Synthesis of [(DPPNCH2CH2)3N]3- molybdenum complexes (DPP = 3,5-(2,5-Diisopropylpyrrolyl)2C 6H3) and studies relevant to catalytic reduction of dinitrogen

Article abstract of DOI:10.1021/ja1008213

Molybdenum complexes that contain a new TREN-based ligand [(3,5-(2,5-diisopropyl-pyrrolyl)₂C₆H₃NCH ₂CH₂)₃N]^3- ([DPPN₃N] ^3-) that are relevant to the catalytic reduction of dinitrogen have been prepared. They are [Bu₄N]{[DPPN₃N]MoN₂}, [DPPN₃N]MoN₂, [DPPN₃N]MoN=NH, {[DPPN ₃N]MoN=NH₂}[BAr^f₄], [DPPN ₃N]Mo≡N, {[DPPN₃N]Mo≡NH}[BAr^f ₄], and {[DPPN₃N]MoNH₃}[BAr^f ₄]. NMR and IR data for [Bu₄N]{[DPPN₃N]MoN ₂} and [DPPN₃N]MoN₂ are close to those reported for the analogous [HIPTN₃N]^3- compounds (HIPT = hexaisopropylterphenyl), which suggests that the degree of reduction of dinitrogen is virtually identical in the two systems. However, X-ray studies and several exchange studies support the conclusion that the apical pocket is less protected in [DPPN₃N]Mo complexes than in [HIPTN₃N]Mo complexes. For example, ¹⁵N/¹⁴N exchange studies showed that exchange in [DPPN₃N]MoN₂ is relatively facile (t _1/2 a 1 h at 1 atm) and depends upon dinitrogen pressure, in contrast to the exchange in [HIPTN₃N]MoN₂. Several of the [DPPN₃N]Mo complexes, e.g., the [DPPN₃N]MoN₂ and [DPPN₃N]MoNH₃ species, are also less stable in solution than the analogous parent [HIPTN₃N]Mo complexes. Four attempted catalytic reductions of dinitrogen with [DPPN₃N]MoN yielded 2.53 ± 0.35 equiv of total ammonia. These studies reveal more than any other just how sensitive a successful catalytic reduction is to small changes in the triamidoamine supporting ligand.

Full text of DOI:10.1021/ja1008213

Published on Web 05/25/2010
Synthesis of [(DPPNCH2CH2)3N]3- Molybdenum Complexes
(DPP ) 3,5-(2,5-Diisopropylpyrrolyl)2C6H3) and Studies
Relevant to Catalytic Reduction of Dinitrogen
Michael R. Reithofer, Richard R. Schrock,* and Peter Mu¨ller
Department of Chemistry, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139
Received January 29, 2010; E-mail: rrs@mit.edu
Abstract: Molybdenum complexes that contain
a new TREN-based ligand [(3,5-(2,5-diisopropyl-
pyrrolyl)2C6H3NCH2CH2)3N]3- ([DPPN3N]3-) that are relevant to the catalytic reduction of dinitrogen
have been prepared. They are [Bu4N]{[DPPN3N]MoN2}, [DPPN3N]MoN2, [DPPN3N]MoNdNH,
{[DPPN3N]MoNdNH2}[BArf4], [DPPN3N]MotN, {[DPPN3N]MotNH}[BArf4], and {[DPPN3N]MoNH3}[BArf4].
NMR and IR data for [Bu4N]{[DPPN3N]MoN2} and [DPPN3N]MoN2 are close to those reported for the
analogous [HIPTN3N]3- compounds (HIPT ) hexaisopropylterphenyl), which suggests that the degree of
reduction of dinitrogen is virtually identical in the two systems. However, X-ray studies and several exchange
studies support the conclusion that the apical pocket is less protected in [DPPN3N]Mo complexes than in
[HIPTN3N]Mo complexes. For example, 15N/14N exchange studies showed that exchange in [DPPN3N]MoN2
is relatively facile (t1/2 ≈ 1 h at 1 atm) and depends upon dinitrogen pressure, in contrast to the exchange
in [HIPTN3N]MoN2. Several of the [DPPN3N]Mo complexes, e.g., the [DPPN3N]MoN2 and [DPPN3N]MoNH3
species, are also less stable in solution than the analogous “parent” [HIPTN3N]Mo complexes. Four
attempted catalytic reductions of dinitrogen with [DPPN3N]MoN yielded 2.53 ( 0.35 equiv of total ammonia.
These studies reveal more than any other just how sensitive a successful catalytic reduction is to small
changes in the triamidoamine supporting ligand.
Introduction
uncovered principles of reduction of dinitrogen to ammonia at
a single metal center employing Mo(0) and W(0) dinitrogen
Efficient catalytic reduction of dinitrogen at room temperature
and 1 atm is one of the greatest challenges in chemistry. In
nature dinitrogen fixation is carried out by nitrogenases, of which
the best known and most studied is the FeMo nitrogenase.1-4
The FeMo nitrogenase consumes eight protons and eight
electrons to yield 2 equiv of NH3 and a minimum of one H2
per N2 reduced, a 75% yield of ammonia relative to electrons
consumed. Determination of the structure of the FeMo nitro-
genase by X-ray diffraction triggered much discussion and
speculation concerning the mechanism of dinitrogen reduction,5,6
but it is still unclear at what metal site (or sites) and exactly
how dinitrogen is reduced.
Since the report of the first dinitrogen complex
([Ru(NH3)5(N2)]2+) by Allen and Senoff in 1965,7 hundreds of
dinitrogen complexes have been prepared that contain transition
metals belonging to groups 4-10; palladium and platinum are
the only exceptions.8 In the 1960s the groups of Chatt and Hidai
complexes.9-12 They were able to show that dinitrogen can be
reduced to ammonia at a single metal center with the electrons
being supplied by the metal, but catalytic reduction of N2 to
NH3 was never achieved.
To date, only two systems are known that effect catalytic
reduction of dinitrogen under mild conditions. The first was
reported by Shilov.13 It requires a mixture of Mo(III), Mg(OH)2,
and a strong reducing agent such as Ti(OH)3 in a protic solvent
(MeOH). Although the reaction is catalytic in molybdenum,
dinitrogen is not reduced directly to ammonia but to hydrazine,
which is then disproportionated partially to dinitrogen and
ammonia. A typical ratio of ammonia to hydrazine in the product
is 1:10. The second catalytic process has been developed in
our group during the past decade.14-17 Catalytic reduction of
dinitrogen directly to ammonia at room temperature and ambient
pressure was achieved at a single Mo center protected by a
sterically demanding, hexaisopropylterphenyl-substituted triam-
idoamine ligand, [(3,5-(2,4,6-i-Pr3C6H2)2C6H3NCH2CH2N)3N]3-
(1) Burgess, B. K.; Lowe, D. J. Chem. ReV. 1996, 96, 2983.
(2) Hardy, R. W. F.; Bottomley, F.; Burns, R. C. A Treatise on Dinitrogen
Fixation; Wiley-Interscience: New York, 1979.
(9) Chatt, J.; Leigh, G. J. Chem. Soc. ReV. 1972, 1, 121.
(10) Chatt, J.; Dilworth, J. R.; Richards, R. L. Chem. ReV. 1978, 78, 589.
(11) Hidai, M.; Mizobe, Y. Chem. ReV. 1995, 95, 1115.
(3) Veeger, C.; Newton, W. E. AdVances in Nitrogen Fixation Research;
Dr. W. Junk/Martinus Nijhoff: Boston, MA, 1984.
(4) Coughlan, M. P. Molybdenum and Molybdenum-Containing Enzymes;
Pergamon: New York, 1980.
(12) Hidai, M. Coord. Chem. ReV. 1999, 185-186, 99.
(13) Shilov, A. E. Russ. Chem. Bull. Int. Ed. 2003, 2, 2555.
(14) Yandulov, D. V.; Schrock, R. R. J. Am. Chem. Soc. 2002, 124, 6252.
(15) Yandulov, D. V.; Schrock, R. R.; Rheingold, A. L.; Ceccarelli, C.;
Davis, W. M. Inorg. Chem. 2003, 42, 796.
(5) Einsle, O.; Tezcan, F. A.; Andrade, S. L. A.; Schmid, B.; Yoshida,
M.; Howard, J. B.; Rees, D. C. Science 2002, 297, 1696.
(6) Kim, J.; Rees, D. C. Science 1992, 257, 1677.
(7) Allen, A. D.; Senoff, C. W. J. Chem. Soc., Chem. Comm. 1965, 621.
(8) MacKay, B. A.; Fryzuk, M. D. Chem. ReV. 2004, 104, 385.
(16) Yandulov, D. V.; Schrock, R. R. Inorg. Chem. 2005, 44, 1103.
(17) Yandulov, D. V.; Schrock, R. R. Science 2003, 301, 76.
9
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A R T I C L E S
Reithofer et al.
pylphenyl groups in HIPT would be replaced with 2,5-
diisopropylpyrrolyl groups. The amido substituent would then
be 3,5-(2,5-diisopropylpyrrolyl)2C6H3 (dipyrrolylphenyl or DPP).
The DPP group should be effectively slightly smaller than a
HIPT group, since a five-membered (pyrrolyl) ring replaces the
six-membered phenyl ring, and of course could also be
electronically different from HIPT. Since an amido ligand is
protonated in the [HIPTN3N]3- ligand system in the process of
dinitrogen reduction, we also felt that there might be some
possibility that the pyrrolyl rings could compete with the amido
nitrogens as bases, most likely through protonation of a pyrrolyl
at an R or ꢀ carbon atom. Therefore, a protonated ligand might
be formed in the [DPPN3N]3- ligand system that would not be
as prone to being lost from the metal as in the [HIPTN3N]3-
ligand system. The synthesis and structures of several complexes
that contain the [DPPN3N]3- ligand system are reported here.
