Non-redox-assisted oxygen-oxygen bond homolysis in titanocene alkylperoxide complexes: [Cp2TiIV(η1-OO tBu)L]+/0, L = Cl-, OTf-, Br -, OEt2, et3P

Article abstract of DOI:10.1021/om050818z

The titanium(IV) alkylperoxide complex Cp₂Ti(OO^tBu)Cl (1) is formed on treatment of Cp₂TiCl₂ with NaOO ^tBu in THF at -20 °C. Treatment of 1 with AgOTf at -20 °C gives the triflate complex Cp₂Ti-(OO^tBu)OTf (2), which is rapidly converted to the bromide Cp₂Ti(OO^tBu)Br (3) on addition of ⁿBu₄-NBr. The X-ray crystal structures of 1 and 3 both show η¹-OO^tBu ligands. Complex 2 is stable only below -20 °C; ¹H, ¹³C, and ¹⁹F NMR spectra suggest that it also contains an η¹-OO^tBu ligand. Removal of the chloride from 1 with [Ag(Et₂O) ₂]BAr′₄ (Ar′ = 3,5-(CF₃) ₂C₆H₃) yields the etherate complex [Cp ₂-Ti(OO^tBu)(OEt₂)]BAr′₄ (4). Again, coordination of a fourth ligand to the Ti center indicates an η¹-OO^t-Bu ligand in 4. These peroxide complexes do not directly oxidize olefins or phosphines. For instance, the cationic etherate complex 4 reacts with excess Et₃P simply by displacement of the ether to form [Cp₂Ti(η¹-OO^tBu)(Et ₃P)]BAr′₄ (5). Compounds 1-5 all decompose by O-O bond homolysis, based on trapping and computational studies. The lack of direct oxygen atom transfer reactivity is likely due to the η¹ coordination of the peroxide and the inability to adopt more reactive η² geometry. DFT calculations indicate that the steric bulk of the ^tBu group inhibits formation of the hypothetical [Cp ₂Ti(η²-OO^tBu)]⁺ species.

Full text of DOI:10.1021/om050818z

Organometallics 2006, 25, 915-924
915
Non-Redox-Assisted Oxygen-Oxygen Bond Homolysis in Titanocene
Alkylperoxide Complexes: [Cp2TiIV(η1-OOtBu)L]+/0, L ) Cl-, OTf-,
Br-, OEt2, Et3P
Antonio G. DiPasquale, David A. Hrovat,† and James M. Mayer*
Department of Chemistry, Campus Box 351700, UniVersity of Washington,
Seattle, Washington 98195-1700
ReceiVed September 21, 2005
The titanium(IV) alkylperoxide complex Cp2Ti(OOtBu)Cl (1) is formed on treatment of Cp2TiCl2 with
NaOOtBu in THF at -20 °C. Treatment of 1 with AgOTf at -20 °C gives the triflate complex Cp2Ti-
n
(OOtBu)OTf (2), which is rapidly converted to the bromide Cp2Ti(OOtBu)Br (3) on addition of Bu4-
NBr. The X-ray crystal structures of 1 and 3 both show η1-OOtBu ligands. Complex 2 is stable only
below -20 °C; 1H, 13C, and 19F NMR spectra suggest that it also contains an η1-OOtBu ligand. Removal
of the chloride from 1 with [Ag(Et2O)2]BAr′4 (Ar′ ) 3,5-(CF3)2C6H3) yields the etherate complex [Cp2-
Ti(OOtBu)(OEt2)]BAr′4 (4). Again, coordination of a fourth ligand to the Ti center indicates an η1-OOt-
Bu ligand in 4. These peroxide complexes do not directly oxidize olefins or phosphines. For instance,
the cationic etherate complex 4 reacts with excess Et3P simply by displacement of the ether to form
[Cp2Ti(η1-OOtBu)(Et3P)]BAr′4 (5). Compounds 1-5 all decompose by O-O bond homolysis, based on
trapping and computational studies. The lack of direct oxygen atom transfer reactivity is likely due to
the η1 coordination of the peroxide and the inability to adopt more reactive η2 geometry. DFT calculations
t
indicate that the steric bulk of the Bu group inhibits formation of the hypothetical [Cp2Ti(η2-OOtBu)]+
species.
the O-O bond. This can occur in either a homolytic or
heterolytic manner (eqs 1, 2; Fenton/Haber-Weiss-type mech-
anisms).1,6 In both cases, the peroxide cleavage is typically
Introduction
Metal peroxide complexes have been proposed as reactive
intermediates in a variety of oxidation reactions ranging from
industrial to biochemical processes.1,2 Coordination of the
peroxide to the metal center activates the peroxide toward direct
oxidation of substrates and/or toward O-O bond cleavage to
give secondary oxidants. Transition metal alkylperoxide com-
plexes, for instance, are suggested intermediates in catalytic
epoxidation reactions from the industrial production of propylene
oxide to the Sharpless titanium-tartrate chiral epoxidation.1,3,4
In biological systems, many metalloproteins are thought to
utilize hydroperoxide intermediates, formed from H2O2 or from
O2 (+ 2e- + H+).2 Based on these systems, many biomimetic
metal systems have been developed.1,2,5
LnMn+OOR f LnM(n+1)+dO + •OR
LnMn+OOR f LnM(n+2)+dO + -OR
(1)
(2)
viewed as requiring an increase in the oxidation state of the
metal center, to stabilize the resulting metal-oxo species.
Newcomb and Coon have recently proposed that an iron
hydroperoxide species can oxidize C-H bonds by direct OH+
insertion without prior cleavage of the O-O bond (eq 3),7 which
has sparked new debate into metalloenzyme oxidation mecha-
nisms.8 An interesting feature of eq 3 is that the oxidation state
The oxidations of C-H bonds by metal/peroxide systems
have traditionally been viewed as involving initial cleavage of
LnFeIIIOOH + R-H f LnFeIIIO- + ROH2
(3)
+
† Department of Chemistry, University of North Texas, P.O. Box 305070,
Denton, TX 76203-5070.
(1) (a) Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed Oxidation of Organic
Compounds; Academic Press: New York, 1981. (b) Organic Peroxides;
Ando, W., Ed.; Wiley & Sons: New York, 1992. (c) The Chemistry of
Hydroxyl, Ether, and Peroxide Groups (Supplement E2); Patai, S., Ed.; John
Wiley & Sons: New York, 1993. (d) Jones, C. W. Applications of Hydrogen
Peroxide and DeriVatiVes; Royal Society of Chemistry: Cambridge, 1999.
(2) (a) Solomon, E. I.; Brunold, T. C.; Davis, M. I.; Kemsley, J. N.;
Lee, S.-K.; Lehnert, N.; Neese, F.; Skulan, A. J.; Yang, Y.-S.; Zhou, J.
Chem. ReV. 2000, 100, 235-349. (b) Que, L., Jr.; Ho, R. Y. N. Chem.
ReV. 1996, 96, 2607-2624. (c) Klinman, J. P. Chem. ReV. 1996, 96,
1-2561.
(6) (a) Kochi, J. K. Organometallic Mechanisms and Catalysis; Aca-
demic: New York, 1978; Chapter 4. For a few specific examples, see: (a)
Wada, A.; Ogo, S.; Nagatomo, S.; Kitagawa, T.; Watanabe, Y.; Jitsukawa,
K.; Masuda, H. Inorg. Chem. 2002, 41, 616-618. (b) Que, L., Jr.; Ho, R.
Y. N. Chem. ReV. 1996, 96, 2607. (c) Mahadevan, V.; Henson, M. J.;
Solomon, E. I.; Stack, T. D. P. J. Am. Chem. Soc. 2000, 122, 10249-
10250.
(7) (a) Newcomb, M.; Toy, P. H. Acc. Chem. Res. 2000, 33, 449-455.
(b) Chandrasena, R. E. P.; Vatsis, K. P.; Coon, M. J.; Hollenberg, P. F.;
Newcomb, M. J. Am. Chem. Soc. 2004, 126, 115-126. (c) Newcomb, M.;
Hollenberg, P. F.; Coon, M. J. Arch. Biochem. Biophys. 2002, 409, 72-
79. (d) Newcomb, M.; Shen, R.; Choi, S. Y.; Toy, P. H.; Hollenberg, P. F.;
Vaz, A. D. N.; Coon, M. J. J. Am. Chem. Soc. 2000, 122, 2677-2686. (e)
Toy, P. H.; Newcomb, M.; Hollenberg, P. F. J. Am. Chem. Soc. 1998, 120,
7719-7729. (f) Toy, P. H.; Newcomb, M.; Coon, M. J.; Vaz, A. D. N J.
Am. Chem. Soc. 1998, 120, 9718-9719. (g) Toy, P. H.; Dhanabalasingam,
B.; Newcomb, M.; Hanna, I. H.; Hollenberg, P. F. J. Org. Chem. 1997, 62,
9114-9122. (h) Coon, M. J. Annu. ReV. Pharmacol. Toxicol. 2005, 45,
1-25.
(3) Weissermel, K.; Arpe, H.-J. Industrial Organic Chemistry, 3rd ed.;
VCH: New York, 1997; pp 240ff, 267ff.
(4) (a) Katsuki, T.; Sharpless, K. B. J. Am. Chem. Soc. 1980, 102, 5976-
5978. (b) Finn, M. G.; Sharpless, K. B. Asymmetric Synth. 1985, 5, 247-
308. (c) Woodard, S. S.; Finn, M. G.; Sharpless, K. B. J. Am. Chem. Soc.
1991, 113, 106-113. (d) Finn, M. G.; Sharpless, K. B. J. Am. Chem. Soc.
