Cyclometalation and coupling of a rigid 4,5-bis(imino)acridanide pincer ligand on yttrium

Article abstract of DOI:10.1039/c5dt01636c

An extremely rigid NNN-donor proligand, 4,5-bis{(diphenylmethylene)amino}-2,7,9,9-tetramethylacridan, H[AIm₂] was prepared in five steps starting from 5-methyl-2-aminobenzoic acid and 4-bromotoluene. Reaction of intensely orange H[AIm₂

Full text of DOI:10.1039/c5dt01636c

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Cyclometalation and coupling of a rigid
4,5-bis(imino)acridanide pincer ligand on yttrium†
Cite this: DOI: 10.1039/c5dt01636c
Edwin W. Y. Wong and David J. H. Emslie*
An extremely rigid NNN-donor proligand, 4,5-bis{(diphenylmethylene)amino}-2,7,9,9-tetramethylacridan,
H[AIm2] was prepared in five steps starting from 5-methyl-2-aminobenzoic acid and 4-bromotoluene.
Reaction of intensely orange H[AIm2] with LiCH2SiMe3 formed deep blue Lix[AIm2]x (x = 2 in the solid
state), while reaction with [Y(CH2SiMe3)3(THF)2] (0.5 equiv.) afforded deep blue [Y(AIm2)(AIm’ )] (1; AIm’ =
2
2
an AIm2 ligand cyclometalated at the ortho-position of one of the phenyl rings). Compound 1 slowly iso-
merizes to form green-brown 2, which contains a single trianionic, hexadentate ligand that features one
amine, two imine, and three amido donors. The acridanide backbone and one imine group in each of the
original AIm2 ligands is intact, but the two acridanide backbones are now linked by an isoindoline hetero-
cycle. Yttrium in 2 is coordinated to six nitrogen donors and the ortho carbon of an isoindoline phenyl
substituent. The intense colours of H[AIm2], Lix[AIm2]x and 1 were shown by TD-DFT calculations to arise
from charge transfer transitions from the HOMO, which is localized on the acridanide ligand backbone, to
the LUMO and LUMO+1, which are localized on the imine substituents. The conversion of 1 to 2 was
studied by UV-Visible absorption spectroscopy and is first-order with a half-life of 7.8 hours at room
temperature.
Received 30th April 2015,
Accepted 26th May 2015
DOI: 10.1039/c5dt01636c
www.rsc.org/dalton
Introduction
Extremely rigid ligands offer the ability to precisely tailor the
coordination environment around a metal center, providing
potential advantages over more flexible ligands. Additionally,
ligand donor atoms can be arranged so that they are biased to
coordinate to a metal in a predetermined geometry, and the
positions of bulky substituents can be fixed in place in order
to stabilize reactive metal centers, prevent dimerization, and
inhibit ligand decomposition pathways. The fixing of the
coordination environment and steric bulk around a metal
center may also allow for control of substrate orientation and
recognition that is useful for designing new catalyst systems.
However, in the organometallic chemistry of electropositive
elements, these advantages can only be realized if both the
ligand framework and its substituents are resistant to cyclo-
metalation1 and nucleophilic attack.2
Fig. 1 Relationship between the XA2 and AIm2 ligands.
rare examples of neutral and cationic organothorium com-
plexes supported by non-carbocyclic pincer ligands, including
neutral [(XA2)AnR2] (An = Th or U; R = CH2SiMe3, CH2CMe3,
CH2Ph or nBu)3 and cationic [(XA2)ThR(η6-toluene)][B(C6F5)4]
(R = CH2SiMe3 or CH2Ph).4 These promising results in actinide
chemistry motivated us to pursue analogous organo-rare earth
complexes supported by extremely rigid pincer ligands. The
primary targets of this research are cationic rare earth alkyl
complexes due to their potential to catalyze a range of trans-
formations such as ethylene and α-olefin polymerization,5
hydroamination,6 and alkyne dimerization.7 However, com-
pared to tetravalent thorium, trivalent rare earth metals
necessitate the use of a monoanionic rather than a dianionic
ancillary ligand in order to access cationic alkyl complexes.
In an effort to develop a highly rigid, monoanionic pincer
ligand that maintains the beneficial features of the dianionic
The Emslie group has previously employed an extremely
rigid dianionic pincer ligand, 4,5-bis(2,6-diisopropylanilido)-
2,7-di-tert-butyl-9,9-dimethylxanthene (XA2; Fig. 1), to isolate
Department of Chemistry & Chemical Biology, McMaster University, 1280 Main
Street West, Hamilton, Ontario L8S 4M1, Canada. E-mail: emslied@mcmaster.ca
†Electronic supplementary information (ESI) available: UV-Visible spectroscopy
and DFT calculation data, and multinuclear and variable temperature NMR
spectra. CCDC 1062734–1062737. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c5dt01636c
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XA2 ligand, 4,5-bis{(diphenylmethylene)amino}-2,7,9,9-tetra- of the ligand backbone, as opposed to the ligands in Fig. 2
methylacridanide (AIm2), was developed, featuring a rigid tri- where the carbon atom of the imine functional group is
cyclic acridanide anion bearing pendant neutral imine donors attached to, or incorporated into, the ligand backbone. Herein,
(Fig. 1). The high rigidity of xanthene- and acridanide-back- we report the synthesis of H[AIm2], reactions of H[AIm2] with
bone ligands stems from (a) the approximate planarity of the [Y(CH2SiMe3)3(THF)2] to form a cyclometalated and a ligand-
ligand backbone when coordinated to large, electropositive coupled product, deprotonation of H[AIm2] to form Lix[AIm2]x
actinide3,4 or rare earth (vide infra) elements, and (b) the (x = 2 in the solid state), and TD-DFT investigations into the
absence of bonds which can rotate to move the ligand donor origin of the unusual UV-Visible spectroscopic properties of
atoms closer or further away from one another. However, it these compounds.
should be noted that in combination with metals which are
too small to fit well within the ligand binding pocket (e.g. Mg8
or Li; vide infra), significant flexing of the ligand backbone
may be observed.
