Synthesis of chromium(III) bis(benzamidinate) complexes via single electron oxidation

Article abstract of DOI:10.1016/j.jorganchem.2007.01.053

Single-electron oxidation of the known Cr(II) bis(amidinate) Cr[(Me₃SiN)₂CPh]₂ (1) provides synthetic access to neutral Cr(III) complexes. The complexes Cr[(Me₃SiN)₂CPh]₂X were prepared by reaction of 1 with AgO₂CPh (X = O₂CPh, 2), of 1 with iodine in THF (X = I/THF, 3), or of 1 with iodine in pentane, followed by addition of 2-adamantanone (X = I/2-adamantanone, 4). Treatment of 2 or 3 with C₃H₅MgCl resulted in the thermally stable allyl complex (X = η³-C₃H₅, 5). A preliminary kinetics study of the reaction of 1 with excess allyl benzoate and allyl acetate was performed. The molecular structures of 2, 3 and 5 were confirmed by single crystal X-ray diffraction.

Full text of DOI:10.1016/j.jorganchem.2007.01.053

Journal of Organometallic Chemistry 692 (2007) 3183–3190
www.elsevier.com/locate/jorganchem
Synthesis of chromium(III) bis(benzamidinate) complexes via
single electron oxidation
a
a
a
Beth A. Dykerman , Joshua J. Smith , Erin M. McCarvill ,
a
a
a
Amanda J. Gallant , Natasha D. Doiron , Brian D. Wagner ,
b
c
a,
*
Hillary A. Jenkins , Brian O. Patrick , Kevin M. Smith
a
Department of Chemistry, University of Prince Edward Island, 550 University Avenue, Charlottetown, Prince Edward Island, Canada C1A 4P3
b
Department of Chemistry, Saint Mary’s University, Halifax, Nova Scotia, Canada B3H 3C3
Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1
c
Received 12 December 2006; received in revised form 30 January 2007; accepted 30 January 2007
Available online 8 February 2007
Abstract
Single-electron oxidation of the known Cr(II) bis(amidinate) Cr[(Me3SiN)2CPh]2 (1) provides synthetic access to neutral Cr(III) com-
plexes. The complexes Cr[(Me3SiN)2CPh]2X were prepared by reaction of 1 with AgO2CPh (X = O2CPh, 2), of 1 with iodine in THF
(X = I/THF, 3), or of 1 with iodine in pentane, followed by addition of 2-adamantanone (X = I/2-adamantanone, 4). Treatment of 2
or 3 with C3H5MgCl resulted in the thermally stable allyl complex (X = g3-C3H5, 5). A preliminary kinetics study of the reaction of
1 with excess allyl benzoate and allyl acetate was performed. The molecular structures of 2, 3 and 5 were confirmed by single crystal
X-ray diffraction.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Chromium complexes; X-ray structure; Amidinate ligands; Allyl; Oxidation
1. Introduction
complexes when treated with CrCl2 obviates the need for
the metal additives required for less reactive organic halide
substrates, like the catalytic cobalt [5] or nickel [6] salts
added to CrCl2 for reactions involving alkyl halides or
alkenyl and aryl halides, respectively. Not only are addi-
tives not required for CrCl2 activation of allylic halides,
if present Ni(II) salts promote unwanted Wurtz coupling
reactions [7].
The Cr(III) allyl complexes produced by single-electron
oxidative addition exhibit reactivity features that render
them useful for synthetic organic chemists [7]. They prefer-
entially react with aldehydes and generally show good
functional group tolerance. Anti configured allyl alcohol
products are obtained whether the initial substituted allyl
halide has an E or Z configuration [7]. This diastereoselec-
tivity is suggested to be due to the directing effects of steric
interactions between the aldehyde and the substituent on
the g1-allyl ligand in the chair configured transition state
Transition metal allyl complexes are key intermediates
in catalytic applications for organic synthesis [1]. In 1977,
Nozaki, Hiyama and co-workers reported the stoichiome-
tric use of CrCl2 for coupling allylic halides and aldehydes
to produce alcohols [2]. The single-electron oxidative addi-
tion process that generates reactive Cr(III) allyl complexes
from Cr(II) chloride and allyl chloride is illustrated in
Scheme 1 [3]. Chlorine atom abstraction by CrCl2 forms
CrCl3 and an allyl radical, which then reacts with a second
equivalent of CrCl2 to give a Cr(III) allyl species [4]. The
ease with which allyl halides form Cr(III) organometallic
*
Corresponding author. Present address: Department of Chemistry,
University of British Columbia Okanagan, Kelowna, British Columbia,
Canada, V1V 1V7. Tel.: +1 250 807 9933; fax: +1 250 807 8005.
