Efficient and chemoselective hydroboration of organic nitriles promoted by TiIV catalyst supported by unsymmetrical acenaphthenequinonediimine ligand

Article abstract of DOI:10.1016/j.jorganchem.2019.120958

We report the synthesis, characterization, and utilization of a titanium (IV) complex [(η⁵-C₅H₅){L}TiCl₂] (1) supported by a monoanionic ligand (L), N-(2, 6-diisopropyl)acenaphthenequinonediimido, as a molecular pre-catalyst for the hydroboration of nitriles. The unsymmetrical N-silylated N-(2, 6-diisopropyl)-N-(trimethylsilyl)-acenaphthenequinonediimine ligand (LSiMe₃) was obtained upon the completion of a one-pot reaction between N-(2, 6-diisopropyl)iminoacenaphthenone and lithium hexamethyldisilazide in the presence of trimethylsilyl chloride in 1:1:1 M ratio at 90 °C. The reaction of LSiMe₃ with {η⁵-(C₅H₅)TiCl₃) in equal proportion (1:1) at 60 °C afforded the titanium complex [(η⁵-C₅H₅){L}TiCl₂] (1) in good yield. The molecular structures of the N-silyl ligand (LSiMe₃) and Ti(IV) complex 1 were established by single-crystal X-ray analysis. Complex 1 was tested as a pre-catalyst for hydroboration of nitriles with pinacolborane (HBpin) and catecholborane (HBcat) to afford diboryl amines at ambient temperature. Titanium complex 1 exhibited high conversion, superior selectivity, and broad functional group tolerance during hydroboration of nitriles with both HBpin and HBcat under mild conditions.

Full text of DOI:10.1016/j.jorganchem.2019.120958

Journal of Organometallic Chemistry 902 (2019) 120958
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Efficient and chemoselective hydroboration of organic nitriles
promoted by TiIV catalyst supported by unsymmetrical
acenaphthenequinonediimine ligand
Indrani Banerjee, Srinivas Anga, Kulsum Bano, Tarun K. Panda*
Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi, Sangareddy, 502285, Telangana, India
a r t i c l e i n f o
a b s t r a c t
We report the synthesis, characterization, and utilization of a titanium (IV) complex [(h
5-C5H5){L}TiCl2]
Article history:
Received 19 August 2019
Received in revised form
18 September 2019
Accepted 25 September 2019
Available online 28 September 2019
(1) supported by a monoanionic ligand (L), N-(2, 6-diisopropyl)acenaphthenequinonediimido, as a mo-
lecular pre-catalyst for the hydroboration of nitriles. The unsymmetrical N-silylated N-(2, 6-diisopropyl)-
N-(trimethylsilyl)-acenaphthenequinonediimine ligand (LSiMe3) was obtained upon the completion of a
one-pot reaction between N-(2, 6-diisopropyl)iminoacenaphthenone and lithium hexamethyldisilazide
in the presence of trimethylsilyl chloride in 1:1:1 M ratio at 90 ꢀC. The reaction of LSiMe3 with {
h
5-(C5H5)
TiCl3) in equal proportion (1:1) at 60 ꢀC afforded the titanium complex [( 5-C5H5){L}TiCl2] (1) in good
h
Keywords:
yield. The molecular structures of the N-silyl ligand (LSiMe3) and Ti(IV) complex 1 were established by
single-crystal X-ray analysis. Complex 1 was tested as a pre-catalyst for hydroboration of nitriles with
pinacolborane (HBpin) and catecholborane (HBcat) to afford diboryl amines at ambient temperature.
Titanium complex 1 exhibited high conversion, superior selectivity, and broad functional group tolerance
during hydroboration of nitriles with both HBpin and HBcat under mild conditions.
© 2019 Elsevier B.V. All rights reserved.
Titanium (IV) complex
Hydroboration
Nitriles
Pinacolborane
Catecholborane
1. Introduction
vital components in several organic transformations [9]. Never-
theless, the use of titanium complexes in the synthesis of organo-
Recent trends in synthetic chemistry demonstrate the growth in
use of synthetic methods in multicomponent reactions and catal-
ysis using earth-abundant metals. Multicomponent reactions
emphasize the progress of reactions using fewer (compared to
conventional reactions) steps, while the choice of earth-abundant
metals is driven by non-toxicity and economy of cost. Titanium is
the second-most abundant metal on earth and is also non-toxic. It
has therefore been used as a preeminent catalyst in several
important multicomponent reactions [1]. Besides its use as a
catalyst in synthetic organic and inorganic chemistry, titanium has
seen widespread application in biomedicine [2], nanochemistry [3],
aerospace [4], and other interdisciplinary areas [5]. Titanium has
also been an essential metal catalyst for diverse synthetic reactions
such as asymmetric epoxidation [6], polymerization [7], and
coupling reactions [8]. Amongst titanium metal precursors,
cyclopentadienyl-supported TiIV complexes have been important
reagents for a long time. In the present day, they have emerged as
boron compounds has not been explored, although a wide range of
transition metals have been found to act as efficient catalysts in the
hydroboration of unsaturated compounds (Fig. 1).
