Artikel

An Unexpected Diboroxane: Crystal Structure ofMe2HN · (CF3)2BOB(CF3)2 · NHMe2

David J. Brauer, Silke Buchheim-Spiegel, Hans Bürger*, and Gottfried Pawelke

Wuppertal, Anorganische Chemie der Bergischen Universität Wuppertal

Received January 10th, 2002.

Dedicated to Professor Dieter Naumann on the Occasion of his 60th Birthday

Abstract. (CF3)2BNMe2 (1) reacts at room temperature with waterin a 2:1 ratio to form bis(dimethylamine)-tetrakis(trifluoromethyl)-diboroxane Me2HN·(CF3)2BOB(CF3)2·NHMe2 (2), whereas a 1:1ratio at �78 °C had yielded HO(CF3)2B·NHMe2 (3). The consti-tution of 2 has been deduced from multinuclear NMR, IR and

The chemistry of organoboron-oxygen compounds is well devel-oped and some aryl substituted species like C6H5B(OH)2 or(C6H5)2BOB(C6H5)2 are even commercially available. Trifluorome-thyl boranes with boron-oxygen bonds are rather rare, however.Owing to the fact that the common starting material for bis(trifluo-romethyl)-substituted boranes, (CF3)2BNMe2 (1), is very sensitiveto hydrolysis, all of its reactions were so far carried out under rigor-ous exclusion of moisture. Hence, most trifluoromethylboraneswith BOH groups which have so far been characterised, likeHO(CF3)2B·NHMe2 (3) [1, 2] and HO(CF3)2BCHRNHMe2 [3, 4],had resulted from unintentional hydrolysis reactions. Moreover,some hydroxyborate salts like Cs[HO(CF3)2BCH2CN] were foundto decompose vigorously when attempts were made to isolate anhy-drous material [5].

It has been found that bis(trifluoromethyl)aminoboranes(CF3)2BNR2, with R � Me or Et, add H2O across the BN doublebond to form amine-hydroxy-bis(trifluoromethyl)boranesHO(CF3)2B·NHR2 in high yields, eq. (1) [1, 2, 6]. An excess ofwater was used and the reaction was carried out at dry ice tempera-ture.

(CF3)2BNR2 � H2O � HO(CF3)2B·NHR2 (R � Me (3), Et) (1)

The amine-hydroxyboranes are solids at room temperature, ther-mally stable up to ca. 90 °C; the structure of 3 has been determinedby single crystal X-ray diffraction [1]. Surprisingly, the hydrolysisof 1 at 20 °C with half or less than half of an equivalent of waterin ether solution furnished a species which forms trigonal prismsupon crystallisation. This material showed peaks in the mass spec-trum with m/z � 335 and m/z � 285. These two fragments showedan isotopic pattern characteristic of two boron atoms and thuswere assigned to [Me2HN(CF3)BOB(CF3)2NHMe2]� and[Me2HNFBOB(CF3)2NHMe2]�, respectively. An X-ray investi-gation (vide infra) identified this material as bis(dimethyl-amine)�tetrakis(trifluoromethyl)diboroxane Me2HN·(CF3)2-

* Prof. Dr. Hans BürgerAnorganische Chemie, Fachbereich 9,Bergische Universität Wuppertal,D-42097 Wuppertal, GermanyE-mail:Buerger1@uni-wuppertal.de

Z. Anorg. Allg. Chem. 2002, 628, 1731�1733 WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 0044�2313/02/628/1731�1733 $ 20.00�.50/0 1731

mass spectra, and the structure has been determined by single-crys-tal X-ray diffraction. Averaging 1.396(6) A, the B�O bond lengthis short, and the B�O�B bond angle, 150.4(3)°, is very wide.

Keywords: Boron; Trifluoromethyl group; Crystal structure

Fig. 1 A perspective drawing of 2 with 20% probability thermalellipsoids for the non-hydrogen atoms.

Table 1 Selected bond lengths/A and angles/deg in 2

B(1)�O 1.400(4) B(2)�O 1.392(5)B(1)�N(1) 1.637(5) B(2)�N(2) 1.625(5)B(1)�C(1) 1.636(6) B(2)�C(5) 1.645(6)B(1)�C(2) 1.643(6) B(2)�C(6) 1.645(6)O�B(1)�N(1) 107.4(3) O�B(2)�N(2) 107.6(3)O�B(1)�C(1) 113.6(3) O�B(2)�C(5) 114.2(3)O�B(1)�C(2) 116.2(3) O�B(2)�C(6) 115.6(3)N(1)�B(1)�C(1) 105.6(3) N(2)�B(2)�C(5) 105.5(3)N(1)�B(1)�C(2) 105.2(3) N(2)�B(2)�C(6) 106.4(4)C(1)�B(1)�C(2) 108.0(3) C(5)�B(2)�C(6) 106.7(3)B(1)�O�B(2) 150.4(3)

BOB(CF3)2·NHMe2 (2). Compound 2 is a non-hygroscopic, stablesolid at room temperature which is soluble in polar organic sol-vents. The formation of 2 seems to proceed via 3, which furtheronreacts with 1 at room temperature, whereas condensation of twomolecules of 3 has not been observed.