E-mail address: kevin.m.smith@ubc.ca (K.M. Smith).
0022-328X/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.01.053

3184
B.A. Dykerman et al. / Journal of Organometallic Chemistry 692 (2007) 3183–3190
2. Results and discussion
CrCl2
CrCl2
CrCl3
+
+
+
Cl
Much of the research on Cr amidinate complexes has
focused on Cr2(amidinate)4 complexes [20] containing
Cr–Cr quadruple bonds [21]. However, there exists a range
of structural possibilities for Cr(LX)2 (LX = bidentate,
monoanionic ligand) complexes, from dimers with the
common paddlewheel geometry [20] to square planar bis-
ligand monomers [22], to flattened tetrahedral Cr(LX)2
compounds [19a,23]. The relative stabilities of these spe-
cies, as well as their tendency to distort from the ideal
structures, are linked to the steric demands of the ligands,
which are not solely dictated by the size of the donor atom
substituents. The critical role of intraligand steric repulsion
between the amidinate N groups and the substituent on the
central C atom has been demonstrated by several research
groups [22d,24]. The importance of interligand steric inter-
actions in controlling the stereochemistry in an extensive
variety of bis-chelate metal(II) complexes was investigated
in detail in the 1960s and 1970s [22ab, 25], and these con-
siderations remain remarkably useful in evaluating the dis-
tortions from the electronically preferred square planar
geometry observed in some Cr(II) bis-chelate complexes
[23].
Cl2Cr
Scheme 1. Single-electron oxidative addition of allyl chloride to CrCl2 to
form reactive Cr(III) allyl species.
Cr
R
Cr
R
R'
R'
O
O
Scheme 2. Steric interactions in the transition state of carbon-carbon
bond formation responsible for the anti diastereoselectivity of Cr-
mediated coupling of aldehyde and allyl ligands.
of the C–C bond-forming step (Scheme 2) [1a,7,8]. In 1996,
Furstner and co-workers rendered the reaction catalytic in
¨
chromium, using Me3SiCl to break the inert Cr(III)–
oxygen bond, and Mn as the stoichiometric reductant to
regenerate the active Cr(II) species [9]. In the decade since
Furstner’s report of the catalytic reaction, several groups
¨
have reported chiral ligand systems to render the coupling
of allyl halides and aldehydes catalytic and enantioselective
[8,10–16].
As part of our investigation of well-defined paramag-
netic organometallic chromium compounds of relevance
to catalytic carbon-carbon bond formation [17], we were
interested in synthesizing thermally-stable Cr(III) allyl
complexes with tunable ancillary ligands. As previously
reported, reaction of CpCr[(Me3SiN)2CPh]Cl with allyl
Grignard resulted in reduction to the corresponding Cr(II)
complex [18]. In this paper, we use single electron oxidation
reactions of a Cr(II) complex first prepared by Edelmann
and co-workers, Cr[(Me3SiN)2CPh]2 (1) [19], in order to
synthesize Cr(III) bis(benzamidinate) compounds, includ-
ing the thermally stable g3-allyl derivative.
To synthesize the target M[(Me3SiN)2CPh]2(g3-C3H5)
compound for M = Cr, we initially investigated the syn-
thetic route successfully applied for the corresponding
M = Ti [26] and M = V [27] complexes. The reaction of
CrCl3(THF)3 with
2 equivalents of (tmeda)Li[(Me3-
SiN)2CPh] gives the Cr(III) bis(amidinate) ‘‘ate’’ complex,
[PhC(NSiMe3)2]2Cr(l-Cl)2Li(tmeda), where tmeda = N,N,
N0,N0-tetramethylethylenediamine. The structure of [PhC-
(NSiMe3)2]2Cr(l-Cl)2Li(tmeda) [28] is similar to the Ti(III)
complex with identical ligands prepared by Gambarotta
and co-workers [26]. The low yield and slow deposition
of LiCl from the Cr(III) ‘‘ate’’ complex encouraged the
Si
N
Si
N
N
Si
(ii)
I
O
N
Ph
Cr
Ph
Si
Si
N
N
Cr
N
N
1
Si
Ph
Ph
3
Si Si
(i)
(iii)
(i)
(iv)
Ph
O
Cr
I
O
O
Si
Si
Si
Si
N
N
Si
Si
N
Cr N
N
Cr N
(iv)
N
N
N
N
N
N
Ph
Ph
4
Ph
Ph
Ph
Ph
2
Si Si
Si Si
Si Si
5
Scheme 3. Synthesis of Cr(III) bis(amidinate) complexes from 1. Experimental conditions: (i) AgO2CPh, THF; (ii) I2, THF; (iii) (a) I2, pentane, (b) 2-
adamantanone; (iv) C3H5MgCl, THF.

