Synthesis and characterization of niobium(IV) and tantalum(V) guanidinates and benzamidinates

Reactivity and Structure of a Bis-phenolate Niobium NHC Complex

We report the facile synthesis of a rare niobium(V) imido NHC complex with a dianionic OCO-pincer benzimidazolylidene ligand (L ¹ ) with the general formula [NbL ¹ (N ^t Bu)PyCl] 1-Py. We achieved this by in situ deprotonation of the corresponding azolium salt [H ₃ L ¹ ][Cl] and subsequent reaction with [Nb(N ^t Bu)Py ₂ Cl ₃ ]. The pyridine ligand in 1-Py can be removed by the addition of B(C₆F₅)₃ as a strong Lewis acid leading to the formation of the pyridine-free complex 1. In contrast to similar vanadium(V) complexes, complex 1-Py was found to be a good precursor for various salt metathesis reactions, yielding a series of chalcogenido and pnictogenido complexes with the general formula [ NbL ¹ (N ^t Bu)Py(EMes)] (E = O (2), S (3), NH (4), and PH (5)). Furthermore, complex 1-Py can be converted to alkyl complex (6) with 1 equiv of neosilyl lithium as a transmetallation agent. Addition of a second equivalent yields a new trianionic supporting ligand on the niobium center (7) in which the benzimidazolylidene ligand is alkylated at the former carbene carbon atom. The latter is an interesting chemically "noninnocent" feature of the benzimidazolylidene ligand potentially useful in catalysis and atom transfer reactions. Addition of mesityl lithium to 1-Py gives the pyridine-free aryl complex 8, which is stable toward "overarylation" by an additional equivalent of mesityl lithium. Electrochemical investigation revealed that complexes 1-Py and 1 are inert toward reduction in dichloromethane but show two irreversible reduction processes in tetrahydrofuran as a solvent. However, using standard reduction agents, e.g., KC₈, K-mirror, and Na/Napht, no reduced products could be isolated. All complexes have been thoroughly studied by various techniques, including ¹H-, ¹³C{¹H}-, and ¹H-¹⁵N HMBC NMR spectroscopy, IR spectroscopy, and X-ray diffraction analysis.

Guanidinate-Stabilized Monomeric Hafnium Amide Complexes as Promising Precursors for MOCVD of HfO2

Novel guanidinato complexes of hafnium [Hf{eta2-(iPrN)2CNR2}2(NR2)2] (R2 = Et2, 1; Et, Me, 2; Me2, 3), synthesized by insertion reactions of N,N'-diisopropylcarbodiimide into the M-N bonds of homologous hafnium amide complexes 1-3 and {[mu2-NC(NMe2)2][NC(NMe2)2]2HfCl}2 (4) using a salt metathesis reaction, are reported. Single-crystal X-ray diffraction analysis revealed that compounds 1-3 were monomers, while compound 4 was found to be a dimer. The observed fluxional behavior of compounds 1-3 was studied in detail using variable-temperature and two-dimensional NMR techniques. The thermal characteristics of compounds 1-3 seem promising for HfO2 thin films by vapor deposition techniques. Metal-organic chemical vapor deposition experiments with compound 2 as the precursor resulted in smooth, uniform, and stoichiometric HfO2 thin films at relatively low deposition temperatures. The basic properties of HfO2 thin films were characterized in some detail.

Synthesis and Structural Investigation of Tungsten Imido Amidinate and Guanidinate Complexes

The tungsten imido guanidinate and amidinate complexes W(NR)Cl(3)[R'NC(NMe(2))NR'] and W(NR)Cl(3)[R'NC(Me)NR'] (R = Ph, (i)Pr, Cy; R' = (i)Pr, (t)Bu, TMS) were synthesized by reacting the corresponding imido complex W(NR)Cl(4)(OEt(2)) with the appropriate lithium amidinate or guanidinate. Crystallographic structure determination of W(N(i)Pr)Cl(3)[(i)PrNC(NMe(2))N(i)Pr] and W(N(i)()Pr)Cl(3)[(i)PrNC(Me)N(i)Pr] allows comparison of structural features between the guanidinate and amidinate ligand in the presence of an identical ancillary ligand set.

Tantalum complexes with all nitrogen coordination sphere: mixed amido-, imido-, guanidinato complexes of tantalum and their thermal behaviour

Novel mixed amido-/imido-/guanidinato-complexes of tantalum are reported. The complexes were synthesised by insertion reactions of carbodiimides of the type C(NR)2(R =iso-propyl (i-Pr)) into the M-N bonds of tantalum mixed amido/imido complexes. Three representative complexes were synthesised and fully characterised by 1H-NMR, 13C-NMR, CHN-analysis and mass spectroscopy. The crystal structure of the complex [Ta(NMe2){(i-Pr-N)2C(NMe2)}2(N-t-Bu)](1)(Me = methyl, t-Bu =tert-butyl) is presented and discussed. The complexes are solids that can be quantitatively sublimed at moderate temperatures (120-130 degrees C) without any sign of decomposition. The thermal properties of the new compounds were studied (TGA) and preliminary MOCVD-experiments were performed. TaN films were deposited and characterised by XRD and XPS.