Homoleptic Tris-Diphosphine Re(I) and Re(II) Complexes and Re(II) Photophysics and Photochemistry

Article abstract of DOI:10.1021/acs.inorgchem.5b01395

The ligand-to-metal charge transfer state (LMCT) of [(dmpe)₃Re]²⁺ (dmpe = 1,2-bis(dimethylphosphino)ethane) has been demonstrated to be a potent oxidant (E⁰(Re²⁺/Re⁺) = 2.61 V vs standard calomel electrode). This complex has been traditionally prepared by nontrivial routes in low yields, and very little has been achieved in optimizing the ground state and emission energy properties of the general class of complexes [(PP)₃Re]²⁺ (PP = chelating diphosphine) through phosphine modification. Improved syntheses for Re(I) tris-homoleptic diphosphine complexes [(PP)₃Re]⁺ (PP = 1,2-bis(dimethylphosphino)ethane (dmpe), 1,2-bis(diethylphosphino)ethane (depe), bis(dimethylphosphino)methane (dmpm), bis(diphenylphosphino)methane (dppm), Me₂PCH₂PPh₂, 1,3-bis(dimethylphosphino)propane (dmpp), or 1,2-bis(dimethyl-phosphino)benzene (dmpb)) were achieved by single-pot reactions exploiting the reducing potential of the phosphines when reacted with Re^V oxo-complexes in 1,2-dichlorobenzene at 160-180 °C. Single-electron chemical oxidation of [(PP)₃Re]⁺ yields luminescent Re^II analogues; appropriate use of Ph₃C⁺, Cp₂Fe⁺, or (4-BrC₆H₄)₃N⁺ B(C₆F₅)₄^- salts produced [(PP)₃Re]²⁺ complexes in good yields. Crystallographic trends for the Re⁺/Re²⁺ pairs show significantly lengthened Re²⁺-P bonds for [(PP)₃Re]²⁺ relative to the corresponding [(PP)₃Re]⁺ system. The redox and luminescence behavior of the complexes indicates the luminescence is from a ligand P(σ)-to-metal (Re(dπ)) charge transfer (²LMCT) state for all the complexes. Structured luminescence at 77 K is postulated to originate from relaxation of the ²LMCT state into two spin-orbit coupled states: the ground state and a state ～3000 cm^-1 above the ground state. The excited-state reduction potential (Re(II/I)) for [(depe)₃Re]²⁺ was determined from the free energy dependence of luminescence quenching rate constants. Yields for formation of charge separated ions were determined for three of the complexes with a variety of electron donors. Despite favorable electrostatics, no charge separated ions were observed for radical ion pairs for which the energy of back electron transfer exceeded 1.1 V.

Full text of DOI:10.1021/acs.inorgchem.5b01395

Article
pubs.acs.org/IC
Homoleptic Tris-Diphosphine Re(I) and Re(II) Complexes and Re(II)
Photophysics and Photochemistry
Jeramie J. Adams, Navamoney Arulsamy, B. Patrick Sullivan, Dean M. Roddick,* Amelia Neuberger,†
and Russell H. Schmehl*,†
Contribution from the Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071, United States
†Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, United States
S
* Supporting Information
ABSTRACT: The ligand-to-metal charge transfer state (LMCT) of [(dmpe)3Re]2+
(dmpe = 1,2-bis(dimethylphosphino)ethane) has been demonstrated to be a potent
oxidant (E0(Re2+*/Re+) = 2.61 V vs standard calomel electrode). This complex has been
traditionally prepared by nontrivial routes in low yields, and very little has been achieved in
optimizing the ground state and emission energy properties of the general class of
complexes [(PP)3Re]2+ (PP = chelating diphosphine) through phosphine modification.
Improved syntheses for Re(I) tris-homoleptic diphosphine complexes [(PP)3Re]+ (PP =
1,2-bis(dimethylphosphino)ethane (dmpe), 1,2-bis(diethylphosphino)ethane (depe), bis-
(dimethylphosphino)methane (dmpm), bis(diphenylphosphino)methane (dppm),
Me2PCH2PPh2, 1,3-bis(dimethylphosphino)propane (dmpp), or 1,2-bis(dimethyl-
phosphino)benzene (dmpb)) were achieved by single-pot reactions exploiting the
reducing potential of the phosphines when reacted with ReV oxo-complexes in 1,2-dichlorobenzene at 160−180 °C. Single-
electron chemical oxidation of [(PP)3Re]+ yields luminescent ReII analogues; appropriate use of Ph3C+, Cp2Fe+, or (4-
−
BrC6H4)3N+ B(C6F5)4 salts produced [(PP)3Re]2+ complexes in good yields. Crystallographic trends for the Re+/Re2+ pairs
show significantly lengthened Re2+−P bonds for [(PP)3Re]2+ relative to the corresponding [(PP)3Re]+ system. The redox and
luminescence behavior of the complexes indicates the luminescence is from a ligand P(σ)-to-metal (Re(dπ)) charge transfer
(2LMCT) state for all the complexes. Structured luminescence at 77 K is postulated to originate from relaxation of the 2LMCT
state into two spin−orbit coupled states: the ground state and a state ∼3000 cm−1 above the ground state. The excited-state
reduction potential (Re(II*/I)) for [(depe)3Re]2+ was determined from the free energy dependence of luminescence quenching
rate constants. Yields for formation of charge separated ions were determined for three of the complexes with a variety of
electron donors. Despite favorable electrostatics, no charge separated ions were observed for radical ion pairs for which the
energy of back electron transfer exceeded 1.1 V.
[(dmpe)3Re]2+ and its congener [(dmpe)3Tc]2+ exhibit
INTRODUCTION
■
unique luminescence properties in solution, with higher
In comparison to the wide variety and scope of d6 Re(I)
quantum efficiencies (Φem) than [(bpy)3Ru]2+.26,27 In addition,
coordination compounds, relatively few stable low-spin d5 ReII
this class of complexes has been highlighted as potentially
monomeric complexes are known.1−11 Heteroleptic diimine
useful since their excited states are strongly oxidizing; they have
and diphosphine carbonyls [(diimine)(CO)3ReX]+,12,13 trans-
been labeled as highly oxidizing excited states (HOES).28,29 In
[(PP)2(CO)2Re]2+ and trans-[(dppe)2(CO)ReBr]+,14,15 have
the case of [(dmpe)3Re]2+, excited-state quenching by a series
been generated electrochemically and have generally limited
of substituted aromatic hydrocarbons has shown that the
stabilities (PP = 1,2-bis(dimethylphosphino)ethane (dmpe),
photoexcited complex is an extremely strong oxidant
(E0(Re2+*/Re+) = +2.58 V vs standard calomel electrode
1, 2-bis(diethylphosphino)ethane (depe), bis-
( d i m e t h y l p h o s p h i n o ) m e t h a n e ( d m p m ) , b i s -
(SCE)).28,29 [(dmpe)3Re]2+ is unique in having a doublet−
(diphenylphosphino)methane (dppm), Me2PCH2PPh2, 1,3-
bis(dimethylphosphino)propane (dmpp), or 1,2-bis-
(dimethylphosphino)benzene (dmpb); dppe = 1,2-bis-
(diphenylphosphino)ethane). More recently, [Re(CO)2Br4]2−
has been reported as a useful precursor for a variety of
[Re(CO)2Br2(L)2] products.16−19 Very few simple homoleptic
doublet absorption transition that is essentially ligand-to-metal
charge transfer (LMCT) in nature, with the excited-state
electron hole essentially residing in the phosphorus σ-bonding
lone-pair molecular orbital set. In contrast, the majority of
strong photooxidant complexes reported to date are based on
metal-to-ligand charge transfer (MLCT) transitions with
bipyridine-type ligands, such as [(TAP)3Ru]2+ (TAP =
ReII complexes are known: [(bpy)3Re]2+ (bpy = 2,2′-
2 2
bipyridine),2 0 , 2 1 [(2,2′-azobipyridine)3 Re]2 +
,
[(dmpe)3Re]2+,23,24 and most recently the mixed diphosphine
Received: June 23, 2015
chelate systems [(depe)3−x(dmpe)xRe]2+ (x = 0−3).25
© XXXX American Chemical Society
A
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Scheme 1
1,4,5,8-tetraazaphenanthrene),30,31 [(bpy)Re(CO)4]+,32
[(bpz)3Ru]2+,33,34 (bpz = 2,2′-bipyrazine), and [Pt(dppe)(5,6-
Me2phen)]2+.35 Some of these complexes have found
applications in selective DNA cleavage by single-electron
oxidation30 and as “phototriggers” to generate radicals to
study their initiation and propagation in biology.36,37 LMCT
systems are less common and are typically metal oxides, such as
using an additional 5 equiv of dmpe in a methanolic KOH
solution at 155 °C for 12 h (70% yield).24 A subsequent
improvement was reported using the Re(V) precursors
[(py)4ReO2]+ or [(PPh3)2ReCl2(O)OEt] with 4 equiv of
dmpe in the presence of thiophenol as a catalyst to give
[(dmpe)3Re]+ in ∼50% yield.47 Most recently [(dmpe)3Re]+
has been prepared in moderate yield from [(benzil)
(PPh3)2ReCl3] using excess dmpe (11 equiv) as both reductant
and trapping ligand and excess TlPF6 as a chloride acceptor.25
Extending this procedure to the ethyl-substituted chelate depe
with the addition of zinc amalgam as a reducing agent afforded
[(depe)3Re]+ in only 4.7% yield. The mixed phosphine chelate
complexes [(dmpe)2(depe)Re]+ and [(depe)2(dmpe)Re]2+
were prepared from the reactions of [(dmpe)2ReCl2]+ and
[(depe)2ReCl2]+ with excess depe and dmpe, respectively. A
complete reduction of [(PPh3)2ReI(O)2] with excess dppm in
refluxing methanol gives [(dppm)3Re]+ in 30% yield.48
38,39
2+
the uranyl ion UO2
,
and molybdenum or tungsten
polyoxometalate (POM) complexes;40,41 these systems typi-
cally involve higher energy excitation thresholds (≤420 nm)
and have been demonstrated to oxidize a variety of organic
substrates including common environmental pollutants.42,43
Several organic-based HOES systems are known, recent
examples being pyrylogens44 and chloride oxidation by guanine
in DNA.45
We have been interested in expanding this unique class of
[(PP)3Re]2+ photooxidants in an effort to systematically
optimize ESPox and photophysical properties. Besides
[(dmpe)3Re]2+, the only modest extension of this class has
been to the series [(dmpe)3−x(depe)xRe]2+.25 Diphosphines
have good flexibility in terms of pendant group and phosphine-
linker modifications and have a considerable potential for
tuning ground state and emission properties. Here we report
the extension of [(PP)3Re]+ systems to a broader range of tris-
chelate compounds and their associated ReII analogues,
[(PP)3Re]2+. An assessment of HOES potentials based on
ground-state and luminescence studies provide invaluable
insights on how to further optimize and more fully develop
this distinctive class of compounds.
