Unexpected neutral aza-macrocycle complexes of sodium

Article abstract of DOI:10.1039/c4cc01407c

Highly unusual Na⁺ complexes with neutral tri- and tetra-amines are isolable in good yield from the reaction of NaBAr^F with the amine in organic media. Structural characterisation reveals primary Na-N bonding, including an unusual sa

Full text of DOI:10.1039/c4cc01407c

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Unexpected neutral aza-macrocycle complexes of
sodium†
Cite this: Chem. Commun., 2014,
50, 5843
Matthew Everett, Andrew Jolleys, William Levason, David Pugh and Gillian Reid*
Received 24th February 2014,
Accepted 15th April 2014
DOI: 10.1039/c4cc01407c
www.rsc.org/chemcomm
Highly unusual Na+ complexes with neutral tri- and tetra-amines are have been reported: Noth and co-workers coordinated 1,5,9-trimethyl-
isolable in good yield from the reaction of NaBArF with the amine in 1,5,9-trizacyclododecane to NaBH4, and Okuda et al. obtained
organic media. Structural characterisation reveals primary Na–N an unexpected complex of NaI with 1,4,7-trimethyl-1,4,7,10-
bonding, including an unusual sandwich cation [Na(Me3tacn)2]+, tetraazacyclododecane (Me3cyclen).4
¨
derived from homoleptic N6-coordination via two Me3-tacn ligands,
and the distorted 5-coordinate [Na(thf)(Me4cyclam)]+.
As part of our recent work on coordination complexes of
silicon5 and germanium6 as potential precursors for supercritical
fluid electrodeposition (SCFED) processes,7 we have been utilising
Group 1 metals form hard metal cations with a very strong affinity the sodium salt of tetrakis(3,5-bis{trifluoromethyl}phenyl)borate
for both anionic and neutral O-donor ligands. This includes the (hereafter referred to as [BArF]ꢀ) both as a supporting electrolyte
crown ether macrocycles, with the size of the binding cavity in the for use in supercritical fluids7b,c and as a halide abstractor in the
macrocycle dictating which alkali metal cation is preferentially presence of neutral N-donor ligands (Scheme 1). Unexpectedly, we
coordinated.1 These complexes are much more soluble in organic found that these N-donor ligands have a high affinity for Na+
solvents than typical Group 1 salts, hence crown ethers often under certain conditions. This resulted in coordination of the
find application as phase transfer catalysts.1c Additionally, by polyamine to Na+ rather than halide abstraction from SiCl4 or
encapsulating the Group 1 cation inside a hydrophobic shell and [GeCl2(1,4-dioxane)]. From the reaction of SiCl4, NaBArFꢁ2(thf) and
eliminating cation–anion interactions, the nucleophilicity of the N,N,N0,N00,N00-pentamethyldiethylenetriamine (pmdta) we were
associated anion can be increased, leading to increased utility in able to isolate compound 1, [Na(pmdta)(thf)(k1-BArF)], as colour-
organic transformations.1d
less crystals suitable for X-ray diffraction (Fig. 1).
N-donor macrocycles can impart extra stability to alkali metal
It was also possible to synthesise compound 1 directly, by
complexes when compared to the analogous crown ethers. Dye adding a solution of the ligand in CH2Cl2 to a suspension of
et al. used an alkylated N-donor cryptand to encapsulate sodium NaBArFꢁ2(thf) in CH2Cl2.8 Complete dissolution occurred upon
cations as the key part of a system of room temperature stable addition of the ligand and 1 was isolated as a white solid via
electrides.2a It is notable that cryptands containing O-donors precipitation with hexane.
form electrides which are unstable above ꢀ60 1C owing to
Compound 1 crystallises in the monoclinic space group Cc
cleavage of the C–O functionalities.2b Despite this, compounds with evidence for positional disorder present in the coordinated
of neutral N-donor macrocyclic ligands with alkali metal cations thf molecule as well as in some of the CF3 groups of the anion.
other than Li+ are not well studied. Several examples exist where The latter is not unusual in weakly coordinating anions which
N-donor macrocycles have been functionalised with anionic or contain CF3 groups, but the [BArF]ꢀ anion in particular is
neutral donor pendant arms and coordinated to alkali metal notorious for being disordered in its solid state structures.9
cations,3 but there are no structurally authenticated examples of Nonetheless, the structure of the molecule is unambiguous with
neutral N-donor macrocycles coordinating to K+, Rb+ and Cs+.
