4,5-Diazafluorene and 9,9'-Dimethyl-4,5-Diazafluorene as Ligands Supporting Redox-Active Mn and Ru Complexes

Redox properties of [Cp*Rh] complexes supported by mono-substituted 2,2'-bipyridyl ligands

The redox properties of half-sandwich rhodium complexes supported by 2,2'-bipyridyl (bpy) ligands can be readily tuned by selection of an appropriately substituted derivative of bpy, but the influences of single substituents on the properties of such complexes are not well documented, as disubstituted bpy variants are much more common. Here, the synthesis, characterization, and redox properties of two new [Cp*Rh] complexes (where Cp* is η5-1,2,3,4,5-pentamethylcyclopentadienyl) supported by the uncommon mono-substituted ligands 4-chloro-2,2'-bipyridyl (mcbpy) and 4-nitro-2,2'-bipyridyl (mnbpy) are reported. Single-crystal X-ray diffraction studies and related spectroscopic experiments confirm installation of the single substituents (-Cl and -NO2, respectively) on the bipyridyl ligands; the precursor monosubstituted ligands were prepared via a divergent route from unsubstituted bpy. Electrochemical studies reveal that each of the complexes undergoes an initial net two-electron reduction at potentials more positive than that associated with the parent unsubstituted complex of bpy, and that the complex supported by mnbpy can undergo a third, chemically reversible reduction at -1.62 V vs. ferrocenium/ferrocene. This redox behavior is consistent with inductive influences from the substituent groups on the supporting ligands, although the nitro group uniquely enables addition of a third electron. Spectrochemical studies carried out with UV-visible detection confirm the redox stoichiometry accessible to these platforms, highlighting the rich redox chemistry and tunable behavior of [Cp*Rh] complexes supported by bpy-type ligands.

Evidence for Uranium(VI/V) Redox Supported by 2,2'-Bipyridyl-6,6'-dicarboxylate

The 2,2'-bipyridyl-6,6'-dicarboxylate ligand (bdc) has been shown in prior work to effectively capture the uranyl(VI) ion, UO22+, from aqueous solutions. However, the redox properties of the uranyl complex of this ligand have not been addressed despite the relevance of uranium-centered reduction to the nuclear fuel cycle and the presence of a bipyridyl core in bdc, a motif long recognized for its ability to support redox chemistry. Here, the bdc complex of UO22+ (1-UO2) has been synthetically prepared and isolated under nonaqueous conditions for the study of its reductive chemical and electrochemical behavior. Spectrochemical titration data collected using decamethylcobaltocene (Cp*2Co) as the reductant demonstrate that 1e- reduction of 1-UO2 is accessible, and companion near-infrared and infrared spectroscopic data, along with theoretical findings from density functional theory, provide evidence that supports the accessibility of the U(V) oxidation state. Data obtained for control ruthenium complexes of bdc and related polypyridyl dicarboxylate ligands provide a counterpoint to these findings; ligand-centered reduction of bdc in these control compounds occurs at potentials more negative than those measured for reduction of 1-UO2, further supporting the generation of uranium(V) in 1-UO2. Taken together, these results underscore the usefulness of bdc as a ligand for actinyl ions and suggest that it could be useful for further studies of the reductive activation of these unique species.

Efficient emission of Zn(II) and Cd(II) complexes with nopinane-annelated 4,5-diazafluorene and 4,5-diazafluoren-9-one ligands: how slight structural modification alters fluorescence mechanism

Zinc(II) and cadmium(II) chlorido complexes with an N,N-chelating nopinane-annelated 4,5-diazafluoren-9-one ligand (LO) were synthesized. While the zinc(II) complex is mononuclear and adopts a tetrahedral ZnN₂Cl₂ coordination geometry, its cadmium(II) analogue features a 1D polymeric structure due to the bridging coordination of chlorido ligands with Cd²⁺ ions having an octahedral CdN₂Cl₄ coordination geometry. The photophysical properties of the oxygen-containing LO ligand and its zinc(II) and cadmium(II) complexes were studied in solution and in the solid state and matched against the properties of its oxygen-free 4,5-diazafluorene congener L and its complexes of the same metal ions. Comprehensive experimental and theoretical studies revealed the impact of the oxygen atom in the ligand core on the luminescence of the ligands and the complexes. For the oxygen-free L ligand and L-based complexes, the structural differences between the S₀ and S₁ geometries are small, which leads to fluorescence with extraordinarily small Stokes shifts. The emission of these compounds is of locally excited character for L and of mixed locally excited + ligand-to-halide charge transfer character for the L-based complexes. The introduction of the oxygen atom in the ligand core results in a drastic red-shift of the emission band due to short-range charge transfer. The differences between the S₀ and S₁ geometries are much more pronounced for LO and LO-based compounds than those of their oxygen-free analogues, leading to an order of magnitude larger Stokes shifts. On going from solution to the solid state, LO and its complexes exhibit aggregation-induced emission (AIE) behaviour with photoluminescence quantum yields (PLQYs) reaching tens of percent.

