Aluminium and gallium trihalide adducts of 2,4,6-triorganylborazines and the synthesis of triorganylborazinium tetrabromoaluminates 1

Borazines of type R 3 B 3 N 3 H 3 (R = Me, Et, i-Pr, t -Bu, Ph) add AlBr 3 and GaCl 3 in a 1:1 ratio to one of its N atoms. During this process the planarity of the borazine is lost. The ring system of the adducts show a semi chair conformation. HBr gas reacts with R 3 B 3 N 3 H 3. AlBr 3 in toluene with formation of hitherto unknown borazinium tetrabromo aluminates (R 3 B 3 N 3 H 4 ) + (AlBr 4 ). The ring system of the cation is almost planar but it shows like the adducts three different B-N bond lengths.


Introduction
3][4][5] This is due to the inherent B-N bond polarity which places a partial negative charge on the nitrogen atoms and a partial positive charge on the boron atoms.For example: borazine (HB=NH) 3 adds 3 equivalents of HBr or HCl to form the cycloborazane [H(X)B-NH 2 ] 3 6 and HCl adds to (ClB=NMe) 3 with formation of (Cl 2 B-NHMe) 3 . 7hile BCl 3 or BBr 3 give no adducts with borazines of type (HB=NH) 3 , (HB=NR) 3 or (R´B=NR) 3 they induce substitution reactions at the boron atoms with formation of Bhaloborazines. 8In contrast, AlX 3 (X = Cl, Br) forms stable 1:1 adducts with (MeB=NMe) 3. 9 while SnCl 4 reacts with (HB=NR) 3 by an H/Cl exchange. 10One can speculate whether the addition of HCl to a borazine occurs stepwise with the first step being the formation of a borazinium cation R 3 B 3 N 3 H 4 +.So far, this kind of borazinium cation is presently unknown.As we will show here, cations of this type are readily accessible.
Synthesis and characterization of EX 3 adducts of borazines Synthesis.Adducts 1 to 10 can be prepared by reacting equivalent amounts of the respective borazine with either AlBr 3´o r GaCl 3 in toluene as shown in Eq. (1).Yields ranged from 45 % to 100 % (see experimental part Compounds 2 -5 and 8 crystallized in tiny needles or very thin plates.These crystals were, however, unsuitable for a crystal structure determination in contrast to well shape crystals of compounds 1, 6, 7 and 10. supported by the 1 H NMR data: only one set of signals for the B-organyl groups is observed instead of the expected two.At -80°C, however, two 11 B NMR signals at δ = 38.6 and 42.4 ppm and two 1 H NMR signals at δ = 0.18 and 0.36 ppm (intensity ratio1 : 2) are observed for compound 1.
Table 1.Chemical shifts for 11 B, 27 Al and 1 H (in ppm) of the parent borazines and their AlBr 3 or GaCl 3  The protons of the alkyl groups are deshielded relative to the parent borazines.This is to be expected because the EX 3 group removes electron density from the borazine rings.Except for compounds 1 and 6 the protons of the GaCl 3 adducts are less deshielded than the AlBr 3 adducts supporting the already mentioned conclusion that AlBr 3 is the stronger Lewis acid.However, in case of adducts 1 and 6 this order is reversed.A similar behaviour has been observed for AlBr 3 and GaCl 3 adducts of aminoboranes. 11he Al nuclei of the adducts 1 to 5 are deshielded by 10 to 20 ppm with respect to Al 2 Br 6. . 11he fairly large difference in the shielding of the 27 Al nucleus between (MeB=NH) 3-AlBr 3 and the other members of the (RB=NH) 3 AlBr 3 series is most likely not a steric effect because in this case the shift difference should increase in the order Et < i-Pr < t-Bu.It may be that the N atoms of trimethylborazine unit are more basic than in the other trialkylborazines.This would fit with the NMR shift for the triphenylborazine-AlBr 3 adduct 2.
IR spectroscopy.The IR spectra of adducts 1 to 10 show many bands.The most intense bands are cited in table 2. Two or three NH stretching bands are expected for the adducts.The NH band of the tetra-coordinated nitrogen atom should be at lower wave number than those of the tri-co-ordinated N atom.One can note that the positions of these bands are almost unaffected whether AlBr 3 or GaCl 3 is coordinated.Characteristic IR bands for the BN vibrations of borazines have already been summarized. 2They are found in the region from 1480 to 1300 cm -1 .For each pair of (RB=NH) 3 EX 3 (EX 3 = AlBr 3 , GaCl 3 ) compounds the strongest band at highest wave numbers (1562 to 1506 cm -1 ) stems from a GaCl 3 adduct.The largest difference is found for the t-butyl derivatives 5 and 10.However, we could not find a proper assignment for the B 2 N vibrations of the tetra-coordinated N1 atoms.Also, only a tentative assignment is made for the N-E-stretching bands.The difference in the wave number between pairs of AlBr 3 /GaCl 3 compounds is largest with 99 cm -1 while the difference for most others is in the order of 50 to 60 cm -1 .MePh.They contain a site disordered toluene molecule.The addition of AlBr 3 or GaCl 3 to borazines (RB=NH) 3 changes the ring planarity of the borazines into a semichair conformation with the EX 3 coordinated N1 atom moving above the B1 to B3 five-membered part of the six-membered ring.Particularly noticeable is compound 10 where these 5 atoms are slightly twisted out of the mean plane through B1 to B3 as shown by the respective torsion angles.The interplanar angles between B1N1B3 and B1N2B2N3B3 vary from 22.1° for 6 to 28.0° for 10.This locates the Al atom or the Ga atoms 0.323 (1), 0.292 (6), 0.353 (7) and 0.375 (10) Å above the B1N2B2N3B3 plane, respectively.Adduct formation at N1 leads to a B-N bond lengthening from 1.504 up to 1.526 Å as expected for tetra-coordinated N atoms.Within the 3σ criterion these bonds can be considered ofalmost equal lengths.This formation of the tetra-coordinated N1 atom induces two different pairs of B-N bond lengths for B1-N2 and B3-N3 on the one hand and N2-B2 and B2-N3 on the other hand.The first pair is significantly shorter than the second pair whose lengths correspond nicely with BN bond lengths determined in borazines (1.41 to 1.43 Å).Another feature which is a consequence of the asymmetry induced by atom N1 is that the endocylic bond angles at the boron atoms and atoms N2 and N3 deviate significantly from 120°.The B1-N1-B3 bond angles range from 117.2 to 118.8° which deviate by 7.3 to 8.9° from the ideal tetrahedral bond angles.But more significant are the changes for the N-B-N bond angles at the B atoms which are quite acute with 113.4 to 115.4°.On the other hand, B-N-B bond angles at atoms N2 and N3 are wide with an average close to 126°.
The semichair conformation has still another consequence.The carbon atoms bonded to atoms B1 and B3 are sitting below the B1N2B2N3 plane as shown by the interplanar angle between the planes N3B3C11N1 and N1B1N2B2N3B3 of 7.0° for 1, 11.0° for 6, 8.5° for 7 and 17.7 ° for 10.
In the solid state the molecules of 1 are associated via a N-H … Br bridges to two different AlBr 3 molecules forming a chain structure (see Figure 1b) and similar N-H … Cl bridge bonds are found in compound 6 (Figure 2b).There is no hydrogen bond interaction between the NH group and a Cl atom in (PhB=NH) 3 GaCl 3 , 7, but there are three in compound 10.One shows a H .. Cl distance of 2.77 Å and an N-H-Cl bond angle of 170.5°, the other H atoms binds to two Cl atoms of the same GaCl 3 unit with H .. Cl distances of 2.98 and 2.72 Å and N-H .. Cl angles of 133.9 and 138.6°.

