Submitted to Organometallics
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Prying open a reactive site for allylic arylation by phosphine ligated geminally-diaurated aryl complexes.
Journal: Organometallics
Manuscript ID: Draft
Manuscript Type: Article
Date Submitted by the Author: n/a
Complete List of Authors: Vikse, Krista; ETH Zurich - Chemistry, Lab. f. org. Chemie Zavras, Athanasios; School of chemistry, Bio21 Institute Thomas, Tudor; Bio21 Institute, Chemistry Ariafard, Alireza; University of Tasmania, School of Chemistry Khairallah, George; Bio21 Institute, Chemistry Canty, Allan; University of Tasmania, Chemistry Yates, Brian; University of Tasmania, School of Chemistry O'Hair, Richard; Inst of Molecular Science/ Biotechnology, School of Chemistry
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1 2 3 4 Submitted to Organometallics 5 Version: 6 April 2015 6 7 8 9 10 11 Prying open a reactive site for allylic 12 13 14 15 arylation by phosphine ligated 16 17 18 geminally diaurated aryl complexes. 19 20 21 22 23 24 a-c a-c a-c d 25 Krista L. Vikse, Athanasios Zavras, Tudor H. Thomas, Alireza Ariafard,* George N. 26 27 Khairallah,* a-c Allan J. Canty, d Brian F. Yates d and Richard A. J. O’Hair* a-c 28 29 30 31 32 (a) School of Chemistry, The University of Melbourne, Victoria 3010, Australia. Fax: +61 3 33 34 9347 5180; Tel: +61 3 8344 2452; Email: [email protected] 35 36 (b) Bio21 Institute of Molecular Science and Biotechnology, The University of Melbourne, 37 38 Melbourne, Victoria 3010, Australia. 39 40 (c) ARC Centre of Excellence for Free Radical Chemistry and Biotechnology. 41 42 43 (d) School of Physical Sciences University of Tasmania Private Bag 75, Hobart, Tasmania 44 45 7001, Australia. 46 47 48 49 * Corresponding Authors: 50 51 52 E mails: [email protected]; [email protected]; [email protected] 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 1 Submitted to Organometallics Page 2 of 23
1 2 3 4 ABSTRACT: 5 6 Gas phase ion molecule reaction experiments, theoretical kinetic modelling and 7 8 9 computational chemistry were used to examine the role of a second gold center in promoting
10 + 11 allylic arylation. Geminally di aurated complexes, [(L)nAu 2Ph] are demonstrated to 12 13 participate in C C bond formation reactions with allyl halides, CH 2=CHCH 2X (X = Cl, Br 14 15 + and I) given a favorable phosphine ligand architecture. Thus while [(Ph 3P)2Au 2Ph] , 1 is 16 17 unreactive towards the allyl halides, [(dppm)Au Ph]+, 2 (where dppm = 18 2 19 20 bis(diphenylphosphino)methane, (Ph 2P)2CH 2) reacts via C C bond coupling to produce 21 + 22 [(dppm)Au 2X] . The reaction kinetics (efficiencies) follows the expected leaving group 23 24 ability, X = : I (58%) > Br (2%) > Cl (0.02%). 25 26 DFT calculations were carried out to examine potential mechanism for the C C bond 27 28 29 coupling reactions of 2 with each of the three allyl halides. The most favorable mechanism 30 31 for C C bond coupling of 2 requires the active participation of both gold centres in a redox 32 33 couple mechanism wherein the allylic halide oxidatively adds across the gold centres to form 34 35 a Au IAu III complex lacking a Au···Au interaction, followed by intramolecular reductive 36 37 elimination of allyl benzene from the Au III centre. 38 39 40 41 42 43 TITLE RUNNING HEAD: Gold promoted allylic arylation 44 45 46 47 KEYWORDS: geminally diaurated aryl gold complex, bisphosphine ligand, metal 48 49 carboxylates, mass spectrometry, decarboxylation, ion molecule reactions, allyl halides, DFT 50 calculation. 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 2 Page 3 of 23 Submitted to Organometallics
1 2 3 Introduction: 4 5 6 There has been a flurry of interest in developing homogeneous gold catalyzed carbon 7 8 carbon bond forming reactions. This interest arises from the fact that gold(I) complexes have 9 10 a number of synthetically useful modes of reactivity including oxidation/reduction chemistry, 11 12 [1] 13 significant π Lewis acid character, and the ability to insert into C H bonds. In addition to 14 15 the chemistry available at a single gold center new modes of reactivity are now being 16 17 uncovered by employing digold catalysts.[2,3] A recent example is the gold catalyzed 18 19 allylation of arylboronic acids (Scheme 1 A) for which Levin and Toste have suggested a 20 21 22 mechanism involving oxidative addition across both gold atoms of a digold complex to form 23 [2g] 24 a Au(II) Au(II) intermediate. 25 26 (A) Cl Cl (B) 27 Au Au AuPPh3 28 Ph P P Ph AuPPh Ph N Ph Fe 3 29 iPr 30 31 PhB(OH)2 Cs2CO3 Nesmeyanov 32 33 Ph Ph 34 Au Au Ph P P Ph 35 Ph N Ph 36 iPr 37 Br Au Au 38 PrI IPr 39 Hashmi Ph Ph 40 II II 41 Au Au Br Ph P P Ph AuL 42 Ph N Ph 43 iPr AuL
44 Gray (L = PPh3); Nolan (L = IPr ) 45
46 Cl Cl Ph 47 Au Au Ph Au P P 48 Ph P Ph (CH2)n Ph N Ph 49 Au P iPr Ph 50 Ph 51 Ph Ph Ph 52 biaryl side product Schmidbaur (n = 2 - 4) 53 54 Scheme 1. Examples of di gold complexes: (A) proposed mechanism for allylic arylation 55 [2g] 56 involving digold intermediates; (B) gem diaurated complexes (IPr = 1,3 bis(2,6 diisopropylphenyl) imidazol 2 ylidene). 57 58 59 60 ACS Paragon Plus Environment 3 Submitted to Organometallics Page 4 of 23
