3,3‘-Bis(triphenylsilyl)biphenoxide as a Sterically Hindered Ligand on Fe(II), Fe(III), and Cr(II)

The structural coordination chemistry of the sterically hindered 3,3‘-bis(triphenylsilyl)-1,1‘-bi-2-phenoxide ligand
([TPSLO2]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 (TPSL(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 (Na2[TPSLO2], 3) can be obtained by NaH deprotonation and was crystallographically
characterized with two THF ligands bound to each cation. Protonolysis of Fe[N(SiMe3)2]2 or Cr[N(SiMe3)2]2(THF)2
with biphenol 2 in arene solvent gives [Fe(μ-TPSLO2)]2 (4) or (TPSLO2)Cr(THF)2 (9), respectively, while anion metathesis
of FeCl2 or FeCl3/bipy with biphenoxide salt 3 in THF yields (TPSLO2)Fe(THF)2 (8) or ferric monomer (TPSLO2)FeCl(bipy) (10), respectively. Dimer 4 reacts with exogenous ligands to form monomeric ligand adducts of Fe(II):  with
py, (TPSLO2)Fe(py)2 (5); with bipy, (TPSLO2)Fe(bipy) (6); with XyNC (Xy = 2,6-xylyl), (TPSLO2)Fe(CNXy)4 (7); and
with THF, 8. Complexes were characterized in solution by 1H 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 π-interaction from one of the phenyl groups of a SiPh3 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.