Organometallics
Communication
photodimerizations,7c,10 ring-closing and opening isomeriza-
tion,11 trans−cis photoisomerization12), and enabled the
development of resilient solids with unique properties (e.g.,
photochromism,13 porosity,14 conductivity,15 and chemical
patterning16). In contrast to a relatively large number of recent
reports of SCSC reactivity17 involving metal−ligand coordina-
tion, single-crystal reactivity involving a B atom has only been
observed in a tricoordinate B←N aminoborane.18 To our
knowledge, SCSC reactivity in tetracoordinate B←N boronic
ester based adducts is unknown.
Initially, we focused to evaluate the reactivity of pure pte in
the solid state (Figure S3 in the Supporting Information).19
Single-crystal X-ray diffraction (SCXRD) analysis revealed pte
to crystallize in the monoclinic space group Pc as yellow plates
(Figure 1). The pte molecules self-assemble in a herringbone
Figure 1. X-ray structure thiophene pte: (a) herringbone arrange-
ment; (b) hydrogen-bonded layers.
Figure 2. X-ray structure of adduct (pbe)·(pte): (a) ORTEP view;
arrangement with nearest-neighbor carbon−carbon double
(CC) bonds separated by 6.09 Å (Figure 1a). The pte
molecules define layers sustained by C−H···N interactions in
the bc plane (Figure 1b). Upon exposure to UV radiation (72
h, 450 W, medium-pressure Hg lamp), pte was photostable, as
confirmed by 1H NMR spectroscopy (Figure S1 in the
Supporting Information).
(b) herringbone arrangement; (c) dimeric assemblies.
disappearance of the olefinic signals (Figure S2 in the
Supporting Information). A powder X-ray diffraction analysis
was supportive of a change in the phase of the solid (Figures
To promote solid-state reactivity, B←N coordination was
achieved by combining pbe (20 mg, 0.010 mmol) with pte
(19.1 mg, 0.010 mmol) in hot acetonitrile (2 mL). Single
crystals in the form of pale yellow prisms suitable for X-ray
diffraction (SCXRD) formed upon slow solvent evaporation
after a period of 2 days.
SCXRD analysis revealed the components (pbe)·(pte) to
undergo a SCSC [2 + 2] photodimerization to form 2(pbe)·
(4p2tcb) regio- and stereoselectively, in quantitative yield
(Figure 3). Importantly, the crystal-to-crystal photoreaction
generated a ditopic B adduct with two pbe units that effectively
act as “hinges” and with two pendant α-thiophenyl rings
(Figure 3a). The bipyridine 4p2tcb is disordered over two sites
(site occupancies: 0.637(4) and 0.363(4)). Whereas the B←N
bond length does not differ considerably (1.659(4) Å) from
that of (pbe)·(pte), the THC of boron (75%) increased after
the photoreaction, which is indicative of a stronger
coordination bond.4b The increase in the THC is consistent
with an increase in electron density. The formation of the
electron-donating cyclobutyl ring, which acts as an efficient
through-bond donor,23 can be ascribed to the increase in
THC. The B···B distance in 2(pbe)·(4p2tcb) (13.27 Å) is
slightly shorter than that of the (pbe)·(pte) dimers (13.64 Å),
consistent with a decrease in carbon−carbon atom distances
conferred by the cyclobutyl ring (Figure 3b). The conforma-
tional change enables the formation of intermolecular C−H···
O contacts between the cyclobutyl ring and pbe. The
formation of the adduct is also accompanied by tilting of the
thiophenyl ring (18.4°) and a slight rotation of the aryl ring
(1.3°) (Figure 3c). To our knowledge, (pbe)·(pte) is the first
example of a solid with a boronic ester that participates in a
SCSC reaction.
SCXRD analysis of (pbe)·(pte) reveals the components to
crystallize in the monoclinic space group P21/n (Figure 2).
The asymmetric unit consists of a T-shaped adduct composed
of pbe coordinated to pte via a B←N bond (1.646(8) Å). The
thiophene molecule pte in the adduct is disordered over two
sites (site occupancies 0.603 and 0.397). The THC of the B
atom (68%) is slightly weaker in comparison to similar boronic
ester based adducts.3,20 The pyridyl and thiophenyl rings adopt
a nearly coplanar conformation (twist angle 6.3°). Two (pbe)·
(pte) molecules self-assemble into discrete head-to-tail dimers
through π···π and C−H···π forces (Figure 2a). The (pbe)·
(pte) molecules form a herringbone-arrangement in the ab
plane through C−H···S and C−H···O interactions (Figure 2b).
The dimers further organize into sheets parallel to the b axis in
the bc plane. The alkenes stack with C···C distances of 3.70 Å,
which satisfies the criteria for a [2 + 2] photocycloaddition
reaction.21 CC bonds of neighboring dimers are separated
by 10.4 Å (Figure 2c).5a
When single crystals of (pbe)·(pte) were exposed to UV
radiation (UV light gel nail dryer) for 2 h,22 a H NMR
1
spectroscopic analysis confirmed the complete conversion of
pte to a cyclobutane, as evidenced by the appearance of
cyclobutyl resonance signals at δ 4.86 and 4.45 ppm and
2(pbe)·(4p2tcb) acts as a channel former for the solid-state
confinement of chloroform.24 Recrystallization of 2(pbe)·
(4p2tcb) (30 mg, 0.039 mmol) in chloroform (2.5 mL)
B
Organometallics XXXX, XXX, XXX−XXX
Campillo-Alvarado, Gonzalo
