Boron complexes of redox-active diimine ligand

Article abstract of DOI:10.1039/c3dt33055a

Boron complexes (dpp-bian)BCl₂ (1) and (dpp-bian)BX (X = Cl, 2; Br, 3) (dpp-bian = 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene) have been prepared by reacting mixtures dpp-bian-BX₃ (1:1) with one (1) and two (2 and 3) equivalents of sodium correspondingly in toluene. Complexes 2 and 3 reveal a moderate stability against ambient oxygen and moisture. The reaction of complex 2 with PhCCLi gave compound (dpp-bian)B-CCPh (4). Treatment of 2 with potassium hydroxide afforded complexes (dpp-bian)B-OH (5) and (dpp-bian)B-OK (6). Boron amide (dpp-bian)B-NH₂ (7) has been isolated from the reaction of compound 1 with sodium in liquid ammonia. Borane (dpp-bian)B-H (8) can be prepared by the reactions of complexes 2 and 3 with LiAlH₄. Diamagnetic compounds 2-8 have been characterized by IR, ¹H and ¹¹B NMR spectroscopy; paramagnetic complex 1 has been studied by the ESR method. Molecular structures of 2, 5, 7 and 8 have been determined by X-ray crystallography.

Full text of DOI:10.1039/c3dt33055a

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Boron complexes of redox-active diimine ligand†
Igor L. Fedushkin,*a Olga V. Markina,a Anton N. Lukoyanov,a Alexander G. Morozov,a
Evgenii V. Baranov,a Maxim O. Maslova and Sergey Yu. Ketkova,b
Cite this: Dalton Trans., 2013, 42, 7952
Boron complexes (dpp-bian)BCl2 (1) and (dpp-bian)BX (X = Cl, 2; Br, 3) (dpp-bian = 1,2-bis[(2,6-diiso-
propylphenyl)imino]acenaphthene) have been prepared by reacting mixtures dpp-bian–BX3 (1 : 1) with
one (1) and two (2 and 3) equivalents of sodium correspondingly in toluene. Complexes 2 and 3 reveal a
moderate stability against ambient oxygen and moisture. The reaction of complex 2 with PhCuCLi gave
compound (dpp-bian)B–CuCPh (4). Treatment of 2 with potassium hydroxide afforded complexes (dpp-
bian)B–OH (5) and (dpp-bian)B–OK (6). Boron amide (dpp-bian)B–NH2 (7) has been isolated from the
reaction of compound 1 with sodium in liquid ammonia. Borane (dpp-bian)B–H (8) can be prepared by
the reactions of complexes 2 and 3 with LiAlH4. Diamagnetic compounds 2–8 have been characterized
by IR, 1H and 11B NMR spectroscopy; paramagnetic complex 1 has been studied by the ESR method.
Molecular structures of 2, 5, 7 and 8 have been determined by X-ray crystallography.
Received 20th December 2012,
Accepted 24th January 2013
DOI: 10.1039/c3dt33055a
www.rsc.org/dalton
(dpp-bian)Ga–Ga(dpp-bian) as catalyst.6b Looking for new
examples of an unusual chemical bonding and reactivity we
Introduction
Nitrogen-based σ-donor ligands have provided progress in started synthesis and investigation of the dpp-bian derivatives
many fields of modern coordination chemistry. Among most of boron as the smallest ion in the group 13 metals.
fundamental results obtained are (i) isolation of stable com-
The first 1,3,2-diazaborolines were prepared almost 40 years
pounds of low-valent main group elements, e.g. N-heterocyclic ago by Niedenzu and Merriam7a,b and later by Weber and
carbenes (NHC)1a,b plus their Si,1c,d Ge,1c,e Sn,1c,e P,1f–i and As1f Schmid.7c Synthesis of boron analogues of NHCs have been
analogues; (ii) synthesis of complexes featuring direct metal– first attempted at the start of the 21st century,7d just immedi-
metal bonds.2,3 We have contributed to these fields by synthe- ately after the pioneering work of Schmidbaur and co-work-
sizing Ge(II)4a,b species as well as compounds comprising of ers8a on the preparation of potassium salts of the 1,3,2-
M–M bonds (Zn–Zn,4c Ga–Ga,4d Zn–Ga,4d Al–Al,4e Ga–Li4f and diazagalol [{(tBuNCH)2}Ga{K(tmeda)}]2 including solvent sep-
Ga–La4g), whose remarkable stability is granted by redox-active arated gallen-anion [{(tBuNCH)2}Ga]−.8b However, the first
N-ligand, 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene efforts to prepare metal boranides failed due to side reactions.
(dpp-bian). One of the most important features of the dpp- Difficulties have already arisen at the stage of synthesis of
bian is variability of its coordinating facilities in the metal diboranes. Thus, reduction of boron precursor {(tBuNCH)2}-
complexes: usually, the dpp-bian serves as neutral,5a radical- B–Br with K/Na alloy in DME, TMEDA or toluene resulted
anionic5b–d or dianionic5b chelating ligand, although the for- 1,3,2-diazaboroles with substituents at the boron atom that
mation of the tri-anion and tetra-anion of dpp-bian have also were abstracted from the respective reaction media.7d First
been documented.5b Further, metal complexes of the dpp-bian metal boranide, boryllithium [{(ArNCH)2}B–Li(μ-DME)]2 (Ar =
reveal an amazing reactivity. For instance, complexes (dpp- 2,6-diisopropylphenyl), has been reported by Segawa,
bian)M–M(dpp-bian) (M = Al,4f Ga6) are able to bind different Mashita and Nozaki in 2006.9a It has been prepared by
alkynes. In the case of gallium this process is reversible. the reduction of N,N′-bis(2,6-diisopropylphenyl)-2-bromo-2,3-
It allows hydroamination (hydroarylation) of alkynes using dihydro-1H-1,3,2-diazaborole with Li[C10H8]. This compound
has already found an application as a nucleophile in organic
synthesis9b as well as in the syntheses of compounds with
direct metal–boron bonds.10 Among the amazing achieve-
ments in the field of boron chemistry in the last decade is
preparation of mono- and dianionic diboranes with one- and
two-electron B–B π bonds accessed by the groups of Berndt
and Power through the population of the empty π-bonding
orbital between the boron atoms in R2B–BR2 (R = alkyl, aryl,
aG. A. Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences,
Tropinina str. 49, 603950 Nizhny Novgorod, Russian Federation.
