Aromatic PCN Palladium Pincer Complexes. Probing the Hemilability

Apr 22, 2015 - 1H, 13C, and 31P NMR spectra were recorded on a Varian Unity INOVA 500 spectrometer operating at 499.77 MHz (1H) using C6D6 ..... The s...
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Aromatic PCN Palladium Pincer Complexes. Probing the Hemilability through Reactions with Nucleophiles André Fleckhaus,† Abdelrazek H. Mousa, Nasir Sallau Lawal, Nitsa Kiriakidou Kazemifar, and Ola F. Wendt* Centre for Analysis and Synthesis, Department of Chemistry, Lund University, P.O. Box 124, S-221 00 Lund, Sweden

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S Supporting Information *

ABSTRACT: A series of unsymmetrical PCN pincer ligands (1-(3-((di-tert-butylphosphino)methyl)phenyl)-N,N-dialkylmethanamine) were cyclometalated with palladium to generate a series of new PCN supported Pd(II) chloro complexes, (PCN)PdCl (4−6), where alkyl = methyl, ethyl, and n-propyl, which were fully characterized by NMR spectroscopy and Xray crystallography. The N,N-dimethyl complex 4 reacts with methyl lithium to give the corresponding methyl and dimethyl complexes (PCN)PdMe (12) and Li[(PCN)PdMe2] (13), which could not be isolated but were characterized in solution. The substitution reactions of (PCN)PdCl (4−6) with iodide to form the corresponding iodo complexes (PCN)PdI (7−9) were investigated by use of UV−vis stopped-flow spectrophotometry. The experiments were performed in methanol over a temperature range from 293 to 325 K. The reactions are reversible and were shown to proceed exclusively via the solvento complex in two reversible consecutive steps. Activation parameters for both the forward and reverse reactions were determined, and they, together with reactivity trends, support an associative pathway. No displacement of the nitrogen donor was detected, and overall this points to a limited hemilability of the ligands on palladium.

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INTRODUCTION

of the kinetics and mechanism for the halide for halide substitution.

Transition metal complexes with tridentate, so-called pincer ligands have been shown to be extremely versatile catalysts in many transformations over the past decades.1 Thus, they have attracted considerable interest in organometallic chemistry and they mediate a number of interesting both stoichiometric and catalytic transformations, including dehydrogenations,2 C−C coupling reactions,3 and the activation of small molecules.4 They can easily be immobilized via attachment of linkers on the ligands backbone,5 and the trans effect of the carbanionic ligand increases the reactivity in the free coordination site.6 Most examples include symmetric ECE type of ligands, but also examples of unsymmetric ligands with typically one soft phosphorus donor and one hard oxygen or nitrogen donor exist. The most common type is the PCN ligand,7 but also, for example, POCN,8 PCO,9 PSiN,10 and PNN11 ligands have been reported. These complexes often show a hemilabile character which opens up one more coordination site and gives rise to interesting reactivity. Surprisingly, the PCN pincer ligands have not been investigated with palladium. Here we report on the synthesis and characterization of a number of PCN palladium pincer complexes with different substituents on the amine functionality. To probe the hemilability, the reactivity of the chloride in the fourth position toward a number of nucleophiles was investigated and in particular we report a detailed investigation © 2015 American Chemical Society

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EXPERIMENTAL SECTION

General Procedures and Materials. All experiments were carried out under an atmosphere of argon or nitrogen using standard Schlenk or high vacuum line techniques12 unless otherwise noted. All solvents were distilled under vacuum directly to the reaction vessel from sodium/benzophenone ketyl radical, except dichloromethane, which was dried over calcium hydride and methanol, which was dried over magnesium. All chemicals were purchased from Acros, Alfa-Aesar, or Sigma-Aldrich. 1H, 13C, and 31P NMR spectra were recorded on a Varian Unity INOVA 500 spectrometer operating at 499.77 MHz (1H) using C6D6 and J. Young NMR tubes unless otherwise stated. Chemical shifts are given in ppm downfield from TMS using residual solvent peaks (1H and 13C) or H3PO4 (31P) as reference. Multiplicities are abbreviated as follows: (s) singlet, (d) doublet, (t) triplet, (q) quartet, (m) multiplet, (br) broad, (v) virtual. Mass spectra were recorded on a MICROMASS Q-Tof mass spectrometer using dilute solutions in acetonitrile/water (50:50) containing 1% of formic acid. Elemental analyses were performed by H. Kolbe Microanalytisches Laboratorium, Mülheim an der Ruhr, Germany. The ligand 2 was prepared according to a literature procedure,7c as were complexes PdCl2(MeCN)2,13 {2,6-bis[(di-tert-butylphosphino)-methyl]phenyl} palladium(II)chloride (10),14 and the corresponding iodide 11.15 Received: December 11, 2014 Published: April 22, 2015 1627

