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Multiple bonding between group 13
metals was unknown until a report from our group and two other groups in
Germany
disclosed the first stable examples in
1991. This work gave details of the first structural characterizations of
compounds having multiple bonds between aluminum or gallium atoms.
Essentially, the addition of an electron to a neutral tetraorganodimetallane,
results in the creation of an M–M bond order of 1.5 between the two metals in
the radical anion. We have shown that the addition of another electron
leads not to a doubly bonded dianion [Ar2M=MAr2]2– but to a rearrangement to the species [M(MAr2)3]2– which features a central metal bound to three –MAr2
substituents and an M–M bond order of 1.33. In addition, we are
investigating the bonding in other M–M bonded group 13 species.
The use of the rather larger terphenyl (i.e. a very bulky aryl) group
LiC6H3-2,6-Trip2 (Trip = C6H2-2,4,6,-i-Pr3) with InCl or TlCl
results in the monomers :MC6H3-2,6-Trip2 which
were the only examples of one coordinate metals in the solid state. These
also possess a lone pair of electrons at the metal so they can function as
donor ligands as in the complex 2,6-Trip2H3C6InMn(h5-C5H5)(CO)2.
Current synthetic results and plans involve M–M bonded species with formally
doubly bonded derivatives of the type RMMR and the exploration of their
chemistry. Also, projects involve the synthesis of compounds with the
group13 metals Al, Ga or In multiply bonded to C, N. O, S or P as in species
such as R–Al=O.
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There are several
outstanding problems involving multiple bonding to the elements Si, Ge, Sn, or
Pb. Examples include the synthesis of compounds of the type RMMR (M = Si,
Ge, Sn or Pb) which are formal
analogues
of acetylene. In 2000, we succeeded in making the first such compound ArPbPbAr
(Ar = C6H3-2,6-Trip2) which has a trans-bent
rather than a linear structure owing to the inert pair effect. In 2002
and 2003, we synthesized the corresponding germanium and tin compounds.
In 2002, we succeeded in synthesizing ArGeGeAr and ArSnSnAr analogs. Closely related to this work is the synthesis
and characterization of compounds with triple bonds between the heavier group
14 elements and transition elements first reported by us in 1996. Several examples of compounds having
triple bonding between germanium and elements such as chromium, molybdenum or
tungsten have been characterized by our group and current efforts are directed
at the synthesis of their silicon or tin analogues. Other examples of
multiply bonded group 14 species currently under investigation are the dianions
(M = Ge or Sn). In addition, we are investigating the synthesis of the
compounds R2M=O with double bonding to oxygen, ie., the ketone
analogues from which there are no isolated stable examples.
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Low
coordinate transition metal complexes have long been regarded as
interesting curiosities. However, it is becoming increasingly clear
that such complexes have an important role to play in various chemical
transformations. At least two metalloproteins, one involving copper,
the other involving iron and molybdenum, may possess metals in
three-coordinate or quasi-three coordinate environments. For
example, the FeMoCo cofactor in the enzyme nitrogenase involves 6 iron
atoms, each nominally bound to three sulfurs and plus a central atom,
nitrogen. Currently the only transition metal complexes, in which
iron is bound to three sulfurs, were synthesized in this
laboratory. These complexes have the formula [Fe(SR)3]– (where R = Large
group
such as C6H2-2,4,6-t-Bu3) and are confined to
iron in the oxidation state +2. Current efforts are directed toward the synthesis
of three-coordinate Fe3+ thiolates. A problem is the tendency
of Fe3+ to oxidize the [SR]– ligand to the disulfide RSSR
while becoming reduced to Fe2+.
However, it is known that reactions of this type can be prevented by
using bulky ligands -- at least for four-coordinate iron
complexes. It is probable that similar techniques can be applied
to three-coordinate iron species. Another current project involves the
synthesis of hydrocarbon soluble FeO clusters. We are attempting
to use large hydrophilic ligands, eg., –N(SiMe3)2 or –C(SiMe3)3 to
synthesize clusters of the general formula Rn(FeO)m (R =
hydrophilic substituent, m > n). The key step in such compounds is
controlling the introduction of oxygen. A number of approaches are being
investigated including the use of N2O as oxidant and partially
hydrated iron salts. Other projects
include the synthesis of low-coordinate early transition metals including Tl2+ and V2+.
Recent work has focused on metal-metal bonding and thus has resulted in the synthesis of an RCrCrR (R = C6H3-2,6(-C6H3-2,6-Pri3)2)
compound in which there is a quadruple CrCr bond. Current efforts are
designed to extend the known range of these compounds and the
exploration of their chemistry.
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| Various techniques are
being used to synthesize silicon, germanium, and boron nanoparticles. In
collaboration with the groups of my colleagues M. Augustine and S. M.
Kauzlarich, we are investigating the controlled reduction of various
organosilicon or organogermanium precursors in which the size of the
nanoparticle is determined by organic ligands employed. In addition, we
are investigating the use of zintl salts such as NaSi, NaSn or Mg2Ge
as precursors for these materials. Their reactions with element halides
and organoelement halides such as RGeCl or RSnCl (R = terphenyl ligand) in
order to generate clusters of the type MnRm (M = Si, Ge
or Sn, n > m) is also being investigated. Another approach to the
synthesis of these clusters involves the decomposition of hydride derivatives
of divalent or tetravalent metals. Stable, divalent group 14 element
hydrides, eg., RGeH or RSnH were unknown until they were recently synthesized
in our laboratories. Preliminary results show that careful heating of
these species affords clusters such as Sn9R3 (R = –C6H3-2,6-Trip2).
Other clusters of tin and germanium have been synthesized by the direct
reduction of RGeCl or RSnCl species with sodium or potassium. We have
shown that Boron nanoparticles can also be synthesized by reduction of
boron halides with alkali metals. With suitable surface capping, they
may have considerable potential application in neutron capture therapy.
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A
central theme in this area is to gain a better understanding of the
process involved in the regeneration of element-hydrogen bonds in
connection with hydrogen storage. The investigations are focused on the
use of element formate derivatives for this purpose.
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