Aluminium alkyl and aryloxide complexes of pyrazine and bipyridines: Synthesis and structure

Article abstract of DOI:10.1039/b410662h

The reaction of AlMe₃ and [(^tBu)₂Al(μ- OPh)]₂ with pyrazine (pyz), 4,4′-bipyridine (4-4′-bipy), 1,2-bis(4-pyridyl)ethane (bpetha) and 1,2-bis(4-pyridyl)ethylene (bpethe) yields (Me₃Al)₂(μ-pyz) (1), (Me₃Al) ₂(μ-4,4′-bipy) (2), (Me₃Al)₂(μ- bpetha) (3), (Me₃Al)₂(μ-bipethe) (4), Al( ^tBu)₂(OPh)(pyz) (5), [(^tBu)₂Al(OPh)] ₂(μ-4,4-bipy) (6a), [(^tBu)₂Al(OPh)] ₂(μ-bpetha) (7a), [(^tBu)₂Al(OPh)] ₂(μ-bipethe) (8a). Compounds 1-4, 6a and 7a have been confirmed by X-ray crystallography. In solution compounds 1-4 undergo a rapid ligand-dissociation equilibrium resulting in a time-average spectrum in the ¹H NMR. In contrast, the solution equilibria for compounds 5-8a are sufficiently slow such that the mono-aluminium compounds may be observed by ¹H NMR spectroscopy: Al(^tBu)₂(OPh)(4,4-bipy) (6b), Al(^tBu)₂(OPh)(bpetha) (7b) and Al( ^tBu)₂(OPh)(bpethe) (8b). The inability to isolate [( ^tBu)₂Al(OPh)]₂(μ-pyz) and the relative stability of each complex is discussed with respect to the steric interactions across the bridging ligand (L) and the electronic effect on one Lewis acid-base interaction by the second Lewis acid-base interaction on the same ligand.

Full text of DOI:10.1039/b410662h

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F U L L P A P E R
Aluminium alkyl and aryloxide complexes of pyrazine and
bipyridines: synthesis and structure†
Doug Ogrin,a Laura H. van Poppel,a Simon G. Bottb and Andrew R. Barron*a
a
Department of Chemistry and Center for Nanoscale Science and Technology,
Rice University, MS-60, 6100 Main Street, Houston, Texas 77005, USA.
E-mail: arb@rice.edu; www.rice.edu/barron
Department of Chemistry, University of Houston, Houston, Texas 77204, USA
b
Received 13th July 2004, Accepted 22nd September 2004
First published as an Advance Article on the web 7th October 2004
The reaction of AlMe3 and [(tBu)2Al(l-OPh)]2 with pyrazine (pyz), 4,4-bipyridine (4-4-bipy), 1,2-bis(4-
pyridyl)ethane (bpetha) and 1,2-bis(4-pyridyl)ethylene (bpethe) yields (Me3Al)2(l-pyz) (1), (Me3Al)2(l-4,4-bipy)
(2), (Me3Al)2(l-bpetha) (3), (Me3Al)2(l-bipethe) (4), Al(tBu)2(OPh)(pyz) (5), [(tBu)2Al(OPh)]2(l-4,4-bipy) (6a),
[(tBu)2Al(OPh)]2(l-bpetha) (7a), [(tBu)2Al(OPh)]2(l-bipethe) (8a). Compounds 1–4, 6a and 7a have been confirmed
by X-ray crystallography. In solution compounds 1–4 undergo a rapid ligand-dissociation equilibrium resulting
in a time-average spectrum in the 1H NMR. In contrast, the solution equilibria for compounds 5–8a are sufficiently
slow such that the mono-aluminium compounds may be observed by 1H NMR spectroscopy: Al(tBu)2(OPh)(4,4-
bipy) (6b), Al(tBu)2(OPh)(bpetha) (7b) and Al(tBu)2(OPh)(bpethe) (8b). The inability to isolate [(tBu)2Al(OPh)]2(l-
pyz) and the relative stability of each complex is discussed with respect to the steric interactions across the bridging
ligand (L) and the electronic effect on one Lewis acid–base interaction by the second Lewis acid–base interaction on
the same ligand.