Results and Discussion
Synthesis of (3,5-(2,5-Diisopropylpyrrolyl)2C6H3NHCH2
CH2)3N (H3[DPPN3N]). The protonated ligand (H3[DPPN3N])
was synthesized in five steps from 1,3-dinitrobenzene and
3-methyl-2-butanone. 1,3-Dinitrobenzene was brominated and
reduced to yield 1,3-diamine-5-bromobenzene (15),25,26 while
3-methyl-2-butanone was converted via a radical C-C coupling
to 2,7-dimethyloctane-3,6-dione (16).27 Both reactions can be
carried out readily on a multigram scale. Subsequent Paal-Knorr
reaction between 15 and 16 yielded the desired 3,5-dipyrro-
lylterphenyl bromide 17 (DPPBr, eq 1). The reaction was
performed in a Dean-Stark apparatus using p-TSA as catalyst
and toluene as solvent. DPPBr could be obtained in good yield,
although it had to be separated by column chromatography from
a product that contains only one pyrrolyl group. Finally,
H3[DPPN3N] was obtained through a palladium-catalyzed C-N
cross-coupling as shown in eq 2. The reaction was performed
in toluene at 80 °C (yield, >95%). The H3[DPPN3N] product
(18) was purified through column chromatography and recrys-
tallized from hexane in 74% yield.
Figure 1. Proposed intermediates in the reduction of dinitrogen at a
[HIPTN3N]Mo (Mo) center (HIPT ) hexaisopropylterphenyl) through
stepwise addition of protons and electrons.
([HIPTN3N]3-). Careful treatment of [HIPTN3N]MoN2 with
[2,6-lutidinium][BArf4] (Arf ) 3,5-(CF3)2C6H3) and CrCp*2 led
to catalytic reduction of dinitrogen to ammonia; approximately
1 equiv of dihydrogen was formed per dinitrogen reduced. No
hydrazine was detected. Catalytic reduction is limited to
approximately four turnovers (∼8 equiv of total ammonia), most
likely as a consequence of protonation and loss of the
[HIPTN3N]3- ligand from the metal.18
Eight of the proposed intermediates (1, 2, 3, 4, 7, 8, 12, and
13; Figure 1) in the catalytic cycle have been characterized
crystallographically.14–16 The species that contains no ligand
in the apical position (14) is not an intermediate in the catalytic
reaction. (Compound 14 still has not been observed, although
it is proposed as an intermediate in slow (t1/2 ≈ 35 h) dinitrogen
exchange in 1.) Instead, ammonia in 13 is displaced by
dinitrogen to re-form 1. Several isolated species could be
employed for catalytic N2 reduction. Synthesis and investigation
of several variations of the [HIPTN3N]3- ligand system19-21
have suggested that sterically less demanding ligands can lead
to a decrease in the efficiency of dinitrogen reduction or even
loss of catalytic activity entirely.22,23 [HIPTN3N]Mo complexes
are currently the most efficient catalysts. DFT calculations with
the full ligand support the proposed mechanism for dinitrogen
reduction.24
In our quest for alternatives to the [HIPTN3N]3- ligand we
entertained the idea of a variation in which the 2,4,6-triisopro-
(18) Schrock, R. R. Angew. Chem., Int. Ed. 2008, 47, 5512.
(19) Smythe, N. C.; Schrock, R. R.; Mu¨ller, P.; Weare, W. W. Inorg. Chem.
2006, 45, 7111.
The reaction between MoCl4(THF)2 and H3[DPPN3N] in THF
led to a red-brown solution that we propose contains an
unknown type of adduct, similar to what is observed in syntheses
in which H3[HIPTN3N] is employed.28,29 The final complex,
[DPPN3N]MoCl (19), was formed after addition of 3.1 equiv
(20) Smythe, N. C.; Schrock, R. R.; Mu¨ller, P.; Weare, W. W. Inorg. Chem.
2006, 45, 9197.
(21) Ritleng, V.; Yandulov, D. V.; Weare, W. W.; Schrock, R. R.; Hock,
A. S.; Davis, W. M. J. Am. Chem. Soc. 2004, 126, 6150.
(22) Weare, W. W.; Dai, X.; Byrnes, M. J.; Chin, J. M.; Schrock, R. R.;
Mu¨ller, P. Proc. Nat. Acad. Sci. U.S.A. 2006, 103, 17099.
(23) Weare, W. W.; Schrock, R. R.; Hock, A. S.; Mu¨ller, P. Inorg. Chem.
2006, 45, 9185.
(25) Miroshnikova, O. V.; Hudson, T. H.; Gerena, L.; Kyle, D. E.; Lin,
A. J. J. Med. Chem. 2007, 50, 889.
(26) Ishida, Y.; Jikei, M.; Kakimoto, M. Macromolecules 2000, 33, 3202.
(27) Ito, Y.; Konoike, T.; Saegusa, T. J. Am. Chem. Soc. 1975, 97, 2912.
(28) Greco, G. E.; Popa, A. I.; Schrock, R. R. Organometallics 1998, 17,
5591.
(24) (a) Reiher, M.; Le Guennic, B.; Kirchner, B. Inorg. Chem. 2005, 44,
9640. (b) Schenk, S.; Le Guennic, B.; Kirchner, B.; Reiher, M. Inorg.
Chem. 2008, 47, 3634. (c) Schenk, S.; Kirchner, B.; Reiher, M.
Chem.sEur. J. 2009, 15, 5073.
(29) Greco, G. E.; Schrock, R. R. Inorg. Chem. 2001, 40, 3850.
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Molybdenum Complexes and Reduction
A R T I C L E S
Figure 3. Thermal ellipsoid drawing of [Bu4N]{[DPPN3N]MoN2}·5[C6H6]
with ellipsoids at 50% probability. Isopropyl groups, [Bu4N]+, hydrogen
atoms, and solvent molecules are omitted for clarity.
Figure 2. Thermal ellipsoid drawing of [DPPN3N]MoCl·2[(C2H5)2O] with
ellipsoids at 50% probability. Hydrogen atoms and solvent molecules are
omitted for clarity.
cm-1. For comparison, the [Bu4N]{[HIPTN3N]MoN2} complex
shows a νNN stretch at 1855 cm-1 (C6D6) and the analogous
15N2-labeled species shows a ν N stretch at 1794 cm-1.15 Two
of Li[N(SiMe3)2] (eq 3). Analytically pure 19 was obtained in
57% yield after extraction of the crude product with diethyl
ether and crystallization of it from a mixture of diethyl ether
and pentane.
15N15
relatively broad resonances were found in the 15N NMR
spectrum of 20 at 385 ppm (NR) and 368 ppm (Nꢀ). In
[Bu4N]{[HIPTN3N]Mo15N2}, the NR signal was found at 389
ppm while the Nꢀ resonance was found at 368 ppm.15 [Na(15-
crown-5)]{[HIPTN3N]MoN2} also showed a νNN stretch at 1853
cm-1, essentially the same as 20. The similarity of the IR and
NMR data for [DPPN3N]3- and [HIPTN3N]3- species suggests
that the electronic difference between the [DPPN3N]3- and
[HIPTN3N]3- ligands is minimal as far as binding dinitrogen is
concerned.
1
Compound 19 has paramagnetically shifted backbone H
NMR resonances similar to what are found in the other
[ArylN3N]MoCl compounds of this general type.28,29 The
methylene protons of the TREN backbone in particular are
shifted toward higher field (-15.5 and -86.6) and broadened,
while the proton resonances of the isopropyl groups as well as
the pyrrolyl protons are relatively sharp and in the expected
diamagnetic region. Only one of the aryl proton resonances
could be detected.
An X-ray diffraction study of 19·2[(C2H5)2O] showed it to
have the structure shown in Figure 2. The molybdenum(IV)
atom exhibits approximate trigonal bipyramidal coordination
geometry, with the substituted TREN ligand coordinated via
the three equatorial amido nitrogen atoms (N(1), N(4), and N(7))
plus the axial amine nitrogen (N(10)). The second axial site is
occupied by chloride. As usually is found in triamidoamine
compounds of this general type,15 the Mo sits slightly above
(0.330(5) Å) the plane defined by the three equatorial amido
nitrogens. As expected, the 2,5-diisopropylpyrrolyl rings lie
perpendicular to the phenyl ring.
The structure of 20 was determined through X-ray diffraction
(Figure 3). The molybdenum(IV) atom has a trigonal bipyra-
midal coordination geometry with the dinitrogen ligand located
in the apical pocket. The [Bu4N]+ cation does not interact with
the N2- ligand, as one would expect. The N-N bond length of
the N2 ligand (1.152(4) Å) is identical within experimental error
with that found in {Mg(DME)3}0.5{[HIPTN3N]MoN2 } (N-N
) 1.150(5) Å), in which the cation also does not interact with
the N2 ligand.15
The sodium salt of {[DPPN3N]MoN2}- was prepared, dis-
solved in benzene, and treated with a slight excess of AgOTf.
The reaction mixture was shaken for ∼1 min, and the mixture
was filtered through Celite. [DPPN3N]MoN2 (22) was isolated
from the filtrate in 60% yield. If the reaction that yields 22 is
not worked up immediately, then only decomposition products
could be observed. No 22 could be isolated when either
30
15
Synthesis of [Bu4N]{[DPPN3N]MoN2} and [DPPN3N]MoN2.