DOI: 10.1021/om501231k Organometallics 2015, 34, 1627−1634

Article

Organometallics Ligand Synthesis. The precursors for the synthesis of ligands 1− 3, the 3-(dialkylamino)benzyl bromide·HBr salts, were synthesized using literature procedures.16 Bromination of methyl-m-methyltoluate followed by a reduction with DiBAl-H produces 3-(bromomethyl)benzyl alcohol, which reacts with the corresponding dialkylamines (aqueous solution (40%) in the case of dimethylamine) to give the 3(dialkylamino)benzyl alcohols as exemplified for the di-n-propyl amine below. Preparation of 3-(N,N-Di-n-propylaminomethyl)benzylalcohol. In a round-bottom flask 2.01 g of 3-(bromomethyl)benzyl alcohol (10.0 mmol) was stirred with 6.9 mL (50.0 mmol) of di-n-propylamine for 15 h at room temperature. After removal of the volatile compounds under reduced pressure, Et2O (100 mL) was added and the solution was washed with aqueous KOH (10%) and brine and dried over MgSO4 leading to 2.31 g (95%) of the crude product in a purity of 90%. Attempts to purify it by column chromatography led to decomposition. Therefore, it was used in the next step without purification. 1H NMR (CDCl3): δ = 7.40−7.21 (m, 4H, Ar), 4.68 (s, 2H,CH2OH), 3.55 (s, 2H,CH2N), 2.55 (t, 3JHH = 7.3 Hz, 4H, N(CH2CH2CH3)2), 1.79 (br s, 1H, CH2OH), 1.56−1.42 (m, 4H, N(CH 2 CH 2 CH 3 ) 2 ), 0.86 (t, 3 J HH = 7.4 Hz, 6H, N(CH2CH2CH3)2). Preparation of 3-(N,N-Dimethylaminomethyl)benzyl bromide·HBr. In a round-bottom flask, 1.02 g of 3-(N,Ndimethylaminomethyl)benzyl alcohol (6.10 mmol) was mixed with 16 mL of HBr (48% in water) and stirred for 16 h at room temperature. Evaporation of the solvents under high vacuum led to a brown oil, and after stirring in 50 mL of Et2O overnight, the product precipitated as a brown solid (1.70 g, 90%). 1H NMR (CD3OD) δ = 7.60 (s, 1H, H1), 7.57 (d, 3JHH = 7.1 Hz, 1H, H3), 7.50 (t, 3JHH = 7.9 Hz, 1H, H4), 7.47 (d, 3JHH = 7.6 Hz, 1H, H5), 4.62 (s, 2H, CH2Br), 4.35 (s, 2H, CH2N), 2.87 (s, 6H, N(CH3)2). 13C{1H} NMR: δ = 141.1, 132.6, 132.0, 131.9, 131.6, 130.9, 61.7, 43.0, 33.1. Preparation of (PtBuCNMe)H (1). In the glovebox, 1.52 g (10.4 mmol) of di-tert-butyl phosphine was added to a solution of 1.60 g (5.20 mmol) of 3-(dimethylaminomethyl)benzyl bromide·HBr salt in 20 mL of dry methanol. The reaction mixture was stirred for 18 h at 100 °C in a sealed Straus flask. After allowing the mixture to cool down to room temperature, 5 mL of dry triethylamine was added and it was stirred for 30 min. The solvents were evaporated under high vacuum, and the flask was reintroduced into the glovebox. The solid was treated with diethyl ether (2 × 20 mL), the suspension was filtered, and the filtrate was evaporated under high vacuum, giving 1.35 g (89%) of the product as a pale-yellow oil. 1H NMR: δ = 7.52 (s, 1H, H1), 7.30 (m, 1H, H3), 7.14 (m, 2H, H4, H5), 3.27 (s, 2H,CH2N), 2.73 (d, 2JHP = 1.9 Hz, 2H,CH2P), 2.08 (s, 6H, N(CH3)2), 1.03 (d, 3 JHP = 10.6 Hz, 18H, C(CH3)3). 13C{1H} NMR: δ = 142.0 (d, 2JCP = 12.4 Hz, C2), 139.9 (s, C6), 130.6 (d, 3JCP = 8.6 Hz, C1), 128.8 (d, 3 JCP = 8.9 Hz, C3), 128.4 (s, C5), 126.4 (d, 4JCP = 2.0 Hz, C4), 64.7 (s, CH2N), 45.5 (s, N(CH3)2), 31.8 (d, 1JCP = 24.5 Hz, C(CH3)3), 29.9 (d, 2JCP = 13.6 Hz, C(CH3)3), 29.1 (d, 1JCP = 25.5 Hz, CH2P). 31 1 P{ H} NMR: δ = 33.2 (s). Preparation of 3-(N,N-Di-n-propylaminomethyl)benzyl bromide·HBr. Using the same procedure as above with 2.00 g of 3-(N,Ndi-n-propylaminomethyl)benzyl alcohol (9.05 mmol) and 25 mL of HBr (48% in water) gave a brown solid, which was recrystallized from MeOH/Et2O giving the product (2.56 g, 77%) as a light-brown powder. 1H NMR (CD3OD): δ = 7.61 (s, 1H, H1), 7.57 (d, 3JHH = 7.5 Hz, 1H, H3), 7.50 (t, 3JHH = 7.6 Hz, 1H, H4), 7.47 (d, 3JHH = 7.6 Hz, 1H, H5), 4.63 (s, 2H, CH2Br), 4.38 (s, 2H,CH2N), 3.14−3.03 (m, 4H, N(CH2CH2CH3)2), 1.88−1.69 (m, 4H, N(CH2CH2CH3)2), 0.99 (t, 3 JHH = 7.3 Hz, 6H, N(CH2CH2CH3)2). 13C{1H} NMR: δ = 141.2 (s), 132.9 (s), 132.0 (s), 131.8 (s), 131.4 (s), 130.9 (s), 57.9 (s), 55.4 (s), 33.1 (s), 18.2 (s), 11.2 (s). Preparation of (PtBuCNnPr)H (3). In the glovebox 1.09 g of 3(N,N-di-n-propylaminomethyl)benzyl bromide·HBr salt (3.00 mmol) and 875 mg of di-tert-butylphosphine (6.00 mmol) were dissolved in 5 mL of dry MeOH in a Straus flask. The sealed flask was heated to 100 °C for 38 h. After addition of 3 mL of triethylamine, it was stirred for 30 min, the volatiles were removed, and the residue filtered with Et2O

