Arsenic, Antimony and Bismuth

13 Arsenic, Antimony and Bismuth J. L. WARDELL University of Aberdeen

13.1 INTRODUCTION 13. 1.1 History 13.1.2 General Features and Trends of the Group V Organometallic Compounds 13.1.3 Nuclear Properties 13.1.4 Naturally Occurring Organoarsenicals 13.1.5 Literature

682 682 682 684 684 684

13.2 PREPARATION OF ORGANOMETAL(III) COMPOUNDS 13.2.1 General Formation of Carbon-Element {111) Bonds 13.2.1.1 Direct synthesis 13.2.1.2 Arsenation 13.2.1.3 Transmetallation 13.2.1.4 Insertion or addition reactions 13.2.1.5 Reactions of organic halides with Group V alkali metal compounds 13.2.1.6 Formation from diazonium salts 13.2.2 Other Routes to Triorganometal(III) Species 13.2.2.1 Thermal decompositions oftetra- and penta-organometal( V) compounds 13.2.2.2 Reduction oftri- and tetra-organometal(V) compounds 13.2.3 Other Routes to Diorganometal(III) Compounds 13.2.3.1 Cleavage of triorganometal(III) compounds 13.2.3.2 Disproportionation reactions 13.2.3.3 Thermolysis of triorganometal{ V) dihalides 13.2.3.4 Reduction of diorganometal{ V) trihalides and arsinic and stibinic acids, Ar^M V{O)OH 13.2.3.5 From triorganoarsenic oxides and sulphides 13.2.4 Other Routes to Monoorganometal(III) Compounds 13.2.4.1 Disproportionation reactions 13.2.4.2 Cleavage of triorganometal(III) compounds 13.2.4.3 Reduction of arsonic and stibonic acids, ArM VO{OH)2, and also RM VX$ 13.2.4.4 Thermal decomposition of diorganometal(V) trihalides

684 684 684 684 685 686 687 687 687 687 688 688 688 689 689 689 689 690 690 690 690 690

13.3

PROPERTIES AND REACTIONS OF ORGANOMETAL(III) COMPOUNDS, R3M AND R K M X B - * (X = HALIDE) 13.3.1 Structure 13.3.2 Stability 13.3.2.1 Configurational stability 13.3.2.2 Oxidative stability 13.3.2.3 Thermal stability 13.3.3 Quaternary Salt Formation 13.3.4 Lewis Acid-Base Character 13.3.5 Cleavage of Triorganometal(III) Compounds

13.4

690 690 691 691 691 691 691 692 692

GROUP V HETEROBENZENES, C 5 H 5 M (M = As, Sb OR Bi)

692

13.5 PREPARATION OF ORGANOMETAL(V) COMPOUNDS 13.5.1 Monoorganometal( V) Compounds 13.5.2 Diorganometal(V) Compounds 13.5.3 Triorganometal(V) Compounds 13.5.4 Tetraorganometal(V) Compounds 13.5.4.1 Reactions of triorganometal(III) compounds with organic halides 13.5.4.2 Reaction of arenediazonium salts and triorganometal(III) compounds 13.5.4.3 Reaction of triorganometal( V) compounds with Grignard reagents 13.5.4.4 Cleavage of pentaorganometal( V) compounds 13.5.5 Preparation of Pentaorganometal(V) Compounds

693 693 693 694 694 694 694 695 695 695

13.6

695

PROPERTIES OF PENTAORGANOMETAL(V) AND ORGANOMETAL(V) HALIDES

681

682

Arsenic, Antimony and Bismuth

13.6.1 Structure 13.6.2 Stability 13.6.2.1 Configurational stability 13.6.2.2 Thermal stability 13.6.2.3 Oxidative stability 13.6.3 Lewis Acid Properties 13.6.4 Cleavage of Organo-Element Bonds in Pentaorganometal(V) Compounds 13.7

YLIDES, R 3 M—CH 2

695 696 696 697 698 698 698 698

13.8 GROUP V OXIDES, HYDROXIDES AND ALKOXIDES 13.8.1 Triorganometal(V) Oxides 13.8.2 Organometal(V) Alkoxides and Hydroxides /3.8.3 Arsinic and Stibinic Acids, R2M VO(OH) 13.8.4 Arsonic and Stibonic Acids, RM VO(OH)2 13.8.5 Organometal(III) Oxides and Alkoxides

699 699 700 700 700 701

13.9

701

ORGANOMETAL-SULPHUR BONDED COMPOUNDS

13.10 ORGANOMETAL HYDRIDES

701

13.11 ORGANOMETAL-NITROGEN BONDED COMPOUNDS 13.11.1 Organometal( V)-Nitrogen Bonded Compounds 13.11.2 Organometal(III)-Nitrogen Bonded Compounds 13.12 COMPOUNDS CONTAINING METAL-METAL BONDS 13.12.1 Element-Element Bonded Compounds 13.12.2 Element-Main Group Metal Bonded Compounds 13.12.3 Element-Transition Metal a-Bonded Compounds

702 702 702 702 702 703 704

REFERENCES

704

13.1 INTRODUCTION 13.1.1 History The first organoarsenic compound was probably obtained by L. C. Cadet de Glassicourt in 1760 as a product of the reaction between arsenic trioxide and potassium acetate, although it was not until the mid-19th century that the compound was established as Me2AsAsMe2. It was also in the mid-19th century that studies of organo-antimony and -bismuth chemistry were begun by Lowig and Schweizer. The discovery of the pharmacological activity of organoarsenicals in 1910 led to a rapid expansion of the work on arsenic derivatives and to a dominant position for arsenic in the organometallic chemistry of the Group V elements — a situation still effectively maintained today. In recent times less attention has been paid to chemotherapy roles and more to structures, stereochemistry and donor properties of these compounds.

13.1.2 General Features and Trends of the Group V Organometallic Compounds The data presented in Table 1 enable some comparisons to be made of the properties of the elements and their organic compounds. Bonds to the phenyl groups are stronger than the corresponding element-methyl bonds. As with bonds to carbon, so bonds to hydrogen (M—H) and element-element bonds (M—M) become progressively weaker in the sequence M = As, Sb and Bi. This sequence accounts for the extensive number of known arsenic-arsenic bonded compounds, including (AsR)^ ring compounds, in comparison to Me2BiBiMe2 being the only really well-defined bismuth derivative. As expected from the electronic configuration of the elements, the +III and +V oxidation states are most frequently met, with compounds having all organic or mixed organic-inorganic ligands. Compounds of the type R3M of all three elements are readily obtained; resolution of R1R2R3M111 (M = As or Sb) into optical isomers has been achieved; for the pentavalent organometallic compounds the ease of formation and stability is reported to be greatest for antimony. Halides, R n M i n X 3 _ n (n = 1 or 2, M = As, Sb or Bi) and R rt M v X 5 _ rt (n = 1, 2, 3 or 4, M = As or Sb; n = 3 or 4, M = Bi), have been prepared and well defined. Oxygen derivatives, such as R M m O , (R 2 M IH )2O, R 3 M V O, R 2 M V (O)OH and RM V O(OH), are only generally well established for M = As or Sb.

Arsenic, Antimony and Bismuth

683

Table 1 Comparative Data for Arsenic, Antimony and Bismuth and their Compounds Property

As

Element electronic configuration Electronegativities of elements Pauling (Allred-Rochow) Mean bond dissociation energies £>(Me—M) in Me 3 M (kJ mol" 1 ) Z)(Ph—M) in Ph 3 M (kJ mol" 1 ) Zero-point average Me—M • bond length (pm) in Me 3 M

10

Sb 2

[Ar]3
3

[Kr]4
Ref.

Bi 4

]O

2

[Xe]4f 5d 6s 6p

3

1.9(1.7) 1

238

224

140

280

267

200

197.5

216.5

225.9

2

2.63(0.193) 8.65

2.18(0.145) 8.48

1.82(0.120)

3 4

Force constants for the Me—M bond in Me3M (N cm"1) stretch (bend) First I.P. (eV) of Me 3 M

1. W. V. Steele, Annu. Rep. Prog. Chem., Sect. A, 1974, 71, 118; H. A. Skinner, Adv. Organomet. Chem., 1964, 2,49. 2. B. Beagley and A. R. Medwid, J. Mol. Struct., 1977, 38, 229. 3. H. Siebert, Z. Anorg. Allg. Chem., 1953, 273, 161. 4. S. Elbel, H. Bergmann and W. Ensslin, J. Chem. Soc, Faraday Trans. 2, 1974, 555.