To this end, we have synthesized aluminium aryloxide and
Introduction
alkyl derivatives of pyrazine and bipyridines; the synthesis
and structural characterization of the chosen compounds is
reported herein. The choice of AlMe3 and (tBu)2Al(OPh) as the
Lewis acids was based on our prior experience in determining
the bond dissociation energies of Lewis acid–base complexes.6
Pyridine-like compounds, as the bidentate ligands (L), were
chosen because of the observation that pyridine complexes of
the Group 13 metals are amongst the strongest Lewis acid–base
interactions and thus should allow for the ready isolation of the
desired complexes.7
Over the last decade we have been interested in the effect of
Lewis acid complexation on a range of small molecules. Group
13 compounds are well known catalysts for the Friedel–Crafts
reaction, in which the highly Lewis acidic center activates an
alkyl or acyl halide, via complexation by placing a positive
charge on the b-substituent (Scheme 1(a)). The increase of
positive charge on the b-substituent is a general effect of the
coordination of aluminium Lewis acids to organic carbonyls
(Scheme 1(b))1 and the increased acidity of coordinated water
and alcohols (Scheme 1(c)).2 Group 13 aryloxides have also been
found to promote a change in regioselectivity of alkyl additions
of quinonel ethers, by placing a positive charge in the c-position
of the substituent (Scheme 1(d)).3 The presence of longer-range
effects has not been as well investigated. We are interested in
determining how far r-bond donor activation extends.
Results and discussion
The reaction of AlMe3 and [(tBu)2Al(l-OPh)]2 with pyrazine
(pyz), 4,4-bipyridine (4,4-bipy), 1,2-bis(4-pyridyl)ethane
(bpetha) and 1,2-bis(4-pyridyl)ethylene (bpethe) (see Scheme 28)
yields the expected di-aluminium compounds: (Me3Al)2(l-pyz)
(1), (Me3Al)2(l-4,4-bipy) (2), (Me3Al)2(l-bpetha) (3),
(Me3Al)2(l-bpethe) (4), [(tBu)2Al(OPh)]2(l-4,4-bipy) (6a),
[(tBu)2Al(OPh)]2(l-bpetha) (7a) and [(tBu)2Al(OPh)]2(l-
bpethe) (8a). The compounds vary in color from white to
orange-red depending on the ligand combination. In the
case of pyrazine and [(tBu)2Al(l-OPh)]2 the di-aluminium
compound is not observed (even under conditions of
excess [(tBu)2Al(l-OPh)]2); only the monometallic complex,
(tBu)2Al(OPh)(pyz) (5), being isolated. The AlMe3 complexes
are all moisture sensitive, while the phenoxide derivatives can
be handled in air for short periods of time. Although these
ligands have been used extensively as linear linkage groups
in crystal engineering, compounds 1–4, 6a and 7a are the first
Scheme 1
Other researchers have investigated effects of multiple Lewis
acid centers on the same Lewis base to enhance activity.4
We have been interested in the result of multiple Lewis acid
centers on small molecules. Part of this study has centered
on the indirect activation of molecules coordinated to a weak
Lewis acid (e.g., Hg) by the action of a strong Lewis acid (e.g.,
Al or Ga).5 As an extension of this work we have been interested
in the effect of binding two Lewis acid centers to different Lewis
base sites on a single molecule, i.e., X3M–L–MX3.
† Electronic supplementary information (ESI) available: Elemental
analysis and IR spectroscopy data; ab initio calculated structures
and structural parameters of AlMe3(py) and (tBu)2Al(OPh)(py). See
http://www.rsc.org/suppdata/dt/b4/b410662h/
Scheme 2
T h i s j o u r n a l i s
©
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
D a l t o n T r a n s . , 2 0 0 4 , 3 6 8 9 – 3 6 9 4
3 6 8 9

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structurally characterized aluminium bridging compounds
for these ligands. To our knowledge, the only other Group 13
derivative is the ferrocene substituted boron derivative.9
The molecular structures of (Me3Al)2(l-pyz) (1), (Me3Al)2(l-
4,4-bipy) (2), (Me3Al)2(l-bpetha) (3), (Me3Al)2(l-bpethe)
(4), [(tBu)2Al(OPh)]2(l-4,4-bipy) (6a) and [(tBu)2Al(OPh)]2(l-
bpethe) (7a) as confirmed by X-ray crystallography are shown
in Figs. 1–6; selected bond lengths and angles for compounds
1–4 are given in Table 1; those for compounds 6a and 7a along
with the ab initio calculated structural parameters (see Experi-
mental) for Al(tBu)2(OPh)(py) are given in Table 2. The Al–C,
Al–N and Al–O bond lengths are within the ranges expected.10
The bond lengths within the ligands are within the ranges
observed for the free ligands [pyrazine,11 4,4-bipy,12 1,2-bis(4-
pyridyl)ethane,13 and 1,2-bis(4-pyridyl)ethylene14]. The only
unusual structural parameter is the ethane bridge in compound
7a. This shows a crystallographic disorder of the ethane bridge;
similarly distorted bridges have been reported previously.15,16
Fig. 3 Molecular structure of (Me3Al)2(l-bpetha) (3). Thermal
ellipsoids shown at the 30% level and hydrogen atoms are omitted
for clarity.