in the glovebox. After evaporation, the product was obtained as a colorless oil (783 mg, 75%). 1H NMR: δ = 7.61 (s, 1H, H1), 7.35 (d, 3 JHH = 6.4 Hz, 1H, H3), 7.24−7.20 (m, 2H, H4, H5), 3.49 (s, 2H,CH2N), 2.80 (d, 2JHP = 2.0 Hz, 2H,CH2P), 2.35 (t, 3JHH = 7.3 Hz, 4H, N(CH2CH2CH3)2), 1.45 (tq, 3JHH = 7.3, 7.3 Hz, 4H, N(CH2CH2CH3)2), 1.08 (d, 3JHP = 10.5 Hz, 18H, C(CH3)3), 0.87 (t, 3JHH = 7.4 Hz, 6H, N(CH2CH2CH3)2). 13C{1H} NMR: δ = 141.8 (d, 2JCP = 12.5 Hz, C2), 140.9 (s, C6), 130.5 (d, 3JCP = 8.7 Hz, C1), 128.5 (d, 3JCP = 8.8 Hz, C3), 128.3 (s, C5), 126.3 (d, 4JCP = 2.1 Hz, C4), 59.4 (s, CH2N), 56.3 (s, N(CH2CH2CH3)2), 31.8 (d, 1JCP = 24.5 Hz, C(CH3)3), 29.9 (d, 2JCP = 13.5 Hz, C(CH3)3), 29.2 (d, 1JCP = 25.6 Hz, CH2P), 20.9 (s, N(CH2CH2CH3)2), 12.2 (s, N(CH2CH2CH3)2). 31 1 P{ H} NMR: δ = 33.4 (s). Synthesis of (PtBuCNMe)Pd-Cl (4). To a suspension of 683 mg of (MeCN)2PdCl2 (2.63 mmol) in 40 mL of toluene in a Straus flask, 738 mg of 1 (2.50 mmol) and 520 mg of K2CO3 (3.75 mmol) were added in the glovebox. The flask was sealed and the reaction mixture stirred for 22 h at 120 °C. After cooling to room temperature, the flask was opened, the mixture was filtered, and the solvents were removed under reduced pressure, giving 1.04 g (95%) of 4 as a pale-yellow solid. 1H NMR (C6D6): δ = 7.00 (t, 3JHH = 7.5 Hz, 1H, H4), 6.89 (d, 3JHH = 7.5 Hz, 1H, H3), 6.69 (d, 3JHH = 7.3 Hz, 1H,H5), 3.42 (s, 2H,CH2N), 2.90 (d, 2JHP = 9.3 Hz, 2H, CH2P), 2.55 (d, 4JHP = 2.2 Hz, 6H, N(CH3)2), 1.30 (d, 3JHP = 14.0 Hz, 18H, C(CH3)3). 13C{1H}NMR: δ = 160.5 (s, C1), 148.6 (s, C6), 147.9 (d, 2JCP = 15.2 Hz, C2), 124.7 (s, C4), 122.2 (d, 3JCP = 21.3 Hz,C3), 120.4 (s,C5), 72.6 (d, 3JCP = 2.3 Hz, CH2N), 49.7 (d, 3JCP = 2.3 Hz, N(CH3)2), 35.2 (d, 1JCP = 28.2 Hz, CH2P), 34.8 (d, 1JCP = 16.6 Hz, C(CH3)3), 29.2 (d, 2JCP = 4.5 Hz, C(CH3)3). 31 1 P{ H} NMR: δ = 93.8 (s). Elemental analysis found (calcd for C18H31ClNPPd): C, 49.79 (49.78); H, 7.16 (7.19); N, 3.22 (3.23). ESI-MS: m/z 439.2 [M − Cl + MeCN]+, 398.2 [M − Cl]+. Synthesis of (PtBuCNEt)Pd-Cl (5). The synthesis of 5 was performed as above using 6.4 mg (0.02 mmol) of 2, 4.4 mg (0.02 mmol) of (MeCN)2PdCl2, and triethylamine (2.7 μL, 0.02 mmol) as a base, giving 7.3 mg (79%) of 5 as a pale-yellow solid. 1H NMR (C6D6): δ = 6.99 (t, 3JHH = 7.5 Hz, 1H, H4), 6.87 (d, 3JHH = 7.5 Hz, 1H, H3), 6.69 (d, 3JHH = 7.4 Hz, 1H, H5), 3.55 (s, 2H, CH2N), 3.47− 3.39 (m, 2H, N(CH2CH3)2), 2.89 (d, 2JHP = 9.4 Hz, 2H, CH2P), 2.33−2.25 (m, 2H, N(CH2CH3)2), 1.37 (t, J = 7.1 Hz, 6H, N(CH2CH3)2), 1.30 (d, 3JHP = 13.9 Hz, 18H, C(CH3)3). 13C{1H} NMR: δ = 159.6 (s,C1), 151.2 (s, C6), 147.9 (d, 2JCP = 15.6 Hz, C2), 124.5 (s, C4), 121.8 (d, 3JCP = 21.6 Hz, C3), 119.4 (s, C5), 65.2 (d, 3 JCP = 2.4 Hz, CH2N), 55.4 (s, N(CH2CH3)2), 35.2 (d, 1JCP = 22.1 Hz, CH2P), 35.0 (d, 1JCP = 16.5 Hz, C(CH3)3), 29.2 (d, 2JCP = 4.4 Hz, C(CH3)3), 13.6 (s, N(CH2CH3)2). 31P{1H} NMR: δ = 90.8 (s). ESIMS: m/z 462.0 [M + H]+, 426.0 [M − Cl]+. Elemental analysis found (calcd for C20H35ClNPPd): C, 51.83 (51.96); H, 7.97 (7.63); N, 3.18 (3.03). Synthesis of (PtBuCNnPr)Pd-Cl (6). The synthesis of 6 was performed as above using 175 mg (0.50 mmol) of 3 and 109 mg (0.50 mmol) of (MeCN)2PdCl2 without the use of a base, giving 160 mg (65%) of 6 as a pale-yellow solid. 1H NMR (C6D6): δ = 7.01 (t, 3JHH = 7.2 Hz, 1H, H4), 6.88 (d, 3JHH = 7.5 Hz, 1H, H3), 6.73 (d, 3JHH = 7.2 Hz, 1H, H5), 3.67 (s, 2H, CH2N), 3.37 (td, 3JHH = 11.9, 3.4 Hz, 2H, N(CH2CH2CH3)2), 2.90 (d, 2JHP = 9.4 Hz, 2H, CH2P), 2.53−2.42 (m, 2H, N(CH2CH2CH3)2), 2.32−2.23 (m, 2H, N(CH2CH2CH3)2), 1.71−1.58 (m, 2H, N(CH2CH2CH3)2), 1.30 (d, 3JHP = 13.9 Hz, 18H, C(CH3)3), 0.81 (t, 3JHH = 7.4 Hz, 6H, N(CH2CH2CH3)2). 13 C{1H} NMR: δ 159.6 (s, C1), 151.2 (s, C6), 147.9 (d, 2JCP = 15.6 Hz, C2), 124.6 (s, C4), 121.8 (d, 3JCP = 21.6 Hz, C3), 119.4 (s, C5), 66.5 (d, 3 J CP = 2.4 Hz, CH2 N), 63.7 (d 3 J CP = 2.1 Hz, N(CH2CH2CH3)2), 35.2 (d, 1JCP = 9.6 Hz, CH2P), 35.0 (d, 1JCP = 1.5 Hz, C(CH3)3), 29.2 (d, 2JCP = 4.4 Hz, C(CH3)3), 21.9 (s, N(CH2CH2CH3)2), 11.8 (s, N(CH2CH2CH3)2). 31P{1H} NMR: δ = 91.0 (s). ESI-MS: m/z 454.2 [M − Cl]+. Elemental analysis found (calcd for C22H39ClNPPd): C, 54.75 (53.88); H, 8.17 (8.02); N, 2.94 (2.86). Synthesis of (PtBuCNMe)Pd-I (7). To a solution of 10.9 mg (0.025 mmol) of 4 in methanol, 44.3 mg (0.30 mmol) of NaI was added. 1628

