Main Group Metal Chemistry Vol. 25, No. 10, 2002

(POLYFLUOROORGANO)HALOBORANES AND (POLYFLUOROORGANO)FLUOROBORATE SALTS: PREPARATION, NMR SPECTRA AND REACTIVITY

Vadim V. Bardin1 and Hermann-Josef Frohn2 *

1 Ν. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry SB RAS, 630090 Novosibirsk, Russia 2 Institute of Chemistry, Inorganic Chemistry, Gerhard-Mercator-Universität Duisburg, Lotharstr. 1, D-47048 Duisburg, Germany

ABSTRACT New developments concerning the preparation and reactivity of (polyfluoroorgano)haloboranes RFBX2, (RF)2BX (X = F, CI, Br), some of their adducts (RF)NBX,. · Base (n = 1 - 3), and their related fluoroborate salts Μ [(RF)nBF4.„] were reviewed. Significant 'B and F NMR spectroscopic patterns of all three classes were described.

CONTENTS 1. Introduction 2. Preparation of (polyfluoroorgano)haloboranes RFBX2 and (RF)2BX (X = F, CI, Br) 2.1. From (polyfluoroorgano)metallic precursors 2.2. From other (polyfluoroorgano)boron compounds 2.3. By various procedures 3. Preparation of (polyfluoroorgano)fluoroborate salts Μ [(RF)nBF4_n] 4. The NMR spectra of (polyfluoroorgano)haloboranes, their complexes with bases and (polyfluoroorgano)fluoroborate salts 5. "The reactivity of (polyfluoroorgano)haloboranes 5.1. Thermal stability 5.2. Reactions with bases and nucleophiles 5.3. Reactions with anhydrous HF 5.4. Conversion of(RF)„BX3.n into (RF)nBF3_n (X = CI, Br) 5.5. Reactions of (polyfluoroorgano)haloboranes with hypervalent fluorine-element-fluorine bonds in xenon fluorides, bromine trifluoride, and organoiodine difluorides 5.6. Reactions of (polyfluoroorgano)haloboranes with selected organoelement compounds. 6. The reactivity of (polyfluoroorgano)fluoroborate salts 7. Conclusions 8. References

1. INTRODUCTION (Organo)dihaloboranes RBX2 and di(organo)haloboranes R2BX (X = F, CI, Br, I) are well-established compounds and their preparations and reactivities were reviewed in fundamental monographs [1, 2]. However, the replacement of all or the majority of hydrogen atoms in the organic group R by fluorine caused significant changes of properties. In fact, the synthetic approaches to the polyfluorinated organoboranes and -borate salts as well as the reactivity of these compounds have a number of peculiarities which combine the specific properties of both roots, organofluorine and organoboron chemistry, and their co-operation. The intensive development of the organofluorine chemistry in 1960s also initiated the research in the field of polyfluorinated organometallics and organo element compounds including the borderline element boron. In 1960 - 1970, the first representatives of perfluorinated alkyl-, alkenyl-, and arylboron compounds were obtained but during the next two decades they remained only "chemical exotics". The situation changed in the 1980s when the high efficiency of (C6F5)3B and of some Μ [(C6F5)4B] salts as co-catalysts in the homogeneous olefin polymerisation was discovered. This caused an "explosive" number of publications on the chemistry of tris(pentafluorophenyl)borane, tetrakis(pentafluorophenyl)borate salts, and related (perfluoroaryl)boron compounds. The application of perfluorinated tri(aryl)boranes as versatile Lewis acids and suitable co-catalysts in the olefin polymerization stimulated the research on this class of acids. Consequently, the tetrakis(polyfluoroaryl)borate anions (often attributed as "non-coordinating anions") attracted interest as counter anions of low nucleophilicity for stabilizing salts with electrophilic cations and similar purposes. The recent advances in these fields (synthesis, reactivity, and practical application) were surveyed in reviews [3,4, 5], actual original papers [6, 7, 8, 9 ], and in the references cited there. Hermann-Josef Frohn et al. (Polyfluoroorgano)haloboranes and (Polyfluororgano) fluoroborate Salts: Preparation, NMR Spectra and Reactivity

The initial achievements in the chemistry of (poiyfluoroorgano)boron compounds were collected in reviews [1, 3, 10, 11] which covered the literature until 1983. Advances in the chemistry of the (trifluoromethyl)boron compounds R(CF3)nBX2_ and Μ [R(CF3)„BX3_„] (η = 1 - 3, X = F, CI, ...) were discussed by Pawelke and Buerger [5], and the preparation of the first tetrakis(trifluoromethyl)borate salts was reported in 2001 [12], When our review had been finished, a paper of Chivers [13] on pentafluorophenylboron halides appeared. The recent advances in the synthesis and reactivity of polyfluorinated mono- and di(organo)haloboranes

(Rf^BXj^, their adducts (RF)„BX3_n · Base, and the related salts Μ [(Rf)„F4_„] where RF represents polyfluorinated alkyl-, alk-1-enyl-, and aryl groups, and X fluorine, chlorine, and bromine, are the scope of this review. Poly- and perfluorinated (alkyl)haloboranes RFBX2, (RF)2BX (X = CI, Br, I), (alk-1- ynyl)haloboranes (RFC-C)„BX3_n (X = F, CI, Br, I), the corresponding fluoroborates Μ [(RFC-C)„BF^„] (n = 1,2), and (polyfluoroorgano)haioborates Μ [(RF)nBX4_J (X = CI, Br, I) are still unknown compounds.

2. PREPARATION OF (POLYFLUOROORGANO)HALOBORANES RFBX2 AND (RF)2BX (X = F, CI, Br) 2.1. From (polyfluoroorgano)metallic precursors A common approach to (organo)haloboranes is the reaction of boron trihalides with organotin or organomercury compounds (borodemetalation) [1, 2]. This route was successfully used to prepare some polyfluorinated (aryl)- and (ethenyl)haloboranes. The treatment of trimethylpentafluorophenyltin or dimethylbis(pentafluorophenyl)tin with boron trichloride in excess gave (pentafluorophenyl)dichloroborane in a high yield [14, 15]. In a similar way, (pentafluorophenyl)dibromoborane (low yield) [16] and (pentafluorophenyl)difluoroborane [15] were obtained (Eqs. (1) - (3)).

BC13 (excess) C6F5SnMe2R * C6F5BCI2 + Me2RSnCl (1) -20 °C, 12 h

R = CH3 (96 %), C6F5 (74 %)

BBr3 (excess) (C6Fs)2SnBu2 » C6F5BBr2 + Bu2SnBr2 (2) 90 °C, 3 h 22 %

BF3 (excess), CC14 C6F5SnMe3 » C6F5BF2 + FSnMe3 + "[Me3Sn] [BF4]" (3) 20 °C, 60 h

The preparation of perfluorinated di(aryl)chloro- and di(aryl)bromoboranes and related compounds required the use of stoichiometric amounts of boron trihalides. The continuous heating of (C6F5)2SnMe2 with BC13 (1 : 1) at 100 to 120 °C led to the formation of bis(pentafluorophenyl)chloroborane (Eq. (4)) [15, 17], A similar replacement of the >SnMe2 fragment by >BBr occurred in the reaction of 9,9-dimethylperfluoro-9- stannafluorene with BBr3 (Eq.(5)) [9]. The reagent (C6F5)4Sn did not react either with boron tribromide (reflux, 24 h) [16] or boron trifluoride (130 °C, 65 h) [18]. This is in contrast to the hydrocarbon analogues, tetraalkyl- and tetraphenyltin, which were easily converted into the corresponding (organo)dihaloboranes when heated with BX3 (X = Br, I) [19].

(C6Fs)2SnMe2 + BC13 • (C6F5)2BC1 + Me2SnCl2 (4) 100 °C, 2 h 36% [15] or 120 °C, 48 h 68 % [17]

590 Main Group Metal Chemistry Vol. 25, No. 10, 2002

benzene SnMs2 + BBr3 BBr (5) -78 to 20 °C 4 h

The first (polyfluoroalk-l-enyl)haloboranes, (trifluoroethenyl)difluoroborane, (trifluoroethenyl)dichloro- borane, and bis(trifluoroethenyl)chloroborane, were prepared by borodestannylation of bis(trifluoroethenyl)- dimethyltin with BX3 (X = F, CI). The formation of (trifluoroethenyl)dichloroborane proceeded in a high yield whereas the corresponding difluoroborane was obtained in 18 % yield only (Eq. (6)). The successful synthesis of di(perfluoroethenyl)chloroborane (CF2=CF)2BC1 was performed by heating of (CF2=CF)2SnMe2 with one equivalent of boron trichloride (Eq. (7)) [20].

(CF2=CF)2SnMe2 + 2BX3 2 CF,=CFBX, + Me,SnX, (6) 20 to 70 °C, 1 h X = F(18 %), CI (93 %)

(CF2=CF)2SnMe2 + BC13 (CF2=CF)2BC1 + Me2SnCl2 (7) 20 to 70 °C, 1 h 85%

In some cases, the isolation of the (polyfluoroorgano)haloboranes from the reaction mixtures was hindered because of the similar boiling points of the boranes and the resulting alkyltin halides. This problem could be overcome by using other (organo)metal precursors, polyfluorinated aryl- or ethenylmercury derivatives. The reaction of the latter with boron trihalides resulted in less or non-volatile organomercury halides.

(Pentafluorophenyl)dihaloboranes C6F5BC12 and C6F5BBr2 were obtained from pentafluorophenyl- mercury derivatives C6F5HgR and boron trihalide (Eqs. (8) - (10)) [15,21, 22],

BC13 (excess) C6F5HgMe C6F5BC12 + MeHgCl (8) 20 °C, 24 h 84%

BBr3, CH2C12 C6F5HgEt C6F5BBr2 + EtHgBr (9) -50 to 20 °C 82%

BBr3, toluene C6F5HgBr C6F5BBr2 + HgBr2 (10) reflux, 2 days 80%

Borodemercuration of the cyclic trimer f(C6F4)Hg]3 with BBr3 at 120 °C led to 1,2-bis(dibromoboryl)- tetrafluorobenzene in 82 % yield (Eq. (11)) [23], Surprisingly, boron trichloride reacted with [(C6F4)Hg]3 under the same conditions [23] or with l,2-bis(trimethylstannyl)tetrafluorobenzene at 180 eC to give decafluoro-9,10-dichloro-9,10-diboraanthracene (53 % yield) (Eq. (12)) [6] instead of the expected 1,2- bis(dichloroboryl)tetrafluorobenzene.

BBr2 BBr3, benzene (11) 120 °C, 1 h BBr2 82%

591 Hermann-Josef Frohn et al. (Polyfluoroorgano)haloboranes and (Polyfluororgano) fluoroborate Salts: Preparation, NMR Spectra and Reactivity

HG ψHg BCI3, benzene 5 120°C, 1 h Hg Φ: "" » 180 B^^^ C, 18 h SnMe3 CI The treatment of bis(trifluoroethenyl)mercury with BC13 smoothly led to (trifluoroethenyl)dichloro- borane (Eq. (13)) [24],

xylene

(CF2=CF)2Hg + 2 BC13 • 2 CF2=CFBC12 + HgCl2 (13) 4 °C, 1 h 41 %

Attempts to use other polyfluorinated organoelement compounds as precursors for polyfluorinated (aryl)haloboranes gave unsatisfactory results. Bochmann et al reported that the fluorine-pentafluorophenyl substitution on BF3 · OEt, with two equivalents of C6F5MgBr in ether resulted in a mixture of products which included the adduct (C6F5)2BF · OEt, (70 %). BC13 and BBr3 did not react selectively with 2 equivalents of C6F5MgBr to (C6F5)2BX (X = CI, Br). In both cases (C6F5)3B was the major product instead of (C6F5)2BX [22]. The general experience showed, that organometallic nucleophiles MRF (M = Li, MgX; RF = polyfluoroaryl, polyfluoroethenyl) reacted with BF3 · OEt2, BC13 or BBr3 often deviating from the given stoichiometry to tri(organo)boranes or tetra(organo)borate salts [25, 26, 27, 28, 29, 30, 31, 32, 33, 34], Haubold et al have prepared some mono-substituted (phenyl)dihaloboranes in good yields by reacting RC6H4SiMe3 with either boron trichloride or tribromide [35]. Later it was shown that also the borodesilylation of the partially fluorinated arylsilane 3,5-C6H3F2SiMe3 (no fluorine atoms in the ortho positions to the trimethylsilyl group) with BBr3 (heptane, reflux, 24 h) resulted in the formation of (3,5- difluorophenyl)dibromoborane in 31 % yield [36], However, no substitution of the trimethylsilyl group by the dihaloboryl group occurred when trimethylsilylpentafluorobenzene was reacted with BX3 (X = CI [23], Br [21]) at elevated temperatures. This negative result was explained by the significantly decreased nucleophilicity of the aryl group in C6F5SiMe3 with respect to its hydrocarbon or partially fluorinated analogues [37], In contrast to the borodestannylation of perfluoroaryl- and perfluoroethenyltin derivatives with boron trihalides, the reaction of CF3SnMe3 with BF3 [38], BC13 or BBr3 [39] resulted in the formation of (trifluoromethyl)haloborate salts (see Section 3) but no (trifluoromethyl)haloboranes were obtained in this reaction.

