Synthesis of mono-, di- and tripalladated 1,3,5-benzenetristyryl complexes. CO insertion to give a dipalladated indenone

Abstract

The tribrominated arenes 1,3,5-C₆(E-CH=CHAr)₃Br₃ (Ar = Ph, (I), p-To (I′)), add oxidatively to [Pd(dba)₂] ([Pd₂(dba)₃]·dba) in the presence of two equivalents of a phosphine (PPh₃ or PMe₂Ph) to form the monopalladated complexes trans-[Pd{C₆(E-CH=CHAr)₃Br₂}Br(L)₂] (Ar = Ph, L = PPh₃ (1a), Ar = p-To, L = PPh₃ (1a′), Ar = Ph, L = PMe₂Ph (1b)), while the reaction in a 1 : 2 : 4 arene : Pd : PMe₂Ph molar ratio affords the dipalladated complex [{trans-PdBr(PMe₂Ph)₂}₂{μ₂-C₆(E-CH=CHPh)₃Br}] (2b). Both I and I′ add oxidatively to 3 equivalents of [Pd(dba)₂] in the presence of the chelating N-donor ligand tmeda (N,N,N′,N′-tetramethylethylenediamine) to form the tripalladated complexes [{PdBr(tmeda)}₃{μ₃-C₆(E-CH=CHAr)₃}] (Ar = Ph, (3c), p-To (3c′)). Complex 3c reacts with PMe₃ to form [{trans-PdBr(PMe₃)₂}₃{μ₃-C₆(E-CH=CHPh)₃}] (3d). Compound 3c also reacts with CO to give the novel dipalladated indenone [2-Ph-4,6-{PdBr(tmeda)}₂-5,7-(E-CH=CHPh)₂-inden-1-one] (4). The crystal structures of 1a′ and 1b were determined by X-ray diffraction studies.

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Introduction

Arylpalladium complexes continue to be a subject of great interest because they are involved in many palladium-catalysed carbon–carbon and carbon–heteroatom bond-forming reactions.^1–17 Consequently, there are many publications related to their synthesis, characterization, and reactivity.^18–36 In particular, the presence of organic substituents ortho to the Pd atom has been shown to provide a very rich and varied chemistry, because they frequently participate in the reactivity of these complexes, resulting in interesting new structures and the formation of novel and potentially useful organic compounds.^{20–29,33,34,37,38} Some time ago, we started to explore the possibility of extending this chemistry to di-^25,26 and tripalladated^39–41 benzene derivatives, ortho-substituted at each Pd(II) centre. Thus, in 2001 some of us reported the first tripalladated benzene derivative, resulting from the 3-fold oxidative addition of 1,3,5-triiodomesitylene to 3 equivalents of [Pd(dba)₂].³⁹ This complex was shown to act as a metallaligand towards Tl(I) (Scheme 1).⁴¹ We later reported tripalladated derivatives of 1,3,5-triformylbenzene.⁴⁰ Since then, there have been (to the best of our knowledge) no further reports in this area, despite the structural and chemical interest of these complexes. In fact, most of the reports involving polymetalated derivatives of benzene with general formula C₆R_6−nM_n (n = 3–6) involve main-group elements, mainly Hg,^42–46 Li,^44,46–49 Mg,⁴⁴ Ge,^50,51 and Sn.^{44,46,52–56} There are also reports of transition metal derivatives, which mostly involve metal clusters with face-capping arene ligands coordinated to three metal atoms, such as Co,⁵⁷ Ru,^58,59 Rh,^60,61 and Os.^58,61–63 Murahashi and co-workers have reported several sandwich-type complexes with Pd₃ to Pd₅ clusters bridging two hydrocarbon rings.^64–68 However, for σ-bonded polymetalated derivatives of benzene, the only examples with Pd are those reported by our group,^39–41 plus the 3-fold cyclopalladation of a 1,3,5-tris(di-2-pyridylamino)benzene ring.⁶⁹ There is also a report of a tripalladated arylamine and a tetrapalladated pyrene, obtained by mechanochemical solvent-free oxidative addition of aryl halides to a Pd(0) complex.⁷⁰ Regarding other transition metals, only one trimetalated benzene with Mn, 1,3,5-C₆H₃[Mn(CO)₅]₃,^71–73 and two further examples with Fe, 1,3,5-C₆H₃[Fe(η⁵-Cp)(CO)₂]₃ ^71–74 and 1,3,5-C₆H₃[Fe(η⁵-C₅H₄Me)(CO)₂]₃,⁷⁵ have been reported so far.

Scheme 1 The first tripalladated benzene derivative, acting as a metallaligand towards Tl(I).

In view of the lack of progress in this area, we have resumed our research^39–41 on polypalladated benzene derivatives and now report the synthesis of mono-, di- and tripalladated 1,3,5-benzenetristyryl complexes, a set of compounds with a great potential as a branch point in the area of metalladendrimer chemistry.⁷⁶ Their chemistry towards unsaturated molecules is also potentially promising, as it could lead to the Pd-mediated synthesis of organic polycyclic compounds. We have started to investigate this chemistry, and we report here the formation of a novel dipalladated indenone, as a result of a CO insertion into one of the aryl–Pd bonds, followed by a depalladation reaction.

Results and discussion

Synthesis of the complexes

For the synthesis of the new 1,3,5-benzenetristyryl Pd(II) complexes, we have used oxidative addition reactions of the starting tribrominated arenes 1,3,5-C₆(E-CH=CHAr)₃Br₃ (Ar = Ph, (I), p-tolyl (I′)), to 1, 2 or 3 equivalents of [Pd(dba)₂] ([Pd₂(dba)₃]·dba),^77,78 in the presence of phosphine ligands (PPh₃ or PMe₂Ph, Scheme 2) or a chelating N-donor ligand (tmeda = N,N,N′,N′-tetramethylethylenediamine, Scheme 3).¹⁹

Scheme 2 Oxidative addition reactions of I, I′ to [Pd(dba)₂] in the presence of phosphines, to form mono- (1a, 1a′, 1b) and dipalladated (2b) complexes.

Scheme 3 Oxidative addition reactions of I, I′ to [Pd(dba)₂] in the presence of tmeda, to form the tripalladated complexes 3c, 3c′.

