An unusual imine (N=C) alkylalumination reaction

Article abstract of DOI:10.1039/a701185g

Addition of 3 equiv. of AlMe₃ to a heptadentate Schiff-base ligand made from salicylaldehyde and bis(2-aminoethyl)-ethylenediamine (H₃L¹) results in an unusual alkylalumination of one of the imine bonds; addition of 2 equiv. of B(OMe)₃ or AlMe₃ to one of these ligands leads to the formation of bimetallic derivatives containing four- and five-coordinate metals, respectively.

Full text of DOI:10.1039/a701185g

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An unusual imine (NNC) alkylalumination reaction
Pingrong Wei and David Atwood*
Center for Main Group Chemistry, Department of Chemistry, North Dakota State University, Fargo, North Dakota 58105, USA
Addition of 3 equiv. of AlMe3 to a heptadentate Schiff-base
ligand made from salicylaldehyde and bis(2-aminoethyl)-
ethylenediamine (H3L1) results in an unusual alkylalumina-
tion of one of the imine bonds; addition of 2 equiv. of
B(OMe)3 or AlMe3 to one of these ligands leads to the
formation of bimetallic derivatives containing four- and five-
coordinate metals, respectively.
clear (Fig. 1). The formation of 1 follows similar reactions for
bimetallic salen derivatives.7 This is not the case for 2 for which
the 27Al NMR data indicates that only five-coordinate alumin-
ium is present (d 65.40). In the 1H NMR there are two Al–Me
shifts in a 1:2 ratio. This data can tentatively be attributed to the
structure shown in Scheme 1. Surprisingly, in reactions with 3
equiv. of the aluminium reagent, one of the NNC bonds
undergoes an unsual alkyalumination reaction in the formation
of trimetallic derivative 3 (Scheme 1, Fig. 2).
In the course of our explorations of group 13 Schiff-base chelate
complexes we have discovered an unusual imine (NNC)
alkylalumination reaction. Schiff-base ligands (and the salen
class, in particular) form a wide range of complexes with
transition1 and main-group metals.2 One of the reasons why
these ligands are so useful is because they do not undergo
further reactions after coordinating a metal. In fact, transition-
metal complexes of the reduced form of the salen ligand (the
salan class)3 are difficult to isolate because they dehydrogenate
back to the Schiff-base derivative when exposed to air.4
In turning our attention to ligands possessing multiple (more
than four) binding sites we examined those derived from the
condensation of 3 equiv. of salicylaldehyde (with H or But at the
2,4-positions, or Cl at the 4-position, of the aryl ring) with bis(2-
aminoethyl)ethylenediamine. The resulting ligands are denoted
H3L1, H3L2 and H3L3, respectively. We had intended to prepare
neutral bi- and tri-metallic derivatives of these ligands for
potential catalytic applications. Previously reported group 13
complexes with this5 and related systems6 have focused on
charged compounds useful in medicinal applications.
In the structure‡ one portion of the molecule coordinates an
AlMe2 unit by an oxygen and imine nitrogen, forming a six-
O(2)
O(3)
B(2)
O(1)
O(6)
N(4)
N(1)
O(5)
B(1)
O(7)
N(3)
N(2)
In the present work, two neutral bimetallic derivatives,
(HL2)[B(OMe)2]2 1 and L1AlMe(AlMe2) 2, could be prepared
in good yields using the appropriate stoichiometry (Scheme 1).†
The X-ray data for 1 was such that a full solution could not be
obtained. However, the overall morphology of the molecule is
O(4)
Fig. 1 Preliminary structure of 1 demonstrating the overall morphology of
the molecule
R
R
N
N
N
N
R
OMe
MeO
B
B
O
O
2 B(OMe)3
HO
OMe
R′
OMe
R′
–2 MeOH
(R = R′ = But)
R′
R′
R
OH
N
1
2
3
R
N
N
N
N
O
N
Al
Al
2 AlMe3
Me
Me
R
O
R
Me
–3 MeH
(R = R′ = H)
O
N
R′
R′
R′
HO
OH
R′
R
N
Me
R′
N
N
N
N
O
3 AlMe3
Al
Al
Me
Me
–3 MeH
(R = Cl, R′ = H)
R
O
R
Me
O
R
Al
R′
Me
R′
R′
R
Me
Scheme 1 General syntheses of group 13 compounds with H3Ln (n = 1–3)
Chem. Commun., 1997
1427

