Synthesis and Properties of “Sandwich” Diimine-Coinage Metal Ethylene Complexes

Abstract

Synthesis and full characterization of cationic isostructural “sandwich” diimine-coinage metal ethylene complexes are reported. Ethylene self-exchange kinetics proceeds by an associative exchange mechanism for Cu and Au complexes. The fastest ligand exchange was observed for Ag complex 8a. The upper limit of ΔG^‡, assuming associative ligand exchange, was found to be ca. 5.0 kcal/mol. Ethylene self-exchange in Cu complex 7b proceeds with ΔG₂₉₈^‡ = 12.9 ± 0.1 kcal/mol, while the exchange is the slowest in Au complex 9, with ΔG₂₉₈^‡ = 16.7 ± 0.1 kcal/mol. Copper complex 7b is unusually stable and can survive in air for years.

Graphical abstract

1. INTRODUCTION

Transition-metal-alkene complexes are important intermediates in catalysis. Many processes, such as olefin polymerization, hydrogenation, metathesis, and cyclopropanation, involve the intermediacy of metal-alkene complexes.^1,2a The olefin complexes of group 11 metals are less common and become increasingly rare moving from copper to silver to gold despite their importance in cyclopropanation and carbon–hydrogen bond functionalization.² In fact, the first gold-ethylene complex was structurally characterized only recently.³ Furthermore, direct comparison of all three coinage metal-ethylene complexes is difficult, as very few have been synthesized with analogous supporting ligands.⁴ Ligand exchange kinetic data have not been obtained for isostructural group 11 metal olefin complexes. Moreover, few monomeric cationic coinage metal-ethylene complexes have been crystallographically characterized.^4a–e,5

We have developed a general synthesis of 8-aryl-1-naphthylamines based on C–H bond functionalization.⁶ In collabo ration with the Brookhart and Coates groups, palladium and nickel complexes of “sandwich” diimines derived from 8-aryl-1-naphthylamines were used for olefin polymerization (Figure 1).⁷ Interestingly, complex 2a catalyzes living polymerization of ethylene at room temperature. This was attributed to the increased axial steric bulk provided by the capping arene groups.^7b Consistent with living polymerization and increased axial bulk, the rate of associative ethylene exchange in 2b is exceedingly slow compared to other palladium diimine systems.^7b

Figure 1.

“Sandwich” Diimine Ligands and Their Palladium Complexes.

The axial aryl substituents on the ligand provide steric protection for the coordinated metal center and substantially retard associative ligand exchange. We speculated that coinage metal “sandwich” diimine complexes with ethylene should possess increased stability. We report here the synthesis, characterization, and ethylene exchange kinetics of “sandwich” diimine copper, silver, and gold complexes.

2. SYNTHESIS OF COMPLEXES

2A. Ligand Synthesis

8-(3,5-Dichlorophenyl)naphthalen-1-amine was synthesized by using C–H bond functionalization methodology developed earlier.⁶ The synthetic procedure is summarized in Scheme 1. Picolinic acid 1-naphthylamide was coupled with 3,5-dichloroiodobenzene to yield N-(8-(3,5-dichlorophenyl)-naphthalen-1-yl)picolinamide (4) in 77% yield. The reaction is easily scalable and can be performed on at least an 80 mmol scale. The reaction requires a Pd(OAc)₂ catalyst, AgOAc as a base, and a halide removal agent and proceeds in the absence of solvent. Amide hydrolysis yields 8-(3,5-dichlorophenyl)naphthalen-1-amine (5). Synthesis of 2,3-butanedione diimine ligand 6 was accomplished in 86% yield.

Scheme 1.

Synthesis of Diimine Ligand 6

2B. Synthesis and Properties of Copper Complexes 7a and 7b

Treatment of a dichloromethane solution of ligand 6 with (CuOTf)₂PhH in the presence of ethylene afforded triflate 7a in 85% yield as a red solid (Scheme 2). The corresponding hexafluoroantimonate salt 7b was synthesized by the reaction of Cu(C₂H₄)₃SbF₆^4a with 6 in ethylene-saturated CH₂Cl₂. Complex 7b was obtained in 81% yield as an orange solid after crystallization by layering pentane over the solution of 7b in CH₂Cl₂ at −20 °C. Pure 7a and 7b are relatively stable and can be stored outside the glovebox at room temperature under an inert atmosphere for years without any signs of decomposition, as evident by ¹H NMR spectroscopy and single-crystal crystallography. Furthermore, solid 7b is stable if kept on the bench under air for years, and it does not lose complexed ethylene if kept under vacuum for at least 30 min. The unusual stability of 7a and 7b relative to other known Cu-ethylene complexes may be related to steric protection provided by the axially positioned aryl groups on the ligand.

