A new series of (CH 2 ) n -linked N-heterocyclic biscarbene (bis-NHC) complexes of rhodium(I) have been synthesized via in situ generation of the silver carbene complexes from bis(imidazolium) halide salts and silver(I) oxide followed by transmetalation to [Rh(COD)Cl] 2 (COD ) 1,5-cyclooctadiene). With a large t-Bu N-substituent, short linkers favor the chelate and long linkers favor the 2:1 complex, contrary to the previous findings with the small n-Bu substituent. In addition, the use of a noncoordinating anion for the AgNHC favors chelate formation. The chelate complex [Rh(COD)L 2 ]PF 6 (L 2 ) [methylenebis(N-(tert-butyl)imidazol-2-ylidene)]) and its dicarbonyl analogue [Rh(CO) 2 L 2 ]PF 6 were structurally characterized. They showed distortion both of the square plane, an electronic effect, and of the metal-NHC moiety, a steric effect.
Equilibrium dialysis of methionyl aminopeptidase from Escherichia coli (EcMetAP) monitored by atomic absorption spectrometry and magnetic circular dichroism (MCD) shows that the enzyme binds up to 1.1 +/- 0.1 equiv of Co(2+) in the metal concentration range likely to be found in vivo. The dissociation constant, K(d), is estimated to be between 2.5 and 4.0 microM. Analysis of the temperature and magnetization behavior of the two major peaks in the MCD spectrum at 495 and 567 nm suggests that these transitions arise from Co(2+) with different ground states. Ligand field calculations using AOMX are used to assign the 495 nm peak to Co(2+) in the 6-coordinate binding site and the 567 nm peak to Co(2+) in the 5-coordinate site. This is further supported by the fact that the binding affinity of the Co(2+) associated with the 567 nm peak is enhanced when the pH is increased from 7.5 to 9.0, consistent with having an imidazole ligand from a histidine amino acid residue. On the basis of the MCD intensities, it is estimated that, when the 5-coordinate site is fully occupied, 0.1 equiv of cobalt is in the 6-coordinate site. Even when the cobalt concentration is very low, there is a small fraction of binuclear sites in EcMetAP formed through cooperative binding between the 5- and 6-coordinate Co(2+) ions. The magnetization behavior of the 6-coordinate Co(2+) MCD peak is consistent with an isolated pseudo-Kramer doublet ground state, suggesting that the cobalt ions in the binuclear sites are not magnetically coupled.
Primary alcohols can be coupled with secondary benzylic alcohols by an air-stable catalytic system involving terpyridine ruthenium or iridium complexes.
Many transition-metal complexes mediate DNA oxidation in the presence of oxidizing radiation, photosensitizers, or oxidants. The DNA oxidation products depend on the nature of the metal complex and the structure of the DNA. Earlier we reported trans-d,l-1,2-diaminocyclohexanetetrachloroplatinum (trans-Pt(d,l)(1,2-(NH(2))(2)C(6)H(10))Cl(4), [Pt(IV)Cl(4)(dach)]; dach = diaminocyclohexane) oxidizes 2'-deoxyguanosine 5'-monophosphate (5'-dGMP) to 7,8-dihydro-8-oxo-2'-deoxyguanosine 5'-monophosphate (8-oxo-5'-dGMP) stoichiometrically. In this paper we report that [Pt(IV)Cl(4)(dach)] also oxidizes 2'-deoxyguanosine 3'-monophosphate (3'-dGMP) stoichiometrically. The final oxidation product is not 8-oxo-3'-dGMP, but cyclic (5'-O-C8)-3'-dGMP. The reaction was studied by high-performance liquid chromatography, (1)H and (31)P nuclear magnetic resonance, and matrix-assisted laser desorption ionization time-of-flight mass spectrometry. The proposed mechanism involves Pt(IV) binding to N7 of 3'-dGMP followed by nucleophilic attack of a 5'-hydroxyl oxygen to C8 of G and an inner-sphere, 2e(-) transfer to produce cyclic (5'-O-C8)-3'-dGMP and [Pt(II)Cl(2)(dach)]. The same mechanism applies to 5'-d[GTTTT]-3', where the 5'-dG is oxidized to cyclic (5'-O-C8)-dG. The Pt(IV) complex binds to N7 of guanine in cGMP, 9-Mxan, 5'-d[TTGTT]-3', and 5'-d[TTTTG]-3', but no subsequent transfer of electrons occurs in these. The results indicate that a good nucleophilic group at the 5' position is required for the redox reaction between guanosine and the Pt(IV) complex.
The mechanism of the hydroarylation reaction between unactivated olefins (ethylene, propylene, and styrene) and benzene catalyzed by [(R)Ir(m-acac-O,O,C 3 )-(acac-O,O) 2 ] 2 and [R-Ir(acac-O,O) 2 (L)] (R = acetylacetonato, CH 3 , CH 2 CH 3 , Ph, or CH 2 CH 2 Ph, and L = H 2 O or pyridine) Ir(III) complexes was studied by experimental methods. The system is selective for generating the anti-Markovnikov product of linear alkylarenes (61 : 39 for benzene + propylene and 98 : 2 for benzene + styrene). The reaction mechanism was found to follow a rate law with first-order dependence on benzene and catalyst, but a non-linear dependence on olefin. 13 C-labelling studies with CH 3 13 CH 2 -Ir-Py showed that reversible b-hydride elimination is facile, but unproductive, giving exclusively saturated alkylarene products. The migration of the 13 C-label from the a to b-positions was found to be slower than the C-H activation of benzene (and thus formation of ethane and Ph-d 5 -Ir-Py). Kinetic analysis under steady state conditions gave a ratio of the rate constants for CH activation and b-hydride elimination (k CH : k b ) of~0.5. The comparable magnitude of these rates suggests a common rate determining transition state/intermediate, which has been shown previously with B3LYP density functional theory (DFT) calculations. Overall, the mechanism of hydroarylation proceeds through a series of pre-equilibrium dissociative steps involving rupture of the dinuclear species or the loss of L from Ph-Ir-L to the solvento, 16-electron species, Ph-Ir(acac-O,O) 2 -Sol (where Sol refers to coordinated solvent). This species then undergoes trans to cis isomerization of the acetylacetonato ligand to yield the pseudo octahedral species cis-Ph-Ir-Sol, which is followed by olefin insertion (the regioselective and rate determining step), and then activation of the C-H bond of an incoming benzene to generate the product and regenerate the catalyst.
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