Organoboron compounds are applied widely e in pharmaceu-
ticals [10], agrochemicals [11], synthesis of natural products [12],
material synthesis, as well as several organic transformations [13].
Hydroboration of unsaturated compounds such as alkenes [14],
alkynes [15], and carbonyl compounds [16] has been widely prac-
ticed, as it provides many simple and useful precursors for organic
reactions. Synthetic chemists find it advantageous to use organo-
boron compounds because of they are easy to handle and are atom
efficient. However, hydroboration of nitriles is more resource rich,
since the resultant free amines are beneficial for many industrial
processes [17]. Over the past few years, numerous methods have
been reported, including with the use of transition metals and
alkaline metals (Fig. 1). Ruthenium(II)-based metal complexes were
used as efficient catalysts by Gunanathan et al. and Szymczak et al.
for the hydroboration of nitriles and imines for the synthesis of
diboryl amines [18]. A FeeIn complex was reported by the Naka-
zawa working group in the efficient hydroboration of both aryl and
* Corresponding author.
E-mail address: tpanda@iith.ac.in (T.K. Panda).
https://doi.org/10.1016/j.jorganchem.2019.120958
0022-328X/© 2019 Elsevier B.V. All rights reserved.

2
I. Banerjee et al. / Journal of Organometallic Chemistry 902 (2019) 120958
Fig. 1. Selected transition metal catalysts for hydroboration of nitriles.
alkyl nitriles [19]. Transition metalebased complexes such as Co(I)
[20], Co(II) [21], and Ni(II) [22] have also been reported in nitrile
hydroboration by both pinacolborane (HBpin) and catecholborane
(HBcat) by the Fout, Trovitch, and Shimada groups respectively.
Nikonov and colleagues demonstrated the hydroboration of
acetonitrile and benzonitrile by HBcat, catalyzed by molybdenu-
m(IV) [23]. Further, Mg(II) complex was reported by Hill et al. as the
only alkaline earth metal known till date for nitrile hydroboration
[24]. Very recently, the Eisen working group demonstrated the use
of Th(IV)-amide metal catalysts in the rapid hydroboration of ni-
triles and imines [25].
Therefore, in recent years, synthetic chemists have frequently
used less toxic earth-abundant metal catalysts in novel catalytic
reactions under mild conditions. Our research group has conducted
hydroboration reactions on carbonyl compounds using efficient
alkali metal catalysts [26]. Very recently, our group also reported
the chemoselective hydroboration of an organic nitrile with an
earth-abundant aluminum alkyl metal complex [27]. This moti-
vated us to conduct further research into the efficient hydro-
boration of organic nitriles by a titanium-based metal complex.
In this paper, we report the synthesis as well as structural as-
pects of a titanium(IV) complex of N-(2, 6-diisopropylphenyl)-
acenaphthenequinonediimido ligand, which was further tested as
an efficient catalyst for the chemoselective hydroboration of a large
array of nitriles by HBpin and HBcat under mild conditions.
were performed on a BRUKER EURO EA at the Indian Institute of
Technology Hyderabad. Hydrocarbon solvents (n-hexane, toluene)
were distilled in nitrogen atmosphere from sodium benzophenone
and LiAlH4 and stored inside the glove box. All the starting mate-
rials e acenapthequinone, 2,6-diisopropylaniline, lithium bis(-
trimethylsilyl)amide, trimethylsilyl chloride, (cyclopentadienyl)
titanium trichloride, and organic nitriles e were purchased from
Sigma Aldrich India and used without further purification. HBpin
and HBcat were purchased from Sigma Aldrich India and distilled
prior to use. The ligand N-(2, 6-diisopropyl)iminoacenaphthenone
was prepared in accordance with methods reported in literature
[28].
