Borane-catalysed dinitrogen borylation by 1,3-B-H bond addition

Article abstract of DOI:10.1039/d1dt00317h

The borylation of ligated dinitrogen by 1,3-B-H bond addition over a W-N-N unit using various hydroboranes has been examined. In a previous study, we have shown that Piers’ borane (1) reacted with the tungsten dinitrogen complex2to afford a boryldiazenido-hydrido-tungsten species. The ease and mildness of this reaction have encouraged us to extend its scope, with the working hypothesis that1could potentially catalyse the 1,3-B-H bond addition of other hydroboranes. Under productive reaction conditions, dicyclohexylborane (HBCy₂) and diisopinocampheylborane (HBIpc₂) underwent retro-hydroboration to give cyclohexylborane (H₂BCy) or isopinocampheylborane (H₂BIpc), respectively; these monoalkylboranes act as N₂-borylating agents in the presence of a catalytic amount of1. Under similar conditions, 9-borabicyclononane (9-BBN) slowly adds over the W-N-N unit without rearrangement to a monoalkylborane. Catecholborane (HBcat) undergoes the 1,3-B-H bond addition without the need for a catalyst. We were not able to build more than one covalent B-N bond between the terminal N of the N₂ligand and the boron reagent with this methodology.

Full text of DOI:10.1039/d1dt00317h

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Borane-catalysed dinitrogen borylation by
1,3-B–H bond addition†
Cite this: Dalton Trans., 2021, 50,
5582
Anaïs Coffinet, Dan Zhang, Laure Vendier, Sébastien Bontemps
and
Antoine Simonneau
*
The borylation of ligated dinitrogen by 1,3-B–H bond addition over a W–NuN unit using various hydro-
boranes has been examined. In a previous study, we have shown that Piers’ borane (1) reacted with the
tungsten dinitrogen complex 2 to afford a boryldiazenido–hydrido–tungsten species. The ease and mild-
ness of this reaction have encouraged us to extend its scope, with the working hypothesis that 1 could
potentially catalyse the 1,3-B–H bond addition of other hydroboranes. Under productive reaction con-
ditions, dicyclohexylborane (HBCy2) and diisopinocampheylborane (HBIpc2) underwent retro-hydrobora-
tion to give cyclohexylborane (H2BCy) or isopinocampheylborane (H2BIpc), respectively; these monoalk-
ylboranes act as N2-borylating agents in the presence of a catalytic amount of 1. Under similar conditions,
9-borabicyclononane (9-BBN) slowly adds over the W–NuN unit without rearrangement to a monoalkyl-
borane. Catecholborane (HBcat) undergoes the 1,3-B–H bond addition without the need for a catalyst.
We were not able to build more than one covalent B–N bond between the terminal N of the N2 ligand
and the boron reagent with this methodology.
Received 29th January 2021,
Accepted 16th March 2021
DOI: 10.1039/d1dt00317h
rsc.li/dalton
W–NuN unit (2→3, Scheme 1),5b a complementary N2 boryla-
tion method to 1,3-B–X bond addition.7 The proposed mecha-
Introduction
The chemistry of dinitrogen complexes has attracted important nism to account for this transformation, based on experi-
research efforts over almost 60 years.1 Indeed, they can serve mental observations, involved two molecules of 1, one being
as platforms to transform the inert but abundant N2 mole- the N2 borylating agent and the other assuming the role of B–
cules, and since the early 2000s several catalysts for the H bond cleavage and hydride shuttling to the metal centre,
reduction of dinitrogen into ammonia2 at ambient tempera-
ture and pressure have been devised. This contrasts with the
harsh conditions found in the Haber–Bosch plant,3 where N2
is converted into ammonia by hydrogenolysis at the industrial
level. N2 complexes can also serve as models for the active
sites of nitrogenases,4 the enzymes responsible for biological
nitrogen fixation. Beyond ammonia and hydrazine, the quest
for the direct conversion of dinitrogen into more complex
nitrogen-containing compounds has fuelled interest towards
the construction of N–E bonds (E = C, B, Si, Al, Ga) at ligated
dinitrogen.1a In this perspective, our team has reported N2 sily-
lation and borylation methods inspired by the chemistry of
frustrated Lewis pairs.5 We have notably shown that in the
presence of Piers’ borane [HB(C6F5)2, 1],6 the compound trans-
[W(N2)2(depe)2] [2, depe = 1,2-bis(diethylphosphino)ethane]
underwent a formal 1,3-addition of the B–H bond across the
LCC-CNRS, Université de Toulouse, CNRS, UPS, 205 route de Narbonne, BP44099,
F-31077 Toulouse cedex 4, France. E-mail: antoine.simonneau@lcc-toulouse.fr
†Electronic supplementary information (ESI) available: Detailed experimental
procedures and crystallographic data. CCDC 2058894–2058896. For ESI and crys-
tallographic data in CIF or other electronic format see DOI: 10.1039/d1dt00317h
Scheme 1 Previously reported5b 1,3-B–H bond addition of Piers’
borane (1) to dinitrogen complex 2 and the proposition of a borane-cat-
alysed mechanism (L = 21 depe).
