Exploiting σ/π coordination isomerism to prepare homologous organoalkali metal (Li, Na, K) monomers with identical ligand sets

Article abstract of DOI:10.1002/chem.201003493

Tetraamine Me₆TREN has been used as a scaffold support to provide coordinative saturation in the complexes PhCH₂M· Me₆TREN (M=Li, Na, K). The Li derivative displays a Li-C σ interaction with a pyramidalized CH₂ both in the solid state and in solution, and represents the first example of I·⁴ coordination of Me₆TREN to lithium. In the sodium derivative, the metal cation slips slightly towards the delocalized π electrons whilst maintaining a partial σ interaction with the CH₂ group. For the potassium case, coordinative saturation successfully yields the first monomeric benzylpotassium complex, in which the anion binds to the metal cation exclusively through its delocalized π system resulting in a planar CH ₂ group. Easy as pi: Me₆TREN acts as a coordinatively saturating scaffold for the complexes PhCH₂M (M=Li, Na, K). As the size of the alkali metal increases, the solvated metal cation gradually slips from the lateral 'carbanionic' carbon to the aromatic π system, enabling the complex to maintain its monomeric constitution. Me₆TREN=tris(N,N- dimethyl-2-aminoethyl)amine. Copyright

Full text of DOI:10.1002/chem.201003493

DOI: 10.1002/chem.201003493
Exploiting s/p Coordination Isomerism to Prepare Homologous
Organoalkali Metal (Li, Na, K) Monomers with Identical Ligand Sets
Matthew G. Davidson,*[a] Daniel Garcia-Vivo,[a] Alan R. Kennedy,[b]
Robert E. Mulvey,*[b] and Stuart D. Robertson*[b]
3364
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Chem. Eur. J. 2011, 17, 3364 – 3369

FULL PAPER
Abstract: Tetraamine Me6TREN has
been used as a scaffold support to pro-
vide coordinative saturation in the
complexes PhCH2M·Me6TREN (M=
Li, Na, K). The Li derivative displays a
lithium. In the sodium derivative, the
metal cation slips slightly towards the
delocalized p electrons whilst maintain-
ing a partial s interaction with the CH2
group. For the potassium case, coordi-
native saturation successfully yields the
first monomeric benzylpotassium com-
plex, in which the anion binds to the
metal cation exclusively through its de-
localized p system resulting in a planar
CH2 group.
ꢀ
Li C s interaction with a pyramidal-
Keywords: alkali metals · benzyl
anion · coordination isomerism ·
monomer · polyamines
ized CH2 both in the solid state and in
solution, and represents the first exam-
ple of h4 coordination of Me6TREN to
Long indispensable to synthetic practitioners of chemistry,
simple organolithium (and increasingly organo alkali metal)
compounds have a rich, diverse structural chemistry.[1] In so-
lution, where these compounds are routinely employed as
reagents,[2] multiple species involving aggregration and/or
solvation as well as assorted dynamic processes can be in ex-
istence,[3] making structure–reactivity correlation extremely
complex and difficult to untangle. Such complexes, of gener-
al formula [RM]x·nL, can adopt a prodigious variety of
chain or cyclic structures falling between the two extremes
of monomer (x=1) and polymer (x=1), with the degree of
aggregation generally being inversely proportional to the re-
activity.[4] The factors governing the extent of aggregation
include the identity of the cation (M) and the anion (R), the
presence or absence of a neutral Lewis donating ligand (L),
the degree of ligation (n) and the size and denticity of R
and L. Given this number of influential components, it is
clear that changing just one variable can have a profound
effect on the others and thus on the final structure. This is
eloquently illustrated by n-butyl- and tert-butyllithium; on
changing from the linear to the more bulky branched alkyl
group, Stalke and Kottke showed that the solid-state struc-
ture of the unsolvated derivatives alters from a hexamer
(x=6) to a tetramer (x=4).[5] Further, the introduction of a
Lewis donor to the latter complex (L=Et2O) invokes a de-
[nBuLi·TMCDA]2ACHTNGUTERNNU[G nBuLi]2 (TMCDA=N,N,N’,N’-tetrame-
thylcyclohexane-1,2-diamine) from varying such ratio.[9] Of
course it is not prudent to surmise that larger anions or
larger polydentate donors will automatically result in small-
er aggregates since electronic factors will also play an impor-
tant role. While lithium complexes are often smaller (and
consequently more soluble) oligomers, heavier sodium and
potassium derivatives have a tendency to give larger insolu-
[10]
ble polymers; for example, hexameric LiCH2SiMe3
is
commercially available as a hexane solution yet its K conge-
ner is insoluble in the same medium.[11] Whereas judicious
choice of anion and Lewis donor can disrupt the polymeri-
zation of lithium species, it is much more arduous to do so
with Na or K complexes of simple organic anions.[12] As a
consequence of this there is a paucity of successful structural
studies pertaining to simple organopotassium complexes in
the literature. This general dichotomy between Li and K
chemistry is principally due to the fundamental differences
between these two monocations—among them, lithium has
a considerably greater charge density, forms s bonds to car-
banionic species, and can only accommodate
a small
number of neutral Lewis donors around it; whereas potassi-
um is softer, more polarizable, has a larger coordination
sphere and possesses a considerable affinity for p-electron
density.[13] The behavior of sodium can be considered inter-
mediate between these two and can fall into either category.
