crystallinity [8, 9] and which was extended to by Cohen and later Ohashi et al.
[10, 11] to propose the introduction of reaction cavities to facilitate solid-state
reactions. Maximising the size of the reaction cavity around the switchable ligand
allows the structure to accommodate the changes during isomerisation with minimal
change in the overall structure, thereby minimising crystal strain and facilitating
higher conversion.
The first linkage isomer system to show 100% conversion to the metastable state
in the single crystal, designed using this principle, was the Ni(II)-nitrite complex [Ni
(dppe)(η
1 -NO 2 )Cl] (dppe ¼ 1,2-bis(diphenylphosphino)ethane) [12], which shows
complete nitro ! nitrito conversion within 90 min when irradiated with 400 nm
LED light. The sterically demanding and photo-inert dppe ligand dominates the
crystal packing and maximises the reaction cavity around the nitrite groups, such that
photoactivation results in a small 0.77% volume expansion.
The same principle was subsequently applied to design other neutral complexes
with 100% conversion, including [Ni(dppe)(η
1 -NO 2 ) 2 ], [Ni(dcpe)(η
1 -NO 2 ) 2 ]
(dcpe ¼ 1,2-bis(dicyclohexylphosphine)ethane) [13] and [Ni(Et 4 dien)(η
2 -O,ON)
(η
1 -NO 2 )] (Et 4 dien ¼ N,N,N
0 ,N
0 -tetraethyldiethylenetriamine) [14]. Multicomponent
crystals with photoactive cations and counter-anions are also known, and the
possibility of incorporating sterically demanding counterions expands the design
space. For example, the [Pd(Bu 4 dien)(NO 2 )]BPh 4 complex (Bu 4 dien ¼ N,N,N
0 ,N
0 -
tetrabutyldiethylenetriamine, BPh 4 ¼ tetraphenylborate) achieves 100% nitro !
nitrito photoconversion in under 15 min when illuminated with low-power LEDs,
with just 0.14% change in the unit-cell parameters, and stands out as an example of a
relatively fast single-crystal-to-single-crystal photoreaction [15] (Fig. 4).
While the “reaction cavity” approach has proved extremely successful in achieving high photoconversion in the solid state, comparison of isostructural materials has
highlighted additional influences from electronic and kinetic factors.
The
[Pd(Et 4 dien)(NO 2 )]OTf
and
[Pt(Et 4 dien)(NO 2 )]OTf
(OTf ¼ trifluoromethanesulfonate) complexes adopt the same crystal structure but
show markedly different behaviour on irradiation at low temperature [16]. Irradiation
at 100 K produces a mixture of the metastable endo-nitrito (η
1 -ONO) and exo-nitrito
(η
1 -ONO) isomers, with different conversion levels in the two systems, whereas
irradiation at higher temperatures leads to 95 and 93% conversion to the endo-nitrito
(η
1 -ONO) isomer in the Pd(II) and Pt(II) systems, respectively. These differences
can be rationalised by thermal expansion at higher temperatures both enlarging the
reaction cavity and weakening hydrogen bonding interactions to the nitro (η
1 -NO 2 )
group in the ground-state structure. The importance of intermolecular interactions
involving the isomerising ligand has also been highlighted in other systems [17–19].
Within the series of isostructural [M(Et 4 dien)(NO 2 )]OTf systems, isomerisation
of the M ¼ Pd(II) completes in 1 h but is dramatically slowed to 3 h in the M ¼ Pt
(II) material using the same excitation source. This appears to be a common feature
of nitrite linkage isomer systems based on third-row transition metals [20] and may
be rationalised in terms of the decrease in kinetic lability of the ligands in transition
metal complexes when descending a group [21].
Watching Photochemistry Happen: Recent Developments in Dynamic Single-Crystal. . .
205
[10, 11] to propose the introduction of reaction cavities to facilitate solid-state
reactions. Maximising the size of the reaction cavity around the switchable ligand
allows the structure to accommodate the changes during isomerisation with minimal
change in the overall structure, thereby minimising crystal strain and facilitating
higher conversion.
The first linkage isomer system to show 100% conversion to the metastable state
in the single crystal, designed using this principle, was the Ni(II)-nitrite complex [Ni
(dppe)(η
1 -NO 2 )Cl] (dppe ¼ 1,2-bis(diphenylphosphino)ethane) [12], which shows
complete nitro ! nitrito conversion within 90 min when irradiated with 400 nm
LED light. The sterically demanding and photo-inert dppe ligand dominates the
crystal packing and maximises the reaction cavity around the nitrite groups, such that
photoactivation results in a small 0.77% volume expansion.
The same principle was subsequently applied to design other neutral complexes
with 100% conversion, including [Ni(dppe)(η
1 -NO 2 ) 2 ], [Ni(dcpe)(η
1 -NO 2 ) 2 ]
(dcpe ¼ 1,2-bis(dicyclohexylphosphine)ethane) [13] and [Ni(Et 4 dien)(η
2 -O,ON)
(η
1 -NO 2 )] (Et 4 dien ¼ N,N,N
0 ,N
0 -tetraethyldiethylenetriamine) [14]. Multicomponent
crystals with photoactive cations and counter-anions are also known, and the
possibility of incorporating sterically demanding counterions expands the design
space. For example, the [Pd(Bu 4 dien)(NO 2 )]BPh 4 complex (Bu 4 dien ¼ N,N,N
0 ,N
0 -
tetrabutyldiethylenetriamine, BPh 4 ¼ tetraphenylborate) achieves 100% nitro !
nitrito photoconversion in under 15 min when illuminated with low-power LEDs,
with just 0.14% change in the unit-cell parameters, and stands out as an example of a
relatively fast single-crystal-to-single-crystal photoreaction [15] (Fig. 4).
While the “reaction cavity” approach has proved extremely successful in achieving high photoconversion in the solid state, comparison of isostructural materials has
highlighted additional influences from electronic and kinetic factors.
The
[Pd(Et 4 dien)(NO 2 )]OTf
and
[Pt(Et 4 dien)(NO 2 )]OTf
(OTf ¼ trifluoromethanesulfonate) complexes adopt the same crystal structure but
show markedly different behaviour on irradiation at low temperature [16]. Irradiation
at 100 K produces a mixture of the metastable endo-nitrito (η
1 -ONO) and exo-nitrito
(η
1 -ONO) isomers, with different conversion levels in the two systems, whereas
irradiation at higher temperatures leads to 95 and 93% conversion to the endo-nitrito
(η
1 -ONO) isomer in the Pd(II) and Pt(II) systems, respectively. These differences
can be rationalised by thermal expansion at higher temperatures both enlarging the
reaction cavity and weakening hydrogen bonding interactions to the nitro (η
1 -NO 2 )
group in the ground-state structure. The importance of intermolecular interactions
involving the isomerising ligand has also been highlighted in other systems [17–19].
Within the series of isostructural [M(Et 4 dien)(NO 2 )]OTf systems, isomerisation
of the M ¼ Pd(II) completes in 1 h but is dramatically slowed to 3 h in the M ¼ Pt
(II) material using the same excitation source. This appears to be a common feature
of nitrite linkage isomer systems based on third-row transition metals [20] and may
be rationalised in terms of the decrease in kinetic lability of the ligands in transition
metal complexes when descending a group [21].
Watching Photochemistry Happen: Recent Developments in Dynamic Single-Crystal. . .
205