Vibrational spectroscopy of reduced Re(I) complexes of 1,10-phenanthroline and substituted analogues

Article abstract of DOI:10.1021/jp056614d

IR spectroscopy in concert with DFT calculations and resonance Raman spectroelectrochemistry has been used to identify the molecular orbital nature of the singly occupied molecular orbital (SOMO) in reduced [Re(CO) ₃Cl(L)] and [Re(CO)₃(4-Mepy)(L)]⁺ complexes, where L = 1,10-phenanthroline and its 4,7-diphenyl-and 3,4,7,8-tetramethyl- substituted analogues. The SOMO of each reduced species considered was found to be of bi symmetry, rather than the close lying orbital of a₂ symmetry (within a C_2ν symmetry description of the phenanthroline moiety). This was deduced in a number of ways. First, the average carbonyl band force constants (Δl_av -k_av{reduced complex} -k _av{parent complex}) range from -57 to -41 N m^-1 for the series of compounds studied. The value of Δk_av relates to the extent of orbital overlap between the ligand MO and the metal dπ MO. These values are consistent with population of a b₁ MO because the wave function amplitude at the chelating nitrogens for this MO is significantly greater than that for a₂ MO. Second, calculations on singly reduced [Re(CO)₃(4-Mepy)(phen)]⁺ and [Re(CO)₃(4-Mepy) (tem)]⁺ predict population of a b₂ SOMO. The spectra predicted for these species are in close agreement with the vibrational spectroscopic data; for the IR data the shifts in the CO bands are predicted to 6 cm-¹ and the mean absolute deviation between calculated and measured Raman bands was found to be 10 cm^-1.

Full text of DOI:10.1021/jp056614d

4880
J. Phys. Chem. A 2006, 110, 4880-4887
Vibrational Spectroscopy of Reduced Re(I) Complexes of 1,10-Phenanthroline and
Substituted Analogues
Sarah L. Howell and Keith C. Gordon*
MacDiarmid Institute for AdVanced Materials and Nanotechnology, Department of Chemistry, UniVersity of
Otago, Union Place, Dunedin, New Zealand
ReceiVed: NoVember 15, 2005; In Final Form: February 13, 2006
IR spectroscopy in concert with DFT calculations and resonance Raman spectroelectrochemistry has been
used to identify the molecular orbital nature of the singly occupied molecular orbital (SOMO) in reduced
[Re(CO)3Cl(L)] and [Re(CO)3(4-Mepy)(L)]+ complexes, where L ) 1,10-phenanthroline and its 4,7-diphenyl-
and 3,4,7,8-tetramethyl-substituted analogues. The SOMO of each reduced species considered was found to
be of b1 symmetry, rather than the close lying orbital of a2 symmetry (within a C2V symmetry description of
the phenanthroline moiety). This was deduced in a number of ways. First, the average carbonyl band force
constants (∆kav ) kav{reduced complex} - kav{parent complex}) range from -57 to -41 N m-1 for the
series of compounds studied. The value of ∆kav relates to the extent of orbital overlap between the ligand
MO and the metal dπ MO. These values are consistent with population of a b1 MO because the wave function
amplitude at the chelating nitrogens for this MO is significantly greater than that for a2 MO. Second, calculations
on singly reduced [Re(CO)3(4-Mepy)(phen)]+ and [Re(CO)3(4-Mepy)(tem)]+ predict population of a b2 SOMO.
The spectra predicted for these species are in close agreement with the vibrational spectroscopic data; for the
IR data the shifts in the CO bands are predicted to 6 cm-1 and the mean absolute deviation between calculated
and measured Raman bands was found to be 10 cm-1.
Introduction
1,10-Phenanthroline (phen) and substituted analogues are
prevalent polypyridyl ligands.1 Complexes of these ligands have
been widely studied because of their interesting photophysical
properties, such as long-lived metal-to-ligand charge transfer
(MLCT) excited states.2 Metal-to-ligand charge-transfer transi-
tions as a first approximation involve the removal of an electron
from a metal-based orbital (dπ) to one localized on the ligand
(π*). Therefore, the nature of the ligand acceptor molecular
orbital in the MLCT excited state is of interest.
Phen and substituted analogues have two close lying unoc-
cupied molecular orbitals that are potentially populated in an
optical transition. Kaim unambiguously demonstrated using EPR
Figure 1. Atom labeling of phen along with the LUMO (b1) and
LUMO+1 (a2). Hydrogen atoms have been omitted for clarity.
studies on the radical anion that the LUMO and LUMO+1 of
phen have symmetries of b1 and a2 (Figure 1), respectively,
to a W(dπ) f phen(b1) transition, and at 1.21 eV, corresponding
to a W(dπ) f phen(a2) transition. The calculation of [W(CO)4-
(tem)] showed the accepting ligand π* molecular orbitals to
swap order. The lowest energy transition W(dπ) f phen(a2)
was predicted at 1.14 eV and the next highest energy transition
W(dπ) f phen(b1) at 1.26 eV.
within a C2V symmetry description.3 Since then, further EPR
studies have been carried out by Kaim et al.4 and Vlcˇek et al.5
on substituted phen, its derivatives and their complexes.
Kaim et al. showed that the correct ordering of the unoccupied
molecular orbitals could be computationally predicted using
Hartree-Fock and the 6-31G(d,p) basis set.4 However, we
have previously shown that a larger basis set is required
(6-311+G(d,p)) when using the density functional theory hybrid
method B3LYP.6 In a study by Vlcˇek et al. of [W(CO)4(phen)]
and [W(CO)4(tem)], TDDFT was used to calculate the lowest
energy singlet and triplet state excitations.7 Previously, Vlcˇek
et al. had shown through EPR experiments that the LUMO’s
of these complexes were of b1 and a2 symmetry, respectively.5
They also used TDDFT to predict the transitions of these
complexes. For [W(CO)4(phen)] two low energy MLCT transi-
tions are predicted. These occurred at 1.14 eV, corresponding
The nature of the LUMO’s of some polypyridyl ligands has
been established using IR of the carbonyl region. Rhenium
carbonyl complexes are ideal candidates to be studied by IR
spectroscopy.8 Carbonyl ligands have strong infrared bands at
frequencies unobstructed by most solvents and the frequencies
are indicative of the electron density at the metal. The
assignment of the nature of the LUMO of a complex can be
determined using IR spectroelectrochemistry of the reduced
species. Different ligand based molecular orbitals have differing
extents of communication with the metal moiety. Because the
carbonyl frequencies are sensitive to the electron density at the
metal center, the IR spectrum in the carbonyl region will differ
depending on the molecular orbitals populated by reduction.
