Intramolecular Arene C–H to C–P Functionalization Mediated by Nickel(II) and Palladium(II)

A tris­(phosphine) ligand with a triarylbenzene backbone was employed to support mono-nickel­(II) and -palladium­(II) complexes. Two phosphine arms coordinated to the metal center, while the third phosphine was found to form a C–P bond with dearomatization of the central arene. Deprotonation effected the rearomatization of the central ring and metal reduction from M­(II) to M(0). The overall conversion corresponds to a functionalization of an unactivated arene C–H bond to a C–P bond. This transformation represents a rare type of mechanism of C–H functionalization, facilitated by the interactions of the group 10 metal with the arene π system. This conversion is reminiscent of and expands the scope of recently reported intramolecular rearrangements of biaryl phosphine ligands common in group 10 catalysis

Intramolecular Arene C–H to C–P Functionalization Mediated by Nickel(II) and Palladium(II)

A tris­(phosphine) ligand with a triarylbenzene backbone was employed to support mono-nickel­(II) and -palladium­(II) complexes. Two phosphine arms coordinated to the metal center, while the third phosphine was found to form a C–P bond with dearomatization of the central arene. Deprotonation effected the rearomatization of the central ring and metal reduction from M­(II) to M(0). The overall conversion corresponds to a functionalization of an unactivated arene C–H bond to a C–P bond. This transformation represents a rare type of mechanism of C–H functionalization, facilitated by the interactions of the group 10 metal with the arene π system. This conversion is reminiscent of and expands the scope of recently reported intramolecular rearrangements of biaryl phosphine ligands common in group 10 catalysis

Cyclometalated Tantalum Diphenolate Pincer Complexes: Intramolecular C−H/M−CH₃ σ-Bond Metathesis May Be Faster than O−H/M−CH₃ Protonolysis

A diphenol linked at the ortho positions to a benzene ring was metalated with TaCl2(CH3)3. Deuterium labeling of the phenol hydrogens and of the linking 1,3-benzenediyl ring reveals an unexpected mechanism involving protonolysis of a methyl group, followed by C−H/Ta−CH3 σ-bond metathesis, leading to cyclometalation of the linking ring and finally protonation of the cyclometalated group by the pendant phenol

A Terminal Fe^III–Oxo in a Tetranuclear Cluster: Effects of Distal Metal Centers on Structure and Reactivity

Tetranuclear Fe clusters have been synthesized bearing a terminal FeIII–oxo center stabilized by hydrogen-bonding interactions from pendant (tert-butylamino)­pyrazolate ligands. This motif was supported in multiple Fe oxidation states, ranging from [FeII2FeIII2] to [FeIII4]; two oxidation states were structurally characterized by single-crystal X-ray diffraction. The reactivity of the FeIII–oxo center in proton-coupled electron transfer with X–H (X = C, O) bonds of various strengths was studied in conjunction with analysis of thermodynamic square schemes of the cluster oxidation states. These results demonstrate the important role of distal metal centers in modulating the reactivity of a terminal metal–oxo

Thermodynamics of Proton and Electron Transfer in Tetranuclear Clusters with Mn–OH₂/OH Motifs Relevant to H₂O Activation by the Oxygen Evolving Complex in Photosystem II

We report the synthesis of site-differentiated heterometallic clusters with three Fe centers and a single Mn site that binds water and hydroxide in multiple cluster oxidation states. Deprotonation of FeIII/II3MnII–OH2 clusters leads to internal reorganization resulting in formal oxidation at Mn to generate FeIII/II3MnIII–OH. 57Fe Mössbauer spectroscopy reveals that oxidation state changes (three for FeIII/II3Mn–OH2 and four for FeIII/II3Mn–OH clusters) occur exclusively at the Fe centers; the Mn center is formally MnII when water is bound and MnIII when hydroxide is bound. Experimentally determined pKa (17.4) of the [FeIII2FeIIMnII–OH2] cluster and the reduction potentials of the [Fe3Mn–OH2] and [Fe3Mn–OH] clusters were used to analyze the O–H bond dissociation enthalpies (BDEO–H) for multiple cluster oxidation states. BDEO–H increases from 69 to 78 and 85 kcal/mol for the [FeIIIFeII2MnII–OH2], [FeIII2FeIIMnII–OH2], and [FeIII3MnII–OH2] clusters, respectively. Further insight of the proton and electron transfer thermodynamics of the [Fe3Mn–OHx] system was obtained by constructing a potential–pKa diagram; the shift in reduction potentials of the [Fe3Mn–OHx] clusters in the presence of different bases supports the BDEO–H values reported for the [Fe3Mn–OH2] clusters. A lower limit of the pKa for the hydroxide ligand of the [Fe3Mn–OH] clusters was estimated for two oxidation states. These data suggest BDEO–H values for the [FeIII2FeIIMnIII–OH] and [FeIII3MnIII–OH] clusters are greater than 93 and 103 kcal/mol, which hints to the high reactivity expected of the resulting [Fe3MnO] in this and related multinuclear systems

