Soft diphosphine and diarsine complexes of niobium(V) and tantalum(V) fluorides: synthesis, properties, structures and comparisons with the corresponding chlorides

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.

Stabilisation of [WV F4 ]+ by N- and P-Donor Ligands: Second-Order Jahn-Teller Effects in Octacoordinate d1 Complexes

The first isolated examples of cationic fluoridotungsten(V) complexes are reported as octacoordinate [WF₄ (L)₄ ]⁺ (L=C₅ H₅ N, P(CH₃ )₃ ). The [WF₄ (NC₅ H₅ )₄ ]⁺ cation is synthesised as its [O₃ SCF₃ ]^- salt upon reaction of WF₅ (NC₅ H₅ )₂ with [(CH₃ )₃ Si(NC₅ H₅ )][O₃ SCF₃ ] in excess C₅ H₅ N, whereas [WF₄ {P(CH₃ )₃ }₄ ]⁺ is accessed directly from WF₆ upon reaction with (CH₃ )₃ SiO₃ SCF₃ and excess P(CH₃ )₃ . These salts were characterised by X-ray crystallography and Raman spectroscopy in the solid state. New geometry indices for octacoordinate complexes (τ₈ and τ₈ ') are introduced, allowing for the facile differentiation of trigonal-dodecahedral (TD) and square-antiprismatic (SA) geometries. This has disambiguated the SA geometries of [WF₄ (L)₄ ]⁺ and the geometries of a series of previously reported d⁰ and d¹ MA₄ B₄ complexes. Computational (DFT-B3LYP) studies of [WF₄ (PH₃ )₄ ]^2+/+ and related model systems demonstrate the occurrence of a second-order Jahn-Teller (SOJT) distortion from TD in d⁰ complexes to SA in d¹ complexes, with the degree of SOJT stabilisation being most significant in 5d complexes containing fluorido ligands and monodentate neutral donors.

Coordination of a Neutral Ligand to a Metal Center of Oxohalido Anions: Fact or Fiction?

Can a neutral ligand bond to a metal center of a square pyramidal oxohalido anion at the available sixth octahedral position? Crystal structures of some compounds indeed suggest that ligands, such as THF, pyridine, H₂O, NH₃, and CH₃CN, can interact with the central metal atom, because they are oriented with their heteroatom toward the metal center with distances being within the bonding range. However, this assumption that is based on chemical intuition is wrong. In-depth analysis of interactions between ligands and oxohalido anions (e.g., VOX₄^–, NbOCl₄^–) reveals that the bonding of a neutral ligand is almost entirely due to electrostatic interactions between the H atoms of a ligand and halido atoms of an anion. Furthermore, ab initio calculations indicate that the ligand–VOF₄^– interactions represent only about one-quarter of the total binding of the ligand within the crystal structure, whereas the remaining binding is due to crystal packing effects. The current study therefore shows that relying solely on the structural aspects of solved crystal structures, such as ligand orientation and bond distances, can lead to the wrong interpretation of the chemical bonding.

Some crystal structures suggest that ligands interact chemically with the metal center of oxohalido anions. However, analysis shows that the attraction between the heteroatom of the ligand and the metal center is counteracted by the repulsion between the heteroatom and the electronegative F atoms, making electrostatic F···H interactions and crystal packing effects the dominant force holding the ligand and anion together. This is the reason why some ligands rotate with their heteroatom away from the metal center.

Tertiary Phosphine and Arsine Complexes of Phosphorus Pentafluoride: Synthesis, Properties, and Electronic Structures

The reaction of PMe₃ or PPh₃ with PF₅ in anhydrous CH₂Cl₂ or hexane forms the white, moisture-sensitive complexes [PF₅(PR₃)] (R = Me, Ph). Similar reactions involving the diphosphines o-C₆H₄(PR₂)₂ afford the complexes [PF₄{o-C₆H₄(PR₂)₂}][PF₆]. The X-ray structures of [PF₅(PR₃)] and [PF₄{o-C₆H₄(PMe₂)₂}][PF₆] show pseudo-octahedral fluorophosphorus centers. Multinuclear NMR spectra (¹H, ¹⁹F{¹H}, ³¹P{¹H}) show that in solution in CH₂Cl₂/CD₂Cl₂ the structures determined crystallographically are the only species present for [PF₅(PMe₃)] and [PF₄{o-C₆H₄(PMe₂)₂}][PF₆] but that [PF₅(PPh₃)] and [PF₄{o-C₆H₄(PPh₂)₂}][PF₆] exhibit reversible dissociation of the phosphine at ambient temperatures, although exchange slows at low temperatures. The complex ¹⁹F{¹H} and ³¹P{¹H} NMR spectra have been analyzed, including those of the cation [PF₄{o-C₆H₄(PMe₂)₂}]⁺, which is a second-order AA'XX'B₂M spin system. The unstable [PF₅(AsMe₃)], which decomposes in a few hours at ambient temperatures, has also been isolated and spectroscopically characterized; neither AsPh₃ nor SbEt₃ forms similar complexes. The electronic structures of the PF₅ complexes have been explored by DFT calculations. The DFT optimized geometries for [PF₅(PMe₃)], [PF₅(PPh₃)], and [PF₄{o-C₆H₄(PMe₂)₂}]⁺ are in good agreement with their respective crystal structure geometries. DFT calculations on the PF₅-L complexes reveal the P-L bond strength falls with L in the order PMe₃ > PPh₃ > AsMe₃, consistent with the experimentally observed stabilities, and in the PF₅-L complexes, electron transfer from L to PF₅ on forming these complexes also follows the order PMe₃ > PPh₃ ≈ AsMe₃.

