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Journal of Spectroscopy
Volume 2013, Article ID 982832, 8 pages
http://dx.doi.org/10.1155/2013/982832
Research Article

1H, 13C, and 15N NMR Studies of Au(III) and Pd(II) Chloride Complexes and Organometallics with 2-Acetylpyridine and 2-Benzoylpyridine

1Faculty of Chemistry, Nicolaus Copernicus University, Gagarina 7, 87100 Toruń, Poland
2Center of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90363 Lodz, Poland

Received 21 June 2012; Accepted 20 September 2012

Academic Editor: Fumiyuki Ito

Copyright © 2013 Daria Niedzielska et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Au(III) and Pd(II) chloride complexes with N(1),O-chelating 2-acetylpyridine (2apy) and N(1)- monodentately binding 2-benzoylpyridine (2bz′py)-[Pd(2apy)Cl2], [Au(2bz′py)Cl3], trans-[Pd(2bz′py)2Cl2], as well as Au(III) chloride organometallics with monoanionic forms of 2apy or 2bz′py, deprotonated at the acetyl or benzyl side groups (2apy*, 2bz′py*)-[Au(2apy*)Cl2], [Au(2bz′py*)Cl2], were studied by 1H, 13C, and 15N NMR. 1H, 13C, and 15N coordination shifts (i.e., differences between the respective , , and chemical shifts of the same atom in the complex and ligand molecules: , , ) were discussed in relation to the molecular structures and coordination modes, as well as to the factors potentially influencing nuclear shielding. Analogous NMR measurements were performed for the new (2bz′pyH)[AuCl4] salt.

1. Introduction

2-Substituted acyl derivatives or pyridine, for example, 2-acetylpyridine (2apy) and 2-benzoylpyridine (2bz′py) (Scheme 1) can coordinate transition metal ions in several ways. In case of Au(III) and Pd(II) chloride complexes three binding modes are known: N(1)-monodentate (e.g., [Au(2bz′py)Cl3] [1] and trans-[Pd(2bz′py)2Cl2] [27]), N(1),O-chelation (e.g., [Pd(2apy)Cl2] [6]) and N(1), - or N(1),C(2′)-chelation of 2apy* or 2bz′py* monoanionic ligands, formed by 2apy or 2bz′py deprotonation at the acetyl group or at the ortho-carbon within the phenyl ring (e.g., [Au(2apy*)Cl2] [8] and [Au(2bz′py*)Cl2] [1, 9]).

982832.sch.001
Scheme 1: 2-acyl derivatives or pyridine: 2-acetylpyridine (2apy) and 2-benzoylpyridine (2bz′py).

Some of these compounds exhibit catalytic properties (trans-[Pd(2bz′py)2Cl2] in hydrogenation of unsaturated functional groups [4, 5]) and biological activity ([Au(2bz′py*)Cl2] is an inhibitor of cathepsin cysteine proteases [9]). Although two X-ray structures were reported for these compounds (trans-[Pd(2bz′py)2Cl2]—CCDC 782241 [7] and [Au(2bz′py*)Cl2]—PUKYAV [1]; all refcodes derived from the Cambridge Structural Database [10]), they remain insufficiently characterized from the spectroscopic point of view. The 1H NMR spectra were reported for [Au(2bz′py)Cl3], trans-[Pd(2bz′py)2Cl2], [Au(2apy*)Cl2], and [Au(2bz′py*)Cl2] [1, 79], but in case of the two former compounds the published data are doubtful as they decompose in the applied DMSO-d6 solvent (vide infra). The 13C NMR measurements were performed for [Au(2apy*)Cl2] and [Au(2bz′py*)Cl2] but without signals assignments [8, 9], while the 15N NMR spectra were never described.

