The chemistry of heavier main-group elements such as aluminium, silicon and phosphorus is very different from that of the lighter ones such as boron, carbon and nitrogen, yet discussions of this topic have been dominated by comparisons with the light elements. Philip Power's review focuses on advances in chemistry of the heavier main-group elements that reveal them as having more in common with the transition metals than the lighter members of the main groups. The concept of heavier main-group elements as 'transition metals' is supported by recent work showing that many of the new compounds react with small molecules such as H2, NH3, C2H4 and CO under mild conditions and display potential as catalysts. The last quarter of the twentieth century and the beginning decade of the twenty-first witnessed spectacular discoveries in the chemistry of the heavier main-group elements. The new compounds that were synthesized highlighted the fundamental differences between their electronic properties and those of the lighter elements to a degree that was not previously apparent. This has led to new structural and bonding insights as well as a gradually increasing realization that the chemistry of the heavier main-group elements more resembles that of transition-metal complexes than that of their lighter main-group congeners. The similarity is underlined by recent work, which has shown that many of the new compounds react with small molecules such as H2, NH3, C2H4 or CO under mild conditions and display potential for applications in catalysis.

Main-group elements as transition metals

Persistent and stable radicals of the heavier main group elements and related species.

Persistent carbon-centered radicals

Oxidative addition and reductive elimination are key steps in a wide variety of catalytic reactions mediated by transition-metal complexes. Historically, this reactivity has been considered to be the exclusive domain of d-block elements. However, this paradigm has changed in recent years with the demonstration of transition-metal-like reactivity by main-group compounds. This Review highlights the substantial progress achieved in the past decade for the activation of robust single bonds by main-group compounds and the more recently realized activation of multiple bonds by these elements. We also discuss the significant discovery of reversible activation of single bonds and distinct examples of reductive elimination at main-group element centers. The review consists of three major parts, starting with oxidative addition of single bonds, proceeding to cleavage of multiple bonds, and culminated by the discussion of reversible bond activation and reductive elimination. Within each subsection, the discussion is...

Oxidative Addition and Reductive Elimination at Main-Group Element Centers

The synthesis and characterization of several m-terphenyl heavier main group 15 (P, As, Sb, or Bi) dihalides, together with their reduction to give a homologous series of double-bonded dipnictenes, are reported. Reaction of LiC6H3-2,6-Mes2 (Mes = C6H2-2,4,6-Me3) or LiC6H3-2,6-Trip2 (Trip = C6H2-2,4,6-iPr3) with the appropriate trihalide affords 2,6-Mes2H3C6ECl2 (E = As, 1; Sb, 2; Bi, 3) and 2,6-Trip2H3C6ECl2 (E = P, 4; As, 5; Sb, 6; Bi, 7). The compounds 1−7 were characterized by 1H and 13C NMR spectroscopy as well as by 31P NMR spectroscopy in the case of 4. In addition, the structures of 3, 5, and 6 were determined. Reduction of the phosphorus species 4 with potassium in hexane gives a mixture of the diphosphene 2,6-Trip2H3C6PPC6H3-2,6-Trip2, 12, and the phosphafluorene species, 1-(2,4,6-triisopropylphenyl)-5,7-diisopropyl-9-phosphafluorene, 11. The compound 11, which results from the insertion of a phosphorus into a C(Ar)−C(i-Pr) bond was synthesized in higher yield by the reduction of 4 with magnesium...

Homologous Series of Heavier Element Dipnictenes 2,6-Ar2H3C6E=EC6H3-2,6-Ar2 (E = P, As, Sb, Bi; Ar = Mes = C6H2-2,4,6-Me3; or Trip = C6H2-2,4,6-iPr3) Stabilized by m-Terphenyl Ligands

Interaction of LiR [R = CH(SiMe3)2 or N(SiMe3)2] and MCl3 in appropriate stoicheiometry affords the following new compounds: M[CH(SiMe3)2]Cl2 or M[CH(SiMe3)2]2Cl (M = P, As, or Sb), M[N(SiMe3)2]Cl2 or M[N-(SiMe3)2]2Cl (M = As or Sb), or Bi[N(SiMe3)2]3. Reaction of P(NPri2)Cl2 with Li[N(SiMe3)2]·OEt2, Li[N(CMe3)(SiMe3)], Li[CH(SiMe3)2], MgButBr, or NHPri2 yields the corresponding new compound P(NPri2)RCl, while Li[CH(SiMe3)2] with P(NMe2)Cl2 affords P[CH(SiMe3)2](NMe2)Cl. Reduction of the appropriate phosphorus(III) or arsenic(III) monochloride in toluene by photolysis with the olefin (Et[graphic omitted]Et gives the persistent (t½= 3 days to > 1 year in PhMe at 300 K) phosphorus(II) or arsenic(II) alkyl or amide: ·M[CH(SiMe3)2]2(M = P or As), ·M[N(SiMe3)2]2, ·P[CH(SiMe3)2](NR2)(R = Pri or SiMe3), ·P(NPri2)[N(SiMe3)2], ·P[N(CMe3)(SiMe3)]2, or ·P(NPri2)[N(CMe3)(SiMe3)]. Electron spin resonance parameters are discussed.