In addition to element-carbon single-bonded compounds, there are a number of compounds having element-carbon double bonds, e.g. the heterobenzenes (1), the stability of which markedly decreases in the sequence M = As > Sb > Bi. For arsenic, species having isolated arsenic-carbon double bonds have also been characterized,1 e.g. PhAs=C(X)Bu l (X = OSiMe3 or N(R)SiMe3) and 2,4,6-Me 3 C 6 H 2 As=CPh 2 .

(1)

Compounds in the +III oxidation state are known having coordination number 2 (e.g. the heterobenzenes), 3 (e.g. R 3 M) and 4 (e.g. RSbCl^"), while in the +V oxidation state the coordination number can be 4 (e.g. R4M + X~, the arsonium, stibonium or bismuthonium salts, and ylides, R 3 M—CR' 2 ), 5 (e.g. R 5 M) and 6 (e.g. M+Ph 6 Sb-). In addition to the +III and +V oxidation states, others of much more limited stability and range have been reported; these are +1, +11 and +IV states. (a) + / Oxidation state, e.g. phenylarsanediyl, PhAs1, has been stabilized2 in the form of a complex with two Cr(CO)s units (Scheme 1). Cr(CO)6

PhAsH2

>

(CO)5CrAsPhH2

- ^

(CO)5CrAsPhLi2 ^ ^

{(CO)5Cr|2AsPh

Scheme 1 (b) + / / Oxidation state, e.g. Ph2M* (M = As or Sb), has been produced by UV or X-irradiation3 of Ph 3 M. In contrast to the transient nature of Ph2M% the hindered radical [(Me 3 Si) 2 CH]2As«, which is formed by photolysis of [(Me 3 Si) 2 CH] 2 AsCl in the presence of particular alkenes, has a half-life of greater4 than 1 month at 20 °C. (c) +IV Oxidation state, e.g. Ph4As% has been obtained by 60 Co 7-irradiation of Ph 4 As + in methanol 5 at 77 K, while Ph3AsCl has been formed by irradiation6 of a single crystal of Ph3AsMe+Cl".

684

Arsenic, Antimony and Bismuth

13.1.3 Nuclear Properties Both NQR spectra (involving 75As, 121Sb, 123Sb and 209Bi) and 121Sb Mossbauer spectra have been studied and have proved particularly useful in structure determinations.

13.1.4 Naturally Occurring Organoarsenicals The water-soluble arsenobetaine Me3AsCH2CO^ has been isolated both from the tail of the western rock lobster7 and from the muscle and liver of a shark, Prionace glaucus.s

13.1.5 Literature Monographs concerned with the general organometallic chemistry of arsenic, antimony and bismuth have been written by Aylett9 in 1979, Samaan 10 in 1978 and Doak and Freedman1 * in 1970. In addition, Cullen12 in 1966 reviewed the organic chemistry of arsenic, Poller13 that of antimony and bismuth compounds (in 1979) and Wieber14 of organobismuth species (in 1977). Other relevant publications are (i) a comprehensive compilation up to 1968 of organo-arsenic, -antimony and -bismuth compounds,15 and (ii) an annual series of reports of the current literature, 16 In addition to these general publications there are a number of more specialized accounts. Some topics covered are stereochemistry of organoarsenic compounds,17 heterobenzenes,18 and other heterocyclic derivatives,19 cyclopolyarsines,20 and the vibrational spectra of Group V organometallic compounds.21

13.2 PREPARATION OF ORGANOMETAL(III) COMPOUNDS 13.2.1 General Formation of Carbon-EIement(III) Bonds 13.2.1.1 Direct synthesis Organometal(III) compounds have been prepared from the element and organic halides. Thus heating arsenic or antimony with halides RX (R = alkyl, phenyl or vinyl), particularly in the presence of copper,22 provided mixtures of RrtMX3_M (M = As or Sb), e.g. equation (1). Perfluoromethyl iodide23 also reacts with As or Sb to give mixtures of products (equation 2), although with antimony the compound (CFs^Sb is the major product. For the most metallic element, bismuth, electrochemical methods have been developed, e.g. electrolysis of RI (R = Me or CNCH 2 CH 2 ) using a bismuth cathode.24 MeBr CF3I

+

As - ^

+

As

(CF3)3As

-£+• Me2AsBr +

(CF3)2AsI

+

MeAsBr2 + (CF3)AsI2

(0 +

Asl3

(2)

13.2.1.2 Arsenation Aryl-arsenic bonds may be formed from reaction of arenes with arsenic halides or oxygen derivatives. Such reactions are examples of electrophilic aromatic substitution and work particularly well with arenes bearing hydroxyl or amino substituents (equation 3). 25 For less reactive arenes, e.g. benzene,26 heat and a Lewis acid catalyst, such as AICI3, are necessary for reaction. Heterocyclic compounds may be prepared by related means (equations 4, 5). Similar reactions apparently do not occur27 with SbX3.

Arsenic, Antimony and Bismuth R

2N~\O/ \

+

AsCl3

( R 2 N ^(Oy~) AsC13-«

~*

/

685

\

\

/

n = 1 or 3

In

(3)

H

Ph2NH

+ AsCl3 —* f O T T O I

(4)

I

Cl

Cl 2

I Cl

13.2.1.3 Transmetallation By far the most extensively used preparative methods are reactions involving Grignard reagents or organolithium compounds and the metal halides in ethereal solvents.28 Derivatives of all three elements can be synthesized by these routes (equations 6, 7). Under controlled conditions and/or with sterically hindered Grignard or lithium reagents, reaction of MX3 can be stopped before complete substitution (equations 8,29 9 30 ). Mixed triorganometallic (equation 10)31 and dimetallated compounds (equation II) 3 2 are among the types of compound obtainable from Grignard and lithium reagents. In addition to metal halides, metal-oxygen and metal-sulphur bonded compounds have been used, e.g. equation (12). 33

3MeMgI

+

3PhLi

MC13 — •

+

MCI3

BufMgCl

—

Me3M Ph3M

+

SbBr3

1

:

1

Bu MgCl

+

SbCl3

2

:

1

l

(0-PhC6H4)(/>-MeC6H4)SbCl

+

M = As, Sb or Bi M = As,SborBi

"50°C>

-^^

PhMgCl

B^SbBrz

F

(9)

—*- (o-PhC6H4)(p-MeC6H4)PhSb

F

F

lor

/y^(F

CI3 — j n r / j o e

F

(10)

F

+ M

jpc

(7) (8)

Bu^SbCl

F

(6)

F

(n)

F

M = As, Sb or Bi /

bx

V

+

EtMgBr

-^

PhAsEt2

(12)

As"°

I

Ph

Routes using organozinc reagents were taken in earlier times but are scarcely followed now. Other electropositive metal compounds have been employed recently: these include organoaluminium reagents, e.g. equation (13).34 Use has also been made of sodium in Wurtz-style reactions, e.g. equation (14). 35 ' 36

686

Arsenic, Antimony and Bismuth LiAlEt4

+

MX 3 — • Et3M

Z1

7

(CH2)IIC1

(13)

/(CH2)«-1

Na/THF

RAs

M = As, Sb or Bi

> RAs / CH 2

R = Me or Ph; n = 3 or 4

(14)

n = 3; 1-R-arsetane n = A\ 1-R-arsenane

Derivatives with less electropositive metals can also be used.18'37'38 Thus transfer from mercury, silicon, tin or lead oforganic groups such as alkyl, alkenyl or alkynyl as well as functionally substituted groups has led to a variety of organo-arsenic, -antimony and -bismuth derivatives (equations 15-17).

MeMCl2

+

MC13

Ph(CH2)2Hg +

— • MeM(CH2Ph)Cl

CpSiMe3

O |

— - CpMCl2

M = As or Sb

M = AsorSb

(16)

+ MX3 — ( M l

Bu 2

(15)

(17)

I X M = As. Sb or Bi

13,2,1.4 Insertion or addition reactions Several types of reaction fall into this category. (a) Arsenic and antimony hydrides39"42 will add tocarbon-carbon multiple bonds (equations 18-20). In a number of cases, base catalysis has been necessary. In contrast to equations (18) and (19), the reaction of Me2AsH with CF3C=CH was neither regio- nor stereo-specific.42 Addition of metal hydrides to carbonyl compounds also occurs.