Results and discussion
Ligand synthesis and structure
Multidentate ligands incorporating both anionic amido
A six-step synthesis of the 4,5-dibromo-2,7,9,9-tetramethyl-
acridan ligand precursor, H[ABr2], was previously reported by
Du and co-workers.16 However, a more efficient four-step route
to H[ABr2] (Scheme 1) which avoids the use of diazomethane
was pursued beginning from commercially available 5-methyl-
2-aminobenzoic acid. This compound was esterified to form
methyl 2-amino-5-methylbenzoate,17 followed by Buchwald–
Hartwig coupling with 4-bromotoluene to afford the previously
reported ligand backbone intermediate, methyl 2-(N-(4-methyl-
phenyl)amino)-5-methylbenzoate.16 Following the published
procedure with minor modifications, this intermediate was
cyclized via tertiary alcohol formation and an intramolecular
Friedel–Crafts alkylation to give 2,7,9,9-tetramethylacridan,
H[AH2]. Finally, H[AH2] was brominated to provide H[ABr2]
according to the literature method.16
and neutral imine donors have previously been shown to be
able to stabilize both neutral and cationic organo-rare earth
complexes without inhibiting productive reactivity. For
example, the ligands in Fig. 29–15 have been used to develop
neutral and cationic hydroamination catalysts (A, D, G and
K),10 cylic ester polymerization catalysts (B–D, G, J and I),11
methyl methacrylate polymerization catalysts (G),12 cationic
isoprene polymerization catalysts (D, G, H, K and L),13 cationic
ethylene polymerization catalysts (D, F),14 and isolable rare
earth alkyl cations (A, D).14,15
The new AIm2 ligand shares common functional elements
with the amido/imine ligands in Fig. 2 but differs in that the
substituent on the nitrogen atom of each imine group is part
Synthesis of H[AIm2] was accomplished in 75% isolated
yield via a Pd-catalyzed imination of H[ABr2] with benzo-
1
phenone imine (Scheme 1).18 The H and 13C NMR spectra of
H[AIm2] in CD2Cl2 are consistent with the expected structure,
and the empirical formula and bulk purity of H[AIm2] were
Fig. 2 Selected amido/imine ligands used to stabilize neutral and cat-
ionic organo-rare earth complexes. Only one resonance structure is
shown for C–E.
Scheme 1 Synthetic route to the H[AIm2] proligand.
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Fig. 3 Single crystal X-ray structure of H[AIm2]. Thermal ellipsoids are
set at 50% probability. Selected bond lengths [Å]: N1–C1 1.3782(17), N1–
C13 1.3828(17), N2–C2 1.4155(16), N2–C18 1.2833(16), N3–C12
1.4250(16), N3–C31 1.2839(17).
Scheme 2 Synthesis of 1, 2 and Li2[AIm2]2. In the structure of 2, an
interaction between yttrium and an ortho carbon atom of one of the
isoindoline phenyl rings is not shown.
confirmed by elemental analysis. The identity of H[AIm2] was
further established by single crystal X-ray diffraction (Fig. 3),
highlighting a planar acridan backbone with the N–H bond
and the imine nitrogen atoms located in the plane, and bond
distances within expected ranges.19
Synthesis and structure of yttrium complexes
Alkane elimination reactions between [Y(CH2SiMe3)3(THF)2]20
and H[AIm2] were found to provide an effective ligand attach-
ment method, with full consumption of the yttrium starting
material requiring two equivalents of H[AIm2]. In benzene-d6,
this reaction cleanly produced an intermediate dark blue
product (1) which converted to a dark green-brown compound
(2) at room temperature over the course of three days
(Scheme 2).
The 1H NMR spectrum of 1 does not contain signals
attributable to a CH2SiMe3 group, indicating that ligand cyclo-
metalation occurs rapidly with concomitant elimination of
SiMe4, either (a) directly from undetected [(AIm2)2Y-
(CH2SiMe3)], or (b) from undetected [(AIm2)Y(CH2SiMe3)2]
prior to reaction with a second equivalent of H[AIm2]; no inter-
mediates were observed when the reaction was performed at
−78 °C in d8-toluene and warmed slowly to room temperature
in the NMR spectrometer. Additionally, three Ar–CH3 signals
Fig. 4 Single crystal X-ray structure of 1·(pentane)0.5(toluene)2. Hydro-
gen atoms and lattice solvent have been removed. Except for nitrogen
atoms, the unactivated AIm2 ligand is depicted in wireframe for clarity.
Thermal ellipsoids are set at 50% probability. Selected bond lengths [Å]:
Y1–N1 2.367(4), Y1–N2 2.529(4), Y1–N3 2.662(4), Y1–N4 2.358(4), Y1–
N5 2.494(4), Y1–N6 2.475(5), Y1–C20 2.466(5), N1–C1 1.391(7), N1–C13
1.374(6), N2–C2 1.438(6), N2–C18 1.303(7), N3–C12 1.445(6), N3–C31
1.303(6), N4–C44 1.374(7), N4–C56 1.385(7), N5–C45 1.423(7), N5–C61
1.305(7), N6–C55 1.417(6), N6–C74 1.317(6).
1
are observed for 1 in the H and 13C NMR spectra, indicating
that one AIm2 ligand has retained side-to-side symmetry
while the second AIm2 ligand has not, consistent with
cyclometalation.
Small single crystals of [(AIm2)(AIm′ )Y]·(pentane)0.5
-
2
(toluene)2 [1·(pentane)0.5(toluene)2; AIm′ = an AIm2 ligand
2
highly distorted pentagonal bipyramidal geometry. In keeping
with the 1H and 13C NMR spectra of 1, one AIm2 ligand is
bound to the metal center in the expected tridentate fashion,
while the second AIm2 ligand has been cyclometalated at the
cyclometalated at the ortho-position of one of the phenyl rings]
were obtained by layering a concentrated toluene solution of 1
with n-pentane and cooling to −25 °C. The crystal structure of
1 (Fig. 4) revealed a seven-coordinate yttrium complex with a
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ortho position of an imine phenyl ring, generating a dianionic,
tetradentate ligand, AIm′ , that lacks a C2 axis.
2
The intact AIm2 ligand backbone is essentially planar, with
yttrium and the two imine nitrogen atoms located in the plane
of the acridanide backbone. By contrast, the AIm′ dianion
2
deviates slightly from planarity with an N(3)–N(1)–N(2)–C(20)
torsion angle of 27.0°. The Y–C(20) distance of 2.466(5) Å is
typical for an Y–C linkage to an ortho-cyclometalated phenyl
ring; for example, the Y–Caryl distances in [Cp2Y{o-C6H4-
(CH2CH2NMe2)}],21 [{o-NC5H3(C6H4)(CHvCMe–NAr)}Y(NHAr)-
(THF)],22 [{o-C6H4(NAr)(Ph(C6H4)PvNMes)}Y(η3-CH2CMeCHMe)-
(THF)],23 [{(o-NC5H3(C6H4)(CMe2NAr))Y(μ-H)(THF)}2],24 and
[Y{o-C6H4(CH2CH2NMe2)}3]25 (Ar
=
2,6-diisopropylphenyl;
cyclometalated phenyl rings are shown in italics) range from
2.41 to 2.50 Å. The Y–Ndiarylamido bonds in 1 are 2.358(4) and
2.367(4) Å, lying towards the upper end of the typical range.
