Platinum(II) Monomer and Dimer Complexes with a Bis(oxazolinyl)phenyl Pincer Ligand

Article abstract of DOI:10.1021/acs.inorgchem.5b01022

A series of platinum(II) complexes with the formulas Pt(phebox)(L)⁺ (phebox^- = 1,3-bis(4,4′-dimethyl-2′-oxazolinyl)phenyl anion; L = pyridine (py), 4-phenylpyridine, quinoline, acridine) and Pt₂(phebox)₂(μ-L′)

Full text of DOI:10.1021/acs.inorgchem.5b01022

Article
pubs.acs.org/IC
Platinum(II) Monomer and Dimer Complexes with a
Bis(oxazolinyl)phenyl Pincer Ligand
Daoli Zhao, Jeanette A. Krause, and William B. Connick*
Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, Ohio 45221-0172, United States
S
* Supporting Information
ABSTRACT: A series of platinum(II) complexes with the
formulas Pt(phebox)(L)+ (phebox− = 1,3-bis(4,4′-dimethyl-2′-
oxazolinyl)phenyl anion; L = pyridine (py), 4-phenylpyridine,
quinoline, acridine) and Pt2(phebox)2(μ-L′)2+ (L′ = pyrazine,
4,4′-bipyridine, 1,2-bis(4-pyridyl)ethane) was prepared. Crystal-
lographic data establish that the metal center is bonded to the
tridentate phebox− and monodentate pyridyl ligands. The five-
membered oxazoline rings favor a CH2−CMe2 twist conforma-
tion. Pt(phebox)Cl and Pt(phebox)(py)+ undergo a ligand-based
chemically reversible redox reaction, whereas the electrochemistry
of the other complexes is chemically and electrochemically less
reversible. In contrast to complexes with the 1,3-bis-
(piperdylmethyl)phenyl anion ligand (pip2NCN−) or related
pincer ligands, each of the phebox− complexes described here
exhibits intense emission in room-temperature methylene chloride solution, which is assigned as originating from a lowest,
predominantly phebox− ligand-centered excited state. In acetonitrile, the complexes undergo solvolysis resulting in displacement
of the pyridyl ligands. The accumulated data demonstrate that subtle variations in the nature of the NCN and ancillary ligands of
platinum(II) complexes provide access to at least five orbitally distinct emissive excited states.
Scheme 1. Selected NCN Pincer Ligands
INTRODUCTION
■
Cyclometalated platinum(II) complexes with tridentate pincer
type ligands, especially NCN complexes, have attracted
widespread interest because of their potential to function as
catalysts for allylation,1−7 Diels−Alder reactions,8 Michael
addition,9−12 aldol reactions,13,14 hydrosilylation,15 conjugate
reduction,16−18 and Heck reactions,19−22 as well as hydro-
genation of ketones.23 There also is growing interest in these
systems because of their intriguing photophysical properties
and potential application in optoelectronic devices such as
organic light-emitting devices24 and chemical sensors.25 One
interesting class of compounds is composed of Pt(R4NCN)-
(L)n+ complexes, such as Pt(Me4NCN)Cl, which were
developed by van Koten and co-workers (Scheme 1).1,26,27 In
a series of previous studies, we reported the structural and
electronic properties of a series of Pt(pip2NCN)(L)n+
monomers (L = halide, pyridyl ligand, n = 0, 1) and
Pt2(pip2NCN)2(μ-L′)2+ (L′ = bridging pyridyl ligands).28−30
Variations in the ancillary L ligand were shown to provide
exquisite control over the electronic structures and lumines-
cence properties of these complexes, allowing for tuning of the
emission maximum from 430 to 640 nm. For example, the
emission from Pt(pip2NCN)(L)n+ originates from a predom-
inantly spin-forbidden ligand field state when L = py or halide,
whereas the emission originates from a lowest pyridyl ligand-
centered state (3LC) when L is 4-phenylpyridine or L′ is 4,4′-
bipyridine in the case of Pt2(pip2NCN)2(μ-L′)2+. When L′ is a
bridging pyrazine (pyz), the emission has significant triplet
metal-to-ligand (pyz) charge-transfer (3MLCT) character.
These conclusions also are consistent with density functional
theory (DFT) calculations,31 which suggest that MLCT states
Received: May 6, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b01022
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 2. Syntheses of Platinum(II) Monomer and Dimer Complexes
involving the bridging L′ ligand are stabilized by introduction of
either electron-releasing substituents on the phenyl group of
the NCN ligand or electron-withdrawing substituents on L′, as
well as by extending the length of the conjugated bridging
ligand. Though the preceding systems are essentially non-
emissive in room-temperature (RT) solution, Batema et al.
have elegantly demonstrated that Pt(Me4NCN)Cl complexes
exhibit intense fluid-solution fluorescence when functionalized
with a 4′-substituted styryl group.32 The resulting stilbene
group largely retains its luminescence properties albeit with
decreased quantum yield due to the attached platinum center.
Complexes with the chiral bis(oxazolinyl)phenyl ligand
(ox2NCN−; Scheme 1) were developed by Nishiyama,5
Denmark,21 and Richard22 and have been extensively studied
as catalysts for asymmetric synthesis.4,5,19,20,33−37 Despite
considerable interest in the chemistry of platinum complexes
with this class of NCN ligand, there have been no reports on
the luminescence properties of platinum(II) complexes with
oxazoline pincer ligands, and comparatively little is known
about their electronic structures. However, there have been
reports on the luminescence properties of related NCN
complexes with imine groups, such as Pt(bib)Cl (bibH = 1,3-
bis(N-methylimino)benzene)38 and Pt(bpb)Cl (bpbH = 1,3-
bis(2-pyridyl)benzene) (Scheme 1).39−41 For example, Pt(bib)
Cl exhibits RT emission in CH2Cl2 solution assigned to a
lowest state with mixed MLCT and ligand-to-ligand charge-
transfer (LLCT) character with the lowest unoccupied
molecular orbital mainly localized on the tridentate ligand.