2.2. Synthesis of N-(2, 6-diisopropyl)-N-(trimethylsilyl)-
acenaphthenequinonediimine (LSiMe3)
N-(2, 6-diisopropyl)iminoacenaphthenone (1.0 g, 0.003 mmol)
and Li(NSiMe3)2 (0.490 g, 0.003 mmol) were taken in a dried 25 ml
Schlenk tube inside the glove box, and 5 ml toluene was added. The
reaction mixture was stirred for 2 h. Trimethylsilyl chloride
(0.37 ml, 0.003 mmol) was then added to the reaction mixture
under inert conditions and heated for 18 h at 90 ꢀC. During the
course of the reaction, a yellow precipitate was observed. The
resulting solution was evaporated under reduced pressure to afford
a solid yellow residue. The solid product was extracted from hexane
(3 ꢁ 15 ml) under inert conditions. The ligand LSiMe3 was obtained
as a yellow crystalline product after removing the solvent under
reduced pressure, and was further purified by recrystallization
from toluene at ꢂ35 ꢀC.
2. Experimental section
2.1. Materials
Yield: 1.0 g, 83%. FT-IR (selected frequency in cmꢂ1):
n
¼ 2961
All manipulations involving air- and moisture-sensitive com-
pounds were carried out with argon, using the standard Schlenk
technique or argon-filled MBraun glove box. CDCl3 and C6D6 were
distilled and stored in the glove box. 1H NMR (400 MHz), 13C{1H}
NMR (100 MHz), and 11B{1H} (128 MHz) spectra were measured on
a BRUKER AVANCE III-400 spectrometer. 1H NMR spectra were
recorded on a 400 MHz spectrometer at 298 K in CDCl3, and
(sp3 CeH str.),1625 (C]N str.),1585,1460,1435 (aromatic C]C str.),
849 (CeH deforming), 1322 (sp3 CeH bending). 1H NMR (400 MHz,
CDCl3): dH 7.93 (d, J ¼ 8.0 Hz, 1H, AreH), 7.86 (d, J ¼ 8.0 Hz, 1H,
AreH), 7.79 (d, J ¼ 8.0 Hz, 1H, AreH), 7.62 (t, J ¼ 8.0 Hz, 1H, AreH),
7.23e7.14 (m, 4H, AreH), 6.42 (d, J ¼ 4.0 Hz, 1H, AreH), 2.80e2.69
(sept, 2H, CH), 1.08 (d, J ¼ 8.0 Hz, 6H, CH3), 0.77 (d, J ¼ 8.0 Hz, 6H,
CH3), 0.36 (s, 9H, Si(CH3)3) ppm. 13C{1H} NMR (100 MHz, CDCl3): dC
166.5 (Ar C]N), 159.3 (Si C]N), 146.8, 141.9, 137.7 (d, J ¼ 57.0 Hz),
130.9, 129.0 (d, J ¼ 64.0 Hz), 127.8 (d, J ¼ 76.0 Hz), 124.0 (d,
J ¼ 55.0 Hz), 122.9, 119.0, 28.0 (CH(CH3)2), 24.1 (CH(CH3)2), 23.9
(CH(CH3)2), 1.4 (Si(CH3)3) ppm. Elem. anal. Calcd for C27H32N2Si
(412.64 g molꢂ1): C, 78.59; H, 7.82; N, 6.79. Found: C, 78.33; H, 7.41;
N, 6.52.
chemical shifts (d ppm) and coupling constants (Hz) are reported in
standard fashion with reference to internal standard tetrame-
thylsilane (TMS)
recorded on a 100 MHz spectrometer at 298 K in CDCl3, and
(
dH ¼ 0.00 ppm). 13C{1H} NMR spectra were
chemical shifts
(d
ppm) are reported relative to CHCl3
[
dC ¼ 77.00 ppm (central line of the triplet)]. Elemental analyses

I. Banerjee et al. / Journal of Organometallic Chemistry 902 (2019) 120958
3
2.3. Synthesis of [(
h
5-C5H5){L}TiCl2] (1)
2.5. General procedure for catalytic hydroboration of organic
nitriles to diboryl amines from HBcat (3ae3m)
To a solution of LSiMe3 (200 mg, 0.485 mmol) in toluene (15 ml),
(cyclopentadienyl)titanium trichloride (106 mg, 0.485 mmol) was
added and the reaction mixture was heated for 12 h at 60 ꢀC. The
yellow solution gradually turned dark red. The resulting solution
was then evaporated under reduced pressure to give a dark red
solid residue. This residue was further purified by washing with
cold hexane and recrystallized from a toluene/n-pentane mixture
(4:1) at ꢂ35 ꢀC to give red crystals.
Catalyst 1 (10 mol%) and catecholborane (0.44 mmol) were
added to a Schlenk flask inside the glove box, followed by organic
nitriles (0.2 mmol). The reaction mixture was heated continuously
at 65 ꢀC under neat condition or under toluene for a stipulated time,
as mentioned in Table 2. The progress of the reaction was moni-
tored by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene
(50 mol%) as an internal standard.
