Jeff W. Kampf

Aryl−CF3 Bond-Forming Reductive Elimination from Palladium(IV)

This communication describes oxidatively induced Ar-CF(3) bond-forming reductive elimination from new Pd(II) complexes of general structure (L approximately L)Pd(II)(Ar)(CF(3)). The electrophilic fluorinating reagent N-fluoro-2,4,6-trimethylpyridinium triflate promotes these reactions in good to excellent yields. The palladium(IV) intermediate ((t)Bu-bpy)Pd(IV)(CF(3))(F)(OTf)(C(6)H(4)F) has been isolated, characterized, and demonstrated to undergo high yielding Ar-CF(3) coupling upon thermolysis. This work provides an attractive conceptual framework for the development of Pd(II/IV)-catalyzed arene trifluoromethylation reactions.

Assembly of Near‐Infrared Luminescent Lanthanide Host(Host–Guest) Complexes With a Metallacrown Sandwich Motif

Optical devices and biomedical imaging probes increasingly utilize the long lifetimes and narrow linewidths of luminescent lanthanide (Ln) ions. Near-infrared (NIR) emitting Ln ions draw particular interest because of the transparency of biological tissue in this spectral range and applications in telecommunications. Ln ions are typically sensitized through ligand absorptions by the antenna effect because the low extinction coefficients of the Laporte-forbidden f–f transitions preclude direct excitation. The major hindrance in realizing efficient Ln ion luminescence in the NIR region is non-radiative quenching by high energy X H (X = C, N, O) vibrations in the ligand. Vibrational quenching has limited luminescence lifetimes to less than 6 ms in protic solvents. While careful ligand design can exclude N H and O H oscillators, C H bonds are difficult to eliminate from organic substrates without relying on synthetically cumbersome deuterated or fluorinated ligands. Herein we present a self-assembly approach to realizing long-lived Ln luminescence in the NIR region by utilizing the unique metallacrown (MC) topology to eliminate high energy X H oscillators from within 6.7 of the lanthanide ion. We report the synthesis, solution stability, and remarkable luminescence properties of a unique host(host–guest) complex in which a Ln[12-MC4]2 3+ sandwich complex is a guest encapsulated by a [24-MC8] host (Ln-1, Figure 1). MCs are inorganic analogues of crown ethers. Much of the interest in MCs has focused on the exceptional solid-state architectures, magnetic properties, and molecular recognition capabilities that arise from their metal-rich topologies. Ln MCs have been prepared that display singlemolecule magnetism and selectively encapsulate anions in monomeric cavitands or dimeric compartments. Chiral Ln[15-MC-5] complexes can serve as building blocks for mesoporous solids, resolved helices, and noncentrosymmetric solids that display second-harmonic generation. To date, Ln MCs have been prepared only with ring metals that contain partially filled d orbitals, which could provide a quenching pathway for luminescence. For this work, the Zn ion was judiciously chosen as the ring metal because its d electronic configuration precludes quenching through a d–d transition. To the best of our knowledge, no Ln MCs with Zn ring metals have been reported. Picoline hydroxamic acid (picHA) was selected as the ligand because it contains no N H or O H oscillators when bound in a Ln MC. The reaction between picHA, sodium hydroxide, zinc(II) triflate, and terbium(III) nitrate in methanol provided the complex formulated as Tb[12-MCZnII,N,picHA-4]2 [24MCZnII, N,picHA-8]·(pyridine)8·(triflate)3 (Tb-1, Figure 1) upon crystallization from the reaction solution with added pyridine. Single crystal X-ray crystallographic analysis shows two concave [12-MCZnII, N, picHA-4] units that sandwich an eightcoordinate Tb central metal. This sandwich complex (Figure 2A, B) is encapsulated in the cavity of a [24-MCZnII, N,picHA8] unit (Figure 2C). The Tb[12-MC-4]2 [24-MC-8] comFigure 1. X-ray crystal structure of Tb-1 shown a) perpendicular to the C4 axis, b) down the C4 axis, and c) highlighting the MC macrocycle. Color scheme: bronze= [12-MC-4], purple= [24-MC-8], green= Tb. Pyridine ligands are displayed as thin purple lines.

