The syntheses and structures of some main group complexes of the sterically hindered N,N′-bis(2,6-diisopropylphenyl)-4-toluamidinate ligand

Article abstract of DOI:10.1039/b409086a

The stoichiometric reaction of the bulky benzamidine N,N′-bis(2,6- diisopropylphenyl)-4-toluamidine (HDippAm) with the metal alkyls Bu ⁿLi (1:1 in THF), Bu₂Mg (2:1 in THF) and Me₃Al (1:1 in Et₂O) is presented. This provides the mononuclear dihapto benzamidinate compounds [Li(DippAm)(THF)₂] (1), [Mg(DippAm) ₂] (2) and [Al(DippAm)Me₂] (3), respectively. Compound 3 was also obtained by salt elimination using dimethylaluminium chloride and 1. All three compounds exhibit sterically strained geometries that are maintained in solution at increased temperatures. Compound 3 displays exceptional thermal and aerobic stability, while 2 constitutes a rare example of non-porphyrin supported square planar magnesium.

Full text of DOI:10.1039/b409086a

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P A P E R
The syntheses and structures of some main group complexes of the
sterically hindered N,N0-bis(2,6-diisopropylphenyl)-4-toluamidinate
ligand
a
b
b
´
´
Rene T. Boere, Marcus L. Colew and Peter C. Junk*
a Department of Chemistry and Biochemistry, University of Lethbridge, Lethbridge, Alberta,
Canada T1K 3M4
b School of Chemistry, Monash University, Victoria, 3800 Australia.
E-mail: peter.junk@sci.monash.edu.au
Received (in Durham) 15th June 2004, Accepted 21st September 2004
First published as an Advance Article on the web 14th December 2004
The stoichiometric reaction of the bulky benzamidine N,N0-bis(2,6-diisopropylphenyl)-4-toluamidine
(HDippAm) with the metal alkyls BunLi (1 : 1 in THF), Bu2Mg (2 : 1 in THF) and Me3Al (1 : 1 in Et2O) is
presented. This provides the mononuclear dihapto benzamidinate compounds [Li(DippAm)(THF)2] (1),
[Mg(DippAm)2] (2) and [Al(DippAm)Me2] (3), respectively. Compound 3 was also obtained by salt
elimination using dimethylaluminium chloride and 1. All three compounds exhibit sterically strained
geometries that are maintained in solution at increased temperatures. Compound 3 displays exceptional
thermal and aerobic stability, while 2 constitutes a rare example of non-porphyrin supported square
planar magnesium.
between a group 1 amidinate (typically generated by path (i) or
Introduction
(ii)) and a metal halide,34 have been tailored to introduce
supplementary donors, e.g. pendant amides35 or amidinates25
(Fig. 1, B and C respectively), heteroatoms36 (D) and sterically
bulky substituents9,10,12,17,21 (E) to the amidinate frame. Owing
to our interest in ligands of type A (Fig. 1) developments
toward the last of these interest us most.
Since we reported the first N,N0-bis(2,6-diisopropylphenyl)
amidines in 1998 (Fig. 1A),1 the area of metal amidinate
chemistry has undergone a significant expansion in scope and
application.2–5 In part, this can be attributed to a report from
Jordan in 1997 describing the treatment of dimethylaluminium
amidinates ([R1NC(R2)NR3]ꢀ) with methide abstraction re-
agents,6 e.g. tris(pentafluorophenyl)borane, to afford cationic
ethene polymerisation catalysts. Since, interest in amidinate
supported alkylaluminium species has understandably flour-
ished.2,7–14
Despite the implications of this landmark discovery, at-
tempts to soundly identify the catalytic species have not been
conclusive.2,15 However, dinuclear trisalkyl species have been
put forward as candidates.2 If indeed these are the active
species, the substituents about the amidinate will have a
considerable influence on the performance of this system not
