Heterocycl. Commun., Vol. 16(4-6), pp. 261–268, 2010 • Copyright © by Walter de Gruyter • Berlin • New York. DOI 10.1515/HC.2010.008 A theoretical multinuclear NMR study of pyrazolylborates* Ibon Alkorta 1, **, José Elguero 1 , Rosa M. Claramunt 2 , Concepción López 2 and Dionisia Sanz 2 1 Instituto de Química Médica, CSIC, Juan de la Cierva, 3, E-28006 Madrid, Spain 2 Departamento de Química Orgánica y Bio-Orgánica, Facultad de Ciencias, UNED, Senda del Rey 9, E-28040 Madrid, Spain **Corresponding author e-mail: ibon@iqm.csic.es Abstract The experimental chemical shifts and coupling constants of five borates of general formula BH n Pz 4–n [from the borohy- dride to tetrakis(pyrazol-1-yl)borate] anions were compared with calculations carried out at the B3LYP/6-311 ++G(d,p) level (GIAO for absolute shieldings), in general with satisfy- ing results. The most stable conformations of pyrazolylborate anions are similar to those of neutral pyrazolylmethanes. Keywords: B3LYP/6-311 ++G(d,p); coupling constants; GIAO; NMR; pyrazolylborates; pyrazolylmethanes. Introduction Pyrazolylborates are fascinating molecules that formally originate from the borohydride anion (tetrahydroborate) ( 1 in Figure 1) replacing up to four H atoms for pyrazol-1-yl rings (Trofimenko, 1999). When the number of pyrazoles is 3, like in 4, these compounds are named scorpionates (Pettinari, 2008). Tris(pyrazolyl)borates and their analogues can be con- sidered as the tripodal equivalent of cyclopentadiene ligands and have been used in catalysis, bioinorganic models systems, metal extraction, and biomedicine. Discovered by Swiatoslaw Trofimenko (1966) they en- joyed an enormous success as ligands in coordination chem- istry. Alone or in collaboration with Trofimenko we have devoted several papers to these compounds (Aubagnac et al., 1991; López et al., 1994; Aubagnac et al., 1995; López et al., 1995; Janiak et al. 1996; Sanz et al., 1996; Claramunt et al., 2004a,b; Santa María et al., 2007; Trofimenko et al., 2007). One of the most significant of our contributions was entitled ‘Structure of bis-, tris- and tetrakispyrazolylborates in the solid state (X-ray crystallography) and in solution’ (López et al., 1990). On the other hand, we have been interested in the ab initio calculation of NMR properties, chemical shifts * Dedicated to Professor C. Foces-Foces on his untimely death on December 2010. (Alkorta and Elguero, 1998; Alkorta et al., 2010a) and cou- pling constants (Alkorta and Elguero, 2003a, 2010b) for sev- eral years. A natural extension of this work was to calculate the conformations of minimum energy as well as the chemical shifts and coupling constants reported in (López et al., 1990). This paper concerns the five compounds of Figure 1. Results and discussion Only the X-ray structure of tetrakis(pyrazol-1-yl)borate ( 5) has been determined (sodium and potassium salts) (López et al., 1990). For all of them, we have calculated the mini- mum energy conformations. The calculations were car- ried out at the B3LYP/6-311 ++G(d,p) level verifying in all cases that the structures were minima (number of imaginary frequencies =0). On these geometries we calculated the abso- lute shieldings ( σ, ppm, within the GIAO approximation) and the coupling constants ( J, Hz) (see Computational details). B3LYP/6-311 ++G(d,p) calculations yield acceptable results for computed NMR properties (Alkorta and Elguero, 1998, 2003a,b; Alkorta et al., 2009, 2010a; Jacob et al., 2010). It appears that no conformational analysis has been carried out on polypyrazolylborates ( 3, 4 and 5 and their C-substituted derivatives) while the conformation of their metal complexes have been widely examined (Trofimenko, 1971; Calderon et al., 1972; DaCruz and Zimmer , 1998; De Bari and Zimmer, 2004; Agrifoglio and Capparelli, 2005; Fraser et al., 2007; Mutseneck et al., 2010). Conformational analysis We carried out theoretical studies (MM2 and MNDO) of the conformational analysis of bis, tris and tetrakis(pyrazol- 1-yl)methanes, related to the compounds of this work but with a central C atom instead of a B - one (Claramunt et al., 1989). We also studied, at B3LYP/6-31G(d) computational level, the conformational space of tris(2-methylbenzimi- dazol-1-yl)methane, a compound related to 4 (Alkorta and Elguero, 2010c). In Table 1 are the energies associated with the different minima and in Figure 2 the corresponding geo- metries; to characterise the geometries we have used the same torsion angles ( φ A , φ B ,....) defined in our pyrazolyl- methanes paper (Claramunt et al., 1989). It is interesting to compare the minima found in the XH n Pz 4-n series (n =0–4) with X =B - (Figure 1) and C (Figure 3) both calculated at the B3LYP/6-311 ++G(d,p) level. For n =4, tetrahydroborate or borohydride ( 1) and meth- ane ( 1 ′), there is a single minimum in both cases with T d symmetry. For n =3, (pyrazol-1-yl)trihydroborate ( 2) and 1-meth- ylpyrazole ( 2 ′), there is a single minimum in both cases