An unexpected deboronation of [tBuB(3-tBuC3H2N2)3]− in a reaction with iron(II) chloride, and the X-ray crystal structure of trans-FeCl2(tBuC3H3N2)4

Synthesis, Characterization and Assessment of the Antioxidant Activity of Cu(II), Zn(II) and Cd(II) Complexes Derived from Scorpionate Ligands

Seeking to enrich the yet less explored field of scorpionate complexes bearing antioxidant properties, we, here, report on the synthesis, characterization and assessment of the antioxidant activity of new complexes derived from three scorpionate ligands. The interaction between the scorpionate ligands thallium(I) hydrotris(5-methyl-indazolyl)borate (TlTp^4Bo,5Me), thallium(I) hydrotris(4,5-dihydro-2H-benzo[g]indazolyl)borate (TlTp^a) and potassium hydrotris(3-tert-butyl- pyrazolyl)borate (KTp^tBu), and metal(II) chlorides, in dichloromethane at room temperature, produced a new family of complexes having the stoichiometric formula [M(Tp^4Bo,5Me)₂] (M = Cu, 1; Zn, 4; Cd, 7), [M(Tp^a)₂] (M = Cu, 2; Zn, 5; Cd, 8), [Cu(Hpz^tBu)₃Cl₂] (3), [Zn(Tp^tBu)Cl] (6) and [Cd(Bp^tBu)(Hpz^tBu)Cl] (9). The obtained metal complexes were characterized by Fourier transform infrared spectroscopy, proton nuclear magnetic resonance and elemental analysis, highlighting the total and partial hydrolysis of the scorpionate ligand Tp^tBu during the synthesis of the Cu(II) complex 3 and the Cd(II) complex 9, respectively. An assessment of the antioxidant activity of the obtained metal complexes was performed through both enzymatic and non-enzymatic assays against 1,1-diphenyl-2-picryl- hydrazyl (DPPH^·), 2,2′-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS^+·), hydroxyl (HO^·), nitric oxide (NO^·), superoxide (O₂⁻) and peroxide (OOH^·) radicals. In particular, the complex [Cu(Tp^a)₂]⋅0.5H₂O (2) exhibited significant antioxidant activity, as good and specific activity against superoxide (O₂^−·), (IC50 values equal to 5.6 ± 0.2 μM) and might be identified as auspicious SOD-mimics (SOD = superoxide dismutase).

Homoleptic Zinc(II) Complexes with First and Second Coordination Shells of 5-tert-Butylpyrazole

Reaction of hydrated Zn[NO3]2 or Zn[BF4]2 with four or more equivalents of 3{5}-tert-butylpyrazole (L(tBu)) yields [Zn(L(tBu))4]X2 (X- = NO3- or BF4-). The nitrate complex contains C2-symmetric four-coordinate zinc(II) centers with a slightly flattened tetrahedral geometry, and each nitrate anion hydrogen bonds to two pyrazole N-H groups. Similar reactions with Zn[ClO4]2 or ZnCl2 in the presence of 2 equiv of AgPF6 or AgSbF6 yield instead [{Zn(L(tBu))4}(L(tBu))4][ClO4]2 and [{Zn(L(tBu))4}(L(tBu))2]Y2 (Y- = PF6- or SbF6-). Crystals of [{Zn(L(tBu))4}(L(tBu))4][ClO4]2 are composed of discrete [{Zn(L(tBu))4}(L(tBu))4]2+ supramolecules that are formed from N-H...N hydrogen bonding between zinc-bound and uncoordinated pyrazole rings. The [{Zn(L(tBu))4}(L(tBu))4]2+ moieties are linked into planar 4(4) nets by hydrogen bonding to bridging ClO4- anions. The ClO4- ions are almost perfectly encapsulated in near-spherical cavities of approximate dimensions 5.0 x 5.0 x 4.5 A that are formed by two interlocked supramolecular dications. Similarly, [{Zn(L(tBu))4}(L(tBu))2][PF6]2 crystallizes as discrete supramolecules in the crystal with the PF6- anions occupying a shallow bowl-shaped cavity on the surface of the complex that is formed by two zinc-bound and one uncoordinated pyrazole ligands. (1)H NMR and IR studies of [{Zn(L(tBu))4}(L(tBu))4][ClO4]2 in CD2Cl2 imply that the second-sphere L(tBu) ligands dissociate from the [Zn(L(tBu))4]2+ center in this solvent and that free and metal-bound L(tBu) are in rapid chemical exchange.