E-mail: igorfed@iomc.ras.ru; Fax: +7 831 4627495; Tel: +7 831 4629631
bDepartment of Chemistry, N. I. Lobachevsky State University of Nizhny Novgorod, 23
Gagarin Avenue, Nizhny Novgorod, 603950, Russian Federation
†CCDC 916524 (2), 916525 (5), 916526 (7) and 916527 (8). For crystallographic
data in CIF or other electronic format see DOI: 10.1039/c3dt33055a
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amino, alkoxy).11 To stabilize a double boron–boron bond in
illusive diborene HBvBH, Robinson and co-workers used
NHC donor.12 The most recent landmark in the chemistry of
boron is the preparation of diboryne, {(ArNCH)2}BuB-
{(ArNCH)2} (Ar = 2,6-diisopropylphenyl) reported by Braunsch-
weig and co-workers.13 This ambient temperature stable com-
pound has been synthesized by the reduction of a bis(NHC)-
stabilized tetrabromodiborane with four equivalents of sodium
naphthalenide.
Scheme 2 Synthesis of compounds 2 and 3.
A sole crystallographically characterized dpp-bian derivative
of boron reported so far is the ionic complex [(dpp-bian)BCl2]- the coupling of unpaired electron with 14N, 10B, 11B, 35Cl, 37Cl
[BCl4] formed in the reaction of dpp-bian with BCl3.5a The and 1H nuclei. Also the absorption in the range
dpp-bian in this complex acts as neutral chelating ligand. 1500–1600 cm−1 in the IR spectrum is indicative for the ses-
Attempts to reduce [(dpp-bian)BCl2][BCl4] with alkali metals in quialteral C⋯N bonds of the dpp-bian radical-anion.5b–d
THF were made, but no pure compounds could be isolated.5a
The reactions of the dpp-bian–BX3 mixtures (1 : 1) (X = Cl,
Here we report on the syntheses and characterisation of a Br) with two equivalents of sodium in toluene (80 °C, 24 h,
series of boron derivatives of anionic dpp-bian ligands.
stirring) afford new boron complexes of the dianionic dpp-
bian ligand (dpp-bian)B–Cl (2) and (dpp-bian)B–Br (3)
(Scheme 2). Although the syntheses of complexes 2 and 3 can
be carried out in other solvents (Et2O or 1,2-dimethoxyethane)
the preferable reaction media is toluene since the products
2 and 3 can be easily crystallised from this solvent after
filtration from sodium chloride. When the reactions between
dpp-bian, BX3 and sodium have been tried in THF the
formation of a gel has been observed, probably, due to the
polymerisation of THF. Note that pure 2 and 3 do not react
with THF and can be re-crystallised from this solvent.
In contrast to all the metal complexes of the dpp-bian
dianion compounds 2 and 3 are rather stable towards moisture
and oxygen: in the crystalline state no decomposition in air
could be observed over seven days. For comparison, alu-
minium and gallium complexes with dpp-bian dianion react
immediately even with traces of the oxygen and water.5d
Noticeable oxidation of 2 with oxygen occurred only in solu-
tion: in air in toluene compound 2 (1.3 mmol L−1) oxidized
completely within 24 hours while stirring at ambient tem-
perature to give free dpp-bian.
Results and discussions
Synthesis and properties of compounds 1–8
Reduction of the dpp-bian with sodium metal in the presence
of boron halides allows preparation of acenaphthene-1,2-
diimine derivatives of boron. The reaction of equimolar
amounts of the dpp-bian, Na and BCl3 in toluene at 80 °C at
stirring within 24 h afforded air- and moisture sensitive com-
pound (dpp-bian)BCl2 (1) in good yield (Scheme 1).
Compound 1 is paramagnetic due to the presence of the
radical-anion of the dpp-bian. In solution complex 1 exhibits
an ESR signal (Fig. 1), whose hyper-fine structure is caused by
Being impressed by reactivity of (dpp-bian)Ga–Ga(dpp-bian)
towards alkynes we treated complex 2 with phenylacetylene.
Unfortunately these substances do not interact. On the other
hand compound 2 readily reacts with Li–CuCPh in THF
resulting in compound (dpp-bian)B–CuCPh (4) (Scheme 3). In
contrast to 2 and 3 compound 4 reacts with water affording the
boronic acid derivative (dpp-bian)B–OH (5). Compound 5 can
be also obtained by reacting compound 2 with potassium
hydroxide. However, this reaction requires prolonged
heating of the reaction mixture accompanied by ultra-soni-
cation treatment (Scheme 3). In its turn compound 5 reacts
with an excess of potassium hydroxide yielding (dpp-bian)BOK
(6) (Scheme 3).
Scheme 1 Synthesis of compound 1.
Due to the lack of suitable single crystals the molecular
structures of compounds 4 and 6 could not be determined by
X-ray crystallography. However both complexes have been satis-
factory characterized by spectroscopic methods and elemental
analysis. The IR spectrum of compound 4 reveals absorption
at 2182 cm−1, which corresponds to the stretching vibrations
Fig. 1 ESR signal of complex 1 (toluene, 293 K): (a) experimental spectrum;
(b) spectrum simulated using WinEPR Simfonia (g = 2.00243; A(14N) = 0.40 (2 N),
A(11B) = 0.50 (1 B), A(10B) = 0.167 (1 B), A(35Cl) = 0.45 (2 Cl), A(37Cl) = 0.375
(2 Cl), A(1H) = 0.108 (4 H) mT).
