Multinuclear NMR study of Au(I), Pd(II) and Ag(I) pyrazole complexes to investigate the coordination mode

The coordination effects induced on the 1 H, 13 C, and 15 N-NMR chemical shifts of the 3,5-di-(4-butoxyphenyl)-1H-pyrazole, HPzbp2 ( 1 ) have been quantified for the following complexes:- [Au(HPz bp2 )(PPh 3 )](NO 3 )

The 1 H-, 13 C-, and 15 N-NMR spectra were obtained in DMSO-d 6 , CDCl 3 or CD 2 Cl 2 solutions, and the chemical shifts and coupling constants are reported in Tables 1-3.The 15 N-NMR chemical shifts referenced to external nitromethane were obtained by 2D-( 1 H-15 N) gs-HMBC and 2D ( 1 H-15 N) gs-HMQC experiments, save for the cases of the labeled derivatives 15 N 2 -HPz bp2 ( 15 N 2 -1) and [Pd(η 3 -C 3 H 5 )( 15 N 2 -HPz bp2 ) 2 ](BF 4 ) ( 15 N 2 -3) where the data were determined by the Inverse Gated 1 H-Decoupling Technique. 5olid state 13 C-and 15 N-NMR experiments were achieved by using the CPMAS sequence; in the 13 C-NMR the non-quaternary suppression sequence (NQS) was applied to identify the quaternary carbon atoms C-3, C-5, C-ipso, and C-para. 6In 15 N-NMR, solid ammonium chloride was used as reference and the chemical shift data were transformed to external nitromethane through the equation: δ (NO 2 CH 3 ) = δ(NH 4 Cl) -338.1 ppm. 7hen analyzing the NMR parameters reported in Table 1 for 1 H-NMR, Table 2 for 13 C-NMR, and Table 3 for 15 N-NMR, the most relevant features are as follows.
Coordination with silver(I) does not affect the 1 H-NMR chemical shifts, while in the case of palladium(II), coordination induces an upfield shift of ~ -0.16 ppm on the H-4, H-ortho and Hmeta protons; with gold(I) only a downfield shift on the H-4 and H-ortho protons, of about +0.16 ppm, was observed.In solution, the 13 C-NMR data for C-3 and C-5 of HPz-(1) cannot be compared with those of the corresponding complexes because a mean value of 148.5 ppm, corresponding to the two degenerate forms a and b in fast exchange equilibrium, is observed at 300 K. 8 The coordination effect on C-4 decreases when going from gold [+3.1 ppm] to palladium [+2.7 ppm] and to silver [+2.1 ppm] and the coupling constant value 1 J (C-4, H-4) increases with coordination [~ 5 Hz].Although to a lesser extent, a similar effect takes place on C-ortho.
However, the 13 C-CPMAS NMR of the pyrazole 1 presents two distinct values for C-3 and C-5 (150.3 and 144.4 ppm) as well as for the carbon atoms which correspond to the two 4butoxyphenyl groups, meaning that the tautomeric equilibrium between forms a and b is frozen-in agreement with its X-ray structure. 1In this way the metal-interaction effects could be evaluated: on C-4, the coordination induces a similar effect in the three complexes [~ +4 ppm], on C-3 the effect is smaller for gold-[+1.2ppm] than for palladium-and silver-[~ +4 ppm]complexes and, finally, δC-5 is almost unaltered.The splittings observed for the NMR carbon signals at the solid state can be explained as due to the presence of several independent molecules in the asymmetric unit and intermolecular packing effects. 9he 15 N-CPMAS NMR chemical shifts presented in Table 4 are the most useful ones for determining the coordination effects on the HPz bp2 (1) ligand.The 3,5-di-(4-butoxyphenyl)-1H-pyrazole, HPz bp2 (1) gives two signals at -183.6 and -98.0 ppm (these are different from the value of -143.5 ppm found in CDCl 3 solution) indicating that the exchange between the a and b tautomers is blocked.The chemical shift of the pyrazole N-2, that bonds to the metal, changes by about -55 ppm; this effect can be compared to that observed when pyrazole derivatives are protonated, i.e., -111.6 ppm for N-methylpyrazole. 10As expected the coordination with the metal does not significantly affect the chemical shift value of N-1.
A recent attempt to use 15 N-NMR studies in solution to probe the nature of electronic metalligand interactions in some organoplatinum complexes containing azines resulted in similar coordination effects [~ -54 ppm] on the nitrogen which binds the metal.The 15 N-CPMAS NMR data for the compound 15 N 2 -3 deserve special comment because it shows three different signals for N-1 and four signals for N-2, owing to spin-spin couplings between 15 N-19 F and 15 N-105 Pd. 12 The corresponding spectrum of 3 shows only two pairs of signals for each nitrogen.As we have already described, this derivative crystallizes in the monoclinic space-group P21/n as three Pd(η 3 -C 3 H 5 )( 15 N 2 -HPz bp2 ) 2 ] + species, 3 showing different dihedral angles between the phenyl and pyrazole groups which are associated through BF 4 groups in the asymmetric unit.Each BF 4 -group is bonded through strong N-H … F hydrogenbonds, with two N-H pyrazole groups bridging two neighboring complexes.Probably the splitting of around 40 Hz observed for N-1 corresponds to the coupling between 15 N-and 19 Fthrough the hydrogen-bond (N-H +… F-BF 3 ) in compound 3 in the solid state, even though there are no literature data for such types of coupling.Solid-state 13 C-(100.73MHz) and 15 N-(40.59MHz) CPMAS-NMR spectra were obtained with a Bruker WB-400 spectrometer at 300 K with a wide-bore 4-mm DVT probe-head at rotational frequencies of ca.5-10 kHz.Samples were carefully packed in ZrO 2 rotors, and the standard CPMAS pulse sequence and NQS technique (Non-Quaternary Suppression to observe only the quaternary C-atoms) were employed. 5

a
For the phenyl groups, the C-para and C-meta are nearly unaffected, and the C-ortho signals are shifted downfield [+2.8 to +1.5 ppm]: the C-ipso carbons move upfield upon coordination[Au > Pd > Ag].Metal complexation also shifts upfield the carbon atoms of the butoxy chains (See Table2).

Table 3 .
13C-NMR chemical shifts (δ in ppm) and coupling constants (J in Hz) for ring-carbons.In brackets are the coordinative chemical shift effects a Broad signal. 13
a Not detected.© ARKAT USA, Inc