M. Muller, E. Eversheim, U. Englert, R. Boese, P. Paetzold 99

Protolysis of Tri-tert-butylazadiboriridine: Formation of a B -H - B Bridge in Unusual Coordination Matthias Mullera, Ellen Eversheha, Ulli Englerta, Roland Boeseb, and Peter Paetzold*a

Institut fur Anorganische Chemie der Technischen Hochschule Aachena, D-52056 Aachen

Institut fur Anorganische Chemie der Universitat-Gesamthochschule Essenb, D-45 1 17 Essen

Received August 18, 1994

Key Words: Tri-tert-butylazadiboriridine / (Hydroboryl)(methoxyboryl)amine / 2,3-pL-Hydro-l,2,3-azoniadiborata-1 -cyclo- propenes

The B-B bond of tri-tert-butylazadiboriridine [-NR- A B atom is identified in the products 3a-f to be planarly BR-BR-] (R = tBu; 1) is oxidized by MeOH to give the (hy- coordinated by four atoms, two of which are forming a BHB droboryl)(methoxyboryl)amine H-BR=NR-BR(0Me) (2a). three-center bond with that B atom. This is a novel bonding p-2,3-Hydroazoniadiborata-l-cyclopropenes [ =NR-BR(X) - situation in boron chemistry. The structures of the products pH-BR=] (3a, 3c-e) are formed by the action of the acids are deduced from IH-, IlB-, and 13C-NMR spectra and are HX upon 1. A similar hydrogen bridge as in 3a-e is formed confirmed by X-ray structural analyses of 2a, 3a, and 3c. during the hydroboration of 1 by catecholborane, yielding 3f.

Tri-tert-butylazadiboriridine (1) is well known to undergo unexpected reactions, which are mainly accompanied by an opening of the B-B bond"]. In this paper we report on the reaction of 1 with Brransted acids HX. These reactions will proceed by protolytic opening of the B-B bond, if the group X is able to act as a x-electron donor. They then involve reduction of the acidic proton to a hydridic hydro- gen atom by boron. An example is the methanolysis of 1 to give the diborylamine 2a, according to Eq. (1). Acids with a poor x-donation power of X behave differently. The group X adds to one of the B atoms, a B-N double bond is formed by the second B atom, and the proton enters into a B-H-B bridging position, according to Eq. (2). Trifluoro- methanesulfonic acid is an example; the electron-attracting sulfonyl group hinders the B-bound 0 atom to donate x electrons. We have discovered the H-N bond of azido-p- amino-nido-decaboraner21, azidoaza-ara~hno-nonaborane[~], and aza-nido-undecaborane[2] to give reaction (2); no 7t elec- trons are available for external B-N bonding in these cases. [Note that the BHB three-membered ring in the structural formula of 3a-e, Eq. (2), represents a BHB (3c2e) bond!].

The proposed structure of 2a in solution is in accord with the NMR spectra. Three nonequivalent tBu groups are pre- sent. One broad "B-NMR peak at 24°C [6 = 44.8, h1/2 =

240 Hz (undecoupled), h1,2 = 200 Hz (H-decoupled)] can be resolved into two overlapping peaks at 80°C [6 = 45.5 (d, J = 140 Hz), 44.7 (s)]. The terminal H atom at Bb is represented by a sharp 'H{"B}-NMR signal at 6 = 4.90; the quadruplet structure of the undecoupled signal is not visible at 24"C, but emerges from the broad singlet (hlR =

190 Hz at -26°C) on slowing down the relaxation rate of the "B nucleus by heating the solution up to 80°C ('J= 144 Hz)I4l. The structure of 2a in the crystal is determined by X-ray structural analysis (Figure 1). A Bb-N1 double bond of 140.3 pm and a B,-N1 single bond of 149.2 pm afford torsion angles C3-Bb-Nl -B, and Bb-Nl -B,-C2 close to 0 and 90", respectively, in accord with the observed values of -15.8 and 96.2". The B,-0 bond length of 135.8 pm means a considerable B-0 n-bond contribution. As a prerequisite for the two double bonds in the chain Bb-N1-B,-O1, the atoms B,, Bb and N1 are planarly configurated.

