Functionalization and cleavage of coordinated dinitrogen via hydroboration using primary and secondary boranes

Article abstract of DOI:10.1139/v04-161

The reaction of the side-on, end-on ditantalum dinitrogen complex ([NPN]Ta)₂(μ-η¹:η²-N ₂)(μ-H)₂ (where NPN = PhP(CH₂SiMe ₂NPh)₂) with a variety of secondary and primary boranes is reported. With 9-BBN, hydroboration of the Ta₂N₂ unit occurs via B-H addition, which in turn triggers a cascade of reactions that result in N-N bond cleavage, ancillary ligand rearrangement involving silicon group migration, and finally elimination of benzene from the N-Ph group and a B-H moiety to generate the imide-nitride derivative. In the presence of excess 9-BBN, the Lewis acid - base adduct of the imide-nitride ([NPμ-N]Ta(=NBC ₈H4₁₄)(μ-NB(H)C₈H₁₄)Ta[NPN]) is formed. A similar set of reactions is observed for dicyclohexylborane (Cy ₂BH), which hydroborates the dinitrogen complex to generate [NPN]Ta(H)(μ-η¹:η²-NNBCy₂)(μ-H) ₂Ta[NPN], followed by loss of H₂ and silicon group migration to yield the imidenitride [NPμ-N]Ta(=NBCy₂)(μ-N) (Ta[NPN]. With thexyl borane (H₂BCMe₂CHMe₂), a similar sequence of reactions is suggested starting with hydroboration to generate [NPN]Ta(H)(μ-η¹:η²-NNB(H)C ₆H₁₃)(μ-H)₂Ta[NPN], followed by loss of H₂ and ancillary ligand rearrangement. When bis(pentafluorophenyl) borane (HB(C₆F₅)₂) is used, no hydroboration of coordinated N₂ is observed, rather simple adduct formation to give ([NPN]Ta)₂(μ-η¹:η²-NN-B(H)(C ₆F₅)₂)(μ-H)₂ occurs.

Full text of DOI:10.1139/v04-161

315
Functionalization and cleavage of coordinated
dinitrogen via hydroboration using primary and
secondary boranes1
Bruce A. MacKay, Samuel A. Johnson, Brian O. Patrick, and Michael D. Fryzuk
Abstract: The reaction of the side-on, end-on ditantalum dinitrogen complex ([NPN]Ta)2(µ-η1:η2-N2)(µ-H)2 (where
NPN = PhP(CH2SiMe2NPh)2) with a variety of secondary and primary boranes is reported. With 9-BBN, hydroboration
of the Ta2N2 unit occurs via B-H addition, which in turn triggers a cascade of reactions that result in N—N bond
cleavage, ancillary ligand rearrangement involving silicon group migration, and finally elimination of benzene from the
N-Ph group and a B-H moiety to generate the imide–nitride derivative. In the presence of excess 9-BBN, the Lewis
acid – base adduct of the imide–nitride ([NPµ–N]Ta(=NBC8H14)(µ-NB(H)C8H14)Ta[NPN]) is formed. A similar set of
reactions is observed for dicyclohexylborane (Cy2BH), which hydroborates the dinitrogen complex to generate
[NPN]Ta(H)(µ-η1:η2-NNBCy2)(µ-H)2Ta[NPN], followed by loss of H2 and silicon group migration to yield the imide–
nitride [NPµ–N]Ta(=NBCy2)(µ-N)(Ta[NPN]. With thexyl borane (H2BCMe2CHMe2), a similar sequence of reactions is
suggested starting with hydroboration to generate [NPN]Ta(H)(µ-η1:η2-NNB(H)C6H13)(µ-H)2Ta[NPN], followed by loss
of H2 and ancillary ligand rearrangement. When bis(pentafluorophenyl)borane (HB(C6F5)2) is used, no hydroboration of
coordinated N2 is observed, rather simple adduct formation to give ([NPN]Ta)2(µ-η1:η2-NN-B(H)(C6F5)2)(µ-H)2 occurs.
Key words: dinitrogen, tantalum, hydroboration, N—N bond cleavage.
Résumé : On rapporte les résultats de la réaction du complexe de diazote ditantale ([NPN]Ta)2(µ-η1:η2-N2)(µ-H)2 (dans
lequel NPN = PhP(CH2SiMe2NPh)2) avec une variété de boranes secondaires et primaires. Avec le 9-BBN,
l’hydroboration de l’unité Ta2N2 se produit par une addition B-H, qui provoque à son tour une cascade de réactions
conduisant à une rupture de la liaison N—N, un réarrangement du ligand impliquant une migration du groupe du sili-
cium et finalement l’élimination du benzène du groupe N-Ph et une portion B-H pour générer la formation du dérivé
imide–nitrure ([NPµ–N]Ta(=NBC8H14)(µ-NB(H)C8H14)Ta[NPN]). On observe un ensemble de réactions semblables avec
le dicyclohexylborane (Cy2BH) qui provoque une hydroboration du complexe de diazote pour générer le [NPN]Ta(H)(µ-
η1:η2-NNBCy2)(µ-H)2Ta[NPN], suivie d’une perte de H2 et de la migration du groupe du silicium pour conduire à la
formation de l’imide nitrure [NPµ–N]Ta(=NBCy2)(µ-N)Ta[NPN]. Avec le thésylborane (H2BCMe2CHMe2), la réaction
suivrait un cours semblable qui débuterait avec une hydroboration pour générer le [NPN]Ta(H)(µ-η1:η2-NNB(H)C6H13)-
(µ-H)2Ta[NPN], suivie d’une perte de H2 et d’un réarrangement de ligand. Avec le bis(pentafluorophényl)borane
(HB(C6F5)2), on n’observe pas l’hydroboration du N2 coordiné, uniquement la formation d’un adduit simple qui génère
le ([NPN]Ta)2(µ-η1:η2-NN-B(H)(C6F5)2)(µ-H)2.
Mots clés : diazote, tantale, hydroboration, scission de la liaison N—N.
[Traduit par la Rédaction] MacKay et al. 323
Introduction
global need for NH3-derived fertilizers (6). While discovery
of a homogeneous version of the Haber–Bosch process is the
goal of a number of groups around the world (7, 8), another
worthwhile goal is to develop a process that converts molec-
ular nitrogen to high-value organonitrogen materials, such as
amines, substituted hydrazines, and N-heterocycles (9–15).
