Reaction of the Lewis Acids B(C6F5)3 and (AlMe2Cl)2 with Azazirconacycles

Article abstract of DOI:10.1021/om9902378

B(C₆F₅)₃ opens the ring of the N-^tBu azazirconacyclobutane 2 by abstracting the carbon from the zirconium; the resulting amido cation 3 reacts slowly with ethylene to form a chelating γ-iminoalkyl zirconocene cation, 4. Similarly, B(C₆F₅)₃ removes carbon from the Zr of the N-Ph azazirconacyclopentane 5a and the N-SiMe₃ azazirconacyclopentane 5b, forming amido cations that are stabilized in the solid state by coordination of phenyl substituents on N (6, from 5a) or C (8, from 5b); 8 slowly loses hydrogen, forming an azaallyl cation 9. In contrast AlMe₂Cl coordinates the N of the zirconaaziridines 10, resulting in an sp² N coordinated to Zr through a p orbital. The structures of 4, 6, 8, and 11a have been established by X-ray crystallography.

Full text of DOI:10.1021/om9902378

Organometallics 1999, 18, 3827-3834
3827
Rea ction of th e Lew is Acid s B(C6F 5)3 a n d (AlMe2Cl)2 w ith
Aza zir con a cycles
C. J eff Harlan, Brian M. Bridgewater, Tony Hascall, and J ack R. Norton*
Department of Chemistry, Columbia University, New York, New York 10027
Received April 6, 1999
B(C6F5)3 opens the ring of the N-tBu azazirconacyclobutane 2 by abstracting the carbon
from the zirconium; the resulting amido cation 3 reacts slowly with ethylene to form a
chelating γ-iminoalkyl zirconocene cation, 4. Similarly, B(C6F5)3 removes carbon from the
Zr of the N-Ph azazirconacyclopentane 5a and the N-SiMe3 azazirconacyclopentane 5b,
forming amido cations that are stabilized in the solid state by coordination of phenyl
substituents on N (6, from 5a ) or C (8, from 5b); 8 slowly loses hydrogen, forming an azaallyl
cation 9. In contrast AlMe2Cl coordinates the N of the zirconaaziridines 10, resulting in an
sp2 N coordinated to Zr through a p orbital. The structures of 4, 6, 8, and 11a have been
established by X-ray crystallography.
In tr od u ction
Because of our interest in zirconaaziridines,7 we
decided to investigate the reaction of B(C6F5)3 and (Me2-
AlCl)2 with these and other azazirconacycles. As the
methyl and amide ligands in Cp2Zr(Me)(NR2) are ab-
stracted competitively by B(C6F5)3,8 we expected that
B(C6F5)3 would abstract from the zirconium in eq 2
either the carbon ligand (giving A) or the nitrogen ligand
(giving the potential zwitterionic catalyst B). We have
found that B(C6F5)3 treatment of several azazircona-
cycles removes the carbon ligand exclusively, while
AlMe2Cl coordinates to the nitrogen of zirconaaziridines.
1
The ability of B(C6F5)3 to abstract an alkyl group
from metallocene dialkyls is well-known from numerous
studies relating to Ziegler/Natta polymerization of
olefins.2-4 For example, the abstraction of a methylene
group from a zirconacyclopentene, illustrated in eq 1,
leads to an active olefin polymerization catalyst with
the counteranion tethered to the end of the growing
chain.5,6
(1) The preparation and properties of this and related boranes have
been reviewed: Piers, W. E.; Chivers, T. Chem. Soc. Rev. 1997, 26,
345-354.
(2) (a) Elder, M. J .; Ewen, J . A. Can. Pat. Appl. 2,027,145, 1991;
Chem. Abstr. 1991, 115, 136998g. Ewen, J . A.; Elder, M. J . U.S. Patent
5 561 092, 1996; Chem. Abstr. 1996, 125, 301814. (b) Yang, X.; Stern,
C. L.; Marks, T. J . J . Am. Chem. Soc. 1991, 113, 3623-3625. (c) Yang,
X.; Stern, C. L.; Marks, T. J . Angew. Chem., Int. Ed. Engl. 1992, 31,
1375-1377. (d) Giardello, M. A.; Eisen, M. S.; Stern, C. L.; Marks, T.
J . J . Am. Chem. Soc. 1993, 115, 3326-3327. (e) Yang, X.; Stern, C. L.;
Marks, T. J . J . Am. Chem. Soc. 1994, 116, 10015-10031. (f) Deck, P.
A.; Marks, T. J . J . Am. Chem. Soc. 1995, 117, 6128-6129. (g) Giardello,
M. A.; Eisen, M. S.; Stern, C. L.; Marks, T. J . J . Am. Chem. Soc. 1995,
117, 12114-12129. (h) Chen, Y. X.; Marks, T. J . Organometallics 1997,
16, 3649-3657. (i) Deck, P. A.; Beswick, C. L.; Marks, T. J . J . Am.
Chem. Soc. 1998, 120, 1772-1784. (j) Lanza, G.; Fragala`, I. L.; Marks,
T. J . J . Am. Chem. Soc. 1998, 120, 8257-8258.
(3) (a) Bochmann, M.; Lancaster, S. J .; Hursthouse, M. B.; Malik,
K. M. A. Organometallics 1994, 13, 2235-2243. (b) Bochmann, M.;
Green, M. L. H.; Powell, A. K.; Sassmannshausen, J .; Triller, M. U.;
Wocadlo, S. J . Chem. Soc., Chem. Commun. 1999, 43-49. (c) Wu, Z.;
J ordan, R. F.; Peterson, J . L. J . Am. Chem. Soc. 1995, 117, 5867-
5868. (d) Horton, A. D. Organometallics 1996, 15, 2675-2677.