Emissions from Pt(bpb)Cl and Pt(bpb)(CN(C6H3-2,6-
Me2))+ have been assigned as originating from a mixed LC/
MLCT state.39−43 Understanding of the electronic structures of
such systems is expected to guide efforts to tailor the optical
properties and reactivity of these systems. Because of the
relative basicities (e.g., pKb values: oxazoline, 4.70; piperidine,
11.12)44,45 and π-acidities of the N-donor groups, the
comparatively rigid ox2NCN− ligand is reasonably expected
to be a slightly weaker σ-donor and much stronger π-acceptor
than either Me4NCN− or pip2NCN−. Therefore, MLCT states
involving the lowest π* level of ox2NCN−, as well as pincer LC
states with ox2NCN− π−π* character, are expected to be
stabilized. To assess the influence of the oxazoline groups on
the molecular geometries, electronic structures, and spectro-
scopic properties of platinum complexes with NCN ligands, we
undertook the synthesis and characterization of a series of
monomer and dimer complexes with the phebox− ligand
(Scheme 1): Pt(phebox)(L)n+ (L = Cl, pyridyl ligand) and
Pt2(phebox)2(μ-L′)2+ (L′ = bridging pyridyl ligand). In contrast
to their pip2NCN− counterparts, the complexes are emissive in
RT fluid solution.
EXPERIMENT SECTION
■
K2PtCl4 was obtained from Pressure Chemical Co. 1,3-Dicyanoben-
zene and AgBF4 were obtained from Sigma-Aldrich. AgPF6 was
obtained from Alfa Aesar. All other reagents were purchased from
Acros. Glacial acetic acid was dried by distilling from P2O5 and acetic
anhydride. Methylene chloride was distilled over CaH2. Tetrabutyl-
ammonium hexafluorophosphate (TBAPF6) was recrystallized three
times from boiling ethanol and dried under vacuum prior to use. All
B
DOI: 10.1021/acs.inorgchem.5b01022
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 1. ORTEP diagram of the cations in crystals of the Pt(phebox)(pyridyl)+ monomers: (a) 1, (b) Molecule A of 2b, (c) 3-m, (d) Molecule A of
4·3/4C6H14·1/2H2O, and (e) 5. Ellipsoids plotted at 50% probability, H atoms omitted for clarity. (ORTEP diagrams of the cations of Molecule B
for 2b, 3-o, and 4·3/4C6H14·1/2H2O are in Figure S1).
other chemicals were used as received. Pt(phebox)Cl (1) was prepared
as previously described.34 Argon was predried using activated sieves,
and trace oxygen was removed with activated R3−11 catalyst from
Schweizerhall. The synthesis of 1,3-bis(4,4′-dimethyl-2-oxazolinyl)-
benzene (pheboxH) and its metal complexes is described in the
Supporting Information.
to TMS (δ = 0 ppm) for CDCl3 or relative to the protic solvent
impurity in the case of CD3CN (δ = 1.94 ppm for CD2HN).
Elemental analyses were performed by Atlantic Microlab, Inc.
(Norcross, GA). UV−visible absorption spectra were recorded using
a HP8453 UV−visible diode array spectrometer. Emission spectra
were recorded using a SPEX Fluorolog-3 fluorimeter equipped with a
double-emission monochromator (385 nm cutoff filter) and a single
excitation monochromator. Glassy solutions (77 K) were prepared by
inserting a quartz electron paramagnetic resonance tube containing a
4:1 ethanol−methanol solution of the respective complex into a
1H NMR spectra were recorded at RT using a Bruker AC 400 MHz
instrument. Deuterated solvents CDCl3 (0.03% (v/v) tetramethyl-
silane (TMS)) and CD3CN were purchased from Cambridge Isotope
Laboratories. Spectra are reported in parts per million (ppm) relative
C
DOI: 10.1021/acs.inorgchem.5b01022
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 2. ORTEP diagram of the cations in crystals of Pt2(phebox)2(μ-L′)2+ (a) 6·2C3H6O, (b) 7b·2C3H6O and (c) 8-c·C4H10O. Ellipsoids plotted
at 50% probability. H atoms omitted for clarity. (ORTEP diagram of the cation in 8-p·5/4C3H6O is in Figure S1).
series of monomers and dimers with the phebox− pincer ligand
quartz-tipped finger Dewar. Emission spectra were corrected for
instrumental response. Mass spectra were obtained by electrospray
were readily prepared by treatment with a slight excess of a
ionization (ESI) of acetonitrile or methanol solutions using a
soluble silver(I) salt, followed by addition of a pyridyl ligand
Micromass Q-TOF-2 instrument. Experimental details of the
(Scheme 2). The complexes were isolated as the pale yellow
crystallography, as well as selected metrics for the complexes, are
hexafluorophosphate salts. In the cases of the pyridyl and
provided in the Supporting Information.
−
−
bipyridyl complexes, both the PF6 (2a and 7a) and the BF4
Cyclic voltammetry measurements were performed using a standard
three-electrode cell and a 100 B/W electrochemical workstation from
Bioanalytical Systems. Scans were recorded of 1−1.5 mM CH2Cl2
solutions containing 0.1 M TBAPF6. All scans were recorded using a
platinum wire auxiliary electrode and a 0.79 mm2 platinum working
electrode. Between measurements, the working electrode was polished
with 0.05 μm alumina, rinsed with distilled water, and wiped dry using
a Kimwipe. Reported potentials are referenced versus Ag/AgCl (3.0 M
NaCl). Peak currents (ip) were estimated with respect to the
extrapolated baseline current, as described by Kissinger and Heine-
man.46 Under these conditions, the ferrocenium/ferrocene (Fc+/Fc)
couple occurs at 0.50 V. The values of (Epc + Epa)/2, which is an
approximation of the formal potential for a redox couple, are referred
to as E°′.
(2b and 7b) salts were prepared. The products were
characterized by a combination of 1H NMR spectroscopy,
elemental analysis, mass spectrometery, and X-ray crystallog-
raphy. The salts are soluble in polar organic solvents (e.g.,
dichloromethane) but readily undergo solvolysis in acetonitrile
resulting in displacement of the pyridyl ligand (vide infra).
1
With the exception of 4, the patterns of resonances in the H
NMR spectra are consistent with C2v symmetric complexes,
indicating that any distortion of the five-membered oxazolinyl
ring is not long-lived. However, at RT, the phebox− methyl
protons of 4 appear as two equally intense resonances (rather
than a single resonance), indicating that rotation of the
quinoline ligand about the Pt−N bond is slow. The α-protons
of the pyridyl/pyrazinyl ligand occur at ≥8.5 ppm, significantly
shifted downfield from those of the free ligand. Similarly, the
methyl proton resonance for 1 is shifted by 0.3 ppm downfield
from the free ligand resonances. By contrast, the methyl
resonances are shifted 0.2−0.8 ppm upfield in the spectra 2−8
due to the ring current of the nitrogen heterocycle.
