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Martin Haehnel,
Martin Ruhmann,
Oliver Theilmann, Subhendu Roy,
Torsten Beweries,
Perdita Arndt,
Anke Spannenberg,
Alexander Villinger,
Eluvathingal D Jemmis,
Axel Schulz,
Uwe Rosenthal
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ABSTRACT: The reaction of the low valent metallocene(II) sources Cp'(2)Ti(η(2)-Me(3)SiC(2)SiMe(3)) (Cp' = η(5)-cyclopentadienyl, 1a or η(5)-pentamethylcyclopentadienyl, 1b) with different carbodiimide substrates RN═C═NR' 2-R-R' (R = t-Bu; R' = Et; R = R' = i-Pr; t-Bu; SiMe(3); 2,4,6-Me-C(6)H(2) and 2,6-i-Pr-C(6)H(3)) was investigated to explore the frontiers of ring strained, unusual four-membered heterometallacycles 5-R. The product complexes show dismantlement, isomerization, or C-C coupling of the applied carbodiimide substrates, respectively, to form unusual mono-, di-, and tetranuclear titanium(III) complexes. A detailed theoretical study revealed that the formation of the unusual complexes can be attributed to the biradicaloid nature of the unusual four-membered heterometallacycles 5-R, which presents an intriguing situation of M-C bonding. The combined experimental and theoretical study highlights the delicate interplay of electronic and steric effects in the stabilization of strained four-membered heterometallacycles, accounting for the isolation of the obtained complexes.
Journal of the American Chemical Society 08/2012; 134(38):15979-91. · 9.91 Impact Factor
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ABSTRACT: Connect five: Theoretical studies reveal that a Group 4 metal can significantly stabilize an unusual four-membered metallacycloallene. The role of the substituents R that donate two electrons to the complex is decisive. The systems contain an unprecedented crucial MC bond in the MC(3) ring (see scheme; Cp=cyclopentadienyl, NHC=N-heterocyclic carbene).
Angewandte Chemie International Edition 04/2012; 51(22):5347-50. · 13.45 Impact Factor
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ABSTRACT: In this paper, we compare the electronic structure of the hafnacycloallene complex Cp2HfC4RR′2R′′ (5Hf), which was previously described by Erker et al., with those of the titanium, zirconium, and hafnium complexes Cp2M(η4-RHC4HR) (3M; i.e. metallacyclopent-2,3,4-trienes, metallacyclocumulenes) and Cp2M(η4-R2C4R2) (4M; i.e. 1-metallacyclopent-3-ynes) using density functional theory (BP86/LANL2DZ) calculations. Moreover, the η3-phenylallenyl zirconocene complex 7Zr, which was synthesized by Wojcicki et al., is included for the comparison. These calculations and extended Hückel calculations show that the bonding in complex 5Hf is remarkably similar to that of complexes 4M and 7Zr. An analysis of the structural parameters and bonding reveals that the unique interaction of the internal carbon atoms along with the terminal carbon atoms with the bent-metallocene moiety is the reason for the unusual stability of these metallacycles. The molecular orbital analysis further suggests that complex 5Hf can react with another metal fragment to give the bimetallic complexes 9 and 10. The electronic structures of complexes 3M, 4M, 5Hf, and 7Zr have been comparatively studied to get a general understanding of the bonding in these metallacycles.
04/2011;
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ABSTRACT: The reaction of the low-valent metallocene(II) sources Cp(2)Ti(η(2)-Me(3)SiC(2)SiMe(3)) (7) and Cp(2)Zr(py)(η(2)-Me(3)SiC(2)SiMe(3)) (11, Cp = η(5)-cyclopentadienyl, py = pyridine) with carbodiimides RN═C═NR (R = Cy, i-Pr, p-Tol) leads to the formation of five membered hetero-metallacycloallenes Cp(2)M{Me(3)SiC═C═C[N(SiMe(3))(R)]-N(R)} (9M-R) (M = Ti, R = i-Pr; M = Zr, R = Cy, i-Pr, p-Tol). Elimination of the alkyne (as the hitherto known reactivity of titanocene and zirconocene alkyne complexes would suggest) was not observed. The molecular structures of the obtained complexes were confirmed by X-ray studies. Moreover, the structure and bonding of the complexes 9Zr-Cy and 9Zr-p-Tol was investigated by DFT calculations.
Journal of the American Chemical Society 03/2011; 133(14):5463-73. · 9.91 Impact Factor
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ABSTRACT: Cobalt(II) complexes of terpyridine bases [Co(L)₂](ClO₄)₂ (1-3), where L is 4'-phenyl-2,2':6',2''-terpyridine (ph-tpy in 1), 4'-(9-anthracenyl)-2,2':6',2''-terpyridine (an-tpy in 2) and 4'-(1- pyrenyl)-2,2':6',2''-terpyridine (py-tpy in 3), are prepared and their photo-induced DNA and protein cleavage activity and photocytotoxic property in HeLa cells studied. The 1 : 2 electrolytic and three-electron paramagnetic complexes show a visible band near 550 nm in DMF-Tris-HCl buffer. The complexes 1-3 show emission spectral bands at 355, 421 and 454 nm, respectively, when excited at 287, 368 and 335 nm. The quantum yield values for 1-3 in DMF-H₂O (2 : 1 v/v) are 0.025, 0.060 and 0.28, respectively. The complexes are redox active in DMF-0.1 M TBAP. The Co(III)-Co(II) and Co(II)-Co(I) couples appear as quasi-reversible cyclic voltammetric responses near 0.2 and -0.7 V vs. SCE, respectively. Complexes 2 and 3 are avid binders to calf thymus DNA giving K(b) value of ∼10⁶ M⁻¹. The complexes show chemical nuclease activity. Complexes 2 and 3 exhibit oxidative cleavage of pUC19 DNA in UV-A and visible light. The DNA photocleavage reaction of 3 at 365 nm shows formation of singlet oxygen and hydroxyl radical species, while only hydroxyl radical formation is evidenced in visible light. Complexes 2 and 3 show non-specific photo-induced bovine serum albumin protein cleavage activity at 365 nm. The an-tpy and py-tpy complexes exhibit significant photocytotoxicity in HeLa cervical cancer cells on exposure to visible light giving IC₅₀ values of 24.2 and 7.6 μM, respectively. Live cell imaging study shows accumulation of the complexes in the cytosol of HeLa cancer cells.
Dalton Transactions 02/2011; 40(6):1233-42. · 3.84 Impact Factor
