Preparation and structural characterization of three types of homo- and heterotrinuclear boron complexes: Salen{[B-O-B][O2BOH]}, salen{[B-O-B][O2BPh]}, and salen{[B-O-B][O2P(O)Ph]}

Article abstract of DOI:10.1021/ic048862e

Three types of homo- and heterotrinuclear boron complexes have been obtained in moderate to good yields from reactions of salen-type ligands with boric acid and combinations of boric acid with phenylboronic and phenylphosphonic acid. The products are air-stable and have relatively high melting points (>290 °C) but are poorly soluble or insoluble in common organic solvents. They have been characterized as far as possible by elemental analysis, mass spectrometry, IR, ¹H, ¹¹B, and ³¹P NMR spectroscopy, and X-ray crystallography. Furthermore, theoretical calculations have been performed for representative examples to permit a complete comparison of the different structure types. A detailed analysis of the molecular structures showed that the complexes are constructed around a central B₃O₃ or B₂PO₃ ring. The salen ligands are attached to two boron atoms of these rings, which have therefore tetrahedral coordination geometries. The complexes contain seven- and eight-membered heterocycles of the B₂C_nON₂ (n = 2, 3) type with chair or twisted-chair and boat-chair or chair-chair conformations, respectively. In the homotrinuclear complexes one of the three boron atoms is three-coordinate and can therefore still act as Lewis acid, thus making these products interesting for catalytic applications, e.g. in asymmetric synthesis. Depending on the substitutents attached to the boron atoms, these complexes show a relationship with either trimetaboric acid, boroxine, or the tetraborate dianion found in Borax.

Full text of DOI:10.1021/ic048862e

Inorg. Chem. 2004, 43, 8490−8500
Preparation and Structural Characterization of Three Types of Homo-
and Heterotrinuclear Boron Complexes: Salen [B B][O2BOH]
Salen [B B][O2BPh] , and Salen [B B][O2P(O)Ph]
{
−O
−
},
{
−O
−
}
{
−O
−
}
Gabriela Vargas,† Ira´n Herna´ndez,† Herbert Hopfl,*,† Mar´ıa-Eugenia Ochoa,‡ Dolores Castillo,‡
1
Norberto Farfa´n,‡ Rosa Santillan,‡ and Elizabeth Go´mez§
Centro de InVestigaciones Qu´ımicas, UniVersidad Auto´noma del Estado de Morelos, AV.
UniVersidad 1001, C.P. 62210 CuernaVaca, Me´xico, Departamento de Qu´ımica, Centro de
InVestigacio´n y de Estudios AVanzados del IPN, Apdo. Postal 14-740,
C.P. 07000 Me´xico D.F., Me´xico, and Instituto de Qu´ımica, UniVersidad Nacional Auto´noma de
Me´xico, Circuito Exterior, Ciudad UniVersitaria, Me´xico D.F. 04510, Me´xico
Received August 17, 2004
Three types of homo- and heterotrinuclear boron complexes have been obtained in moderate to good yields from
reactions of salen-type ligands with boric acid and combinations of boric acid with phenylboronic and phenylphosphonic
acid. The products are air-stable and have relatively high melting points (>290 °C) but are poorly soluble or insoluble
in common organic solvents. They have been characterized as far as possible by elemental analysis, mass
spectrometry, IR, 1H, 11B, and 31P NMR spectroscopy, and X-ray crystallography. Furthermore, theoretical calculations
have been performed for representative examples to permit a complete comparison of the different structure types.
A detailed analysis of the molecular structures showed that the complexes are constructed around a central B3O3
or B2PO3 ring. The salen ligands are attached to two boron atoms of these rings, which have therefore tetrahedral
coordination geometries. The complexes contain seven- and eight-membered heterocycles of the B2CnON2 (n
)
2, 3) type with chair or twisted-chair and boat-chair or chair-chair conformations, respectively. In the homotrinuclear
complexes one of the three boron atoms is three-coordinate and can therefore still act as Lewis acid, thus making
these products interesting for catalytic applications, e.g. in asymmetric synthesis. Depending on the substitutents
attached to the boron atoms, these complexes show a relationship with either trimetaboric acid, boroxine, or the
tetraborate dianion found in Borax.
1. Introduction
apical site is occupied by an interchangeable ligand. In hexa-
and heptacoordinate complexes the ligands are usually
located in the equatorial plane, leaving two or three sites
for the coordination of additional ligands.2 Due to these
geometrical particularities salen complexes have found
applications as catalytically active species for a series of
chemical reactions, including asymmetric synthesis.3
Complexes with ligands of the Salen class (salenH2 )
N,N′-ethylenebis(salicylideneimine)) have been studied ex-
tensively, in particular with transition metals.1 Generally,
salen type ligands feature two covalent and two coordinate-
covalent sites situated in a planar array, which makes them
ideal ligands for the generation of specific metal polyhedra.
Complexes with pentacoordinate metal centers usually have
square pyramidal coordination environments, in which the
The huge majority of salen complexes are mononuclear,
an exception being compounds with group 13 elements,
which can also be di-, tri-, and tetranuclear, especially in
* Author to whom correspondence should be addressed. E-mail: hhopfl@
buzon.uaem.mx. Fax: (+52) 777 329 79 97.
(2) Sa´nchez, M.; Harvey, M. J.; Nordstrom, F.; Parkin, S.; Atwood, D.
A. Inorg. Chem. 2002, 41, 5397.
(3) (a) Zhang, W.; Loebach, J. L.; Wilson, S. R.; Jacobsen, E. N. J. Am.
Chem. Soc. 1990, 112, 2801. (b) Jacobsen, E. N.; Zhang, W.; Muci,
A. R.; Ecker, J. R.; Deng, L. J. Am. Chem. Soc. 1991, 113, 7063. (c)
Konsler, R. G.; Karl, J.; Jacobsen, E. N. J. Am. Chem. Soc. 1998,
120, 10780.
† Universidad Auto´noma del Estado de Morelos.
‡ Centro de Investigacio´n y de Estudios Avanzados del IPN.
§ Universidad Nacional Auto´noma de Me´xico.
(1) (a) Holm, R. H.; Everett, G. W., Jr.; Chakravorty, A. Prog. Inorg.
Chem. 1966, 7, 83. (b) Hobday, M. D.; Smith, T. D. Coord. Chem.
ReV. 1972, 9, 311.
8490 Inorganic Chemistry, Vol. 43, No. 26, 2004
10.1021/ic048862e CCC: $27.50
© 2004 American Chemical Society
Published on Web 12/03/2004

Homo- and Heterotrinuclear Boron Complexes
Chart 1. Reaction between a Salen Derivative as Ligand and a Boric,
Boronic, or Borinic Acid Yielding Dinuclear (I, II), Trinuclear (III),
and Tetranuclear (IV) Products
possible applications in catalytic processes, where a Lewis
acid is required. The circumstance that the two tetracoordi-
nate boron atoms are chiral and that it is relatively facile to
introduce further chiral functional groups in these ligands
makes them interesting for asymmetric synthesis.7 Until now,
only two type III derivatives have been described in the
literature.5b
The present contribution enhances the knowledge on the
preparation and structural characterization of homotrinuclear
boron-salen complexes as well as on the transformation of
dinuclear oxo-bridged borates to heterotrinuclear species
containing two boron atoms and one phosphorus atom.
2. Experimental Section
Instrumentation. 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 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 HP 5989A and JEOL JMS 700
equipment. Elemental analyses have been carried out on Perkin-
Elmer Series II 2400 and Elementar Vario ELIII instruments. It
should be mentioned that elemental analyses of boronic acid
derivatives are complicated by incombustible residues (boron
carbide) and therefore not always in the established limits of
exactitude, especially with respect to carbon.8 Therefore, only the
values for hydrogen and nitrogen are indicated.
Preparative Part. Commercial starting materials and solvents
have been used. The salen, salen(tBu), acen, salpen, salpen(tBu),
acpen, salphen, salphen(tBu), acphen, salcen, salcen(tBu), and accen
ligands 1a-l have been prepared according to a method reported
in the literature.9
Preparation of the Salen{[B(OH)-O-B(OH)]} Complexes.
Compounds 2b,g have been prepared by similar methods; therefore,
the experimental procedure of the preparation is only described in
detail for the first case.
tetrahedral coordination environments.4,5 Since the boron
atom very rarely exceeds the coordination number of 46 and
forms relatively strong covalent bonds with oxygen and
coordinate-covalent bonds with nitrogen atoms, this element
is an excellent candidate for the study of such species. So
far, four different types of boron-salen complexes are known
(Chart 1).
Considering that the two salicylideneimino groups of the
ligands coordinate to different boron atoms, the composition
of complexes I-IV can be described as follows: In I two
individual BR2 or B(OR)2 groups are complexed,4 while in
II and III it is a dinuclear boroxane group, RB-O-BR,5
and
a
trinuclear boroxine moiety, (B-O-B)-
(O2BPh).5b In IV two diboradisiloxane rings, (B-O-SiR2-
O)2, are connected through a pair of ligands to form a
molecule with a large cylinder-shaped cavity.4g
SalentBu{[B(OH)-O-B(OH)]} (2b). Compound 2b was pre-
pared from 1 equiv of ligand 1b (2.00 g, 4.06 mmol) and 2 equiv
of boric acid (0.50 g, 8.12 mmol) in 15 mL of acetonitrile. After
the solution was stirred for 15 min, a yellow precipitate of 2b had
formed that was collected by filtration and dried. Yield: 60%.
