Bisimino-functionalized dibenzo[a,c]acridines as highly conjugated pincer frameworks for palladium(II): synthesis, characterization and catalytic performance in Heck coupling

Abstract

A new pair of highly conjugated ligands, 10-[1-(arylimino)ethyl]-14-[(arylimino)methyl]dibenzo[a,c]acridine (aryl = 2,6-Me₂Ph L1, 2,6-Et₂Ph L2) incorporating both aldimine and ketimine units, have been prepared by a straightforward sequence of organic transformations from phenanthrene-9,10-dione. Cyclopalladation occurs readily at ambient temperature on treating L1 or L2 with PdCl₂(NCCH₃)₂ in aprotic solvents to afford exclusively (N_ketimineNC)Pd(II) chloride pincer complexes Pd1 or Pd2, respectively. By contrast in methanol, Pd1 or Pd2 are isolated as the minor product with aldehyde-containing Pd3 or Pd4 as the major one, the result of hydrolysis of the pendant aldimine units in Pd1 and Pd2, respectively. All the ligands and palladium complexes have been characterized by FT-IR, ¹H and ¹³C NMR spectroscopy, mass spectrometry and elemental analysis; the molecular structures for L1, Pd1, Pd2, Pd3 and Pd4 are also reported. Using low catalyst loadings (0.0005–0.002 mol%) and elevated temperatures (140–200 °C), Pd1–Pd4 are able to efficiently mediate the coupling of haloarenes with vinyl-containing substrates with turnover numbers as high as 174 000; the effects of steric/electronic variation within the substrate and NNC-pincer complex on catalyst performance are examined.

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Introduction

Palladium complexes bearing neutral and monoanionic pincer ligands have established themselves as a powerful tool for a wide range of organic transformations such as the aldol reaction, Michael addition, allylation of aldehydes and imines as well as a range of cross-coupling reactions.^1–4 With regard to Heck-type coupling, numerous pincer catalysts have been developed incorporating two neutral donors and an anionic aryl-C donor within the terdentate ligand set. In particular those based on symmetrical EC⁻¹E type (E = neutral donor) (A, Chart 1)⁵ framework have been at the forefront of research while those involving unsymmetrical ligand frames such as EC⁻¹E′ (B, Chart 1)⁶ and EEC⁻¹ (C, Chart 1)⁷ have also started to emerge as efficient catalysts for this transformation. The presence of a Pd–C σ-bond as the central or exterior donor no doubt imparts the complex with a high level of thermal stability and tolerance of many functional groups, hence inhibiting facile deactivation pathways and thus leading to highly productive catalysts.⁸ Indeed, pincer complexes have proved exceptional in determining this necessary balance between stability and reactivity.⁹ Consequently, it can be viewed that an ideal pincer catalyst would deliver high turnover numbers, while allowing low catalyst loading and exhibiting good thermal stability.¹⁰ Furthermore, it needs to be amenable to rational design, allowing for systematic tuning of the ligand to enable subtle control over the catalytic properties of the central metal atom.

Chart 1 Monoanionic pincer frameworks reported (A–C) along with our current work; E and E′ refer to neutral donors.

With a view to enhancing the thermal stability further of an NNC-type palladium pincer complex, we have been interested in developing a means of increasing the conjugation of the ligand backbone through the introduction of suitably fused arene and heteroarene rings. In this report we devise an implement a synthetic approach to develop novel bisimino-functionalized benzo[a,c]acridines that are conducive to ready steric and electronic variation (see Chart 1). Owing to the planarity of the pentacyclic skeleton, cyclopalladation proceeds efficiently generating NNC complexes which have been used in the coupling of haloarenes to vinyl-containing substrates. Full synthetic details are reported as is an in depth discussion of the catalytic evaluation of the complexes and substrate scope.

Results and discussion

Synthesis of L1 and L2

Ligands L1 and L2 have been prepared using a six step procedure in reasonable overall yield from commercially available phenanthrene-9,10-dione (Scheme 1). The intermediate compound, 10-ethyl-14-methyldibenzo[a,c] acridine (2), has been previously reported using a rather cumbersome low yielding route.¹¹ We have found that 2 can be more conveniently prepared by firstly treating phenanthrene-9,10-dione with a slight excess of 2,6-diethylaniline in the presence of para-toluenesulfonic acid to form spiro-[7-ethyl-3-methylindoline-2,10-phenanthren-9-one] (1) and then completing the heterocyclization by heating 1 at reflux in 1,2-dichlorobenzene. Mono-bromination at both the ethyl and methyl groups in 2 with two equivalents of N-bromosuccinimide (NBS) in carbon tetrachloride gave 3 in near quantitative yield.¹² Substitution of the two bromide groups in 3 for OH groups to give 4 occurred readily by using a silver nitrate assisted hydrolysis in acetone/water (3 : 1 ratio),¹² which when followed by a pyridinium chlorochromate (PCC) oxidation afforded the mixed aldehyde-ketone 5 in excellent yield.¹³ Finally, the target ligands, 10-[1-(arylimino)ethyl]-14-[(arylimino)methyl]dibenzo[a,c]acridine (aryl = 2,6-Me₂Ph L1, 2,6-Et₂Ph L2), could be obtained by the Schiff base condensation of 5 with either 2,6-dimethylaniline or 2,6-diethylaniline in toluene, respectively in reasonable yield.^14,15 All organic compounds have been characterized by ¹H, ¹³C NMR, IR spectroscopy, HR mass spectrometry or elemental analysis.

Scheme 1 Reagents and conditions: (a) p-TsOH, toluene, reflux, 12 h; (b) p-TsOH, 1,2-dichlorobenzene, reflux, 12 h; (c) NBS, dibenzoyl peroxide, CCl₄, reflux, 6 h; (d) AgNO₃, acetone/water, 50 °C, 6 h; (e) PCC, CH₂Cl₂, molecular sieves, RT, 6 h.

It is noteworthy that the formation of 1 was unexpected and, to the knowledge of the authors, has not been observed as a product (ca. 38%) in the condensation reaction of phenanthrene-9,10-dione with 2,6-substituted anilines; only small quantities of the expected mono- and bis-imine derivatives of phenanthrene-9,10-dione could be detected under these conditions.^16,17 The thermally driven rearrangement and ring closing steps involved in the formation of 2 (70%) from 1 are likely mediated by a combination of the relief in ring strain in 1 and the thermodynamic stability of the highly conjugated aromatic 2. A proposed mechanism for the formation of 2 is depicted in Scheme 2 highlighting the role of acid and involving the elimination of water. Further confirmation of the structures of 1 and 2 was provided in the form of single crystal X-ray structures (see ESI†).

Scheme 2 Proposed mechanism for the formation of 2 from 1.

The ¹H NMR spectra of L1 and L2 recorded in CDCl₃ feature prominent singlets for their –CH=N protons at δ 8.84_av. and –CMe=N protons at δ 2.51_av., while the ¹³C NMR spectra show –C=N peaks at δ 171.5_av. (ketimine) and δ 163.4_av. (aldimine) for L1 and for L2. In their ToF mass spectra peaks corresponding to their protonated molecular ions are seen for each. In addition, the structure of L1 has been the subject of a single X-ray diffraction study. Suitable crystals for the study were grown by layering a dichloromethane solution of L1 with heptane. A view of L1 is shown in Fig. 1; selected bond distances and angles are collected in the figure caption.

