Single-site N-N bond cleavage by Mo(iv): Possible mechanisms of hydrazido(1-) to nitrido conversion

Article abstract of DOI:10.1039/c2dt32643d

Mo(NMe₂)₄ and the tridentate, dipyrrolyl ligand H₂dpma^mes were found to form 5-coordinate Mo(NMe ₂)₂(dpma^mes) (1), which exhibits spin-crossover behaviour in solution. The complex is a ground state singlet with a barrier of 1150 cm^-1 for production of the triplet in d₈-toluene. The complex reacts with 1,1-disubstituted hydrazines or O-benzylhydroxylamine to produce nitrido MoN(NMe₂)(dpma^mes). The mechanism of the 1,1-dimethylhydrazine reaction with 1 was examined along with the mechanism of substitution of NMe₂ with H₂NNMe₂ in a diamagnetic zirconium analogue. The proposed mechanism involves production of a hydrazido(1-) intermediate, Mo(NMe₂)(NHNMe₂)(dpma ^mes), which undergoes an α,β-proton shift and N-N bond cleavage with metal oxidation to form the nitrido. The rate law for the reaction was found to be -d[1]/dt = k_obs[1][hydrazine] by initial rate experiments and examination of the full reaction profile. This conversion from hydrazido(1-) to nitrido is somewhat analogous to the proposed mechanism for O-O bond cleavage in some peroxidases.

Full text of DOI:10.1039/c2dt32643d

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Single-site N–N bond cleavage by Mo(IV): possible
mechanisms of hydrazido(1–) to nitrido conversion†
Cite this: Dalton Trans., 2013, 42, 2530
Stephen A. DiFranco, Richard J. Staples and Aaron L. Odom*
Mo(NMe2)4 and the tridentate, dipyrrolyl ligand H2dpmames were found to form 5-coordinate Mo-
(NMe2)2(dpmames) (1), which exhibits spin-crossover behaviour in solution. The complex is a ground state
singlet with a barrier of 1150 cm−1 for production of the triplet in d8-toluene. The complex reacts with
1,1-disubstituted hydrazines or O-benzylhydroxylamine to produce nitrido MoN(NMe2)(dpmames). The
mechanism of the 1,1-dimethylhydrazine reaction with 1 was examined along with the mechanism of
substitution of NMe2 with H2NNMe2 in a diamagnetic zirconium analogue. The proposed mechanism
involves production of a hydrazido(1–) intermediate, Mo(NMe2)(NHNMe2)(dpmames), which undergoes
an α,β-proton shift and N–N bond cleavage with metal oxidation to form the nitrido. The rate law for the
reaction was found to be −d[1]/dt = kobs[1][hydrazine] by initial rate experiments and examination
of the full reaction profile. This conversion from hydrazido(1–) to nitrido is somewhat analogous to the
proposed mechanism for O–O bond cleavage in some peroxidases.
Received 5th November 2012,
Accepted 20th November 2012
DOI: 10.1039/c2dt32643d
www.rsc.org/dalton
Introduction
The reduction of dinitrogen is arguably one of the most impor-
tant reactions ever discovered1 and is the starting point for the
production of the majority of nitrogen-containing compounds
with applications from fertilizers to pharmaceuticals. In
accordance with the importance of the reaction, numerous
studies have been carried out on the biological systems respon-
sible (nitrogenases),2 the industrial process3 for production of
ammonia (Haber–Bosch), and other systems capable of nitro-
gen reduction.1
The naturally occurring systems can, but do not always,
contain molybdenum in the active site of the cofactor but do
include an iron–sulfur cluster.4
The mechanism of the N–N cleavage has been divided into
two general forms depending largely on the sites of proto-
nation, which have been dubbed the Distal and Alternating
Mechanisms. In Scheme 1 are some of the steps commonly
attributed to these cycles.5
Michigan State University, Department of Chemistry, 578 S. Shaw Ln, East Lansing,
MI 48824, USA. E-mail: odom@chemistry.msu.edu
†Electronic supplementary information (ESI) available: Determination of rate
constants in Table 1, initial rate experiments on H2NNMe2 with 1, initial rate
experiments on concentration of 1, details for thermal conversion of syn-2 and
anti-2, UV-Vis trace of anti-2 formation using H2NNMe2 with 1, X-ray powder
diffraction on samples of 1, Eyring Plot for H2NNMe2 with 1, additional mecha-
nistic discussions, kinetics on the reaction of H2NNMe2 with 3, and details
for the single crystal X-ray diffraction experiment. CCDC 909421–909426. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2dt32643d
Scheme 1 Two catalytic cycles often discussed for N–N bond cleavage. The
exact steps for electron transfer are left ambiguous.
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Frequently in molybdenum- and tungsten-based systems, N1–Mo1–N2. The value for τ from τ = (α − β)/60 = 0.60, where a
the cleavage of the nitrogen–nitrogen single bond required in value of 1 is for tbp and a value of 0 is for sp.12
these reactions is proposed to occur through a hydrazido(2–)6
Due to weaker donation of the pyrroles to the Mo center
intermediate that becomes protonated to an ammonium relative to the dimethylamidos,13 metal–N(pyrrole) distances
imido (hydrazidium) complex (Distal Cycle of Scheme 1). If the are usually significantly longer than metal–NMe2 distances,
metal center has the two electrons required for N–N bond clea- and that is the case here. The average Mo–N(pyrrole) distance
vage, this can occur through simple N–N bond scission in in the structure was determined to be 2.091(2) Å. The average
Mo(IV) and W(IV) hydrazidium intermediates.