(4) For abstraction from mono-Cp complexes see: (a) Pellecchia, C.;
Pappalardo, D.; Oliva, L.; Zambelli, A. J . Am. Chem. Soc. 1995, 117,
6593-6594, and references therein. (b) Ewart, S. W.; Sarsfield, M. J .;
J eremic, D.; Tremblay, T. L.; Williams, E. F.; Baird, M. C. Organo-
metallics 1998, 17, 1502-1510, and references therein.
(5) (a) Karl, J .; Erker, G.; Fro¨hlich, R. J . Am. Chem. Soc. 1997, 119,
11165-11173. (b) Karl, J .; Erker, G.; Fro¨hlich, R.; Bickelhaupt, F.;
Goedheijt, M. S.; Akkerman, O. S.; Binger, P.; Stannek, J . Angew.
Chem., Int. Ed. Engl. 1997, 36, 2771-2774. (c) Karl, J .; Erker, G.
Chem. Ber./ Recl. 1997, 130, 1261-1267.
(6) For mono-Cp zirconacyclopentene systems see: (a) Pindado, G.
J .; Thornton-Pett, M.; Bochmann, M. J . Chem. Soc., Dalton Trans.
1997, 3115-3127. (b) Pindado, G. J .; Thornton-Pett, M.; Bouwkamp,
M.; Meetsma, A.; Hessen, B.; Bochmann, M. Angew. Chem., Int. Ed.
Engl. 1997, 36, 2358-2361. (c) Pindado, G. J .; Lancaster, S. J .;
Thornton-Pett, M.; Bochmann, M. J . Am. Chem. Soc. 1998, 120, 6816-
6817. (d) Pindado, G. J .; Thornton-Pett, M.; Bochmann, M. J . Chem.
Soc., Chem. Commun. 1997, 609-610.
(7) (a) Gately, D. A.; Norton, J . R. J . Am. Chem. Soc. 1996, 118,
3479-3489. (b) Gately, D. A.; Norton J . R.; Goodson, P. A. J . Am. Chem.
Soc. 1995, 117, 986-996. (c) Tunge, J .; Gately, D. A.; Norton, J . R. J .
Am. Chem. Soc. 1999, 121, 4520-4521.
10.1021/om9902378 CCC: $18.00 © 1999 American Chemical Society
Publication on Web 08/18/1999

3828 Organometallics, Vol. 18, No. 19, 1999
Harlan et al.
Sch em e 1
Resu lts
Rea ction of th e Aza zir con a cyclobu ta n e 2 w ith
B(C6F 5)3. The azazirconacyclobutane 2 is available from
the reversible reaction of ethylene with the terminal
imido complex 1.9 The addition of 2 equiv of B(C6F5)3 to
an ethylene-saturated solution of 1 in C6D6 results in
an immediate color change from light yellow to purple;
the B(C6F5)3 adduct 3 is formed, along with B(C6F5)3-
(THF) (Scheme 1).
We have not been able to isolate 3, but the room-
temperature NMR data in C6D6 are consistent with the
structure drawn in Scheme 1. (We found no solvent
suitable for low-temperature spectra in which 3 was
soluble and stable.) The 13C NMR resonance of the CH2
group attached to B(C6F5)3 is not observed in a standard
spectrum because of residual coupling to the boron (an
effect noticed in similar B(C6F5)3 complexes5a) but can
be found at δ 7.9 in the HMQC spectrum of 3. There is
no evidence for a Zr-F interaction (the 19F spectrum of
3 shows only one set of o, m, and p resonances,
respectively, for the B(C6F5)3 moiety). There is no
evidence of an agostic interaction between the Zr and a
methylene C-H bond (such interactions can result in
an upfield shift of the methylene 1H NMR resonances6a).
At room temperature solutions of 3 containing excess
ethylene slowly form (Scheme 1) 4, which has been
isolated as a yellow crystalline solid. When examined
by X-ray crystallography, crystals of 4 grown from
toluene/hexanes contained a molecule of toluene and a
molecule of hexane in each unit cell; both solvent
molecules were disordered. (Complex 4 can be obtained
solvent free by recrystallization from CH2Cl2/hexanes.)
The molecular structure of 4 in these solvent-containing
F igu r e 1. Molecular structure of 4. Selected bond lengths
(Å) and angles (deg): Zr-N 2.3313(17), Zr-C4 2.262(2),
N-C2 1.308(2), C2-C3 1.509(3), C3-C4 1.529(3), C2-C1
1.489(3), C1-B 1.685(3), Zr-N-C2 115.35(13), Zr-C4-
C3 105.53(14), N-C2-C1 127.62(18), N-C2-C3 115.94(17),
C2-C1-B 121.87(15), C2-C3-C4 115.33(16), N-Zr-C4
76.19(7), C1-C2-C3 116.03(16).
Complex 4 is formally the result of the addition of two
ethylenes to 1, along with the loss of two hydrogens.
1H NMR shows that the conversion of 3 to 4 occurs in
high yield, but the solution thickens and small amounts
of polyethylene precipitate. However, at room temper-
ature ethylene-saturated solutions of 4 in C6D6 or CD2-
Cl2 do not yield polyethylene, so its formation during
the conversion of 3 to 4 is due to a small amount of
highly active impurity or decomposition product. Addi-
tion of diphenylacetylene to a freshly prepared solution
of 3 almost completely suppressed polyethylene forma-
tion, presumably by trapping the active polymerization
catalyst, without affecting the formation of 4.