1991, 113, 113-126.
(5) Biomimetic Oxidations Catalyzed by Transition Metal Complexes;
Meunier, B., Ed.; Imperial College Press, 1998.
10.1021/om050818z CCC: $33.50 © 2006 American Chemical Society
Publication on Web 01/21/2006

916 Organometallics, Vol. 25, No. 4, 2006
DiPasquale et al.
of the metal does not change. Therefore if such reactions are
possible, they could occur even with nonoxidizable metal centers
such as d0 ions. d0-Metal ions are widely used to activate
peroxides but for oxygen atom transfer rather than C-H bond
oxidation. The classic example is the selective epoxidation of
terminal alkenes rather than oxidation of their weak allylic C-H
bonds.1,3 These reactions usually involve η2-peroxide or η2-
alkylperoxide complexes, from which oxygen atom transfer
directly gives an oxo or alkoxide complex (eq 4). η2-Peroxide
complexes are more reactive than their η1-isomers because they
are more electronically activated and more sterically accessible.1b,c
alkenes. Instead, they decompose at room temperature via
homolysis of the peroxide bond even though formal oxidation
of the metal center is not possible (eq 5). Homolytic O-O bond
LnMn+(OOR) f LnMn+(O•) + •OR
(5)
cleavage has previously been found in the decomposition of
Cp*2Hf(OOtBu)R compounds.9 The lack of reactivity of the Cp2-
Ti(η1-OOtBu)L complexes is attributed to their inability to form
the more reactive and accessible η2 conformer, even with very
weakly binding ligands L. A preliminary account of part of this
work has appeared.17
Results
I. Synthesis of Cp2TiIV(OOtBu)L Complexes. Titanocene
dichloride (Cp2TiCl2, Cp ) η5-C5H5) reacts with 4 equiv of
sodium tert-butylperoxide (NaOOtBu) in THF at -20 °C to give
the new titanocene tert-butylperoxide complex Cp2Ti(OOtBu)-
Cl (1) (eq 6). Low temperatures are required to prevent the
Reported in this article are the synthesis, characterization,
and reactivity of new titanocene(IV) d0 η1-alkylperoxide
complexes Cp2Ti(OOtBu)L, where L is an anionic or neutral
ligand. This class of compounds was studied in order to probe
the reactivity of peroxide complexes containing a nonoxidizable
metal center. Related hafnium complexes Cp*2Hf(OOtBu)R
(Cp* ) η5-C5(CH3)5, R ) alkyl) have been reported by Bercaw
and co-workers,9 and Cp2Zr(OOtBu)Cl has been very briefly
mentioned by Schwartz and co-workers.10,11 Titanium species
have been widely used to catalyze peroxide reactions, and
various intermediates have been suggested.12 For example, the
active oxidants “TiOOH” and even “TidO” have been proposed
for reactions of titanium silicate molecular sieves using H2O2.13
Homogeneous titanium/tBuOOH systems epoxidize alkenes and
convert sulfides into sulfoxides, in some cases with high
enantiomeric excess.4,14 Closest to the chemistry described here,
Cp2TiCl2 is reported to catalyze the tBuOOH oxidation of bis-
homoallylic alcohols into tetrahydrofuranols and tetrahydropy-
ranols.15 In few of these systems, however, has the reactive
species been isolated and characterized in great detail.4,14,16
The Cp2TiIV(η1-OOtBu)L compounds described here are not
direct oxygen atom transfer reagents toward phosphines and
THF
Cp2TiCl2 + NaOOtBu
8
+ NaCl
t
Cp2Ti(OO Bu)Cl
-20 °C
1
(6)
decomposition of 1 as described below. Reactions using fewer
than 4 equiv of NaOOtBu result in a reduced yield and starting
material still present in the reaction mixture. Complex 1 is
isolated in 84% yield by removal of the THF solvent and
extraction with hexanes to leave unreacted NaOOtBu and NaCl
behind. The yellow solid is >98% pure by 1H NMR (using C6-
Me6 as an internal standard). This synthetic route follows the
briefly mentioned Cp2Zr(OOtBu)Cl;10,11 we have not been able
to locate a procedure or characterization for this zirconium
compound, and our attempts to produce it have not been
successful (as indicated by 1H NMR). The related Cp*2Hf(OOt-
Bu)R was prepared by proteolysis of Cp*2Hf(H)R with anhy-
drous tert-butylhydroperoxide.9
Complex 1 has been characterized by NMR, IR, and mass
spectroscopies, elemental analysis, and X-ray crystallography.
1H and 13C{1H} NMR spectra in THF-d8 show the expected
(8) (a) Auclair, K.; Hu, Z.; Little, D. M.; Ortiz de Montellano, P. R.;
Groves, J. T. J. Am. Chem. Soc. 2002, 124, 6020-6027. (b) Ogliaro, F.; de
Visser, S. P.; Cohen, S.; Sharma, P. K.; Shaik, S. J. Am. Chem. Soc. 2002,
124, 2806-2817. (c) Schoeneboom, J. C.; Neese, F.; Thiel, W. J. Am. Chem.
Soc. 2005, 127, 5840-5853. (d) Pfister, T. D.; Ohki, T.; Ueno, T.; Hara,
I.; Adachi, S.; Makino, Y.; Ueyama, N.; Lu, Y.; Watanabe, Y. J. Biol. Chem.
2005, 280, 12858-12866. (e) Cerny, M. A.; Hanzlik, R. P. Arch. Biochem.
Biophys. 2005, 436, 265-275. (f) Puchkaev, A. V.; Ortiz de Montellano,
P. R. Arch. Biochem. Biophys. 2005, 434, 169-177. (g) Hlavica, P. Eur. J.
Biochem. 2004, 271, 4335-4360. (h) Nam, W.; Ryu, Y. O.; Song, W. J. J.
Biol. Inorg. Chem. 2004, 9, 654-660. (i) Schoeneboom, J. C.; Cohen, S.;
Lin, H.; Shaik, S.; Thiel, W. J. Am. Chem. Soc. 2004, 126, 4017-4034. (j)
Bathelt, C. M.; Ridder, L.; Mulholland, A. J.; Harvey, J. N J. Am. Chem.
Soc. 2003, 125, 15004-15005.
t
singlets for the Cp and Bu groups. IR spectra in CH2Cl2
solutions show a moderate intensity band at 819 cm-1 not
present in Cp2TiCl2 or NaOOtBu, which is tentatively assigned
as the O-O stretch.
Crystals of 1 suitable for X-ray diffraction were grown from
a saturated toluene solution at -5 °C. Structure solution showed
a typical bent-metallocene structure (Tables 1, 2; Figure 1). The
tert-butylperoxo ligand is bound to the titanium center through
only one oxygen, as indicated by the long Ti‚‚‚O(2) distance
of 2.952(2) Å and the open Ti-O(1)-O(2) angle of 121.5(1)°.
Overall the structure is similar to that of the ethoxide analogue,
Cp2Ti(OEt)Cl.18 The Ti-O(1) bond distance in 1 of 1.9090-
(14) Å is somewhat longer than the 1.855(2) Å Ti-OEt bond
distance. The orientation of both the OOtBu and OEt ligands
has the R-substituent (OtBu, Et) out of the equatorial plane,
which allows π-donation from the oxygen p orbital to the empty
1a1 orbital of the Cp2Ti(Cl) metallocene fragment.19 π-Donation
(9) van Asselt, A.; Santarsiero, B. D.; Bercaw, J. E. J. Am. Chem. Soc.
1986, 108, 8291-8293.
(10) Blackburn, T. F.; Labinger, J. A.; Schwartz, J. Tetrahedron Lett.
1975, 35, 3041-3044.
(11) Blackburn, T. F. Transition Metal-Assisted Oxidation of Organic
Compounds. Ph.D. Thesis, Princeton University, 1977.
(12) (a) Fugiwara, M.; Wessel, H.; Hyung-Suh, P.; Roesky, H. W.
Tetrahedron 2002, 58, 239-242. (b) Crocker, M.; Herold, R. H. M.; Orpen,
A. G.; Overgaag, M. T. A. J. Chem. Soc., Dalton Trans. 1999, 3791-
3804. (c) Gallot, J. E.; Kaliaguine, S. Can. J. Chem. Eng. 1998, 76, 833-
852. (d) Moller, M.; Husemann, M.; Boche, G. J. Organomet. Chem. 2001,
624, 47-52.
(13) Ratnasamy, P.; Srinivas, D.; Knoezinger, H. AdV. Catal. 2004, 48,
1-169.
(15) (a) Della Sala, G.; Giordano, L.; Lattanzi, A.; Proto, A.; Scettri, A.
Tetrahedron 2000, 56, 3567-3573. (b) Lattanzi, A.; Della Sala, G.; Russo,
M.; Scettri, A. Synlett 2001, 9, 1479-1481.
(14) (a) The Chemistry of Sulfur-containing Functional Groups; Patai,
S., Rappoport, Z., Eds.; John Wiley & Sons: New York, 1993. (b) Di Furia,
F.; Licini, G.; Modena, G.; Motterle, R.; Nugent, W. A. J. Org. Chem.
1996, 61, 5175-5177. (c) Bonchio, M.; Calloni, S.; Di Furia, F.; Licini,
G.; Modena, G.; Moro, S.; Nugent, W. A. J. Am. Chem. Soc. 1997, 119,
6935-6936. (d) Bonchio, M.; Licini, G.; Modena, G.; Bortolini, O.; Moro,
S.; Nugent, W. A. J. Am. Chem. Soc. 1999, 121, 6258-6268.