A deep-green solution was obtained after several hours upon
reduction of [DPPN3N]MoCl with 10 equiv of sodium sand in
THF under a dinitrogen atmosphere. After 24 h the solution
was filtered and [Bu4N]Cl was added. Deep-green, diamagnetic
[Bu4N]{[DPPN3N]MoN2}could then be isolated (20, eq 4). In
Zn(OAc)2 or ZnCl2 was employed as the oxidizing agent.
When the oxidation of {[DPPN3N]MoN2}- with Zn(OAc)2 was
followed by IR spectroscopy, the expected N2 compound could
be observed as the reaction proceeded, but over a period of
several hours it decomposed roughly as rapidly as it was formed.
C6D6 the νNN stretch in 20 was found at 1853 cm-1. In the
(30) Hetterscheid, D. G. H.; Hanna, B. S.; Schrock, R. R. Inorg. Chem.
2009, 48, 8569.
15
15N15
N
analogous N-labeled compound ν
was found at 1792
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A R T I C L E S
Reithofer et al.
a half-life for the conversion of [DPPN3N]Mo15N2 to
[DPPN3N]Mo14N2 was found to be 56 ( 4 min (k ) (2.1 (
1.5) × 10-4 s-1). Similar studies were carried out in a Schlenk
flask at ∼1.5 and 2 atm (7.5 and 15 psi overpressure of
dinitrogen, respectively). At 1.5 atm t1/2 was found to be 40
min (k ) 2.9 × 10-4 s-1), whereas at 2 atm t1/2 was found to be
12-13 min (k ) (10.1 ( 2.4) × 10-4 s-1). (Only four data
points over a period of ∼2 half-lives could be obtained at 2
atm, so the error could be larger than apparent.) The 15N2/14N2
exchange is clearly pressure dependent, probably to the first
order in dinitrogen. A facile exchange and a pressure dependence
in the [DPPN3N]3- system contrasts to what is found in
[HIPTN3N]Mo15N2, where t1/2 for N2 exchange is ∼35 h at 1
atm and the exchange rate is independent of pressure at up to
∼4 atm.22 Apparently, since the five-membered 2,5-diisopro-
pylpyrrolyl ring is operationally smaller than the 2,4,6-triiso-
propylphenyl ring, dinitrogen can attack [DPPN3N]MoN2 and
displace the bound dinitrogen through a six-coordinate inter-
mediate. Conversely, since associative exchange of coordinated
nitrogen for bound nitrogen in [HIPTN3N]MoN2 is not facile,
a (slow) unimolecular dissociation of the dinitrogen ligand to
give unobservable [HIPTN3N]Mo is the next option for dini-
trogen exchange.
Figure 4. Cyclic voltammogram of [Bu4N]{[DPPN3N]MoN2} (20) in PhF
(0.1 M [Bu4N][BArf4]) at different scan rates between 50 and 900 mV/s
(vertical axis ) current in µA).
Therefore, it appears that a fast-acting oxidizing agent is required
in order to reduce the reaction time to a minimum.
IR and NMR studies are fully in agreement with the proposed
nature of [DPPN3N]MoN2. In C6D6 the νNN stretch is found at
1993 cm-1, which is virtually the same stretching frequency as
is found in [HIPTN3N]MoN2 (νNN ) 1990 cm-1). 15N labeled
[HIPTN3N]MoN2 can be prepared via reduction of
[DPPN3N]MoCl under 15N2 followed by oxidation with AgOTf.
However, it also can be synthesized through 14N2/15N2 exchange,
since that exchange is relatively facile (vide infra). The ν
Synthesis of [DPPN3N]ModN-NH. Addition of a solution
of [Et3NH][BArf4] in diethyl ether at -30 °C to solid
[Bu4N]{[DPPN3N]MoN2} resulted in a rapid color change and
formation of an orange-brown solution. A proton NMR spectrum
verified formation of the diamagnetic [DPPN3N]ModN-NH
complex (δ ) 8.54, NdNH), while 15N NMR revealed two
double doublets at 407.4 and 234.3 ppm. The resonance at 407.4
15N15
N
1
ppm is assigned to the NR nitrogen (2JNH ) 7.4 Hz, JNN
)
stretch in [DPPN3N]Mo15N2 was found at 1927 cm-1 (cf. 1924
cm-1 in [HIPTN3N]Mo15N2, ref 15). The 1H NMR spectrum of
[DPPN3N]MoN2 reveals characteristic paramagnetic shifted
ligand backbone proton resonances at 21.2 and -31.5 ppm.
Electrochemical oxidation of [Bu4N]{[DPPN3N]MoN2} takes
place reversibly in PhF (0.1 M[Bu4N][BArf4]) at scan rates
between 50 and 900 mV/s (Figure 4). The E° value for the
[DPPN3N]MoN2/{[DPPN3N]MoN2}- couple is -1.76 V, which
15.4 Hz), while that at 234.3 ppm is assigned to the Nꢀ nitrogen
(1JNH ) 55.1 Hz, JNN ) 15.4 Hz). All data strongly support
1
the protonation of {[DPPN3N]MoN2}- to yield diamagnetic
[DPPN3N]Mo-NdNH (eq 5).
0/-
should be compared to -2.01 V for the [HIPTN3N]MoN2
couple.31 Further oxidation of [DPPN3N]MoN2 to
{[DPPN3N]MoN2}+ is irreversible, even at a scan rate of 104
mV/s (Ipa ) -0.34 V at a scan rate of 500 mV/s). Both reduction
and oxidation are reversible under similar conditions in the
[HIPTN3N]MoN2 system.16
Reduction of [DPPN3N]MoCl followed by titration at -30
°C with a solution of a slight excess of [Et3NH]OTf yielded an
orange-brown solution from which an orange-brown crystalline
solid was obtained. Proton NMR studies suggest that the orange-
brown product is [DPPN3N]Mo-NdNH contaminated with
2-3% of [DPPN3N]MoN2 (22). (The amount of 22 was
determined through integration of the pyrrolyl resonances at 5.83
ppm for 22 and 6.19 ppm for 21.) Compounds 21 and 22 appear
to cocrystallize and therefore are unlikely to be separable
through recrystallizations. We propose that 22 is formed through
direct reduction of protons by 21. Some attempted protonations
in the parent system also led to formation of the dinitrogen
complex instead of the [HIPTN3N]MoNdNH complex, or to
mixtures of the two.15
We reported that [HIPTN3N]MoNdNH decomposes slowly
in benzene to [HIPTN3N]MoH via a first-order “ꢀ-elimination”
process (k ) 2.2 × 10-6 s-1, t1/2 ) 90 h at 61 °C);15 it is stable
indefinitely in C6D6 at 22 °C. In contrast, [DPPN3N]MoNdNH
in C6D6 decomposes over a period of 14 days to the extent of
about 10-20%, but no decomposition product could be identi-
When taking into account the similarity of the 15N NMR and
IR data for [DPPN3N]MoN2 and [HIPTN3N]MoN2, we were
surprised that [DPPN3N]MoN2 is easier to reduce than
[HIPTN3N]MoN2 by 0.25 V. Substituting the DPP groups for
the HIPT groups clearly significantly alters the potential at which
an electron is transferred to the metal center, even though the
nature of dinitrogen activation is essentially the same in
[DPPN3N]MoN2 and [HIPTN3N]MoN2 (vide supra).
It is clear from qualitative observations that 15N/14N exchange
in 20 takes place to a significant degree in 1 h in solution at
22 °C. In order to quantify the rate of exchange of dinitrogen,
a 15 mM solution of [DPPN3N]Mo15N2 in PhF was prepared in
an argon-filled glovebox at 22 °C. The solution was then
transferred to an dinitrogen-filled glovebox and opened periodi-
cally to the atmosphere. Aliquots were removed and examined
over a period of 2-3 h while the solution was stirred at 1 atm.
The rate of formation of [DPPN3N]Mo14N2 was followed, and
(31) Heinze, J. Angew. Chem., Int. Ed. Engl. 1984, 23, 831.
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A R T I C L E S
fied. After 24 h at 60 °C the NdNH proton resonance could no
longer be observed, but again, no decomposition product could
be identified. Addition of collidine (2,4,6-trimethylpyridine) to
[DPPN3N]MoNdNH did not lead to rapid decomposition, but
over the course of
2 weeks at room temperature,
[DPPN3N]MoNdNH decomposed completely, in contrast to
only 10-20% in the absence of collidine. Therefore, decom-
position does appear to be accelerated to some degree in the
presence of collidine.
In the [CF3hybrid]Mo15Nd15NH system, in which one of the
three HIPT groups in [HIPTN3N]3- is replaced by a 3,5-
(CF3)2C6H3 group, 15N/14N exchange takes place with a half-
life of 4.5 h.23 A similar exchange in [HIPTN3N]Mo15Nd15NH
is estimated to be ∼100 times slower.22 The rate of exchange
for [CF3hybrid]Mo15Nd15NH may not be totally reliable, since
the compound could be prepared only in situ; measurements
with a pure isolable species would have been desirable.
