Lewis base adducts derived from transfer hydrogenation catalysts: Scope and selectivity

The coordination tendencies of the unsaturated 16e Lewis acid [Cp*Ir(TsDPEN)]⁺ ([1H]⁺), where TsDPEN is H ₂NCHPhCHPhNTs^-, are surveyed, together with parallel studies on analogous complexes such TsDACH (TsDACH = H₂NC ₆H₁₀NTs^-) and Tsen (Tsen = H₂NC ₂H₄NTs^-) derivatives as well as Rh analogues. Crystallographic analyses of the adducts of [Cp*IrL(TsDPEN)]⁺, where L = NCMe, NH₃, PPh₃, and CO, and [Cp*Ir(CO)(R,R-TsDACH)]⁺ are described. In the TsDPEN system, the Lewis base adducts contain an absolute configuration that is opposite that for the TsDPEN ligand and feature equatorial phenyl groups. In the case of [Cp*Ir(CO)(R,R-TsDACH)]⁺, both R and S metal centers cocrystallize. Isomerization of the R to the S metal center was first order in [Cp*(R-Ir)(CO)(R,R-TsDACH)]⁺ with minimal solvent effects. The pK_a of the amine of the Lewis base adducts correlated linearly with the pK_a of the free ligand in MeCN and the pK_a of the amine (H₂NCHPhCHPhNTs) of the Lewis base adduct in MeCN. Amines with pK_a < 16 (MeCN scale), in the absence of additional hydrogen bonding to the TsDPEN ligand set, do not to bind to [1H]⁺, whereas bulky bases with pK_a > 20 deprotonated the iridium amine.