5582 | Dalton Trans., 2021, 50, 5582–5589
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thus closing a catalytic cycle (Scheme 1). As a continuation of increased the amount of HBCy2. 31P NMR analysis of the reac-
this work, we wanted to check whether this 1,3-B–H addition tion mixture run with four equivalents thereof for 2 hours at
can be extended to other hydroboranes and whether Piers’ 60 °C revealed the conversion to compound 7 (Scheme 2,
1
borane would be a competent catalyst for this reaction.8
bottom). The H NMR spectrum of the crude reaction mixture
showed a hydride quintet coupling to four magnetically equi-
2
valent phosphorus nuclei (δ −3.38, quintet, JHP = 30 Hz).
Diagnostic signals for a four-coordinated boron species carry-
Results
1
ing 2 hydrogens were observed in the H and 11B NMR spectra
1
(1H: δ 3.29, broad singlet; H{11B}: δ 3.29, singlet; 11B: δ −30.0,
Four different organo-hydroboranes, namely dicyclohexyl-
borane (HBCy2), diisopinocampheylborane (HBIpc2), 9-borabi-
cyclononane (9-BBN) and catecholborane (HBcat), were exam-
ined. Three types of reactivities have been observed, as
detailed below.
triplet, JBH = 91 Hz). Layering a concentrated toluene solution
with pentane afforded a crystalline material (76% yield) suit-
able for an X-ray diffraction study. This allowed us to confirm
that the targeted formal 1,3-B–H bond addition had been
achieved, with H2BCy being the actual borylating agent
(Fig. 1), thus explaining the need for an excess of HBCy2 to
achieve the full conversion of 2.
Reactions of 2 with dicyclohexylborane (HBCy2) and diisopino-
campheylborane (HBIpc2)
In the molecular structure, a second tetracoordinated boron
moiety is found. Its presence is explained by the capture of
another equivalent of H2BCy, forming a Lewis pair with the
distal Lewis basic nitrogen of the boryldiazenido moiety [N2–
B2 1.618(6) Å]. The latter could not be observed in the 11B
NMR spectrum and is characterised by a B–N linkage having a
double bond character [N2–B1 1.405(6) Å].10 The distal N atom
adopts a trigonal planar geometry (∑angles = 360°), yet the wide
B–N–B angle (126°), presumably originating from steric repul-
sions between the boron substituents, deviates it from ideality.
The N–N bond length [1.346(3) Å] lies halfway between those
of double (ca. 1.25 Å) and single (ca. 1.45 Å) N–N bonds. This
noticeable elongation (dN–N 1.1233(17) Å in 2) results from the
formal two-electron reduction of the N2 ligand upon functiona-
lisation, as well as the effect of H2BCy complexation pulling
the electron density from the metal into the antibonding orbi-
tals of the boryldiazenido ligand.11 The W–N bond is 1.789(2)
Å long, indicative of a double bond character,12 and the W
centre lies in a distorted octahedron (all P–W–N angles >90°
and up to 106°). Likewise, the W–N–N unit deviates from line-
arity [W1–N1–N2 170°]. These deviations from ideal geome-
tries are, again, probably of steric origin. Overall, these struc-
tural features compare well with those of the previously
reported compound 6 (Scheme 1).5b
We reacted trans-[W(N2)2(depe)2] 2 with one equivalent of
dicyclohexylborane (HBCy2) at room temperature in C6D6.
