Phosphorus-based functional groups as hydrogen bonding templates for rotaxane formation

Article abstract of DOI:10.1021/ja2049786

We report on the use of the hydrogen bond acceptor properties of some phosphorus-containing functional groups for the assembly of a series of [2]rotaxanes. Phosphinamides, and the homologous thio- and selenophosphinamides, act as hydrogen bond acceptors t

Full text of DOI:10.1021/ja2049786

ARTICLE
pubs.acs.org/JACS
Phosphorus-Based Functional Groups as Hydrogen Bonding
Templates for Rotaxane Formation
Rehan Ahmed,‡ Andrea Altieri,† Daniel M. D’Souza,† David A. Leigh,*,† Kathleen M. Mullen,†
Marcus Papmeyer,† Alexandra M. Z. Slawin,‡ Jenny K. Y. Wong,† and J. Derek Woollins*,‡
†School of Chemistry, EaStCHEM, University of Edinburgh, The King’s Buildings, West Mains Road, Edinburgh EH9 3JJ,
United Kingdom
‡School of Chemistry, EaStCHEM, University of St. Andrews, St. Andrews, Fife KY16 9ST, United Kingdom
S Supporting Information
b
ABSTRACT: We report on the use of the hydrogen bond
acceptor properties of some phosphorus-containing functional
groups for the assembly of a series of [2]rotaxanes. Phosphina-
mides, and the homologous thioÀ and selenophosphinamides,
act as hydrogen bond acceptors that, in conjunction with an
appropriately positioned amide group on the thread, direct the
assembly of amide-based macrocycles around the axle to form
rotaxanes in up to 60% yields. Employing solely phosphorus-
based functional groups as the hydrogen bond accepting groups on the thread, a bis(phosphinamide) template and a phosphine
oxideÀphosphinamide template afforded the corresponding rotaxanes in 18 and 15% yields, respectively. X-ray crystallography of
the rotaxanes shows the presence of up to four intercomponent hydrogen bonds between the amide groups of the macrocycle and
various hydrogen bond accepting groups on the thread, including rare examples of amide-to-phosphinamide, -thiophosphinamide,
and -selenophosphinamide groups. With a phosphine oxideÀphosphinamide thread, the solid-state structure of the rotaxane is
remarkable, featuring no direct intercomponent hydrogen bonds but rather a hydrogen bond network involving water molecules
that bridge the H-bonding groups of the macrocycle and thread through bifurcated hydrogen bonds. The incorporation of
phosphorus-based functional groups into rotaxanes may prove useful for the development of molecular shuttles in which the
macrocycle can be used to hinder or expose binding ligating sites for metal-based catalysts.
’ INTRODUCTION
significant hydrogen bond basicity such as N+ÀOÀ and
S+ÀOÀ.9,10 Accordingly, we decided to determine the effective-
ness of employing various PdO, PdS, and PdSe functional
groups as a hydrogen bonding template for rotaxane formation
(II, Scheme 1). The hydrogen bond basicity of PdS and PdSe
functional groups is not well established, and introducing phos-
phorus-based functional groups into a rotaxane architecture14
could be useful for developing molecular shuttles that can expose
or conceal metal binding sites by changing the position of the
rotaxane ring on the thread.
Recently, templates for rotaxane formation based on sulf-
oxides10 and nitrone9 functional groups have been reported. The
related phosphorus analogues have proven to be valuable ligands
for metal coordination15 and have found application in both
organocatalysis16 and transition metal catalysis17 due to their
Lewis basicity and hydrogen bonding18 ability. Accordingly,
we investigated the efficacy of some phosphinic, thiophosphinic,
and selenophosphinic amides (PdX)NHR; X = O, S, Se)19 and
phosphine oxides as hydrogen bonding templates in rotaxane
formation.
