O,N,N-Pincer ligand effects on oxidatively induced carbon–chlorine coupling reactions at palladium

Abstract

The syntheses of two families of sterically tuneable O,N,N pro-ligands are reported, namely the 2-(phenyl-2′-ol)-6-imine-pyridines, 2-(C₆H₄-2′-OH),6-(CMe=NAr)C₅H₃N [Ar = 4-i-PrC₆H₄ (HL1_a), 2,6-i-Pr₂C₆H₃ (HL1_b)] and the 2-(phenyl-2′-ol)-6-(amino-prop-2-yl)pyridines, 2-(C₆H₄-2′-OH),6-(CMe₂NHAr)C₅H₃N [Ar = 4-i-PrC₆H₄ (HL2_a), 2,6-i-Pr₂C₆H₃ (HL2_b)], using straightforward synthetic approaches and in reasonable overall yields. Interaction of HL1_a/c and HL2_a/b with palladium(II) acetate affords the O,N,N-pincer complexes, [{2-(C₆H₄-2′-O)-6-(CMe=NAr)C₅H₃N}Pd(OAc)] (Ar = 4-i-PrC₆H₄ (1a), 2,6-i-Pr₂C₆H₃ (1b)) and [{2-(C₆H₄-2′-O)-6-(CMe₂NHAr)C₅H₃N}Pd(OAc)] (Ar = 4-i-PrC₆H₄ (2a), 2,6-i-Pr₂C₆H₃ (2b)), which can be readily converted to their chloride derivatives, [{2-(C₆H₄-2′-O)-6-(CMe=NAr)C₅H₃N}PdCl] (Ar = 4-i-PrC₆H₄ (3a), 2,6-i-Pr₂C₆H₃ (3b)) and [{2-(C₆H₄-2′-O)-6-(CMe₂NHAr)C₅H₃N}PdCl] (Ar = 4-i-PrC₆H₄ (4a), 2,6-i-Pr₂C₆H₃ (4b)), respectively, on reaction with an aqueous sodium chloride solution. Treating each of 3a, 3b, 4a and 4b with two equivalents of di-p-tolyliodonium triflate at 100 °C in a toluene/acetonitrile mixture affords varying amounts of 4-chlorotoluene along with the 4-iodotoluene by-product with the conversions highly dependent on the steric and backbone properties of the pincer complex employed (viz.4a > 3a > 4b > 3b); notably, the least sterically bulky and most flexible amine-containing 4a reaches 90% conversion to 4-chlorotoluene in 15 h as opposed to 17% for imine-containing 3b. In the case of 3a, the inorganic palladium species recovered from the reaction has been identified as the Pd(II) salt [{2-(C₆H₄-2′-O)-6-(CMe=N(4-i-PrC₆H₄)C₅H₃N}Pd(NCMe)][O₃SCF₃] (5a), which was independently prepared by the reaction of 3a with silver triflate in acetonitrile. Single crystal X-ray structures are reported for HL1_a, HL2_a, 1a, 1b, 2a, 2b, 3a and 5a.

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Introduction

While hypervalent iodine salts of the type [Ar₂I][X] (X = OTf, BF₄) have been widely used in Pd(0)/(II) cross coupling reactions,¹ their application in Pd(II)/(IV) and/or Pd(II)/(III) chemistry has only started to emerge over the last decade.^2,3 With regard to the Pd(II)/(IV) couple, stable palladium(IV) species have been characterised,⁴ computationally modelled⁵ and highlight the ability of the I(III) reagent to transfer an “Ar⁺” group to the palladium(II) centre; decomposition can ensue via reductive elimination of an aryl-containing product. The chlorination of Pd(II)–C and Pd(II)–Cl containing complexes with PhICl₂ represents another transformation that has been more extensively studied and these reactions are considered to proceed via a facile C–Cl bond forming reductive elimination from a Pd(IV) intermediate.^6,7 For example, van Koten has spectroscopically characterised a transient Pd(IV) species from the reaction of an Pd(II) chloride N,C_ph,N-pincer complex with PhICl₂, which is presumed to then undergo C–Cl bond forming reductive elimination with the phenyl moiety of the pincer ligand.⁸ Indeed, a variety of pincer ligand frameworks including symmetrical (e.g., N,C,N^5,8) and unsymmetrical (e.g., C,N,N,⁹O,N,C,¹⁰O,N,N¹¹) variations have proved conducive to promoting the formation of related electron deficient Pd(IV) intermediates, a feature that is likely to be attributable to the electron supplying nature of the tridentate manifold.

In this article we report the stoichiometric reactivity of a range of palladium(II) chloride O,N,N-pincer complexes towards di-p-tolyliodonium triflate with a view to monitoring the effect that the O,N,N-spectator ligand has on the anticipated formation of 4-chlorotoluene. In particular, we target two families of pyridine-based O,N_py,N pincers in order to investigate how structural features within their respective O,N_py,N ligand manifold influence the C–Cl bond forming process; the effects of imine (L1) vs. amine (L2) nitrogen donor and steric factors within the N-aryl group (Ar = 4-i-PrC₆H₄, 2,6-i-Pr₂C₆H₃) will be examined (Fig. 1). Full details of the synthetic and characterisation data for the pro-ligands, 2-(phenyl-2′-ol)-6-ketimine-pyridines (HL1) and 2-(phenyl-2′-ol)-6-(amino-prop-2-yl)pyridines (HL2), will be reported as will the corresponding data for their palladium(II) acetate (1 and 2) and chloride (3, 4) complexes.

Fig. 1 Monoanionic 2-(phenyl-2′-olate)-6-ketimine-pyridine (L1) and 2-(phenyl-2′-olate)-6-(amino-prop-2-yl)pyridine (L2) pincer ligands.

Results and discussion

(a) Preparation of pro-ligands HL1 and HL2

The 2-(phenyl-2′-ol)-6-imine-pyridines, 2-(C₆H₄-2′-OH),6-(CMe=NAr)C₅H₃N [Ar = 4-i-PrC₆H₄ (HL1_a), 2,6-i-Pr₂C₆H₃ (HL1_b)], have been prepared in modest to good yield via sequential Suzuki coupling and condensation reactions from 2-hydroxyphenylboronic acid and 2-bromo-6-acetyl pyridine (Scheme 1). As a slight modification to the reported synthesis of ketone precursor, 2-(C₆H₄-2′-OH),6-(CMe=O)C₅H₃N, it was found that the cross coupling proceeds more efficiently and over a shorter reaction time using a catalyst composed of Pd(OAc)₂ and PPh₃ in a reaction vessel open to the air.¹² Treatment of HL1_a and HL1_b with trimethylaluminium in toluene at elevated temperature followed by hydrolysis gave the 2-(phenyl-2′-ol)-6-(amino-prop-2-yl)pyridines, 2-(C₆H₄-2′-OH),6-(CMe₂NHAr)C₅H₃N [Ar = 4-i-PrC₆H₄ (HL2_a), 2,6-i-Pr₂C₆H₃ (HL2_b)], in good yield. The new compounds, HL1_a, HL2_a and HL2_b, have been characterised by a combination of ¹H, ¹³C{¹H} NMR, IR spectroscopy and ESI mass spectrometry (see Experimental).

Scheme 1 Reagents and conditions: (i) 2-Br-6-{MeC(O)}C₅H₃N, cat. Pd(OAc)₂/PPh₃, toluene, 90 °C, 12 h; (ii) ArNH₂, MeOH, cat. CH₃COOH, reflux; (iii) AlMe₃, toluene, 110 °C, 12 h; (iv) H₂O.

Compounds, HL1_a, HL2_a and HL2_b, all display protonated molecular ions peaks in their electrospray mass spectra and downfield shifted signals for the phenolic protons (range: δ 14.18–14.60) in their ¹H NMR spectra. For HL1_a, the imine methyl substituent is seen as a singlet at δ 2.32 in the ¹H NMR spectrum while the IR spectrum reveals a characteristic ν(C=N)_imine stretch at 1635 cm⁻¹. For amine-containing HL2_a and HL2_b, broad singlets are visible for the NH protons between δ 3.3–4.0 in their ¹H NMR spectra along with sharp singlets for the equivalent gem-dimethyl protons. Further confirmation of the composition of HL1_a and HL2_a was achieved using single crystal X-ray diffraction.

Perspective views of HL1_a and HL2_a are depicted in Fig. 2a and b; selected bond distances and angles for both structures are listed in Table 1. Each structure consists of a central pyridine ring that is substituted at its 2-position by a phenyl-2′-ol group but differs at the 6-position with a trans-configured N-arylimine unit for HL1_a [C(12)–N(2) 1.2692(19) Å] or a saturated CMe₂NH(4-i-PrC₆H₄) unit for HL2_a [C(11)–C(12)–N(2) 108.97(16)°]. In general, the pyridine nitrogen atoms adopt a cis conformation with respect to the neighbouring phenol oxygen as a result of a hydrogen-bonding interaction between the phenol hydrogen atom and the pyridine nitrogen [O(1)⋯N(1) 2.563 (HL1_a), 2.537 Å (HL2_a)], a conformation that has been observed in related structures.^12–14

Fig. 2 (a) Molecular structure of HL1_a, including a partial atom numbering scheme. All hydrogen atoms, apart from H1, have been omitted for clarity. (b) Molecular structure of HL2_a, including a partial atom numbering scheme. All hydrogen atoms, apart from H1 and H2, have been omitted for clarity.