2.2. From other (polyfluoroorgano)boron compounds There are many routes to (organo)haloboranes RBX2 and R2BX starting from (organo)boron compounds RnBY^ where Y represents an other halogen atom, organic group, oxygen- or nitrogen-bonded group [1]. However, only few related synthetic methods are known for the preparation of (polyfluoroorgano)haloboranes. The replacement of chlorine or bromine atoms bonded to boron by fluorine will be discussed in Section 5. It has been reported that (trifluoromethyl)difluoroborane was prepared by the dismutation reaction of (trifluoromethyl)dibutylborane and boron trifluoride (Eq. (14)) [40, 41]. The product was characterized by a vapour-phase molecular weight measurement and an IR spectrum.

K/Na, NEt3 1. CF3I, NEt3 BF3

Bu2BC1 • Bu2BK · NEt3 • CF3BBu2 » CF3BF2 (14) 63% 2. HClgas 30% 5%

Unlike (alkylamino)boranes R„B(NMe2)3.n [42], (trifluoromethyl)-, and bis(trifluoromethyl)- (dialkylamino)boranes (CF3)„B(NAlk2)3_„ [43] cannot act as starting material for the synthesis of (trifluoromethyl)haloboranes. Because the reaction of CF3B(NMe2)2 with anhydrous HCl and HBr in ether only gave the 1 : 2 adducts in quantitative yields (Eq. (15)). When the reaction with HBr was carried out in

592 Main Group Metal Chemistry Vol. 25, No. 10, 2002

the absence of a solvent, the decomposition products BF3, CHF3, CHF,Br, and black tar were obtained only. Anhydrous HF (aHF) reacted with CF3B(NMe2)2 in the absence of solvents yielding CF3BF2 · NHMe2 again with a four-coordinated boron atom (Eq. (16)). Bis(trifluoromethyl)(dialkylamino)boranes displayed a similar reactivity towards anhydrous hydrogen halides (Eq. (17)) [44, 45]. The different reaction routes with aHF or HX (X = CI, Br) were explained by the strength of the BF bond favoured over the BX bond [44],

ether

CF3B(NMe2)2 + 2 HX » [CF3B(NHMe2)2] [X]2 (15) X = C1, Br -78 °C to 20 °C 100%

no solvent

CF3B(NMe2)2 + 3+n HF » CF3BF2 · NHMe, + [NH2Me2] [F(HF)„] (16) 85% ether

(CF3)2B(NR2) + 3 HX • (CF3)2BX · NHR2 + [NH2R2] [X] (17) R = Et, i-Pr; X = F, CI, Br -78 °C 80 - 98 %

It is important that the donor-acceptor complexes of (trifluoromethyl)haloboranes with amines cannot be converted into base-free (trifluoromethyl)dihaloboranes by the reaction with protic acids. For instance, the use of a larger (> 3) molar excess of aHF in reaction (16) resulted in a diminished yield of CF3BF2 · NHMe2 19 and the formation of by-products [CF3BF3]", BF3 · NHMe2, [BF4]~, and F ( F NMR) [44], The treatment of CF3BX(NEt2) with hydrogen halides HX (X = CI, Br) in ethereal solution gave the new adduct CF3BX2 · NHEtj (Eq. (18)) [45]. This result has a close similarity to the resistance of the adduct CF3BF2 · NMe3 towards gaseous HCl at 60 °C [39] which makes doubtful the early reports of Parsons et al [40, 41] on the recovery of free (trifluoromethyl)difluoroborane from CF3BF2 · NR3 (R = Me, Et) under the action of anhydrous HCl. ether

CF3BX(NEt2) + HX • CF3BX2 · NHEt2 (18) X = CI, Br -78 °C to 20 °C 80 - 98 %

Similarly, the attempted preparation of (C6F5)2BF by the vacuum-distillation of its adduct (C6F5)2BF · OEtj (120 to 130 °C / 5 mm Hg) gave (C6F5)2BOEt in 40 % yield instead of the desired base-free fluoroborane. Attempts to transform (C6F5)2BOEt into the corresponding di(organo)haloboranes by reactions with SiCl4, TiCl4, A1C13, or BBr3 failed [22]." Recently, a simple and convenient route to (poly-) and (perfluoroorgano)difluoroboranes was elaborated, consisting in the abstraction of one fluoride anion from the corresponding (fluoroorgano)trifluoroborate salts Μ [RFBF3] by boron trifluoride or arsenic pentafluoride in neutral non-basic media (hydrocarbons, chlorohydrocarbons, chlorofluorocarbons) at normal pressure (Eq. (19)) [21,46,47].

CH,CU

Κ [ArBF3](sohd) + BF3(gas) • ArBF2 + K[BF4]| (19) -40 to -50 °C 65- 100%

Ar = 2-C6H4F, 3-C6H4F, 4-C6H4F, 2,6-C6H3F2, 3,5-C6H3F2, 2,4,6-C6H2F3, 3,4,5-C6H2F3, 2,3,4,5-C6HF4, C6F5

CH2C12 or CC13F Κ [RCF=CFBF3](SO|jd) + BF3(gas) • RCF=CFBF2 + Κ [BF4U (20) -40 to -60 °C 80 - 90 %

R = F, cis-, trans-C 1, cis-C2F5, c«-C6F13, trans-C4F9, trans-C4H9, trans-C6H5

593 Hermann-Josef Frohn et cd. (Polyfluoroorgano)haloboranes and (Polyfluororgano) fluoroborate Salts: Preparation, NMR Spectra and Reactivity

In contrast to Κ [C6HNF5_nBF3] and Κ [RCF=CFBF3], fluoride cannot be abstracted from potassium (perfluoroalkyl)trifluoroborates by BF3 in CH2C12 but the use of arsenic pentafluoride was successful and gave the corresponding (perfluoroalkyl)difluoroboranes (Eq. (21)) [47],

CFCI3 or CH2C12 Κ [CnF2n+1BF3](solid) + ASF5 » QF2n+1BF2 + K[AsF6]| (21) η = 3,6 -55 °C >90 %

The general character of this approach to (organo)fluoroboranes should be emphasized. This was demonstrated by the abstraction of fluoride in arylxenon(II) di(aryl)difluoroborates (in the presence of an electrophilic cation, Eqs. (22), (23)) [48] and by the syntheses of representative (organo)difluoroboranes RBF2 with non-fluorinated organic groups R = phenyl, hex-l-enyl, butyl, and octyl [49],

ASF5, CH,C12

IQFjXe] KC6F5)2BF2] » [C6F5Xe] [AsF6]j + (C6F5)2BF (22) -50 °C to 20 °C

AsF5 · NCCH3, CH,CI,

[C6F5Xe] [(C6F5)2BF2] » [C6F5Xe][AsF6]| + (C6F5)2BF-NCCH3 (23) -30 °C, 3 h

CH2C12, CHC13, CFC13 or pentane

Κ [RBF3](soljd) + BF3(gas) » RBF2 + Κ [BFJj (24) -40 to 20 °C

R = C4H9 (80 %), C8H17 (63 %), cis-, trans-C4H9CH=CH (53 %), C6HS (86 %)

The resulting solutions of the organodifluoroboranes can be easily separated from the solid by-product by decantation, and the yield of the desired (polyfluoroorgano)fluoroboranes can be determined by 'H or 19F NMR spectroscopy using a quantitative standard. This procedure is convenient especially for the preparation of the very volatile, highly moisture sensitive, and fuming (organo)fluoroboranes in small-scales (a 0.5 mmol). The solutions of the (organo)fluoroboranes can be directly used for many synthetic purposes. Alternatively, the individual (organo)fluoroboranes can be isolated without difficulty when an inert solvent with the appropriate boiling point was used [49],

2.3. By various preparations In addition to the syntheses of (polyfluoroorgano)haloboranes discussed before, there exist less-common methods that have more specific than a general practical importance. When tetrafluoroallene and boron trifluoride were combined at -196 °C in a Pyrex ampoule and warmed slowly to ambient temperature, (perfluoroprop-2-enyl)difluoroborane was obtained [50].

Pyrex ampoule

CF2=C=CF2 + BFj • CF2=C(CF3)BF2 (25) -196 °C to 20 °C 70%

Tetrachlorodiborane reacted slowly with fluoro- and trifluoroethenes to form fluoro-containing (alk-1- enyl)dichIoroboranes together with numerous by-products [51].

B2CI4 CF2=CFH » CF2=CHBCI2 + CFC1=CHBC12 + BF3 + BC13 + HCl (26) 20 °C, 18 days

B2C14 CH2=CFH » CH2=CHBC12 + C12BCH2CH(BC12)2 + BF3 + BClj (27) 20 °C, 31 days

No (organo)boranes were found among the reaction products from CF2=CH2, CH3CF=CH2, and

594 Main Group Metal Chemistry Vol. 25, No. 10, 2002

CF3CH=CH2 and B2Cl4(some days at 20 °C). CHF=CHF was not reactive at 20 °C even for several months.

Tetrafluoroethene reacted quickly and smoothly with B3F5 to yield cis-1,2-bis(difluoroboryl)-difluoroethene in a "fair yield" (Eq. (28)) [52].

F,BBFBF2

CF2-CF2 • CJJ-F2BCF=CFBF2 (28) -100 °C

3. PREPARATION OF (POLYFLUOROORGANO)FLUOROBORATE SALTS Μ [(RF)nBF<_„] (Polyfluoroorgano)fluoroborate salts can be prepared by the addition of carbon nucleophiles to trihaloboranes BX3 (carbon-boron bond formation) with subsequent replacement of X by F, by the fluoride addition to (polyfluoroorgano)boranes (RF)„BFJ_„, or by the transformation of an organic group R in salts Μ [RJBF^J (η = 1 - 3) into Μ [(Rp^BF^J. Recently methods of conversion of adducts R(CF3)2B · NMej and Μ [R(CF3)2B · Base] into salts Μ [R(CF3)2BF] were elaborated. The latter and the carbon-boron bond formation have preparative importance and they are discussed in this Section. The other ones are given in Sections 5 and 6. The direct formation (Eq. (29)) of the (trifluoromethyl)trifluoroborate salt by the reaction of trifluoromethane with boron trifluoride in the presence of the base NMe3 was reported but the yield was not determined [53].

1. CHF3 excess, -83 °C

CHF3 + BF3 + NMe3 • [Me^H] [CF3BF3] (29)

2. H20

An alternative and more convenient route to Μ [CF3BF3] was based on the borodestannylation reaction. Analogous to the reaction of perfluorinated aryl- and alkenyltin compounds with boron trihalides which yielded the corresponding (perfluoroorgano)haloboranes (Section 2.1) the reaction of trifluoromethyl- trimethyltin with BF3 and subsequent treatment of the primary product with KF in water ended with potassium (trifluoromethyl)trifluoroborate (Eq. (30)) [38],

BF3, CC14 aq. KF

CF3SnMe3 • "[Me3Sn] [CF3BF3]" • Κ [CF3BF3] (30)

- Me3SnF

Later this method was modified, and besides the potassium (trifluoromethyl)trifluoroborate salt additionally salt Κ [(CF3)2BF2] was isolated [54]. It is reasonable to assume that the reaction proceeded via the intermediate formation of Me3SnF and CF3BF2 or (CF3),BF which interact via fluoride bridges. Addition of fluoride anions allows to open this bridges. No analogous reaction is known for preparing polyfluorinated (aryl)- and (alkenyl)fluoroboranes. This route is limited to the preparation of trifluoromethyl derivatives only.