Boron Clusters as Highly Stable Magnesium-Battery Electrolytes†

Boron clusters are proposed as a new concept for the design of magnesium-battery electrolytes that are magnesium-battery-compatible, highly stable, and noncorrosive. A novel carborane-based electrolyte incorporating an unprecedented magnesium-centered complex anion is reported and shown to perform well as a magnesium-battery electrolyte. This finding opens a new approach towards the design of electrolytes whose likelihood of meeting the challenging design targets for magnesium-battery electrolytes is very high.

Nonamorphism in Flufenamic Acid and a New Record for a Polymorphic Compound with Solved Structures

The unprecedented polymorphism of the non-steroidal anti-inflammatory drug (NSAID) flufenamic acid (FFA) is described here. Nine polymorphs were accessed through the use of polymer-induced heteronucleation (PIHn) and solid-solid transformation at low temperature. Structural elucidation of six of these forms, in addition to the two previously known forms, makes FFA indisputably octamorphic. Although the structure of at least one other form of FFA remains elusive, the occurrence of most of these polymorphs under one crystallization condition through PIHn illustrates that a fine interplay exists among the kinetic factors that lead to phase selection in this NSAID.

A Boronium Ion with Exceptional Electrophilicity

During our studies on aromatic borylation,[1] we considered the combination of a highly electrophilic R2BNTf2 reagent with a base that would neutralize the HNTf2 byproduct of borylation without deactivating the electrophile. In principle, these requirements might be satisfied by 1,8-bis(dimethylamino)naphthalene (1), a hindered and exceptionally basic aniline that finds numerous applications as a basic catalyst or reagent due to its legendary lack of nucleophilicity.[2, 3] Strong electrophiles interact weakly, if at all, with the amine nitrogens, and very few examples are known where stable bonds to nitrogen can be formed between 1 and electrophilic groups larger than hydrogen.[2, 4-7] Among these exceptional cases, cyclic boronium structures 2 and 3 are relatively stable because the subunits BH2 and BF2 have minimal steric requirements.[5] However, the more hindered BMe2 derivative 4 has not been detected and no analogous BR2 structures are known.[5a, 8] In view of this long history, we were somewhat surprised to find that an adduct is readily formed simply upon mixing 1 with the 9-BBN bistriflimide reagent 5a despite the transannular steric demands of the 9-BBN core and the need to form adjacent quaternary bonds to boron as well as nitrogen.[9, 10] The remarkable structural features and unusual reactivity of this adduct are the subject of the following communication.

A mixed 3d-4f 14-metallacrown-5 complex that displays slow magnetic relaxation through geometric control of magnetoanisotropy.

We describe the synthesis and magnetic properties of a unique mixed 3d-4f 14-metallacrown-5 complex. This is the first metallacrown family to feature μ-O and μ-OH bridges as well as to incorporate a Ln(III) ion into the ring. Alternating-current SQUID magnetometry of the Tb, Dy, and Ho derivatives reveals slow magnetic relaxation, a hallmark property of single-molecule magnets. For the Dy structure (4), an effective energy barrier U(eff) of 16.7 K and a relaxation time of 4.9 × 10(-8) s were calculated. Because of the relatively small total spin, this behavior most likely results from a large magnetoanisotropy, which is controlled through geometric constraints.

A Detailed Study of Acetate-Assisted C–H Activation at Palladium(IV) Centers

This report describes a detailed investigation of acetate-assisted C-H activation at Pd(IV) centers supported by the tris(2-pyridyl)methane (Py3CH) ligand. Mechanistic information about this transformation has been obtained through the following: (i) extensive one- and two-dimensional NMR analysis, (ii) reactivity studies of a series of substituted analogues, and (iii) isotope effect studies. These experiments all suggest that C-H activation at [(Py3CH)Pd(IV)(biphenyl)Cl2](+) occurs via a multistep process involving chloride-to-acetate ligand exchange followed by conformational and configurational isomerization and then C-H cleavage. The data also suggest that C-H cleavage proceeds via an acetate-assisted mechanism with the carboxylate likely serving as an intramolecular base. The viability of acetate-assisted C-H activation at high valent palladium has important implications for the design and optimization of catalytic processes involving this transformation as a key step.

