Reaction of Ketones with 1,4-Diaza-1,3-diene Zirconium and Hafnium Complexes: First Example of a 1,3-Dipolar Cycloaddition Reaction of 1,4-Diaza-1,3-diene Complexes of Early Transition Metals

The novel complexes
M[O(R)PhCH{CHN(tBu)}N(tBu)]2
[M = Zr, R = Me (4a), R = Ph (4b) and M
= Hf,
R = Me (5a), R = Ph (5b)] have been prepared
in almost quantitative yield by reaction of
(tBu-DAD)2Zr (1) and
(tBu-DAD)2Hf (2) [tBu-DAD
= (tBu)NCHCHN(tBu)] with 2 equiv of
the ketones MeCOPh (3a) or PhCOPh
(3b). The reaction proceeds via a 1,3-dipolar
cycloaddition of the CO bond across the MNC unit of
the
DAD complexes. The molecular structures of the complexes
4a and 5b have been determined by a
single-crystal
X-ray diffraction study (4a, triclinic, space group
P1̄; a = 10.395(2) Å, b
= 10.865(2) Å, c = 16.842(3) Å, α
=
93.80(3)°, β = 99.84(3)°, γ = 106.12(3)°,
V = 1787.4(6) Å3, Z = 2, R1
= 0.035 (wR2 = 0.101) for 6963
reflections with I > 2σ(I);
5b, monoclinic, space group
P21/c, a =
19.961(4) Å, b = 10.482(2) Å, c =
20.150(4)
Å, β = 91.30(1)°, V = 4215(1)
Å3, Z = 4, R1 = 0.036 (wR2 =
0.097) for 4650 reflections with I >
2σ(I)). The
metal atoms in 4a and 5b adopt a pseudooctahedral
coordination sphere consisting of four nitrogen and two
oxygen atoms. The distortion is a consequence of the [2.2.1]
bicyclic structure of the newly formed tridentate
ligands which are not able to span three regular octahedral
positions.