Mp: >300 °C. IR (KBr): ν˜ ) 3429 (br, m, B-OH), 2959 (s),
Since type III boron complexes possess a three-coordinate
boron atom, they are particularly interesting in view of
(4) (a) Hohaus, E. Fresenius Z. Anal. Chem. 1983, 315, 696. (b) Ghose,
B. N. Synth. React. Inorg. Met.-Org Chem. 1986, 16, 1383. (c) Kliegel,
W.; Amt, H.; Becker, H.; Lauterbach, U.; Lubkowitz, G.; Rettig, S.
J.; Trotter, J. Can. J. Chem. 1994, 72, 2118. (d) Atwood, D. A.; Jegier,
J. A.; Remington, M. J.; Rutherford, D. Aust. J. Chem. 1996, 49, 1333.
(e) Wei, P.; Atwood, D. A. Inorg. Chem. 1997, 36, 4060. (f) Wei, P.;
Atwood, D. A. Chem. Commun. 1997, 1427. (g) Wei, P.; Keizer, T.;
Atwood, D. A. Inorg. Chem. 1999, 38, 3914. (h) Agustin, D.; Rima,
G.; Gornitzka, H.; Barrau, J. Organometallics 2000, 19, 4276. (i)
Woodgate, P. D.; Horner, G. M.; Maynard, N. P.; Rickard, C. E. F. J.
Organomet. Chem. 2000, 595, 215. (j) Kunkely, H.; Vogler, A. Inorg.
Chim. Acta 2001, 321, 171. (k) Keizer, T. S.; DePue, L. J.; Parkin,
S.; Atwood, D. A. J. Am. Chem. Soc. 2002, 124, 1864.
(7) (a) Corey, E. J.; Helal, C. J. Angew. Chem., Int. Ed. 1998, 37, 1986.
(b) Corey, E. J.; Link, J. O. Tetrahedron Lett. 1992, 33, 4141. (c)
Lohray, B. B.; Bhushan, V. Angew. Chem., Int. Ed. Engl. 1992, 31,
729. (d) Caze, C.; El Moualij N.; Hodge, P.; Lock, C. J.; Ma, J. J.
Chem. Soc., Perkin Trans. 1 1995, 345. (e) Ferey, V.; Vedrenne, P.;
Toupet, L.; Le Gall, T.; Mioskowski, C. J. Org. Chem. 1996, 61, 7244.
(f) Petasis, N. A.; Zavialov, I. A. J. Am. Chem. Soc. 1997, 119, 445.
(g) Davies, C. D.; Marsden, S. P.; Stokes, E. S. E. Tetrahedron Lett.
1998, 39, 8513. (h) Petasis, N. A.; Zavialov, I. A. J. Am. Chem. Soc.
1998, 120, 11798. (i) Batey, R. A.; MacKay D. B.; Santhakumar, V.
J. Am. Chem. Soc. 1999, 121, 5075. (j) Li, Y.; Liu, Y.; Bu, W.; Guo,
J.; Wang, Y. Chem. Commun. 2000, 1551. (k) Barringhaus, K.-H.;
Matter, H.; Kurz, M. J. Org. Chem. 2000, 65, 5031. (l) Bandini, M.;
Cozzi, P. G.; Monari, M.; Perciaccante, R.; Selva, S.; Umani-Ronchi
A. Chem. Commun. 2001, 1318. (m) Chan, K.-F.; Wong, H. N. C.
Org. Lett. 2001, 3, 3991.
(5) (a) Sa´nchez, M.; Ho¨pfl, H.; Ochoa, M.-E.; Farfa´n, N.; Santillan, R.;
Rojas, S. Inorg. Chem. 2001, 40, 6405. (b) Sa´nchez, M.; Keizer, T.
S.; Parkin, S.; Ho¨pfl, H.; Atwood, D. A. J. Organomet. Chem. 2002,
654, 36.
(6) For hypervalent boron complexes, see: (a) Hillier, A. C.; Jacobsen,
H.; Gusev, D.; Schmalle, H. W.; Berke, H. Inorg. Chem. 2001, 40,
6334. (b) Yamashita, M.; Yamamoto, Y.; Akiba, K. Nagase, S. Angew.
Chem., Int. Ed. 2000, 39, 4055.
(8) James, T. D.; Sandanayake, K. R. A. S.; Shinkai, S. Angew. Chem.,
Int. Ed. Engl. 1996, 35, 1910 and references therein.
(9) Dubsky, J. V.; Sokol, A. Collect. Czech. Chem. Commun. 1931, 3,
548.
Inorganic Chemistry, Vol. 43, No. 26, 2004 8491

Vargas et al.
2871 (m), 1640 (CdN, s), 1567 (m), 1442 (s), 1396 (s, B-O),
1308 (m), 1245 (m), 1189 (m), 1141 (m), 971 (w), 876 (w), 831
(w), 768 (w), 679 (w), 641 (w), 516 (w) cm-1. 1H NMR (400 MHz,
CDCl3): δ ) 8.01 (s, 2H, C(H)dN), 7.47, 7.00 (d, 4H, H-3, H-5),
ppm. MS (20 eV, EI): m/z (%) ) 348 (0.4) [M - BO2H], 329
(100), 315 (7), 304 (16), 286 (12), 245 (5), 185 (47). Anal. Calcd
for C18H19B3N2O6 (Mr ) 391.80): H, 4.85; N, 7.15. Found: H,
5.58; N, 7.74.
t
4.21 and 3.71 (ABCD, 4H, NCH2, NCH2′), 1.47, 1.28 (s, 36H, -
Salpen{[B-O-B][O2BOH]} (3d). Yield: 93%. Mp: >350 °C.
IR (KBr): ν˜ ) 3387 (br, m, OH), 1648 (s, CdN), 1563 (m), 1484
(m), 1397 (m, B-O), 1313 (m), 1238 (m), 1148 (s), 995 (m), 924
Bu) ppm. 11B NMR (96 MHz, CDCl3): δ ) 5.0 (h1/2 ) 270 Hz)
ppm. MS (20 eV, EI): m/z (%) ) 563 (23) [M + 1], 546 (31) [M
- OH], 529 (46), 514 (58), 501 (66), 492 (100).
(w), 858 (w), 824 (w), 760 (m), 711 (m), 625 (w), 458 (w) cm-1
.
1H NMR (400 MHz, DMSO-d6): δ ) 8.60 (s, 2H, C(H)dN), 7.48
(m, 4H, H-3, H-5), 6.87 (m, 4H, H-2, H-4), 6.6 (br, s, 1H, OH),
3.86 and 3.80 (AB, 4H, N-CH2, N-CH2′), 2.45 and 2.04 (AB,
2H, H-9) ppm. 13C NMR (100 MHz, DMSO-d6): δ ) 163.4 (Cd
N), 159.6 (C-1), 136.4 (C-3), 131.6 (C-5), 118.4, 118.2 (C-2, C-4),
116.2 (C-6), 54.1 (N-CH2), 31.8 (C-9) ppm. 11B NMR (128 MHz,
DMSO-d6): δ ) 20.7 (h1/2 ) 380 Hz), 2.3 (h1/2 ) 190 Hz) ppm.
MS (20 eV, EI): m/z (%) ) 319 (100) [M - BO3], 305 (4), 290
(14), 277 (9), 263 (4), 250 (2), 236 (18), 187 (19), 159 (24), 132
(7), 117 (4), 91 (9), 77 (8). Anal. Calcd for C17H17B3N2O6 (Mr )
377.78): H, 4.50; N, 7.42. Found: H, 4.78; N, 7.25.
Salphen{[B(OH)-O-B(OH)]} (2g). Yield: 32%. Mp: >300
°C (dec). IR (KBr): ν˜ ) 3357 (br, m, OH), 1625 (CdN, s), 1555
(s), 1481 (m), 1452 (m), 1371 (m, B-O), 1311 (m), 1190 (m),
1117 (s), 1022 (w), 925 (w), 916 (w), 866 (w), 808 (m), 764 (m),
684 (w), 571 (w), 456 (w) cm-1. 1H NMR (200 MHz, DMSO-d6):
δ ) 8.65 (s, 2H, C(H)dN), 7.60 (m, 8H, H-10, H-9, H-3, H-5),
6.94 (m, 4H, H-2, H-4). 13C NMR (50 MHz, DMSO-d6): δ ) 164.7
(CdN), 159.5 (C-1), 138.5, 137.9 (C-4, C-8), 133.1 (C-5), 129.8
(C-10), 126.0 (C-9), 118.5 (C-2, C-4), 115.9 (C-6). 11B NMR (64
MHz, DMSO-d6): δ ) 3.6 (h1/2 ) 1430 Hz) ppm.
Preparation of the Salen{[B-O-B][O2BOH]} Complexes.
Compounds 3a-g have been prepared by similar methods; there-
fore, the experimental procedure of the preparation is only described
in detail for the first case.
