Insertion reactions of small unsaturated molecules in the N-B bonds of boron guanidinates

Article abstract of DOI:10.1039/c7dt02081c

We report here 1,1- and 1,2-insertion reactions of small unsaturated molecules in the N-B bonds of two boron guanidinates, (Me₂N)C(NⁱPr)₂BCy₂ (1) and {ⁱPr(H)N}C(NⁱPr){N(p-^tBu-C₆H₄)}BCy₂ (2), and two bisboron guanidinates(2-), {ⁱPr(BCy₂)N}C(NⁱPr){N(p-^tBu-C₆H₄)}BCy₂ (3) and {ⁱPr(C₈H₁₄B)N}C(NⁱPr){N(p-Me-C₆H₄)}BC₈H₁₄ (4), the latter being prepared for the first time by double deprotonation of the corresponding guanidine with the 9-borabicyclo[3.3.1]nonane dimer, (H-BC₈H₁₄)₂. Compounds 1-4 easily insert aromatic isonitriles, XylNC (Xyl = 2,6-Me₂-C₆H₃) and (p-MeO-C₆H₄)NC, to give the expected diazaboroles 5-12, some of them being structurally characterised by X-ray diffraction. Interestingly, the BC₈H₁₄ derivatives 11 and 12 are in a fast temperature-dependent equilibrium with the de-insertion products, whose thermodynamic parameters are reported here. A correlation between these equilibria and the puckered heterocyclic structure found in the solid state for 11, and confirmed by DFT calculations, is also established. Reactions of the aforementioned guanidinates with CO are more sluggish or even precluded, and only one product, {ⁱPr(H)N}C{N(p-^tBu-C₆H₄)}(NⁱPr)(CO)BCy₂ (13), could be isolated in moderate yields. The 1,2-insertions of benzaldehyde in compounds 1, 2 and 4 are reversible reactions in all cases, and only one of the insertion products, {ⁱPr(H)N}C{N(p-^tBu-C₆H₄)}(NⁱPr)(PhHCO)BCy₂ (16a), was isolated and diffractrometrically characterised. Likewise, CO₂ reversibly inserts into a N-B bond of 2 to give {ⁱPr(H)N}C{N(p-^tBu-C₆H₄)}(NⁱPr)(CO₂)BCy₂ (19) with a conversion of ca. 9%. In all these equilibria, de-insertion is always favoured upon increasing the temperature.

Full text of DOI:10.1039/c7dt02081c

Dalton
Transactions
View Article Online
View Journal
PAPER
Insertion reactions of small unsaturated molecules
in the N–B bonds of boron guanidinates†
Cite this: DOI: 10.1039/c7dt02081c
b
Alberto Ramos, *a Antonio Antiñolo, *b Fernando Carrillo-Hermosilla,
Rafael Fernández-Galán,b María del Pilar Montero-Rama,b Elena Villaseñor,b
Antonio Rodríguez-Diéguez c and Daniel García-Vivó
d
We report here 1,1- and 1,2-insertion reactions of small unsaturated molecules in the N–B bonds of two
boron guanidinates, (Me2N)C(NiPr)2BCy2 (1) and {iPr(H)N}C(NiPr){N(p-tBu-C6H4)}BCy2 (2), and two bis-
boron guanidinates(2–), {iPr(BCy2)N}C(NiPr){N(p-tBu-C6H4)}BCy2 (3) and {iPr(C8H14B)N}C(NiPr){N(p-Me-
C6H4)}BC8H14 (4), the latter being prepared for the first time by double deprotonation of the corres-
ponding guanidine with the 9-borabicyclo[3.3.1]nonane dimer, (H-BC8H14)2. Compounds 1–4 easily
insert aromatic isonitriles, XylNC (Xyl = 2,6-Me2-C6H3) and (p-MeO-C6H4)NC, to give the expected diaza-
boroles 5–12, some of them being structurally characterised by X-ray diffraction. Interestingly, the BC8H14
derivatives 11 and 12 are in a fast temperature-dependent equilibrium with the de-insertion products,
whose thermodynamic parameters are reported here. A correlation between these equilibria and the
puckered heterocyclic structure found in the solid state for 11, and confirmed by DFT calculations, is also
established. Reactions of the aforementioned guanidinates with CO are more sluggish or even precluded,
and only one product, {iPr(H)N}C{N(p-tBu-C6H4)}(NiPr)(CO)BCy2 (13), could be isolated in moderate
yields. The 1,2-insertions of benzaldehyde in compounds 1, 2 and 4 are reversible reactions in all cases,
and only one of the insertion products, {iPr(H)N}C{N(p-tBu-C6H4)}(NiPr)(PhHCO)BCy2 (16a), was isolated
and diffractrometrically characterised. Likewise, CO2 reversibly inserts into a N–B bond of 2 to give {iPr(H)
N}C{N(p-tBu-C6H4)}(NiPr)(CO2)BCy2 (19) with a conversion of ca. 9%. In all these equilibria, de-insertion
is always favoured upon increasing the temperature.
Received 7th June 2017,
Accepted 10th July 2017
DOI: 10.1039/c7dt02081c
rsc.li/dalton
N-donor ligands. Among them, guanidinato anions have
Introduction
played an important role.1 These ligands display a planar ‘CN3’
In the last 20 years, the search for alternatives for cyclopenta- core and offer great electronic flexibility due to variable contri-
dienyl-based ligands in the fields of organometallic and butions of two main resonance forms: 1,3-diazaallyl (I and II
coordination chemistry has led to the development of new in Scheme 1) and iminium-diamide (III in Scheme 1), depend-
ing on the electronic needs of the metal. Additionally, the
steric and electronic properties of guanidinates can be easily
aDepartamento de Química Inorgánica, Orgánica y Bioquímica, Instituto Regional de
Investigación Científica Aplicada, Universidad de Castilla-La Mancha, Campus
Universitario, E-13071 Ciudad Real, Spain. E-mail: Alberto.Ramos@uclm.es
bDepartamento de Química Inorgánica, Orgánica y Bioquímica-Centro de Innovación
en Química Avanzada(ORFEO-CINQA), Universidad de Castilla-La Mancha, Campus
Universitario, E-13071 Ciudad Real, Spain. E-mail: Antonio.Antinolo@uclm.es
cDepartamento de Química Inorgánica, Facultad de Ciencias, Universidad de
Granada, Avenida de la Fuente Nueva S/N, 18071 Granada, Spain
tuned by the sensible choice of substituents. These outstand-
ing features have contributed to the development of a rich
coordination chemistry with most metals of the periodic table,
in which the guanidinato ligand can display different coordi-
nation modes: κ1-monodentate (not very common), κ2-biden-
tate chelate or μ–κ1:κ1-bridge (types A, B and C in Fig. 1).
dDepartamento de Química Orgánica e Inorgánica/IUQOEM, Universidad de Oviedo,
E-33071 Oviedo, Spain
†Electronic supplementary information (ESI) available: A CIF file containing
crystallographic data for compounds 4, 5, 8a, 10, 11 and 16a, plots, tables and
NMR spectra used to determine thermodynamic parameters for isonitrile de-
insertion reactions of compounds 11 and 12, NMR spectra for new compounds,
optimised structures of compounds 9–12 and an XYZ file containing Cartesian
coordinates for all computed species. CCDC 1554010–1554015. For ESI and crys-
tallographic data in CIF or other electronic format see DOI: 10.1039/c7dt02081c
Scheme 1 Resonance structures for guanidinato ligands.
This journal is © The Royal Society of Chemistry 2017
Dalton Trans.

View Article Online
Paper
Dalton Transactions
Fig. 1 Coordination modes for the guanidinato ligands.
Guanidinato compounds have also found applications in
key technological areas such as homogeneous catalysis (olefin
polymerisation, ring-opening polymerisation or amine guany-
lation) or materials science and nanotechnology (precursors
for atomic layer deposition –ALD– or chemical vapour depo-
sition –CVD– processes),1b,d–g,i,k and can be conveniently pre-
pared by three main methods: (i) carbodiimide insertion in
M–N bonds; (ii) guanidine deprotonation with metal alkyls or
hydrides; (iii) salt metathesis between metal halides and alkali
metal guanidinates.
Scheme 2 FLP-type reactivity of boron amidinates HC(NR)2B(C6F5)2.8
In the last few years our group has focused on the synthesis
of novel guanidinato compounds with early,2 late3 and s-block
metals,4 as well as the catalytic guanylation reaction.2c,3a,4,5 As
part of this study, we recently turned our attention to the rela-
tively less explored chemistry of boron guanidinates.6 Thus, in
our previous report we prepared new symmetrical and asym-
metrical dialkylboron guanidinates, as well as the first bis-
boron guanidinate(2–), by salt metathesis from the corres-
ponding lithium guanidinates and chloroboranes, employing
commercially available or readily prepared reagents.
Diffractometric studies revealed a chelate coordination (type B,
Fig. 1) for the guanidinato ligands in these compounds. Their
thermal stability was also tested revealing that, in some
instances, carbodiimide de-insertion reactions took place even
at room temperature in solution. These uncommon reactions
(carbodiimide insertion reactions are usually favourable, being
the most general method to prepare guanidinato compounds)
were the subject of kinetic and thermodynamic studies, and
a reaction mechanism was postulated, involving a switch of
the coordination mode of the guanidinato, from κ2-chelate to
κ1-monodentate, in the first step.
alkyl, aryl; R″ = alkyl), with less electron-withdrawing groups
on boron, were also able to react under mild conditions with
isocyanates, aldehydes, aminoboranes (1,2-insertion reac-
tions)9 or isonitriles (1,1-insertion reactions)10 but failed to
react under harsher conditions with carbodiimides11 or
nitriles.9
With these precedents in mind, here we report the systema-
tic reactivity study towards selected unsaturated molecules
(isonitriles, CO, benzaldehyde and CO2) of two dialkylboron
guanidinates, a symmetrical and an asymmetrical one, (Me2N)
C(NiPr)2BCy2 (1) and {iPr(H)N}C(NiPr){N(p-tBu-C6H4)}BCy2 (2),
Since there is a limited number of examples of related
boron(III) guanidinates in the literature and their chemistry
remains largely unexplored,7 we decided to focus now on the
B–N insertion reactions of our dialkylboron guanidinates with
small unsaturated molecules. In this sense, we were first
inspired by the outstanding results recently published by
Stephan and co-workers8 on the FLP-type reactivity of related
i
boron amidinates of the formula [HC(NR)2B(C6F5)2] (R = Pr,
tBu), towards small unsaturated molecules (Scheme 2) to give
1,1-insertions (with CO and isonitriles), 1,2-insertions (with
CO2, carbodiimides, benzaldehyde or acetonitrile) or heteroly-
tic C–H cleavage (terminal alkynes). Previously, related work by
Dorokhov and Mikhailov, who have extensively studied the
reactivity of borylamidines, showed that N,N′-disubstituted
boron amidinates of the formula [PhC(NR)(NR′)BR″2] (R,R′ = Fig. 2 Guanidinato compounds 1–4.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2017

View Article Online
Dalton Transactions
Paper
and two bisboron guanidinates(2–), {iPr(BCy2)N}C(NiPr) detection of a monodentate guanidinate isomer for 1 is rather
{N( p-tBu-C6H4)}BCy2 (3) and {iPr(C8H14B)N}C(NiPr) important for the insertion reactions we present herein from a
{N(p-Me-C6H4)}BC8H14 (4) (Fig. 2), the latter being prepared mechanistic point of view, since Stephan postulated an open-
for the first time by double deprotonation of the correspond- chain form of the strained chelate as the active species for the
ing guanidine with the 9-borabicyclo[3.3.1]nonane dimer, FLP-type reactivity of their boron amidinates, which they failed
(H-BC8H14)2. All these species were found to react with to detect in solution even after increasing the temperature up
aromatic isonitriles, and some of them with CO to give 1,1- to 80 °C.8
insertion products. Likewise, the addition of benzaldehyde
and CO2 led, in some instances, to 1,2-insertion products, in
Synthesis and structural characterisation of compound 4
an equilibrium with the reagents.
As mentioned in Introduction, compounds 1–3 were prepared
from a metathetical synthetic route, therefore generating
lithium chloride as an undesired side product. In order to test
an alternative, more atom-efficient route to prepare boron gua-
nidinates we tried the direct deprotonation of a guanidine
with a commercial secondary borane. Pleasingly, double depro-
tonation of (p-Me-C6H4N)C(NHiPr)2 with one equivalent of the
dimer (H-BC8H14)2 proceeded smoothly in toluene under mild
conditions (room temperature, 4 h) to give the novel bisboron
guanidinate(2–) 4 in good yields.
Results and discussion
Solvent-dependent equilibrium for the isomers of compound 1
As mentioned above, four boron guanidinates with different
steric and electronic properties were chosen to study their reac-
tivity (Fig. 2). Although the synthesis and structural character-
isation of compounds 1–3 were already described in our pre-
vious report,6 we would like to note an important feature
regarding compound 1, which was unfortunately overlooked
The molecular structure of compound 4 was determined by
a diffractometric analysis (Fig. 3, Table 1). This compound
1
before. A careful analysis of different H NMR spectra in C6D6
ˉ
crystallised in the triclinic P1 space group. The main structural
solution of this compound revealed the presence of small
amounts (ca. 11%) of another species with inequivalent CH
(iPr) groups, attributed to the monodentate isomer, 1-M. The
latter seems to be in a fast solvent-dependent equilibrium with
the major species, the chelate isomer 1-C, since upon chan-
ging the solvent to CD2Cl2, the relative amount of 1-M
increased slightly (Scheme 3). The presence of this monoden-
tate isomer in solution, proposed as an intermediate in the
carbodiimide de-insertion reaction,6 is not so surprising even
when this is the least abundant coordination mode for such
ligands in the literature.1c In fact, Dorokhov and Mikhailov
already detected this type of solvent-dependent equilibrium in
dialkylboron amidinates of the formula [PhC(NMe)2BR2] (R =
Pr, Bu).9 Moreover, a monodentate coordination was also pos-
tulated for the first boron guanidinates reported in the litera-
ture by Lappert and co-workers, [(Et2N)C{N(p-tol)2}BXY] (XY =
Ph2; Ph, SBu; Ph, NEt2; o-O2C6H4), on the basis of IR spectra,7a
as well as for the boron amidinate [HC(NCy)2BC8H14], on the
basis of 11B NMR,12 overall indicating that the monodentate
coordination of amidinato/guanidinato ligands to boron
centres is not that unusual. Finally, it should be noted that the
features of this compound are reminiscent of those found for
compound 3, previously described.6 Thus, two N atoms (N1
Fig. 3 Molecular structure of compound 4: H atoms are omitted for
clarity and ORTEP ellipsoids are plotted at the 50% probability level.
Table 1 Selected bond lengths (Å) and angles (°) for compound 4
B1–N1
B1–N2
C9–N1
C9–N2
C9–N3
N3–B2
N1–C17
N2–C10
N3–C20
1.631(2)
1.637(2)
1.318(2)
1.335(2)
1.404(2)
1.416(2)
1.464(2)
1.421(2)
1.502(2)
C1–B1–C5
108.5(1)
79.4(1)
N1–B1–N2
N1–C9–N2
N1–C9–N3
N2–C9–N3
C9–N3–B2
C20–N3–B2
C9–N3–C20
C15–C10–N2–C9
103.7(1)
128.5(1)
127.8(1)
120.8(1)
126.1(1)
113.1(1)
49.6(2)
Scheme 3 Solvent-dependent equilibrium for compound 1.
This journal is © The Royal Society of Chemistry 2017
Dalton Trans.