Consequently these differences encountered as one navi-
gates Group 1 means, to the best of our knowledge, there
are no known crystallographically characterized isomeric
molecular organo alkali-metal species in the literature.[14] To
that end, we were interested in the challenge of preparing a
family of isomeric Lewis donor stabilized monomers (i.e.,
homologues which only varied in M), which required careful
consideration over the component parts. As reported herein,
this challenge has now been unequivocally met to reveal
facets of the fundamental structure and bonding proclivities
of Li, Na, and K summarized above.
crease in aggregation yielding a dimer [tBuLi·Et2O]2.[5]
A
fine example of variable reactivity was unveiled by Mitzel
and co-workers who demonstrated that nBuLi and tBuLi de-
protonate 1,3,5-trimethyl-1,3,5-triazacyclohexane (TMTAC)
at the doubly deactivated aminal (NCH2N) position,[6] while
the same ligand acts only as a donor[7] to polymeric MeLi.[8]
The influence of complex to donor ratio (i.e. n) was exposed
most recently by a pair of structures [nBuLi·TMCDA]2 and
[a] Prof. M. G. Davidson, Dr. D. Garcia-Vivo
Department of Chemistry, University of Bath
Bath, BA2 7AY (UK)
E-mail: m.g.davidson@bath.ac.uk
The potential to coordinate via a s or p interaction made
the benzyl (PhCH2 ) anion appear attractive for such a
ꢀ
[b] Dr. A. R. Kennedy, Prof. R. E. Mulvey, Dr. S. D. Robertson
WestCHEM, Department of Pure and Applied Chemistry
University of Strathclyde, Glasgow, G1 1XL (UK)
Fax : (+44)141-548-4787
quest. While PhCH2Li has been shown to exist as a mono-
mer in the presence of either the tridentate cyclic donor
N,N’,N’’-trimethyl-1,4,7-triazacyclononane (Me3TACN)[15] or
the mixed donor system N,N,N’,N’-tetramethyl-1,2-ethylene-
diamine/tetrahydrofuran (TMEDA/THF),[16] the real chal-
E-mail: r.e.mulvey@strath.ac.uk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201003493.
Chem. Eur. J. 2011, 17, 3364 – 3369
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3365

