Three paramagnetic heterobimetallic lantern complexes of the form [PtM(tba) 4 (OH 2 )] (M ¼ Fe, 1; Co, 2; Ni, 3; tba ¼ thiobenzoate) have been prepared in a single-step, bench-top procedure. In all three cases, a lantern structure with Pt-M bonding is observed in solution and in the solid state. Compound 1 is a monomer whereas 3 exists as a dimer in the solid state via a Pt/Pt metallophilic interaction. Compound 2 has been characterized in forms with (2a, purple) and without (2b, yellow) Pt/Pt metallophilic interactions. The dimers 2a (J ¼ À10 cm À1 , based on the spin Hamiltonian Ĥ ¼ À2J (S A $S B )) and 3 (J ¼ À60 cm À1 ) exhibit antiferromagnetic coupling between the two first-row metal ions in the solid state via a Pt/Pt non-covalent metallophilic interaction. The electronic structure ofNi, units have been studied with DFT calculations, confirming the relative spin-state energies observed and the antiferromagnetic exchange pathway through four d z 2 orbitals. The compounds 2a and 3 are the first examples of antiferromagnetic coupling through an unbridged M/M contact.

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A series of Pt-based heterobimetallic lantern complexes of the form [PtM(SAc)4(OH2)] (M = Co, 1; Ni, 2; Zn, 3) were prepared using a facile, single-step procedure. These hydrated species were reacted with 3-nitropyridine (3-NO2py) to prepare three additional lantern complexes, [PtM(SAc)4(3-NO2py)] (M = Co, 4; Ni, 5; Zn, 6), or alternatively dried in vacuo to the dehydrated species [PtM(SAc)4] (M = Co, 7; Ni, 8; Zn, 9). The Co- and Ni-containing species exhibit Pt-M bonding in solution and the solid state. In the structurally characterized compounds 1-6, the lantern units form dimers in the solid state via a short Pt···Pt metallophilic interaction. Antiferromagnetic coupling between 3d metal ions in the solid state through noncovalent metallophilic interactions was observed for all the paramagnetic lantern complexes prepared, with J-coupling values of -12.7 cm(-1) (1), -50.8 cm(-1) (2), -6.0 cm(-1) (4), and -12.6 cm(-1) (5). The Zn complexes 3 and 6 also form solid-state dimers, indicating that the formation of short Pt···Pt interactions in these complexes is not predicated on the presence of a paramagnetic 3d metal ion. These contacts and the resultant antiferromagnetic coupling are also not unique to heterobimetallic lantern complexes with axially coordinated H2O or the previously reported thiobenzoate supporting ligand.

Readily exchangeable water molecules are commonly found in the active sites of oxidoreductases, yet the overwhelming majority of studies on small-molecule mimics of these enzymes entirely ignores the contribution of water to the reactivity. Studies of how these enzymes can continue to function in spite of the presence of highly oxidizing species are likewise limited. The mononuclear Mn complex with the potentially hexadentate ligand N-(2-hydroxy-5-methylbenzyl)-N,N',N'-tris(2-pyridinylmethyl)-1,2-ethanediamine (L) was previously found to act as both a HO-responsive MRI contrast agent and a mimic of superoxide dismutase (SOD). Here, we studied this complex in aqueous solutions at different pH values in order to determine its (i) acid-base equilibria, (ii) coordination equilibria, (iii) substitution lability and operative mechanisms for water exchange, (iv) redox behavior and ability to participate in proton-coupled electron transfer (PCET) reactions, (v) SOD activity and reductive activity toward both oxygen and superoxide, and (vi) mechanism for its transformation into the binuclear Mn complex with L-L and its hydroxylated derivatives. The conclusions drawn from potentiometric titrations, low-temperature mass spectrometry, temperature- and pressure-dependent O NMR spectroscopy, electrochemistry, stopped-flow kinetic analyses, and EPR measurements were supported by the structural characterization and quantum chemical analysis of proposed intermediate species. These comprehensive studies enabled us to determine how transiently bound water molecules impact the rate and mechanism of SOD catalysis. Metal-bound water molecules facilitate the PCET necessary for outer-sphere SOD activity. The absence of the water ligand, conversely, enables the inner-sphere reduction of both superoxide and dioxygen. The L complex maintains its SOD activity in the presence of OH and Mn-oxo species by channeling these oxidants toward the synthesis of a functionally equivalent binuclear Mn species.

More than half a century of intense investigation in coordination compounds has laid a firm foundation for our understanding of the ligand fields in transition-metal complexes. [1] Complexes of the heavier 4d and 5d metals are generally low spin, whereas the spin of 3d metal complexes can be high or low, depending on ligand characteristics. The number and type of donor atoms, ligand substituents, and the presence or absence of chelate rings all influence metal spin states. A combination of data-mining and detailed computational study have quantified recently these empirical observations. [2,3] In spite of such variety, there are still some types of metal complexes that are rarely observed.The stereospinomers [3] of high-spin, square-planar complexes, for example, are extremely rare because the large separation of the d [7] is known whose high-spin examples are subtly dependent on ligand substitution. Until the structure was confirmed [8] as tetrameric with octahedral coordination at the Co center, [Co(acac) 2 ] was postulated to have square-planar geometry based on magnetic and spectroscopic data that differed from tetrahedral complexes.[9] A search of the Cambridge Structural Database (V 5.32) [10] [22] Ni. [20] Spectroscopic [20] and computational [18] work have shown that these fluorinated ligands are medium field ligands, on par with OH À and F À , and stronger than NCO À . The electron-withdrawing power of extensively fluorinated ligands reduces the p-donor character of the O atom, such that bridging is not observed and mononuclear species are readily prepared.More recently, we have begun studies of the chelating perfluoropinacolate ligand, ddfp 2À . [23] Magnetic susceptibility and elemental analysis data were reported for K 2 [M(ddfp) 2 ], (M = Mn, Ni, Cu) for which square-planar geometry was proposed.[24] An octahedral bis-H 2 O adduct, (Me 4 N) 2 [Co-(OH 2 ) 2 (ddfp) 2 ] has been proposed based on elemental analysis data. [25] Despite the relative ease in making the [M(ddfp) 2 ] 2À complexes with first-row transition metals, no examples of M = Co II or Fe II have been published. We now report a highspin, square-planar Co II complex, {K(DME) 2 } 2 [Co(ddfp) 2 ] (1), and the analogous high-spin, square-planar Fe II complex {K(DME) 2 } 2 [Fe(ddfp) 2 ] (2). We also provide a discussion of three other square-planar {MO 4 } species from the recent literature whose composition and spin-state characteristics clarify the ligand requirements for the highly unusual highspin, square-planar combination in late row 3d metals.Compound 1 has been prepared as pale pink crystals as shown in Equation (1), and is stable in an inert atmosphere and in various organic solvents, but yields a brown oil upon prolonged exposure to air. Iron-containing 2, and the Zn II derivative, {K(DME) 2 } 2 [Zn(ddfp) 2 ] (3), were similarly prepared as purple-pink, and colorless crystals, respectively. No