Exploiting aurophilic interactions in a [2 + 2] photocycloaddition: Single-crystal reactivity with changes to surface morphology

Article abstract of DOI:10.1021/acs.inorgchem.9b01306

A single-crystal-to-single-crystal (SCSC) photodimerization is achieved using Au(I) coordination and aurophilic interactions. The rigid Au₂(dppbz)(CF₃COO)₂ precursor (dppbz = 1,2-bis(diphenylphosphino)benzene) self-assembles with 1,2-trans-bis(4-pyridyl)ethylene (bpe) to afford a discrete [Au₄(dppbz)₂(bpe)₂]⁴⁺ macrocycle in the solid state. The alkene undergoes a [2 + 2] photocycloaddition reaction. The photoreaction proceeds via a rare SCSC transformation in quantitative yield that generates [Au₄(dppbz)₂(4,4′-tpcb)]⁴⁺ (4,4′-tpcb = rctt-tetrakis(4-pyridyl)cyclobutane) stereoselectively. Mechanical strain induced by the photoreaction is evidenced by the formation of ramp features on single-crystal surfaces using scanning electron microscopy.

Full text of DOI:10.1021/acs.inorgchem.9b01306

Communication
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Exploiting Aurophilic Interactions in a [2 + 2] Photocycloaddition:
Single-Crystal Reactivity with Changes to Surface Morphology
Changan Li, Gonzalo Campillo-Alvarado, Dale C. Swenson, and Leonard R. MacGillivray*
Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States
*
S Supporting Information
feasibility of using a rigid phosphine ligand (i.e., 1,2-
bis(diphenylphosphino)benzene (dppbz)) to sustain auro-
philic interactions and sustain the formation of a rectangular
macrocycle involving 1,2-trans-bis(4-pyridyl)ethylene (bpe).
We expected the stacking of bpe in the macrocycle to be
ABSTRACT: A single-crystal-to-single-crystal (SCSC)
photodimerization is achieved using Au(I) coordination
12
and aurophilic interactions. The rigid Au (dppbz)-
2
(
(
CF COO) precursor (dppbz
diphenylphosphino)benzene) self-assembles with 1,2-
=
1, 2-bis-
3
2
amenable for an intermolecular photocycloaddition according
13
trans-bis(4-pyridyl)ethylene (bpe) to afford a discrete
to Schmidt, in particular, gold coordination and aurophilic
4+
[Au (dppbz) (bpe) ] macrocycle in the solid state. The
4 2 2
interactions (i.e., Au(I)···Au(I)), direct stacking of bpe in the
alkene undergoes a [2 + 2] photocycloaddition reaction.
The photoreaction proceeds via a rare SCSC trans-
formation in quantitative yield that generates
4+
macrocycle [Au (dppbz) (bpe) ] (1) (Scheme 1) wherein
4
2
2
two bpe molecules react in the solid state to generate
4+
[
Au (dppbz) (rctt-tpcb)] (where: tpcb = rctt-tetrakis(4-
4 2
4
+
[
Au (dppbz) (4,4′-tpcb)] (4,4′-tpcb = rctt-tetrakis(4-
4
2
pyridyl)cyclobutane) (2).
pyridyl)cyclobutane) stereoselectively. Mechanical strain
induced by the photoreaction is evidenced by the
formation of ramp features on single-crystal surfaces
using scanning electron microscopy.
Scheme 1. Use of Au(I)···Au(I) to Direct Intra-macrocycle
[
2 + 2] Photocycloaddition in the Solid State
1
oncovalent interactions (e.g., hydrogen bonding,
2
3
N
halogen bonding, and metal/boron coordination )
have been exploited to direct the formation of covalent
bonds in photoactive solids. The strategy relies on auxiliary
molecules or “templates” to organize reactants (i.e., olefins) in
4
a suitable geometry for a [2 + 2] photodimerization. In this
context, noncovalent interactions of gold continue to draw
interest as a means to construct exceptionally diverse
5
supramolecular architectures. Gold(I) complexes involving
aurophilic interactions have been shown to exhibit intriguing
optical, electrical, magnetic, and catalytic properties for
6
promising applications as advanced materials. However, the
use of aurophilic interactions to achieve photoreactivity in the
solid state has rarely been investigated, and single-crystal
reactivity has still not been demonstrated.
7
12
The preparation of 1 was adapted from Deak et al. The
́
preparation afforded single crystals as colorless irregular prisms
by the slow diffusion of n-hexane into a CH Cl solution (1:3)
of Au (dppbz)(CF COO) and bpe over a period of 2 days.
Powder X-ray diffraction revealed a diffractogram consistent
with that reported. Whereas the single crystals of Deak et al.
́
readily decay, the crystals reported here were sufficiently stable
to support full X-ray data collection. Repeated experiments also
showed the crystals to be highly photoactive. Single-crystal X-
Herein we report the use of gold coordination and
aurophilic forces to achieve a quantitative [2 + 2] photo-
cycloaddition via a rare single-crystal-to-single-crystal (SCSC)
2
2
2
3
2
8
12
transformation. We describe what is to our knowledge the first
example of aurophilic forces directing a photodimerization in
an SCSC transformation. Scanning electron microscope
(
SEM) images reveal submicron features in the form of