Building upon these prior syntheses, we successfully
converted Re(V) oxo complexes [(PPh3)2ReI(O)2],
[(PPh3)2ReI2(O)OEt], or [(py)4Re(O)2]+OTf−, directly to
[(PP)3Re]+ products in moderate to excellent yields (49−93%)
in a one-pot reaction using 5−6 equiv of the appropriate
diphosphine (dmpe, dmpm, depe, Me2PCH2PPh2, dmpb (o-
phenylenebis(dimethylphosphine)) in 1,2-dichlorobenzene at
170−180 °C. The propyl-bridged dimethylphosphine chelate
dmpp was found to react only with [(py)4Re(O)2]+ under
similar conditions to give the desired product [(dmpp)3Re]+.
An improved synthesis of previously reported48 [(dppm)3Re]+
from [(PPh3)2ReI(O)2] was achieved by extending the reflux
time in methanol from 3 to 24 h (Scheme 1). [(PP)3Re]+/2+
complexes often suffer from poor solubility, which hampers full
characterization. Accordingly, all initially formed [(PP)3Re]+
complexes were treated with K[B(C6F5)4] in CH2Cl2 to give
the highly soluble corresponding borate anion products,
[(PP)3Re]B(C6F5)4.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of [(PP)3Re]+ Com-
plexes. Broader access to [(PP)3Re]+ complexes requires a
more generally applicable synthetic approach. The initial
synthesis of [(dmpe)3Re]+ involved the reduction of [ReO4]−
to Re(III) with 10 equiv of dmpe and HCl under pressure at
155 °C to produce [(dmpe)2ReCl2]+ in good yield.46 Further
reduction of [(dmpe)2ReCl2]+ to [(dmpe)3Re]+ was performed
[(PP)3Re]+ complexes were characterized by NMR, X-ray
diffraction, and elemental analysis. 31P NMR resonances for all
the complexes, except for that of Me2PCH2PPh2, appear as
B
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Scheme 2
Table 1. Selected Metrical Parameters for Structurally-Characterized (PP)3Ren+ Complex Pairs
P−Re−P (deg) ave,
intrachelate
P−Re−P (deg) ave,
interchelate
a
complex
[(dmpp)3Re]+ (5)
Re−P, ave (Å)
Δ (Å)
2.420
88.3
[(dmpp)3Re]2+ (12)
[(dmpe)3Re]+ (1)
2.381
82.4
80.8
80.9
80.2
68.9
68.5
68.5
68.7
82.2
81.9
92.7
0.067
0.054
0.046
[(dmpe)3Re]2+ (8)
2.448
93.3
[(depe)3Re]+ (2)
2.405
93.3
[(depe)3Re]2+ (9)
2.459
93.6
[(dmpm)3Re]+ (3)
2.371
2.417
98.1
[(dmpm)3Re]2+ (10)
fac-[(Me2PCH2PPh2)3Re]+ (4)
fac-[(Me2PCH2PPh2)3Re]2+ (11)
[(dmpb)3Re]+ (7)
98.2
2.376 (PMe2) 2.419 (PPh2)
2.399 (PMe2) 2.465 (PPh2)
2.372
98.1
0.023 (PMe2) 0.046 (PPh2)
0.053
b
105.7, 93.2
92.8
[(dmpb)3Re]2+ (14)
2.425
92.9
a
b
Δ is defined as Δ = [(Re2+−P)ave − (Re+−P)ave]. The three Re−PMe2 and the three Re−PPh2 bonds are symmetry-related by a crystallographic
C3 axis and thus are not averaged.
broad singlets: δ −50.3 (ν1/2 = 6 Hz) (dppm), 16.8 (ν1/2 = 225
Hz) (depe), 2.1 (ν1/2 = 790 Hz) (dmpe), 1.5 (ν1/2 = 345 Hz)
(dmpb), −53.8 (ν1/2 = 165 Hz) (dmpp), and −66.2 (ν1/2 = 50
Hz) (dmpm). The significant broadening of 31P NMR signals is
tentatively attributed to quadrupolar relaxation by 185Re and
187Re in an octahedral coordination environment.49 For the
unsymmetrical complex fac-[(Me2PCH2PPh2)3Re]+, 31P NMR
spectra showed two doublets centered at −45.9 and −63.3 ppm
origin of this discrepancy is not clear, but the spectroscopic data
all support a level of purity in excess of 95%.
[(PP)3Re]2+ Syntheses. Single-electron chemical oxidations
of [(dmpe)3Re]+ or [(depe)3Re]+ have been performed using
H2O2 as the oxidant to give the corresponding Re(II) products
in low yields.50,23 Kirchhoff has more recently shown that
[(dmpe) (depe)2Re]2+ can be prepared in situ by the reduction
of the Re(III) precursor [(depe)2ReCl2]+ in the presence of
excess dmpe with Zn/Hg amalgam, but this approach has not
been extended to other systems.25
2
2
with JPP(trans) = 94 Hz, which show unresolved JPP(cis)
coupling. Proton methyl resonances for [(PP)3Re]+, where PP
= dmpe, dmpm, dmpp, and dmpb, appear as two distinct broad
singlets due to reduced symmetry imposed by the three-
chelating ligands. Previous 1H NMR spectra for (dmpe)3Re+ do
not report CH2CH2 ligand backbone resonances,47 probably
due to significant overlap in the lower resolution spectra (250
In light of the general reversibility of [(PP)3Re]2+/+ redox
couples (vide infra), simple stoichiometric one-electron
chemical oxidants were used to prepare the corresponding
[(PP)3Re]2+ complexes in good yield (66−95%, Scheme 2).
For the more readily oxidized complexes [(PP)3Re]+ (PP =
dmpe, depe, dmpm, Me2PCH2PPh2, or dmpp), the oxidants
1
MHz) with the more intense methyl resonances. H NMR
Ph3C+B(C6F5)4 (−0.11 mV vs Cp2Fe+, CH3CN) or [Cp2Fe]-
−
spectra for [(depe)3Re]+ show one broad unresolved methyl
resonance at δ 1.15 with the pendant diastereotopic ethyl CH2
groups appearing as two separate resonances at δ 1.61 and 1.89.
F o r t h e a s y m m e t r i c d i p h o s p h i n e c o m p l e x
[(Me2PCH2PPh2)3Re]+ (4) a mixture of isomers may be
anticipated with mutually facial or meridional sets of Me2P and
Ph2P groups. Under the conditions employed in the synthesis
of 4, only the facial isomer was observed and the stereo-
chemistry was confirmed by X-ray diffraction (vide infra).
Elemental analyses of the [(PP)3Re]2+ complexes were
satisfactory, although a few exhibited deviations of slightly
more than 0.5% in analysis of one element in the complex. The
51
B(C6F5)4 readily gave the corresponding [(PP)3Re]2+
complexes. The more powerful oxidant (4-BrC6H4)3N+B-
(C6F5)4 (+0.67 V vs Cp2Fe+, CH3CN) was required for the
−
more electron-poor complexes [(dmpb)3Re]+ and
[(dppm)3Re]+. Oxidation of more electron-rich [(PP)3Re]+
systems with (4-BrC6H4)3N+ led to overoxidation and the
formation of uncharacterized orange solids, which are
presumed to be ReIII products.
Crystallographic Studies. As a general class of coordina-
tion compounds, structurally characterized six-coordinate
phosphine complexes [(R3P)6M] have been reported for
C
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Mn,52 Re,25,53,54 Fe,55−57 Rh,58 Co,59 Cr,60−64 Mo,60,63,65−67
W,56,60,63,68,69 V,63,70 Nb and Ta,63 Zr,71 and Ru.72 Structural
Table 2. UV−vis Absorption Maxima and Absorptivities for
[(PP)3Re]2+ Complexes in Deoxygenated 0.23 mM CH3CN
data for tris phosphine chelate Re complexes have been
λmax
,
ε,
P−Re−P chelate
54
reported for [(dppm)3Re]+
and for the redox pairs
[(PP)3Re]2+
dmpp, 12
nm (cm−1
)
M−1 cm−1
angle
[(dmpe)2(depe)Re]2+ and [(dmpe)2(depe)Re]+.25 Direct iso-
structural comparisons of metals in different oxidation states are
not widely available, and to our knowledge this isostructural
Re+/2+ pair is the only known example for a homoleptic
polyphosphine complex.