In the past, we studied 1H, 13C, and 15N NMR spectroscopic properties of some Au(III) and Pd(II) chloride complexes and organometallics with 2-phenylpyridine (2ppy: [Au(2ppy)Cl3], trans- and cis-[Pd(2ppy)2Cl2], [Au(2ppy*)Cl2] [11, 12]), using 1H-13C and 1H-15N HMQC/HMBC. Very recently, we have applied the same NMR techniques for analogous compounds with 2-benzylpyridine (2bzpy: [Au(2bzpy)Cl3], trans-[Pd(2bzpy)2Cl2], [Au(2bzpy*)Cl2] [13]). In this paper we report similar results for the following species: [Au(2apy*)Cl2], [Pd(2apy)Cl2], [Au(2bz′py)Cl3], trans-[Pd(2bz′py)2Cl2] and [Au(2bz′py*)Cl2]. Additionally, we describe the new (2bz′pyH)[AuCl4] salt, analogous to that of (2bzpyH)[AuCl4] (CCDC 873251 [13]).

2. Experimental

2.1. Materials

2apy and 2bz′py (98% purity) were purchased from Fluka, Au (99.99%) from Polish Mint, and PdCl2 (99.9%) from POCh Gliwice (Poland). Aqueous HAuCl4 solution (ca. 0.05 M) was prepared by dissolving Au in aqua regia followed by the removal of HNO3 and HCl excess by boiling. Solid NaAuCl4 and K2PdCl4 were prepared in water by the reactions of HAuCl4 with NaOH and PdCl2 with KCl, respectively, and evaporation to dryness.

2.2. Syntheses

[Au(2apy*)Cl2] was obtained by the method of Fan et al. (from NaAuCl4 and 2apy) [8], [Pd(2apy)Cl2] by that of Kovala-Demertzi et al. (from K2PdCl4 and 2apy) [6], [Au(2bz′py)Cl3] by that of Fuchita et al. (from NaAuCl4 and 2bz′py) [1], trans-[Pd(2bz′py)2Cl2] by that of Kovala-Demertzi et al. and Małecki and Maroń (from K2PdCl4 and 2bz′py) [6, 7], whereas [Au(2bz′py*)Cl2] by that of Zhu et al. (from NaAuCl4 and 2bz′py in propionitrile, in the presence of CF3SO3Ag [9]; it is worth noting that an attempt to synthesize this organometallic compound according to the report of Fuchita et al., that is, by refluxing [Au(2bz′py)Cl3] in propionitrile in the presence of CF3COOAg [1], has failed). The yields were ca. 60–80% for [Au(2apy*)Cl2], [Pd(2apy)Cl2], [Au(2bz′py)Cl3], trans-[Pd(2bz′py)2Cl2], and ca. 25% for [Au(2bz′py*)Cl2]. Their composition was confirmed by elemental analysis: [Au(2apy*)Cl2]—C 21.9%, H 1.7%, N 3.7% (calculated for AuC7H6NOCl2: C 21.7%, H 1.6%, N 3.6%); [Pd(2apy)Cl2]—C 28.3%, H 2.5%, N 4.8% (calculated for PdC7H7NOCl2: C 28.1%, H 2.4%, N 4.7%); [Au(2bz′py)Cl3]—C 29.6%, H 2.3%, N 3.0% (calculated for AuC12H9NOCl3: C 29.6%, H 1.9%, N 2.9%); trans-[Pd(2bz′py)2Cl2]—C 53.0%, H 3.3%, N 5.3% (calculated for PdC24H18N2O2Cl2: C 53.0%, H 3.3%, N 5.1%); [Au(2bz′py*)Cl2]—C 32.1%, H 1.9%, N 3.2% (calculated for AuC12H8NOCl2: C 32.0%, H 1.8%, N 3.1%). Their IR spectral data were as follows: [Au(2apy*)Cl2]— 1715 cm−1, 365, 356 cm−1; [Pd(2apy)Cl2]— 1685 cm−1, 367, 343 cm−1 (368, 342 cm−1 [6]); [Au(2bz′py)Cl3]— 1670 cm−1, 372, 360, 353 cm−1 (1671 cm−1 and 353 cm−1 [1]); trans-[Pd(2bz′py)2Cl2]— 1676 cm−1, 349 cm−1 (1670–1676 cm−1 and 345 cm−1 [3, 6, 7]); [Au(2bz′py*)Cl2]— 1675 cm−1, 353, 302 cm−1 (1674–1677 cm−1 and 362, 300 cm−1 [1, 9]). The trans-geometry of [Pd(2bz′py)2Cl2] was proved by the presence of only one stretching vibration ( ) typical for trans-MX2Y2 square-planar molecules ( symmetry), in consistency with the known X-ray structure (CCDC 782241 [7]).

