Mono-and Bimetallic Aluminum Alkyl, Alkoxide, Halide and Hydride Complexes of a Bulky Conjugated Bis-Guanidinate(CBG) Ligand and Aluminum Alkyls as Precatalysts for Carbonyl Hydroboration

Article abstract of DOI:10.1021/acs.inorgchem.9b03778

Tetra-aryl-substituted symmetrical conjugated bis-guanidine (CBG) ligands such as L^1-3 (3H) [L(3H) = {(ArHN)(ArHN)C=N-C=NAr(NHAr)}; Ar = 2,6-Me₂-C₆H₃ (L¹(3H)), 2,6-Et₂-C₆H₃ (L²(3H)), and 2,6-ⁱPr₂-C₆H₃ (L³(3H))] have been employed to synthesize a series of four-and six-membered aluminum heterocycles (1-8) for the first time. Generally, aluminum complexes bearing N,N′-chelated guanidinate and β-diketiminate/dipyrromethene ligand systems form four-and six-membered heterocycles, respectively. However, the conjugated bis-guanidine ligand has the capability of forming both four-and six-membered heterocycles possessing multimetal centers within the same molecule; this is due to the presence of three acidic protons, which can be easily deprotonated (at least two protons) upon treatment with metal reagents. Both mono-and dinuclear aluminum alkyls and mononuclear aluminum alkoxide, halide, and hydride complexes have been structurally characterized. Further, we have demonstrated the potential of mononuclear, six-membered CBG aluminum dialkyls in catalytic hydroboration of a broad range of aldehydes and ketones with pinacolborane (HBpin).

Full text of DOI:10.1021/acs.inorgchem.9b03778

pubs.acs.org/IC
Article
Mono- and Bimetallic Aluminum Alkyl, Alkoxide, Halide and Hydride
Complexes of a Bulky Conjugated Bis-Guanidinate(CBG) Ligand and
Aluminum Alkyls as Precatalysts for Carbonyl Hydroboration
Thota Peddarao, Nabin Sarkar, and Sharanappa Nembenna*
Cite This: https://dx.doi.org/10.1021/acs.inorgchem.9b03778
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: Tetra-aryl-substituted symmetrical conjugated bis-guanidine
(CBG) ligands such as L1−3 (3H) [L(3H) = {(ArHN)(ArHN)CN−C
NAr(NHAr)}; Ar = 2,6-Me2-C6H3 (L1(3H)), 2,6-Et2-C6H3 (L2(3H)), and
2,6-iPr2−C6H3 (L3(3H))] have been employed to synthesize a series of four- and
six-membered aluminum heterocycles (1−8) for the first time. Generally,
aluminum complexes bearing N,N′- chelated guanidinate and β-diketiminate/
dipyrromethene ligand systems form four- and six-membered heterocycles,
respectively. However, the conjugated bis-guanidine ligand has the capability of
forming both four- and six-membered heterocycles possessing multimetal centers
within the same molecule; this is due to the presence of three acidic protons,
which can be easily deprotonated (at least two protons) upon treatment with
metal reagents. Both mono- and dinuclear aluminum alkyls and mononuclear
aluminum alkoxide, halide, and hydride complexes have been structurally
characterized. Further, we have demonstrated the potential of mononuclear, six-
membered CBG aluminum dialkyls in catalytic hydroboration of a broad range of aldehydes and ketones with pinacolborane
(HBpin).
INTRODUCTION
■
Lappert and co-workers reported the first guanidinate
stabilized transition metal complex in 1970.1 Since then, the
N,N′,N″,N‴-tetra-substituted guanidinate anions (unsubsti-
tuted guanidine core, N3C1H5) have found extensive utility
as supporting ligands for the main group as well as the
transition or lanthanide metal complexes in organometallic
chemistry.2 However, synthesis and catalytic application of
guanidinate aluminum complexes are very limited in the
literature.3 On the other hand, the synthesis and coordination
chemistry of biguanides, a guanylate derivative of guanidine
(N5C2H7), date back to the late 19th century.4 As far as
coordination chemistry of biguanides with aluminum is
concerned, in the year 1964, Nandi et al. reported a complex
Figure 1. Selected literature examples of N,N′-chelated aluminum
alkyl heterocycles, A−F.7
of Al(III) biguanide, [C2N5H7)2Al(OC2H5)Cl]Cl. To our
knowledge, this is the only example reported in the literature,
which is confirmed by IR and CHN analyses.5 Surprisingly, to
date, there are no reports on N,N′,N″,N‴-tetra-aryl substituted
biguanides or conjugated bis- guanidinate (N5C2H3Ar4 core)
metal complexes in the literature. Recently, our group
developed a series of tetra-aryl substituted conjugated bis-
guanidines (CBG);6 these are analogues of popular β-
diketimine or NacNac ligands (see Figure 1).7
Received: December 30, 2019
Generally, N,N′-chelated β-diketiminate8/dipyrromethene9
and triazapentadienate10 ligands, upon coordination with any
metal (element) center or aluminum, form C3N2M or
https://dx.doi.org/10.1021/acs.inorgchem.9b03778
© XXXX American Chemical Society
Inorg. Chem. XXXX, XXX, XXX−XXX
A

Inorganic Chemistry
pubs.acs.org/IC
Article
C3N2Al11 and C2N3M or C2N3Al six-membered heterocycles,
respectively. Tetra substituted guanidinate anions12 form
CN2M or CN2Al four-membered heterocycles. However, Wei
and co-workers reported structurally characterized mononu-
clear six-membered bis-guanidinate supported aluminum
dialkyl complexes, by two synthetic routes: (i) insertion of
carbodiimide into a tetra-substituted guanidinate aluminum
dialkyl complex and (ii) reaction of the lithium salt of
guanidine with AlMe2Cl followed by the treatment of
carbodiimide.7e Very recently, our group reported four-
membered dinuclear aluminum dialkyl complexes bearing a
dinucleating tetra-substituted guanidinate anion (see Figure
1).7f
trimethylaluminum solution. When the reaction mixture of
L1(3H) and trimethylaluminum in a 1:1 molar ratio was stirred
at room temperature for 15 h, formation of compound 1
(Scheme 1) was observed as a colorless compound in good
Scheme 1. Synthesis of Aluminum Alkyl and Halide
Complexes of CBG (1−4)
However, guanidine ligands with two monoanionic N,N′-
chelating sites (ditopic) and, upon coordination with
aluminum, that can lead to the formation of both four- and
six-membered heterocycles containing CN2Al and C2N3Al
core, respectively, so far have not been documented in the
literature.
Thus, herein, we report well-defined mono- and dinuclear
aluminum alkyls and mononuclear aluminum alkoxide, halide,
and hydride complexes bearing a tetra-aryl substituted
conjugated bis-guanidinate (CBG) anion for the first time. It
is worth mentioning that, compared to the synthesis of Wei’s
bis-guanidinate aluminum complex, E, we employed a simple
deprotonation method, i.e., treatment of CBG with an AlMe3.
More importantly, we have shown that CBG can be used as a
dinucleating ligand with two anionic N,N′-chelating sites.
Moreover, CBG supported aluminum dialkyls perform as
active precatalysts for the reduction of numerous aldehydes
and ketones with pinacolborane (HBpin).
yield (65%). Similarly, the reaction between L2(3H) and
AlMe3 in toluene in a 1:1 stoichiometric ratio led to the
formation of compound 2 in a 70% yield. However, a 1:3
molar ratio of L1(3H) and trimethylaluminum solution at
room temperature for 15 h allowed the formation of
compound 3 (Scheme 1). Further, a reaction of aluminum
dialkyl complex 1 with two equivalents of molecular I2 in C6D6
at 60 °C for 6 h was performed, which affords compound 4
[L1(2H)AlI2] (Scheme 1).
Compounds 1−4 have been spectroscopically characterized.
Both mono- and dinuclear aluminum alkyl and halide
complexes 1−4 exhibit the anticipated resonances in the
NMR spectra (1H and 13C{1H}) and the same with their
configuration. The 1H NMR spectra of aluminum dialkyl
complexes 1 and 2 display sharp signals at −0.48 ppm and
−0.45 ppm, respectively, assigned to the six hydrogens from
the Al(CH3)2 group. Moreover, complete disappearance of N−
H−N resonances of free ligands L1(3H) and L2(3H) at 12.91
and 12.97 ppm in C6D6, respectively suggests the formation of
compounds 1 and 2.
RESULTS AND DISCUSSION
■
Synthesis and Spectroscopic Characterization of
Aluminum Complexes. A new type of conjugated bis-
guanidine (CBG) ligand of varying steric bulk such as
L1−3(3H) [L(3H) = {(ArHN)(ArHN)CN−CNAr-
(NHAr)}; Ar = 2,6-Me2-C6H3 (L1(3H)), 2,6-Et2-C6H3
(L2(3H)), and 2,6-iPr2-C6H3 (L3(3H))] have been employed
for the preparation of aluminum complexes of CBG (see
Figure 2).
1
The H NMR spectrum of 3 reveals two singlets at −0.54
ppm and −0.77 ppm in C6D6 corresponding to six protons of
each of the Al(CH3)2 groups, which implies that compound 3
is a dinuclear aluminum alkyl complex. And also, there was a
complete disappearance of the N-H-N resonance at 12.91 ppm
of L1(3H) and one H of the side arm (ArN−H)2 resonances at
1
4.62 ppm. Moreover, the H NMR spectrum of compound 3
shows one singlet at 5.0 ppm, which corresponds to side arm
1
ArN−H. The H NMR spectrum of compound 4 reveals a
complete removal of one singlet resonance of Al(CH3)2 at
−0.48 ppm. In 13C{1H} NMR spectra, a distinctive peak for
the carbon atom of the N3C core of the CBG ligand was
observed at 157.5, 158.3, and 158.0 ppm in C6D6, for
compounds 1, 2, and 4, respectively. However, compound 3
shows two peaks for the N3C at 154.9 (six-membered ring)
and 164.0 (four-membered ring) ppm; this is due to the
unsymmetrical nature of the dinuclear aluminum complex. The
13C{1H} NMR spectra of 1 and 2 exhibit a typical signal for
Al(CH3)2 at −7.5 and −8.5 ppm, respectively, while
compound 3 displays two peaks at −7.51 and −7.91 ppm.
