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).

A detailed investigation was conducted on the reaction of a 1,1′-bi-2-naphthol–coumarin-based fluorescent probe with amino acids. On the basis of the studies, including fluorescence spectroscopy, ¹H NMR, UV–vis, mass spectroscopy, single-crystal X-ray analysis, and molecular modeling, it was found that the distinctively different fluorescent responses of the probe toward the amino acid at the two excitation wavelengths are due to two different reaction pathways that generate different intermediates and products.

A second-row transition metal complex {MoTp(NO)(DMAP)} (DMAP = 4-(dimethylamino)pyridine; Tp = tris(pyrazolyl)borate) is shown to form dihapto-coordinate complexes with a range of substituted pyridines bearing both electron-withdrawing and electron-donating substituents. Subsequent reactivity of the pyridine ligand is demonstrated by protonation and nucleophilic addition reactions.

Dihapto-coordinate 1,2-dihydropyridine complexes of the metal fragment {WTp(NO)(PMe₃)} (Tp = tris(pyrazolyl)borate), derived from pyridine, are demonstrated to undergo protonation at C6 followed by regioselective amination at C5 with a variety of primary and secondary amines. The addition takes place stereoselectively anti to the metal center, producing exclusively cis-disubstituted products. The resulting 1,2,5,6-tetrahydropyridines can be successfully liberated by oxidation, providing a route to novel molecules of potential medicinal interest.

Interest in beryllium, the lightest member of group 2 elements, has grown substantially within the synthetic community. Herein, we report the synthesis and crystal structure of a heteroleptic haloberyllium borohydride bis(1-isopropyl-3-methyl-benzimidazol-2-ylidene)methane ‘carbodicarbene’ (CDC) complex [(CDC)BeCl(BH₄)]. Crystallographic data: Triclinic space group P1̅, a = 8.8695(14), b = 12.394(2), c = 16.844(3) Å, α = 102.395(4), β = 96.456(4), γ = 99.164(4)°, wR2 (all data) = 0.2706 for 6720 unique data and 357 refined parameters.