Synthesis and reversible reductive coupling of cationic, dinitrogen-derived diazoalkane complexes

Article abstract of DOI:10.1021/ic900495s

A series of cationic diazoalkane complexes [4-RC₆H ₄C(H)NNMo(N[t-Bu]Ar)₃][AICI₄], [1-R][AICI ₄] (R = NMe₂, Me,H,Br,CN; Ar = 3,5-C₆H ₃Me₂) has been prepared

Full text of DOI:10.1021/ic900495s

Inorg. Chem. 2009, 48, 7181–7193 7181
DOI: 10.1021/ic900495s
Synthesis and Reversible Reductive Coupling of Cationic, Dinitrogen-Derived
Diazoalkane Complexes
John J. Curley, Tetsuro Murahashi,† and Christopher C. Cummins*
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Room 6-435,
Cambridge, Massachusetts 02139. †Current address: Department of Applied Chemistry, Graduate School of
Engineering, Osaka University, Suita, Osaka, 565-0871, Japan.
Received March 12, 2009
A series of cationic diazoalkane complexes [4-RC6H4C(H)NNMo(N[t-Bu]Ar)3][AlCl4], [1-R][AlCl4] (R = NMe2, Me, H, Br,
CN; Ar = 3,5-C6H3Me2) has been prepared by treatment of the N2-derived diazenido complex Me3SiNNMo(N[t-Bu]Ar)3
with 4-RC6H4CHO and 2 equiv of AlCl3. The structures of [1-H][AlCl4] and [1-NMe2][AlCl4] were determined by X-ray
crystallography. The C-N and N-N stretching modes were identified by a combined IR and Raman spectroscopy study,
and other physical properties are discussed in detail. The electrochemical reduction potential for [1-R][AlCl4] was shown
to be linear with the Hammett σ parameter. This reduction process forms the C-C bonded dimer, μ-(4-RC6H4C-
(H)NN)2[Mo(N[t-Bu]Ar)3]2, that was characterized by X-ray crystallography for R=H. Possible mechanisms for the
formation ofthis dimer are presented. Both electrochemical investigations and quantum chemical calculations are used to
describe the odd-electron complex 4-RC6H4C(H)NNMo(N[t-Bu]Ar)3, 1-R, that is an intermediate in the formation of
[1-R]2. The C-C bond in [1-R]2 is redox-noninnocent and is broken upon oxidation. This reaction was used to prepare
[1-H][A] (A = PF6-, OTf-), and possible uses of this property in charge-storage devices are discussed.
Introduction
fragments without the intermediacy of ammonia.9-18 Such a
synthetic protocol could be designed to avoid the need for H2,
which currently is formed by steam reforming of CH4,
producing the greenhouse gas CO2. Therefore, the develop-
ment of new chemical methods to incorporate nitrogen atoms
into organic molecules without a need for the reduction of N2
to ammonia would effect both a more efficient and environ-
mentally benign production of nitrogen-containing organic
molecules. For these reasons, this area of research is under
active exploration.11-19
A great deal of energy has been expended optimizing
methods for the chemical fixation of dinitrogen.1-7 Of
these methods, the Haber-Bosch process, which forms
NH3 from N2 and H2, is the most well-studied and currently
the most widespread process for nitrogen fixation.8 Ammo-
nia produced by this process may be incorporated into
organic molecules in a subsequent synthetic step. However, a
more synthetically efficient route to nitrogen-containing
organic molecules would directly incorporate N2 into organic
In order to bypass traditional schemes for nitrogen fixa-
tion, it is important that the product of N2 transfer be a useful
synthetic precursor for organic reactions. One class of com-
pounds that meets this criteria is that of the diazoalkanes,
which are important precursors to nitrogen-containing or-
ganic molecules. These molecules react by 1,3-dipolar addi-
tion reactions to incorporate both nitrogen atoms into
organic functional groups.20-22 Thereby, diazoalkanes are
*To whom correspondence should be addressed. E-mail: ccummins@mit.edu.
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r
2009 American Chemical Society
Published on Web 07/06/2009
pubs.acs.org/IC

7182 Inorganic Chemistry, Vol. 48, No. 15, 2009
Curley et al.
used as synthons for pyrazolines,23-27 compounds used in
transition metal complex, one might expect that Ph2CN2
would add to Mo(N[t-Bu]Ar)3 to form a diazoalkane com-
plex, in analogy to the odd-electron compound PhCNMo-
(N[t-Bu]Ar)3 (Ar=3,5-C6H3Me2).53-55 However, we found
that both Ph2CN2 and N2O engage in facile reactions with
Mo(N[t-Bu]Ar)3 that form NMo(N[t-Bu]Ar)3.56,57 As a
result, odd-electron diazoalkane complexes cannot be pre-
pared by simple coordination of a free diazoalkane to Mo-
(N[t-Bu]Ar)3. This finding motivated us to develop alter-
native routes for the synthesis of the complexed diazoalkane
functional group in order to prepare complexes of the
formula 4-RC6H4C(H)NNMo(N[t-Bu]Ar)3, 1-R (R=NMe2,
Me, H, Br, CN). The N2-derived diazenido compound
Me3SiNNMo(N[t-Bu]Ar)3 is formed in high yields by the
treatment of Mo(N[t-Bu]Ar)3 with a mixture of 1% Na/Hg
and Me3SiCl under an atmosphere of N2.58 We thought that the
distal nitrogen of the trimethylsilyl diazenido would be suffi-
ciently nucleophilic to participate in reactions that form N-C
bonds, and therefore, protocols were developed to convert this
diazenido fragment into a coordinated diazoalkane.
asymmetric ligand25 and drug design.28,29
Diazoalkane complexes may be made from coordinated N2,
such as one N2 ligand in (N2)2M(diphos)2 (M = Mo, W;
diphos = chelating diphosphine),1,17,20,30-32 or by the treat-
ment of transition-metal fragments with an organic diazoalk-
ane to form a coordination complex.33-40 For instance,
photolysis of gem-dibromides (i.e., Me2CBr2) in the presence
of (N2)2W(diphos)2 has been shown to afford [Me2CNNW(di-
phos)2Br]þ.31 An alternative route to diazoalkane complexes
of this type has been accomplished by the sequential treat-
ment of (N2)2M(diphos)2 with a strong acid followed by an
aldehyde. In those diazoalkane complexes that are prepared
by the addition of an organic diazoalkane to a transiton-
metal complex, either hydrazones or diazenes may serve as
precursors to the diazoalkane ligand.41,42 In many cases,
compounds arrived at by either synthetic route are stable with
regard to N2 loss, which would lead to the formation of a
carbene compex.1,20,33,43-46 However, diazoalkanes also re-
main synthons of choice to generate metal carbenes.33-37,44-52
Given that diazoalkane complexes may be made by the
addition of a diazoalkane to a coordinatively unsaturated
Results and Discussion
Synthesis and Structure of Diazoalkane Complexes.
Inspired by organic condensation reactions between alde-
hydes and amines,59 we explored the treatment of
Me3SiNNMo(N[t-Bu]Ar)3 with aldehydes. A related strat-
egy has been successful in the case of [H2NNM(diphos)2F]-
[BF4] (M=Mo, W), which undergoes reactions with ketones
to lose 1 equiv of H2O, forming [R2CNNM(diphos)2F]-
[BF4].17,30,32 Because Brønsted acids are incompatible with
our ancillary ligands, strong Lewis acids were used in
place of protic conditions. Accordingly, Me3SiNNMo(N-
[t-Bu]Ar)3 was treated with 1 equiv of AlCl3 and PhCHO in
Et2O, upon which the product diazoalkane complex
[PhC(H)NNMo(N[t-Bu]Ar)3][AlCl4], [1-H][AlCl4], preci-
pitates from the reaction mixture as a red powder
(Scheme 1). However, half of the starting diazenido complex
was recovered from the reaction mixture when this proce-
dure was followed. When 2 equiv of AlCl3 were used,
[1-H][AlCl4] was isolated from the reaction mixture in
90% yield by filtration. We presume that the soluble
byproduct of this synthesis is [μ-(Me3SiO)AlCl2]2.60,61 The
salt, [1-H][AlCl4], is stable with regard to N2 loss. In fact,
[1-H][AlCl4] may be heated to 60 °C in CDCl3 for several
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Article
Inorganic Chemistry, Vol. 48, No. 15, 2009 7183
Scheme 1. a
a Synthesis of diazoalkane complexes. Yields and Hammett σ para-
meters for the aryl substituent are given in the scheme.
Figure 1. ORTEP drawing of [1-H][AlCl4] with 50% probability ellip-
soids. Hydrogenatoms havingcalculated atomic positions are omitted for
clarity. Selected bond lengths and angles are given in the text.
Employing N2 as the synthon for the diazoalkane ligand
of [1-H][AlCl4] made it convenient to isotopically label the
diazoalkane fragment via use of 15N2. The 15N NMR
spectrum of 15N2-[1-H][AlCl4] depicts two doublets of doub-
lets centered at 384 and 417 ppm. These values agree well
with the chemical shifts of 397 and 422 ppm obtained from
density functional theory (DFT) calculations (Table S1,
Supporting Information). The deshielding of these nitrogen
nuclei has been attributed to low-energy nN f π* circula-
This strategy allows us to draw upon linear free energy
relationships established within the field of physical
organic chemistry as a guide for comparison.35,64,65 We
found that, with only minor modifications in the protocol,
the synthesis of [1-H][AlCl4] could be successfully applied
for the synthesis of four other diazoalkane complexes:
[4-RC6H4C(H)NNMo(N[t-Bu]Ar)3][AlCl4], [1-R][AlCl4]
(R=NMe2, 51%; Me, 90%; Br, 97%; CN, 72% isolated
yield; Scheme 1) by combination of the appropriate
aldehyde, AlCl3, and Me3SiNNMo(N[t-Bu]Ar)3.
