Synthetic and structural investigations of alkali metal diamine bis(phenolate) complexes

Article abstract of DOI:10.1039/b718186h

Two lithium and one sodium diamine bis(phenolate) complexes have been prepared and characterised by X-ray crystallography and NMR spectroscopy. Two parent diamine bis(phenol) ligands were utilised in the study (1-H₂ and 2-H₂). Dimeri

Full text of DOI:10.1039/b718186h

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www.rsc.org/dalton | Dalton Transactions
Synthetic and structural investigations of alkali metal diamine
bis(phenolate) complexes†‡§
William Clegg,a Matthew G. Davidson,b David V. Graham,c Gemma Griffen,c Matthew D. Jones,b
Alan R. Kennedy,c Charles T. O’Hara,*c Luca Russoa and Calum M. Thomsonc
Received 26th November 2007, Accepted 2nd January 2008
First published as an Advance Article on the web 22nd January 2008
DOI: 10.1039/b718186h
Two lithium and one sodium diamine bis(phenolate) complexes have been prepared and characterised
by X-ray crystallography and NMR spectroscopy. Two parent diamine bis(phenol) ligands were utilised
in the study (1-H2 and 2-H2). Dimeric (1-Li2)2 was prepared by treating 1-H2 with two molar
equivalents of n-butyllithium in hydrocarbon solvent. It adopts a ladder-like structure in the solid state,
which appears to deaggregate in C6D6 solution. The monomeric (hence, dinuclear) TMEDA-solvated
species [2-Li2·(TMEDA)] has two chemically unique Li atoms in the solid state and is prepared by
reacting 2-H2 with two molar equivalents of n-butyllithium in hydrocarbon solvent, in the presence of
N,N,Nꢀ,Nꢀ-tetramethylethylenediamine (TMEDA). Finally, the dimeric sodium-based [2-Na2·(OEt2)]2
was prepared by reacting 2-H2 with two molar equivalents of freshly prepared n-butylsodium in a
hydrocarbon–diethyl ether medium. The complex adopts a Na4O4 cuboidal structure in the solid state,
which appears to remain intact in C6D6 solution.
Introduction
Group 1 metal phenolates are currently attracting considerable
interest in several distinct fields. For instance, they can be utilised
as precursors to other non-alkali metal phenolates via metathesis
reactions,1 as initiators in the ring opening polymerisation of cyclic
esters such as e-caprolactone and L-lactide2–5 and for controlling
network assembly in the synthesis of coordination polymers.6
Subtle changes in ligand structure and solvent medium have
highlighted interesting differences in the structural and reaction
chemistry of the resultant complexes.7–9 A subclass of the metal
Fig. 1 Amine bis(phenolate) ligands 1–3.
phenolate complexes which have come to prominence recently are
(Fig 2).14 The diamine bis(phenol) ligands employed in these
the amine bis(phenolate) complexes. Of particular interest to us
syntheses are isomeric—the first of which is 1-H2, whilst the
are ligands 1 and 2 (Fig. 1).
second is Me2NCH2CH2N(CH2ArOH)2, 3-H2 where Ar = 3,5-
C6H2-tBu2]. The Li complexes obtained are both dioxane adducts;
however, the former, [1-Li2·(dioxane)1.5]2, possesses a dimeric,
tetranuclear structure whereby the dinuclear units are bridged by a
molecule of dioxane. In the latter [3-Li2·(dioxane)]∞, the structure
adopts a polymeric arrangement in the solid state. To provide
further insight into the chemistry of alkali metal bis(phenolate)
complexes, we report the synthesis and characterisation of three
new diamine bis(phenolate) complexes.
We have recently reported the homoleptic dimeric Mg10 and
trinuclear Ba11 salts of 1. By treating M(OiPr)4 (where, M = Ti,
Zr or Hf) with an equimolar quantity of 1 and 2, several Group 4
heteroleptic Ti, Zr and Hf salts were prepared and characterised.12
Mountford and Duchateau et al. have recently reported the
structure of several Sm amine bis(phenolate) complexes.13 Most
pertinent to this particular study, Kerton et al. have published
the structures of two dilithium diamine bis(phenolate) complexes
aSchool of Natural Sciences (Chemistry), Newcastle University, Newcastle-
upon-Tyne, UK NE1 7RU
Results and discussion
bDepartment of Chemistry, University of Bath, Bath, UK BA2 7AY
Syntheses and solid-state structures
cDepartment of Pure and Applied Chemistry, University of Strathclyde,
WestCHEM, Glasgow, UK G1 1XL. E-mail: charlie.ohara@strath.ac.uk;
Tel: +44 (0)141 548 2667
† Dedicated to Professor Ken Wade—a truly inspirational inorganic
chemist.