Cyclometalated Tantalum Diphenolate Pincer Complexes: Intramolecular C−H/M−CH₃ σ-Bond Metathesis May Be Faster than O−H/M−CH₃ Protonolysis

A diphenol linked at the ortho positions to a benzene ring was metalated with TaCl2(CH3)3. Deuterium labeling of the phenol hydrogens and of the linking 1,3-benzenediyl ring reveals an unexpected mechanism involving protonolysis of a methyl group, followed by C−H/Ta−CH3 σ-bond metathesis, leading to cyclometalation of the linking ring and finally protonation of the cyclometalated group by the pendant phenol

Modulation of Proton-Coupled Electron Transfer through Molybdenum–Quinonoid Interactions

An expanded series of π-bound molybdenum–quinonoid complexes supported by pendant phosphines has been synthesized. These compounds formally span three protonation–oxidation states of the quinonoid fragment (catechol, semiquinone, quinone) and two different oxidation states of the metal (Mo0, MoII), notably demonstrating a total of two protons and four electrons accessible in the system. Previously, the reduced Mo0–catechol complex 1 and its reaction with dioxygen to yield the two-proton/two-electron oxidized Mo0–quinone compound 4 was explored, while, herein, the expansion of the series to include the two-electron oxidized MoII–catechol complex 2, the one-proton/two-electron oxidized Mo–semiquinone complex 3, and the two-proton/four-electron oxidized MoII–quinone complexes 5 and 6 is reported. Transfer of multiple equivalents of protons and electrons from the Mo0 and MoII catechol complexes, 1 and 2, to H atom acceptor TEMPO suggests the presence of weak O–H bonds. Although thermochemical analyses are hindered by the irreversibility of the electrochemistry of the present compounds, the reactivity observed suggests weaker O–H bonds compared to the free catechol, indicating that proton-coupled electron transfer can be facilitated significantly by the π-bound metal center

Tetranuclear Fe Clusters with a Varied Interstitial Ligand: Effects on the Structure, Redox Properties, and Nitric Oxide Activation

A new series of tetranuclear Fe clusters displaying an interstitial μ₄-F ligand was prepared for a comparison to previously reported μ₄-O analogues. With a single nitric oxide (NO) coordinated as a reporter of small-molecule activation, the μ₄-F clusters were characterized in <i>five</i> redox states, from Fe^II₃{FeNO}⁸ to Fe^III₃{FeNO}⁷, with NO stretching frequencies ranging from 1680 to 1855 cm^–1, respectively. Despite accessing more reduced states with an F^– bridge, a two-electron reduction of the distal Fe centers is necessary for the μ₄-F clusters to activate NO to the same degree as the μ₄-O system; consequently, NO reactivity is observed at more positive potentials with μ₄-O than μ₄-F. Moreover, the μ₄-O ligand better translates redox changes of remote metal centers to diatomic ligand activation. The implication for biological active sites is that the higher-charge bridging ligand is more effective in tuning cluster properties, including the involvement of remote metal centers, for small-molecule activation

Dioxygen Reduction by a Pd(0)–Hydroquinone Diphosphine Complex

A novel p-terphenyl diphosphine ligand was synthesized with a noninnocent hydroquinone moiety as the central arene (1-H). Pseudo-tetrahedral 4-coordinate Ni0 and Pd0–quinone (2 and 3, respectively) complexes proved accessible by metalating 1-H with the corresponding M­(OAc)2 precursors. O2 does not react with the Pd0–quinone species (3) and protonation occurs at the quinone moiety indicating that the coordinated oxidized quinonoid moiety prevents reactivity at the metal. A 2-coordinate Pd0–hydroquinone complex (4-H) was prepared using a one-pot metalation with PdII followed by reduction. The reduced quinonoid moiety in 4-H shows metal-coupled reactivity with small molecules. 4-H was capable of reducing a variety of substrates including dioxygen, nitric oxide, nitrous oxide, 1-azido adamantane, trimethylamine n-oxide, and 1,4-benzoquinone quantitatively producing 3 as the Pd-containing reaction product. Mechanistic investigations of dioxygen reduction revealed that the reaction proceeds through a η2-peroxo intermediate (Int1) at low temperatures followed by subsequent ligand oxidation at higher temperatures in a reaction that consumed half an equivalent of O2 and produced water as a final oxygenic byproduct. Control compounds with methyl protected phenolic moieties (4-Me), displaying a AgI center incapable of O2 binding (7-H) or a cationic Pd–H motif (6-H) allowed for the independent examination of potential reaction pathways. The reaction of 4-Me with dioxygen at low temperature produces a species (8-Me) analogous to Int1 demonstrating that initial dioxygen activation is an inner sphere Pd-based process where the hydroquinone moiety only subsequently participates in the reduction of O2, at higher temperatures, by H+/e– transfers