1 2 3 Historically, bimetallic catalysis has been a particularly fruitful area of research, albeit 4 5 not often well understood. [4] Thus we were interested in further exploring the reactivity of 6 7 digold systems. Based on our earlier observation of allylic alkylation reactions promoted by 8 9 [5] 10 cationic coinage metal clusters, we speculated that cationic geminally diaurated complexes 11 [6] 12 similar to the ones shown in (Scheme 1B) might be capable of promoting allylic arylation 13 14 reactions. We were further encouraged by the fact that in recent years cationic gold 15 16 complexes supported by phosphine or N heterocyclic carbene ligands have gained attention 17 18 [7] 19 as potential catalytic intermediates in the functionalization of C C multiple bonds. 20 21 Nesmeyanov et al. reported the synthesis, structural characterization and limited 22 23 reactivity of gem diaurated complexes, [6a] but these fascinating species were largely ignored 24 25 for 30 years until similar intermediates were detected in homogeneous gold catalyzed 26 27 reactions. Since then, several gem diaurated aryls have been isolated and structurally 28 29 [8] 30 characterized. While gem diaurated aryl and vinyl species have been implicated repeatedly 31 [6b,9] 32 as off cycle reservoirs in gold catalyzed C C bond forming reactions, almost nothing is 33 34 known about the inherent reactivity of gem diaurated complexes towards organic substrates. 35 36 Indeed, the only mode of reactivity that has been reported is the “SN2 like” ligand exchange 37 38 [6a,10] n 39 at gold (eq. 1). Here we use multistage mass spectrometry (MS ) techniques to examine 40 41 whether cationic gem diaurated complexes themselves can directly facilitate C C bond 42 43 forming reactions in the gas phase. Ion molecule reactions of the gem diaurated complexes 44 45 + [(L)nAu 2Ph] 1 (n = 2, L = Ph 3P) and 2 (n = 1, L = dppm = bis(diphenylphosphino)methane, 46 47 (Ph P) CH ) with allyl halides, CH =CHCH X (X = Cl, Br and I) were examined.[11] A key 48 2 2 2 2 2 49 50 finding is that the ligand plays a pivotal role in promoting C C bond coupling. Thus while 1 51 52 does not undergo C C bond coupling with the allyl halides, 2 does. The origin of this ligand 53 54 effect is examined through computational chemistry methods. 55 56 [(Ph P) Au Ar] + + Nu → Ph PAuAr + [Ph PAuNu] + (1) 57 3 2 2 3 3 58 59 60 ACS Paragon Plus Environment 4 Page 5 of 23 Submitted to Organometallics
1 2 3 Experimental: 4 5 6 Reagents 7 8 Reagents were purchased from Aldrich and used without further purification. Methanol and 9 10 chloroform were purchased from RCI Labscan. Chloro(triphenylphosphine)gold(I) was 11 12 13 purchased from STREM. Triphenylphosphine was purchased from Riedel de Haën. Silver(I) 14 15 benzenesulfinate was prepared by a literature method. [12] 16 17 18 19 Mass spectrometry experiments 20 21 22 All mass spectrometry experiments were carried out using a Finnigan LTQ FT hybrid linear 23 24 ion trap (Finnigan, Bremen, Germany) fitted with the standard factory electrospray ionization 25 26 source and modified as described previously to allow the introduction of gaseous neutral 27 28 reagents into the ion trap. [13] A previous study has demonstrated that ions in the linear ion 29 30 [14] 31 trap are essentially quasi thermalized to room temperature. Accurate mass measurements 32 33 using the FT ICR allowed unambiguous assignment of the observed signals. 34 35 36 37 + Preparation and isolation of complexes 1 and 3: [(Ph 3P) 2Au 2Ph] and 38 39 [(Ph P) Au (O SPh)] +. 40 3 2 2 2 41 42 Silver(I) benzenesulfinate (50 µmol in 5 mL 1:1 MeOH/CHCl 3) was added to a solution of 43 44 chloro(triphenylphosphine)gold(I) (10 µmol in 5 mL 1:1 MeOH/CHCl 3) and the solution was 45 46 made up to 20 mL with 1:1 MeOH/CHCl 3 (v/v). The solution was stirred for 5 minutes before 47 48 being transferred via syringe to the electrospray source at 3 µL min 1. Typical electrospray 49 50 51 source conditions were: Positive ion mode, Sheath Gas = 5 arbitrary units, Auxiliary Gas = 0 52 53 arbitrary units, Sweep Gas = 0 arbitrary units, Spray Voltage = 4.5 kV, Capillary Temp. = 54 55 300 °C, Capillary Voltage = 15 V, Tube Lens Voltage = 65 V. The most abundant peak of the 56 57 58 59 60 ACS Paragon Plus Environment 5 Submitted to Organometallics Page 6 of 23