B.A. Dykerman et al. / Journal of Organometallic Chemistry 692 (2007) 3183–3190
3185
development of alternative synthetic routes to the desired
allyl complex [29].
Single-electron oxidation has been shown to be a useful
synthetic strategy to prepare well-defined, mixed ligand
complexes [17,30]. In order to obtain neutral Cr(III)
bis(amidinate) complexes, the oxidation of the known
Cr(II) complex 1 was investigated (Scheme 3). Treatment
of 1 with AgO2CPh generated Ag metal and the Cr(III)
benzoate complex, Cr[(Me3SiN)2CPh]2(O2CPh) (2)
(Fig. 1). The viability of 2 had been presaged by the isola-
tion and structural characterization of a related complex,
Cr[(Me3SiN)2CPh]2[(HN)2CPh] [31], an unexpected minor
byproduct in the preliminary reactivity studies performed
on CpCr[(Me3SiN)2CPh]Cl [18]. The bidentate coordina-
tion of the benzoate ligand is consistent with the marked
affinity of Cr(III) for the 6-coordinate, pseudo-octahedral
geometry.
Oxidation of 1 with I2 in THF gives the 6-coordinate
solvated complex Cr[(Me3SiN)2CPh]2(I)(THF) (3) in mod-
erate yields. The synthesis of Mn[(Me3SiN)2CPh]2I has pre-
viously been reported using a similar synthetic strategy: salt
metathesis by reaction of MnI2(THF)2 with Mg[(Me3-
SiN)2CPh]2(THF)2, followed by oxidation with iodine
[32]. Crystals of 3 suitable for X-ray diffraction were
obtained by recrysallization from a mixture of pentane
and THF (Fig. 2). Reaction of the Cr(III) iodide 3 with
AgO2CPh provided an alternative synthetic route to the
Cr(III) benzoate 2 via formation of the insoluble AgI salt.
Treatment of 1 with I2 in pentane followed by addition of
Fig. 2. ORTEP diagram of Cr[(Me3SiN)2CPh]2(I)(THF) (3; 50% ellipsoids).
˚
Hydrogen atoms have been omitted for clarity. Selected bond lengths (A) and
angles (ꢁ): Cr(1)–I(1) = 2.7664(5), Cr(1)–N(1) = 2.023(3), Cr(1)–N(2) =
2.081(3), Cr(1)–N(3) = 2.050(3), Cr(1)–N(4) = 2.083(3), Cr(1)–O(1) =
2.103(2), N(1)–Cr(1)–N(2) = 66.21(11), N(3)–Cr(1)–N(4) = 65.93(10), I(1)–
Cr(1)–O(1) = 90.45(6),
I(1)–Cr(1)–N(1) = 90.57(7),
I(1)–Cr(1)–N(2) =
87.83(7), I(1)–Cr(1)–N(3) = 168.02(8), I(1)–Cr(1)–N(4) = 102.10(7), Cr(1)–
N(1)–Si(1) = 139.95(16), Cr(1)–N(2)–Si(2) = 139.04(16), Cr(1)–N(3)–Si(3) =
139.43(16),
Cr(1)–N(4)–Si(4) = 141.87(15),
N(1)–C(1)–N(2) = 115.4(3),
N(3)–C(21)–N(4) = 121.9(3).
2-adamantanone (OAd, C10H14O) provides access to the
ketone adduct of the Cr(III) iodide, Cr[(Me3SiN)2C-
Ph]2(I)(OAd) (4). Efforts to obtain X-ray quality crystals
of 4 were unsuccessful. However, the corresponding Cr(III)
chloride complex of 2-adamantanone, obtained by a simi-
lar oxidation of 1 in pentane with PbCl2 followed by addi-
tion of the ketone, was amenable to characterization by
single crystal X-ray diffraction (see Supplementary data).
