New boronates prepared from 2,4-pentanedione derived ligands of the NO2 and N2O2 type - Comparison to the complexes obtained from the corresponding salicylaldehyde derivatives

Article abstract of DOI:10.1016/j.jorganchem.2003.11.032

2,4-Pentanedione (=acetylacetone) has been reacted with 2-aminoethanol, 1,2-diaminoethane and 1,3-diaminopropane to give the NO₂ and N₂O₂ type ligands named acacaminolH ₂, acacenH₂ and acpenH₂, which are structurally and electronically related to the corresponding ligands derived from salicylaldehyde (salaminolH₂ and salenH ₂). On reaction of acacaminolH₂ with phenylboronic acid a dinuclear monomeric complex has been obtained containing one three- and one four-coordinate boron atoms as well as one six-membered and one seven-membered heterocyclic ring. Since with salaminolH₂ a dimeric complex with a central 10-membered heterocycle had been reported, it becomes apparent that there may be differences in reactivity when comparing 2,4-pentanedione and salicylaldehyde derived ligands. The molecular compositions of the boron complexes prepared from acacenH₂ and acacpenH₂ are analogous to the corresponding salen and salpen derivatives, however, the presence of two methyl groups in the six-membered chelate rings generates some structural changes, the most important being the distortion of the boat conformation of the central heterocyclic ring. This was predicted by computational methods and confirmed experimentally for one of the complexes. A further important observation was that the products described in here are much more soluble than the salicylaldehyde derivatives. As lateral product the adduct of acacenH₂ with 1,3,5-triphenylboroxine was crystallized. Elemental analysis, IR and NMR (¹H, ¹³C, ¹¹B) spectroscopy, mass spectrometry, ab intio theoretical calculations and X-ray crystallography have been applied to carry out this study.

Full text of DOI:10.1016/j.jorganchem.2003.11.032

Journal of Organometallic Chemistry 689 (2004) 811–822
www.elsevier.com/locate/jorganchem
New boronates prepared from 2,4-pentanedione derived ligands
of the NO2 and N2O2 type – comparison to the complexes
obtained from the corresponding salicylaldehyde derivatives
a
a
a,
b
Mario Sanchez , Obdulia Sanchez , Herbert Hopfl *, Maria-Eugenia Ochoa ,
ꢀ
ꢀ
€
b
Dolores Castillo , Norberto Farfan , Susana Rojas-Lima
b
c
ꢀ
a
ꢀ
ꢀ
ꢀ
Centro de Investigaciones Quımicas, Universidad Autonoma del Estado de Morelos, Av. Universidad 1001, C.P. 62210 Cuernavaca, Mexico
b
Departamento de Quꢀımica, Centro de Investigacioꢀn y de Estudios Avanzados del IPN, Apdo. Postal 14-740, C.P. 07000 Mꢀexico D.F., Mꢀexico
c
Centro de Investigaciones Quꢀımicas, Universidad Autꢀonoma del Estado de Hidalgo, Carretera Pachuca-Tulancingo km 4.5, Ciudad Universitaria,
C.P. 42076 Pachuca de Soto, Hidalgo, Mꢀexico
Received 14 October 2003; accepted 26 November 2003
Abstract
2,4-Pentanedione ( ¼ acetylacetone) has been reacted with 2-aminoethanol, 1,2-diaminoethane and 1,3-diaminopropane to give
the NO2 and N2O2 type ligands named acacaminolH2, acacenH2 and acpenH2, which are structurally and electronically related to
the corresponding ligands derived from salicylaldehyde (salaminolH2 and salenH2). On reaction of acacaminolH2 with phenylbo-
ronic acid a dinuclear monomeric complex has been obtained containing one three- and one four-coordinate boron atoms as well as
one six-membered and one seven-membered heterocyclic ring. Since with salaminolH2 a dimeric complex with a central 10-mem-
bered heterocycle had been reported, it becomes apparent that there may be differences in reactivity when comparing 2,4-pentan-
edione and salicylaldehyde derived ligands. The molecular compositions of the boron complexes prepared from acacenH2 and
acacpenH2 are analogous to the corresponding salen and salpen derivatives, however, the presence of two methyl groups in the six-
membered chelate rings generates some structural changes, the most important being the distortion of the boat conformation of the
central heterocyclic ring. This was predicted by computational methods and confirmed experimentally for one of the complexes. A
further important observation was that the products described in here are much more soluble than the salicylaldehyde derivatives.
As lateral product the adduct of acacenH2 with 1,3,5-triphenylboroxine was crystallized. Elemental analysis, IR and NMR (1H, 13C,
11B) spectroscopy, mass spectrometry, ab intio theoretical calculations and X-ray crystallography have been applied to carry out this
study.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Salen; 2,4-Pentanedione; Phenylboronic acid; Boronates; X-ray crystallography; Ab initio calculations
1. Introduction
For both ligand types a series of different structures is
possible, depending on the steric bulk of the substituents
(R-R00) and the spatial distribution of the donating at-
oms in the ligand, the substituents on the boric acid, as
well as the solvent and the conditions used for the
reaction. As outlined in Scheme 2, in the case of the
salaminol derivatives (salaminolH2 ¼ N-2-(salicylidenei-
mino)ethanol) four different types of reaction products
have been identified so far, two being monomeric (I and
II) [1d,1e,1j], one being dimeric (III) [1e,1g–1i] and an-
other one being polymeric (IV) [1i]. From an applicative
point of view structure types II and III are the most
During the last few years we and others have explored
the chemistry between boric acid derivatives and tri-
dentate NO2 [1,2] as well as tetradentate N2O2 type li-
gands derived from salicylaldehyde (Scheme 1) [3,4].
* Corresponding author. Tel.: +52-777-329-79-97; fax: +52-777-329-
79-97.
€
E-mail address: hhopfl@buzon.uaem.mx (H. Hopfl).
0022-328X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2003.11.032

812
M. Sꢀanchez et al. / Journal of Organometallic Chemistry 689 (2004) 811–822
R
R''
R = H, tBu
R' = H, Me
R'
O
R
B
R
R''
R'
O
R'
O
R'
O
N
OH
N
R'' = -(CH2)n- (n = 2-6)
R
R
-C(Me)2CH2-, -C(Me)HC(Ph)H-,
o-C6H4, o-C6H4CH2-, m-C6H4CH2-,
o-C6H4CH2CH2-, p-C6H4CH2CH2-,
-CH(R''')C(O)- (R''' = H, Me, iPr, Ph)
N
B
N
B
N
B
R'
R''
OH
R
O
R
R
Ar
Ar
R
R
Salaminol derivatives
V
VI
R''
R'
R'
R = H, tBu
R' = H, Me
R
R'
R
R
O
R
O
N
N
R'' = -(CH2)n- (n = 2-6),
O
O
N
O
N
O
N
B
-C(Me)HCH2-,
N
B
B
B
X
B
B
OH
HO
o-C6H10-, o-C6H4,
O
O
Si
O
O
B
R
R
R
R
R
R
R
R
R
R
O
O
Si
Si
Si
O
Salen derivatives
O
O
O
N
X
Scheme 1. Salaminol and salen type ligands used so far for the com-
plexation of boric acid derivatives.
