Poly{guanidinium} salts: Application in the preparation of a coordinatively saturated aluminium cation

Article abstract of DOI:10.1039/b611343e

Protonation of the linked bis(guanidine), H₂C{hpp}₂, affords isolated guanidinium salts that have been used to prepare a coordinatively saturated aluminium cation. The Royal Society of Chemistry.

Full text of DOI:10.1039/b611343e

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COMMUNICATION
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Poly{guanidinium} salts: application in the preparation of a
coordinatively saturated aluminium cation
Pedro J. Arago´n Sa´ez,a Sarah H. Oakley,b Martyn P. Coles*b and Peter B. Hitchcockb
Received (in Cambridge, UK) 7th August 2006, Accepted 19th October 2006
First published as an Advance Article on the web 13th November 2006
DOI: 10.1039/b611343e
Scheme 1) thereby making it more susceptible to decomposition or
Protonation of the linked bis(guanidine), H2C{hpp}2, affords
isolated guanidinium salts that have been used to prepare a
coordinatively saturated aluminium cation.
formation of catalytically inert Lewis base adducts. In the context
of the work reported herein, this is perhaps best illustrated by the
activation of dialkyl aluminium amidinate compounds using
ammonium borate salts, [HNR3][B(C6F5)4], where formation of
stable amine adducts result.1 In this contribution, we describe a
fundamentally different approach to the generation of cationic
main group compounds via the reaction of a protonated ligand
precursor with a neutral metal alkyl reagent (Pathway 2, Scheme 1).
Using this protocol, the cationic charge is generated at the same
time that the neutral ligand-set is introduced, and the resultant
metal species has a higher intrinsic degree of coordinative
saturation than the corresponding cation generated in Pathway 1.
Reaction of the linked bis(guanidine), H2C{hpp}2 (1,2 (hppH =
1,3,4,6,7,8-hexahydro-2Hpyrimido[1,2-a]pyrimidine) with triethyl-
amine hydrochloride afforded the guanidinium salt, [H2C{hpp}
{hppH}][Cl] (2a). The low solubility of 2a in hydrocarbon solvents
limited its use as a reagent, and hence anion metathesis was
conducted with NaBPh4 to afford the tetraphenylborate reagent,
[H2C{hpp}{hppH}][BPh4] (2b). The IR spectrum of 2b (Nujol
mull) shows an N–H stretch at 3384 cm21, with a shift of u(CLN)
from 1611 cm21 in 1 to a broad absorption centred at 1584 cm21
in the protonated species. Key features of the 1H NMR spectrum{
include a highly deshielded NH resonance at 14.00 ppm and a
broad peak centred at y4.5 ppm for the exocyclic bridging
methylene group. Upon cooling to 203 K, this latter resonance is
The chemistry of the main group metals has been undergoing
somewhat of a renaissance in the last decade, fuelled in part by the
realization that complexes of these elements are capable of
catalyzing a range of chemical transformations previously thought
to lie exclusively within the remit of the transition and lanthanide
metals. In these metal-mediated processes, a basic requirement of
the ancillary ligand set is that a balance between (i) offering
sufficient steric support to prevent rearrangement and/or decom-
position and (ii) allowing access of the substrate to the metal
centre, be maintained throughout the catalytic cycle. For catalysis
purported to proceed via a positively charged metal centre, the
‘play-off’ between (i) and (ii) becomes even more critical as
the process whereby the cation is generated typically involves the
removal of a ligand from a neutral ‘pre-catalytic’ state. This
necessarily reduces the steric environment at the metal (Pathway 1,
2
resolved into an AB pattern{ (d 5.44 and 3.64, JHH = 15.3 Hz),
although only one set of resonances for the corresponding
methylene protons of each hpp group is observed over this
temperature range, suggesting rapid exchange of the NH proton
between the guanidine and guanidinium components.
