Coordination-Driven Self-Assembly of Cyclometalated Iridium Squares Using Linear Aromatic Diisocyanides

Article abstract of DOI:10.1021/acs.inorgchem.1c00312

Here, we demonstrate facile [4 + 4] coordination-driven self-assembly of cyclometalated iridium(III) using linear aryldiisocyanide bridging ligands (BLs). A family of nine new [Ir(C^N)2(μ-BL)]44+ coordination cages is described, where C^N is the cyclometa

Full text of DOI:10.1021/acs.inorgchem.1c00312

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Coordination-Driven Self-Assembly of Cyclometalated Iridium
Squares Using Linear Aromatic Diisocyanides
Morris E. Olumba, Hanah Na, Alan E. Friedman, and Thomas S. Teets*
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ABSTRACT: Here, we demonstrate facile [4 + 4] coordination-driven self-assembly of
cyclometalated iridium(III) using linear aryldiisocyanide bridging ligands (BLs). A family
4+
of nine new [Ir(C^N)2(μ-BL)]4 coordination cages is described, where C^N is the
cyclometalating ligand2-phenylpyridine (ppy), 2-phenylbenzothiazole (bt), or 1-
phenylisoquinoline (piq)and BL is the diisocyanide BL, with varying spacer lengths
between the isocyanide binding sites. These supramolecular coordination compounds are
1
prepared via a one-pot synthesis, with isolated yields of 40−83%. H NMR spectroscopy
confirms the selective isolation of a single product, which is affirmed to be the M4L4 square
by high-resolution mass spectrometry. Detailed photophysical studies were carried out to reveal the nature of the luminescent triplet
states in these complexes. In most cases, phosphorescence arises from the [Ir(C^N)2]+ nodes, with the emission color determined by
the cyclometalating ligand. However, in two cases, the lowest-energy triplet state resides on the aromatic core of the BL, and weak
phosphorescence from that state is observed. This work shows that aromatic diisocyanide ligands enable coordination-driven
assembly of inert iridium(III) nodes under mild conditions, producing supramolecular coordination complexes with desirable
photophysical properties.
enables the facile assembly of complex frameworks with
predictable geometries in high yields. The structures that are
formed by this route are often categorized using the “MxLy”
notation, where x is the number of metal nodes and y is the
number of BLs that span these nodes. Several luminescent
platinum assemblies have been prepared via this methodology,
largely due to the ease of self-assembly of square-planar
platinum complexes.8,16 Ruthenium and rhenium have also
been used as the luminescent nodes in several self-assembly
schemes.3,6,17,18 Iridium(III) coordination cages have been
previously reported, including some with half-sandwich iridium
building blocks that were not reported to be photoluminescent
or photochemically active.19,20 Recently, there have been
reports of coordination-driven self-assembly of cyclometalated
iridium(III) coordination cages, where the building blocks are
known to exhibit efficient, highly tunable phosphorescence.
Using a strategy that had previously been adopted to prepare
square tetranuclear assemblies of several other metals,21−24
Baranoff et al. prepared luminescent tetranuclear M4L4
complexes where four bis-cyclometalated iridium fragments
were bridged by cyanide ligands.11 Chepelin et al. accessed
enantiopure three-dimensional hexanuclear M6L4 capsules
INTRODUCTION
■
The synthesis of photoactive two- and three-dimensional
polynuclear metallocage assemblies has become a particular
area of focus in photochemistry, owing to the unique excited-
state dynamics of these systems which provide insights into
excited-state energy and charge-transfer phenomena.1,2 Some
of the most well-studied and characterized complexes of this
nature are ruthenium-, rhenium-, and platinum-based
assemblies with polypyridyl or metalloporphyrin bridging
ligands (BLs).3−6 Platinum coordination cages have become
especially popular for their use as molecular sensors, while
ruthenium, rhenium, and palladium coordination cages have
seen use in photocatalytic applications.6−10 Comparatively few
examples of iridium coordination assemblies have been
reported, despite the robust luminescence properties of many
iridium complexes.11−14 One factor contributing to this lack of
development is the significant substitutional inertness of
iridium(III), which can necessitate high temperatures and
long reaction times for the assembly of cages with iridium(III)
building blocks.15 However, due to the trans-effect labilization
brought on by the cyclometalated aryl groups, ligand
substitution chemistry on bis-cyclometalated iridium com-
plexes can be readily achieved, and they have the potential to
be versatile building blocks for luminescent supramolecular
assemblies.
Received: January 30, 2021
Published: March 30, 2021
Arguably the most efficient synthetic approach for supra-
molecular photoactive cages or metallacycles is coordination-
driven self-assembly, in which molecules organize into ordered,
well-defined structures as a consequence of directional metal−
ligand bond formation.8 Coordination-driven self-assembly
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a
Scheme 1. Synthesis of Cyclometalated Iridium Squares via Coordination-Driven Self-Assembly
a
Isolated yields are shown in parentheses.
where the bis-cyclometalated iridium nodes were bridged by
1,3,5-tricyanobenzene linkers, studied the photoluminescence
of these cages, and demonstrated host−guest chemistry of
noncoordinating anions inside the cationic hexairidium cage.12
Most recently, Oldknow et al. reported a three-dimensional
M3L2 cage using a chiral tripodal cyclotriguaiacylene-derived
BL, functionalized with photoisomerizable azo-aromatic link-
ers.13 These cages all make use of the well-studied cyano and
pyridyl building blocks as the BL between the cyclometalated
iridium metal centers.
