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Published on: March 20, 2017
3,3'-Bis(triphenylsilyl)biphenoxide as a sterically hindered ligand on Fe(II), Fe(III), and Cr(II)
1Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA.
Abstract:
The structural coordination chemistry of the sterically hindered 3,3'-bis(triphenylsilyl)-1,1'-bi-2-phenoxide ligand ([(TPS)LO(2)](2)(-)), an isosteric homologue of a chiral binaphthoxide ligand used in asymmetric induction, has been investigated on Fe(II), Fe(III), and Cr(II). The ligand diol ((TPS)L(OH)(2), 2) can be prepared on a multigram scale from 2,2'-dimethoxybiphenyl via a convenient, two-step ortho-metalation/silylation/deprotection sequence; the sodium salt of the ligand dianion (Na(2)[(TPS)LO(2)], 3) can be obtained by NaH deprotonation and was crystallographically characterized with two THF ligands bound to each cation. Protonolysis of Fe[N(SiMe(3))(2)](2) or Cr[N(SiMe(3))(2)](2)(THF)(2) with biphenol 2 in arene solvent gives [Fe(mu-(TPS)LO(2))](2) (4) or ((TPS)LO(2))Cr(THF)(2) (9), respectively, while anion metathesis of FeCl(2) or FeCl(3)/bipy with biphenoxide salt 3 in THF yields ((TPS)LO(2))Fe(THF)(2) (8) or ferric monomer ((TPS)LO(2))FeCl(bipy) (10), respectively. Dimer 4 reacts with exogenous ligands to form monomeric ligand adducts of Fe(II): with py, ((TPS)LO(2))Fe(py)(2) (5); with bipy, ((TPS)LO(2))Fe(bipy) (6); with XyNC (Xy = 2,6-xylyl), ((TPS)LO(2))Fe(CNXy)(4) (7); and with THF, 8. Complexes were characterized in solution by (1)H NMR and in the solid state by single-crystal X-ray diffraction. The 4-coordinate complexes (5, 6, 8, 9) adopt skew-distorted tetrahedral (for Fe(II)) or square planar (for Cr(II)) geometries; the 5- and 6-coordinate complexes (10 and 7) assume more typical distorted square pyramidal/trigonal bipyramidal and cis-octahedral stereochemistries. Dimer 4 possesses an unusual structure where each Fe(II) center is pseudo-4-coordinate and each biphenoxide ligand provides one terminal phenoxide donor, one bridging phenoxide, and a weak aromatic pi-interaction from one of the phenyl groups of a SiPh(3) substituent. The steric influence of the hindered biphenoxide ligand within the coordination sphere is revealed structurally through distortion of coordination polyhedra in the 4-coordinate species and through conformational and deformational changes within the biphenoxide ligand itself.
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