R ea ct ion of t h e Aza zir con a cyclop en t a n es 5
(Cp 2Zr (NRCHP h CH2CH2), R ) P h , 5a ; R ) SiMe3,
5b) w ith B(C6F 5)3. Addition of 1 equiv of B(C6F5)3 to
an orange solution of the azazirconacyclopentane 5a in
benzene results (eq 3) in the formation of 6, which
precipitates as brown crystals of 6‚1.5(C6H6) after the
addition of hexanes. Pure 6, without molecules of
solvation, can be obtained by synthesis in or recrystal-
1
crystals is shown in Figure 1. The H NMR and HMQC
spectra of solutions of 4 imply that its solid-state
structure is retained in solution. Again, the 13C NMR
resonance of the CH2 group attached to B(C6F5)3 is not
observed in a standard spectrum but is readily apparent
in the HMQC spectrum (δ 36.5).
(8) Temme, B.; Erker, G. J . Organomet. Chem. 1995, 488, 177-182.
(9) (a) Walsh, P. J .; Hollander, F. J .; Bergman, R. G. J . Am. Chem.
Soc. 1988, 110, 8729. (b) Walsh, P. J .; Hollander, F. J .; Bergman, R.
G. Organometallics 1993, 12, 3705-3723.

Reactions of Lewis Acids with Azazirconacycles
Organometallics, Vol. 18, No. 19, 1999 3829
-2.12 at 223 K; CD2Cl2) of one of the methylene protons
of the CH2B(C6F5)3 group suggests an agostic interaction
with the zirconium that increases as the temperature
is lowered;6c however, the 13C NMR of the methylene
carbon resonance in 6 (δ 20.3) remains about the same
in the THF adduct 7 (δ 19.5) (see below). (Erker has
reported a case in which displacement of an agostic
CH2B(C6F5)3 by THF from a zirconocene cation resulted
in a downfield shift of ca. 25 ppm.5c)
Addition of ethylene (2-3 atm) to toluene solutions
of 6 (25-70 °C) results (after a short induction period)
in low yields of polyethylene; again, a decomposition
product is probably responsible. Addition of THF to
crystalline 6 or slurries of 6 in toluene causes a rapid
color change from brown to yellow and the formation of
7 (eq 4), with coordination of the THF replacing coor-
dination of the two aromatic carbons in 6. Complex 7 is
completely insoluble in aromatic solvents and CD2Cl2
but dissolves readily in THF. Its 1H NMR shows two
phenyl resonances slightly upfield from the others;
however, COSY, HMQC, and HMBC experiments sug-
gest that they belong not to the ortho N-Ph protons but
to the meta N-Ph protons (δ 6.31, m) and ortho C-Ph
protons (δ 6.45, d). Furthermore, no 13C NMR reso-
nances, including those of the carbons to which these
protons are attached (δ 129.4 and 130.0, respectively,
at 294 K), are shifted upfield; all remain unchanged
even at low temperature (203 K). It is clear that neither
aromatic ring in 7 is coordinated.
F igu r e 2. Molecular structure of 6. Selected bond lengths
(Å) and angles (deg): Zr1-N1 2.112(5), Zr1-C31 2.638(7),
Zr1-C32 2.711(8), Zr1-C49 3.002(7), N-C31 1.384(8),
N1-C47 1.466(8), C49-B1 1.668(11), Zr1-N1-C31 95.7(4),
Zr1-N1-C47.
lization from toluene/hexanes. Crystals of both 6 and
6‚1.5(C6H6) did not give X-ray diffraction data of high
quality; solution of the best data set gave the connectiv-
ity displayed in Figure 2 and drawn in eq 3, with one
N-phenyl double bond weakly coordinated to the Zr (the
Zr1-C31 bond length is 2.638(7) Å and the Zr1-C32
bond length is 2.711(8) Å).
This interaction persists in solution. One aromatic
ring of 6 shows five inequivalent H and five inequiva-
Addition of 1 equiv of B(C6F5)3 to a yellow solution of
5b in aromatic solvents immediately changes its color
to purple (eq 5). From toluene a purple oil precipitates
that eventually crystallizes as Cp2Zr(N(SiMe3)C(Ph)-
HCH2CH2B(C6F5)3)‚2C6H5CH3 (8‚2C6H5CH3). Crystals
of 8 prepared in benzene contain no benzene molecules
in the crystal. The structure of 8‚2C6H5CH3 has been
determined by X-ray crystallography (Figure 3); as just
seen with 6, two carbons of the phenyl of 8 are weakly
coordinated to the Zr. In 8 the Zr-C71 bond length is
1
lent 13C resonances at low temperature (223 K) in CD2-
Cl2. Solution coordination of the carbons pictured in eq
3 is implied by the upfield NMR shift of one ortho proton
(δ 6.41 at 223 K) and its associated carbon (δ 83.9) (the
ipso carbon has not been located); the other ortho proton
and its carbon appear in normal locations (δ 6.82 and
119.0, respectively). As the sample is warmed to room
temperature, the two pairs of ortho and meta proton
resonances coalesce, presumably as a result of the
dissociation of the coordinated carbons. In C6D6 the
averaged 1H signal (δ 5.87) observed for both ortho
protons at room temperature is shifted even further
upfield.
(10) (a) Latesky, S. L.; McMullen, A. K.; Niccolai, G. P.; Rothwell,
I. P. Organometallics 1985, 4, 902-908. (b) J ordan, R. F.; LaPointe,
R. E.; Bajgur, C. S.; Echols, S. F.; Willet, R. J . Am. Chem. Soc. 1987,
109, 4111-4113. (c) Hlatky, G. G.; Turner, H. W.; Eckman, R. R. J .
Am. Chem. Soc. 1989, 111, 2728-2729. (d) Horton, A. D.; Frijns, J . H.
