THE CHEMISTRY OF ORGANO-ARSENIC, ANTIMONY AND BISMUTH COMPOUNDS: AN OVERVIEW Shivram S. Garje1 and Vimal K. Jain*2 1 Department of Chemistry, University of Pune, Pune - 411 007, India 2 Chemistry Division, Bhabha Atomic Research Centre, Mumbai - 400 085, India Abstract The chemistry of organo-arsenic, -antimony and -bismuth compounds has been reviewed. General features of group 15 elements and the stereochemistries adopted by these elements are discussed briefly. This review covers the chemistry of tri- and pentavalent organometallic compounds of arsenic, antimony and bismuth as well as metal-metal and multiply bonded, heterocyclic and coordination compounds. Pharmaceuticals and environmental aspects of these compounds have also been discussed. Emerging trends in the field have been brought out in the end. Introduction The field of organoarsenic chemistry has a long history that started as early as 1760 when Cadet de Glaussicourt isolated tetramethyldiarsine (Me2As-AsMe2), generally known as Cacodyl, in 1760 [1]. It took about a century to establish the composition of Cadet's compound. Trimethylantimony [2] and triethylbismuth [3] were the first organometallics of antimony and bismuth reported by Löwig and Schweizer in 1850. Considerable attention was paid when the potential of organoarsenicals such as 3,3'-diamino-4,4'- dihydroxy arsinobenzene (Salvarsan base) as therapeutic agent against the treatment of syphilis was reported by Paul Ehrlich in 1910 [3a], As a natural extension to this development, compounds of higher homologues (Sb and Bi) were also investigated. However, with the discovery of Penicillin as an effective cure for syphilis in 1943 and the inhibition of organoarsenicals in war, interest in these compounds declined for quite some time. During this period, the progress in this field was rather slow and steady but gained momentum only in the last 15 years or so when their potential as MOCVD precursors in material science was realized. Besides these developments, compounds of these elements have biocidal properties and find extensive applications in organic synthesis [4] and industry. Apart from their diverse applications, the area of basic research has also made great stride in recent years due to their interesting structural features and synthetic challenges [5-7], This review intends to bring out some salient features of the chemistry of organoarsenic, antimony and bismuth compounds and to find out emerging trends in this field. The review is an overview of the field and is by no means comprehensive. References to the more detailed review articles which cover specific aspects are given in the text. General features of group '15' elements Nitrogen, phosphorus, arsenic, antimony and bismuth constitute the 'group 15' of the periodic table. These members are sometimes referred as "pnictides", but this name is not widely used since it is not 2 approved by IUPAC. They have the following general outer electronic configuration: ns , ηρΛ ηpy\ ηpz\ Some of the physical properties of these elements are given in Table 1. Although, there is a progressive variation in physical properties from nitrogen to bismuth, some properties such as bond energies, electronegativities, etc. vary irregularly. Nitrogen differs considerably from the remaining group 15 elements in that (i) it can form strong pK-p„ multiple bonds, and (ii) its inability to increase coordination number beyond four. Other group 15 elements may have coordination number five or six by employing one or two outer d orbitals in bonding. On descending the group, there is a gradual increase in the metallic character. A slow increase in the electropositive character and a gradual decrease in the ionization potential from phosphorus to bismuth leads to the formation of bismuth cation, Bi'+. All the group 15 elements form trivalent compounds of the types R3M, R2MX, RMX2 and MX3 (R = organic group and X = inorganic lieand) with an inert pair of electrons. Nitrogen and phosphorus compounds are generally tetrahedral (sp hybridization) with a stereochemically active electron pair usually referred as "lone pair". In the case of bismuth compounds, the lone pair is stereochemically inactive and remains almost always in the 6s orbital. It is usually called as "inert pair". Compounds of arsenic and antimony can adopt either of these configurations [8]. 