O
R' = -CH2CH2-,
-CH 2CH(Me)-
X = CH2, NH
R = Ph
VII
VIII
R''
R''
Scheme 3. From the reaction between a salen derivative as ligand and a
phenylboronic derivative acid dinuclear (IV, V), trinuclear (VI) and
tetranuclear (VIII) products can be obtained.
R'
R'
O
B
N
O
N
O
Ph
B
B
O
Ph
R
R
O
Ph
II
I
R
R
R'
H
R
R'
H
R
R
N
O
N
O
R''
N
Ar
O
O
N
R
R'
O
R''
B
R'
R''
B
N
O
R
R'
O
Ar
B
O
Scheme 4. Electronic and structural analogy between imines derived
from saliylaldehyde and 2,4-pentanedione.
R''
III
R
n
Ar
R
IV
R
2,4-Pentanedione ( ¼ acetylacetone) is like salicylalde-
hyde planar and one of its tautomers possesses a similar
distribution of the p-electron density (Scheme 4). De-
spite of this similarity, 2,4-pentanedione based ligands
have been used to a much less extent than the above
mentioned salicylaldehyde derivatives. Due to the un-
ique structural features of phenylboronates derived from
salaminol [1c] and salen [3,4] type ligands in comparison
to complexes with other metal ions, we decided to ex-
pand this chemistry using herein the related 2,4-pen-
tanedione ligands and explored the reactivity of three
representative ligands with phenylboronic acid. Acaca-
minolH2 (acacaminolH2 ¼ N-2-(acetylacetimine)- etha-
nol) is a tridentate ligand comparable to salaminolH2,
while acacenH2 (acacenH2 ¼ N,N0-ethylenebis(acetyl-
acetimine)) and acacpenH2 (acacpenH2 ¼ N,N0-propyl-
enebis(acetylacetimine)) are fourdentate N2O2 ligands
comparable to salenH2 and salpenH2 (salpenH2 ¼ N,N0-
propylenebis(salicylideneimine)). The latter two ligands
have been used previously for the preparation of metal
complexes [7,8].
Scheme 2. From the reaction between a salaminol derivative as ligand
and a phenylboronic acid monomeric (I, II), dimeric (III) and poly-
meric (IV) products can be obtained.
interesting ones, type II because of the presence of the
Lewis acidic, tricoordinate boron atom that may be
useful as catalytic center in polymerization processes or
in asymmetric synthesis [2f], and type III because of the
formation of a central macrocyclic ring (10–18 members
so far), which might be useful for host–guest chemistry
[5].
In the case of the salen derivatives (salenH2 ¼ N,N0-
ethylenebis(salicylideneimine)) four different reaction
products have been identified until now (Scheme 3), two
being dinuclear (V,VI) [3,4], one being trinuclear (VII)
[3b] and another one being tetranuclear containing a
huge cylinder-shaped cavity in its interior (VIII) [4g]. In
this context it should be mentioned that such a struc-
tural variety in the complexes formed from salaminol
and salen type ligands is unique for the boron element,
comparing its chemistry with that of the rest of the
group 13 family and the whole series of other repre-
sentative and transition metal elements [6].
In what follows the results of these reactions are
presented in a comparative way to the complexes ob-
tained from the salicylaldehyde derivatives.

M. Sꢀanchez et al. / Journal of Organometallic Chemistry 689 (2004) 811–822
813
2. Experimental
the solution was cooled down two room temperature,
whereupon a colorless precipitate of the product formed
that was filtered off under vacuum and washed with
benzene. Complex 1 is soluble in benzene, toluene,
chloroform, dichloromethane and THF. Yield: 80%;
m.p. 154–156 °C.
2.1. Instrumental
NMR studies were carried out with Varian Gemini
200, Jeol GSX 270, Bruker 300 and Varian Inova 400
instruments. Standards were TMS (internal, 1H, 13C)
and BF3 ꢀ OEt2 (external, 11B). Chemical shifts are stated
in parts per million; they are positive, when the signal is
shifted to higher frequencies than the standard. COSY,
HMQC and NOESY experiments have been carried out
in order to assign the 1H and 13C spectra completely. IR
spectra have been recorded on a Bruker Vector 22 FT
spectrophotometer. Mass spectra were obtained on a
HP 5989A equipment. Elemental analyses have been
carried out on Perkin–Elmer Series II 2400 and Ele-
mentar Vario ELIII instruments.
2.4.1. Spectroscopic data
IR (KBr) mmax: 3068 (w), 3012 (w), 2958 (w), 2888 (w),
1624 (m), 1534 (s), 1470 (w), 1439 (w), 1419 (m), 1375
(w), 1361 (m), 1344 (w), 1323 (m), 1301 (m), 1264 (m),
1250 (m), 1182 (m), 1154 (m), 1132 (m), 1098 (m), 1049
1
(m), 1019 (m) cmꢁ1; H NMR (270 MHz, CDCl3) d:
2.06 (3H, s, H-1), 2.14 (3H, s, H-5), 3.44, 3.74, 3.86 and
4.29 (2H, ABCD, H-6, H-7), 5.23 (1H, s, H-3), 7.26 (3H,
m, m-H, p-H), 7.41 (3H, m, m0-H, p0-H), 7.47 (2H, dd, o-
H), 8.01 (2H,dd, o0-H) ppm; 13C NMR (67.9 MHz,
CDCl3) d: 20.4 (C-5), 23.2 (C-1), 49.8 (C-6), 63.9 (C-7),
97.7 (C-3), 127.2 (C-p), 127.5 (C-m0Þ, 127.6 (C-m), 130.2
(C-p0Þ, 131.2 (C-o), 134.9 (C-o0Þ, 168.6 (C-4), 176.2 (C-2)
ppm; 11B NMR (96 MHz, CDCl3) d: 3 (h1=2 ¼ 160 Hz,
2.2. Preparative part
Commercial starting materials and solvents have been
used. The acacenH2 and acacpenH2 ligands have been
prepared according to a method reported in the litera-
ture [8].
