Ytterbocene Charge-Transfer Molecular Wire Complexes

A systematic study of the novel charge-transfer [(f)14-(π*)0-(f)14 → (f)13-(π*)2-(f)13] electronic state
found in 2:1 metal-to-ligand adducts of the type [(Cp*)2Yb](BL)[Yb(Cp*)2] [BL = tetra(2-pyridyl)pyrazine
(tppz) (1), 6‘,6‘ ‘-bis(2-pyridyl)-2,2‘:4‘,4‘ ‘:2‘ ‘,2‘ ‘‘-quaterpyridine (qtp) (2), 1,4-di(terpyridyl)-benzene (dtb) (3),
Cp* = (C5Me5)] has been conducted with the aim of determining the effects of increased Yb−Yb separation
on the magnetic and electronic properties of these materials. The neutral [(f)13-(π*)2-(f)13], cationic
[(f)13-(π*)1-(f)13] and dicationic [(f)13-(π*)0-(f)13] states of these complexes were studied by cyclic voltammetry,
UV−vis−NIR electronic absorption spectroscopy, NMR, X-ray crystallography, and magnetic susceptibility
measurements. The spectroscopic and magnetic data for the neutral bimetallic complexes is consistent
with an [(f)13(π*)2(f)13] ground-state electronic configuration in which each ytterbocene fragment donates
one electron to give a singlet dianionic bridging ligand with two paramagnetic Yb(III) centers. The
voltammetric data demonstrate that the electronic interaction in the neutral molecular wires 1−3, as
manifested in the separation between successive metal reduction waves, is large compared to analogous
transition metal systems. Electronic spectra for the neutral and monocationic bimetallic species are
dominated by π−π* and π*−π* transitions, masking the f−f bands that are expected to best reflect the
electronic metal−metal interactions. However, these metal-localized transitions are observed when the
electrons are removed from the bridging ligand via chemical oxidation to yield the dicationic species, and
they suggest very little electronic interaction between metal centers in the absence of π* electrons on the
bridging ligands. Analysis of the magnetic data reveals that the qtp complex displays antiferromagnetic
coupling of the type Yb(α)(αβ)Yb(β) at ∼13 K.