Probing the Mechanistic and Energetic Basis for the Weak-Link Approach to Supramolecular Coordination Complexes

Through the Weak-Link Synthetic Approach, the unsymmetric hemilabile ligand, 1,4-bis(2-diphenylphosphinoethoxy)naphthalene (7), has been used to prepare binuclear Rh(I)
macrocycles in a two-step fashion. The intermediate structure, [(μ2,η1:η4:η1-(1,4-bis(2-(diphenylphosphino)ethoxy)naphthalene))2Rh2][BF4]2 (8), and several macrocyclic complexes,
[(μ2-1,4-bis(2-(diphenylphosphino)ethoxy)naphthalene)2Rh2(CH3CN)4][BF4]2 (9), [(μ2-1,4-bis(2-(diphenylphosphino)ethoxy)naphthalene)2Rh2(CO)6][BF4]2 (10), and [(μ2-1,4-bis(2-(diphenylphosphino)ethoxy)naphthalene)2(CO)2Rh2(CH3CN)2][BF4]2 (11), are reported herein. Additionally, the mononuclear Rh(I) complexes, [(η1:η4:η1-1,4-bis(2-(diphenylphosphino)ethoxy)naphthalene)Rh][BF4] (12) and [(η1:η1-1,4-bis(2-(diphenylphosphino)ethoxy)naphthalene)(CO)Rh(CH3CN)][BF4] (13), were isolated and fully characterized. Single-crystal X-ray
diffraction structures of 8, 9 (cis-, anti-), 11, and 12 were determined. 2-D solution NMR
studies and the solid-state crystal structures of 8 and 12 suggest that the arenes bind to
Rh(I) in predominantly an η4-fashion. The conversion of dimer 8 into monomer 12 was
monitored under a variety of conditions. The kinetic investigation of this process as a function
of temperature with a series of phosphine-based ligands present has afforded the thermochemical parameters of this conversion, which are consistent with an associative mechanism.
Significantly, this work represents the first study aimed at gaining mechanistic insight into
the conversion of the binuclear intermediates into the mononuclear products formed in the
Weak-Link Approach. The high activation barrier and associative nature of this step is what
allows one to efficiently synthesize targeted macrocyclic and higher-ordered structures via
the approach.