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Scheme 4 Synthesis of compound 7.
Scheme 3 Synthesis of compounds 4, 5 and 6.
Scheme 5 Synthesis of compound 8.
that intermediate boron species formed under reduction with
alkali metals react further with reaction media affording an
intractable mixture of products. Therefore, reduction of com-
pounds 2 and 3 with sodium dissolved in liquid ammonia at
−45 °C has been attempted. According to the literature data at
that temperature the formation of NaNH2 does not happen.14
Addition of 2 or 3 into a solution of sodium in liquid NH3 was
accompanied by a color change from red to green-brown
within a few minutes. Evaporation of ammonia resulted in a
yellow-brown powder, which being dissolved in toluene
gave a violet solution. Crystallisation from toluene afforded
amide (dpp-bian)B–NH2 (7) as violet needle-like crystals
(Scheme 4).
Compounds 2 and 3 react with LiAlH4 in THF to give boron
hydride (dpp-bian)B–H (8) (Scheme 5). In contrast to reported
syntheses of boron hydrides using LiAlH4,15 these reactions
proceed only at high temperatures in the presence of an excess
of LiAlH4.
Fig. 2 The 1H NMR spectra of 5 (top) and 6 (bottom) (400 MHz, C6D6, 293 K).
Compound 8 is quite stable towards oxygen. Also, it is
unreactive towards compounds with multiple C–C and C–O,
e.g. phenylacetylene or benzylideneacetone. The presence of
the B–H is confirmed by absorption at 2311 cm−1 in the IR
spectrum.
of the C–C triple bond of the phenylethynyl ligand. The 1H
NMR spectrum of 4 also reveals the expected signal set for the
organic fragments. The 1H NMR spectra of 5 and 6 are
depicted in Fig. 2. In both spectra the methyl groups of the
isopropyl substituents give rise to two doublets (each 12 H).
In the spectrum of compound 6 these signals are overlapped
Molecular structures and DFT calculations of compounds 2, 5,
7, and 8
with signals of the diethyl ether. The four methine protons of Molecular structures of compounds 2, 5, 7, and 8 have been
the dpp-bian ligand in 5 and 6 appear in each case as septets determined by X-ray crystallography and are depicted in
at δ 3.50 and 3.68 ppm, respectively. The singlet signal at Fig. 3–6, respectively. Crystal data and structure refinement
1
δ 2.77 ppm in the H NMR spectrum of 5 confirms the pres- details are collected in Table 1. The selected bond lengths
ence of the OH group in the compound.
determined by single crystal X-ray diffraction together with
Reduction of compounds 2 and 3 with alkali metals in corresponding distances calculated by DFT B3LYP/6-31G(d) are
hexane, toluene and THF aimed at the preparation of com- given in Table 2. All four compounds represent monomeric
pounds with boron–boron and alkali metal–boron bonds has three-coordinate boron species. The sums of the boron bond
been attempted. However, the syntheses of the desired com- angles (360°) in each compound indicate sp2-hybridization of
pounds were failed. The use of Li[C10H8] as reducing agent9a the boron atoms. It provides delocalisation of p-electrons
in THF also resulted in no isolable products. One can suggest over five atoms in the boron cycles. Probably due to this
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Fig. 6 Molecular structure of complex 8 at 30% probability level. Hydrogen
Fig. 3 Molecular structure of compound 2 at 30% probability level. Hydrogen
atoms except atom H(1) are omitted.
atoms are omitted.
form π-complexes, for example (η5-diazaborolidine)Cr(CO)3.16
In contrast, compounds 2 and 3 do not react with Cr(CO)6,
probably due to the presence of bulky isopropyl groups in
close proximity to the 1,3,2-diazaborole moiety. According to
X-ray diffraction analyses of 2, 5, 7 and 8, the five-membered
boron cycles N(1)N(2)C(1)C(2)B are planar. Mean deviations of
the atoms B, N(1), N(2), C(1) and C(2) from the plane are in the
range of 0.001–0.006 Å. The Cl atom in 2 is disposed virtually
in the plane of the boron cycle: the displacement from the
plane is 0.077 Å. A similar situation is observed for O(1), N(3)
and H(1) atoms in 5, 7 and 8: deviations of the atoms O(1), N(3)
and H(1) from the boron cycle are 0.033, 0.011 and 0.091 Å,
respectively. The B–N bond lengths in compounds 2, 5, 7, and
8 fall into the narrow range 1.429(3)–1.450(2) Å (av. 1.436 Å).
They are remarkably shorter than Al–N and Ga–N
bonds in other group 13 metal complexes of dpp-bian dianion,
e.g. (dpp-bian)AlI(Et2O) (av. Al–N 1.836 Å)17a and (dpp-bian)-
Ga–Ga(dpp-bian) (av. Ga–N 1.860 Å)4d (Table 2). The observed
differences are in good agreement with the difference in
covalent radii of trivalent B, Al and Ga (0.85, 1.26 and 1.24 Å,
respectively).17b The geometries of N,B-heterocycles in 2, 5, 7,
and 8 can be compared with that of acenaphthene-1,2-diimine
derived carbene, (dpp-bian)C1b (Table 2). As expected the
N–C(carbene) bonds in (dpp-bian)C (av. 1.378 Å)1b are shorter
than B–N bonds in 2, 5, 7, and 8 (av. 1.436 Å). Since in com-
pound 5 the hydroxyl group is connected to the aromatic cycle
this compound can be considered as “heterocyclic phenol”.