Yield: 229 mg, 88%. 1H NMR (400 MHz, C6D6): dH 7.43 (d,
J ¼ 8.0 Hz, 1H, AreH), 7.10 (d, J ¼ 8.0 Hz, 2H, AreH), 7.03e7.01 (m,
1H, AreH), 6.94 (t, J ¼ 4.0 Hz, 2H, CH3), 6.87e6.80 (m, 3H, AreH),
6.49e6.47 (m, 2H, AreH), 6.15 (s, 5H, Cp-H), 2.80e2.70 (sept, 2H,
CH), 0.94 (d, J ¼ 4.0 Hz, 6H, CH3), 0.67 (d, J ¼ 8.0 Hz, 6H, CH3) ppm.
13C{1H} NMR (100 MHz, C6D6): dC 160.8 (Ar C]N), 148.1 (Si C]N),
141.9, 136.0 (d, J ¼ 87.0 Hz), 131.6, 130.6, 129.4 (d, J ¼ 36.0 Hz), 125.6,
124.1 (d, J ¼ 26.0 Hz), 122.8, 119.7, 118.3 (Cp-C), 29.2 (CH(CH3)2),
23.9 (CH(CH3)2), 23.4 (CH(CH3)2) ppm. Elem. anal. Calcd for
2.6. X-ray crystallography
The X-ray data of LSiMe3 and catalyst 1 were collected in an
Agilent Supernova X-Calibur Eos CCD detector with graphite-
monochromatic Mo-Ka (0.71073 Å) (LSiMe3) or Cu-Ka (1.54184 Å)
(1) radiation, either at room temperature (LSiMe3) or at 150 K (1).
The structures of both were solved by direct methods (SIR2004)
[29] and refined on F [2] by full-matrix least-squares methods using
SHELXL-2016/6 [30]. Non-hydrogen atoms were anisotropically
refined. H-atoms were included in the refinement on calculated
C
30H31N2Cl2Ti (538.35): C, 66.93; H, 5.80; N, 5.20. Found: C, 66.65;
H, 5.44; N, 4.96.
positions riding on their carrier atoms. The function minimized was
P
[
w(Fo2 - F2c)2] (w ¼ 1/[
s
[2](Fo2) þ (aP) [2] þ bP]), where P ¼
2.4. General procedure for catalytic hydroboration of organic
nitriles to diboryl amines from HBpin (2ae2s)
(Max(Fo2,0) þ 2F2c)/3 with
s
[2](Fo2) from counting statistics. The
function R1 and wR2 were (
s
jjFoj-jFcjj)/
sjFoj and [
s
w(Fo2-F2c)2/
s
(wFo4)]1/2 respectively. Crystallographic data (excluding structure
Catalyst 1 (10 mol%) and HBpin (0.44 mmol) were added to a
Schlenk flask inside the glove box, followed by the addition of
organic nitriles (0.2 mmol). The reaction mixture was heated
continuously at 65 ꢀC under neat condition or under toluene for a
stipulated time, as mentioned in Table 2. Toluene was then added,
and the reaction mixture was filtered through a short plug of Celite
and evaporated under reduced pressure to obtain a solid residue.
The diboryl amines are moisture- and air-sensitive, and hence
experimental procedures were conducted and NMR samples were
prepared inside the glove box. All products were characterized
using multi-nuclear NMR spectroscopy, and details are given in the
supporting information.
factors) for the structures reported in this paper have been
deposited in the Cambridge Crystallographic Data Centre as sup-
plementary publication no. CCDC 1941995 (1) and 1941996
(LSiMe3). Copies of the data can be obtained free of charge on
application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax: þ
(44)1223-336-033; email: deposit@ccdc.cam.ac.uk). The crystal
data are displayed in Table 1.
3. Results and discussion
3.1. Synthesis and structures
The unsymmetrical N-(2, 6-diisopropyl)-N-(trimethylsilyl)-ace-
naphthenequinonediimine (LSiMe3) ligand was readily prepared,
yielding 83% of a highly pure product, by the one-pot reaction be-
tween N-(2, 6-diisopropyl)iminoacenaphthenone and anhydrous
lithium hexamethyldisilazide in toluene, followed by the addition
of trimethylsilyl chloride in equimolar (1:1:1) ratio at 90 ꢀC
(Scheme 1).
Table 1
Single crystal data and structure refinement parameters for LSiMe3 and 1.
Compound
LSiMe3
1
CCDC No.