just electronically but also sterically. This is borne out by
[Al(Benzamidinate)R2] (R ¼ alkyl) species with large aryl
substituents at the NCN backbone carbon. These generate
catalytically inactive species upon alkide abstraction.12 Similar
studies for dialkylaluminiums bearing amidinates with encum-
bered substituents at nitrogen have not been reported.8
As a side benefit of the above, the metalloamidinate chemis-
tries of groups 1–5,3,4,16–20 especially that of group 3,21–26 have
received significant interest. This has been bolstered by the size-
charge characteristics of amidinates, which can be considered
comparable to cyclopentadienides.27 Here, a major aim has
been the steric and electronic tuning of the amidine/ate. To
attain this the traditional synthetic paths to such species,28,29
these being: (i) protonolysis of a neutral amidine using a metal
alkyl30 or organoamide31 of appropriate basicity; (ii) insertion
of a carbodiimide into a metal alkyl32 or pnictide33 (the latter
to render ‘guanidinate’ type species); and (iii) salt elimination
So far, the introduction of bulky substituents has been
executed by two means. The first of these, the method that
we chose in our original contribution,1 is to generate neutral
amidines with bulky substituents at nitrogen using classical
amidine synthetic protocols.37 However, from the prodigious
work of Power et al.,38 preparative routes to terphenyl halides
have been popularised giving ready access to lithiated meta-
terphenyls.39 These include the lithiated 2,6-Tripp2C6H3, 2,6-
Mes2C6H3, 2,4,6-Ph3C6H2 and 2,6-(4-ButC6H4)C6H3 species
(Tripp ¼ 2,4,6-triisopropylphenyl, Mes ¼ 2,4,6-trimethyl-
phenyl), which have been applied to amidinate synthesis path (ii)
(the insertion of a carbodiimide into a metal alkyl or pnicitide),
to generate amidinates with terphenyl groups at carbon (see E,
Fig. 1).9,10,12,17,21 According to the group of Arnold, this
generates ‘bowl-like’ metal coordination environments with
steric encumbrance coplanar and orthogonal to the diazaallyl
donor.21 A space-fill depiction of one of these coordinated
ligands (that of [Al(PriNC(2,6-MesC6H3)NPri)(CH3)2]),12
viewed both above and in the plane of the MNCN metalla-
cycle, can be seen in Fig. 2. Recent reports of lithium,17,21
magnesium17 and aluminium9–10,12 complexes bearing this
ligand-type provide a firm basis for comparison with the
bulk of our N,N0-bis(2,6-diisopropylphenyl)amidinates,
represented here by N,N0-bis(2,6-diisopropylphenyl)-4-tolua-
midinate (DippAm).1 Herein we describe the synthesis
of [Li(DippAm)(THF)2] (1), [Mg(DippAm)2] (2) and
[Al(DippAm)(CH3)2] (3), generated by stoichiometric treat-
ment of the appropriate metal alkyl with HDippAm, and
relate the observed solution/solid-state behaviour of these
species to relevant compounds from the groups of Arnold
and Jordan.
w Present address: Department of Chemistry, University of Adelaide,
South Australia 5005, Australia.
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 a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 5
128
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 2 8 – 1 3 4

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Fig. 1 Recently developed amidinate ligand types (Ar ¼ alkyl sub-
stituted or unsubstituted aryl, M ¼ metal).
Scheme 1 Reagents and conditions: (i) 1.0 nBuLi, ꢀ1.0 nBuH, THF,
ꢀ30 1C to ambient temperature, overnight; (ii) 0.5 Bu2Mg, ꢀ1.0 nBuH,
THF, ꢀ30 1C to ambient temperature, 2 h; (iii) 1.0 Me3Al, ꢀ1.0 CH4,
Et2O, ꢀ50 1C to ambient temperature, overnight; (iv) 1.0 Me2AlCl,
ꢀ1.0 LiCl, Et2O/hexane, 0 1C to ambient temperature, ca. 5 h.