Mo–NMe2 distance was 1.917(2) Å. The much weaker
Conversely, some molecular iron-based systems for nitrogen donor nitrogen of the dpmames had an Mo(1)–N(3) distance of
reduction often have been suggested to proceed through 2.401(2) Å.
diazene intermediates (Alternating Cycle of Scheme 1).7
Magnetic susceptibility measurements in solution (Evan’s
In this report, we demonstrate facile single-site cleavage of method at 29.7 °C in d8-toluene)14 provided a μeff = 1.18 μB,
an N–N bond through production of a Mo(IV) hydrazido(1–). well below the spin-only moment for high-spin d2 of 2.87 μB
The available data suggest that molybdenum systems can and well below the value for the high spin tert-butylisonitrile
proceed from hydrazido(1–) to nitrido by way of α,β-proton adduct of the same compound 1·CNBut (vide infra). SQUID
migration. Consequently, pathways that include complexed magnetometry on 1 in the solid state suggests the compound
diazene,8 hydrazido(1–), and nitrido may be viable for is a ground-state singlet. Even at room temperature, the com-
Mo-based N–N bond cleavage.
pound exhibited no detectable paramagnetism as a solid. This
suggests that the compound has a thermally accessible triplet
state in solution but not in the solid state, likely because an
isomerization is required to access the higher spin state.
Examination of the magnetism in solution was done over
the accessible temperature range (∼230–350 K), bound on the
lower end by the solubility of the compound and on the higher
end by its stability in solution. The data were fit using the
expression of Gütlich and coworkers (eqn (1)).15 In our case,
Results and discussion
Synthesis and characterization of compounds
The ancillary ligand chosen was a sterically more substantial
version of the pyrrole-based N,N-di(pyrrolyl-α-methyl)-N-methyl-
amine, dpma, which we have employed in several catalytic and
stoichiometric studies previously.9 For this chemistry, mesityl
groups were installed10 into the remaining α-positions of
the pyrroles. The synthesis of the new ligand, H2dpmames, is
shown in Fig. 1 along with the synthesis and structure of the
molybdenum dimethylamido derivative prepared by trans-
amination on Mo(NMe2)4.11
The structure of 5-coordinate 1 is very nearly halfway
between trigonal bipyramidal (tbp) and square pyramidal (sp).
The largest angle subtended at molybdenum is 164.9(1)° (α)
for N3–Mo1–N5 and the second largest is 129.1(1)° (β) for
1
none of the resonances in the H NMR could be followed over
the entire temperature range due to broadening. Consequently,
we used an internal standard of PhSiMe3 in the solution with
a capillary containing d8-toluene and reference PhSiMe3 to
follow the contact shift due to the paramagnetic species.
C
δ ¼
þ δ′
ð1Þ
Tð1 þ eΔG
Þ
SC=RT
The value δ is the difference in ppm between the chemical
shift of the methyl groups in the PhSiMe3 reference (inside the
capillary) and PhSiMe3 in solution with 1. In eqn (1), C is a
constant, T is the temperature in Kelvin, and R is the gas
constant.
Fitting the data to this equation gives the plot shown in
Fig. 2. From the fit, ΔGSC, the free energy associated with the
spin crossover, was found to be 3.3 kcal mol−1 (1150 cm−1).
This is similar to values reported by Rothwell and coworkers
for a large series of W(IV) complexes that exhibited spin cross-
over in solution.16 For these cyclometallated 2,6-diphenyl-
phenol compounds, W(OC6H3Ph–C6H4–)2L2, the energy difference
varied from 358–1205 cm−1, where L was a variety of different
pyridine derivatives. Similarly, Schrock and co-workers have
reported singlet–triplet spin crossover for the Mo(IV) species
[(Me3SiNCH2CH2)3N]MoNMe2.17 The ΔGSC value reported
for this related system was ∼1800 cm−1 based on reported
enthalpy and entropy values.
Interestingly, there are now three different magnetic behav-
iours for reported Mo(IV) bis(dimethylamido) complexes
bearing derivatives of the dpma ligand. In our paper using the
Fig. 1 Synthesis and structure of Mo(NMe2)2(dpmames) (1). Hydrogens and a
toluene solvent molecule found in the lattice are omitted from the structure.
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Fig. 2 Fit of the Me3SiPh methyl group chemical shifts to eqn (1). The values
for the fit parameters are C = (2.31 0.61) × 106, ΔG = 3278 202 cal mol−1
and δ’ = 18.3 2.0 Hz.
,
Scheme 2 Synthesis of anti-2 and syn-2 by N–N or N–O bond cleavage with
isolated yields for the complexes. See Table 1 for hydrazines and yields of anti-2.
Yields of by-products in the O-benzylhydroxylamine reaction are by GC-FID.
less substituted pyrrolyl-based ligand,18 6-coordinate
Mo(NMe2)2(HNMe2)(dpma) was reported as a paramagnetic
complex with a magnetic moment close to the spin-only value
for two unpaired electrons. For this study, we confirmed that
measurement on a freshly prepared sample; the value for this
Mo(IV) 6-coordinate compound was found to be 2.48–2.40 μB
from 210–300 K. Schrock and coworkers recently reported a
All of the 6-coordinate Mo(IV) dpma compounds observed
thus far have had high spin ground states. The 5-coordinate
Mo(IV) dpma complexes so far reported apparently have been
spin crossover compounds with singlet ground states.
In an attempt to prepare the Mo(IV) terminal hydrazido(2–)
complex, dimethylhydrazine was added to 1. A diamagnetic
product was obtained, the nitrido complex anti-Mo(N)(NMe2)-
(dpmames) (anti-2) shown in Scheme 2.20 The product has
the nitrido nitrogen and methyl of the dpmames ligand on
opposite sides of the plane defined by the Mo–N1(pyrrolyl)–
N2(pyrrolyl) atoms. The yield of anti-2 in this reaction by
1H NMR was 75%, and the isolated yield was 74% (Table 1).
