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Inorganica Chimica Acta 311 (2000) 133–137

Note

A new oxo-bridged borate derived from dimeric boron complexes

Victor Barba, Gabriela Vargas, Elizabeth Gomez, Norberto Farfan *Departamento de Quımica, Centro de In6estigacion y de Estudios A6anzados del IPN, Apdo. Postal 14-740, 07000 Mexico D.F., Mexico

Received 10 March 2000; accepted 28 August 2000

Abstract

Reaction of 2-salicylideneamino hydroxyethane (H2SAE) with B(NMe2)3, B(OH)3 and C6F5B(OH)2 leads to the dimeric boroncompounds [B(SAE)(NMe2)]2 (2), [B(SAE)(OH)]2 (3) and [B(SAE)(C6F5)]2 (4) in moderate to good yields. Compound 2 hydrolysesslowly at room temperature (r.t.) to give the dimeric oxo-bridged boron compound m-oxo-[B(SAE)]2 (5). Dehydration of 3 orhydrolysis of 4 with elimination of the aryl group conduced to the same derivative. The X-ray crystallographic analysis confirmedthe molecular structure for compound 5. © 2000 Elsevier Science B.V. All rights reserved.

Keywords: Boron complexes; Tridentate ligands; Dimeric complexes; Oxo-bridged complexes

1. Introduction

Although boron compounds have been widely inves-tigated in both organic and inorganic chemistry [1–3],there is still great interest in their study due to the factthat they show interesting properties such as fluores-cence [4], photoconductivity [5], and electrolumines-cence [6]. Moreover, they also find wide applications insynthesis and medicinal chemistry [7,8]. Recently wehave reported that 2-salicylideneamino hydroxyethane1 (H2SAE) reacts with arylboronic acids to producedimeric complexes containing six- and ten-memberedheterocycles [9,10] which could be used in host–guestchemistry. Therefore, the preparation of dimeric com-pounds using a series of boron reagents could be inter-esting for future applications.

Herein, we report the isolation and characterizationof a novel oxo-bridged dimeric borate (5), its synthesisand X-ray analysis. Oxo-bridged units are importantstructural features in the chemistry of the transitionmetals present in many active sites of biological systemssuch as metalloproteins and metalloenzymes, where the

oxo-bridged moieties are responsible for a wide rangeof reactions [11,12].

2. Experimental

2.1. Instrumentation

NMR spectra were recorded in CDCl3 and DMSO-d6

solutions on Bruker Avance DPX 300 and Jeol Eclipse+400 spectrometers. Chemical shifts (ppm) are relativeto (CH3)4Si for 1H and 13C and BF3OEt2 for 11B.Coupling constants are quoted in Hz. Infrared spectrawere recorded on a Perkin–Elmer 16F-PC FT-IR spec-trophotometer. Mass spectra were recorded on aHewlett-Packard 5989A spectrometer. Melting pointswere obtained on a Gallenkamp MFB-595 apparatusand are uncorrected.

The X-ray diffraction study was determined on anEnraf-Nonius CAD4 diffractometer (lMo Ka=0.71073A, , monochromator: graphite, T=293 K, v−2u scan)and the crystal was mounted in a LINDEMAN tube.Absorption correction was not necessary; correctionswere made for Lorentz and polarization effects. Solu-tion and refinement: direct methods (SHELXS-86) for

* Corresponding author. Tel.: +52-5-747 3721; fax: +52-5-7477113.

E-mail address: jfarfan@mail.cinvestav.mx (N. Farfan).

0020-1693/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 0 -1693 (00 )00282 -6

V. Barba et al. / Inorganica Chimica Acta 331 (2000) 133–137134

structure solution and the SHELXS (version 1.8, 1993)software package for refinement and data output. Hy-drogen atoms were determined by difference Fouriermaps and their positions as well as one overall isotropicthermal parameter were refined (I\4s(I); R=S(��Fo�− �Fc��)/S�Fo�; Rw= [S w(�Fo�− �Fc�)2/S wFo

2]1/2).Elemental microanalysis was performed by Oneida

Research Services, Whitesboro, New York, 13492.

2.2. Reagents

All starting materials were commercially available(Aldrich). Solvents were used without further purifica-tion, but single crystals were grown from spectrophoto-metric grade solvents.

The reagent H2SAE (1) was prepared as described inthe literature [13,14].

2.3. General procedure for the preparation ofcompounds 2, 3 and 4

The following procedure is representative for thepreparation of boron compounds 2, 3 and 4 describedin this study. Equimolecular quantities of H2SAE (1)and the boron reagent: (B(NMe2)3, B(OH)3 orC6H5B(OH)2) in dry THF were stirred at r.t. for 30 minunder nitrogen atmosphere. After the reaction wascompleted, the precipitate was collected by filtration toyield the corresponding boronates.