‡ CCDC reference numbers 669061–669063. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b718186h
§ Electronic supplementary information (ESI) available: NMR spectra
data for (1-Li2)2, [2-Li2·(TMEDA)]and [2-Na2·(OEt2)]2. See DOI: 10.1039/
b718186h
Complexes (1-Li2)2, [2-Li2·(TMEDA)] and [2-Na2·(OEt2)]2
were prepared using conventional deprotonation procedures
(Scheme 1).
For the lithium amine bis(phenolate) complexes, a hexane–
toluene solution of 1-H2 was treated with two molar equivalents of
nBuLi. By reducing the volume in vacuo and cooling the mixture to
−27 ◦C a crop of colourless, crystalline 1-Li2 was deposited (yield
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Scheme 1 Preparation of (1-Li2)2, [2-Li2·(TMEDA)] and [2-Na2·(OEt2)].
nBuNa (two molar equivalents) was treated with diethyl ether,
colourless 2-Na2·(OEt2) was formed in 34% yield.
The X-ray-determined structure of (1-Li2)2 is shown in Fig. 3,
and its key structural dimensions are listed in Table 1. Solvent-free
(1-Li2)2 is a discrete tetranuclear dimer which adopts a ladder-
like conformation. The Li atoms which occupy positions in the
outer rungs [Li(1) and Li(4)] are formally five-coordinate, and
are coordinated by one l2-O, one l3-O, two N and one ipso-C
from an adjacent benzene ring. Those occupying the central rungs
[Li(2) and Li(3)] are three-coordinate and the attached atoms
adopt a distorted trigonal pyramidal environment [endo-ladder
angles: O(2)–Li(3)–O(4), O(3)–Li(3)–O(4), O(1)–Li(2)–O(2) and
O(2)–Li(2)–O(4) are 93.5(2), 103.2(2), 104.0(2) and 93.8(2)◦; exo-
ladder angle◦s: O(2)–Li(3)–O(3) and O(1)–Li(1)–O(4) are 158.7(3)
Fig.
2
Previously published structures of two lithium diamine
bis(phenolate) complexes. For simplicity no stereochemistry is shown.14
of first batch, 42%). A similar reaction was attempted using 2-H2;
however, no crystalline material could be isolated from solution.
More success was forthcoming on introducing the coordinating
diamine TMEDA to the mixture, producing colourless, crystalline
2-Li2·(TMEDA) in a yield of 48%. Akin to its Li analogue, a base-
free Na complex of 2-H2 could not be isolated—instead when
a hexane/toluene solution of 2-H2 (one molar equivalent) and
˚
and 156.5(3) respectively]. The Li–O bonds range from 1.844(4) A
˚
[for Li(3)–O(3)] to 1.978(4) A [for Li(4)–O(4)]. As expected the
Fig. 3 Molecular structure of (1-Li2)2, showing selected atom labelling with hydrogen atoms omitted for clarity. The Li · · · Cipso interactions are denoted
by dashed lines.
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◦
◦
˚
˚
Table 1 Bond lengths (A) and angles ( ) for (1-Li2)2
Table 2 Key bond lengths (A) and angles ( ) for [2-Li2·(TMEDA)]
Li(1)–O(1)
Li(1)–O(2)
Li(1)–N(1)
Li(1)–N(2)
Li(1)–C(21)
Li(2)–O(1)
Li(2)–O(2)
Li(2)–O(4)
Li(3)–O(2)
Li(3)–O(3)
Li(3)–O(4)
Li(4)–O(3)
Li(4)–O(4)
Li(4)–N(3)
Li(4)–N(4)
Li(4)–C(55)
1.857(5)
1.975(5)
1.999(5)
2.054(5)
2.397(5)
1.860(5)
1.934(5)
1.931(4)
1.934(4)
1.844(4)
1.940(5)
1.852(5)
1.978(4)
2.003(5)
2.046(5)
2.437(5)
O(1)–Li(1)–O(2)
O(1)–Li(2)–O(2)
O(1)–Li(2)–O(4)
O(2)–Li(2)–O(4)
O(2)–Li(3)–O(3)
O(2)–Li(3)–O(4)
O(3)–Li(3)–O(4)
N(3)–Li(4)–N(4)
N(3)–Li(4)–O(3)
N(3)–Li(4)–O(4)
N(4)–Li(4)–O(3)
N(4)–Li(4)–O(4)
O(3)–Li(4)–O(4)
Li(1)–O(1)–Li(2)
Li(1)–O(2)–Li(2)
Li(1)–O(2)–Li(3)
Li(2)–O(2)–Li(3)
Li(3)–O(3)–Li(4)
Li(2)–O(4)–Li(3)