Attempts to obtain salts Μ [C„F2n+1BF3] from the longer chain perfluoroalkyltrimethyltin compounds CnF2„+iSnMe3 (n = 3, 4, 6) and BF3 in analogy to ref. [38, 54] failed under various conditions. The reason of the significant influence of the length of the perfluorinated chain in C„F2n+1SnMe3 on the borodestannylation is not known [47], When boron trichloride and boron tribromide were used instead of boron trifluoride in reactions with

CF3SnMe3 in pentane at -90 °C, a fast precipitation occurred. The initially formed white solids, presumably "[Me3Sn] [CF3BX3]" (Eq. (31)), decomposed at -70 °C. When the decomposition proceeded in the presence of trimethylamine, the formation of adducts CF,YBF2 · NMe3, and salts Μ [(CFjY^BF^J was detected (Eq. (32)) [39],

pentane or i'-hexane

CF3SnMe3 + BC13 (or BBr3) • <"[Me3Sn] [CF3BC13]"> (or

-90 °C <"[Me3Sn] [CF3BBr3]">) (31)

595 Hermann-Josef Frohn et al. (Polyfluoroorgano)haloboranes and (Polyfluororgano) fluoroborate Salts: Preparation, NMR Spectra and Reactivity

NMea, -45 to 20 °C

<"[Me3Sn] [CF3BX3]"> • CF2YBF2 · NMe3 + Μ [(CF2Y)nBF4_n]

+ BF3 · NMe3 + Me3SnCl (32) X = CI, Br; Y = F, CI, Br; Μ = NHMe,, CHFjNM^; η = 1,2

Adducts CF2YBF2 · NMe3 are low-melting solids, which were purified by vacuum-sublimation. U 19 Ammonium salts Μ [(CF2Y)„BF4_n] were proved by their 'H, B, and F NMR spectra but not isolated. Although protic acids HX cannot be used to obtain the corresponding fluoroboranes and fluoroborates from R(CF3)2B · NMe3 (Section 2.2), the heating of a series of adducts R(CF3)2B · NMe3 with Et3N · 3 HF provided fluoroborates [Et3NH] [R(CF3)2BF] in good yields (Eq. (33)) [55].

Et3N · 3 HF

R(CF3)2B · NMe3 • [Et3NH] [R(CF3)2BF] (33) 155- 190 °C

R = Et, PhCH2, Ph, C6F5, CF2=CF, cis- and ira/w-CF3CF=CF

Treatment of the triethylammonium salt (R = CF2=CF) with Cs2C03 in acetone led to the corresponding cesium salt (86 %). Tetramethylammonium (trifluoroethenyl)bis(trifluoromethyl)fluoroborate was directly obtained from (CF2=CF)(CF3)2B · NMe3 and [Me?N] F (54 %) [56]. Recently cesium tris(trifluoromethyl)-N,N-dihaloaminoborate was prepared by halogenation of (CF3)3B · NH3 · 4 H20 with chlorine and bromine in aqueous solution of CsOH in 60 - 65 % yield [57], It was shown that the thermolysis (Eq. (34)) resulted in Cs [(CF3)3BX] with X = CI (55 %) and Br (40 %). Furthermore, the fluorination of Cs [(CF3)3BNC12] with AgF2 (Eq. (35)) or F2 (Eq. (36)) gave Cs [(CF3)3BF] [57],

Cs t(CF3)3BNX2] • Cs [(CF3)3BX] (34) 190 °C, 10 h X = CI (55 %), Br (40 %) no solvent

Cs [(CF3)3BNC12] + AgF, • Cs [(CF3)3BF] (35) 20 °C, 2 d 60 % aHF

Cs [(CF3)3BNC12] + F, / He » Cs [(CF3)3BF] + Cs [(CF3)nBF^] (n = 1, 2) (36) -50 °C 50 %

There are several less common routes to tris(polyfluoroorgano)fluoroborates. Cs [(CF3)3BF] was obtained in a mixture with Cs [(CF3)3BC2F5] (1 : 1) by the photolysis of Cs [(CF3)3BN=NC2F5] [12], Reactions of xenon difluoride or pentafluorophenyliodine difluoride with an excess of tris(pentafluorophenyl)boron in MeCN gave salts with the tris(pentafluorophenyl)fluoroborate anion. The subsequent cation exchange with KF resulted in Κ [(C6F5)3BF] [58] (no yield was given). In 1995 Vedejs et al reported a convenient conversion of (aryl)dihydroxyboranes and (aryl)dialkoxy- boranes (arylboronic acids and their esters, respectively) into potassium (aryl)trifluoroborates by reaction with Κ [HF2] in aqueous methanol [59]. This method was modified and extended to the preparation of a wide variety of potassium (polyfluoroorgano)trifluoroborates [21, 47, 60]. Recently it was also applied to the synthesis of Κ [RBF3] salts with non-fluorinated alkyl-, alkenyl-, and alkynyl groups [61, 62,63], For both non-fluorinated as well as fluorinated organic groups R, the general route to Κ [RBF3] includes the following steps: generation of an reactive nucleophilic agent MR, its addition to trialkoxyborane and subsequent alkoxy-fluorine substitution on the (organo)trialkoxyborate (or its mixture with the (organo)dialkoxyborane) with Κ [HF2] (Scheme 1).

Scheme I

B(OAlk)3 Κ [HF2] RX » MR » Μ [RB(OAlk)3] + RB(OAlk), • K[RBF3| R = alkyl, alk-l-enyl, and aryl group, polyfluorinated or non-fluorinated; Μ = Li, MgX

596 Main Group Metal Chemistry Vol. 25, No. 10, 2002

Despite the common character of this approach, the preparation of potassium (polyfluoroorgano)- trifluoroborates showed specific aspects for each type of polyfluorinated organic group RF. Reactions of pentafluorophenylmagnesium bromide with B(OAlk)3 (2 - 2.3 equivalents) in ether at -40 to 0 °C showed a decreasing reactivity of trialkoxyboranes in the series B(OCH3)3 > BiOQH,^ > B(0-i- QH^j. The addition of the Grignard reagent to the solution of B(OCH3)3 at -40 °C caused an immediate 19 reaction indicated by precipitation and after 1 h all C6F5MgBr was consumed ( F NMR). Tripropoxyborane started to react with C6F5MgBr at -5 to 0 °C. Tris(isopropoxy)borane was the less reactive alkoxyborane: the total consumption of C6F5MgBr was achieved at room temperature after 3 - 4 h. Hydrolysis of the reaction suspensions with diluted HCl and the subsequent treatment of the crude product with Κ [HF2] in aqueous methanol gave Κ [C6F5BF3] and Κ [(C6F5)2BF2] in the ratio 85 : 15 %, respectively. The separation of the pure salt Κ [C6F5BF3] was achieved by washing of the crude product with water saturated ether. The series of Κ [C6H„F5_nBF3] salts was obtained in 60 - 90 % yield using this procedure (Eq. (37)) [21].

B(OAIk)3 l.aq. HCl MArF » Μ [ArFB(OAlk)3] + ArFB(OAlk)2 • Κ [ArFBF3] (37)

2. Κ [HF2]

Μ = MgX, Li; ArF = 2-C6H4F, 3-C6H4F, 4-C6H4F, 2,6-C6H3F2, 3,5-C6H3F2) 2,4,6-C6H2F3) 3,4,5-C6H2F3,

2,3,4,5-C6HF4, C6F5

By using chloro(methoxy)boranes BCl(OMe)2 and BCl2(OMe) instead of B(OMe)3, the pure (aryl)fluoroborate salts Κ [C6F5BF3] and Κ [(C6F5)2BF2], respectively, were synthesised [64]. At the first glance the preparation of (polyfluoroalk-l-enyl)trifluoroborates Κ [RCF=CFBF3] looks like the synthesis of Κ [C6H5_nF„BF3]. Nevertheless, there are two remarkable distinctions worth to mention. First, no di(organyl)methoxyboranes or di(organyl)dimethoxyborates were detected in the reaction of polyfluoroalkenyllithium with B(OMe)3 in contrast to the related polyfluoroarylation of trimethoxyborane. Second, the methoxy-fluorine substitution on RCF=CFB(OMe)2 and Li [RCF=CFB(OMe)3] with Κ [HF2] in aqueous MeOH proceeded incompletely and gave a mixture of Κ [RCF=CFBFn(OMe)3_„] while the analogous substitution on both alkoxy species ArB(OAlk)2 and [ArB(OAlk)3]~ [21, 59, 62] or on acids ArCH=CHB(OH)2 [62, 63] ended unambiguously with the corresponding potassium (aryl)trifluoroborates. In general, it is obvious, that the replacement of the organic group X in the borane XB(OAlk)2 (X = Ar, ArCH=CH) by a higher electron-withdrawing fluoroorgano group Y is accompanied by an increase in the Lewis acidity of borane YB(OAlk)2 which hinders the elimination of the anion [OAlk]" from the corresponding borate [YBFn(OAlk)3_J"\ Fortunately, a higher degree of protonation of the oxygen atom by acidification of the reaction mixture with HF^ facilitates the leaving of the OAlk group from the boron atom as AlkOH (or its protonated form) and allows to obtain the desired trifluoroborate salts in good yields (Eq. (38)) [60],

B(OMe)3 Κ [HF2], aq. MeOH Li [CF=RCF] » RCF=CFB(OMe)2 + Li [RCF=CFB(OMe)3] • 40 % HF, aq. MeOH

Κ [RCF=CFBF„(OMe)3_J » Κ [RCF=CFBF3] (38)

R = F (59 %), cis- and trans-C 1 (54 %), m-C2F5 (43 %), m-C6F13 (49 %), trans-C4F9 (72 %), trans-CAH9 (51 %), trans-C6H5 (47 %) (the position of substituent R is related to the BF3 group; yields based on the precursor RCF=CFH)

The influence of acidity on the alkoxy-fluorine substitution in alkoxyboranes is well demonstrated by the model reaction of B(OMe)3 with Κ [HF2] in aqueous MeOH. Even in excess of Κ [HF2], the salt Κ [BF3(OMe)| is the predominant product and Κ [BF4] the minor one only (Eq. (39)) [60].

> 3 Κ [HF2] B(OMe)3 » Κ [BFJ + Κ [BF3(OMe)] + Κ [BFn(OMe)^] (39)

MeOH, H20 4 : 96 minor (n = 1,2)

597 Hermann-Josef Frohn et al. (Polyfluoroorgano)haloboranes and (Polyfluororgano) fluoroborate Salts: Preparation, NMR Spectra and Reactivity

With the above consideration in mind, the preparation of potassium (perfluoropropyl)- and (perfluorohexyl)trifluoroborates was successfully performed (Eq. (40)) [47],

EtMgBr, ether B(OMe)3 Κ [HF,] CtfW • QF^.Mgl • [Mgl] [C„F2n+1B(OMe)3] •

40 % HF

ή [C„F2n+1BF2(OMe)] • Κ [C„F2n+1BF3] (40) η = 3 (64 %), 6 (55 %) (based on QF^I)

It should be noted that the intermediate formation of (polyfluoroalkyl)-, (alk-l-enyl)-, and (aryl)alkoxyboranes and alkoxyborates in the reaction mixture was monitored by 19F NMR spectroscopy [21, 47, 60]. Some individual (polyfluoroorgano)trialkoxyborates [65] and -dialkoxyboranes [24] were recently isolated.