Results and discussion
The treatment of HDippAm with nbutyllithium (1 : 1) or
dibutylmagnesium (2 : 1) in THF at low temperature results
in clean alkane elimination to give compounds devoid of the
FTIR stretches (Nujol) and 1H NMR resonances (C6D6)
attributable to the N–H moiety of the parent ligand (see
Scheme 1). These ‘trademark’ stretches and signals occur at
3428 cmꢀ1, 3366 cmꢀ1 (N–H stretch)1 and 5.98 ppm, respec-
tively and are associated with a shift in the NCN backbone
C–N stretching frequencies from five strong absorbances at 1650,
1611, 1584, 1567 and 1510 cmꢀ1 to two distinct stretches at
1610 and 1573 cmꢀ1 for lithium compound 1, and 1609 and
1573 cmꢀ1 for magnesium compound 2. In addition, the FTIR
spectrum of 2, unlike that of 1, is free of stretches associated
with THF, e.g. a broad C–O stretch at ca. 1050 cmꢀ1, indicat-
ing the absence of included solvent. 1H and 13C NMR Spectra
confirm this and, from singular NCN resonances located at
154.3 and 158.9 ppm (1 and 2 respectively, HDippAm; 141.5
ppm), infer the presence of a singular benzamidinate isomer in
solution. Furthermore, for both 1 and 2, the number of
aromatic singlets in the 13C NMR spectra (8 for both) suggests
symmetrical binding about the NCN donor. This arrangement
invokes isopropyl methyl substituents with two separate che-
mical shifts (1; 1.20 and 1.30 ppm, 2; 1.16 and 1.34 ppm). The
existence of one broadened septet methyne resonance for both
1 and 2 (1; virtual septet 3.84 ppm, 2; virtual septet 3.79 ppm)
suggests these methyl environments emanate from restricted
rotation about the aryl–Pri bond rather than two distinct
isopropyl environments. Also, a ratio of 2 THF donors : 1
fluxional processes in solution as is consistent with deproto-
nated ligands of this type. Presuming the steric bulk of the 2,6-
diisopropyl N-substituents is too great to permit a Z-syn41
DippAm ligand, the symmetrical spectra of 1 and 2 must result
from E-anti isomerism,41 wherein placement of the 2,6-diiso-
propylphenyl rings orthogonal to the metal–NCN metallacycle
demands projection of one isopropyl methyl toward and the
other away from the NCN backbone. To gain unequivocal
proof of this and to elucidate the nuclearity of 1 and 2, both
compounds were recrystallised to provide samples suitable for
single crystal X-ray structure determination. The outcome of
these can be seen in Figs. 3 and 4a (POV-RAY illustrations,
40% thermal ellipsoids, see captions for selected bond lengths
and angles), while unit cell and refinement parameters are listed
in Table 1. As illustrated, 1 and 2 do indeed exhibit symme-
trical DippAm ligands, in which both chelate a single metal
centre. For compound 1, two THF donors complete four-
coordination of the lithium centre, while delocalisation of the
anionic charge across the bulky benzamidinate is evidenced by
the NCN backbone carbon–nitrogen lengths of 1.327(4) and
1.330(3) A (identical within experimental error) and a NCN
angle of 116.7(3)1 (HDippAm; 1.317(3), 1.344(3) A and
119.4(2)1 resp.).1 These parameters are similar to those of the
monomeric four-coordinate TMEDA complex; [Li(PriNC{2,6-
1
DippAm ligand can be deduced from the H NMR spectrum
of compound 1. This gives 1 and 2 the empirical formulae
[Li(DippAm)(THF)2] and [Mg(DippAm)2].
While HDippAm exists as a dynamic equilibrium of at least
two isomers at ambient temperature in deutero benzene solu-
tion,40 the NMR spectra of 1 and 2 indicate the absence of
(4-ButC6H4)2C6H3}NPri)(TMEDA)]
(NCN
parameters;
1.329(5), 1.327(5) A and 116.3(4)1),17 which displays a dihaptic
benzamidinate. This ‘bowl-like’ complex possesses Li-amidi-
nate nitrogen bond lengths of 1.995(9) and 1.998(9) A,17
suggesting greater ligand–metal proximity than the DippAm
ligand of 1 (Li–N; 2.032(6) and 2.057(6) A). Presumably this
arises from the bite of the TMEDA donor for the former
(86.5(4)1), which eases approach of the benzamidine relative to
the THF donors of 1 (O–Li–O bite 96.8(3)1). By contrast, the
lithium–benzamidinate nitrogen bond length of the three co-
ordinate lithium species [Li(PriNC(2,6-Tripp2C6H3)NPri)
(TMEDA)] is expectedly shorter due to decreased lithium
coordination (1.978(8) A). Here the significant bulk of the
benzamidinate forces a singular (monohapto) lithium–amide
nitrogen contact, and retention of discrete C–N and C
N
Q
bond character across the backbone (1.309(5) and 1.361(5) A,
respectively).17
Magnesium compound 2 (see Fig. 4) crystallises with one
half molecule in the asymmetric unit, wherein the magnesium
lies on a two-fold rotation axis perpendicular to the metalla-
cyclic plane. This generates a planar ‘‘Mg(NCN)2’’ core. The
Fig. 2 Space-fill illustration of [Al(PriNC(2,6-MesC6H3)NPri)(CH3)2]
above (left) and in (right) the metallacyclic plane. Isopropyl and
dimethylaluminium carbons coloured teal and yellow respectively
(POV-RAY illustration, 100% van der Waals radii).
