Development of Nonclassical Photoprecursors for Rh2 Nitrenes.

Characterization of reactive intermediates in C-H functionalization is challenging due to the fleeting lifetimes of these species. Synthetic photochemistry provides a strategy to generate post-turnover-limiting-step intermediates in catalysis under cryogenic conditions that enable characterization. We have a long-standing interest in the structure and reactivity of Rh2 nitrene intermediates, which are implicated as transient intermediates in Rh2-catalyzed C-H amination. Previously, we demonstrated that Rh2 complexes bearing organic azide ligands can serve as solid-state and in crystallo photoprecursors in the synthesis of transient Rh2 nitrenoids. Complementary solution-phase experiments have not been available due to the weak binding of most organic azides to Rh2 complexes. Furthermore, the volatility of the N2 that is evolved during in crystallo nitrene synthesis from these precursors has prevented the in crystallo observation of C-H functionalization from lattice-confined nitrenes. Motivated by these challenges, here we describe the synthesis and photochemistry of nonclassical nitrene precursors based on sulfilimine ligands. Sulfilimines bind to Rh2 carboxylate complexes more tightly than the corresponding azides, which has enabled the full solid-state and solution-phase characterization of these new complexes. The higher binding affinity of sulfilimine ligands as compared with organic azides has enabled both solution-phase and solid-state nitrene photochemistry. Cryogenic photochemical studies of Rh2 sulfilimine complexes confined within polystyrene thin films demonstrate that sulfilimine photochemistry can be accomplished at low temperature but that C-H amination is rapid at temperatures compatible with N═S photoactivation. The potential of these structures to serve as platforms for multistep in crystallo cascades is discussed.

Copper Complexes with Diazoolefin Ligands and their Photochemical Conversion into Alkenylidene Complexes.

Homometallic copper complexes with alkenylidene ligands are discussed as intermediates in catalysis but the isolation of such complexes has remained elusive. Herein, we report the structural characterization of copper complexes with bridging and terminal alkenylidene ligands. The compounds were obtained by irradiation of CuI complexes with N-heterocyclic diazoolefin ligands. The complex with a terminal alkenylidene ligand required isolation in a crystalline matrix, and its structural characterization was enabled by in crystallo photolysis at low temperature.

Oxygen-atom transfer photochemistry of a molecular copper bromate complex.

We report the synthesis and oxygen-atom transfer (OAT) photochemistry of [Cu(tpa)BrO3]ClO4. In situ spectroscopy and in crystallo experiments indicate OAT proceeds from a Cu-O fragment generated by sequential Cu-O bond cleavage and OAT from BrOx to [Cu(tpa)]+. These results highlight synthetic opportunities in M-O photochemistry and demonstrate the utility of in crystallo experiments to evaluating photochemical reaction mechanisms.

C-H Activation of Inert Arenes using a Photochemically Activated Guanidinato-Magnesium(I) Compound.

UV irradiation of solutions of a guanidinate coordinated dimagnesium(I) compound, [{(Priso)Mg}2 ] 3 (Priso=[(DipN)2 CNPri 2 ]- , Dip=2,6-diisopropylphenyl), in either benzene, toluene, the three isomers of xylene, or mesitylene, leads to facile activation of an aromatic C-H bond of the solvent in all cases, and formation of aryl/hydride bridged magnesium(II) products, [{(Priso)Mg}2 (µ-H)(µ-Ar)] 4-9. In contrast to similar reactions reported for ß-diketiminate coordinated counterparts of 3, these C-H activations proceed with little regioselectivity, though they are considerably faster. Reaction of 3 with an excess of the pyridine, p-NC5 H4 But (pyBut ), gave [(Priso)Mg(pyBut H)(pyBut )2 ] 10, presumably via reduction of the pyridine to yield a radical intermediate, [(Priso)Mg(pyBut ⋅)(pyBut )2 ] 11, which then abstracts a proton from the reaction solvent or a reactant. DFT calculations suggest two possible pathways to the observed arene C-H activations. One of these involves photochemical cleavage of the Mg-Mg bond of 3, generating magnesium(I) doublet radicals, (Priso)Mg⋅. These then doubly reduce the arene substrate to give "Birch-like" products, which subsequently rearrange via C-H activation of the arene. Circumstantial evidence for the photochemical generation of transient magnesium radical species includes the fact that irradiation of a cyclohexane solution of 3 leads to an intramolecular aliphatic C-H activation process and formation of an alkyl-bridged magnesium(II) species, [{Mg(µ-Priso-H )}2 ] 12. Furthermore, irradiation of a 1 : 1 mixture of 3 and the ß-diketiminato dimagnesium(I) compound, [{(Dip Nacnac)Mg}2 ] (Dip Nacnac=[HC(MeCNDip)2 ]- ), effects a "scrambling" reaction, and the near quantitative formation of an unsymmetrical dimagnesium(I) compound, [(Priso)Mg-Mg(Dip Nacnac)] 13. Finally, the EPR spectrum (77 K) of a glassed solution of UV irradiated 3 is dominated by a broad featureless signal, indicating the presence of a doublet radical species.

Iodine-Iodine Cooperation Enables Metal-Free C-N Bond-Forming Electrocatalysis via Isolable Iodanyl Radicals.