The structure of anti-2 is shown in Fig. 3 (top). The struc-
ture of the Mo(VI) nitrido is best approximated as square pyra-
midal (τ = 0.02).
related 5-coordinate compound Mo(NMe2)2(tpaAr), where tpaAr
=
tris[2-(3,5-trifluoromethylphenyl)pyrrolylmethyl)amine.19 In
this compound, which is structurally similar to 1, two pyrrolyl
substituents are bound to the metal center and the third is a
“dangling” NH-pyrrole group. Interestingly, this compound is
reported to have a mixture of broad and sharp lines in the
NMR that were “slightly paramagnetically shifted”. Conse-
quently, this complex, contrary to 1, seems to have a prepon-
derance of the singlet complex in solution. In this report, 1
is a spin-crossover compound in solution and apparently
diamagnetic in the solid state.
The expectation in proceeding from the formally Mo(IV)
complex 1 to the Mo(VI) complex 2 is that the bond distances
should shorten. However, the Mo1–N(pyrrolyl) average dis-
tance in 2 is 2.127(3) Å, slightly longer than the 2.091(2) Å
found in 1. This lengthening of the pyrrolyl distance in 2 vs. 1
may be due to the rigidity of the dpmames ligand in this square
planar derivative and the widening of the N(pyrrolyl)–Mo–N-
(pyrrolyl) angle from 129.06(7)° in 1 to 145.0(1)° in anti-2. The
Mo1–N3(donor amine) distance in higher valent 2 was found
to be 2.274(3) Å, whereas in 1 it was a much longer 2.401(2) Å.
The Mo1–NMe2 distance in 2 is 1.906(3) Å, which is not
statistically different from the 1.917(2) Å average distance in 1.
The Mo–N(nitrido) distance in 2 was found to be 1.647(3) Å.
Other 1,1-disubstituted hydrazines react with 1 to give the
same product (Scheme 2 and Table 1).
ð2Þ
To examine the relationship between the electronic struc-
ture of 1 and its coordination number further, we prepared
(eqn (2)) the tert-butylisonitrile adduct Mo(NMe2)2(CNBut)-
(dpmames) (1·CNBut). This adduct is quite similar structurally
to 1 with the CNBut ligand trans to one of the dimethylamido
ligands and with the donor amine of the dpmames trans to the
other NMe2 group. The complex is high spin with μeff = 2.47 μB
like the previously reported 6-coordinate Mo(NHMe2)-
(NMe2)2(dpma). The isonitrile adduct of 1 was also structurally
characterized (see the ESI† for details).
Addition of O-benzylhydroxylamine (Scheme 2) led to for-
mation of the syn-isomer of 2, where the amine donor methyl
of the dpmames and nitrido nitrogen are on the same side of
the Mo–N1(pyrrolyl)–N2(pyrrolyl) plane. The isomer syn-2 was
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Table 1 Rates and yields of the reactions of various substrates with 1 to form 2
Substrate (H2NX)
H2NNMe2
% Yield of 2a (1H NMR)
By-product % yield HX (GC-FID)
kobsc (M−1s−1 10−3
)
b
75
18
—
873
684
5
2
31
b
81
—
146.8 0.1
H2NN(Ph)Me
H2NNPh2
H2NOBn
82
76
31
91
135.8 0.2
21.1 0.1
—
98
51e
d
a The product is 2-anti except for the reaction with H2NOBn which gave 2-syn. b By-product yield was not determined. c Errors are from the fits
and then propagated through the equations. d Reaction was too fast to measure using the methods employed here. e Yield given is for benzyl
alcohol, but 3 by-products were identified. See the Experimental and Scheme 2.
the functional a similar value of 2 kcal mol−1 was obtained
with syn more stable than anti. Experimentally, heating syn-2
in toluene at 100 °C for 24 h led to some conversion to the
anti-2 isomer (see the ESI† for more details); however, the con-
version did not continue to completion and some decompo-
sition also occurred. Only about 9% anti was produced during
the heating of the syn isomer. Alternatively, heating anti-2 did
not result in detectable (1H NMR) amounts of syn-2; only
decomposition was observed. It seems that the energies of the
isomers are very comparable but kinetic barriers hamper the
equilibrium. The isomer, syn or anti, produced in the reaction
of 1 and hydrazine or hydroxylamine is determined kinetically.
Since the hydrazido(1–) derivatives are unstable intermedi-
ates in the case of Mo(IV), in order to examine their structure,
we prepared the Zr(IV) analogues where no bond cleavage can
occur. The zirconium bis(dimethylamido) complex Zr(NMe2)2-
(dpmames) (3) is cleanly produced by addition of H2dpmames to
Zr(NMe2)4; 3 was also structurally characterized (see the ESI†).
The addition of one equivalent of H2NNMe2 to 3 provides
mixtures of the bis(hydrazido(1–)) complex Zr(NHNMe2)2-
(dpmames) (4) and a trace of a compound not fully character-
Fig. 3 ORTEP diagram at the 50% probability level for the structures of anti-
Mo(N)(NMe2)(dpmames (anti-2, top) and syn-Mo(N)(NMe2)(dpmames
) ) (syn-2,
1
ized that has a H NMR spectrum as expected for the mono-
bottom) as determined by single crystal X-ray diffraction. H-atoms omitted.
(hydrazido(1–)) complex. It appears that the second addition of
hydrazine may have a similar rate constant to the first. The
complex 4 was prepared cleanly by addition of 2 equivalents of
the hydrazine (Fig. 4).
isolated in 31% yield. Also found in the reaction mixture were
benzyl alcohol (51% yield), benzaldehyde (11% yield), and 1,2-
diphenylethane (6% yield), where the yields are relative to
internal standard (dodecane) from GC-FID.
Mechanistic investigations
The syn-isomer also was structurally characterized (Fig. 3
bottom). The structure of syn-2 is, like the structure of 1, in
between sp and tbp with τ = 0.61. The Mo–N(pyrrolyl) and
Mo–NMe2 distances in syn-2 are the same within error as in 1.