View Article Online
Paper
Dalton Transactions
and N2) of the guanidinato ligand in compound 4 are chelat- and 24.1°, respectively. This is probably due to the steric con-
ing one of the boron centres (B1), defining an almost planar gestion on the BN2C plane induced by the cyclic C8H14 frag-
four-membered CN2B heterocycle as denoted by the low ment bound to B1, also causing a staggered conformation of
torsion angle between the CN2 and N2B planes of 5.5°. the aryl ring with respect to the latter plane, with a C15–C10–
Conversely, the exocyclic N3 presents a κ1-monodentate coordi- N2–C9 torsion of 49.6(2)° compared to 22.2(4)° measured for
nation mode to the other tricoordinate B atom. The C–N dis- the same torsion angle in 3.6
tances within the guanidinato core are intermediate between
The solution NMR characterisation of 4 is in good agree-
the typical single and double C–N bonds, as expected due to ment with the solid state structure of the compound (see
the contribution of the different resonance forms (Scheme 1). Table 2). The absence of symmetry elements in the molecule is
However, C9–N3 is somewhat longer than the other two, 1.40 Å evident in the multinuclear NMR spectra recorded for 4. For
vs. 1.33 and 1.32 Å, which fits well with a much lower contri- example, two couples of diastereotopic methyls attributed to
bution of the “iminium diamide” resonance form, also the N–iPr groups give rise to four different doublets, at 1.32
observed for compound 3. As in the latter case, this is consist- and 1.23 ppm, and 0.95 and 0.82 ppm, respectively. Each pair
ent with the involvement of the lone pair on N3 on a π inter- is coupled to a methyne proton, giving rise to the corres-
action with B2, confirmed by the B2–N3 distance of 1.416(2) Å, ponding septets at 3.83 and 3.54 ppm. The lack of symmetry of
which falls in the range found for B–N distances in aminobor- 4 is also evident in the 13C{1H} NMR, which displays four and
anes with significant
π
(BvN) bonding (1.41–1.45 Å).13 two signals, respectively, for the CH3 and CH groups of these
Moreover, the sum of angles at B2 and N3 is practically 360°, N–iPr moieties. Additionally, up to 12 different signals
so that an sp2 hybridisation can be safely assigned to both between 35 and 22 ppm could be assigned to both BC8H14
atoms, which further confirms our hypothesis. Focusing on groups, instead of the 16 expected, probably due to the acci-
the exocyclic borylamine group, a strongly staggered confor- dental degeneracy of some of the signals. The 11B NMR spec-
mation with respect to the heterocycle can be clearly detected, trum shows two broad signals, ca. 53 and 13 ppm, respectively,
with a C20–N3–B2/N1–C9–N2 torsion angle of 62.5°, somewhat attributed to the tri- and tetracoordinate boron, and compar-
smaller than that measured for compound 3 of 77.9°. In any able to those found for the isostructural compound 3.6
case, this arrangement also hampers the electron overlapping
1,1-Insertion reactions of compounds 1–4 with isonitriles
and CO
with the rest of the guanidinato ligand, which, alongside the
elongated C9–N3 distance, agrees well with an almost negli-
gible contribution of the “iminium diimide” resonance struc-
ture. There is some degree of pyramidalisation on the N atoms
chelating B1, not observed for compound 3. This is denoted by
the sum of angles at N1 and N2, 355.8 and 353.1° and the fact
that the N–iPr and N–Ar groups are pointing out of the hetero-
cycle plane on opposite directions, with average angles of 21.8
The reactivity of the guanidinato compounds 1–4 was tested
towards isonitriles in the first place. Interestingly, they all
t
failed to react with BuNC even at 80 °C, but instead proved
to be quite reactive towards less sterically-demanding
aromatic isonitriles, such as XylNC (Xyl = 2,6-Me2-C6H3) or
(p-MeO-C6H4)NC even at room temperature to give the
Table 2 Selected IR and NMR data for new compounds
δH (ppm) [JHH in Hz]a
Compound
ν (cm−1
)
CH-iPr
NH
CH3-iPr
δBa (ppm)
4
5
6
—
3.83 [6.7], 3.54 [6.9]
3.57 [6.8], 3.32 [6.8]
3.59 [6.8], 3.30 [6.8]
5.25 [br], 2.97 [9.2, 6.3]
3.82 [7.2], 2.74 [8.9, 6.4]
5.20 [br], 2.96 [9.4, 6.3]
3.78 [7.2], 2.82 [9.2, 6.4]
4.10 [6.7], 3.40 [br]
4.07 [6.7], 3.46 [6.9, 6.6]
4.15 [6.7], 2.97 [6.7]
4.10 [6.7], 2.97 [6.7]
4.59 [br], 3.06 [9.3, 6.4]
3.74 [6.7], 2.93 [6.7]
3.98 [≈7], 3.35 [≈7]
3.87 [6.8], 3.52 [9.4, 6.4]b
4.07 [≈7], 3.14
—
—
—
1.32 [6.7], 1.23 [6.7], 0.95 [6.9], 0.82 [6.9]
1.60 [6.8], 1.14 [6.8]
1.60 [6.8], 1.12 [6.8]
1.22 [br], 0.60 [6.3]
1.14 [7.2], 0.52 [6.4]
1.33 [6.8], 0.61 [6.4]
1.12 [7.2], 0.61 [6.4]
1.79 [6.7], 1.75 [6.7], 1.2 [vbr], 0.9 [vbr]
1.78 [6.7], 1.77 [6.7], 1.25 [6.9], 0.92 [6.6]
1.79 [6.7], 1.72 [6.7], 0.95 [6.7], 0.41 [6.7]
1.78 [6.7], 1.71 [6.7], 0.98 [6.7], 0.35 [6.7]
1.19 [≈7], 0.61 [6.4]
1.60 [6.7], 1.53 [6.7], 0.94 [6.7], 0.40 [6.7]
1.19 [≈7], 1.10 [≈7], 0.94 [≈7], 0.86 [≈7]
1.27 [≈7], 1.10 [6.4], 1.05 [6.8], 0.94 [6.4]b
—
53.2, 12.6
1.6
1.6
0.6
—
1638 (s, CvN)
1638 (s, CvN)
3453 (w, N–H), 1649 (s, CvN)
—
3346 (w, N–H), 1638 (s, CvN)
—
1634 (m, CvN)
1652 (m, CvN)
1648 (m, CvN)
1633 (m, CvN)
3294 (m, N–H) 1674 (s, CvO)
—
7a
7b
8a
8b
9
10
11
12
13
14
15
16a
16b
17
18
19
3.86 [9.2]
3.55 [8.9]
3.79 [9.4]
3.69 [9.2]
—
0.7
—
49.5, 1.5
51, −2
56.3, 2.5
52.5, 2.7
2.1
—
—
4.05 [9.3]
—
—
—
—
—
3389 (w, N–H)
3.63 [9.4]b
3.89 [≈7]
—
8.0b
—
—
—
—
—
4.27 [6.9], 3.22 [6.7]
3.76 [≈7], 3.20
3.88 [6.7], 2.99 [9.2, 6.3]
1.31 [6.9], 0.92 [6.7], 0.48 [6.9], 0.40 [6.7]
1.22 [≈7], 0.78 [≈7], 0.59 [≈7], 0.58 [≈7]
1.56 [6.7], 0.47 [6.3]
—
—
—
3.85 [8.7]
3.44 [9.2]
a In C6D6 at 298 K at 400 or 500 MHz (1H), or 128 MHz (11B) unless otherwise stated. b In CD2Cl2.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2017

View Article Online
Dalton Transactions
Paper
expected 1,1-insertion products. Thus, compound 1 reacted in isonitriles, which, upon mixing, led to the insertion products
ca. 2 h with the latter isonitriles to give compounds (Me2N)C {iPr(BC8H14)N}C{N(p-Me-C6H4)}(NiPr)(CNAr)BC8H14 [Ar = Xyl
(NiPr)2(CNAr)BCy2 [Ar = Xyl (5), p-MeO-C6H4 (6)], which were (11), p-MeO-C6H4 (12)] (Fig. 4). These compounds turned out
isolated as microcrystalline solids in good yields (Fig. 4). The to be in a temperature-dependent equilibrium with the
reaction of the asymmetrical guanidinato compound 2 towards reagents, shifted towards the insertion products at room temp-
the same isonitriles took also place at room temperature in ca. erature, which will be discussed in more detail later.
2 h. However, in this case, two regioisomers were possible,
The reactivity of compounds 1–4 was also tested towards
depending on which N–B bond the insertion took place. CO. In general terms, these reactions turned out to be more
Monitoring the reactions by 1H NMR in C6D6 (Fig. S13 and sluggish than their counterparts with isonitriles, especially for
S15, ESI†) revealed that, indeed, a mixture of the two possible the BCy2 derivatives. This observation goes in line with that
regioisomers was obtained with both isonitriles, {iPr(H)N}C{N reported by Stephan and co-workers for the more active boron
i
t
(p-tBu-C6H4)}(NiPr)(CNAr)BCy2 [Ar = Xyl (7a), p-MeO-C6H4 (8a)], amidinates [HC(NR)2B(C6F5)2] (R = Pr, Bu).8 Thus, we were
and {iPr(H)N}C(NiPr){N(p-tBu-C6H4)}(CNAr)BCy2 [Ar = Xyl (7b), only able to isolate one carbonylation product in moderate
p-MeO-C6H4 (8b)] (Fig. 4). In both instances, the major isomer yields, this being {iPr(H)N}C{N(p-tBu-C6H4)}(NiPr)(CO)BCy2
i
is the one in which the insertion takes place in the PrN–B (13), obtained after exposing compound 2 to an atmosphere of
bond, with ratios 7a : 7b and 8a : 8b of 6 : 1 and 3 : 1, respect- CO (ca. 4 atm) at 40 °C for 3 days (Scheme 4). Interestingly, CO
ively. It is worth mentioning as well that these isomers do not insertion was not observed for compound 1 at room tempera-
seem to be in a temperature-dependent equilibrium in solu- ture, and instead the expected carbodiimide de-insertion reac-
tion, since there are no changes in their relative ratio upon tion took place progressively, accelerated upon increasing the
warming the solutions up to 60 °C. Of the latter isomer mix- temperature (50 °C).6 As for compound 3, when subjected to
tures, only the major species, 7a and 8a, could be isolated as similar reaction conditions (3 days, 50 °C, 4 atm), the major
crystalline solids, the others being only detected by some compound in solution was still unreacted 3, and only small
characteristic signals in the 1H NMR spectra (see Table 2). amounts of other species were detected by NMR.
Interestingly, the reaction of bisboron guanidinate(2–)
3
Unfortunately, signals attributed to the expected insertion
towards these isonitriles was slower, probably owing to the product could not be unambiguously assigned. However, one
higher steric hindrance and reduced basicity of the chelate of the species detected was compound 13, suggesting that, to a
N atoms induced by the exocyclic N(iPr)BCy2 group, but much lesser extent, carbonylation also took place, but the product
more regioselective, and insertions only took place in the obtained must be very reactive under these conditions to give
i
endocyclic PrN–B bond to give compounds {iPr(BCy2)N}C{N compound 13 after partial hydrolysis of the exocyclic N–B
(p-tBu-C6H4)}(NiPr)(CNAr)BCy2 [Ar = Xyl (9), p-MeO-C6H4 (10)] bond. Remarkably, as opposed to compounds 1–3, compound
almost quantitatively after at least 2 days at room temperature. 4 reacted immediately upon exposure to CO (1 atm) to give the
Similar regioselectivity was achieved in the reactions of 4 with insertion product {iPr(BC8H14)N}C{N(p-Me-C6H4)}(NiPr)(CO)
BC8H14 (14), which is in equilibrium with the de-insertion
compounds, 4 and CO, clearly shifted towards the reagents at
Fig. 4 Compounds 5–12.
Scheme 4 Reactions of compounds 2 and 4 towards CO.
This journal is © The Royal Society of Chemistry 2017
Dalton Trans.

View Article Online
Paper
Dalton Transactions
room temperature (ratio 4 : 14 ca. 9 : 1). Structural nuances reactive species in these 1,1-insertion reactions, which
between the borabicyclic BC8H14 and the BCy2 seem to be suggests that the presence of an open-chain isomer in solution
behind the difference in reactivity between compounds 1–3 vs. is a not sufficient condition for this type of reactivity, and
compound 4 towards isonitriles and CO, as will be addressed other electronic and steric factors may be influential as well
in the next section.
(e.g., sufficient degree of basicity/acidity at the N/B centres, or
At this stage, some mechanistic considerations should be less steric hindrance around the chelating N centres and the
briefly commented regarding the 1,1-insertion reactions of iso- tetracoordinate boron).
nitriles and CO in boron guanidinates 1–4. We assume that, in
Structural characterisation of 1,1-insertion products 5–14
a first stage, an open-chain intermediate (detected in the case
of 1-M) is required in order to react with the incoming unsatu-
rated molecule. This was also assumed by Stephan and co-
workers8 for FLP-type reactions of boron amidinates and later
corroborated by DFT calculations.14 After this step, two path-
ways are possible, one involving a concerted insertion mechan-
ism14 and the other a two-step process involving the formation
of the EC–borane adduct (E = O, NR) and subsequent nucleo-
philic attack by the N atom to the C atom of the isonitrile or
CO to give the five-membered heterocycle. Although we have
not detected an intermediate suggesting the formation of such
an adduct, Erker and co-workers did find them in the reactions
of intramolecular P/B FLPs of the formula Ph2PC(p-Tol)vCRB
(C6F5)2 (R = C6F5, Me) with CNtBu, which turned out to be in
temperature-dependent equilibria with the 1,1-insertion pro-
ducts and the reagents.15 As for the regioselectivity of the 1,1-
insertion reactions herein studied with the asymmetrical gua-
nidinates 2–4, as mentioned before, in all cases the major (or
only) product detected is the one resulting from the insertion
The solid-state structures of the isonitrile insertion products 5,
8a, 10 and 11 were determined by X-ray diffraction analyses
(Fig. 5, Table 3). These compounds crystallised in the mono-
clinic space groups P21/c (5) and P21/n (11), and the triclinic
ˉ
space group P1 (8a and 10). As can be seen, they all share
many common features and will be therefore discussed
together. However, some differences are also evident between
the molecule of 11 and the other three compounds. Thus,
despite being all built around five-membered heterocycles (dia-
zaboroles), compounds 5, 8a and 10 display a roughly planar
arrangement as indicated by the sum of internal angles of ca.
540°, whereas this ring is clearly puckered for 11 (530.7°).
Indeed, the latter compound displays a half chair configur-
ation in which the C2 and B1 atoms are located at different
sides of the plane defined by the CN3 core of the guanidine, as
denoted by the N2–C1–N1/B1–C2–N2 torsion angle of 25.7°. In
contrast, compounds 5, 8a, and 10 only show slight deviations
from planarity (N2–C1–N1/B1–C2–N2 < 8°). The half-chair con-
formation of 11 could be explained on steric grounds, as the
cyclic C8H14 group on B1 seems to induce higher steric
pressure on the neighbouring bulky groups of the diazabora-
zole ring [N(p-tolyl) and CvNXyl], as opposed to two Cy
i
in the endocyclic PrN–B bond. This could be explained in
terms of the enhanced donor ability of the more basic N
centre [N(iPr) > N(Ar)]. Interestingly, compound 1, for which
the open chain isomer was detected, 1-M, was not the most
Fig. 5 Molecular structures of compounds 5 (top left), 8a (top right), 10 (bottom left) and 11 (bottom right): H atoms are omitted for clarity and
ORTEP ellipsoids are plotted at the 50% probability level. Colour code: C, grey; B, pink; N, blue; O, red.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2017