NMR spectroscopy analysis revealed the absence of new tung-
sten species, and raising the temperature to 80 °C overnight
did not change this outcome. This contrasts with the reaction
of 2 with the electrophilic hydroborane 1, occurring under
milder conditions and during which the Lewis acid–base
adduct thereof (4, Scheme 1) could be detected.5b 11B NMR
analysis revealed the presence of tricyclohexyldiborane and
BCy3 (see Fig. S1†). This substituent scrambling presumably
arose from a retro-hydroboration, a reaction known for thexyl-
and isopinocampheyl-substituted boranes that readily occurs
in the absence of 2 (Scheme 2, top, and Fig. S1†).9 To verify
whether HB(C6F5)2 (1) could catalyse 1,3-B–H bond addition of
HBCy2 (or in situ-formed H2BCy), we repeated the experiment
in the presence of 10 mol% of 1 at 60 °C in C6D6. After a day,
NMR analysis revealed again the presence of BCy3 as well as
tricyclohexyldiborane. We also noticed the formation of two
new metal complexes according to 31P{1H} NMR with singlets
at δ 45.2 and 41.7 ppm and 2 remaining the major product of
the mixture. Hoping to achieve higher conversion, we
In comparison with HBCy2, diisopinocampheylborane
(HBIpc2) did not react with 2 even under forcing conditions. At
room temperature and in C6D6, HBIpc2 also underwent a sub-
stituent rearrangement (Scheme 2, top). Indeed, its dimer
evolves into triisopinocampheyldiborane and α-pinene (see
Fig. S6†).9a Having established from the above-discussed reac-
tion involving HBCy2 that dihydroboranes seem to add more
easily to the W–N2 unit than monohydroboranes, we adjusted
the number of equivalents of HBIpc2 in order to generate the
right amount of H2BIpc in the reaction mixture, and added a
catalytic amount (10 mol%) of 1. With two equivalents of
HBIpc2, full and selective conversion of 2 to compound 8 was
achieved after 15 min at 60 °C (Scheme 2, bottom). Although
the newly formed compound 8 could not be crystallised, its
NMR characterisation is very similar to that of compound 7:
the 1H NMR spectrum showed a hydride signal coupling to the
Scheme 2 Generation of monoalkylboranes from HBR2 (top) and reac-
tions of trans-[W(N2)2(depe)2] (2) in the presence of dicyclohexyl- or dii-
sopinocampheylborane catalysed by 1 (bottom).
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Fig. 1 Solid-state molecular structures of compounds 7, 10 and 11, with ellipsoids drawn at the 50% level of probability. All hydrogens have been
omitted for clarity except those bound to B or W. Selected distances (Å) and angles (°) for 7: W1–N1, 1.789(2); N1–N2, 1.346(3); N2–B1, 1.405(6);
N2–B2, 1.618(6); W1–H56, 1.77(6); W1–N1–N2, 170.3(3); B1–N2–B2, 126.4(3). 10: W1–N1, 1.783(6); N1–N2, 1.383(9); N2–B1, 1.56(1); N2–B2, 1.41(1);
W1–H17, 1.82; W1–N1–N2, 175.2(5); B1–N2–B2, 129.3(7). 11: W1–N1, 1.807(3); N1–N2, 1.295(4); N2–B1, 1.545(6); N2–B1i, 1.536(6); W1–H5, 1.87(5);
W1–N1–N2, 178.6(3); B1–N2–B1, 94.1(3); N2–B1–N2, 85.9(3).
four phosphorus (δ −3.33, quintet, 2JHP = 30.6 Hz), and the 11
B
NMR spectrum featured a triplet at −30.0 ppm, leaving little
doubt on the identity of 8.
Reactions with 9-borabicyclo[3.3.1]nonane (9-BBN)
The reactivity of trans-[W(N2)2(depe)2] 2 with one equivalent of
9-borabicyclo[3.3.1]nonane (9-BBN) at room temperature in
C6D6 was also examined. Likewise, no reaction occurred even
under forcing conditions (2 d, 80 °C). Hence, we added
10 mol% of hydroborane 1 to an equimolar solution of 2 and
9-BBN. 31P{1H} NMR monitoring of the reaction after a few
days at 60 °C revealed that two new products 9 and 10 started
to form (δ 45.0 and 40.7 ppm, respectively), both showing a
corresponding hydride quintet in 1H NMR (δ −3.33 and
−4.10 ppm, respectively). Upon prolonged heating, compound
9 formed predominantly, reaching ca. 70% of the mixture
according to 31P NMR integration after 14 days at 70 °C, and
on the basis of NMR spectroscopy, it was assigned to be the
product of 1,3-B–H bond addition (Scheme 3). The other tung-
sten-containing components of the reaction mixture were 2
Scheme 3 Reactions of trans-[W(N2)2(depe)2] (2) with 9-BBN and cata-
lytic or stoichiometric amounts of HB(C6F5)2 (1).
and unidentified minor impurities (see Fig. S12†), while latter assumption, as well as the identity of 9, for which we
several unidentified boron species seemed to be present could not ascertain the structure with X-ray diffraction.
