Photolytic zirconium benzyl bond cleavage and subsequent aryl C-F activation in zirconium complexes of fluorinated aryl diamides

Article abstract of DOI:10.1021/om0204181

Photolysis of Zr(C₆F₅NCH₂CH₂OCH₂)₂ (CH₂Ph)₂ at 435 nm results in formation of the metalated dimer {Zr[C₆F₄NCH₂CH₂OCH₂CH ₂OCH₂CH₂NC₆F₅][CH₂ Ph]}₂·(C₇H₈)₂ (1), the bridging difluoride dimer {Zr[C₆F₅NCH₂CH₂OCH₂CH ₂OCH₂CH₂NC₆F₅][F]}₂ [μ-F]₂ (2), and bibenzyl. Complex 1 was characterized by NMR spectroscopy (¹H, ¹⁹F, ¹³C) and X-ray crystallography. In the solid state 1 adopts a pentagonal bipyramidal geometry at each zirconium center with a η¹-benzyl group occupying one of the axial positions. The bridging difluoride 2 was characterized by ¹H and ¹⁹F NMR spectroscopy. For comparison purposes, Zr(C₆F₅NCH₂CH₂OCH₂)₂ Me₂ (3) was synthesized and characterized by NMR spectroscopy and X-ray crystallography. Photolysis of 3 at 435 nm does not result in any reaction.

Full text of DOI:10.1021/om0204181

Organometallics 2002, 21, 3947-3954
3947
P h otolytic Zir con iu m Ben zyl Bon d Clea va ge a n d
Su bsequ en t Ar yl C-F Activa tion in Zir con iu m
Com p lexes of F lu or in a ted Ar yl Dia m id es
Paul E. O’Connor,† David J . Berg,*,† and Tosha Barclay‡
Department of Chemistry, University of Victoria, P.O. Box 3065,
Victoria, British Columbia, Canada V8W 3V6, and Chemistry Department,
North Harris College, 2700 W.W. Thorne Drive, Houston, Texas 77073
Received May 28, 2002
Photolysis of Zr(C6F5NCH2CH2OCH2)2(CH2Ph)2 at 435 nm results in formation of the
metalated dimer {Zr[C6F4NCH2CH2OCH2CH2OCH2CH2NC6F5][CH2Ph]}2‚(C7H8)2 (1), the
bridging difluoride dimer {Zr[C6F5NCH2CH2OCH2CH2OCH2CH2NC6F5][F]}2[µ-F]2 (2), and
bibenzyl. Complex 1 was characterized by NMR spectroscopy (1H, 19F, 13C) and X-ray
crystallography. In the solid state 1 adopts a pentagonal bipyramidal geometry at each
zirconium center with a η1-benzyl group occupying one of the axial positions. The bridging
1
difluoride 2 was characterized by H and 19F NMR spectroscopy. For comparison purposes,
Zr(C6F5NCH2CH2OCH2)2Me2 (3) was synthesized and characterized by NMR spectroscopy
and X-ray crystallography. Photolysis of 3 at 435 nm does not result in any reaction.
In tr od u ction
plicated by the formation of more than one photoproduct
and by rapid thermal reactions, so a definitive under-
The photochemical decomposition of alkyl zirconium
complexes was first observed more than 30 years ago
during the preparation of ZrR4 species.1 This reac-
tion is a serious problem during the preparation of
Zr(CH2Ph)4 in the absence of coordinating solvents,
although the exact nature of the decomposition products
is still not known. The photochemical formation of low-
valent zirconium species, either Zr(III) or Zr(II), is well
documented,2,3 although it is unclear in many instances
whether the Zr(II) oxidation state is formed directly
during photolysis2 or by disproportionation of an un-
stable trivalent state.3i The situation is generally com-
standing of these processes is lacking.4
Although C-F bond activation is quite common for
electron-rich transition metals,5 it is far less common
with electron-deficient early transition metals despite
the exceptional thermodynamic stability of M-F bonds
for these elements.6 In their highest oxidation states,
these metals seldom react with fluorocarbons, and many
complexes containing fluorinated alkyls and ancillary
ligands are known. Perhaps the most significant ex-
-
ample of this is the stability of B(C6F5)4 and B(p-
-
C6H4F)4 counterions in the presence of extremely
electron-deficient Cp2ZrR+ cations, even in the latter
case where direct coordination of F to the metal occurs.7
Most examples of early transition metal C-F activation
* To whom correspondence should be addressed. E-mail: djberg@
uvic.ca. Tel: 250-721-7161. Fax: 250-721-7147.
† University of Victoria.
involve a metal in a reduced oxidation state (e.g., Eu2+
Sm2+, Yb2+, U3+, Ti3+, Zr2+, or Zr3+).8
,
‡ North Harris College.
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K.; Kru¨ger, C. Chem. Ber. 1982, 115, 3300. (e) Erker, G.; Kropp, K.;
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In a previous contribution, we reported the synthesis
and structural characterization of several complexes of
the type Zr(C6F5NCH2CH2OCH2)2(X)(Y) (X ) Cl, CH2Ph,
N(SiMe3)2; Y ) Cl, CH2Ph).9 The alkyl derivatives of
this series (X and/or Y ) CH2Ph or CH2SiMe3) show
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1977, 142, 289. (b) Bamford, C. H.; Puddephatt, R. J .; Slater, D. M. J .
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Organomet. Chem. 1981, 221, C21. (d) J ones, S. B.; Petersen, J . L. J .
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R. J . Chem. Soc., Chem. Commun. 1983, 374. (f) Bajgur, C. S.; J ones,
S. B.; Petersen, J . L. Organometallics 1985, 4, 1929. (g) Lappert, M.
F.; Raston, C. L. J . Chem. Soc., Chem. Commun. 1981, 173. (h) Czisch,
P.; Erker, G.; Korth, H.-G.; Sustmann, R. Organometallics 1984, 3,
945. (i) Wielstra, Y.; Gambarotta, S.; Meetsma, A.; Spek, A. Organo-
metallics 1989, 8, 2948. (j) Cacciola, J .; Reddy, K. P.; Petersen, J . L.
Organometallics 1992, 11, 665. (k) Lappert, M. F.; Raston, C. L.;
Skelton, B. W.; White, A. H. J . Chem. Soc., Dalton Trans. 1997, 2895.
(l) Casty, G. L.; Lugmair, C. G.; Radu, N. S.; Tilley, T. D.; Walzer, J .
F.; Zargarian, D. Organometallics 1997, 16, 8. (m) Dioumaev, V. K.;
Harrod, J . F. Organometallics 2000, 19, 583. (n) Chien, J . C. W.; Wu,
J .-C.; Rausch, M. D. J . Am. Chem. Soc. 1981, 103, 1180. (o) Rausch,
M. D.; Boon, W. H.; Alt, H. G. J . Organomet. Chem. 1977, 141, 299.
(4) Alt, H. G. Angew. Chem., Int. Ed. Engl. 1984, 23, 766.
(5) Kiplinger, J . L.; Richmond, T. G.; Osterburg, C. E. Chem. Rev.
1994, 94, 373.
(6) (a) Doherty, N. M.; Hoffman, N. W. Chem. Rev. 1991, 91, 553.