A key challenge in devising any homogeneous catalytic
cycle involving molecular nitrogen is coordination of N2 to
the metal centre. Because dinitrogen is intrinsically unre-
active, formation of N2 complexes is not straightforward.
The activation of small molecules by metal complexes is a
mature area of inorganic chemistry. Nevertheless, molecular
nitrogen, one of the most abundant small molecules in the
biosphere, continues to frustrate inorganic chemists because
of its intrinsic lack of reactivity (1–5). Under extreme condi-
tions, N2 will react with H2 over an activated iron surface to
generate ammonia; this energy-intensive catalytic transfor-
mation, known as the Haber–Bosch process, supplies the
Received 15 September 2004. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 16 March 2005.
B.A. MacKay, S.A. Johnson, B.O. Patrick, and M.D. Fryzuk.2 Department of Chemistry, University of British Columbia,
2036 Main Mall, Vancouver, BC V6T 1Z1, Canada.
1This article is part of a Special Issue dedicated to Dinitrogen Chemistry.
2Corresponding author (e-mail: fryzuk@chem.ubc.ca).
Can. J. Chem. 83: 315–323 (2005)
doi: 10.1139/V04-161
© 2005 NRC Canada

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Can. J. Chem. Vol. 83, 2005
Scheme 1.
Scheme 2.
Since the first report of a dinitrogen complex in 1965 (16),
many N2 derivatives have been synthesized (17–20). The
most common method of synthesis generally requires
strongly reducing conditions, which unfortunately is incom-
patible with many functionalization protocols. Recently, we
reported (21) a facile method for the formation of a ditan-
talum dinitrogen complex that involves reaction of N2 with
the ditantalum tetrahydride 1 (Scheme 1). This reaction pro-
ceeds smoothly to generate the side-on, end-on dinitrogen
complex 2 without the necessity of added reducing agents. It
occurred to us that multiple additions of some simple hy-
dride reagent (E-H) across the Ta2N2 core of 2 could regen-
erate the starting tetrahydride 1 and produce a functionalized
hydrazine moiety (N2E4). Since addition of N2 to 1 reforms
the dinitrogen complex 2, this could result in a catalytic se-
quence.
To test this hypothesis, we have examined the reaction of
2 with a number of simple hydride reagents, and preliminary
communications have already been published (22, 23). In the
case where E-H is 9-borabicyclo[3.3.1]nonane (9-BBN), one
E-H addition occurs cleanly; however, a catalytic cycle was
precluded because of an ancillary ligand dominated rear-
rangement. Nevertheless, this work did allow the discovery
of a new type of N—N bond cleavage process that results
from functionalization of coordinated N2. In this paper, we
report the full details of the reaction of the dinitrogen com-
plex 2 with some primary and secondary boranes.
Attempted second hydroboration — Synthesis and
characterization of [NPN]Ta(H)(␮-N2-B(C6H11)2)(␮-
H)2Ta[NPN] (6)
Having established that hydroboration of 2 with 9-BBN
leads to N—N bond cleavage and the formation of reactive
nitrides, we examined multiple additions of 9-BBN to fur-
ther test the catalytic plan shown in Scheme 1. Addition of 2
or more equiv. of 9-BBN to 2 resulted in a complicated mix-
ture of products as observed by 31P NMR spectroscopy. Al-
though none of the products were immediately identifiable,
there did seem to be a major product present; unfortunately,
it could not be separated from the other materials present.
Fortuitously, addition of 1 equiv. of 9-BBN to imide–nitride
5 resulted in the clean formation of this same major product
unencumbered by impurities. This new complex 6 has two
resonances (1:1 integration) in its 31P NMR spectrum that
1
are distinct from those of 5, and the H NMR spectrum does
not feature resonances indicative of tantalum hydrides. To
determine its identity, crystals of 6 were obtained from a
cooled THF solution and subjected to X-ray diffraction anal-
ysis.
Results and discussion
As summarized in Scheme 2, our initial attempt at E-H
addition to 2 using 9-BBN resulted in the first example of a
hydroborated dinitrogen complex (3). It was found that
addition product 3 is thermally unstable and undergoes H2
elimination and N—N bond cleavage, presumably via the
unobserved intermediate A, followed by silicon migration
from the ancillary ligand to the bridging nitride to generate
4. We presume that the final step is elimination of benzene
via the B—H bond and the phenyl of the Ta=N-Ph unit to
generate the final imide-nitride 5. Postulation of this pro-
posed scheme was facilitated by the X-ray crystal structures
of intermediates 3 and 4, and final product 5, along with de-
tailed labeling experiments (22).
The solid-state molecular structure of 6 (Fig. 1) shows
that a simple Lewis acid – base reaction between the bridg-
ing nitride in 5 and the B atom of 9-BBN has taken place.
This second equiv. of 9-BBN occupies a position roughly
equal to that of the single 9-BBN fragment in intermediate 4
(Scheme 2). The N3—B2 bond length of 1.516(5) Å sug-
gests a N—B single bond (24). The borylimido group is de-
flected towards the phosphine donor Pl by the presence of
the second equiv. of 9-BBN. Other than this, complex 6 is
very similar in structure to 5.
In solution, the presence of the boron hydride is implied
1
by a broad H NMR resonance integrating to one proton
at δ 4.32 ppm. In the solid state, hydrogen atom H71 was re-
© 2005 NRC Canada

MacKay et al.
