Oxidative halogenation of dinuclear N-heterocyclic dicarbene gold(I) complexes

Journal of Organometallic Chemistry 723 (2013) 108e114

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Oxidative halogenation of dinuclear N-heterocyclic dicarbene gold(I) complexes Marco Baron a, Cristina Tubaro a, *, Marino Basato a, Marta M. Natile b, Claudia Graiff c a

Dipartimento di Scienze Chimiche, Università di Padova, via Marzolo 1, 35131 Padova, Italy ISTM-CNR, INSTM, Dipartimento di Scienze Chimiche, Università di Padova, via Marzolo 1, 35131 Padova, Italy c Dipartimento di Chimica Generale e Inorganica, Chimica Analitica, Chimica Fisica, Università di Parma, Viale delle Scienze 17/A, 43100 Parma, Italy b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 July 2012 Received in revised form 12 September 2012 Accepted 1 October 2012

Cl2, Br2, and I2 are effective agents for the oxidative addition of carbene-coordinated Au(I) centres in [Au2(MeIm-xylylene-ImMe)2](PF6)2. The resulting complexes depend on the type of xylylene bridge between the two carbene units. With m- and p-xylylene, the obtained products are always the Au(III)/ Au(III) complexes [Au2X4(MeIm-m,p-xylylene-ImMe)2](PF6)2 (X ¼ Cl, Br, I). By contrast, with the oxylylene bridge only in the reaction with iodine the Au(III)/Au(III) complex is isolated as the unique compound, while with chlorine or bromine a mixture of two complexes, the usual Au(III)/Au(III) and the Au(II)/Au(II) [Au2X2(MeIm-o-xylylene-ImMe)2](PF6)2 (X ¼ Cl, Br), is obtained. This different behaviour is attributable to a greater tendency of o-xylylene bridge to favour an approach of the gold centres compared to the m- and p- analogues. Ó 2012 Elsevier B.V. All rights reserved.

Keywords: Gold(III) N-heterocyclic dicarbene Halogen oxidative addition Dinuclear complexes

1. Introduction The chemistry of gold complexes has gained in the recent years an impressive development due to the discovery of unparallel characteristics in very broad fields like catalysis, energy storage and bioapplications [1]. The ligand set involves a variety of donor atoms, ranging from nitrogen or phosphorous to oxygen or sulphur. Very interesting results in the above cited fields were obtained also with N-heterocyclic carbene ligands by Nolan (catalysis and bioapplications) [2,3], Nocera (energy storage) [4], and Baker (bioapplications) [5]. Most effort was concentrated on gold(I) complexes, due to a limited stability of the þIII oxidation state for this metal. We have been interested since some years in the coordination chemistry of N-heterocyclic carbenes and in the catalytic applications of the corresponding Pd(II), Cu(I) and Ag(I) metal complexes [6]. We have also recently shown that Nheterocyclic dicarbenes efficiently coordinate to gold(I) centres [7] and that the resulting [Au2(MeIme(CH2)neImMe)2](PF6)2 (n ¼ 1e 4) dinuclear complexes can undergo oxidative addition by chlorine or bromine to afford stable and well characterised products [8]. The reaction outcome markedly depends on the length of the alkylene (CH2)n bridge linking the two carbenes moieties, so that with n ¼ 1, 2 and 4 the oxidation with X2 (X ¼ Cl, Br) gives only the dinuclear bis-dicarbene Au(III)eAu(III) complexes [Au2X4(MeIme (CH2)neImMe)2](PF6)2, while with the propylene bridging group

* Corresponding author. Tel.: þ39 049 8275655; fax: þ39 049 8275223. E-mail address: [email protected] (C. Tubaro). 0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jorganchem.2012.10.002

the major product is the dinuclear Au(II)eAu(II) complex [Au2X2(MeIme(CH2)3eImMe)2](PF6)2. In some cases, in the attempt to obtain crystals of these complexes, evolution products have been isolated, like a mixed valence Au(I)eAu(III) complex or an Au(III)eAu(III) metallopolymer. In order to generalise the behaviour of this type of gold complexes towards oxidative addition, we have drastically changed the nature of the bridge by employing ortho-, meta- and paraxylylene bridged dicarbenes (Chart 1), and extended the range of oxidants to iodine. 2. Results and discussion The gold(I) dicarbene complexes (1o, 1m, and 1p) have been synthesised via reaction in dimethylformamide of the diazolium salt, in the presence of a mild base (NaOAc), with the Au(I) precursor AuCl(SMe2), followed by anion metathesis in a methanol/ water mixture with KPF6 (Scheme S1, Supporting Information). The synthesis and characterisation of complexes 1o and 1m have been already reported by us [7], while complex 1p is a new compound. In all cases, the formation of the dicarbene complexes is confirmed by the absence of the C2-H signal in the 1H NMR spectrum of the reaction products, which indicates the deprotonation of the diimidazolium salt, and by the presence of the C2 carbon resonance at about 180 ppm, in a range typical of N-heterocyclic carbenes bonded to Au(I) centres. The reactivity of complexes 1 towards halogen addition is markedly different for 1o, so that it will be described in a separate session.