Further, only two structurally authenticated examples with Na+
School of Chemistry, University of Southampton, Highfield, Southampton,
SO17 1BJ, UK. E-mail: G.Reid@soton.ac.uk; Tel: +44 (0)23 8059 3609
† Electronic supplementary information (ESI) available: Full experimental details,
plus all the crystallographic and analytical data for all products. CCDC 987718–
Scheme 1 Unexpected formation of 1 from the attempted use of NaBArF
as a halide abstractor. B = remainder of [BArF]ꢀ anion (omitted for clarity).
987721. For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4cc01407c
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Fig. 1 ORTEP representation of 1 with ellipsoids at 50% probability. H atoms
and positional disorder of three CF3 groups and the thf ring are omitted for
clarity. The CF3 group interacting with Na1 is disordered and only one refined
position for the CF3 group (containing F15a) is shown. Selected bond lengths
(Å): N1–Na1 2.473(5), N2–Na1 2.452(5), N3–Na1 2.477(5), O1–Na1 2.262(5),
F15a–Na1 2.60(1), F15b–Na1 2.349(6) (not shown).
Fig. 2 ORTEP representation showing one of two crystallographically distinct
cations in the asymmetric unit of compound 2b. A picture of the major disorder
component from the other cation is shown in the ESI† (Fig. S1). Thermal
ellipsoids at 50% probability. The BArF anion, which does not interact with the
Na+ centre, and the H atoms are omitted for clarity. Selected bond lengths (Å):
N1–Na1 2.554(5), N2–Na1 2.527(5), N3–Na1 2.545(5) (this cation not shown),
N4–Na2 2.507(4), N5–Na2 2.600(5), N6–Na2 2.567(5). Range of cis N–Na–N
angles within the same macrocyclic ring (1): 70.5(2)–72.0(2), between
neighbouring rings: 108.1(2)–109.5(2). Symmetry code: ꢀx + 1/2, ꢀy + 1/2, ꢀz.
one tridentate pmdta ligand, one thf molecule and one k1 Naꢁ ꢁ ꢁF
interaction from a CF3 group completing a 5-coordinate, distorted
square-based pyramidal geometry at Na+. The Na–N bond distances
are significantly longer than the Na–O bond distance, reflecting the half cations and one disordered anion comprising the asymmetric
strong oxophilicity of Na+, but they are among the shorter range of unit. One half-cation also contained positional disorder of the
known Na–N bond lengths in the CSD.10 The k1 interaction ethylene linkers within the macrocyclic ring, but the nitrogen
between a [BArF]ꢀ CF3 group and Na+ is only the second example atoms were not disordered and the coordination environment
of [BArF]ꢀ interacting with any metal,11 although the positional around both half cations is clearly that of a sandwich complex
disorder of the CF3 group in compound 1 precludes an accurate with two macrocycles surrounding one Na+ cation. Despite the
comparison of NaꢁꢁꢁF bond distances. However, the observed oxophilicity of the alkali metal cations, no thf is retained in the
NaꢁꢁꢁF distances in 1 are only slightly longer than the sum of the structure, presumably due to the strong preference for the Me3tacn
ionic radii for Na+ and Fꢀ (1.02 and 1.33 Å respectively), and well ligand for tridentate coordination (Fig. 2).
within the sum of van der Waals radii for Na and F (3.96 Å).12
The Na–N bond distances for 2b are slightly longer (by B0.1 Å)
Analysis of the product (1H and 13C{1H} NMR spectroscopy, than those in 1. This may simply be due to the increased coordina-
elemental analysis) supported the formulation observed crystallo- tion number at Na+. The solution 23Na NMR spectrum shows a
graphically, although it is noteworthy that the signals for the singlet at +6.2 ppm.
triamine ligand in the 1H NMR spectrum were very broad at room
It is notable that 2b is only the second structurally
temperature as a result of the amine-coordinated cations under- characterised example of a sandwich complex of any metal
going fast ligand exchange in solution, as is typical of labile alkali with Me3tacn as ligand. The other was reported by Wieghardt
metal complexes. Solution 23Na NMR spectroscopy showed a signal et al. with Ag(I), which has a larger ionic radius than Na(I)
at +2.1 ppm (relative to a 0.1 mol dmꢀ3 solution of NaCl in D2O), (1.15 Å and 1.02 Å, respectively, for 6-coordination).13 The cis
which is B6 ppm to high frequency of NaBArFꢁ2(thf) (ꢀ4.8 ppm in N–Na–N angles around Na are significantly distorted away from
CH2Cl2) and reflects the difference between a homoleptic O-donor the ideal 901; those within a ring are about 401 smaller than
environment at Na and one with significant N-donation.
those between neighbouring macrocycles and this is almost
Following the new procedure,8 two complexes of NaBArF with the certainly due to steric repulsion between the methyl groups on
macrocycle 1,4,7-trimethyl-1,4,7-triazacyclononane (Me3tacn) were alternating macrocyclic rings causing the Na–N bond distance to
also synthesised using a 1 : 1 ratio of macrocycle to Na (2a) and a increase, thus reducing the intra-macrocycle bond angles. This is
2:1 ratio (2b). Whilst crystals of compound 2a did not give a viable consistent with the Ag(I) structure reported by Wieghardt which
X-ray data set, spectroscopic analysis indicated that a product with also showed a large disparity between the inter-ring and intra-
similar composition to compound 1 was obtained and elemental ring N–M–N angles.
analysis supported the formulation [Na(Me3tacn)(thf)][BArF].