2.4. [Bis(dimethylamino)][bis[m-[(2-hydroxy-kO)-ethyl]imine-kN]methylphenolate-(2-)-kO]]diboron (2)

Compound 2 was prepared from H2SAE (3.03 mmol,0.50 g) and tris-dimethylamineborane (3.03 mmol, 0.43g ). The product was obtained as an unstable yellowsolid (1.07 mmol, 0.47 g), m.p. 117–119°C, 71% yield.1H NMR (300 MHz, CDCl3) d : 8.44 (1H, s, H-7), 7.35(1H, t, J=8.0 Hz, H-4), 7.24 (1H, d, J=8.0 Hz, H-6),6.96 (1H, d, J=8.0 Hz, H-3), 6.89 (1H, t, J=8.0 Hz,H-5), 3.93 (2H, t, J=5.2 Hz, H-9a, 8a), 3.76 (2H, t,J=5.2 Hz, H-9b, 8b), 2.17 (6H, s, 2Me) ppm. 13CNMR (75 MHz, CDCl3) d : 167.2 (C-7), 161.3 (C-2),132.7 (C-4), 131.7 (C-6), 118.9 (C-3), 118.8 (C-1), 117.3(C-5), 62.4 (C-8), 62.0 (C-9), 31.2 (CH3) ppm. 11BNMR (96 MHz, CDCl3) d : 4.8 (s, br, h1/2=94 Hz)ppm. IR (KBr) n(C�N) 1644 s.

2.5. Dihydroxy-[bis[m-[(2-hydroxy-kO)ethyl]imine-kN]-methylphenolate-(2-)-kO]]diboron (3)

Compound 3 was prepared from H2SAE(3.03 mmol,0.50 g) and (3.03 mmol, 0.19 g) of boric acid. Theproduct was obtained as an unstable white solid (1.24mmol, 0.47 g), m.p. 206–208°C, 82% yield. 1H NMR(300 MHz, CDCl3) d : 8.43 (1H, s, H-7), 7.89 (1H, s,

OH), 7.55 (1H, td, J=7.5, 1.8 Hz, H-4), 7.51 (1H, dd,J=7.5, 1.8 Hz, H-6), 6.98 (1H, d, J=7.5 Hz, H-3),6.94 (1H, t, J=7.5 Hz, H-5), 3.86 (1H, t, J=10.6 Hz,H-8a), 3.81 (1H, t, J=10.6 Hz, H-9a), 3.70–3.75 (2H,m, H-9b, 8b) ppm. 13C NMR (75 MHz, CDCl3), d :167.5 (C-7), 161.9 (C-2), 133.0 (C-4), 132.5 (C-6), 119.5(C-3), 119.1 (C-5), 117.4 (C-1), 61.8 (C-8), 61.4 (C-9)ppm. 11B NMR (96 MHz, CDCl3) d : 1.5 (s, br, h1/2=87 Hz) ppm. IR (KBr) n(OH) 3204 m, n(C�N) 1644 s.

2.6. Dipentafluorophenyl-[bis[m-[(2-hydroxy-kO)ethyl]-imine-kN]methylphenolate-(2-)-kO]]diboron (4).

Compound 4 was prepared from H2SAE (3.03 mmol,0.50 g) and pentafluorophenylboronic acid (3.03 mmol,0.64 g). The product was obtained as a pale yellow solid(1.17 mmol, 0.79 g), m.p. 277–279°C, 77% yield. 1HNMR (400 MHz, CDCl3) d : 8.38 (1H, s, H-7), 7.42(1H, t, J=7.7 Hz, H-4), 7.26 (1H, d, J=7.7 Hz, H-6),6.97 (1H, d, J=7.7 Hz, H-3), 6.88 (1H, t, J=7.7 Hz,H-5), 4.47 (1H, t, J=11.1 Hz, H-8a), 4.24 (1H, t,J=12.6 Hz, H-9a), 3.93 (1H, d, J=12.6 Hz, H-9b),3.50 (1H, d, J=11.1 Hz, H-8b) ppm. 13C NMR (100MHz, CDCl3) d : 166.9 (C-7), 161.1 (C-2), 137.0 (C-4),136.7 (d, JC�F=25 Hz, C-para), 130.9 (C-6), 129.0 (d,JC�F=25 Hz, C-meta), 128.2 (d, JC�F=25 Hz, C-or-tho), 119.5 (C-3), 118.1 (C-5), 115.0 (C-1), 61.9 (C-8),61.1 (C-9) ppm. 11B NMR (128 MHz, CDCl3) d : 2.0 (s,br, h1/2=58 Hz). IR (KBr) n(C�N) 1644 s.

