Synthesis of α-amino acidato derivatives of niobium and tantalum pentahalides and their conversion into iminium salts

Observing Similarities and Differences in the Properties of Isostructural Niobium(V)/Tantalum(V) Coordination Compounds with Strong Pi-Donor Ligands

While niobium and tantalum are found together in their mineral ores, their respective applications in technology require chemical separation. Nb/Ta separations are challenging due to the similar reactivities displayed by these metals in the solution phase. Coordination complexes of these metals have been studied in the contexts of catalysis, small-molecule activation, and functional group insertion reactivity; relatively few studies exist directly comparing the properties of isostructural Nb/Ta complexes. Such comparisons advance the development of Nb/Ta separation chemistry through the potential for differential reactivity. Here, we explore fundamental physicochemical properties in extensively characterized Nb/Ta coordination complexes [Na(DME)3][MClamp], (Clamp6- = tris-(2-(3',5'-di-tert-butyl-2'-oxyphenyl)amidophenyl)amine; M = Nb, Ta) to advance the understanding of the different electronic, optical, and excited-state properties that these metals exhibit in pi-loaded coordination complexes.

Decarbonylation of phenylacetic acids by high valent transition metal halides

The unusual decarbonylation of α-phenyl carboxylic acids with suitable substituents is a general reaction promoted at room temperature by homoleptic halides of high valent transition metals.

Ubiquity of cis-Halide → Isocyanide Direct Interligand Interaction in Organometallic Complexes

We recently reported a density functional theory (DFT) analysis of the Nb(V)-C bond in various NbCl₅(L) complexes, discovering that the carbon ligand L receives electronic density from the metal (classical back-donation) and from the chlorides in the cis position (direct interligand interaction). Here we report the synthesis and the structural characterization of two new coordination compounds of niobium pentahalides, i.e., NbX₅(CNXyl) (X = Cl, Br; Xyl = 2,6-C₆H₃Me₂), and the corresponding DFT analyses of the Nb(V)-C bond using the Natural Orbitals for Chemical Valence-Charge Displacement (NOCV-CD) approach, confirming the presence of a cis-halide → isocyanide direct interligand interaction. To verify whether the latter is limited to Nb complexes or not, we performed a NOCV-CD analysis on a series of several organometallic complexes based on Ti(IV), Nb(V), Ta(V), Rh(III), Pd(II), and Au(III), all of which bear one halide ligand and m-xylyl-isocyanide in a mutual cis position, revealing that the cis-halide → isocyanide interaction is always present.

Stable coordination complexes of α-diimines with Nb(v) and Ta(v) halides

Uncommon coordination compounds of high valent transition metal halides with N-aryl α-diimines have been obtained, without the need for reducing agents to quench the activation power of the metal centre.

Activation of C-H Bonds of Alkyl- and Arylnitriles by the TaCl5-PPh3 Lewis Pair

A new pathway of activation of C-H bonds of alkyl- and arylnitriles by a cooperative action of TaCl₅ and PPh₃ under mild conditions is reported. Coordination of nitriles to the highly Lewis acidic Ta(V) center resulted in an activation of their aliphatic and aromatic C-H bonds, allowing nucleophilic attack and deprotonation by the relatively weak base PPh₃. The propensity of Ta(V) to form multiple bonds to nitrogen-containing ligands is an important driving force of the reaction as it led to a sequence of bond rearrangements and the emergence of, in the case of benzonitrile, a zwitterionic enediimido complex of Ta(V) through C═C double bond formation between two activated nitrile fragments. These transformations highlight the special role of the high-valent transition metal halide in substrate activation and distinguish the reactivity of the TaCl₅-PPh₃ system from both non-metal- and late transition metal-based frustrated Lewis pairs.

The reactions of α-amino acids and α-amino acid esters with high valent transition metal halides: synthesis of coordination complexes, activation processes and stabilization of α-ammonium acylchloride cations

We describe the synthesis of rare coordination compounds of early transition metals with α-amino acids and α-amino acid esters, the unusual C–C dimerization ofl-proline, and the stabilization of reactive α-ammonium acylchloride cations.

Synthesis and structural characterization of mixed halide–N,N-diethylcarbamates of group 4 metals, including a case of unusual tetrahydrofuran activation

A straightforward synthesis of dichloride-dicarbamates of group 4 metals consists of the metathesis reaction of the respective tetrahalides with tetracarbamates.

Coordination complexes of niobium and tantalum pentahalides with a bulky NHC ligand

The reactivity of niobium and tantalum pentahalides with a bulky NHC ligand is described, including the first crystallographic characterization of a Ta complex with a monodentate NHC ligand.

Synthesis of a highly reactive form of WO2Cl2, its conversion into nanocrystalline mono-hydrated WO3 and coordination compounds with tetramethylurea

A new form of WO₂Cl₂ was prepared from WCl₆. Conversion of WO₂Cl₂ into nanocrystalline WO₃·H₂O occurred upon air exposure at room temperature. The first coordination complexes of WO₂Cl₂ with an alkylurea are reported.