However, a comparable Y–Ndiarylamido bond distance of 2.340(3)
Å was reported for [{κ3-N(o-C6R4(NHC))2}Y{N(SiMe3)2}2] {NHC =
CN(NCH2Ph)(CH)2}.26
Fig. 5 Single crystal X-ray structure of 2·(benzene)3.35. Hydrogen atoms
and lattice solvent have been removed, and phenyl substituents [except
C(86)] are depicted in wireframe for clarity. Thermal ellipsoids are set at
50% probability. Selected bond lengths [Å]: Y1–N1 2.3281(16), Y1–N2
2.5236(18), Y1–N3 2.2020(18), Y1–N4 2.3258(17), Y1–N5 2.5388(17), Y1–
N6 2.5858(18), Y1–C86 2.904(2), N1–C1 1.365(3), N1–C13 1.396(3), N2–
C2 1.424(2), N2–C18 1.300(3), N3–C12 1.393(3), N3–C31 1.457(3), N4–
C44 1.384(3), N4–C56 1.386(3), N5–C45 1.445(2), N5–C61 1.295(3),
N6–C31 1.569(3), N6–C55 1.461(3), N6–C74 1.556(3).
The Y–Nimine bonds in the AIm2 monoanion in 1 are 2.475(5)
and 2.494(4) Å. Crystallographically characterized rare earth
imine complexes in which the nitrogen atom of the imine
serves as the attachment point to the remainder of a multi-
dentate ligand framework are [Cp″(PhCN)Y(μ–κ2:κ1-PhCH2N–
CPhvN)(μ–κ1:κ3-NvCPh–NvCPh–N–CHPh–NvCHPh)YCp″]
{Cp″
=
C5Me4(SiMe3)},27
[Cp*2Sm{κ2-N(SiMe3)–CHPh–
NvCHPh}],28 and 9-coordinate [Sm{κ5-NH(CH2CH2NvCH-
(2-NC4H3))2}{κ4-NH(CH2CH2NvCH(2-NC4H3))(CH2CH2NvCH-
(2-NHC4H3)}],29 with Ln–Nterminal-imine bonds of 2.517(3),
2.548(7) and 2.83(1) Å, respectively. With the exception of the
latter bond distance, which is long due to the high coordi-
nation number, these distances are very similar to those in
compound 1, taking into account the larger ionic radius of
Sm3+ versus Y3+ (0.96 vs. 0.90 Å for C.N. = 6).30 Compared with
the Y–Nimine distances in the AIm2 monoanion in 1, the corres-
single trianionic, hexadentate ligand that features one amine,
two imine, and three amido donors. The acridanide backbone
and one imine group of each of the original AIm2 ligands is
intact, but the two acridanide backbones are now linked by an
isoindoline ring (Scheme 2). The yttrium center is coordinated
to six nitrogen donors in an arrangement which leaves one
face of the metal center open, possibly leading to an inter-
action with the ortho carbon [C(86)] of an isoindoline phenyl
substituent positioned 2.904(2) Å from yttrium.
ponding distances in the AIm′ dianion are significantly
2
elongated [Y–N(2) = 2.529(4) Å; Y–N(3) = 2.662(4) Å], apparently
due to steric restrictions imposed by the anionic amido and
The hexadentate, trianionic ligand contains two chiral
centers in the isoindoline 5-membered ring, at N(6) and C(31),
and crystals of 2·(benzene)3.35 contain a racemic mixture of SS
and RR enantiomers. Diastereomers were not observed crystallo-
graphically or in the 1H NMR spectrum, indicating that the
constrained geometry of the yttrium-bound ligands during the
rearrangement process (vide infra) only allows formation of the
rac diastereomer. The CvN bonds of the unrearranged imines
[1.295(3) and 1.300(3) Å] and the Caryl–Namido distances
[1.365(3) to 1.396(3) Å] in 2 are very similar to the corres-
aryl donors of the rigid AIm′ ligand framework; N(2) is located
2
between the two anionic donors, while N(3) is on the peri-
phery. Nevertheless, all of the Y–Nimine bonds in 1 fall within
the broad range of Y–Nimine distances reported in the literature;
for example 2.429(2) and 2.457(2) Å in [{N(o-C6H4(CHvNAr))2}-
Y(CH2SiMe3)2] {L = bis(imino)diphenylamido},5c 2.483(2) Å in
[{XylN–CH2–C(CH2SiMe3)vNXyl}Y-(CH2SiMe3)2(THF)] (Xyl
2,6-dimethylphenyl),6a and 2.665(4)
in [{NC9H5(8-O)-
disubstituted
=
Å
(2-CMevNMes)}YCl2(dmso)2]
(NC9H5
=
quinoline).31
ponding bond distances in 1 (CvN = 1.30–1.32 Å; C–Ndiarylamido
=
Compound 2 was prepared via the reaction of [Y(CH2Si-
Me3)3(THF)2] with two equivalents of H[AIm2] in benzene fol-
lowed by stirring for three days at room temperature. A total of
8 methyl (Ar–Me and CMe2) signals are observed in the 1H and
13C NMR spectra of 2, indicating the formation of a highly
asymmetric product. X-ray quality crystals of 2·(benzene)3.35
(Fig. 5) were grown by layering a saturated benzene solution
with pentane. Compound 2 is an isomer of 1 containing a
1.38–1.40 Å). The Caryl–Nimine distances in 2 [1.424(2) and
1.445(2) Å] are also comparable with those in 1 (1.41–1.45 Å),
and are similar to the C(31)sp3–N(3)amido and C(55)aryl–N(6)sp3
bonds to the isoindoline ring in 2 [1.457(3) and 1.461(3) Å,
respectively]. By contrast, the C(31)–N(6) and C(74)–N(6) dis-
tances within the isoindoline ring in 2 are substantially
elongated at 1.569(3) and 1.556(3) Å, respectively. Presumably
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for steric reasons, these distances are longer than the corres-
ponding C–N distances of 1.50–1.53 Å in [{O(o-C6R4)CHvN–
CHPh–CHPh–N(CH2)2(o-C6H4)}MXn] {MXn = TiCl3 and NiCl},
which contain an isoindoline heterocycle free from substitu-
ents on carbon.32
The Y–Ndiarylamido distances in 2 [Y(1)–N(1) = 2.3281(16) Å
and Y(1)–N(4) = 2.3258(17) Å] are slightly shorter than the Y–
Ndiarylamido distances in 1, while the Y–Nimine bonds [Y(1)–N(2)
= 2.5236(18) Å and Y(1)–N(5) = 2.5388(17) Å] are slightly longer
than the Y–Nimine bonds to the unaltered AIm2 ligand in 1.
These differences are ostensibly the result of geometric con-
straints imposed by the hexadentate ligand framework in 2.