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 2 8 – 1 3 4
129

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Fig. 3 X-ray crystal structure of 1 (POV-RAY illustration, 40%
thermal ellipsoids). All hydrogen atoms omitted for clarity. Selected
bond (A) lengths and angles (1): Li(1)–N(1) 2.032(6), Li(1)–N(2)
2.057(6), Li(1)–O(1) 1.955(6), Li(1)–O(2) 1.966(6), N(1)–C(25)
1.327(4), N(2)–C(25) 1.330(3), C(25)–C(26) 1.508(4), N(1)–Li(1)–N(2)
67.2(2), N(1)–Li(1)–O(1) 126.3(3), N(1)–Li(1)–O(2) 119.9(3), O(1)–
Li(1)–O(2) 96.8(3), N(1)–C(25)–N(2) 116.7(3), tolyl plane : metalla-
cyclic plane 49.6(1).
Fig. 4 (a) X-ray crystal structure of 2 (POV-Ray illustration, 40%
thermal ellipsoids). All hydrogen atoms and lesser occupancy disor-
dered atoms (C(24B)) omitted for clarity. Selected bond (A) lengths
and angles (1): Mg(1)–N(1) 2.047(2), Mg(1)–N(2) 2.069(2), N(1)–C(25)
1.347(3), N(2)–C(25) 1.343(3), C(25)–C(26) 1.497(4), N(1)–Mg(1)–N(2)
65.5(1), N(1)–Mg(1)–N(1)# 171.2(1), N(1)–Mg(1)–N(2)# 114.9(1),
N(2)–Mg(1)–N(2)# 174.6(1), N(1)–C(25)–N(2) 111.8(2), tolyl plane : -
metallacyclic plane 34.1(1). Symmetry transformation used to generate
# atoms: 1 ꢀ x, y, 1/2 ꢀ z. (b) Space-fill illustration of 2 (same
orientation as 4a). Tolyl carbons coloured teal (POV-RAY illustration,
100% van der Waals radii).
bond lengths and angles within the metallacycles, in particular
the NCN parameters, suggest the metal centre is more con-
gested (NCN C–N lengths and NCN angle; 1.347(3), 1.343(3)
A and 111.8(2)1, the former two are identical within experi-
mental error) than its lithium analogue. Furthermore, the
square planar geometry adopted is highly unorthodox42 (N(1)–
Mg(1)–N(2) 65.5(1)1, N(1)–Mg(1)–N(2)# 114.9(1)1, sum of
magnesium metal centres and two unusual compounds from
Lappert and Sachdev; the 1-azallyl species [Mg(Me3SiNC
angles about Mg
been observed before in rigidly held porphyrin coordinated43
¼
360.91). This geometry has only
(But)C(H)SiMe3)2]44
and
the
hydrazide
compound
[Mg{PhNN(SiMe3)2}2].45 The former of these exhibits
Table 1 Summary of crystal data for compounds 1–3. (Parameters for 3 have been amended to encompass the observed ‘‘site-symmetry’’ of
this compound)
[Li(DippAm)(THF)2] (1)
C40H57N2O2Li
[Mg(DippAm)2] (2)
C32H42N2Mg0.5
[Al(DippAm)(CH3)2] (3)
C34H47N2Al
Formula
Formula Weight
Temperature (K)
604.82
123(2)
ꢀ
P1
465.82
123(2)
C2/c
510.72
123(2)
P31
Space Group
a/A
10.922(3)
11.627(2)
15.562(4)
80.772(9)
77.567(13)
77.758(11)
23.4090(11)
10.6758(4)
25.0402(12)
90
15.2283(4)
15.2283(4)
12.0910(2)
90
b/A
c/A
a/1
b/1
g/1
114.337(3)
90
90
120
Volume/A3
Z
1872.3(8)
2
5701.7(4)
8
1.085
2428.26(10)
3
1.048
Dc/g cmꢀ3
m/mmꢀ1
1.073
0.064
0.072
0.085
Reflections collected
Unique reflections
Parameters varied
R(int)
22064
8589
415
30267
6614
332
17025
7271
365
0.1829
0.0719
0.1563
0.1902
0.0666
0.1387
0.0674
0.0477
0.0946
R1
wR2
130
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 2 8 – 1 3 4

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considerable closing of the planes created by the NCC 1-azallyl
donor and the MgN2C2 metallacycle (ca. 461),44 while the latter
displays amide nitrogen to magnesium contacts (mean value;
1.92 A) that radically contrast with the mean Mg–N amine
contacts (2.42 A).45 These ‘secondary’ lengths are outside of
the combined covalent radii of magnesium and nitrogen (ca.