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of 3 is through an intra- or intermolecular alkylation remains to
be determined.
Cl(3)
Cl(1)
C(4)
This work was supported by the National Science Foundation
(Grant 9452892) and the donors of the Petroleum Research
Fund, administered by the American Chemical Society (Grant
31901-AC3). Partial support was provided by Union Carbide.
The receipt of an NSF-CAREER (CHE-9625376) award is also
gratefully acknowledged.
C(27)
C(32)
C(31)
Al(2)
C(1)
C(24)
O(3)
C(14)
O(2)
O(1)
C(30)
N(1)
C(21)
C(17)
Al(3)
N(3)
C(19)
C(11)
N(4)
C(33)
C(13)
Al(1)
C(7)
C(22)
Footnotes
Cl(2)
C(29)
C(20)
* E-mail: emgc@plains.nodak.edu
C(12)
N(2)
† Full experimental details will be published in due course. Only a partial
list of data is given here. 1: mp 182–184 °C. 1H NMR (CDCl3): d 1.13–1.41
(m, 54 H, CCH3), 2.85 (s, 6 H, OCH3), 3.16 (s, 6 H, OCH3) 2.72, 2.93, 3.35,
3.50 (m, 12 H, NCH2), 3.81 (s, 1H, NCHN), 6.84 (d, 1 H, Ph H), 7.06 (d, 2
H, Ph H), 7.17 (d, 1 H, Ph H), 7.51 (d, 2 H, Ph H), 8.07 (s, 2 H, NNCH).
Analysis, Calc: C, 70.34; H, 9.38. Found: C, 69.95; H, 9.28%. 2: mp >
260 °C. 1H NMR (CDCl3): d 20.92 to 20.73 (m, 9 H, AlCH3), 2.46–3.02,
3.28–3.71 (m, 12 H, NCH2), 3.86 (s, 1 H, NCHN), 6.63–7.41 (m, 12H, Ph
H), 8.14 (s, 1 H, NNCH), 8.21 (s, 1 H, NNCH). 27Al (CDCl3): d 65.40.
Analysis, Calc: C, 65.00; H, 6.49. Found: C, 65.13; H, 6.24%.
3·1.5C6H5Me: mp > 260 °C. 1H NMR (CDCl3): d 20.99 to 20.53 (m, 15
H, AlCH3), 1.38 (d, 3 H, CCH3), 2.35 (s, 3 H, PhCH3), 2.73–2.77, 3.08, 3.48
(m, 12 H, NCH2), 3.53 (s, 1 H, NCHN), 4.14 (m, 1H, NCHCH3), 7.11 (m,
9 H, Ph H), 7.94 (s, 1 H, NNCH). 27Al (CDCl3): d 54.50, 163.95. Analysis
Calc. for 3·C6H5Me: C, 58.46; H, 6.08. Found: C, 58.15; H, 6.26%.
C(8)
C(9)
C(10)
Fig. 2 Molecular structure and atom numbering scheme for 3. Selected bond
distances (Å) and angles (°): Al(1)–O(1) 1.769(3), Al(1)–C(30) 1.945(5),
Al(1)–C(29) 1.947(5), Al(1)–N(1) 1.961(3), Al(2)–O(3) 1.816(3), Al(2)–
O(2) 1.884(3), Al(2)–C(31) 1.935(5), Al(2)–C(32) 1.943(5), Al(3)–N(4)
1.822(3), Al(3)–O(2) 1.912(3), Al(3)–O(3) 1.944(3), Al(3)–C(33) 1.966(4),
Al(3)–N(3) 2.091(3), N(4)–C(21) 1.459(5), C(21)–C(22) 1.524(6);
O(1)–Al(1)–C(30) 110.6(2), O(1)–Al(1)–C(29) 112.2(2), C(30)–Al(1)–
C(29) 119.1(2), O(1)–Al(1)–N(1) 95.77(14), C(30)–Al(1)–N(1) 110.5(2),
C(29)–Al(1)–N(1) 106.1(2), O(3)–Al(2)–O(2) 79.95(11), O(3)–Al(2)–
C(31) 115.9(2), O(2)–Al(2)–C(31) 108.8(2), O(3)–Al(2)–C(32) 110.6(2),
O(2)–Al(2)–C(32) 116.9(2), C(31)–Al(2)–C(32) 118.7(2), N(4)–Al(3)–
O(2) 120.37(13), N(4)–Al(3)–O(3) 90.82(13), O(2)–Al(3)–O(3) 76.15(11),
N(4)–Al(3)–C(33) 127.4(2), O(2)–Al(3)–C(33) 111.6(2), O(3)–Al(3)–
C(33) 93.9(2), N(4)–Al(3)–N(3) 83.96(13), O(2)–Al(3)–N(3) 89.82(11),
O(3)–Al(3)–N(3) 159.98(12), C(33)–Al(3)–N(3) 104.7(2), Al(2)–
O(2)–A(31) 100.78(12), Al(2)–O(3)–Al(3) 102.05(13), N(4)–C(21)–C(22)