Deprotonation of compound 5 with KOH to “phenolate” 6 sup-
ports this conclusion. The B–Cl, B–OH, B–NH2, and B–H bond
lengths in compounds 2, 5, 7, and 8 match well to those
distances in related boron compounds.18
Fig. 4 Molecular structure of compound 5 at 30% probability level. Hydrogen
atoms except atom H(1) are omitted.
Electronic structures of the free dpp-bian2− dianion and
compounds 2, 5, 7, 8, A, C, D were studied by DFT at the
B3LYP/6-31G(d) level of theory. The DFT calculations for
complex B have been performed earlier.4d The optimized bond
lengths agree well with the X-ray data (Table 2). The calculated
C–C distance in the diimine fragment of the dpp-bian ligand
in the complexes studied (1.374–1.393 Å, Table 2) is shorter
Fig. 5 Molecular structure of compound 7 at 30% probability level.
delocalisation compounds 2, 5, 7, and 8 reveal remarkable
stability against oxygen and moisture. The aromatic character
of 1,3,2-diazaboroles is further evident from their ability to
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Table 1 Crystal structure data for 2, 5, 7 and 8
2
5
7
8
Formula
Mr
C
36H40BClN2
C36H41BN2O
528.52
C36H42BN3
527.54
C36H41BN2
512.52
546.96
T/K
100(2)
100(2)
100(2)
100(2)
Crystal size/mm3
0.72 × 0.39 × 0.07
Monoclinic
P2(1)/n
0.58 × 0.07 × 0.05
Monoclinic
P2(1)/n
0.48 × 0.19 × 0.07
Triclinic
ˉ
P1
0.28 × 0.14 × 0.10
Monoclinic
P2(1)/n
Crystal system
Space group
a/Å
b/Å
c/Å
α/°
12.9857(16)
19.268(2)
13.4096(17)
90
110.856(2)
90
3135.3(7)
4
1.159
0.148
1168
−15 ≤ h ≤ 15
−22 ≤ k ≤ 22
−15 ≤ l ≤ 15
1.98 to 25.00
99.6
12.8866(12)
18.7947(17)
13.3897(12)
90
110.201(2)
90
3043.5(5)
4
1.153
0.068
1136
−15 ≤ h ≤ 15
−23 ≤ k ≤ 23
−16 ≤ l ≤ 16
2.17 to 26.00
—
10.9275(7)
12.8248(9)
13.0157(9)
94.550(2)
113.4610(10)
108.675(2)
1539.63(18)
2
1.138
0.066
568
−13 ≤ h ≤ 13
−15 ≤ k ≤ 15
−16 ≤ l ≤ 16
2.11 to 26.00
—
12.8332(13)
18.739(2)
13.4197(14)
90
109.425(3)
90
3043.6(5)
4
1.119
0.064
1104
−15 ≤ h ≤ 15
−23 ≤ k ≤ 23
−16 ≤ l ≤ 16
1.90 to 26.00
—
β/°
γ/°
V/Å3
Z
Dcalcd/g cm−3
μ/mm−1
F(000)/e
hkl range
θ range for data collection/°
Completeness to θ = 25.00° (%)
Completeness to θ = 26.00° (%)
Max. and min. transmission
Refl. measured
—
99.2
99.2
99.8
0.9897 and 0.9007
23 998
0.9966 and 0.9616
25 678
0.9954 and 0.9692
13 225
0.9936 and 0.9824
25 945
Refl. unique
Rint.
5491
0.0779
5936
0.0950
6010
0.0332
5975
0.0965
Param. refined
327
383
398
398
R(F2)/wR(F2) [I > 2σ(I)]
R(F2)/wR(F2) [all data]
GoF (F2)
0.1287/0.3196
0.1548/0.3389
1.246
0.0852/0.1954
0.1645/0.2267
1.008
0.0591/0.1384
0.1023/0.1536
1.016
0.0807/0.1914
0.1685/0.2279
0.934
Δρfin (max/min)/e Å−3
1.669/−0.583
0.418/−0.306
0.249/−0.194
0.363/−0.255
Table 2 Selected bond lengths (Å) for 2, 5, 7, 8, (dpp-bian)AlI(Et2O)17a (A), (dpp-bian)Ga–Ga(dpp-bian)4d (B), (dpp-bian)C1b (C) and (dpp-bian)2− (D) according to
single-crystal X-ray diffraction and DFT calculationb (italic)
Compound
2
5
7
8
A
B
C
D
N–C
1.385(3)
1.395(3)
1.400
1.395(3)
1.408(3)
1.400
1.395(2)
1.403(2)
1.400
1.398(2)
1.391(2)
1.394
1.394(8)
1.418(9)
1.413
1.370(3)
1.371(3)
1.380
1.387(5)
1.389(5)
1.388
1.368
1.364
1.397
1.362(4)
1.374
1.433(4)
1.429(4)
1.440
1.400
1.348(3)
1.374
1.429(3)
1.437(3)
1.453
1.400
1.357(2)
1.374
1.443(2)
1.450(2)
1.458
1.394
1.357(3)
1.376
1.435(3)
1.436(3)
1.445
1.416
1.39(1)
1.393
1.833(6)
1.839(5)
1.847
1.380
1.383(3)
1.390
1.918(1)
1.924(1)
1.894
1.388
1.373(5)
1.372
1.379(5)
1.377(6)
1.386
C(1)–C(2)
N–Ea
1.421
—
1.440
2.313
2.331
1.449
2.313
2.336
1.458
2.317
2.337
1.445
2.302
2.318
1.850
2.718
2.759
1.894
2.678
2.645
1.386
2.157
2.164
N⋯N
3.020
a E = B for 2, 5, 7, 8; E = Al for A; E = Ga for B; E = С for B. b DFT results for all complexes except compound B3e were obtained in this work.