(b) Simo˜es, J . A. M.; Beauchamp, J . L. Chem. Rev. 1990, 90, 629.
(7) Horton, A. D.; Orpen, A. G. Organometallics 1991, 10, 3910.
(8) Ln2+ (Ln ) Eu, Sm, Yb): (a) Deacon, G. B.; MacKinnon, P. I.;
Tuong, T. D. Aust. J . Chem. 1983, 36, 43. (b) Deacon, G. B.; Koplick,
A. J .; Raverty, W. D.; Vince, D. G. J . Organomet. Chem. 1979, 182,
121. (c) Burns, C. J .; Andersen, R. A. J . Chem. Soc., Chem. Commun.
1989, 136. (d) Watson, P. L.; Tulip, T. H.; Williams, I. Organometallics
1990, 9, 1999. U3+: (e) Weydert, M.; Andersen, R. A.; Bergman, R. G.
J . Am. Chem. Soc. 1993, 115, 8837. Ti3+: (f) Burk, M. J .; Staley, D. L.;
Tumas, W. J . Chem. Soc., Chem. Commun. 1990, 809. M2+ or M3+ (M
) Zr, Ti): (g) Kiplinger, J . L.; Richmond, T. G. J . Am. Chem. Soc. 1996,
118, 1805.
(9) O’Connor, P. E.; Morrison, D. J .; Steeves, S.; Burrage, K. Berg,
D. J . Organometallics 2001, 20, 1153.
10.1021/om0204181 CCC: $22.00 © 2002 American Chemical Society
Publication on Web 08/20/2002

3948 Organometallics, Vol. 21, No. 19, 2002
O’Connor et al.
{Zr [C6F 5NCH 2CH 2OCH 2CH 2OCH 2CH 2NC6F 5][F ]}2[µ-
F ]2 (2). Method 1 (in situ generation during photolysis): A
sample of Zr(C6F5NCH2CH2OCH2)2(CH2Ph)2 (0.010 g, 13 µmol)
dissolved in benzene-d6 was placed in a sealable NMR tube
under argon. The NMR tube was placed in glass cooling jacket
and irradiated with filtered light (435 nm cutoff) from a 150
W incandescent light bulb for 14 h. At the end of this period
no trace of starting material was detectable by 1H NMR
spectroscopy and red crystals of 1 were seen coating the wall
of the tube. Compound 2 was characterized in situ by NMR
spectroscopy. Larger scale photolysis resulted in precipitation
of 2 as a white solid that was very difficult to redissolve.
Method 2 (using n-Bu3SnF): Solid n-Bu3SnF (2 equiv) was
added to a vigorously stirred solution of Zr(C6F5NCH2CH2-
OCH2)2(CH2Ph)2 in toluene. After 24 h, solid 2 was filtered
away from the toluene supernatant containing n-Bu3Sn-
(CH2Ph). The NMR spectrum of 2 generated in this manner
was identical to that observed by method 1. Unfortunately, 2
produced by this method was contaminated with unreacted
n-Bu3SnF. NMR (benzene-d6): 1H δ 3.90 (m, 2H), 3.80 (m, 2H),
3.60 (m, 2H), 3.18 (m, 2H), 2.94 (m, 2H), 2.65 (m, 2H); 19F{1H}
δ +109.2 (s, Zr-Fterm), -51.4 (s, Zr-µ-F-Zr), -150.1 (br s,
o-arylF), -165.8 (t, p-arylCF, 3J FF ) 20 Hz), -166.6 (t, m-arylF,
3J FF ) 21 Hz). Anal. Calcd for C36H24N4O4F24Zr2: C, 35.59; H,
1.99; N, 4.61. Found: C, 35.05; H, 1.74; N, 4.55.
remarkable thermal stability provided they are pro-
tected from ambient light. However, in the presence of
light, these compounds darken slowly at room temper-
ature and much more rapidly at elevated temperatures.
In the case of the dibenzyl derivative, we observed that
a sparingly soluble red crystalline byproduct formed
slowly during recrystallization. In this paper, we report
the structure of this product and show that it results
from the clean photochemical cleavage of a benzyl
zirconium bond in Zr(C6F5NCH2CH2OCH2)2(CH2Ph)2 to
form a Zr(III) complex followed by intramolecular aryl
C-F bond activation to yield a metalated Zr(IV) product.
Exp er im en ta l Section
Gen er a l P r oced u r es. All manipulations were carried out
under an argon atmosphere, with the rigorous exclusion of
oxygen and water, using standard glovebox (Braun MB150-
GII) or Schlenk techniques. Tetrahydrofuran (THF), hexane,
and toluene were dried by distillation from sodium benzo-
phenone ketyl under argon immediately prior to use. Zr(C6F5-
NCH2CH2OCH2)2Cl2 and Zr(C6F5NCH2CH2OCH2)2(CH2Ph)2
were prepared as previously reported.9 Tributyltin fluoride was
purchased from Aldrich and used as received.
Zr (C6F 5NCH2CH2OCH2)2Me2 (3). A stirred suspension of
Zr[CH2OCH2CH2N(C6F5)]2Cl2 (0.485 g 0.76 mmol) in dry
diethyl ether (60 mL) was cooled in a dry ice-acetone bath.
MeLi (1.4 M, 1.1 mL, 1.5 mmol) was added via syringe, and
the mixture was allowed to stir for 75 min. The Schlenk flask
was then transferred to an ice bath and the solvent removed
under vacuum. Toluene (2 × 10 mL) was added to the residue,
and the resulting suspension was filtered through Celite. The
filtrate was concentrated to ca. 4 mL and cooled at -30 °C to
afford 3 as colorless crystals. Repeated recrystallization from
toluene afforded crystals suitable for X-ray diffraction. Yield:
0.219 g (48%). Mp: 153 °C dec. NMR (benzene-d6): 1H δ 3.33
(t, 4H, NCH2 or OCH2CH2N, 3J HH ) 5.5 Hz), 3.11 (t, 4H, NCH2
1H (360 MHz), 13C (90.55 MHz), and 19F (338.86 MHz) NMR
spectra were recorded on a Bruker AMX-360 MHz spectrom-
eter. Benzene-d6 was dried over activated 4 Å molecular sieves,
and THF-d8 was distilled from sodium benzophenone ketyl and
stored over 4 Å molecular sieves prior to use. NMR spectra
were recorded using 5 mm tubes fitted with a Teflon valve
(Brunfeldt) at room temperature unless otherwise specified.
1H and 13C NMR spectra were referenced to residual solvent
resonances. 19F NMR spectra were referenced to external
CCl3F. Melting points were recorded using a Bu¨chi melting
point apparatus and are not corrected. Elemental analyses
were performed by Canadian Microanalytical, Delta, B.C.
