Molecular and electronic structure of MM quadruply bonded complexes containing O2CC6H4N(Ph)2 supporting ligands

Article abstract of DOI:10.1016/j.poly.2013.06.002

The complexes trans-M₂(TⁱPB)₂(O ₂CC₆H₄-4-NPh₂)₂ where M = Mo (I), and M = W (II) were prepared from the reaction between M ₂(TⁱPB)₄ and the carboxylic acid Ph ₂N-4-C₆H₄CO₂H (～2 equiv.) in toluene (TⁱPB = 2,4,6-triisopropylbenzoate). The compounds were isolated as microcrystalline powders that are yellow, I or purple II, and are air-sensitive. Compound I was characterized by single-crystal X-ray crystallography and shown to have a centrosymmetric structure and a planar central C₆H₄-CO₂M₂O ₂C-C₆H₄ unit. Calculations indicate that the HOMO is principally M₂δ with mixing of the filled π-orbitals of the N,N-diphenyl-4-aminobenzoate. Oxidation of I and II yield cationic species that have been characterized by EPR and UV-Vis-NIR spectroscopy. The radical cations show EPR spectra characteristic of M₂⁵⁺ centers having the MM configuration σ²π⁴δ ¹ and compound I⁺ shows a low energy NIR transition assignable to a ligand to metal charge transfer transition. The homoleptic compound Mo₂(O₂CC₆H₄-4-N-Ph ₂)₄, III, was prepared from the reaction between Mo(CO)₆ and Ph₂N-4-C₆H₄CO ₂H in refluxing ortho-dichlorobenzene and is shown to exhibit similar spectroscopic features to those of I.

Full text of DOI:10.1016/j.poly.2013.06.002

Polyhedron 64 (2013) 339–345
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Molecular and electronic structure of MM quadruply bonded complexes
containing O CC H N(Ph) supporting ligands
2
6
4
2
⇑
Malcolm H. Chisholm , Christopher B. Durr, Sharlene A. Lewis
Department of Chemistry and Biochemistry, The Ohio State University, 100 W. 18th Ave., Columbus, OH 43210, USA
a r t i c l e i n f o
a b s t r a c t
i
Article history:
The complexes trans-M (T PB) (O CC H -4-NPh ) where M = Mo (I), and M = W (II) were prepared from
2
2
2
6
4
2 2
Received 30 January 2013
Accepted 6 June 2013
Available online 20 June 2013
i
the reaction between M
(
2
(T PB)
4
and the carboxylic acid Ph
2
N-4-C
6
H
4
CO
2
H (ꢀ2 equiv.) in toluene
i
T PB = 2,4,6-triisopropylbenzoate). The compounds were isolated as microcrystalline powders that are
yellow, I or purple II, and are air-sensitive. Compound I was characterized by single-crystal X-ray crys-
tallography and shown to have a centrosymmetric structure and a planar central C -CO C-C
unit. Calculations indicate that the HOMO is principally M d with mixing of the filled -orbitals of the
N,N-diphenyl-4-aminobenzoate. Oxidation of I and II yield cationic species that have been characterized
6
H
4
2
M
O
2 2
6 4
H
Keywords:
2
p
MM quadruply bonded complexes
Molecular and electronic structure
Mixed valence
5+
by EPR and UV–Vis–NIR spectroscopy. The radical cations show EPR spectra characteristic of M
2
centers
and compound I shows a low energy NIR transition assignable to a
ligand to metal charge transfer transition. The homoleptic compound Mo (O CC -4-N-Ph , III, was
prepared from the reaction between Mo(CO) and Ph N-4-C CO H in refluxing ortho-dichlorobenzene
and is shown to exhibit similar spectroscopic features to those of I.
2
4
1
+
having the MM configuration
r p d
2
2
6
H
4
2 4
)
6
2
6
H
4
2
Ó 2013 Elsevier Ltd. All rights reserved.
1
. Introduction
Kubiak has shown, may be both solvent and temperature depen-
dent [5,6].