In consideration of the above examples, there is room to
expand the coordination-driven self-assembly of cyclometa-
lated iridium(III). Here, we introduce linear bidentate aryl
isocyanides as bridging ligands for self-assembled M4L4
cyclometalated iridium squares, varying the cyclometalating
ligand on iridium and the length of the bridging ligand. The
cyclometalating ligands were chosen to vary the triplet energy
of the bis-cyclometalated iridium node, descending in the
order 2-phenylpyridine (ppy), 2-phenylbenzothiazole (bt), and
1-phenylisoquinoline (piq). Linear diisocyanides of the type
described here have been used in surface chemistry25−27 and as
bridging ligands for multimetallic structures,28−32 but their
application in coordination-driven self-assembly is underex-
plored. We used three different linear aryl diisocyanides with
varying spacer lengths in this study. Commercially available
1,4-phenylene diisocyanide (PDI) and the known compound
4,4′-diisocyano-1,1′-biphenyl (BPDI) have been previously
described,25−32 and we also introduce here the new ligand 1,1′-
(1,2-ethynediyl)bis(4-isocyanobenzene) (EDI) with a longer
π-conjugated linker between the two isocyanides. Our
hypothesis is that bridging isocyanides offer several potential
synthetic advantages in the assembly of polynuclear complexes
with cyclometalated iridium building blocks. The nitrile- and
pyridine-based bridging ligands that are ubiquitous in
coordination-driven self-assembly often require high temper-
atures and long reaction times to assemble substitutionally
inert metal nodes,12,33 but isocyanides are stronger σ-donors
and thus should assemble more rapidly and with milder
conditions. Moreover, aryl isocyanide moieties are easily
installed in two steps from aniline precursors,34 so it should
be possible to prepare a large variety of directionally oriented
polynucleating ligands to template a wide range of structures.
Beyond these synthetic advantages, previous works from our
group have demonstrated some beneficial photophysical
consequences when aryl isocyanides are paired with cyclo-
metalated iridium. In a series of mononuclear bis-cyclo-
metalated iridium complexes with aryl isocyanide ancillary
ligands, we showed that the π-acidic isocyanides stabilize the
iridium-centered dπ HOMOs, which results in greater redox
stability and blue-shifted photoluminescence relative to many
other cyclometalated iridium complexes.35−37 High quantum
yields and long lifetimes, on the order of 10−5 s, were noted for
the blue- and green-phosphorescent members of these series.
The work described here demonstrates these aforemen-
tioned synthetic benefits, where coordination-driven self-
assembly of labile cyclometalated precursors with linear
bidentate aryl isocyanides enables rapid preparation of M4L4
squares under mild reaction conditions. The M4L4 squares are
characterized by NMR and high-resolution mass spectrometry,
which evince clean, high-yielding formation of the products.
The photophysical properties of the tetranuclear complexes are
all described, with a combination of UV−vis absorption and
steady-state and time-resolved photoluminescence measure-
ments providing insights into the nature of the excited states in
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these compounds. The isocyanide can have photophysical
consequences, with the longer, more conjugated bridging
ligands allowing energy transfer from the bis-cyclometalated
iridium-localized triplet state to a bridging-ligand-centered
state, which can influence the observed photoluminescence.
This work shows that linear aryl diisocyanides are very effective
bridging ligands for [4 + 4] self-assembly of luminescent
tetranuclear cyclometalated iridium complexes. The synthetic
advances and photophysical properties described here support
the continued use of this ligand class in the design of
photoactive constructs for fundamental studies of molecular
photophysics and applications in optoelectronics and sensing.
exhibit subtle shifts when coordinated in the squares. The self-
assembled products Ir1−Ir9 were recalcitrant to forming single
crystals, but high-resolution electrospray ionization mass
spectrometry (ESI-MS) demonstrates selective formation of
the [4 + 4] M4L4 squares. Intact molecular ion peaks were
observed in each complex, the most intense being the [M −
3PF6]3+ ion observed in all nine complexes, with representative
examples shown in Figure 1 for the BL = PDI series, Ir1−Ir3.
Mass spectrometry data for all nine complexes are shown in
Supporting Information, Figures S20−S28. In Ir1−Ir6, the [M
− 2PF6]2+ ion is also resolved, in addition to the 3+ ion. There
is a good match between the observed and theoretical m/z
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The three-step syn-
thetic procedure for the new complexes described in this report
is summarized in Scheme 1. Synthesis begins with the
ubiquitous chloride-bridged dimers [Ir(C^N)2(μ-Cl)]2 (C^N
= cyclometalating ligand), prepared by refluxing IrCl3·nH2O
with 2.5 equivalents cyclometalating ligand following the
known Nonoyama procedure.38 The chloro-bridged dimer is
subsequently cleaved by refluxing the dimer with 2.5
equivalents silver hexafluorophosphate (AgPF6) in acetonitrile
at 60 °C.39,40 This forms a cationic mononuclear iridium bis−
acetonitrile complex [Ir(C^N)2(NCMe)2](PF6), a synthon
previously used to construct cationic monometallic cyclo-
metalated iridium complexes.41 The [Ir(C^N)2(NCMe)2]-
(PF6) intermediate is refluxed in a 1:1 ratio with one of the
linear aromatic diisocyanides in dichloromethane at 40 °C for
5 h, which produces the M4L4 squares. Purification of all
complexes was achieved by precipitation of the complex from
dichloromethane with diethyl ether. Three different cyclo-
metalating ligands (C^N) and three different diisocyanide BLs
were used, producing nine complexes (Ir1−Ir9) using the
same general route, with moderate to good isolated yields
ranging between 40 and 83%. Crude NMR spectra indicate
that the M4L4 products are the major products formed, so in
cases where the isolated yield is modest, there was some
product loss during the precipitation step. The linear bridging
diisocyanide ligands (BPDI and EDI) were synthesized using a
known two-step procedure.34 A linear aryldianiline precursor is
formylated using acetic anhydride and formic acid at 0 °C in
tetrahydrofuran, which forms acetic formic anhydride in situ as
the formylating agent. The resulting formylamide is then
dehydrated with phosphoryl chloride and diisopropyl amine to
yield the linear diisocyanides.
The synthesized complexes were characterized by 1H NMR,
1
infrared (IR) spectroscopy, and mass spectrometry. The H
NMR spectra are reproduced in the Supporting Information.
Figures S1−S3 show the spectra of the mononuclear
acetonitrile-bound precursors, and Figures S4−S12 show the
spectra for the M4L4 squares Ir1−Ir9. NMR spectra of the
squares are consistent with the isolation of a single product in
each case, and the number of aromatic peaks and their
integration suggest localized C2 symmetry at each iridium
node. Comparison of the NMR spectra of Ir1−Ir9 with those
of the acetonitrile precursors in the same solvent, as shown in
Figures S13−S15, allows us to assign which peaks originate
from the C^N ligands and which come from the BLs. 1H NMR
peaks associated with the BLs typically overlap with peaks from
the C^N ligands and thus are difficult to conclusively identify,
but comparison with the free ligand spectra (Figures S16 and
S18) suggests that the ortho C−H resonances in the BLs only
Figure 1. ESI-MS of Ir1−Ir3, with each plot showing the
experimental (top) and simulated (bottom) isotopologue pattern
for the [M − 3PF6]3+ fragment. Peak positions are marked.
