A series of new titanium complexes of the general type Ti(mbmp)X2 (X = Br, I; OMe, OiPr, OtBu), containing a bidentate bis(phenoxy) ligand derived from 2,2′-methylenebis(6-tert-butyl-4-methylphenol) (mbmpH2), was prepared. The 1H-chemical shifts for the protons of the methylene link were found to vary considerably as a function of the ligand X. Reaction of Ti(OiPr)4 with two equivalents of mbmpH2 gave the C2-symmetric spiro-titanate complex Ti(mbmp)2. The crystalline labile solvent adducts of the dichloro complex Ti(mbmp)Cl2 with THF and DME were isolated and characterized. A single-crystal X-ray structural analysis revealed the THF adduct to be a mono(THF) adduct with a trigonal-bipyramidal configuration, the bidentate mbmp ligand spanning an apical and equatorial site and the THF ligand occupying the remaining apical position.

Synthesis and Characterization of Mononuclear Titanium Complexes Containing a Bis(phenoxy) Ligand Derived from 2,2′-Methylene-bis(6-tert-butyl-4-methylphenol)

The synthesis and characterization of 2-{1-{3,5-bis(1,1-dimethylethyl)-2-{[2,4,8,10-tetrakis(1,1-dimethylethyl)dibenzo[d,f][1,3,2]dioxaphosphepin-6-yl]oxy}phenyl}ethyl}-4,6-bis(1,1-dimethylethyl)phenyl diphenyl phosphite (6) is described. In the 31P-NMR spectrum (1H-decoupled) of 6, an unprecedented eight-bond P,P coupling of J = 72.8 Hz is observed. In the X-ray crystal structure of 6, an intramolecular P–P distance of 3.67 A is found, which is within the sum of the van-der-Waals radii of the P-atoms. The observed intramolecular P–P distance suggests that a through-space coupling mechanism is operative. The solid-state conformation of 6 is compared to the conformation obtained by semi-empirical MO geometry optimizations (PM3 method). The calculated geometry suggests that the solid-state structure is near a true energy minimum, but that crystal-packing forces decrease the intramolecular P–P distance in the solid state. In the absence of crystal-packing forces, however, the collisional and vibrational energy available in solution may lead to the population of states with a shortened intramolecular P–P distance in 6. The proximity of the P-atoms in 6 is due to restricted conformational freedom resulting from steric congestion within the molecule. The free energy of activation (ΔG* = 10.2 and 10.8 kcal/mol for unequal populations of exchanging conformers) for ring inversion of the dibenzo[d,f][1,3,2]dioxaphosphepin ring in 6 is determined by variable-temperature 31P-NMR spectroscopy. Semi-empirical MO calculation on model compounds suggest that the structure of the transition state for ring inversion has the two aryl rings and O-atoms in a common plane, with the P-atom lying above this plane.

Sterically Congested Phosphite Ligands: Synthesis, crystallographic characterization, and observation of unprecedented eight-Bond 31P,31P coupling in the 31P-NMR spectra

A Sensitive Spectroscopic Probe for Monitoring Changes in the Coordination Sphere of Titanium: Eight-Membered Dioxatitanacycles and Their Organometallic Derivatives†

12H-dibenzo[d,g][1,3,2]dioxaphosphocins: Synthesis, structures and “through-space” spin-spin coupling in their NMR spectra

The neutral pentacoordinate Ge complexes 4 and 5 were prepared and characterized by crystallographic and NMR spectra analysis. The transannular Se atoms in the eight-membered rings of both 4 and 5 are bonded to Ge. The geometry at Ge is displaced 50.3% from tetrahedral to trigonal bipyramidal in 4, and 83.9% to octahedral in 5.

Neutral Pentacoordinate and Hexacoordinate Germanium(IV) Complexes: Valence Expansion At Gemanium By Transannular Bonding Of Selenium in an Eight-Membered Ring1

The syntheses of novel bridged dibenzo[d,f][1,3,2]dioxaphosphepin and dibenzo[d,g][1,3,2]dioxaphosphocin ring systems are described. The 1H NMR spectral data of the eight-membered bridged dioxaphosphocins suggest the observation of a single conformational isomer. The 1H NMR spectrum of the dibenzo[d,g][1,3,2]dioxaborocin 5 requires that either a single nonplanar conformation is being observed or that ring inversion is slow on the NMR time scale. A lower limit of δG ‡ for ring inversion of 5 has been calculated to be 14.4 Kcal/mole. The 1H NMR of the C-12 methyl substituted dioxaphosphocins 4h-i showed the presence of a cis-trans isomer mixture.

Dario Bini

Papers

The Reaction of Seven- and Eight- Membered Cyclic Phosphorochloridites with Polyols

Conformation of seven- and eight-membered organosilicon heterocycles in solution

Bridged heterocycyles: the reaction of seven- and eight-membered cyclic phosphorochloridites with polyols