Table 1 Selected bond distances (Å) and angles (°) for HL1_a and HL2_a

	HL1a	HL2b
Bond lengths
C(1)–O(1)	1.3455(19)	1.353(2)
C(12)–N(2)	1.2692(19)	1.460(2)
C(6)–C(7)	1.466(2)	1.480(2)
C(11)–C(12)	1.482(2)	1.530(3)
Bond angles
C(11)–C(12)–N(2)	115.71(15)	108.97(16)
C(12)–N(2)–C(14)	123.06(15)	125.80(16)

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(b) Palladium(II) complexes of L1 and L2

Interaction of HL1_a/b and HL2_a/b with palladium(II) acetate affords the O,N,N-pincer complexes, [{2-(C₆H₄-2′-O)-6-(CMe=NAr)C₅H₃N}Pd(OAc)] (Ar = 4-i-PrC₆H₄ (1a), 2,6-i-Pr₂C₆H₃ (1b)) and [{2-(C₆H₄-2′-O)-6-(CMe₂NHAr)C₅H₃N}Pd(OAc)] (Ar = 4-i-PrC₆H₄ (2a), 2,6-i-Pr₂C₆H₃ (2b)), in good yield (Scheme 2). Compounds 1 and 2 can be readily converted to their chloride analogues [{2-(C₆H₄-2′-O)-6-(CMe=NAr)C₅H₃N}PdCl] (Ar = 4-i-PrC₆H₄ (3a), 2,6-i-Pr₂C₆H₃ (3b)) and [{2-(C₆H₄-2′-O)-6-(CMe₂NHAr)C₅H₃N}PdCl] (Ar = 4-i-PrC₆H₄ (4a), 2,6-i-Pr₂C₆H₃ (4b)) by treating their chloroform or dichloromethane solutions with aqueous sodium chloride. Alternatively, 1a can be prepared more conveniently by the template reaction of 2-(C₆H₄-2′-OH),6-(CMe=O)C₅H₃N, Pd(OAc)₂ and 4-isopropylaniline in toluene. Complexes 1–4 are air stable and have been characterised using a combination of mass spectrometry (FAB, ESI and ToF), IR and NMR (¹H and ¹³C) spectroscopy and elemental analyses (see Experimental section). In addition, crystals of 1a, 1b, 2a, 2b and 3a have been the subject of single crystal X-ray diffraction studies.

Scheme 2 Reagents and conditions: (i) Pd(OAc)₂, toluene, 75–80 °C; (ii) NaCl(aq.), CHCl₃ or CH₂Cl₂, RT.

The molecular structures of imine-based 1a, 1b and 3a are closely related and will be discussed together; amine-containing 2a and 2b will be discussed later. Views of 1b and 3a are given in Fig. 3 and 4; selected bond distances and angles are collected for all three structures in Table 2. There are four independent molecules for 1a in the unit cell (molecules A–D) which differ most noticeably in the relative inclinations of the adjacent phenolate and pyridine rings (vide infra). The structures (1a, 1b and 3a) each consist of a single palladium(II) centre bound by a tridentate monoanionic 2-(phenyl-2′-olate)-6-ketimine-pyridine ligand along with a monodentate O-bound acetate (1) or chloride (3) to complete a distorted square planar geometry. Both 5- and 6-membered chelate rings are present within the complexes with the bite angle for the 6-membered ring being slightly more compatible with the geometrical requirements of the palladium(II) centre [O(1)–Pd(1)–N(2)_6-membered: 96.4(4)_av. (1a), 94.4(1) (1b), 93.8 (2)° (3a) vs. N(2)–Pd(1)–N(1)_5-membered 82.1(4)_av. (1a), 81.7(1) (1b), 81.7(2)° (3a)]. In all cases some twisting of the phenolate unit with respect to the pyridyl plane is apparent [tors. N(2)–C(13)–C(14)–C(15) 0.0(3)_A, 2.5(3)_B, 5.7(3)_C, 9.5(3)_D (1a), 14.1(3) (1b), 22.1(3)° (3a)]. In general, the Pd–N_imine bond distance is the longest of the three metal–ligand interactions involving the O,N,N-ligand followed by the Pd–N_pyridine distance and then by the Pd–O_phenolate distance which is best exemplified for complex 3a [Pd(1)–N(1)_imine 2.011(4) > Pd(1)–N(2)_pyridine 1.972(4) > Pd(1)–O(1)_phenolate 1.961(3) Å]. Replacing an O-bound acetate for a chloride has little effect on the trans Pd–N_pyridine distance [1.972(4) Å (3a) vs. 1.980(10)_av. (1a)]. The N-aryl group in 1b is inclined towards orthogonality with regard to the neighbouring C=N_imine vector [tors. C(7)–N(2)–C(1)–C(2) 86.1(3)°], while in the less sterically bulky 1a and 3a the aryl group is tilted [tors. C(7)–N(2)–C(1)–C(2) 66.4(4)_av (1a), 57.8(6) (3a)°]. There are no intermolecular contacts of note. The structural features resemble related aldimine-based palladium complexes [{2-(3-C₁₂H₈-2-O)-6-(CH=NAr)C₅H₃N}PdX] (X = OAc, Cl) reported elsewhere.^14,15

Fig. 3 Molecular structure of 1b including a partial atom numbering scheme. All hydrogen atoms have been omitted for clarity.

Fig. 4 Molecular structure of 3a including a partial atom numbering scheme. All hydrogen atoms have been omitted for clarity.

Table 2 Selected bond distances (Å) and angles (°) for 1a, 1b and 3a

	1a				1b	3a
	Molecule A	Molecule B	Molecule C	Molecule D
Bond lengths
Pd(1)–O(1)	1.947(7)	1.928(8)	1.951(8)	1.934(8)	1.953(3)	1.961(3)
Pd(1)–N(1)	1.972(9)	1.980(9)	1.978(10)	1.961(10)	2.006(3)	2.011(4)
Pd(1)–N(2)	1.961(9)	1.972(9)	1.980(9)	2.005(10)	1.969(3)	1.972(4)
Pd(1)–Cl(1)	—	—	—	—	—	2.3039(14)
Pd(1)–O(2)	2.038(8)	2.033(8)	2.016(8)	2.025(8)	2.036(3)	—
C(7)–N(1)	1.319(12)	1.295(13)	1.303(13)	1.302(14)	1.292(5)	1.301(6)
C(7)–C(8)	1.484(13)	1.496(14)	1.515(15)	1.514(15)	1.509(5)	1.497(7)
C(15)–O(1)	1.306(12)	1.310(13)	1.347(13)	1.321(14)	1.317(5)	1.317(6)
Bond angles
N(1)–Pd(1)–N(2)	82.9(4)	82.2(4)	81.8(4)	81.9(4)	81.68(13)	81.65(17)
N(1)–Pd(1)–O(1)	177.5(4)	177.5(4)	177.2(4)	178.2(4)	174.49(12)	174.56(16)
N(2)–Pd(1)–O(1)	95.2(4)	96.1(4)	96.2(4)	96.4(4)	94.35(12)	93.84(16)
N(2)–Pd(1)–Cl(1)	—	—	—	—	—	177.97(13)
N(2)–Pd(1)–O(2)	176.9(3)	175.0(4)	175.8(4)	176.1(4)	172.47(12)	—

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A view of amine-based 2a is given in Fig. 5; selected bond distances and angles are given for both 2a and 2b in Tables 3. The structures are similar to imine-containing 1a and 1b with a distorted square planar palladium(II) centre bound by a monoanionic O,N,N ligand and a monodentate O-bound acetate. In this case the more flexible 2-(phenyl-2′-olate)-6-(amino-prop-2-yl)pyridine acts as the O,N,N ligand again forming both 5-membered and 6-membered chelate rings. The presence of both a gem-dimethyl sp³-hybridised carbon (N(1)–C(7)–C(10) 108.9(8) (2a) and 109.7(2)° (2b)) and secondary amine nitrogen donor results in some puckering of the 5-membered chelate ring while the 6-membered chelate ring shows similar properties to those observed in 1a, 1b and 3a with some twisting of the phenolate unit with respect to the pyridyl plane evident [tors. N(2)–C(14)–C(15)–C(16) 18.3(3) (2a), 21.6° (2b)]. The Pd–O_phenolate and Pd–N_pyridine distances are comparable to those in 1a, 1b and 3a while the Pd–N_amine length is ca. 0.05 Å longer than the average Pd–N_imine distance in 1a, 1b and 3a consistent with the poorer donor characteristics of an amine. The pendant oxygen atom on the acetate ligand undergoes an intramolecular hydrogen bond interaction with the amine hydrogen atom [O(3)⋯N(1) 2.750 (2a), 2.895 (2b) Å]. It is worthy of note that the isopropyl group on C(2) in 2b occupies a position above the axial site of the N(1)–N(2)–O(1)–Pd(1) square plane (vide infra). There are no intermolecular contacts of note.