{Zr [C6F 4NCH2CH2OCH2CH2OCH2CH2NC6F 5][CH2P h ]}2‚
(C7H8)2 (1). Zr(C6F5NCH2CH2OCH2)2(CH2Ph)2 (1.00 g, 1.33
mmol) was weighed into an Erlenmyer flask in the glovebox
and dissolved in 120 mL of toluene. The solution was allowed
to stand exposed to ambient light for 11 days, during which
time red crystals of 1 deposited from solution. The supernatant
was decanted off and allowed to stand for a further 7 days,
producing a second crop of 1. The crystals were washed with
toluene and dried under reduced pressure. Total yield: 0.098
g (17%). Further crops of 1 can be obtained, but these are
invariably contaminated with increasing amounts of 2. Mp:
3
or OCH2CH2N, J HH ) 5.5 Hz), 2.73 (s, 4H, OCH2CH2O), 0.47
1
(s, 6H, CH3); 13C δ 143.1 (d, o- or m-aryl CF, J CF ) 234 Hz),
1
138.5 (d, o- or m-aryl CF, J CF ) 250 Hz), 133.3 (d, p-aryl CF,
1
1J CF ) 271 Hz), 73.5 (t, OCH2CH2N, J CH ) 143 Hz), 68.9 (t,
1
1
OCH2CH2O, J CH ) 148 Hz), 52.5 (t, NCH2, J CH ) 136 Hz),
45.2 (q, CH3, J CH ) 114 Hz); 19F{1H} δ -152.0 (d, o-arylF,
1
3J FF ) 22 Hz), -165.5 (t, m-arylF, J FF ) 22 Hz), -167.3 (t,
3
3
p-arylF, J FF ) 22 Hz). UV (CH2Cl2): λmax 277 nm (ꢀ 10 000
M-1 cm-1). Anal. Calcd for C20H18N2O2F10Zr: C, 40.06; H, 3.03;
N, 4.67. Found: C, 39.71; H, 3.04; N, 4.59.
P h otolysis Exp er im en ts. Toluene or benzene-d6 solutions
of the samples were irradiated with a 150 W incandescent light
bulb. Samples were cooled during irradiation by means of a
water jacket. The light was filtered through filters with low-
wavelength cutoffs of 375, 435, and 550 nm. Initial experi-
ments showed that only the 435 nm filter afforded clean photo-
products with Zr(C6F5NCH2CH2OCH2)2(CH2Ph)2. Irradiation
using the 375 nm filter resulted in very complex 1H NMR
spectra consistent with formation of several products. Irradia-
tion at 550 nm did not result in any significant reaction over
a period of 28 h. As a control experiment, a solution of
Zr(C6F5NCH2CH2OCH2)2(CH2Ph)2 in toluene was wrapped in
foil and kept at room temperature for several months; no
detectable formation of 1 or 2 was observed, and the starting
zirconium complex was recovered unchanged. Irradiation of
Zr(C6F5NCH2CH2OCH2)2(CH2Ph)2 in THF-d8 at 435 nm re-
sulted in the formation of bibenzyl and a complex mixture of
products that did not contain 1.
3
193 °C (dec). NMR (THF-d8): 1H δ 6.91 (t, 2H, m-arylH, J HH
3
) 7.7 Hz), 6.53 (t, 1H, p-arylH, J HH ) 7.3 Hz), 6.43 (d, 2H,
3
o-arylH, J HH ) 8.2 Hz), 4.38 (m, 1H), 4.31 (m, 2H), 4.24 (m,
1H), 4.12 (m, 2H), 4.05 (m, 1H), 4.00 (m, 2H), 3.77 (m, 2H),
2
3.11 (m, 1H), 1.85 (d, 1H, CHaPh, J HH ) 10.4 Hz), 1.72 (d,
2
1H, CHbPh, J HH ) 10.4 Hz); 13C{1H} δ 151.2 (quat-arylC),
1
1
146.2 (arylCF, J CF ) 228), 145.0 (arylCF, J CF ) 238), 139.4
(arylCF, 1J CF ) 256), 136.6 (arylCF, 1J CF ) 233), 134.2 (arylCF,
1J CF ) 252), 130.0 (quat-arylC), 129.3 (arylCF, J CF ) 228),
1
129.2 (quat-arylC), 128.1 (m-arylC), 126.3 (o-arylC), 120.3 (p-
arylC), 77.4 (NCH2CH2O), 76.9 (NCH2CH2O), 73.1 (OCH2-
CH2O), 71.6 (OCH2CH2O), 68.0 (CH2Ph), 56.4 (NCH2CH2O),
50.2 (NCH2CH2O); 19F{1H} δ -114. 9 (dd, Zr-CCF), -147.1
3
(d, o-arylF, J FF ) 21 Hz), -156.2 (t, NCCF), -163.8 (t,
3
m-arylCF, J FF ) 21 Hz), -165.7 (dd, NCCCF), -167.0 (t,
3
p-arylF, J FF ) 22 Hz), -170.1 (m, ZrCCCF). Anal. Calcd for
C
57H46N4O4F18Zr2 (one molecule of toluene per dimer):10 C,
49.77; H, 3.37; N, 4.07. Found: C, 48.50; H, 3.36; N, 4.09.
X-r a y Cr ysta llogr a p h ic Stu d ies. X-ray quality crystals
of 1 deposited directly from the reaction mixture (toluene
solution). The crystals were placed in mineral oil under an
atmosphere of argon and sealed in a glass capillary. Data were
(10) Prolonged exposure of this compound to vacuum resulted in
partial loss of the toluene of solvation; both 1H NMR and elemental
analysis showed that only one molecule of toluene per dimer remains
after several hours under vacuum.
collected on
a Siemens Smart 1000 CCD diffractometer

Zr Complexes of Fluorinated Aryl Diamides
Organometallics, Vol. 21, No. 19, 2002 3949
equipped with graphite-monochromated Mo KR radiation (λ
) 0.71073 Å) at 293 K. Structure solutions were carried out
using SHELXS-97,11 and refinement was done on F2. An
absorption correction was applied in both cases (abs range: 1
0.72-1.00; 3 0.74-1.00). The final Fourier difference maps
showed maximum and minimum peaks of -0.33/+0.33 (1) and
-0.50/+0.29 (3) e Å-3. Thermal ellipsoid plots were drawn with
ORTEP3.12
Resu lts
During a period of several days’ exposure to ambient
light, a yellow toluene solution of Zr(C6F5NCH2CH2-
OCH2)2(CH2Ph)2 slowly reacted to form insoluble red
crystals of {Zr[C6F4NCH2CH2OCH2CH2OCH2CH2NC6-
F5][CH2Ph]}2‚(C7H8)2 (1). At this point the solution
contained bibenzyl, identified by NMR spectroscopy,13
and a zirconium difluoride species 2 discussed in more
detail below (eq 1). Control experiments clearly showed
that the reaction was photochemical in nature, as
toluene solutions of Zr(C6F5NCH2CH2OCH2)2(CH2Ph)2
wrapped in foil were recovered unchanged after several
months at room temperature. In addition, the reaction
was found to proceed to completion in 14 h when
photolysis was carried out using a 150 W incandescent
light source masked with a 435 nm filter. Photolysis
using a longer wavelength filter (550 nm) resulted in
no detectable reaction over a period of days. Photolysis
at shorter wavelengths (375 nm) produced a complex
mixture of products. This strong wavelength dependence
confirms the photochemical nature of the reaction in eq
1.