* Corresponding author. E-mail: kgordon@alkali.otago.ac.nz.
10.1021/jp056614d CCC: $33.50 © 2006 American Chemical Society
Published on Web 03/17/2006

Spectroscopy of Re(I) 1,10-Phenanthroline Complexes
J. Phys. Chem. A, Vol. 110, No. 14, 2006 4881
complex) or dichloromethane (tem complex) (100 mL). Pentane
(100 mL) was added to this and the solution was placed in the
refrigerator until fine needlelike crystalline solid formed from
the solution.
[Re(CO)3Cl(phen)]. Yellow crystalline solid. Yield: 62%.
1H NMR (CDCl3, 300 MHz): δ 9.42 (dd, 2H, J1 ) 3.6 Hz,
J2 ) 1.5 Hz), 8.57 (dd, 2H, J1 ) 6.9 Hz, J2 ) 1.5 Hz), 8.04
(s, 2H), 7.89 (dd, 2H, J1 ) 8.6 Hz, J2 ) 5.1 Hz). m/z 451
({M - Cl}+). Found: C, 37.38; H, 1.74; N, 5.78. Calcd for
C15H8N2O3ClRe: C, 37.08; H, 1.66; N, 5.77. E°(1st reduction)
) -1.3 V (vs DMFc/DMFc+).
[Re(CO)3Cl(dip)]. Orange crystalline solid. Yield: 80%. 1H
NMR (CDCl3, 300 MHz): δ 9.44 (d, 2H, J ) 5.4 Hz), 8.05 (s,
2H), 7.80 (d, 2H, J ) 5.4 Hz), 7.62-7.52 (m, 10H). Found:
C, 50.89; H, 2.71; N, 4.28. Calcd for C27H16N2O3ClRe: C,
50.82; H, 2.53; N, 4.39. E°(1st reduction) ) -1.2 V (vs DMFc/
DMFc+).
Figure 2. LUMO (phenazine-based) and LUMO+2 (phenanthroline-
based) of dppz. Hydrogen atoms have been omitted for clarity.
It is also possible to examine the vibrational spectrum of the
reduced ligand and thus establish the molecular orbital nature
of the SOMO. This may be accomplished using resonance
Raman spectroscopy on the reduced complex in which the
excitation wavelength used to generate the Raman scattering is
coincident with a chromophore of the ligand radical anion. The
signature from the radical anion may be used to ascertain the
structure of the species and infer the nature of the SOMO. This
has been accomplished in a study of dppz complexes. In dppz
there are two unoccupied molecular orbitals that lie close in
energy and are quite different in terms of their disposition across
the ligand framework. There is a phenanthroline-based molecular
orbital that has significant wave function amplitude at the
chelating nitrogen atoms (LUMO+2) and a phenazine based
molecular orbital with no amplitude at the chelating nitrogen
atoms but significant amplitude at the phenazine nitrogens
(LUMO) (Figure 2). In a study of the [Re(CO)3Cl(L)] complex
of dppz and its partially and fully deuterated isotopomers,9 the
resonance Raman spectra of the reduced species were part of
the evidence for a phenazine-based SOMO.
Computational chemistry can offer further insight into the
structure and vibrational spectra of reduced complexes. Gordon
et al. have carried out a number of DFT studies to model the
Raman and IR spectra of singly reduced polypyridyl complexes
in an attempt to establish the structural changes that occur upon
reduction.9-12
In this study we have examined the vibrational spectra of a
series of substituted phenanthroline complexes and their reduced
species to ascertain the nature of the SOMO in each. The
spectroscopic findings have been modeled using DFT calcula-
tions.
[Re(CO)3(4-Mepy)(phen)]CF3SO3. Yellow crystalline solid.
1
Yield: 95%. H NMR (300 MHz, CDCl3): δ 9.55 (2H, dd,
J1 ) 5.1 Hz, J2 ) 1.2 Hz), 8.84 (2H, dd, J1 ) 8.4 Hz, J2 ) 1.5
Hz), 8.20 (2H, s), 8.18 (2H, dd, J1 ) 8.1 Hz, J2 ) 4.8 Hz),
8.07 (2H, d, J ) 6.6 Hz), 7.08 (2H, d, J ) 6.0 Hz), 2.22 (3H,
s). m/z 544 ({M - CF3SO3}+). Found: C, 38.84; H, 2.30; N,
5.93. Calcd for C22H15N3O6F3SRe.(C5H12)0.15: C, 38.84; H, 2.41;
N, 5.97%; E°(1st reduction) ) -1.3 V (vs DMFc/DMFc+).
[Re(CO)3(4-Mepy)(tem)]CF3SO3. Pale yellow crystalline
1
solid. Yield: 55%. H NMR (300 MHz, CDCl3): δ 9.11 (2H,
s), 8.30 (2H, s), 7.98 (2H, d, J ) 6.6 Hz), 7.11 (2H, d, J ) 6.3
Hz), 2.88 (6H, s), 2.71 (6H, s), 2.22 (3H, s). Found: C, 41.65;
H, 3.03; N, 5.42. Calcd for C26H23N3O6F3SRe: C, 41.70; H,
3.10; N, 5.61. E°(1st reduction) ) -1.5 V (vs DMFc/DMFc+).
Physical Measurements. Spectroscopic grade solvents were
used for all spectroscopic measurements.
Spectral data were analyzed using Galactic Industries GRAMS/
32 AI software.
1H NMR spectra were recorded at 25 °C, using either a Varian
300 MHz NMR spectrometer. Chemical shifts are given relative
to residual solvent peaks. Microanalyses were performed at the
Campbell Microanalysis Laboratory at the University of Otago.
Mass spectrometry measurements were obtained from a Mi-
cromass LCT instrument for electrospray measurements or using
a Shimadzu QP8000 alpha with ESI probe.
FT-IR spectra were collected, using a Perkin-Elmer Spectrum
BX FT-IR system with Spectrum v.2.00 software.
