Tetrahedral enforcers

In contrast, the more sterically demanding [TpBut]MgR derivatives of the tetrahedral enforcer system are stable to such redistributions, presumably due to the inability to form the corresponding [TpBut]2Mg complex. For this reason, the reactivity of the [TpBut]MgR complexes has been studied more extensively than that of the less sterically demanding [TpMe2]MgR system. 

The hydridotris(pyrazolyl)borate (Tp) ligand has become well known and well established as a formal analogue of the cyclopentadienyl (Cp) ligand (43). Unlike Cp, however, when appropriately substituted, Tp can become what has been described (44, 45) as a tetrahedral enforcer ligand, producing complexes that are constrained to be tetrahedral even when other factors might allow octahedral coordination. 

Bulky Tp ligands, sometimes considered tetrahedral enforcers, strongly promote a tetrahedral geometry even when this results in a highly unusual structure, such as 15e paramagnetic Co(ll) complex 5.40. 

The metallation should proceed via the formation of a chelated tetrahedral magnesium enolate complex, with a (Z)-geometry. The conformational rigidity would be enforced by chelation of both the imide enolate and bis(sulfonamide) ligand to the tetrahedral magnesium ion. 

Furthermore, the sulfonamide bond is expected to possess enhanced metabolic stability with structural similarities to the tetrahedral transition state involved in amide bond enzymatic hydrolysis, thus making sulfonamide peptides interesting candidates in the development of protease inhibitors and new drugs. The oligomers and polymers should also be interesting molecular scaffolds, with specific secondary structures enforced by hydrogen bonding)100,101 ... 

As one of the special features, MOMEC has a plane twist function. This has been included to limit the tetrahedral twist in four-coordinate compounds, where 1,3-nonbonded interactions lead to a preference for a tetrahedral arrangement (see Section 3.6). That is, the plane twist potential can be used to induce a square-planar arrangement or, using constraints, any intermediate structure can be enforced. The same potential can in principle be used for other structural features (see Fig. 17.14.1), such as the Bailar twist of six-coordinate complexes or for computing the rotational barrier of metallocenes. However, at present it has only been implemented in MOMEC for the tetrahedral twist and no parameters have been included as yet. 

Model the other Pt complexes described in Section 17.6 using a tetrahedral twist function rather than the previously used out-of-plane functions. Establish what values of the force constant ky are necessary to enforce planarity in the different complexes. 

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