The structure of 2 is displayed in Fig. 1, and selected bond lengthsand angles are given in Table 1. While no crystallographic sym-metry is imposed on the molecule, its molecular symmetry roughly

D. J. Brauer, S. Buchheim-Spiegel, H. Bürger, G. Pawelke

corresponds to C2 in the solid state � the twofold axis lying alongthe bisector of the B(1)�O�B(2) angle. As is typical foramine�trifluoromethylboranes, the NHMe2 and CF3 substituentsare roughly staggered with respect to the bonds radiating from theboron atom. Interestingly, the amines are so oriented that antiper-iplanar H�N�B�O fragments arise whereas a synclinal confor-mation was found for the corresponding fragment in 3 [1]. Thesynclinal conformation allows 3 to form centrosymmetric N�H···Ohydrogen-bonded dimers in the solid state.

The wide B�O�B angle, 150.4(3)°, is the most conspicuous featureof the structure. A comparably large B�O�B angle of 151.0(3)°was reported for the polycyclic boroxane A.

The size of this angle in A was attributed to the spanning of the boronatoms by two biimidazole moieties [7]. Rather large B�O�B bondangles are also observed in unsupported bridges, however; an angleof 139.6(5)°, e.g., was found in the [(C6F5)3B(µ�OH)B(C6F5)3]�

anion despite protonation [8]. The large angle in 2 might result fromfrontal strain. The opening of this angle lengthens F···F contacts ac-ross the oxo linkage so that the three shortest, 2.930(5)�3.000(5) A,conform to twice the van der Waals radius of a fluorine atom, 2.94 A[9]. Additional widening would compress the distance between theC(3) and C(7) methyl groups, and the tightest H···H contact betweenthese groups, 2.20 A [10], is shorter than twice the van der Waals ra-dius of a hydrogen atom, 2.40 A [9]. The average B�O bond lengthin 2, 1.396(6) A, is slightly but significantly shorter than the anal-ogous value of A, 1.422(6) A, but similar to that reported for bicyclicdifluoro-di-µ-acetato-diboroxane B (1.392(10) A [11]), which exhib-its an approximately tetrahedral B�O�B bond angle of 108.1(6)°.In the latter compound the oxo bridge might have been shortenedinductively considering the high electronegativity of the boron sub-stituents. That the B�O linkage in 3, 1.432(2) A, is significantlylonger than in 2, conforms to the idea, that an opening of theB�O�B angle leads to an increase of oxygen 2s character in itsbonds and thus shortens these valencies. This strengthening appearsto occur at the expense of the B�C and B�N bonds. Thus, the aver-age values for these bond lengths in 2 � 1.642(4) and 1.631(8) A,respectively � are slightly but significantly longer than the corre-sponding linkages in 3 � 1.624(4) and 1.602(3) A.

In order to estimate the importance of hydrogen bonding in crystalsof 2, interatomic distances involving the hydrogen atoms were cal-culated after the hydrogen coordinates were idealised [10]. NoN�H···O hydrogen bonding is present. An intramolecular contactinvolving a methyl hydrogen atom is the shortest H···O interaction,and it must be much weaker than the 0.6 A shorter N�H···O hy-drogen bond of 3. Structural details of the six shortest H…F con-tacts are listed in Table 2. Whereas the N�H fragments are ex-pected to be the best proton donors in 2, the two N�H···F interac-tions are markedly bent at both the F and H atoms, and only oneof them � H(1)···F(2) (y-1, x, 2-z), 2.31 A � is short enough to be

Z. Anorg. Allg. Chem. 2002, 628, 1731�17331732

Table 2 Intermolecular H···F contacts in 2

Donor Acceptora) H···F (A) M�H···F (°) H···F�C (°)

N(1)�H(1) F(2)1 2.31(4) 129(3) 109(1)N(2)�H(2) F(8)2 2.52(4) 124(3) 107(1)C(3)�H(3B) F(3)3 2.50(2) 136(3) 157.7(8)C(4)�H(4B) F(6)3 2.46(2) 143(2) 154.9(6)C(7)�H(7B) F(9)4 2.39(1) 157(2) 156.2(5)C(8)�H(8B) F(12)4 2.47(1) 144(2) 151.3(7)

a Symmetry codes: (1) �1 � y, x, 2 � z; (2) 1 � x, 1 � x � y, 5/3 �z; (3) �1 � x, y, z;(4) x, 1 � y, z.