Bulky alkyls, amides, and aryloxides of main Group 5 elements. Part 1. Persistent phosphinyl and arsinyl radicals ·MRR′ and their chloroprecursors MRR′Cl and related compounds

Photolysis of toluene solutions of dialkyl-phosphorus or -arsenic chlorides [(Me3Si)2CH]2MCl (M = P or As) or bisamido-phosphorus or -arsenic chlorides [(Me3Si)2N]2MCl (M = P or As) in the presence of an electron-rich olefin leads to solutions containing persistent (t½ > 1 month at 20 °C) two-co-ordinate phosphorus or arsenic-centred radicals [(Me3Si)2CH]2M· or [(Me3Si)2N]2M·(M = P or As) characterised by their e.s.r. spectra.

Synthesis and electron spin resonance study of stable dialkyls and diamides of phosphorus and arsenic, R12M· and (R22N)2M·

A number of solution-stable species of general formula MR3˙[R = CH(SiMe3)2: M = Si. Ge. or Sn], M(NR′2)3˙ and M (NR′R″)3˙(R′= SiMe3, R″= CMe3, M = Ge or Sn) have been prepared and characterised by e.s.r. spectroscopy. Most of the radicals have been generated by photolysis of the bivalent Group 4 species MR, M(NR′2)2 or M(NR′R″)2 when available; others have been obtained by alternative photochemical experiments. The e.s.r. parameters indicate that the radicals have non-planar structures similar to those of analogous transient species such as MMe3˙. The mechanism of formation of the radicals is discussed : their unusual stability (e.g. SnR3˙ has a halflife of ca. 1 year at 20 °C) is attributed mainly to steric hindrance to dimerisation.

Subvalent Group 4B metal alkyls and amides. Part 4. An electron spin resonance study of some long-lived photochemically synthesised trisubstituted silyl, germyl, and stannyl radicals

Photolysis of R2Sn [R =(Me3Si)2CH] with visible light in benzene at ambient temperature yields the stable radical R3Sn·, which has an e.s.r. solution spectrum showing coupling with the methine protons and 119Sn and 117Sn nuclei.

Tris[bis(trimethylsilyl)methyl]tin(III), R3Sn·: an unusually stable stannyl radical, from photolysis of R2Sn

The reaction of MCl2(M = Ge or Sn) or Si2Cl6 with R1Li or (R22N)Li [R1=(Me3Si)2CH, R2= Me3Si] and subsequent irradiation affords the stable metal-centred radicals R13Si·, R13M·, or (R22N)3M·(e.g. R13Ge· has t½ > 4 months in C6H6 at 20 °C), the solution e.s.r. spectra of which show well defined hyperfine splittings [e.g. for (R22N)3Ge·, a decet of septets, due to coupling with 73Ge (I= 9/2) and 14N (I= 1)].

Photochemical synthesis and electron spin resonance characterisation of stable trivalent metal alkyls (Si, Ge, Sn) and amides (Ge and Sn) of Group IV elements

Andrew Hudson

Papers

Bulky alkyls, amides, and aryloxides of main Group 5 elements. Part 1. Persistent phosphinyl and arsinyl radicals ·MRR′ and their chloroprecursors MRR′Cl and related compounds

Synthesis and electron spin resonance study of stable dialkyls and diamides of phosphorus and arsenic, R12M· and (R22N)2M·

Subvalent Group 4B metal alkyls and amides. Part 4. An electron spin resonance study of some long-lived photochemically synthesised trisubstituted silyl, germyl, and stannyl radicals

Tris[bis(trimethylsilyl)methyl]tin(III), R3Sn·: an unusually stable stannyl radical, from photolysis of R2Sn

Photochemical synthesis and electron spin resonance characterisation of stable trivalent metal alkyls (Si, Ge, Sn) and amides (Ge and Sn) of Group IV elements