Novel Copper(II) Complexes with Dipinodiazafluorene Ligands: Synthesis, Structure, Magnetic and Catalytic Properties

The reactions of CuX₂ (X = Cl, Br) with dipinodiazafluorenes yielded four new complexes [CuX₂L₁]₂ (X = Cl (1), Br (2), L₁ = (1R,3R,8R,10R)-2,2,9,9-Tetramethyl-3,4,7,8,9,10-hexahydro-1H-1,3:8,10-dimethanocyclopenta [1,2-b:5,4-b’]diquinolin-12(2H)-one) and [(CuX₂)₂L₂]n (X = Cl (3), Br (4), L₂ = (1R,3R,8R,10R,1’R,3’R,8’R,10’R)-2,2,2’,2’,9,9,9’,9’-Octamethyl-1,1’,2,2’,3,3’,4,4’,7,7’,8,8’,9,9’,10,10’-hexadecahydro-1,3:1’,3’:8,10:8’,10’-tetramethano-12,12’-bi(cyclopenta [1,2-b:5,4-b’]diquinolinylidene). The complexes were characterized by IR and EPR spectroscopy, HR-ESI-MS and elemental analysis. The crystal structures of compounds 1, 2 and 4 were determined by X-ray diffraction (XRD) analysis. Complexes 1–2 have a monomeric structure, while complex 4 has a polymeric structure due to additional coordinating N,N sites in L₂. All complexes contain a binuclear fragment {Cu₂(μ-X)_2×2} (X = Cl, Br) in their structures. Each copper atom has a distorted square-pyramidal coordination environment formed by two nitrogen atoms and three halogen atoms. The Cu-N_ax distance is elongated compared to Cu-N_eq. The EPR spectra of compounds 1–4 in CH₃CN confirm their paramagnetic nature due to the d⁹ electronic configuration of the copper(II) ion. The magnetic properties of all compounds were studied by the method of static magnetic susceptibility. For complexes 1 and 2, the effective magnetic moments are µ_eff ≈ 1.87 and 1.83 µ_B (per each Cu²⁺ ion), respectively, in the temperature range 50–300 K, which are close to the theoretical spin value (1.73 µ_B). Ferromagnetic exchange interactions between Cu(II) ions inside {Cu₂(μ-X)₂X₂} (X = Cl, Br) dimers (J/k_B ≈ 25 and 31 K for 1 and 2, respectively) or between dimers (θ′ ≈ 0.30 and 0.47 K for 1 and 2, respectively) were found at low temperatures. For compounds 3 and 4, the magnetic susceptibility is well described by the Curie–Weiss law in the temperature range 1.77–300 K with µ_eff ≈ 1.72 and 1.70 µ_B for 3 and 4, respectively, and weak antiferromagnetic interactions (θ ≈ −0.4 K for 3 and −0.65 K for 4). Complexes 1–4 exhibit high catalytic activity in the oxidation of alkanes and alcohols with peroxides. The maximum yield of cyclohexane oxidation products reached 50% (complex 3). Based on the data on the study of regio- and bond-selectivity, it was concluded that hydroxyl radicals play a decisive role in the oxidation reaction. The initial products in reactions with alkanes are alkyl hydroperoxides.

Evidence for Charge Delocalization in Diazafluorene Ligands Supporting Low-Valent [Cp*Rh] Complexes

Ligands based upon the 4,5-diazafluorene core are an important class of emerging ligands in organometallic chemistry, but the structure and electronic properties of these ligands have received less attention than they deserve. Here, we show that 9,9'-dimethyl-4,5-diazafluorene (Me₂ daf) can stabilize low-valent complexes through charge delocalization into its conjugated π-system. Using a new platform of [Cp*Rh] complexes with three accessible formal oxidation states (+III, +II, and +I), we show that the methylation in Me₂ daf is protective, blocking Brønsted acid-base chemistry commonly encountered with other daf-based ligands. Electronic absorption spectroscopy and single-crystal X-ray diffraction analysis of a family of eleven new compounds, including the unusual Cp*Rh(Me₂ daf), reveal features consistent with charge delocalization driven by π-backbonding into the LUMO of Me₂ daf, reminiscent of behavior displayed by the workhorse 2,2'-bipyridyl ligand. Taken together with spectrochemical data demonstrating clean conversion between oxidation states, our findings show that 9,9'-dialkylated daf-type ligands are promising building blocks for applications in reductive chemistry and catalysis.

Synthesis, structural studies, and redox chemistry of bimetallic [Mn(CO)3] and [Re(CO)3] complexes

Manganese ([Mn(CO)3]) and rhenium tricarbonyl ([Re(CO)3]) complexes represent a workhorse family of compounds with applications in a variety of fields. Here, the coordination, structural, and electrochemical properties of a family of mono- and bimetallic [Mn(CO)3] and [Re(CO)3] complexes are explored. In particular, a novel heterobimetallic complex featuring both [Mn(CO)3] and [Re(CO)3] units supported by 2,2'-bipyrimidine (bpm) has been synthesized, structurally characterized, and compared to the analogous monomeric and homobimetallic complexes. To enable a comprehensive structural analysis for the series of complexes, we have carried out new single crystal X-ray diffraction studies of seven compounds: Re(CO)3Cl(bpm), anti-[{Re(CO3)Cl}2(bpm)], Mn(CO)3Br(bpz) (bpz = 2,2'-bipyrazine), Mn(CO)3Br(bpm), syn- and anti-[{Mn(CO3)Br}2(bpm)], and syn-[Mn(CO3)Br(bpm)Re(CO)3Br]. Electrochemical studies reveal that the bimetallic complexes are reduced at much more positive potentials (ΔE≥ 380 mV) compared to their monometallic analogues. This redox behavior is consistent with introduction of the second tricarbonyl unit which inductively withdraws electron density from the bridging, redox-active bpm ligand, resulting in more positive reduction potentials. [Re(CO3)Cl]2(bpm) was reduced with cobaltocene; the electron paramagnetic resonance spectrum of the product exhibits an isotropic signal (near g = 2) characteristic of a ligand-centered bpm radical. Our findings highlight the facile synthesis as well as the structural characteristics and unique electrochemical behavior of this family of complexes.