1 2 3 isotope pattern ( m/z 995 for 1 and m/z 1059 for 3) was isolated within the ion trap for further 4 5 investigation. 6 7 8 9 + 10 Preparation and isolation of complexes 2 and 4: [(dppm)Au 2Ph] and 11 + 12 [(dppm)Au 2(O2CPh)] . 13 14 Dppm (10 µmol in 3 mL 1:1 MeOH/CHCl 3) was added to a mixture of benzoic acid (0.2 15 16 mmol in 2 mL MeOH) and chloro(triphenylphosphine)gold(I) (10 µmol in 3 mL 1:1 17 18 19 MeOH/CHCl 3) and the solution was made up to 20 mL with 1:1 MeOH/CHCl 3 (v/v). The 20 21 solutions were allowed to sit for at least 15 minutes before being transferred via syringe to the 22 23 electrospray source at 5 10 µL min 1. Typical electrospray source conditions were: Positive 24 25 ion mode, Sheath Gas = 10 arbitrary units, Auxiliary Gas = 5 arbitrary units, Sweep Gas = 0 26 27 arbitrary units, Spray Voltage = 6.00 kV, Capillary Temp. = 250 °C, Capillary Voltage = 49 28 29 30 V, Tube Lens Voltage = 205 V. The most abundant peak of the isotope pattern ( m/z 855 for 2 31 32 and m/z 899 for 4) was isolated within the ion trap for further investigation. 33 34 35 36 Preparation and isolation of complex 5: [Au(Ph)(TPPMS)] 37 38 39 Gold(III) acetate (5.7 µmol; 2 mg) was added to [(Ph 3P)2N)][TPPMS] (TPPMS = 40 41 triphenylphosphine monosulfonate; 0.5 µmol; 0.5 mL, 1 mM in MeOH) and benzoic acid (10 42 43 µmol; 0.5 mL, 20 mM in MeOH). The solution was transferred immediately via syringe to 44 45 the electrospray source at 5 10 µL min 1. Typical electrospray source conditions were: 46 47 Negative ion mode, Sheath Gas = 10 arbitrary units, Auxiliary Gas = 0 arbitrary units, Sweep 48 49 50 Gas = 0 arbitrary units, Spray Voltage = 4.00 kV, Capillary Temp. = 200°C, Capillary 51 52 Voltage = 45 V, Tube Lens Voltage = 250 V. The most abundant peak of the isotope pattern 53 54 (m/z 615) was isolated within the ion trap for further investigation. 55 56 57 58 59 60 ACS Paragon Plus Environment 6 Page 7 of 23 Submitted to Organometallics
1 2 3 Ion molecule reactions and rate constant determinations 4 5 The neutral allyl halide reagent was infused at various concentrations into the ion trap via a 6 7 heated modification in the helium inlet line. [13] There, it was allowed to interact with the 8 9 10 reactant ion. Rates of reaction between the neutral reagent and the reactant ion were 11 12 measured by varying the time delay between isolation of the reactive ion within the ion trap 13 14 containing the neutral reagent and mass analysis of the contents of the ion trap (referred to as 15 16 ‘reaction delay’, RD). The decay of the reactant ion was monitored over at least 7 values of 17 18 19 RD. 20 21 The intensity of the reactant ion at each RD was determined by integration of its ion count. 22 23 Relative pseudo first order rate constants were obtained from the slope of plots of lne(signal 24 25 intensity) vs. RD plot. Absolute pseudo first order rate constants were estimated by dividing 26 27 the relative rate constant by the calculated concentration of allyl halide in the ion trap. The 28 29 30 rate constants reported are an average of three independent experiments conducted on 31 32 separate days. Theoretical collision rates were calculated for each reaction via the Average 33 34 Dipole Orientation theory of Su and Bowers using the COLRATE program [15], and ‘reaction 35 36 efficiency’ (φ) values are reported as: (experimental absolute rate)/(theoretical collision rate) 37 38 39 × 100%. 40 41 42 43 Computational details . [16 21] Gaussian 09 [16] was used to fully optimize all the structures 44 45 reported in this paper at the M06 level of density functional theory (DFT). [17] The effective 46 47 core potential of Hay and Wadt with a double ξ valence basis set (LANL2DZ) was chosen to 48 49 50 describe Au, Cl, Br, I, and P. The 6 31G(d) basis set was used for other atoms. Polarization 51 52 functions were also added for Au (ξ f = 1.05), Cl (ξ d = 0.64), Br (ξ d = 0.428), I (ξ d = 0.289), 53 54 and P (ξ d = 0.387). This basis set combination will be referred to as BS1. Frequency 55 56 calculations were carried out at the same level of theory as those for the structural 57 58 59 60 ACS Paragon Plus Environment 7 Submitted to Organometallics Page 8 of 23
1 2 3 optimization. Transition structures were located using the Berny algorithm. Intrinsic reaction 4 5 coordinate (IRC) calculations were used to confirm the connectivity between transition 6 7 structures and minima. To further refine the energies obtained from the M06/BS1 8 9 10 calculations, we carried out single point energy calculations for all of the structures with a 11 12 larger basis set (BS2) at the B3LYP D3BJ level of theory. BS2 utilizes the def2 TZVP basis 13 14 set on all atoms. Effective core potentials including scalar relativistic effects were used for 15 16 gold and iodine atoms. The B3LYP D3BJ calculations were used to overcome the deficiency 17 18 19 of the M06 level in incorporating long range correlation for dispersion forces. To estimate the 20 21 corresponding enthalpy, H, and Gibbs energies, G, the corrections were calculated at the 22 23 M06/BS1 level and finally added to the single point energies. We have used the corrected 24 25 enthalpy and Gibbs free energies obtained from the B3LYP D3BJ/BS2//M06/BS1 26 27 calculations throughout the paper unless otherwise stated. 28 29 30 31 32 33 34 35 36 Results and Discussion 37 38 39 40 Gas phase formation and DFT calculated structures of phosphine ligated 41 42 gem diaurated aryl complexes 43 44 + 45 1 (m/z 995) can be formed via desulfination of [(Ph 3P) 2Au 2(O 2SPh)] , 3 (eq. 2, X = S, 46 47 n = 2, L = Ph P) under collision induced dissociation (CID) conditions [22] within the ion trap 48 3 49 50 mass spectrometer (Supporting Information Figures S1 and S2). 2 (m/z 855) can be formed 51 + 52 via decarboxylation of [(dppm)Au 2(O 2CPh)] , 4 (eq. 2, X = C, n = 1, L = dppm) under CID 53 54 conditions [23] within an ion trap mass spectrometer, or it can be isolated directly from 55 56 electrospray ionization (ESI) of a solution containing AuClPPh 3, dppm and benzoic acid in 57 58 59 60 ACS Paragon Plus Environment 8 Page 9 of 23 Submitted to Organometallics