1
tions.62 The observed JNN =17 Hz coupling is consistent
with literature data for metal diazenido compounds in which
the metal-N-N angle is approximately linear.62 We assign
the more downfield signal as the nitrogen bound to Mo on
the basis of both proton coupling and comparison to related
compounds such as Me3SiNNMo(N[t-Bu]Ar)3. The pro-
ton-coupled 15N NMR spectrum revealed N-H couplings
Crystals of [1-H][AlCl4] suitable for X-ray diffraction
were grown from THF/n-pentane at -35 °C. The X-ray
crystal structure of [1-H][AlCl4] shows that the three
anilide ligands are oriented in the same direction, impart-
ing a pseudo-C3 symmetry on the lower hemisphere of the
molecule that is broken by the Cs-symmetric diazoalkane
3
2
of JNH = 6 Hz and JNH = 3 Hz, consistent with the
expectation that the magnitude of the 2JNH coupling through
an sp2 hybridized atom will be smaller than that of the 3JNH
coupling for a planar diazoalkane ligand.63
˚
ligand (Figure 1). The Mo-N4 distance of 1.7326(18) A
suggests π bonding between Mo and the diazoalkane N.
The synthesis of [1-H][AlCl4] requires both that an
oxygen atom be removed from the aldehyde, breaking
the CdO double bond, and that a CdN double bond be
formed during the reaction. It is also a redox reaction in
which molybdenum goes from the þ4 to the þ6 formal
oxidation state. We found it interesting that such a
transformation gave a diazoalkane-containing product
in high yields. Moreover, the thermal stability of [1-H]-
[AlCl4] contrasts with previous use of this diazoalkane,
PhC(H)N2, to study carbene transfer reactions which
occur via N2 loss.35,41 Therefore, we sought to expand
the synthetic methodology in a manner suitable for
systematic comparison of the properties of such diazoalk-
ane complexes.41 For this purpose, we chose to vary the
substituent in the 4-position of the diazoalkane aryl ring.
˚
The N4-N5 distance of 1.336(2) A and the N5-C4
distance of 1.290(3) A are consistent with the description
˚
that the diazoalkane ligand is reduced by backbonding
from Mo. Accordingly, the N-N distance is substantially
longer than what one would expect for a free diazoalkane
66,67
˚
(cf. 1.13(4) A for H2CN2).
internuclear distances was found in the single-crystal
A slightly different set of
˚
X-ray structure of [1-NMe2][AlCl4]: Mo-N4, 1.730(2) A;
˚
˚
N4-N5, 1.317(3) A; and N5-C4, 1.312(4) A (Figure 2).
The NMe2 group is coplanar with the aryl ring, maximizing
the electron donation of this substituent into the π network
of the diazoalkane. Presumably, this electronic perturba-
tion is responsible for the small alteration in internuclear
distances in [1-NMe2][AlCl4] compared to [1-H][AlCl4].
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7184 Inorganic Chemistry, Vol. 48, No. 15, 2009
Curley et al.
Figure 2. ORTEP drawing of [1-NMe2][AlCl4] with 50% probability
ellipsoids. Hydrogen atoms having calculated atomic positions are
omitted for clarity. Selected bond lengths and angles are given in the text.
Figure 3. IR (above) and Raman (below) spectra of [1-H][AlCl4] (solid
blue line) and 15N2-[1-H][AlCl4] (dashed green line) in CDCl3 at 20 °C.
Both sets of metrics compare well with those obtained for
other terminally bound, singly bent diazoalkanes coordinated
to Mo and W.17,68-72 Diazoalkane ligands are described as
singly bent when the C-N-N angle is approximately 120°
and the metal-N-N angle is close to 180°,33 as is the case for
the compounds reported here. The N4-N5-C4 and Mo1-
N4-N5 angles are 117.86(19)° and 174.21(16)° in [1-H]-
[AlCl4] and 116.9(3)° and 176.0(2)° in [1-NMe2][AlCl4].
Electronic and Vibrational Spectroscopy. Diazoalkane
complexes typically have characteristic stretching modes
that are suitable for study by vibrational spectroscopy.20,33
To gain information about the vibrational structure of the
coordinated diazoalkane ligand, infrared and Raman spec-
troscopy data were collected for complexes [1-R][AlCl4]
(R=NMe2, Me, H, Br, CN). Due to the low symmetry of
the five cations (approximately Cs), most vibrational modes
of the diazoalkane ligand give rise to bands in both the
Raman and IR spectra (Figures S15-S20, Supporting
Information). The Raman spectrum of [1-H][AlCl4] con-
tains five intense bands that are attributed to vibrational
modes of the PhC(H)N2 ligand located at 1528, 1446, 1366,
1193, and 1173 cm-1. The band located at 1528 cm-1 in
both the IR and Raman spectra is in the spectral region for
which the predominantly CdN stretching mode of the
η1-diazoalkane ligand is expected.20,33 This assignment
was supported by a 15N2-labeling experiment in which the
corresponding band for 15N2-[1-H][AlCl4] was found at
1520 cm-1 (Figure 3). The 8 cm-1 shift of the CdN
stretching mode is smaller than would be predicted by a
harmonic oscillator model (24 cm-1), indicating that this
vibrational mode couples to others. Coupling of the C-N
and N-N vibrational modes is observed for both coordi-
nated and free diazoalkanes.73,74 Additionally, for aryl
diazenido complexes, it has been demonstrated that the aryl
C-H modes couple into the N-N oscillator.75,76
Numeric frequency calculations carried out on the model
complex [PhC(H)NNMo(NH2)3]þ, [1-m]þ, support the
assignment of the band at 1528 cm-1 as a primarily CdN
oscillator (Table S3, Supporting Information). These com-
putational results predicted a vibration at 1459 cm-1 due to
a C-H wagging motion of hydrogen atoms bound to the
Ph group. Accordingly, a band for [1-H][AlCl4] was found
at 1446 cm-1. Moreover, this band shifts just 2 cm-1 upon
substitution of 15N2 for 14N2, consistent with nature of the
described vibrational mode.
A band that is only intense in the Raman spectrum of
[1-H][AlCl4] was located at 1366 cm-1. The Raman activity
of this band suggested to us that it is largely composed of
N-N stretching character, which is expected to give rise to a
smaller change in dipole moment than the CdN stretching
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Article
Inorganic Chemistry, Vol. 48, No. 15, 2009 7185
Table 1. Vibrational Data for the Diazoalkane Ligand in cm-1
complex
ν1
ν2
ν3
a
a
[1-NMe2][AlCl4]
[1-Me][AlCl4]
[1-H][AlCl4]
1498
1526
1528
1520
1525
1524
1367
1366
1346
1367
1361
1197
1193
1164
1192
1193
15N2-[1-H][AlCl4]
[1-Br][AlCl4]
[1-CN][AlCl4]
a Sample fluorescence prevented the acquisition of Raman data.
motion. When 15N2 is incorporated into the diazoalkane
ligand, this band shifts to 1346 cm-1. The 20 cm-1 shift that
occurs upon isotopic substitution is twice as large as the shift
found for the predominantly CdN stretching mode but is
less than half the predicted value of 46 cm-1 for a harmonic
N-N oscillator. Calculations carried out on [1-m]þ found a
band at 1363 cm-1 that is described by a mixture of both the
C-N and N-N stretching modes combined with a wagging
of the hydrogen atom attached to the diazoalkane carbon.
For this reason, the band at 1366 cm-1 is attributed to a
highly coupled vibrational mode that contains both CdN
and N-N stretching character.
Figure 4. Normalized UV-vis spectra of [1-R][AlCl4] (R = Me, H, Br,
CN) in CH2Cl2 at 20 °C.
In the Raman spectrum of [1-H][AlCl4], the two bands at
1193 and 1173 cm-1 collapse into a single band located at
1164 cm-1 upon 15N substitution.77 We suggest that this is
because the two signals overlap in the spectrum of the
labeled compound. The band located at 1193 cm-1 is both
sharp and intense in the Raman spectrum, consistent
with an N-N stretching mode. Moreover, this band shifts
by 29 cm-1 upon isotopic substitution compared with the
40 cm-1 shift predicted by a harmonic oscillator model,
further supporting the notion that the origin of this band is a
stretching mode of the N2 unit. DFT calculations support
the description of this vibration as a predominately N-N
stretching mode. For [1-m]þ, a band of this type was found
at 1148 cm-1, which is in close agreement with experimental
data given the simplicity of the model system.
Figure 5. UV-vis spectrum of [1-NMe2][AlCl4] in CH2Cl2 at 20 °C.