Experimental Section
A continuous-wave Melles Griot OmNichrome 543-MAP
argon-ion laser was used to generate resonance Raman scatter-
ing. Typically, the laser output was adjusted to give 20-60 mW
at the sample. The incident beam and the collection lens were
arranged in a 135° backscattering geometry to reduce Raman
intensity reduction by self-absorption.16 An aperture-matched
lens was used to focus scattered light through a narrow band
line-rejection (notch) filter (Kaiser Optical Systems) and a quartz
wedge (Spex) and onto the 100 µm entrance slit of a spec-
trograph (Acton Research SpectraPro 500i). The collected light
was dispersed in the horizontal plane by a 1200 grooves/mm
ruled diffraction grating (blaze wavelength 500 nm) and detected
by a liquid nitrogen cooled back-illuminated Spec-10:100B CCD
controlled by a ST-133 controller and WinSpec/32 (version
2.5.8.1) software (Roper Scientific). Wavenumber calibration
was performed using Raman bands from a 1:1 (by volume)
mixture of acetonitrile and toluene sample.17,18 Peak positions
were reproducible to within 1-2 cm-1. Spectra were obtained
with a resolution of 5 cm-1. Freshly prepared samples were
Synthesis. Preparation of [Re(CO)3Cl(L)] complexes was
based on the method by Moya et al.13 [Re(CO)5Cl], phen, dip
and tem were used as supplied (Aldrich).
[Re(CO)3Cl(L)]: [Re(CO)5Cl] (0.298 g, 0.824 mmol) and
polypyridyl ligand (0.824 mmol) were combined in argon purged
methanol (100 mL) and refluxed under an argon atmosphere
for 21/2 h. The resulting solution was cooled and the precipitate
collected. The solid was washed with methanol, diethyl ether
and finally pentane.
[Re(CO)3(4-Mepy)(L)]CF3SO3 (where 4-Mepy is 4-meth-
ylpyridine) was made from [Re(CO)3Cl(L)] on the basis of
literature methods.14,15
[Re(CO)3(4-Mepy)(L)]CF3SO3: [Re(CO)3Cl(L)] (0.4 mmol)
and AgCF3SO3 (124 mg, 0.48 mmol) were dissolved in
acetonitrile (100 mL). 4-Mepy was added (200 µL, 2.00 mmol)
and the resulting mixture was then refluxed for 4 days. The
solution was cooled and filtered to remove any AgCl. The
solvent was removed from the filtrate by rotary evaporation,
resulting in an oil. The oil was dissolved in either THF (phen

4882 J. Phys. Chem. A, Vol. 110, No. 14, 2006
Howell and Gordon
TABLE 1: Calculated, Observed and Literature Frequencies
of Some Re(I) Polypyridyl Complexes
held in a spinning NMR tube. Concentrations used were
typically 5-20 mmol dm-3
.
Electronic absorption spectra were recorded on a Varian Cary
kav/
∆kav/
ν˜/cm-1
1929b
complex
N m-1 N m-1 a
500 scan UV-vis-NIR spectrophotometer, with Cary WinUV
software. Samples were typically ∼10-4 mol dm-3
.
[Re(CO)3(phen)(4-Mepy)]+
calculated
2035 1559
1942 1953 2020
1896 1889 2099 1502 -57
Cyclic voltammetry samples were freshly prepared in aceto-
nitrile (1.0 mmol dm-3) with 0.1 mol dm-3 tetrabutylammonium
perchlorate. Samples were purged with nitrogen gas for 15 min
prior to taking measurements. The electrochemical cell consisted
of a 1.0 mm diameter platinum working electrode embedded
in a Kel-F cylinder with a platinum auxiliary electrode and a
Ag/AgCl reference electrode. The cyclic votammograms were
recorded using a Powerlab/4SP potentiostat with EChem v.1.5.2
software. Potentials were referenced to decamethylferrocene
(DMFc).
[Re(CO)3(phen•-)(4-Mepy)]
calculated
1899 1917 1990
[Re(CO)3(tem)(4-Mepy)]+
calculated
1925b
2032 1553
1927 1948 2017
[Re(CO)3(tem•-)(4-Mepy)]
calculated
1896b
2012 1513 -41
1895 1913 1986
1901 1918 2023 1533
1910 1931 1999
[Re(CO)3Cl(phen)]
calculated
[Re(CO)3Cl(phen•-)]-
calculated
1871b
1999 1480 -52
1873 1887 1969
1901 1917 2022 1531
1908 1929 1997
1869 1886 2000 1487 -44
1876 1891 1971
[Re(CO)3Cl(dip)]
calculated
Applied potentials reported are versus Ag/Ag+. The electronic
absorption spectra of reduced species were measured using an
OTTLE cell with a platinum grid as the working electrode.19
Spectra were acquired by applying the first reduction potential
of the complex to the sample and measuring approximately
every two minutes. Difference spectra were found by subtracting
the first spectrum from subsequent measurements for each
compound. These were then added to the electronic absorption
spectrum of the parent compound. For Raman spectra the
OTTLE cell used had a Pt grid working electrode with a section
cut out.20 The Raman scattering was measured in this part of
the cell to avoid laser reflection. The applied potential was set
beyond the first reduction potential of the complex, and data
were collected after sufficient time that the probed volume
contained only the reduced product. The OTTLE cell used for
IR spectroscopy is detailed elsewhere.21 For some samples the
spectra were measured using the minimum number of scans
due to degradation.
[Re(CO)3Cl(dip•-)]-
calculated
[Re(4,4′-(CO2Et)2bpy)(CO)3(4-Etpy)]+ c 1935b
[Re(4,4′-(CO2Et)2bpy•-)(CO)3(4-Etpy)]c 1905b
[Re(4,4′-(CO2Et)2bpy)(CO)3(MQ+)]2+ c 1939b
[Re(4,4′-(CO2Et)2bpy)(CO)3(MQ+)]2+ c 1939b
[Re(4,4′-(CO2Et)2bpy)(CO)3(MQ•)]+ c 1928b
2038 1567
2015 1524 -44
2039 1572
2039 1572
2032 1557 -15
[Re(4,4′-bpy)2(CO)3Cl]d
2026 1922 1895 1533
[Re(4,4′-bpy•-)2(CO)3Cl]- d
2012 1903 1882 1509 -24
a ∆kav upon reduction. b Bands appear broad and are assigned as e
symmetry, thus doubly degenerate. c Reference 28. d Reference 31.