(16) Boche, G.; Mo¨bus, K.; Harms, K.; Marsch, M. J. Am. Chem. Soc.
1996, 118, 2770-2771.
(17) DiPasquale, A. G.; Kaminsky, W.; Mayer, J. M. J. Am. Chem. Soc.
2002, 124, 14534-14535.
(18) Huffman, J. C.; Moloy, K. G.; Marsella, J. A.; Caulton, K. G. J.
Am. Chem. Soc. 1980, 102, 3009-3014.

Non-Redox-Assisted Oxygen-Oxygen Bond Homolysis
Organometallics, Vol. 25, No. 4, 2006 917
Table 1. Single-Crystal X-ray Diffraction Collection and
Refinement Data for Cp2Ti(OOtBu)X
Complex 1 reacts rapidly with 1 equiv of silver triflate
(AgOTf) in THF-d8 at -20 °C to quantitatively form the orange
1
triflate complex Cp2Ti(OOtBu)OTf (2) by H NMR (eq 7).
Cp2Ti(OOtBu)Cl (1)
Cp2Ti(OOtBu)Br (3)
empirical formula
fw
temperature (K)
wavelength (Å)
cryst description
color
cryst syst
space group
unit cell dimens
a (Å)
C14H19Cl1O2Ti1
302.64
130(2)
0.71073
prism
orange
C14H19Br1O2Ti1
347.10
130(2)
0.71073
prism
orange
THF
t
+ AgOTf -20 °C8
Cp2Ti(OO Bu)Cl
1
t
+ AgCl (7)
Cp2Ti(OO Bu)OTf
2
monoclinic
P21/c
monoclinic
P21/c
Warming solution or solid samples of 2 above 0 °C results in
decomposition to black material within minutes. This instability
has prevented the isolation of pure 2. H NMR spectra of 2
6.4150(2)
12.0410(5)
19.0220(10)
90
108.7020(14)
90
1391.74(10)
4
6.6130(3)
11.8830(5)
19.0890(10)
90
109.0891(19)
90
1417.57(11)
4
1
b (Å)
c (Å)
show singlets for the Cp and tBu groups downfield from those
of 1 at the same temperature. The triflate ligand is observed as
a quartet at δ 120.4 ppm in the 13C{1H} NMR (JC-F ) 318
Hz) and as a singlet at δ 79.2 ppm by 19F NMR (THF-d8, -20
°C), similar to values for other triflate complexes.23 19F NMR
spectra of solutions containing both 2 and excess AgOTf show
two signals (δ 79.2, 78.6 ppm). Since AgOTf is completely
dissociated in THF,24 this indicates that the triflate is bound to
the titanium center in 2 rather than free in solution. The bound
OTf- strongly suggests that the peroxide is still η1-bound to
Ti.
R (deg)
â (deg)
γ (deg)
volume (Å3)
Z
density (Mg/m3)
1.444
0.798
632
1.626
3.414
704
abs coeff (mm-1
F(000)
)
cryst size (mm)
no. of reflns for
indexing
θ range for data
collection (deg)
index ranges
0.48 × 0.26 × 0.14
0.10 × 0.10 × 0.10
741
269
3.57-28.28
3.26-28.30
Treatment of in situ-generated 2 with 1 equiv of nBu4NBr at
-20 °C forms the bromide analogue of 1, Cp2Ti(OOtBu)Br (3),
in 62% yield based on 1 (eq 8). The highest yields of 3 are
-6 e h e 6
-14 e k e 15
-25 e l e 25
5103
3017
0.0447
-8 e h e 8
-13 e k e 14
-25 e l e 25
5637
3311
0.0508
no. of reflns collected
no. of unique reflns
Rint
completeness to
θ ) 25.00°
abs corr
max./min. transmn
refinement method
no. of data
no. of params
(restraints)
THF
+ nBu NBr -20 °C8
t
Cp2Ti(OO Bu)OTf
4
96.7%
98.8%
2
+ nBu NOTf (8)
t
Cp2Ti(OO Bu)Br
4
semiempirical
0.8965/0.7007
semiempirical
0.7265/0.7265
3
full-matrix least-squares
3017
166 (0)
3311
166 (0)
obtained when pentane and Et2O are used as solvents for
preparative reactions (THF-d8 is used for NMR reactions) and
when the solutions are kept cold throughout. Complexes 1 and
3 are isomorphous (Table 2) and have very similar spectra. The
formation of 3 from 1 via 2 confirms the characterization of 2
and shows that ligand metathesis occurs readily without
disruption of the tert-butylperoxo ligand.
goodness-of-fit on
1.047
0.969
F2 (S)
final R1 (I > 2σ)
wR2 (all data)
largest diff peak and
hole (e/Å3)
0.0393
0.1083
0.384/-0.427
0.0391
0.0741
0.528/-0.540
To avoid the anion coordination to titanium observed for 2,
is supported by the Ti-O(1) bond distance in 1 being ∼0.1 Å
shorter than that predicted for a Ti-O σ bond on the basis of
covalent radii (1.99-2.05 Å20). This π-donation may also be
the cause of the longer Ti-Cl distances in 1 and Cp2Ti(OEt)Cl
(2.396(1), 2.405(1) Å) versus that in Cp2TiCl2 (2.364(3) Å).21
The O(1)-O(2) distances of 1.467(2) Å in 1 and 1.489(12) Å
in Cp*2Hf(OOtBu)Et9 are both within the range of typical
peroxide bond distances (1.42 to 1.50 Å).22 The Cl-Ti-O(1)-
O(2) dihedral angle in 1 of 79.71(11)° is larger than the C(1)-
Hf-O(1)-O(2) dihedral angle of 70.9(7)° in the hafnium
analogue. The only other structurally characterized titanium
alkylperoxide complex is an [(η2-tert-butylperoxo)titanatrane]2
dimer,16 which has Ti-O(1) and O-O bond distances [1.913-
(3) and 1.469(3) Å] that are nearly identical to those in 1.
However, the Ti-O(2) bond distance of 2.269(2) Å and the
Ti-O(1)-O(2) angle of 83.2(2)° for the η2-peroxide are much
smallersby nearly 0.7 Å and 40°sthan in the η1-bound
titanocene complex 1.
-
we have explored compounds with the B[3,5-(CF3)2C6H3)]4
anion (BAr′4-). Reaction of 1 with 1 equiv of AgBAr′4‚xEt2O25
in CD2Cl2 at -20 °C quantitatively forms the ionic species [Cp2-
Ti(OOtBu)(OEt2)]BAr′4 by 1H NMR (4; eq 9). The coordinated
CD2Cl2
t
+ AgBAr ′‚xEt O -20 °C8
Cp2Ti(OO Bu)Cl
4
2
1
t
BAr ′ + AgCl (9)
[Cp2Ti(OO Bu)OEt2]
4
4
ether in 4 [(δ 3.61 (br q, 4H), 1.29 (t, 6H)] derives from the
AgBAr′4‚xEt2O; in our hands ether cannot be removed from
this reagent without decomposition.26 1H NMR spectra of
reaction mixtures at -20 °C show separate Et2O resonances
(23) 13C{1H} NMR spectra and shifts for (a) AgOTf, nBu4NOTf, and
TMS-OTf from: http://www.aist.go.jp/RIODB/SDBS/menu-e.html, Inte-
grated Spectral Data Base System for Organic Compounds (accessed Mar
2005). (b) NaOTf and MeOTf from: Sigma-Aldrich Chemical Co. Web
Site. http://www.sigmaaldrich.com (accessed Mar 2005).
(19) Lauher, J. W.; Hoffman, R. J. Am. Chem. Soc. 1976, 98, 1729-
1742.
(20) Pauling, L. The Nature of the Chemical Bond, 2nd ed.; Cornell
University Press: New York, 1942; p 346.
(21) Clearfield, A.; Warner, D. K.; Saldarriaga-Molina, C. H.; Ropal,
R.; Bernal, I. Can. J. Chem. 1975, 53, 1622-1629.
(22) Reference 1b, p 150.
(24) Cf. Lawrance, G. A. Chem. ReV. 1986, 86, 17-33.
(25) (a) Reger, D. L.; Wright, T. D.; Little, C. A.; Lamba, J. J. S.; Smith,
M. D. Inorg. Chem. 2001, 40, 3810-3814. (b) Hayashi, Y.; Rohde, J. J.;
Corey, E. J. J. Am. Chem. Soc. 1996, 118, 5502-5503.
(26) Ether-free solutions of AgBArF4 in CH2Cl2 at low temperature have
been described,25b but we were not able to prepare such solutions.

918 Organometallics, Vol. 25, No. 4, 2006
Table 2. Selected Bond Distances (Å) and Angles (deg) for Cp2Ti(OOtBu)X (X ) Cl, 1; Br, 3)a
DiPasquale et al.
