Enhancing the Magnetic Anisotropy of Linear Cr(II) Chain Compounds Using Heavy Metal Substitutions.

Magnetic properties of the series of three linear, trimetallic chain compounds Cr2Cr(dpa)4Cl2, 1, Mo2Cr(dpa)4Cl2, 2, and W2Cr(dpa)4Cl2, 3 (dpa = 2,2'-dipyridylamido), have been studied using variable-temperature dc and ac magnetometry and high-frequency EPR spectroscopy. All three compounds possess an S = 2 electronic ground state arising from the terminal Cr(2+) ion, which exhibits slow magnetic relaxation under an applied magnetic field, as evidenced by ac magnetic susceptibility and magnetization measurements. The slow relaxation stems from the existence of an easy-axis magnetic anisotropy, which is bolstered by the axial symmetry of the compounds and has been quantified through rigorous high-frequency EPR measurements. The magnitude of D in these compounds increases when heavier ions are substituted into the trimetallic chain; thus D = -1.640, -2.187, and -3.617 cm(-1) for Cr2Cr(dpa)4Cl2, Mo2Cr(dpa)4Cl2, and W2Cr(dpa)4Cl2, respectively. Additionally, the D value measured for W2Cr(dpa)4Cl2 is the largest yet reported for a high-spin Cr(2+) system. While earlier studies have demonstrated that ligands containing heavy atoms can enhance magnetic anisotropy, this is the first report of this phenomenon using heavy metal atoms as "ligands".

Heterometallic Second-Row Transition Metal Chain Compounds in Two Charge States: Syntheses, Properties, and Electronic Structures of [Mo-Mo-Ru](6+/7+) Chains.

Reaction of Mo2(dpa)4 (dpa = 2,2'-dipyridylamido) with (1)/2 equiv of [Ru(CO)3Cl2]2 in molten naphthalene at 250 °C provides facile access to the first all-second-row transition metal heterometallic chain compound, MoMoRu(dpa)4Cl2 (1). The one-electron oxidized compound [MoMoRu(dpa)4Cl2](OTf) (2) is synthesized by reaction of 1 with FeCp2(OTf). X-ray crystallography reveals a contraction of the Mo-Ru bond distance from 2.38 Å in 1 to 2.30 Å in 2, and an elongation of the Mo-Mo bond distance from 2.12 Å in 1 to 2.21 Å in 2. The short Mo-Ru bond distances indicate significant electron delocalization along the Mo-Mo-Ru chain, which is quantified by density functional theory (DFT) calculations. Molecular orbital analyses of both compounds based on DFT results reveal full delocalization of the orbitals of σ and π symmetry for both compounds. Additionally, δ orbital delocalization is observed in 2.

Heterometallic multiple bonding: delocalized three-center σ and π bonding in chains of 4d and 5d transition metals.

Heterotrimetallic compounds Mo2Ru(dpa)4Cl2 (1) and W2Ru(dpa)4Cl2 (2) are prepared by reactions of M2(dpa)4 (M = Mo or W; dpa = 2,2'-dipyridylamide) with [Ru(CO)3Cl2]2. Crystallographic studies reveal short Mo-Ru and W-Ru distances, 2.38 Å (1) and 2.39 Å (2), suggestive of delocalized Mo-Mo-Ru and W-W-Ru bonding. In contrast to the σ bonding found in the corresponding iron compounds, density functional theory calculations reveal both a three-center/two-electron σ bond and two three-center/four-electron π bonds in the M-M-Ru compounds.

Oxidative stretching of metal-metal bonds to their limits.

Oxidation of quadruply bonded Cr2(dpa)4, Mo2(dpa)4, MoW(dpa)4, and W2(dpa)4 (dpa = 2,2'-dipyridylamido) with 2 equiv of silver(I) triflate or ferrocenium triflate results in the formation of the two-electron-oxidized products [Cr2(dpa)4](2+) (1), [Mo2(dpa)4](2+) (2), [MoW(dpa)4](2+) (3), and [W2(dpa)4](2+) (4). Additional two-electron oxidation and oxygen atom transfer by m-chloroperoxybenzoic acid results in the formation of the corresponding metal-oxo compounds [Mo2O(dpa)4](2+) (5), [WMoO(dpa)4](2+) (6), and [W2O(dpa)4](2+) (7), which feature an unusual linear M···M≡O structure. Crystallographic studies of the two-electron-oxidized products 2, 3, and 4, which have the appropriate number of orbitals and electrons to form metal-metal triple bonds, show bond distances much longer (by >0.5 Å) than those in established triply bonded compounds, but these compounds are nonetheless diamagnetic. In contrast, the Cr-Cr bond is completely severed in 1, and the resulting two isolated Cr(3+) magnetic centers couple antiferromagnetically with J/kB= -108(3) K [-75(2) cm(-1)], as determined by modeling of the temperature dependence of the magnetic susceptibility. Density functional theory (DFT) and multiconfigurational methods (CASSCF/CASPT2) provide support for "stretched" and weak metal-metal triple bonds in 2, 3, and 4. The metal-metal distances in the metal-oxo compounds 5, 6, and 7 are elongated beyond the single-bond covalent radii of the metal atoms. DFT and CASSCF/CASPT2 calculations suggest that the metal atoms have minimal interaction; the electronic structure of these complexes is used to rationalize their multielectron redox reactivity.

Assessing metal-metal multiple bonds in Cr-Cr, Mo-Mo, and W-W compounds and a hypothetical U-U compound: a quantum chemical study comparing DFT and multireference methods.

To gain insights into the trends in metal-metal multiple bonding among the Group 6 elements, density functional theory has been employed in combination with multiconfigurational methods (CASSCF and CASPT2) to investigate a selection of bimetallic, multiply bonded compounds. For the compound [Ar-MM-Ar] (Ar=2,6-(C(6)H(5))(2)-C(6)H(3), M=Cr, Mo, W) the effect of the Ar ligand on the M(2) core has been compared with the analogous [Ph-MM-Ph] (Ph=phenyl, M=Cr, Mo, W) compounds. A set of [M(2)(dpa)(4)] (dpa=2,2'-dipyridylamide, M=Cr, Mo, W, U) compounds has also been investigated. All of the compounds studied here show important multiconfigurational behavior. For the Mo(2) and W(2) compounds, the σ(2)π(4)δ(2) configuration dominates the ground-state wavefunction, contributing at least 75%. The Cr(2) compounds show a more nuanced electronic structure, with many configurations contributing to the ground state. For the Cr, Mo, and W compounds the electronic absorption spectra have been studied, combining density functional theory and multireference methods to make absorption feature assignments. In all cases, the main features observed in the visible spectra may be assigned as charge-transfer bands. For all compounds investigated the Mayer bond order (MBO) and the effective bond order (EBO) were calculated by density functional theory and CASSCF methods, respectively. The MBO and EBO values share a similar trend toward higher values at shorter normalized metal-metal bond lengths.

Not all density functionals are created equal: the case of the missing electron in the oxidized [W-W≡O](7+) core.

The location of the unpaired electron in the new mixed-valent (W2)(IV,V) trication [W2O(dpa)4](3+) presents a challenge for DFT methods. EPR spectroscopy confirms the unpaired electron to be in the W(V)-oxo unit, in agreement with the predictions of hybrid functionals B3LYP and TPSSh, but contrary to the predictions of non-hybrid functionals.