However, the 13C{1H} NMR spectrum of compound 4 reveals
a complete disappearance of the Al(CH3)2 peak, which
suggests the alkyl halide exchange in compound 4. Further,
[L1(2H)AlI2] 4 was confirmed by solid-state structural
Figure 2. N−H and N3C resonances of L1−3(3H) CBG ligands.
These ligands are synthesized by using the protocol
developed in our laboratory.6
The synthesis of compounds 1 [L1(2H)AlMe2], 2 [L2(2H)-
AlMe2], and 3 [L1(H)(AlMe2)2] was accomplished by the
reaction of L1(3H) or L2(3H) with trimethylaluminum in
toluene at ambient temperature. The construction of mono- or
dinuclear Al(III) dialkyl complexes of L1−2(3H) was found to
be dependent on the stoichiometric ratio of the metal reagent,
as it leads to different products with different molar ratios of
B
https://dx.doi.org/10.1021/acs.inorgchem.9b03778
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
analysis, which clearly illustrates the existence of two iodide
moieties which are bonded to the Al metal atom (vide infra).
In the case of less bulky CBGs, i.e., L1(3H) and L2(3H), the
formation of Al(III) dialkyl complexes was controlled by the
stoichiometric ratio of the reagent. In contrast, the bulkier
CBG ligand, i.e., L3(3H) supported Al(III) dialkyl complexes,
was monitored by reaction temperatures. Overall, the reactions
of CBGs with an aluminum reagent can be tuned by the bulky
nature of the ligand, stoichiometric ratios, and the reaction
temperature conditions. The stoichiometric ratios play a
crucial role in the formation of products for less bulky ligands,
while for reaction temperatures for bulkier ligands, the latter
case is evidenced by the formation of compounds 5 and 6.
Treatment of L3(3H) with trimethylaluminum in a 1:3
molar ratio in toluene afforded an unsymmetrical aluminum
complex bearing a four-membered metallacycle [L3(2H)-
AlMe2] (5). The crude product was crystallized from toluene
to give 5 as colorless crystals in an isolated yield of 65%. When
the same reaction was performed at an elevated temperature,
110 °C for 24 h, the formation of a mixture of compounds 5
Scheme 3. Synthesis of Aluminum Hydride and Alkoxide
Complexes of CBG (7 and 8)
1
and 6 (Scheme 2) was observed by H NMR spectroscopy.
followed by crystallization from toluene to give 7 as colorless
crystals in good yield (65%).
The N−H−N resonance of L3(3H) at 13.02 ppm has
completely vanished. The side arm ArN−H resonance at 4.96
ppm, which corresponds to two protons in L3(3H), was shifted
to the downfield region at 5.76 ppm in 7. The resonances of
Al−H were not detected in 1H NMR spectroscopy of
compound 7 in C6D6 because of the quadrupolar broadening
on the 27Al center (nuclear spin = 5/2). In the IR spectrum of
7, two sharp bands at 1809 and 1841 cm−1 were noticed,
attributed to the symmetric and asymmetric stretching
frequencies of the Al−H bonds, respectively, and thus
confirming the presence of Al−H bonds. Carbon resonance
“N3C” at 156.5 ppm in L3(3H) was shifted to 159.8 ppm in 7.
When the reaction mixture of 7 and benzyl alcohol in a 1:2
molar ratio was stirred at room temperature in toluene for 12
h, the alkoxide molecule [L3(2H)Al(OCH2Ph)2] (8) was
produced, and the crude product was crystallized from toluene
to give 8 as colorless crystals in good yield (60%). The side
arm ArN−H resonance at 5.76 ppm, which corresponds to two
protons in 7, was shifted to the downfield region at 6.05 ppm
in 8. The methylene (−CH2−) protons of (C6H5CH2O) in 8
resonate at 5.04 ppm. The N3C carbon resonance was
observed at 159.6 ppm of 8 in C6D6.
Scheme 2. Synthesis of Aluminum Alkyl Complexes of CBG
(5 and 6)
Further, a pure compound of 6, i.e., symmetrical [L3(2H)-
(AlMe2)], was withdrawn as colorless crystals in 37% yield by
crystallization of the n-hexane solution containing both 5 and
6.
The 1H NMR spectra of 5 and 6 show sharp singlet peaks at
−0.16 and −0.30 ppm, respectively, for the six protons of the
Al(CH3)2 group. Complete disappearance of the N−H−N
resonance at 13.02 ppm of L3(3H) was noticed in both
compounds 5 and 6. The two equivalent ArN−H resonances at
4.94 ppm in L3(3H) became chemically unequivalent and
resonated at different chemical shift positions as two singlets at
4.86 ppm (ArN−H) and 6.51 ppm (ArN−H) in compound 5;
this is due to the unsymmetric structure of compound 5
(Scheme 2). However, in the case of compound 6, one singlet
resonates at 5.91 ppm, which corresponds to the two
equivalent ArN-H moieties, suggesting the symmetric structure
of compound 6. The 13C{1H} NMR spectra display a
diagnostic peak for the N3C at 168.03 and 159.5 ppm, for
compounds 5 and 6, respectively. Another characteristic peak
for Al(CH3)2 appeared at −0.16 ppm for compound 5, and
such an expected peak is silent in compound 6.
Crystallographic Information. Although the NMR
spectra of these complexes 1−8 are giving conclusive evidence
for their formation, for further confirmation, single-crystal X-
ray structures were also obtained. The composition and
connectivity in all these complexes 1 and 3−8 were established
clearly by the X-ray studies. Single crystals suitable for X-ray
diffraction of all the compounds 1, 3, 4, and 6−8 were grown
from concentrated toluene solution, while for compound 5, the
crystals were grown from the n-hexane solution. Compounds 1,
4, and 7 were crystallized in the monoclinic system with the Pc,
P2(1)/c, and C2/c, space groups, respectively, whereas
compounds 3, 5, 6, and 8 were crystallized in the triclinic
Next, the first well-defined aluminum dihydride complex
bearing an N,N′-chelated conjugated bis-guanidinate anion has
been reported. CBG supported aluminum dihydride can be
synthesized by two synthetic methods (methods A and B;
Scheme 3).
In the first method, the reaction between L3(3H) and
LiAlH4 in a 1:1.8 ratio at reflux temperature in toluene for 5
days led to the formation of [L3(2H)AlH2] (7) in 43% yield.
However, compound 7 can also be synthesized in better yield
by the reaction of L3(3H) with commercially available alane,
AlH3·NMe2Et. The reaction mixture of a 1:1 molar ratio of
L3(3H) and alane in toluene was stirred at 80 °C for 12 h,
̅
with the space group P1. Molecular structures of these
complexes 1 and 3−8 are shown in Figures 3−5.
The crystal data and structure refinement details of
complexes (1 and 3−8) are summarized in (Table S1). In
all cases, the aluminum metal center adopts a distorted
tetrahedral geometry, connected to a guanidinate anion in
N,N′-chelate fashion and two alkyls (1 and 3, 5−6) or two
C
https://dx.doi.org/10.1021/acs.inorgchem.9b03778
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Figure 3. Molecular structures of 1 (left), 3 (middle) and 4 (right).
The thermal ellipsoids are shown at 30% probability, and all the
hydrogen atoms except those bound to nitrogen atoms are deleted for
clarity. Selected bond lengths (Å) and angles (deg), for 1 (left): Al1−
C1 1.969(11), Al1−C2 1.985(9), Al1−N1 1.915(8), Al1−N2
1.895(7), N1−C3 1.354(10), N3−C3 1.317(11), N3−C4
1.353(11), N2−C4 1.317(10); N2−Al1−N1 94.0(3), N2−Al1−C1
111.6(4), N1−Al1−C2 114.0(4), C1−Al1−C2 114.85, C3−N3−C4
124.6(7). For 3 (middle): Al1−C37 1.958(3), Al1−C38 1.953(3),
Al2−C1 1.963(3), Al2−C2 1.957(3), Al1−N4 1.926(2), Al2−N1
Al2−N2 1.920(2), N1−C19 1.336(3), N3−C19 1.404(3), N3−C20
1.352(3), N2−C20 1.336(3), N4−C19 1.329(3), N5−C20 1.346(3);
N1−Al2−N2 93.18(10), N4−Al1−N3 67.80(10), N4−Al1−C37
111.17(12), C38−Al1−C37 118.87(16), C38−Al1−N3 114.43(12),
N1−Al2−C2 109.95(12), C20−N3−C19 125.6(2), N4−C19−N3
106.3(2). For 4 (right): Al1−I1 2.507(2), Al1−I2 2.521(2), Al1−N1
1.853(6), C2−N2 1.351(9), C2−N3 1.342(9), C1−N3 1.341(9),
C1−N1 1.347(9), C2−N5 1.348(9); N1−Al1−N2 97.4(3), N2−
Al1−I1 108.8(2), N1−Al1−I2 109.1(2), I1−Al1−I2 104.51(8), N3−
C2−N2 126.3(6), C1−N3−C2 124.2(6).
Figure 5. Molecular structures of 7 (left) and 8 (right). The thermal
ellipsoids are shown at 30% probability, and all the hydrogen atoms
except those bound to nitrogen atoms are deleted for clarity. Selected
bond lengths (Å) and angles (deg), for 7 (left): Al1−H 1.510(3),
Al1−N1 1.8834(18), N2−C2 1.344(2), N1−C2 1.341(3), N3−C2
1.364(3); N1i−Al1−N1 94.40(11), N1i−Al1−H 109.5(11), C2i−
N2−C2 122.8(2). For 8 (right): Al1−O8 1.7214(13), Al1−O24
1.7079(15), Al1−N1 1.8698(15), Al1−N2 1.8761(15), C1−N1
1.355(2), C1−N3 1.3419(19), C2−N3 1.348(2), C2−N2 1.345(2),
C1−N4 1.363(2), C2−N5 1.364(2), N4−H4 0.860, N5−H5a 0.860;
N1−Al1−N2 94.82(7), N1−Al1−O8 110.28(7), N1−Al1−O24
115.33(7), N2−Al1−O8 113.16(6), N2−Al1−O24 113.06(7), O8−
Al1−O24 109.59(7), N1−C1−N3 118.29(13), C1−N3−C2
122.76(15), N3−C2−N2 126.85(14), N3−C1−N4 116.83(15),
N3−C2−N5 115.29(15).