1
3
1
3
Ti-Cl/Br
2.396(1)
1.909(2)
1.467(2)
1.458(2)
1.522(3)
1.520(3)
1.525(3)
29.6(2)
2.568(1)
1.922(2)
1.472(3)
1.457(3)
1.520(4)
1.522(4)
1.515(4)
29.3(2)
Cp(1)-Ti-Cl/Br
Cp(2)-Ti-Cl/Br
Cp(1)-Ti-Cp(2)
Cp(1)-Ti-O(1)
Cp(2)-Ti-O(1)
Cl/Br-Ti-O(1)
Ti-O(1)-O(2)
O(1)-O(2)-C(11)
O(2)-C(11)-C(12)
O(2)-C(11)-C(13)
O(2)-C(11)-C(14)
105.2(2)
106.2(3)
132.7(3)
110.4(2)
99.7(2)
97.31(4)
121.45(10)
107.65(13)
110.38(16)
101.53(16)
110.34(16)
105.1(2)
106.2(3)
132.7(2)
110.7(3)
100.0(3)
96.43(7)
120.65(14)
107.23(19)
110.3(2)
101.4(2)
110.6(3)
Ti-O(1)
O(1)-O(2)
O(2)-C(11)
C(11)-C(12)
C(11)-C(13)
C(11)-C(14)
Cp(1)-Ti-O(1)-O(2)
Cp(2)-Ti-O(1)-O(2)
Cl/Br-Ti-O(1)-O(2)
Ti-O(1)-O(2)-C(11)
172.4(3)
79.7(3)
156.6(3)
172.7(2)
79.5(3)
155.9(2)
a Cp(#) is the centroid of the Cp ring.
Figure 2. Plot of [1] versus time (using the average of both Cp
t
1
and Bu peaks from H NMR) at 303 K in C6D12 (b, red), C6D6
(9, blue), and CD2Cl2 ([, green) with exponential fits.
Figure 1. ORTEP diagram of Cp2Ti(OOtBu)Cl (1), with thermal
ellipsoids drawn at 30% probability.
II. Decomposition of Cp2TiIV(OOtBu)L Complexes. All of
the Cp2TiIV(OOtBu)L complexes decompose at ambient tem-
peratures in solution. The decompositions and all the reactions
described in this account were done under anaerobic and
anhydrous conditions. Complex 1 decays in CD2Cl2 over a
couple of hours to give tert-butyl alcohol (96%) and a number
of Cp-containing products in low yield, including Cp2TiCl2
(confirmed by spiking with authentic material and by mass
for 4 and for free ether in solution. Removal of the volatiles
and addition of fresh CD2Cl2 results in loss of the free Et2O
peaks. Exchange of bound and free Et2O is thus slow on the
NMR time scale, which is surprising since ether is typically
taken as a poor ligand.1c The observation of a single, simple
quartet for the methylene hydrogens of the bound ether indicates
that there is rapid rotation about the Ti-OEt2 bond. As found
for the triflate complex 2, binding of a weak ligand is favored
over an alternative structure with an η2-peroxide.
1
spectrometry). Products were identified and quantified by H
NMR, integrating versus a C6Me6 internal standard. tert-Butyl
Generated in situ, 4 reacts with 1 equiv of Et3P to give the
alcohol was further confirmed by vacuum transferring the
phosphine complex [Cp2Ti(OOtBu)(Et3P)]BAr′4 (5, eq 10).
1
volatiles from a reaction, acquiring a H NMR spectrum, and
spiking the sample with tBuOH. The hydroxyl resonance of the
CD2Cl2
1
tBuOH was not observed in any reaction mixture in either H
t
BAr ′ + PEt -20 °C8
[Cp2Ti(OO Bu)OEt2]
4
3
or 2H NMR spectra, even after vacuum transfer, so it could not
4
t
be determined whether the product is BuOH or tBuOD. Other
t
BAr ′ + Et O (10)
[Cp2Ti(OO Bu)PEt3]
4
2
titanium products could not be identified. The tert-butoxide
complex Cp2Ti(OtBu)Cl is not among the products; it has been
prepared independently from Cp2TiCl2 and NaOtBu (see Sup-
porting Information). Treatment of reaction mixtures with HCl(aq)
or Me3SiCl did not yield additional Cp2TiCl2. The decomposi-
tions of 2-5 in CD2Cl2 are quite similar to that of 1, except
that a ∼50% yield of Cp2TiBr2 is observed in the case of 3.
The decompositions of 1, 3, 4, and 5 in CD2Cl2 all follow
5
Surprisingly, no oxidation of the phosphine is seen at -20 °C
by 1H or 31P NMR, even in the presence of excess PEt3. Instead,
Et3P simply displaces the bound Et2O. The triflate analogue of
5 is similarly formed on treatment of 2 with PEt3. Like 2 and
4, thermal instability in solution and in the solid state has
precluded isolation of pure 5, which has been characterized by
1
first-order kinetics, as monitored by H NMR (Figure 2; 2 has
1
NMR. The Cp resonance for 5 in the H NMR is a doublet
been generated only in THF). The relative rate constants fall in
the order 4 > 3 = 5 > 1 (Table 3). Overall, there is remarkably
little variation among the four compounds, as the range in kdec
is less than an order of magnitude and the activation parameters
3
with JH-P ) 3 Hz, similar to other titanocene alkylphosphine
complexes.27 A gradient selected-HMBC 2D NMR experiment
1
showed a correlation between the Cp H NMR resonance at δ
6.27 and the 31P NMR resonance at δ 27.6, confirming this
are quite similar. Decomposition of 1 is slower in C6D12 (t1/2
)
assignment.
2.1 h) and C6D6 (1.0 h) than in CD2Cl2 (0.5 h, Figure 2).
III. Reactivity of Cp2TiIV(OOtBu)L Complexes. In the
presence of 1 equiv of Ph3P, the decay of 1 occurs at roughly
the same rate as the decomposition of 1 (both in CD2Cl2; see
(27) (a) Van de Heisteeg, B. J. J.; Schat, G.; Akkerman, O. S.;
Bickelhaupt, F. Tetrahedron Lett. 1987, 51, 6493-6496. (b) Fussing, I. M.
M.; Pletcher, D.; Whitby, R. J. J. Organomet. Chem. 1994, 470, 119-125.

Non-Redox-Assisted Oxygen-Oxygen Bond Homolysis
Organometallics, Vol. 25, No. 4, 2006 919
Table 3. Kinetic Data for the Decompositions of Cp2TiIV(OOtBu)L in CD2Cl2
compound
kdec (s-1) (303 K)
∆Hqa
∆Sqb
∆Gq (298 K)a
range in Tc
Cp2Ti(OOtBu)Cl (1)
(4.1 ( 0.2) × 10-4
(3.2 ( 0.3) × 10-4
(7.1 ( 0.3) × 10-3
(3.2 ( 0.2) × 10-4
24 ( 2
23 ( 2
22 ( 2
24 ( 2
5 ( 2
1 ( 2
3 ( 2
4 ( 2
22 ( 2
22 ( 2
21 ( 2
23 ( 2
283-313
283-318
266-306
286-326
Cp2Ti(OOtBu)Br (3)
Cp2Ti(OOtBu)(OEt2)+ (4)
Cp2Ti(OOtBu)(PEt3)+ (5)
a kcal mol-1
.
b cal K-1 mol-1
.
c Temperature range (K) of rate constants used in Eyring analysis.
a
Table 4. Products and Rate Constants for Reactions of Cp2Ti(OOtBu)Cl (1) in CD2Cl2
reaction
k × 104 (s-1) (303 K)
R3PO
R2POtBu
tBuOHb
Me2CdCH2
tBuCl
Cp2TiCl2
1
4.1 ( 0.2
5.9 ( 0.3
7.2 ( 0.6
6.9 ( 0.5
2.8 ( 0.8
5.2 ( 0.6
96%
24%
29%
26%
98%
72%
n/o
n/o
5%
50%
48%
44%
4%
1 + 1 Ph3P
1 + 5 Ph3P
98%
96%
97%
n/o
n/o
n/o
59%
56%
57%
n/o
12%
11%
11%
n/d
1 + 20 Ph3P
1 + 20 nBu3SnH
1 + 1 Ph3P +
20 nBu3SnH
1 + 1 Et3P
23%
n/o
n/o
n/d
4%
1.6 ( 0.7
3.2 ( 0.7
3.5 ( 0.5
n/o
n/o
∼95%c
3%
28%
95%
tr
8%
tr
tr
3%
tr
27%
28%
34%d
1 + 1 P(OPh)3
1 + CBr4
50%
1
1
2
a n/d ) not determined; n/o ) not observed by H or 31P{1H} NMR; tr ) trace amount (<1%). b Hydroxyl resonance not observed in H or H NMR.
c Et2POtBu grows in to a maximum of 70% yield but is concurrently consumed; the yield reported is for Et2POtBu plus its decomposition products.29 d May
contain Cp2TiClBr or Cp2TiBr2.
n
n
the kinetic studies below). Ph3PO is formed in 98% yield, by
1H and 31P{1H} NMR and by mass spectrometry. Cp2TiCl2 is
produced in 50% yield, together with a number of other
unidentified Cp-containing products. The total Cp integral in
The reaction of 1 with Bu3P forms both Bu2POtBu (78%)
n
1
and Bu3PO (19%, by H NMR integration). These structural
isomers are easily distinguished by 31P{1H} NMR.30 Reaction
of 1 and (PhO)3P gives the mixed phosphite (PhO)2P(OtBu) by
31P NMR and MS (50%);31 the phosphate (PhO)3PO was not
detected.
1
the H NMR (δ 7.0-5.8) remains constant over the course of
the reaction, indicating that little paramagnetic material is formed
t
t
(<5%). The Bu group in 1 is converted to BuOH (23%),
isobutylene (CH2dCMe2, 59%), and tBuCl (12%). These volatile
products were most easily identified by vacuum transferring the
The kinetics of the phosphine reactions were monitored by
1H NMR in CD2Cl2 at 303 K (Table 4). The decay of 1 in the
presence of 1 equiv of Ph3P (both 40 mM) follows first-order
1
volatiles to a new NMR tube, obtaining a H NMR spectrum,
kinetics over 6 half-lives, with k ) (5.9 ( 0.3) × 10-4 s-1
.
and then spiking with authentic materials (tBuCl was also
confirmed by EI-MS). The product yields do not change
substantially when the amount of Ph3P is increased from 1 to
20 equiv. Complexes 2-5 react similarly with excess Ph3P in
CD2Cl2 (THF-d8 for 2) to give high yields of Ph3PO. Excess
PPh3 added to solutions of 5 does not displace the PEt3 bound
to the Ti center.