M. Baron et al. / Journal of Organometallic Chemistry 723 (2013) 108e114

Me

N

N

N

N

Me

ortho, meta and para substituted Chart 1. Dicarbene ligands employed in this study.

2.1. Oxidative addition of X2 to complexes 1m and 1p p The gold(III) complexes 2m X or 2X were synthesised by addition of halogen (Cl2, Br2 or I2) to an acetonitrile solution of the corresponding gold(I) dicarbene complexes in Au/X2 1/1.2 molar ratio (Scheme 1), exploiting an experimental procedure already successfully adopted for bridged dinuclear dicarbene gold(I) complexes [8]; the chlorine source was the safe and easy to handle PhICl2. Oxidative addition resulted to be selective towards the formation of Au(III)eAu(III) dinuclear complexes, which were isolated as yellow solids after partial evaporation of the acetonitrile solution and precipitation with diethyl ether. Addition of halogen at lower X2/Au ratios (0.5 and 0.75) did not modify the reaction output, in particular no Au(II)/Au(II) complexes were observed. In general, the best way to establish if the oxidation of the metal centre has occurred is the comparison of the carbene carbon atom chemical shift: in fact, it moves from ca. 180 ppm for NHCe Au(I) to 145e155 ppm for NHCeAu(III) complexes [8,9]. This upfield shift can be justified with the higher Lewis acidity of the Au(III) vs. Au(I) metal centre, which induces a greater delocalisation of the electron density from the C4eC5 double bond of the imidazole ring to the carbene carbon atom. The 1H NMR spectra of the gold(III) complexes are similar to those of the starting gold(I) compounds, thus indicating that the dinuclear and highly symmetric structure is maintained upon oxidation. This conclusion is also supported by the presence of the fragments [Au2X4(MeImeYeImMe)2(PF6)n](2  n)þ (n ¼ 0, 1) in the ESI-MS spectra of the gold(III) complexes. The general features of the 1H NMR spectra remain unchanged p in the series of complexes 2m X or 2X with the different anions X, thus suggesting that the steric hindrance of the coordinated halide scarcely affects the structure of the complexes. The most significant difference in the NMR spectra is represented by the chemical shift value of the carbene carbon atom, which changes from 154 ppm (Cl) to 151 ppm (Br) and 145 ppm (I). This trend can be tentatively attributed to the different electronegativity of the halides (Cl > Br > I) and it has been already observed in similar transdihalide bis(carbene) gold(III) complexes [10]. The dinuclear nature of the oxidised complexes has been further p confirmed by single-crystal X-ray analysis on 2m Cl (Fig. 1) and 2Cl. Suitable crystals have been obtained by slow diffusion of diethyl ether in an acetonitrile solution of the corresponding complexes.

109

Complex 2m Cl is isostructural with its bromine analogue, already reported by the authors [8a]. It has a centrosymmetric structure with the two aromatic rings of the xylylene bridge parallel. The bond angles CcarbeneeAueCcarbene and CleAueCl are close to linearity (C1eAueC16 ¼ 177.57(10) , Cl1eAueCl2 ¼ 178.80(3) ), as expected for a metal centre coordinated in a square planar geometry. The bond distances AueCl (2.2811(8) and 2.2842(7)  A) and A) are comparable to those reAueCcarbene (2.044(3) and 2.036(3)  ported for monomeric dichloro bis-NHC gold(III) complexes [9b,11]. The dihedral angle between the imidazol-2-ylidene rings coordinated to the Au(III) centre is 48.20(5) . The Au/Au distance is 8.854 Å (8.874  A for the bromine complex) [8a] and significantly longer than the Au/Au distance in the corresponding Au(I) complex (4.741  A) [5a]. A dinuclear structure has been observed also for complex 2pCl (Figure S1, Supporting Information). In fact, although the crystals do not allow a detailed refinement of the structure, the main features, as nuclearity, atoms connectivity and ligand arrangement, can be unequivocally established. 2.2. Oxidative addition of X2 to complex 1o A rather different behaviour has been observed for the Au(I) complex 1o with the o-xylylene bridge. In this case, only with iodine the oxidative addition afforded quantitatively the expected dinuclear Au(III)eAu(III) complex 2oI , whereas with chlorine or bromine the isolated solids resulted to be a mixture of the two complexes 2oCl/3oCl and 2oBr/3oBr, respectively. Complexes 2o are the usual Au(III) species, whereas complexes 3o are Au(II)eAu(II) derivatives. The characterisation and behaviour in solution for the chloro and bromo complexes is very similar and it will be discussed in details only for 2oBr/3oBr (Scheme 2). Their 1H NMR spectra differ for the methylene hydrogens signals of the o-xylylene bridge: a broad singlet for 2oBr and an AB system for 3oBr, thus indicating a rigid structure in solution for the last complex (see Fig. 2). The carbene carbon resonance in complex 3oBr (154.1 ppm) is close to the one reported for a similar propylene bridged Au(II) complex (154.7 ppm) [8b]. On these bases for complex 3oBr is proposed the same dinuclear structure determined via X-ray single-crystal analysis for the complex with the propylene bridge. It should be underlined that similar short Au/Au bond distances are observed in dicarbene gold(I) dinuclear complexes with these bridges (3.27  A for the propylene one and 3.05  A for a dinuclear oxylylene dicarbene complex similar to 1o) [5,7]. In the present case we were unable to isolate the gold(II) complex in pure form because, while the mixture 2oBr/3oBr is stable in the solid state, in deuterated acetonitrile solution it evolves in few hours to give a mixture of products 1o/2oBr/4oBr in a 4:12:3 molar ratio (Fig. 2). Treatment of the 1o/2oBr/4oBr mixture with a further excess of bromine allows the synthesis of pure 2oBr.