(2bz′pyH)[AuCl4] was obtained by boiling aqueous HAuCl4 solution with 2bz′py in ethanol (1 : 1; 3 h). After concentration, the overnight-formed yellow precipitate was washed with ethanol (yield ca. 90%); elemental analysis: C 28.0%, H 2.4%, N 3.2% (calculated for AuC12H10NOCl4: C 27.6%, H 1.9%, N 2.7%). IR spectrum: 1670 cm−1, 355 cm−1, with these values being comparable to those for free 2bz′py ( 1668 cm−1; our data) and [AuCl4] ( 350 cm−1 [14]).

2.3. Spectroscopic Measurements

IR spectra were measured by a Perkin-Elmer Spectrum 2000 FT-IR spectrometer with a triglycine sulphate detector, in KBr and polyethylene for 400-400 cm−1 and 400–100 cm−1 ranges, respectively. NMR spectra were measured at 303 K in various solvents, by a Bruker Avance III 700 MHz NMR spectrometer. The 1H-13C HMQC, 1H-13C HMBC and 1H-15N HMBC experiments were adjusted for  Hz,  Hz,  Hz, with the following parameters: pulse lengths for 1H: 9-10 μs, 13C: 12-13 μs, 15N: 23-24 μs; acquisition time for 1H–13C HMQC and HMBC: 0.2–0.3 s, for 1H-15N HMBC: 0.07–0.08 s; relaxation delay for 1H-13C HMQC and HMBC: 1.5 s, for 1H-15N HMBC: 2 s. As references were used: TMS for 1H and 13C (with residual 1H and 13C solvent signals as primary references—in CDCl3: 7.24 ppm and 77.2 ppm, in CD2Cl2: 5.32 ppm and 54.0 ppm, in CD3CN: 1.94 ppm and 1.4 ppm, in DMSO-d6: 2.50 ppm and 39.5 ppm); neat nitromethane for 15N.

3. Results and Discussion

3.1. IR Spectroscopy

The shift of the C=O stretching vibration absorption band is sometimes used to determine the coordination mode in 2-acylpyridines complexes; thus we have tested this method for the studied known 2apy/2apy* and 2bz′py/2bz′py* compounds. In fact, the comparison of their IR spectra to those for free ligands reveals the decrease in [Pd(2apy)Cl2] (1700 cm−1 → 1685 cm−1) and the increase in [Au(2apy*)Cl2] (1700 cm−1 → 1715 cm−1) or [Au(2bz′py)Cl3], trans-[Pd(2bz′py)2Cl2], [Au(2bz′py*)Cl2] (1668 cm−1 → 1670, 1676, 1675 cm−1). Such a difference might indicate the presence or absence of the coordination bonding between the carbonyl group and the metal atom; however, our review of the literature of the IR data for some other 2apy/2apy* and 2bz′py/2bz′py* species has suggested this criterion is uncertain—even for the complexes having the same coordination sphere the changes are variable (e.g., 16 cm−1 decrease for [Ag(2apy-N1, CO)2] CF3SO3—PURYOR [15]: 1700 cm−1 → 1684 cm−1 [15] versus 0 cm−1 change for [Ag(2apy-N1, CO)2] ClO4—RAWVAN, RAWVAN01 [16, 17]: 1700 cm−1 → 1700 cm−1 [16]). Thus, much a better tool for such studies is NMR spectroscopy.

3.2. 1H NMR Spectroscopy

[Au(2apy*)Cl2] and [Pd(2apy)Cl2] are insoluble in CDCl3, CD2Cl2, and CD3CN, but soluble in DMSO-d6. However, [Au(2apy*)Cl2] is stable in the latter solvent, whereas [Pd(2apy)Cl2] immediately decomposes to free 2apy, with its solubility in the other media (e.g., DMF-d7) being insufficient even for 1H NMR measurements. For all studied compounds, the 1H chemical and coordination shifts are collected in Table 1.

tab1
Table 1: 1H chemical and coordination/protonation shifts (in parentheses) for Au(III) and Pd(II) chloride complexes, organometallics, and salts with 2apy or 2bz′py.

