Coordination Chemistry with Sterically Hindered Hydrotris(pyrazolyl)borate Ligands: Organometallic and Bioinorganic Perspectives

Crystal structure of chlorido{hydridotris[3-phenyl-5-methylpyrazol-1-yl-κN 3]borato}copper(II), C30H28BClCuN6

C30H28BClCuN6, triclinic, P 1 ‾ $P‾{1}$ (no. 2), a = 11.549(3) Å, b = 12.321(3) Å, c = 12.348(4) Å, α = 111.785(1)°, β = 116.664(3)°, γ = 97.513(1)°, V = 1361.3(7) Å3, Z = 2, R gt (F) = 0.0440, wR ref (F 2) = 0.1063, T = 197 K.

Comparison of thallium(I) complexes with mesityl-substituted tris(pyrazolyl)hydroborate ligands, [Tl{HB(3-Ms-5-Mepz)3}] and [Tl{HB(3-Ms-5-Mepz)2(3-Me-5-Mspz)}]

Tris(pyrazolyl)borate (scorpionate) ligands can be considered as the most prolific ligands in contemporary coordination chemistry due to the availability of various steric and electronic substituents at the pyrazolyl rings that allow fine-tuning of the open-coordination site for metal centres. The thallium(I) complexes of anionic tridentate-chelating scorpionate ligands, namely [tris(3-mesityl-5-methyl-1H-pyrazol-1-yl-κN2)hydroborato]thallium(I) monohydrate, [Tl(C39H46BN6)]·H2O, (I), and [bis(3-mesityl-5-methyl-1H-pyrazol-1-yl-κN2)(5-mesityl-3-methyl-1H-pyrazol-1-yl-κN2)hydroborato]thallium(I), [Tl(C39H46BN6)], (II), show a {TlIN3} coordination, with average TlI—N bond lengths of 2.53 and 2.55 Å in (I) and (II), respectively. The overall TlIcoordination geometry is distorted trigonal pyramidal, with the average N—TlI—N angle being approximately 73° for both. The dihedral angle between the planes of the pyrazolyl and benzene rings of the mesityl group is 82° in (I), while the corresponding angles in (II) are in the range 64–104°. The structural differences between the two ligands are expected to contribute to the different reactivities of the transition metal coordination complexes towards activation of small molecules such as dioxygen and ethylene.

Molecular structures of tris(2-mercapto-1-tert-butylimidazolyl)hydroborato and tris(2-mercapto-1-adamantylimidazolyl)hydroborato sodium complexes: analysis of [Tm(R)] ligand coordination modes and conformations

The tris(mercaptoimidazolyl)hydroborato complexes, [κ(3)-S2H-Tm(Bu(t))]Na(THF)3 and [κ(3)-S2H-Tm(Ad)]Na(THF)3, which feature t-butyl and adamantyl substituents, have been synthesized via the reactions of the respective 1-R-1,3-dihydro-2H-imidazole-2-thiones with NaBH4 in THF (R = Bu(t), 1-Ad). X-ray diffraction studies indicate that the compounds are monomeric and that the [Tm(R)] ligands coordinate to the metal in a κ(3)-S2H manner via two of the sulfur donors and the hydrogen attached to boron, a combination that is unprecedented for sodium derivatives. Analysis of the tris(mercaptoimidazolyl)hydroborato compounds that are listed in the Cambridge Structural Database has allowed for the formulation of a set of criteria that enables κ(x)-S(x) and κ(x+1)-S(x)H coordination modes to be identified. Furthermore, the various κ(x)-S(x) and κ(x+1)-S(x)H coordination modes have also been analyzed with respect to the conformations of the [Tm(R)] ligands, which differ by rotation of the imidazolethione moieties about the B-N bond.

Gallium hydride and monovalent indium compounds that feature tris(pyrazolyl)hydroborate ligands

The tris(pyrazolyl)hydroborate compounds [tris(3,5-dimethyl-1H-pyrazol-1-yl-κN2)hydroborato]indium(I), [In(C15H22BN6)], abbreviated as [TpMe2]In, and [tris(3-tert-butyl-5-methyl-1H-pyrazol-1-yl-κN2)hydroborato]indium(I), [In(C24H40BN6)], abbreviated as [TpBut,Me]In, represent well defined examples of three-coordinate monovalent indium. In both compounds, the geometry at indium is pyramidal and natural bond orbital (NBO) calculations indicate that the indium lone pair occupies an orbital that is primarily 5sin character. The trivalent gallium hydride compound hydrido[tris(3-tert-butyl-5-methyl-1H-pyrazol-1-yl-κN2)hydroborato]gallium(III) tetrachloridogallium(III), [Ga(C24H40BN6)H][GaCl4], abbreviated as {[TpBut,Me]GaH}[GaCl4], is obtainedviareaction of [TpBut,Me]Tl with [HGaCl2]2, and the Ga—H bond length of 1.49 (6) Å compares favorably with the mean value of 1.50 Å for structurally characterized gallium hydride compounds that are listed in the Cambridge Structural Database.

Synthesis, characterization, thermal and antioxidant studies of potassium dihydrobisphenothiazinyl borate and its transition metal complexes

The bidentate borate anion H(2)B(ptz)(2)(-) and its transition metal complexes have been synthesized and characterized by elemental analyses, magnetic susceptibility, electronic, IR, (1)H and (13)C NMR data. The molar conductance of 10(-3) M solution for all the complexes supports their non-ionic nature. The TGA profile of borate anion shows a single stage unlike that of two stage decomposition plot of the metal complexes. On the basis of spectroscopic studies the geometry of all the complexes have been proposed to be distorted-tetrahedral. The in vitro antioxidant and lipid oxidation inhibition of the ligand and its complexes have also been studied. The Cu[B(ptz)(2)](2) complex was found to be most effective in all the studies.

Stabilization of Unusual Substrate Coordination Modes in Dinuclear Macrocyclic Complexes

The steric protection offered by the macrobinucleating hexaazaditiophenolateligand (L) allows for the preparation of the first stable dinuclear nickel(II) borohydride bridged complex, which reacts rapidly with elemental sulphur producing a tetranuclear nickel(II) complex [{(L)Ni2}2(μ-S6)]2+ bearing a helical μ4-hexa- sulfide ligand. The [(L)CoII 2]2+ fragment have been able to trap a monomethyl orthomolybdate in the binding pocket. Unusual coordination modes of substrate in dinuclear macrocyclic compounds was demonstrated.