As far as the cluster part of the chiral molecules 3c-e is concerned, there is no structural problem, since the struc- tures of the corresponding molecules HX have been charac- t e r i~ed[~ ,~ ] . The NMR data of 3c-e are in accord with the reported structures and are conclusive themselves (Table 1); the mirror plane of 7-aza-nido-undecaborane NB10H13, however, is no longer present in 3e. The more important moiety in the molecules 3c-e is the group R3HNB2. Three different tBu groups and two different B atoms are easily identified in the NMR spectra (Table 1). The H atom from the acid HX is detected to be largely deshielded by 'H- NMR shifts at 6 = 4.44, 4.92, and 4.10 for 3c-e, respec- tively. 2D-' 'B/'H-NMR experiments reveal crosspeaks be- tween these H atoms and both B atoms; the crosspeaks with Bb are more intensive than those with B,. The product 3b has been described earlierf5]. Without 2D-"B/'H-NMR evi- dence, we have suggested the 'H-NMR signal at 6 = 4.86 to represent an ammonium N-H proton and the structure

Chem. Ber. 1995,128,99- 103 0 VCH Verlagsgesellschaft mbH, D-69451 Weinheim, 1995 0009-2940/95/0202-0099 $10.00+ .25/0

100 M. Muller, E. Eversheim, U. Englert, R. Boese, P. Paetzold

R I + HOMe

R

N Me0 R

2a

R

I [R=J

R I

3a-e

1 H-X

R3HNB, \ AH

36 3c

3d

/ endo-H

Figure 1. Molecular structure of 2a (thermal ellipsoids at 40% pro- bability); selected bond lengths [pm] and angles ["]: B,-NI 149.2(3), Bb-NI 140.3(4), B,-01 135.8(3), N1 -C1 153.4(3), B,-C2 161.1(4), Bb-C3 159.8(4); B,-Nl-Bb 122.8(2), Ba-N1-Cl 118.3(2), Bb-Nl-Cl 118.7(2), Nl-B,-Ol 120.9(2), NI-Ba-C2 124.2(2), 01-Ba-C2 114.9(2), B,-01-C4 124.4(2),

Nl-Bb-C3 129.7(2)

c11

c4

of the R3HNB2 group to be the unopened three-membered ring of la; the azaborane NBgHll(N3) would then have

been added to a B-N and not to a B-B bond of la . Ac- cording to more recent 2D-l1B/'H-NMR evidence, 3b must now be considered to contain the same R3HNB, group as 3c-e.

The same structural conclusions can be drawn from the NMR spectra of 3a in [D8]toluene, which exhibit four 'H- NMR signals in the ratio 9:9:9:1 at -85°C and two I'B- NMR signals in the typical shift range [6 = 3.5 (AllZ = 650 Hz, undecoupled), 26.4 (hllZ = 1050 Hz, undecoupled)] at -60°C. At 24"C, however, there is only one peak for both of the B-bound tBu groups and both of the B atoms (6 = 15.8). We assume a fluxional process of the triflate group, according to Eq. (3), which is rapid on the NMR time scale. Presumably, a six-membered ring [-0-S-0-B-N-B-] is formed during the degenerate rearrangement equilibrium (3). A 2D-"B/'H-NMR experiment at -60°C reveals two crosspeaks for the 'H-NMR signal at 6 = 4.81 that clearly prove an H atom to bridge B, and Bb.

The structures of 3a and 3c in the crystal are confirmed by X-ray structural analyses (Figures 2, 3). The product 3c crystallizes in twins and requires a particular procedure for the structure solution (see Experimental). There is a strik- ing similarity of the structures of 3a and 3c with respect to the dimensions of the N-B - B three-membered ring, in- cluding the three tBu groups. The considerable shortness of the double bond N1-Bb (134.61135.2 pm) can partly be attributed to a "small-ring effect"; the shortest known B-N double bond has been found in an N-B-C three-mem- bered ring (132.9 pm)r61. A long B-B bond (188.0/189.6 pm) contrasts with the short B-B bond of type-1 three- membered rings, e.g. [-B(tBu)-B(tBu)-N(Mes)-] (1 55.8 pm)"]. The bond N1-B, (149.91151.0 pm) represents a sin- gle bond involving a planar three-coordinated N atom. The bridging H atoms are unequivocally localized in both struc- tures, though with large standard deviations.

