The syntheses and structures of group 1 expanded dipyrrolides: The formation of a 12-rung amidolithium circular ladder

Article abstract of DOI:10.1039/b303611a

Elaboration of meso-disubstituted dipyrromethanes to incorporate imino or amino donors results in ligands that adopt unique, aggregated structures with Group 1 metals.

Full text of DOI:10.1039/b303611a

The syntheses and structures of Group 1 expanded dipyrrolides: the
formation of a 12-rung amidolithium circular ladder†
Jason B. Love,*a Alexander J. Blake,a Claire Wilson,a Stuart D. Reid,a Andrew Novaka and Peter B.
Hitchcockb
a School of Chemistry, University of Nottingham, University Park, Nottingham, UK NG7 2RD.
E-mail: jason.love@nottingham.ac.uk; Fax: +44 115 9513563; Tel: +44 115 8467167
b Department of Chemistry, University of Sussex, Falmer, Brighton, UK BN1 9QJ
Received (in Cambridge, UK) 2nd April 2003, Accepted 20th May 2003
First published as an Advance Article on the web 5th June 2003
Elaboration of meso-disubstituted dipyrromethanes to incor-
porate imino or amino donors results in ligands that adopt
unique, aggregated structures with Group 1 metals.
two coordinated THF molecules. Slow cooling of a solution of
2 in THF resulted in the deposition of X-ray quality crystals; the
solid state structure of 2 is shown in Fig. 1. Unlike the solution
data, the solid state structure reveals different coordination
environments for the potassium ions. Both K1 and K2 are bound
to the dipyrrolide ligand in the same manner; the metal is p-
bound to one pyrrolide ring and s-bound to the adjacent
pyrrolide nitrogen and the associated imine nitrogen. This
bonding motif has been observed for other transition metals and
f-element meso-dipyrrolide systems and promotes effective
aggregate formation.2,3 While the coordination environment of
K1 is completed by two THF molecules, K2 undergoes further
p-bonding to a pyrrolide group of an adjacent molecule, so
forming a polymeric zigzag chain (Fig. 1a). The K2–pyrrolide–
K1B interaction deviates considerably from linearity (K2–cent–
K1B = 164.6°) and is asymmetric (K2–cent = 3.103, K1B–
cent = 2.968 Å). Similar polymeric chain motifs have been
seen in a potassium octaalkylporphyrinogen structure,7 and in
the alkali metal cyclopentadienyls [K(THF)n(C5Me5)]H, al-
though in the latter the K–cent–KA interactions approach
linearity.8
The dipyrrolides (L, R = alkyl, aryl) are proving to be versatile
dianionic ligands for the electropositive metals.1,2 While the
combination of both cyclopentadienyl and amido ligand
characteristics encourages the stabilisation of highly reactive
metal species through aggregation,3 the creation of reactive,
multimetallic compounds is still difficult to control. In order to
generate ligand sets that have well-defined donor compart-
ments, a strategy to increase the denticity of the dipyrromethane
ligand is being pursued by us. Similar methodology has been
used in the preparation of expanded porphyrins.4 Herein, we
describe the first syntheses of N-donor-elaborated meso-
disubstituted dipyrromethanes and the syntheses and structures
of Group 1 complexes incorporating these ligands. Such
compounds can provide insight into the structural chemistry of
these ligand sets and also afford salt elimination routes to the
incorporation of transition metals.5 The binding of alkali metals
to the pyrrolic framework can also moderate the redox
potentials of the resulting transition metal complex.6
The reaction between H4[L4] and BunLi (4 equiv.) in Et2O
resulted in the formation of the amidopyrrolides 3a and 3b in a
combined, isolated yield of 75%; 3a precipitated from Et2O,
while 3b was isolated from the supernatant liquors and was
crystallised from pentane.† The 1H and 7Li{1H} NMR spectra
The Vilsmeier–Hack reactions between H2[L] and POCl3–
DMF result in the carbonylation of the dipyrromethanes in the
5,5A-positions, so forming 1a and 1b (Scheme 1).† Dialdehyde
1b was crystallographically characterised, and adopts a dimeric
structure formed by hydrogen bonding between the pyrrole
protons and carbonyl oxygens of discrete monomers.‡ Reaction
of 1 with excess ButNH2 in MeOH in the presence of anhydrous
carbonate yields the diiminodipyrromethanes H2[L1] and
H2[L2] in good yield; H2[L1] is volatile and is best purified by
sublimation at 150 °C/1022 mbar. Reduction of the imines with
NaBH4 in methanol results in the formation of the diaminodi-
pyrromethanes H4[L3] and H4[L4].
The reaction between H2[L2] and KH in THF caused rapid
gas evolution and the generation of the dipotassium salt
K2(THF)2[L2] 2 in good yield. NMR data are consistent with a
symmetrical ligand arrangement and confirmed the presence of
Scheme 1 Synthesis of diimino- and diamidodipyrrolides.
† Electronic supplementary information (ESI) available: full experimental
details and crystallographic data. See http://www.rsc.org/suppdata/cc/b3/
b303611a/
Fig. 1 X-Ray crystal structure of 2 [50% ellipsoids, But and THF carbons
removed for clarity].
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This journal is © The Royal Society of Chemistry 2003