R. E. Mulvey, M. G. Davidson, S. D. Robertson et al.
lenge would be to prevent polymerization of the sodium[17]
and potassium derivative by effecting coordinative satura-
tion.[18] Due to its versatile (h3[19] or h4[20]) denticity, and
recent application in Group 1 metal chemistry,[19c] tris(N,N-
dimethyl-2-aminoethyl)amine (Me6TREN)[21] seemed a logi-
cal choice of donor as it could act as a protective scaffold,
covering but not completely encapsulating a range of alkali
metals thus leaving space for coordination of the anion. Syn-
thetically, this was achieved by adding the tetraamine to a
colorless solution of LiTMP (TMP=2,2,6,6-tetramethylpi-
peridide) in toluene. The activating presence of the heteroa-
tomic additive induced metallation[22] of the solvent, which
was evident via an immediate color change to yellow/
orange. Slow evaporation of the solvent yielded a crop of
orange crystals of PhCH2Li·Me6TREN (1), which were sub-
jected to a single-crystal X-ray diffraction study.[23]
shorter Li Cipso [2.722(5) ꢁ] and Li Cortho [3.366(6) and
ꢀ
ꢀ
3.486(6) ꢁ] distances. The -CH2CH2NMe2 arms are inequi-
valent in the solid state due to the nonlinearity at C20 with
the aromatic ring displaced towards N2 and N3 and away
ꢀ
from N1. The Li N distances lie in the range 2.162(3)–
2.356(3) ꢁ with the average (2.250 ꢁ) noticeably greater
than
PhCH2Li·TMEDA·THF (2.148 ꢁ)[16] and PhCH2Li·DABCO
(2.10 ꢁ, DABCO=1,4-diazabicyclo
[2.2.2]octane),[25] which
that
in
PhCH2Li·TACN
(2.10 ꢁ),[15]
AHCTUNGTRENNUNG
can be attributed to the greater number of Lewis donors per
cation in 1.
For the heavier metal derivatives, addition of Me6TREN
to a toluene suspension of PhCH2M (M=Na, K, prepared
via a superbasic tBuOM/nBuLi mixture[26]) resulted in a ho-
mogeneous red solution. Slow diffusion of hexane into this
toluene solution at 58C yielded a crop of red crystals of
PhCH2Na·Me6TREN (2); cooling the toluene solution to
ꢀ358C was sufficient to yield a large crop of red crystals of
PhCH2K·Me6TREN (3). Elucidated by X-ray crystallogra-
phy, the molecular structure of 2 is displayed in Figure 2.[23]
The solid-state structure (Figure 1, pertinent bond param-
eters are presented in Table 1) shows that complex 1 consists
of discrete monomeric units as seen previously in
Figure 1. Molecular structure of one of the independent molecules of 1
with hydrogen atoms and toluene of crystallization omitted and thermal
ellipsoids drawn at 50% probability.
Figure 2. Molecular structure of 2 with hydrogen atoms and toluene of
crystallization omitted and thermal ellipsoids drawn at 50% probability.
Table 1. Selected bond parameters of complexes 1–3 (values for second
independent molecule of 1 are in brackets).
Like 1, the molecular structure of 2 is monomeric with a
five-coordinate trigonal-bipyramidal metal center, represent-
ing the first crystallographically characterized monomeric
benzylsodium complex.[27] The principal difference however,
is that although the sodium cation appears to be s-bonded
to the ꢂcarbanionicꢃ center, it has, together with its poly-
amine scaffold, started to migrate towards the aromatic ring.
This is clearly evidenced by the much less obtuse angle at
C20 [103.3(1)8], which brings the metal into much closer
proximity of the ipso-carbon atom. Indeed, while comparing
absolute bond lengths with 1 is futile due to the presence of
1
2
3
ꢀ
M Ca [ꢁ]
2.352(3) [2.335(3)]
3.467(3) [3.362(3)]
1.47 [1.44]
1.400(3) [1.400(2)]
133.4(2) [126.4(1)]
113.4(2) [113.4(1)]
2.556(1)
3.183(1)
1.24
1.397(2)
103.3(1)
113.4(1)
3.893(4)
3.098(4)
0.80
1.390(6)
46.2(2)
113.6(4)
ꢀ
M Cipso [ꢁ]
ꢀ
ꢀ
M Cipso/M Ca
ꢀ
Ca
Cipso [ꢁ]
M-Ca-Cipso [8]
Cortho-Cipso-Cortho [8]
PhCH2Li·Me3TACN but with the lithium center in an unusu-
al five-coordinate (trigonal-bipyramidal) environment, with
such coordinative saturation preventing polymerization.
This represents the first example of h4 coordination of
Me6TREN to a lithium cation, presumably facilitated by the
lack of steric bulk at the anionic moiety.[24] The structure
ꢀ
different alkali metals, comparing the ratios of the M
ꢀ
C
ipso:M Ca bond lengths is revealing. This value is 1.47 in 1
(1.44 for the second independent molecule) but is reduced
to 1.24 in 2, clearly demonstrating the movement of the
ꢀ
ꢀ
contains a Li C s bond and importantly shows no signifi-
alkali metal to a position where M p-bonding interactions
cant interactions between the metal and the phenyl ring, in
contrast to the mixed solvate PhCH2Li·TMEDA·THF,[16]
which has a considerably tighter C-C-Li angle [94.6(2)8] and
may play a role. Again, nonlinearity at C20 forces the
phenyl ring towards one of the arms of the ligand (N1)
making these three pendant groups inequivalent.
