Julie S. Palmer

p-Block metallocenes: the other side of the coin

Although transition metal metallocenes {such as ferrocene, [(C5H5)2Fe]} have been a cornerstone in the development of modern organometallic chemistry and continue to be a focus for chemical and structural studies, in comparison the chemistry of the main group metal counterparts has remained relatively undeveloped. The recent resurgence of interest in p-block (Groups 13–15) metallocenes in particular has given fresh insights into the structural preferences, bonding requirements and reactivity of these under-publicised species, which in many ways represent ‘the other side of the coin’. The more varied (ionic and covalent) character of the metal-ligand bonds and the less restricted electronic requirements of p-block metals leads to greater structural diversity and radically different reactivity than is found for the transition metal relatives. This short review focuses on the remarkable range of p-block complexes that has so far been uncovered and attempts to unravel some of the electronic and structural trends in these species.

Thallium(I)- and organothallium(III)-substituted mono-, bis- and tris-tetrazoles: synthesis and supramolecular structures

Six thallium(I) tetrazolates have been synthesized by either the addition of an alcoholic solution of the appropriate tetrazole to an ethanolic solution of thallium ethoxide, or from the sodium salt of the tetrazole with Tl2SO4. Seven diphenylthallium-substituted mono- and bis-tetrazoles have been synthesized by a condensation route involving the appropriate tetrazole and diphenylthallium hydroxide. 1-Diphenylthallyl-5-phenyltetrazole has also been prepared by a cycloaddition reaction between diphenylthallium azide and benzonitrile. The structures of [Tl(18-crown-6)+]2[C2N102−], Ph2TlN4CPh·MeOH, 1,4-(Ph2TlN4C)2C4H8·2MeOH and 1,2-(Ph2TlN4C)2C6H4·Ph2TlCl·2MeOH·2H2O have been determined. The last three compounds generate supramolecular structures which incorporate six-membered Tl2N4 rings. The last two contain the first examples of tetrazoles which utilise all four available ring nitrogens for co-ordination.

The hydrogen bonded polymer structures of [{Mn(2-mbiH)2·TMEDA}·A–A]∞[2-mbiH2 = 2-mercaptobenzimidazole; A–A = TMEDA (Me2NCH2CH2NMe2) or DABCO (N{CH2CH2}3N)]

The reaction of [MnCp2] with 2-mbiH2 (1:2 equiv.) in the presence of TMEDA gives [{Mn(2-mbiH)2·TMEDA}·TMEDA]∞1. A similar reaction employing TMEDA and DABCO (1:1 equiv.) yields [{Mn(2-mbiH)2·TMEDA}·DABCO]∞2. In the crystal, both complexes adopt similar polymeric structures in which the bifunctional [Mn(2-mbiH)2·TMEDA] units are associated by hydrogen bonds to lattice-bound TMEDA or DABCO ligands. The structural motif adopted by both complexes can be regarded as resultingfrom a fusion of separate helical elements.

STRUCTURED ASSEMBLY OF HETEROMETALLIC ARRAYS

A range of heterometallic cage complexes can be prepared by step-wise metallation reactions of monolithiated primary amines and phosphines ([REHLi]n; E= N, P) with various Group 13, 14 and 15 metal reagents. The p block imido and phosphinidine anions present in these species are novel ligand systems to main group and transition metals. This simple synthetic approach thereby provides a basis for structured assembly of a range of heterometallic cage and ring compounds.

Metal selection of ligand functionality in[(mes)2P(O)2Li·2thf]2 and[{(Me3Si)2N}Cd{(mes)2PO}2Li·2thf] (mes =C6H2Me3-2,4,6)

The reaction of LiBu n with [(mes) 2 P(H)O] affords the diorganophosphinate complex [(mes) 2 P(O) 2 Li·2thf] 1 and [(mes) 2 PLi], however, addition of [Cd{N(SiMe 3 ) 2 } 2 ] to the mixture gives the diorganophosphinite complex [{Me 3 - Si) 2 N}Cd{(mes) 2 PO} 2 Li·2thf] 2; the formation of these complexes illustrates that the ligand functionality of 1 can be controlled by the character of the coordinated metal ion.

The paramagnetic, heterometallic manganese cubanes [{E2(NCy)4}(MnCp)2] (Cy = C6H11, Cp = C5H5, E = As, Sb)

The heterometallic cubanes [{E2(NCy)4}(MnCp)2] (E = Sb 1, As 2; Cy= C6H11, Cp= C5H5) are the first examples of complexes in which a paramagnetic metal ion has been incorporated into a p block ligand framework.

Synthesis and structure of [{ButP)2H}K·pmdeta]2, containing an organo diphosphido ligand [pmdeta = (Me2NCH2CH2)2NMe]

Reduction of [ButP]4 with K metal (1:5 equiv.) followed by the addition of pmdeta and stoichiometric hydrolysis gives [{(ButP)2H}K·pmdeta]2, a unique complex containing a [ButP(H)PBut]– anion ligand homologous with a phosphido anion (R2P–).

Synthesis and structure of [Sn(mit)6Cu4]; a [SnCu4] cage supported by Sn–Cu cluster bonding [mit = (CH)2N(Me)C(S)N]

The reaction of [Sn(mit)2]n with CuCl gives the heterobimetallic complex [{Sn(mit)6Cu4] 1, in which the hypho-hexagonal-bipyramidal [SnCu4] core is stabilised by SnII–CuI cluster bonding between the endo lone pair of the Sn centre and the tetrahedral Cu4 fragment.

Stabilisation of unusual metal co-ordination geometries using an oxo-cubane ligand; syntheses and structures of [{Sn4(NtBu)3O}3LiCl]·3thf and [{Sn4(NtBu)3O}3FeCl2]·3thf

Complexation of LiCl and FeCl2 with the oxo-cubane [Sn4(NtBu)3O] 1 (3 equivalents) gave [{Sn4(NtBu)3O}3LiCl]· 3thf 2 and [{Sn4(NtBu)3O}3FeCl2]·3thf 3, respectively; the co-ordination of three sterically demanding, ‘ether-like’ [Sn4(NtBu)3O] ligands to the Li+ and Fe2+ centres of 2 and 3 results in unusual geometries for these metals. The geometry in 2 can be described as a distorted trigonal bipyramid with one vertex missing, while in 3 it is trigonal bipyramidal. Model semi-empirical calculations combined with the structural findings stress that although sterically encumbered 1 is a highly effective ligand for oxophilic metals.

Heterometallic Complexes of Sn(II)

We report here some new approaches to heterometallic complexes containing Sn(II) and transition metals. These are exemplified by the syntheses and structures of the complexes [Sn(mit)6Cu4] (1), [{Sn4(NBu′)3O}Cr(mit)Cl2(NMe2H)] (2) (mit= CH2N(MeC(...S)...N] and [(thf)(Me2NH)2Cl2Cr]2{ClSn(μ-O)}]2 (3).