Fig. 5 Molecular structure of 2a including a partial atom numbering scheme. All hydrogen atoms, apart from H1, have been omitted for clarity.

Table 3 Selected bond distances (Å) and angles (°) for 2a and 2b

	2a	2b
Bond lengths
Pd(1)–O(1)	1.951(6)	1.9541(19)
Pd(1)–N(1)	2.061(6)	2.045(2)
Pd(1)–N(2)	1.983(7)	1.972(2)
Pd(1)–O(2)	2.008(6)	2.034(2)
C(7)–C(8)	1.519(11)	1.533(4)
C(7)–C(9)	1.557(11)	1.529(4)
C(7)–N(1)	1.467(10)	1.525(4)
Bond angles
N(1)–Pd(1)–N(2)	81.8(3)	84.46(9)
N(1)–Pd(1)–O(1)	176.1(3)	179.16(9)
N(2)–Pd(1)–O(1)	94.5(3)	94.99(9)
N(1)–Pd(1)–O(2)	96.2(3)	94.71(9)
N(2)–Pd(1)–O(2)	176.8(3)	174.90(8)
O(1)–Pd(1)–O(2)	87.6(2)	85.90(8)
N(1)–C(7)–C(10)	108.9(8)	109.7(2)

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Complexes 1–4, display either molecular ion peaks and/or fragmentation peaks corresponding to the loss of an acetate or a chloride in their mass spectra. For imine-based 1 and 3, the ν(C=N)_imine stretch shifts by ca. 35 cm⁻¹ to lower wavenumber when compared to those for the corresponding free HL1, supportive of imine coordination.¹⁶ In 1b and 3b two distinct doublets are seen for the isopropyl methyl groups in their ¹H NMR spectra consistent with restricted rotation about the N-aryl or Ar–i-Pr bonds in solution. In contrast, there are four distinct doublets in 2b and 4b implying all four methyl groups are now inequivalent in the amine-based pincer complexes. The N–H protons in 2a and 2b are downfield shifted (between δ 8.7–9.9) consistent with the NH⋯O_acetate hydrogen bonding as seen in the solid state, whilst in their chloride derivatives, 4a and 4b, the corresponding protons are found more upfield (between δ 6.1–6.7). The acetate methyl groups in 1 and 2 can be seen at δ ca. 1.6 in their ¹H NMR spectra with the MeC(O)O carbon atoms observable at δ ca. 178.8 in their ¹³C NMR spectra. In addition strong bands assignable to the symmetric and asymmetric ν(COO) vibrations in 1 and 2, are in agreement with those expected for monodentate acetate ligands.¹⁷

(c) Reactivity of 3 and 4 towards [p-tolyl₂I][O₃SCF₃]

All four palladium(II) chloride pincer complexes, 3a, 3b, 4a and 4b, were assessed on their ability to undergo oxidation with a hypervalent iodonium reagent and mediate the formation of a carbon–chlorine coupled product. Typically, 3 and 4 were treated with two equivalents of di-p-tolyliodonium triflate at 100 °C in a mixture of toluene–acetonitrile and their reaction mixtures monitored by gas chromatography using an internal standard to quantify the conversions (Scheme 3).

Scheme 3 Oxidation of 3 and 4 with di-p-tolyliodonium triflate to give 4-chlorotoluene and 4-iodotoluene.

The results of the screening are collected in Table 4. Several points emerge from inspection of the data. Firstly, all the palladium pincer complexes screened afford 4-chlorotoluene in varying amounts along with the expected 4-iodotoluene by-product. Secondly, two structure/reactivity relationships are apparent namely: (i) within each N,N,O family the least sterically bulky N-aryl group promotes the highest conversions to 4-chorotoluene, e.g., 4a (93%, entry 6) vs.4b (26%, entry 7) and 3a (80%, entry 4) vs.3b (17%, entry 5); (ii) amine-containing 4a and 4b yield higher conversions than their direct imine counterparts 3a and 3b, respectively. Thirdly, periodic monitoring of the conversion for 3a reveals a rapid initial reaction (33% in 1 h, entry 1) which reaches a plateau over time.

Table 4 Percentage conversion to 4-chorotoluene and 4-iodotoluene on reaction of 3 or 4 with [(p-tolyl)₂I][O₃SCF₃]^a

Entry	Pd(II) chloride pincer	Time/h	Conversion/% to 4-chlorotoluene^b	Conversion/% to 4-iodotoluene^b
a Conditions: 3 or 4 (0.05 mmol), [(p-tol)2I][OTf] (0.1 mmol), ([Pd]/[(p-tol)2IOTf] = 2), toluene/MeCN, 100 °C. b Determined using gas chromatography using naphthalene as an internal standard.
1	3a	1	33	27
2	3a	2.5	57	42
3	3a	6	74	67
4	3a	15	80	71
5	3b	15	17	7
6	4a	15	93	89
7	4b	15	26	10

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It is uncertain as to the origin of these ligand effects but it would seem likely that the sterically bulky 2,6-i-Pr₂Ph substitution pattern in 3b and 4b is inhibiting the oxidative transfer of the aryl group to the palladium centre. Indeed, work-up of the reaction between imine-containing 3b and di-p-tolyliodonium triflate at 100 °C over 15 hours (entry 5) gave unreacted starting materials as the major identifiable inorganic components. The increased flexibility of the ligand manifold in amine-containing 4 may, in part, contribute to the improved performance over the corresponding imine.

Unfortunately we were unable to prove or disprove the involvement of a transient Pd(IV) species (e.g., [(ONN)PdCl(p-tolyl)(NCMe)][O₃SCF₃]) by NMR spectroscopy due to the poor solubility of the reaction mixtures at lower temperatures. Nevertheless, we were able, in one case, to identify the palladium-containing decomposition product of the presumed reductive elimination event. Solid residues isolated from the reaction of 3a with di-p-tolyliodonium triflate (entry 4) could be extracted into acetonitrile and found to contain unreacted di-p-tolyliodonium triflate and the Pd(II) salt [{2-(C₆H₄-2′-O)-6-(CMe=N(4-i-PrC₆H₄)C₅H₃N}Pd(NCMe)][O₃SCF₃] (5a). Confirmation of the presence of 5a was obtained through spiking an ¹H NMR solution of the mixture with a genuine sample of 5a (prepared from the reaction of 3a with AgO₃SCF₃ in acetonitrile). Indeed 5a has been fully characterised by mass spectrometry, IR and NMR (¹H, ¹⁹F and ¹³C) spectroscopy and has been the subject of a single crystal X-ray diffraction study.

A view of 5a is given in Fig. 6a; selected bond distances and angles are collected in Table 5. There are two independent cations and associated anions in the unit cell with the main differences between the cations being the inclinations of N-aryl groups. The structure of 5a comprises a cationic palladium(II) unit charged balanced by a non-coordinating triflate anion. The cationic unit adopts a distorted square planar geometry [max. distortion: N(1)–Pd(1)–N(2) 82.0(3)_A, 82.5(3)_B°] with the 2-(phenyl-2′-olate)-6-ketimine-pyridine ligand occupying three coordination sites and the η¹-N acetonitrile molecule the fourth. The structural parameters displayed by the pincer ligand closely mirror the features observed in neutral precursor 3a with the Pd–N_imine distance again the longest [Pd(1)–N(1) 2.017(8) Å, 1.997(8) Å] of the three donor atoms. Interestingly, the independent cations assemble in such a way as to maintain the Pd(II) centres in close proximity (Pd(1)⋯Pd(1A) 3.313 Å) and only slightly further apart than the sum of the van der Waals radii (3.26 Å) (Fig. 6b). Further confirmation of the salt-like nature of 5a comes from the positive ESI mass spectrum (recorded in MeCN) which reveals peaks corresponding to the cationic unit while the negative spectrum the triflate anion. The ¹⁹F NMR spectrum (in CD₃CN) displays a single peak at δ −79.3 comparable with that observed in related triflate salts of Pd-acetonitrile species.¹⁸

Fig. 6 (a) Molecular structure of the cationic unit in 5a including a partial atom numbering scheme. All hydrogen atoms have been omitted for clarity. (b) Intermolecular packing of the two independent cationic units in 5a.