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Chem. Eur. J. 2011, 17, 3364 – 3369

Monomeric Alkali Metal Benzyl Complexes
FULL PAPER
Figure 3. Molecular structure of one of the independent molecules of 3
with hydrogen atoms omitted and thermal ellipsoids drawn at 50% prob-
ability. K···C p-arene contacts are denoted by dashed lines.
Figure 4. Superimposition of solid-state structures of
(yellow), and 3 (orange) with methyl carbon atoms and all protons omit-
ted from the Me6TREN molecule for clarity.
1 (purple), 2
The molecular structure of 3[23] (Figure 3) was also experi-
mentally determined and revealed the presence of two inde-
pendent molecules, one was disordered over the benzyl
anion and the Me6TREN molecule, however the other
showed no disorder whatsoever.
Mimicking 1 and 2, the Me6TREN ligand in 3 ligates to
the cation in a h4 manner via its four nitrogen lone pairs,
with the coordinative saturation successfully preventing
polymerization such as that previously found for the
PMDETA-[28] or THF-solvated[29] derivatives of benzylpotas-
sium. Remarkably, the potassium cation shows no interac-
tion with the CH2 arm [K···Ca separation distance=
3.893(4) ꢁ] but rather has slipped round accompanied by its
multidentate Lewis donor scaffold to bind exclusively to the
of the negative charge on Ca into the aryl ring. Delocaliza-
tion is generally negated by the charge localizing influence
of a highly charged cation, which is in close proximity to Ca
(i.e., as in 1) and is favored by larger cations (i.e., as in 3).
Such ring deformations have been comprehensively and elo-
quently described by Harder and co-workers.[30a,b,31] In par-
ꢀ
ticular, delocalized structures should display shortened Ca
ꢀ
ꢀ
Cipso and Cortho Cmeta bonds along with elongated Cipso Cortho
ꢀ
and Cmeta Cpara bonds. This consequently influences bond
angles, giving Cipso and Cpara angles smaller than the expect-
ed value of 1208 and larger angles at Cortho and Cmeta. Such
deformation clearly takes place in complexes 1–3 (Figure 5)
with the greater degree of deformation, and thus charge de-
localization, occurring in 3 as expected due to the lesser
proximity of the metal to Ca in accord with the NMR data
(vide infra).
delocalized
p system of the aromatic ring [3.097(4)–
3.250(4) ꢁ, av 3.156 ꢁ] resulting in a naked CH2 head.
While K–p interactions of this nature are precedented due
to the affinity of potassium for adopting multihapto environ-
ments,[30] this structure represents, to the best of our knowl-
edge, the first example of a benzylpotassium complex free
NMR spectral data suggest the structures of 1–3 are main-
tained in solution, with appropriate resonances for the aro-
matic, carbanionic, and ligand groups all accounted for
(Table 2). Despite the inequivalence of the -CH2CH2NMe2
arms in the solid state (vide supra), each environment is
of
a
cation–Ca
interaction.
For
example,
[PhCH2K·PMDETA]1 shows h3 coordination to the carba-
nionic [3.171(2) ꢁ], ipso-carbon [3.218(2) ꢁ] and one ortho-
carbon [3.297(2) ꢁ].[28] The bulkier triaryl-substituted
equivalent in solution due to free rotation around the M C
ꢀ
bond; indeed the two Me6TREN CH2 environments in 2 are
1
completely indistinguishable by H NMR spectroscopy.
ꢀ
Ph3CK·PMDETA·THF shows no K Ca interaction but this
may also be due to extensive electron delocalization pro-
moted by the presence of two other aromatic rings.[13b] The
potassium center of 3 lies in a distorted trigonal-bipyramidal
Table 2. NMR resonances of starting materials and complexes 1, 2, and 3
environment with the center of the benzyl
p system
(all values in ppm).
