Neglected bidentate sp2 N-donor carrier ligands with triazine nitrogen lone pairs: platinum complexes retromodeling cisplatin guanine nucleobase adducts

Rapid rotation of guanine base derivatives about Pt-N7 bonds results in fluxional behavior of models of the key DNA intrastrand G-G cross-link leading to anticancer activity of Pt(II) drugs (G = deoxyguanosine). This behavior impedes the characterization of LPtG2 models (L = one bidentate or two cis-unidentate carrier ligands; G = guanine derivative not linked by a phosphodiester group). We have examined the formation of LPtG2 adducts with G = 5'- and 3'-GMP and L = sp(2) N-donor bidentate carrier ligands [5,5'-dimethyl-2,2'-bipyridine (5,5'-Me2bipy), 3-(4'-methylpyridin-2'-yl)-5,6-dimethyl-1,2,4-triazine) (MepyMe2t), and bis-3,3'-(5,6-dialkyl-1,2,4-triazine) (R4dt)]. NMR spectroscopy provided conclusive evidence that these LPt(5'-GMP)2 complexes exist as interconverting mixtures of head-to-tail (HT) and head-to-head (HH) conformers. For a given G, the rates of G base rotation about the Pt-N7 bonds of LPtG2 models decrease in the order Me4dt > Et4dt > MepyMe2t > 5,5'-Me2bipy. This order reveals that the pyridyl ring C6 atom + H atom grouping is large enough to impede the rotation, but the equivalently placed triazine ring N atom + N lone pair grouping is sterically less impeding. For the first time, the two possible HH conformers (HHa and HHb) in the case of an unsymmetrical L have been identified in our study of (MepyMe2t)Pt(5'-GMP)2. Although O6-O6 clashes involving the two cis G bases favor the HT over the HH arrangement for most LPtG2-type complexes, the HH conformer of (R4dt)Pt(5'-GMP)2 adducts has a high abundance (approximately 50%). We attribute this high abundance to a reduction in O6-O6 steric clashes permitted by the overall low steric effects of R4dt ligands. Under the reaction conditions used, 3'-GMP forms a higher abundance of the LPt(GMP)2 adduct than does 5'-GMP, a result attributable to more favorable second-sphere communication in the LPt(3'-GMP)2 adduct than in the LPt(5'-GMP)2 adduct.     

Dinuclear nickel complexes with bidentate N,O ligands: synthesis, structure, and catalytic oligomerization of ethylene

The new dicationic dinuclear complexes [Ni(micro-Cl)(2)(N,OH)(2)]Cl(2) (11, N,OH = 2-(4,4-dimethyl-4,5-dihydrooxazol-2-yl)-propan-2-ol; 12, N,OH = 2-pyridin-2-yl-propan-2-ol) were prepared in good yields and evaluated as precatalyts in the oligomerization of ethylene, using MAO or AlEtCl(2) as cocatalyst. These paramagnetic complexes were characterized by single-crystal X-ray diffraction in the solid state and in solution with the help of the Evans method, which revealed agreement between the octahedral coordination spheres found in solution and in the solid state. The N donor atoms of each chelating ligand are in mutual cis position, and the OH donors are mutually trans situated. Selectivities for 1-butene within the C(4) fraction of 61% (11) and 58% (12) were observed in the presence of 200 equiv of MAO, but better turnover frequencies (28 300 (11) and 20 400 (12) mol of C(2)H(4)/(mol of Ni.h)) were obtained when 800 equiv of MAO was used. In the presence of 6 equiv of AlEtCl(2), the activities were considerably increased, up to 174 300 (11) and 97 100 (12) mol of C(2)H(4)/(mol of Ni.h), and the selectivity for C(4) olefins was 70% and 64%, respectively.     

Dinuclear vanadium complexes with rigid phenylpolycarboxylate ligands: synthesis,structure, and catalytic bromination reaction with potential detection of hydrogen peroxide

Vanadium complexes (VO)₂(2,2′-bipy)₂(bta)(H₂O)₂ (1) and (VO)₂(1,10-phen)₂(bta)(H₂O)₂ (2) (H₄bta?=?1,2,4,5-benzenetetracarboxylic acid, 2,2′-bipy?=?2,2-bipyridine and 1,10-phen?=?1,10-phenanthroline) have been synthesized by the reaction of V₂(SO₄)₃, H₄bta, 2,2′-bipy (for 1) and 1,10-phen (for 2) by hydrothermal methods. The complexes were characterized by elemental analysis, IR, UV–vis, thermogravimetric analyses, and single-crystal X-ray diffraction. Structural analyses indicate that 1 and 2 are both VO-bta-N-heterocycle system complexes. The central vanadium is coordinated by N₂O₄ donors to form a distorted octahedral geometry. The complexes exhibit catalytic bromination activity in a single-pot reaction with conversion of phenol red to bromophenol blue in a mixed solution of H₂O-DMF at 30?±?0.5?°C with pH 5.8, indicating that they can be considered as a functional model of vanadium-dependent haloperoxidases. The practical application of H₂O₂ detection has also been studied.     

Ate complexes of Ge(II) and Sn(II) with bidentate ligands [LiE(OCH2CH2NMe2)3]2 (E=Ge, Sn): synthesis and structure

The reaction of equimolar quantities of LiOCH₂CH₂NMe₂ and E¹⁴(OCH₂CH₂NMe₂)₂ (E¹⁴=Ge, Sn) in ether yielded new ate complexes [LiE¹⁴(OCH₂CH₂NMe₂)₃]₂ (E¹⁴=Ge (1), Sn (2)) with bidentate ligands. The compounds 1 and 2 are white crystalline substances which are highly soluble in THF and pyridine and very sensitive to the traces of oxygen and moisture. The structures of these compounds are studied by X-ray diffraction analysis. The ate complexes 1 and 2 are powerful nucleophiles and may be employed as ligands (neutral) in the coordination chemistry of the transition metals. The electronegative O-substituents at the divalent E¹⁴ atoms render them less oxidizable than alkyl- or aryl-substituted derivatives, and the bidentate ligands, owing to intramolecular donor-acceptor interactions, make them more thermodynamically stable compared to monodentate ligands.