Main-Group
and Main-Group Metal Chemistry
Our work centers around main-group chemistry, the
interaction of main-group elements with transition metals, and the
control of such interactions.
Variable
Coordination Around a Central Main-Group Element
Calixarenes are a unique class of compounds that posssess an unusual
combination of features: constraint and flexibility. The
size of the central cavity supplies the constraining feature, while the
ability of the phenolic groups to rotate provides flexibility.
This
is best illustrated in Scheme 1 summarizing our work on variable
coordination
of a single phosphorus atom within the framework of a calix[4]arene. Initially, insertion of
the phosphorus into
para-Rcalix[4]arene
(R = tert-butyl, H) yields the
six-coordinate zwitterionic
compound 1
via loss of two moles of dimethylamine. Removal of the third mole
of
dimethylamine, either by heat or treatment with acid, yields the
three-coordinate phosphite 2.
Methylation of 2 gives the
four-coordinate 3.
Finally, deprotonation of 3 yields the five-coordinate
phosphorane 4.
The size of the calix[4]arene cavity is ideal to support a single main-group atom, while the flexibility of the backbone adopts to the coordination requirements of the central atom. This is best illustrated by examining the structures of the six-coordinate and five-coordinate derivatives, 1 and 4, respectively.
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| In 1 (R = H), the calix[4]arene backbone adopts the cone conformation, ideal for the six-coordinate phosphorus atom. | In 4 (R = tert-butyl), one of the phenolic units "flips" to accommodate the five-coordinate phosphorus atom in an almost perfect trigonal bipyramidal geometry, with O(1) and O(3) at the axial positions and O(2), O(4), and C(1) at the equatorial sites. |
Control of Ligand/Metal Interaction
The calix[4]arene appears best to support a single main-group atom. In efforts to incorporate two atoms, i.e., a main-group (ligand) atom and a transition metal, we chose the larger calix[5]arene. The small increase in cavity size should provide room for both atoms, while still providing some constraint to control the ligand/metal interaction. Treatment of para-tert-butylcalix[5]arene with one equivalent of tris(dimethylamino)phosphine yields the mono-phosphorus compound 5 (Scheme 2). Insertion of tungsten proceeds smoothly to yield complex 6. The structure of 6 reveals that the phosphorus lone pair is pointing directly at the vacant coordination site on tungsten. However, the P---W distance is 3.15 Å, well outside the longest bond lengths reported for P-W bonds (although a small phosphorus-tungsten coupling constant is observed in the NMR spectrum, indicative of a weak interaction). Several factors may account for the long distance. The calix[5]arene backbone might prevent approach of the phosphorus to the tungsten. In addition, amido and imido ligands are known to be excellent π-donors to high oxidation-state-metals; thus, the tungsten may not want any further electron density. If the latter is the determining factor, then replacing one of the nitrogen ligands with a ligand that places less density on the metal might allow the phosphorus to bind to the tungsten. In fact, this is exactly what occurs. Treatment of 6 with trifluoromethanesulfonic acid yields 7 via replacement of tert-butylamido with triflate. The PW distance decreases to 2.74 Å (with a corresponding increase in the phosphorus-tungsten coupling constant), indicating bond formation. In addition, the calixarene backbone reorients to adopt the cone conformation. We see below that sometimes a conformational change alone can lead to a drastic alteration in the ligand/metal interaction.
Scheme 2
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| In 6, although the phosphorus lone pair is pointing directly at the vacant coordination site on tungsten, the distance is too long for a "bond." | In 7, the excellent π-donor amido ligand is replaced with the much weaker binder triflate resulting in formation of the P-W bond. In addition, the calixarene backbone reorients to adopt the cone conformation. In some cases, a conformational change alone can lead to a drastic alteration in the ligand/metal interaction (see below). |
The importance of the calixarene geometry in controlling the phosphorus/metal interaction is illustrated when 5 is treated with tetrakis(dimethylamino)titanium (Scheme 3). In this reaction, two isomers are formed, 8-cone and 8-1,2-alt, that differ only in the calix[5]arene conformation (with the calix[5]arene adopting an approximate cone conformation in one and an approximate 1,2-alternate conformation in the other). The P---Ti distance in 8-cone is 3.69 Å, well outside the range of any conceivable interaction. However, the 1.2-alternate conformation allows a significantly closer approach of the phosphorus (P---Ti distance = 2.90 Å). Part of the reason for this decreased distance in 8-1,2-alt may be the reduced steric repulsion of the dimethylamino groups in this conformation; however, this is not the only reason, since the tungsten complex 7 exhibits the cone conformation for the calix[5]arene, and it has the shorter P---W distance compared to 6 in which the calix[5]arene adopts an approximate 1,2-alternate conformation.
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| In 8-cone, the P---Ti distance is 3.69 Å, well outside the range of any conceivable interaction. | In 8-1,2-alt, the 1.2-alternate conformation allows a significantly closer approach of the phosphorus (P---Ti distance = 2.90 Å). |
Efforts are currently underway to further understand the constraints of
calix[5]arenes with a single phosphorus ligand (5)
as well as derivatives containing two phosphorus ligands.