TABLE 2: Calculated and Observed Band Frequencies for
Singly Reduced [Re(CO)3(4-Mepy)(phen)]+
[Re(CO)3(4-Mepy)(phen•-)] [Re(CO)3(4-Mepy)(phen•-)]
B3LYP/6-31G(d)c
experimentald
νa symmetryb
ν˜/cm-1
ν˜/cm-1
69 a1(4-Mepy)
72 a1(4-Mepy)
74 a1
77 a1
78 a1
82 a1
85 b2(4-Mepy)
86 a1
89 a1(4-Mepy)
90 b2
91 a1
1008
1064
1076
1139
1185
1225
1272
1292
1399
1403
1421
1460
1471
1507
1532
1605
1626
1018
1062
1093
1146
1161
1221
1278
1309
1389
1410
1431
1445
1489
1511
1534
1578
1626
Calculations
The geometry, vibrational frequencies and their IR and Raman
intensities were calculated using DFT calculations (B3LYP
functional) with the basis sets: 6-31G(d) and LANL2DZ (on
Re atoms). These were implemented with the Gaussian 03W22
program package. No negative frequencies were predicted
indicating that the structures obtained were at energy minima.
The visualization of the vibrational modes was provided by
GaussViewW (Gaussian Inc.).
95 a1
97 a1(4-Mepy)
99 a1(4-Mepy)
100 a1
104 a1
105 a1(4-Mepy)
Results
a Mode numbers are from the calculation. b Symmetries are based
on C2V symmetry of phen and are phen localized unless otherwise
indicated. c The LANL2DZ basis set was used for the Re atom.
d Resonance Raman data were collected with 514.5 nm excitation. Due
to the large number of bands observed only those with intensities of
>20% of the most intense band (ν100) are tabulated.
The spectroscopic studies of the Re(I) complexes studied
herein fall into two categories; first the carbonyl bands are
probed using IR spectroelectrochemistry and second the ligand-
based and ligand radical anion vibrations are examined with
resonance Raman spectroscopy. DFT calculations have been
carried out on the parent and reduced Re(I) complexes presented
herein. The calculated frequencies are presented in Tables 2
and 3 and are considered in further detail in the discussion
section. The DFT calculations carried out predicted the singly
reduced Re(I) complexes considered in this study to be species
have the labels a1 and e. Upon reduction or excitation the
coordinated nitrogens become inequivalent, reducing the sym-
metry to Cs and hence lifting the degeneracy of the modes.24,25
The ∆A IR spectrum of [Re(CO)3(4-Mepy)(phen•-)] -
[Re(CO)3(4-Mepy)(phen)]+, is shown in Figure 3a. Parent
complex carbonyl bands are evident at 1929 and 2035 cm-1 as
bleaches in the spectrum (Table 1). Upon reduction of [Re-
(CO)3(4-Mepy)(phen)]+ the carbonyl bands shift to lower
frequencies. The 1929 cm-1 band of the parent species is
resolved into two bands, at 1886 and 1889 cm-1, in the spectrum
of the reduced complex. The carbonyl band in the spectrum of
the parent complex at 2035 cm-1 is shifted to 2009 cm-1 in the
spectrum of the reduced complex. This decrease in frequency
of the carbonyl bands upon reduction is consistent with the
2
with B1 symmetry.
I. IR Spectra. In Re(I) tricarbonyl complexes with a facial
arrangement of carbonyls about the Re(I) center and with Cs
symmetry such as in the case of [Re(CO)3Cl(L)] complexes
(L ) bidentate ligand), three carbonyl bands are expected in
the IR spectrum. From highest to lowest wavenumbers these
have the symmetry labels a′(1), a′′, and a′(2), respectively.23 In
complexes with three coordinated nitrogens, such as [Re(CO)3-
(4-Mepy)(L)]+, the complex has pseudo-C3V symmetry, and
therefore two of the modes become degenerate and the bands

Spectroscopy of Re(I) 1,10-Phenanthroline Complexes
J. Phys. Chem. A, Vol. 110, No. 14, 2006 4883
TABLE 3: Calculated and Observed Band Frequencies for Reduced [Re(CO)3(4-Mepy)(tem)]+
[Re(CO)3(4-Mepy)(tem•-)]
B3LYP/6-31G(d)c
[Re(CO)3(4-Mepy)(tem•-)]
experimentald
experimentala
symmetryb
a1(4-Mepy)
a1
a1
a1
a1
a1
a1(tem + 4-Mepy)
a1(tem + 4-Mepy)
a1(tem + 4-Mepy)
a1
ν˜/cm-1
ν˜/cm-1
96
102
104
114
116
117
118
119
128
130
132
133
1211
1285
1338
1435
1464
1465
1469
1469
1509
1548
1601
1626
1208
1307
1339
1427
1444
1525
1557
1599
1616
a1
a1(4-Mepy)
a Mode numbers are from the calculation. b Symmetries are based on C2V symmetry of the phenanthroline moiety of tem and are tem localized
unless otherwise indicated. c The LANL2DZ basis set was used for the Re atom. d Resonance Raman data were collected with 457.9 nm excitation.
Figure 3. ∆A IR spectra upon reduction of (a) [Re(CO)3(4-Mepy)-
(phen)]+ and (b) [Re(CO)3(4-Mepy)(tem)]+.
occupation of a π* molecular orbital of phen. Increase in the
electron density in the π* molecular orbital of phen increases
the back-bonding to the π* molecular orbital of the carbonyl
Figure 4. Resonance Raman spectrum of singly reduced (a) [Re(CO)3-
ligands, resulting in a decrease in frequency.
(4-Mepy)(phen)]+ (λex ) 514.5 nm) and (b) [Re(CO)3(4-Mepy)(tem)]+
The ∆A IR spectrum measured of the complex [Re(CO)3(4-
(λex ) 457.9) in acetonitrile (0.1 M TBAP as supporting electrolyte).
Mepy)(tem•-)] - [Re(CO)3(4-Mepy)(tem)]+ upon application
of a reduction potential is shown in Figure 3b. Similar to the
of [Re(CO)3(4-Mepy)(phen•-)] (Figure 4a). The spectrum of the
data for [Re(CO)3(4-Mepy)(phen)]+, bleaching of the carbonyl
parent species (spectrum not shown) at 514.5 nm is dramatically
bands of the parent complex is observed at 1925 and 2032 cm-1
different to that of the reduced species. The parent species is
(Table 1). Concomitant with this, new features grow in at 1896
not in resonance at this excitation wavelength so the spectrum
and 2012 cm-1. The 1925 and 1896 cm-1 bands have signifi-
is dominated by solvent bands. The spectrum has two weak
cantly greater bandwidth than those at 2032 and 2012 cm-1
.
bands at 1061 and 1305 cm-1. Although in the spectrum of the
reduced complex there are bands close to these positions (1062
and 1309 cm-1), the intensities of these bands have clearly
increased relative to the solvent bands and there is growth of a
large number of new bands as the sample is reduced. A number
of bands are observed in the resonance Raman spectrum of
[Re(CO)3(4-Mepy)(phen•-)], these have been tabulated in
Table 2.
