Tri- and Tetravalent and Mixed-Valence Niobium Complexes Supported by a Tripodal Tripyrrolylmethane Trianion

Abstract

The reaction of Nb2Cl6(TMEDA)2 with the potassium salt of tripyrrolylmethane in a 1:1 ratio afforded two products which have been isolated and characterized. The first compound is the dinuclear {[HC(C4H3N)3]Nb(THF)}2·2THF (1), with two trivalent and diamagnetic metal centers bridged by two tripyrrolyl trianions. Each of the two ligands adopted a rather unusual bridging mode, with two rings each σ-bonding one of the two metal centers and the third both π-bonded to one of the two niobium atoms and σ-bonded to the other. The second product of the reaction is {([H(C4H3N)3]2NbK)2}{Nb4Cl11[K(THF)2]2}·2THF (2), which displays some unusual features. The complex is ionic, with the cationic {Nb4Cl11[K(THF)2]2}2+ unit containing the metal in a mixed-valence state. The two positive charges of the cationic moiety are balanced by two identical anionic {([HC(C4H3N)3]2NbK)2}- units, each containing Nb in the tetravalent state. Each anion is connected to an identical one by the bridging potassium atom, thus assembling a linear and anionic polymeric array. A similar reaction carried out with the lithium salt of the tripyrrolide anion led instead to the simple monomeric and tetravalent complex {[HC(C4H3N)3]2Nb}{Li(THF)4}2 (3).

Symmetry and Geometry Considerations of Atom Transfer: Deoxygenation of (silox)3WNO and R3PO (R = Me, Ph,tBu) by (silox)3M (M = V, NbL (L = PMe3, 4-Picoline), Ta; silox =tBu3SiO)

“…Even the silox methyls shift dramatically to δ 83.15, and no resonance could be confidently assigned to the PMe 3 ligand; no resonance was observed in the 31 P{ 1 H} spectrum either. Niobium(III) is typically found in dimeric or cluster derivatives, , so this simple coordination compound is quite unusual.…”

“…The new coordination complexes described herein are relatively unexceptional given previous studies, but a few derivatives are worth comment. The Nb(III) derivative, (silox) 3 NbPMe 3 ( 1 -NbPMe 3 ), is a rare monomeric example of this uncommon oxidation state. , While it is tempting to conclude that its distorted tetrahedral geometry, which approaches that of a trigonal monopyramid, is a natural consequence of the severe steric constraints imposed by the silox groups, the PMe 3 ligand actually has a substantially larger cone angle . An increase in the O(pπ)→Nb(dπ) interactions as the O 3 Nb core approaches a trigonal planar geometry may help offset the phosphine/silox sterics, which are also somewhat alleviated by the subtle canting of the Nb−O−Si linkages.…”

Synthesis and structures of transition metal pacman complexes of heteroditopic Schiff-base pyrrole macrocycles

“…In both K 4 (L) and in the above imine-pyrrole salts the K-η 5 -centroid distances of 2.962 and 2.903 Å, respectively, are appreciably shorter, with the longest reported K-η 5 -pyrrole distance of 3.309 Å for a niobium complex of a tripodal ligand. 27 The majority of K-O distances (2.750(2)-3.134(3) Å) are of a standard length though μ 2 -O3 is only weakly bonding to K1 (3.540 Å) compared to K2 (2.908(2) Å), which is compensated by the relatively short K1-O4 distance of 2.625(2) Å. 28 Intermolecular interactions between potassium and the arenebackbones (K-centroid-C16′-C21′, 3.386 and 3.590 Å) are also weak, with reported K-η 6 -arene-centroid distances ranging from ca.…”

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“…A similar reaction with the lithium salt led to the simple monomer, [Li(thf) 4 ] 2 [{H(C 4 H 3 N) 3 } 2 Nb], for which the crystal structure was also determined. 45 The new imido complexes, [{Nb(L) 2 Cl 3 } 2 (m-Y)] (L = MeCN, 4-Bu t py; Y = 1,2-NC 6 H 4 N, 1,3-NC 6 H 4 N, 1,4-NC 6 H 4 N), have been prepared. These compounds can react with alkylating agents to give the corresponding alkyl complexes, [{NbLR 3 } 2 Y] (R = CH 2 SiMe 3 , CH 2 CMe 3 ; L = MeCN, thf; Y = 1,4-NC 6 H 4 N).…”