Scheme 2.

Synthesis of Copper Complexes 7a and 7b

Complexes 7a and 7b were characterized by ¹H and ¹³C NMR spectroscopy and elemental analysis. The ¹H NMR spectrum of 7a in CDCl₃ shows a broad resonance centered at 4.01 ppm that is assigned to the signal of the ethylene ligand protons. In contrast, complex 7b shows two resolved signals for ethylene protons at 4.01−3.93 (m, 2H) and 3.86−3.77 ppm (m, 2H) in CD₂Cl₂. A carbon-13 signal for complexed C₂H₄ was not observed in the ¹³C NMR spectra of 7a from −20 to +40 °C. However, the shift for bound C₂H₄ can be located using 2D-HMQC NMR spectra, and it appears at 88.9 ppm. For 7b, the bound ethylene signal in the ¹³C spectrum appears at 88.0 ppm. The assignment of the signal was verified by 2D- HMQC NMR spectra. The complexed ethylene signals in the ¹H and ¹³C spectra are substantially upfield compared with those of other monomeric cationic copper-ethylene complexes and are similar to shifts observed in neutral complexes.^4a,f,5,8

Crystals of 7a suitable for X-ray analysis were grown by layering Et₂O over the solution of the complex in CH₂Cl₂ at −20 °C. The ORTEP diagram of complex 7a is shown in Figure 2. The solid-state structure of 7a shows an ethylene ligand coordinated to copper in a typical η² fashion. Ethylene is nearly coplanar with the α-diimine ligand and the ligand chelates the copper ion with a bite angle of 81.94(16)°. The complexed ethylene C═C distance is 1.346(8) Å, which is slightly lengthened compared to the C═C distance of free ethylene (1.337 Å) and is fairly typical for the values observed in other copper(I)-ethylene complexes with bidentate nitrogen- donating ligands.^4f,5 The Cu–C(ethylene) distances for 7a are 1.972(5) and 2.016(5) Å. The aryl groups of the ligand are centered over the five-membered chelate ring. Interestingly, the ortho-carbons of the dichlorophenyl groups (C22, C38) show a relatively close approach to Cu(1) at 3.17 and 3.14 Å.

Figure 2.

ORTEP view of the molecular structure of 7a. Thermal ellipsoids are drawn to encompass 50% probability. Hydrogens and the triflate counterion are omitted for clarity. Selected bond distances (Å) and angles (deg): C(1)–C(2) 1.346(8), Cu(1)–C(1) 2.016(5), Cu(1)–C(2) 1.972(5), Cu(1)–N(1) 1.970(4), Cu(1)−N(2) 1.986(4), N(1)–Cu(1)–N(2) 81.94(16).

2C. Synthesis and Properties of Silver Complexes 8a and 8b

Silver-ethylene complexes are substantially rarer than the corresponding copper complexes, presumably due to their instability that is caused by the more oxidizing nature of Ag(I) and weaker binding of olefins to Ag.⁹ Reaction of AgSbF₆ with ligand 6 in the presence of ethylene in CH₂Cl₂ at −75 °C afforded complex 8a as a dark orange, air-, temperature-, and light-sensitive solid (Scheme 3). It decomposes at room temperature within 1 day as a solid and within a few hours in solution under an inert atmosphere. It can be stored for several months as a solid under an inert atmosphere at −20 °C. The corresponding tetrafluoroborate complex 8b was obtained similarly in 88% yield as an orange-brown, sensitive solid.

Scheme 3.

Synthesis of Silver Complexes 8a and 8b

Complexes 8a and 8b were characterized by ¹H and ¹³C NMR spectroscopy and elemental analysis. The signal of the ethylene ligand protons appears as a singlet at 4.94 ppm in CDCl₃ (8a) or CD₂Cl₂ (8b) solvents. Carbon-13 signals for complexed C₂H₄ appear at 105.4 (8a) and 105.9 (8b) ppm. The assignment of complexed ethylene ¹³C signals was verified by DEPT-135 or 2D-HMQC experiments. The complexed ethylene signals in ¹H and ¹³C spectra are shifted somewhat upfield compared with those observed for other monomeric cationic silver-ethylene adducts.^4a–c,f However, the dearth of well-characterized cationic Ag complexes and their structural differences with 8 do not allow for meaningful comparisons.