Structures. The structures of 1−8 were confirmed by X-ray
crystallography (Figures 1, 2, and S1; Tables S1−S4). Each
platinum center is bonded to a tridentate phebox− ligand and a
pyridyl ligand, resulting in a distorted square planar
coordination geometry similar to those found for other
platinum(II) complexes with oxazolinyl pincer li-
gands.20,33,34,49 The pyridyl groups are approximately perpen-
dicular to the coordination plane, as expected for the steric
demands of the oxazoline methyl groups, whose proximity
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The preparation of
protonated oxazoline pincer ligand precursors, such as
pheboxH, has typically been accomplished in four steps, using
thionyl chloride in a moisture-free environment.47 Although
Button and co-workers reduced the synthesis to two steps using
anhydrous ZnCl2, the preparation of the catalyst is
cumbersome, requiring three cycles of crystallization from a
melt under vacuum and cooling under Ar.48 We found that
pheboxH is readily prepared in a single step by refluxing 1,3-
dicyanobenzene and 2-methyl-2-amino-1-propanol in the
presence of Zn(OAc)2·2H2O in air without prior drying or
purification of the solvent. Using this product, Pt(phebox)Cl
was prepared as previously described.34 From that complex, a
D
DOI: 10.1021/acs.inorgchem.5b01022
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
restricts rotation of the pyridyl group about the Pt−N bond
Fc), in keeping with its positive charge and strong tpy(π*)/
Pt(6pz) mixing.61 Therefore, we conclude that the first
reduction potentials follow the order Pt(tpy)Cl+ ≫ Pt-
(phebox)Cl > Pt(Mebpb)Cl.
1
and results in upfield shifts in the H NMR spectra. The
intramolecular Pt···Pt distances for dimers 6 (7.0285(5) Å), 7b
(11.3489(8) Å), and 8-c (9.1385(6) Å) and 8-p (9.1884(6) Å)
are typical for these bridging ligands.29,30,50−58 In crystals of 8-p
and 8-c, the 1,2-bis(4-pyridyl)ethane (bpa) ligand adopts the
less energetically favorable gauche conformation, presumably
due to packing effects. It is interesting that the Pt−N(pyridyl)
bond lengths for 1−8 are significantly shorter than those found
for pip2NCN− analogues,29,30 as well as Pt(Me4NCN)-
(pyOH)+.59,60 The accumulated data indicate a stronger
interaction between the platinum center and pyridyl ligand in
the phebox− series and suggests that phebox− is a weaker trans/
cis directing ligand than pip2NCN− or Me4NCN−.
We also note that the puckering of the five-membered
oxazolinyl rings for these and seven related complexes was
characterized based on previously developed descriptions of
five-member rings (Supporting Information).20,33,34,49 Those
results show that the oxazolinyl groups do not lie on a single
pseudorotation pathway with a unique puckering amplitude and
that complexes with large puckering amplitudes show a decided
preference for a CH2−CMe2 twist conformation.
By contrast, compounds 5−7 undergo pyridyl ligand-based
reductions at greater than −1.3 V. For example, the −1.19 V
anodic peak potential for 5 is close to that observed for
protonated acridine (−1.20 V; 1 M KNO3 in 0.01 M HCl).66
The bridging ligand reduction processes for 6 and 7 are nearly
reversible and anodically shifted by ∼0.1 V from those of their
Pt2(pip2NCN)2(μ-L)2+ counterparts (pyz: −0.88 V, ΔEp = 66
mV;30 bpy: −1.07 V, ΔEp = 80 mV). Thus, it is evident that the
Pt(phebox)+ unit has a slightly stronger stabilizing effect on the
bridging ligand π* level than Pt(pip2NCN)+. This observation
is consistent with the notion that the weaker σ-donor and
stronger π-acceptor properties of phebox− conspire to make it
an overall weaker donor than pip2NCN−.
Electronic Spectroscopy. The electronic spectrum of 1
exhibits maxima near 295, 340, and 380 nm (∼5000 M−1 cm−1;
Figure 3 and Table S5), spanning a region where pheboxH
Electrochemistry. To better understand the electronic
structures of these complexes, cyclic voltammograms (CVs)
were recorded in 0.1 M TBAPF6 methylene chloride solution
(Table 1). None of the complexes showed oxidation waves at
Table 1. Electrochemical Data for Pt(phebox)Ln+ (1−5) and
2+
Pt2(phebox)2(μ-L′) (6−8)
a
L (compound no.)
E°′, V (ΔEp, mV)
Cl (1)
−1.50 (100)
−1.38 (85)
−1.46 (67)
−1.31 (100)
−1.19 (irr)
py (2a)
phpy (3)
quin (4)
ac (5)
Figure 3. RT electronic absorption spectra of 1 (black), 2 (red), 3
(blue), 4 (green), and 5 (dashed line) in CH2Cl2.
pyrazine (6)
bpy (7a)
bpa (8)
−0.79 (80)
−1.07 (80), −1.49(140)
−1.33 (100)
absorbs only weakly. The two longest wavelength maxima
occur in a region where transitions with significant MLCT and/
or phebox LC character are expected.38−42,64 Both bands are
somewhat solvent-sensitive, shifting to longer wavelength with
decreasing polarity (Figure S5). The longer wavelength
transition near 380 nm likely has significant MLCT character
because it is absent from the spectra of 2−4 and 6−8,
presumably because it is shifted by at least 1300 cm−1 to shorter
wavelength (Figures 3 and S5). This is consistent with the
expectation that MLCT states in the latter complexes lie at
higher energy than in 1, because the nitrogen heterocyclic
ligands are weaker donors than the chloro ligand. For example,
the longest wavelength MLCT band of Ru(tpy)(bpy)Cl+ (550
nm) is red-shifted from that of Ru(tpy)(bpy)(py)2+ (465 nm)