B
tetrac:), 27 (h ¼ 520 Hz, Btric:) ppm; MS (70 eV) m=z
(%): 334 (Mþ1+=21, 0.3), 333 (Mþ, 0.1), 256 (Mþ ) C6H5,
41), 152 (Mþ ) C12H10BO, 100), 126 (8), 110 (4), 104(3),
91(1), 77 (11), 51 (10). Elemental analysis (%): Calc.: C,
68.43; H, 6.35; N, 4.20. Found: C, 68.25; H, 6.43, N,
4.79.
2.3. Preparation of acacaminolH2
The acacaminolH2 ligand was prepared by reaction
of acetylacetone (3.50 g, 35.0 mmol) with 2-ethanol-
amine (2.14 g, 35.0 mmol) in ethanol (20 ml). After 30
Min. of reflux in presence of a Dean-Stark trap part of
the solvent was eliminated through distillation. Under
cooling to room temperature a precipitate of the ligand
formed, which was filtered off under vacuum and wa-
shed with small amounts of chloroform. The colorless
product is soluble in all common organic solvents.
Yield: 79%; m.p. 94–96 °C.
2.5. Preparation of acacen[B(Ph)–O–B(Ph)] (3)
Compound 3 was prepared from one equivalent of
acacenH2 (0.25 g, 1.11 mmol) and two equivalents of
phenylboronic acid (0.27 g, 2.22 mmol) in benzene (8
ml). After 1 h of reflux in presence of a Dean-Stark trap
the solution was cooled down two room temperature,
whereupon a colorless precipitate of the product formed
that was filtered off under vacuum and washed with
benzene. Complex 3 is soluble in all common organic
solvents except for hexane. Yield: 41%; m.p. > 300 °C.
2.3.1. Spectroscopic data
IR (KBr) mmax: 3277 (br, m), 2962 (w), 2933 (w), 2879
(w), 1607 (m), 1551 (s), 1437 (m), 1374 (m), 1353 (m),
1310 (m), 1245 (m), 1197 (w), 1117 (w), 1080 (w), 1057
(w), 1027 (w) cmꢁ1. 1H NMR (300 MHz, CDCl3) d: 1.94
(3H, s, H-5), 1.96 (3H, s, H-1), 3.39 (dd, 2H, H-6), 3.73
(2H, t, H-7), 4.95 (1H, s, H-3) ppm; 13C NMR (75 MHz,
CDCl3) d: 19.5 (C-5), 28.9 (C-1), 45.8 (C-6), 61.7 (C-7),
96.0 (C-3), 164.4 (C-4), 195.2 (C-2) ppm. MS (70 eV)
m=z (%): 143 (Mþ, 88), 128 (50), 112 (87), 100 (76), 94
(50), 84 (69), 70 (52), 58 (61), 43 (100).
2.5.1. Spectroscopic data
IR (KBr) mmax: 3069 (w), 3052 (w), 3002 (w), 2958 (w),
2959 (w), 1619 (s), 1529 (s), 1461 (m), 1429 (m), 1410
(m), 1370 (m), 1342 (m), 1316 (s), 1198 (s), 1129 (m),
1118 (s), 1051 (m), 1016 (m) cmꢁ1; 1H NMR (400 MHz,
CDCl3) d: 1.79 (6H, s, H-5), 2.09 (6H, s, H-1), 3.19 and
3.48 (4H, AA0BB0, H-6), 5.17 (2H, s, H-3), 7.14 (2H, d,
H-p), 7.20 (4H, dd, H-m), 7.44 (4H, d, H-o) ppm; 13C
NMR (100 MHz, CDCl3) d: 19.8 (C-5), 23.4 (C-1), 46.7
(C-6), 97.9 (C-3), 126.0 (C-p), 126.8 (C-m), 132.4 (C-o),
167.1 (C-4), 176.4 (C-2) ppm; 11B NMR (128 MHz,
CDCl3) d : 4 (h1=2 ¼ 230 Hz, Btetrac:) ppm; MS (70 eV)
m=z (%): 415 (Mþ+1, 0.3), 337 (Mþ-C6H5, 100), 259 (4),
233 (54), 151 (6), 130 (17), 77(2). Elemental analysis (%):
Calc.: C, 69.53; H, 6.81; N, 6.76. Found: C, 69.76; H,
6.80, N, 6.60.
2.4. Preparation of acacaminol[B(Ph)–O–B(Ph)] (1)
Compound 1 was prepared from one equivalent of
acacaminolH2 (0.18 g, 1.25 mmol) and two equivalents
of phenylboronic acid (0.30 g, 2.46 mmol) in benzene (8
ml). After 1 h of reflux in presence of a Dean-Stark trap

814
M. Sꢀanchez et al. / Journal of Organometallic Chemistry 689 (2004) 811–822
2.6. Preparation of acacpen[B(Ph)–O–B(Ph)] (4)
complex 7 the N–H hydrogen atoms have been localized
by difference Fourier maps. In the case of compounds 4
and 7 two independent molecules are present in the
asymmetric unit. Additionally, in the crystal lattice of
compound 7 one acacenH2 ligand molecule is present
per asymmetric unit. Molecular structures were created
by the CRYSTALS software package [13,14]. Crystal-
lographic data for the structures reported in this paper
have been deposited with the Cambridge Crystallo-
graphic Data Centre as supplementary publications no.
CCDC-221127-221129. Copies of the data can be ob-
tained free of charge on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK (fax: (+44)1223-336-
033; email: deposit@ccdc.cam.ac.uk, www: http://www.
ccdc.cam.ac.uk).
Compound 4 was prepared from one equivalent of
acacpenH2 (0.50 g, 2.10 mmol) and two equivalents of
phenylboronic acid (0.51 g, 4.20 mmol) in benzene (8
ml). After 1 h of reflux in presence of a Dean-Stark trap
the solution was cooled down to room temperature,
whereupon a colorless precipitate of the product formed
that was filtered off under vacuum and washed with
benzene. Complex 4 is soluble in all common organic
solvents except for hexane. Yield: 68%; m.p. 248–251
°C.
2.6.1. Spectroscopic data
IR (KBr) mmax: 3051 (w), 1627 (m), 1547 (s), 1428 (w),
1368 (w), 1325 (w), 1204 (m), 1128 (w), 1033 (w), 964
(w), 906 (w), 745 (w), 704 (w) cmꢁ1
.