A number of monohydroboranes XX'BH are known, that do not dimerize to give diborane(6) derivatives, pro- vided at least one of the ligands X, X' will donate z elec- trons; the product 2a is an appropriate example. In 3a-e, however, the H atom in a molecule of the type XX'BH maintains a hydrogen bridge directed to a B atom in the backbone of one of the ligands X, X'. To the best of our knowledge, only one borane, the bridged three-membered ring A, has previously been reported[*], where one of four atoms, which are planarly coordinating a B atom, is a bridg- ing H atom, as in 3a-e.

The B-B bond of 1 can be hydroborated by the amino- borane H2B=Nz'PrZ, according to Eq. (4)['1. In spite of the formal analogy with the addition of HOMe to 1 [Eq. (l)], Eq. (4) does not describe a protolysis, but a "borolysis", according to the polarity of the adding H atom. The prod- uct 2b of Eq. (4), however, is comparable to the product 2a of Eq. (1). We have now hydroborated 1 by catecholborane, but instead of an open-chain product of type 2, we have obtained the product 3f, which contains a B-H-B bridge. We deduce the structure of 3f from the NMR spectra. The "B-NMR signals at 6 = -3.1 (BJ, 22.7 (Bb), 37.2 (Bexo) are not far away from those of 3a-e (Table 1) and of

Chem. Ber. 1995,128, 99-103

Formation of a B-H-B Bridge in Unusual Coordination 101

Table 1. "B- and 'H-NMR chemical shifts and coupling constants 'J(BH) [Hz] of 3c-e

B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B y ] B,lil

3~ 6 0.5 -29.4 10.3 1 0 . 4 -7.4 15.2 -24.2 3.6 5.3 13.8 I 0.5 24.3 J 140 116 147 153 147 I 140[cI 171 183 159 I

J 177 152 152 I I 140 153 134 152 I I

J 153 153 160 153 153 153 I 165 165 165 153

3d 6 -25.2 -43.5 31.0 I 8.6 -49.1 1.6 1 9 . 3 -5.6 I I 3.0 25.6

3efd1 6 -22.7 -2.8 - 1 . 1 -28.0 9.1 -28.0 I -6.5 -17.1 -18.2 -2.8 5.5 24.8

HI H2 H3 H-l H5 H6 H7 H8 H9 H10 H11 3 tBu

3c[d 6 2.70 1.21 3.55 0.13 2.39 I 1.66 2.98 3.56 3.89 I 0.82 1.18 1.42 3dlq 6 1.86 0.50 0.50 I I 0.87 3.62 0.50 3.17 I I 0.88 1.25 1.46 3ergl 6 1.71 2.33hl 2.86 0.92 3.03 0.92 I 2.78 1.88 1.88 2.78p1 0.99 1.22 1.48

Assignment of "B-NMR shifts by 2D-"B/"B-NMR spectra; no crosspeaks were detected for B-B bonds that are bridged by H (exceptions: B5-B6, B8-B9 of 3d) or are edges in BBN trian les (exception: BI-B5 of 3d), also not for the extra-long bonds B5-B10 and B7-B8 of 3c, for B6-B7 of 3c, and for B3-B4 of 3e. -qbl Assignment of 'H-NMR shifts of exo-, endo-, and p-H by 2D-"B/'H- NMR spectra. - 1'1 J(B-Hend,) = 55 Hz. - Cd] In C6D6; in contrast to the accidental degeneracy of B2/Bll, the degeneracy B4/B6 is resolved in CD2C12 into peaks at 6 = -28.8 and -28.6, attributed to B4 and B6, respectively; the assignment of B2/B3 may be vice versa. - K e ] lH-NMR data in addition: 6 = -2.78 (1-H, 9- lO), -2.68 (p-H, 8-9), 4.44 (p-H, hli2 = 70 Hz, a-b), -0.16 (NH; no change of peak shape on 'H{"B}decoupling), 0.54 (Hendo at B7); 13C NMR: 6 = 22.8, 29.9, 32.7 (3 9); 53.1 (s). - [fl 'H-NMR data in addition: 6 = -2.20 (p-H, 8-9, or Hendo at B8; no "BI'H crosspeak detectable with respect to B9), -0.63 (2 p-H, 6-7, 7-8), -0.54 (p-H, 5-6), 4.92 (p-H, hl12 = 70 Hz, a-b); I3C NMR: 6 = 29.5, 30.9, 33.3 (3 4); 53.4 (s). - [g] 'H-NMR data in addition: 6 = -2.27 (p-H, 8-9), -2.04 (p-H, 10- ll), 4.22 (p-H, h i2 = 80 Hz, a-b); I3C NMR (C6D6): 6 = 29.7, 31.8, 32.5 (3 9); 53.2 (s). - Ih] Attributions H2/Hll and H2/H3 might be inverted. - riI hIi2 = 160 (B& 320 Hz (Bb) (undecoupled).