The 1H NMR spectral pattern of the [Au(2apy*)Cl2] organometallic corresponds well to the N(1),CH2-cyclometallated structure, revealing 5 proton resonances (2 doublets: H(3), H(6); 2 triplets: H(4), H(5); 1 singlet: CH2). The heterocyclic ring protons are significantly deshielded (mean value: 0.55 ppm), with the largest effect concerning the nitrogen-adjacent H(6) proton (  ppm); high deshielding occurs also for the deprotonated acetyl group (  ppm). The reason is probably an inductive effect of two electronegative chlorides and, in case of the acetyl substituent, the CH3 → CH2 transition followed by metallation of the respective carbon.

The review of NMR data for some other d8 square-planar transition metal ions reveals similarly large H(6) and CH2 deshielding effects in N(1), -chelated 2apy* organometallics (e.g., for [Pd(2apy*)(P(C6H5)3)Cl] in CDCl3:  ppm,  ppm, as  ppm,  ppm [18]), whereas in N(1), O-chelated 2apy complexes they occurred only for H(6) but not for CH3 (e.g., for [Rh(2apy)(CO)Cl], [Rh(2apy)(COCH3)Cl], and [Rh(2apy)(COC2H5)Cl] in CDCl3: –0.54 ppm, to −0.10 ppm, as  ppm,  ppm [19]); free 2apy in CDCl3:  ppm,  ppm [20].

The [Au(2bz′py)Cl3] and trans-[Pd(2bz′py)2Cl2] complexes are soluble and stable in CDCl3, CD2Cl2, and CD3CN. In all solvents, the latter molecule exhibits no rotational isomerism even upon cooling to −40°C; the only changes concern the H(6) peak, appearing at 20°C as a broadened singlet ( = ca. 20 Hz) and becoming at 0°C a well-shaped multiplet. The occurrence of trans-[Pd(2bz′py)2Cl2] in one form only is surprising as for many other trans-[PdL2Cl2] complexes with 2-alkyl- or 2-arylpyridines (L = 2-methylpyridine [21], 2,3-dimethylpyridine and 2,4-dimethylpyridine [22], 2-(2-chloroethyl)pyridine [23], 2bzpy [13, 24], and 2-(1-methylbenzyl)pyridine [25]) two distinct rotamers were observed by 1H NMR, even at ambient conditions. Most likely, in case of trans-[Pd(2bz′py)2Cl2] the steric hindrance allows for one stable conformation only, analogous to that known from the solid phase (CCDC 782241 [7]), that is, with phenyl rings in both 2bz′py molecules positioned at the largest distance.

For [Au(2bz′py)Cl3] the pyridine ring protons are moderately deshielded (mean : 0.29–0.49 ppm, depending on the solvent), similarly to [Au(2ppy)Cl3] and [Au(2bzpy)Cl3] (mean : 0.36 ppm and 0.32 ppm, both in CDCl3 [11, 13]). However, H(6) deshielding is larger than in both latter complexes ( : 0.22–0.39 ppm versus 0.15 ppm and 0.14 ppm [11, 13]).

For trans-[Pd(2bz′py)2Cl2] the pyridine ring protons are moderately shielded (mean : from −0.26 to −0.19 ppm, depending on the solvent), in contrast to trans-[Pd(2ppy)2Cl2] and trans-[Pd(2bzpy)2Cl2], where they were variously affected (mean : −0.04 ppm and 0.10 ppm, both in CDCl3 [11, 13]). In CDCl3, a relatively large H(6) shielding contrasts to much weaker effect for trans-[Pd(2ppy)2Cl2] and the deshielding for trans-[Pd(2bzpy)2Cl2] ( : −0.44 ppm versus −0.12 ppm and 0.44 ppm [11, 13]).

The differences between both complexes are probably caused by the fact that in [Au(2bz′py)Cl3] an inductive deshielding effect of three chlorides is present only, while in trans-[Pd(2bz′py)2Cl2] an analogous influence of two chlorides is overcome by anisotropic interactions of the adjacent heterocyclic rings, which can either increase or decrease 1H shielding constants, depending on the orientation of all -electron ring systems. The variations of an anisotropic effect caused by the C=O double bond must be also taken into account.