The alkylarylamido donor, N3, resulting from the ligand coup-
ling and rearrangement is trigonal planar [sum of bond angles
around N3
= 358.88(25)°] with a Y(1)–N(3) distance of
2.2020(18) Å. This Y–Nalkylarylamido distance is substantially
shorter than the Y–Ndiarylamido bonds in 1 or 2 [2.3258(17)–
2.367(4) Å], consistent with the enhanced donor ability of an
alkylarylamido versus a diarylamido group; for comparison the
Y–Nalkylarylamido distance in [{κ3-ArNCMeCHCMeN(CH2)2NMe2}-
Y(NHAr){N(Xyl)CH2Ph}] {Xyl = 2,6-dimethylphenyl; Ar = 2,6-
diisopropylphenyl} is 2.213(2) Å.33
The amine donor, N6, is pyramidalized with the sum of the
C–N–C bond angles around N6 equal to 338.38(27)°. There are
no crystallographic reports of an isoindoline donor bound to a
rare earth element. However, the Y–N distances to the trialkyla-
mine donors in [{κ3-ArNCMeCHCMeN(CH2)2NMe2}Y(NHAr)(κ2-
Fig. 6 (a) Absorption spectra of H[AIm2] (black) and Li[AIm2] (gray) in
benzene; calculated high oscillator strength transitions for H[AIm2] and
Li[AIm2] are shown as orange and blue lines, respectively. (b) Absorption
spectra of 1 (black) and 2 (grey) in benzene; for 1, calculated transitions
with high oscillator strengths are shown as vertical black bars. In each
plot, the y-axis on the left is the experimental extinction coefficient
while the y-axis on the right is the calculated oscillator strength for each
transition.
amiPr)] {Ar
= =
2,6-diisopropylphenyl; amiPr
CH(NiPr)2}33
and [{κ4-N(CH2(o-C5H4N))-(CH2CH2NSiMe3)2}Y(κ2-amTMS,Ph)]
{amTMS,Ph = CPh(NSiMe3)2}34 are 2.511(3) and 2.557(3) Å,
respectively, approaching the Y(1)–N(6) distance of
2.5858(18) Å in 2. The metrical parameters of 2 are therefore in
agreement with the identification of the hexadentate ligand as
a tris(amido)-mono(amino)-bis(imino) donor.
functional, CAM-B3LYP, with the default parameters (µ = 0.33,
α = 0.19, β = 0.46) and the 6-31G* basis set, yielding a structure
which closely mirrors the X-ray crystal structure. The µ, α, and
β parameters of the CAM-B3LYP functional were then tuned
according to the method reported by Nakano et al.38 to more
accurately reproduce the excited state energies of H[AIm2];
values of µ = 0.15, α = 0.15, and β = 0.85 for CAM-B3LYP
resulted in energies for the transitions that are in good agree-
ment with the experimental spectrum (Fig. 6). The TD-DFT cal-
culations predict two intense electronic transitions from the
HOMO to the LUMO and the LUMO+1. The HOMO is primar-
ily centered on the acridan ligand backbone, with the largest
atomic contribution (25%) from the central nitrogen atom. By
contrast, the LUMO and LUMO+1 are mostly centered on the
NvCPh2 substituents with the largest atomic contributions
from the imine carbon atoms (10.5% and 11.5%), indicating
that charge transfer transitions are responsible for the intense
orange colour of H[AIm2] (Fig. 7). This assignment is in
keeping with the molecular structure of H[AIm2], which bears
resemblance to a donor–(π-bridge)–acceptor organic dye, such
as those used in dye-sensitized solar cells.39
In an attempt to disfavor ligand cyclometalation and
rearrangement, a bulkier variant of AIm2 with the phenyl sub-
stituents replaced with mesityl substituents was pursued.
However, a suitable catalyst system18b,35 for the Pd-catalyzed
imination
of
H[ABr2]
with
2,2′,4,4′,6,6′-hexamethyl-
benzophenoneimine36 could not be found. The reaction of
[Y(CH2SiMe2Ph)3(THF)2]37 with one equivalent of H[AIm2] in
benzene-d6 was also performed. However, 1 formed rapidly as
the major product, indicating that the bulkier CH2SiMe2Ph
ligand is not able to favor the formation of a mono-ligated bis-
(alkyl) complex or to prevent AIm2 ligand cyclometalation at
room temperature.
UV-visible absorption spectroscopy and TD-DFT calculations
H[AIm2], has an intense orange colour in solution and in the
solid state, and the UV-vis absorption spectrum of H[AIm2] in
benzene exhibits a broad absorption band in the visible region
with an absorption maximum at 438 nm (ε = 4640 L mol−1
cm−1; Fig. 6). Time-dependent density functional theory calcu-
lations (TD-DFT, PCM benzene) were used to probe the origin
of the intense orange colour of H[AIm2]. The ground-state geo-
metry of H[AIm2] was optimized using a long range corrected
The intense blue and green-brown colours of 1 and 2 are
highly unusual for d0 yttrium(III) complexes; in benzene, 1
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Fig. 8 Single crystal X-ray structure of Li2[AIm2]2·toluene. Hydrogen
atoms and lattice solvent have been removed, and phenyl substituents
are depicted in wireframe for clarity. Thermal ellipsoids are set at 50%
probability. Selected bond lengths [Å]: Li1–Li1* 2.474(6), Li1–N1 2.134(3),
Li1*–N1 2.140(3), Li1–N2 2.095(3), Li1*–N3 2.089(3).
Fig. 7 Molecular orbital isosurfaces and TD-DFT transition energies for
(a) H[AIm2], (b) Li[AIm2], (c) Li2[AIm2]2, and (d) yttrium compound 1.
Li(1)*, Li(1)–N(1)–C(1), Li(1)*–N(1)–C(13) and C(1)–N(1)–C(13)
angles of 70.74(14)°, 85.87(12)°, 86.56(12)° and 112.15(13)°,
respectively. The angle between the Li(1)–N(1)–Li(1)* and C(1)–
N(1)–C(13) planes is 24.5°, and the Li(1)–C(1)–C(13)–Li(1)*
torsion angle between the four atoms bound to N(1) is just
2.0°. The unusual geometry at N(1) is suggestive of a 3c-2e
Li(1)–N(1)–Li(1)* bonding interaction, and long Li–Namido dis-
tances support this conclusion; the Li(1)–N(1)amido and Li(1)*–
N(1)amido distances are 2.140(3) and 2.134(3) Å, respectively,
while the Li(1)–N(2)imine and Li(1)*–N(3)imine bonds are
2.095(3) and 2.089(3) Å. These bond distances follow the same
trend as the average Li–Nbridging [2.250(5) Å] and Li–Nnon-bridging
shows a broad absorption maximum at 637 nm (ε = 7911 L
mol−1 cm−1), while 2 shows a broad absorption that spans the
visible range with a λmax at 898 nm (ε = 3299 L mol−1 cm−1
)
