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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.
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 calixarene. Initially, insertion of the phosphorus into para-Rcalixarene (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 calixarene 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.
|In 1 (R = H), the calixarene 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 calixarene 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 calixarene. 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-butylcalixarene 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 calixarene 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.
|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 calixarene conformation (with the calixarene 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 calixarene, and it has the shorter P---W distance compared to 6 in which the calixarene adopts an approximate 1,2-alternate conformation.
|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 calixarenes with a single phosphorus ligand (5) as well as derivatives containing two phosphorus ligands.