(Fig. S13†). The slow rate of this B–H bond addition incited us
An X-ray diffraction study on single crystals allowed us to
to perform the stoichiometric reaction between complex 2, 1 confirm the proposed structure of 10 (Fig. 1), with the covalent
and 9-BBN. The analysis of the reaction mixture by 31P NMR vs. dative bonding between the boron atoms and the terminal
showed full and selective conversion to compound 10 N being confirmed by comparison of B–N bond lengths [N2–
(Scheme 3) after 4 h at 60 °C. Traces of 10 were also observed B1, 1.405(6) Å vs. N2–B2, 1.617(6) Å]. The structure of 10
in the 31P and 19F NMR spectra acquired at the end of the cata- resembles that of 7, with a tungsten centre in a distorted octa-
lytic reaction (see Fig. S14†). The analysis of the 19F and 11B hedron and the W–N–N unit deviating from linearity (W1–N1–
NMR spectra of 10 suggested, in accordance with previous N2, 175°). While W–N bond lengths are similar in 7 and 10,
results,5b that it is formed by the Lewis acid–base interaction the N–N bond in 10 is slightly longer (+0.03 Å), probably as a
of 1 with the boryldiazenido moiety of 9. The reaction of 9 result of the higher electrophilicity of Piers’ borane than that
with 1 to give 10 brought convincing evidence supporting the of H2BCy.
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Reaction with catecholborane (HBcat)
reason is most probably of kinetic origin, since monoalkylbor-
anes are less sterically hindered than their disubstituted
parents, making the interaction with the N2 complex as well as
the B–H bond activation by 1 easier. 9-BBN, which does not
undergo retro-hydroboration, reacts much slower and also
needs catalytic amounts of 1. Interestingly, when the reaction
of 9-BBN with the dinitrogen complex is conducted in the pres-
ence of an equimolar amount of 1, the B–H bond addition pro-
ceeds very rapidly and selectively. Indeed, the 1,3-B–H bond
addition product of Piers’ borane5b (3, Scheme 1) is not
observed. The following experiment allowed us to shed light
on this fact: when product 3 was treated with an equimolar
amount of 9-BBN at RT, 10 was obtained quantitatively in a
few hours (Scheme 5). This result suggests that in the catalytic
1,3-B–H bond addition mechanism with H2BCy, H2BIpc or
9-BBN, the addition of Piers’ borane to give 3 may precede the
interaction of the alkylhydroborane with the M–NuN unit.
After complexation of the latter with the thus-formed C6F5-sub-
stituted boryldiazenido moiety in 3, boron-to-boron hydride
transfer then affords a Piers’ borane-complexed 1,3-B–H bond
addition compound such as 10. The dissociation of 1 followed
by reaction with dinitrogen complex 2 would then close the
catalytic sequence. However, further experimental work cast
doubts on this conjecture, at least in the case of 9-BBN.
Attempts to dissociate 1 in compound 10 in the presence of
various Lewis bases such as PMe3, pyridine, tetramethyl-
ethylenediamine (TMEDA) or 2 at elevated temperatures for
several hours proved unsuccessful (Scheme 5, top).14 We have
checked the integrity of Piers’ borane (1) in the presence of a
10-fold excess of 9-BBN: at RT, a new species forms in C6D6
within a few hours, which we believe is the mixed diborane 12
(Scheme 5, bottom). Upon heating to a temperature relevant to
the above-described experiments, the putative dimer seems to
get involved in an equilibrated substituent rearrangement
process with two other compounds, one being 9-C6F5-BBN15
(13), while the second one is presumably a mixed diborane
composed of 9-BBN and in situ-formed H2BC6F5 (see
Fig. S22–23† and Scheme 5). The 19F NMR spectrum recorded
at the end of the reaction affording 9 does not feature the spec-
tral signature of 1, but unambiguously shows the presence of
10 along with another unidentified compound (see Fig. S13
and S14†). These data suggest that 1 is a precatalyst in the
reaction involving 9-BBN, but the presence of 10 at the end of
the reaction also implies that this species might be a catalyst
resting state.
Next, we checked the reactivity of trans-[W(N2)2(depe)2] 2 with
one equivalent of catecholborane (HBcat) at room temperature
in C6D6 (Scheme 4). Quite contrastingly, NMR analysis revealed
that a reaction occurred in a selective manner. Indeed, we
observed a new diamagnetic compound 11 having a singlet
resonance at δ 43.8 ppm in 31P{1H} NMR with a corresponding
hydride quintet at −3.6 ppm in 1H NMR after a few minutes at
room temperature. Almost full conversion was obtained after 3
days at 60 °C (>90% according to 31P NMR). Compared to the
previous examples, Piers’ borane 1 was not necessary to
perform the targeted formal 1,3-B–H bond addition.
