Influence of Constitution and Charge on Radical Pairing Interactions in Tris-radical Tricationic Complexes

Article abstract of DOI:10.1021/jacs.6b04343

The results of a systematic investigation of trisradical tricationic complexes formed between cyclobis(paraquat-p-phenylene) bisradical dicationic (CBPQT^2(?+)) rings and a series of 18 dumbbells, containing centrally located 4,4′-bipyridinium radical cationic (BIPY^?+) units within oligomethylene chains terminated for the most part by charged 3,5-dimethylpyridinium (PY⁺) and/or neutral 3,5-dimethylphenyl (PH) groups, are reported. The complexes were obtained by treating equimolar amounts of the CBPQT⁴⁺ ring and the dumbbells containing BIPY²⁺ units with zinc dust in acetonitrile solutions. Whereas UV-Vis-NIR spectra revealed absorption bands centered on ca. 1100 nm with quite different intensities for the 1:1 complexes depending on the constitutions and charges on the dumbbells, titration experiments showed that the association constants (K_a) for complex formation vary over a wide range, from 800 M^-1 for the weakest to 180 000 M^-1 for the strongest. While Coulombic repulsions emanating from PY⁺ groups located at the ends of some of the dumbbells undoubtedly contribute to the destabilization of the trisradical tricationic complexes, solid-state superstructures support the contention that those dumbbells with neutral PH groups at the ends of flexible and appropriately constituted links to the BIPY^?+ units stand to gain some additional stabilization from C-H?π interactions between the CBPQT^2(?+) rings and the PH termini on the dumbbells. The findings reported in this Article demonstrate how structural changes implemented remotely from the BIPY^?+ units influence their non-covalent bonding interactions with CBPQT^2(?+) rings. Different secondary effects (Coulombic repulsions versus C-H?π interactions) are uncovered, and their contributions to both binding strengths associated with trisradical interactions and the kinetics of associations and dissociations are discussed at some length, supported by extensive DFT calculations at the M06-D3 level. A fundamental understanding of molecular recognition in radical complexes has relevance when it comes to the design and synthesis of non-equilibrium systems.

Full text of DOI:10.1021/jacs.6b04343

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Subscriber access provided by UNIV OF NEBRASKA - LINCOLN
Article
Influence of Constitution and Charge on Radical Pairing
Interactions in Trisradical Tricationic Complexes
Chuyang Cheng, Tao Cheng, Hai Xiao, Matthew D. Krzyaniak, Yuping Wang,
Paul R. McGonigal, Marco Frasconi, Jonathan C. Barnes, Albert C Fahrenbach,
Michael R. Wasielewski, William A. Goddard III, and J. Fraser Stoddart
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b04343 • Publication Date (Web): 06 Jun 2016
Downloaded from http://pubs.acs.org on June 11, 2016
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Journal of the American Chemical Society is published by the American Chemical
Society. 1155 Sixteenth Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Page 1 of 35
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
Influence of Constitution and Charge on Radical Pairing
Interactions in Trisradical Tricationic Complexes
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Chuyang Cheng,† Tao Cheng,§ Hai Xiao,§ Matthew D. Krzyaniak,†, ‡ Yuping Wang,†
†
Paul R. McGonigal, Marco Frasconi,† ┴ Jonathan C. Barnes,† # Albert C. Fahrenbach,† ○
∥
⧫
Michael R. Wasielewski,†,‡ William A. Goddard III,§ J. Fraser Stoddart†*
† Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois
60208, United States
§ Materials and Process Simulation Center (MC 139ꢀ74), California Institute of Technology,
Pasadena, California 91125, United States
‡ ArgonneꢀNorthwestern Solar Energy Research (ANSER) Center, Northwestern University,
2145 Sheridan Road, Evanston, Illinois 60208, United States
∥ Department of Chemistry, Durham University, South Road, Durham DH1 3LE, United
Kingdom
┴ Department of Chemistry, University of Padova, Via Marzolo 1, Padova 35131, Italy
#
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue,
Cambridge, Massachusetts 02139, United States
○
Howard Hughes Medical Institute, Department of Molecular Biology and Center for
Computational and Integrative Biology, Massachusetts General Hospital, 185 Cambridge
Street, Boston, Massachusetts 02114, United States
⧫
EarthꢀLife Science Institute, Tokyo Institute of Technology, 2ꢀ12ꢀ1ꢀIEꢀ1 Ookayama, Meguroꢀ
ku, Tokyo 152ꢀ8550, Japan
Abstract The results of a systematic investigation of trisradical tricationic complexes formed
between cyclobis(paraquatꢀpꢀphenylene) bisradical dicationic (CBPQT2(•+)) rings and a series of
18 dumbbells, containing centrally located 4,4′ꢀbipyridinium radical cationic (BIPY•+) units
within oligomethylene chains terminated for the most part by charged 3,5ꢀdimethylpyridinium
ACS Paragon Plus Environment