1 2 3 methanol/chloroform (Supporting Information Figures S3 and S4). The latter method is most 4 5 efficient. High resolution mass spectrometry experiments show that this is the monocation 6 7 and not a macrocylic dication, {[(dppm)Au Ph]+} , 6. 8 2 2 9 10 11 12 [(L) Au (O XPh)] + → [(L) Au Ph] + + XO (2) 13 n 2 2 n 2 2 14 15 16 17 Ph Ph 2 18 Ph Ph 19 P P 20 Au Au 21 22 Au Au 23 P P 24 Ph Ph 25 Ph Ph 26 27 6 28 29 30 DFT calculations were performed to determine the lowest energy isomers of the 31 32 geminal digold complex in the gas phase. As depicted in Figure 1a, the geminal complex 33 34 exists as two isomers 2 and 2', which differ in their coordination modes. The lower energy 35 36 37 isomer ( 2) can be interpreted in terms closely related to that by Weber and Gagné in 38 [24] 39 describing the bonding in a binuclear gold vinyl complex. Owing to the parallel orbitals at 40 41 both gold centres, the orientations of the phenyl bridge in both 2 and 2' are skewed.[25] In 42 43 2′(σ diaurated isomer) the Au C(Ph) σ orbital is involved in interaction with the second gold 44 45 empty orbital, while in 2(σ π diaurated isomer), both the Au C(Ph) σ orbital and the phenyl π 46 47 [7] 48 orbital interact simultaneously with the empty orbital (Figure 1b). The presence of 49 50 interaction between the Au C(Ph) σ orbital and the second gold empty orbital in 2 is 51 52 indicated by NBO analysis. The second order perturbation energy, E(2), for 2′ shows an 53 54 energy of 33.5 kcal/mol for interaction of the Au C(Ph) σ bond with the second gold empty 55 56 57 orbital. This interaction becomes weaker in 2 with an E(2) value of 20.0 kcal/mol due to the 58 59 60 ACS Paragon Plus Environment 9 Submitted to Organometallics Page 10 of 23
1 2 3 contribution of the π bond in the structure of 2. The weaker interaction between the Au C(Ph) 4 5 σ orbital and the second gold orbital in 2 results in a longer Au Au distance of 2.925 Å with a 6 7 smaller Wiberg bond index of 0.169. The Au Au distance in 2′ is calculated to be shorter 8 9 10 (2.756 Å) with a larger Wiberg bond index (0.205). As shown in Figure 1b, the geometry of 11 12 dppm results in a larger orbital overlap for A and a smaller orbital overlap for B. The lower 13 14 stability of isomer 2' compared to 2 can be attributed to the smaller orbital overlap between 15 16 the Au C(Ph) σ orbital and the second gold empty orbital. 17 18 19 (a) (b) 20 21 empty empty 22 orbital π−orbital orbital Au Au Au Au σ−orbital 23 Au Au 24 Au Au P P Ph Ph P P 25 Ph Ph P Ph Ph Ph Ph Ph Ph P P P 26 2 2' Ph Ph Ph Ph 27 Ph Ph 0.0 (0.0) 2.1 (0.9) A B 28 29 30 Figure 1. Schematics showing (a) the equilibrium between two isomers 2 and 2′ for the 31 32 geminal digold complex (the relative energies ∆G (∆H) in kcal/mol were obtained from 33 B3LYP D3BJ/def2 TZVP//M06/LANL2DZ* 6 31G* calculations), and (b) orbital 34 representations depicting the interaction of the second gold empty orbital with a π orbital of 35 the phenyl ring ( A) and with the Au C(Ph) σ orbital ( B). 36 37 38 The CID fragmentation patterns of the geminal digold complexes are entirely 39 40 consistent with their calculated structures (see Supporting Information Figures S5 7 for the 41 42 accurate mass analysis, CID spectra of 1 and 2, and associated reaction equations). 43 44 45 46 47 48 Gas phase ion molecule reactions of allyl halides with phosphine ligated 49 50 51 gem diaurated aryl complexes 52 53 54 To investigate the inherent reactivity of the geminal digold complexes 1 and 2 in 55 56 allylic arylation, we examined their gas phase reactivity towards the allyl halides, C 3H5X (X= 57 58 Cl, Br and I). In these experiments, 1 and 2 were generated in a linear ion trap by one of the 59 60 ACS Paragon Plus Environment 10 Page 11 of 23 Submitted to Organometallics
1 2 3 two methods described above, purified in the gas phase via mass selection and then allowed 4 5 to interact with a known concentration of gaseous allyl halide introduced into the ion trap [10] 6 7 for a specified length of time (the reaction delay, RD), after which any resulting products or 8 9 10 remaining reactant were ejected from the ion trap, detected and quantified. 11 12 These experiments highlight that both the ligand and the substrate play a role in the 13 14 + 15 reactions of ligated gem diaurated aryls with allyl halides. [(Ph 3P) 2Au2Ph] m/z 995 ( 1) is 16 17 completely unreactive towards all allyl halides C 3H5X (X = Cl, Br, I, eq. 3), even at 10 18 19 seconds, which is the longest reaction time we can probe (data not shown). In contrast, an 20 21 examination of Figure 2 shows that ion molecule reactions (IMRs) between mass selected 22 23 [(dppm)Au Ph]+ (2, m/z 855) and allyl halides results in allylic alkylation to yield the ionic 24 2 25 + 26 product, [(dppm)Au 2X] (eq. 4). 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 11 Submitted to Organometallics Page 12 of 23