Crystals of 8b suitable for X-ray analysis were grown by layering methylcyclohexane over the solution of 8b in CH₂Cl₂ at −20 °C. The ORTEP diagram of 8b is shown in Figure 3. The bound ethylene C═C bond length is 1.348(10) Å, which is slightly lenghthened compared to the C═C distance of free ethylene (1.337 Å), and falls within the values expected for silver-ethylene complexes. Less than 20 silver-ethylene complexes have been structurally characterized.^4a–c,10,11 Among them, the only examples of cationic complexes are tris(ethylene)Ag cations reported by Dias and Krossing.^4a–c

Figure 3.

ORTEP view of the molecular structure of 8b. Thermal ellipsoids are drawn to encompass 50% probability. Hydrogens, CH₂Cl₂, methylcyclohexane, and the tetrafluoroborate counterion are omitted for clarity. Selected bond distances (Å) and angles (deg): C(1)–C(1) 1.348(10), Ag(1)–C(1) 2.237(4), Ag(1)–N(4) 2.241(3), N(4)–Ag(1)–N(4) 72.46(13).

2D. Synthesis and Properties of Gold Complex 9

Well-characterized gold-ethylene adducts are particularly rare. Only six complexes appear to be characterized by X-ray crystallog raphy.^3,4d–f,12 Specifically, three fluorinated Tp-Au(ethylene) complexes and one complex supported by a fluorinated triazapentadiene have been reported by Dias.^3,4f,12 Two Au(C₂H₄)₃ cationic complexes were structurally characterized by Dias and Krossing.^4d,e Cationic gold-ethylene complexes supported by nitrogen ligands have not been structurally characterized.

Complex 9 was synthesized by treating tris(ethylene)-gold hexafluoroantimonate^4d with ligand 6 (Scheme 4). It was isolated in 42% yield as a dark red solid. Complex 9 decomposes in solution within an hour at room temperature. Therefore, it is imperative that 9 is prepared at low temperatures to obtain pure material. Solid 9 can be stored at −20 °C under an inert atmosphere for several months. Crystals of 9 suitable for X-ray analysis were grown by layering pentane over the solution of 9 in CH₂Cl₂ at −20 °C. The ORTEP diagram is shown in Figure 4.

Scheme 4.

Synthesis of Gold Complex 9

Figure 4.

ORTEP view of the molecular structure of 9. Thermal ellipsoids are drawn to encompass 50% probability. Hydrogens and the hexafluoroantimonate counterion are omitted for clarity. Selected bond distances (Å) and angles (deg): C(37)–C(38) 1.455(13), Au(1)–C(37) 2.118(8), Au(1)–C(38) 2.094(8), Au(1)–N(1) 2.199(5), Au(1)–N(2) 2.196(5), N(2)–Au(1)–N(1) 73.16(18).

The bound ethylene C═C bond length is 1.455(13) Å, which is longer than the values observed for other structurally characterized Au-ethylene complexes at 1.351 to 1.387 Å. However, meaningful comparison of structures is difficult due to the scarcity of well-characterized gold-ethylene complexes.

3. ETHYLENE SELF-EXCHANGE STUDIES

3A. Copper Complex 7b

Ligand exchange in 16-electron complexes usually proceeds by an associative substitution mechanism.¹³ A two-term rate law is often observed: rate = k_obs[complex], where k_obs = k₁ + k₂[ligand]. The k₁ term arises from associative substitution by solvent, counterion, or impurities. First, the mechanism of ethylene exchange was verified by the dependence of the bound ethylene ¹H NMR line shape on added free C₂H₄. Broadening of the complexed ethylene signal at 3.77–4.01 ppm was observed upon addition of ethylene, showing that ligand exchange is indeed associative.

Second, media-promoted ethylene loss (k₁ term) was analyzed by variable-temperature ¹H NMR spectroscopy. For C₂-symmetric complex 7b, the k₁ term can be determined by analysis of the complexed C₂H₄ AA′BB′ system in the ¹H NMR spectrum that should collapse to a singlet upon media- promoted ethylene loss (followed by rapid reassociation). Thus, media-promoted ethylene loss in 7b was studied by variable-temperature ¹H NMR in C₂D₂Cl₄. Data were analyzed by line shape analysis of the ethylene region of spectra. First-order rate constants (k_1obs, s⁻¹) were obtained by matching observed ¹H NMR spectra with those simulated using the WinDNMR program during coalescence of the bound ethylene signals.¹⁴ An Eyring analysis¹⁵ of the rate data between 303 and 373 K (see Supporting Information for details) gives ΔG₂₉₈^‡ = 15.9 ± 0.1 kcal/mol, ΔH^‡ = 9.9 ± 0.2 kcal/mol, and ΔS^‡ = −20.1 ± 1.5 eu. The negative value of the entropy of activation is consistent with associative ligand displacement.¹⁵