Reaction with phosphines. 2,4,6-Tribromo-1,3,5-tris-(E-styryl)benzene (1,3,5-C₆(E-CH=CHPh)₃Br₃, I, Scheme 2) adds oxidatively to one equivalent of [Pd(dba)₂] in the presence of two equivalents of phosphine (PPh₃ or PMe₂Ph) to form trans-[Pd{C₆(E-CH=CHPh)₃Br₂}Br(L)₂] (L = PPh₃ (1a), PMe₂Ph (1b)). Similar reactions with a 1 : 2 : 4 arene : Pd : phosphine molar ratio only afford a dipalladated complex, [{trans-PdBr(PMe₂Ph)₂}₂{μ₂-C₆(E-CH=CHPh)₃Br}] (2b), with PMe₂Ph, as the reaction with PPh₃ again gives complex 1a. Our previous work had shown that the trihaloarenes 1,3,5-C₆Me₃I₃ ³⁹ and 1,3,5-C₆(CHO)₃Br₃ ⁴⁰ also form dipalladated Pd(II) complexes only with the more basic and less sterically demanding phosphine PMe₂Ph, and not with PPh₃, so this seems to be a general trend. Also similarly to those arenes,^39,40 the 3-fold oxidative addition of I to [Pd(dba)₂] in the presence of any of the phosphine ligands (PPh₃, PMe₂Ph, or PMe₃) was unsuccessful. The related arene 1,3,5-C₆(E-CH=CHTo)₃Br₃ (To = p-tolyl, I′) reacts with [Pd(dba)₂] and PPh₃ in a 1 : 1 : 2 ratio to give a monopalladated complex 1a′ (Scheme 2), which was characterized by X-ray crystallography, as was 1b (Fig. 1 and 2). Oxidative addition reactions of I′ in the presence of PMe₂Ph or PMe₃ were not investigated.

Fig. 1 Thermal ellipsoid plot (50% probability level) of 1b. Selected bond lengths (Å) and angles (°): Pd(1)–C(1) = 2.010(3), Pd–P(1) = 2.3138(9), Pd(1)–P(2) = 2.3116(9), Pd(1)–Br(1) = 2.5180(4), Br(2)–C(3) = 1.916(3), Br(2)–C(5) = 1.916(3), C(1)–Pd(1)–P(1) = 89.85(8), C(1)–Pd(1)–P(2) = 90.17(8), P(2)–Pd(1)–P(1) = 177.54(3), C(1)–Pd(1)–Br(1) = 177.05(9), P(1)–Pd(1)–Br(1) = 90.00(2), P(2)–Pd(1)–Br(1) = 90.10(2).

Fig. 2 Thermal ellipsoid plot (30% probability level) of 1a′. Selected bond lengths (Å) and angles (°): Pd(1)–C(1) = 2.022(2), Pd–P(1) = 2.3334(6), Pd(1)–P(2) = 2.3680(6), Pd(1)–Br(1) = 2.5201(3), Br(2)–C(3) = 1.915(2), Br(2)–C(5) = 1.919(2), C(1)–Pd(1)–P(1) = 87.39(6), C(1)–Pd(1)–P(2) = 91.05(6), P(1)–Pd(1)–P(2) = 174.70(2), C(1)–Pd(1)–Br(1) = 176.24(6), P(1)–Pd(1)–Br(1) = 88.987(18), P(2)–Pd(1)–Br(1) = 92.648(18).

Reactions with tmeda. The arene I reacts with [Pd(dba)₂] and the chelating N-donor ligand tmeda to form the tripalladated complex [{PdBr(tmeda)}₃{μ₃-C₆(E-CH=CHPh)₃}] (3c, Scheme 3), even if substoichiometric amounts of Pd and tmeda are used (1 : 1 : 1 or 1 : 2 : 2 arene : Pd : tmeda ratio, instead of the required 1 : 3 : 3 ratio, which, nonetheless, results in a cleaner reaction). A similar tripalladated complex, [{PdBr(tmeda)}₃{μ₃-C₆(E-CH=CHTo)₃}] (3c′), can be isolated from the reaction of 1,3,5-C₆(E-CH=CHTo)₃Br₃ (I′) with [Pd(dba)₂] and tmeda, although 3c′ forms alongside minor amounts of another complex, presumably a lower nuclearity analogue. Curiously, we have found that a substoichiometric arene : Pd : tmeda ratio (such as 1 : 2 : 2) increases the yield of 3c′, which was purified by TLC (see Experimental). Our previous work^39,40 had shown that similar reactions with the arene 1,3,5-C₆Me₃I₃ ³⁹ also afforded tripalladated complexes [{PdI(N^N)}₃(μ₃-C₆Me₃)] (N^N = bpy, tbbpy), even with substoichiometric arene : Pd : N^N ratios, while the oxidative addition of 1,3,5-C₆(CHO)₃Br₃ ⁴⁰ to [Pd(dba)₂] in the presence of N^N ligands afforded mixtures of mono-, di- or tripalladated complexes (depending on the stoichiometric reactant ratio), from which the separate complexes could be isolated and characterized (with N^N = bpy the solubility was very low, and only a monopalladated complex was isolated, while with N^N = tmeda and tbbpy the mono-, di- and tripalladated complexes could be separated by TLC and fully characterized). Clearly, the strong electron-withdrawing character of the formyl groups in 1,3,5-C₆(CHO)₃Br₃ did not favour the successive oxidative additions,⁴⁰ allowing a certain degree of selectivity in the palladation of the arene, in contrast to 1,3,5-C₆Me₃I₃,³⁹ and the arenes I and I′ described here.

Reactions of I and I′ with [Pd(dba)₂] in the presence of other chelating N^N ligands such as tbbpy (4,4′-di-tert-butyl-2,2′-bipyridine) or bpy (2,2′-bipyridine), or a chelating phosphine ligand, dppe, resulted in mixtures of compounds, which could not be characterized.

Reactions with complex 3c. The tripalladated complex [{PdBr(tmeda)}₃{μ₃-C₆(E-CH=CHPh)₃}] (3c) reacts with excess PMe₃ (1 : 12 ratio) at 50 °C to form [{trans-PdBr(PMe₃)₂}₃{μ₃-C₆(E-CH=CHPh)₃}] (3d, Scheme 4), which could not be prepared by direct oxidative addition (see above). This reactivity is also consistent with previous observations,^39,40 showing that the failure to obtain tripalladated complexes by oxidative addition in the presence of PMe₃ is probably the result of kinetic, and not thermodynamic, effects. A similar ligand displacement reaction with 3c′ was not pursued.

Scheme 4 Ligand displacement reaction on 3c.