Single crystal X-ray diffraction§ (Fig. 1) confirmed the
formation of the mono-protonated guanidinium salt, consisting
of isolated [H2C{hpp}{hppH}]+ cations and non-coordinating
[BPh4]2 anions. Both ‘hpp’ groups in the cation are aligned, with
Scheme 1 Synthetic routes to cationic main group alkyl complexes,
illustrated for a group 13 element (M) supported by ligand set (L), in the
presence of Lewis base (LB).
aDepartamento de Qu´ımica Inorga´nica, Orga´nica y Bioqu´ımica,
Facultad de Ciencias Qu´ımicas, Universidad de Castilla-La Mancha,
Avenida Camilo Jose´ Cela, 10 CP 13071, Ciudad Real, Spain.
E-mail: PedroJose.Aragon@uclm.es; Fax: +34 926 295318;
Tel: +34 926 295300
bDepartment of Chemistry, University of Sussex, Falmer, Brighton,
BN1 9QJ, UK. E-mail: m.p.coles@sussex.ac.uk;
Fax: +44 (0)1273 677196; Tel: +44 (0)1273 877339
Fig. 1 ORTEP of the cationic component of 2b (ellipsoids drawn at
30%).
816 | Chem. Commun., 2007, 816–818
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the exocyclic methylene group located above the plane defined by
N1/2/4/5. The hydrogen on N1 was located and refined, showing
+
…
that it is associated within an asymmetric [N–H N] intramole-
cular hydrogen bridge (IHB).3,4 This structural motif has been
observed in related bis{guanidine} systems in which the two
nitrogen atoms involved are constrained in the 1,8-positions of di-
substituted naphthalene, resulting in non-linear bridges with angles
of 152 and 142u. In 2b, the higher degree of flexibility inherent in
the methylene bridge allows a considerably more linear IHB of
167u to be adopted.
The similar r values" for the guanidinium and guanidine
components of 2b (0.98 and 0.95, respectively) indicate ‘partial
protonation’5 of the neutral moiety via the proton bridge,4 in
1
agreement with the equivalent H NMR resonances for the two
hpp groups. Further examination of the carbon-nitrogen distances,
however, show significant differences between the two compo-
nents. For example, the DCN value" for the protonated guanidine
˚
is 0.03 A, whilst within the formally neutral hpp-unit, a larger
difference between the C–N single and CLN double bonds of
6
0.08 A is noted. There is also a significant reduction in the C(1)–
˚
Fig. 3 (a) ORTEP of one of the cationic components of 3b (ellipsoids
drawn at 30%). (b) Dimeric arrangement of [H2C{hppH}2][Cl][BPh4]
…
˚
N(3) distance [1.341(2) A] compared with the corresponding value
showing NH Cl hydrogen bonds.
˚
in the neutral guanidine [1.362(2) A], indicating a greater
delocalization of the Namide lone pair into the sp2 carbon of the
framework for the protonated guanidine. Considering the different
resonance forms possible within a protonated hpp moiety, this is
consistent with a large contribution from resonance form B, Fig. 2.
Using the same synthetic methodology, we have demonstrated
that 1 can also be doubly protonated to afford the bis{guanidi-
nium} dication. Exchanging one of the chloride anions for [BPh4]2
allowed isolation of the mixed anion salt, [H2C{hppH}2][BPh4][Cl]
(3b). The IR spectrum (Nujol mull) shows a broad absorption at
2296 cm21, in the region associated with u(NH) for weakly
hydrogen bonded systems,7 as indicated in the crystal structure
(vide infra). The 1H NMR spectrum{ in CD3CN displays a single
sharp resonance for the bridging methylene protons at d 5.07, with
a broad peak at d 8.43 for NH suggesting a symmetric structure in
solution. The X-ray crystal structure of 3b§ (two independent
molecules) shows that the two guanidinium components within the
dication [H2C{hppH}2]2+ are positioned with the N–H moieties
spatially removed from one another (Fig. 3a), rationalized in terms
of electrostatic repulsion between the two positively charged
centres. The distribution of C–N distances differs substantially
from that in 2a, with r values of 0.99 and shorter bonds to both
… …
arrangement of cations bridged by two NH Cl HN bridges,
with angles of 118.6 and 101.1u at the chloride (Fig. 3b).