Hydrogen bonding has previously been used1À12 to assemble
benzylic amide macrocycles around various amide,1,2 ester,1h,3
squaraine,4 phenolate,5 urea,6 pyridone,7 azodicarboxamide,8
nitrone,9 sulfoxide10 and ion-pair11 templates to generate rotax-
anes and catenanes.12 Although threading protocols using pre-
formed macrocycles have been successful in some cases,1j,6,7 the
poor solubility of most benzylic amide macrocycles in the
nonpolar solvents needed to promote intercomponent hydrogen
bonding has meant that the template assembly of building blocks
about the thread to form the macrocycle is most often used to
construct such rotaxanes.13 These five-component ‘clipping’
reactions involve multipoint hydrogen bonding between the
open-chain precursor 1 (which in the absence of a suitable
template preferentially adopts a linear synÀanti conformation)
and the thread 2 promotes a conformational change that
brings the reactive end groups close together, leading to rapid
cyclization of 1 about the axle (I, Scheme 1).1c,d,h The majority
of neutral hydrogen bond accepting groups employed in
such threads to date have been amides, squaraine units (which
have significant oxocarbon anion character), and esters.1À4
The hydrogen bond acceptors in the thread have recently
been extended to noncarbonyl-based functional groups with
Received: June 5, 2011
Published: June 30, 2011
r
2011 American Chemical Society
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Scheme 1. Hydrogen Bonding Modes of Dipeptide and
BisphosphorusÀDerived Templates in Rotaxane Synthesis
Scheme 2. Synthesis of Phosphinamide, Thiophosphina-
mide, and Selenophospinamide [2]Rotaxanes 6À8a
a Reagents and conditions: (i) isophthaloyl dichloride (5-fold excess),
p-xylylenediamine (5-fold excess), Et3N, CHCl3, RT, 4 h, 60% (6), 18%
(7), 20% (8). The intercomponent hydrogen bonding shown for the
[2]rotaxanes is one of several intramolecular hydrogen bonding modes that
are likely to be present for the rotaxane molecules in solution.1d,fÀh,n,q,t,8À10 It
does not necessarily correspond to the hydrogen bond motifs found in the
solid-state structures (see Figures 1À5) where intermolecular hydrogen
bonding modes (and, in Figure 5, the participation of solvate molecules) are
also available.
’ RESULTS AND DISCUSSION
Suitable phosphinamide (3), thiophosphinamide (4), and
selenophosphinamide (5) threads, which feature the chalcogen
atoms in a spatial orientation similar to that of the carbonyl
groups in a glycylglycine motif (Scheme 1), were prepared in
three steps from commercially available glycylglycine ethyl ester
hydrochloride (see Supporting Information). Pleasingly, upon
addition of a 5-fold molar excess of isophthaloyl dichloride,
p-xylylenediamine and triethylamine in anhydrous chloroform
each thread afforded the corresponding [2]rotaxane, 6À8,
through the hydrogen bond-directed five-component ‘clipping’
reaction (Scheme 2). The phosphinamide [2]rotaxane 6 was
obtained in 60% yield, similar to that (62%) obtained for the
previously reported1d glycylglycine analogue, indicating that the
phosphinamide group (with a large, sp3-hybridized P+ÀOÀ
group) is able to function almost as effectively as one of the
amide groups in the hydrogen bond-directed template reaction.
Somewhat unexpectedly, the thiophosphinamide and selenopho-
sphinamide [2]rotaxanes 7 and 8 were also formed in reasonable
yield, 18 and 20% respectively, from the reactions with the
corresponding PdS and PdSe threads (4 and 5).
Figure 1. X-ray crystal structure of phosphinamide rotaxane 6. Selected
bond lengths [Å]: P37ÀO37, 1.48. Intercomponent hydrogen bond
distances [Å]: O37ÀH2N, 1.87; O40ÀH20N, 2.20; O40ÀH29N, 2.05.
Intercomponent hydrogen bond angles [deg]: N2ÀHÀO37, 161.4;
N20ÀHÀO40, 170.5; N29ÀHÀO40, 159.7.
phosphorus functional group rather than the ester moiety, which
is a poor hydrogen bond acceptor.1h,20 The macrocycle adopts a
distorted boat-like conformation with one amide hydrogen bond-
ing to the O/S/Se atom of the phosphorus functional group with
the other isophthalamide unit adopting bifurcated hydrogen
bonds to the single amide group of the thread. The fourth amide
Single crystals of each of the three rotaxanes (6À8) were
obtained by slow diffusion of hexane vapor into solutions of the
rotaxanes in chloroform and the solid-state structures deter-
mined by X-ray crystallography (Figures 1À3). The three crystal
structures show broadly similar features: the macrocycle hydro-
gen bonds to the amide carbonyl and the O/S/Se atom of the
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Scheme 3. Synthesis of Bis(phosphinamide) [2]Rotaxane
12 (and Attempted Synthesis of the Corresponding Bis-
(thiophosphinamide) and Bis(selenophospinamide)
[2]Rotaxanes 13 and 14)a
Figure 2. X-ray crystal structure of thiophosphinamide rotaxane 7.