N2C ligation from a second Li4[L4] unit (analogous to K–imine
bonding in 2). Unlike 2, the structure is further complicated by
the presence of the second Li environment, Li1, which
coordinates to both amido-N2 and pyrrolide-N1, and to amido-
N2A of a second monomer, so generating the circular ladder.
This ladder has a pronounced and unusual twist,9 (Fig. 2c)
which results in distorted pyramidal Li1 (S N–Li–N = 318.2,
cf. Li2 S = 349.8°) and rhombohedral Li1–N2–Li1a–N2a
[internal angles = 104.9 and 69.6°, Li1–N2 2.074(4), Li–N2A
2.002(4) Å], and is a likely consequence of amido-arm
geometrical constraint. Li-amide circular ladders that are
derived from monodeprotonated amines, e.g. [LiN(H)But]8,10
usually form by straightforward edge-to-edge LiN ring associa-
tion between dimeric units. However, with the tetradeproto-
nated tetraamine 3b, the formation of simple LiN repeating
units in the monomer is precluded, and results instead in a
laterally-shifted LiN chain (Fig. 3). The combination of three of
these chains, coupled with cyclisation would result in the
observed structure. Like other LiN ladders, 3b contains
approximately alternating long–short edge bonds with an
intermediate rung length.11
Fig. 3 Possible mechanism for the formation of 3b.
We thank the Royal Society (J.B.L., University Research
Fellowship), the University of Nottingham, the EPSRC
(S.D.R.), and the Nuffield Foundation (A.N., Undergraduate
Research Bursary) for support.
Notes and references
‡ CCDC 207484–207486. See http://www.rsc.org/suppdata/cc/b3/
b303611a/ for crystallographic data in .cif format.
1 T. Dubé, S. Conoci, S. Gambarotta and G. P. A. Yap, Organometallics,
2000, 19, 1182; G. Aharonian, S. Gambarotta and G. P. A. Yap,
Organometallics, 2002, 21, 4257; Y. Shi, C. Hall, J. T. Ciszewski, C.
Cao and A. L. Odom, Chem. Commun., 2003, 586.
2 A. Novak, A. J. Blake, C. Wilson and J. B. Love, Chem. Commun.,
2002, 2796.
3 M. Ganesan, S. Gambarotta and G. P. A. Yap, Angew. Chem., Int. Ed.,
2001, 40, 766.
4 A. Jasat and D. Dolphin, Chem. Rev., 1997, 97, 2267.
5 F. Antolini, P. B. Hitchcock, M. F. Lappert and P. Merle, Chem.
Commun., 2000, 1301; K. Gregory, P. von Ragué Schleyer and R.
Snaith, Adv. Inorg. Chem., 1991, 37, 47; R. E. Mulvey, Chem. Soc. Rev.,
1991, 20, 167.
Fig. 2 The X-ray crystal structure of 3b [50% ellipsoids].
6 J. Guan, T. Dubé, S. Gambarotta and G. P. A. Yap, Organometallics,
2000, 19, 4820; M. Ganesan, C. D. Bérubé, S. Gambarotta and G. P. A.
Yap, Organometallics, 2002, 21, 1707; C. Floriani, E. Solari, G. Solari,
A. Chiesi-Villa and C. Rizzoli, Angew. Chem., Int. Ed., 1998, 37,
2245.
7 L. Bonomo, E. Solari, R. Scopelliti and C. Floriani, Chem. Eur. J., 2001,
7, 1322.
8 M. L. Cole, C. Jones and P. C. Junk, J. Chem. Soc., Dalton Trans., 2002,
896; W. J. Evans, J. C. Brady, C. H. Fujimoto, D. G. Giarikos and J. W.
Ziller, J. Organomet. Chem., 2002, 649, 252.
9 A. R. Kennedy, R. E. Mulvey and A. Robertson, Chem. Commun., 1998,
89; N. Kuhn, G. Henkel and J. Kreutzberg, Angew. Chem., Int. Ed.,
1990, 29, 1143.
10 N. D. R. Barnett, W. Clegg, L. Horsburgh, D. M. Lindsay, Q.-Y. Liu, F.
M. Mackenzie, R. E. Mulvey and P. G. Williard, Chem. Commun., 1996,
2321.
for 3a and 3b are very similar and are consistent with both
complete deprotonation of the aminopyrrole ligand H4[L4] and
the formation of complexes that contain two distinct lithium
environments [e.g. 3b: dLi 0.08 (br s), 27.65 (br s)]. The
simplicity of the NMR data for 3b is consistent with, but not
reflected by, the X-ray structure (Fig. 2), where it is im-
mediately apparent that a remarkable structural aggregation has
occurred to maximise electrostatic interactions between the Li
ions and N-donors. In the solid state, 3b exists as three
interlocked Li4[L4] units, the core of which is a unique 12-rung
lithium amide circular ladder (Fig. 2c) which is delimited by a
helical ligand periphery (Fig. 2b). As with 2, the dipyrrolide
moiety adopts alternating s-N/p-C4H2N bonding to Li2 (and
Li2B); the coordination sphere of Li2 is completed by amido-
11 R. E. Mulvey, Chem. Soc. Rev., 1998, 27, 339.
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