Antonio Zucca

Synthesis and characterization of gold(III) adducts and cyclometallated derivatives with 6-benzyl- and 6-alkyl-2,2′-bipyridines

The reaction of a series of 6-substituted-2,2′-bipyridines HL (N2C10H7R, R = CH2Ph, CHMePh, CMe2Ph, CH2Me, CMe3 or CH2CMe3) with HAuCl4 or Na[AuCl4] has been investigated. Under different experimental conditions, salts [H2L][AuCl4], adducts [Au(HL)Cl3] or cyclometallated derivatives [Au(L)Cl][X](X = AuCl4, BF4 or PF6) have been isolated. The cyclometallated species arise from direct activation of a C–H bond either of a phenyl or a methyl substituent. The structures of an adduct [Au{N2C10H7(CHMePh)-6}Cl3] and two metallated species [Au{N2C10H7(CMe2C6H4)-6}Cl][AuCl4] and [Au{N2C10H7(CMe2CH2)-6}Cl][BF4]·0.5H2O have been determined by X-ray diffraction. In the adduct the gold atom is bonded to the nitrogen atom of the unsubstituted pyridine ring: a long-range interaction with the other nitrogen atom is observed, Au ⋯ N 2.758(4)A. In the two cyclometallated species the bipyridines act as tridentate N,N,C anions, giving a [5,6] and a [5,5]-fused ring system, respectively. In [Au{N2C10H7(CMe2C6H4)-6}Cl]+ the hexaatomic N,C ring adopts a boat-like conformation: the structure reveals a rather short interaction between one hydrogen of the Me group in axial position and the gold atom, Au ⋯ H 2.62 A. The pentaatomic N,C ring in [Au{N2C10H7(CMe2CH2)-6}Cl]+ is not planar: the co-ordination around the gold atom is essentially square planar. The new cyclometallated complexes [Au(N,N,C)Cl]+ are compared with those derived from similarly substituted pyridines described previously.

Platinum(II) N-heterocyclic carbene complexes: coordination and cyclometallation.

Four different NHC ligands have been coordinated to Pt(II) centres; for the first time cyclometallation of the NHC ligand was observed, but only when the platinum centre had a DMSO and two methyl co-ligands. Cyclometallation resulted in the exclusive formation of five-membered rings, and the absence of any double cyclometallation reactions with appropriate ligands rules out the possibility of an oxidative addition type mechanism for the cyclometallation reaction.

Palladium(II) and platinum(II)-C( 3 )-substituted 2,2′-bipyridines

Reaction of Pd(OAc)2 with HL1 and HL2 (HL1=6-iso-propyl-2,2′-bipyridine; HL2=6-neo-pentyl-2,2′-bipyridine), followed by treatment with LiCl or KI, gives [PdCl(L1)]2, (1), [PdCl(L2)]2 (2), and [PdI(L2)]2 (3), respectively. The chloride bridge in complexes1 and2 is split by PPh3 to give the mononuclear species PdCl(L1)(PPh3) (4) and PdCl(L2)(PPh3) (5). Spectroscopic data provide evidence for coordination of the deprotonated ligands through a nitrogen and the C(3) atom of the 6-substituted pyridine. An analogous platinum complex PtCl(L3)(SMe2) (6) (HL3=6-tert-butyl-2,2′-bipyridine) was obtained from trans-PtClMe(SMe2)2 and HL3. The crystal structures of compounds1 and6 have been solved by X-ray diffraction analysis.

Metallation of unactivated methyl groups: platinum(II) derivatives with 6-alkyl-2,2′-bipyridines

The reaction of K 2 [PtCl 4 ] with three 6-alkyl-2,2′-bipyridines HL, (N 2 C 10 H 7 R; R = CH 2 Me, HL et ; CHMe 2 , HL ip ; CH 2 CMe 3 , HL np ) affords five- and six-membered cyclometallated complexes [Pt(L)Cl] where L is a terdentate N , N , C anionic ligand which originates from HL through direct activation of a C(sp 3 )−H bond. In the case of HL ip , metallation generates a stereogenic carbon atom β to the metal. Complex 5, [Pt(L np )Cl], having an uncommon six-membered ring containing a Pt−C(sp 3 ) bond, was studied in depth in solution, by means of 1- and 2- dimensional NMR spectroscopy, and in the solid state by X-ray diffraction. Differences are observed in the behaviour of these ligands: thus with HLet an adduct, [Pt(HL et )Cl 2 ], was also isolated and its structure was determined by X-ray diffraction. In the metallated species, the fourth ligand, Cl, can easily be substituted by neutral or anionic ligands (CO, PPh 3 , CN − ). The metallated species react with potentially bidentate ligands L′−L′′ [Ph 2 PCH 2 PPh 2 (dppm), Ph 2 P(CH 2 ) 2 PPh 2 (dppe), Ph 2 P(CH 2 ) 2 AsPh 2 (dppae)] to give, with different Pt:L ratios, mononuclear [Pt(L)(L′−L′′)] + or dinuclear [(L)Pt(μ−L′−L′′)Pt(L)] ++ cationic species. In the second case an uncommon unsupported L′−L′′ bridge is present. The cyclometallated species are stable in air and moisture, and the N , N , C system is rather robust.

