Generally applicable NMR titration methods for the determination of equilibrium constants for coordination complexes: Syntheses and characterizations of metallacrown ethers with α,ω-bis(phosphite)- Polyether ligands and determination of equilibrium binding constants to li+

摘要

A new metallacrown ether, cis-PdCl_2{(2,20-C_(12)H _8O_2)P(OCH_2CH_2)_4OP(2,20- O_2H_8C_(12))}, has been synthesized and fully characterized by NMR spectroscopy and X-ray crystallography. A new synthetic method and complete NMR characterization for the previously reported and closely related cis-Mo(CO)_4(2,20-C_(12)H_8O _2)P(OCH_2CH_2)_4OP(2,20-O _2H_8C_(12)) metallacrown ether are also reported. Both complexes are monomeric, exhibiting cis coordination geometries both in solution and in the solid state, and the cis-Mo(CO)_4(2,20-C _(12)H_8O_2)P(OCH_2CH_2) _4OP(2,20-O_2H_8C_(12)) metallacrown ether does not appear to undergo cis-trans isomerization in the presence of HgCl _2. These results are surprising because the closely related PdCl _2(Ph_2P(CH_2CH_2O)_(n-) CH _2CH_2PPh_2) (n = 3-5) metallacrown ethers exhibit both cis-trans and monomer-cyclic oligomer equilibria in solution, and cis-Mo(CO)_4 (Ph_2P(CH_2CH_2O) _nCH_2CH_2PPh_2) (n = 3-5) metallacrown ethers rapidly undergo cis-trans isomerization in the presence of HgCl _2. The coordination of Li~+ cations to the cis-Mo(CO) _4(2,20-C_(12)H_8O_2)P(OCH _2CH_2)_4OP(2,20-O_2H_8C _(12)) metallacrown ether has been evaluated using ~(31)P{1H} NMR spectroscopy. A ~(31)P{~1H} NMR continuous variation Job experiment of LiB(C_6F_5)_4 3 2Et_2O and cis-Mo(CO)_4(2,20-C_(12)H_8O_2)P(OCH _2CH_2)_4OP(2,20-O_2H_8C _(12)) indicates that the binding stoichiometry is 1:1. Accurate binding constants were determined by performing multiple ~(31)P{~1H} NMR titrations at varying concentrations of cis-Mo(CO)_4(2,20-C _(12)H_8O_2)P(OCH_2CH_2) _4OP(2,20-O_2H_8C_(12)) using a binding density and McGhee-von Hippel analysis in an optimized mixture of dichloromethane-d_2 and tetrahydrofuran. This yielded an observed binding affinity, K_(obs), of 0.077 ± 0.005 mM~(-1) under nonstoichiometric, equilibrium conditions. The observed binding affinity was then related to the concentration of tetrahydrofuran (log K_(obs) = -3.58 log [THF] + log K_a) using a solvent back-titration, where the negative slope, 3.58, represents the average number of tetrahydrofuran molecules displaced from the alkali cation upon coordination to the metallacrown ether. An FT-IR titration was also performed by monitoring ligand carbonyl stretching frequencies under high affinity, stoichiometric conditions. This study developed and demonstrated the use of solvent back-titrations and that cation binding to the metallacrown ether reduces π-back-bonding between the Mo metal and the CO ligands, increasing the Cotton-Kraihanzel stretching force constants (k _(cis) and k_(trans)) for all CO ligands by approximately 10%.