2.1 A)46 and do not suggest explicit four-coordination of the
metal. Accordingly, neither ‘non-porphyrin’43–45 example de-
monstrates the ‘symmetrical’ planarity of 2, wherein the
MgNCN metallocycles exhibit a maximum rms deviation from
planarity of below 0.001 A (N(1) and N(2) exhibit deviations of
0.0001(11) A). Further, there are no obvious Mgꢁ ꢁ ꢁH–C agos-
tic contacts that could rationalise the adopted geometry, the
closest Mgꢁ ꢁ ꢁC and Mgꢁ ꢁ ꢁH distances being those of C(7) and
H(7) (3.888(3) and 3.07 A, respectively).47 Aside from magne-
sium bis(amidinate) species bearing apical THF donors,18,48
the known Mg(Amidinate)2 species49–53 possess ligands in
which the NCN units are disposed near orthogonal to one
another to minimise steric interaction. In the absence of
any close intermolecular interactions (including p–p arene
stacking), we propose that the DippAm ligand prohibits a
tetrahedral geometry by unfavourable interaction of the 2,6-
diisopropyl substituents.42 This coerces metallacycle to metal-
lacycle coplanarity. A space-fill diagram of 2 is displayed in
Fig. 4b (same orientation as Fig. 4a). From this it is apparent
that the tolyl ring (in teal) sits coplanar to the diazaallyl
fragment, thereby inducing significant buttressing with the
2,6-diisopropylphenyl rings. This impedes aryl–Pri bond rota-
tion and NCN backbone lengths (see above) that are extended
relative to those of the homoleptic tetrahedral magnesium
bis(amidinate) species [Mg(PriN(C(2,6-Mes2C6H3)NPri)2]
(both C–N NCN lengths 1.31(2) A),17 [Mg(ButNC(Ph)NBut)2]
(mean C–N NCN length; 1.33 A)49 and [Mg(MesNC
(But)NMes)2] (mean C–N NCN length; 1.33 A).52 Likewise,
the Mg–N lengths of 2 (2.047(2) and 2.069(2) A) are longer
than those of these compounds (mean values 2.04, 2.04 and
2.04 A respectively),17,49,52 which in turn are considerably
shorter than those of five/six coordinate magnesium bis(ami-
dinates), e.g. [Mg{(4-MeC6H4)NC(H)N(4-MeC6H4)}2(THF)2]
(mean Mg–N; 2.153 A).18
gests considerable hindrance about the aryl–Pri bond. As
suggested above, it is apparent from this and the solid-state
data that the driving force for decreased bond mobility in both
1 and 2 is the non-orthogonal placement of the 4-tolyl group
relative to the NCN backbone (see Fig. 4b).54 A similar
interaction is sterically forbidden for the benzamidinates of
Arnold (Fig. 1E), i.e. placement of the backbone phenyl
perpendicular to the diazaallylic plane is favoured.
Like 1 and 2, the reaction of trimethylaluminium with one
equivalent of HDippAm at low temperature results in clean
alkane elimination to form the dimethylaluminium compound
[Al(DippAm)(CH3)2] (3). Compound 3 was also prepared via
salt elimination using dimethylaluminium chloride and 1 under
similar conditions (see Scheme 1). The FTIR and 1H, 13C
NMR spectra of solvent donor free 3 display DippAm
stretches and resonances consistent with those for 1 and 2 (C–N
stretches at 1612, 1576 cmꢀ1, Pri methyl doublets at 1.09 and
1.39 ppm, virtual methyne septet at 3.75 ppm, discrete reso-
nances maintained at 70 1C without coalescence) while the N
CN backbone resonance is shifted upfield by ca. 20 ppm to
173.5 ppm. This placement compares well to the NCN reso-
nances of [Al(PriNC(2,6-Tripp2C6H3)NPri)(CH3)2]12 and
[Al(AdNC(CH3)NAd)(CH3)2]8 (Ad ¼ 1-adamantyl), which
appear at 169.5 and 172.6 ppm respectively. Likewise, the
location of the ‘‘Al(CH3)2’’ 1H, 13C and 27Al NMR resonances
at 0.04, ꢀ9.5 and 69.5 ppm (Al2Me6 27Al NMR resonance in
toluene; 157 ppm)55 compare well to those of the above species
(Tripp benzamidinate; ꢀ0.37, ꢀ4.34,12 adamantyl acetamidi-
nate; ꢀ0.82, ꢀ9.6 ppm,8 27Al NMR not reported) and a closely
related dimethylaluminium N,N0-bis(2,6-diisopropylphenyl)
pivalamidinate (ꢀ0.76 and ꢀ6.8 ppm respectively, 27Al NMR
not reported).8
Like compounds 1 and 2, the ready availability of crystalline
samples of 3 permitted a single crystal X-ray structure deter-
mination. As can be seen in Fig. 5 (POV-RAY illustration,
40% thermal ellipsoids, see Table 1 for unit cell and refinement
parameters), 3 crystallises as a monomeric [Al(DippAm)
(CH3)2] unit with a dihaptic DippAm. Akin to 1 and 2, the
NCN C–N bond lengths suggest significant delocalisation
of the anionic charge across the backbone (1.338(3) and
1.338(3) A, HDippAm; 1.317(3) and 1.344(3) A).1 Meanwhile,
the NCN angle of 110.3(2)1 and Al–N bond lengths of 1.942(2)
and 1.942(2) A are reasonably typical for bulky amidinates
coordinated to dimethylaluminium (analogous angles and
lengths for [Al(PriNC(2,6-Mes2C6H3)NPri)(CH3)2]; 108.2(3)1,
1.953(4) and 1.951(4) A)12 and thus open and extended (resp.)