Selected bond lengths [Å]: P37ÀS37, 1.95. Intercomponent hydrogen
bond distances [Å]: S37ÀH2N, 2.40; O40ÀH20N, 2.19; O40ÀH29N,
2.07. Intercomponent hydrogen bond angles [deg]: N2ÀHÀS37,
168.6; N29ÀHÀO40, 153.6; N20ÀHÀO40, 173.5.
a Reagents and conditions: (i) isophthaloyl dichloride (5-fold excess),
p-xylylenediamine (5-fold excess), Et3N, CHCl3, RT, 4 h, 18% (12),
0% (13), 0% (14).
Figure 3. X-ray crystal structure of selenophosphinamide rotaxane 8.
Selected bond lengths [Å]: P37ÀSe37, 2.10. Intercomponent hydrogen
bond distances [Å]: Se37ÀH11N, 2.54; O40ÀH20N, 2.09; O40ÀH29N,
2.12. Intercomponent hydrogen bond angles [deg]: N11ÀHÀSe37,
153.2; N20ÀHÀO40, 173.3; N29ÀHÀO40, 169.4.
Figure 4. X-ray crystal structure of bis(phosphinamide) rotaxane 12.
Selected bond lengths [Å]: P31ÀO31 = P31aÀO31a, 1.45. Intercompo-
nent hydrogen bond distances [Å]: O31ÀHN2 = O31AÀHN2a, 2.20;
O31ÀH11 = O31aÀHN11a, 2.38. Intercomponent hydrogen bond
angles [deg]: N2ÀHÀO31 = N2aÀHÀO31a, 160.0; N11ÀHÀ
O31 = N11aÀHÀO31a, 165.6.
group of the macrocycle engages in intermolecular hydrogen
bonding to an adjacent rotaxane molecule in the crystal. In com-
parison to the short PdO HN hydrogen bond (O37ÀH2A,
synthesis of the PdS and PdSe rotaxanes (Scheme 2). A search
of the CCDC database revealed only one other example21 of
amide-to-phosphinamide hydrogen bondingand oneexample22 of
amide-to-thiophosphinamide hydrogen bonding. As far as we are
aware, there are no previous examples of amides hydrogen
bonding to PdSe functional groups.23
Having shown that one amide carbonyl of the rotaxane-
forming glycylglycine template (Scheme 1) can be success-
fully substituted by phosphinamide, thiophosphinamide and
3 3 3
1.87 Å) in the phosphinamide rotaxane 6, the corresponding
PdS HN and PdSe HN hydrogen bonds in the thiopho-
3 3 3
3 3 3
sphinamide and selenophosphinamide rotaxanes are relatively
long (S37ÀH2A, 2.40 Å (7); Se37ÀH11N, 2.54 Å (8)). While
the differences in hydrogen bond length can be largely attributed
to the increasing atomic radius of sulfur and selenium (O2À
1.26 Å; S2À 1.70 Å; Se2À 1.84 Å), weaker intercomponent hydro-
gen bonding is consistent with the lower yields obtained in the
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Scheme 4. Synthesis of PhosphinamideÀPhosphine Oxide
[2]Rotaxane 17 (and Attempted Synthesis of Bis(phosphine
oxide) [2]Rotaxane 18)a
Figure 5. X-ray crystal structure of phosphinamideÀphosphine oxide
rotaxane 17. Selected bond lengths [Å]: P37ÀO37, 1.50; P42ÀO42, 1.50.