View Article Online
Dalton Transactions
Paper
Stephan’s diazaboroles HC(NiPr)2(CE)B(C6F5)2 (E = O, NtBu)8
or Warren’s oxazaborole HC(NAr′)(O)(NXyl)B(C6F5)2 (Ar′ =
4-tBu-2,6-Ph2-C6H2)16 displayed planar five-membered rings
similar to 5, 8a, and 10, which makes compounds 11 and 12
rather unique examples.
Table 3 Selected bond lengths (Å) and angles (°) for compounds 5, 8a,
10 and 11a
Compound
5
8a
10
11
B1–N1
N1–C1
C1–N2
C1–N3
N2–C2
C2–B1
C2–N4
N4–C6
1.618(4)
1.294(3)
1.368(4)
1.403(3)
1.432(3)
1.658(4)
1.281(4)
1.409(3)
—
96.0(2)
106.0(2)
111.5(2)
114.5(2)
123.3(2)
122.2(2)
111.5(2)
5.0
1.629(2)
1.313(2)
1.358(2)
1.372(2)
1.436(2)
1.654(2)
1.279(2)
1.416(2)
—
94.7(1)
107.8(1)
110.7(1)
114.3(1)
121.5(1)
124.1(1)
112.1(1)
7.5
1.645(6)
1.310(6)
1.362(5)
1.419(6)
1.449(6)
1.637(7)
1.282(5)
1.421(6)
1.432(5)
94.6(3)
108.0(4)
111.2(4)
113.5(4)
121.5(4)
124.9(4)
112.6(3)
3.6
1.624(2)
1.317(2)
1.350(2)
1.414(1)
1.463(2)
1.642(2)
1.275(2)
1.410(2)
1.417(2)
94.9(1)
102.5(1)
110.3(1)
113.5(1)
122.2(1)
124.2(1)
109.5(1)
25.7
The other structural parameters found in the solid state for
the four compounds are very similar. For example, the two new
bonds formed upon insertion of the isonitrile in one of the
N–B bonds, i.e., N2–C2 and C2–B2, with distances ca. 1.43–1.46 Å
and 1.64–1.66 Å, respectively, fall in the ranges expected for
single N–C and C–B bonds. The inserted isonitrile has now
become an exocyclic imine (E isomer in all cases), as denoted
by the C2–N6 distances, ca. 1.28 Å, consistent with the formu-
lation of double CvN bonds. There is less π delocalisation
along the C–N bonds of the parent guanidinato moiety, with
distances following this sequence: C1–N1 (1.29–1.32 Å) <
C1–N2 (1.35–1.37 Å) < C1–N3 (1.37–1.42 Å), suggesting higher
localisation of the π electrons on the C1–N1 bond (i.e., more
double CvN bond character), and almost negligible for the
exocyclic C1–N3 (i.e., essentially a single C–N bond), especially
so for the bisboron derivatives 10 and 11 (C1–N3 distances ca.
1.42 Å). This is in good agreement with the almost perpendicu-
lar arrangement of the substituents at the exocyclic N3 within
these compounds, (B2–N3–C4/N1–C1–N2 torsion angles of
75.2 and 87.7° for 10 and 11, respectively). Moreover, the sum
of angles about N3 and B2 atoms in these compounds (ca.
360°), as well as the N3–B2 distances, are consistent with a
strong π NvB bonding, like in the parent guanidinates 3 and
4. Finally, in all four structures, the planar arrangement of the
N3–B2
N1–B1–C2
B1–C2–N2
C2–N2–C1
N2–C1–N1
N2–C1–N3
N3–C1–N1
C1–N1–B2
N2–C1–N1/B1–C2–N2b
a Values according to the labelling shown in the picture below.
b Torsion between the planes.
groups, which can be arranged perpendicularly at both sides CN3 core of the guanidinato moiety is evidenced by the trigo-
of the heterocyclic plane reducing the steric impact on the nal planar arrangement around the central C1 atom (sum of
closer groups. Thus, in order to minimise the steric congestion angles ca. 360°), indicative of an sp2 hybridisation.
in 11, the BC8H14 is pointing out of the N1–C1–N2 plane,
whereas the p-tolyl and CvNXyl groups are projecting out- NMR in solution and by ATR-IR in the solid state (Table 2),
wards on the opposite side of the plane.
except for those species that could not be isolated, which were
Compounds 5–14 were also characterised by multinuclear
In order to rule out the possibility of packing effects only detected by 1H NMR (7b, 8b and 14). In the first place,
accounting for the puckered ring found for compound 11, the the IR spectra of compounds 5–12 display a medium-to-strong
structures of compounds 9–12 were optimised using Density band in the frequency range expected for the stretch of double
Functional Theory (DFT) methods (Fig. S28 in the ESI†). CvN bonds (1634–1652 cm−1), attributed to the imine func-
Overall, the structural parameters of all these compounds are tionality formed upon insertion of the isonitrile and compar-
essentially identical to those experimentally measured for 10 able to those observed for related diazaborazoles of the
and 11, including the presence of significantly puckered five- formula [RC(NR′)2(CNPh)BR″2] (R, R′ = alkyl, aryl; R″ = alkyl).10
membered rings in the two BC8H14 derivatives, 11 and 12, and As expected, compound 13 exhibits a C–O stretch band at
five-membered planar rings for the two BCy2 derivatives com- slightly higher frequencies, 1674 cm−1, attributed to the
puted, 9 and 10. This implies that the puckering in the BC8H14 double CvO bond, although >40 cm−1 below those found for
derivatives has
a genuinely steric or electronic origin. related diazaborazoles with the more electron withdrawing
Moreover, a constrained optimisation of the geometry of com- C6F5 groups on boron.8,17 Additionally, the N–H stretch was
pound 11 to retain a planar central ring akin to that observed also assigned to a weak band in the range 3294–3453 cm−1 in
for the BCy2 derivatives (Fig. S29 in the ESI†) yielded a struc- the IR spectra of compounds 7a, 8a, and 13.
ture located 7.3 kcal mol−1 above the corresponding puckered
The NMR spectroscopic data for these compounds are,
(not-restrained) structure. Interestingly, the cyclic B atom in overall, in good agreement with the solid-state structures of
the forcedly planar structure of 11 displays quite asymmetric the diazaborazoles commented before. For example, the 11B
bond distances (B–C = 1.686 Å and B–N = 1.619 Å), this further NMR spectra exhibit either one or two signals for the tetra-
confirming the idea that an unbearable steric pressure is and tricoordinate boron atoms (the latter, of course, only for
exerted by the BC8H14 group on planar dispositions. Examples bisboron derivatives 9–12), at the expected chemical shifts, ca.
of structurally related species in the literature, such as 0 and 50 ppm, respectively. We should note here that tetracoor-
This journal is © The Royal Society of Chemistry 2017
Dalton Trans.

View Article Online
Paper
Dalton Transactions
dinate borons resonate some 10 ppm upfield from those of the in these interactions. In order to shed some light on the
parent boron guanidinates 1–4, perhaps due to the replace- nature of these dynamic processes, variable temperature 1H
ment of one B–N bond by a new B–C bond, an effect also NMR experiments were carried out for 7a in toluene-d8 in the
observed in Stephan’s amidinates.18 1H and 13C{1H} NMR range 253–353 K (see Fig. S5 in the ESI†), as well as for 13 at
spectra for the bisboron derivatives, 9–12 and 14, are consist- 333 K in C6D6 (Fig. S21(a) in the ESI†) which showed that,
ent with the low symmetry (C1) observed in the previous diffr- upon increasing the temperature, these signals sharpened up
actometric studies. Thus, each iPr group gives rise to two progressively to give a somewhat broad septet and a well-
signals for the diastereotopic CH3 moieties in both spectra. defined doublet, for the CH and CH3 groups of 7a and 13, at
Likewise, up to sixteen different signals could be assigned to 353 and 333 K, respectively. Upon cooling down the tempera-
two different BC8H14 groups in the carbon-13 spectra for com- ture below 298 K for 7a, an overall broadening of all signals
pounds 11 and 12, whereas four inequivalent Cy groups gener- takes place, especially affecting the aforementioned CH and
ated more complex spectra with some broad overlapping CH3 resonances, with the former shifting to higher frequen-
signals for compounds 9 and 10, also indicating some cies, up to 5.66 ppm at 253 K. Unfortunately, below 253 K pre-
dynamic behaviour as will be commented below.
cipitation of the product precluded further recording at lower
In contrast, compounds 5–8, and 13 show a higher apparent temperatures and kinetic parameters concerning these pro-
symmetry, most likely due to fast rotation in the NMR time- cesses could not be calculated. In any event, we believe that,
scale of the exocyclic amino groups (NMe2 or NHiPr) in solu- upon cooling down the temperature, the fast rotation of the exo-
tion at room temperature. This would generate a false sym- cyclic C–NHiPr bond would slow down in the NMR time scale
metry plane containing the diazaborole ring, thus making the and, at the same time the CH⋯N interaction would be strength-
i
i
Me groups within each Pr equivalent. Indeed, each Pr group ened,20 possibly accounting for the latter observations for 7a.
1
generates only one signal due to the CH3 moieties in their H
In the case of bisboron derivatives 9 and 10, broad signals
and 13C NMR spectra. Moreover, the now equivalent Cy groups were also detected for one of the Pr groups at 298 K in the H
i
1
originate up to six signals (sometimes less due to accidental NMR. Moreover, compound 9 also showed broad signals for
degeneracy or overlapping) in the carbon-13 spectra for com- the Cy groups and the inequivalent Me groups of the xylyl frag-
1
pounds 5, 6, 7a, 8a and 13. Other significant signals in the 13
C
ment, in both the H and 13C NMR spectra. Therefore, a vari-
NMR spectra for compounds 5–13 are those assigned to the able temperature NMR study was also performed on 9 in
iminic carbon of the exocyclic imine groups [CvNXyl or CvN toluene-d8, in the range of temperatures 193–353 K (Fig. S6 in
(p-MeO-C6H4)], which give rise to broad signals in the range the ESI†). As observed before for 7a, warming up led to a pro-
177.6–188.0 ppm, only marginally below those reported by gressive sharpening of the mentioned broad signals. However,
Erker and co-workers for the zwitterionic phosphaboroles cooling down below 298 K revealed a more complex situation.
CRvC(p-tol)PPh2(CvNtBu)B(C6F5)2 (R
= C6F5, Me), ca. At 273 K, the methyl resonances of the xylyl group become
192 ppm. On the other hand, the carbonyl moiety in 13 gives broader, but below this temperature they sharpen up again,
rise to a resonance at 203.2 ppm, somewhat more deshielded and a new set of signals is clearly evident at 233 K, at similar
than those reported for similar diazaborolones by Rojas and chemical shifts, most likely attributed to a minor isomer
collaborators (ca. 193 ppm).17 Similarly, signals in the range (rotamer) of 9 in a 1 : 3 ratio with respect to the major one. The
158–165 ppm could be easily attributed to the iminic carbon resonances of one of the iPr groups undergo substantial
of the endocyclic imine in these compounds.
changes as well, not only in line shape, but also in chemical
Dynamic behaviour was observed in the NMR spectra shift. For example, the methyne resonance which gives rise to
recorded at 298 K for compounds 7a, 8a, 9, 10 and 13, that is, a septet at 353 K, ca. 3.5 ppm, progressively shifts upfield and
those derived from the asymmetrical boron and bisboron gua- broadens upon cooling down to 298 K. Below this temperature
nidinates, 2 and 3. Thus, the compounds with an exocyclic it sharpens up again down to 233 K, to finally broaden again
NHiPr group (i.e. 7a, 8a, and 13) displayed rather broad signals before splitting into two broad signals at 193 K, at 3.22 and
assigned to the CH (1H, 13C) and CH3 (1H) fragments of the 3.12 ppm, respectively, in a 1 : 4 ratio. At the same time, the
i
other Pr group at 298 K. Moreover, the methyne signals are Me groups coupled to the latter CH undergo a similar process
particularly downfield for 7a and 8a, at ca. 5.25 ppm, and shifting from 1.25 and 0.96 ppm at 353 K to ca. 1.05 and
slightly less so for 13 (4.59 ppm), compared to those of other 0.77 ppm, respectively, at 233 K, before broadening up again
compounds herein reported (usually in the range 3.0–4.2 ppm, down to 193 K. All these data clearly indicate the existence of
see Table 2). This could be explained by weak CH⋯N hydrogen several dynamic processes in solution for compounds 9 and
bond interactions, probably between the methyne group and 10. In this sense, short CH⋯N contacts were also measured for
the exocyclic amine NH(iPr). In line with this, a short (C5) compound 10 in the solid state, both between the methyne
H⋯N3 distance of 2.43 Å was measured for 8a in the solid group and the exocyclic N atom, with a (C5)H⋯N3 distance of
state, well below the sum of van der Waals radii for these 2.48 Å, and also between a Me group and the exocyclic imine,
atoms (1.20 Å for H, 1.55 Å for N).19 Evidence of similar intra- with a (C5)CH3⋯N4 distance of 2.44 Å. These interactions
molecular CH⋯N interactions was detected as well for related alongside inhibited rotation around N–Aryl bonds at low temp-
heterocycles by X-ray diffraction,20 and/or 1H NMR,20,21 the eratures in the NMR timescale may account for the rotamers
latter confirming the deshielding effect of the H atom involved detected for compound 9 in solution.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2017

View Article Online
Dalton Transactions
Paper
Thermodynamic studies on isonitrile de-insertion in
compounds 11 and 12
benzaldehyde to solutions of compound 1 only led to 30% con-
version to the insertion product (Me2N)C(NiPr)2(PhHCO)BCy2
(15), which could not be isolated and was only identified by 1H
NMR (Scheme 6). The reaction of 2 with 2 equivalents of benz-
aldehyde proceeded slowly at room temperature until an equi-
librium was reached after ca. 48 h consisting of the two poss-
ible N–B insertion products: {iPr(H)N}C{N(p-tBu-C6H4)}(NiPr)
(PhHCO)BCy2 (16a) and {iPr(H)N}C(NiPr){N(p-tBu-C6H4)}
(PhHCO)BCy2 (16b), in a 7 to 1 ratio with a conversion slightly
over 50%. Increasing the temperature up to 60 °C shifted the
equilibrium towards the de-insertion products. Luckily, the
major insertion product 16a could be isolated in high yields by
carrying out the reaction with 5 equivalents of benzaldehyde
in a non-polar solvent such as pentane and crystallisation at
−20 °C. Compound 4 reacts with 10 equivalents of benz-
aldehyde to give the insertion product {iPr(BC8H14)N}C{N(p-
Me-C6H4)}(NiPr)(PhHCO)BC8H14 (17), as well as {iPr(H)N}C{N
(p-Me-C6H4)}(NiPr)(PhHCO)BC8H14 (18), in a 2 to 1 ratio
approx., and ca. 60% conversion based on compound 4. The
formation of 18 probably follows from the partial hydrolysis of
the exocyclic N–B bond induced by trace amounts of water in
the benzaldehyde as increasing the amount of the latter also
increased the relative amount of this product (ratio 17 : 18 ca.
1 : 2 upon addition of 30 equivalents of benzaldehyde).
Unfortunately, none of these products could be isolated. We
note as well that the insertion of aldehydes in dialkylboron
amidinates reported by Mikhailov and co-workers9 gave also
thermally unstable insertion products, which seems to be
related with the relative acidity of the boryl fragment, as
As mentioned earlier, a fast equilibrium was found in solution
between compounds 11 and 12 and the isonitrile de-insertion
products, CNAr (Ar = Xyl, p-MeO-C6H4) and the parent boron
guanidinates 3 and 4 (Scheme 5). In both cases, the equili-
brium was clearly shifted towards the insertion product at
room temperature, which allowed the isolation of these pro-
ducts in high yields. Therefore, we decided to study the effect
of temperature on these equilibria by recording 1H NMR
spectra in C6D6 in the range 298–353 K, which allowed us to
calculate the equilibrium constants (Keq) and, by means of
Van’t Hoff plots (ln Keq/T vs. 1/T), the thermodynamic para-
meters, ΔH° and ΔS°, for the isonitrile de-insertion reaction
(Tables S1, S2 and Fig. S1–S4 in the ESI†). Linear regressions
nicely fit with our data, giving standard enthalpy and entropy
values of 66.6 1.9 kJ mol−1 and 175.6 6.0 J mol−1 K−1 for 11
and 72.2 1.2 kJ mol−1 and 170.9 3.7 J mol−1 K−1 for 12,
respectively. These values are consistent with our experimental
observations, as the expected strong entropic contribution
implies a shift of the equilibrium towards the de-insertion pro-
ducts upon increasing the temperature.
Some final remarks should be made about these reversible
isonitrile and carbonyl insertion/de-insertion equilibria found
for the borabicycle derivatives. The optimised structures of 11
and 12, with a half-chair configuration for the five-membered
heterocycle, induced by the steric demands of the BC8H14
moiety, must be less energetically favoured than the pseudo-
planar structures found for the BCy2 derivatives. This will
account for the equilibria found for the former BC8H14 deriva-
tives, as opposed to the thermally-stable insertion products
obtained for the BCy2 guanidinates 1–3 (no sign of de-inser-
tion products was detected up to 353 K).
1,2-Insertion reactions of compounds 1–4 with benzaldehydes
and CO2
All the guanidinato compounds, except compound 3, showed
reactivity towards benzaldehyde to give the expected 1,2-inser-
tion products, reminiscent of those reported by Stephan et al.8
and Mikhailov et al.9 with boron amidinates. However, in all
cases, the reactions turned out to be in equilibria between the
insertion and de-insertion products, most of the times shifted
towards the reagents even after employing a great excess of
benzaldehyde. For instance, addition of ca. 10 equivalents of
Scheme 6 Reversible 1,2-insertion reactions of 1,
2 and 4 with
Scheme 5 Isonitrile de-insertion equilibrium for compounds 11 and 12.
benzaldehyde.
This journal is © The Royal Society of Chemistry 2017
Dalton Trans.