The first-order kinetics is surprising under these apparently
second-order conditions. This k is only 44% faster than that
found for decomposition of 1 at 303 K [(4.1 ( 0.2) × 10-4
s-1]. With 5 and 20 equiv of Ph3P, k ) (7.2 ( 0.6) × 10-4 s-1
and (6.9 ( 0.5) × 10-4 s-1, which are only ∼20% faster than
the k observed for 1 equiv. Thus the reaction is not first-order
in Ph3P. In the presence of 1 equiv of Et3P or (PhO)3P, the
first-order rate constants for decay of 1 are slower or comparable
to the decomposition of 1 without added reagents: (1.6 ( 0.7)
× 10-4 s-1 [PEt3], (3.2 ( 0.7) × 10-4 s-1 [(PhO)3P] versus
kdec(1) ) (4.1 ( 0.2) × 10-4 s-1. In sum, while the presence of
phosphine or phosphite affects the rate of decay of 1, the
dependence on [R3P] is closer to zero-order than first-order.
As discussed below, the data are consistent with rate-limiting
O-O bond homolysis that does not involve the substrate. To
probe the involvement of a radical chain pathway, the decom-
In contrast, the reaction of 1 with 1 equiv of Et3P in CD2Cl2
does not form any triethylphosphine oxide by 31P{1H} NMR.
Instead, the major phosphorus-containing product is the phos-
phinite Et2POtBu, identified by EI-MS and its characteristic 31
P
chemical shift of δ 109.28 Et2POtBu was independently syn-
t
thesized from Et2PCl, BuOH, and Et3N, confirming the 31P
NMR chemical shift. The origin of this unusual product is
discussed below. Et2POtBu is slowly consumed as the reaction
proceeds, yielding other unidentified phosphorus products.29 The
maximum observed yield of Et2POtBu is 70%, at which point
there is roughly a 25% yield of its apparent decay products, so
1 + PEt3 appears to form the phosphinite quantitatively. No
substantial difference is observed in the Cp region of reaction
1H NMR spectra when compared to spectra for the decomposi-
tion of 1. Reactions of 3 and 5 with Et3P also give high yields
of Et2POtBu and its subsequent decomposition products. The
decomposition of the phosphine complex 5 in the absence of
added Et3P does not form either phosphinite or phosphine oxide.
Apparently the Et3P initially present in 5 remains bound to
titanium throughout decomposition.
n
position of 1 was examined in the presence of Bu3SnH and
CBr4; neither had a substantial effect on the rate constant.
Added cyclohexene, norbornene, trans-stilbene, dimethyl
sulfide, or allyl alcohol does not significantly affect the
decomposition of 1. No products of oxidation of these substrates
were observed by 1H NMR or by GC-FID. There are only slight
changes in the ratios of the products of decomposition of 1,
and the rate constant for decay of 1 is essentially unchanged.
Similarly, the decomposition products from the triflate complex
2 are unaffected by the presence of cyclohexene, norbornene,
or trans-stilbene.
n
(28) Phosphorus-31 NMR: principles and applications; Gorenstein, D.,
Ed.; Academic Press: Orlando, 1984.
(30) 31P{1H} NMR: nBu2POtBu, δ 105 and Bu3PO, δ 47 (identical to
that reported for nBu3PO in CDCl3: Albright, T. A.; Freeman, W. J.;
Schweizer, E. E. J. Org. Chem. 1975, 40, 3437-3441).
(31) Ko¨nig, T.; Habicher, W. D.; Ha¨hner, U.; Pionteck, J.; Ru¨ger, C.;
Schwetlick, K. J. Prakt. Chem./Chem.-Ztg. 1992, 334, 333-349. EI-MS-
[(PhO)2P(OtBu)]: M+ ) 290 m/z.
(29) The consumption of Et2POtBu in reactions of Et3P with 1 likely
t
results from addition of BuO• to Et2POtBu. The apparent products of this
addition appear in the 31P NMR spectrum from δ 70-30 ppm, suggesting
that they are phosphorus(V) oxides.

920 Organometallics, Vol. 25, No. 4, 2006
DiPasquale et al.
Table 5. Calculated Gas-Phase Bond Lengths (Å) and Angles (deg) at B3LYP/6-31G*-Optimized Geometries
Cp2Ti(OOtBu)Cla
Cp2Ti(O•)Clb
Cp2Ti(OOH)+
Cp2Ti(OOMe)+
Cp2Ti(OOtBu)+
Ti-O(1)
1.880 (1.909)
2.838 (2.952)
2.380 (2.396)
2.110 (2.052)
2.136 (2.072)
1.449 (1.467)
118.2 (121.5)
95.36 (97.31)
108.8 (107.7)
157.9 (156.6)
n/a
1.699
n/a
2.384
2.078
2.111
n/a
n/a
101.4
n/a
1.936
2.085
n/a
2.044
2.050
1.460
74.2
1.928
2.082
n/a
2.052
2.054
1.460
74.4
1.917
2.069
n/a
2.055
2.065
1.462
74.1
Ti-O(2)
Ti-Cl
Ti-Cp(1)c
Ti-Cp(2)c
O(1)-O(2)
Ti-O-O
O-Ti-Cl
O-O-R
n/a
n/a
n/a
102.7
-113.6
3.4
109.4
-131.9
7.2
113.5
-148.2
11.7
Ti-O-O-Rd
twist anglee
n/a
n/a
a Solid-state values from X-ray crystallography in parentheses. b UB3LYP. c Distance to Cp centroid. d Torsion angle. e Deviation from orthogonality
between the Cp#-Ti-Cp# plane (Cp# ) Cp centroid) and the O(1)-Ti-O(2) plane for η2-bound peroxides (see Figure 5).
IV. Computational Studies. Cp2Ti(OOtBu)Cl (1), its ho-
molysis to [Cp2Ti(O•)Cl + tBuO•], and several [Cp2Ti(OOR)+]
cations (R ) H, Me, tBu) have been studied at the B3LYP level
of density functional theory (DFT), with the 6-31G* basis set.
Selected metrical data from computed gas-phase structures are
given in Table 5 (see Supporting Information for complete
details and for computations using the LANL2DZ basis set,
which give results similar to those using the 6-31G* basis set).
There is very good agreement between the calculated structure
of 1 and that observed in the solid state. The largest discrepancy
in a bond length is between the calculated Ti-O distance, being
0.03 Å shorter than that found in the X-ray structure. The
calculated gas-phase enthalpy for O-O bond homolysis in 1 is
21.6 kcal mol-1. This is a lower limit to the enthalpic barrier
and is therefore in good agreement with the experimental
solution value ∆Hq ) 24 ( 2 kcal mol-1. The calculations are
discussed in more detail below.
Addition of tBuO• to Ph3P, however, is known to proceed by
â-scission in [Ph3POtBu] to give the phosphine oxide Ph3PO.
Presumably the greater strength of the Ph-P bond disfavors
R-scission.32 nBu3P reacts with BuO• to give 80% Bu2POtBu
and 20% nBu3PO,33 and (PhO)3P + tBuO• gives solely (PhO)2-
POtBu.34 In each case, these products from tBuO• quantitatively
match the products observed on reaction of 1 with the respective
R3P. Unfortunately, no products resulting from the ethyl radical
(eq 12), such as ethane or chloroethane, were detected in the
1H NMR spectra of reaction mixtures. Thus the reactions of 1
with R3P involve tBuO• as the active oxidant; 1 does not oxidize
the phosphines directly.29 The formation of Et2POtBu in
reactions of 3 and 5 with Et3P indicates a similar pathway in
t
n
t
these cases. It should be noted that BuOO• is not involved in
the reactions of 1 since this peroxyl radical is known to
quantitatively oxidize (PhO)3P to the phosphate (PhO)3PO,35
which is not observed. tBuOO• in principle could be formed by
Ti-O bond homolysis, as reported recently for Cp2Ti(TEMPO)-
(Cl).36
Discussion
t
As a further test for the intermediacy of BuO•, reactions of
1 with Ph3P were run in the presence of tri-n-butyl tin hydride.
Given the broad interest in peroxide complexes, and titanium
peroxide complexes specifically, it is surprising that few
examples have been isolated and studied in detail.1,2,4,13,14 In
this work, a series of titanocene tert-butylperoxide complexes
have been prepared starting from Cp2TiCl2 and NaOOtBu
(Scheme 1). With well-characterized materials in hand, we have
been able to probe the mechanisms of decomposition and various
reactions.
nBu3SnH reduces tBuO• to tBuOH with a rate constant of 2.2 ×
108 M-1 s-1 37
almost an order of magnitude slower than the
,
rate constant for Ph3P + tBuO• (1.9 × 109 M-1 s-1).32 Reaction
of 1 + Ph3P + 20 nBu3SnH yields 23% Ph3PO, as compared to
n
t
98% in the absence of Bu3SnH, and the amount of BuOH
increases from 24% to 72% (Table 4). Consistent with the
known rate constants, the BuO• is trapped mostly, but not
completely, by the 20-fold excess of Bu3SnH over the Ph3P.
