Synthesis and Reactivity of Mixed Arene Halides of Niobium(I). Crystal and Molecular Structure of Nbmes₂ and [Nbmes₂(CO)][Nb₂(μ‐I)₃(CO)₈]

Abstract

Bis(mesitylen)niobium(O), Nbmes,, prepared via the NbC15/A1/ AlCl,/mes system by exaustive reduction in the presence of 1,2-dimethoxyethane or tetrahydrofuran as previously reported, has been studied by X-ray diffraction methods. The sandwich molecule has parallel, substantially planar, aromatic rings, in an eclipsed conformation. Nbmes, is oxidized to NbmeszX by reaction with l,lf-dimethy1-4,4'-bipyridinium halides [methylviologen halides, mvX, (X = C1, I)]. The reaction of both halo compounds with CO yielded the niobium (1) derivatives [Nbmes,(CO) Recently, we reported that the reaction of NbXs with Al/ AlX3 in mesitylene followed by low-temperature treatment of the reaction mixture with 1,2-dimethoxyethane and/or tetrahydrofuran afforded the bis-mesitylene derivative of niobium(O), Nbmes2. The reactivity of this compound has been investigated: the niobium(0) derivative is prone to oxidation to niobium(1) and can be easily reduced to niobium( -I), especially in the presence of carbon monoxide ['].In the framework of our studies concerning the low-valent derivatives of group 5 metals, we now report on the crystal and molecular structure of the parent compound, Nbmes,, the preparation of mixed halide-arene derivatives of niobium(1) and their reaction with CO affording the dinuclear anion [Nb2(p-I)j(CO)s] -as the [Nbmes2(CO)] + derivative. Results and DiscussionAfter several attempts, large red crystals of the paramagnetic [h8 (21°C) = 1.87 BM] Nbmes, were grown from pentane at -30°C, and the structure of the compound was solved. It consists of a sandwich molecule where the niobium atom is bound to two almost parallel (dihedral angle, 2.9 ") and planar (maximum deviation of the carbon atoms from the least-squares plane 0.007 A) mesitylene rings (Figure 1).The methyl substituents, which are in an eclipsed conformation (Figure 2) show only small deviations from the ring plane, the value of 0.104 A being the maximum deviation.The central niobium atom exhibits distances of 1.849(1) and 1.850(1) A from the ring planes, the Nb-C mean distance ranging from 2.305(5) to 2.362(5) 8. The mean C-C distances within the aromatic rings are 1.410(7) and 1.508(7) A. Several zerovalent bis(arene)metal complexes, mainly concerning chromium, have been structurally characterizedm. It is interesting to note that most of them show planar and parallel aromatic rings[2",b~d-nnl and adopt an eclipsed conformation both for benzene and alkylbenzene complexcribed[2d1 vanadium complex follow the same trend. Although in the niobium complex the interatomic repulsion is certainly lower than in 3d systems due to the larger size of the central metal atom, the eclipsed conformation in both the neutral and the cationic [Nbmes,(CO)] + complex (vide infra) is somewhat surprising. This problem, which is outside the scope of this paper, certainly deserves further attention.es P-e,h,ml. . the niobium derivative as well as a recently des-

Diniobium Inverted Sandwich Complexes with μ-η6:η6-Arene Ligands: Synthesis, Kinetics of Formation, and Electronic Structure

“…The two Nb–N BDI distances are 2.246(4) Å, which are similar to those observed in related BDI complexes of Nb(III) and Nb(V). ,, The Nb–N imido –CMe 3 bond angle (175.3(4)°) is close to linear, and the Nb–N imido distance is 1.775(3) Å, which is also typical of values reported in the literature. ,, The puckered C 6 H 6 ligand of 2a is characteristic of niobium–arene complexes. − ,,,, The fold angle at C3···C6 is 22.8°, such that the Nb(1)–C(3) (2.280(5) Å) and Nb(1)–C(6) (2.284(5) Å) distances are shorter than observed for the other four Nb–C bonds (average 2.471(4) Å). These Nb–C distances compare well with Nb–C distances observed previously for arene compounds (from 2.198(5) Å to 2.513(4) Å). − The C–C bond lengths of the benzene ring vary, with C(1)–C(2) (1.337(6) Å) and C(4)–C(5) (1.384(6) Å) being significantly shorter than bonds involving the C(3) and C(6) atoms (average of 1.450(5) Å).…”

“…Nevertheless, rapid crystallization of 2a from a mixture of toluene/hexanes formed material suitable for X-ray crystallographic analysis (Figure , Tables and S1). Although several niobium arene compounds have been reported, − to the best of our knowledge 2a is the first niobium–benzene complex that has been structurally characterized. In 2a the BDI and imido ligands occupy facial positions in a pseudo-octahedral geometry.…”

Substituent effects in bis(arene)chromium compounds containing a CN group in the aromatic ring

“…(2), R = CN (3), R = CH 3 (4), R = CF 3 of the CH 3 group in (3) is shifted by 0.066 Å from the C 6 -ring plane away from the Cr atom. A similar structural feature was observed for other bis-arene complexes of transition metals containing -electron-donating substituents [CH 3 (Braga & Sabatino, 1990;Calderazzo et al, 1992), NMe 2 (Elschenbroich et al, 2003)] in the phenyl rings. Unexpectedly, unlike the previously studied neutral bisarene complexes containing -electron-withdrawing groups (Cl, COOH, CF 3 ) in aromatic ligands, in which these substituents are virtually coplanar with the phenyl rings, the CN groups in (2)-(4) are shifted towards Cr from the plane of the rings.…”

Site Selectivity in Electrophilic (H+, CH3+) Abstraction on Os(H)2X2(PiPr3)2

“…A survey of available structural data shows that ∠M−(μ 2 -X)−M becomes more acute for the heavier halides X. For iodide, this angle is in the range 55−87° . This would aggravate X/P i Pr 3 and P i Pr 3 /P i Pr 3 steric repulsions in (L 2 OsH 2 ) 2 (μ-I) 3 + , and may explain why the iodo derivative shows consistently different behavior when OsH 2 X 2 L 2 undergoes electrophilic attack.…”