The Al−C bond lengths in compounds 1, 3, 5, and 6 lie in
the range 1.949(3) Å to 1.985(9) Å and are similar to those
reported N,N′-chelated aluminum complexes (A−F). A:
7a
LAlMe2 (L = HC[C(Me)N(2,6-Me2C6H3)]2 (Al(1)−
C(22) 1.959(2) Å, Al(1)−C(23) 1.964(2) Å). B: [L″AlMe2]
[L″ = {(NArCPh)2N; Ar = Dipp]7b (Al(1)−C(3) 1.958(2) Å,
Al(1)−C(4) 1.950(3) Å). C: [LaAlMe2; La = meso-
mesityldipyrromethene]7c (Al(1)−C(19) 1.962(2) Å). D:
LbAlMe2 [Lb = {ArNC(N(iPr2)N(Ar)} (Ar = Dipp)]7d
(Al(1)−C(32) 1.9688(19) Å, Al(1)−C(33) 1.9726(19) Å).
E: [{N(C6H5)C(NAr)-N(C6H5)C(NEt2)N(Ar)}AlMe2; Ar =
Xyl]7e (Al(1)−C(35) 1.956(3) Å, Al(1)−C(33) 1.9950(3) Å).
F: [L′(AlMe2)2] [L′ = {ArNCNAr}2{μ-N(C2H4)2N}; Ar =
Mes]7f (Al(1)−C(20) 1.949(6) Å, Al(1)−C(21) 1.953(6) Å).
And also, Al−N bond distances are similar to those of reported
aluminum dialkyl complexes (A−F). The Al−I bond lengths
for compound 4, i.e., Al1−I1, are 2.507(2) Å, and Al1−I2 is
2.521(2) Å. These are consistent with the disclosed Al−I bond
lengths of N,N′-chelated aluminum diiodide complexes
[LaAlI2; La = meso-mesityl dipyrromethene Al(1)−I(1) 2.506
(1) Å and Al(1)−I(2) 2.524(1) Å].13 The Al1−H bond
distance in 7 is 1.51(3) Å, and this value is falling in the range
of reported bond lengths Al(1)−H(1) = 1.51(4) and Al(1)−
H(2) = 1.54(4) Å of LaAlH2; La = meso-mesityl dipyrrome-
thene.7c The Al−O bond distances are Al1−O8 = 1.7214(13)
Å and Al1− O24 = 1.7079(15) Å in 8 and are consistent with
that of compound LaAlOtBu2[Al(1)−O(1) = 1.706(2)Å and
Al(1)−O(2) = 1.701(2)Å].9c The N−Al−N bite angles in
compounds 1, 4, and 6−8 in the range of 93.18−97.4° that are
comparable with six-membered aluminum heterocycles [N−
Al−N bite angles in compounds A−F: 95.24(7)° (A),
93.05(8)° (B), 92.1(1)° (C), 91.44(9)° (E), while N−Al−N
bite angles in compounds 3 and 5 are 67.80(10) (for four-
membered ring) and 68.49(7)°, respectively, which are close to
four-membered guanidinatealuminum dimethyl complexes]
[N−Al−N bite angles in compounds D (69.06(6)°) and F
(68.75(16)°) and (69.15(17)°)].
Figure 4. Molecular structures of 5 (left) and 6 (right). The thermal
ellipsoids are shown at 30% probability, and all the hydrogen atoms
except those bound to nitrogen atoms are deleted for clarity. Selected
bond lengths (Å) and angles (deg), for 5 (left): Al1−C26 1.965(2),
Al1−C27 1.949(3), Al1−N1 1.9487(19), Al1−N2 1.9145(17), N4−
C52 1.367(3), N3−C52 1.303(3), N3−C25 1.363(2), N1−C25
1.364(2), N5−C52 1.364(2), N2−C25 1.338(3); N2−Al1−N1
68.49(7), N2−Al1−C27 112.54(9), N1−Al1−C26 115.52(9),
C27−Al1−C26 117.57(10), N3−C52−N4 126.45(17), C52−N3−
C25 124.32(17), N3−C25−N1 130.47(18), N3−C52−N5
119.24(18), N2−C25−N3 122.30(17). For 6 (right): Al1−C1
1.961(3), Al1−C2 1.976(3), N1−Al1 1.912(2), N2−Al1
1.9208(18), C4−N1 1.353(3), C4−N3 1.337(3), C3−N3 1.348(3),
C3−N2 1.340(3), C4−N4 1.367(3), C3−N5 1.366(3), N4−H77
0.860(3), N5−H76 0.870(3); N1−Al1−N2 93.94(8), N1−Al1−C1
111.56(10), N2−Al1−C1 113.67(10), N1−Al1−C2 107.28(10),
N2−Al1−C2 110.00(10), C(1)−Al1−C2 117.69(12), N3−C4−N1
125.62(19), C4−N3−C3 123.94(19), N2−C3−N3 126.86(18), N3−
C4−N4 116.28(19), N3−C3−N5 113.78(19).
iodides (4) or two hydrides (7) or two alkoxide (8) moieties;
thus the aluminum coordination number is four. Further, solid-
state structures revealed that all compounds 1, 4, and 6−8
display six-membered metallacycles containing a C2N3Al ring,
while compound 5 comprises a four-membered metallacycle
with CN2Al ring. Interestingly, compound 3 features both four-
and six-membered metallacycles within a single molecule.
D
https://dx.doi.org/10.1021/acs.inorgchem.9b03778
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Three bonding modes (I−III), as illustrated in Chart 1, are
Table 1. Hydroboration of Benzaldehyde with HBpin
Catalyzed by 1 and 2
a
observed for aluminum(III) complexes bearing either L1(3H)
b
entry
cat.
cat. (mol %)
temp
rt
rt
60 °C
60 °C
60 °C
rt
time
conv(%)
Chart 1. Three Bonding Modes
1
2
3
4
5
6
7
2
2
2
2
2
1
1
5%
3%
1%
1%
1%
3%
1%
12 h
12 h
8 h
6 h
4 h
>99
>99
>99
88
76
>99
>99
12 h
10 h
60 °C
a
All reactions were done on 0.3 mmol scales. Benzaldehyde (1 equiv),
pinacolborane (1 equiv), L1−2(2H)AlMe2 (x mol %), neat.
b
1
Conversion was determined by H NMR spectroscopy.
or L3(3H) ligands. The bonding mode I has been noticed for
the mononuclear aluminum dialkyl (1 and 6), dihalide (4),
dihydride (7), and dialkoxide (8) complexes, while II is seen
for the aluminum dialkyl complex (5). In these bonding modes
(I and II), the N,N′- chelating scaffold is characterized as an
LX-type ligand comprising both sigma- and pi-donor proper-
ties. However, a six-membered metallacycle is noticed in
bonding mode I, while a four-membered metallacycle is
observed in II, which is attributed to the steric nature of the
ligand. Moreover, the bonding mode III type has been
observed for the dinuclear aluminum(III) dialkyl complex, in
which it exhibits double chelation; the L2X2 type ligand
consists of two sigma and two pi donor properties. It is worth
mentioning that in this bonding type, both four- and six-
membered metallacycles within a single molecule have been
detected.
Scheme 4. Hydroboration of Aldehydes using Al-alkyl
Complex (2) as a Catalyst
a
Catalysis: Hydroboration of Aldehydes and Ketones
Catalyzed CBG Aluminum Alkyls. The sustainable and
environmentally friendly aluminum-based reagents/molecules
are ideal for applications in catalysis.14 Hence, the develop-
ment of well-defined aluminum-based catalysts is quite
attractive.15 In 2015, Roesky and co-workers reported the β-
diketiminate supported aluminum hydride as a catalyst for
hydroboration of carbonyl compounds.16 Previously we
reported molecular aluminum hydride catalyzed selective
reduction of a broad range of aldehydes and ketones.17 To
date, there have been few reports on the aluminum catalyzed
reduction of carbonyl compounds.18 However, the transition
metal catalyzed hydroboration of carbonyl compounds has
been extensively investigated.19 Given this, we decided to test
the catalytic activity of newly synthesized aluminum alkyls 1
and 2 as precatalysts for hydroboration of several aldehydes
and ketones.
a
Reaction conditions: aldehyde (0.3 mmol), pinacolborane (0.3
mmol), catalyst 2 (1 mol %), 8 h at 60 °C under N2. Reported
numbers are the NMR conversion.
boronate esters (10a−10l). We noticed the tolerance of halide,
nitrile, ester, alkene, and alkyne functionalities.
Further, we explored the catalytic activity of compounds 1
and 2 for the hydroboration of ketones with HBpin. Initially,
we chose acetophenone as a model substrate. We observed
that 2 mol % of catalyst 1 or 2 at 60 °C afforded the best
results (Table 2).
We began our investigation by using compound 2 as a
catalyst, HBpin as a reducing agent, and benzaldehyde 9a as a
model substrate (Table 1).
We noticed the formation of boronate ester in quantitative
yield in 12 h at room temperature with a catalyst loading of 5
mol % under neat conditions. Further, the same reaction was
carried out by using lower catalyst loadings (3 mol % and 1
mol %). We observed quantitative conversion in both cases;
however, an elevated temperature (60 °C) is required for 1
mol % catalyst loading and less time when compared to 3 mol
%. Catalyst 1 also gave almost similar results to those of
catalyst 2.