Formula
1941996
C27H32N2Si
412.63
293(2)
Triclinic
P-1
11.7251(9)
13.0093(9)
17.3664(14)
79.035(7)
79.131(7)
79.251(6)
4
2522.6(3)
1.086
1.108
1.614e25.817
18963
9465
9465
555
0.0433
0.0688
0.1090, 0.2204
1.286
0.131/-0.268
1941995
C29H28N2Cl2Ti
523.33
150(2)
Triclinic
P-1
11.2164(16)
11.3860(2)
11.8302(16)
73.364(14)
61.820(14)
84.315(14)
2
1274.8(4)
1.363
4.919
4.056e70.656
8067
4596
4596
311
0.0631
0.0513
0.0851, 0.1226
1.003
0.321/-0.338
Formula weight
Temperature(K)
Crystal system
Space group
a (Å)
The nucleophilic reaction of hexamethyldisilazide occurs at the
carbonyl carbon center of N-(2, 6-diisopropyl)iminoacenaph-
thenone, followed by rearrangement of eN(SiMe3)2 group and
elimination of lithium chloride and hexamethyldisiloxane, afford-
ing the desired product, i.e. LSiMe3. In the 1H NMR spectrum of
LSiMe3, the signal for the amine Si(CH3)3 hydrogen atoms appears
as a singlet at dН ¼ 0.36 ppm. The resonance of the isopropyl CH3
protons appears as two doublets, at dН ¼ 0.77 ppm and 1.08 ppm.
The methine proton (CH) of the ligand appear at dН ¼ 2.74 ppm as a
septate signal. The solid-state structure of LSiMe3 was established
by single-crystal X-ray diffraction. Its molecular structure is
depicted in Fig. 2. LSiMe3 crystallizes in the triclinic space group P-1
with four molecules in the unit cell. Details of the structural and
refinement parameters of the crystal structure of LSiMe3 are given
in Table 1. The NeSi bond distance of 1.740(4) Å in LSiMe3 is very
similar to that previously reported for 1,2-bis[(trimethylsilyl)imino]
acenaphtheneesupported complexes (1.791e1.797 Å) [28a]. The
C1eN1 bond distance of 1.261(5) Å is consistent with distances
measured in other N,N’-(bisaryl)acenaphthenequinonediimine li-
gands, and is hence indicative of N]C double bonds [28a].
b (Å)
c (Å)
a
b
g
Z
(ꢀ)
(ꢀ)
(ꢀ)
Volume A [3]
dcalc (g cm-3)
m
(mm-1)
2
q
range (o)
Total Reflections
Unique Reflections
Observed data[I > 2
No. of parameters
R(int)
s(I)]
R1 [I > 2
s
(I)]
R1, wR2 (all data)
GooF (all data)
Max. peak/hole

4
I. Banerjee et al. / Journal of Organometallic Chemistry 902 (2019) 120958
Scheme 1. Synthesis of N-silyl ligand LSiMe3.
were grown at ꢂ35 ꢀC from a concentrated toluene solution. The
solid-state structure of complex 1 was determined by single crystal
x-ray diffraction analysis. Details of the structural and refinement
parameters of the crystal structure of complex 1 are given in Table 1
and molecular structure of complex 1 is shown in Fig. 3. Complex 1
crystallizes in the triclinic space group P-1 and has two indepen-
dent molecules in the unit cell. The molecular structure of Ti(IV)
complex (1) confirms the attachment of the N-(2, 6-diisopropyl)
acenaphthenequinonediiminato ligand to the titanium center.
Complex 1 is monomeric and the coordination polyhedron is
formed by the k1 chelation of one iminato nitrogen atom of mon-
oanionic ligand L, h
5 -coordination of one cyclopentadienyl moiety,
Fig. 2. The molecular solid-state structure of the N-silyl ligand, LSiMe3.
Hydrogen atoms except 25c and 26a are omitted for clarity. Selected bond lengths (Å)
and angles (ꢀ): C1eN1 1.261(5), N1eSi1 1.740(4), C2eN2 1.270(5), N2eC13 1.432(5),
C1eC2 1.538(6), C1eN1eSi1 136.4(3), C1eC2eN2 121.1(4), C2eN2eC13 119.8(4).