Fig. 1 ORTEP diagram of L1 with the thermal ellipsoids set at 50% probability level; all hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): N(1)–C(12) 1.324(2), N(1)–C(21) 1.353(2), N(3)–C(22) 1.271(2), N(3)–C(24) 1.427(2), C(32)–N(2) 1.274(2), N(2)–C(33) 1.426(2), C(11)–C(10) 1.405(2), C(10)–C(9) 1.378(3), C(12)–N(1)–C(21) 119.07(14), C(32)–N(2)–C(33) 117.86(14), C(22)–N(3)–C(24) 121.21(15), N(3)–C(22)–C(20) 116.38(15), N(3)–C(22)–C(23) 125.38(16), C(9)–C(10)–C(11) 120.63(18).

The structure consists of a slightly twisted dibenzo[a,c] acridine polycycle based on five fused aromatic rings. At the C15- and C20-carbons of the dibenzo[a,c]acridine are located the aldimine and ketimine units, respectively, with the ketimine fragment positioned on the same side as the acridine nitrogen. The two imino C–N bond distances are quite typical for C=N double bond distances and are indeed similar to one another [C32–N2 1.274(2); C22–N3 1.271(2) Å]. The N-aryl ring of the aldimine unit is oriented towards orthogonality with respect to the heterocyclic ring, N1–C12–C13–C15–C16–C21, with a dihedral angle of ca. 77.53°, whilst the N-aryl ring of the ketimine adopts a shallower angle of ca. 32.85°.

Synthesis of NNC Pd pincer complexes

With a view to preparing organo-palladium complexes based on L1 or L2, we envisaged that, due to the presence of more than one nitrogen-based directing group, two sites for C_aryl–H activation on the dibenzo[a,c]acridine could conceivably occur, namely at the C₂-position or the C₁₁-position (see Scheme 3 for numbering system). To explore these possibilities, L1 and L2 were treated with PdCl₂(NCMe)₂ under different reaction conditions (Table 1). Hence, the equimolar reaction of L1 or L2 with PdCl₂(NCCH₃)₂ in aprotic solvents such as dichloromethane or toluene at room temperature gave Pd1 and Pd2, respectively, as the only isolable product in reasonable yield (Scheme 3). However, on reacting L1 or L2 in a mixture of methanol/toluene at 50 °C, two types of complexes were isolable for each namely Pd1/Pd3 or Pd2/Pd4, respectively.¹⁸ Indeed, Pd3 and Pd4 were formed as the major products while Pd1 and Pd2 the minor ones. Addition of sodium bicarbonate to the reaction of L1 with PdCl₂(NCMe)₂ in toluene gave some modest increase in the yield of Pd1. Complexes Pd1, Pd2, Pd3 and Pd4 were characterized by ¹H NMR, ¹³C NMR, FT-IR spectroscopy, ESI mass spectrometry and by elemental analysis. In addition, all four complexes were the subject of single crystal X-ray diffraction studies.

Scheme 3 Synthesis of NNC-Pd pincer complexes: (a) dichloromethane or toluene, RT; (b) toluene, NaHCO₃ (cat.), 60 °C, 6 h; (c) methanol/toluene, 50 °C, 6 h.

Table 1 Reaction condition effects on product formation

Entry	Ligand	Solvent	Base	Temp. (°C)	Product 1 (%)	Product 2 (%)
1	L1	Dichloromethane	—	25	Pd1 (42)	—
2	L2	Dichloromethane	—	25	Pd2 (38)	—
3	L1	Toluene	—	25	Pd1 (36)	—
4	L2	Toluene	—	25	Pd2 (30)	—
5	L1	Toluene	NaHCO3 (cat.)	60	Pd1 (52)	—
6	L1	Methanol/toluene	—	50	Pd1 (12)	Pd3 (46)
7	L2	Methanol/toluene	—	50	Pd2 (15)	Pd4 (41)

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Single crystals of Pd1 and Pd2 were grown by layering chloroform solutions of the complex with heptane. The molecular structures of both are similar and will be discussed together. As a representative example a view of Pd1 is depicted in Fig. 2; selected bond distances and angles are listed in Table 2. Each structure reveals a mononuclear species in which the palladium(II) center occupies a tridentate cavity within the ligand frame with the ketimine nitrogen, the acridine nitrogen and an aryl carbon serving as the donor atoms. The geometry of the palladium can be best described as distorted square planar with a chloride ligand filling the fourth coordination site. Within the NNC-palladium unit both five- and six-membered chelate rings are present with the carbon donor incorporated into the smaller metallacycle. Due to the constraints of the NNC ligand the C1–Pd1–N2 angles are less than the desired 180°: 167.25(19)° for Pd1, 167.09(13)° for Pd2. The Pd(1)–C(1) bond distances in Pd1 [1.979(5) Å] and Pd2 [1.986(3) Å], are comparable with those found in related NNC pincer complexes¹⁹ and are the shortest of the three NNC contacts to palladium; this reflects the ionic contribution to the Pd–C bond. Little significant bond variation is seen between the bound C=N_ketimine bond length and the C=N_aldimine bond lengths in either structure and indeed similar to that found in L1. Some puckering of the six-membered chelate ring is apparent with the bound C=N_ketimine unit some way of co-planar with the pyridine plane (dihedral angle = 30.1° Pd1, 34.6° Pd2); the corresponding angle for the pendant aldimine is close to 45° (dihedral angle = 44.8° Pd1, 43.8° Pd2). For both complexes the N-aryl rings are inclined towards perpendicular (Pd1 88.28°, 85.59°; Pd2 81.63°, 89.44°) with respect to the C1–N1–N2–Cl1 coordination plane, presumably to minimize steric interactions.

Fig. 2 ORTEP diagram of Pd1 with thermal ellipsoids set at 50% probability level; all hydrogen atoms are omitted for clarity.

Table 2 Selected bond lengths (Å) and angles (°) for Pd1, Pd2, Pd3 and Pd4

	Pd1	Pd2	Pd3	Pd4
Pd(1)–C(1)	1.979(5)	1.986(3)	1.978(3)	1.9834(19)
Pd(1)–N(1)	2.028(4)	2.027(3)	2.027(2)	2.0254(16)
Pd(1)–N(2)	2.123(4)	2.115(3)	2.111(2)	2.1276(16)
Pd(1)–Cl(1)	2.3100(17)	2.3009(9)	2.3039(8)	2.3030(8)
C(32)–N(3)aldimine	1.264(7)	1.275(4)	—	—
C(22)–N(2)ketimine	1.294(7)	1.281(4)	—	—
C(32)–O(1)aldehyde	—	—	1.205(4)	1.207(2)
C(1)–Pd(1)–N(1)	82.38(19)	82.98(13)	82.94(10)	82.96(7)
N(1)–Pd(1)–N(2)	90.44(17)	89.57(10)	91.55(8)	91.11(6)
C(1)–Pd(1)–N(2)	167.25(19)	167.09(13)	170.18(10)	166.88(7)
N(1)–Pd(1)–Cl(1)	173.29(12)	173.55(8)	172.84(6)	170.39(4)
C(1)–Pd(1)–Cl(1)	93.17(16)	92.48(11)	92.05(8)	92.80(6)
N(2)–Pd(1)–Cl(1)	94.83(13)	95.71(8)	94.11(6)	94.73(5)

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The ¹H NMR spectra of Pd1 and Pd2 are supportive of their solid state structures being maintained in solution. The presence of only 16 protons in the aromatic region is consistent with cyclopalladation having occurred in L1 or L2 (cf. 17 Ar–H protons). The bound CMe=N resonance is shifted ca. 0.3 ppm upfield on coordination for each complex while the chemical shift of the pendant CH=N remains relatively unchanged. Surprisingly, no discernible difference in ν(C=N)_imine stretching frequency in the IR spectrum could be observed for the two types of imine. The ESI mass spectra for each complex recorded in acetonitrile show strong [M − Cl]⁺ peaks as their MeCN adducts.