317
Fig. 1. ORTEP drawing of the solid-state molecular structure of
6 as determined by X-ray crystallography (ellipsoids at 50%
probability). Silyl methyls and phenyl ring carbons other than
ipso are omitted for clarity. H71 was refined isotropically. Se-
lected bond lengths (Å), bond angles (°), and torsion angles (°):
Ta1—N1 1.824(3), N1—B1 1.404(5), Ta1—N2 2.175(3), N2—
Ta2 1.947(3), Ta2—N3 1.854(3), N3—Ta1 2.156(3), N3—B2
1.516(5), N2—Si1 1.738(3), Ta1—N4 2.126(3), Ta1—P1
2.5825(10), Ta2—N5 2.091(3), Ta2—N6 2.048(3), Ta2—P2
2.7737(10); Ta1-N2-Ta2 93.87(11), N2-Ta2-N3 91.05(12), Ta2-
N3-Ta1 97.23(13), N3-Ta1-N2 77.58(11), Ta1-N1-B1 179.3(3),
Ta1-N3-B2 88.6(3), N1-Ta1-P1 86.99(10), N1-Ta1-N4 123.12(13),
Ta1-N2-Si1 139.05(16), N3-Ta2-N5 101.25(13), N3-Ta2-N6
103.04(12), N3-Ta2-P2 171.01(9), Ta1-N2-Ta2-N3 3.87(11), Ta1-
Ta2-N2-Si1 6.9(6), N1-Ta1-N2-Ta2 –117.54(13), P1-Ta1-Ta2-
P2 13.46(4), Ta2-N2-Ta1-N4 98.92(14).
by hydroboration or simple Lewis acid adduct formation.
This precludes hydroboration as the appropriate E-H addi-
tion to completely satisfy the proposal of Scheme 1.
A homologous series of complexes prepared with
dicyclohexylborane (Cy2BH) and thexyl borane
(H2BCMe2CHMe2)
We wondered about the generality of the cascade of reac-
tions shown in Scheme 2. The ability to cleave coordinated
N2 and to generate a new N—B bond was incentive to exam-
ine whether or not other borane reagents would engage in
similar outcomes or in fact open up new reaction pathways.
Given the ready availability of different organoborane re-
agents, this seemed worthwhile.
The reaction of 1 equiv. of dicyclohexylborane (Cy2BH)
with 2 in toluene gives [NPN]Ta(H)(µ-η1:η2-NNBCy2)(µ-
H)2Ta[NPN] (7) in 93% yield after stirring for 12 h (eq. [2]).
fined isotropically, and thus its position as shown in Fig. 1 is
likely based on the structures obtained for 4 as well as the
position and coordination geometry of B2. This reaction is
shown in eq. [1]. Significantly, the intramolecular reaction
to give 5 from 3 proceeds whether additional hydroboration
1
Identification of 7 was facilitated by the similarity of its H
and 31P NMR spectra to those of the initial 9-BBN adduct
(3). Thus, the H NMR spectrum of 7 indicates C1 solution
1
symmetry (eight silyl methyl resonances and two separate
one-proton resonances associated with bridging hydrides are
present), and a new resonance at δ 16.04 ppm implies the ex-
istence of a new terminal hydride, as was found for 3. The
solid-state molecular structure of 7 was not established.
Does 7 decompose in the same manner as 3 (Scheme 2)?
Toluene or THF solutions of 7 show conversion to [NPµ-
N]Ta(=NPh)(µ-NB(H)Cy2)Ta[NPN] (8) in similar yield and
on a time scale comparable to the decomposition of 3 into 4.
Therefore, hydroboration and N—N bond cleavage are both
possible using Cy2BH interchangeably with 9-BBN. The
solid-state molecular structure of 8 has been determined and
it is shown in Fig. 2. The relative orientations of the phenyl-
imido ligand and H85 are similar to 4. The fact that N—N
bond scission and silyl group migration from an [NPN]
ligand amide to the new dinitrogen-derived nitrido ligand
have occurred in an exactly analogous fashion is evident.
Bond lengths and angles are comparable to those of 4, ex-
cept that the [NPN] ancillary ligand bound to Ta2 of com-
plex 8 is rotated by 70° about the Ta-Ta axis as compared to
its position in 4.
reagent is present or not; as mentioned above, the 2:1 reac-
tion between 9-BBN and 2 also gives mostly 6 when this re-
action is monitored by 31P NMR spectroscopy, indicating
that 9-BBN is not competent to intercept the nascent nitrido
intermediate postulated in the transformation of 4 to 5, either
Like its 9-BBN analogue 4, 8 is observed via NMR spec-
troscopy to eliminate benzene in d8-THF solution, and at the
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Can. J. Chem. Vol. 83, 2005
Fig. 2. ORTEP drawing (spheroids at 50% probability) of 8
([NPµ-N]Ta(=NPh)(µ-NB(H)Cy2)Ta[NPN]). Silyl methyl and
phenyl ring carbons other than ipso omitted for clarity. H85 was
located in the electron difference map and refined isotropically.
Selected bond lengths (Å), bond angles (°), and torsion angles
(°): Ta1—N1 2.142(3), Ta1—N2 2.191(3), Ta1—N3 1.788(3),
Ta1—N4 2.146(3), Ta1—P1 2.6370(11), N1—B1 1.532(7), B1—
H1 1.59(6), Ta1—H1 1.90(8), N2—Si1 1.738(3), N1—N2 2.675(3),
Ta2—N5 2.070(3), Ta2—N6 2.066(3), Ta2—P2 2.7897(10); N1-
Ta1-N2 76.31(12), Ta1-N2-Ta2 94.88(12), N2-Ta2-N1 90.40(13),
Ta2-N1-Ta1 98.41(13), Ta1-N1-B1 89.8(2), N1-B1-H1 90.2(6),
N1-Ta1-N3 117.17(13), N1-Ta1-N4 114.08(11), N1-Ta1-P1 136.84(8),
N1-Ta2-N5 114.35(12), N1-Ta2-N6 117.00(13), N1-Ta2-P2 99.46(9),
Ta1-N2-Ta2-N1 0.25(11), P1-Ta1-Ta2-P2 –143.96(4), N1-Ta1-N2-
Si1 –176.8(2), N2-Ta1-N1-B1 –169.8(2).
Scheme 3.
mary borane. The presence of another boron hydride for
intramolecular reactions after an initial hydroboration
suggested the possibility of different post-hydroboration
reaction paths including another potential opportunity to
form N—H bonds. The 1:1 reaction between 2 and thexyl
borane (H2BCMe2CHMe2) proceeds to completion over
12 h as shown in eq. [3] to give [NPN]Ta(H)(µ-η1:η2-
NNB(H)C6H13)(µ-H)2Ta[NPN] (10) in 92% yield.
same time, 31P NMR resonances of a new species (9) arise.