2,4,6-Triorganylborazinium tetrabromoaluminates
Synthesis.There are several options for generating borazinium salts: i) by replacement of a boron bonded halogen atom as a halide by a strong neutral nucleophile as shown in Eq. ( 2), -ii) by heterolytic cleavage of a boron halogen bond with a strong halide acceptor as depicted in Eq. ( 3); and iii) by addition of a proton to an N atom of neutral borazines as indicated in Eq. ( 4).
Amongst these reactions (3) is most likely not favored because a borazinium ion with a dicoordinated boron atom would be generated and this will be accompanied by an increased steric strain.A reaction as shown in Equ.(2) has been observed in the reaction of 2,4,6trichloroborazine with pyridine.Up to 4 molecules of pyridine can add to trichloroborazine (ClB=NH) 3 .6][17] except for the 1:5 adduct which has been structurally characterized as a penta(pyridine)borazinium 3+ trichloride. 17It is also well known that borazines add hydrogen halides up to a 1 : 3 ratio.The latter are cyclotriborazanes [R(Hal)B-NH 2 ] 3 . 18Therefore, the formation of borazinium ions is obviously not favored.However, its formation can be expected when the halide anion is replaced by a less nucleophilic and larger anion.From this point of view one may expect that trifluromethylsulfonic acid would be a candidate for borazinium salt formation, as well as HSbF 6 or H[Al(Hal) 4 ].In this study we reacted HBr with the borazine aluminium tribromide adducts 3 and 5 in toluene in order to generate the comparatively large tetrabromoaluminate.We observed a reaction according to Eq. ( 4).Compounds 11 and 12 were obtained as single crystals suitable for an X-ray structure determination.
NMR and IR spectra.The IR spectra of the borazinium salts 11 and 12 (see table 4) show more bands in the NH and NB region than the EX 3 adducts 1 -10.In 11 and 12 we have one NH 2 group and two NH groups.The former should lead to two IR bands (ν as and ν sym ) , the other two may either be degenerate or -if combined -can also lead to two bands at higher wave numbers.Actually, only two bands were located for 11 but four for 12.In the region of the BN valence bonds four strong band were observed for 11 but only three for 12.We assign the bands at lower wave numbers to the NB 2 vibration of the tetracoordinated N atom, the others to the remaining BN ring atoms.The strongest bands for the parent borazines are 1485 cm -1 for (RB=NH) 3 (R = Me, Et, Bu) and 1472 cm -1 for (PhB=NH) 3 . 5So, the high frequency data at 1533, 1520 for 11 and 1518 cm -1 for 12 indicates stronger BN bonding for the planar B1 to B3 part of the borazinyl cation.The strong band at 423 cm -1 is typical for the AlBr 4 anion.This observation fits with a fluxional behaviour.For both borazinium cations only a single broad proton resonance for the NH groups is observed at 4.65 and 4.38 for 11 and 12 respectively.The tetrabromoaluminate ion is represented by a sharp signal at 84.5 ppm. 13lecular structures.The borazinium salt 11 crystallizes in rectangular platelets.They are orthorhombic, space group Pca2 1 , Z = 4.The colorless prisms of compound 12 are also orthorhombic, space group Pna2 1 , Z = 4. Relevant bonding parameters for both compounds are listed in Table 6.Figures 5a and 6a show the molecular units.The six membered borazinium rings are almost planar.The B1-N1-B3 plane forms an interplanar angle with the other BN atoms of 8.9° for 11 and 4.2° for 12.The arrangement of the Et groups in 11 are asymmetric and reduces the point group symmetry to C 1 .The angles for N2-B2-C3 and N3-B2-C3 differ by 4.3°, while those of N1-B3-C5 and N3-B3-C5 differ by 11.1° similar to the difference of bond angles for N1-B1-C1 and N2-B1-C1 of 12.2°.While the ethyl group at atoms B1 and B2 are almost coplanar with the ring the torsion angle C6-C5-B3-N1 is 53.5° and the torsion angle C6-C5-B3-N3 122.9°.
The effect of the protonation of the borazine rings is the same as for the addition of EX 3 molecules: The N1-B bonds are long (1.56 Å), the adjacent BN bonds are short and the B-N bond to the para-B2 atom is typical for borazines.This indicates that the borazinium cation has lost its pseudoaromatic character.Also the endocyclic ring angles correspond with those of the adducts.The alterations are larger for the 2,4,6-tri(t-butyl)borazinium cation than for the ethyl derivative, most likely a steric effect.The N1 atom in 11 lays 0.119Å above the ring plane, and in case of 12 0.085 Å.All other ring atoms are less than 0.09 Å apart from the mean plane through atoms B1N2B2N3B3.