Me2AsH Ph2SbH

+ CF 3 CF=CF 2 +

PhC=CH

10

° °C>

Me2AsCF2CFHCF3

— • /ra«5-Ph2SbCH=CHPh

(18) (19)

Ar

HC=C-C(R)OMeC=CH

+

ArAsH2

H 1Jrg¥ n^>

.As

|

}

(20)

R OMe

(b) Arsenic halides add to carbon-carbon multiple bonds;43'44 examples of both Lewis acid catalysis and photochemical initiation are known (equations 21-23). (c) Diarsines add tomultiple bonds under irradiation orinthe presence ofa radical initiator;45 the reactions are not stereoselective (equation 24). (d) Reaction of diazoalkanes with metal-halogen bonds provides halomethyl derivatives of arsenic and antimony (equation 25).46

Arsenic, Antimony and Bismuth

AsCl3

+

AsF3

HC=CH

+

^ ^

CF 2 =CF 2

687

(ClCH==CH)ttAsCl3-w

J^ave*

(CF3CF2)/?AsF3-/?

/i=l,2or3 n = 2 and 3

Me2As Me2AsCl

+

CF 3 C=CCF 3

C=C^

F3C

R 4 As 2

AsCl3

+

PhC=CH

+

CH 2 N 2

- j ^

-^^

(22)

Cl X

-^^

(21)

(23) CF3

(£)- and (Z)-R 2 AsCPh=CHAsR 2

(ClCH 2 ) n AsCl 3 - lf

n= 1,2 or 3

(24)

(25)

13.2A.5 Reactions of organic halides with Group V alkali metal compounds As well as being used for the formation of monometallated compounds, these reactions have been frequently used in preparation of dimetallated compounds, useful chelating ligands in transition metal complexes (equations 26, 27). 47 ' 48

...

„.

.

,

x 2Me2SbNa

2Ph2AsK

^ \

+ +

|V~VT

.Br

[QT

C1(CH2)WC1

liquid NH3

—

dioxane

>

^ ^ ^ .SbMe2

•

(r\\

[Ml

Ph2As(CH2)/7AsPh2

(26) (27)

13.2.1.6 Formation from diazonium salts The reaction of an arenediazonium salt with MX3 is a source of organoelement(III) compounds. This is a particularly useful reaction for bismuth. Various recipes have been advocated; the best for Ar3Bi involves diazonium tetrafluoroborates.49 13.2.2 Other Routes to Triorganometal(III) Species These methods have organometal(V) compounds as starting materials. 13.2.2.1 Thermal decompositions of tetra- and penta-organometal( V) compounds Heating either covalent or ionic tetraorganometal(V) compounds, R4MX, can produce triorganometal(III) derivatives (equations 28, 29 and Scheme 2).5O~55 While several of these reactions lead to trivial products or products more readily prepared by other routes, others provide compounds not easily obtained in other ways. As Scheme 2 shows,51 these thermal reactions can be used as steps in overall conversions of one triorganometallic(III) derivative to another; this has proved particularly useful for arsenic. Both non-radical and radical mechanisms have been proposed for the various decompositions of R4MX. However, not all R4MVX compounds provide R 3 M m on heating; thus while reaction of R4SbX (X = Br,52 I,53 OR 54 and SR 50 ) all yield R3Sb, Ar4Sb v OH thermally decomposes55 to Ar3Sb v O and ArOH in a radical chain reaction. Pentaorganometal(V) compounds similarly can provide triorganoelement(III) derivatives;56 thus PI15M (M = As, Sb or Bi) on heating gives PI13M. More elaborate thermal decompositions are illustrated in Scheme 3.

688

Arsenic, Antimony and Bismuth Me4As+I~ Ph4SbSPh

—•

Ph3Sb

T ^^.AsMe2

+

foT

AsMe2

+

—*•

[Ph2S Rr y

MeaAs +

+

Ph2S2

Mel +

PhH

Me

1

-^ f o r +

9 *)2 Br

J

+

Ph2

+

• • •]

(29)

2

^^JKs

( H

(28)

i 2Br "

As Me2 A

i,(CH 2 Br) 2 ii, A

Scheme 2

As—C6H4Ph-o

R = Me orPh

or Bu

Scheme 3

13.2.2.2 Reductions oftri- and tetra-organometal( V) compounds Reduction of R4MX (by hydride reagents), 57 of R3MX2 (by zinc, aluminium or boron hydrides, bisulphite or hydrazine 58 ) and of R3MO 5 8 all provide R3M compounds.

13.2.3 Other Routes to Diorganometal(III) Compounds 13.2.3.1 Cleavage of triorganometal(III)

compounds

HI

Hydrogen halides cleave R — M bonds, with aryl groups being more readily removed than alkyl (equation 30). 5 9 The ease of this electrophilic cleavage is in the sequence M = Bi > Sb > As. For trialkylbismuth, RaBi, cleavage even by halogens, X2, to give R2BiX can take preference 60 to addition of the halogen which would have provided RaBiX2; for trialkyl-arsines and -stibines and for all triaryl derivatives, addition of halogen normally results. However, as thermolysis of R3MX2 leads 30 ' 61 to R 2 M X (M = As, Sb or Bi), reaction of R 3 M (M = As, Sb or Bi) with halogens can still be used to prepare R2MX for all three elements, even if a two-step procedure is necessary (Scheme 4).

Me2AsPh R3M

+

X2

- ^

—•>

Me2AsI

R3MX2 —*Scheme 4

+

Phi

R2MX

(30) +

RX

Arsenic, Antimony and Bismuth

689

Other electrophilic species, including interhalogens and metal halides such as HgCb and MC13 (M = As, Sb or Bi, see Section 13.2.3.2), also cleave R—M bonds. As well as cleavage by electrophilic species, cleavage can occur by nucleophiles, especially of electron withdrawing groups, e.g. cleavage of trifluoro- and alkynyl-arsenic bonds by hydroxide ion,23a'62 and by radicals, e.g. cleavage of R 3 M (M = Sb or Bi; R = Me, Et or Ph) in reaction with benzenethiol. However, in the latter reaction,63 two R groups are normally cleaved.

13.2.3.2 Disproportionation reactions Reaction of R3M with MX3 (M = As, Sb or Bi) in a 2:1 mole ratio theoretically should lead to R2MX (equation 31). Examples can be found where this occurs;64'65 however, in a number of cases a mixture of products — difficult to separate — attains and in others the major product is not the anticipated one, e.g. the product from (o-MeC6H4)3Bi and BiBr3 (2:1 mole ratio) is (o-MeC 6 H 4 )BiBr 2 . 2R 3 M

+

MX 3

—•

3R2MX

(31)

13.2.3.3 Thermolysis of triorganometal(V) diha I ides This was mentioned in Section 13.2.3.1. It has been established that the ease of departure of R from R 3 AsX 2 (equation 32) is in the sequence R = Ph < alkyl < CF 3 < PhCH 2 , 61 and that the thermal stability of Ar3BiX2 decreases in the order X = F > Cl > Br > I.66 At the temperatures employed, some disproportionation of R2MX may result. R3MX2

-^

R 2 MX

+

RX

(32)

13.2.3.4 Reduction of diorganometal( V) trihalides and arsinic and stibinic acids, ArzM V(O)OH Both types of compound are readily reduced67 by tin(II) dichloride and sulphur dioxide in hydrochloric acid (equations 33, 34). R 2 MX 3 Ar 2 M(O)OH

—+

R2MX

—*

(33)

Ar2MX

(34)

13.2.3.5 From triorganoarsenic oxides and sulphides Russian workers68 have shown that R3ASY (Y = O or S) gives R2As m YR / on reaction with alkyl halides (equation 35). Acyl halides, RCOX, also react with R3ASY to give R 2 AsYOCR. EtPh2AsO

+

RX

—* [Ph2EtAsYR]+X-

—> Ph2AsYR

(35)

690

Arsenic, Antimony and Bismuth

13.2.4 Other Routes to Monoorganometal(III) Compounds 13.2.4.1 Disproportionation reactions As stated in Section 13.2.3.2, disproportionation reactions in theory and practice may be quite different. However, in this category there are a number of useful reactions69 for the preparation of RMX2 (equation 36). R3M

+

2MX3

—*• 3RMX2

(36)

13.2.4.2 Cleavage of triorganometal(IH) compounds It is possible to cleave two groups from triorganometal(III) compounds under specific conditions (equations 37, 38; 59 ' 63 see Section 13.2.3.1). Ph2MeAs Ph3Bi

+

+

HI

PhSH

—* MeAsI2 —•

PhBi(SPh)2

(37) (38)