F igu r e 1. ORTEP3 drawing12 (thermal ellipsoids at 30%
probability) of 1. The toluene of solvation is omitted for
clarity.
Ta ble 1. Su m m a r y of Cr ysta llogr a p h ic Da ta
1
3
formula
fw
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
C
64H54N4O4F18Zr2
C
20H18N2O2F10Zr
1467.55
triclinic
P1h (No. 2)
8.9913(11)
11.0332(14)
15.2867(19)
82.974(2)
79.912(2)
87.465(2)
1481.4(3)
1
599.58
triclinic
P1h (No. 2)
8.8830(18)
10.415(2)
13.156(3)
83.147(4)
74.269(4)
82.043(4)
1156.0(4)
2
Z
F(calcd) (g cm-3
)
1.64
0.46
Mo KR, 0.7107
293
1.72
0.58
Mo KR, 0.7107
293
µ (mm-1
λ (Å)
)
T (K)
2θmax (deg)
no. obsd reflns
50
11 259
50
6268
no. of unique reflns
5201
4065
F000
740
0.031
0.067
596
0.037
0.072
Ra
b
Rw
a
b
2
R ) ∑(|Fo| - |Fc|)/∑|Fo|. Rw ) [∑w(|Fo| - |Fc| )/∑w(|Fo|)2]1/2
.
crystallography. The ORTEP3 drawing of 1 (Figure 1)
shows a centrosymmetric dimer containing amido ni-
trogen bridges between the two zirconium centers. Data
collection parameters are given in Table 1, and selected
bond distances and angles are listed in Table 2. Each
half of the dimer consists of a metalated C6F4NCH2CH2-
OCH2CH2OCH2CH2NC6F5 ligand bound to the metal by
two anionic amido nitrogens, two neutral ether oxygens,
and the anionic ortho carbon of the metalated C6F4 ring.
The five bonded atoms and the zirconium atom lie
approximately in a plane (maximum deviations: N(1)
+0.35 Å and O(1) -0.26 Å). This leads to a pentagonal
bipyramidal geometry at the metal center when the
axial benzyl group and bridging amido nitrogen from
the other ligand are included.14 The bridging amido
group of each ligand is that which is bonded to the
metalated C6F4 ring. Presumably this amido nitrogen
bridges because metalation constrains the C6F4 ring to
be nearly coplanar with the pentagonal plane (dihedral
angle 24°), thus minimizing steric repulsions incurred
by face-to-face dimerization of monomeric units. In
The red crystals of 1 are very insoluble in hydrocarbon
solvents and do not recrystallize well from ethereal
solvents. Fortunately, the crystals obtained from the
reaction mixture were of sufficient quality to allow us
to establish the structure of this compound by X-ray
(11) Sheldrick, G. SHELX-97: Programs for Crystal Structure
Determination; University of Go¨ttingen, Germany, 1997.
(12) Farrugia, L. J . ORTEP3 for Windows. J . Appl. Crystallogr.
1997, 30, 565.
(13) Bibenzyl was identified by its characteristic 1H and 13C NMR
CH2 resonances: 1H NMR (CDCl3) δ 2.90; 13C NMR 37.9 (t) ppm.
Literature: Pouchert, C. J .; Behnke, J . Aldrich Library of 13C and 1H
FT NMR Spectra, Edition 1; Aldrich Chemical Co.: 1993; Vol. 2, p
8A.

3950 Organometallics, Vol. 21, No. 19, 2002
O’Connor et al.
Ta ble 2. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for 1
Zr(1)′-N(1) 2.401(3) Å). Asymmetry in Zr-N bond
lengths is normally observed in bridging dimethylamido
complexes, although the difference observed in 1 is
considerably less than that usually observed (∆ ) 0.09
Å for 1 compared with ∆ ) 0.18 Å for Zr2(µ-NMe2)15a-f).
This discrepancy is entirely accounted for by a length-
ening of the “short” Zr-N distance within the pentago-
nal plane (i.e., between each Zr and the bridging amido
nitrogen of its own chelating ligand) by ca. 0.09 Å in
comparison to the distance predicted from bridging
dimethylamido zirconium complexes.15a-f In contrast the
“long” bridging Zr(1)′-N(1) distance is very similar to
that observed in bridging dimethylamido complexes
(av 2.396 Å).15a-f The observed trends in bond lengths
are consistent with significant crowding within the
pentagonal plane but less crowding in the axial posi-
tions.
Bond Distances
Zr(1)-N(1)
Zr(1)-N(1)′
Zr(1)-O(2)
Zr(1)-C(19)
2.308(2)
2.401(2)
2.285(2)
2.314(3)
Zr(1)-N(2)
Zr(1)-O(1)
Zr(1)-C(1)
2.163(2)
2.286(2)
2.326(3)
Bond Angles
N(1)-Zr(1)-N(2)
N(1)-Zr(1)-O(1)
N(1)-Zr(1)-C(1)
150.29(8) N(1)-Zr(1)-N(1)′
74.85(7)
133.18(7)
91.93(9)
138.92(7)
93.08(9)
90.02(6)
127.20(8)
88.34(7)
98.44(9)
80.89(9)
105.15(7)
111.2(2)
69.70(7)
61.51(8)
N(1)-Zr(1)-O(2)
N(1)-Zr(1)-C(19)
N(2)-Zr(1)-O(1)
N(2)-Zr(1)-C(1)
O(1)-Zr(1)-N(1)′
O(1)-Zr(1)-C(1)
O(2)-Zr(1)-N(1)′
N(2)-Zr(1)-N(1)′ 94.10(7)
N(2)-Zr(1)-O(2) 72.40(7)
N(2)-Zr(1)-C(19) 99.30(9)
O(1)-Zr(1)-O(2) 66.89(6)
O(1)-Zr(1)-C(19) 81.81(8)
O(2)-Zr(1)-C(1)
C(1)-Zr(1)-N(1)′
165.23(8) O(2)-Zr(1)-C(19)
95.56(8) C(1)-Zr(1)-C(19)
C(19)-Zr(1)-N(1)′ 166.28(8) Zr(1)-N(1)-Zr(1)′
Zr(1)-N(1)-C(6)
Zr(1)′-N(1)-C(6)
90.8(2)
114.8(2)
Zr(1)-N(1)-C(7)
The benzyl group in 1 is bonded η1 to the zirconium
center (Zr(1)-C(19)-C(20) 118.52(18)°). The benzyl
Zr-C bond distance (Zr(1)-C(19) 2.314(3) Å) compares
well with the distance observed in Zr{[CH2OCH2CH2N-
(C6F5)2]2}[CH2Ph]Cl (2.292(11) Å)9 and those reported
in the literature for a wide range of benzyl zirconium
complexes.16 The Zr-C bond distance to the metalated
aryl ring (Zr(1)-C(1) 2.326(3) Å) is well within the range
normally observed for Zr-phenyl bonds,17 although it
is significantly longer than that found in the only other
structurally characterized zirconium complex containing
a four-membered ZrNC2 ring (2.289(4) Å).16d This may
also reflect steric crowding within the pentagonal plane.