Novel tert-butyl-tris(3-hydrocarbylpyrazol-1-yl)borate ligands: synthesis, spectroscopic studies, and coordination chemistry

The lithium (1) and thallium (2) salts of five new tert-butyl-tris(3-hydrocarbylpyrazol-1-yl)borate ligands [t-BuTp(R)]- (R = H, a; Me, b; i-Pr, c; t-Bu, d; Ph, e) have been synthesized and characterized. Because of steric congestion at B, the reaction between t-BuBH3Li x 0.5 Et2O and excess 2,5-dimethylpyrazole Hpz(Me2) afforded the bis-pz(Me2) derivative, Tl[t-BuBH(3,5-Me2pz)2] (3) after metathesis with TlNO3. The compounds were characterized by elemental analysis and NMR spectroscopy. The Li salts 1a and 1c exhibit fluxional behavior on the NMR time scale in solution at room temperature. The solid-state 7Li and 11B NMR spectra of 1c suggest that this salt exists as a mixture of axial and equatorial isomers. The partial hydrolysis of 1d afforded the dimeric Li complex {Li[t-BuB(pz(t-Bu))2(mu-OH)]}2 (4). The crystal structure of 4 shows two Li cations encapsulated by the heteroscorpionate [t-BuB(OH)(3-t-Bupz)2]- ligands. A salt elimination reaction between FeCl2(THF)1.5 and 2 equiv of Li[t-BuTp(R)] (R = H, Me) followed by an in situ one-electron oxidation produced good yields of the homoleptic, paramagnetic low-spin iron(III) complexes [Fe(t-BuTp)2]PF6 (5) and [Fe(t-BuTp(Me))2]PF6 (6) that were characterized by elemental analyses, magnetic susceptibility measurements in solution and the solid phase, 1H NMR, high-resolution mass spectrometry, Mössbauer spectroscopy, and single-crystal X-ray diffraction. The crystals are composed of discrete molecular units with the central Fe(III) ion in an almost perfectly octahedral coordination to six nitrogen atoms. Compound 5 has the shortest Fe-N bond lengths ever reported for [Fe(RTp(R)')2]+-type compounds.

Fluorinated Tris(pyrazolyl)borate Ligands without the Problematic Hydride Moiety: Isolation of Copper(I) Ethylene and Copper(I)−Tin(II) Complexes Using [MeB(3-(CF3)Pz)3]-

The thallium derivative of a fluorinated, B-methylated, tris(pyrazolyl)borate ligand, [MeB(3-(CF3)Pz)3]-, has been synthesized via a two-step process using the corresponding pyrazole, Li[MeBH3], and thallium(I) acetate. Reaction of [MeB(3-(CF3)Pz)3]Tl with CuBr in the presence of ethylene leads to [MeB(3-(CF3)Pz)3]Cu(C2H4). It is a thermally stable solid. [MeB(3-(CF3)Pz)3]Cu(C2H4) reacts with [(Bn)2ATI]SnCl to yield [MeB(3-(CF3)Pz)3]Cu<--Sn(Cl)[(Bn)2ATI], featuring an unsupported Cu(I)-Sn(II) bond [2.4540(4) A].

Silver(i) carbonyl and silver(i) ethylene complexes of a B-protected fluorinated tris(pyrazolyl)borate ligand

B-methylated ligand [MeB(3-(C2F5)Pz)3]-enables the isolation of a lithium adduct [MeB(3-(C2F5)Pz)3]Li with fac-N3F3 coordination, and rare isolable silver carbon monoxide and silver ethylene complexes, [MeB(3-(C2F5)Pz)3]AgCO and [MeB(3-(C2F5)Pz)3]AgC2H4.