We have found that 15N/14N exchange also takes place in
[DPPN3N]]Mo15Nd15NH. Under 1 atm of dinitrogen a first-
order reaction (in Mo) was found to have k ) 1.1 × 10-6 s-1
(t1/2 ≈ 7 days) at a concentration of 4 mM in C6D6. The 15N/
14N exchange reaction was also investigated at 2 atm of
dinitrogen, but no pressure dependence was found (k ) 1.4 ×
10-6 s-1, t1/2 ≈ 6.6 days). When 1 equiv of [Et3NH][OTf] was
present, the exchange was complete in ∼24 h at 1 atm of
dinitrogen pressure. However, in the presence of 1 equiv of
[Et3NH][OTf], [DPPN3N]]MoNdNH also decomposes to a
significant degree to form [DPPN3N]]MoN2 (∼25-30% in
24 h); after ∼4 days the NNH peak could no longer be detected
Figure 5. Thermal ellipsoid drawing of {[DPPN3N]LMoN2H2}-
[BArf4]·[C6H6]·2.5[C5H12] with ellipsoids at 50% probability. Isopropyl
groups, [BArf4]- anions, hydrogen atoms (except for H12a and H12b), and
solvent molecules are omitted for clarity.
1
by H NMR spectroscopy and [DPPN3N]]MoN2 (>80%) was
identified as the main decomposition product. The precise nature
of the acid-catalyzed exchange/decomposition reaction is not
yet known.
Protonation of [DPPN3N]MoNdNH in diethyl ether with
[H(Et2O)2][BArf4] led to formation of {[DPPN3N]-
MoNdNH2}[BArf4] (23) as red needles in 77% yield. Com-
pound 23 is likely to be an intermediate in a mechanism of
dinitrogen reduction analogous to that shown in Figure 1 for
the HIPT system. Crystals of 23 suitable for X-ray diffraction
were grown from a mixture of benzene and pentane at -30 °C.
The result of the structural determination is shown in Figure 5.
The molybdenum(IV) has a trigonal pipyramidal coordination
geometry, with the NdNH2 ligand located in the apical pocket.
The position of the NH2 atoms could be identified from the
difference Fourier synthesis map. Refinement of the NH2
positions led to NdN-H12a ) 119(3)°, NdN-H12b ) 93(3)°,
and H12a-N-H12b ) 110(4)°. Although the errors are
understandably large, the sum of the three angles is considerably
less than 360°, even if the largest possible values are summed
(332°); we conclude that the ꢀ nitrogen is not planar in 23.
Syntheses and Structures of [DPPN3N]MotN and
{[DPPN3N]MotNH}[BArf4]. Heating a toluene solution of
[DPPN3N]MoCl and 2 equiv of Me3SiN3 at 90 °C for 96 h results
in the formation of [DPPN3N]MotN (24), which crystallizes as
yellow-brown needles from a mixture of benzene and pentane at
-30 °C. A similar reaction between 50% terminally 15N-labeled
sodium azide and [DPPN3N]MoCl in dioxane and 5 equiv of DME
gave 50% 15N-labeled [DPPN3N]Mot15/14N in 80% yield. The
nitride resonance is found at 905 ppm in a 15N NMR spectrum,
which is ∼7 ppm to lower field than where it is found in
[HIPTN3N]MotN.
Figure 6. Thermal ellipsoid drawing of the first crystallographically
independent molecule of 24 with ellipsoids at 50% probability. Isopropyl
groups, hydrogen atoms, and solvent molecules are omitted for clarity.
5S in the Supporting Information for distances and angles.) The
asymmetric unit contains two molecules of 24, and since one
of them shows significant disorder, bond distances and angles
are discussed only for the ordered molecule. (For details, see
Experimental Details.) The MotN bond distance of 1.662(3)
Å agrees well with known MotN distances of 1.652(5) Å in
[HIPTN3N]MotN and 1.658(5) Å in [HTBTN3N]MotN
(HTBT ) hexa-tert-butylterphenyl).15,32 As reported for the
[HIPTN3N]Mo compounds, the N(11)-Mo(1)-N-Cipso dihe-
dral angles are relatively small (0.6-4.69°), and the
Mo(1)-N-Cipso-Cortho dihedral angles vary from -35.7° to
-36.2°. As a result, one of the pyrrolyl rings points toward the
metal’s coordination pocket while the other one points away.
(32) Laplaza, C. E.; Johnson, M. J. A.; Peters, J. C.; Odom, A. L.; Kim,
E.; Cummins, C. C.; George, G. N.; Pickering, I. J. J. Am. Chem.
Soc. 1996, 118, 8623.
The crystal structure of 24 revealed the expected trigonal
bipyramidal coordination geometry at Mo (Figure 6). (See Table
9
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A R T I C L E S
Reithofer et al.
Figure 8. Cyclic voltammogram of {[DPPN3N]MoNH3}[BArf4] in PhF
(0.1 M [Bu4N][BArf4]) at different scan rates (measured under an argon
atmosphere; vertical axis ) current in µA).
reaction was successful only when ammonia was first condensed
onto sodium and the mixture was stirred for 1 h at
-75 °C. The reaction was complete after
3 h, and
{[DPPN3N]MoNH3}[BArf4] could be isolated in good yield as
a red-brown solid. In contrast to what was found for the
analogous [HIPTN3N]3- salt, the [BPh4]- analogue could not
be prepared analogously. In terms of NMR spectra, the most
characteristic features of {[DPPN3N]MoNH3}[BArf4] are the
paramagnetically shifted backbone proton resonances at -19
and -107 ppm.
Figure 7. Thermal ellipsoid drawing of the first crystallographically
independent molecule of {[DPPN3N]MotNH}+ with ellipsoids at 50%
probability. Isopropyl groups, [BArf4]- anions, hydrogen atoms (except for
H11), and solvent molecules are omitted for clarity.
In comparison to the [HIPTN3N]Mo compounds, where the
planes of the TRIP rings are within 5-10° of being perpen-
dicular to the phenyl rings, the pyrrolyl rings are 10-23° away
from being perpendicular to the plane of the phenyl ring. We
propose this to be the consequence of a pyrrolyl’s isopropyl
substituents in the 2 and 5 positions not being as sterically
demanding as the 2,4,6-triisopropylphenylgroup’s isopropyl
substituents in the 2 and 6 positions.
The electrochemical reduction of {[DPPN3N]MoNH3}[BArf4]
under argon was investigated via cyclic voltammetry. Reduction
occurs quasireversibly in PhF (0.1 M[Bu4N][BArf4] electrolyte)
at scan rates from 50 to 1900 mV/s (Figure 8). Similar
experiments under dinitrogen also did not lead to a reversible
couple. Therefore, we propose that [DPPN3N]MoNH3 is unstable
under either argon or dinitrogen. Nevertheless, reduction of
{[DPPN3N]MoNH3}[BArf4] with 1 equiv of CrCp*2 under
dinitrogen gave [DPPN3N]MoN2 in moderate yield (40%). In
contrast, reduction of {[HIPTN3N]MoNH3}+ occurs reversibly
in PhF as well as in THF (E° ) -1.63 in PhF; E° ) -1.51 in
THF).16 Interestingly, in THF at low scan rates, NH3/N2
exchange can be observed in intermediate [HIPTN3N]MoNH3
that is formed in the CV experiment. Reduction of
{[HIPTN3N]WNH3}+ was also found to be completely irrevers-
ible, and no turnover was observed in an attempted catalytic
reduction.33
Catalytic Reduction of N2. The ability of the [DPPN3N]3-
complexes to reduce dinitrogen catalytically was investigated
in a manner analogous to that employed for the [HIPTN3N]3-
system.17 Catalytic runs employing [DPPN3N]MoN, 48 equiv
of [2,4,6-Me3C6H2N][BArf4], and 36 equiv CrCp*2 were per-
formed four times; the average yield of total ammonia was 2.53
( 0.35 equivalents. If we assume that 1 equiv of ammonia is
derived through reduction of the nitride, then at most 1.88 equiv
of ammonia are derived from dinitrogen.17 Since less than 2
equiv of ammonia are formed from dinitrogen, it could be argued
that this system therefore is not catalytic. On the other hand,
formation of more than 2 equiv of total ammonia implies that
the system essentially returned to and passed the [DPPN3N]MoN
starting point; from this viewpoint one could argue that reduction
is catalytic. In contrast, the use of [HIPTN3N]MoN as a
“catalyst” yields ∼7.0 equiv of ammonia from dinitrogen.
Although we cannot pinpoint the cause(s) of the borderline
Protonation of 24 with [H(Et2O)2][BArf4] in diethyl ether at
-30 °C yielded
a
deep-red solution that contains
{[DPPN3N]MotNH}[BArf4] (25), in addition to other diamag-
netic products in significant and variable amounts. In order to
verify that {[DPPN3N]MotNH}+ is formed in this reaction,
the 50% 15N-labeled compound was synthesized; the 15N
resonance in partially labeled {[DPPN3N]MotNH}+ was
detected
at
428.7
ppm
(cf.
427.7
ppm
in
[[HIPTN3N]MotNH}[BArf4]).15 The NH proton resonance was
observed at 6.12 ppm and its connectivity confirmed via 1H,15N
HSQC NMR spectroscopy. In the partially labeled species this
resonance is split by 74 Hz (JHN), as was observed in
{[HIPTN3N]Mot15NH}[BArf4].15
The result of the X-ray study of compound 25 is shown in
Figure 7. The asymmetric unit contains two molecules of 25,
and both [BArf4]- anions as well as one of the
{[DPPN3N]MotNH}+ cations are disordered. Therefore, all
numerical values listed in Table 4S refer only to the ordered
{[DPPN3N]MotNH}+ ion. The MotNH bond (1.722(2) Å)
is slightly longer than the MotN in 24 (1.662(3) Å), as
expected. The position for the NH hydrogen atom could be
identified unequivocally from the difference Fourier synthesis;
the Mo-N-H angle is 178(2)°. The pyrrolyl rings are es-
sentially perpendicular (7-0.2°) to the phenyl rings.