(Fig. 6b). Furthermore, it was found that deprotonation of
H[AIm2] with one equivalent of LiCH2SiMe3 in toluene gave
dark blue Li2[AIm2]2 (Scheme 2) with λmax at 654 nm (ε =
7889 L mol−1 cm−1) in benzene (Fig. 6a).
X-ray quality crystals of Li2[AIm2]2·toluene were grown by
layering pentane onto a toluene solution. The single crystal
X-ray structure (Fig. 8) shows that Li[AIm2] dimerizes in the
solid state with a center of inversion midway between N(1)–
N(1)*, rendering each monomeric unit crystallographically
equivalent. Compared to the relatively flat ligand backbones of
H[AIm2], 1, and 2, the ligand backbone in Li2[AIm2]2 has a
139° angle between the two aryl rings, and the central nitro-
gen-containing ring adopts a boat conformation. The Li(1)–
Li(1)* distance of 2.474(6) Å is shorter than the reported dis-
tance of 2.79(1) Å40 for [Li2{μ-1κ2:2κ2-N(PPh2NSiMe3)2}2], which
exhibits similar connectivity to Li2[AIm2]2, but is similar to the
Li–Li distance of 2.494(6) Å in [Li2{μ-1κ2:2κ2-N(SiMe2NEt2)2}2]
which features an acute Li–Si–Si–Li torsion angle of 38°
around the central anionic nitrogen donors.41
[1.997(5)
Å]
distances
reported
for
[Li2{μ-1κ2:2κ2-
N(PPh2NSiMe3)2}2].40
TD-DFT calculations using the CAM-B3LYP functional (µ =
0.15, α = 0.15, and β = 0.85) and 6-31G* basis set for the
Li[AIm2] monomer and Li2[AIm2]2 dimer predict a red-shift of
λmax relative to H[AIm2]. However, only monomeric Li[AIm2]
afforded computed transition energies in reasonable agree-
ment with experimental results (Fig. 6 and 7). In solution, it is
likely that compound Li[AIm2] exists as a monomer and/or a
dimer, rather than a well-separated ion pair, and from these
two possibilities, TD-DFT calculations suggest that a mono-
meric species is dominant. The experimental λmax for solutions
of Li[AIm2] is also nearly identical in benzene and THF
(654 vs. 652 nm), providing support for the predominance of a
The geometry at lithium is distorted tetrahedral with a
63.5° angle between the N(1)–Li(1)–N(2) and the N(1)*–Li(1)–
N(3)* planes. The acridanide nitrogen donor, N(1), adopts an
unusual distorted square planar geometry with Li(1)–N(1)–
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monomeric species in both solvents. As with H[AIm2], calcu-
lations on all three species predict intense charge transfer
transitions from the HOMO to the LUMO and the LUMO+1/
LUMO+2; the HOMO is centered primarily on the ligand back-
bone and the LUMO and LUMO+1/LUMO+2 are centered on
the imine substituents (Fig. 6 and 7).
TD-DFT calculations were also carried out on 1 using the
tuned CAM-B3LYP functional with LanL2DZ (Y) and 6–31G*
(C, N, H) basis sets, yielding computed electronic transition
energies close to experimental values (Fig. 6). The HOMO is
mostly based on the ligand backbone of the unactivated AIm2
ligand, whereas the LUMO and LUMO+1 are located primarily
on the NvCPh2 substituents of this ligand, each with a small
contribution from an yttrium d orbital (Fig. 7). Therefore, the
colour of 1 originates primarily from charge transfer within
the intact AIm2 ligand in 1.
Fig. 9 Absorption spectra for the conversion of 1 to 2 recorded from
0.5 to 60 h in benzene.
Kinetics and proposed mechanism for the formation of 2
In the conversion of 1 to 2, it is likely that the imine carbon of
the intact AIm2 ligand in 1 inserts into the Y–C bond to form
1′. Compound 2 can then be formed by a second 1,2-insertion
involving the newly formed amido nitrogen and the imine
carbon bound to the cyclometalated phenyl substituent,
thereby generating the isoindoline ring that links the two acri-
danide moieties (Scheme 3).
Conversion of 1 to 2 was monitored by recording an absorp-
tion spectrum every 30 minutes from 0.5 hours to 60 hours
after 1 was dissolved in benzene (Fig. 9). The absorption
spectra were deconvoluted (see ESI†) to extract the absorbance
of 1 at 630 nm since the absorbance of 2 at 630 nm is non-
zero. Since, no intermediates are observed spectroscopically,
k2 ≫ k1 in Scheme 3, a first-order rate law can be applied as a
close approximation for the rearrangement of 1 to 2. The
observed rate constant for the reaction is 0.089 h−1, giving a
half-life of 7.8 hours at room temperature (see Fig. S1 in ESI†
for a plot of ln[A1] vs. time (A1 = absorbance of 1); Eyring analy-
sis was not attempted since the reaction is slow at room temp-
erature and does not proceed cleanly above room
temperature).
A unimolecular process is consistent with
the proposed intramolecular mechanism, ruling out inter-
molecular ligand transfer processes.
Conclusions
An extremely rigid acridanide proligand, H[AIm2], has been
prepared and deprotonated to form Li[AIm2]. The proligand is
intensely orange coloured while the lithiated ligand is deep
blue; these colours were found to originate from charge trans-
fer transitions from the ligand backbone to the imine substitu-
ents, highlighting the structural relationship between H[AIm2]
and Li[AIm2] and donor-pi-acceptor dyes. Addition of two
equivalents of H[AIm2] to [Y(CH2SiMe3)3(THF)2] rapidly gener-
ated cyclometalated bis(ligand) complex 1. This deep blue
compound undergoes a two-step intramolecular ligand coup-
ling process to form green-brown 2 which contains a hexa-
dentate tris(amido)-mono(amino)-bis(imino) trianion featuring
an isoindoline moiety bridging between the two acridanide
ligand backbones. As with H[AIm2] and Li[AIm2], the intense
colour of 1 was calculated to originate from ligand-based
charge transfer transitions. Attempts to synthesize stable
dialkyl yttrium AIm2 complexes using AIm2 proved unsuccess-
ful, even using bulkier CH2SiMe2Ph ligands. The 2,7,9,9-tetra-
methylacridanide ligand backbone is expected to provide a
versatile platform for the development of new highly rigid
pincer ligands. Future work will target acridanide pincer
ligands incorporating neutral donor substituents that are less
Scheme 3 Proposed reaction pathway for conversion of 1 to 2.
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prone to cyclometalation and nucleophilic attack, with a view which phenyl rings in the structure of 2 correspond to rings
towards the synthesis of robust neutral and cationic organo- A–C, or which acridanide backbone aryl rings correspond to α,
rare earth complexes.
β, γ and δ.
UV-Vis absorption spectra were recorded at ambient temp-
erature on a Varian Cary 50 Probe Spectrophotometer in a
1 cm quartz cell equipped with a Chemglass CHEM-VAC™
CHEM-CAP® valve. Elemental analyses were performed on a
Thermo (Carlo Erba) Flash 2000 Elemental Analyzer at London
Experimental
General procedures
All manipulations of oxygen- or moisture-senstive materials Metropolitan University.
were carried out under an atmosphere of argon in an Innova-
tive Technology PureLab HE glovebox, MBraun UNIlab glove-
box, or using standard Schlenk techniques. All glassware was
Computational methods
rigorously dried overnight at 120 °C and cooled under vacuum DFT and TD-DFT calculations were performed using Gaussian
before use.