SalpentBu{[B-O-B][O2BOH]} (3e). Yield: 78%. Mp: >350
°C. IR (KBr): ν˜ ) 3410 (br, m, OH), 2958 (s), 2871 (m), 1644 (s,
CdN), 1566 (m), 1395 (s, B-O), 1310 (m), 1259 (m), 1184 (m),
1130 (m), 936 (m), 901 (m), 821 (w), 771 (m), 681 (m), 609 (w)
Salen{[B-O-B][O2BOH]} (3a). Compound 3a was prepared
from 1 equiv of ligand 1a (2.00 g, 7.46 mmol) and 3 equiv of boric
acid (1.38 g, 22.38 mmol) in 15 mL of acetonitrile. The mixture
was refluxed for 4 h using a Dean-Stark trap, whereupon a yellow
precipitate of 3a had formed that was collected by filtration and
dried. Recrystallization from acetone gave crystals suitable for X-ray
crystallography. Yield: 98%. Mp: >350 °C. IR (KBr): ν˜ ) 3309
(br, m, OH), 3057 (w), 2928 (w), 1639 (s, CdN), 1570 (m), 1492
(m), 1458 (m), 1408 (m, B-O), 1287 (m), 1226 (m), 1150 (m),
1069 (m), 976 (m), 940 (w), 857 (m), 812 (m), 751 (m), 647 (w),
465 (w) cm-1. 1H NMR (400 MHz, DMSO-d6): δ ) 8.64 (s, 2H,
C(H)dN), 7.54 (m, 4H, H-3, H-5), 6.93 (m, 4H, H-2, H-4), 6.37
(s, 1H, OH), 4.08 and 3.98 (ABCD, 4H, N-CH2, N-CH2′) ppm.
11B NMR (128 MHz, DMSO-d6): δ ) 20.6 (h1/2 ) 380 Hz), 2.7
(h1/2 ) 190 Hz) ppm. MS (20 eV, EI): m/z (%) ) 305 (100) [M
- BO3], 277 (4), 250 (4), 236 (21), 173 (9), 152 (20), 91 (13), 77
(9). Anal. Calcd for C16H15B3N2O6 (Mr ) 363.74): H, 4.15; N,
7.69. Found: H, 4.31; N, 7.71.
1
cm-1. H NMR (200 MHz, CDCl3): δ ) 8.02 (s, 2H, C(H)dN),
7.49, 7.00 (d, 4H, H-3, H-5), 4.19 and 3.70 (AB, 4H, N-CH2),
t
1.43, 1.24 (s, 36H, Bu) ppm. 11B NMR (64 MHz, DMSO-d6): δ
) 18.5 (h1/2 ) 680 Hz), 1.4 (h1/2 ) 610 Hz) ppm. MS (20 eV, EI):
t
m/z (%) ) 560 (100) [M - C3H6], 545 (4) [M - Bu], 506 (26),
489 (7), 446 (9), 273 (15), 260 (85), 247 (32), 232 (16), 230 (10),
219 (10), 204 (12), 190 (16). Anal. Calcd for C33H49B3N2O6 (Mr
) 602.43): H, 8.20; N, 4.65. Found: H, 8.58; N, 4.71.
Acpen{[B-O-B][O2BOH]} (3f). Yield: 92%. Mp: >350 °C.
IR (KBr): ν˜ ) 3351 (br, s, OH), 3047 (m), 2990 (m), 1618 (s,
CdN), 1558 (m), 1481 (m), 1399 (s, B-O), 1328 (s), 1266 (m),
1110 (s), 1024 (m), 944 (m), 919 (m), 843 (m), 806 (m), 759 (m),
1
673 (m), 586 (w), 500 (w), 468 (m) cm-1. H NMR (400 MHz,
DMSO-d6): δ ) 7.75 (dd, 2H, H-5), 7.45 (ddd, 2H, H-3), 6.86
(m, 4H, H-2, H-4), 3.90 and 3.80 (AB, 4H, NCH2), 2.58 (s, 3H,
C(Me)dN), 2.24 and 2.04 (AB, 2H, CH2-9) ppm. 13C NMR (100
MHz, DMSO-d6): δ ) 170.4 (CdN), 159.0 (C-1), 135.0 (C-3),
129.4 (C-5), 119.4 (C-6), 118.4 (C-2), 118.1 (C-4), 45.9 (NCH2),
21.9 (C-9), 16.5 (C-(Me)dN) ppm. 11B NMR (64 MHz, CDCl3):
δ ) 20.0 (h1/2 ) 540 Hz), 1.90 (h1/2 ) 330 Hz) ppm. MS (20 eV,
EI): m/z (%) ) 344 (23), 329 (18), 318 (36), 303 (27), 199 (100),
185 (70). Anal. Calcd for C19H21B3N2O6 (Mr ) 405.82): H, 5.22;
N, 6.90. Found: H, 5.28; N, 7.05.
SalentBu{[B-O-B][O2BOH]} (3b). Yield: 64%. Mp: >350
°C. IR (KBr): ν˜ ) 3435 (br, m, OH), 2959 (s), 2872 (w), 1639 (s,
CdN), 1567 (w), 1446 (m), 1396 (s, B-O), 1306 (w), 1245 (w),
1141 (m), 1066 (w), 962 (w), 875 (w), 825 (w), 766 (w), 679 (w)
1
cm-1. H NMR (300 MHz, CDCl3): δ ) 13.6 (br, s, 1H, OH),
8.44, (s, 2H, C(H)dN), 7.42 (d, 2H, H-3), 7.14 (d, 2H, H-5), 3.93
and 3.75 (ABCD, 2H, N-CH2, N-CH2′), 1.47 and 1.33 (s, 36H,
tBu) ppm. 13C NMR (75 MHz, CDCl3): δ ) 168.5 (CdN), 158.4
(C-1), 140.6, 137.1 (C-2, C-4), 127.5, 126.4 (C-3, C-5), 118.1 (C-
6), 62.6, 62.2 (N-CH2, N-CH2′), 35.4, 34.5 (tBu-C), 31.9, 29.8
(tBu-Me) ppm. 11B NMR (64 MHz, CDCl3): δ ) 19.6 (h1/2 ) 380
Hz), 0.8 (h1/2 ) 380 Hz) ppm. MS (20 eV, EI): m/z (%) ) 530
(53) [M - tBuH], 515 (61), 473 (19), 285 (12), 236 (13), 213 (10),
57 (100). Anal. Calcd for C32H47B3N2O6 (Mr ) 588.17): H, 8.05;
N, 4.76. Found: H, 8.50; N, 4.75.
Salphen{[B-O-B][O2BOH]} (3g). Yield: 77%. Mp: >350
°C. IR (KBr): ν˜ ) 3339 (br, m, OH), 3058 (m), 1627 (s, CdN),
1556 (m), 1481 (m), 1454 (m), 1408 (m, B-O), 1371 (m), 1312
(m), 1220 (m), 1192 (m), 1161 (m), 1118 (m), 1021 (m), 953 (m),
916 (m), 875 (m), 810 (m), 764 (m), 684 (m), 572 (m), 472 (w)
1
cm-1. H NMR (400 MHz, DMSO-d6): δ ) 8.66 (s, 2H, C(H)d
N), 7.61 (m, 8H, m, H-3, H-5, H-9, H-10), 6.94 (m, 4H, H-2, H-4)
ppm. 13C NMR (100 MHz, DMSO-d6): δ ) 164.9 (CdN), 159.7
(C-1), 138.8, 138.2 (C-3, C-8), 133.3 (C-5), 130.0 (C-10), 126.2
(C-9), 118.9 (C-2), 118.6 (C-4), 116.1 (C-1) ppm. 11B NMR (128
MHz, DMSO-d6): δ ) 20.6 (h1/2 ) 260 Hz), 3.0 (h1/2 ) 450 Hz)
ppm. MS (20 eV, EI): m/z (%) ) 328 (24), 326 (5), 235 (3), 221
(10). Anal. Calcd for C20H15B3N2O6 (Mr ) 411.79): H, 3.64; N,
6.80. Found: H, 3.79; N, 7.05.
Acen{[B-O-B][O2BOH]} (3c). Yield: 98%. Mp: >350 °C.
IR (KBr): ν˜ ) 3411 (br, m, OH), 1618 (s, CdN), 1557 (s), 1397
(s, B-O), 1282 (s), 1240 (m), 1115 (s), 983 (m), 924 (m), 856
(m), 810 (m), 755 (s), 696 (m), 671 (m), 562 (w), 463 (m) cm-1
.
1H NMR (400 MHz, DMSO-d6): δ ) 7.77 (d, 2H, H-5), 7.45 (ddd,
2H, H-3), 6.87 (m, 4H, H-2, H-4), 4.11 and 4.05 (ABCD, 4H,
N-CH2, N-CH2′), 2.59 (s, 6H, C(Me)dN) ppm. 11B NMR (64
MHz, DMSO-d6): δ ) 19.6 (h1/2 ) 690 Hz), 0.8 (h1/2 ) 320 Hz)
Preparation of the Salen{[B-O-B][O2BPh]} Complexes.
Compounds 4a-g have been prepared by similar methods; there-
8492 Inorganic Chemistry, Vol. 43, No. 26, 2004

Homo- and Heterotrinuclear Boron Complexes
fore, the experimental procedure of the preparation is only described
in detail for the first case.
m-BPh, p-BPh), 7.02 (d, 2H, H-5), 4.15 and 3.61 (AB, 4H, NCH2),
t
1.44, 1.27 (s, 36H, Bu) ppm. 11B NMR (64 MHz, DMSO-d6): δ
) 19.4 (h1/2 ) 250 Hz), 1.1 (h1/2 ) 390 Hz) ppm. MS (70 eV, EI):
m/z (%) ) 662 (2) [M], 647 (1), 618 (1), 585 (1) [M - Ph], 543
(3), 460 (55), 446 (29), 404 (8), 306 (21), 264 (9), 216 (13). Anal.
Calcd for C39H53B3N2O5 (Mr ) 662.53): H, 8.06; N, 4.22. Found:
H, 8.33; N, 3.73.