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Figure 2. Overlaid UV−vis absorption spectra (black solid lines) in CH2Cl2 at 293 K and photoluminescence spectra in CH2Cl2 at 293 K (red solid
lines) and 1:3 CH2Cl2/toluene at 77 K (blue dashed lines) for complexes Ir1−Ir9.
Table 1. Summary of the Photophysical Properties of Complexes Ir1−Ir9
photoluminescence
UV−vis absorption
λ/nm (ε × 10−3/M−1 cm−1
λmax/nm
CH2Cl2/toluene at 77 K
)
CH2Cl2 at 293 K
ΦPL
τ/μs
b
Ir1
Ir2
Ir3
Ir4
Ir5
Ir6
Ir7
Ir8
251 (153), 275 (134), 311 (121)
267 (146), 313 (182), 356 (62)
277 (200), 388 (40)
312 (222)
312 (264), 359 (62)
283 (265), 311 (212), 386 (13)
325 (450), 348 (394)
325 (380), 348 (311), 373 (57)
282 (454), 327 (572), 350 (508), 389 (96)
258 (60), 310 (sh,16), 343 (9)
315 (40), 361 (20), 376 (sh,16)
286 (52), 354 (19), 406 (9.6)
452, 483, 512
500, 534, 572
628
454, 492, 533, 573
499, 536, 571
625
509, 540, 572
487, 537, 570
576, 616
454, 486, 512
500, 536, 576
587 (sh), 625
449, 481, 507
508, 549, 575
574, 621, 669 (sh)
492, 532, 568
493, 530, 573, 622
582, 628
0.018
0.29
0.050
0.012
0.14
10
15
17
b
b
44
22
12
0.048
a
b
517
N.D.
15
534, 572, 605
581, 624, 675
449, 481, 509, 529 (sh)
497, 535, 580, 631
0.11
0.077
0.24
0.33
0.096
20
10
30
36
6.0
Ir9
35
[Ir(ppy)2(CNdmp)2]PF6
35
[Ir(bt)2(CNdmp)2]PF6
42
[Ir(piq)2(CNdmp) (py)]PF6
577, 625, 683
a
b
Photoluminescence is too weak at room-temperature for an accurate determination. Estimated upper limit is 0.001. Emission decay trace was the
best fit as a biexponential. The reported lifetime is a weighted average of the two time constants. The last three rows include data for the
mononuclear bis-cyclometalated iridium complexes [Ir(C^N)2(CNdmp)2]PF6 (C^N = ppy or bt, CNdmp = 2,6-dimethylphenyl isocyanide) and
[Ir(piq)2(CNdmp) (py)]PF6 (py = pyridine), previously published by our group.35,42
peak positions and isotopologue distribution patterns for each
molecular ion peak. IR spectra, as shown in Figures S29−S40
of Supporting Information for the three free ligands (PDI,
BPDI, and EDI) and Ir1−Ir9, are consistent with the
formulated structures. Assuming a localized D4h symmetry
environment for the M4L4 squares, ignoring the influence of
the C^N ligands on each iridium, there should be two IR-
active Eu stretching modes, and for all complexes, there is a
strong νCN stretching band observed, along with a second band
̃
as a shoulder at slightly higher frequency. The strongest νCN
̃
band occurs between 2126 and 2144 cm−1 in the nine Ir
squares, slightly higher frequency than the same band in the
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free ligands (2125, 2124, and 2125 cm−1 for PDI, BPDI, and
EDI, respectively).
lifetimes for mononuclear bis-isocyanide complexes with the
same C^N ligand. For complex Ir2, which uses the phenylene-
spaced bridging isocyanide PDI, the photoluminescence
quantum yield (ΦPL) is 0.29, almost identical to typical
mononuclear [Ir(bt)2(CNAr)2]+ complexes. However, the ΦPL
value drops slightly with the longer, more conjugated BLs,
BPDI (Ir5, ΦPL = 0.14) and EDI (Ir8, ΦPL = 0.11). As
discussed in detail below, we believe this decrease is due to the
interference of nearby weakly luminescent triplet states
Photophysical Properties. The UV−vis absorption and
photoluminescence emission spectra of complexes Ir1−Ir9 are
shown in Figure 2. The UV−vis absorption spectra were
recorded at 293 K in CH2Cl2, and photoluminescence was
recorded both at 293 K in CH2Cl2 and at 77 K in 1:3 CH2Cl2/
toluene glass. All photophysical data are summarized in Table
1. The absorption spectra of phenylene and biphenylene-
bridged Ir1−Ir6 show two main features, falling in line with
several other characterized bis-cyclometalated iridium com-
plexes with aryl isocyanide ancillary ligands.35,36,42 In the UV
(λ < 350 nm), there are intense absorption bands occurring at
similar wavelengths for all six of these compounds; these are
assigned to localized, ligand-centered (1LC or 1π → π*)
transitions on the cyclometalating and BLs. In addition, a less
intense, lower-energy shoulder is resolved in the spectra of the
complexes with C^N = bt and piq, assigned to a metal-to-
ligand-charge-transfer (1MLCT) excitation. This band is
presumably obscured in the C^N = ppy complexes (Ir1 and
Ir4) by the more intense 1LC bands, but it occurs at ∼360 nm
when C^N = bt (Ir2 and Ir5) and at ∼390 nm when C^N =
piq (Ir3 and Ir6). Whereas the positions of these bands are
quite similar to mononuclear isocyanide-ligated cyclometalated
iridium complexes,35,36,42 the molar absorptivity values are ca.
4× larger, as expected given the tetranuclear nature of these
complexes. The UV−vis absorption spectra of the diphenyle-
thynyl-spaced complexes Ir7−Ir9 (BL = EDI) are markedly
different, dominated in the UV by intense transitions
originating from the BL. All three BL = EDI complexes have
nearly identical and very intense bands at ca. 325 and 350 nm,
assigned to 1π → π* transitions on the EDI ligand, with the ca.