With the optimized conditions in hand, we decided to
explore the hydroboration of a wide range of aldehydes
(Scheme 4).
We were satisfied to see, in all cases, a quantitative
conversion of aldehydes (9a−9l) into the corresponding
Accordingly, under these optimized reaction conditions, by
using catalyst 2, hydroboration of ketones (11a−11l) to yield
their corresponding boronate esters (12a−12l) was inves-
tigated, which produced at a quantitative yield for all the cases
within 12 h (Scheme 5).
The intramolecular chemoselective reduction of ketones in
the presence of other functionalities such as halide, nitrile, and
ester was noticed.
Furthermore, we have tested the catalytic activity of other
newly reported aluminum-based compounds such as dinuclear
aluminum alkyl (3), aluminum dihalide (4), mononuclear
bulky aluminum dialkyl (6), and aluminum dihydride (7) for
the hydroboration of carbonyl compounds with HBpin. Both
compounds 3 and 7 exhibit similar catalytic activity to those of
E
https://dx.doi.org/10.1021/acs.inorgchem.9b03778
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Table 2. Hydroboration of Acetophenone with HBpin
Catalyzed by 1 and 2
Scheme 6. Proposed Mechanism for the Hydroboration of
Carbonyl Compounds Catalyzed by Aluminumdialkyl
Complex (2)
a
b
entry
cat.
cat. (mol %)
temp
rt
rt
60 °C
60 °C
60 °C
rt
time
conv (%)
1
2
3
4
5
6
7
2
2
2
2
2
1
1
6%
4%
2%
2%
2%
4%
2%
12 h
12 h
12 h
10 h
8 h
>99
>99
>99
78
66
>99
>99
12h
12 h
60 °C
a
All reactions were done on 0.3 mmol scales. Acetophenone (1
equiv), pinacolborane (1 equiv), L1−2(2H)AlMe2 (x mol %), neat.
b
1
Conversion was determined by H NMR spectroscopy.
Scheme 5. Hydroboration of Ketones using Al Complex (2)
a
as a Catalyst
hydride complexes bearing an N,N′-chelated conjugated bis-
guanidinate ligand are reported for the first time. We have
investigated the difference in reactivity and coordination
properties of ligands L1(3H) and L3(3H) toward the reagent
trimethylaluminum depending upon reaction conditions. We
noticed the formation of mono-[L1(2H)AlMe2] (1) and
dinuclear [L1(H)(AlMe2)2] (3) compounds in the case of
ligand L1(3H) when treated with trimethylaluminum (1.0
equiv) and (3.0 equiv), respectively, at room temperature.
Whereas, in the case of L3(3H), the formation of only
mononuclear unsymmetrical (5) or symmetrical (6) com-
pounds was observed. Compound 1 upon treatment with two
equivalents of iodine in C6D6 led to the formation of
compound 4, in which alkyl-halide interchange took place.
Deprotonation of L3(3H) upon treatment with alane afforded
the aluminum dihydride complex, [L3(2H)AlH2] (7).
Compound 7 was further treated with 2 equiv of benzyl
alcohol, which allowed the formation of aluminum dialkoxide
complex L3(2H)Al(OCH2Ph)2 (8). More importantly, com-
pounds 1 and 2 effectively catalyze the reduction of various
aldehydes and ketones. Compounds 4 and 7 are suitable
starting materials for the preparation of six-membered low
valent aluminum(I) heterocycles.20 At present, such inves-
tigations are ongoing in our laboratory.
a
Reaction conditions: ketone (0.3 mmol), pinacolborane (0.3 mmol),
catalyst 2 (2 mol %), 12 h at 60 °C under N2. Reported numbers are
the NMR conversion.
catalysts 1 and 2 for the hydroboration of benzaldehyde with
HBpin, while compounds 4 and 6 are less active when
compared to aluminum alkyls or hydride catalysts (see Table
S2). Compound 7 shows a similar catalytic activity to those of
catalysts 1 and 2 for the hydroboration of acetophenone with
HBpin, while catalysts 4 and 6 show low conversion of
acetophenone to the corresponding boronate ester (see Table
S3). Thus, both aluminum alkyls and hydride-based catalysts
are ideal for the hydroboration of carbonyl compounds
The mechanism of aluminum alkyl catalyzed hydroboration
of carbonyl compounds was previously reported by other
research groups. The aluminum dialkyl reacts with HBpin and
generates active catalytic species aluminum alkyl-hydride. Next,
the insertion of a carbonyl functionality into the Al−H bond
occurs, which leads to the generation of an aluminum alkoxide
intermediate.
EXPERIMENTAL SECTION
■
General Methods. All air and moisture sensitive reactions were
carried out by using standard Schlenk line and glovebox techniques
under an inert atmosphere of dinitrogen. The precursors L1−3 (3H)
[L(3H) = {(ArHN)(ArHN)CN−CNAr(NHAr)}; Ar = 2,6-
Me2−C6H3,(L1(3H)), 2,6-Et2−C6H3,(L2(3H)), and 2,6-iPr2−C6H3
(L3(3H))] were prepared by using a method developed in our
laboratory.6 AlMe3 (2.0 M in toluene) and alane-N,N-dimethyl ethyl
amine complex (0.5 M) in toluene were purchased from Sigma-
Aldrich and used as received. Anhydrous solvents such as benzene, n-
hexane, and toluene were collected from the solvent purification
system (MBraun), freeze−thawed twice, and stored under an
atmosphere of dinitrogen before use. NMR spectra were recorded
on a Bruker AV 400 MHz spectrometer {1H (400 MHz) and 13C
(100 MHz)} at ambient temperature. Dry deuterated benzene (C6D6)
and chloroform (CDCl3) solvents were utilized for NMR analyses.
Chemical shifts (δ in ppm) in the 1H and 13C{1H} NMR spectra were
referenced to the residual signals of the deuterated solvents.21 High-
Finally, the reaction of the aluminum alkoxide with HBpin
afforded the boronate ester product and regeneration of
aluminum alkyl-hydride catalyst for the further catalytic cycle
(Scheme 6).
CONCLUSION
■
The structurally characterized mono- and dimetallic aluminum
alkyls and monometallic aluminum alkoxide, halide, and
F
https://dx.doi.org/10.1021/acs.inorgchem.9b03778
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
resolution mass spectra (HRMS) were recorded on Bruker micrO-
TOF-Q II spectrometer. IR spectra were recorded on the PerkinElmer
FTIR spectrometer.
The solvent was removed completely, then washed with n-hexane (10
mL). The crude product was crystallized from toluene at 0 °C to give
5 as colorless crystals (0.175 g, 0.219 mmol, 65%). Mp: 190−195 °C.
1H NMR (400 MHz, C6D6, 25 °C): δ −0.16 (s, 6H, Al(CH3)2), 0.72
(d, J = 8.0 Hz, 12H, CH(CH3)2), 1.06 (d, J = 8.0 Hz, 6H,
CH(CH3)2), 1.10 (d, J = 8.0 Hz, 12H, CH(CH3)2), 1.14 (d, J = 8.0
Hz, 6H, CH(CH3)2), 1.28 (d, J = 8.0 Hz, 12H, CH(CH3)2), 2.62
(sept, 2H, CH(CH3)2), 2.88−2.93 (m, 2H, CH(CH3)2), 3.77 (sept,
4H, CH(CH3)2), 4.86 (s, 1H, NH), 6.51 (s, 1H, NH), 6.86 (d, J = 8.0
Hz, 2H, ArH), 6.95 (d, J = 8.0 Hz, 2H, ArH), 7.00 (d, J = 8.0 Hz, 2H,
ArH), 7.09 (s, 6H, ArH). 13C{1H} NMR (100 MHz, C6D6, 25 °C): δ
−8.3 (Al(CH3)2), 23.0 (Ar-iPrC), 24.0 (Ar-iPrC), 26.1 (Ar-iPrC),
26.7 (Ar-iPrC), 28.2 (Ar-iPrC), 28.4 (Ar-iPrC), 28.8 (Ar-iPrC), 123.1
(ArC), 124.4 (ArC), 124.7(ArC), 125.2 (ArC), 130.0 (ArC), 130.5
(ArC), 132.0 (ArC), 138.8 (ArC), 144.3 (ArC), 146.5 (ArC), 148.8
(ArC), 150.0 (ArC), 168.0 (N3C). ESI-TOF HRMS: m/z 798.6163
([M + H]+, calcd. for C52H76AlN5 + H+: 798.5989).
Synthesis of [L1(2H)AlMe2] (1). To a solution of L(3H) (0.25 g,
0.482 mmol, 1.0 equiv) in toluene (20 mL), trimethylaluminum (2.0
M in toluene, 0.25 mL, and 0.504 mmol, 1.0 equiv) was added at 0
°C. The solution was allowed to attain the room temperature, and the
stirring was extended for 15 h. The solvent was removed completely
then washed with n-hexane and was added to the crude solid toluene
(∼10 mL), heated to 70 °C, and slowly cooled to room temperature
to give 1 as colorless crystals (0.180 g, 0.314 mmol, 65%). Mp: 290−
300 °C. 1H NMR (400 MHz, C6D6, 25 °C): δ −0.48 (s, 6H,
Al(CH3)2), 1.85 (s, 12H, CH3), 2.48 (s, 12H, CH3), 4.83 (s, 2H,
ArNH), 6.55 (d, J = 4 Hz, 4H, ArH), 6.74 (t, J = 8 Hz, 2H, ArH),
6.94−6.98 (m, 2H, ArH), 7.01 (d, J = 8 Hz, 4H, ArH). 13C{1H} NMR
(100 MHz, C6D6, 25 °C): δ 8.0 (Al(CH3)2), 18.4 (Ar-CH3), 18.5 (Ar-
CH3), 126.0 (ArC), 126.3 (ArC), 127.1 (ArC), 129.1 (ArC), 135.4
(ArC), 135.5 (ArC), 135.8 (ArC), 139.6 (ArC), 157.1 (N3C). ESI-
TOF: m/z 574.3522 ([M + H]+, calcd. for C36H44AlN5 + H+:
574.3485).