Discussion
Borazines of type (RB=NH) 3 add readily AlBr 3 and GaCl 3 in a 1:1 ratio.But even more interesting is the fact that the AlBr 3 adducts react with HBr to form the hitherto unknown borazinium salts [R 3 B 3 MN 3 H 4 ]AlBr 4 .There is a close structural relationships between the adducts and the borazinium cations.The BN bonds to the tetra-coordinated nitrogen atom N1 are long (aver.1.56 Å).The adjacent two BN bonds are rather short (aver.1.38 Å) while the BN ARKAT USA, Inc.
5][16][17][18][19] However, the B 3 N 3 rings of the EX 3 adducts possess a semichair conformation while the rings of the two borazinyl cations are almost planar.This corresponds with ab initio calculations for the cation H 3 B 3 N 3 H 4 + . 20The N1-B bond lengths of the borazinium cations are about 3% larger than in the adducts and fall already into the range for amine borane adducts (1.58 -1.64 Å), while those of B1-N2 and B3-N3 are already close to aminoboranes (1.36 Å).
As pointed out, the B1-N1-B3 bond angle is rather wide for a tetracoordinated N atom, but they are close to the calculated value for the parent borazinium cation (118.0°). 20The molecular parameters of the cation in compound 12 deviate approximately by 3° from the calculated values.This is due to the influence of the rather bulky t-butyl group.
A consequence of the semichair conformation of the adducts 1 to 10 is that the boron bonded C atoms are bent in the opposite direction of atom N1.The organyl groups at atoms B1 and B3 in the AlBr 3 adducts are more strongly bent backwards than in the GaCl 3 adducts.This may be due to the longer Ga-N bonds, although the lengths difference to the Al-N bond is fairly small.The longer B-N bonds to atom N1 and the open B1-N1-B3 bond angle results in an increase of the B1-B3 distance.Consequently the B1-N2 and B3-N3 bonds are no longer parallel to one another.
The ready formation of compounds 11 and 12 suggests that borazinium ions of type R 3 B 3 N 3 H 4 + are readily accessible, provided that a suitable anion is present.Such anions can be