than that in the free dpp-bian2− dianion (1.421 Å). The NBO of the boron atom as compared to aluminium and gallium
approach describes this C–C bond as a double bond. The corres- results in a more covalent character of the N–B bond.
ponding ellipticity parameter (0.380) found from the AIM Indeed, the AIM analysis shows that on going from (dpp-bian)-
analysis19 appears to be very close to that for the CvC bond in AlCl to 2 the electron density in the (3, −1) critical point of
ethylene (0.375). On the other hand, the calculated C–N dis- the N–E bond (E = Al, B) increases from 0.090 to 0.195 a.u.
tances increases on going from the dianion to the complexes. Accordingly, the local orbital locator20 (LOL) profile demon-
These changes are caused by the formation of the coordinative strates an increase in the LOL value and a substantial shift of
bond (dpp-bian)–E (E = B, C, Al, Ga). A higher electronegativity the LOL maximum corresponding to the N–E bond towards
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Fig. 7 The LOL contour map (the 0.55–0.70 a.u. range, step 0.05 a.u.) in the
plane orthogonal to the metallacycle for the N–Al bond in (dpp-bian)AlCl (right)
and the N–B bond in 2 (left).
Fig. 9 UV-Vis spectra of compounds 2 and B (toluene, 298 K).
the UV-Vis spectra on going from compound B to compound 2
(Fig. 9).
Conclusions
In conclusion, we have prepared a series of 1,3,2-diazaboroles
derived from acenaphthene-1,2-diimine – compounds 2–8. Due
to the delocalisation of the p-electrons over the five-membered
heterocycles and shielding of boron atom with isopropyl groups
the title compounds reveal good stability against oxygen and
moisture. In contrast, (dpp-bian)BCl2, which contains dpp-bian
radical-anion, is air- and moisture sensitive. Attempted syntheses
of diboranes starting from chlorine and bromine boron precur-
sors 2 and 3 failed, probably due to the instability of the B–B
bonds arising from the sterical hindrances caused by bulky dpp-
bian ligands. In contrast to our expectation compound 2 is not
able to add phenylacetylene across B–N–C section in the five-
membered cycle in the way observed in Ga and Al complexes of
the dpp-bian dianion. We believe the energy of the HOMO in
compound 2 (−4.87 V) is too low to interact effectively with the
LUMO of the alkyne to furnish the formation of the cycloadduct.
Fig. 8 Frontier orbitals of compound 2.
Table 3 The frontier orbital energies and HOMO–LUMO energy gaps (eV) in
the dpp-bian complexes of boron, aluminium and gallium
Complex
HOMO
LUMO
ΔELUMO–HOMO
(dipp-bian)B–Cl (2)
(dipp-bian)B–OH (5)
(dipp-bian)B–NH2 (7)
(dipp-bian)B–H (8)
(dipp-bian)Al–Cl
−4.88
−4.50
−4.25
−4.78
−4.43
−4.13
−1.75
−1.56
−1.47
−1.64
−1.59
−1.58
3.13
2.94
2.78
3.14
2.84
2.55
(dipp-bian)2Ga2 (B)
the boron atom when one goes from (dpp-bian)AlCl to
complex 2 (Fig. 7). The LOL increase implies a reduction in the
quantum kinetic energy density of electrons caused by the
covalent bonding.21
Experimental
General procedures
All manipulations were carried out using Schlenk techniques.
Diimine dpp-bian22 has been prepared according to literature
procedure. Melting points were measured in sealed capillaries
and are uncorrected. The solvents were distilled from sodium-
The HOMO and LUMO shapes for the boron compounds
investigated are very similar. Those of 2 are depicted in Fig. 8.
The HOMO is N–B bonding but B–Y (Y = Cl, OH, NH2) anti-
bonding. The LUMO is localized mainly on the dpp-bian
ligand. Similar MO isosurfaces were found for the dpp-bian
gallium complexes.4d The frontier orbital energies are given in
Table 3. The HOMO energies of the boron complexes decrease
in the order 7 > 5 > 8 > 2. They are lower than that of digallane
B.4d It provides a higher stability of the boron derivatives
against oxygen. The HOMO–LUMO energy gap increases on
going from compound B to the boron dpp-bian complexes
(Table 3). For example, the ΔELUMO–HOMO value for the 2 mole-
1
benzophenone prior to use. The H NMR spectra were recorded
on Bruker DPX-200 NMR and Bruker Advance III 400 spectro-
meters; IR spectra – on a FSM-1201 spectrometer; ESR spectra
– on a Bruker EMX instrument. The yields of the products calcu-
lated on the amount of the dpp-bian used in the syntheses.
1,2-Bis[(2,6-diisopropylphenyl)imino]acenaphtheneboron-
dichloride (dpp-bian)BCl2 (1)
cule is 0.58 eV higher than that for compound B. It correlates To a solution of 0.5 g of dpp-bian (1.0 mmol) in toluene
well with a 0.38 eV blue shift of the lowest absorption band in (40 mL) 0.023 g (1.0 mmol) of sodium metal and 0.12 g
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(1.0 mmol) of BCl3 were added. The mixture was stirred at 936s, 905w, 893w, 814vs, 803vs, 766vs, 760vs, 755s, 688s, 638s,
80 °С till the sodium dissolved completely. The precipitated 613s, 573w, 554s, 540w, 519m, 481m, 463w.
sodium chloride was filtered off. From the concentrated
toluene solution compound (dpp-bian)BCl2 was isolated
as
a red microcrystalline powder. Yield 0.38 g (65%).