Li(2)–O(4)–Li(4)
Li(3)–O(4)–Li(4)
102.5(2)
104.0(2)
156.5(3)
93.8(2)
158.7(3)
93.5(2)
103.2(2)
90.7(2)
105.4(2)
136.9(3)
124.6(2)
101.0(2)
101.5(2)
77.8(2)
73.3(2)
150.3(2)
86.1(2)
79.3(2)
86.0(2)
153.1(2)
74.0(2)
Li(1)–O(3)
Li(1)–O(22)
Li(1)–N(11)
Li(1)–N(14)
Li(2)–O(3)
Li(2)–O(22)
Li(2)–N(29)
Li(2)–N(32)
1.854(2)
1.850(2)
2.018(2)
2.056(2)
1.923(2)
1.902(2)
2.112(17)
2.245(13)
N(11)–Li(1)–O(22)
N(14)–Li(1)–O(3)
N(14)–Li(1)–O(22)
O(3)–Li(1)–O(22)
N(29)–Li(2)–N(32)
N(29)–Li(2)–O(3)
N(29)–Li(2)–O(22)
N(32)–Li(2)–O(3)
N(32)–Li(2)–O(22)
O(3)–Li(2)–O(22)
134.80(13)
126.67(12)
102.74(10)
97.49(9)
83.4(4)
113.5(3)
123.9(3)
135.4(3)
106.5(3)
97.49(9)
N(11)–Li(1)–N(14)
N(11)–Li(1)–O(3)
88.90(9)
105.52(10)
N(1)–Li(1)–N(2)
N(1)–Li(1)–O(1)
N(1)–Li(1)–O(2)
N(2)–Li(1)–O(1)
N(2)–Li(1)–O(2)
90.9(2)
105.4(2)
137.4(3)
120.6(2)
101.8(2)
longest Li–O bonds are those involving the l3-O atoms [O(2) and
O(4)], but consequently this gives rise to long (and by implication,
weak) Li–O bonds to the three-coordinate Li atoms [Li(2) and
˚
Li(3)]. The mean Li–(l2-O) bond distance (1.853 A) in (1-Li2)2 is
identical to the equivalent bond in Kerton’s dioxane solvate of (1-
14
˚
Li2)2 (1.853 A), as are the Li–N distances. As alluded to earlier,
the outermost Li atoms p-bond to an adjacent benzene ring in
1
˚
an g -manner (mean distance, 2.417 A). These bonds are in the
range for those found in other related Li phenoxide systems15–22
˚
(2.268–2.596 A). The ladder-like core of (1-Li2)2 comprises three
essentially planar Li2O2 rhombi [sum of endocyclic angles for
Li(1)–O(1)–Li(2)–O(2), O(2)–Li(2)–O(4)–Li(3) and Li(3)–O(4)–
Li(4)–O(3) rings are 357.6, 359.5 and 358.0◦ respectively]. Overall,
the molecule adopts a convex cisoidal geometry [dihedral an-
gles between: Li(1)–O(2)–Li(2)–O(1) and Li(2)–O(2)–Li(3)–O(3);
and Li(2)–O(2)–Li(3)–O(3) and Li(3)–O(3)–Li(4)–O(4) planes are
23.7(2) and 17.7(4)◦ respectively] (Fig. 4). Ladder motifs akin to
that in (1-Li2)2 are common in lithium chemistry.23–26
Fig. 5 The molecular structure of [2-Li2·(TMEDA)], showing selected
atom labelling with hydrogen atoms omitted for clarity.
C p-bonding in [2-Li2·(TMEDA)] [shortest Li · · · Caryl distance:
˚
2.596(2) A for Li(1) · · · C(4)], although the aryl rings tend to
tilt considerably more toward Li(1) than Li(2) {c.f. Li(1)–O(3)–
C(4) [108.52(9)◦] with Li(2)–O(3)–C(4) [162.30(10)◦]; and Li(1)–
O(22)–C(21) [109.51(9)◦] with Li(2)–O(22)–C(21) [168.51(10)◦]}.
The shortest Li–O and Li–N distances in [2-Li2·(TMEDA)] are
those involving Li(1)—highlighting the excellent metal-binding
properties of the N2O2 chelating ligand. For instance, the Li(2)–
˚
NTMEDA bonds (mean distance: 2.179 A) are significantly longer
than the Li(1)–N bonds associated with the amine bis(phenolate)
˚
ligand (mean distance: 2.037 A). An identical scenario is found
Fig. 4 The ladder-like core of (1-Li2)2 showing its gentle cisoidal
for the Li–O bonds: mean Li–O bond distance is 1.852 and
geometry.
˚
1.913 A for Li(1) and Li(2) respectively. The TMEDA bite
The X-ray-determined structure of [2-Li2·(TMEDA)] is shown
in Fig. 5, and its key structural dimensions are listed in Table 2.
As for (1-Li2)2, there are two chemically distinct Li atoms
within the dinuclear, monomeric structure of [2-Li2·(TMEDA)];
however, their overall chemical environments are more similar.