The Mo1–N3(donor) and Mo1–N5(nitrido) distances are the
same as in anti-2.
A computational study was carried out on the two isomers
using Density Functional Theory with the LANL2DZ basis
set as implemented in Gaussian09.21 The difference in energy
(ΔH°) between the syn and anti derivatives was calculated to be
extremely small, with syn-2 being more stable than anti by
2 kcal mol−1 using B3LYP as the functional. Using B3PW91 as
The reaction between molybdenum-containing 1 and hydr-
azines was not amenable to typical pseudo-1st order con-
ditions for the examination of the reaction kinetics. Using
either the metal complex or the hydrazine in large excess led
to very low yields of the nitrido product, and we were unable to
isolate and characterize the products under these conditions.
However, nitrido product 2 does not react with excess hydr-
azine on the timescale of the hydrazine reactions with 1.
Excess hydrazine or 1 in the N–N cleavage reactions leads to
unidentified by-product formation; however, we were able to
vary the hydrazine concentrations and examine initial rates
for the loss of 1. These experiments suggest a 1st order
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with starting material, which is likely the cause of the low
yields for this particular substrate. All other amine by-products
do not react on the timescales of nitrido formation with either
the starting material or product.
No product inhibition was found for the hydrazine reac-
tions except for addition of piperidine to reactions of N-amino-
piperidine with 1. Other by-products were tested with up to
10 equivalents of the corresponding amine and gave the
same rate constant for disappearance of 1 and provided clean
formation of 2.
The reaction with O-benzylhydroxylamine liberates benzyl
alcohol as the major by-product. The nitrido product 2 does
not react with benzyl alcohol. The starting material 1 does
react rapidly with benzyl alcohol using a radical pathway.24
We examined the temperature behaviour of the rate of 1,1-
dimethylhydrazine reactions with 1. An Eyring plot of ln(kobs/T)
vs. 1/T was linear and provided ΔH‡ = +7 kcal mol−1 and ΔS‡ =
−35 cal mol−1 K. These parameters are consistent with a very
modest enthalpic barrier and a very ordered activated complex.
The parameters are similar to many known activation para-
meters for ligand additions to metal complexes.25
The data above did not conclusively identify the rate-
determining step in the reaction. In order to further investi-
gate the NMe2 for NHNMe2 exchange as a possible rate-
determining step, we used the zirconium complex 3 and its
reaction with H2NNMe2 as a model. The reaction between 3
and two equivalents of dimethylhydrazine was followed by
1H NMR and showed 2nd-order kinetics like its molybdenum
analogue. Examination of the 2nd order rate constant versus
temperature for the zirconium reaction gave activation
parameters, ΔH‡ = +6.4 kcal mol−1 and ΔS‡ = −45 cal mol−1 K,
similar to the molybdenum system.
Fig. 4 Synthetic route to the hydrazido(1–) zirconium complex 4 and ORTEP
diagram at the 50% probability level for the structure of Zr(η2-NHNMe2)2(dp-
mames) (4) as determined by single crystal X-ray diffraction. H-atoms (pink
spheres) are omitted except on the hydrazido(1–) nitrogens N4 and N6.
dependence on hydrazine concentration. Similar initial rate
experiments changing metal concentration suggest a 1st order
dependence on the concentration of 1.
Kinetics using 1 : 1 hydrazine to 1 provided clean 2nd order
behaviour. Considering the 1st order dependence of the reac-
tion on hydrazine, 1st order dependence on 1, and 2nd order
dependence overall, the rate law is assigned as −d[1]/dt =
kobs[1][dimethylhydrazine].
The reaction to form 2 was carried out with a variety of
different substrates (Table 1). With all the hydrazine deriva-
tives investigated, anti-2 was the product. There was a dramatic
affect of hydrazine substituents on the rate of nitrido for-
mation; however, the cause of that dependence seems complex
and is likely due to a mixture of factors including steric con-
straints of the incoming reactant. Reactions with all of the
hydrazines were followed by UV-Vis absorption spectroscopy
and fit 2nd order kinetics.
We propose that the rate-determining step in the hydrazine
reaction with 1 is the dimethylamido substitution step. In
zirconium-containing 3, the second replacement of NMe2 has
a similar rate as the first replacement with dimethylhydrazine
(vide supra). A second NMe2 replacement is not observed in the
reaction with the molybdenum(IV) analogue. Since the N–N
bond cleavage in the reaction of 1 with dimethylhydrazine
would then be faster than the substitution of dimethylamido,
reaction of the first equivalent of dimethylhydrazine with
1 gives the mono(dimethylamido) complex 2. The nitrido 2 is
then inert to NMe2 replacement by dimethylhydrazine. In
other words, the unimolecular N–N bond cleavage occurs
much faster than the bimolecular reaction of hydrazine and
the unobserved hydrazido(1–) intermediate Mo(NHNMe2)-
(NMe2)(dpmames).
In light of the data above, we propose the N–N cleavage
mechanism illustrated in Scheme 3. One of the dimethylamido
ligands in 1 is protolytically replaced with a hydrazido(1–)
ligand. We speculate that the unobserved hydrazine adduct A
adopts a geometry reminiscent of previously reported18 Mo-
(NMe2)2(NHMe2)(dpma) where the donor nitrogen of NHMe2
is trans to the donor nitrogen of the dpma ancillary.
Using the G3 method22 implemented in Gaussian09, we calcu-
lated the Bond Dissociation Enthalpies (BDEs) associated
with some of the substrates in Table 1. The BDE of Me2NNH2
for the N–N bond was calculated as 60.2 kcal mol−1, whereas
the experimental BDE for this compound is 59.0 2.23 The
N–N and N–O BDEs for N-aminopyrrole and H2NOMe (as a
model for H2NOCH2Ph) were calculated as 34.9 and 54.4 kcal
mol−1, respectively. As a result, it appears that the rate of bond
cleavage is not correlated with the N–N or N–O BDE.
