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Article
Size-Matched Radical Multivalency
Mark C. Lipke, Tao Cheng, Yilei Wu, Hasan Arslan, Hai Xiao,
Michael R. Wasielewski, William A. Goddard, and J. Fraser Stoddart
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b09892 • Publication Date (Web): 07 Feb 2017
Downloaded from http://pubs.acs.org on February 10, 2017
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Size-Matched Radical Multivalency
Mark C. Lipke,
1
Tao Cheng,
2
Yilei Wu,
1
Hasan Arslan,
1
Hai Xiao,
2
Michael R. Wasielewski,
1
William A. Goddard III,
2
J. Fraser Stoddart
1*
1
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston,
IL 60208 (USA)
2
Materials and Process Simulation Center, , California Institute of Technology, 1200 California
Blvd, Pasadena, CA 91125 (USA)
*E-mail:
stoddart@northwestern.edu
MAIN TEXT
Professor J. Fraser Stoddart
Department of Chemistry
Northwestern University
2145 Sheridan Road
Evanston, IL 60208 (USA)
Email: stoddart@northwestern.edu
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ABSTRACT: Persistent -radicals such as MV
+•
(MV refers to methyl viologen, i.e., N,Nꞌ-
dimethyl-4,4ꞌ-bipyridinum) engage in weak radical-radical interactions. This phenomenon has
been utilized recently in supramolecular chemistry with the discovery that MV
+
•
and
[cyclobis(paraquat-p-phenylene)]
2(
+
•)
(CBPQT
2(+•)
) form a strong 1:1 host-guest complex
[CBPQT⊂MV]
3(+•)
. In this full paper, we describe the extension of radical-pairing-based
molecular recognition to a larger, square-shaped diradical host, [cyclobis(paraquat-4,4ꞌ-
biphenylene)]
2(
+
•)
(MS
2(+•)
). This molecular square was evaluated for its ability to bind an
isomeric series of possible diradical cyclophane guests, which consist of two radical viologen
units that are linked by two ortho-, meta-, or para-xylylene bridges to provide different spacing
between the planar radicals. UV-Vis-NIR Measurements reveal that only the m-xylylene-linked
isomer (m-CBPQT
2(+•)
) binds strongly inside of MS
2(+•)
, resulting in the formation of a tetra-
radical complex [MS⊂m-CBPQT]
4(+•)
. Titration experiments and variable temperature UV-Vis-
NIR and EPR spectroscopic data indicate that, relative to the smaller trisradical complex
[CBPQT⊂MV]
3(+•)
, the new host-guest complex forms with a more favorable enthalpy change
that is offset by a greater entropic penalty. As a result, the association constant
(K
a
= (1.12 +/- 0.08) x 10
5
M
-1
) for [MS⊂m-CBPQT]
4(+•)
is similar to that previously determined
for [CBPQT⊂MV]
3(+•)
. The (super)structures of MS
2(+•)
, m-CBPQT
2(+•)
, and [MS⊂m-
CBPQT]
4(+•)
were examined by single-crystal X-ray diffraction measurements and DFT
calculations. The solid-state and computational structural analyses reveal that m-CBPQT
2(+•)
is
ideally sized to bind inside of MS
2(+•)
. The solid-state superstructures also indicate that localized
radical-radical interactions in m-CBPQT
2(+•)
and [MS⊂m-CBPQT]
4(+•)
disrupt the extended
radical-pairing interactions that are common in crystals of other viologen radical cations. Lastly,
the formation of [MS⊂m-CBPQT]
4(+•)
was probed by cyclic voltammetry, demonstrating that
the radical states of the cyclophanes are stabilized by the radical-pairing interactions.
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■ INTRODUCTION
The study of synthetic hosts for guest recognition, which was initiated with the synthesis of
crown ethers that bind Group IA and IIA metal cations,
1
has subsequently undergone expansion
to incorporate the investigation of inclusion complexes based on a diverse array of molecular
recognition motifs.
2-7
Complexes have been formed using electrostatic attraction,
3
the
hydrophobic effect,
4
- stacking,
5
hydrogen bonding,
6
and numerous other noncovalent
bonding interactions.
2
A noteworthy feature of many recognition motifs is the importance of
size-complementarity between the host and the guest, as noted early on in size-matched crown
ethers and cations.
1,7
Other size-based binding recognition phenomena include Rebek’s 55-
percent rule
8
for the inclusion of guests inside hydrophobic capsules, and the necessity for a 3.2 –
3.5 Å spacing between the -surfaces of donor-acceptor - stacks.
9
This latter consideration has
been a defining feature of the tetracationic host, cyclobis(paraquat-p-phenylene)
10
(CBPQT
4+
),
which has two electron-deficient viologen units — i.e., 4,4ꞌ-bipyridinium dications — that are
ideally spaced (Scheme 1, upper left) for interaction with planar, electron-rich aromatic guests
such as tetrathiafulvene
10c
(TTF). More recently, the spacing of viologen units has been found
11
to be ideal (Scheme 1, upper right) for the diradical dication CBPQT
2(+•)
to bind the methyl
viologen radical cation (MV
+•
) as a unique tricationic trisradical complex held together by
radical-pairing interactions. The respective abilities of CBPQT
4+
and CBPQT
2(+•)
to bind
aromatic and -radical guests has made this redox-active cyclophane one of the most studied of
supramolecular hosts.
2m,l,5b,12
Recognition motifs of these types have been used to template the
formation of a wide variety of mechanically interlocked molecules
13
(MIMs) and artificial
molecular machines
14
(AMMs).
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Despite the long history of CBPQT
4+
, it is only recently that much attention has been given to
size-homologues of this host,
12,15
a situation which stands in contrast to other common
macrocyclic hosts — e.g., cucurbiturils,
16
cyclodextrins,
17
and crown ethers
1,7
— for which many
differently sized variants have been investigated. Early efforts at expanding the CBPQT
4+
motif
include the synthesis of the square-shaped tetracationic cyclobis(paraquat-4,4ꞌ-biphenylene)
18
(MS
4+
), which provides a significantly increased separation between its viologen units. This
increase creates enough space to form a 1:1 complex with ferrocene
18
or to host planar aromatic
guests in a 1:2 host-guest ratio,
19
despite the fact that both types of interactions are weak in the
absence of supporting [C—H---O] hydrogen bonding interactions.
18,19c
Nevertheless, these 1:2
complexes have been useful for templating the formation of [3]catenanes,
19a,b
[3]rotacatenanes,
20
and higher order oligocatenanes.
21
More recent efforts efforts
12,15
to expand the CBPQT
4+
host
have focused primarily on extending the electron poor -surface in order to accommodate larger
two-dimensional guests along with those that deviate only slightly from planarity, e.g., helicene,
corannulene. These two-dimensionally extended variants of CBPQT
4+
now abound, while in
contrast, only one recent study
15e
addresses the binding of a three-dimensional guest, namely C
60
,
by a larger homologue of CBPQT
4+
.
Investigations of expanded homologues of CBPQT
4+
have so far overlooked possible radical-
based host-guest chemistry of the reduced states of these cyclophanes. As a result, the small
tricationic trisradical [CBPQT
⊂MV]
3(+•)
, and simple derivatives thereof,
22
have stood as
singular examples of radical-paired host-guest complexes since their introduction more than half
a decade ago. In this time period, this interaction has formed the basis of a diverse array of MIMs
and AMMs such as multistate redox-actuated switches,
23
nanopumps,
14d
and molecular
muscles.
24
The development of additional, larger examples of these radical-based recognition
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