Aluminium complexes of a phenoxyimine ligand with a pendant imidazolium moiety: Synthesis, characterisation and evidence for hydrogen bonding in solution

Article abstract of DOI:10.1002/ejic.200800742

Novel alkylaluminium complexes (phim)AlMe₂ (1) and (phimid)AlR₂⁺Br^- [R = Me (2), R = iBu (3)] bearing the Schiff base ligands 3,5-tBu₂-2-(OH)C₆H ₂CH=NiPr (phim-H) and 3,5-tBu₂-2-(OH)C₆H ₂CH=NCH₂CH₂[CH(NCHCHNiPr)]Br (phimid-H·Br) have been prepared and fully characterised. Complexes 1-3 each have a tetrahedral structure, with the aluminium atom surrounded by the oxygen and nitrogen atoms of the chelating ligand and two alkyl groups. The structures of phimid-H·Br and of complex 1 have been determined by X-ray diffraction studies. Investigation of the solution structures of 1-3 by ¹H NMR spectroscopy revealed that the coordinated phimid ligand is involved in hydrogen bonding with bromide anion. Treatment of 1 with B(C ₆F₅)₃ led smoothly to (phim)Al(C ₆F₅)Me (4) by transfer of a C₆F₅ group from MeB(C₆F₅)₃^- to the initially formed coordinatively unsaturated cationic intermediate. In contrast, treatment of 2 with one equiv. of B(C₆F₅)₃ afforded the cationic monomethyl species (phimid)AlMeBr⁺MeB(C ₆F₅)₃^- (5), stabilised by the coordination of the bromide anion acting as a Lewis base. Wiley-VCH Verlag GmbH & Co. KGaA, 2009.

Full text of DOI:10.1002/ejic.200800742

FULL PAPER
DOI: 10.1002/ejic.200800742
Aluminium Complexes of a Phenoxyimine Ligand with a Pendant Imidazolium
Moiety: Synthesis, Characterisation and Evidence for Hydrogen Bonding in
Solution
Stefano Milione,*[a] Fabia Grisi,*[a] Roberto Centore,[b] and Angela Tuzi[b]
Keywords: Aluminium / N,O ligands / Hydrogen bonds
Novel alkylaluminium complexes (phim)AlMe2 (1) and
(phimid)AlR2+Br– [R = Me (2), R = iBu (3)] bearing the Schiff
base ligands 3,5-tBu2-2-(OH)C6H2CH=NiPr (phim-H) and
3,5-tBu2-2-(OH)C6H2CH=NCH2CH2[CH(NCHCHNiPr)]Br
(phimid-H·Br) have been prepared and fully characterised.
Complexes 1–3 each have a tetrahedral structure, with the
aluminium atom surrounded by the oxygen and nitrogen
atoms of the chelating ligand and two alkyl groups. The
structures of phimid-H·Br and of complex 1 have been deter-
mined by X-ray diffraction studies. Investigation of the solu-
tion structures of 1–3 by 1H NMR spectroscopy revealed that
the coordinated phimid ligand is involved in hydrogen bond-
ing with bromide anion. Treatment of 1 with B(C6F5)3 led
smoothly to (phim)Al(C6F5)Me (4) by transfer of a C6F5 group
from MeB(C6F5)3 to the initially formed coordinatively un-
saturated cationic intermediate. In contrast, treatment of 2
–
with one equiv. of B(C6F5)3 afforded the cationic monomethyl
–
species (phimid)AlMeBr+ MeB(C6F5)3 (5), stabilised by the
coordination of the bromide anion acting as a Lewis base.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
Moreover, the increasing interest being focused on the
utilisation of ionic liquids, many of which are imidazolium
salts,[4] as solvents for catalysis has prompted studies
centred on the interaction between imidazolium cations and
metals.[5]
We were interested in exploring the coordination behav-
iour of a modified phenoxyimine ligand containing a pen-
dant imidazolium moiety. In particular we considered
the salicylaldimine-functionalised imidazolium bromide
ligand [3,5-tBu2-2-(OH)C6H2CH=NCH2CH2(CH{NCH-
CHNiPr})]Br (phimid-H·Br), previously employed in the
synthesis of N-heterocyclic carbene complexes of late tran-
sition and lanthanide metals.[6]
Here we describe the synthesis, solution properties and
reactivity of novel alkylaluminium complexes bearing the
phimid-H·Br ligand. The distinctive behaviour of these com-
plexes is compared with that of an analogous alkylalumin-
ium complex bearing a salicylaldimine ligand lacking the
pendant imidazolium moiety: 3,5-tBu2-2-(OH)C6H2CH=
NiPr (phim-H).
Introduction
Schiff bases have been widely employed as heterobident-
ate ligands in the coordination chemistry of both transition
and p-block metal complexes.[1] Among these, aluminium-
based alkyl complexes have been reported and have been
shown to catalyse a variety of reactions.[2] These investi-
gations were generally centred on the metal first-coordina-
tion sphere, with scarce attention having been paid to the
influence of the second-sphere bonding interactions on the
reactivity and properties of these complexes. Non-coordi-
nating active sites may play important roles in molecular
recognition and activation processes involving catalysts sup-
ported by Schiff bases. However, exploration of noncova-
lent interactions exhibited by this group of compounds is
still an emerging area of research. In a recent paper by Lew-
inski et al.,[3] intra- and intermolecular noncovalent interac-
tions such as the C–Himino···O, C–Haryl···O, C–Haliph···O
and C–H···π hydrogen bonds and π stacking of group 13
Schiff base complexes were revised, with various structural
motifs being delineated and correlated to the ligand archi-
tecture and nature of the metal centre.