relative to less bulky species like [Al{(c-C6H11)NC(But)N
Variable temperature 1H NMR experiments in the tempera-
ture range 0–70 1C were conducted for 1 and 2 to assess the
observed impeded rotation. For 1, the two isopropyl-methyl
doublets coalesce to one broadened singlet between 60 and
65 1C. A similar measurement for 2 could not be made in the
temperature range used (b.p. of C6D6 79.1 1C), with distinct
isopropyl-methyl doublets plainly evident at 70 1C. This sug-
Fig. 5 X-ray crystal structure of 3. Depiction at an angle to (left) and in the plane of the metallocycle (right) (POV-RAY illustration, 40% thermal
ellipsoids). All hydrogen atoms and lesser occupancy disordered atoms (C(29A) and C(32A)) omitted for clarity. Selected bond (A) lengths and
angles (1): Al(1)–N(1) 1.942(2), Al(1)–N(2) 1.941(2), Al(1)–C(33) 1.951(3), Al(1)–C(34) 1.952(3), N(1)–C(25) 1.338(3), N(2)–C(25) 1.338(3), C(25)–
C(26) 1.476(3), N(1)–Al(1)–N(2) 68.9(1), N(1)–Al(1)–C(33) 117.2(2), N(1)–Al(1)–C(34) 112.8(1), C(33)–Al(1)–C(34) 118.4(1), N(1)–C(25)–N(2)
110.3(2), tolyl plane: : metallacyclic plane 39.9(1).
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 2 8 – 1 3 4
131

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flame-dried glassware. Infrared spectra were recorded as Nujol
mulls using sodium chloride plates on a Nicolet Nexus FTIR
1
spectrophotometer. H NMR spectra were recorded at 300.1
MHz, 13C NMR spectra at 75.5 MHz and 27Al NMR at 78.2
MHz using a Bruker DPX 300 spectrometer. Chemical shifts
were referenced to the 13C or residual 1H resonances of the
1
d6-benzene solvent employed (13C and H NMR respectively),
or an external [Al(H2O)6]31 standard (1.1 M [Al(H2O)6][NO3]3
in H2O/D2O) (27Al NMR). Melting points were determined in
sealed glass capillaries under dinitrogen and are uncorrected.
Microanalyses (C, H and N) were undertaken at the University
of Otago, P.O. Box 56, Dunedin, New Zealand.
Fig. 6 Space-fill illustration of compound 3 above (left) and in (right)
the metallacyclic plane (as per Fig. 2). Tolyl and dimethylaluminium
carbons coloured teal and yellow respectively (POV-RAY illustration,
100% van der Waals radii).
[Li(g2-N,N0-DippAm)(THF)2] (1)
nButyllithium (0.70 cm3, 1.12 mmol) was added dropwise to a
stirred colourless solution of HDippAm (0.50 g, 1.10 mmol) in
THF (30 cm3) at ꢀ30 1C. The resulting solution, which
developed a yellow hue upon addition, was gradually warmed
to ambient temperature and stirred overnight. Removal of all
volatiles under reduced pressure rendered a light yellow pow-
der that was washed with cold (0 1C) hexane (2 ꢂ 3 cm3).