Fig. 1 Molecular structure of (Me3Al)2(l-pyz) (1). Thermal ellipsoids
shown at the 30% level and hydrogen atoms are omitted for clarity.
Fig. 4 Molecular structure of (Me3Al)2(l-bpethe) (4). Thermal
ellipsoids shown at the 30% level and hydrogen atoms are omitted
for clarity.
Fig. 2 Molecular structure of (Me3Al)2(l-4,4-bipy) (2). Thermal
ellipsoids shown at the 30% level and hydrogen atoms are omitted for
clarity.
The AlMe3 moieties within the structures of compounds
1–4 are arranged in a staggered conformation with respect
to each other (Figs. 1–4), irrespective of the identity of the
bridging ligand. For compounds 2–4 this is the result of
crystallographically imposed symmetry, but compound 1’s
conformation is not symmetry controlled (see Fig. 1). In a
similar manner, compounds 6a and 7a adopt anti conforma-
tions (except for one of the independent molecules in the
asymmetric unit of 7a), without symmetry restriction
(Figs. 5 and 6). Thus, the conformation around aluminium is
independent of the identity of the substituents on aluminium.
Fig. 5 Molecular structures of one of the two crystallographically
independent molecules of [(tBu)2Al(OPh)]2(l-4,4-bipy) (6a). Thermal
ellipsoids are shown at the 15% level and hydrogen atoms are omitted
for clarity.
3 6 9 0
D a l t o n T r a n s . , 2 0 0 4 , 3 6 8 9 – 3 6 9 4

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Table 1 Selected bond lengths (Å) and angles (°) for compounds 1–4 as determined from X-ray crystallography in comparison with the calculated
structural parameters for AlMe3(py)
Compound
(Me3Al)2(l-pyz) (1)
(Me3Al)2(l-4,4-bipy) (2)
(Me3Al)2(l-bpetha) (3)
(Me3Al)2(l-bpethe) (4)
AlMe3(py)a
Al–N
Al–C
2.073(2)
1.960(3)–1.963(3)
2.032(2)
1.966(3)–1.973(3)
2.020(2)
1.971(3), 1.972(2)
2.033(2)
1.974(3)–1.982(3)
2.115
2.001–2.002
C–Al–N
C–Al–C
100.2(1)–102.0(1)
115.2(2)–118.3(2)
100.5(1)–104.0(1)
114.3(2)–116.4(2)
101.9(1), 103.94(7)
115.30(8), 114.1(2)
101.4(1)–103.7(1)
113.6(1)–116.5(2)
101.14–102.46
115.99–116.09
a Calculated structure, see Experimental section.
Table 2 Selected bond lengths (Å) and angles (°) for compounds 6a and 7a as determined from X-ray crystallography in comparison with the
calculated structural parameters for (tBu)2Al(OPh)(py)
[(tBu)2Al(OPh)]2(l-4,4-bipy) (6a)
Molecule 1a
Molecule 2b
[(tBu)2Al(OPh)]2(l-bpetha) (7a)
(tBu)2Al(OPh)(py)c
Al–O
Al–N
Al–C
O–C
1.719(5), 1.755(4)
2.004(5), 2.000(5)
1.962(7)–1.980(7)
1.327(8), 1.348(6)
1.732(4)
2.005(4)
1.997(7), 1.981(7)
1.355(6)
1.703(4), 1.735(3)
1.998(3), 2.000(3)
1.964(5)–1.982(6)
1.300(6), 1.334(5)
1.727
2.029
2.008, 2.009
1.348
O–Al–C
C–Al–C
C–Al–N
O–Al–N
C–O–Al
110.0(3)–114.3(3)
122.3(3), 123.1(3)
102.3(3)–107.7(3)
95.6(2), 94.8(2)
112.0(3), 112.2(3)
122.9(4)
103.5(2), 105.9(3)
96.1(2)
108.3(2)–113.8(2)
121.3(2), 121.4(3)
103.9(2)–108.5(2)
96.2(2), 102.6(2)
162.4(4), 146.5(3)
112.7
121.4
104.1, 106.3
95.9
159.1
147.9(5), 141.4(4)
138.9(4)
a Whole molecule in asymmetric unit. b Half of a centrosymmetric molecule in asymmetric unit. c Calculated structure, see Experimental section.
1–4 and the calculated structural parameters for AlMe3(py),
see Table 1. This suggests that the presence of the second Lewis
acid center (in the di-aluminium derivatives) does not alter
the structural aspects of the other Lewis acid–base interac-
1
tion. However, H and 13C NMR spectroscopic measurements
suggest that there is an effect of one Lewis acid on the second
within the same molecule.