1H NMR (400
2.9. Theoretical calculations
MHz, CDCl3) d: 1.62 (2H, m, H-7), 1.85 (6H, s, H-5),
1.95 (6H, s, H-1), 3.25 and 3.48 (4H, AA0BB0, H-6), 4.90
(2H, s, H-3), 7.10 (2H, m, H-p), 7.18 (4H, m, H-m), 7.62
(4H, d, H-o) ppm. 13C NMR (50 MHz, CDCl3) d: 19.9
(C-5), 23.7 (C-1), 29.0 (C-7), 46.5 (C-6), 96.4 (C-3), 126.3
(C-p), 126.9 (C-m), 131.7 (C-o), 166.8 (C-4), 175.8(C-2)
ppm; 11B NMR (64 MHz, CDCl3) d: 5 (h1=2 ¼ 200 Hz,
HF/6-31G(d, p) geometry optimizations were done on
a PC with a Pentium III processor using the PC GA-
MESS software [15]. Structures were visualized with
Molekel 4.3 [16] and Mercury 1.1.2 [17]. All geometry
optimizations were followed by frequency calculations,
using the same basis set, to characterize the stationary
points as true minima.
B
tetrac:) ppm; MS (70 eV) m=z (%): 351 (Mþ-Ph, 100),
273 (16), 247 (52), 137 (39), 77 (4). Elemental analysis
(%): Calc.: C, 70.09; H, 7.01; N, 6.54. Found: C, 70.62;
H, 7.39, N, 6.66.
3. Results and discussion
2.7. Preparation of complex 7
3.1. Preparation and characterization of acacami-
nol[B(Ph)–O–B(Ph)]
Compound 7 was obtained as a crystalline product,
when compound 3 was recrystallized slowly from ben-
zene. Apart from complex 7 the crystals contained 0.5
equivalents of the acacenH2 ligand. Yield: 58%; m.p.
165–167 °C.
AcacaminolH2 was prepared through a condensation
reaction between 2,4-pentanedione and 2-ethanolamine
in absolute ethanol with a yield of 79%. When this li-
gand is refluxed in benzene and in the presence of a
Dean-Stark trap with an equimolar amount of phen-
ylboronic acid, complex 1 is isolated as the only
solid product in a yield of 32%. Using the ligand and
phenylboronic acid in a 1:2 ratio, the yield increases to
80% (Scheme 5).
Elemental analysis (%): Calc.: C, 65.69; H, 6.94; N,
7.42. Found: C, 64.98; H, 6.12; N, 7.40.
2.8. X-ray crystallography
X-ray diffraction studies were performed on Bruker-
AXS Smart 6000 (compounds 1 and 7) and APEX
(compound 4) diffractometers with CCD area detectors
A comparative analysis of the 1H and 13C NMR
spectroscopic data between the ligand and the boron
compound shows that the distribution of the p-electron
density changes. It is well known that for uncoordinated
acetylacetimines there exists a tautomeric equilibrium
between three species in solution, the Schiff base A, the
ꢁ
(kMo Ka ¼ 0:71073 A, monochromator: graphite).
Frames were collected at T ¼ 293 K (compounds 1 and
7) and T ¼ 100 K (compound 4) via x–rotation
(D=x ¼ 0:3°) at 5 and 10 s per frame (SMART [9]). The
measured intensities were reduced to F 2 and corrected
for absorption with SADABS (SAINT-NT [10]). Cor-
rections were made for Lorentz and polarization effects.
Structure solution, refinement and data output were
carried out with the SHELXTL-NT program package
[11,12]. Non hydrogen atoms were refined anisotropi-
cally, while hydrogen atoms were placed in geometri-
cally calculated positions using a riding model. For
7
5
6
H3C
H3C
O
B
o'
OH
N
H
4
N
m'
p'
Benzene, ∆
+
2 PhB(OH)2
3
B
O
-3 H2O
O
2
O
H3C
o
H3C
1
m
p
acacaminolH2
1
Scheme 5. Preparation of complex 1.

M. Sꢀanchez et al. / Journal of Organometallic Chemistry 689 (2004) 811–822
815
system in the 1H NMR spectrum with signals at
d ¼ 3:44 and 3.74 ppm for the NCH2 methylene group
and at d ¼ 3:86 and 4.29 ppm for the OCH2 group. It
R
R
R'
R
R
R'
R
R'
H
N
O
N
O
N
O
H
H
H
H
H
1
should be mentioned that the H and 13C NMR spectra
R
have been completely assigned using 2D NMR experi-
ments like COSY, HMQC and NOESY.
B
C
A
The fact that a dinuclear boron complex has been
formed was deduced from the 11B NMR spectrum
showing two signals at d ¼ 3 and 27 ppm, the first being
in the shift range typical for a tetra-coordinate boron
and the second one being typical for a three-coordinate
boron atom [19].
Besides elemental analysis and mass spectrometry,
the molecular structure of 1 was confirmed by X-ray
crystallography. The most relevant crystallographic data
are summarized in Table 1. Selected bond lengths, bond
angles and torsion angles are listed in Table 2.
As can be seen from the molecular structure shown in
Fig. 1, compound 1 contains two boron heterocycles,
the first being a C3BNO heterocycle consisting of
six members and the second one being a C2B2NO2
Scheme 6. Species present in the tautomeric equilibria of acetylaceti-
mines are the Schiff base A, the cetamine B and the enimine C.
cetamine B and the enimine C (Scheme 6), which is
shifted towards the tautomer containing the acid proton
at the nitrogen atom (B) [18].
1
According to the H and 13C NMR data the coor-
dinated ligand in complex 1 possesses a distribution of
the p-electron density characteristic for the enimine
species C. This is evident from the high-field shift of the
signal for the carbonyl carbon C-2, Dd ¼ 19:0 ppm, and
the simultaneous low-field-shift of the signal for C-4,
Dd ¼ 4:2 ppm, upon formation of the boronate.
The threefold coordination of the ligand to boron
atoms can be deduced from the appearance of an ABCD
Table 1
Crystallographic data for compounds 1, 4 and 7
Crystal data
1a
4b
C25H30B2N2O3
7a
Formula
Crystal size (mm)
Mw (g molꢁ1
Space group
C19H21B2NO3
0.18 ꢂ 0.20 ꢂ 0.22
332.99
C62H78B6N6O10
0.29 ꢂ 0.50 ꢂ 0.70
1132.16
0.16 ꢂ 0.18 ꢂ 0.42
)
428.13
P21
ꢂ
P21=c
P1
Cell parameters
ꢁ
a (A)
11.053(2)
13.586(3)
12.884(3)
90
13.452(2)
7.4004(9)
23.038(3)
90
12.162(2)
16.114(3)
18.325(4)
110.49(3)
101.41(3)
92.86(3)
3269.9(11)
4
ꢁ
b (A)
ꢁ
c (A)
a (°)
b (°)
c (°)
106.550(5)
90
96.767(2)
90
ꢁ3
V (A )
1854.6(7)
4
0.078
2277.6(5)
4
0.080
Z
l (mmꢁ1
)
0.076
1.15
qcalcd (g cmꢁ3
)
1.19
1.25
Data collection
h limits (°)
hkl limits
No. collected refl.