Crystals of Pd3 and Pd4 suitable for the X-ray determination were grown by layering dichloromethane solutions with heptane at room temperature. A perspective view of representative Pd4 is given in Fig. 3; selective bond lengths and bond angles for Pd3 and Pd4 are collected alongside Pd1 and Pd2 in Table 2. The structures closely resemble Pd1 and Pd2 with the main difference being that an aldehyde group is linked to the para-position of the pyridine unit rather than an aldimine group. The presence of the aldehyde on the back of the ligand frame has little effect on the NNC bond parameters with the Pd–N_imine distance again the longest [Pd3 2.111(2) Å, Pd4 2.1276(16) Å] and the Pd–C distance the shortest [Pd3 1.978(3) Å, Pd4 1.9834(19) Å]. As with the aldimine unit in Pd1 and Pd2, the aldehyde unit is tilted with regard to the pyridine plane (dihedral angle = 42.1° Pd3, 54.8° Pd4). The presence of the aldehyde groups is further confirmed in the ¹H NMR spectra for Pd3 and Pd4 with downfield signals for the –CHO protons visible at δ 10.63 and δ 10.62 respectively, while in the ¹³C NMR spectra the –CHO carbons appear at δ 193.2 and 192.1 ppm. In the IR spectra bands corresponding to both ν(C=O)_aldehyde and ν(C=N)_imine at 1697 and 1614 cm⁻¹, respectively are clearly observable for both. As with Pd1 and Pd2 the ESI mass spectrum for each complex recorded in acetonitrile show strong [M − Cl]⁺ peaks as their MeCN adducts.

Fig. 3 ORTEP diagram of Pd4 with thermal ellipsoids set at 50% probability level; all hydrogen atoms and are omitted for clarity.

The preferential C–H activation of the C₂–H proton (see Scheme 3 for numbering system) of the dibenzo[a,c]acridine unit over the C₁₁–H position is likely due to the extra chelate ring stability afforded when Pd1 and Pd2 are formed. Attempts to react Pd1 or Pd2 with a further equivalent of PdCl₂(NCMe)₂, with a view to initiating a second C–H activation, were unsuccessful. It is assumed the C–H activation step to form Pd1 or Pd2 follows an AMLA type mechanism with a chloride ligand acting as the intramolecular base.²⁰ Nevertheless the ready activation of this C–H bond at room temperature with palladium(II) chloride is worthy of note and is seems likely that this is made more facile by the close and fixed proximity of the aryl-H bond that is imposed by the conjugated ligand framework. With regard to the unexpected formation of Pd3 and Pd4, it would seem likely that the use of protic MeOH as solvent is initiating the hydrolysis of the corresponding aldimine groups.

Heck coupling reactions

To probe the capacity of NNC pincer complexes Pd1–Pd4 to mediate a range of Heck-type coupling reactions, we initially explored the use of Pd1 to establish the most suitable conditions for the coupling of bromobenzene with styrene. Using a 0.002 mol% catalyst loading we explored the effect of base, solvent, temperature and reaction duration on the conversion to stilbene; the results are collected in Table 3.

Table 3 Optimization of reaction conditions using Pd1 as catalyst^a

Entry	Base	Solvent	Temp. (°C)	Time (h)	Conv.^b (%)	TON^c
a Reaction conditions: 4 × 10^−5 mmol Pd1, 2.0 mmol bromobenzene, 2.4 mmol styrene, 2.2 mmol base, 4.0 mL solution. b Determined by GC. c TON: mol stilbene/mol Pd. d 2.0 mmol iodobenzene used in place of bromobenzene. e 1 × 10^−5 mmol Pd1 (0.0005 mol%), 2.0 mmol iodobenzene used in place of bromobenzene.
1	Na2CO3	DMA	60	8	0	0
2	Na2CO3	DMA	100	8	0	0
3	Na2CO3	DMA	120	8	0	0
4	Na2CO3	DMA	140	4	21	10 500
5	Na2CO3	DMA	140	8	29	14 500
6	Na2CO3	DMA	140	12	41	20 500
7	K2CO3	DMA	140	8	6	3000
8	NaHCO3	DMA	140	8	21	10 500
9	NaOH	DMA	140	8	6	3000
10	Na2CO3	Toluene	100	8	0	0
11	Na2CO3	DMF	140	8	67	33 500
12	Na2CO3	CH3CN	60	8	0	0
13^d	Na2CO3	DMF	140	8	90	45 000
14^d	Na2CO3	DMF	200	8	88	44 000
15^e	Na2CO3	DMF	140	8	87	174 000

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On inspection of the data, the most suitable reaction parameters were found when the coupling was performed in dimethylformamide (DMF) at 140 °C in the presence of Na₂CO₃ for eight hours resulting in a 67% conversion to stilbene (Table 3, entry 11). Some other solvents such as dimethylacetamide (DMA) (entries 1–6, Table 3), toluene (entry 10, Table 3) and acetonitrile (entry 12, Table 3) were also examined under similar conditions but these all proved less effective. Likewise, a variety of different bases such as K₂CO₃ (entry 7, Table 3), NaHCO₃ (entry 8, Table 3) and NaOH (entry 9, Table 3) were screened but these again gave less promising results. Notably, the high temperature of 140 °C proved vital as performing the reactions at temperatures below 120 °C gave no conversion (entries 1–3, Table 3).

As expected, using more activated arenes led to higher conversions. For example with iodobenzene 90% conversion (entry 13, Table 3) was achieved, while with 4-iodotoluene close to quantitative conversion was noted (Table S1a†). Prolonging the reaction time gave only modest improvements in the conversion (Table S1b†). Most notably, Pd1 allowed 88% conversion to stilbene even at 200 °C (entry 14, Table 3) which highlights the exceptional balance between thermal stability and reactivity displayed by this catalyst.^9,10 Reducing the catalyst loading from 0.002 to 0.0005 mol%, showed only a slight drop in conversion (87%, TON = 174 000) (Table 3, entry 15). Compared with previous reports that show similarly high conversions in Heck reactions, Pd1 is at the lower end of the range in terms of catalyst loading; its high thermal robustness represents a notable advantage.^6e,7b–d

To examine the role of electronic and steric effects on the catalytic efficiency, Pd2, Pd3 and Pd4 were additionally evaluated under the optimized reaction conditions established for Pd1; the results are compiled in Table 4. Several points emerge from examining the data. Firstly, both aldehyde-containing complexes Pd3 and Pd4 (entries 3 and 4 respectively) gave improved catalytic efficiency when compared with their imine counterparts Pd1 and Pd2, respectively. Secondly, the more sterically protected Pd2 gave a lower conversion to stilbene (entry 2, Table 4) compared to Pd1. Thirdly, the use of palladium(II) chloride alone gave only 20% conversion; albeit at higher catalyst loading. These observations highlight the role of the ligand in influencing catalytic performance. Both steric and electronic factors present within the ligand frame further affect turnover with a combination of good electron withdrawing groups and less sterically hindered aryl groups leading to optimal performance.