As shown in Scheme 2, the distinct selectivity profile of
the CrCl2-mediated coupling of allyl halides and aldehydes
is attributed to the nature of the transition state for the key
carbon-carbon bond-forming reaction step [1a,7,8]. This
mechanism requires the aldehyde to coordinate cis to the
g1-allyl ligand. It is significant to note that in all of the
6-coordinate Cr bis(amidinate) complexes we have struc-
turally characterized, the two amidinates are related by
an approximate C2 axis and the remaining two coordina-
tion sites posses a cis orientation. While this geometry is
presumably required for the chelating benzoate 2, and is
likely reinforced by the bridging Cr(l-Cl)2Li structural unit
in [(Me3SiN)2CPh]2Cr(l-Cl)2Li(tmeda) [26,28], its recur-
rence in the halide/solvent and halide/ketone structures is
encouraging [29b]. This situation is in contrast to some of
the chiral ligand systems currently used with Cr in cata-
lytic, asymmetric reactions, where the trans configurations
of neutral and anionic ligands has led to mechanistic pro-
posals involving the bimolecular reaction of two Cr com-
plexes in the C–C bond formation step [10,33].
Fig. 1. ORTEP diagram of Cr[(Me3SiN)2CPh]2(O2CPh) (2; 50% ellipsoids).
˚
Hydrogen atoms have been omitted for clarity. Selected bond lengths (A) and
angles (ꢁ): Cr(1)–N(1) = 2.0364(13), Cr(1)–N(2) = 2.0620(13), Cr(1)–
Reaction of the Cr(III) iodide 3 with C3H5MgCl in THF
led to the formation of the allyl complex, Cr[(Me3-
SiN)2CPh]2(g3-C3H5) (5). Related alkylation reactions
have been reported in the patent literature, where the
treatment of Cr[(Me3SiN)2CPh]2Cl with stoichiometric
N(3) = 2.0395(14), Cr(1)–N(4)
= 2.0450(14), Cr(1)–O(1) = 2.0584(12),
Cr(1)–O(2) = 2.0553(11), N(1)–Cr(1)–N(2) = 66.28(5), N(3)–Cr(1)–N(4) =
66.42(6), O(1)–Cr(1)–O(2) = 64.22(5), Cr(1)–N(1)–Si(1) = 139.69(8), Cr(1)–
N(2)–Si(2) = 135.23(7), Cr(1)–N(3)–Si(3) = 137.91(8), Cr(1)–N(4)–Si(4) =
136.59(8), N(1)–C(1)–N(2) = 114.90(14), N(3)–C(21)–N(4) = 114.67(15).

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B.A. Dykerman et al. / Journal of Organometallic Chemistry 692 (2007) 3183–3190
LiCH2SiMe3 produced an active catalyst for olefin oligo-
merization [29a]. The relatively low recrystalized yield of
5 (25–40%) is presumably due to the high pentane solubility
of the complex: while the Cr(III) allyl complex could also
be synthesized by reaction of Cr(III) benzoate with the allyl
Grignard, or sequential addition of I2 then C3H5MgCl to
Cr(II) complex 1, the isolated recrystallized yield of 5 from
pentane fell in the same range for each synthetic route.
Addition of 1,4-dioxane appeared to assist in the separa-
tion of byproduct Mg salts, but did not lead to higher
yields of the allyl product. The X-ray structure of 5
(Fig. 3) exhibited the allyl ligand disordered over two posi-
tions. The same disorder was reported for the V(III) analog
of 5 [27], whose authors suggested that the unusual distor-
tions reported previously for the allyl ligand of the Ti(III)
complex might also be due to this type of disorder [26].
Despite the widespread interest in the synthesis of
Cr(III) alkyl complexes due to their involvement in Cr-
based olefin polymerization catalysts [34], examples of
Cr(III) allyl complexes amenable to systematic study at
room temperature are rare. Compared to other
[CrR3(THF)n] species, the homoleptic allyl complex is sec-
ond in thermal stability only to the venerable tris(aryl)
compounds [35], and is more robust than the correspond-
ing benzyl, alkyl or vinyl complexes [36]. However, the pro-
posed bimolecular thermal decomposition of Cr(C3H5)3 to
give (C3H5)Cr(l-C3H5)2Cr(C3H5) and 1,5-hexadiene is a
recurring reactivity motif in Cr(III) allyl chemistry [36].