Further reaction of LSiMe3 with h5 -cyclopentadienyl titaniu-
m(IV) trichloride [(h5 -C5H5)TiCl3] in toluene at 60 ꢀC resulted in
the formation of a corresponding mixed ligand titanium complex
[(h5 -C5H5){L}TiCl2] (1) in 88% yield, through elimination of the
volatile trimethylsilyl chloride (Scheme 2). Titanium complex 1
demonstrated good solubility in common organic solvents such as
THF and toluene and was characterized using standard spectro-
scopic and analytical techniques. The solid-state structure of com-
pound 1 was also established by single-crystal X-ray diffraction
analysis. In the 1H spectrum of titanium complex 1 measured in
C6D6, the cyclopentadienyl C5Hꢂ5 hydrogen atoms can be seen at
dН ¼ 6.15 ppm as a sharp singlet. Two doublet (at dН ¼ 0.67 ppm and
0.94 ppm) and one septate (at dН ¼ 2.75 ppm) resonance signals
confirm the presence of isopropyl groups of the ligand moiety L.
X-ray quality air- and moisture-sensitive crystals of complex 1
Fig. 3. Representation of the molecular solid-state structure of [(
(1).
h [5]-C5H5){L}TiCl2]
All hydrogen atoms (except H26) are omitted for clarity. Selected bond lengths (Å):
Ti1eN1 1.854(3), Ti1eCl1 2.2817(13), Ti1eCl2 2.2529(14), Ti1eC25 2.329(4), Ti1eC26
2.341(4), Ti1eC27 2.349(4), Ti1eC28 2.362(4), Ti1eC29 2.362(4), C1eN1 1.260(4),
C2eN2 1.275(5), C1eC2 1.534(5), C1eC2eN2 119.9(3), C2eN2eC13 118.4(3),
C2eC1eN1 125.5(3), C1eN1eTi1 163.4(3), N1eTi1eCl1 104.91(10), N1eTi1eCl2
103.60(10), Cl1eTi1eCl2 101.86(5).
Scheme 2. Synthesis of titanium (IV) complex 1.

I. Banerjee et al. / Journal of Organometallic Chemistry 902 (2019) 120958
5
and two chloride ions. The monoanionic iminato ligand L was
initiated by the elimination of trimethylsilyl chloride (Scheme 2)
and thus readily bonded to the Ti center to form the Ti(IV) metal
complex 1 in a monodentate fashion. This monodentate coordi-
nation of the ligand L to the TiIV ion is presumably due to the
presence of the cyclopentadienyl ring which may prevent the
bidentate coordination due to steric factor. The geometry around
the titanium ion may best be described as pseudo tetrahedral, in
Results of the catalytic hydroboration reaction are set out in Table 3.
With the optimized condition as the benchmark for the
hydroboration reaction with unsubstituted benzonitrile, which
afforded a yield of 99% (Table 3, entry 2a), we employed a range of
aryl nitriles possessing electron-withdrawing and electron-
donating groups, as well as heterocyclic nitriles and aliphatic ni-
triles, for hydroboration with HBpin. The reactions were smooth in
all cases and the results are set out in Table 3. The reaction between
HBpin and 4-fluoro, 4-chloro, and 4-bromo benzonitrile afforded a
near-quantitative yields of 98%, 96%, and 91% of the diboryl product
respectively (Table 3, entries 2be2d). The reaction of substituted 4-
trifluoromethyl benzonitrile with HBpin also afforded an excellent
yield of 93% (Table 3, entry 2e) under optimal conditions. The re-
action protocol was also found to be very effective for benzonitriles
with electron-donating groups under optimized conditions. 4-
Methylbenzonitrile produced a quantitative yield of 98% while 4-
(tert-butyl)benzonitrile afforded a slightly lower yield of 87%
(Table 3, entries 2fe2g). 4-Methoxy, 4-(methylthio), and 4-
dimethylamino benzonitrile also provided the diboryl product in
good yields e 89%, 82%, and 85% respectively (Table 3, entries
2he2j).
view of the h
5 -C5Hꢂ5 ring being a pseudo-monodentate ligand. The
TieN distance [1.854(3) Å] is close to the Ti-Niminato distance [1.765
(3) Å] and [1.824(2) Å] observed in iminazolin-2-iminato com-
plexes [(ImtBuN)Ti(h5 -Cp)Cl2] [31] and [(ImtBuN)Ti(NMe2)3] [32]
(ImRN ¼ Imidazolin-2-iminato) complexes respectively. However,
this distance is much shorter than TieN covalent bond observed in
literature (1.928(2) Å in [h5 -CpTi((Dipp)2DAD)Cl]) [33]. The imi-
nato nitrogen of ligand L coordinates to the titanium ion nearly in
linear fashion [TieN1eC1 ¼163.4(3)ꢀ]. The TieC(Cp) distances,
ranging from 2.329(4) to 2.362(4) Å, are in agreement with previ-
ously reported TieC(Cp) values [31].