13.2.4.3 Reduction ofarsonic and stibonic acids, ArM vO(Off)2, and also RM v Xt Reducing agents available for the reduction67'70 of ArM v O(OH)2 and RMVX4 compounds include SO2 and SnCl2, both in the presence of hydrochloric acid (equations 39,40). The reduction of ArM v O(OH)2 is particularly useful since these acids are readily prepared from diazonium salt-MC^ reactions (see Section 13.5.1). As most RMX4 compounds are not thermally stable and are usually prepared from RMO(OH)2, there appears to be little advantage in using RMX4 in preference to RMO(OH)2 if a choice is possible. Ar

ArMVO(OH)21 ^ ~ T

RMCU

MIXHCI'

MCl 2

RMC 2

'

(39) (40)

13.2.4.4 Thermal decomposition of diorganometal( V) trihalides Both arsenic and antimony compounds have been obtained by thermolysis of R2MX3 compounds (equation 41). R2MX3

-+

RMX2

+

RX

(41)

13.3 PROPERTIES AND REACTIONS OF ORGANOMETAL(III) COMPOUNDS, R3M AND R n MX3_ n (X = HALIDE) 13.3.1 Structure It has been established, by a variety of techniques, including X-ray crystallography, that the structures of R3M111 (R = alkyl and aryl) are pyramidal,71 with the average valence bond angle becoming smaller (^90°) from arsenic to bismuth, e.g. /CMC in (p-CXC^H^M is 102, 97 and

Arsenic, Antimony and Bismuth

691

93° for M = As, Sb and Bi, respectively.72 Various RWMX3_« (X ^ F), e.g. Me2AsX, Ph2AsX and MeAsl2, also have pyramidal structures,73 in contrast to the fluorine-bridged trigonal bipyramidal Ph 2 SbF. 74

13.3.2 Stability 133.2.1 Configurational stability The barriers to inversion at arsenic and antimony are sufficiently high to enable resolution of asymmetric compounds to be made;75 indeed, the barriers are higher for As than for P, e.g. the barrier to inversion of MeEtPhAs111 is 177 kJ mol" 1 , compared to ca. 140 kJ mol"1 for MeEtPhP111.76 No racemization of Ph(l-naphthyl)(/?-HO 2 CC 6 H 4 )Sb m occurred even after refluxing in p-xylene for 2 h.77 Routes to resolution of asymmetric compounds have included crystallization78 of internal diastereoisomeric complexes, e.g. [PtCl((±)-MeEtPhAs}|(—)-stilbenediamine}]Cl, and salts, e.g. [(±)-p-MeC6H4(H 3 NC 6 H4)EtAs, (H-)-tartrate], as well as reduction of chiral arsonium salts. As with other main group cyclopentadienyl derivatives, Cp—As and Cp—Sb exhibit fluxionality, namely metallotropic 1,2-migrations.38'79

13.3.2.2 Oxidative stability All the trialkylmetal(III) compounds are unstable to oxygen. Some differences in products have however been recognized. On standing in air in solution, Et3As m gave80 diethylarsinic acid, Et 2 As v O(OH). Other R2R/As (R = alkyl) reacted similarly, unlike Ph 2 MeAs m which was reported to give Pli2MeAsvO. Both trialkyl-stibine and -bismuth derivatives inflame in air. However, controlled oxidation of EtsBi at —50 °C yielded Et2BiOEt;81 products from trialkylstibine have not been well-defined so far. The triarylmetal compounds, Ar3M m , are much more stable to air and oxidation by oxygen, but oxidation to Ar3M v O does occur with suitable oxidizing agents, e.g. peroxides, with Ph^Sb111 being more easily oxidized than is PlvjAs111.82 Various other reagents, including halogens (see Section 13.5.3) and free radicals, e.g. (CF3)2N—O« (equation 42), 83 can also oxidize R3M111 to metal(V) derivatives. (C 6 F 5 ) 3 M m

+

2(CF 3 ) 2 N-O —

(C6F5)3MV{ON(CF3)2)2

(42)

M = As or Sb

13.3.2.3 Thermal stability Ipatiev and Rasuwajew84 established from decomposition temperatures that the thermal stability of PI13M falls in the order As > Sb > Bi. A similar sequence holds for alkyl derivatives.

13.3.3 Quaternary Salt Formation This is discussed in some detail in Section 13.5.4.1. The rates of reaction of Mel and Me3M (M = As or Sb) in methanol solution (equation 43) clearly indicate the greater reactivity of the arsenic compound.85 In general, quaternization of (alkyl)3_/t(aryl)/7M becomes increasingly difficult in the sequence n = 0, 1, 2 and 3.

R3M

+

R'X

—*

R3RrM+X-

(43)

692

Arsenic, Antimony and Bismuth

13.3.4 Lewis Acid-Base Character Triorganometal(III) compounds normally act as Lewis bases or donors (soft or Class b donors) and a truly vast array of complexes, particularly with transition metal species, is known. Sequences of donor ability have been established from thermodynamic data. Towards AgClC>4 in DMSO at 25 °C the relative complexing abilities of PI13M, based onenthalpy data as well as equilibrium constants, decreased as M = (P > ) As > Sb > Bi( > N). 8 6 The enthalpies of interaction of MesM with the main group acceptors BX3 (X = F or Cl) were also 87 in the order M = (P > ) As > Sb. 87 Alkyl derivatives are stronger donors than aryl derivatives. Interaction with proton acids shows that R3M can also act as Br^nsted bases. Thus conductivity measurements in liquid hydrogen chloride suggest equilibrium (44). 88 Ph3As

+

2HC1 = ^ Ph 3 AsH + HCL 2 -

(44)

While R3M normally act as donors, examples have been provided of their ability to act as acceptors, e.g. with C6Me6.89 When R in R3M is a strongly electron-withdrawing group, reasonably strong Lewis acids can beobtained; thus complexes of (CF3)3M (M = As or Sb) with pyridine have been isolated. 90 Progressive replacement of organic groups by halides in R3M leads to stronger Lewis acids.

13.3.5 Cleavage of Triorganometal(III) Compounds As mentioned in Sections 13.2.3.1 and 13.2.4.3, cleavage of organo-metal bonds in R3M111 compounds canoccur on reaction with electrophiles, free radicals andbases, including alkali metals.

13.4 GROUP V HETEROBENZENES, C5H5M (M = As, Sb OR Bi) 18 All three heterobenzenes have been prepared asshown in Scheme 5. Various routes have now been established forthe preparation of substituted derivatives, especially forarsenic. These heterobenzenes have properties which satisfy the usual conditions for aromaticity, e.g. diamagnetic ring current, planarity and equal carbon-carbon bond lengths. In addition, electrophilic substitution by MeCOCl has been reported with arsabenzene.

(HC=C)2CH2 -U kf^\

si/

JU kf ^ ] JiU (P*) k

Bu2

ivr

ivr

I X i, Bu2SnH2; ii, MX 3 (M = As, Sb or Bi); iii, DBU (-HX) Scheme 5

The thermal stability of (1) markedly decreases in the sequence M = As > Sb > Bi. In the case of bismabenzene, forwhich a dimer exists at low temperature, isolation has not yet been achieved but it can bedetected spectroscopically and can betrapped, e.g. by CF3C=CCF3 (equation 45). The rates of reaction of (1) with CF3O=CCF3 to give heterobarrelenes are in the sequence M = (P < ) As < Sb
Arsenic, Antimony and Bismuth

|

J

+

CF3CSCCF3 - >

693

/ ^ - /

M

*

(45)

//MX-CF3

(1) M = P, As, Sb or Bi

13.5 PREPARATION OF ORGANOMETAL(V) COMPOUNDS 13.5.1 Monoorganometal(V) Compounds The most readily available monoorganometal(V) species are the arsonic and stibonic acids, RMVO(OH)2- Several named procedures can be used for arsonic acids (equations 46-50),92 while for stibonic acids the Bart reaction and a modification of the Scheller reaction are usually used. Bart Scheller

ArN2+X" ArN2+X"

+

Bechamp

+

As(ONa)3

AsCl3

PhX

+

+

—* ArAsO(ONa)2

CuCl

H3As04

+

H2O

—•

ArAsO(OH)2

—• p-XC6H4As0(0H)2

(46) (47) (48)

X = NH2 or OH Meyer

RX

+

As(ONa)3

+

H2O —*

RAsO(OH)2

(49)

R = alkyl Rosenmund

ArX

+

As(ONa)3

+

Cu11

+

H2O

—• ArAsO(OH)2

(50)

The arsonic and stibonic acids can be converted to thermally unstable arylmetal tetrahalides, e.g. on reaction with concentrated hydrochloric acid or with SF4.93 Another preparation of tetrachlorides is by oxidation of RM ni Cl2 (M = As or Sb) using chlorine.