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 2 35 Figure 2. MS spectra showing IMRs between 2 and C 3H5X, for X =: (a) Cl (concentration of 9 3 9 3 36 9 × 10 molecules/cm , RD = 10,000 ms), (b) Br (concentration of 6 × 10 molecules/cm , 37 RD = 10,000 ms), (c) I (concentration of 6 × 10 9 molecules/cm 3, RD = 1,000 ms). The mass 38 selected ion is marked with an asterisk. 39 40 41 + [(Ph 3P) 2Au 2Ph] + C3H5X → no reaction (3) 42 43 [(dppm)Au Ph] + + C H X → [(dppm)Au X] + + C H Ph (4) 44 2 3 5 2 3 5 45 46 47 To quantify the kinetic order of reactivity, rate measurements were carried out and a 48 49 comparison of the rate constants and reaction efficiencies (Table 1) highlights that the 50 51 52 reactivity of the substrates follow their expected leaving group ability, X = : I (58%) > Br 53 54 (2%) > Cl (0.02%). 55 56 57 58 59 60 ACS Paragon Plus Environment 12 Page 13 of 23 Submitted to Organometallics
1 2 3 Table 1. Gas phase kinetics associated with the ion molecule reactions of mono and bi 4 nuclear gold ions with C 3H5X, X = Cl, Br, I. 5 [a][b] [c] 6 Ion Molecule kexptl φ 7 + [d,e] -13 [(dppm)Au Ph] C H Cl 2.1 × 10 0.02 8 2 3 5 + -11 9 [(dppm)Au 2Ph] C3H5Br 1.9 × 10 2.0 10 + -10 [(dppm)Au 2Ph] C3H5I 5.0 × 10 58 11 - [d,f] -14 12 [(TPPMS)AuPh] C3H5I 2.2 × 10 <0.001 13 + [(Ph 3P) 2Au 2Ph] C3H5I No reaction 0 14 3 1 1 15 [a] In units of cm molecule s . [b] Experimental rates for consumption of reactant ion. [c] 16 Reaction efficiency (φ) = ( kexpt /ktheor ) × 100%. The theoretical ion molecule collision rate was 17 obtained from average dipole orientation (ADO) theory, calculated using the Colrate 18 program. [15b] [d] Reaction rate too slow to measure accurately. A maximum possible rate was 19 calculated by estimating no more than: [e] 2% product formation under the reaction 20 conditions; [f] 1% product formation under the reaction conditions. 21 22 23 24 Previous experiments have shown that the mononuclear organo gold(I) anion 25 26 [11c] 27 [CH 3AuCH 3] reacts only sluggishly with allyl iodide (eq. 5, reaction efficiency of 0.07%) 28 + [26] 29 but that the monoligated gold cation, [Ph 3PAu] , reversibly binds allyliodide (eq. 6). Thus, 30 31 both the second gold centre and the nature of the ligand in 2 play a crucial role in C C bond 32 33 coupling reactions with the allyl halides. To test the role of a single gold centre with a more 34 35 36 relevant mononuclear phenyl gold phosphine complex, the fixed charge phosphine complex 37 [11e] 38 [(TPPMS)AuPh] (5) was “synthesized” in the gas phase as reported previously, mass 39 40 selected, and exposed to allyl iodide. While a very small signal of the ionic product 41 42 [(TPPMS)AuI] consistent with C C bond coupling (eq. 7) is observed (Supporting 43 44 45 Information, Figure S8) this is an extremely slow reaction (reaction efficiency <0.001%, 46 47 Table 1); hence, the presence of a second gold centre plays a key role in promoting C C 48 49 bond formation. 50 51 52 53 54 [CH AuCH ] + C H I → I + [(CH ) Au(C H )] (5) 55 3 3 3 5 3 2 3 5 56 + + 57 [Ph 3PAu] + C3H5I → [Ph 3PAu(C3H5I)] (6) 58 59 60 ACS Paragon Plus Environment 13 Submitted to Organometallics Page 14 of 23
1 2 3 [(TPPMS)AuPh] + C3H5I → [(TPPMS)AuI] + C3H5Ph (7) 4 5 6 7 8 9 10 11 Mechanism of allylic arylation 12 13 14 Having established the inherent reactivity of the geminal digold complex towards the 15 16 allyl halides we next turned to DFT calculations to provide a feasible mechanism of this 17 18 allylic arylation reaction. Two different classes of mechanisms were considered: reactions 19 20 which are isohypsic with respect to the oxidation state of gold; [27] and reactions that involve 21 22 23 redox changes to one of the gold atoms. Based on a comprehensive DFT survey of possible 24 25 C C bond forming pathways, four key potential mechanisms emerged, and these are 26 27 illustrated in Figure 3 for allyliodide and depicted for the other allylic substrates in the SI 28 29 (Figures S9, S10). 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 14 Page 15 of 23 Submitted to Organometallics
1 2 3 4 (a) I 5 Au Au Au Au I 6 P P Ph Ph Au Au Ph P P Ph 7 Ph Ph Ph Ph P P TS'7-8_I 2 Ph Ph 8 0.0 (0.0) Ph Ph -0.9 (-13.5) 9 TS7-8_I 10 -3.3 (-15.4) 11 I I I 12 Au Au 13 Ph P P Ph Ph Ph 14 Ph 9_I -45.9 (-45.2) 15 -12.6 (-25.0) -12.6 (-25.0)
16 47.7 (-60.9) 47.7 (-60.9) 17 I I 18 I Au Au I I Au Au 19 Au Au P P P Ph P Ph Ph P P Ph Ph Ph 20 Ph Ph Ph Ph Ph 8_I Ph 7_I 8_I 21 22 pathway (i) pathway (ii) 23 concerted isohypsic mechanism 24 25 (b) 26 27 I I 28 Au Au Au Au 29 Ph P P Ph P P Ph Ph Au Ph Ph Au 30 Ph Ph TS7-10_I Ph P P Ph I 3.4 (-9.9) Ph 31 TS11-8_I Ph 2 I Au 32 0.3 (-13.2) 0.0 (0.0) Au 33 Au Au Ph P P Ph -0.5 (-12.5) Ph Ph Ph P P Ph 34 TS _I I I Ph Ph 12-8 35 TS7-12_I -5.7 (-19.1) I -5.1 (-16.4) I -7.6 (-21.4) Au Au 36 I I Au Au I P 37 Ph P Ph P P Ph Ph III I Ph Ph 38 Ph Ph Au Au 9_I 11_I Ph Ph P P Ph -12.1 (-23.7) 39 -12.6 (-25.0) -12.6 (-25.0) -45.9 (-45.2) Ph Ph 40 10_I -47.7 (-60.9) I -47.7 (-60.9) 41 I I III I Au Au I Au Au 42 I I Au Au Au Au Ph P P Ph 43 Ph P P Ph P P Ph Ph Ph Ph Ph Ph 12_I Ph P P Ph 44 8_I Ph Ph 7_I Ph Ph 45 8_I 46 pathway (iii) pathway (iv) redox couple mechanism 47 48 Figure 3. Four competing mechanisms for C C bond coupling between 2 and allyl iodide to 49 + 50 yield allyl benzene and [(dppm) Au 2I] calculated at the B3LYP D3BJ/def2 51 TZVP//M06/LANL2DZ* 6 31G* level of theory, shown as diagrams comparing the 52 energetics of the various mechanism. (a) illustrates concerted isohypsic reactions involving 53 C C bond formation (i) at the α carbon via TS’ 7 8_I , and (ii) at the γ carbon via TS 7 8_I . (b) 54 shows the redox couple pathways involving allyl iodide oxidative addition (iii) at one gold 55 56 center, and (iv) across both gold centers as the preferred mechanism. The relative Gibbs and 57 enthalpy energies (in parentheses) are given in kcal/mol. 58 59 60 ACS Paragon Plus Environment 15 Submitted to Organometallics Page 16 of 23
1 2 3 In the higher energy concerted isohypsic mechanisms in which the gold core remains 4 5 in the Au IAu I state throughout, (i) and (ii), C C bond formation occurs at the α and γ carbon 6 7 [28] 8 atoms by S N2 and S N2' processes, respectively. As shown in Figure 3, the reaction begins 9 10 with the coordination of allyl iodide to 2 to yield 7_I in which the phenyl group is no longer a 11 12 bridging ligand. We were unable to find a transition state between 2 and 7_I and a relaxed 13 14 PES scan between the two structures shows that it is an entirely exothermic pathway (see SI 15 16 Figure S11). From 7_I , formation of allyl benzene is achieved through TS _I (pathway (ii)), 17 7 8 18 19 which involves a rocking motion of the allyl fragment between the iodide and the ipso carbon 20 21 of the phenyl group. This forms the product complex, 8_I , in which allyl benzene is 22 23 coordinated to a single gold atom through a phenyl π bond. A related concerted isohypsic 24 25 mechanism in which C C bond formation occurs at the α carbon of the allyl halide has a 26 27 28 higher barrier via transition structure TS′7 8_I . 29 30 The lowest energy pathway for all of the allylic substrates is the redox couple 31 32 33 mechanism (pathway (iv)). In this mechanism, the reaction starts with direct oxidative 34 35 addition of allyl iodide across the gold(I) centres via transition structure TS 7 12 _I to give a 36 37 Au IAu III complex ( 12_I ) followed by reductive elimination at the Au III centre leading directly 38 39 to the products. An alternative higher energy redox couple mechanism (iii) involves 40 41 intramolecular oxidative addition at one Au I center in 7_I to give 10_I followed by reductive 42 43 44 elimination across both gold centres. Oxidative addition through TS 7 12 _I is an easier process 45 46 due to the electron donating phenyl group which makes gold more prone to oxidation. This 47 48 interpretation is further supported by the higher stability of the Au IAu III complex 12_I 49 50 compared to 10_I (~7.0 kcal/mol). 51 52 53 As mentioned above, for all of the allylic substrates, the pathway involving C C bond 54 55 formation via transition structure TS 12 8_X is the most favourable (mechanism (iv) in Figure 56 57 3). The relative energies of all the stationary points on this pathway are summarized in Table 58 59 60 ACS Paragon Plus Environment 16 Page 17 of 23 Submitted to Organometallics
1 2 3 2. We note that in comparing the reactivity of allyl X (X = Cl, Br, I) (Table 2), the reaction 4 5 energies for coordination of allyl X to form 7_X follow the order I > Br > Cl, i.e. 12.6, 8.5, 6 7 5.1 kcal/mol, respectively. This is consistent with the greater ability of iodine as an electron 8 9 [29] 10 donor, resulting from the higher energy of iodine lone pairs. Although the most favourable 11 12 interaction in the initial adduct formation is that of the iodide with a gold centre, the 13 14 reactivity trend observed results from energies of TS 12 8_X relative to 2. Indeed, for the most 15 16 reactive substrate (X= I), the DFT manifold lies entirely below 2, in contrast to X = Cl where 17 18 19 TS 12 8_Cl lies slightly above 2, and for which there is negligible reaction detected by mass 20 21 spectrometry. These results suggest that the higher reactivity of the adduct 7_I compared to 22 23 7_Br and 7_Cl can be traced back to its higher stability. 24 25 26 27 28 Table 2. DFT predicted Gibbs relative energies for the reaction of 2 with C 3H5X (X=Cl, Br, 29 + I) to yield allyl benzene and [(dppm)Au 2X] via the redox couple mechanism reaction 30 III 31 involving C C bond formation at a Au centre via TS 12 8_X (mechanism (iv) in Figure 3). 32 [a] Structure C H I C H Br C H Cl 33 3 5 3 5 3 5 34 2 0.0 0.0 0.0 35 7_X -12.6 -8.5 -5.1 36 37 TS 7-12 _X -7.6 -4.1 0.0 38 12_X -12.1 -10.2 -5.9 39 40 TS 12-8_X -5.7 -2.3 0.7 41 9_X -45.9 -39.4 -33.8 42 43 [a] Structures are calculated at B3LYP D3BJ/def2 TZVP//M06/LANL2DZ* 6 31g* level. 44 Energies are in kcal/mol relative to respective separated reactants. 45 46 47 48 As noted above, the related cation containing Ph 3P as a non bridging ligand in 49 50 + [(Ph 3P)2Au 2Ph] (1) is unreactive towards the allyl halides examined in mass spectrometric 51 52 studies. Consistent with this observation, we find in DFT computation that the analogous 53 54 + 55 reaction to that of [(dppm)Au 2Ph] (2) with allyl iodide is endergonic (eq. 8). 56 57 58 59 60 ACS Paragon Plus Environment 17 Submitted to Organometallics Page 18 of 23
