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)]center dot CH2Cl2 (2c), depending on the reaction conditions. The dication in the hydrolysis product [Tl-4(mu(3)-OH)(2)][H2N{B(C6F5)(3)}(2)](2)center dot 4CH(2)Cl(2) consists of two bridging and two terminal Tl+ ions bound to triply bridging hydroxides. Heating Et2O complexes in toluene afforded [Tl(eta(6)-toluene)(n)][H2N{B(C6F5)(3)}(2)] (4, n = 2, 3), while C6Me6 addition gave the first thallium-C6Me6 adduct, [Tl(eta(6)-C6Me6)(2)][H2N{B(C6F5)(3)}(2)]center dot 1.5CH(2)Cl(2) (5a), a bent sandwich complex with very short Tl center dot center dot center dot centroid distances. These arene complexes show no close contacts between cations and anions. Displacement of toluene ligands by ferrocene gave [Tl-2(FeCp2)(3)][H2N{B(C6F5)(3)}(2)](2)center dot 5CH(2)Cl(2) (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)]center dot 2CH(2)Cl(2) (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 degrees, 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.