13.5.2 Diorganometal(V) Compounds Diorganometal(V) trihalides, R 2 M V X 3 (M = As or Sb; X ^ I), have been prepared by reaction of halogen94 with R 2 M m X (R = alkyl or aryl; equation 51) (as well as with R 2 M m H and R2MMR2), by reaction of hydrogen halide with arsinic and stibinic acids, R2MVO(OH) (equation 52), or by reactions of diazonium salts 95 ' 96 with SbCb or SbCls. Ph2Mn'X R2MVO(OH)

+

+

X2

—•

3HX —-

Ph2MvX3 M = AsorSb

R2MX3

+

2H2O M = AsorSb

(51) (52)

Arsinic and stibinic acids, ArAr'M v O(OH), may be prepared from arenediazonium salts, ArNjX", and Ar r MX 2 (X = halide or OH) in modified Meyer, Rosenmund, Bart or Scheller reactions (equation 53). In addition, symmetric R2MO(OH) are often obtained as by-products in the preparation of arsonic and stibonic acids.92 ArN2+X~

+

Ar'MX2 - ^ V

ArAr'MO(OH)

M = As or Sb

(53)

694

Arsenic, Antimony and Bismuth

13.5.3 Triorganometal(V) Compounds Triorganometal(V) dihalides, R 3 M V X 2 (R = aryl, M = As, Sb or Bi; R = alkyl, M = As or Sb), are most readily prepared by addition of halogens to R3M111 (equation 54); even fluorine, in very dilute solution, has been used.97 For trialkylbismuth, however, cleavage of an alkyl-bismuth bond by halogens occurs in preference to addition (see Section 13.2.3.1). Other reagents used have included PhIF 2 , 93 XeF 2 93 and SF 4 98 (for formation of R 3 MF 2 ) as well as SO2C12, PbCl4, CuCl 2 , and T1C13 (for chlorides).99 R3Mm

X2 —* R3MVX2 X = F,C1, Brorl

+

(54)

13.5.4 Tetraorganometal(V) Compounds 13.5.4.1 Reactions oftriorganometaI{III) compounds with organic ha Iides The rates of reaction of triorganometal(III), R3M, and alkyl halides, R'X, are in the sequence M = (P >) As > Sb » Bi, with RsBi being extremely unreactive. All trialkyl- and triaryl-arsines react readily with alkyl halides,100 with the reactivity of (aryl) n (alkyl) 3-n As decreasing as n increases (this sequence also holds for stibines). As expected for 5^2-type reactions,101 methyl halides are more reactive than ethyl halides and, generally, for R'X the reactivity decreases as X = I > Br > Cl. Aryl and vinyl halides react on heating in the presence of aluminium chloride.26 R3M

+

—*- R3RrM+X-

R'X

(55)

Trialkyl-, but not triaryl-, stibines react with alkyl halides.102 However, from triarylstibines, stibonium salts can be obtained on reaction with the more reactive trimethyloxonium tetrafluoroborate (equation 56). Treatment of the stibonium tetrafluoroborate with KI, for example, produces the stibonium iodide.103 Ph3Sb

+

Me3O+BF4

—* Ph3MeSb+BF4

(56)

For tetraarylstibonium halides, a route similar to that used for tetraarylarsonium halides is available (equation 57). 104 Ph3Sb

+

PhCl

+

AlCb —*

Ph4SbCl

(57)

13.5.4.2 Reaction of arenediazonium salts and triorganometal{IIt) compounds A route to tetraarylstibonium salts is the copper catalysed reaction of arenediazonium tetrafluoroborate with ArsSb (equation 58); conversion of the stibonium tetrafluoroborate to the halide can be made. This method has been used to obtain unsymmetric compounds.105

Ph3Sb

+

/?-MeC 6 H 4 N 2 + BF 4 -

-^>

Ph 3 (/?-MeC 6 H 4 )Sb+BF 4 -

(58)

Arsenic, Antimony and Bismuth

695

13.5.4.3 Reaction of triorganometaI(V) compounds with Grignardreagents Both triorganometal(V) dichlorides and oxides react with Grignard reagents to provide arsonium and stibonium salts (equations 59, 60). 106 Ph3SbCl2 Ph3AsO

+ +

PhMgX PhMgX

—• —-

Ph4SbX

(59)

Ph4AsX

(60)

13.5.4.4 Cleavage ofpentaorganometal(V) compounds Cleavage of pentaorganometal(V) compounds to give tetraorganometal(V) salts has been achieved by, among other reagents, halogens, hydrogen halides and other Br^nsted acids, triphenylboron and mercury chloride (equations 61-63). l07 Ph5M Ph5M

+

+

HA — Ph5M

X2

—

Ph4MA +

Ph4MX +

PhX

( 61 )

PhH e.g. A = RCO2 and halide

(62)

Ph3B —-

+

Ph4M+BPh4

(63)

13.5.5 Preparation of Pentaorganometal(V) Compounds Pentaorganometal( V) compounds have been obtained by reaction of organolithium reagents with R 3 MX 2 , R4MX and R 3 M=NSO 2 Ph (M = As, Sb or Bi).108 For antimony, Li+SbR^ species which are produced, and which can be isolated, have to be decomposed by addition of water to give RsSb. 108 Use has been made of Ar 4 SbF compounds. Thermal decomposition of the organoantimony sulphonates, R4SbC>2SAr, also provides pentaorganoantimony compounds (equation 64). 109

R4SbO2SAr

-^

R4ArSb

+

SO2

(64)

13.6 PROPERTIES OF PENTAORGANOMETAL(V) AND ORGANOMETAL(V) HALIDES 13.6.1 Structure For RsAs (R = Me and Ph), trigonal bipyramidal structures have been indicated from vibrational spectral studies;110 PI15AS has the same structure in solution. This is also the case for MesSb Table 2

Details of the Structures of Pentaarylantimony Compounds, obtained by X-ray Crystallography

Compound Ph5Sb

Structure square pyramid

Details Sb—C ax

211.5 pm; zCax—Sb—CbaSai

Sb—Cbasal 221.6 pm; ZCbasal—Sb—Cbasal

Ph5Sb-cyclo-C6Hi2 (/7-MeC6H4)5Sb

trigonal bipyramid Sb—C ax Sb—C eq trigonal bipyramid Sb—C ax Sb—C eq

Ref. 96.4-106.0°

1

86.7-88.6°

223.6-225.1 212.0-215.0 223.8-225.4 214.5-218.0

1. A. L. Beauchamp, M. J. Bennett and F. A. Cotton, J. Am. Chem. Soc, 1968, 90, 6675. 2. C. Brabant, B. Blanck and A. L. Beauchamp, J. Organomet. Chem., 1974, 82, 231. 3. C. Brabant, J. Hubert and A. L. Beauchamp, Can. J. Chem., 1973, 51, 2952.

2 3

696

Arsenic, Antimony and Bismuth Table 3

Structures of Organometallic(V) Halides Structure

Ref.

Ionic:a tetrahedral Me4As+ Ionic:a tetrahedral Ph 4 As + Bridging fluorines:a polymeric chains: six-coordinate Sb Trig. bipy:a 2Ph and Me equatorial Trig. bipyb

1 2 3 4 5

Trig. bipy:a Cl axial Ionic:a Me 3 AsBr + Trig. bipy:a discrete molecules Trig. bipy:a X axial

6 6 7 5,8

Trig. bipy:a X axial (X = Cl) a (X = F, Cl, Br and I) b Trig. bipy:a Cl axial Two forms: (1) covalent with bridging Cl: Sb—Cl 235.5 pm (terminal) Sb—Cl 280.0 pm (bridging) Sb—C av 213.3pm (2) ionic: Me4Sb+SbCl^ Sb—C av 211.5 pm Sb—Cl 238.2 pm Dimer:a bridging Cl Ph axial: Cl equat. Weakly bridging Bra

5,9

Compound Me4AsBr Ph4AsI Me 4 SbF Ph 3 MeSbF Ph4SbX (X = F, Cl or Br) Me3AsCl2 Me3AsBr2 Ph3AsF2 Me 3 SbX 2 (X = F, Cl, Br and I) Ph 3 SbX 2 (X = F, Cl, Br and I) Ph3BiCl2 (Me2SbCl3)2