We have started to explore the reactivity of 3c towards unsaturated organic molecules, which could potentially insert into the C−Pd bonds, resulting in novel polynuclear complexes or polycyclic organic compounds (see the Introduction). Frequently, this reactivity is promoted by TlOTf, but when 3c was reacted with alkynes (RC≡CR, R = Me, Ph, COOMe) in the presence of TlOTf, the product was the same as in the reaction with TlOTf alone, and was presumably a complex between 3c and TlOTf, similar to that shown in Scheme 1 (Introduction),⁴¹ but which we have not been able to characterize fully. Reactions of 3c with alkynes in the absence of TlOTf yielded only the starting material, and the use of AgClO₄ instead of TlOTf resulted in mixtures of compounds, which could not be characterized. Reactions of 3c with XyNC also failed to yield a major characterizable product, but when 3c was reacted with CO, the dipalladated indenone, [2-Ph-4,6-{PdBr(tmeda)}₂-5,7-(E-CH=CHPh)₂-inden-1-one] (4, Scheme 5), formed as a major product, which was purified from the minor impurities by preparative TLC and isolated with a very good yield (76%). Complex 4 is the result of the insertion of CO into only one of the three aryl–Pd(II) bonds in 3c, followed by depalladation in that position, and has been fully characterized by NMR. No further CO insertions into the two remaining aryl–Pd(II) bonds (to form a C3-symmetric tri-indenone derivative) were achieved, even if the reaction was carried out in excess CO and heating at 60 °C (see Experimental). Attempts to further react 4, once isolated, with CO have also been unsuccessful. Thus, the formation of the first indenone ring seems to deactivate the complex towards successive insertions. Although the palladium-catalyzed synthesis of indenones by annulation of alkynes and CO with ortho-functionalized arenes has been described before,^79–82 in those reports the intermediate ortho-alkenylarylpalladium complexes were not isolated. Moreover, this is the first time that such a CO insertion reaction is carried out in a polypalladated complex, thus affording a dipalladated indenone, a type of dinuclear Pd(II) complex that has not been described before. The reactivity of the PMe₃ complex 3d towards CO was not similar to that of 3c and provided no clean result. Complex 3c′ was not considered a suitable starting material for this research, because of the difficulties related to its purification (see above). The reactivity of the mono- and dipalladated complexes 1a, 1a′, 1b and 2b towards unsaturated molecules has not been investigated so far.

Scheme 5 CO insertion in 3c to form the dipalladated indenone 4.

Structure of the complexes

The ¹H, ¹³C and ³¹P NMR data of complexes 1–4 agree with the structures proposed in Scheme 2–5 (see Table 1 in the ESI† for a comparative assignment). The 16 Hz value for the ³J_HH coupling constant between the alkenyl protons confirms the E geometry for all the complexes. The tripalladated complexes 3c and 3c′ show a 1 : 2 pattern for all the NMR resonances, as one of the [PdBr(N^N)] groups is oriented in the opposite direction to the other two with respect to the aryl plane (the rotation around the Pd−C bonds is hindered). The mono- (1a, 1a′, 1b) and dipalladated (2b) phosphine complexes also show a 1 : 2 pattern in the ¹H and ¹³C-NMR spectra, as a result of the substitution pattern around the aryl ring. In contrast, the tripalladated PMe₃ complex 3d shows a single set of resonances, both in the ¹H and in the ¹³C spectra, as this molecule has a 3-fold axis perpendicular to the aryl plane.

The phosphine complexes, 1a, 1a′, 1b, 2b and 3d, have an all-trans geometry and thus they have a symmetry plane in the aryl ring and show a single ³¹P resonance. As usual, the ³¹P chemical shift decreases in the order PPh₃ (δ ca. 21 ppm for 1a,a′) > PMe₂Ph (δ −9.9 ppm for 1b and −12.4 ppm for 2b) > PMe₃ (δ −19.2 ppm for 3d).^39,40

The ¹³C NMR resonances of complexes 1–4 were fully assigned with the help of 2D NMR experiments (see Table 1 in the ESI†). The highest chemical shifts correspond to the aryl carbon atoms directly bonded to Pd: ca. 152 ppm for the tmeda complexes and slightly higher (161–165 ppm) for the phosphine complexes, as is usual.^19,40,83 The aryl carbon atoms bonded to the alkenyl substituents appear in the range 136–146 ppm, similar to the starting arenes I, I′ (139.6 ppm), with the highest chemical shifts when this carbon is ortho to two [PdBrL₂] moieties (L = phosphine, complexes 2b and 3d). The C−Br carbon atoms resonate at lower frequencies, 122–126 ppm (similar to 124 ppm in I, I′). See the ESI† for a more detailed discussion of the NMR data.

The crystal and molecular structures of the monopalladated complexes 1a′ (Fig. 1) and 1b (Fig. 2) were determined by X-ray diffraction studies (Table 1 in ESI†). The structures show somewhat distorted square planar coordination around the Pd atoms. R.m.s. deviations from the best plane through Pd and the four donor atoms are 0.056 Å for complex 1a′ and 0.021 Å for 1b. The Pd–C bond distances are 2.022(2) Å for 1a′ and 2.010(3) Å for 1b, similar to the value found for the related complex trans-[Pd{C₆(CHO)₃Br₂}Br(PPh₃)₂] (2.0163(15) Å).⁴⁰ The Pd–P (2.3426(4) Å and 2.3279(4) Å) distances in that complex⁴⁰ are also similar to those found in 1a′ (2.3334(6) Å, 2.3680(6) Å) and 1b (2.3138(9) Å, 2.3116(9) Å). In contrast, the Pd–Br distance, 2.4865(2) Å,⁴⁰ is slightly shorter than in 1a′ (2.5201(3) Å) and 1b (2.5180(4) Å), an indication that the 1,3,5-benzenetristyryl ring has a larger trans influence than the 1,3,5-benzenetricarboxaldehyde. The styryl substituents of the central aromatic ring C1–6 of the tristyrylbenzene moieties are differently disposed in the two structures; for 1a′ all the styryl double bonds point to the same side of the ring, whereas for 1b, C7=C8 and C9=C10 point in the opposite direction to C11=C12 [cf. (a) torsion angles C1–C2–C7–C8, C3–C4–C9–C10 and C5–C6–C11–C12, which are 51.5, 61.9 and 48.7° for 1a′, but 29.8, 91.9 and −156.3° for 1b and (b) interplanar angles to the central ring: C41–46 72°, C51–56 29°, C61–66 79° for 1a′ but 3°, 84°, 9° for 1b]. The bromine atom Br1 of 1b lies appreciably outside the ring plane (by 0.36 Å, in the opposite direction to the styryl groups).

