Thallium(I) Sandwich, Multidecker, and Ether Complexes Stabilized by Weakly-Coordinating Anions: A Spectroscopic, Structural, and Theoretical Investigation

The reaction of thallium ethoxide with [H(OEt2)2][H2N{B(C6F5)3}2] in diethyl ether afforded [Tl(OEt2)3][H2N{B(C6F5)3}2] (2a), [Tl(OEt2)4][H2N{B(C6F5)3}2] (2b), or [Tl(OEt2)2][H2N{B(C6F5)3}2]·CH2Cl2 (2c),
depending on the reaction conditions. The dication in the hydrolysis product [Tl4(μ3-OH)2][H2N{B(C6F5)3}2]2·4CH2Cl2 consists of two bridging and two terminal Tl+ ions bound to triply bridging hydroxides. Heating
Et2O complexes in toluene afforded [Tl(η6-toluene)n][H2N{B(C6F5)3}2] (4, n = 2, 3), while C6Me6 addition
gave the first thallium−C6Me6 adduct, [Tl(η6-C6Me6)2][H2N{B(C6F5)3}2]·1.5CH2Cl2 (5a), a bent sandwich
complex with very short Tl···centroid distances. These arene complexes show no close contacts between
cations and anions. Displacement of toluene ligands by ferrocene gave [Tl2(FeCp2)3][H2N{B(C6F5)3}2]2·5CH2Cl2 (6) which contains the multidecker cations [Tl(FeCp2)]+ and [Tl(FeCp2)2]+ in a 1:1 ratio. By contrast,
decamethylferrocene leads to electron transfer; the isolable thallium−ferrocene complexes may therefore
be viewed as precursor complexes for this redox step. With 18-crown-6 the complexes [Tl(18-crown-6)2][H2N{B(C6F5)3}2] (11a) and [Tl(18-crown-6)][H2N{B(C6F5)3}2]·2CH2Cl2 (11b) were isolated. The structure
of the latter shows an eight-coordinate thallium ion, where the coordination to the six oxygen donors in
equatorial positions is completed by axial contacts to two F atoms of the counter anions. The bonding
between thallium(I) and arenes was explored by density-functional theory (DFT) calculations. The optimized
geometry of [Tl(tol)3]+ converged to a structure very similar to that obtained experimentally. Calculations
on [Tl(C6Me6)2]+ (5b) to establish whether a linear or bent geometry is the most stable revealed a very flat
potential-energy surface for distortions of the Ctr(3)−Tl−Ctr(4) angle. Overall, there is very little energetic
preference for one particular geometry over another above about 140°, in good agreement with the
crystallographic geometry. The calculated Tl−arene interaction energies increase from 73.7 kJ mol-1 for
toluene to 121.7 kJ mol-1 for C6Me6.