View Article Online
Paper
Dalton Transactions
Scheme 7 Reversible 1,2-insertion reaction of 2 and CO2.
similar insertion products with more acceptor BAr2 groups
(Ar = Ph,9 C6F5 ), showed higher chemical and thermal stability.
8
The reactivity of solutions of guanidinato compounds 1–4
was also tested towards CO2 within sealed NMR tubes.
Compounds 1 and 3 did not show evidence of reaction with CO2
under ambient conditions (1 atm, room temperature). However,
the 1,2-insertion product {iPr(H)N}C{N(p-tBu-C6H4)}(NiPr)(CO2)
BCy2 (19), was detected by 1H NMR in the reaction of 2 with CO2
under the same conditions, with a conversion of ca. 9% after
15 h at room temperature (Scheme 7). Once more, this reaction
turned out to be in equilibrium, and increasing the temperature
up to 80 °C resulted in full CO2 de-insertion to give the parent
compound 2. In this sense, plausible evidence of the formation
of small amounts of insertion products (<5%) was also obtained
in the reactions of 4 towards CO2 (1 atm) after ca. 15 h at room
temperature, which also appear to be in equilibrium with the
de-insertion products rather shifted towards the latter. However,
these products could be neither isolated nor unambiguously
identified. The latter equilibria are not surprising since Stephan
and Erker found similar reversible temperature-dependent CO2
binding in P/B intra- and intermolecular FLP systems.18
Fig. 6 Molecular structure of compound 16a: H atoms are omitted for
clarity, except for those bound to C1 and N3, and ORTEP ellipsoids are
plotted at the 50% probability level.
Table 4 Selected bond lengths (Å) and angles (°) for compound 16a
B–O
O–C1
1.474(7)
1.382(6)
1.495(6)
1.379(6)
1.308(6)
1.677(6)
1.355(7)
1.505(6)
1.450(6)
B–O–C1
O–C1–N1
C1–N1–C2
N1–C2–N2
N1–C2–N3
N2–C2–N3
C2–N2–B
112.6(4)
109.5(4)
110.1(4)
120.0(4)
119.1(4)
120.9(5)
124.5(4)
102.5(4)
56.4
C1–N1
N1–C2
C2–N2
N2–B
C2–N3
N1–C3
N2–C15
N2–B–O
N1–C2–N2/O–C1–N1a
a Torsion between the planes.
As already commented for the 1,1-insertion reactions, the
1,2-insertion reactions seem to follow a concerted mechanism
from an open-chain form of these boron guanidinates, accord-
ing to the DFT studies of CO2 additions to related intra-
molecular N/B14 or P/B18 FLPs. This implies the concomitant
formation of N/P–C and B–O bonds. So it can be concluded
that increasing the donor capacity of the basic centre and/or
the acceptor capacity of the boron centre would lead to more
stable insertion products and, in general, more reactive
systems. Efforts towards this end are underway in our group,
using simple, straightforward synthetic methods for the prepa-
ration of new boron amidinates and guanidinates from in-
expensive reagents, circumventing the use of C6F5 groups on
boron. We must note that recent reports prove the validity of
this approach, as in the H2 activation by an “inverse” FLP con-
sisting of a bulky organic superbase and a moderate-to weak
boron-based Lewis acid,22 or the FLP-reactivity of an intra-
molecular BCy2-based compound obtained by 1,1-hydrobora-
tion of an isonitrile.23
crystallised in the tetragonal space group P43. As can be seen,
the molecular structure is the expected result of the 1,2-inser-
tion of the double CvO bond in the (iPr)N–B bond to form a
non-planar six-membered heterocycle. Thus, two new single
N–C and O–B bonds have been formed, as denoted by the
N1–C1 and B–O distances of 1.495(6) and 1.474(7) Å, respec-
tively, whereas the C1–O distance, 1.382(6) Å, is now also con-
sistent with the formulation of a single endocyclic C–O bond.
All these bond distances are reminiscent of those found for
the related complex [HC(NtBu2)2(PhCOH)B(C6F5)2] reported by
Stephan.8
The puckered conformation of the six-membered hetero-
cyclic ring is denoted by the sum of the internal angles, ca.
680°, far from the theoretical 720° of a planar hexagon. This
could be described more accurately as a somewhat distorted
half-chair configuration in which the C1 atom is puckered out
of the pseudo-planar N1–C2–N2–B–O skeleton (N1–C2–N2–B
and C2–N2–B–O torsion angles of only 14.4(7) and 3.9(6)°,
respectively). This is corroborated by the wide torsion angle
Structural characterisation of compounds 15–18
The solid-state structure of compound 16a was determined by between the N1–C2–N2/O–C1–N1 planes of 56.4°. The C2–N2
an X-ray diffraction analysis (Fig. 6, Table 4). This compound distance, of 1.308(6) Å, is consistent with the formulation of a
Dalton Trans.
This journal is © The Royal Society of Chemistry 2017

View Article Online
Dalton Transactions
Paper
double CvN bond, although there is still some π delocalisa- 17 and 18. Moreover, the asymmetry of these molecules origi-
i
tion within the other C–N bonds of the guanidine “CN3” core, nated three different signals for each Pr group: one for the
as indicated by C2–N1 and C2–N3 distances, of 1.379(6) and methyne, CH, in the expected region, ca. 3–4 ppm, and two for
1.355(7) Å, intermediate between single and double C–N the methyl groups, ca. 1.4–0.4 ppm, detected in all instances,
bonds. This observation goes in line with the sum of bond except for the CH3 groups of the minor isomer 16b, which
angles about the N atoms: ca. 360° for N2, suggesting a sp2 were masked by the signals of the major isomer.
hybridisation, but ca. 346° and 354° for N1 and N3, respect-
ively, indicating some degree of pyramidalisation.
In the case of the reaction of 2 with CO2 to give the inser-
tion product 19, despite its low conversion, ca. 9%, a new set
Despite the fact that compound 16a slowly de-inserts benz- of signals could be easily assigned to the proposed product,
aldehyde in solution at room temperature, it could be satisfac- with an apparent C2-symmetry, like the parent guanidinate 2,
torily characterised by multinuclear NMR spectroscopy and most likely due to fast rotation of the exocyclic NHiPr group as
the spectra recorded are in quite good agreement with the well. As for the regioselectivity of this reaction, we propose that
solid-state structure commented above. Thus, the 11B NMR the CO2 insertion in the iPrN–B bond based on previous
presented a signature broad signal at 8.0 ppm, suggestive of a results commented in this paper indicates that this regio-
tetracoordinate boron. Additionally, the C1-symmetric struc- isomer is the most stable product in the 1,1- and 1,2-insertion
ture of the molecule of 16a gives rise to three distinct signals reactions of 2.
i
1
for each Pr group in the H NMR spectrum at room tempera-
ture (Table 2), a septet (CH) and two doublets (CH3) for the iPr
group bound to the endocyclic N atom, and a doublet of
Conclusions
i
septets (CH) and two doublets (CH3) for the Pr group bound
to the exocyclic N atom. Likewise, distinct signals were also We have studied the N–B insertion reactions of two boron
i
observed in the carbon-13 NMR spectrum for the Pr groups, guanidinates, (Me2N)C(NiPr)2BCy2 (1) and {iPr(H)N}C(NiPr)
except for two isochronous Me groups at 23.6 ppm, therefore {N(p-tBu-C6H4)}BCy2 (2), and two bisboron guanidinates (2–),
accounting for a total of five, instead of six signals. Signature {iPr(BCy2)N}C(NiPr){N(p-tBu-C6H4)}BCy2 (3) and {iPr(C8H14B)N}
singlets at 5.97 ppm (1H) and 82.7 ppm (13C) were attributed C(NiPr){N(p-Me-C6H4)}BC8H14 (4), all of them with (cyclo)alkyl
to the CH(Ph) of the inserted benzaldehyde, in the expected groups on boron, towards small unsaturated molecules.
range for similar products of benzaldehyde addition in intra- Compound 4 was prepared for the first time by deprotonation
molecular N/B8 and P/B24 FLPs. However, the presence of of a guanidine with a secondary borane, proving the feasibility
broad signals for the Cy groups in the 1H and 13C NMR of this atom-efficient synthetic route instead of the salt meta-
spectra, especially in the latter, in which only five signals were thesis route employed in the other cases. Compounds 1–4
detected out of the twelve expected, denoted again the exist- display a chelate coordination mode of the guanidinato ligand
ence of dynamic processes in solution. These observations to one boron centre, and only compound 1 displayed a fast
prompted us to carry out variable-temperature NMR experi- solvent-dependent equilibrium in solution between a minor
ments for 16a in CD2Cl2 in the temperature range 290–193 K monodentate isomer 1-M and the major chelate 1-C, relevant
(Fig. S7 in the ESI†). Thus, upon cooling down from 290 K, an from a mechanistic point of view to account for the N–B inser-
overall broadening of all the signals takes place in the 1H NMR tion reactions.
spectra, and at 193 K the splitting of some of the latter gives
1,1-Insertion of aromatic isonitriles, CNXyl and CN(p-
rise to two sets of signals attributed to two isomers (probably MeO-C6H4), takes place in all instances to give novel diazabor-
rotamers) of 16a. Thus, focusing on the singlet at 5.96 ppm, it oles, and in the case of the asymmetrical guanidinates 2–4, the
broadens progressively down to 228 K (coalescence tempera- insertion in the endocyclic (iPr)N–B bond is always favoured,
ture, Tc), before splitting up into two different sharp signals at probably because of the more donor character of this N centre.
193 K, at 5.97 and 5.83 ppm in a 4 to 1 ratio, respectively. Four different compounds were structurally characterised in
Likewise, the signal attributed to the p-tBu group at 1.32 ppm the solid state by X-ray diffraction. Of all the structures, only
at 290 K splits into two singlets at 1.27 and 1.23 ppm roughly that derived from 4, {iPr(BC8H14)N}C{N(p-Me-C6H4)}(NiPr)
in the same ratio at 193 K. Other signals seem to undergo (CNXyl)BC8H14 (11), displayed a puckered five-membered ring
similar processes, like those from aromatic protons or one of as opposed to a pseudo-planar one displayed by BCy2 deriva-
the CH-iPr moieties. The presence of two isomers at low temp- tives. DFT-optimised structures confirmed that the half-chair
eratures probably follows from “freezing” some of the bond arrangement for BC8H14-derived diazaboroles 11 and 12 has a
rotation processes (exocyclic C–NHiPr, N–aryl, etc.) typically steric origin induced by the clash of the boron bicyclic group
fast at room temperature in the NMR time scale.
on the neighbouring NAr and CvNAr moieties, a circumstance
As already stated before, the other benzaldehyde insertion which seems to be accompanied by a decreased stability of
1
products were mostly identified by the H NMR spectra of the these compounds. This seems to favour the fast temperature-
reaction crude and the structural proposal was safely assigned dependent equilibria found in solution between 11 and 12 and
based on the similarity to the fully-characterised compound the isonitrile de-insertion products, lying far towards the inser-
16a. For example, signature singlets ca. 6 ppm were assigned tion products at room temperature, but being increasingly dis-
in all cases to the HC(Ph) of the inserted aldehyde for 15, 16b, placed towards de-insertion upon increasing the temperature
This journal is © The Royal Society of Chemistry 2017
Dalton Trans.