The changes are not due to any direct reaction of 1 and Bu3-
t
n
I. Pathway for Decomposition of Peroxide Complexes:
O-O Bond Homolysis. The formation of the mixed phosphin-
ites (Et2POtBu from Et3P, nBu2POtBu from nBu3P, and (PhO)2-
POtBu from (PhO)3P) are to our knowledge unprecedented
reactions of metal alkylperoxo complexes. Typically, peroxo
species rapidly oxidize phosphines and phosphites to phosphine
oxides and phosphates, respectively.1 The only reasonable way
to form R2POtBu compounds in these reactions is via phosphine
trapping of the tert-butoxyl radical. tBuO• adds to Et3P and other
phosphines at close to the diffusion limit to form metastable
phosphoranyl radicals [R3POtBu]. The radical from Et3P
decomposes on the millisecond time scale by R-scission (R to
the radical center), forming the phosphinite and Et• (eqs 11,
12).32 R-Scission is the only observed pathway for [Et3POtBu],
n
SnH, as the decay of 1 is actually decelerated by the addition
n
of the 20 equiv of Bu3SnH (Table 4).
tert-Butoxyl radicals are reduced by Ph3P to give tBu•.32a tBu•
t
can form BuCl by abstracting Cl•sfrom the CD2Cl2 solvent,
1, Cp2TiCl2, and/or other titanium productssor can form
isobutylene by donating a H• to some mild oxidant in the
solution (tBu• has very weak C-H bonds). tBuCl and isobutylene
(32) (a) kPR3+tBuO• ) 1.2 and 1.9 × 109 M-1 s-1 for Et3P and Ph3P,
respectively: Griller, D.; Ingold, K. U.; Patterson, L. K.; Scaiano, J. C.;
Small, R. D., Jr. J. Am. Chem. Soc. 1979, 101, 3780-3785. (b) Davies, A.
G.; Dennis, R. W.; Griller, D.; Roberts, B. P. J. Organomet. Chem. 1972,
40, C33-C35.
(33) Chatgilialoglu, C.; Ingold, K. U.; Scaiano, J. C. J. Am. Chem. Soc.
1981, 103, 7739-7742.
t
even though the alternative â-scission to give Et3PO and Bu•
is much more thermodynamically favorable.
(34) Kochi, J. K.; Krusic, P. J. J. Am. Chem. Soc. 1969, 91, 3944-
39466.
(35) Pobedimskii, D. G.; Kirpichnikov, P. A. J. Polym. Sci., Polym.
Chem. Ed. 1980, 18, 815-825.
(36) (a) Huang, K. W.; Waymouth, R. M. J. Am. Chem. Soc. 2002, 124,
8200-8201. (b) Huang, K. W.; Han, J. H.; Cole, A. P.; Musgrave, C. B.;
Waymouth, R. M. J. Am. Chem. Soc. 2005, 127, 3807-3816.
(37) Scaiano, J. C. J. Am. Chem. Soc. 1980, 102, 5399-5400.
-1
M-1 s
ktrap) 1.2 × 109
PEt3 + tBuO•
8 [Et3P•OtBu] (11)
-1
s
kR-scission∼104
[Et3POtBu]
8 Et2POtBu + Et•
(12)

Non-Redox-Assisted Oxygen-Oxygen Bond Homolysis
Organometallics, Vol. 25, No. 4, 2006 921
Scheme 1. Synthetic Routes to Complexes 2-5 from 1
t
lower than ν(OO) in BuOOtBu (920 cm-1 39). Structural
Scheme 2. Mechanism for the Decomposition of
Cp2TiIV(OOtBu)L in the Presence of Et3P or Ph3P
parameters, however, do not show much difference in 1 versus
tBuOOtBu, either in the O-O distance (1.467(2) versus 1.478-
(3) Å) or the X-O-O-C torsion angle (156.6(1)° versus 164.1-
(7)°).40 The DFT-calculated gas-phase enthalpy for O-O bond
homolysis in 1 of 21.6 kcal mol-1 is also consistent with the
experimental enthalpic barrier in solution of 24 ( 2 kcal mol-1
.
This barrier to homolysis is 10 kcal mol-1 lower than the 34
kcal mol-1 determined for BuOOtBu and 16-19 kcal mol-1
t
lower than organic hydroperoxides, which typically have a bond
strength of 40-43 kcal mol-1 1b
.
O-O bond homolysis is a common mode of decomposition
of metal alkylperoxide complexes, as in the Haber-Weiss
mechanism (and the Fenton reaction for Fe2+ + H2O2).1,6a Que
and co-workers have generated Fe(TPA)(OOtBu) at low tem-
perature and determined that it undergoes homolysis to BuO•
t
are the dominant C4 products observed from 1 + Ph3P (Table
4). In the absence of phosphines, the tBuO• abstracts hydrogen
from solvent or some other source to give high yields of tBuOH/
tBuOD. Apparently, this abstraction is faster than decomposition
of tBuO• to acetone, as none of this product is observed.38 The
and an iron(IV)-oxo intermediate (TPA ) tris(2-pyridylmethyl)-
amine).41 A series of cobalt(III) alkylperoxide complexes have
been prepared by Mascharak et al. and shown to undergo
homolysis upon mild heating.42 In contrast, the copper(II) tert-
butylperoxide complex Cu(OOtBu)(HB(3-tBu-5-iPrpz)3) does not
appear to undergo homolysis (which is calculated to have a high
barrier) but rather acts as an electrophilic oxidant.43 The only
previous examples of d0 peroxide complexes undergoing ho-
molysis are the hafnocene derivatives Cp*2Hf(OOtBu)Cl and
Cp*2Hf(OOtBu)(C6H5) reported by Bercaw et al.9 In the
presence of 9,10-dihydroanthracene, the latter compound con-
verts to Cp*2Hf(C6H5)OH and tBuOH.9 The barrier for homoly-
sis in Cp*2Hf(OOtBu)Cl is ∆Hq ) 22.6 kcal mol-1, similar to
what is seen for 1-5.
t
24% yield of BuOH in the presence of Ph3P likely results not
from this H atom abstraction pathway, but rather from oxidation
of tBu• by 1 or some other species. In the presence of Et3P, the
tBu groups are retained in the Et2POtBu product and very low
t
t
yields of BuOH, BuCl, and CH2dCMe2 are found.
The data all indicate the presence of BuO• as the primary
t
oxidant, formed by homolytic cleavage of the O-O bond
(Scheme 2). Unimolecular O-O bond homolysis is the rate-
limiting step, as the rate constants for decomposition (kdec) with
various added substrates over a range of concentrations vary
by less than a factor of 2 (Table 4). The variation in kdec is
likely due to changes in the stoichiometry of decomposition.
For instance, if the tBu• radical generated from 1 in the presence
of Ph3P itself consumes 1 equiv of 1, then the observed rate
constant for the decay of 1 would be twice the rate of homolysis.
Rate-limiting homolysis is further supported by the small
positive ∆Sq values for the decompositions of 1, 3, 4, and 5
(Table 3). A radical chain mechanism is not likely because of
the reproducible simple first-order kinetic behavior and because
the decomposition of 1 is little affected by the oxidative and
Homolyses of the O-O bonds in Cp2TiIV(OOtBu)L (1-5)
and Cp*2HfIV(OOtBu)L are surprising because, unlike the cases
above, the oxyl-metal product cannot be stabilized by oxidation
of the metal center. Homolysis of 1 generates tBuO• and a titanyl
species “Cp2Ti(O•)Cl”, following eq 5 above. While the fate of
(39) Vacque, V.; Sombret, B.; Huvenne, J. P.; Legrand, P.; Suc, S.
Spectrochim. Acta, Part A 1997, 53, 55-66.
(40) Slovokhotov, Y. L.; Timofeeva, T. V.; Antipin, M. Y.; Struchkov,
Y. T. J. Mol. Struct. 1984, 112, 127.
(41) (a) Zang, Y.; Kim, J.; Dong, Y.; Wilkinson, E. C.; Appelman, E.
H.; Que, L., Jr. J. Am. Chem. Soc. 1997, 119, 4197-4205. (b) Lehnert, N.;
Ho, R. Y. N.; Que, L., Jr.; Solomon, E. I. J. Am. Chem. Soc. 2001, 123,
8271-8290.
(42) (a) Chavez, F. A.; Nguyen, C. V.; Olmstead, M. M.; Mascharak, P.
K. Inorg. Chem. 1996, 35, 6282-6291. (b) Chavez, F. A.; Briones, J. A.;
Olmstead, M. M.; Mascharak, P. K. Inorg. Chem. 1999, 38, 1603-1608.
(c) Chavez, F. A.; Mascharak, P. K. Acc. Chem. Res. 2000, 33, 539-545.
(43) Chen, P.; Fujisawa, K.; Solomon, E. I. J. Am. Chem. Soc. 2000,
122, 10177-10193, and references therein.
n
reductive radical traps CBr4 and Bu3SnH (Table 4). The ease
of homolysis in 1 is consistent with the tentatively assigned
O-O stretching frequency, 819 cm-1, since this is significantly
t
(38) The fate of BuO• in methylene chloride with added cyclohexane
(as a H• source) has been documented: Russell, G. A. J. Org. Chem. 1959,
24, 300-302.

922 Organometallics, Vol. 25, No. 4, 2006
DiPasquale et al.
t
Figure 3. Geometry optimizations for [Cp2Ti(η2-OOR)+], R ) H, Me, and Bu (B3LYP/6-31G*).
the tert-butoxyl radical is amply documented (see above), there
is essentially no experimental information about the titanyl
radical product Cp2Ti(O•)Cl. The titanyl might have been
expected to add to olefins, to transfer its oxygen atom to Et3P
to form Et3PO, or to be trapped by nBu3SnH, but these are not
observed. This contrasts with the trapping of putative “Cp*2-
Hf(O•)Cl” by 9,10-dihydroanthracene to give the hydroxide.9
It is possible that Cp2Ti(O•)Cl is highly reactive and is rapidly
converted to a myriad of products, making it difficult to trap
with external reagents.