These reactions have been pursued in detail by Jolly and
co-workers for CpCr(g3-C3H5)2 and various substituted
derivatives [37]. In 2005, Stryker and co-workers reported
the single-electron oxidation of the 18e Cr(II) CpCr(g3-
C3H5)(CO)2 to the corresponding 17e Cr(III)[CpCr(g3-
C3H5)(CO)2][PF6] complex, which is indefinitely stable as
a solid at room temperature [38]. The thermal stability of
Gabba¨ı’s Cp*Cr(C6F5)(g3-CH2Ph) benzyl complex [39]
suggests that with a suitable monodentate anionic ligand,
15e CpCr(X)(g3-allyl) spin quartet complexes may be
accessible, in contrast with the reduction to Cr(II) observed
for the putative CpCr[(Me3SiN)2CPh](C3H5) intermediate
[18]. The thermal stability of 5 in the solid state and in
solution is presumably due to the steric protection of the
g3-allyl provided by the bulky N,N0-bis(trimethylsilyl)-
benzamidinate ligands, leading to an increased barrier to
bimolecular decomposition pathways.
Previously, UV–visible spectroscopy had been employed
to monitor the progress of the oxidative addition reaction
of iodomethane with CpCr[(ArNCMe)2CH] (Ar = 2,6-
diisopropylphenyl, 2,6-(CHMe2)2C6H3) under pseudo-
first-order conditions [40]. Initial reactions of Cr(II)
bis(amidinate) 1 and excess H2C@CHCH2Cl proceeded
too rapidly for the same procedure to be followed: the
color change that accompanied the reaction was essentially
complete before the sealable UV–vis cell could be loaded
and removed from the glovebox. Given the manifest stabil-
ity of the Cr(III) benzoate complex 2, it seemed probable
that the reaction of 1 with allyl benzoate to give 2 and 5
should be thermodynamically favorable. Dilute pentane
solutions of 1 were prepared (typically 20 mg of 1 in
100 mL of pentane, [1] = 3.5 · 10ꢀ4 M), excess H2C@
CHCH2O2CPh was added, and the growth of the absor-
bance band at 439 nm was monitored by UV–vis spectros-
copy. Plots of kobs vs [C3H5OBz] were linear under the
pseudo-first-order conditions employed, consistent with
the generally-accepted mechanism for single-electron oxi-
dative addition of organic substrates by Cr(II) illustrated
in Scheme 1. The second order rate constant obtained by
this preliminary kinetics study was calculated to be
(4.4 0.4) · 10ꢀ2 Mꢀ1sꢀ1. The reaction of 1 and allyl ace-
tate was investigated by the same procedure, with a result-
ing second order rate constant of (2.8 0.2) ·
10ꢀ2 Mꢀ1sꢀ1. The slightly higher reactivity of the benzoate
may be attributed to a decrease in the C–O bond dissocia-
tion energy in H2C@CHCH2–O2CR for R = Ph compared
to R = Me [3].
3. Conclusions
The known Cr(II) bis(amidinate) complex Cr[(Me3-
SiN)2CPh]2 [19] was demonstrated to be a useful synthetic
precursor to new Cr(III) complexes, including a thermally
stable g3-allyl complex. A preliminary kinetics study on
the rate of reaction of Cr[(Me3SiN)2CPh]2 with allylic carb-
oxylates was performed. As previously noted in other sys-
tems, addition of electron donating ligands with N donor
atoms increases the rate at which Cr(II) reacts with organic
substrates in one-electron oxidative addition reactions
[4,40]. In this case, complex 1 reacts in pentane with allylic
esters, a class of reactants outside of the scope typically
employed in organic synthetic applications of CrCl2, where
Fig. 3. ORTEP diagram of Cr[(Me3SiN)2CPh]2(g3-C3H5) (5; 50% ellipsoids).
Hydrogen atoms and amidinate methyl groups have been omitted for clarity.