505 (19 800)
528 (18 900)
531 (18 800)
541 (18 500)
539 (18 550)
551 (18 150)
589 (17 000)
1850
1530
1880
550
dmpe, 8
80.8
80.2
81.9
68.5
68.7
depe, 9
dmpb, 14
dmpm, 10
Me2PCH2PPh2, 11
dppm, 13
700
Interestingly, Re−P bond lengths for [(dmpe)2(depe)Re]+/2+
indicate a lengthening of the bonds of ∼0.05 Å following
oxidation. The result is in agreement with earlier
[(dmpe)3Re]2+/+ EXAFS data.23 Structural data for all
obtainable [(PP)3Re]2+/+ pairs in the present study (Table 1)
also show significantly longer Re2+−P bonds than their Re+−P
counterparts (average Re−P bond length difference defined as
Δ): Δ(dmpe) ≈ 0.07 Å, depe ≈ 0.05 Å, dmpm ≈ 0.05 Å, dmpb
≈ 0.05 Å, and for Me2PCH2PPh2 the two distances are ∼0.02
for Me2P−Re and ∼0.05 Å for Ph2P−Re. No standard ionic
radii for Re+ and Re2+ have been established; nevertheless, the
simplistic expectation is that Re+−P bond lengths would be
longer, not shorter, than the corresponding Re2+−P bond
lengths. We performed density functional theory (DFT)
calculations for the [(dmpe)3Re]2+/+ pair to further explore
this phenomenon. Optimized structures confirm the observed
bond length trend: the Re−P average bond distances for
[(dmpe)3Re]+ (2.409, 2.381 Å observed) and [(dmpe)3Re]2+
(2.475, 2.448 Å observed) are systematically lengthened by
0.027 and 0.028 Å but have a virtually identical calculated
Δ(dmpe) of 0.066 Å (0.067 Å observed). Spin-corrected Mayer
bond order indexes73 reflect a lower effective bond order for
Re2+ relative to Re+ (0.829 vs 0.896). A natural bond orbital
analysis also reveals a significant increase in phosphorus lone
pair p-character for Re2+−P (sp1.56) relative to Re+−P (sp1.37),
1400
1300
Figure 1. Absorption spectrum of [Re(depe)3]2+ in acetonitrile at
room temperature.
which may at least partially explain the bond lengthening.74
A
increase in the absorptivity. For [(dmpe)3Re]2+ this band has
been assigned as a P(σ) → Re(dπ) LMCT transition.27 The
behavior, however, is also appropriate for a Re localized d−d
transition. For the CH2−linked phosphine derivatives, the
phenyl containing phosphines have characteristic high energy
phenyl localized π→π* transitions in the UV; the visible
transitions exhibit a shift to lower energies with increased
phenyl substitution (Figure S2). The more electron-poor 1,2-
aryl-bridged dmpb complex 14 shows a slight shift of the lowest
energy transition to lower energy relative to dmpe and depe
qualitative conclusion based on “Bent’s Rule” may be stated:
The higher effective electronegativity of Re2+ results in a shift in
p-character to the phosphorus lone pair, resulting in greater
atomic orbital radial extension and a longer associated Re−P
bond.
The average Re−P bond length for [(PP)3Re]+ complexes
́
modestly increases in the following order: dmpb (2.37 Å) ≈
́
́
dmpm (2.37 Å) ≈ dmpe (2.38 Å) ≈ Me2P−Re of
́
́
Me2PCH2PPh2 (2.38 Å) < depe (2.41 Å) ≈ Ph2P−Re of
́
Me2PCH2PPh2 (2.42 Å) and qualitatively follows the increased
derivatives, which, despite a significant difference in E0(Re2+/+
)
steric demand of the PR2 groups. As expected, the largest
distortions from a metal octahedral environment are in the
complexes containing one CH2 phosphine linker. For the Re2+
analogues, except for the Me2PCH2PPh2 complexes, the bite
angles become slightly more acute due to the lengthened Re−P
bond lengths.
Electronic Absorption Spectra. While UV−vis properties
for [(dmpe)3−x(depe)xRe]2+ complexes have been studied by
absorption spectroelectrochemistry,25−27 readily isolated
[(PP)3Re]2+ systems in this study allow for straightforward
UV−vis characterization for a wide range of phosphine chelate
derivatives (Table 2). Significant changes in the [(PP)3Re]2+
absorption spectra are observed depending on the nature of the
phosphine substituents, the chelate linking groups, and the
chelate bite angle. A representative absorption spectrum is
shown in Figure 1. Comparing alkyl phosphines with methyl
substituents, an increase in the bite angle dmpm < dmpe <
dmpp results in a blue shift in the absorption maximum and an
potentials (vide infra), have very similar energies (Figure S3).
Luminescence Behavior. In earlier work, room-temper-
ature luminescence was observed for [(dmpe)3Re]2+. The
emission emanated from the P(σ) → Re(dπ) LMCT doublet
excited state of the complex.27,29 Some of the complexes here
also have ambient temperature emission with maxima and
Stokes shifts similar to the LMCT luminescence of the dmpe
complex. Figure 2 illustrates two examples of room-temperature
complex absorption and luminescence in acetonitrile. Spectra of
all the complexes in solution at room temperature and in
propionitrile/butyronitrile glasses at 77 K are shown in
Supporting Information (Figures S5−S9). Steady-state and
time-resolved luminescence maxima, lifetimes, and quantum
yields, along with the E0 energy obtained from fits of 77 K
emission, are summarized in Table 3. The luminescence in each
case arises from excitation into the LMCT absorption and
higher energy bands. The complexes having a methane bridge
D
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Table 4. Summary of Results for 77 K Emission Spectra
compound
Emax, cm−1 (nm) ΔE (cm−1
)
rel inten
[Re(dmpp)3]2+, 12
[Re(dmpe)3]2+, 8
17 125 (583)
16 860 (592)
16 890 (592)
17 200 (570)
16 240 (614)
16 600 (602)
15 730 (634)
2725
3130
3160
3220
2970
2958
3310
0.44
0.14
0.14
0.75
0.14
0.30
0.15
[Re(depe)3]2+, 9
[Re(dmpm)3]2+, 10
[Re(dmpb)3]2+, 14
[Re(dppm)3]2+, 13
[Re(Ph2PCH2PMe2)3]2+, 11
Figure 2. Room-temperature absorption (solid line) and fluorescence
(dashed line) for acetonitrile solutions of [Re(dmpe)3]2+ (upper) and
[Re(dmpp)3]2+ (lower).
(PCH2P) exhibit weak or no emission in solution at room
temperature.
The room-temperature excited-state lifetimes for the
complexes that could be measured ranged between 6 and 25
ns. Since both the ground and excited states are doublets,
radiative decay rate constants could be obtained (Table 3)
without consideration of intersystem crossing efficiency, as is
often the case with Re(I), Ru(II), Os(II), and other
luminescent transition metal complexes. For the dmpp, dmpe,
and depe complexes radiative decay rate constants are on the
order of 1 × 106 s−1, more than 20 times greater than typical
MLCT radiative decay rate constants for the spin-forbidden
radiative relaxation. The dmpb complex, with very weak room-
temperature luminescence, appears to have a smaller radiative
decay rate constant, but this will not be firmly established until
an accurate luminescence quantum yield is determined.
Since room-temperature luminescence was not observed for
some complexes, and the luminescence was broad and
structureless for the others (except for the dmpb and dmpp
complexes), spectra were obtained in glasses at 77 K for most
of the complexes (Table 4). All of the 77 K spectra exhibit two
clearly resolved bands. The emission maxima are nearly
identical to those obtained in solution at room temperature,
which is unusual for charge transfer transitions.
Figure 3. 77 K fluorescence spectra of [Re(dmpe)3]2+ (upper) and
[Re(dmpp)3]2+ (lower) in 4:1 ethanol/methanol glasses.
complexes measured, the average medium frequency mode is
typically 1200−1400 cm−1, and the much larger frequencies
obtained here are compatible only with C−H stretching modes
of the molecules studied; it is unlikely that these modes
contribute to excited-state relaxation.
This large separation between the luminescent components
of the 77 K emission spectrum, approaching and sometimes
exceeding 3000 cm−1 (Table 4), may be rationalized by
considering spin−orbit coupling effects of the Re(II) center. In
earlier work on Re(II) tris(diphosphine) complexes, Sullivan
and co-workers noted that time-dependent density functional
calculations on [Re(dmpe)3]2+ revealed a low-energy excited
state ∼2400 cm−1 above the ground state.29 The state arises
because of splitting of the 2T ground state (assuming a pseudo-
octahedral geometry) into two states resulting from spin−orbit
coupling interactions (twofold and fourfold degenerate states).
The energy gap between the two observed maxima is greater
than 2700 cm−1 for all the complexes, and, as a result, a
Franck−Condon analysis to determine medium frequency
modes associated with excited-state decay is not really
appropriate. For nearly all Ru(II) diimine and Re(I) diimine
Table 3. Absorption and Room-Temperature Emission Properties for Selected (PP)3Re2+ Complexes in Deoxygenated CH3CN
and in 77 K CH3CH2CN/CH3CH2CH2CN Matrices
a
abs max
(cm−1
Em (cm−1
)
E0 (cm−1
77 K
)
stokes shift
(cm−1
Φem
τ (ns)
298 K
kr (s−1
)
knr (s−1
× 107
)
complex
)
298 K
)
298 K
× 106
[Re(dmpp)3]2+, 12
[Re(dmpe)3]2+, 8
19 800
18 900
18 800
18 500
18 550
18 150
17 000
17 240
16 800
16 750
16 750
17 500
17 150
16 900
16 890
16 290
17 200
15 760
16 620
2560
2100
2050
1750
1050
0.069
25
12
19
6
2.7
3.7
5.8
5.0
>2
0.066
5.5
[Re(depe)3]2+, 9
0.043
2.3
[Re(dmpb)3]2+, 14
[Re(dmpm)3]2+, 10
[Re(Ph2PCH2PMe2)3]2+, 11
[Re(dppm)3]2+, 13
<0.001
<0.001
<0.2
b
c
NE
2390
b
c
NE
380
a
b
c
Not corrected for detector response. NE: no emission observed. Stokes shift calculated using 77 K emission maximum.
E
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Given the free ion spin−orbit coupling of nearly 3000 cm−1 for
Re, the state splitting should result in an absorption transition
in the mid-infrared. In fact, the infrared spectra of two of the
chromophores, [(dmpe)3Re]2+ and [(dmpp)3Re]2+, both
exhibit a broad, weak absorption between 2500 and 2700
change of 38 mV per pendant substituent. For the ethyl-linked
phosphines depe (+0.19 V) and dmpe (+0.28 V), the relatively
modest electronic effect of replacing a methyl by an ethyl group
results in a decrease of E1/2 of 0.09 V, or 15 mV per Me/Et
change. E0(Re2+/+) for the phenylene-bridged complex
[(dmpb)3Re]+ (+0.46 V) corresponds to a 180 mV increase
in potential from [(dmpe)3Re]+ (30 mV/P−C(phenyl)), which
is comparable to the effect of replacing pendant Me groups with
Ph. For Me2P(CH2)nPMe2 chelates varying n = 1, 2, 3, the
ordering: dmpm (n = 1, + 0.13 V) < dmpp (n = 3, + 0.20 V) <
dmpe (n = 2, + 0.28 V) does not reflect a simple chelate bite
angle trend, and is likely due to a combination of inductive and
ring-strain effects. Cyclic voltammograms (CVs) for the Re2+/+
waves of representative complexes are shown in Figure 4 (CVs
of other complexes are provided in Supporting Information,
Figures S11 and S12).