Mixed valence molecular species arise when two redox active
Although major attention has been devoted to inorganic sys-
tems, the phenomenon is common to organic chemistry and some
prime examples of recent studies are shown in Scheme 1. The
amine based cations have been extensively studied by Lambert
[7] and the anions by Nelsen et al. [8]. The 1,4-phenylene bridged
cation represents a relatively rare example of a fully delocalized
organic cation and this probably accounts for the fact that N,N -
tetraaryl-1,4-phenylenediamines are employed in optoelectronic
devices as hole transport materials [9].
metal centers are linked by a bridging group and there is an overall
odd charge. In inorganic chemistry this has been a topic of consid-
erable interest for over four decades and the Creutz-Taube ion [1]
5+
[
(NH
3
5
) -Ru(
l
-pyrazine)Ru-(NH
3
)
5
]
represents perhaps the classic
case for the question of where the odd positive charge rests. On the
Robin and Day classification [2] a fully delocalized ion, one where
the two redox centers share the odd charge equally, is class III
while one where the charge is trapped is class I. Class II represents
a situation where the charge is localized, but thermal energy or
light may induce charge transfer. The two redox centers are to
some degree coupled electronically. Here Hush theory [3] predicts
the appearance of an optical transition that is solvent dependent;
since this inter-valence charge transfer, IVCT, is photoinduced
and is accompanied by a dipole change. As the electronic coupling
becomes stronger the energy required to promote this electron
exchange becomes less and the distinction between class II and
class III becomes a matter of the timeframe of the experimental
technique employed. Indeed, much attention in the past decade
has been directed to the study of mixed valence, MV, ions that
are close to this II/III border. The optical properties of these ions
have distinct features. [4] Also the rate of electron exchange can
be monitored on the IR time-scale in certain instances and, as
0
We have recently been employing MM quadruply bonded units to
study the properties of MV ions close to the II/III border [10]. Com-
pounds of the form [M
sents a conjugated organic group coupled to the M
2
]-L-[M
2
] and trans-L-M
2
-L, where ‘‘L’’ repre-
center(s),
2
provide upon oxidation and reduction, respectively, MV ions that
are metal or ligand centered. The spectral feature of the anions
formed upon reduction of the isonicotinate complexes trans-M
(T PB) [O CC H N-B(C H ) ] where T PB = 2,4,6-triisopropylbenzo-
2 2 5 4 6 5 3 2
2
i
i
ate are shown in Fig. 1 and represent MV anions that are class III (fully
delocalized) for M = W and close to the II/III border for M = Mo [11].
Following this earlier study we were curious concerning the possible
nature of the dimetallated amine shown schematically in Scheme 2
below in relation to the phenylenediamine cation shown in Scheme 1.
We describe here our preparation of both the bis–bis complexes
i
trans-M
2
(T PB)
2
[O
2
CC
6
H
4
-4-N(Ph)
[O
their electrochemical and spectroscopic properties.
2
]
2
(where M = Mo and W) and
the homoleptic complex Mo
2
2
CC -4-N(Ph) together with
H
6 4
2 4
]
⇑
Corresponding author. Tel.: +1 614 688 3625; fax: +1 614 292 0368.
E-mail address: chisholm@chemistry.ohio-state.edu (M.H. Chisholm).
0
277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.06.002

3
40
M.H. Chisholm et al. / Polyhedron 64 (2013) 339–345
Scheme 1. Organic mixed valence systems.
2.2. Electronic absoption spectra
UV–Vis–NIR electronic spectra in THF solutions were measured
at room temperature using
spectrometer.
a Perkin–Elmer Lambda 900
2.3. Electrochemical studies
Cyclic voltammograms were collected at a scan rate of 50 mV/s
using a Princeton Applied Research (PAR) 173A potentiostat-galva-
nostat equipped with a PAR 176 current-to-voltage converter. The
measurements were performed under an argon atmosphere in a
n
0
.1 M solution of Bu
4
NPF
6
in THF or CH
2
Cl
2
inside a single-com-
partment voltammetry cell that was equipped with a platinum
wire auxiliary electrode, a platinum working electrode and a pseu-
n
doreference electrode consisting of a silver wire in 0.1 M Bu
4
NPF
6
/
n
4 6 2 2
THF or 0.1 M Bu NPF /CH Cl separated from the bulk solution by
i
+
Fig. 1. Taken from reference [10]Spectral features of reduced trans-M
2
(T PB)
2
[O2-
a Vycor tip. The potentials are referenced to the Cp
2 2
Fe/Cp Fe
CC N-B(C where M = Mo (black) and M = W (red). (Color Online.)