The only species observed by UV-Vis absorption spectro-
scopy for all of the hydrazine substrates, except N-aminopiper-
idine, over the course of the reactions are the starting material
1 and the product 2. These reactions show a clean isosbestic
point (see the ESI†). However, the reaction with N-aminopiper-
idine is complicated by reactions of the piperidine by-product
It appears that it is this bimolecular coordination of the
hydrazine (or hydroxylamine) derivative to the metal that is
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In the next step, the β-nitrogen of the hydrazido acts as a
proton acceptor during the α,β-proton shift. The N–N bond
cleavage could occur concomitant with proton migration (Path
A) or through an intermediate ammonium hydrazido(2–),
sometimes called a hydrazidium (Path B). It is unknown if the
actual α,β-proton shift occurs with the aid of another ligand,
such as the dimethylamido, or with some other species in
solution, such as amine by-product. However, such catalysis
in the proton migration is certainly possible considering
the computationally derived value for unassisted α,β-proton
migration in Mo phosphine systems was assigned a “lower
limit” of 17.5 kcal mol−1 1g
.
The reaction mechanism proposed here is an oxidative
elimination from a metal-appended nitrogen atom, where
the metal is oxidized by elimination of substituents to form a
metal ligand multiple bond.26 Tuczek and coworkers have pro-
posed a similar mechanism in their studies on the Chatt cycle,
where the N–N bond cleavage occurs with an α,β-proton shift
for molybdenum phosphine complexes.1g
In a process that might involve the microscopic reverse of
the hydrazine cleavage reaction described, the nucleophilic
addition of an amine to a nitrido with concomitant metal
reduction has been reported by Meyer and coworkers.27 In eqn
(3) is shown one example, where tpm = tris(1-pyrazolyl)-
methane.28 Several related reductive additions, where the
metal center is reduced by addition of substituents to metal
ligand multiple bonds have been reported.29
ð3Þ
Whether the nitrido ends up syn or anti with respect to the
methyl on the dpmames donor nitrogen may be determined by
which dimethylamido is kinetically preferred for protolytic
replacement in intermediate A of Scheme 3. Considering the
similarity between the two dimethylamidos, we postulate
that this is determined by steric interactions between the
complex and hydrazine/hydroxylamine substrate leading to the
difference in products observed when using hydrazines
and O-benzylhydroxylamine. Alternatively, the sites of initial
coordination for the larger H2NNR2 compounds could be as
shown in A, while the smaller H2NOBn may bind in a site
similar to the isonitrile in 1·CNBut, trans to a dimethylamido
(eqn (2)).
Scheme 3 Proposed mechanism for the reaction of 1 with H2NNMe2. Mesityl
groups on the dpmames ligand were omitted for clarity.
rate determining. The alternative rate determining steps are
proton migration (conversion from A to B) or the coordination
and proton migration occurring in a concerted fashion. We
assign the RDS as the coordination based on the similarity of
the activation parameters to other associative substitutions.24
In examining various donor ligands with 1, flat and cylin-
drically symmetric donors (CNBut, pyridine, DMAP, and 2-pico-
line) react extremely quickly with reactions being done faster
than samples can be taken. Larger donors such as the hydr-
azone formed from benzaldehyde and 1,1-dimethylhydrazine,
Me2NNvC(H)Ph, reacted very slowly over the course of days as
judged by disappearance of the UV-Vis bands in 1. Again,
steric constraints of the incoming donor ligand are one factor
in the rate of reaction in the system.
Conclusions
We have described a new 5-coordinate Mo(IV) complex with
spin crossover behaviour in solution. The free energy barrier
for the spin state change was measured as 1150 cm−1. The
singlet is the ground state in solution and was the only species
observed in the solid, suggesting that molecular dynamics
unavailable in the lattice are required for the spin equilibrium.
After protolytic cleavage of a dimethylamido and dimethyl-
amine loss, formation of the hydrazido(1–) ligand follows. The
experiments with the zirconium hydrazido model suggest that
the hydrazido(1–) is η2 in this intermediate.
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The Mo(IV) compound reacts with hydrazines and O-alkyl
Dalton Transactions
hydroxylamines to give the Mo(VI) nitrido complex, NMo-
(NMe2)(dpmames) (2). Depending on the nature of the nitrogen
atom donor molecule (hydrazines or O-benzylhydroxylamine),
two different isomers of 2 were isolated, one isomer where the
methyl group of the dpmames ligand is syn to the nitrido and
one where the methyl is anti.
We propose a mechanism where the hydrazido(1–) complex
undergoes an α,β-proton shift either concerted with N–N bond
cleavage or in a stepwise fashion with an unstable hydr-
azidium, ammonium hydrazido(2–), intermediate.
While the N–N cleavage mechanism proposed in Scheme 3
does not fall into either of the commonly discussed pathways
in Scheme 1, there is precedent for this type of mechanism out
of the peroxidase literature for O–O bond cleavage
(Scheme 4).30 It is proposed that peroxidase uses an α,β-proton
shift in a heme iron peroxide to generate a histidine-stabilized
hydrogen isoperoxide complex. Cleavage of the O–O bond,
which computationally occurs simultaneously with proton
migration (cf. Path A in Scheme 3), liberates water and gene-
rates the ferryl iron(IV) with a porphyrin radical cation (Com-
pound I).31
Scheme 4 Poulos–Kraut mechanism30 for heterolytic O–O cleavage and ferryl
generation in peroxidase.