by 1500 cm−1.62
For comparison, we note that the electronic spectrum of
Pt(bib)Cl exhibits two longest wavelength absorption band
maxima at 386 (4.1 × 103 M−1 cm−1) and 400 nm (4.4 × 103
M−1 cm−1).38 Similarly, Pt(bpb)Cl shows two long-wavelength
bands at 380 (8.7 × 103 M−1 cm−1) and 401 nm (7.0 × 103 M−1
cm−1).39 Several studies of these and related complexes with
substituted bpb− ligands support the consensus view that
several bands having significant LC(π−π*) and MLCT
character occur in this region with the longest wavelength
band having substantial MLCT character.39−42,64 For six
a
Cyclic voltammograms were recorded in 0.1 M TBAPF6/CH2Cl2 at
0.10 V/s and referenced vs Ag/AgCl.
<1.5 V versus Ag/AgCl. However, excepting 5, each of the
monomers exhibited a reduction wave in the −1.3 to −1.5 V
range, which is tentatively assigned to a phebox− ligand-based
process. Notably, the ∼0.12 V anodic shift of the pyridine
complex relative to 1 is consistent with the expected relative
donor properties of the ancillary pyridyl and chloro ligands;
similar shifts occur for the mainly polypyridyl ligand-based
reductions of Pt(tpy)Ln+ (0.22 V; tpy = 2,2′:6′,2″-terpyr-
idine),61 Ru(tpy)(2,2′-bpy)Ln+ (0.20 V; 2,2′-bpy = 2,2′-
bipyridine),62 and Ru(2,2′-bpy)2(Cl)(L) (0.11 V).63 Also in
keeping with this assignment, we note that 2−4 and 8 were
reduced at more positive potentials than the chloro complex
(1). An interesting comparison comes from the observation
that the reduction process for 1 (−2.0 V vs Fc+/Fc) is shifted
by 0.1−0.3 V to more positive potentials compared to that of
Pt(bpb)Cl (CH2Cl2, −2.14 V; dimethylformamide (DMF),
−2.18 V vs Fc+/Fc)64 and Pt(Mebpb)Cl (Mebpb− = 5-methyl-
1,3-di(2-pyridyl)phenyl anion; CH2Cl2, −2.27 V vs Fc+/Fc),65
despite the expected increased delocalization in these latter
systems. By contrast, cationic Pt(tpy)Cl+ is reduced at
significantly more positive potentials (DMF: −0.74 V vs Fc+/
E
DOI: 10.1021/acs.inorgchem.5b01022
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
solvents investigated, the 340 and 380 nm bands in the
spectrum of Pt(phebox)Cl are approximately twice as sensitive
as the band maxima for Pt(bpb)Cl, in keeping with the notion
that the 380 nm band of 1 also has significant MLCT character.
From these comparisons and the spectrum of Pt(tpy)Cl+ (405
nm),67 it is evident that the relative energies of the lowest spin-
allowed MLCT states are Pt(phebox)Cl > Pt(bib)Cl ≈
Pt(bpb)Cl ≈ Pt(tpy)Cl+. The similarity in energies for the
latter three compounds is attributable to several factors. First,
the results suggest that the marked stabilization of the π*(tpy)
level of Pt(tpy)Cl+ (vide supra) is largely offset by stabilization
of the Pt 5d orbitals due to reduced donation by the tridentate
tpy ligand, as compared to NCN ligands. Likewise, although the
π-system of Pt(bib)Cl is smaller than that of Pt(bpb)Cl, the
strong π-acceptor properties of the imine groups of Pt(bib)Cl
are expected to stabilize MLCT states, as noted for pyridine
carboxaldimine complexes.68 However, the ∼0.15 eV destabi-
lization of the lowest 1MLCT state of 1 as compared to
Pt(bib)Cl is attributable to the relative donor properties of the
ether and methyl substituents on the ligand π* level. An
additional consideration is variation in the extent of LC/MLCT
mixing, though detailed knowledge of this effect is currently
lacking.
Titration of a methylene chloride solution of 3 with
acetonitrile showed a gradual decrease in intensity of the
phpy-centered 278 nm absorption band, confirming ligand
dissociation (Figure 4). The absence of large changes at >330
Figure 4. Apparent molar absorptivity (ε) of a 70 μM solution of
Pt(phebox)(phpy)+ in methylene chloride during titration with 1.2 ×
105 equiv of acetonitrile.
At long wavelengths, the absorption profiles of the dimers
(6−8; Figure S6) are similar to those of 2−4. For example, the
spectrum of 8 is almost identical to that of Pt(phebox)(py)+
(2) except with ∼1.5 times the molar absorptivity. This is
consistent with the notion that the electronic structures of the
Pt(phebox)(pyridyl)+ and Pt(phebox)(pyz)+ units are only
weakly unperturbed by dimer formation. Also note that 6 (257
nm, 13.3 × 103 M−1 cm−1) and 7 (268 nm, 15.3 × 103 M−1
cm−1) exhibit shorter wavelength bands tentatively assigned to
bridging ligand transitions.29,30,69,70 Likewise, compounds 3 and
4 also show contributions from the N-heterocyclic ligands at λ
≤ 320 nm, which are found in spectra of related
molecules.29,71−76 At wavelengths greater than 320 nm, the
absorption profile of 5 is distinctly different and more intense
than that observed for the other compounds; the observed
progression in the 330−360 nm range and the band at 415 nm
also appear in the spectrum of acH+.77
nm is consistent with this region being dominated by mainly
transitions involving the phebox− ligand in the Pt(phebox)-
(phpy)+ and Pt(phebox)(CH3CN)+ complexes (Figure 4).
During titration with acetonitrile (0−45% by volume),
isosbestic points were not well-defined, presumably because
of the sensitivity of the spectra to changes in solvent dielectric.
Under the assumption of the solvolysis equilibrium in eq 1, the
equilibrium constant was estimated to be ∼2 × 10−4, indicating
a strong preference for coordination of phpy over acetonitrile.