1
The solution H and 13C NMR spectra of compounds 1–4
show a single set of resonances due to the ligand and AlMe3
groups. The chemical shifts are temperature-dependant con-
sistent with the presence of solution equilibria typical of such
Group 13 Lewis acid–base complexes. An indication of the
extent of dissociation may be obtained from the 13C NMR
signal of the Al–CH3 group. We have shown that the 13C NMR
shift of the aluminium methyl group in AlMe3L is controlled
by the steric bulk of the ligand (L) and as a consequence the
equilibrium constant (Keq in eqn. (1)).19
Fig. 6 Molecular structure of [(tBu)2Al(OPh)]2(l-bpetha) (7a).
Thermal ellipsoids are shown at the 30% level. Hydrogen atoms are
omitted and carbon atoms shown as shaded spheres for clarity.
Keq


AlMe L
AlMe + L
(1)
3
3
In a similar manner, as may be seen from Figs. 2 and 5, the
4,4-bipy rings in compounds 2 and 6a are coplanar (as are
the rings in compound 4, see Fig. 4), suggesting that the sub-
stituents on the aluminium appear to have little effect on the
geometry of the ligand. However, the bridging bpetha ligands
in compounds 3 and 7a adopt different geometries, possibly
as a result of the lack of conjugation between the ligand rings
allowing for more flexible packing of the terminal AlR3−x(OR)x
groups (see Figs. 3 and 6). In both compounds the two pyridine
rings adopt anti conformations about the central C2 unit, rather
than a gauche conformation.15 However, while the rings in
compound 3 are coplanar (although not with the C2H4 unit) in
a manner that has been observed previously,17 the bpetha ligand
in compound 7a adopts a twist geometry reminiscent of the
orientations reported for the bis(4-pyridyl)propane ligand.16,18
While we do not have directly analogous monomeric deri-
vatives for comparison, it is worth noting that the ab initio
calculated structure of Al(tBu)2(OPh)(py) faithfully reproduces
the structural features of compounds 6a and 7a, in particular
the Al–N distances, the terminal phenoxide Al–O distances and
Al–O–C angles (Table 2). A similar comparison may be made
between the observed X-ray structure data for compounds
For a series of compounds with identical steric bulk of L, the
13C NMR shift of the aluminium methyl group will be deter-
mined by the position of the equilibrium (eqn. (1)).20 Thus for
the series of compounds 1–4 the 13C NMR shift is going to
be identical for the complex. However, due to the equilibrium
(eqn. (1)) the actual observed 13C NMR shift will be a time
average between uncomplexed AlMe3 and AlMe3(L). The value
for Keq can be calculated if the shifts of the free and complexed
AlMe3 are known. Unfortunately, since the equilibrium is
sufficiently facile even at low temperature we can only compare
relative shifts as a measure of relative Keq values. However, from
the 13C NMR shifts of the methyl groups (see Experimental
section) it may be concluded that at the same temperatures
and concentrations, the order of the extent of ligand dissocia-
tion follows the trend 1 (pyz) > 2 (4.4-bipy) > 3 (bpetha)  4
(bpethe), i.e., K1 is larger for compound 1 than compound 4.
Thus the BDE for the second Al–N bond is less in compound
1 than in compound 4. We can conclude that the greater the r-
bond distance between the two nitrogen (or aluminium) atoms
the less the deactivation of one Al–N bond by the other Al–N
interaction. The similarity of compounds 3 and 4 confirms that
there is no effect of unsaturation but rather r-bond distance is
D a l t o n T r a n s . , 2 0 0 4 , 3 6 8 9 – 3 6 9 4
3 6 9 1

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the important parameter in determining the communication
between remote Lewis acid centers (see below).
Pyrazine, 4,4-bipyridyl, 1,2-bis(4-pyridyl)ethane and trans-
1,2-bis(4-pyridyl)ethylene were purchased from Sigma–Aldrich
and used as received. [(tBu)2Al(l-OPh)]2 was prepared as
previously reported.21 Solution NMR spectra were recorded on
a Bruker Avance 400 and 500 spectrometer using either CDCl3 or
C6D6 as internal locks. Proton and carbon spectra were collected
using a 5 mm broadband probe. Chemical shifts are relative to
TMS (1H and 13C). All spectra are reported at 298 K unless
otherwise stated. Melting points were determined in sealed
capillaries and are uncorrected. IR spectra were recorded on a
Thermo Nicolet FTIR spectrometer. IR samples were analyzed
using a diamond cell ATR. Elemental analysis was performed by
Prevalere Life Sciences, Inc., Whitesboro, NY.