Synthesis of {[DPPN3N]MoNH3}+ and Its Reduction and
Formation of [DPPN3N]MoN2. The reaction between
[DPPN3N]MoCl and 4 equiv of NH3 in CH2Cl2 in the presence
of Na[BArf4] yielded {[DPPN3N]MoNH3}[BArf4]. This reaction
is extremely sensitive to small quantities of impurities in the
solvent (CH2Cl2), Na[BArf4], or especially in ammonia. The
(33) Yandulov, D. V.; Schrock, R. R. Can. J. Chem. 2005, 83, 341.
9
8354 J. AM. CHEM. SOC. VOL. 132, NO. 24, 2010

Molybdenum Complexes and Reduction
A R T I C L E S
catalytic activity in the [DPPN3N]3- system, we suspect that
the additional problems that limit turnover result (at least in
part) from the only slightly reduced steric protection of the metal
that is afforded with the [DPPN3N]3- ligand system relative to
the [HIPTN3N]3- ligand system.
spectrometer in a demountable solution cell (0.2 mm Teflon spacer,
KBr windows). FT-ICR-MS (ESI) spectra was measured by the
Department of Chemistry Instrumentation Facility at the Mas-
sachusetts Institute of Technology. Electrochemical measurements
were carried out in an argon or dinitrogen filled glovebox using a
CHI 620C potentiostat, 0.1 M [Bu4N][BArf4]/PhF electrolytes, and
a standard three-electrode cell assembly with a glassy carbon (3.0
mm diameter) disk working electrode, a platinum wire auxiliary
electrode, and a reference electrode consisting of a AgCl-coated
silver wire submerged in 0.1 M [Bu4N][BArf4]/PhF electrolyte. All
measurements were referenced externally and/or internally to the
ferrocene/ferricinium couple. Elemental analyses were performed
by Midwest Microlabs, Indianapolis, IN.
Conclusions
A new TREN-based ligand containing 2,5-disubstituted pyrrolyl
groups in place of 2,4,6-triisopropylphenyl groups has been
synthesized and compounds containing it prepared. Seven inter-
mediates in a potential dinitrogen reduction cycle have been isolated
and characterized; they are ([Bu4N]{[DPPN3N]MoN2},
[DPPN3N]MoN2, [DPPN3N]MoNdNH, {[DPPN3N]MoNd
NH2}[BArf4], [DPPN3N]MotN, {[DPPN3N]MotNH}[BArf4],
and {[DPPN3N]MoNH3}[BArf4]). 15N NMR and IR data of
[Bu4N]{[DPPN3N]MoN2} and [DPPN3N]MoN2 (also 15N la-
beled) are close to those reported for the analogous
[HIPTN3N]3- compounds, a fact that leads us to conclude that
N2 activation by the [DPPN3N]Mo system is comparable to that
found in the [HIPTN3N]Mo system. X-ray structural studies bear
out the prediction that the apical pocket is less protected in
[DPPN3N]Mo complexes than in [HIPTN3N]Mo complexes. The
15N/14N exchange studies in [DPPN3N]MoN2 showed that the
rate of exchange depends upon dinitrogen pressure. Exchange
of N2 into [DPPN3N]Mo-15Nd15NH was also facile and
consistent with a more open coordination environment in
[DPPN3N]3- complexes. Significant differences in redox po-
tentials and stabilities of intermediates were observed for
[DPPN3N]Mo complexes versus [HIPTN3N]Mo complexes. An
especially important example of the latter is the instability of
the [DPPN3N]MoNH3 species. Finally, catalytic reduction of
dinitrogen by [DPPN3N]MoN is borderline, depending upon
one’s definition of “catalytic.” These studies reveal just how
sensitive a successful catalytic reduction can be to small steric
and electronic changes in the triamidoamine supporting ligand.
Synthesis of DPPBr (17). A mixture of 3,5-diaminobromoben-
zene (8.7 g, 46 mmol), 2,7-dimethyloctane-3,6-dione (17 g, 100
mmol), p-toluenesulfonic acid monohydrate (40 mg, 0.21 mmol),
and 120 mL of toluene was refluxed under dinitrogen in a
Dean-Stark apparatus for 24 h. The expected amount of water (4
mol equiv) separated from the reaction mixture. The solution was
filtered through silica gel, and the solvent was removed under
reduced pressure. The pure product was obtained after column
chromatography (LM/toluene) and recrystallization from hexane,
yield 15.2 g (72%). 1H NMR (CDCl3, 400 MHz, 297 K) δ 7.50 (d,
JHH ) 1.8 Hz, 2H, 4,6-Ar), 7.20 (t, JHH ) 1.8 Hz, 1H, 2-Ar), 5.98
(s, 4H, Py), 2.69 (sept, JHH ) 6.8 Hz, 4H, CHMe2), 1.12 (d, JHH
)
6.8 Hz, 12H, CH(CH3)2), 1.05 (d, JHH ) 6.8 Hz, 12H, CH(CH3)2)
ppm. FT-ICR-MS (ESI): calcd m/z 455.2056 [M + H+]+, found
m/z 455.2051 [M + H+]+. Anal. Calcd (%) for C26H35BrN2: C,
68.56; H, 7.75; N, 6.15. Found: C, 68.36; H, 7.67; N, 6.14.
Synthesis of H3[DPPN3N] (18). A 250 mL round-bottom flask
equipped with a magnetic stir bar was charged with 17 (15 g, 32.9
mmol), NaO-t-Bu (4.43 g, 46.1 mmol), and TREN (1.605 g, 10.98
mmol). The Pd catalyst solution was prepared by vigorously stirring
Pd2(dba)3 (302 mg, 0.33 mmol) and rac-BINAP (615 mg, 1 mmol)
in 20 mL of toluene overnight at room temperature. The mixture
was filtered through Celite into the main reaction mixture, and the
Celite was rinsed with another 20 mL of toluene. The flask was
closed and stirred for 24 h at 85 °C. After the reaction mixture had
cooled, the solid NaBr was filtered off through a bed of Celite.
The solvent was removed under reduced pressure, and the crude
product was purified (twice) by column chromatography (EtOAc/
toluene, 1/30). The pure product was obtained as a white solid after
crystallization from hexanes, yield 10.4 g (74%). 1H NMR (C6D6,
400 MHz, 297 K) δ 6.50 (t, JHH ) 1.7 Hz, 3H, 4-Ar), 6.31 (d, JHH
) 1.7 Hz, 6H, 2,6-Ar), 6.22 (s, 12H, Py), 3.46 (t, JHH ) 5.2 Hz,
3H, NH), 2.93 (sept, JHH ) 6.8 Hz, 12H, CHMe2), 2.60 (m, 6H,
Experimental Details
General. All manipulations of air- and moisture-sensitive
compounds were carried out using standard Schlenk and glovebox
techniques under an atmosphere of nitrogen or argon in oven-dried
glassware, including NMR tubes. Diethyl ether, pentane, methylene
chloride, THF, and toluene were purged with nitrogen and passed
through activated alumina columns. Heptane, C6D6, and THF-d8
were distilled from a dark-purple Na/benzophenone ketyl solutions,
and PhF was dried over CaH2, degassed, and vacuum distilled prior
to use. All dried and deoxygenated solvents were stored over 4 Å
Linde-type molecular sieves prior to use. Prior to use ammonia
was condensed onto sodium and the mixture stirred for 1 h at -75
°C. Li[N(SiMe3)2] was purified by sublimation. [Bu4N][BArf4] was
prepared by salt metathesis of [Bu4N]Cl with Na[BArf4] in Et2O
and recrystallization from CH2Cl2/pentane. [HNEt3][BArf4] was
synthesized by treating NEt3 ·HCl with Na[BArf4] in diethyl ether.
1,3-Diamine-5-bromobenzene and 2,7-dimethyloctane-3,6-dione
were synthesized according to standard literature procedure.25–27
The catalytic reduction of dinitrogen was performed according to
a previously reported procedure.17 NMR spectra were recorded
either on a Bruker Avance 400 MHz or on a Bruker Avance 600
MHz spectrometer and referenced to the residual protio solvent
peaks. Directly measured 15N NMR and 1H,15N-HSQC spectra were
recorded on a Varian Inova 500 MHz spectrometer and referenced
externally to benzamide (15N, 105.33 ppm relative to neat NH3).34
IR spectra were measured on a Nicolet Avatar 360 FT-IR
NHCH2), 1.23 (d, JHH ) 6.8 Hz, 36H, CH(CH3)2), 1.16 (d, JHH
)
6.8 Hz, 36H, CH(CH3)2) ppm. FT-ICR-MS (ESI): calcd m/z
1269.9770 [M + H+]+, found m/z 1269.9742 [M + H+]+. Anal.
Calcd (%) for C84H120N10: C, 79.45; H, 9.52; N, 11.03. Found: C,
79.19; H, 9.59; N, 11.02.