The lower frequency pair (1925 and 1896 cm-1) are attributed
to the e modes of the carbonyl system and those at 2032 and
2012 cm-1 are a1 modes.
The IR spectrum of [Re(CO)3Cl(phen)] has carbonyl bands
at 1901, 1917, and 2022 cm-1 (Table 1). The reduced species
has bands at 1871 and 1999 cm-1, with the former being of
greater bandwidth than the latter.
Upon application of a reduction potential the IR spectrum of
[Re(CO)3Cl(dip)] has bleaches at 1901, 1917, and 2022 cm-1
(Table 1) due to the carbonyl modes of the parent species. The
IR spectrum of [Re(CO)3Cl(dip•-)]- has carbonyl bands at 1869,
An excitation wavelength of 457.9 nm was used to collect
the resonance Raman spectrum of singly reduced [Re(CO)3(4-
Mepy)(tem)]+. The spectrum of the parent complex is of an
emission background with only one weak discernible features
a band at 1376 cm-1 due to the solvent (spectrum not shown).
The resonance Raman spectrum of [Re(CO)3(4-Mepy)(tem•-)]
1886 and 2000 cm-1
.
II. Resonance Raman Spectra. An excitation wavelength
of 514.5 nm was used to measure the resonance Raman spectrum

4884 J. Phys. Chem. A, Vol. 110, No. 14, 2006
Howell and Gordon
has eight bands observed in the 1000-1650 cm-1 region of the
N m-1. Upon reduction there is a ∆kav of -15 N m-1. The
smaller ∆kav upon reduction is indicative of a smaller change
in back-bonding compared to the complex [Re(4,4′-(CO2Et)2-
bpy)(CO)3(4-Etpy)]+.
spectrum (Figure 4b, Table 2).
Discussion
For the complex [Re(4,4′-bpy)2(CO)3Cl], the IR spectrum of
I. IR Spectra. The carbonyl band frequencies are sensitive
to the environment at the metal center due to back-bonding from
the metal center to the carbonyl ligands. Changes in the
frequencies of the carbonyl bands upon reduction in the phen,
dip and tem complexes may be used to determine the nature of
the SOMO. Occupation of the b1 molecular orbital will allow
the reducing electron to have greater overlap with the metal
center compared to the a2 orbital due to the greater electron
density on the coordinating nitrogens.
One way to quantify the extent of back-bonding in these types
of complexes is to calculate the average carbonyl force constants
(kav) for the parent and reduced species and examine the change
upon reduction (∆kav). kav is calculated using eq 1,26,27 and the
values are presented in Table 1. This approach, developed by
Stufkens et al.,27 has recently been employed by Gordon et al.
to investigate the localization of the SOMO in reduced Re(I)
tricarbonyl complexes of biquinoline and other closely related
polypyridyl ligands.21
the parent and reduced complex have been measured.31 The ∆kav
has been calculated at -24 N m-1 21
,
from the data collected
by George et al.,31 approximately half that of [Re(bpy)(CO)3Cl],
which was calculated as -44 N m-1 27
The small ∆kav for
.
reduction of [Re(4,4′-bpy)2(CO)3Cl] is attributed to the localiza-
tion of the acceptor molecular orbital on the monodentate ligand
4,4′-bpy, which can communicate with the metal via a single
coordinated nitrogen, hence transfer from the reduced ligand
to the metal center is less.
Singly reduced phen had been shown to be a species with
2B1 symmetry.3 The presence of a metal center leads to a
preferential stabilization of the b1 unoccupied molecular orbital
over that of a2 symmetry. This stabilization reduces the
configuration interaction between the close lying unoccupied
molecular orbitals.4,32 [Re(CO)3Cl(phen)] and [Re(CO)3(4-
Mepy)(phen•-)] are species with 2B1 symmetry, therefore should
have a ∆kav for reduction comparable to that calculated for
[Re(4,4′-(CO2Et)2bpy)(CO)3(4-Etpy)]+ or for [Re(bpy)(CO)3Cl].
Upon the reduction of [Re(CO)3(4-Mepy)(phen)]+ kav decreases
from 1559 to 1502 N m-1, a decrease of 57 N m-1. For
g ν 2
∑
i
i
i
kav ) 4.0383 × 10-4
(1)
[Re(CO)3Cl(phen)], the ∆kav was calculated to be -52 N m-1
.
Both phen complexes have ∆kav values for reduction of slightly
greater magnitude than the ∆kav upon reduction of [Re(4,4′-
(CO2Et)2bpy)(CO)3(4-Etpy)]+ or [Re(bpy)(CO)3Cl].
g
∑
i
i
The ordering of the molecular orbitals can be changed by
varying the substituents on phen. Kaim et al. studied the
potassium salts for the reduced ligands, 2,2′-bipyridine, 4,4′-
dimethyl-2,2′-bipyridine 1,10-phenanthroline, and 4,7-dimethyl-
1,10-phenanthroline. From the EPR data of these reduced species
they invoked b1 orbital symmetry for the SOMO.3 The radical
anion of several diaza derivatives of phen and their {M(CO)4}
complexes (M ) Cr, Mo, W) have been considered,4 as well
as methyl substituted phen and their Pt(II) complexes,33 to
determine the b1/a2 molecular orbital ordering. The nature of
the LUMO of tem and some of its complexes has been
investigated by the groups of Kaim32,33 and Vlcˇek.5,7 Substitution
at the 3, 4, 7, and 8 positions of the phenanthroline moiety cause
a switching of molecular orbitals from unsubstituted phen: tem•-
where gi ) degeneracy of the ith CO-stretching mode and νi is
the frequency of the ith CO-stretching mode (cm-1).
The change in kav (∆kav) values upon reduction (kav(reduced)
- kav(parent)) of the substituted phen complexes and a number
of reference compounds can be used to qualitatively ascertain
the extent of communication between the ligand and metal in
each reduced complex and therefore deduce the nature of the
SOMO.