In contrast to that observed for the AlMe3 complexes in
1
solution, the H NMR spectrum of compound 5 shows peaks
associated with free pyrazine and [(tBu)2Al(l-OPh)]2. In a
similar manner, solution NMR for compounds 6a–8a show
multiple sets of resonances associated with the 2:1 com-
plexes (i.e., 6a–8a), the 1:1 complexes [i.e., Al(tBu)2(OPh)(L)]
(6b–8b), the free ligand and [(tBu)2Al(l-OPh)]2. For example,
1
the solution H and 13C NMR spectra of compound 6a dis-
solved in CDCl3 shows four sets of resonances (Fig. 7), the
first can be assigned to the Lewis acid–base complex (6a),
while the other two are consistent with uncoordinated 4,4-
bipy and the dimeric species [(tBu)2Al(l-OPh)]2. The fourth
set of resonances is assigned to the formation of the new
complex, (tBu)2Al(OPh)(4,4-bipy) (6b). Variable-temperature
NMR confirms that equilibria are present, although the
presence of the multiple sets of resonances (as opposed to a
time average signal) indicates that the reaction is slow on the
(Me3Al)2(l-pyz) (1)
AlMe3 (0.50 mL, 5.2 mmol) was injected into a cooled (−78 °C)
solution of pyrazine (0.209 g, 2.61 mmol) in hexane (50 mL).
The solution was allowed to warm to room temperature, and was
stirred for an additional 1.5 h. All volatiles were then removed
in vacuo leaving a yellow powder. Yield: 0.56 g, 96%. Mp: 90 °C
(decomp.). 1H NMR (CDCl3): d 9.01 (4H, s, NCH), −0.72 (18H,
s, CH3). 13C{1H} NMR (CDCl3): d 144.38 (NC), −8.45 (CH3).
1
NMR time scale (10−5 s). Based upon the H NMR spectra
{and the lack of observation of [(tBu)2Al(OPh)]2(l-pyz)} the
relative order for the K1 (cf., Scheme 3) to be 1 (pyz)  2 (4.4-
bipy) < 3 (bpetha)  4 (bpethe), i.e., the extent of ligand dis-
sociation is greater for 1 than compound 4. Both of these trends
are consistent with a r induction effect of the first Lewis acid
on the second Lewis base (nitrogen) site.
(Me3Al)2(l-4,4-bipy) (2)
The compound was prepared by a method analogous to that
for compound 1 using AlMe3 (0.50 mL, 5.2 mmol) and 4,4-
bipyridyl (0.407 g, 2.61 mmol). All volatiles were removed in
vacuo leaving an off-white powder. X-Ray quality crystals were
obtained through slow evaporation of a CDCl3 solution. Yield:
1
0.77 g, 98%. Mp: 135 °C (decomp.). H NMR (CDCl3): d 8.84
[4H, dd, J(H–H) = 3.3 Hz, J(H–H) = 6.6 Hz, NCH], 7.84 [4H,
dd, J(H–H) = 3.3 Hz, J(H–H) = 6.6 Hz, CH], −0.75 (18H, s,
CH3). 13C{1H} NMR (CDCl3): d 148.6 (NC), 147.7 (C-ipso),
123.4 (NCC), −8.5 (CH3).
Fig. 7 1H NMR spectra of the 4,4-bipy resonances, showing the
presence of [(tBu)2Al(OPh)]2(l-4,4-bipy) (6a), (tBu)2Al(OPh)(4,4-bipy)
(6b) and uncoordinated 4,4-bipy (*).
(Me3Al)2(l-bpetha) (3)
The compound was prepared by a method analogous to that
for compound 1 using AlMe3 (0.50 mL, 5.2 mmol) and 1,2-
bis(4-pyridyl)ethane (0.480 g, 2.61 mmol). All volatiles were re-
moved in vacuo leaving a white powder. Yield: 0.83 g, 97%. Mp:
138 °C (decomp.). 1H NMR (CDCl3): d 8.56 [4H, dd, J(H–H) =
3.4 Hz, J(H–H) = 6.5 Hz, NCH], 7.42 [4H, dd, J(H–H) =
3.4 Hz, J(H–H) = 6.5 Hz, NCCH], 3.14 (4H, s, CH2) −0.80
(18H, s, CH3). 13C{1H} NMR (CDCl3): d 153.8 (C-ipso), 147.5
(NC), 125.2 (NCC), 35.2 (CH2), −8.6 (CH3).
