Advances in Geminal Organodimetallics: Synthesis, Structural Characterization, and Reactivity

The geminal organodimetallic complexes [{{Ph2P(Me3Si)N}2C}2M4], L2M4, where M4 = Na4; Li2Na2; LiNa3; Li2K2; Na2K2; Na3K, have been prepared through a variety of methods including: direct or sequential deprotonation of the neutral ligand with strong bases (t-BuLi, n-BuNa, (Me3Si)2NNa, PhCH2K or (Me3Si)2NK), transmetallation of the homometallic derivatives (M4 = Li4 or Na4) with t-BuONa or t-BuOK, and by cation exchange upon mixing the homometallic complexes in arene solution. The dianionic complexes have been characterized by single-crystal X-ray diffraction and found to form a homologous series of dimeric structures in the solid-state, in accord with the previously reported structure of L2Li4. Each complex is composed of an M4 plane of metals, with the ligands adopting capping positions to form distorted M4C2 octahedral cores. The metals in homometallic complexes L2Li4 and L2Na4 define an approximate square, whereas the heterometallic derivatives have distinctly rhombic arrangements. The lighter metals in these heterometallic compounds interact strongly with the carbanions and the heavier metals are pushed towards the periphery of the structures. 1H, 13C, 7Li, 31P and 29Si multinuclear NMR spectroscopic studies, cryoscopic measurements, and electrospray ionization-mass spectroscopic studies are consistent with the dimers being retained in solution. Dynamic solution behavior was discovered for L2Li2Na2, where all five possible tetrametallic derivatives L2Li4, L2Li3Na, L2Li2Na2, L2LiNa3 and L2Na4 coexist. Density functional theory (DFT) and natural bond order (NBO) calculations in association with natural population analyses (NPA) reveal significant differences in the electronic structures of the variously metallated dianions. The smaller cations are more effective in localizing the double negative charge on the carbanion (in the form of two lone pairs), leading to differences in the distribution of the electron density within the ligand backbones. In turn, a complex interplay of hyperconjugation, electrostatics and metal-ligand interactions is found to control the resulting electronic structures of the geminal organodimetallic complexes. The lithium-based geminal organodimetallic [{{Ph2P(Me3Si)N}2CLi2}2], L2Li4, undergoes major structural changes in polar media. Upon dissolution of L2Li4 in THF, a new charge-separated species [L2Li3]-[Li(thf)4]+ is immediately formed. In turn, the ion pair slowly (hours) undergoes dismutation to form a neutral monomer LLi2(thf)n. The identities of these two solution complexes were confirmed through a series of experiments including variable concentration Keq measurements, 7Li NMR, and diffusion-ordered NMR. The ion pair [L2Li3]-[Li(thf)4]+ was also crystallized from THF solution, and the structure was elucidated in the solid state. Attempts to further charge separate the lithium cations from the ligand backbone with additives such as 12-crown-4 or HMPA generally resulted in decomposition of the dianion system. Density Functional Theory calculations of L2Li4, [L2Li3]-[Li(thf)4]+, and LLi2(thf)n were also performed. Two possible structures for the monomer, LLi2(thf)3 and LLi2(thf)4, were found and are different in energy by only 1.41 kcal/mol. 1H NMR rate studies of the [L2Li3]-[Li(thf)4]+ to LLi2(thf)n equilibration process were performed (DHÌ¢‰âÂåÁ = 12.6 Ìâå± 0.5 kcal/mol, DSÌ¢‰âÂåÁ = -33.6 Ìâå± 1.8 kcal/mol̢蠉ã¢K). We furthermore explored the reactivities of [L2Li3]-[Li(thf)4]+ and LLi2(thf)n. Addition of the protic amine (Me3Si)2NH to an equilibrium mixture of the two complexes at room temperature showed immediate consumption of LLi2(thf)4 (forming LiN(SiMe3)2(thf)n and L(H)Li(thf)n) followed by the slow disappearance of [L2Li3]-[Li(thf)4]+. Room-temperature rate studies using 1H NMR spectroscopy reveal first-order decay of [L2Li3]-[Li(thf)4]+ with rate constants independent of the concentration of (Me3Si)2NH. This decay of [L2Li3]-[Li(thf)4]+ primarily occurs through dismutation to LLi2(thf)n (which in turn reacts with (Me3Si)2NH). 1H NMR rate studies could be directly performed on the reaction between LLi2(thf)n and (Me3Si)2NH by performing experiments in the temperature range of -75 to -45 Ìâå¼C. The observed rates for LLi2(thf)n from measurements between -75 and -45 Ìâå¼C were extrapolated to 25 Ìâå¼C. Comparison of the reaction rates of LLi2(thf)n and [L2Li3]-[Li(thf)4]+ provide a reactivity ratio on the order of 8x105!!!