Coordination Adducts of Niobium(V) and Tantalum(V) Azide M(N3)5 (M=Nb,Ta) with Nitrogen Donor Ligands and their Self‐Ionization

Several new donor–acceptor adducts of niobium and tantalum pentaazide with N‐donor ligands have been prepared from the pentafluorides by fluoride–azide exchange with Me₃SiN₃ in the presence of the corresponding donor ligand. With 2,2′‐bipyridine and 1,10‐phenanthroline, the self‐ionization products [MF₄(2,2′‐bipy)₂]⁺[M(N₃)₆]^?, [M(N₃)₄(2,2′‐bipy)₂]⁺[M(N₃)₆]^? and [M(N₃)₄(1,10‐phen)₂]⁺[M(N₃)₆]^? were obtained. With the donor ligands 3,3′‐bipyridine and 4,4′‐bipyridine the neutral pentaazide adducts (M(N₃)₅)₂?L (M=Nb, Ta; L=3,3′‐bipy, 4,4′‐bipy) were formed.     

Organometallic Niobium and Tantalum Complexes with Primary Phosphine Ligands: Syntheses and Molecular Structures of [Cp*MCl4(PH2R)] (M = Nb,Ta; Cp* = C5Me5;R = But,Ad, Cy,Ph, 2, 4, 6‐Me3C6H2 (Mes))

The reaction of [Cp*MCl₄] (M = Nb, Ta; Cp* = C₅Me₅) with PH₂R in toluene at room temperature gives the primary phosphine complexes [Cp*MCl₄(PH₂R)] [Cp* = C₅Me₅; M = Nb: R = Bu^t ( 1a ), Ad ( 2a ), Cy ( 3a ), Ph ( 4a ), 2, 4, 6‐Me₃C₆H₂ (Mes) ( 5a ); M = Ta: R = Bu^t ( 1b ), Ad ( 2b ), Cy ( 3b ), Ph ( 4b ), Mes ( 5b )] in high yield. 1—5 were characterized spectroscopically (NMR, IR, MS) and by crystal structure determinations. The starting material [Cp*TaCl₄] is monomeric in the solid state, as shown by crystal structure determination.     

Icosahedral gold cage clusters: M@Au12- (M=V, Nb, and Ta)

We report the observation and characterization of a series of stable bimetallic 18-valence-electron clusters containing a highly symmetric 12-atom icosahedral Au cage with an encapsulated central heteroatom of Group VB transition metals, M@Au(12) (-) (M=V,Nb,Ta). Electronic and structural properties of these clusters were probed by anion photoelectron spectroscopy and theoretical calculations. Characteristics of the M@Au(12) (-) species include their remarkably high binding energies and relatively simple spectral features, which reflect their high symmetry and stability. The adiabatic electronic binding energies of M@Au(12) (-) were measured to be 3.70+/-0.03, 3.77+/-0.03, and 3.76+/-0.03 eV for M=V, Nb, and Ta, respectively. Comparison of density-functional calculations with experimental data established the highly symmetric icosahedral structures for the 18-electron cluster anions, which may be promising building blocks for cluster-assembled nanomaterials in the form of stoichiometric [M@Au(12) (-)]X(+) salts.     

Synthese,Struktur und Eigenschaften der Tetraarsenidometallate(V) M7[TAs4] (M = K,Rb; T = Nb,Ta)

Synthesis, Structure, and Properties of the Tetraarsenidometallates(V) M₇[TAs₄] (M = K, Rb; T = Nb, Ta) The tetraarsenidometallates(V) M₇[TAs₄] (M = K, Rb; T = Nb, Ta) have been prepared from RbAs, KAs, Rb₃As, K₃As, and Nb or Ta in sealed Nb(Ta) ampoules at T = 1100 K. They crystallize in a new structure type oP24 (Pmn2₁, no. 31); K₇[NbAs₄]: a = 1019.2(2) pm, b = 916.2(2) pm, c = 830.6(1) pm; K₇[TaAs₄]: a = 1017.3(2) pm, b = 915.5(2) pm, c = 830.5(2) pm; Rb₇[NbAs₄]: a = 1059.2(4) pm, b = 952.8(4) pm, c = 860.4(4) pm; Z = 2 formula units per unit cell). The compounds form dark red crystals and they are sensitive against air and moisture. They are semiconductors with E_g = 1.80 eV. The thermal decomposition in dynamical vacuum gives evidence for the existance of K₄TAs₃ and K₂TAs₂ (T = Nb, Ta). Main structural units are polar oriented tetrahedra [TAs₄] with d (T – As) = 252.2(1) pm; 251.3(1) pm; 253.0(4) pm, respectively. The As atoms are trigonal prismatically coordinated by M and T atoms. These trigonal prisms form anionic and cationic layers [M₄As₂]^2? and _∞²[M₃TAs₂]²⁺ alternating along the b axis. The structure is comparable with that of Co₂P and can be described as a stuffed shear variant of the Na₆□ZnO₄ type of structure.     