(Me3Al)2(l-bpethe) (4)
The compound was prepared by a method analogous to
that for compound 1 using AlMe3 (0.50 mL, 5.2 mmol) and
1,2-bis(4-pyridyl)ethylene (0.475 g, 2.61 mmol). All volatiles
were removed in vacuum leaving a pale-yellow powder. Yield:
1
0.817 g, 96%. Mp: 155 °C (decomp.). H NMR (CDCl3): d 8.66
[4H, dd, J(H–H) = 2.8 Hz, J(H–H) = 5.3 Hz, NCH], 7.70 [4H,
dd, J(H–H) = 2.8 Hz, J(H–H) = 5.3 Hz, NCCH], 7.40 (2H, s,
CH), −0.77 (18H, s, CH3). 13C{1H} NMR (CDCl3): d 148.0
(NC), 146.5 (C-ipso), 131.9 (NCC), 122.9 (CH), −8.6 (CH3).
Scheme 3
The concentration dependence of the equilibria is confirmed
from the UV-visible spectra. For example, compound 6a is
bright orange, but when the sample is dilute enough to see
absorptions in the UV/visible, the spectra of shows two peaks
at 271 and 278 nm which are due to uncoordinated 4,4-bipy
in solution.
Al(tBu)2(OPh)(pyz) (5)
[(tBu)2Al(l-OPh)]2 (0.500 g, 1.07 mmol) and pyrazine (0.085 g,
1.07 mmol) were dissolved in hexane (70 mL). The solution
was stirred for 1.5 h yielding a bright yellow solution. All
volatiles were removed in vacuo yielding a yellow powder. Yield:
Experimental
1
0.604 g, 90%. Mp: 93 °C (decomp.). H NMR (C6D6): d 8.05
[2H, dd, J(H–H) = 1.5 Hz, J(H–H) = 4.5 Hz, NCH], 7.85 [2H,
dd, J(H–H) = 1.5 Hz, J(H–H) = 4.5 Hz, NCH], 7.30 (2H, m,
o-CH), 7.15 (2H, m, p-CH), 6.93 (1H, m, p-CH), 0.30 [18H, s,
C(CH3)3].
All reactions were performed under an inert atmosphere of
purified nitrogen using standard inert-atmosphere techniques.
Solvents were distilled and degassed prior to use. CDCl3 was
dried by storage over activated 4 Å Linde molecular sieves.
3 6 9 2
D a l t o n T r a n s . , 2 0 0 4 , 3 6 8 9 – 3 6 9 4

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[(tBu)2Al(OPh)]2(l-4,4-bipy) (6a)
A 100 mL round bottom flask was charged with [(tBu)2Al(l-
OPh)]2 (0.25 g, 0.53 mmol) and 4,4-bipy (0.16 g, 1.06 mmol).
After 5 minutes, the reactants changed color from white to
orange. CH2Cl2 (30 mL) was added to the reaction mixture,
giving a bright orange solution that was stirred for 18 h and
cooled to −30 °C. After 20 days, all solvent was removed
and the resulting orange oil was dissolved degassed toluene
(15 mL). This was cooled to −30 °C for two days giving bright
orange crystals suitable for single crystal X-ray analysis. Yield:
1
0.326 g, 98%. H NMR (CDCl3): d 9.07 [2H, dd, J(H–H) =
3.6 Hz, J(H–H) = 6.6 Hz, 2-CH], 8.02 [2H, dd, J(H–H) =
3.6 Hz, J(H–H) = 6.6 Hz, 3-CH], 7.21 (5H, m, CH), 0.95 [18H,
s, C(CH3)3]. 13C CPMAS: d 165.4 (OC), 152.1 (2-CH), 133.7 (3-
CH), 125.6 (4-CH) 125.4 (OCCH), 35.8 [C(CH3)3]. 27Al MAS
NMR: d 52.2, 27.0, 5.8 (W1/2 = 6760 Hz).
[(tBu)2Al(OPh)(4,4-bipy)] (6b)
1H NMR (CDCl3): d 8.95 [2H, dd, J(H–H) = 3.5 Hz, J(H–H) =
6.7 Hz, 2-CH], 8.88 [2H, dd, J(H–H) = 2.9 Hz, J(H–H) = 6.1 Hz,
7-CH], 7.94 [2H, dd, J(H–H) = 3.5 Hz, J(H–H) = 6.7 Hz, 3-
CH], 7.64 [2H, dd, J(H–H) = 2.9 Hz, J(H–H) = 6.1 Hz, 6-CH],
6.86 (2H, m, OCH), 6.85 (2H, m, m-CH), 6.77 (1H, m, p-CH),
0.96 [18H, s, C(CH3)3].
[(tBu)2Al(OPh)]2(l-bpetha) (7a)
[(tBu)2Al(l-OPh)]2 (0.500 g, 1.07 mmol) and 1,2-bis(4-pyridyl)-
ethane (0.200 g, 1.07 mmol) were dissolved in hexane (70 mL).
