Tp ligands

One purpose of our work is to mimic the chiral environment of the enzymes. Therefore, we thought it a reasonable goal to supply chiral models for the active sites of metalloenzymes. This was achieved before by Alsfasser et al. 113) or Vahrenkamp et al. 114) via amino acids that have been incorporated into the ligand systems. Modification of Tp ligands by chiral pyrazoles derived from the chiral pool is another way to chiral W,W,iV tripod ligands and has been achieved before by W. B. Tolman and coworkers (115). Thus, first we focused on the synthesis of a racemic mixture of a chiral NJtl,0 scorpionate... 

Due to otherwise extraordinary long abbreviations for compound 3d, its precursor 2d, and the ligands 8 and 25 the abbreviation system for Tp ligands introduced by S. Trofimenko was... 

Modification of Tp ligands by chiral pyrazoles derived from the chiral pool is another way to chiral N,N,N tripod ligands and has been achieved before by Tolman and coworkers (115). 

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. 

In contrast, the analogous reaction with NaTp affords exclusively the monomeric complex 461, in which fluxionality of the K2-Tp ligand was noted in the temperature range 28 to —31 °C.142 Moreover, free rotation of the 2-picolyl ligand was inferred from the equivalence of the methylenic protons (cf. inequivalent in 459, 460, and the parent), and a discernible 3lP—1H coupling of 5 Hz confirmed a cis disposition relative to the PPh3 ligand. 

Cp deriva ti ves.163 In the sold state, the two Tp ligands of 510 bind in an asymmetric k3 fashion, such that two pyrazolyl donors are strongly bound (d(Pd—N) 2.16 A), while the third interacts weakly (d(Pd—N) 2.64 A). This was interpreted as a formal 5-coordinate geometry correlating with the presumed intermediate responsible for equilibration of all pyr-azolyl groups in the solution phase. The metal metal bond of 510 was considered to be weak, on the basis of full occupancy of the a and n orbitals of the Pd2(CNXyl)2 core. 

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