View Article Online
Paper
Dalton Transactions
up to 80 °C. Reactions with CO were more sluggish in general, methyne signals of each isomer (1-C : 1-M = 89 : 11 in C6D6 and
1
but followed the same tendency found with isonitriles in 83 : 17 in CD2Cl2). Partial spectroscopic data for 1-M: H NMR
terms of regioselectivity and equilibria found for the (400 MHz, C6D6): δ 3.82 (sept, 1H, JHH = 6.7 Hz, CH-iPr), 3.49
1
carbonylation product of the BC8H14 derivative 4.
(sept, 1H, JHH = 6.0 Hz, CH-iPr), 2.67 (s, 6H, NMe2). H NMR
As for the 1,2-insertion reactions with benzaldehydes and (400 MHz, CD2Cl2): δ 3.88 (sept, 1H, JHH = 6.7 Hz, CH-iPr), 3.32
CO2, when they took place, all turned out to be in equilibria, (sept, 1H, JHH = 6.0 Hz, CH-iPr), 2.72 (s, 6H, NMe2). The rest of
often shifted towards the de-insertion products. Only one the signals of 1-M were masked by those of the major isomer
product could be isolated and fully characterised, this being 1-C, although cross-peaks between the CH–CH3 couples were
{iPr(H)N}C{N(p-tBu-C6H4)}(NiPr)(PhHCO)BCy2 (16a).
detected in 1H–1H 2D COSY experiments (see Fig. S8 and S9 in
From these results it can be finally concluded that 1,1- and the ESI†).
1,2-insertion reactions similar to the FLP-type reactivity
Preparation of {iPr(BC8H14)N}C{N(p-Me-C6H4)}(NiPr)BC8H14 (4)
described by Stephan and co-workers8 with boron amidinates
of the formula HC(NR)2B(C6F5)2 can also be obtained with
A
solution of the 9-borabicyclo[3.3.1]nonane dimer,
boron guanidinates straightforwardly prepared from affordable (H-BC8H14)2, (0.625 g, 2.56 mmol) in toluene (10 mL) was
commercial reagents in high yields, with less electron with- added to a solution of (p-Me-C6H4N)C(NHiPr)2 (0.586 g,
drawing groups on boron. The price to pay by reducing the 2.51 mmol) in toluene (10 mL). The resulting solution was
acceptor capacity in the boron centre seems to be a somewhat stirred for 4 h at room temperature, at which time the solvent
reduced reactivity of these systems, often detected in the form was removed under vacuum. The remaining solid was washed
of insertion/de-insertion equilibria, or precluded insertions with pentane (3 × 10 mL) and dried under vacuum to give com-
under mild conditions. In any event, we believe that increasing pound 4 as a white solid (0.700 g, 59%). Crystals of 4 suitable
the donor ability of the chelate N atoms of the guanidinato for an X-ray diffraction study were grown from a concentrated
ligands, often more basic than their amidinate counterparts solution of 4 in hexane kept at −20 °C. Anal. calcd for
due to the presence of an additional exocyclic NR2 group, and C30H49B2N3: C, 76.12; H, 10.43; N, 8.88. Found: C, 76.24;
additional fine-tuning of the steric crowding around the B–N H, 10.23; N, 8.71. 1H NMR (500 MHz, C6D6): δ 7.09, 6.93
bonds could expand the reactivity repertoire of these systems (AA′XX′, 4H, C6H4), 3.83 (sept, 1H, JHH = 6.7 Hz, CH-iPr), 3.54
moving away from the C6F5 groups on boron, ubiquitous so far (sept, 1H, JHH = 6.9 Hz, CH-iPr), 2.50–1.20 (m, 28H, 2 × C8H14),
in the FLP chemistry. Efforts towards this end are currently 2.04 (s, 3H, p-CH3-C6H4), 1.32 (d, 3H, JHH = 6.7 Hz, CH3-iPr),
underway in our laboratory.
1.23 (d, 3H, JHH = 6.7 Hz, CH3-iPr), 0.95 (d, 3H, JHH = 6.9 Hz,
CH3-iPr), 0.82 (d, 3H, JHH = 6.9 Hz, CH3-iPr). 13C{1H} NMR
(101 MHz, C6D6): δ 160.1 (s, CN3), 141.0 (s, ipso-C-C6H4),
135.0 (s, p-C-C6H4), 129.7, 127.9 (2 × s, o,m-C-C6H4), 53.0,
45.4 (2 × s, 2 × CH-iPr), 34.6, 34.3, 33.5, 33.4, 32.8, 32.6 (6 × s,
CH2-BC8H14), 25.9 (br, CH-BC8H14), 25.1 (s, CH2-BC8H14),
24.5 (br, CH-BC8H14), 24.4, 24.2 (2 × s, 2 × CH3-iPr), 24.1
(br, CH-BC8H14), 23.8 (s, CH3-iPr), 23.4 (s, CH2-BC8H14),
22.7 (s, CH3-iPr), 22.2 (s, CH2-BC8H14), 21.0 (s, p-CH3-C6H4).
11B NMR (128 MHz, tol-d8): δ 53.2 (br, Δν1/2 ca. 1100 Hz,
tricoordinate B), 12.6 (br, Δν1/2 ca. 400 Hz, tetracoordinate B).
Experimental
General procedures
All manipulations were carried out under dry nitrogen using
standard Schlenk and glovebox techniques. Solvents were dis-
tilled from appropriate drying agents and stored under N2 in
Schlenk tubes equipped with J. Young-type Teflon stoppers
and containing activated molecular sieves (4 Å). Microanalyses
were carried out with a LECO CHNS-932 analyser. ATR-FTIR
spectra were recorded on a Bruker Tensor 27 spectrophoto-
meter. NMR spectra were recorded on Varian FT-400 and Inova
FT-500 spectrometers using standard VARIAN-FT software.
Chemical shifts (δ) are given in ppm and coupling constants
(J) in Hz. All reagents were purchased from the usual com-
mercial suppliers, except for the guanidine (p-Me-C6H4N)
Preparation of (Me2N)C(NiPr)2(CNXyl)BCy2 (5)
Solid CNXyl (0.066 g, 0.50 mmol) was added to a solution of 1
(0.175 g, 0.50 mmol) in pentane (10 mL). A white suspension
containing compound 5 was formed after stirring for 5 min at
room temperature. The suspension was further stirred for 3 h
to ensure complete reaction. Then, the supernatant was dec-
anted and the solid was washed with cold pentane (2 × 10 mL)
and dried under vacuum to give compound 5 as a white solid
(0.120 g, 50%). Crystals of 5 suitable for X-ray diffraction were
grown from a concentrated solution of 5 in hexane kept at
−20 °C. Anal. calcd for C30H51BN4: C, 75.29; H, 10.74; N, 11.71.
C(NHiPr)2 and boron guanidinates (Me2N)C(NiPr)2BCy2 (1),
3a
{iPr(H)N}C(NiPr){(p-tBu-C6H4)}Cy2 (2) and {iPr(Cy2B)N}C(NiPr)
{N(p-tBu-C6H4)}BCy2 (3),6 which were prepared according to
literature procedures.
Solvent-dependent equilibrium between isomers 1-C and 1-M
Found: C, 74.85; H, 10.48; N, 11.64. IR (cm−1): ν 1638 (s,
1
Two samples containing compound 1 (0.024 g, 0.07 mmol CvN). H NMR (500 MHz, C6D6): δ 7.12 (d, 2H, JHH = 7.4 Hz,
each) were dissolved in ca. 0.8 mL of C6D6 and CD2Cl2, respec- m-C6H3), 6.94 (t, 1H, JHH = 7.4 Hz, p-C6H3), 3.57 (sept, 1H,
tively, charged into NMR tubes equipped with a J. Young valve JHH = 6.8 Hz, CH-iPr), 3.32 (sept, 1H, JHH = 6.8 Hz, CH-iPr), 2.46 (s,
and their 1H NMR spectra were immediately recorded. The 6H, Me2-Xyl), 2.08 (s, 6H, NMe2), 1.95–0.60 (m, 22H, Cy), 1.60
relative isomer ratio was thus measured integrating the (d, 6H, JHH = 6.8 Hz, CH3-iPr), 1.14 (d, br, 6H, JHH = 6.8 Hz,
Dalton Trans.
This journal is © The Royal Society of Chemistry 2017

View Article Online
Dalton Transactions
Paper
CH3-iPr). 13C{1H} NMR (101 MHz, C6D6): δ 182.6 (s, br, C = 298 K): δ 7.26, 7.18 (AA′XX′, 4H, C6H4), 7.06 (d, 2H, JHH
=
NXyl), 163.5 (s, CN3), 150.3 (s, ipso-C-Xyl), 128.1 (s, m-C-Xyl), 7.3 Hz, m-C6H3), 6.85 (t, 1H, JHH = 7.3 Hz, p-C6H3), 5.28 (br,
126.9 (s, br, o-C-Xyl), 121.4 (s, p-C-Xyl), 47.4, 47.3 (2 × s, 1H, N-CH-iPr, Δν1/2 ca. 75 Hz), 3.86 (d, 1H, JHH = 9.2 Hz, NH),
2 × CH-iPr), 41.0 (s, NMe2), 30.6, 29.8, 29.4, 28.4 (4 × s, 2.95 (dsept, 1H, JHH = 9.2, 6.4 Hz, NH-CH-iPr), 2.44 (s, 6H, Me2-
CH2-Cy), 22.6 (s, CH3-iPr), 20.6 (s, br, CH3-iPr), 20.0 (s, br, Xyl), 1.85–0.80 (m, 22H, Cy), 1.22 (br, 6H, CH3-iPr) 1.17 (s, 9H,
Me2-Xyl). 11B NMR (128 MHz, C6D6): δ 1.6 (br, Δν1/2 ca. 240 Hz, B). CH3-tBu), 0.63 (d, 6H, JHH = 6.4 Hz, NH-CH3-iPr). 1H NMR
(500 MHz, tol-d8, 253 K): δ 7.25, 7.16 (AA′XX′, 4H, C6H4), 7.11
Preparation of (Me2N)C(NiPr)2{CN(p-MeO-C6H4)}BCy2 (6)
(d, 2H, JHH = 7.3 Hz, m-C6H3), 6.91 (t, 1H, JHH = 7.3 Hz,
Compound 6 was prepared in an analogous manner to 5 using p-C6H3), 5.66 (br, 1H, N-CH-iPr, Δν1/2 ca. 160 Hz), 3.86 (br, 1H
compound 1 (0.105 g, 0.30 mmol) and CN(p-MeO-C6H4) NH), 2.92 (br, 1H, NH-CH-iPr), 2.49 (s, 6H, Me2-Xyl), 2.00–0.85
(0.040 g, 0.30 mmol). It was obtained as a white-orangish solid (m, 22H, Cy), 1.22 (br, 6H, CH3-iPr) 1.14 (s, 9H, CH3-tBu), 0.59
(0.097 g, 67%). Anal. calcd for C29H49BN4O: C, 72.48; H, 10.28; (d, br, 6H, JHH ca. 6.4 Hz, NH-CH3-iPr). The signal for the Me
i
N, 11.66. Found: C, 72.64; H, 10.06; N, 11.64. IR (cm−1): ν 1638 groups of one of the Pr groups was not detected, obscured by
1
(s, CvN). H NMR (500 MHz, C6D6): δ 6.99, 6.95 (AA′BB′, 4H, the signals owing to Cy groups. 13C{1H} NMR (101 MHz, C6D6):
C6H4), 3.59 (sept, 1H, JHH = 6.8 Hz, CH-iPr), 3.38 (s, 3H, OCH3), δ 177.6 (s, br, CvNXyl), 158.0 (s, CN3), 149.6, 149.5 (2 × s, p-C-
3.30 (sept, 1H, JHH = 6.8 Hz, CH-iPr), 2.08 (s, 6H, NMe2), C6H4 + ipso-C-Xyl), 140.0 (s, ipso-C-C6H4), 128.1 (s, m-C-Xyl),
2.0–0.7 (m, 22H, Cy), 1.60 (d, 6H, JHH = 6.8 Hz, CH3-iPr), 1.12 127.1 (s, o/m-C-C6H4), 126.6 (s, o-C-Xyl), 125.9 (s, m/o-C-C6H4),
(d, 6H JHH = 6.8 Hz, CH3-iPr). 13C{1H} NMR (126 MHz, C6D6): 121.2 (s, p-C-Xyl), 46.1 (s, NH-CH-iPr), 41.8 (br, N-CH-iPr), 34.5
δ 182.8 (s, br, CvNAr), 163.5 (s, CN3), 155.5 (s, p-C-Ar), (s, C-tBu), 31.4 (s, CH3-tBu), 30.8 (s, br, CH-Cy), 29.8, 29.5, 29.4,
146.1 (s, ipso-C-Ar), 121.9 (s, m-C-Ar), 113.7 (s, o-C-Ar), 55.1 28.5 (4 × s, 4 × br, CH2-Cy), 23.1 (s, NH-CH3-iPr), 21.3 (s, N-
(s, OCH3), 47.6, 47.5 (2 × s, 2 × CH-iPr), 40.9 (s, NMe2), 31.9 CH3-iPr), 20.1 (s, Me2-Xyl). 11B NMR (128 MHz, tol-d8): δ 0.6
(s, CH-Cy), 31.3, 30.0, 30.0, 29.9, 28.6 (5 × s, CH2-Cy), 22.4, 20.0 (br, Δν1/2 ca. 600 Hz, B). Partial spectroscopic data for 7b:
(2 × s, 2 × CH3-iPr). 11B NMR (128 MHz, C6D6): δ 1.6 (br, Δν1/2 1H NMR (500 MHz, C6D6): δ 3.82 (sept, 1H, JHH = 7.2 Hz,
ca. 340 Hz, B).
N-CH-iPr), 3.55 (d, 1H, JHH = 8.9 Hz, NH), 2.74 (dsept, 1H,
JHH = 8.9, 6.4 Hz, NH-CH-iPr), 2.07 (s, 6H, Me2-Xyl), 1.14 (d,
6H, JHH = 7.2 Hz, N-CH3-iPr), 1.12 (s, 9H, CH3-tBu), 0.52 (d, 6H,
Reaction of 2 with CNXyl
Method A: Compound 2 (0.045 g, 0.10 mmol) and CNXyl JHH = 6.4 Hz, NH-CH3-iPr).
(0.014 g, 0.010 mmol) were dissolved in ca. 0.8 mL of C6D6 and
transferred to an NMR tube equipped with a J. Young valve.
The reaction was monitored by H NMR, and after 2 h, com- Method A: Compound
Reaction of 2 with CN(p-MeO-C6H4)
1
2 (0.045 g, 0.10 mmol) and
plete transformation of 2 into a mixture of the insertion com- CN(p-MeO-C6H4) (0.014 g, 0.10 mmol) were dissolved in
pounds {iPr(H)N}C{N(p-tBu-C6H4)}(NiPr)(CNXyl)BCy2 (7a) and ca. 0.8 mL of C6D6 and transferred to an NMR tube
{iPr(H)N}C(NiPr){N(p-tBu-C6H4)}(CNXyl)BCy2 (7b) in
a 6 : 1 equipped with a J. Young valve. The reaction was monitored by
ratio was observed. Only compound 7a could be isolated as a 1H NMR, and after 2 h at room temperature, complete trans-
white crystalline solid after removal of the solvent under formation of 2 into a mixture of the insertion compounds
vacuum and crystallisation in hexane at −20 °C. Method B: {iPr(H)N}C{N(p-tBu-C6H4)}(NiPr){CN(p-MeO-C6H4)}BCy2 (8a) and
Alternatively, compound 7a could be isolated in higher quan- {iPr(H)N}C(NiPr){N(p-tBu-C6H4)}{CN(p-MeO-C6H4)}BCy2 (8b) in
tities after stirring a yellow pentane solution (5 mL) containing a 3 : 1 ratio was observed. Only 8a could be isolated as a white-
2 (0.092 g, 0.20 mmol) and CNXyl (0.027 g, 0.21 mmol) for 4 h. yellowish crystalline solid after crystallisation in a concentrated
Storage of the latter solution at −20 °C yielded 7a as a white solution in pentane of the reaction mixture kept at −20 °C
crystalline solid (0.045 g, 39%). Anal. calcd for C38H59BN4 (7a): (0.020 g, 34%). These crystals were also suitable for an X-ray
C, 78.32; H, 10.21; N, 9.61. Found: C, 78.12; H, 10.31; N, 9.69. diffraction analysis. Method B: Alternatively, 8a can be pre-
Spectroscopic data for 7a: IR (cm−1): ν 3453 (w, N–H), 1649 (s, pared in a 0.20 mmol scale with similar yields by the reaction
CvN). 1H NMR (500 MHz, C6D6): δ 7.31, 7.23 (AA′XX′, 4H, of 2 and CN(p-MeO-C6H4) in pentane, isolated as well by
C6H4), 7.14 (d, 2H, JHH = 7.4 Hz, m-C6H3), 6.94 (t, 1H, JHH
7.4 Hz, p-C6H3), 5.25 (br, 1H, N-CH-iPr), 3.86 (d, 1H, JHH
=
=
crystallisation at −20 °C. Anal. calcd for C37H57BN4O (8a):
C, 76.01; H, 9.83; N, 9.58. Found: C, 75.72; H, 9.70; N, 9.39.
9.2 Hz, NH), 2.97 (dsept, 1H, JHH = 9.2, 6.3 Hz, NH-CH-iPr), 2.51 Spectroscopic data for 8a: IR (cm−1): ν 3346 (w, N–H), 1638 (s,
(s, 6H, Me2-Xyl), 1.95–0.85 (m, 22H, Cy), 1.22 (br, 6H, CH3-iPr) CvN). 1H NMR (400 MHz, C6D6): δ 7.28, 7.23 {AA′XX′, 4H,
1.17 (s, 9H, CH3-tBu), 0.60 (d, 6H, JHH = 6.3 Hz, NH-CH3-iPr). C6H4(tBu)}, 7.15, 6.98 {AA′XX′, 4H, C6H4(OMe)}, 5.20 (br, 1H,
1H NMR (500 MHz, tol-d8, 353 K): δ 7.26, 7.18 (AA′XX′, 4H, N-CH-iPr), 3.79 (d, 1H, JHH = 9.4 Hz, NH), 3.38 (s, 3H, OCH3),
C6H4), 7.00 (d, 2H, JHH = 7.5 Hz, m-C6H3), 6.79 (t, 1H, JHH
=
2.96 (dsept, 1H, JHH = 9.4, 6.4 Hz, NH-CH-iPr), 2.0–0.9 (m, 22H,
7.5 Hz, p-C6H3), 5.23 (sept, br, JHH ca. 7.0 Hz, 1H, N-CH-iPr), Cy), 1.33 (d, br, 6H, JHH = 6.8 Hz, CH3-iPr), 1.16 (s, 9H,
3.88 (d, 1H, JHH = 9.2 Hz, NH), 2.99 (dsept, 1H, JHH = 9.2, CH3-tBu), 0.61 (d, JHH = 6.4 Hz, CH3-iPr). 13C{1H} NMR
6.4 Hz, NH-CH-iPr), 2.38 (s, 6H, 2,6-Me2-X), 1.80–0.75 (m, 22H, Cy), (101 MHz, C6D6): δ 180.0 {s, br, CvN-(p-MeO-C6H4)}, 158.2 (s,
1.25 (d, 6H, JHH = 7.2 Hz, CH3-iPr), 1.16 (s, 9H, CH3-tBu), 0.68 CN3), 155.7 {s, p-C-C6H4(OMe)}, 149.9 {s, p-C-C6H4(tBu)}, 145.8
1
(d, 6H, JHH = 6.4 Hz, NH-CH3-iPr). H NMR (500 MHz, tol-d8, {s, ipso-C-C6H4(OMe)}, 140.1 {s, ipso-C-C6H4(tBu)}, 127.8, 126.0
This journal is © The Royal Society of Chemistry 2017
Dalton Trans.