The solution was stirred for 1.5 h, during which time a pre-
cipitant was observed. The supernatant was decanted and the
precipitant dried in vacuo leaving a white solid. The solid was
dissolved in toluene and kept at −30 °C for 10 days giving
X-ray quality crystals. Yield: 0.68 g, 96%. Mp: 125 °C. 1H NMR
(CDCl3): d 8.75 [4H, dd, J(H–H) = 2.8 Hz, J(H–H) = 5.3 Hz,
N–CH], 7.50 [4H, dd, J(H–H) = 2.8 Hz, J(H–H) = 5.3 Hz,
NCCH], 7.18 (2H, m, o-CH), 6.82 (2H, m, m-CH), 6.76 (1H, m,
p-CH) 3.23 (4H, s, CH2) 0.90 [18H, s, C(CH3)3]. 13C{1H} NMR
(CDCl3): d 160.3 (OC), 154.6 (C-ipso), 147.7 (NC), 129.3 (o-
CH), 125.3 (NCCH), 119.7 (m-CH), 117.6 (p-CH), 35.2 (CH2),
30.4 [C(CH3)3], 14.9 [C(CH3)3].
(tBu)2Al(OPh)(bpetha) (7b)
1H NMR (CDCl3): d 8.68 [2 H, dd, J(H–H) = 3.5 Hz, J(H–
H) = 6.6 Hz, N–CH], 8.53 [2H, m, J(H–H) = 5.8 Hz, N–CH],
7.42 [2 H, m, J(H–H) = 6.6 Hz, NCCH], 7.18 (2H, m, o-CH),
6.81 (2H, m, m-CH), 6.77 (1H, m, p-CH), 3.12 (2 H, m, CH2),
3.03 (2 H, m, CH2), 0.89 [18H, s, C(CH3)3].
[(tBu)2Al(OPh)]2(l-bpethe) (8a)
The compound was prepared by a method analogous to that
for compound 7 using [(tBu)2Al(l-OPh)]2 (0.500 g, 1.07 mmol)
and 1,2-bis(4-pyridyl)ethylene (0.195 g, 1.07 mmol). The
supernatant was decanted and the precipitant dried in
vacuum leaving a red–orange solid. Yield: 0.683 g, 98%.
1
Mp: 135 °C (decomp.). H NMR (CDCl3): d 8.86 [4H, dd,
J(H–H) = 3.7 Hz, J(H–H) = 6.7 Hz, N–CH], 7.81 [4H, dd,
J(H–H) = 3.7 Hz, J(H–H) = 6.7 Hz, N–C–CH], 7.48 (2H, s,
CH), 7.20 (2H, m, o-CH), 6.83 (2H, m, m-CH), 6.77 (1H, m,
p-CH), 0.93 [18H, s, (CH3)3]. 13C{1H} NMR (CDCl3): d 160.3
(OC), 148.1 (NC), 147.2 (C-ipso), 132.4 (CH), 129.3 (o-CH),
123.1 (NCCH), 119.7 (m-CH), 117.7 (p-CH), 30.4 [C(CH3)3],
15.1 [C(CH3)3].
(tBu)2Al(OPh)(bpethe) (8b)
1H NMR (CDCl3): d 8.80 [2 H, dd, J(H–H) = 1.3 Hz, J(H–
H) = 6.6 Hz, N–CH], 8.73 [2H, dd, J(H–H) = 2.2 Hz, J(H–H) =
4.4 Hz, 7-CH], 7.77 [2 H, dd, J(H–H) = 3.2 Hz, J(H–H) =
D a l t o n T r a n s . , 2 0 0 4 , 3 6 8 9 – 3 6 9 4
3 6 9 3

View Article Online
5 A. S. Borovik, S. G. Bott and A. R. Barron, Angew. Chem. Int. Ed.,
2000, 39, 4117; A. Borovik, S. G. Bott and A. R. Barron, J. Am.
Chem. Soc., 2001, 123, 11219; A. Borovik and A. R. Barron, J.
Am. Chem. Soc., 2002, 124, 3743; C. S. Branch and A. R. Barron,
J. Am. Chem. Soc., 2002, 124, 14156.
6.6 Hz, NCCH], 7.49 (2H, s, CH), 7.20 (2H, m, o-CH), 6.84
(2H, m, m-CH), 6.77 (1H, m, p-CH), 0.94 [18H, s, C(CH3)3].
Crystallographic studies
6 M. B. Power, J. R. Nash, M. D. Healy and A. R. Barron, Organo-
metallics, 1992, 11, 1830; L. H. van Poppel, S. G. Bott and A. R.
Barron, J. Am. Chem. Soc., 2003, 125, 11006.