2
cm−1, close to the energy splitting of the T state predicted
through DFT analysis (Figure S10). The lower energy
transition is not observed in the room-temperature emission
spectra except for the dmpp complex. This is in part due to the
larger bandwidth at room temperature that serves to obscure
the weaker lower energy luminescence.
For the complex series dmpm, Me2PCH2PPh2, dppm, the
absorption energy decreases with increasing degree of phenyl
substitution, while the emission energies do not vary system-
atically. It is also important to note that the dppm complex has
a very small experimental Stokes shift when comparing the
room-temperature absorption maximum and the 77 K
luminescence maximum. Since, for other complexes, relatively
minor changes are observed between room-temperature and 77
K luminescence maxima, the observation for the dppm complex
is likely a realistic estimate of the room-temperature Stokes
shift. For fluorescence, the Stokes shift is directly related to the
degree of excited-state distortion; thus, the implication is that
phenyl substitution in place of methyl groups leads to a
significantly smaller excited-state distortion.
Redox Behavior. [(dmpe)3Re]+, [(dmpe)2(depe)Re]+,
[(dmpe)(depe)2Re]+, and [(depe)3Re]+ have previously been
studied by cyclic voltammetry;26−29,75 the complexes all have
reversible single-electron E0(Re2+/+) waves and partially
reversible Re3+/2+ waves that are clearly accompanied by
some decomposition.25 Results for the reversible one-electron
potentials in CH3CN/TBAPF6 (TBAPF6 = tetrabutylammo-
nium hexafluorophosphate) for the series of complexes
[(PP)3Re]+ are given in Table 5. For the E0(Re2+/+) redox
couples ipc/ipa ratios were essentially unity, whereas for the
Re3+/2+ couples, the ratios were less than one and varied with
sweep rate.
Figure 4. Cyclic voltammetry for (PP)3Re+ complexes (PP = dmpm
(red), dmpe (magenta), depb (blue) in 1 mM CH3CN, scan rate 100
mV, 0.1 M TBAPF6, vs Ag/Ag+(MeCN).
E0(Re3+/2+) potentials are less reversible than the E0(Re2+/+
)
couples and are similar to those previously reported for
[(dmpe)3Re]+, [(dmpe)2(depe)Re]+, and [(dmpe) (de-
pe)2Re]+.25 The decrease in reversibility has been attributed
by Kirchhoff to phosphine-arm dissociation followed by
CH3CN coordination.26 The most electron-rich complex
[(depe)3Re]+ exhibits the greatest chemical reversibility and
an ipc/ipa ratio near unity (0.92) at a sweep rate of 100 mV/s.
The Re3+/2+ waves for complexes with more positive
E0(Re3+2/+) potentials were typically irreversible or outside
the potential window of the solvent. Chemical oxidation of
Table 5. E0(Re2+/+) and E0(Re3+/2+) Potentials for the Series
a
[(PP)3Re]+ in 1 mM CH3CN
b
E0(Re2+/+
)
E0(Re3+/2+
E1/2, V
)
E0(Re2+*/+), V
E1/2, V
(ΔP, mV)
PP
dmpp, 12
(ΔP, mV)
[(dmpm)3Re]+ with 2 equiv of (4-BrC6H4)3N+SbF6 in
−
0.20 (86)
0.28 (89)
0.19 (62)
0.46 (98)
0.13 (82)
0.36 (69)
0.57 (65)
2.32
2.38
2.21
2.48
2.06
2.31
2.63
CH3CN produced an orange solid, which was shown by X-
ray diffraction to be a seven-coordinate ReIII complex having a
coordinated solvent molecule. The crystallographic data of this
seven-coordinate complex, [(dmpm)3Re(MeCN)]3+(15), is
given in the Supporting Information.
dmpe, 8
1.18 (104)
1.09 (80)
depe, 9
dmpb, 14
dmpm, 10
Ph2PCH2PMe2, 11
dppm, 13
Absorption and Luminescence Spectral Trends. The
variation in absorption and luminescence energies in the series
dmpm, dmpe, and dmpp can be rationalized by the relative
degree of ligand field splitting expected for the complexes based
upon the bite angle of the ligands and the energies of the
phosphorus P(σ) orbitals. Assuming the absorption and
luminescence arises from an LMCT transition, a semi-
quantitative picture emerges if the relative E0(Re2+/+) potentials
are viewed as a measure of the Re(dπ) energies (relative
ionization energies) and the emission energy reflects the
relative energy of relaxing from the Re(dπ) to the P(σ) SOMO
formed upon creation of the excited state, (P(σ)1Re(dπ)6 →
P(σ)2Re(dπ)5), as illustrated in Figure 5. The idea that the
LMCT transitions are affected by a variety of subtle features is
a
Scan rate 100 mV, 0.01 M TBAPF6, Ag/Ag+(MeCN) reference
electrode. Also excited-state Re2+*/+ potential derived using E00 values
b
from Table 3. E1/2(Re2+*/+) = E00 + E1/2(Re2+/+).
The following overall order for the E0(Re2+/+) for
[(PP)3Re]+ was observed: PP = dmpm < depe < dmpp <
dmpe < Me2PCH2PPh2 < dmpb < dppm. For the CH2−
bridged phosphines, replacing methyl substituents with more
electron-withdrawing phenyl groups results in a consistent
increase in the E0(Re2+/+) potential. The successive +0.23 and
+0.21 V increases in E1/2 accompanying the replacing of a total
of six methyl pendant groups with phenyls corresponds to a
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Figure 5. Relative energies of the Re(dπ) and P(σ) levels for
complexes having methyl, ethyl, and propyl bridges between the
dimethylphosphine moieties.
Figure 6. Relative energies of the Re(dπ) and P(σ) levels for
[Re(R2PCH2PR2)3]2+ complexes having zero, three, and six
diphenylphosphino moieties. Absorption energies, using fixed Re(dπ)
levels, are represented by the dashed (blue) lines.
indicated by the fact that plots of the absorption maximum
versus E0(Re2+/+) vary considerably from linearity for the series
studied here (see Figure S13). The energies of the dπ levels
vary by 0.25 V between the dmpm and dmpe complexes and by
0.08 V between dmpe and dmpp. With this approach the
dmpm ligand, despite the obviously poorer overlap with Re
orbitals because of the small bite angle (∼68°; Table 1), is the
best electron donor of the group. Note that the Re−P bond
distance is shortest for dmpm. In going from dmpm to dmpp
the Re−P bond length increases and the intraligand P−Re−P
bite angle increases; these changes oppose one another in the
context of influencing the E0(Re2+/+) and may account for the
differences observed. Assuming the relative energies of the P(σ)
levels of the three ligands are directly related to the energy of
the Re(dπ) and the emission energy as shown in Figure 5, the
P(σ) levels vary by only 60 mV, suggesting the predominant
factor in determining this level is the relative ionization energy
of the coordinated P(σ) orbitals. Overall, the differences in
emission energies are small (∼700 cm−1), and any explanation
of the behavior for this series is likely to reflect subtle
differences in the orbital overlap that cannot easily be inferred
from bite angle and bond distance changes.
dmpm. Presumably the most easily ionized P center of the
ligand, P(Me)2, serves as the donor, and the neighboring
P(Ph)2 moiety has a small impact on the relative ionization
energy of the phosphorus of the P(Me)2 group. This serves to
provide an explanation for the observation that the tris-
(Ph)2PCH2P(Me2) complex exhibits luminescence at lower
energy than either the tris-dmpm or tris-dppm complexes.
Another peculiar aspect of the behavior of the phenyl-
phosphine-containing complexes is the observed Stokes shifts.
Since the ground and excited states are both doublets, no spin
change occurs between the absorbing and emitting states, and a
systematic trend in the Stokes shifts would be expected. In
considering the observed Stokes shifts (Table 3), drawn from
energies for solution absorption at room-temperature and 77 K
luminescence in nitrile solvent matrices, the value for
(Me)2PCH2P(Ph)2 is by far the largest, while that for the
dppm complex is very small (<400 cm−1). Examination of
absorption spectra (Figure S2) clearly shows that the
phenylphosphine-containing complexes have larger absorptiv-
ities (2×) than the dmpm complex and distinctly different
absorption bandshapes. If it is assumed that, for the
(Me)2PCH2P(Ph)2 complex, the lowest energy allowed
absorption transition is dominated by the P(Ph)2 levels
(P(Ph)2(σ) → Re(dπ)), but that luminescence is assigned as
Re(dπ) → P(Me)2(σ), the large Stokes shift observed can then
be rationalized. A semiquantitative depiction of this difference
in absorption and luminescence transition energies is shown in
Figure 6. One final note on the Stokes shifts is that, for the
dppm complex, the implication is that the excited-state
distortion is very small, while for the (Me)2PCH2P(Ph)2
complex, significant displacement exists between the ground-
and excited-state potential surfaces. Re−P bond distance
differences between the Re(II) and Re(I) complexes of
(Me)2PCH2P(Ph)2 indicate that the change observed for the
Re−P(Me)2 bond is smaller than that observed for the Re−
P(Ph)2 bond. While data are not available for the dppm
complex, the results for the (Me)2PCH2P(Ph)2 complex alone
suggest that, if Re−P displacement is an important contributor
to excited-state distortion, the distortion should be larger for
the Re−P(Ph)2 linkages.
The series of complexes having an increasing number of
diphenylphosphine moieties exhibit progressively more positive
E0(Re2+/+) potentials as the net σ donation from the
coordinated phosphine decreases with decreasing basicity
(increasing number of diphenylphosphine moieties) and the
degree of π-back-donation to the phosphines in the Re
coordination environment increases. A plot of the E0(Re2+/+
)
as a function of the number of diphenylphosphine substituents
is linear, but the emission energy does not exhibit this same
linearity. Employing the approach used for the dmpx (x = m, e,
p) above, the relative orbital energies are presented in Figure 6.