Table 5 Selected bond distances (Å) and angles (°) for 5a

	Molecule A	Molecule B
Bond lengths
Pd(1)–N(1)	2.017(8)	1.997(8)
Pd(1)–N(2)	1.953(8)	1.951(8)
Pd(1)–N(3)	2.007(9)	1.994(10)
Pd(1)–O(1)	1.959(7)	1.979(7)
C(7)–N(1)	1.277(13)	1.297(13)
C(9)–C(7)	1.515(14)	1.473(15)
C(23)–N(3)	1.138(13)	1.176(14)
Range S(1)–Otriflate	1.416(9)–1.434(11)
Bond angles
N(1)–Pd(1)–N(2)	82.0(3)	82.5(3)
N(1)–Pd(1)–O(1)	175.0(3)	176.1(3)
N(1)–Pd(1)–N(3)	95.9(3)	94.5(3)
N(2)–Pd(1)–O(1)	94.5(3)	95.0(3)
N(2)–Pd(1)–N(3)	177.3(3)	174.9(3)

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Conclusions

Two families of palladium(II) chloride O,N,N pincer complexes (3 and 4), differing in the type of exterior nitrogen donor and, within each family, the steric properties of the N-aryl ring, have been prepared via their respective acetate analogues (1 and 2) and fully characterised. Oxidation of 3 and 4 with di-p-tolyliodonium triflate leads in all cases to carbon–chloride coupling to give 4-chlorotoluene with the conversion highly dependent on the O,N,N pincer framework employed; the recovery of 5a with an intact pincer framework highlights the robustness of the ligand manifold to oxidation. Notably, the least sterically hindered member of each family (3a and 4a) leads to the highest conversion with amine-containing 4a the highest. These observations set the stage for an investigation of these and related pincer systems in various Pd(II)/(IV)-mediated C–X coupling reactions. These results will be reported in due course.

Experimental

General

All operations, unless otherwise stated, were carried out under an inert atmosphere of dry, oxygen-free nitrogen using standard Schlenk and cannular techniques or in a nitrogen purged glove box. Solvents were distilled under nitrogen from appropriate drying agents¹⁹ or were employed directly from a Solvent Purification System (Innovative Technology, Inc). The electrospray (ESI) mass spectra were recorded using a micromass Quattra LC mass spectrometer with acetonitrile or methanol as the matrix. FAB mass spectra (including high resolution) were recorded on a Kratos Concept spectrometer with NBA as matrix or on a Water Xevo QToF mass spectrometer equipped with an atmospheric solids analysis probe (ASAP). The infrared spectra were recorded in the solid state with Universal ATR sampling accessories on a Perkin Elmer Spectrum One FTIR instrument. NMR spectra were recorded on a Bruker DPX 300 spectrometer operating at 300.03 (¹H) and 75.4 MHz (¹³C) or a Bruker DRX400 spectrometer at 400.13 (¹H), 376.46 (¹⁹F) and 100.61 MHz (¹³C) or a Bruker Avance III 500 spectrometer at 125 MHz (¹³C), at ambient temperature unless otherwise stated; chemical shifts (ppm) are referred to the residual protic solvent peaks and coupling constants are expressed in hertz (Hz). Melting points (mp) were measured on a Gallenkamp melting point apparatus (model MFB-595) in open capillary tubes and were uncorrected. Elemental analyses were performed at the Science Technical Support Unit, London Metropolitan University. The reagents 2,6-diisopropylaniline, 4-isopropylaniline, silver triflate and trimethylaluminium (2 M solution in toluene) were purchased from Aldrich Chemical Co. and used without further purification. The compounds 2-hydroxyphenylboronic acid,¹² 2-bromo-6-acetyl pyridine²⁰ and di-p-tolyliodonium triflate²¹ and HL1_b¹² were prepared using literature procedures. All other chemicals were obtained commercially and used without further purification.

Synthesis of 2-(phenyl-2′-ol)-6-acetyl-pyridine

A round-bottomed flask equipped with stirrer bar and reflux condenser, open to the air, was loaded with 2-bromo-6-acetylpyridine (2.10 g, 10.00 mmol), Pd(OAc)₂ (0.047 g, 0.21 mmol), triphenylphosphine (0.110 mg, 0.42 mmol) and 2-hydroxyphenyl boronic acid (1.88 g, 13.7 mmol). Toluene (40 mL), ethanol (22 mL) and aqueous 2 M K₂CO₃ (13 mL, 26.00 mmol) were added and the mixture heated to 90 °C for 12 h. The resultant black reaction mixture was cooled to room temperature followed by the addition of 1 mL H₂O₂ (30% in water) and stirred for a further 30 min. The organic phase was separated and the aqueous phase washed with toluene (3 × 10 mL). The combined organic extracts were washed with water (3 × 30 mL) and brine (10 mL) and concentrated to afford a brown solid. This solid was slurried in methanol (10 mL) for 1 h and the resultant solid filtered and washed with methanol (3 mL) and dried under reduced pressure. 2-(Phenyl-2′-ol)-6-acetyl-pyridine was collected as a yellow solid (1.885 g, 84%). ¹H NMR (CDCl₃, 400 MHz): δ 2.71 (s, 3H, CH₃C=O), 6.90 (ddd, ³J_HH 8.4, ³J_HH 7.4, ⁴J_HH 1.4, 1H, Ar–H), 7.00 (dd, ³J_HH 8.3, ⁴J_HH 1.3, 1H, Ar–H), 7.30 (ddd, ³J_HH 8.5, ³J_HH 7.5, ⁴J_HH 1.7, 1H, Ar–H), 7.78 (dd, ³J_HH 8.1, ⁴J_HH 1.7, 1H, Ar–H), 7.94 (m, 2H, Py–H), 8.06 (dd, ³J_HH 7.1, ⁴J_HH 2.1, 1H, Py–H), 13.64 (s, 1H, O–H). ESIMS m/z: 214 [M + H]⁺. The data was consistent with that reported in ref. 13.

Synthesis of 2-(C₆H₄-2′-OH),6-{CMe=N(4-i-PrC₆H₄)}C₅H₃N (HL1_a)

2-(Phenyl-2′-ol)-6-acetyl-pyridine (0.405 g, 1.90 mmol), 4-isopropyl aniline (0.473 g, 3.50 mmol) and MgSO₄ (2.76 g, 23.0 mmol) were suspended in bench methanol (10 mL) and one drop of acetic acid added. The mixture was stirred and heated at reflux for 9 days whereupon a further drop of acetic acid was added and the mixture stirred at reflux for an additional 12 h. On cooling to room temperature the reaction mixture was filtered and the MgSO₄ washed with chloroform (30 mL) and the filtrate concentrated under reduced pressure. The resultant solid was heated in MeOH (10 mL), cooled to room temperature and the suspension collected by filtration and dried under reduced pressure affording HL1_a as yellow solid (0.381 g, 61%). Single crystals suitable for an X-ray determination were grown by slow cooling of a saturated solution of HL1_a in EtOH. Mp: 123–125 °C. ¹H NMR (CDCl₃, 300 MHz): δ 1.20 (d, ³J_HH 7.1, 6H, CHMe₂), 2.32 (s, 3H, CH₃C=N), 2.84 (sept, ³J_HH 7.1, 1H, CHMe₂), 6.69 (d, ³J_HH 8.4, 2H, Ar_mipp–H), 6.85 (app. td, ³J_HH 8.1, ⁴J_HH 1.2, 1H, Ar_phenol–H), 6.96 (dd, ³J_HH 8.2, ⁴J_HH 1.2, 1H, Ar_phenol–H), 7.16 (d, ³J_HH 8.3, 2H, Ar_mipp–H), 7.25 (app. td, ³J_HH 8.2, ⁴J_HH 1.5, 1H, Ar_phenol–H), 7.75 (dd, ³J_HH 8.1, ⁴J_HH 1.4, 1H, Ar_phenol–H), 7.79–7.91 (m, 2H, Py–H), 8.12 (dd, ³J_HH 7.6, ⁴J_HH 1.1, 1H, Py–H), 14.18 (s, 1H, O–H). ¹³C{¹H} NMR (CDCl₃, 75 MHz): δ 15.5 (CH₃C=N), 23.1 (CHMe₂), 32.6 (CHMe₂), 117.4 (CH), 117.7 (C), 118.0 (CH), 118.3 (CH), 118.8 (CH), 119.1 (CH), 125.4 (CH), 125.9 (CH), 130.6 (CH), 137.2 (CH), 143.6 (C), 147.2 (C), 152.7 (C), 155.6 (C), 158.6 (C), 163.6 (C=N_imine). IR (cm⁻¹): ν(C=N)_imine 1635, ν(C=N)_pyridine 1587. ESIMS m/z: 331 [M + H]⁺, 329 [M − H]. HRMS (ASAP): Calc. for C₂₂H₂₃N₂O [M + H]⁺ 331.1810, found 331.1803. Anal calc. for (C₂₂H₂₂N₂O) C 79.97, N 8.48, H 6.71. Found: C 79.97, N 8.41, H 6.64%.