Scheme 1. Oxidative addition of halogen (Cl2, Br2 and I2): synthesis of the gold(III) dicarbene complexes with m- and p-xylylene bridging groups.

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M. Baron et al. / Journal of Organometallic Chemistry 723 (2013) 108e114

   Fig. 1. ORTEP view of complex 2m Cl; hydrogens, PF6 anions and solvent molecules are omitted for clarity. Selected bond distances (A) and angles ( ): C1eAu 2.044(3), C16eAu 2.036(3), Cl1eAu 2.2811(8), Cl2eAu 2.2842(7); C16eAueC1 177.57(10), C16eAueCl1 90.91(8), C1eAueCl1 91.13(8), C16eAueCl2 87.91(8), C1eAueCl2 90.06(8), Cl1eAueCl2 178.80(3). Symmetry code: (‘) ex, ey, ez.

Complex 4oBr is a mixed valence Au(I)/Au(III) complex, formally deriving from a disproportionation process of complex 3oBr. The identification of this species has been carried out through 1H, 13C and 2D NMR experiments; in particular two sets of signals relative to the two different imidazol-2-ylidene rings are present. More information on the obtained complexes derives from XPS analysis of the solid samples 1o, 2oBr and the mixture of two complexes, 2oBr/3oBr, which reveals the presence of gold in different oxidation states (Fig. 3). The binding energies (BEs) of the Au 4f core level for 1o complex (85.3 and 89.0 eV for Au 4f7/2 and 4f5/2, respectively) are characteristic of Au(I) [8a,12].

Concerning the 2oBr sample, the higher BEs of the main peaks at 87.7 and 91.4 eV (for Au 4f7/2 and 4f5/2, respectively) are indicative of the presence of Au(III) centres [8a,12], while the small contributions at lower BEs (85.6 and 89.3 eV for Au 4f7/2 and 4f5/2, respectively) are due to Au(I) deriving from the low stability of Au(III) under the X-ray beam. XPS measurements as a function of exposure time, in fact, were used in the past to differentiate between Au(I) and Au(III) [12]. Three different Au 4f doublets are evident in the sample 2oBr/3oBr: besides the small contributions of Au(I) due to the above cited sample instability to the X-ray beam, the main peaks are an overlap of two doublets (see Fig. 3 and

2+

1

o

Me

Br2

o

2

acetonitrile, RT Au/Br2 1/1.2

Br

N

Me

2 PF6-

N

Me

Y

Br

+

N

N Au

Au

Br

Y N

N

N

N

Me

3oBr

acetonitrile RT, 12 h

2+

Me o

2

Br

Br2

o

o

1 + 2

acetonitrile, RT

Br

+

N

N

Y

Br Me

N

N Au

Br

Me

Au Y

N

2 PF6-

N

N 4oBr

Scheme 2. Oxidative addition of bromine to complex 1o.

N

Me

M. Baron et al. / Journal of Organometallic Chemistry 723 (2013) 108e114

111

Table 1 Au 4f peak positions (Binding Energy, eV) and atomic %. Sample

Au 4f7/2

Au 4f5/2

%

1o 2oBr

85.3 85.6 e 87.7 85.4 86.9 87.9

89.0 89.3 e 91.4 89.1 90.6 91.6

100 13.4

2oBr/3oBr

Au(I) Au(I) Au(III) Au(I) Au(II) Au(III)

Au(I) Au(I) Au(III) Au(I) Au(II) Au(III)

86.6 8.5 43.9 47.6

species are also obtained from XPS analyses and are summarised in Table 1. It is worth mentioning that the atomic ratio between Au(II) and Au(III) in 2oBr/3oBr is around 1 confirming the presence of similar amount of the two complexes as above evidenced by NMR results. Fig. 2. 1H NMR spectra of the gold complexes obtained in the reaction steps described in Scheme 2. Complexes 1o (*), 2oBr (:), 3oBr (>) and 4oBr (C).