We observed that Me3tacn was readily protonated in the
1H NMR spectroscopy at room temperature showed broad presence of even trace water; one attempted synthesis of 2a was
resonances for the macrocycle protons. The 23Na NMR shift inadvertently exposed to the atmosphere forming the hydrolysis
of +3.7 ppm is similar to that observed from compound 1, product [Me3tacnH][BArF] (see ESI†). This suggests that judicious
consistent with N-donor atom coordination to Na+.
choice of conditions (solvent and anion) is required in order to
Compound 2b, [Na(Me3tacn)2][BArF], crystallises in the access the potentially very rich coordination chemistry of alkali
monoclinic space group C2/c with two symmetry-independent metal cations with a wider range of Lewis bases.
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NaBArF readily forms complexes with N-donor ligands in pre-
ference to abstracting a halide from Lewis acids such as SiCl4
and [GeCl2(1,4-dioxane)], and the complexes have also been
synthesised without the Lewis acid. The complexes exhibit a
rich range of structural interactions, including a very rare
example of two Me3tacn macrocycles forming a sandwich
complex and only the second example of a [BArF]ꢀ anion
interacting with a metal. The successful formation of these
new species may be a result of the low lattice energy of NaBArF
when compared to other Group 1 ionic salts, coupled with
the high solubility of the [BArF]ꢀ anion in organic media which
do not compete with the N-donor ligand, and suggests that
judicious choice of solvent and anion may lead to the unveiling
of a rich area of coordination chemistry of softer donor
ligands with hard Group 1 cations. Further investigations into
extending this study to other alkali metal cations and ligands
are underway.
Fig. 3 ORTEP representation of the cation of compound 3. Thermal ellipsoids
at 50% probability. The BArF anion, which does not interact with the Na+ centre,
and the H atoms are omitted for clarity. Selected bond lengths (Å) and angles (1):
N1–Na1 2.438(2), N2–Na1 2.454(2), N3–Na1 2.451(2), N4–Na1 2.473(2),
O1–Na1 2.313(2). N1–Na1–N3 123.91(7), N2–Na1–N4 150.33(7), N1–Na1–N2
77.94(7), N1–Na1–N4 89.49(7), N2–Na1–N3 88.00(6), N3–Na1–N4 76.82(6).
This work was funded by EPSRC (through a Programme
Grant EP/I033394/1 and also through EP/I010890/1). The
Reaction of NaBArFꢁ2(thf) with the tetradentate N-donor SCFED Project (www.scfed.net) is a multidisciplinary collabora-
macrocycle, Me4cyclam (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclo- tion of British universities investigating the fundamental and
tetradecane) yielded colourless crystals of 3. Structural analysis applied aspects of supercritical fluids.
(Fig. 3) revealed a tetradentate coordination mode for the macro-
Notes and references
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molecule completing a distorted 5-coordinate geometry at sodium.
The sodium cation lies above the N4-plane by 0.889(1) Å, reflecting
a mismatch between the Na+ cation size and the macrocyclic
binding cavity. The t value14 of 0.44 indicates that the geometry
at Na+ is almost half-way between ideal square based pyramidal
and ideal trigonal bipyramidal. The Na–O bond length is about
0.05 Å longer than the corresponding distance in compound 1,
while the Na–N bond lengths are identical to 1 (within experimental
error) and they are also in accord with the Na–N bond lengths
found in [Na(Me3cyclen)I], the only other neutral tetradentate
macrocyclic ligand coordinated to sodium.4b
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8 A typical procedure is as follows: to a suspension of NaBArFꢁ2(thf)
(240 mg, 0.23 mmol) in anhydrous CH2Cl2 (5 mL) was added a
solution of the ligand (1 mol eq.) in CH2Cl2 (2 mL). The solution was
stirred under N2 for 4 h, then layered with anhydrous hexane
(30 mL) to produce colourless crystals. Full experimental details,
crystallographic and analytical data for all products are contained in
the ESI†.
We have shown that neutral alkylated multidentate N-donor
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5846 | Chem. Commun., 2014, 50, 5843--5846
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