2200 cm−1 spacing suggestive of a vibronic progression
involving the alkyne CC stretching motion. Consistent
with this assignment, the spectrum of free EDI (Figure S50)
shows two similarly spaced sharp bands, blue-shifted by ca.
2500 cm−1 in the free ligand relative to the M4L4 squares.
These bands are considerably more intense than the ligand-
localized transitions in Ir1−Ir6, by a factor of 1.5 or more. In
3
centered on the BL, which are thermally populated at room
temperature. Although we do not have a direct bis-isocyanide
analogue to compare to, we do note that the photo-
luminescence spectra of piq squares Ir3, Ir6, and Ir9, in
addition to strongly resembling one another, are quite similar
to cationic [Ir(piq)2(CNAr) (pyridine)]+ complexes we have
previously described.42 In the piq series, there is no systematic
dependence of the lifetime or quantum yield on the BL.
Quantum yields are modest, as is typically the case for cationic
red-phosphorescent complexes, and the lifetimes of all three
are similar, suggesting that in the piq series, the bridging-ligand
triplet states do not interfere. Photoluminescence spectra were
also collected at 77 K in rigid solvent glass, as shown in Figure
2, and in all cases, there is a more pronounced vibronic
structure at low temperature but minimal shifts in peak
wavelengths, consistent with the T1 state being predominately
C^N 3LC. To summarize, for the M4L4 squares with C^N = bt
or piq, the photoluminescence arises from a mixed
3LC/3MLCT state centered on one of the [Ir(C^N)2]+
nodes, and the photophysical properties of the nodes are
retained upon assembling the cyclometalated iridium frag-
ments into a metalacyclic structure.
In contrast, for the C^N = ppy series (Ir1, Ir4, and Ir7), the
photoluminescence behavior is strongly dependent on the
bridging isocyanide ligand. To visualize the differences more
clearly between these three complexes, their spectra at both
293 and 77 K are overlaid, as shown in Figure 3. In the
4+
phenylene-spaced [Ir(ppy)2(μ-PDI)2]4 square (Ir1), the
photoluminescence is in the blue−green region and strongly
resembles mononuclear [Ir(ppy)2(CNAr)2]+ complexes.35
A
1
Ir8 (C^N = bt) and Ir9 (C^N = piq), MLCT transitions at
sharp vibronic structure is noted, and the peak maxima in Ir1
are within 3 nm (130 cm−1) of the mononuclear analogues
(see Table 1). That said, the quantum yield of Ir1 is quite
modest (ΦPL = 0.018), in contrast to the brightly luminescent
mononuclear analogues (ΦPL = 0.24−0.37), and the lifetime of
Ir1 (10 μs) is more than a factor of 2 smaller. The
photoluminescence spectra in Ir4 (BL = BPDI) and Ir7 (BL
= EDI) are dramatically different, as shown in Figures 2 and 3.
The spectra are significantly red-shifted from that of Ir1, again
with the well-resolved vibronic structure and quantum yields
lower energy are resolved, occurring at very similar wave-
lengths as the corresponding bands in the other complexes
with the same cyclometalating ligands.
The photoluminescence spectra of Ir1−Ir9 are also
displayed in Figure 2. For the complexes with C^N = bt and
piq, which respectively luminesce in the green and red regions,
the photoluminescence profiles are very similar for a given
C^N ligand, and strongly resemble mononuclear cationic bis-
cyclometalated iridium complexes with similar coordination
environments. In these cases, phosphorescence arises from a
triplet state centered on a [Ir(C^N)2]+ fragment of mixed C^N
that are modest (Ir4; ΦPL = 0.012) or negligible (Ir7; ΦPL
<
0.001). Complex Ir4 has the longest lifetime in the series at 44
μs, and despite its very weak photoluminescence, the lifetime
of Ir7 is also quite long at 15 μs. The spectra at 77 K are also
distinct from one another, which provides further evidence that
the three C^N = ppy complexes all luminesce from different
triplet states.
3
3LC and MLCT character, as is typical for cyclometalated
iridium complexes.43 In particular, we have previously studied
several mononuclear [Ir(bt)2(CNAr)2]+ complexes,35,36 which
exhibit photoluminescence profiles nearly identical to the
4+
[Ir(bt)2(μ-BL)]4 squares presented here (Ir2, Ir5, and Ir8).
The peak emission wavelengths of the M4L4 squares and their
mononuclear analogues fall within 4 nm (140 cm−1) of one
another, and in all cases, there is a discernible vibronic
structure at room temperature with three maxima observed.
There is some effect of the BLs on the photoluminescence
quantum yield and lifetime. The lifetimes of Ir2, Ir5, and Ir8
are similar to one another and slightly shorter than typical
Although Ir1 and all C^N = bt and piq complexes clearly
luminesce from the typical [Ir(C^N)2]+ 3LC/3MLCT state, we
propose that the T1 states in Ir4 and Ir7 are centered on the
3
bridging isocyanide ligands, denoted here as BL states. The
free ligands BPDI and EDI are not luminescent at room
temperature or at 77 K in transparent solvent glass. Thus, we
do not have phosphorescence spectra of BPDI to compare to,
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lifetime in Ir4, compared to the other M4L4 squares, is
consistent with luminescence arising from an aromatic triplet
state on the BL. Similarly, while the EDI ligand also does not
produce phosphorescence on its own, we can compare the
phosphorescence of Ir7 to that of the conjugated hydrocarbon
core. As also shown in Figure 4, the phosphorescence of Ir7 is
similar to that of diphenylacetylene,45 again with a significant
red-shift (ca. 2000 cm−1) brought on by the introduction of
isocyano groups. The vibronic spacing of ca. 1100 cm−1 in Ir7
is consistent with totally symmetric modes within the
diphenylacetylene core.45 Thus, in Ir4 and Ir7, the lowest-
energy triplet state is centered on the aromatic BL and not on
the [Ir(ppy)2]+ node. This switching of phosphorescence from
the normal C^N-centered 3LC/3MLCT state to the isocyanide
ancillary ligand has been observed before in mononuclear
complexes. In our own previous work, we showed that
phosphorescence can originate from a naphthyl-centered
triplet state in certain bis-cyclometalated iridium complexes
with 2-naphthyl isocyanide ancillary ligands,35 and Favale et al.
have shown similar behavior in complexes with oligoaryl
isocyanide ancillary ligands.46
Figure 3. Overlaid emission spectra of C^N = ppy complexes Ir1, Ir4,
and Ir7, recorded in CH2Cl2 at 293 K (top) and in 1:3 CH2Cl2/
toluene at 77 K (bottom). Data are the same as that shown in Figure
2, showing these three complexes to illustrate the effect of the BL on
their photoluminescence. Symbols were added to help distinguish the
overlaid plots.