Identification of this complex as ([NPµ–N]Ta(=NBCy2)(µ-
1
N)Ta[NPN] was made by H and 31P NMR spectroscopy in
analogy to that already shown in Scheme 2. This new
imide–nitride 9 forms cleanly from 8 in an overall yield of
83% as measured by 31P NMR spectroscopy against an in-
ternal reference. This is an improvement over the conversion
of 4 to 5, in which many other 31P NMR-active side-
products were detected. Although the change from 9-BBN to
dicyclohexylborane does not significantly attenuate
hydroboration of 1 or N—N bond cleavage, it did increase
the yield of 9 vs. the yield of 5. In Scheme 3, this chemistry
is summarized starting with hydroboration adduct 7; loss of
H2 triggers N—N bond cleavage, presumably through an un-
observed intermediate such as B, which then undergoes sili-
con migration from the ancillary [NPN] ligand to generate 8
and finally 9 via loss of benzene. Presumably, addition of
another equivalent of dicyclohexylborane to 9 would pro-
duce an adduct similar to 6 in eq. [1]; however, this was not
pursued.
Although crystals suitable for X-ray diffraction were not
isolated, the H NMR spectrum of 10 shows the expected C1
symmetry and the new resonance characteristic of a terminal
hydride at δ 15.52 ppm, analogous to that already observed
for 3 and 7. The remaining boron hydride is evident as a
broad singlet at δ 4.42 ppm that integrates to one proton.
1
As for complexes 3 and 7, solutions of 10 are thermally
unstable. The rearranged product (11) has no resonances
1
suggestive of bridging or terminal hydride ligands in its H
NMR spectrum, and the 31P NMR resonances of 11 do not
unequivocally make it a sister complex to 4 or 8. The
1H NMR spectrum is indicative of C1 symmetry, and the
resonances can be assigned as [NPµ-N]Ta(=NPh)(µ-
NB(H)2C6H13)Ta[NPN], as shown in eq. [4].
Since hydroboration of 2 and N—N bond cleavage oc-
curred for the secondary boranes 9-BBN and Cy2BH, it was
of interest to examine the course of the reactions with a pri-
© 2005 NRC Canada

MacKay et al.
319
borane HB(C6F5)2. The 15N NMR spectrum of the 15N2-
enriched complex 15N2-12, which was prepared by the reac-
tion of 15N2-2 with HB(C6F5)2, contains two resonances, at
δ −46.9 and δ 24.0. The resonance at δ −46.9 is a doublet of
2
1
doublets, with a JNP value of 25.9 Hz and a JNN value of
15.3 Hz. As for 15N2-2, the chemical shift of this resonance
and the large coupling to 31P indicates that this is the end-on
nitrogen. The chemical shift of the second 15N resonance
(δ = 24.0) is much closer to that observed in the B(C6F5)3
adduct of 15N2-2 (δ 2.4) than to 15N2-3 (δ –114.8), where a
tantalum hydride bond is formed. This reaction product is,
therefore, not related to complexes 3, 7, and 10, which are
the products of B-H addition. Rather, this spectral informa-
tion indicates that 12 is more closely related to the Lewis
acid – base adducts of 2 with B(C6F5)3, GaMe3, and AlMe3,
(28) and can be assigned as [NPN]Ta(µ-η1:η2-NN-
B(H)(C6F5)2)(µ-H)2Ta[NPN]. As for these complexes and
parent complex 2, the solution symmetry of 12 is Cs as evi-
1
denced by the four unique silyl methyl resonances in its H
NMR spectrum. The formation of complex 12 is illustrated
in eq. [5].
Satisfactory elemental analysis was obtained for this formu-
lation, but the solid-state molecular structure of 11 has not
been established. The only indication that 11 might be other
than that suggested is that it does not degrade via elimina-
tion of benzene to a congener of 5 or 9. Instead it can be ob-
served by 31P NMR spectroscopy to convert into a number
of different products over a week. None of these were spec-
trally related to 5 or 9. Hydroboration of 2 with thexyl bor-
ane may lead to new rearrangements for 8 that were not
observed with secondary boranes, but characterization of the
resulting complexes has not been possible. It is likely that
the additional B-H functionality coupled with the already
documented propensity for ancillary ligand rearrangements
in this system renders this combination too reactive. With
group 6 dinitrogen complexes, the reaction with thexyl bor-
ane results in a totally different and distinct outcome than
described herein (25) (Table 1).
Synthesis of ([NPN]Ta)2(␮-␩1:␩2-NN-B(H)(C6F5)2)(␮-H)2
(12)
Although the dinitrogen unit of 2 can be derivatized by
hydroboration, it is not clear how to remove the functiona-
lized dinitrogen moiety or the dinitrogen-derived atoms in
the descendant complexes from the bimetallic core, because
the Ta–N interactions remain quite strong. It was postulated
that a more Lewis acidic borane might react in a similar
manner to 9-BBN, but would remove additional electron
density from the dinitrogen moiety, and thus greatly weaken
the Ta—N bonds. A straightforward synthesis of the strongly
Lewis acidic secondary borane HB(C6F5)2 has recently been
reported (26, 27), and so the reactivity of this borane with 2
was investigated.
This assignment was confirmed by X-ray crystallography.
An ORTEP drawing of the solid-state molecular structure of
12 is shown in Fig. 3. Hydrides were not located in the dif-
fraction experiment, but the Ta2N2 core bonding and the
relative disposition of the ancillary [NPN] ligands is remi-
niscent of parent complex 2 and of its Lewis acid adducts.