Results and Discussion
[a] Dipartimento di Chimica, Università di Salerno,
Via Ponte don Melillo, Fisciano, 84084 Salerno, Italy
E-mail: smilione@unisa.it
Synthesis and Characterisation of the Ligands
fgrisi@unisa.it
The phenoxyimine ligands 3,5-tBu2-2-(OH)C6H2CH=
NiPr (phim-H) and [3,5-tBu2-2-(OH)C6H2CH=NCH2CH2-
(CH{NCHCHNiPr})]Br (phimid-H·Br) were prepared in
high yields by modified literature procedures.[6,7] Well re-
[b] Dipartimento di Chimica, Università di Napoli “Federico II”,
Complesso Universitario di Monte S. Angelo,
Via Cinthia, 80126 Napoli, Italy
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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Novel Alkylaluminium Complexes
solved 1H NMR spectra of these ligands are reported in the gen bonding acceptors with acidic H atoms of the imid-
Supporting Information; the assignments of the protonic azolium rings and imino H atoms as donors. Several inter-
resonances were based on previously reported data, and in actions of this type that agree with cut-off rules suggested
the case of phimid-H·Br were corroborated by long-range for weak hydrogen bonding[8] can be found in the packing
1H–1H COSY experiments. Recrystallisation of phimid- [C11B–H11b···O2 2.263 Å 151.8°, C10A–H10a···O2(i)
H·Br from hexane/diethyl ether afforded crystals suitable 2.727 Å 136.3°, C12A–H12a···Br2 2.930 Å 120.9°, C10B–
for X-ray analysis.
H10b···Br2 2.944 Å 149.2°, C10A–H10a···Br1(ii) 2.963 Å
The X-ray molecular structure of phimid-H·Br is shown 136.5°, with (i) = –x + 1, –y + 1, –z + 1 and (ii) = –x, –y,
in Figure 1, and selected geometrical parameters are given –z + 1]. Not unexpectedly, the closest H···Br contact is
in Table 1. The crystallographically independent unit con- shown by H12, the most acidic H atom of the molecule.
tains two ligand molecules, two bromide counterions and a
water molecule. In the heteroaromatic ring, bond lengths
C12A–N2A and C12A–N3A (C12B–N2B and C12B–N3B)
are equivalent to each other, consistently with the resonance
structures of the imidazolium ion. The H atom of the ortho
hydroxy group points toward the lone pair of the imino N
atom as the result of strong intramolecular hydrogen bond-
ing[8] [O1A···N1A 2.611(4) Å 142.9°; O1B···N1B 2.577(5) Å
152.2°]. Each independent molecule assumes a non-elon-
gated, bent conformation, mainly as the result of a gauche-
type torsion angle around the C8A–C9A and C8B–C9B
bond.
Figure 1. X-ray structure of one crystallographically independent
imidazolium cation of [3,5-tBu2-2-(OH)C6H2CH=NCH2CH2-
(CH{NCHCHNiPr})]Br (phimid-H·Br). Displacement ellipsoids
are drawn at 30% probability level.
Figure 2. Crystal packing of [3,5-tBu2-2-(OH)C6H2CH=NCH2-
CH2(CH{NCHCHNiPr})]Br (phimid-H·Br) viewed down a. H
atoms are shown for water molecules only.
Table 1. Selected lengths [Å], bond angles [°] and torsion angles [°]
for phimid-H·HBr.
Another weak hydrogen bonding acceptor is the phenyl
ring (π acceptor[8]). The packing shows a C–H···Cg (Cg is
the centroid of the phenyl ring) interaction falling in the
range of weak hydrogen bonding[8] [C14(i)–H14f(i)···Cg
2.979 Å 173.0° with (i) = –x + 1, –y + 1, –z + 1].
C7A–N1A
1.283(5)
1.313(5)
1.323(5)
116.5(3)
C7B–N1B
1.282(5)
1.318(5)
1.322(6)
119.1(4)
C12A–N3A
C12A–N2A
C7A–N1A–C8A
C12B–N3B
C12B–N2B
C7B–N1B–C8B
N1A–C8A–C9A–N2A 53.7(5)
N1B–C8B–C9B–N2B 54.3(5)
Synthesis and Characterisation of the Complexes 1–3
Treatment of toluene solutions of anhydrous phim-H or
In the crystal packing (Figure 2), molecules form layers
in the ab plane, piled up along c. Each layer is better de-
scribed as a double layer with polar imidazolium and bro- phimid-H·Br with 1 equiv. of AlR3 readily afforded the di-
mide ions on the outer regions and phenyl and tert-butyl alkylaluminium derivatives (phim)AlMe2 (1) and (phimid)-
groups in the inner ones. Water molecules are sandwiched AlR2+Br– [R = Me (2), R = iBu (3)], with elimination of
between the polar surfaces of adjacent (double) layers along the corresponding alkane (Scheme 1). The reactions were
c. They are possibly involved in weak bonding[8] with bro- fast on the NMR scale and quantitative. 1H NMR monitor-
mide anions [O2···Br1(i) 3.354(4) Å O2–H2A···Br1(i) 163.6°; ing of the reaction of phimid-H·Br in the NMR tube
O2···Br2(ii) 3.272(4) Å O2–H2B···Br2(ii) 150.0°, with (i) = x showed that deprotonation occurred only at the phenolic
+ 1, y + 1, z and (ii) = –x + 1, –y + 1, –z + 1]. Bromide group and did not involve the imidazolium NC(H)N moi-
anions (and O atoms of water molecules) also act as hydro- ety. As a matter of fact, the signal of the NC(H)N proton
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S. Milione, F. Grisi, R. Centore, A. Tuzi
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was still present, giving rise to resonances at δ = 11.12 and
11.05 ppm for 2 and 3, respectively: only slightly shifted rel-
ative to that observed for phimid-H·Br. Heating of a [D8]-
toluene solution of 2 at 373 K for a few hours resulted in
decomposition. Complex 1–3 are very soluble in haloge-
nated and aromatic solvents; crystals of 1 suitable for X-ray
analysis were grown from hexane. In the case of complex 2,
poor crystal quality and weak diffraction prevented struc-
tural analysis.