Polymer-Induced Heteronucleation of Tolfenamic Acid: Structural Investigation of a Pentamorph

The nonsteroidal anti-inflammatory drug (NSAID) tolfenamic acid (TA), previously thought to be dimorphic, is demonstrated to have at least five polymorphs. The new forms were uncovered through the emerging screening technique of polymer-induced heteronucleation (PIHn). The presence of conformational changes among forms, whole molecule disorder, space group diversity, and varying number of molecules in the asymmetric unit occurring within a very narrow free energy window (approximately 0.3 kcal/mol) make the solid-state chemistry of this molecule uniquely complex among pharmaceuticals. These aspects make it a particularly suitable benchmark compound for crystal structure prediction methods.

Ga3+/Ln3+ Metallacrowns: A Promising Family of Highly Luminescent Lanthanide Complexes That Covers Visible and Near-Infrared Domains

Luminescent lanthanide(III)-based molecular scaffolds hold great promises for materials science and for biological applications. Their fascinating photophysical properties enable spectral discrimination of emission bands that range from the visible to the near-infrared (NIR) regions. In addition, their strong resistance to photobleaching makes them suitable for long duration or repeated biological experiments using a broad range of sources of excitation including intense and focalized systems such as lasers (e.g., confocal microscopy). A main challenge in the creation of luminescent lanthanide(III) complexes lies in the design of a ligand framework that combines two main features: (i) it must include a chromophoric moiety that possesses a large molar absorptivity and is able to sensitize several different lanthanide(III) ions emitting in the visible and/or in the near-infrared, and (ii) it must protect the Ln(3+) cation by minimizing nonradiative deactivation pathways due to the presence of -OH, -NH and -CH vibrations. Herein, a new family of luminescent Ga(3+)/Ln(3+) metallacrown (MC) complexes is reported. The MCs with the general composition [LnGa4(shi)4(C6H5CO2)4(C5H5N) (CH3OH)] (Ln-1, Ln = Sm(3+)-Yb(3+)) were synthesized in a one pot reaction using salicylhydroxamic acid (H3shi) with Ga(3+) and Ln(3+) nitrates as reagents. The molecular structure of [DyGa4(shi)4(C6H5CO2)4(C5H5N) (CH3OH)] was obtained by X-ray analysis of single crystals and shows that the complex is formed as a [12-MCGa(III)shi-4] core with four benzoate molecules bridging the central Dy(3+) ion to the Ga(3+) ring metals. The powder X-ray diffraction analysis demonstrates that all other isolated complexes are isostructural. The extended analysis of the luminescence properties of these complexes, excited by the electronic states of the chromophoric ligands, showed the presence of characteristic, sharp f-f transitions that can be generated not only in the NIR (Sm, Dy, Ho, Er, Yb) but also in the visible (Sm, Eu, Tb, Dy, Tm). All Ln-1 complexes possess very high quantum yield values with respect to other literature compounds, indicating a good sensitization efficiency of the [12-MCGa(III)shi-4] scaffold. Especially, as of today, the Yb-1 complex exhibits the highest NIR quantum yield reported for a lanthanide(III) complex containing C-H bonds with a value of 5.88(2)% in the solid state. This work is a significant step forward toward versatile, easily prepared luminescent lanthanide(III) complexes suitable for a variety of applications including highly in demand biological imaging, especially in the NIR domain.

Pseudohalide complexation by manganese 12-metallacrowns-4 complexes

Abstract The reaction of manganese chloride, sodium or potassium thiocyanate and salicylhydroxamic acid in dimethylformamide–methanol solution leads to the formation of the 12-membered metallacrowns [Na(dmf)2]2(SCN)2{[12-MCMn(III)N(shi)-4](dmf)4} (1) and [K(dmf)2]2(SCN)2{[12-MCMn(III)N(shi)-4](dmf)4} (2). The crystal structure analyses of 1 and 2 show that pseudohalide ligands are bound to the ring manganese ions through the N atoms, while the alkaline ions, Na+ or K+, are accommodated at the cavity of the metallacrown ring. The alkali cations are bound to four oxygen atoms of the metallacrown ring and a single axial dmf. The binding of the pseudohalides (SCN−, OCN− and N3−) to the manganese ions of the metallacrown ring is very similar to that observed previously for (NaBr)2 and (KBr)2 metallacrowns; however, unlike the previously described halide complexes, the thiocyante does not form a bridge between the ring and central metal ions. Furthermore, the pseudohalide ligands do not form a second bond to an adjacent metallacrown, thus, single metallacrowns are isolated rather than chains or columns. The affinity of thiocyanate for the metallacrown is approximately equal to chloride and significantly greater than bromide.