Small molecule redox mediators convey interfacial electron transfer events into bulk solution and can enable diverse substrate activation mechanisms in synthetic electrocatalysis. Here, we report that 1,2-diiodo-4,5-dimethoxybenzene is an efficient electrocatalyst for C-H/E-H coupling that operates at as low as 0.5 mol % catalyst loading. Spectroscopic, crystallographic, and computational results indicate a critical role for a three-electron I-I bonding interaction in stabilizing an iodanyl radical intermediate (i.e., formally I(II) species). As a result, the optimized catalyst operates at more than 100 mV lower potential than the related monoiodide catalyst 4-iodoanisole, which results in improved product yield, higher Faradaic efficiency, and expanded substrate scope. The isolated iodanyl radical is chemically competent in C-N bond formation. These results represent the first examples of substrate functionalization at a well-defined I(II) derivative and bona fide iodanyl radical catalysis and demonstrate one-electron pathways as a mechanistic alternative to canonical two-electron hypervalent iodine mechanisms. The observation establishes I-I redox cooperation as a new design concept for the development of metal-free redox mediators.

Nitrogen Atom Transfer Catalysis by Metallonitrene C-H Insertion: Photocatalytic Amidation of Aldehydes.

C-H amination and amidation by catalytic nitrene transfer are well-established and typically proceed via electrophilic attack of nitrenoid intermediates. In contrast, the insertion of (formal) terminal nitride ligands into C-H bonds is much less developed and catalytic nitrogen atom transfer remains unknown. We here report the synthesis of a formal terminal nitride complex of palladium. Photocrystallographic, magnetic, and computational characterization support the assignment as an authentic metallonitrene (Pd-N) with a diradical nitrogen ligand that is singly bonded to PdII . Despite the subvalent nitrene character, selective C-H insertion with aldehydes follows nucleophilic selectivity. Transamidation of the benzamide product is enabled by reaction with N3 SiMe3 . Based on these results, a photocatalytic protocol for aldehyde C-H trimethylsilylamidation was developed that exhibits inverted, nucleophilic selectivity as compared to typical nitrene transfer catalysis. This first example of catalytic C-H nitrogen atom transfer offers facile access to primary amides after deprotection.

Electronic Structure Analysis and Reactivity of the Bimetallic Bis-Titanocene Vinylcarboxylate Complex, [(Cp2Ti)2(O2C3TMS2)]†.

Multimetallic redox cooperativity features heavily in both bioinorganic and synthetic reactions. Here, the electronic structure of the bimetallic Ti/Ti complex 11, [(Cp2Ti)2(O2C3TMS2)] has been revisited with EPR, confirming a predominantly TiIII/TiIII electronic structure. Reactions of 11 with 2,6-dimethylphenyl isocyanide (CNXyl), TMSCl, MeI, and BnCl were explored, revealing differential redox chemistry of the bimetallic core. In reactions with CNXyl and TMSCl, the metallacyclic TiIII center remained unperturbed, with reactions taking place at the pendent κ2(O,O)-titanocene fragment, while reaction with MeI resulted in remote oxidation of the metallacyclic Ti center, indicative of a cooperative redox process. All structures were studied via X-ray diffraction and EPR spectroscopic analysis, and their electronic structures are discussed in the context of the covalent bond classification (CBC) electron counting method.

Nitrene Photochemistry of Manganese N-Haloamides*.

Manganese complexes supported by macrocyclic tetrapyrrole ligands represent an important platform for nitrene transfer catalysis and have been applied to both C-H amination and olefin aziridination catalysis. The reactivity of the transient high-valent Mn nitrenoids that mediate these processes renders characterization of these species challenging. Here we report the synthesis and nitrene transfer photochemistry of a family of MnIII N-haloamide complexes. The S=2 N-haloamide complexes are characterized by 1 H NMR, UV-vis, IR, high-frequency and -field EPR (HFEPR) spectroscopies, and single-crystal X-ray diffraction. Photolysis of these complexes results in the formal transfer of a nitrene equivalent to both C-H bonds, such as the α-C-H bonds of tetrahydrofuran, and olefinic substrates, such as styrene, to afford aminated and aziridinated products, respectively. Low-temperature spectroscopy and analysis of kinetic isotope effects for C-H amination indicate halogen-dependent photoreactivity: Photolysis of N-chloroamides proceeds via initial cleavage of the Mn-N bond to generate MnII and amidyl radical intermediates; in contrast, photolysis of N-iodoamides proceeds via N-I cleavage to generate a MnIV nitrenoid (i.e., {MnNR}7 species). These results establish N-haloamide ligands as viable precursors in the photosynthesis of metal nitrenes and highlight the power of ligand design to provide access to reactive intermediates in group-transfer catalysis.

In Crystallo Snapshots of Rh2-Catalyzed C-H Amination.

While X-ray crystallography routinely provides structural characterization of kinetically stable pre-catalysts and intermediates, elucidation of the structures of transient reactive intermediates, which are intimately engaged in bond-breaking and -making during catalysis, is generally not possible. Here, we demonstrate in crystallo synthesis of Rh2 nitrenoids that participate in catalytic C-H amination, and we characterize these transient intermediates as triplet adducts of Rh2. Further, we observe the impact of coordinating substrate, which is present in excess during catalysis, on the structure of transient Rh2 nitrenoids. By providing structural characterization of authentic C-H functionalization intermediates, and not kinetically stabilized model complexes, these experiments provide the opportunity to define critical structure-activity relationships.