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+

Academic Article

Abstract

  • A new metallacrown ether, cis-PdCl2{(2,20-C12H 8O2)P(OCH2CH2)4OP(2,20- O2H8C12)}, 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-C12H8O 2)P(OCH2CH2)4OP(2,20-O 2H8C12) 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 12H8O2)P(OCH2CH2) 4OP(2,20-O2H8C12) 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(Ph2P(CH2CH2O)n- CH 2CH2PPh2) (n = 3-5) metallacrown ethers exhibit both cis-trans and monomer-cyclic oligomer equilibria in solution, and cis-Mo(CO)4 (Ph2P(CH2CH2O) nCH2CH2PPh2) (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-C12H8O2)P(OCH 2CH2)4OP(2,20-O2H8C 12) metallacrown ether has been evaluated using 31P{1H} NMR spectroscopy. A 31P{1H} NMR continuous variation Job experiment of LiB(C6F5)4 3 2Et2O and cis-Mo(CO)4(2,20-C12H8O2)P(OCH 2CH2)4OP(2,20-O2H8C 12) indicates that the binding stoichiometry is 1:1. Accurate binding constants were determined by performing multiple 31P{1H} NMR titrations at varying concentrations of cis-Mo(CO)4(2,20-C 12H8O2)P(OCH2CH2) 4OP(2,20-O2H8C12) using a binding density and McGhee-von Hippel analysis in an optimized mixture of dichloromethane-d2 and tetrahydrofuran. This yielded an observed binding affinity, Kobs, of 0.077 ± 0.005 mM-1 under nonstoichiometric, equilibrium conditions. The observed binding affinity was then related to the concentration of tetrahydrofuran (log Kobs = -3.58 log [THF] + log Ka) 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 ktrans) for all CO ligands by approximately 10%. © 2011 American Chemical Society.
  • Published In

  • Organometallics  Journal
  • Digital Object Identifier (doi)

    Pubmed Id

  • 10204871
  • Author List

  • Sheff JT; Lucius AL; Owens SB; Gray GM
  • Start Page

  • 5695
  • End Page

  • 5709
  • Volume

  • 30
  • Issue

  • 21