Synthesis of 2-(C₆H₄-2′-OH),6-(CMe₂NHAr)C₅H₃N (HL2)

(a) Ar = 4-i-PrC₆H₄ (HL2_a): A Schlenk flask equipped with stir bar was evacuated and backfilled with nitrogen. The vessel was loaded with HL1_a (0.510 g, 1.50 mmol) and toluene (20 ml) and trimethylaluminium (2.0 ml, 4.00 mmol, 2 M solution in toluene) introduced dropwise. The solution was then stirred and heated to reflux for 12 h before being cooled to room temperature and concentrated under reduced pressure. Petroleum ether (20 ml, 40/60) was added and the solution cooled to 5 °C prior to the slow addition of water (20 ml). The mixture was then stirred for 1 h at room temperature before the organic phase was isolated. The aqueous phase was extracted with chloroform (4 × 50 ml) and the combined organic extracts washed with water (3 × 10 mL) and brine (1 × 10 mL) and then dried over MgSO₄. The solvent was removed under reduced pressure to provide HL2_a as an orange oil which solidified slowly over time (0.500 g, 96%). Single crystals suitable for an X-ray determination were grown by slow cooling of a saturated solution of HL2_a in ethanol. Mp: 109–112 °C. ¹H NMR (CDCl₃, 400 MHz): δ 1.07 (d, ³J_HH 7.0, 6H, CHMe₂), 1.66 (s, 6H, N–C(CH₃)₂), 2.66 (sept, ³J_HH 7.0, 1H, CHMe₂), 3.97 (br s, 1H, N–H), 6.19 (d ³J_HH 8.6, 2H, Ar_mipp–H), 6.81 (d, ³J_HH 8.6, 2H, Ar_mipp–H), 6.86 (app. td, ³J_HH 8.1, ⁴J_HH 1.2, 1H, Ar–H), 6.96 (dd, ³J_HH 8.3, ⁴J_HH 1.2, 1H, Ar–H), 7.25 (ddd, ³J_HH 8.5, ³J_HH 7.2, ⁴J_HH 1.6, 1H, Ar–H), 7.49–7.53 (1H, m, Ar–H), 7.69–7.73 (2H, m, Ar–H), 7.77 (dd, ³J_HH 8.0, ⁴J_HH 1.6, 1H, Py–H), 14.55 (s, 1H, O–H). ¹³C{¹H} NMR (CDCl₃, 100 MHz): δ 23.06 (CHMe₂), 28.2 (N–C(CH₃)₂), 32.0 (CHMe₂), 56.5 (C–N), 114.3 (CH), 115.8 (CH), 117.4 (CH), 117.7 (C), 177.8 (CH), 118.1 (CH), 125.2 (CH), 125.7 (CH), 130.4 (CH), 137.1 (C), 137.6 (CH), 142.2 (C), 155.9 (C), 158.9 (C), 162.8 (C). IR (cm⁻¹): 1592 (C=N)_pyridine. ESIMS m/z: 347 [M + H]⁺. HRMS (EI): Calc. for: C₂₃H₂₇N₂O [M + H]⁺ 347.2123, found: 347.2140.

(b) Ar = 2,6-i-Pr₂C₆H₃ (HL2_b): A similar procedure to that described for HL2_a was followed using HL1_b (0.601 g, 2.70 mmol), toluene (20 ml) and trimethylaluminium (3.40 ml, 6.70 mmol 2 M solution in toluene). On work-up, HL2_b was afforded as an orange oil which solidified slowly over time (0.549 g, 88%). Mp: 70–72 °C. ¹H NMR (CDCl₃, 400 MHz): δ 0.98 (d, ³J_HH 7.0, 12H, CHMe₂), 1.49 (s, 6H, N–C(CH₃)₂), 2.95 (sept, ³JHH 7.0, 2H, CHMe₂), 3.34 (br s, 1H, N–H), 6.85 (ddd, ³J_HH 8.2, ³J_HH 7.4, ⁴J_HH 1.3, 1H, Ar–H), 6.94 (dd, ³J_HH 8.2, ⁴J_HH 1.2, 1H, Ar–H), 6.98 (m (app. s), 3H, Ar–H), 7.23 (ddd, ³J_HH 8.4, ³J_HH 7.2, ⁴J_HH 1.6, 1H, Ar–H), 7.59 (dd, ³J_HH 7.4, ⁴J_HH 1.2, 1H, Py–H), 7.72–7.79 (3H, m, Ar–H), 14.60 (s, 1H, O–H). ¹³C{¹H} NMR (CDCl₃, 100 MHz): δ 22.8 (CHMe₂), 27.4 (CHMe₂), 28.2 (N–C(CH₃)₂), 58.1 (C–N), 115.7 (CH), 117.2 (CH), 117.4 (CH), 117.7 (CH), 118.1 (C), 122.1 (CH), 123.5 (CH), 125.3 (CH), 130.3 (CH), 137.0 (CH), 138.7 (C), 144.3 (C), 155.5 (C), 159.0 (C), 165.1 (C). IR (cm⁻¹): 1591 (C=N)_pyridine. ESIMS m/z: 389 [M + H]⁺. HRMS (EI): Calc. for C₂₆H₃₃N₂O [M + H]⁺ 389.2593, found 389.2606.

Synthesis of [{2-(C₆H₄-2′-O)-6-(CMe=NAr)C₅H₃N}Pd(OAc)] (1)

(a) Ar = 4-i-PrC₆H₄ (1a): A Schlenk flask equipped with stir bar was evacuated and backfilled with nitrogen. The vessel was loaded with HL1_a (0.100 g, 0.300 mmol), Pd(OAc)₂ (0.068 g, 0.300 mmol) and toluene (10 ml) and then stirred and heated at 80 °C for 12 h. On cooling to room temperature the volatiles were removed under reduced pressure. The resultant solid was dissolved in dichloromethane (5 mL) and hexane (100 mL) introduced affording 1a as a red solid (0.136 g, 90%). Single crystals suitable for an X-ray determination were grown by slow diffusion of hexane into a solution of 1a in chloroform. Mp: >240 °C (decomp.). ¹H NMR (CDCl₃, 400 MHz): δ 1.22 (d, ³J_HH 6.9, 6H, CHMe₂), 1.48 (s, 3H, CH₃C(O)O–), 2.07 (s, 3H, CH₃C=N), 2.90 (sept, ³J_HH 6.9, 1H, CHMe₂), 6.61 (ddd, ³J_HH 8.2, ³J_HH 6.6, ⁴J_HH 1.5, 1H, Ar_phenolate–H), 7.04 (dd, ³J_HH 8.5, ⁴J_HH 1.3, 1H, Ar–H), 7.09–7.14 (m, 4H, Ar–H), 7.23 (d, ³J_HH 8.2, 2H, Ar_mipp–H), 8.02 (d, ³J_HH 8.5, 1H, Py–H), 8.06 (dd, ³J_HH 8.5, ³J_HH 8.5, 1H, Py–H), 8.97 (d, ³J_HH 8.7, 1H, PyH). ¹³C{¹H} NMR (CDCl₃, 100 MHz): δ 16.5 (CH₃C=N), 21.7 (CH₃C(O)O–), 22.9 (CHMe₂), 32.9 (CHMe₂), 114.5 (CH), 118.2 (C), 122.3 (CH), 122.3 (CH), 122.4 (CH), 125.6 (CH), 126.2 (CH), 128.5 (CH), 130.6 (CH), 137.5 (CH), 141.2 (C), 147.5 (C), 150.0 (C), 162.0 (C), 172.4 (C=N_imine), 177.0 (C=O). IR (cm⁻¹): 1613 (C=N)_imine, 1590 (COO_asymm/C=N_pyridine), 1456 (COO_symm). FABMS m/z: 435 [M − OAc]⁺. Anal calc. for (C₂₄H₂₄N₂O₃Pd): C 58.25; H 4.89; N 5.66 Found: C 58.12; H 4.83; N 5.67%.

(b) Ar = 2,6-i-Pr₂C₆H₃ (1b): A similar procedure to that described for 1a was followed using HL1_b (0.100 g, 0.27 mmol), Pd(OAc)₂ (0.061 g, 0.27 mmol) afforded 1b as a red solid (0.135 g, 93%). Crystals suitable for an X-ray determination were grown by slow diffusion of hexane into a solution of 1b in chloroform. Mp: >240 °C (decomp.). ¹H NMR (CDCl₃, 400 MHz): δ 1.04 (d, ³J_HH 6.9, 6H, CHMe₂), 1.41 (d, ³J_HH 6.7, 6H, CHMe₂), 1.43 (s, 3H, CH₃C(O)C–), 2.24 (s, 3H, CH₃C=N), 3.22 (sept, ³J_HH 6.8, 2H, CHMe₂), 6.64 (ddd, ³J_HH 8.3, ³J_HH 6.3, ⁴J_HH 1.9, 1H, Ar_phenolate–H), 7.14–7.22 (4H, m, under CHCl₃), 7.28 (dd, ³J_HH 8.2, ³J_HH 7.3, 1H, Ar–H), 7.60 (dd, ³J_HH 7.5, ⁴J_HH 1.0, 1H, Py–H), 7.79 (d, ³J_HH 8.6, 1H, Ar_phenolate–H), 8.08 (dd, ³J_HH 8.8, ³J_HH 7.5, 1H, Py–H), 8.43 (d, ³J_HH 8.7, 1H, Py–H). ¹³C{¹H} NMR (CDCl₃, 125 MHz): δ 18.5 (CHMe₂), 22.5 (CHMe₂), 23.7 (CH₃C(O)O–), 24.4 (CH₃C=N), 28.8 (CHMe₂), 115.8 (CH), 119.2 (C), 122.6 (CH), 123.8 (CH), 123.9 (CH), 126.6 (CH), 128.4 (CH), 128.5 (CH), 132.3 (CH), 137.1 (CH), 139.5 (C), 140.8 (C), 152.7 (C), 154.2 (C), 164.1 (C), 174.2 (C=N_imine), 177.3 (C=O). IR (cm⁻¹): 1600 (C=N_imine/COO_asymm/C=N_pyridine), 1456 (COO_symm). ESIMS m/z: 477 [M − OAc]⁺, 518 [(M − OAc + MeCN]⁺. HRMS (ASAP): Calc. for: C₂₇H₃₀N₂O₃Pd [M]⁺ 536.1291 Found 536.1333.