Salen{[B-O-B][O2BPh]} (4a). For the preparation of com-
pound 4a a mixture of 1 equiv of ligand 1a (1.00 g, 3.73 mmol)
and 2 equiv of boric acid (0.46 g, 7.46 mmol) was refluxed in 15
mL of acetonitrile until a yellow precipitate formed. Then 1 equiv
of phenylboronic acid (0.46 g, 3.73 mmol) was added and the
mixture was refluxed for 4 h using a Dean-Stark trap. The solid
was collected by filtration and dried. Recrystallization from DMF
gave crystals suitable for X-ray crystallography. Yield: 88%. Mp:
290-293 °C. IR (KBr): ν˜ ) 3058 (w), 2930 (w), 1651(s, CdN),
1559 (w), 1478 (w), 1443 (w), 1359 (m), 1299 (m), 1232 (w), 1116
(s), 1047 (m), 973 (m), 854 (w), 800 (w), 766 (m), 698 (w), 610
(w), 466 (w) cm-1. 1H NMR (400 MHz, DMSO-d6): δ ) 8.70 (s,
2H, C(H)dN), 7.58 (m, 6H, m, H-3, H-5, o-BPh), 7.29 (d, 1H,
p-BPh), 7.19 (dd, 2H, m-BPh), 6.98 (m, 4H, H-2, H-4), 4.01 and
4.11 (2H, ABCD, NCH2, NCH2′) ppm. 13C NMR (100 MHz,
DMSO-d6): δ ) 163.3 (CdN), 162.3 (i-BPh), 159.4 (C-1), 136.8
(C-3), 134.2 (o-BPh), 132.0 (C-5), 129.8 (p-BPh), 127.1 (m-BPh),
118.7 (C-2), 118.6 (C-4), 116.3 (C-6), 54.7 (NCH2) ppm. 11B NMR
(64 MHz, DMSO-d6): δ ) 19.9 (h1/2 ) 380 Hz), 1.4 (h1/2 ) 190
Hz) ppm. MS (70 eV, EI): m/z (%) ) 424 (54) [M], 395 (12), 380
(11), 347 (95) [M - Ph], 319 (26), 303 (72), 277 (45), 249 (14),
200 (10), 174 (100), 152 (87), 132 (33). Anal. Calcd for
C22H19B3N2O5 (Mr ) 424.07): H, 4.51; N, 6.60. Found: H, 4.72;
N, 6.71.
Acpen{[B-O-B][O2BPh]} (4f). Yield: 78%. Mp: >350 °C.
IR (KBr): ν˜ ) 3070 (w), 2927 (w), 1619 (s, CdN), 1556 (m),
1486 (m), 1443 (m), 1336 (s), 1277 (m), 1168 (s), 1121 (s), 978
(m), 886 (w), 838 (w), 759 (m), 712 (m), 669 (m), 608 (w) cm-1
.
1H NMR (400 MHz, DMSO-d6): δ ) 7.78, 7.69 (d, 4H, H-5,
o-BPh), 7.46 (dd, 2H, H-3), 7.37 (dd, 1H, p-BPh), 7.28 (dd, 2H,
m-BPh), 6.88 (m, 4H, H-2, H-4), 3.96 and 3.84 (AB, 4H, NCH2),
2.60 (6H, s, C(Me)dN), 2.33 and 1.97 (AB, 2H, H-9) ppm. 11B
NMR (96 MHz, DMSO-d6): δ ) 19.5 (h1/2 ) 370 Hz), 1.0 (h1/2
)
100 Hz) ppm. MS (70 eV, EI): m/z (%) ) 466 (29) [M], 389 (5)
[M - Ph], 344 (77) [M - C6H5BO2], 329 (40), 318 (85), 308 (1),
303 (61), 248 (27), 221 (23), 77 (100). Anal. Calcd for C25H25B3N2O5
(Mr ) 466.15): H, 5.40; N, 6.00. Found: H, 5.49; N, 6.25.
Salphen{[B-O-B][O2BPh]} (4g). Yield: 31%. Mp: 316-318
°C. IR (KBr): ν˜ ) 3059 (m), 2924 (w), 1625 (s, CdN), 1554 (s),
1481 (m), 1452 (m), 1369 (s), 1312 (s), 1114 (s), 1021 (m), 953
(m), 916 (w), 865 (w), 809 (m), 765 (m), 684 (w), 570 (w), 459
(w) cm-1 1H NMR (200 MHz, DMSO-d6): δ ) 8.64 (s, 2H,
.
C(H)dN), 7.60 (m, 10H, H-3, H-5, H-9, H-10, o-BPh), 6.94 (m,
7H, H-2, H-4, m-BPh, p-BPh) ppm. 11B NMR (64 MHz, DMSO-
d6): δ ) 20.5 (h1/2 ) 720 Hz), 2.4 (h1/2 ) 410 Hz) ppm. MS (70
eV, EI): m/z (%) ) 472 (2) [M], 429 (10), 402 (11), 353 (25)
[M-C6H5BO2], 325 (63), 312 (37), 296 (60), 260 (24), 221 (100).
Anal. Calcd for C26H19B3N2O5 (Mr ) 472.12): H, 4.05; N, 5.93.
Found: H, 3.92; N, 7.02.
SalentBu{[B-O-B][O2BPh]} (4b). Yield: 62%. Mp: 310-
312 °C. IR (KBr): ν˜ ) 2958 (m), 2869 (w), 1643 (s, CdN), 1564
(m), 1515 (w), 1445 (m), 1364 (m), 1304 (m), 1258 (w), 1186 (w),
1136 (m), 1050 (w), 975 (w), 827 (w), 774 (w), 706 (w), 669 (w)
1
cm-1. H NMR (200 MHz, CDCl3): δ ) 8.03 (s, 2H, C(H)dN),
7.75 (d, 2H, o-BPh), 7.58, 7.07 (d, 4H, H-3, H-5), 7.18 (dd, 3H,
m-BPh, p-BPh), 4.18 and 3.91 (ABCD, 4H, NCH2, NCH2′), 1.46,
1.28 (s, 36H, tBu) ppm. 11B NMR (64 MHz, DMSO-d6): δ ) 20.5
(h1/2 ) 370 Hz), 1.4 (h1/2 ) 40 Hz) ppm. MS (70 eV, EI): m/z (%)
) 648 (1) [M], 604 (10), 546 (2), 529 (11), 501 (7), 460 (27), 446
(6), 346 (43), 264 (43). Anal. Calcd for C38H51B3N2O5 (Mr )
648.51): H, 7.92; N, 4.31. Found: H, 8.56; N, 4.33.
Preparation of Compound 5. Crystals of compound 5 were
obtained during attempts to crystallize acphen{[B-O-B][O2BOH]}
from acetonitrile. IR (KBr): ν˜ ) 3480 (m), 3434 (m), 3365 (m),
1614 (s, CdN), 1556 (m), 1483 (s), 1442 (s), 1389 (s, B-O), 1271
(m), 1208 (m), 1071 (m), 992 (m), 947 (m), 848 (m), 759 (m), 706
1
(m), 698 (m), 608 (w), 543 (w), 474 (w). H NMR (400 MHz,
Acen{[B-O-B][O2BPh]} (4c). Yield: 62%. Mp: 314-316 °C.
IR (KBr): ν˜ ) 3068 (w), 1617 (s, CdN), 1555 (m), 1485 (m),
1445 (m), 1324 (s), 1118 (s), 1050 (m), 983 (m), 882 (m), 842
(m), 763 (m), 711 (m), 669 (m), 572 (m) cm-1. MS (70 eV, EI):
m/z (%) ) 452 (28) [M], 409 (22), 375 (12) [M - Ph], 345 (3),
331 (100), 305 (20), 292 (25). Anal. Calcd for C24H23B3N2O5 (Mr
) 452.13): H, 5.13; N, 6.19. Found: H, 5.09; N, 6.36.
Salpen{[B-O-B][O2BPh]} (4d). Yield: 85%. Mp: >310 °C.
IR (KBr): ν˜ ) 3058 (w), 1626 (s, CdN), 1555 (s), 1482 (m), 1453
(m), 1409 (m), 1370 (m), 1312 (m), 1191 (w), 1161 (m), 1117 (s),
1021 (w), 953 (w), 916 (w), 868 (w), 808 (m), 766 (m), 685 (w),
572 (w) cm-1. 1H NMR (400 MHz, DMSO-d6): δ ) 8.54 (s, 2H,
C(H)dN), 7.55 (dd, 2H, o-BPh), 7.12 (m, 5H, H-3, m-BPh, p-BPh),
7.04 (dd, 2H, H-5), 6.48 (dd, 2H, H-4), 6.35 (d, 2H, H-2), 3.49
and 3.36 (AB, 4H, NCH2), 2.20 (m, 2H, H-9) ppm. 11B NMR (96
MHz, DMSO-d6): δ ) 20.1 (h1/2 ) 830 Hz), 1.0 (h1/2 ) 50 Hz)
ppm. MS (70 eV, EI): m/z (%) ) 438 (1) [M], 395 (100) [M -
C3H7], 394 (45), 336 (1), 319 (6), 291 (59), 248 (5), 235 (6), 188
(9), 159 (42). Anal. Calcd for C23H21B3N2O5 (Mr ) 438.10): H,
4.82; N, 6.39. Found: H, 5.16; N, 6.28.
DMSO-d6): δ ) 7.86 (d, 2H, H-5), 7.61 (dd, 2H, H-3), 7.06-6.85
(m, 10H, H-2, H-9, H-10, H-11, H-12), 6.66 (dd, 2H, H-4), 4.90
(br, s, 4H, NH2), 2.35 (s, 6H, C(Me)dN) ppm.