Complex 1 dissolves sufficiently in THF-d8 to obtain
NMR spectra. The 1H NMR spectrum shows a low-
symmetry environment where the inequivalent ligand
backbone protons appear as a series of overlapping
multiplets between 3.1 and 4.4 ppm. The benzylic
protons are also inequivalent and appear as AB doublets
at 1.85 and 1.72 ppm. As expected, all six carbons of
(2)Zr(1)′-N(1)-C(7) 116.3(2)
(2)Zr(1)-N(2)-C(13) 134.1(2)
Zr(1)-N(2)-C(12) 115.5(2)
Zr(1)-C(1)-C(6)
N(1)-C(6)-C(1)
90.5(2)
113.8(2)
Zr(1)-C(19)-C(20)
C(6)-N(1)-C(7)
118.5(2)
115.0(2)
C(12)-N(2)-C(13) 110.1(2)
contrast, the C6F5 ring lies nearly perpendicular (di-
hedral angle 98°) to the pentagonal plane, and this
would clearly hinder dimerization through the amido
nitrogen bonded to it.
The geometry at the two amido nitrogens differs
markedly. The bridging amido nitrogen is pyramidal,
with bond angles ranging from 105.15(7)° (Zr(1)-N(1)-
Zr(1)′) to 116.27(15)° (C(7)-N(1)-Zr(1)′); as might be
expected, the angle involving the aryl carbon of the
metalated ring is significantly distorted from normal
tetrahedral values (Zr(1)-N(1)-C(6) 90.8(2)°). The other
amido nitrogen displays the planar coordination geom-
etry (sum of angles about N(2) ) 359.7°) commonly
found in complexes of early transition metals. Consist-
ent with the difference in geometry, the bridging amido
nitrogen has a longer Zr-N distance than the nonbridg-
ing amido nitrogen (Zr(1)-N(1) 2.308(2) Å; Zr(1)-N(2)
2.163(2) Å). The terminal Zr-N bond length found in
this structure is significantly longer than those observed
in Zr{[CH2OCH2CH2N(C6F5)2]2}[N(SiMe3)2]Cl (2.102(5),
2.131(5), and 2.100(5) Å),9 Zr{[CH2OCH2CH2N(C6F5)2]2}-
[CH2Ph]Cl (2.076(9) and 2.076(10) Å),9 or zirconium
complexes containing a terminal dimethylamido group
(average Zr-N 2.055 Å)15a-f and alkyl aryl amides (av
) 2.08 Å).15g-l There is also significant asymmetry in
the bridging Zr-N distances (Zr(1)-N(1) 2.308(2) Å,
(15) Zirconium complexes containing both bridging and terminal
dimethylamides: (a) Daniele, S.; Hitchcock, P. B.; Lappert, M. F.;
Merle, P. G. J . Chem. Soc., Dalton Trans. 2001, 13. (b) Chisholm, M.
H.; Hammond, C. E.; Huffman, J . C. Polyhedron 1988, 7, 2515. (c) Liu,
X.; Wu, Z.; Peng, Z.; Wu, Y. Xue, Z. J . Am. Chem. Soc. 1999, 121, 5350.
(d) Kempe, R.; Hillenbrand, G.; Spannenberg, A. Z. Kristallogr.-New
Cryst. Struct. 1997, 212, 490. (e) Wu, Z.; Diminnie, J . B.; Xue, Z. Inorg.
Chem. 1998, 37, 2570. (f) Petz, W.; Weller, F.; Avtomonov, E. V. J .
Organomet. Chem. 2000, 598, 403. For some representative zirconium
complexes containing terminal alkyl aryl amides see: (g) Zhang, X.;
Zhu, Q.; Guzei, I. A.; J ordan, R. F. J . Am. Chem. Soc. 2000, 122, 8093.
(h) Mehrkhodavandi, P.; Bonitatebus, P. J .; Schrock, R. R. J . Am.
Chem. Soc. 2000, 122, 7841. (i) McMullen, A. K.; Rothwell, I. P.;
Huffman, J . C. J . Am. Chem. Soc. 1985, 107, 1072. (j) Polamo, M.;
Mutikainen, I.; Leskela, M. Z. Kristallogr. 1996, 211, 641. (k) Kasani,
A.; Gambarotta, S.; Bensimon, C. Can. J . Chem. 1997, 75, 1494. (l)
Scollard, J . D.; McConville, D. H.; Vittal, J . J . Organometallics 1995,
14, 5478.
(14) Structurally characterized seven-coordinate zirconium com-
plexes adopting a pentagonal bipyramidal geometry include zirconium
fluoride derivatives14a-h and complexes containing polydentate alkoxy-
and aminoethers.14i-l (a) Bukvetskii, B. V.; Gerasimenko, A. V.;
Davidovich, R. L.; Medkov, M. A. Koord. Khim. 1985, 11, 77. (b) Nelson,
M. J .; Girolami, G. S. J . Organomet. Chem. 1999, 585, 275. (c)
Bukvetskii, B. V.; Gerasimenko, A. V.; Kondratyuk, I. P.; Davidovich,
R. L.; Medkov, M. A. Koord. Khim. 1987, 13, 77. (d) Gao, Y.; Guery,
J .; J acobini, C. Acta Crystallogr. C 1993, 49, 963. (e) Alcock, N. W.;
Errington, W.; Golby, S. L.; Patterson, S. M. C.: Wallbridge, M. G. H.
Acta Crystallogr. C 1994, 50, 226. (f) Il’in, E. G.; Roesky, H. W.;
Aleksandrov, G. G.; Kovalev, V. V.; Sergeev, A. V.; Yagodin, V. G.;
Sergienko, V. S.; Shchelokov, R. N.; Buslaev, Y. A. Dokl. Akad. Nauk
SSSR 1997, 355, 349. (g) Errington, W.; Ismail, M. A. Acta Crystallogr.
C 1994, 50, 1540. (h) Prinz, H.; Bott, S. G.; Atwood, J . L. J . Am. Chem.
Soc. 1986, 108, 2113. (i) Alvanipour, A.; Atwood, J . L.; Bott, S. G.; J unk,
P. C.; Kynast, U. H.; Prinz, H. J . Chem. Soc., Dalton Trans. 1998, 1223.
(j) Polamo, M.; Leskela, M. J . Chem. Soc., Dalton Trans. 1996, 4345.
(k) Male, N. A. H.; Thornton-Pett, M.; Bochmann, M. J . Chem. Soc.,
Dalton Trans. 1997, 2487. (l) Schweder, B.; Walther, D.; Dohler, T.;
Klobes, O.; Gorls, H. J . Prakt. Chem.-Chem.-Zeitung 1999, 341, 736.
(16) For
a representative sample of structurally characterized
zirconium benzyl complexes containing multidentate ancillary ligands
please see: (a) Tshura, E. Y.; Goldberg, I.; Kol, M.; Weitman, H.;
Goldschmidt, Z. J . Chem. Soc., Chem. Commun. 2000, 379. (b) Guerin,
F.; McConville, D. H.; Vittal, J . J .; Yap, G. A. P. Organometallics 1998,
17, 5172. (c) Ziniuk, Z.; Goldberg, I.; Kol, M. Inorg. Chem. Commun.
1999, 2, 549. (d) Qian, B.; Scanlon, W. J .; Smith, M. R.; Motry, D. H.