Figure 3. X-ray structure of 1.
Table 2. Selected lengths (Å), bond angles [°] and torsion angles [°]
for compound 1.
Al–O1
Al–C2
Al–C1
Al–N1
O1–C3
N1–C9
C2–Al–C1
O1–Al–N1
C2–Al–N1
1.769(1)
1.949(2)
1.955(2)
1.963(2)
1.326(2)
1.291(2)
117.9(1)
95.15(6)
110.93(8)
C2–Al–N1
C1–Al–N1
C3–O1–Al
C9–N1–C10
C9–N1–Al1
110.93(8)
109.79(8)
131.8(1)
116.2(2)
120.6(1)
C10–N1–Al1 123.2(1)
N1–C9–C8
Al–O1–C3
C2–Al–N1
127.9(2)
171.9(1)
110.93(8)
the phenyl ring (π acceptor[8]). In this case, accessibility to
intermolecular contacts is possible, but the strength as ac-
ceptor is lower than with O1. Nonetheless, the packing
shows a C–H···Cg (Cg is the centroid of the phenyl ring)
interaction falling in the range of weak H bonding[8] con-
sistently with the d(H···Cg)Ͻ3.0 Å cut-off criterion sug-
gested by Braga et al.[10] [H12b(i)···Cg 2.92 Å 148.3° with (i)
= 1.5 – x, 0.5 – y, z – 0.5].
It is worth noting that the imino H atom, the most acidic
one in 1, is not significantly involved in H bonding in the
crystal packing. This, again, is presumably a consequence
of the steric hindrance for intermolecular contacts around
the H bonding acceptor O1.
Scheme 1.
The X-ray structure of 1 is shown in Figure 3; selected
geometric data are given in Table 2. The complex is mono-
meric, with the salicylaldiminato moiety acting as a chelat-
ing ligand through its N and O atoms. Two methyl groups
complete a distorted tetrahedral coordination geometry
around the Al atom. The Al–O1 [1.769(1) Å] bond length
is significantly shorter than Al–N1 [1.963(2) Å]. This effect,
as well as the enlargement of the Al–O1–C3 valence angle
[131.8(1)°], has been observed in other salicylaldiminato
AlIII complexes.[9] Of the bond angles around Al, the small-
est is N1–Al–O1 [95.15(6)°], which is associated with the
bite angle of the chelating ligand. The six-membered chelate
ring is not planar, mainly because Al is a little out of the
plane of the other five atoms [0.219(2) Å].
The strongest hydrogen bonding acceptor in 1 is the
phenoxy oxygen atom (O1). However, the presence of the
bulky tert-butyl group in the ortho position on the phenyl
moiety, together with that of the two methyl groups bound
to Al, make O1 largely inaccessible to close intermolecular
contacts, which are indeed not found in the packing.[3] On
the other hand, weak intramolecular hydrogen bonding in-
teractions are found with O1 as acceptor and the H atoms
of the adjacent tert-butyl group as donors (H18a···O1
2.386 Å 122.5°; H19c···O1 2.312 Å 124.8°). These interac-
The molecular structure of 2 in solution was determined
by long-range COSY, HSQC and NOESY experiments. The
NMR spectroscopic data for 1–3 were consistent with a tet-
rahedral Cs symmetric structure resulting from the κ2-N,O
coordination of the ligands to the metal centre through the
1
phenoxy group and the imino nitrogen. In the H NMR
spectra of 1 and 2 the methyl groups bound at the alumin-
ium centres appear as sharp singlets at –0.22 and
1
–0.35 ppm, respectively, whereas in the H NMR spectrum
of 3 the isobutyl groups appear as an AAЈMX3X3Ј system
with two multiplets centred at 2.19 (3JM,X = 6.7 Hz) and
3
0.30 ppm (1JA,AЈ = 14.1 Hz; JA,M = 7.1 Hz) and two dou-
blets centred at δ = 1.14 and 0.86 ppm.
Hydrogen Bonding Interactions in Solution
It is well documented that the three ring protons in 1,3-
tions agree with result found by Lewinski for weak H- dialkylimidazolium cations engage in hydrogen bonding
bonding in sterically overcrowded salicylaldiminato Al with halide anions.[11] It is reasonable to assume that the
complexes.[3] The other hydrogen bonding acceptor in 1 is pendant imidazolium moieties in the coordinated phimid li-
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Novel Alkylaluminium Complexes
gands of 2 and 3 are involved in hydrogen bonding with the
bromide anion, as already found in the solid structure of
1
phimid-H·Br. As a matter of fact, the H NMR resonances
of the imine HC=N and the imidazolium protons are
shifted downfield if compared with analogous literature
data; the imine HC=N protons, for example, resonate at δ
= 7.38 and 9.82 ppm in the cases of 1 and 2, respectively. In
order to probe the presence of these interactions we added a
Lewis acidic compound to a solution of 2. As expected, the
addition of AlMe3 to a [D6]benzene solution of 2 caused
significant upfield shifts of the resonances of the coordi-
nated ligand, with the largest shifts being those seen for
the imine HC=N and the imidazolium NC(H)N resonances
(Figure 4). This behaviour can be explained by considering
that the addition of AlMe3 to the solution of 2 results in
–
the replacement of Br– with BrAlMe3 . This anion shows a
lower affinity for bonding with the “acidic” hydrogen of the
phimid ligand and generates a dissociated ion pair as shown
in the following equilibrium (1):
Figure 5. Plot of chemical shifts of H1 (᭿), H2 (᭺), H3 (ᮀ) and H4
(᭡) (Scheme 2) for (phimid)AlMe2Br (2) versus [AlMe3]0/[(phimid)-
AlMe2Br]0 (C6D6, 298 K).