09 (Revision D.01).44 Geometry optimizations were performed
Caesium carbonate was dried overnight at 70 °C under using the CAM-B3LYP functional,45 the 6-31G* basis set (C, H,
vacuum. Methyl-2-amino-5-methylbenzoate was prepared N, Li),46 and LanL2DZ basis set (Y).47 Frequency calculations
according to a published procedure.17 1,8-Dibromo-3,6,9,9- at the same level of theory confirmed that the optimized struc-
tetramethyl-9,10-dihydroacridine was synthesized from 3,6,9,9- tures were located at a minimum on the potential energy
tetramethyl-9,10-dihydroacridine using a reported procedure.16 surface. The same basis sets were used for single-point calcu-
20
37
Y(CH2SiMe3)3(THF)2 and Y(CH2SiMe2Ph)3(THF)2 were syn- lations, but the parameters of the CAM-B3LYP functional were
thesized according to published procedures. LiCH2SiMe3 was modified to µ = 0.15, α = 0.15, and β = 0.85 from the default
received as a 1.0 M solution in pentane, volatiles were removed parameters of µ = 0.33, α = 0.19, β = 0.46. The energies and
in vacuo, and the remaining solid was collected and stored at intensities of the 10 lowest energy electronic transitions were
−30 °C in a glovebox. All other reagents were purchased from calculated by TD-DFT48 using the same functional/basis set
commercial sources and used as received.
combinations employed for the single-point calculations and a
Pentane, benzene, and THF were dried and distilled at polarized continuum model (PCM) for benzene.49 AOMix50 was
atmospheric pressure from sodium benzophenone ketyl; used for determining atomic orbital compositions employing
sodium was used to dry toluene. Unless otherwise noted, all Mulliken Population Analysis.
anhydrous solvents were stored over an appropriate drying
agent prior to use (THF, toluene, and C6D6 over Na/Ph2CO; APEX II diffractometer with a 3 kW Sealed tube Mo generator
pentane over Na/Ph2CO/tetraglyme). in the McMaster Analytical X-Ray (MAX) Diffraction Facility.
X-ray crystallographic analyses were performed on a SMART
All NMR spectra were collected on a Bruker DRX-500 or an Temperature was regulated using an Oxford Cryosystems Cryo-
AV-600 spectrometer. Chemical shifts for 1H NMR spectra were stream. Suitable crystals were coated in Paratone oil inside a
referenced to residual 1H NMR resonances of the deuterated glovebox (Li2[AIm2]2, 1, and 2) or in air (H[AIm2]), mounted on
solvent and 13C NMR spectra were referenced to 13C NMR reso- a MiTeGen head, and then placed in the cold stream of the
nances of the deuterated solvent.42 All chemical shifts are diffractometer. Details of the data reduction and absorption
reported relative to tetramethylsilane.
correction can be found in the CIF. Structures were solved
Peak assignments for all new compounds were made using using SIR9251 and refined by least-squares procedures in
1H, 13C{1H}, DEPT-135, DEPT-q, COSY, HSQC and HMBC NMR CRYSTALS.52 The hydrogen bound to the nitrogen in H[AIm2]
experiments. TOCSY NMR was also used to aid in the assign- was located in the difference map and refined isotropically
1
ment of the H NMR spectra of 1 and 2. In the 13C NMR spec- without other restraints. All other hydrogen atoms were placed
trum of compound 1, 19 of the 25 expected Ar–CH peaks and in idealized geometric positions and linked to their respective
18 of the 19 expected quaternary Ar peaks were located. In the atoms using a riding model during refinement. The isotropic
13C NMR spectrum of compound 2, 30 of the 39 expected Ar– temperature factor of each hydrogen atom was initially set to
CH peaks, and 23 of the 25 expected quaternary Ar peaks were 1.2 times that of the atom it is bonded to and then the temp-
located. The remaining peaks were presumably obscured by erature factors of the hydrogen atoms were linked during
coincidental overlap with other aryl peaks, given the large refinement. Crystals of 1 diffracted poorly and the data set for
number of inequivalent aryl carbon environments (44 for 1 1 was 87.5% complete at θ = 25.267°, limiting the accuracy of
and 64 for 2). These missing peaks could not be located in 2D the metrical parameters. Toluene molecules in
1 and
HSQC or HMBC NMR experiments. Li2[AIm2]2 were disordered and modeled accordingly (see CIF
Assignments of the resonances in the NMR spectra were for details). One benzene molecule in 2 was positionally dis-
made according to the numbering scheme for acridines, as ordered over two overlapping positions with 59 : 41 occupancy
illustrated in Scheme 1.43 For compound 2, C6H5 ring A, and and refined anisotropically. Another benzene molecule in 2
aryl rings B and C are rings in which all protons are inequiva- exhibited partial occupancy and the occupancy of this mole-
lent, and rings labeled α, β, γ and δ indicate the four aryl rings cule was refined freely (see CIF for details). All crystal structure
of the acridanide backbones. We were not able to identify diagrams were generated by ORTEP-3 for Windows (v. 2.02)53
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Table 1 Crystallographic data for H[AIm2], 1, 2, and Li2[AIm2]2
Compound
Lattice Solvent
H[AIm2]
—
1
2
Li2[AIm2]2
Toluene
(Pentane)0.5 (toluene)2