Organometallics 1999, 18, 1693. (e) Shao, P.; Gendron, R. A. L.; Berg,
D. J .; Bushnell, G. W. Organometallics 2000, 19, 509. (f) Morton, C.;
Munslow, I. J .; Sanders, C. J .; Alcock, N. W.; Scott, P. Organometallics
1999, 18, 4608. (g) Giesbrecht, G. R.; Shafir, A.; Arnold, J . J . Chem.
Soc., Chem. Commun. 2000, 2135. (h) Morton, C.; Gillespie, K. M.;
Sanders, C. J .; Scott, P. J . Organomet. Chem. 2000, 606, 141. (i)
Aizenberg, M.; Turculet, L.; Davis, W. M.; Schattenmann, F.; Schrock,
R. R. Organometallics 1998, 17, 4795. (j) Schrock, R. R.; Seidel, S. W.;
Schrodi, Y..; Davis, W. M. Organometallics 1999, 18, 428. (k) Graf, D.
D.; Schrock, R. R.; Davis, W. M.; Stumpf, R. Organometallics 1999,
18, 843. (l) Schrock, R. R.; Baumann, R.; Reid, S. M.; Goodman, L. T.;
Stumpf, R.; Davis, W. M. Organometallics 1999, 18, 3649.

Zr Complexes of Fluorinated Aryl Diamides
Organometallics, Vol. 21, No. 19, 2002 3951
the ligand backbone are inequivalent in the 13C NMR
spectrum and the benzylic carbon appears at 68.0 ppm
(observable in the DEPT spectrum). The 19F NMR shows
seven inequivalent fluorine resonances. Interestingly,
the resonance due to the fluorine ortho to the metalated
carbon appears far downfield (δ -114.9 ppm) of the
other resonances (δ -147 to -171 ppm). It is noteworthy
that this fluorine is positioned over the face of the C6F5
ring of the same ligand in the crystal structure (F(1)-C
distances (Å): C(13) 3.089(4), C(14) 3.548(4), C(15)
3.782(4), C(16) 3.546(4), C(17) 3.066(4), C(18) 2.819(4)).
This may provide some evidence that the solid state
structure of 1 is maintained in solution, although it is
possible that the same relative positioning of this
fluorine with respect to the C6F5 ring could be main-
tained even if the dimer were to fragment into mono-
mers (with THF occupying the coordination site held
by the bridging amido group, for example).
The observation that bibenzyl was produced along
with 1 and that no benzyl fluoride was detectable by
NMR spectroscopy strongly suggested formation of a
zirconium fluoride species. When the photolysis reaction
was repeated in benzene-d6 solution in an NMR tube, a
single soluble zirconium product (2) containing aryl-
diamide ligand resonances was observed (1 deposits as
benzene-insoluble crystals on the walls of the tube). This
product also proved to be sparingly soluble in hydro-
carbons, which prevented adequate characterization by
13C NMR spectroscopy; however, we were able to obtain
1H and 19F NMR data. While the 1H spectrum of 2
simply contains six multiplets for the ligand backbone,
the 19F spectrum is far more informative. In addition
to the multiplets due to the C6F5 ring between -150
and -167 ppm, two new singlets are observed at +109.2
and -51.4 ppm. The downfield resonance is well outside
the range found for fluorinated aryl groups18 but is
consistent with a terminal Zr-F.19 The resonance at
-51.4 ppm is also too far downfield to arise from an aryl-
CF group, but it agrees quite well with the chemical
shifts normally observed for bridging fluorides of the
type Zr-F-Zr.20 Although these data are not sufficient
to unambiguously assign the structure of 2, they are
consistent with the structure shown in eq 1. Complex 2
slowly precipitates from solution as a pale yellow glassy
solid that is often contaminated with 1. Unfortunately,
2 does not redissolve once it precipitates (a feature
F igu r e 2. ORTEP3 drawing12 (thermal ellipsoids at 30%
probability) of 3.
common to many metal fluorides due to strong bridging
interactions), so we were unable to confirm the structure
of this compound by X-ray diffraction.
Complex 2 can be independently prepared by the
reaction of Zr(C6F5NCH2CH2OCH2)2(CH2Ph)2 with 2
equiv of solid n-Bu3SnF in toluene (eq 2). Resonances
1
due to 2 were observed in the H and 19F NMR of the
supernatant after stirring overnight. In addition,
n-Bu3Sn(CH2Ph) was identified as the major soluble
1
product by H NMR of the supernatant. However, the
low solubility of both 2 and n-Bu3SnF made it impossible
to obtain pure 2 by this route.
To compare the behavior of various zirconium alkyls,
we decided to synthesize the methyl complex Zr(C6F5-
NCH2CH2OCH2)2Me2 (3). The synthesis proceeded
straightforwardly according to eq 3 starting from
Zr(C6F5NCH2CH2OCH2)2Cl2 and MeLi (2 equiv). The
structure of 3 was determined by X-ray crystallography;
an ORTEP3 drawing is shown in Figure 2. Data
collection parameters are given in Table 1, and selected
bond distances and angles are collected in Table 3.
Complex 3 adopts a distorted octahedral geometry with
cis amido nitrogens and cis dialkyls, similar to other
(17) Typical Zr-phenyl bond lengths may be found in the following
references: (a) Amor, F.; Butt, A.; du Plooy, K. E.; Spaniol, T. P.;
Okuda, J . Organometallics 1998, 17, 5836. (b) Erker, G.; Czisch, P.;
Mynott, R.; Tsay, H.; Kruger, C. Organometallics 1985, 4, 1310. (c)
Alcock, N. W.; Clase, H. J .; Duncalf, D. J .; Hart, S. L.; McCamley, A.;
McCormack, P. J .; Taylor, P. C. J . Organomet. Chem. 2000, 605, 45.
(d) Kraft, B. M.; Lachicotte, R. J .; J ones, W. D. J . Am. Chem. Soc. 2001,
123, 10973. (e) Schock, L. E.; Brock, C. P.; Marks, T. J . Organometallics
1987, 6, 232. (f) Fryzuk, M. D.; Mylvaganam, M.; Zaworotko, M. J .;
MacGillivray, L. R. J . Am. Chem. Soc. 1993, 115, 10360. (g) Clegg,
W.; Horsburgh, L.; Lindsay, D. M.; Mulvey, R. E. Acta Crystallogr. C
1998, C54, 315. (h) Giannini, L.; Caselli, A.; Solari, E.; Floriani, C.;
Chiesi-Villa, A.; Rizzoli, C.; Re, N.; Sgamelotti, A. J . Am. Chem. Soc.
1997, 119, 9198. (i) Scott, M. J .; Lippard, S. J . Organometallics 1997,
16, 5857. (j) J effrey, J .; Lappert, M. F.; Luong-Thi, N. T.; Atwood, J .
L.; Hunter, W. E. J . Chem. Soc., Chem. Commun. 1978, 1081. (k)
Fryzuk, M. D.; Haddad, T. S.; Rettig, S. J . Organometallics 1989, 8,
1723. (l) Yue, N.; Hollink, E.; Guerin, F.; Stephan, D. W. Organo-
metallics 2001, 20, 4424. (m) Lee, H.; Bridgewater, B. M.; Parkin, G.