Both Li(1) and Li(2) are in a distorted tetrahedral arrangement
(mean angle: 110.1 and 110.0◦ respectively) and are bound
to two l2-O and two N atoms, where the N atoms belong
to internal 2 for Li(1) and external TMEDA for Li(2). The
central four-membered Li2O2 ring is almost planar (sum of
endocyclic angles: 358.6◦). There appears to be no appreciable Li–
angle [83.4(4)◦] is 5.5◦ more acute than the corresponding angle
attributed to the amine bis(phenolate) ligand. To the best of our
knowledge, three other TMEDA solvates of lithium phenolates
have been crystallographically characterised: a hexanuclear adduct
of dilithiated 2-tert-butyl-5-methylphenol, lithiated TMEDA and
TMEDA (in a ratio of 2 : 2 : 2);15 a dodecanuclear adduct of
trilithiated 2,5-dimethylphenol and TMEDA (in a 4 : 4 ratio);16 and
octanuclear [(salen)Li2]3·Li2O·(TMEDA)2·H2O] where salen-H2 =
N,Nꢀ-ethylenebis(salicylideneimine).27 Additionally, the crystal
structure of a TMEDA-solvated mixed lithium–magnesium phe-
nolate has also been published.28
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The X-ray-determined structure of [2-Na2·(OEt2)]2 is shown
in Fig. 6 and and its key structural dimensions are listed in
Table 3. Table 4 collects together the crystal and structure re-
finement data for (1-Li2)2, [2-Li2·(TMEDA)] and [2-Na2·(OEt2)]2.
[2-Na2·(OEt2)]2 exhibits a tetranuclear distorted cubane structure.
Each corner-positioned Na atom is coordinated by three l3-O
atoms. Na(1) and Na(2) are further stabilised by the coordination
of a molecule of diethyl ether, whilst Na(3) and Na(4) are further
stabilised by internal chelation by two N atoms [N(1) and N(2),
and N(3) and N(4) respectively for Na(3) and Na(4)]. Due
to the presence of disorder in the diethyl ether molecules the
pseudo-tetrahedral geometry around Na(1) and Na(2) can not be
discussed in detail. Interestingly, the geometries around the five-
coordinate Na(3) and Na(4) atoms are significantly different in the
◦
Fig. 6 The molecular structure of [2-Na2·(OEt2)]2, showing selected atom
labelling with hydrogen atoms omitted for clarity. Dashed bonds represent
the Na–O(ether) bonding.
˚
Table 3 Key bond lengths (A) and angles ( ) for [2-Na2·(OEt2)]2
Na(1)–O(3)
Na(1)–O(5)
Na(1)–O(6)
Na(2)–O(3)
Na(2)–O(4)
Na(2)–O(5)
Na(3)–O(3)
Na(3)–O(4)
Na(3)–O(6)
Na(4)–O(4)
Na(4)–O(5)
Na(4)–O(6)
Na(3)–N(1)
Na(3)–N(2)
Na(4)–N(3)
Na(4)–N(4)
2.263(4)
2.285(4)
2.307(3)
2.350(4)
2.268(3)
2.311(3)
2.389(3)
2.259(4)
2.275(4)
2.309(4)
2.304(4)
2.332(4)
2.349(4)
2.522(4)
2.427(4)
2.435(4)
O(3)–Na(2)–O(5)
O(4)–Na(2)–O(5)
O(3)–Na(3)–O(4)
O(3)–Na(3)–O(6)
O(3)–Na(3)–N(1)
O(3)–Na(3)–N(2)
O(4)–Na(3)–O(6)
O(4)–Na(3)–N(1)
O(4)–Na(3)–N(2)
O(6)–Na(3)–N(1)
O(6)–Na(3)–N(2)
N(1)–Na(3)–N(2)
O(4)–Na(4)–O(5)
O(4)–Na(4)–O(6)
O(4)–Na(4)–N(3)
O(4)–Na(4)–N(4)
O(5)–Na(4)–O(6)
O(5)–Na(4)–N(3)
O(5)–Na(4)–N(4)
O(6)–Na(4)–N(3)
O(6)–Na(4)–N(4)
N(3)–Na(4)–N(4)
92.50(13)
93.49(12)
91.48(12)
91.51(12)
86.44(13)
132.63(14)
96.36(13)
152.65(17)
86.55(14)
110.94(16)
135.77(14)
75.16(15)
92.59(12)
93.44(13)
163.47(15)
104.78(14)
89.61(12)
88.83(12)
162.46(15)
103.05(14)
86.99(14)
75.23(14)
solid state. Firstly, the Na(3) atom appears to adopt a distorted
trigonal bipyramidal arrangement where N(1) and O(4) adopt the
axial positions [N(1)–Na(3)–O(4) angle: 152.65(17)◦]. Secondly,
the Na(4) atom appears to adopt a distorted square pyramidal
arrangement. Unlike the Na(3) case, there are two angles which
are tending towards 180◦—O(4)–Na(4)–N(3) and O(5)–Na(4)–
N(4) are 163.47(15)◦ and 162.46(15)◦ respectively—hence O(6)
adopts the axial coordination site. The Na–(l3-O) bond distances
˚
range from 2.259(4) to 2.389(3) A, the shortest and longest are
attributed to Na(3)–O(4) and Na(3)–O(3) respectively. Turning to
˚
the Na–N bonds in [2-Na2·(OEt2)]2, the Na(3)–N(1) [2.349(4) A]
˚
and Na(3)–N(2) [2.522(4) A] distances differ signifcantly. On the
other hand the Na(4)–N bonds are more uniform [Na(4)–N(3) and
O(3)–Na(1)–O(5)
O(3)–Na(1)–O(6)
O(5)–Na(1)–O(6)
O(3)–Na(2)–O(4)
95.52(13)
94.03(12)
90.73(12)
92.28(13)
˚
