Organotransition-Metal Metallacarboranes. 44. Construction of Pentadecker and Hexadecker Sandwiches from Triple-Decker Building Blocks

Directed syntheses of multidecker sandwich complexes of the title classes, incorporating C2B3 bridging rings and C₅Me₅ end rings, have been achieved based on the tailored triple-decker synthon Cp*Co(Et₂C₂B₃H₂Me)-Co(Et₂C₂B₃H₃)^2– (1^2–), the neutral (diprotonated) form of which is described in the preceding paper. Three-way reactions of 1^2–, the double-decker anions Cp*Co(Et₂C₂B₃H₂-5-A)^2– (A = H or Me), and Co²⁺ ion in cold THF, with workup of the products in air, afforded the Co₄ pentadecker sandwiches Cp*Co(Et₂C₂B₃H₂A)M(Et₂C₂B₃H₃)-CoH(Et₂C₂B₃H₂Me)CoCp* (3, M = Co, A = H;4, M = CoH, A = Me; 5, M = Co, A = Me). A similar reaction employing Ni²⁺ gave the heterometallic Co₃Ni pentadecker [Cp*Co(Et₂C₂B₃H₂Me)]₂NiCoH(Et₂C₂B₃H₃) (6). Hexadecker sandwiches were obtained via treatment of l^2– or 1^– with Co²⁺ or Pt²⁺ ions, generating respectively the Co₅ species [Cp*Co(Et₂C₂B₃H₂Me)Co(Et₂C₂B₃H₃)]₂H_xCo (7, x = 1; 8, x = 2) and the Co₄Pt complex [Cp*Co-(Et₂C₂B₃H₂Me)Co(Et₂C₂B₃H₃)]₂Pt (9). The pentadecker and hexadecker products were obtained as black, moderately air-stable solids in 5-20% individual isolated yields and characterized from their ESR or NMR, UV–visible, and mass spectra, elemental analysis, and an X-ray crystal structure determination on 8. Except for diamagnetic 4, all of these complexes are paramagnetic as shown by ESR spectroscopy. In several species the unpaired electrons appear to be extensively delocalized, while in others they are restricted to the region of the interior metal atoms. ESR and electronic spectra are consistent with assignments of the pentadeckers 3, 4, and 5 as 53, 54, and 53 valence electron (ve) systems, respectively, with each of the 53 ve species having one unpaired electron. Compound 6, though a 54 ve system isoelectronic with 4, is nevertheless paramagnetic. ESR data on the hexadecker sandwiches 7 and 9 are consistent with 64 ve systems having two unpaired electrons, and strong spin–spin interaction is indicated between neighboring Co(IV) centers in 7. Complex 8, with two CoH protons, is formulated as a 65 ve species with one unpaired electron and is the largest multidecker sandwich to be confirmed by X-ray crystallography. The stack is somewhat bowed, with Co–Co–Co angles that deviate by 10–12° from linearity. One of the two cobalt-bound protons was located and refined, and was revealed to be an η¹-H ligand on Co, with no apparent bonding interaction with other cluster atoms. Crystal data for 8: space group P1; a = 13.434(2) Å, b = 17.418(3) Å, c = 13.430(2) Å, α = 100.82(1)°, β = 97.11(1)°, γ = 102.65(1)°; Z = 2; R = 0.069 for 4690 independent reflections having I > 3σ(I).