Third, ethylene exchange between the bound ethylene of 7b and free C₂H₄ was studied by variable-temperature ¹H NMR in C₂D₂Cl₄. Ethylene was added to the solution of 7b via syringe, and the exact quantity of added C₂H₄ (8.8 equiv) was determined by integration of NMR signals. Matching the observed ¹H NMR spectra with those simulated using the WinDNMR program until coalescence of bound and free ethylene signals occurred gave first-order rate constants (k_2obs, s⁻¹). An Eyring analysis of the calculated second-order rate data between 256 and 304 K (see Supporting Information for details) gives ΔG₂₉₈^‡ = 12.9 ± 0.1 kcal/mol, ΔH^‡ = 6.2 ± 0.5 kcal/mol, and ΔS^‡= −22.5 ± 1.7 eu. As expected, activation parameters are consistent with those expected for associative ligand displacement. The k₁ term for media-promoted ethylene exchange does not provide a significant contribution to k_obs since k_1obs ≪ k_2obs.

3B. Silver Complex 8a

Free and complexed ethylene signals in 8a are coalesced into a sharp singlet at all temperatures above ca. 156 K. However, at 146 K in CDCl₂F solvent some broadening can be observed as decoalescence appears to be beginning. An approximate upper limit of ΔG^‡ can be calculated at 146 K using the fast exchange approximation and assuming that the observed broadening of the coalesced signal is due to a slowed exchange rate. The upper limit of ΔG^‡, assuming associative ligand exchange by analogy with 7b, was estimated to be ca. 5.0 kcal/mol (see Supporting Information for details). The number is approximate due to the following reasons. First, the equation used for equally populated sites was employed. Second, some broadening of other signals was also observed at 146 K, indicating broadening could be due to other effects. Hence, the calculated ΔG^‡ value was considered as a higher limit of ΔG^‡.

3C. Gold Complex 9

Ethylene exchange between 9 and C₂H₄ is slow on the NMR time scale at temperatures up to 50 °C. At those and higher temperatures, complex 9 is unstable and decomposes to colloidal gold. Consequently, the exchange rate could not be determined by NMR line-broadening experiments. Complex 9-d₄ was prepared in 81% yield by purging a solution of 9 in CH₂Cl₂ with C₂D₄ at −30 °C. ¹H NMR analysis indicated 97% conversion to the labeled complex. An excess of C₂H₄ was added, the exact concentration of C₂H₄ was measured by NMR, and the rate of incorporation of unlabeled ethylene into 9 was monitored by ¹H NMR spectroscopy at 199, 204, 209, and 214 K (Table 1). An Eyring analysis of the rate data (Figure 5) gives ΔG₂₉₈^‡ = 16.7 ± 0.1 kcal/mol, ΔH^‡ = 10.0 ± 1.4 kcal/mol, and ΔS^‡ = −22.5 ± 4.8 eu. The negative value of the activation entropy is consistent with an associative ligand displacement mechanism.

Table 1.

Ethylene Exchange between (Cl₂ArN(CMe))₂Au(C₂D₄)SbF₆ (9-d₄) and C₂H₄ in CD₂Cl₂

entry	T (K)	[C2H4] (equiv)	[C2H4] (M)	kobs (10−4 s−1)	kexch (10−4 M−1 s−1)	activation paramsa
1	199	15.0	0.30	1.38 ± 0.10	4.59 ± 0.6	ΔG298‡ = 16.7 ± 0.1
2	204	13.5	0.27	2.59 ± 0.16	9.59 ± 1.1	ΔH‡ = 10.0 ± 1.4
3	209	7.80	0.16	2.58 ± 0.24	16.3 ± 2.5	ΔS‡ = −22.5 ± 4.8
4	214	7.90	0.16	4.73 ± 0.48	30.1 ± 4.4

^aΔG₂₉₈^‡ (kcal/mol), ΔH^‡ (kcal/mol), ΔS^‡ (eu).

Figure 5.

Eyring plot for ethylene exchange in 9-d₄.

