Unprecedented reversible migration of amide to schiff base ligands attached to tin: Latent single-site initiators for lactide polymerization

Article abstract of DOI:10.1021/ja0470315

In an unprecedented transformation, amide ligands are found to attack the imine carbon centers of tridentate Schiff base ligands attached to tin. The process is reversible, and the resultant (masked) amide species can be exploited as latent single-site in

Full text of DOI:10.1021/ja0470315

Published on Web 10/05/2004
Unprecedented Reversible Migration of Amide to Schiff Base Ligands
Attached to Tin: Latent Single-Site Initiators for Lactide Polymerization
Nonsee Nimitsiriwat,† Edward L. Marshall,† Vernon C. Gibson,*,† Mark R. J. Elsegood,‡ and
Sophie H. Dale‡
Departments of Chemistry, Imperial College London, South Kensington Campus, London SW7 2AZ, U.K., and
Loughborough UniVersity, Loughborough, LE11 3TU, U.K.
Received May 20, 2004; E-mail: V.Gibson@imperial.ac.uk
Schiff base ligands represent one of the most widely utilized
classes of ligand in metal coordination chemistry.1 Their complexes
find many important catalytic applications, ranging from asymmetric
epoxidation,2 Lewis acid assisted organic transformations,3 to
various types of polymerization,4-7 as well as their widespread use
as model compounds for the study of the active sites of metalloen-
zymes.8 While an attractive feature of Schiff base ligands is their
relative inertness to ligand-centered reaction chemistry, the partially
positively charged imine unit can sometimes render them susceptible
to nucleophilic attack. This usually arises from irreversible in-
tramolecular attack by donors incorporated within the Schiff base
ligand leading to macrocyclic ring contractions.9 Radical mediated
alkylations are also known to occur.10,11 Herein, we describe an
unprecedented attack by amide at the imine carbon atoms of
Figure 1. Crystal structure of 3a with H atoms omitted for clarity. Selected
tridentate Schiff base ligands to give unusual tetradentate ligand
systems. A reverse process disassembles the so-formed tetradentate
ligands to afford single-site initiators for lactide polymerization.
The iminophenols 1-3 containing O, S, and P donor arms were
synthesized by condensation of 3,5-But2-salicylaldehyde with the
appropriate amine and reacted with Sn(NMe2)2 to give the crystal-
line products 1a-3a in good yields (Scheme 1). The same products
could also be obtained, first by treatment of SnCl2 with the in-
situ-generated lithium salts derived from 1-3 to afford the mono-
chloride complexes 1b-3b, followed by their reactions with
LiNMe2.
bond lengths (Å) and angles (deg): Sn(1)-P(1) 2.8400(4), Sn(1)-N(1)
2.1649(12), Sn(1)-N(2) 2.4490(13), Sn(1)-O(1) 2.0628(9), N(1)-C(15)
1.4672(17), N(2)-C(15) 1.4948(19), P(1)-Sn(1)-N(2) 130.10(3),
N(1)-Sn(1)-N(2) 59.40(4), N(2)-Sn(1)-O(1) 82.54, N(1)-C(15)-N(2)
101.82(11), Sn(1)-N(1)-C(15) 98.16(8), Sn(1)-N(2)-C(15) 86.16(7).
Scheme 1
NMR spectra for 1a-3a revealed chemical shifts in the range δ
5.35-5.60, considerably upfield of the usual region for the imine
HCdN resonances of salicylaldiminato ligands (typically ca. δ
7.50-8.50) while the 13C NMR spectrum showed corresponding
upfield shifts for the associated carbon nuclei, at δ 87-90 (cf. ca.
δ 165-170 for salicylaldiminato complexes). These are indicative
of a saturated carbon center rather than an unsaturated imino moiety.
In addition, a 2D HMBC NMR spectrum12 showed the presence
of coupling between the methyl hydrogens of the dimethylamino
unit and the carbon of the ligand CH moiety implying a close
association of these two groups.
It proved possible to obtain crystals of 3a suitable for an X-ray
structure determination. A view of the structure is shown in Figure
1; selected bond lengths and angles are given in the figure caption.