G. Angew. Chem., Int. Ed. Eng. 1991, 30, 1152-1154. (e) Pellechia,
C.; Grassi, A.; Immirzi, A. J . Am. Chem. Soc. 1993, 115, 1160-1162.
(f) Green, M. L. H.; Sassmannshausen, J . J . Chem. Soc., Chem.
Commun. 1999, 115-116.
Such upfield shifts can be attributed to the interaction
of an aryl group with a zirconium center.10 A temper-
ature-dependent high-field shift (δ -0.94 at 298 K, δ

3830 Organometallics, Vol. 18, No. 19, 1999
Harlan et al.
F or m a tion of th e Aza a llyl Com p lex 9. When
purple solutions of 8 in CD2Cl2 remain at room tem-
perature for several hours, a yellow product forms which
appears to be the azaallyl complex 9 (eq 6).7c,11 Although
1
we have been unable to isolate 9, its H and 13C NMR
spectra make a convincing case for the structure drawn
in eq 6. The CH2B(C6F5)3 protons are diastereotopic,
1
appearing as broad humps in the H NMR at δ 0.62 and
-1.15 (raising the possibility that this methylene in-
teracts directly with the Zr). The C2 methine resonance
(δ 4.93) is downfield from that of a zirconaaziridine and
near that (δ 5.02) in the allyl complex Cp2Zr(η3-CH2C-
(Ph)CHCH2B(C6F5)3),5a and the carbon resonances C1,
C2, and C3 (δ 154.6, 107.7, and 22.0) are remarkably
similar to the corresponding ones in Cp2Zr(η3-CH2C(Ph)-
CHCH2B(C6F5)3) (δ 147.4, 107.0, and 24.7, respectively).
F igu r e 3. Molecular structure of 8. Selected bond lengths
(Å) and angles (deg): Zr-N 2.062(3), Zr-C71 2.842(4), Zr-
C72 2.580(4), N-C3 1.501(4), C1-B 1.646(5), C3-C71
1.506(5), C72-C71 1.398(6), Zr-N-C3 112.2(2), N-C3-
C71 110.0(3), N-C3-C2 111.5(3), C3-C71-C72 119.9(3),
Zr-N-Si 130.9(2), Si-N-C3 116.0(2).
2.842(4) Å and the Zr-C72 bond length is 2.580(4) Å.
Rea ction of Zir con a a zir id in es w ith Lew is Acid s.
Reaction of 2 equiv of B(C6F5)3 with the zirconaaziri-
dines 10 resulted in complex reaction mixtures. Addition
of 1 equiv (2 equiv Al) of (AlMe2Cl)2 to hexane slurries
of the zirconaziridines 10a and 10b results in the
formation of AlMe2Cl adducts 11 and 1 equiv of AlMe2-
Cl(THF) (eq 7). The complexes 11 were isolated in low
yield as orange-yellow crystals. The reaction of 10a with
(AlMe2Cl)2 occurs in moderate yield (1H NMR), and only
a small amount of 11a has been isolated. The reaction
of 10b with (AlMe2Cl)2 is extremely clean (1H NMR),
but difficulties in separating 11b from AlMe2Cl(THF)
have resulted in low isolated yields.
Purple crystalline 8 and 8‚2C6H5CH3 dissolve in CD2-
Cl2 to form a purple solution that is unstable at room
temperature (see below). As with 6, the low-temperature
1
(193 K) H NMR of 8 or 8‚2C6H5CH3 in CD2Cl2 shows
five separate resonances for the phenyl protons, and
these resonances coalesce pairwise as the temperature
is raised. However, there is no upfield shift of any of
1
the H and 13C NMR resonances of the phenyl in 8, so
there is (in contrast to the situation with 6) no evidence
that the Zr-Ph interaction persists in solution.
Cyclop en ta d ien yl Liga n d Exch a n ge. The pairs of
cyclopentadienyl ligands in 5, 6, 7, and 8 are diaste-
reotopic, and separate 1H NMR resonances are expected
for these ligands. The cyclopentadienyl resonances of 6
are well separated at room temperature in C6D6, but
appear as a single extremely broad peak in CD2Cl2,
indicating exchange; slower Cp exchange is observed for
7. For 6 in CD2Cl2, from measurements between -40
and -10 °C, ∆Hq ) 12.8(6) kcal/mol and ∆Sq ) +2(3)
eu, with an extrapolated rate constant of about 7(2) ×
103 s-1 at 25 °C; for 7 (THF-d8), ∆Hq ) 10.6(1) kcal/mol
and ∆Sq ) -14.5(4) eu, with a rate constant of 66(1)
s-1 at 25 °C.

Reactions of Lewis Acids with Azazirconacycles
Organometallics, Vol. 18, No. 19, 1999 3831
F igu r e 5. Various “π agostic” interactions with zirco-
nium: compound 11a , this work; compound 12, ref 13a;
various compounds 13, ref 14.
Al-N-C3 angles is 355°. The hybridization of the N is
approximately sp2, with a filled p orbital interacting
with the electrophilic Zr in a “π agostic” fashion (Figure
5).13a Similar interactions are known between planar
carbon and zirconocene Zr in 12,13a in cyclopentadienyl
structures such as 13 (Figure 5),14 and in structures
with bridging methyls (which have a Zr on each side of
the planar carbon).15
F igu r e 4. Molecular structure of 11a . Selected bond
lengths (Å) and angles (deg): Zr-N1 2.366(6), Zr-C3
2.297(7), Zr-Cl 2.680(3), N1-C3 1.462(9), N1-Al 1.952(7),
Al-Cl 2.325(4), N1-Zr-Cl 74.3(2), Zr-N1-C3 69.2(3), Zr-
Cl-Al 81.22(10), N1-C3-Zr 74.3(4).