Table 4 Heck reaction of bromobenzene with styrene catalyzed by Pd1–Pd4^a

Entry	Complex	Conversion^b (%)	TON^c
a Reaction conditions: 4 × 10^−5 mmol complex, 2.0 mmol bromobenzene, 2.4 mmol styrene, 2.2 mmol Na2CO3, 4.0 mL DMF, 140 °C for 8 h. b Determined by GC. c TON: mol stilbene/mol Pd. d 2 × 10^−3 mmol Pd.
1	Pd1	67	33 500
2	Pd2	62	31 000
3	Pd3	70	35 000
4	Pd4	66	33 000
5^d	PdCl2	20	200

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On the basis of the encouraging results obtained for the Heck coupling of bromobenzene with styrene, we decided to explore the potential of Pd3 for the coupling reaction of other haloarenes with styrene and N-tert-butylacrylamide; the results are documented in Table 5. Under similar conditions (viz., 0.002 mol% of catalyst, 8 h reaction at 140 °C, DMF as solvent and Na₂CO₃ as base), Pd3 showed a broad substrate scope with respect to the haloarenes employed (Table 5). In all the reactions involving styrene (entries 1–5 and 7–9), good to very good TON values were observed²¹ with the iodoarenes the best. However 2,6-dimethylbromobenzene appeared the least active substrate (entry 6) which can be attributed to the detrimental effect of the ortho-methyl substituents due to their steric properties, similar observations have been seen elsewhere.^22,23 No negative effects on conversion were observed with either electron withdrawing (–CN) or donating (–NH₂) groups positioned on the ortho-position of the bromoarenes, in fact some conversion rates were improved (entries 3 and 7). Meanwhile electron donating groups on the para-position enhanced the coupling reaction (entries 1, 4, 8 and 9). Using N-tert-butylacrylamide as the vinyl substrate with Pd3 also resulted in good to excellent conversions albeit with a narrower range of turnover numbers (entries 10–16). 4-Iodotoluene gave the highest activity (essentially quantitative conversion) of all the reactions screened (entry 16) and slightly higher than observed when styrene was employed as the vinyl-containing substrate (entry 8), an observation that has been noted previously.²⁴

Table 5 Heck reaction of aryl halides with styrene and N-tert-butylacrylamide using Pd3 as catalyst^a

Entry	Ar–X	Vinyl substrate	Conv.^b (%)	Yield^c (%)	TON^d
a Reaction conditions: 4 × 10^−5 mmol Pd3, 2.0 mmol Ar–X, 2.4 mmol styrene or N-tert-butylacrylamide, 2.2 mmol Na2CO3, 4.0 mL DMF, 140 °C for 8 h. b Determined by GC. c Isolated yield, their ^1H and ^13C NMR spectra were compared with literature reports.^25–27 d TON: mol product/mol Pd.
1		Styrene	49	38	24 500
2		Styrene	60	51	30 000
3		Styrene	45	36	24 500
4		Styrene	78	70	39 000
5		Styrene	50	44	25 000
6		Styrene	19	12	9500
7		Styrene	42	35	21 000
8		Styrene	97	90	45 000
9		Styrene	97	91	48 500
10			50	41	25 000
11			78	70	34 000
12			43	30	21 500
13			55	40	27 500
14			57	45	28 500
15			94	87	47 000
16			>99	93	49 500

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Conclusions

In summary, we have reported the synthesis of two highly conjugated mixed aldimine-ketimine ligands, L1 and L2, based on the dibenzo[a,c]acridine motif by a series of simple steps including heterocyclization of an unusual spirocycle. N_ketimineNC–palladium pincer complexes, Pd1 and Pd2, are readily accessible at room temperature on reaction of L1 or L2 with PdCl₂(NCMe)₂ in aprotic solvents while the introduction of methanol leads to hydrolysis of the non-coordinated aldimine unit in Pd1 and Pd2 to give aldehyde-containing Pd3 and Pd4. All four complexes display good to very good performance for the metal-mediated coupling of bromobenzene with styrene at elevated temperatures (ca. 140 °C) with Pd3 the standout performer. The effectiveness of Pd3 to catalyze the coupling of a series of electronically and sterically distinct bromobenzenes with styrene and N-tert-butylacrylamide has also shown the general applicability of this NNC pincer complex. Further studies are underway to determine the scope of these thermally robust ligand frames to support other metal-mediated catalytic processes.

Experimental

General consideration

All manipulations of ligands were carried out under an open air atmosphere while those involving complexes were performed using standard Schlenk techniques under a nitrogen atmosphere. Solvents were distilled under nitrogen from appropriate drying agents immediately prior to use. ¹H and ¹³C NMR spectra were recorded on a Bruker DMX 400 MHz spectrometer (400 MHz for ¹H, 100 MHz for ¹³C) at ambient temperature using TMS as an internal standard; δ values were given in ppm and J values in Hz. Fourier Transform IR spectra were recorded on a Perkin-Elmer System 2000 FT-IR spectrometer. Elemental analysis was performed using an HPMOD 1106 microanalyzer. GC measurements were performed on a VARIAN CP-3800 GC containing a chrompack capillary column (cp-sil 5 CB). The electrospray (ESI) mass spectra for the complexes and the high resolution mass spectra (MALDI TOF) for all new organic compounds were measured using Bruker 9.4 T Solarix (FT-ICR-MS) instrument. Phenanthrene-9,10-dione, 2,6-diethylaniline, 2,6-dimethylaniline, the haloarenes, styrene and N-tert-butylacrylamide were purchased from Aldrich and Acros and used without further purification. PdCl₂(NCCH₃)₂ was prepared using the literature method.²⁸ Other general reagents were purchased from Aldrich, Acros or local suppliers.

Synthesis of pincer Ligands

Preparation of spiro-[7-ethyl-3-methylindoline-2,10-phenanthren-9-one] (1). To a solution of phenanthrene-9,10-dione (10.00 g, 48.03 mmol) and p-toluenesulfonic acid (1.83 g, 20 mol%) in dry toluene (200 mL) was added dropwise 2,6-diethylaniline (14.34 g, 96.06 mmol) and the reaction mixture stirred and heated to reflux for 12 h. After cooling to room temperature, the mixture was concentrated on the rotary evaporator and purified by silica gel column chromatography using petroleum ether/ethyl acetate (125/1) as the eluent to give 1 as a yellow crystalline solid (6.23 g, 38%). The mono-imine¹⁶ and bis-imine^16,17 derivatives of phenonthrene-9,10-dione were discarded. Mp: 118–119 °C. ¹H NMR (400 MHz, CDCl₃, TMS): δ 7.97 (d, J = 8 Hz, 1H, Ph–H), 7.90–7.87 (m, 2H, Ph–H), 7.79 (d, J = 7.6 Hz, 1H, Ph–H), 7.69 (t, J = 8 Hz, 1H, Ph–H), 7.43–7.30 (m, 3H, Ph–H), 7.03 (d, J = 7.2 Hz, 1H, Ph–H), 6.81–6.73 (m, 2H, Ph–H), 4.64 (s, 1H, NH), 3.46 (q, J = 7.2 Hz, 1H, CH), 2.75–2.69 (m, 2H, CH₂), 1.38 (t, J = 7.8 Hz, 3H, CH₃), 0.98 (d, J = 7.2 Hz, 3H, CH₃). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 201.4, 152.7, 148.7, 143.0, 137.6, 134.7, 131.4, 130.8, 130.7, 129.5, 128.7, 128.1, 127.4, 127.2, 127.0, 126.3, 124.2, 123.1, 121.1, 120.5, 54.7, 24.5, 17.6, 13.6. FT-IR (KBr, cm⁻¹): 3070 (w), 2964 (s), 2927 (w), 2870 (w), 1676 (ν(–C=O) s), 1596 (s), 1478 (s), 1478 (s), 1393 (m), 1267 (w), 1064 (m), 987 (w), 898 (m), 733 (s). HRMS (MALDI) (m/z) calcd for C₂₄H₂₁NO [M + H]⁺ 340.17014; found 340.16953.