J . Chem. Soc., Dalton Trans. 2000, 4490.
(19) 19F NMR chemical shifts for series of cyclopentadienyl zirconium
complexes containing terminal Zr-F groups range from +20 to +110
ppm (δ relative to CCl3F): (a) Shah, S. A. A.; Dorn, H.; Voigt, A.;
Roesky, H. W.; Parisini, E.; Schmidt, H.; Noltemeyer, M. Organo-
metallics 1996, 15, 3176. (b) Shah, S. A. A.; Dorn, H.; Roesky, H. W.;
Parisini, E. Schmidt, H.; Noltemeyer, M. J . Chem. Soc., Dalton Trans.
1996, 4143. (c) Shah, S. A. A.; Dorn, H.; Gindl, J .; Noltemeyer, M.;
Schmidt, H.; Roesky, H. W. J . Organomet. Chem. 1998, 550, 1. (d)
Herzog, A.; Liu, F.; Roesky, H. W.; Demsar, A.; Keller, K.; Noltemeyer,
M.; Pauer, F. Organometallics 1994, 13, 1251. (e) Murphy, E. F.;
Lubben, T.; Herzog, A.; Roesky, H. W.; Demsar, A.; Noltemeyer, M.;
Schmidt, H. Inorg. Chem. 1996, 35, 23. (f) Herzog, A.; Roesky, H. W.;
J ager, F.; Steiner, A. J . Chem. Soc., Chem. Commun. 1996, 29. (g)
Kraft, B. M.; Lachilotte, R. J .; J ones, W. D. Organometallics 2002, 21,
727.
(18) 19F NMR chemical shifts for perfluorinated aryl groups range
from -110 to -180 ppm (δ relative to CCl3F): (a) Mann, B. E. In NMR
of the Periodic Table; Harris, R. K., Mann, B. E., Eds.; Academic
Press: London, 1978. (b) J ameson, C. J . In Multinuclear NMR; Mason,
J ., Ed.; Plenum Press: New York, 1987.
(20) 19F NMR chemical shifts for series of cyclopentadienyl zirconium
complexes containing bridging Zr-F groups range from -19 to -112
ppm (δ relative to CCl3F): Please see ref 19c-f and: Chen, Y.; Metz,
M. V.; Li, L.; Stern, C. L.; Marks, T. J . J . Am. Chem. Soc. 1998, 120,
6287.

3952 Organometallics, Vol. 21, No. 19, 2002
O’Connor et al.
Ta ble 3. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for 3
recent years there have been a number of photochemical
studies on Cp2ZrR2 and related complexes that clearly
show the formation of both Zr(III) and the free organic
radicals by EPR spectroscopy.3,23 Indeed, in many cases
the Zr(III) species are stable enough to isolate24 from
the reaction mixture, although they are often prone to
subsequent thermal disproportionation to Zr(II) and
Zr(IV).3i A broad and weak EPR signal (g ) 2.001) was
observed during the photolysis of Zr(C6F5NCH2CH2-
OCH2)2(CH2Ph)2, providing some support for initial
formation of a benzyl radical and a Zr(III) benzyl species
4 (eq 4).25 The energy available from 435 nm light (276
kJ mol-1) is similar to the measured strength of Zr-
benzyl carbon bonds (263 kJ mol-1),26 lending further
support to homolytic Zr-C bond cleavage as the initial
photochemical reaction. The lack of reaction for 3 is
probably due to the lack of absorption in the visible
spectrum27 and the greater bond strength of Zr-Me
bonds (285 kJ mol-1).26
Bond Distances
Zr(1)-N(1)
Zr(1)-O(1)
Zr(1)-C(1)
2.133(3)
2.411(2)
2.256(3)
Zr(1)-N(2)
Zr(1)-O(2)
Zr(1)-C(2)
2.103(3)
2.296(2)
2.251(3)
Bond Angles
N(1)-Zr(1)-N(2)
N(1)-Zr(1)-O(2)
N(1)-Zr(1)-C(2)
N(2)-Zr(1)-O(2)
N(2)-Zr(1)-C(2)
O(1)-Zr(1)-C(1)
O(2)-Zr(1)-C(1)
C(1)-Zr(1)-C(2)
116.66(10) N(1)-Zr(1)-O(1) 69.05(9)
101.74(10) N(1)-Zr(1)-C(1) 133.56(13)
87.34(12)
70.82(10)
89.48(12)
77.59(11)
95.05(12)
91.03(13)
N(2)-Zr(1)-O(1) 139.54(10)
N(2)-Zr(1)-C(1) 109.73(13)
O(1)-Zr(1)-O(2) 68.88(10)
O(1)-Zr(1)-C(2) 130.81(12)
O(2)-Zr(1)-C(2) 160.30(12)
Zr(1)-N(1)-C(3) 115.7(2)
C(3)-N(1)-C(15) 117.0(3)
Zr(1)-N(2)-C(9) 122.7(2)
Zr(1)-N(1)-C(15) 123.3(2)
Zr(1)-N(2)-C(8)
C(8)-N(2)-C(9)
123.3(2)
112.3(3)
zirconium complexes of this ligand that we have re-
ported.9 As in earlier structures, one of the oxygen atoms
(O1) is considerably further from the metal than the
other, reflecting a distortion toward a five-coordinate
geometry (Zr(1)-O(1) 2.411(2) Å, Zr(1)-O(2) 2.296(2) Å).
The extent of this distortion is less in 3 than in previous
structures, most likely due to the smaller methyl groups
present here. Other metrical parameters are similar to
previous structures in this family.9
Assuming that photolysis initially produces 4, the
question still remains as to how 4 converts to the
(21) There is more convincing evidence for ligand-induced thermal
reductive elimination from Zr(IV) dialkyls: (a) Gell, K. I.; Harris, T.
V.; Schwartz, J . Inorg. Chem. 1981, 20, 481. (b) Gell, K. I.; Schwartz,
J . J . Am. Chem. Soc. 1981, 103, 2687. (c) Yoshifuji, M.; Gell, K. I.;
Schwartz, J . J . Organomet. Chem. 1978, 153, C15. (d) Gell, K. I.;
Schwartz, J . J . Organomet. Chem. 1978, 162, C11. (e) Gell, K. I.;
Schwartz, J . J . Chem. Soc., Chem. Commun. 1979, 244.
Photolysis of a colorless solution of 3 using a 435 nm
filter resulted in no detectable reaction over a period of
14 h. Photolysis at a shorter wavelength (375 nm)
resulted in photochemical decomposition similar to that
observed for the dibenzyl complex, although the rate of
decomposition was slower for 3. A UV/vis spectrum of
3 showed no significant absorption in the visible region.
(22) Some evidence for photochemical reductive elimination from
a
Cp2Zr(R)(H) type compounds (Cp ) C5Me5, R ) Ph and CH2CH(CH3)2
;
Cp ) C5H5, R ) CH2PPh2b) has been presented, although the possible
involvement of radical pathways was not investigated: (a) Miller, F.