Hydrogen bond distances [Å]: O37ÀH60a, 1.94; O37ÀH60b, 2.68;
O60ÀH20N, 2.35; O60ÀH29N, 2.17; O42ÀH61a, 1.95; O42ÀH61b,
2.68; O61ÀHN2, 2.35; O61ÀHN11, 2.17. Hydrogen bond angles [deg]:
O60ÀH60aÀO37, 128.5; O60ÀH60bÀO37, 79.3; N20ÀHÀO60,
175.1; N29ÀHÀO60, 161.5; O61ÀH61aÀO42, 128.4; O61À
H61bÀO42, 79.2; N2ÀHÀO61, 175.1; N11ÀHÀO61, 161.5.
a Reagents and conditions: (i) isophthaloyl dichloride (5-fold excess),
p-xylylenediamine (5-fold excess), Et3N, CHCl3, RT, 4 h, 15% (17),
0% (18).
Scheme 5. Synthesis of Phosphine Oxide-Based
[2]Rotaxanes 21 and 22a
selenophosphinamide functionalities, we investigated the effi-
cacy of employing two such groups in the thread. Symmetrical
bis(phosphinamide) thread 9, and the analogous thio- and
seleno- analogues, 10 and 11, were synthesized from 1,2-diami-
noethane and the corresponding chalcogen chlorodiphenylpho-
sphine derivatives (see Supporting Information) and subjected
to the rotaxane-forming reaction conditions employed pre-
viously (Scheme 3). Although no rotaxane could be detected
in the reactions involving the bis(thiophosphinamide) or bis-
(selenophosphinamide) threads, the bis(phosphinamide) rotax-
ane 12 was isolated in 18% yield. Single crystals of the rotaxane
were obtained by layering a saturated CDCl3 solution with
hexanes and the solid state structure determined by X-ray
crystallography (Figure 4).
The X-ray crystal structure of 12 shows that, somewhat
remarkably, given the size and (tetrahedral) arrangement of
the phosphorus functional groups, the rotaxane molecules adopt
a co-conformation in which both phosphinamide moieties are
able to form bifurcated hydrogen bonds (2.20À2.38 Å) with the
macrocycle isophthalamide groups (Figure 4). The macrocycle is
able to satisfy this hydrogen bonding arrangement while adopt-
ing a low-energy chair conformation.
a Reagents and conditions: (i) isophthaloyl dichloride (5-fold excess),
p-xylylenediamine (5-fold excess), Et3N, CHCl3, RT, 4 h, 42% (21),
6% (22).
We next turned our attention to employing phosphine oxide
groups in the template site. Keeping the interhydrogen bond-
acceptor distance similar to the bis(phosphinamide) system, the
phosphinamideÀphosphine oxide thread 15 and bis(phosphine
oxide) thread 16 were prepared (see the Supporting Information)
and each subjected to the standard rotaxane-forming conditions
(Scheme 4). Although no rotaxane could be detected using the
bis(phosphine oxide) thread 16, we were delighted to isolate the
[2]rotaxane 17 in 15% yield from the reaction featuring the
phosphinamideÀphosphine oxide template (Scheme 4). This is
consistent with other studies24 that suggest that phosphinamides
are more strongly polarized than phosphine oxides and therefore
the phosphinamide phosphoryl oxygen is more negatively
charged and should be a better hydrogen bond acceptor. How-
ever, the different steric requirements of the groups adjacent to
the PdO unit in the thread—sp2-NH for the phosphinamide;
sp3-CH2 in the phosphine oxide—may also contribute to the
differences in the effectiveness of the templates orienting the
open-chain macrocycle precursor (II in Scheme 1) about the
thread.
Single crystals of rotaxane 17 were obtained by layering a
saturated CDCl3 solution with hexanes, and the solid-state
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in solution (Figure 6). Comparison of the spectra in Figure 6
shows that in both rotaxanes the co-conformations of the
mechanically interlocked components are dominated by hydro-
gen bonding that locates the xylylene groups of the macrocycles
over the ethylene spacer (note the large shifts in protons Ha
and Hb). The strength of hydrogen bonding between the
components of 21 and 22, however, is very different, reflected
in the relative chemical downfield shifts of the macrocycle amide
protons (HD) in 21 (δ 7.79) and 22 (δ 7.31). Furthermore, the
HC protons of the macrocycle which are positioned in the
shielding region of a carbonyl group when bifurcated hydrogen
bonding occurs, are shielded to a greater degree in 21 (δ 8.33)
than in 22 (δ 8.12).