To summarize the photoluminescence properties of these
compounds, Figure 5 shows qualitative energy−level diagrams
for the complexes presented here. The UV−vis absorption
but the phosphorescence of the biphenyl core is well-studied.44
As shown in Figure 4, the phosphorescence in Ir4 bears a
striking resemblance to that of biphenyl, with a similar profile
and vibronic structure, albeit ca. 800 cm−1 red-shifted on
account of the isocyano groups, which are electron-with-
drawing and extend the conjugation. Moreover, the longer
Figure 4. Overlaid spectra of (a) phosphorescence of biphenyl and
Ir4 and (b) phosphorescence of diphenylacetylene (recorded at 77 K)
and Ir7. Symbols were added to help distinguish the overlaid plots.
The spectrum for biphenyl is adapted with permission from ref 44.
Copyright 2020 American Chemical Society. The spectrum for
diphenylacetylene is adapted from ref 45. Copyright 2003 American
Chemical Society.
Figure 5. Qualitative excited-state energy-level diagram for M4L4
squares Ir1−Ir9. The top diagram pertains to Ir1−Ir3, Ir5, Ir6, Ir8,
and Ir9. The bottom diagram pertains to Ir4 and Ir7.
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spectra of all compounds suggest that singlet states associated
involved in the photophysics, with the lowest-energy triplet
state localizing on the BL and producing weak luminescence
from that state. This work shows that isocyanide-based BLs
offer advantages in the synthetic chemistry of self-assembled
polynuclear cyclometalated iridium complexes and can also
influence the photophysics, motivating continued pursuit of
isocyanide-bridged cyclometalated iridium cages with diversi-
fied structures and exploration of their applications in
optoelectronics, sensing, and photochemistry.
1
with both the BL and the [Ir(C^N)2]+ nodes (1LC and
1MLCT) can be populated, the former being the most evident
when BL = EDI, the latter when C^N = bt and piq. Though we
cannot specify the exact internal conversion/intersystem
crossing pathways that populate the emissive T1 state, we do
observe that the excitation spectra (Figures S41−S49 of
Supporting Information) overlay well with the UV−vis
absorption, indicating that the same T1 state is populated no
matter which singlet state is excited. In complexes with C^N =
bt and piq, along with Ir1, the T1 state is a 3LC/3MLCT state
centered on the [Ir(C^N)2]+ nodes (top diagram of Figure 5).
The energy of this state depends on the cyclometalating ligand,
as has been documented in numerous other studies. Consistent
with this, in all of these compounds, both the room-
temperature and 77 K photoluminescence (Figure 2) strongly
resemble many other mononuclear [Ir(C^N)2(CNAr)2]+
complexes. The energy of the bridging-ligand triplet state in
these compounds, 3BL, depends on which BL is used, but in all
EXPERIMENTAL SECTION
■
Materials. All reagents and solvents were commercially obtained
and used as received, unless otherwise noted. Reactions were
performed in an open atmosphere fume hood, unless otherwise
stated. BPDI and EDI were prepared using known literature
procedures for converting aniline precursors into isocyanides,34,47
and PDI was commercially obtained from Acros Organics and used as
obtained.
Physical Methods. 1H NMR spectra were recorded at room
temperature using a JEOL ECA-500 and JEOL ECA-600
spectrometer with frequencies of 500 and 600 MHz, respectively.
All NMR data were processed using Delta software version 5.3.1 and
presented with MestReNova software version 14.1.2-25024. Mass
spectra were acquired with a Bruker Daltonics SolariX 12T Fourier
Transform Ion Cyclotron Resonance Mass Spectrometer and
analyzed using Bruker Compass DataAnalysis 4.2 software. ESI-MS
results given in the experimental section are monoisotopic masses,
and full experimental and simulated traces are presented in Figures
S20−S28 in Supporting Information. UV−vis spectra were collected
using an Agilent Carey 8454 UV−vis spectrophotometer. Emission
spectra were collected using a Horiba FluoroMax-4 spectrofluor-
ometer. Excitation spectra were collected in 1 cm quartz cuvettes
using the Horiba FluoroMax-4 spectrofluorometer. Each cuvette was
prepared in a glovebox with deoxygenated dichloromethane. The
excitation spectrum was collected by monitoring at a fixed detection
wavelength (corresponding to the emission maximum of each sample)
while varying the excitation wavelength. Phosphorescence lifetimes
were measured on a Horiba DeltaFlex Lifetime System, using 330 nm
excitation and appropriate long-pass filters. All room-temperature
photophysical measurements were carried out in 1 cm quartz cuvettes
prepared in a glovebox with deaerated dichloromethane to exclude
oxygen from measurements. Low-temperature measurements were
performed with custom electron paramagnetic resonance tubes with
high vacuum Teflon valve seals, in a 3:1 mixture of toluene to
dichloromethane in a finger Dewar cooled to 77 K with liquid
nitrogen. Emission quantum yields were determined relative to a
standard of tetraphenylporphyrin in toluene (ΦF = 0.11).
General Procedure for Synthesis of [Ir(C^N)2(NCMe)2]PF6.
[Ir(C^N)2(μ-Cl)]2 (1.0 equiv) and silver hexaflurophosphate (2.5
equiv) were dissolved in dry acetonitrile in a glovebox. After stirring
for 2 h at room temperature, the mixture was removed from the
glovebox and refluxed for an additional 2 h at 60 °C. After cooling to
room temperature, the mixture was filtered through Celite to collect
AgCl formed in the reaction. The solvent was then removed under
vacuum. The product was dissolved in the minimum amount of
CH2Cl2 and precipitated with diethyl ether. The solid was then
filtered, washed with excess diethyl ether, and dried under vacuum
with gentle heating at 30 °C for 24 h.