Upon hydroboration, the N—N bond was elongated from
1.319(4) Å in 2 to 1.411(15) Å in 3. The same bond in 12 is
only marginally elongated as compared to the starting
dinitrogen complex and the other metric parameters of the
Ta2N2 core are similar. Therefore, derivatizaton of the coor-
dinated N2 unit of 2 with strongly Lewis acidic boranes does
not affect core bonding. Like 2 and its Lewis adducts, 12 is a
stable molecule as compared to 3, 7, and 10, and it shows no
propensity to undergo the reductive elimination and N—N
bond scission common to these complexes. It seems that ad-
dition of the H—B bond across the exposed Ta=N bond of 2
is a precondition for these processes. The theoretically pre-
dicted reversal of the polarity of this bond in HB(C6F5)2 vs.
dialkylboranes may preclude this reaction, implying that E—
H bond polarity is a important consideration in selecting
The reaction of HB(C6F5)2 with complex 2 occurs imme-
diately; however, the colour of the solution changes from the
red-brown of 2 to a dark yellow-brown colour, rather than to
an orange colour as observed for the B-H addition adducts 3,
7, and 10. Two new resonances are observed in the 31P NMR
spectrum at δ 10.0 and δ 22.8 with a JPP value of 25.1 Hz. A
1
broad peak at δ 4.7 in the H NMR spectrum integrates to a
single proton. The location and broadness of this resonance
identifies it as a hydrogen atom bound to boron. A similar
1
peak is observed in the H NMR spectrum of the starting
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Can. J. Chem. Vol. 83, 2005
Table 1. X-ray crystallographic information for complexes 6, 8, and 11.3
[NPµ-N]Ta(=N-BC8H14)(µ-
N-B(H)C8H14)Ta[NPN] (6)
[NPµ-N]Ta(=NPh)(µ-N-
B(H)(C6H11)2)Ta[NPN] (8)
([NPN]Ta)2(µ-NN-
B(H)(C6H5)2)(µ-H)2 (11)
CCDC registry
Formula
250128
C58H86N6P2Ta2 Si4B2[C6H6]
250129
C67H93N6Ta2P2Si4B
250130
C60H65BF10N6P2Si4Ta2(C6H6)2.5
FW
1581.39
1518.70
1802.46
Colour, habit
Crystal dimensions (mm)
Crystal system
Space group
Yellow, chip
0.20 × 0.15 × 0.15
Monoclinic
Yellow, chip
0.35 × 0.20 × 0.10
Triclinic
Dark, block
0.32 × 0.24 × 0.16
Monoclinic
P21/n (No. 14)
P1(No. 2)
P21/n (No. 14)
a (Å)
b (Å)
c (Å)
α (°)
11.7634(4)
25.6823(8)
24.0178(9)
90
96.180(2)
90
7213.9(4)
4
1.46
12.0875(4)
13.7220(4)
20.9574(8)
91.266(3)
90.784(3)
84.952(3)
3461.5(2)
2
17.2272(3)
24.5044(2)
18.4711(3)
90
95.071(1)
90
7766.9(2)
4
1.54
β (°)
γ (°)
V (Å3)
Z
ρcalcd (g/cm3)
1.46
F(000)
3208.00
3.181
0.6368–1.0000
55.8
1538.00
3.312
0.7278–1.0000
55.7
3596.00
2.988
0.8192–1.0000
56.0
µ(MoKα) (mm–1)
Transmission factors
2θmax (°)
Total no. of reflns.
No. of unique reflns.
Rmerge
62 376
16 351
0.053
31 854
14 230
0.062
46 043
15 345
0.043
No. reflns with I ≥ nσ(I)
No. of variables
12 735 (n = 2)
775
0.043
11 678 (n = 2)
720
0.039
15 345 (n = 2)
881
0.036
R (F2, all data)
Rw (F2, all data)
0.068
0.074
0.076
R (F, I > nσ(I))
Rw (F, I > nσ(I))
0.029
0.062
0.029
0.071
0.046
0.081
GOF
0.94
0.098
1.088
Note: Rigaku/ADSC CCD diffractometer, R = Σ ||Fo | – |Fc2||/Σ|Fo |; Rw = (Σw(|Fo | – |Fc2|)2/Σw|Fo | )1/2
.
2
2
2
2 2
reagents for any scheme bent on using 2 for catalytic
functionalization of N2.
the starting dinitrogen complex 2 in the same manner as the
alkylboranes; instead, a stable adduct is formed, likely a re-
sult of the enhanced Lewis acidity of the boron centre owing
to the presence of the perfluorophenyl substituents. This is
summarized in Scheme 4.
Summary
The initial study of the hydroboration of a dinuclear tanta-
lum dinitrogen complex with 9-BBN has been extended to
other monoalkyl- and dialkylboranes. In all cases, the N2
complexes, derivatized by H—B bond addition across the
exposed Ta=N bond, spontaneously undergo reductive elimi-
nation of two bridging hydrides as H2 coupled to N—N
bond cleavage without loss of the new B—N bond. This cas-
cade of reactions is only somewhat attenuated by different
substituents on the hydroboration reagent. Subsequent addi-
tion reactions across new Ta=N moieties do not occur, but
the additional equivalents of hydroboration reagent can be
incorporated into the complex as alkylborohydride adducts.
Bis(pentafluorophenyl)borane HB(C6F5)2 fails to react with
Experimental section
General considerations
Unless otherwise stated, all manipulations were performed
under an atmosphere of dry oxygen-free dinitrogen by
means of standard Schlenk or glovebox techniques (Vacuum
Atmospheres HE-553-2 glovebox equipped with a MO-40-
2H purification system and a –60 °C freezer). Anhydrous
hexanes and toluene were purchased from Aldrich, sparged
with dinitrogen, and passed through columns containing ac-
tivated alumina and Ridox catalyst before use. Anhydrous
diethylether was stored over sieves and distilled from so-
3 Supplementary data may be purchased from the Directory of Unpublished Data, Document Delivery, CISTI, National Research Council
Canada, Ottawa, ON K1A 0S2, Canada (http://cisti-icist.nrc-cnrc.gc.ca/irm/unpub_e.shtml for information on ordering electronically).
CCDC 250128–250130 contain the crystallographic data for this manuscript. These data can be obtained, free of charge, via
ww.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax +44 1223 336033; or deposit@ccdc.cam.ac.uk).
© 2005 NRC Canada

MacKay et al.
321
Fig. 3. ORTEP depiction of the solid-state molecular structure of
([NPN]Ta)2(µ-η1:η2-NN-B(H)(C6F5)2)(µ-H)2 (12) as determined
by X-ray crystallography. The bridging hydrides and the hydro-
gen bound to boron were not located. The silyl methyl groups
and fluorine atoms are omitted for clarity, and only the ipso car-
bons of the PPh and NPh groups are shown. Selected bond
lengths (Å) and bond angles (°): N5—N6 1.368(4), Ta1—Ta2
2.8903(2), Ta1—N5 2.186(3), Ta1—N6 1.964(3), Ta2—N5
1.878(3), N6—B1 1.557(5), Ta1—P1 2.6089(10), Ta2—P2
2.6265(10); Ta1-N5-Ta2 90.36(12), Ta1-N6-N5 79.8(2), Ta1-N6-
B1 155.6(3).