No. ind. refl. (Rint
2 < h < 23
)12, 12; )14, 15; )14, 12
9511
2 < h < 23
)14, 14; )8, 8; )25, 25
18544
2 < h < 26
)13, 14; )19, 18; )18, 22
21026
)
2659 (0.07)
1111
6297 (0.05)
5950
12787 (0.05)
4541
No. observed refl.c
Refinement
Rc;d
Rw
No. of variables
GoF
0.041
0.098
228
0.064
0.141
574
0.054
0.148
789
e;f ;g
0.80
)0.10
0.12
1.20
)0.27
0.30
0.81
)0.16
0.19
ꢁ3
ꢁ
Dqmin (e Aꢁ3
)
ꢁ
Dqmax (e A
)
a Data collection on a Bruker Smart 6000 diffractometer.
b Data collection on a Bruker Apex diffractometer.
c I > 2rðIÞ.
P
P
d R ¼ ðFo2 ꢁ Fc2Þ= Fo2.
e All data.
P
P
2
2 1=2
f Rw ¼ ½ wðFo2 ꢁ Fc2Þ = wðFo2Þ ꢃ
.
2
g wꢁ1 ¼ r2Fo2 þ ðX ꢄ PÞ þ Y ꢄ P; P ¼ ðFo2 þ 2Fc2Þ=3; X ¼ 0:0365 for 1, 0.0565 for 4, 0.0587 for 7; Y ¼ 0 for 1, 1.18 for 4, 0 for 7.

816
M. Sꢀanchez et al. / Journal of Organometallic Chemistry 689 (2004) 811–822
Table 2
reaches almost the N ! B bond length found in cubic
ꢁ
Selected bond lengths (A), bond angles (°) and torsion angles (°) for
compound 1
ꢁ
boron nitride, 1.56 A [20b]. On the other hand, the B1–
ꢁ
O1 bond is rather long [1–4], 1.508(4) A, indicating that
Bond lengths
there is still some reminiscent character of the cetamine
tautomer in the coordinated ligand. That the equilib-
rium has been displaced in direction of the enimine
tautomer can be seen from the C2–C3, C3–C4 and C4–
B1–N1
B1–O1
B1–O2
B1–C14
B2–O2
B2–O3
B2–C8
1.574(4)
1.508(4)
1.435(4)
1.605(4)
1.326(4)
1.375(4)
1.562(4)
N1–C4
N1–C6
O1–C2
O3–C7
C2–C3
C3–C4
C6–C7
1.302(3)
1.473(3)
1.302(3)
1.422(3)
1.339(4)
1.413(4)
1.501(4)
ꢁ
N1 bond lengths of 1.339(4), 1.413(4) and 1.302(3) A,
respectively. The B1–O2 bond length is in the range
ꢁ
observed for related species, 1.435(4) A [1–4]. In com-
Bond angles
O1–B1–N1
O1–B1–O2
O1–B1–C14
O2–B1–C14
O2–B1–N1
N1–B1–C14
O2–B2–O3
O2–B2–C8
O3–B2–C8
B1–O2–B2
parison, for the structurally characterized, related
complex 2, the corresponding N ! B, B–Oph and B–OB
108.2(2)
106.3(2)
108.2(3)
113.9(3)
109.9(3)
110.0(2)
126.1(3)
119.7(3)
114.1(3)
137.3(3)
B1–O1–C2
B1–N1C4
124.5(3)
123.7(3)
114.0(3)
127.5(3)
121.5(3)
123.0(3)
118.9(3)
122.1(3)
113.0(3)
116.5(3)
ꢁ
bond lengths are 1.629(5), 1.470(6) and 1.431(5) A [1e].
B1–N1–C6
B2–O3–C7
O1–C2–C3
C2–C3–C4
C3–C4–N1
C4–N1–C6
N1–C6–C7
C6–C7–O3
Me
Ph
O
N
B
B
O
O
Torsion angles
B1–N1–C6–C7
N1–C6–C7–O3
C6–C7–O3–B2
C7–O3–B2–O2
O3–B2–O2–B1
B2–O2–B1–N1
O2–B1–N1–C6
)81.7(3)
71.9(4)
)43.2(5)
10.8(5)
22.8(6)
)43.4(5)
61.9(3)
O1–C2–C3–C4
C2–C3–C4–N1
C3–C4–N1–B1
C4–N1–B1–O1
N1–B1–O1–C2
B1–O1–C2–C3
)2.3(5)
1.1(5)
2
)1.6(5)
2.8(4)
As expected, for the tricoordinate boron atom the B–
O bonds are significantly shorter due to pp–pp interac-
tions, 1.326(4) A for B2–O2 and 1.375(4) A for B2–O3.
Interestingly, the B2–C8 bond is also significantly
shorter than the corresponding B1–C14 bond,
)3.9(4)
4.0(5)
ꢁ
ꢁ
ꢁ
1.562(4) $ 1.605(4) A, indicating that there is probably
some delocalization of the aromatic p-electron density
to the boron atom. This observation is confirmed by the
fact that the B-phenyl ring is localized almost in the
same plane as the BO2 group, the O2–B2–C8–C9 torsion
angle being )9.3(5)°. Similar results have been also
found for complex 2 [1e].
Finally, it should be mentioned that the B–O–B bond
angle is relatively large, 137.3(3)°, however not unex-
pected, since similar values have been reported also for
other complexes containing a B–O–B bond, e.g.,
131.4(3)° for 2 [1e,21].
The conformation of the seven-membered heterocy-
clic ring in 1 can be described as distorted chair, whereby
the plane of the chair is formed by atoms O2, B1 C6 and
C7. Atoms B2 and O3 deviate less from this plane than
ꢁ
atom N1, Dd ¼ 0:49, 0.52 and )0.73 A, respectively.
Fig. 1. Perspective view of the molecular structure of compound 1.
Ellipsoids are shown at the 20% probability level.
Considering that with the related ligand salaminolH2
a dimeric complex of type III (Scheme 2) has been ob-
tained instead of the monomeric dinuclear species 1
(type II), the question arises, why different products are
formed? Comparing the molecular models of the two
possible products, neither a steric repulsion nor an an-
gular strain that might disfavor the dimeric structure
can be found. Therefore, we suppose that both struc-
tures might be possible, nevertheless, the dimeric prod-
uct, being the kinetic product [1e], cannot be isolated
under the reaction conditions applied in here, because
heterocycle of seven members. The coordination num-
bers and geometries of the two boron atoms are differ-
ent, B1 being distorted tetrahedral with bond angles
between 106.3(2)° and 113.9(3)°, and B2 being trigonal
planar with bond angles between 114.1(3)° and
126.1(3)°.