Emission Spectroscopy. Whereas Pt(pip2NCN)(L)+
complexes are essentially nonemissive in fluid solution,
compounds 1−8 exhibit intense yellow-orange emission in
methylene chloride solution. Thus, the influence of the more
conjugated NCN ligand is immediately obvious. Lowest-lying
excited states in such systems have been extensively
discussed,29,38−42,64,78,79 and the most logical candidates in
the present case are those with LC and/or MLCT character
involving the phebox− ligand; excited-state mixing will be
favored when the parent states approach in energy and there is
strong metal−ligand interaction. Consistent with this notion,
we note that the 1400−1500 cm−1 vibronic spacings in the
emission profile of Pt(phebox)Cl are indicative of involvement
of the phebox− ligand (Figure 5a and Table 2). The emission
lifetime is concentration-dependent, consistent with bimolecu-
lar self-quenching, as noted previously for platinum polypyridyl
complexes.80 Presumably because of the steric demands of the
oxazolinyl methyl substituents, the self-quenching rate constant
(ksq, 1.7(3) × 109 M−1 s−1) is slightly less than that found for
Pt(bpb)Cl (ksq, 5.3 × 109 M−1 s−1).40 The lifetime at infinite
dilution (3.0(1) μs) confirms that the emission originates from
a predominantly spin-forbidden excited state. With the
exception of 5, the structured emission profiles of the remaining
compounds are nearly superposable with 1400−1500 cm−1
vibronic spacings similar to those of 1. However, as compared
to the spectrum of 1, the emission band is shifted by ∼800
cm−1 to longer wavelength. In other words, it is evident that the
emission energy is mildly sensitive to the donor properties of
the ancillary ligand (e.g., Cl− vs pyridine), which is consistent
Ligand Dissociation. Whereas compound 1 is stable in
acetonitrile, 2−8 are not, and the observed behavior is
consistent with solvolysis resulting in displacement of the
pyridyl ligands. This is illustrated for 3 by the reaction
Pt(phebox)(phpy)+ + CH3CN ⇆ Pt(phebox)(NCCH3)+ + phpy
(1)
For example, in RT ∼2 mM CD3CN solutions, the pyridyl
1
resonances in the H NMR spectra of 3 were coincident with
those of the free ligand, indicating that the ligand was fully
dissociated. The pattern of phebox− resonances is consistent
with formation of C2v symmetric Pt(phebox)(CD3CN)+, which
was also identified in the mass spectrum. Notably, the oxazoline
CH2 and C(Me)2 proton resonances appeared at 4.76 and 1.49
ppm, respectively. Coordination of acetonitrile was suggested
by the downfield shift of the solvent CD2HCN resonance. With
the addition of pyridine (Figure S7), a new set of resonances
appeared, including the α-proton resonance of coordinated
pyridine at 9.00 ppm and a new set of CH2 and C(Me)2 proton
resonances at 4.66 and 1.10 ppm, respectively. Evaporation of
the solvent and recording of the 1H NMR spectrum in CD2Cl2
confirmed complete recovery of the original Pt(phebox)-
(phpy)+ complex.
3
with a predominantly LC excited state with partial MLCT
F
DOI: 10.1021/acs.inorgchem.5b01022
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Ru(tpy)(bpy)(py)2+ (585 nm) in 77 K frozen acetonitrile
solution is shifted by ∼2000 cm−1 from that of Ru(tpy)(bpy)-
(Cl)+ (662 nm).62 Thus, we propose that the lowest excited
3
state has less, but not inconsequential, MLCT character. For
example, variations in the extent of MLCT mixing also are
suggested by the fact that values of S (1.3−1.4) for 2−4 are
slightly greater than that for 1 (1.0; Figure 5b and Table 3).
Table 3. Emission Spectroscopic Data for Pt(phebox)Cl and
1−8 in 77 K Ethanol−Methanol (4:1) Glassy Solution (λex =
350 nm)
compound
emission (λmax, nm)
497, 537, 586, 640
I0,0/I1,0 (S)
1
1.0
1.4
1.4
1.3
a
2a
484, 523, 572, 628
a
3
489, 526, 572, 628
a
4
485, 523, 570, 627
a
5
390, 414, 441, 465, 497, 533, 584, 638, 657sh
487, 522, 566, 619
6
1.4
1.4
1.4
7a
8
490, 528, 572, 626
486, 528, 572, 626
a
With >50 equiv of pyridyl ligand.
Figure 5. (a) Emission spectra of 1 (black), 2 (red), and 3 (blue) in
RT CH2Cl2 solution and (b) emission spectra of 1 (black) and 2 (red,
with excess pyridine), as well as the excitation spectrum of 1 (red), in
77 K 4:1 ethanol/methanol glassy solution.
Note that the excitation spectrum of Pt(phebox)Cl, which is in
good agreement with the absorption spectrum (Figure 5b), also
shows two weak low-energy features near 460 and 490 nm
(1300 cm−1 spacing). Similar features appear in the 77 K
excitation spectrum of Pt(tpy)Cl+ (433, 464 nm)83 and the
absorption spectrum of Pt(bpb)Cl (485 nm, 145 M−1 cm−1).39
The narrow, lower-energy feature overlaps well with the first
vibronic feature at 497 nm in the emission spectrum.
Accordingly, we assign this absorption to the lowest-energy
Table 2. Emission Spectroscopic Data for 1−5 and 8 in
Room-Temperature CH2Cl2 Solution (λex = 350 nm)
compound
emission (λmax, nm)
I0,0/I1,0 (S)
1
510, 552, 600 (sh), 654 (sh)
490, 528, 570(sh), 629 (sh)
490, 529, 570(sh), 629(sh)
490, 527, 570(sh), 629(sh)
491, 530, 570(sh), 629 (sh)
491, 532, 575(sh), 632(sh)
1.2
1.4
1.4
1.4
2a
3
3
spin-forbidden mainly LC transition.
From these results and based on the shortest wavelength
emission vibronic band, we can draw conclusions concerning
the relative energies of the lowest excited states of Pt(tpy)Cl
(77 K, butyronitrile, 470 nm),67 Pt(bpb)Cl (RT, CH2Cl2, 491
nm; 77 K, 2-methyl-tetrahydrofuran, 486 nm),41 Pt(phebox)Cl
(RT, CH2Cl2, 510 nm; 4:1 EtOH/MeOH 497 nm), and
Pt(bib)Cl (RT, CH2Cl2, 562 nm). The accumulated data
suggest that the energy decreases along the series Pt(tpy)Cl+ >
Pt(bpb)Cl > Pt(phebox)Cl > Pt(bib)Cl. The experimental data
contradict estimates from DFT calculations, which suggest
Pt(bpb)Cl > Pt(tpy)Cl+.84 The states for Pt(tpy)Cl+, Pt(bpb)-
Cl, and Pt(phebox)Cl lie within a narrow range (∼1200 cm−1),
whereas that of Pt(bib)Cl is stabilized by ∼1800 cm−1 (∼0.2
eV) below that of Pt(phebox)Cl. The destabilization of the
lowest excited state of 1 as compared to Pt(bib)Cl is
qualitatively consistent with the electron donor properties of
the oxazolinyl ether group, which is expected to have a
destabilizing influence on the lowest phebox− π* level and
therefore reduce MLCT character in the lowest excited state.