The reaction of excess (Me2AlCl)2 (C6D6 solution) with
other azazirconacycles such as 5 in C6D6 resulted in
immediate cleavage of the zirconacycle ligand and the
formation of Cp2ZrMeCl and Cp2ZrCl2. These metath-
esis reactions probably proceed via intermediates struc-
turally related to 11.15a
Small yellow crystals of 11a , of poor quality, from
CH2Cl2/hexanes solution at -20 °C, have been examined
by X-ray crystallography (Figure 4). The AlMe2Cl moiety
is connected by the coordination of the zirconaaziridine
N to Al and by the coordination of the Cl (of the AlMe2-
Cl) to the Zr. The Zr-C and C-N bond lengths of the
aziridine ligand are not substantially affected by Al
coordination; in 10a 7b they are Zr-C ) 2.299(5), C-N
) 1.431(7) Å, and in 10b20 they are Zr-C ) 2.26(1),
C-N ) 1.41(1) Å, while in 11a they are Zr-C )
2.297(7), C-N ) 1.462(9) Å. The Zr-N bond lengthens
substantially upon coordination: it is 2.113(4) Å in 10a
and 2.11(1) Å in 10b, but 2.366(6) Å in 11a . The Zr-Cl
bond in 11a (2.680(3) Å) is significantly longer than the
typical Zr-Cl distance (ca. 2.44 Å) in zirconocene alkyl
chloride complexes,12 but within the range of Zr-Cl dis-
tances observed for other chlorides bridging Al and Zr.13
The N of 11a is in the plane of C3, C31, and the Al in
Figure 4; the sum of the C3-N-C31, C31-N-Al, and
Discu ssion
The conversions of 3 to 4 (Scheme 1) and of 8 to 9 (eq
6), and the exchange of the Cp ligands in 6, probably
all begin with a â hydrogen elimination16 like that in
eq 8.
(11) Another zirconocene azaallyl complex has been reported: Coles,
N.; Harris, M. C. J .; Whitby, R. J .; Blagg, J . Organometallics 1994,
13, 190-199.
(12) Hunter, W. E.; Hrncir, D. C.; Bynum, R. V.; Penttila, R. A.;
Atwood, J . L. Organometallics 1983, 2, 750-755, and references
therein.
(13) (a) Erker, G.; Noe, R.; Kru¨ger, C.; Werner, S. Organometallics
1992, 11, 4174-4177. (b) Kaminsky, W.; Kopf, J .; Sinn, H.; Vollmer,
H.-J Angew. Chem., Int. Ed. Engl. 1976, 15, 629-630. (c) Kopf, J .;
Kaminsky, W.; Vollmer, H.-J . Cryst. Struct. Commun. 1980, 9, 197-
201.
(14) (a) Diamond, G. M.; Chernega, A. N.; Mountford, P.; Green, M.
L. H. J . Chem. Soc., Dalton Trans. 1996, 921-938. (b) Corradi, M. M.;
Duncalf, D. J .; Lawless, G. A.; Waugh, M. P. Chem. Commun. 1997,
203-204.
For example, the formation of 4 from 3 can be
rationalized by the route shown in Scheme 2.17
Such â hydride abstractions have been previously
reported with the cationic zirconocene system in eq 918
and after Cp2Zr(Me)(N(CH2)5) is treated with B(C6F5)3.8
(15) (a) Waymouth, R. W.; Potter, K. S.; Schaefer, W. P.; Grubbs,
R. H. Organometallics 1990, 9, 2843-2846. (b) Waymouth, R. M.;
Santarsiero, B. D.; Coots, R. J .; Bronikowski, M. J .; Grubbs, R. H. J .
Am Chem. Soc. 1986, 108, 1427-1441. (c) Waymouth, R. M.; Santar-
siero, B. D.; Grubbs, R. H. J . Am Chem. Soc. 1984, 106, 4050-4051.
(16) Negishi, E.; Takahashi, T. Acc. Chem. Res. 1994, 27, 124-130.
(17) A cationic iminoacyl complex like that shown in Scheme 2 has
been structurally characterized: Temme, B.; Erker, G.; Fro¨hlich, R.;
Grehl, M. Angew. Chem., Int. Ed. Engl. 1994, 33, 1480-1482.
(18) (a) J ordan. R. F.; Taylor, D. F. J . Am. Chem. Soc. 1989, 111,
778-779. See also: (b) Nugent, W. A.; Ovenall, D. W.; Holmes, S. J .
Organometallics 1983, 2, 161-162. (c) Planalp, R. P.; Andersen, R.
A.; Zalkin, A. Organometallics 1983, 2, 16-20.
After 14 is formed from 8 by â hydrogen elimination,
the azaallyl 9 is plausibly formed by γ hydrogen
(19) Such substituents increase the rate of â hydrogen abstraction
in the formation of zirconaaziridines from zirconocene alkyl amides
(see ref 11).
(20) Buchwald, S. L.; Watson, B. T.; Wannamaker, M. W.; Dewan,
J . C. J . Am. Chem. Soc. 1989, 111, 4486-4494.

3832 Organometallics, Vol. 18, No. 19, 1999
Sch em e 2. Sp ecu la tive Mech a n ism for F or m a tion of 4 fr om 3
Harlan et al.
abstraction and H2 loss (eq 10).
by steric constraints (bulky substituents are present on
all of these nitrogens).