Experimental Section
General Procedures.All experiments have been performed under anhydrous conditions using the Schlenk technique with dinitrogen as the protecting gas.The borazines have been prepared by procedures described in the literature. 5Commercial GaCl 3 and AlBr 3 were sublimed before use.Solvents were dried by conventional methods.NMR spectra were recorded on JEOL 270 and 400 instruments using TMS ( 1 H, 13 C) as an internal standard, and BF 3 OEt 2 and a 1M AlCl 3 aqueous solution as external standards.C 6 D 6 was used as solvent.A Siemens P4 diffractometer equipped with an area detector and a low temperature device was used for data collection.If not otherwise stated data collection was performed at -80 °C with MoKα radiation and a graphite monochromator.The thermal ellipsoids shown in the figures represent a 25 % probability.Elemental analysis was performed at the microanalytical laboratory of the department.(1).To a stirred solution of (MeB=NH) 3 (0.92g, 7.44 mmol) in toluene (50 mL) was added a solution of AlBr

Figure 2 .
Figure 2. ORTEP plot of compound 6.Adduct formation at N1 leads to a B-N bond lengthening from 1.504 up to 1.526 Å as expected for tetra-coordinated N atoms.Within the 3σ criterion these bonds can be considered ofalmost equal lengths.This formation of the tetra-coordinated N1 atom induces two different pairs of B-N bond lengths for B1-N2 and B3-N3 on the one hand and N2-B2 and B2-N3 on the other hand.The first pair is significantly shorter than the second pair whose lengths correspond nicely with BN bond lengths determined in borazines (1.41 to 1.43 Å).13

13
Figure 2. ORTEP plot of compound 6.Adduct formation at N1 leads to a B-N bond lengthening from 1.504 up to 1.526 Å as expected for tetra-coordinated N atoms.Within the 3σ criterion these bonds can be considered ofalmost equal lengths.This formation of the tetra-coordinated N1 atom induces two different pairs of B-N bond lengths for B1-N2 and B3-N3 on the one hand and N2-B2 and B2-N3 on the other hand.The first pair is significantly shorter than the second pair whose lengths correspond nicely with BN bond lengths determined in borazines (1.41 to 1.43 Å).13

Figure 5b .
Figure 5b.Stereoplot of the unit cell of compound 11 showing N-H .. Br interaction down the baxis.

Figure 6b .
Figure 6b.Part of the chain structure of compound 11, view down.

Table 5 summarizes
NMR shifts for the two borazinium tetrabromoaluminates. Compared with the δ 11 B data for the respective AlBr 3 adducts 3 and 5 the boron nuclei of 11 and 12 are better shielded by 5.7 and 4.7 ppm in 11 and -1.8 and 4.5 ppm for 12.In contrast, the 1 H NMR data reveal two different CH 2 B groups in 11 but only one CH 3 group.The intensity ratio for these three signals is 4 : 2 : 9. Obviously the signals for the CH 3 groups show the same chemical shift.On the other hand, only a single 1 H NMR signal is found for the tert.-butylgroups in 12.

Table 6 .
Selected bond lengths and bond angles of the borazinium tetrabromoaluminates 11 and 12