1,2-Bis[(2,6-diisopropylphenyl)imino]acenaphtheneboron-
M.p. 290–295 °С. Anal. calcd for C36H40BCl2N2: C 74.24,
H 6.92, Cl 12.17; found: C 74.28, H 6.95, Cl 12.20. IR (Nujol,
cm−1): ν = 3071m, 3046m, 1938w, 1868w, 1813w, 1680w,
1655w, 1619s, 1596w, 1563s, 1539s, 1501vs, 1452w, 1428w,
1405m, 1370m, 1345s, 1331m, 1314s, 1260m, 1250s, 1227s,
1200s, 1184w, 1115w, 1063m, 1048w, 997m, 967w, 941s, 852s,
828w, 822s, 810vs, 771vs, 758m, 751m, 695m, 656s, 631m. ESR
(toluene, 293 K): g = 2.00243; A(14N) = 0.40 (2 N), A(11B) = 0.50
(1 B), A(10B) = 0.167 (1 B), A(35Cl) = 0.45 (2 Cl), A(37Cl) = 0.375
(2 Cl), A(1H) = 0.108 (4 H) mT.
phenylacetylenide (dpp-bian)BCuCPh (4)
To a solution of 0.55 g (1.0 mmol) of (dpp-bian)BCl in THF
(30 mL) 0.25 g (2.5 mmol) of phenylacetylene and 0.014 g
(2.0 mmol) of lithium were added. The mixture was stirred at
80 °С till the metal dissolved completely. The precipitated
lithium chloride was filtered off. Solvent was removed in
vacuum and the residue was dissolved in hexanes. Compound
(dpp-bian)BСuCPh was isolated from hexanes as needle like
deep red crystals. Yield 0.2 g, (33%). M.p. 270–275 °С. Anal.
calcd for C44H45BN2: C 86.26, H 7.40; found C 86.13, H 7.51.
1H NMR (400 MHz, C6D6, ppm): δ 7.31 (dd, 2 Harom, 6.8 and
8.3 Hz), 7.23 (2 d, 4 Harom, CH 6.8 and 8.3 Hz), 7.19 (d, 2
1,2-Bis[(2,6-diisopropylphenyl)imino]acenaphtheneboron-
chloride (dpp-bian)BCl (2)
H
H
arom, 8.3 Hz), 6.89 (dd, 2 Harom, 6.8 and 8.3 Hz), 6.71 (d, 2
arom, 6.8 Hz), 3.43 (sept, 4 H, CH(CH3)2, 6.8 Hz), 1.33 (d, 12
To a solution of 0.5 g of dpp-bian (1.0 mmol) in toluene
(40 mL) 0.05 g (2.2 mmol) of sodium metal and 0.12 g H, CH(CH3)(CH3), 6.8 Hz), 1.05 (d, 12 Н, CH(CH3)(CH3), 6.8
(1.0 mmol) of BCl3 were added. The mixture was stirred at Hz). 11B NMR (128.39 MHz, C6D6, ppm): δ 24.8 (s, 1 B). IR
80 °С till the sodium dissolved completely. The precipitated (Nujol, cm−1): ν = 3065m, 3046m, 3020m, 2182m, 1929w,
sodium chloride was filtered off. From toluene compound 1862w, 1794w, 1613s, 1596m, 1491w, 1400m, 1350w, 1344m,
(dpp-bian)BCl was isolated in the form of red crystals. Yield 1335w, 1277w, 1254m, 1217m, 1187w, 1175m, 1107w, 1068w,
0.43 g (79%). M.p. 247–253 °С. Anal. calcd for C36H40BClN2: 1058m, 1030w, 937m, 916w, 816s, 806m, 768s, 754vs, 690s,
C 79.05, H 7.37, Cl 6.48; found: C 78.65, H 7.50, Cl 6.40. 665m, 613m, 579w, 561w, 537m, 494w, 437w.
1H NMR (400 MHz, C6D6, ppm): δ 7.31 (dd, 2 Harom, 6.8 and
8.3 Hz), 7.23 (2 d, 4 Harom, 6.8 and 8.3 Hz), 7.19 (d, 2 Harom
,
8.3 Hz), 6.89 (dd, 2 Harom, 6.8 and 8.3 Hz), 6.71 (d, 2 Harom, 6.8
Hz), 3.43 (sept, 4 H, CH(CH3)2, 6.8 Hz), 1.33 (d, 12 H, CH(CH3)-
(CH3), 6.8 Hz), 1.05 (d, 12 Н, CH(CH3)(CH3), 6.8 Hz). 11B NMR
(128.39 MHz, C6D6, ppm): δ 24.8 (s, 1 B). IR (Nujol, cm−1):
ν = 3067w, 2725w, 2672w, 1931w, 1860w, 1786w, 1719w, 1613s,
1589m, 1533s, 1399m, 1339m, 1307m, 1244s, 1221s, 1194s,
1177m, 1107w, 1059s, 961w, 936s, 814vs, 804vs, 764vs, 753m,
689m, 675w, 650m, 644m, 613m, 575s.
1,2-Bis[(2,6-diisopropylphenyl)imino]acenaphtheneboron-
hydroxide (dpp-bian)BOH (5)
(a) To a solution of 0.55 g (1.0 mmol) of (dpp-bian)BCl in THF
(30 mL) 0.056 g (1.0 mmol) of potassium hydroxide was added.
Over the course of stirring at 80 °С for 24 h the reaction
mixture turned violet. The precipitate of potassium chloride
was filtered off. THF was removed under vacuum and the
residue was dissolved in diethyl ether. From the concentrated
solution compound (dpp-bian)BOH was isolated as violet pris-
matic crystals. Yield 0.36 g (68%). (b) To a solution of 0.61 g
(1 mmol) of (dpp-bian)BCuCPh in diethyl ether 0.018 g
1,2-Bis[(2,6-diisopropylphenyl)imino]acenaphtheneboron-
bromide (dpp-bian)BBr (3)
To a solution of 0.5 g of dpp-bian (1.0 mmol) in toluene (1 mmol) of distilled water was added by condensation. From
(40 mL) 0.05 g (2.2 mmol) of sodium metal and 0.25 g the concentrated solution compound (dpp-bian)BOH was iso-
(1.0 mmol) of BBr3 were added. The mixture was stirred at lated as violet prismatic crystals. Yield 0.32
g (61%).