In both complexes the changes for phenyl ring protons are of variable sign and small absolute magnitude (mean : −0.02 to 0.05 ppm for [Au(2bz′py)Cl3], 0.02–0.05 ppm for trans-[Pd(2bz′py)2Cl2], depending on the solvent). On average, all heterocyclic ring protons in [Au(2bz′py)Cl3] are deshielded (mean : 0.12–0.25 ppm), while in trans-[Pd(2bz′py)2Cl2] they are shielded (mean : −0.10 to −0.06 ppm).

In DMSO-d6 both complexes immediately decompose, yielding uncoordinated 2bz′py, as their NMR spectra are identical with that for the free ligand; it is worth noting that some previous authors reported the 1H chemical shifts for [Au(2bz′py)Cl3] [1] and trans-[Pd(2bz′py)2Cl2] [7] just in this NMR solvent, assuming they concerned the complex molecules, although they were nearly the same as for 2bz′py. This problem is often for Au(III) and Pd(II) chloride-azine complexes, as exemplified by [Au(2bzpy)Cl3] and trans-[Pd(2bzpy)2Cl2] [13].

The [Au(2bz′py*)Cl2] organometallic is insoluble in CDCl3 but soluble in some other NMR solvents: slightly in CD3CN, moderately in CD2Cl2, and well in DMSO-d6, where, in contrast to [Au(2bz′py)Cl3], it does not decompose. A similar behavior was reported for [Au(LL*)Cl2] organometallics with many other 2-arylpyridines (LL = 2ppy [12, 2628], 2bzpy [13, 29, 30], 2-phenoxypyridine and 2-phenylsulfanylpyridine [31], 2-phenylaminopyridine, and 2-phenyl(N-methyl)aminopyridine [31, 32]).

In all solvents the 1H NMR pattern of [Au(2bz′py*)Cl2] corresponds well to the N(1),C(2′)-cyclometallated structure, revealing 8 proton resonances (4 doublets: H(3), H(6), H(3′), H(6′) and 4 triplets: H(4), H(5), H(4′), H(5′)), with the absence of H(2′) and inequivalency of all other phenyl ring hydrogens. The pyridine ring protons are significantly deshielded (mean value: 0.48−0.52 ppm, depending on the solvent), with the largest effect occurring for H(6) ( = 0.75−0.88 ppm versus = 0.33–0.48 ppm, depending on the solvent). In CDCl3, the H(6) deshielding is much larger than for [Au(2bz′py)Cl3] ( : 0.88 ppm versus 0.39 ppm); the same relation was noted in various media for [Au(2ppy*)Cl2] and [Au(2ppy)Cl3] (0.86–0.87 ppm versus 0.15 ppm [11, 12, 2628]), as well as for [Au(2bzpy*)Cl2] and [Au(2bzpy)Cl3] (0.66–0.79 ppm versus 0.00–0.33 ppm [13, 29, 30]). In DMSO-d6, H(6) deshielding is close to [Au(2ppy*)Cl2] and [Au(2bzpy*)Cl2] ( : 0.75 ppm versus 0.86–0.87 ppm [12, 2628] and 0.69 ppm [13, 30]). It seems to be related to the presence of chlorides in the Au(III) coordination sphere, because in [Au(2bz′py*)2]+ cations this proton was shielded (  ppm, as  ppm, in DMSO-d6, counterions [1]).

The changes for phenyl ring protons in [Au(2bz′py*)Cl2] are of variable sign and moderate or small absolute magnitude, although rather shielding (mean : from −0.08 to −0.02 ppm, depending on the solvent). The H(3′) atom, adjacent to the metallated C(2′) carbon, is moderately deshielded while the other protons are weakly shielded ( = 0.22–0.38 ppm versus = −0.28 to −0.05 ppm, depending on the solvent). In DMSO-d6, the same dependency was observed for [Au(2ppy*)Cl2] ( = 0.36 ppm versus = −0.11 to 0.02 ppm [12]) and [Au(2bzpy*)Cl2] ( = 0.13 ppm versus = −0.11 to −0.02 ppm [13]). On average, all heterocyclic ring protons in [Au(2bz′py*)Cl2] are moderately deshielded (mean value: 0.21–0.25 ppm, depending on the solvent), again similarly to [Au(2ppy*)Cl2] and [Au(2bzpy*)Cl2]   (mean : 0.30 ppm and 0.23–0.30 ppm [12, 13]).