Meyer et al. investigated the complexes [Re(4,4′-(CO2Et)2-
bpy)(CO)3(4-Etpy)]+ and [Re(4,4′-(CO2Et)2bpy)(CO)3(MQ+)]2+
(4,4′-(CO2Et)2bpy ) 4,4′-bis(ethylethanoate)-2,2′-bipyridine;
4-Etpy ) 4-ethylpyridine; MQ+ ) N-methyl-4,4′-bipyrindinium
cation).28 The reduced forms of each of these two complexes
exemplify the two extremessgood and poor communication to
the metal center. The reduced species were formed upon by
photoexcitation followed by reductive quenching with 10-
methylphenothiazine (10-MePTZ). In the resulting singly re-
duced [Re(4,4′-(CO2Et)2bpy)(CO)3(4-Etpy)]+, a bpy-based mo-
lecular orbital is occupied. The bpy-based molecular orbital is
of b1 symmetry and closely resembles that of the b1 LUMO of
phen.2,29,30 The resulting singly reduced [Re(4,4′-(CO2Et)2bpy)-
(CO)3(MQ+)]2+, formed by reaction of the photoproduct with
a reductive quencher, has the reducing electron on the MQ+
ligand. This reducing electron is localized on the remote
pyridinium ring, resulting in a Re(I) center bridged by a pyridyl
to a reduced pyridinium. The separation of the reduced electron
from the metal center results in very little communication and
hence a small change in the extent of back-bonding is expected.
In the case of [Re(4,4′-(CO2Et)2bpy)(CO)3(4-Etpy)]+ the kav
is calculated at 1567 N m-1, on the basis of a doubly degenerate
mode at 1935 cm-1 and a singly degenerate mode at 2038
2
is a species with A2 symmetry, as determined by Kaim et al.
through EPR studies.32 Complexation can potentially induce
orbital swapping as the metal center will stabilize the b1 orbital
more than the a2 orbital. Coordination of tem to {Pt(Mes)2}2+
(Mes ) mesityl),32,33 {Cr(CO)4}5 and {W(CO)4}5 was inves-
tigated and these complexes were all found to have LUMO’s
of a2 symmetry. Solvent-induced orbital switching was found
for the Pt(II) complex upon changing the solvent from THF to
the polar solvent DMF, evidenced by EPR studies.33
For [Re(CO)3(4-Mepy)(tem)]+, the ∆kav upon reduction is
calculated at -41 N m-1, less than that calculated for [Re(CO)3-
(4-Mepy)(phen)]+, but comparable to that calculated for [Re-
(4,4′-(CO2Et)2bpy)(CO)3(4-Etpy)]+ and much larger than for
[Re(4,4′-(CO2Et)2bpy)(CO)3(MQ+)]2+. Therefore, it is likely that
the LUMO of the tem complex is predominantly b1.
Although dip does not appear to have been studied, it has
been postulated that because the localization of the molecular
orbital at the 4 and 7 positions are similar for both the b1 and
a2 molecular orbitals (Figure 1), the stabilization of the molecular
orbitals induced by substitution at these positions would be
similar, therefore no orbital switching would occur.5 This would
cm-1 28 For [Re(4,4′-(CO2Et)2bpy•-)(CO)3(4-Etpy)] the kav is
.
1524 N m-1, calculated from modes at 1905 (e) and 2015 (a1)
cm-1. ∆kav upon reduction of this complex is -44 N m-1. For
[Re(4,4′-(CO2Et)2bpy)(CO)3(MQ+)]2+, kav is calculated at 1572

Spectroscopy of Re(I) 1,10-Phenanthroline Complexes
J. Phys. Chem. A, Vol. 110, No. 14, 2006 4885
suggest that dip•- is a species with 2B1 symmetry. The ∆kav for
the reduced species, but not the parent complexes, are enhanced.
The appropriate excitation wavelength was chosen by consider-
ing the electronic absorption spectra of the reduced species (see
Supporting Information).
reduction of [Re(CO)3Cl(dip)] was calculated to be -44 N m-1
.
This is a very similar value to that observed for the phen
complex and it supports the assignment of the SOMO of
[Re(CO)3Cl(dip)] as a b1 molecular orbital.
The resonance Raman spectra of the reduced [Re(CO)3Cl(L)]
complexes were unable to be considered in using the experi-
mental protocol described in this paper as reduction of the
[Re(CO)3Cl(L)] complexes of phen, dip and tem via application
of a reduction potential results in the rapid formation of a
number of potential byproducts.35 Although electronic absorption
and resonance Raman spectra of the reduced species takes a
number of minutes to collect, the IR spectrum of the reduced
complexes can be collected within several seconds. This
acquisition time is sufficiently short to measure the signal due
to the reduced complexes before appreciable secondary reactions
take place. The resulting spectra are comparatively noisy;
however, the frequencies of the carbonyl bands were able to be
identified. In the first several seconds the growth of 2-3 bands
due to carbonyl modes are observed, suggesting the formation
of a single species. Over a longer period of time (a couple of
minutes) the spectra show a change in the number of bands of
positive? ∆A (not shown). This is indicative of the formation
Modeling was undertaken to determine how well DFT
calculations are able to predict the changes in electron density
at the metal center. Frequency calculations were used as these
provide ready comparison between calculated and experimental
observables. Due to the overestimate of vibrational frequencies
by an average of over 100 cm-1 by the calculations it is
appropriate to introduce a scale factor.34 Due to the higher
anharmonicity of carbonyl modes relative to C-C ligand
modes,26 the two spectral regions require different scale factors.
For the polypyridyl region (1000-1650 cm-1) a scale factor of
0.97 is appropriate; however, for the carbonyl bands a scale
factor of 0.95 minimizes the absolute deviation between
experimental and calculated frequencies.
The mean absolute deviation between observed and calculated
carbonyl mode frequencies for the parent and reduced complexes
was found to be 16 cm-1. More importantly, the shifts in the
positions of the carbonyl bands upon reduction of the complexes
were predicted with a mean absolute deviation between observed
of more than one species in solution. This does not occur for
and calculated shifts of 6 cm-1
.
the [Re(CO)
+
3(4-Mepy)(L)] complexes as the 4-Mepy ligand
is less labile than Cl-.
For [Re(CO)3(4-Mepy)(phen)]+ ground-state carbonyl bands
are observed at 1929 and 2035 cm-1 with modes predicted at
1942, 1953, and 2020 cm-1 (Table 1). Upon reduction the bands
are observed to decrease by 43, 40 and 26 cm-1, respectively.
The predicted decreases are within 4 cm-1 of those observed.