Table 1). The contributions at higher BEs are the same as those observed in the 2oBr complex suggesting the presence of Au(III). The contributions at 86.9 and 90.6 eV (for Au 4f7/2 and 4f5/2, respectively) are comparable with those reported in literature for Au(II) complexes [13]. The atomic amounts of different gold

3. Conclusions All the three employed halogens (Cl2, Br2, and I2) are effective agents for the oxidative addition to the Au(I) centres. The resulting Au(III) complexes are stable compounds, also with the less oxidising iodine. This confirms the ability of dicarbene ligands in stabilising both low and high oxidation states of the metals. The reaction output depends on the nature of the bridge between the two carbene units. With the m- and p-xylylene bridges, the obtained products are always the Au(III)/Au(III) complexes. By contrast, the type of complex isolated with the o-xylylene bridge appears to be dependent on the used oxidant. With iodine the only product is the Au(III)/Au(III) complex, while with chlorine or bromine a mixture of two complexes, Au(III)/Au(III) and Au(II)/Au(II) is obtained; the Au(II)/Au(II) complex slowly undergoes disproportionation in solution to the corresponding Au(I)/Au(I), Au(I)/Au(III) and Au(III)/ Au(III) complexes. Only a second treatment of this mixture with halogen in excess gives the pure Au(III)/Au(III) complex. Recently, we have tentatively attributed the tendency to give Au(II)/Au(II) complexes to a relatively short Au/Au distance; in this view it is not surprising that an o-xylylene bridge would favour an approach of the metal centres compared to the m- and p- analogues. For the same reason the more hindering iodine could limit this approach between the two gold atoms and favour their direct oxidative

Table 2 Selected crystallographic data for complex 2m Cl.

Fig. 3. Au 4f XP peaks: 1o (black solid line), 2oBr (grey solid line), 2oBr/3oBr (light grey solid line). Fitting of Au 4f peaks of 2oBr and 2oBr/3oBr were also reported.

Complex Formula Molecular weight Crystal system Space group a/ A b/ A c/ A a/ b/ g/ Volume,  A3 T (K) Z Dcalc/g cm3 F(000) m(Mo-Ka)/mm1 Reflections collected Unique reflections Observed reflections [I > 2s(I)] R [I > 2s(I)] R [all data]

2m Cl/2CH3CN C36 H42 Au2 Cl4 F12 N10 P2 1440.47 Triclinic P-1 7.9435(8) 11.9782(12) 13.5287(13) 74.3020(10) 83.0540(10) 73.9670(10) 1189.5(2) 203(2) 1 2.011 692 6.539 19504 7282 [Rint ¼ 0.0280] 6645 R1 ¼ 0.0236, wR2 ¼ 0.0593 R1 ¼ 0.0271, wR2 ¼ 0.0611

R1 ¼ SjFo  Fcj/jS(Fo); wR2 ¼ [S[w(F2o  F2c )2]/S[w(F2o )2]]1/2.

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M. Baron et al. / Journal of Organometallic Chemistry 723 (2013) 108e114