An analysis of the bond lengths in the two hetero-
cycles reveals some interesting features: the coordinative
ꢁ
B1–N1 bond is extremely short [20a], 1.574(4) A, and

M. Sꢀanchez et al. / Journal of Organometallic Chemistry 689 (2004) 811–822
817
the products formed from acacenH2 are more soluble
and no precipitation occurs during the reaction. For the
salicylaldehyde derivative the dimeric product precipi-
tates during the reaction.
mirror plane in the cis-isomer and a C2 axis in the trans-
isomer – a differentiation by NMR spectroscopy was not
possible. Nevertheless, based on X-ray crystallographic
studies it has been proposed that 5 and 6 have cis-con-
figuration [3].
3.2. Preparation and characterization of acacen[B(Ph)–
O–B(Ph)] 3 and acacpen[B(Ph)–O–B(Ph)] 4
N
B
N
B
N
B
N
B
AcacenH2 and acacpenH2 are known ligands and
have been prepared as reported [8]. On reaction of
acacenH2 and acacpenH2 with phenylboronic acid in a
1:2 stoichiometry the dinuclear complexes 3 and 4
are obtained in yields of 41% and 68%, respectively
(Scheme 7). Interestingly, both complexes are well sol-
uble in a series of solvents like chloroform, ethyl acetate,
acetone and DMSO, while the analogous salen and
salpen derivatives have very low solubility. The products
have been identified by elemental analysis, IR and NMR
(1H, 13C, 11B) spectroscopy, mass spectrometry and
additionally by X-ray crystallography in the case of
complex 4.
As in the case of complex 1 the distribution of the p-
electron density in the ligand changes upon coordination
to the boron atoms, Dd(13C) ¼ 19.1 ppm for C2,
Dd ¼ 4:3 ppm for C-4 in the case of 3 and Dd(13C) ¼ 19.4
ppm for C2, Dd ¼ 3:6 ppm for C-4 in the case of 4. For
both complexes the formation of a central heterocycle
containing two boron atoms involved in a N ! B bond
is confirmed by (i) the appearance of AB systems for the
NCH2 methylene hydrogen atoms in the 1H NMR
spectra, with signals at d ¼ 3:2 and 3.5 ppm, (ii) the
integration of the 1H NMR spectra indicating the
presence of two B-phenyl groups per ligand, and (iii) a
signal in the 11B NMR spectrum characteristic for a
tetracoordinate boron atom at d ¼ 4 ppm for 3 and
d ¼ 5 ppm for 4.
O
O
O
O
O
O
5
6
In order to evaluate, which isomer is thermodynam-
ically more favored in the case of complexes 3 and 4, we
optimized the molecular structures by computational
methods using HF/6-31G(d, p) (PC GAMESS software
[15]). In previous studies it has been shown that this
basis set is adequate for the calculation of boron com-
pounds having a coordinative N ! B bond [22]. The
calculated molecular structures of the cis- and trans-
isomers of 3 and 4 are shown in Fig. 2, confirming that
the heterocyclic rings in the cis-isomers possess a boat-
conformation and in the trans-isomers a chair or a
twisted conformation. The calculated energy differences
indicate that in both cases the cis-isomer is slightly more
stable than the trans-isomer, DDHf ¼ 1:78 kcal/mol for 3
and DDHf ¼ 4:32 kcal/mol for 4. These results agree
with the structural studies realized for the corresponding
salen and salpen derivatives [3].
Fortunately, crystals suitable for X-ray crystallogra-
phy could be grown for complex 4, so that in this
case the computational results can be supported by
the experimentally determined molecular structure. The
most relevant crystallographic data are summarized in
On the basis of the available spectroscopic data it is
not possible to determine the correct conformation of
the seven- and eight-membered heterocyclic rings in 3
and 4, and neither the configuration of the boron atoms.
For the analogous salen[B(Ph)–O–B(Ph)] and sal-
pen[B(Ph)–O–B(Ph)] complexes 5 and 6 it has been re-
ported previously that the B-phenyl groups may be in
cis- or trans-orientation, giving in the first case a boat
and in the second case a chair conformation. Due to the
fact that both isomers possess molecular symmetry – a
5
6
CH3
H3 C
4
CH3
H3C
H3C
(CH2 )n
(CH2 )n
N
N
N
N
Benzene,
- 3 H2O
∆
H
H
3
2
+
2 PhB(OH)2
B
B
O
o
O
O
O
O
H3 C
CH3
CH3
1
acacenH2 (n = 2)
acacpenH2 (n = 3)
m
p
3 (n = 2)
4 (n = 3)
Fig. 2. Calculated molecular structures of compounds 3 and 4 (cis and
trans-isomers).
Scheme 7. Preparation of complexes 3 and 4.