Cyclometallated derivatives of platinum(II) derived from 6-(tert-butyl)-2,2′-bipyridine (HL). Crystal and molecular structure of [Pt(L)CI]

The reaction of K2[PtCI4] with 6-(tert-butyl)-2,2′-bipyridine (HL) gave [Pt(L)CI]1a a cyclometallated species resulting from direct activation of a methyl C–H bond. The structure has been determined by X-ray diffraction. The crystals contain two independent molecules: average values for the principal parameters are Pt–CI 2.300, Pt–C 2.003, Pt–N (trans to C) 2.114 and Pt–N (trans to CI) 1.948 A. The chlorine atom in 1a can easily be displaced by neutral ligands (L′); in the presence of NaBF4 the cationic species [Pt(L)(L′)][BF4](L′= PPh32, CO 3 or 3,5-dimethylpyridine 4) were isolated in good yields. With potentially bidentate neutral ligands, L″–L″[Ph2P(CH2)2PPh2(dppe), Ph2P(CH2)2AsPh2(dadpe) or Ph2PCCPPh2(dppa)], [LPt(µ–L″–L″)PtL]2+(L″–L″= dppe 5, dadpe 6 or dppa 7) or [PtL(L″–L″)]+ species (L″–L″= dppe 8 or dadpe 9) can be obtained by adequate choice of the L″–L″ : Pt molar ratio (1 : 2 or 1 : 1, respectively). The new complexes 1–9 were characterized in solution by 1H, 13C-{1H} NMR spectra.

Reactivity of 6-(2-tolyl)- and 6-(2,6-xylyl)-2,2′-bipyridines with palladium(II) derivatives. Selective C(sp3)H vs. C(sp2)H activation

Two new 6-substituted 2,2′-bipyridines, L, 6-(2-tolyl)bipy, L1, and 6-(2,6-xylyl)bipy, L2, have been synthesized. Their reactions with Na2[PdCl4] or {Pd(OAc)2} afford either 1:1 adducts [Pd(L)X2] (X=Cl, OAc) or five-membered cyclometallated derivatives [Pd(L1-H)X] arising from C(sp2)H activation. From the chloro-alkyl intermediates [Pd(L)(Me)Cl], in the presence of Na[BAr′4] (Ar′=3,5-(CF3)2C6H3), cationic species [Pd(L)(Me)(S)]+ (L=L1, L2; S=CH3CN) can be obtained. At variance, in less coordinating solvents, e.g. dichloromethane, unexpected activation of a C(sp3)H bond occurs with loss of methane, to afford 6-membered cyclometallated derivatives. The latter species were isolated as [Pd(L-H)(PPh3)][BAr′4].

Electrochemical behaviour of cyclometallated gold(III) complexes. Evidence of transcyclometallation in the fate of electroreduced species

Abstract The electrochemical behaviour of a series of neutral and cationic N–C and N–N–C cyclometallated gold(III) species: ([Au(py1)(Cl)2], Hpy1=2-benzylpyridine; [Au(bipyn)(Cl)][PF6], Hbipy1=6-benzyl-2,2′-bipyridine, Hbipy2=6-(1-methylbenzyl)-2,2′-bipyridine, Hbipy3=6-(1,1-dimethylbenzyl)-2,2′-bipyridine, Hbipy4=6-phenyl-2,2′-bipyridine) has been investigated in different solvent systems using cyclic voltammetry and controlled-potential coulometry. All the species considered show, in cyclic voltammetry, an irreversible one-electron reduction in the potential range −0.9/−1.2 V versus Fc+/0, using a Pt electrode. On the other hand, exhaustive coulometries have shown the consumption of a number of Faraday per mole within the range 1–2. The very complex decomposition of the electrogenerated species affords elemental gold and significant amounts of different gold by-products. In some situations (e.g. reduction of [Au(bipy4)(Cl)][PF6]), working with 0.1 mol dm−3 Na[PF6], CH3CN solvent system, it has been possible to characterise the most abundant gold(III) compound, by NMR, FAB-MS, and elemental analysis. The analytical and spectroscopic data provide evidence for the N–C bis-cyclometallated gold(III) derivative (e.g. [Au(bipy4)2][PF6]); the result entails quite an unusual transcyclometallation process. The same compound can also be obtained, albeit in very minor yield, by chemical reduction of [Au(bipy4)(Cl)][PF6].