Given the wealth of data available for the IR-active, CdN
stretching mode of coordinated diazoalkanes, information
for the N-N stretching mode remains sparse in compari-
son to information concerning the same stretch for free
diazoalkanes.45 The NdN stretching mode of PhC(H)N2
is 2062 cm-1, which is between the values for H2CN2,
The compounds [1-R][AlCl4] (R=Me, H, Br, CN) are all
dark red in color, but solutions of [1-NMe2][AlCl4] are a
vibrant fuchsia color. This observation prompted us to
compare the UV-vis spectral features of these five com-
plexes. The UV-vis spectra for [1-R][AlCl4] (R=Me, H, Br,
CN) all show an absorption near 450 nm and contain no
intense absorption bands at lower energies (Figure 4). How-
ever, [1-NMe2][AlCl4] was found to possess an intense
absorption with a maximum at 538 nm (Figure 5). The
absorption band is assigned as a ligand-to-metal charge
transfer band (LMCT). This assignment is in accord with
both the formally d0 metal center and the observation that
less electron-donating R groups do not give rise to strong
absorptions in the visible region. This absorption has a
pronounced feature on the hypsochromic side of the band,
indicating that the net absorption is the sum of overlapping
spectral features. The absorption may be fit to the sum of
three Gaussian distributions with spacing of approximately
1180 cm-1. This value likely corresponds to the vibrational
mode of the Mo-N bond in the excited state.80,81
2097 cm-1, and Ph2CN2, 2042 cm-1 74,78,79
The observed
.
N-N stretch in [1-H][AlCl4] is consistent with a weakening
of this bond in comparison to the free diazoalkane.
All of the diazoalkane complexes studied have similar
vibrational structures (Table 1). In view of the isotopic
substitution results obtained for [1-H][AlCl4], the predomi-
nantly CdN and N-N stretching modes were located in the
IR and Raman spectra for each of [1-R][AlCl4] (R=NMe2,
Me, Br, CN; Table 1). There is no obvious Hammett trend
present in the vibrational data, which may reflect the
coupling of the multiple vibrational modes. Vibrational
modes that have weak intensity in the infrared spectrum
were not located for [1-NMe2][AlCl4] because sample fluor-
escence interfered with the acquisition of Raman data.
(77) Due to the strong kinematic coupling between oscillators in
[1-H][AlCl4], the origin of the IR- and Raman-active band found at
1173 cm-1 is uncertain.
(78) Foffani, A.; Pecile, C.; Ghersetti, S. Tetrahedron 1960, 11, 285–289.
(79) Moore, C. B.; Pimentel, G. C. J. Chem. Phys. 1964, 40, 329–341.
(80) Wigley, D. E. Prog. Inorg. Chem. 1994, 42, 239–482.
(81) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds; Wiley:
New York, 1988; pp 33-34.

7186 Inorganic Chemistry, Vol. 48, No. 15, 2009
Curley et al.
Scheme 2
Irradiation of CHCl3 solutions of [1-NMe2][AlCl4] with
visible light gave rise to an emission with a Stokes shift of
15 600 cm-1 (λabs=541 nm, λem=573 nm). A number of
workers have recently shown that the LMCT excited state
may give rise to fluorescence in d0 complexes that contain
metal-ligand multiple bonds.82-84 Our case is distinct in
that both absorption and emission of the LMCT excited
state occur in the visible region; therefore, UV irradiation
is not required to observe emission from [1-NMe2][AlCl4].
Indeed, the NMe2 substituent is found in many efficient
organic fluorophores that emit from a CT state.85 This is
desirable for applications such as photooxidants that may
be developed for use with a visible light source.84
Reductive Dimerization of Diazoalkane Ligands via C-C
Bond Formation. Reduction of the diazoalkane salt, [1-H]-
[AlCl4], by one electron (1e) was anticipated to generate
the odd-electron complex PhC(H)NNMo(N[t-Bu]Ar)3,
1-H. However, this species was found to be unstable with
regard to spontaneous dimerization at carbon to form the
dimeric, diazo complex, μ-(PhC(H)NN)2[Mo(N[t-Bu]-
Ar)3]2, [1-H]2 (Scheme 2). The addition of solid Cp2Co
to [1-H][AlCl4] dissolved in CHCl3 leads to the rapid and
quantitative formation of [1-H]2 with the precipitation of
[Cp2Co][AlCl4]. Filtration of the reaction mixture re-
moves [Cp2Co][AlCl4], leaving only [1-H]2 in the filtrate,
which is then collected in 97% yield. This was found to be
the most convenient protocol for separating the products
Figure 6. An ORTEP drawing of [1-H]2 shown with 50% probability
ellipsoids. Hydrogen atoms having calculated atomic positions are
omitted for clarity. Selected bond lengths and angles are given in the text.
despite the observation that Cp2Co is unstable in CHCl3
for prolonged periods of time; the formation of [1-H]2 is
essentially complete upon mixing. Dimeric [1-H]2 has low
solubility in solvents such as C6H6, Et2O, or n-alkanes but
forms solutions readily in THF, MeCN, or halocarbons.
Upon the reduction of [1-H][AlCl4], the characteristic
1
downfield shift at 8.92 ppm in the H NMR spectrum
for the hydrogen atom bound to the diazoalkane carbon
moves to 6.17 ppm. This indicates that the unsaturation at
that carbon is removed upon reduction, consistent with
1
dimer formation. Examining the H NMR spectrum, it
intrigued us that only one set of resonances predomi-
nated; one would expect an unselective radical-radical
coupling reaction to form a 1:1 mixture of rac and meso
isomers with each isomer having a unique set of chemical
shifts. To rationalize this observation, we concluded that
the reductive dimerization of [1-H][AlCl4] is selective for
the formation of either the meso or rac product mixtures.
To identify the preferred stereochemistry of C-C bond
formation, crystals suitable for X-ray diffraction were
grown of [1-H]2. The molecule crystallizes on a C2 axis
and is therefore assigned as the rac isomer (Figure 6).
Anomalous scattering data confirmed that the crystal-
lographic model with (R,R) stereochemistry corresponds
(82) Williams, D. S.; Korolev, A. V. Inorg. Chem. 1998, 37, 3809–3819.
(83) Williams, D. S.; Thompson, D. W.; Korolev, A. V. J. Am. Chem. Soc.
1996, 118, 6526–6527.
(84) Heinselman, K. S.; Hopkins, M. D. J. Am. Chem. Soc. 1995, 117,
12340–12341.
(85) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Springer:
New York, 2006; pp 206-207.

Article
Inorganic Chemistry, Vol. 48, No. 15, 2009 7187
to the enantiomer contained within the crystal.86-88 The
preferential formation of the rac isomer is in qualitative
agreement with a radical-radical coupling in which a
transition state leading to the rac product is favored. This
is the only transition state that accommodates C-C bond
formation with C41-C4-C40-C410 and N5-C4-C40-
N50 angles that approximate 180°, minimizing unfavor-
able steric interactions between the two reaction partners.
Likewise, reduction of both [1-Br][AlCl4] and [1-CN]-
[AlCl4] with Cp2Co in d8-THF leads to the formation of
only one set of signals in the 1H NMR spectrum, which we
presume to be the rac isomer as for [1-H]2. However,
reduction of [1-Me][AlCl4] or [1-NMe2][AlCl4] gives a
functional group (generically, M-N-N-H) and found
a codependence of the metal-N-N and N-N-H an-
gles.91 These calculations concluded that, when the latter
angle is 120°, the M-N-N angle is predicted to be 174°.
Electrochemical dimerization of coordinated diazoalk-
anes has been reported for two systems that contain W,
eqs 1 and 2.92,93 In only one case was the product soluble
enough for spectroscopic and structural characterization,
eq 1.93 In this case, the reduction of [Me2CNNWCl(CO)-
(PMe2Ph)3]þ is accompanied by loss of a phosphine
ligand; therefore, the reaction was carried out in the
presence of CO to cleanly generate the C-C bonded
product. Moreover, there is no possibility of forming
diastereomeric products because the two faces of the
diazoalkane ligand are identical. Reductive dimerization
of d0 diazoalkane complexes of W can also lead to the
formation of a W-W bond with the terminal N atom of
the diazoalkane bridging the two W atoms, as opposed to
C-C bond formation, eq 3.94
1
mixture of the meso and rac isomers, as judged by H
NMR spectroscopy.
Highly selective formation of the rac product for the
dimerization of 1-R (R=H, Br, CN) is consistent with a
number of mechanisms. If C-C bond formation were
reversible, an equilibrium process involving [1-R]2 and
2 equiv of 1-R could drive the dimer to the more stable
stereoisomer. However, on the basis of the large differ-
2½Me2CNNWClðCOÞðPMe2PhÞ3ꢀ½ZnCl3ðTHFÞꢀþMg
þðxsÞ CO f μ-ðMe2CN2Þ2½WðCOÞ2ðPMe2PhÞ2ꢀ2 ð28%Þ
0/þ
ence in EP for [1-R]0/þ and [1-R]2
of approximately
900 mV (vide infra), this explanation appears unlikely. A
þ
related scenario is that the formation of [1-R]2 by a
þ 2PMe2Ph
ð1Þ
combination of 1-R and [1-R]þ is rapid and reversible
with respect to the second electron transfer. In such a
mechanism, the stereochemical outcome of C-C bond
formation might be explained by the relative stability of
the two stereoisomers of [1-R]2þ. An equally plausible
possibility is that the two neutral radicals, 1-R, couple in
such a way as to minimize steric repulsion, which leads to
a preferential formation of rac isomers. Electrochemical
studies of the pinacol coupling involving 4-subsituted
acetophenones show a modest preference for the forma-
tion of the rac isomers in basic media.89 Although
these results were attributed mainly to hydrogen bond-
ing, similar electrodimerization of benzaldehyde did not
discriminate between the formations of rac or meso
isomers.89,90
2½H2CNNWFðdppeÞ2ꢀ½BF4ꢀ
þ 2e f μ-ðH2CNNÞ2½WFðdppeÞ2ꢀ2
ð2Þ
2Ph2CNNWðcalix½4ꢀareneÞ
þ 2Na f Na2½μ-ðPh2CNNÞ2½Wðcalix½4ꢀareneÞꢀ2ꢀ ð3Þ
Reductive dimerization of [1-R][AlCl4] proceeds thr-
ough the intermediacy of the odd-electron, diazoalkane
complex 1-R, which is the initial product of 1e reduction.