Preparation of 10-ethyl-14-methyldibenzo[a,c]acridine (2). A solution of 1 (5.00 g, 14.74 mmol) and p-toluenesulfonic acid (0.56 g, 20 mol%) in dichlorobenzene (40 mL) was stirred and heated to reflux for 12 h. After the mixture was cooled to room temperature, the solvent was evaporated under reduced pressure. The crude product was purified by silica gel column chromatography using petroleum ether/ethyl acetate (250/1) as the eluent, yielding 2 as a white solid (3.31 g, 70%). Mp: 100–101 °C. ¹H NMR (400 MHz, CDCl₃, TMS): δ 9.68–9.44 (m, 1H, Ph–H), 8.55 (d, J = 8 Hz, 1H, Ph–H), 8.48–8.45 (m, 1H, Ph–H), 8.30 (d, J = 8 Hz, 1H, Ph–H), 8.09 (d, J = 8.8 Hz, 1H, Ph–H), 7.71–7.69 (m, 2H, Ph–H), 7.63 (t, J = 6.8 Hz, 2H, Ph–H), 7.55 (t, J = 7.2 Hz, 2H, Ph–H), 3.60–3.55 (m, 2H, CH₂), 3.28 (s, 3H, CH₃), 1.54 (t, J = 7.6 Hz, 3H, CH₃). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 146.6, 144.7, 143.7, 140.8, 132.3, 132.2, 131.6, 130.6, 130.1, 129.4, 128.0, 127.9, 127.6, 127.6, 126.7, 126.1, 126.0, 123.7, 123.4, 122.5, 122.4, 25.4, 20.4, 15.5. FT-IR (KBr, cm⁻¹): 3066 (w), 2963 (s), 2935 (m), 2880 (w), 1424 (m), 1398 (s), 1352 (m), 793 (s), 753 (s). HRMS (MALDI) (m/z) calcd for C₂₄H₁₉N [M + H]⁺ 322.15957; found 322.15902.

Preparation of 10-(1-bromoethyl)-14-(bromomethyl)dibenzo[a,c] acridine (3). 2 (3.00 g, 9.34 mmol), N-bromosuccinimide (3.32 g, 18.68 mmol) and dibenzoyl peroxide (0.23 g, 10 mol%) were suspended in CCl₄ (100 mL). The suspension was stirred and heated to reflux in a vessel open to the air for 6 h. On completion of the reaction, the mixture was filtered whilst hot. The filtrate was collected and the solvent removed under reduced pressure. The residue was recrystallized from dichloromethane and heptane forming 3 as a light yellow solid (4.25 g, 95%). Mp: 174–175 °C. ¹H NMR (400 MHz, CDCl₃, TMS): δ 9.42 (s, 1H, Ph–H), 8.82 (d, J = 7.6 Hz, 1H, Ph–H), 8.58 (d, J = 8.0 Hz, 1H, Ph–H), 8.47 (d, J = 7.6 Hz, 2H, Ph–H), 8.10 (d, J = 6.0 Hz, 1H, Ph–H), 7.78–7.67 (m, 5H, Ph–H), 7.05 (q, J = 7.2 Hz, 1H, CH), 5.37 (s, 2H, CH₂), 2.32 (broad singlet, 3H, CH₃). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 147.5, 141.8, 137.6, 132.2, 132.2, 131.5, 130.3, 130.2, 128.8, 128.7, 128.4, 127.5, 127.2, 126.9, 126.7, 126.2, 124.7, 124.1, 122.6, 44.3, 31.9, 26.4. FT-IR (KBr, cm⁻¹): 3069 (w), 3031 (w), 2971 (m), 1374 (s), 1348 (s), 789 (s), 764 (m), 748 (s). HRMS (MALDI) (m/z) calcd for C₂₄H₁₇Br₂N [M + H]⁺ 477.98060; found 477.98005.

10-(1-Hydroxyethyl)-14-(hydroxymethyl)dibenzo[a,c]acridine (4). 3 (1.00 g, 2.10 mmol) was added to a solution of silver nitrate (1.25 g, 7.34 mmol, 3.5 equiv.) in 100 mL of 70% aqueous acetone at 50 °C, and the resultant suspension was stirred for 50 °C for 3 h. On completion of the reaction, the mixture was filtered through sintered glass, diluted with water (50 mL) and extracted with dichloromethane (3 × 30 mL). The combined dichloromethane extracts were washed twice with water, dried over anhydrous MgSO₄ and concentrated under reduced pressure to give 4 as a brown powder (0.71 g, 96%). No further purification was performed. Mp: 177–178 °C. ¹H NMR (400 MHz, DMSO, TMS): δ 9.24 (s, 1H, Ph–H), 8.87 (d, J = 8.0 Hz, 1H, Ph–H), 8.75 (d, J = 7.2 Hz, 1H, Ph–H), 8.69 (d, J = 6.8 Hz, 1H, Ph–H), 8.45 (d, J = 8.4 Hz, 1H, Ph–H), 8.00 (d, J = 7.2 Hz, 1H, Ph–H), 7.80–7.71 (m, 5H, Ph–H), 6.22–6.19 (m, 2H, CH₂), 5.38 (d, J = 4.4 Hz, 1H, OH), 5.29 (t, J = 4.4 Hz, 2H, OH, CH), 1.63 (s, 3H, CH₃). ¹³C NMR (100 MHz, DMSO, TMS): δ 145.6, 145.2, 142.8, 141.9, 131.6, 130.7, 130.3, 130.2, 129.9, 128.5, 128.4, 128.1, 127.0, 126.8, 126.3, 125.6, 125.0, 123.7, 123.5, 123.0, 122.7, 64.3, 58.9, 25.8. FT-IR (KBr, cm⁻¹): 3226 (broad, OH), 3074 (w), 2963 (w), 2921 (w), 1706 (s), 1550 (s), 1278 (w), 760 (s), 730 (s). HRMS (MALDI) (m/z) calcd for C₂₄H₁₉NO₂ [M + H]⁺ 354.14940; found 354.14885.