Here, the energies of the d(π) orbitals lower as a result of the
combined decreased ligand σ donation and increased Re-to-P
back bonding. The energies of the P(σ) orbitals reflect the
nature of the phosphine donor moieties. For the dppm
complex, the basicity is weaker and the ionization energy of the
phosphorus is more positive than the dmpm phosphorus, and
thus both the Re(dπ) and P(σ) levels decrease in energy with
increasing phosphine aryl substitution. The complex of
(Me)2PCH2P(Ph)2, however, exhibits a change in the Re(dπ)
levels, reflecting a net change in the electron density of the
Re(II) center, but the P(σ) level that results from inclusion of
the emission energy (as above) is nearly the same as that of
Excited-State Redox Potentials and Photoinduced
Electron-Transfer Reactions. One of the characteristics of
these complexes that makes them extremely interesting is the
potential of the 2LMCT states to act as very strong oxidants. An
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estimate of the excited-state potentials can be obtained from
the emission energies and the ground-state redox potentials.
The Re3+/2+ potentials for [Re(dmpe)3]2+ and the depe
complex are ∼1.1 V versus Ag+/Ag, and the E0 energies
obtained from fits of the 77 K emission spectra are 2.1 eV,
yielding an E0(Re3+/2+*) of ca. −1 V versus Ag+/Ag. However,
the E0(Re2+*/+), approximately equal to the sum of E0(Re2+/+
)
and E0, is in excess of 2.0 V for all the chromophores.28,29 The
E0 energies for the series of complexes of dmpm, depe, dmpe,
and dmpp are 1.93 < 2.10−2.10 < 2.13, respectively. Using
E0(Re2+/+) values from Table 5, the estimated E0(Re2+*/+)
values for these complexes are all greater than 2.0 V versus
Ag+/Ag and are included in Table 5. Thus, all these complexes
are extraordinary oxidants in their emissive excited states. The
phenyl-substituted derivatives, with more positive E0(Re2+/+
)
Figure 8. Transient spectra obtained 1 μs following pulsed laser
excitation (λex = 530 nm) of [Re(depe)3]2+ with (A) 14DMB and (B)
124TMB. Absorption spectrum of [Re(depe)3]2+ included below B.
potentials and comparable E0 energies, are even stronger
excited-state oxidants.
Earlier work on the dmpe complex illustrated that the excited
state was quenched by aromatic hydrocarbons with very
positive E0(Ar+/0) potentials, and Rehm−Weller analysis of
quenching rate constants provided a measure of the
E0(Re2+*/0) potential that did not differ significantly from the
value estimated from the emission energy and ground-state
redox potential.29
We continued this work by looking at the excited-state
quenching of [Re(depe)3]2+ by a series of amines, alkox-
ybenzenes, and simple aromatics. The dependence of the rate
constant on the redox potential of the quencher is shown in
Figure 7 below. While this is a Rehm−Weller-type plot, the
cation radical spectrum summed with the Re(I) complex
absorption and the ground-state absorption (Δελ = ε(MB)λ
+
ε(Re(I))λ − ε(Re(II))λ). The absence of ground-state bleaching of
the Re complex in the transient spectra reflects the fact that the
14DMB and 124TMB radical cations have much larger molar
absorptivities (ε > 10 000 M−1 cm−1) than the Re(II)
complexes, and thus, the bleaching of the [Re(LL)3]2+
absorption is in the noise. The back electron transfer yielded
the starting reactants and was diffusion-limited in all cases
examined.
Transient spectra could not be obtained for all of the
quenchers used, especially for quenchers having very positive
Q+/Q potentials. Despite the fact that the luminescence of the
chromophore used was nearly completely quenched in each
case studied, the expected radical ion spectra were not observed
for quenchers such as anisole, xylene, or toluene for which the
back reaction free energies are >1.5 V. Given the surprising
failure to observe radical ions in some cases, we decided to
determine charge separation yields for reactions of [Re(PP)3]2+
(PP = dmpe, depe, and dmpp) complexes with 14DMB and
124TMB. The results are summarized in Table 6, showing
Table 6. Summary of Results from Charge Separation Yield
Experiments in Acetonitrile Solution, Including the Free
Energy of the Back Electron Transfer Reaction
chromophore ΔGBET
% ηCS with 1,2,4-TMB
% ηCS with 1,4-DMB
a
(V)
(1.12 V)
(1.34 V)
[Re(dmpe)3]2+
91
3
3
2
53
3
Figure 7. Free energy dependence of luminescence quenching rate
constants for reaction of photoexcited [Re(depe)3]2+ with a series of
electron donors (tabulated quenching rate constants and quenchers in
Table S21).
ΔGBET (V)
0.84
68
1.06
6
[Re(depe)3]2+
ΔGBET (V)
4
1.0
16
1.22
0
[Re(dmpp)3]2+
ΔGBET (V)
0.92
1.14
paucity of data at the point where the quenching rate constant
decreases precipitously stopped us from fitting the data with
typical expressions; the falloff occurs at ∼2.1 V versus Ag+/Ag,
representing a best guess estimate of the excited-state potential.
This is in reasonably good agreement with that estimated from
luminescence and redox data (2.2 V).
a
Determined from the sum of E0(Q+/Q) − E0(Re2+/+); the energy
associated with separation of the ions from the geminate pair is not
considered, but should be small and invariant among all the pairs
studied here.
Transient absorption spectra of the radical ions produced in
the photoredox reactions for several of the quenchers were also
obtained. Representative spectra are shown in Figure 8 for the
quenching of [Re(depe)3]2+ with 1,4-dimethoxybenzene
(14DMB) and 1,2,4-trimethoxybenzene (124TMB). The
spectra represent the difference between the alkoxybenzene
charge separation efficiencies for 124TMB and 14DMB with
back reaction exergonicities ranging from 0.84 to 1.22 V.
Additional charge separation experiments were performed
utilizing the [Re(PP)3]2+ complexes with dimethylaniline
(E0(DMA+/DMA) = 0.76 V) and N-methylphenothiazine
(E0(MPT+/MPT) = 0.54 V) as quenchers. The charge
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separation yields were very high, in some cases on the order of
100% with these donors; back electron transfer exergonicities
ranged from 0.26 to 0.64 V. With quenchers having more
positive E0(Q+/Q) potentials (1,2-dimethoxybenzene (1.54 V)
or anisole (1.76 V)), the charge separation yield was zero with
all three of the chromophores studied; the exergonicities in this
group ranged from 1.26 to 1.64 V. This complete absence of
charge separated ions in cases where the free energy of back
electron transfer is nearly activationless has been the subject of
earlier work, and the postulate has been that, for such highly
exergonic sytems, back electron transfer can occur outside the
contact ion pair.76,77
phino groups. The emission energies for these complexes vary
in a way that suggests the nature of the half-occupied
phosphine orbital (alkyl or aryl) is the determining factor as
the total number of diphenylphosphino substituents on the
complex increases from zero to three to six; here, both the
Re(dπ) and P(σ) levels exhibit large changes as the basicity of
the coordinating phosphorus centers decreases.
Because the excited-state energies are ∼2 eV, the photo-
excited complexes are extraordinary oxidizing agents. The free
energy dependence of the quenching of [Re(depe)3]2+ with a
series of electron donors was evaluated, and the excited-state
E0(Re2+*/+) potential was found to be in reasonable agreement
with that estimated from the emission energy and ground-state
potential. Transient absorption spectra of the photoredox
products with arylamine and alkoxybenzene donors indicate
that the back reaction is complete and diffusion-limited.
Surprisingly, charge separation of geminate photoredox pairs
Looking at the results for the three complexes with 124TMB
and 14DMB, it is clear that, for each chromophore, as the free
energy for back electron transfer (ΔGBET) becomes more
exergonic the charge separation efficiency decreases. The
charge separation efficiency reflects the relative rates of the
radical ions escaping the solvent cage and the rate of back
electron transfer within the solvent cage. The rate of the back
electron transfer process should follow Marcus electron transfer
dynamics, since, in each case, the process is an outer-sphere
electron transfer. Given this, the free energy of activationless
electron transfer will depend intimately on the reorganizational
energy associated with the [Re(LL)3]+/2+ and the Q+/0
processes. The crystallographic results indicate that substantial
nuclear reorganization is required for reoxidation of the Re
complexes and that the dmpe complex Re−P bond length
decrease for the process is somewhat larger than the depe
complex. With the overall observations that (a) easily oxidized
quenchers have nearly 100% charge separation efficiencies and
(b) the yield of ions steadily decreases as the Q+/0 potential
becomes more positive (back electron transfer becomes more
exergonic in each case), the implication is that the free energy
for activationless back electron transfer is at potentials
somewhere between 1.1 and 1.2 V for the three complexes
used in the charge separation measurements. For the more
easily oxidized quenchers, back electron transfer is in the
Marcus normal region, and cage escape rate constants compete
more and more effectively with back electron transfer as ΔGBET
becomes less exergonic, ultimately reaching ηcs ≈ 1 when E0 for
the [Re(PP)3]+/Q+ reaction is less than ∼0.6−0.8 V versus
Ag+/Ag. Given that the apparent activationless free energies are
relatively low, the possibility exists to observe charge separated
ions for back reactions that are more exergonic than any of the
systems studied here.
is nearly zero for reactions involving donors with E0(D+/0
)
potentials greater than ca. 1.5 V versus AgCl/Ag, implying that
the back electron transfer reactions may have reorganizational
energies on the order of 1.1 to 1.2 V, making them nearly
activationless when the back electron transfer process is highly
exergonic. This aspect of the photoredox behavior of these
complexes will be investigated in greater detail.
EXPERIMENTAL SECTION
■
Photophysical and Electrochemical Methods. Absorption
spectra were recorded on an Agilent 8453 diode-array spectropho-
tometer at room temperature in distilled and degassed CH3CN.