Thermodynamics, kinetics, and mechanism of (silox)3M(olefin) to (silox)3M(alkylidene) rearrangements (silox = tBu3SiO; M = Nb, Ta)

Olefin complexes (silox)(3)M(ole) (silox = (t)Bu(3)SiO; M = Nb (1-ole), Ta (2-ole); ole = C(2)H(4), C(2)H(3)Me, C(2)H(3)Et, C(2)H(3)C(6)H(4)-p-X (X = OMe, H, CF(3)), C(2)H(3)(t)Bu, (c)C(5)H(8), (c)C(6)H(10), (c)C(7)H(10) (norbornene)) rearrange to alkylidene isomers (silox)(3)M(alk) (M = Nb (1=alk), Ta (2=alk); alk = CHMe, CHEt, CH(n)Pr, CHCH(2)C(6)H(4)-p-X (X = OMe, H, CF(3) (Ta only)), CHCH(2)(t)Bu, (c)C(5)H(8), (c)C(6)H(10), (c)C(7)H(10) (norbornylidene)). Kinetics and labeling experiments suggest that the rearrangement proceeds via a delta-abstraction on a silox CH bond by the beta-olefin carbon to give (silox)(2)RM(kappa(2)-O,C-OSi(t)Bu(2)CMe(2)CH(2)) (M = Nb (4-R), Ta (6-R); R = Me, Et, (n)Pr, (n)Bu, CH(2)CH(2)C(6)H(4)-p-X (X = OMe, H, CF(3) (Ta only)), CH(2)CH(2)(t)Bu, (c)C(5)H(9), (c)C(6)H(11), (c)C(7)H(11) (norbornyl)). A subsequent alpha-abstraction by the cylometalated "arm" of the intermediate on an alpha-CH bond of R generates the alkylidene 1=alk or 2=alk. Equilibrations of 1-ole with ole' to give 1-ole' and ole, and relevant calculations on 1-ole and 2-ole, permit interpretation of all relative ground and transition state energies for the complexes of either metal.     

Niobium and tantalum tris(methimazolyl)borate complexes [M(=NC6H3iPr2-2,6)Cl2{HB(mt)3}] (M = Nb, Ta; mt = methimazolyl)

The first early transition metal tris(methimazolyl)borate com-plexes [M(=NR)Cl2{HB(mt)3}] (M = Nb, Ta; R = C6H3(i)Pr(2)-2,6; mt = methimazolyl) have been obtained from the reactions of [Nb(=NR)Cl3(DME)] or [Ta(=NR)Cl3(THF)2] (DME = dimethyl ether; THF = tetrahydrofuran) with Na[HB(mt)3] and structurally characterized, illustrating that the HB(mt)3 ligand can indeed be compatible with "hard" metals in high oxidation states.     

Heteronuclear Trimetallic MFe2 and M2Fe (M=V,Nb, and Ta) Clusters for Dinitrogen Activation

Catalysts with heteronuclear metal active sites may have high performance in the nitrogen reduction reaction (NRR), and the in-depth understanding of the reaction mechanisms is crucial for the design of related catalysts. In this work, the dissociative adsorption of N₂ on heteronuclear trimetallic MFe₂ and M₂Fe (M=V, Nb, and Ta) clusters was studied with density functional theory calculations. For each cluster, two reaction paths were studied with N₂ initially on M and Fe atoms, respectively. Mayer bond order analysis provides more information on the activation of N−N bonds. M₂Fe is generally more reactive than MFe₂. The coordination mode of N₂ on three metal atoms can be end-on: end-on: side-on (EES) for both MFe₂ and M₂Fe. In addition, a unique end-on: side-on: side-on (ESS) coordination mode was found for M₂Fe, which leads to a higher degree of N−N bond activation. Nb₂Fe has the highest reactivity towards N₂ when both the transfer of N₂ and the dissociation of N−N bonds are taken into account, while Ta-containing clusters have a superior ability to activate the N−N bond. These results indicate that it is possible to improve the performance of iron-based catalysts by doping with vanadium group metals.