4. NMR SPECTRA OF (POLYFLUOROORGANO)HALOBORANES, THEIR ADDUCTS WITH BASES, AND (POLYFLUOROORGANO)FLUOROBORATE SALTS The NMR spectroscopy is one the most powerful and fast tools for the characterisation of organoelement compounds. Therefore, it is reasonable to present here the "B and 19F NMR spectral data of some typical (polyfluoroorgano)haloboranes (RF)„BX3_n, their donor-acceptor complexes with bases (R^BX^,, · Base and sbme related salts Μ [(R^BF^]. A detailed spectral analysis of "B as well as l9F spectral data of all three classes was beyond the scope of this discussion. Comprehensive critical reviews on these fields were given in relevant books [66,67], Here the main patterns and important tendencies of the spectra are presented only. The "B NMR signals of (polyfluoroorgano)difluoroboranes RpBF, are located in the narrow region of 16 to 24 ppm. The replacement of the fluorine atoms at boron by chlorine and bromine causes a high-frequency shift to 30 to 55 ppm. This region is overlapping with that of (diorgano)chloroboranes and -bromoboranes (RF)2BX (49 to 62 ppm) (Table 1). (Polyfluoroorgano)boron compounds RfBF2 · N-Base, Μ [RpBF3], and Μ [(Rf)2BF2] with a four-coordinated boron centre are characterized by the "B chemical shifts from -2 to 7 ppm. The "B resonance in the neutral adduct (C6F5)2BF · OEt, is less shielded with respect to the corresponding negatively charged [(C6F5)2BF2]~ anion. The most shielded "B resonances were observed in spectra of Μ [R(Rp)2BX] salts [X = F, δ(Β~) =-3 to -7 ppm; X = CI, Br, δ (Β) = -13 to -15 ppm], A slightly shielding of the 11Β resonances (1 to 3 ppm) is common for all structurally related (organo)boron compounds wühen the hydrogen atoms in the organic groups are replaced by fluorine atoms. 19 The F NMR spectra of (organo)difluoroboranes show the resonance of the BF2 group at -74 to -92 ppm, whereas in the di(organo)fluoroborane (C6F5)2BF a broad BF signal appears at -28.9 ppm. It is important to emphasize that in boranes the observed chemical shift of the fluorine atoms bonded to boron strongly depends on the purity of the sample. Under the action of traces of basic impurities, e.g., from hydrolysis the signal becomes broad and moves to lower frequencies caused by admixtures of adduct RfBF2 · Base, which exchanged in a fast reaction the base with the free difluoroborane RfBF2. For instance, the BF2 resonances of C3F7BF2, CF2=CFBF2, and C6F5BF2 are located at -76, -87 and -74 ppm, respectively, while 19 the corresponding F resonances of adducts CF3BF2 · NMe3, CF2=CFBF2 · OEt2, and C6F5BF2 • NCCH3 are displayed at -173, -149 and -136 ppm, respectively (Table 1). The conversion of bis(pentafluorophenyl)fluoroborane into the adducts (C6F5)2BF · OEt, or (C6F5)2BF · NCCH3 is accompanied by a shielding of Δδ 121 or 147 ppm, respectively. A similar significant shielding of the fluorine atoms bonded to boron is observed when (polyfluoroorgano)fluoroboranes were converted into (polyfluoroorgano)fluoroborate anions (Table 1). Unlike in the "B NMR spectra, the position of the BF resonances in the 19F NMR spectra depend on the nature of the fluoroorgano group RF. For example, salts Μ [RfBF3] are characterized by chemical shifts of the BF3 group at -133, -143, and -152 ppm for RF = C6F5, CF2=CF, and C3F7, respectively (Table 1). Furthermore, in (fluorophenyl)trifluoroborates Κ [C6H5_nF„BF3] and -difluoroboranes C6H^nFnBF: the positions of l9F resonances of the fluorine atoms bonded to boron depend on the presence of ortho-fluorine l9 atoms in the phenyl group. In the F NMR spectra of Κ [ArBF3] (Ar = C6H5, 3-C6H4F, 4-C6H4F, 3,5-C6H3F2, 3,4,5-C6H2F3) (no ortho-fluorine atoms) the signal of the BF3 group is located at -141 to -143 ppm. In (phenyl)trifluoroborates Κ [2-C6H4FBF3] and Κ [2,3,4,5-C6HF4BF3] (one ortho-fluorine) the BF3 signal is shifted to -139.06 ± 0.36 ppm. When both ortho-positions were occupied by fluorine atoms (K [2,6- C6H3F2BF3], Κ [2,4,6-C6H2F3BF3], and Κ [C6F5BF3]) the resonance of the BF3-group is found at -133.30 ±

598 Main Group Metal Chemistry Vol. 25, No. 10, 2002

0.30 ppm [21]. A related phenomenon was found in the series of (l,2-difluoroalk-l-enyl)trifluoroborates Κ [RCF=CFBFj] [60], (polyfluoroalkyl)trifluoroborates [M] [XCF2BF3] (M = NHMe3, X = F, CI, Br [39]; Μ = Κ, X = C2F5, C5Fn [47]) and adducts XCF2BF2 · NMe3 (X = F, CI, Br) [39]. A remarkable deshielding of the BF3 resonance in the anions [RCF=CFBF3]~ takes place when the electron-withdrawing substituents R = F, C4F9, C6F13 were substituted by chlorine or C4H9, C6H5 groups [60]. The boron-bonded fluorine atoms in the anions [cij-RCF=CFBF3]~ (R = CI, C4F9, C6F13) are slightly deshielded with respect to the corresponding frans-isomers. The replacement of X = F by X = CI and Br in XCF2BF2 · NMe3 led to a deshielding of the BF2 resonance of 3 and 2 ppm, respectively (Table 1). The BF resonances of (polyfluoroorgano)fluoroboranes in both "B and 19F NMR spectra are broadened signals without distinct boron-fluorine couplings '7(B,F). Coordination of a neutral base or the fluoride anion resulted in a tetra-coordinated boron centre and allowed the direct determination of the J(B,F) values. A small selection of these couplings are presented in Table 2.

Table 1. The "B and 19F NMR chemical shifts (in ppm) of selected (polyfluoroorgano)boron compounds Compound Solvent δ(Β) 6(F) (BF) Refs.

C3F7BF2 CD2C12 (-50 °C) 16.0 -76.50 47 CF3BF, · NMe3 ether -0.60 -173.4 39 CFoCIBFo · NMe3 ether -0.37 -170.1 39 CF2BrBF2 · NMe3 ether -0.16 -168.0 39 Κ [CF3BF3] MeCN -1.7 -155.5 57 a Μ [CF2C1BF3] CH3CN -156.7 39 a Μ [CF,BrBF3] CH3CN -155.0 39 κ [C3F;BF3] CD3CN -0.64 -152.12 47 CH,=CHBF, C6D5CD3 22.8 -88.6 67,68 CF,=CFBF, CH,C1, 21.8 -87.06 46,68 CF2=CFBCI2 C6D5C"D3 31.3 68 CF,=CFBBr2 C6D5CD3 49.6 68 CF,=CFBF2 · OEu CH2CI2 -149.04 46 Κ [CF2=CFBF3] acetone 3.4 -143.36 60 Κ [CH,=CHBF3] acetone 3.4 -145.9 62 C6H5BF, neat 23.8 -91.7 67,69 QHjBCI, neat 54.5 69 C6H5BBr, neat 56.1 69 Κ [C6H5BF3] acetone 4.4 -143.3 62 C6F5BF2 CH,CI, 22.84 -74.43 21 C6F5BF, · NCCH3 CD,C1, -136.54 70" Κ [C6F5BF3] CD3CN 1.81 -133.43 21 (C6F5)2BF CHICL·) -28.9 71 (C6F5)2BC1 C6D6 " 59.1 17 (C6F5)2BBr CDC13 61.2 22 (C^BF-OEt, C6D6 12.4 -149.95 22

(C6F5)2BFNCCH3 CH2CI2 -176.0 72 c Μ [(C6F5)2BF2] MeCN (-30 °C) 7.14 -144.6 72 Κ I(CF3)2BF2] MeCN -2.2 -180.5 57 (CF2=CF)2BC1 C6D5CD3 49.9 68 (CF,=CF),BBr C6D5CD3 57.0 68 Μ [(C6F5)3BF] " MeCN -188.9 58 Cs [(CF3)3BF] MeCN -7.3 -230.0 57 Cs [(CF3)3BC1] MeCN -12.7 57 Cs [(CF3)3BBr] MeCN -14.9 57 a Μ = Με,ΝΗ; b δ(Η) 2.15 (s); 6(F) -136.28 (F-2,6), -155.80 (t, V (F,F) 19.6, F-4), -164.40 (F-3,5); c Μ C6F5Xe.

599 Hermann-Josef Frohn et al. (Polyfluoroorgano)haloboranes and (Polyfluororgano) fluoroborate Salts: Preparation, NMR Spectra and Reactivity

Table 2. The "B-'9F coupling constants in selected (polyfluoroorgano)boron compounds. Compound Solvent 'J(B,F) (Hz) 27(B,F) (HZ) Refs.

CF3BF2 · NMe3 ether 51.2 32.6 39

CF2C1BF2 · NMe3 ether 48.5 25.5 39

CF2BrBF2 · NMe3 ether 47.0 23.7 39

CF3BCL · NHEU ether 38.4 45 Κ [CF3BF3] MeCN 39.7 33.6 57 a Μ [CF2C1BF3] CH3CN 38.6 24.9 39 a Μ [CF,BrBF3] CH3CN 38.5 22.6 39 κ [C3F;BF3] CD3CN 41 20 47 Κ [CF,=CFBF3] CH3CN 41 25 60 Κ [C6F5BF3] CD3CN 43 21 b Μ [(C6F5)2BF2] MeCN (-30 °C) 57.6 72 Κ f(CF3)2BF2] MeCN 57.2 30.6 57 a b Μ = Me3NH. Μ = C6F5Xe.

5. THE REACTIVITY OF (POLYFLUOROORGANO)HALOBORANES 5.1. Thermal stability The present information on the thermal stability of (polyfluoroorgano)haloboranes is not always reliable because of the high reactivity of these compounds. It was reported that "CF3BF2 decomposed quantitatively to BF3 and a white, non-volatile residue in the presence of oxygen gas, moisture, glyptal resin, and other catalysts of undetermined nature, but it was stable for periods of months in vacuum in the absence of these substances" [41]. The (trifluoroethenyl)haloboranes (CF2=CF)„BX3_n (X = F, CI; η = 1 - 3) were characterized as colourless, air sensitive substances which burned with a sooty flame on contact with air [20, 24] and occasionally detonated under ill-definite conditions. In the gas phase at ambient temperatures CF2=CFBF2 (b.p. -14 °C) decomposed to form BF3 at a rate of about 5 % per week. (Trifluoroethenyl)dichloroborane (b.p. 48 °C) was stable at 100 °C for 5 h but underwent partial decomposition on standing for a few days at room temperature providing BF3 [20], Chambers and Chivers have reported the slow conversion of C6F5BF2 to (C6F5)2BF and BF3 at room temperature (40 % conversion within one month) [14] or at higher temperature (77 % conversion after 16 h at 95 °C followed by 18 h at 195 °C) [15]. Heating of C6F5BC12 for a longer time (250 °C, 25 h) led to (C6F5)2BC1 and BC13 [15]. The boranes C6F5BF2 (b. p. 104 to 105 °C) [21], C6F5BC12 (b.p. 150 °C, extrapolated) [15], C6F5BBr2 (b.p. 83 to 88 °C / 20 mm Hg) [16], CF2=CFBC12 (b.p. 48 °C), and (CF2=CF)2BC1 (b.p. 100.5 °C) [20] could be purified by distillation in a flame-dried glass equipment without thermal decomposition or dismutation. Solutions of poly- and perfluorinated (alk-l-enyl)- and (aryl)difluoroboranes in inert solvents (CH2C12, CFC13) showed no decompositions or dismutation reactions at 20 °C during weeks under an atmosphere of dry argon [21,46],

5.2. Reactions with bases and nucleophiles Replacement of all or the majority of hydrogen atoms in (organo)haloboranes R„BX3_n by fluorine caused a significant increase of the Lewis acidity at the boron centre. Consequently, (polyfluoroorgano)haloboranes display a high reactivity towards basic and nucleophilic reagents. The primary products of these reactions are the neutral donor-acceptor complexes (RF)„BX3_„ · Base or the negatively charged borate anions |(RF)nBX3_nY]", which remained unchanged or underwent further transformations depending on the nature of the formed species and on the reaction conditions. The reactivity of the free (polyfluoroalkyl)dihaloboranes is still unstudied. Walker and Leffler have estimated the relative Lewis acidity of some (trifluoroethenyl)boranes by measuring the Δδ(Η) values in the Ή NMR spectra of (CF2=CF)nBX3_n · 0(C//2CH3)2 (X = F, CI; η = 0 - 3) with respect to base 0(C/i,CH3), itself [73]: BCI3 > CF2=CFBC12 > (CF2=CF)2BC1 > (CF2=CF)3B > BF3 The treatment of (trifluoroethenyl)- and (c/s-heptafluorobut-l-enyl)difluoroborane with diethyl ether in dichloromethane gave the corresponding etherates (quantitative yield), which were characterized by "F NMR spectroscopy [46]. CH2CI2 RCF=CFBF2 + OEt, » RCF=CFBF2 · OEt, (R = F, m-C2Fs) (41) -40 to 20 °C

600 Main Group Metal Chemistry Vol. 25, No. 10, 2002

The formation of CF2=CFBC12 · NMe3 by reacting the (alkenyl)dichloroborane with trimethylamine was used for the identification of CF2=CFBC12 but only the analytical data of this adduct were reported [20], Similarly, adducts CFX=CHBF2 · NMe3 were isolated after treatment of the (alkenyl)difluoroborane with an excess of trimethylamine (Eq. (42)) [51].

CFX=CHBF2 + NMe3 CFX=CHBF, · NMe, (X = F, CI) (42)

The interaction of (pentafluorophenyl)difluoroborane and -dichloroborane with an equimolar quantity of anhydrous pyridine or acetonitrile in an inert solvent (dibutyl ether, dry petroleum or dichloromethane) resulted in the precipitation of C6F5BX2 · Base adducts (Eqs. (43), (44)) [15, 70].