Luminescence spectra were recorded on a PTI-Felix Fluorometer with
photomultiplier detector. Samples were prepared as acetonitrile
solutions and were studied without degassing. Transient absorption
spectra were acquired with a system described elsewhere.78
Luminescence quantum yields of fluorescence were measured
utilizing [Ru(bpy)3](PF6)2 as a reference. Reference and sample were
absorbance-matched at the wavelength of excitation. After obtaining
the emission spectra for both sample and reference, the quantum yield
of the sample was determined by
IR(1 − 10−Abs
)
)
S
ΦS = ΦR
IS(1 − 10−Abs
R
where Ir and Is are the integrated intensity of the emission, Abs
represents the absorbance at the wavelength where the reference and
sample were matched, and ΦR is the quantum yield for the reference.
Lifetime measurements were performed by time-correlated single-
photon counting, exciting with a 377 nm pulsed diode laser (IBH) and
detecting emitted light at right angles with an IBH-cooled PMT and
using a Tennelec TAC/Ortec Easy-MCA for acquisition of the
independent delay times. The digitized data of the MCA was
processed using PTI deconvolution software. Samples were prepared
as acetonitrile solutions and investigated without degassing. Initially,
the laser scatter was acquired at a wavelength near the excitation
SUMMARY
■
This work expands on earlier reports of the photochemistry and
photophysics of tris(diphosphine) Re2+ complexes in solution.
A range of fully characterized [Re(R2P(CH2)xPR2)3]2+ (x = 1−
3; R = Me, Ph) complexes were prepared in which changes in
the photophysical behavior were examined as a function of the
size of the chelate ring as well as the degree of aryl versus alkyl
substitution on the chelating phosphines. Increasing the chelate
ring size from four to six results in a blue shift in the
luminescence and increases in the luminescence lifetime and
quantum yield, attributable to changes in the ligand field
splitting and stabilization of the P(σ) bonding orbitals upon
increasing the bite angle of the diphosphine ligand. The redox
behavior for the complexes shows that the E(Re2+/+) potential
is small and positive for all the tris-dialkylphosphinoalkyl
complexes, but becomes steadily more positive with increasing
replacement of dialkylphosphino moieties with diphenylphos-
frequency. A bandpass filter (600
10) was utilized during the
acquisition of sample emission to eliminate scatter signal.
Electrochemical measurements were performed under N2 in
CH3CN solutions with 0.1 M Bu4N+PF6− as the supporting electrolyte
with 1 mM (PP)3Re+ analyte using a model ED401 computer-
controlled potentiostat (eDAQ). A three-electrode configuration with
a glassy carbon working electrode with a nonaqueous Ag/Ag+
reference electrode (containing 0.01 mM Ag/AgNO3 and 0.1 M
TBAPF6 in a CH3CN solution) and a platinum wire auxiliary electrode
was used. The potential values were referenced to an internal
ferrocenium/ferrocene couple, which is reported to be +0.40 V versus
51
+
−
SCE in NBu4 PF6 in CH3CN.
General Synthetic Procedures. All manipulations were con-
ducted under N2 or vacuum using high-vacuum line and glovebox
I
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techniques unless otherwise noted. All ambient-pressure syntheses
were performed under a pressure of ∼590 Torr (elevation ∼2195 m).
All solvents were dried using standard procedures and stored under
vacuum. Aprotic deuterated solvents used in NMR experiments were
dried over activated 3 Å molecular sieves. Elemental analyses were
performed by Columbia Analytical Services. NMR spectra were
obtained with a Bruker DRX-400 instrument using 5 mm NMR tubes
fitted with Teflon valves (Chemglass, CG-512). 31P NMR spectra were
referenced to an 85% H3PO4 external standard. The phosphines dmpp,
dppe, and all other reagents were purchased from Aldrich and were
used without further purification except sulfuryl chloride, which was
purchased from Acros Organics. (PPh3)2ReO2I and (PPh3)2ReI2(O)-
OEt,79 (dppm)3Re+I−,48 [(py)4Re(O)2]+OTf−,47 dmpe and depe,80
dmpm,81 Me2PCH2PPh2,81,82 and dmpb83 were prepared according to
literature methods. For the synthesis of dmpp the dihalide Br(CH2)3Br
was used instead of Cl(CH2)2Cl.84 Ph3C+B(C6F5)4−, Cp2Fe+B-
pure white microcrystalline 1 that was collected by filtration (0.98 g,
77% yield).
Alternate Preparation of [(dmpe)3Re](B(C6F5)4) (1).
(PPh3)2ReI2(O)OEt (2.000 g, 1.949 mmol) and dmpe (1.95 mL,
1.755 g, 11.70 mmol) and 12 mL of o-dichlorobenzene were combined
in a 18 mL Teflon-valved medium-wall Pyrex reaction tube with a
magnetic stirbar. The reaction mixture was heated with stirring at 180
°C for 24 h, during which time the reaction mixture changed from red,
to yellow, and finally to colorless along with a large amount of a white
crystalline precipitate. The reaction mixture was cooled to ambient
temperature, the suspension was filtered, the solid was rinsed with 20
mL of CH2Cl2, and the filtrates were combined. [(dmpe)3Re]I was
precipitated by slowly adding Et2O to the combined filtrates to give a
nearly quantitative yield of the product (1.480 g). This iodide complex
and K+B(C6F5)4− (1.540 g, 2.145 mmol) were suspended in 25 mL of
CH2Cl2 and stirred at ambient temperature for 18 h. The resulting
white suspension was filtered to remove KI, and 60 mL of MeOH was
added to the filtrate to precipitate the product. The volume was
reduced to ca. 15 mL to induce further precipitation, and the product
was collected under ambient air, giving 1 as a pure white
microcrystalline solid (2.340 g, 91% yield). Crystals suitable for
single-crystal X-ray diffraction were grown from the diffusion of
MeOH into a saturated CH2Cl2 solution of 1. Anal. Calcd for
C42H48P6F20BRe: C, 38.29%; H, 3.68%. Found: C, 38.35%; H, 3.70%.
1H NMR (CD2Cl2, 400.13 MHz, 20 °C): δ 1.65 (m, 6H;
P(CHaHb)2P), 1.57 (s, 18H; P(CH3)a(CH3)b), 1.46 (s, 18H;
P(CH3)a(CH3)b), 1.41 (m, 6H; P(CHaHb)2P) 31P{1H} NMR
(CD2Cl2, 161.97 MHz, 20 °C): δ 2.1 (br. s, ν1/2 = 790 Hz).
−
−
(C6F5)4 , and (4-BrC6H4)3N+B(C6F5)4 were prepared by modified
procedures from the literature (see below).51,85
Cp2Fe+B(C6F5)4−. Modified from the literature procedure:51 instead
+
of using FeCl3 as the oxidant and NH4 PF6− as the metathesizing salt,
SO2Cl2 was used as the oxidant, and salt exchange employed
−
K+B(C6F5)4 . Cp2Fe+Cl− was prepared by adding SO2Cl2 (8.0 mL,
13.3 g, 99 mmol) dropwise to a solution of Cp2Fe (10.0 g, 53.8 mmol)
in 60 mL of CH2Cl2. The orange reaction mixture immediately turned
dark blue, and the product precipitated as a dark blue crystalline solid
that was collected as a first crop (8.16 g) via filtration under ambient
air and rinsed three times with 25 mL of diethyl ether. A second crop
was collected from the filtrate by the addition of ca. 100 mL of diethyl
ether (1.66 g, 82% overall yield). Method A. Cp2Fe+Cl− (0.500 g, 2.25
mmol) was dissolved in 20 mL of H2O giving a dark blue solution. A
[(depe)3Re](B(C6F5)4) (2). Complex 2 was prepared similarly to 1:
(PPh3)2ReO2I (0.600 g, 0.690 mmol), depe (0.885 mL, 0.782 g, 3.793
mol), and 6 mL of o-dichlorobenzene were heated at 170 °C with
stirring for 8 h to give a yellow solution. After filtration 10 mL of Et2O
and 10 mL of hexanes were added, and the mixture was cooled to −20
°C for 24 h, giving a dark greenish-yellow oily precipitate. The filtrate
was collected, and the volatiles were removed under vacuum to give a
brown oil, which was dissolved in 10 mL of CH2Cl2, and pure
(depe)3Re+I− was precipitated as a white solid by the addition of 20
mL of Et2O followed by 60 mL of hexanes (0.480 g, 75% yield). Anal.
Calcd for C30H72P6IRe: C, 38.62%; H, 7.78%. Found: C, 38.61%; H,
−
solution of K+B(C6F5)4 (1.470 g, 2.05 mmol) dissolved in 15 mL of
H2O and 2 mL of CH3CN was added dropwise to the blue solution
producing a blue precipitate. The precipitate was collected via
filtration, rinsed with twice with 20 mL of H2O, and dried under
vacuum overnight (1.61 g, 91% yield). Method B. The product was
obtained by stirring a suspension of K+B(C6F5)4− (1.78 g, 2.48 mmol)
and Cp2Fe+Cl− (0.500 g, 2.25 mmol) in 80 mL of CH2Cl2 at ambient
temperature for 5 h. The blue solution was filtered to remove KCl, and
the volatiles were removed under vacuum. The blue residue was
triturated with 25 mL of hexane three times, and during the last
trituration the solution was cooled to −78 °C giving large blue flakes
of the product, which were collected via filtration (1.80 g, 92% yield).
(4-BrC6H4)3N+B(C6F5)4−. This complex was prepared by adapting a
literature procedure:85 instead of using I2 as the oxidant, SO2Cl2 was
−
7.40%. (depe)3Re+I− (0.400 g, 0.429 mmol) and K+B(C6F5)4 (0.400
g, 0.557 mmol) were stirred in 10 mL of CH2Cl2 for 3 h, giving a
colorless solution and a white KI precipitate. After filtration, 20 mL of
MeOH was added, and the volume of the filtrate was reduced to 10
mL, giving 2 as a white crystalline solid, which was collected by
filtration (0.503 g, 79% yield). Anal. Calcd for C54H72P6F20BRe: C,
−
used as the oxidant, and K+B(C6F5)4 was used instead of
1
43.66%; H, 4.89%. Found: C, 43.55%; H, 4.66%. H NMR (CD3CN,
85
−
Ag+B(C6F5)4 . SO2Cl2 (1.0 mL, 1.67 g, 12.4 mmol) was added
dropwise to a suspension of (4-BrC6H4)3N (1.06 g, 2.21 mmol) and
K+B(C6F5)4− (1.59 g, 2.21 mmol) in 10 mL of CH2Cl2. Upon addition
of SO2Cl2 a dark blue solution was obtained along with KCl
precipitate, and the reaction mixture was stirred overnight at ambient
temperature. The precipitate was removed by filtration, volatiles were
removed under vacuum, and the residue was triturated with ca. 20 mL
of hexane, giving dark blue flakes, which were collected via filtration
(2.40 g, 94% yield).