R R

H O \ ,s=o

0' ' C F 3

3)

Figure 3. Molecular structure of 3c (thermal ellipsoids at 30% pro- bability); selected bond lengths [pm] and angles ["]: B,-HI 136(3), Bb-HI 128(3), Ba-Bb 189.6(4), B,-N1 151.0(3), Bb-NI 135.2(3), B,-N2 158.0(3), N2-B5 158.2(4), N2-B6 153.1(3), N2-H2 96(4), NI-Cl 147.5(3), B,-C2 161.8(4), Bb-C3 158.3(4); B,-Hl-Bb 92(2), B,-NI-Bh 82.8(2), B,-NI-Cl 138.2(2), Bh-NI-CI 139.0(2), Nl-B,-Bb 45.0(1), NI-Bb-B, 52.2(1), NI-Bb-C3

148.1(3), Ba-Bb-C3 159.5(3)

Figure 2. Molecular structure of 3a (thermal ellipsoids at 40% pro- bability); selected bond lengths [pm] and angles ["]: B,-HI 140(7), Bb-HI 133(9), B,-Bb 188.8(14), B,-NI 150.1(9), Bb-NI 134.7(12). B,-01 153.5(8). S1-01 150.2(5). S1-02 140.6(6). S1-03 '141.!9(7), Sl-C4 '181.4(9), NI-Cl" 148.3(10), B,-C2 158.4(1 l), Bb-C3 158.2(14); B,-HI-Bb 88(5), B,-NI-Bb 82.9(6), Ba-N1-CI 136.2(7), Bb-NI-CI 140.7(7), Nl-B,-Bb 45.1(4), Nl-Bb-B, 52.1(5), NI-Bb-C3 148.7(7), Ba-Bb-C3

159.1(6)

F1

H Me3Si, 1 ,SiMe3

C I n

R--8-B-R

A

0

1,1:2,2-bis(catecholdiyl)diborane(4) (6 = 30.7r41), respec- tively, but are distinctly different from those of 2a, b; the bond between B, and B,, is indicated by a crosspeak in the 2D-I1B/"B-NMR spectrum. The 'H-NMR signal of the bridging H atom (6 = 3.74) gives a strong and a weak

Chem. Ber 1995, 128, 99-103

crosspeak with the "B-NMR signals at 6 = 22.7 and -3.1, respectively [hllZ = 300, 360 Hz (undecoupled); 290, 330 Hz ('H-decoupled)]. We do not understand the structural dif- ference between the two aminodiborane(4) derivatives 2b and 3f with respect to B-H-B bridging; the arguments

102 M. Muller, E. Eversheim, U. Englert, R. Boese, P. Paetzold

given for the open-chain structure of 2a cannot for 2b.

R

+ HqB=NiPr2 I I

2b

1 R I

be used

(4 )

3f -0 We gratefully acknowledge the support of this work by the Stg-

tung Stipendienfonds - Fonds des Verbands der Chemischen Indu- strie e. % (M.M.) and by the Deutsche Forschungsgemeinschaft.

