give a pinkish purple solid. Removal of the volatiles under reduced pressure afforded the desired pure product as a reddish brown powder without further purification.