Reaction of 2b with one equivalent of AlMe3 proceeded
smoothly with evolution of methane and generation of a colourless
crystalline solid that was purified by crystallization from THF. 1H
NMR and elemental analysis{ were consistent with formation of
the cationic aluminium alkyl, [H2C{hpp}2AlMe2][BPh4] (4b), with
the predicted high field 1H NMR resonance for the two equivalent
AlMe2 groups at 20.86 ppm. The signal for the protons of the
bridging methylene group is broad, suggestive of conformational
changes within the eight-membered C3N4Al metallacycle.
The crystal structure§ of 4b (Fig. 4) consists of two molecules in
the unit cell that differ slightly in their bond lengths and angles;
there are also two molecules of THF within the lattice. The cation
˚
the protonated [N2/N5: 1.328(3) and 1.329(3) A] and the amide
˚
[N3/N6: 1.329(3) and 1.332(3) A] nitrogen atoms. This suggests an
almost equal contribution from resonance forms A and B in the
guanidinium components of 3b. Hydrogen bonding between the
…
NH and the chloride anion is observed in the solid-state [H Cl
˚
distances in the range 2.27–2.36 A] generating a dimeric
Fig. 4 (a) ORTEP of one of the independent molecules of 4b (ellipsoids
drawn at 30%). (b) Crystal packing for 4b showing the presence of non-
coordinated THF molecules within the lattice.
Fig. 2 Resonance contributions to the bicyclic guanidinium cation,
[hppRH]+.
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137.0 (m-C6H5), 126.7 (q, J = 3, o-C6H5), 122.9 (p-C6H5), 64.6
(H2C{hppH}2), 49.3 (hpp-CH2), 48.6 (hpp-CH2), 46.6 (hpp-CH2), 39.7
(hpp-CH2), 21.9 (hpp-CH2), 21.5 (hpp-CH2). [H2C{hpp}2AlMe2][BPh4]
(4b): Anal. Calc. for C41H52AlBN6 (C4H8O): C 73.16, H 8.19, N 11.38%.
Found: C 73.80, H 7.97, N, 12.51%. 1H NMR (300 MHz, CD2Cl2, 298 K):
d 7.33 (m, 8H, m-C6H5), 7.05 (t, J = 7.4, 8H, o-C6H5), 6.90 (t, J = 7.2, 4H,
p-C6H5), 4.65 (br s, 2H, H2C{hpp}2), 3.15 (m, 8H, hpp-CH2), 3.06 (m, 4H,
hpp-CH2), 2.96 (m, 4H, hpp-CH2), 1.84 (m, 8H, hpp-CH2), 20.86 (s, 6H,
AlMe2). 13C{1H}NMR (75 MHz, CD2Cl2, 298 K): d 164.3 (q, J = 49,
i-C6H5), 158.4 (CN3), 136.2, (m-C6H5), 125.9 (q, J = 3, o-C6H5), 122.1 (p-
C6H5), 70.0 (H2C{hpp}2), 48.5 (hpp-CH2) 47.8 (hpp-CH2), 47.0 (hpp-
CH2), 41.9 (hpp-CH2), 23.6 (hpp-CH2), 22.4 (hpp-CH2), 29.4 (br, AlMe2).
{ The Gibbs free energy associated with this process is calculated at
51.4 kJ mol21, according to the equation: DG{ = [22.96 + ln (Tc/Du)].