–
Br–[(phimid)AlMe2]+ + AlMe3 i [(phimid)AlMe2]+ + BrAlMe3
(1)
Scheme 2.
The constant for equilibrium (1) (Keq) was determined by
1
standard H NMR titrations in which the concentration of
2 was kept constant while the concentration of AlMe3 was
varied. A plot of the measured change in chemical shift of
the NC(H)N proton with reference to that of 2 (∆δ) versus
[AlMe3]0/[2]0 from a series of solutions containing constant
[2]0 is given in Figure 6. The titration data were analysed by
nonlinear regression analysis with use of the WinEQNMR
Figure 4. Expanded region of the 1H NMR spectra of (phimid)-
AlMe2Br at different AlMe3/(phimid)AlMe2Br molar ratios. Proton
resonances are labeled as in Scheme 2. The asterisk indicates the
satellite peak of the deuterated solvent (C6D6, 298 K).
program[13] and afforded
a
value for Keq of
3.2Ϯ0.1ϫ103 –1. On repetition of the titration of 2 with
the weaker Lewis acid AliBu3, a lower value for the associa-
tion constant was obtained (Keq = 1.2Ϯ0.1ϫ103 –1).
It is worth noting that similar behaviour had already
The free and hydrogen-bonded ion pairs exchange been observed for alkylhalogenoaluminate(III) ionic li-
rapidly: as the AlMe3/(phimid)AlMe2Br ratio increases, the quids. In mixtures of 1,3-dialkylimidazolium chloride with
concentration of the hydrogen-bonded complex decreases, aluminium chloride the chemical shifts of the protons on
so the population of the more shielded proton increases and the cations are highly dependent on the proportions of alu-
the signal moves upfield. Through inspection of a plot of minium chloride and organic chloride salt; in particular, the
the chemical shifts of the resonances for 2 versus AlMe3/2 NC(H)N proton is very sensitive, moving significantly up-
molar ratio (Figure 5) it is reliable to assume that the hydro- field with increasing aluminium halide mole fraction.[14]
gen-bond donor sites are the NC(H)N proton of the imid-
To probe the possibility of formation of bonding between
azolium moiety, the hydrogen atom of the imine group and hydrogen-bond donor sites in 1 and potential electron pair
H3 of the aryl group (Scheme 2). The shifts of the other donors in solution, nBu4NBr was added to a [D6]benzene
resonances are best attributed to changes in electronic dis- solution of 1 (halide anions act as strong hydrogen bond
tribution in the ligand.[12]
acceptors).[8]
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S. Milione, F. Grisi, R. Centore, A. Tuzi
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tored at room temperature by 1H NMR spectroscopy,
35 mol-% conversion of the reagents was observed after
10 min, with quantitative conversion being reached in a few
hours. The 19F NMR spectrum revealed inter alia the pres-
–
ence of MeB(C6F5)3 , suggesting that the species 4 is
formed through the transfer of a C6F5 group from
–
MeB(C6F5)3 by the initially formed coordinatively unsatu-
rated cationic intermediate.
Treatment of 2 with one equiv. of B(C6F5)3 afforded the
cationic monomethyl species (phimid)AlMeBr+MeB-
–
1
(C6F5)3 (5). H NMR monitoring of the reaction in the
NMR tube showed this to be fast and quantitative in a few
minutes. The 19F NMR spectrum revealed only the presence
–
of the MeB(C6F5)3 anion and was unchanged over 24 h at
room temperature. Complex 5 is very soluble in haloge-
nated and aromatic solvents, but on standing it slowly de-
composes to unknown species, hampering its crystallisa-
tion.
Figure 6. Plot of the measured change in chemical shift of the
NC(H)N proton of (phimid)AlMe2Br (2) with reference to that of
the uncomplexed 2 (∆δ) versus [AlMe3]0/[(phimid)AlMe2Br]0 from a
series of solutions containing constant [2]0. ([2]0 = 2.5 m, [AlMe3]0
= 0–70 m, C6D6, 298 K).
The molecular structure of 5 in solution was determined
1
by NMR spectroscopy. In the H NMR spectrum of 5 the
methyl group bound at the aluminium centre appeared as a
sharp singlet at –0.14 ppm and, except for protons 5 and 6
(see Scheme 2), the resonances for the phimid fragment were
shifted to higher field with respect to the neutral complex,
with the largest shifts being those for the imine HC=N (∆δ
= 3.64 ppm) and the imidazolium NC(H)N resonances
(∆δ=2.69 ppm). The shifts of these protons were similar to
those observed for 2 in the presence of AlMe3 in large ex-
This only caused a small downfield shift of the imine
HC=N resonance,[15] indicating the involvement of this
group in labile hydrogen bonding.
Reactivity Studies: Reactions between Al Complexes 1 and
2 and B(C6F5)3
There has been growing interest in the synthesis of cat- cess, indicating that the bromide anion is no longer involved
ionic aluminium complexes, because the enhanced Lewis in the hydrogen bonding with the imidazolium moiety. The
acidity of the aluminium centre should result in higher inertness of 5 toward the transfer of a C6F5 group from
catalytic activity and may lead to new applications.[16] MeB(C6F5)3– suggests that the electrophilic aluminium cen-
B(C6F5)3 has been used successfully to generate reactive tre is stabilised by coordination of the bromide anion, act-
alkylaluminium cations LnAlR+ from LnAlR2 precursors. ing as a Lewis base.