112.0(3), N(4)–C(21)–C(23) 113.6(3), C(22)–C(21)–C(23) 110.3(3).
‡ Crystallographic data: 1, monoclinic, P21/c, a
b = 20.6409(19), c = 22.7704(20) Å, b = 98.779(2)°, U = 7168.5(5) Å3,
Z = 4, 2670 observed [I > 4.0s(I)], R1 = 0.1932, wR2 = 0.5439.
3·1.5C6H5Me, monoclinic, P21/n, a
c = 13.7396(7) Å, b = 115.006(1)°, U = 4667.0(4) Å3, Z = 4, 6024
observed [I > 2.0s(I)], R1 = 0.0620, wR2 = 0.1218. CCDC 182/515.
=
15.4330(14),
= 11.8994(6), b = 31.498(2),
membered ring. This is a familiar coordination mode for Schiff-
base ligands. The second portion of the molecule contains two
aluminium atoms linked by bridging oxygens. The terminal
AlMe2 group adopts a geometry that is structurally similar to the
other AlMe2 group. The central, five-coordinate aluminium
atom is in a distorted trigonal-bipyramidal geometry. The
equatorial bond distances are systematically shorter than the
axial distances. This is exemplified in the distances for axial,
Al(3)–N(3) [2.091(3) Å] compared to equatorial, Al(3)–N(4)
[1.822(3) Å] atoms. The oxygens follow a similar trend [Al(3)–
O(2) 1.912(3), Al(3)–O(3) 1.944(3) Å]. Closely related to 3 is
the trimetallic complex (salean)AlMe(AlMe2)2 [salean = N,NA-
bis(o-hydroxybenzyl)-1,2-diaminoethane].8 It also contains
four- and five-coordinate (trigonal bipyramidal) aluminiums
bridged by oxygen atoms. In this complex the axial atoms are
oxygen and form an angle of 163.0(1)° at distances of 1.982(3)
and 1.959(3) Å. The differences in the NNC and N–C(Me)
linkages are reflected in the N–C bond distances of 1.290(5) and
1.459(5) Å, respectively. Furthermore, the imine group appears
as a singlet in the 1H NMR (d 7.94) while the amine NCH(Me)
appears as a multiplet (d 4.14).
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The use of group 13 reagents in the reduction of alkynes,
alkenes and carbonyl substituents is a well known, yet still
expanding area of chemistry.9 Alkylalumination reactions
generally require the intermediacy of a group 4 catalyst.10 The
catalyst acts to activate the substrate in a manner that is still
being defined. That an aluminium species is being produced in
3 that can alkylate carbon–nitrogen multiple bonds in the
absence of a metal catalyst is an intriguing prospect. Based upon
the fact that a compound containing two five-coordinate
aluminium atoms can be isolated (2), the third equivalent of
AlMe3 presumably effects the alkylation in 3. The coordination
of the first two AlRn units makes the imine linkage vulnerable
to this addition. This type of reaction bears some resemblance to
the alkylation of nitriles in which the first step is the formation
of an organoaluminium–nitrile adduct.11 Whether the formation
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Received in Columbia, MO, USA; 19th February 1997; 7/01185G
1428
Chem. Commun., 1997