Fig. 2 An R22 (10) hydrogen-bonded ring formed by molecules

separated by a primitive translation along the a axis.

deemed a hydrogen bond by conservative standards [12]. The re-sulting molecular aggregation may be described as a C(5) hydrogen-bonded chain [13]. Four intermolecular H···F contacts were foundin the 2.39�2.50 A range which involve the CH3 groups. Theseoccur in pairs between molecules displaced by one lattice trans-lation in the a and b directions, respectively, and lead to formationof two R2

2 (10) hydrogen-bonded rings [13] (Figure 2). All of theseintermolecular interactions cross-link the molecules into slabswhich lie perpendicular to the c axis. Objection could be taken tothe description of the above-mentioned interactions as hydrogenbonds because eight intramolecular H···F contacts are found tohave lengths between 2.18(3) to 2.45(4) A. However, these appar-ently result from unavoidable interactions between fourth and fifthnearest neighbours; therefore, they have no obvious structuralconsequences. Furthermore, each is characterised by electro-statically unfavourable, sharply bent geometries at both the F(~90°) and H (99�125°) atoms.

Experimental

Crystal structure determination

X-ray data were measured at 24 °C with a P3 diffractometer em-ploying CuKα radiation. C8H14B2F12N2O crystallises in the trig-onal space group P3221 with a � 7.6682(4), c � 46.159(5) A,Z � 6 and Dc � 1.712 g/cm3. A total of 6492 intensities were gath-ered (4° � 2θ � 114°), of which 2135 are unique � 1902 having I> 2σ(I). They were corrected empirically for absorption. The struc-ture was solved by direct methods. Refinement converged withR1 � 0.052 (all F) and wR2 � 0.108 (all F) using the SHELXTLsuite of programs. Further crystallographic data, except for struc-ture factors, have been deposited with the Cambridge Crystallo-

An Unexpected Diboroxane:

graphic Data Centre by the deposit number CCDC 176976. Copiesof the data can be obtained free of charge from The Director,CCDC, 12 Union Road, Cambridge CD2 1EZ, UK (Fax: �44-1223-336033; e-mail: deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).

Bis(dimethylamine)�tetrakis(trifluoromethyl)diboroxane (2)

To a stirred solution of 1.3 g (6.7 mmol) (CF3)2BNMe2 (1) in 30ml of ether, 0.06 g (3.36 mmol) of water were added at 20 °C. Stir-ring was continued for 1 h and all volatile material removed in va-cuo. The residue was crystallised from CH2Cl2 to give 2 in 89%yield; m.p. 110 °C. C8H14B2F12N2O (403.81); H 3.4 (calc. 3.49); C24.3 (23.80); N 7.1 (6.94)%.

MS (70 eV): m/z 44(100%) [(CH3)2N]�, 185(95%) [M-C2F5-C2F4]�,92(90%) [HOFBNH(CH3)2]�, 94(60%) [F2BNH(CH3)2]�,235(23%) [M-C2F4-CF3]�, 140(15%) [M-C2F5-C2F4-NH(CH3)2]�,285(6%) [M-C2F5]�, 335(2%) [M-CF3]�.

IR (KBr): 3300 cm�1 w (NH), 2989 w (CH), 1277 s (δCH), 1117 vs1056 s (CF3), 711 s, 688 s (δsCF3).

Raman: 3299 cm�1 w (NH), 3064 s, 2996 vs (CH), 1609 m (δNH),1476 m, 1318 m, 1253 m (δCH), 902 s (BO), 713 vs (δsCF3).

NMR: Bruker ARX 400 (400 MHz, 100.6 MHz and 376.5 MHz,for 1H, 13C and 19F, respectively), Bruker AC 250 (79.8 MHz for11B). � IR/Raman: Bruker Equinox 55. � MS: Varian MAT 311.

1H NMR in CD2Cl2 (CDHCl2 � 5.35 ppm): 2.77 ppm d N(CH3)2,4.0 ppm NH.

Z. Anorg. Allg. Chem. 2002, 628, 1731�1733 1733

13C NMR (CD2Cl2 � 53.8 ppm): 38.5 ppm. � 19F NMR (externalstandard CFCl3) �68.0 ppm. � 11B NMR (external standardBF3·OEt2): �6.9 ppm.

The Fonds der Chemischen Industrie is thanked for financial sup-port.

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