80 °С till the sodium dissolved completely. The precipitated M.p. 225–230 °С. Anal. calcd for C36H41BN2O: C 81.81, H 7.82;
1
sodium bromide was filtered off. From toluene compound found: C 81.78, H 7.77. H NMR (400 MHz, C6D6, ppm): δ 7.30
(dpp-bian)BBr was isolated in the form of red crystals. Yield (dd, 2 Harom, 6.5 and 8.8 Hz), 7.23 (2d, 4 Harom, 6.5 and 8.8
0.49 g (83%). M. p. 295 °С. Anal. calcd for C36H40BBrN2: Hz), 7.16 (d, 2 Harom, 8.3 Hz), 6.89 (dd, 2 Harom, 7.0 and 8.3
C 73.11, H 6.82, Br 13.51; found: C 73.17, H 6.73, Br 13.43. Hz), 6.66 (d, 2 Harom, 7.0 Hz), 3.50 (sept, 4 H, CH(CH3)2, 6.8
1H NMR (200 MHz, C6D6, ppm): δ 7.45–7.19 (m, 8 Harom), 6.99 and 7.0 Hz), 2.77 (s, 1 H, OH), 1.27 (d, 12 H, CH(CH3)(CH3), 6.8
(dd, 2 Harom, 6.8 and 8.2 Hz), 6.81 (d, 2 Harom, 6.8 Hz), 3.52 Hz), 1.13 (d, 12 Н, CH(CH3)(CH3), 7.0 Hz). 11B NMR
(sept, 4 H, CH(CH3)2, 6.8 Hz), 1.45 (d, 12 H, CH(CH3)(CH3), (128.39 MHz, C6D6, ppm): δ 24.0 (s, 1 B). IR (Nujol, cm−1):
6.8 Hz), 1.19 (d, 12 Н, CH(CH3)(CH3), 6.8 Hz). IR (Nujol, cm−1): ν = 3636m, 3175w, 2724w, 2672w, 1937w, 1910w, 1868w,
ν = 3067m, 3049m, 3040m, 2361w, 1958w, 1929m, 1912w, 1792w, 1717w, 1667w, 1611m, 1587m, 1539s, 1425w, 1397w,
1861w, 1798w, 1788w, 1720w, 1614s, 1589w, 1531s, 1508w, 1253m, 1219w, 1208w, 1173m, 1154w, 1102w, 1058m, 1044w,
1422m, 1399s, 1364s, 1337s, 1300s, 1254s, 1238s, 1220s, 995m, 969w, 947w, 937w, 925w, 916w, 900w, 835w, 814s, 806s,
1186s, 1140m, 1108m, 1059s, 1041m, 1032m, 1000w, 960m, 766s, 750m, 735s, 723s, 695m, 675w, 660m, 613m.
7958 | Dalton Trans., 2013, 42, 7952–7961
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Potassium-1,2-bis[(2,6-diisopropylphenyl)imino]acenaph-the
neborolate (dpp-bian)BOK (6)
LiAlH4 was filtered off. From concentrated toluene solution
compound (dpp-bian)BH was isolated in the form of needle
like prismatic crystals. Yield 0.46 g (90%). M.p. 298 °С. Anal.
calcd for C36H41BN2: C 84.36, H 8.06; found: C 84.28, H 8.12.
1H NMR (200 MHz, C6D6, ppm): δ 7.33 (dd, 2 Harom, 9.2 and
To a solution of 0.55 g (1.0 mmol) of (dpp-bian)BCl in THF
(30 mL) 0.2 g (3.6 mmol) of potassium hydroxide was added.
Over the course of stirring at 80 °С for 24 h the reaction
mixture turned deep blue. The precipitated solid was filtered
off. THF was removed under vacuum and the residue was dis-
solved in diethyl ether. From the concentrated solution com-
pound (dpp-bian)BOK was isolated as deep blue prismatic
crystals. Yield 0.36 g (63%). M.p. > 300 °С. Anal. calcd for
C36H40BN2OK: C 76.31, H 7.12; Found: C 76.23, H 7.19.
1H NMR (400 MHz, C6D6, ppm): δ 7.14 (m, 8 Harom), 6.92 (dd,
2 Harom, 6.8 and 8.3 Hz), 6.59 (d, 2 Harom, 6.8 Hz), 3.68 (sept,
4 H, CH(CH3)2, 7.0 Hz), 3.24 (q, 4 H, (CH3CH2)2O, 7.0 Hz), 1.16
(d, 12 H, CH(CH3)(CH3), 7.0 Hz), 1.13 (d, 12 Н, CH(CH3)(CH3),
7.0 Hz), 1.01 (t, 6 H, (CH3CH2)2O, 7.0 Hz) – 11B NMR
5.4 Hz), 7.26 (2 d, 4 Harom, 9.2 and 5.4 Hz), 7.21 (d, 2 Harom
,
8.2 Hz), 6.92 (dd, 2 Harom, 8.2 and 6.8 Hz), 6.74 (d, 2 Harom, 6.8
Hz), 3.47 (sept, 4 H, CH(CH3)2, 6.8 Hz), 1.28 (d, 12 H, CH(CH3)-
(CH3), 6.8 Hz), 1.14 (d, 12 Н, CH(CH3)(CH3), 6.8 Hz). 11B NMR
(128.39 MHz, ppm): δ 24.8 (s, 1 B). 13C NMR (100.62 MHz,
C6D6, ppm): 146, 137, 135, 130, 128, 128, 127, 124, 118, 68, 29,
25, 25, 23. IR (Nujol, cm−1): ν = 3069m, 3042m, 2724w, 2585s,
2311s, 1920w, 1859w, 1796w, 1613s, 1589m, 1532s, 1422m,
1400s, 1364m, 1335m, 1254s, 1231w, 1219s, 1175s, 1169s,
1140w, 1122w, 1109m 1094m, 1059m, 1044m, 999w, 965w,
937s, 893m, 848m, 814vs, 805vs, 781w, 765vs, 760vs, 670m,
625m, 550w, 499m.
