Cu(i)-O2 oxidation reactions in a fluorinated all-O-donor ligand environment

Synthesis, Characterization, and the Effect of Lewis Bases on the Nuclearity of Iron Alkoxide Complexes

Inspired by the potential of alkoxides as weak-field ligands and their ability to bridge, we report herein a series of high-spin iron complexes supported by a bis-alkoxide framework PhDbf. A diiron complex [Fe2(PhDbf)2] (1a) is obtained upon metalation of the ligand, whereas addition of substituted pyridines affords five-coordinate mononuclear iron complexes [(R-Py)2Fe(PhDbf)] (2a-4a, R = H, p-tBu, p-CF3). The potential for nuclearity control of the metal complexes via auxiliary ligands is highlighted by the formation of asymmetric diiron species [(p-CF3-Py)Fe2(PhDbf)2] (5a) and [(m-CF3-Py)Fe2(PhDbf)2] (6a) with trifluoromethyl substituted pyridines, while electron-rich pyridines only produced monomeric species. Electronic properties analysis via UV-vis, electron paramagnetic resonance, 57Fe Mössbauer spectroscopy, and time-dependent density functional theory, along with redox capabilities of these complexes are reported to illustrate the effect of nuclearity on reactivity and the potential of these complexes to access higher oxidation states relevant in oxidative chemistry. Species 1a-5a, [(THF)2Fe(PhDbf)][PF6] (7), [PyFe(PhDbf)Cl] (2b), and [Py2Fe(PhDbf)][PF6] (2c) were characterized via SCXRD. Indirect evidence for the formation of dimeric Fe(III) species (1b, 5b, and 6b) is discussed.

Combining Donor Strength and Oxidative Stability in Scorpionates: A Strongly Donating Fluorinated Mesoionic Tris(imidazol-5-ylidene)borate Ligand

Strongly donating scorpionate ligands support the study of high-valent transition metal chemistry; however, their use is frequently limited by oxidative degradation. To address this concern, we report the synthesis of a tris(imidazol-5-ylidene)borate ligand featuring trifluoromethyl groups surrounding its coordination pocket. This ligand represents the first example of a chelating poly(imidazol-5-ylidene) mesoionic carbene ligand, a scaffold that is expected to be extremely donating. The {NiNO}10 complex of this ligand, as well as that of a previously reported strongly donating tris(imidazol-2-ylidene)borate, has been synthesized and characterized. This new ligand's strong donor properties, as measured by the υNO of its {NiNO}10 complex and natural bonding orbital second-order perturbative energy analysis, are at par with those of the well-studied alkyl-substituted tris(imidazol-2-ylidene)borates, which are known to effectively stabilize high-valent intermediates. The good donor properties of this ligand, despite the electron-withdrawing trifluoromethyl substituents, arise from the strongly donating imidazol-5-ylidene mesoionic carbene arms. These donor properties, when combined with the robustness of trifluoromethyl groups toward oxidative decomposition, suggest this ligand scaffold will be a useful platform in the study of oxidizing high-valent transition-metal species.

HAA by the first {Mn(iii)OH} complex with all O-donor ligands

There is considerable interest in MnOHx moieties, particularly in the stepwise changes in those O-H bonds in tandem with Mn oxidation state changes. The reactivity of aquo-derived ligands, {MOHx}, is also heavily influenced by the electronic character of the other ligands. Despite the prevalence of oxygen coordination in biological systems, preparation of mononuclear Mn complexes of this type with all O-donors is rare. Herein, we report several Mn complexes with perfluoropinacolate (pinF)2- including the first example of a crystallographically characterized mononuclear {Mn(iii)OH} with all O-donors, K2[Mn(OH)(pinF)2], 3. Complex 3 is prepared via deprotonation of K[Mn(OH2)(pinF)2], 1, the pKa of which is estimated to be 18.3 ± 0.3. Cyclic voltammetry reveals quasi-reversible redox behavior for both 1 and 3 with an unusually large ΔEp, assigned to the Mn(iii/ii) couple. Using the Bordwell method, the bond dissociation free energy (BDFE) of the O-H bond in {Mn(ii)-OH2} is estimated to be 67-70 kcal mol-1. Complex 3 abstracts H-atoms from 1,2-diphenylhydrazine, 2,4,6-TTBP, and TEMPOH, the latter of which supports a PCET mechanism. Under basic conditions in air, the synthesis of 1 results in K2[Mn(OAc)(pinF)2], 2, proposed to result from the oxidation of Et2O to EtOAc by a reactive Mn species, followed by ester hydrolysis. Complex 3 alone does not react with Et2O, but addition of O2 at low temperature effects the formation of a new chromophore proposed to be a Mn(iv) species. The related complexes K(18C6)[Mn(iii)(pinF)2], 4, and (Me4N)2[Mn(ii)(pinF)2], 5, have also been prepared and their properties discussed in relation to complexes 1-3.