5
H
4
6 5 3 2
H ) ]
couple.
2.4. Mass spectrometry
Matrix assisted laser desorption ionization time-of-flight (MAL-
DI-TOF) mass spectra were obtained on a Bruker Microflex mass
spectrometer. Dithranol was used as the matrix.
2.5. Electronic structure calculations
The model complexes were optimized in the gas-phase using
density functional theory (DFT) utilizing the GAUSSIAN09 suite of pro-
grams [12,13]. The B3LYP [14,15] functional was used in conjunc-
tion with the SDD energy consistent pseudopotentials and the SDD
energy consistent basis set for for molybdenum and tungsten and
the 6-31G⁄ basis set for H, C, N and O atoms. Vibrational frequency
analysis was used to confirm that the optimized structures were
minima on the potential energy surface. GAUSSVIEW isosurface con-
tour plots are shown with an isovalue of 0.02 [15]. Electronic
absorption spectra were calculated using the time dependent
DFT (TDDFT) method.
2.6. Crystallographic information
Scheme 2. Structure of the dimetallated amine.
Single crystals of I were isolated as bright yellow plates and
handled under a pool of fluorinated oil. Examination of the diffrac-
tion pattern was done on a Nonius Kappa CCD diffractometer with
2
. Materials and methods
Mo Ka radiation. All work was done at 200 K using an Oxford Cryo-
2.1. NMR
systems Cryostream Cooler.
Data integration was done with Denzo, and scaling and merging
of the data was done with Scalepack [16]. The structure was solved
by the direct methods program in SHELXS-97 [17]. Full-matrix least-
NMR spectra were recorded on a 400 MHz Bruker DPX Advance
00 spectrometer. All H NMR chemical shifts are in ppm relative
1
4
2
to the protio impurity in THF-d
8
at 3.58 ppm.
squares refinements based on F were performed in SHELXL-97, as

M.H. Chisholm et al. / Polyhedron 64 (2013) 339–345
341
incorporated in the WINGX package [18]. For each methyl group, the
hydrogen atoms were added at calculated positions using a riding
model with U(H) = 1.5Ueq (bonded carbon atom). The rest of the
hydrogen atoms were included in the model at calculated positions
using a riding model with U(H) = 1.2Ueq (bonded atom). Neutral
atom scattering factors were used and include terms for anoma-
lous dispersion [19].
benzene (20 mL) and THF (5 mL). The solution went from yellow
to dark ruby red. The THF was removed in vacuo and the precipitate
was washed with copious amounts of hexanes to yield an orange-
brown solid. The solid was filtered and recrystallized from THF to
yield orange crystals. (Yield 86%) NMR (THF-d
(d, 8H, JHH = 8.5 Hz), 7.26 (t, 16H, JHH = 7.3 Hz), 7.11 (d, 16H,
HH = 8.0 Hz), 7.04 (m, 16H). UV–Vis (THF, 293 K) 448, 328 nm.
8
, 400 MHz): 8.11
J
i
Several supporting isopropyl groups on the T PB moiety as well
2 2 6 4 2 4
MALDI-TOF: Found: 1345.5 m/z. Mo [O CC H N(Ph) ] Requires:
as the axially coordinated THF molecules were disordered and
modeled appropriately. A detailed discussion of their refinement
can be found embedded in the accompanying CIF.
1348.22 m/z.
3
. Results and discussion
2.7. EPR studies
3.1. Syntheses
EPR measurements were carried out at X-band (9.5 GHz). The
spectra were obtained using a Bruker EMX + EPR spectrometer.