3.2. Catalysis
Additionally, we obtained good yields (up to 80%) from the
hydroboration reaction of 1-napthonitrile, picolinonitrile, and
thiophene-2-carbonitrile (Table 3, entries 2ke2m). The scope of
hydroboration was also extended to 4-(fluorophenyl)acetonitrile
and aliphatic nitriles, such as acetonitrile, chloro-, and methoxy
acetonitrile, and good yields of up to 75% were obtained for all the
substrates under optimized conditions (Table 3, entries 2ne2q).
Additionally, we checked the chemoselectivity of the TiIV catalyst 1
towards hydroboration of methyl-4-cyanobenzoate and 4-
cyanobenzaldehyde. In 4-cyanobenzaldehyde, both aldehyde and
nitrile groups underwent smooth reduction under optimum con-
ditions, exhibiting no chemoselectivity of the functional groups
(Table 3, entry 2r). However, chemoselective reduction of the nitrile
group occurred from methyl-4-cyanobenzoate when 2.2 equivalent
HBpin was used, keeping the ester group intact (Table 3, entry 2s).
3.2.1. Hydroboration of nitriles with HBpin
At the beginning of our study, we chose the model reaction
condition: 5 mol% of the Ti complex for the hydroboration of ben-
zonitrile and HBpin (2.2 equivalents) at 65 ꢀC, under neat condition.
When carried out at room temperature, the reaction yielded 48% of
the corresponding diboryl amine product (Table 2, entry 2). We
were, however, delighted to observe that complex 1 exhibited good
catalytic activity toward the hydroboration of benzonitrile when
heated to a temperature of 65 ꢀC, yielding 61% of the product, in the
formation of the corresponding boryl amine (Table 2, entry 3). With
a higher catalyst loading e up to 10 mol% e under neat condition at
65 ꢀC for 12 h, the desired product was obtained with a yield of 99%
(Table 2, entry 4). In addition, we screened the hydroboration re-
action separately with no catalyst loading (Table 2, entry 1) and
using catalyst at room temperature (Table 2, entry 2). In both cases,
we observed no product formation. The efficacy of the catalyst was
examined with solvents such as toluene and THF. We observed that
toluene produced a good yield e 90% e while THF produced a
drastic decrease in yield e 50% of the product (Table 2, entries 5e6).
Thus, with a temperature of 65 ꢀC and neat condition, we set out to
examine the scope of substrates of various aryl and alkyl nitriles
and HBpin in the presence of 10 mol % of the titanium catalyst 1.
3.2.2. Hydroboration of nitriles with HBcat
In order to extend the protocol of our reaction, the efficacy of the
titanium catalyst 1 was further examined for the hydroboration of
nitriles with HBcat. Under the optimized conditions indicated in
Table 1, when 10 mol% of complex 1 was used for the reaction of 1
equivalent of benzonitrile with 2.2 equivalent of HBcat under neat
condition, an excellent yield of 99% was observed (Table 4, entry
3a).
The yield was calculated from NMR in the presence of 1,3,5-
trimethoxybenzene as an internal standard. With this optimized
condition, we re-examined the catalyst efficiency of the substrate
scope of nitriles with electron-withdrawing and electron-donating
groups, as well as heterocyclic and aliphatic nitriles. In the case of
electron-withdrawing halogen substitution, the use of 4-fluoro, 4-
chloro, and 4-bromo benzonitrile gave excellent yields e 99%
each (Table 4, entries 3be3d). When the substitution was carried
out using the 4-trifluoromethyl group, excellent yield was observed
which was in accordance with the other halogen functional groups
on benzonitrile (Table 4, entry 3e).
Table 2
Catalyst screening for hydroboration of benzonitrile with HBpin.
Thereafter, efficiency of the catalyst was investigated through
electron-donating substitutions on benzonitriles, such as 4-
methylbenzonitrile, and a yield of 83% was obtained (Table 4, en-
try 3f). Surprisingly, when 4-methoxybenzonitrile was reacted with
HBcat, we observed an increase (to 99%) in the yield (Table 4, entry
3h), whereas the reaction with 4-methylthiobenzonitrile resulted
in a slight decrease in the yield, at 82% (Table 4, entry 3i). Further
decrease of yield (75%) was observed when the hydroboration was
carried out on 4-dimethylamino benzonitrile (Table 4, entry 3j),
Entry Catalyst loading (mol%) Time (h) T (ꢀC) Solvent Isolated yielda (%)
1
2
3
4
5
6
e
12
12
12
12
12
12
65
r.t
65
65
65
65
neat
neat
neat
neat
Toluene 91
THF 50
0
5
5
10
10
10
48
61
99
Reaction conditions: catalyst (x mol%), benzonitrile (1 eq.), HBpin (2.2 eq.). The
reaction mixture was heated to 65 ꢀC. aIsolated yield.