Layering the reaction mixture with pentane allowed us to
grow single crystals of 11 (33% yield) suitable for X-ray diffrac-
tion analysis, which revealed a dimeric solid-state structure of
D2h symmetry for 7 (Fig. 1): two boryl groups are bridging two
W–N2 units in an almost square BNBN arrangement (N–B
bond lengths ca. 1.54 Å, B–N–B angles 94°, and N–B–N angles
86°). Compared to the structures of 3 and 6, the octahedron
formed by the W ligands is less distorted, and the W–N–N
arrangement is linear (W1–N1–N2, 179°). The N–N bond is
also shorter [1.295(4) Å]. Similar dimeric structures have been
obtained from the reactions of group 13 chlorides with end-on
dinitrogen complexes.13 The dimeric nature of 11 in the solid,
which contrasts with that of 7 or 9 (as suggested by diffusion-
ordered NMR spectroscopy, see Fig. S15†), is likely to result
from the diminished steric hindrance of the flat Bcat group.
Discussion
The above-described experiments have confirmed the possi-
bility to use Piers’ borane as a catalyst for the borylation of N2
bound to a group 6 metal centre by 1,3-B–H bond addition
when using dialkylhydroboranes (HBCy2, HBIpc2 and 9-BBN).
Within this family of compounds, two different behaviours
have been observed depending on the propensity of the dia-
lkylhydroborane to undergo retro-hydroboration under the
reaction conditions to give a monoalkylborane. The latter is
the active borylating agent, and in the case of HBCy2 and
HBIpc2, only the products of the 1,3-B–H bond addition of
H2BCy and H2BIpc over the W–N–N unit were observed. The
HBcat stands out against the 3 other boranes examined.
This boron reagent is known to be a much milder hydrobora-
tion reagent than HBCy2 or HBIpc2 due to its quenched Lewis
acidity.16 As such, HBcat is also a weak hydride donor com-
pared to 9-BBN on the basis of the calculated hydride donor
abilities [ΔGH−(298 K) = 159.2 vs. 99.0 kcal mol−1, respectively],
but this trend reverses upon Lewis base [for example,
NHC-HBcat, ΔGH−(298 K) = 44.8 kcal mol−1 and NHC-9-BBN,
ΔGH−(298 K) = 47.0 kcal mol−1 with NHC = 1,3-di(isopropyl)
imidazole-2-ylidene] or hydride coordination {[H2Bcat]−,
ΔGH−(298 K) = 26.3 kcal mol−1 and [9-H2BBN]−, ΔGH−(298 K) =
Scheme 4 Reaction of trans-[W(N2)2(depe)2] (2) with catecholborane
(HBcat).
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Scheme 5 Reaction of 3 with 9-BBN and attempts to dissociate 1 from 10 with various Lewis bases (top) and reaction of 1 with 9-BBN in the
absence of a dinitrogen complex (bottom).
32.8 kcal mol−1}.17 Although the flatness of HBcat and its ylating agents. None of the boranes examined have allowed
monomeric nature in non-coordinating solvents also offer a building more than one covalent N–B bond. According to the
kinetic advantage, we believe the peculiarity of HBcat reactivity experimental observations, we believe that the mechanism of
lies in its electronic properties. We propose a similar mecha- these catalytic reactions starts with the 1,3-B–H bond addition
nism for the 1,3-B–H bond addition of HBcat to that with of 1 to give 3, followed by complexation of the alkylhydrobor-
Piers’ borane (Scheme 1): Lewis acid–base adduct formation of ane to the terminal N of the resulting W-ligated boryldiazenido
HBcat with 2 increases the polarisation of the B–H bond to an ligand and a boron-to-boron hydride transfer ensues. While it
extent that a second equivalent of HBcat is now able to abstract is reasonable to think that the dissociation of 1 should close
the hydride, generating the [H2Bcat]− anion. The latter is a the catalytic cycle, such an elementary step could not be vali-
competent hydride donor (vide supra), especially when com- dated in the case of 9-BBN because 1 is likely to be a pre-
pared to [H2B(C6F5)2]− [ΔGH−(298 K) = 61.5 kcal mol−1], which catalyst in this specific reaction. This urges us to explore in
also bears a steric penalty due to its C6F5 substituents. This detail the reactivity of H2BC6F5 but also L·BH3 (L = THF, SMe2,
may explain why in the reactions with HBcat we were not able etc.) or diborane (B2H6) with zero-valent group 6 dinitrogen
to detect a species resembling 5 (Scheme 1), the W–H bond complexes. We will share the results of this future work in due
formation event being probably too fast under the experi- time.