General Procedure for Synthesis of [Ir(C^N)2(μ-BL)]4(PF6)4
Complexes (Scheme 1). Complexes Ir1−Ir9 were synthesized by
treating the respective [Ir(C^N)2(NCMe)2]PF6 complex (1.0 equiv)
with the linear diisocyanide BL (1.0 equiv) in 10−30 mL of
dichloromethane, heating at 40 °C in a Schlenk flask under a N2
atmosphere for 5 h. After cooling to room temperature, the solvent
was then removed under vacuum. The product was dissolved in the
minimum amount of CH2Cl2 and precipitated with diethyl ether. The
solid was then filtered and washed with excess diethyl ether and finally
dried under vacuum with gentle heating at 30 °C for 24 h.
3
cases, it is higher in energy than LC/3MLCT and does not
contribute to the observed luminescence. That said, the
abnormally low quantum yield in some compounds,
particularly evident in Ir1 and to a lesser extent in C^N = bt
complexes Ir5 and Ir8, suggests that the weakly luminescent
3BL state may be thermally accessible in these compounds at
room temperature, which results in a decrease in the quantum
yield. In contrast, for the C^N = piq complexes, Ir3, Ir6, and
Ir9, there is larger energy separation between the 3LC/3MLCT
3
and BL states, and in these compounds, the quantum yields
are not dependent on the bridge. In Ir4 and Ir7, the 3BL state
is lowest in energy, and weak phosphorescence originates from
that state (bottom diagram of Figure 5). The phosphorescence
in these compounds strongly resembles that of the aromatic
hydrocarbon core, though red-shifted as a result of the
isocyano groups. In these compounds, it is likely that the C^N-
centered 3LC/3MLCT state can be initially populated
following intersystem crossing, but internal conversion to the
3BL state occurs prior to luminescence.
CONCLUSIONS
■
In this work, we described a synthetic strategy using
coordination-driven self-assembly to synthesize nine new
phosphorescent iridium(III) M4L4 coordination cages with
cyclometalated iridium ([Ir(C^N)2]+) nodes. Self-assembly of
bis-cyclometalated iridium complexes is rare, in large part due
to the substitutional inertness of Ir(III), and here, we introduce
linear aryl diisocyanide BLs, whose strong coordination ability
facilitates rapid, facile, and selective assembly of the M4L4
squares. Reactions proceed under milder conditions (ca. 40
°C) and with shorter reaction times (5 h) than that is often
required for Ir(III) self-assembly, indicating some synthetic
advantages of using isocyanide-based ligands in self-assembly.
The photophysical consequences of the isocyanide BLs are also
apparent. For most compounds, the photoluminescence
strongly resembles that of mononuclear isocyanide-bound
cyclometalated iridium complexes, originating from the
[Ir(C^N)2]+ node. In these cases, the electronic consequences
of the isocyanide are apparent, with a sharper vibronic
structure, bluer-shifted peak maxima, and longer lifetimes
than that is typically observed in iridium cyclometalates.
However, in two of the compounds with extended π-
conjugation on the aryl diisocyanide, the BL is directly
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pubs.acs.org/IC
Article
Synthesis of [Ir(ppy)2(μ-PDI)]4(PF6)4 (Ir1). Prepared by the
general procedure above using [Ir(ppy)2(NCMe)2]PF6 (63 mg) and
PDI (12 mg). Yield: 39 mg, 58%. 1H NMR (600 MHz, CD3CN): δ =
9.16 (s, 8H, ppy, Ar H), 8.13 (s, 16H, ppy, Ar H), 7.75 (s, 8H, ppy, Ar
H), 7.59−7.32 (m, 24H, ppy/PDI, Ar H), 7.01 (s, 8H, ppy, Ar H),
6.88 (s, 8H, ppy, Ar H), 6.11 (s, 8H, ppy, Ar H). HRMS (ESI) m/z:
[M − 2PF6]2+ calcd, 1403.22708; found, 1403.26009; [M − 3PF6]3+
(q, J = 9.0, 8.2 Hz, 8H, bt, Ar H), 7.76 (t, J = 7.6 Hz, 8H, bt, Ar H),
7.71−7.62 (m, 8H, bt, Ar H), 7.59−7.49 (m, 16H, EDI, Ar H), 7.45−
7.26 (m, 16H, EDI, Ar H), 7.08 (dd, J = 9.9, 5.6 Hz, 8H, bt, Ar H),
6.92 (q, J = 7.4 Hz, 8H, bt, Ar H), 6.24 (d, J = 7.4 Hz, 8H, bt, Ar H).
HRMS (ESI) m/z: [M − 3PF6]3+ calcd, 1169.79707; found,
1169.85281. FT-IR (cm−1): 2139 (νCN
), 2168 (sh) (νCN).
̃ ̃
Synthesis of [Ir4(piq)8(μ-EDI)4](PF6)4 (Ir9). Prepared by the
general procedure using [Ir(piq)2(NCMe)2]PF6 (29 mg) and EDI (8
mg). Yield: 22 mg, 63%. 1H NMR (500 MHz, CD3CN): δ = 9.14 (t, J
= 6.0 Hz, 8H, piq, Ar H), 8.97 (d, J = 8.7 Hz, 8H, piq, Ar H), 8.33 (d,
J = 7.7 Hz, 8H, piq, Ar H), 8.18 (t, J = 6.4 Hz, 8H, piq, Ar H), 8.01−
7.77 (m, 24H, piq, Ar H), 7.62−7.30 (m, 32H, EDI, Ar H), 7.11 (t, J
= 6.6 Hz, 8H, piq, Ar H), 6.86 (t, J = 7.7 Hz, 8H, piq, Ar H), 6.19 (q,
J = 6.9 Hz, 8H, piq, Ar H). HRMS (ESI) m/z: [M − 3PF6]3+ calcd,
calcd, 887.16314; found, 887.17682. FT-IR (cm−1): 2131 (νCN
̃
), 2167
(sh) (νCN).