Treatment of dialkylaluminium complexes bearing biden-
The methyl group of MeB(C6F5)3– in the 1H NMR spec-
tate N,N- or N,O-ligands with B(C6F5)3 leads to unstable trum of 5 appeared at δ = 1.16 ppm, indicating that the
tricoordinate alkylaluminium cations, which react quite anion is not substantially coordinated to the aluminium
rapidly by abstracting a C6F5 group from the MeB- centre. This was further confirmed by the small chemical
–
(C6F5)3 anion to form neutral LnAlR(C6F5) products.[17] shift difference (∆δ = 2.8 ppm) between the m- and p-fluor-
Several studies have shown that the presence of a Lewis ine 19F NMR resonances.[20]
base (L1) can stabilise the aluminium cation through the
formation of {LX}Al(R)(L1)+ adducts.[18] Gibson et al.
Conclusions
showed that a pendant donor group that in the neutral alu-
minium complexes is weakly bonding or nonbonding be-
New alkylaluminium complexes 2 and 3 bearing a modi-
comes a normal donor group in the cationic derivatives fied phenoxyimine ligand with a pendant imidazolium moi-
upon treatment with B(C6F5)3, thus stabilising these spe- ety (phimid-H·Br) are reported. The coordination behaviour
cies.[19]
of these compounds was investigated and compared with
The addition of 1 equiv. of B(C6F5)3 to a [D6]benzene that of an analogous novel alkylaluminium complex (1) in-
solution of 1 resulted in the selective and quantitative for- corporating a salycilaldimine ligand lacking the pendant
1
mation of (phim)Al(C6F5)Me (4). The H NMR spectrum imidazolium portion (phim-H). The structural features of
exhibited a triplet at δ = 0.05 ppm (JHF = 1.4 Hz) character- both these types of complexes were considered with regard
istic of an Al-Me resonance coupled to the α-fluorines of to the hydrogen bond interactions.
a coordinated C6F5 group.[17] The resonances of the phim
The ability of 1 to engage in intermolecular hydrogen
fragment were generally shifted downfield with respect to bonding was detected by 1H NMR solution experiments
the relative resonances of the precursor 1. Two quintets in conducted in the presence of a source of halide anions such
1
the same H NMR spectrum at 1.33 and 0.96 ppm indi- as nBu4NBr: in this case, 1 is able to enter into a labile
cated the presence of MeB(C6F5)2 and Me2B(C6F5) in a 4:1 hydrogen bond with bromide anion through the imine pro-
molar ratio. When treatment of 1 with B(C6F5)3 was moni- ton.
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Novel Alkylaluminium Complexes
1H NMR (400 MHz, C6D6, 25 °C): δ = 7.68 (d, 1 H, Ph–H), 7.38
(s, 1 H, CH=N), 6.79 (d, 1 H, Ph–H), 2.99 [m, 1 H, CH(CH3)2],
1.60 [s, 9 H, C(CH3)3], 1.33 [s, 9 H, C(CH3)3], 0.95 [d, 6 H,
CH(CH3)2], –0.22 [s, 6 H, Al(CH3)2] ppm. 13C{1H} NMR spectro-
scopic data (100 MHz, C6D6, 25 °C): –7.2, 23.5, 29.9, 31.9, 34.5,
35.9, 60.2, 119.3, 129.3, 131.9, 139.1, 141.2, 162.4, 170.3 ppm.
C20H34AlNO (331.53): calcd. C 72.46, H 10.36, N 4.23; found C
72.95, H 10.94, N 4.83.
More significantly, treatment of 2 and 3 with a Lewis
acid such as AlMe3 in solution, monitored by H NMR
1
spectroscopy, gave evidence of strong involvement of the
NC(H)N proton of the imidazolium moiety, the hydrogen
atom of the imine group and H3 of the aryl group in hydro-
gen bonding interactions with bromide anion. The presence
of an imidazolium tail thus strengthens the hydrogen donor
sites on the skeleton of the salicylaldimine ligand. The
manifestation of these hydrogen interactions implies that
the imidazolium pendant moiety of the coordinated ligand
is forced to assume a bent conformation to allow the hydro-
gen bond donor sites to surround the bromide anion.
Reactivity studies of Al complexes 1 and 2 with
B(C6F5)3, with the goal of generating monoalkyl cationic
species as potential active catalysts from the corresponding
neutral dialkyl precursors, revealed different behaviour. Be-
cause of the instability of the initially formed tricoordinate
alkylaluminium cation, complex 1 afforded the neutral
(phim)Al(C6F5)Me product through the abstraction of a
Synthesis of (phimid)AlMe2Br (2):
A solution of AlMe3
(0.71 mmol) in toluene (5.0 mL, 0.142 ) was added at room tem-
perature to a suspension of phimid-H·Br (0.320 g, 0.71 mmol) in
toluene (40 mL). Evolution of methane was observed. The resulting
pale yellow solution was stirred for one hour. The solution was
concentrated and cooled to –30 °C, affording a yellow solid
(0.190 g, 59%). Spectroscopic data for (phimid)AlMe2Br: 1H NMR
(400 MHz, C6D6, 25 °C): δ = 11.12 (s, 1 H, NCHN), 9.82 (s, 1 H,
CH=N), 8.04 (d, 1 H, Ph–H), 7.66 (d, 1 H, Ph–H), 6.26 (s, 1 H,
NCH), 5.57 (s, 1 H, NCH), 4.66 (m, 2 H, NCH2), 4.38 [m, 1 H,
CH(CH3)2], 4.32 (m, 2 H, NCH2), 1.58 [s, 9 H, C(CH3)3], 1.37 [s,
9 H, C(CH3)3], 1.08 [d, 6 H, CH(CH3)2], –0.35 [s, 6 H, Al-
(CH3)2] ppm. Selected 13C{1H} NMR spectroscopic data
(100 MHz, C6D6, 25 °C): δ = –8.4, 22.6, 29.9, 31.8, 49.5, 53.7, 55.8,
119.4, 123.0, 130.7, 133.7, 135.2, 176.2 ppm. C25H41AlBrN3O
(506.58): calcd. C 59.27, H 8.17, N 8.30; found C 59.83, H 8.74, N
8.12.