˚
Selected bond lengths (A) and angles (ꢁ): Cr(1)–N(1) = 2.0733(19),
Cr(1)–N(2) = 2.105(2), Cr(1)–N(3) = 2.0835(19), Cr(1)–N(4) = 2.0726(19),
Cr(1)–C(1) = 2.272(3), Cr(1)–C(2) = 2.256(4), Cr(1)–C(3) = 2.229(3), N(1)–
Cr(1)–N(2) = 65.09(8), N(3)–Cr(1)–N(4) = 65.36(8), C(1)–Cr(1)–C(3) =
65.21(15), C(1)–C(2)–C(3) = 129.4(6), Cr(1)–N(1)–Si(1) = 137.09(12),
Cr(1)–N(2)–Si(2) = 142.15(12), Cr(1)–N(3)–Si(3) = 140.98(11), Cr(1)–
N(4)–Si(4) = 137.91(12), N(1)–C(16)–N(2) = 115.9(2), N(3)–C(36)–
N(4) = 115.5(2).

B.A. Dykerman et al. / Journal of Organometallic Chemistry 692 (2007) 3183–3190
3187
allyl bromides and chlorides are normally used [7,10–15],
although use of allyl tosylates and mesylates have also been
reported [7].
immediately turned a teal blue color. After stirring over-
night, the reaction mixture was greenish-blue to incident
light and green to transmitted light. The solvent was
removed in vacuo. The reaction mixture was extracted with
pentane, filtered through Celite, and concentrated in vacuo.
To the solution was added 200 lL of tmeda. The solution
was filtered through Celite again, and cooled to ꢀ30 ꢁC
overnight to give dark crystals of [(Me3SiN)2CPh]2Cr(l-
Cl)2Li(tmeda) (101.1 mg, 26% yield). Elemental analysis:
calculated (found) C: 49.72% (49.47%); H: 8.08%
(8.42%); N: 10.87% (11.24%).
4. Experimental
4.1. General considerations
All compounds were synthesized and stored under N2
using standard Schlenk and glovebox techniques. Sam-
ples for X-ray diffraction, melting point determination,
elemental analysis, and infrared spectroscopy were pre-
pared in a glovebox. Melting point determinations were
performed in flame-sealed tubes and are uncorrected.
Infrared spectra were obtained from KBr pellets on a
Perkin–Elmer 1600 Series FTIR. All UV–visible mea-
surements were conducted in pentane solution in a spe-
4.3. Synthesis of Cr[(Me3SiN)2CPh]2(O2CPh) (2)
To a solution of 1 (573.8 mg, 0.990 mmol) in 40 mL
THF, AgO2CPh (230.0 mg, 1.004 mmol) was added as a
solid. The reaction mixture immediately darkened, and a
precipitate was visible. After stirring overnight, the solvent
was removed in vacuo. The reaction mixture was extracted
with pentane and filtered through Celite to remove the grey
metallic precipitate from the red-brown solution. The sol-
vent was again removed in vacuo, redissolved in a mini-
mum of pentane, filtered through Celite, and cooled to
ꢀ30 ꢁC overnight to give black metallic crystals of 2
(447.3 mg, 65% yield).
cially constructed cell for air-sensitive samples:
a
Kontes Hi-Vacꢂ Valve with PTFE plug was attached
by a professional glassblower to a Hellma 10 mm path
length quartz absorption cell with a quartz-to-glass
graded seal. The kinetics measurements were collected
using a Varian Cary 50 Bio UV–visible spectrophotome-
ter, and the data were fit to a first order growth equation
using the instrument software. Elemental analyses were
performed by Guelph Chemical Laboratories, Guelph,
ON, Canada. Spectroscopic and physical data for com-
plexes 2–5 are collected in Table 1.
4.4. Synthesis of Cr[(Me3SiN)2CPh]2(I)(THF) (3)
Anhydrous THF and pentane were dried by passage
through a column of activated alumina [41]. Anhydrous
reagents (2.0 M C3H5MgCl in THF, LiN(SiMe3)2, PhCN,
tmeda N,N,N0,N0-tetramethylethylenediamine, AgO2CPh)
were purchased from Aldrich and used as received. Iodine
(Aldrich) was sublimed prior to use. 2-Adamantanone
(Aldrich) was used as received. Allyl acetate (Aldrich)
and allyl benzoate (TCI America) were degassed by three
freeze-pump-thaw cycles, and then dried over molecular
sieves prior to use. Cr[(Me3SiN)2CPh]2 (1) was synthesized
in a modification of the literature procedure [19a,22d],
from CrCl2(tmeda) [42] and (tmeda)Li[(Me3SiN)2CPh]
[26].