Experimental

The starting arenes, trans-1,3,5-C₆(E-CH=CHAr)₃Br₃, (Ar = Ph, (I), p-Tol (I′)) were prepared by a 3-fold HWE (Horner–Wadsworth–Emmons) reaction on the phosphonate 2,4,6-tribromo-1,3,5-tris(diethoxyphosphorylmethyl)benzene,⁸⁴ which, in turn, was prepared from 1,3,5-tribromo-2,4,6-tris(hydroxymethyl)benzene⁸⁵ by reaction with P(OEt)₃. See ESI† for a depiction of the synthetic route. The conditions for the HWE were analogous to those described in the literature for related, non-brominated aromatic phosphonates.⁸⁶

Nuclear magnetic resonance (NMR) spectra (¹H, ¹³C and ³¹P) were recorded on 400 MHz and 600 MHz Bruker Avance spectrometers at room temperature. Chemical shifts are given in ppm (δ) relative to TMS (¹H, ¹³C) or H₃PO₄ (³¹P). Infrared spectra were recorded on a PerkinElmer 16F-PC-FT spectrometer with Nujol mulls between polyethylene sheets. Melting points were determined on a Reichert apparatus and are uncorrected. Elemental analyses were carried out with a Carlo Erba 1106 microanalyzer. All experiments were conducted under N₂ atmosphere using Schlenk techniques. CH₂Cl₂ was distilled before use. “[Pd(dba)₂]”^77,78 was prepared according to literature procedures. Tmeda (Fluka), bpy (Fluka), PPh₃ (Fluka), PMe₂Ph (Aldrich) and PMe₃ (Aldrich) were used as received.

Synthesis of trans-1,3,5-C₆(E-CH=CHPh)₃Br₃ (I)

2,4,6-Tribromo-1,3,5-tris(diethoxyphosphorylmethyl)benzene (1.53 g, 2.00 mmol) and benzaldehyde (0.61 mL, 6.00 mmol) were mixed in THF (20 mL) under N₂. KO^tBu (673 mg, 6.00 mmol) was added in small portions and the mixture was stirred for 3 h at RT. On hydrolysis with water (70 mL) a thick white precipitate formed, and the suspension was further stirred for 30 min. After centrifugation the precipitate was separated by decantation and washed with water (2 × 50 mL) and EtOH (2 × 5 mL). Finally, filtration yielded 2,4,6-tribromo-1,3,5-tristyrylbenzene as colourless feathery crystals. Yield: 1.12 g (90%). Mp: 226–228 °C. IR: no relevant signals. ¹H NMR (400 MHz, CDCl₃):δ 7.56 (d, 6H, o-H Ph, ³J_HH = 7 Hz), 7.40 (t, 6H, m-H Ph, ³J_HH = 8 Hz), 7.32 (t, 3H, p-H Ph, ³J_HH = 8 Hz), 7.00 (AB, 3H, α-CH, ³J_HH = 16 Hz), 6.79 (AB, 3H, β-CH, ³J_HH = 16 Hz). ¹³C{¹H} NMR (75.5 MHz, CDCl₃):δ 139.6 (3C, C-alkenyl), 136.8 (3C, β-CH=), 136.5 (3C, i-C Ph), 129.0 (6C, m-CH Ph), 128.63 and 128.59 (3C each, α-CH= and p-CH Ph), 127.0 (6C, o-CH Ph), 124.1 (3C, C–Br). Anal. Calcd for C₃₀H₂₁Br₃: C, 58.00; H, 3.41. Found: C, 57.81; H, 3.35.

Synthesis of trans-1,3,5-C₆(E-CH=CH(p-Tol))₃Br₃ (I′)

2,4,6-Tribromo-1,3,5-tris(diethoxyphosphorylmethyl)benzene (3.62 g, 4.73 mmol) and p-tolylaldehyde (1.69, 14.2 mmol) were mixed in THF (60 mL) under N₂. KO^tBu (1.59 g, 14.2 mmol) was added in small portions and the mixture was stirred for 6 h at RT. On hydrolysis with water (200 mL) a thick white precipitate formed, and the suspension was further stirred for 30 min. After centrifugation the precipitate was separated by decantation and washed with water (2 × 50 mL) and EtOH (2 × 5 mL). Finally, filtration yielded 2,4,6-tribromo-1,3,5-tris-(E-2-(p-tolyl)ethenyl)benzene as an off-white solid. Yield: 1.75 g (56%). Mp: 185–187 °C. IR: no relevant signals. ¹H NMR (600 MHz, CDCl₃):δ 7.43 (AA′BB′, 6H, o-H Tol, ³J_HH = 8 Hz), 7.17 (AA′BB′, 6H, m-H Tol, ³J_HH = 8 Hz), 6.93 (AB, 3H, α-CH, ³J_HH = 16 Hz), 6.75 (AB, 3H, β-CH, ³J_HH = 16 Hz), 2.36 (s, 9H, CH₃). ¹³C{¹H} NMR (150.9 MHz, CDCl₃):δ 139.6 (3C, C-alkenyl), 138.5 (3C, p-C Tol), 136.7 (3C, β-CH=), 133.8 (3C, i-C Tol), 129.6 (6C, m-CH Tol), 127.8 (3C, α-CH=), 126.9 (6C, o-CH Tol), 124.0 (3C, C–Br), 21.5 (3C, Me). Anal. Calcd for C₃₃H₂₇Br₃: C, 59.76; H, 4.10. Found: C, 59.77; H, 3.94.

Synthesis of trans-[Pd{C₆(E-CH=CHPh)₃Br₂}Br(PPh₃)₂] (1a)