DOI: 10.1021/om501231k Organometallics 2015, 34, 1627−1634

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Organometallics Scheme 1. Synthesis of (PCN)PdCl (4−6). Complex 10 and 11 are Shown for Comparisiona

a

without the use of base. found (calcd for C22H39INPPd): C, 45.55 (45.41); H, 6.66 (6.76); N, 2.37 (2.41). Crystallography. XRD quality crystals of compounds 4−9 were obtained through vapor diffusion from dichloromethane/hexane at 6 °C. Intensity data were collected with an Oxford Diffraction Excalibur 3 system, using ω-scans and Mo Kα (λ = 0.71073 Å) radiation.17 The data were extracted and integrated using Crysalis RED.18 The structures were solved by direct methods and refined by full-matrix least-squares calculations on F2 using SHELXL,19 JANA2006,20 and OLEX2.21 Molecular graphics were generated using Diamond 3.2i.22 CCDC deposition numbers 1032326−1032331. Kinetic Measurements. Reactions of 4−6 with iodide in methanol were monitored using an Applied Photophysics Bio Sequential SX-17 MX stopped-flow spectrophotometer. The substitution of chloride by iodide was studied in methanol by observing the increase in absorbance at 316 nm. The complex solution (0.1 mM) contained chloride (1−10 mM) and was mixed with at least a 10-fold excess of iodide (2.5−50 mM), assuring the reaction is under pseudofirst-order conditions. The kinetic traces were fitted to single exponentials using the software provided by Applied Photophysics spectrophotometer.23 This gave observed rate constants at different concentrations of leaving and incoming ligands. Rate constants are given as an average of at least 5 runs. Variable temperature measurements were made between 20 and 52 °C. Rate laws were fitted using KaleidaGraph 4.1. The spectra of the products were in agreement with those prepared separately and characterized. The reaction of 10 to give 11 was treated similarly but using a Cary 100 Bio UV−visible spectrophotometer and following the increase in absorbance at 330 nm. Reaction of 4 with MeLi. Method A (formation of (PCNMe)PdMe (12)): In a J. Young NMR tube, 38 μL of a solution of MeLi (1.6 M in diethyl ether, 0.06 mmol, 1.2 equiv) were evaporated at the high vacuum line. The NMR tube was reintroduced into the glovebox, where 0.5 mL of C6D6 and 21.7 mg of 4 (0.05 mmol, 1.0 equiv) were added. After 1 day, complete conversion to the product could be observed according to the 1H and 31P NMR spectra. Isolation of the product was not possible, so 12 was characterized in situ by NMR spectroscopy only. 1H NMR (C6D6): δ = 7.19−7.17 (m, 1H, H4), 7.14 (dd, 3JHH = 7.5, 1.1 Hz, 1H, H3), 6.98 (dd, 3JHH = 7.0, 1.1 Hz, 1H, H5), 3.69 (s, 2H, CH2N), 3.27 (d, 2JHP = 9.0 Hz, 3H, CH2P), 2.40 (s, 3H, N(CH3)2), 2.39 (s, 3H, N(CH3)2), 1.25 (d, 3JHP = 13.3 Hz, 18H, C(CH3)3), −0.01 (s, 3H, Pd-CH3). 31P{1H} NMR: δ = 93.2 (s). Method B: In a J. Young NMR tube, 76 μL of a solution of MeLi (1.6 M in diethyl ether, 0.12 mmol, 2.4 equiv) were evaporated at the high vacuum line. The NMR tube was reintroduced into the glovebox, where 0.5 mL of C6D6 and 21.7 mg of 4 (0.05 mmol, 1.0 equiv) were

After evaporation of the solvent, the residue was dissolved in benzene and filtered through a plug of Celite. Evaporation gave 7 (12.6 mg, 96%) as a yellow solid. 1H NMR (C6D6): δ = 7.01 (t, 3JHH = 7.5, 1H, H4), 6.88 (d, 3JHH = 7.5 Hz, 1H, H3), 6.69 (d, 3JHH = 7.4 Hz, 1H, H5), 3.42 (s, 2H,CH2N), 2.95 (d, 2JHP = 9.5 Hz, 2H, CH2P), 2.64 (d, 4JHP = 2.2 Hz, 6H, N(CH3)2), 1.29 (d, 3JHP = 14.0 Hz, 18H, C(CH3)3). 13 C{1H} NMR: δ = 165.0 (s, C1), 148.7 (s, C6), 147.6 (d, 2JCP = 15.1 Hz, C2), 125.0 (s, C4), 121.9 (d, 3JCP = 21.3 Hz, C3), 120.5 (s, C5), 72.4 (d, 3JCP = 2.8 Hz, CH2N), 51.5 (d, 3JCP = 2.8 Hz, N(CH3)2), 36.0 (d, 1JCP = 28.2 Hz, CH2P), 35.0 (d, 1JCP = 16.8 Hz, C(CH3)3), 29.6 (d, 2 JCP = 4.4 Hz, C(CH3)3). 31P{1H} NMR: δ = 97.2 (s). Elemental analysis found (calcd for C18H31INPPd): C, 40.91 (41.12); H, 5.66 (5.94); N, 2.68 (2.66). Synthesis of (PtBuCNEt)Pd-I (8). The synthesis of 8 was performed as above using 23.12 mg (0.05 mmol) of 5 and 74.94 mg (0.5 mmol) of NaI, giving 26.3 mg (95%) of 8 as a yellow solid. 1H NMR (500 MHz, C6D6): δ = 7.00 (t, 3JHH = 7.5, 1H, H4), 6.86 (d, 3JHH = 7.4 Hz, 1H, H3), 6.65 (d, 3JHH = 7.4 Hz, 1H, H5), 3.82−3.71 (m, 2H, N(CH2CH3)2), 3.57 (s, 2H,CH2N), 2.93 (d, 2JHP = 9.5 Hz, 2H, CH2P), 2.36−2.24 (m, 2H, N(CH2CH3)2), 1.32 (t, 3JHH = 7.0 Hz, 6H, N(CH2CH3)2), 1.30 (d, 3JHP = 14.0 Hz, 18H, C(CH3)3). The peaks of the methyl groups of N(CH2CH3)2 and P(C (CH3)3)2 are partially overlapping. 13C{1H} NMR: δ = 162.8 (s, C1), 152.4 (s, C6), 147.7 (d, 2JCP = 15.4 Hz, C2), 124.8 (s, C4), 121.6 (d, 3JCP = 21.7 Hz, C3), 119.0 (s, C5), 65.0 (d, J = 2.2 Hz, CH2N), 57.3 (s, N(CH2CH3)2), 35.9 (d, 1JCP = 27.7 Hz, CH2P), 35.3 (d, 1JCP = 16.5 Hz, C(CH3)3), 29.6 (d, 2JCP = 4.1 Hz, C(CH3)3), 13.9 (s, N(CH2CH3)2). 31P{1H} NMR: δ = 93.8 (s). Elemental analysis found (calcd for C20H35INPPd): C, 43.51 (43.38); H, 6.22 (6.37); N, 2.48 (2.53). Synthesis of (PtBuCNnPr)Pd-I (9). The synthesis of 9 was performed as above using 24.52 mg (0.05 mmol) of 6 and 74.94 mg (0.5 mmol) of NaI, giving 27.9 mg (96%) of 9 as a yellow solid. 1H NMR (C6D6): δ = 7.02 (t, 3JHH = 8.1 Hz, 1H, H4), 6.88 (d, 3JHH = 7.4 Hz, 1H, H3), 6.69 (d, 3JHH = 7.5 Hz, 1H, H5), 3.73 (td, J = 12.1, 4.4 Hz, 2H, N(CH2CH2CH3)2), 3.68 (s, 2H,CH2N), 2.94 (d, 2JHP = 9.5 Hz, 2H,CH2P), 2.58−2.47 (m, 2H, N(CH2CH2CH3)2), 2.30−2.23 (m, 2H, N(CH2CH2CH3)2), 1.55−1.43 (m, 2H, N(CH2CH2CH3)2), 1.30 (d, 3JHP = 14.0 Hz, 18H C(CH3)3), 0.80 (t, 3JHH = 7.3 Hz, 6H, N(CH2CH2CH3)2). 13C{1H} NMR: δ = 162.8 (s, C1), 152.4 (s, C6), 147.7 (d, 2JCP = 15.3 Hz, C2), 124.8 (s, C4), 121.6 (d, 3JCP = 21.6 Hz, C3), 119.1 (s, C5), 66.5 (s, CH2N), 65.4 (s, N(CH2CH2CH3)2), 35.9 (d, 1JCP = 27.7 Hz, CH2P), 35.3 (d, 1JCP = 16.6 Hz, C(CH3)3), 29.6 (d, 2 JCP = 4.1 Hz, C(CH3)3), 22.2 (s, N(CH2CH2CH3)2), 11.5 (s, N(CH2CH2CH3)2). 31P{1H} NMR: δ= 93.9 (s). Elemental analysis 1629