The nature of the implicated titanyl intermediate, Cp2Ti(O•)-
Cl, has been probed by DFT calculations using the UB3LYP
functional (Table 5). The computed Ti-O distance of 1.699 Å
is 0.18 Å shorter than the Ti-O bond in 1 and is a reasonable
value for a “normal” titanyl (LnTiIVdO) complex. Cp*2Ti(dO)-
(4-phenylpyridine), for instance, has d(TidO) ) 1.665(3) Å,
as measured by X-ray crystallography.44 This suggests the
presence of some Ti-O π bonding in Cp2Ti(O•)Cl, although
no specific π-bonding orbital is evident in the valence MOs.
The Mulliken atomic spin populations (R spin-â spin; see
Supporting Information) indicate that the oxyl oxygen carries
the largest spin density (0.58), followed by the titanium (0.22)
and four of the Cp carbon atoms (0.12-0.19). These four carbon
atoms are close to the Ti-O π orbital perpendicular to the Ti-
O-Cl plane, and such an interaction may be responsible for
their spin densities. The Ti-Cp interaction in Cp2Ti(O•)Cl
appears to be perturbed relative to 1, based on the ∼0.03 Å
increase in the Ti-Cp(centroid) distances (although as above
this is not evident from the valence MOs). In sum, the
calculations suggest that the putative titanyl intermediate could
be described as a mixture of resonance structures Cp2Ti(O•)Cl
and (Cp•)(Cp)Ti(dO)(Cl).
and other early metals,1-5 so there must be some feature(s)
specific to the metallocene structure that disfavor η2-coordina-
tion. Electronically, η2-coordination prevents π-donation from
the peroxide ligand because the frontier orbitals of the bent
metallocene are localized in the “wedge” plane.19 Thus a species
such as [Cp2Ti(η2-OOR)+] could only achieve a 16e- config-
uration. Perhaps more important is the unfavorable steric
t
interaction between the Cp ligands and the Bu substituent in
[Cp2Ti(η2-OOtBu)+]. This steric clash is indicated by the DFT-
calculated structures for [Cp2Ti(η2-OOR)]+ with R ) H, Me,
t
and Bu. The optimized geometries (Figure 3, Table 5) show
increasing distortions of the peroxide ligand with increasing
steric bulk of the R group. The peroxide twists out of the
equatorial plane of the metallocene: the deviations from
orthogonality between the O-Ti-O and Cp#-Ti-Cp# planes
(Cp# ) Cp centroid) are 3.4°, 7.2°, and 11.7° for R ) H, Me,
and tBu, respectively. In addition, the R group is pushed away
from the metal, with increasing Ti-O-O-R torsion angles of
114°, 132°, and 148°. Remarkably, the η2-form [Cp2Ti(η2-
OOtBu)]+ is calculated to be 1 kcal mol-1 higher in enthalpy
than the unsaturated [Cp2Ti(η1-OOtBu)]+ isomer. This is
consistent with even very weak ligands giving [Cp2Ti(η1-
OOtBu)(L)]+/0 rather than an η2 form. In essence, the bulk of
the tBu group prevents the formation of the η2 isomer. In light
of this, catalytic systems using titanocene dichloride and
tBuOOH seem likely to involve cleavage of a Cp ring to allow
for η2-coordination and the resulting reactivity observed.12,15
Conclusions
A series of titanocene complexes with η1-tert-butylperoxide
ligands, Cp2TiIV(η1-OOtBu)L, have been prepared [L ) Cl (1),
OTf (2), Br (3), Et2O (4), PEt3 (5)]. Compounds 1-5 all
decompose at ambient temperature, and phosphine-trapping
studies indicate a mechanism of O-O bond homolysis for 1,
3, and 5. Homolysis generates tBuO•, which reacts with Et3P to
give Et2POtBu. Facile O-O homolysis is surprising since the
titanium(IV) centers are d0 and cannot be oxidized; peroxide
bond homolyses in transition metal complexes typically are
facilitated by concomitant oxidation of the metal. Compounds
1-5 do not oxidize alkenes or phosphines, which was unex-
pected in light of the wide use of titanium compounds as
catalysts for oxidations where peroxides are the terminal
oxidants. Homolysis and the lack of direct oxygen atom
reactivity in 1-5 are likely due to the inability of the complexes
to form more reactive η2 conformers, apparently for steric
reasons.
II. Attempted Conversion to η2-Alkylperoxide Com-
plexes: The Limited Reactivity of 1-5. Compounds 1-5 are
remarkably inert toward oxygen atom transfer for a peroxide
complex, particularly as a complex of a Lewis-acidic, early
transition metal. We attribute this lack of reactivity to the η1
coordination mode of the alkylperoxide ligand. These com-
pounds all have a 16-electron countsconsidering the peroxide
as a σ-only ligandsand could in principle convert to an η2
structure without ligand loss. Instead, they approach an 18e-
configuration through π-donation from the peroxide R oxygen,
as indicated by the short Ti-O distances and the orientation of
the η1 peroxide ligand. Removal of the chloride ligand in 1 does
not form a cationic η2 complex: even the weak ligands triflate
and Et2O bind to the titanium centers in 2 and 4 in preference
to an η2 complex.
η2 Complexes of tBuOO- are thought to be common
intermediates in reactions mediated by compounds of titanium
Experimental Section
General Procedures. All manipulations were performed under
an argon or nitrogen atmosphere, using standard high-vacuum-line
(44) Smith, M. R.; Matsunaga, P. T.; Andersen, R. A. J. Am. Chem.
Soc. 1993, 115, 7049-7050.

Non-Redox-Assisted Oxygen-Oxygen Bond Homolysis
Organometallics, Vol. 25, No. 4, 2006 923
or inert atmosphere glovebox techniques unless otherwise noted.
All glassware was flame-dried under vacuum immediately before
use. Protio (Fisher Scientific) and deuterio solvents (Cambridge
Isotope) were dried and degassed over Na/Ph2CO (pentane,
cyclohexane, hexanes, benzene, toluene, Et2O, THF) or CaH2 (CH2-
Cl2) and vacuum transferred immediately before use. Cp2TiCl2
(99+%) and AgOTf (99%) (both Strem) were used as received.
AgBAr′4‚xEt2O was synthesized following literature procedures.25
nBu4NBr (Aldrich) was ground finely in a mortar and pestle and
dried under vacuum. Following a related procedure,45 NaOOtBu
was synthesized by precipitation from equimolar amounts of
HOOtBu (∼5.5 M in decane over 4 Å molecular sieves, >97%)
and NaOtBu (>97%, both from Fluka) in THF, filtered, dried in
vacuo, and stored in a desiccator.
mg, 0.33 mmol) was added. The reaction mixture was stirred at
-20 °C for 2 h, and the THF was removed in vacuo at ambient
temperature. Et2O (25 mL) was added, and the resulting solids were
filtered and washed with excess Et2O until the filtrate was no longer
yellow. The volume of Et2O was reduced to ∼5 mL, and pentane
(5 mL) was added, yielding 71 mg of yellow 3 (62% based on 1).
X-ray quality crystals were obtained by the slow evaporation of a
saturated CH2Cl2 solution of 3 at -5 °C for 48 h. 1H NMR (THF-
d8, -20 °C): 6.40 (s, 10H, C5H5), 1.08 (s, 9H, C(CH3)3). 13C{1H}
NMR (THF-d8, -20 °C): 117.1 (s, C5H5), 82.7 (s,OOCMe3), 27.0
(s, OOC(CH3)3). IR (CH2Cl2, cm-1): 2985 m (CH); 1449 w, 1361
12
w, 1188 w (tBu); 1016 w (CO); 819 m (OO). MS for
C
1H19-
14
79Br16O248Ti (M+): calcd, 346.00478; found, 346.00486. Anal.
Calcd for C14H19BrO2Ti: C, 48.45; H, 5.52. Found: C, 48.23; H,
5.48.
NMR spectra were obtained at 300 K (unless otherwise noted)
on Bruker Avance DRX-499, AV-500, or DMX-750 spectrometers.
NMR spectra were obtained using either J. Young-valved sealable
or flame-sealed NMR tubes and are referenced to residual solvent
[Cp2Ti(OOtBu)OEt2][BAr′4] (4). An NMR tube was charged
with 10.0 mg (0.033 mmol) of 1 and 36.9 mg (0.033 mmol) of
AgBAr′4‚2Et2O, and ∼0.7 mL of CD2Cl2 was vacuum transferred
in at low temperature. The NMR tube was sealed and, keeping it
at or below -20 °C, shaken thoroughly and then placed vertically
1
peaks for H and 13C, external 85% H3PO4 (in a sealed capillary)
for 31P, or external CFCl3 for 19F. Mass spectra were performed in
EI+ ionization mode using a direct inlet probe (hot stage) on Kratos
Profile HV-3 (low-resolution) or JEOL HX-110 (high-resolution)
mass spectrometers. FT-IR spectra were obtained on a Bruker
VECTOR 22/N-C spectrophotometer using a NaCl solution cell.
Elemental analyses were performed by Atlantic Microlab (Norcross,
GA).
