| Abstract
| - A systematic study of the novel charge-transfer [(f)14-(π*)0-(f)14 → (f)13-(π*)2-(f)13] electronic statefound 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 separationon 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 susceptibilitymeasurements. The spectroscopic and magnetic data for the neutral bimetallic complexes is consistentwith an [(f)13(π*)2(f)13] ground-state electronic configuration in which each ytterbocene fragment donatesone electron to give a singlet dianionic bridging ligand with two paramagnetic Yb(III) centers. Thevoltammetric data demonstrate that the electronic interaction in the neutral molecular wires 1−3, asmanifested in the separation between successive metal reduction waves, is large compared to analogoustransition metal systems. Electronic spectra for the neutral and monocationic bimetallic species aredominated by π−π* and π*−π* transitions, masking the f−f bands that are expected to best reflect theelectronic metal−metal interactions. However, these metal-localized transitions are observed when theelectrons are removed from the bridging ligand via chemical oxidation to yield the dicationic species, andthey suggest very little electronic interaction between metal centers in the absence of π* electrons on thebridging ligands. Analysis of the magnetic data reveals that the qtp complex displays antiferromagneticcoupling of the type Yb(α)(αβ)Yb(β) at ∼13 K.
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