Chemistry of Ti(OiPr)Cl-3 with chloride and oxygen-containing ligands: The roles of alkoxide and solvents in the six-coordinate titanium complexes

Abstract

Ti((OPr)-Pr-i)Cl-3 reacts easily with various ligands to form a series of six-coordinate complexes, [Ti((OPr)-Pr-i)Cl-5](2-)(HAm+)(2) (Am = NEt(3) (7a) or NC5H5 (7b)), Ti((OPr)-Pr-i)Cl(3)L(2) (L = THF (8) or PhCHO (9)), Ti((OPr)-Pr-i)-Cl-3(PhCHO)(Et(2)O) (10), and [Ti((OPr)-Pr-i)Cl-2(mu-Cl)(PhC(O)OMe)](2) (11). Upon dissolution of 7a in THF, 8 was obtained. When 1 mol equiv of HNEt(3)Cl was added to 8, [Ti((OPr)-Pr-i)Cl-4(THF)](-)(HNEt(3))+ (12) was obtained. With the addition of another 1 mol equiv of HNEt(3)Cl, 12 was converted to 7a. One THF in 8 can be removed in vacuo to give the chloride-bridged dimer [Ti((OPr)-Pr-i)Cl-2(mu-Cl)(THF)](2) (13) which can be converted back to 8 by dissolving in THF. 13 was found to react with 2 mol equiv of HNEt(3)Cl or PhCHO to give 12 and Ti((OPr)-Pr-i)Cl-3(PhCHO)(THF) (14), respectively. The molecular structures of 7b, 8, and 10-13 show short Ti-(OPr)-Pr-i distances, and the relative bonding order of (OPr)-O---Pr-i > Cl-, THF > Et(2)O > PhCHO > mu-Cl- > RC(O)OMe is discussed based on the solid state structures. This bonding sequence is very useful for the prediction of the geometry for six-coordinate complexes of early transition metals with the following principle: The strongest ligand prefers a trans position to the weakest ligand, and the second strongest ligand favors a trans position to the second weakest ligand in the complex. Kinetically, the trans position to the isopropoxide is rather labile for substitution, and the lability of the trans ligand ensures the effectiveness of titanium alkoxides for subsequent reactions or as catalysts in many asymmetric organic syntheses.