6
I. Banerjee et al. / Journal of Organometallic Chemistry 902 (2019) 120958
Table 3
Substrate scope of hydroboration of nitriles with HBpin using TiIV catalyst 1.
Reaction conditions: catalyst (10 mol%), nitrile substrates (1 eq.), HBpin (2.2 eq.). The reaction mixture was heated to 65 ꢀC. aIsolated yield. b2l, the 2ne2s reaction was carried
out in toluene, c3 eq. HBpin was used.
probably due to increase in the electron-donating character. It is
noteworthy that reaction with the heterocyclic substrate e namely
thiophene-2-carbonitrile e again afforded an excellent yield of 99%
(Table 4, entry 3k). Hydroboration with HBcat was extended to
aliphatic nitriles such as chloro- and methoxy acetonitrile, and
excellent yields of up to 95% were obtained for these substrates
under optimized conditions (Table 4, entries 3le3m). The current
protocol for hydroboration of nitriles using TiIV catalyst, is
competitive to the existing catalytic system such as Ru [18a], Fe-In
[19], Co(I) [20], Co(II) [21], Mo(IV) [23], Mg(II) [24], Th (IV) [25] with
respect to substrate scope, yield and reaction time reported by
other research groups.

I. Banerjee et al. / Journal of Organometallic Chemistry 902 (2019) 120958
7
Table 4
The substrate scope of hydroboration of nitriles with HBcat using TiIV catalyst 1.
Reaction conditions: catalyst (10 mol%), nitrile substrates (1 eq.), HBcat (2.2 eq.). The reaction mixture was heated at 65 ꢀC. aYields were calculated based on 1H NMR (400 MHz)
integration of characteristic product peak using 50 mol% of 1,3,5-trimethoxybenzene as an internal standard. bFor 3l, 3m the reaction was carried out in toluene.
After the hydroboration of organic nitriles, we performed hy-
drolysis of diboryl amines for a few substrates to give the corre-
sponding amines. The diborylated amines were treated with 0.05 M
aqueous HCl for 4 h at room temperature. The derivatives of benzyl
ammonium chloride were obtained as white powder (Table 5, en-
tries 4ae4c) in moderate yields of 70e80%.
For the hydroboration of aryl nitriles with HBpin, the reaction
progress with various substrates ranging from electron-
withdrawing to electron-donating and heterocyclic groups e was
monitored by in situ 1H NMR. The reactions were carried out in a
screw-cap NMR tube charged with C6D6 which were sealed and
heated to 65 ꢀC at regular intervals.
e
The conversion of diboryl products was carried out on 1
equivalent of benzonitrile (1a), p-tolylbenzonitrile (1f), p-methox-
ybenzonitrile (1h), p-(methylthio)benzonitrile (1i), and thiophene-
2-carbonitrile (1m), each with 2.2 equivalent of HBpin in the
presence of the TiIV catalyst, followed by 1,3,5-trimethoxybenzene
(20 mol%) charged with C6D6. The characteristic product formation
was not observed at room temperature. Thereafter the NMR tubes
with the reaction mixture were heated to 65 ꢀC at regular intervals
of time, such as 30 min. Although complete conversion of the
products was not achieved within 5 h, we observed a gradual in-
crease in the formation of characteristic products e the increase
progressed in linear fashion with the passage of time.The most
plausible mechanism has been proposed based on previous litera-
ture [18a,27]. We propose the formation of titanium hydride [TieH]
complex (I) as the active species through the reaction of catalyst 1
and HBpin. Fig. 4 in ESI describes the proposed mechanism.
Table 5
Hydrolysis of diboryl amines.

8
I. Banerjee et al. / Journal of Organometallic Chemistry 902 (2019) 120958
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To summarize, we have reported the synthesis of a titanium (IV)
metal complex supported by a monoanionic, N-(2, 6-diisopropyl)
acenaphthenequinonediiminato ligand. We have also established
the molecular structure of the complex. We have utilized TiIV
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alkyl nitriles with HBpin and HBcat to afford corresponding diboryl
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nitriles, producing excellent to good yields. The corresponding
primary amines were isolated in good yields for a few substrates.
The mechanism involves the formation of a titanium hydride in-
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This work was financially supported by the JICA FRIENDSHIP
Project under the Collaboration Kick-starter Programme (CKP). I.B.
thanks the Ministry of Human Resource and Development (MHRD),
Government of India, and S.A. and K.B. thank CSIR for their
respective Ph.D. fellowships.
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