Dizirconium Complexes Supported by Preorganized Binucleating Bis(amidinates)

Several new dizirconium complexes have been prepared using preorganized bis(amidinate)
ligands featuring rigid 9,9-dimethylxanthene backbones. Reaction of the bis(amidine)
iPrLXanH2 with 2 equiv of Zr(NMe2)4 forms the hexa(amido) derivative iPrLXanZr2(NMe2)6 (1)
in good yield. In a similar reaction, the unsymmetrical bis(amidine) tBu,EtLXanH2 reacts with
Zr(NMe2)4 to form tBu,EtLXanZr2(NMe2)6 (2) as a 9:1 mixture of C2- and Cs-symmetrical
rotational diastereomers. At elevated temperatures the two diastereomers interconvert, and
the C2-form is slightly preferred thermodynamically (ca. 0.3 kcal/mol). Metathesis reaction
of iPrLXanLi2 with 2 equiv of CpZrCl3 affords the mixed amidinate-cyclopentadienyl anion
derivative iPrLXanZr2Cp2Cl4 (3) in good yield. Solutions of 3 are readily converted to the σ-alkyl
derivatives iPrLXanZr2Cp2Me4 (4) and iPrLXanZr2Cp2(CH2SiMe3)4 (5) by treatment with dialkylmagnesium and organolithium reagents. At slightly elevated temperatures, solutions of 5
react with H2 to form yellow tetrahydride iPrLXanZr2Cp2H4 (6). The solid-state structure of 6
reveals that three of the four hydrido ligands bridge the two metal centers, resulting in a
short intermetal separation of 3.1050(6) Å. In solution, 6 undergoes a rapid fluxional process
(observed by 1H NMR spectroscopy) that exchanges the four hydrides. At low temperature,
three distinct hydrides can be observed, which is consistent with either a dibridged structure
or a tribridged species undergoing rapid exchange of the terminal hydride with only one of
the three bridging hydrides.