Hydrogen–deuterium exchange (H/D exchange) is a method commonly used for studying catalytic activation of C–H(D) bonds by transition metal complexes. In this study, a series of additives were studied for H/D exchange of toluene-d₈ with acetic acid (HOAc) using (^RPNP)Rh(X) complexes (R = phosphine substituents including cyclohexyl, isopropyl, and tert-butyl; X = trifluoroacetate or acetate) as the precatalysts. Cu(OAc)₂ and AgOAc additives were found to benefit Rh-mediated C–H(D) activation of toluene with meta–para selectivity by facilitating the conversion to active (^RPNP)Rh species and stabilizing the Rh catalysts from decomposition to inactive Rh(s). In contrast, nonoxidizing Lewis acid additives, such as B(OMe)₃ or NaOAc, were not effective at facilitating Rh-catalyzed toluene C–H activation. The complexes (^RPNP)Rh^III(H)(X)₂ and [(^RPNP)Rh^I(CO)][X] (X = TFA or OAc) were found to be intermediates of the catalytic the H/D exchange.

Herein, we describe the syntheses and structural characterization of bis(carbene)- and tris(carbene)-stabilized organomagnesium cations. The reaction of the N-heterocyclic carbene (NHC) stabilized Grignard reagent (^iPrNHC)₂Mg(Me)(Br) (1) and Na[BAr^F₄] (^iPrNHC = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene, Ar^F = 3,5-bis(trifluoromethyl)phenyl) in chlorobenzene yields exclusively the bis(NHC)-stabilized dication [{(^iPrNHC)₂Mg}₂(μ-Me)₂][(BAr^F₄)₂] (2). If the reaction is performed in ethereal or nonpolar arene solvents, 2 undergoes Schlenk-type rearrangements to tris(NHC)-stabilized cations [(^iPrNHC)₃Mg(Me)][BAr^F₄] (3[BAr^F₄]) and [(^iPrNHC)₃Mg(Br)][BAr^F₄] (4[BAr^F₄]). These monomeric cations 3[A] and 4[A] (A = BAr^F₄, BPh₄) can be independently prepared as single pure products in high yields using common hydrocarbon solvents. The electronic influence of tris(carbene) stabilization is further evidenced by an NHC-mediated ionization of magnesium bromide in the absence of abstraction reagents. The reaction between the sterically unencumbered 1,3,4,5-tetramethylimidazol-2-ylidene (^MeNHC) ligand and (^MeNHC)₂MgBr₂ (7) resulted in two geometrically unique cations of the type [(^MeNHC)₃MgBr][Br]: complex 8a bearing a weakly coordinating bromide anion resulting in a trigonal bipyramidal magnesium center, and complex 8b bearing a noncoordinating bromide anion where the magnesium atom resides in a tetrahedral coordination environment. All isolated complexes were characterized by NMR spectroscopy and single-crystal X-ray diffraction, and their bonding was investigated by density functional theory (DFT).

Charge‐separated metal–organic frameworks (MOFs) are a unique class of MOFs that can possess added properties originating from the exposed ionic species. A new charge‐separated MOF, namely, UNM‐6 synthesized from a tetrahedral borate ligand and Co²⁺ cation is reported herein. UNM‐6 crystalizes into the highly symmetric P43n space group with fourfold interpenetration, despite the stoichiometric imbalance between the B and Co atoms, which also leads to loosely bound NO₃⁻ anions within the crystal structure. These NO₃⁻ ions can be quantitatively exchanged with various other anions, leading to Lewis acid (Co²⁺) and Lewis base (anions) pairs within the pores and potentially cooperative catalytic activities. For example, UNM‐6‐Br, the MOF after anion exchange with Br⁻ anions, displays high catalytic activity and stability in reactions of CO₂ chemical fixation into cyclic carbonates.

The exceptionally π-basic metal fragments {MoTp(NO)(DMAP)} and {WTp(NO)(PMe₃)} (Tp = tris(pyrazolyl)borate; DMAP = 4-(N,N-dimethylamino)pyridine) form thermally stable η²-coordinated complexes with a variety of electron-deficient arenes. The tolerance of substituted arenes with fluorine-containing electron withdrawing groups (EWG; −F, −CF₃, −SF₅) is examined for both the molybdenum and tungsten systems. When the EWG contains a π bond (nitriles, aldehydes, ketones, ester), η² coordination occurs predominantly on the nonaromatic functional group. However, complexation of the tungsten complex with trimethyl orthobenzoate (PhC(OMe)₃) followed by hydrolysis allows access to an η²-coordinated arene with an ester substituent. In general, the tungsten system tolerates sulfur-based withdrawing groups well (e.g., PhSO₂Ph, MeSO₂Ph), and the integration of multiple electron-withdrawing groups on a benzene ring further enhances the π-back-bonding interaction between the metal and aromatic ligand. While the molybdenum system did not form stable η²-arene complexes with the sulfones or ortho esters, it was capable of forming rare examples of stable η²-coordinated arene complexes with a range of fluorinated benzenes (e.g., fluorobenzene, difluorobenzenes). In contrast to what has been observed for the tungsten system, these complexes formed without interference of C–H or C–F insertion.