The samples were prepared by partially oxidizing approximately
i
The compounds trans-M
2
(T PB)
2
(O
2
CC
6
H
4
-4-NPh
2
)
2
where
i
M = Mo or W and T PB = 2,4,6-triisopropylbenzoate, were prepared
from the reaction between the homoleptic compounds M
and 2 equivalents of the carboxylic acid Ph N-4-C CO H in tolu-
i
2 4
(T PB)
1
0 mg of compounds I–III in a 5 mL solution of THF with a trace
. The samples were enclosed in a thin
2
H
6 4
2
amount (ꢀ2 mg) of AgPF
6
ene. The compounds I (M = Mo) and II (M = W) precipitated from
solution during the reaction and were collected by centrifugation.
The microcrystalline powders were washed with toluene and
hexanes and dried under a dynamic vacuum. Compound I is yellow
and compound II is purple. Both compounds are soluble in THF and
are air-sensitive. Crystals of I were grown by layering a saturated
THF solution with hexanes. All reactions and procedures involved
the use of an argon atmosphere with dried and degassed solvents.
EPR tube sealed with a Kontes top. All samples were run at room
temperature.
2
.8. Syntheses
N,N-diphenyl-4-aminobenzoic acid was prepared according to
literature protocols by Vilsmeier formylation of triphenylamine
[
20] followed by oxidation of the monoaldehyde with potassium
i
i
The homoleptic compound Mo (O CC H -4-NPh ) was prepared
from the reaction between Mo(CO)
permanganate [21]. Mo
2
(T PB)
4
[22] and W
2
(T PB)
4
[23] were pre-
2
2
6
4
2 4
6
and Ph
2 6 4 2
N-4-C H CO H in
pared by reported procedures.
refluxing ortho-dichlorobenzene and THF. After 3 days this yielded
a red solution. THF was removed under vacuum and the resulting
precipitate was washed with hexanes to yield an orange-brown
microcrystalline solid.
i
2
.9. Mo
2
(T PB)
2
[O
2
CC
6
H
4
-4-N(Ph)
2
]
2. (I)
i
Mo
2
(T PB)
4
(0.200 g, 0.169 mmol) was dissolved in toluene (ca.
CC N(Ph)
2
5
mL). This solution was added to a suspension of HO
2
H
6 4
(
0.100 g, 0.345 mmol) in toluene (ca. 5 mL). The orange solution
i
3
.2. Single crystal and molecular structure of trans-Mo
2
(T PB)
2
formed initially changed shortly afterwards to a bright yellow sus-
pension. The reaction was stirred for 3 days. The solid was col-
lected by centrifugation and washed twice with toluene (ca.
[O
2
CC N(Ph) , I
6
H
4
2 2
]
ꢀ
Compound I crystallized in the space group P1 and contains a
5
mL) and once with hexanes (ca. 10 mL) to yield a canary yellow
powder. (Yield 65%) Crystals were grown by layering a saturated
THF solution with hexanes. NMR (THF-d 400 MHz): 8.26 (d, 4H,
HH = 8.6 Hz), 7.38 (t, 8H, JHH = 8.0 Hz), 7.20 (m, 16H), 7.07 (s, 4H),
center of inversion. There are two disordered solvent THF mole-
cules that are coordinated along the MM axis. Two of the isopropyl
moieties also showed disorder over two positions and were mod-
eled accordingly. An ORTEP drawing of I can be seen below in
Fig. 2. From the ORTEP plot we can clearly see the trans nature of
the ligands which are related by a center of inversion. Furthermore,
8
J
3
(
2
1
.12 (septet, 4H, JHH = 7.7 Hz), 2.94 (septet, 2H, JHH = 7.1 Hz) 1.30
d, 12H, JHH = 6.8 Hz), 1.15 (d, 24H, JHH = 6.8 Hz). UV–Vis (THF,
93 K) 438, 326 nm. MALDI-TOF: Found: 1262.3 m/z. Requires
267.30 m/z.
i
it is clear that the aryl ring of the T PB moiety is twisted signifi-
cantly out of the plane of its CO
it from conjugation. In contrast, the C
is virtually planar to its CO
for extensive L -Mo d-L
been seen for compounds of the form trans-M
2
group; thus effectively removing
moiety of the TPA ligand
6 4
H
i
2
.10. W
2
(T PB)
2
[O CC H
2 6 4
2
N(Ph) ]
2. (II)
2
group (torsion angle ꢀ7°) which allows
p
2
p
conjugation. A similar geometry has
i
W
2
(T PB)
4
(0.220 g, 0.162 mmol) was dissolved in toluene (ca.