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.

Modifying bis(triflimide) ionic liquids by dissolving early transition metal carbamates

The authors report the first modification of ionic liquids with metal carbamates. A selection of homoleptic N,N-dialkylcarbamates of group 4 and 5 metals, M(O₂CNR₂)_n, were dissolved in bis(trifluoromethylsulfonyl)imide-based ionic liquids, i.e. [bmim][Tf₂N] and [P(oct)₄][Tf₂N], at 293 K. The resulting solutions were characterized by means of IR, UV and NMR spectroscopy, and the data were compared to those of the respective metal compounds. Notably, the dissolution process did not proceed with the release of any of the original carbamato ligands, thus preserving the intact coordination frame around the metal centre. The solvation process of Ti(O₂CNⁱPr₂)₄, as a model species, in [bmim][Tf₂N] was rationalized by DFT calculations. As a comparative study, solutions of NbF₅ and MCl₅ (M = Nb, Ta) in [bmim][Tf₂N] were also investigated, revealing the possible occurrence of solvent anion coordination to the metal centres.

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.

Neutral and cationic tungsten(vi) fluoride complexes with tertiary phosphine and arsine coordination

The dodecahedral [WF₄{o-C₆H₄(EMe₂)₂}₂]²⁺ dications (E = P, As) present the highest oxidation state metal fluoride complexes with soft donor ligands.

Niobium tetrahalide complexes with neutral diphosphine ligands

The reactions of NbCl4 with diphosphine ligands o-C6H4(PMe2)2, Me2PCH2CH2PMe2 or Et2PCH2CH2PEt2 in a 1 : 2 molar ratio in MeCN solution produced eight-coordinate [NbCl4(diphosphine)2]. [NbBr4(diphosphine)2] (diphosphine = o-C6H4(PMe2)2 or Me2PCH2CH2PMe2) were made similarly from NbBr4. X-ray crystal structures show that [NbCl4{o-C6H4(PMe2)2}2] has a dodecahedral geometry, but the complexes with dimethylene-backboned diphosphines are distorted square antiprisms. The Nb-P distances and <P-Nb-P angles are very similar in the two types, but Nb-Cl distances are ∼0.1 Å longer in the square antiprismatic complexes. These paramagnetic (d(1)) complexes were also characterised by microanalysis, magnetic measurements, IR and UV-visible spectroscopy. Using a 1 : 1 molar ratio of NbCl4 : diphosphine (diphosphine = Me2PCH2CH2PMe2, Et2PCH2CH2PEt2, Cy2PCH2CH2PCy2 and Ph2PCH2CH2CH2PPh2) afforded [NbCl4(diphosphine)], and [NbBr4(Me2PCH2CH2PMe2)] was obtained similarly. These 1 : 1 complexes are unstable in solution, preventing X-ray crystallographic study, but based upon their diamagnetism, IR, UV-visible and (31)P{(1)H} NMR spectra, they are formulated as halide-bridged dimers [(diphosphine)X2Nb(μ-X)4NbX2(diphosphine)], with single Nb-Nb bonds and chelating diphosphines. The Nb(iv) complexes are prone to hydrolysis and oxidation in solution and the structures of the Nb(v) complexes [NbBr4(Me2PCH2CH2PMe2)2][NbOBr4(MeCN)] with a dodecahedral cation, and [{NbOCl3{Et2P(CH2)2PEt2}}2{μ-Et2P(CH2)2PEt2}] which contains seven-coordinate Nb(v) centres with a symmetrical diphosphine bridge, are reported. The structure of niobium tetrabromide, conveniently made from NbCl4 and BBr3, is a chain polymer with edge-linked NbBr6 octahedra and alternating long and short Nb-Nb distances, the latter ascribed to Nb-Nb bonds.

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.

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.