r a y d i ff r a c t i o n c o n fi r m e d
a
f o r m u l a o f
ramps to appear on the surfaces of the single crystals, which is
consistent with a mechanical strain being produced during the
[
Au (dppbz) (bpe) ] (CF COO) ·2CH Cl ·2H O.
4
2
2 3
3
12
2
2
2
The macrocycle crystallizes in the monoclinic space group
P2 /n. The asymmetric unit consists of one and one-half
macrocycles (assemblies A and B), six CF CO ions, and
[
2 + 2] photocycloaddition.
Our work is inspired by pioneering studies of Puddephatt,
1
−
who has explored the applications of aurophilic interactions to
3
2
6
c
design supramolecular networks such as macrocycles,
9
10
11
catenanes, oligomers, and polymers. Our approach is
based on observations by Deak et al., who demonstrated the
Received: May 6, 2019
́
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b01306
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
CH Cl /H O solvent molecules. The X-ray structure reveals
the stacked pairs of bpe molecules to be parallel and separated
by (A) 3.86 and (B) 3.28 Å (Figure 1).
4,4′-tpcb in quantitative yield (Figure 2). The X-ray analysis
2
2
2
showed the generation of the cyclobutane ring to result in a
Figure 2. X-ray structure of 2: (a) Overlay views of 1 (light gray) and
2
(gray) and (b) extended view of assemblies A and B. Counter ions
Figure 1. X-ray structure of 1: (a) macrocycle (assembly A) and (b)
extended view of discrete assembles. Counter ions and solvent atoms
are omitted for clarity.
and solvent atoms are omitted for clarity.
slight weakening of the aurophilic forces (Au(1)···Au(2)
T h e s e l f - a s s e m b l y o f t h e c o m p o n e n t s o f
3
.159(2), Au(3)···Au(4) 3.110(2), Au(5)···Au(6) 3.142(2)
4+
2+
[
Au (dppbz) (bpe) ] give rise to [Au (dppbz) ] as corners
4 2 2 4 2
Å). The results contrast previous observations using
argentophilic (i.e., Ag···Ag) forces, where the formation of
the cyclobutane is accompanied by the complete breaking of
and bpe as edges (Figure 1a). The Au(I) centers are
coordinated linearly (assembly A (deg): P(1)−Au(1)−N(1)
1
1
67.0(4), P(2)−Au(2)−N(2) 166.6(4), P(3)−Au(3)−N(3)
67.8(5), P(4)−Au(4)−N(4) 167.4(4); assembly B: P(5)−
16
the supramolecular force. Free movement of the Au(I) atoms
is likely restricted by the rigid dppbz ligand. The photo-
dimerization also resulted in the lengthening of the b axis and
Au(5)−N(5) 169.9(4), P(6)−Au(6)−N(6) 166.3(4)) and are
engaged in significant aurophilic forces (Au(1)···Au(2)
the expansion of the cell volume by ca. 2% (i.e., 13493(2) to
2
3
.982(1) Å, Au(3)···Au(4) 3.043(1) Å, Au(5)···Au(6)
3
1
3760(4) Å ). The expansion contrasts with previous studies
14
.049(1) Å) (Figure 1b). The CC bonds of the alkene
of coordination complexes in which the unit-cell volume
in assembly A are disordered over two sites, displaying a
crisscross arrangement with a separation distance of 3.86 Å.
The CC bonds of assembly B, which sits on a center of
inversion, are parallel and separated by 3.28 Å. We note that
there is evidence of the CC bonds of assembly B having
undergone a photodimerization, as demonstrated by the peaks
in the final difference electron density map consistent with a
cyclobutane ring (Figure S2). The refinement of the peaks,
however, was not possible owing to severe disorder. The
distances of the stacked CC bonds in both cases are within
8,16,17
decreases following the photocycloaddition.
Unusual features appeared on the surfaces of the single
crystals during the course of the photoreaction. SEM revealed
the unreacted and reacted single crystals to display smooth
surfaces and ramps with arrowheads on uneven surfaces,
respectively (Figure 3). Ramp-like features are commonly
13
the range of Schmidt for a [2 + 2] photodimerization. The
assemblies pack end-to-end with CC bonds of neighboring
assemblies separated on the order of 10 Å. Both assemblies
pack through combinations of edge-to-face π−π interactions as
well as C−H···O and C−H···F contacts with surrounding
−
CF CO counterions.
3
2
The photoactivity of 1 was evaluated by exposing the single
15
crystals to UV radiation (UV light gel nail dryer) for 17 h.
The formation of 4,4′-tpcb was evidenced by the emergence of
1
a cyclobutane peak at 4.82 ppm on the H NMR spectrum.
Optical microscopy showed the transparency of the single
crystals to be maintained during the photoreaction.
A single-crystal structure analysis of 1 exposed to UV
radiation revealed all stacked CC bonds to react to generate
Figure 3. SEM images of the crystal surface before photo-
cycloaddition (top) and after photocycloaddition (bottom).
B
DOI: 10.1021/acs.inorgchem.9b01306
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
1
8
observed in single crystals of ZSM-5 crystals.
The
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