Scheme 4.
C7D7H (δ 2.09 ppm). 31P NMR spectra were referenced to
external P(OMe)3 (δ 141.0 ppm with respect to 85% H3PO4
at δ 0.0 ppm) or internal P(OMe)3 flame-sealed inside a
1 mm × 25 mm glass capillary tube if required, 13C NMR
spectra to 13CC5D6 (δ 128.4 ppm) and 13CD2Cl2 (δ 54.0 ppm),
11B spectra to neat BF3·Et2O (δ 0.0 ppm), 15N spectra to ex-
ternal nitromethane at 0.0 ppm, and 29Si to Me4Si 50% in
CDCl3 (δ 0.0 ppm). Elemental analyses were performed by
Mr. P. Borda and Mr. M. Lakha, of The University of Brit-
ish Columbia Department of Chemistry.
The syntheses of complexes 1, 2 (21), 3, 4, and 5 (22),
dicyclohexylborane (29), thexyl borane (30), and HB(C6F5)2
(27) were performed as described in the literature.
[NP␮-N]Ta(=N-BC8H14)(␮-NB(H)C8H14)Ta[NPN] (6)
To a stirred 15 mL toluene solution of 5 (298 mg,
0.229 mmol) was added dropwise 0.25 mL (1.1 equiv.) of
1.0 mol/L 9-BBN solution in THF. The resulting mixture
was stirred overnight and the 31P NMR spectrum of a small
portion indicated that the reaction had proceeded to consume
all of complex 5, giving exclusively resonances of 6. After
evaporation of solvent and precipitation from pentane,
241 mg (0.169 mmol, 74% yield) of 6 was recovered on a
frit. 1H NMR (C6D6, 300 K, 400 MHz) δ: –0.69, –0.21, 0.05,
0.13, 0.25, 0.34, 0.43, (s, 3H each) SiCH3; 0.84, 0.97 (d, 1H
each) PCH2, complicated overlapping multipets from 0.70 to
2.26 (total 39H), B-C8H14 and PCH2 (solvent was contami-
nated with a small amount of hexanes) 4.32 (b, 1H), B-H;
6.73, 6.79, 6.81, 6.85, 6.93, 7.07, 7.09, 7.20 (s, d, t, some
peaks obscured by solvent) C6H5-N and C6H5-P; (7.51 (d,
2H, JHH = 6.95 Hz), 7.94 (d, 2H, JHH = 7.05 Hz), o-C6H5-P.
31P NMR (C6D6, 300 K, 161.9 MHz) δ: –12.52 (b), 27.26
(s). Anal. calcd. for C58H86B2N6P2Si4Ta2: C 48.88, H 6.08,
N 5.90; found: C 49.12, H 6.23, N 5.81.
dium benzophenone ketyl under argon. Pentane was stored
over sieves and distilled from sodium benzophenone ketyl
solublized by tetraglyme under dry dinitrogen prior to stor-
age over a potassium mirror. Tetrahydrofuran was heated at
reflux over CaH2 prior to distillation from sodium benzophe-
none ketyl under argon. Nitrogen gas was dried and
deoxygenated by passage through a column containing acti-
vated molecular sieves and MnO.
Deuterated benzene was dried by heating at reflux with
sodium–potassium alloy in a sealed vessel under partial
pressure, then trap-to-trap distilled, and freeze–pump–thaw
degassed three times. Deuterated tetrahydrofuran and tolu-
ene were dried by refluxing with molten potassium metal in
a sealed vessel under vacuum, then trap-to-trap-distilled, and
freeze–pump–thaw degassed three times. Unless otherwise
[NPN]Ta(H)(␮-␩1:␩2-NNBCy2)(␮-H)2Ta[NPN](7)
Toluene (25 mL) was added to an intimate mixture of dry
2 (0.412 g, 0.327 mmol) and white solid dicyclohexylborane
(58.2 mg, 1 equiv.) in a glove box. The resulting mixture
was stirred vigorously overnight, the solvent was evaporated,
and the residues were triturated under hexanes, giving
0.438 mg (0.305 mmol, 93.2% yield) of 7. 1H NMR
(500.1 MHz, C6D6, 300 K) δ: –0.37, –0.22, –0.17, –0.05,
–0.10, –0.01, 0.09, 0.14 (s, 24H total), SiCH3; 0.28, 0.40,
0.64, 0.73, 0.77, 0.86, 1.09, 1.15 to 1.34 (broad), 1.47, 1.59,
1
1
stated, H, 31P, H{31P}, 13C, 11B, 15N, 29Si-DEPT, and two-
dimensional NMR spectra were recorded on either a Bruker
AMX-500 instrument (5 mm BBI probe) operating at
1
500.1 MHz for H or a Bruker AVA-400 instrument (5 mm
BBI probe) operating at 400.1 MHz for 1H. 1H NMR spectra
were referenced to residual proton in deuterated solvent as
follows: C4D7HO (δ 3.58 ppm), C6D5H (δ 7.15 ppm), and
© 2005 NRC Canada

322
Can. J. Chem. Vol. 83, 2005
[NPN]Ta(H)(␮-␩1:␩2-NNB(H)C6H13)(␮-H)2Ta[NPN] (10)
To a stirred toluene solution of 1 (0.336 g, 0.266 mmol)
was added 0.54 mL (0.27 mmol, 1.02 equiv.) of freshly pre-
pared thexyl borane (0.5 mol/L in THF). After stirring over-
night the solvents were evaporated, leaving an orange
residue that was triturated under hexanes and recovered on a
frit, giving 0.332 g (0.244 mmol, 91.8% yield) of solid pale
orange 10. 1H NMR (400 MHz, C6D6, 300 K) δ: –0.32,
–0.14, –0.05, 0.11, 0.17, 0.19, 0.25, 0.34 (s, 3H each),
SiCH3; 1.21, 1.41, 1.45, 1.59, 1.61, 1.67, 1.78, 1.81 (d, 1H
each) PCH2; 0.82, 0.89 (d, 3H each) 1.11, 1.23 (s, 3H each),
B-C(CH31)2CH(CH3)2; 1.64 (m, 1H) B-C(CH3)2CH(CH3)2;
4.42 (b, H) B-H; 5.96, 6.07, 6.22, 6.28, 6.35, 6.66, 6.91,
6.94, 7.01, 7.12, 7.20, 7.22, 7.25 (d, t, total 26H), P-C6H5
and N-C6H5; 7.71, 7.92 (d, 2H each) o-P-C6H5; 10.2, 11.6
(d, 1H each) TaµH; 15.52 (s, 1H) TaH. 31P NMR
(161.9 MHz, C6H6, 300 K) δ: 8.73 (d, JPP = 15.3 Hz), 24.16
(d, JPP = 15.3 Hz). Anal. calcd. for C54H79BN6P2Si4Ta2: C
47.72, H 5.86, N 6.18; found: C 47.32, H 6.26, N 6.12.