addition to gold(III). DFT calculations are underway to model this behaviour. 4. Experimental section 4.1. General remarks All manipulations were carried out using standard Schlenk techniques under an atmosphere of argon or dinitrogen. The reagents were purchased by Aldrich as high-purity products and generally used as received; all solvents were technical grade and used as received. The gold(I) complexes 1o and 1m and the gold(III) complex 2m Br were prepared according to literature procedures [5a,7,8a]. PhICl2 and the imidazolium salt 1,10 -dimethyl-3,30 -(pxylylene)diimidazolium dibromide were prepared following the literature procedures [14,15]. NMR spectra were recorded on a Bruker Avance 300 MHz (300.1 MHz for 1H and 75.5 for 13C); chemical shifts (d) are reported in units of ppm relative to the residual solvent signals. ESI-MS spectra were recorded on a Finnigan Thermo LCQ-Duo ESI-MS. XPS spectra were recorded using a PerkineElmer PHI 5600ci spectrometer with a standard Al-Ka source (1486.6 eV) working at 200 W. The working pressure was less than 1  108 Pa. The spectrometer was calibrated by assuming the binding energy (BE) of the Au 4f7/2 line to lie at 84.0 eV with respect to the Fermi level. Extended spectra (survey) were collected in the range 0e1350 eV (187.85 eV pass energy, 0.5 eV step, 0.025 s step1). Detailed spectra were recorded for the following regions: Au 4f, Br 3d, N 1s and C 1s (11.75 eV pass energy, 0.1 eV step, 0.2 s step1). The standard deviation in the BE values of the XPS line is 0.10 eV. The peak positions were corrected for the charging effects by considering the C 1s peak at 285.0 eV and evaluating the BE differences. The powder for the XPS analysis was evacuated for 12 h at ca. 1  103 Pa before measurement. 4.2. Synthesis of bis(1,10 -dimethyl-3,30 -(p-xylylene)diimidazol-2,20 diylidene)digold(I) bis(hexafluorophosphate) (1p) A mixture of sodium acetate (1.80 mmol), the diimidazolium salt 1,10 -dimethyl-3,30 -(p-xylylene)diimidazolium dibromide (0.80 mmol) and AuCl(SMe2) (0.81 mmol) in DMF (25 mL) was heated and maintained at 120  C for 2 h. Addition of n-hexane (10 mL) and dichloromethane (1 mL) afforded a white solid, which was filtered and dried under vacuum. The dibromide complex was dissolved in methanol (10 mL) and a solution of KPF6 (5 equiv.) in water (3 mL) was added, affording the precipitation of the desired product. The solid was filtered, washed with H2O (3 mL), methanol (2  3 mL) and finally dried under vacuum. White solid (yield 61%). Anal. Calcd for C32H36Au2F12N8P2: C, 31.57; H, 2.98; N, 9.21%. Found: C, 31.35; H, 2.31; N, 9.13%. 1H NMR (CD3CN): d 3.88 (s, 12H, CH3), 5.27 (s, 8H, CH2), 7.11 (s, 8H, CH), 7.20 (s, 8H, CH). 13C NMR (CD3CN): d 38.6 (CH3), 54.4 (CH2), 122.9 (CH), 124.4 (CH), 128.6 (CH), 137.9 (C), 185.3 þ (NCN). ESI-MS (positive ions, CH3CN): m/z 1071.37 [Au(I) 2 L2PF6] , 2þ L ] . 463.46 [Au(I) 2 2 4.3. General procedure for the oxidative addition of halogen (PhICl2, Br2 or I2) to the gold(I) complexes: synthesis of complexes 2oX, 2m X and 2pX A mixture of the halogen (0.48 mmol) and the gold(I) bis(hexafluorophosphate) complex 1 (0.20 mmol) in acetonitrile (15 mL) was stirred at room temperature overnight and then concentrated under vacuum. Addition of diethyl ether (15 mL) affords the desired product, which was filtered, washed with diethyl ether (3  3 mL) and dried under vacuum.