Synthesis of [L3(2H)(AlMe2)] (6). To a solution of L3(3H) (0.25 g,
0.337 mmol, 1.0 equiv) in toluene (15 mL), trimethylaluminum (2.0
M in toluene, 0.595 mL, 1.19 mmol, 3.0 equiv) was added at 0 °C,
and then the reaction mixture was stirred at 110 °C for 24 h.
Formation of a mixture of products was observed. The crude product
was crystallized from n-hexane at 0 °C to give 6 as colorless crystals
Synthesis of [L2(2H)AlMe2] (2). This compound was prepared by
the method described for [L1(2H)AlMe2] (1) by using L2(3H) (0.3 g,
0.48 mmol) and trimethylaluminum (2.0 M in toluene, 0.25 mL, and
0.49 mmol). Compound 2 (0.230 g, 0.335 mmol, 70%). Mp: 270−
290 °C. 1H NMR (400 MHz, C6D6, 25 °C): δ −0.45 (s, 6H,
Al(CH3)2), 0.96 (t, J = 8.0 Hz, 12H, CH3), 1.32 (t, J = 8.0 Hz, 12H,
CH3), 2.16−2.25 (m, 4H, CH2), 2.30−2.37 (m, 4H, CH2), 2.83−2.93
(m, 4H, CH2), 3.18−3.28 (m, 4H, CH2), 5.08 (s, 2H, ArNH), 6.62−
6.64 (d, J = 8.0 Hz, 4H, ArH), 6.76−6.78 (d, J = 8.0 Hz, 2H, ArH),
6.87−6.91 (m, 2H, ArH), 6.98−7.02 (m, 1H, ArH), 7.05−7.11 (m,
3H, ArH). 13C{1H} NMR (100 MHz, C6D6, 25 °C): δ 8.5
(Al(CH3)2), 13.7 (Ar−CH2CH3), 14.0 (Ar−CH2CH3), 23.3 (Ar-
CH2CH3), 24.6 (Ar-CH2CH3), 125.0 (ArC), 125.9 (ArC), 126.6
(ArC), 126.5 (ArC), 134.5 (ArC), 138.9 (ArC), 140.5 (ArC), 140.9
(ArC), 158.3 (N3C).
1
(0.1 g, 0.125 mmol, 37%). Mp: 235−240 °C. H NMR (400 MHz,
C6D6, 25 °C): δ −0.30 (s, 6H, Al(CH3)2), 0.89−0.90 (m, 24H,
CH(CH3)2), 1.33 (d, J = 4 Hz, 12H, CH(CH3)2), 1.38 (d, 8 Hz, 12H,
CH(CH3)2), 3.01−3.06 (sept, 4H, CH(CH3)2), 3.83−3.90 (m, 4H,
CH(CH3)2), 5.91 (s, 2H, NH), 6.79 (d, J = 8 Hz, 4H, ArH), 6.95−
6.98 (m, 2H, ArH), 7.16−7.22 (m, 4H, ArH). 13C{1H} NMR (100
MHz, C6D6, 25 °C): δ 24.9 (Ar-iPrC), 27.1 (Ar-iPrC), 28.5 (Ar-
iPrC), 123.0 (ArC), 125.4 (ArC), 127.07 (ArC), 132.8 (ArC), 137.8
(ArC), 145.2 (ArC), 146.7 (ArC), 159.5 (N3C). ESI-TOF HRMS: m/
z 798.6150 ([M + H]+calcd. for C52H76AlN5 + H+; 798.5989).
Synthesis of [L3(2H)AlH2] (7). Method A. To a mixture of L3(3H)
(2.0 g, 2.694 mmol, 1.0 equiv) and LiAlH4 (0.184 g, 4.850 mmol, 1.8
equiv), toluene (15 mL) was added, and the reaction mixture was
heated at 110 °C for 5 days. Then, the compound was crystallized
from toluene to get colorless crystals of compound 7 with a yield of
0.9 g, 1.169 mmol, 43%.
Synthesis of [L1(H)(AlMe2)2] (3). To a solution of L1(3H) (0.25 g,
0.482 mmol, 1.0 equiv) in toluene (10 mL), trimethylaluminum (2.0
M in toluene, 0.72 mL, 1.44 mmol, 3.0 equiv) was added at 0 °C. The
solution was allowed to warm to room temperature, and the stirring
was extended for 15 h. The solvent was removed completely, then
washed with n-hexane (5 mL). The crude product was crystallized
from toluene (∼10 mL) at 5 °C to give 3 as colorless crystals (0.168
g, 0.267 mmol, 55%). Mp: 205−210 °C. 1H NMR (400 MHz, C6D6,
25 °C): δ −0.77 (s, 6H, Al(CH3)2), −0.54 (s, 6H, (Al(CH3)2)), 2.04
(s, 6H, CH3), 2.21 (s, 6H, CH3), 2.33 (s, 6H, CH3), 2.43 (s, 6H,
CH3), 5.00 (s, 1H, NH), 6.63−6.67 (m, 5H, ArH), 6.76−6.78 (d, J =
8 Hz, 2H, ArH), 6.83−6.87 (m, 1H, ArH), 6.95−7.01 (m, 4H, ArH).
13C{1H} NMR (100 MHz, C6D6, 25 °C): δ −8.2 (Al(CH3)2), −7.9
(Al(CH3)2), 18.4 (Ar-CH3), 18.5 (Ar-CH3), 18.9 (Ar-CH3), 19.05
(Ar-CH3), 19.09 (Ar-CH3), 19.6 (Ar-CH3), 124.1 (Ar-C), 124.5 (Ar-
C), 129.0 (Ar-C), 129.1 (Ar-C), 129.4 (Ar-C), 129.5 (Ar-C), 131.6
(Ar-C), 133.2 (Ar-C), 133.7 (Ar-C), 135.7 (Ar-C), 137.6 (Ar-C),
138.3 (Ar-C), 140.1 (Ar-C), 142.0 (Ar-C), 154.5 (N3C), 163.6 (N3C).
ESI-TOF HRMS: m/z 630.2511 ([M + H]+, calcd. for C38H49Al2N5 +
H+: 630.5211).
Method B. To a solution of L3(3H) (0.25 g, 0.336 mmol, 1.0
equiv) in toluene (10 mL), alane-N,N-dimethylethylamine complex
solution (0.5 M in toluene, 0.7 mL, 0.353 mmol, 1.0 equiv) was added
at room temperature and heated at 80 °C for 12 h. Then, the
compound was crystallized from toluene, to give 7 as colorless crystals
(0.168 g, 0.219 mmol, 65%). Mp: 220−230 °C. 1H NMR (400 MHz,
C6D6, 25 °C): δ 0.81 (d, J = 8.0 Hz, 12H, CH(CH3)2), 1.08 (d, J =
8.0 Hz, 12H, CH(CH3)2), 1.33 (d, J = 8.0 Hz, 12H, CH(CH3)2), 1.50
(d, J = 4.0 Hz, 12H, CH(CH3)2), 3.06−3.13 (m, 4H, CH(CH3)2),
3.78−3.84 (m, 4H, CH(CH3)2), 5.76 (s, 2H, NH), 6.81 (d, J = 8.0
Hz, 3H ArH), 6.94−6.99 (m, 4H ArH), 7.15−7.19 (m, 5H, ArH).
13C{1H} NMR (100 MHz, C6D6, 25 °C): δ 22.3 (Ar-iPrC), 24.3 (Ar-
iPrC), 24.9 (Ar-iPrC), 26.4 (Ar-iPrC), 28.3 (Ar-iPrC), 28.35 (Ar-
iPrC), 122.6 (ArC) 125.0 (ArC), 127.1 (ArC), 132.2 (ArC), 135.8
(ArC), 145.4 (ArC), 146.6 (ArC), 159.4 (N3C). IR (Nujol mull,
cm−1): 3793 (br), 3392(s), 2921, 2726, 2672, 1841s (Al-H), 1809s
(Al-H), 1453, 1376, 1311, 1260, 1094, 1020, 797, 721, 559, 520.
Synthesis of [L3(2H)Al (OCH2Ph)2] (8). To a solution of
[L3(2H)AlH2] (7) (0.15 g, 0.194 mmol, 1.0 equiv) in toluene (10
mL), benzyl alcohol (0.044 g, 43 μL, and 0.409 mmol, 2.0 equiv) was
added at room temperature, and the reaction mixture was stirred at
room temperature for 12 h. The crude compound was crystallized
from toluene to give 8 as colorless crystals (0.115 g, 0.116 mmol,
Synthesis of [L1(2H)AlI2] (4). To a solution of [L1(2H)AlMe2] (1;
0.020 g, 0.0348 mmol, 1.0 equiv) in C6D6 (0.6 mL), iodine (0.018 g,
0.0697 mmol, 2.0 equiv) was added and then the NMR tube was
heated at 60 °C for 6 h. The compound was crystallized from toluene
at 0 °C to give 4 as colorless crystals (0.0194 g, 0.0244 mmol, 70%).
1H NMR (400 MHz, C6D6, 25 °C): δ 1.77 (s, 12H, CH3), 2.68 (s,
12H, CH3), 5.07 (s, 2H, ArNH), 6.48 (d, J = 8.0 Hz, 4H, ArH), 6.68
(t, J = 8 Hz, 2H, ArH), 6.93−6.99 (m, 6H, ArH). 13C{1H} NMR (100
MHz, C6D6, 25 °C): δ 18.3 (Ar-CH3), 21.0 (Ar-CH3), 126.6 (ArC),
127.2 (ArC), 127.5 (ArC), 129.7 (ArC), 130.1 (ArC), 134.6 (ArC),
135.2 (ArC), 135.8 (ArC), 136.9 (ArC), 157.7 (N3C).