Experiment a1 NMR: Varian Unity 500 at 499.843 ('H; standard: TMS), 160.364 ("B; standard: BF3 . OEtz), 125.639 MHz (I3C; standard: TMS) in [D,]toluene (2a, 3a), CD2Cl2 (3b-e), C6D6 (3f). - H,N Analy- ses: Carlo-Erba Elemental Analyzer 1106. - All experiments were carried out under dry N2. Anhydrous solvents were used.

tert-Butylltert-butyl(hydro) boryl][tert-butyl(methoxy) boryll- amine (2a): A 1 M solution of methanol in hexane (2.50 ml) was added to 1 (0.53 g, 2.56 mmol) in Et20 (15 ml) at -78°C. The solution was stirred at room temp. for 12 h. Volatile components were then removed at 10 mbar. The product 2a (0.59 g, 960/0), m.p. ca. OT, was condensed into a cooled receiver at 24"C/0.001 mbar. - 'H NMR: 6 = 0.99, 1.08, 1.23, 3.35,4.90 ( 5 S, 9:9:9:3:1). - I3C NMR: 6 = 28.1, 29.9, 32.7, 53.2 (4 9); 54.1 (s). - C13H31BZNO (239.0): calcd. H 13.07, N 5.86; found H 12.44, N 5.78.

1,2,3- Tri-tert-butyl-p-2.3-hydro-3- (trifluoromethanesu~o nato) - 1,2,3-azoniadiborata-1 -cyclopropene (3a): Trifluoromethanesulfonic acid (0.11 g, 0.73 mmol) was added to 1 (0.16 g, 0.77 mmol) in pentane (10 ml) at -78°C. The solution was brought to room tem- perature. Volatile components were removed at 0.001 mbar. The product 3a (0.21 g, 79%), m.p. ca. -5"C, crystallized from pentane at -78°C. A solution of 3a in toluene could be stored at room temp. for several days without decomposition. - 'H NMR: F = 0.96 (s, 18H), 1.18 (s, 9H), 4.82 (s, hi/2 = 80 Hz, 1H); (-85°C): 6 = 0.85 (s, 9H), 0.92 (s, 9H), 1.01 (s, 9H), 4.81 (s, hi12 = 10 Hz, 1 H). - I3C NMR: F = 28.7 (q, BtBu), 32.1 (q, NtBu), 53.3 (s, NC). - CI3Hz8BZF3NO3S (357.1): calcd. H 7.90, N 3.92; found H 7.79, N 3.93.

6-Azido-8,9:9,lO-di-p-hydro-1,2,3,4,5,7,7,8,9,1O-decahydro-S, 6-p- [ (I,2,3-tri-tert-butyl-2,3-p-hydro-1,2,3-azoniadiborata-1-cyclo- propene-3-yl)amino]-nido-decarborane (3c): BloH12(N3)(NH2)[2] (0.24 g, 1.35 mmol) was added to 1 (0.26 g, 1.26 mmol) in pentane (8 ml) at -78°C. During rigorous stirring at -78°C for 12 h, most of the insoluble crystalline BIOHI2(N3)(NHZ) disappeared, while the product 3c precipitated. More polar solvents, e.g. THF, or bringing the solution to room temp. as long as 1 was still present gave prod- ucts different from 3c. The suspension of 3c in pentane was re-

moved by means of a syringe, leaving the small excess of BI0Hl2(N3)(NH2) on the bottom of the flask. The solid product was then filtered, washed with several portions of pentane at room temp., dried in vacuo, and recrystallized twice from CH2CIZ at -78"C, yielding 0.31 g (62%). - C12H41B12N5 (385.2): calcd. H 10.73, N 18.18; found H 11.19, N 18.47.

5-Azido-S, 6: 6,7: 7,8: 8, 9-tetra-p-hydro-1 ,2,3,6,7,8,9-hexahydro-4- (l,2,3-tri-tert-butyl-2,3-pu-hydro-1,2,3-azoniadiborata-l -cycle- propene-3-yl) -4-aza-arachno-nonaborane (3d): Compound 1 (0.16 g, 0.78 mmol) was added to a suspension of arachno-NB8Hl2(N3) (0.12 g, 0.78 mmol) in hexane (10 ml) at -78°C; the azanonabo- rane was prepared from nido-NBgHl1(N3) (1 mmol)[21 by degra- dation with methanol (3 mmol) in CH2ClZ at -78"C[31. After stir- ring at -78°C for 5 h, volatile materials were removed at room temp. in high vacuo. The residue was dissolved again in pentane (10 ml), and the solution was filtered from the insoluble sideprod- ucts. After removal of pentane in vacuo, a yellow oil (0.21 g) re- mained which contained 3d (yield ca. 67%) with about 90% purity, according to NMR data.

