Divalent Lanthanide Metallocene Complexes with a Linear Coordination Geometry and Pronounced 6s-5d Orbital Mixing.

A small but growing number of molecular compounds have been isolated featuring divalent lanthanides with 4fn5dz21 electron configurations. While the majority of these possess trigonal coordination geometries, we previously reported the first examples of linear divalent metallocenes Ln(CpiPr5)2 (Ln = Tb, Dy; CpiPr5 = pentaisopropylcyclopentadienyl). Here, we report the synthesis and characterization of the remainder of the Ln(CpiPr5)2 (1-Ln) series (including Y and excluding Pm). The compounds can be synthesized through salt metathesis of LnI3 and NaCpiPr5 followed by potassium graphite reduction for Ln = Y, La, Ce, Pr, Nd, Gd, Ho, and Er, by in situ reduction during salt metathesis of LnI3 and NaCpiPr5 for Ln = Tm and Lu, or through salt metathesis from LnI2 and NaCpiPr5 for Ln = Sm, Eu, and Yb. Single crystal X-ray diffraction analyses of 1-Ln confirm a linear coordination geometry with pseudo-D5d symmetry for the entire series. Structural and ultraviolet-visible spectroscopy data support a 4fn+1 electron configuration for Ln2+ = Sm, Eu, Tm, and Yb and a 4fn5dz21 configuration for the other lanthanides ([Kr]4dz21 for Y2+). Characterization of 1-Ln (Ln = Y, La) using electron paramagnetic resonance spectroscopy reveals significant s-d orbital mixing in the highest occupied molecular orbital and hyperfine coupling constants that are the largest reported to date for divalent compounds of yttrium and lanthanum. Evaluation of the room temperature magnetic susceptibilities of 1-Ln and comparison with values previously reported for trigonal Ln2+ compounds suggests that the more pronounced 6s-5d mixing may be associated with weaker 4f-5d spin coupling.

Hydrogen-Atom Noninnocence of a Tridentate [SNS] Pincer Ligand.

Double deprotonation of bis(2-mercapto-4-methylphenyl)amine ([SNS]H3) followed by addition to NiCl2(PR3)2 in air-free conditions afforded [SN(H)S]Ni(PR3) (1a, R = Cy; 1b, R = Ph) complexes, characterized as diamagnetic, square-planar nickel(II) complexes. When the same reaction was conducted with 3 equiv of KH, the diamagnetic anions K{[SNS]Ni(PR3)} were obtained (K[2a], R = Cy; K[2b], R = Ph). In the presence of air, the reaction proceeds with a concomitant one-electron oxidation. When R = Cy, a square-planar, S = 1/2 complex, [SNS]Ni(PCy3) (3a), was isolated. When R = Ph, the bimetallic complex {[SNS]Ni(PPh3)}2 ({3b}2) was obtained. This bimetallic species is diamagnetic; however, in solution it dissociates to give S = 1/2 monomers analogous to 3a. Complexes 1-3 represent a hydrogen-atom-transfer series. The bond dissociation free energies (BDFEs) for 1a and 1b were calculated to be 63.9 ± 0.1 and 62.4 ± 0.2 kcal mol-1, respectively, using the corresponding p Ka and E°' values. Consistent with these BDFE values, TEMPO• reacted with 1a and 1b, resulting in the abstraction of a hydrogen atom to afford 3a and 3b, respectively.

A Heterobimetallic W-Ni Complex Containing a Redox-Active W[SNS]2 Metalloligand.

The tungsten complex W[SNS]2 ([SNS]H3 = bis(2-mercapto-4-methylphenyl)amine) was bound to a Ni(dppe) [dppe = 1,2-bis(diphenylphosphino)ethane] fragment to form the new heterobimetallic complex W[SNS]2Ni(dppe). Characterization of the complex by single-crystal X-ray diffraction revealed the presence of a short W-Ni bond, which renders the complex diamagnetic despite formal tungsten(V) and nickel(I) oxidation states. The W[SNS]2 unit acts as a redox-active metalloligand in the bimetallic complex, which displays four one-electron redox processes by cyclic voltammetry. In the presence of the organic acid 4-cyanoanilinium tetrafluoroborate, W[SNS]2Ni(dppe) catalyzes the electrochemical reduction of protons to hydrogen coincident with the first reduction of the complex.

One octarepeate expansion to the human prion protein alters both the Zn2+ and Cu2+ coordination environments within the octarepeate domain.

The influence of a single octarepeat expansion on the Cu(II) and Zn(II) coordination environments within the octarepeat domain of the human prion protein is examined. Using X-ray absorption spectroscopy and diethyl pyrocarbonate labeling studies, we find that at low copper concentrations the "normal" octarepeat domain (four PHGGGWGQ repeats) coordinates Zn(II) in an (N/O)(6) coordination environment with two histidine residues and Cu(II) in a redox-inactive (N/O)(4) coordination environment using one imidazole residue. Expansion of the octarepeat region by one repeat (five PHGGGWGQ repeats) yields a three-histidine (N/O)(6) coordination environment for Zn(II) and a two-histidine (N/O)(4) coordination environment for Cu(II) at low copper concentrations. This Cu(II)[(N/O)(2)-histidine(2)] coordination motif is redox-active and capable of generating H(2)O(2) under reducing aerobic conditions.

[4+4]-Cycloaddition of Isoprene for the Production of High-Performance Bio-Based Jet Fuel.

Isoprene was efficiently converted to 1,6-dimethyl-1,5-cyclooctadiene (DMCOD) by selective [4+4]-cycloaddition with a catalyst formed by in situ reduction of [(MePI)FeCl(µ-Cl)]2 (MePI = [2-(2,6-(CH3)2-C6H3-N=C(CH3))-C4H5N]). DMCOD was isolated in 92% yield, at the preparative scale, with a catalyst loading of 0.025 mol%, and a TON of 3680. Catalytic hydrogenation of DMCOD yielded 1,4-dimethylcyclooctane (DMCO). The cyclic structure and ring strain of DMCO afforded gravimetric and volumetric net heats of combustion 2.4 and 9.2% higher, respectively, than conventional jet fuel. In addition, the presence of methyl branches at two sites resulted in a -20 °C kinematic viscosity of 4.17 mm2 s-1, 48 % lower than the maximum allowed value for conventional jet fuel. The ability to derive isoprene and related alcohols readily from abundant biomass sources, coupled with the highly efficient [Fe]-catalyzed [4+4]-cycloaddition described herein, suggests that this process holds great promise for the economical production of high-performance, bio-based jet fuel blendstocks.

Heterobimetallic complexes of palladium and platinum containing a redox-active W[SNS]2 metalloligand.

Complexes of the general formula W[SNS]2M(dppe) (M = Pd, Pt; [SNS]H3 = bis(2-mercapto-p-tolyl)amine; dppe = 1,2-bis(diphenylphosphino)ethane) were prepared by combining the corresponding (dppe)MCl2 synthon with W[SNS]2 under reducing conditions. X-ray diffraction studies revealed the formation of a heterobimetallic complex supported by a single thiolate bridging ligand and a short metal-metal bond between the tungsten and palladium or platinum. Electrochemical and computational results show that the frontier orbitals lie predominantly on the W[SNS]2 fragment suggesting that it behaves as a redox-active metalloligand in these complexes.