7 M. D. Healy, J. W. Ziller and A. R. Barron, J. Am. Chem. Soc.,
1990, 112, 2949.
8 L. Carlucci, G. Ciani, A. Gramaccioli, D. M. Proserpio and
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10 K. M. Nykerk and D. P. Eyman, Inorg. Chem., 1967, 6, 1461;
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Trans., 2002, 3327.
Single crystal diffraction data for compounds 1–4, 6a and
7a were collected at ambient temperature on a Bruker CCD
SMART system, equipped with graphite-monochromated
Mo-Ka radiation (k = 0.71073 Å) and corrected for Lorentz
and polarization effects. The structures were solved using the
direct methods program XS22 and difference Fourier maps
and refined by using full-matrix least squares methods. All
non-hydrogen atoms were refined with anisotropic thermal
parameters. Hydrogen atoms were introduced in calculated
positions and allowed to ride on the attached carbon atoms
[d(C–H) = 0.95 Å]. Refinement of positional and anisotropic
thermal parameters led to convergence (see Table 3).
CCDC reference numbers 244715–244720.
See http://www.rsc.org/suppdata/dt/b4/b410662h/ for crystal-
lographic data in CIF or other electronic format.
Computational methods
Density functional calculations on AlMe3(py) and Al(tBu)2-
(OPh)(py) were carried out using a Gaussian-98 suite.23 Com-
plete geometry optimizations were performed at B3LYP24
level using the 6–31G** basis set for C and H and Stuttgart
RLC ECP basis set for Al. Vibrational frequencies were then
evaluated to verify the existence of the true potential minimum
and to determine zero-point energies. Molecular mechanics
calculations were performed using Spartan (02 Windows)
running on a PC. The final structures are optimized using
molecular mechanics method MMFF94.
11 P. J. Wheatley, Acta Cryst., 1957, 10, 182; G. de With, S. Harkema
and D. Feil, Acta Crystallogr., Sect. B, 1976, 32, 3178.
12 N. M. Boag, K. M. Coward, A. C. Jones, M. E. Pemble and J. R.
Thompson, Acta Crystallogr., Sect. C, 1999, 55, 672.
13 S. Ide, N. Karacan and Y. Tufan, Acta Crystallogr., Sect. C, 1995,
51, 2304.
14 J. Vansant, G. Smets, J. P. Declercq, G. Germain and M. van
Meerssche, J. Org. Chem., 1980, 45, 1557.
15 T. L. Hennigar, D. C. MacQuarrie, P. Losier, R. D. Rogers and
M. J. Zaworotko, Angew. Chem., Int. Ed. Engl., 1997, 36, 972.
16 Z.-Y. Fu, S.-M. Hu, J.-C. Dai, J.-J. Jhang and X.-T. Wu, Eur. J.
Inorg. Chem., 2003, 2670.
Acknowledgements
Financial support for this work is provided by the ACS-PRF
and NSF (CHE-0243575). The Bruker CCD Smart System
Diffractometer of the Texas Center for Crystallography at Rice
University was funded by the Robert A. Welch Foundation and
the Bruker Avance 200 NMR spectrometer was purchased with
funds from ONR Grant N00014-96-1-1146.
17 P. S. Mukherjee, S. Konar, E. Zangrando, T. Mallah, J. Ribas
and M. R. Chudhuri, Inorg. Chem., 2003, 42, 2695; L. Carlucci,
G. Ciani, D. M. Proserpio and S. Rizzato, J. Chem. Soc., Dalton
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18 L. Carlucci, G. Ciani, D. M. Proserpio and S. Rizzato, CrystEng-
Comm., 2003, 5, 190.
19 A. R. Barron, J. Chem. Soc., Dalton Trans., 1988, 3047.
20 J. A. Francis, C. N. McMahon, S. G. Bott and A. R. Barron,
Organometallics, 1999, 18, 4399.
21 C. L. Aitken and A. R. Barron, J. Chem. Crystallogr., 1996, 26, 293.
22 G. M. Sheldrick, SHELXTL. Bruker AXS, Inc. Madison, WI, 1997.
23 Gaussian 98, Revision A.5: M. J. Frisch, G. W. Trucks, H. B.
Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G.
Zakrzewski, J. A. Montgomery, R. E. Stratmann, J. C. Burant,
S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C.
Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi,
B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski,
G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski,
J. V. Ortiz, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz,
I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith,
M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez,
M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong,
J. L. Andres, M. Head-Gordon, E. S. Replogle, and J. A. Pople,
Gaussian, Inc., Pittsburgh PA, 1998.
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3 6 9 4
D a l t o n T r a n s . , 2 0 0 4 , 3 6 8 9 – 3 6 9 4