Preparation of 10-acetyl-14-formyldibenzo[a,c]acridine (5). Pyridinium chlorochromate (1.13 g, 5.25 mmol) was added to solution of 4 (1.0 g, 2.83 mmol) in dichloromethane (50 mL) at 0 °C. Crushed molecular sieves (1 g) were also added to adsorb sticky by-products. The mixture was stirred for 6 h at ambient temperature. On completion of the reaction (as monitored by TLC), the mixture was filtered through a silica plug. The filtrate was diluted by the addition of water (50 mL) and extracted with dichloromethane (3 × 15 mL). The combined extracts were dried over anhydrous magnesium sulfate and, following solvent evaporation, the residue was recrystallized from dichloromethane and heptane affording 5 as a yellow solid (0.89 g, 90%). Mp: 196–197 °C. ¹H NMR (400 MHz, CDCl₃, TMS): δ 10.58 (s, 1H, CHO), 9.31–9.28 (m, 1H, Ph–H), 9.03–8.99 (m, 1H, Ph–H), 8.67 (d, J = 10.4 Hz, 1H, Ph–H), 8.58–8.55 (m, 1H, Ph–H), 8.12–8.10 (m, 1H, Ph–H), 7.93–7.90 (m, 1H, Ph–H), 7.87–7.73 (m, 5H, Ph–H), 3.14 (s, 1H, CH₃). ¹³C NMR (100 MHz, DMSO, TMS): δ 202.5, 194.9, 149.5, 147.3, 143.4, 139.2, 136.9, 131.7, 131.3, 130.9, 130.9, 130.1, 129.6, 129.5, 128.6, 127.8, 127.7, 127.3, 126.0, 125.6, 124.3, 123.3, 123.2, 121.6. FT-IR (KBr, cm⁻¹): 3075 (w), 2855 (w), 1682 (ν(C=O) m), 1391 (s), 1344 (m), 797 (s), 760 (m), 739 (s). HRMS (MALDI) (m/z) calcd for C₂₄H₁₅NO₂ [M + H]⁺ 350.11810; found 350.11755.

Preparation of 10-[1-(2,6-dimethylphenylimino)ethyl]-14-[(2,6-dimethylphenylimino)methyl]dibenzo[a,c]acridine (L1). To a mixture of 5 (0.85 g, 2.43 mmol) and catalytic amount of p-toluenesulfonic acid (0.13 g, 10 mol%) in dry toluene (30 mL) was added dropwise 2,6-dimethylaniline (0.74 g, 6.08 mmol). The mixture was stirred and heated to reflux for 12 h. On cooling to room temperature, all volatiles were removed under reduced pressure. The crude product was purified by column chromatography with petroleum ether/ethyl acetate (125 : 1, v/v) as the eluent affording L1 as a deep yellow powder (0.60 g, 45%). Mp: 195–196 °C. ¹H NMR (400 MHz, CDCl₃, TMS): δ 9.69 (d, J = 8.8 Hz, 1H, Ph–H), 9.33 (d, J = 7.6 Hz, 1H, Ph–H), 8.84 (s, 1H, CH=N), 8.59 (d, J = 8.0 Hz, 1H, Ph–H), 8.52 (d, J = 8.0 Hz, 1H, Ph–H), 8.22 (d, J = 8.0 Hz, 1H, Ph–H), 8.17 (d, J = 8.0 Hz, 1H, Ph–H), 7.82–7.686 (m, 4H, Ph–H), 7.53 (t, J = 8.0 Hz, 1H, Ph–H), 7.18 (t, J = 6.4 Hz, 4H, Ph–H), 7.08–6.99 (m, 2H, Ph–H), 2.52 (s, 3H, CH₃C=N), 2.37 (s, 12H, 4 × CH₃). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 171.5, 163.7, 151.0, 148.8, 147.4, 145.4, 141.6, 135.3, 132.1, 131.8, 131.2, 130.0, 128.9, 128.8, 128.6, 128.3, 128.2, 128.1, 128.0, 127.6, 127.0, 126.6, 126.3,126.0, 124.5, 124.4, 124.2, 123.9, 123.0, 122.7, 31.9, 28.9, 23.4, 22.7, 18.5, 18.1, 17.5, 14.0. FT-IR (KBr, cm⁻¹): 2959 (w), 2918 (w), 2855 (m), 1646 (ν(HC=N) s), 1628 (ν(CH₃C=N) s), 1375 (s), 1347 (s), 1292 (m), 1202 (m), 854 (s), 797 (s), 768 (s). Anal. Calcd for C₄₀H₃₃N₃ (555): N, 7.56; C, 86.45; H, 5.99. Found: N, 7.61; C, 86.38; H, 6.09. HRMS (MALDI) (m/z) calcd for C₄₄H₃₃N₃ [M + H]⁺ 556.27527; found 556.27472.

Preparation of 10-[1-(2,6-diethylphenylimino)ethyl]-14-[(2,6-diethylphenylimino)methyl]dibenzo[a,c]acridine (L2). L2 was prepared using a similar procedure and molar ratios to that described for L1 affording a deep yellow solid (0.61 g, 41%). Mp: 207–208 °C. ¹H NMR (400 MHz, CDCl₃, TMS): δ 9.76 (d, J = 8.4 Hz, 1H, Ph–H), 9.34 (d, J = 8.0 Hz, 1H, Ph–H), 8.83 (s, 1H, CH=N), 8.59 (d, J = 8.0 Hz, 1H, Ph–H), 8.52 (d, J = 7.6 Hz, 1H, Ph–H), 8.19 (d, J = 8.0 Hz, 1H, Ph–H), 8.13 (d, J = 8.0 Hz, 1H, Ph–H), 7.82–7.76 (m, 2H, Ph–H), 7.73–7.68 (m, 2H, Ph–H), 7.51 (t, J = 8.0 Hz, 1H, Ph–H), 7.23–7.21 (m, 4H, Ph–H), 7.17–7.13 (m, 2H, Ph–H), 2.88–2.69 (m, 8H, 4 × CH₂), 2.52 (s, 3H, CH₃C=N), 1.34 (t, J = 7.6 Hz, 6H, 2 × CH₃), 1.26 (t, J = 7.6 Hz, 6H, 2 × CH₃). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 171.4, 163.3, 149.9, 147.8, 147.5, 145.4, 141.8, 134.8, 133.3, 132.2, 132.0, 131.9, 131.8, 131.2, 130.0, 129.0, 128.8, 128.2, 128.1, 127.7, 127.1, 126.7, 126.6, 126.4, 125.9, 124.8, 124.3, 123.9, 123.3, 12.7, 53.4, 24.6, 23.6, 15.2, 14.0. FT-IR (KBr, cm⁻¹): 2963 (w), 2925 (w), 2866 (m), 1652 (ν(HC=N) s), 1620 (ν(CH₃C=N) s), 1456 (w), 1347 (s), 1290 (m), 1256 (m), 856 (s), 799 (s), 766 (s). Anal. Calcd for C₄₄H₄₁N₃ (611): N, 6.87; C, 86.38; H, 6.75. Found: N, 6.76; C, 86.07; H, 6.86. HRMS (MALDI) (m/z) calcd for C₄₄H₄₁N₃ [M + H]⁺ 612.33787; found 612.33732.

Synthesis of NNC pincer complexes

(a) Typical procedure in either toluene or dichloromethane solution. Equimolar amounts of L1 (0.200 g, 0.36 mmol) and PdCl₂(NCCH₃)₂ (0.093 g, 0.036 mmol) were added to either dichloromethane (10 mL) or toluene solution (10 mL) and the reaction mixture stirred for 10 h at ambient temperature. The resultant precipitate was collected by filtration, washed with diethyl ether and heptane. The respective yields are given in Table 1.

(b) Typical procedure in toluene solution along with base. To a solution of L1 (0.200 g, 0.36 mmol) in toluene (25 mL), was added PdCl₂(NCCH₃)₂ (0.093 g, 0.36 mmol) under a nitrogen atmosphere. A catalytic amount of NaHCO₃ was also added and the reaction mixture stirred and heated at 60 °C for 6 h. On cooling to room temperature, all volatiles were removed under reduced pressure. The residue was re-dissolved in CH₂Cl₂ (20 mL) and filtered to remove any insoluble material. Following filtration, the filtrate was concentrated and the residue recrystallized from heptane to give Pd1 as an orange solid (0.130 g, 52%).