Na(4)–N(3) distances: 2.427(4) and 2.435(4) A]; however, the mean
˚
˚
Na(3)–N (2.436 A) and Na(4)–N (2.431 A) distances are almost
identical. From a search of the Cambridge Structural Database,29
Table 4 Crystal data and structure refinement for (1-Li2)2, [2-Li2·(TMEDA)] and [2-Na2·(OEt2)]2
Compound
(1-Li2)2
[2-Li2·(TMEDA)]
[2-Na2·(OEt2)]2
Molecular formula
Formula mass
T/K
C82H124Li4N4O4
1257.61
123(2)
Triclinic
¯
P1
15.1350(6)
16.7141(6)
18.3935(7)
88.527(2)
66.122(2)
69.106(2)
3936.4(3)
2
C28H46Li2N4O2
484.57
150(2)
Monoclinic
P21/n
12.151(3)
15.012(3)
16.937(4)
90
109.913(3)
90
2904.7(10)
4
1.108
C52H80N4Na4O6
949.16
123(2)
Monoclinic
P21/n
16.2626(4)
16.8683(6)
21.4569(5)
90
111.185(2)
90
5488.3(3)
4
1.149
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
Z
Dc/Mg m−3
1.061
0.063
l/mm−1
0.069
0.101
Crystal size/mm
Reflections collected
Independent reflections (Rint
Refined parameters
Goodness-of-fit on F2
R (F, F2 > 2r)
0.28 × 0.10 × 0.06
0.80 × 0.60 × 0.60
0.40 × 0.30 × 0.30
26003
25601
16661
)
13822 (0.0636)
896
1.010
0.0617
0.1521
7055 (0.0184)
412
1.039
0.0460
0.1375
8619 (0.0757)
604
1.105
0.0839
0.2350
Rw (F2, all data)
−3
˚
Max, min difference/e A
)
0.33, −0.30
0.40, −0.42
0.51, −0.42
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it appears that [2-Na2·(OEt2)]2 is the first structurally characterised
Na amine bis(phenolate) complex. However, several Na phenolate
complexes9,30–38 adopt structures with a similar cubanoid core to
that of [2-Na2·(OEt2)]2. Perhaps most pertinent to our work is
the study of sodium Schiff base complexes presented by Deacon
et al.38 Like [2-Na2·(OEt2)]2, the complexes reported in this study—
[Na2(salpn)(OEt2)]2 and [Na2(salpn)(DME)]2 (where salpn is [(o-
spectra normally give an indication of the purity of the complex
(because of the generally low number and good separation of
the resonances). This complexity presumably arises due to a
high degree of dynamic transformations in solution, and/or the
existence of multiple solution oligomers being present. In an effort
to simplify the spectrum, the experiment was repeated using the
strong r-donating solvent D5-pyridine. The spectrum obtained
was significantly simplified. As well as uncoordinated TMEDA,
it appears that the only lithium phenolate complex present is
a pyridine-solvated dinuclear monomer [other than the residual
pyridine signals, only two resonances in the aryl region of the
spectrum (7.07 and 6.90 ppm) were observed]. The CH2 resonances
=
OC6H4CH NCH2)2CH2] and DME is dimethoxyethane)—have
a Na4O4 core and two of the Na atoms are solvated by an ethereal
solvent molecule. The Na–O bonds in the diethyl ether-solvated
˚
Schiff base complex [range of bond distances, 2.235(3)–2.347(3) A]
are similar to those in [2-Na2·(OEt2)]2 [range of bond distances,
1
˚
2.235(3)–2.347(3) A].
in the H spectrum appeared broad. Likewise, their respective
1
resonances in the 13C{ H} NMR spectrum are also broad.
The 1H NMR spectrum obtained for [2-Na2·(OEt2)]2 shows
that no diethyl ether was lost on isolation despite the crystals
being subjected to vacuum during the process. In contrast to the
NMR spectra of the previous compounds reported here, it appears
that the cuboidal framework of [2-Na2·(OEt2)]2 remains intact
in solution. O(4) and O(6) are in significantly different chemical
environments from O(3) and O(5) (Fig. 8); therefore, the associated
benzene rings are different, giving rise to four resonances in the
aryl region of the 1H spectrum. However, the entire structure
does not stay intact in C6D6 solution—the chemical shifts for
diethyl ether molecules are identical to those of uncoordinated
solvent, indicating that it is probably labile in C6D6. Due to the
complication that there are two distinct phenolate environments
present in the solution structure of [2-Na2·(OEt2)]2 this should
result in the appearance of eight diastereotopic CH2 resonances—
Solution structures
Crystalline samples of (1-Li2)2, [2-Li2·(TMEDA)] and [2-
Na2·(OEt2)]2 are soluble in C6D6 solution; hence, a NMR spec-
troscopic study of the complexes was possible.