400.13 MHz, 20 °C): δ 1.89 (m, 18H; overlapping P(CHaHb)2P and
2
2
3
P(CHaHbCH3)2), 1.61 (ddq, JHP = 22.4 Hz, JHH = 14.9 Hz, JHH
=
7.5 Hz, 12H; P(CHaHbCH3)2), 1.34 (m, 6H; P(CHaHb)2P), 1.15 (br.
m, 36H; P(CHaHbCH3)2). 31P{1H} NMR (CD3CN, 161.97 MHz, 20
°C): δ 16.8 (br. s, ν1/2 = 225 Hz). Crystals suitable for single-crystal X-
ray diffraction were grown from the diffusion of MeOH into a
saturated CH2Cl2 solution of 2.
Alternate Preparation of [(depe)3Re](B(C6F5)4) (2).
(PPh3)2ReI2(O)OEt (0.900 g, 0.877 mmol), depe (1.13 mL, 0.995
g, 4.84 mmol), and 6 mL of o-dichlorobenzene were combined in a 12
mL Teflon-valved medium-wall Pyrex reaction tube with a magnetic
stirbar. The reaction mixture was heated at 170 °C for 24 h, during
which time the reaction mixture changed from red to pale yellow with
an accompanying yellow precipitate, which decreased during the
course of the reaction. The reaction mixture was cooled to ambient
temperature, the solution was transferred to a round-bottom flask, and
the volatiles were removed under vacuum to give an oily yellow
suspension. Addition of 30 mL of diethyl ether to the suspension and
stirring overnight gave a yellow solution with a yellow solid, which was
isolated. Addition of K+B(C6F5)4− (0.693 g, 0.965 mmol) to this solid
suspended in 15 mL of CH2Cl2 and stirring at ambient temperature for
3 h gave a precipitate of KI, which was removed. Addition of 30 mL of
MeOH to the filtrate and reduction of the volume to ca. 15 mL gave 2
as a pure white microcrystalline solid (0.652 g, 50% yield).
[(dmpe)3Re](B(C6F5)4) (1). (PPh3)2ReO2I (0.600 g, 0.690 mmol),
dmpe (0.633 mL, 0.569 g, 3.79 mol), and 6 mL of o-dichlorobenzene
were added to a 12 mL Teflon-valved medium-wall Pyrex reaction
tube with a magnetic stirbar. The reaction mixture was heated with
stirring for 6 h at 170 °C to give a clear solution with a small amount
of gray precipitate. After it cooled, the solid was filtered off under
ambient air. The product (dmpe)3ReI was precipitated and isolated as
a microcrystalline solid from the filtrate by slowly adding ca. 40 mL of
Et2O (0.470 g, 89% yield). The product was judged to be 95+% pure
by NMR. The crude product (dmpe)3Re+I− (0.470 g, 0.969 mmol)
−
and K+B(C6F5)4 (0.904 g, 1.259 mmol) were dissolved in 25 mL of
CH2Cl2 and stirred at ambient temperature for 3 h to give a white
−
suspension of KI and excess K+B(C6F5)4 . The salts were filtered away,
and 40 mL of MeOH was added to the filtrate. CH2Cl2 was slowly
removed by reducing the volume of the filtrate by ∼50%, producing
J
DOI: 10.1021/acs.inorgchem.5b01395
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
[(dmpm)3Re](B(C6F5)4) (3). Complex 3 was prepared according to
the alternate preparation of 2: (PPh3)2ReI2(O)OEt (1.200 g, 1.17
mmol), dmpm (1.02 mL, 0.877 g, 6.44 mmol), and 10 mL of o-
dichlorobenzene were combined in a 20 mL storage tube. The reaction
mixture was heated at 170 °C and the work up procedure followed the
alternate preparation for 2. After it stirred in Et2O and was filtered,
giving the crude product as a pale brown solid (80−90% pure by
NMR), it was then combined with K+B(C6F5)4− (0.923 g, 1.28 mmol),
and the product was isolated (0.725 g, 49% yield). Crystals suitable for
single-crystal X-ray diffraction were grown from the diffusion of
MeOH into a saturated CH2Cl2 solution. Anal. Calcd for
C39H42P6F20BRe: C, 36.73%; H, 3.32%. Found: C, 36.58%; H,
to ca. 10 mL, giving a pale yellow precipitate (0.303 g, 63% yield). The
1H and 31P NMR spectra were consistent with reported values. Anal.
Calcd for C99H66BF20P6Re: C, 58.91%; H, 3.30%. Found: C, 58.79%;
H, 3.21%.
[(dmpb)3Re](B(C6F5)4) (7). (PPh3)2Re(O) (OEt)I2 (1.00 g, 0.975
mmol), dmpb (1.215 g, 6.10 mmol), and 10 mL of o-dichlorobenzene
were combined in an 18 mL Teflon-valved medium-wall Pyrex
reaction tube with a magnetic stirbar. The reaction mixture was heated
at 180 °C for 24 h giving a clear solution with a white precipitate. The
solid was filtered away, and the volatiles were removed under vacuum
to give a white residue. Et2O (30 mL) was added to the residue, and it
was triturated at ambient temperature overnight giving crude
(dmpb)3Re+I− as a white solid, which was collected via filtration and
rinsed three times with 20 mL of Et2O (0.656 g). (dmpb)3Re+I−
(0.656 g, 0.721 mmol) and K+B(C6F5)4− (0.545 g, 0.759 mmol) were
suspended in 15 mL of CH2Cl2 and stirred for 18 h at ambient
temperature. The solution was filtered away from KI, and 50 mL of
MeOH was added to the filtrate. The volume of the filtrate was
reduced to ca. 8 mL giving a white precipitate that was collected by
filtration under ambient air (0.459 g). A second fraction of
(dmpb)3Re+I− was extracted from the initial filtered solids with 10
mL of CH2Cl2, the volatiles were removed, and the solid triturated
with 25 mL of Et2O. (dmpb)3Re+I− (0.400 g, 0.441 mmol) and
1
2.96%. H NMR (acetone-d6, 400.13 MHz, 20 °C): δ 4.15 (m, 6H;
(CH3)2PCH2P(CH3)2), 1.77 (br. s, 18H; (CH3)a(CH3)bP), 1.76 (br.
s, 18H; (CH3)a(CH3)bP). 31P{1H} NMR (acetone-d6, 161.97 MHz, 20
°C): δ −66.2 (br. s, ν1/2 = 50 Hz).
[fac-(Me2PCH2PPh2)3Re](B(C6F5)4) (4). (PPh3)2ReI2(O)OEt (0.600
g, 0.585 mmol), Me2PCH2PPh2 (0.700 g, 2.69 mmol), and 6 mL of o-
dichlorobenzene were combined in a 12 mL Teflon-valved medium-
wall Pyrex reaction tube with a magnetic stirbar. The reaction mixture
was heated with stirring at 170 °C for
3 d, giving
(Me2PCH2PPh2)3Re+I− as a pure crystalline solid, which was rinsed
with 20 mL of MeOH and 20 mL of Et2O (0.532 g, 83% yield). Anal.
Calcd for C45H54P6IRe: C, 49.35%; H, 4.97%. Found: C, 49.13%; H,
4.60%. fac-(Me2PCH2PPh2)3Re+I− (0.500 g, 0.457 mmol) and
−
K+B(C6F5)4 (0.348 g, 0.485 mmol) were suspended in 15 mL of
CH2Cl2 and stirred for 18 h at ambient temperature, and the product
was worked up as described above (0.550 g, 71%). Crystals suitable for
single-crystal X-ray diffraction were grown by the slow diffusion of
MeOH (1.5 mL) layered above a solution of 7 in CH2Cl2 (0.5 mL).
The elevated hydrogen analysis may indicate that iodide metathesis
was incomplete. Anal. Calcd for C54H48P6BF20Re: C, 44.43%; H,
3.31%. Found: C, 44.93%; H, 4.26%. 1H NMR (CD2Cl2, 400.13 MHz,
20 °C): δ 7.81 (br. m, 6H; meta-C6H4(P(CH3))2), 7.58 (dd, 3J = 5 Hz,
3J = 3 Hz, 6H; ortho-C6H4(P(CH3))2), 2.01 (s, 18H; C6H4(P-
(CH3))2), 1.52 (s, 18H; C6H4(P(CH3))2). 31P{1H} NMR (CD2Cl2,
161.97 MHz, 20 °C): δ 1.5 (br. s, ν1/2 = 345 Hz, 6P; C6H4(P(CH3))2).
[(dmpe)3Re](B(C6F5)4)2 (8). Complex 1 (0.900 g, 0.684 mmol) and
−
K+B(C6F5)4 (0.328 g, 0.457 mmol) were suspended in 25 mL of
CH2Cl2 and stirred for 3 h at ambient temperature. The yellow
solution was filtered away from KI, and 50 mL of MeOH was added to
the filtrate. The volume of the filtrate was reduced to ca. 15 mL giving
a yellow precipitate that was collected by filtration under ambient air
(0.702 g, 93% yield). Crystals suitable for single-crystal X-ray
diffraction were grown by the slow evaporation from a saturated 1:1
CH2Cl2/MeOH solution. Anal. Calcd for C69H54BF20P6Re: C, 50.35%;
1
H, 3.31%. Found: C, 50.20%; H, 3.93%. H NMR (CD2Cl2, 400.13
MHz, 20 °C): δ 7.43 (br. s, 15H; (C6H5)a(C6H5)bP), 7.32 (t, 3JHH = 7
3
Hz, 3H; para-(C6H5)a(C6H5)bP), 7.23 (t, JHH = 7 Hz, 6H; meta-
3
−
Ph3C+B(C6F5)4 (0.694 g, 0.752 mmol) were dissolved in 25 mL of
(C6H5)a(C6H5)bP), 7.07 (pseudo t, JHH = 8 Hz, 6H; ortho-
2
2
(C6H5)a(C6H5)bP), 5.06 (br. dt, JHH = 15 Hz, JHP = 7 Hz, 3H;
CH3CN. When mixed, the solution immediately turned red. The
reaction mixture was stirred for 5 h at ambient temperature, and the
volatiles were removed under vacuum. The residue was taken up in 50
mL of Et2O and stirred at ambient temperature for 24 h to give a
suspension of the red product, which was collected via filtration (1.00
g, 73% yield). Anal. Calcd for C66H48P6B2F40Re: C, 39.70%; H, 2.42%.
