Reduction of dinitrogen to ammonia at a well-protected reaction site in a molybdenum triamidoamine complex

Article abstract of DOI:10.1021/ja020186x

We have synthesized a triamidoamine ligand ([(RNCH₂CH₂)₃N]^3-) in which R is 3,5-(2,4,6-i-Pr₃C₆H₂)₂C₆H₃ (HexaIsoPropylTerphenyl or HIPT). The reaction between MoCl₄(THF)₂ and H₃[HIPTN₃N] in THF followed by 3.1 equiv of LiN(SiMe₃)₂ led to formation of orange [HIPTN₃N]MoCl. Reduction of [HIPTN₃N]MoCl with magnesium in THF under dinitrogen led to formation of salts that contain the {[HIPTN₃N]Mo(N₂)}^- ion. The {[HIPTN₃N]Mo(N₂)}^- ion can be oxidized by zinc chloride to give [HIPTN₃N]Mo(N₂) or protonated to give [HIPTN₃N]Mo-N=N-H. Other relevant compounds that have been prepared include {[HIPTN₃N]Mo-N=NH₂}⁺, [HIPTN₃N]Mo≡N, {[HIPTN₃N]Mo=NH}⁺, and {[HIPTN₃N]Mo(NH₃)}⁺. (The anion is usually {B(3,5-(CF₃)₂C₆H₃)₄}^- = {BAr'₄}^-.) Reduction of [HIPTN₃N]Mo(N₂) with CoCp₂ in the presence of {2,6-lutidinium}BAr'₄ in benzene leads to formation of ammonia and {[HIPTN₃N]Mo(NH₃)}⁺. Preliminary X-ray studies suggest that the HIPT substituent creates a deep, three-fold symmetric cavity that protects a variety of dinitrogen reduction products against bimolecular decomposition reactions, while at the same time the metal is left relatively open toward reactions near the equatorial amido ligands. Copyright

Full text of DOI:10.1021/ja020186x

Published on Web 05/10/2002
Reduction of Dinitrogen to Ammonia at a Well-Protected Reaction Site in a
Molybdenum Triamidoamine Complex
Dmitry V. Yandulov and Richard R. Schrock*
Contribution from Department of Chemistry, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139
Received February 5, 2002
Scheme 1. Synthesis of H3[HIPTN3N] and [HIPTN3N]MoCl
Over 30 years of research have been devoted to the synthesis
and study of transition-metal complexes that contain dinitrogen.1
However, reports of catalytic reduction of dinitrogen to ammonia
are extremely rare,1-5 and in no instance is the catalyst a “well-
defined” species. With that goal in mind we have chosen to explore
the chemistry of triamidoamine ([(RNCH2CH2)3N]3- ) [RN3N]3-
)
molybdenum complexes, especially those in which R is an ordinary
aryl group6,7 (cf. R ) SiMe38-10 or C6F511). We found (in the form
of electrochemical evidence) that when R is a phenyl substituted
in the 3- and 5-positions with phenyl or p-t-BuPhenyl rings (e.g.,
m-terphenyls), the relatively stable [RN3N]Mo-NdN-Mo[RN3N]
complex could not form for steric reasons. To further protect the
site at which dinitrogen binds we turned to the synthesis of ligands
in which the amido substituent is the 3,5-(2,4,6-i-Pr3C6H2)2C6H3
(HIPT ) HexaIsoPropylTerphenyl) group. We find that this ligand
protects the metal to a degree that has not been achievable before,
one that allows dinitrogen to be reduced with protons and electrons
to ammonia.
Scheme 2. Synthesis of [HIPTN3N]Mo Complexesa
The synthesis of HIPT bromide follows the method employed
to prepare other m-terphenyls, as shown in Scheme 1.6,12,13 The
Pd-catalyzed coupling14,15 of the HIPT group to triethylenetetramine
proceeded without complications to give H3[HIPTN3N] in 87%
yield on an 82-g scale. (See Supporting Information for all
experimental details.) The reaction between MoCl4(THF)2 and
H3[HIPTN3N] in THF for 1 h produced a dark red solution that we
assume to contain an adduct (or adducts),7 the exact nature(s) of
which is(are) not known. Addition of 3.1 equiv of LiN(SiMe3)2
over a period of 1.5 h to this red solution led to formation of orange
[HIPTN3N]MoCl (Mo-Cl).