From the crystal structure of (1-Li2)2, it is evident that, if the
solid-state structure is maintained in solution, two chemically
distinct phenoxide groups (pertaining to l2-O and l3-O bonding
modes), should be observed in the 1H NMR spectrum. In practice,
only one type of phenoxide is observed in C6D6 solution at
concentrations of 5 mg l−1 and 15 mg l−1 (only two aromatic H
1
resonances are seen, c.f. the H NMR spectrum for 1-H2). This
observation is consistent with the dimeric (tetranuclear) solid-state
structure deaggregating in solution to give monomeric (dinuclear)
oligomers which may be C6D6-solvated (Fig. 7). In D8-toluene
1
solution at 228 K, the H NMR spectrum revealed that there
10
a similar scenario was noticed in the solution structure of (1-Mg)2
were still only two aryl resonances corresponding to the complex;
hence, these data suggest that the ladder undergoes dissociation
in solution rather than being subject to rapid intramolecular
dynamic exchange. Alkali metal–p interactions like those alluded
to here have recently been subject to considerable research39–41 and
their importance has been realised in fields as diverse as zeolite-
supported processes,42 in the interaction of alkali metal cations
with aromatic side chains of proteins,43–46 and in the selectivity of
superbases,47 via coordination of an arene molecule to a Group 1
metal centre. In (1-Li2)2 the ligand has two sets of diastereotopic
and in a dimeric calcium bis(phenolate) complex reported by
Bochmann et al.48 However, presumably due to dynamic behaviour
within the ligand framework, no obvious CH resonances could be
1
observed in the H spectrum of [2-Na2·(OEt2)]2; instead a broad
region (from 4.2 to 1.8 ppm) just above the spectrum’s baseline
is observed. This type of dynamic behaviour has previously been
noted for (1-Ba)3·(THF)2.11
1
CH2 groups (Fig. 1). In the H NMR spectrum of (1-Li2)2 four
distinct CH2 resonances can be observed. This appears to indicate
that the Li–O bonding in the solution oligomers is strong, resulting
in a fixed stereochemistry of the ligand and hence the appearance
of the diastereotopic H atoms in the NMR spectrum. These data
are in agreement with those obtained by Kerton et al. for (1-Li2)2
in D5-pyridine.14
Fig. 8 View of [2-Na2·(OEt2)]2 cubane showing the different connectiv-
ities of the phenoxide ligands. (For clarity all C and H atoms have been
omitted.)
Fig. 7 Proposed symmetrical deaggregation of (1-Li2)2 in C6D6 solution.
Conclusions
The 1H NMR spectrum of crystalline [2-Li2·(TMEDA)] in C6D6
was found to be extremely complex (due to the large number of
overlapping signals in both the aromatic and aliphatic regions of
the spectrum—see supporting information§) and no useful data
could be extrapolated from the spectrum. The aryl regions of such
Three new alkali metal diamine bis(phenolate) complexes have
been prepared and characterised. In the solid state, the donor-free
Li complex (1-Li2)2 is dimeric (hence tetranuclear) and adopts a
‘ladder-like’ arrangement; in C6D6 solution, the dimer appears
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to deaggregate. [2-Li2·(TMEDA)] is monomeric in the solid state.
Perhaps surprisingly, in C6D6 solution the complex is subject to a
high degree of dynamic activity, which can be suppressed by using
a more polar solvent such as D5-pyridine. Finally, [2-Na2·(OEt2)]2
is dimeric (tetranuclear) in the solid state, but unlike (1-Li2)2 it
adopts a cubanoid arrangement. 1H NMR spectroscopy appears
to indicate that the cubanoid core remains intact in C6D6.
