Tm ligands

The Tm^Me Ligand was first reported by Reglinski and Spicer (J. Chem. Soc. Chem. Commun. , 1996, 1975) and was prepared by reacting Methimazole (1-methylimidazole-2-thione) with sodium borohydride in a solvent-free melt. Both lithium and potassium salts have also been prepared. Other workers (e.g. Parkin, Vahrenkamp, Rabinovich) have extended the family of ligands (Tm^R) by replacing the methyl group with other organic functional groups (phenyl, 2-tolyl, 3-tolyl, 4-tolyl, cumyl, t-butyl, benzyl etc), while species BH₂(mt)₂^- (Bm^Me) and BH₃(mt)^- have also been prepared. In contrast to the original syntheses, many of these ligand preparations are carried out in THF, toluene or xyxlene as solvent. The Tm^Me anion is a tridentate, tripodal sulfur donor ligand which is broadly similar to the Tp ligands. The donor atoms in this ligand class are similar to those in thioureas. Several research groups worldwide including those of John Reglinski and Mark Spicer, Gerard Parkin, Tony Hill [1], Heinrich Vahrenkamp, Daniel Rabinovich and Claudio Petinnari have been working on this ligand class. These ligands are an example of the scorpionate ligands.

While in many resects the Tm^R ligands behave like the Tp ligands (many directly analogous metal complexes have been obtained) there are also many differences observed. These differences arise from three main factors:

1. The "soft" sulfur donor atoms;
2. The large 8-membered chelate rings formed on complexation to a metal (cf 6-membered rings for Tp); and
3. The apparently greater reactivity of the borohydride group.

The soft donor atoms allow, for instance, formation of stable lower p-block complexes, whereas the N-donor Tp ligands only form very moisture sensitive species. The larger chelate rings introduce a greater ligand flexibility, allowing many "inverted" structures in which the ligand coordinates through two S atoms and via the borohydride. This in turn leads to the formation of boratrane complexes (discussed below).

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Ruthenium, rhodium, osmium and related metals

• It was shown that the reaction of a 16VE ruthenium vinyl [RuCl(CO)(CH=CHPh)(PPh₃)] with NaTm forms a zero valent ruthenium complex [B(mt)₃Ru(CO)(PPh₃)] which has a boron metal bond. If the ruthenium starting material is replaced with a osmium complex [OsHCl(CO)(PPh₃)₃] then an intermediate is formed which decomposes into [B(mt)₃Os(CO)(PPh₃)].

Here it can be seen that the boron binds to the metal, the osmium complex is an 18 VE complex, where the metal is formally in the zero oxidation state. The carbonyl stretching frequency is very low for this complex because the metal is so electron rich. The ruthenium complex is not shown because it has the same structure.

M.R.StJ.Foreman, A.F.Hill, A.J.P.White and D.J.Williams, Organometallics, 2004, 23, 913.

A.F.Hill, G.R.Owen, A.J.P.White and D.J.Williams, Angew. Chem., Int. Ed. Engl., 1999, 38, 2759.

• In the case of the reaction of [RuHCl(CO)(PPh₃)₃] with NaTm, it is possible to isolate [RuTmH(CO)(PPh₃)₃] which on treatment with phenylacetylene forms the zerovalent [B(mt)₃Ru(CO)(PPh₃)] complex.

Here it can be seen that the hydrogen atom attached to the boron is being transferred to the metal, it is thought that if the hydrogen is transferred totally to the metal that a reductive elimination reaction (opposite of oxidative addition) can occur to form the zero valent metal borane complex.

M.R.StJ.Foreman, A.F.Hill, G.R.Owen, A.J.P.White and D.J.Williams, Organometallics, 2003, 22, 4446.

• If the ruthenium is replaced with rhodium then the corresponding compound is a chloride complex rather than a carbonyl complex.

This complex should be comapired with the osmium complex, here to provide the metal with 18 valence electrons one fewer electrons is needed, so as a result the carbonyl seen in the ruthenium and osmium complexes has been replaced with a chloride ligand.

I.R.Crossley, M.R.St. J.Foreman, A.F.Hill, A.J.P.White and D.J.Williams, Chem. Comm., 2005, 221.

Molybdenum

A large number of molybdenum complexes have been made, many of these mirror in some ways the chemistry of the Tp and cyclopentadienyl ligands. These very sulfur rich molybdenum complexes might be possible models for a molybdenum sulfide surface used in Hydrodesulfurization.

M.R.StJ.Foreman, A.F.Hill, N.Tshabang, A.J.P.White, D.J.Williams, Organometallics, 2003, 22, 5593.

M.Garner, M.-A.Lehmann, J.Reglinski and M.D.Spicer, Organometallics, 2001, 20, 5233.

Tungsten

It is possible by the reaction of [WBrL₂(CO)₂(CN-i-Pr₂)] to form a Tm complex [WTm(CO)₂(CN-i-Pr₂)].

M.R.St. J.Foreman, A.F.Hill, A.J.P.White and D.J.Williams, Organometallics, 2003, 22, 3831.

Zinc and cadmium complexes

A large number of zinc and cadmium complexes of these Tm class ligands have been made as models for enzymes.

An example of a cadmium complex, here the zinc is bonded to by the Tm liagnd and a thiolate ligand.

S.Bakbak, C.D.Incarvito, A.L.Rheingold and D.Rabinovich, Inorganic Chemistry, 2002, 41, 998.

Actinide complexes

A uranium complex of Bm has been reported, to the uranium are attached three THF ligands and two Bm ligands. Note that the hydrides attached to the boron atoms are much closer to the uranium atom than the two phenyl groups. This suggests that the hydrides are partway between being attached to the boron and the metal.

L.Maria, A.Domingos, I.Santos, Inorganic Chemistry, 2001, 40, 6863.

Action as a nucleophile

In addition to acting as a ligand, Tm and Bm ligands can react with electrophiles such as dichloromethane to form cationic S, S' alkylated products.

I.R.Crossley, A.F.Hill, E.R.Humphrey, M.K.Smith, N.Tshabang and A.C.Willis, Chem. Comm., 2004, 1878.