Reduction of Mo-Cl with Mg in THF under a dinitrogen
atmosphere produces crude “Mo-NdN-Mg(THF)3Cl”, which
upon recrystallization from pentane yields red, diamagnetic Mo-
NdN-MgCl(THF)3 (in Nujol νNN ) 1810 cm-1, ν15N15N ) 1751
cm-1; in C6D6 νNN ) 1812 cm-1; Scheme 2). If the Mg reduction
is followed by treatment with Bu4NCl and dioxane, bright green,
benzene-soluble, diamagnetic {Bu4N}{Mo-NdN} is formed (in
THF νNN ) 1859 cm-1, ν15N15N ) 1794 cm-1). THF solutions of
isolated Mo-NdN-MgCl(THF)3 also show an IR absorption at
1856 cm-1, characteristic of the “free anion,” {Mo-NdN}-.
Oxidation of crude “Mo-NdN-Mg(THF)3Cl” (or crystalline
Mo-NdN-MgCl(THF)3 or {Bu4N}{Mo-NdN}) yields Mo(N2)
in 80% yield. The IR spectrum of Mo(N2) in benzene (νNN ) 1990
cm-1) should be compared with νNN ) 1934 cm-1 in [TMSN3N]-
Mo(N2) in pentane.9 In benzene at room temperature (∼22 °C)
several days are required for the 15N in Mo(15N2) to exchange with
atmospheric 14N2.
a Pertinent ν(14N14N) and ν(15N15N) (italicized) IR frequencies (cm-1
,
in C6D6), and 15N NMR chemical shifts (underlined, ppm in C6D6) are listed
next to each compound.
Protonation of {Mo-NdN}- with {Et3NH}{BAr′4} (Ar′ ) 3,5-
(CF3)2C6H3) afforded red-orange, diamagnetic Mo-NdN-H in
50% yield. Proton NMR spectra show the diazenido proton
resonance at 8.57 ppm, which is split into a doublet of doublets in
2
Mo-15Nd15N-H (1J(15N-H) ) 53.6 Hz, J(15N-H) ) 7.9 Hz).
Further protonation of Mo-NdNH with [H(OEt2)2][BAr′4] afforded
dark red, diamagnetic {ModN-NH2}{BAr′4} in 68% yield. The
hydrazido protons appear as a singlet at 6.66 ppm in the proton
NMR spectrum, while the 15N2-analogue features a doublet of
2
doublets at 6.67 ppm (1J(15N-H) ) 90.5 Hz, J(15N-H) ) 1.4
Hz). Proton and 15N NMR data (Scheme 2) are comparable to those
* To whom correspondence should be addressed. E-mail: rrs@mit.edu.
of the closely related {[ArylN3N]Mo} species.7
9
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J. AM. CHEM. SOC. 2002, 124, 6252-6253
10.1021/ja020186x CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Treatment of Mo-Cl with TMS-N3 in toluene at 90 °C for 65
h afforded bright yellow MotN in 77% yield. Protonation of Mot
N with HBAr′4 yields dark red {Mo)NH}{BAr′4} in 93% yield.
course.) In MotN each 3,5-disubstituted aryl ring on an equatorial
amido nitrogen is turned ∼35° from lying in the same plane as the
amido nitrogen, which places one of the Trip rings (each of which
is approximately perpendicular (83-89°) to the phenyl ring to which
it is attached) “away” from the metal center, and one “up”, as shown
schematically for one HIPT group in Mo-Cl in Scheme 1. The
three Trip rings pointing up form a relatively compact, deep (∼7
Å) three-sided cavity. However, the metal still appears to be
relatively open to attack from the “side,” that is, near the equatorial
amido nitrogens, for example, by {LutH}BAr′4.
The findings presented here suggest that the [HIPTN3N]3- ligand
dramatically protects a variety of complexes that contain dinitrogen
in various stages of reduction, perhaps most importantly against
bimolecular decomposition, thereby allowing several to be isolated.
We propose that the six that we have isolated are among the dozen
or so intermediates that one might expect to form in a Chatt-type
reduction of end-on bound dinitrogen.1d,g A second likely important
feature of [HIPTN3N]3- complexes is that they should resist
degradation by multiple protonations of amido nitrogens. On the
basis of what we have observed so far it would seem that dinitrogen
reduction consists of a protonation to give a cationic species
followed by a one-electron reduction to give a neutral species. We
are in the process of attempting to couple dinitrogen reduction with
reduction of the cationic ammonia complex and subsequent
displacement of ammonia with dinitrogen to achieve a catalytic
cycle.
