Preparation and characterization of immobilized constrained-geometry hafnium complexes

A Tris(pyrazolyl) η6-Arene Ligand That Selects Cu(I) over Cu(II)

1,3,5-Tris{2'-[(pyrazol-1-yl)methyl]phenyl}benzene, 4, and its complexes with Cu(I) and Ag(I) have been prepared and characterized. Both CuI4 and AgI4 triflate crystallize in the rhombohedral space group R3, with the cations and anions each exhibiting crystallographically imposed 3-fold (C3) symmetry. In both complexes, 4 behaves as a tris(pyrazolyl) eta6-arene ligand whose arms act as three-pronged tweezers to form chiral, propeller-like cations with pyramidal MN(pyrazole)3 coordination geometries. Centers of symmetry in the space group ensure that the crystals are racemates, with equal numbers of P,P,P and M,M,M enantiomers. In broad outline, each cation is shaped like a three-legged stool, with the metal ion centered at the top and pointed downward from a triangular N(pyrazole) plane toward the center of gravity (Cg) of the central benzene ring (a metal-endo conformation), which constitutes the bottom shelf of the stool. The Cu(I)...Cg and Ag(I)...Cg distances, 3.195(2) and 3.165(2) A, respectively, support the existence of an eta6 bonding interaction with Ag(I) and, to a lesser extent, with Cu(I). NMR data for AgI4 suggest rapid interconversion of this cation in solution between P,P,P and M,M,M enantiomers. Our inability to prepare any Cu(II) complexes with 4 is consistent with cyclovoltammetric results, which suggest that the ligand is more easily oxidized than Cu(I).

A Geometrically Constraining Bis(benzimidazole) Ligand and Its Nearly Tetrahedral Complexes with Fe(II) and Mn(II)

2,2'-Bis[2-(1-propylbenzimidazol-2-yl)]biphenyl), 4, and its bis complexes with Fe(II) and Mn(II) have been prepared and characterized structurally and spectroscopically. Ligand 4 adopts an open, "trans" conformation in the solid state with the benzimidazole (BzIm) groups on opposite sides of the biphenyl unit. In its complexes with metal ions, a "cis" conformation is observed, and 4 behaves as a geometrically constraining bidentate ligand with four planar groups connected by three "hinges". Reaction of 4 with Fe(II) or Mn(II) yielded isomorphous crystals (space group Pnn2) of Fe(II)(4)2.(ClO4)2 and Mn(II)(4)2.(ClO4)2, in which the M(II)(4)2 cations exhibit distorted-tetrahedral coordination geometries (N-M-N angles, 109 +/- 11 degrees ) enforced by rigid, chiral nine-membered M(4) rings in the twist-boat-boat conformation. Individually, the cations show R,R or S,S stereochemistry, and the crystals are racemates. Mn(II)(4)2.(ClO4)2 exhibits a quasi-reversible Mn(II) --> Mn(III) oxidation at E(1/2) = 0.64 V; the corresponding Fe(II) --> Fe(III) oxidation occurs at E(1/2) = 1.76 V. The electrochemical stability of the Fe(III) oxidation state in this system suggests the possibility of isolating an unusual pseudotetrahedral Fe(III)N(BzIm)(4) species. Ultraviolet spectra of the iron and manganese complexes are dominated by absorptions of the ligand 4 blue-shifted by approximately 2000-3000 cm(-1). Ligand-field absorptions were observed for the Fe(II) complex; those for the Mn(II) complex were obscured by tailing ultraviolet absorptions. Electron paramagnetic resonance and magnetic susceptibility measurements are consistent with a high-spin Mn(II) complex, while for the Fe(II) complex, the falloff of the magnetic moment with decreasing temperature is indicative of zero-field splitting with D approximately 4 cm(-1).