View Article Online
Paper
Dalton Transactions
{2 × s, o,m-C-C6H4(tBu)}, 122.2 {s, m-C-C6H4(OMe)}, 113.8 {s, o- 2 × 3H, 2 × CH3-iPr). Broad signals at 3.22 (CH-iPr), 2.70 and
C-C6H4(OMe)}, 55.1 (s, OCH3), 46.3 (s, NH-CH-iPr), 42.9 (br, N- 2.62 ppm are attributed to a minor rotamer (ratio minor/major
CH-iPr), 34.5(s, C-tBu), 32.1 (br, CH-Cy), 31.4 (s, CH3-tBu), 30.3, rotamer ca. 1 : 4 at 193 K). 13C{1H} NMR (101 MHz, C6D6):
30.1, 30.0, 29.9, 28.7 (5 × s, CH2-Cy), 23.2 (s, NH-CH3-iPr), 21.1 δ 181.7 (s, br, CvNXyl), 164.9 (s, CN3), 150.4, 149.7 (2 × s, p-C-
(s, N-CH3-tiPr). 11B NMR (128 MHz, C6D6): δ 0.7 (br, Δν1/2 ca. C6H4 + ipso-C-Xyl), 138.9 (s, ipso-C-C6H4), 128.3 (s, o,m-C-
580 Hz, B). Partial spectroscopic data for 8b: 1H NMR C6H4), 127.9 (s, m-C-Xyl), 125.5 (s, m,o-C-C6H4), 121.7 (s, p-C-
(500 MHz, C6D6): δ 7.35–7.25 {AA′XX′, 4H, C6H4(tBu)}, 6.92, Xyl), 55.3 (br, CH-iPr), 48.9 (s, CH-iPr), 34.9–27.2 (Cy), 34.5 (s,
6.83 {AA′XX′, 4H, C6H4(OMe)}, 3.78 (sept, 1H, JHH = 7.2 Hz, C-tBu), 31.3 (s, CH3-tBu), 24.5, 24.0, 22.0 (3 × br, CH3-iPr), 20.8,
N-CH-iPr), 3.69 (d, 1H, JHH = 9.2 Hz, NH), 3.30 (s, 3H, OCH3), 19.7 (2 × br). 11B NMR (128 MHz, tol-d8): δ 49.5 (br, Δν1/2 ca.
2.82 (dsept, 1H, JHH = 9.2, 6.4 Hz, NH-CH-iPr), 1.13 (s, 9H, 500 Hz, tricoordinate B), 1.5 (br, Δν1/2 ca. 600 Hz, tetracoordi-
CH3-tBu), 1.12 (d, 6H, JHH = 7.2 Hz, CH3-iPr), 0.61 (d, JHH = 6.4 nate B).
Hz, CH3-iPr).
Preparation of {iPr(BCy2)N}C{N(p-tBu-C6H4)}(NiPr){CN(p-
Preparation of {iPr(BCy2)N}C{N(p-tBu-C6H4)}(NiPr)(CNXyl)BCy2 MeO-C6H4)}BCy2 (10)
(9)
Compound 10 was prepared in a similar manner to 9, employ-
Neat CNXyl (0.014 g, 0.10 mmol) was added to a suspension of ing compound 3 (0.090 g, 0.14 mmol), CN(p-MeO-C6H4)
compound 3 (0.063 g, 0.10 mmol) in hexane (5 mL). The (0.022 g, 0.16 mmol) and hexane (4 mL). Compound 10 was
mixture was stirred for 2 days at room temperature and a white thus obtained as a white solid (0.065 g, 60%) after stirring for
suspension containing 9 was finally obtained. After decanting 4 days at room temperature and a similar workup to that used
the mother liquor, washing with more cold hexane (2 × 5 mL), for compound 9. Crystals of 10 suitable for an X-ray diffraction
and drying under vacuum, compound 9 was obtained as a analysis were grown from a concentrated solution of 10 in
white solid (0.050 g, 66%). Anal. calcd for C50H80B2N4: C, hexane at −20 °C. Anal. calcd for C49H78B2N4O: C, 77.36; H,
79.14; H, 10.63; N, 7.38. Found: C, 79.13; H, 10.39; N, 7.07. IR 10.33; N, 7.36. Found: C, 77.47; H, 10.15; N, 7.17. IR (cm−1):
(cm−1): ν 1634 (m, CvN). 1H NMR (400 MHz, C6D6): δ 7.33, ν 1652 (m, CvN). 1H NMR (400 MHz, C6D6): δ 7.23 {s, 4H,
7.26 (AA′XX′, 4H, C6H4), 7.16–7.10 (m, 2H, m-C6H3), 6.95 (t, JHH C6H4(tBu)}, 7.08, 6.99 {AA′XX′, 4H, C6H4(OMe)}, 4.07 (sept, 1H,
= 7.4 Hz, 1H, p-C6H3), 4.10 (sept, 1H, JHH = 6.7 Hz, CH-iPr), JHH = 6.7 Hz, CH-iPr), 3.46 (m, 1H, JHH = 6.9, 6.6 Hz, CH-iPr),
3.40 (br, 1H, CH-iPr), 2.59, 2.52 (2 × s, 2 × 3H, Me2-Xyl), 3.39, (s, 3H, OCH3), 2.10–0.35 (m, 44H, Cy), 1.78, 1.77 (2 × d,
2.20–0.50 (m, 44H, Cy), 1.79, 1.75 (2 × d, 2 × 3H, JHH = 6.7 Hz, 2 × 3H, JHH = 6.7 Hz, 2 × CH3-iPr), ca. 1.25 (d, br, 3H, JHH
2 × CH3-iPr), ca. 1.2 (vbr, 3H, CH3-iPr), 1.11 (s, 9H, CH3-tBu), ca. 6.9 Hz, CH3-iPr), 1.12 (s, 9H, CH3-tBu), 0.92 (d, br, 3H, JHH
=
=
0.9 (vbr, 3H, CH3-iPr). 1H NMR (500 MHz, tol-d8, 353 K): δ 7.28, 6.6 Hz, CH3-iPr). 13C{1H} NMR (101 MHz, C6D6): δ 181.9 {s, br,
7.25 (AA′XX′, 4H, C6H4), 7.03–6.99 (m, 2H, m-C6H3), 6.81 (t, 1H, CvN-(p-MeO-C6H4)}, 164.5 (s, CN3), 155.9 {s, p-C-C6H4(OMe)},
JHH = 7.5 Hz, p-C6H3), 4.12 (sept, 1H, JHH = 6.7 Hz, CH-iPr), 150.5 {s, p-C-C6H4(tBu)}, 145.5 {s, ipso-C-C6H4(OMe)}, 139.0 {s,
3.49 (≈sept, 1H, JHH ≈ 6.8 Hz, CH-iPr), 2.49, 2.44 (2 × s, 2 × 3H, ipso-C-C6H4(tBu)}, 128.5, 125.4 {2 × s, o,m-C-C6H4(tBu)}, 121.8
Me2-Xyl), 2.10–0.65 (m, 44H, Cy), 1.79, 1.74 (2 × d, 2 × 3H, {s, m-C-C6H4(OMe)}, 113.8 {s, o-C-C6H4(OMe)}, 55.2 (s, OCH3),
JHH = 6.7 Hz, CH3-iPr), 1.25 (d, 3H, JHH = 6.7 Hz, CH3-iPr), 1.12 54.3 (br, CH-iPr), 49.3 (s, CH-iPr), 34.8 (s, CH-Cy), 34.5 (s,
1
(s, 9H, CH3-tBu), 0.96 (d, JHH = 6.4 Hz, 3H, CH3-iPr). H NMR C-tBu), 33.6, 32.9, 32.1 (3 × s, 3 × CH-Cy), 31.4 (s, CH3-tBu),
(500 MHz, tol-d8, 298 K): δ 7.29, 7.24 (AA′XX′, 4H, C6H4), 30.6–27.3 (CH2-Cy), 25.2, 23.5 (2 × s, 2 × br, 2 × CH3-iPr), 21.7,
7.09–7.03 (m, 2H, m-C6H3), 6.87 (t, JHH = 7.5 Hz, 1H, p-C6H3), 21.2 (2 × s, 2 × CH3-iPr). 11B NMR (128 MHz, C6D6): δ ca. 51
4.10 (sept, 1H, JHH = 6.7 Hz, CH-iPr), 3.40 (br, Δν1/2 ca. 50 Hz, (vbr, tricoordinate B), ca. −2 (vbr, tetracoordinate B).
1H, CH-iPr), 2.55, 2.49 (2 × s, 2 × br, 2 × 3H, Me2-Xyl),
Preparation of {iPr(BC8H14)N}C{N(p-Me-C6H4)}(NiPr)(CNXyl)
2.30–0.40 (m, 44H, Cy), 1.79, 1.74 (2 × d, 2 × 3H, JHH = 6.7 Hz,
BC8H14 (11)
CH3-iPr), 1.10 (s, 9H, CH3-tBu). Broad signals attributed to
CH3-iPr could not be detected due to overlapping with Cy A yellow hexane suspension (4 mL) containing compound 4
signals. H NMR (500 MHz, tol-d8, 233 K): δ 7.40–7.20 (m, br, (0.095 g, 0.20 mmol) and CNXyl (0.030 g, 0.22 mmol) was
1
4H, C6H4), 7.17–7.09 (m, 2H, m-C6H3), 6.97 (t, JHH = 7.5 Hz, stirred for 2 h at room temperature to yield a suspension con-
1H, p-C6H3), 4.05 (sept, br, 1H, JHH ca. 6.5 Hz, CH-iPr), 3.22 (br, taining compound 11. After a similar workup to that of com-
Δν1/2 ca. 30 Hz, 1H, CH-iPr), 2.67, 2.51 (2 × s, 2 × br, 2 × 3H, pound 9, compound 11 was obtained as a light yellow solid
Me2-Xyl), 2.40–0.40 (m, 44H, Cy), 1.77, 1.73 (2 × d, JHH ca. 6.5 (0.106 g, 88%). Crystals of 11 suitable for an X-ray diffraction
Hz, 2 × CH3-iPr), 1.06 (s, 9H, CH3-tBu), 1.05 (d, br, 3H, CH3-iPr), analysis were grown from a concentrated solution of 11 in
0.77 (d, br, 3H, JHH ca. 6 Hz, CH3-iPr). Signals at 2.68 and 2.49 hexane at −20 °C. Anal. calcd for C39H58B2N4: C, 77.49; H,
(Me2-Xyl) and 1.09 (CH3-tBu) are attributed to a minor rotamer 9.67; N, 9.27. Found: C, 77.11; H, 9.59; N, 9.08. IR (cm−1):
of this compound (ratio minor/major rotamer ca. 1 : 3 at ν 1648 (m, CvN). 1H NMR (500 MHz, C6D6, 298 K): δ 7.27–6.85
1
233 K). H NMR (500 MHz, tol-d8, 193 K): δ 7.45–6.90 (m, br, (m, 7H, Ar), 4.15 (sept, 1H, JHH = 6.7 Hz, CH-iPr), 2.97 (sept,
7H, Ar), 4.02, 3.12 (2 × br, 2 × 1H, 2 × CH-iPr), 2.73, 2.55 (2 × 1H, JHH = 6.7 Hz, CH-iPr), 2.54, 2.52 (2 × s, 2 × 3H, Me2-Xyl),
br, 2 × 3H, Me2-Xyl), 2.50–0.50 (m, 44H, Cy), 1.76, 1.74 (2 × br, 2.00 (s, 3H, p-Me-C6H4), 2.25–1.10 (m, 28H, 2 × C8H14), 1.79,
2 × 3H, 2 × CH3-iPr), 1.05 (s, 9H, CH3-tBu), 1.00, 0.74 (2 × br, 1.72, 0.95, 0.41 (4 × d, 4 × 3H, JHH = 6.7 Hz, 4 × CH3-iPr).
Dalton Trans.
This journal is © The Royal Society of Chemistry 2017