Therefore, it seemed natural to interrogate this process
by electrochemical methods. Of initial interest was a com-
parison of the energy required for reduction of the cation
to the neutral radical. A graph of the redox potential versus
the Hammett parameter shows a large change in redox
potential as a function of the substitution on the 4-R group
of the diazoalkane ligand (Figure 7). A fit to the Hammett
plot gave a F=-283 mV for the series R=NMe2, Me, H,
Br, and CN. Therefore, complexes having a diazoalkane
bearing an electron-donating group in the 4-position of
the aryl ring deliver a more negative reduction potential
than those that contain an electron-withdrawing group.
A parallel trend has been described for the reduction
potential of [4-RC6H4C(H)NNM(dppe)2F][BF4] (M =
Mo, W).95
With respect to [1-H][AlCl4], the C-C bond formation
forces the cumulene system to reverse the long and short
internuclear distances in [1-H]2. This structural change
occurs in conjunction with the conversion of molybde-
num from the þ6 to the þ4 oxidation state in the reaction
during reductive coupling. The Mo-N distance in [1-H]2,
˚
1.769(2) A, is longer than that of [1-H][AlCl4]. Corre-
˚
spondingly, the N-N bond is contracted, 1.243(4) A, and
˚
the C-N bond lengthened, 1.478(4) A, compared to the
cationic complex. The newly formed C-C linkage has an
internuclear distance of 1.553(6) A ,which is typical for
˚
such a C-C single bond. In the X-ray crystal structure of
[1-H]2, the N-N-C bond angle of 119.4(3)° is consistent
with sp2 hybridization at nitrogen, as the Mo-N-N
angle of 167.2(2)° deviates from linear. This angle is
expected to be less than 180° in metal diazenido com-
plexes.62 Hoffmann has described the bonding in this
In cyclic voltammetry, the ratio of cathodic-to-anodic
peak currents reflects the concentration in solution for the
generation and decay of electroactive species. Therefore,
(91) Dubois, D. L.; Hoffmann, R. Nouv. J. Chem. 1977, 1, 479–492.
(92) Pickett, C. J.; Tolhurst, J. E.; Copenhaver, A.; George, T. A.;
Lesterb, R. K. J. Chem. Soc., Chem. Commun. 1982, 1071–1072.
(93) Aoshima, T.; Tanase, T.; Mizobe, Y.; Yamamotob, Y.; Hidai, M.
J. Chem. Soc., Chem. Commun. 1992, 586–588.
(94) Guillemot, G.; Solari, E.; Floriani, C.; Rizzoli, C. Organometallics
2001, 20, 607–615.
(86) A Flack x parameter of -0.050(27) was calculated by SHELXL.
::
(87) Muller, P. Crystal Structure Refinement: A Crystallographer’s Guide
to SHELXL; Oxford: New York, 2006, p 103.
(88) Flack, H. D. Acta Crystallogr. 1983, A39, 876–881.
(89) Stocker, J. H.; Jenevein, R. M.; Kern, D. H. J. Org. Chem. 1969, 34,
2810–2813.
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(90) Stocker, J. H.; Jenevein, R. M. J. Org. Chem. 1968, 33, 2145–2146.

7188 Inorganic Chemistry, Vol. 48, No. 15, 2009
Curley et al.
Figure 7. Potentials (E1/2 vs SCE) at which the reduction of [1-R][AlCl4]
occurs plotted against the Hammett parameter for R. The fit shown has a
F = -283 mV (R2 = 0.998).
Figure 9. Plots of cathodic/anodic peak currents versus log of the scan
rate (mV/s) showing that the dimerization is not diffusion-limited. Fits to
the data in the form ic/ia = a þ b log(ν), where ν is the scan rate, are shown
as follows: [1-Br][AlCl4] (dashed line), a = 7.56(26), b = -1.69(10); [1-
H][AlCl4] (solid line), a = 5.95(10), b = -1.46(4); [1-Me][AlCl4] (dotted
line), a = 3.67(6), b = -0.76(3).
under the same conditions (1 mM solutions at a scan rate
of 100 mV/s), the wave corresponding to [1-Me]þ/0 app-
ears more reversible than that for [1-H]þ/0 because the
ratio of cathodic-to-anodic peak currents is closer to
unity. Assuming that the odd-electron diazoalkane com-
plexes, 1-R, have similar diffusion behaviors, we would
conclude that this ratio should be the same for all three
substituents. This result demonstrates that the dimeriza-
tion process is not diffusion-limited.98
The dimerization reaction that forms [1-H]2 as a pro-
duct by the reduction of [1-H]þ has two limiting mechan-
isms. A total of 2 equiv of the neutral, odd-electron
compound 1-H may dimerize, forming a C-C bond.
Alternatively, 1-H may react with [1-H]þ to form an
intermediate complex, [1-H]2þ, that is subsequently re-
duced (either by the addition of 1e from an electrode or a
disproportionation reaction). These two mechanisms
may be discriminated by monitoring the changes in the
cathodic peak potential as a function of scan rate. In the
radical-radical coupling case, a slope of -19 is expected
for such a plot, while a slope of -29 is expected for a radi-
cal-cation coupling.96,99 Our attempts to make this
distinction were not reliable, presumably due to the high
resistance of anhydrous media. Except for in the immedi-
ate vicinity of the electrode surface, the concentration of
[1-H]þ is always expected to be higher than the concen-
tration of 1-H, raising the question as to whether chemical
reduction of these cations would necessarily follow the
same mechanism that predominates under electrochemi-
cal conditions. We favor the notion that this dimerization
proceeds by radical-radical coupling, by analogy with
the well-studied radical-radical dimerization of the per-
sistent radical, PhCNMo(N[t-Bu]Ar)3,53-55,100 but do
not rule out a mechanism in which [1-R]2 is formed by a
radical-cation dimerization followed by fast electron
transfer.
Figure 8. Stack plot of cyclic voltammograms of 1 mM [1-H][AlCl4] in
0.2 M [N(n-Bu)4][PF6]/THF solution at 20 °C. Potentials shown are
relative to SCE.
monitoring this ratio as a function of the scan rate is a
sensitive marker of how much electroactive species is in
solution.96 The height of the cathodic wave that produces
1-H varies linearly with the square-root of scan rate as
expected for a 1e reduction (Figure 8).96,97 However, the
anodic current corresponding to the oxidation of 1-H
grows in more quickly than predicted by the scan rate
variation. This behavior shows that, in a 1 mM solution,
1-H is long-lived enough to be oxidized prior to dimeriza-
tion at sufficiently fast scan rates. The low anodic peak
current for slow scan rates indicates that depletion of 1-H
in solution, via dimerization, is fast relative to the time
scale of the electrochemical experiment. Consistent with
a bimolecular dimerization reaction, cyclic voltammo-
grams of [1-R][AlCl4] were concentration-dependent. In-
creasing the concentration led to less reversible waves,
while decreasing the concentration gave more reversible
behavior.
Solutions of [1-R][AlCl4] (R=Br, H, Me) at concentra-
tions of 1 mM have nearly the same cathodic peak
current, indicating that the diffusion behavior of the three
compounds is similar. A variable scan rate experiment
showed that the ratio of cathodic-to-anodic peak current
is not equal for the series (Figure 9). Upon comparing the
cyclic voltammograms of [1-H][AlCl4] and [1-Me][AlCl4]
(98) Olmstead, M. L.; Hamilton, R. G.; Nicholson, R. S. Anal. Chem.
1969, 41, 260–267.
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(100) McDonough, J. E.; Weir, J. J.; Sukcharoenphon, K.; Hoff, C. D.;
Kryatova, O. P.; Rybak-Akimova, E. V.; Scott, B. L.; Kubas, G. J.;
Mendiratta, A.; Cummins, C. C. J. Am. Chem. Soc. 2006, 128, 10295–10303.
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(96) Saveant, J.-M. Elements of Molecular and Biomolecular Electrochem-
istry; Wiley: Hoboken, NJ, 2006; pp 148-150.
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Article
Inorganic Chemistry, Vol. 48, No. 15, 2009 7189
Figure 10. Relative energies of the calculated frontier orbitals for 1-m.
ꢀ
Saveant has described the electrodimerization of aro-
matic aldehydes in great detail.96,101-105 The mechanism
of dimerization has been shown to be a radical-radical
coupling of two pinacolate radical anions generated at the
electrode. The reaction proceeds at rates much slower
than the diffusion limit, and specific solvation by water
has a large effect on the measured rate of this reaction.105
The activation barrier to this reaction has been attri-
buted to entropy requirements in the C-C bond-forming
step.96,102,103 A reduced spin density on the carbon atom
brought about by delocalization of the unpaired electron
onto the aromatic group both stabilizes the intermediate
and raises the entropic requirement for C-C bond for-
mation. Therefore, increasing the amount of radical
character at the carbon participating in C-C bond for-
mation generally increases the rate of dimerization.