(Benzene)3.35
Empirical formula
C
43H37N3
C95.50H85N6Y
1405.67
Triclinic
ˉ
P1
C106.09H91.09N6Y
1538.91
Triclinic
ˉ
P1
C46.50H40LiN3
647.79
Triclinic
ˉ
P1
Formula weight
Crystal system
Space group
Crystal habit
a (Å)
b (Å)
c (Å)
595.76
Triclinic
ˉ
P1
Block
Plate
Block
Block
7.9754(3)
12.3322(5)
17.8966(8)
104.062(3)
101.490(2)
95.503(2)
1653.85(12)
2
13.744(2)
14.389(2)
20.904(3)
82.938(4)
71.274(4)
74.886(4)
3776.3(9)
2
13.055(3)
14.122(3)
24.089(5)
106.793(3)
94.276(3)
98.405(3)
4174.0(16)
2
11.8264(6)
12.7464(6)
12.7973(7)
101.517(3)
108.568(3)
94.633(3)
1770.11(16)
2
α (°)
β (°)
γ (°)
V (Å3)
Z
T (K)
150
1.196
0.070
100
1.236
0.825
150
1.224
0.752
100
1.215
0.070
ρcalcd (g cm−3
)
µ (mm−1
)
R [I0 ≥ 2σ(I0)]
0.0387
0.0841
1.0015
0.0697
0.1614
0.9210
0.0447
0.1104
0.9701
0.0640
0.1479
0.9830
Rw [I0 ≥ 2σ(I0)]
Goodness of fit
and rendered using POV-Ray (v. 3.6.1).54 The crystallographic NaOH solution and extracted with Et2O (2 × 50 mL). All of the
data are summarized in Table 1. organic phases were combined, dried with anhydrous Na2SO4,
Methyl 2-(N-(4-methylphenyl)amino)-5-methylbenzoate. This and gravity filtered. Volatiles were removed from the filtrate
procedure was adapted from a previous report of a Buchwald– under reduced pressure to yield a brown solid suitably pure for
Hartwig coupling using similar substrates.55 A solution of further reactions. Yield: 1.946 g (82.1%). Analytical and
4-bromotoluene (3.452 g, 20.18 mmol) and methyl-2-amino- spectroscopic data for this compound have been published
5-methylbenzoate (3.280 g, 19.86 mmol) in toluene (20 mL) previously.16
was added to a mixture of palladium(II) acetate (0.244 g,
H[AIm2]. The catalyst mixture of Pd2dba3 (0.0927 g,
1.09 mmol), tri-tert-butylphosphine (0.658 g, 3.25 mmol), and 0.101 mmol), rac-BINAP (0.1261 g, 0.2025 mmol), and sodium
caesium carbonate (8.840 g, 27.13 mmol) in toluene (4 mL) tert-butoxide (0.6324 g, 6.580 mmol)18a in toluene (5 mL) was
and refluxed for 18.5 hours. The reaction mixture was allowed heated to 110 °C. A solution of HABr2 (1.000 g, 2.531 mmol)
to cool to room temperature and then gravity filtered. The and benzophenone imine (1.10 mL, 6.58 mmol) in toluene
orange filtrate was concentrated under reduced pressure and (20 mL) was then added to the catalyst mixture and refluxed
the crude product was purified by silica gel column chromato- for 16 hours. The reaction mixture was allowed to cool to room
graphy (hexanes followed by 50 : 1 hexanes : ethyl acetate) to temperature, extracted with Et2O (100 mL), washed with H2O
obtain a yellow oil. Yield: 3.682 g (72.6%). Analytical and spec- (150 mL), and the organic layer was collected. The aqueous
troscopic data for this compound have been published layer was extracted with Et2O (100 mL) twice more. The organic
previously.16
layers were combined and dried with MgSO4, gravity filtered,
3,6,9,9-Tetramethyl-9,10-dihydroacridine (HAH2). The syn- and the filtrate was concentrated under reduced pressure to
thesis of HAH2 has been reported,16 however, minor modifi- obtain a dark orange oil. The crude product was purified by
cations to the procedure are detailed here. MeMgBr (3.18 M in silica gel column chromatography (CH2Cl2) to obtain an
Et2O, 12.6 mL, 40.0 mmol) was added dropwise to a solution orange oil. A crystalline orange solid was obtained by adding a
of
methyl
2-(N-(4-methylphenyl)amino)-5-methylbenzoate small amount of EtOH to a concentrated Et2O solution of the
(2.554 g, 10.00 mmol) in THF (60 mL) at 0 °C, warmed to room product. The solid was washed with hexanes, collected by
temperature, and stirred for 15 hours. The reaction solution gravity filtration, and air dried. Yield: 1.130 g (74.8%). Crystals
was added to H2O (150 mL) dropwise and the product was suitable for X-ray analysis were obtained by slow evaporation of
extracted with Et2O. The organic layer was collected, dried over 1 : 2 Et2O/EtOH solution of the product. 1H NMR (600 MHz,
anhydrous Na2SO4, and gravity filtered. The filtrate was con- CD2Cl2, 298 K): δ 7.66 (m, 4H, PhH), 7.41 (br s, 1H, NH),
centrated under reduced pressure to give a yellow oil, which 7.30–7.35 (m, 8H, PhH), 7.18 (m, 4H, PhH), 7.11 (m, 4H, PhH),
was used in the next step without further purification. 85% 6.84 (br s, 2H, CH1,8), 6.00 (m, 2H, CH3,6), 2.02 (s, 6H, ArMe),
H3PO4 (10 mL) was added to the yellow oil and heated to 1.50 (s, 6H, CMe2). 13C{1H} NMR (151 MHz, CD2Cl2, 298 K):
120 °C for 2.25 hours. The green reaction mixture was diluted δ 169.0 (NvC), 139.7 (NvC–C), 137.2 (NvC–C), 135.8 (C12,13),
with H2O, (80 mL) extracted with Et2O, and the organic phase 130.82 (PhC), 130.78 (C4,5), 129.5 (C11,14), 129.3 (PhC), 129.2
was collected. The aqueous phase was neutralized using a 1 M (PhC), 129.0 (PhC), 128.5 (PhC), 128.3 (C2,7), 121.7 (CH1,8),
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118.4 (CH3,6), 36.8 (CMe2), 30.0 (CMe2), 21.1 (ArMe). UV-vis 6.46 (s, 1H, CH1/8 γ), 6.33 (s, 1H, CH1/8 δ), 6.22 (s, 1H, CH3/6 α),
(benzene): λmax (ε) = 438 nm (4640). Anal. Calcd for C43H37N3: 5.84 (s, 1H, CH3/6 β), 5.82 (s, 1H, CH3/6 δ), 5.15 (s, 1H, CH3/6 γ),
C, 86.69; H, 6.26; N, 7.05. Found: C, 86.45; H, 6.38; N, 6.97.