mental conditions. In the case of Piers’ borane, we surmise
that an important factor for W–H bond formation (5→6,
Scheme 1) is the nature of the trans ligand, NNB(C6F5)2, whose
Experimental section
General considerations
electron-withdrawing properties might counter-balance the
weak hydride donor ability of [H2B(C6F5)2]−.18
All reactions were performed in flame- or oven-dried glassware
with rigorous exclusion of air and moisture, using a nitrogen
filled Jacomex glovebox (O2 < 1 ppm, H2O < 1 ppm). Solvents
Conclusions
used were pre-dried (toluene and n-pentane by passing
In the present article, we expand the scope of 1,3-B–H bond through a PureSolv MD 7 solvent purification machine;
addition over a W–N2 unit. This mild method for N2 boryla- n-hexane and hexamethyldisiloxane (HMDSO) by distillation
tion, which complements 1,3-B–X bond addition,7 can be over CaH2), degassed by freeze–pump–thaw cycles, dried over
attractive for who seeks to synthesise nitrogen–boron com- molecular sieves and stored in a glovebox. C6D6 (purchased
pounds directly from dinitrogen. Having this goal in mind, we from Eurisotop) was degassed by freeze–pump–thaw cycles,
have examined the reactivity of 4 different boranes. Only HBcat dried over molecular sieves and stored in a glovebox. HB
19
20
reacts spontaneously with the dinitrogen complex 2, affording (C6F5)2 (1),6a HBCy2 and HBIPc2 were synthesized accord-
the 1,3-B–H bond addition product 11. Hydroboranes HBCy2, ing to reported procedures and stored in a glovebox. 9-BBN
HBIpc2 and 9-BBN required catalytic amounts of Piers’ borane and HBcat were purchased from Sigma-Aldrich and used as
1
(1) to react with 2. In the case of HBCy2 and HBIpc2, monoalk- received in a glovebox. H, 11B, 19F and 31P NMR spectra were
ylboranes H2BCy and H2BIpc formed in situ are the actual bor- recorded in C6D6 or toluene-d8 using NMR tubes equipped
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with J. Young valves on a Bruker Avance III 400 spectrometer.
8: 1H NMR (400 MHz, C6D6) δ: 3.38 (br s, 3H), 2.59–2.50 (m,
Chemical shifts are in parts per million (ppm) downfield from 2H), 2.43–2.34 (m, 4H), 2.27–2.13 (m, 12H), 2.06–1.97 (m, 3H),
tetramethylsilane and are referenced to the residual solvent 1.82 (m, 2H), 1.74–1.65 (m, 3H), 1.48 (d, J = 7.2 Hz, 8H), 1.45
resonance as the internal standard (C6HD5: δ reported = (s, 6H), 1.37 (s, 6H), 1.20–1.10 (m, 16H), 0.80–0.68 (m, 20H),
7.16 ppm; C7HD7: δ reported = 2.08 ppm for 1H NMR). 11B, 19F, −3.33 (quint, JHW = 13.4 Hz, JHP = 30.7 Hz, 1H). 31P{1H} NMR
29Si and 31P NMR spectra were calibrated according to the (162 MHz, C6D6) δ: 41.2 (mc, JWP = 282 Hz, second order
IUPAC recommendation using a unified chemical shift scale pattern). 31P NMR (162 MHz, C6D6) δ: 41.2 (mc, second order
based on the proton resonance of tetramethylsilane as the pattern). 11B NMR (128 MHz, C6D6) δ: −3.32 (br s), −30.0 (t,
primary reference.21 Data are reported as follows: chemical JBH = 88.9 Hz).
shift, multiplicity (br = broad, s = singlet, d = doublet, t =
IR (ATR) ν/cm−1 = 2962, 2936, 2878, 2357, 2081, 1634, 1501,
triplet, q = quartet, quint = quintet, m = multiplet, mc = centro- 1453, 1416, 1293, 1104, 1074, 1027, 961, 869, 806, 751, 735,
symmetric multiplet), coupling constant (Hz), and integration. 683.
Infrared (IR) spectra were recorded in a glovebox on an Agilent
Elem. anal. calcd for C40H86B2N2P4W·0.55C6D6: C, 53.58; H,
Cary 630 FT-IR spectrophotometer equipped with ATR or trans- 8.93; N, 2.89. Found: C, 54.09; H, 9.55; N, 2.56.
mission modules and are reported in wavenumbers (cm−1).