BU20 or petroleum C6F5BX2 + Py C6F5BX2 · Py (43) X = F, CI 20 °C

CH2C12

C6F5BF2 + CHJCN CFTF,BF, · NCCH, (44) 20 °C

The primary adducts in reactions of (pentafluorophenyl)dichloroborane and bis(pentafluorophenyl)chloroborane with secondary silylamines (C6F5)nBCl3_„ · NHR(SiMe3) were not detected because of the easy conversion into (pentafluorophenyl)chloro(amino)boranes and bis(pentafluorophenyl)(amino)boranes, respectively (Eqs. (45), (46)) [74, 75]. Alkali amides Μ [NR(SiMe3)] (M = Li, R = mesityl [74], Μ = Na, R = SiMe3 [75]) reacted in a similar way (Eqs. (47), (48)).

hexane C6F5BC12 + 2 NHR(SiMe3) C6F5BCl(NRSiMe3) + NH2R + ClSiMe3 (45) 20 °C R = SiMe3 (67 %), r-Bu (68 %) toluene (C6F5)2BC1 + NH(SiMe3)2 (C6F5)2B(NHSiMe3) + ClSiMe3 (46) 70 °C, 1 h 79% hexane

C6F5BC12 + Li [N(2,4,6-C6H,Me3)(SiMe3)] » C6F5BCl[N(2,4,6-C6H2Me3)(SiMe3)J (47) - LiCl 82% petroleum

(C6F5)2BC1 + Na [N(SiMe3)2] » (C6F5)2BN(SiMe3)2 + NaCl (48) 20 °C, 0.5 h 94%

In general, the reaction of C6F5BC12 with substituted anilines ArNH2 (Ar = 4-MeOC6H4, 2,4,6-C6H2Me3) in refluxing benzene (2 days) also proceeded via the related intermediates C6F5BCl(NHAr) which easily lost HCl to form C6F5B=NAr [76]. However, the latter compounds were unstable with respect to dimerization [74,76], For comparison it should be mentioned that the secondary amine adducts CF3BX2 · NHAlk2 did not eliminate HX spontaneously [5]. C6F5BC12 reacted with two equivalents of trimethylsilylazide to form C6F5B(N3)2 which trimerizes in the solid state to give [C6F5B(N3)2]3, the first example of an azido substituted Ν,Ν',Ν"- tris(diazo)triazatriboratacyclohexane. Its monomer was trapped by pyridine yielding C6F5B(N3)2 · Py [77], Similarly, the nucleophilic replacement of chlorine bonded to boron was found in the reaction of (C6F5),BC1 with Me3SiN3 (Eq. (49)).

601 Hermann-Josef Frohn et al. (Polyfluoroorgano)haloboranes and (Polyfluororga.no) fluoroborate Salts: Preparation, NMR Spectra and Reactivity

toluene

(C6F5)2BC1 + Me3SiN3 • (C6F5)2BN3 + ClSiMe3 (49) -78 to 20 °C, 12 h 65 %

In the presence of pyridine reaction (49) produced the adduct (C6F5)2BN3 · Py in 80 % yield [78]. In the interaction of (polyfluoroorgano)haloboranes with water, likely, the coordination of the oxygen atom at boron is the first step but the final product depends strongly on the reaction conditions. For instance, the quantitative hydrodeborylation (the formal replacement of the three-coordinated boron atom by hydrogen) of CF2=CFBF2 took place at 120 °C (15 h) when treated with water yielding trifluoroethylene [20]. The hydrodeborylation of difluoroborane CF2=C(CF3)BF2 forming the polyfiuorinated olefin CF2=CHCF3 was also reported but the conditions were not specified [50]. When dichloromethane solutions of RCF=CFBF2 were reacted with water at 0 °C (R = F) or at a -78 °C (R = cis- and trans-CI), the anions 19 l9 [RCF=CFBF3]~ and [BF4]~ were detected in the aqueous phase ( F NMR). No F NMR signals which could be attributed to the alkenes RCF=CFH were observed. Also the boronic acids RCF=CFB(OH)2 were not observed, neither in the aqueous nor in the organic phase [46]. However, (pentafluorophenyl)dihydroxyborane and bis(pentafluorophenyl)hydroxyborane were obtained in good yields by the careful hydrolysis of (pentafluorophenyl)dichloroborane and bis(pentafluorophenyl)- chloroborane, respectively (Eqs. (50), (51)) [15, 79], The latter resulted also in the hydrolysis of the di(organo)chloroborane with 2 Ν HCl, but no details were reported [80],

acetone, -78 °C

C6F5BC12 + 2 H20 » C6F5B(OH)2 (50) - 2 HCl 89 % acetone, -20 °C

(C6F5)2BC1 + H20 » (C6F5)2BOH (51) - HCl 49 %

The formation of (pentafluorophenyl)-l,3,2-dioxaborolandione-4,5 from C6F5BC12 and oxalic acid (Eq. (52)) has a close similarity to the hydrolysis of (pentafluorophenyl)dichloroborane to C6F5B(OH)2 [79].

(X .,0 C6F5BC12 + (OOOH)2 -—2 HCl"", toluen" '"»e VI " BCgF; (52) 20 °C,35 h Ο 0 95%

In contrast, the heating of CF2=CHBC12 and CFC1=CHBC12 with acetic acid (100 °C, 16 h) led to the complete hydrodeborylation of both boranes and formation of 1,1-difluoro- and 1-chloro-l-fluoroethene, respectively [51]. The information about the addition of negatively charged species to (polyfluoroorgano)haloboranes is limited to reactions of (pentafluorophenyl)difluoroborane with alkali fluorides, which led to alkali (pentafluorophenyl)trifluoroborates [70, 81]. This route is useful mainly for preparing salts with desired cations (Eqs. (53), (54)). Η-,Ο QF5BF2 + KF » Κ [QF5BF3] (53) 62% MeCN C6F5BF2 + CsF » Cs [C6F5BF3] (54) 85 - 95 %

The interaction of perfluoro-l,2-di[bis(pentafluorophenyl)boryl]benzene with fluoride donors yielded interesting μ-bridged fluoroborate salts with two equivalent boron centres (Eq. (55)) [23,82].

602 Main Group Metal Chemistry Vol. 25, No. 10, 2002

FA, Cf CHJCI, s s K£)I + [F r — [Mr (55) B(C6F5)2 20 °C 10F A' CfiFs

[F]": (KF + 18-crown-6) (4 h), M= [K(18-crown-6)] (90 %); ([Ph3C] [BF4])(20 min), Μ = Ph3C (95 %)

5.3. Reactions with anhydrous HF Reactions of (polyfluoroorgano)haloboranes with anhydrous HF (aHF) have some peculiarities because aHF combines the properties of a strong protic acid and a donor of the fluoride anion. The substitution of chlorine or bromine by fluorine in C6F5BX2 was achieved in the heterogeneous system aHF-CCl3F or aHF-CH^Cl, with 50 to 60 % yields of (pentafluorophenyl)difluoroborane (Eqs. (56), (57)) [21],

CCljF

C6F5BC12 + 2 HF » C6F5BF2 + 2 HCl (56) -50 to 20 °C

CHJCU

C6F5BBr2 + 2 HF • C6F5BF2 + 2 HBr (57) -50 to -5 °C

In contrast to boron trifluoride, (perfluorohexyl)difluoroborane, (perfluoro-irans-hex-l-enyl)difluoro- borane, (perfluoro-c/i-but-l-enyl)difluoroborane, and (pentafluorophenyl)difluoroborane were well soluble in aHF at room temperature, but below -20 to -30 °C their solubility significantly decreased and white suspensions were formed. As Lewis acids of graded strength, these boranes RFBF2 interacted with the 19 fluoride donor aHF in different extent. Thus, the F NMR spectrum of C6F,3BF2 in aHF (-20 °C) contained the B-F resonance at -150.60 ppm, which was attributed to a BF3 group, and its relative intensity was equal to that of the trifluoromethyl group (-80.71 ppm) of the perfluorinated hexyl chain. The 19F NMR spectra of the less acidic (organo)difluoroboranes franj-C4F9CF=CFBF2, cis-C2F5CF=CFBF2, and C6F5BF2 showed no 19 resonances of the boron-bonded fluorine atoms whereas the F NMR spectra of CF2=CFBF2 and C1CF=CFBF2 in aHF (-40 °C) contained a BF2 resonance and thereby the spectra did not differ from those in CH2C12 [46]. It means, that the less acidic (alkenyl)boranes RFBF2 (RP = CF2=CF, CC1F=CF) did not abstract the fluoride ion from aHF and the more acidic boranes (RF = cis-C2F5, trans-C4F9, C6F5) underwent a fast fluorine exchange with HF. In the case of the most acidic (alkyl)borane, C6F13BF2 the equilibrium was practically completely shifted to the trifluoroborate anion (Scheme 2) [83],

Scheme 2

+ RFBF2 + HF [RFBF2(F · H )]

RfBF2 + 2 HF [H2F] [RfBF3]

In the system RFBF2 / aHF two main aspects should be emphasized: (a) the significant acidification of the media after the dissolution of boranes RFBF2 in aHF ("acidified" aHF) and (b) the presence of + trifluoroborate anions |RFBF3|" or the donor-acceptor complexes [RFBF2(F • H )] in these solutions. Both factors play a significant role in the reactions of (polyfluoroorgano)difluoroboranes with aHF. For instance, after shaking the less acidic boranes CF2=CFBF2 in CH2C12 solution with aHF at -40 °C, the acidic phase contained both CF2=CFH and residual (trifluoroethenyl)difluoroborane. A similar picture was observed in the reaction of (2-chlorodifluoroethenyl)difluoroborane with aHF (Eq. (58)).

CH2CI2 RCF=CFBF2 + aHF » RCF=CFH + BF3 (58) R = F, cis- and trans-C 1

603 Hermann-Josef Frohn et cd. (Polyfluoroorgano)haloboranes and (Polyfluororgano) fluoroborate Salts: Preparation, NMR Spectra and Reactivity

Alternatively, the addition of aHF to the more acidic borane /rans-C4F9CF=CFBF2 in CH2C12 solution at -20 °C followed by wanning up to room temperature did not show the formal hydrodeborylation reaction [46],

5.4. Conversion of(RF)ßX3_ into (RF)„BF3_„ (X = CI, Br) Polyfluorinated (alkenyl)- and (aryl)dichloroboranes and -dibromoboranes were converted into the corresponding -difluoroboranes by treatment with antimony trifluoride. This procedure was sometimes accompanied by side-reactions. For instance, chlorofluoro- and (trifluoroethenyl)difluoroboranes were prepared in moderate yields by reacting the (alkenyl)dichloroboranes with SbF3 at low temperature (Eqs. (59), (60)) [20, 51]. Surprisingly, (trifluoroethenyl)difluoroborane was also obtained from bis(trifluoroethenyl)chloroborane under the same conditions (Eq. (61)) [20].

SbF3 CF2=CFBC12 * CF2=CFBF2 (59) -23 °C, 12 h 59 %

SbF3

CFX=CHBC12 * CFX=CHBF2 (60) -78 °C, 18 h

SbF3 (CF2=CF)2BC1 » CF2=CFBF2 (61) -23 °C, 12 h

Chlorine-fluorine substitution on (pentafluorophenyl)dichloroborane with SbF3 at -15 °C led to the formation of C6F5BF2 [15] while at ambient temperature the yield of RFBF2 was significantly diminished because of the transfer of the pentafluorophenyl group from boron to the antimony atom (Eqs. (62), (63)) [15, 21]. SbF3 C6F5BC12 » C6F5BF2 (62) -15 °C, 4 h 73%

SbF3 C6F5BCI2 » C6F5BF2 + (C6F5)2SbCl (63) 45 °C, 1.5 h 17% 55%

To our knowledge, the opposite conversion of RFBF2 and (RF)2BF into the corresponding (polyfluoroorgano)chloroboranes and -bromoboranes was not reported.

5.5. Reactions of (polyfluoroorgano)haloboranes with hypervalent fluorine-element-fluorine bonds in xenon fluorides, bromine trifluoride and organoiodine difluorides Reactions of this type are not numerous but they play an important role in the synthesis of organoxenon, organobromine(III), and organoiodine(III) compounds. In these processes weakly nucleophilic polyfluorinated organic groups were transferred from the boron atom to the strongly oxidizing xenon(II or IV), bromine(III) or iodine(III) centres. Indeed, the interaction of polyfluorinated (aryl)difluoroboranes with xenon difluoride in dichloromethane led to the formation of arylxenon(II) tetrafluoroborates. Pentafluorophenylxenon(II) tetrafluoroborate was also obtained from C6F5BC12 and XeF2 although it required two equivalent of xenon difluoride because of the primary oxidation of negatively polarized chlorine to Cl2. The general character of the xenodeborylation reaction (the formal replacement of the three-coordinated boron atom by xenon) was shown by the representative preparations of pentafluorophenyldifluoroxenon(IV) tetrafluoroborate and trifluoroethenylxenon(II) tetrafluoroborate (Eqs. (64) - (66)).