Compound 5 with excess acridine exhibits three emission
bands in 77 K frozen solution (Figure S10). The shorter
wavelength progression matches that of free acridine under
these conditions, whereas the midrange progression (495, 533,
584, 638 nm) is similar to that of Pt(phebox)Cl. Interestingly,
at longer wavelengths there is unexpected additional intensity
and a distinct shoulder at 657 nm, which are consistent with
acridine phosphorescence.85 It is possible that the latter
originates from the Pt(phebox)(ac)+ complex or energy transfer
to free acridine.
4
5
8
1.4
character. In support of this assignment, we note that the
Huang−Rhys ratio (S = I(1,0)/I(0,0)) increases from 1.2 to 1.4
upon substitution of the chloro donor ligand with the more
weakly donating N-heterocycle ligands, indicating an increase in
the excited-state geometric distortion. For platinum(II)
polypyridyl complexes having emissive states with mixed LC/
MLCT character, the tendency of S to increase with increasing
LC character is well-documented.79 For example, the Huang−
Rhys ratios for the chloro and cyano derivatives of Pt(tpy)X+ in
77 K butyronitrile solution are 0.6 and 1.2, respectively.67,81
The vibronic structure is sharpened in 4:1 ethanol/methanol
77 K glassy solution, and the emission bands are slightly blue-
shifted from the RT spectra (Figure 5b). Spectra of 2−4 under
these conditions were excitation wavelength-dependent as
expected for dissociation of the pyridyl ligand in alcohol
solution (Figure S8); the resulting structured emission from
Pt(phebox)(ROH)+ (R = methyl, ethyl) occurs at slightly
longer wavelength than that of the pyridyl complexes. To
suppress ligand dissociation, spectra of the latter complexes
were recorded with excess pyridyl ligand. The resulting spectra
are distinctly sharper under these conditions and independent
of excitation wavelength (Figure S9).82 Similar to the RT
results, the bands for 2−4 are red-shifted by 300−500 cm−1
from that of 1. For comparison, MLCT emission from
G
DOI: 10.1021/acs.inorgchem.5b01022
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Crystallography at Advanced Light Source (SCrALS) project
at Beamline 11.3.1 at the Advanced Light Source (ALS),
Lawrence Berkeley National Laboratory. The ALS is supported
by the U.S. Department of Energy, Office of Energy Sciences,
under Contract No. DE-AC02-05CH11231.
CONCLUSIONS
■
To place these findings in context, it is useful to compare the
preceding observations with those obtained for the Pt-
(pip2NCN)Ln+ analogues. Notably, the latter give rise to
3
3
emissions in frozen solution from lowest LF, pyridyl LC, or
3MLCT states involving the pyridyl ligand. By contrast, and as a
result of the low-lying π*(phebox−) level, 1−4 and 6−8 give
rise to emissions from distinctly different lowest excited states
having 3LC and 3MLCT character involving the phebox− ligand
(Figure 5b and Figure S11). Thus, subtle variations in the
nature of the NCN and ancillary ligands provide access to at
least five orbitally distinct emissive excited states. Interestingly
and as also evident for Pt(bpb)Cl39−41 and Pt(bib)Cl,38 the LC
and MLCT states involving the NCN ligand are much less
susceptible to nonradiative deactivation in fluid solution than
Pt(pip2NCN)Ln+ complexes. Likewise, although Pt(tpy)Ln+
complexes have lowest excited states with tpy LC/MLCT
character, those complexes tend to be nonemissive in fluid
solution due to population of low-lying LF states resulting in
nonradiative deactivation.86,87 Thus, it would appear that the
strong donor properties of the phenyl donor groups of
phebox−, bpb−, and bib− ligands increase the gap between
the lowest state and higher-lying LF states, thereby reducing the
latter’s deactivating effect. Lastly, we note that, for chloro
complexes with different meridional tridentate ligands, we have
found that the lowest excited-state energy decreases along the
series Pt(tpy)Cl+ > Pt(bpb)Cl > Pt(phebox)Cl > Pt(bib)Cl.
Taken all together, the results presented here suggest a rather
synthetically convenient means of modulating the orbital
character and energies of the lowest excited states of
platinum(II) complexes with the 1,3-bis(imino)benzene ligand
skeleton.
REFERENCES
■
(1) van Koten, G. Top. Organomet. Chem. 2013, 40, 1−20.
(2) Nishiyama, H.; Ito, J.-i. Chem. Commun. 2010, 46, 203−212.
(3) Nishiyama, H. Chem. Soc. Rev. 2007, 36, 1133−1141.
(4) Motoyama, Y.; Okano, M.; Narusawa, H.; Makihara, N.; Aoki, K.;
Nishiyama, H. Organometallics 2001, 20, 1580−1591.
(5) Motoyama, Y.; Makihara, N.; Mikami, Y.; Aoki, K.; Nishiyama, H.
Chem. Lett. 1997, 951−952.
(6) Motoyama, Y.; Narusawa, H.; Nishiyama, H. Chem. Commun.
1999, 131−132.
(7) Motoyama, Y.; Nishiyama, H. Synlett 2003, 1883−1885.
(8) Motoyama, Y.; Koga, Y.; Nishiyama, H. Tetrahedron 2001, 57,
853−860.
(9) Motoyama, Y.; Koga, Y.; Kobayashi, K.; Aoki, K.; Nishiyama, H.
Chem. - Eur. J. 2002, 8, 2968−2975.
(10) Fossey, J. S.; Richards, C. J. J. Organomet. Chem. 2004, 689,
3056−3059.
(11) Stol, M.; Snelders, D. J. M.; Godbole, M. D.; Havenith, R. W. A.;
Haddleton, D.; Clarkson, G.; Lutz, M.; Spek, A. L.; van Klink, G. P.
M.; van Koten, G. Organometallics 2007, 26, 3985−3994.
(12) Bugarin, A.; Connell, B. T. Organometallics 2008, 27, 4357−
4369.
(13) Inoue, H.; Kikuchi, M.; Ito, J.-i.; Nishiyama, H. Tetrahedron
2008, 64, 493−499.
(14) Mizuno, M.; Inoue, H.; Naito, T.; Zhou, L.; Nishiyama, H.
Chem. - Eur. J. 2009, 15, 8985−8988.
(15) Tsuchiya, Y.; Uchimura, H.; Kobayashi, K.; Nishiyama, H.
Synlett 2004, 2099−2102.