The tin amide group is found to have attacked the imine carbon of
the salicylaldimine to give a new dianionic tetradentate ligand in
which the O, N, and P donors of the original salicylaldimine ligand
and the nitrogen of the dimethylamino group are bonded to the
tin(II) center. As a result, the original CdN double bond is now a
CsN single bond, with a length of 1.467(17) Å. The SnsN bond
lengths reflect the changes in bonding that have occurred upon
generating the new ligand system. The SnsN(2) bond length, at
2.4490(13) Å, is consistent with a dative covalent interaction, while
the Sn-N(1) bond distance, at 2.1649(12) Å, is indicative of an
amide ligand containing a significant degree of lone pair donation
to the tin center. Consistently, N(1) adopts an almost planar
geometry. The overall outcome of the ligand rearrangement is a
metal-centered geometry that is best viewed as being derived from
a trigonal bipyramid in which one of the equatorial coordination
sites is occupied by the stereochemically active lone pair of
† Imperial College London.
‡ Loughborough University.
9
13598
J. AM. CHEM. SOC. 2004, 126, 13598-13599
10.1021/ja0470315 CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
attributable to the diastereotopic methyl units of the NMe2 end-
group. Consistently, a MALDI-TOF mass spectrum on a sample
of PLA prepared using 3a and 20 equiv of rac-lactide revealed
PLA chains bearing Me2N end-groups.12
In summary, we have described an unprecedented migration of
an amino group to the imine carbon atom of a family of tridentate
salicylaldiminato complexes to give highly unusual tetradentate
ligands. The so-formed complexes act as latent single-site initiators
for the controlled polymerization of rac-lactide.
Scheme 3
Acknowledgment. This work was supported by the EPSRC and
a scholarship from the Institute for the Promotion of Teaching
Science and Technology (IPST), Thailand (to N.N.)
Supporting Information Available: Full experimental details for
the synthesis of all complexes, including crystallographic data for 3a
and polymerization data (PDF and CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
References
(1) See: ComprehensiVe Coordination Chemistry II; McCleverty, J. A., Meyer,
T. J., Eds.; Elsevier: London, 2004.
(2) Samsel, E. G.; Srinivasan, K.; Kochi, J. K. J. Am. Chem. Soc. 1985, 107,
7606. Zhang, W.; Loebach, J. L.; Wilson, S. R.; Jacobsen, E. N. J. Am.
Chem. Soc. 1990, 112, 2801. Irie, R.; Noda, K.; Ito, Y.; Matsumoto, N.;
Katsuki, T. Tetrahedron Lett. 1990, 31, 7345. Sigman, M. S.; Jacobsen,
E. N. J. Am. Chem. Soc. 1998, 120, 4901.
(3) (a) Lewis Acids in Organic Synthesis; Yamamoto, H. Ed.; Wiley-VCH:
Weinheim, Germany, 2000, and references therein. (b) Atwood, D. A.;
Jegier, J. A.; Rutherford, D. J. Am. Chem. Soc. 1995, 117, 6779.
(4) (a) Younkin, T. R.; Connor, E. F.; Henderson, J. I.; Friedrich, S. K.;
Grubbs, R. H.; Bansleben, D. A. Science 2000, 287, 460. (b) Starzewski,
K. A. O.; Younkin, T. R.; Connor, E. F.; Henderson, J. I.; Friedrich, S.
K.; Grubbs, R. H.; Bansleben, D. A. Science 2000, 288, 1749.
(5) (a) Mitani, M.; Mohri, J.; Yoshida, Y.; Saito, J.; Ishii, S.; Tsuru, K.; Matsui,
S.; Furuyama, R.; Nakano, T.; Tanaka, H.; Kojoh, S.; Matsugi, T.;
Kashiwa, N.; Fujita, T. J. Am. Chem. Soc. 2002, 124, 3327. (b) Saito, J.;
Mitani, M.; Matsui, S.; Mohri, J.; Kojoh, S.; Kashiwa, N.; Fujita, T. Angew.