DOI: 10.1021/om501231k Organometallics 2015, 34, 1627−1634

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Organometallics

reported for the palladium PCO complexes.9 The tridentate coordination mode of the ligand was confirmed by X-ray crystallography, and the so determined molecular structures are shown in Figure 1. Crystal data and collection and refinement

added. After 1 h, a mixture of two compounds was formed in a ratio of approximately 85:15 according to 1H and 31P NMR spectroscopy, and this stayed unchanged over a period of several days. Addition of another 76 μL of methyl lithium solution in the glovebox led to full conversion to the minor product which decomposed after evaporation of the solvents. Method C: In a J. Young NMR tube, 152 μL of methyl lithium (1.6 M in diethyl ether, 0.24 mmol, 4.8 equiv) were evaporated at the high vacuum line. The NMR tube was reintroduced into the glovebox, where 0.5 mL of C6D6 and 21.7 mg of 4 (0.05 mmol, 1.0 equiv) were added. After 1 h, a mixture of the tridentate complex 12 and the anionic, bidentate complex 13 were formed in a ratio of approximately 30:70 according to 1H and 31P NMR spectroscopy. Addition of a drop of Et2O in the glovebox led to full conversion to the anionic, bidentate complex 13. 1H NMR (C6D6): δ = 7.20 (d, J = 7.1 Hz, 1H), 7.01 (t, J = 7.1 Hz, 1H), 6.80 (d, J = 7.2 Hz, 1H), 4.49 (br s, 2H), 2.59 (br s, 2H), 2.05 (br s, 3H), 1.80 (br s, 3H) 1.41 (br s, 18H), 0.60 (d, J = 5.2 Hz, 3H), 0.21 (s, 3H). 31P{1H} NMR (C6D6): δ = 67.6.

Figure 1. Molecular structure of 4, 5, and 6 at the 30% probability level. Hydrogen atoms are omitted for clarity.

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details are presented in Supporting Information, Table S4, and geometrical data are given in Table 1. The Pd(II) center has a

RESULTS AND DISCUSSION Synthesis of (PCN)Pd Complexes. The new (PCN)H ligands (1 and 3) were synthesized following the literature procedure7c for the previously described ligand (PtBuCNEt)H (2) with only slight modifications. After radical bromination of methyl-m-methyltoluate and reduction of the ester group, a sequence of three nucleophilic substitutions leads to the desired phosphine−amine ligands. The use of different secondary alkyl amines is tolerated, allowing for the synthesis of the differently substituted ligands. Reaction of these ligands with PdCl2(MeCN)2 or PdCl2(PhCN)2 at 120 °C in toluene afforded the new (PCN)Pd-Cl complexes 4−6 in moderate yields. Addition of base improved the reaction significantly, leading to almost quantitative formation of the desired products. All reactions are described in Scheme 1. The 31P{1H} NMR spectra of the (PCN)Pd chloride complexes contain only a single resonance at 93.8 ppm for 4, 90.8 ppm for 5, and 91.0 ppm for 6. Compounds 4 and 5 are pure as judged by microanalysis. However, satisfactory microanalysis could not be obtained for compound 6 despite repeated recrystallization and prolonged drying. Although these results are outside the range viewed as establishing analytical purity, they are provided to illustrate the best values obtained. The 1H NMR spectrum is provided as evidence of identity and purity (see Supporting Information, Figure S28). The chemical shift of the phosphorus atom is thus situated between the shift in (PCPtBu)Pd-Cl complexes (72.5 ppm)24 and (PtBuCOMe)PdCl (106.1 ppm)9 indicating that the properties of the Pd−N bond is also somewhere in between the Pd−P and the Pd−O bond. All the 1H NMR spectra are typical for unsymmetrically substituted pincer complexes. The spectrum of 4, as an example, contains three signals in the aromatic region, two doublets at 6.89 and 6.69 ppm with a 3JHH coupling of 7.5 and 7.3 ppm for the protons in 3- and 5-position, and a pseudotriplet at 6.99 ppm with a 3JHH coupling of 7 ppm. The benzylic protons on the amine arm give rise to a singlet, and on the phosphine arm they give a doublet with a 2JHP coupling of 9.4 Hz. The N-dimethyl group is split into a doublet by a 4JPH coupling as confirmed by a 31P decoupling experiment. The tert-butyl protons at 1.30 ppm couple with the phosphorus atom, leading to a doublet with 3JHP = 14.0 Hz. The presence of only three aromatic protons, each showing 3J coupling and the downfield shift of the phosphorus, all clearly indicate a mer-tridentate coordination of the PCN ligand with no indication of any dimer−monomer equilibria as previously

Table 1. Selected Bond Lengths (Å) and Bond Angles (deg) with Estimated Standard Deviations for Complexes 4−6 4 bond (Å) Pd−C1 Pd−N1 Pd−P1 Pd−Cl1 angles (deg) C1−Pd1−N1 C1−Pd1−P1 C1−Pd1−Cl1 N1−Pd1−P1 N1−Pd1−Cl1 P1−Pd1−Cl1