–
C6F5 group from the MeB(C6F5)3 counterion.
In contrast, the presence of the imidazolium bromide
moiety as a pendant arm in 2 allowed the formation of a
cationic monomethyl species (phimid)AlMeBr+MeB-
–
(C6F5)3 in which the bromine was no longer involved in
Synthesis of (phimid)AlMe2Br (2; NMR Tube Reaction): In a glove-
box, AlMe3 (1.3 µL, 0.013 mmol) dissolved in C6D6 (0.3 mL) was
added to a suspension of phim-H·Br (6 mg, 0.013 mmol) in C6D6
(0.3 mL) at room temperature. The resulting pale yellow solution
was transferred to a NMR tube (10 mm o.d.) and analysed.
the hydrogen bonding with the imidazolium moiety.
The influence of the noncovalent interactions observed
in the second coordination spheres of 2 and 3 on their cata-
lytic abilities in ring-opening polymerisation of cyclic esters
is currently under investigation.
1
1H NMR Titration: The H NMR titration was performed by ad-
dition of increasing amounts of AlMe3 to a [D6]benzene solution
of 2 (2.50 m, 0.6 mL). During titration the concentration of
AlMe3 was varied over the 0.25–56.0 m range. The appearance of
Experimental Section
–
a singlet at δ = 0.01 ppm was attributed to the BrAlMe3 anion.
The chemical shift of the imidazolium proton at δ = 11.12 ppm was
followed and plotted against the concentration of AlMe3 added.
The titration data were analysed by nonlinear regression analysis
with use of the WinEQNMR program.[13]
General: All experiments were performed under nitrogen by stan-
dard Schlenk-type techniques or in a glove-box (type MBRAUN).
Toluene and hexane were distilled from sodium/benzophenone.
C6D6 and C7D8 were degassed under N2 flow and stored over acti-
vated molecular sieves (4 Å) in a glove-box prior to use. NMR spec-
tra were recorded on Bruker AM 300 and Bruker AVANCE 400
instruments operating at 300 MHz and 400 MHz for 1H, respec-
tively. The 1H and 13C chemical shifts are referenced to SiMe4 with
use of the residual protio impurities of the deuterated solvents as
external references. 11B and 19F chemical shifts are reported versus
BF3(OEt2) and CFCl3, respectively. Elemental analyses were per-
formed with a Perkin–Elmer 240-C instrument. AlMe3 and Al-
Synthesis of (phimid)AliBu2Br (3):
A solution of AliBu3
(1.00 mmol) in toluene (5.0 mL, 0.20 ) was added at room tem-
perature to a suspension of phimid-H·Br (0.450 g, 1.00 mmol) in
toluene (40 mL). Evolution of isobutene was observed. The re-
sulting pale yellow solution was stirred for one hour. The solution
was concentrated and cooled to –30 °C, affording a yellow solid
1
(0.28 g, 47%). Spectroscopic data for (phimid)AliBu2Br: H NMR
(400 MHz, C6D6, 25 °C): δ = 11.05 (s, 1 H, NCHN), 9.81 (s, 1 H,
CH=N), 8.04 (d, 1 H, Ph–H), 7.68 (s, 1 H, Ph–H), 6.42 (s, 1 H,
NCH), 5.68 (s, 1 H, NCH), 4.81 (m, 2 H, NCH2), 4.41 [m, 1 H,
CH(CH3)2], 4.36 (m, 2 H, NCH2), 2.19 [m, 2 H, AlCH2CH-
(CH3)2], 1.61 [s, 9 H, C(CH3)3], 1.35 [s, 9 H, C(CH3)3], 1.26 [d,
6 H, CH(CH3)2], 1.14 [d, 3 H, AlCH2CH(CH3)2], 0.86 [m, 3 H,
1
(iBu)3 (Aldrich) were checked for purity by H NMR and used as
received. B(C6F5)3 was purchased from Boulder Scientific and used
as received. 3,5-tBu2-2-(OH)C6H2CH=NiPr (phim-H) and
[3,5-tBu2-2-(OH)C6H2CH=NCH2CH2(CH{NCHCHNiPr})]Br
(phimid-H·Br) were synthesised by modified literature pro-
cedures.[6,7] All other chemicals were obtained commercially and
used as received unless stated otherwise.
AlCH2CH(CH3)2], 0.30 [m,
4
H, AlCH2CH(CH3)2] ppm.
C31H53AlBrN3O (590.76): calcd. C 63.02, H 9.06, N 7.11; found C
63.54, H 9.72, N 7.47.
Synthesis of (phim)AlMe2 (1): A solution of AlMe3 (1.09 mmol) in
hexane (5.0 mL, 0.218 ) was added at room temperature to a solu-
tion of (phim-H) (0.300 g 1.09 mmol) in hexane (20 mL, 0.0545 ).