(Ph 2 SbCl 3 ) 2 Ph2SbBr3 a

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

10 11

12 13

b

X-ray crystal structural data. Mossbauer spectral data. E. Collins, D. J. Suter and F. G. Mann, / . Chem. Soc. (London), 1963, 4051. R. C. L. Mooney, J. Am. Chem. Soc, 1940, 62, 2955. W. Schwarz and H. J. Guder, Z. Anorg. Allg. Chem., 1978, 444, 105. J. Bordiner, B. C. Andrews and G. G. Long, Cryst. Struct. Commun., 1976, 5, 804. J. N. R. Ruddick, D. Sams and J. C. Scott, Inorg. Chem., 1974,13, 1503; G. G. Long, J. G. Stevens, R. J. Tullbane and L. H. Bowen, J. Am. Chem. Soc, 1970, 92, 4230. M. B. Hursthouse and I. A. Steer, / . Organomet. Chem., 1971, 27, Cl 1. A. Augustine, G. Ferguson and F. C. Marsh, Can. J. Chem., 1975, 53, 1647. A. F. Wells, Z. Kristallogr., 1938, 99, 367. T. N. Polynova and M. A. Poraikoshits, Zh. Strukt. Khim., 1966,12, 742. D. M. Hawley and G. Ferguson, J. Chem. Soc. (A), 1968, 2539. W. Schwarz and H. J. Guder, Z. Naturforsch., Teil B, 1978, 33, 485. J. Bordner, G. O. Doak and J. R. Peters, Jr., / . Am. Chem. Soc, 1974, 96, 6763. S. P. Bone and D. B. Sowerby, J. Chem. Soc, Dalton Trans., 1979, 718.

(from the vibrational spectrum),111 (/?-MeC6H4)5Sb112 and the solvate PhsSb-icyclohexane113 (the latter two by X-ray crystallography); (p-MeC^H^Sb also has the same structure in solution. Unsolvated solid PhsSb, however, has a square pyramidal structure,114 which persists in solution in methylene chloride. As no specific interactions exist between the cyclohexane and PhsSb molecules in the solid solvate, the small lattice energy differences between PhsSb-^cyclohexane and unsolvated PhsSb crystals are sufficient to change the stereochemistry. Some features of these structures are listed in Table 2. For the halides Rs-^MX^, a variety of structures has been identified (see Table 3) involving four, five and six coordinate metal.

13.6.2 Stability 13.6.2.1 Configurational stability Even though trigonal bipyramidal structures occur in solution, the methyl groups in MesM (M = As 110 or Sb115) and in (p-MeC 6 H 4 ) 5 M (M = As or Sb) 116 have been shown to be equivalent by ] H NMR spectroscopy even at low temperatures, e.g. at —100 °C for MesSb. These equivalences are due to rapid exchanges of the methyl and tolyl groups between axial and equatorial positions, involving square pyramidal intermediates or transition states.

Arsenic, Antimony and Bismuth

697

Mixed Me r t Et5_ n Sb are also fluxional molecules, 117 with rapid alkyl exchanges occurring even at —80 °C. For more rigid molecules, such as the trigonal bipyramidal arsorane and stiborane (2), higher temperatures are necessary for equilibration of the methyl groups; 118 the free energies of activation of the pseudorotational process in bromobenzene are 72.2 and 64.6 kJ mol" 1 for M = As and Sb, respectively. The exchange process of (2) involves trigonal bipyramidal transition states with diequatorial biarylylene groups.

(2) M = As or Sb Arsonium salts, e.g. M e E t P h ( P h C H 2 ) A s + X ~ , 1 1 9 and stibonium salts, e.g. (MeEtPr PhSb) + X~, 1 2 0 have been resolved, resolutions being via crystallization of diastereoisomeric salts of optically active anions. In addition, formation of arsonium salts from asymmetric arsines occurs with retention. Racemization of arsonium perchlorate and sulphate has been found to occur less readily than of halides. 121 19 F N M R spectroscopy indicates that rapid fluorine exchange occurs in Ph 2 AsF 3 even at - 9 0 °C. 1 2 2 1

13,6.2.2 Thermal stability All pentaorganometal(V) and organometal(V) halides decompose on heating, as referred to in sections concerned with the formation of organometal(III) compounds. At about 100 °C, MesAs decomposes to trimethylarsine, methane and ethylene, possibly via an ylide intermediate. The thermal stability of PI15M is in the sequence Ph 5 As > PhsSb > Ph 5 Bi. A non-radical intramolecular mechanism has been indicated 108 ' 123 for PhsSb, with the products being Ph 3 Sb and Ph 2 . Both Ph 5 As and Ph 5 Bi provide, as well as Ph 3 M (M = As or Bi) and Ph 2 , some benzene probably due to the formation of Ph-. Heating in solvents, e.g. R 5 M (M = As or Sb) in CC1 4 (Scheme 6) 1 2 4 ' 1 2 5 or PhsBi in pyridine, leads to different mechanisms and products. Bu4Sb- +

Bu5Sb Bu- +

BuCl

CCI4

Bu4Sb Bu4SbCCl3

Bu CC13

Bu4SbCCl

CC1 Bu4SbCl

[:CC12]

Scheme 6 For organometal(V) halides the following thermal stability sequences have been obtained: (i) (aryl) n MX 5 _ w > (alkyl)«MX 5 - n and (ii) R 3 M X 2 > R 2 M X 3 > RMX 4 ; very few R M X 4 compounds are known because of their thermal instability, but complexation renders them more stable, e.g. R S b C l 4 L (R = Me, Ph or p-Tol; L = H M P T , py or DMSO) 1 2 6 have appreciable stability. The products of decomposition of R^MXs-^ are normally RX and R n _iMX4_ r t , e.g. equation (65), especially on heating in closed systems. However, heating R n MXs_ r t {n = 1 or 2) in a stream of an inert gas, e.g. CO 2 , can lead to loss of X 2 , e.g. equation (66).

698

Arsenic, Antimony and Bismuth Ph2AsCl3

—-

PhAsCl2

+

PhCl

(65)

Ph2AsCl3

—•

Ph2AsCl

+

Cl2

(66)

13.6.2.3 Oxidative stability Pentamethylantimony has been shown to be more stable to air than is trimethylstibine.

13.6.3 Lewis Acid Properties Such properties have been best studied with the organoantimony halides. As expected, successive replacements of organic groups by halogens result in increasing acceptor strengths. Thus Popov and Kondratenko 93 obtained the sequence PhSbF4 > Ph2SbF3 > Pli3SbF2. As well as neutral complexes, such as PhSbCLfDMSO, various hexacoordinate anionic species have also been produced. These include Me 4 N + Ph 2 SbCl 3 X- (X = Cl or Br) 127 and R 4 As+PhSbXJ (R = Ph or Me; X = F, Cl or Br). 128 In addition, several neutral complexes of the types RrtSbX4_/7(oxime)129 and RrtSbX4_rt(acac) 130 ' 131 (Hoxime = 8-quinolinol, Hacac = acetylacetone) have been studied in solution and in the solid state. All the RrtSb4_,j(oxime) and R rt Sb 4 _ rt (acac) type complexes are monomeric and all except Me3SbCl(oxime) contain six-coordinate antimony with chelating oxime and acac groups. In the Me3SbCl(oxime) complex the oxime is apparently acting as a monodentate oxygen ligand, thus making the antimony five-coordinate. Equilibria involving isomers of R2SbCl2(acac) occur in solution. RrtSbX5_rt

+

Hacac

—•

RwSb4_«(acac)

(67)

The Lewis acidity of R/TSbCl4_rt(acac) and the strength of the Sb—acac interaction increase with decreasing «; this is reflected in the IR and ! H NMR spectra of those compounds.