[DPPN3N]MoCl (19). A 250 mL Schlenk flask was charged with
18 (4 g, 3.15 mmol), MoCl4(THF)2 (1.144 g, 3.06 mmol), and 80
mL of THF. The resulting solution was stirred for 1 h. Solid
Li[N(SiMe3)2] (1.59 g, 9.5 mmol) was added all at once, and the
resulting solution was stirred for 1 h. The solvent was removed
under reduced pressure, and the residue was dried for 3 h at
50-60 °C in vacuo. The solid residue was extracted several times
with diethyl ether (total volume, 150 mL), and the extracts were
filtered through Celite. The volume was reduced to 50 mL, and an
amount of 50 mL of pentane was added. The resulting suspension
was stored overnight at -30 °C. The product was filtered off and
rinsed with pentane to afford the product as an orange powder,
yield 2.45 g (57%). 1H NMR (THF-d8, 400 MHz, 297 K) δ 13.76
(brs, Ar), 5.79 (s, 12H, Py), 2.54 (brs, 12H, CHMe2), 1.12 (brs,
72H, CH(CH3)2), -15.48 (brs, NCH2), -86.64 (brs, NCH2) ppm.
Anal. Calcd (%) for C84H117ClMoN10: C, 72.15; H, 8.43; N, 10.02;
Cl, 2.54. Found: C, 72.31;h 8.20; N 9.99; Cl 2.49.
(34) Sadek, M.; Brownlee, R. T. C. J. Magn. Reson., Ser. B 1995, 109,
70.
9
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A R T I C L E S
Reithofer et al.
Crystals for the X-ray study were grown from a diethyl ether
solution, which was covered by a layer of pentane, and the mixture
was stored for several days at -30 °C.
THF (20 mL), the green solution was filtered through Celite, cooled
in a -30 °C freezer for 2 h, and titrated dropwise with a cooled
THF (10 mL) solution of [Et3NH][OTf] (18.7 mg, 75.1 µmol) until
a steady dark-orange-brown color was obtained (1.05 equiv of
[Et3NH][OTf] was required). The solution was stirred for 10 min
at room temperature. The solvent was removed under reduced
pressure, and the residue was dried at 40-50 °C for 4 h. The residue
was extracted with benzene (15-20 mL), and the extract was
filtered through Celite and concentrated in vacuo (3-4 mL), covered
by a layer of pentane, and stored overnight at room temperature
and for another day at -30 °C. The product was filtered off, washed
with pentane, and dried in vacuo. The mother liquor was concen-
[Bu4N]{[DPPN3N]MoN2} (20). [DPPN3N]MoCl (200 mg, 143
µmol) and sodium sand (33 mg, 1.43 mmol) were suspended in 20
mL of THF, and the mixture was stirred with a glass-coated stir
bar for 24 h under a dinitrogen atmosphere. Within the first hour,
the solution became dark-green. After 24 h, the reaction mixture
was filtered, and the filtrate was treated with solid [Bu4N]Cl (48
mg, 171 µmol). The mixture was stirred for an additional 18 h.
The solvent was removed under reduced pressure, and the residue
was heated at 50 °C in vacuo for 3 h. The residue was extracted
with benzene, and the extract was filtered through Celite, concen-
trated to ∼10 mL, and covered by a layer of pentane. A bright-
green oil formed after standing the solution at room temperature
overnight. The solvent was decanted off, the oil was dissolved in
diethyl ether, and the solvent was removed under reduced pressure.
After the residue was heated in vacuo, an amorphous solid was
1
trated and a second crop obtained, yield 194 mg (65%). H NMR
(C6D6, 400 MHz, 297 K) δ 8.54 (s, 1H, NdNH), 7.11 (s, 6H, 2,6-
Ar), 6.74 (s, 3H, 4-Ar), 6.19 (s, 12H, Py), 3.28 (brs, 6H, NCH2),
2.91 (sept, JHH ) 6.7 Hz, 12H, CHMe2), 1.84 (brs, 6H, NCH2),
1.18 (d, JHH ) 6.7 Hz, 72H, CH(CH3)2) ppm. Because of impurities
of [DPPN3N]MoN2 (about 2%), EA was not determined.
1
obtained, yield 157 mg (67%). HNMR (C6D6, 400 MHz, 297 K)
[DPPN3N]Mo15N2H. [Bu4N]{[DPPN3N]Mo15N2} (190 mg, 116.2
µmol) was dissolved in 5 mL of diethyl ether, and the solution
was chilled at -30 °C for 30 min. At this temperature solid
[Et3NH][OTf] (30.7 mg, 122 µmol) was added and the resulting
reaction mixture was stirred for 10 min. The solvent was removed
under reduced pressure, and the solid was dried in vacuo for 2 h.
The residue was extracted with benzene (15-20 mL). The extract
was filtered through Celite, concentrated in vacuo (3-4 mL),
covered by a layer of pentane, and stored overnight at room
temperature and for another day at -30 °C. The product was filtered
off, washed with pentane, and dried in vacuo, yield 68 mg (40%).
δ 7.60 (d, JHH ) 1.5 Hz, 6H, 2,6-Ar), 6.54 (t, JHH ) 1.6 Hz, 3H,
4-Ar), 6.18 (s, 12H, Py), 3.61 (brs, 6H, NCH2), 3.31 (sept, JHH
)
6.7 Hz, 12H, CHMe2), 2.25 (brs, 8H, TBA), 1.83 (brs, 6H, NCH2),
1.36 (d, JHH ) 6.7 Hz, 36H, CH(CH3)2), 1.30 (d, JHH ) 6.7 Hz,
36H, CH(CH3)2), 0.95 (m, 8H, TBA), 0.85 (m, 8H, TBA), 0.76 (t,
JHH ) 6.9 Hz, 12H, TBA) ppm. IR (C6D6, cm-1): 1853 (νNN). Anal.
Calcd (%) for C100H153MoN13: C, 73.54; H, 9.44; N, 11.15. Found:
C, 73.17; H, 9.18; N, 11.15. Crystals for the X-ray study were grown
from a benzene solution, which was covered by a layer of pentane.
The mixture was stored overnight at room temperature and for
several days at -30 °C.
1
1HNMR (C6D6, 400 MHz, 297 K) δ 8.55 (dd, JNH ) 55.4 Hz,
[Bu4N]{[DPPN3N]Mo15N2}. In
a
250 mL Schlenk flask
2JNH ) 7.4 Hz, 1H, NdNH), 7.12 (s, 6H, 2,6-Ar), 6.74 (s, 3H,
[DPPN3N]MoCl (400 mg, 286 µmol) and sodium sand (66 mg,
2.8 mmol) were suspended in 40 mL of THF. The mixture was
degassed in vacuo and filled with 15N2 (100 Torr, ∼200 mL). The
reaction mixture was stirred using a glass-coated stir bar for 24 h,
degassed, and again filled with 15N2 (100 Torr, ∼200 mL). After
24 h, the reaction mixture was filtered through Celite, Bu4NCl (95
mg, 343 µmol) was added to the filtrate, and the resulting solution
was stirred overnight. The procedure was the same as described
above for the unlabeled compound, yield 320 mg (68%). 1H NMR
(C6D6, 400 MHz, 297 K) δ 7.59 (brs, 6H, 2,6-Ar), 6.53 (brs, 3H,
4-Ar), 6.20 (s, 12H, Py), 3.27 (brs, 6H, NCH2), 2.92 (sept, JHH )
6.8 Hz, 12H, CHMe2), 1.82 (brs, 6H, NCH2), 1.18 d, JHH ) 6.8
Hz, 72H, CH(CH3)2) ppm; 15N NMR (C6D6, 50.7 MHz, 297 K) δ
2
407.4 (dd, JNN ) 15.4 Hz, JNH ) 7.4 Hz, 1N, NNH), 232.6 (dd,
JNH ) 55.1 Hz, JNN ) 15.4 Hz, 1N, NNH) ppm.
[DPPN3N]MoN2 (22). Method A. A benzene solution of
{[DPPN3N]MoNH3}[BArf4] (100 mg, 44.6 µmol) and CrCp2* (14.3
mg, 44.6 µmol) was stirred at room temperature for 24 h. After a
couple of minutes a yellow precipitate formed. In order to complete
the NH3/N2 exchange, the flask was opened several times during
the reaction. The solution was filtered off, and the solvent was
removed under reduced pressure. The residue was dried in vacuo
for 3 h and extracted with benzene. The extract was filtered through
Celite, concentrated, covered by a layer of pentane, and stored
overnight at room temperature and for another day at -30 °C. The
green-brown product was filtered off, washed with pentane, and
dried in vacuo. The mother liquor was concentrated and a second
crop obtained, yield 25 mg (40%). 1H NMR (C6D6, 400 MHz, 297
K) δ 21.2 (brs, 6H, NCH2), 6.21 (s, 3H, 4-Ar), 5.83 (s, 12H, Py),
1.71 (s, 36H, CH(CH3)2), 0.83 (s, 36H, CH(CH3)2), -5.6 (brs, 6H,
4-Ar), 6.18 (s, 12H, Py), 3.60 (brs, 6H, NCH2), 3.31 (sept, JHH
)
6.4 Hz, 12H, CHMe2), 2.35 (brs, 8H, TBA), 1.83 (brs, 6H, NCH2),
1.36 (d, JHH ) 6.7 Hz, 36H, CH(CH3)2), 1.30 (d, JHH ) 6.7 Hz,
36H, CH(CH3)2), 0.98 (m, 8H, TBA), 0.90 (m, 8H, TBA), 0.77 (t,
JHH ) 6.9 Hz, 12H, TBA) ppm; 15N NMR (C6D6, 50.7 MHz, 297
K) δ 385 (brs), 368 (brs) ppm. IR (C6D6; cm-1): 1792 (ν N).