[Pd(dba)₂] (277 mg, 0.48 mmol), PPh₃ (252 mg, 0.96 mmol) and C₆(E-CH=CHPh)₃Br₃ (300 mg, 0.48 mmol) were mixed under N₂ atmosphere in dry degassed toluene (20 mL). The mixture was stirred at 50 °C for 5 h until the dark red colour of [Pd(dba)₂] was no longer observed. The brownish suspension was then evaporated to dryness and the residue was extracted with CH₂Cl₂ (20 mL). The extract was filtered through Celite and the filtrate was evaporated to dryness in vacuo. Et₂O (20 mL) was added and a solid formed, which was filtered off, thoroughly washed with Et₂O and dried in vacuo to give 1a as a pale yellow solid. Yield: 278 mg (46%). Mp: 223–225 °C (dec). IR: no relevant signals. ¹H NMR (600 MHz, CDCl₃):δ 7.53 (d, 2H, o-H Ph, ³J_HH = 8 Hz), 7.40–7.26 (several m, 9H, m,p-H), 7.40–7.26 (very broad, 18H, o,p-H PPh₃), 7.24 (AB, 2H, α-CH=, ³J_HH = 16 Hz), 7.19 (d, 4H, o-H Ph, ³J_HH = 8 Hz), 7.13 (very broad, 12H, m-H PPh₃), 6.77 (AB, 1H, α-CH=, ³J_HH = 16 Hz), 6.59 (AB, 2H, β-CH=, ³J_HH = 16 Hz), 6.47 (AB, 1H, β-CH=, ³J_HH = 16 Hz). ¹³C{¹H} NMR (150.9 MHz, CDCl₃):δ 165.4 (m, 1C, C1–Pd), 140.4 (t, 2C, C2-alkenyl, ³J_PC = 3 Hz), 137.5 (s, 2C, i-C To), 137.2 (s, 1C, i-C To), 136.1 (t, 1C, C4-alkenyl, ⁵J_PC = 1 Hz), 135.1 (s, 1C, β-CH=), 134.3 (s, 2C, β-CH=), 131.2 (t, 2C, α-CH=, ⁴J_PC = 2 Hz), 130.9 (vt, 6C, i-C PPh₃, ¹J_PC + ³J_PC = 46 Hz), 130.6 (s, 1C, α-CH=), 130.7 (very broad, 12C, o-CH PPh₃), 128.9 (s, 2C, m-CH To), 128.8 (s, 4C, m-CH To), 128.0 (s, 1C, p-CH To), 128.0 (very broad, 12C, m-CH PPh₃), 127.9 (s, 2C, p-CH To), 127.0 (s, 4C, o-CH To), 126.7 (s, 2C, o-CH To), 122.3 (s, 2C, C3–Br) (resonance for p-CH PPh₃ not observed). ³¹P{¹H} NMR (161.9 MHz, CDCl₃):δ 20.7 (s). Anal. Calcd for C₆₆H₅₁Br₃P₂Pd: C, 63.31; H, 4.11. Found: C, 63.48; H, 4.49.

Synthesis of trans-[Pd{C₆(E-CH=CHTo)₃Br₂}Br(PPh₃)₂] (1a′)

[Pd(dba)₂] (132 mg, 0.23 mmol), PPh₃ (121 mg, 0.46 mmol) and C₆(E-CH=CHTo)₃Br₃ (150 mg, 0.23 mmol) were mixed under N₂ atmosphere in dry degassed toluene (15 mL). The mixture was heated at 60 °C for 3.5 h until the dark red colour of [Pd(dba)₂] was no longer observed. The brownish suspension was then evaporated to dryness and the residue was extracted with CH₂Cl₂ (20 mL). The extract was filtered through Celite and the filtrate was evaporated to dryness in vacuo. A mixture of Et₂O (5 mL) and hexane (15 mL) was added and a solid formed, which was filtered off, washed with boiling hexane (2 × 10 mL) and dried in vacuo to give 1a′ as a pale yellow solid. Yield: 101 mg, 34%. Single crystals of 1a′ were obtained by slow diffusion of hexane into an Et₂O solution of 1a′. Yield: 77 mg, 26% Mp: 248–250 °C. IR: no relevant signals. ¹H NMR (400 MHz, CDCl₃):δ 7.7–7.0 (very broad, 30H, PPh₃), 7.43–7.07 (several m, 14H, o,m-H Ph, α-CH=), 6.72 (AB, 1H, α-CH=, ³J_HH = 16 Hz), 6.54 (AB, 2H, β-CH=, ³J_HH = 16 Hz), 6.42 (AB, 1H, β-CH=, ³J_HH = 16 Hz), 2.46 (s, 6H, Me To), 2.37 (s, 3H, Me To). ¹³C{¹H} NMR (100.8 MHz, CDCl₃):δ 164.9 (m, 1C, C1–Pd), 140.4 (t, 2C, C2-alkenyl, ³J_PC = 3 Hz), 137.9 (s, 1C, p-C Ph), 137.7 (s, 2C, p-C Ph), 136.2 (t, 1C, C4-alkenyl, ³J_PC = 1 Hz), 134.9 (s, 1C, β-CH=), 134.7 (s, 2C, i-C Ph), 134.4 (s, 1C, i-C Ph), 134.2 (s, 2C, β-CH=), 131.0 (vt, 6C, i-C PPh₃, ¹J_PC + ³J_PC = 46 Hz), 130.3 (very broad, 12C, o-CH PPh₃), 130.2 (t, 2C, α-CH=, ³J_PC = 2 Hz), 129.7 (s, 1C, α-CH=), 129.5 (s, 2C, m-CH Ph), 129.4 (s, 4C, m-CH Ph), 127.9 (very broad, 12C, m-CH PPh₃), 127.0 (s, 4C, o-CH Ph), 126.6 (s, 2C, o-CH Ph), 122.2 (s, 2C, C3–Br), 21.6 (s, 2C, Me To), 21.5 (s, 1C, Me To). ³¹P{¹H} NMR (161.9 MHz, CDCl₃):δ 20.7 (s). Anal. Calcd for C₆₉H₅₇Br₃P₂Pd: C, 64.03; H, 4.44. Found: C, 64.04; H, 4.20.

Synthesis of trans-[Pd{C₆(E-CH=CHPh)₃Br₂}Br(PMe₂Ph)₂] (1b)