Synthesis, characterization, and alkoxide transfer reactivity of dimeric Tl2(OR)2 complexes

Reaction of LiOC^tBu₂Ph with TlPF₆ forms the dimeric Tl₂(OC^tBu₂Ph)₂ complex, a rare example of a homoleptic thallium alkoxide complex demonstrating formally two-coordinate metal centers. Characterization of Tl₂(OC^tBu₂Ph)₂ by ¹H and ¹³C NMR spectroscopy and X-ray crystallography reveals the presence of two isomers differing by the mutual conformation of the alkoxide ligands, and by the planarity of the central Tl-O-Tl-O plane. Tl₂(OC^tBu₂Ph)₂ serves as a convenient precursor to the formation of old and new [M(OC^tBu₂Ph)_n] complexes (M = Cr, Fe, Cu, Zn), including a rare example of T-shaped Zn(OC^tBu₂Ph)₂(THF) complex, which could not be previously synthesized using more conventional LiOR/HOR precursors. The reaction of [Ru(cymene)Cl₂]₂ with Tl₂(OC^tBu₂Ph)₂ results in the formation of a ruthenium(ii) alkoxide complex. For ruthenium, the initial coordination of the alkoxide triggers C-H activation at the ortho-H of [OC^tBu₂Ph] which results in its bidentate coordination. In addition to Tl₂(OC^tBu₂Ph)₂, related Tl₂(OC^tBu₂(3,5-Me₂C₆H₃))₂ was also synthesized, characterized, and shown to exhibit similar reactivity with iron and ruthenium precursors. Synthetic, structural, and spectroscopic characterizations are presented.

Reversible PCET and Ambient Catalytic Oxidative Alcohol Dehydrogenation by {V=O} Perfluoropinacolate Complexes

A new air-stable catalyst for the oxidative dehydrogenation of benzylic alcohols under ambient conditions has been developed. The synthesis and characterization of this compound and the related monomeric and dimeric V(IV)- and V(V)-pin^F (pin^F = perfluoropinacolate) complexes are reported herein. Monomeric V(IV) complex (Me₄N)₂[V(O)(pin^F)₂] (1) and dimeric (μ-O)₂-bridged V(V) complex (Me₄N)₂[V₂(O)₂(μ-O)₂(pin^F)₂] (3a) are prepared in water under ambient conditions. Monomeric V(V) complex (Me₄N)[V(O)(pin^F)₂] (2) may be generated via chemical oxidation of 1 under an inert atmosphere, but dimerizes to 3a upon exposure to air. Complexes 1 and 2 display a perfectly reversible V^IV/V couple at 20 mV (vs Ag/AgNO₃), whereas a quasi-reversible V^IV/V couple at -865 mV is found for 3a. Stoichiometric reactions of 3a with both fluorenol and TEMPOH result in the formation of (Me₄N)₂[V₂(O)₂(μ-OH)₂(pin^F)₂] (4a), which contains two V(IV) centers that display antiferromagnetic coupling. In order to structurally characterize the dinuclear anion of 4a, {K(18C6)}⁺ countercations were employed, which formed stabilizing K···O interactions between the counterion and each terminal oxo moiety and H-bonding between the oxygen atoms of the crown ether and μ-OH bridges of the dimer, resulting in {K(18C6)}₂[V₂(O)₂(μ-OH)₂(pin^F)₂] (4b). The formal storage of H₂ in 4a is reversible and proton-coupled electron transfer (PCET) from crystals of 4a regenerates 3a upon exposure to air over the course of several days. Furthermore, the reaction of 3a (2%) under ambient conditions with excess fluorenol, cinnamyl alcohol, or benzyl alcohol resulted in the selective formation of fluorenone (82% conversion), cinnamaldehyde (40%), or benzaldehyde (7%), respectively, reproducing oxidative alcohol dehydrogenation (OAD) chemistry known for VO_x surfaces and demonstrating, in air, the thermodynamically challenging selective oxidation of alcohols to aldehydes/ketones.

Formation of monomeric Sn(ii) and Sn(iv) perfluoropinacolate complexes and their characterization by 119Sn Mössbauer and 119Sn NMR spectroscopies

The synthesis and characterization of a series of Sn(ii) and Sn(iv) complexes supported by the highly electron-withdrawing dianionic perfluoropinacolate (pin^F) ligand are reported herein. Three analogs of [Sn^IV(pin^F)₃]^2- with NEt₃H⁺ (1), K⁺ (2), and {K(18C6)}⁺ (3) counter cations and two analogs of [Sn^II(pin^F)₂]^2- with K⁺ (4) and {K(15C5)₂}⁺ (5) counter cations were prepared and characterized by standard analytical methods, single-crystal X-ray diffraction, and ¹¹⁹Sn Mössbauer and NMR spectroscopies. The six-coordinate Sn^IV(pin^F) complexes display ¹¹⁹Sn NMR resonances and ¹¹⁹Sn Mössbauer spectra similar to SnO₂ (cassiterite). In contrast, the four-coordinate Sn^II(pin^F) complexes, featuring a stereochemically-active lone pair, possess low ¹¹⁹Sn NMR chemical shifts and relatively high quadrupolar splitting. Furthermore, the Sn(ii) complexes are unreactive towards both Lewis bases (pyridine, NEt₃) and acids (BX₃, Et₃NH⁺). Calculations confirm that the Sn(ii) lone pair is localized within the 5s orbital and reveal that the Sn 5p_x LUMO is energetically inaccessible, which effectively abates reactivity.