15N15
[Na(15-crown-5)]{[DPPN3N]MoN2}. [DPPN3N]MoCl (500 mg,
358 µmol) and sodium sand (82 mg, 3.58 mmol) were suspended
in 30 mL of THF and stirred with a glass-coated stir bar for 24 h
under an dinitrogen atmosphere. The color of the solution changed
to dark green in approximately 1 h. After 24 h, the solution was
filtered off, the solvent was removed, and the violet residue was
dried in vacuo for 2 h. The solid was dissolved in 20 mL of diethyl
ether, and 15-crown-5 ether (78.8 mg, 358 µmol) dissolved in 5
mL of diethyl ether was added dropwise. The resulting bright-green
solution was stirred for 24 h. The solvent was removed under
reduced pressure. The crude product was extracted with benzene,
and the filtrate was layered with pentane. After standing the mixture
at room temperature and then for 18 h at -30 °C, a bright-green
2,6-Ar), -31.5 (brs, 6H, NCH2) ppm. IR (C6D6, cm-1): 1993 (νNN
)
cm-1. Anal. Calcd (%) for C84H117MoN12: C, 72.54; H, 8.48; N,
12.08. Found: C, 72.22; H, 8.11; N; 11.74.
Method B (Preferred). [DPPN3N]MoCl (200 mg, 143 µmol)
and sodium sand (33 mg, 1.43 mmol) were suspended in 20 mL of
THF, and the mixture was stirred with a glass-coated stir bar for
24 h under a dinitrogen atmosphere. The pressure in the flask was
periodically adjusted to the pressure in the N2-filled glovebox.
Within the first hour, the color of the solution changed to dark green.
After 24 h, the solution was filtered off, the solvent was removed,
and the violet residue was dried in vacuo for 2 h. The solid was
dissolved in 10 mL of benzene, and solid AgOTf (40.4 mg, 157
µmol) was added. The resulting mixture was shaken for ∼1 min
and filtered immediately through Celite. The resulting dark-brown
solution was concentrated and layered with pentane. The product
was isolated as an olive green solid in 60% yield.
1
powder was isolated, yield 484 mg (84%). H NMR (C6D6, 400
MHz, 297 K) δ 7.58 (brs, 6H, 2,6-Ar), 6.49 (brs, 3H, 4-Ar), 6.15
(s, 12H, Py), 3.59 (brs, 6H, NCH2), 3.25 (brs, 12H, CHMe2), 2.85
(brs, 20H, 15-crown-5), 1.82 (brs, 6H, NCH2), 1.35 (s, 36H,
CH(CH3)2), 1.25 (s, 36H, CH(CH3)2) ppm. IR (C6D6, cm-1): 1855
(νNN). Anal. Calcd (%) for C93H135MoN12NaO5: C, 68.95; H, 8.40;
N, 10.37. Found: C, 68.81; H, 8.10; N, 10.16.
[DPPN3N]MoN2H (21). Following the reduction of
[DPPN3N]MoCl (100 mg, 71.5 µmol) by Na (16 mg, 715 µmol) in
[DPPN3N]Mo15N2. An amount of 40 mL of degassed THF was
vacuum-transferred onto a mixture of [DPPN3N]MoCl (800 mg,
9
8356 J. AM. CHEM. SOC. VOL. 132, NO. 24, 2010

Molybdenum Complexes and Reduction
A R T I C L E S
572 µmol) and sodium sand (131 mg, 5.72 mmol). The Schlenk
flask was refilled with 15N2 (840 Torr, ∼50 mL), and the mixture
was stirred with a glass-coated stir bar for 24 h. Oxidation of the
diazenido salt with AgOTf and workup of the compound were done
as reported for the unlabeled complex in an argon filled glovebox,
297 K) δ 8.34 (brs, 8H, C6H3-3,5-(CF3)2), 7.66 (brs, 4H, C6H3-
3,5-(CF3)2), 6.80 (s, 6H, 2,6-Ar), 6.43 (s, 3H, 4-Ar), 6.12 (s, 1H,
NH), 6.09 (s, 12H, Py), 3.46 (t, JHH ) 4.9 Hz, 6H, NCH2), 2.51
(sept, JHH ) 6.8 Hz, 12H, CHMe2), 2.30 (t, JHH ) 4.9 Hz, 6H,
NCH2), 1.06 (d, JHH ) 6.8 Hz, 36H, CH(CH3)2), 0.95 (d, JHH
6.8 Hz, 36H, CH(CH3)2) ppm. Anal. Calcd (%) for
116H130BF24MoN11: C, 62.17; H, 5.85; N, 6.88. Found: C, 62.20;
)
yield 426 mg (53%). IR (C6D6, cm-1) 1927 (ν N).
15N15
{[DPPN3N]MoNNH2}[BArf4] (23). An ether solution of
[DPPN3N]MoN2H (90.6 mg, 65.1 µmol) was treated with solid
[H(Et2O)2][BArf4] (68.5 mg, 67.7 µmol) at -30 °C. The mixture
was allowed to warm to room temperature. Within the first minutes
the solution turned dark red. After 90 min the solvent was removed
under reduced pressure and the residue was exposed to vacuum
for 2 h. The solid was extracted with pentane. The extract was
filtered through Celite and concentrated to 5 mL, and 1 mL benzene
was added. The product crystallized at -30 °C as red needles, yield
113 mg (77%). 1H NMR (C6D6, 400 MHz, 297 K) δ 8.33 (brs, 8H,
C6H3-3,5-(CF3)2), 7.65 (brs, 4H, C6H3-3,5-(CF3)2), 6.65 (s, 3H,
4-Ar), 6.57 (s, 3H, 4-Ar), 6.40 (s, 2H, N-NH2), 6.08 (s, 12H, Py),
3.39 (s, 6H, NCH2), 2.56 (sept, JHH ) 6.8 Hz, 12H, CHMe2), 2.27
(s, 6H, NCH2), 1.04 (d, JHH ) 6.8 Hz, 36H, CH(CH3)2), 0.96 (d,
JHH ) 6.8 Hz, 36H, CH(CH3)2) ppm. Anal. Calcd (%) for
C116H131BF24MoN12: C, 61.76; H, 5.85; N, 7.45. Found: C, 62.02;
H, 5.81; N, 7.22.
C
H, 5.86; N, 6.90.
Crystals for the X-ray study were grown from a C6D6 solution
in a J-Young tube.
{[DPPN3N]MoNH}[BArf4] (50% 15N-Labeled). The synthesis
was carried out as described for {[DPPN3N]MoNH}[BArf4], yield
61 mg (74%). 1H NMR (C6D6, 400 MHz, 297 K) δ 8.31 (brs, 8H,
C6H3-3,5-(CF3)2), 7.63 (brs, 4H, C6H3-3,5-(CF3)2), 6.68 (s, 6H, 2,6-
Ar), 6.45 (s, 3H, 4-Ar), 6.12 (d, JHN ) 74 Hz), 6.09 (s, 12H, Py),
3.44 (t, JHH ) 5.3 Hz, 6H, NCH2), 2.50 (sept, JHH ) 6.8 Hz, 12H,
CHMe2), 2.24 (t, JHH ) 5.3 Hz, 6H, NCH2), 1.06 (d, JHH ) 6.8
Hz, 36H, CH(CH3)2), 0.95 (d, JHH ) 6.8 Hz, 36H, CH(CH3)2) ppm;
15N{1H} NMR (C6D6, 50.7 MHz, 297 K) δ 428.7 ppm.
{[DPPN3N]MoNH3}[[BArf4] (26). Ammonia (38 mL, ∼413
Torr, 858 µmol) was vacuum-transferred onto a frozen CH2Cl2 (5
mL) solution of [DPPN3N]MoCl (300 mg, 215 µmol) and Na[BArf4]
(209 mg, 236 µmol). The mixture was thawed and stirred for 3 h
at room temperature under partial vacuum. The solvent was
removed under reduced pressure, and the residue was dried in vacuo
for 1 h. The solid was extracted with pentane (total, ∼30 mL), and
the extracts were filtered through Celite and stored at -30 °C
overnight. A brown-red solid was filtered off, yield 301 mg (62%).
1H NMR (C6D6, 400 MHz, 297 K) δ 8.21 (brs, 8H, C6H3-3,5-
(CF3)2), 7.62 (brs, 4H, C6H3-3,5-(CF3)2), 6.10 (brs, 12H, Py), 2.72
(brs, 12H, CHMe2), 1.03 (brs, 74H, CH(CH3)2), -3.17 (brs, Ar),
-19 (brs, 6H, NCH2), -107 (brs, 6H, NCH2) ppm. Anal. Calcd
(%) for C116H132BF24MoN11: C, 62.11; H, 5.93; N, 6.87. Found: C,
62.19; H, 5.90; N, 6.81.
Crystals for the X-ray study were grown from a benzene solution,
which was covered by a layer of pentane and stored at room
temperature for 1 day and for several days at -30 °C.