Addition of fresh THF (ca. 10 cm3) followed by filtration and
placement at ꢀ10 1C rendered 1 as colourless rectangular
blocks (0.57 g, 86%), m.p. 161 1C. 1H NMR (d6-benzene,
(c-C6H11)}(CH3)2] (analogous angles and lengths; 107.81,
1.927(2) and 1.912(1) A).7
Aside from these structural and spectroscopic characteris-
tics, the most striking aspect of compound 3 is its exceptional
aerobic and thermal stability. Solid samples show no sign of
decomposition up to 360 1C (limit of apparatus used) under a
dinitrogen atmosphere56 and, similarly, no decomposition (as
1
evidenced by H NMR) upon exposure to air for periods in
3
excess of 2 h. This stability is also evident in deutero benzene
solution where, in the absence of moisture (sample stored over
3 A molecular sieves), there is no decomposition after exposure
to air overnight. It appears this robustness is borne out of
considerable steric bulk about the AlNCN metallacycle (see
Fig. 6, aspects chosen same as Fig. 2). The significant bending
of the 2,6-diisopropylphenyl ipso-carbons out of the AlNCN
metallacyclic plane (see Fig. 5) testifies to this bulk (ipso-
carbons sit 0.341(3) and 0.343(3) A out of the AlNCN plane).
303 K): d 1.20 (br d, 12H, CH(CH3)(CH3), JHH ¼ 6.0 Hz),
1.30 (br d, 12H, CH(CH3)(CH3), 3JHH ¼ 5.9 Hz), 1.36 (m, 8H,
CH2 THF), 1.86 (br s, 3H, 4-CH3), 3.48 (m, 8H, OCH2 THF),
3.84 (br septet, 4H, CH(CH3)(CH3), 3JHH ¼ 6.1 Hz), 6.66–7.38
(br m, 8H, Ar–H), 7.66 (br d, 2H, Ar–H, JHH ¼ 7.6 Hz). 13C
NMR (d6-benzene, 303 K): d 21.4 (CH(CH3)(CH3)), 24.9 (CH
(CH3)(CH3)), 25.4 (4-CH3), 23.9 (CH2 THF), 28.7 (CH(CH3)
(CH3)), 68.4 (OCH2 THF), 121.3, 123.4, (Ar–CH), 128.0 (Ar–
C), 129.7, 131.0 (Ar–CH), 138.7, 141.5, 143.6 (Ar–C), 154.3 (N
CN). FTIR (Nujol)/cmꢀ1: 1610 (sh w), 1573 (sh m), 1464 (br s),
1356 (sh m), 1316 (s), 1254 (s), 1234 (s), 1184 (sh m), 1174
(sh m), 1142 (sh w), 1098 (sh m), 1072 (m), 1048 (sh s), 941 (m),
895 (m), 835 (sh w), 818 (sh m), 808 (w), 796 (w), 760 (sh s), 731
(sh w), 686 (w), 652 (w), 632 (w). Anal. Calc. For Li1C40H57N2O2:
C, 79.43; H, 9.50; N, 4.63. Found: C, 80.21; H, 9.76; N, 4.90.
Conclusion
We have demonstrated that the ligand DippAm is capable of
exerting considerable steric influence when complexed to
lithium, magnesium and aluminium. In the special instance
of magnesium; this results in a rare ‘square planar’ magnesium
compound. For dimethylaluminium, the steric protection af-
forded by the DippAm ligand manifests as exceptional thermal
and aerobic stability. As demonstrated by Figs. 2 and 6, the
protection afforded by ligands of type A may rival that of the
‘Arnold-type’ ligands; E (Fig. 1).56 All three compounds
suggest the 4-tolyl backbone group, placed such that it sits
near coplanar to the metallacycles generated, increases the
hindrance experienced at the metal.54 This incites heavily
impeded rotation about the aryl–isopropyl bonds, and is
maintained at increased temperatures.
[Mg(g2-N,N0-DippAm)2] (2)
Dibutylmagnesium (0.50 cm3, 0.50 mmol) was added dropwise
to a stirred colourless solution of HDippAm (0.50 g, 1.10
mmol) in THF (30 cm3) at ꢀ30 1C. The resulting colourless
solution was gradually warmed to ambient temperature over a
period of ca. 2 hours, concentrated in vacuo (to ca. 3 cm3),
filtered and left for several days. During this period small light
yellow blocks of 2 deposited. These were collected by filtration
and dried under a flow of dinitrogen after washing with cold
hexane (0 1C, 1 ꢂ 2 cm3) (0.16 g, 34%), m.p. 278 1C (decom-
Our use of the ligand frame A (see Fig. 1) in the stabilisation
and preparation of NCP, PCP and NCAs anionic donor sets
will form the basis of forthcoming publications.