4. DISCUSSION

We have synthesized and characterized a complete set of group 11 metal complexes possessing the same “sandwich” diimine supporting ligand. Only a few such complete series of coinage metal complexes have been structurally characterized. Krossing and Dias have reported a series of M(C₂H₄)₃⁺X⁻ complexes, where M = Cu, Ag, and Au and X⁻ = noncoordinating counterion.^4a–e Bound ethylene C═C bond lengths vary from 1.33 to 1.36 Å (M = Cu) to 1.32–1.33 Å (M = Ag) to 1.364 Å (M = Au). Dias has disclosed complexes of the general structure [PhB(3-C₂F₅)Pz)₃]M(C₂H₄), where M = Cu, Ag, and Au.^4f In this series, d_C═C for complexed ethylene is 1.354(7) Å (M = Cu), 1.311(5) (M = Ag), and 1.366(12) (M = Au). For our series, bound ethylene C═C bond distances are 1.346(8) Å (M = Cu), 1.348(10) Å (M = Ag), and 1.455(13) Å (M = Au). The trend is the same for all three sets, with the greatest complexed ethylene C═C bond lengthening observed for Au and the smallest for Ag and Cu complexes. Results are summarized in Table 2.

Table 2.

Summary of the Properties of Complexes

complex	1H NMR δ (C2H4), ppma	13C NMR δ (C2H4), ppma	ΔG298‡ (kcal/mol)	complex	dC═C (Å)	dM–C (Å)	dM–N (Å)	N–M–N (deg)
7b	4.01−3.93; 3.86−3.77	88.0	12.9 ± 0.1	7a	1.346(8)	1.972(5) 2.016(5)	1.970(4) 1.986(4)	81.94(16)
8a	4.94	105.4	<5.0b	8b	1.348(10)	2.237(4)	2.241(3)	72.46(13)
9	3.31−3.30; 3.28−3.27	65.4	16.7 ± 0.1	9	1.455(13)	2.118(8) 2.094(8)	2.199(5) 2.196(5)	73.16(18)

^aMeasured in CD₂Cl₂ for 7b and in CDCl₃ for 8a and 9.

^bΔG₁₄₆^‡.

Furthermore, we have determined ethylene self-exchange energetics for complexes 7b, 8a, and 9. This study represents the first kinetic study of olefin exchange in isostructural coinage metal complexes. The ethylene self-exchange rate for Ag complex 8a is too fast to be measured even at 146 K. Estimation of the upper limit of ΔG^‡ gives a value of 5.0 kcal/mol. In contrast, the corresponding values for copper and gold complexes 7b and 9 are ΔG₂₉₈^‡ = 12.9 ± 0.1 kcal/mol, ΔH^‡ = 6.2 ± 0.5 kcal/mol, ΔS^‡= −22.5 ± 1.7 kcal/mol (7b) and ΔG₂₉₈^‡ = 16.7 ± 0.1 kcal/mol, ΔH^‡ = 10.0 ± 1.4 kcal/mol, ΔS^‡= −22.5 ± 4.8 kcal/mol (9). The exchange rate shows the following trend: Ag (second transition series) ≫ Cu (first transition series) > Au (third transition series).

Inspection of the literature shows several other cases where the ligand exchange rates follow the order 4d metal >3d metal >5d metal. Carbon monoxide substitution by phosphine ligands in CpM(CO)₂ (M = Co, Rh, Ir) systems shows first-order kinetics for both the metal complex and incoming ligand, suggesting an associative substitution mechanism.¹⁶ The substitution rates follow the trend Rh > Co > Ir. Interestingly, the tendency observed in CpM(CO)₂ and “sandwich” diimine-coinage metal-ethylene complexes is consistent with two studies where dissociative ligand substitution was investigated. Nickel triad [(EtO)₃P]₄M complexes show the fastest exchange for Pd and the slowest for Pt species, with the Ni complex demonstrating an intermediate substitution rate.¹⁷ Further more, substitution reactions of group VI metal hexacarbonyls show the fastest exchange rates for the molybdenum complex.¹⁸ Metal–ligand bond energies in 14-electron M(C₂H₄)₂⁺ species also follow the trend Au > Cu > Ag.¹⁹ Results obtained in our study provide another example of a case where the complex of the second transition series element shows the fastest ligand exchange rates.

5. CONCLUSIONS

We have synthesized and structurally characterized isostructural “sandwich” diimine-coinage metal ethylene complexes. Ethylene self-exchange kinetics in these complexes proceeds by an associative exchange mechanism for Cu and Au complexes. The fastest ligand exchange was observed for Ag complex 8a, where the exchange rate is too fast to be accurately measured even at 146 K. Ethylene self-exchange in Cu complex 7b proceeds with ΔG₂₉₈^‡ = 12.9 ± 0.1 kcal/mol, while the exchange is the slowest in Au complex 9, with ΔG₂₉₈^‡ = 16.7 ± 0.1 kcal/mol. Copper complex 7b is unusually stable and can survive in air for years.