We propose a very close nitrogen analogue of the peroxidase
mechanism is the low energy pathway that leads to facile N–N
bond cleavage in our system. Considering, the Tuczek and co-
workers proposed similar steps for the Chatt cycle and that the
microscopic reverse has also been observed in mid- to late
transition metals, this is an important possible pathway for N2
activation in biological and industrial systems.
8.01 (s br, 2H, N–H), 6.89 (s, 4H, aromatic C–H), 6.07–6.05 (m,
2H, pyrrole C–H), 5.92–5.90 (m, 2H, pyrrole C–H), 3.53 (s, 4H,
CH2), 2.28 (s, 6H Ar-p-CH3), 2.18 (s, 3H, NCH3), 2.11 (s, 12H,
Ar-o-CH3). 13C{1H} NMR (125 MHz, CDCl3): 138.3, 137.4, 130.9,
129.1, 128.0, 127.9, 108.0, 107.7, 53.5, 41.9, 21.0, 20.6. Anal.
Calcd for C29H35N3: C, 81.84; H, 8.29; N, 9.87. Found: C, 81.53;
H, 8.32; N, 9.67. Mp: 82–84 °C.
Experimental
Mo(NMe2)2(dpmames) (1)
General experimental details, tables for the X-ray diffraction
experiments, a more thorough discussion on how the kinetic
data were collected and the results can be found in the ESI.†
NMR data were collected in the Max T. Rogers NMR facility.
X-ray diffraction data were collected at the Center for Crystallo-
graphic research at MSU.
In a glove box under an N2 atmosphere, a 100 mL Schlenk
tube was loaded with a stir bar and a solution of Mo(NMe2)4
11
(1.00 g, 36.7 mmol,
1 equiv.) in toluene–hexane (1 : 4,
5 : 20 mL). To the Schlenk tube, H2dpmames (1.56 g,
3.67 mmol, 1 equiv.) in toluene (8 mL) was added. The head-
space was evacuated, and the vessel was sealed with a Teflon
stopcock. The tube was removed from the box and was placed
H2dpmames
In a 125 mL Erlenmeyer flask methylamine hydrochloride into a 55 °C oil bath for 10 h, while stirring vigorously. After
(0.911 g, 13.5 mmol, 1 equiv.) was dissolved in formaldehyde this time, the vessel was allowed to cool to room temperature
solution (37% v/v) (2.19 g, 27.0 mmol, 2 equiv.) and EtOH and was taken back inside the dry box. The volatiles were
(20 mL). The solution was transferred to a 100 mL Schlenk removed in vacuo, and the solids were washed with hexane
tube, sealed, and stirred for 10 min in a 55 °C oil bath. 2-Mesityl- (10 mL). The solids were dissolved in a minimal amount of
pyrrole (5.00 g, 27.0 mmol, 2 equiv.) in EtOH (30 mL) was toluene and held at −35 °C yielding 1 as bright green crystals
added to the Schlenk tube, and the headspace was evacuated. (1.78 g, 2.94 mmol, 80%). Magnetic susceptibility (Evan’s
The solution continued to stir at 55 °C for 7 h, during which a method, 29.7 °C): μeff = 1.178 μB. TOF-MS ES+ calcd (found):
white precipitate formed. The Schlenk tube was cooled to 608.68 (609.2). UV-Vis [toluene, 25 °C] λmax in nm (ε in cm−1
room temperature, and the precipitate was collected on a glass M−1): 643.9 (498.7), 786.9 (187.4). Mp: 138–144 °C (d). The
frit and washed with EtOH (3 × 20 mL). The solids were basi- molecule contains a disordered toluene in the lattice as crystal-
fied with aq. NaOH (1 M, 150 mL) and extracted with CH2Cl2 lized. Attempts to obtain elemental analysis were not satisfac-
(3 × 100 mL). The organic layers were combined and dried tory unless toluene was included with occupancy of 0.3. Anal.
under reduced pressure, yielding H2dpmames as a white Calcd for C34H45MoN5·0.3C7H8: C, 66.36; H, 7.52; N, 11.02.
1
powder (3.97 g, 9.32 mmol, 69%). H NMR (500 MHz, CDCl3): Found: C, 66.60; H, 7.26; N, 11.39.
2536 | Dalton Trans., 2013, 42, 2530–2539
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Dalton Transactions
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Mo(NMe2)2(CNBut)(dpmames) (1·CNBut)
aromatic C–H), 6.63 (s, 2H, aromatic C–H), 6.32 (d, JHH =
3.0 Hz, 2H, pyrrole C–H), 6.26 (d, JHH = 3.0 Hz, 2H, pyrrole
C–H), 3.66 (dd, JHH = 0.5 Hz, JHH = 14.0 Hz, 2H, CH2), 3.50 (dd,
JHH = 0.5 Hz, JHH = 14.0 Hz, 2H, CH2), 3.20 (d, JHH = 1.0 Hz,
3H, N(CH3)2), 2.74 (s, 3H, NCH3), 2.60 (s, 6H, Ar-p-CH3), 2.50
(d, JHH = 1.0 Hz, 3H N(CH3)2), 2.11 (s, 12H, Ar-o-CH3). 13C{1H}
NMR (125 MHz, CDCl3): 139.72, 139.62, 137.23, 136.86, 136.54,
134.77, 128.42, 127.45, 112.15, 106.10, 63.88, 59.31, 53.13,
47.56, 21.35, 20.98, 20.84. Mp: 117–123 °C (dec). By-products
of this reaction include benzyl alcohol (51.2%), benzaldehyde
(10.7%) and 1,2-diphenylethane (5.9%) as determined by
GC-MS/GC-FID; the yields are relative to dodecane internal
standard for calibrated samples of those compounds.