The transient radical intermediate, 1-H, is unstable
with respect to dimerization to form a C-C bond in the
product [1-H]2. Dimerization at carbon suggested to us
that the unpaired electron resides partially on the dia-
zoalkane carbon atom. To investigate the electronic
structure of the transient radical intermediate, DFT
calculations were performed on the model complex PhC-
(H)NNMo(NH2)3, 1-m. For a starting geometry, the
half-molecule that occupies the asymmetric unit in the
crystal structure of [1-H]2 was used, and the anilide
ligands were replaced with NH2 donors. During the
geometry optimization, the initially pyramidal diazo-
alkane carbon atom became planar, as expected for an
sp2 hybridization at carbon. In 1-m, the SOMO is de-
stabilized by approximately 1.5 eV with respect to the
SOMO-1 (Figure 10).
Figure 11. Two views of the SOMO for 1-m shown at an isosurface
value of 0.05.
Figure 12. Cyclic voltammogram of [1-H]2 in a 0.2 M [N(n-Bu)4][PF6]
THF solution acquired at 250 mV/s and 20 °C. Potentials shown are
relative to SCE.
was split into a quintet by a hyperfine interaction of
2.6 mT from 14N nuclei. Delocalization of the SOMO of
1-m onto the aryl ring suggests that a para substituent could
influence the stability of the SOMO and the relative spin
densities for atoms along the diazoalkane fragment. This
provides a model for the variation in reduction potential and
lifetime of 1-R with variation of the substituent.
Electrochemical oxidation of the dimers [1-R]2 gives a
broad anodic wave in the cyclic voltammogram with a
peak current approximately twice the magnitude of the
wave corresponding to the cathodic [1-R]þ/0 couple
(Figure 12). Therefore, the oxidation of [1-R]2 was as-
signed as a 2e wave that occurs as a result of two
consecutive 1e oxidation events.96 Behavior of this kind
is expected for an electrochemical oxidation in which the
second electron is removed more easily than the first.106
Consistent with this reasoning is the difference between
the oxidation potential of [1-R]2 and that of the neutral
1-R, as the potential of the latter lies approximately 1 V
negative of the former. A frequently invoked mechanism
for reactions of this type is the ECE reaction, wherein
oxidation of the dimer, [1-H]2, leads to the formation of
both [1-H]þ and 1-H at a potential at which the neutral
compound undergoes rapid oxidation to form a second
equivalent of [1-H]þ.99,106
Approximately 15% of the SOMO resides on the
diazoalkane carbon atom and 43% on Mo (Figure 11).
Experimental evidence for the delocalization of unpaired
spin density onto diazoalkane ligands has been provided
by study of the reduction of [4-RC6H4C(H)NNMo-
(dppe)2F][BF4] in an electron paramagnetic resonance
(EPR) probe.95 An EPR resonance centered at g=2.01
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(101) Saveant, J.-M.; Tessler, D. J. Phys. Chem. 1978, 83, 1723–1727.
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1991, 113, 8811–8817.

7190 Inorganic Chemistry, Vol. 48, No. 15, 2009
Curley et al.
that formally stores two holes.107-109 The complemen-
tary function, electron storage, is accomplished by [1-H]2.
Combination of hole- and electron-storing molecules
is a goal in the development of molecular batteries.109
Only a few systems have been described that carry out
both functions, such as Floriani’s [M(salophen)]n (n=2,
0, -2).110-113 Therefore, it has become desirable to find
mixed-molecule systems for charge storage. These sys-
tems expand the scope of molecular charge storage
technology and make it possible to tune the electro-
chemical difference between the hole- and charge-storage
compartments of the molecular battery.
Conclusions
Figure 13. Potentials (Ep vs SCE) at which oxidative cleavage of [1-R]2
occurs are plotted against the Hammett parameters for R. The fit to four
datapointshas a F=-160 mV (R2 = 0.994). Data for [1-NMe2]2 were not
fit to the line.
A new synthetic route to diazoalkane ligands from N2 has
been described. Characterization of the complexes, [1-R]-
[AlCl4], included Raman spectroscopy to elucidate the N-N
stretching frequency of the coordinated diazoalkane, for
which literature data are scarce. Reductive dimerization of
[1-R]þ coupled the diazoalkane ligands at carbon to form a
bridging ligand. One such dimer, [1-H]2, was crystallo-
graphically characterized. Electrochemical studies showed
that the reduction of [1-R][AlCl4] forms a transient odd-
electron complex, 1-R. The structure of this odd-electron
intermediate was probed using quantum chemical calcula-
tions. Future work might be aimed at liberation of the
coordinated diazoalkane from the metal scaffold and the
application of C-C coupled diazoalkane ligands in charge-
storage devices.
Interested in the electronic effects of the para substi-
tuent, we investigated oxidative cleavage for [1-R]2. One
would expect the electronic influence of the substituent to
be less dramatic in the case of dimer oxidative cleavage
than was observed for reductive coupling of [1-R]þ
because of the absence of a conjugated π network en-
abling communication between the aromatic ring and
molybdenum. The potential (Epþ/0) for oxidative clea-
vage of [1-R]2 was measured by differential scanning
voltammetry and fit to a Hammett plot with F=-160 mV
for four points (R = Me, H, Br, CN; Figure 13). The
potential for oxidation of [1-NMe2]2 was nearly that of
[1-CN]2. In order to test for a V-shaped Hammett beha-
vior, the compound [1-OMe][AlCl4] was made and briefly
investigated. Oxidation of the dimeric species [1-OMe]2
occurred at Ep=417 mV. This value for R=OMe fit well
to a line when plotted with the four other points (R=Me,
H, Br, CN). It is unclear why the potential for [1-NMe2]2
does not fit to the Hammett trend; however, it is note-
worthy that this compound has other nearby oxidation
events at more positive potentials that may correspond
to oxidation of the dimethylamine functional group
(Figure S23, Supporting Information).
Experimental Procedures
General Remarks. All manipulations were carried out under
an atmosphere of purified nitrogen in a Vacuum Atmospheres
model MO-40 M glovebox equipped with the QP-30 accessory,
or by standard Schlenk techniques.114,115 Inside the MO-40 M
glovebox, the ambient temperature ranged from 18 to 22 °C. All
glassware was oven-dried at a temperature above 150 °C for at
least 12 h and allowed to cool under a dynamic vacuum prior to
˚
use. Celite, alumina, and 4 A sieves were activated by heating to
a temperature greater than 180 °C under a dynamic vacuum for
2 days (Celite) or 5 days (alumina and 4 A sieves). Et2O, n-
˚
hexane, n-pentane, and toluene were bubble-degassed with
nitrogen and forced, under positive pressure, through a column
of activated alumina followed by a column of activated Q5.116
CH2Cl2 was bubble-degassed with nitrogen and forced, under
positive pressure, through two columns of activated alumina.116
After the transient radical 1-H dimerizes to form a
C-C bond, two electrons are formally stored in a C-C
bond. We sought to engage these electrons in a reaction
that would return the starting material, [1-H]þ. Accord-
ingly, oxidative cleavage of [1-H]2 effected by either the
addition of AgOTf or [Cp2Fe][PF6] to [1-H]2 gives [1-
H][OTf] (68%) and [1-H][PF6] (78%) as products. This
demonstrates a two-step route for counterion exchange
that converts the initial diazoalkane complex [1-H][AlCl4]
into [1-H][A] (A=PF6-, OTf-; eqs 4 and 5).
(107) Venkatesan, K.; Blacque, O.; Fox, T.; Alfonso, M.; Schmalle,
H. W.; Berke, H. Organometallics 2004, 23, 1183–1186.
(108) Venkatesan, K.; Blacque, O.; Fox, T.; Alfonso, M.; Schmalle,
H. W.; Kheradmandan, S.; Berke, H. Organometallics 2005, 24, 920–932.
(109) Venkatesan, K.; Blacque, O.; Berke, H. Organometallics 2006, 25,
5190–5200.
(110) Franceschi, F.; Solari, E.; Floriani, C.; Rosi, M.; Chiesi-Villa, A.;
Rizzoli, C. Chem. Eur. J. 1999, 5, 708–721.
½1-Hꢀ½AlCl4ꢀþ½Cp2Coꢀf½1-Hꢀ2 ð97%Þþ½Cp2Coꢀ½AlCl4ꢀ
ð4Þ
(111) THF is coordinated by both Ti and V compounds when n=þ2 and
by Ti when n=0.
(112) Rosi, M.; Sgamellotti, A.; Franceschi, F.; Floriani, C. Chem.;Eur.
J. 1999, 5, 2914–2920.
(113) Gallo, E.; Solari, E.; Re, N.; Floriani, C.; Chiesi-Villa, A.; Rizzoli,
C. J. Am. Chem. Soc. 1997, 119, 5144–5154.
½1-Hꢀ2 þ½Cp2Feꢀ½PF6ꢀf½1-Hꢀ½PF6ꢀ ð78%ÞþCp2Fe ð5Þ
(114) Shriver, D. F.; Drezdon, M. A. The Manipulation of Air-Sensitive
Compounds; 2nd ed.; John Wiley & Sons: New York, 1986.