1
60%). Mp: 190−200 °C. H NMR (400 MHz, C6D6, 25 °C): δ 0.76
(s, 12H, CH(CH3)2), 1.07 (s, 12H, CH(CH3)2), 1.28 (t, J = 4 Hz,
25H, CH(CH3)2), 3.02−3.09 (sept, 4H, CH(CH3)2), 3.86−3.93
(sept, 4H, CH(CH3)2), 5.04 (s, 4H, OCH2Ph), 6.05 (s, 2H, NH),
6.78 (d, J = 8.0 Hz, 4H, ArH), 6.94 (t, J = 8.0 Hz, 2H, ArH), 7.04−
7.07 (m, 2H, ArH), 7.12−7.14 (m, 10H, ArH), 7.19 (d, J = 8.0 Hz,
4H, ArH). 13C{1H} NMR (100 MHz, C6D6, 25 °C): δ 24.8 (Ar-
iPrC), 26.3 (Ar-iPrC), 28.7 (Ar-iPrC), 28.8 (Ar-iPrC), 65.1 (O-
Synthesis of [L3(2H)AlMe2] (5). To a solution of L3(3H) (0.25 g,
0.337 mmol, 1.0 equiv) in toluene (10 mL), trimethylaluminum (2.0
M in toluene, 0.51 mL, 1.02 mmol, 3.0 equiv) was added at 0 °C, and
then the reaction mixture was stirred at room temperature for 15 h.
G
https://dx.doi.org/10.1021/acs.inorgchem.9b03778
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
CH2Ph), 123.0 (ArC), 125.4 (ArC), 125.6 (ArC), 126.0 (ArC), 127.2
(ArC), 132.2 (ArC), 136.7 (ArC), 145.1 (ArC), 145.8 (ArC), 147.2
(ArC), 159.6 (N3C).
Notes
The authors declare no competing financial interest.
General Procedure for the Catalytic Hydroboration of
Aldehydes and Ketones. Aldehyde or ketone (0.3 mmol),
pinacolborane (0.3 mmol), and catalyst 1 or 2 (1 mol % or 2 mol
%) were charged in a screw cap vial inside the glovebox. Next, the vial
was taken out from the glovebox and placed in a preheated oil bath at
60 °C for 8 or 12 h. The course of the reaction was checked by 1H (in
dry CDCl3) NMR spectroscopy. The 1H NMR spectrum explains the
full conversion of the starting material and the existence of a unique
CH2 or CH peak.
X-ray Crystallography. Crystals of 1 and 3−8 were removed
from the Schlenk flask under a stream of dinitrogen and immediately
covered with a thin layer hydrocarbon oil. A suitable crystal was
selected, attached to the glass fiber, and quickly placed in a low-
temperature stream of dinitrogen. The X-ray diffraction data were
collected on a Bruker four-circle Kappa Apex II diffractometer
equipped with a charge-coupled detector using graphite monochro-
mated Mo Kα radiation (λ = 0.71073 Å) at 100 K. Data collection
was observed with Apex II software. The data sets were integrated and
scaled using Apex II.22 All structures were determined using direct
methods employed in ShelXT23 and OleX,24 and refinement was
carried out using least-squares minimization implemented in
ShelXL.25 All non-hydrogen atoms were refined with anisotropic
displacement parameters. Hydrogen atoms were fixed geometrically in
idealized positions and were refined using a riding model. The crystal
data, data collection, and refinement parameters for compounds 1 and
3−8 are summarized in Table S1.
ACKNOWLEDGMENTS
■
The authors thank the NISER, HBNI, Bhubaneswar, Depart-
ment of Atomic Energy, Government of India for supporting
this work. T.P. acknowledges the University Grant Commis-
sion (UGC) Govt. of India for his fellowship.
REFERENCES
■
(1) Chandra, G.; Jenkins, A. D.; Lappert, M. F.; Srivastava, R. C.
Amido-derivatives of metals and metalloids. Part X. Reactions of
titanium(IV), zirconium(IV), and hafnium(IV) amides with unsatu-
rated substrates, and some related experiments with amides of boron,
silicon, germanium, and tin(IV). J. Chem. Soc. A 1970, 2550−2558.
(2) (a) Francos, J.; Cadierno, V. The chemistry of guanidinate
complexes of the platinum group metals. Dalton Trans. 2019, 48 (25),
9021−9036. (b) Cui, X.-Y.; Tan, C.-H.; Leow, D. Metal-catalysed
reactions enabled by guanidine-type ligands. Org. Biomol. Chem. 2019,
17 (19), 4689−4699. (c) Chlupaty, T.; Ruzicka, A. Hybrid amidinates
and guanidinates of main group metals. Coord. Chem. Rev. 2016, 314,
103−113. (d) Kurek, A.; Gordon, P. G.; Karle, S.; Devi, A.; Barry, S.
T. Recent advances using guanidinate ligands for chemical vapour
deposition (CVD) and atomic layer deposition (ALD) applications.
Aust. J. Chem. 2014, 67 (7), 989−996. (e) Collins, S. Polymerization
catalysis with transition metal amidinate and related complexes.
Coord. Chem. Rev. 2011, 255 (1−2), 118−138. (f) Jones, C. Bulky
guanidinates for the stabilization of low oxidation state metallacycles.
Coord. Chem. Rev. 2010, 254 (11−12), 1273−1289. (g) Edelmann, F.
T. Lanthanide amidinates and guanidinates: from laboratory
curiosities to efficient homogeneous catalysts and precursors for
rare-earth oxide thin films. Chem. Soc. Rev. 2009, 38 (8), 2253−2268.
(h) Coles, M. P. Application of neutral amidines and guanidines in
coordination chemistry. Dalton Trans. 2006, No. 8, 985−1001.
(3) (a) Han, H.-F.; Zhang, S.-F.; Guo, Z.-Q.; Tong, H.-B.; Wei, X.-
H. Three asymmetric guanidinato metal complexes: Synthesis, crystal
structures and their use as pre-catalysts in the Meerwein-Ponndorf-
Verley reduction. Polyhedron 2015, 99, 71−76. (b) Cole, M. L.;
Davies, A. J.; Jones, C.; Junk, P. C.; McKay, A. I.; Stasch, A.
Aluminum and Indium Complexes derived from Guanidines,
Triazenes, and Amidines. Z. Anorg. Allg. Chem. 2015, 641 (12−13),
2233−2244. (c) Obenauf, J.; Kretschmer, W. P.; Kempe, R. Efficient
Synthesis of Aluminium-Terminated Polyethylene by Means of
Irreversible Coordinative Chain-Transfer Polymerisation Using a
Guanidinatotitanium Catalyst. Eur. J. Inorg. Chem. 2014, 2014 (9),
1446−1453. (d) Barry, S. T. Amidinates, guanidinates and
iminopyrrolidinates: Understanding precursor thermolysis to design
a better ligand. Coord. Chem. Rev. 2013, 257 (23−24), 3192−3201.
(e) Koller, J.; Bergman, R. G. Synthesis, Characterization, and
Reactivity of Aluminum Alkyl/Amide Complexes Supported by
Guanidinate and Monoanionic OCO-Pincer Ligands. Organometallics
2010, 29 (15), 3350−3356. (f) Bonyhady, S. J.; Collis, D.; Frenking,
G.; Holzmann, N.; Jones, C.; Stasch, A. Synthesis of a stable adduct of
dialane(4) (Al2H4) via hydrogenation of a magnesium(I) dimer. Nat.
Chem. 2010, 2 (10), 865−869. (g) Zhang, W.-X.; Li, D.; Wang, Z.; Xi,
Z. Alkyl Aluminum-Catalyzed Addition of Amines to Carbodiimides:
A Highly Efficient Route to Substituted Guanidines. Organometallics
2009, 28 (3), 882−887. (h) Chang, C.-C.; Hsiung, C.-S.; Su, H.-L.;
Srinivas, B.; Chiang, M. Y.; Lee, G.-H.; Wang, Y. Carbodiimide
Insertion into Organoaluminum Compounds and Thermal Rearrange-
ment of the Products. Organometallics 1998, 17 (8), 1595−1601.
(i) Aeilts, S. L.; Coles, M. P.; Swenson, D. C.; Jordan, R. F.; Young, V.
G., Jr. Aluminum Alkyl Complexes Containing Guanidinate Ligands.
Organometallics 1998, 17 (15), 3265−3270.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b03778.
13C{1H} NMR spectra for compounds 1, 3−8, and
boronate esters; crystallographic data and structure
refinement summary (PDF)
Accession Codes
CCDC 1559771−1559777 contains the supplementary
crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.uk/data_request/
CIF, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax:+ 44 1223 336033
AUTHOR INFORMATION
■
Corresponding Author
Sharanappa Nembenna − School of Chemical Sciences,
National Institute of Science Education and Research (NISER),
Homi Bhabha National Institute (HBNI), Bhubaneswar 752
050, India; orcid.org/0000-0001-8856-4561;
Phone: 0091-6742494174; Email: snembenna@niser.ac.in
Authors
Thota Peddarao − School of Chemical Sciences, National
Institute of Science Education and Research (NISER), Homi
Bhabha National Institute (HBNI), Bhubaneswar 752 050,
India
Nabin Sarkar − School of Chemical Sciences, National Institute
of Science Education and Research (NISER), Homi Bhabha
National Institute (HBNI), Bhubaneswar 752 050, India
(4) (a) Kathuria, D.; Bankar, A. A.; Bharatam, P. V. ″What’s in a
structure?″ The story of biguanides. J. Mol. Struct. 2018, 1152, 61−78.
(b) Ray, P. Complex Compounds of Biguanides and Guanylureas
with Metallic Elements. Chem. Rev. 1961, 61 (4), 313−359.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.9b03778
H
https://dx.doi.org/10.1021/acs.inorgchem.9b03778
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
(5) Nandi, S. D.; Banerjee, P. Complexes of aluminum(III) and
beryllium(II) with biguanide. Z. Naturforsch., B: J. Chem. Sci. 1974, 29
(5−6), 347−348.
(6) Peddarao, T.; Baishya, A.; Sarkar, N.; Acharya, R.; Nembenna, S.