[DPPN3N]MoN (24). A toluene solution of [DPPN3N]MoCl (300
mg, 215 µmol) and Me3SiN3 (50 mg, 429 µmol) in a Schlenk flask
was heated at 90 °C for 4 days. The solvents were removed from
the dark-yellow-brown solution under reduced pressure, and the
residue was dried at 50 °C in vacuo. The solid was extracted with
benzene. The extract was filtered through Celite, concentrated to
∼5 mL in vacuo, and layered with 40 mL of pentane. A yellow-
brown crystalline solid formed after standing the solution at room
temperature overnight and for several hours at -30 °C, yield 176
mg (60%). 1H NMR (C6D6, 400 MHz, 297 K) δ 7.78 (d, JHH ) 1.6
Hz, 6H, 2,6-Ar), 6.65 (t, JHH ) 1.6 Hz, 3H, 4-Ar), 6.19 (s, 12H,
Py), 3.14 (t, JHH ) 4.8 Hz, 6H, NCH2), 2.84 (sept, JHH ) 6.8 Hz,
Crystallographic Details. Low temperature diffraction data were
collected on a Siemens Platform three-circle diffractometer coupled
to a Bruker-AXS Smart Apex CCD detector with graphite-
monochromated Mo KR radiation (λ ) 0.710 73 Å) for the structure
of compound 24 and on a Bruker D8 three-circle diffractometer
coupled to a Bruker-AXS Smart Apex CCD detector with graphite-
monochromated Cu KR radiation (λ ) 1.541 78 Å) for the other
four structures (φ- and ω-scans). The structures were solved by
direct methods using SHELXS35 and refined against F2 on all data
by full-matrix least-squares with SHELXL-9736 using established
refinement techniques.37 All non-hydrogen atoms were refined
anisotropically. All hydrogen atoms (except the Mo-N-H hydro-
gen atoms in the structures of 23 and 25, which were located in
the difference Fourier synthesis) were included into the model at
geometrically calculated positions and refined using a riding model.
The isotropic displacement parameters of all hydrogen atoms were
fixed to 1.2 times the Ueq value of the atoms they are linked to (1.5
times for methyl groups). Details of the data quality and a summary
of the residual values of the refinements for all structures are given
in the Supporting Information. Descriptions of the individual
refinements follow below.
12H, CHMe2), 1.73 (t, JHH ) 4.9 Hz, 6H, NCH2), 1.20 (d, JHH
)
6.8 Hz, 36H, CH(CH3)2), 1.14 (d, JHH ) 6.8 Hz, 36H, CH(CH3)2)
ppm. Anal. Calcd (%) for C84H117MoN11: C, 73.28; H, 8.57; N,
11.19. Found: C, 73.09; H, 8.56; N, 10.98.
Crystals for the X-ray study were grown from a benzene solution,
which was covered by a layer of pentane and stored at room
temperature.
[DPPN3N]MoN (50% 15N-Labeled). A dioxane solution of
[DPPN3N]MoCl (200 mg, 143 µmol), NaN3 (1-15N) (19 mg, 286
µmol), and DME (74 µL, 715 µmol) was stirred at 85 °C for 96 h.
The solvent was removed under reduced pressure, and the residue
was dried at 50 °C in vacuo. The solid was extracted with benzene.
The extract was filtered through Celite, concentrated to ∼3 mL in
vacuo, and layered with 30 mL of pentane. A yellow-brown
crystalline solid formed after standing the solution at room
temperature overnight and for several hours at -30 °C, yield 157
1
mg (80%). H NMR (C6D6, 400 MHz, 297 K) δ 7.77 (s, 6H, 2,6-
j
Compound 19 crystallizes in the triclinic space group P1 with
Ar), 6.65 (s, 3H, 4-Ar), 6.19 (s, 13H, Py, NH), 3.14 (t, JHH ) 4.9
Hz, 6H, NCH2), 2.84 (sept, JHH ) 6.8 Hz, 12H, CHMe2), 1.73 (t,
JHH ) 5.0 Hz, 6H, NCH2), 1.20 (d, JHH ) 6.8 Hz, 36H, CH(CH3)2),
1.14 (d, JHH ) 6.8 Hz, 36H, CH(CH3)2) ppm; 15N NMR (C6D6,
50.7 MHz, 297 K) δ 905 ppm.
one molecule of 19 and two diethyl ether molecules per asymmetric
unit. Similarity restraints on 1-2 and 1-3 distances and displace-
ment parameters as well as rigid bond restraints for anisotropic
displacement parameters were applied to the two solvent molecules;
the model contains no restraints involving the target molecule.
{[DPPN3N]MoNH}[BArf4] (25). A diethyl ether solution of
[DPPN3N]MoN (150 mg, 109 µmol) was treated with solid
[H(Et2O)2][BArf4] (114 mg, 112 µmol) at -30 °C. The mixture
was allowed to warm to room temperature. In the first 10 min the
solution turned deep red. After 1 h the solvent was removed in
vacuo and the residue was extracted with pentane. The extract was
filtered through Celite and concentrated to ∼20 mL in vacuo, and
5 mL benzene was added. The product crystallized at -30 °C as
deep-red needles, yield 187 mg (77%). 1H NMR (C6D6, 400 MHz,
j
Compound 20 crystallizes in the triclinic space group P1 with
one molecule of 20 and four benzene molecules per asymmetric
unit. Two isopropyl groups of the DPP ligands show disorder and
two of the benzene molecules are disordered over four positions.
(35) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.
(36) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.
(37) Mu¨ller, P. Crystallogr. ReV. 2009, 15, 57.
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J. AM. CHEM. SOC. VOL. 132, NO. 24, 2010 8357

A R T I C L E S
Reithofer et al.
Similarity restraints on 1-2 and 1-3 distances and displacement
parameters as well as rigid bond restraints for anisotropic displace-
ment parameters were applied to all disordered atoms. In addition
all benzene molecules were restrained to be planar within 0.02 Å3
and similarity restraints were applied to geometrically relate the
three DPP fragments to one another. For both isopropyl group
disorders the ratios between the two components were refined freely,
while the sum of occupancies for every two related components
was constrained to unity. The occupancies for the four components
of the two-molecule benzene disorder were refined individually,
and the sum of all four occupancies was restrained to 2.
Compound 23 crystallizes in the monoclinic space group P21/n
with one molecule of 23, 2.5 pentane molecules, and one benzene
molecule per asymmetric unit. The half occupied pentane molecule
is disordered over two crystallographically independent positions,
involving an inversion center (resulting in four disorder components
for the full pentane molecule). The other two pentane molecules
show comparatively large anisotropic displacement parameters, but
modeling a disorder was not stable. In addition one CF3 group of
the [BArf4] anion was modeled as disordered over two positions.
The ratios between the two components of all disorders were refined
freely, while the sum of occupancies for every two related
components was constrained to unity. Similarity restraints on 1-2
and 1-3 distances and displacement parameters as well as rigid
bond restraints for anisotropic displacement parameters were applied
to all solvent molecules and to all fluorine atoms. In addition the
atoms of one of the pentane molecules were restrained to behave
approximately isotropic (within 0.04 Å3). In spite of the disorders
described above, the cores around the metal atom is well-defined
and coordinates for the two Mo NdNH2 hydrogen atoms could be
taken from the difference Fourier synthesis. These hydrogen atoms
were subsequently refined semifreely with the help of distance
restraints while constraining their isotropic displacement parameter
to 1.2 times the value of Ueq of the corresponding nitrogen atoms.
The presence of a half occupied pentane molecule in the asymmetric
unit leads to a noninteger number for carbon in the empirical
formula.
related components was constrained to unity. None of the three
solvent molecules were noticeably disordered, and similarity and
planarity restraints were applied to the solvent molecules in order
to stabilize the convergence of the model.
j
Compound 25 crystallizes in the triclinic space group P1 with
two molecules of 25 and six benzene molecules per asymmetric
unit. One of the two independent 25 cations is well behaved (no
disorders needed to be resolved), while the other cation shows
extensive disorder of the three DPP arms, only some of which could
be resolved. Most CF3 groups in both [BArf4] anions are disordered;
for a total of six of them (three in each [BArf4]) the disorder could
be resolved. Only one of the benzene molecules shows disorder
over two positions. The ratios between the two components of all
disorders were refined freely, while the sum of occupancies for
every two related components was constrained to unity. Similarity
restraints on 1-2 and 1-3 distances and displacement parameters
as well as rigid bond restraints for anisotropic displacement
parameters were applied to all atoms where applicable. In addition,
planarity restraints were applied to the solvent models and the
anisotropic displacement parameters of all fluorine atoms were
restrained to behave approximately isotropic within 0.05 Å3. In spite
of the many disorders, the cores around the metal atoms of both
independent 25 cations are well-defined and coordinates for both
Mo-N-H hydrogen atoms could be taken from the difference
Fourier synthesis. These two hydrogen atoms were subsequently
refined semifreely with the help of distance restraints while
constraining their isotropic displacement parameter to 1.2 times the
value of Ueq of the corresponding nitrogen atoms.
Acknowledgment. Research support from the National Institutes
of Health (Grant GM 31978) is gratefully acknowledged. M.R.R.
is grateful to the Austrian Science Foundation for an Erwin-
Schro¨dinger fellowship (Grant J2822-N19), to Dr. Thomas Kupfer,
Jia Min Chin, and Brian S. Hanna for helpful discussions, and to
Keith M. Wampler for advice on practical aspects of electrochem-
istry.
Compound 24 crystallizes in the monoclinic space group P21/c
with two molecules of 24 and three benzene molecules per
asymmetric unit. One of the two independent molecules of 24 is
well behaved (only one isopropyl group is disordered), while the
other one shows extensive disorder of all atoms of two of the three
DPP arms. Similarity restraints on 1-2 and 1-3 distances and
displacement parameters as well as rigid bond restraints for
anisotropic displacement parameters were applied to all atoms where
applicable. The ratios between the two components of all disorders
were refined freely, while the sum of occupancies for every two
Supporting Information Available: Crystal data and structure
refinement tables for all X-ray structural studies and selected
bond lengths and angles for [DPPN3N]MotN and
{[DPPN3N]MotNH}[BArf4]; five CIF files containing crystal-
lographic data. This material is available free of charge via the
Internet at http://pubs.acs.org.
JA1008213
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8358 J. AM. CHEM. SOC. VOL. 132, NO. 24, 2010