To a solution of 1 (584.7 mg, 1.01 mmol) in 35 mL THF,
I2 (128.1 mg, 0.505 mmol) was added as a solid. After stir-
ring overnight, the solvent was removed in vacuo. The res-
idue was extracted with a mixture of pentane and THF
(5:1). The solution was concentrated in vacuo, filtered,
and cooled to ꢀ30 ꢁC for several days to give green crystals
of 3 (494.1 mg, 63% yield).
4.5. Synthesis of Cr[(Me3SiN)2CPh]2(I)(2-
adamantanone) (4)
To a solution of 1 (152.9 mg, 0.264 mmol) in 15 mL pen-
tane, I2 (35.6 mg, 0.140 mmol) was added as a solid. The
purple solution turned yellow-brown to incident light, red
to transmitted light. After the solution was stirred for
45 min, 2-adamantanone (39.3 mg, 0.262 mmol) was added
to the reaction mixture as a solution in 5 mL of pentane. As
the pentane solution of 2-adamantanone was added, the
4.2. Synthesis of [(Me3SiN)2CPh]2Cr(l-Cl)2Li(tmeda)
To a solution of (tmeda)Li[(Me3SiN)2CPh] (390.4 mg,
1.01 mmol) in 30 mL of THF, CrCl3(THF)3 (189.1 mg,
0.505 mmol) was added as a solid. The reaction mixture
Table 1
Spectroscopic and physical data for Cr[(Me3SiN)2CPh]2X
Compound # (X)
M.p. (ꢁC)
UV–visible
Elemental analysis: calculated (found)
k (e Mꢀ1cmꢀ1
)
C (%)
H (%)
N (%)
2 (O2CPh)
3 (I/THF)
4 (I/OAd)
5 (C3H5)
181–182
168–170
179–181
164–165
437 nm (166)
460 nm (237)
401 nm (640)
432 nm (786)
56.61 (56.92)
46.31 (46.14)
50.51 (50.87)
56.17 (55.93)
7.34 (7.66)
7.00 (6.89)
7.06 (7.37)
8.29 (8.61)
8.00 (8.17)
7.20 (7.39)
6.54 (6.88)
9.03 (9.28)

3188
B.A. Dykerman et al. / Journal of Organometallic Chemistry 692 (2007) 3183–3190
solution turned green. The solution was concentrated in
vacuo, filtered, and cooled to ꢀ30 ꢁC for several days to
give dark crystals of 4 in two crops (119.7 mg, 54% yield).
4.8. Kinetics measurements
In a glovebox, a sample of 1 (20.4 mg, 0.0352 mmol) was
dissolved in pentane and diluted to the mark in a 100 mL
volumetric flask (concentration 3.52 · 10ꢀ4 M). An excess
of allyl benzoate (128.9 mg, 0.795 mmol, 22.6 mmol per
mmol of 1) was added to the volumetric flask, and the solu-
tion was thoroughly mixed. An aliquot was loaded into the
UV–visible absorption cell for air-sensitive samples
(described above), the cell was sealed and removed from
the glovebox, and the sample was then transferred to the
spectrophotometer. The absorption at 439 nm was
recorded every 5 min for 300 min. The resulting kinetics
trace of absorbance vs time was fit to a first-order growth
curve to give the rate constant kobs = 0.0179 minꢀ1. Three
other experimental runs were performed similarly, with
20.6 mg, 20.4 mg, and 20.1 mg of 1, 100.0 mL, 50.0 mL,
and 100.0 mL of pentane, and 95 mg, 100 mg, and
191 mg of allyl benzoate, respectively. The kobs obtained
4.6. Synthesis of Cr[(Me3SiN)2 CPh]2(g3-C3H5) (5)
To a solution of 5 (377.9 mg, 0.486 mmol) in 35 mL THF,
C3H5MgCl (0.28 mL of 2.0 M solution in THF, 0.56 mmol)
was added by syringe. The solution immediately turned to a
red-brown color. After stirring for 90 min, the solvent was
removed in vacuo. The residue was extracted with 30 mL
of pentane, filtered through Celite, and the solvent was
again removed in vacuo. The residue was extracted with a
minimum amount of pentane, filtered through Celite, and
cooled ꢀ30 ꢁC for several days to give dark crystals of 5 in
two crops (111.8 mg, 37% yield).