1.9809(19) 2.1802(16) 2.2428(5) 2.4090(5) 81.11(7) 82.95(6) 175.69(6) 161.77(5) 94.74(5) 101.32(2)

5 1.9685(17) 2.1873(18) 2.2325(4) 2.4061(5) 82.85(8) 82.54(6) 175.85(6) 165.26(5) 94.53(5) 100.170(18)

6 1.981(10) 2.154(10) 2.222(3) 2.396(3) 83.4(4) 82.0(3) 175.6(3) 165.1(3) 92.7(3) 102.01(12)

distorted square planar coordination geometry, with P and N being positioned trans to each other with slight distortion (P− Pd−N 162°; C1−Pd−Cl 176°). The P−Pd bond is shortened to 2.24 Å compared to 2.30 in 10,25 whereas the Pd−N bond is elongated to 2.18 Å (compared to 2.10 Å in (NCN)Pd−Cl),26 showing the weaker trans effect of N compared to P. The Pd− C1 bond length is 1.98 Å, in-between the lengths of these complexes as well (ca. 2.03 in 10, 1.92 Å in (NCN)Pd−Cl) The reason may be increased electrophilicity of the Pd-center or steric in origin due to the decreased size of N compared to P. The Pd−Cl bond of 2.41 Å (2.40 Å in 10, 2.42 Å in (NCN)Pd−Cl), on the other hand, is almost unaffected, and this bond also shows a variability of ca. 0.02 Å between polymorphs of 10.25b A significant shortening of the Pd−N distance for the n-propyl substituted chloro complex is observed; this trend is, however, not reproduced in the iodo complexes and probably stems from a slight disorder in the nitrogen atom in 6 rather than any underlying chemical difference. Investigation of the Hemilabile Properties of the (PCNMe)Pd Complex. To probe the possible hemilabile properties of the (PCN)Pd complexes, we studied potential decomplexation reactions starting with the reaction with strong nucleophiles. The reactions were conveniently monitored by 31 1 P{ H} NMR spectroscopy. Immediately after the addition of one equivalent of methyl lithium to a solution of 4 in C6D6, we observed two new signals in addition to the starting material. 1630

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Organometallics One signal at 93.2 ppm was assigned to the expected reaction product, the (PCN)Pd-Me complex (12). The other signal at 67.6 ppm indicated that the Pd−N bond was already cleaved under this mild conditions, leading to the anionic, dimethyl species 13, in strong analogy to Milstein’s observations for platinum complexes, cf. Scheme 2.7b Notably, for the (PCN)Pd Scheme 2. Reaction with Strong Nucleophiles

Figure 2. Molecular structure of 7, 8, and 9 at the 30% probability level. Hydrogen atoms are omitted for clarity.

and collection and refinement details are presented in Supporting Information, Table S5, and geometrical data are given in Table 2. The structures of compounds 8 and 9 contain Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) with Estimated Standard Deviations for Complexes 7−9

complex, it was not necessary to further weaken the M−N bond by expansion of the metallacycle from a 5-membered to a 6-membered ring.7c Keeping the reaction for a longer time showed that the Pd−N bond cleavage was reversible by equilibration of the reaction, leading to formation of the (PCN)Pd-Me complex as a single product. Addition of an excess of methyl lithium moved the equilibrium to the dimethyl product with complete conversion to the anionic complex 13 as could be seen by a single resonance in 31P NMR at 67.56 ppm. Attempts to isolate this anionic complex failed so far. Both filtration and evaporation led to decomposition forming (PCN)Pd−Cl, a monomethyl species and the free ligand 1. Performing the reaction in the absence of diethyl ether to get a higher quality 1H NMR spectrum of 13 did not lead to complete conversion of 12 to 13. The use of 4.8 equiv of methyl lithium resulted in a mixture of 30% of the pincer Pd methyl complex 12 and 70% of the anionic species 13. This ratio stays unchanged for more than 10 days, whereas the addition of one drop of diethyl ether led to complete conversion to 13, the more polar solvent, presumably favoring charge separation. Performing the reaction in THF-d8, instead, did not lead to quantitative formation of 13. Twenty minutes after addition of 2.4 equiv of methyl lithium, a mixture of 36% of 12 and 64% of 13 was obtained. Keeping the reaction for a longer time led to decomposition of the anionic complex into several phosphorus containing species. Nucleophilic Substitution with NaI. As an additional probe of the hemilabile character of the PCN ligand, we decided to study nucleophilic substitution with weaker nucleophiles. This is a type of reaction that has been studied extensively for group 10 square planar complexes, and it generally involves associative pathways with five-coordinate transition states with parallel solvolytic and direct pathways.27 We reasoned that hemilability could open up possibilities for more stable anionic intermediates, thereby favoring the direct pathway and more closed transition states. Adding an excess of sodium iodide to a methanol solution of 4−6 transformed them into the corresponding iodides 7−9, which were isolated and fully characterized. The NMR spectra of 7 show the same features as those of the chloride complex (4). The chemical shifts of all protons are almost exactly the same with a maximum downfield shift of Δδ = 0.1 ppm. The biggest differences occur in the 31P and 13C NMR spectra, with the phosphorus signal being shifted downfield to 97.2 ppm (Δδ = 3.4 ppm) and the ipso carbon (C1) being shifted to 165.0 ppm (Δδ = 4.6 ppm). The molecular structures were determined using X-ray diffraction and are given in Figure 2. Crystal data

7

8

9

2.007(12)/ 1.975(13) 2.228(10)/ 2.211(10) 2.258(4)/2.254(4)

1.979(6)/1.992(5)

bonds (Å) Pd−C1

1.971(5)

Pd−N1

2.187(4)

Pd−P1

2.2444(12)

Pd−I1

2.6928(5)

2.7173(12)/ 2.7151(13)

2.2463(15)/ 2.2422(16) 2.7271(7)/ 2.7282(6)

80.40(18)

80.0(5)/80.4(5)

81.9(2)/82.3(2)

82.79(14)

81.6(4)/81.9(4)

175.64(13)

177.1(4)/176.7(4)

162.47(13)

161.5(3)/162.3(3)

95.55(12)

97.1(3)/96.3(3)

101.40(3)

101.34(10)/ 101.41(11)