1
for 30 min to allow the AgCl to settle. H NMR (CD2Cl2, -20
°C): 7.73 (br s, 8H, o-Ar′), 7.58 (s, 4H, p-Ar′), 6.51 (s, 10H, C5H5),
3
3
3.61 (br q, 4H, JHH ) 7 Hz, O(CH2CH3)2), 1.29 (t, 6H, JHH ) 7
Hz, O(CH2CH3)2), 1.16 (s, 9H, C(CH3)3). 13C{1H} NMR (CD2Cl2,
-20 °C): 161.5 (q, JC-B ) 50 Hz, B-ipso-Ar′), 134.8 (s, o-Ar′),
129.0 (qq, JC-F ) 32, 3 Hz, m-Ar′), 124.8 (q, JC-F ) 272 Hz,
CF3), 118.6 (s, p-Ar′), 117.8 (s, C5H5), 85.0 (s, OOCMe3), 66.8 (s,
Ti-O(CH2CH3)2), 26.1 (s, OOC(CH3)3), 15.7 (br s, Ti-O(CH2CH3)2).
CAUTION: All the metal peroxide complexes described here
should be prepared and handled in small quantities, stored in an
inert atmosphere below 0 °C, and manipulated with Teflon-coated
spatulas. Complex 1 has been observed to undergo spontaneous
exothermic decomposition in the solid state.
[Cp2Ti(OOtBu)Et3P][BAr′4] (5). Complex 4 was prepared as
above except that the reaction was run in the glovebox freezer in
a 1 dram vial with the reagents, solvent, and apparatus precooled
to -20 °C. CD2Cl2 (∼0.7 mL) was added to the solids, and the
reaction was mixed and after 10 min filtered through two 13 mm
nylon-membrane syringe filters (0.45 µm pore size, Gelman
Acrodisc) using a 1 mL glass syringe. The filtrate was injected
into a J. Young-valved sealable NMR tube containing Et3P (5 µL,
Cp2Ti(OOtBu)Cl (1). In the air, Cp2TiCl2 (100 mg, 0.40 mmol)
and NaOOtBu (180 mg, 1.60 mmol) were ground together carefully
using a mortar and pestle. The mixed solids were placed in a swivel-
frit assembly and evacuated on a vacuum line. THF (50 mL) was
vacuum transferred onto the solids at -78 °C, and the reaction
mixture was stirred at -20 °C for 2 h. The THF was removed in
vacuo at ambient temperature and hexanes (50 mL) vacuum
transferred in. The resulting solids were filtered and washed with
excess hexanes until the filtrate was no longer yellow in color.
Removal of the hexanes, addition of 10 mL of pentane, and filtration
gave yellow 1 (102 mg, 84%). X-ray quality crystals were obtained
by cooling a saturated, filtered toluene solution of 1 at -5 °C for
1
0.033 mmol) at -20 °C. H NMR (CD2Cl2, -20 °C): 7.73 (br s,
8H, o-Ar′), 7.58 (s, 4H, p-Ar′), 6.27 (d, JH-P ) 3 Hz, 10H, C5H5),
1.96 (br m, 6H, P(CH2CH3)3), 1.10 (dt, JH-P ) 7 Hz, JH-H ) 8
Hz, 9H, P(CH2CH3)3), 1.16 (s, 9H, C(CH3)3). 13C{1H} NMR (CD2-
Cl2, -20 °C): 162.0 (q, JC-B ) 50 Hz, B-ipso-Ar′), 135.0 (s, o-Ar′),
129.0 (qq, JC-F ) 31, 3 Hz, m-Ar′), 124.8 (q, JC-F ) 272 Hz,
CF3), 117.8 (s, p-Ar′), 115.6 (d, JC-P ) 4 Hz, C5H5), 85.0 (s,
OOCMe3), 26.0 (s, OOC(CH3)3), 17.3 (d, JC-P ) 13 Hz, P(CH2-
CH3)3), 9.0 (br s, P(CH2CH3)3). 31P{1H} NMR (CD2Cl2, -20 °C):
27.6 (s).
1
24 h. H NMR (THF-d8, -20 °C): 6.35 (s, 10H, C5H5), 1.09 (s,
9H, C(CH3)3). 13C{1H} NMR (THF-d8, -20 °C): 117.3 (s, C5H5),
82.4 (s, OOCMe3), 27.0 (s, OOC(CH3)3). IR (CH2Cl2, cm-1): 3008
s (CH); 1451 s, 1445 s, 1206 s (tBu); 1018 w (CO); 815 m (OO).
Et2POtBu. Et2PCl (100 mg, 0.80 mmol, Acros Organics) was
added to a stirring solution of Et3N (809 mg, 8.00 mmol) and tBuOH
(593 mg, 8.00 mmol) in dry benzene (10 mL). After stirring
overnight, a white precipitate (presumably Et3NHCl) was filtered
off, and the volatiles were removed in vacuo, leaving Et2POtBu as
a thick oil. 31P{1H} NMR (CD2Cl2): 109.6 ppm. MS: 162 m/z.
1H NMR Kinetics. In a typical reaction, a vial was charged with
1 (10.0 mg, 0.033 mmol), C6Me6 (1.0 mg) as an internal standard,
and Ph3P (8.7 mg, 0.033 mmol). CD2Cl2 (0.5 mL) was added, and
the resulting solution was rapidly transferred to a J. Young-valved
sealable NMR tube, sealed, and quickly frozen in liquid N2. The
NMR tube was warmed to room temperature at the spectrometer,
and spectra were acquired every 5 min for at least 3 h, employing
a 10 s delay between pulses for accurate integration. Peaks in each
spectrum were integrated individually versus the internal standard
and the residual solvent peak using WinNuts.
12
MS for
C
1H1935Cl16O248Ti (M+): calcd, 302.05530; found,
14
302.05544. Anal. Calcd for C14H19ClO2Ti: C, 55.62; H, 6.34.
Found: C, 55.71; H, 6.33.
Cp2Ti(OOtBu)OTf (2). THF (25 mL) was vacuum transferred
onto 1 (100 mg, 0.33 mmol) and AgOTf (85 mg, 0.33 mmol) at
-78 °C, and the reaction mixture was stirred at -20 °C for 2 h.
The THF was removed in vacuo at 0 °C, and Et2O (50 mL) was
added. The suspension was stirred at -20 °C for 0.5 h and filtered
at -78 °C to remove the AgCl and any residual AgOTf, and
immediately the Et2O was removed in vacuo at -20 °C, yielding
2 as an orange solid. Complex 2 is unstable at temperatures over
0 °C, decomposing quickly both in solution and as a solid. 1H NMR
(THF-d8, -20 °C): 6.58 (s, 10H, C5H5), 1.17 (s, 9H, C(CH3)3).
13C{1H} NMR (THF-d8, -20 °C): 120.4 (q, JC-F ) 318 Hz,
OSO2CF3), 118.8 (s, C5H5), 83.6 (s,OOCMe3), 26.7 (s, OOC(CH3)3).
19F NMR (THF-d8, -20 °C): 79.2 (s, OSO2CF3).
Cp2Ti(OOtBu)Br (3). Complex 2 was generated as above,
X-ray Crystallographic Studies. Crystals were mounted on a
glass capillary with oil at -143 °C. Intensity data were collected
on an Enraf-Nonius KappaCCD diffractometer equipped with a fine
focus Mo-target X-ray tube. The data were integrated and scaled
using hkl-SCALEPACK (hkl-2000 for 3).46 These programs apply
n
quickly dissolved in THF (25 mL), and at -78 °C Bu4NBr (106
(45) NaOEt + HOOtBu f NaOOtBu: Lobanova, G. N. Visn. L’ViV.
Politekh. Inst. 1971, 58, 11-14.

924 Organometallics, Vol. 25, No. 4, 2006
DiPasquale et al.
a multiplicative correction factor (S) to the observed intensities
I to give the corrected intensity (Icorr). Solution by direct methods
(SIR 97) produced a complete heavy atom-phasing model.47 All
hydrogen atoms were placed using a riding model. All non-
hydrogen atoms were refined anisotropically by full-matrix least
squares (SHELXL-97, Table 2).48
Computational Studies. Geometries were optimized and vibra-
tional analyses were performed at the density functional (DFT) level
of theory using the 6-31G* basis set.49 The hybrid B3LYP
functional was employed, which combines Becke’s gradient-
corrected exchange functional50 with the gradient-corrected cor-
relation functional of Lee, Yang, and Parr.51 The vibrational analyses
were used to confirm the nature of the stationary points, and the
unscaled vibrational frequencies were used to compute thermal
contributions to enthalpies. All of the calculations were carried out
with the Gaussian 03 suite of programs.52
2
2
(I): (e-2B(sin θ)/λ )/scale. S is calculated from the scale and the B
factor, which is determined for each frame and is then applied to
(46) Otinowski, Z.; Minor, W. Processing of X-ray Diffraction Data
Collected in Oscillation Mode. In Methods in Enzymology; Carter, C. W.,
Jr., Sweet, R. M., Eds.; Academic Press: New York, 1996; pp 307-326.
(47) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Burla, M. C.; Polidori,
G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435-442.
(48) Sheldrick, G. M. SHELXL-97: Program for the Refinement of
Crystal Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(49) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213-
222.
(50) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.
(51) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785-789.
(52) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K.
N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A.
D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A.
G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian
03, Revisions B.05; Gaussian, Inc.: Wallingford, CT, 2004.
Acknowledgment. We are grateful to the U.S. National
Institutes of Health (grant R01 GM50422 to J.M.M.) and the
U.S. National Science Foundation (grant CHE-0239304 to W.
T. Borden) for support of this work.
Supporting Information Available: Metrical data for DFT-
calculated structures and intermediates, Eyring plots for the
decomposition of 1 and 3-5 in CD2Cl2, and CIF files for 1 and 3.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM050818Z