4.7. X-ray crystallographic data
Crystal data for C33H51CrN4O2Si4 (2): M = 700.17,
for runs 2, 3, and 4 with allyl benzoate were 0.0132 minꢀ1
,
Monoclinic, space group P2ð1Þ=c, a = 11.6635(5) A,
0.0342 minꢀ1, and 0.0286 minꢀ1, respectively. From the
slope of the straight line plot of kobs vs C3H5O2CPh con-
centration for the four experiments (R2 = 0.9782), the sec-
ond order rate constant k for the reaction with allyl
˚
˚
˚
b = 17.6196(7) A,
c = 19.5093(9) A,
b = 98.526(2)ꢁ,
a,c = 90ꢁ, U = 3965.0(3) A , Z = 4, Dc = 1.173 g cmꢀ3, l
(Mo Ka) = 0.71069 A, T = 173(2) K, Rigaku/ADSC
3
˚
˚
CCD, total reflections 35996, unique reflections
I > 0.00r(I) 8441, observed reflections 8441 (Rint = 0.044).
The structure was solved by direct methods [43] and
expanded using Fourier techniques. Full-matrix least-
squares refinement was conducted using SHELXL-97 [44].
The non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were included but not refined.
benzoate was determined to be 0.044 0.004 Mꢀ1 sꢀ1
.
A series of four experiments using allyl acetate instead
of allyl benzoate were also performed using the procedure
described above. In each case, a volume of 100.0 mL of
pentane was used, and the absorption at 439 nm was mon-
itored every 5 min for 300 min. In the four runs, the masses
of 1 used were 20.0 mg, 21.0 mg, 20.1 mg, and 20.4 mg and
the volumes of allyl acetate added by 250 lL syringe were
60 lL, 45 lL, 72 lL, and 102 lL, respectively. The
observed kobs values obtained for the four runs were
0.0089 minꢀ1, 0.0048 minꢀ1, 0.0104 minꢀ1, and 0.0155
minꢀ1, respectively. From the slope of the straight line plot
of kobs versus C3H5O2CMe concentration for the four
experiments (R2 = 0.9788), the second order rate constant
k for the reaction with allyl acetate was determined to be
Crystal data for C30H54CrIN4OSi4 (3): M = 778.03,
˚
Orthorhombic, space group Pbca, a = 20.6181(8) A,
˚
˚
b = 17.6960(6) A, c = 21.3316(9) A, a, b, c = 90ꢁ, U =
7783.0(5) A , Z = 8, Dc = 1.328 g cmꢀ3
, l (Mo Ka)
3
˚
˚
= 0.71069 A, T = 173(2) K, Rigaku/ADSC CCD, total
reflections 63931, unique reflections I > 0.00r(I) 7289,
observed reflections 7289 (Rint = 0.102). The structure
was solved by direct methods [43] and expanded using Fou-
rier techniques. Full-matrix least-squares refinement was
conducted using SHELXL-97 [44]. Some non-hydrogen atoms
were refined anisotropically, while the rest were refined iso-
tropically. Hydrogen atoms were included but not refined.
Crystal data for C29H51CrN4Si4 (5): M = 620.10, Tri-
0.028 0.002 Mꢀ1 sꢀ1
.
Acknowledgement
This work was supported by the University of Prince
Edward Island and the Natural Sciences and Engineering
Research Council of Canada (NSERC).
˚
˚
ꢀ
clinic, space group P1, a = 10.942(2) A, b = 11.921(2) A,
˚
c = 15.426(3) A, a = 68.131(4)ꢁ, b = 78.321(3)ꢁ, c =
87.146(6)ꢁ, U = 1827.8(6) A , Z = 2, Dc = 1.127 g cmꢀ3, l
3
˚
˚
(Mo Ka) = 0.71073 A, T = 291(2) K, Siemens 1K
Appendix A. Supplementary material
SMART/CCD, total reflections 35996, unique reflections
I > 2r(I) 6361, observed reflections 9773 (Rint = 0.0208).
Direct methods and Fourier techniques were used to solve
the structure; refinement was conducted by full-matrix
least-squares methods on F2 using SHELXTL-PC V5.03. All
non-hydrogen atoms were refined anisotropically. The allyl
ligand was disordered over two positions. All hydrogen
atoms were included at geometrically idealized positions
that were updated with each refinement cycle.
CCDC 634934, 634935, 634936, 634937 and 635124 con-
tain the supplementary crystallographic data for this paper.
These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or
e-mail: deposit@ccdc.cam.ac.uk. Supplementary data

B.A. Dykerman et al. / Journal of Organometallic Chemistry 692 (2007) 3183–3190
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