formaldehyde solution (38 wt% in H2O) (8.7 mL, 110 mmol),
and 2,4-dimethylphenol (12.2 g, 100 mmol) in methanol.53 The
resultant mixture was heated to reflux for 18 h, and 2-H2 was
collected by filtration and washed with cold methanol. Typical
1
yield: 14.8 g (83%). H NMR (400.13 MHz, C6D6, 300 K): d
10.39 (br s, 2H, OH), 6.87 (d, 2H, JHH = 2.4 Hz, Ar–H), 6.54 (d,
2H, JHH = 2.4 Hz, Ar–H), 3.24 (s, 4H, ArCH2N), 2.42 (s, 6H,
Ar–CH3), 2.20 [br s, 4H, N(CH2)2], 2.15 (s, 6H, Ar–CH3), 1.80
1
(s, 6H, N–CH3). H NMR (400.13 MHz, D5-pyridine, 300 K):
Experimental
d 10.76 (br s, 2H, OH), 6.93 (d, 2H, Ar–H), 6.72 (d, 2H, Ar–
H), 3.60 (s, 4H, ArCH2N), 2.55 (s, 6H, Ar–CH3), 2.36 [br s, 4H,
N(CH2)2], 2.23 (s, 6H, Ar–CH3), 2.13 (s, 6H, N–CH3). 13C NMR
(100.63 MHz, C6D6, 300K): d 154.4 [C(aryl)–OH], 131.2 [C(aryl)–Me],
Materials and methods
All experimental manipulations (except ligand preparations) were
performed under an atmosphere of dry, oxygen-free argon, using
standard Schlenk techniques and a glovebox. 1.6 M n-Butylithium
in hexane was purchased from Aldrich. n-Butylsodium was pre-
pared using previously published methods.49 Diethyl ether, hexane
and toluene were freshly distilled from sodium benzophenone
ketyl, and TMEDA was distilled from CaH2 prior to use. 1H
127.4 [C(aryl)–H], 127.0 [C(aryl)–Me], 125.1 [C(aryl)–H], 121.1 [C(aryl)
–
CH2], 61.9 (NCH2CH2), 54.0 (NCH2–Ar), 41.2 (N–CH3), 20.6
(Ar–CH3), 16.0 (Ar–CH3).
Preparation of (1-Li2)2
1
and 13C{ H} NMR spectra were recorded at 400.13 MHz and
A flame-dried Schlenk tube was charged with 1-H2 (0.53 g, 1 mmol)
and dissolved in 5 mL of toluene. Two molar equivalents of n-
butyllithium solution (1.25 mL of 1.6 M solution in hexanes,
2 mmol) were added to the colourless solution. The reaction
mixture was stirred for 30 minutes, then approximately 50% of
the solution was removed in-vacuo. On cooling the solution to
−27 ◦C for 15 h, a crop of large, colourless, block-shaped crystals
of 1-Li2 were obtained (unoptimised yield: 0.23 g, 42%). 1H NMR
(400.13 MHz, C6D6, 296.2 K): d 7.45 (d, 2H, JHH = 2.4 Hz, Ar–H),
6.95 (d, 2H, JHH = 2.4 Hz, Ar–H), 3.97 (d, 2H, JHH = 11.6 Hz, Ar–
100.06 MHz respectively using a Bruker AMX400 or DPX400.
1
The chemical shifts for residual H and 13C resonances of the
C6D6 solvent were referenced to 7.16 and 128.0 ppm respectively.
X-Ray crystallography
Crystal data and other information on the structure refinements
are given in Table 4. Data for (1-Li2)2 and [2-Na2·(OEt2)]2 were
collected on a Nonius KappaCCD diffractometer, and those for
[2-Li2·(TMEDA)] on a Bruker SMART 1K diffractometer, all with
CH2), 2.76 (d, 2H, JHH = 11.6 Hz, Ar–CH2), 2.63 (d, 2H, JHH
=
˚
Mo-Ka radiation (k = 0.71073 A), using standard procedures and
8.9 Hz, Ar–CH2–N–CH2), 1.60 (s, 6H, N–CH3), 1.44 [s, 18H, Ar–
C(CH3)3], 1.41 [s, 18H, Ar–C(CH3)3], 1.28 (d, 2H, JHH = 11.6 Hz,
software.50,51 The structures were solved by direct methods and
refinement was on F2 values for all unique data in each case.52
Ar–CH2). 13C NMR (100.63 MHz, C6D6, 300 K): d 161.6 [C(aryl)
–
O], 138.5 [C(aryl)–tBu], 136.8 [C(aryl)–tBu], 128.0 [C(aryl)–H, observed
using HSQC experiment], 126.4 [C(aryl)–CH2], 124.4 [C(aryl)–H], 63.2
(NCH2–Ar), 50.3 (NCH2CH2), 42.6 (N–CH3), 35.3 [Ar–C(CH3)3],
34.1 [Ar–C(CH3)3], 32.1 [Ar–C(CH3)3], 30.5 [Ar–C(CH3)3].
Preparation of N,Nꢀ-bis(2-hydroxy-3,5-tert-butyl)-N,Nꢀ-
dimethylethane-1,2-diamine (1-H2)
1-H2 was prepared using a modified Mannich reaction53,54 by
combining N,Nꢀ-dimethylethylenediamine (5.32 mL, 50 mmol),
formaldehyde solution (38 wt% in H2O) (8.7 mL, 110 mmol),
and 2,4-di-tert-butylphenol (20.6 g, 100 mmol) in methanol.53
The resultant mixture was heated to reflux for 18 h, and 1-
H2 was collected by filtration and washed with cold methanol.