Chem., Int. Ed. 2001, 40, 2918.
electrons. Such a geometry is consonant with the predictions of
the vibronic coupling model13 as well as simple valence shell
electron pair repulsion (VSEPR) theory.
To learn more about the reactivity of the NR2-bridged species,
the Et2N derivative 1a′ was prepared and treated with excess
Me2NH. This led to clean formation of the Me2N-bridged product
1a. Conversely, treatment of 1a with 12 equiv of Et2NH led to a
1.6:1 mixture of 1a and 1a′ (K298 0.021) revealing the greater
thermodynamic stability of products containing smaller amine
bridges. A postulated pathway for this exchange process is via a
terminal amide species (Scheme 2). That such terminal amide
species are viable is supported by the observation that the NiPr2
and N(SiMe3)2 derivatives12 contain terminal rather than bridging
R2N units. Related alkoxide derivatives, 1c-3c (Scheme 1),
obtained by treatment of 1a-3a with Ph2MeCOH, also have
terminal alkoxide ligands.
(6) Jones, D. J.; Gibson, V. C.; Green, S. M.; Maddox, P. J. Chem. Commun.
2002, 10, 1038.
(7) O’Reilly, R. K.; Gibson, V. C.; White, A. J. P.; Williams, D. J. J. Am.
Chem. Soc. 2003, 125, 8450.
(8) Examples include the following. (a) Vitamin B12 cofactors: Dolphin, D.
Vitamin B12; Wiley: New York, 1984. (b) Methylreductase (MCR):
Berkassel, A. Bioorg. Chem. 1991, 19, 101. (c) Carbon monoxide
dehydrogenase (COdH): Lu, Z.; Crabtree, R. H. J. Am. Chem. Soc. 1995,
117, 3994. (d) Mn clusters of photosystem II: Feichtinger, D.; Plattner,
D. A. Angew. Chem., Int. Ed. Engl. 1997, 36, 1718.
Sn(II) alkoxide and amide complexes play a central role in the
production of polylactide (PLA),14 a biosustainable material derived
from lactic acid. The potential formation of terminal amide species
from 1a-3a suggested that they may act as latent single-site
initiators for lactide polymerization. Treatment of 1a-3a with rac-
lactide afforded PLA with good control (see Scheme 3). The O, S,
and P donor derivatives 1a-3a gave quite similar rates (kapp) of
polymerization. In all cases, a linear increase in molecular weight
(Mn) with conversion was observed as well as a linear relationship
(9) Guerreiro, P.; Tamburini, S.; Vigato, V. A. Coord. Chem. ReV. 1995, 139,
17.
(10) Floriani, C.; Solari, E.; Corazza, F.; Chiesi-Villa, A.; Guastini, C. Angew.
Chem., Int. Ed. 1989, 28, 64. Solari, E.; Floriani, C.; Chiesi-Villa, A.;
Rizzoli, C. Dalton Trans. 1992, 367.
(11) Knight, P. D.; Clarke, A. J.; Kimberley, B. S.; Jackson, R. A.; Scott, P.
Chem. Commun. 2002, 352. Knight, P. D.; O’Shaughnessy, P. N.;
Munslow, I. J.; Kimberley, B. S.; Scott, P. J. Organomet. Chem. 2003,
683, 103.
1
between Mn and monomer/initiator ratio.12 Examination of the H
(12) See Supporting Information.
(13) Reinen, D.; Atanasov, M. in ComprehensiVe Coordination Chemistry II;
McCleverty, J. A., Meyer, T. J., Eds.; Elsevier: London, 2004; Vol. I,
pp. 661-667.
NMR spectrum of a sample of 1a treated with 10 equiv of lactide
showed resonances attributable to the imine hydrogens of the
salicylaldiminato ligand along with the dimethylamino moiety
attached to the end of the growing polymer chains. For the isolated
polymer, two singlet resonances, at δ 2.94 and 3.03 (CDCl3), are
(14) O’Keefe, B. J.; Hillmyer, M. A.; Tolman, W. B. Dalton Trans. 2001,
2215.
JA0470315
9
J. AM. CHEM. SOC. VOL. 126, NO. 42, 2004 13599