82.95(18)/ 81.93(18) 175.33(18)/ 172.39(17) 164.81(15)/ 164.01(15) 93.47(15)/ 93.49(15) 101.72(4)/ 102.47(4)

angles (deg) C1− Pd1−N1 C1− Pd1−P1 C1− Pd1−I1 N1− Pd1−P1 N1− Pd1−I1 P1− Pd1−I1

2.194(5)/2.186(5)

two molecules in the asymmetric unit. As for the chloride complexes, the (PCN)Pd-I complexes show a distorted square planar complexation of the palladium by the tridentate ligand and iodide. Compared to the chloride complexes, all bonds and angles stay nearly unchanged, with only the Pd−halogen bond being 0.2 Å longer for I than Cl. The equilibrium reaction in Scheme 3 was conveniently studied in methanol using UV/vis spectroscopy. The equilibrium constants for the reactions were determined using spectrophotometry by fitting eq 1 to the absorbance at different temperatures and concentrations of iodide and chloride Scheme 3. Substitution Reaction of the PCN Complexesa

a

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Organometallics A = A 0[Cl−] + A∞Keq[I−] −

−

[Cl ] + Keq[I ]

Scheme 4. Suggested Mechanism for the Substitution Reactions in Schemes 3 and 5a

(1)

where A = equilibrium absorbance, A0 the absorbance of the chloride complexes (4−6, 10), and A∞ the absorbance of the iodide complexes (7−9, 11).28 The plotted fits (Supporting Information, Figure S1) and all data (Supporting Information, Table S1) are given in the Supporting Information. A van ’t Hoff plot (Supporting Information, Figure S2) gave the enthalpy and entropy of reaction for the equilibria involving complexes 4−6. Equilibrium constants and thermodynamic parameters are reported in Table 3. Table 3. Thermodynamic Data in Methanol for the Equilibria Involving 4−6 starting complex

K298 (M)

4 5 6

0.91 ± 0.30 1.7 ± 0.9a 1.5 ± 0.7a

a

ΔH° (kJ mol−1)

ΔS° (J K−1 mol−1)

17.2 ± 2.5 2.3 ± 3.0 23 ± 7

57 ± 8 12 ± 10 82 ± 22

a

The contribution from the direct path (k12/k21) is negligible.

kobs = k12[I −] + k 21[Cl−] +

k 23k 31[Cl] + k13k 32[I ] k 31[Cl] + k 32[I ]

(2)

Because the equilibrium constant is known, eq 2 can be rearranged to a form containing only three unknowns, eq 3, where, Keq = (k12/k21) = (k13/k23) (k32/k31) .

Determined from the van ’t Hoff plot. Estimated standard error from the standard error of the individual equilibrium constants in the van ’t Hoff plot.

a

kobs = k12([I ] + (1/Keq[Cl]))

Time-resolved spectra (Supporting Information, Figure S3) of the reaction in Scheme 3 display well-defined isosbestic points, showing that it is a single reaction without intermediates with significant concentrations. The reaction was studied under pseudo-first-order conditions, and the so-obtained observed rate constants as a function of iodide concentration are given in Figure 3 and Supporting Information, Figures S4 and S5. It is

+

((k13/Keq)(k 32/k 31)[Cl]) + (k13(k 32/k 31)[I ]) [Cl] + ((k 32/k 31)[I ]) (3)

It is quite clear, though, that neither eq 2 nor eq 3 will give rise to a saturation behavior because the k12-term will continue to grow with increasing iodide concentrations. Accordingly, fitting eq 3 to the data in Figure 3 gave a very poor fit with negative fitted rate constants, large errors, and other inconsistencies. Instead, we realized that the term for the solvent path will level off toward k13 at high iodide concentrations. Assuming a mechanism where only the solvent path contributes to the reactivity (k12 and k21 are negligibly small) gives the rate law in eq 4. Using equilibrium constants obtained from the van ’t Hoff plot, eq 4 was fitted to the data in Figures 3, Supporting Information, S4 and S5, giving good− excellent fits with consistent values of k13 and k32/k31 at different concentrations of chloride. The rate constant k23 could also be calculated from the equilibrium constants. A variable temperature study (at a single chloride concentration) as shown in Figure 4 and Supporting Information, Figures S6−S7 gave activation parameters for k13 and k23 from Eyring plots (Supporting Information, Figures S8−S10). Key kinetic data are given in Table 4, and all data are given in the Supporting Information.

Figure 3. Observed rate constants as a function of iodide concentration for the reaction of 5 with iodide to form 8 in methanol at T = 25 °C. [PdTOT] = 0.1 mM. The solid lines denote the best fit to eq 4. Different [Cl−] are indicated by colors (black, 1 mM; red 2.5 mM; blue 5 mM; green 10 mM).

kobs =

((k13/Keq)(k 32/k 31)[Cl]) + (k13(k 32/k 31)[I ]) [Cl] + ((k 32/k 31)[I ]) (4)

clear that there is saturation behavior where the rate constants level off at higher iodide concentrations. This cannot be explained if the mechanism is a direct reversible substitution. On the other hand, assuming a reversible involvement of a solvent path (Scheme 4), as previously found in many similar reactions, will give a rate law of the form in eq 2. This is based upon steady-state conditions for the solvento intermediate. The direct pathway is responsible for the first two terms and the solvent path for the third term.29

For comparison, we decided to perform a limited study on the substitution behavior of a (PCP)Pd complex. Thus, 10 was reacted with iodide in methanol solution to form 11 in an equilibrium reaction, cf. Scheme 5. Both are known compounds, and the formation of 11 was corroborated by NMR spectroscopy.14,15 The reaction was monitored by UV− vis spectrometry and the equilibrium constant for the reaction was determined as described above by fitting eq 1 to the absorbance at different iodide concentrations, cf. Supporting 1632

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Organometallics

Scheme 5. Substitution Reaction of the PCP Complexa

a

No further substitution was observed.

Figure 4. Observed rate constants as a function of iodide concentration for the reaction of 5 with iodide to form 8 in methanol. [PdTOT] = 0.1 mM. [Cl−] = 5.0 mM. The solid lines denote the best fit to eq 4. Different temperatures (in K) are indicated by colors (black, 293.1; red 297.1; blue 301.4; green 307.8; purple 315.2).