2.7. m-Oxo-[bis[m-[(2-hydroxy-kO)ethyl]imine-kN]-methylphenolate-(2-)-kO]]diboron (5)

2.7.1. Method (a)To a solution of 2 (1.15 mmol, 0.5 g) in THF (50 ml),

was added an equivalent of water and the solution wasstirred for 30 min at r.t. After the reaction was com-pleted the yellow solid that precipitated was collectedby filtration (1.01 mmol, 0.37 g) m.p.=309°C, 88%yield.

2.7.2. Method (b)A solution of 3 (1.31 mmol, 0.5 g) in THF (50 ml)

was refluxed for 3 h. After the reaction was completedthe yellow solid that precipitated was collected by filtra-tion (0.94 mmol, 0.34 g, 72% yield).

2.7.3. Method (c)To a solution of 4 (0.73 mmol, 0.5 g) in THF (50 ml),

was added an equivalent of water and the reactionmixture was refluxed for 4 h. After the reaction wascompleted the yellow solid that precipitated was col-lected by filtration (0.46 mmol, 0.16 g, 59% yield).

1H NMR (300 MHz, CDCl3) d : 8.07 (1H, s, H-7),7.44 (1H, ddd, J=8.2, 7.1, 1.5 Hz, H-4), 7.27 (1H, dd,

V. Barba et al. / Inorganica Chimica Acta 311 (2000) 133–137 135

J=8.2, 1.5 Hz, H-6), 6.98 (1H, dd, J=7.1, 1.0 Hz,H-3), 6.81 (1H, td, J=8.2, 1.0 Hz, H-5), 4.51 (1H, dt,J=11.2, 2.0 Hz, H-8a), 4.27 (1H, dt, J=12.8, 2.6 Hz,H-9a), 3.97 (1H, dd, J=11.2, 2.0 Hz, H-9b), 3.54 (1H,dd, J=12.8, 2.6 Hz, H-8b) ppm. 13C NMR (75 MHz,CDCl3) d : 161.7 (C-7), 160.6 (C-2), 137.3 (C-4), 131.2(C-6), 119.7 (C-3), 118.4 (C-5), 115.2 (C-1), 62.1 (C-8),61.4 (C-9) ppm. 11B NMR (96 MHz, CDCl3) d : 5.6 (s,br, h1/2=39 Hz) ppm. IR (KBr) n(C�N) 1646 s ; MS(DIP) m/z 364 [M+−1]. Anal. Calc.: C, 59.34; H, 4.95;N, 7.69. Found: C, 59.28; H, 4.68; N, 7.61%.

3. Results and discussion

The present investigation describes the preparation ofthe dimeric boron compounds 2, 3 and 4, by reaction ofH2SAE with B(NMe2)3, B(OH)3 and C6F5(OH)2, re-spectively, in THF at r.t. (Scheme 1). Compound 2 isunstable to air leading to the formation of 5, as de-scribed below. The structures of 2, 3 and 4 were as-signed based on the results of the reaction of H2SAEwith arylboronic acids [9,10]. The 1H NMR spectra of2 to 4 show that the signal corresponding to the azome-thine group is shifted to lower field (Dd:0.30 ppm) incomparison with H2SAE. In the 13C NMR spectra, thesignal for the same group is also shifted to lower fieldby Dd=0.2, 0.5 and 0.7 ppm for 2, 3 and 4, respec-tively, in comparison with H2SAE. The 11B NMR

spectra showed the tetrahedral environment for theboron atoms giving rise to signals between +1.5 andd= +4.8 ppm, the latter corresponding to complex 2.

Oxo-bridged derivative 5 was obtained from the threedimeric compounds using the synthetic routes shown inScheme 1, however, the best yield (88%) was obtainedby stirring 2 for 30 min at r.t. with an equivalent ofwater. Alternatively, exposure of 2 to air leads tocompound 5 after several hours due to hydrolysis. Thesame compound was also isolated in approximately72% yield by heating compound 3 during 3 h or afterrefluxing 4 with H2O for 4 h, although in approxi-mately 59% yield. The formation of 5 is faster from 2than from 4 suggesting that the presence of waterfavours rupture of the covalent N�B bond in preferenceto hydrolysis of the B�C bond owing to easier loss ofthe HNMe2 moiety in comparison with C6F5H.

The spectroscopic data established the compositionfor diborate compound 5. Thus, the mass spectrumshows a peak at m/z=364 corresponding to the molec-ular ion with the composition C18H18B2N2O5. In the 1Hand 13C NMR spectra only one set of signals wasobserved due to the presence of a C2 symmetry axis inthe molecule, the azomethine group is shifted to higherfields for compound 5 compared to dimeric compounds2, 3 and 4, while the protons of the ethylene unitshow an AA%XX% system in contrast to H2SAE whereonly two triplets were observed. In the 11B NMRspectrum only one broad signal is observed which is

Scheme 1. Synthesis of compound 5.