45 Vol. 22, No. 1, 1999 The Chemistry of Organo-Arsenic,Antimony and Bismuth Compounds an Overview These elements exhibit a higher valency of five. However, arsenic(V) and bismuth(V) compounds are less common as compared to phosphorus(V) and antimony(V) compounds. This is attributed to the effect of electron penetration [9], As compared to 3s electrons of phosphorus (where 3d shell is empty), the 4s electrons of arsenic penetrate within the 3d10 shell and are held more firmly. Similarly, the 6s electrons in bismuth penetrate within the 4/4 shell and are held firmly compared to the 5s electrons of antimony, where the 4/ shell is empty. Pentavalent compounds usually show trigonal bipyramidal geometry with sp3d hybridization. In contrast, pentaphenylantimony has a square pyramidal structure [10]. Some typical coordination geometries exhibited by the group 15 organometallics are shown in Scheme 1. Trivalent organo-arsenic, -antimony and -bismuth compounds Relatively few primary (RMH:) and secondary (R:MH) hydrides have been reported possibly due to their high toxicity, high volatility and tendency to decompose. The number of hydrides known for these metals follow a trend, As > Sb » Bi. These hydrides can be prepared in several ways; reduction being the most common method [11-13], Thus, reduction of arsonic, arsinic acids and their salts with a variety of reducing agents such as zinc dust with hydrochloric acid [11], amalgamated zinc [12] readily afford primary and secondary arsines. Halo-arsines and -stibines can be reduced readily by LiAlH4 or Li/NaBH4 [13]. Table 1. Some selected physical properties of group 15 elements Physical property Ν Ρ As Sb Bi Atomic number 7 15 33 51 83 Atomic weight 14 30.97 74.92 121.75 208.98 4 2 Electronic configuration [He]2sV [Ne]3sV [Ar]3i/° [Ktw; [Xe]4/ 5i/°6s 4s2 4p3 5s 5ρ 6p3 1st Ionization 14.5 11.0 9.8 8.6 7.3 potential Electronegativity 3.0 2.1 (2.18)* 2.0 (2.19)* 1.9 (2.06)* 1.9 (2.14)* (Pauling) Radii: Ionic 1.71 (N"3) 2.12 2.22 2.45 (Sb"3) 1.08 (Bi+3) (Ρ"3) (As"3) 0.92 (Sb+3) Covalent 0.70 1.10 1.20 1.36 1.46 (for trivalent state) Boiling Point (°C) -196 280 633 (s) 1380 1560 Melting point (°C) -210.1 44.1 (wh.) 817 (gr., 36 630.5 (gr) 271.3 590 (r) atm.) Density (g/cmJ) 0.81 1.82 (wh.) 5.73 (gr.) 6.67 9.80 Oxidation states -3 to +5 ** 3, 5 3, 5 3, 5 3, 5 Abudance in earth's crust 19 1120 1.8 0.20 0.008 by weight (in ppm) Where wh. = white, r = red, gr. = grey, s = sublimes, atm. = atmospheric pressure; *= recent values computed by Batsanov; ** = e.g. -Ill in NH3, -II in NH2NH2, -I in NH2OH, 0 in N2, +1 in N20 (nitrous oxide), +11 in NO (nitric oxide), +111 in HN02 (nitrous acid), +IV in N02 (nitrogen dioxide), +V in HN03 (nitric acid). A very large number of tertiary -arsines, -stibines and -bismuthines have been synthesized and several of them have been fully characterized [14, 15], The most common method for synthesizing these derivatives is the reaction of the Grignard reagent [16] with MX3 (eqn. 1). However, when the Grignard reagent is difficult to obtain, organolithium reagents are employed [17], Apart from these reagents, a number of other transmetallation reactions have been employed to prepare R3M. The reagents which have been used in transmetallation include organo-aluminium [18], -zinc [19], -cadmium, [20], -mercury [21] and -tin [22] compounds. The Wurtz-Fittig reaction is seldomly used to prepare alkyl derivatives, however, it has been utilized to prepare aryl or cyclic derivatives [23] (eqn. 2). 3RMgX + MXj f R3M + 3MgX2 (1) 46 Shivram S. Garje and Vimal K. Jain Main Group Metal Chemistry M(lll) Miio^. (Pk Pyramidal Tetrahedral Trigonal bipyramidal 2 3 sp SP3 sp dz2 e.g. RQM R3M-^M Ph2Bi(S2COPr') (3>< Square pyramidal Octahedral Pentagonal bipyramidal 3 3 3 sp dx2.y2 spV sp d e.g. PhSb(S2PPh2)2 Sb[S2P(OEt)2]3 Bi(S2COPr')3 M(IV) yM lllO^.. Tetrahedral Trigonal bipyramidal Square pyramidal 3 3 3 sp sp dz2 sp dx2.y2 + e.g. [R4M] X" R5M, R3MX2 Ph5Sb "V Octahedral spV [SbPh6]", [SbMe4F]n Scheme 1 47 Vol. 22, No. 1, 1999 The Chemistry of Organo-Arsenic,Antimony and Bismuth Compounds an Overview ,CH2Br MeAsCU sMe (2) 4 Na CH2Br Most of the trialkyl derivatives are volatile liquids with disagreable odours and are air sensitive. The lower members are spontaneously flammable in air. On the other hand, the triaryl derivatives are solids and are stable in air. Tertiary-arsines, -stibines and -bismuthines, R3M all have pyramidal structure with the average bond angle (CMC) becoming smaller from arsenic to bismuth. For example, ZCMC in (p- CICsH^M is 104°, 97°, 93° for Μ = As, Sb, Bi, respectively [24]. These molecules have high inversion barriers (~ 200 KJmor1). Chiral arsines RR'R"As have been isolated using both chemical and biological methods [25], In recent years, several of the alkyl derivatives have found wide applications as precursors in the formation of compound semiconductors such as GaAs, InSb, etc.[26-29]. Mono- and di-organometal halides of arsenic, antimony and bismuth R3.nMX,, (η = 1 or 2; Μ = As, Sb or Bi) are well known [1], They can be prepared in several ways and each method of preparation is rather specific to the metal.