Gold(III) six-membered N boolean AND C boolean AND N pincer complexes: synthesis, structure, reactivity and theoretical calculations

The first six-membered gold(III) N^C^N pincer complex was obtained in good yield, under very mild conditions, by transmetalation of [Hg(κC-N^C^N)Cl] (N^CH^N = 1,3-bis(pyridin-2-ylmethyl)benzene, HL(1)) with Na[AuCl(4)]. The X-ray crystal structure of [Au(N^C^N)Cl][PF(6)] showed that the fused six-membered metallacycles each exist in a strongly puckered boat conformation. As shown by the (1)H NMR spectra in various solvents, the same structure is also retained in solution: no inversion of the six-membered metallacycles is observed in DMSO up to 95 °C. This correlates well with a reaction barrier of 17.5 kcal/mole, as determined by quantum chemical calculations. The reactivity of the present pincer complex is compared to that of the analogous 1,3-bis(2-pyridyl)benzene, HL(2), derivative, which has five-membered fused metallacycles. Sharp differences are found in the reactions with phosphines, such as PPh(3) and dppe (1,2-bis-diphenylphosphino-ethane), and with silver salts. Theoretical calculations were carried out on the two pincer complexes in order to try to understand these differences, and we found that the gold-chlorine bond is significantly stronger in the case of the complex containing five-membered metallacyclic rings.

Platinum complexes with NNC ligands. Syntheses, electrochemical and spectroscopic characterisations of platinum(II) and relevant electroreduced species

Abstract A series of cyclometallated platinum(II) species with NNC ligands (NNC= ortho -C-deprotonated form of 6-benzyl-substituted 2,2′-bipyridines) and different fourth monodentate ligands (Cl − , I − , CN − , CO, PPh 3 , Py, MeCN) has been investigated electrochemically, with particular emphasis to the analysis of the relatively stable one-electron reduced species. EPR spectra have been recorded at different temperatures on the electroreduced solutions. The analyses were supported by a systematic comparison with simulated spectra and allowed a definition of the interaction of the additional unpaired electron with the 195 Pt, 14 N, and 1 H nuclei present in the molecule. Attention has been also paid to the effects of the ligands on the thermodynamics ( E 1/2,r ) of the reduction, as well as to some reactivity aspects of the substrates that could be shown by voltammetric tests. A number of these compounds have been synthesised for the first time, so that a full characterisation has been performed.

Cyclometallated derivatives of platinum(II) derived from 1,4-benzodiazepin-2-ones. Crystal and molecular structure of Pt(L)(HL)Cl(HL7-chloro-1,3-dihydro-1-methyl-5-phenyl-2H-1,4-benzodiazepin-2-one, DIAZEPAM), a molecule containing a neutral and a deprotonated 1,4-benzodiazepin-2-one

Abstract The complexes [Pt(L)Cl]2 (1), Pt(L)(HL)Cl (2), (two conformers, 2a and 2b), Pt(L)(Ph3P)Cl (3), Pt(L)(3,5-Me2t-py)Cl (4) and Pt(L)(CO)Cl (5) (HL=7-chloro-1,3-dihydro-1-methyl-5-phenyl-2H-1,4-benzodiazepin-2-one, DIAZEPAM) have been prepared and characterized by IR, MS and 1H, 13C, 31p and two-dimensional correlation NMR spectra. Complexes 1-5 contain a deprotonated DIAZEPAM coordinated to the metal through the N(4) atom and the ortho-carbon of the 5-phenyl substituent. In complex 2, as shown by a sing]e crystal X-ray structure determination, carried out on conformer 2a, in addition to the cyclometallated system, a neutral molecule of DIAZEPAM is coordinated through the N(4) atom. The crystals of compound 2a, C32H25Cl3N4O2Pt, are monoclinic, space group P21/n with a = 13.601(3), b = 15.951(5), c = 13.837(3) A, β = 96.38(2)°, Z = 4. The structure was refined to R = 0.021 and Rw = 0.032 on the basis of 4922 unique reflections with I>3σ(I). The platinum atom is in a square planar geometry with the carbon atom trans to the chloride ligand: PtC=1.983(3), PtCl=2.402(1), PtN=2.031(3) and 2.009(3) A.