13.6.4 Cleavage of Organo-Element Bonds in Pentaorganometal(V) Compounds Pentaorganometal(V) compounds are readily cleaved by a variety of electrophilic reagents, including PI13B, halogens and proton acids (see also Section 13.5.4.4). In Me2SbEt3, Me is more readily cleaved by carboxylic acids, alcohols,132 etc., while in PIi4SbC6H4Me-/? it was established105'133 that a Ph group is cleaved ca. 5 times more easily than a /?-MeC6H4 group by several alcohols (ROH). The mechanisms of these alcoholyses are termed Swl-Sb for ROH and S N 2(Sb-ate) for RONa. Rates of solvolysis of Ph 4 SbC 6 H 4 X-p correlated with a values, with a p value of 1.12. Reaction of PhsBi with P^OH proceeds somewhat differently, since Pt^Bi is obtained.134

13.7 YLIDES, R3M—CH2 The number of ylidic species known decreases in the sequence P > As » Sb > Bi. The preparations of arsenic, antimony and bismuth ylides have been achieved (i) by treatment of R3(R'CH2)M + X~ with bases (equations 68, 69); 135 ' 136 (ii) by reaction of diazo compounds with R3M (equation 70); 137 (iii) by reaction of activated methylene groups with R3ASO or R3ASX2 (X = halide) (equation 71); 137 and (iv) by /ra«s-ylidation (equations 72, 73). 136 Evidence points to a considerably greater contribution to the overall structure by R3AS—CRr2 than R3As=CR2- Trimethylarsenic methylene or trimethylarsenic methanediyl, Me3As—CH2, is unstable to air, heat and water, hydrolysis producing Me 4 As + OH~. At low temperatures the axial and equatorial methyl groups in Me3As—CH2 can be distinguished, which is not the case for Me3§b—CH2. In R3AS—CH2 (R = Me or Ph) the ylidic carbons are sp3 hybridized,135 in contrast to the planar sp2 carbon 138 in ylides of the type R3AS—CHC(R)=O. 13C NMR spectroscopy also indicates that silyl substitution causes flattening of the pyramidal structure. 139

Arsenic, Antimony and Bismuth

Ph,MeAs+Br-

NaNH

Me3(Me.,SiCH2)As+Cl-

'/ T H F ,

PhjAs-CH,

BuL /E 2

L/

2 = \Tf Ph"

(69)

P Ph

L/

N

(68)

Me3A+s—CHSiMe3

' ' °»

Pi h Ph3M +

699

ph

~*

(70

3M ~\^[

? h

Ph

'

Ph

M = As (yellow), Sb (deep yellow) or Bi (deep blue) Ph 3 As=O

+

CH3NO2

Ph3As—CHR Me3As—CHSiMe3

+ +

P2 5/Et3N

°

R/COX MeOH

>

—

—•

Ph3As—CHNO2

(71)

Ph 3 As- CR(COR')

(72)

Me3As—CH2

+

Me3SiOH

(73)

Compared to the phosphorus analogue there is a reduced 7r-bond contribution to the metal-carbon bond in an arsenic ylide. The crystal structure of the ylide l-acetyl-2,3,4-triphenyl-5-(triphenylarsonio)cyclopentadienide (3) consists of a distorted trigonal bipyramid about arsenic due to considerable As—O dipolar attraction. 140 Arsenic ylides undergo the Wittig reaction with carbonyl compounds; however, the reactions are not always as clean as with the phosphorus ylides.103

°AsPh

(3)

13.8 GROUP V OXIDES, HYDROXIDES AND ALKOXIDES 13.8.1 Triorganometal(V) Oxides R3AsvO, prepared by gentle oxidation35 of R3AS, e.g. by H2O2 or HgO, are good oxygen donors. The arsenic-oxygen bond is intermediate between R 3 As=O and R3AS—O. They readily form hydrates, such as PI13ASOH2O, which has a centrosymmetric dimeric structure (4).141 Complexes with hydrogen halides, R3ASOHX, are obtained either from partial hydrolysis of R3AsX2 or by addition of HX to R 3 AsO. In Ph 3 AsOHX (5; X = Cl or Br) the HX is hydrogen-bonded to the oxygen with no arsenic-halogen bonding.142 In general, however, equilibrium (74) can be considered. The position of the equilibrium depends on X and R, e.g. (5; X = CIO4, Br3) is a strong electrolyte in MeOH solutions,143 so structure (6) is important. Triorganoantimony(V) oxides are polymeric compounds and are obtained from R3Sb or R3SbX2 by complete hydrolysis. Partial hydrolysis of Pri3SbCl2 provides molecular (Pr^SbCl^O; other molecular (Ph3SbX)2O compounds as well as ionic [(Ph3Sb-L)2O]2+ 2C1O^ (L = py or DMSO) are known. 144 Similar compounds have been obtained for bismuth, e.g. [(Pti3BiOPPh3)2]O2+2ClO4 and /i-oxo-bis[perchloratotriphenylbismuth(V)], (Ph3BiOClO3)2O. The structure of the latter has distorted145 trigonal pyramids about each antimony with axial oxygens (bridging and from OCIO3). Some use has been made of Pl^BiO as an oxidant, e.g. equation (75). 146

700

Arsenic, Antimony and Bismuth

A Ph3As—Q

;o—AsPh3

(4) As—O= 164 pm Ph 3 As—O-H

-X

(Ph 3 AsOH) + X-

^=^

(5) X = Cl or Br

(74)

(6)

A s - O = 170 pm

Ph3BiO

+

C 2 H 5 OH

—*

Ph3Bl

+

CH3CHO

+

H2O

(75)

13.8.2 Organometal(V) Alkoxides and Hydroxides Tetraorganometal(V) hydroxides are basic compounds. The general preparations of R n M v (OR/)5-« include cleavage of RsM v and exchange reactions of halides with R/OH (or R'ONa) (equations 76, 77). R5MV

+

R3MVX2

R'OH

+

R'ONa

—*

R4MVOR/

—*

R3M(OR')2

(76) (77)

The compounds Ph4SbOR (R = H or Me) and Pli3Sb(OMe)2 are trigonal bipyramid molecules in the solid state, with axial OR groups. An additional feature of the Pr^SbOH structure is the hydrogen bonding of hydroxyl groups to give dimer units; 147 ' 148 Me4AsOMe has also been shown to be a covalent molecule.136 Fluxional behaviour in the series of compounds R,jAs(OMe)5_ n (R = Me or Ph, n = 1, 2 or 3) was shown by NMR spectroscopy.149

13.8.3 Arsinic and Stibinic Acids, R2MVO(OH) The preparations of these compounds from diazonium compounds have been mentioned in Section 13.5.2. Combined oxidation and hydrolysis of R2MX also produces R2M V O(OH). While R2As(O)OH in solution act as monobasic acids (the pATa of Me2AsO(OH) is 6.27), they can be protonated by strong acids. In contrast to the polymeric stibinic acids, Me2AsO(OH) (7) in the solid state exists as centrosymmetric hydrogen-bonded dimers, having tetrahedral arsenic. 150

M

As u

/

O-H-Ox ,Me /As

V-H-O

S

Me

(7) As—O= 162 pm

13.8.4 Arsonic and Stibonic Acids, RMVO(OH)2 The major preparative routes to these compounds were given in Section 13.5.1. In water, RAsO(OH)2 are reasonably strong acids, e.g. for R = Me, pA^i = 2.66 and p/^2 = 5.89. In the solid state the structure of PhAsO(OH)2 consists of dimer molecules hydrogen-bonded into infinite

Arsenic, Antimony and Bismuth

701

chains with approximately tetrahedral arsenic.151 The stibonic acids are polymeric; a Mossbauer study of these compounds indicated trigonal bipyramidal antimony with the bridging oxygens in apical positions.152

13.8.5 Organometal(III) Oxides and Alkoxides Bis(diorganometal) oxides, (R 2 M) 2 O (M = As or Sb), are conveniently prepared from R 2 MX on alkaline hydrolysis. Both (Ph2As)2O and (Ph2Sb)2O are pyramidal molecules, with the MOM angle being 137 and 122° for M = As and Sb, respectively.153 Organoarsine oxides, arseno-alkanes and -arenes, (RAsO)M, are formed either from RAsX 2 on hydrolysis or by reduction of arsonic acids. In solution they exist as mixtures of oligomers and/or polymers, e.g. for RAsO (R = Et, Pr or Bu) cyclic trimers predominate.154 The polymeric antimony analogues are prepared similarly. The alkoxides R rt M(OR')3- M can be obtained155 by alcoholysis of R n MX 3 _« (M = As, Sb and Bi) or (RMO) n , as well as from reaction of R3AsO and R'X.

13.9 ORGANOMETAL-SULPHUR BONDED COMPOUNDS Triorganoarsine sulphides, R3ASS, and triorganostibine sulphides, RsSbS, are formed by reaction of R3MVO or R3MVX2 with H 2 S or by reaction of R3M11 with sulphur. The latter reaction with chiral arsines proceeds with retention of configuration (i.e. step i in Scheme 7). 156 The triorganostibine sulphides are also monomeric157 and are prepared by similar routes. (p-HO 2 CC 6 H 4 )PhEtAs

+

S8

-^

(p-HO 2 CC 6 H 4 )PhEtAsS

-^

(/?-HO2CC6H4)PhAsSPr

i, retention; ii, inversion Scheme 7

Tetraorganoantimony thiolates, R 4 SbSR', have been obtained from RSH and R4SbX (X = halide50 or OMe) or RsSb.158 They are thermally unstable, particularly the tetraalkyl compounds, but reactions with electrophiles, such as halogens and organic halides, have been studied. Various attempts to prepare R3Sb(SR')2 have always failed, probably due to their thermal instability; in contrast, bisthiocarboxylates, R3Sb(SCSR')2> can be isolated. Several general types of metal(III)-sulphur bonded compound have been studied. These include (R 2 M) 2 S, RMS and R w M I1I (SR / )3-n, which can be obtained from the appropriate oxide, halide or alkoxide. Additional procedures for R w M III (SR / )3-« are the reactions of R'SH with R 3 M (M = Sb or Bi) 63 and that of ROC with R 3 AsS (step ii, Scheme 7).

