Monodentate coordination of the normally chelating chiral diamine (R,R)-TMCDA

Article abstract of DOI:10.1039/C6CC07190B

After isolating an unusual binuclear, but monosolvated NaHMDS complex [{(R,R)-TMCDA}·(NaHMDS)₂]_∞ which polymerises via intermolecular electrostatic Na...Me_HMDS interactions, further (R,R)-TMCDA was added to produce the dis

Full text of DOI:10.1039/C6CC07190B

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Monodentate coordination of the normally chelating chiral
diamine (R,R)-TMCDA†
Received 00th January 20xx,
Accepted 00th January 20xx
Ana I. Ojeda-Amador, Antonio J. Martínez-Martínez, Alan. R. Kennedy, David R. Armstrong and
Charles T. O’Hara*
DOI: 10.1039/x0xx00000x
www.rsc.org/
After isolating an unusual binuclear, but monosolvated NaHMDS with a recent study noting that it ‘displays no tendency to bind
complex [{(R,R)-TMCDA}·(NaHMDS)2]∞ which polymerises via as a monodentate ligand.’14 This has been attributed to the κ1
intermolecular electrostatic Na···MeHMDS interactions, further (or by implication η1) form of (R,R)-TMCDA inducing severe
(R,R)-TMCDA was added to produce the discrete binuclear amide steric strain due to the juxtaposition of the metal-NMe2 with
[κ2-{(R,R)-TMCDA}·(NaHMDS)2{κ1-(R,R)-TMCDA}], whose salient the uncoordinated NMe2 group. The structural chemistry of
feature is the unique monodentate coordination of one of the alkali metal amide complexes continues to be an important
chiral diamine ligands.
topic of research.15 We have recently discovered that lithium
and sodium 1,1,1,3,3,3-hexamethyldisilazide (LiHMDS and
NaHMDS) can capture alkali metal halide salts in the presence
of donor ligands to form ion pair metal anionic crown (MAC)
complexes, for example [Li·{(R,R)-TMCDA}2]+[Li5HMDS5Cl]−.4f, 16
A key starting material which remained hitherto elusive in our
studies involving sodium is the (R,R)-TMCDA-solvated NaHMDS
complex. Crystallisation of other donor ligated [e.g.,
Me6TREN17 and (−)-sparteine18] NaHMDS complexes has
Chiral diamine ligands, for example (−)-sparteine, its (+)-
and
sparteine
surrogate
N,N,N',N'-(1R,2R)-
tetramethylcyclohexane-1,2-diamine [(R,R)-TMCDA] have
attracted considerable attention in asymmetric synthesis in a
whole host of transition metal catalysed methodologies.1 From
an s-block perspective, when paired with an organolithium
reagent it can be envisaged that ‘chiral carbanions’ are
created, which can be used in subsequent enantioselective
syntheses.2 Focusing particularly on the C2-symmetric ligand
(R,R)-TMCDA, it has come to prominence recently as the
availability of the historically more widely utilised diamine (−)-
sparteine, has been unreliable over the past few years.3 In
terms of its coordination chemistry, (R,R)-TMCDA has
worldwide interest and has been well studied. Over 50 metal
complexes containing its ligated form have been reported,
spanning both the s- (Li,4 Na,4e K,4e and Mg,5) and d-block
metals (Cu,6 Zn,7 Ru,8 Pd,9 Pt10 and Hg11). Within s-block
chemistry and germane to this work, Strohmann has
comprehensively studied (R,R)-TMCDA complexes of
synthetically important organolithium reagents (such as
tBuLi4a, MeLi,4b iPrLi,4b, sBuLi4b, nBuLi4c, BH3P(Ph)(Me)CH2Li4d,
MeLi4g, PhLi,4h (allyl)Li4h and (benzyl)Li4i derivatives). An all-
encompassing feature of all known structures is that the chiral
diamine ligand adopts exclusively a κ2-bidentate chelating
mode. Due to the less flexible, fixed bite angle in (R,R)-TMCDA,
with respect to that of N,N,N',N'-tetramethylethylenediamine
(TMEDA),12 it is a stronger chelating ligand than the latter,13
proven difficult, although the polymeric TMEDA [(μ-
19
TMEDA)·(NaHMDS)2]∞
and
N,N,N′,N′-
tetramethylpropanediamine
(TMPDA)
[(μ-
20
TMPDA)·(NaHMDS)2]∞ complexes, which propagate via the
non-chelating diamine ligand, are known (Fig. 1). These have
similar structural motifs to Williard’s lithium diisopropylamide
(LDA) complex [(μ-TMEDA)·(LDA)2]∞.19arlie
In an effort to prepare the (R,R)-TMCDA complex of
NaHMDS, an equimolar mixture of NaHMDS and (R,R)-TMCDA
was combined in n-hexane medium and left to stir at ambient
temperature for 1 hour (Scheme 1). The reaction mixture was
then cooled to −33°C and crystals suitable for X-ray
crystallographic analysis deposited after 48 hours (27% non-
optimised, crystalline yield; maximum yield 50% based on
(R,R)-TMCDA consumption). X-ray data reveal the mono-(R,R)-
Fig.