(115) Burger, B. J.; Bercaw, J. E. Experimental Organometallic Chemistry;
Wayda, A. L., Darensbourg, M. Y., Eds.; American Chemical Society:
Washington, DC, 1987; pp 79-98.
Related examples of redox-noninnocent C-C bonds
have been studied for their capacity to act as charge-
storage devices. Metallocumulenes such as Cp-
(Et2PCH2CH2PEt2)MndCdCH2 participate in oxida-
tive coupling reactions to form a C-C bonded dimer
(116) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518–1520.

Article
Inorganic Chemistry, Vol. 48, No. 15, 2009 7191
THF was dried over sodium metal and distilled from dark
purple solutions of sodium benzophenone ketyl. MeCN was
taken from an Aldrich Sure-Seal bottle, filtered through a
column of activated alumina (4 ꢁ 3 cm), and degassed under a
dynamic vacuum. O(SiMe3)2 was distilled from dark purple
solutions of sodium benzophenone ketyl. CHCl3 was refluxed
over CaH2 for 48 h prior to distillation. All solvents were stored
hydrogen-atom-labeled H1 in the structures for [1-H][AlCl4], [1-
NMe2][AlCl4], and [1-H]2. The atom labeled H1 was located in a
Fourier difference map, and the C-H distance was refined upon.
Quantum Chemical Calculations. All DFT calculations were
carried out using the Amsterdam Density Functional (ADF)
program package, versions 2004.01 or 2008.01.121-123 The
geometry of 1-m was optimized using the local exchange-
correlation potential of Vosko et al.124 augmented self-consis-
tently with gradient-corrected functionals for electron exchange
according to Becke125 and for electron correlation according to
Perdew.126,127 This nonlocal density functional is termed BP86
in the literature.128 All other calculations were carried out using
the OLYP functional, which is a combination of the OPTX
exchange functional of Handy and Cohen used with Lee, Yang,
and Parr’s nonlocal correlation function (LYP).129-131 For all
calculations, relativistic effects were included using the zero-
order regular approximation (ZORA).132,133 The basis set used
was the all-electron ADF ZORA/TZ2P (triple-ζ with two
polarization functions) basis. NMR spectroscopy chemical
shifts and coupling constants were obtained from optimized
geometries of the full molecules [1-H]þ and [1-H]2. Crystallo-
graphically determined atomic positions were used as the start-
ing point for all geometry optimizations.
15N NMR Data for 15N2-Me3SiNNMo(N[t-Bu]Ar)3. This
compound was prepared from Mo(N[t-Bu]Ar)3 and 15N2 foll-
owing literature procedures.16 15N{1H} NMR (50 MHz, CDCl3,
20 °C): δ 400 (d, 1JNN=12 Hz, NMo), 278 (d, 1JNN=12 Hz, NSi).
Identical chemical shifts and coupling constants were obtained
at 50 MHz and 20 °C in C6D6. When 1H decoupling is not
applied, the resonance at 280 ppm is unobserved, consistent with
weakened intensity when the resonance is split by the protons on
the trimethylsilyl group.
˚
over 4 A sieves. C6D6 was degassed by three freeze-pump-
thaw cycles and stored over 4 A sieves for 3 days prior to use.
CDCl3 and CD2Cl2 were refluxed over CaH2 for 24 h then
˚
1
distilled and stored over 4 A sieves. H and 13C NMR shifts are
˚
referenced to residual solvent resonances (for CD2Cl2, 5.32 and
54.00 ppm). All 15N NMR spectra are externally referenced
to neat H3CC15N (δ=245 ppm) in comparison to liquid NH3
(δ=0 ppm).117 UV-vis spectra were obtained on HP8453
spectrophotometers in 1 cm quartz cells manufactured by
Starna. Infrared spectra were obtained on a Nicolet Magna
860 FT-IR in the transmission mode. Positive ion electrospray
ionization mass spectra (ESI-MS) were obtained using a Bruker
Daltonics APEXIV, 4.7 T Fourier Transform Ion Cyclotron
Resonance Mass Spectrometer. Combustion analysis was per-
formed by Midwest Microlab, LLC (Indianapolis, Indiana).
Raman Spectroscopy. An Invictus solid-state laser at 785 nm
manufactured by Kaiser Optics was routed through fiber optic
cables to a Hololab series 5000 Raman Microscope. The sample
was placed in an NMR tube under the objective (10ꢁ) of the
microscope. The Stokes-shifted Raman scattering was observed
via 180° reflectance through the objective of the Raman micro-
scope at a laser power of 30 mW. Reported Raman spectra are
an average of eight spectra, each collected for 2 s using an
automated routine in Hololab software to automatically correct
for the dark current. The averaged spectrum has the solvent
signals subtracted and was subsequently baseline-corrected
using GRAMS software. Raman shifts are reported in reference
to an external standard of neat cyclohexane (801.3 cm-1).
Electrochemical Measurements. A Bioanalytic Systems CW-
50 potentiostat, 1-mm-diameter Pt disk working electrode,
Pt wire counter electrode, and Ag wire pseudoreference elec-
trode were used for all measurements. All electrochemical
measurements were made in 0.2 M [N(n-Bu)4][PF6] THF solu-
tions. To measure potentials, Cp*2Fe was used as an internal
standard. The Cp*2Fe0/þ couple is referenced to 117 mV versus
SCE. Reversible redox potentials of [1-R][AlCl4] were measured
by cyclic voltammetry of a 50 μM solution at several scan rates.
Oxidative peak potentials of in situ formed [1-R]2 were mea-
sured by differential pulse voltammetry (DPV) acquired in
5 mM solutions of [1-R][AlCl4]. DPV data were acquired with
a single scan from ca. -1 to þ1 V taken at 20 mV/s using the
following data collection parameters: pulse height, 50 mV; pulse
length, 50 ms; sampling intervals, 20 ms; and cycle repeat time,
200 ms. For R=OMe, all potentials were measured at arbitrary
concentrations. Isolated samples of [1-H][AlCl4] and [1-H]2
show the same features in the cyclic voltammogram.
Preparation of [4-RC6H4C(H)NNMo(N[t-Bu]Ar)3][AlCl4], [1-
R][AlCl4]. The procedure used for [1-H][AlCl4] is given. Minor
modifications of this procedure were used in the synthesis of all
other diazoalkane salts, [1-R][AlCl4], discussed herein. See the
Supporting Information for complete preparative details.
At 20 °C, 5 mL of Et2O was added dropwise to a stirred
suspension of AlCl3 (755 mg, 5.655 mmol, 2.08 equiv) in 10 mL
of n-pentane to form a solution. The so-obtained solution was then
added dropwise to a mixture of Me3SiNNMo(N[t-Bu]Ar)3 (2.00 g,
2.755 mmol) and PhCHO (293 mg, 2.764 mmol, 1 equiv) in 80 mL
of Et2O. Upon addition, a red precipitate formed. The mixture was
additionally stirred for 2 h before the solids were collected by
filtration of the reaction mixture. The so-obtained solids were
washed with Et2O (15 mL ꢁ 6) to remove a yellow filtrate. Drying
of the solids under a dynamic vacuum afforded the product as a red
powder (2.333 g, 2.491 mmol, 90.4%). Crystals suitable for X-ray
diffraction studies were grown from THF/n-pentane at -35 °C.
Characterization data are listed below.
(121) te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Fonseca Guerra,
C.; van Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001,
22, 931–967.
(122) Fonseca Guerra, C.; Snijders, J. G.; te Velde, G.; Baerends, E. J.
Theor. Chem. Acc. 1998, 99, 391–403.
(123) Baerends, E. J. et al. ADF2004.01; SCM, Vrije Universiteit:
Amsterdam, 2004.
(124) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58,
1200–1211.
(125) Becke, A. D. Phys. Rev. A 1988, 38, 3098–3100.
(126) Perdew, J. P. Phys. Rev. B 1986, 34, 7406.
(127) Perdew, J. P. Phys. Rev. B 1986, 34, 7406–7406.
(128) Deng, L. Q.; Schmid, R.; Ziegler, T. Organometallics 2000, 19,
3069–3076.
Single Crystal X-Ray Diffraction. X-ray diffraction data were
collected using a Siemens Platform three-circle diffractometer
coupled to a Bruker-AXS Smart Apex CCD detector with
˚
graphite-monochromated Mo KR radiation (λ = 0.71073 A),
performing φ and ω scans. The molecular structure was obtained
by direct methods using SHELXS118 and refined against F2 on
all data by full-matrix least-squares with SHELXL-97.119,120 All
non-hydrogen atoms were refined anisotropically; all hydrogen
atoms were included into the model at their geometrically calcu-
lated positions and refined using a riding model except for the
(129) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789.
(130) Handy, N. C.; Cohen, A. J. Mol. Phys. 2001, 99, 403–412.
(131) Baker, J.; Pulay, P. J. Comput. Chem. 2003, 24, 1184–1191.
(132) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1993,
99, 4597–4610.
(133) van Lenthe, E.; Snijders, J. G.; Baerends, E. J. J. Chem. Phys. 1996,
105, 6505–6516.
(117) Binsch, G.; Lambert, J. B.; Roberts, B. W.; Roberts, J. D. J. Am.
Chem. Soc. 1964, 86, 5564–70.
(118) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.
::
::
(119) Sheldrick, G. M. SHELXL-97; University of Gottingen: Gottingen,
Germany, 1997.
(120) Sheldrick, G. Acta Crystallogr. 2008, A64, 112–122.