CC -4-
(0.088 g, 0.304 mmol) in toluene (ca. 5 mL). The purple sus-
i
2
(T BP)
2
L
2
where L
5
mL). This solution was added to a suspension of HO
2
6 4
H
is para-cyanobenzoate [24], vinyl-thienyl carboxylate [25], and
-carboethoxy-2-azulene carboxylate [26]. Full structural data
N(Ph)
2
6
pension formed initially darkened to a dark purple color over time.
The reaction was stirred for 3 days. The solid was collected by centri-
fugation and washed twice with toluene (ca. 5 mL) and once with
hexanes (ca. 10 mL) to yield a purple powder. (Yield 51%) NMR
are given in the Supporting information.
3.3. Electronic structure calculations
(
THF-d
Hz), 7.20 (m, 12H), 7.12 (t, 4H, JHH = 7.4 Hz), 7.07 (s, 4H), 2.93 (m,
H), 1.29 (d, 12H, JHH = 7.0 Hz), 1.09 (d, 24H, JHH = 7.0 Hz). UV–Vis
THF, 293 K) 609, 413, 334, 293 nm. MALDI-TOF: Found: 1438.6 m/
z. Requires: 1443.1 m/z.
8
400 MHz): 8.04 (d, 4H, JHH = 8.4 Hz), 7.36 (t, 8H, JHH = 8.1 -
In order to assist in the interpretation of the spectroscopic data
0
0
6
(
we have carried out calculations on the model compounds I and II ,
i
respectively, where formate substitutes for T PB ligands. This saves
on computational time and since the aryl groups of the T PB ligands
i
are twisted, extensive conjugation is not possible as it is for the
0
0
2.11. Mo [O CC
2 2 6
4 2
H N(Ph) ]4. (III)
trans-O
2
CC
6
H
4 2
-4-NPh ligands. The calculations for I and II con-
verged in C
1
symmetry. The frontier orbitals are shown in Fig. 3
0
HO CC
2
6 4
H N(Ph)
2
(0.90 g, 3.11 mmol) and Mo(CO)
6
(0.411 g,
for both the molybdenum and tungsten model compounds, I and
0
1
.56 mmol) were refluxed for 3 days in a solution of 1,2-dichloro-
II . The calculated orbital energies are shown in Table 1.

3
42
M.H. Chisholm et al. / Polyhedron 64 (2013) 339–345
Table 1
Calculated orbital energies of the frontier.
Orbital energies (eV)
0
0
II
Molecular orbital
I
LUMO+2
LUMO+1
LUMO
HOMO
HOMO-1
HOMO-2
ꢁ1.10
ꢁ1.38
ꢁ1.61
ꢁ4.67
ꢁ5.23
ꢁ5.39
ꢁ0.87
ꢁ1.07
ꢁ1.45
ꢁ4.24
ꢁ5.22
ꢁ5.32
in-phase ligand based combination which cannot interact with
the M d orbital whereas the HOMO-2 is the out-of-phase counter-
part that has a symmetry match with the M d. Again the orbital
2
2
mixing in the HOMO-2 appears greater for the molybdenum com-
plex and this is further supported by the relative energy differences
0
calculated for the HOMO-1 and HOMO-2: 0.16 eV for I versus
0
0
.10 eV for II . This reflects the closer energy levels of the Mo
2
d
Fig. 2. Thermal ellipsoid plot of I shown at 50% probability. Hydrogen atoms,
solvent and disorder omitted for clarity. (Mo = Green, N = Blue, O = Scarlet,
C = Gray). (Color Online.)
0
and the ligand
p
orbitals relative to the W
2
d. The HOMO in I is
0
at ꢁ4.67 eV and at ꢁ4.24 eV for II .