Preparation of Salen{[B-O-B][O2P(O)Ph]} (6a). Compound
6a was prepared from 1 equiv of ligand 1a (1.00 g, 7.46 mmol)
and 2 equiv of boric acid (0.92 g, 14.92 mmol) in 15 mL of
acetonitrile. The mixture was refluxed for 4 h using a Dean-Stark
trap, whereupon 1 equiv of phenylphosphonic acid (1.17 g, 7.46
mmol) was added. The yellow precipitate of 6a that was formed
after 8 h of reflux was collected by filtration and dried. Yield: 92%.
Mp: 289-292 °C. IR (KBr): ν˜ ) 3057 (w), 1640 (s, CdN), 1561
(m), 1479 (m), 1447 (m), 1356 (m), 1304 (w), 1236 (m), 1198 (s),
1142 (s), 985 (m), 944 (m), 820 (w), 752 (m), 705 (w), 590 (w),
538 (m) cm-1. 1H NMR (400 MHz, DMSO-d6): δ ) 8.81 (s, 2H,
3
C(H)dN), 7.76 (ddd, 2H, JH-P ) 14 Hz, o-PPh), 7.62 (dd, 2H,
H-5), 7.56 (ddd, 2H, H-3), 7.51 (d, 1H, p-PPh), 7.46 (m, 2H,
m-PPh), 6.98 (dd, 2H, H-4), 6.91 (d, 2H, H-2), 4.40 and 4.13
(ABCD, 4H, NCH2, NCH2′) ppm. 11B NMR (128 MHz, DMSO-
d6): δ ) 2.0 (h1/2 ) 320 Hz). 31P NMR (81 MHz, DMSO-d6): δ
) 5.51 ppm. MS (FAB+): m/z (%) ) 460 (4) [M], 307 (20), 289
(11), 154 (100), 136 (71), 107 (20), 77 (18). Anal. Calcd for
C22H19B2N2O6P (Mr ) 460.00): H, 4.16; N, 6.08. Found: H, 4.85;
N, 6.92.
SalpentBu{[B-O-B][O2BPh]} (4e). Yield: 72%. IR (KBr):
ν˜ ) 2961 (w), 1641 (s, CdN), 1438 (s), 1389 (s), 1365 (s), 1168
(w), 1102 (m), 912 (w), 863 (w), 813 (w), 745 (w), 703 (w), 607
(w) cm-1
.
1H NMR (200 MHz, DMSO-d6): δ ) 8.02 (s, 2H,
X-ray Crystallography. X-ray diffraction studies were per-
formed on a Bruker-APEX diffractometer with a CCD area detector
C(H)dN), 7.68 (m, 2H, o-BPh), 7.50 (d, 2H, H-3), 7.41 (m, 3H,
Inorganic Chemistry, Vol. 43, No. 26, 2004 8493

Vargas et al.
Chart 2. Salen Ligands Used for the Reactions Described in This
Contribution
(λΜïΚR ) 0.710 73 Å; monochromator, graphite). Frames were
collected at T ) 293 K (compound 3a) and T ) 100 K (compounds
4a and 5) via ω- and φ-rotation at 10 s/frame (SMART).10a The
measured intensities were reduced to F2 and corrected for absorption
with SADABS (SAINT-NT).10b Corrections were made for Lorentz
and polarization effects. Structure solution, refinement, and data
output were carried out with the SHELXTL-NT program package.10c,d
Non-hydrogen atoms were refined anisotropically, while hydrogen
atoms were placed in geometrically calculated positions using a
riding model. All O-H and N-H hydrogen atoms have been
localized by difference Fourier maps. Solvent molecules are present
in each of the crystal lattices (acetone for 3a, DMF for 4a, and
acetonitrile for 5). Half of the acetonitrile molecules in the crystal
lattice of 5 are located on crystallographic C2-symmetry axes. A
reflections-to-parameter ratio of 5:1 has been considered sufficient
for the studies performed herein. Molecular structures were
illustrated by the SHELXTL-NT software package.10c,d Crystal-
lographic data for the structures reported in this paper have been
deposited with the Cambridge Crystallographic Data Centre as
supplementary publication nos. CCDC-247664-247666. Copies of
the data can be obtained free of charge on application to CCDC,
12 Union Road, Cambridge CB2 1EZ, U.K. (fax, (+44)1223-336-
033; e-mail, deposit@ccdc.cam.ac.uk; www, http://www.ccdc.ca-
m.ac.uk).
Scheme 1. Preparation of the Dinuclear and Trinuclear Complexes
2a-g and 3a-g Using Ligands 1a-g and Boric Acid as Starting
Materials
Theoretical Calculations. HF/6-31G(d,p) geometry optimiza-
tions were done on a PC with a Pentium III processor using the
PC GAMESS software.11 Structures were visualized with Molekel
4.312 and Mercury 1.1.2.13 All geometry optimizations were
followed by frequency calculations, using the same basis set, to
characterize the stationary points as true minima.
3. Results and Discussion
3.1. Preparation and Characterization of Homodi- and
Homotrinuclear Salen-Derived Boron Complexes. Two
different synthetic methods have been reported for the
preparation of the trinuclear boron complexes III shown in
Chart 1. The first method consists of the transformation of
a dinuclear boroxane II with phenylboronic acid to the
trinuclear derivative under elimination of benzene and
requires 12 h of reflux in acetonitrile. The second method
starts from the intermediate formed between the ligand and
2 equiv of boric acid, to which after 4-5 h phenylboronic
acid is added without isolation of the dinuclear intermediate.5b
One aim of this study was to optimize the reaction
conditions and to obtain information on the reaction mech-
anism. To reach this goal, the ligands were first reacted with
2 and then with 3 equiv of boric acid. A total of 12 known
ligands 1a-l have been used for this purpose to study also
the influence of the substituents on the synthetic process and
the structure of the products (Chart 2).
Interestingly, using acetonitrile as solvent the dinuclear
boron complexes 2 could be isolated only in two cases,
namely with salen(tBu) (1b) and salphen (1g). With salen,
acen, salpen, salpen(tBu), and acpen precipitation of the
trinuclear species 3a,c,d-f occurred in yields ranging from
30 to 32% (Scheme 1). In contrast, with salphen(tBu),
acphen, salcen, salcen(tBu), and accen only unseparable
product mixtures were obtained that contained the trinuclear
compounds, which have been identified by mass spectrom-
etry, and oligo- or polymeric species, which have not been
further characterized. As expected, the yields of the trinuclear
metaboric acid esters 3a-g could be increased significantly,
when 3 instead of 2 equiv of boric acid were added to the
initial reaction mixture (64-98% for 3a-g).
As indicated in Scheme 2 the reaction between the di-
and trinuclear boron complexes is apparently reversible. This
conclusion can be drawn from the observation that the di-
and trinuclear boric acid esters could be transformed in all
cases to the trinuclear phenylboroxin derivatives 4a-g in
yields ranging from 31 to 88%, adding after 4 h 1 equiv of
(10) (a) SMART: Bruker Molecular Analysis Research Tool, versions 5.057
and 5.618; Bruker Analytical X-ray Systems: Madison, WI, 1997,
2000. (b) SAINT + NT, versions 6.01 and 6.04; Bruker Analytical
X-ray Systems: Madison, WI, 1999, 2001. (c) Sheldrick, G. M.
SHELX86, Program for Crystal Structure Solution; University of
Go¨ttingen: Go¨ttingen, Germany, 1986. (d) SHELXTL-NT, versions
5.10 and 6.10; Bruker Analytical X-ray Systems: Madison, WI, 1999,
2000.
(11) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon,
M. S.; Jensen, J. J.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su,
S.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. J. Comput. Chem.
1993, 14, 1347.
(12) (a) Flu¨kiger, P.; Lu¨thi, H. P.; Portmann, S.; Weber. J. Molekel 4.3;
Swiss Center for Scientific Computing: Manno, Switzerland, 2000-
2002. (b) Portmann, S.; Lu¨thi, H. P. Molekel: An interactive molecular
graphics tool. Chimia 2000, 54, 766.
(13) Mercury, version 1.1.2; Cambridge Crystallographic Data Center:
Cambridge, U.K., 2002.
8494 Inorganic Chemistry, Vol. 43, No. 26, 2004

Homo- and Heterotrinuclear Boron Complexes
Scheme 2. Transformation of the Trinuclear B-OH Derivatives
3a-g to the B-Ph Derivatives 4a-g with an Equilibrium for the
Dinuclear Species 2a-g Being Proposed
As can be seen from Figure 1, the molecular structures of
compounds 3a and 4a contain a central six-membered B3O3
ring, in which two of the three boron atoms have tetrahedral
and one has a trigonal planar coordination environment.
Apparently, the salen ligand has the perfect bite to coordinate
to the B3O3 moiety through the formation of two chelate
rings, thus forming an additional seven-membered B2C2N2O
heterocycle. These heterocycles possess twisted-chair con-
formations. The B3O3 rings are not completely planar but
have an envelope conformation, since the oxygen O3 atoms
are slightly deviated from the mean planes of the remaining
five atoms (0.36 Å for 3a and 0.35 Å for 4a). The O4 and
O5 oxygen atoms have cis-configuration with respect to the
tetrahedral boron atoms. It has been demonstrated earlier for
salen[B(R)-O-B(R)] complexes (type II in Chart 1) that
cis-configurated derivatives are thermodynamically more
stable than trans-configurated derivatives.5
phenylboronic acid to the initial 1:2 mixtures of ligand and
boric acid.
Complexes 3a-g may be considered as neutral derivatives
of the tetraborate dianion [B4O5(OH)4]2- found in Borax, in
which one of the two rings in the bicycle is closed by a
N-(C)n-N (n ) 2, 3) instead of a O-B(OH)-O bridge
(Figure 1, Chart 3).
