2 Homometallic Alkoxides

560 Alkoxo and Aryloxo Derivatives of Metals
Table 6.23 Hafnium aryloxides
Bond Bond
length ( ˚A) angle ⊲Ž⊳ Compound Aryloxide M–O M–O–Ar Ref.
[Hf(OAr) 3Cl] OC6H3But tetrahedral Hf
2-2,6 1.925 (2)
1.917 (3)
152
156
i, ii
1.938 (3) 160
[Cp2Hf(OAr) 2]
pseudo-tetrahedral Hf
OC6H3Cl2-2,6 2.004 (7)
2.022 (8)
147
149
iii
[Hf(OAr) 2⊲CH2py-6-Me) 2 ] OC6H3But 6-coord. Hf
2-2,6 1.996 (3)
2.190 (1)
173
175
iv
[Hf(OAr) 2⊲ 2-PhNDCMe⊳2] OC6H3But 6-coord. Hf
2-2,6 1.978 (4) 171 v
[Hf(di-OAr)Cl2⊲THF⊳] 7-coord. Hf, trans arrangement of Cl
N,N0-o-phenylene bis(salicylaldiminato)
2.008 (3)
2.048 (4)
143
140
vi
[Hf(OAr) 2⊲CH2Ph⊳⊲THF⊳][BPh4]
6-coord. Hf cation
OC6H4⊲c-
CCOCH2CMe2N⊳-2
2.01 (1)
1.99 (1)
146
137
vii
iL.R. Chamberlain, J.C. Huffman, J. Keddington, and I.P. Rothwell, J. Chem. Soc., Chem.
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24, 995 (1985).
iiiDou Shiqi and Chen Shoushan, Gaodeng Xuexiao Huaxue Xuebao (Chem. J. Chin. Uni.), 5,
812 (1984).
ivS.M. Beshouri, P.E. Fanwick, I.P. Rothwell, and J.C. Huffman, Organometallics, 6, 2498
(1987).
vL.R. Chamberlain, L.D. Durfee, P.E. Fanwick, L. Kobriger, S.L. Latesky, A.K. McMullen, I.P.
Rothwell, K. Folting, J.C. Huffman, W.E. Streib, and R. Wang, J. Am. Chem. Soc., 109, 390
(1987).
viF. Corazza, E. Solari, C. Floriani, A. Chiesi-Villa, and C. Guastini, J. Chem. Soc., Dalton
Trans., 1335 (1990).
viiP.G. Cozzi, E. Gallo, C. Floriani, A. Chiesi-Villa, and C. Rizzoli, Organometallics, 14, 4994
(1995).
The dichlorides [(ArO)2TiCl2] are an important class of starting materials for entry
into the organometallic chemistry of titanium. 358 For 2,6-disubstituted-phenoxides,
structural studies show a monomeric, pseudo-tetrahedral geometry in the solid state
(Table 6.21). The dichlorides themselves can be used as Lewis acid catalysts for
Diels–Alder reactions. 359 Careful alkylation can lead to the corresponding dialkyls,
[(ArO)2TiR2] (RD Me, benzyl). 360,361 An alternative approach involves addition of
the parent phenol to the alkyl substrate [TiR4] in hydrocarbon solvent. For R D Me,
Ph these alkyls are generated in situ whereas for R D CH2Ph, CH2SiMe3 the relatively
stable tetra-alkyls can be isolated. The stoichiometry of the product depends strongly
on reaction conditions, the bulk of the aryloxide and nature of the alkyl group.
In the case of zirconium and hafnium the lack of accessible dichlorides means that
alkyl derivatives are generated via the corresponding homoleptic alkyl intermediates.
With bulky 2,6-di-tert-butylphenoxide the tris-benzyls [(ArO)ZrR3] can be isolated
and studied. 134 The dialkyls [(ArO)2MR2] (MD Zr, Hf; R D Me, benzyl) can also
be isolated for bulky aryloxides. In the case of 2,6-diphenylphenols the monomethyl
species [(ArO)3ZrMe] are readily formed by simple addition of parent phenol to the
dimethyl compound.