[K···centroid distance=2.8234(7) ꢁ] and N4 in the axial po-
C7H8/Me6TREN
1
2
3
1H
13C
1H
13C
1H
13C
1H
13C
ꢀ
sitions and N1 N3 in the equatorial positions. For a more
complete list of bond parameters of complexes 1–3 see the
Supporting Information. The slippage of the Me6TREN-pro-
tected metal cation is clearly illustrated in Figure 4, which
shows a superimposition of the three structures with the
benzyl anion kept in a constant position.
para
meta
ortho
ipso
7.05
7.13
7.02
–
125.6
128.5
129.3
137.8
21.4[a]
54.0
6.23 105.7 5.93 101.1 5.24 94.1
7.08 128.8 6.93 129.8 6.56 130.4
6.76 117.6 6.57 114.0 6.16 111.3
–
161.3
–
158.4
–
153.0
ꢀ
CH2
2.12[a]
2.08 37.8
1.86 51.3
1.97 57.3
2.00 45.5
2.51 41.2
1.78 50.8
1.78 57.0
2.00 45.1
3.21 56.2
2.02 52.3
2.08 57.4
2.04 45.1
CH2 (TREN) 2.40
CH2 (TREN) 2.66
CH3 (TREN) 2.14
A structural consequence of this movement by the alkali
metal away from the CH2 group and towards the aromatic p
system is a deformation of the C6 ring due to delocalization
58.7
46.0
[a] CH3.
Chem. Eur. J. 2011, 17, 3364 – 3369
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3367

R. E. Mulvey, M. G. Davidson, S. D. Robertson et al.
ꢀ
(M=Li, Na, K). The first product displays a Li C s interac-
tion with a pyramidalized CH2 group both in the solid state
and in solution and represents the first example of h4 coordi-
nation of Me6TREN to lithium. Moving to sodium, the
metal cation slips slightly towards the delocalized p elec-
trons whilst maintaining a partial s interaction with the CH2
group. Coordinative saturation successfully yields the first
monomeric benzylpotassium complex which, to the best of
our knowledge, is the smallest molecular weight molecular
organo-potassium compound ever to be characterized crys-
tallographically.[34] The anion binds to the metal cation ex-
clusively through its delocalized p system resulting in a
planar CH2 group. This begs the question, “is the principal
interaction in polymers of type PhCH2K·donor actually the
p interaction with polymerization facilitated via a secondary
s interaction rather than the other way round as convention-
ally thought?” We now intend to investigate if these mono-
mers display increased reactivity with respect to their oligo-
meric counterparts.
Experimental Section
All reactions were carried out under a protective argon atmosphere using
standard Schlenk techniques. Full details, including complete characteri-
zation, are given in the Supporting Information.
Synthesis of PhCH2Li·Me6TREN (1): LiTMP (0.294 g, 2 mmol) was dis-
solved in toluene (10 mL) and Me6TREN (0.53 mL, 2 mmol) was added
by using a syringe with stirring to yield an orange solution. This was
dried in vacuo to give an oily product. Fresh toluene (5 mL) was added
and this was removed very slowly under reduced pressure until a crop of
orange crystals slowly precipitated (0.395 g, 53%).
Figure 5. Bond parameters (in ꢁ and o) for the benzyl anion of 1 (top), 2
(middle), and 3 (bottom). The parameters of the second independent
molecule of 1 are within experimental error of those given and have thus
been omitted for clarity.
Synthesis of PhCH2Na·Me6TREN (2): PhCH2Na (0.114 g, 1 mmol) was
weighed into a clean dry Schlenk flask and suspended in toluene (8 mL).
Me6TREN was added dropwise until a homogeneous orange solution re-
sulted (0.26 mL, 1 mmol). Slow diffusion of hexane into this solution at
58C yielded a crop of red crystals (0.104 g, 30%).
NMR data, in particular 1J(13C–1H) coupling constants,
ACHTUNGTRENNUNG
Synthesis of PhCH2K·Me6TREN (3): PhCH2K (0.130 g, 1 mmol) was
weighed into a clean dry Schlenk flask and suspended in toluene (8 mL).