13.10 ORGANOMETAL HYDRIDES Organometal(III) hydrides, R rt MH3_ n , have been prepared from a variety of organometal-(III) and -(V) oxygen and halide compounds. Reductants have included zinc/acid (for arsenic derivatives only),159 and hydrides such as LiAlH4, NaBH 4 and LiBH(OMe) 3 , 160 as well as electrolytic reductions of arsonic acids.161 The arsenic hydrides are stable to water and dilute acid; however, antimony and bismuth hydrides do react with Br^nsted acids, such as HC1 and carboxylic acids (equation 78). The sequences of thermal stabilities are, for R n MH 3 _ w : M = As > Sb > Bi and n = 1 > 2 > 3. Very low temperatures are necessary for the preparation of bismuth hydrides by LiAlH 4 , 94 while Me 2 SbH only slowly decomposes at room temperature (equation 79). 160 All the hydrides react with oxygen;161 the products are dependent on the hydride, e.g. equation (80). Arsenic and antimony hydrides react with diborane to give complexes such as MeAsH 2 BH3. Additions to C = C , C = C and C = O have been mentioned (Section 13.2.1.4); additions to N = N also occur.162 Organometal(V) hydrides are unknown.

702

Arsenic, Antimony and Bismuth Me2SbH

+

HC1

2Me2SbH Me2AsH2

- ^

—•

(MeAs)x

—•

Me2SbCl

Me2SbSbMe2 +

(MeAsO)«

+

+ +

H2 H2 MeAsO(OH)2

(78) (79) (80)

13.11 ORGANOMETAL-NITROGEN BONDED COMPOUNDS 13.11.1 OrganometaK V(-Nitrogen Bonded Compounds Oxidative addition of NH2CI to Ph3As provides Ph3As(NH2)Cl, which can lose HC1 to form Ph3As=NH. 163 Other routes to triorganoiminoarsoranes are reactions of arsonium salts with NaNH2, e.g. equation (81), 164 and of PI13AS with nitrene precursors.165

Me4AsX

+

NaNH 2

—•

Me 3 As=NH

(81)

13.11.2 OrganometaI(III)-Nitrogen Bonded Compounds A number of aminoorganoarsenic(III) compounds have been produced by reactions of haloarsines with primary or secondary amines (equation 82).166 For antimony and bismuth derivatives, lithium amides have been used (equations 83, 84). As with most main group amines, the As—N, Sb—N and Bi—N bonds readily undergo protolytic reactions, e.g. with H2O, ROH and RSH, and for aminostibines, reaction also occurs with cyclopentadiene and diazomethane. +

R'2NH

Me2SbCl

+

LiNMe2

—-

Me2SbNMe2

(83)

LiN(Me)SiMe3

—

Me2BiN(Me)SiMe3

(84)

Me2BiBr

+

—

RAs(NR/2)2

RAsCl2

(82)

13.12 COMPOUNDS CONTAINING METAL-METAL BONDS 13.12.1 Element-Element Bonded Compounds The strength of the element-element bond decreases markedly from arsenic to bismuth. A large number of compounds containing arsenic-arsenic bonds are known, all containing As111. These compounds range in type from tetraorganodiarsines, R2ASASR2, to cyclic oligomers and polymers. Formation of R2ASASR2 (R = Me, CF3 or Ph) has been achieved in a variety of ways, including reduction of R2As(O)OH either by H3PO2 or electrolytically, and from reaction167 of R2ASCI with R2ASH (equation 85). The considerable reactivity of R2ASASR2 is illustrated by reaction with air [—*• Pri2AsO(OH)], sulphur, alkali metal and with carbon-carbon multiple bonds (all leading to cleavage of the arsenic-arsenic bond).168 They also thermally decompose to R3AS and As. The compounds R2ASASR2 can also interact with transition metal compounds to form new complexes in which the As—As bond can still be intact or is cleaved, e.g. equations (86) and (87). 169 An interesting triarsenic compound is 4-methyl-l,2,6-triarsatricyclo[2.2.1.02'6]heptane (8); its preparation is given in Scheme 8. It is very easily oxidized, e.g. by the oxygen from THF. 170 Polyarsines, (RAs)rt, have been prepared by a variety of methods,20 including reduction of arsonic acids and other monoorgano-arsenic(III) and -arsenic(V) species as well as by reaction of RASX2 with alkali metals. For poly(methylarsine), yellow cyclic pentameric and purple-black ladder-like structural polymeric structures have been determined by X-ray methods.171 A hexamer, (PhAs)6, has been found for poly(phenylarsine). Disproportionation and exchange processes among the oligomers/polymers can occur in solution.

Arsenic, Antimony and Bismuth R2AsCl

Me2AsAsMe2

Me2AsAsMe2

+

+

+

R2AsH

Mn2(CO)10

Mn(CO)3Cp

—

—•

R2AsAsR2 Me2 As (CO)4Mn ^ Mn(CO)4 As Me2

703 (85)

(86)

—* [Cp(CO)2Mn^—Me2AsAsMe2—>Mn(CO)2Cp]

(87)

CH 2 AsI 2 MeC^CH 2 AsI 2 CH 2 AsI 2 (8)

Scheme 8

Far fewer antimony-antimony bonded compounds exist. Various tetraorganodistibines, R.2Sb-SbR2 (R = Me, CF3, Et, Bul or Ph), have been prepared by similar methods used for arsenic. The distibines are very reactive with oxidation of Me2SbSbMe2 and Ph2SbSbPh2 leading to Me2Sb(O)OH and Ph2SbOOSbPh2, respectively. All are cleaved by halogens. However, they have appreciable thermal stability, e.g. Me2SbSbMe2 survives for 1 week at 100 °C in a sealed tube. 172 The best characterized antimony oligomer is the red air-sensitive tetramer, (ButSb)4, which is formed by the reduction of Bu2SbCl by LiAlH^ 1 7 3 All other (RSb)^ compounds have not been characterized. Tetramethyldibismuth, Me2BiBiMe2, has probably been prepared from methyl radicals and a heated bismuth mirror. 174 13.12.2 Element-Main Group Metal Bonded Compounds 169175 Formation of alkali metal derivatives can be achieved by reaction of alkali metals with (aryl) w (alkyl)3_^M (n ^ 1), R n MX3_ n (« = 1 or 2) and R2MH in solvents such as ammonia and THF (equations 88-90). In addition, use can be made of reactions of organolithium with metal hydrides (equation 91). Other Group V-metal bonded compounds can be prepared, e.g. involving aluminium, gallium and indium 176 and Group IVB metals. 177 Routes to the Group IVB bonded compounds are illustrated in equations (92)-(94).

PI13M Ph2MCl

Na

+

—* Ph2MNa

Na —•

Ph2MH

+

Na

PhMH2

+

2BuLi

R'2AsLi Et3M

+

+

+

—•

R3MCI —

R'MH

—-

Ph3SnLi

Ph2MNa Ph2MNa

—•

M = As or Sb M = As, Sb or Bi

(89)

M = As or Sb

(90)

PhMLi2 M = As or Sb

R'2AsMR3

M = Si, Ge, Sn or Pb

(R3MO3M M = Sb or Bi; M' = Si, Ge or Sn +

Ph2SbCl

—

(88)

Ph3SnSbPh2

(91) (92) (93) (94)

704

Arsenic, Antimony and Bismuth

13.12.3 Element-Transition Metal (T-Bonded Compounds TT-Bonded complexes involving R3M as a ligand are not considered here. Examples of the routes to a-bonded compounds are given in equation (95) 178 and in Scheme 9. 179

Ph2BiCl NaCpCr(CO) 3

+ -^

NaRe(CO)5

—» Ph2BiRe(CO)5

Cp(CO)3CrAsMe2

^-

(95)

CpCr(CO)3SiMe3

••• 111 >r

Cp(CO) 3 CrAsMe 3 + I" i, Me2AsCl, -NaCl; ii, Me2AsCl, -Me 3 SiCl; iii, Mel Scheme 9

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