The facile formation of 7 from the reaction of Cp2ZrMe2
and 3 is thought to be initiated by interaction of the Lewis
acidic B center with the Zr-bound methyl group. This prompts
simultaneous formation of MeBpin and transfer of the boryl-
oxide ligand to Zr. Presumably this process repeats to give 7.
All attempts to intercept the intermediate in this process were
unsuccessful. This chemistry reflects both the acidity of the B
centers and the reactivity of the B–O bonds in 3. Thus, while
the reactivity shown herein affords a new synthetic route to
a B-based anion, the lability of the B–O bonds makes these
boryloxides unsuitable for use as activators or non-coordinating
anions. Efforts are underway to utilize this unique synthetic
route to prepare related boryloxide salts in which the B–O bond
strengths are enhanced by the introduction of electronically
favorable and sterically demanding substituents. The results of
these efforts will be reported in due course.
Fig. 3 ORTEP drawing of 7, 30% thermal ellipsoids are shown.
Hydrogen atoms are omitted for clarity, one orientation of the disor-
˚
dered chelate carbons are shown. Selected distances (A) and angles
(◦): Zr(1)–O(4) 2.011(9), Zr(1)–O(1) 2.025(8), O(1)–B(1) 1.291(16),
O(2)–B(1) 1.311(17), O(3)–B(1) 1.400(17), O(4)–B(2) 1.280(15),
O(5)–B(2) 1.360(16), O(6)–B(2) 1.357(16); O(4)–Zr(1)–O(1) 97.6(3),
B(1)–O(1)–Zr(1) 156.0(9), B(2)–O(4)–Zr(1) 154.2(9), O(2)–B(1)–O(3)
108.6(13), O(5)–B(2)–O(6) 108.3(12).
Acknowledgements
˚
˚
Zr–O bond distances of 2.011(9) A and 2.025(8) A with a
O–Zr–Oꢀ angle of 97.6(3)◦. In comparison, the Zr–O bond
distances and O–Zr–Oꢀ bond angles in the zirconocene–alkoxide
species, Cp2Zr(l-OCH2CMe2CH2O)2ZrCp2 and Cp2Zr(l◦-
Financial support from NSERC of Canada, NOVA Chemicals
Corporation is gratefully acknowledged. SH is grateful for the
award of an OGS scholarship.
˚
˚
OCH2C6H4CH2O)2ZrCp2 are 1.945(6) A, 1.946(6) A, 101.4(3)
and 99.4(1)◦, respectively.55 The longer Zr–O bond length in
7 is consistent with the presence of the Lewis acidic boron
center. The larger angles at Zr in the zirconocene–alkoxides may
be an artifact of the macrocyclic nature of these complexes.
References
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˚
The B–O bond distances were determined to be 1.291(16) A
◦
˚
and 1.280(15) A with Zr–O–B bond angles of 156.0(9) and
154.2(9)◦. In addition, the Bpin units are canted with respect to
the ZrO2 plane by only 18.0◦ and 5.7◦ respectively. This geometry
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ligands to form metal complexes of Li, Co, Mn, Fe, Al, Zn
and Cd.56–62 In addition only two examples of Zr complexes
containing boryloxide ligands have been reported. Balkwill et al.
have described the bimetallic complexes (Cp2Zr(l2-O2BAr))2
(Ar = Ph, C6H2-2,4,6-Me3, C6F5) (Scheme 5).63 synthesized via
reaction of Cp2ZrMe2 with ArB(OH)2 generated via hydrolysis
of (OBAr)3. These macrocyclic species exhibit shorter average
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12 R. Roesler, B. J. N. Har and W. E. Piers, Organometallics, 2002, 21,
˚
Zr–O (1.985(2) A) bonds and longer average B–O (1.350(6)
4300.
˚
A) bonds. At the same time, the Zr–O–B angles range from
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141.8(2)◦ to 156.7(2)◦ for the derivatives with Ar = Ph, C6H2-
2,4,6-Me3. The upper limit of this range is similar to the Zr–
O–B angles seen in 7, but presumably the macrocyclic nature
of these complexes accounts for the lower end of this range.
More recently, Tilley’s group has also reported the structure of
the related boryloxide derivative Cp2ZrMe(OB(OSi(Ot-Bu)3)2)
(Scheme 5).64 The Zr–O distance of 1.974(4) A is slightly shorter
˚
than those in 7 while the bridging B–O bond distance and Zr–O–
◦
˚
B angle are slightly larger at 1.329(3) A and 160(2) , respectively.
These metric perturbations are consistent with the steric de-
mands of the boryloxide substituents in Cp2ZrMe(OB(OSi(Ot-
Bu)3)2).
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Scheme 5 Known Zr–boryloxide species.
2 1 8 6
D a l t o n T r a n s . , 2 0 0 5 , 2 1 8 2 – 2 1 8 7