The (2bz′pyH)[AuCl4] salt is insoluble in CDCl3 and CD2Cl2 but soluble in CD3CN. In the latter solvent its 1H NMR spectrum is noticeably different from that of unprotonated 2bz′py; however, addition of the free ligand results in only one set of 1H signals at intermediate chemical shifts, indicating fast proton exchange between 2bz′pyH+ and 2bz′py. In DMSO-d6 it immediately decomposes yielding unprotonated 2bz′py, probably due to the hydrolysis of 2bz′pyH+ cations by residual water (in the past we determined its amount in this NMR solvent by the Karl-Fischer method, as ca. 0.05% by weight [33]). It is worth noting that analogous 2bzpyH+ cations (from (2bzpyH)[AuCl4]—CCDC 873251) remained protonated in DMSO-d6 [13], which suggests that 2bz′py is a weaker base than 2bzpy.

In CD3CN the 2bz′py protonation results in the moderate 1H deshielding effect (mean for 2bz′pyH+: 0.24 ppm), much stronger for the pyridine ring (mean : 0.48 ppm) than the phenyl one (mean : 0.05 ppm). The deshielding of H(6) is weaker than for more far-distant pyridine ring protons ( = 0.21 ppm versus = 0.37–0.69 ppm). The same effects and dependencies were noted in various solvents for (2bzpyH)[AuCl4] (mean : 0.28–0.35 ppm; mean : 0.53–0.66 ppm versus mean : 0.08–0.11 ppm; = 0.00–0.33 ppm versus = 0.58–0.83 ppm [13]).

3.3. 13C and 15N NMR Spectroscopy

13C and 15N NMR chemical and coordination/protonation shifts for all studied species (except for [Pd(2apy)Cl2]) are collected in Tables 2 and 3, respectively.

tab2
Table 2: 13C chemical and coordination/protonation shifts (in parentheses) for Au(III) and Pd(II) chloride complexes, organometallics and salts with 2apy or 2bz′py.
tab3
Table 3: 15N chemical and coordination/protonation shifts (in parentheses) for Au(III) and Pd(II) chloride complexes, organometallics, and salts with 2apy or 2bz′py.

In [Au(2apy*)Cl2] the nitrogen-adjacent C(2) and C(6) atoms are shielded up to ca. 3 ppm, while the other heterocyclic carbons are deshielded up to ca. 7 ppm; the average effect for the whole pyridine ring is deshielding (mean value: 2.4 ppm). The Au-bonded acetyl carbon is deshielded by as much as ca. 32 ppm, whereas the uncoordinated carbonyl group remains nearly unaffected (shielding by ca. 2 ppm), which confirms the suggested N(1), -chelation mode.

The Au(III) or Pd(II) complexation of 2bz′py results in the predominant deshielding of pyridine ring carbons, up to ca. 5 ppm; the average effects (mean : 3.3 ppm for [Au(2bz′py)Cl3] and 2.4 ppm for trans-[Pd(2bz′py)2Cl2]) are nearly exactly the same like for [Au(2bzpy)Cl3] (3.2 ppm) and trans-[Pd(2bzpy)2Cl2] (2.4 ppm) [13]. Within the phenyl ring, the quarternary C(1′) atoms have not been detected, while those of C(2′)-C(6′) are deshielded, up to ca. 2 ppm and up to ca. 1 ppm, respectively.

For [Au(2bz′py*)Cl2] the changes are variable. The coordinated C(2′) atom is deshielded by ca. 6-7 ppm only, with this phenomenon being weaker than for [Au(2bzpy*)Cl2] (ca.12-13 ppm) [13] and [Au(2ppy*)Cl2] (ca. 26 ppm) [12]. So large differences between relatively similar compounds are probably caused by the fact that C(2′) deshielding is the overall result of carbon deprotonation and its subsequent auration, both effects being of opposite sign and comparable magnitude. The other heterocyclic carbons are variously affected: the quaternary C(2) and C(1′) atoms are shielded by ca. 8-9 ppm, while the CH ones are rather deshielded, up to ca. 7 ppm. On average, the 13C deshielding is noted (pyridine ring—mean : 1.5–1.6 ppm; phenyl ring—mean : 0.6–1.1 ppm; both heterocyclic rings-mean : 1.0–1.3 ppm). The carbonyl group is shielded by ca. 5 ppm.