7192 Inorganic Chemistry, Vol. 48, No. 15, 2009
Curley et al.
Characterization Data for [4-Me2NC6H4C(H)NNMo(N[t-
Bu]Ar)3][AlCl4], [1-NMe2][AlCl4]. 1H NMR (600 MHz, CDCl3,
20 °C): δ 8.56 (1 H, s, CHN2), 7.84 (2 H, br, Me2NC3H2), 6.98
(3 H, s, para-Ar), 6.81 (2 H, d, H2C3C(H)N2), 5.78 (6 H, s, ortho-
Ar), 3.24 (6 H, s, NCH3), 2.18 (18 H, s, Ar-CH3), 1.26 (27 H, s,
C(CH3)3) ppm. 13C NMR (125 MHz, CDCl3, 20 °C): δ 163.8
(N2C), 156.1 (Me2NC), 145.5 (ipso-Ar), 138.6, 130.7, 129.3, 116.6,
112.9, 68.0 (CMe3), 40.7 (NMe2), 32.2 (CMe3), 21.6 (Ar-Me) ppm;
one carbon was obscured. 27Al NMR (125 MHz, CDCl3, 20 °C): δ
103.4 ppm. ESI-MS: m/z787.4348 (787.4339, Mþ). Anal. Calcdfor
C45H65N6MoAlCl4: C, 56.61; H, 6.86; N, 8.80; Cl, 14.85. Found:
C, 56.42; H, 6.84; N, 8.85; Cl, 14.83. UV-vis: λ (ε) 538 (45400), 509
(35300, shoulder) nm (M-1cm-1). MP=125-135 °C (dec).
Characterization Data for [4-MeOC6H4C(H)NNMo(N[t-
br, 133.6, 131.2, 130.9, 128.5, 69.9 (CMe3), 32.0 (CMe3), 21.6
(Ar-Me) ppm; one carbon was obscured. 27Al NMR (125 MHz,
CDCl3, 20 °C): δ 103.4 ppm. ESI-MS: m/z 769.3870 (769.3869,
Mþ). Anal. Calcd for C44H59AlCl4MoN6: C, 56.39; H, 6.35;
N, 8.97; Cl, 14.94. Found: C, 56.04; H, 6.40; N, 8.65; Cl, 14.71%.
MP=112-120 °C (dec).
Thermal Stability of [PhC(H)NNMo(N[t-Bu]Ar)3][AlCl4], [1-H]-
[AlCl4]. Into 1.0 mL of CDCl3 was dissolved [1-H][AlCl4] (50 mg,
0.055 mmol) to form a red solution. The solution was then heated to
50 °C for 5 days, over which time no color change was observed.
Analysis by 1H NMR spectroscopy showed only [1-H][AlCl4] and
no evidence of decomposition.
Preparation of μ-(PhC(H)NN)2[Mo(N[t-Bu]Ar)3]2, [1-H]2. At
-35 °C, to a solution of [1-H][AlCl4] (1.00 g, 1.24 mmol) in
30 mL of CHCl3 was added Cp2Co (234 mg, 1.24 mmol, 1 equiv)
as a solid. The vessel that contained Cp2Co was rinsed with
10 mL of n-pentane that was also added to the reaction mixture.
The resulting mixture rapidly changed in color from the initial
dark red to yellow. The mixture was stirred for 20 min while
warming to 20 °C. Filtration of the reaction mixture through
Celite removed yellow solids that were washed with n-pentane
(30 mL ꢁ 2). The filtrate was then filtered through Celite a
second time and the solvent removed under a dynamic vacu-
um. The dried filtrate was collected as a yellow powder (764 mg,
1.20 mmol, 96.7%), MP=156-160 °C (dec). 1H NMR (500 MHz,
CDCl3, 20 °C): δ 7.28 (4 H, t, meta-Ph), 7.20 (6 H, m, ortho- and
para-Ph), 6.74 (6 H, s, para-Ar), 6.16 (2 H, s, CHN2), 5.73 (12 H, s,
ortho-Ar), 2.10 (36 H, s, Ar-CH3), 1.27 (54 H, s, C(CH3)) ppm. 1H
NMR (500 MHz, d8-THF, 20 °C): δ 7.28 (4 H, t, meta-Ph), 7.13
(6 H, m, ortho- and para-Ph), 6.75 (6 H, s, para-Ar), 6.2 (2 H, s,
CHN2), 5.78 (12 H, s, ortho-Ar), 2.09 (36 H, s, Ar-CH3), 1.26
(54 H, s, C(CH3)) ppm. 13C NMR (125 MHz, CDCl3, 20 °C): δ
149.1 (ipso-Ar), 141.6, 136.8, 130.9, 130.1, 127.5, 126.1, 126.0, 73.8
(CMe3), 62.4 (N2C), 32.9 (CMe3), 21.4 (Ar-Me) ppm. FT-IR
(CDCl3, 20 °C): νNN 1453 cm-1. Anal. Calcd for C86H120Mo2N10:
C, 69.52; H, 8.14; N, 9.43. Found: C, 69.34; H, 8.19; N, 9.36. A
single crystal suitable for X-ray crystallographic analysis was
obtained by storing a THF solution at -35 °C for several days.
For 15N4-[1-H]2, 15N NMR (50 MHz, CDCl3, 20 °C): 428 (d,
1JNN=17 Hz, NMo), 304 (d, 1JNN=17 Hz, NC) ppm.
1
Bu]Ar)3][AlCl4], [1-OMe][AlCl4]. H NMR (600 MHz, CDCl3,
20 °C): δ 8.78 (1 H, s, CHN2), 8.03 (2 H, d, H2C3C(H)N2), 7.10 (2
H, d, MeOC3H2), 7.00 (3 H, s, para-Ar), 5.74 (6 H, s, ortho-Ar),
3.94 (3H, s, OCH3), 2.18 (18 H, s, Ar-CH3), 1.26 (27 H, s,
C(CH3)3) ppm. 13C NMR (125 MHz, CDCl3, 20 °C): δ 166.4
(MeOC), 164.8 (N2C), 145.2 (ipso-Ar), 138.7, 133.4, 130.9,
128.9, 122.0, 115.8, 68.8 (CMe3), 56.2 (OMe), 32.0 (CMe3),
21.3 (Ar-Me) ppm. 27Al NMR (125 MHz, CDCl3, 20 °C): δ
103.4 ppm. Anal. Calcd for C44H62AlCl4MoN5O: C, 56.12; H,
6.64; N, 7.44; Cl, 15.05. Found: C, 55.86; H, 6.26; N, 7.53; Cl,
15.00%.
Characterization Data for [4-MeC6H4C(H)NNMo(N[t-
Bu]Ar)3][AlCl4], [1-Me][AlCl4]. 1H NMR (600 MHz, CDCl3,
20 °C): δ 8.87 (1 H, s, CHN2), 7.96 (2 H, d, H2C3C(H)N2), 7.40
(2 H, d, MeC3H2), 7.02 (3 H, s, para-Ar), 5.75 (6 H, s, ortho-Ar),
2.50 (3 H, s, C6H4CH3), 2.19 (18 H, s, Ar-CH3), 1.28 (27 H, s,
C(CH3)3) ppm. 13C NMR (125 MHz, CDCl3, 20 °C): δ 165.6
(N2C), 147.7, 145.3 (ipso-Ar), 138.8, 131.0, 131.0, 130.8, 128.9,
127.5, 69.2 (CMe3), 32.1 (CMe3), 22.5 (C6H4Me), 21.6 (Ar-Me)
ppm. 27Al NMR (125 MHz, CDCl3, 20 °C): δ 103.4 ppm. ESI-
MS: m/z 758.4081 (758.4073, Mþ). Anal. Calcd for
C44H62AlCl4MoN5: C, 57.06; H, 6.75; N, 7.57; Cl, 15.12. Found:
C, 57.03; H, 6.76; N, 7.36; Cl, 14.78%. MP=191-193 °C.