Synthesis of [{2-(C₆H₄-2′-O)-6-(CMe₂NHAr)C₅H₃N}Pd(OAc)] (2)

(a) Ar = 4-i-PrC₆H₄ (2a): A Schlenk flask equipped with a stir bar was evacuated, back-filled with nitrogen and then loaded with HL2_a (0.040 g, 0.12 mmol), Pd(OAc)₂ (0.026 g, 0.12 mmol) and toluene (4 mL). After stirring and heating at 75 °C for 12 h, the reaction mixture was allowed to cool to room temperature and the volatiles removed under reduced pressure. The residue was dissolved in dichloromethane (1 mL) before hexane (20 mL) was added to precipitate the product. The product was collected on a Celite plug, washed with hexane (10 mL) before being dissolved in dichloromethane (10 mL) and the solution collected. On evaporation of the volatile components, 2a was obtained as a red powder (0.057 g, 93%). Single crystals suitable for an X-ray determination were grown by slow diffusion of hexane into a solution of 2a in chloroform. Mp: >240 °C (decomp.). ¹H NMR (CDCl₃, 400 MHz): δ 1.09 (d, ³J_HH 6.9, 6H, CHMe₂), 1.36 (s, 3H, N–C(CH₃)₂), 2.00 (s, 3H, CH₃C(O)C–), 2.40 (s, 3H, N–C(CH₃)₂), 2.72 (sept, ³J_HH 6.9, 1H, CHMe₂), 6.60 (ddd, ³J_HH 8.1, ³J_HH 6.5, ⁴J_HH 1.8, 1H, Ar_phenolate–H), 6.67 (d, ³J_HH 8.4, 2H, Ar_mipp–H), 6.87 (dd, ³J_HH 6.1, ⁴J_HH 2.3, 1H, Py–H), 6.94 (d, ³J_HH 8.3, 2H, Ar_mipp–H), 7.06–7.14 (m, 2H, Ar_phenolate–H), 7.57 (d, ³J_HH 8.5, 1H, Ar_phenolate–H), 7.80 (d, ³J_HH 8.5, 1H, Py–H), 7.82 (dd, ³J_HH 8.5, ³J_HH 6.2, 1H, Py–H), 9.92 (br s, 1H, NH). ¹³C{¹H} NMR (CDCl₃, 100 MHz): δ 23.8 (CHMe₂), 24.1 (CH₃C(O)O–), 24.4 (N–C(CH₃)₂), 33.6 (CHMe₂), 33.6 (N–C(CH₃)₂), 70.2 (C–N), 116.0 (CH), 116.3 (CH), 121.4 (CH), 121.8 (C), 122.9 (CH), 123.0 (CH), 127.5 (CH), 128.9 (CH), 132.3 (CH), 138.9 (C), 139.7 (C), 147.2 (C), 153.5 (C), 164.4 (C), 168.0 (C), 181.6 (C=O). IR (cm⁻¹): 3400 (NH), 1574 (COO_asymm/C=N_pyridine), 1448 (COO_symm). ESIMS: m/z 510 [M]⁺, 592 [M − OAc + MeCN]⁺. HRMS (FAB): m/z Calc. for C₂₅H₂₈N₂O₃Pd [M]⁺ 510.6296. Found 510.1125.

(b) Ar = 2,6-i-Pr₂C₆H₃ (2b): A similar procedure to that outlined for 2a was employed using HL2_b (0.024 g, 0.61 mmol) and Pd(OAc)₂ (0.014 g, 0.061 mmol) gave 2b as a yellow solid (0.033 g, 98%). Single crystals suitable for an X-ray determination were grown by slow diffusion of hexane into a solution of 2b in dichloromethane. Mp: >240 °C (decomp.). ¹H NMR (CDCl₃, 400 MHz): δ 0.72 (d, ³J_HH 6.9, 3H, CHMe₂), 1.18 (s, 3H, NC(CH₃)₂), 1.19 (d, ³J_HH 6.9, 3H, CHMe₂), 1.22 (d, ³J_HH 6.9, 3H, CHMe₂), 1.54 (d, ³J_HH 6.7, 3H, CHMe₂), 1.91 (s, 3H, CH₃C(O)O–), 2.31 (s, 3H, N–C(CH₃)₂), 3.16 (sept, ³J_HH 6.7, 1H, CH(Me)₂), 3.72 (sept, ³J_HH 6.8, 1H, CH(Me)₂), 6.61 (ddd, ³J_HH 8.5, ³J_HH 6.4, ⁴J_HH 2.0, 1H, Ar_phenolate–H), 6.84 (dd, ³J_HH 6.9, ⁴J_HH 2.1, 1H, Py H), 7.02–7.18 (m, 5H, Ar–H), 7.54 (d, ³J_HH 8.3, 1H, Ar_phenolate–H), 7.74–7.80 (m, 2H, Py–H), 8.66 (br s, 1H, NH). ¹³C{¹H} NMR (CDCl₃ 100 MHz): δ 21.9 (CHMe₂), 22.3 (CH₃C(O)O–), 23.7 (CH₃), 24.1 (CH₃), 24.5 (CHMe₂), 24.6 (CH₃), 27.4 (CHMe₂), 27.7 (CHMe₂), 32.3 (N–C(CH₃)₂), 70.8 (C–N), 115.0 (CH), 115.6 (CH), 110.0 (CH), 120.8 (C), 121.5 (CH), 124.2 (CH), 124.7 (CH), 126.9 (CH), 127.9 (CH), 131.4 (CH), 134.5 (C), 137.8 (CH), 143.0 (C), 143.5 (C), 152.3 (C), 163.1 (C), 169.3 (C), 179.2 (C=O). IR (cm⁻¹): 3064 (NH), 1590 (COO_asymm/C=N_pyridine), 1450 (COO_symm). TOFMS (ASAP): m/z 553 [M + H]⁺, 493 [M − OAc]⁺. Anal. calc. for (C₂₈H₃₄N₂O₃Pd·3CH₂Cl₂): C 46.09, H 4.99 N 3.47% Found: C 46.00, H 4.64, N 3.61%.

Synthesis of [{2-(C₆H₄-2′-O)-6-(CMe=NAr)C₅H₃N}PdCl] (3)

(a) Ar = 4-i-PrC₆H₄ (3a): A round bottomed flask equipped with stirrer bar and open to the air was loaded with 1a (0.568 g, 1.15 mmol), chloroform (30 mL) and brine (30 mL). After stirring vigorously at room temperature for 1 h the organic phase was separated, washed with water (3 × 30 ml) and filtered through a Celite plug. The plug was washed with chloroform (10 mL) and the solution concentrated to a smaller volume (ca. 5 mL) before hexane (100 mL) was added to precipitate the title compound as dark red solid (0.537 g, 99%). Single crystals suitable for an X-ray determination were grown by slow diffusion of hexane into a solution of 3a in chloroform. Mp: >240 °C (decomp). ¹H NMR (CDCl₃, 400 MHz): δ 1.22 (d, ³J_HH 6.9, 6H, CHMe₂), 2.28 (s, 3H, CH₃C=N), 2.88 (sept, ³J_HH 6.9, 1H, CHMe₂), 6.67 (ddd, ³J_HH 8.3, ³J_HH 6.1, ⁴J_HH 2.0, 1H, Ar_phenolate–H), 7.04 (d, ³J_HH 8.4, 2H, Ar_mipp–H), 7.16–7.24 (m, 4H, Ar–H), 7.61 (dd, ³J_HH 7.6, ⁴J_HH 1.0, 1H, Py–H), 7.68 (d, ³J_HH 8.4, 1H, Ar_phenolate–H), 7.89 (dd, ³J_HH 8.6, ³J_HH 7.5, 1H, Py–H), 8.18 (d, ³J_HH 8.7, 1H, Py–H). ¹³C{¹H} NMR (CDCl₃, 125 MHz): δ 18.5 (CH₃C=N), 23.9 (CHMe₂), 33.7 (CHMe₂), 116.0 (CH), 119.1 (C), 123.2 (CH), 123.6 (CH), 124.1 (CH), 125.7 (CH), 126.5 (CH), 128.9 (CH), 132.1 (CH), 138.0 (CH), 143.7 (C), 148.2 (C), 150.5 (C), 154.7 (C), 162.4 (C), 175.7 (C=N_imine). IR (cm⁻¹): ν(C=N)_imine 1598. FABMS m/z: 470 [M]⁺, 435 [M − Cl]⁺. Anal calc. for (C₂₂H₂₁N₂OPdCl): C 56.07; H 4.49; N 5.94. Found: C 55.99; H 4.38; N 6.01%.