Under an N2 atmosphere, a scintillation vial was loaded with a
stir bar, 1 (0.1 g, 0.165 mmol, 1 equiv.) and toluene (5 mL). To
this, tert-butylisonitrile (0.1 g, 0.165 mmol, 1 equiv.) in toluene
(1 mL) was added. The solution was stirred and rapidly turned
dark red. After 1 h, the volatiles were removed in vacuo. The
residue was extracted with toluene (2 mL). The solution was
filtered through Celite and layered with an equal volume of
pentane. Crystallization at −35 °C gave dark red crystals of
1·CNBut in 40% yield (0.045 g, 0.066 mmol). Magnetic suscep-
tibility (Evan’s method, 28.2 °C): μeff = 2.469 μB. The molecule
contains a toluene in the lattice as crystallized. Attempts to
obtain elemental analysis were not satisfactory unless toluene
was included with full occupancy. Anal. Calcd for
C38H54MoN6·C7H8: C, 69.03; H, 7.98; N, 10.73. Found: C, 68.89;
H, 8.17; N, 10.61. Mp: 128–130 °C (dec). UV-Vis [toluene,
25 °C] λmax in nm (ε in cm−1 M−1): 372.2 (6588), 490 (3042).
Crystals for X-ray diffraction grown from toluene gave poor
structural results, and the crystals were regrown from Et2O.
Zr(NMe2)2(dpmames) (3)
Under an N2 atmosphere, a 100 mL Schlenk tube was loaded
with a stir bar, Zr(NMe2)4 (0.851 g, 3.18 mmol, 1 equiv.),
toluene (6 mL), and Et2O (1 mL). To the pressure tube was
added H2dpmames (1.35 g, 3.18 mmol, 1 equiv.) in toluene
(1 mL). The headspace was evacuated, and the tube was sealed
with a Teflon stopcock and removed from the dry box. The
solution was stirred in a 70 °C oil bath for 48 h. The tube was
taken back into the dry box, and the solution was filtered
through Celite. The volatiles were removed in vacuo yielding 3
as a yellow powder (1.75 g, 2.89 mmol, 91% yield). 1H NMR
(500 MHz, C6D6): 6.82–6.81 (m, 2H, aromatic C–H), 6.74–6.73
(m, 2H, aromatic C–H), 6.30 (dd, JHH = 2.75 Hz, JHH = 0.5 Hz,
2H, pyrrole C–H), 6.16 (d, JHH = 3.0 Hz, 2H, pyrrole C–H), 4.05
(d, JHH = 13.5 Hz, 2H, CH2), 3.40 (d, JHH = 13.5 Hz, 2H, CH2)
2.59 (s, 6H, N(CH3)2), 2.28 (s, 6H, N(CH3)2), 2.26 (s, 3H, NCH3),
2.12 (s, 6H, Ar-o-CH3), 2.10 (s, 6H, Ar-o-CH3), 2.02 (s, 6H, Ar-p-
CH3). 13C{1H} NMR (125 MHz, CDCl3): 139.07, 138.61, 138.26,
136.03, 135.94, 135.92, 128.02, 127.47, 127.37, 109.25, 104.71,
59.39, 43.25, 40.63, 39.16, 21.17, 21.01, 20.75. Mp: 262–270 °C.
Mo(N)(NMe2)(dpmames) (anti-2)
Under an N2 atmosphere, a scintillation vial was loaded with a
stir bar, 1 (0.650 g, 1.07 mmol, 1 equiv.) and toluene (8 mL).
To the stirring solution of 1, a solution of N,N-dimethylhydr-
azine (0.64 g, 1.07 mmol, 1 equiv) in toluene (1 mL) was added.
Upon addition, the solution turned brown and an orange pre-
cipitate formed. After 1 h, the volatiles were removed in vacuo.
The residue was stirred in pentane (2 mL) for 5 min, and the
suspension was filtered on a glass frit. The solids were col-
lected and dried in vacuo yielding the title compound as an
orange powder (0.459 g, 0.795 mmol, 75%). Diffraction quality
crystals were grown from a concentrated toluene solution
1
layered in pentane held at −35 °C. H NMR (500 MHz, C7D8):
6.70 (s, 2H, aromatic C–H), 6.58 (s, 2H, aromatic C–H), 6.37 (d,
JHH = 3.0 Hz, 2H, pyrrole C–H), 6.17 (dd, JHH = 0.5 Hz, JHH = 3.0 Zr(η2-NHNMe2)2(dpmames) (4)
Hz, 2H, pyrrole C–H), 4.53 (d, JHH = 13.0 Hz, 2H, CH2), 3.52 (s,
Under an N2 atmosphere, a scintillation vial was loaded with 3
3H, NCH3), 3.34 (d, JHH = 12.5 Hz, 2H, CH2), 2.43 (s, 6H, Ar-p-
CH3), 2.18 (s, 3H N(CH3)2), 2.12 (s, 3H N(CH3)2), 2.05 (s, 6H,
Ar-o-CH3), 2.04 (s, 6H, Ar-o-CH3). 13C{1H} NMR (125 MHz,
C6D6): 140.8, 140.2, 139.2, 138.4, 136.8, 136.6, 111.2, 106.8,
63.4, 46.5, 43.6, 21.2, 21.0, 20.9. Anal. Calcd for C31H39MoN5:
C, 64.46; H, 6.81; N, 12.12. Found: C 64.35; H, 6.72; N, 12.08.
Mp: 264–270 °C (dec).
