From Clusters to Ionic Complexes: Structurally Characterized Thallium Titanium Double Alkoxides

A series of sterically varied titanium alkoxides {[Ti(OR)4]n, n = 4, OR = OCH2CH3 (OEt); n = 1, OCH(CH3)2
(OPri); n = 2, OCH2C(CH3)3 (ONep); n = 1, OC6H3(CH3)2-2,6 (DMP)} were reacted with a series of thallium
alkoxides {[Tl(OR)]x (x = 4, OR = OEt, ONep; n = ∞, DMP)}. The resultant products of the [Tl(μ3-OEt)]4-modified [Ti(OR)4]n (OR = OEt, OPri, ONep) were found by X-ray analysis to be Tl4Ti2(μ-O)(μ3-OEt)8(OEt)2 (1),
Tl4Ti2(μ-O)(μ3-OPri)5(μ3-OEt)3(OEt)2 (2), and TlTi2(μ3-OEt)2(μ-OEt)(μ-ONep)2(ONep)4 (3), respectively. The reaction
of [Tl(μ3-OEt)]4, 12HOEt, and 4[Ti(μ-ONep)ONep)3]2 to generate 3 in a higher yield resulted in the isolation of
TlTi2(μ3-OEt)(μ3-ONep)(μ-OEt)(μ-ONep)2(ONep)4 (4). Compounds 1 and 2 possess an octahedral (Oh) arrangement
of two Ti and four Tl metal atoms around a μ-O central oxide atom (the Tl−O distance is too long to be considered
a bond). For both compounds, each Ti atom adopts a distorted Oh geometry with one terminal OEt ligand. The Tl
atoms are formally 4-coordinated, adopting a distorted pyramidal geometry using four μ3-OR (OR = OEt or OPri)
ligands to complete their coordination sphere. The Tl atoms reside ∼1.4 Å below the basal plane of oxygens. In
contrast to these structures, both 3 and 4 utilize ONep ligands and display reduced oligomerization yielding trinuclear
complexes without oxo formation. The two Ti cations are Oh, and the single Tl cation is in a formal distorted
pyramidal (PYD) arrangement. If the lone pair of the Tl cations are considered in the geometry, each Tl adopts a
square base pyramidal geometry. Two terminal ONep ligands are bound to each Ti with the remainder of the
molecule consisting of μ3- and μ-ONep ligands. The reaction of [Tl(μ3-ONep)]4 with two equivalents of [Ti(μ-ONep)(ONep)3]2 also led to the isolation of the homoleptic trinuclear complex TlTi2(μ3-ONep)2(μ-ONep)3(ONep)4
(5) which is analogous in structure to the mixed ligand species of 3 and 4. Each Ti is Oh coordinated with six
ONep ligands, and the single Tl is PYD bound by ONep ligands. A further increase in the steric bulk of the pendant
ligands, using [Tl(μ-DMP)]∞ and [Ti(μ-ONep)(ONep)3]2, resulted in a further decrease in the nuclearity yielding the
dinuclear species TlTi(μ-DMP)(μ-ONep)(DMP)(ONep)2 (6). For 6, the two metals are bound by a μ-ONep and a
μ-DMP ligand. The Tl metal center was solved in a bent geometry while the Ti adopted a distorted trigonal bipyramidal
(TBP) geometry using three ONep and two DMP ligands to fill its coordination sphere. Further increasing the steric
bulk of the ancillary ligands using Ti(DMP)4 and [Tl(μ-DMP)]∞ led to the formation of [Tl+][-(η2-3-DMP)Ti(DMP)4]
(7). The Ti metal center is in a TBP geometry, and the “naked” Tl cation resides unencumbered by solvent molecules
but was found to have a strong π-interaction with four DMP ligands of neighboring Ti(DMP)5- anions. For this
novel set of compounds, 205Tl NMR spectroscopy was used to investigate the solution behavior of these compounds.
Multiple 205Tl resonances were observed for the solution spectra of the crystalline material of 1−6, and a broad
singlet was observed for 7. The large number of minor resonances noted for these compounds was attributed to
sensitivity of the Tl cation based on small variations due to ligand rearrangement. However, the major resonance
noted in the 205Tl NMR solution spectra of 1−7 are in agreement with their respective solid-state structures.