Exp er im en ta l Section
Manipulations were conducted under N2 or Ar using stan-
dard Schlenk techniques and Vacuum Atmospheres inert
atmosphere boxes; solvents were dried and degassed. Com-
plexes 1,9 2,9 5b,20 and 107b,20 were prepared according to
published procedures. Complex 5a was prepared from 10a in
an analogous manner as for 5b from 10b and recrystallized
from toluene/hexanes. Cp2ZrCl2, generously provided by Boul-
der Scientific, (AlMe2Cl)2 (Texas Alkyls), and polymer grade
ethylene (Matheson) were used as received. B(C6F5)3 (Boulder
Scientific) was used as received or purified via recrystalliza-
tion/sublimation (additional purification did not affect results).
HMQC, HMBC, and COSY experiments were performed at 500
MHz; 19F NMR spectra were recorded at 282 MHz; 13C NMR
spectra were recorded at 75 MHz; 1H NMR spectra were
recorded at 500 MHz unless otherwise noted. NMR spectra
were taken at room temperature unless otherwise noted;
The exchange of the Cp ligands in 6 and 7 requires
that â hydrogen elimination remove the chiral center,
as shown in eq 11. The ease with which this process
occurs may be due to the phenyl substituents on both
nitrogen and carbon.19
1
HMQC results are reported as δ 13C/δ H.
Cp 2Zr N(tBu )CH2CH2B(C6F 5)3 (3). Cp2Zr(NtBu)(THF) (1)
(0.040 g, 0.11 mmol) was dissolved in C6D6 (1 mL); B(C6F5)3
(0.11 g, 0.215 mmol) was dissolved in C6D6 (1.5 mL). Both
solutions were saturated with ethylene and combined under
an ethylene atmosphere, giving a dark blue-violet color im-
mediately. 1H NMR (C6D6): δ 5.81 (s, 10 H), 2.14 (br, 2 H),
2.02 (br, 2 H), 0.57 (s, 9 H). 13C NMR (C6D6): δ 115.7 (C5H5),
32.0/2.14 (CH2) 59.6 (C(CH3)3), 29.4 (C(CH3)3), 7.9/2.02 (CH2).
19F NMR (C6D5CD3): δ -132.1 (d, J FF ) 20 Hz) -158.2 (t, J FF
) 20 Hz), -163.3 (m).
Attempts to obtain low-temperature NMR spectra in toluene-
d8 resulted in separation of 3 as an oil and loss of signal.
Attempts at preparing 3 in CD2Cl2 resulted in rapid decom-
position.
P r ep a r a tion of 4 fr om 1 via 3. Cp2Zr(NtBu)(THF) (1) (0.20
g, 0.55 mmol) in toluene (18 mL) and B(C6F5)3 (0.56 g, 1.09
mmol) in toluene (12 mL) were saturated with ethylene and
combined as above; the solution was allowed to sit overnight
at room temperature and turned a light yellow/green. After
some toluene was removed (to a volume of ca. 8 mL), hexane
(ca. 12 mL) was added and the solution cooled to -20 °C for 3
weeks; a night at room temperature gave yellow crystalline
product containing toluene and hexanes. Yield: 80 mg (17%).
Higher yields of analytically pure material were obtained by
recrystallization of similarly prepared reaction mixtures from
CH2Cl2/hexanes. 1H NMR (CD2Cl2; 300 MHz): δ 6.28 (s, 10
H), 2.92 (br, 2 H), 2.76 (t, 2 H, J ) 6.6 Hz.), 1.35 (t, 2 H, J )
6.6 Hz.), 0.92 (s, 9 H). 13C NMR (CD2Cl2): δ 209.5 (CdN), 113.2
(C5H5), 59.7 (CCH3), 45.2/1.35 (CH2), 41.9/2.76 (CH2), 36.5/2.92
(CH2B(C6F5)3), 27.1 (CCH3). 19F NMR (CD2Cl2): δ -131.0 (br),
Con clu sion s
While Me2AlCl coordinates to the nitrogen of the
zirconaaziridine 10, B(C6F5)3 exclusively abstracts the
carbon (possibility A in eq 2 in the Introduction) rather
than the nitrogen (possibility B) from the zirconium of
the azazirconacyclobutane 2 and the azazirconacyclo-
pentanes 5. Carbon abstraction can be explained either
by electronic stabilization of the cationic products (by π
donation from the nitrogen of the amido cations A) or

Reactions of Lewis Acids with Azazirconacycles
Organometallics, Vol. 18, No. 19, 1999 3833
Ta ble 1. Su m m a r y of Cr ysta llogr a p h ic Da ta for Com p ou n d s 4, 6, 8, a n d 11a
4
6
8
11a
empirical formula
fw
temperature, K
crystal system
space group
a, Å
C
42.50H36BF15NZr
C52H34BF15NZr
1059.83
213(2)
monoclinic
P21/c
20.9775(14)
21.3518(14)
20.6737(14)
90
95.4880(10)
90
C54H45BF15NSiZr
1123.03
203(2)
triclinic
P1h
13.0474(6)
13.4594(6)
16.9094(8)
87.3920(10)
69.1730(10)
66.5280(10)
2529.7(2)
2
C25H27AlClNZr
495.13
293(2)
orthorhombic
Pbca
13.2367(11)
15.576(2)
23.120(2)
90
947.75
203(2)
monoclinic
P21/c
12.8543(6)
21.9755(10)
15.8173(7)
90
113.8070(10)
90
4087.9(3)
4
b, Å
c, Å
R, deg
â, deg
90
90
γ, deg
V, Å3
9217.5(11)
8
1.527
4766.8(8)
8
1.380
Z
d
calc, g/cm3
1.540
1.474
λ (Mo KR), Å
0.71073
0.370
28337
9064
9064/16/599
1.033
0.71073
0.337
69017
21376
21376/0/1057
1.062
0.71073
0.334
17633
10520
10519/0/664
1.039
0.71073
0.621
13269
5032
5031/0/263
1.034
µ, mm-1
no. of data collected
no. of unique data
no. of data/restraints/params
GOF on F2
R1, wR2 (I > 2σ(I))
R1, wR2(all data)
0.0357, 0.0907
0.0455, 0.0960
0.0958, 0.1748
0.2809, 0.2225
0.0640, 0.1361
0.1028, 0.1556
0.0892, 0.1471
0.2478, 0.1861
-161.3 (t, J FF ) 20 Hz), -165.4 (m). Anal. Calcd for C36H25
BF15NZr (858.6): C, 50.36; H, 2.93; N, 1.63. Found: C, 50.52;
H, 3.30; N, 1.34.