CH,CU C6H^,FnBF2 + XeF2 » [C6H„F^nXe] [BF4] (64) η = 1 - 5 -40 °C 50 - 80 %

604 Main Group Metal Chemistry Vol. 25, No. 10, 2002

CHjClo

C6F5BF2 + XeF4 —^ [C6F5XeF2] [BF4] (65) -55 °C 100 %

CH2C12

CF2=CFBF2 + XeF2 » [CF2=CFXe] [BF4] (66) -50 °C 85 %

In contrast to the reaction of XeF2, the interaction of C6F5XeF with C6F5BF2 in dichloromethane gave the salt [C6F5Xe] [C6F5BF3]. No transfer of the boron-bonded C6F5 group to the xenon atom proceeded to obtain the covalent compound (C6F5)2Xe [84]. More details about related reactions of (polyfluoroorgano)difluoroboranes with xenon fluorides are given in a review on organoxenon compounds [85]. The replacement of the inert solvent dichloromethane by the strong acid, aHF, resulted in significant changes of the reaction route caused by the specific interaction of both reactants RFBF2 and XeF, with aHF (see Section 5.4). In contrast to the reaction of C6F5BF2 with XeF2 in CH2C12, a mixture of perfluorinated phenyl- and cyclohexa-l,4-dienylxenonium cations, (cyclohexa-l,4-dienyl)trifluoroborate, and tetrafluoro- borate anions were formed after the quantitative conversion of C6F5BF2 and XeF2 in aHF solution (Eq. (67)) [83].

XeF2, aHF (67) + [BF4] -30 C, - Xe 10 : 13 (molar ratio)

No organoxenon compounds were formed in the reaction of xenon difluoride with (periluoro-ris-but-1- enyl)difluoroborane and (perfluoro-ira/is-hex-l-enyl)difluoroborane. Essentially fluorine addition across the carbon-carbon double bond occurred at -30 to -20 °C to produce (perfluoroalkyl)trifluoroborate anions (Eqs. (68), (69)) [83],

+ XeF2, -Xe° Κ [HF2] ck-C2F,CF=CFBF, [H2F] [C4F9BF3] Κ [C4F9BF3] (68) aHF, -30 °C 70 to 75 %

+ XeF2, -Xe° ira«5-C4FqCF=CFBF, [H2F] [C6F13BF3] (69) aHF, -30 °C

During the last years Frohn et al have demonstrated the successful application of (polyfluoroorgano)- difluoroboranes as transfer reagents of polyfluoroorgano groups for the synthesis of polyfluorinated organobromine(III) and organoiodine(III) compounds. Bis(pentafluorophenyl)bromine(III) tetrafiuoroborate was prepared by the reaction of C6F5BF2 or its adduct C6F5BF2 · NCCH3 with bromine trifluoride. It should be noted that the same result was obtained using an equivalent amount of (C6F5)2BF (Eqs. (70), (71)) [70, 86].

BrF3, CH2C12

2 C6F5BF2 or C6F5BF2 · NCCH3 » [(C6F5)2Br| [BF4] (70) -78 to 20 °C 38 to 50 %

BrF3, CH,CI,

(CfiF5)2BF » [(C6F5)2Br] [BF4] (71) -78 to 20 °C, 1 h 93 %

A series of fluorine-containing symmetric and asymmetric diaryliodine(III) [87), arylalken-1-

605 Hermann-Josef Frohn et al. (Polyfluoroorgano)haloboranes and (Polyfluororgano) fluoroborate Salts: Preparation, NMR Spectra and Reactivity enyliodine(III), and arylalkyliodine(III) salts (Eqs. (72), (73)) [88] were prepared in good yields using organodifluoroboranes.

CH->C1'>

ArBF, + C6F5IF2 [Ar(C6F5)I] [BF4 (72) 20 °C

Ar = C6H5, 2-, 3- and 4-FC6H4, 2,6-C6F2H3, 3,5-C6F2H3, 2,4,6-C6F3H2, 3,4,5-C6F3H2, C6F5

CH2C12 or CC13F CftF,BF, + RJF, [Rp(C6F5)I] [BF4] (73) -50 to 0 °C

Rf = C6F13, (CF3)2CF, rrans-(CF3)2CFCF=CF

5.6. Reactions of (polyfluoroorgano)haloboranes with selected organoelement compounds There are some examples where polyfluoroaryl groups were transferred from a more electropositive metal atom to the boron atom of (polyfluoroaryl)haloboranes. Organolithium reagents are well known powerful carbon nucleophiles and their reactions with boron trihalides and (organo)haloboranes led to tri(organo)boranes and tetra(organo)borate salts depending on the reaction conditions (stoichiometry, temperature, solvent etc.) [1, 2]. Tris(pentafluorophenyl)boron was obtained from pentafluorophenyllithium and BX3 (X = CI [32, 33], Br [34]) in pentane or hexane. With Li [C6F5] as fluoroorgano nucleophile the reaction ended with Li [(C6F5)4B] [32, 34]. Under strongly regulated conditions it is possible to substitute halogen atoms in (polyfluoroaryl)haloboranes by other organic groups and to avoid the final addition of a forth organo group to boron and to produce only polyfluorinated tri(organo)boranes. For example, bis(pentafluorophenyl)chloroborane reacted with nonafluoro-2- biphenyllithium to yield tris(perfluoroaryl)borane (Eq. (74)) [89], When this reaction was performed in non- coordinating pentane, the product was isolated free of coordinating solvent. In contrast, the substitution of both chlorine atoms in C6F5BC12 by octafluoro-2,2'-dilithiobiphenyl in diethyl ether gave the donor-acceptor complex of the tri(aryl)borane with OE^ (Eq. (75)) and the O-base could not be removed without decomposition of the borane [9]. pentane

(C6F5)2BC1 + Li [2-C6F5C6F4] 2-C6F5C6F4B(C6F5)2 (74) -40 to 20 °C 37%

OEt,

C^BClj + B(C6F5).OEt2 (75) - 2 LiCl

The use of organic derivatives of some transition metals instead of the highly reactive organolithium reagents was more preferable. These reactions are related to the formation of (R^BX^ described in Section 2.1. Indeed, heating of (pentafluorophenyl)dibromoborane with pentafluorophenylmercury bromide gave bis(pentafluorophenyl)bromoborane (Eq. (76)) [22]. toluene C6F5BBr2 + C6F5HgBr • (C6F5)2BBr (76) reflux, 2 days 35 %

Several unsymmetrical organotin derivatives were involved in reactions with (polyfluoroorgano)halo- boranes. In this way, tris(trifluoroethenyl)boron (Eq. (77)) [20] and a series of bis(pentafluorophenyl)- (alkynyl)boranes [90] were prepared in high yields (Eq. (78)).

2 (CF2=CF)2BC1 + (CF2=CF)2SnMe2 (CF2=CF)3B + Me,SnCl2 (77) 50 °C 100%

606 Main Group Metal Chemistry Vol. 25, No. 10, 2002

hexane

2 (C6F5)2BC1 + (ROC)2SnMe2 (C6F5)2B(C-CR) + Me2SnCl2 (78) -78 to 25 °C

R = t-Bu (85 %), Ph (75 %), C6F5 (not isolated because of instability)

Perfluorinated borafluorene and diboraanthracene derivatives were synthesized by reactions of (perfluoroaryl)chloroboranes with perfluoroaryl(dimethyl)tin (Eqs: (79), (80)) [6,9],

benzene CsFjBClz + SnMe2 BC6F5 (79) 70 °C, 7 days

CI C F ι I 6 5 • ο toluene (80) (C6F5)2SnMe2 W 140 °C, 72 h I Β' CI c6F5

80%

A formal similarity to the halogen-organyl substitution was found in the replacement of chlorine in bis(pentafluorophenyl)chloroborane by hydrogen or deuterium under the action of chloro(dimethyl)silane (Eq. (81)) or tributyldeuterotin (Eq. (82)), respectively [91].

η (C6FS)2BC1 + η Me,SiClH (excess) [(C6F5)2BHJ„ + η Me,SiCl, (81) -78 to 20 °C, 1 h 96%

hexane

η (C6F5)2BC1 + η Bu3SnD [(C6F5):BD]„ + nBujSnCl (82) 20 °C, 1 h 81 %

In addition to organic derivatives of tin and mercury, organozinc and organozirconium compounds were examined for the substitution of boron-bonded bromine by organic groups. For example, the heating of 1,2- bis(dibromoboryl)tetrafluorobenzene with bis(pentafluorophenyl)zinc in toluene gave perfluoro-1,2- bis(diphenylboryl)benzene (Eq. (83)) [23].

Β Br? toluene, 80 °C, 12 h + 2Zn(C6F5)2 (83) • 2 ZnBr2 Β Br, B(C6F5)2 55%

9-Methyl-9-boraoctafluorofluorene was obtained in the reaction of the corresponding bromoborane and

Cp,ZrMe2 (Eq. (84)) [9],

607 Hermann-Josef Frohn et al. (Polyfluoroorgano )haloboranes and (Polyfluororgano) fluoroborate Salts: Preparation, NMR Spectra and Reactivity

It should be noted that the attempt to perform a ligand redistribution between B(C6F5)3 and boron tribromide (toluene / hexane, -78 °C to 20 °C) failed (the preliminary communication [92] on the synthesis of (C6F5)2BBr using this procedure was wrong [22]) although similar processes are well known in hydrocarbon (organo)borane chemistry [1,2].

6. THE REACTIVITY OF (POLYFLUOROORGANO)FLUOROBORATE SALTS The thermal stability of Μ [RFBF3] salts was not systematically studied. In an early publication [38] the formation of tetrafluoroethene was reported as the main product in the vacuum pyrolysis of Κ [CF3BF3] or [NH4] [CF3BF3] (Eq. (85)). Salt Κ [C6F5BF3] melted at 324 °C and decomposed in vacuum at 300 °C to yield perfluoro(polyphenylenes) and Κ [BF4] (Eq. (86)) [81].

vacuum

Μ [CF3BF3] » » CF2=CF, + cyclo-C3F6 (traces) (85) - Μ [BFJ

Μ = NH4 (175 °C), Κ (330 to 350 °C) vacuum, 300 °C Κ [C6F5BF3] * * (C6F4-)„ (86)

- Κ [BF4]

The abstraction of a fluoride anion from (polyfluoroorgano)trifluoroborate salts by the action of Lewis acids leading to (polyfluoroorgano)difluoroboranes was discussed in Section 2.2. However, taking into account the heterogeneous character of the studied reactions between Κ [RFBF3] salts and boron trifluoride or arsenic pentafluoride in weakly polar solvents (CH2C12, CFC13, pentane or benzene) the driving forces are complex. In addition to the difference in the Lewis acidity of RFBF2 and BF3 or AsF5, the distinguished solubilities and lattice energies of Κ [RFBF3] and Κ [BF4] or Κ [AsF6] play an important role in this process which is still not investigated.

The majority of the known Μ |RFBF3] salts were prepared with Μ = K. "[N^Sn] [CF3BF3]" reacted with KF in water to yield Κ [CF3BF3] and underwent cation exchange to give the aqueous solution of the acid Η

[CF3BF3] (Eq. (87)) [38],

Η-cation exchange resin

"[Me3Sn] [CF3BF3]" » H^ [CF3BF3]aq (87) H2O

Barium and ammonium (trifluoromethyl)trifluoroborates were obtained by neutralization of H^ |CF3BF3|aq with BaC03 and aqueous ammonia, but the properties of these salts were not investigated [38], Treatment of |Et3NH] [R(CF3)2BF] with Cs2C03 or K2C03 in chloroform resulted in the corresponding cesium and potassium salts, respectively |55]. Potassium (polyfluoroalk-l-enyl)trifluoroborates were recently involved in isomerisation reactions on the carbon-carbon double bond. Solutions of Κ |rrans-C4F9CF=CFBF3], Κ [c«-C2F5CF=CFBF3], and Κ [m-C6F13CF=CFBF3] in acetone were irradiated with a high-pressure mercury lamp (λ > 280 nm) to produce a mixture of the cis- and trans- isomers in a ratio of 3 to 7 (Eq. (88)). Further irradiation did not increase the relative ratio of isomers, only decomposition of (polyfluoroalk-l-enyl)trifluoroborates to unknown products proceeded |93|.