[Pd(dba)₂] (92 mg, 0.16 mmol), PMe₂Ph (45 μL, 0.32 mmol) and C₆(E-CH=CHPh)₃Br₃ (99 mg, 0.16 mmol) were mixed under N₂ atmosphere in dry degassed toluene (20 mL). The mixture was stirred at 50 °C for 16 h until the dark red colour of [Pd(dba)₂] was no longer observed. The brownish suspension was then evaporated to dryness and the residue was extracted with CH₂Cl₂ (20 mL). The extract was filtered through Celite and the filtrate was evaporated to dryness in vacuo. Et₂O (10 mL) was added and a clear solution was obtained, to which cold hexane (10 mL) was added. A solid formed, which was filtered off, washed with a small amount of cold Et₂O (2 mL) and dried in vacuo to give 1b as a beige solid. Single crystals of 1b were grown by slow diffusion of hexane into an Et₂O solution of 1b. Yield: 69 mg (43%). Mp: 220–222 °C. IR: no relevant signals. ¹H NMR (600 MHz, CDCl₃):δ 8.13 (AB, 2H, β-CH=, ³J_HH = 16 Hz), 7.55 (d, 2H, o-H Ph, ³J_HH = 7 Hz), 7.46 (AB, 2H, α-CH=, ³J_HH = 16 Hz), 7.43–7.38 (m, 6H, o,m-H Ph), 7.35 (t, 4H, m-H Ph), 7.33–7.26 (m, 7H, o-H PPh + p-H Ph), 7.21 (t, 2H, p-H PPh, ³J_HH = 7 Hz), 7.13 (t, 4H, m-H PPh, ³J_HH = 7 Hz), 6.87 (AB, 1H, α-CH=, ³J_HH = 16 Hz), 6.54 (AB, 1H, β-CH=, ³J_HH = 16 Hz), 1.61 (vt, 12H, PMe, ²J_PH + ⁴J_PH = 7 Hz). ¹³C{¹H} NMR (150.9 MHz, CDCl₃):δ 161.8 (t, 1C, C1–Pd, ²J_PC = 2 Hz), 139.8 (t, 2C, C2-alkenyl, ³J_PC = 2 Hz), 137.4 (s, 2C, i-C Ph), 137.1 (s, 1C, i-C Ph), 135.6 (t, 1C, C4-alkenyl, ⁵J_PC = 1 Hz), 135.5 (s, 1C, β-CH=), 133.8 (vt, 2C, i-C PMe₂Ph, ¹J_PC + ³J_PC = 45 Hz), 132.7 (t, 2C, α-CH=, ⁴J_PC = 1 Hz), 131.8 (t, 2C, β-CH=, ⁵J_PC = 2 Hz), 130.5 (vt, 4C, o-CH PMe₂Ph, ²J_PC + ⁴J_PC = 10 Hz), 130.2 (s, 1C, α-CH=), 129.6 (s, 2C, p-CH PMe₂Ph), 129.1 (s, 4C, m-CH Ph), 128.9 (s, 2C, m-CH Ph), 128.1 (s, 1C, p-CH Ph), 128.02 (s, 2C, p-CH Ph), 127.99 (vt, 4C, m-CH PMe₂Ph, ³J_PC + ⁵J_PC = 9 Hz), 126.8 (s, 4C, o-CH Ph), 126.7 (s, 2C, o-CH Ph), 123.7 (s, 2C, C3–Br), 14.0 (vt, 4C, PMe, ¹J_PC + ³J_PC = 31 Hz). ³¹P{¹H} NMR (161.9 MHz, CDCl₃):δ −9.9 (s). Anal. Calcd for C₄₆H₄₃Br₃P₂Pd: C, 55.03; H, 4.32. Found: C, 54.67; H, 4.43.

Synthesis of [{trans-PdBr(PMe₂Ph)₂}₂{μ₂-C₆(E-CH=CHPh)₃Br}] (2b)

[Pd(dba)₂] (737 mg, 1.28 mmol), PMe₂Ph (364 μL, 2.56 mmol) and C₆(E-CH=CHPh)₃Br₃ (398 mg, 0.64 mmol) were mixed under N₂ atmosphere in dry degassed toluene (20 mL). The mixture was heated at 50 °C for 16 h until the dark red colour of [Pd(dba)₂] was no longer observed. The brownish suspension was then evaporated to dryness and the residue was extracted with CH₂Cl₂ (20 mL). The extract was filtered through Celite and the filtrate was evaporated to dryness in vacuo. Et₂O (5 mL) was added and a clear solution was obtained to which cold hexane (10 mL) was added. A solid formed, which was filtered off, washed with a small amount of cold Et₂O (2 mL) and dried in vacuo to give 2b as a beige solid. Yield: 234 mg (26%). Mp: 206–208 °C. IR: no relevant signals. ¹H NMR (400 MHz, CDCl₃):δ 8.46 (AB, 1H, β-CH=, ³J_HH = 16 Hz), 8.26 (AB, 1H, α-CH=, ³J_HH = 16 Hz), 8.12 (AB, 2H, β-CH=, ³J_HH = 16 Hz), 7.81 (AB, 2H, α-CH=, ³J_HH = 16 Hz), 7.66–7.60 (m, 8H, Ph), 7.4–6.9 (several m, 27H, Ph), 1.48 and 1.27 (vt, 12H each, PMe, ²J_PH + ⁴J_PH = 7 Hz). ¹³C{¹H} NMR (75.5 MHz, CDCl₃):δ 160.7 (m, 2C, C1–Pd), 146.0 (m*, 1C, C2-alkenyl), 138.2 (m, 2C, C3-alkenyl), 137.8 (s, 2C, i-C Ph), 137.6 (s, 1C, i-C Ph), 137.4 (m, 1C, α-CH=), 134.7 (vt, 4C, i-C PMe₂Ph, ¹J_PC + ³J_PC = 44 Hz), 134.0 (m, 2C, β-CH=), 131.7 (vt, 8C, o-CH PMe₂Ph, ²J_PC + ⁴J_PC = 12 Hz), 130.5 (m, 2C, α-CH=), 129.9 (s, 5C, β-CH= + p-CH PMe₂Ph), 129.0 (s, 4C, m-CH Ph), 128.5 (s, 2C, m-CH Ph), 128.4 (vt, 8C, m-CH PMe₂Ph, ³J_PC + ⁵J_PC = 10 Hz), 127.6 (s, 2C, p-CH Ph), 127.0 (s, 1C, p-CH Ph), 126.74 (s, 2C, o-CH Ph), 126.70 (s, 4C, o-CH Ph), 126.3 (s, 1C, C4–Br), 15.8 (vt, 4C, PMe₂Ph, ¹J_PC + ³J_PC = 30 Hz), 13.3 (vt, 4C, PMe₂Ph, ¹J_PC + ³J_PC = 30 Hz). ³¹P{¹H} NMR (161.9 MHz, CDCl₃):δ −12.4 (s). Anal. Calcd for C₆₂H₆₅Br₃P₄Pd₂: C, 53.70; H, 4.72. Found: C, 53.88; H, 4.80.