-
1H NMR (CD2Cl2; 500 MHz; 193 K): δ 8.10 (pt, meta, 1 H,
J obs ) 7.55), 7.84 (d, ortho, 1 H, J H-H ) 7.65), 7.78 (t, para, 1
H, J H-H ) 7.45), 7.75 (d, ortho, 1 H, J H-H ) 7.95), 7.62 (pt,
meta, 1H; J obs ) 7.00), 6.72 (s, 5 H), 5.68 (s, 5 H), 4.06 (m, 1
H), 1.26 (br, 1 H), 0.84 (br, 2 H), 0.45 (br, 1 H), -0.17 (s, 9 H).
HMQC (CD2Cl2, 253 K): δ 117.1/(6.72, 5.69), 70.6/4.06, 39.0/
(1.26, 0.84), 16.7/(0.84, 0.45), 2.5/-0.17. 19F NMR (CD2Cl2): δ
-131.9 (d, J FF ) 20 Hz), -163.3 (t, J FF ) 20 Hz), -166.3 (m).
Anal. Calcd for C40H29BF15NSiZr (938.76): C, 51.18; H, 3.11;
N, 1.49. Found: C, 51.35; H, 2.82; N, 1.24.
Cp2Zr NP h CHP h CH2CH2B(C6F5)3 (6). A solution of B(C6F5)3
(1.0 g, 1.95 mmol) in toluene (15 mL) was added to a solution
of 5a (0.80 g, 1.9 mmol) in toluene (20 mL) with stirring for
20 min at room temperature; the mixture turned dark brown.
About half the toluene was removed under vacuum, hexanes
were added (ca. 10 mL), and the solution was allowed to stand
overnight, forming reddish brown crystalline 6 that was
isolated by filtration and dried under vacuum. Yield: 1.0 g
(54%). Carrying out the reaction in benzene resulting in the
formation of brown crystalline 6‚1.5(C6H6) (by X-ray diffrac-
tion), which lost benzene under vacuum (by 1H NMR). 1H NMR
(C6D6; 300 MHz): δ 7.02 (m, 2 H), 6.90 (m, 3 H), 6.67 (m, 2
H), 6.21 (m, 1 H), 5.87 (br, obs, 2 H), 5.75 (s, 5 H), 4.98 (s, 5
H), 4.04 (d, 1 H), 2.00 (m, 1 H), 1.84 (m, 1 H), 1.23 (br, 1 H),
F or m a tion of th e Aza a llyl 9 fr om 8. Purple solutions of
8 in CH2Cl2 at room temperature turn yellow and cleanly
convert to 9 (>90%) after several hours. (At higher tempera-
tures multiple products form.) Attempts at crystallizing 9 from
CH2Cl2 or toluene by cooling or addition of hexanes resulted
in formation of a yellow oil. 1H NMR (CD2Cl2): δ 7.44 (m, 4
H), 7.14 (m, 1 H), 6.36 (s, 5 H), 5.86 (s, 5 H), 4.93 (m, 1 H),
0.62 (m, 1 H), 0.03 (s, 9 H), -1.14 (m, 1 H). HMQC (CD2Cl2):
δ 112.0/6.36, 108.9/5.86, 107.7/4.93, 22.0/(0.62, -1.14), 3.7/0.03.
13C NMR (CD2Cl2): δ 154.6 (C-Ph). 19F NMR (CD2Cl2): δ
-131.7 (d, J FF ) 20 Hz), -159.6 (t, J FF ) 20 Hz), -164.1 (m).
AlMe2Cl Ad d u ct 11a . To a suspension of 10a (0.57 g, 1.2
mmol) in hexanes (30 mL) was added a solution of (AlMe2Cl)2
(2.5 mL, 2.75 mmol, 1.1 M in hexanes). Over 30 min the 10a
dissolved and formed a yellow solution. Cooling it to -78 °C
resulted in the formation of an oily yellow-orange solid, which
was isolated by filtration. The solid was dissolved in CH2Cl2
(5 mL, pretreated with 0.1 mL of 1.1 M [AlMe2Cl]2 solution)
and kept cold as hexanes (ca. 15 mL) were added. Cooling the
solution to -78 °C overnight resulted in the formation of an
orange oil; warming the oil and solution to -20 °C over 3-4 h
gave crystalline orange 11a , for which adequate analysis could
not be obtained. Yield: 50 mg (8.4%). 1H NMR (C6D6; 300
MHz): δ 7.29 (br, 4 H), 7.0 (m, 4 H), 6.83 (t, 2 H), 5.54 (s, 5
H), 5.42 (s, 5 H), 4.40 (s, 1 H), - 0.15 (s, 3 H), - 0.35 (s, 3 H).
13C NMR (C6D6): δ 112.4 (C5H5), 110.3 (C5H5), 73.5 (CHPh),
0.2 (AlCH3), -1.9 (AlCH3).
AlMe2Cl Ad d u ct 11b. To a suspension of 10b (0.52 g, 1.1
mmol) in hexanes (40 mL) was added (AlMe2Cl)2 (2.1 mL, 2.3
mmol, 1.1 M in hexanes). Over 5 min the 10b dissolved and
formed a yellow solution. After filtering and concentrating (to
ca. 15 mL) the solution was cooled to -20 °C for several hours.
