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Diniobium Inverted Sandwich Complexes with μ‑η6:η6‑Arene Ligands: Synthesis, Kinetics of Formation, and Electronic Structure
journal contribution
posted on 2013-02-27, 00:00 authored by Thomas
L. Gianetti, Grégory Nocton, Stefan G. Minasian, Neil C. Tomson, A. L. David Kilcoyne, Stosh A. Kozimor, David
K. Shuh, Tolek Tyliszczak, Robert G. Bergman, John ArnoldMonometallic niobium arene complexes [Nb(BDI)(NtBu)(R-C6H5)] (2a: R = H
and 2b: R = Me, BDI = N,N′-diisopropylbenzene-β-diketiminate) were synthesized
and found to undergo slow conversion into the diniobium inverted arene
sandwich complexes [[(BDI)Nb(NtBu)]2(μ-RC6H5)] (7a: R
= H and 7b: R = Me) in solution. The kinetics of this
reaction were followed by 1H NMR spectroscopy and are in
agreement with a dissociative mechanism. Compounds 7a-b showed a lack of reactivity toward small molecules,
even at elevated temperatures, which is unusual in the chemistry of
inverted sandwich complexes. However, protonation of the BDI ligands
occurred readily on treatment with [H(OEt2)][B(C6F5)4], resulting in the monoprotonated cationic
inverted sandwich complex 8 [[(BDI#)Nb(NtBu)][(BDI)Nb(NtBu)](μ-C6H5)][B(C6F5)4] and the dicationic complex 9 [[(BDI#)Nb(NtBu)]2(μ-RC6H5)][B(C6F5)4]2 (BDI# = (ArNC(Me))2CH2).
NMR, UV–vis, and X-ray absorption near-edge structure (XANES)
spectroscopies were used to characterize this unique series of diamagnetic
molecules as a means of determining how best to describe the Nb–arene
interactions. The X-ray crystal structures, UV–vis spectra,
arene 1H NMR chemical shifts, and large JCH coupling constants provide evidence for donation of
electron density from the Nb d-orbitals into the antibonding π
system of the arene ligands. However, Nb L3,2-edge XANES
spectra and the lack of sp3 hybridization of the arene
carbons indicate that the Nb → arene donation is not accompanied
by an increase in Nb formal oxidation state and suggests that 4d2 electronic configurations are appropriate to describe the
Nb atoms in all four complexes.
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Keywords
electron densitysandwich complexes2 bantibonding π systemElectronic StructureMonometallic niobium arene complexes1 H NMR spectroscopydiamagnetic moleculesXANESBDI ligandsUV7 bDiniobium Inverted Sandwich Complexesarene ligandsarene 1 H NMR chemical shiftsmonoprotonated cationicdissociative mechanismNb atomsJCHsp 3 hybridizationoxidation statearene carbons
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