818
M. Sꢀanchez et al. / Journal of Organometallic Chemistry 689 (2004) 811–822
Table 3
ꢁ
Selected bond lengths (A), bond angles (°) and torsion angles (°) for compounds 3 and 4 (theoretical data for compounds 3 and 4, X-ray data for
compound 4)
cis-3, trans-3a;b (calculated data)
cis-4, trans-4a;b (calculated data)
4c (X-ray data)
Bond lengths
B1–N1
B1–O1
1.621/1.619
1.529/1.522
1.389/1.392
1.635/1.633
1.298/1.304
1.479/1.475
1.278/1.267
1.388/1.373
1.421/1.422
1.561/1.538
1.643/1.648
1.547/1.532
1.386/1.401
1.608/1.624
1.322/1.303
1.492/1.465
1.288/1.265
1.360/1.373
1.405/1.422
1.554/1.541
1.624(6)
1.530(5)
1.397(5)
1.613(6)
1.297(5)
1.479(5)
1.300(5)
1.363(6)
1.410(6)
1.516(6)
B1–O3
B1–C14
N1–C3
N1–C6
O1–C1
C1–C2
C2–C3
C6–C7
Bond angles
O1–B1–N1
O1–B1–O3
O1–B1–C14
O3–B1–C14
O3–B1–N1
N1–B1–C14
B1–O1–C1
B1–N1–C3
B1–N1–C6
B1–O3–B2
O1–C1–C2
C1–C2–C3
C2–C3–N1
C3–N1–C6
N1–C6–C7
C6–C7–C8
104.9/107.2
111.2/106.5
107.8/104.9
111.1/117.1
110.3/111.0
111.4/110.6
124.3/126.3
123.5/121.7
114.0/118.8
131.6/133.0
123.9/122.4
121.2/120.6
120.9/121.7
122.5/118.9
112.7/118.3
–/–
107.1/105.6
108.3/105.6
106.5/105.4
115.1/116.4
112.6/111.0
106.9/111.9
125.3/124.1
120.9/119.9
116.2/117.8
126.4/141.3
121.7/121.8
123.1/121.1
121.0/121.2
122.5/120.8
112.5/116.7
114.4/115.9
105.4(3)
112.0(3)
107.5(3)
113.1(3)
110.7(3)
107.9(3)
125.1(3)
124.0(4)
114.3(3)
130.9(3)
122.1(4)
121.8(4)
119.8(4)
121.5(4)
115.1(4)
116.0(4)
Torsion angles
B1–N1–C6–C7
84.7/6.8
65.7/)85.0
)107.1(4)
)102.8(4)
N1–C6–C7–C8/N2
)25.2/)63.2
47.7/98.8
65.0(5)
59.4(5)
C6–C7–N2–B2
C6–C7–C8–N2
59.8/19.4
–/–
–/–
)67.9/)64.9
–/–
)60.8(5)
)67.9(5)
C7–N2–B2–O3
C7–C8–N2–B2
54.0/54.6
–/–
–/–
)40.8/)27.8
–/–
105.4(4)
106.6(4)
)56.4(5)
)70.8(5)
)60.6(5)
)43.9(5)
62.1(5)
N2–B2–O3–B1
C8–N2–B2–O3
B2–O3–B1–N1
O3–B1–N1–C6
O1–C1–C2–C3
C1–C2–C3–N1
C2–C3–N1–B1
C3–N1–B1–O1
N1–B1–O1–C1
39.8/)47.1
–/–
45.1/)33.9
67.2/90.1
)56.4/)29.7
)30.9/58.4
)61.3/)48.0
)53.6/81.1
56.9(5)
52.5(4)
63.5(4)
7.2, )2.8
)4.3, )5.0
)11.7, 2.2
2.2, 4.2
2.7, )0.7
2.3, 4.0
)5.7(6), 0.9(7)
)0.7(7), 5.1(7)
2.8(6), )0.4(7)
)4.7(7), )5.6(7)
9.2(6), 4.7(6)
1.8(6), )8.7(6)
)15.9(5), )8.1(5)
4.8(5), 20.2(5)
13.1(5), 8.6(5)
)10.3(5), )21.2(5)
)4.2, )0.4
)5.6, )9.4
)4.9, 1.5
9.2, )6.0
13.0, )1.4
23.3, 22.4
)15.1, 0.4
)27.8, )29.3
)6.6, 4.1
)0.6, )1.8
24.8, )8.6
0.7, 0.1
)29.4, 8.3
)2.9, )0.9

M. Sꢀanchez et al. / Journal of Organometallic Chemistry 689 (2004) 811–822
819
Table 3 (continued)
cis-3, trans-3a;b (calculated data)
cis-4, trans-4a;b (calculated data)
4c (X-ray data)
B1–O1–C1–C2
16.4, )3.6
4.9, 3.4
8.5/0.5
17.3/18.3
)4.0(6), )6.0(6)
9.1(6), 10.9(6)
a Mean values except for the torsion angles (see note b).
b The two values in each line correspond to the torsion angles of one isomer (first line cis-isomer, second line trans-isomer).
c Mean values for the two molecules in the asymmetric unit (in the case of the torsion angles each of the singular values is listed: the values in the
same line correspond to analogous angles in the same molecule).
C3BNO heterocycle in complex 1 show only variations
between )2.3(5)° and 4.0(5)°, in the analogous hetero-
cycles of 4 there are variations between )21.2(5)° and
+20.2(5)°.
The bond angles in the eight-membered heterocycle
of 4 vary from 110.7(3)° to 130.9(3)° and are similar to
the ones found for complex 6, with the exception that
the B–N–C and B–O–B bond angles are smaller,
114.3(3)° $ 118.9(2)° for B–N–CH2 and 130.9(3)° $
134.6(2)° for B–O–B.
While in comparison to the molecular structure of
complex 1 the bond lengths in the acetylacetimine frag-
ment are practically the same in complex 4, characteristic
changes in the bond lengths around the boron atom are
occurring, i. e. the N ! B and B1–O1 bonds are longer,
ꢁ
ꢁ
1.625(6) $ 1.574(4) A and 1.527(5) $ 1.508(4) A, re-
spectively, and the B1–O2 bond is shorter, 1.397(4) $
Fig. 3. Perspective view of the molecular structure of compound 4.
Ellipsoids are shown at the 50% probability level.
ꢁ
1.435(4) A. Significant variations in the bond angles are
not observed, but the B–O–B bond angle is smaller in
comparison to 1, 130.9(3)° $ 137.3(3)°.
Table 1. Selected bond lengths, bond angles and torsion
angles are listed in Table 3.
A comparison of the theoretical and experimental
geometric data of complex cis-4 in Table 3 shows a
reasonably well agreement with respect to bond lengths
and bond angles. In the case of the bond lengths, mayor
differences are only observed for the N ! B,
As can be seen from the molecular structure shown
in Fig. 3, the central eight-membered heterocycle has a
distorted boat-conformation with the B-phenyl groups
having cis-orientation. Nevertheless, while in the cor-
responding salpen[B(PH)–O–B(Ph)] complex 6, the sal-
icylidene groups have an almost parallel orientation in 4
a mutual displacement of the acetylacetimine groups is
observed, thus causing that the boat is more distorted
(see torsion angles in Table 3). This mutual displace-
ment of the acetylacetimine moieties most probably
results from the transannular steric repulsion between
the methyl groups. Moreover, the conformation of the
eight-membered heterocyclic ring in 4 is significantly
different for the two independent molecules present in
the asymmetric unit of the crystal lattice. Comparing
the torsion angles in Table 3, the most significant
variations are related to twists around the C6–C7, C7–
C8, N1–B1, N2–B2, B1–O3 and B2–O3 bonds. Ap-
parently, also the torsion angles in the six-membered
heterocycles are affected by these distortions, since there
are large variations comparing the corresponding angles
for the two heterocycles present in one and the same
molecule as well as between the two independent
molecules in the asymmetric unit (Table 3). While the
torsion angles in the almost planar, six-membered
ꢁ
1.643 $ 1.624(6) A, and the C6–C7 bonds, 1.554 $
ꢁ
1.516(6) A. In the case of the bond angles, the largest
differences occur for O1–B1–O3, B1–N1–C3 and B1–
O3–B2 with differences of 3.7, 3.1 and 4.5°. The varia-
tions are larger for the torsion angles, especially for the
BNCC and NCCC bonds, however, due to the fact that
there also significant variations between the torsion
angles in the two independent molecules present in the
asymmetric unit of the crystal lattice, it can be supposed
that the conformation of the eight-membered heterocy-
clic ring presents some flexibility.