and 1.77 (broad overlapping resonances), cyclohexyl and
PCH2; 6.61, 6.66, 6.73, 6.82, 6.88, 6.96, 7.13 to 7.26 (over-
lapping), 7.29, 7.38, and 7.70 (overlapping doublets, triplets,
20H total, some resonances obscured by solvent) P-C6H5
and N-C6H5; 8.14 (d, JPH = 6.38 Hz), 8.24 (d, JPH
6.82 Hz), o-P-C6H5; 10.70 and 11.51 (d, 1H each, JHH
=
=
10.0 Hz) µ-H; 16.04 (d, JPH = 14.6 Hz), Ta-H. 13C NMR
was not recorded. 31P NMR (202.5 MHz, C6D6, 300 K) δ:
7.98 (d, JPP = 10.2 Hz), 20.0 (d, JPP = 10.2 Hz). Anal. calcd.
for C60H87BN6P2Si4Ta2: C 50.07, H 6.09, N 5.84; found: C
50.35, H 6.34, N 5.48.
Decomposition of 7 to [NP␮-N]Ta(=NPh)(␮-
NB(H)Cy2)Ta[NPN] (8)
A capped THF solution of 396 mg (0.275 mmol) 7 was
left in a glove box at ambient temperature for 3 weeks and
then cooled in a –60 °C freezer, giving crystals of 8. Yield
1
0.186 g, 0.129 mmol, 47%. H NMR (400.1 MHz, C7D8,
300 K) δ: –1.13, –0.90, –0.09, –0.15, –0.36, 0.18, 0.34 (s,
24H total), SiCH3; 0.50, 1.16 (d, 1H each) PCH2; –0.01,
0.68, 0.87, 1.06, 1.33, 1.45, 1.48 (broad overlapping
multiplets, total 26H) B-C6H11 and PCH2; 4.09 (b, FWHM
32 Hz, 1H) BH; 6.28, 6.30, 6.34, 6.40, 6.50, 6.74, 6.83,
6.94, 7.00, 7.11, 7.18 (overlapping doublets, triplets, 20H
total, some resonances obscured by solvent) P-C6H5 and N-
C6H5; 7.84 (d, JPH = 6.59 Hz), 7.49 (d, JPH = 7.06 Hz), (10H
total) o-P-C6H5. 13C NMR (100.6 MHz, C7D8, 300 K) δ:
–1.77, –0.80, –0.64, 0.30, 2.67, 2.75, 3.30, 5.39, SiCH3;
32.85 (b), B-Cipso of cyclohexyl; 11.14, 15.45, 18.19, 25.93,
26.05, CH2 of cyclohexyl; 20.99, 25.32, 26.05, 26.41, 27.16,
29.99, 30.49, 31.58, P-CH2; 118.27, 119.03, 120.98, 122.79,
123.16, 123.29, 125.70, 126.98, 127.15, 127.51, 127.74,
128.56, 133.56, 135.84 (some resonances obscured by sol-
vent), P-C6H5 and N-C6H5; 127.62, 131.96, o-P-C6H5;
151.24, 158.19, ipso-P-C6H5. 31P NMR (161.9 MHz, C7D8,
300 K) δ: –2.30 (d, JPP = 3.1 Hz), 28.30 (d, JPP = 3.1 Hz).
Anal. calcd. for C60H85BN6P2Si4Ta2: C 50.14, H 5.96, N
5.85; found: C 50.28, H 6.13, N 5.46.
[NP␮-N]Ta(=NPh)(␮-NB(H)2C6H13)Ta[NPN] (11)
A 15 mL toluene solution of 0.328 g (0.241 mmol) 11
was allowed to stand in a glove box at ambient temperature
for 8 days. The 31P NMR spectrum of a portion of this solu-
tion showed no remaining 10, and solvent was evaporated.
Solid 11 (134 mg, 41% yield) was recovered on a frit after
trituration under hexanes. 1H NMR (400 MHz, C4D8O,
300 K) δ: –0.48, –0.43, –0.08, –0.02, 0.08, 0.11, 0.27, 0.34
(s, 3 H each, 24H total), SiCH3; 0.55, 0.78, 1.20, 1.30, 1.57,
1.81, 2.22, 2.36 (d, 1H each), PCH2; 0.73, 0.81, 1.40, 1.57
(s, 3H each), B-C(CH3)2CH(CH3)2; 3.58, 4.32 (b, 1H each),
B-H; 6.76, 6.80, 6.96, 7.02, 7.05, 7.06, 7.09, 7.12, 7.14,
7.16, 7.19, 7.30, 7.33 (d, t, 1 and 2H each, 26H total) P-
C6H5 and N-C6H5; 7.61, 8.02 (d, 2H each) P-C6H5. 31P
NMR (161.9 MHz, C4D8O, 300 K) δ: –6.68 (b), 18.55 (s).
Anal. calcd. for C54H77BN6P2Si4Ta2: C 47.79, H 5.72, N
6.19; found: C 47.42, H 6.10, N 6.38.
([NPN]Ta)2(␮-␩1:␩2-NNB(H)(C6F5)2)(␮-H)2 (12)
To a solution of 2 (0.7589 g, 0.6017 mmol) in 10 mL of
C6H6 was added solid HB(C6F5)2 (0.2081 g, 0.6017 mmol,
1.0 equiv.). The solution was allowed to stir for 3 h, and the
product was then allowed to crystallize by slow evaporation.