Conjugated Bis-Guanidines (CBGs) as β- Diketimine Analogues:
Synthesis, Characterization of “CBGs” and Their Lithium Salts.
Manuscript Submitted.
M.; Dias, H. V. R. Fluorinated triazapentadienyl ligand supported
ethyl zinc(II) complexes: reaction with dioxygen and catalytic
applications in the Tishchenko reaction. Dalton Trans. 2016, 45
(11), 4896−4906. (d) Tian, D.; Xie, Q.; Yan, L.; Tong, H.; Zhou, M.
Zinc and aluminum complexes derived from 2, 4-N, N′-disubstituted
1, 3, 5-triazapentadienyl ligands: Synthesis, characterization and
catalysis of the ring-opening polymerization of rac-lactide. Inorg.
Chem. Commun. 2015, 58, 35−38. (e) Kopylovich, M. N.; Karabach,
Y. Y.; Guedes da Silva, M. F. C.; Figiel, P. J.; Lasri, J.; Pombeiro, A. J.
L. Alkoxy-1,3,5-triazapentadien(e/ato) Copper(II) Complexes: Tem-
plate Formation and Applications for the Preparation of Pyrimidines
and as Catalysts for Oxidation of Alcohols to Carbonyl Products.
Chem. - Eur. J. 2012, 18 (3), 899−914. (f) Zhou, M.; Gong, T.; Qiao,
X.; Tong, H.; Guo, J.; Liu, D. Magnesium(II) Complexes of 2,4-N,N′-
Disubstituted 1,3,5-Triazapentadienyl Ligands: Synthesis and Char-
acterization of [N(Dipp)C(NMe2)NC(NMe2)N(SiMe3)MgBr]3 and
[N(R)C(R′)NC(R′)N(SiMe3)]2Mg (Dipp = 2,6-iPr2-C6H3, R = Ph,
SiMe3; R′ = NMe2, 1-piperidino). Inorg. Chem. 2011, 50 (5), 1926−
1930. (g) Gushchin, P. V.; Tyan, M. R.; Bokach, N. A.; Revenco, M.
D.; Haukka, M.; Wang, M.-J.; Lai, C.-H.; Chou, P.-T.; Kukushkin, V.
Y. Novel Tailoring Reaction for Two Adjacent Coordinated Nitriles
Giving Platinum 1,3,5-Triazapentadiene Complexes. Inorg. Chem.
2008, 47 (24), 11487−11500.
(11) (a) Keyes, L. K.; Todd, A. D. K.; Giffin, N. A.; Veinot, A. J.;
Hendsbee, A. D.; Robertson, K. N.; Geier, S. J.; Masuda, J. D.
Reaction of sterically encumbered phenols, TEMPO-H, and organo-
carbonyl insertion reactions with L-AlH2 (L = HC(MeCNDipp)2,
Dipp = 2,6-diisopropylphenyl). RSC Adv. 2017, 7 (59), 37315−
37323. (b) Caise, A.; Kolychev, E. L.; Hicks, J.; Fuentes, M. A.;
Goicoechea, J. M.; Aldridge, S. Reversible borohydride formation
from aluminum hydrides and {H(9-BBN)}2: structural, thermody-
namic and reactivity studies. Dalton Trans. 2019, 48 (29), 10845−
10852. (c) Caise, A.; Jones, D.; Kolychev, E. L.; Hicks, J.; Goicoechea,
J. M.; Aldridge, S. On the Viability of Catalytic Turnover via Al-O/B-
H Metathesis: The Reactivity of β-Diketiminate Aluminum Hydrides
towards CO2 and Boranes. Chem. - Eur. J. 2018, 24 (51), 13624−
13635. (d) Liu, Y.; Li, J.; Ma, X.; Yang, Z.; Roesky, H. W. The
chemistry of aluminum(I) with β-diketiminate ligands and pentam-
ethylcyclopentadienyl-substituents: Synthesis, reactivity and applica-
tions. Coord. Chem. Rev. 2018, 374, 387−415. (e) Gong, S.; Ma, H. β-
Diketiminatealuminium complexes: synthesis, characterization and
ring-opening polymerization of cyclic esters. Dalton Trans. 2008,
3345−3357.
(7) (a) Yang, Y.; Li, H.; Wang, C.; Roesky, H. W. Studies of the
Ligand Effect on the Synthesis of Dialuminoxanes by Various β-
Diketiminato Ligands. Inorg. Chem. 2012, 51 (4), 2204−2211.
(b) Bakthavachalam, K.; Reddy, N. D. Synthesis of Aluminum
Complexes of Triaza Framework Ligands and Their Catalytic Activity
toward Polymerization of ε-Caprolactone. Organometallics 2013, 32
(11), 3174−3184. (c) Gianopoulos, C. G.; Kirschbaum, K.; Mason,
M. R. Mono- and Bimetallic Aluminum Alkyl, Aryl, and Hydride
Complexes of a Bulky Dipyrromethene Ligand. Organometallics 2014,
33 (17), 4503−4511. (d) Koller, J. r.; Bergman, R. G. Highly Efficient
Aluminum-Catalyzed Hydro-amination/-hydrazination of Carbodii-
mides. Organometallics 2010, 29 (22), 5946−5952. (e) Han, H.-F.;
Guo, Z.-Q.; Zhang, S.-F.; Li, J.; Wei, X.-H. Guanidinatoaluminum
complexes: synthesis, crystal structures and reactivities. RSC Adv.
2016, 6 (103), 101437−101446. (f) Peddarao, T.; Baishya, A.; Hota,
S. K.; Nembenna, S. Bimetallic aluminum alkyl and iodide complexes
stabilized by a bulky bis-guanidinate ligand. J. Chem. Sci. 2018, 130
(7), 1−7.
(8) (a) Webster, R. L. β-Diketiminate complexes of the first row
transition metals: applications in catalysis. Dalton Trans. 2017, 46
(14), 4483−4498. (b) Camp, C.; Arnold, J. On the non-innocence of
″Nacnacs″: ligand-based reactivity in β-diketiminate supported
coordination compounds. Dalton Trans. 2016, 45 (37), 14462−
14498. (c) Chen, C.; Bellows, S. M.; Holland, P. L. Tuning steric and
electronic effects in transition-metal β-diketiminate complexes. Dalton
Trans. 2015, 44 (38), 16654−16670. (d) Tsai, Y.-C. The chemistry of
univalent metal β-diketiminates. Coord. Chem. Rev. 2012, 256 (5−8),
722−758. (e) Sarish, S. P.; Nembenna, S.; Nagendran, S.; Roesky, H.
W. Chemistry of Soluble β-Diketiminatoalkaline-Earth Metal
Complexes with M-X Bonds (M = Mg, Ca, Sr; X = OH, Halides,
H). Acc. Chem. Res. 2011, 44, 157−170. (f) Stasch, A.; Jones, C.
Stable dimeric magnesium(I) compounds: from chemical landmarks
to versatile reagents. Dalton Trans. 2011, 40 (21), 5659−5672.
(g) Nagendran, S.; Roesky, H. W. The Chemistry of Aluminum(I),
Silicon(II), and Germanium(II). Organometallics 2008, 27 (4), 457−
492. (h) Bourget-Merle, L.; Lappert, M. F.; Severn, J. R. The
Chemistry of β-Diketiminatometal Complexes. Chem. Rev. 2002, 102
(9), 3031−3066. (i) Desat, M. E.; Kretschmer, R. Dinuclear
Aluminum Halide Complexes Based on Bis(β-diketiminate) Ligands:
Synthesis, Structures, and Electrochemical Characterization. Inorg.
Chem. 2019, 58 (23), 16302−16311.
(9) (a) Hennessy, E. T.; Betley, T. A. Complex N-Heterocycle
Synthesis via Iron-Catalyzed, Direct C-H Bond Amination. Science
2013, 340 (6132), 591−595. (b) Wilding, M. J. T.; Iovan, D. A.;
Betley, T. A. High-Spin Iron Imido Complexes Competent for C−H
Bond Amination. J. Am. Chem. Soc. 2017, 139 (34), 12043−12049.
(c) Gianopoulos, C. G.; Kumar, N.; Zhao, Y.; Jia, L.; Kirschbaum, K.;
Mason, M. R. Aluminum alkoxide, amide and halide complexes
supported by a bulky dipyrromethene ligand: synthesis, character-
ization, and preliminary ε-caprolactone polymerization activity. Dalton
Trans. 2016, 45 (35), 13787−13797. (d) Iovan, D. A.; Wrobel, A. T.;
McClelland, A. A.; Scharf, A. B.; Edouard, G. A.; Betley, T. A.
Reactivity of a stable copper−dioxygen complex. Chem. Commun.
2017, 53 (74), 10306−10309.
(10) (a) Kirubakaran, B.; Beesam, R.; Nareddula, D. R. Zinc(II)
complexes of Triaza and Amidinate ligands: Efficient initiators for the
ring-opening polymerization of ε-Caprolactone and rac-Lactide. Appl.
Organomet. Chem. 2017, 31 (12), e3833. (b) Guo, B.-B.; Lin, Y.-J.;
Jin, G.-X. Controllable construction of half-sandwich octanuclear
complexes based on pyridyl-substituted ligands with conjugated
centers. Dalton Trans. 2017, 46 (25), 8190−8197. (c) Kulkarni, N. V.;
Das, A.; Ridlen, S. G.; Maxfield, E.; Adiraju, V. A. K.; Yousufuddin,
(12) (a) Zheng, P.; Hong, J.; Liu, R.; Zhang, Z.; Pang, Z.; Weng, L.;
Zhou, X. Synthesis and Reactivities of GuanidinateDianion Com-
plexes of Heterobimetallic Lanthanide-Lithium Cp2Ln-
[(CyN)2CNPh]Li(THF)3. Organometallics 2010, 29 (5), 1284−
1289. (b) Lu, Z.; Yap, G. P. A.; Richeson, D. S. TetrasubstitutedG-
uanidinate Anions as Supporting Ligands in Organoyttrium
Chemistry. Organometallics 2001, 20 (4), 706−712.