Oxidative addition and reductive elimination reactions are central steps in many catalytic processes, and controlling the energetics of reaction intermediates is key to enabling efficient catalysis. A series of oxidative addition and reductive elimination reactions using (^RPNP)RhX complexes (R = tert-butyl, isopropyl, mesityl, phenyl; X = Cl, I) was studied to deduce the effect of the size of the phosphine substituents. Using (^RPNP)RhCl as the starting material, oxidative addition of MeI was observed to produce (^RPNP)Rh(Me)(I)Cl, which was followed by reductive elimination of MeCl to form (^RPNP)RhI. The thermodynamics and kinetics vary with the identity of the substituent R on phosphorus of the PNP ligand. The presence of large steric bulk (e.g., R = tert-butyl, mesityl) on the phosphine favors Rh(I) in comparison to the presence of two smaller substituents (e.g., R = isopropyl, phenyl). An Eyring plot for the oxidative addition of MeI to (^tBuPNP)RhCl in THF-d₈ is consistent with a polar two-step reaction pathway, and the formation of [(^tBuPNP)Rh(Me)I]I is also consistent with this mechanism. DFT calculations show that the steric bulk affects the reaction energies of addition reactions which generate six-coordinate complexes by tens of kcal mol^–1. The ligand steric bulk is calculated to have a reduced effect (a few kcal mol^–1) on S_N2 addition barriers, which only require access to one side of the square plane.

The alkaline-earth elements (Be, Mg, Ca, Sr, and Ba) strongly favor the formation of diamagnetic compounds in the +2 oxidation state. Herein we report a paramagnetic beryllium radical cation, [(CAAC)₂Be]^+• (2) [CAAC = cyclic (alkyl)(amino)carbene], prepared by oxidation of a zero-valent beryllium complex with 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO). Compound 2 was characterized by EPR spectroscopy, elemental analysis, X-ray crystallography, and DFT calculations. Notably, the isolation of 2 represents the first s-block charged radical and the first crystalline beryllium radical.

Using a bis(N-heterocyclic carbene) ligand system, we have synthesized magnesium complexes bearing redox-active α-diimines and observed structural rearrangements promoted by dynamic N-heterocyclic carbene (NHC) dissociation. The reduction of a bis(NHC)-stabilized magnesium dihalide (iPrNHC)2MgBr2 (1; iPrNHC = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) with KC8 in the presence of the respective diimine, affords the doubly reduced compounds (iPrNHC)2Mg(MesDABMe) (2), (iPrNHC)2Mg(MesDABH) (3), and (iPrNHC)2Mg(DippDABMe) (4) (MesDABMe = N,N′-bis(2,4,6-trimethylphenyl)-2,3-dimethyl-1,4-diaza-1,3-diene, MesDABH = N,N′-
bis(2,4,6-trimethylphenyl)-1,4-diazabutadiene, DippDABMe = N,N′-bis-(2,6-diisopropylphenyl)-2,3-dimethyl-1,4-diaza-1,3-diene) as mononuclear five-membered magnesacycles. In contrast to the κ2-diamide coordination in 2−4, (iPrNHC)2Mg(DippDABH) (5; DippDABH = N,N′-bis(2,6-diisopropylphenyl)-1,4-diazabutadiene), prepared under similar conditions, crystallizes as the dinuclear 10-membered magnesacycle [(iPrNHC)Mg(μ DippDABH)]2 (6), where the bridging η1:η1-enediamide ligands are involved in cooperative bonding interactions with the NHC ligands. The diradical complex Mg(DippDABH)2 (7) was also obtained from a solution of 5, which supports an equilibrium between 5 and 6. The rearrangement of 6 to 5 results in an Mg(DAB)2− species that is not stabilized by a Lewis base, which can undergo a disproportionation reaction to form the stable Mg(DAB•−)2 diradical (7). The mechanism for the formation of 6 was evaluated, and a comparative mono(NHC) stabilization of the methylated DAB analogue Mg(DippDABMe) afforded the solid-state coordination polymer [(iPrNHC)Mg-(DippDABMe)·KBr]n (8). The observation of a KBr interaction with the magnesacycle highlights the accessibility to a more Lewis acidic magnesium center upon carbene dissociation from bis(NHC)-stabilized species.

N‐Heterocyclic carbene (NHC)‐ and cyclic (alkyl)(amino)carbene (CAAC)‐stabilized borafluorene radicals have been isolated and characterized by elemental analysis, single‐crystal X‐ray diffraction, UV/Vis absorption, cyclic voltammetry (CV), electron paramagnetic resonance (EPR) spectroscopy, and theoretical studies. Both the CAAC–borafluorene radical (2) and the NHC–borafluorene radical (4) have a considerable amount of spin density localized on the boron atoms (0.322 for 2 and 0.369 for 4). In compound 2, the unpaired electron is also partly delocalized over the CAAC ligand ^carbeneC and N atoms. However, the unpaired electron in compound 4 mainly resides throughout the borafluorene π‐system, with significantly less delocalization over the NHC ligand. These results highlight the Lewis base dependent electrostructural tuning of materials‐relevant radicals. Notably, this is the first report of crystalline borafluorene radicals, and these species exhibit remarkable solid‐state and solution stability.

The synthesis and reactivity study of the first isolable boraphosphaketene, cyclic(alkyl)(amino) carbene (CAAC)-borafluorene-P=C=O (2), is described. Photolysis of compound 2 results in the formation of CAAC-stabilized BP-doped phenanthryne (3) through tandem decarbonylation, monoatomic phosphide insertion, and ring-expansion. Notably, while BN-doped phenanthryne was previously discussed as a reactive intermediate which could not be isolated, the heavier BP-doped analogue exhibits remarkable solution and solid-state stability. The reactivity of 2 with stable carbenes was also explored. Addition of CAAC to 2 led to migration of the original CAAC ligand from boron to phosphorus and coordination of the added CAAC to carbon, affording compound 4. Reaction of 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (NHC) with 2 resulted in N-C bond activation to give the unusual spiro-heterocyclic compound (5).