Tris hydroborato ligands

Bis and tris(pyrazolyl)hydroborato ligands are generally prepared as the potassium derivatives by the direct reaction of KBH4 with the appropriate pyrazole (RR pzH), as illustrated in Scheme 1 for the parent system (13). The reaction is extremely general and has been applied to a number of different derivatives, which have included the incorporation of chiral (14, 15) and tethered (16) substituents. However, if the alkyl substituents of the pyrazole are inequivalent (i.e., R + R ), a potential problem may be encountered with the formation of isomeric products, in which either of the R and R substituents may occupy the 3-position of the poly(pyrazolyl)borato ligand. Nevertheless, if the difference in steric demands of R and R is large, there is a strong... 

Fig. 15. Cone angles (6) and wedge angles (at) of tris(pyrazolyl)hydroborato ligands.

In contrast to the chemistry of the zinc complexes [TpRR]ZnR, the cadmium alkyl derivatives have not been used to prepare an extensive series of half-sandwich [TpRR]CdX complexes. However, several [Tp JCdX complexes supported by sterically demanding tris(pyrazolyl)hydroborato ligands, e.g., [TpBut]CdCl (98), [TpBut]CdI (91), [Tp lCdl (91), and [TpBut,Me]Cd(Tj2-02N0) (91), have been synthesized by metathesis between CdX2 and either K[TpRR ] or Tl[TpRR ] [Eq. (15)]. 

Scheme 28. Modification of a tris(pyrazolyl)hydroborato ligand to a receptor for binding anions.

Finally, the tris(pyrazolyl)hydroborato ligand system has also provided an interesting example in which a crystallographic site is disordered between a vacancy and a chain of three atoms. Thus, the x-ray structure of the cobalt complex [TpAnt]CoNCS (Ant = 9-anthryl) revealed the presence of the cocrystallized thallium derivative... 

TpAnt]Tl, and that the composition of the crystal studied was approximately [TpAnt](CoNCS)0.94TI0.06. As a consequence of the cocrystallization, the crystallographic site occupied by the CoNCS moiety was disordered with a thallium atom (Fig. 57). The ability of [TpAnt]CoNCS and [TpAnt]Tl, which differ substantially in the number of atoms at the disordered site, to cocrystallize is presumably due to the fact that the disordered site is enclosed within the pocket that is provided by the tris(pyrazolyl)hydroborato ligand. 

Fig. 57. Cocrystallized [TpAnt]CoNCS and [TpAnt]Tl. Reprinted from Polyhedron, 14, Han, R., Parkin, G., and Trofimenko, S The tris[3-(9-anthryl)pyrazol-l-yl]hydroborato ligand, [TpAnt] Compositional disorder between a vacancy and a chain of three atoms, p. 387, copyright 1995, with kind permission from Elsevier Science Ltd, The Boulevard, Langford Lane, Kidlington 0X5 1GB, UK.

Tris(pyrazolyl)hydroborato ligands are also known to adopt bidentate and uniden-tate coordination to a lesser degree. For example, a rare example of unidentate coordination is provided by [i7l-TpBu,]Ni(PMe3)2(p-C6H4X) (X = H, Me, OMe). See Gutierrez, E. Hudson, S. A. Monge, A. Nicasio, M. C. Paneque, M. Carmona, E. 

In some recently discovered tris(pyrazolyl)hydroborato ligands, the 3-phenyl substituent is tethered to the 4-position of the pyrazolyl ring, e.g., tris(2//-benz[G]inda-zol-2-yl)hydroborato. See... 

An S4 coordination sphere of the type seen in the Ada protein, where four cysteines coordinate, was achieved with the tridentate tris(2-mercapto-l-phenylimidazolyl)hydroborato ligand. The zinc thiophenolate derivative showed reactivity of the thiolate linkage (Figure 8).506... 

Monomeric tetrahedral compounds can also be achieved by the use of tris-chelating ligands. The crystal structures of several [tns(pyrazolyl)hydroborato]alkylmagnesium have shown that they have shorter Mg—C bond lengths (2.10-2.18 A) and a typical example is given [Compound 30] [30]. 

The reactions usually proceeded to approximately 90% completion over a period of days, at which point the authors believe the redistribution reached equilibrium. However, substitution of the tris(3,5-dimethylpyrazolyl)hydroborato (>/ -HB(3,5-Me2pz)3) by the more sterically demanding tris(t-butylpyrazoyl)hydroborato ligand ( -HB(3- -Bupz)3) in magnesium complexes 13 prevents ligand redistribution after 7 days at 120°C. 

Tris(pyrazolyl)hydroborato ligands [HB(3,5-RR pz)3] (RR pz = C3N2RR H), which were introduced by Trofimenko in 1966 [10], are readily obtained as the potassium derivatives by the direct reaction of KBH4 with the appropriate pyrazole RR pzH [Eq. (2) 9],... 

Figure 2 Cone angles of tris(pyrazolyl)hydroborato ligands.

The molecular structures of several [Tp ]MgR derivatives have been determined by single-crystal X-ray diffraction, which confirms that in each case the molecules are solvent-free monomers, and that the tris(pyrazolyl)hydroborato ligand is -coordinated to magnesium. The coordination environment about the magnesium may be described as... 

Furthermore, the resonances attributable to the tris(pyrazolyl)hydroborato ligand are also... 

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