4.3.1. Tetrachlorobis(1,10 -dimethyl-3,30 -(o-xylylene)diimidazol-2,20 diylidene)digold(III) bis(hexafluorophosphate) (2oCl) The NMR spectra of the isolated yellow solid in CD3CN present two different sets of signals, relative to two distinct gold complexes, 2oCl and 3oCl, in ca. 12/7 molar ratio. Anal. Calcd for C32H36Au2Cl4F12N8P2 (63% 2oCl) þ C32H36Au2Cl2F12N8P2 (37% 3oCl): C, 28.87; H, 2.72; N, 8.50; Cl, 8.61%. Found: C, 28.99; H, 2.73; N, 7.81; Cl, 7.81%. (2oCl). 1H NMR (CD3CN): d ¼ 4.05 (s, 12H, CH3), 5.45 (s, 8H, CH2), 6.72 (m, 4H, xylylene), 7.25 (m, 8H, CH þ xylylene), 7.48 (m, 4H, CH). 13 C NMR (CD3CN): d 38.5 (CH3), 51.6 (CH2), 125.3 (CH), 127.3 (CH), 127.7 (2CH), 129.8 (CH), 133.1 (C), 154.9 (NCN). (3oCl). 1H NMR (CD3CN): d 3.57 (s, 6H, CH3), 5.02 and 5.34 (AB system, 4H, CH2), 7.05 (s, 2H, CH), 7.43 (s, 2H, CH), 7.71 (m, 4H, xylylene). The 13C resonances, which are not detectable in the 1D spectrum, are clearly observed in the 1H, 13C-HMBC NMR spectra in CD3CN; in particular it was possible to identify the carbene carbon resonance at 155.5 ppm. The above mentioned mixture 2oCl/3oCl evolves in CD3CN solution and after 3 h the 1H NMR spectrum presents three set of signals relative to complexes 1o, 2oCl and 4oCl in ca. 1/8/2 molar ratio. (4oCl). 1H NMR (CD3CN): d 3.78 (s, 6H, CH3), 4.01 (s, 6H, CH3), 5.22 and 5.23 (2s, 8H, CH2), 6.92 (m, 4H, CH), 7.10 (m, 4H, CH), 7.16 (m, 4H, CH), 7.42 (m, 4H, CH). It was possible to identify complex (4oCl) as a dinuclear gold(I)/gold(III) complex, as a consequence of the presence of the carbene carbon resonance at 154.2 and 185.6 ppm in the 1H, 13C-HMBC NMR spectra. The 2oCl/3oCl mixture was dissolved in acetonitrile (15 mL), kept under stirring for 12 h and then treated with PhICl2 in large excess for 24 h; the volatiles were removed under vacuum and the yellow residue was treated with diethyl ether, filtered and dried in vacuum. This treatment was repeated twice, finally giving a yellow solid, which was identified as pure 2oCl. Anal. Calcd for C32H36Au2Cl4F12N8P2: C, 28.29; H, 2.67; N, 8.25%. Found: C, 28.84; H, 2.81; N, 7.88%. 4.3.2. Tetrachlorobis(1,10 -dimethyl-3,30 -(m-xylylene)diimidazol2,20 -diylidene)digold(III) bis(hexafluorophosphate) (2m Cl ) Light yellow solid (yield 44%). Anal. Calcd for C32H36Au2Cl4F12N8P.22CH3CN: C, 30.00; H, 2.94; N, 9.72%. Found: C, 29.91; H, 2.92; N, 10.02%. 1H NMR (CD3CN): d 3.98 (s, 12H, CH3), 5.00 (s, 8H, CH2), 7.01 (m, 4H, xylylene), 7.23 (m, 4H, CH), 7.35 (s, 2H, xylylene), 7.38 (m, 6H, CH and xylylene). 13C NMR (CD3CN): d 38.5 (CH3), 53.8 (CH2), 125.4, 126.6, 128.7, 129.4, 131.3, 136.5 (CH and Ar), 154.1 (NCN). 4.3.3. Tetrachlorobis(1,10 -dimethyl-3,30 -(p-xylylene)diimidazol2,20 -diylidene)digold(III) bis(hexafluorophosphate) (2pCl) Light yellow solid (yield 92%). Anal. Calcd for C32H36Au2Cl4F12N8P2: C, 28.29; H, 2.67; N, 8.25%. Found: C, 27.74; H, 3.13; N, 7.95%. 1H NMR (CD3CN): d 3.97 (s, 12H, CH3), 5.09 (s, 8H, CH2), 7.16 (s, 8H, xylylene), 7.25 (d, 4H, CH), 7.37 (d, 4H, CH). 13C NMR (CD3CN): d 38.5 (CH3), 53.9 (CH2), 125.3 (CH), 126.6 (CH), 129.7 (CH), 136.4 (C), 154.2 (NCN). ESI-MS (positive ions, CH3CN): m/z (III) þ (I) þ 2þ 1213 [Au(III) 2 L2Cl4PF6] , 1071 [Au 2L2PF6] , 533 [Au2 L2Cl4] , 463 [Au(I)L]þ. 4.3.4. Tetrabromobis(1,10 -dimethyl-3,30 -(o-xylylene)diimidazol2,20 -diylidene)digold(III) bis(hexafluorophosphate) (2oBr) The NMR spectrum of the isolated yellow solid in CD3CN presents two different sets of signals, relative to two distinct gold complexes, 2oBr and 3oBr, in 1/1 molar ratio. Anal. Calcd for C32H36Au2Br4F12N8P2 (50% 2oBr) þ C32H36Au2Br2F12N8P2 (50% 3oBr): C, 26.47; H, 2.50; N, 7.72; Br, 16.21%. Found: C, 26.71; H, 2.81; N, 7.63; Br, 16.17%.

