Wesley A. Hoffert

Antiferromagnetic coupling across a tetrametallic unit through noncovalent interactions

Three paramagnetic heterobimetallic lantern complexes of the form [PtM(tba)4(OH2)] (M = Fe, 1; Co, 2; Ni, 3; tba = 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 = −10 cm−1, based on the spin Hamiltonian Ĥ = −2J(SA·SB)) and 3 (J = −60 cm−1) 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 of C4v [PtM(tba)4], C2 [PtM(tba)4(OH2)], (M = Fe, Co, Ni) and D2 symmetry [PtM(tba)4(OH2)]2 M = Co, Ni, units have been studied with DFT calculations, confirming the relative spin-state energies observed and the antiferromagnetic exchange pathway through four dz2 orbitals. The compounds 2a and 3 are the first examples of antiferromagnetic coupling through an unbridged M⋯M contact.

Crystallographic coincidence of two bridging species in a dinuclear CoIII ethynyl­benzene complex

In the title compound, trans,trans-[μ-(m-phenylene)bis(ethyne-1,2-diyl)]bis[chlorido(1,4,8,11-tetraazacyclotetradecane)cobalt(III)]–trans,trans-[μ-(5-bromo-m-phenylene)bis(ethyne-1,2-diyl)]bis[chlorido(1,4,8,11-tetraazacyclotetradecane)cobalt(III)]–tetraphenylborate–acetone (0.88/0.12/2/4), [Co2(C12H4)Cl2(C10H24N4)2]0.88[Co2(C10H3Br)Cl2(C10H24N4)2]0.12(C24H20B)2·4C3H6O, with the exception of the acetylene and bromine groups, all atomic postitions are the same in the two compounds and are modeled at full occupancy. The CoIII ions are six-coordinate with acetylide and chloride ligands bound to the axial sites and the N atoms from the cyclam rings coordinated at the equatorial positions. N—H⋯O and N—H⋯Cl hydrogen-bonding interactions help to consolidate the crystal packing.

Unusual electronic effects imparted by bridging dinitrogen: an experimental and theoretical investigation.

We describe the preparation, structural and magnetic characterizations, and electronic structure calculations for a redox-related family of dinitrogen-bridged chromium acetylide complexes containing the [RC(2)Cr(μ-N(2))CrC(2)R](n+) (R = Ph-, (i)Pr(3)Si-; n = 0, 1, 2) backbone: [(dmpe)(4)Cr(2)(C(2)Ph)(2)(μ-N(2))] (1), [(dmpe)(4)Cr(2)(C(2)Si(i)Pr(3))(2)(μ-N(2))] (2), [(dmpe)(4)Cr(2)(C(2)Si(i)Pr(3))(2)(μ-N(2))]BAr(F)(4) (3), and [(dmpe)(4)Cr(2)(C(2)Si(i)Pr(3))(2)(μ-N(2))](BAr(F)(4))(2) (4). Compounds 3 and 4 are synthesized via chemical oxidation of 2 with [Cp(2)Co](+) and [Cp*(2)Fe](+), respectively. X-ray structural analyses show that the alteration of the formal Cr oxidation states does not appreciably change the Cr-N-N-Cr skeletal structures. Magnetic data collected for 2 and 4 are consistent with high-spin triplet and quintet ground states, respectively. The mixed-valent complex 3 exhibits temperature dependent magnetic behavior consistent with a quartet ⇌ doublet two-center spin equilibrium. Electronic structure calculations (B3LYP) performed on the full complexes in 2 and 4 suggest that the high-spin states arise from singly occupied orthogonal π* orbitals coupled with a variable occupation of dδ orbitals. Significant N-N and Cr-N π-bonding pins the occupation of the π manifold, leading to variable occupation of the dδ space. In contrast, mixed-valent 3 is not well described by a B3LYP hybrid density functional model. A [9,11] CAS-SORCI study on a simplified model of 3 reproduces the observed Hunds rule violation for the S = 1/2 ground state and places the lowest quartet 1.45 kcal/mol above the doublet ground state.