Evolution of methane was observed. The resulting yellow solution
was stirred for one hour. The solution was filtered and cooled to
–30 °C, affording a crystalline, yellow solid (0.22 g, 61%). Single
crystals suitable for X-ray analysis were grown from a saturated
hexane solution at –30 °C. Spectroscopic data for (phim)AlMe2 (1):
Generation of (phim)Al(C6F5)Me (4): In a glove-box, equimolar
amounts of (phim)AlMe2 (1, 7 mg, 0.021 mmol) and B(C6F5)3
(11 mg, 0.021 mmol) were placed in a sample vial and dissolved in
C6D6 (0.7 mL). The resulting pale yellow solution was transferred
to a NMR tube (10 mm o.d.) and analysed at 25 °C. Spectroscopic
1
data for (phim)Al(C6F5)Me: H NMR (400 MHz, C6D6, 25 °C): δ
= 7.72 (d, 1 H, Ph–H), 7.47 (s, 1 H, CH=N), 6.85 (d, 1 H, Ph–H),
Eur. J. Inorg. Chem. 2008, 5532–5539
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5537

S. Milione, F. Grisi, R. Centore, A. Tuzi
FULL PAPER
3.02 [m, 1 H, CH(CH3)2], 1.53 [s, 9 H, C(CH3)3], 1.29 [s, 9 H,
by difference Fourier map and riding on carrier atoms. Max. resid-
C(CH3)3], 0.82 [dd, 6 H, CH(CH3)2], 0.05 [s, 3 H, Al(CH3)] ppm. ual electronic density was 0.306 (–0.348) eÅ–3 for complex 1 and
19F NMR (376 MHz, C6D6, 25 °C): δ = –122.3 (q, 2 F, o-C6F5), 0.340 (–0.659) eÅ–3 for (phimid-H·Br). Some crystal and collection
–154.3 (t, 1 F, p-C6F5), –161.6 (m, 2 F, o-C6F5) ppm. Spectroscopic
data for [MeB(C6F5)2]: 1H NMR (400 MHz, C6D6, 25 °C): δ = 1.33
(t, 3 H, B–CH3) ppm. 19F NMR (376 MHz, C6D6, 25 °C): δ =
–130.2 (m, 4 F, o-C6F5), –147.3 (m, 2 F, p-C6F5), –162.2 (m, 4 F, m-
C6F5) ppm. 11B NMR (–25.19 MHz, C6D6, 25 °C): δ = 71.5 [s, 1B,
MeB(C6F5)2] ppm. Spectroscopic data for [Me2B(C6F5)]: δ = 0.96
(t, 3 H, B-CH3) ppm. 19F NMR (376 MHz, C6D6, 25 °C): δ =
–131.3 (d, 2 F, o-C6F5), –151.7 (m, 1 F, p-C6F5), –162.8 (m, 2 F, m-
C6F5) ppm. 11B NMR (–25.19 MHz, C6D6, 25 °C): δ = 80.4 [s, 1B,
Me2B(C6F5)] ppm.
data are reported in Table 3.
CCDC-689736 (for 1) and -689737 (for phimid-H·Br) contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supporting Information (see also the footnote on the first page of
this article): NMR spectra and 2D NMR experiments for 1–5.
[1] a) D. A. Atwood, M. J. Harvey, Chem. Rev. 2001, 101, 37–52;
b) S. Yamada, Coord. Chem. Rev. 1999, 190–192, 537–555; c)
C.-M. Che, J.-S. Huang, Coord. Chem. Rev. 2003, 242, 97–113.
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tore, A. Tuzi, Organometallics 2006, 25, 266–274; g) N. No-
mura, T. Aoyama, R. Ishii, T. Kondo, Macromolecules 2005,
38, 5363–5366.
Generation of (phimid)AlMeBr+MeB(C6F5)3 (5): In a glove-box,
–
equimolar amounts of (phimid)AlMe2Br (2, 11 mg, 0.021 mmol)
and B(C6F5)3 (11 mg, 0.021 mmol) were placed in a sample vial
and dissolved in C6D6 (0.7 mL). The resulting yellow solution was
transferred to a NMR tube (10 mm o.d.) and analysed at 25 °C.
Spectroscopic data for (phimid)AlMeBr+: 1H NMR (400 MHz,
C6D6, 25 °C): δ = 7.70 (d, 1 H, Ph–H), 7.48 (s, 1 H, NCHN), 7.13
(s, 1 H, CH=N), 6.79 (d, 1 H, Ph–H), 6.46 (s, 1 H, NCH), 6.06 (s,
1 H, NCH), 4.04 [m, 1 H, CH(CH3)2], 3.36 (m, 2 H, NCH2), 2.98
(m, 2 H, NCH2), 1.47 [s, 9 H, C(CH3)3], 1.21 [s, 9 H, C(CH3)3],
0.51 [dd, 6 H, CH(CH3)2], –0.14 [s, 3 H, Al(CH3)2] ppm. Spectro-
scopic data for [MeB(C6F5)3]–: 1H NMR (400 MHz, C6D6, 25 °C):
δ = 1.16 (BCH3) ppm. 19F NMR (376 MHz, C6D6, 25 °C): δ =
–132.5. (d, 2 F, o-C6F5), –163.9 (t, 2 F, m-C6F5), –166.8 (t, 1 F, p-
C6F5) ppm. 11B NMR (–25.19 MHz, C6D6, 25 °C): δ = –14.7 [s,
[3] J. Lewin´ski, J. Zachara, I. Justyniak, M. Dranka, Coord. Chem.
Rev. 2005, 249, 1185–1199.
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237; b) T. Welton, Chem. Rev. 1999, 99, 2071–2084.
[5] a) M. Niehues, G. Kehr, R. E. Fröhlich, G. Erker, Z. Natur-
forsch., Teil B 2003, 58, 1005–1008; b) S. K. Chattopadhyay,
T. C. W. Mak, B.-S. Luo, L. K. Thompson, A. Rana, S. Ghosh,
Polyhedron 1995, 14, 3661–3667.
[6] a) W. Li, H. Sun, M. Chen, Z. Wang, D. Hu, Q. Shen, Y.