View Article Online
Dalton Transactions
Paper
1H NMR (500 MHz, CD2Cl2, 253 K): δ 7.20–7.08 (m, 3H, C6H4), liquor was decanted and the solid was washed with cold
6.99–6.93 (m, 2H, m-C6H3), 6.92 (m, 1H, C6H4), 6.75 (t, 1H, hexane (2 × 5 mL). After drying under vacuum, compound 13
JHH = 7.4 Hz, p-C6H3), 4.22 (sept, 1H, JHH = 6.7 Hz, CH-iPr), was obtained as a white solid (0.030 g, 23%). Anal. calcd for
3.21 (sept, 1H, JHH = 6.7 Hz, CH-iPr), 2.34 (s, 3H, p-Me-C6H4), C30H50BN3O: C, 75.14; H, 10.51; N, 8.76. Found: C, 75.21; H,
2.26 (s, 6H, Me2-Xyl), 2.20–0.60 (m, 28H, 2 × C8H14), 1.74 (d, 10.20; N, 8.59. IR (cm−1): ν 3294 (m, br, N–H), 1674 (s, CvO).
3H, JHH = 6.7 Hz, CH3-iPr), 1.69 (d, 3H, JHH = 6.7 Hz, CH3-iPr), 1H NMR (400 MHz, C6D6): δ 7.26, 7.23 (AA′BB′, 4H, p-tBu-
1.24 (d, 3H, JHH = 6.9 Hz, CH3-iPr), 0.50 (d, 3H, JHH = 6.7 Hz, C6H4), 4.59 (br, 1H, N-CH-iPr), 4.05 (d, 1H, JHH = 9.3 Hz, NH),
CH3-iPr). 13C{1H} NMR (126 MHz, CD2Cl2, 253 K): δ 188.0 (s, 3.06 (dsept, 1H, JHH = 9.3, 6.4 Hz, NH-CH-iPr), 2–0–0.9 (m,
br, CvNXyl), 162.7 (s, CN3), 149.2 (s, ipso-C-Xyl), 139.8 (s, ipso- 22H, Cy), 1.19 (d, br, 6H, JHH ca. 7 Hz, CH3-iPr), 1.18 (s, 9H,
1
C-C6H4), 137.0 (s, p-C-C6H4), 129.6, 129.5 (2 × s, o,m-C-C6H4), CH3-tBu), 0.61 (d, JHH = 6.4 Hz, CH3-iPr). H NMR (400 MHz,
128.1 (s, m-C-Xyl), 127.9 (s, o,m-C-C6H4), 127.7 (s, m-C-Xyl), C6D6, 333 K): δ 7.25, 7.25 (AA′BB′, 4H, p-tBu-C6H4), 4.56 (sept,
127.3 (s, o,m-C-C6H4), 127.2 (s, o-C-Xyl), 126.1 (s, o-C-Xyl), JHH ca. 7.0 Hz, 1H, N-CH-iPr), 3.94 (d, 1H, JHH = 9.3 Hz, NH),
121.1 (s, p-C-Xyl), 54.4, 48.2 (2 × s, 2 × CH-iPr), 34.4, 33.5, 33.1, 3.08 (dsept, 1H, JHH = 9.3, 6.4 Hz, NH-CH-iPr), 2–0–0.9 (m,
32.1, 32.0, 32.0, 31.4, 30.6 (8 × s, CH2-BC8H14), 29.3, 24.8 (2 × 22H, Cy), 1.20 (d, 6H, JHH ca. 7 Hz, CH3-iPr), 1.19 (s, 9H,
br, CH-BC8H14), 24.4 (s, CH2-BC8H14), 24.3 (br, CH-BC8H14), CH3-tBu), 0.63 (d, JHH = 6.4 Hz, CH3-iPr). 13C{1H} NMR
24.3 (s, CH3-iPr), 24.0 (s, CH2-BC8H14), 24.0 (s, CH3-iPr), 23.2 (126 MHz, C6D6): δ 203.2 (s, br, CvO), 158.2 (s, CN3), 149.7 (s,
(s, CH2-BC8H14), 22.5 (s, CH3-iPr), 22.3 (s, CH2-BC8H14), 21.0 (s, p-C-C6H4), 139.8 (s, ipso-C-C6H4), 126.4, 126.1 (2 × s, o,m-C-
p-CH3-C6H4), 20.7 (br, CH-BC8H14), 20.2 (s, CH3-iPr), 19.5, 19.4 C6H4), 46.5 (s, N-CH-iPr), 41.0 (br, NH-CH-iPr), 34.6 (s, C-tBu),
(2 × s, Me2-Xyl). 11B NMR (128 MHz, CD2Cl2, 298 K): δ 56.3 (br, 31.4 (s, CH3-tBu), 30.8 (br, CH-Cy), 30.1, 29.8, 29.8, 29.6, 28.5
Δν1/2 ca. 1200 Hz, tricoordinate B), ca. 2.5 (br, Δν1/2 ca. 400 Hz, (5 × s, CH2-Cy), 23.1 (s, NH-CH3-iPr), 21.2 (s, N-CH3-iPr).
tetracoordinate B).
11B NMR (128 MHz, C6D6): δ 2.1 (br, Δν1/2 ca. 540 Hz, B).
Preparation of {iPr(BC8H14)N}C{N(p-Me-C6H4)}(NiPr){CN(p-
Reaction of compound 4 with CO
MeO-C6H4)}BC8H14 (12)
Compound 4 (0.030 g, 0.06 mmol) was dissolved in ca. 0.8 mL
Compound 12 was prepared in a similar manner to that of 11, of C6D6 and charged into an NMR tube equipped with a
from compound 4 (0.095 g, 0.20 mmol) and CN(p-MeO-C6H4) J. Young valve. Then, CO (ca. 1 atm) was added through the
(0.030 g, 0.22 mmol), and isolated as a pale yellow solid freeze–pump–thaw procedure. Monitoring of the reaction by
(0.100 g, 83%). Anal. calcd for C38H56B2N4O: C, 75.25; H, 9.31; 1H NMR revealed that an equilibrium was already reached
N, 9.24. Found: C, 74.94; H, 9.25; N, 9.10. IR (cm−1): ν 1633 (m, after 15 min between 4 and the insertion product {iPr(BC8H14)
CvN). 1H NMR (400 MHz, C6D6): δ 7.26 {m, 1H, o,m- N}C{N(p-Me-C6H4)}(NiPr)(CO)BC8H14 (14) with a ratio ca. 9 : 1.
C6H4(Me)}, 7.10–6.90 (m, 7H, Ar), 4.10 (sept, J = 6.7 Hz, 1H), Partial spectroscopic data for 14: 1H NMR (500 MHz, C6D6):
3.39 (s, 3H, OCH3), 2.87 (sept, 1H, JHH = 6.7 Hz, CH-iPr), δ 3.74 (sept, 1H, JHH = 6.7 Hz, CH-iPr), 3.31 (m, 1H, C8H14),
2.40–1.10 (m, 28H, 2 × C8H14), 2.01 (s, 3H, p-Me-C6H4), 1.78, 2.93 (sept, 1H, JHH = 6.7 Hz, CH-iPr), 1.97 (s, 3H, p-Me-C6H4),
1.71, 0.98, 0.35 (4 × d, 4 × 3H, JHH = 6.7 Hz, 4 × CH3-iPr). 13C 1.60, 1.53, 0.94, 0.40 (4 × d, 4 × 3H, JHH = 6.7 Hz, 4 × CH3-iPr).
{1H} NMR (101 MHz, C6D6): δ 185.4 (s, br, CvNAr′), 162.7 (s,
Reaction of compound 1 with benzaldehyde
CN3), 156.0 {s, p-C-C6H4(OMe)}, 145.9 {s, ipso-C-C6H4(OMe)},
140.7 {s, ipso-C-C6H4(Me)}, 136.9 {s, p-C-C6H4(Me)}, 129.6, Benzaldehyde (50 μL, 0.50 mmol) was added to a solution of
129.6, 128.7, 128.5 {4 × s, o,m-C-C6H4(Me)}, 121.2 {s, br, m-C- compound 1 (0.018 g, 0.05 mmol) in C6D6 (ca. 0.8 mL) and the
1
C6H4(OMe)}, 114.5 {s, o-C-C6H4(OMe)}, 55.2 (s, OCH3), 54.3, reaction was monitored by H NMR in an NMR tube equipped
48.6 2 (2 × s, 2 × CH-iPr), 34.5, 33.6, 33.5, 32.6, 32.4, 32.1, 31.9, with a J. Young valve. After 4 h at room temperature an equili-
31.5 (8 × s, CH2-BC8H14), 30.9, 25.1 (2 × br, CH-BC8H14), 25.0, brium mixture was reached between the insertion product
24.6 (2 × s, CH2-BC8H14), 24.5 (br, CH-BC8H14), 24.3, 23.7 (2 × (Me2N)C(NiPr)2(PhHCO)BCy2 (15), compound 1 and benz-
s, 2 × CH3-iPr) 23.2 (s, CH2-BC8H14), 22.8 (s, CH3-iPr), 22.6 (s, aldehyde, with an approximate ratio of 4 : 9 : 87. Partial spec-
1
CH2-BC8H14), 21.1 (br, CH-BC8H14), 20.9 (s, p-CH3-C6H4), 19.9 troscopic data for 15: H NMR (500 MHz, C6D6): δ 5.99 (s, 1H,
(s, CH3-iPr). 11B NMR (128 MHz, C6D6): δ 52.5 (br, Δν1/2 ca. H-CPh), 3.98 (sept, 1H, JHH ≈ 7 Hz, CH-iPr), 3.35 (sept, 1H,
1200 Hz, tricoordinate B), ca. 2.7 (br, Δν1/2 ca. 600 Hz, tetra- JHH ≈ 7 Hz, CH-iPr), 2.29 (s, 6H, NMe2), 1.19, 1.10, 0.94, 0.86
coordinate B).
(4 × d, 4 × 3H, JHH ≈ 7 Hz, 4 × CH3-iPr).
Preparation of {iPr(H)N}C{N(p-tBu-C6H4)}(NiPr)(CO)BCy2 (13)
Reaction of compound 2 with benzaldehyde
A toluene solution (10 mL) containing compound 2 (0.120 g, Method A: Benzaldehyde (15 μL, 0.15 mmol) was added to a
0.27 mmol) was charged into a glass vessel equipped with a solution of compound 2 (0.034 g, 0.075 mmol) in C6D6 (ca.
J. Young stopper, placed under a CO atmosphere (ca. 4 atm) by 0.8 mL) and the reaction was monitored by 1H NMR in an
the freeze–pump–thaw procedure, and stirred at 40 °C for 3 NMR tube equipped with a J. Young valve. After 48 h at room
days. Then, the solvent was removed under vacuum, and the temperature an equilibrium was reached between the insertion
remaining viscous solid was dissolved in hexane and kept at products {iPr(H)N}C{N(p-tBu-C6H4)}(NiPr)(PhHCO)BCy2 (16a)
−20 °C. A white precipitate was thus obtained; the mother and {iPr(H)N}C(NiPr){N(p-tBu-C6H4)}(PhHCO)BCy2 (16b) and
This journal is © The Royal Society of Chemistry 2017
Dalton Trans.

View Article Online
Paper
Dalton Transactions
the reagents, with relative ratios 16a : 16b : 2 : benzaldehyde ca. Reaction of compound 2 with CO2
21 : 3 : 17 : 60 at 25 °C. De-insertion is favoured at higher temp-
Compound 2 (0.030 g, 0.07 mmol) was dissolved in ca. 0.8 mL
of C6D6 and charged into an NMR tube equipped with a
J. Young valve. Then, CO2 (ca. 1 atm) was added through the
freeze–pump–thaw procedure and the reaction was monitored
eratures, and the product ratio changes to 11 : 2 : 28 : 59 upon
heating up to 60 °C. Method B: The major insertion product,
16a, could be isolated as a crystalline solid following this pro-
cedure. Benzaldehyde (100 μL, 0.98 mmol) was added to a
pentane solution (5 mL) of compound 2 (0.090 g, 0.20 mmol)
to form a white suspension which was stirred for 3 h. After
this time, pentane was removed under vacuum and ca. 0.5 mL
of pentane was added to the remaining viscous residue, con-
taining mostly benzaldehyde and a mixture of compounds
16a, 16b and 2. White crystals of 16a (0.071 g, 63%), which
were suitable for an X-ray analysis, were obtained after keeping
the latter mixture at −20 °C. Anal. calcd for C36H56BN3O (16a):
C, 77.54; H, 10.12; N, 7.54. Found: C, 77.19; H, 9.72; N, 7.29.
Spectroscopic data for 16a: IR (cm−1): ν 3389 (w, N–H).
1H NMR (500 MHz, CD2Cl2, 298 K): δ 7.61, 7.39 (AA′MM′X, 2 × 2H,
o,m-Ph), 7.38 (AA′XX′, 2H, o,m-C6H4), 7.31 (AA′MM′X, 1H,
p-Ph), 7.00 (AA′XX′, 2H, m,o-C6H4), 5.97 (s, 1H, H-CPh), 3.87
(sept, JHH = 6.8 Hz, 1H, N-CH-iPr), 3.63 (d, 1H, JHH = 9.4 Hz,
NH), 3.52 (m, 1H, JHH = 9.4, 6.4 Hz, NH-CH-iPr), 1.7–0.0 (m,
22H, Cy), 1.32 (s, 9H, CH3-tBu), 1.27 (d, br, 3H, JHH ≈ 7 Hz,
1
by H NMR. After 20 h an equilibrium is reached between the
insertion product {iPr(H)N}C{N(p-tBu-C6H4)}(NiPr)(CO2)BCy2
(19) and compound 2, in a ratio ca. 9 : 91. Partial spectroscopic
data for 19: 1H NMR (500 MHz, C6D6): δ 7.13, 7.01 (AA′XX′, 4H,
C6H4), 3.88 (sept, 1H, JHH = 6.7 Hz, N-CH-iPr), 3.44 (d, 1H,
JHH = 9.2 Hz, NH), 2.99 (dsept, 1H, JHH = 9.2, 6.3 Hz,
NH-CH-iPr), 1.56 (d, JHH = 6.7 Hz, CH3-iPr), 1.15 (s, 9H,
CH3-tBu), 0.47 (d, JHH = 6.3 Hz, CH3-iPr).
General procedure for thermodynamic measurements
In a typical procedure, C6D6 solutions (ca. 0.8 mL) containing
compound 11 or 12 and a known concentration of an internal
standard, tetrakis(trimethylsilyl)silane (TKS), were charged
into an NMR tube equipped with a J. Young valve. Then, 1H
NMR spectra for these solutions were recorded at different
temperatures, between 298 and 353 K and the concentrations
of 11/12 and 4 were calculated by measuring the integrals of
their methyne (CH) signals relative to that of the SiMe3 groups
of the internal standard (Fig. S2 and S4 in the ESI†).
CH3-iPr), 1.10 (d, 3H, JHH = 6.4 Hz, CH3-iPr), 1.05 (d, 3H, JHH
=
6.8 Hz, CH3-iPr), 0.94 (d, 3H, JHH = 6.4 Hz, CH3-iPr). 13C{1H}
NMR (126 MHz, CD2Cl2, 298 K): δ 160.5 (s, CN3), 150.2 (s, p-C-
C6H4), 142.0 (s, ipso-C-Ph), 141–0 (s, ipso-C-C6H4), 128.5 (s,
o/m-C-Ph), 127.9 (s, o/m-C-C6H4), 127.8 (s, p-C-Ph), 127.3 (s, m/o-
C-Ph), 126.1 (s, m/o-C-C6H4), 82.7 (s, HC-Ph), 53.8, 50.9 (2 × s,
2 × CH-iPr), 34.8 (s, C-tBu), 32.6 (br, CH-Cy), 31.5 (s, CH3-tBu),
30.7, 29.8, 29.6, 28.4 (4 × br, CH2-Cy), 23.6 (s, 2 × CH3-iPr),
22.5, 22.1 (2 × s, 2 × CH3-iPr). 11B NMR (128 MHz, CD2Cl2):
δ 8.0 (br, Δν1/2 ca. 400 Hz, B) ppm. Partial spectroscopic data
X-ray crystal determination
X-ray data collection of suitable single crystals was carried out
at 100(2) K on a Bruker VENTURE area detector equipped with
graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) by
applying the ω-scan method. The data reduction was per-
formed with the APEX225 software and corrected for absorp-
tion using SADABS.26 Crystal structures were solved by direct
methods using the SIR97 program27 and refined by full-matrix
least-squares on F2 including all reflections using anisotropic
displacement parameters by means of the WINGX crystallo-
graphic package.28 All hydrogen atoms were included as fixed
contributions riding on attached atoms with isotropic thermal
displacement parameters 1.2 times or 1.5 times those of their
parent atoms for the organic ligands. In compound 10, lattice
solvent molecules could not be refined owing to their dis-
ordered disposition in the voids of the structures, so the elec-
tron density at the voids corresponding to one hexane mole-
cule was subtracted from the reflection data by the SQUEEZE
procedure as implemented in the PLATON program29 during
the refinement. Details of the structure determination and
refinement of compounds 4, 5, 8a, 10, 11 and 16a are summar-
ised in Table 5. Crystallographic data (excluding structure
factors) for the structures reported in this paper have been de-
posited with the Cambridge Crystallographic Data Center as
supplementary publication numbers CCDC 1554010–1554015.
1
for 16b: H NMR (500 MHz, C6D6): δ 6.36 (s, 1H, H-CPh), 4.07
(sept, 1H, JHH ≈ 7 Hz, N-CH-iPr), 3.89 (d, 1H, JHH = 7.9 Hz,
NH), 3.14 (m, 1H, NH-CH-iPr).
Reaction of compound 4 with benzaldehyde
Benzaldehyde (60 μL, 0.60 mmol) was added to a solution of
compound 4 (0.030 g, 0.06 mmol) in C6D6 (ca. 0.8 mL) and the
1
reaction was monitored by H NMR in an NMR tube equipped
with a J. Young valve. A mixture consisting mostly of four com-
pounds: {iPr(BC8H14)N}C{N(p-Me-C6H4)}(NiPr)(PhHCO)BC8H14
(17), {iPr(H)N}C{N(p-Me-C6H4)}(NiPr)(PhHCO)BC8H14 (18),
benzaldehyde and compound 4 was detected after 15 min at
room temperature in a ratio ca. 6 : 3 : 86 : 5. Partial spectro-
scopic data for 17: 1H NMR (500 MHz, C6D6): δ 6.35 (s, 1H,
H-CPh), 4.27 (sept, 1H, JHH = 6.9 Hz, CH-iPr), 3.22 (sept, 1H,
JHH = 6.7 Hz, CH-iPr), 2.03 (s, 3H, p-Me-C6H4), 1.31 (d, JHH
6.9 Hz, CH3-iPr), 0.92 (d, JHH = 6.7 Hz, CH3-iPr), 0.48 (d, JHH
=
=
6.9 Hz, CH3-iPr), 0.40 (d, JHH = 6.7 Hz, CH3-iPr). Partial spectro-
scopic data for 18: 1H NMR (500 MHz, C6D6): δ 6.13 (s, 1H,
H-CPh), 3.85 (d, 1H, JHH = 8.7 Hz, NH), 3.76 (sept, 1H, JHH ≈ 7
Theoretical calculations
Hz, N-CH-iPr), 3.20 (m, 1H, NH-CH-iPr), 2.00 (s, 3H, p-Me- All DFT computations were carried out using the Gaussian09
C6H4), 1.22, 0.78, 0.59, 0.58 (4 × d, 4 × 3H, JHH ≈ 7 Hz, package,30 in which the hybrid method B3LYP was used with
CH3-iPr).
the Becke three-parameter exchange functional31 and the Lee–
Dalton Trans.
This journal is © The Royal Society of Chemistry 2017