(16) Kanazawa, Y.; Nishiyama, H. Synlett 2006, 3343−3345.
(17) Kanazawa, Y.; Tsuchiya, Y.; Kobayashi, K.; Shiomi, T.; Itoh, J.;
Kikuchi, M.; Yamamoto, Y.; Nishiyama, H. Chem. - Eur. J. 2006, 12,
63−71.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b01022. CCDC Nos. 1058633−1058642 contain the
crystallographic data for 1, 2b, 3-m, 3-o, 4−6, 7b, 8-c, and 8-p.
These data can be obtained free of charge from the Cambridge
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
■
S
(18) Tsuchiya, Y.; Kanazawa, Y.; Shiomi, T.; Kobayashi, K.;
Nishiyama, H. Synlett 2004, 2493−2496.
(19) Stark, M. A.; Jones, G.; Richards, C. J. Organometallics 2000, 19,
1282−1291.
(20) Motoyama, Y.; Kawakami, H.; Shimozono, K.; Aoki, K.;
Nishiyama, H. Organometallics 2002, 21, 3408−3416.
(21) Denmark, S. E.; Stavenger, R. A.; Faucher, A.-M.; Edwards, J. P.
J. Org. Chem. 1997, 62, 3375−3389.
(22) Stark, M. A.; Richards, C. J. Tetrahedron Lett. 1997, 38, 5881−
5884.
(23) Ito, J.-i.; Ujiie, S.; Nishiyama, H. Chem. Commun. 2008, 1923−
1925.
Synthesis details, complete details of the crystallographic
study, tables of selected bond distances and angles,
oxazolinyl puckering analysis, analytical and spectro-
1
scopic data, and H NMR spectra of 2 during titration
with pyridine. (PDF)
(24) Williams, J. A. G. Chem. Soc. Rev. 2009, 38, 1783−1801.
(25) Li, K.; Chen, Y.; Lu, W.; Zhu, N. Y.; Che, C. M. Chem. - Eur. J.
2011, 17, 4109−4112.
AUTHOR INFORMATION
Corresponding Author
*E-mail: bill.connick@uc.edu. Fax: (+1) 513-556-9239.
Notes
■
(26) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed. 2001, 40,
3750−3781.
(27) van der Boom, M. E.; Milstein, D. Chem. Rev. 2003, 103, 1759−
1792.
(28) Jude, H.; Krause Bauer, J. A.; Connick, W. B. Inorg. Chem. 2002,
The authors declare no competing financial interest.
41, 2275−2281.
ACKNOWLEDGMENTS
■
(29) Jude, H.; Krause Bauer, J. A.; Connick, W. B. Inorg. Chem. 2004,
43, 725−733.
This research was supported the National Science Foundation
(Grant No. CHE-1152853). Funding for the SMART6000
CCD diffractometer was through NSF-MRI Grant No. CHE-
0215950. D.Z. thanks the Univ. of Cincinnati Univ. Research
Council for a summer fellowship. We also thank Drs. L. Sallans
and S. Macha for assistance with MS measurements.
Synchrotron data were collected through the Service
(30) Jude, H.; Krause Bauer, J. A.; Connick, W. B. Inorg. Chem. 2005,
44, 1211−1220.
(31) Shi, L.-L.; Liao, Y.; Yang, G.-C.; Su, Z.-M.; Zhao, S.-S. Inorg.
Chem. 2008, 47, 2347−2355.
(32) Batema, G. D.; van de Westelaken, K. T. L.; Guerra, J.; Lutz, M.;
Spek, A. L.; van Walree, C. A.; de Mello Donega, C.; Meijerink, A.; van
H
DOI: 10.1021/acs.inorgchem.5b01022
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Klink, G. P. M.; van Koten, G. Eur. J. Inorg. Chem. 2007, 2007, 1422−
1435.
(33) Motoyama, Y.; Mikami, Y.; Kawakami, H.; Aoki, K.; Nishiyama,
(65) Mroz, W.; Botta, C.; Giovanella, U.; Rossi, E.; Colombo, A.;
Dragonetti, C.; Roberto, D.; Ugo, R.; Valore, A.; Williams, J. A. G. J.
Mater. Chem. 2011, 21, 8653−8661.
(66) Komorsky-Lovric, S.; Mirceski, V.; Scholz, F. Microchim. Acta
1999, 132, 67−77.
H. Organometallics 1999, 18, 3584−3588.
(34) Fossey, J. S.; Richards, C. J. Organometallics 2004, 23, 367−373.
(35) Ito, J.-i.; Ujiie, S.; Nishiyama, H. Organometallics 2009, 28, 630−
638.
(67) Michalec, J. F.; Bejune, S. A.; Cuttell, D. G.; Summerton, G. C.;
Gertenbach, J. A.; Field, J. S.; Haines, R. J.; McMillin, D. R. Inorg.
Chem. 2001, 40, 2193−2200.
(36) Chuchuryukin, A. V.; Huang, R.; Lutz, M.; Chadwick, J. C.;
Spek, A. L.; van Koten, G. Organometallics 2011, 30, 2819−2830.
(37) Stol, M.; Snelders, D. J. M.; de Pater, P. J. J. M.; van Klink, K. G.
P. M.; Kooijman, H.; Spek, A. L.; van Koten, G. Organometallics 2005,
24, 743−749.
(38) Blackburn, O. A.; Coe, B. J.; Helliwell, M.; Raftery, J.
Organometallics 2012, 31, 5307−5320.
(39) Williams, J. A. G.; Beeby, A.; Davies, E. S.; Weinstein, J. A.;
Wilson, C. Inorg. Chem. 2003, 42, 8609−8611.
(40) Farley, S. J.; Rochester, D. L.; Thompson, A. L.; Howard, J. A.
K.; Williams, J. A. G. Inorg. Chem. 2005, 44, 9690−9703.
(41) Wang, Z.; Turner, E.; Mahoney, V.; Madakuni, S.; Groy, T.; Li,
J. Inorg. Chem. 2010, 49, 11276−11286.
(42) Sotoyama, W.; Satoh, T.; Sato, H.; Matsuura, A.; Sawatari, N. J.
Phys. Chem. A 2005, 109, 9760−9766.
(43) Chen, Y.; Lu, W.; Che, C.-M. Organometallics 2013, 32, 350−
353.
(68) Ball, P. J.; Shtoyko, T. R.; Krause Bauer, J. A.; Oldham, W. J.;
Connick, W. B. Inorg. Chem. 2004, 43, 622−632.