Reaction of 2 with 9-BBN catalysed by 1
Elemental analyses were performed on samples sealed in tin
capsules under N2 by the analytical service of the Laboratoire In a glovebox, trans-[W(N2)2(depe)2] (2, 26 mg, 40 µmol, 1
de Chimie de Coordination; results are the average of two inde- equiv.), 9-BBN (4.9 mg, 40 µmol, 1 equiv.) and HB(C6F5)2 (1,
pendent measurements.
1.4 mg, 4.0 µmol, 0.1 equiv.) were weighed in a 4 mL vial. After
addition of C6D6 (0.5 mL), the dark orange solution was
heated at 70 °C for 14 days, the time after which compound 9
became the main component of the reaction mixture (ca. 70%
according to 31P NMR) that still contained 2 and other un-
identified boron species. Attempts to isolate 9 in an analyti-
cally pure form have failed.
Reaction of 2 with HBCy2 catalysed by 1
In a glovebox, trans-[W(N2)2(depe)2] (2, 52 mg, 80 µmol, 1
equiv.), HBCy2 (57 mg, 32 µmol, 4 equiv.) and HB(C6F5)2 (1,
3 mg, 8 µmol, 0.1 equiv.) were weighed in a 4 mL vial. After
addition of C6D6 (0.6 mL), the orange-red solution was heated
for 2 hours at 60 °C. The resulting orange solution was concen-
trated to ca. 0.3 mL under reduced pressure and layered with
pentane before storage at −40 °C. After a week, orange crystals
of 7 were recovered by decantation and dried under vacuum
(50 mg, 61 µmol, 76% yield). Single crystals suitable for an
X-ray diffraction study were obtained from the same crop.
7: 1H NMR (400 MHz, C6D6) δ: 3.29 (br s, 2H), 2.21–2.02
(mc, 8H), 2.06 (t, J = 11.6 Hz, 8H), 1.94 (s, 2H), 1.82–1.63 (m,
5H), 1.63–1.38 (m, 14H), 1.39–1.18 (m, 10H), 1.14 (quint, J =
7.6 Hz, 12H), 0.73 (p, J = 7.4 Hz, 12H), −3.38 (quint, JHW = 15.8
Hz, JHP = 30.4 Hz, 1H). 31P{1H} NMR (162 MHz, C6D6) δ: 41.7
9: 1H NMR (400 MHz, C6D6) δ: 2.08–1.99 (m, 12H),
1.89–1.85 (m, 3H), 1.80–1.71 (m, 6H), 1.68–1.62 (m, 3H), 1.51
(s, 2H), 1.47–1.34 (m, 12H), 1.17 (dt, J = 14.8, 7.6 Hz, 12H),
0.87 (dt, J = 13.6, 7.5 Hz, 12H), −3.33 (quint, JHW = 26.1 Hz, JHP
= 26.3 Hz, 1H). 31P{1H} NMR (162 MHz, C6D6) δ: 45.03 (JWP
282 Hz). 11B NMR (128 MHz, C6D6) δ: 45.9.
=
Reaction of 2 with 9-BBN and 1
In a glovebox, trans-[W(N2)2(depe)2] (2, 20 mg, 30 µmol, 1
equiv.), 9-BBN (3.8 mg, 30 µmol, 1 equiv.) and HB(C6F5)2 (1,
10 mg, 30 µmol, 1 equiv.) were weighed in a 4 mL vial. After
addition of C6D6 (0.6 mL), the dark orange solution was
heated at 60 °C for 4 hours. The resulting orange solution was
concentrated under vacuum and pentane (2 mL) was added
before storage at −40 °C. After one day, orange crystals of 10
were recovered by decantation and dried under vacuum
(23 mg, 21 µmol, 70% yield). Single crystals suitable for X-ray
diffraction crystallography were obtained from the same crop.
(JPW = 282 Hz). 11B NMR (128 MHz, C6D6) δ: −30.0 (t, JBH
=
91.0 Hz).
IR (ATR) ν/cm−1 = 2958, 2900, 2783, 1603, 1503, 1456, 1362,
1335, 1274, 1188, 1163, 1124, 1095, 1081, 1029, 982, 867, 802,
755, 732, 710, 693, 686, 662.