Me6TREN was added dropwise until a homogeneous orange solution re-
sulted (0.53 mL, 2 mmol). This solution was cooled to ꢀ308C overnight
to yield a crop of red crystals (0.236 g, 64%).
can be utilized to predict the amount of delocalization
1
ACTHNUTRGNEUNG
through the benzyl anion.[30b,32] The J(13C–1H) coupling con-
stant is extremely sensitive to the hybridization of the de-
protonated carbon atom. A pyramidalized (sp3) benzyl carb-
anion is expected to show such a coupling constant of
ꢀ
125 Hz, while the expected value for a planar (sp2) CH2
group is 167 Hz.[16] Complex 1 displayed a coupling constant
at the sp3 end of the scale (132 Hz), suggesting a pyramidal-
ized Ca in solution, consistent with the solid-state structure,
while that of complex 3 was 151 Hz[33] clearly more towards
the planar end of the scale. The value for 2 was 142 Hz, in-
termediate between the Li and K values.
Acknowledgements
This work was generously sponsored by the UK Engineering and Physi-
cal Science Research Council (award no. EP/F063733/1), the Royal Soci-
ety/Wolfson Foundation (research merit award to R.E.M.), and the Euro-
pean Commission by means of a grant from the Marie Curie Intra-Euro-
pean Postdoctoral Fellowship (PIEF-2009-235173) program (D.G.-V.).
We warmly thank Dr. Jan Klett for composing the frontispiece.
ꢀ
The chemical shift of the para C H is also highly indica-
tive of charge delocalization,[31] since as the metal migrates
away from the CH2 group and towards the aromatic ring,
the interaction between the metal and the aromatic system
increases. This is clearly the case in this series as Table 2
shows, with a considerable upfield shift occurring for the
para C and H resonances on going from 1 to 3.
[1] a) B. J. Wakefield, The Chemistry of Organolithium Compounds,
Pergamon, Oxford, 1974; b) A.-M. Sapse, P. von R. Schleyer, Lithi-
um Chemistry: A Theoretical and Experimental Overview, Wiley,
New York, 1995; c) J. Clayden, Organolithiums: Selectivity for Syn-
thesis Elsevier, Oxford, 2002; d) The Chemistry of Organolithium
Compounds, Part 2 (Eds.: Z. Rappoport, I. Marek), Wiley, New
York, 2004.
In summary, we have successfully used the tetraamine
Me6TREN as a scaffold support to provide coordinative sat-
uration in the congeneric complexes PhCH2M·Me6TREN
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Chem. Eur. J. 2011, 17, 3364 – 3369

Monomeric Alkali Metal Benzyl Complexes
FULL PAPER
[2] L. Brandsma, H. Verkruijsse, Preparative Polar Organometallic
Chemistry 1, Springer, Berlin, 1987.
ACHTUNGTRENNUNG
[3] For a seminal review of donor effects on lithium enolate aggrega-
tion, see: D. Seebach, Angew. Chem. 1988, 100, 1685; Angew. Chem.
Int. Ed. Engl. 1988, 27, 1624.
[4] B. Walfort, L. Lameyer, W. Weiss, R. Herbst-Irmer, R. Bertermann,
J. Rocha, D. Stalke, Chem. Eur. J. 2001, 7, 1417.
[5] T. Kottke, D. Stalke, Angew. Chem. 1993, 105, 619; Angew. Chem.
Int. Ed. Engl. 1993, 32, 580.
[6] D. Bojer, I. Kamps, X. Tian, A. Hepp, T. Pape, R. Frçhlich, N. W.
Mitzel, Angew. Chem. 2007, 119, 4254; Angew. Chem. Int. Ed. 2007,
46, 4176.
[23] Crystal data for 1: C20.75H39LiN4; an orange plate gave the triclinic
¯
space group P1, a=9.2836(3), b=14.3353(5), c=17.4903(6) ꢁ, a=
97.155(3), b=103.094(3), g=93.311(3)8, V=2240.6(1) ꢁ3, T=
123(2) K, Z=4, 1calcd =1.042 Mgmꢀ3, MoKa l==0.71073 ꢁ, R1=
0.0460 (for 5414 reflections with I>2s(I)) wR2=0.1104 and S=
0.890 for 507 parameters and 9771 independent reflections. Min/
Max residual electron density 0.284/ꢀ0.207 eꢁꢀ3. Crystal data for 2:
C26H45NaN4; a red plate gave the orthorhombic space group P212121,
a=9.3986(3), b=14.8466(5), c=19.2843(6) ꢁ, V=2690.9(1) ꢁ3, T=
123(2) K, Z=4, 1calcd =1.078 Mgmꢀ3
, MoKa l=0.71073 ꢁ, R1=
[7] I. Kamps, B. Neumann, H.-G. Stammler, N. W. Mitzel, Organometal-
lics 2010, 29, 4746.