⁄
For molybdenum the LUMO is the Mo
2
d
orbital and the
LUMO+1 and LUMO+2 orbitals are the in-phase and out-of-phase
⁄
For both molecules the HOMO has significant M
with some ligand contribution from the O CC -4-NPh
This mixing is more pronounced for molybdenum and involves a
ligand to metal antibonding interaction from the filled CO orbi-
tals which also mix with a filled ‘e’ type orbital of the phenyl group
and the nitrogen lone pairs. A view of this orbital interaction is
shown in Fig. 4 which is taken at right angles to the one in Fig. 3.
2
d character
ligand
p
orbitals. For tungsten the in-phase and out-of-phase li-
⁄
2
H
6 4
2
ligands.
gand
p
combinations represent the LUMO and LUMO+1, respec-
tively and the LUMO+2 is the W
the correct symmetry to interact (back-bond) with the out-of-
phase ligand
⁄
2
d
2
orbital. The M d orbital has
2
p
⁄
p
orbital and, as seen in Fig. 3, this orbital has some
2
M d character. Moreover, the energy separation between the in-
⁄
and out-of-phase ligand
ing from the M center. This is greater for tungsten than molybde-
num: 0.23 eV for I versus 0.33 for II .
p orbitals is a reflection of the back-bond-
The HOMO-1 and HOMO-2 are both ligand-based filled
p
2
0
0
orbitals which also involve the –NPh groups. The HOMO-1 is an
2
0
0
Fig. 3. Frontier MOs and calculated energies of model compounds I (left) and II (right).

M.H. Chisholm et al. / Polyhedron 64 (2013) 339–345
343
0
0
Fig. 4. Orbital interactions in the HOMO of model compounds I (left) and II (right).
Fig. 5. Select GAUSSVIEW isosurface contour plots and energies of compound III.
Calculations were also performed on the homoleptic compound
III where no approximations were introduced. The HOMO is again
reversible one electron oxidations waves at 106 and 630 mV. The
splitting between the two peaks is 523 mV. No reduction peaks
are observed within the solvent window.
a Mo
gand filled
filled ligand
tal, the HOMO-4 has a significant Mo
Mo
interactions with the filled ligand
2
d orbital but now has a significant admixture from all four li-
-orbitals as shown in Fig. 5. Below the HOMO lie four
combinations of which only the lowest energy orbi-
d contribution. The HOMO
p
p
Compounds I, II and the ligand Ph
by cyclic voltammetry in CH Cl solutions. The voltammograms of
Ph N-4-C CO H showed one oxidative peak at 754 mV. This cor-
2 6 4 2
N-4-C H CO H were studied
2
2
2
2
H
6 4
2
2
d orbital is raised in energy to ꢁ4.29 eV by its anti-bonding
responds to the removal of one of the lone pair electrons on the
nitrogen atom. There are two reductive peaks occurring at 622
and 466 mV. Oxidation of triphenylamine yields a radical cation.
In triphenylamine complexes in which the para phenyl positions
are partially or completely unsubstituted, this radical cation
rapidly dimerizes to form the tetraphenylbenzidine (TPB) dimer
[27]. The CV of this dimer exhibits two reversible one electron
oxidations which occur at redox potentials below the oxidation
p
orbitals. The calculated energy
0
is closer to that calculated for the tungsten complex II than for the
0
⁄
molybdenum complex I . The LUMO is the Mo
2
d
orbital and the
⁄
next four orbitals are ligand based
the four ligand based filled
HOMO-5. Only the LUMO+4 has a significant Mo
p
orbitals that complement
p
orbitals, the HOMO-1 through
2
d character. It is
the metal-ligand antibonding complement of the HOMO which
⁄
2
has Mo d to ligand
p
bonding character. The HOMO, HOMO-4
2 6 4 2
of triphenylamine [28]. The CV of Ph N-4-C H CO H suggests that
and LUMO, LUMO+4 orbitals are shown in Fig. 5 and the other fron-
tier orbitals are shown in the Supporting information along with
their calculated energies.
the compound dimerizes upon oxidation and the two peaks seen
during the down sweep are due to the two one electron reductions
of the oxidized dimer. Compounds I and II both show two oxidative
peaks. The first oxidation occurs at E1/2 of 9 and ꢁ555 mV for I and
II respectively (Fig. 7). This corresponds to the removal of an elec-
3.4. Electrochemical studies
tron from the M
compound II than compound I by 546 mV which is consistent with
the higher energy of the W d orbital. The second oxidation occurs
2
d orbital. This oxidation occurs more easily in
1
,4-Bis(diphenylamino)benzene was studied by cyclic voltam-
metry in CH Cl solutions (Fig. 6). The voltammograms show two
2
2
2
2 2 2
Fig. 6. Cyclic voltammogram showing the oxidative (left) and reductive (right) waves of 1,4-bis(diphenylamino)benzene in CH Cl . Potentials are referenced to the Cp
Fe0/+1
couple.