1
position). H NMR (d6-benzene, 303 K): d 1.16 (d, 12H, CH
3
(CH3)(CH3), JHH ¼ 7.0 Hz), 1.34 (d, 12H, CH(CH3)(CH3),
3JHH ¼ 6.9 Hz), 1.78 (s, 3H, 4-CH3), 3.79 (virtual septet, 4H, C
Experimental
3
H(CH3)(CH3), JHH ¼ 7.0 Hz), 6.78 (d, 2H, Ar–H, JHH ¼ 7.8
General
Hz), 6.94–7.32 (m, 8H, Ar–H)HH. 13C NMR (d6-benzene, 303
K): d 19.8 (CH(CH3)(CH3)), 22.6 (CH(CH3)(CH3)), 25.6
(4-CH3), 31.3 (CH(CH3)(CH3)), 122.2, 123.4, 125.9 (Ar–CH),
126.6 (Ar–C), 130.7 (Ar–CH), 136.2, 139.9, 143.1 (Ar–C),
158.9 (NCN). FTIR (Nujol)/cmꢀ1: 1609 (sh m), 1573 (sh m),
1458 (br s), 1383 (s), 1361 (s), 1319 (m), 1278 (m), 1232 (s), 1194
(sh m), 1159 (m), 1109 (s), 1059 (m), 1045 (m), 1022 (w), 965 (sh
m), 952 (sh m), 932 (sh w), 840 (m), 823 (sh m), 795 (m), 775
(m), 761 (m), 738 (w), 698 (w), 688 (w), 676 (w), 657 (w), 636
(w). Anal. Calc. for Mg1C64H82N4: C, 82.51; H, 8.87; N, 6.01.
Found: C, 82.41; H, 9.38; N, 6.14.
N,N0-Bis(2,6-diisopropylphenyl)-4-toluamidine (HDippAm)
was prepared by a literature procedure.1,37 nButyllithium (1.6
M in hexane), dibutylmagnesium (1.0 M in heptane), trimethy-
laluminium (2.0 M in hexanes) and dimethylaluminium chlor-
ide (1.0 M in hexanes) were purchased from Aldrich and used
as received. Tetrahydrofuran (THF), diethyl ether and hexane
were dried over sodium, freshly distilled from sodium benzo-
phenone ketyl and freeze-thaw degassed prior to use. All
manipulations were performed using conventional Schlenk
techniques under an atmosphere of high purity dinitrogen in
132
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 2 8 – 1 3 4

View Article Online
absences as P3(1))61 was attempted. This failed to provide
satisfactory refinement parameters.
[Al(g2-N,N0-DippAm)(CH3)2] (3)
Method (i): A solution of HDippAm (0.30 g, 0.66 mmol) in
diethyl ether (20 cm3) was added dropwise to a cooled (ꢀ50 1C)
stirred solution of trimethylaluminium (0.40 cm3, 0.80 mmol),
also in diethyl ether (40 cm3) over a period of 30 minutes. The
resulting colourless solution was stirred overnight and per-
mitted to warm to ambient temperature. Filtration and re-
moval of all volatiles in vacuo gave a colourless powder. This
was washed with cold hexane (3 ꢂ 1.50 cm3, ꢀ10 1C) to remove
traces of trimethylaluminium, and the remaining colourless
solid recrystallised from fresh diethyl ether (ca.10 cm3) at 0 1C.
This gave the title compound as irregular colourless
plates around the periphery of the solution (0.14 g, 42%),
m.p. 4360 1C.
Crystallographic data (excluding structure factors) for the
structures reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre. CCDC reference
numbers 242105–242107. See http://www.rsc.org/suppdata/nj/
b4/b409086a/ for crystallographic data in .cif or other electro-
nic format.
Acknowledgements
The authors would like to thank the Australian Research
Council for continued financial support. RTB acknowledges
the ARC for a Linkage-International award and Prof. Alan
M. Bond for a fruitful research visit to Monash University. Dr
Craig M. Forsyth and Ms Kristina Konstas are sincerely
thanked for their assistance during the collection of 27Al
NMR data, and Mr Jason D. Masuda is thanked for perform-
ing some preliminary experiments.
Method (ii): A diethyl ether solution of 1 (0.30 g, 0.50 mmol,
10 cm3) was added to a cooled (0 1C) stirred solution of
dimethylaluminium chloride (0.50 cm3, 0.50 mmol) in hexane
(10 cm3). After stirring for several hours, volatiles were re-
moved in vacuo to render a light yellow solid. This was
extracted into diethyl ether (ca. 20 cm3) and separated from
the remnant lithium chloride by filtration. Concentration (ca.
10 cm3), followed by placement at ꢀ10 1C gave 3 as colourless
irregular plates (0.21 g, 82%), m.p. 4360 1C. 1HNMR (d6-
benzene, 303 K): d 0.04 (s, 6H, Al(CH3)2), 1.09 (d, 12H, CH
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