0.0318 (for 5413 reflections with I>2s(I)) wR2=0.0682 and S=
0.950 for 294 parameters and 6582 independent reflections. Min/
Max residual electron density 0.167/ꢀ0.183 eꢁꢀ3. Crystal data for 3:
C19H37KN4; a red plate gave the monoclinic space group P21/c, a=
15.8222(5), b=18.4801(4), c=16.1656(5) ꢁ, b=109.962(4)8, V=
[8] MeLi forms a tetrameric heterocubane, which is linked to other het-
erocubanes in three dimensions by weak Li···Me interactions: E.
Weiss, G. Hencken, J. Organomet. Chem. 1970, 21, 265.
[9] C. Strohmann, V. H. Gessner, J. Am. Chem. Soc. 2008, 130, 11719.
[10] B. Tecle, A. F. M. M. Rahman, J. P. Oliver, J. Organomet. Chem.
1986, 317, 267.
[11] W. Clegg, B. Conway, D. V. Graham, E. Hevia, A. R. Kennedy,
R. E. Mulvey, L. Russo, D. S. Wright, Chem. Eur. J. 2009, 15, 7074.
[12] a) For an outstanding review of the structural chemistry of organo-
lithium compounds, see: W. N. Setzer, P. von R. Schleyer, Adv. Orga-
nomet. Chem. 1985, 24, 353. b) The structural chemistry of the
organo complexes of the heavier alkali metals has also received a
comprehensive treatise: C. Schade, P. von R. Schleyer, Adv. Organo-
met. Chem. 1987, 27, 169.
[13] M. Schlosser, “Organoalkali Reagents” in Organometallics in Syn-
thesis: A Manual (Ed.: M. Schlosser), Wiley, New York, 1994.
[14] The more complicated hetero alkali metal cluster series
[(tBuO)8Li4M4] (M=Na, K, Rb, Cs) are isomeric: D. R. Armstrong,
W. Clegg, A. M. Drummond, S. T. Liddle, R. E. Mulvey, J. Am.
Chem. Soc. 2000, 122, 11117.
4442.7(2) ꢁ3, T=123(2) K, Z=8, 1calcd =1.078 Mgmꢀ3
, MoKa l=
0.71073 ꢁ, R1=0.0735 (for 5210 reflections with I>2s(I)) wR2=
0.2145 and S=1.050 for 425 parameters and 8719 independent re-
flections. Min/Max residual electron density 0.888/ꢀ0.495 eꢁꢀ3
.
CCDC-790265 (1), CCDC-790266 (2), and CCDC-790267 (3) con-
tain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[24] The similar tetraamine tris((2-pyridyl)methyl)amine has been shown
by a molecular structure determination to bind in a h4 fashion to LiI
although this is not a suitable ligand for organo-alkali metal com-
plexes due to the high CH acidity of the ligand backbone: S. K.
Brownstein, P.-Y. Plouffe, C. Bensimon, J. Tse, Inorg. Chem. 1994,
33, 354.
[25] S. P. Patterman, I. L. Karle, G. D. Stucky, J. Am. Chem. Soc. 1970,
92, 1150.
[26] M. Schlosser, J. Hartmann, Angew. Chem. 1973, 85, 544; Angew.
Chem. Int. Ed. Engl. 1973, 12, 508.
[15] J. Arnold, V. Knapp, J. A. R. Schmidt, A. Shafir, J. Chem. Soc.
Dalton Trans. 2002, 3273.
[27] F. H. Allen, Acta Crystallogr. Sect. A 2002, 58, 380.
[16] W. Zarges, M. Marsch, K. Harms, G. Boche, Chem. Ber. 1989, 122,
2303.
[28] D. Hoffmann, W. Bauer, F. Hampel, N. J. R. van Eikema Hommes,
P. von R. Schleyer, P. Otto, U. Pieper, D. Stalke, D. S. Wright, R.
Snaith, J. Am. Chem. Soc. 1994, 116, 528.