̃
Synthesis of [Ir(bt)2(μ-PDI)]4(PF6)4 (Ir2). Prepared by the
general procedure using [Ir(bt)2(NCMe)2]PF6 (59 mg) and PDI (9
1
mg). Yield: 33 mg, 53%. H NMR (600 MHz, CD3CN): δ = 8.39−
8.13 (m, 16H, bt, Ar H), 7.90−7.56 (m, 24H, bt, Ar H), 7.52−7.24
(m, 16H, PDI, Ar H), 7.04 (s, 8H, bt, Ar H), 6.89 (s, 8H, bt, Ar H),
6.17 (s, 8H, bt, Ar H). HRMS (ESI) m/z: [M − 2PF6]2+ calcd,
1627.11536; found, 1627.16055; [M − 3PF6]3+ calcd, 1036.42110;
1153.91328; found, 1153.96859. FT-IR (cm−1): 2132 (νCN
̃
), 2155
(sh) (νCN).
̃
found, 1036.44181. FT-IR (cm−1): 2126 (νCN
), 2171 (sh) (νCN).
̃ ̃
General Procedure for Synthesis of Diisocyanide Ligands.
The linear BLs were synthesized via a known two-step procedure for
converting aniline precursors into isocyanides.47 The dianiline
precursor is formylated using acetic formic anhydride, prepared in
situ through reaction of acetic anhydride and formic acid. Formic acid
(32 equiv) and acetic anhydride (33 equiv) are allowed to stir
together at 0 °C for 30 min. Afterward, the mixture is allowed to
reflux at 60 °C for 3 h. After cooling to room temperature the
prepared acetic formic anhydride is added to the aniline precursor
(1.0 equiv) dissolved in tetrahydrofuran and allowed to stir overnight.
The solvent was then removed under vacuum, yielding a white or tan
solid. The solid was collected in a vial and dried under vacuum with
gentle heating at 30 °C for 24 h. The resulting formylamide is then
dehydrated with phosphoryl chloride and diisopropyl amine to yield
the linear diisocyanides. Without further purification, the formylamide
(1.0 equiv) from the first step was added to dichloromethane forming
a suspension. Diisopropylamine (7.0 equiv) and phosphorous
oxychloride (2.8 equiv) were added to the suspension. The mixture
was cooled to 0 °C and allowed to stir for 30 min, followed by 8 h of
stirring at room temperature. The reaction was quenched by adding
saturated aqueous Na2CO3 and stirred for 24 h. A biphasic extraction
with dichloromethane was performed with two brine washes. The
organic layer was dried over MgSO4, then filtered, and the solvent was
removed under vacuum yielding the crude product. The product was
purified by performing a silica flash column eluted with dichloro-
methane, yielding the pure diisocyanide.
Synthesis of [Ir(piq)2(μ-PDI)]4(PF6)4 (Ir3). Prepared by the
general procedure using [Ir(piq)2(NCMe)2]PF6 (37 mg) and PDI (6
mg). Yield: 20 mg, 51%. 1H NMR (500 MHz, CD3CN): δ = 9.04 (s,
8H, piq, Ar H), 8.92 (s, 8H, piq, Ar H), 8.28 (s, 8H, piq, Ar H), 8.14
(s, 8H, piq, Ar H), 8.00−7.71 (m, 24H, piq, Ar H), 7.64−7.24 (m,
24H, piq/PDI, Ar H), 7.07 (s, 8H, piq, Ar H), 6.83 (s, 8H, piq, Ar H),
6.11 (s, 8H, piq, Ar H). HRMS (ESI) m/z: [M − 2PF6]2+ calcd,
1603.28968; found, 1603.30502; [M − 3PF6]3+ calcd, 1020.53821;
found, 1020.55971. FT-IR (cm−1): 2127 (νCN
), 2170 (sh) (νCN).
̃ ̃
Synthesis of [Ir(ppy)2(μ-BPDI)]4(PF6)4 (Ir4). Prepared by the
general procedure using [Ir(ppy)2(NCMe)2]PF6 (30 mg) and BPDI
1
(8 mg). Yield: 16 mg, 46%. H NMR (500 MHz, CD3CN): δ = 9.26
(s, 8H, ppy, Ar H), 8.14 (d, J = 24.8 Hz, 16H, ppy, Ar H), 7.79 (s, 8H,
ppy, Ar H), 7.73−7.63 (m, 16H, BPDI, Ar H), 7.61−7.38 (m, 24H,
ppy/BPDI, Ar H), 7.02 (d, J = 7.3 Hz, 8H, ppy, Ar H), 6.92 (s, 8H,
ppy, Ar H), 6.19 (d, J = 7.7 Hz, 8H, ppy, Ar H). HRMS (ESI) m/z:
[M − 2PF6]2+ calcd, 1555.28968; found, 1555.32318; [M − 3PF6]3+
calcd, 988.53821; found, 988.55519. FT-IR (cm−1): 2131 (νCN
̃
), 2162
(sh) (νCN).
̃
Synthesis of [Ir(bt)2(μ-BPDI)]4(PF6)4 (Ir5). Prepared by the
general procedure using [Ir(bt)2(NCMe)2]PF6 (30 mg) and BPDI (8
mg). Yield: 14 mg, 40%. 1H NMR (500 MHz, CD3CN): δ = 8.42 (d, J
= 8.2 Hz, 8H, bt, Ar H), 8.23 (s, 8H, bt, Ar H), 7.86 (d, J = 8.6 Hz,
8H, bt, Ar H), 7.75 (s, 8H, bt, Ar H), 7.70−7.52 (m, 24H, bt/BPDI,
Ar H), 7.41−7.30 (m, 16H, BPDI, Ar H), 7.07 (t, J = 7.1 Hz, 8H, bt,
Ar H), 6.93 (t, J = 7.7 Hz, 8H, bt, Ar H), 6.25 (d, J = 7.6 Hz, 8H, bt,
Ar H). HRMS (ESI) m/z: [M − 2PF6]2+ calcd, 1779.17796; found,
1779.20868; [M − 3PF6]3+ calcd, 1137.79707; found, 1137.81026.