Typical yield: 20.3 g (77%). 1H NMR (400.13 MHz, C6D6, 300 K):
d 10.77 (br s, 2H, OH), 7.50 (d, 2H, JHH = 2.5 Hz, Ar–H),
6.88 (d, 2H, JHH = 2.5 Hz, Ar–H), 3.26 (s, 4H, ArCH2N),
2.12 [br s, 4H, N(CH2)2], 1.78 (s, 6H, N–CH3), 1.70 [s, 18H,
C(CH3)3], 1.36 [s, 18H, C(CH3)3]. 13C NMR (100.63 MHz, C6D6,
Preparation of [2-Li2·(TMEDA)]
A flame-dried Schlenk tube was charged with 2-H2 (0.36 g, 1 mmol)
and dissolved in 3 mL of hexane. Two molar equivalents of n-
butyllithium solution (1.25 mL of 1.6 M solution in hexanes,
2 mmol) were added to the colourless solution. The reaction
mixture was stirred for 2 h. TMEDA (0.31 mL, 2 mmol) was
added to the mixture to produce a thick white suspension before
the majority of the volatiles were removed in-vacuo. Toluene
(5 mL) was added and the suspension was strongly heated to
produce a colourless homogeneous solution. The solution was
cooled slowly in a Dewar flask filled with hot water. After 15 h,
large, colourless, block-shaped crystals of [2-Li2·(TMEDA)] were
obtained (unoptimised yield: 0.23 g, 48%). 1H NMR (400.13 MHz,
D5-pyridine, 300 K): d 7.07 (br s, 2H, Ar–H), 6.90 (br s, 2H, Ar–
H), 2.90–2.68 (br, 2H, CH), 2.49 (br, 2H, CH), 2.40 (s, 4H, CH2,
TMEDA), 2.34 (s, 6H, Ar–CH3), 2.29 (br, 2H, CH), 2.21 (s, 6H,
Ar–CH3), 2.19 (s, 12H, N–CH3, TMEDA), 2.10 (s, 6H, N–CH3),
2.03 (br, 2H, CH). 13C NMR (100.63 MHz, D5-pyridine, 300 K):
298 K): d 155.4 [C(aryl)–OH], 141.2 [C(aryl)–tBu], 136.6 [C(aryl)
–
tBu], 124.0 [C(aryl)–H], 123.7 [C(aryl)–H], 122.0 [C(aryl)–CH2], 63.2
(NCH2CH2), 54.0 (NCH2–Ar), 41.4 (N–CH3), 35.7 [Ar–C(CH3)3],
34.7 [Ar–C(CH3)3], 32.4 [Ar–C(CH3)3], 30.4 [Ar–C(CH3)3]. Mass
spectroscopic data: (FAB positive mode) m/z, 524.7 (M+, 100%),
261.7 (37%), 218.7 (83%), 202.7 (11%).
Preparation of N,Nꢀ-bis(2-hydroxy-3,5-methyl)-N,Nꢀ-
dimethylethane-1,2-diamine (2-H2)
2-H2 was prepared using a modified Mannich reaction53,54 by
combining N,Nꢀ-dimethylethylenediamine (5.32 mL, 50 mmol),
d 165.6 [C(aryl)–O], 136.0 [C(aryl)–Me], 131.7 [C(aryl)–H], 130.7 [C(aryl)
–
H], 126.4 [C(aryl)–Me], 118.5 [C(aryl)–CH2], 61.7 (CH2), 58.5 (CH2,
1300 | Dalton Trans., 2008, 1295–1301
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TMEDA), 55.8 (CH2), 46.2, 42.5 (CH3, TMEDA), 21.1 (Ar–CH3),
19.0 (Ar–CH3).
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Preparation of [2-Na2·(OEt2)]2
22 M. R. Haneline and A. F. Heyduk, J. Am. Chem. Soc., 2006, 128,
8410–8411.
A flame-dried Schlenk tube was charged with freshly prepared
BuNa (0.16 g, 2 mmol), 2-H2 (0.36 g, 1 mmol) and hexane (3 mL).
The resultant suspension was cooled to −78 ◦C, prior to the
addition of diethyl ether (2 mL). This solution was allowed to
stir for 2 h at ambient temperature. Small colourless crystals
were deposited (unoptimised yield: 0.33 g, 34%) after allowing
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1
the solution to stand for 15 h. H NMR (400.13 MHz, C6D6,
296.2 K): d 7.08 (br s, 2H, Ar–H), 7.02 (br s, 2H, Ar–H), 6.85 (br s,
2H, Ar–H), 6.83 (br s, 2H, Ar–H), broad region above baseline
4.2–1.8, 3.19 [q, 8H, OCH2CH3 (diethyl ether)], 2.37 [s, 6H, Ar–
CH3], 2.31 [s, 6H, Ar–CH3], 2.11, 2.02 (s, 6H, Ar–CH3), 1.95 (s,
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©