The air-stable products 2b,g, 3a-g, and 4a-g have been
characterized as far as possible by elemental analysis, mass
1
spectrometry, spectroscopic methods (IR, H, 13C, and 11B
NMR), and X-ray crystallography. Most of the products have
low solubility in common organic solvents and have rela-
tively high melting points (>290 °C).
The IR spectra for compounds 2b,g, 3a-g, and 4a-g
show that the absorptions that can be attributed to the νCdN
stretching vibrations (ν˜ ) 1618-1651 cm-1) are shifted to
higher wavenumbers compared to the free ligands (∆ν˜ )
4-22 cm-1).
The NfB bond lengths in 3a and 4a range from 1.597(6)
to 1.625(3) Å and are therefore among the strongest NfB
bonds known.15 Generally, in tetrahedral boron coordination
environments B-OPh bonds are significantly longer than
B-OB bonds.5,16 Although the experimental values for 3a
and 4a,c range from 1.458(6) to 1.474(3) Å for B-OPh and
1.403(3) to 1.420(6) Å for B-O3B, the B1-O4 and B2-
O5 bond lengths do not follow this trend: 1.449(6)-1.458-
(3) Å.
The most conclusive evidence that molecules with a
complex system of three (2b,g) and four boron-containing
heterocyclic rings (3a-g and 4a-g) have formed is provided
As expected, B-O bonds with three-coordinate boron
atoms have pπ-pπ contributions and are therefore much
shorter than the B-O bond lengths discussed above for the
tetrahedral boron atoms. The corresponding values for B3-
O4 and B3-O5 in 3a and 4a,c range from 1.352(6) to 1.370-
(3) Å. The same is true for the B-C bond lengths:
1.564(3)-1.567(4) Å for 4a and 4c compared to 1.596(4)-
1.627(6) Å for II (Chart 1).5 The Bπ-Cπ interaction can be
also evidenced by the O-B-C-C torsion angles of 9.4(3)
and 11.5(4)° for 4a,c, respectively, since otherwise an almost
perpendicular orientation would be expected. An interesting
result for the homotrinuclear boron complexes containing
B3O3 moieties is that the B-O-B bond angles formed
between the tetrahedral boron atoms are much smaller than
the ones found in complexes of type II, 119.2(2)-
120.3(2)° for 3a and 4a,c compared to 128.6(2)-137.8(5)°
for II.
1
by H and 11B NMR spectroscopy. The coordination of the
nitrogen to the boron atoms and the simultaneous formation
of a B-O-B bridge makes the boron atoms chiral, thus
generating a diastereotopic environment for the methylene
groups in 3a-f and 4a-f, which form part of the central
heterocyclic ring. For the dinuclear derivatives 2b,g only
signals typical for tetracoordinate boron atoms are detected
in the 11B NMR spectra,14 δ ) 5.0 ppm for 2b and δ ) 3.6
ppm for 2g. For the trinuclear complexes 3a-g and 4a-g
this signal is slightly high-field-shifted (δ ) 1-3 ppm) and
accompanied by a less intense signal typical for a three-
coordinate boron atom (δ ) 18-21 ppm).14
For each of the two series of trinuclear complexes one
representative member could be crystallized (3a and 4a), so
that accurate geometric data are available. Details of the
crystal data and a summary of data collection parameters
for the complexes are given in Table 1. Selected bond
lengths, bond angles, and torsion angles are listed in Table
2. The crystal structure of 4c has already been reported,5b
and selected geometric parameters of this molecule have been
included in Table 2 for comparison. Figure 1 shows the
molecular structures for compounds 3a and 4a.
To obtain more detailed structural information on the
compounds that could not be prepared in pure form or
crystallized, we optimized the molecular structures of
complexes 3a,d,g,j and 4a,d,g,j by computational methods
(15) Ho¨pfl, H. Struct. Bonding 2002, 103, 1.
(16) (a) Ho¨pfl, H.; Sa´nchez, M.; Barba, V.; Farfa´n, N.; Rojas, S.; Santillan,
R. Inorg. Chem. 1998, 37, 1679. (b) Norman, D. W.; Edwards, J. P.;
Vogels, C. M.; Decken, A.; Westcott, S. A. Can. J. Chem. 2002, 80,
31. (c) Yalcin, N.; Kenar, A.; Arici, C.; Atakol, O.; Tastekin, M. Main
Group Met. Chem. 2001, 24, 247.
(14) No¨th, H.; Wrackmeyer, B. NMR Basic Principles and Progress; Diehl,
P., Fluck, E., Kosfeld, R., Eds.; Springer-Verlag: Berlin, 1978; Vol.
14.
Inorganic Chemistry, Vol. 43, No. 26, 2004 8495

Vargas et al.
Table 1. Crystallographic Data for Compounds 3a, 4a, and 5
3a
4a
5
Crystal Data
formula
C16H15B3N2O6‚CH3C(O)CH3
C22H19B3N2O5‚DMF
0.19 × 0.25 × 0.27
496.92
C28H27B3N4O6‚1.5CH3CN
0.19 × 0.21 × 0.27
609.55
cryst size (mm3)
fw
0.02 × 0.18 × 0.31
421.81
space group
C2/c
P212121
C2/c
Cell Parameters
9.9136(12)
a (Å)
b (Å)
c (Å)
â (deg)
V (Å3)
Z
25.148(3)
12.005(2)
13.546(2)
99.232(3)
4036.7(10)
8
19.790(3)
14.437(2)
22.334(3)
109.441(2)
6017.2(14)
8
10.1719(12)
23.553(3)
90
2375.1(5)
4
µ (mm-1
calcd (g cm-3
)
0.103
1.388
0.097
1.390
0.093
1.346
F
)
Data Collection
θ limits (deg)
2 < θ < 23
2 < θ < 25
2 < θ < 23
hkl limits
-27, 27; -13, 13; -14, 14
16 058
-11, 11; -11, 11; -28, 28
16 668
-21, 21; -15, 15; -24, 24
18 103
no. of collcd reflns
no. of indep reflns (Rint
no. of obsd reflnsa
)
2815 (0.100)
1834
4127 (0.039)
3965
4204 (0.048)
3827
Refinement
Ra,b
Rw
0.080
0.185
283
1.081
-0.22
0.24
0.040
0.085
337
0.091
0.181
432
c,d
no. of variables
GOF
1.18
1.29
∆Fmin (e Å-3
)
)
-0.15
0.20
-0.29
0.77
∆Fmax (e Å-3
2
2
2
2
2
2
a I > 2σ(I) b R ) ∑(Fo - Fc )/∑Fo . c All data. d Rw ) [∑w(Fo - Fc )2/∑w(Fo )2]1/2
.
using the HF/6-31G(d,p) basis set. In previous studies it has
been shown that this basis set is adequate for the calculation
of boron compounds having a coordinative NfB bond.17
Selected geometric parameters for the calculated compounds
are listed in Table 2, and the calculated molecular structures
for 4a,d,g,j are shown in Figure 2.18
O2-B2-O3, O3-B1-N1, O3-B2-N2, B1-O1-C1, and
B2-O2-C17 angles with differences of -3.4, +4.3, +3.8,
-4.9, -4.6, +4.5, and +6.2° (mean values). For 4a the
largest deviations correspond to the O1-B1-O3, O3-B1-
N1, B1-O1-C1, B2-O2-C17, and N2-C10-C9 angles
with differences of +4.1, -4.7, +5.1, +9.2, and +4.6° (mean
values), respectively. Interestingly, although the overall
conformations of the six- and seven-membered heterocyclic
rings do not change, there are larger variations for the torsion
angles (Table 2 and Supporting Information); therefore, it
can be supposed that the conformations of the seven-and
eight-membered heterocyclic rings present some flexibility,
which allows for an accommodation according to attractive
or repulsive intermolecular interactions in the solid state that
are absent in the calculated gaseous phase.
In the case of complexes 3a and 4a the quality of the
computational results could be evaluated by a comparison
with the experimentally determined values, which showed a
reasonably good agreement with respect to the bond lengths
and bond angles. In the case of the bond lengths major
differences are only observed for the NfB bonds (1.689/
1.695 T 1.597(6)/1.612(7) Å for 3a and 1.678/1.690 T
1.620(3)/1.625(3) Å for 4a); however, similar differences
have been observed for other boron complexes containing a
coordinate-covalent NfB bond and have been attributed to
the fact that the calculated molecular structures correspond
to molecules in the gasphase.17 Smaller differences occur
for the B-O bonds formed between the tetrahedral and
trigonal planar boron atoms (1.420/1.421 T 1.449(6)/
1.458(6) Å for 3a and 1.424/1.425 T 1.453(3)/1.458(3) Å
for 4a) and the CPh-O bonds (1.299/1.299 T 1.332(5)/
1.342(5) Å for 3a and 1.293/1.301 T 1.332(3)/1.343(3) Å
for 4a). In the case of the bond angles, for 3a the largest
deviations are observed for the O1-B1-N1, O1-B1-O3,
In a comparison of the calculated geometric parameters
within the two series of homotrinuclear boron complexes
3a,d,g,j and 4a,d,g,j in Table 2, only very small differences
are observed for the B-OH/B-Ph pairs of molecules 3a/
4a, 3d/4d, 3j/4j, and 3g/4g. Somewhat larger variations are
only observed for that part of the torsion angles that describe
the conformation of the six-membered BC3NO heterocycles
in the pair 3a/4a, thus proving that there is some confor-
mational flexibility in the system. The NfB bond lengths
are shortest for complexes 3a and 4a (1.678-1.695 Å) and
longest for complexes 3d (1.714 and 1.721 Å) and 3g (1.732
and 1.738 Å). The variations in the bond angles are small,
with exception of those affected by the presence of substit-
uents in the N-Cn-N bridge (see Supporting Information).