The protonation of 2bz′py results in variable effects within both heterocyclic rings: from ca. 5 ppm shielding of C(6) to ca. 11 ppm deshielding of C(4) (the C(2) signal was not detected); a similarly complicated 13C NMR pattern was noted for 2bzpyH+ cations [13]. The carbonyl group is surprisingly shielded by as much as ca. 7 ppm, which may suggest its partial protonation and an equilibrium between N(1)- and CO-protonated forms of 2bz′pyH+.

The Au(III) or Pd(II) complexation of 2bz′py results in large N(1) with shielding, this effect being ca. 16 ppm weaker for [Au(2bz′py)Cl3] than trans-[Pd(2bz′py)2Cl2], as illustrated by absolute magnitudes of the 15N coordination shifts ( : 87.3 ppm versus 103.5 ppm). The same dependency was noted for many other [AuLCl3] and trans-[PdL2Cl2] complexes with N(1)-monodentately bonded azines (L = pyridine; 2-, 3-, 4-methylpyridine; 2,3-, 2,4-, 2,6-, 3,5-dimethylpyridine; 2,4,6-trimethylpyridine; 2-, 3-, 4-phenylpyridine; 2bzpy [11, 13, 21, 22, 34, 35]). Generally, for 2-arylpyridines the parameters decrease in the 2bz′py → 2bzpy → 2ppy order: 87.3 ppm → 83.9 ppm → 77.6 ppm for [AuLCl3] and 103.5 ppm → 94.4 ppm → 90.4 ppm for trans-[PdL2Cl2] [11, 13].

In [Au(2bz′py*)Cl2] the N(1) atom is ca. 17 ppm more shielded than in [Au(2bz′py)Cl3] ( : 104.7 ppm versus 87.3 ppm), with this difference being much larger than in case of similar pairs [Au(2bzpy*)Cl2]–[Au(2bzpy)Cl3] (90.9 ppm versus 83.9 ppm) [13] and [Au(2ppy*)Cl2]–[Au(2ppy)Cl3] (77.0 ppm versus 77.6 ppm) [11, 12]. For all studied [Au(LL*)Cl2] organometallics the parameters again decrease in the 2bz′py → 2bzpy → 2ppy order: 104.7 ppm → 90.9 ppm → 77.0 ppm [12, 13]. The N(1) shielding is observed also for [Au(2apy*)Cl2], being, however, ca. 10 ppm smaller than for [Au(2bz′py*)Cl2] (94.1 ppm versus 104.7 ppm).

The protonation of 2bz′py results in significant N(1) shielding as well, being ca. 13 ppm larger than for 2bzpy ( : 111.1 ppm versus 97.9 ppm [13]; the change of solvent-CD3CN versus DMSO-d6 must be taken into account). This effect is typical for azines [36] and proves that 2bz′pyH+ cations remain stable in CD3CN.

4. Conclusions

2-Acetylpyridine may coordinate Au(III) and Pd(II) central atoms in two different ways: by N(1), -chelation of the deprotonated 2apy* anion and by N(1),O-chelation of the neutral 2apy molecule. 2-Benzoylpyridine forms three various types of compounds with these metal ions: complexes with N(1)-monodentately bonded 2bz′py ligands, organometallics with the N(1),C(2′)-cyclometallated 2bz′py* anions, and salts containing protonated 2bz′pyH+ cations. Their 1H, 13C and 15N NMR spectra reveal some interesting dependencies between the respective , , coordination shifts and the molecular structures. For [Au(2apy*)Cl2] and [Au(2bz′py*)Cl2] the most characteristic phenomenon is the large deshielding of the nitrogen-adjacent H(6) proton and the metallated CH2 or C(2′) carbons, as well as even more significant shielding of the coordinated N(1) nitrogen; the latter effect occurs also for [Au(2bz′py)Cl3], trans-[Pd(2bz′py)2Cl2], and 2bz′pyH+.

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