1 2 3 4 Grxn = 24.5 I 5 I 6 + kcal/mol Au + Au (8) 7 Au Au 8 Ph P PPh PPh3 PPh3 9 3 3 10 1 13 14 11 12 13 14 15 16 Conclusions 17 18 19 20 Mass spectrometry has emerged as an important tool in monitoring metal catalyzed 21 22 reactions [30], resolving mechanistic controversies in gold chemistry [31] and directing the 23 24 [32] 25 synthesis of new coinage metal compounds . Here we have shown that multistage mass 26 27 spectrometry experiments can be used to discover new C C bond forming reactions. In 28 29 particular we have demonstrated that gem diaurated aryls can participate in allylic arylation 30 31 reactions in the gas phase when the gold(I) centres are bridged by 32 33 34 bis(diphenylphosphino)methane. While descriptors have been developed to better understand 35 [33] 36 the role of chelating P,P donor ligands in the reactivity of mononuclear metal complexes, 37 38 a full understanding of how these ligands influence reactivity in binuclear complexes is a 39 40 subject of ongoing research in the organometallic and catalysis community. [34] Our DFT 41 42 calculations show that the dppm ligand plays a key role in allowing the gem diaurated aryl to 43 44 45 act as a “molecular hinge”. Indeed the ring strain induced by the dppm partially “pries open” 46 47 a reactive site, which further “opens” upon approach of the allyl halide (Figure S11). The 48 49 dppm ligand also controls the lowest energy pathway, which involves regiospecific bond 50 51 formation at the γ carbon of the allyl halide. Whether this effect can be translated to solution 52 53 54 remains an open question. The ring strain induced by the dppm may cause the preferential 55 56 57 58 59 60 ACS Paragon Plus Environment 18 Page 19 of 23 Submitted to Organometallics
1 2 3 formation of a macrocyclic complex 6, or alternatively a solvent molecule, S, might open up 4 5 the gem diaurate to form the binuclear complex 15 . 6 7 8 9 10 11 12 S 13 Au Au 14 15 P P 16 Ph Ph Ph 17 Ph 18 15 19 20 21 22 23 Acknowledgments 24 25 26 We thank the ARC for financial support via grant DP110103844 (to RAJO and 27 28 GNK), DP1096134 (to GNK) and DP150101388 (to RAJO and AJC). The authors gratefully 29 30 acknowledge the support of the University of Tasmania for a Visiting Scholarship (to AA), 31 32 and the generous allocation of computing time from the Victorian Partnership for Advanced 33 34 35 Computing (VPAC) Facility. 36 37 38 39 Supporting Information 40 41 42 Additional supporting information, which includes: the Cartesian coordinates of all 43 44 species examined; a full citation of reference 16; mass spectra showing the formation of 1 – 4 45 46 and the fragmentation reactions of 1 and 2; formation and ion molecule reaction of 47 48 [(TPPMS)AuPh] , 5, with C3H5I; DFT calculated potential energy diagrams for the reactions 49 50 of 2 with C H Br and C H Cl; relaxed PES scan for reaction of allyl iodide with 2. This 51 3 5 3 5 52 53 material is available free of charge via the Internet at http://pubs.acs.org . 54 55 56 57 58 59 60 ACS Paragon Plus Environment 19 Submitted to Organometallics Page 20 of 23
1 2 3 References and footnotes 4 5 6 (1) (a) Wegner, H. A.; Auzias, M. Angew. Chem. Int. Ed. 2011 , 50 , 8236 8247; (b) For a 7 recent comprehensive overview of modern gold catalysis: Modern Gold Catalyzed 8 Synthesis ; A. S. K. Hashmi, A. S. K., Toste, F. D., Eds.; WILEY VCH: Weinheim, 9 2012. 10 (2) (a) Bandini, M.; Eichholzer, A. Angew. Chem. Int. Ed. 2009 , 48 , 9533 9537; (b) 11 Gómez Suárez, A.; S. Nolan, S. P. Angew. Chem. Int. Ed. 2012 , 51 , 8156 8159; (c) 12 Roithová, J.; Janková, Š.; Jašíková, L.; Váňa, J.; Hybelbauerová, S. Angew. Chem. Int. 13 Ed. 2012 , 51 , 8378 8382; (d) Hashmi, A. S. K.; Lauterbach, T.; Nösel, P.; Vilhelmsen, 14 M. H.; Rudolph, M.; Rominger, F. Chem. Eur. J. 2013 , 19 , 1058 1065; (e) Briones, J. 15 16 F.; Davies, H. M. L. J. Am. Chem. Soc. 2012 , 134 , 11916 11919; (f) Cao, Z.; Wang, X.; 17 Tan, C.; Zhao, X.; Zhou, J.; Ding, K. J. Am. Chem. Soc. 2013 , 135 , 8197 8200; (g) 18 Levin, M. D.; Toste, F. D. Angew. Chem. Int. Ed. 2014 , 53 , 6211 6215; (h) Wieteck, 19 M.; Tokimizu, Y.; Rudolph, M.; Rominger, F.; Ohno, H.; Fujii, N.; Hashmi, A. S. K. 20 Chem. Eur. J. 2014 , 20 , 16331 16336; (i) For a recent review of dual gold catalysis see: 21 Hashmi, A. S. K. Acc. Chem. Res. , 2014 , 47 , 864 876. (i) Ito, S.; Nanko, M.; Mikami, 22 K. ChemCatChem , 2014 , 6, 2292 2297; (j) Barreiro, E. M.; Boltukhina, E. V.; White, 23 A. J. P.; Hii, K. K. (M.) Chem. Eur. J ., 2015 , 21 , 2686 2690. 24 (3) For key references on the role of the second metal centre in the gas phase reactions of 25 26 bimetallic ions see: (a) Koszinowski, K.; Schröder, D.; Schwarz, H. J. Am. Chem. Soc. 27 2003 , 125 , 3676 3677; (b) Koszinowski, K.; Schröder, D.; Schwarz, H. Angew. Chem. 28 Int. Ed. 2004 , 43 , 121 124; (c) Ma, J. B.; Wang, Z. C.; Schlangen, M.; He, S. G.; 29 Schwarz, H. Angew. Chem. Int. Ed. , 2012 , 51 , 5991 5994; (d) Waters, T.; O'Hair, R. A. 30 J.; Wedd, A. G. J. Am. Chem. Soc. , 2003 , 125 , 3384 3396; (e) Waters, T.; O'Hair, R. A. 31 J.; Wedd, A. G. Inorg. Chem. , 2005 , 44 , 3356 3366; (f) Khairallah, G. N.; da Silva, G.; 32 O’Hair R. A. J. Angew. Chem. Int. Ed. , 2014 , 53 , 10979 10983. 