- Tm ligands
The TmMe 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) withsodium 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 (TmR) 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- (BmMe) 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 TmMe anion is a tridentate, tripodal sulfur donor ligand which is broadly similar to theTp ligands . The donor atoms in this ligand class are similar to those inthiourea s. Several research groups worldwide including those of John Reglinski and Mark Spicer, Gerard Parkin, Tony Hill [http://rsc.anu.edu.au/research/hill.php] , Heinrich Vahrenkamp, Daniel Rabinovich and Claudio Petinnari have been working on this ligand class. These ligands are an example of thescorpionate ligand s.While in many resects the TmR 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:
#The "soft" sulfur donor atoms;
#The large 8-membered chelate rings formed on complexation to a metal (cf 6-membered rings for Tp); and
#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).
Ruthenium ,rhodium ,osmium and relatedmetals * 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 aboron metal bond. If the ruthenium starting material is replaced with aosmium 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 areductive elimination reaction (opposite ofoxidative addition ) can occur to form the zero valent metalborane 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 acarbonyl 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 thecarbonyl seen in the ruthenium and osmium complexes has been replaced with achloride 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 andcadmium complexes of these Tm class ligands have been made as models forenzyme s.An example of a cadmium complex, here the zinc is bonded to by the Tm liagnd and a
thiol ate ligand.S.Bakbak, C.D.Incarvito, A.L.Rheingold and D.Rabinovich, "
Inorganic Chemistry ", 2002, 41, 998.Actinide complexesA uranium complex of Bm has been reported, to the
uranium are attached threeTHF ligands and two Bm ligands. Note that the hydrides attached to theboron atoms are much closer to the uranium atom than the two phenyl groups. This suggests that thehydride s 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 withelectrophiles such asdichloromethane to formcation ic "S, S' "alkyl ated products.I.R.Crossley, A.F.Hill, E.R.Humphrey, M.K.Smith, N.Tshabang and A.C.Willis, "Chem. Comm.", 2004, 1878.
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