Forming a chromium-based interstrand DNA crosslink: Implications for carcinogenicity.

The reduction of the carcinogen chromate has been proposed to lead to three Cr(III)-containing DNA lesions: binary adducts (Cr(III) and DNA), interstrand crosslinks, and ternary adducts (Cr(III) linking DNA to a small molecule or protein). Although the structures of binary adducts have recently been elucidated, the structures of interstrand crosslinks and ternary adducts are not known. Analysis of Cr(III) binding to an oligonucleotide duplex containing a 5'-CG site allows elucidation of the structure of an oxide- or hydroxide-bridged binuclear Cr(III) assembly bridging the two strands of DNA. One Cr(III) is directly coordinated by the N-7 atom of a guanine residue, and the complex straddles the helix to form a hydrogen bond between another guanine residue and a Cr(III)-bound aquo ligand. No involvement of the phosphate backbone was observed. The properties and stability of this Cr-O(H)-Cr-bridged complex differ significantly from those reported for Cr-induced interstrand crosslinks, suggesting that interstrand crosslinks resulting from chromate reduction may be organic in nature.

Synthetic Factors Determining the Curvature and Nuclearity of the Giant Mn70 and Mn84 Clusters with a Torus Structure of ∼4 nm Diameter.

New members of the Mn70 and Mn84 torus-like cluster family have been prepared from a hybrid comproportionation-alcoholysis reaction of [Mn12O12(O2CR)16(H2O)4] in alcohol in the presence of R'CO2H with NnBuMnO4 or MnII salts as initiators. Reactions using MeCO2H in nPrOH or nBuOH gave [Mn70O60(O2CMe)70(OnPr)20(nPrOH)18.5(H2O)21.5] (3) and [Mn70O60(O2CMe)70(OH)3(OnBu)17(nBuOH)7.5(H2O)32.5] (4), respectively, whereas EtCO2H in nPrOH gave [Mn84O72(O2CEt)84(OnPr)24(nPrOH)16(H2O)32] (5). They consist of alternating near-linear [Mn3(µ3-O)4]+ and distorted-cubane [Mn4(µ3-O)2(µ3-OR)2]6+ units bridged by syn,syn-µ-RCO2- and µ3-O2- groups and overall are [Mn14]5 and [Mn14]6 oligomers, the repeating unit containing two Mn3 and two Mn4 units. 3/4/5 possess external diameters (including organic ligands) of 4.0/4.1/4.6 nm, respectively, and crystallize as supramolecular nanotubes but with different packing arrangements. Considering all Mn70/Mn84 tori now available, we conclude that the Mn70 vs Mn84 nuclearity is determined by the relative bulk of the carboxylates vs the alkoxides, their increasing bulk favoring Mn84 and Mn70, respectively, with carboxylates larger than acetate giving Mn84. Alternating current (ac) magnetic susceptibility studies revealed frequency-dependent χ″M signals below ∼2.4 K, indicating 3-5 to be new members of the giant [Mn14]n torus family of giant single-molecule magnets (SMMs), in which Mn84 and Mn70 are the largest homometallic Mn/O clusters and SMMs to date.

Validation of the Green's Function Approximation for the Calculation of Magnetic Exchange Couplings.

In this work, we assess the potential of the Green's function approximation to predict isotropic magnetic exchange couplings and to reproduce the standard broken-symmetry energy difference approach for transition metal complexes. To this end, we have selected a variety of heterodinuclear, homodinuclear, and polynuclear systems containing 3d transition metal centers and computed the couplings using both the Green's function and energy difference methods. The Green's function approach is shown to have mixed results for the cases tested. For dinuclear complexes with large strength couplings (≳50 cm-1), the Green's function method is unable to reliably reproduce the energy difference values. However, for weaker dinuclear couplings, the Green's function approach acceptably reproduces broken-symmetry energy difference couplings. In polynuclear cases, the Green's function approximation worked remarkably well, especially for FeIII complexes. On the other hand, for a NiII polynuclear complex, qualitatively wrong couplings are predicted. Overall, the evaluation of exchange couplings from local rigid magnetization rotations offers a powerful alternative to time-consuming energy differences methods for large polynuclear transition metal complexes, but to achieve a quantitative agreement, some improvements to the method are needed.

Magnetic Properties of High-Nuclearity Fex-oxo (x = 7, 22, 24) Clusters Analyzed by a Multipronged Experimental, Computational, and Magnetostructural Correlation Approach.

The synthesis, structure, and magnetic properties of three related iron(III)-oxo clusters are reported, [Fe7O3(O2CPh)9(mda)3(H2O)] (1), [Fe22O14(OH)3(O2CMe)21(mda)6](ClO4)2 (2), and [Fe24O15(OH)4(OEt)(O2CMe)21(mda)7](ClO4)2 (3), where mdaH2 is N-methyldiethanolamine. 1 was prepared from the reaction of [Fe3O(O2CPh)6(H2O)3](NO3) with mdaH2 in a 1:2 ratio in MeCN, whereas 2 and 3 were prepared from the reaction of FeCl3/NaO2CMe/mdaH2 in a 2:∼13:2 ratio and FeCl3/NaO2CMe/mdaH2/pyridine in a 2:∼13:2:25 ratio, respectively, both in EtOH. The core of 1 consists of a central octahedral FeIII ion held within a nonplanar Fe6 loop by three µ3-O2- and three µ2-RO- arms from the three mda2- chelates. The cores of the cations of 2 and 3 consist of an A:B:A three-layer topology, in which a central Fe6 (2) or Fe8 (3) layer B is sandwiched between two Fe8 layers A. The A layers structurally resemble 1 with the additional Fe added at the center to retain virtual C3 symmetry. The central Fe6 layer B of 2 consists of a {Fe4(µ4-O)2(µ3-OH)2}6+ cubane with an Fe on either side attached to cubane O2- ions, whereas that of 3 has the same cubane but with an {Fe3(µ3-O)(µ-OH)} unit attached on one side and a single Fe on the other. Variable-temperature dc and ac magnetic susceptibility studies revealed dominant antiferromagnetic coupling in all complexes leading to ground-state spins of S = 5/2 for 1 and S = 0 for 2 and 3. All Fe2 pairwise exchange parameters (Jij) for 1-3 were estimated by two independent methods: density functional theory (DFT) calculations using broken symmetry methods and a magnetostructural correlation previously developed for high-nuclearity FeIII/O complexes. The two approaches gave satisfyingly similar Jij values, and the latter allowed rationalization of the experimental ground states by identification of the spin frustration effects operative and the resultant relative spin vector alignments at each FeIII ion.

Using Hyperoptimized Tensor Networks and First-Principles Electronic Structure to Simulate the Experimental Properties of the Giant {Mn84} Torus.

The single-molecule magnet {Mn84} is a challenge to theory because of its high nuclearity. We directly compute two experimentally accessible observables, the field-dependent magnetization up to 75 T and the temperature-dependent heat capacity, using parameter-free theory. In particular, we use first-principles calculations to derive short- and long-range exchange interactions and compute the exact partition function of the resulting classical Potts and Ising spin models for all 84 Mn S = 2 spins to obtain observables. The latter computation is made possible by using hyperoptimized tensor network contractions, a technique developed to simulate quantum supremacy circuits. We also synthesize the magnet and measure its heat capacity and magnetization, observing qualitative agreement between theory and experiment and identifying an unusual bump in the heat capacity and a plateau in the magnetization. Our work also identifies some limitations of current theoretical modeling in large magnets, such as sensitivity to small, long-range exchange couplings.