1
The imido proton resonance at 6.59 ppm in H NMR spectrum is
flanked by satellites in the spectrum of the 50% 15N-labeled
analogue (1J(15N-H) ) 73.7 Hz), while a doublet is found at 427.7
ppm (1JNH ) 73.7 Hz) in the 15N NMR spectrum of the 50% 15N-
labeled analogue. These 15N NMR data fall within the general
ranges established for complexes of this general type.16,17
Last, reaction of Mo-Cl with NaBAr′4 in the presence of NH3
afforded dark red, paramagnetic {Mo(NH3)}{BAr′4} in 80% yield.
No reaction is observed between Mo-Cl and NaBAr′4 in the
absence of ammonia, and addition of a few equivalents of NH3
alone to Mo-Cl only results in formation of a small amount of
the free, protonated ligand. The related salt, {Mo(NH3)}{BPh4},
was isolated in 71% yield in an analogous reaction with NaBPh4.
While the 1H NMR spectrum of {Mo(NH3)}{BAr′4} is unchanged
in the presence of NEt3 at 22 °C, addition of Bu4NCl to a C6D6
solution of recrystallized {Mo(NH3)}{BAr′4} reforms Mo-Cl and
free ammonia nearly quantitatively (95(2)%; three determinations
using the indophenol method18).
We found that reactions involving cobaltocene as the reducing
agent (E1/2 ≈ -1.3 V vs ferrocene/ferrocenium or Fc/Fc+)19 and
{2,6-lutidinium}BAr′4 as the proton source can be carried out in
benzene, since the reduction of only slightly soluble {LutH}BAr′4
with CoCp2 in benzene to give hydrogen is a relatively slow process
(hours) at room temperature. The addition of 1.0 equiv of {LutH}-
BAr′4 and 2.0 equiv CoCp2 in benzene to Mo(N2) yields Mo-Nd
NH essentially quantitatively. If {LutH}BAr′4 or CoCp2 is added
separately to Mo(N2), essentially no reaction is observed by either
1H NMR or IR. Addition of 7.0 equiv of {LutH}BAr′4 and 8.2
equiv of CoCp2 in benzene to Mo(N2) yields (by NMR) ∼60%
{Mo(NH3)}{BAr′4} and ∼10% of free ligand (H3[HIPTN3N]).
When Bu4NCl and NEt3 are added to this mixture and the volatile
components analyzed for total ammonia using the indophenol
method, the amount was found to be 1.09(2) equiv (three
determinations). Upon treating MotN with 3.5 equiv of {LutH}-
BAr′4 and 4.2 equiv of CoCp2 in benzene, {Mo(NH3)}{BAr′4} is
formed in ∼80% yield (by NMR); further treatment of the reaction
mixture with Bu4NCl and NEt3 affords 0.88(2) equiv of ammonia
(three determinations). We cannot exclude the possibility that some
dinitrogen may be lost as free N2 at some point during the reduction
of Mo(N2) to {Mo(NH3)}{BAr′4}.
The missing steps that are needed to make the process potentially
catalytic are reduction of {Mo(NH3)}{BAr′4} to Mo(NH3) and
displacement of ammonia from Mo(NH3) by dinitrogen. In fact,
{Mo(NH3)}{BAr′4} reacts with 3 equiv of CoCp2 in C6D6, to give
an equilibrium mixture of {Mo(NH3)}{BAr′4} (90%) and Mo(N2)
(10%) after 18 h; the ratio remains unchanged in the closed system
for another 22 h. The same reaction in the presence of 2 equiv of
BPh3 (to scavenge NH3) affords Mo(N2) nearly quantitatively (by
NMR) in 12 h at room temperature. Furthermore, the analogous
reaction between Cp2Co and {Mo(NH3)}{BPh4} in the presence
of BPh3 quantitatively yields Mo(N2) in 1 h or less, perhaps as a
consequence of the very low solubility of [Cp2Co][BPh4] in
benzene.
Acknowledgment. R.R.S. is grateful to the National Institutes
of Health (GM 31978) for research support. We thank Dr. W. M.
Davis, Dr. C. Ceccarelli, Professor A. L. Rheingold, and Professor
S. J. Lippard for carrying out or assisting with crystallographic
studies that will be reported elsewhere.
Supporting Information Available: Experimental details for the
synthesis of all compounds and spectroscopic data for all compounds
(PDF). This material is available free of charge via the Internet at
http://pubs.acs.org.
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X-ray data (Mo radiation) collected on several compounds
(Mo(N2), various {Mo(N2)}- salts, and MotN) were relatively
weak, we believe largely perhaps because of the relatively large
amount of organic material present; no structure could be refined
below 12%. (Structures will be reported in detail elsewhere in due
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