(69) Halverson, F.; Hirt, A. R. C. J. Chem. Phys. 1951, 19, 711−718.
(70) Krumholz, P. J. Am. Chem. Soc. 1951, 73, 3487−3492.
(71) Favini, G. Gazz. Chim. Ital. 1963, 93, 635−648.
(72) Tastan, S.; Krause, J. A.; Connick, W. B. Inorg. Chim. Acta 2006,
359, 1889−1898.
(73) Ryu, C. K.; Kyle, K. R.; Ford, P. C. Inorg. Chem. 1991, 30,
3982−3986.
(74) Giordano, P. J.; Wrighton, M. S. J. Am. Chem. Soc. 1979, 101,
2888−2897.
(75) Yip, H. K.; Che, C. M.; Peng, S. M. J. Chem. Soc., Dalton Trans.
1993, 179−187.
(76) Da, T. T.; Vu, D. B.; Dinh, N. H. J. Coord. Chem. 2004, 57,
485−496.
(77) Turnbull, N. H. J. Chem. Soc. 1945, 441−444.
(78) Fleeman, W. L.; Connick, W. B. Comments Inorg. Chem. 2002,
23, 205−230.
(44) Elguero, J.; Gonzalez, E.; Imbach, J. L.; Jacquier, R. Bull. Soc.
Chim. Fr. 1969, 11, 4075−4077.
(79) Connick, W. B.; Miskowski, V. M.; Houlding, V. H.; Gray, H. B.
Inorg. Chem. 2000, 39, 2585−2592.
(45) Frenna, V.; Vivona, N.; Consiglio, G.; Spinelli, D. J. Chem. Soc.,
Perkin Trans. 2 1985, 1865−1868.
(80) Connick, W. B.; Geiger, D.; Eisenberg, R. Inorg. Chem. 1999, 38,
3264−3265.
(46) Kissinger, P. T.; Heineman, W. R. J. Chem. Educ. 1983, 60, 702−
706.
(81) Wilson, M. H.; Ledwaba, L. P.; Field, J. S.; McMillin, D. R.
Dalton Trans. 2005, 2754−2759.
(47) Harris, T. D.; Neuschwander, B.; Boekelheide, V. J. Org. Chem.
1978, 43, 727−730.
(48) Button, K. M.; Gossage, R. A.; Phillips, R. K. R. Synth. Commun.
2002, 32, 363−368.
(49) Fossey, J. S.; Jones, G.; Motevalli, M.; Nguyen, H. V.; Richards,
C. J.; Stark, M. A.; Taylor, H. V. Tetrahedron: Asymmetry 2004, 15,
2067−2073.
(50) Han, Y.; Huynh, H. V.; Tan, G. K. Organometallics 2007, 26,
6447−6452.
(51) Stang, P. J.; Cao, D. H.; Saito, S.; Arif, A. M. J. Am. Chem. Soc.
1995, 117, 6273−6283.
(82) The excitation spectra for 2−8 are consistent with the
absorption spectra and support the conclusion that the emissions
originate from different chromophores rather than, for example, a
shared impurity.
(83) Bailey, J. A.; Hill, M. G.; Marsh, R. E.; Miskowski, V. M.;
Schaefer, W. P.; Gray, H. B. Inorg. Chem. 1995, 34, 4591−4599.
(84) Shi, L. L.; Li, T.; Zhao, S. S.; Li, H.; Su, Z. Theor. Chem. Acc.
2009, 124, 29−36.
(85) Morawski, O.; Prochorow, J. Chem. Phys. Lett. 1995, 242, 253−
258.
(86) Miskowski, V. M.; Houlding, V. H.; Che, C. M.; Wang, Y. Inorg.
Chem. 1993, 32, 2518−2524.
(87) Aldridge, T. K.; Stacy, E. M.; McMillin, D. R. Inorg. Chem. 1994,
33, 722−727.
(52) Komeda, S.; Kalayda, G. V.; Lutz, M.; Spek, A. L.; Yamanaka, Y.;
Sato, T.; Chikuma, M.; Reedijk, J. J. Med. Chem. 2003, 46, 1210−1219.
(53) Willermann, M.; Mulcahy, C.; Sigel, R. K. O.; Cerda, M. M.;
Freisinger, E.; Sanz Miguel, P. J.; Roitzsch, M.; Lippert, B. Inorg. Chem.
2006, 45, 2093−2099.
(54) Janzen, D. E.; Patel, K. N.; VanDerveer, D. G.; Grant, G. J.
Chem. Commun. 2006, 3540−3542.
(55) Sawada, T.; Yoshizawa, M.; Sato, S.; Fujita, M. Nat. Chem. 2009,
1, 53−56.
(56) Hirahara, E.; Takaishi, S.; Yamashita, M. Chem. - Asian J. 2009,
4, 1442−1450.
(57) Rao, Y. L.; Wang, S. N. Inorg. Chem. 2009, 48, 7698−7713.
(58) Sun, S. S.; Anspach, J. A.; Lees, A. J.; Zavalij, P. Y.
Organometallics 2002, 21, 685−693.
(59) Albrecht, M.; Lutz, M.; Schreurs, A. M. M.; Lutz, E. T. H.; Spek,
A. L.; van Koten, G. J. Chem. Soc. Dalton.Trans. 2000, 3797−3804.
(60) Chuchuryukin, A. V.; Chase, P. A.; Mills, A. M.; Lutz, M.; Spek,
A. L.; van Klink, G. P. M.; van Koten, G. Inorg. Chem. 2006, 45, 2045−
2054.
(61) Hill, M. G.; Bailey, J. A.; Miskowski, V. M.; Gray, H. B. Inorg.
Chem. 1996, 35, 4585−4590.
(62) Tsai, C. N.; Allard, M. M.; Lord, R. L.; Luo, D. W.; Chen, Y. J.;
Schlegel, H. B.; Endicott, J. F. Inorg. Chem. 2011, 50, 11965−11977.
(63) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.;
von Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85−277.
(64) Rossi, E.; Murphy, L.; Brothwood, P. L.; Colombo, A.;
Dragonetti, C.; Roberto, D.; Ugo, R.; Cocchi, M.; Williams, J. A. G. J.
Mater. Chem. 2011, 21, 15501−15510.
I
DOI: 10.1021/acs.inorgchem.5b01022
Inorg. Chem. XXXX, XXX, XXX−XXX