D.; Sanner, R. D. Organometallics 1988, 7, 818. (b) Raoult, Y.;
Choukroun, R.; Blandy, C. Organometallics 1992, 11, 2443.
(23) Bonomo, L.; Toraman, G.; Solari, E.; Scopelliti, R.; Floriani, C.
Organometallics 1999, 18, 5198.
(24) (a) Williams, G. M.; Schwartz, J . J . Am. Chem. Soc. 1982, 104,
1122. (b) Samuel, E. Inorg. Chem. 1983, 22, 2967. (c) Atkinson, J . M.;
Brindley, P. B.; Davies, A. G.; Hawari, J . A. J . Organomet. Chem. 1984,
264, 253. (d) Choukroun, R.; Gervais, D. J . Chem. Soc., Chem.
Commun. 1985, 224. (e) Samuel, E.; Guery, D.; Vedel, J .; Basile, F.
Organometallics 1985, 4, 1073. (f) Choukroun, R.; Basso-Bert, M.;
Gervias, D. J . Chem. Soc., Chem. Commun. 1986, 1317. (g) Blandy,
C.; Locke, S. A.; Young, S. J .; Shore, N. E. J . Am. Chem. Soc. 1988,
110, 7540. (h) Choukroun, R.; Gervais, D.; Raoult, Y. Polyhedron 1989,
8, 1758. (i) Fryzuk, M. D.; Mylvaganam, M.; Zaworotko, M. J .;
MacGillivray, L. R. Polyhedron 1996, 15, 689.
Discu ssion
The pathway for the formation of 1 in the presence
of light is an interesting question. Since bibenzyl is
observed as a byproduct in this reaction and benzyl
fluoride is not, simple abstraction of an ortho aryl
fluorine to afford the monomer of 1 directly can be ruled
out. Bibenzyl could arise either by direct reductive
elimination to afford a Zr(II) species or by homolytic
Zr-C bond cleavage to afford a benzyl radical and a
Zr(III) benzyl complex. There have been many claims
for photochemical reductive elimination to form Zr(II)
in the literature, mostly for Cp2ZrR2 complexes.2 How-
ever, in most instances reductive elimination was as-
sumed to occur based on the observation of R-R
products, and few mechanistic studies supporting this
idea have been presented.21,22 On the other hand, in
(25) Given the long photolysis time required to convert all of the
starting material (as monitored by NMR), the steady state concentra-
tion of benzyl radicals is likely to be very low, making observation
difficult.
(26) Some pertinent group 4 metal-carbon bond dissociation en-
thalpies (kJ mol-1) from ref 6b: Zr-CH2Ph 263 (Zr(CH2Ph)4); Zr-Me
285 (Cp2ZrMe2), 284 (Cp2/ZrMe2); Ti-CH2Ph 237 (Cp2Ti(CH2Ph)2);
Ti-Me 298 (Cp2TiMe2).
(27) The benzyl complex Zr(C6F5NCH2CH2OCH2)2(CH2Ph)2 shows
a shoulder at 259 nm with an ꢀ value of 70 700 M-1 cm-1. Unlike the
methyl derivative, this absorption tails significantly into the visible
region, resulting in the yellow color of the complex.

Zr Complexes of Fluorinated Aryl Diamides
Organometallics, Vol. 21, No. 19, 2002 3953
Sch em e 1
observed products 1 and 2. Two distinctly different
routes can be envisioned: (a) disproportionation of
Zr(III) complex 4 into Zr(IV) and Zr(II) species followed
by oxidative addition of an ortho aryl C-F bond (Scheme
1) or (b) electron transfer to the electron-poor C6F5 ring
followed by C-F bond cleavage (Scheme 2).28 Both of
these processes also require a ligand redistribution
reaction (eqs 5, 6) in order to account for the observed
products. Redistributions such as those shown in eqs 5
and 6 are very well known in zirconium chemistry. It
should also be pointed out that equivalent transforma-
tions within a dimer or higher oligomeric structure can
also be proposed that would account for these transfor-
mations equally well.
Sch em e 2
At this point, we have no evidence to distinguish
between the pathways shown in Schemes 1 and 2. There
is ample precedence for disproportionation of Zr(III)
species into Zr(II) and Zr(IV) under thermal conditions,3i
and while there is no precedent for oxidative addition
of an aryl C-F bond to Zr(II) that we are aware of, there
are many examples with low-valent metals of later
transition metal triads.5 Electron transfer processes
similar to that shown in Scheme 2 have been proposed
to account for C-F bond activation reactions by divalent
(28) Important fundamental studies of the thermal C-F bond
lanthanides with both aryl and aliphatic fluorocarbons.8
Interestingly, C-F bond activation by divalent lan-
thanides was found to be strongly solvent dependent,
proceeding at much slower rates in coordinating sol-
vents. This was attributed to the necessity for coordina-
tion of the C-F bond to the metal center prior to
electron transfer. In our experiments, we found that 1
did not form at all in THF-d8 solution, so it is tempting
to speculate that C-F coordination is also required here.
Of course, it is also possible that a coordinating solvent
cleavage mechanism for both alkyl and aryl C-F bonds in Cp/2Zr(X)2
systems (X ) H, C6F5) have been reported by J ones et al. These studies
suggest that alkyl C-F bond cleavage proceeds by a radical pathway,28b,c
while aryl C-F cleavage can occur either by radical processes28a or
via formation of tetrafluorobenzyne.19g,28c In the present work, it is
possible to postulate a mechanism involving formation of tetrafluoro-
benzyne and a zirconium imide that subsequently react to form the
metalated C6F4 unit. However, this seems unlikely to us as it
necessitates C-N bond cleavage and the formation of two high-energy
intermediates. (a) Edlebach, B. L.; Kraft, B. M.; J ones, W. D. J . Am.
Chem. Soc. 1999, 121, 10327. (b) Kraft, B. M.; Lachicotte, R. J .; J ones,
W. D. J . Am. Chem. Soc. 2000, 122, 8559. (c) Kraft, B. M.; Lachicotte,
R. J .; J ones, W. D. J . Am. Chem. Soc. 2001, 123, 10973.

3954 Organometallics, Vol. 21, No. 19, 2002
O’Connor et al.
may have an effect on the disproportion of Zr(III) and
on C-F oxidative addition, so this fact alone does not
provide any distinction between the pathways in Schemes
1 and 2.
Attempts to trap a putative Zr(II) species by carrying
out the photolysis reaction in the presence of alkynes
or phosphines failed: 1, 2, and bibenzyl were obtained.
Photolysis of Zr(C6F5NCH2CH2OCH2)2(CH2Ph)2 in a
5:1 benzene-d6-C6F6 mixture at 435 nm did not re-
sult in any evidence for intermolecular aryl C-F activa-
tion. It would appear that whatever mechanism is
operative, intramolecular C-F activation is strongly
preferred.
Ack n ow led gm en t. The support of the Natural
Sciences and Engineering Research Council of Canada
is gratefully acknowledged.
Su p p or tin g In for m a tion Ava ila ble: Tables of atomic
coordinates, bond distances and angles, and anisotropic ther-
mal parameters for 1 and 3. This material is available free of
charge via the Internet at http://pubs.acs.org.
OM0204181