(128.39 MHz, C6D6, ppm):
δ 21.0 (s, 1
B). 13C NMR
(100.62 MHz, ppm): 147.8, 140.69, 133.16, 130.63, 127.14,
126.37, 123.37, 123.08, 116.13, 65.55, 28.31, 23.77, 23.16,
15.21. IR (Nujol, cm−1): ν = 3032w, 2722w, 1607w, 1555m,
1543w, 1514s, 1497s, 1393m, 1360w, 1342w, 1333w, 1250m,
1213w, 1200m, 1175w, 1163w, 1148m, 1118w, 1075w, 1055m,
974m, 936w, 808vs, 792m, 780w, 760vs, 743m, 722w, 704s,
673s, 614m.
DFT calculations
The DFT calculations were carried out with the Gaussian03
program package23 using the 6-31G(d) basis set24 and the
B3LYP hybrid functional.25 The molecular geometries were
fully optimized and the vibration frequencies were analytically
computed. The absence of imaginary frequencies testified for
the energy minimum. The NBO26 and AIM19 analyses were
also performed. For calculation of the AIM charges the
AIMALL program27 was employed. The LOL values were
obtained with the Multiwfn multifunctional wavefunction
analyzer.28
1,2-Bis[(2,6-diisopropylphenyl)imino]acenaphtheneboron-
amide (dpp-bian)BNH2 (7)
To a solution of 0.023 g (1.0 mmol) of sodium in liquid
ammonia (20 mL) at −45 °С 0.55 g (1.0 mmol) of (dpp-bian)-
BCl was added. Within several minutes stirring at −45 °С the
reaction mixture changed from red to green-brown. Ammonia
was evaporated at reduced pressure and the residue was dis-
solved in toluene (20 mL) resulting in a violet solution. Precipi-
tated sodium chloride was filtered off. From concentrated
toluene solution complex (dpp-bian)BNH2 was isolated as
needle like violet crystals. Yield 0.34 g (65%). M.p. 230–233 °С.
Single-crystal X-ray structure determination
The data for 2, 5, 7 and 8 were collected on a Bruker SMART
APEX diffractometer, using graphite monochromated MoKα
radiation (ω-scan technique, λ = 0.71073 Å). The structures
were solved by direct methods using SHELXS-9729a and were
refined on F2 using SHELXL-97.29b SADABS29c was used to
perform area-detector scaling and absorption corrections. All
non-hydrogen atoms were refined anisotropically. Hydrogen
atoms were placed in calculated positions and refined in the
“riding-model” (Uiso(H) = 1.5Ueq.(C) in CH3-groups and Uiso(H)
= 1.2Ueq.(C) in other ligands), except for those of OH and NH2
groups in 5, 7 and H(1) at the boron atom in 8, which were
obtained from differential Fourier-synthesis and refined isotro-
pically. In complexes 5, 7 and 8 the i-Pr groups are disordered
over two positions. CCDC 916524 (2), 916525 (5), 916526 (7)
and 916527 (8) contain the supplementary crystallographic
data for this paper.
Anal. calcd for C36H42BN3: C 81.96, H 8.02; found: C 82.08,
1
H 7.94. H NMR (400 MHz, C6D6, ppm): δ 7.32 (dd, 2 Harom
,
6.5 and 8.8 Hz), 7.25 (2d, 4 Harom, 6.5 and 8.8 Hz), 7.15 (d, 2
H
H
arom, 8.3 Hz), 6.89 (dd, 2 Harom, 7.0 and 8.3 Hz), 6.64 (d, 2
arom, 7.0 Hz), 3.56 (sept, 4 H, CH(CH3)2, 7.0 Hz), 1.37 (s, 2 H,
NH2), 1.28 (d, 12 H, CH(CH3)(CH3), 7.0 Hz), 1.14 (d, 12 Н, CH
(CH3)(CH3), 7.0 Hz). 11B NMR (128.39 MHz, C6D6, ppm):
δ −30.2 (s, 1 B). IR (Nujol, cm−1): ν = 3507m, 3426m, 3066m,
1933w, 1864w, 1784w, 1611m, 1586vs, 1540s, 1399m, 1364w,
1346w, 1308w, 1253m, 1217w, 1208w, 1177m, 1152w, 1107w,
1059m, 1042w, 978w, 947w, 935m, 895w, 812vs, 806s, 762vs,
710w, 681w, 671w, 654s, 614m, 578w, 561w, 528w.
1,2-Bis[(2,6-diisopropylphenyl)imino]acenaphtheneborane
(dpp-bian)BH (8)
Acknowledgements
To a solution of 0.55 g (1.0 mmol) of (dpp-bian)BCl in THF
(30 mL) 0.1 g (2.63 mmol) of LiAlH4 was added. The reaction This work was supported by the Russian Foundation for Basic
mixture was stirred at reflux for 60 h. To the solid left after Research (Grants No. 12-03-01016, 11-03-97051 and 13-03-
evaporation of THF toluene (25 mL) was added. An excess of 00542).
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