(c) Typical procedure in methanol solution. PdCl₂(NCCH₃)₂ (0.093 g, 0.36 mmol) was added to a solution of L1 (0.200 g, 0.36 mmol) in a mixture of methanol/toluene (10 mL/15 mL) under a nitrogen atmosphere. After stirring and heating the reaction mixture at 50 °C for 6 h, all volatiles were removed under reduced pressure. The resultant residue was purified by a short column of silica gel. Elution with petroleum ether/ethyl acetate (100/1) firstly afforded Pd1 as an orange solid on rotary evaporation. Pd3 could then be collected using petroleum ether/ethyl acetate 50/1 as an orange solid on evaporation. The respective yields are given in Table 1.

Pd1: Based on either of the procedures described in (a), (b) and (c), Pd1 was obtained as an orange solid in the corresponding yield given in Table 1. ¹H NMR (400 MHz, CDCl₃, TMS): δ 9.75 (d, J = 8.4 Hz, 1H, Ph–H), 8.83 (s, 1H, CH=N), 8.48 (d, J = 8.0 Hz, 1H, Ph–H), 8.41 (d, J = 7.6 Hz, 1H, Ph–H), 8.19 (d, J = 7.2 Hz, 1H, Ph–H), 8.03–7.98 (m, 2H, Ph–H), 7.78 (t, J = 8.4 Hz, 1H, Ph–H), 7.71 (t, J = 7.2 Hz, 1H, Ph–H), 7.50–7.42 (m, 2H, Ph–H), 7.21–7.11 (m, 6H, Ph–H), 2.40 (s, 6H, 2 × CH₃), 2.37 (s, 6H, 2 × CH₃), 2.34 (s, 3H, CH₃). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 169.4, 162.3, 156.7, 150.3, 148.6, 141.5, 140.2, 137.2, 136.6, 133.4, 132.6, 132.5, 132.2, 131.5, 131.2, 130.0, 128.8, 128.1, 127.9, 126.9, 126.6, 126.5, 126.4, 125.7, 125.1, 124.3, 118.3, 53.3, 52.9, 23.0, 18.9, 18.5. FT-IR (KBr, cm⁻¹): 2961 (w), 2914 (w), 2850 (w), 1618 (ν(HC=N), (CH₃C=N) broad), 1588 (m), 1567 (m), 1368 (m), 1268 (m), 1235 (m), 1186 (w), 849 (w), 756 (m), 698 (s). ESI MS (MeCN): m/z 701 [M − Cl + MeCN]. Anal. Calcd for C₄₀H₃₂N₃PdCl (695): N, 6.03; C, 68.97; H, 4.63. Found: N, 6.11; C, 68.91; H, 4.55.

Pd2: Based on either of the procedures outlined in (a) and (c), Pd2 was obtained as an orange solid in the corresponding yield given in Table 1. ¹H NMR (400 MHz, CD₂Cl₂, TMS): δ 9.78 (d, J = 8.4 Hz, 1H, Ph–H), 8.81(s, 1H, CH=N), 8.5 (d, J = 8.0 Hz, 1H, Ph–H), 8.43 (d, J = 8.0 Hz, 1H, Ph–H), 8.05–8.01(m, 3H, Ph–H), 7.79 (t, J = 8.4 Hz, 1H, Ph–H), 7.72 (t, J = 7.6 Hz, 1H, Ph–H), 7.49 (t, J = 7.6 Hz, 1H, Ph–H), 7.41 (t, J = 7.6 Hz, 1H, Ph–H), 7.23–7.14 (m, 6H, Ph–H), 2.97–2.88 (m, 2H, CH₂), 2.7–2.6 (m, 6H, 3 × CH₂), 2.3 (s, 3H, CH₃), 1.29–1.21 (m, 12H, 4 × CH₃). ¹³C NMR (100 MHz, CD₂Cl₂, TMS): δ 170.3, 162.1, 149.3, 147.8, 140.3, 137.6, 136.4, 133.4, 133.3, 132.6, 132.6, 131.4, 131.0, 130.1, 127.3, 126.7, 126.6, 125.7, 125.4, 125.3, 124.2, 118.3, 24.5, 24.5, 23.7, 15.2, 13.3. FT-IR (KBr, cm⁻¹): 2961 (w), 2927 (w), 2869 (w), 1612 (ν(HC=N), (CH₃C=N) broad), 1564 (ν(CH₃C=N) w), 1350 (w), 1269 (s), 1239 (s), 1177 (m), 857 (w), 759 (s), 709 (s). ESI MS (MeCN): m/z 757 [M − Cl + MeCN]. Anal. Calcd for C₄₄H₄₀N₃PdCl (751): N, 5.36; C, 70.21; H, 5.58. Found: N, 5.28; C, 70.91; H, 5.55.

Pd3: Based on the procedure outlined in (c), Pd3 was obtained as an orange solid in the yield given in Table 1. ¹H NMR (400 MHz, CD₂Cl₂, TMS): δ 10.63 (s, 1H, CHO), 8.88 (d, J = 8.0 Hz, 1H, Ph–H), 8.57 (d, J = 8.0 Hz, 1H, Ph–H), 8.44 (d, J = 7.6 Hz, 1H, Ph–H), 8.05 (t, J = 7.6 Hz, 2H, Ph–H), 7.88–7.78 (m, 3H, Ph–H), 7.69 (t, J = 7.6 Hz, 1H, Ph–H), 7.43 (t, J = 7.6 Hz, 1H, Ph–H), 7.17 (t, J = 7.6 Hz, 3H, Ph–H), 2.33 (s, 6H, 2 × CH₃), 2.29 (s, 3H, CH₃). ¹³C NMR (100 MHz, CD₂Cl₂, TMS): δ 193.1, 169.9, 156.8, 148.8, 138.3, 137.9, 133.1, 132.9, 132.5, 132.1, 130.8, 129.8, 128.7, 128.1, 127.9, 127.2, 126.7, 125.5, 125.3, 124.3, 123.6, 118.5, 29.3, 22.9, 18.5. FT-IR (KBr, cm⁻¹): 2973 (m), 2916 (w), 2851 (w), 1696 (ν(C=O) s), 1614 (ν(C=N) s), 1567 (w), 1349 (w), 1265 (m), 1190 (m), 1050 (m), 853 (w), 754 (s), 724 (s). ESI MS (MeCN): m/z 598 [M − Cl + MeCN]. Anal. Calcd for C₃₂H₂₃N₂OPdCl (593): N, 4.72; C, 64.77; H, 3.91 Found: N, 4.72; C, 64.91; H, 4.90.