8,9: 10,ll -Di-p-hydro-l,2,3,4,S,6,8,9,10,11 -decahydro-7-(1,2,3- tri-tert-butyl-2,3-p-hydro-~,2,3-azoniadiborata-l-cyclopropene-3-yl) - 7-aza-nido-undecaboune (3e): Compound 1 (0.180 g, 0.87 mmol) was added to a suspension of NBloH13 (0.115 g, 0.85 mm01)[~] in hexane (5 ml) at -78°C. After stirring at -78°C for 2 h and sub- sequent stirring of the clear solution at room temp. for 3 h, volatile materials were removed in high vacuo. The remaining solid was dissolved in hexane (4 ml), the solution was filtered and then con- centrated to a volume of 2 ml, from which the product crystallized at -78°C. A second crystallization from hexane gave pure 3e (0.21 g, 72%). - C12H40Bl2N2 (342.2): calcd. H 11.78, N 8.19; found H 12.49, N 8.19.

1,2,3- Tri-tert-butyl-2,3-p-hydro-3-( (phenylene-1,2-dioxy) boryll- 1,2,3-nzoniadiborata-l-cyclopropene (3Q: Catecholborane (0.27 g, 2.22 mmol) in hexane (2 ml) was added to 1 (0.46 g, 2.22 mmol) in hexane (6 ml) at -78°C. The reaction mixture was slowly brought to room temperature. Volatile products were removed in high vacuo at room temp., finally for a few minutes at 60°C. Then hexane (10 ml) was added to the oily residue. After filtration through silica gel, the clear solution was concentrated in vacuo, thus affording crystalline 3f (0.36 g, 50%) at -78"C, m.p. ca. -10°C. - 'H NMR: 6 = 1.10, 1.16, 1.20 (3 S, 9:9:9); 3.74 (s, h1/2 = 50 Hz, 1H); 6.77-7.13 (m, 4H). - I3C NMR: 6 = 29.1, 30.4, 32.7 (3 9); 52.0 (s); 112.6, 122.5 (2 d); 149.0 (s). - C18H32B3N02 (326.9): calcd. H 9.87, N 4.28; found H 9.70, N 4.28.

X-ray Structure Determinations: Diffraction parameters and crystal data for 2a and 3a are collected in Table 2. - Crystals of compound 3c showed a unit cell with a = 853, b = 1257, c = 2321 pm, and all angles very close to 90". The systematic absences sug- gested the presence of a diagonal glide plane in only one direction. Space group Pmn2' (no. 31) with a multiplicity of 4 in the general position was preferred to Pmmn (no. 59): According to approxi- mate space-filling criteria"'], the volume of the unit cell indicated the presence of only four molecules per cell and the molecules were not expected to occupy special positions. In a recent survey on rare space groups["], P m m n had been classified as "equally balanced" between symmorphic and antimorphic and P m r ~ 2 ~ as "tending towards antimorphism". This means that both space groups do not belong to the most common ones, and indeed no example of an organic molecule in their general positions was known by 1992. Furthermore, the axial reflections normal to the diagonal glide plane seemed to indicate the presence of a screw axis and gave additional reason to doubt the above orthorhbombic space groups.

Chem. Ber. 1995, 128, 99-103

Formation of a B-H-B Bridge in Unusual Coordination 103

Table 2. Experimental X-ray diffraction parameters and crystal data for 2a and 3aCa

2a 3n

Space group (no.) P2,lc (14) P2,Jc (14) a ipml 995.2(3) 870.6(3) b [pml 1157.4(4) 2805.0(12) c [pml 1389.8(4) 902. 8(4)

90.21(2) 115.57(3) 1.6007(8) 1.989( 1)

P [“I v “ n 3 l Z 4 4 Calculated density [g c ~ n - ~ ] 0.992 1.136 Crystal dimensions [ m d ] 0.674.).53.0.47 0.3Lbl Radiation: K, MO MO

Absorption ccetfcicnt [mm-’] 0.06 0.18 Independent reflect. (2 e,,,,,) 2077 2569 Observed reflect. [F, > 4 oQ1 1684 1682 g in iv-1 = &F,) + gF,Z 0.00109 0.007 No. of refined parameters 163 215 R 0.0515 0.0808 R w 0.0559 0.0938

Temperature [K] 125 I10

Max. res. electr. dens. [e nm”] 240 375

pal Nicolet R3mN diffractometer; structure solution by direct meth- ods using SHELXTL-PLUS. - cb] Crystal growth on the diffracto- meter at 263 K by a micro-zone melting process, making use of focussed infrared radiation[g].

