Metal-organic frameworks (MOFs) are the subject of intense research focus due to their potential applications in gas storage and separation, biomedicine, energy, and catalysis. Recently, low-valent MOFs (LVMOFs) have been explored for their potential use as heterogeneous catalysts, and multitopic phosphine linkers have been shown to be a useful building block for the formation of LVMOFs. However, the synthesis of LVMOFs using phosphine linkers requires conditions that are distinct from those in the majority of the MOF synthetic literature, including the exclusion of air and water and the use of unconventional modulators and solvents, making it somewhat more challenging to access these materials. This work serves as a general tutorial for the synthesis of LVMOFs with phosphine linkers, including information on the following: 1) the judicious choice of the metal precursor, modulator, and solvent; 2) the experimental procedures, air-free techniques, and required equipment; 3) the proper storage and handling of the resultant LVMOFs; and 4) useful characterization methods for these materials. The intention of this report is to lower the barrier to this new subfield of MOF research and facilitate advancements toward novel catalytic materials.

Low-valent metal-organic frameworks (LVMOFs) and related materials have gained interest due to their potential applications in heterogeneous catalysis. However, of the few LVMOFs that have been reported, none have shown catalytic activity. Herein, a low-valent metal-organic material constructed from phosphine linkers and Ir-I nodes is reported. This material is effectively a crystalline, insoluble analogue of Vaska's complex. As such, the material reversibly binds O-2 and catalyzes the reductive formation of enamines from amides.

ABSTRACT: The influence of a series of N,N-chelating guanidinate ligands on the reactivity of in situ generated tantalum complexes for the intermolecular hydroaminoalkylation of amines has been explored. Increased conversion was observed with the sodium salt of 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBDNa) paired with the tantalum precursor Ta(CH2SiMe3)3Cl2 at 110 degrees C. Terminal alkenes underwent hydroaminoalkylation with a variety of secondary amine substrates to give substituted secondary amine products. Mono- and bis(guanidinate) tantalum complexes were prepared, and their structures were investigated. Importantly, the guanidinate ligand could be alkylated in the beta position using Ta(NMe2)5 as a precatalyst to give the linear regioisomer as the major product, and this substituted ligand gave improved hydroaminoalkylation conversion in comparison to the parent ligand.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

The synthesis, structure, and reactivity of vanadium pyridonate complexes are described. Vanadium(III) pyridonate complexes were accessed through protonolysis and reduction of a tetrakis(amido)vanadium(IV) starting material. Bis(pyridonate) vanadium(IV) precursors could be isolated depending on the amount of proteoligand added. The targeted vanadium(III) species tend to form dimers, but monomeric complexes can be achieved in the presence of neutral donors such as amines or pyridine derivatives or through the use of sterically demanding proligands. The reduction process is proposed to involve dimeric intermediates and be mediated by the amine released from protonolysis, thereby forming the corresponding imine as a byproduct. Isolated amine complexes of vanadium(III) are presented. In contrast, bis(amidate)vanadium(IV) complexes were not found to undergo a similar reduction. This work informs on design principles for the synthesis and application of new vanadium pyridonate catalysts for transformations involving dimerization and PCET for changes in oxidation state.

A zirconium-catalyzed hydroaminoalkylation of alkynes to access α,β,γ-substituted allylic amines in an atom-economic fashion is reported. The reaction is compatible with N-(trimethylsilyl)benzylamine and a variety of N-benzylaniline substrates, with the latter giving the allylic amine as the sole organic product. Various internal alkynes with electron-withdrawing and electron-donating substituents were tolerated. Model intermediates of the reaction were synthesized and structurally characterized. Stoichiometric studies on key intermediates revealed that the open coordination sphere at zirconium, imparted by the tethered bis(ureate) ligand, is crucial for the coordination of neutral donors. These complexes may serve as models for the inner-sphere protonolysis reactions required for catalytic turnover.

The reductive coupling of alcohols using vanadium pyridonate catalysts is reported. This attractive approach for C(sp3)-C(sp3) bond formation uses an oxophilic, earth-abundant metal for a catalytic deoxygenation reaction. Several pyridonate complexes of vanadium were synthesized, giving insight into the coordination chemistry of this understudied class of compounds. Isolated intermediates provide experimental mechanistic evidence that complements reported computational mechanistic proposals for the reductive coupling of alcohols. In contrast to previous mononuclear vanadium(V)/vanadium(III)/vanadium(IV) cycles, this pyridonate catalyst system is proposed to proceed by a vanadium(III)/vanadium(IV) cycle involving bimetallic intermediates.

The intermolecular hydroamination of alkenes with alkylamines has been a long-standing challenge in catalysis, partially due to the near-thermoneutral nature of this transformation. Consistent with this understanding, we report the direct observation of reversible C–N bond formation in hydroamination. A bis(ureate) zirconium complex catalyzed the intermolecular hydroamination of 2-vinylpyridine. Reversible C–N bond formation was characterized by variable-temperature NMR spectroscopy, and thermodynamic parameters were determined using van’t Hoff plots. Isolated intermediates support an aza-Michael-addition mechanism. Sensitivity to steric bulk in the C–N bond forming step provided further evidence for the kinetically accessible but limited thermodynamic driving force for this transformation.