2.26 (s, 3H, ArMe δ), 2.01 (s, 3H, ArMe α), 1.73 (s, 3H, ArMe β),
Compound 1. Y(CH2SiMe3)3(THF)2 (0.050 g, 0.101 mmol) 1.66 (s, 3H, ArMe γ), 1.87, 1.34 (s, 2 × 3H, one CMe2), 1.57, 1.33
and H[AIm2] (0.120 g, 0.202 mmol) were stirred in benzene (s, 2 × 3H, one CMe2). 13C{1H} NMR (151 MHz, C6D6, 298 K):
(6 mL) at room temperature to give a dark blue solution. Vola- δ 180.63, 164.91 (2 × NvC), 149.34, 148.64, 147.76, 147.47,
tiles were removed in vacuo after 30 minutes and a dark blue 143.82, 143.39, 142.74, 142.56, 140.73, 140.27, 140.17, 140.07,
solid was collected. A saturated solution of the crude product 137.85, 135.63, 135.35, 132.91, 130.08, 126.31, 126.13, 124.91,
was prepared in toluene and filtered through Celite to remove 124.54, 121.56, 121.47 {23 × quaternary Ar (expected 25)},
any undissolved solids. The crude product was recrystallized 134.71 (m-C6H5 A), 133.99 (CH3/6 γ), 131.21, 131.10, 130.50,
by layering pentane onto the saturated toluene solution in 130.26, 130.17, 130.03, 129.87, 129.52, 129.46, 128.73, 128.70,
several sealed culture tubes and cooling to −30 °C. Yield: 128.34, 128.11, 127.82, 127.72, 127.38, 127.19, 126.95, 126.90,
1
0.110 g (84.9%). H NMR (500 MHz, d8-toluene, 238 K): δ 8.83 126.59, 126.55, 126.02 {22 × Ar–CH (expected 31)}, 124.26
3
(d, JH,H 7 Hz, 1H, C6H4 ortho to metalated carbon atom), 7.56 (CH1/8 β), 122.01 (CH3/6 α), 121.73 (CH1/8 γ), 121.05 (CH3/6 β),
3
3
(d, JH,H 7 Hz, 4H, o-PhH intact AIm2), 7.28 (t, JH,H 7 Hz, 1H, 113.89 (CH3/6 δ), 113.86 (CH1/8 δ), 100.38, 84.45 (isoindoline
3
C6H4 meta to metalated carbon atom), 7.25 (d, JH,H 7 Hz, 2H, CPh2 and CPhNAr), 39.97, 27.56 (2 × one CMe2), 36.38, 36.32
o-PhH metalated AIm2), 7.2–6.7 (m, 29H, PhH), 6.91 (2H, CH1,8 (2 × CMe2), 36.25, 31.01 (2 × one CMe2), 22.31 (ArMe δ), 21.42
intact AIm2), 6.81, 6.72 (2 × 1H, CH1 and CH8 on metalated (ArMe α), 20.61 (ArMe β), 20.40 (ArMe γ). UV-vis (benzene): λmax
3
AIm2), 6.60 (d, JH,H 7 Hz, 2H, o-PhH metalated AIm2), 5.94 (s, (ε) = 898 nm (3299). Anal. Calcd for C104H89N6Y (2·3benzene):
2H, CH3,6 intact AIm2), 5.92, 5.42 (s, 2 × 1H, CH3 and CH6 on C, 82.63; H, 5.93; N, 5.56. Found: C, 82.51; H, 6.03; N, 5.58.
metalated AIm2), 2.04, 1.84 (s, 2 × 3H, CMe2 intact AIm2), 1.99
Lix[AIm2]x (x = 2 in the solid state). LiCH2SiMe3 (0.0158 g,
{s, 9H, ArMe intact AIm2 (6H) + ArMe metalated AIm2 (3H)}, 0.168 mmol) and H[AIm2] (0.1000 g, 0.1679 mmol) were
1.80 {s, 9H, CMe2 metalated AIm2 (6H) + ArMe metalated AIm2 stirred in toluene (10 mL) at room temperature to give a dark
(3H)}. 13C{1H} NMR (125 MHz, d8-toluene, 238 K): δ 194.20 (d, blue solution. Volatiles were removed in vacuo after 1.5 hours
1J13C,89Y 42.7 Hz, ipso Y–C6H4), 178.47, 174.21, 169.51 (3 × and a dark blue solid was collected. A concentrated solution of
NvC), 139.89 (Ar–CH ortho to metalated carbon), 140.81, the crude product was prepared in toluene and filtered
131.15, 130.87, 130.60, 130.42, 129.86, 129.52, 128.68, 128.44, through Celite to remove any undissolved solids. The crude
127.56, 127.23, 126.69, 125.77, 124.58, 124.48, 124.07, 123.16, product was recrystallized by layering pentane onto the
122.79 {18 × Ar–CH (expected 24)}, 151.39, 145.81, 144.55, toluene solution and cooling to −30 °C. Yield: 73.3 mg
142.84, 142.50, 141.71, 140.71, 139.82, 139.41, 137.70, 134.06, (72.6%). 1H NMR (600 MHz, C6D6, 298 K): δ 7.17 (d, 3JH,H 7 Hz,
130.59, 130.01, 126.92, 123.81, 122.82, 120.60 {17 × quaternary 4H, o-PhH B), 7.15 (s, 2H, CH1,8), 7.06 (d, 3JH,H 7 Hz, 4H, o-PhH
3
3
Ar (expected 18)}, 40.59, 28.43 (2 × CMe2 intact AIm2), 36.57 A), 6.98 (t, JH,H 7 Hz, 2H, p-PhH A), 6.95 (t, JH,H 7 Hz, 2H,
3
3
(CMe2 intact AIm2), 36.02 (CMe2 metalated AIm2), 35.88 (CMe2 p-PhH B), 6.91 (t, JH,H 7 Hz, 4H, m-PhH B), 6.82 (t, JH,H 7 Hz,
metalated AIm2), 21.60, 20.50 (ArMe metalated AIm2), 21.22 4H, m-PhH A), 6.27 (s, 2H, CH3,6), 2.14 (s, 6H, ArMe), 1.88 (s,
(ArMe intact AIm2). UV-vis (benzene): λmax (ε) = 637 nm (7911). 6H, CMe2). 13C{1H} NMR (151 MHz, C6D6, 298 K): δ 164.01
Anal. Calcd for C91H83N6Y (1·pentane): C, 80.99; H, 6.20; N, (NvC), 146.96 (C12,13), 142.85 (ipso-PhC A), 137.88 (ipso-PhC
6.23. Found: C, 80.91; H, 6.24; N, 6.19.
B), 137.18 (C4,5), 132.15 (C11,14), 130.16 (o-PhC B), 129.44
Compound 2. Y(CH2SiMe3)3(THF)2 (0.050 g, 0.101 mmol) (p-PhC A), 128.98 (m-PhC A), 128.95 (p-PhC B), 128.35 (down
and H[AIm2] (0.120 g, 0.202 mmol) were stirred in benzene m-PhC), 128.02 (up o-PhC), 125.07 (CH1,8), 121.43 (C2,7), 120.23
(5 mL) at room temperature to give a dark blue solution that (CH3,6), 37.60 (C(CH3)2), 31.85 (CMe2), 21.64 (ArMe). UV-vis
slowly changed to dark green-brown over the course of three (benzene): λmax (ε) = 654 nm (7889). Anal. Calcd for C50H44LiN3
days. Volatiles were removed in vacuo and a dark green-brown (Li[AIm2]·toluene): C, 86.55; H, 6.39; N, 6.06. Found: C, 86.35;
crystalline solid was collected. A saturated solution of the H, 6.46; N, 6.14.
crude product was prepared in benzene and filtered through
Celite to remove any undissolved solids. The crude product
was recrystallized by layering pentane onto the saturated
Acknowledgements
benzene solution in several sealed culture tubes. Yield:
117.5 mg (91.3%). 1H NMR (600 MHz, C6D6, 298 K): δ 8.79 D. J. H. E. thanks NSERC of Canada for a Strategic Project Grant.
3
3
(broad d, JH,H 7 Hz, 1H, o-C6H5 A), 8.21 (broad d, JH,H 6 Hz,
3
1H, o-ArH B), 8.13 (d, JH,H 7.5 Hz, 1H, o-ArH C), 7.79 (broad t,
3JH,H 7 Hz, 1H, m-C6H5 A), 7.51 (broad s, 2H, ArH), 7.45 (broad
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