Synthesis of [{PdBr(tmeda)}₃{μ₃-C₆(E-CH=CHPh)₃}] (3c)

[Pd(dba)₂] (576 mg, 1.00 mmol), tmeda (150 μL, 1.00 mmol) and C₆(E-CH=CHPh)₃Br₃ (205 mg, 0.33 mmol) were mixed under N₂ atmosphere in dry degassed toluene (30 mL). The mixture was heated at 65 °C for 2.5 h until the dark red colour of [Pd(dba)₂] was no longer observed. The dark green suspension was then concentrated in vacuo and the residue was extracted with CH₂Cl₂ (40 mL). The extract was filtered through Celite and the filtrate was evaporated to dryness in vacuo. Et₂O (20 mL) was added and a solid formed, which was filtered off, thoroughly washed with Et₂O and dried in vacuo to give 3c as a yellow solid. Yield: 252 mg (59%). Mp: 209–210 °C (dec). IR: no relevant signals. ¹H NMR (600 MHz, CDCl₃):δ 9.63 (AB, 1H, β-CH=, ³J_HH = 16 Hz), 9.25 (AB, 2H, α-CH=, ³J_HH = 16 Hz), 9.23 (AB, 1H, α-CH=, ³J_HH = 16 Hz), 8.85 (AB, 2H, β-CH=, ³J_HH = 16 Hz), 7.80 (d, 4H, o-H Ph, ³J_HH = 8 Hz), 7.71 (d, 2H, o-H Ph, ³J_HH = 8 Hz), 7.34 (t, 4H, m-H Ph, ³J_HH = 8 Hz), 7.30 (t, 2H, m-H Ph, ³J_HH = 8 Hz), 7.18 (t, 2H, p-H Ph, ³J_HH = 7 Hz), 7.11 (t, 1H, p-H Ph, ³J_HH = 7 Hz), 2.73, 2.61, 2.54, 2.33, 2.27 and 2.20 (s, 6H each, Me), 2.6–2.2 (several m, 12H, CH₂). ¹³C{¹H} NMR (150.9 MHz, CDCl₃):δ 152.1 (2C, C3–Pd), 151.7 (1C, C1–Pd), 141.4 (2C, C2-alkenyl), 141.2 (1C, i-C Ph), 140.52 (1C, C4-alkenyl), 140.49 (2C, i-C Ph), 138.9 (1C, α-CH=), 137.7 (2C, α-CH=), 128.8 (4C, m-CH Ph), 128.7 (2C, m-CH Ph), 128.3 (2C, β-CH=), 126.12 (2C, o-CH Ph), 126.08 (4C, o-CH Ph), 125.9 (2C, p-CH Ph), 125.6 (1C, p-CH Ph), 125.2 (1C, β-CH=), 63.2 (1C, CH₂), 63.0 (2C, CH₂), 58.32 (2C, CH₂), 58.26 (1C, CH₂), 51.7 (4C, Me), 51.0, 49.2, 48.55 and 48.50 (2C each, Me). Anal. Calcd for C₄₈H₆₉Br₃N₆Pd₃: C, 44.72; H, 5.40; N, 6.52. Found: C, 44.40; H, 5.43; N, 6.31.

Synthesis of [{PdBr(tmeda)}₃{μ₃-C₆(E-CH=CHTo)₃}] (3c′)

[Pd(dba)₂] (173 mg, 0.30 mmol), tmeda (45 μL, 0.30 mmol) and C₆(E-CH=CHTo)₃Br₃ (100 mg, 0.15 mmol) were mixed under N₂ atmosphere in dry degassed toluene (15 mL). The mixture was heated at 70 °C for 1.5 h until the dark red colour of [Pd(dba)₂] was no longer observed. The dark green suspension was then concentrated in vacuo and the residue was extracted with CH₂Cl₂ (20 mL). The extract was filtered through Celite and the filtrate was evaporated to dryness in vacuo. Et₂O (15 mL) was added and a solid formed, which was filtered off and thoroughly washed with Et₂O. This solid was purified by a preparative TLC on silica gel using a mixture of acetone/hexane (1 : 1.25) as eluent. The yellow band with R_f = 0.6 was collected, and the product was extracted with acetone (30 mL). Evaporation of the acetone and addition of Et₂O (15 mL) precipitated a solid, which was filtered off, thoroughly washed with Et₂O and dried in vacuo to give 3c′ as a yellow solid. Yield: 77 mg, 58%. Mp: 210–212 °C (dec). IR: no relevant signals. ¹H NMR (400 MHz, CDCl₃):δ 9.57 (AB, 1H, β-CH=, ³J_HH = 16 Hz), 9.20 (AB, 2H, α-CH=, ³J_HH = 16 Hz), 9.16 (AB, 1H, α-CH=, ³J_HH = 16 Hz), 8.82 (AB, 2H, β-CH=, ³J_HH = 16 Hz), 7.71 (d, 4H, o-H Ph, ³J_HH = 8 Hz), 7.62 (d, 2H, o-H Ph, ³J_HH = 8 Hz), 7.18–7.10 (m, 6H, m-H Ph), 2.73, 2.62 and 2.54 (s, 6H each, Me tmeda), 2.36 (s, 9H, Me To), 2.34, 2.32 and 2.26 (s, 6H each, Me tmeda), 2.75–2.15 (several m, 12H, CH₂). ¹³C{¹H} NMR (75.5 MHz, CDCl₃):δ 151.6 (2C, C3–Pd), 151.1 (1C, C1–Pd), 141.3 (2C, C2-alkenyl), 140.4 (1C, C4-alkenyl), 138.4 (1C, p-C To), 138.0 (1C, α-CH=), 137.7 (2C, p-C To), 136.6 (2C, α-CH=), 135.4 (2C, i-C To), 135.1 (1C, i-C Tl), 129.51 (4C, m-CH To), 129.45 (2C, m-CH To), 128.1 (2C, β-CH=), 126.0 (6C, o-CH To), 124.9 (1C, β-CH=), 63.1 (1C, CH₂), 63.0 (2C, CH₂), 58.25 (2C, CH₂), 58.20 (1C, CH₂), 51.7 (4C, Me tmeda), 51.0 and 49.1 (2C, Me tmeda), 48.5 (4C, Me tmeda), 21.40 (2C, Me To), 21.36 (1C, Me To). Anal. Calcd for C₅₁H₇₅Br₃N₆Pd₃: C, 46.02; H, 5.68; N, 6.31. Found: C, 45.97; H, 5.36; N, 6.11.