Information, Figure S11. Plotting observed rate constants as a function of iodide concentration at two different chloride concentrations again gives a saturation behavior (Figure 5) as for the PCN complexes, and eq 4 was used to evaluate the kinetics. As expected, we therefore conclude that also for the sterically more hindered PCP complex the substitution goes entirely through the solvent path in Scheme 4. Kinetic and thermodynamic data are given in Table 4. Thus, the substitution reactions for all the complexes investigated are first-order in the metal complex and zeroorder in the nucleophile for both the forward and reverse reaction. As usual for d8 square−planar complexes, we interpret this as a nucleophilic attack by the solvent which is supported by the fairly low activation enthalpies and negative activation entropies. It is well-known that bulky cis-ligands and strongly labilizing trans-ligands give rise to a lower nucleophilic discrimination, but in platinum(II) chemistry, it is still unusual that the solvent path completely dominates with fairly strong nucleophiles as iodide.28,30 It can be noted that previous examples of substitution in cyclometalated (NNC) platinum complexes show a strong preference for the direct pathway but no evidence for hemilability of the nitrogen donor.31 Compared to the overwhelming amount of data on substitution reactions in platinum(II), there is a scarcity of reports on palladium(II), but the general trend is probably the same. Therefore, it seems that the PCN and PCP frameworks induce an unexpected overall decreased nucleophilic discrimination and there is no sign that the PCN ligand opens up for direct attack by iodide or chloride. On the contrary, the k32/k31 values indicate a lower discriminating ability for the PCN complexes compared to 10. This is in line with the higher reactivity of the PCN complexes, indicating that this is not only explained by the lower steric bulk (which should increase the nucleophilic discrimination) but also by a more electrophilic metal center.

Figure 5. Observed rate constants as a function of iodide concentration for the reaction of 10 with iodide to form 11 in methanol at T = 25 °C. [PdTOT] = 0.1 mM. The solid lines denote the best fit to eq 4. Different [Cl−] are indicated by colors (black, 1 mM; red 2.5 mM).

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CONCLUSION In conclusion, we present the synthesis of a number of PCN palladium pincer complexes. With weak nucleophiles such as iodide, there is no tendency to hemilability. On the contrary, the PCN palladium metal centers show a low nucleophilic discrimination where reaction with the solvent is completely dominating over nucleophilic attack by the halide. With strong nucleophiles, the nitrogen arm is readily displaced, more readily than in the corresponding platinum complexes.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Selected NMR spectra, all kinetic data and figures, and crystallographic tables and crystal data (CIF). This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected]. Present Address †

University of Applied Sciences, Hochschule Emden/Leer, Constantiaplatz 4, DE-26723 Emden, Germany. Notes

The authors declare no competing financial interest.

Table 4. Kinetic Data for the Reactions in Scheme 3 and 4 in Methanola starting complex 4 5 6 10b a

k13 (s−1) 30 0.542 0.50 (5.4

± ± ± ±

4 0.021 0.06 0.2)−310

k23 (s−1) 13.2 0.189 0.155 (1.5

± ± ± ±

1.9 0.007 0.016 0.2)−310

k32/k31

ΔH⧧13 (kJ mol−1)

ΔS⧧13 (J K−1 mol−1)

ΔH⧧23 (kJ mol−1)

ΔS⧧23 (J K−1 mol−1)

± ± ± ±

37 ± 4 65.7 ± 1.1 65.6 ± 1.5

−93 ± 12 −30 ± 4 −32 ± 5

51 ± 4 70.6 ± 1.6 60.7 ± 1.2

−52 ± 12 −25 ± 6 −58 ± 4

0.41 0.61 0.47 0.28

0.13 0.06 0.06 0.04

Rate constants are given at 298 K. bK = 1.0 ± 0.8. 1633

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(11) (a) Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D. J. Am. Chem. Soc. 2005, 127, 12429. (b) Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D. Angew. Chem., Int. Ed. 2006, 45, 1113. (12) Burger, B. J.; Bercaw, J. E. In New Developments in the Synthesis, Manipulation and Characterization of Organometallic Compounds; Wayda, A., Darensbourg, M. Y., Eds.; American Chemical Society: Washington, DC, 1987; Vol. 357. (13) Wayland, B. B.; Schramm, R. F. Inorg. Chem. 1969, 8, 971. (14) Johansson, R.; Wendt, O. F. Dalton Trans. 2007, 488. (15) Johnson, M. T.; Dzolic, Z.; Cetina, M.; Wendt, O. F.; Ö hrstrom, L.; Rissanen, K. Cryst. Growth Des. 2012, 12, 362. (16) O’Hanlon, P. J.; Rogers, N. H. Eur. Pat. Appl. AN 406065, 1982. (17) Crysalis CCD; Oxford Diffraction Ltd.: Abingdon, Oxfordshire, UK, 2005. (18) Crysalis RED; Oxford Diffraction Ltd.: Abingdon, Oxfordshire, UK, 2005. (19) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, A64, 112. (20) Petricek, V.; Dusek, M.; Palatinus, L. Z. Kristallogr. 2014, 229, 345. (21) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H. J. Appl. Crystallogr. 2009, 42, 339. (22) Crystal Impact Putz H., Brandenburg K., Eds.; GbR: Kreuzherrenstr. 102, 53227 Bonn, Germany, 2015. (23) Bio Sequential SX-17MV Stopped-Flow ASVD Spectrophotometer, Software Manual; Applied Photophysics Ltd.: 203/205 Kingston Road, Leatherhead KT22 7PB, UK, 1994. (24) Moulton, C. J.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1976, 1020. (25) (a) Kimmich, B. F. M.; Marshall, W. J.; Fagan, P. J.; Hauptman, E.; Bullock, R. M. Inorg. Chim. Acta 2002, 330, 52. (b) Johnson, M. T.; Dzolic, Z.; Cetina, M.; Lahtinen, M.; Ahlquist, M. S. G.; Rissanen, K.; Ö hrstrom, L.; Wendt, O. F. Dalton Trans. 2013, 42, 8484. (26) Liu, B.-B.; Wang, X.-R; Guo, Z.-F.; Lu, Z.-L. Inorg. Chem. Commun. 2010, 13, 814. (27) Tobe, M. L.; Burgess, J. Inorganic Reaction Mechanisms; Longman: Amsterdam, NY, 1999; Chapter 3.3. (28) Wendt, O. F.; Elding, L. I. Inorg. Chem. 1997, 36, 6028. (29) Kuznik, N.; Wendt, O. F. J. Chem. Soc., Dalton Trans. 2002, 3074. (30) (a) Cusumano, M.; Marrichi, P.; Romeo, R.; Ricevuto, V.; Belluco, U. Inorg. Chim. Acta 1979, 34, 169. (b) Faraone, G.; Ricevuto, V.; Romeo, R.; Trozzi, M. Inorg. Chem. 1970, 9, 1525. (31) Romeo, R.; Plutino, M. R.; Scolaro, L. M.; Stoccoro, S.; Minghetti, G. Inorg. Chem. 2000, 39, 4749.

ACKNOWLEDGMENTS Financial support from the Swedish Research Council, the Knut and Alice Wallenberg Foundation, the Crafoord Foundation, and the Royal Physiographic Society in Lund is gratefully acknowledged.

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DOI: 10.1021/om501231k Organometallics 2015, 34, 1627−1634