λ > 280 nm

Κ |c/s-C„F2n+1CF=CFBF3] » Κ [/ram-CnF2n+,CF=CFBF3] (88) η = 2,4, 6 acetone

608 Main Group Metal Chemistry Vol. 25, No. 10, 2002

When solutions of potassium (2-R-difluoroethen-l-yl)trifluoroborates (R — CI, C4H9, C3F7O) were irradiated in acetone, a similar ratio trans- to cis-isomers was again obtained as the predominant product (Eq. (89)). However, the isomerisation was accompanied by the formation of solid by-products and unrecognised minor organofluorine compounds.

λ > 280 nm

Κ [frans-RCF=CFBF3] • Κ [CK-RCF=CFBF3] + ... (89) acetone

R = CI, C4F9, C3F70

The photo induced isomerisation of Κ [fraws-C4F9CF=CFBF3] in methanol or acetonitrile proceeded slowly and after 2 h the l9F NMR spectra of the MeOH and MeCN solutions showed only 10 % conversion of the rrart5-isomer into Κ [c«-C4F9CF=CFBF3]. The remarkable acceleration of the photo induced cis-, frans-isomerisation of Κ [RCF=CFBF3] salts in acetone with respect to methanol or acetonitrile solutions unambiguously pointed out the sensibilisation of the process by acetone. The addition of bromine (1 equivalent) into the solution of Κ [irans-C4F9CF=CFBF3] in MeCN also accelerated the isomerisation and formed a mixture of trans- and cis-isomers (4 : 6, after 2 h) while without bromine the ratio was 9 : 1 (after 2 h, λ > 280 nm). Finally the reaction mixture had the typical 19 red coloration of Br; and the yield of potassium (perfluorohex-l-enyl)trifluoroborate was above 86 % ( F NMR) (the bromo-containing by-products were not analysed). A similar photoreaction of Κ [trans- C4F9CF=CFBF3] in MeOH (1 equivalent of bromine) gave a mixture of trans- and m-isomers (44 : 56, after 2 h). Further irradiation for additional 3 h led to discoloration of the solution but did not influence either the yield nor the ratio of isomers. Probably, the observed discoloration was due to the photo oxidation of methanol by bromine. The use of a super-high-pressure mercury lamp (λ > 365 nm) for the isomerisation gave isomeric mixtures of the same composition as mentioned above, but the processes of isomerisation proceeded more slowly (s 20 h). In the case of Κ [fra«j-C6H5CF=CFBF3] a total decomposition took place in acetone under UV irradiation (λ > 365 nm) within 12 h [93]. The halogen addition to the carbon-carbon double bond was found in the reaction of potassium (trifluoroethenyl)- and (chlorodifluoroethenyl)trifluoroborates with molecular halogen in dichloromethane (Eq. (90)) [94],

CH2CU Κ [RCF=CFBF3] + X2 • Κ [RCFX-CFXBF3] (90) 20 °C R = F, CI; X = CI, Br

The hydrodeboration (here the formal replacement of the four-coordinated boron atom by hydrogen) of the (fluoroorgano)trifluoroborates Κ [RFBF3] [Rf = C6F5, XCF=CF (X = F, cis- and trans-CI, -C3F70, cis- C2F5, trans-CtF9, -C4H9) and C6F13] with acids of different strength HX (100 % acetic acid, (H0 ca. 0), HF^ (25 % HF*,: (H0 = 0.88; 40 % HF^: (H0 = 2.08; 50 % HF^: (H0 = 3.84), 100 % CF3C02H ((H0 3.0), or anhydrous HF (aHF) ((H0 15 ) was established [95]. The rate of hydrodeboration was determined by the nature of the group Rf, by the acidity of HX and by the temperature (Eq. (91)). For instance, potassium (trans-1,2-difluorohex-1 -enyl)trifluoroborate is slightly soluble in 100 % acetic acid but did not react at 20 °C within 24 h. Stronger acids like HF^, 100 % CF3C02H, or aHF caused a fast carbon-boron bond cleavage to form trans- 1,2-difluorohex-l-ene in a quantitative yield.

+ HX

Κ [frarts-C4H9CF=CFBF3] » frans-C4H9CF=CFH (91)

- Κ [BF4]

HX = 27 % HF^ (20 °C, 1 h), 100 % CF3C02H (20 °C, 1 - 3 h), aHF (-20 °C, s 1 h)

The potassium salts of perfluorinated (trans-hex-1 -enyl)trifluoroborate and (cis-but-l- enyl)trifluoroborate are well soluble in aHF at -50 to 20 °C and their solutions were stable for some days.

609 Hermann-Josef Frohn et cd. (Polyfluoroorgano)haloboranes and (Polyfluororgano) fluoroborate Salts: Preparation, NMR Spectra and Reactivity

Partial hydrodeboration was detected after 3 days in the case of Κ [c/i-C2F5CF=CFBF3] and after 30 days for Κ [fra»s-C4F9CF=CFBF3] (Eq. (92)) [95].

Κ [RFCF=CFBF3] + aHF » RFCF=CFH + Κ [BF4] (92) 20 °C

RF = cis-C2F5: 3 days, s 10 % conversion RF = trans-C4F9: 3 days, no reaction; 30 days, 28 % conversion

In an early publication [81], the hydrolysis of Κ [C6F5BF3] in a hot aqueous solution was reported resulting in C6F5H, KF, HF and H3B03. Later it was found that this salt did not react with 40 % HF^ at 20 °C within 4 days, but on heating (85 to 95 °C) it underwent a slow hydrodeboration with a half of life of ca. 1 h. The total decomposition under those conditions was completed within 6 h to yield pentafluorobenzene and Κ 19 [BF4], The solution of borate Κ [C6F5BF3] in aHF at -40 °C displayed a gradual broadening of the F resonance of the fluorine atoms bonded to boron and after 15 to 30 minutes the signal disappeared finally [21]. At 20 °C the conversion into C6F5H and Κ [BF4] was completed within s 18 h (Eq. (93)). It is interesting that the addition of Κ [HF2] to the aHF solution ("basic HF" of Κ [C6F5BF3]) has no effect on the l9F NMR spectrum, but it accelerated strongly the hydrodeboration [95].

Κ [C6F5BF3] + aHF » C6F5H + Κ [BFJ (93) 20 °C, s 18 h

No changes were found in the solution of potassium (perfluorohexyl)trifluoroborate in aHF after 10 days at 20 °C as well as in Κ [HF2]-aHF solutions (20 °C, 7 days) (Eq. (94)).

20 °C Κ [C6F13BF3] + aHF or Κ [HF2] in aHF * no reaction (94) 10 or 7 days

These results allow to build the following sequences of reactivity of (organo)trifluoroborate salts towards hydrodeboration by acids: Κ [/rans-C4H9CF=CFBF3] > Κ [cis- and rra«s-C3F7OCF=CFBF3] - Κ [CF2=CFBF3] a Κ [cis- and irans-ClCF=CFBF3] - Κ [C6F5BF3] > Κ [cw-C2F5CF=CFBF3] a Κ [trans- C4F9CF=CFBF3] > Κ [C6F13BF3] [95], It should be emphasized that the hydrodeboration of the salts Κ [RCF=CFBF3] proceeded stereo specifically: the borates Κ [IRA«s-C4H9CF=CFBF3], Κ [IRARTJ-C4F9CF=CFBF3], and Κ [ris-C2F5CF=CFBF3] gave exclusively the alkenes IRA/IS-C4H9CF=CFH, rram-C4F9CF=CFH, and CIS-C2F5CF=CFH, respectively. The cis / trans ratio of the alkenes RCF=CFH was the same as in the corresponding borates Κ [cis- and trans-

RCF=CFBF3] (R = CI, C3F70). 19 The F NMR spectra of Κ [RFBF3] in acids showed strong interactions of the BF3 group with protons or acid molecules. The observed spectral data were interpreted in terms of protonation of the fluorine atoms bonded to boron, which bear the majority of the negative charge or as a strong donor-acceptor interaction of these fluorine atoms with acid molecules (Scheme 3).

Scheme 3 In media of high acidity:

+ [RBF3J- + H [RBF2(F-H)1 RBF2 + HF

In media of low acidity: [RBF3r + HX - [RBF2(F—H-X)]"

The dissolution of (organo)trifluoroborates in aHF resulted primarily in the protonation of the fluorine atoms bonded to boron (formation of "basic" HF). This led to a fluoroborate / fluoroborane equilibrium and a decreased acidity of HF. These circumstances played the key role in reactions of potassium (polyfluoroorganyl)trifluoroborates with xenon difluoride in aHF [83]. Indeed, the reaction of potassium (pentafluorophenyl)trifluoroborate with xenon difluoride in aHF gave predominantly pentafluorophenylxenon(II) tetrafluoroborate (Eq. (95)). The

610 Main Group Metal Chemistry Vol. 25, No. 10, 2002

oxidative fluorination of [C6F5BF3] - despite of its anionic nature - occurred in a minor extent with respect to that in reaction of (pentafluorophenyl)difluoroborane (Section 5.5).

+ Xe [BF4]"

XeF2, aHF

+ C6F6 (traces) (95) -30 to 20 °C, - Xe° 6 (molar ratio)

However, a decrease of the acidity of the media did not influence the reaction route of potassium (perfluoroalk-1 -enyl)trifluoroborates with xenon difluoride. At -30 °C salts Κ [R'CF=CFBF3] quickly underwent fluorine addition across the carbon-carbon double bond to form the potassium (perfluoroalkyl)trifluoroborates Κ [R'CF2CF2BF3] (Eq. (96)).

aHF, -30 °C

Κ [R'CF=CFBF3] + XeFj «· Κ [R'CF2CF2BF3] + Xe° (96) 15 to 20 min. 84 -95

R' = F, cis-C2F5) trans-C4F9

The fluorine addition to the pentafluorophenyl group bonded to boron was also detected in reactions of xenon difluoride with tris(pentafluorophenyl)borane or Cs [(C6FS)3BF] in aHF [96] (see also Section 5.5). Potassium (perfluorohexyl)trifluoroborate did not react with XeF2 in aHF at room temperature within 1.5 hours. The mechanism of the xenodeborylation of Κ [C6F5BF3] with xenon difluoride in aHF and reasons of the fluorine addition to the (alkenyl)trifluoroborates Κ [R'CF=CFBF3] instead of the xenodeborylation were discussed in [83].

7. CONCLUSIONS The recent state of chemistry of (polyfluoroorgano)haloboranes and (polyfluoroorgano)fluoroborate salts was presented here and is mainly focused on the following aspects: 1. There exists a general and convenient method of preparation of Κ [RFBF3] salts where RF represents a periluorinated alkyl, alk-l-enyl and aryl group:

+ B(OAlk)3 K[HF2], H

MRF • Μ [RFB(OMe)3] • Κ [RFBF3] Μ = Li, MgX.

2. The corresponding perfluorinated (alkyl)-, (alk-l-enyl)-, and (aryl)difluoroboranes can be prepared by the fluoride abstraction from salts Κ [RFBF3] using appropriate fluoride anion acceptors like BF3 or AsF5. Individual entities of RFBF2 can be obtained from the reaction solutions by distillation. 3. The best-known way to (polyfluoroethenyl)- and (polyfluoroaryl)chloroboranes and -bromoboranes is based on the reaction of the corresponding tin or mercury derivatives with boron trichloride and boron tribromide. Routes to (polyfluoroalk-l-ynyl)haloboranes were not yet investigated.

RFSnMe3 + BX3 • RFBX, + XSnMe3

RFHgR' + BX3 • RFBX2 + R'HgX

Rf = CF2=CF, polyfluoroaryl; X = CI, Br

4. The reactivity of polyfluorinated (organo)haloboranes and (organo)fluoroborate salts is only partially investigated. However, the present achievements in this field show a high synthetic potential of these substances and illustrate in some cases their ability as unique polyfluoroorgano group transfer reagents. The application of (organo)boron compounds in synthetic chemistry is actually expanding in new promising areas.

611 Hermann-Josef Frohn et al. (Polyfluoroorgano)haloboranes and (Polyfluororgano) fluoroborate Salts: Preparation, NMR Spectra and Reactivity

ACKNOWLEDGEMENTS We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft, the Russian Foundation for Basic Research and the Fonds der Chemischen Industrie.

