Stable Formally Zerovalent and Diamagnetic Monovalent Niobium and Tantalum Complexes Based on Diazadiene Ligands

Abstract

This paper describes the efficient synthesis and full characterization of rare formally zerovalent and diamagnetic monovalent pseudooctahedral niobium and tantalum complexes. The reaction of NbCl(4)(thf)(2) with 4 equiv of Na and 3.5 equiv of iPr(2)-dad (1,4-diisopropyl-1,4-diaza-1,3-diene) yields Nb(iPr(2)-dad)(3) in 52% yield. Ta(iPr(2)-dad)(3) is also obtained in 33% yield from 5 equiv of Na/naphthalene and 3.5 equiv of iPr(2)-dad. Oxidation of these complexes with AgBPh(4) yields the diamagnetic complexes [M(iPr(2)-dad)(3)][BPh(4)] (M = Nb, Ta). X-ray and spectroscopic data indicate that the unpaired electron density is localized on the ligands. DFT calculations reveal that, in the prevailing D(3) symmetry, a very strong splitting of t(2g) metal-based orbitals occurs, leading to the diamagnetism of the 16e M(iPr(2)-dad)(3)(+). This strong splitting allows a M-N nonbonding, ligand-based orbital to accommodate additional electrons, as a result of which the formally zerovalent complexes, M(iPr(2)-dad)(3), are in fact correctly formulated as M(+)iPr(2)-dad (-), that is, (16e + 1e).

A Critical Review of Electronic Effects in Enediamido and α‐Diimino Complexes of the Group 4 Metals

“…Curiously, it should be noted that the electron richer d 5 complex also has 18 electrons since experimental and computational studies for the neutral [Ta(iPr 2 -DAD) 3 ] complex confirm that the SOMO has exclusively ligand-based character. [20] This is consistent with the EPR spectra. Moreover, a significantly larger energy gain is computed for singly populating a 2 instead of a 1 , while the C-C bond and C-N distances are comparatively shorter and longer, respectively, in the neutral species than in the monocation.…”

“…[18] Zr, p-TolNC(Ph)C(Ph)N-p-Tol YOBYAP [19] Hf, p-TolNC(Ph)C(Ph)N-p-Tol YOBYET [19] A density functional analysis of an equivalent group 5 metal complex, namely the redox species [M(iPrNC(H)=C(H)NiPr) 3 ] 0,+1 (M = Nb, Ta), is available. [20] Based on the MO architecture, it has been suggested that conversion between the two redox complexes is ligand-based. We have examined analogous group 4 complexes and have optimized the model [Ti{MeNC(H)=C(H)-NMe} 3 ] (1; Figure 1).…”

“…Moreover, a significantly larger energy gain is computed for singly populating a 2 instead of a 1 , while the C-C bond and C-N distances are comparatively shorter and longer, respectively, in the neutral species than in the monocation. [20] In spite of the ligand-based redox chemistry, it might be improper to formulate a prevailing dianionic character of the ligands as this would imply intra-ligand repulsion. Although the additional electron in [Ta(iPr 2 -DAD) 3 ] is assigned to the ligand, there is no evidence of Ta-N bond elongation.…”

Ta(CNDipp)6: An Isocyanide Analogue of Hexacarbonyltantalum(0)

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Hexakis(2,6-diisopropylphenylisocyanide)tantalum is the first isocyanide analogue of the highly unstable Ta(CO) 6 and represents the only well-defined zerovalent tantalum complex to be prepared by conventional laboratory methods. Tw op rior examples of homoleptic Ta 0 complexes are known, Ta(benzene) 2 and Ta(dmpe) 3 ,d mpe = 1,2-bis(dimethylphosphano)ethane,but these have only been accessed via ligand cocondensation with tantalum vapor in as ophisticated metalatom reactor.C onsistent with its 17-electron nature,T a-(CNDipp) 6 undergoes facile one-electron oxidation, reduction, or disproportionation reactions.I nt his sense,i tq ualitatively resembles V(CO) 6 ,t he only paramagnetic homoleptic metal carbonyl isolable under ambient conditions.

Incontrast to vanadium and niobium, [1] isolable homoleptic complexes of zerovalent tantalum are quite rare,w ith only two examples presently known:T a(benzene) 2

[2] and Ta-(dmpe) 3 , [3] dmpe = 1,2-bis(dimethylphosphano)ethane.A lso, these species are very poorly accessible because they are only available via tantalum vapor synthesis methods,which require specialized metal-atom reactors.

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Hexakis(2,6-diisopropylphenylisocyanide)tantalum is the first isocyanide analogue of the highly unstable Ta(CO) 6 and represents the only well-defined zerovalent tantalum complex to be prepared by conventional laboratory methods. [4] Although Ta(bipy) 3 ,bipy = a,a'-bipyridine, [5] and Ta(iPr 2 -dad) 3 , iPr 2 -dad = 1,4-diisopropyl-1,4-diazabuta-1,3-diene, [6] were originally suggested to be zerovalent tantalum complexes,later experimental and computational studies indicate that both compounds contain high-valent Ta V . [4] Although Ta(bipy) 3 ,bipy = a,a'-bipyridine, [5] and Ta(iPr 2 -dad) 3 , iPr 2 -dad = 1,4-diisopropyl-1,4-diazabuta-1,3-diene, [6] were originally suggested to be zerovalent tantalum complexes,later experimental and computational studies indicate that both compounds contain high-valent Ta V .

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