Macrochelation, Cyclometallation and G-Quartet Formation: N3- and C8-Bound PdII Complexes of Adenine and Guanine

Abstract

The reactions of Pd(II) ions with a series of chelate-tethered derivatives of adenine and guanine have been studied and reveal a difference in the reactivity of the purine bases. Reactions of [PdCl2(MeCN)2] and A-alkyl-enH x Cl (alkyl = propyl or ethyl, A adenine, en = ethylenediamine) yield the monocationic species [PdCl(A-N3-Et-en)]+ (1) and [PdCl(A-N3-Pr-en)]+ (2). Both involve co-ordination at the minor groove site N3 of the nucleobase as confirmed by single-crystal X-ray analysis. Reactions with the analogous G-alkyl-enH x Cl derivatives (G=guanine, alkyl = ethyl or propyl) were more complex with a mixture of species being observed. For G-Et-en HCI a product was isolated which was identified as [PdCl(G-C8-Et-en)]+ (3). This compound contains a biomolecular metal-carbon bond involving C8 of the purine base. Crystallography of a product obtained from reaction of G-Pr-enH x Cl and [Pd(MeCN)4][NO3]2 reveals an octacationic tetrameric complex (4), in which each ligand acts to bridge two metal ions through a combination of a tridentate binding mode involving the diamine and N3 and monodentate coordination at N7.

[Au2(μ-G)(μ-dmpe)]·(KBr)0.75·2H2O, a cyclic dinuclear gold(I) complex with an N3,N9-bridging coordination mode of guanine and aurophilic interactions: synthesis, X-ray crystal structure and luminescence properties (dmpe=1,2-bis(dimethylphosphino)ethane and G=guaninato dianion)

“…In keeping with this, the 1 H NMR spectrum of 1 shows no low field NH signals and the C8-H resonance undergoes an upfield shift of about 0.9 ppm with respect to its position in the spectra of metal complexes of neutral 9-methylguanine [29]. Similar upfield shifts of the C8-H proton have been observed for the tetrameric complex [{(g 5 -C 5 Me 5 )Ir(G)(H 2 O)} 4 ](CF 3 SO 3 ) 4 (G ¼ N7, N9-guaninate bridging dianion) [14] and for [Au 2 (l-HX)(l-dmpe)] (HX ¼ N3,N9-xanthinate bridging dianion) [27]. These relatively large shifts may be a consequence of the shielding effect promoted by the large amount of electron density delocalised around the pyrimidine and imidazole rings.…”

“…In two instances, MCl 3 (H 3 G) Á 2H 2 O (M ¼ Zn(II) [11] and Cu(II) [12]) guanine is protonated, as the guaninium cation, and coordinated in monodentate fashion through the N9 atom to the metal ions. In the two remaining structures, Na 10 [Pt(l-PO 4 )(G)] Á 22H 2 O [13] and [Ir(g 5 -C 5 Me 5 )(G)(H 2 O) 4 ] 4 (CF 3 SO 3 ) 4 [14] the dianion of guanine (G) is coordinated to the metal ions in N9-monodentate and N7,N9-bridging forms, respectively.…”

Complexes Bearing Protic N-Heterocyclic Carbenes: Synthesis and Applications

“…-allyltheophylline 57, can be oxidatively added to low-valent group 10 metals (Scheme 24). 61,62 Reaction of 8-chlorocaffeine with [M(PPh 3 ) 4 ] (M = Pd and Pt) in the absence of a proton source yields the platinum complex [58] or the palladium complex [59] possessing a caffeine-derived anionic ylidene ligand. In the presence of a proton source, the cationic complexes [61] and [62] have been obtained from the same reaction.…”

“…In the presence of a proton source, the cationic complexes [61] and [62] have been obtained from the same reaction. These complexes can also be obtained by the protonation of [58] and [59] using HBF 4 . The complexes are rather robust and tolerate the strong acid HBF 4 in the N-protonation reaction.…”

“…57 A similar approach was used for the C 8 H oxidative addition of N-ethylenediamine-functionalized guanine to ruthenium 58 and palladium. 59 Precoordination of the diaminetether was essential in both cases for the C 8 H oxidative addition to proceed, since purine bases generally prefer to coordinate as Werner-type ligands via the N 7 /N 9 atoms of the five-membered heterocycle.…”