33 (4) Pérez Temprano, M. H.; Casares, J. A.; Espinet, P. Chem. Eur. J . 2012 , 18 , 1864 1884. 34 (5) (a) Khairallah, G. N.; O’Hair, R. A. J. Angew. Chem. Int. Ed . 2005 , 44 , 728 731; (b) 35 36 Khairallah, G. N.; Waters, T.; O’Hair, R. A. J. Dalton Trans. , 2009 , 2832 2836; (c) 37 Khairallah, G. N.; Williams, C. M.; Chow, S.; O’Hair, R. A. J. Dalton Trans. , 2013, 42 , 38 9462 9467 ; (d) Al Sharif, H.; Vikse, K. L.; Khairallah, G. N.; O’Hair, R. A. J. 39 Organometallics , 2013 , 32 , 5416 5427. 40 (6) (a) Nesmeyanov, A. N.; Perevalova, E. G.; Grandberg, K. I.; Lemenovskii, D. A.; 41 Baukova, T. V.; Afanassova, O. B. J. Organomet. Chem . 1974 , 65 , 131 144; (b) Weber, 42 D.; Tarselli, M. A.; Gagné, M. R. Angew. Chem. Int. Ed. 2009 , 48 , 5733 5736; (c) 43 Hashmi, A. S. K.; Braun, I.; Rudolph, M.; Rominger, F. Organometallics , 2012 , 31 , 44 644 661; (d) Heckler, J. E.; Zeller, M.; Hunter, A. D.; Gray, T. G. Angew. Chem. Int. 45 46 Ed . 2012 , 51 , 5924 5928; (e) Gómez Suárez, A.; Dupuy, S.; Slawin, A. M. Z.; Nolan, 47 S. P. Angew. Chem. Int. Ed . 2013 , 52 , 938 942. 48 (7) For a review of cationic two coordinate gold π complexes see: Brooner, R. E. M.; 49 Widenhoefer, R. A. Angew. Chem. Int. Ed . 2013 , 52 , 11714 11724. 50 (8) (a) Schmidbaur, H.; Inoguchi, Y.; Chem. Ber . 1980 , 113 , 1646 1653; (b) Usón, R.; 51 Laguna, A.; Fernández, E. J.; Media, A.; Jones, P. G. J. Organomet. Chem . 1988 , 350 , 52 129 138; (c) Porter, K. A.; Schier, A.; Schmidbaur, H. Organometallics , 2003 , 22 , 53 4922 4927. (d) Browne, A. R.; Deligonul, N.; Anderson, B. L.; Rheingold, A. L.; Gray, 54 T. G. Chem. Eur. J ., 2014 , 20 , 17552 17564. 55 56 (9) (a) Seidel, G.; Lehmann, C. W.; Fürstner, A. Angew. Chem. Int. Ed. 2010 , 49 , 8466 57 8470; (b) Brown, T. J.; Weber, D.; Gagné, M. R.; Widenhoefer, R. A. J. Am. Chem. 58 Soc. 2012 , 134 , 9134 9137; (c) Hashmi, A. S. K.; Braun, I.; Nösel, P.; Schädlich, J.; 59 60 ACS Paragon Plus Environment 20 Page 21 of 23 Submitted to Organometallics
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1 2 3 Am. Chem. Soc., 1971 , 93 , 6847 6854), its use here nicely highlights the emerging role 4 of gold as a Lewis acid. 5 (28) Hoseini, S. J.; Nasrabadi, H.; Nabavizadeh, S. M.; Rashidi, M.; Puddephatt, R. J. 6 Organometallics , 2012 , 31 , 2357 2366. 7 (29) For iodine effects in transition metal catalyzed reactions see: Maitlis, P. M.; Haynes, 8 9 A.; James, B.R.; Catellani, M.; Chiusoli, G.P. Dalton Trans. 2004 , 21 , 3409 3419. 10 (30) Reactive Intermediates. MS Investigations in Solution ; Santos, L. S., Ed; Wiley VCH: 11 Weinheim, 2009. 12 (31) Robinson, P. S. D.; Khairallah, G. N.; da Silva, G.; Lioe, H.; O’Hair, R. A. J. Angew. 13 Chem. Int. Ed ., 2012 , 51 , 3812 3817. 14 (32) (a) Zavras, A.; Khairallah, G. N.; Connell, T.U.; White, J. M.; Edwards, A. J.; 15 Donnelly, P. S.; O’Hair, R. A. J. Angew. Chem. Int. Ed ., 2013 , 52 , 8391 8394; (b) 16 Zavras, A.; Khairallah, G. N.; Connell, T. U.; White, J. M.; Edwards, A. J.; Mulder, R. 17 J.; Donnelly, P. S.; O’Hair, R. A. J. Inorg. Chem., 2014, 53 , 7429 7437. 18 19 (33) (a) Dierkes, P.; van Leeuwen, P. W. N. M., J. Chem. Soc., Dalton Trans. , 1999 , 1519; 20 (b) J. Jover, J; Fey, N.; Harvey, J. N.; Lloyd Jones, G. C.; Orpen, A. G.; Owen Smith, 21 G. J. J.; Murray, P.; Hose, D. R. J.; Osborne, R.; Purdie, M. Organometallics 2012 , 31 , 22 5302. 23 (34) A wide range of homo and hetero bimetallic complexes of dppm have been formed and 24 characterized. For reviews, see: (a) Puddephatt, R. J. Chem. Soc. Rev. , 1983 , 99; (b) 25 Chaudret, B.; Delavaux, B.; Poilblanc, R. Coord. Chem. Rev. , 1988 , 86 , 191. Dppm 26 and related ligands have been widely used to synthesize gold complexes and clusters. 27 For a review: (c) Laguna, A.; Laguna, M. J. Organomet. Chem. , 1990 , 394 , 743. 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 22 Page 23 of 23 Submitted to Organometallics
1 2 3 Table of Contents Graphic: 4 5 6
7 8 X 9 X X I I 10 Au Au I I I AuI I AuIII 11 Au Au Au Au Ph P P Ph 12 Ph P P Ph Ph P P Ph Ph P P Ph Ph Ph Ph Ph Ph Ph Ph 13 Ph 14 15 concerted redox couple mechanism 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 23
Minerva Access is the Institutional Repository of The University of Melbourne
Author/s: Vikse, KL; Zavras, A; Thoinas, TH; Ariafard, A; Khairallah, GN; Canty, AJ; Yates, BF; O'Hair, RAJ
Title: Prying open a Reactive Site for Allylic Arylation by Phosphine-Ligated Geminally Diaurated Aryl Complexes
Date: 2015-07-13
Citation: Vikse, K. L., Zavras, A., Thoinas, T. H., Ariafard, A., Khairallah, G. N., Canty, A. J., Yates, B. F. & O'Hair, R. A. J. (2015). Prying open a Reactive Site for Allylic Arylation by Phosphine- Ligated Geminally Diaurated Aryl Complexes. ORGANOMETALLICS, 34 (13), pp.3255-3263. https://doi.org/10.1021/acs.organomet.5b00287.
Persistent Link: http://hdl.handle.net/11343/57498
File Description: Submitted version