One further exception are the B1-O1-C1 and B2-O2-
C17 bond angles, whose values range from 121.9 to 129.9°.
A comparison of the torsion angles describing the conforma-
(17) (a) Howard, S. T.; Foreman, J. P.; Edwards, P. G. Can. J. Chem. 1997,
75, 60. (b) Ho¨pfl, H. J. Mol. Struct. (THEOCHEM) 1998, 427, 1. (b)
Rayo´n, V. M.; Sordo, J. A. J. Mol. Struct. (THEOCHEM) 1998, 426,
171. (c) Hirao, H.; Omoto, K.; Fujimoto, H. J. Phys. Chem. A 1999,
103, 5807. (c) Sa´nchez, M.; Sa´nchez, O.; Ho¨pfl, H.; Ochoa, M.-E.;
Castillo, D.; Farfa´n, N.; Rojas-Lima, S. J. Organomet. Chem. 2004,
689, 811.
(18) The molecular structure for compounds 3a,d,g,j are presented in the
Supporting Information.
8496 Inorganic Chemistry, Vol. 43, No. 26, 2004

Homo- and Heterotrinuclear Boron Complexes
Table 2. Selected Experimental and Calculated Bond Lengths (Å), Bond Angles (deg), and Torsion Angles (deg) for Compound 3a,d,g,j and 4a,d,g,j
3a
3a
4a
4a
4c6b
3d
4d
3g
4g
3j
4j
6a
(exptl)
(calcd)
(exptl)
(calcd)
(exptl)
(calcd) (calcd)
(calcd)
(calcd)
(calcd)
(calcd)
(calcd)
Bond Lengthsa
B1-N1
1.597(6)
1.612(7)
1.458(6)
1.474(6)
1.420(6)
1.404(6)
1.458(6)
1.449(6)
1.352(6)
1.365(6)
1.365(6)
1.695
1.689
1.455
1.458
1.395
1.394
1.420
1.421
1.351
1.364
1.360
1.620(3)
1.625(3)
1.471(3)
1.471(3)
1.415(3)
1.407(3)
1.458(3)
1.453(3)
1.365(3)
1.363(3)
1.690
1.678
1.460
1.463
1.393
1.394
1.425
1.424
1.356
1.357
1.603(3)
1.624(3)
1.468(3)
1.474(3)
1.418(3)
1.403(3)
1.458(3)
1.451(3)
1.354(3)
1.370(3)
1.714
1.679
1.681
1.461
1.460
1.398
1.397
1.418
1.419
1.355
1.355
1.732
1.738
1.470
1.468
1.388
1.389
1.430
1.421
1.373
1.357
1.349
1.705
1.705
1.457
1.457
1.388
1.389
1.417
1.417
1.355
1.355
1.713
1.705
1.450
1.452
1.401
1.396
1.418
1.420
1.350
1.365
1.360
1.714
1.705
1.451
1.453
1.396
1.395
1.423
1.418
1.355
1.358
1.667
1.656
1.445
1.449
1.390
1.384
1.452
1.456
1.572
1.572
1.465
1.799
B2-N2
1.721
1.450
1.450
1.398
1.402
1.412
1.411
1.363
1.356
1.358
B1-O1
B2-O2
B1-O3
B2-O3
B1-O4
B2-O5
B3/P1-O4
B3/P1-O5
B3/P1-O6
B3/P1-C18
1.564(3)
1.580
1.567(4)
1.577
1.579
1.576
Bond Anglesa
O1-B1-N1
O2-B2-N2
O1-B1-O3
O2-B2-O3
O1-B1-O4
O2-B2-O5
O3-B1-O4
O3-B2-O5
O3-B1-N1
O3-B2-N2
N1-B1-O4
N2-B2-O5
B1-O3-B2
B1-O4-B3/P1
B2-O5-B3/P1
O4-B3/P1-O5
B1-O1-C1
B2-O2-C17
109.1(4)
106.5(4)
108.7(4)
109.3(4)
110.4(4)
109.9(4)
113.1(4)
114.9(4)
110.4(4)
109.4(4)
105.1(4)
106.5(4)
120.0(4)
120.6(4)
120.8(4)
121.4(5)
123.1(4)
120.3(4)
105.7
105.6
113.0
113.1
111.1
109.5
115.2
115.7
105.5
104.8
105.5
107.5
118.9
118.8
119.4
120.7
127.6
126.5
107.6(2)
106.2(2)
109.0(2)
110.7(2)
110.6(2)
109.8(2)
114.8(2)
115.8(2)
109.9(2)
107.8(2)
104.7(2)
105.9(2)
119.2(2)
119.1(2)
120.6(2)
121.4(2)
123.1(2)
120.7(2)
105.4
105.9
113.1
111.7
111.1
110.0
115.1
115.6
105.2
106.5
106.0
106.6
119.8
119.7
120.7
120.1
128.2
129.9
108.2(2)
105.5(2)
109.0(2)
109.5(2)
110.4(2)
109.5(2)
114.2(2)
115.6(2)
108.1(2)
109.2(1)
106.7(2)
107.0(2)
120.3(2)
119.6(2)
119.7(2)
122.1(2)
123.0(2)
117.7(2)
103.3
104.1
104.1
112.7
112.8
110.2
110.2
115.7
115.7
106.1
106.0
107.0
107.1
122.1
121.0
121.0
120.7
122.9
123.0
103.8
104.3
114.4
115.0
111.5
110.5
114.9
115.4
106.9
107.0
104.1
103.2
119.3
119.4
119.4
121.7
129.6
129.3
105.6
105.5
112.0
112.3
111.0
110.8
116.1
116.1
106.2
105.8
105.1
105.5
118.2
120.2
120.3
120.2
129.0
128.9
105.2
105.3
113.4
113.0
111.5
109.5
114.4
115.5
104.8
104.4
106.7
108.4
118.5
119.1
119.0
120.7
126.2
125.7
105.2
105.0
113.6
113.4
111.0
109.6
114.6
115.7
105.2
104.3
106.3
108.1
118.1
119.3
120.1
120.2
126.1
125.2
106.2
105.0
111.8
113.8
110.4
109.4
114.8
115.5
108.5
107.2
104.4
104.9
123.3
125.1
128.2
103.0
124.9
120.9
103.2
113.1
113.1
110.4
110.5
115.6
115.7
104.7
103.9
108.7
109.5
121.1
119.8
120.0
121.2
121.9
122.6
Torsion Anglesa
71.2(3)
B1-N1-C9 -C10
B2-N2-C10-C9
O3-B1-N1-C9
O3-B2-N2-C10
B1-O3-B2-N2
B2-O3-B1-N1
N1-C9-C10-N2
64.1(5)
45.7(6)
-49.7(5)
25.7(6)
79.6
-17.3
64.6(2)
45.9(2)
77.8
2.8
-64.0
57.5
84.1
-98.1
-61.9
37.1
86.5
52.8
-53.9
-74.2
71.9
102.3
-100.5
1.4
51.5
-51.5
-72.1
72.9
95.7
-96.1
-0.3
84.5
-23.6
-74.1
76.7
84.1
-25.0
-74.4
77.9
62.2
48.5
-45.2
23.9
-92.1
81.2
-84.5
46.3(3)
-52.1(2)
26.5(3)
-29.7
-93.4
92.2
-87.2
-87.6
86.9
81.4
-80.7
-68.9 -48.7(2)
71.9
87.0
28.3(2)
92.7(2)
89.1(5)
94.1(2)
84.2
89.6
90.2
-80.9(5) -101.6 -81.4(2)
-82.3(6) -48.4 -84.8(2)
-84.8(3)
-91.1(2)
-86.0
-102.8 -103.0
-45.7 -44.7
a For numeration, please see X-ray structures in Figure 1.
tion of the seven-membered heterocyclic rings in compounds
3a,g,j and 4a,g,j shows again that the substituents at the
N-Cn-N bridge have a significant influence. The calculated
structures of all complexes with seven-membered heterocy-
clic rings have chair conformations (Figure 3). However, the
distribution of the atoms within the chair is different: in the
case of compounds 3a, 4a, 3j, and 4j the base of the chair
is formed by the B1, O3, N2, and C10 atoms, while in
compounds 3g and 4g it is formed by the nitrogen and boron
atoms. An explanation is that the latter are obtained from
the more rigid salphen ligand. It should be noticed that the
experimentally determined molecular structures of com-
pounds 3a and 4a have twisted-chair conformations. As
shown by the theoretical calculations, in the case of the eight-
membered heterocycles in compounds 3d and 4d the central
methylene group can have two different orientations, exo or
endo, which gives rise to a boat-chair or chair-chair
conformation, respectively. It can be expected that there is
only a very small energetic difference between these two
conformations, so that a conformational equilibrium can be
supposed in solution.
During the attempts to prepare compound 3i crystals of a
decomposition product 5 could be grown that were suitable
for X-ray crystallography (Chart 4). Details of the crystal
data of 5 and a summary of data collection parameters for
this complex are given in Table 1. Selected bond lengths,
bond angles, and torsion angles are listed in Table 3. The
molecular structure of compound 5 is shown in Figure 4.