Synthesis of BPDI. Prepared by general procedure using
benzidine (513 mg, 28 mmol, 1.0 equiv) with acetic anhydride (9
mL, 92 mmol, 33 equiv) and formic acid (3.5 mL, 90 mmol, 32
equiv). Yield: 524 mg, 78%. N,N′-[1,1′-biphenyl]-4,4′-diyl-bis-
(formamide) (227 mg, 0.94 mmol, 1.0 equiv) with diisopropylamine
(0.5 mL, 62 mmol, 6.0 equiv) and phosphorous oxychloride (0.4 mL,
38 mmol, 4.0 equiv). Yield: 82 mg, 38%. 1H NMR (600 MHz,
CD2Cl2): δ = 7.59 (d, J = 8.3 Hz, 4H), 7.46 (d, J = 8.2 Hz, 4H).
13C{1H} NMR (151 MHz, CD2Cl2): δ (ppm) 165.7, 140.5, 128.2,
FT-IR (cm−1): 2144 (νCN
), 2170 (sh) (νCN).
̃ ̃
Synthesis of [Ir(piq)2(μ-BPDI)]4(PF6)4 (Ir6). Prepared by the
general procedure using [Ir(piq)2(NCMe)2]PF6 (24 mg) and BPDI
1
(6 mg). Yield: 21 mg, 78%. H NMR (500 MHz, CD3CN): δ = 9.16
(s, 8H, piq, Ar H), 8.97 (d, J = 7.3 Hz, 8H, piq, Ar H), 8.32 (d, J = 9.5
Hz, 8H, piq, Ar H), 8.22−8.09 (m, 8H, piq, Ar H), 8.00−7.76 (m,
24H, piq, Ar H), 7.72−7.53 (m, 16H, BPDI, Ar H), 7.53−7.32 (m,
16H, BPDI, Ar H), 7.10 (d, J = 9.0 Hz, 8H, piq, Ar H), 6.86 (s, 8H,
piq, Ar H), 6.19 (d, J = 7.6 Hz, 8H, piq, Ar H). HRMS (ESI) m/z: [M
− 2PF6]2+ calcd, 1755.35228; found, 1755.36982; [M − 3PF6]3+
127.1, 126.4. FT-IR (cm−1): 2124 (νCN
̃
).
Synthesis of EDI. Prepared by general procedure of diisocyanide
ligands using 4,4′-ethyne-1,2-diyldianiline (500 mg, 2.4 mmol, 1.0
equiv) with acetic anhydride (8 mL, 79 mmol, 33 equiv) and formic
acid (3 mL, 76 mmol, 32 equiv). Yield: 568 mg, 89%. 4,4′-Ethyne-1,2-
diyl-bis(formamide) (268 mg, 1.0 mmol, 1.0 equiv) with diisopropyl-
amine (0.5 mL, 71 mmol, 7.0 equiv) and phosphorous oxychloride
calcd, 1121.91328; found, 1121.92311. FT-IR (cm−1): 2140 (νCN
̃
),
2166 (sh) (νCN).
̃
Synthesis of [Ir(ppy)2(μ-EDI)]4(PF6)4 (Ir7). Prepared by the
general procedure using [Ir(ppy)2(NCMe)2]PF6 (28 mg) and EDI (8
mg). Yield: 27 mg, 83%. 1H NMR (500 MHz, CD3CN): δ = 9.24 (s,
8H ppy, Ar H), 8.19−8.09 (m, 16H ppy, Ar H), 7.79 (d, J = 8.0 Hz,
8H ppy, Ar H), 7.64−7.53 (m, 16H EDI, Ar H), 7.47−7.35 (m, 24H
ppy/EDI, Ar H), 7.03 (t, J = 7.7 Hz, 8H ppy, Ar H), 6.92 (t, J = 7.4
Hz, 8H ppy, Ar H), 6.18 (d, J = 8.0 Hz, 8H ppy, Ar H). HRMS (ESI)
m/z: [M − 3PF6]3+ calcd, 1020.53821; found, 1020.58226. FT-IR
1
(0.3 mL, 28 mmol, 2.8 equiv). Yield: 85 mg, 36%. H NMR (600
MHz, CD2Cl2): δ = 7.55 (d, J = 8.3 Hz, 4H), 7.37 (d, J = 8.2 Hz, 4H).
13C{1H} NMR (151 MHz, CD2Cl2): δ (ppm) 166.5, 132.7, 126.7,
126.2, 123.9, 90.2. FT-IR (cm−1): 2125 (νCN
̃
).
(cm−1): 2136 (νCN
), 2165 (sh) (νCN).
̃ ̃
ASSOCIATED CONTENT
■
Synthesis of [Ir(bt)2(μ-EDI)]4(PF6)4 (Ir8). Prepared by the
general procedure using [Ir(bt)2(NCMe)2]PF6 (24 mg) and EDI (6
mg). Yield: 14 mg, 48%. 1H NMR (500 MHz, CD3CN): δ = 8.39 (d, J
= 8.7 Hz, 8H, bt, Ar H), 8.23 (q, J = 7.9, 6.0 Hz, 8H, bt, Ar H), 7.86
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00312.
5905
https://doi.org/10.1021/acs.inorgchem.1c00312
Inorg. Chem. 2021, 60, 5898−5907

Inorganic Chemistry
NMR spectra, mass spectrometry data, infrared spectra,
pubs.acs.org/IC
Article
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AUTHOR INFORMATION
■
Corresponding Author
Thomas S. Teets − Department of Chemistry, University of
Houston, Houston, Texas 77204-5003, United States;
orcid.org/0000-0002-7471-8467; Email: tteets@uh.edu
Authors
Morris E. Olumba − Department of Chemistry, University of
Houston, Houston, Texas 77204-5003, United States
Hanah Na − Department of Chemistry, University of Houston,
Houston, Texas 77204-5003, United States
Alan E. Friedman − Department of Materials, Design, and
Innovation, University at Buffalo, The State University of
New York, Buffalo, New York 14260, United States;
orcid.org/0000-0002-4764-8168
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00312
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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The authors acknowledge the Army Research Lab (Cooper-
ative Agreement W911NF1920196) and the Welch Founda-
tion (grant no E-1887) for funding.
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