A yellow crystalline product formed that was separated from
the supernatant liquid, dissolved in hexanes (40 mL), filtered,
and cooled to -20 °C; repeating this process gave analytically
pure 11b. Yield: 80 mg (14.8%). 1H NMR (C6D6; 300 MHz): δ
7.24 (t, 2 H), 7.08-6.85 (br, 3 H), 5.64 (s, 5 H), 5.43 (s, 5 H),
4.31 (s, 1 H), 0.36 (s, 9 H), 0.08 (s, 3 H), -0.13 (s, 3 H). 13C
NMR (C6D6): δ 112.1 (C5H5), 108.3 (C5H5), 68.2 (CHPh), 5.4
1
-0.71 (br, 1 H). H NMR (6‚1.5(C6H6); CD2Cl2; 500 MHz; 223
K): δ 7.65 (br, 1 H), 7.36 (br, obs, 1 H), 7.34 (s, 1 H, residual
C6H6), 7.20 (m, 3 H), 7.12 (m, 2 H), 7.01 (m, 2 H), 6.83 (br, 1
H), 6.63 (s, 5 H), 6.41 (br, 1 H), 5.49 (s, 5 H), 4.22 (m, 1 H),
1.60 (m, 2 H), 1.49 (m, 1 H), -2.12 (br, 1 H). HMQC (CD2Cl2;
223 K): δ 139.4/7.65, 139.2/7.36, 129.0/7.20, 127.7/7.12, 127.0/
7.01, 126.5/7.20, 119.0/6.83, 116.5/6.63, 115.7/5.49, 83.9/6.41,
70.4/4.22, 38.5/(1.60, 1.49), 20.3/(1.60, -2.12). 19F NMR:
-130.5 (br), -159.0 (t, J F-F ) 20 Hz), -163.4 (br). Anal. Calcd
for 43H25BF15NZr (942.68): C, 54.79; H, 2.67; N, 1.49.
δ
C
Found: C, 55.00; H, 2.33; N, 1.28.
Cp 2Zr NP h CHP h CH2CH2B(C6F 5)3(THF ) (7). Addition of
THF to solid 6 or slurries of 6 in toluene produced a rapid
color change from brown to yellow and clean formation of 7.
Compound 7 is insoluble in aromatic solvents, and CH2Cl2 and
was recrystallized from THF/toluene or THF/CH2Cl2 to yield
1
yellow blocks. H NMR (THF-d8): δ 7.06 (m, 6 H), 6.68 (s, 5
H), 6.45 (d, 2 H, J H-H ) 7.14), 6.31 (m, 2 H), 6.13 (s, 5 H), 5.65
(m, 1 H), 1.71 (m, obs, 1 H), 1.50 (m, 1 H), 1.28 (m, 1 H), 0.29
(m, 1 H). HMQC (THF-d8): δ 130.0/6.45, 129.4/6.45, 115.5/
6.13, 115.0/6.68, 80.5/5.65, 34.8/(1.71, 1.28), 19.5/(1.50, 0.29).
19F NMR (THF-d8): δ -133.0 (d, J F-F ) 22.6 Hz), -167.2 (t,
J F-F ) 19.7 Hz), -169.4 (m).
Cp 2Zr N(SiMe3)CHP h CH2CH2B(C6F 5)3 (8). A solution of
B(C6F5)3 (0.18 g, 0.35 mmol) in toluene (10 mL) was added to
a solution of 5b (0.15 g, 0.35 mmol) in toluene (10 mL) with
stirring at room temperature; the mixture turned purple, and
after several minutes a purple oil formed. After several hours
the oil crystallized into purple blocks of 8‚2(C6H5CH3) (by X-ray
diffraction and 1H NMR) that were isolated by filtration,
washed with hexanes, and dried under vacuum. Yield: 0.3 g
(83%). Carrying out the reaction in benzene proceeded simi-
larly to give crystalline 8 (by 1H NMR and elemental analysis).
(SiCH3), 2.6 (AlCH3), 1.0 (Al-CH3). Anal. Calcd for C22H31
-

3834 Organometallics, Vol. 18, No. 19, 1999
Harlan et al.
ClNSiZr (491.23): C, 53.79; H, 6.36; N, 2.85. Found: C, 53.75;
H, 6.26; N, 2.73.
Ack n ow led gm en t. Financial support for this work
was provided by Dr. J ohn Birmingham (Boulder Scien-
tific Co.) and NSF Grant CHE-98-96151. The authors
thank J . Tunge and G. Parkin for helpful discussions,
G. Parkin for help with X-ray diffraction studies, and
D. Ramage for initial studies on the interaction of 5a
with aluminum Lewis acids.
X-r a y Str u ctu r e Deter m in a tion s. Crystal data collection
and refinement parameters are summarized in Table 1. Data
were collected on a Bruker P4 diffractometer equipped with a
SMART CCD detector. The structures were solved using direct
methods and standard difference map techniques and refined
by full matrix least-squares procedures using SHELXTL.21
Hydrogen atoms on carbon were included in calculated posi-
tions.
Su ppor tin g In for m ation Available: Tables giving atomic
coordinates, bond lengths and angles, anisotropic displacement
parameters and hydrogen coordinates for 4, 6, 8, and 11a . This
material is available free of charge via the Internet at
http://pubs.acs.org.
(21) Sheldrick, G. M. SHELXTL, An Integrated System for Solving,
Refining and Displaying Crystal Structures from Diffraction Data;
University of Go¨ttingen: Go¨ttingen, Federal Republic of Germany,
1981.
OM9902378