This product is a partial ester of trimetaboric acid and has
structural features which can be related to complexes 3a-
g. The central part of the molecule consists of an almost
planar B3O3 ring, in which two boron atoms have tetrahedral
and one boron atom has trigonal planar geometry. The
tetrahedral boron atoms are each chelated by a fragment of
the initial acphen ligand, which should have formed by partial
hydrolysis. Since the coordinated ligands have opposite
orientations in relation to the B3O3 ring, approximate C2-
symmetry can be expected in solution, whereby atoms O1,
B3, and O4 are lying on the symmetry axis. Considering
that atom B3 may still act as Lewis acid, this type of
compound might be useful as a catalyst for asymmetric
synthesis.7
Inorganic Chemistry, Vol. 43, No. 26, 2004 8497

Vargas et al.
Figure 3. Seven-membered heterocycles in complexes 3a,g,j, and 4a,g,f
possessing chair and twisted-chair conformations. Please note the difference
between the calculated and experimental conformation of the heterocycles
in 3a and 4a, thus indicating conformational flexibility. The eight-membered
heterocycles in 3d and 4d have boat-chair and chair-chair conformations,
respectively.
Chart 4. Compound 5 as a Decomposition Product of a
Homotrinuclear Complex and Considered as Derivative of Trimetaboric
Acid
Figure 1. Perspective views of the molecular structures of compound 3a
(top) and compound 4a (bottom). Ellipsoids are shown at the 30% (3a)
and 50% (4a) probability level.
Table 3. Selected Bond Lengths (Å), Bond Angles (deg), and Torsion
Angles (deg) for Compound 5
Bond Lengths
B1-N1
B1-O1
B1-O3
B1-O5
B3-O2
B3-O4
1.624(6)
1.403(6)
1.447(6)
1.466(6)
1.374(6)
1.360(6)
B1-N2
B2-O1
B2-O2
B2-O6
B3-O3
1.628(6)
1.424(6)
1.455(6)
1.454(6)
1.344(6)
Bond Angles
O1-B1-N1
O3-B1-N1
O5-B1-N1
O1-B1-O3
O1-B1-O5
O3-B1-O5
O2-B3-O3
O3-B3-O4
111.6(3)
104.3(3)
104.7(3)
115.4(3)
110.4(4)
109.7(4)
121.7(4)
116.6(4)
O1-B2-N2
O2-B2-N2
O6-B2-N2
O1-B2-O2
O1-B2-O6
O2-B2-O6
O2-B3-O4
106.0(4)
108.2(4)
105.8(4)
114.8(4)
113.3(4)
108.3(4)
121.6(4)
Figure 2. Calculated molecular structures of compounds 4a,d,g,j.
Chart 3. Homotrinuclear Complexes 3a-g Considered as Derivatives
of the Tetraborate Anion Found in Borax
Torsion Angles
B1-O1-B2-O2
B2-O2-B3-O3
B3-O3-B1-O1
B1-N1-C9-C10
N1-C9-C10-N3
-4.0(6)
-3.9(6)
10.9(6)
112.3(4)
5.7(7)
O1-B2-O2-B3
O2-B3-O3-B1
O3-B1-O1-B2
B2-N2-C23-C24
N2-C23-C24-N4
-4.0(6)
-3.7(6)
10.8(6)
90.1(5)
5.8(7)
acidic diols, a number of heterotrinuclear derivatives of the
B2EO3 type could be prepared.
In a first approach we experimented with phenylphospho-
nic acid, and from Scheme 3 it can be seen that the synthetic
pathway established for the preparation of compounds 4a-g
can be extended to the preparation of the boron-phosphorus
complex 6a. Complex 6a was obtained from the reaction
between the salen ligand 1a and 2 equiv of boric acid, to
3.2. Preparation and Characterization of a Heterotri-
nuclear Salen-Derived Boron-Phosphorus Complex. In
the first part of this report it could be shown that it is possible
to prepare homotrinuclear boron complexes containing either
a three-coordinate B-OH (3a-g) or B-Ph (4a-g) moiety.
If this method could be extended to reactions with other
8498 Inorganic Chemistry, Vol. 43, No. 26, 2004

Homo- and Heterotrinuclear Boron Complexes
Figure 5. Calculated molecular structures of compounds syn-6a and anti-
6a. Two configurations are possible for the heterotrinuclear phosphonate
6a; however, the syn conformers are thermodynamically more stable.
the steric repulsion that arises form the interaction of the
P-phenyl group with the N-C-C-N backbone of the
seven-membered heterocycle. The calculated molecular
structures for syn- and anti-6a are shown in Figure 5.
A comparison of the bond lengths, bond angles and torsion
angles in the molecular structure of 6a (Table 2) with the
values calculated for 3a and 4a shows differences for the
NfB bond, which is significantly shorter (1.656/1.667 Å
for 6a T 1.678-1.695 Å for 3a and 4a), the B-O-B bond
angle, which is larger (123.3° for 6a T 118.9 for 3a and
119.8° for 4a), and the torsion angles in the six- and seven-
membered heterocycles (Table 2). These variations indicate
again the conformational flexibility of these heterocycles.
The P-O, PdO, and P-C bond lengths are 1.572, 1.465,
and 1.799 Å, and agree with the values reported for
compounds containing P(O)-O-B bonds.19,20
Figure 4. Perspective view of the molecular structure of compound 5.
Ellipsoids are shown at the 50% probability level.
Scheme 3. Preparation of the Heterotrinuclear Complex 6a Using
Complex 2a and Phenylphosphonic Acid as Starting Materials
4. Conclusions
This contribution has shown that salen ligands and boric
acid can be combined to homodinuclear and homotrinuclear
boron complexes; apparently, these reactions are reversible
in polar solvents. Both boron atoms in the dinuclear species
are tetrahedral, while the homotrinuclear derivatives contain
additionally a three-coordinate boron atom. In the presence
of acidic dioles such as phenylboronic or phenylphosphonic
acid, the B-OH moiety can be interchanged by a B-Ph or
a P(O)Ph group and probably also by other functional groups
such as SiR2 and SnR2.
which after 4 h phenylphosphonic acid was added without
isolation of the dinuclear intermediate (yield: 92%). The
molecular structure of compound 6a was established by
elemental analysis and mass spectrometry as well as 1H, 11B,
and 31P NMR spectroscopy. Unfortunately, crystals could
not be grown for this derivative.
1
As expected, the chemical shifts measured in the H and
11B NMR spectra of compound 6a are very similar to the
ones observed for 4a. The signals for the methylene hydrogen
atoms that are forming part of the seven-membered central
heterocycle of the heterotrinuclear complex are diaste-
reotopic, and the 11B NMR shift is δ ) 2.0 ppm. The 31P
NMR shift is δ ) 5.51 ppm, and similar shift differences
have been measured for a series of borophosphonates [RPO3-
BR′]4 (R, R′ ) alkyl, aryl).19
The homo- and heterotrinuclear boron derivatives dis-
cussed herein contain an almost planar B3O3 or B2PO3
heterocycle; therefore, these complexes can be considered
as derivatives of trimetaboric acid, B3O3(OH)3, boroxine,
B3O3R3, or the tetraborate dianion found in Borax,
[B4O5(OH)4]2-. The seven-membered B2C2N2O heterocycles
possess chair or twisted-chair conformations, while the eight-
membered B2C3N2O heterocycles prefer boat-chair and chair-
chair conformations.
Since theoretically two configurations are possible for
compound 6a (Figure 5), ab initio calculations at the HF/
6-31G(d,p) level have been performed to determine their
relative energies. The energy difference between the ther-
modynamic more stable syn and the less stable anti config-
uration is 5.9 kcal/mol. This difference can be attributed to
According to theoretical calculations, for the heterotri-
nuclear boron-phosphorus derivatives the syn configuration,
(20) (a) Bontchev, R. P.; Junghwan, D.; Jacobson, A. J. Inorg. Chem. 1999,
38, 2231. (b) Bontchev, R. P.; Junghwan, D.; Jacobson, A. J. Angew.
Chem., Int. Ed. 1999, 38, 1937. (c) Zhao, Y.; Zhu, G.; Zou, Y.; Pang,
W. Chem. Commun. 1999, 2219. (d) Yang, G.-Y.; Sevov, S. C. Inorg.
Chem. 2001, 40, 2214. (e) Asnani, M.; Ramanan, A.; Vittal, J. J. Inorg.
Chem. Commun. 2003, 6, 589.
(19) (a) Walawalkar, M. G.; Murugavel, R.; Roesky, H. W.; Schmidt, H.-
G. Inorg. Chem. 1997, 36, 4202. (b) Diemert, K.; Englert, U.; Kuchen,
W.; Sandt, F. Angew. Chem., Int. Ed. Engl. 1997, 36, 241. (c)
Walawalkar, M. G.; Murugavel, R.; Roesky, H. W.; Schmidt, H.-G.
Organometallics 1997, 16, 516.
Inorganic Chemistry, Vol. 43, No. 26, 2004 8499

Vargas et al.
Supporting Information Available: Figures for the calculated
structures of compounds 3a,d,g,j, atomic coordinates for all
calculated structures, additional geometric data for the calculated
structures, and complete lists of geometric data for the structures
determined by X-ray crystallography in CIF format. This material
is available free of charge via the Internet at http://pubs.acs.org.
This material is also available directly from the corresponding
author.
in which the P-phenyl group is oriented in opposite direction
to the salen ligand, is thermodynamically more stable.
The three-coordinate boron atoms in the salen{[B-O-
B][O2BR]} derivatives and in (acphen′)2{[B-O-B][O2B-
(OH)]} should still have Lewis acidic properties and are
embedded in environments that makes them interesting
catalysts for asymmetric reactions.
Acknowledgment. The authors thank the CONACyT for
financial support.
IC048862E
8500 Inorganic Chemistry, Vol. 43, No. 26, 2004