(13) Gianopoulos, C. G.; Kumar, N.; Zhao, Y.; Jia, L.; Kirschbaum,
K.; Mason, M. R. Aluminum alkoxide, amide and halide complexes
supported by a bulky dipyrromethene ligand: synthesis, character-
ization, and preliminary ε-caprolactone polymerization activity. Dalton
Trans. 2016, 45 (35), 13787−13797.
(14) Aldridge, S.; Downs, A. J. The Group 13 Metals Aluminium,
Gallium, Indium and Thallium: Chemical Patterns and Peculiarities;
John Wiley & Sons Ltd., 2011.
(15) (a) Nikonov, G. I. New Tricks for an Old Dog: Aluminum
Compounds as Catalysts in Reduction Chemistry. ACS Catal. 2017, 7
(10), 7257−7266. (b) Dagorne, S.; Wehmschulte, R. Recent
Developments on the Use of Group 13 Metal Complexes in Catalysis.
ChemCatChem 2018, 10 (12), 2509−2520. (c) Li, W.; Ma, X.;
Walawalkar, M. G.; Yang, Z.; Roesky, H. W. Soluble aluminum
hydrides function as catalysts in deprotonation, insertion, and
activation reactions. Coord. Chem. Rev. 2017, 350, 14−29.
(16) Yang, Z.; Zhong, M.; Ma, X.; De, S.; Anusha, C.;
Parameswaran, P.; Roesky, H. W. An Aluminum Hydride That
I
https://dx.doi.org/10.1021/acs.inorgchem.9b03778
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Functions like a Transition-Metal Catalyst. Angew. Chem., Int. Ed.
2015, 54 (35), 10225−10229.
2,6−iPr2C6H3): A Stable Aluminum Analogue of a Carbene. Angew.
Chem., Int. Ed. 2000, 39 (23), 4274−4276. (b) Ekkert, O.; White, A. J.
P.; Toms, H.; Crimmin, M. R. Addition of aluminium, zinc and
magnesium hydrides to rhodium(III). Chem. Sci. 2015, 6 (10), 5617−
5622. (c) Li, X.; Cheng, X.; Song, H.; Cui, C. Synthesis of
HC[(CBut)(NAr)]2Al (Ar = 2,6-Pri2C6H3) and Its Reaction with
Isocyanides, a Bulky Azide, and H2O. Organometallics 2007, 26 (4),
1039−1043.
(17) Jakhar, V. K.; Barman, M. K.; Nembenna, S. Aluminum
Monohydride Catalyzed Selective Hydroboration of Carbonyl
Compounds. Org. Lett. 2016, 18 (18), 4710−4713.
(18) (a) Woodside, A. J.; Smith, M. A.; Herb, T. M.; Manor, B. C.;
Carroll, P. J.; Rablen, P. R.; Graves, C. R. Synthesis and
Characterization of a TripodalTris(nitroxide) Aluminum Complex
and Its Catalytic Activity toward Carbonyl Hydroboration. Organo-
metallics 2019, 38 (5), 1017−1020. (b) Zhang, G.; Wu, J.; Zeng, H.;
Neary, M. C.; Devany, M.; Zheng, S.; Dub, P. A. Dearomatization and
Functionalization of Terpyridine Ligands Leading to Unprecedented
ZwitterionicMeisenheimer Aluminum Complexes and Their Use in
Catalytic Hydroboration. ACS Catal. 2019, 9 (2), 874−884.
(c) Lebedev, Y.; Polishchuk, I.; Maity, B.; DinisVelosoGuerreiro,
M.; Cavallo, L.; Rueping, M. Asymmetric Hydroboration of
Heteroaryl Ketones by Aluminum Catalysis. J. Am. Chem. Soc.
2019, 141 (49), 19415−19423. (d) Franz, D.; Jandl, C.; Stark, C.;
Inoue, S. Catalytic CO2 Reduction with Boron- and Aluminum
Hydrides. ChemCatChem 2019, 11 (21), 5275−5281. (e) Pollard, V.
A.; Orr, S. A.; McLellan, R.; Kennedy, A. R.; Hevia, E.; Mulvey, R. E.
Lithium diamidodihydridoaluminates: bimetallic cooperativity in
catalytic hydroboration and metallation applications. Chem. Commun.
2018, 54 (10), 1233−1236. (f) Bismuto, A.; Cowley, M. J.; Thomas,
S. P. Aluminum-Catalyzed Hydroboration of Alkenes. ACS Catal.
2018, 8 (3), 2001−2005. (g) Prashanth, B.; Bhandari, M.; Ravi, S.;
Shamasundar, K. R.; Singh, S. Electronically Unsaturated Three-
Coordinate Aluminum Hydride and Organoaluminum Cations. Chem.
- Eur. J. 2018, 24 (19), 4794−4799. (h) Pollard, V. A.; Fuentes, M. A.;
Kennedy, A. R.; McLellan, R.; Mulvey, R. E. Comparing Neutral
(Monometallic) and Anionic (Bimetallic) Aluminum Complexes in
Hydroboration Catalysis: Influences of Lithium Cooperation and
Ligand Set. Angew. Chem., Int. Ed. 2018, 57 (33), 10651−10655.
(i) Jin, D.; Ma, X.; Liu, Y.; Peng, J.; Yang, Z. Novel aluminium
compounds derived from Schiff bases: Synthesis, characterization and
catalytic performance in hydroboration. Appl. Organomet. Chem. 2019,
33 (3), e4637. (j) Liu, Y.; Ma, X.; Ding, Y.; Yang, Z.; Roesky, H. W.
N-TosylHydrazone Precursor for Diazo Compounds as Intermediates
in the Synthesis of Aluminum Complexes. Organometallics 2018, 37
(21), 3839−3845. (k) Franz, D.; Sirtl, L.; Poethig, A.; Inoue, S.
Aluminum Hydrides Stabilized by N-Heterocyclic Imines as Catalysts
for Hydroborations with Pinacolborane. Z. Anorg. Allg. Chem. 2016,
642 (22), 1245−1250. (l) Lemmerz, L. E.; McLellan, R.; Judge, N. R.;
Kennedy, A. R.; Orr, S. A.; Uzelac, M.; Hevia, E.; Robertson, S. D.;
Okuda, J.; Mulvey, R. E. Donor-influenced Structure-Activity
Correlations in Stoichiometric and Catalytic Reactions of Lithium
Monoamido-Monohydrido-Dialkylaluminates. Chem. - Eur. J. 2018,
24 (39), 9940−9948. (m) Liu, Y.; Liu, X.; Liu, Y.; Li, W.; Ding, Y.;
Zhong, M.; Ma, X.; Yang, Z. Synthesis, characterization, and catalytic
performance of aluminum and Tin(II) compounds supported by β-
diketiminato ligands. Inorg. Chim. Acta 2018, 471, 244−248.
(n) Ding, Y.; Ma, X.; Liu, Y.; Liu, W.; Ni, C.; Yan, B.; Yan, L.;
Yang, Z. Synthesis of organoaluminum chalcogenides and their
applications in Lewis acid catalysis. Inorg. Chim. Acta 2019, 497,
119091. (o) Ding, Y.; Liu, X.; Ma, X.; Liu, Y.; Zhong, M.; Li, W.;
Yang, Z.; Yang, Y. Synthesis, characterization, and catalytic perform-
ance of Aluminum and Tin Compounds with β-diketiminato ligand. J.
Organomet. Chem. 2018, 868, 55−60.
(21) (a) Gottlieb, H. E.; Kotlyar, V.; Nudelman, A. NMR chemical
shifts of common laboratory solvents as trace impurities. J. Org. Chem.
1997, 62 (21), 7512−7515. (b) Fulmer, G. R.; Miller, A. J. M.;
Sherden, N. H.; Gottlieb, H. E.; Nudelman, A.; Stoltz, B. M.; Bercaw,
J. E.; Goldberg, K. I. NMR Chemical Shifts of Trace Impurities:
Common Laboratory Solvents, Organics, and Gases in Deuterated
Solvents Relevant to the Organometallic Chemist. Organometallics
2010, 29 (9), 2176−2179.
(22) APEX2; SAINT; SAINT-PLUS; SADABS; Bruker AXS Inc.:
Madison, WI, 2001.
(23) Sheldrick, G. M. Crystal structure refinement with SHELXL.
Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71 (1), 3−8.
(24) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A.
K.; Puschmann, H. OLEX2: a complete structure solution, refinement
and analysis program. J. Appl. Crystallogr. 2009, 42 (2), 339−341.
(25) (a) Sheldrick, G. M. A short history of SHELX. Acta
Crystallogr., Sect. A: Found. Crystallogr. 2008, 64 (1), 112−122.
(b) Sheldrick, G. M. SHELXT - Integrated space-group and crystal-
structure determination. Acta Crystallogr., Sect. A: Found. Adv. 2015,
71 (1), 3−8.
(19) (a) Shegavi, M. L.; Bose, S. K. Recent advances in the catalytic
hydroboration of carbonyl compounds. Catal. Sci. Technol. 2019, 9
(13), 3307−3336. (b) Chong, C. C.; Kinjo, R. Catalytic Hydro-
boration of Carbonyl Derivatives, Imines, and Carbon Dioxide. ACS
Catal. 2015, 5 (6), 3238−3259. (c) Chakraborty, S.; Bhattacharya, P.;
Dai, H.; Guan, H. Nickel and Iron Pincer Complexes as Catalysts for
the Reduction of Carbonyl Compounds. Acc. Chem. Res. 2015, 48 (7),
1995−2003.
(20) (a) Cui, C.; Roesky, H. W.; Schmidt, H.-G.; Noltemeyer, M.;
Hao, H.; Cimpoesu, F. Synthesis and Structure of a Monomeric
Aluminum(I) Compound [{HC(CMeNAr)2}Al] (Ar
=
J
https://dx.doi.org/10.1021/acs.inorgchem.9b03778
Inorg. Chem. XXXX, XXX, XXX−XXX