(b) Ar = 2,6-i-Pr₂C₆H₃ (3b): A similar procedure to that described for 3a was employed using 1b (0.289 g, 0.54 mmol) affording 3b as a red solid (0.221 g, 80%). Mp: >240 °C (decomp.). ¹H NMR (CDCl₃, 400 MHz): δ 1.06 (d, ³J_HH 6.9, 6H, CHMe₂), 1.39 (d, ³J_HH 6.8, 6H, CHMe₂), 2.22 (s, 3H, CH₃C=N), 3.06 (sept, ³J_HH 6.8, 2H, CHMe₂), 6.69 (ddd, ³J_HH 8.4, ³J_HH 6.8, ⁴J_HH 1.5, 1H, Ar_phenolate–H), 7.16 (d, ³J_HH 7.9, 2H, Ar_dipp–H), 7.20–7.32 (m, 3H, Ar–H), 7.71 (dd, ³J_HH 7.5, ⁴J_HH 1.0, 1H, Py–H), 7.82 (dd, ³J_HH 8.6, ⁴J_HH 1.4, 1H, Ar_phenolate–H), 8.13 (dd, ³J_HH 8.8, ³J_HH 7.6, 1H, Py–H), 8.47 (d, ³J_HH 8.8, 1H, Py–H). ¹³C{¹H} NMR (CDCl₃, 125 MHz): δ 18.2 (CHMe₂), 23.7 (CHMe₂), 23.9 (CH₃C=N), 28.9 (CHMe₂), 116.2 (CH), 118.7 (C), 122.8 (CH), 123.8 (CH), 124.2 (CH), 127.0 (CH), 128.4 (CH), 128.6 (CH), 132.6 (CH), 137.2 (CH), 139.8 (C), 141.3 (C), 152.3 (C), 154.1 (C), 163.5 (C), 175.2 (C=N_imine). IR (cm⁻¹): ν(C=N)_imine 1607. FABMS: m/z 512 [M]⁺, 477 [M − Cl]⁺. TOFMS (ASAP): m/z 513 [M + H]⁺, 477 [M − Cl]⁺. Anal. calc. for (C₂₅H₂₇N₂OPdCl): C 58.49, H 5.30, N 5.46 Found: C 58.38, H 5.27, N 5.52%.

Synthesis of [{2-(C₆H₄-2′-O)-6-(CMe₂NHAr)C₅H₃N}PdCl] (4)

(a) Ar = 4-i-PrC₆H₄ (4a): A round bottomed flask equipped with stirrer bar and open to the air was loaded with 2a (0.281 g, 0.55 mmol), dichloromethane (20 mL) and brine (20 mL). After stirring vigorously at room temperature for 12 h the organic phase was separated, washed with water (3 × 30 ml) and filtered through a Celite plug. Hexane (100 mL) was added to precipitate the product which was trapped on a Celite plug and washed with hexane (20 mL). The product was dissolved in dichloromethane and the solution collected. All volatiles were removed under reduced pressure affording 4a as a yellow solid (0.219 g, 82%). Mp: >240 °C (decomp.). ¹H NMR (CDCl₃, 400 MHz): δ 1.19 (d, ³J_HH 7.0, 6H, CHMe₂), 1.53 (s, 3H, N–C(CH₃)₂), 2.51 (s, 3H, N–C(CH₃)₂), 2.83 (sept, ³J_HH 7.0, 1H, CHMe₂), 6.67 (br, s, 1H, NH), 6.69–6.73 (m, 1H, Ar–H), 6.94–6.99 (m, 3H, Ar–H), 7.07 (d, ³J_HH 8.7, 2H, Ar–H), 7.21 (d, ³J_HH 4.3, 2H, Ar–H), 7.69 (d, ³J_HH 8.4, 1H, Ar–H), 7.92–8.00 (m, 2H, Ar–H). ¹³C{¹H} NMR (CDCl₃, 125 MHz): δ 23.8 (CHMe₂), 24.2 (N–C(CH₃)₂), 33.6 (CHMe₂), 33.8 (N–C(CH₃)₂), 72.0 (C–N), 116.1 (CH), 116.3 (CH), 121.6 (CH), 121.9 (C), 123.0 (CH), 123.3 (CH), 127.5 (CH), 129.0 (CH), 132.4 (CH), 139.1 (CH), 139.4 (C), 147.5 (C), 152.6 (C), 164.2 (C), 167.0 (C). IR (cm⁻¹): ν(C=N_pyridine) 1573, ν(NH) 3171. FABMS: m/z 486 [M]⁺, 451 [M − Cl]⁺. HRMS (ASAP): m/z Calc. for C₂₃H₂₆N₂OPdCl [M + H]⁺ 487.0768. Found 487.0792. Calc. for C₂₃H₂₅N₂OPd [M − Cl]⁺ 451.002. Found 451.1026. Calc. for (C₂₃H₂₅N₂OPdCl·CHCl₃): C 47.51; H 4.32; N 4.62 Found: C 47.54; H 4.19; N 4.71%.

(b) Ar = 2,6-i-Pr₂C₆H₃ (4b): A similar procedure to that described for 4a was employed using 2b (0.221 g, 0.40 mmol) affording 4b as a yellow solid (0.154 g, 73%). Mp: >240 °C (decomp). ¹H NMR (CDCl₃, 400 MHz): δ 0.83 (d, ³J_HH 6.9, 3H, CHMe₂), 1.23 (s, 3H, N–C(CH₃)₂), 1.27 (d, ³J_HH 6.8, 3H, CHMe₂), 1.41 (d, ³J_HH 6.6, 3H, CHMe₂), 1.51 (d, ³J_HH 6.7, 3H, CHMe₂), 2.16 (s, 3H, N–C(CH₃)₂), 3.02 (sept, ³J_HH 6.7, 1H, CHMe₂), 3.35 (sept, ³J_HH 6.8, 1H, CHMe₂), 6.10 (br, s, 1H, NH), 6.61 (ddd, ³J_HH 8.2, ³J_HH 6.3, ⁴J_HH 2.1, 1H, Ar_phenolate–H), 6.86 (dd, ³J_HH 7.5, ⁴J_HH 1.2, 1H, Py–H), 7.05–7.08 (m, 2H, Ar–H), 7.03–7.17 (m, 3H, Ar–H), 7.55 (d, ³J_HH 8.3, 1H, Ar_phenolate–H), 7.81 (dd, ³J_HH 8.3, ³J_HH 7.4, 1H, Py–H), 7.88 (d, ³J_HH 8.4, 1H, Py–H). ¹³C{¹H} NMR (CDCl₃, 100 MHz): δ 22.6 (CHMe₂), 24.3 (N–C(CH₃)₂), 24.4 (CHMe₂), 24.9 (CHMe₂), 25.5 (CHMe₂), 28.9 (CHMe₂), 29.3 (CHMe₂), 34.3 (N–C(CH₃)₂), 72.0 (C–N), 116.1 (CH), 116.1 (CH), 121.4 (C), 123.1 (CH), 124.5 (CH), 125.7 (CH), 128.0 (CH), 129.0 (CH), 132.3 (CH), 135.7 (C), 138.9 (CH), 142.2 (C), 143.0 (C), 153.4 (C), 164.0 (C), 169.0 (C). IR (cm⁻¹): ν(C=N_pyridine): 1573. FABMS: m/z 528 [M]⁺, 493 [M − Cl]⁺. HRMS (ASAP) m/z: Calc. for C₂₆H₃₂N₂OPdCl [M + H]⁺ 529.1238. Found 529.1235. Calc. for C₂₆H₃₁N₂OPd [M − Cl]⁺ 493.1471 Found 493.1413. Anal calc. for (C₂₆H₃₁N₂OPdCl·0.5CHCl₃): C 54.03; H 5.39; N 4.76 Found: C 54.44, H 5.75, N 4.78%.