Characterization Data for [PhC(H)NNMo(N[t-Bu]Ar)3]-
[AlCl4], [1-H][AlCl4]. 1H NMR (600 MHz, CDCl3, 20 °C):
δ 8.96 (1 H, s, CHN2), 8.10 (2 H, d, ortho-Ph), 7.70 (1 H, t,
para-Ph), 7.61 (2 H, t, meta-Ph), 7.04 (3 H, s, para-Ar), 5.75 (6 H,
s, ortho-Ar), 2.20 (18 H, s, Ar-CH3), 1.29 (27 H, s, C(CH3)3)
ppm. 13C NMR (125 MHz, CDCl3, 20 °C): δ 165.5 (NNC), 145.3
(ipso-Ar), 135.8, 131.1, 130.9, 130.9, 130.2, 129.9, 128.8, 69.4
(CMe3), 32.1 (CMe3), 21.6 (Ar-CH3) ppm. 27Al NMR (125
MHz, CDCl3, 20 °C): δ 103.4 ppm. ESI-MS: m/z 744.3939
(744.3917, Mþ). Anal. Calcd for C43H60AlCl4MoN5: C, 56.62;
H, 6.64; N, 7.68; Cl, 15.35. Found: C, 56.77; H, 6.73; N, 7.36; Cl,
15.33%. For 15N2-[1-H][AlCl4], 15N NMR (50 MHz, CDCl3,
Preparation of μ-(4-RC6H4C(H)NN)2[Mo(N[t-Bu]Ar)3]2, [1-
R]2, by Cp2Co reduction of [4-R-C6H4C(H)NNMo(N[t-Bu]Ar)3]-
[AlCl4], [1-R][AlCl4]. Into 0.5 mL of d8-THF was dissolved
[1-R][AlCl4] to form a red solution. To this solution was added
approx 1 equiv of Cp2Co in the proportions given below. Upon
addition, the solution took on a yellow hue immediately, and a
light yellow precipitate formed, presumably [Cp2Co][AlCl4],
after stirring the mixture for 3 min. The reaction mixture was
filtered and the filtrate analyzed by 1H NMR spectroscopy. For
[1-NMe2][AlCl4] (50 mg, 0.052 mmol) and Cp2Co (10 mg,
0.054 mmol), 1H NMR (500 MHz, d8-THF, 20 °C): δ 7.12
(4 H, d, Me2NC3H2), 6.74 (6 H, s, para-Ar), 6.55 (4 H, d,
H2C3C(H)N2), 6.09 (2 H, s, CHN2), 5.76 (12 H, s, ortho-Ar),
2.91 (6 H, s, Me), 2.08 (36 H, s, Ar-CH3), 1.28 (54 H, s,
C(CH3)3) ppm and δ 7.66 (4 H, d, Me2NC3H2), 7.31 (4 H, d,
H2C3C(H)N2), 6.74 (6 H, s, para-Ar), 5.25 (2 H, s, CHN2), 5.74
(12 H, s, ortho-Ar), 2.93 (6 H, s, Me), 2.07 (36 H, s, Ar-CH3),
1.29 (54 H, s, C(CH3)3) ppm formed in a 1.0:0.4 ratio. For [1-
Me][AlCl4] (48mg, 0.052mmol) and Cp2Co(10mg, 0.054mmol),
1H NMR (500 MHz, d8-THF, 20 °C): δ 7.50 (4 H, br s,
MeC3H2), 7.15 (6 H, s, para-Ar), 6.96 (4 H, s, H2C3C(H)N2),
6.14 (2 H, s, CHN2), 5.84 (6 H, s, C6H4CH3), 5.78 (12 H, s, ortho-
Ar), 2.09 (36 H, s, Ar-CH3), 1.26 (54 H, s, C(CH3)3) ppm and
δ 7.37 (4 H, br s, MeC3H2), 7.08 (6 H, s, para-Ar), 6.75 (4 H, s,
H2C3C(H)N2), 6.32 (2 H, s, CHN2), 5.84 (6 H, s, C6H4CH3),
5.78 (12 H, s, ortho-Ar), 2.24 (36 H, s, Ar-CH3), 1.35 (54 H, s,
C(CH3)3) ppm formed in a 1.0:0.85 ratio. For [1-H][AlCl4]
(50 mg, 0.053 mmol) and Cp2Co (11 mg, 0.058 mmol), 1H
NMR data are identical to that given above. For [1-Br][AlCl4]
1
2
20 °C): 384 (dd, JNN=17 Hz, JNH=3 Hz, Mo-N), 417 (dd,
1JNN=17 Hz, 3JNH=6 Hz, N-C) ppm. MP=192-194 °C.
Characterization Data for [4-BrC6H4C(H)NNMo(N[t-Bu]-
Ar)3][AlCl4], [1-Br][AlCl4]. 1H NMR (600 MHz, CDCl3, 20 °C):
δ 8.98 (1 H, s, CHN2), 8.00 (2 H, d, H2C3CHN2), 7.73 (2 H, d,
H2C3Br), 7.02 (3 H, s, ortho-Ar), 5.75 (6 H, s, para-Ar), 2.20 (18 H,
s, Ar-CH3), 1.28 (27 H, s, C(CH3)3 ppm. 13C NMR (125 MHz,
CDCl3, 20°C): δ164.4 (NNC), 145.3 (ipso-Ar), 138.8, 133.3, 132.0,
131.1, 131.0, 128.9 (C-Br), 128.8, 69.5 (CMe3), 32.1 (CMe3), 21.6
(Ar-Me) ppm; one carbon was obscured. 27Al NMR (125 MHz,
CDCl3, 20 °C): δ 103.4 ppm. ESI-MS: m/z 822.3026 (822.3022,
Mþ). Anal. Calcd for C43H59AlBrCl4MoN5: C, 52.17; H, 6.01; N,
7.07; Br, 7.97; Cl, 14.14. Found: C, 51.79; H, 5.92; N, 6.97; Br, 7.57;
Cl, 13.43%. MP=189-192 °C.
Preparation of [4-NCC6H4C(H)NNMo(N[t-Bu]Ar)3][AlCl4],
[1-CN][AlCl4]. 1H NMR (600 MHz, CDCl3, 20 °C): δ 9.12 (1 H,
s, CHN2), 8.28 (2 H, d, H2C3CHN2), 7.84 (2 H, d, H2C3CN),
7.03 (6 H, s, para-Ar), 5.75 (6 H, s, ortho-Ar), 2.20 (18 H, s, Ar-
CH3), 1.29 (27 H, s, C(CH3)3) ppm. 13C NMR (125 MHz,
CDCl3, 20 °C): δ 163.0 (N2C), 145.3 (ipso-Ar), 138.9 br, 134.6

Article
Inorganic Chemistry, Vol. 48, No. 15, 2009 7193
1
(52 mg, 0.053 mmol) and Cp2Co (10 mg, 0.054 mmol), 1H NMR
(500 MHz, d8-THF, 20 °C): δ 7.33 (4 H, br s, BrC3H2), 7.22 (4 H,
s, H2C3C(H)N2), 6.76 (6 H, s, para-Ar), 6.13 (2 H, s, CHN2),
5.73 (12 H, s, ortho-Ar), 2.09 (36 H, s, Ar-CH3), 1.26 (54 H, s,
C(CH3)3). For [1-CN][AlCl4] (50 mg, 0.053 mmol) and Cp2Co
31P NMR (162 MHz, CDCl3, 20 °C): δ -144 (sept, JPF
715 Hz) ppm.
=
Oxidative Cleavage of μ-(PhC(H)NN)2[Mo(N[t-Bu]Ar)3]2, [1-
H]2, by AgOTf. The [1-H]2 (45.1 mg, 0.0304 mmol) was dissolved
in Et2O (20 mL). To the solution was added AgOTf (15.6 mg,
0.0607 mmol), and the mixture was stirred for 2 h at room
temperature. The mixture was filtered, and the volatiles in the
filtrate were removed in vacuo. Crystallization from benzene/
THF layered with n-pentane gave bright orange microcrystals
of [(PhCHNN)Mo(N[t-Bu]Ar)3][OTf] (37.0 mg, 68% yield). 1H
1
(11 mg, 0.058 mmol), H NMR (500 MHz, d8-THF, 20 °C):
δ 7.54 (4 H, br s, NCC3H2), 7.43 (4 H, s, H2C3C(H)N2), 6.79
(6 H, s, para-Ar), 6.32 (2 H, s, CHN2), 5.78 (12 H, s, ortho-Ar),
2.10 (36 H, s, Ar-CH3), 1.26 (54 H, s, C(CH3)3).
Oxidative Cleavage of μ-(PhC(H)NN)2[Mo(N[t-Bu]Ar)3]2, [1-
H]2, by [Cp2Fe][PF6]. A 5 mL THF solution of [1-H]2 (300 mg,
202 mmol) was frozen in a liquid-nitrogen-cooled cold well. To
the frozen solution was added [Cp2Fe][PF6] (135 mg, 408 mmol,
2 equiv) as a solid. Upon thawing of the initially yellow solution,
an immediate color change to red-orange occurred. The mixture
was allowed to stir at 20 °C for 20 min. The mixture was filtered
through Celite, and the Celite pad was washed with 5 mL of
THF. The combined filtrate was dried under a dynamic vacuum
to afford a red-orange powder. The powder was stirred in 15 mL
of n-pentane at 20 °C for 15 min to form a yellow solution,
presumably containing the Cp2Fe byproduct, which was re-
moved by filtration. The red solids that remained atop the frit
were washed with n-pentane (5 mL ꢁ 2) until the washings
were colorless and dried under a dynamic vacuum (280 mg,
315 mmol, 78%), MP=186-192 °C (dec). 1H NMR data
are identical to those reported for [1-H][AlCl4]. 19F NMR
NMR data are identical to those reported for [1-H][AlCl4]. 19
NMR (282 MHz, CDCl3, 20 °C): -76.9 ppm. Anal. Calcd for
C44H60F3MoN5O3S: C, 59.25; H, 6.78; N, 7.85; Found: C,
59.01; H, 6.63; N, 7.65%.
F
Acknowledgment. Emily McLaurin is thanked for acquiring
the fluorescence spectrum of [1-NMe2][AlCl4]. We thank Pro-
fessor Daniel G. Nocera for insightful discussions and experi-
mental assistance. Professors Bill Geiger and Jean-Michel
ꢀ
Saveant are thanked for helpful discussions. We gratefully
thank the NSF for funding this work (Grant CHE-719157).
1
Supporting Information Available: ESI-MS, H NMR, IR,
Raman, and CV data for compounds [1-R][AlCl4] in addition to
computational details. This material is available free of charge
via the Internet at http://pubs.acs.org.
1
(376 MHz, CDCl3, 20 °C): δ -73 (d, JFP = 715 Hz) ppm.