A detailed inspection of the averaged results from merging equiva- lent reflections with respect to the supposed three perpendicular mirror planes in the Laue class mmm revealed significantly better results in one direction, i.e. better agreement with the lower Laue symmetry 2/m. Crystals of 3c are twins of the TLQS type, accord- ing to the sytematics of Donnay and Donnay[I2I (“pseudo- perohedry” in Friedel’s nomenclature). The twin matrix [l ,O,O; 0,1,0; O,O,l] was used to generate the orientation of the second do- main. For the subsequent structure determination, a crystal with one predominant and one much smaller domain was used. - Crys- tal data of 3c: Space group P2Jn (no. 14); a = 853.2(5), b = 2320.6(4), c = 1257(1) pm; p = 90.06(6)’; V = 2.489(3) nm3; 2 = 4; Dber = 1.028 g . ~ r n - ~ ; Fooo = 832; ~(CU-IY,) = 3.741 cm-’. Due to the very small twin obliquity, data collection could be performed in a conventional manner with an ENRAF-NONIUS CAD4 dif- fractometer with Cu-K, radiation (graphite monochromator) at 203

K. 5906 reflections were registered with the a-scan type in the range 5” < 0 C 74”. The structure solution with direct methods (SHELXS 86) was obtained without considering the twinning. For the refinement, all 4595 independent reflections with non-zero intensities were treated as overlapping data of two domains related by the above matrix. 428 parameters were refined with the SHELXL 93 program: Non-hydrogen atoms were refined with an- isotropic and hydrogen atoms in the borane fragments with iso- tropic displacement parameters. A slight disorder in the tBu group at Bb was taken into account. The refinement converged at R, = 0.239, this factor being weighted with respect to intensities { w-l = 02(e) + 0.03353 P2 + 0.2711 P with P = [max(e, 0) + 2e]/3}; the corresponding conventional factor is R = 0.085. The ratio of the volumes of the two twin domains was refined to ca. 7.0. - Further details of the crystal structure investigations of 2a, 3a, 3c may be obtained from the Fachinformationszentrum Karlsruhe, Gesellschaft fur technisch-wissenschaftliche Information rnbH, D- 76344 Eggenstein-Leopoldshafen (FRG), on quoting the deposi- tory numbers CSD-401126 (3a), -401 127 (2a), -401 128 (3c), the names of the authors, and the journal citation.

M. Muller, T. Wagner, U. Englert, P. Paetzold, Chem. Ber. 1995, 128, 1-9. J. Muller, P. Paetzold, R. Boese, Heterntom Chem. 1990, I ,

H.-P. Hansen, Dissertation, Technische Hochschule, Aachen, 1994. B. Wrackmeyer, Ann. Rep. N M R Spectrosc. 1988,20, 61-203. J. Muller, P. Paetzold, U. Englert, J. Runsink, Chem. Ber. 1992, 125, 97- 102. P. Paetzold, B. Redenz-Stormanns, R. Boese, Angew. Chem. 1990, 102, 910-911; Angew. Chem. Int. Ed. Engl. 1990, 29, 900- 902. E. Eversheim, U. Englert, R. Boese, P. Paetzold, Angew. Chem. 1994, 106, 211-213; Angew. Chem. Int. Ed. Engl. 1994, 33, 201 -202. R . Wehrmann, H. Meyer, A. Berndt, Angew. Chem. 1985, 97, 779-781; Angew. Chem. Int. Ed. Engl. 1985,24, 788-790. D. Brodalla. D. Mootz. R. Boese. W. ODwald. J: ADDI. Crvs-

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