Hm. Gau et al., CHEMISTRY OF TI(OIPR)CL-3 WITH CHLORIDE AND OXYGEN-CONTAINING LIGANDS- THE ROLES OF ALKOXIDE AND SOLVENTS IN THE 6-COORDINATE TITANIUM COMPLEXES, Journal of the American Chemical Society, 118(12), 1996, pp. 2936-2941
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 Ph
CHO (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 anoth
er 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 dissolvi
ng in THF. 13 was found to react with 2 mol equiv of HNEt(3)Cl or PhCH
O to give 12 and Ti((OPr)-Pr-i)Cl-3(PhCHO)(THF) (14), respectively. Th
e molecular structures of 7b, 8, and 10-13 show short Ti-(OPr)-Pr-i di
stances, 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 st
ate structures. This bonding sequence is very useful for the predictio
n of the geometry for six-coordinate complexes of early transition met
als with the following principle: The strongest ligand prefers a trans
position to the weakest ligand, and the second strongest ligand favor
s a trans position to the second weakest ligand in the complex. Kineti
cally, the trans position to the isopropoxide is rather labile for sub
stitution, and the lability of the trans ligand ensures the effectiven
ess of titanium alkoxides for subsequent reactions or as catalysts in
many asymmetric organic syntheses.