M. Baron et al. / Journal of Organometallic Chemistry 723 (2013) 108e114

(2oBr). 1H NMR (CD3CN): d 3.99 (s, 12H, CH3), 5.43 (s, 8H, CH2), 6.75 (m, 4H, xylylene), 7.25 (m, 4H, CH), 7.27 (m, 4H, xylylene), 7.50 (m, 4H, CH). 13C NMR (CD3CN): d 38.5 (CH3), 51.6 (CH2), 125.3 (CH), 127.7 (2CH), 129.8 (CH), 133.1 (C), 152.4 (NCN). (3oBr). 1H NMR (CD3CN): d 3.54 (s, 6H, CH3), 5.02 and 5.32 (AB system, 4H, CH2), 7.03 (s, 2H, CH), 7.43 (s, 2H, CH), 7.71 (m, 4H, xylylene). The 13C resonances, which are not detectable in the 1D spectrum, are clearly observed in the 1H, 13C-HMBC NMR spectra in CD3CN; in particular it was possible to identify the carbene carbon resonance at 154.1 ppm. The above mentioned mixture 2oBr/3oBr evolves in CD3CN solution and after 3 h the 1H NMR spectra presents three set of signals relative to complexes 1o, 2oBr and 4oBr in 4/12/3 molar ratio. (4oBr). 1H NMR (CD3CN): d 3.75 (s, 6H, CH3), 3.96 (s, 6H, CH3), 5.22 and 5.23 (2s, 8H, CH2), 7.00 (m, 4H, CH), 7.15 (m, 4H, CH), 7.23 (m, 4H, CH), 7.43 (m, 4H, CH). It was possible to identify (4oBr) as a dinuclear gold(I)/gold(III) complex, for the presence of the carbene carbon resonances at 151.8 and 185.4 ppm in the 1H, 13C-HMBC NMR spectra. The 2oBr/3oBr mixture was dissolved in acetonitrile (15 mL), kept under stirring for 12 h and then treated with bromine in large excess for 24 h; the volatiles were removed under vacuum and the yellow residue was treated with diethyl ether, filtered and dried in vacuum. The yellow solid was identified as pure 2oBr. Anal. Calcd for C32H36Au2Br4F12N8P2: C, 25.00; H, 2.36; N, 7.29%. Found: C, 24.75; H, 2.58; N, 6.96%. 4.3.5. Tetrabromobis(1,10 -dimethyl-3,30 -(p-xylylene)diimidazol2,20 -diylidene)digold(III) bis(hexafluorophosphate) (2pBr) Yellow solid (yield 76%). Anal. Calcd for C32H36Au2Br4F12N8P2: C, 25.00; H, 2.36; N, 7.29%. Found: C, 24.81; H, 2.69; N, 7.06%. 1H NMR (CD3CN): d 3.90 (s, 12H, CH3), 5.13 (s, 8H, CH2), 7.16 (s, 8H, xylylene), 7.28 (d, 4H, CH), 7.39 (d, 4H, CH). 13C NMR (CD3CN): d 38.8 (CH3), 54.1 (CH2), 125.6 (CH), 126.8 (CH), 129.8 (CH), 136.0 (CH), 151.5 þ (NCN). ESI-MS (positive ions, CH3CN): m/z 1391 [Au(III) 2 L2Br4PF6] , (I) þ (I) þ 1071 [Au2 L2PF6] , 463 [Au L] . 4.3.6. Tetraiodobis(1,10 -dimethyl-3,30 -(o-xylylene)diimidazol-2,20 diylidene)digold(III) bis(hexafluorophosphate) (2oI ) Orange solid (yield 72%). Anal. Calcd for C32H36Au2F12I4N8P2: C, 22.29; H, 2.10; N, 6.49%. Found: C, 22.15; H, 2.32; N, 6.46%. 1H NMR (CD3CN): d 3.89 (s, 12H, CH3), 5.36 (s, 8H, CH2), 6.83 (m, 4H, xylylene), 7.29 (m, 8H, CH), 7.53 (m, 4H, CH and xylylene). 13C NMR (CD3CN): d ¼ 39.0 (CH3), 51.7 (CH2), 125.6 (CH), 128.0 (2CH), 129.5 (CH), 133.1 (C), 145.8 (NCN). ESI-MS (positive ions, CH3CN): m/z þ (III) þ Au(I)L2I2PF6]þ, 1071 [Au(I) 1578 [Au(III) 2 L2I4PF6] , 1324 [Au 2 L2PF6] , 463 [Au(I)L]þ. 4.3.7. Tetraiodobis(1,10 -dimethyl-3,30 -(m-xylylene)diimidazol-2,20 diylidene)digold(III) bis(hexafluorophosphate) (2m I ) Orange solid (yield 86%). Anal. Calcd for C32H36Au2F12I4N8P2: C, 22.29; H, 2.10; N, 6.49%. Found: C, 22.64; H, 1.96; N, 6.71%. 1H NMR (CD3CN): d 3.81 (s, 12H, CH3), 4.98 (s, 8H, CH2), 7.20 (m, 4H, xylylene), 7.44 (m, 4H, CH), 7.48 (m, 8H, CH and xylylene). 13C NMR (CD3CN): d 39.5 (CH3), 54.8 (CH2), 126.1 (CH), 127.0 (CH), 129.8 (CH), 130.7 (CH), 131.7 (CH), 135.8 (CH), 145.0 (NCN). ESI-MS (positive þ (III) Au(I)L2I2PF6]þ, ions, CH3CN): m/z 1578 [Au(III) 2 L2I4PF6] , 1324 [Au þ (I) þ 1071 [Au(I) 2 L2PF6] , 463 [Au L] . 4.3.8. Tetraiodobis(1,10 -dimethyl-3,30 -(p-xylylene)diimidazol-2,20 diylidene)digold(III) bis(hexafluorophosphate) (2pI ) Orange solid (yield 92%). Anal. Calcd for C32H36Au2F12I4N8P2: C, 22.29; H, 2.10; N, 6.49%. Found: C, 21.66; H, 2.21; N, 6.30%. 1H NMR (CD3CN): d 3.80 (s, 12H, CH3), 5.14 (s, 8H, CH2), 7.20 (s, 8H, xylylene), 7.33 (d, 4H, CH), 7.42 (d, 4H, CH). 13C NMR (CD3CN): d 40.0 (CH3), 55.0 (CH2), 126.1 (CH), 127.0 (CH), 130.7 (CH), 136.0 (CH), 145.0

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þ (NCN). ESI-MS (positive ions, CH3CN): m/z 1578 [Au(III) 2 L2I4PF6] , þ (I) þ L PF ] , 463 [Au L] . 1324 [Au(III)Au(I)L2I2PF6]þ, 1071 [Au(I) 2 2 6