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Journal of the American Chemical Society
Page 2 of 35
1
2
(PY+) and/or neutral 3,5ꢀdimethylphenyl (PH) groups, are reported. The complexes were
obtained by treating equimolar amounts of the CBPQT4+ ring and the dumbbells containing
BIPY2+ units with zinc dust in acetonitrile (MeCN) solutions. Whereas UVꢀVISꢀNIR spectra
revealed absorption bands centered on ca. 1100 nm with quite different intensities for the 1:1
complexes depending on the constitutions and charges on the dumbbells, titration experiments
show that the association constants (Ka) for complex formation vary over a wide range from Ka
values of 800 Mꢀ1 for the weakest to 180000 Mꢀ1 for the strongest. While Coulombic repulsions
emanating from PY+ groups located at the ends of some of the dumbbells undoubtedly contribute
to the destabilization of the trisradical tricationic complexes, solidꢀstate superstructures support
the contention that those dumbbells with neutral PH groups at the ends of flexible and
appropriately constituted links to the BIPY•+ units stand to gain some additional stabilization
from C‒H···π interactions between the CBPQT2(•+) rings and the PH termini on the dumbbells.
The findings reported in this full paper demonstrate how structural changes implemented
remotely from the BIPY•+ units influence their noncovalent bonding interactions with CBPQT2(•+)
rings. Different secondary effects (Coulombic repulsions versus C‒H···π interactions) are
uncovered and their contributions to both binding strengths associated with trisradical
interactions and the kinetics of associations and dissociations are discussed at some length and
are supported by extensive DFT calculations at the M06ꢀD3 level. A fundamental understanding
of molecular recognition in radical complexes has relevance when it comes to the design and
synthesis of nonꢀequilibrium systems.
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ꢀ INTRODUCTION
Noncovalent bonding interactions have become an important consideration, along with
molecular recognition when designing functional materials during the past couple of decades.1
The bottomꢀup approach of supramolecular chemistry2 has contributed to many areas with
potential for applications such as chemical sensors3, responsive materials4, drug delivery
vehicles5, catalysis6 etc. Weak interactions such as hydrogen bonding7, metal coordination8,
hydrophobic forces9, van der Waals interactions10, πꢀπ stacking11 and electrostatic effects12 have
all been investigated widely in the context of supramolecular systems. Radicalꢀradical
interactions, however, have found only limited attention13 in such systems.
1,1′ꢀDialkylꢀ4,4′ꢀbipyridinium (BIPY2+) dications are commonly used units14 in supramolecular
2
ACS Paragon Plus Environment

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Page 3 of 35
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
chemistry, mostly as πꢀelectron poor acceptors in πꢀπ stacking and in hostꢀguest complexes. By
contrast, the radicalꢀradical dimerization of the reduced form of BIPY2+ ‒ namely BIPY•+ ‒ was
discovered15 much earlier than the charge transfer complexation16 of BIPY2+ dications with
electron donors. The radicalꢀradical dimerization17 of BIPY•+ radical cations, also known as
pimerization18, however, was not widely employed19 in the design of supramolecular systems
because of the low binding ability17a,17b,17d of radical pairs, resulting in pimerization only being
observable at significantly high concentrations. Cyclobis(paraquatꢀpꢀphenylene)20 (CBPQT4+) as
a higher homologue of BIPY2+, containing two BIPY2+ units connected in a rigid fashion by two
paraꢀxylylene linkers, has been exploited as an electron deficient host20c,21 during the past quarter
century. Recently, we discovered (Figure 1) that BIPY2+ forms a trisradical tricationic complex22
‒ namely BIPY•+⊂CBPQT2(•+) ‒ with the CBPQT4+ ring under reducing conditions. The binding
constant23 (Ka ~ 104 Mꢀ1 in MeCN) associated with this 1:1 complex is comparable or even
stronger than those (Ka = 103–105 Mꢀ1 in MeCN) involving donor–acceptor complexes24 between
aromatic crown ethers and CBPQT4+. The strength of the trisradical tricationic complex outstrips
that (Ka < 103 Mꢀ1 in MeCN) involving dimerization15e of BIPY•+ units on account of the
macrocyclic effect.25 Subsequently, we have introduced radicalꢀradical interactions into
templating the synthesis of rotaxanes26 and catanenes27, as well as into foldamers28, daisy
chains29, molecular switches30, molecular motors31, molecular pumps32, and semiconducting
materials33.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Although there are a large number of molecular recognition motifs from which to choose when
designing artificial molecular machines31b, there are few, if any, that can be switched back and
forth between repulsion and attraction without making and breaking covalent bonds, while also
being orthogonal/complementary to other common nonbonded interactions. Our recent study31a,32
has revealed that the radicalꢀradical interactions between BIPY•+ units and CBPQT2(•+) rings are
not only capable of meeting the above criteria but also, more importantly, the kinetics of
association and dissociation can be modulated, a key factor designing nonꢀequilibrium systems.
In this full paper, an inꢀdepth investigation brings to light a simple way of modulating the
thermodynamics of the trisradical interactions. We assess the binding aptitudes of the CBPQT2(•+)
ring towards a series of dumbbells (Table 1) in which oligomethylene chains incorporate BIPY•+
units in their midriffs and carry variously (i) two positively charged 3,5ꢀdimethylpyridinium
(PY+) termini, (ii) two neutral 3,5ꢀdimethylphenyl (PH) termini, and (iii) a PY+ terminus at one
3
ACS Paragon Plus Environment