Experimental and Computational Study of the Structure, Steric Properties, and Binding Equilibria of...

Steric properties of crystallographically and computationally determined structures of linear palladium(0) and square planar palladium(II) complexes of di(tert-butyl)neopentylphosphine (P(t-Bu)₂Np), tert-butyldineopentylphosphine (P(t-Bu)Np₂), and trineopentylphosphine (PNp₃) have been determined. Structures of linear palladium(0) complexes show that steric demand increases as tert-butyl groups are replaced with neopentyl groups (P(t-Bu)₂Np < P(t-Bu)Np₂ < PNp₃). In square planar palladium(II) complexes, PNp₃ gives the smallest steric parameters, whereas P(t-Bu)Np₂ has the largest steric demand. The change in the steric demand of PNp₃ compared to P(t-Bu)₂Np and P(t-Bu)Np₂ results from a significant conformational change in PNp₃ depending on the coordination number of the metal. The steric properties of these ligands were also probed by measuring the equilibrium constant for coordination of free phosphine to dimeric [(R₃P)Pd(μ-Cl)Cl]₂ complexes. Binding equilibria follow the same trend as the steric parameters for square planar complexes with PNp₃ having the highest binding constant. In contrast to the normal trend, the neopentylphosphines show increased pyramidalization at phosphorus with increasing steric demand. We hypothesize that this unusual dependence reflects the low back side strain of the neopentyl group, which allows the ligand to be more pyramidalized while still exerting a significant front side steric demand.