The solid was collected and dried, to yield
([(NPN]2TaH)2N2(HB(C6F5)2) as a brown solid (0.943 g,
Decomposition of 8 to [NP␮-N]Ta(=NBCy2)(␮-
N)Ta[NPN] (9)
A d8-THF solution of 8 suitable for NMR spectroscopy in
a Wilmad NMR tube capped with a plastic stopper and
sealed with ParaFilm laboratory film and bearing a sealed
capillary containing internal standard was left in a glove box
for 3 weeks. Spectra were acquired intermittently. After this
1
97%). H NMR (500 MHz, C7D8, 350 K) δ: –0.28, –0.07,
–0.03, 0.03 (s, 24H total, SiCH3), 0.61, 1.45, 1.68, 2.03
(AMX, 8H total, CH2 ring), 4.7 (br, W1/2 = 250 Hz, 1H,
BH), 6.77, 6.87, 6.94, 6.98, 7.06, and 7.20 (overlapping m,
26 H, NPh and PPh), 7.02 and 7.57 (m, 4H total, PPh o-H),
1
time, the 31P and H NMR spectra were exclusively that of
8, and the integration with respect to internal standard al-
lowed evaluation of 83% yield. The total integration of other
1
2
2
1
31P NMR active resonances was 6%. H NMR (400.1 MHz,
11.56 (dd, JHP = 20.9 Hz, JHP = 17.3 Hz, 2H, TaH). H
NMR (500 MHz, C7D8, 245 K) δ: –0.26, –0.26, –0.24,
–0.17, 0.00, 0.09, 0.11, 0.41 (s, 24H total, SiCH3), 0.63,
1.05, 1.20, 1.28, 1.36, 1.39, 1.67, 2.73 (AMX, 8H total, CH2
ring), 4.7 (br, 1H, BH), 5.80, 6.16, 6.51, 6.75, 6.85 (m, 1H
each, NPh or PPh) 6.91–7.37 (overlapping m, 18 H, NPh
and PPh), 7.49 (m, 2H, PPh-o-H), 7.53 (m, 2H, PPh or
C7D8, 300 K) δ: –1.10, –0.88, –0.34, –0.13, –0.02, 0.18,
0.20, 0.33 (s, 3H each), SiCH3; 0.49, 0.75, 0.89, 1.17, 1.21,
1.31, 1.55 (d, 1H each), P-CH2; 0.6–1.6 (complicated over-
lapping multiplets, 22H total), B-(C6H11)2; 3.81 (b, FWHM
28 Hz, 1H) B-H; 6.27, 6.34, 6.51, 6.57, 6.63, 6.67, 6.77,
6.83, 6.86, 6.95, 6.99, 7.03, 7.11, 7.16, 7.42, 7.57 (d, t, over-
lapping, 26H total) P-C6H5 and N-C6H5; 7.47, 7.83 (d, 2H
each), o-P-C6H5. 31P NMR (161.9 MHz, C7D8, 300 K) δ:
0.76 (d, JPP = 5.94 Hz), 17.41 (d, JPP = 5.94 Hz). Elemental
analysis was not obtained.
2
NPh), 7.88 (m, 2H, PPh-o-H), 11.00 (br m, JHH = 14.2 Hz,
2
2
1H, TaH), 11.67 (ddd, JHH = 14.2 Hz, JHP = 14.7 Hz,
2JHP2 = 31.9 Hz, 1H, TaH). 31P NMR (C7D8, 299 K) δ: 10.0
2
(d, JPP = 25.1, [NPN] ligand), 22.8 (d, JPP = 25.1, [NPN]
© 2005 NRC Canada

MacKay et al.
323
12. M. Akashi, Y. Sato, and M. Mori. J. Org. Chem. 66, 7873
(2001).
13. K. Komori, H. Oshita, Y. Mizobe, and M. Hidai. J. Am. Chem.
ligand). Anal. calcd. for C60H65BF10N6P2Si4Ta2: C 44.84, H
4.08, N 5.23; found: C 44.77, H 4.13, N 5.10.
Soc. 111, 1939 (1989).
([NPN]Ta)2(␮-␩1:␩2-15N15NB(H)(C6F5)2)(␮-H)2 (15N2-12)
The 15N-labeled analogue was prepared in a manner iden-
tical to that used for 12, except for using the 15N-labeled
14. M. Mori, K. Hori, M. Akashi, M. Hori, Y. Sato, and M.
Nishida. Angew. Chem. Int. Ed. 37, 636 (1998).
15. K. Ueda, Y. Sato, and M. Mori. J. Am. Chem. Soc. 122,
10 722 (2000).
16. A.D. Allen and C.V. Senoff. Chem. Commun. 621 (1965).
17. M.P. Shaver and M.D. Fryzuk. Adv. Synth. Catal. 345, 1061
(2003).
precursor 15N2-2. 31P NMR (C7D8, 299 K) δ: 10.0 (dd, JPP
=
=
2
25.1 Hz, JPN = 17.2 Hz, [NPN] ligand), 22.8 (d, JPP
25.1 Hz, [NPN] ligand). 15N NMR (C7D8, 299 K) δ: −48.1
(dd, JNN = 16.2 Hz, JNP = 25.1 Hz), 11.5 (d, JNN
1
2
1
=
16.2 Hz).
18. M. Hidai and Y. Mizobe. Metal Ions Biol. Syst. 39, 121
(2002).
19. S. Gambarotta. In Inorganic chemistry highlights. Edited by
G. Meyer, D. Naumann, and L. Weseman. John Wiley and
Sons, New York. 2002. p. 285.
20. J. Chatt, J.R. Dilworth, and R.L. Richards. Chem. Rev. 78, 589
(1978).
21. M.D. Fryzuk, S.A. Johnson, B.O. Patrick, A. Albinati, S.A.
Mason, and T.F. Koetzle. J. Am. Chem. Soc. 123, 3960 (2001).
22. M.D. Fryzuk, B.A. MacKay, S.A. Johnson, and B.O. Patrick.
Angew. Chem. Int. Ed. 41, 3709 (2002).
Acknowledgements
The authors thank the Natural Sciences and Engineering
Research Council of Canada (NSERC) for funding (PGS-A,
PGS-B, and PDF awards to B.A.M. and S.A.J., Discovery
Grant to M.D.F.).
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© 2005 NRC Canada