8. REFERENCES 1. Methoden der organischen Chemie (Houben-Weyl). Bd 13 / 3b. Organobor-Verbindungen 11. Thieme, Stuttgart, 1983. 2. Β. M. Mikhailov and Y. U. Bubnov, Organoboron compounds in organic synthesis. /; Bell & Bain, Glasgow, 1984. 3. W. Ε. Piers and T. Chivers, Chem. Soc. Rev., 26 (1997) 345. 4. Ε. Y.-X. Chen and Τ. J. Marks, Chem. Rev., 100 (2000) 1391. G. Pawelke and H. Buerger, Coord. Chem. Rev., 215 (2001) 243. 6. Μ. V. Metz, D.J . Schwartz, C. L. Stern, P. N„ Nickias and T. J. Marks, Angew. Chem. Int. Ed., 39 (2000) 1312. 7. L. Li., Stern and T. J. Marks, Organometallics, 19 (2000) 3332. 8. W. E. Piers, G. J. Irvine and V. C. Williams, Eur. J. Inorg. Chem., (2000) 2131. 9. P. A. Chase, W. E. Piers and B. O. Patrick, J. Am. Chem. Soc., 122 (2000) 12911. 10. R. D. Chambers and T. Chivers, Organomet. Chem. Rev., 1 (1966) 279. ίΐ. I. L. Knunyants, R. N. Sterlin and V. L. Isaev, Zh. Vses. Khim. Obshchest., 15 (1970) 25 (in Russian); Chem. Abstr., 72 (1970) 132825. 12. E. Bernhardt, G. Henkel, Η. Willner, G. Pawelke and H. Buerger, Chem. Eur. J., 7 (2001) 4696. 3. T. Chivers, J. Fluorine Chem., 115 (2002) 1. 4. R. D. Chambers and T. Chivers, Proc. Chem. Soc., (1963) 208. 15. R. D. Chambers and T. Chivers, J. Chem. Soc., (1965) 3933. 16. J. L. Pohlman, F. E. Brickman, G., Tesi and R. E. Donadio, Z. Naturforsch., B20 (1965) 1. 17. P. J. Parks, W. E. Piers and G. P. A. Yap, Organometallics, 17 (1998) 5492. 18. D. W. A. Sharp and J. M. Winfield, J. Chem. Soc., (1965) 2278. 19. H. Nöth and H. Vahrenkamp, J. Organomet. Chem., 11 (1968) 399. 0. S. L. Stafford and F. G. Stone, J. Am. Chem. Soc., 82 (1960) 6238. 1. H.-J. Frohn, H. Franke, P. Fritzen and V. V. Bardin, J. Organomet. Chem., 598 (2000) 127. 22. R. Duchteau, S. J. Lancaster, M. Thornton-Pett and M. Bochmann, Organometallics, 16 (1997) 4995. 23. V. C. Williams, W. E. Piers, W. Clegg, M. R. J. Elsegood, S. Collins and Τ. B. Marder, J. Am. Chem. Soc., 121 (1999) 3244. 24. R. N. Sterlin, V. L. Isaev, G. M. Zakharov, B. Martin and I. L. Knunyants, Zh. Vses. Khim. Obshchest., 12 (1967) 475 (in Russian); Chem. Abstr., 68 (1968) 13025. 25. J. L. Pohlman, F. E. Brickman, G., Tesi and R. E. Donadio, Z. Naturforsch., B20 (1965) 5. 26. A. G. Massey and A. J. Park, Tris(pentafluorophenyl)boron and its adducts in Organometallic Synthesis; Eds. R. B. King, J. J. Eisch, Vol. 3, Elsevier; New York, 1986, p. 461 - 462. 27. F. A. R. Kaul, G. T. Puchta, H. Schnider, M. Grosche, D. Mihalios and W. A. Herrmann, J. Organomet. Chem., 621 (2001) 177. 28. L. Jia, X. Yang, C. L. Stern and T. J. Marks, Organometallics, 16 (1997) 842. 29. G. Rodriguez and P. Brant, Organometallics, 20 (2001) 2420. 30. Υ. X. Chen, Μ. V. Metz, L. Li, C. L. Stern and T. J. Marks, J. Am. Chem. Soc., 120 (1998) 6287. 31. L. Li and T. J. Marks, Organometallics, 17 (1998) 3996. 32. A. G. Massey and A. J. Park, J. Organomet. Chem., 2 (1964) 245. 33. J. B. Lambert, S. Zhang and S. M. Ciro, Organometallics, 13 (1994) 2430. 34. A. N. Chernega, A. J. Graham, M. L. H. Green, J. Haggitt, J. Lloyd, C. P. Mehnert, N. Metzler and J. Souter, J. Chem. Soc., Dalton Trans., (1997) 2293. 35. W. Haubold, J. Herdtie, W. Gollinger and W. Einholz, J. Organomet. Chem., 315 (1986) 1. 36. P. A. Deck, C. L. Beswick and T. J. Marks, J. Am. Chem. Soc., 120 (1998) 1784. 37. H.-J. Frohn, A. Lewin and V. V. Bardin, J. Organomet. Chem., 570 (1998) 255. 38. R. D. Chambers, H. C. Clark and C. J. Willis, J. Am. Chem. Soc., 82 (1960) 5298. 39. H. Buerger, M. Grunwald and G. Pawelke, J. Fluorine Chem., 28 (1985) 183. 40. T. D. Parsons, D. Baker, A. B. Burg and G. L. Juvinall,7. Am. Chem. Soc., 83 (1961) 250. 41. T. D. Parsons, J. M. Self and L. H. Schaad, J. Am. Chem. Soc., 89 (1967) 3446. 42. H. Nöth and P. Fritz, Z. Anorg. Allg. Chem., 322 (1963) 297. 43. H. Buerger, M. Grunwald and G. Pawelke, J. Fluorine Chem., 31 (1986) 89. 44. D. J. Brauer, Η. Buerger, G. Pawelke, W. Weuter and J. Wilke, J. Organomet. Chem.. 329 (1987) 293. 45. D. J. Brauer, Η. Buerger, F. Dörrenbach, G. Pawelke and W. Weuter, J. Organomet. Chem., 378 (1989) 125. 46. H.-J. Frohn and V. V. Bardin, J. Organomet. Chem., 631 (2001) 54.

612 Main Group Metal Chemistry Vol. 25, No. 10, 2002

47. H.-J. Frohn and V. V. Bardin, Z. Anorg. Allg. Chem., 627 (2001) 15. 4«. H.-J. Frohn, A. Klose, Th. Schroer, G. Henkel, V. Buss, D. Opitz and R. Vahrenhorst, Inorg. Chem., 37 (1998) 4884. 49. H.-J. Frohn, F. Bailly and V. V. Bardin, Z. Anorg. Allg. Chem., 628 (2002) 723. 50. R. D. Wilson, W. Maya, D. Pilipovich and K. O. Christe, Inorg. Chem., 22 (1983) 1355. 51. J. J. Ritter, Τ. D. Coyle and J. M. Beilama, J. Organomet. Chem., 42 (1972) 25. 52. P. L. Timms, J. Am. Chem. Soc., 89 (1967) 1629. 53. J. Jander and H. Nagel, Lieb. Ann. Chem., 669 (1963) 1. 54. G. Pawelke, F. Heyder and H. Buerger, J. Organomet. Chem., 178 (1979) 1. 55. D. J. Brauer, Η. Buerger, R. Huebinger, G. Pawelke, Z. Anorg. Allg. Chem., 627 (2001) 679. 56. D. J. Brauer, G. Pawelke, J. Organomet. Chem., 604 (2000) 43. 57. D. J. Brauer, Η. Buerger, Y. Chebude and G. Pawelke, Inorg. Chem., 38 (1999) 3972. 58. D. Naumann, W. Tyrra, J. Chem. Soc., Chem. Commun., (1989) 47. 59. E. Vedejs, R. W. Chapman, S. C. Fields, S. Lin and M. R. Schrimpf, J. Org. Chem., 60 (1995) 3020. 60. H.-J. Frohn and V. V. Bardin, Z. Anorg. Allg. Chem., 627 (2001) 2499. 61. G. A. Molander and T. Ito, Org. Lett., 3 (2001) 393. 62. S. Darses, G. Michaud and J.-P. Genet, Eur. J. Org. Chem., (1999) 1875. 63. N. A. Petatis, A. K. Yudin, I. A. Zavialov, G. K. S. Prakash and G. A. Olah, Synlett, (1997) 606. 64. P. Fritzen and H.-J. Frohn, 13th Eur. Symp. on Fluorine Chemistry. Bordeaux (France), July 15-20 (2001), 1-P63. 65. H.-J. Frohn, N. Yu. Adonin and V. V. Bardin, Main Group Metal Chem., 24 (2001) 845. 66. H. Nöth and B. Wrackmeyer, Nuclear magnetic resonance spectroscopy of boron compounds. Springer, Berlin, 1978. 67. S. Berger, S. Braun and H.-O. Kalinowski, NMR-Spektroskopie von Nichtmetallen. Bd 4. ,9F-NMR- Spektroskopie. Thieme. Stuttgart, 1994. 68. E. J. Stampf and J. D. Odom, J. Organomet. Chem., 108 (1976) 1. 69. F. C. Nahm, Ε. F. Rothergy and Κ. Niedenzu, J. Organomet. Chem., 35 (1972) 9. 70. H.-J. Frohn and D. Welting, unpublished results. 71. H.-J. Frohn and A. Klose, unpublished results. 72. H.-J. Frohn, St. Jakobs and G. Henkel, Angew. Chem. Int. Ed., 28 (1989) 1506. 73. Ν. Walker and A. J. Leffler, Inorg. Chem., 13 (1974) 484. 74. P. Paetzold, A. Richter, Τ. Thijssen and S. Wuertenberg, Chem. Ber., 112 (1979) 3811. 75. J. R. Galsworthy, M. L. H. Green, V. C. Williams and A. N. Chernega, Polyhedron, 17 (1998) 119. 76. P. I. Paetzold and W. M. Simson, Angew. Chem., 78 (1966) 825. 77. W. Fraenk, Th. M. Klapötke, Β. Krumm, Η. Nöth, Μ. Suter, Μ. Warchhold, J. Chem. Soc., Dalton Trans., (2000) 4635. 78. W. Fraenk, Τ. M. Klapoetke, B. Krumm and P. Mayer, Chem. Commun., (2000) 667. 79. P. I. Paetzold, W. Scheibitz and E. Scholl, Ζ. Naturforsch., B26 (1971) 646. 80. Κ. Ishihara, Η. Kurihara and Η. Yamamoto, J. Org. Chem., 62 (1997) 5664. 81. R. D. Chambers, T. Chivers and D. A. Pyke, J. Chem. Soc., (1965) 5144. 82. V. C. Williams, G. J. Irvine, W. E. Piers, Z. Li, S. Collins, W. Clegg, M. R. J. Elsegood, and Τ. B. Marder, Organometallics, 19 (2000) 1619. 83. H.-J. Frohn and V. V. Bardin, Z. Anorg. Allg. Chem., 628 (2002) in press. 84. H.-J. Frohn and M. Theissen, Angew. Chem. Int. Ed., 39 (2000) 4591. 85. H.-J. Frohn and V. V. Bardin, Organometallics, 20 (2001) 4750. 86. H.-J. Frohn, M. Giesen, D. Welting and G. Henkel, Eur. J. Solid State Inorg. Chem., 33 (1996) 841. 87. F. Bailly, P. Barthen, H.-J. Frohn and M. Köckerling, Ζ. Anorg. Allg. Chem., 626 (2000) 2419. 88. H.-J. Frohn and F. Bailly, unpublished results. 89. L. Li, C. L. Stern and T. J. Marks, Organometallics, 19 (2000) 3332. 90. K. Koeler, W. E. Piers, A. P. Jarvis, S. Xin, Y. Feng, A. M. Bravakis, S. Collins, G. P. A. Yap and T. P. Marder, Organometallics, 17 (1998) 3557. 91. D. J. Parks, R. E. von H. Spence and W. E. Piers, Angew. Chem. Int. Ed., 34 (1995) 809. 92. Μ. Bochmann, S. J. Lancaster and Ο. V. Robinson, J. Chem. Soc., Chem. Commun., (1995) 2081. 93. H.-J. Frohn and V. V. Bardin, Z. Anorg. Allg. Chem., 628 (2002) 721. 94. H.-J. Frohn and V. V. Bardin, 13th Eur. Symp. on Fluorine Chemistry. Bordeaux (France), July 15-20 (2001), 1-P61. 95. H.-J. Frohn, S. G. Idemskaya and V. V. Bardin, Z. Anorg. Allg. Chem., 628 (2002) 883. 96. H.-J. Frohn and Th. Schroer, J. Fluorine Chem., 112 (2001) 259.

613