Excited-state properties of cyclopentadienylytterbium complexes

Abstract The excited-state properties of species derived from tris(η 5 -cyclopentadienyl)ytterbium(III), Cp 3 Yb, and from bis(η 5 -pentamethylcyclopentadienyl)ytterbium(II), (Me 5 C 5 ) 2 Yb, have been investigated. Electronic spectra of several adducts of Cp 3 Yb have been examined in benzene solution. When added to solutions of this Lewis acid, bases such as Et 3 P, pyrrolidine (pl), THF, and tetrahydrothiophene (THT) cause substantial changes in the rich Cp 3 Yb absorption spectrum. Titrations reveal that these changes are consistent with the formation of 1:1 adducts whose formation constants, log K f , range from ∼3.5 for THF and THT to ∼5.0 for Et 3 P and pl. The magnitude of K f roughly correlates with the position of a visible absorption band previously assigned as a ligand-to-metal charge-transfer (LMCT) transition, and with the gas-phase basicities of the bases employed. Luminescence is observed in the near-IR (∼1000–1100 nm), ff spectral region from Cp 3 Yb and its adducts upon excitation into their LMCT bands. Spectral changes accompanying base addition are consistent with 1:1 adduct formation with apparent excited-state adduct formation constants, log K * app , equal to the corresponding log K f values within experimental error. The coincidence in K f and K * app values probably reflects the excitation conditions employed rather than the establishment of excited-state equilibrium. Electronic spectra of adducts of (Me 5 C 5 ) 2 Yb have been examined in toluene solution. The solid has been isolated as a mono-THF adduct (P. L. Watson, J. Chem. Soc., Chem. Comm. , 652 (1980)) and as a mono-Et 2 O adduct (R. A. Anderson, et al., Inorg. Chem. , 19, 299 (1980)), orange and yellow-green compounds, respectively. Toluene solutions retain these colors. The THF adduct exhibits bands at 445, 505, and 790 nm in toluene with absorptivities of ∼410, 320, and 190 M −1 cm −1 , respectively; corresponding values for the Et 2 O adduct are 460 and 690 nm with absorptivities of ∼410 and 180 M −1 cm −1 , respectively. Both species are reluctant to add a second equivalent of base to their coordination sphere; a K f value of ∼10 M −1 for formation of the bis-THF adduct was measured from spectral changes accompanying titration of the mono-THF adduct with THF. For adducts of (Me 5 C 5 ) 2 Yb involving one equivalent of base, the lowest-energy absorption band appears particularly sensitive to the identity of the base, red-shifting as the base is changed from Et 2 O to THF to pyridine (λ max ∼ 800 nm). The direction of the shift is consistent with an assignment for the band of Yb → π*(Me 5 C 5 ), i.e. , metal-to-ligand charge transfer (MLCT). Supporting this assignment are photolysis experiments in which irradiation of the THF adduct in toluene, either alone with UV excitation or in C 6 H 5 Cl/toluene mixture with visible and near-IR excitation, leads to Yb(III) products with quantum efficiencies of ∼10 −1 . The mono-THF adduct emits at 295 K in toluene solution when excited with visible light; the emission band is broad (∼90 nm fwhm) with a maximum at 935 nm. Additionally, the adduct exhibits a novel chemiluminescent reaction indicative of the strong reducing power of these Yb(II) species: upon addition of O 2 -saturated toluene to a toluene solution of the adduct, a sharp emission band is observed at ∼985 nm. This band is also observed in photoluminescence of the product solution and is characteristic of an Yb(III) species. The species and mechanism responsible for the chemiluminescence will be discussed.