View Article Online
Dalton Transactions
Paper
Table 5 Crystallographic data and structure refinement details for all compounds
Compound
4
5
8a
10
11
16a
Chem. form.
CCDC
Form. weight
C30H49B2N3
1554013
473.34
C30H50BN4
1554014
477.55
C37H57BN4O
1554015
584.67
C55H92B2N4O
1554010
846.94
C39H58B2N4
1554011
604.51
C36H56BN3O
1554012
557.64
Cryst. system
Space group
Triclinic
P1
Monoclinic
P21/c
Triclinic
P1
Triclinic
P1
Monoclinic
P21/n
Tetragonal
P43
ˉ
ˉ
ˉ
a (Å)
b (Å)
c (Å)
α (°)
9.858(3)
10.828(3)
14.100 (4)
89.37
71.462(53)
81.03
1408.2(6)
2
1.041
9.452(2)
29.608(3)
10.930(3)
90
112.215(4)
90
2831.7(11)
4
1.110
10.6730(7)
11.9049(9)
16.1199(11)
68.820(2)
85.544(2)
64.423(2)
1715.2(2)
2
11.4481(18)
14.078(2)
16.683(2)
69.089(3)
85.835(3)
75.449(4)
2430.7(6)
2
13.906(3)
11.690(3)
21.696(3)
90
92.946(4)
90
3522.3(12)
4
1.024
16.181(4)
16.181(3)
12.989(3)
90
90
90
3400.1(18)
4
1.078
β (°)
γ (°)
V (Å3)
Z
GOFa
1.042
1.073
Rint
0.0746
0.0621/0.1278
0.1118/0.1461
0.1313
0.1079/0.2586
0.1441/0.2840
0.0853
0.0521/0.1080
0.0924/0.1243
0.1071
0.0586/0.1420
0.1068/0.1629
0.0369
0.0521/0.1374
0.0619/0.1478
0.1418
0.0463/0.0946
0.0707/0.1043
R1 b/wR2 c [I > 2σ(I)]
R1 b/wR2 c (all data)
a S = [∑w(Fo − Fc2)2/(Nobs − Nparam)]1/2
.
b R1 = ∑||Fo| − |Fc||/∑|Fo|. c wR2 = [∑w(Fo − Fc2)2/∑wFo2 1/2; w = 1/[σ2(Fo2) + (aP)2 + bP] where P =
]
2
2
(max(Fo2, 0) + 2Fc2)/3.
Yang–Parr correlation functional.32 An accurate numerical
integration grid (99 590) was used for all the calculations via
the keyword Int = Ultrafine. The light elements (B, N, C and H)
were described with the 6-31G* basis.33 Geometry optimis-
ations used X-ray data of the compounds or related species as
the starting point and were performed under no symmetry
restrictions. Frequency analyses were performed at all the
stationary points to ensure that minimum structures with no
imaginary frequencies were achieved. Molecular orbitals and
vibrational modes were visualised using the Molekel
programme.34
( j) M. P. Coles, Chem. Commun., 2009, 3659–3676;
(k) F. T. Edelmann, Adv. Organomet. Chem., 2008, 57, 183–
352; (l) W. E. Piers and D. J. H. Emslie, Coord. Chem. Rev.,
2002, 233–234, 131–155; (m) P. J. Bailey and S. Pace, Coord.
Chem. Rev., 2001, 214, 91–141.
2 (a) R. Fernández-Galán, J.-A. Navarro, F. Carrillo-
Hermosilla, A. Antiñolo, I. López-Solera and E. Villaseñor,
Aust. J. Chem., 2014, 67, 1063; (b) D. Elorriaga, F. Carrillo-
Hermosilla, A. Antiñolo, I. López-Solera, R. Fernández-
Galán and E. Villaseñor, Dalton Trans., 2014, 43, 17434–
17444; (c) D. Elorriaga, F. Carrillo-Hermosilla, A. Antiñolo,
F. J. Suárez, I. López-Solera, R. Fernández-Galán and
E. Villaseñor, Dalton Trans., 2013, 42, 8223–8230;
(d) D. Elorriaga, F. Carrillo-Hermosilla, A. Antiñolo,
I. López-Solera, R. Fernández-Galán and E. Villaseñor,
Chem. Commun., 2013, 49, 8701–8703; (e) D. Elorriaga,
F. Carrillo-Hermosilla, A. Antiñolo, I. López-Solera,
R. Fernández-Galán, A. Serrano and E. Villaseñor,
Eur. J. Inorg. Chem., 2013, 2940–2946; (f) R. Fernández-
Galán, A. Antiñolo, F. Carrillo-Hermosilla, I. López-Solera,
A. Otero, A. Serrano-Laguna and E. Villaseñor,
Organometallics, 2012, 31, 8360–8369; (g) R. Fernández-
Galán, A. Antiñolo, F. Carrillo-Hermosilla, I. López-Solera,
A. Otero, A. Serrano-Laguna and E. Villaseñor,
J. Organomet. Chem., 2012, 711, 35–42; (h) D. Elorriaga,
F. Carrillo-Hermosilla, A. Antiñolo, I. López-Solera,
B. Menot, R. Fernández-Galán, E. Villaseñor and A. Otero,
Organometallics, 2012, 31, 1840–1848.
Acknowledgements
We gratefully acknowledge financial support from the
Ministerio de Economia y Competitividad (MINECO), Spain
(grant numbers CTQ2016-77614-R and CTQ2014-51912-
REDC). A. R. acknowledges a postdoctoral contract funded by
the “Plan Propio de I + D + i” of the Universidad de Castilla-La
Mancha, co-funded by the European Social Fund.
Notes and references
1 (a) T. Chlupatý and A. Růžička, Coord. Chem. Rev., 2016,
314, 103–113; (b) A. Kurek, P. G. Gordon, S. Karle, A. Devi
and S. T. Barry, Aust. J. Chem., 2014, 67, 989;
(c) F. T. Edelmann, Adv. Organomet. Chem., 2013, 61, 55–
374; (d) S. T. Barry, Coord. Chem. Rev., 2013, 257, 3192–
3201; (e) F. T. Edelmann, Chem. Soc. Rev., 2012, 41, 7657–
7672; (f) S. Collins, Coord. Chem. Rev., 2011, 255, 118–138;
(g) A. A. Trifonov, Coord. Chem. Rev., 2010, 254, 1327–1347;
(h) C. Jones, Coord. Chem. Rev., 2010, 254, 1273–1289;
(i) F. T. Edelmann, Chem. Soc. Rev., 2009, 38, 2253–2268;
3 (a) C. Alonso-Moreno, F. Carrillo-Hermosilla, A. Garcés,
A. Otero, I. López-Solera, A. M. Rodríguez and A. Antiñolo,
Organometallics, 2010, 29, 2789–2795; (b) J. Francos,
P. J. González-Liste, L. Menéndez-Rodríguez, P. Crochet,
V. Cadierno, J. Borge, A. Antiñolo, R. Fernández-Galán and
F. Carrillo-Hermosilla, Eur. J. Inorg. Chem., 2016, 2016,
393–402; (c) L. Menéndez-Rodríguez, E. Tomás-Mendivil,
This journal is © The Royal Society of Chemistry 2017
Dalton Trans.

View Article Online
Paper
Dalton Transactions
J. Francos, P. Crochet, V. Cadierno, A. Antiñolo, 16 A. C. McQuilken, Q. M. Dao, A. J. P. Cardenas, J. A. Bertke,
R.
Fernández-Galán
and
F.
Carrillo-Hermosilla,
S. Grimme and T. H. Warren, Angew. Chem., Int. Ed., 2016,
55, 14335–14339.
Organometallics, 2015, 34, 2796–2809.
4 M. De Tullio, A. Hernán-Gómez, Z. Livingstone, W. Clegg, 17 A. R. Cabrera, R. S. Rojas, M. Valderrama, P. Pluss,
A. R. Kennedy, R. W. Harrington, A. Antinolo, A. Martínez,
F. Carrillo-Hermosilla and E. Hevia, Chem. – Eur. J., 2016,
22, 17646–17656.
5 (a) D. Nieto, S. Bruña, A. M. González-Vadillo, J. Perles,
F. Carrillo-Hermosilla, A. Antiñolo, J. M. Padrón,
H. Berke, C. G. Daniliuc, G. Kehr and G. Erker, Dalton
Trans., 2015, 44, 19606–19614.
18 C. M. Momming, E. Otten, G. Kehr, R. Frohlich,
S. Grimme, D. W. Stephan and G. Erker, Angew. Chem., Int.
Ed., 2009, 48, 6643–6646.
G. B. Plata and I. Cuadrado, Organometallics, 2015, 34, 19 R. Taylor and O. Kennard, J. Am. Chem. Soc., 1982, 104,
5407–5417; (b) J. Romero-Fernández, F. Carrillo- 5063–5070.
Hermosilla, A. Antiñolo, C. Alonso-Moreno, 20 S. Fukumoto, T. Nakashima and T. Kawai, Angew. Chem.,
A. M. Rodríguez, I. López-Solera and A. Otero, Dalton Int. Ed., 2011, 50, 1565–1568.
Trans., 2010, 39, 6419–6425; (c) A. Antiñolo, F. Carrillo- 21 (a) F. Sánchez-Viesca and M. Berros, Heterocycles, 2002, 57,
Hermosilla, R. Fernández-Galán, J. Martínez-Ferrer,
C. Alonso-Moreno, I. Bravo, S. Moreno-Blázquez,
M. Salgado, E. Villaseñor and J. Albaladejo, Dalton Trans.,
2016, 45, 10717–10729.
1869–1879; (b) A. V. Afonin, D. V. Pavlov, A. I. Albanov,
O. A. Tarasova and N. A. Nedolya, Magn. Reson. Chem.,
2013, 51, 414–423.
22 S. Mummadi, D. K. Unruh, J. Zhao, S. Li and C. Krempner,
J. Am. Chem. Soc., 2016, 138, 3286–3289.
6 A. Antiñolo, F. Carrillo-Hermosilla, R. Fernández-Galán,
M. P. Montero-Rama, A. Ramos, E. Villaseñor, R. S. Rojas 23 B. R. Barnett, C. E. Moore, A. L. Rheingold and
and A. Rodríguez-Diéguez, Dalton Trans., 2016, 45, 15350–
15363.
7 (a) R. Jefferson, M. F. Lappert, B. Prokai and B. P. Tilley,
J. Chem. Soc. A, 1966, 1584–1590; (b) D. J. Brauer,
S. Buchheim-Spiegel, H. Bürger, R. Gielen, G. Pawelke and
J. Rothe, Organometallics, 1997, 16, 5321–5330;
J. S. Figueroa, Chem. Commun., 2015, 51, 541–544.
24 (a) C. M. Momming, G. Kehr, B. Wibbeling, R. Frohlich
and G. Erker, Dalton Trans., 2010, 39, 7556–7564;
(b) L. M. Elmer, G. Kehr, C. G. Daniliuc, M. Siedow,
H. Eckert, M. Tesch, A. Studer, K. Williams, T. H. Warren
and G. Erker, Chem. – Eur. J., 2017, 23, 6056–6068.
(c) G. A. Pierce, N. D. Coombs, D. J. Willock, J. K. Day, 25 Bruker Apex2, Bruker AXS Inc., Madison, Wisconsin, USA,
A. Stasch and S. Aldridge, Dalton Trans., 2007, 4405–4412; 2004.
(d) M. Findlater, N. J. Hill and A. H. Cowley, Dalton Trans., 26 G. M. Sheldrick, SADABS, Program for Empirical
2008, 4419–4423; (e) H. Braunschweig, R. D. Dewhurst,
K. Schwab and K. Wagner, Acta Crystallogr., Sect. E: Struct.
Adsorption Correction, Institute for Inorganic Chemistry,
University of Gottingen, Germany, 1996.
Rep. Online, 2010, 66, o610; (f) C. Jones, D. P. Mills, 27 A. Altomare, M. C. Burla, M. Camalli, G. L. Cascarano,
A. Stasch and W. D. Woodul, Main Group Chem., 2010, 9,
23–30.
C. Giacovazzo, A. Guagliardi, A. G. G. Moliterni, G. Polidori
and R. Spagna, J. Appl. Crystallogr., 1999, 32, 115–119.
8 M. A. Dureen and D. W. Stephan, J. Am. Chem. Soc., 2010, 28 G. M. Sheldrick, SHELX-2014, Program for Crystal Structure
132, 13559–13568.
9 B. M. Mikhailov, V. A. Dorokhov and L. I. Lavrinovich, Russ.
Chem. Bull., 1978, 27, 2304–2310.
Refinement, University of Göttingen, Göttingen, Germany,
2014.
29 A. L. Spek, J. Appl. Crystallogr., 2003, 36, 7–13.
10 V. A. Dorokhov, L. I. Lavrinovich and B. M. Mikhailov, Dokl. 30 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
Akad. Nauk SSSR, 1979, 245, 121–125.
11 V. A. Dorokhov, I. P. Yakovlev and B. M. Mikhailov, Russ.
Chem. Bull., 1980, 29, 485–491.
12 R. Boese, R. Koester and M. Yalpani, Z. Naturforsch.,
B: Chem. Sci., 1994, 49, 1453–1458.
13 M. F. Lappert, P. P. Power, A. R. Sanger and
R. C. Srivastava, Metal and metalloid amides: syntheses,
structures, and physical and chemical properties, ed. E.
Horwood, 1980.
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato,
X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng,
J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, T. Vreven, J. A. Montgomery, J. E. Peralta,
F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers,
K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand,
K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar,
J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene,
J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,
R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin,
K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador,
J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas,
J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox,
Gaussian 09, Revision B.01, Wallingford CT, 2009.
14 L. Yang, X. Ren, H. Wang, N. Zhang and S. Hong, Res.
Chem. Intermed., 2011, 38, 113–133.
15 (a) A. J. P. Cardenas, Y. Hasegawa, G. Kehr, T. H. Warren
and G. Erker, Coord. Chem. Rev., 2016, 306, 468–482;
(b) O. Ekkert, G. G. Miera, T. Wiegand, H. Eckert,
B. Schirmer, J. L. Petersen, C. G. Daniliuc, R. Fröhlich,
S. Grimme, G. Kehr and G. Erker, Chem. Sci., 2013, 4,
2657.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2017

View Article Online
Dalton Transactions
Paper
31 A. D. Becke, J. Chem. Phys., 1993, 98, 5648–5652.
32 C. Lee, W. Yang and R. G. Parr, Phys. Rev. B: Condens.
Matter, 1988, 37, 785–789.
M. A. Al-Laham, W. A. Shirley and J. Mantzaris, J. Chem.
Phys., 1988, 89, 2193–2218; (c) G. A. Petersson and M. A. Al-
Laham, J. Chem. Phys., 1991, 94, 6081–6090.
33 (a) P. C. Hariharan and J. A. Pople, Theor. Chim. Acta, 1973, 34 S. Portmann and H. P. Lüthi, Chimia, 2000, 54,
28, 213–222; (b) G. A. Petersson, A. Bennett, T. G. Tensfeldt,
766–770.
This journal is © The Royal Society of Chemistry 2017
Dalton Trans.