4.4. X-ray crystal structure determination of complexes 2m Cl Data for complex 2m Cl were collected at 203 K on a Bruker APEX II single-crystal diffractometer, working with Mo-Ka graphite A) and equipped with an monochromatic radiation (l ¼ 0.71073  area detector [16]. Details for the X-ray data collection are reported in Table 2. The structure was solved by direct methods with SHELXS-97 and refined against F2 with SHELXL-97 [17], with anisotropic thermal parameters for all non-hydrogen atoms. The hydrogen atoms were placed in the ideal geometrical positions. Acknowledgements C. T. thanks University of Padova for financial support (CPDA085452 and HELIOS project). Appendix A. Supplementary material Supplementary material related to this article can be found at http://dx.doi.org/10.1016/j.jorganchem.2012.10.002. References [1] (a) S.P. Nolan, Nature 445 (2007) 496; (b) Z. Li, C. Brouwer, C. He, Chem. Rev. 108 (2008) 3239; (c) A. Arcadi, Chem. Rev. 108 (2008) 3266; (d) D.J. Gorin, B.D. Sherry, F.D. Toste, Chem. Rev. 108 (2008) 3351; (e) M.M. Díaz-Requejo, P.J. Perez, Chem. Rev. 108 (2008) 3379; (f) A.S.K. Hashmi, M. Rudolph, Chem. Soc. Rev. 37 (2008) 1766; (g) See the special issue “Bioinorganic and Biomedical Chemistry of Gold”, Coord. Chem. Rev. 253 (2009) 1597e1708. [2] (a) N. Marion, S.P. Nolan, Chem. Soc. Rev. 37 (2008) 1776; (b) S.P. Nolan, Acc. Chem. Res. 44 (2011) 91; (c) S. Diez-Gonzalez, N. Marion, S.P. Nolan, Chem. Rev. 109 (2009) 3612. [3] J. Weaver, S. Gaillard, C. Toye, S. Macpherson, S.P. Nolan, A. Riches, Chem. Eur. J. 17 (2011) 6620. [4] T.S. Teets, D.G. Nocera, J. Am. Chem. Soc. 131 (2009) 7411. [5] (a) P.J. Barnard, M.V. Baker, S.J. Berners-Price, B.W. Skelton, A.H. White, Dalton Trans. (2004) 1038; (b) P.J. Barnard, L.E. Wedlock, M.V. Baker, S.J. Berners-Price, D.A. Joyce, B.W. Skelton, J.H. Steer, Angew. Chem. Int. Ed. 45 (2006) 5966. [6] (a) C. Tubaro, A. Biffis, R. Gava, E. Scattolin, A. Volpe, M. Basato, M.M. DíazRequejo, P.J. Perez, Eur. J. Org. Chem. (2012) 1367; (b) A. Biffis, L. Gazzola, P. Gobbo, G. Buscemi, C. Tubaro, M. Basato, Eur. J. Org. Chem. (2009) 3189; (c) A. Biffis, C. Tubaro, E. Scattolin, M. Basato, C. Santini, G. Papini, E. Alvarez, S. Conejero, Dalton Trans. (2009) 7223; (d) G. Buscemi, A. Biffis, C. Tubaro, M. Basato, Catal. Today 140 (2009) 84; (e) A. Biffis, C. Tubaro, G. Buscemi, M. Basato, Adv. Synth. Catal. 350 (2008) 189; (f) C. Tubaro, A. Biffis, E. Scattolin, M. Basato, Tetrahedron 64 (2008) 4187; (g) A. Biffis, G. Gioia Lobbia, G. Papini, M. Pellei, C. Santini, E. Scattolin, C. Tubaro, J. Organomet. Chem. 693 (2008) 3760; (h) C. Tubaro, A. Biffis, C. Gonzato, M. Zecca, M. Basato, J. Mol. Catal. A Chem. 248 (2006) 93. [7] M. Baron, C. Tubaro, A. Biffis, M. Basato, C. Graiff, A. Poater, L. Cavallo, N. Armaroli, G. Accorsi, Inorg. Chem. 51 (2012) 1778. [8] (a) M. Baron, C. Tubaro, M. Basato, A. Biffis, M.M. Natile, C. Graiff, Organometallics 30 (2011) 4607; (b) M. Baron, C. Tubaro, M. Basato, A. Biffis, C. Graiff, J. Organomet. Chem. 714 (2012) 41. [9] (a) D. Fremont, P.R. Singh, E.D. Stevens, J.L. Petersen, S.P. Nolan, Organometallics 26 (2007) 1376; (b) S. Gaillard, A.M.Z. Slawin, A.T. Bonura, E.D. Stevens, S.P. Nolan, Organometallics 29 (2010) 394. [10] H.G. Raubenheimer, P.J. Olivier, L. Lindeque, M. Desmet, J. Hrusak, G.J. Kruger, J. Organomet. Chem. 544 (1997) 91. [11] S. Gaillard, X. Bantreil, A.M.Z. Slawin, S.P. Nolan, Dalton Trans. (2009) 6967. [12] A. McNeillie, D.H. Brown, W.E. Smith, M. Gibson, L. Watson, J. Chem. Soc. Dalton Trans. (1980) 767. [13] (a) A.P. Koley, R. Nirmala, L.S. Prasad, S. Ghosh, P.T. Manoharan, Inorg. Chem. 31 (1992) 1764; (b) H. Schmidbaur, J.R. Mandl, F.E. Wagner, D.F. Van de Vodel, G.P. Van der Kelen, Chem. Commun. (1976) 170. [14] A. Zielinska, L. Skulski, Tetrahedron Lett. 45 (2004) 1087.

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(c) G.M. Sheldrick, SADABS, Bruker Analytical X-ray Systems, Madison, WI, 1999; (d) APEX II Software User Guide, SAINT Version 7.06a, SADABS, Version 2.01, Bruker AXS Inc., Madison, Wisconsin, USA, 2008. [17] G.M. Sheldrick, SHELX-97, Programs for Crystal Structure Analysis (Release 97-2), Göttingen, Germany, 1997.