1 Structures of previously known polymeric [(μ-TMEDA)·(NaHMDS)2]∞
and[(μ-TMPDA)·(NaHMDS)2]∞
.
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with a methyl group from each HMDS ligand [Na1···C12
2.987(4) and Na1···C22 2.987(4) Å]. The second Na metal
centre (Na2) remains only two-coordinate with respect to the
bridging amido N atoms. To satisfy this electron deficiency,
Na2 engages
a
solitary intermolecular Na···Me(SiMe2)
electrostatic interaction (Fig. 2b), which is short in comparison
to known literature examples [range Na···Me(SiMe2) 2.763(4)-
2.901(3) Å].21 This sole intermolecular Na···Me interaction
induces propagation of binuclear units in a zigzag polymer
chain. This change in the coordination chemistry of (R,R)-
TMCDA in
1 with respect to the bridging TMEDA and TMPDA
ligands in the aforementioned polymeric sodium amides
emphasises the propensity for the chiral 1,2-diamine to remain
as a chelating ligand rather than binding in a monodentate
fashion. As a consequence of this coordination mismatch,
significantly shorter Na2-NHMDS bonds (mean distance, 2.356 Å)
are observed when compared with Na1-NHMDS bonds (mean
Scheme
TMCDA}·(NaHMDS)2{κ1-(R,R)-TMCDA}] 2.
1 Syntheses of [{(R,R)-TMCDA}·(NaHMDS)2]∞ 1
and [{κ2-(R,R)-
distance, 2.530 Å). Despite utilising
NaHMDS:(R,R)-TMCDA in this synthesis, it is clearly evident
that the ultimate ratio in is 2:1. When this optimised ratio is
was again the sole product isolated
a 1:1 ratio of
TMCDA, binuclear [{(R,R)-TMCDA}·(NaHMDS)2]∞
There are six crystallographically distinct but essentially
chemically equivalent molecules of [{(R,R)-
TMCDA}·(NaHMDS)2] in the structure of , thus for brevity only
one is discussed here. Interestingly, the empirical formula of
1 (Fig. 2a).
1
used in the synthesis,
(36% crystalline yield).
1
1
1,
Complex
which contains an unsolvated Na site. Bochmann revealed the
mono(tetrahydrofuran), mono(THF), complex
1 is a rare example of a solvated sodium amide
i.e., [(donor)·(NaHMDS)2] is identical to that for the
aforementioned TMEDA and TMPDA derivatives; however, in
keeping with previously known (R,R)-TMCDA complexes, the
diamine adopts a chelating bonding mode, and with respect to
the N donor atoms, renders one Na metal centre (Na1) four-
coordinate in a distorted tetrahedral arrangement (bond
angles range from 68.70(9) to 151.55(10)°, see ESI† for full
details). Additionally, Na1 has two long Na···Me interactions
[(THF)·(NaHMDS)2] where one Na atom is two coordinate
whilst the other binds to the ether to render it three
coordinate.22 Interestingly, seven years prior to this report
Dehnicke published the bis(THF) analogue [(THF)2·(NaHMDS)2]
where both Na atoms are three coordinate.23 This begged the
question: ‘could the coordinatively unsaturated (Lewis acidic)
Na atom in 1, act as a host for another Lewis base?’