[29] M. Westerhausen, W. Schwarz, Z. Naturforsch. B 1998, 53, 625.
[30] a) F. Feil, S. Harder, Organometallics 2000, 19, 5010; b) F. Feil, S.
Harder, Organometallics 2001, 20, 4616; c) H. Schumann, D. M. M.
Freckmann, S. Dechert, Organometallics 2006, 25, 2696.
[31] a) S. Harder, C. Ruspic, N. Nꢄ Bhriain, F. Berkermann, M. Schꢆr-
mann, Z. Naturforsch. B 2008, 63, 267; b) S. Harder, Z. Anorg. Allg.
Chem. 2010, 636, 2205.
[17] The only known structure of benzylsodium is a TMEDA-solvated
cyclic tetramer (C. Schade, P. von R. Schleyer, H. Dietrich, W.
Mahdi, J. Am. Chem. Soc. 1986, 108, 2484) ,whereas an analogous
mixed alkali metal (Li2Na2) derivative has also been reported (D. R.
Baker, W. Clegg, L. Horsburgh, R. E. Mulvey, Organometallics
1994, 13, 4170).
[18] The
much
sterically
bulkier
complex
Ph3CK·PMDETA
(PMDETA=N,N,N’,N’’,N’’-pentamethyldiethylenetriamine) is
a
polymer [a) H. Viebrock, T. Panther, U. Behrens, E. Weiss, J. Orga-
nomet. Chem. 1995, 491, 19] but addition of THF causes coordina-
tive saturation yielding the monomeric mixed solvate
Ph3CK·PMDETA·THF [b) D. Hoffmann, W. Bauer, P. von R.
Schleyer, U. Pieper, D. Stalke, Organometallics 1993, 12, 1193].
[19] a) G. Ferguson, M. Parvez, Acta Crystallogr. Sect. A 1979, 35, 2207;
b) C. Slugovcl, C. Gemel, J.-Y. Shen, D. Doberer, R. Schmid, K.
Kirchner, K. Mereiter, Monatsh. Chem. 1999, 130, 363; c) D. M.
Cousins, M. G. Davidson, C. J. Frankis, D. Garcꢄa-Vivꢅ, M. F.
Mahon, Dalton Trans. 2010, 39. d) It has been suggested on the
strength of 13C spectroscopic evidence that Me6TREN ligates tBuLi
in a h3 fashion although no structural evidence was presented: H.
Luitjes, M. Schakel, M. P. Aarnts, R. F. Schmitz, F. J. J. de Kanter,
G. W. Klumpp, Tetrahedron 1997, 53, 9977.
[32] For a review of the effect of alkali metal identity on “carbanions”,
see: C. Lambert, P. von R. Schleyer, Angew. Chem. 1994, 106, 1187;
Angew. Chem. Int. Ed. Engl. 1994, 33, 1129.
[33] The coupling constant could not be determined from the 13C spec-
trum due to overlap with the more intense signals of Me6TREN. A
sample of 3 was prepared from 13C-a enriched toluene, however K–
H exchange occurred between benzyl K and toluene solvent mean-
ing the final product was not enriched. The coupling constant was
ultimately determined from 13C satellites in the 1H spectrum since
the resonance representing the CH2 group is sufficiently isolated for
such satellites to be visible in a strong sample.
[34] A search of the Cambridge Crystallographic Database[27] suggests
that organopotassium complexes require either a bulky anion or a
bulky donor to force formation of a monomer. The lowest molecular
weight example reported thus far seems to be CpK·[18]crown-6
(MW =368.51 amu): S. Neander, F. E. Tio, R. Buschmann, U. Beh-
rens, F. Olbrich, J. Organomet. Chem. 1999, 582, 58.
[20] For a review of tripodal tetraamine ligands, see: A. G. Blackman,
Polyhedron 2005, 24, 1.
[21] For the synthesis of Me6TREN, see: G. J. P. Britovsek, J. England,
A. J. P. White, Inorg. Chem. 2005, 44, 8125.
[22] LiTMP does not deprotonate toluene on its own, indeed a sample of
LiTMP in C7D8 shows no change in its 1H NMR spectrum after
16 h. For an illuminating review containing examples of heteroatom-
Received: December 3, 2010
Published online: February 21, 2011
Chem. Eur. J. 2011, 17, 3364 – 3369
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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