3
44
M.H. Chisholm et al. / Polyhedron 64 (2013) 339–345
2 6 4 2
Fig. 9. Electronic absorption spectra of the ligand, HO CC H -4-NPh (black), and
Fig. 7. Cyclic voltammograms showing the oxidation of the metal centers of
compounds I (green) and II (purple) in CH Cl . Potentials are referenced to the
Cp Fe / couple. (Color Online.)
2
compounds (green), II (purple), and III (red). Spectra were taken at room
temperature in THF. (Color Online.)
I
2
2
0
+1
molybdenum compound and the MLCT of compound III is more in-
tense than that of compound I.
at E1/2 of 774 and 753 mV for I and II respectively (Fig. 8). This cor-
responds to the removal of one of the nitrogen lone pair electrons
on the ligand. The CV of compounds I and II show no reductions
within the solvent window.
3.5.2. Oxidized compounds
ꢁ
The absorption spectra of compounds I–III and the PF salts of
6
+
+
+
I , II , and III are shown in Fig. 10. In addition to the absorption
bands observed in the neutral complexes, compounds I and III+
show broad absorptions centered at 818 and 752 nm, respectively.
+
3
3
.5. Electronic absorption spectra
.5.1. Neutral compounds
These are assignable to ligand
2
p to M d transitions due to the hole
The room temperature absorption spectra of compounds I–III
on the Mo center. The higher energy absorption bands are red
2
and the ligand HO
displays two absorption bands which are due to pp transitions.
Compounds I–III all show intense absorptions in the visible region
2
CC
6
H
4
-4-NPh
2
are displayed in Fig. 9. The ligand
shifted relative to their position in the neutral compounds.
⁄
+
Compound II shows no new transitions. The MLCT band is not ob-
served and the remaining absorption bands are red shifted relative
to their positions in the neutral compound.
of the spectrum. For compounds I and III these are attributed to
⁄
2
Mo d to O
CC H
2 6 4
-4-NPh
2
p
transitions. Compound II shows two
absorption bands in the visible region. The lower energy band is as-
3.6. EPR spectra
⁄
signed to the W
2
d to O
2
CC H
6 4
-4-NPh
2
p
transition and the higher
⁄
i
energy band is assigned to the W
2
d to CO
2
p
transition of the T PB
In order to investigate the nature of a possible mixed valance
ligand. The transitions in the UV range are assigned to O
2
CC
6
H
4
-4-
species all three compounds were chemically oxidized with AgPF
6
yielding the PF6 salts of I , II , and III . According to the molecular
⁄
ꢁ
+
+
+
NPh
2
centered pp transitions in compounds I–III. The energy of
the tungsten MLCT is lower than that of the molybdenum which
is expected due to the higher energy of the W d orbitals. The MLCT
of the tungsten compound is more intense than that of the
orbital calculations the HOMO of all compounds contained both
2
MMd orbitals and nitrogen/ligand
p orbitals. By analysis of the
EPR of the oxidized species we can determine where the positive
2 6 4 2 2 2 2
Fig. 8. Cyclic voltammograms of HO CC H -4-NPh (black), compound I(green) and compound II (purple) in CH Cl . Potentials are referenced to the Cp
Fe0/+1 couple. (Color
Online.)

M.H. Chisholm et al. / Polyhedron 64 (2013) 339–345
345
Appendix A. Supplementary data
CCDC 908656 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2013.06.002.
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Inorg. Chem. 44 (2005) 1061.
We thank the National Science Foundation, grant number CHE
957191 for support of this work and the Ohio Supercomputer
0
Center for computational resources.