Elem. anal. calcd for C32H74B2N2P4W: C, 47.08; H, 9.14; N,
3.43. Found: C, 47.10; H, 8.90; N, 3.26.
1
10: H NMR (400 MHz, C6D6) δ: 3.98 (d, J = 107.1 Hz, 1H),
2.15–1.98 (m, 4H), 1.97–1.85 (m, 4H), 1.85–1.77 (m, 2H), 1.72
(q, J = 4.7, 4.2 Hz, 4H), 1.65–1.44 (m, 10H), 1.39–1.26 (m, 6H),
Reaction of 2 with HBIpc2 catalysed by 1
In a glovebox, trans-[W(N2)2(depe)2] (2, 13 mg, 20 µmol, 1 1.22–1.07 (m, 8H), 1.00 (quint, J = 7.4 Hz, 12H), 0.62 (quint,
equiv.), HBIpc2 (12 mg, 40 µmol, 2 equiv.) and HB(C6F5)2 (1, JHW = 9.8 Hz, JHP = 7.4 Hz, 12H), −4.10 (quint, JHP = 34.1 Hz,
0.7 mg, 2.0 µmol, 0.1 equiv.) were weighed in a 4 mL vial. After 1H). 31P{1H} NMR (162 MHz, C6D6) δ: 40.7 (JPW = 287 Hz). 11B
the addition of C6D6 (0.6 mL), the orange-red solution was NMR (128 MHz, C6D6) δ: −10.9 (s); the boron shift of the tri-
heated at 60 °C for 15 minutes, the time after which the full coordinated boryl group could not be detected. 19F NMR
conversion of 2 into 8 was ascertained by NMR analysis. After (377 MHz, C6D6) δ: −132.7 (dd, JFF = 25.5, 9.3 Hz, 4Fortho),
evaporation to dryness in a glovebox, the oily residue was tritu- −162.5 (t, JFF = 20.2 Hz, 2Fpara), −166.0 to −166.5 (m, 4Fmeta).
rated in pentane. The supernatant was discarded, and the oily
IR (ATR) ν/cm−1 = 2965, 2937, 2917, 2881, 2354, 1639, 1507,
residue was dried under vacuum. Compound 8 could not be 1452, 1375, 1332, 1264, 1228, 1108, 1091, 1075, 1039, 1028,
isolated otherwise than as a C6D6-containing clathrate.
964, 936, 887, 869, 844, 803, 736, 712, 680, 662.
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Elem. anal. calcd for C40H64B2F10N2P4W: C, 43.98; H, 5.91;
Dalton Transactions
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N, 2.65. Found: C, 44.00; H, 6.08; N, 2.48.
Reaction of 2 with HBcat
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In a glovebox, trans-[W(N2)2(depe)2] (2, 52 mg, 80 µmol, 1
equiv.) and HBcat (8.6 µL, 80 µmol, 1.0 equiv.) were weighed
in a 4 mL vial. After addition of C6D6 (0.6 mL), the orange solu-
tion was heated at 60 °C for 3 days. The resulting solution was
then concentrated under vacuum before adding hexane
(2 mL). The resulting turbid solution was centrifuged, giving a
yellow liquid that was stored at −40 °C. After a week, a mixture
of crystals and powder of 11 were recovered by filtration and
dried under vacuum (26.6 mg, 18 µmol, 45% yield). Single
crystals suitable for an X-ray diffraction study were obtained by
layering pentane onto a saturated C6D6 solution of 11 before
storage at −40 °C (33% yield).
11: 1H NMR (400 MHz, C6D6) δ: 6.90–6.86 (m, 4H),
6.83–6.79 (m, 4H), 2.10–1.87 (m, 16H), 1.70 (ddd, J = 11.1, 9.2,
5.5 Hz, 8H), 1.40–1.23 (m, 24H), 1.15 (quint, J = 7.4 Hz, 24H),
0.82–0.70 (m, 24H), −3.60 (quint, JHW = 18.1 Hz, JHP = 29.2 Hz,
2H). 31P{1H} NMR (162 MHz, C6D6) δ: 43.8 (JPW = 286 Hz). 11B
NMR (128 MHz, C6D6) δ: 5.4 (s).
IR (ATR) ν/cm−1 = 2958, 2900, 2839, 2272, 1604, 1503, 1457,
1378, 1362, 1335, 1274, 1188, 1163, 1124, 1095, 1081, 1029,
982, 867, 802, 755, 732, 710, 693, 686, 661.
Elem. anal. calcd for C52H106B2N4O4P8W2: C, 41.96; H, 7.18;
N, 3.76. Found: C, 42.20; H, 7.40; N, 3.70.
Conflicts of interest
The authors declare no conflict of interest.
Acknowledgements
A. C. is grateful to the French Ministry of National and
Superior Education and Research (MENESR) for a PhD fellow-
ship. CSC is acknowledged for the PhD fellowship of
D. Z. A. S. acknowledges the European Research Council (ERC)
for funding (Grant Agreement 757501). C. Bijani is acknowl-
edged for assistance with additional NMR experiments during
the revision process.
Notes and references
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