Found: C, 39.62%; H, 2.34%. Crystals suitable for X-ray diffraction
were grown from a saturated solution of CH3CN.
2
2
PCHaHbP), 4.57 (br. dt, JHH = 15 Hz, JHP = 7 Hz, 3H; PCHaHbP),
1.87 (d, 2JHP = 5 Hz, 9H; P(CH3)a(CH3)b), 1.04 (d, 2JHP = 6 Hz, 9H;
P(CH3)a(CH3)b). 31P{1H} NMR (CD2Cl2, 161.97 MHz, 20 °C): δ
−45.9 (d, 2JPPtrans = 94 Hz; (C6H5)2PCH2P(CH3)2), −63.3 (d, 2JPPtrans
= 94 Hz; (C6H5)2PCH2P(CH3)2).
[(dmpp)3Re](B(C6F5)4) (5). [(py)4Re(O)2]+OTf− (0.500 g, 0.732
mmol), dmpp (0.625 g, 3.81 mmol), and 6 mL of o-dichlorobenzene
were combined in a 12 mL Teflon-valved medium-wall Pyrex reaction
tube with a magnetic stirbar. The reaction mixture was heated at 170
°C for 2 h, and (dmpp)3Re+OTf− was precipitated as an off-white solid
by the slow addition of ca. 50 mL of Et2O and collected via filtration
(0.398 g, 66% yield). (dmpp)3Re+OTf− (0.526 g, 0.481 mmol) and
[(depe)3Re](B(C6F5)4)2 (9). Oxidation of complex 2 to give 9 was
performed analogously to 8 except that the reaction mixture was
stirred for 3.5 h, and a second crop was isolated from the filtrate by
removing the volatiles and triturating the residue with Et2O: 2 (0.503
−
g, 0.339 mmol), Ph3C+B(C6F5)4 (0.343 g, 0.373 mmol), (first crop
−
K+B(C6F5)4 (0.382 g, 0.532 mmol) were suspended in 20 mL of
0.455 g, second crop 0.240 g, 95% total yield). Anal. Calcd for
C78H72P6B2F40Re: C, 43.31%; H, 3.36%. Found: C, 44.02%; H, 3.13%.
Crystals suitable for single-crystal X-ray diffraction were grown from a
1:2 CH2Cl2/MeOH solution.
CH2Cl2 and stirred at ambient temperature for 3 h, the resulting
K+OTf− was removed by filtration, 40 mL of MeOH was added to the
filtrate, and the volume reduced to ca. 10 mL. The product
precipitated as a white solid and was collected via filtration (0.460 g,
70% yield). Crystals suitable for single-crystal X-ray diffraction were
grown by slow evaporation from a saturated 1:1 CH2Cl2/MeOH
solution. Anal. Calcd for C45H54BF20P6Re: C, 39.81%; H, 4.01%.
[(dmpm)3Re](B(C6F5)4)2 (10). Complex 3 (0.150 g, 0.118 mmol)
and Cp2Fe+B(C6F5)4− (0.102 g, 0.118 mmol) were dissolved in 20 mL
of CH2Cl2, and the reaction mixture was stirred for 18 h. The product
precipitated as a microcrystalline red solid, which was collected by
filtration and rinsed once with 10 mL of CH2Cl2 (0.178 g, 77% yield).
The product was recrystallized by evaporation from a 1:2 mixture of
CH3CN/chlorobenzene. Anal. Calcd for C63H42P6B2F40Re: C, 38.71%;
H, 2.17%. Found: C, 38.22%; H, 2.42%. Crystals of the PF6− salt of 10
suitable for single-crystal X-ray diffraction, prepared analogously using
1
Found: C, 39.73%; H, 4.30%. H NMR (CD2Cl2, 400.13 MHz, 20
°C): δ 2.07 (br. m, 6H; PCH2CH2CH2P), 1.90 (br. m, 12H;
PCH2CH2CH2P), 1.67 (br. s, 18H; P(CH3)a(CH3)b), 1.60 (br. s, 18H;
P(CH3)a(CH3)b). 31P{1H} NMR (CD2Cl2, 161.97 MHz, 20 °C): δ
−53.8 (br. s, ν1/2 = 165 Hz).
[(dppm)3Re](B(C6F5)4) (6). The yellow solid (dppm)3Re+I− was
prepared from (PPh3)2ReO2I according to literature methods,54 except
that the reflux time in methanol was extended from 3 to 24 h (0.350 g,
−
Cp2Fe+PF6 , were grown by slow evaporation from a saturated
CH2Cl2 solution.
[fac-(Me2PCH2PPh2)3Re](B(C6F5)4)2 (11). Complex 4 (0.300 g, 0.182
mmol) and Cp2Fe+B(C6F5)4− (0.158 g, 0.183 mmol) were dissolved in
20 mL of CH2Cl2 and stirred at ambient temperature for 3 h. The
volatiles were removed from the resulting purple solution, and the
residue was triturated with 25 mL of Et2O. The residue was
−
69%). The (dppm)3Re+I− and K+B(C6F5)4 (0.206 g, 0.287 mmol)
were suspended in 15 mL of CH2Cl2 at ambient temperature and
stirred for 2 h. The resulting KI was filtered away, 30 mL of MeOH
was added to the filtrate, and the volume was reduced under vacuum
K
DOI: 10.1021/acs.inorgchem.5b01395
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
reprecipitated twice from CH2Cl2/hexane, and the purple product was
collected via filtration (0.274 g, 66% yield). Anal. Calcd for
C93H54P6B2F40Re: C, 48.04%; H, 2.34%. Found: C, 47.89%; H,
Notes
The authors declare no competing financial interest.
−
3.20%. Crystals of the PF6 salt of 11 suitable for single-crystal X-ray
ACKNOWLEDGMENTS
−
diffraction were prepared by the oxidation of 4 with Cp2Fe+PF6 in
■
This work was funded by the Division of Chemical Sciences,
Geosciences, and Biosciences, Office of Basic Energy Sciences
of the U.S. Department of Energy through Grant Nos. DE-
FG02-08ER15979 (D.M.R.) and DE-FG-02-96ER14617
(R.H.S.) and synthetic work through NSF Grant No. CHE-
1213903 (D.M.R.). The authors are especially grateful to one
reviewer for insightful comments relating to the interpretation
of the structured low-temperature luminescence.
CH3CN followed by slow evaporation.
[(dmpp)3Re](B(C6F5)4)2 (12). Complex 5 (0.350 g, 0.258 mmol) and
Cp2Fe+B(C6F5)4 (0.223 g, 0.258 mmol) were dissolved in 20 mL of
−
CH2Cl2 and stirred for 18 h at ambient temperature. The product
precipitated as an orange microcrystalline solid, which was collected
via filtration (0.450 g, 86% yield). The solid was recrystallized by the
slow evaporation from a 1:1 solution of 1,2-difluorobenzene and
toluene in 90% yield. Anal. Calcd for C66H54P6B2F40Re·C12H8F4: C,
42.95%; H, 2.76%. Found: C, 43.10%; H, 3.02%.
[(dppm)3Re](B(C6F5)4)2 (13). Complex 6 (0.350 g, 0.173 mmol) and
(4-BrC6H4)3N+B(C6F5)4− (0.200 g, 0.172 mmol) were dissolved in 20
mL of CH2Cl2 and stirred at ambient temperature for 24 h. The
volatiles were removed from the resulting blue/purple solution, and
the residue was triturated with 25 mL of Et2O. The residue was
reprecipitated twice from CH2Cl2/hexane, and the dark blue product
was collected via filtration (0.411 g, 88% yield). Anal. Calcd for
C123H66P6B2F40Re: C, 54.72%; H, 2.47%. Found: C, 55.31%; H,
2.72%.
DEDICATION
■
Dedicated to B. Patrick Sullivan, in recognition of his
contributions to inorganic photochemistry.
REFERENCES
■
(1) Comprehensive Coordination Chemistry II, Vol. 5: Transition Metal
Groups 7 and 8; Constable, E. C., Dilworth, J. R., Eds.; Elsevier Ltd:
Amsterdam, The Netherlands, 2004.
(2) Comprehensive Coordination Chemistry, Vol. 4: Middle Transition
Elements; Gillard, R. D., McCleverty, J. A., Eds.; Pergmanon Press:
New York, NY, 1987; Vol. 4.
[(dmpb)3Re](B(C6F5)4)2 (14). Complex 7 (0.275 g, 0.189 mmol) and
(4-BrC6H4)3N+B(C6F5)4− (0.218 g, 0.189 mmol) were dissolved in 20
mL of CH2Cl2, and the mixture was stirred at ambient temperature for
3 h. Upon addition of CH2Cl2 the solution turned immediately from
dark blue to red. Removal of the volatiles and addition of 25 mL of
Et2O to the residue followed by stirring for 1 h and standing for an
additional 15 min caused the product to settle as a red oil. The Et2O
layer was removed from the product oil, and the volatiles were
removed. The resulting residue was suspended in hexane and stirred
overnight at ambient temperature to precipitate the product as a red/
purple powder, which was collected via filtration (0.313 g, 78% yield).
Anal. Calcd for C78H48P6B2F40Re·CH2Cl2: C, 42.67%; H, 2.27%.
Found: C, 42.80%; H, 3.04%. Crystals suitable for single-crystal X-ray
diffraction were grown from the [(dmpb)3Re](SbF6)2 salt, prepared
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b01395.
■
244.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: russ@tulane.edu. (R.H.S.)
*E-mail: dmr@uwyo.edu. (D.M.R.)
■
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