Synthesis of [{2-(C₆H₄-2′-O)-6-(CMe=N{(4-i-PrC₆H₄)}C₅H₃N}Pd(NCMe)][O₃SCF₃] (5a)

A Schlenk flask was loaded in the glovebox and 3a (0.124 g, 0.264 mmol) along with AgOSO₂CF₃ (68 mg, 0.264 mmol) introduced. On removal from the glovebox, MeCN (10 mL) was added and the reaction mixture stirred at room temperature for 12 h in the absence of light. The resultant slurry was allowed to settle before the insoluble components were removed by cannular filtration and the filtrate collected in a second dry Schlenk flask. The solvent was removed under reduced pressure affording 5a as a hygroscopic orange solid (0.160 g, 97%). Single crystals suitable for an X-ray determination were obtained by layering of a solution of 5a in MeCN–toluene (5 : 95 v/v) with hexane. ¹H NMR (CD₃CN, 400 MHz): δ 1.31 (d, ³J_HH 7.0, 6H, CHMe₂), 2.45 (s, 3H, CH₃C=N), 3.05 (sept, ³J_HH 7.0, 1H, CHMe₂), 6.93 (ddd, ³J_HH 8.4, ³J_HH 7.0, ⁴J_HH 1.3, 1H, Ar_phenolate–H), 7.13 (dd, ³J_HH 8.6, ⁴J_HH 1.3, 1H, Ar_phenolate–H), 7.26 (d, ³J_HH 8.5, 2H, Ar_mipp–H), 7.40 (ddd, ³J_HH 8.5, ³J_HH 6.8, ⁴J_HH 1.5, 1H, Ar_phenolate–H), 7.49 (d, ³J_HH 8.5, 2H, Ar_mipp–H), 8.08–8.13 (2H, m, Ar–H), 8.39 (dd, ³J_HH 8.7, ³J_HH 7.5, 1H, Py–H), 8.67 (d, ³J_HH 8.8, 1H, Py–H), the coordinated CH₃CN ligand was not observed due to rapid exchange with bulk CD₃CN. ¹³C{¹H} NMR (CD₃CN, 100 MHz): δ 17.4 (CH₃C=N), 22.8 (CHMe₂), 33.3 (CHMe₂), 116.9 (CH), 118.5 (C), 120.7 (CH), 122.4 (CH), 125.6 (CH), 126.8 (CH), 127.2 (CH), 129.5 (CH), 132.9 (CH), 139.4 (CH), 142.9 (C), 149.5 (C), 150.3 (C), 155.2 (C), 160.0 (C), 177.8 (C=N_imine), CF₃SO₃⁻ not observed. ¹⁹F NMR (CD₃CN, 376 MHz): δ −79.3 (s, 3F, CF₃SO₃). IR (cm⁻¹): ν(C=N)_imine 1597. ESIMS (+ve): m/z 476 [M − CF₃SO₃]⁺. ESIMS (−ve): m/z 149 [CF₃SO₃]⁻. HRMS (ASAP): m/z Calc. for C₂₃H₂₁N₂O₄SF₃Pd [M − MeCN]⁺ 584.0218 Found 584.0482.

General procedure for reactions of Pd–Cl complexes with the iodonium salt

A microwave vessel equipped with stirrer bar and open to the air was loaded with 3 or 4 (0.05 mmol) and di-p-tolyliodonium triflate (0.10 mmol, 2 eq.) and the contents suspended in toluene (4.5 mL) and MeCN (0.5 mL) before the system was sealed. The mixture was then stirred and heated to 100 °C for the specified time period. On cooling to room temperature the internal standard naphthalene (1 eq.) was added in hexane (2 mL). 1 mL of this reaction mixture was removed, diluted with a further 2 mL of hexane and the solids removed by filtration through a silica plug. The plug was washed with hexane (1 mL) and the filtrate was subject to analysis by GC. GC conditions: Hold oven temperature at 40 °C for 2 min; ramp 10 °C min⁻¹ for 10 min; hold oven temperature at 180 °C for 12 min; injection temperature 250 °C; injection volume 1 μL; split ratio: 50 : 1. All reactions were repeated in triplicate.

Crystallographic studies

Data for HL1_a, HL2_a, 1a, 1b, 2a, 2b, 3a and 5a were collected on a Bruker APEX 2000 CCD diffractometer. Details of data collection, refinement and crystal data are listed in Table 6. The data were corrected for Lorentz and polarisation effects and empirical absorption corrections applied. Structure solution by direct methods and structure refinement based on full-matrix least-squares on F² employed SHELXTL version 6.10.²² Hydrogen atoms were included in calculated positions (C–H = 0.93–1.00 Å) riding on the bonded atom with isotropic displacement parameters set to 1.5U_eq(C) for methyl H atoms and 1.2U_eq(C) for all other H atoms. All non-H atoms were refined with anisotropic displacement parameters. Disordered solvent was omitted using the SQUEEZE option in PLATON for 1b and 2a.²³

Table 6 Crystallographic and data processing parameters for HL1_a, HL2_a, 1a, 1b, 2a, 2b, 3a and 5a^a

Complex	HL1a	HL2a	1a	1b
Formula	C22H22N2O	C23H26N2O	C96H96N8O12Pd4·7CHCl3·H2O	C27H30N2O3Pd·0.75C6H14
M	330.42	346.46	2833.00	623.10
Crystal size (mm^3)	0.41 × 0.35 × 0.20	0.35 × 0.30 × 0.26	0.43 × 0.24 × 0.15	0.31 × 0.24 × 0.13
Temperature (K)	150(2)	150(2)	150(2)	150(2)
Crystal system	Monoclinic	Monoclinic	Monoclinic	Monoclinic
Space group	P2(1)/c	P2(1)/c	P2(1)/c	C2/c
a (Å)	7.6425(19)	9.166(6)	27.533(6)	26.910(8)
b (Å)	11.027(3)	16.955(11)	19.525(4)	14.159(4)
c (Å)	20.590(5)	13.033(9)	23.435(5)	15.463(5)
α (°)	90	90	90	90
β (°)	93.528(5)	102.965(11)	111.63(3)	110.788(6)
γ (°)	90	90	90	90
U (Å^3)	1731.8(7)	1974(2)	11 711(4)	5508(3)
Z	4	4	4	8
D c (Mg m^−3)	1.267	1.166	1.607	1.503
F(000)	704	744	5696	2608
μ(Mo-Kα)(mm^−1)	0.078	0.071	1.144	0.712
Reflections collected	13 311	13 931	22 937	21 197
Independent reflections	3410	3471	22 937	5402
R int	0.0574	0.0595	0.000	0.0834
Restraints/parameters	0/229	0/239	1134/1358	0/304
Final R indices (I > 2σ(I))	R 1 = 0.0489	R 1 = 0.0528	R 1 = 0.0958	R 1 = 0.0490
	wR2 = 0.1054	wR2 = 0.1289	wR2 = 0.1419	wR2 = 0.1019
All data	R 1 = 0.0705	R 1 = 0.0708	R 1 = 0.2822	R 1 = 0.0713
	wR2 = 0.1147	wR2 = 0.1385	wR2 = 0.1947	wR2 = 0.1084
Goodness of fit on F^2 (all data)	0.981	1.030	0.822	0.959

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Complex	2a	2b	3a	5a
a Data in common: graphite-monochromated Mo-Kα radiation, λ = 0.71073 Å; R1 = ∑||Fo| − |Fc||/∑|Fo|, wR2 = [∑w(Fo^2 − Fc^2)^2/∑w(Fo^2)^2]^1/2, w^−1 = [σ^2(Fo)^2 + (aP)^2], P = [max(Fo^2, 0) + 2(Fc^2)]/3, where a is a constant adjusted by the program; goodness of fit = [∑(Fo^2 − Fc^2)2/(n − p)]^1/2 where n is the number of reflections and p the number of parameters.
Formula	C25H28N2O3Pd·1.5CHCl3	C28H34N2O3Pd·CH2Cl2	C22H21ClN2OPd·CHCl3	C25H24F3N3O4PdS·MeCN
M	6889.95	637.90	590.63	666.99
Crystal size (mm^3)	0.23 × 0.15 × 0.04	0.37 × 0.24 × 0.20	0.35 × 0.29 × 0.07	0.45 × 0.43 × 0.04
Temperature (K)	150(2)	150(2)	150(2)	150(2)
Crystal system	Monoclinic	Monoclinic	Monoclinic	Triclinic
Space group	P2(1)/c	P2(1)/c	P2(1)/c	P1̄
a (Å)	16.155(4)	16.640(6)	17.785(4)	13.264(11)
b (Å)	13.910(3)	10.960(4)	8.6156(19)	13.822(11)
c (Å)	13.360(3)	17.137(6)	16.469(4)	17.160(14)
α (°)	90	90	90	80.989(15)
β (°)	109.643(5)	116.252(5)	110.168(4)	78.907(15)
γ (°)	90	90	90	64.369(13)
U (Å^3)	2827.5(11)	2803.1(16)	2368.8(9)	2774(4)
Z	4	4	4	4
D c (Mg m^−3)	1.621	1.512	1.656	1.597
F(000)	1396	1312	1184	1352
μ(Mo-Kα)(mm^−1)	1.113	0.866	1.253	0.805
Reflections collected	22 017	21 332	18 002	21 653
Independent reflections	5551	5505	4659	10 758
R int	0.1884	0.0497	0.1264	0.1073
Restraints/parameters	277/285	0/341	0/283	36/740
Final R indices (I > 2σ(I))	R 1 = 0.0695	R 1 = 0.0373	R 1 = 0.0554	R 1 = 0.0999
	wR2 = 0.1575	wR2 = 0.0935	wR2 = 0.0830	wR2 = 0.2351
All data	R 1 = 0.1940	R 1 = 0.0439	R 1 = 0.1026	R 1 = 0.1570
	wR2 = 0.1815	wR2 = 0.0966	wR2 = 0.0938	wR2 = 0.2619
Goodness of fit on F^2 (all data)	0.725	1.059	0.897	1.033

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CCDC reference numbers 1040521–1040528.

Acknowledgements

We thank the University of Leicester for financial assistance. Johnson Matthey PLC are thanked for their generous loan of palladium salts.

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Footnote

† CCDC 1040521–1040528. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c5dt00062a

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