’ CONCLUSIONS
Replacing one or both amide groups of the classical glycylgly-
cine template for rotaxane formation with hydrogen bond
acceptors based on various phosphorus-containing functional
groups can result in effective motifs for the hydrogen bondÀ
directed synthesis of [2]rotaxanes. In addition to PdO-based
phosphinamide and phosphine oxide functional groups, thio-
and selenophosphinamides can act as sufficiently good hydrogen
bond acceptors to form rotaxanes in combination with an amide
group in the template site. Phosphinamides can successfully
substitute for both amide groups, even though this results in a
significant change in both steric size and shape (sp2 to sp3) of the
hydrogen bond-accepting groups at the template site. X-ray
crystal structures and 1H NMR spectroscopy reveal information
about the intramolecular hydrogen bonding networks between
the components in both solution and the solid state. As well as
increasing the diversity of templates available for the assembly of
benzylic amide macrocycle rotaxanes, such templates may prove
useful for the assembly of rotaxanes and molecular shuttles in
which the macrocycle can be used to hinder or expose binding
ligating sites for metal-based catalysts.
Figure 6. 1H NMR spectra (CDCl3, 400 MHz, 298 K) of (a) phosphine
oxideÀamide thread 19, (b) phosphine oxideÀamide-based [2]-
rotaxane 21, (c) phosphine oxideÀester thread 20, (d) phosphine
oxideÀester-based [2]rotaxane 22. The assignments correspond to
the lettering shown in Scheme 5.
structure was determined by X-ray crystallography (Figure 5).
Remarkably, the hydrogen bonding motifs and relative positions
of the isophthalamide macrocycle and the phosphinami-
deÀamide based thread in the crystal structure of rotaxane 17
are very different from the other phosphorus rotaxanes and,
indeed, to all other amide-based rotaxanes reported to date.
There are no direct intercomponent hydrogen bonds between
the macrocycle and thread in the solid-state structure of 17
(Figure 5). Rather, both isophthalamide units of the macrocycle
engage in bifurcated hydrogen bonding with water molecules
which, in turn, act as hydrogen bond donors in a set of bifurcated
hydrogen bonds to the oxygen atom of the phosphinamide and
phosphine oxide groups on the thread. Although rotaxanes 6À8
and 12 all have water molecules as solvates in their X-ray crystal
structures, only 17 sacrifices direct intracomponent hydrogen
bonding and employs water as a ‘bridge’ to hydrogen bonding in
a manner reminiscent of the role water often plays to connect
hydrogen bonding groups in the binding of substrates in the
active sites of enzymes and antibodies.25
’ ASSOCIATED CONTENT
S
Supporting Information. Detailed experimental proce-
b
dures and spectroscopic data for all compounds synthesized,
and full crystallographic data for rotaxanes 6À8, 12, and 17. This
material is available free of charge via the Internet at http://pubs.
acs.org.
’ AUTHOR INFORMATION
Corresponding Author
David.Leigh@ed.ac.uk; jdw3@st-and.ac.uk
Finally, we investigated the efficacy of a phosphine oxide unit
with an amide or ester group as the other hydrogen bond-
accepting moiety in the template site (Scheme 5). Reaction of
phosphine oxideÀamide thread 19 or phosphine oxideÀester
thread 20 under the rotaxane-forming reaction conditions
afforded the corresponding [2]rotaxanes 21 and 22 in 42% and
6% yields, respectively (Scheme 5). Esters are much poorer
hydrogen bond acceptors than amides,1h,20 and so the modest
yield of 22 is unsurprising.
’ ACKNOWLEDGMENT
This work was supported by the EPSRC. We thank the
Deutsche Akademia der Naturforscher Leopoldina (BMBF LPD
9901/8-166) and Peter und Traudl Engelhorn-Stiftung for
postdoctoral fellowships to D.M.D’S.
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intercomponent hydrogen bonding in the interlocked structures
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