V. Barba et al. / Inorganica Chimica Acta 331 (2000) 133–137136

Fig. 1. Molecular structure for compound 5.

shifted to lower field (d=5.6 ppm) in comparison withcomplexes 2, 3 and 4. In the IR spectrum a strongabsorption at 1646 cm−1, assigned to the asymmetricstretching of the C�N unit, is shifted to higher energyby 2 cm−1 relative to those observed for compounds 2,3 and 4.

Compound 5 is stable upon exposure to air, due tothe presence of covalent B�O and coordinative N�Bbonds. A crystal suitable for X-ray diffraction wasobtained from a mixture of ethanol–hexane allowing to

establish the dimeric structure (Fig. 1). Details of thecrystal and data collection parameters are given inTable 1 and selected bond distances and angles arelisted in Table 2. The molecule displays pseudo C2

symmetry with a two fold axis passing through theoxygen atom and the midpoint of the B(1)···B(1%) vec-tor. The H2SAE ligand is joined to the boron centersvia two oxygen and one nitrogen atoms. The environ-ments of the two boron atoms are very similar showinga distorted tetrahedral geometry with a tetrahedralcharacter (THC) of 86% [15].

The seven-membered heterocyclic rings in borate 5containing the BNCCOBO atoms have a chair confor-mation, in contrast to the twisted boat conformationfor a similar heterocycle previously reported [16]. Thesix-membered heterocycles in 5 are approximately pla-nar with deviations from the best plane being less than0.21 A, corresponding to the boron atoms.

The N�B bond lengths of 1.611(3) and 1.603(3) A, areconsistent with the N�B lengths for an oxo-bridged

Table 1Crystallographic data for compound 5

Chemical formula C18H18B2N2O5

Formula weight (g mol−1) 363.96Space group P21/nCrystal size (mm) 0.22×0.19×0.18

9.026(2)a (A, )10.588(2)b (A, )17.840(4)c (A, )

b (°) 101.69(3)V (A, 3) 1669.6(6)Formula units per cell (Z) 4Dcalc. (g cm−3) 1.448F(000) 760Temperature of measurement (K) 293u Range (°) 4.5–49.9Reflections collected 3042Independent reflections 2943[Rint=0.0264]

1612(Fo)2\4s(Fo)2Observed reflectionsFinal R indices [I\4s(I)] R1=0.0330

wR2=0.0881R indices (all data) R1=0.0997

wR2=0.1124Goodness-of-fit (s) 1.016Parameters 244Maximum D/s −0.002Dr min (e A, −3) −0.197Dr max (e A, −3) 0.157

Table 2Selected bond distances (A, ) and bond angles (°) for compound 5

Bond distances1.487(3)O(1)�B(1) 1.493(3)O(1%)�B(1%)1.437(3) O(2)�B(1%)O(2%)�B(1) 1.438(3)

1.603(3)N(1%)�B(1%)N(1)�B(1) 1.611(3)1.414(3)1.416(3) O(3)�B(1%)O(3)�B(1)

Bond anglesO(1%)�B(1%)�N(1%) 107.6(2)O(1)�B(1)�N(1) 107.9(2)

109.0(2) O(1%)�B(1%)�O(3)O(1)�B(1)�O(3) 109.3(2)O(1%)�B(1%)�O(2) 109.5(2)109.5(2)O(1)�B(1)�O(2%)O(2)�B(1%)�N(1%) 104.4(2)O(2%)�B(1)�N(1) 104.4(2)

116.9(2)O(2)�B(1%)�O(3)O(2%)�B(1)�O(3) 117.4(2)108.2(2) N(1%)�B(1%)�O(3)N(1)�B(1)�O(3) 108.6(2)124.4(2)B(1)�O(3)�B(1%)

V. Barba et al. / Inorganica Chimica Acta 311 (2000) 133–137 137

diboron complex previously reported by Niedenzu andWang [6,17,18]. The B�O bond lengths (1.414(3)–1.493(3) A, ) are also in the range for these compounds,however, it is important to mention that the B�O bondsin the oxo-bridged unit are considerably shorter thanthose observed for 5. The B�O�B angle (124.4(2)°) isshorter than other seven-membered heterocycles in sim-ilar boronates [16–20].

4. Conclusions

We think that the versatility of compounds 2, 3 and4 in the formation of 5 could have a full range ofapplications. So far, to show the labile nature of thesegroups further investigations on the chemical reactivityof these complexes are in progress.

5. Supplementary material

Crystallographic data for the structural analysis hasbeen deposited with the Cambridge CrystallographicData Center, CCDC No. 136217 for compound X.Copies of this information may be obtained free ofcharge from The Director, CCDC, 12 Union Road,Cambridge, CB2 1EZ, UK (Fax: +44-1223-336-033;e-mail: deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).

Acknowledgements

The authors wish to thank CONACYT for financialsupport and M.L. Rodrıguez for the NMR spectra.

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