A logical route to address this question would be to utilise
monodentate donors such as THF and diethylether, in an
attempt to saturate the deficient metal centre; but, it is highly
likely that these strong σ–donors would also displace the
chelating (R,R)-TMCDA ligand. Therefore to maintain synthetic
simplicity, we repeated the preparation of
1 but employing an
excess (two molar equivalents) of (R,R)-TMCDA with respect to
NaHMDS in an attempt to coordinate a second molecule of the
Lewis base ligand to the donor-free metal centre. High quality
crystals (39 % crystalline yield) were obtained by storing the
resultant solution at −33°C for 24 h, which were analysed by X-
ray crystallography and were pleasingly found to be the target
bis(solvated) derivative [{κ2-(R,R)-TMCDA}·(NaHMDS)2{κ1-(R,R)-
TMCDA}]
sphere of Na1 in
66.90(6) to 151.05(8), see ESI†) is essentially identical to that
found in , exhibiting additional long contacts with a methyl
2
(Fig. 3). The distorted tetrahedral coordination
2
(bond angles around Na1 range from
1
group from each HMDS amido ligand [Na1···C27 2.968(3) and
Na···C24 2.976(3) Å]. However, the second sodium metal
centre, Na2, is additionally coordinated to an extra molecule of
(R,R)-TMCDA, giving rise to
geometry. As such there are two distinct coordinated diamine
ligands within the structure of . Undoubtedly, the most eye-
a distorted trigonal planar
Fig. 2 a) Molecular structure of [{(R,R)-TMCDA}·(NaHMDS)2]∞ 1 showing one
molecule from the asymmetric unit. Hydrogen atoms omitted for simplicity and
thermal ellipsoids are displayed at 35% probability. b) Section of the zigzag
polymeric chain of 1. The dashed lines illustrate Na···Me(SiMe2) interactions. The
symmetry operation used to generate the atoms labelled with ' is -x+1,y+1/2,-
z+1.
2
catching feature is that one (R,R)-TMCDA ligand adopts a
previously unseen κ1-coordination mode. To change from a κ2-
2 | J. Name., 2012, 00, 1-3
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structure of
2
was to be retained in solution, two unique sets
of (R,R)-TMCDA resonances would be expected. In reality a
single set of resonances (at δ2.06, 1.99, 1.51 and 0.80 in C6D6
solution) is observed. This indicates that a single (R,R)-TMCDA
environment exists at 300K in arene solution, indeed, a
variable temperature NMR spectroscopic study of
2 in [D8]-
toluene solution unveiled that this situation was maintained
even at low temperature (down to 206 K, see ESI†). In
1
addition, H and 13C NMR spectra obtained in non-polar [D12]-
cyclohexane also reveal this situation (see ESI†). Therefore due
to the steric bulk of the HMDS ligands within the molecule
[thus precluding a dual κ2- situation for the (R,R)-TMCDA
ligands], it is likely that the spectra show a time-averaged
situation between dynamic κ1- and κ2- coordinated (R,R)-
TMCDA ligands.
Fig. 3 Molecular structure of [κ2-{(R,R)-TMCDA}·(NaHMDS)2{κ1-(R,R)-TMCDA}] 2.
Hydrogen atoms and one disordered component of the mono-dentate (R,R)-
TMCDA ligand are omitted for simplicity. Thermal ellipsoids are displayed at 35%
probability.
In closing, we have shown that counter to previous studies
which suggested otherwise, (R,R)-TMCDA can indeed bind to
an alkali metal in a non-chelating κ1-manner.
to a κ1-coordination mode, it appears that inversion of the N1
atom of the (R,R)-TMCDA has occurred, no longer allowing the
ligand to chelate to Na2 (Fig. 3).
Complex 2 is a discrete dimeric entity, despite the potential
availability for N2 to coordinate further. In theory, this could
be achieved if this N atom could also invert thus allowing an
additional exo-coordination site; however, it is unlikely that
this would occur due to high steric strain (buttressing).14 The
κ1-coordinated (R,R)-TMCDA is disordered over two domains,
but its atomic connectivity and geometry are unequivocal. The
κ2- and the hitherto unseen κ1-coordination mode (R,R)-
Notes and references
This work was supported by the EPSRC (through a Career
Acceleration Fellowship to CTOH, EP/J001872/1 and
EP/L001497/1). The authors would like to thank Professors
Mulvey and Hevia, and Dr Robertson for useful discussions. The
research data associated with this paper is openly available at
http://dx.doi.org/10.15129/103ad89d-550d-44ad-8aa8-
62d54c47e3ac.
TMCDA observed in
2 can be compared with DFT calculations
(at the B3P86/6-311+G∗ level) performed for its diamine
relative (−)-sparteine (Fig. 4).24 It has been shown that when
(−)-sparteine binds to a metal complex, it always adopts a
chelating ‘cis’ configuration. However, in the absence of a
metal complex, it is actually slightly more stable (by 3.4 kcal
mol−1) in a ring-flipped ‘trans’ configuration [akin to our κ1-
coordinated (R,R)-TMCDA] where the lone pairs of electron
present on the N atoms are not adjacent to each other. We
have performed similar DFT studies (ESI†) on (R,R)-TMCDA and
have shown that there is negligible difference (less than 1 kcal
mol-1) between the potentially κ1- and κ2-coordination modes.
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solution appeared to correspond to a metallo-coordinated
ligand (see ESI† for full details). For
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appears at δ0.31 in the same solvent. If the solid state
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Fig. 4 Relative stabilities of cis and trans isomers of uncoordinated (−)-
sparteine.24
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