The bands observed in the resonance Raman spectrum of
[Re(CO)3(4-Mepy)(phen•-)] can also be compared to those in
the resonance Raman spectra of Li+phen•-, reported by Leroi
et al.36 and Schoonover et al.37 It must be noted that these
previously published studies were carried out at different
excitation wavelengths to the 514.5 nm excitation considered
herein. The spectrum collected by Schoonover et al.37 used 406.7
nm excitation and the spectra collected by Leroi et al.36 were
measured with a series of excitation wavelengths: 351.1, 363.8,
406.8, 568, and 647 nm. The resonance enhancement pattern
for those Li+phen•- spectra is expected to be different to that
observed using 514.5 nm excitation of [Re(CO)3(4-Mepy)-
(phen•-)]. Hence, comparison of band intensities would be
invalid and some bands may be enhanced in one spectrum and
not appear at all in another. However, the vibrational frequencies
are not modulated allowing the comparison of frequencies to
be carried out. The bands at 1221, 1410, 1445, 1534, and 1578
cm-1 in the resonance Raman spectrum of [Re(CO)3(4-
Mepy)(phen•-)] are likely to be due to the same modes that
For the tem complex, [Re(CO)3(4-Mepy)(tem)]+, the bands
are slightly decreased in frequency, compared to [Re(CO)3(4-
Mepy)(phen)]+, at 1925 and 2032 cm-1. This decrease is
predicted by the calculation with modes at 1937, 1948, and 2017
cm-1 (Table 1). A decrease in the frequencies of the bands by
29 and 20 cm-1 is observed upon reduction. The calculated shifts
are all too large at 42, 35, and 31 cm-1 for the three modes.
The smaller than predicted shifts observed may be indicative
of the molecular orbital occupied by the reducing electron having
some a2 character.
For the [Re(CO)3Cl(phen)] and [Re(CO)3Cl(dip)] complexes,
carbonyl bands are observed at the same frequency, within
experimental uncertainty, at 1901, 1918, and 2023 cm-1. These
are predicted at 1910, 1931, and 1999 cm-1 for [Re(CO)3Cl-
(phen)] (Table 1). The calculation of the dip complex predicts
each band within 2 cm-1 of that calculated for the phen complex.
In the reduced form the carbonyl bands of the phen complex
shift to 1871 and 1999 cm-1. The dip complex has two bands
within experimental uncertainty at 1869 and 2000 cm-1, with
an additional band at 1886 cm-1. The calculated frequencies of
the carbonyl bands for [Re(CO)3Cl(phen•-)]- are 1873, 1887,
and 1969 cm-1. The reduced dip complex has the lowest and
highest energy carbonyl bands predicted within 3 cm-1 of those
for the phen complex. However, the middle band is predicted
at 1891 cm-1. This predicted difference of 4 cm-1 manifests as
splitting of the two degenerate modes for the dip complex,
resulting in three, not two, bands in the reduced complex.
produce bands at 1221, 1405, 1449, 1539, and 1576 cm-1
,
respectively, in the spectra of Li+phen•-, measured by Leroi et
al.36 The band at 1431 cm-1 in the spectrum of [Re(CO)3(4-
Mepy)(phen•-)] can be correlated to the Li+phen•- band
observed by Schoonover et al. at 1421 cm-1 37
Both studies
.
report a number of additional Li+phen•- bands that cannot be
readily correlated to the bands observed in the resonance Raman
spectrum of [Re(CO)3(4-Mepy)(phen•-)], for example the
prominent bands at 1130, 1272, and 1585 cm-1 observed by
Schoonover et al.37 However, the lack of enhancement of these
modes in the resonance Raman spectrum of [Re(CO)3(4-
Mepy)(phen•-)] may be due to an absence of enhancement of
these bands at this particular wavelength.38
II. Resonance Raman Spectra. Resonance Raman spectros-
copy has been used to probe the 1000-1650 cm-1 region, as
bands from the supporting electrolyte swamps infrared spectra
in this region. Because the electrolyte bands are not resonantly
enhanced at the excitation wavelengths used, the electrolyte is
spectroscopically silent in the region of interest and the spectra
of the reduced species more readily collected than with IR
spectroscopy. The excitation wavelength is chosen such that
Gordon et al. reported the resonance Raman spectrum of the
metal-to-ligand charge-transfer excited state of [Cu(phen)-
(PPh3)2]+, which can be denoted as [CuII(phen•-)(PPh3)2]+*.
Three bands were observed, which were assigned to phen•-
modes. These occurred at 1453, 1551, and 1585 cm-1. The
lowest energy band may be correlated to the band at 1445 cm-1
in the resonance Raman spectrum of [Re(CO)3(4-Mepy)(phen•-)]

4886 J. Phys. Chem. A, Vol. 110, No. 14, 2006
Howell and Gordon
Figure 5. Eigenvectors for selected normal modes of [Re(CO)3(4-Mepy)(phen•-)] and [Re(CO)3(4-Mepy)(tem•-)].
and the highest energy band correlated to one at 1578 cm-1
.
The combination of IR and resonance Raman spectroscopy
of the reduced complex, in concert with the DFT calculations
support the assignment of the SOMO of [Re(CO)3(4-Mepy)-
(phen•-)] being b1 rather than a2 in nature.
Although there is also a band at 1511 cm-1 in the resonance
Raman spectrum of [Re(CO)3(4-Mepy)(phen•-)], this has been
assigned as a 4-Mepy mode (vide supra).
The appropriateness of use of B3LYP/6-31G(d) (with
LANL2DZ ECP for Re atoms) calculations for the polypyridyl
ligand area of the vibrational spectra of Re(I) complexes was
established by comparing the IR and Raman spectra of
[Re(CO)3Cl(phen)] and [Re(CO)3(4-Mepy)(phen)]+ to the fre-
quencies calculated (see Supporting Information). For both
complexes the mean absolute deviation between calculated and
observed frequencies is 5 cm-1. Only small shifts in frequency
(<7 cm-1) and relative intensities are observed for bands
common to both complexes, suggesting that substitution of the
Cl- ligand for 4-Mepy causes little perturbation to the poly-
pyridyl ligand.
DFT theory calculations have allowed the bands observed in
the resonance Raman spectrum of [Re(CO)3(4-Mepy)(phen•-)]
to be assigned. A number of bands are identified as 4-Mepy-
localized modes. These appear at 1018, 1062, 1389, 1489, and
1626 cm-1 in the resonance Raman spectrum and are calculated
at 1008 (ν69), 1064 (ν72), 1399 (ν89), 1471 (ν97), 1511 (ν99),
and 1626 (ν105) cm-1, respectively. The remaining bands are
assigned to phen•- localized modes. These occur at 1093, 1146,
1161, 1221, 1278, 1309, 1410, 1431, 1445, 1511, 1534, 1507,
and 1578 cm-1 (Table 2). Most of the bands observed are
attributed to bands with a1 symmetry (assuming C2V point group
for phen). The exceptions are the bands at 1278 (ν85) and 1410
(ν90) cm-1, which have medium intensity at this excitation
wavelength and are 4-Mepy- and phen-localized modes, re-
spectively. The most intense band in the spectrum is observed
at 1534 cm-1, predicted at 1532 cm-1 (ν100). This is a
delocalized mode, involving atoms over the three rings in phen.