(0.100 g, 0.166 mmol, 1 equiv.), a stir bar, and a mixture of
toluene and Et2O (1 : 1 v/v, 8 mL). The solution was rapidly
stirred and a 0.712 M toluene solution of 1,1-dimethylhydr-
azine was added dropwise (466 μL, 0.332 mmol, 2 equiv.). DME
(1 mL) was added. The solution stirred for 16 h, and the vola-
tiles were removed in vacuo. 4 was obtained in 86% yield as an
off-white powder (0.090 g, 1.42 mmol, 86% yield). Diffraction
quality crystals of 4 were obtained from a −35 °C concentrated
Mo(N)(NMe2)(dpmames) (syn-2)
1
toluene solution layered with Et2O. H NMR (500 MHz, C7D8):
Under an N2 atmosphere, a scintillation vial was loaded with a 6.83 (br s, 2H, aromatic C–H), 6.74 (br s, 2H, aromatic C–H),
stir bar, 1 (0.123 g, 0.202 mmol, 1 equiv.), and toluene (8 mL). 6.25 (d, JHH = 3.0 Hz, 2H, pyrrole C–H), 6.14 (dd, JHH = 1.0 Hz,
To the stirring solution of 1, a solution of O-(benzyl)hydroxyl- JHH = 3.0 Hz, 2H, pyrrole C–H), 4.30 (s, 1H, NH), 4.17 (d, JHH
=
amine (0.750 mL, 0.269 M, 0.202 mmol, 1 equiv.) in toluene 13.5 Hz, 2H, CH2), 4.10 (s, 1H, NH), 3.68 (d, JHH = 13.5 Hz, 2H,
(1 mL) was added. Upon addition, the solution turned brown. CH2), 2.48 (s, 3H, NCH3), 2.30 (s, 6H N(CH3)2), 2.22 (s, 6H,
After 2 h, the volatiles were removed in vacuo. The residue was N(CH3)2), 2.12 (s, 6H, Ar-o-CH3), 1.91 (s, 3H, Ar-o-CH3), 1.57 (s,
taken up in Et2O (5 mL) and filtered through Celite. The solu- 6H, Ar-p-CH3). 13C{1H} NMR (125 MHz, C6D6): 140.12, 139.04,
tion was concentrated in vacuo to 2 mL and held at −35 °C, 138.91, 138.89, 136.41, 135.63, 128.54, 111.29, 108.21, 104.08,
which crystallized 2-syn as orange blocks (0.0356 g, 62.57, 53.49, 50.91, 42.62, 22.44, 21.49, 21.07. Mp: 216–218 °C
0.062 mmol, 30.7%). 1H NMR (500 MHz, C6D8): 6.79 (s, 2H, (dec).
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 2530–2539 | 2537

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Dalton Transactions
Chem., 2012, 51, 439; (b) C. T. Saouma, C. E. Moore,
A. L. Rheingold and J. C. Peters, Inorg. Chem., 2011, 50,
11285; (c) L. D. Field, H. L. Li and A. M. Magill, Inorg.
Chem., 2009, 48, 5.
Acknowledgements
The authors would like to thank Kathy Severin and Dan Jones
for help in data collection. ALO would like to thank Jim
McCusker, Mitch Smith, and Reza Ghiladi for helpful discus-
sions. The financial support of the National Science Foun-
dation CHE-1012537 for this work is greatly appreciated.
8 For some representative examples of Mo and W diazenes
see: (a) M. R. Smith III, T.-Y. Cheng and G. L. Hillhouse,
Inorg.Chem., 1992, 31, 1535; (b) M. R. Smith III, T.-Y. Cheng
and G. L. Hillhouse, J. Am. Chem. Soc., 1993, 115, 8638 For
a few selected recent examples of diazene complexed with
other metals see: (c) L. D. Field, H. L. Li and S. J. Dalgarno,
Inorg. Chem., 2010, 49, 6214; (d) C. T. Saouma, P. Muller
and J. C. Peters, J. Am. Chem. Soc., 2009, 131, 10358;
(e) L. D. Field, H. L. Li, S. J. Dalgarno and P. Turner, Chem.
Commun., 2008, 1680.
9 (a) J. T. Ciszewski, J. F. Harrison and A. L. Odom, Inorg.
Chem., 2004, 43, 3605; (b) Y. Li, Y. Shi and A. L. Odom,
J. Am. Chem. Soc., 2004, 126, 1794; (c) Y. Li, A. Turnas,
J. T. Ciszewski and A. L. Odom, Inorg. Chem., 2002, 41,
6298; (d) C. Cao, J. T. Ciszewski and A. L. Odom, Organome-
tallics, 2001, 20, 5011; (e) S. A. Harris, J. T. Ciszewski and
A. L. Odom, Inorg. Chem., 2001, 40, 1987.
Notes and references
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1980, 19, 3704 Here we replace their (Δν/Δ0)LS with
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20 For an example of hydrazine cleavage in a dinuclear tant-
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6 Hydrazido(2–) and isodiazene are two different resonance 21 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
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very similar to Fischer versus Schrock carbenes and
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M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
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(a) J. L. Crossland, C. G. Balesdent and D. R. Tyler, Inorg.
2538 | Dalton Trans., 2013, 42, 2530–2539
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Paper
J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene,
J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,
alcohol to
a
nitrido has also been reported.
(c) M. H. V. Huynh, P. S. White, K. D. John and T. J. Meyer,
Angew. Chem., Int. Ed., 2001, 40, 4049.
R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, 28 Example in eqn (3) is from ref. 26a. The hydrazido(1–),
K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador,
J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas,
J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox,
compound 4b in the paper, was characterized as a mixture
with its deprotonated hydrazido(2–) derivative in 54%
yield.
GAUSSIAN 09 (Revision C.01), Gaussian, Inc., Wallingford, 29 For some selected examples see: (a) D. C. Ware and
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H. Taube, Inorg. Chem., 1991, 30, 4605; (b) T. J. Crevier and
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26 Compare with a related reaction where a formal M(II) nitro-
syl complex is formally oxidized by addition of highly redu-
cing V(III) through intraligand bond cleavage and
formation of nitrido.(a) A. L. Odom, C. C. Cummins and
J. D. Protasiewicz, J. Am. Chem. Soc., 1995, 117, 6613;
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neutral hydroamination mechanistic proposals where elec-
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