§ Crystallographic 2b: C39H47BN6, M = 610.64, T = 173(2) K, triclinic,
consists of a distorted tetrahedral aluminium dimethyl centre
supported by two donor nitrogen atoms from the H2C{hpp}2
ligand, forming an eight-membered metallacycle in a distorted
˚
‘twist-boat’ configuration. The Al–N distances [1.907(2) A–
˚
1.922(2) A] are relatively short compared to other Al–Nimine
bonds,8 and a contraction of the Al–Me bond lengths from
1.985(2) A/1.993(2) A in the related [Al{hpp}Me2]2 dimer9 to
˚
˚
˚
˚
1.964(3) A–1.976(3) A in 4b is commensurate with a positive
charge located at the metal. However, whilst formally neutral, the
C–N bond lengths within the guanidine components are not
localised into single and double bonds to a great extent (DCN
=
˚
0.03 A), suggesting the charge is shared between the metal and the
ligand. It is of interest to note that there are no close contacts
between the THF molecules and the metal centre within the unit
cell of 4b, suggesting a crowded environment at aluminium. The
effects of this steric protection on potential catalytic applications
form part of an ongoing study into the chemistry of this system.10
In summary we have synthesized and characterized mono- and
di-protonated guanidium salts of the linked bis(guanidine)
precursor, H2C{hpp}2. The application of the former as a reagent
for the direct synthesis of cationic main group complexes has been
illustrated for the dialkyl aluminium system, affording the stable
cation, [H2C{hpp}2AlMe2][BPh4]. Unfortunately, limited solubility
of the product species has, to date, hampered extension of this
work using compound 3b as a method of potentially generating
dicationic metal complexes.
¯
˚
space group P1 (no. 2), a = 10.2719(3), b = 12.4868(3), c = 14.3087(5) A, a =
3
˚
109.992(2), b = 99.770(1), c = 100.197(2)u, U = 1644.02(8) A , Z = 2, Dc =
1.234 Mg m23, m (Mo-Ka) = 0.073 mm21, independent reflections = 5727
[Rint = 0.051], R1 [for 4569 reflections with I . 2s(I)] = 0.046, wR2 (all
data) = 0.115. 3b C39H48BClN6, M = 647.09, T = 173(2) K, triclinic, space
¯
˚
group P1 (No.2), a = 13.8838(3), b = 16.0260(3), c = 17.2417(4) A, a =
3
˚
114.885(1), b = 93.524(1), c = 91.947(1), U = 3466.80(13) A , Z = 4, Dc =
1.24 Mg m23, m (Mo-Ka) = 0.15 mm21, independent reflections = 13606
[Rint = 0.059], R1 [for 9575 reflections with I . 2s(I)] = 0.056, wR2 (all
data) = 0.140. 4b C41H52AlBN6?(C4H8O), M = 738.78, T = 173(2) K,
¯
triclinic, space group P1 (no. 2), a = 10.8106(2), b = 19.9865(4), c =
˚
21.7023(3) A, a = 66.307(1), b = 87.632(1), c = 74.326(1), U =
4122.18(13) A , Z = 4, Dc = 1.19 Mg m23, m (Mo-Ka) = 0.09 mm21
3
˚
,
independent reflections = 16005 [Rint = 0.055], R1 [for 11181 reflections
with I . 2s(I)] = 0.061, wR2 (all data) = 0.167. CCDC 617259–617261. For
crystallographic data in CIF or other electronic format see DOI: 10.1039/
b611343e
" The r value is defined as the ratio of the doubled CLN distance (a) and
C–NR2 distances (b, c): r = 2a/(b + c), and has been used as a measure of
the degree of protonation of a guanidine. See ref. 4 and 5. The DCN value is
defined as the difference between the formally CLN double and C–N single
bonds within the amidine unit: DCN = d(C–N) 2 d(CLN). See ref. 6.
We wish to acknowledge the University of Sussex for financial
support and the Junta de Comunidades de Castilla-La Mancha for
a travel scolarship (PJAS). Dedicated to the memory of Dr
Anthony G. Avent.