Medium intensity bands observed at 1431 and 1445 cm-1 are
predicted at 1421 (ν91) and 1460 (ν95) cm-1, respectively. ν91
can also be described as a delocalized phen mode, whereas ν95
is based in the central ring of phen. The shifts observed on going
from parent to reduced species are consistent with the population
of a b1 SOMO.
Most of the bands in the resonance Raman spectrum of
[Re(CO)3(4-Mepy)(tem•-)] have been assigned as tem•- based
modes (Table 3). All have a1 symmetry, assuming C2V symmetry
of the phenanthroline moiety of the tem ligand. The mean
absolute deviation, between calculated (B3LYP/6-31G(d)) and
observed frequencies, is 10 cm-1. The bands at 1208 and 1616
cm-1 have been assigned as 4-Mepy modes, predicted at 1211
(ν96) and 1626 (ν133) cm-1, respectively. The strongest bands
in the region 1000-1650 cm-1 are at 1307, 1427, and 1444
cm-1, predicted at 1285 (ν102), 1435 (ν114), and 1464(ν116) cm-1
.
The band at 1444 cm-1, which has been attributed to ν116, may
actually be due to any or all of the four modes predicted between
1464 and 1469 cm-1
.
As reduced [Re(CO)3(4-Mepy)(tem)]+ and [Re(CO)3(4-
2
Mepy)(phen)]+ are both B2 species, it might be expected that
they have bands in common due to the reduced phenanthroline
moiety.
The bands at 1307, 1427, 1444, and 1525 (ν128) cm-1 in the
resonance Raman spectrum of [Re(CO)3(4-Mepy)(tem•-)] are
also seen in the resonance Raman spectrum of [Re(CO)3-
(4-Mepy)(phen•-)] at 1309, 1431, 1445, and 1534 cm-1
,
respectively. The band at 1534 cm-1 in the resonance Raman
spectrum of [Re(CO)3(4-Mepy)(phen•-)] exhibits the largest
shift from the phen to tem complex. This may be due to the
change in character of the mode. In the phen complex, the mode
is due to the phen moiety only, whereas in the tem complex
there is come contribution to the mode from motion of atoms
in the 4-Mepy ligand (Figure 5). Visualization of the modes
reveals that ν102 and ν114, of [Re(CO)3(4-Mepy)(tem•-)], are
delocalized over the three rings of the phenanthroline moiety
of tem•- and involve some motion of the methyl groups in
tem•-. The modes of [Re(CO)3(4-Mepy)(phen•-)] at 1309 and
1431 cm-1 are delocalized over the three aromatic rings of
phenanthroline.

Spectroscopy of Re(I) 1,10-Phenanthroline Complexes
J. Phys. Chem. A, Vol. 110, No. 14, 2006 4887
(20) Waterland, M. R.; Gordon, K. C.; McGarvey, J. J.; Jayaweera, P.
M. J. Chem. Soc., Dalton Trans. 1998, 609.
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Conclusions
This study has shown that the nature of the acceptor molecular
orbital of phenanthroline-based Re(I) complexes can be deduced
from IR spectra by determining the ∆kav for the carbonyl bands
upon reduction. These assignments have been supported by
resonance Raman spectroelectrochemistry and DFT calculations,
which have been used to investigate the polypyridyl ligand anion
vibrational modes. DFT calculations predict the complexes
considered as 2B2 species. The mean absolute deviation between
calculated and observed vibrational frequencies in the 1000-
1650 cm-1 region was found to be 10 cm-1. The singly reduced
[Re(CO)3Cl(phen)], [Re(CO)3Cl(dip)], [Re(CO)3Cl(tem)], [Re-
(CO)3(4-Mepy)(phen)]+, and [Re(CO)3(4-Mepy)(tem)]+ com-
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N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
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D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A.
G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
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(26) Braterman, P. S. Metal carbonyl spectra; Academic Press Inc.
London Ltd: London, 1975.
2
plexes were all found to be B2 species.
Supporting Information Available: Electronic absorption
spectra of the reduction product of [Re(CO)3(4-Mepy)(phen)]+
and the reduction product of [Re(CO)3(4-Mepy)(tem)]+. Table
of calculated and observed vibrational frequencies of [Re(CO)3-
Cl(phen)]. This material is available free of charge via the
Internet at http://pubs.acs.org.
References and Notes
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(30) The b1 LUMO of phen has no electron density on at the C5 and
C6 positions. This molecular orbital only has density on the bpy moiety of
phen.
(31) George, M. W.; Johnson, F. P. A.; Westwell, J. R.; Hodges, P. M.;
Turner, J. J. J. Chem. Soc., Dalton Trans. 1993, 2977.
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(38) The electronic absorption spectrum of phen•- has been previously
reported and there are a number of bands.39,40 As a lithium salt in THF,
these lie at 394, 582 and 631 nm with a shoulder at 538 nm as reported by
Schoonover et al.37 Our electronic absorption spectrum of [Re(CO)3(4-
Mepy)(phen•-)] has a broad absorption in the visible with a shoulder at
∼540 nm and extensive absorption out to 650 nm. The resonance Raman
data we report are in resonance with the low energy shoulder rather than
the stronger 390 nm band. In our case measurements within the higher
energy chromophore would be complicated by the fact that the parent species
also absorbs and thus gives resonance Raman scattering at those shorter
wavelengths.
(16) Shriver, D. F.; Dunn, J. B. R. Appl. Spectrosc. 1974, 28, 319.
(17) McCreery, R. L. Raman Spectroscopy for Chemical Analysis; John
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(18) The ASTM subcommitte on Raman spectroscopy has adopted eight
materials as Raman shift standards (ASTM E 1840). The band wavenumbers
for these standards are available at http://chemistry.ohio-state.edu/∼rmccreer/
shift.html.
(39) Kato, T.; Shida, T. J. Am. Chem. Soc. 1979, 101, 6869.
(40) Malueg, V. J. M.A. Thesis, University of Texas at Austin, 1981.
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