Zhang, Organometallics 2005, 24, 5925–5928; b) J. Zhang, H.
Yao, Y. Zhang, H. Sun, Q. Shen, Organometallics 2008, 27,
2672–2675.
–
1B, MeB(C6F5)3 ] ppm.
Crystal Structure Determinations: Data collection was performed
at low temperature (–100 °C) for (phim)AlMe2 (1) and at 20 °C for
(phimid-H·Br) on a Bruker–Nonius kappa CCD diffractometer
(graphite monochromated Mo-Kα radiation, phi scans + omega
scans to fill the asymmetric unit). Cell parameters were obtained
from a least-squares fit of the θ angles of 188 reflections in the
range 3.460°ՅθՅ21.129° for complex 1 and of 98 reflections in
the range 3.919°ՅθՅ18.044° for (phimid-H·Br). A semiempirical
absorption correction (multiscan, SADABS[21]) was applied in both
cases. Both structures were solved by direct methods and anisotrop-
ically refined by the full-matrix, least-squares method on F2 against
all independent measured reflections (SIR97[22] and SHELX-97[23]
programs). H atoms were placed in calculated positions or located
[7] V. T. Kasumov, F. Köksal, A. Sezer, Polyhedron 2005, 24, 1203–
1211.
[8] G. Desiraju, T. Steiner in The Weak Hydrogen Bond in Struc-
tural Chemistry and Biology, Oxford University Press, 1999.
[9] a) P. A. Cameron, V. C. Gibson, C. Redshaw, J. A. Segal, G. A.
Solan, A. J. P. White, D. J. Williams, J. Chem. Soc., Dalton
Trans. 2001, 1472–1476; b) J. Lewin´ski, J. Zachara, K. B. Sta-
rowieyski, Z. Ochal, I. Justyniak, T. Kopec´, P. Stolarzewicz, M.
Dranka, Organometallics 2003, 22, 3773–3780.
Table 3. Crystal, collection and refinement data.
[10] D. Braga, F. Grepioni, E. Tedesco, Organometallics 1998, 17,
2669–2672.
(phim)AlMe2
phimid-H·Br
[11] a) A. G. Avent, P. A. Chaloner, M. P. Day, K. R. Seddon, T.
Welton, J. Chem. Soc., Dalton Trans. 1994, 3405–3413; b) A.
Elaiwi, P. B. Hitchcock, K. R. Seddon, N. Srinivasan, Y. Tan,
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3472.
Chemical formula
Formula weight
T [K]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
C20H34NOAl
331.46
173
orthorhombic
Pbcn
25.018(4)
14.638(4)
11.902(4)
90
(C23H36N3O) Br·1/2H2O
459.47
293
triclinic
¯
P1
[12] The addition of AlMe3 to a [D6]benzene solution of complex
1 at room temperature produces no change in the correspond-
10.347(1)
11.830(2)
21.585(4)
101.85(1)
91.36(2)
106.81(2)
2465.5(7)
4, 1.238
1.687
1
ing H NMR spectrum, excluding the possibility of formation
of aggregates between 1–3 and free AlMe3 through the oxygen
of the phenolate ligand (see Supporting Information).
[13] M. J. Hynes, J. Chem. Soc., Dalton Trans. 1993, 311–312.
[14] A. A. Fannin, L. A. King, J. A. Levisky, J. S. Wilkes, J. Phys.
Chem. 1984, 88, 2609–2614.
[15] For an [nBu4NBr]/[(phim)AlMe2] molar ratio equal to 1.6, the
observed change in chemical shift of the HC=N proton of
(phim)AlMe2 (1) is 0.03 ppm.
90
90
γ [°]
V [Å3]
4359(2)
8, 1.010
0.098
3.26°–27.50°
4966/218
0.0548
Z, dcalc [gcm–3]
µ [mm–1]
Theta range
Data/parameters
R1 [IϾ2σ(I)]
wR2 (all data)
2.27°–27.50°
10350/526
0.0591
[16] a) D. A. Atwood, Coord. Chem. Rev. 1998, 176, 407–430; b)
D. A. Atwood, B. C. Yearwood, J. Organomet. Chem. 2000,
600, 186–197.
0.1308
0.1479
5538
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 5532–5539

Novel Alkylaluminium Complexes
[17] B. Qian, D. L. Ward, M. R. Smith, Organometallics 1998, 17,
3070–3076.
[18] a) E. Ihara, V. G. Young, R. F. Jordan, J. Am. Chem. Soc. 1998,
[20] A. D. Horton, J. de With, A. J. Van der Linder, H. Van de Weg,
Organometallics 1996, 15, 2672–2674.
[21] SADABS, Bruker-Nonius, Delft, The Netherlands, 2002.
120, 8277–8278; b) C. E. Radzewich, I. A. Guzei, R. F. Jordan, [22] A. Altomare, M. C. Burla, M. Camalli, G. Cascarano, C. Gia-
J. Am. Chem. Soc. 1999, 121, 8673–8674; c) S. Dagorne, L.
Lavanant, R. Welter, C. Chassenieux, P. Haquette, G. Jaouen,
Organometallics 2003, 22, 3732–3741.
covazzo, A. Guagliardi, A. G. G. Moliterni, G. Polidori, R.
Spagna, J. Appl. Crystallogr. 1999, 32, 115–119.
[23] G. M. Sheldrick, SHELX-97, University of Göttingen, Ger-
many, 1997.
[19] P. Cameron, V. C. Gibson, C. Redshaw, J. A. Segal, M. D.
Bruce, A. J. P. White, D. J. Williams, J. Chem. Soc., Dalton
Trans. 2002, 415–422.
Received: July 25, 2008
Published Online: October 31, 2008
Eur. J. Inorg. Chem. 2008, 5532–5539
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5539