1 S. Dagorne, I. A. Guzei, M. P. Coles and R. F. Jordan, J. Am. Chem.
Soc., 2000, 122, 274.
Notes and references
2 S. H. Oakley, M. P. Coles and P. B. Hitchcock, Inorg. Chem., 2004, 43,
7564.
3 V. Raab, J. Kipke, R. M. Gschwind and J. Sundermeyer, Chem.–Eur.
J., 2002, 8, 1682.
4 V. Raab, K. Harms, J. Sundermeyer, B. Kovacevic and Z. B. Maksic,
J. Org. Chem., 2003, 68, 8790.
5 B. Kovacevic and Z. B. Maksic, Chem.–Eur. J., 2002, 8, 1694.
6 G. Ha¨felinger and F. K. H. Kuske, The Chemistry of Amidines and
Imidates, Wiley, Chichester, 1991.
7 J. Emsley, Chem. Soc. Rev., 1980, 9, 91.
8 S. Dagorne, S. Bellamin-Laponnaz and R. Welter, Organometallics,
2004, 23, 3053.
9 S. L. Aeilts, M. P. Coles, D. C. Swenson, R. F. Jordan and V. G.
Young, Jr., Organometallics, 1998, 17, 3265.
10 Preliminary studies have indicated that cation 4b is remarkably stable,
showing no reactivity with olefins (1-hexene) or weak acids (phenyl-
acetylene). P. J. Arago´n Sa´ez and M. P. Coles, unpublished results.
{ Selected analytical data: [H2C{hpp}{hppH}][BPh4] (2b): Anal. calc. for
C39H47BN6: C 76.71, H 7.76, N 13.76%. Found: C 76.71, H 7.64, N
13.89%. 1H NMR (500 MHz, CD2Cl2, 298 K): d 14.00 (s, 1H, NH), 7.32
(m, 8H, m-C6H5), 7.03 (t, J = 7.4, 8H, o-C6H5), 6.89 (m, 4H, p-C6H5), 4.54
(br s, 2H, H2C{hpp}{hppH}), 3.25 (m, 4H, hpp-CH2), 3.17 (m, 4H, hpp-
CH2), 3.11 (m, 4H, hpp-CH2), 3.05 (m, 4H, hpp-CH2), 1.86 (m, 4H, hpp-
CH2), 1.82 (m, 4H, hpp-CH2). 13C{1H} NMR (125 MHz, CD2Cl2, 298 K):
d 164.3 (q, J = 49, i-C6H5), 151.7 (CN3), 136.3 (m-C6H5), 125.9 (q, J = 3, o-
C6H5), 122.1 (p-C6H5), 66.8 (H2C{hpp}{hppH}), 48.1 (hpp-CH2), 47.7
(hpp-CH2), 47.6 (hpp-CH2), 40.9 (hpp-CH2), 22.3 (hpp-CH2), 21.9 (hpp-
CH2). 11B NMR (160 MHz, CD2Cl2, 298 K):
d 12.2 (m).
[H2C{hppH}2][Cl][BPh4] (3b): Anal. calc. for C39H48BClN6: C 72.39,
H 7.48, N 12.99%. Found: C 72.45, H 7.49, N 12.81%. 1H NMR
(300 MHz, CD3CN, 298 K): d 8.43 (br s, 2H, NH), 7.13 (m, 8H, m-C6H5),
6.86 (t, J = 7.4, 8H, o-C6H5), 6.71 (t, J = 7.2, 4H, p-C6H5), 5.07 (s, 2H,
H2C{hppH}2), 3.14 (m, 12H, hpp-CH2), 1.81 (m, 12H, hpp-CH2). 13C{1H}
NMR (75 MHz, CD3CN, 353 K): d 165.2 (q, J = 49, i-C6H5), 152.8 (CN3),
818 | Chem. Commun., 2007, 816–818
This journal is ß The Royal Society of Chemistry 2007