Synthesis of [{trans-PdBr(PMe₃)₂}₃{μ₃-C₆(E-CH=CHPh)₃}] (3d)

3c (100 mg, 0.077 mmol) and PMe₃ (924 μL of a 1 M solution in toluene, 0.924 mmol) were mixed under N₂ atmosphere in dry degassed THF (15 mL). The mixture was heated at 50 °C for 6 h, during which the precipitation of a pale green solid was observed. The reaction mixture was then evaporated to dryness and the residue was extracted with CH₂Cl₂ (30 mL). The extract was filtered through Celite and the filtrate was evaporated to dryness in vacuo. Et₂O (20 mL) was added and a solid formed, which was filtered off, thoroughly washed with Et₂O and dried in vacuo to give 3d as a pale green solid. Yield: 92 mg (86%). Mp: 208–209 °C (dec). IR: no relevant signals. ¹H NMR (600 MHz, CDCl₃):δ 8.76 (AB, 3H, β-CH=, ³J_HH = 16 Hz), 8.59 (AB, 3H, α-CH=, ³J_HH = 16 Hz), 7.63 (d, 6H, o-H Ph, ³J_HH = 7 Hz), 7.39 (t, 6H, m-H Ph, ³J_HH = 7 Hz), 7.23 (t, 3H, p-H Ph, ³J_HH = 7 Hz), 1.29 (vt, 54H, PMe₃, ²J_PH + ⁴J_PH = 7 Hz). ¹³C{¹H} NMR (150.9 MHz, CDCl₃):δ 162.8 (m, 3C, C1–Pd), 143.6 (m, 3C, C2-alkenyl), 139.3 (m, 3C, α-CH=), 138.4 (s, 3C, i-C Ph), 129.4 (s, 6C, m-CH Ph), 127.2 (s, 3C, p-CH Ph), 126.3 (s, 6C, o-CH Ph), 125.2 (m, 3C, β-CH=), 15.7 (vt, 18C, PMe₃, ¹J_PC + ³J_PC = 30 Hz). ³¹P{¹H} NMR (121.4 MHz, CDCl₃):δ −19.2 (s). Anal. Calcd for C₄₈H₇₅Br₃P₆Pd₃: C, 41.27; H, 5.41. Found: C, 41.62; H, 5.43.

Synthesis of 2-Ph-4,6-{PdBr(tmeda)}₂-5,7-(E-CH=CHPh)₂-inden-1-one (4)

CO was bubbled for 5 min through a solution of complex 3c (60 mg, 0.047 mmol) in dry degassed THF (25 mL) in a Carius tube, and the solution was then heated at 60 °C for 2 h. The dark red solution was then filtered through MgSO₄ and the filtrate was evaporated to dryness in vacuo. The residue was redissolved in the minimum amount of CH₂Cl₂ and the solution was chromatographed by preparative TLC on silica gel using a mixture of acetone/hexane (2 : 3) as eluent. The brick-red band with R_f = 0.75 was collected and extracted with acetone (30 mL). The acetone solution was concentrated to a small volume and addition of Et₂O (15 mL) precipitated a solid, which was filtered off and dried in vacuo to give 4 as a brick-red solid. Yield: 36 mg (76%). Mp: 208–209 °C (dec). IR: ν(CO): 1676 cm⁻¹. ¹H NMR (300 MHz, CDCl₃):δ 9.54 (d, 1H, β^II-CH=, ³J_HH = 16 Hz), 9.31 (d, 1H, α^I-CH=, ³J_HH = 16 Hz), 8.87 (d, 1H, β^I-CH=, ³J_HH = 16 Hz), 8.59 (d, 1H, α^II-CH=, ³J_HH = 16 Hz), 8.28 (s, 1H, H7), 7.86–7.83 (m, 2H, o-H Ph^I), 7.83–7.80 (m, 2H, o-H Ph^III), 7.72 (d, 2H, o-H Ph^II, ³J_HH = 7 Hz), 7.44–7.39 (m, 2H, m-H Ph^I), 7.39–7.36 (m, 2H, m-H Ph^III), 7.36–7.32 (m, 2H, m-H Ph^II), 7.30–7.27 (m, 1H, p-H Ph^I), 7.27–7.24 (m, 1H, p-H Ph^III), 7.24–7.20 (m, 1H, p-H Ph^II), 2.75–2.2 (several m's, 8H, CH₂), 2.70 (s, 9H, Me), 2.59, 2.36 and 2.31 (s, 3H each, Me), 2.63 (s, 6H, Me). ¹³C{¹H} NMR (75.4 MHz, CDCl₃):δ 199.8 (CO), 152.3 (C3–Pd), 151.4 (C2-alkenyl), 148.6 (CH7), 145.6 (C6), 145.0 (C1–Pd), 139.2 (i-C Ph^I), 138.8 (i-C Ph^II), 137.6 (α^I-CH=), 136.5 (C4-alkenyl), 133.4 (β^II-CH=), 133.2 (i-C Ph^III), 132.4 (C8), 130.9 (β^I-CH=), 128.9 (2C, m-CH Ph^I), 128.6 (2C, m-CH Ph^II), 128.2 (2C, m-CH Ph^III), 127.4 (2C, o-CH Ph^III), 127.3 (α^II-CH=), 127.1 (p-CH Ph^II), 127.0 (p-CH Ph^III), 126.8 (3C, o-CH Ph^II and p-CH Ph^I), 126.2 (2C, o-CH Ph^I), 125.1 (C5), 62.92, 62.86, 58.4 and 58.3 (CH₂), 51.8, 51.6, 51.5, 51.2, 49.1, 48.7, 48.6 and 48.3 (Me). Anal. Calcd for C₄₃H₅₂Br₂N₄OPd₂: C, 50.96; H, 5.17; N, 5.53. Found: C, 51.03; H, 5.56; N, 5.59. Exact mass (ESI+ TOF):m/z calculated for [4-Br]⁺ (C₄₃H₅₂BrN₄OPd₂): m/z: 933.1407, found: 933.1394, Δ = 1.3 ppm.

Conclusions

We have described a novel family of mono-, di-, and tripalladated benzene derivatives, with alkenyl substituents in ortho position to the Pd(II) atoms. These complexes possess a great chemical potential regarding their reactivity towards organic molecules. Thus, a dipalladated indenone has been obtained by CO insertion into one of the aryl–Pd bonds of a tripalladated complex, followed by depalladation in that position. Further research in this area will be pursued.

Author contributions

Rashmi V. Shenoy: investigation and writing – original draft preparation. Peter G. Jones: X-ray crystallography and manuscript editing. José Vicente: conceptualization, methodology, project administration, funding acquisition. Eloísa Martínez-Viviente: methodology, supervision, writing – original draft preparation, reviewing and editing, visualisation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support from the Spanish Ministry of Science and Innovation/FEDER (grant PGC2018-100719-B-100) and Séneca Foundation of the Region of Murcia (Ayudas a Grupos de Excelencia, grant 19890/GERM/15) is gratefully acknowledged. R. S. is grateful to the European Commission for a Marie Curie Individual Fellowship (PIIF-GA-2008-219687).

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Footnote

† Electronic supplementary information (ESI) available: NMR data for complexes 1–4 and arenes I, I′ (table, comments and spectra), X-ray crystallographic data, structure refinements and CIF files for complexes 1a′ and 1b. CCDC 2195995 and 2195996. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3dt00091e

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