The preference of the cis-configuration over the trans-
configuration in complexes 3 and 4 can be explained by
the fact that the angular and conformational strains are
less in the boat conformer when compared to the chair
conformer. This can be recognized comparing the bond
angles in each pair of corresponding isomers (Table 3).
For complex 3 the O3–B1–N1, B1–N1–C6, N1–C6–C7
and B1–O3–B2 bond angles have larger deviations from
the ideal tetrahedral angle in the case of the trans-
isomer. The same tendency is observed for the isomers

820
M. Sꢀanchez et al. / Journal of Organometallic Chemistry 689 (2004) 811–822
Table 4
of complex 4, where the calculated B1–O3–B2 bond
angle in trans-4 reaches a value of 141.3°.
ꢁ
Selected bond lengths (A), bond angles (°) and torsion angles (°) for
compound 7 (mean values)
Bond lengths
3.3. Preparation and characterization of the 1,3,5-trip-
henylboroxine derivative 7
B1–N1
B1–O1
B1–O3
B1–C1
B2–O1
B2–O2
B2–C13
B3–O2
B3–O3
1.626(4)
1.470(4)
1.474(4)
1.604(4)
1.341(4)
1.386(4)
1.569(5)
1.390(4)
1.342(4)
B3–C7
1.566(4)
1.492(3)
1.506(4)
1.448(4)
1.340(4)
1.382(4)
1.407(4)
1.270(4)
N1–C19
C19–C20
N2–C20
N2–C21
C21–C23
C23–C24
O4–C24
In an attempt to grow crystals of complex 3 suitable
for an X-ray crystallographic study, in several occasions
a crystalline material containing the 1,3,5-triphenylbo-
roxine derivative 7 and the acacenH2 ligand in a 2:1
proportion was obtained. The composition of these
crystals was analyzed by elemental analysis, NMR (1H,
13C, 11B) spectroscopy and X-ray diffraction. A possible
path for the hydrolysis of 3 is shown in Scheme 8, where
it is proposed that three equivalents of the ligand are
partially hydrolyzed by two equivalents of water to give
two equivalents of the triboroxine derivative 7, two
equivalents of 2,4-pentanedione and one equivalent of
acacenH2. Apparently, between two equivalents of 7
and one equivalent of the ligand a crystal lattice of
sufficient stability is formed to displace the reaction
equilibrium in this direction. Such 1,3,5-triphenylbo-
Bond angles
O1–B1–N1
O1–B1–O3
O1–B1–C1
O3–B1–C1
O3–B1–N1
N1–B1–C1
104.1(2)
113.1(3)
112.7(3)
112.5(3)
104.2(2)
109.7(2)
O1–B2–O2
O2–B3–O3
B1–O1–B2
B1–O3–B3
B2–O2–B3
121.3(3)
120.5(3)
121.9(3)
122.5(3)
120.1(3)
Torsion angles
N1–C19–C20–N2 66.9(3)
roxine adducts are well-known [23]. The 1H, 13C and 11
B
NMR data of 7 indicate that the boroxine adduct is
dissociated in solution or that there exists a fast dynamic
exchange equilibrium between the 4-(2-aminoethyle-
neamino)-pent-3-en-2-one and the B3O3 heterocycle.
Such equilibria have been already reported for related
systems [23d,23f].
The most relevant crystallographic data are summa-
rized in Table 1. Selected bond lengths, bond angles and
torsion angles are listed in Table 4. The molecular
structure shown in Fig. 4 proves the existence of the
ꢁ
N ! B coordinative bond in the solid-state, 1.626(4) A.
The molecular geometry of the 1,3,5-triphenylboroxine
adduct is similar to that observed for a series of related
adducts with amines as Lewis basic ligands [23d]. This
CH3
H3C
N
N
3
B
B
O
O
O
H3C
CH3
Fig. 4. Perspective view of the molecular structure of compound 7.
Ellipsoids are shown at the 20% probability level.
+ 2 H2O
X-ray structure gives also evidence for the predomina-
tion of the cetamine tautomeric form for the ligands
used in this contribution, since the positions of the
acetylacetimine hydrogen atoms in the uncoordinated
chelate rings could be localized by a difference Fourier
H3C
H3C
H3C
CH3
CH3
N
H
NH2
N
H
N
H
O
2
B
O
B
O
+
2
+
B
O
O
O
O
O
H3 C
ꢁ
ꢁ
map (N–H ꢀ ꢀ ꢀ O ¼ 1.98 A, N ꢀ ꢀ ꢀ O ¼ 2.67 A). The C24–
7
O4, C23–C24, C21–C23 and C21–N2 bond lengths of
ꢁ
1.270(3), 1.406(4), 1.382(4) and 1.340(4) A, respectively,
support this observation. Similar values are found for
Scheme 8. Possible mechanism for the hydrolysis of complex 7 in the
presence of water.

M. Sꢀanchez et al. / Journal of Organometallic Chemistry 689 (2004) 811–822
821
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[3] (a) M. Sanchez, H. Hopfl, M.-E. Ochoa, N. Farfan, R. Santillan,
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the uncoordinated acacenH2 molecules present in the
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ꢀ
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4. Conclusions
(c)
W. Kliegel, H. Amt, H. Becker, U. Lauterbach,
This contribution has shown that the 2,4-pentanedi-
one derived ligands acacaminolH2, acacenH2 and acac-
penH2 react with phenylboronic acid in a similar way as
the corresponding salicylaldehyde derivatives salami-
nolH2, salenH2 and salpenH2. However, the products
prepared in here are much more soluble than the sali-
cylaldehyde derivatives. Furthermore, the presence of
two methyl groups in the six-membered chelate ring
enhances the steric bulk of this part of the ligand,
causing changes in the molecular composition or the
molecular structure of the product: with acacaminolH2 a
dinuclear monomeric instead of a dimeric complex was
obtained, while in the case of acacenH2 and acacpenH2
a significant distortion of the boat conformation of the
central heterocyclic ring was predicted by computational
methods and confirmed experimentally for one of the
complexes.
G. Lubkowitz, S.J. Rettig, J. Trotter, Can. J. Chem. 72 (1994)
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