Pd4: Based on the procedure outlined in (c), Pd4 was obtained as an orange solid in the yield given in Table 1. NMR (400 MHz, CD₂Cl₂, TMS): δ 10.62 (s, 1H, CHO), 8.87 (d, J = 8.0 Hz, 1H, Ph–H), 8.55 (d, J = 8.0 Hz, 1H, Ph–H), 8.43 (d, J = 7.6 Hz, 1H, Ph–H), 8.04 (d, J = 7.6 Hz, 2H, Ph–H), 7.86–7.77 (m, 3H, Ph–H), 7.68 (d, J = 7.6 Hz, 1H, Ph–H), 7.41(d, J = 7.6 Hz, 1H, Ph–H), 7.20 (s, 3H, Ph–H), 2.94–2.85 (m, 2H, CH₂), 2.6–2.56 (m, 2H, CH₂), 2.29 (s, 3H, CH₃), 1.28–1.24 (m, 6H, 2 × CH₃). ¹³C NMR (100 MHz, CD₂Cl₂, TMS): δ 193.2, 170.2, 156.9, 147.7, 141.2, 140.0, 138.3, 138.0, 136.4, 133.3, 132.8, 132.0, 131.4, 131.2, 131.2, 130.8, 127.5, 127.1, 126.6, 125.8, 124.3, 123.7, 118.5, 24.5, 23.6, 13.2. FT-IR (KBr, cm⁻¹): 2962 (w), 2912 (w), 2839 (w), 1689 (ν(C=O) s), 1611 (ν(C=N) s), 1584 (m), 1346 (m), 1265 (w), 1178 (m), 1096 (s), 854 (m), 755 (s), 724 (m). ESI MS (MeCN): m/z 626 [M − Cl + MeCN]. Anal. Calcd for C₃₄H₂₇N₂OPdCl (620): N, 4.51; C, 65.71; H, 4.38 Found: N, 4.52; C, 65.91; H, 4.38.

General procedure for Heck reaction

The Heck coupling reactions of haloarenes with styrene or N-tert-butylacrylamide were carried out in the presence of catalytic amounts of the palladium pincer complex. In a typical procedure (entry 11, Table 3) a mixture of bromobenzene (0.21 mL, 2.0 mmol), styrene (0.25 mL, 2.4 mmol), DMF (4 mL), anhydrous Na₂CO₃ (0.23 g, 2.2 mmol) was loaded into an oven dried Schlenk flask under a nitrogen atmosphere. Pd1 (100 μL, 4 × 10⁻⁵ mmol, 2 × 10⁻³ mol%), taken from a 4 × 10⁻⁴ M stock solution in DMF (10 mL), was added to the Schlenk flask via syringe and the reaction mixture stirred and heated at 140 °C. After the desired reaction time, the mixture was allowed to cool to room temperature, diluted with water and ethyl acetate. The organic layers were combined, washed with brine and then dried over anhydrous MgSO₄. All volatiles were removed under reduced pressure and the residue purified by flash chromatography on silica gel using petroleum ether/ethyl acetate as the eluent affording stilbene (0.24 g, 67%).

X-Ray structure determination

X-ray data were collected with a Rigaku MM007-HF Saturn 724 + CCD diffractometer using Mo Kα radiation (λ = 0.71073 Å), employing a confocal mirror monochromator as a radiation source for L1 whilst graphite-monochromator as a radiation source was employed for complexes Pd1, Pd2, Pd3 and Pd4. Cell parameters were obtained by global refinement of the positions of all collected reflections. Intensities were corrected for Lorentz and polarization effects and empirical absorption. The structures were determined by direct methods and refined by full-matrix least squares on F². All the hydrogen atoms were placed in calculated positions. The molecular structure determination and their refinement were performed by the SHELXL-97 package.²⁹ The disorder behavior of two ethyl groups in complex Pd2 was also processed by the SHELXL-97 software. During the structure refinement, free solvent (chloroform) was squeezed (Pd2) with PLATON software.³⁰ Details of the crystal data and structure refinements for all five structures are shown in Table 6.

Table 6 Crystal data and structure refinements for L1, Pd1, Pd2, Pd3 and Pd4

	L1	Pd1·2CHCl3	Pd2	Pd3·CH2Cl2	Pd4
Empirical formula	C40 H33N3	C42H34N3PdCl7	C44H40N3PdCl	C33H25N2OPdCl3	C34H27N2OPdCl
Formula weight	555.69	935.29	751.63	678.30	621.43
Temperature/K	173(2)	173(2)	173(2)	173(2)	173(2)
Wavelength/Å	0.71073	0.71073	0.71073	0.71073	0.71073
Crystal system	Monoclinic,	Triclinic	Trigonal	Monoclinic	Monoclinic
Space group	P21/c	P1̄	R3̄	P21/c	P21/n
a/Å	11.910(2)	10.641(2)	36.490(5)	11.611(2)	14.187(3)
b/Å	32.793(7)	10.978(2)	36.490(5)	19.188(4)	12.380(3)
c/Å	9.4655(19)	17.744(3)	21.305(4)	13.466(3)	14.674(3)
α/°	90	100.04(3)	90.00	90.00	90.00
β/°	113.37(3)	106.43(3)	90.00	113.15(3)	92.59(3)
γ/°	90	93.41(3)	120.00	90.00	90.00
Volume/Å^3	3393.6(12)	1944.7(6)	24 568(7)	2758.5(10)	2574.7(9)
Z	4	2	18	25	4
D calcd/(g cm^−3)	1.088	1.597	0.916	2.782	1.603
μ/mm^−1	0.064	0.994	0.412	4.624	0.857
F(000)	1176	944	6984	2125	1264
Crystal size/mm	0.23 × 0.14 × 0.10	0.54 × 0.38 × 0.12	0.33 × 0.33 × 0.30	0.32 × 0.21 × 0.12	0.20 × 0.19 × 0.11
θ range (°)	1.96 to 27.50	1.22 to 27.47	2.24 to 54.82	4.48 to 54.96	3.9 to 54.98
Limiting indices	−15 ≤ h ≤ 15	−13 ≤ h ≤ 13	−47 ≤ h ≤ 47	−14 ≤ h ≤ 15	−18 ≤ h ≤ 18
	−42 ≤ k ≤ 42	−14 ≤ k ≤ 1	−47 ≤ k ≤ 47	−24 ≤ k ≤ 24	−16 ≤ k ≤ 16
	−11 ≤ l ≤ 12	−23 ≤ l ≤ 23	−27 ≤ l ≤ 27	−17 ≤ l ≤ 17	−19 ≤ l ≤ 19
No. of rflns collected	23 288	24 363	102 155	19 114	32 464
No. unique rflns	7780	8870	12 432	6237	5894
R(int)	0.0518	0.0370	0.0899	0.0262	0.0422
No. of params	393	483	469	364	355
Completeness to θ	99.6	99.5	100	98.8	99.7
Goodness of fit on F^2	1.047	1.024	1.072	1.107	1.084
Final R indices [I > 2σ(I)]	R 1 = 0.0633, wR2 = 0.1406	R 1 = 0.0765, wR2 = 0.2212	R 1 = 0.0652, wR2 = 0.2055	R 1 = 0.0369, wR2 = 0.0834	R 1 = 0.0281, wR2 = 0.0705
R indices (all data)	R 1 = 0.0814, wR2 = 0.1510	R 1 = 0.0811, wR2 = 0.2352	R 1 = 0.0755, wR2 = 0.2162	R 1 = 0.0392, wR2 = 0.0850	R 1 = 0.0286, wR2 = 0.0708
Largest diff. peak, and hole (e Å^−3)	0.219/−0.164	1.946/−2.206	0.72/−0.92	1.02/−0.84	0.64/−0.58

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Acknowledgements

This work is supported by National Natural Science Foundation of China (No. 51273202 and U1362204). QM thanks CAS-TWAS president's fellowship for financial support. GAS is grateful to the Chinese Academy of Sciences for a Visiting Scientist Fellowship.

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Footnote

† Electronic supplementary information (ESI) available: X-ray crystallographic data. CCDC 1496370 (1), 1496371 (2), 1496372 (Pd1), 1496373 (Pd2), 1496374 (Pd3), 1496375 (Pd4) and 1496529 (L1). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6qo00469e

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